vol49_1_2006def 103 annals of geophysics, vol. 49, n. 1, february 2006 key words fourier transform imaging spectrometer – stationary sagnac interferometer – spectral calibration – hyperspectral remote sensing 1. introduction recent advances in optics technology and on-board pre-elaboration data facilities have driven the attention of the international scientific community towards the development of the so called «stationary imaging interferometers» for earth remote sensing purposes (junttila, 1992; horton, 1996). these interferometers do not employ any moving part to scan the imaged field of view (fov) and generate the entire interference pattern simply moving over the surface of the observed target (junttila, 1991; descour, 1996; horton et al., 1997). the launch of the first fourier transform hyperspectral imager (fthsi) on board of us department of defence (dod) technological satellite mightysat ii.1 was an attempt to overcome the main drawbacks that limit the use of push-broom and whisk-broom imaging spectrometers for environment investigation (meigs et al., 1997; otten iii et al. 1997, 1998). these limitations concern sensor calibration, electronics complexity and the circumstance that spectral parameters such as spectral resolution and sampling step cannot be changed during the flight. system precursors of fthsi had been the hypercam and the ircam developed by kestrel corporation (u.s.a.) for airborne applications, and the spatially modulated fourier transform spectrometer (smifts) developed by hawaii university. the main properties of these instruments are listed in table i. in a stationary interferometer all the impinging radiation is collected by the detector for interferogram sampling (jacquinot and fellgett advantages) (jacquinot, 1954; bennett et al., 1993; persky, 1995). this is a significant imrecent advances in earth remote sensing: fourier transform stationary hyperspectral imagers alessandro barducci, paolo marcoionni and ivan pippi istituto di fisica applicata «nello carrara» (ifac), cnr, firenze, italy abstract future trends for the development of new remote sensing imagers have being defined since the launch of the first fourier transform hyperspectral imager (fthsi) on board of dod technological satellite mightysat ii.1. starting from the analysis of fthsi optical configuration we have proposed an interesting modification which produces an image of the observed surface superimposed to a stationary interference pattern. this new optical arrangement together with the possibility to accommodate the spectral resolution by changing the device optical aperture and the sensor sampling step make the new instrument interesting for earth remote sensing purposes. in this paper we present some preliminary results obtained from a laboratory prototype developed at our institute. some hints are discussed about the use of such an instrument on board of airborne and satellite platforms. mailing address: dr. ivan pippi, istituto di fisica applicata «nello carrara» (ifac), cnr, via madonna del piano 10, 50019 sesto fiorentino (fi), italy; e-mail: i.pippi@ifac.cnr.it 104 alessandro barducci, paolo marcoionni and ivan pippi provement of the signal-to-noise ratio (snr) because all the photons coming from a pixel of the source contribute to the interferogram power, no matter what their wavelength is. on the contrary, filters, gratings and prisms of a standard spectrometer reject most of the electromagnetic radiation field. the principal critical points of an imaging interferometer are connected with the spreading of the observed target pixels originated by the not-entirely compensated instrument motion, and the high data-rate requested. starting from the analysis of fthsi optical configuration we have developed a laboratory prototype of the imaging interferometer. in section 2 we briefly discuss the adopted optical configuration and the experimental activity we have performed in order to calibrate the instrument response. in section 3 we give our conclusions. 2. experimental activity the laboratory device uses a new image plane interferometer geometry to produce «autocorrelation function modulation» in the image plane of the two dimensional array such as the phase offset of the modulation linearly varies across the images. the interferogram is imaged onto a ccd array as a spatial distribution of intensity rather than as a function of time. the typical optical layout of the developed interferometer, called sagnac configuration, is shown in fig. 1. the characteristics of the ccd used to spatially sample the interferogram are listed in table ii. as a two dimensional image is formed by the fore-optics, an entire frame is recorded for each autocorrelation phase offset. the three dimensional array of this data is processed to generate table i. the main properties of fthsi and its precursors. instrument fthsi hypercam ircam smifts spectral range 350-1050 nm 450-1050 nm 1700-5000 nm 1000-5200 nm 3000-5000 nm spectral resol. 85.4 cm–1 87 cm–1 45 cm–1 95 cm–1 (1.7 nm@450 nm) (50 nm@3000 nm) 35 cm–1 channels 256 180 55 256 field-of-view 1.75° 13° 14° 13° altitude 570 km, 3 km 3 km sun-synchronous orbit fig. 1. layout of the stationary interferometer arranged in the triangular geometry. the light from the object is first collimated by the objective l, it enters the device, it exits from it at 90° though two folding mirrors (m1 and m2) after being split into two coherent beams by the beam-splitter bs put at 45° with respect to the optical axes. p collocates the lens which focuses the energy on the ccd plane. 105 recent advances in earth remote sensing: fourier transform stationary hyperspectral imagers an autocorrelation function data-cube which is fourier transformed to yield a wavenumber hyperspectral data-cube. the fundamental law, which describes the intensity i(opd) falling on the photosensitive element (neglecting the optical losses) is (2.1) where i0(m) is the intensity of the ray before entering the interferometer, opd is the optical path difference introduced by the beam splitter for every direction of propagation, and k = 1/m is the wavenumber relative to m. as can be seen from eq. (2.1), the interferogram is an oscillating function which has a maximum at opd = 0 and decays for large opd’s. actually, an interferogram is constituted by a pair of values p, dn(p) which respectively indicate the pixel position on the matrix and the corresponding electronic signal expressed in digital number. as shown in eq. (2.2), the spatial coordinate p of the pixel can be linked to the sampling step determined as the distance p (pitch) between the centres of two adjacent pixels (2.2) j being the index of the acquired interferogram and j0 the position of the pixel corresponding to the null opd. as we have stated, the relation between opd and the propagation direction j of a single ray entering the interferometer is linear, as long as the imaged fov is small enough ( )j j p0= -p ( ) ( ) ( )opd cos opdi i k k dk 2 1 2 0 = + r6 @# (2.3) f being the focal length of the lens which focuses the interference image, and a expresses the direct proportionality between opd and j. in order to calibrate in wavenumber (or in opd values) the response of the interferometer we employed a he-ne laser (mexc = 632.8 nm) illuminating a double planar diffuser to produce uniform (homoge( ) ( )opd a f a f a j j p0= = = -j j p . table ii. principal characteristics of the ccd array used to sample the interferogram. type ccd frame-transfer with anti-blooming, th7887a number of pixel 1024 × 1024 pixel size 14 nm × 14 nm spectral range 430 nm-1000 nm @qe>3% responsivity 23dn/(nj cm2) @450 nm 18% @680 nm dynamic range 3200:1 maximum frame/rate 60 fps digitalization 12 bit fig. 2. raw (level 0) image (in gray-scale) obtained illuminating a double planar diffuser with a he-ne laser. we observe a pattern of vertical fringes superimposed to the target image. 106 alessandro barducci, paolo marcoionni and ivan pippi neous and isotropic) radiation field at the instrument entrance. figure 2 shows a single imageframe obtained with the laser source. as can be seen, the instrument fov is entirely filled with straight stripes (also called «fringes of equal thickness»), due to electromagnetic interference overlapping the flat input image. the shape of this pattern is due to the circumstance that all pixels on a vertical line go through the same opd, so that they undergo the same amount of interference for the same wavelength of light. conversely, the high number of these fringes is due to the high intrinsic coherence-degree of the employed source. the image here presented was pre-processed according to the following scheme: fig. 4. un-calibrated spectral radiance retrieved from the interferogram shown in fig. 3 after offset subtracting. fig. 3. interferogram acquired onto the ccd plane relative to an impulse-like source, having a bandwidth far below the spectral resolution of the imaging system. the employed source is a he-ne laser. the solid line indicated the fitting function used to retrieve the pixel position corresponding to the null opd. 107 recent advances in earth remote sensing: fourier transform stationary hyperspectral imagers – dark signal subtraction; – instrument spatial response compensation; – geometrical distortion correction (vignetting, spatial shift of the fringes pattern). then, from the single frame an interferogram averaged over all pixels was extracted. its central part was fitted with a sinc-like function in order to estimate and subtract the dc offset as expressed in eq. (2.1) by the term i0(k) / 2. an ideal interferogram is symmetric with respect to zero path difference and contains cosine contributions only. however, additional phase errors caused by the misaligned sampling grid, which does not match the region of zero path difference, result in an evident asymmetry in a real interferogram shape. figure 3 shows an average interferogram and the fitting function, which best fits its central part. in this plot a strong asymmetry is evident with respect to the pixel scale. after offset subtracting, the new interferogram is fourier transformed to retrieve the uncalibrated spectral radiance of the employed source. figure 4 shows the result of inverse cosine transform obtained without adopting any apodization. 3. conclusions in this paper we have presented preliminary images acquired with a new stationary imaging interferometer operating in sagnac configuration. we have discussed the experimental activity carried out in order to calibrate the interferometer response. in addition we have analyzed a methodology to pre-process the acquired raw interferogram for dark-signal subtraction, instrument spatial response compensation and geometrical distortion correction (vignetting, spatial shift of the fringes pattern). future investigations will address the option to employ this device on board of an airplane for earth remote sensing purposes. references bennet, c.l., m. carter, d. fields and j. hernandez (1993): imaging fourier transform spectrometer, spie proc., 1937, 191-200. cabib, d., r.a. buckwald, y. garini and d.g. soenksen (1996): spatially resolved fourier transform spectroscopy (spectral imaging): a powerful tool for quantitative analytical microscopy, spie proc., 2678, 278291. descour, m.r. (1996): the throughput advantage in imaging fourier-transform spectrometers, spie proc., 2819, 285-290. horton, r.f. (1996): optical design for a high-etendue imaging fourier transform spectrometer, spie proc., 2819, 300-315. horton, r.f., c.a. conger and l.s. pellegrino (1997): high etendue imaging fourier transform spectrometer – initial results, spie proc., 3118, 380-390. jacquinot, p. (1954): the luminosity of spectrometers with prisms, grating, or fabry-perot etalons, j. opt. soc. am., 44, 761-765. junttila, m.l. (1991): performance limits of stationary fourier spectrometers, j. opt. soc. am., a8, 14571462. junttila, m.l. (1992): stationary fourier transform spectrometer, appl. opt., 31, 4106-4112. meigs, a.d., l.j. otten iii, t.yu. cherezova, j.b. rafert and r.g. sellar (1997): lwir and mwir ultraspectral fourier transform imager, spie proc., 3221, 421428. otten iii, l.j., a.d. meigs, b.a. jones, p. prinzing, s. don fronterhouse, r.g. sellar, j.b. rafert and c. hodge (1997): the engineering model for the mightysat ii.1 hyperspectral image, spie proc., 3221, 412420. otten iii, l.j, a.d. meigs, b.a. jones, p. prinzing and s. don fronterhouse (1998): payload qualification and optical performance test results for the mightysat ii.1 hyperspectral imager, spie proc., 3498, 231-238. persky, m.j. (1995): a review spaceborne infrared fourier transform spectrometer for remote sensing, rev. sci. instrum., 66 (10), 4763-4797. 155_161 adg v5 n01 pole 1.pdf annals of geophysics, vol. 45, n. 1, february 2002 155 long-term trends in f0 f2 over grahamstown using neural networks allon w.v. poole and martin poole department of physics and electronics, rhodes university, grahamstown, republic of south africa abstract many authors have claimed to have found long-term trends in ƒ0f2, or the lack thereof, for different stations. such investigations usually involve gross assumptions about the variation of ƒ 0 f 2 with solar activity in order to isolate the long-term trend, and the variation with magnetic activity is often ignored completely. this work describes two techniques that make use of neural networks to isolate long-term variations from variations due to season, local time, solar and magnetic activity. the techniques are applied to ƒ 0 f 2 data from grahamstown, south africa (26 e, 33 s). the maximum long-term change is shown to be extremely linear, and negative for most hours and days. the maximum percentage change tends to occur in summer in the afternoon, but is noticeably dependent on solar activity. the effect of magnetic activity on the percentage change is not marked. mailing address: dr. allon w.v. poole, department of physics and electronics, rhodes university, g r a h a m s t ow n 6 1 4 0 , r e p u b l i c o f s o u t h a f r i c a ; e mail: a.poole@ru.ac.za 1. introduction the ionospheric quantity ƒ 0 f 2 is well known to vary with season (day number, dn); diurnally (hour lt, hr); and with solar activity and magnetic activity. we need now to consider a fifth variation, which we can call long-term trend (ltt). we approached the problem using two techniques. 2. the techniques technique 1 the methods of training a neural network (nn) have been described elsewhere (poole and mckinnell, 2000) and will not be repeated in detail here. briefly, the nn was trained with all usable hourly ƒ 0 f 2 data from 1973-2000 as output or target data, and the four variables dn, hr, f2 and a16 as concomitant input data. f2 is a two month running mean of the solar 10.7 cm flux, used as a measure of solar activity, and a16 is a two day running mean of the 3 hourly magnetic index, a k , used to measure magnetic activity. after training, the nn produces a value of ƒ 0 f 2 for any combination of the input variables. the choice of two months for f2 and two days for a16 was based on the results of an independent investigation in which nns were trained with input variables of different lengths, the optimum length being chosen as that length which produced the minimum rms error (williscroft and poole, 1996). the nn produces the function f 1 such that ƒ 0 f 2 = f 1 (dn, hr, f2, a16). we will call ƒ 0 f 2 evaluated in this way ƒ 0 f 2 (nn). the function f1 thus embodies the variation of ƒ0f2 for all combinations of the four input variables, so that the residuals r evaluated according to r = ƒ 0 f 2 (measured) ƒ 0 f 2 (nn) will key words long-term trends neural networks f 0 f 2 ionosphere 156 allon w.v. poole and martin poole fig. 1a,b. the residual r plotted as a function of time for hr = a) 12 h 00 and b) 00 h 00, each with a fitted linear regression line. a b long-term trends in f 0 f 2 over grahamstown using neural networks 157 be independent of dn, hr, f2 and a16. the residuals are due to short term, seemingly random and chaotic deviations of measured ƒ0 f2 from the model f 1 . however, the residuals will contain information about long-term variation, if it exists, since a variable representing ltt was not included in the input to the nn. accordingly, we computed the residuals r for each datum used in the training. these r were then grouped by hour and plotted against time. the results are shown in figs. 1a,b for 12 h 00 and 00 h 00 respectively. a trendline has been fitted to both, the slope of which gives the average rate of change of the residuals with time in mhz/year. this method of plotting the residuals versus time is similar in principle to that used by foppiano et al. (1999), and upadhyay and mahajan (1998). it is also of interest that when the trendline was fitted to the two groups (1973-1986) and (1987-2000) separately, almost identical slopes were obtained, indicating a negligible second derivative with respect to time. technique 2 for this treatment we included the index (1-245448) which measured the chronological position of each hourly datum (1 = 00 h 00, 1 january 1973; 245448 = 23 h 00, 31 december 2000) as an indicator for ltt. we trained a nn with this extra input to create a function f2 (dn, hr, f2, a16, ltt) and then interrogated this network with appropriate synthetic data to determine long-term trends for a variety of situations. to show the linearity of the general decline in ƒ0 f2, the nn was interrogated at 5 equally spaced times during the total period, corresponding to ltt = 10 000, 60 000, 110 000, 160 000 and 210 000, for the 16 combinations of dn = 81, 172, 265, 356 and hr = 00 h 00, 06 h 00, 12 h 00 and 18 h 00, for low solar activity and low magnetic index. these are presented in the 16 graphs in fig. 2. the chosen daynumbers dn = 81, 172, 265 and 356 correspond to autumn equinox, winter solstice, spring equinox and summer solstice respectively. in the diagrams of fig. 2 the index ltt along the x-axis has been converted back to years for clarity. the values of ƒ 0 f 2 and the error bars are formed by taking the mean, and standard deviation of the mean, of 20 neural networks all trained with the same data but with unique, arbitrary and random starting conditions. because nns proceed to their final value by an iterative process involving least squares, they do not provide unique solutions, and need to be averaged to minimize this statistical variation. the calculated uncertainty in the evaluation of ƒ 0 f 2 from the nns varies slightly with the input parameters, but is of the order of 0.03 mhz, well below the long-term changes made evident by this investigation. in this context, «low» is the lower quartile value of all the f2 or a16 data in the period 1973-2000. we have diagrams similar to fig. 2 for the three other combinations of (f2, a16) = (low, high), where «high» is similarly the value of the upper quartile. these diagrams are similar to fig. 2 but differ in the magnitudes of the slopes, and are not presented here. figure 2 is presented to illustrate the extreme linearity of the decreases, where present. because of this linearity, it is meaningful to express the change as a simple difference between the values given by f2 for ltt = 10 000 and ltt = 210 000, a separation in time corresponding to 22.83 years. we calculated the quantity ƒ0f2 (dn, hr) = f2(dn, hr, l, l, 210 000) f 2 (dn, hr, l, l, 10 000) and plotted it in two dimensions against dn (converted to months) and hr in fig. 3a-d. the fig. 3a shows a general negative change in ƒ0 f2 with time, with peaks occurring as shown in table i. there is a small positive change of + 0.07 mhz which peaks at (dn, hr) = (196, 18 h 00). the values of the other input parameters f2 and a16 for each of the figs. 3a to 3d are given in table ii. in table ii, the symbols l and h refer to the lower and upper quartile values of f2 and a16, evaluated over the period 1973-2000, and so represent «low» and «high» values of solar and magnetic activity. figure 3a is thus the response for low solar and low magnetic activity. figures 4a to 4d show the same differences, but presented as a percentage change according to ƒ 0 f 2 % = [ƒ 0 f 2 (nn, ltt = 210 000) ƒ 0 f 2 (nn, ltt = 10 000)] × 100 / ƒ 0 f 2 (nn, ltt = 10 000). 158 allon w.v. poole and martin poole f ig . 2. e va lu at io ns o f th e n eu ra l n et w or k fu nc ti on f 2( d n , h r , f 2, a 16 , l t t ) fo r va ri ou s co m bi na ti on s of d n a nd h r ,w it h f 2 = l ( lo w ), a 16 = l (l ow ). t he f iv e po in ts o n ea ch g ra ph c or re sp on d to f iv e eq ua ll y sp ac ed v al ue s of l t t b et w ee n 10 00 0 an d 21 0 00 0. long-term trends in f 0 f 2 over grahamstown using neural networks 159 fig. 3a-d. contour maps of the function ƒ0f2 versus dn and hr for f2, a16 = a) ll; b) hl; c) lh, and d) hh. a b c d 3. discussion and conclusions the slope of the regression line through the 12 h 00 residuals shown in fig. 1a was found to be 0.01479 ± .00012 mhz/year, calculated using standard techniques. the small value of the uncertainty in the slope attests to the statistical reliability of the result, and is a consequence of the large number (8083) of points in the regression. note that these residuals include every combination of dn, f2 and a16 that was present in the data, and so represent a decrease averaged over all these variables. this is an important point because, as will be shown, the decrease is dependent on all these variables to a greater or lesser extent. the second technique, indeed, gives results that are specific for partidn hr ƒ 0 f 2 /[mhz] mhz/year 23 13 h 00 0.41 0.018 286 15 h 00 0.37 0.016 table i. peak values of negative change. figure f2 a16 3(a) l l 3(b) h l 3(c) l h 3(d) h h table ii. upper and lower quartile value of f2 and a16 160 allon w.v. poole and martin poole fig. 4a-d. contour maps of the function ƒ 0 f 2 versus dn and hr for f2, a16 = a) ll; b) hl; c) lh, and d) hh. a b c d cular values of dn, hr, f2 and a16, and can be regarded as an average long-term behaviour for any chosen set of the four input variables. a comparison of the two techniques is possible by choosing an hour (hr = 12 h 00) and averaging ƒ 0 f 2 (12 h 00) over dn = 1 365 for each of the four combinations of f2, a16 = h,l and dividing by the 22.83 year separation. this can be compared with the figure quoted above, and is found to be 0.015 mhz/year, which shows consistency in the two techniques. the equivalent results for 00 h 00 are 0.00244 ± .00067 mhz/year (technique 1) and 0.00557 mhz/year (technique 2) which agree at least in their order of magnitude. a general result is that, at low solar activity, the largest negative percentage change occurs between 09 h 00 and 20 h 00 during late summer (figs. 4a,c). at high solar activity, there are very pronounced negative peaks at around 21 h 00 near the equinoxes. the effect of increased magnetic activity is not marked (compare figs. 4a with 4c, 4b with 4d). note that the contention by, for instance, danilov (2000), that longterm trends in ƒ0f2 could be explained by changes in the spatial and temporal morphology of magnetic storms would not be revealed by these techniques since the influence of such storms is specifically removed from the residuals in technique 1, and specifically catered for in technique 2. we have not, in this publication, attempted an explanation for these quite large negative trends in ƒ 0 f 2 over grahamstown. they appear long-term trends in f 0 f 2 over grahamstown using neural networks 161 to be amongst the largest reported in the literature (foppiano et al., 1999; upadhyay and mahajan, 1998; chandra et al., 1997). we intend to analyse data from other stations before venturing an explanation. however, the methods we have used, involving neural networks to remove the known dependencies, appear to be reliable, and stress the fact that long-term trends are very dependent on season, local time, solar activity and to a lesser extent, magnetic activity. it is thus not possible to make quantitative statements about long-term trends unless one is specific about geophysical circumstances (dn, hr, f2, a16) under which the comparisons are made. these dependencies should provide valuable clues to the reasons for the changes, when applied to other ionospheric stations. references chandra, h., g.d. vyas and s. sharma (1997): longterm changes in ionospheric parameters over ahmedabad, adv. space res., 20, 2161-2164. danilov, a.d. (2000): f 2 -region response to geomagnetic disturbances, j. atmos. sol.-terr. phys., 63, 441-449. foppiano, a.j., l. cid and v. jara (1999): ionospheric longterm trends for south american mid-latitudes, j. atmos sol.-terr. phys., 61, 717-723. poole, a.w.v and l.a. mckinnell (2000): on the predictability of ƒ 0 f 2 using neural networks, radio sci., 35, 225-234. uphadyay, h.o. and k.k. mahajan (1998): atmospheric greenhouse effect and ionospheric trends, geophys. res lett., 5, 3375-3378. williscroft, l.a. and a.w.v. poole (1996): neural networks, ƒ 0 f 2 , sunspot number and magnetic activity, geophys. res. lett., 23, 3659-3662. annals n.6/2003 ok 23/04 1339 annals of geophysics, vol. 46, n. 6, december 2003 key words volcanic eruption – lower ionosphere – atmospheric waves – radio wave absorption 1. introduction the mt. pinatubo volcano erupted in june 1991. this eruption was accompanied by the largest injection of volcanic aerosols into the stratosphere observed in the 20th century. the cloud of erupted aerosols reached altitudes around 30 km. after the mt. pinatubo eruption the stratosphere warmed due to the absorption of radiation by the new aerosols produced by the eruption, particularly at low latitudes (e.g., labitzke, 1994). the absorbed radiation was missing in the troposphere and, therefore, the troposphere cooled. at middle and higher latitudes, the enhanced concentration of volcanic aerosols resulted (with some delay) in a reduction of ozone concentration in the (lower) stratosphere (e.g., mcgee et al., 1994). some 1-2 years after the eruption the ozone reduction-related cooling overwhelmed the original aerosol heating and a mild cooling of the stratosphere appeared (e.g., randell et al., 1995). the pronounced volcano-related changes in the troposphere and stratosphere allow us assume the possibility of changes in the lower ionosphere due to: – changes in tropospheric sources of upward propagating waves (gravity and planetary). – changes in filtering properties of the stratosphere to the upward propagating waves. here such possible changes in the lower ionosphere are studied with the use of radio wave absorption measurements (monitoring) in the lower ionosphere over central europe. possible effects on gravity and planetary wave activity inferred from absorption measurements and on absorption itself are investigated. the existence of the effect of mt. pinatubo on the gravity wave activity inferred from absorption was demonstrated by laštovička et al. (1998). impact of the mt. pinatubo volcanic eruption on the lower ionosphere and atmospheric waves over central europe jan laštovička institute of atmospheric physics, academy of sciences of czech republic, prague, czech republic abstract the very strong volcanic eruption of mt. pinatubo in june 1991 directly affected the troposphere and lower and middle stratosphere. here we look at its effects in the mesopause region as revealed by the radio wave absorption measurements in the lower ionosphere over central europe and inferred planetary and gravity wave activity. the gravity wave activity inferred from the nighttime lf radio wave absorption displays an evident enhancement for waves of periods of about 2-3 h coinciding with regional measurements of the optical depth of (volcanic) aerosols, while there is no detectable effect for short period waves (t < 1 h). there is no detectable effect in the planetary wave activity inferred from the daytime hf radio wave absorption. as for the absorption itself, the results on the impact of the mt. pinatubo eruption do not provide an observable effect. mailing address: dr. jan laštovička, institute of atmospheric physics, academy of sciences of czech republic, boční ii, 14131 prague 4, czech republic; e-mail: jla@ufa.cas.cz 1340 jan laštovička section 2 describes data and methods. section 3 illustrates effects in gravity wave activity. section 4 deals with possible effects in planetary wave activity and section 5 in the absorption itself. the paper ends with brief conclusions. 2. data and methods the radio wave absorption in the lower ionosphere was measured by the a3 method, which consists in receiving a continuous sky wave transmitted usually by a commercial transmitter with an oblique incidence on the ionosphere. the a3 method exists in two versions, low frequency (lf) version and medium/high frequency (mf/hf) version, which differ by the method of calibration and by antenna systems. the lf version in digital modification used by us was described by laštovička et al. (1993). the hf a3 measurements used in the paper were described by laštovička and jiskra (1978) in the older analog version, which was operating till the early 1990s. the lf absorption was measured at pruhonice on 270 khz (reflection point 49.45on, 16.05oe; transmitter-receiver distance 236 km; nighttime reflection height ~ 95 km). the gravity wave activity (in terms of average gravity wave amplitude) was inferred from well nighttime data ( χ > 100o) by the correloperiodogram method applied to each individual night (for details see laštovička et al., 1993). the well-nighttime data are used to exclude the effect of strong daytime variation of reflection height, i.e. of substantial changes in the height region where the absorption is created, on the derived gravity wave activity. moreover, well in daytime there is no measurable signal on the given frequency due to total absorption. the hf absorption was measured at panska ves on 6090 khz (reflection point 50.07on, 10.30oe; transmitter-receiver distance 610 km; daytime reflection height ~ 97-98 km). daytime absorption data at solar zenith angle χ = 75o (average from morning and afternoon data) are used for inferring the planetary wave activity (in terms of average planetary wave amplitude) from consecutive 2 month planetary wave oscillation spectra by the correloperiodogram method (for details see, e.g., pancheva et al., 1989). at night and high solar zenith angles close to sunrise and sunset, radio waves for this radio path are reflected in the f region, i.e. we cannot measure a pure absorption in the lower ionosphere. therefore, only welldaytime data may be used. the aerosol optical depth is considered as a measure of volcanic aerosols. we use data measured at relatively nearby station geesthacht, germany, 53.4on, 10.4oe (ansman et al., 1997). they published data for august 1991march 1995. the pre-pinatubo level of aerosols was taken to be equal to the essentially undisturbed level of january-march 1995. a possible error in estimating the pre-pinatubo level does not play a role with respect to the large pinatubo-related increase of concentration of volcanic aerosols in the stratosphere, which peaked at geesthacht in 1992 and winter/early spring 1993 (see fig. 1). 3. gravity waves the effect of the mt. pinatubo volcanic eruption on the gravity wave activity was studied in central europe based on the nighttime absorption at 270 khz by laštovička et al. (1998) and laštovička (1999). figure 1 shows the development of gravity wave activity in six period ranges between 10-180 min in the winter half of the year (october-march) for winters 1988/1989-1994/1995 (the last two winters october-december only). the measurements became unreliable after 1 january 1995 due to changes of transmitter regime. they were terminated a couple of years ago. all available wintertime data are shown in fig. 1. an evident increase in gravity wave activity after the mt. pinatubo eruption is revealed at long periods (t > 120 min, top curves) in very good coincidence with the large increase in the volcanic aerosol optical depth. this increase in gravity wave activity is well pronounced despite the decreasing solar activity (laštovička (1999) found an increase in gravity wave activity with increasing solar activity). however, no detectable pinatubo-related change of the gravity wave activity at short periods (t < 60 min, bottom curves) can be observed. 1341 impact of the mt. pinatubo volcanic eruption on the lower ionosphere and atmospheric waves over central europe what might be the origin of the longer-period (2-3 h) gravity wave activity response to the mt. pinatubo eruption? the seasonal variation pattern of the gravity wave activity and its change with altitude in the middle atmosphere is significantly affected by the background wind and temperature fields, which affect propagation and dissipation of gravity waves (e.g., gavrilov and fukao, 1999). on the other hand, the solar cycle dependence of the seasonal variation pattern of the gravity wave activity inferred from the 270 khz nighttime radio wave absorption measurements seems to be substantially affected by a shift of the average storm tracks in the northern atlantic region, i.e. by changes in the gravity wave source (bošková and laštovička, 2001). background temperature fields in the troposphere and stratosphere were affected by the mt. pinatubo eruption. as concerns other parameters, sufficient information is not available. nevertheless, some changes in the wind field and maybe tropospheric storm tracks can be expected, which would mean that both the changes in gravity wave sources and filtering properties of the stratosphere could play a role. however, it is not clear at all why the longer period gravity waves are affected, whereas this is not the case for the short period gravity waves. unfortunately, the available data do not enable us to analyze effect of another strong volcanic eruption, that of el chichon in 1982. as far as i know, the impact of volcanic eruptions fig. 1. gravity wave activity (relative amplitude in %) inferred from the nighttime 270 khz absorption measured in central europe, winters of 1988/1989-1994/1995 (after laštovička, 1999). r sunspot number. winter data points – october-december and january-march averages. gravity wave periods (from bottom to top of the figure): ∆ for 10-30 min, + for 31-60 min, for 61-90 min, + for 91-120 min, for 121-150 min, + for 151180 min. 1342 jan laštovička on the gravity wave activity in the upper mesopause region has not been studied by any other author. 4. planetary waves the daytime 6090 khz absorption is used to investigate the potential effect of the mt. pinatubo eruption on the planetary wave activity in the lower ionosphere in central europe. figure 2 displays the development of the planetary wave activity for the period 1971-1996. each data point is taken from a spectrum computed from a two-month interval. we do not see any observable effect of the mt. pinatubo volcanic eruption on the planetary wave activity as deduced from daytime radio wave absorption in central europe. in the 1990s, the only evident change is a decreasing variability of the planetary wave activity towards the end of the period. on the other hand, the planetary wave activity is very noisy, so a small effect of pinatubo would be undetectable. nevertheless such a clear effect like that in the gravity wave activity evidently has not been observed. if the main reason of the effect of pinatubo in the gravity wave activity is a shift in storm tracks, then we should expect rather no effect of pinatubo on the planetary wave activity, which is not affected significantly by shifts of storm tracks. 5. radio wave absorption the 270 khz measurements are reliable till the end of 1993. since january 1994 they are fig. 2. planetary wave activity (amplitude) inferred from the daytime 6090 khz radio wave absorption measured in central europe at the solar zenith angle of 75o, october 1971-august 1996. 1343 impact of the mt. pinatubo volcanic eruption on the lower ionosphere and atmospheric waves over central europe mostly unreliable due to transmitter problems except for a few months (fortunately those allowing construct fig. 1). therefore only the 6090 khz data are used to examine the effect of the mt. pinatubo eruption in radio wave absorption itself. to find the possible effect, matrices of correlation coefficients and partial correlation coefficients are computed and the principal component analysis without and with rotated axes is applied. monthly mean values of the aerosol optical depth, sunspot number (solar activity), aa index (geomagnetic activity) and absorption are used. the results are presented in tables i-iv. tables i and ii show that neither correlation coefficients, nor partial correlation coefficients reveal an effect of the mt. pinatubo eruption on radio wave absorption. the correlations point to the well-known positive relation between absorption and solar activity (via changes of intensity of ionizing radiation). the correlation between volcanic aerosol and sunspots is accidental due to the occurrence of aerosol recovery at the decay branch of the solar cycle. neither standard (table iii), nor rotated (table iv) principal component analysis provides an effect of mt. pinatubo eruption on radio wave absorption. this corresponds to the results based on correlations shown in tables i and ii. table iv again points to some relation between absorption and solar activity (sunspots) and displays the above-explained accidental correlation of aerosol and sunspot cycle. thus we can say that there is no observable effect of the mt. pinatubo volcanic aerosols on daytime radio wave absorption in the lower ionosphere above about 90 km. 6. conclusions all conclusions are based on the radio wave absorption measurements in the lower ionosphere over central europe. i) no observable effects of the mt. pinatubo volcanic eruption were found in the daytime hf radio wave absorption itself and in the planetary wave activity inferred from this absorption. ii) in the gravity wave activity derived from the nighttime lf radio wave absorption, no effect of mt. pinatubo was found for shortperiod gravity waves (t < 1 h). however, a substantial intensification of the longer-period gravity wave activity (t = 2-3 h) was observed. iii) the origin of the gravity wave activity enhancement may be due to both changes in properties of the middle atmosphere and in gravity wave sources. the real origin of this enhancement and an answer to the question while table i. matrix of correlation coefficients. aerosol sunspot aa absorption aerosol 1 0.61 0.09 0.07 sunspot 0.61 1 0.35 0.51 aa 0.09 0.35 1 0.19 absorption 0.07 0.51 0.19 1 table ii. matrix of partial correlation coefficients. aerosol sunspot aa absorption aerosol 1 0.68 – 0.18 – 0.36 sunspot 0.68 1 0.33 0.57 aa – 0.18 0.33 1 – 0.04 absorption – 0.36 0.57 – 0.04 1 table iii. principal component analysis (f 1 + f 2 + + f 3 = 95%). factor f 1 f 2 f 3 % 50% 25% 20% aerosol 0.67 – 0.69 – 0.12 sunspot 0.93 – 0.11 0.06 aa 0.51 0.52 – 0.67 absorption 0.63 0.48 0.58 table iv. varimax rotated principal component analysis. factor f 1 f 2 f 3 f 4 aerosol 0.96 0 0.02 0.26 sunspot 0.40 0.32 0.20 0.83 aa – 0.04 0.08 0.99 0.13 absorption 0.01 0.97 0.08 0.21 1344 jan laštovička only longer-period gravity waves are enhanced is not clear at present and will be the subject of further investigations. acknowledgements this study was supported by the academy of sciences of the czech republic, projects z3042911 and k3012102. references ansman, a., i. mattis, u. wandinger, f. wagner, j. reichardt and t. deshler (1997): evolution of the pinatubo aerosol: raman lidar observations of particle optical depth, effective radius, mass, and surface area over central europe at 53.4on, j. atmos. sci., 54, 2630-2641. bošková, j. and j. laštovička (2001): seasonal variation of gravity wave activity in the lower ionosphere in central europe, stud. geophys. geod., 45, 85-92. gavrilov, n.m. and s. fukao (1999): a comparison of seasonal variations of gravity wave intensity observed by the mu radar with a theoretical model, j. atmos. solar-terr. phys., 56, 3485-3494. labitzke, k. (1994): stratospheric temperature changes after the mt. pinatubo eruption, j. atmos. terr. phys., 56, 1027-1034. laš tovička, j. (1999): solar activity effects on gravity wave activity inferred from radio wave absorption in the lower ionosphere, stud. geophys. geod., 43, 107-128. laštovička, j. and m. jiskra (1978): oblique incidence a3 measurements of ionospheric absorption at 6090 khz, trav. géophys., 472, 461-476. laštovička, j., j. boška and d. burešová (1993): digital measurements of lf radio wave absorption in the lower ionosphere and inferred gravity wave activity, ann. geophysicae, 11, 937-946. laštovička, j., d. bureš ová and j. boška (1998): effect of the qbo and the mt. pinatubo volcanic eruption on the gravity wave activity in the lower ionosphere, stud. geophys. geod., 42, 170-182. mcgee, t.j., p. newman, m. gross, u. singh, s. godin, a.-m. lacoste and g. megie (1994): correlation of ozone loss with the presence of volcanic aerosols, geophys. res. lett., 21, 2801-2804. pancheva, d., e. apostolov, j. laštovička and j. boška (1989): long-period fluctuations of meteorological origin observed in the lower ionosphere, j. atmos. terr. phys., 51, 381-388. randell, w.j., f. wu, j.m. russell iii, j.w. waters and l. froidevaux (1995): ozone and temperature changes in the stratosphere following the eruption of mt. pinatubo, j. geophys. res., 100, 16753-16764. (received june 27, 2003; accepted october 15, 2003) annals 47, 1, 2004, 01/07def 151 annals of geophysics, vol. 47, n. 1, february 2004 key words baikal rift zone – electromagnetic monitoring – transfer functions – resistivity changes of the fault 1. introduction it is generally accepted that one of the possible precursors of earthquake in the seismically active baikal region may be the change in the electrical resistivity of the saturated porous rock in deep-water rift faults. in accordance with the modern concept of the baikal region geoelectrical structure (merklin et al., 1979), there is a narrow fault there that is galvanically connected with a deep-seated conductor. berdichevsky et al. (1989) showed that for two-dimensional (2d) model of marine deep magnetotelluric investigations the vertical electric field at the ocean bottom is highly sensitive to the resistivity of the underlying cross-section. besides, berdichevsky et al. (1996) showed that certain 2d earth models would generate considerable vertical electrical currents if the models include vertical faults of low resistivity. these authors used horizontal magnetotelluric field as an incident field. the latter work deals with magnetotellurics in the lesser caucasus but the first one deals with deep marine magnetotellurics. meanwhile, baikal water is fresh and much more resistive as compared with the oceanic water, however, shneyer et al. (1998) proved that in 2d model of southern baikal the vertical electric field, ez, is again sensitive to the presence of thin vertical fault of low resistivity. though these results are valuable, we decided to explore the behaviour of ez for a threedimensional (3d) baikal model, bearing in mind that the fault is a body of limited length but not of an infinite one. in this paper we build ez-response as a monitor of a baikal rift fault electrical resistivity: 3d modelling studies oleg v. pankratov, alexei v. kuvshinov, dmitry b. avdeev, vitaly s. shneyer and igor l. trofimov geoelectromagnetic research institute, russian academy of sciences, moscow region, russia abstract 3d numerical studies have shown that the vertical voltage above the baikal deep-water fault is detectable and that respective transfer functions, ez-responses, are sensitive to the electrical resistivity changes of the fault, i.e. these functions appear actually informative with respect to the resistivity «breath» of the fault. it means that if the fault resistivity changed, conventional electromagnetic instruments would be able to detect this fact by measurement of the vertical electric field, ez, or the vertical electric voltage just above the fault as well as horizontal magnetic field on the shore. other electromagnetic field components (ex, ey, hz) do not seem to be sensitive to the resistivity changes in such a thin fault (as wide as 500 m). on the other hand, such changes are thought to be able to indicate a change of a stress state in the earthquake preparation zone. besides, the vertical profile at the bottom of lake baikal is suitable for electromagnetic monitoring of the fault electrical resistivity changes. altogether, the vertical voltage above the deep-water fault might be one of earthquake precursors. mailing address: dr. oleg v. pankratov, geoelectromagnetic research institute, russian academy of sciences, 142190 troitsk, p.o. box 30, moscow region, russia; email: o.pankratov@mtu-net.ru 152 oleg v. pankratov, alexei v. kuvshinov, dmitry b. avdeev, vitaly s. shneyer and igor l. trofimov a simple 3d model of southern baikal and verify: 1) whether ez is detectable over the fault, and 2) whether ez-response is sensitive to the resistivity changes of the fault. 2. model lake baikal is located in the southern part of eastern siberia (see fig. 1). it is the oldest existing freshwater lake on earth (20-25 million years old), being the deepest continental body of water. it is 636 km long and 48 km wide. baikal lies in a deep structural hollow surrounded by rock. the fault that we are interested in, is located at the southern part of lake baikal, not far from the town of slydyanka. a simplified 3d resistivity model of the baikal deep fault is shown in figs. 2 and 3. the geometry of the model was taken mainly from seismic data by merklin et al. (1979), whereas the resistivity data were taken from electromagnetic data by popov (1977), and kieselev and popov (1992). in the model the 2 km thick, 40 ω⋅m water layer is underlain by fig. 1. map of the region of lake baikal. modelling region is marked with a rectangle. the fault is marked with a thick line. 153 ez-response as a monitor of a baikal rift fault electrical resistivity: 3d modelling studies the 1 km thick, 60 ω⋅m sedimentary layer. both layers are surrounded and underlain by 1000 ω⋅m rock. below, at a depth of 15 km, there is a 10 km thick conductive layer of 10 ω⋅m. as for the fault in the model, it is 28 km long in y-direction (parallel to the shore) and 0.5 km wide in x-direction (perpendicular to the shore). the vertical size of the fault is 13 km. the fault outcrop is located at lake bottom exactly where the 35° steep slope of the north-western shore of the lake is ending. the shape of the south-eastern shore slope has been proved not to affect the electromagnetic (em) field calculated in the vicinity of north-western shore. thus, the fault appears to be galvanically connecting the deep conductive layer under the lake and the lake itself. 3. numerical modelling in the 3d resistivity model proposed we have performed a series of simulations of the fig. 2. 3d resistivity model of the baikal fault (side view). line a-b is the profile where behaviour of ez is studied. c is the coast site where components hx and hy are taken to obtain ez response (see details in the text). fig. 3. 3d resistivity model (plane view). 154 oleg v. pankratov, alexei v. kuvshinov, dmitry b. avdeev, vitaly s. shneyer and igor l. trofimov vertical electric field, ez, along vertical profile a-b as well as horizontal magnetic field on the shore. this 2 km long profile (from lake surface to the bottom) is located just over the center of the fault (see fig. 2). the amplitude of the incident plane wave electric field is chosen to be equal to 10 mv/km, resembling typical amplitudes of mid-latitude disturbances. period of the incident field is taken to be 100 s and 1 h. while modelling, we varied the fault resistivity, ρfault, the values taken to be 10, 11 and 20 ω⋅m. to perform the simulations we used x3d code which is based on the solution of modified scattering equation by the krylov subspace iterations (avdeev et al., 1997, 2000). the modelling region of 44 km × 80 km × 15 km is divided into 440 × 100 × 14 cells. figure 4 presents the vertical electric field, ez, along vertical profile a-b for ex-polarized incident plane wave, with fault resistivity being 10 ω⋅m. left and right panels of the figure reveal the results for the periods of 100 s and 1 h respectively. the figure demonstrates that the values of |ez| are ranging, depending on depth, from zero (at the surface) up to 9 mv/km (at the bottom). in practice during the future experiment we are going to measure the vertical voltage, v(a, b), between points a and b. in our model, v(a, b) can be calculated as .,v e z dza b z a b = #^ ]h g . the left plot in fig. 4 implies that the vertical voltage, v(a, b), should exceed 5 mv, which is very promising, since 5 mv could be readily detected by conventional em instruments, their measurement precision accounting for 0.01 mv. in addition, it is also seen from the figure that fig. 4. electric field components ez and ex along profile a-b. the source is ex-polarized plane wave. the results are presented for periods 100 s (left panel) and for 1 h (right panel). the fault resistivity is 10 ω⋅m. 155 ez-response as a monitor of a baikal rift fault electrical resistivity: 3d modelling studies near the fault outcrop, ez-field even dominates the primary field ex. note that only real parts of the components are discussed, since imaginary parts are two orders of magnitude less. it can be seen from fig. 4 ez(z) that can be approximated as follows: h e z e b ez z z b . ] ] ] g g g (0 < z ≤ b) (3.1) when we evaluate integral ,v e z dz e z dza b z z b a b 0 = =# #^ ] ]h g g . here b = 2 km is the bottom depth. also, h = 621 m and h = 586 m for period of 100 s and 1 h respectively. as a consequence, the vertical voltage is directly proportional to the vertical electric field measured at the bottom ,v k e ba b z$=^ ]h g (3.2) where proportionality coefficient is k = h (1− e− b/h). we can show that coefficient k does not depend on the amplitude of the incident field. the other consequence: higher values of the vertical electric field are accumulated near the bottom, so that it is not necessary to locate point a exactly at the surface. we can locate point a, say, 100 or 200 m deeper without significant change in the value of v (a, b). as for the other polarization of the incident field (ey-polarized plane wave), the vertical electric field is at least two orders of magnitude less than that for ex-polarized incident field. so far we demonstrated the amplitudes of the fields themselves. but it is known that while monitoring, the external field should be excluded from consideration. for this purpose, we introduce ez-response as the expansion coefficients in ,v u h u ha b zx x r zy y r = +^ h (3.3) where hxr and hyr are the horizontal magnetic field components at the coastal reference site (site c, see figs. 2 and 3). expansion (3.3) can be used in the following way. let v1(a, b) and v 2(a, b) be the vertical voltage values measured for any two different polarizations of the incident field. let (hx1, hy1) and (hx2, hy2) be the horizontal magnetic fields measured at the coastal reference site for the first and for the second polarization respectively. from expansion (3.3) it follows that: , , v u h u ha b a b zx x zy y zx x zy y 1 1 1 2 2 2 = + .v u h u h= + ^ ^ h h (3.4) therefore, we obtain final formulae for transfer functions uzx and uzy as follows , ,detu u h h h h h v v a b a b 1 zy zx y x y x 2 2 1 1 1 2= d e ^ ^ en o h h o (3.5) where deth = (hx1hx2 − hy1hy2). then these transfer functions uzx and uzy are called the ezresponses because bigger values of the vertical electric field, ez, are accumulated near the bottom and because v(a, b) is proportional to ez (b) (eq. (3.2)). table i presents the absolute value of the ezresponse, |uzy|, shown with respect to the fault resistivity and period. table i shows |uzy| transfer function alone, since response uzx appears to be negligibly small compared to uzy. this is due to the fact that only tm-polarized incident field generates major vertical electrical current through the fault. although electromagnetic field components ex, ey, hx, hy, hz appear to be insensitive to the table i. absolute value of ez-response, |uzy|, with respect to the fault resistivity and period. ez-response (mv/nt) 100 s 1 h ρfault = 10 ω⋅m 776 ⋅10 − 9 186 ⋅10 − 9 ρfault = 11 ω⋅m 743 ⋅10 − 9 180 ⋅10 − 9 ρfault = 20 ω⋅m 526 ⋅10 − 9 128 ⋅10 − 9 precision of 14 ⋅10 − 9 5 ⋅10 − 9 experimental ez-response 156 oleg v. pankratov, alexei v. kuvshinov, dmitry b. avdeev, vitaly s. shneyer and igor l. trofimov resistivity of the fault, we still need components hx and hy on the shore in order to obtain transfer functions uzx and uzy. indeed, transfer functions uzx and uzy do not depend on the polarization of the incident field but the vertical voltage does. the second row of table i shows that for a 100 s period, an operator should measure the values of 776, 743, and 526 and distinguish them from each other having the measurement precision equal to 14. obviously it is possible. for a 1 h period, the measurement precision is just enough to distinguish the 10 ω⋅m fault from the 11 ω⋅m fault; and it is far enough to distinguish the 11 ω⋅m fault from the 20 ω⋅m fault. altogether, for both periods the changes in the fault resistivity lead to detectable changes in ez-responses. more explicitly, 10% and 100% fault resistivity changes result in 4% and 30% ez-response changes, respectively. it should be also stressed that traditional impedance responses (simulated but not shown here) have appeared to be practically insensitive to the changes of the fault resistivity. further numerical modelling (performed but not shown here) reveals that the link between the ez-responses and the fault resistivity holds valid for bigger resistivity values. namely, for the values ρfault = 10, 11, 12, 20, 40, 80, 160 and 320 ω⋅m we calculated the ez-responses and found that |uzy (ρfault)| can be approximated as follows: uzy fault fault$.t o t c _ i (3.6) where γ = γ (t ) and ν =ν (t ) depend on the period, t, of the incident field. approximation (3.6) now follows that though we could hardly distinguish the 10 ω⋅m fault from the 11 ω⋅m fault at 1 h period, the 10 ω⋅m fault can easily be distinguished from the 14 ω⋅m fault at this period. though our model is an estimate, we realize that the link between the ez-responses and the fault resistivity is rough enough and it must be detected while in situ measurements. 4. conclusions 3d numerical studies have shown that the vertical voltage is detectable above the baikal deep fault, and that ez-responses are sensitive to the resistivity changes of the fault, i.e. ez-responses appear actually informative with respect to the resistivity «breath» of the fault. further studies should include more detailed modelling and field operations. it should answer the question whether changes in ez-responses (and ρ fault) are connected with changes in a stress state in the fault vicinity and whether ez-responses can be used as one of earthquake precursors. acknowledgements the research has been partly made possible through grants no. 00-05-64182 and no. 00-0564677 from the russian foundation for basic research. we are also grateful for both reviewers’ valuable comments. references avdeev, d.b., a.v. kuvshinov, o.v. pankratov and g.a. newman (1997): high performance three-dimensional electromganetic modelling using modified neumann series. wide-band numerical solution and examples, j. geomag. geoelectr., 49, 1519-1539. avdeev, d.b., a.v. kuvshinov, o.v. pankratov and g.a. newman (2000): 3d em modelling using fast integral equation approach with krylov subspace accelerator, in extended abstracts of the 62nd meeting of eropean association of geoscientist and engineers, vol. 2, p. 183. berdichevsky, m.n., o.n. zhdanova and m.s. zhdanov (1989): marine deep geoelectricity (nauka, moscow), pp. 90. berdichevsky, m.n., v.p. borisova, n.s. golubtsova, a.i. ingerov, yu.f. konovalov, a.v. kulikov, l.n. solodilov, g.a. chernyavsky and i.p. shpak (1996): an experience of interpretaion of mt soundings in the lesser caucasus mountains, fiz. zemli, 4, 99-117 (in russian). kieselev, a.i. and a.m. popov (1992): asthenospheric diapir beneath the baikal rift: petrological constraints, tectonophysics, 208, 287-295 merklin, l.r., v.e. milanovskyi, v.i. galkin and m.v. zakharov (1979): structure of sedimentary layer and basement relief, in gelologic-hydrophysical and deepwater studies at lake baikal, 104-110. popov, a.m. (1977): high conductive deep layers as they found from magnetotelluric data, in deep structure of the baikal rift (nauka, novosibirsk), (in russian). shneyer, v.s., e.yu. sokolova and i.l. trofimov (1998): two-dimensional electromagnetic field modelling results in the south-baikal depression region, in abstract of 4th international conference: modern methods and techniques of oceanological research, moscow (in russian). 097_103 adg v–5 n01 bradley.pdf annals of geophysics, vol. 45, n. 1, february 2002 97 proposed terminology for the classification and parameters for the quantification of variability in ionosphere morphology peter a. bradley (1) and ljiljana r. cander (2) (1) pandora, farnham common, slough, berks, u.k. (2) rutherford appleton laboratory, chilton, didcot, oxon, u.k. abstract much uncertainty currently exists in the use by different workers of the term ‘variability’ in describing the results of statistical analyses applied to ionospheric measurement data sets and in their relationships with various existing or new ionospheric models. often it is not clear whether data for different time periods or different geographical areas, and if so which, are being used to formulate results. terms are presented in the annex which it is suggested should be used, at least once in every publication addressing this topic, to describe unambiguously what is talked about. the background to the proposed terminology is discussed. options for variability parameters are also addressed. 1. introduction parameters characterising ionospheric phenomena are subject both to periodic and irregular temporal variations. ionospheric characteristics from which the height profiles of electron density may be reconstituted for a given location, and vertical total electron content, which is the height-integrated density within unit cross section, experience systematic changes with time-of-day, season and epoch of the solar cycle together with so-called ‘random’ fluctuations about these regular states. there are also small-scale variations with geographical position as well as major repeatable geographical dependencies. for aeronomy investigations and for applications involving the planning and performance assessment of telecommunication systems there is need to quantify both the regular and irregular changes that take place. in most cases such information comes from past measurements which are incomplete in space and time, and where there is need to use matched empirical expressions to provide complete coverage. it is usual to assume that future conditions may be estimated by extrapolation from the past with long-term predictions involving estimates of monthly median conditions based on some solar index. variations about these monthly median figures are quantified statistically. on the other hand, there also exist requirements for short-term forecasts over various time intervals of up to a few hours from the present where a number of current studies are addressing approaches involving update of recent past ‘instantaneous’ measurements mailing address: dr. ljiljana r. cander, rutherford appleton laboratory, radio communications research unit, chilton, oxon ox11 0qx, u.k.; e-mail: l.cander@rl.ac.uk key words ionosphere – ionospheric terminology – space and time variations 98 peter a. bradley and ljiljana r. cander in terms of some near real-time space weather or geomagnetic index monitor. so, as well as modelling median states and generating forecasting algorithms, various different types of temporal and spatial variability specification are needed. these include the separate applications of quantifying the match of median models to past or future measurement data sets from the same or different locations, the statistical characterisation of the day-to-day changes at a given hour and for a given place about these median models, the degree of match of forecasting algorithms to the data sets used in their formulation, a measure of the ‘goodness’ of a particular forecasting algorithm taken from the match to other data sets, and for aeronomy studies measures of variations in time and space over various continuous periods and distances. unfortunately most relevant models and measurement data sets embody a mixture of the above described temporal and spatial changes, so that in order to obtain the above sorts of variability information various approximations have to be applied, with considerable opportunities for error or confusion. this paper aims to present a standardised classification of the different variability types, to be used in future studies to minimise the above problems that have already been identified in certain past analyses. whilst any varying data set can be specified in terms of its associated amplitude probability distribution, such information is too unwieldy and unnecessary for most ionospheric applications. reference distribution parameters are offered quantifying variability magnitude and distribution skew, from which if needed the full distribution or specific percentile points may be given approximately. 2. requirements ionosphere electron densities vary significantly, both geographically at a given time, and at a given location at separate times. the changes that take place are also different at different heights because of the varying relative production, loss and movement influences, and so this means they are not the same for the individual ionospheric characteristics. how then can profile variability be specified? many efforts are directed at generating electron-density height profile models. most are monthly median models, in which for a particular place the separate relevant ionospheric characteristics are modelled as a function of latitude, longitude, time-of-day, season and epoch of the solar cycle (as defined in terms of an appropriate solar-cycle index). all geographical locations are covered, and profiles are reconstituted in terms of these characteristics values using best-available empirical algorithms. the basic input data to the models come from past vertical-sounding or topside measurements, from theoretical considerations involving mean reaction rates and estimates of ionising fluxes, or from some combination of these various data sources. a different case is where models are wanted for individual occasions, such as a particular day and time. in these instances it is usual to generate values of the separate ionospheric characteristics for the occasion of interest, and then to formulate the corresponding height profile on the assumption that all the various mean algorithms still apply, an exercise that at best has to be approximate. when concerned with past epochs, individual characteristics values, so-called ‘instantaneous values’ may come from maps produced for example through the application of geographical smoothing applied to discrete sounding results (samardjiev et al., 1993). ionospheric characteristics instantaneousvalue maps, including where needed maps of tec, and the associated electron-density height profiles may apply for specific past epochs, or they may incorporate a forecast element, and be for a future time. likewise, monthly median characteristics maps and corresponding profile models may be used to ‘predict’ conditions, past or future, for different epochs than those associated with their formulation. from the foregoing it is seen that a number of different sources of variability exist depending on the time periods and geographical locations being considered. it is usual to categorise changes as ‘periodic’ (or predictable) when the causes can be identified and included in the models, and so-called ‘random’ when the dependencies are 99 proposed terminology for the classification and parameters for the quantification of variability in ionosphere morphology not known, or are believed to arise from the combination of a number of individual factors that cannot be separately quantified. within the frameworks discussed above for a given ionospheric characteristic, we can consider a number of specific scenarios, for each of which we suppose the quoted variability combination is determined: i) past monthly median values and a monthly median model based on measured data for the same epochs and same geographical areas – gives a measure of model accuracy in matching input data, useful in quantifying model uncertainties. ii) past monthly median values and a monthly median model based on measured data for different epochs, or different geographical areas – gives a measure of model consistency for other situations, useful in establishing the extent of model applicability for possible improvement. iii) long-term future prediction and a monthly median model – predicted figures are taken to be monthly median values and the variability combination gives a measure of prediction accuracy. iv) continuous measured data sequence with values quoted at regular time or regular spatial intervals – gives a measure of data variability averaged over whatever period or area is considered. v) past measured instantaneous values and a monthly median model for matched or disparate epochs and geographical areas – gives a measure of model applicability to specifying instantaneous values. vi) extrapolated forecast from recent past measured instantaneous values – gives a measure of forecasting accuracy. further comments can be offered as follows: when considering regular daily-hourly ionosonde measurements, data for successive hours are influenced by the systematic diurnal changes and this should be borne in mind if quoting variability statistics from such information. data sequences over shorter periods may provide useful information, for example concerning the extent of ionospheric disturbances, gravity and planetary waves (forbes et al., 2000). in such cases the sampling duration must always be quoted. measured values for different days of a given month at a particular hour are often treated as an indication of day-to-day variability, yet such data sets also embody the influences of within-the-hour variability because the hourly ionosonde readings are essentially ‘snapshots’ of ionospheric state over the minute or less that a sounding takes. within-the-hour variability is usually, but not always, less than dayto-day variability. this limitation should always be mentioned when discussing any such results. storm and quiet-day values may be included within the variability statistics or treated as base references from which other day or other location values depart. tec data sets taken from orbiting satellites include additional factors contributing to the variability, depending on how frequently measurements are recorded and the analyses procedures that are introduced to give these. when considering the variability of electrondensity distributions, height profiles can be examined to yield electron density statistics at a series of different heights, or height distributions established for a series of different densities. the profiles may come either from direct measurements (ezquer et al., 2002; mosert et al., 2002), or be obtained by reconstitution from the individual ionospheric characteristics values. amalgamation of the variability combinations for the different ionospheric characteristics does not seem feasible because of lack of knowledge of the associated correlation between different characteristics. from the foregoing, the following cases are suggested as particularly appropriate for study: – day-to-day variability of fof2, m(3000)f2, fof1 and foe for selected ionosonde stations and hours, taken from measured hourly vertical soundings and without attempted correction for within-the-hour variability (e.g., kouris et al., 2000a). these statistics are of potential value to the generation of improved propagation prediction procedures, where probability of ionospheric support is sought. correlation between the different characteristics, if any, needs also to be established. definitive figures for the most appropriate time and geographical groupings could usefully complement existing models. – day-to-day variability of tec for selected gps measuring stations and hours, provided by 100 peter a. bradley and ljiljana r. cander fig. 1. maps derived from 24 h ahead forecast of fof2, muf(3000)f2 and tec for 5 june 2001 at 1200 ut (http://ionosphere.rcru.rl.ac.uk). 101 proposed terminology for the classification and parameters for the quantification of variability in ionosphere morphology fig. 2. maps derived from measured values of fof2, muf(3000)f2 and tec for 5 june 2001 at 1200 ut (http://ionosphere.rcru.rl.ac.uk). 102 peter a. bradley and ljiljana r. cander the international gps service for geodynamics (igs). these statistics are also of potential value to the design and operation of improved earthspace communication systems. at present often tec is determined by height profile integration. it may well be that ultimately a global tec model will be created based on direct measurements, perhaps along the lines of those already produced for europe (leitinger and feichter, 2000; cander, 2001). – variability between past monthly median values derived from measurements of the above ionospheric characteristics and values given from model specifications for different epochs and stations. these statistics can be interpreted as providing an indication of model accuracy. – variability among measured data sequences of the above ionospheric characteristics for continuous time periods of a few hours (kouris et al., 2000b). these statistics provide a measure of short-term variations over whatever period or area is considered and are useful confidence figures to augment mapped characteristics values. – variability between past measured instantaneous mapped values of the above ionospheric characteristics and models for matched or disparate epochs and geographical areas (figs. 1 and 2). these statistics provide a measure of model applicability to specifying instantaneous values. – variability of electron density on different days of a given month at a particular hour for a series of different f-region heights. this information should be particularly valuable in establishing those heights at which variability is a minimum, and so aid especially in formulating improved algorithms to specify f-region thickness. 3. variability parameters periodic changes are not usually considered in determining variability factors but it is the random variations that are quantified. a given data set consisting of ‘randomly’ varying scalar quantities x is only fully specified by its complete amplitude-probability distribution. however, such information is unwieldy and not necessary for most applications. the set may though usefully be characterised by its mean value (or median value x m ), together with some measure of the spread about that central figure. one variability parameter is the overall standard deviation σ related to the root-mean-square error (rmse). a more refined specification includes a measure of distribution skew, with upper and lower distribution standard deviations σ u and σ d (or decile deviations d u and d l ) being separately quoted. ways of re-constituting the full distribution from median and standard or decile deviations assuming a particular distribution law exist and are adequate for many purposes. in the past, pairs of half-normal distributions have been adopted for the upper and lower sections, and use of the chi-squared distribution has been advocated (zacharisen and crow, 1970). however, the numerical complexity this involves is considered unjustifiable. simple empirical formulae fig. 3. idealised cumulative-amplitude probability distribution fitted to median and decile deviation values, from bradleyy and bredford (1976). x=variable; x 0 =required value to be exceeded; x m =median value; du=upper decile deviation; dl=lower decile deviation. 103 proposed terminology for the classification and parameters for the quantification of variability in ionosphere morphology approximating to a pair of normal distributions developed by bradley and bedford (1976) and based entirely on use of x m , d u and d l have been adopted by the itu-r (2000) in a number of cases where day-to-day variability of ionospheric propagation parameters needs to be specified (fig. 3). often it is useful to express the spread parameter (ρ or d) as a fraction of the median or mean value. indicated variability figures are obviously influenced by the length of the data series, particularly if long-period systematic changes are present and by sampling rates if a continuous sequence is approximated by a succession of ‘spot values’, as say in standard hourly verticalincidence ionospheric soundings, or regularinterval satellite probe measurements. which quantities to quote will depend on the application, but in many cases it is suggested use of the (x m , d u and d l ) variability combination will provide all that is needed; sometimes the decile range d u -d l is a useful quantity; other requirements call for specification of extreme ‘worst case’ maxima and minima of a particular data set. annex proposed variability terminology monthly median model accuracy variability – quantification of match between monthly median model and measurement data for the same epochs and same geographical areas. monthly median model applicability variability – quantification of match between monthly median model and measurement data for different past epochs and the same geographical area, or for different geographical areas and the same epochs. long-term prediction accuracy variability – quantification of match between monthly median model and measurement data for future epochs and the same or different geographical areas. short-term forecasting accuracy variability – quantification of match between forecast values from recent past measured instantaneous values for the same or different geographical areas. continuous time-period variability – quantification of variations in measurement data sequence for the same location, with values quoted at regular time intervals. spatial variability – quantification of variations in measurement data sequences for the same epoch, with values quoted at regular location separations. composite measurement variability – quantification of combined variations in measurement data sequences for different locations and times. note: in all cases a full specification of the measurement data used should be provided. references bradley, p.a. and c. bedford (1976): prediction of hf circuit availability, electron. lett., 12, 32-33. cander, lj. r. (2001): tec specification over europe: a procedure, in iee eleventh international conference on antennas and propagation, manchester, iee publication, no. 480, 802-805. ezquer, r.g., m. mosert, s.m. radicella and c.a. jadur (2002): study of the electron density variability at fixed heights over san juan and tucuman, adv. space res., 29 (6), 993-997. forbes, j.m., s.e. palo and x. zhang (2000): variability of the ionosphere, j. atmos. sol.-terr. phys., 62, 685693. itu-r (2000): hf propagation prediction method, recommendation itu-r p. 533, international telecommunication union, geneva, switzerland. kouris, s. s, d.n. fotiadis and r. hanbaba (2000a): on the day-to-day variations of the muf over europe, phys. chem. earth, c, 25 (4), 319-325. kouris, s.s., b. zolesi, d.n. fotiadis and c. bianchi (2000b): on the variability within-the-hour and from hour-to-hour of the f-region characteristics above rome, phys. chem. earth, c, 25 (4), 347-351. leitinger, r. and e. feichter (2000): costtec the electron content monthly median map adopted by cost251, phys. chem. earth, c, 25 (4), 311-314. mosert, m., s.m. radicella, d. buresova, r.g. ezquer and c.a. jadur (2002): study of the variations of the electron density at 170 km, adv. space res., 29 (6), 937941. samardjiev, t., p.a. bradley, lj.r. cander and m.i. dick (1993): ionospheric mapping by computer contouring techniques, electron. lett., 29 (20), 1794-1975. z ac h a r i s e n , d. h. and e.l. c row (1970): f itting distributions of telecommunication variables with chisquare distributions, radio sci., 5 (11), 1307-1315. microsoft word 7445-18831-1-ed_marone-peppoloni.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7445 1 ethical dilemmas in geosciences. we can ask, but, can we answer? eduardo marone cem/ufpr, ioi-tc-lac, brazil iapg – international association for promoting geoethics edmarone@gmail.com silvia peppoloni istituto nazionale di geofisica e vulcanologia iapg – international association for promoting geoethics silvia.peppoloni@ingv.it abstract the choices of a geoscientist while carrying out his/her activity are always accompanied by ethical implications, because they can have a strong impact on the population, the natural environment, the economy, the landscape and the cultural resources of the affected area. it is not uncommon for a geoscientist to be faced with ethical dilemmas that are problems with a difficult solution, since options to solve such dilemmas will have negative consequences. how does one make a choice in these circumstances? what is the ethical duty of geoscientists and what has to be their professional attitude? in which cases do they have the duty to take action or the duty to not act (duty of omission)? a brief review of the concepts formulated by philosophers in reference to ethics can serve to provide some answers to the above questions and to understand how geoscientists can best serve society. “among men there are but few who behave according to principles which is extremely good, as it can so easily happen that one errs in these principles, and then the resulting disadvantage extends all the further, the more universal the principle and the more resolute the person who has set it before himself.” (immanuel kant, 1764) 1. introduction thics is a branch of philosophy (moral philosophy) having as many definitions as there are philosophers. one might consider it as the main philosophical “tool” to identify right and wrong conduct (fieser, 2017) and to define which are our ethical duties and obligations, once we have established a shared set of reference values. one of the critical situations we face, as humans in general and as geoscientists in particular, occurs when we are exposed to an ethical dilemma. an ethical dilemma is a problem of difficult solution, which offers an alternative between two or more options, none of which is fully acceptable in practice or both with negative consequences. dilemmas arise because of conflicts between right or wrong for the actions/means and the bad or good of the consequences/ends. they involve a conflict between ethical duties, in which to follow one would result in violating another. moral philosophy tries to provide rules and principles enabling us trying to solve ethical dilemmas. among the many, one of the best known ethical guides is the “golden rule”, or law of reciprocity, a principle suggesting treate annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7445 2 ing the others as one would wish to be treated (flew, 1979). one alleged goal of ethics would be to help us in determining how to solve ethical dilemmas. firstly, we should question: • is it possible to solve such dilemmas? • what must we do if we cannot? it is not always easy or possible to apply the golden rule or other ethical laws to solve a given dilemma. in geosciences, when faced with a (geo)ethical dilemma, geoscientists have to initially answer some questions: • can we solve any of the dilemmas that appear in the practice of our profession? • always? • in some cases? • not at all? • what if we cannot? • must we? in the following, we try to analyse if it is possible to answer the above questions and, if not, what should be the ethical duty of a geoscientist when facing an apparently unsolvable dilemma. 2. problems, questions and geoethical dilemmas ethical behaviour is primarily about making correct choices. thus, we have to keep in mind that, when confronting professional dilemmas, we have to deal with ethical consequences of our work (bobrowsky et al., 2017). ethics is intimately tied to critical thinking, pushing us to use our best “shelter” of logical/scientific tools/methods to solve problems that have ethical implications. but that is not enough. in fact, dilemmas are particular problems that create situations that could put us on difficult grounds, forcing us to decide based on what we think the ethically correct course of action would be. ethical dilemmas, however, need decisions using critical thinking, on what would be a logical/rational way of action. nonetheless, there are no truly ‘right’ solution to such dilemmas, as they often ask us to compare two different ethical or logical imperatives and choose which one we feel is the most important (wall, 2003). dilemmas arise when we do not have a solution satisfying both ethical and logical premises, although many modern philosophers propose theories to resolve such situations. in fact, if a dilemma has a solution that does not violate logical thinking nor ethical principles, it is not a real dilemma. however, we should consider also that dilemmas are solvable using scientific means but may violate ethical principles. for example, the opening of a mine in an economically depressed area could have indisputable benefits for the local population, as this activity would provide new jobs, improve the facilities and infrastructure of the territory, increase community services and foster local microeconomics. nevertheless, at the same time, the mine may have a strong impact on the natural environment, disrupt ecosystems, lead to landscape deterioration, become a source of groundwater pollution, or trigger new hazards in the area. therefore, there are positive and negative aspects to be considered, according to different perspectives: in the short and long term, at both small and large scales. in the short term, the mine can represent a great economic benefit for the local population but, in the long term, it may impoverish the area and its inhabitants from an environmental and aesthetic point of view, by overturning the initial positive effects. considering the local scale, the mine may have negative repercussions on the quality of the environment, whereas in a large-scale perspective it can have positive effects on the country's gdp. how long (in terms of time and space) will those effects (positive and negative) persist? hence, a specific situation, not necessarily a dilemma as in the short term, could become a dilemma in the long term. although we all want dilemmas solved, such a perception however, may be hopeless if it turns out that the nature of dilemmas is to remain dilemmas (grassian, 1992). this does not mean we cannot find an acceptable solution from a scientific/technical point of view. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7445 3 if we focus on geoethical dilemmas, taking into account the challenging question of the philosopher immanuel kant (formulated in 1764): • is it rational for me to will that my choice become a universal law of nature? as geoscientists, we hold the knowledge (our scientific information, data and methods), which is not perfect, thus fallible, and always subject to possible changes and improvements by definition, as in any “truthful” science (popper, 1959). if we have to do what is right, based on our scientific knowledge and a critical thinking, despite potentially bad consequences, then we have to be sure that our knowledge is advanced and scientifically updated, to be considered as true (that is, it possesses truly the qualities attributed to it). • however, is our scientific knowledge such a universal truth? if our knowledge is neither perfect nor absolute, nor a universal truth (and we know that): • can we answer in one or other direction to geoethical dilemmas (in good faith)? if a geoscientist usually makes choices trying to look at the best consequences (or at least not the worst), sometimes bad consequences must be carefully evaluated and even accepted. nevertheless: • who has to decide whether to accept bad consequences? it is not always the duty of a geoscientist to take a decision among those options that appear in a given geoethical dilemma. for example, the final decision on the feasibility and implementation of an engineering geology intervention can depend not only on scientific and/or technological considerations, but also on political matters. however, geoscientists must not replace politicians, but “provide all the concrete and exhaustive elements to take a decision as sustainable as possible for that social and environmental system” (peppoloni and di capua, 2017a). so, a geoscientist is ethically forced to properly inform, with no bias, those who are really in charge of the decision-making process. when the geoscientist also holds the decisionmaking duty, she/he has to look beyond geosciences and consider other reference systems (social, cultural, economic, etc.) in taking the decision 3. the no answer option modern philosophy has shown multiple ideas and reasoning about how to face ethical dilemmas. however, in the path to achieve a high ethical standard, it has been proposed that all individuals have to construct his/her own set of ethical values, targeting the highest level in the kohlberg’s stages of moral development (kohlberg, 1982; kohlberg et al., 1983). these stages (table 1) are planes of moral adequacy conceived to explain the development of ethical reasoning. kohlberg's stages of moral development constitute an adaptation of a psychological theory originally conceived by jean piaget (1896-1980) (piaget, 1932). table 1: kohlberg’s planes of moral adequacy (adapted from kohlberg, 1982) level stage social driver pre-conventional (the morality of actions is judged by its direct consequences) 1 obedience and punishment (blind egoism) 2 self-interest orientation (instrumental egoism) conventional (the morality of actions is judged by comparing them to society's views and expectations) 3 interpersonal accord and conformity (social relationships) 4 law and order morality (social systems) post-conventional (individual’s morality may take precedence over society’s morality: principles include basic human rights as life, liberty, and justice) 5 social contract orientation 6 universal ethical principles (principled conscience: mutual respect) kohlberg's theory holds that moral reasoning, the basis for ethical behavior, has six identifiable deannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7445 4 velopmental stages, each of them more adequate to respond to ethical dilemmas than the preceding stage. at stage 6, action is never a means to a goal, but it is an aim in itself; the individual acts because it is right and not for avoiding punishment, or for complying with social laws; he/she acts in the right way because this is mainly in his/her own interest. although it is not easy to find individuals always acting accordingly with the highest ethical stage, which could be considered somehow utopian, we can consider this highest stage as a spur to push ourselves on that direction. ideally, any individual must climb stages to the top, or at least try to climb, to elevate the ethical quality of his/her behavior. most people rely on stage 5, assuming that following a given social contract (for example a deontological professional code of ethics/conduct) would be enough; others remain at stage 4, following rules because they are in force, not due to deep conviction. the ultimate stage (stage 6) provides individuals with an impeccable ethical conduct, because of their deep conviction and consciousness that their ethical values are the right ones, even if they know those values cannot be considered universal laws. deontological theories may deny that consequences are of any concern, provided the intention was good. however, not all geoscientists will feel comfortable if their actions, although approved by deontological codes and in spite of their good intentions, result in harm to other people (society) or the environment. thus, geoscientists cannot think that deontological codes are the last step for an ethical assurance where they can find all the necessary ways to face ethical dilemmas, whatever the consequences (peppoloni and di capua, 2017b write about the ‘… tendency to confuse “the ethics of responsibility” with “the ethics embodied by the tool” …’). they have to aspire to respond to ethical dilemmas starting from strong values and be conscious of the limits of their knowledge. 4. conclusion: the dilemma of dilemmas the main question we have to consider and answer is: • have ethical dilemmas a “right” solution? according to present knowledge, they do not. as we have shown, a real ethical dilemma is a problem with no perfect solution in absolute terms. we can find only acceptable solutions concerning each specific context. thus, if real ethical dilemmas with a conflict between means and aims cannot simply be solved by a geoscientist: • where is his/her duty? if there is no perfect solution to geoethical dilemmas, because the possibility of a conflict between what would be right and what is acceptable and wise to be done, and considering that in most cases a solution is expected from geoscientists: • what has to be their professional attitude? if we are facing a geoethical real dilemma, our first professional attitude must be accepting we cannot offer a unique right solution, but options and potential outcomes/scenarios. our duty is to explain the choices and the consequences of each choice. we cannot fall on the mistake of considering our geoscience knowledge as a universal law, thinking we might solve any geoethical dilemma based on it and only it. but, we can suggest geoethical decisions by justifying them adequately from a scientific and technical point of view, and by clearly indicating pros and cons of the choice we are proposing, including a cost/benefit analysis also in societal and environmental terms, and including in our scientific analysis both probabilities and uncertainties. this approach is expected to lead to that point of equilibrium among positive and negative consequences (peppoloni and di capua, 2017b). however, geoscientists cannot be always able to propose solutions to real geoethical dilemmas. but, in this case: • what are their professional duties? in which way can geoscientists best serve society? facing real geoethical dilemmas is mostly linked to our duties of omission (not proposing a given unique solution) than to select one because we really think, even in good faith, it is annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7445 5 the lesser of two evils (the principle that when faced with selecting from two unethical/immoral options, the one which is least unethical/immoral should be chosen). we have to present, exclusively, technical-scientific data without influencing the choice of decisionmakers. it is not our duty, as geoscientists, to select among evils. we have to refrain from taking sides in such cases. in any case, our duty of informing remains, which means to clearly explain, to all the players, which are the evils and what consequences have to be expected whatever the decision taken would be. critical situations push us to offer solutions to geoethical dilemmas under great pressures and tight deadlines. this is the golden moment of our professional behaviour, when the highest ethical standards are requested and in which we must refrain from offering a scientific solution to the unsolvable. for example, during an intense seismic swarm in a high-risk area, prone to strong earthquakes, we could be asked to give our advice to evacuate or not a village for the possibility of a mainshock; this is a decision to be taken by decision-makers and not by geoscientists, since earthquakes are not currently predictable with any acceptable accuracy. when we are confronted with a geoethical dilemma, it could be good to think according to the gödel theorem (smith, 2007): the notion of truth in a system is not definable inside the same system. in the case of geosciences, this means that we have to accept the limitation of geosciences (our system) in offering true solutions to real geoethical dilemmas based solely on geoscientific knowledge. we have to let decision-makers take the final decision, based on principles other than geoscientific ones (from other systems). our obligation, however, is to be clear on all the potential (geo)consequences for different options/scenarios, no more, no less. it is time to better examine and rethink the challenge on how to approach geoethical dilemmas, creating consciousness about the values that should guide our professional duties, which do not finish because any deontological code says so. moreover, geoscientists do not have only duties of informing, but in some cases also to refrain from taking a decision. we must not answer what cannot be answered. references bobrowsky p., cronin v.s., di capua g., kieffer s.w., peppoloni s. (2017). the emerging field of geoethics. in: scientific integrity and ethics with applications to the geosciences, edited by l.c. gundersen. special publication american geophysical union, john wiley and sons, inc. fieser j. (2017). “ethics”, the internet encyclopedia of philosophy, issn 2161-0002, http://www.iep.utm.edu/ethics/, accessed 18 october 2017. flew a. (1979). a dictionary of philosophy. london: pan books in association with the macmillan press. p. 134. isbn 0-330-48730-2. grassian v. (1992). moral reasoning, prentice hall. kant i. (1764). observations on the feeling of the beautiful and sublime. trans. john t. goldthwait. university of california press. kohlberg l. (1982). moral development. in: broughton j.m. & freeman-moir d.j. (eds.), the cognitive developmental psychology of james mark baldwin: current theory and research in genetic epistemology, norwood, nj: ablex publishing corp. kohlberg l., levine c., hewer a. (1983). moral stages: a current formulation and a response to critics. basel, ny: karger. isbn: 3805537166. peppoloni s. and di capua g. (2017a). ethics, pp. 1-5, doi: 10.1007/978-3-319-12127-7_115-1. in: bobrowsky p.t. and marker b. (eds.), encyclopedia of engineering geology, encyclopedia of earth sciences series, springer international publishing, isbn: 978-3-319-12127-7. peppoloni s. and di capua g. (2017b). geoethics: ethical, social and cultural implications in geosciences. annals of geophysics, 60, fast track 7, doi: 10.4401/ag-7473. piaget j. (1932). the moral judgment of the child. london: kegan paul, trench, trubner and co., isbn: 0029252407. popper k.r. (1959). the logic of scientific discovery. 2002, routledge, isbn: 978-0-41527844-7. smith p. (2007). an introduction to gödel’s theorems. cambridge university press. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7445 6 wall t.f. (2003). thinking critically about moral problems by. belmont, ca: wadsworth, isbn: 9780534574239. vol50,4,2007 493 annals of geophysics, vol. 50, n. 4, august 2007 key words radon – uranium – indoor air – soil gas – hyblean foreland 1. introduction uranium is present in rocks and soil. the most abundant isotope is 238u that decaying generates 222rn. radon is a noble gas and thus does not undergo chemical reactions which could preclude its free movement within soil. once radon is free to move, when it has left its original matrix through the emanation process, it can give rise to different mechanisms of migration, until it arrives at the soil surface and exhales to the atmosphere. the first mechanism of migration is diffusion. the second one is convection, which can occur when a sufficient thermal gradient is available within the soil, depending on many local parameters, such as viscosity, porosity, permeability. the third one is transport by means of gas carrier (monnin and seidel, 1997; etiope and martinelli, 2002; yang et al., 2003). the major sources for indoor radon gases are the soil under the building, the household water and the building material (swedjmark et al., 1989; ramachandran et al., 1990; jonsson, 1991; kullab, 2005). the principal source of 222rn inside the home is the soil surrounding the building (nason and cohen, 1987; durrani, 1999), so a correlation between the soil and indoor radon could be expected. in recent years, several national indoor surveys have been performed in different countries (abu-jarad and al-jarallah, 1986; diwivedi et al., 1997; yu et al., 1997; iakovleva and karataev, 2001; canoba et al., 2001; srivastava et al., 2001; espinosa and gammage, 2003; al-jarallah et al., 2003) many studies have also been performed on soil gas, particularly in fractured zones (king et al., 1996; mazur et al., 1999; jonsson et al., 1999; choubey et al., 1999; durani, indoor and soil radon measurements in the hyblean foreland (south-east sicily) cristina antoci (1), giuseppina immè (1), santo la delfa (2) (3), salvatore lo nigro (1), daniela morelli (1), giuseppe patanè (2) and giuseppe alessandro (4) (1) dipartimento di fisica e astronomia, università degli studi di catania, italy (2) dipartimento di scienze geologiche, università degli studi di catania, italy (3) irma-omega, via paolo vasta 158/c, acireale (ct), italy (4) provincia regionale di ragusa, settore geologia e geognostica, ragusa (rg), italy abstract indoor radon behavior in two sites of se sicily was studied as a function of the soil radon concentration. the chosen locations were ragusa and modica towns, placed in the hyblean plateau (northern margin of the african plate). soil samples were analysed by gamma spectrometry to determine the amount of radionuclides. indoor air and soil gas radon measurements were simultaneously performed in both sites using active detectors. radon in soil was measured one meter deep. a positive correlation was obtained between indoor radon concentration and the soil gas concentration. mailing address: dr. daniela morelli, dipartimento di fisica e astronomia, università degli studi di catania, via s.sofia 64, 95123 catania, italy; e-mail: daniela.morelli@ct.infn.it 494 cristina antoci et al. 1999; al-tamimi and abumarad, 2001; vaupotiã, 2003). since uranium and radium present in the soil are the main source of indoor radon, in this study, measurements of radionuclides in soil along with radon concentration measurements in the soil gas air and indoors were carried out in se sicily. the relation between soil gas radon concentration and indoor air radon concentration is also shown and commented. 2. measurements a study was carried out in the winter of 2003/2004. it was performed in two different sites: ragusa and modica (a village 8 km southeast of ragusa). simultaneous indoor air and soil gas radon measurements were performed. in order to determine the amount of radionuclides, gamma spectrometry was carried out on soil samples collected in the two sites. 2.1. geology of the area the two investigation sites are located in the central hyblean plateau (south-east sicily) (fig. 1). the hyblean plateau is part of the northern margin of the african plate and has remained a relatively undeformed foreland during the neogene collisional process affecting the african-european convergent belt; it consists of ca. 6000 m of carbonates and marls with intercalations of volcanic horizons, which have occurred at several episodes (patacca et al., 1979; bianchi et al., fig. 1. site location: the hyblean foreland. 495 indoor and soil radon measurements in the hyblean foreland (south-east sicily) 1989; ragg et al., 1999). the rock successions in which ragusa and modica lies are oligomiocene carbonates. 2.2. indoor radon study indoor radon measurements were made in rooms characterized by similar building materials in the basement. the rooms were closed before the measurements beginning for a period of one week and sealed throughout the investigation period, to avoid external influences on the measurements. the edifices were made of cement, sand, bricks and concrete, having one window and one door. the chosen rooms were placed in the basement. continuous measurements were carried out with an alphaguard (genitron instruments) device operating in diffusion mode (the radon enters into the ionization chamber by natural diffusion through a filter that allows only radon to enter). in modica the measurements were performed from 1st december 2003 till 11th december 2003, while in ragusa from 17th january 2004 till 5th february 2004. 2.3. soil radon study soil measurements were curried out over the monitored rooms. measurements were performed with an alphaguard (genitron instruments) device with the soil probe one meter deep, connected by means of a pump with flow rate of 0.05 l min−1 to the detection system operating with an ionisation chamber. before reaching the detector, the soil radon passes through an aqua stop filter to eliminate the moisture, and then to a progeny filter to allow the only 222rn to pass. in both sites continuous soil gas measurements were performed simultaneously with the indoor ones. 2.4. uranium concentration in soil sample gamma-ray spectrometry was used to determine the radionuclides activity concentration in sample of soil of the investigated area. a highpurity germanium e&g ortec (hpge), with an efficiency of 30% was employed. the samples were dried at about 80°c for 4 h to eliminate the humidity, then crushed and homogenized until obtaining a 250 µm powder and dried at about 80°c for 24 h to eliminate the residual humidity. then the samples were weighed and placed in a marinelli beaker of 100 cm3. this kind of beaker allows only a 5 mm thick sample and therefore the gamma self-absorption negligible. each sample was sealed for 4 weeks to reach the secular equilibrium (astm, 1983, 1986). the acquisition time was 18000 s. for determination of the full-energy peak efficiency in the energy range from 60 to 2000 kev a calibration source, with the same geometry as the samples, was prepared by means of a mesh of known activity. 3. experimental results and discussion continuous measurements in soil and indoors, with an integration rate of 10 min, were simultaneously performed. measurements in modica were carried out from 1st december 2003 to 11th december 2003, while in ragusa from 17th january 2004 to 5th february 2005. figure 2a,b reports the concentration of radon in indoor air and soil gas radon for the two investigated sites. to better underline the general trend the daily mean was calculated and reported in fig. 3a,b. from this figure it can be seen that the obtained trends for the modica site show a clear correlation between soil and indoor radon concentration. the same result is less evident for the trends obtained for ragusa. calculating the linear coefficient r, between in-soil and indoor radon, r=0.84 was found for the modica site and r=0.09 for the ragusa. to better investigate the behaviour of the two sites, a cross-correlation of the two signals (soil and indoor radon) was performed. figures 4 and 5 report the results for ragusa and modica, respectively. the soil and indoor daily mean radon concentration trends were normalized to the arithmetic mean value, calculated throughout the investigated period, to make the two series better comparable. 496 cristina antoci et al. ( ( ) ) ( ( ) ) ( ( ) ) ( ( ) x i x y i d y x i x y i d y i i i 2 2 ) − − − − − − ( )r d = r r r r6 @ / / / fig. 2a,b. radon concentration: a) modica december 1st, 2003-december 11th, 2003; b) ragusa january 17th, 2004-february 5th, 2004. a cross correlation analysis was carried out. it is a standard method of estimating the degree to which two series are correlated. considering the two series the cross correlation r at delay d is defined as the following function: a b 497 indoor and soil radon measurements in the hyblean foreland (south-east sicily) where the two series are indicated with x(i) and y(i) respectively; x⎯ and y⎯ are the arithmetic means of the corresponding series; d = 1, 2, ..., n−1 is the delay (days, being the two series daily trend). from figs. 4 and 5 it can seen that the maximum correlation at both sites is for d = 0, indicating that indoor and soil radon concentration have the maximum correlation without delay, even if the correlation is higher in modica than in ragusa. looking at the recorded indoor and soil radon values, the two sites show different characteristics. table i lists the average over the investigation period. it can be observed that: i) the mean soil radon concentration is 7 kbq m−3 in ragusa and 18 kbq m−3 in modica, while the fig. 3a,b. radon daily mean concentration: a) modica; b) ragusa. a b fig. 4. cross-correlation (upper plot) between soil and indoor radon trend (down plot) site of ragusa. fig. 5. cross-correlation (upper plot) between soil and indoor radon trend (down plot) site of modica. 498 cristina antoci et al. table i. indoor and soil radon concentration means all over the investigation period. site soil indoor rn [bq m−3] rn [bq m−3] mean stand. dev. mean stand. dev. modica 18 000 11 000 700 250 ragusa 7 000 2 000 400 290 mean indoor radon is 400 bq m–3 in ragusa and 700 bq m–3 in modica; ii) soil radon values show fewer fluctuations in ragusa than in modica, the relative percentage errors are, in fact, of 28% and 61% respectively; iii) indoor values are have fewer fluctuations in modica than in ragusa, the relative percentage errors, in this case, are 35% and 72% respectively. these differences may be linked to the difference in tectonic disturbance and porosity of the soil, the building materials having been chosen with the same characteristics. looking at the mean values it is also possible to observe that the percentage of indoor radon concentration with respect to the soil is 6% for ragusa and 4% for modica. this first results indicate that the exhalation rate in ragusa soil is higher than that in modica, indoor accumulation is favoured in ragusa. this could indicate that the ragusa area is more fractured than the modica one as the exhalation rate is strictly linked to the degree of soil fracturation. the results obtained by gamma spectrometry (table ii) also indicate different values on radionuclides concentration in the two sites, otherwise the ratio between the amount of the elements post 222rn (214pb and 214bi) and that of 226ra is higher in ragusa than in modica. the larger disequilibrium between 214pb, 214bi and 226ra recorded in modica justifies the higher in-soil values. high disequilibrium, in fact, is due to radon escape that is linked to the soil porosity, favouring the emanation process. under this hypothesis, we can justify the results obtained: i) the rocks in the modica area are more porous, favouring radon emanation (high in soil radon concentration) and the area being less fractured the in soil accumulation is higher with respect to indoor one; ii) the rocks in the ragusa area are less porous so the radon 499 indoor and soil radon measurements in the hyblean foreland (south-east sicily) emanation is not favoured (low in soil radon concentration) and as the area is more fractured the indoor accumulation is higher than the soil one. 4. conclusions radon measurements were performed indoors and in the soil in ragusa and modica (se sicily). different geological features characterize the two sites while the dwellings, where the indoor measurements were done had the same building typology and materials. the results of simultaneous indoor and soil radon measurements in both sites showed a positive correlation. from the gamma-ray analysis it was also possible to explain the obtained differences in values. in particular from the 214pb/226ra and 214bi/226ra ratios it is clear that the modica soil is more porous than the ragusa one. moreover the continuous soil and indoor radon measurements have evidenced that the modica area, as expected, is more fractured. the results obtained indicate that in the modica area the predominant process involved is radon emanation, while in the ragusa area it is exhalation. acknowledgements we thank the provincia regionale di ragusa, assessorato territorio e ambiente, settore geologia e geognostica, directed by dr. s. buonmestieri, for having contributed to this work providing the instruments and the locations for the investigation. references abu-jarad, f. and m.i. al-jarallah (1986): radon activity in saudi houses, radiat. prot. dosim., 14, 243-249. al-jarallah, m.i., x. fazal-ur-rehman and f. abujarad (2003): indoor radon survey in dwellings of some regions in yemen, radiat. meas., 36, 449-451. al-tamimi, m.h. and k.m. abumurad (2001): radon anomalies along faults in north of jordan, radiat. meas., 34, 397-400. astm (american society for testingand materials) (1983): standard method for sampling surface soil for radionuclides c 998-83, annual book of astm standards (philadelphia, pa), 512-514. astm (american society for testingand materials) (1986): recommended practice for investigation and sampling soil and rock for engineering purposes d 420, annual book of astm standards (philadelphia, pa), 109-113. bianchi, f., s. carbone, m. grasso, g. invernizzi, f. lentini, g. longaretti, s. merlini and f. moscarditable ii. radionuclides amount in the soil samples. sample code radioactive element concentration standard weighted standard series (bq kg–1) deviation mean deviation modica 238u th-234 18.37 6.35 pre-radon ra-226 54.11 11.16 238u pb-214 19.97 1.99 post-radon pb-214 4.32 1.20 10.99 0.84 bi-214 13.91 2.68 pb-212 9.46 0.58 232th ac-228 1.11 1.78 8.11 0.34 ragusa 238u th-234 16.03 11.20 pre-radon ra-226 140.56 9.45 238u pb-214 43.27 1.78 post-radon pb-214 43.66 1.62 43.06 1.06 bi-214 41.54 2.29 pb-212 3.10 0.31 232th ac-228 3.11 1.29 2.65 0.27 tl-208 1.05 0.57 500 cristina antoci et al. ni (1989): sicilia orientale: profilo geologico nebrodiiblei, mem. soc. geol. it., 38, 429-458. canoba, a., f.o. lopez, m.i. arnaud, a.a. oliveria, r.s. neman, j.c. hadler, p.j. iunes, s.r. paulo, a.m. osorio, r. aparecido, c. rodriguez, v. moreno, r. vasquez, g. espinosa, j.i. golzarri, t. martinez, m. navarrete, i. cabrera, n. segovia, p. pena, e. tamez, p. pereyra, m.e. lopez-herrera and l. sajo-bohus (2001): indoor radon measurements and methodologies in latin american countries, radiat. meas., 34, 483486. choubey, v.m., k.s. bist, n.k. saini and r.c. ramola (1999): relation between soil-gas radon variation and different lithotectonic units, garhwal himalya, india, 51, 487-592. durani, s.a.(1999): radon concentration values in the field: correlation with underlying geology, radiat. meas., 31, 271-276. dwivedi, k.k., s. ghosh, s. singh, limatimjen, j. satyanarayana, g.s. murthy, j. prasad and a. srivastava (1997): indoor radon measurements in some indian cities, radiat. meas., 28, 647-649. espinosa, g. and r.b. gammage (2003): a representative survey of indoor radon in sixteen regions in mexico city, radiat. prot. dosim., 103, 73-76. etiope, g. and g. martinelli (2002): migration of carrier and trace gases in the geosphere: an overview, phys. earth planet. inter., 129, 185-204. iakovleva, v.s. and v. d. karataev (2001): radon level in tomsk dwellings and correlation with factors of impact, radiat. meas., 34, 501-504. jonsson, g. (1991): solid state nuclear track detectors in radon measurements indoor and in the soil, int. radiat. appl. instr. d, 19 (1-4), 335-338, doi: 10.1016/13590189(91)902008-y. jonsson, g., c. baixeras, r. devantier, w. enge, l.l. font, k. freyer, r. ghose and h-c. treutler (1999): soil radon levels measured with ssntd’s and the soil radium content, radiat. meas., 31, 291-294. king, c.-h., b.-s. king and w.c. evans (1996): spatial radon anomalies on active faults in california, appl. geochem., 11, 497-510. kullab, m. (2005): assessment of radon-222 concentrations in buildings, building materials, water and soil in jordan, appl. radiat. isot., 62, 765-773. mazur, d., m. janik, j. loskiewicz, p. olko and j. swakon (1999): measurements of radon concentration in soil gas by cr-39 detectors, radiat. meas., 31, 295-300. monnin, m.m. and j. seidel (1997): radon and volcanic surveillance, in radon measurements by etched track detectors: application in radiation protection, edited by s.a. durrani and r. ilic (earth science and the environment, world scientific), 301-318. nason, r. and b.l. cohen (1980): correlation between 226ra in soil, 222rn in soil gas, and 222rn inside adjacent houses, health phys., 52, 73-74. patacca, e., p. scandone, g. giunta and v. liguori (1979): mesozoic paleotectonic evolution of the ragusa zone (southeastern sicily), geol. rom., 18, 331-369. ragg, r., m. grasso and b. muller (1999): patterns of tectonic stress in sicily from borehole and breakout observations and finite element modelling, tectonics, 18, 669-685. ramachandran, t.v., m.c. subba rammu and u.c. mishra (1990): a correlation study of radon in dwellings with radium content of soil, current. sci., 59, 979-982. srivastava, a., m.r. zaman, k.k. dwivedi and t.v. ramachandran (2001): indoor radon level in the dwellings of rajshahi and chuadanga regions of bangladesh, radiat. meas., 34, 497-500. swedjemark, g.a., h. wahren, a. makilato and w. tell (1989): experience from indoor radon-daughther schemes in sweden, environ. int., 15, 253-260. vaupotiã, j. (2003): indoor radon in slovenia, nuclear tecnol. rad. prot., 2, 36-43. yang, t.f., c.y. chou, c-h. chen, l.l. chyi and j.h. jiang (2003): exhalation of radon and its carrier gases in sw taiwan, radiat. meas., 36, 425-429. yu, k.n., e.c.m. young, m.j. stokes, z.j. guan and k.w. cho (1997): a survey of radon and thoron progeny for dwellings in hong kong, health phys., 73 (2), 373-377. the muraves telescope front-end electronics and data acquisition annals of geophysics, 60, 1, 2017, s0104; doi:10.4401/ag-7379 s0104 the muraves telescope front-end electronics and data acquisition luigi cimmino1,*, fabio ambrosino1,2, lorenzo bonechi3, roberto ciaranfi3, raffaello d’alessandro3,4, vincenzo masone1, nicola mori3,4, pasquale noli1, giulio saracino1,2, paolo strolin1,2 1 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 2 università di napoli federico ii, dipartimento di fisica, naples, italy 3 istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy 4 università di firenze, dipartimento di fisica e astronomia, sesto fiorentino, florence, italy abstract the muraves detector is a 4 m2 muon tracker equipped with a low power consumption electronic and designed to work in volcanic areas. due to the great amount of channels (~1500) the detector is equipped with a multilayer electronic for data acquisition. it consists of 12 slave boards and 1 master board per square meter of detector and a single raspberry pi board that rules the whole set of one-square-meter detectors. because of this modularity, we can enlarge in principle the detector surface by adding more one-square-meter elements. in the present work, we resume the main features of the muraves detector designed for the muography of volcanoes and, more generally, for the imaging of the underground. we focus on the capability to fine tune every single channel of the detector and the precise measure of the time of flight of the muons. the latter uses a time expansion technique and it should allow us to make a background rejection never obtained until now. 1. introduction the muray detector prototype [ambrosino et al. 2013, 2014] constitutes the basic part of the muraves telescope. the muraves telescope can work in volcanic areas. thus, it has to be modular and easy to install. with respect to the muray detector, the basic plane arrangement has been modified to reduce weight and size. the latter assembles two modules, each consisting of 32 plastic scintillators, inside a solid shell that prevents mechanical stresses, weighing ~60 kg. the muray detector is made of 6 planes of 64 triangular-shaped plastic scintillators, arranged in three xy stations. it acts as a tracker and the angular resolution is about 8 mrad, when the outer stations are 1 m spaced. moreover, it is suitable for precise measurement of the muon time of flight. the latter is one of the tools used to reduce background events. in particular, the measurement of the muon time of flight can determine the track. further background rejection can be achieved by requiring coincident signals in three stations, instead of two. each scintillator is read by a silicon photomultiplier (sipm). a sipm integrated read out chip easiroc is used to manage the output signals of each group of 32 sipms coupled to the modules. 2. the slave board features the extended analogue sipm integrated read out chip (easiroc) [callier et al. 2012] has 32 analogue channels, each one provided with an 8-bit dac (figure 1) for fine-tuning the bias voltage of the connected sipm. the sipm signal is voltage amplified by means of a low noise pre-amplifier capacitively coupled with the photomultiplier. low or high gain amplification are available for all 32 channels and the gain can be adjusted using a shared 4-bit register. two dedicated tunable shapers and a track and hold provide the charge measurement. the track and hold circuit records the signal’s peak and store the values inside a register, converted with a 12-bit adc. the peaking time value depends on the slow control settings (figure 2). the amount of charge released by each sipm is saved in the form of an amplified and shaped signal at the specified peaking time. two programmable potentiometers set the hold time and the timeout. the hold potentiometer regulate the minimum time the sipm article history received july 5, 2016; accepted october 5, 2016. subject classification: muon radiography, time of flight, sipm read-out electronic, raspberry pi, data acquisition control systems. signal is locked at a fixed amplitude value and the timeout setting specifies how long the system wait for a trigger coincidence, depending on the trigger logic. all slave boards include a xilinx spartan iii fpga for logic functions. the slave boards are low power consumption and can work in two modes, run and diag. the power consumption of the front-end electronics is below 25 w and it includes the sipms powering. each board is equipped with a programmable voltage regulator (max1932) that can provide supply voltages in the range 28 v to 75 v. the slave working voltage is 6v and it uses linear and switching regulators to provide all the required voltages. the switching regulators work with a dedicated ground, in order to avoid the spreading of electrical noise through the board. 3. the trigger system every time at least one of the 32 sipm has a signal with an amplitude exceeding a given threshold, a fast shaper followed by a discriminator produces a local trigger, the so-called fastor logic signal. the threshold is set by a 10 bit register and the local trigger is 100ns long. each station is routed to a multiplexer, which is usually programmed to have the logic or of the two slaves of each single plane and the logic and of the two planes belonging to the station. the logic output of each station is put in the coincidence with a fourth multiplexer to determine the and trigger logic of the detector. the fulfillment of the trigger logic produces a global trigger that starts the data acquisition. 4. time expansion technique the time expansion technique is used to magnify the time of flight of the muon. this technique provides the accuracy necessary for digitizing the time, despite the fact that the limit on power consumption imposes a moderate clock frequency. the greater the time expansion, the better the accuracy with which we determine the time of flight. the slave boards have therefore been equipped with a time-to-digital converter (tdc), based on a time expansion technique, that digitizes and records the time cimmino et al. 2 figure 3. the relation between the charge and the discharge of a capacitor (left) and the corresponding enable window (right). with the fast or trigger signal a capacitor starts charging and it stops when the master board produces a global trigger. at the same time the enable signal is activated and it lasts until the same capacitor is discharged. time is digitized by counting the clock pulses. figure 1. the fine tuning of the sipm bias voltage is realized by setting an 8-bit dac (dac8). higher dac8 slow control setting corresponds to a lower voltage adjustment. figure 2. slow shaper time constant (s.s.t.c.) as a function of the slow control register setting. 3 interval between the production of a local trigger and the reception of a stop signal. the stop signal is sent by the master board and represents a signal common to all slave boards. the digital time value is inferred from the discharge of a capacitor with respect to a reference clock. counting the number of clock pulses occurred during the discharge, the time is expanded using a discharge circuit, with greater time constant with respect to the one that previously charged the capacitor. this time expansion technique increases both the accuracy of the digitization and the system latency. a discharge circuit is used to count the number of clock pulses occurred during the discharge. for the last version of the circuit, the expansion factor is e = 20. figure 3 shows the entire process relative to a single slave board. when sizing the discharge circuit, it would be desirable to use low capacitance values, because the steepness of the discharge sharply cuts through the threshold. this event determines the closing of the enable window and its jitter reduces in this case. a small jitter of the enable signal, corresponds to a small error in the counts. however, as it can be seen in figure 4, small values of the capacitance lead to a non-linear response. the time expansion circuit uses a 100 pf capacitor which offers good linearity. the measured rms was 1.8 counts, suitable for a sub-ns time resolution. 5. the master board a master board manages all the slave boards in a muray telescope. the master board is equipped with a credit-card-sized computer, the raspberry pi (rpi). the most interesting features of the rpi is the on-board general purpose input/output (gpio) unit, based on which we implemented a low level protocol with the master board. we obtained a link speed of 20 mbps, the maximum data transfer rate supported by the rpi. the protocol works so that the data acquisition starts when the rpi sets slave boards in run mode; when a trigger occurs, the rpi stops the acquisition and switches the slave boards in diag mode. the rpi interprets physical events by means of hardware interrupts. once a physical event is recognized, the rpi downloads and records the hit by sending 32 clock pulses per time to the master board. in response to each clock train, the rpi receives a 32 bit packet from the master board, up empty the fifo memory of the latter. a handshaking procedure, written in c language, runs at the end of every communication and uses the rpi gpio native libraries. another important feature used during data acquisition, is the possibility to set the number of events to be collected. since the rpi is not a real time processing device, the software stores the data acquired from the master board inside the ram; when the acquisition stops, the program runs a separated thread that writes the data on a usb hard disk drive. then the acquisition restarts, cutting down the dead time (several ms) due to the hard disk drive access. the muraves telescope front-end electronics and data acquisition figure 4. plot of the experimental data relative to the sizing of the time expansion circuit. the value of the capacitance has been chosen in order to have a tdc linear response. figure 5. measured pulse height dark spectrum of a hamamatsu mppc s12825-050p. the peaks of the first, second and third photoelectron distribution are well visible. 6. telescope characterization the sipm used is the hamamatsu mppc s12825050p, whose pulse height spectrum is shown in figure 5. this kind of sipm has a low dark rate and the variation of the operating voltage as function of the temperature is about 50 mv. the operating voltage varies from 66.34 v to 66.60 v. in general, a change of the temperature produces a variation of the breakdown voltage of the sipm, where the gain and the photon detection efficiency (pde) vary of some percent per celsius degree following these changes. moreover, the dark rate increases with increasing temperature [renker 2006]. in figure 6 is reported a recorded dark rate vs threshold scan, relative to a slave of the telescope. we choose a test working point with the or32 dark rates between 50 khz and 100 khz, so considering the fourth photoelectron signals. up to now, we have a trigger efficiency for the coincidence of six planes greater than 80%, with some planes reaching the 99% efficiency. the trigger efficiency was measured with the planes in the horizontal position. the measured muon flux in a steradian is about 40 hz. 7. conclusions muon radiography applications in the field of volcanology requires tracking with high spatial resolution and rejection of backward muons mimicking muons coming from the volcano. a precise time of flight measurement is necessary for the backward muon rejection. in order to satisfy these requirements, the muray prototype was improved and the muraves detector is provided with high spatial/angular resolution, high resolution timing, time of flight measurement and can be used in extreme environment (few tens of watts of power consumption and modularity). while the muraves detector is under construction, we tested and characterized the front-end electronics to obtain the best performances. we studied the 8-bit dac response for the fine tuning of the sipms bias voltage and characterized the tunable settings to improve signal acquisition. the signal formation and its conversion can be adjusted by setting ad hoc values for the preamplification, the slow shaper and the peaking time. the sipm response to the threshold variation and trigger efficiency are actually under investigation, to set the working point of the detector. the high detection efficiency and the good quality of the collected data are suitable for the data analysis. the resolution of the muon time of flight was measured to be better than 400 ps. finally, a complete acquisition software in python programming language was written and runs on a raspberry pi computer. the software has auto control functions and can detect malfunctions, a mandatory requirement for long stand-alone data acquisitions. references ambrosino, f., et al. (2013). the mu-ray detector for muon radiography of volcanoes, nucl. instrum. meth. a, 732, 423-426. ambrosino, f., et al. (2014). the mu-ray project: detector technology and first data from mt. vesuvius, j. instrum., 9, c02029. callier, s., c. de la taille, g. martin-chassard and l. raux (2012). easiroc, an easy & versatile readout device for sipm, physics procedia, 37, 15691576, (tipp 2011). renker, d. (2006). geiger-mode avalanche photodiodes, history, properties and problems, nucl. instrum. meth. a, 567, 48-56. *corresponding author: luigi cimmino, istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy; email: cimmino@na.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. cimmino et al. 4 figure 6. plot of the dark rates of the 32 sipms vs dac10 slow control setting (top) and the corresponding or32 of all dark rates (bottom). higher dac10 values corresponds to lower thresholds. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice the etnaplumelab (epl) research cluster: advance the understanding of mt. etna plume, from source characterisation to downwind impacts annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 the etnaplumelab (epl) research cluster: advance the understanding of mt. etna plume, from source characterisation to downwind impacts. pasquale sellitto1, giuseppe salerno 2, pierre briole3∗ 1 laboratoire de météorologie dynamique (lmd), cnrs-umr8539, institut pierre simon laplace, école normale supérieure, école polytechnique, université pierre et marie curie, paris, france psellitto@lmd.ens.fr 2 istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy 3 laboratoire de géologie, cnrs-umr8538, école normale supérieure, paris, france abstract in 2013, a multidisciplinary research cluster named etnaplumelab (epl) was established, gathering experts from volcanology and atmospheric science communities. target of epl is to advance the understanding of mt. etna’s gas and aerosol emissions and the related processes, from source to its regional climatic impact in the mediterranean area. here, we present the cluster and its three interacting modules: epl-radio (radioactive aerosols and other source parameters for better atmospheric dispersion and impact estimations), smed (sulfur mediterranean dispersion) and med-suv (mediterranean supersite volcanoes) work package 5. first results have provided pioneering highlights on the relevance of mt. etna’s plume impact at the mediterranean regional scale. these results underline that further efforts need to be made to get insight into a synoptic volcanogenic-atmospheric chemistry/climatic understanding of volcanic plumes impact. i. introduction m ount etna is a persistent source of gas and aerosols in the lower and upper mediterranean troposphere. its sulphur emission has been estimated to be about 10 times larger than anthropogenic emissions in the same area [graf et al., 1997]. recently, it has been shown that mt. etna has the potential to modulate the downwind aerosol properties, e.g. by the production of highlyreflective secondary sulphate aerosols from the conversion of sulphur dioxide emission [villani et al., 2006, sellitto et al., 2016]. in its turn, this has revealed the potential to exert a significant direct regional climatic forcing, depending on the environmental conditions and the optical properties of the produced aerosols [sellitto and briole, 2015, sellitto et al., 2016]. though mt. etna is one of the most studied volcanoes in the world, e.g. [bonaccorso et al., 2004], its atmospheric impact, from small-range air quality perturbation to far-range climatic forcing is not yet well understood, or even, its influence might be potentially underestimated. this is due ∗this work has been partially supported by the european union’s horizon 2020 research and innovation programme under grant agreement no 654182 (envri+), the european union’s 7th framework program under the grant no 308665 (med-suv) and the ingv under the grant smed. 1 mailto:psellitto@lmd.ens.fr annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 to the multidisciplinary ancestry of this argument, which require a synoptic investigation framework from inner-volcanology to atmospheric sciences and climatology. this integrated framework would allow to better characterise the volcanic degassing and to refine and tune the atmospheric chemistry/physics and climate sciences modelling, to better understand both tropospheric and stratospheric downwind impacts. based on this undeniable objects, in 2013 a multidisciplinary research cluster named etnaplumelab (epl) was kicked-off. this cluster results from the combination of three different interacting projects. here, we will detail the scopes, rationale and perspectives of epl. the paper is structured as follows: sect. ii reports the general structure of epl, in sect. iii the three different modules that constitute the epl cluster are detailed and finally in sect. iv conclusions and perspectives are given. ii. a multidisciplinary and multiscale modular approach for the characterisation of volcanic plumes the epl research cluster aims at a better understanding of mount etna’s gas and particulate emissions, their generation, evolution and impacts. the target area is the mediterranean basin. the cluster arises from the interacting combination of three different modules/projects: • epl-radio (radioactive aerosols and other source parameters for better atmospheric dispersion and impact estimations); • smed (sulfur mediterranean dispersion); • med-suv (mediterranean supersite volcanoes) wp (work package) 5. figure 1 shows the spatial extent and the volcanic and atmospheric processes tackled by the three epl modules, as well as their interactions. the epl-radio project is specifically devoted to source characterisation, in terms of geochemistry and degassing processes, and the near-source plume characterisation. the smed project investigates the dispersion of the so2 emissions in the central mediterranean region, whereas the med-suv wp5 project, though its multipack volcanology scope, also promotes the study of the impact of mt. etna’s emissions at a broader regional scale. the smed and the med-suv wp5 projects superpose to a certain geographical extent. 2 annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 tropospheretroposphere stratospherestratosphere so 2 ash sicily sicily mediterranean mediterranean z epl-radio geochemistry degassing near-source plume so 2 → sulfate aerosols med-suv wp5 regional radiative forcing smed dispersion cloudaerosol interactions figure 1: spatial extents and interactions of the three modules establishing the epl reserch cluster. figure 2 shows the structure of the epl research cluster into the three modules. the modules are interacting and knowledge gathered from one module (at smaller spatial scale) is fed to the following module (at larger spatial scale). for instance, epl-radio has the potential to provide input source parameters for the dispersion modelling tackled by smed; on the other hand smed has the potential to give plume’s parameters such as to define optical downwind aerosols properties essential for the climatic impact estimation explored by med-suv wp5. the different scientific targets and the methods employed in each of the three modules (e.g., modelling and observations) are also indicated and color-coded. the timeframes of the modules are also shown. the broad split into (a) source epl-radio , (b) evolution smed, and (c) impact med-suv wp5, of mt. etna emissions follows from fig. 1. the epl research cluster relies on high quality direct and remote groundand satellitebased observations of volcanic plume, together with chemistry, transport and radiative transfer modelling. while for the smed and medsuv the employ of both observations and modelling is needed, epl-radio is only based on groundand satellite-based measurements for characterising the aerosol source (including gaseous precursors of secondary aerosols). the med-suv project ended in december 2015, but results are still available for further investigation. the smed project runs from november 2015 to end 2016. the epl-radio activities are foreseen until summer 2017. 3 annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 emission rates of primary aerosols and and of secondary aerosols precursors aerosol near-source properties source impact regional distribution of primary aerosols, secondary aerosol and their precursors plume dispersion chemical and micro-physical evolution evolution downwind aerosol properties perturbations direct radiative forcing = chemistry/transport modelling = radiative transfer modelling = satellite measurements = ground-based measurements 2013 2014 2015 20162015 2017 2018 med-suv wp5med-suv wp5 smedsmed epl-radioepl-radio ep lra di o ep lra di o sm ed sm ed me dsu v me dsu v w p5w p5 cloud-aerosols interactions air quality figure 2: breakdown of the epl research cluster, with indication of scientific targets, methods and timeframes of the three individual modules of the cluster. iii. the different modules iii.1 epl-radio the epl-radio project aims at improving the characterisation of mt. etna as atmospheric aerosols source, gathering multi-domain (solid earth and atmospheric sciences) information [sellitto, 2016]. object of epl-radio is to advance our knowledge on mt. etna’s emissions, targeting emission processes, from inner degassing mechanisms to aerosol near-source characterisation. this project brings together different and original information, such as observations of (a) radioactive disequilibria of radon daughters, (b) coupled direct/remote size-resolved aerosols distribution, (c) primary/secondary near-source sulfate aerosols partitioning. this volcanic aerosols decription allows a detailed size-resolved physicochemical characterisation of the emitted aerosols. the point (a) is more thoroughly discussed by terray et al. (2016). the volcanic aerosol source is characterised by determining the size-resolved aerosol emissions, with a cascade impactor and a microtops-ii optical spectrometers (see, e.g. [mather et al., 2004]), and the primary fraction of the emitted sulfate aerosols (with respect to the secondary sulfate aerosols produced by in-plume conversion of so2 emissions) by ftir (fourier transform infrared) spectroscopy [la spina et al., 2010]. complementary three-dimensional aerosols information is obtained with a scanning ample li4 annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 dar (aerosol multi-wavelength polarization lidar experiment light detection and ranging) system [scollo et al., 2015]. these parameters are crucial to assess the size-dependent plume dispersion and downwind climatic impacts, and are then an input knowledge for the smed and med-suv wp5 modules. the epl-radio project is linked to an extended transnational access at mt. etna’s summit craters area, partially funded by the european union’s horizon 2020 research and innovation programme (envri+ tna). two dedicated campaigns have been scheduled in 2016 (c1: aerosol sampling at the summit craters, c2: distal observations in an area of about 30 km from the craters) and 2 in 2017 (c3 and c4). highest priority is given to c1 and c3, i.e. the field experiments at the summit craters. first, preliminary results of campaigns c1 and c2 have shown how near-source processes (in the first few km downwind the emitting craters) play a major role in determining the volcanic aerosol properties downwind, thus influencing the dispersion and impacts of the plume. iii.2 smed the smed project [salerno et al., 2016] aims at assessing: (a) the spatial distribution of the regional atmospheric dispersion of mt. etna’s volcanic emissions, (b) the impact of moderate volcanic activity and long-lived passive degassing, and (c) lower to upper tropospheric sulphate aerosol formation processes and lifecycle, in the regional area of central-eastern sector of the mediterranean basin. while the project is mostly target to the central-eastern mediterranean, where up to 80% of the etnean plume disperses [sellitto et al., 2017], the less downwindventilated central-southern mediterranean region is also explored [sellitto et al., 2016]. the study is pursued by a comprehensive set of remote groundand satellite-based measurements, such as: (a) the ingv osservatorio etneo flame (flux automatic measurements) ultraviolet scanning spectrometer network [salerno et al., 2009] and ftir [la spina et al., 2010] measurements, (b) low earth orbit (modis moderate resolution imaging spectroradiometer, iasi infrared atmospheric sounding interferometer) and geostationary (seviri spinning enhanced visible and infrared imager) satellite sensors, (c) the aeronet (aerosol robotic network http://aeronet.gsfc.nasa.gov) stations in nicolosi(catania), italy, and in the centraleastern mediterranean, (d) the enea superstation for climate observation in lampedusa. the aeronet stations routinely measure column optical and microphysical properties of the aerosol layer, which are used to assess the downwind impact of mt. etna’s sulphur emissions. additionally, the lampedusa station is equipped with complementary instrumentation, including a lidar which provides height resolved aerosol and clouds information. these data allow the characterization of aerosol layer and their precursors, which is fundamental for identifying downwind volcanic contribution. ground and space observations are complemented with an atmospheric lagrangian model (flexpart flexible particle dispersion model, https://flexpart.eu/) for inspecting the transport and dispersion of volcanic plumes over the mediterranean region. first results have allowed the characterization of the etnean plumes dispersion at the decadal timescale, thus quantifying the frequency of ventilation of different mediterranean sub-sectors and confirming the prevalence (about 80% frequency) of the dispersion of the plumes towards the eastern quadrant (i.e., towards greece) [sellitto et al., 2017]. iii.3 med-suv wp5 the european union’s 7th framework program med-suv project aims at improving the assessment capacity of volcanic hazards in selected supersites (i.e., well instrumented sites) of southern italy. the main targets of this project are the optimisation and integration of existing and new observation/monitoring systems, the systematic study of volcanic pro5 annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 cesses and the enhancement of the coordination between the scientific and end-user communities. work package (wp) 5 of med-suv is devoted to improve the knowledge of processes occurring at mt. etna supersite. the primary aim of med-suv wp5 is the study of inner volcanology processes. nevertheless, the degassing, transport and atmospheric impacts of mt. etna are also addressed. one specific target of med-suv wp5 is to extend the scopes of the vamos seguro (volcanic ash monitoring and forecasting between sicilia and malta area and sharing of the results for aviation safety) project (http://www.vamosseguro.eu/) and to study the downwind impacts of mount etna’s ash and so2 emissions in the central mediterranean. in such a sense, med-suv wp5 extends geographically the scopes of smed (see fig. 1), allowing to investigate the regional impacts of volcanic emissions in the central mediterranean. these interacting studies have shown, for the first time, that even relatively weak eruptions have the potential to modulate the optical and micro-physical properties of the aerosol layer at hundreds of km from mt. etna, thus exerting a significant, episodic radiative forcing [sellitto et al., 2016], which is critically dependent from chemical/micro-physical processes in atmosphere [sellitto and briole, 2015]. the radiative impact is studied with both radiative transfer modelling and satellite observations of radiative fluxes. longer-term analyses have shown that mt. etna’s sulphur emissions can have a significant decadal impact on aerosol optical properties even in weakly ventilated subsectors, like observed at lampedusa (southwestern quadrant) [sellitto et al., 2017]. iv. conclusion and perspectives emissions of volcanic gases and particles profoundly impact on terrestrial environment, atmospheric composition and climate, at various temporal and spatial scales. early research results suggested that the regional impact of mt. etna in the mediterranean might be potentially underestimated. a novel and vigorous multidisciplinary approach is then required to explore this subject. hence, in 2013, the epl multidisciplinary and multiscale research cluster has been established, aiming at a better characterisation of mt. etna’s emissions, from source to downwind impacts. first results have shown pioneering evidences of mt. etna’s impacts in the mediterranean, in terms of the modulation of the aerosol optical properties and of the potential direct radiative forcing at selected locations. the near-craters and near-source distal eplradio campaigns, foreseen for the summers 2016 and 2017, will provide further insights into the degassing and emission processes, and near-source plume characterisation. this will allow to refine the regional dispersion modelling and help the interpretation of satellite data at the regional scale. on the other hand, the smed activities will provide an overall picture of the dispersion and regional distribution of volcanic effluents in the mediterranean, that will give better direct climatic forcing estimations. further important topics, not addressed at present by epl are the modelling and observation of the chemical and micro-physical evolution of the plume, the role of volcanic aerosol in the formation, determination of optical properties and climatic forcing of low and high clouds and, at a smaller spatial scale, the impact of volcanic emissions on local and regional air quality (see fig. 2). further research is ongoing to include these aspects in the research cluster. acknowledgements all participants of the epl-radio, smed and medsuv wp5 projets are gratefully acknowledged, especially: t. caltabiano, e. carboni, s. corradini, a. di sarra, p.-j. gauthier, a. la spina, b. legras, l. merucci, g. puglisi, s. scollo, l. terray. 6 annals of geophysics, fast track 6, 2017; doi:10.4401/ag-7106 references [bonaccorso et al., 2004] bonaccorso, a. et al. (2004). mt. etna: volcano laboratory. american geophysical union publisher, washington, dc, 143. [graf et al., 1997] graf, h. f. et al. (1997). volcanic sulfur emissions: estimates of source strength and its contribution to the global sulfate distribution. j. geophys. res., 102(10), 10727–10738. [la spina et al., 2010] la spina, a. et al. (2010). unravelling the processes controlling gas emissions from the central and northeast craters of mt. etna. j. volcanol. geotherm. res., 198(3-4), 368–376. [mather et al., 2004] mather, t. et al. (2004). characterization and evolution of tropospheric plumes from lascar and villarrica volcanoes, chile. j. geophys. res., 109, d21303. [salerno et al., 2009] salerno, g. et al. (2009). three-years of so2 flux measurements of mt. etna using an automated uv scanner array: comparison with conventional traverses and uncertainties in flux retrieval. j. volcanol. geotherm. res., 183, 76–83. [salerno et al., 2016] salerno, g. et al. (2016). smed sulphur mediterranean dispersion. geophys. res. abstr., 18, egu20168131, 2016. [scollo et al., 2015] scollo, s. et al. (2015). lidar measurements carried out during the 28 february 2013 lava fountain event at mt. etna, in italy. geophys. res. abstr., 17, egu2015-10137. [sellitto and briole, 2015] sellitto, p. and briole, p. (2015). on the radiative forcing of volcanic plumes: modelling the impact of mount etna in the mediterranean. ann. geophys., 58, doi:10.4401/ag-6879. [sellitto, 2016] sellitto, p. (2016). the epl-radio project co-funded by envriplus, envriplus newsletter #3, 2016 http://www.envriplus.eu/wpcontent/uploads/2015/08/epl-radio.pdf. [sellitto et al., 2016] sellitto, p. et al. (2016). synergistic use of lagrangian dispersion and radiative transfer modelling with satellite and surface remote sensing measurements for the investigation of volcanic plumes: the mount etna eruption of 2527 october 2013. atmos. chem. phys., 16, 6841–6861. [sellitto et al., 2017] sellitto, p. et al. (2017). the impact of mount etna sulfur emissions on the atmospheric composition and aerosol properties in the central mediterranean: a statistical analysis over the period 2000-2013 based on observations and lagrangian modelling atmos. environ.,148, 77–88. [terray et al., 2016] terray, l. et al. (2016). degassing dynamics at mount etna inferred from radioactive disequilibria (210pb210bi-210po) in the volcanic plume. geophys. res. abstr., 18, egu2016-10373-3. [villani et al., 2006] villani, m. g. et al. (2006). transport of volcanic aerosol in the troposphere: the case study of the 2002 etna plume. j. geophys. res., 111, d21102. 7 introduction a multidisciplinary and multiscale modular approach for the characterisation of volcanic plumes the different modules epl-radio smed med-suv wp5 conclusion and perspectives vol49_2_2006 deep-sea gravity measurements: geostar-2 mission results valerio iafolla, sergio nozzoli, emiliano fiorenza and vadim milyukov istituto di fisica dello spazio interplanetario (ifsi), cnr, roma, italy abstract a new concept gravity meter with sensitivity close to in the range of 10−5−1 hz intended for observation of the vertical component of the earth gravity and teleseismic waves was implemented at the istituto di fisica dello spazio interplanetario (ifsi), cnr and successfully operated during the geostar-2 mission. the gravimeter has demonstrated a capability to operate for a long time in an autonomous regime and a good reliability for operation in extreme environments; at the same time the experimental measurements gave information for further gravimeter implementation. results of observation and data analysis including the recording of seismic waves excited by global earthquakes and the evaluation of the low frequency modes of free oscillations of the earth are reported. hz8 2-ms10 695 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words seafloor gravimeter – teleseismic waves 1. introduction the rarity of gravity observation in regions that are difficult to access still restricts our knowledge of the detailed gravity field of the earth. a special interest from this point of view is the gravity measurement on the sea or ocean bottom. a gravity meter for deep-sea use was implemented at the istituto di fisica dello spazio interplanetario (ifsi), cnr on the basis of technology developed in the framework of the programs for design and realization of a space-borne high sensitive accelerometer (iafolla et al., 1997, 1998) with financial support of the agenzia spaziale italiana (asi). such technology has also been used for the implementation of tiltmeters, one of them has successfully operated for several years in the infn underground laboratory (gran sasso) (iafolla et al., 2001). the instrument, named «geograv-1», is conceived for measuring the vertical component both the variation of gravitational field and seismic waves and is able to operate in extreme environments (deep-sea level) for a long period without remote control. the gravimeter, together with other scientific instruments, was installed on the autonomous deep-sea observatory named geostar intended for multidisciplinary, longterm monitoring. the geostar-2 observatory was successfully deployed in september 2000 on the bottom of tyrrhenian sea at the depth of 2000 m, near ustica island (italy), and was successfully recovered in march 2001. the full operation time of the observatory was 172 days (favali et al., 2002; gasparoni et al., 2002). 2. seafloor gravimeter the mechanical part of the gravimeter consists of a proof mass which is connected to an external frame by two torsion arms and represents an harmonic oscillator with resonance frequency equal to 15 hz. the mechanical oscillator is obtained machining a single plate of aluminium al 5060. two external plates are faced mailing address: dr. valerio iafolla, istituto di fisica dello spazio interplanetario (ifsi), cnr, via del fosso del cavaliere, 00133 roma, italy; e-mail: valerio.iafolla@ifsi.rm.cnr.it 696 valerio iafolla, sergio nozzoli, emiliano fiorenza and vadim milyukov in its opposite sides to realize a couple of capacitive detectors working in differential mode. this difference should be zero when the proof mass is under the action of the earth gravity (g = 9.8 m/s2). (the minimum detectable acceleration, 10−8 m/s2, produces a displacement of 6.3×10−13m). the read-out system of the instrument is a capacitive bridge biased by a voltagevp = 100 v at the frequency of 100 khz. two arms of the bridge are constituted by the two capacitor detectors in the differential configuration while the other two are external fixed capacitors. the displacement of the proof mass gives the variations of the two sensing capacitors, producing a bridge inbalance and a consequent modulation of the driven voltage at the signal frequency. the output signal due to the unbalancing bridge is amplified by a low noise amplifier, demodulated and sampled at a rate of 10 s. the digital signal is sent to the geostar data acquisition control system (dacs), which also synchronizes the gravimeter acquisition system with the other instruments (iafolla and nozzoli, 2002). the total gravimeter power consumption is 190 mw. the gravimeter is suspended inside special spherical deep-sea glass housing by means of gimbals and is installed on the geostar platform. during the mission there is no possibility for remote control of the gravimeter. the gimbals allow automatic recovery of the verticality of the gravimeter sensitive axis with a precision better than 1° (1.5×10−3 m/s2 in gravity units) when the geostar platform is located at the sea bottom. the arrangement of the gravimeter inside the spherical glass housing is shown in fig. 1. the pre-mission calibration of the gravimeter was done by the standard procedure – high precision inclination in a vertical plane. the value of the calibration factor is equal to (0.98 ± ± 0.26)×10−8 ms−2/adc_count, estimated in the dynamic range of 0.1 m/s2. the instrumental response is linear within 0.5%. 3. estimation of the instrument dynamic during the operation time the gravimeter is underwent the action of the following signals: a variation of the vertical tidal gravity component, seismic waves from global earthquakes, local disturbances of the gravitational field. one can expect the largest signal variation at the level of (2−3)×10−6 m/s2 caused mostly by tidal gravity. the greatest instrumental effects are due to thermal variations and ageing of the gravimeter mechanical springs. the gravimeter thermal dependence, caused mainly by the thermal variation of the spring elastic constant, is estimated at the level of 10−3 ms−2/°c. the daily temperature variation is expected to be less than 10−3 °c, while during the six months of the mission it could be approximately 1°c. the maximum signal variation related to this value is 10−3 m/s2. time dependence of the output signal is caused by ageing of the elastic springs and the consequent changing the proof mass equilibrium position. the preliminary experimental estimation indicates an upper limit of 5×10−4 ms−2/day. during 180 days of the mission this exponential drift gave the signal variation of 9×10−2 m/s2. this value determines the total signal variation during full mission time, and the necessary instrument dynamic range. 4. data analysis the gravimeter operated from september 25, 2000 up to march 16, 2001, almost 172 days. the original data consist of 21 uninterfig. 1. general view of see-floor gravimeter mounted inside of spherical glass housing. 697 deep-sea gravity measurements: geostar-2 mission results rupted runs divided by gaps of the different time duration (table i). during the mission the signal changed in time at a rate of ∆g/∆t = = 2.33×10−4 ms−2/day, that is twice less than the predicted value. the maximum signal variation is 4×10−2 m/s2. removing the time trend reduces the gravity variation from 4×10−2 m/s2 to 0.16× ×10−2 m/s2. the variations of the gravity signal, the temperature and the pressure during the whole mission are shown in fig. 2, indicating the strong correlation between the gravity signal and the temperature. the thermal constant of the gravimeter was estimated using the data of november 2000. the second order polynomial of the temperature data was fitted by the least squares method to the gravity data. to avoid the distortion of the gravity signal due to the high frequency temperature noise, the temperature data were filtered by a low pass filter with a cutoff frequency of 10−4 hz. the estimated experimental value of the linear thermal constant of the gravimeter is (∆g/∆t ) = − 5.478×10−3ms−2/°c. the temperature regression reduces the gravity signal variation to the level of (20−30)×10−6 m/s2. the standard deviation (std) of the residual gravity signal is 9.1×10−6 m/s2, while std of the detrend gravity signal is 179×10−6 m/s2. therefore regression to the temperature reduced the signal variation of almost 20 times. to compare the gravity signal with theoretical tides, the residual signal was filtered with the pass-band filter in the tidal frequency domain (diurnal and semidiurnal waves). theoretical gravity tides were calculated by means of the eterna package (wenzel, 1996) for the same period (november 2000) and for the gravimeter location. the experimental gravity signal and the theoretical tides are shown in fig. 3. std of residual gravity is 4.3×10−6 m/s2, table i. time parameters of the geostar gravimeter data. beginning end duration date time date time hours run 1. 25/09/2000 11:00:00 27/09/2000 12:59:50 50 run 2. 27/09/2000 13:57:40 30/09/2000 12:59:50 71 run 3. 30/09/2000 14:57:30 03/10/2000 02:59:50 60 run 4. 03/10/2000 04:57.30 08/10/2000 11:59:50 127 run 5. 08/10/2000 03:57:30 09/10/2000 07:59:50 18 run 6. 09/10/2000 08:57.30 20/10/2000 19:59:50 275 run 7 20/10/2000 21:57:30 26/10/2000 04:59:50 127 run 8. 26/10/2000 05:57:30 04/11/2000 06:59:50 217 run 9. 04/11/2000 07:57:30 16/11/2000 01:59:50 282 run 10. 16/11/2000 03:57:30 01/12/2000 23:59:50 380 run 11. 02/12/2000 01:57:30 02/12/2000 19:59:50 18 run 12. 02/12/2000 20:57:30 02/12/2000 20:57:30 3 run 13. 03/12/2000 01:21:30 20/12/2000 21:59:50 429 run 14. 20/12/2000 22:57:30 06/01/2001 00:59:50 386 run 15. 06/01/2001 01:57:30 25/01/2001 05.59:40 470 run 16. 25/01/2001 16:57:30 28/01/2001 15:59:50 71 run 17. 28/01/2001 16:57:30 22/02/2001 08:59:50 592 run 18. 22/02/2001 10:57:30 26/02/2001 04:59:50 90 run 19. 26/02/2001 05:57:30 28/02/2001 12:59:50 55 run 20. 28/02/2001 13:57:30 03/03/2001 00:59:50 59 run 21. 03/03/2001 02:57:30 16/03/2001 00:59:50 310 fig. 3. residual gravity signal after temperature regression (november 2000), and gravity tides calculated by eterna program for the same period of time. signal variation is almost one order higher then tidal gravity. 698 valerio iafolla, sergio nozzoli, emiliano fiorenza and vadim milyukov while std of tidal gravity is 0.54×10−6 m/s2. it means that the signal variation in the tidal domain even after temperature reduction is still almost one order higher than tidal gravity. the spectral densities of the gravitational signal, the temperature and pressure data were estimated for the entire set of the observation, from september 2000 to march 2001. the gravfig. 2. gravity, temperature and pressure observations in geostar-2 mission. 699 deep-sea gravity measurements: geostar-2 mission results table ii. list of seismic events registered by the gravimeter in geostar mission. the magnitudes are given moment magnitude units, mw. the last column is the values of the maximal amplitudes of the gravimeter response to the appropriate earthquake. usgs national earthquake information center date origin time utc geographic coordinates depth magnitude region gravimeter h:min sec lat long max amplitude m/s2 2000 ×10−6 1. 28/09 23:23 43 0.215s 80.582w 23 mw 6.6 near coast of ecuador 1.0 2. 02/10 02:25 31 7.977s 30.709e 34 mw 6.5 lake tanganyika region 4.0 3. 04/10 16:58 44 15.421s 166.910e 23 mw 6.7 vanuatu islands 1.0 4. 06/10 04:30 19 35.456n 133.134e 10 mw 6.5 western honshu, japan 4.0 5. 25/10 09:32 24 6.507s 105.604e 38 mw 6.8 sunda strait, indonesia 0.5 6. 07/11 00:18 04 55.627s 29.876w 10 mw 6.6 south sandwich islands 6.0 7. 10/11 20:10 53 36.601n 4.773e 10 mw 5.7 northern algeria region 4.0 8. 16/11 04:54 56 3.980s 152.169e 33 mw 7.6 new ireland region 15.0 9. 16/11 07:42 16 5.233s 153.102e 30 mw 7.4 new ireland region 8.0 10. 17/11 21:01 56 5.496s 151.781e 33 mw 7.6 new britain region 0.5 11. 25/11 18:09 11 40.245n 49.946e 50 mw 6.3 eastern caucasus 12.0 12. 06/12 17:11 06 39.566n 54.799e 30 mw 7.0 turkmenistan 7.0 13. 15/12 16:44 47 38.457n 31.351e 10 mw 6.0 turkey 1.5 14. 20/12 11:23 54 39.01s 74.66w 11 mw 6.5 southern america 1.5 fig. 4. amplitude spectra for gravity and pressure signals for all observation data. two peaks of the pressure spectrum are corresponding to diurnal p1s1 (p = 24.0 h) and semidiurnal m2 (p = 12.41 h) tidal waves. 700 valerio iafolla, sergio nozzoli, emiliano fiorenza and vadim milyukov ity and temperature spectra look like a flickernoise having no outstanding peaks while the pressure spectrum clearly demonstrates two well-pronounced maxima, corresponding to the diurnal p1s1 and semidiurnal m2 tidal waves (the corresponding frequencies are 1.16×10−5 hz and 2.24×10−5 hz). figure 4 shows the gravity and pressure spectra. during the geostar-2 mission the gravimeter working like a vertical seismometer recorded several global earthquakes. the total number of detected events during the six months of the mission is 22, the majority of them with magnitudes of mw 6.5-7.5 (hereafter magnitudes are given in the moment magnitude units, mw). the minimal detected magnitude is mw 4.9 (el salvador, 17/02/2001). the recorded earthquakes are listed in table ii (the earthquake parameters are taken from the national earthquake information center, denver; u.s.a.). the seismograms for some of them obtained from our data are shown in fig. 5. estimate of the gravimeter response to seismic waves – the response of the gravimeter to seismic waves excited by earthquakes differed in amplitude (the maximum amplitudes recorded by the gravimeter are shown in the last column of table ii). according to the gutenberg and richter formula, the empirical relationship between the energy e radiated as seismic waves and the moment magnitude mw is the following: loge=11.8 + 1.5 mw. one can estimate the relationship between the energy of seismic waves and the maximum amplitude response of the gravimeter (fig. 6). even if some of the important earthquake parameters were not taken into account (such, for example, as distance and depth), the rough estimate demonstrates the linear relation between the logarithm of the amplitude and the released energy, which is described by an empirical formula: spheroidal oscillations of the earth – free oscillations of the earth were observed and evaluated for the first time after the historical great earthquakes with magnitudes of mw 8.5 and greater (kamchatka, 1952, mw 9.0; chili, 1960, mw 9.5; the kurile islands, 1963, mw 8.5; and alaska, 1964, mw 9.2). due to evolution of both the instrumentation and analytical methods it becomes possible to observe the eigenfrequencies of many free oscillations of the earth excited by earthquakes also with magnitudes of mw 7-8. nevertheless the lowest order modes with frequencies below 0.8 mhz can very rarely be observed with good signal-to-noise ratio. due to the rather quiet condition the geostar mission provides a ( ) . . ( ) .log ms erga e5 4 0 33 10max 2 23#= +table ii (continued). usgs national earthquake information center date origin time utc geographic coordinates depth magnitude region gravimeter h:min sec lat long max amplitude m/s2 2001 ×10−6 15. 09/01 16:49 28 14.928s 167.170e 103 mw 7.1 vanuatu islands 5.0 16. 10/01 16:02 44 57.078n 153.211w 33 mw 7.0 kodiak island region 4.0 alaska 17. 13/01 17:33 32 13.049n 88.660w 60 mw 7.7 el salvador 60.0 18. 26/01 03:16 40 23.419n 70.232e 16 mw 7.7 southern india 25.0 19. 13/02 14:22 05 13.671n 88.938w 10 mw 6.6 el salvador 3.0 20. 13/02 19:28 30 4.680s 102.562e 36 mw 7.4 southern sumatera 3.0 21. 17/02 20:25 15 13.79n 89.11w 10 mw 4.9 el salvador 0.8 22. 24/02 07:23 48 1.127n 126.249e 35 mw 7.1 nothern molucca sea 3.0 701 deep-sea gravity measurements: geostar-2 mission results fig. 5. seismograms of some of the earthquakes registered by gravimeter. number of seismogram is corresponding to number of the earthquake in table ii. acceleration is given in arbitrary units (au). «zero line» is original time of event. fig. 6. relationship between gravimeter response and energy of seismic waves excited by earthquakes. linear polynomial is fitted to experimental values denoted as (*). 702 valerio iafolla, sergio nozzoli, emiliano fiorenza and vadim milyukov good opportunity to estimate and study the low frequency modes of the earth. for our analysis we selected three earthquakes which followed each other during two days in november 2000: new ireland (november 16, 04:54); new ireland (november 16, 07:42); and new britain (november 17, 21:01). these earthquakes were closely located (coordinates, depth) and had very similar magnitudes: mw 7.6, 7.4 and 7.6. for such a case the energy of seismic waves is expected to be accumulated during following quakes and «lifetime» of excited modes is increased. the length of the record used for the analysis is 182 h after the first quake. the data were filtered by the high pass filter with the cut frequency of 1.8×10−4 hz and then the fast fourier transformation with application of the hanning window of 91 h was performed. figure 7 presents the evaluation of the low-degree spheroidal modes of the free oscillation of the earth. the spectrum peaks can be identified with the most of the fundamental spheroidal modes of degree from 0 till 8. the some overtones of first and second degrees are pronounced too. the estimated periods in comparison with theoretical fig. 7. amplitude spectrum of 182-h-long record of new ireland earthquake. the vertical dashed lines show the degenerate frequencies of selected spheroidal modes as predicted for earth model 1066a. table iii. periods (pexp) of the low frequency main tones and overtones of free oscillation of the earth observed in the new ireland earthquake. pth is spheroidal modes as predicted for earth model 1066a. fundamental tones overtones nsl 0s2 0s3 0s4 0s5 0s7 0s8 1s1 1s2 1s4 1s5 2s2 2s4 pexp, s 3277 2114 1546 1200 801 708 2482 1463 849 733 905 720 pth, s 3230 2136 1547 1191 813 709 2466 1468 853 730 904 722 pexp − pth, s 47 −22 −1 9 −12 −1 16 −5 −4 3 1 −2 703 deep-sea gravity measurements: geostar-2 mission results ones are summarized in table iii. the experimental values demonstrate a good agreement with the theoretical ones. it should be mentioned that, since the barometric pressure corrections were not made, some of the peaks could be misidentified. due to this reason the non identified spectrum peaks could be related to possible pressure influence. fine resolution of the spheroidal mode 0s2 – the spinning of the earth produces the coriolis force, which is spherically asymmetric. this effect as well as the ellipticity of the earth lead to a breakdown of the degeneracy of the eigenfrequencies for 2l+1 values for each spherical harmonic of l degree. the result is called splitting, with the split eigenfrequencies being close together. so, the spheroidal mode 0s2, the longest-period fundamental (n = 0) mode of the earth, is split to five components. the degenerated mode 0s2 is clearly exposed in the spectrum of fig. 7. to resolve the fine structure of this mode the record data of the length of 273 h were resampled with sampling time of 1 min, and filtered with a narrow bandpass filter. figure 8 shows the result obtained for the fine resolution of the quintet 0s2. the vertical lines represent the theoretical values of quintet periods. the three highest peaks can, with reasonable certainty, be identified as, from left to right, m = − 2, m = 0, and m = 2. two others peaks of the quintet corresponded to m = − 1, and m = 1, are not completely resolved. the partial resolution of the quintet and the non symmetrical shapes of the resolved peakes can be explained by the fact that the data is rather contaminated by noise. 6. conclusions the new concept gravity meter for a deepsea measurement with sensitivity close to in the frequency range of 10−5+ −1 hz was developed with the financial support of the istituto nazionale di geofisica e vulcanologia (ingv). the gravimeter was installed on the autonomous deep-see observatory geostar-2 intended for multidisciplinary, ms hz10 8 2fig. 8. fine resolution of fundamental spheroidal mode 0s2 evaluated from 273-h-long record of new ireland earthquake. sampling time is 1 min. the vertical lines represent theoretical periods for 0s2 quintet. 704 valerio iafolla, sergio nozzoli, emiliano fiorenza and vadim milyukov long-term monitoring, and has successfully operated from september 25, 2000 to march 16, 2001, almost 172 days with efficiency of 99%. the large dynamic of the instrument permitted it to operate throughout the mission in an autonomous regime. the trend of the gravimeter output due to the spring aging had a value of 2.33×10−2 ms−2/day and is easily removable. the temperature dependence of the output signal is mainly caused by the thermal effects of the mechanical springs. the experimentally estimated linear thermal constant of the gravimeter is −5.478 × 10−3 ms−2/°c. regression to the temperature reduces the signal variation almost 20 times in the low frequency region; nevertheless it is still one order higher than the expected tidal variation. during the mission 22 global earthquakes with the magnitudes between mw 4.9 and mw 7.5 were recorded. the response of the gravimeter to the seismic waves exited by earthquakes differ ed in amplitude varying from 0.5×10−6 m/s2 to 60×10−6 m/s2. the logarithm of the response amplitude of the gravimeter and the energy of seismic waves released in the earthquakes demonstrate the linear relationship between them. the high sensitivity of the gravimeter and quite environment disclosed some of the low-order spheroidal tones and overtones of free oscillations of the earth below 0.8 mhz. the evaluation was done for the record of the new ireland earthquake (16/11/2000, mw 7.6). the experimental values demonstrate a good agreement with theoretical ones. for the same record the fine structure of the quintet 0s2 was resolved and some of the constituents were estimated. on the whole the gravimeter demonstrated a good capability to perform precise geophysical measurements in extreme environments and provides a good opportunity to record and study the phenomena such as teleseismic waves and free oscillations of the earth. references favali, p., g. smriglio, l. beranzoni, t. braun, m. calcara, g. d’anna, a. de santis, d. di mauro, g. etiope, f. frugoni, v. iafolla, s. monna, c. montuori, s. nozzoli, p. palangio and g. romeo (2002): towards a permanent deep-sea observatory: the geostar european experiment, in science-technology synergy for research in marine environment: challenges for the xxi century, edited by l. beranzoli, p. favali and g. smriglio, development in marine technology series (elsevier, amsterdam), 12, 111-120. gasparoni, f., d. calore and r. campaci (2002): from abel to geostar: development of the first european deep-sea scientific observatory, in science-technology synergy for research in marine environment: challenges for the xxi century, edited by l. beranzoli, p. favali and g. smriglio, development in marine technology series (elsevier, amsterdam), 12, 143-159. iafolla, v. and s. nozzoli (2002): gravimeter for deepsea measurements, in science-technology synergy for research in marine environment: challenges for the xxi century, edited by l. beranzoli, p. favali and g. smriglio, development in marine technology series (elsevier, amsterdam), 12, 183-197. iafolla, v., e. lorenzini and v. milyukov (1997): gizero: new facility for gravitational experiments in free fall, gravitation cosmol. (j. russ. grav. soc.), 3 (2), 145-150. iafolla, v., e. lorenzinil, v. milyukov and s. nozzoli (1998): methodology and instrumentation for testing the weak equivalence principle in stratospheric free fall, rev. sci. instrum., 69, 4146-4151. iafolla, v., v. milyukov and s. nozzoli (2001): tidal tilt observations in gran sasso underground laboratory, nuovo cimento c, 24, 263-271. wenzel, h.-g.(1996): the nano-gal software: earth tide data processing package, eterna 3.30, bull. inf. marees terr., 124, 9425-9439. u l t e r i o r i e l e m e n t i a s o s t e g n o d i u n a c o r r e z i o n e p e r i v a l o r i d e l l a g r a v i t a ' c a r l o m o r e l l i 1. c o n i è n o l o , la m a g g i o r p a r i e d e l l e m i s u r e di g r a v i l a e s e g u i l e s u l l a l e n a s e n o relative, c i o è d e r i v a n o da m i s u r e n e l l e q u a l i è s i a l a d e t e r m i n a l a la differenza di g r a v i l a fra le s t a z i o n i . c i ò p e r c h è le m i s u r e r e l a t i v e p r e s e n t a n o m i n o r i d i f f i c o l t à e s o n o m o l l o p i ù p r e c i s e di q u e l l e a s s o l u t e . ma q u e s t o i m p l i c a c h e a l m e n o u n v a l o r e a s s o l u t o d e v e e s s e r e m i s u r a l o con la m a s s i m a p r e c i s i o n e p o s s i b i l e , p e r s e r v i l e q u a l e ca[tosali lo di riferimento. 2. — q u a l e c a p o s a l d o i n t e r n a z i o n a l e di r i f e r i m e n t o p e r l e m i s u r e di g r a v i t à v e n n e c o n v e n z i o n a l m e n t e a c c e t t a t o n e l 1 9 1 1 il v a l o r e m i s u r a l o a p o t s d a m da k l t i m . n e l ' i r t w g v n o i . k r ( # = 9 8 1 , 2 7 4 c m sec 2 ) , d a l o c h e i a c c u r a t e z z a c o n cui v e n n e r o c o n d o t t e l e e s p e r i e n z e e la l u n g a s e r i e di m i s u r e e s e g u i l e ( 1 8 9 8 1 9 0 6 ) f a c e v a n o r i t e n e r e a l l o r a q u e s t o v a l o r e m o l t o e s a t t o ( e r r o r e m e d i o = 0 , 0 0 3 ) , di g r a n l u n g a più p r e c i s o d e l l e a l t r e m i s u r e a s s o l u t e p r e c e d e n t i . a q u e s t o c a p o s a l d o i n t e r n a z i o n a l e il b o r i ì v s r i f e r ì nel 1 9 0 9 , p r e v i a o p p o r t u n a c o m p e n s a z i o n e . i v a l o r i d e g l i a l t r i c a p i s a l d i n a z i o n a l i a l l o r a e s i s t e n t i , d a n d o così o r i g i n e al c o s i d d e t t o sistema di potsdam. t u t t e l e m i s u r e di g r a v i l a s u c c e s s i v e f u r o n o d a a l l o r a c a l c o l a t e in q u e s t o s i s t e m a . 3 . s o l t a n t o n e l 1 9 3 6 . a l l o r c h é v e n n e p u b b l i c a t o il n u o v o v a l o r e a s s o l u t o m i s u r a l o a w a s h i n g t o n da h i ì y . l cooic ( # = 9 8 0 , 0 8 0 h ) , 0 0 3 ) , si r i s c o n t r ò u n a n o t e v o l e d i f f e r e n z a c o l v a l o r e t r a s p o r t a t o a w a s h i n g t o n ( n e l s i s t e m a di p o t s d a m ) m e d i a n t e una m i s u r a r e l a t i v a dal b i ì o w n nel 1933 (g 9 8 0 , 1 0 0 ) . a n c h e la p u b b l i c a z i o n e ( 1 9 3 9 ) del v a l o r e di una s u c c e s s i v a m i sura a s s o l i l i a e s e g u i t a dal c.i.ark a t e d d i n g l o n ( # = 9 8 1 . 1 8 1 5 0 , 0 0 1 5 ) m i s e in e v i d e n z a un v a l o r e m o l t o più b a s s o di q u e l l o c l i c n e l l o t e s s o p o s t o v e n i v a a c c e ! l a t o eoi s i s t e m a >li p o t s d a m ( # = 9 8 1 , 1 9 5 3 ) . i n v e c e un c o l l e g a m e n t o di w a s h i n g t o n c o n t e d d i n g l o n e f f e t t u a l o i a i ì i . o u o i i i x i i nel 1939 da b r o w n k e 111 ì . i . a r u trovava una d i l f c r e n z a in l n i o n acc o r d o con i r i l i l i a l i d e l l e d u e m i s u r e a s s o l i n e p r e c e d e n t i . 1. d o p o a v e r e s e g u i l o la compensazione (iella relè ii.ternazionale delle stazioni di riferimento ( ' ) e p o s l a così la p r e m e s s a per un c o n l r o n l o più a t t c n d i l i i l e fra i v a l o r i a s o l u l i m i s u r a l i n e l l e v a r i e ( a z i o n i , lo s c r i v e n l e m e l l e v a in e v i d e n z a , sulla b a s e s o p r a t t u t t o d e l l e due m i s u r e m o d e r n e ili w a s h i n g t o n e l e d d i n g t o n , il p r o b a b i l e e r r o r e di cui -i rivelava o r a a f f e t t o il v a l o r e di p o t s d a m ( '). m i e stesse c o n c i u i o n i p e r v e n i v a n o , in m a n i e r a del t u t t o indip e n d e n t e , b k o w n k e b u l l a r d ( " ) in i n g h i l t e r r a , e d u y d k n ( ' ) ( ' ) negli 1 . s . a . c o n t e m p o r a n e a m e n t e lo s c r i v e n t e e s p o n e v a l ' o p p o r t u n i t à di abb a n d o n a r e il v a l o r e c o n v e n z i o n a l e di p o t s d a m q u a l e c a p o s a l d o i n t e r n a z i o n a l e di r i f e r i m e n t o , e di r i c a v a r e q u e s t o v a l o r e non da una o l a . m a da l u l l e le m i u r e a s s o l u t e m o d e r n e , sulla base d e l l e d i f f e r e n z e ira di esse r i s u l t a n t i d a l l a r e c e n t e c o m p e n s a z i o n e ('') ( ' ). in c o n c l u s i o n e , la correzione più probabile da apportarsi ai valori del sistema di potsdam per passare al nuovo sistema internazionale veniva indicala in — 0 , 0 1 6 0 , 0 0 2 c m . s e c -. ( i ) •>. — c e p r o p o s t e stillo stesso a r g o m e n t o a v a n z a l e indi pendentemente s o n o : a ) dal s o l l o c o m i l a l o p e r la g r a v i t à del » \ a l . b e s . c o u n c i l c o m m i l l e e on f u n d a m e n t a l p h y i c a l con<lants », sulla base del ries a m e del r i s u l t a l o di p o t s d a m e f f e t t u a t o dal d r v d e n ( ' ) : o.illt. in una p u b b l i c a z i o n e successiva s . 11a \1mih ( s ) d i m o s t r a v a perù c h e , in s e g u i t o ad un nuovo c o l l e g a i n e u l o ( i 9 4 6 ) di g r a n d e p r e c i s i o n e con g r a v i m e t r o a v y c k o i ' t fra le v a r i e s t a z i o n i di r i f e r i m e n t o a w a s h i n g t o n , la d i f f e r e n z a d e t e r m i n a l a dal b r o v w nel i 9 3 3 è m o l l o p r o b a b i l m e n t e e r r a l a di 2 i n g a l . per cui la c o r r e z i o n e per p o t s d a m d i v e n t a : — 0,015. ( 2 ) b) dal cl.vllk ( , j ) , c h e -lilla ba<e del v a l o r e 9 8 1 . 2 6 2 r i e l a b o r a l o por p o t s d a m dal d r y d i ì n (')• dei v a l o r i a s o l u l i s o p r a m e n z i o n a l i p e r w a s h i n g t o n e t e d d i n g t o n e dello d i f f e r e n z e p o t s d a m w a s h i n g t o n b r o w n 1933, w a s h i n g l o n t e d d i n g o n b r o n w k b u i . l a r d 1939 e t e l i l'ili! ul hi elementi a sostegno di una correzione per 1 valori della graviti 85 d i n g t o n . p o t s d a m b r i . r . a r l i j o r j . l 1 9 3 6 . ricavava col m e t o d o dei min i m i q u a d r a t i : — 0 , 0 1 4 5 ; ( 3 ) menlre. i n t r o d u c e n d o n e i c a l c o l i p o l s d a m col v a l o r e o r i g i n a l e 9 8 1 . 2 7 1 . la c o r r e z i o n e si r i d u r r e b b e a — 0 . 0 0 8 5 . 6 . c o m e >i v e d e , 1 valori ( 2 ) e ( 3 ) confermano pienamente il valore (1) ila noi proposto per la correzione al sistema (li potsdam. l e n e n d o c o n t o p e r ò d e l l a d i f f e r e n z a r i v e l a l a da s . h a i n m e r ( s ) . il v a l o r e m e d i o ira i p r e c e d e n t i ( i ) ( 2 ) ( 3 ) s e m b r a ora il più a d a l l o per la c o r r e z i o n e p r o p o s t a : per cui -i a v r e b b e : s i s t e m a i n t e r n a z i o n a l e s i s t e m a di p o t s d a m 0 , 0 1 5 c m . s e c (_]) •. m e n l r e r i m a n d i a m o i! l e t t o r e a l l e c o n s i d e r a z i o n i già esposle n e l l e p r e c e d e n t i note ( ' | ( " ) ( ' ) s u l l ' o p p o r t u n i t à di una a d o z i o n e del n u o v o s i s t e m a i n t e r n a z i o n a l e di r i f e r i m e n t o p r o p o s t o per le m i sure di g r a v i l a , ed a l l e c o n s e g u e n z e c l i c ne d e r i v e r e b b e r o , o s s e r v i a m o a n c o r a e l l e : a ) la p r o p o s t a s a r e b b e o r i n a i p i e n a m e n t e g i u s t i f i c a t a da q u a n t o s o p r a , e il v a l o r e f i) p r o p o s t o p e r la c o r r e z i o n e al s i s t e m a di p o t s d a m d e r i v a da valori m o l t o c o n c o r d a n t i , t a n t o da r i t e n e r s i s u f f i c i e n t e m e n t e a 11 end i b i l e : b) non s e m b r a p r o b a b i l e c l i c n u o v e m i s u r e a s s o l u t e col p e n d o l o p o s s a n o a v a n z a r e s e n s i b i l m e n t e n e l l a p r e c i s i o n e , e q u i n d i v a r i a r e i r i s u l t a t i p r e c e d e n t i di q u a n t i t à d e g n e di r i l i e v o c"|: e) m e n t r e p e r le m i s u r e d e l l ' a l t e z z a b a r o m e t r i c a , c l i c v a r i a con i a c c e l e r a z i o n e d e l l e g r a v i t à , v i e n e assunto c o m e v a l o r e di r i f e r i m e n t o q u e l l o a l l a l a t i t u d i n e di -15''. al l i v e l l o m e d i o del m a r e ( a c c e t t a t o per c o n v e n z i o n e in 9 8 0 . 6 6 5 d a l l a q u i n t a c o n f e r e n z a i n t e r n a z i o n a l e p e r i pesi e le m i s u r e , 1913), e io stesso v a l o r e v e n n e a d o p e r a t o p e r la def i n i z i o n e della p r e s s i o n e a t m o s f e r i c a c u i r i f e r i r e i punti di e b o l l i z i o n e nella s c a l a i n t e r n a z i o n a l e d e l l a t e m p e r a t u r a ( s e t t i m a e o l i a v a c o n f . g e n . p e s i e m i s u r e , 1927 e 1933), per d e l l o v a l o r e i" i n l e r n a l ionai m e t e o r o l o g i c a l c o m m i l t e e a c c e t t a v a il v a l o r e a r r o t o n d a t o 9 8 0 , 6 2 ( " ) : c o n v e r r e b b e p e r c i ò e l i m i n a r e q u e s t a d i v e r s i t à di valori i n t e r n a z i o n a l m e n t e a c c o l l a l i , t e n e n d o c o n t o p e r es. d e l l a f o r m u l a i n t e r n a z i o n a l e c a s s i n i s s i l v a 1930 a d a t t a l a al nuovo s i s t e m a i n t e r n a z i o n a l e (nel -is i o m a di p o t s d a m essa fornisce a 15" il v a l o r e 9 8 0 , 6 2 9 1 ) . c a r m i m o r k i 1 i la q u e s t i o n o e m i n a q u i n d i o r i n a i m a t u r a p e r la d i s c u s i o n o in e d c i n t e r n a z i o n a l e , p e r c u i è da a u s p i c a r e c l i c essa v e n g a p o r t a l a ad u n a p r o s s i m a \ s s e m l i l e a d e l l ' i 11ione g e o d e t i c a e g e o f i s i c a i n t e r n a z i o n a l e , in m o d o d a r i c h i a m a r e i a t t e n z i o n e sul p r o b l e m a e da p o r t a r e c o n s e g u e n t e m e n t e ad u n a o l u z i o n e d e i i n i t i v a . l'ricslr osseilittorio (jcofisico dell'i.\ai. giuu.no 1917. ri issl \to i valori (iella gravità basati sul sistema in uso (sistema di potsdam j necessitano di una correzione, dell'ordine di — 0 , 0 1 5 em. se e : a (/ucsti) risultato si c pervenuti indipendentemente in invilitici ru. i .s. i. e italia. i s i i ì l i o g r \ l i a l ' i moina.1,1 c . : compensazione della rete internazionale delle stazioni, di rij<rimento per le misure di gravità retatila i i i o l l . .'-ne. wlrialica s c i e n z e v i i . . \ ni. x i i . t r i e s t e 19161. (-1 molila.1.1 ('..: su una correzione per • valori della gravila. i \lli \cc, v./. i . i n e e i . r o m a . s. \ i l i . \ o i . i. f a s e . 7-1!. p g . 8-16-8531. r"'i liltov. ni-: ii. c. e h u i . i . a r i ) li. c.: coni purismi o j lite accelerili imi due lo gravity ut llie -\ational l.iiboratory. '/ ed d in gioii and the ilari nu ni sianilard. washington, l>. c. (proe. r. scie. london. 961). 163. paji. 1111-117. manli i<)!();. l 'i duviikn ii. i..: i ree.xainination oj the l'olsdnm ihsoiule delerminniioii oj gravity. i j o u r . r e s e a r e l i nat. l ì u r e a n ni' s l a n d a r d s . v o i . 2'). n. 5. 1912. pg. 3li.ì:il 11. i l d i ì v d k n i i . i . . : ihsoiule g rari ty~l tot or minai i ou s. < nat. l ì e s e a r r h c o n i l i ! vuierican g e o p h . i n i o n . t r a n s a e t i o n s ni' 1913. l ' a r i . 1. w a s h i n g t o n . i). c . . 1913. p g . 12-131. '''i m o i i k i . i . i c.: l'or un sistema di riferimento i< internazionale >i di'lle misure di grillila. \ota preliminare. i g e o l i i c a pura ed a p p l i c a l a , v o i . n i l i , l'a-c. 3-1. m i l a n o 1 9 1 6 i . l ' i moina.1.1 ( . . : \ mi velie contrilmtion en lavmir d'un sistemo internatiolinl de niesures tic gravitò. ( [itili. g e o d e . il. 3. p a r i s 1917'. l s ! i iaai vil li s . : gntvitncter lies iniueen gravity base slntions in ii ashinglmi ii. ('. i i r a n s a r l i o n s aniericaii g e n p h v i c a l l n i o n . voi. 211. il. 2. a p r i l e i 9 l 7 l . i ' ' i c i . a i i k j . s . : l'he acceleratimi due lo gravity. l a discussimi un l niland s t a n d a r d * . c o l i m i . i r o m the nat. p h > , l a b . . 21 m a r c h 19-161. ( j " l l ' o r s e ( p i a l l i l e n u o v o e l e m e n t o p o t r a n n o a p p o r t a r e le m i s u r e d e l l a gravila e e g u i l e m e d i a n t e l'osserv a z i o n e del t e m p o ili caduta l i b e r a (li gravi ilei vaiolo: una tale m i s u r a è in p r e p a r a z i o n e al l ì u r e a n i n t e r n a i , des p o i d s et m e u r e ai! o p e r a di c u . v o i . k t i s u r la mesurc ahsolue de la gravite. ('.. 11. ae. s e . l ' a r i . 2 2 2 . pg. 373-37."). j1 l e v r . p j i 6 ) . ( 1 ' i o r g a n i s a l i o n m é t é o r o l o g u p i e i n t e r i i a l i o n a l e . c o l i m i . m é t . i n i . , l ' r o . v e l l i . b e r l i n , 1 1 9 3 9 ) : l a u s a n n e , 4 5 , 75, (19-11). moment tensor solutions for the amatrice 2016 seismic sequence annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7240 moment tensor solutions for the amatrice 2016 seismic sequence silvia pondrelli*, simone salimbeni, paolo perfetti istituto nazionale di geofisica e vulcanologia, sezione di bologna, italy *silvia.pondrelli@ingv.it abstract on august 24, 2016 a ml 6.0 earthquake struck central italy region, nearly completely destroying some small ancient towns as amatrice, accumoli, arquata and pescara del tronto. in the following days thousands of aftershocks have been recorded by the ingv national seismometric network, 16 of them with a magnitude greater than 4.0. a quick rcmt solution has been rapidly computed for all of them and made available on the web. within a few weeks a definitive rcmt solution is ready for all of them, plus one. for major events (and not only) of the amatrice seismic sequence, several rapid moment tensor solutions have been produced by various groups, using different methods and dataset. comparing qrcmts with other similar products, it is evident a great similarity of focal mechanisms while on the contrary, the mw have a clear variability. i. introduction hen an earthquake occurs with a magnitude greater than 4.5 in the euromediterranean region, in a short time, between minutes to hours, several moment tensors are made available. among them, the quick rcmt (regional centroid moment tensor; pondrelli et al., 2012) is computed automatically, revised by an operator and then pubblished on the dedicated web page (autorcmt.bo.ingv.it/quicks. html) and on the european mediterranean seismological center (emsc) event web page (www.emsc-csem.org). this preliminary semiautomatic procedure, named pypaver, is applied since 2010 (pondrelli et al., 2012) and after several years of test, also in this last seismic emergency has properly worked. in fact, the 24 of august 2016 at 1:36 am, when a ml 6.0 earthquake struck powerfully amatrice and other small towns of central italy, a quick rcmt (qrcmt) has been rapidly computed and made available. the automatic procedure has been activated by an ingv email alert including the preliminary hypocentral location. the same process activated several times in the following hours and days, allowing the computations of a ten of qrcmts (figure 1 and table 1; http://autorcmt.bo.ingv.it/quicks. html). out of seismic sequences, after 3-4 months the qrcmts have been determined, a revision is done, using the largest amount of seismograms available. then, a definitive rcmt solution is obtained and included in the european mediterranean rcmt catalog (pondrelli et al., 2015 and references therein; www.bo.ingv.it/rcmt/). definitive rcmt solutions for greater events of the amatrice seismic sequence have been computed rapidly, within one month from the main shock (figure 1). rcmts, quick and definitive, are computed using an inversion method derived from the global cmt computation method (ardvisson and ekström, 1998; ekström and nettles, 2014; ekström et al., 2012), that calculates synthetic and inverts seismograms for body and surface waves recorded mainly (but not only) at regional distances and at intermediate period, i.e. between 35 and 150 s (pondrelli et al., 2015). in this brief paper, we will report on the comparison between qrcmts, w annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7240 figure 1. map of seismicity, qrcmts and rcmts of the amatrice region. qrcmts are in blue to light blue (different shades are for different days of occurrence, following the label below the map), revised rcmts are in red. in black are focal mechanisms for previous earthquakes with m > 4.0 (european-mediterranean rcmt catalog, http://www.bo.ingv. it/ rcmt). green squares are historical events, scaled with magnitude (rovida et al., 2016; http://emidius.mi.ingv.it/cpti15). background seismicity is mapped with black circles for events before amatrice main shock, while in different blue shades are mapped events of the amatrice seismic sequence (m>2.0, iside wg, 2016; http://iside.rm.ingv.it/iside/standard/index.jsp). date time lat. long. depth (km) ml mw qrcmt mw final rcmt 2016-08-24 01:36:32 42.698 13.234 8.1 6.0 6.20 6.21 (bw) 6.20 (sw) 6.18 (comb.) 2016-08-24 01:37:26 42.712 13.253 9.0 4.5 ----2016-08-24 01:56:00 42.614 13.275 4.8 4.4 ----2016-08-24 02:33:29 42.794 13.154 8.7 5.4 5.52 5.54 2016-08-24 03:40:11 42.617 13.245 10.6 4.2 ----2016-08-24 04:06:50 42.769 13.125 7.6 4.3 4.74 4.66 2016-08-24 11:50:30 42.819 13.146 8.4 4.7 ----2016-08-24 17:46:09 42.663 13.222 10.0 4.4 4.39 4.49 2016-08-24 23:22:05 42.648 13.213 11.2 3.8 ----2016-08-25 03:17:16 42.753 13.208 9.5 4.5 4.63 4.58 2016-08-25 12:36:05 42.596 13.290 10.0 4.3 4.60 4.60 2016-08-26 04:28:25 42.600 13.290 10.9 4.8 4.93 4.94 2016-08-27 02:50:59 42.839 13.249 8.2 4.0 --4.31 2016-08-28 15:55:35 42.820 13.238 8.7 4.4 4.30 4.32 2016-09-03 01:34:12 42.775 13.130 10.6 4.3 4.38 4.44 2016-09-03 10:18:51 42.866 13.215 9.3 4.5 4.46 4.48 table 1. list of events for which a trial to compute a qrcmt has been done. locations and ml are from ingv cnt first determination (cnt.rm.ingv.it), mw qrcmt are from autorcmt.bo.ingv.it/quicks.html and mw rcmt are for revised solution from this paper. different values for mw of the main shock are from the inversion for different signals: bw – only body waves; sw – only surface waves; comb. – body and surface waves combined. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7240 revised rcmts and other rapid moment tensor solutions for the same group of events. ii. quick and definitive rcmts as already described, immediately after the earthquake of the august 24, a qrcmt solution has been determined, and the same has been tried for all events with a ml 4.0. in table 1 the list of the events for which a trial to compute a rapid solution has been done, shows that we obtained a results in 10 over 16 cases. most of failed attempts are in the few hours after the main shock, when signal of minor events, i.e. m between 4 and 4.5, is completely hidden by the coda of the m6 earthquake. only few hours after the main shock it has been possible again to compute a qrcmt with m 4.2. the ten qrcmts are drawn in figure 1. they show a prevailing extensional pattern, as commonly seen along the apennines, with a tensional direction ne-sw. a couple of focal mechanisms are slightly different. one, located in the neastern part of the aftershocks cloud, is still extensional but rotated nearly 90° with respect to others; another one, again located at ne shows a strike-slip focal solution. being smaller events, they probably nucleated along secondary faults, with different geometry with respect to the principal ne-sw extensional activated structure. definitive rcmts, computed after one month, are either shown in figure 1 (red focal mechanisms). one more solution is avaliable with respect to qrcmts, due to additional seismograms available later. in table 1, mw for definitive rcmt solutions are reported, showing a good agreement with values obtained for qrcmts and focal mechanisms are almost identical (figure 1). a small no-dc component affects some solutions, mainly qrcmts; however it is a common amount, overall during a seismic sequence, where the continuous seismicity makes seismograms often more noisy. an additional test has been done during revision. for the main shock of august 24, we computed several rcmt solutions: one inverting only for body waves, another one using just surface waves and, at the end, a combined solution with both waveforms (see figure in supplementary files). all of them gave a pure, extensional focal mechanism, identical in all three cases. in table 1 we included all three values obtained for the mw, to show we obtain always the same value, mw 6.2. comparison with other quick solutions in figure 2, all qrcmts computed immediately after the occurrence of events belonging to the amatrice seismic sequence are drawn in the first colum, together with other rapid moment tensor solutions we were able to find, computed for the same events by other agencies and using other techniques. this figure allows a visual comparison. a description of data we compare with qrcmts follows. usgs moment tensor solutions are reachable at the event page of any single event (earthquake. usgs.gov/earthquakes) or in the web pages of the archive (http://earthquake.usgs.gov/earth quakes/eventpage/us10006g7d#moment-tensor). usgs moment tensors are computed for earthquakes with m down to 4.0, when possible. for greater events, solutions are obtained inverting the w-phase, a very long period phase (between 100 and 1000 s) arriving with the p-wave, well visible on vertical component of broadband displacement records. the computation method is well described at http://eost.ustrasbg.fr/wphase /method.html. these moment tensor solutions can be computed at both regional (5 to 20 degrees) and, more often, at teleseismic distances. in these cases, the mw reported is mww. it is worth to note that for smaller magnitude events, moment tensors come from inversion of the complete waveforms at regional distances (less than ~13 degrees) and the mw is then named mwr (earthquake.usgs. gov/data/comcat/dataeventterms.php#magt ype). it is worth to note that in the usgs list of the amatrice seismic sequence, all events with m lower than 5.0 have an mwr only. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7240 figure 2. in the first colum, quick rcmts computed for 10 events of the amatrice seismic sequence. on the left, date and time of the earthquakes. other columns are the other rapid focal mechanisms available; for references see the text. the color in the background represents the different cut off values used in each computation methods. solutions reported as gfz are computed at the german research centre for geoscience and are reachable at the following link: geofon.gfzpotsdam.de/eqinfo/list.php?mode =mt. they are obtained for events also with small to moderate magnitudes, with an automatic procedure, applying a spectral analysis technique that, to reach the best fitting source parameters, adopts the appropriate range of phases, frequency and inversion method along computations (saul et al., 2011). for events of the amatrice sequence, being at an epicentral distance greater than 200 km, applied bandpass filters should be 0.01–0.025. gcmt, the global cmt focal mechanisms, are the most used and diffused solutions. the entire catalog is searchable at http://www. global cmt.org. usually cmt solutions are computed for events with m greater than 5, and this is the reason of the only two cmts displayed in figure 2. the computational method is extendedly described in ekström and nettles (2014). the unique ipgp solution comes from the scardec project, active at geoscope observatory (geoscope.ipgp.fr/index.php/en/data/ earthquake-data/latest-earthquakes). the scardec method uses teleseismic body waves (in particular the w-phase) to retrieve the source parameters of earthquakes with magnitudes larger than 5.5 6.0 (vallée et al., 2011). the highpass frequency is dependent on the earthquake magnitude and duration, ranging from 0.0125 hz for a m 6 to 0.003 hz for a very large and long earthquake. tdmt solutions are reachable at cnt.rm.ingv.it/ tdmt. these moment tensors are computed first automatically and then reviewed, for any event with ml greater than 3.5 occurred in italy. the inversion method is an evolution of the dreger and helmberger (1993) system and changes applied to it are described in scognamiglio et al. (2009). slu mt solutions are computed at the saint louis university, using codes described in herrmann (2013), and can be downloaded at www.eas.slu. edu/eqc/eqc_mt/mech.it, where source parameters for italy events in particular are stored. the tdmt and slu mt have in common the crustal velocity model, named cia (herrmann et al., 2011) and typical frequency interval of analysed signal, i.e. 15-40 s, the shortest if compared to others methods listed here. at first sight, in figure 2, a strong correspondence between all focal mechanisms solutions for all events exists, except for few small differences, as for the event of september 3, 10:18 for which usgs obtained an extensional mechanism when all other solutions are strike slip. what is less homogeneous is the mw value obtained by different methods (figure 2 and table 1). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7240 iv. discussion several rapid moment tensor solutions are computed immediately worldwide after the occurrence of a strong earthquake. in the amatrice case, we compared the results obtained with different methods of computation, for 10 events of the sequence, all with m greater than 4.0. focal mechanisms are in general strongly similar except for a couple of solutions (figure 2). what clearly differs is the mw, thus the m0 seismic moment. in particular, it is possible to recognize two main families of seismic moment tensors, one produced using global or regional velocity models and long period seismograms (cmt s.l., usgs, ipgp, figure 2), the other includes solutions computed using local velocity models (i.e., cia, herrmann et al., 2011) and higher frequency signals (tdmt and slu mt). usgs moment tensors are a particular case because obtained with different methods depending on their magnitude: the main shock and the events with m greater than 5 have been elaborated using the w-phase method, while the other events using a regional method. a similar situation exists for gfz solutions for which usually the cut off changes with m; the declared bandpass filter here is 0.01–0.025 hz for all solutions (figure 2, the same color background is used for gfz), but we can also assume that for greater events the cut off may increase. therefore, we consider that usgs and gfz part of solutions belongs to one family and another part to the other. in general, looking to figure 2, we can say that mw values for the “long period” family are equal for great earthquakes and bigger than values given by “shorter period” families. to enhance this pattern, a different background color has been used, scaled with lower period of signal used for inversion. it seems like a local mw exists, determined using “shorter period” methods; it is in relation with the calibrated worldwide mw with a constant difference of -0.2, as described by gasperini et al. (2013). up to now, this difference is still motivated with the difference in methodologies or in the dataset of used seismograms. however, it is worth to note that these differences exist also among rcmt and gfz or usgs results, that actually produce the same value for mw. what make really the difference between what applied by the two groups is the frequency of inverted signal and, consequently, the velocity models. for instance, the velocity model used by tdmt and slu mt is a crustal model, while long period methods need regional to global propagation models. this interesting feature points also the attention on the choice of the mw to use when we include the data in a catalog or in any seismic hazard study. at present, solutions like rcmts are preferred because calibrated with past data, ensuring the homogeneity needed in a catalog. for instance, this characteristic avoids underestimations in seismic hazard evaluations. however, being high frequency techniques, as tdmt and slu mt, able to find out good quality source parameters for smaller events (e.g. m 3.5), assume a fundamental relevance in seismotectonic studies, where also minor events may reveal information otherwise not available. in conclusion, it can be useful to include in catalogs all these values for the mw, with indications of the used bandpass filter, to fix if they can be cosidered local, regional or worldwide magnitude. obviously, to test the hypothesis exposed here for the different value that mw has dependently on methods and signals used, a larger dataset for the comparison is necessary to have a statistic validity, and it is what we suppose to do in the next future. acknowledgements we would like to thank the anonymous reviewer for interesting suggestions and f. loddo for help given in the download of data. references [arvidsson and ekström, 1998] arvidsson, r., ekström, g., 1998. global cmt analysis of moderate earthquakes mw ≥ 4.5 using interannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7240 mediate period surface waves. bull. seismol. soc. am. 88, 1003–1013. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice assessing ‘alarm-based cn’ earthquake predictions in italy annals of geophysics, 59, 6, 2016, s0648; doi:10.4401/ag-6889 s0648 assessing ‘alarm-based cn’ earthquake predictions in italy matteo taroni*, warner marzocchi, pamela roselli istituto nazionale di geofisica e vulcanologia, rome, italy abstract the quantitative assessment of the performance of earthquake prediction and/or forecast models is essential for evaluating their applicability for risk reduction purposes. here we assess the earthquake prediction performance of the cn model applied to the italian territory. this model has been widely publicized in italian news media, but a careful assessment of its prediction performance is still lacking. in this paper we evaluate the results obtained so far from the cn algorithm applied to the italian territory, by adopting widely used testing procedures and under development in the collaboratory for the study of earthquake predictability (csep) network. our results show that the cn prediction performance is comparable to the prediction performance of the stationary poisson model, that is, cn predictions do not add more to what may be expected from random chance. 1. introduction earthquake prediction is one of the ways in which seismologists make statements about the future seismic activity, usually on the basis of the observation of one or more candidate diagnostic precursors [sykes and jaumé 1990, pulinets and davidenko 2014]. a prediction consists in casting an alarm, i.e., a deterministic assertion that one target earthquake of a given magnitude will occur in a specified space-time window. such predictions appear to be prospective deterministic statements, but do not present the full picture. in fact all prediction schemes proposed to date have an intrinsically probabilistic nature [jordan et al. 2011]. to quantify this probability it is necessary to consider the possibility of raising false alarms and/or to miss some target earthquakes. when an alarm is cast for a specific space-time window, we have a hit (true positive) if a target earthquake occurs, otherwise we have a false alarm (false positive). when no alarm is cast for a specific spacetime window, if a target earthquake occurs we have a miss (false negative), otherwise a correct (true) negative. another way in which seismologists make statement about the future seismic activity is through probabilistic forecasting that consists of the estimation of the probability of one or more events in well-defined magnitude-space-time windows (e.g., marzocchi et al. [2014], for the italian region). according to jordan et al. [2011], probabilistic forecasting provides a more complete description of prospective earthquake information than deterministic prediction, and, more important, it separates hazard estimation made by scientists from the public protection role of civil authorities [jordan et al. 2014]. probabilistic forecasts can become predictions once a probability threshold is chosen [zechar and jordan 2008]; however, this threshold does not have any specific scientific meaning, but it has to be related to the kind of mitigation actions that are associated with the prediction [marzocchi 2013]. regardless of these possible shortcomings, it is undoubted that earthquake predictions have a natural and immediate attractiveness for lay people, probably because a prediction is much easier to understand than a probabilistic forecast [gigerenzer et al. 2005]. in italian language, the distinction between the english terms “forecast” and “prediction” cannot be made because only one word “previsione” exists and it has a strong deterministic connotation. the large impact on society of earthquake forecast/prediction is a further motivation for carefully evaluating the performances of each model. this is the main target of an international initiative named collaboratory for the study of earthquake predictability (csep) [jordan 2006, zechar et al. 2010, zechar and zhuang 2014). in essence, csep promotes experiments in which prospective forecasts/predictions are compared to real observations in different testing regions. all experiments are rigidly controlled, i.e., they are replicable by anyone and modelers cannot change their forecasts/predictions in retrospect. besides the statistical evaluation of each single model, these experiments offer also a unique opportunity to compare the relative forecasting/prediction skill of different models. as a article history received october 8, 2015; accepted october 18, 2016. subject classification: earthquake interactions and probability, statistical analysis. matter of fact, claiming that one model is able to predict earthquakes may be misleading from a scientific point of view: it is much more interesting to compare the model performances with the skill of other competing models. in order to achieve a meaningful comparison, models have to provide forecasts/prediction in a proper common format. however, not all forecast/prediction models adopt such a format, so they are not all presently ready to be evaluated by csep experiment. this is the case of the algorithm under study here. since january 1, 1998, a group of researchers began to provide earthquake predictions in italy through the use of a pattern recognition based on cn (californianevada, first regions of application) algorithm [gabrielov et al. 1986, keilis-borok et al. 1990, peresan, et al. 1999, peresan et al. 2005, romashkova and peresan 2013]. this type of prediction allows a quantitative validation of the prediction ability because the method is rigorously applied forward in time. this is certainly the most crucial aspect for the evaluation of any forecast/prediction model, because it guarantees that the results are not affected by any conscious or unconscious adjustment. in this paper we evaluate the results obtained so far from the cn algorithm applied to the italian territory, by adopting widely used testing procedures and under development in csep. 2. the cn model in italy and the target earthquakes the cn algorithm is an earthquake alarm-based model that provides intermediate-term predictions for mainshocks. the algorithm casts an alarm when a time of increase probability (tip) for the occurrence of target earthquakes in one specific region is identified. the identification of a tip is based on the observation of a set of candidate seismic precursory patterns that have been identified analyzing the past seismicity and are kept fixed [keilis-borok et al. 1990]. originally, the model was set up for california-nevada (cn) region, but since then it has been adapted to many other regions of the world. the details of the application of the cn algorithm to the italian territory can be found in peresan et al. [1999, 2005], and romanshkova and peresan [2013] and are not described here. the cn predictions in italy are related to three macro-zones that cover part of the italian territory and are characterized by a different minimum magnitude m0 for identification of the target events. these macrozones (figure 1) are northern italy (n-italy; m0 ≥ 5.4), central italy (c-italy; m0 ≥ 5.6), and southern italy (s-italy; m0 ≥ 5.6). the target earthquakes are only mainshocks, i.e., aftershocks above the magnitude m0 are not considered target events [peresan et al. 1999]. the forward predictions have been routinely performed every two months since january 1, 1998. more recently a new macrozone has been added (adriatic region), but it will be not considered here because of the short time window for testing. the reference magnitude of the target earthquakes is taken from the uci2001 catalog [peresan et al. 2002]. we do not perform any quality check to this catalog and we take it for granted. the cn predictions for italy are available in a public website (http://www.geoscienze.units.it/esperimentodi-previsione-dei-ter remoti-mt/algorithm-cn-initaly/cn-predictions-in-italy.html). taroni et al. 2 figure 1. maps of the three italian macro-zones related to the regionalization proposed by peresan et al. [1999]; (a) northern region; (b) central region; (c) southern region. m0 is the minimum magnitude for the target earthquakes in each macro-zone (for details on magnitudes see peresan et al. [1999]). 3 only the current prediction is released by the authors under a password access that has been given to a list of interested scientists. in table 1 we show the list of target earthquakes that occurred during a defined testing period (from 01/01/1998 to 08/28/2016, including the very recent amatrice earthquake of 08/24/2016) reported on cn web-site. a comparison of this catalog with the data extracted by the neic catalog (national earthquake information center; http://earthquake.usgs.gov/earth quakes/search/) for the same region, depth and testing period (table 2), highlights some inconsistencies; for example, some earthquakes above m0 occurred outside the three macro-zones as shown in figure 2. here, we do not deepen this inconsistency and take for granted the catalog reported in table 1 that will be used for the following analysis. assessing ‘alarm-based cn’ earthquake predictions in italy data (yyyy/mm/dd) lat. (°) lon. (°) depth (km) m0 cn-alarm cn-region 19980412 46.24 13.65 10 6.0 yes north 19980909 40.03 15.98 10 5.7 yes centre, south 20030914 44.33 11.45 10 5.6 yes north 20040712 46.30 13.64 7 5.7 yes north 20041124 45.63 10.56 17 5.5 no north 20090406 42.33 13.33 8 6.3 no centre 20120520 44.90 11.23 8 6.1 yes north 20160824 42.70 13.24 4 6.0 yes centre data (yyyy/mm/dd) lat. (°) lon. (°) depth (km) mw 19980326 43.26 12.97 10.0 5.4 19980412 46.25 13.65 10.0 5.6 19980909 40.04 15.98 10.0 5.6 20020906 38.38 13.70 5.0 6.0 20021031 41.79 14.87 10.0 5.9 20030329 43.11 15.46 10.0 5.5 20081223 44.56 10.41 28.3 5.4 20090406 42.33 13.33 8.8 6.3 20120520 44.89 11.23 6.3 6.0 20160824 42.71 13.17 10.0 6.2 table 1. event list related to cn algorithm website (http://www.geoscienze.units.it/esperimento-di-previsione-dei-terremoti-mt/algorithm-cn-in-italy.html) for earthquakes occurred in italy since 1/1/1998 to 08/28/2016 and related parameters. last two columns indicate if there was a tip (or not) and the macro-zones related, respectively. table 2. event list extracted by the neic website (national earthquake information center; http://earthquake.usgs.gov/earth quakes/search/) for earthquakes occurred in italy since 1/1/1998 to 28/8/2016 and related parameters. figure 2. (a) location map of earthquakes (red stars) listed in table 1. (b) location map of earthquakes (blue stars) listed in table 2. (c) location map of earthquakes (magenta stars) listed in table 2 that did not occur inside the macro-zones considered by the cn model. 3. assessing the cn predictions we test the cn predictions by using two different statistical methods: the molchan test (mt) [molchan 1997, zechar and jordan 2008] for the alarm-based model, and the parimutuel gambling score (pgs) [zechar and zhuang 2014] for the evaluation of the earthquakes forecasts. the mt and pgs methods have some remarkable differences. the mt test is based on the definition of a null hypothesis that can be rejected or not according to the observed data. on the contrary, in pgs test the null hypothesis is not requested, because the score is intended to provide a rank of the models based on their relative predictive skill. molchan test (mt) the mt verifies if the number of hits for the cn model (in the space-time domain) is consistent with the number of hits expected by a reference model (e.g. poisson model). when the observed number of hits is sufficiently bigger than those expected by the poisson model, we conclude that the model under test is significantly better than the reference model [zechar and jordan 2008]. this test is usually represented through a diagram, using different fractions of space-time occupied by alarms. in our case, the cn algorithm sets one specific value for such a variable, so the test collapses into a bernoulli case. specifically, h is the number of hits for the cn model, n is the total number of target events, and the parameter of the bernoulli distribution is given by x, that is the percentage of the space-time covered by alarms (for instance, if the alarms cover the whole interval of time and half space, then x= 0.5). this choice of the parameter is appropriate for our specific case, while it may require much more elaborated estimations in testing other earthquake prediction models [e.g., marzocchi et al. 2003, molchan and romashkova 2010]. the null hypothesis h0 under testing is: the cn model and the poisson model have the same predictive performances. under h0, a binomial distribution describes the probability p of a poisson model to obtain h or more correct predictions, with n observed target events and x is the percentage of the space-time occupied by alarm; that is, x is the probability to observe one target earthquake in a pure random process described by the poisson distribution. the poisson model is chosen because it can be considered the simplest random guess forecast strategy within each zone [kagan 2009]. the binomial distribution reads in the classical neyman-pearson statistical testing framework [neyman and pearson 1933] we can ‘reject’ or ‘not reject’ the null hypothesis, if p is less or larger that a pre-selected significance level. in practice, we set a significance level of 0.05 (this value is commonly used in science) and we reject the null hypothesis if p is less than 0.05; otherwise, we conclude that there is no empirical evidence supporting that the cn prediction capability is superior to the poisson process. parimutuel gambling score (pgs) the pgs method is a useful tool to compare the prediction performances of two models: in our case, cn and poisson models. the pgs method works like each model is a gambler. in the space-time bins there is a gamble between models; each model bets 1 coin on this bin. then, each model gets a win proportional to the probability assigned to the event (earthquake or not) that has occurred on the bin. in mathematical terms where wj is the gain that the j-th model obtains in the bin, k is the total number of the models and pj is the probability of the occurred event (earthquake or not) for the j-th model. the sum of the wj for each bin represents the skill of the models: the bigger the win, the better the model. the sum of the wins and losses for all the k models is always zero. in previous works the pgs method has been used to check the performance of the likelihood-based models [taroni et al. 2014, zechar and zhuang 2014]. in this work, we apply the same procedure to the alarm-based models. in particular, for the cn model we define pm= 1 when a target event happens during an alarm or when no target event happens during no-alarm (i.e. hit and correct negative, respectively). we define pm= 0 when no target event happens during an alarm or when a target event happens during a period when there is no alarm. (i.e. false alarm and miss, respectively). to compute the probability pm related to the poisson model, we use the same macro-zones and the same minimum magnitude used in the cn model. we compute the poisson rate by using the cpti11 catalog [rovida et al. 2011] declustered with the gardner and knopoff algorithm [gardner and knopoff 1974] until 12/31/1997 (the cn prediction experiment started on 1/1/1998). 4. results the molchan test results are summarized in table 3, where we shown that p of equation (1) is never less than the preselected significance level; this means that p n ii h n i x= = t y| 1 nv ix-q nv wj k1wj =+ | p pj mm k pj 1= | taroni et al. 4 (1) (2) 5 we cannot reject the null hypothesis for all three macrozones with the pre-selected significance level of 0.05. the parimutuel gambling score results are shown in table 4; the results indicate that the poisson model is better in all three macro-zones (scoring 32.7 for the north macro-region, 36.7 for center, and 27.7 for south). of course the scoring of the cn model has opposite values. 5. conclusions considering the data available so far, the molchan test does not show that cn prediction performance is significantly better than predictions based on the stationary poisson model. moreover, the results of parimutuel gambling score indicate that the poisson model is even better than cn in predicting earthquakes, as shown by the values associated to each win. this result is similar to what obtained by the testing of other similar pattern recognition models performed at a global scale once a proper null hypothesis is used [marzocchi et al. 2003, zechar and zhuang 2010]. from a practical perspective, the results show that cn predictions do not add significant information that may be used to enhance societal earthquake preparedness. acknowledgements. this work has been carried out within the seismic hazard center (centro di pericolosità sismica, cps) at the istituto nazionale di geofisica e vulcanologia (ingv). we thank the associated editor and the anonymous reviewers for constructive comments. references gabrielov, a.m., o.e. dmitrieva, v.i. keilis-borok, v.g. kosobokov, i.v. kuznetsov, t.a.levshina, k.m. mirzoev, g.m. molchan, s.k. negmatullaev, v.f. pisarenko, a.g. prozorov, w. rinehart, i.m. rotwain, p.n. shebalin, m.g. shnirman and s.y. schreider (1986). algorithms of long-term earthquakes’ prediction, international school for research oriented to earthquake prediction-algorithms, software and data handling, lima, peru. gardner, j.k., and l. knopoff (1974). is the sequence of earthquakes in southern california, with aftershocks removed, poissonian?, b. seismol. soc. am., 64 (5), 1363-1367. gigerenzer, g., r. hertwig, e. van den broek, b. fasolo and k.v. katsikopoulos (2005). “a 30% chance of rain tomorrow”: how does the public understand probabilistic weather forecasts?, risk anal., 25 (3), 623-629. jordan, t.h. (2006). earthquake predictability, brick by brick, seismol. res. lett., 77 (1), 3-6. jordan, t.h., y.t. chen, p. gasparini, r. madariaga, i. main, w. marzocchi, g. papadopoulos, g. sobolev, k. yamaoka and j. zschau (2011). operational earthquake forecasting: state of knowledge and guidelines for utilization, annals of geophysics, 54 (4), 315-391. jordan, t.h., w. marzocchi, a. michael and m. gerstenberger (2014). operational earthquake forecasting can enhance earthquake preparedness, seismol. res. lett., 85, 955-959. kagan, y.y. (2009). testing long-term earthquake forecasts: likelihood methods and error diagrams, geophys. j. int., 177 (2), 532-542. keilis-borok, v.i., i.v. kuznetsov, g.f. panza, i.m. rotwain and g. costa (1990). on intermediate-term earthquake prediction in central italy, pure appl. geophys., 134 (1), 79-92. marzocchi, w., l. sandri and e. boschi (2003). on the validation of earthquake-forecasting models: the case of pattern recognition algorithms, b. seismol. soc. am., 93 (5), 1994-2004. marzocchi, w. (2013). seismic hazard and public safety, eos tran. agu, 94, 240-241. marzocchi, w., a.m. lombardi and e. casarotti (2014). the establishment of an operational earthquake forecasting system in italy, seismol. res. lett., 85 (5), 961-969. molchan, g.m. (1997). earthquake prediction as a decision-making problem, pure appl. geophys., 147 (1), 1-15. molchan, g.m., and l.romashkova (2010). earthquake prediction analysis based on empirical seismic rate: the m8 algorithm, geophys. j. int., 183, 1525-1537. neyman, j., and e.s. pearson (1933). on the problem of the most efficient tests of statistical hypotheses, philos. t. roy. soc. a, 231, 694-706. assessing ‘alarm-based cn’ earthquake predictions in italy zone hit rate (# of hits/ # of eqs) percentage of space-time in alarm p of equation (1) north 4/5 35.71% 0.058 center 2/3 35.71% 0.292 south 0/1 25.00% 1.000 zone cn model poisson model north -32.7 32.7 center -36.7 36.7 south -27.7 27.7 table 3. molchan test results. table 4. parimutuel gambling score results (bold italic for the best model). peresan, a., g. costa and g.f. panza (1999). a proposal of regionalization for the application of the cn earthquake prediction algorithm to the italian territory, annali di geofisica, 42 (5), 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precursor to large earthquakes in the san francisco bay area, nature, 348, 595-599. taroni, m., j.d. zechar and w. marzocchi (2014). assessing annual global m6+ seismicity forecasts, geophys. j. int., 196, 422-431. zechar, j.d., and t.h. jordan (2008). testing alarmbased earthquake predictions, geophys. j. int., 172 (2), 715-724. zechar, j.d., and j. zhuang (2010). risk and return: evaluating rtp earthquake predictions, geophys. j. int., 82, 1319-1326. zechar, j.d., m.c. gerstenberger and d.a. rhoades (2010). likelihood-based tests for evaluating spacerate-magnitude earthquake forecasts, b. seismol. soc. am., 100, 1184-1195. zechar, j.d., and j. zhuang (2014). a parimutuel gambling perspective to compare probabilistic seismicity forecasts, geophys. j. int., 199 (1), 60-68. corresponding author: matteo taroni, istituto nazionale di geofisica e vulcanologia, rome, italy; email: matteo.taroni@ingv.it. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol50,1,2007 111 annals of geophysics, vol. 50, n. 1, february 2007 key words ionosphere – eand f-layers – earthquakes-magnetic storm – fof2 and foes 1. introduction ever since davies and baker (1965) reported the first seismo-ionospheric anomalies related to alaskan earthquake, several workers have attempted to study this problem using ground and satellite based observations. however, some convincing evidences of this phenomenon have come rather recently (pulinets et al., 1991, 1994; ruzhin and depueva, 1996) in which it has been found that the seismic effect on ionospheric f2layer is observed in terms of reduction in morning time fof2 about a week before the occurrence of the earthquakes and this phenomenon may be observed over a wide ionospheric area covered by 30° latitude and 60° longitude. more recently similar studies have been made by liu et al. (2000, 2001), chuo et al. (2002) and singh et al. (2004) in which they have reported afternoon reductions in fof2 also apart from morning time reductions. anomalies in fof2 and total electron content (tec) have also been reported by devi et al. (2001, 2004) prior to the occurrence of earthquakes. they have shown that both positive and negative ionospheric effects develop prior to an earthquake depending on the epicenter position with respect to the observing site. simultaneous ionospheric eand f-layer perturbations caused by some major earthquakes in india birbal singh (1) and om p. singh (2) (1) department of electronics and communication engineering, faculty of engineering and technology, r.b.s. college, bichpuri, agra, india (2) department of physics, faculty of engineering and technology, r.b.s. college, bichpuri, agra, india abstract the variation in nighttime ionospheric eand f-region critical frequencies (foes and fof2) is examined for the months of august 1988, september 1993 and may 1997 in which major earthquakes (6 < m < 6.6) occurred in indian seismic zones of bihar-nepal border, latur in west india, and jabalpur in central india. the ionospheric data are obtained from ahmedabad (latitude 23.01°n, longitude 72.36°e). the percent deviations of fof2 and foes from their monthly median are determined for pre-midnight (18:00-00:00 h lt) and post-midnight (00:0006:00 h lt) periods and studied over a span of 20 days before the occurrence of earthquakes. the results show that fof2 are reduced in both the time sectors prior to the occurrence of main shocks. in pre-midnight sector the reduction is between 24 and 30%, 0 to 4 days before the main shocks and in the post-midnight sector it is between 18 and 26%, 1 to 15 days before the main shocks. the foes show enhancements by 100 to 155% during the same periods. the effects of magnetic storms (kp and dst variations) on the data are identified clearly but they do not vitiate the effects of earthquakes. the earthquake related anomalies are interpreted in terms of electromagnetic coupling between the lithosphere and ionosphere during earthquakes preparation processes. mailing address: dr. birbal singh, department of electronics and communication engineering, faculty of engineering and technology, r.b.s. college, bichpuri, agra-283105, india; e-mail: birbal@sancharnet.in 112 birbal singh and om p. singh several workers have also reported the ionospheric anomalies in e-layers prior to the occurrence of earthquakes (gokhberg et al., 1988; liperovski et al., 1992; pulinets et al., 1994; ondoh, 2000; ogawa et al., 2002). recently, ondoh and hayakawa (1999), ondoh (2003) and singh and singh (2004) have made an elaborate study of this problem and have shown anomalous foes increases 2-20 days before the occurrence of earthquakes. although, many workers have examined the effects of earthquakes on ionospheric eand flayers separately as stated above there is no report of simultaneous observations of eand flayer perturbations related to seismic activities. in the present paper, we examine this problem in detail by considering three major earthquakes that occurred in india in recent years. our results show simultaneous earthquake induced changes in the eand f-layers which are reflected by simultaneous reductions in fof2 and enhancements in foes. 2. ionospheric, earthquake and magnetic storm data the ionospheric data have been obtained from physical research laboratory, ahmedabad (latitude 23.01°n, longitude 72.36°e), india where a digital ionosonde has been in operation for a long time. the data recorded at this station includes fof2, foes, hpf2 and median values for respective months. the earthquake data have been obtained from india meteorological department, new delhi, india. since ionospheric anomalies appear during or after the occurrence of magnetic storms also it is worthwhile to examine the fof2 and foes changes in relation to magnetic storms also. for this purpose, we consider kp index and dst index data, which have been obtained from indian institute of geomagnetism, colaba, mumbai, india and world data center c2, kyoto university, kyoto, japan respectively. 3. results and discussion six major earhquakes have occurred in india during the last 15 years. they include biharnepal earthquake (20 august, 1988), uttarkashi earthquake (20 october, 1991), latur earthquake (30 september, 1993), jabalpur earthquake (22 may, 1997), chamoli earthquake (29 march, 1999) and bhuj earthquake (26 january, 2001). in order to examine the effects of these earthquakes on eand f-regions of the overhead ionosphere we have attempted to analyze the ionospheric data obtained from ahmedabad. unfortunately, foes data are available corresponding to three earthquakes only, which are bihar-nepal earthquake, latur earthquake and jabalpur earthquake, and hence in the present study we consider the fand e-regions anomalies corresponding to these earthquakes only. the locations of these three earthquakes are shown in the map of fig. 1 by solid circles and abbreviated words as bhrnpl, ltr and jbl respectively. the location of ahmedabad from where the ionospheric data have been obtained is also shown in the same figure by abbreviated word ahd. other details of these earthquakes such as the date and time of occurrence, geographic location, magnitude, depth and distance from ahmedabad are shown in table i. it may be noted that the ionospheric parameters show considerable variations following geomagnetic storms. the ionospheric responses to such storms are of negative kind and large at high and middle latitudes while they are modest and of positive kind at low latitudes especially during daytime hours. during nighttime, both positive and negative responses are possible at low and equatorial latitudes (lakshmi et al., 1983, 1997). here, it may also be mentioned that jain and singh (1977) and jain et al. (1979) have studied the low and equatorial latitude ionospheric changes during the period of magnetic storms and have shown that these ionospheric regions are influenced significantly during prolonged magnetic disturbances only and there is almost negligible variation during isolated storms. furthermore, it may be pointed out here that the ionospheric station ahmedabad is located close to the anomaly crest region and shows mixed response to magnetic storms and, sometimes, significant variations even during quiet periods. however, in the present study adequate care has been taken to see that the ionospheric departures due to magnetic storms and unknown reasons do not vitiate the main focus of our discussion. 113 simultaneous ionospheric eand f-layer perturbations caused by some major earthquakes in india fig. 1. map of india showing the locations of three major earthquakes (solid circles). also shown is the location of ionospheric station ahmedabad (ahd). table i. details of the three earthquakes considered. date of location time latitude longitude magnitude depth distance occurrence (ist)* (°n) (°e) from ahd (km) 20/08/1988 bhr-npl 23:09 h 26.72 86.63 6.6 35 2500 border 30/09/1993 latur 03:55 h 18.07 76.62 6.3 12 1100 22/05/1997 jabalpur 03:25 h 23.08 80.06 6.0 35 1600 * ist = ut + 5.50 h. 114 birbal singh and om p. singh fig. 2a. the variations of monthly median of fof2 and foes (shown by solid curves) and hourly measured values (shown by open triangles) for ten days before the occurrence of the earthquakes in the months of august, 1988 (top two panels), september, 1993 (middle two panels) and may, 1997 (bottom two panels). monthly median is repeated on each day. the inverted arrows indicate the days of the earthquakes. 115 simultaneous ionospheric eand f-layer perturbations caused by some major earthquakes in india fig. 2b. temporal variation of monthly median fof2 and foes (solid curves) and hourly measured values (open triangles) on the days of observed anomalies corresponding to the earthquakes under consideration. figure 2a shows the daily variations of fof2 and foes corresponding to three earthquakes under consideration. the fof2 and foes data are shown by triangles whereas monthly medians are shown by solid lines. the data are shown for 10 days before the occurrence of the respective earthquakes. the days of earthquakes are shown by inverted arrows in all the panels. from a 116 birbal singh and om p. singh glance at the top two panels which correspond to bihar-nepal earthquake of 20 august, 1988 we see that there is a maximum reduction in morning time fof2 from median values on 18 august, whereas there is a maximum enhancement in foes on the same day. since there is no major magnetic storm in this month before the earthquake (as shown ahead) it may be inferred that fof2 and foes are influenced by the earthquake giving rise to peak anomalies 2 days before the occurrence of the earthquake. further, it will be shown that fof2 and foes indicate decreasing and increasing trends respectively starting from 03 august itself thereby showing the influence of earthquakes from 18 days before its occurrence. the middle two panels correspond to latur earthquake of 30 september 1993. here, the fof2 is lowest in the morning of 28 september whereas foes is enhanced during the midday and evening hours of 27 and 28 september. there is a reduction in fof2 and corresponding enhancement in foes on 22 september also which may be attributed to a magnetic storm that occurred on 20 september (discussed ahead). the bottom two panels correspond to jabalpur earthquake of 22 may 1997. in this case the fof2 has gone lowest and foes largest on 19 may 1997 about three days before the occurrence of the earthquake. the figure shows enhancements in foes on 13 and 15 may also. the enhancement on 15 may may be attributed to severe magnetic storm (kp = 7, dst = − 116), which occurred on the same day. the reason for the enhancement on 13 may is not known, but it may not be attributed to the storm of 1 may because the effect of such isolated storms cannot affect the ionosphere so late. to identify the times of occurrence of the anomalies (reduction in fof2 and enhancement in foes) occurring on 18 august 1988, 28 september 1993 and 19 may 1997 as shown in fig. 2a we plot the temporal variation of fof2 and foes data along with their median values in fig. 2b. the results of maximum deviation and times of occurrence are presented in table ii. here it may be noted that percent reduction in morning time fof2 and enhancements in foes are the largest as compared to other days. further, the foes data presented in the table ii on 18 august during afternoon hours shows a value of 296% where as in the abstract we have mentioned the maximum enhancement of 155%. this result should not be taken as contradictory because the data presented in the abstract have resulted from the running mean which will be made clear ahead. in order to make the earthquake induced anomalies more clear and distinct from those arising due to storm and other unknown factors we have made a thorough analysis of the data by following a procedure in which we have calculated the percent reduction in hourly fof2 and percent enhancement in hourly foes relative to monthly median values. we have chosen the data for the period 18:00-06:00 h, lt only and divided them into two sectors, pre-midnight (18:0000:00 h) and post-midnight (00:00-06:00 h). this period of the data has been chosen to avoid complications arising due to variations in photoionisation intensity during daytime (michel parrot, personal communication, bhopal, 17 november 2003). since the data are not continuous and there are some intermittent gaps, we have taken table ii. details of times and maximum deviation of fof2 and foes observed during the anomalies. maximum reduction in fof2 maximum enhancement in foes days of morning hours afternoon hours morning hours afternoon hours anomaly time reduction time reduction time enhancement time enhancement (h) (%) (h) (%) (h) (%) (h) (%) 18/08/1988 03:00 26.4 18:00 13.3 03:00 40.0 19:00 296.0 28/09/1993 05:00 32.0 21:00 29.6 04:00 142.8 23:00 105.0 19/05/1997 04:00 37.5 17:00 20.4 05:00 50.0 14:00 146.5 117 simultaneous ionospheric eand f-layer perturbations caused by some major earthquakes in india six days running mean of the peak reduction in fof2 and peak enhancement in foes for both the pre-midnight and post-midnight data. the variations of percent reduction in fof2 and enhancement in foes are shown for 20 days before the occurrence of each earthquake in the top two panels of fig. 3a-c. the solid circles correspond to pre-midnight data and solid squares fig. 3a,b. a) top – six days running mean of the peak percent reductions during pre-and post-midnight sectors in fof2 for 25 days in the month of august 1988. the earthquake (m=6.6, shown by downward arrow) occurred at a ) bihar-nepal border on 20 august 1988. second – same as in top panel but for enhancements in foes. third – kp index variation. bottom – dst index variation on the days under consideration. b) the same as (a) but for latur earthquake (m=6.3) which occurred on 30 september 1993. a b 118 birbal singh and om p. singh correspond to post-midnight data. the days of the earthquakes are shown by inverted arrows with the magnitude of the earthquakes indicated nearby. in the bottom two panels of these figures we show the variation of the magnetic storms by three hour kp indices and corresponding dst indices. here, it may be noted that kp ≤ 4, 4 ≤ kp ≤ 6 and kp≥6 indicate quiet, moderate and severe magnetic storms respectively. the magnetic storm condition is also denoted in terms of dst index and the value of dst ≤ − 50 nt represents minor storms. in general, the low latitude ionosphere is influenced by large reductions in dst (dst≤ − 100) only (b.m. reddy, pers. comm., november 2003). from fig. 3a it may be seen that there are enhancements in percent reduction in fof2 and enhancements in foes from 3 august itself about 18 days before the occurrence of the earthquake and the effect is reflected in both the pre and postmidnight data of fof2 and in pre-midnight data of foes. the largest enhancement in pre-midnight fof2 reduction amounts to 24%, one day before the earthquake whereas the same in postmidnight hours amounts to 26%, four days before the earthquake. the maximum enhancements in foes of 155% on 15 august and 102% on 7 august are observed in pre and post-midnight data respectively. during the periods of these enhancements the magnetic condition was quiet. hence, the anomalies may be attributed to the earthquake only. the magnetic storms occurred on 20 and 21 august, after the occurrence of the earthquake, which may not influence the ionosphere in advance. it may be mentioned here that the days of maximum enhancement in reduction of fof2 and maximum enhancement in foes as seen in this as well as in the next two figures do not match exactly with those shown in fig. 2. this discrepancy has been produced by taking the six days running mean of the data, otherwise the days of anomalies remain unchanged. figure 3b presents the results corresponding to latur earthquake. the top two panels show that there are large enhancements in reduction in fof2 between 17 and 23 september and large enhancements in foes on 15 september (which appears on 13 september because of six days running mean), which may be attributed to the severe magnetic storm of 13 september (kp = 8, σkp = 48). however, the enhancements in reduction in fof2 and enhancements in foes during 25-30 september may be attributed to the earthquake of 30 september only. maximum enhancements in fof2 reductions are observed on 28 september by 18% in post-midnight data and by 30% on 30 septemfig. 3c. the same as fig. 3a but for jabalpur earthquake (m = 6.0) which occurred on 22 may 1997. 119 simultaneous ionospheric eand f-layer perturbations caused by some major earthquakes in india ber in the pre-midnight data. in case of foes, maximum enhancements of 140% on 25 september and of 117% on 27 september are observed in post and pre-midnight data respectively. the moderate magnetic storms (kp = 5) on 20, 23 and 30 september are unlikely to influence the data as stated before that only severe and prolonged magnetic storms are found to influence the low latitude ionosphere. figure 3c shows the results corresponding to the jabalpur earthquake of 22 may 1997. in this month two magnetic storms (kp = 6.7) occurred on 1 and 15 may 1997. the dst variation shows the values −63 and −116 on these two days respectively. as shown in the top panel of the figure, there are increases in fof2 reductions during 4-9 may as well as on 16 and 18 may. these increases in reductions may be attributed to the storms of 1 and 15 may respectively. however, the maximum enhancement in reduction in the pre-midnight data by 25% on 20 may and by 18% on 22 may may possibly be due to the earthquake that occurred on 22 may. the second panel shows that there is a gradual increase in foes in both the pre and post-midnight data starting from 2 may to 21 may, up to one day before the occurrence of the earthquake. the maximum enhancement in post-midnight data on 21 may is 114% whereas in pre-midnight data it is 100%. this gradual increase in foes from nearly 20 days before may be due to the earthquake of 22 may. the transient increase in foes during 16-17 may may be attributed to the storm of 15 may. from the results presented in fig. 3a-c it may be seen that the effects of earthquakes on fof2 and foes data corresponding to latur and jabalpur earthquakes are not as much convincing as that related to bihar-nepal earthquake presented in fig. 3a. one reason for this is that there occurred magnetic storms during the period of analysis in the later two cases, which complicated the data whereas there was no magnetic storm in the first case. however, this ambiguity can be eliminated if we look at the diurnal variation in fof2 and foes of fig. 2 where enhancements and reductions are visible very clearly. in order to give further support to our results mentioned above we have carried out more rigorous statistical analysis. we have employed the past 10 years of data corresponding to the months in which fof2 and foes data have been analyzed in fig. 3a-c. out of the bulk of 10 years data we have picked up only those, which corresponded to the same months and the years in which the sun spot numbers were almost similar. for example, in the case of bihar-nepal earthquake the lowest fof2 was found to be on 18 august 1988 two days before the occurrence of the earthquake. hence, we have considered the same day data in the years 1978, 1984, 1993, 1998 and 1999, which corresponded approximately to similar sun spot numbers. then we determine the deviation of fof2 from monthly median (∆fof2) for each hour of the days. from these data, mean (m) and standard deviation (σ) were calculated. similar calculations have been made for foes also. the variations of m and m ± σ along with ∆fof2 and ∆foes corresponding to each earthquake are shown by different notations in fig. 4. from this figure it is seen that ∆fof2 is deviated out of m + σ in the morning (04:00-07:00 h) in all the cases. the deviation is also seen in the afternoon (17:00-19:00 h) and in the late evening hours (21:00-23:00 h). the foes deviations are mostly in the pre-midnight hours in all the cases except in jabalpur case in which it is deviated during afternoon also. these results are in agreement with the results presented in fig. 3a-c. the anomalous state of the ionosphere during seismic activities as presented above has been interpreted by several workers in terms of lithosphere-ionosphere coupling produced by earthquake-induced electric fields, which are generated from internal gravity waves. these electric fields can penetrate the ionospheric fand e-region heights and bring out changes in the electron density profile due to e ×b drift (kim and hegai, 1999; sorokin and chmyrev, 1999). the enhancement or depression in fof2 values may be attributed to positive or negative direction of electric fields. the effect of distant earthquakes on ionospheric data over ahmedabad may be interpreted in terms of e ×b drift of an electron stream upward over the epicenter of earthquakes which may then move towards or away from ahmedabad depending upon positive or negative direction of electric fields or neutral winds causing enhancement or depres120 birbal singh and om p. singh fig. 4. statistical results showing the deviation of fof2 and foes from the monthly median on the day it was lowest in the month of the earthquake (∆fof2 and ∆foes). the mean (m) and standard deviation (σ) around mean (m ± σ) are computed for the same days in the past ten years of data on which sun spot numbers were nearly identical. 121 simultaneous ionospheric eand f-layer perturbations caused by some major earthquakes in india sion in fof2 data as discussed by other workers (yoshimatsu, 1938; ondoh, 1999). in case of anomalies at the e-layer heights, the upward e ×b drift will be amplified thereby augmenting the density pumping process from the epicentral zone to off epicenter positions in the ionosphere (ruzhin and depueva, 1996). the other possibility of e-layer anomaly is quasi-static heating and production of additional ionization in the sporadic e-layers caused by cloud discharge in the atmosphere over seismic zones induced by seismic electric fields (ondoh and hayakawa, 2002; ondoh, 2003). the foes enhancements prior to the occurrence of large earthquakes have also been suggested due to radon emanations from seismic sources, which are carried upward by the electric fields and create additional ionization at the heights of elayer (ondoh and hayakawa, 2002; ondoh, 2003). over all we expect that the reduction in fof2 and enhancements in foes during the periods of earthquakes may probably be interpreted in terms of lifting of fand e-layers due to e ×b drift caused by a unidirectional electric field. in this mechanism the f2 peak will be lifted to a region of lower electron density whereas es-layer will be lifted upward to a region of higher electron density producing reduction in fof2 and enhancement in foes respectively. acknowledgements the authors are grateful to their research colleagues mr. manoj tomar, mr. vinod kushwah and mr. vikram singh for discussions and help in preparations of the diagrams. one of the authors (bs) is grateful to csir, new delhi, india for financial support under emeritus science scheme. thanks are also due to the director, physical research laboratory, ahmedabad, india for making available the ionospheric data. references chuo, y.j., j.y. liu, s.a. pulinets and y.i. chen (2002): the ionospheric perturbations prior to the chi-chi and chia yi earthquakes, j. geodyn., 33, 509-517. davies, k. and d.m. baker (1965): 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(1938): abnormal variations of earth-current potentials accompanied ith the earthquake off the cape of sioya, may 23, 1938, mem. akioka magn. obs., 1, 1-11. 551_561.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 551 a different approach to the analysis of gps scintillation data biagio forte (1), sandro m. radicella (1) and rodolfo g. ezquer (2) (1) the abdus salam international centre for theoretical physics, trieste, italy (2) laboratory of ionosphere of national university of tucuman tucuman regional school of the national technological university conicet argentina abstract amplitude scintillation data from gps were analyzed. the objective is to estimate the impact of ionospheric scintillations at satellite based augmentation systems ranging and integrity monitoring station (sbas rims) level and at gps user level. for this purpose, a new approach to the problem was considered. data were studied from the point of view of the impact of scintillations on the calculation of vtec at pierce points and ionospheric grid points. an ionospheric grid of 5°× 5° surface squares was assumed. from geometrical considerations and taking into account the basic principle to compute vtec at grid points, with the data analyzed it is shown that scintillations very seldom affect the calculation of a grid point vtec. data from all the rims and for the entire gps satellites network must be analyzed simultaneously to describe a realistic scenario for the impact of scintillations on sbas. finally, gps scintillation data were analyzed at user level: service availability problems were encountered. 1. introduction a radio wave crossing the upper and lower atmosphere of the earth atmosphere suffers a distortion of phase and amplitude. when it traverses drifting ionospheric irregularities, the radio wave experiences amplitude and phase fluctuations which vary widely with its frequency, but also with magnetic and solar activity, time of day, season and location. these effects are called amplitude and phase ionospheric scintillations. irregularities producing scintillations are predominantly in the f layer, at altitudes ranging from 200 to 1000 km with the primary disturbance region for high and equatorial latitude irregularities between 250 and 400 km (aarons, 1982). there are also times when e-layer irregularities in the 90 to 100 km produce scintillation, particularly sporadic e and auroral e layers (aarons, 1982). several techniques have been used to study irregularities giving rise to scintillations. these include: ground, airborne and satellite based hf swept frequency vertical sounders studying electron density structure and observing both bottomside and topside f-layer irregularities. 1) in-situ measurements by rockets and satellites of electron and ion density irregularities, electric fields, and electron and ion flux. 2) coherent (vhf to microwave) radar backscatter. 3) scintillation techniques which directly measure the perturbations of radio signal traveling through the ionosphere (aarons, 1982). mailing address: dr. biagio forte, aeronomy and radiopropagation laboratory, the abdus salam international centre for theoretical physics, strada costiera 11, 34100 trieste, italy; e-mail: bforte@ictp.trieste.it key words ionospheric scintillations gps scintillation monitor sbas 552 biagio forte, sandro m. radicella and rodolfo g. ezquer 2. ionospheric scintillations phenomenology for a wave propagating in an isotropic medium, all the points on the wave front have the same phase, whereas for a wave propagating in a non-isotropic medium the electron density irregularities act as antennas, in such a way that all the points on the wave front have different phase. scintillations are caused by scattering of radio waves by free electrons in the ionospheric plasma: scintillation may involve weak or strong scattering. the strongest scattering is observed in the equatorial and auroral regions, particularly in the equatorial areas; scintillation tends to be weak at mid latitudes. the maximum scintillation occurrence is at night. the geographical pattern of occurrence is suggested in the well known figure by basu et al. (1988) (fig. 1). it is generally agreed that weak mid-latitude scintillations are due to diffractive scattering while strong scintillations (at low latitudes) are caused by refractive scattering, that originates from ionization irregularity in the form of «holes» or «bubbles» that are perpendicular to the line of sight. refractive scattering is considered to be due to irregularities of scale larger than the fresnel length, and diffractive scattering is assumed to involve irregularities having sizes near the fresnel length. in amplitude scintillation studies, two indices have been used: the first one is the index s 4 , representing the standard deviation of received power divided by the mean value and defined as (briggs and parkin, 1963) (2.1) where i is the field intensity. the second index is si, proposed as a convenient approximate measure of scintillation and defined by (2.2) where the p’s represent the received signal power. in order to avoid overemphasizing extreme conditions, it is recommended that the third peak down from the maximum and the third minimum up from the absolute minimum be used to define p max and p min (whitney et al., 1969). s i i i4 2 2 1 2 = / si p p p p = + max min max min fig. 1. pattern of ionospheric scintillation during solar maximum and solar minimum (basu et al., 1988). 553 a different approach to the analysis of gps scintillation data 3. satellite-based augmentation systems (sbas) sbas are augmentation systems to gps and will be used for a navigation precision approach. three systems have been envisaged: a) the u.s. wide area augmentation system (waas); b) the european geostationary navigation overlay system (egnos); c) the japanese mtsat (multifunctional transport satellite) satellite based augmentation system (msas). basic gps service cannot meet the accuracy (the difference between the measured position at any given time to the actual or true position), scintillation activity changes with geomagnetic latitude: it is higher in the region of geomagnetic equator and equatorial anomaly (low latitudes) and in the region of trough, plasmapause and auroral oval (high latitudes), where irregularities appear more frequently. the general behavior of scintillation activity is summarized in tables i, ii and iii where the main results of experimental observations are given. these observations correspond to fixed geometry conditions: only one station at a time looking at one geostationary satellite or localized in situ measurements and gps signal observed from a single station. table i. the high latitude scintillation characteristics. generalities the pattern of the high latitudes scintillations is shown in fig. 1 (basu et al., 1988). the occurrence of scintillation at high latitudes is both during day time and at night. at high latitudes, two regions of peak scintillations are observed. one corresponds to the auroral oval and the other in the region above 80° geomagnetic latitude over the polar cap (aarons, 1982). kind of scintillations auroral polar cap in which periods of the year mainly between february and june: maximum occurrence appears in they occur? the activity increases with increasing months of little or no sunlight at fgeomagnetic activity. region heights. much lower scintillation occurrence appears in sunlight months (aarons et al., 1981). at what time? the scintillation is most intense in the the diurnal variation is weak and nighttime sector, but significant well defined only during the morning (0700-1000 lt) scintillation winter months (aarons et al., 1981). is also observed; scintillation activity, both in daytime and at night, follows the general pattern of local magnetic activity (rino and matthews, 1980). because of what? it has be shown a collocation of two irregularity components in the scintillations patches in the auroral polar cap: antisunward drifting oval and f region ionization irregularities and intense enhancements (irregularity zones both irregularities within the f layer equatorward and poleward of the polar cap arcs (aarons et al., 1981). auroral oval) (vickrey et al., 1980). which is the frequency activity generally decreasing with increasing frequency. dependence? which is the solar activity the probability of scintillations occurrence (and their intensity) increases with dependence? solar activity. the measurements made until now show that scintillation activity is proportional to solar activity (aarons,1982). 554 biagio forte, sandro m. radicella and rodolfo g. ezquer sbas are based on a network of ranging and integrity monitoring ground stations (rims). signals from gps satellites are received by rims that determine the existing error. each rims transfers data to the master station that computes the correction information, assessing the integrity of the system. the correction message is then transmitted to a geostationary satellite, at the same frequency as gps (l1 = 1575.42 mhz). finally, the geostationary satellite broadcasts the correction message to the system user. in this paper attention will focus on the firststep link used by sbas (the rims receive signals the availability (the ability of a system to be used for navigation whenever it is needed by the users, and its ability to provide that service throughout a flight operation), and integrity (the ability of a system to provide timely warnings to users or to shut itself down when it should not be used for navigation), that are requirements critical to safety of flight. sbas is a safety-critical navigation system that will provide a quality of positioning information never before available to the aviation community. it improves the accuracy, integrity and availability of the basic gps signals. table ii. the mid-latitude scintillation characteristics. generalities at mid-latitudes scintillations are weak and their occurrence is very low. the ionospheric scintillations are not a serious problem for the radiopropagation at the mid-latitudes: these represent a problem quite in high and low latitudes (basu et al., 1988). kind of scintillations random qp in which periods of the year they occur mainly in the summer; but they occur mainly in the summer they occur? they occur also during the other (hajkowicz and dearden, 1988). seasons (hajkowicz, 1994). at what time? the activity peak is observed in the between 22.00 lt and 2.00 lt; they summer, between 20.00 lt and 24.00 are observed also between 8.00 lt lt; in the other seasons, instead, they and 10.00 lt, during minimum solar occur between 24.00 lt and 4.00 lt. activity (hajkowicz and dearden, they are observed with much less 1988). frequency also during daytime, between 8.00 lt and 16.00 lt, following solar activity. because of what? the daytime random scintillations they originate from tids, appear related to the presence of e s concerning mainly the f region (particularly the e sc type (hajkowicz et al., 1981). (hajkowicz, 1978). the nighttime ones are caused by spread-f (hajkowicz, 1977). which is the frequency the percentage of occurrence (the number of the observed events) decreases dependence? with the transmission frequency, as depicted in fig. 2 (fujita et al., 1982). usually, the observed dependence is s 4 f n, where f is the frequency, while n 1.38 during nighttime and n 1.52 during daytime. which is the solar activity the probability of scintillation occurrence and their intensity increase with dependence? solar activity. measurements show that scintillation activity is proportional to solar activity (aarons, 1982). 555 a different approach to the analysis of gps scintillation data table iii. the low latitude scintillation characteristics. generalities the pattern of the nighttime equatorial latitudes scintillations is shown in fig. 1, where we can see the fluctuation of their intensity and the occurrence time. at the equatorial latitudes, the scintillations are stronger in the dark area, shaped like a stretched oval, because of the terrestrial rotation (basu et al., 1988). in which periods of the year they show a different pattern with the longitude: for example, in the pacific they occur? sector the scintillation activity peak occurs between may and july, while the minimum occurs between november and december. the opposite pattern is observed in the afro-american sector (aarons, 1982; basu and basu, 1981). at what time? generally during nighttime: they appear between 20.00 lt and 21.00 lt and last 4 h about (basu and basu, 1981). because of what? because of bubble-like irregularities in the f region. the irregularities, causing scintillation of a transmitted signal in vhf band, have an extent of about some kilometres, while that ones, causing scintillation for a trasmitted signal in l band, have an extent of about 102 m (aarons, 1982). which is the frequency it is usually observed that s 4 f n, where f is the frequency and n 1.5 for dependence? s 4 < 0.6. instead for s 4 > 0.6 n decreases monotonically, approaching a value of zero for saturated scintillations (strong scintillations) (rastogi et al., 1990). which is the solar activity the probability of scintillation occurrence and their intensity increase withdependence? solar activity. measurements show that scintillation activity is proportional to solar activity (aarons, 1982). from the gps satellites constellation) and a new approach to investigate the impact of scintillations on sbas will be presented. moreover, a statistical study of signal availability at «user level» will be illustrated. 4. data used the data set used for this study was collected at the institute of physics of national university of tucuman (argentina), by means of a scintillation monitor developed and provided by cornell university (beach et al., 1999). the scintillation monitor is a modified plessey gps system, specifically a version of the receiver with modified software. the monitor receives gps l1 signals from satellites in view, provides power measurements of received signals at a high rate and computes s 4 scintillation index at 1 min intervals. data were collected over 11 months, starting from september 1998 up to november 1999. for each day of a month, the measurements time interval is 12 h during nighttime. the received power data are filtered of multipath fluctuations with a high pass filter (beach, 1998). 5. rims level statistics we assume that the gps receiver in tucuman is a rim station. according to sbas specifications we use the thin shell ionosphere approximation, with a ionospheric grid at 350 km with 5°× 5° surface squares. the objective is to determine the impact of scintillations on each pierce point vertical total electron content (vtec) calculation that, in turn, intervenes in the vtec computation at grid points. pierce points are considered at the ionospheric grid height (350 km). we study the s 4 scintillation 556 biagio forte, sandro m. radicella and rodolfo g. ezquer index for every pierce point to determine the percentage of time where there is a «good» gps signal to be used to compute vtec. this defines the actual ionospheric conditions and computes the ionospheric error message for each ionospheric grid point to be sent to the users of the augmented gps system. figure 2 shows an example of the results for january 1999 for a 5°× 5° surface square of the ionospheric grid for all the events with 0.8 < s4 < 1. the threshold of 0.8 for s4 was chosen, supposing that for s4 > 0.8 the receiver loses the gps signal (strong scintillation regime). for each 5°× 5° surface square, table iv shows the total number of measurements for january 1999 and the number of events with 0.8 < s4 < 1, while table v shows the percentages of these events per total number of measurements in each surface square. in tables iv to ix the numbers at the abscissa and at the ordinate replace the actual longitudes and latitudes of each square of 5°× 5°. the occurrence of these strong scintillation events depends on the satellite position the tucuman receiver is looking at. the probability of observing scintillation events will be greater in the surface square where most pierce points are located, while no events will be observed in zones where no satellites have been in view. nevertheless, from tables iv and v it can be noted that the maximum of occurrence of strong scintillation events is not necessarily located in the square where most pierce points are located. table vi shows a time percentage statistics for each ionospheric grid surface square: the percentage represents the number of single minutes (among all the available information minutes in a particular square) where an s4 between 0.8 and 1 was detected during january 1999. similarly, referring to january 1999 data, table vii shows the percentage of two config. 2. distribution of the information for events with 0.8 < s4 < 1, for january 1999. 557 a different approach to the analysis of gps scintillation data table iv. number of all measurements (upper number) and number of events with 0.8 < s4 < 1 (lower number). table v. percentage of events with 0.8 < s4 < 1 among the total number of measurements. table vi. percentages of single minutes where 0.8 < s4 < 1, for january 1999. 0 0 0 0 0 0 0 0 0 0 0 2.035 3.297 0 0 0 0 0 0.509 1.212 1.535 4.690 0 0 0 0.730 0.569 0.043 0.289 1.249 1.278 0 0 0 0.202 0.059 0.082 0.067 0.264 0 0 0 1.462 0 0 0.116 0.392 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 2.035 3.297 0 0.509 1.212 1.534 4.690 0.725 0.570 0.032 0.211 1.249 1.278 0 0.185 0.031 0.057 0.067 0.264 0 1.462 0 0.116 0.392 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 344 273 0 0 0 0 0 0 7 9 0 0 0 0 19 1179 3136 5150 1450 0 0 0 0 6 38 79 68 0 0 0 276 3863 22065 19946 4082 704 0 0 2 22 7 42 51 9 0 0 105 8650 31922 24694 5981 1136 0 0 0 16 10 14 4 3 0 0 1 1710 0 7 1717 255 0 0 0 25 0 0 2 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 558 biagio forte, sandro m. radicella and rodolfo g. ezquer table vii. percentages of two consecutive minutes where 0.8 < s4 < 1, for january 1999. table viii. percentages of 3 consecutive minutes where 0.8 < s4 < 1, for january 1999. 0 0 0 0 0 0 0 0 0 0 0 0.581 0.733 0 0 0 0 0 0 0.191 0 1.724 0 0 0 0 0.207 0.012 0.131 0.539 0.852 0 0 0 0 0.012 0 0 0.176 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.136 0.207 0 0 0 0 0 0 0.021 0.294 0.568 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 table ix. percentages of 4 consecutive minutes where 0.8 < s4 < 1, for january 1999. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.078 0 0 0 0 0 0 0 0 0 0.568 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 559 a different approach to the analysis of gps scintillation data secutive minutes for each square of the ionospheric grid. the same is for table viii and ix corresponding to different cases of three and four consecutive minutes. there are some squares that are affected by scintillations events lasting several consecutive minutes. in these squares, no vtec computation can be made for the given pierce points. then, no calculation usable for the vtec calculation at the grid points could be made for that square in the interval of time with consecutive minutes strong scintillations. 6. the geometry of the problem all the results presented and the considerations on the figures and tables are valid from the point of view of a single rim station, located at tucuman, near the equatorial anomaly and during high solar activity. in fig. 3 a schematic representation of the sbas geometry can be observed. the ray paths of different satellites can encounter irregularities giving rise to scintillations, but it depends on the position of the ground stations the satellites are looked from. from a ground station the signal of a particular gps satellite suffers disruptive scintillation events, whereas from another ground station (far enough from the previous one) the signal of the same satellite does not suffer disruptive scintillation. this result shows that the impact of ionospheric scintillations on satellite-based augmentation systems can be estimated only considering data from all the rims looking at the entire gps satellites constellation. observing the behavior of scintillation events from one ground station at a time, we can see a scenario valid from the point of view of that particular ground station only. the system indeed uses all rims to compute vtec at grid points to estimate the ionospheric error message to be sent to the user. for instance, if for a given time the computation algorithm sees four pierce points in a particular grid square and for two of these pierce points s4 is greater than a given threshold, the algorithm could select the other two ray paths to compute the vtec at the corresponding grid point. in this way, the scintillation effect could be minimized. 7. user level statistics let us now focus on the scintillation measurement results from the point of view of an hypothetical user of gps system located in tucuman. for each month of observation we analyzed the percentage of time the user can utilize gps system without any problem of scintillating signal. we suppose that the gps system cannot be used by a user in minutes where the number of satellites, seeing s4 less than a given threshold, is less than 4; the system, indeed, can be used in minutes where the number of satellites, seeing s4 less than a given threshold, is greater than 4 (or equal to 4). we compute percentages of time where the system cannot be used over the total number of measurements minutes available for each month. the result is shown in table x. three thresholds were chosen for s 4 : 0.5, 0.8, 0.9. for each threshold, two columns are shown: the first shows the percentage of minutes (among the fig. 3. the geometry of the problem from the point of view of the algorithm; the bubbles along the ray paths represent irregularities at 350 km originating ionospheric scintillation. 560 biagio forte, sandro m. radicella and rodolfo g. ezquer minutes of observation) where the number of satellites, seeing s4 lower than the given threshold is less than 4; the second column shows the percentage of consecutive minutes (two or more consecutive minutes) where the number of satellites, seeing s 4 lower than the given threshold is less than 4. at user level, the gps system could encounter availability problems in the case of a user in the area of the equatorial anomaly and during particular heliogeophysical conditions (high solar activity or ionospheric storms). 8. conclusions the time percentages of scintillation events greater than a given critical threshold show that the scintillation impact on sbas operation at rims level is low. scintillation events seldom total measure % s 4 > 0.5 % s 4 > 0.8 % s 4 > 0.9 minutes in the month % bad min % bad % bad min % bad % bad min % bad min consecutive consecutive consecutive min min min september 1998 20883 0.939% 0.7% 0.163% 0.1% 0.1% 0.048% (196) (146) (34) (22) (21) (10) october 1998 22014 1.921% 1.504% 0.413% 0.232% 0.3% 0.136% (423) (331) (91) (51) (66) (30) november 1998 17103 0.807% 0.596% 0.064% 0% 0.029% 0% (138) (102) (11) (0) (5) (0) december 1998 20969 0.067% 0% 0.014% 0% 0.009% 0% (14) (0) (3) (0) (2) (0) january 1999 19158 0.230% 0.130% 0.026% 0% 0.026% 0% (44) (25) (5) (0) (5) (0) february 1999 19994 2.691% 2.096% 0.155% 0.075% 0.045% 0.01% (538) (419) (31) (15) (9) (2) march 1999 19337 2.317% 2.037% 0.61% 0.455% 0.326% 0.243% (448) (394) (118) (88) (63) (47) april 1999 21443 0.051% 0.037% 0% 0% 0% 0% (11) (8) (0) (0) (0) (0) september 1999 17003 0.623% 0.5% 0.047% 0% 0.029% 0% (106) (85) (8) (0) (5) (0) october 1999 13902 2.144% 1.877% 0.683% 0.532% 0.460% 0.352% (298) (261) (95) (74) (64) (49) november 1999 20669 1.055% 0.721% 0.184% 0.068% 0.121% 0.029% (218) (149) (38) (14) (25) (6) table x. user level time percentages. 561 a different approach to the analysis of gps scintillation data affect the system at this level. the impact becomes still lower if the number of rims is increased for the calculation of vtec at a given grid point. in this case, the time percentages shown for different cases in tables iv-ix could be lower, with improving availability, integrity and safety of sbas. previous analyses of scintillation behavior are based on data collected at single and isolated stations. the results found are not readily applicable to sbas, because such systems have a multi-rim station geometry rather than a single-station geometry. if a particular rims is detecting high scintillation events from a given gps satellite probably from another rims (far from the previous one), the signal from the same satellite is no longer affected by high scintillation events. if the signal to the first rims can go through an irregularity causing scintillations, the signal to the second rims probably does not go through any irregularity, suffering no scintillation problems. the sbas algorithm then could select the second signal data to compute vtec at grid points. the time percentages shown in tables iv to ix were computed assuming loss of lock when s4 > 0.8. as only amplitude scintillations data were available, this threshold was chosen assuming the phase screen approximation to estimate the correlation between phase and amplitude scintillations. the analysis method illustrated represents the starting point to describe a more realistic scenario for the impact of scintillations on sbas: the next step is the simultaneous analysis of data from all the rims. finally, the data analysis made at user level also describes a realistic scenario. the impact of scintillations on a single frequency receiver is evaluated by means of percentages of minutes where the number of satellites, seeing s 4 less than a given threshold, is less than 4 (table x). once again, the thresholds were theoretically fixed. the results show that in some cases system availability problems could be encountered. acknowledgements the data showed in this work were measured by means of a receiver kindly provided by dr. p. kintner (cornell university). references aarons, j. (1982): global morphology of ionospheric scintillations, proc. ieee, 70 (4), 360-378. aarons, j., j.p. mullen and h.e. whitney (1981): uhf scintillation activity over polar latitudes, geophys. res. lett., 8 (3), 277-280. basu, s. and s. basu (1981): equatorial scintillationsa review, j. atmos. terr. phys., 43 (5), 473-489. basu, s., e. mackenzie and su. basu (1988): ionospheric constraints on vhf/uhf communications links during solar maximum and minimum periods, radio sci., 23 (3), 363-378. beach, t.l. (1998): global positioning system studies of equatorial scintillations, ph.d. thesis, cornell university, ithaca, new york. beach, t.l. and p.m. kintner (1999): simultaneous global positioning system observations of equatorial scintillations and total electron content fluctuations, j. geophys. res., 104 (a10), 22,553-22,565. briggs, b.h. and i.a. parkins (1963): on the variation of radio star and satellite scintillations with zenith angle, j. atmos. terr. phys., 25, 339-365. fujita, m., k. sinno and t. ogawa (1982): frequency dependence of ionospheric scintillations and its application to spectral estimation of electron density irregularities, j. atmos. terr. phys., 44 (1), 13-18. hajkowicz, l.a. (1977): morphological and ionospheric aspects of quasiperiodic scintillations, j. atmos. terr. phys., 39, 833-841. hajkowicz, l.a. (1978): multi-satellite scintillations spreadf and sporadic-e over brisbane-2, j. atmos. terr. phys., 40, 99-104. hajkowicz, l.a. (1994): types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum, j. atmos. terr. phys., 56 (3), 391-399. hajkowicz, l.a., and d.j. dearden (1988): observations of random and quasi-periodic scintillations at southern mid-latitudes over a solar cycle, j. atmos. terr. phys., 50 (6), 511-517. hajkowicz, l.a., e.n. bramley and r. browing (1981): drift analysis of random and quasiperiodic scintillations in the ionosphere, j. atmos. terr. phys., 43 (7), 723-733. rastogi, r.g., p.v. koparkar, h. chandra and m.r. deshpande (1990): multifrequency studies of equatorial ionospheric scintillations at ootacamund, j. atmos. terr. phys., 52 (1), 69-76. rino, c.l. and s.j. matthews (1980): on the morphology of auroral zone radio wave scintillation, j. gephys. res., 85 (a8), 4139-4151. vickrey, j.f., c.l. rino and t.a. potemra (1980): chatanika/triad observations of unstable ionization enhancements in the auroral f-region, geophys. res. lett., 7 (10), 789-792. whitney, h.e., c. malik and j. aarons (1969): a proposed index for measuring ionospheric scintillation, planet. space sci., 17, 1069-1073. (received april 8, 2002; accepted august 26, 2002) vol51,2_3,2008 491 annals of geophysics, vol. 51, n. 2/3, april/june 2008 key words microzonation, umbria-marche earthquake, seismic codes 1. introduction the working hypothesis behind microzonation studies is that we can study a limited number of ideal situations prior to an earthquake occurrence to forecast the areas where the damage will be increased by amplification of seismic motion or by secondary site effects (liquefaction, soil settlement, slope instability, etc.). as often it happens in seismology, also for microzonation each earthquake is an occasion to learn new lessons, and the umbria-marche seismic sequence occurred in september 1997 is no exception. in the 10 years elapsed since that event, several microzonation studies were carried out in the area. immediately after the event, the studies were aimed to the most damaged area and to support emergency intervention and reconstruction plans. in the following years, regional and national projects were aimed to transfer the lessons learned to other towns (among the others spoleto, cerreto di spoleto, città di castello, gubbio, fabriano, cagli, treia, serra de’conti, offida, senigallia). havcodes, models and reality: reductionism vs. holism in a review of microzonation studies in the umbria-marche region marco mucciarelli dipartimento di strutture, geotecnica e geologia applicata, università della basilicata, potenza, italy abstract in the 10 years since the umbria-marche earthquake, several microzonation studies were carried out in the two regions. in the immediate aftermath of the event, the focus was on the epicentral area and toward emergency intervention and reconstruction plans. in the following years, regional and national projects aimed to transfer the lessons learned to other towns. usually, those two kinds of microzonation studies are referred to as «simplified» and «detailed». the difference is more subtle, and leads to the question of whether a microzonation study can be tackled following a reductionist approach, i.e. leaving different experts taking care of a limited subject (geology, geophysics, seismology, geotechnics, structural engineering). the impression looking back at 10 years of studies is that a holistic approach would be more appropriate to describe a system (structure-soil-bedrock) that is non-linear, inhomogeneous, and presenting feedback among its components. a second problem that emerges is the link with codes and practitioners. during the past 10 years the seismic code has been changed and a new version is on arrival. the last proposed version of the code is based on a parameter (vs30) that is discussed in the same country where it was first adopted, and introduces a parameter (acclivity) that appears to be a secondor third order problem with respect to others that are completely disregarded (e.g., 2-d site effects). a possible explanation for this mismatch between codes, models and reality is that our knowledge of distribution and amplitude of site effects is biased by selective under-sampling. being driven by damage, and paying less attention to a uniform distribution of studied sites and situations, we act like a drunk man looking of his lost keys under a street lamp, not because he is sure that he lost them there, but because the light is there. mailing address: dr. marco mucciarelli, dipartimento di strutture, geotecnica e geologia applicata, università della basilicata, potenza, italy; e-mail: marco.mucciarelli@unibas.it. vol51,2_3,2008 4-03-2009 10:28 pagina 491 492 m. mucciarelli ing more time (and sometimes more resources) available, the second kind of studies were carried out performing more experimental work in situ and more detailed modelling, while the first studies used a limited amount of geophysical and geotechnical data and rely mostly upon geological maps and 1-d models. usually, those two kinds of microzonation studies are referred to as «simplified» and «detailed». the difference is more subtle, and leads to the question if a microzonation study can be tackled following a reductionist approach, i.e. leaving different experts taking care of a limited subject (geology, geophysics, seismology, geotechnics, structural engineering) and drawing conclusions from that. a further need of microzonation studies is to produce something that can be transferred to codes and practitioners, and here the problem is to (quoting a. einstein) «make things as simple as possible, but not simpler». this problem was born with modern seismology. the year 2007 marks not only the 10th anniversary of the 1997 umbria-marche quake but also the 150th anniversary of the 1857 val d’agri quake. after a field trip in the affected area, robert mallet wrote its book «first principles of observational seismology». the problem of which geological and morphological situations could influence the observed damage distribution was already tackled by previous studies, but mallet is the first to understand the problem of biased distribution of experimental evidence. he wrote: «much pain has been devoted by earthquake describers (hamilton, dolomieu, spallanzani, and others) to the question whether towns situated upon the plain on loose material, or those on the solid rocks on hill tops, suffered most; and their discussions evince much obscurity of thought, having no guiding principle.... were the whole of the facts ... discussed with regard to this question, it would be found, that more places were destroyed upon the rock, than upon loose clay or other such foundations, and more upon hills than upon plains; but this would not conduct us to the whole truth, because in all south italy, there are a great many more places upon rock and upon hills, than upon clays and low plains». 2. review of simplified microzonation studies a work that takes into account the problem outlined by mallet was published by tertulliani (2000). this paper examines all the localities affected by the first two shock of the umbriamarche sequence with a damage reaching vi-vii degree in the european macroseismic scale. a simplified geo-morphological scheme is then proposed, distinguishing town located either a) on alluvium near the basin edge; (b) on fan debris; (c) on debris-alluvium transition; (d–e) on bedrock debris transition; (f) on bedrock-alluvium transition; (g) on bedrock; (h) on alluvium far from the basin edge. the conclusion is that «heavy damage occurred in hamlets settled on alluvial deposits near to the edge of the basin, with a border fault to mark the bedrock limit» while «the severity gradually decreases as we move to villages settled on debris and on a bedrock-debris transition. the lowest intensity is found on bedrock sites and on the inner part of the basin». stratigraphy appears thus to be a governing factor more important than morphology. the most extensive microzonation study aimed to emergency purposes was performed by pergalani et al. (1999). in this work «a team of surveyors were trained to collect field information such as geologic and geomorphologic features and, where possible, pre-existing geotechnic or geophysic information. information was collected and analysed with the aid of dynamic codes to calculate the possible local site effects». using 60 villages as a training set, the authors produced a generalization of the stratigraphic and morphologic situations, synthesized in a table grouping morphology types (valley, ridges or slopes) and lithologic units and thickness, and finally assigning to each zone a frequency-independent amplification factor according with the provision of the then enforced seismic code. the conclusion was that «the procedure ... prove to be a suitable tools for site effects assessment in post-event situation when time and budget constraint are present; in fact about 1000 sites have been surveyed and an estimate of the possible amplification established in less than six months. the average cost was also relatively small: about us$1500 for each site». some years later, marzorati et al. (2003) performed a work aimed to vol51,2_3,2008 4-03-2009 10:28 pagina 492 493 codes, models and reality: reductionism vs. holism in a review of microzonation studies in the umbria-marche region «evaluating the accuracy of that methodology, following a completely different approach. the actual damages, detected by an aerial photo interpretation, are compared to the theoretical damages estimated through empirical curves». the conclusions are particularly interesting from the point of view of the knowledge transfer to practitioners: «most of the errors detected in the comparison are... not ascribed to the methodology, but are simply due to the incorrect application of the reference table proposed for the zonation, or to the inaccurate geologic and geomorphologic surveys. those erroneous evaluations could be justified only if considering that the microzoning had to be carried out so rapidly that several surveyors not adequately trained were employed... other inaccuracies revealed new geomorphological and geological settings that have to be included in the reference table, like the saddle-like morphology and the surface faulting». among the single-technique approaches to microzonation, two methodologies were tested on large data sets for the first time in italy. goretti and dolce (2002) used an objective methodology to identify damage enhancements from observed data, called normalised damage index: «the distribution of the seismic intensity that actually affected buildings is obtained by means of well-assessed vulnerability functions, together with a bayesian approach, and taking into account uncertainties in building type and building damage». mucciarelli and monachesi (1998) performed a series of measurements using the hvsr technique (nakamura, 1989) following the suggestion about how to collect and process data provided by mucciarelli (1998). the measurements were taken in 12 sites arranged in couple of places where a strong difference in intensity occurred within a short distance. then a correlation was sought between the intensity variation and the amplification ratio in different band of frequency. the result was that the higher correlation occurred in the range 1-2 hz. 3. review of detailed microzonation studies from 1997 to 2007, several studies for microzonation were performed in towns located in the umbria and marche regions. the main studies are listed in the following: nocera umbra (cattaneo and marcellini, 2000), fabriano (marcellini and tiberi, 2000), cagli, treia, serra de’conti, offida (mucciarelli and tiberi, 2004), città di castello (bordoni et al. 2003), cerreto di spoleto (martino et al., 2006) senigallia (mucciarelli and tiberi, 2007), spoleto (vuan et al., 2007), gubbio (fiorini et al., 2007). these works mixed in a different degree geological, geophysical and geotechnical data, but are all united by a common feature: the use of recorded earthquakes. the main results of these works concern three points: 1) the importance of 2-d effects in sedimentary basins; 2) mixed experimental and numerical evidence of strong morphologic effects; 3) the importance of soil-building resonance for damage enhancement. 3.1. 2-d effects the strongest amplification recorded for the umbria-marche earthquake occurred in the gubbio basin (castro et al., 2004). in general, alluvial plains were affected by high amplification, as demonstrated also for città di castello basin (bordoni et al., 2003) and colfiorito basin (di giulio et al., 2003). the importance of this kind of effects is well known to seismologist and engineers, but nothing has been done so far for their inclusion in the seismic codes, notwithstanding proposals aimed to this goal (chavez-garcia and faccioli, 2000). 3.2. morphologic effects there has been mixed evidence about the role of morphology in amplifying recorded seismic ground motion in the umbria-marche area. laurenzano et al. (2008) noted no amplification on the treia ridge but a strong amplification in the underlying valley. marsan et al. (2004) analysing the data from two arrays, «observed a large amplification in the basin border of cesi site, in opposite small amplification due to topografic effect in sellano site». martino et vol51,2_3,2008 4-03-2009 10:28 pagina 493 494 m. mucciarelli al. (2006) studied the sharp ridge of cerreto di spoleto. they deployed a seismological array «in order to highlight possible topographic amplification effects all along the ridge; the findings, however, at least with the low energy level of the recorded ground motion, do not point out any topographic amplification» and after the study of the rock mass fracturation, they concluded that «rock sites can show seismic amplification effects other than the topographic one. in particular, the seismic response in a rock mass ridge can be affected by rock mass jointing: adjacent rock masses with significantly different jointing and specific geometries seem to favour trapped wave amplification, if persistence along depth of these characteristics occurs». the amplification due to waves trapped inside a fault zone was already observed and modelled by rovelli et al. (2002) for nocera umbra. the italian seismic code disregarded the problem of amplification at rock sites, abolishing the hard-rock class of the nehrp classification scheme (vs30>1500 m/s). on the contrary, the new provisions introduce topographic amplification for all the slopes with more than 15° inclination. 3.3. soil-building resonance during the umbria-marche sequence, the enhancement of damage due to soil-building resonance was studied for first time in italy with digital instrumentation. some of the most peculiar damage patterns observed were ascribed to soil-building resonance, as for the case of damage to reinforced concrete buildings in a limited area of fabriano (marcellini and tiberi, 2000; mucciarelli et al., 2001;) or the damage to retrofitted buildings in the historical centre of sellano (mucciarelli et al., 2001; nunziata and natale, 2004). the case of these last two works is interesting because they reach similar conclusions starting from independent dataset and models. mucciarelli et al. (2001) performed empirical measurement of ambient vibration on both soil and buildings, concluding that «the amplifcation is mainly due to a 2-d hill-top effect, with variability due to anthropic fills. it is clear that the damage is greater for buildings whose frequency approaches the site transfer function». nunziata and natale (2004) aimed to reproduce accelerometric recording using numerical modelling and concluded that «taking into account the building typology of 2–3 floors, the high damage level might be attributed not only to the degraded conditions of the brick houses but also to 2d amplification effects, as the frequencies of the maximum peaks of the spectral amplification correspond to the eigenfrequencies of the buildings». notwithstanding this evidence, no mention is made in the italian code about the possibility of taking into account the frequency of soil and building for the existing structures to be retrofitted. 4. discussion other events in italy (like the 2002 molise earthquake) were tackled using methodologies that were first applied after the umbriamarche sequence. the data collected during umbria-marche microzonation campaigns were a valuable source of information for further studies. for example, shear waves velocity estimates contributed with those performed in other regions to verify the reliability of vs30 as a proxy of seismic amplification (mucciarelli and gallipoli, 2006; gallipoli and mucciarelli, 2009). these studies concluded that «vs30 ... is not a good proxy of observed amplification effects in about one third of the sites. the reason why in italy vs30 does not provide satisfactory estimates is linked to peculiar geological settings that are widespread in the country. vs30 seems to work fine only if a site has a strictly monotonic velocity profile increasing with depth and a strong impedance contrast in the first dozen meters». the data from umbria-marche helped also to investigate the effect of selective under-sampling mentioned in the introduction (mucciarelli and gallipoli, 2004). from 1997 to 2004 years, our group conducted two kinds of hvsr campaigns: microzonation studies and post earthquake surveys. the aim was different: in the first case, we sampled all the municipalities vol51,2_3,2008 4-03-2009 10:28 pagina 494 495 codes, models and reality: reductionism vs. holism in a review of microzonation studies in the umbria-marche region or sites within the study area without a priori selection; in the second case, we performed measurements aimed at investigating possible resonance phenomena between the fundamental frequency of the soil and the one of damaged buildings. thus in this second case we performed a selection driven by the observed damage. the existence of hidden selection criteria in a sample may introduce unwanted bias in the outcome. we performed all the measurement using the same instrumentation and processing technique. with a database of 540 hvrs measurements, we compared three samples: 407 measurements from microzonation studies in different italian regions (umbria, marche, basilicata, molise), 79 from postearthquake surveys (umbria-marche, slovenia, calabrolucano, izmit) and 54 from our latest postearthquake survey (molise). in this last case, we performed measurements not only at the most damaged sites, but also more uniformly in the investigated area (gallipoli et al., 2004). the variable we considered for our statistics is the highest hvsr value in the frequency range 0.5-10 hz. it is clear from fig. 1 that the data from microzonation studies and post-quake survey have similar distributions but very different parametrisation. the main result from fig. 1 is the probability of observing high hvsr values at randomly sampled sites. the data sampled during microzonation studies have a lower probability than the ones resulting from postquake studies. this means that there are few sites with strong local amplification, and they are those that claim for attention when the damage is driving our researches. the distribution observed after the molise 2002 event shows that a study conducted after an earthquake yield the same distribution of earthquake-independent studies, provided that the sampled location are not pre-selected but the whole area is investigated. fig. 1. comparison between the empirical cumulative distribution functions of different sets of hvsrs, as a function of the largest value observed in the frequency range 0.5-10 hz. see text for details. 100 90 80 70 60 50 40 30 20 10 0 2 4 6 0.5 hz < max hvsr <10 hz microzonation post-earthquake molise 2002 % 80 10 vol51,2_3,2008 4-03-2009 10:28 pagina 495 496 m. mucciarelli 5. conclusions the umbria-marche sequence provided several valuable lessons about microzonation studies. these can be summarised in the following: 1. no simple method is safe enough if the practitioners are not well-trained to use it; 2. the strongest amplifications recorded for the umbria-marche earthquake were due to 2-d basin effects; 3. topographic amplification, if and where present, is a second-order problem with respect to impedance contrast amplification; 4. the most peculiar damage patterns observed were due to soil-building resonance; 5. vs30 is not a good proxy of observed amplification effects in about one third of the cases. very little of this information was taken into account for the formulation of the italian seismic code. a leitmotiv often repeated is that the codes must be simple or the practitioners could have problems in applying them. one may wonder if any of us would oppose the diffusion of medical ultrasonography or computed tomography scans on the basis that old-fashioned doctors are familiar just with their wooden stethoscopes. a wrong diagnosis is never good, not only if the illness is worst than assumed, but also in case of over-conservatism, causing unnecessary expense and routing limited resources where they are not needed. ten years after the umbria-marche quake, but also ten years after the introduction of the nehrp-fema provisions, the world outside italy has treasured the experiences gathered in the meanwhile. – the relationship between acclivity and amplification has been revised by wald and allen (2007) who propose using the topographic map as a first order support for determining amplification via vs30 since «topographic variations should be an indicator of near-surface geomorphology and lithology ... with steep mountains indicating rock, nearly flat basins indicating soil, and a transition between the end members on intermediate slopes». – the presence of amplification in sites with 1500<vs30<800 has been repeatedly described (see, e.g., sokolov et al., 2007). –the fact that soil non-linear behaviour is not always a guarantee of lesser amplification was studied by bonilla et al. (2005), that showed how the hysteretic and dilatant behaviour of cohesionless soils «increases the duration of the strong shaking and in some circumstances produces the maximum acceleration». – the need to overcome the classification based on velocity profiles only has been invoked by studies that propose to include soil fundamental period (rodriguez-marek et al. 2001, pitilakis et al., 2006) as well as 2-d effects (chavez-garcia and faccioli, 2000). – the importance of soil building resonance in microzonation studies was shown by navarro et al. (2004) and recently discussed in an international workshop (cassidy et al., 2008) whose conclusions were that «there is general agreement that soil-building resonance can be a cause of damage enhancement. there are suggestions that peculiar damage patterns (e.g. the collapse of a single building in a set of identical ones) could be due to city-soil interaction. there is the need to better quantify the occurrence of resonance taking into account the time-varying, non-linear behaviour of soil and building subjected to strong ground motion». there is no clearcut conclusion from this list of questions and problems, but the impression is that microzonation studies must be interdisciplinary and no conclusion should be drawn using (outdated) models based on a single technique. to evaluate the virtues of an holistic approach we can find again inspiration in the work of robert mallet. he was an engineer, stated the rules for seismology and wrote beautiful pages about local geology and site effects: disciplinary barriers and fences were introduced much later on, and we can ask ourselves if we had a real payback from that. references bonilla, l.f, r.j. archuleta and d. lavallée (2005): hysteretic and dilatant behavior of cohesionless soils and their effects on nonlinear site response: field vol51,2_3,2008 4-03-2009 10:28 pagina 496 497 codes, models and reality: reductionism vs. holism in a review of microzonation studies in the umbria-marche region data observations and modeling, bull. seism. soc. am., 95, 2373-2395. bordoni, p., g. cultrera , l. margheriti, p. augliera, g. caielli, m. cattaneo, r. de franco, a. michelini and d. spallarossa (2003): a microseismic study in a low seismicity area: the 2001 site-response experiment in the città di castello basin (italy), annals of geophysics, 46 (6), 1345-1360. cattaneo, m. and a. marcellini (editors) (2000): terremoto dell’umbria-marche: microzonazione sismica di nocera umbra e sellano, (cnr-gndt roma, 2000, pp 228. + cd-rom, isbn 88-900449-7-5). chavez-garcia, f.j. and e. faccioli (2000): complex site effects and building codes: making the leap, journal of seismology, 4, 23-40. di giulio, g., a. rovelli, f. cara, r.m. azzara, f. marra, r. basili and a. caserta (2003): long-duration asynchronous ground motions in the colfiorito plain, central italy, observed on a two-dimensional dense array, j. geophys. res., 108 (b10), 2486, doi: 10.1029/2002jb002367. fiorini, e., f. pacor, d. bindi, a. rovelli, f. cara, g.. di giulio, g.. milana, g.. monachesi, d. nieto, g. bohm, d. albarello, v. d’amico, m. picozzi, m. mucciarelli, g. scarascia mugnozza, s. rivellino, m. menichetti, s. parolai, s. richwalski and i. orpsal (2007): geological model of the gubbio basin (italy) for the characterisation of local seismic response, report for the ingv-dpc project s2.3.6, (last download at http://esse3.mi.ingv.it/deliverables/deliverables_s3_d21.pdf). gallipoli, m.r., m. mucciarelli, m. tropeano, s. gallicchio and c. lizza (2004): hvsr measurements in the area damaged by the 2002 molise, italy earthquake, earthq. spectra, 20, s81-s94. gallipoli, m.r. and m. mucciarelli (2009): comparison of site classification from vs30, vs10 and hvsr in italy, bull. seism. soc. am., 99 (1), doi: 10.1785/01200800083. goretti, a. and m. dolce (2002): site effects evaluation from surveyed typological and damage data, proc. 12th european conference on earthquake engineering, (cd-rom edition), pp. 10. laurenzano, g., e. priolo and e. tondi (2008): 2-d numerical simulations of earthquake ground motion: examples from the marche region, italy, in press on journal of seismology. marcellini, a. and p. tiberi (editors) (2000): la microzonazione sismica di fabriano, (biemmegraf, piediripa di macerata), p. 350 + cd-rom. marsan, p., g. milana, a. pugliese and t. sanò (2004): local amplification effects recorded by a local strong motion network during the 1997 umbria-marche earthquake, 13th world conference on earthquake engineering vancouver, (cd-rom, paper no. 1046). marzorati, s., l. luzi, v. petrini, f. meroni and f. pergalani (2003): detection of local site effects through the estimation of building damages, soil dyn. and earthq. engin., 23, 497-511. martino, s., a. minutolo, a. paciello, a. rovelli, g. scarascia mugnozza and v. verrubbi (2006): evidence of amplification effects in fault zone related to rock mass jointing, nat. haz., 39, 419-449. mucciarelli, m. (1998): reliability and applicability range of the nakamura’s technique using microtremors. an experimental approach, journ. earthq. eng., 2, 625-638. mucciarelli, m. and g. monachesi (1998): a quick survey of local amplifications and their correlation with damage observed during the umbro-marchesan earthquake of september 26, 1997, journ. earthq. eng., 2, 1-13. mucciarelli, m., p. contri, g. monachesi, g. calvano and m.r. gallipoli, (2001): an empirical method to assess the seismic vulnerability of existing buildings using the hvsr technique, pageoph, 158, 26352647. mucciarelli, m., m. herak and j. cassidy (editors) (2008): inreasing seismic safety by combining engineering technologies and seismological data, (springer, dordrechts, the netherlands, isbn 978-14020-9194-0), pp. 382. mucciarelli, m. and p. tiberi (editors) (2004): microzonazione sismica di cagli, offida, serra de’. conti, treia. regione marche, (gndt-ingv,), pp. 71+ cd-rom. mucciarelli m. and m.r. gallipoli (2004): the hvsr technique from microtremor to strong motion: empirical and statistical considerations; 13th world conference on earthquake engineering vancouver, (cdrom, paper no. 45). mucciarelli, m. and m.r. gallipoli, (2006): comparison between vs30 and other estimates of site amplification in italy, proc. 1st european conference on earthquake engineering and seismology, cd-rom edition, paper 270. mucciarelli, m. and p. tiberi (editors) (2007): microzonazione sismica di senigallia, (tecnoprint ancona), pp. 420+ 2 cd-rom. navarro, m., f. vidal, m. feriche, t. enomoto f.j. sanchez and i. matsuda (2004): expected ground-r.c building structures resonance phenomena in granada city (southern spain), 13th world conference on earthquake engineering, (vancouver, b.c., canada august 1-6, paper no. 3308). nakamura, y. (1989): a method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, q. rept. rail. tech. res. inst., 30 (1), 25-33. nunziata, c. and m. natale. (2004): spectral amplification effects at sellano, central italy, for the 1997–98 umbria seismic sequence; natural hazards, 33, 365378. pergalani f., r. romeo, l. luzi, v. petrini, a. pugliese and t. sanò (1999): seismic microzoning of the area struck by umbria–marche (central italy) ms 5.9 earthquake of 26 september 1997, soil dyn. and earthq. engin., 18, 279-296. pitilakis, k., c. gazepis and a. anastasiadis (2006): design response spectra and soil classification for seismic code provisions, proc. of the workshop on geotechnical evaluation and application of the seismic eurocode ec8, (athens), pp. 31-46. rodriguez-marek, a., j.d. bray and n.a. abrahamson (2001): an empirical geotechnical seismic site response procedure, earthquake spectra, 17, 6587. rovelli, a., a. caserta, f. marra and v. ruggiero (2002): can seismic waves be trapped inside an inacvol51,2_3,2008 4-03-2009 10:28 pagina 497 498 m. mucciarelli tive fault zone? the case study of nocera umbra, central italy. bull. seism. soc. am., 92, 2217-2232. sokolov, v., y., lohb c-h. and w.-y. jean (2007): application of horizontal-to-vertical (h/v) fourier spectral ratio for analysis of site effect on rock (nehrp-class b) sites in taiwan, soil dyn. earthq. engin., 27, 314323. tertulliani a. (2000): qualitative effects of local geology on damage pattern, bull. seism. soc. am., 90 (6), 15431548. vuan a., m. romanelli, c. barnaba, a. restivo, l. lovisa, e. priolo and g. rossi (2007): relazione scientifica finale. convenzione tra comune di spoleto e ogs per lo studio di microzonazione del centro storico della città di spoleto, (rel. ogs 2007/53-crs/12, last download at http://www.ogs.trieste.it/show/uploadfile/file.aspx?idfile=105&iduniversity=2). wald d.j. and t.i. allen (2007): topographic slope as a proxy for seismic site conditions and amplification; bull. seism. soc. am., 97, 1379-1395. vol51,2_3,2008 4-03-2009 10:28 pagina 498 575_586 abdel fattah 31.pdf annals of geophysics, vol. 45, n. 5, october 2002 575 source characteristics of a moderate earthquake (m 4.9) using empirical green’s function technique ali k. abdel-fattah national research institute of astronomy and geophysics, helwan, cairo, egypt abstract the rupture process of a moderate earthquake (m 4.9) on 28th january 1999 was analyzed using velocity records at local distances less than 80 km. the characterization of the rupture process was obtained from studying aftershocks distribution, azimuthal variations of relative source time functions (rstfs), and a set of spatio-temporal slip models. rstfs were retrieved by deconvolution of small aftershock records from those of the mainshock. in addition, velocity p-wave records of the respective event were inverted to recover slip distribution on the fault plane using the records of aftershocks as empirical green functions (egfs). the waveform inversion was adopted using three egfs. in the inversion, the rupture propagation velocity was fixed and assumed to be eight-tenths of the local shear wave velocity. the total seismic moment was estimated to range from 0.011 e + 18 nm (mw = 4.6) to 0.017 e + 18 nm (mw = 4.8). the hypocentral distribution of the aftershocks, azimuthal variations of rstfs, and the set of slip distribution models were exhibited bilateral rupture propagation along the strike and dip of the fault plane. the presence of two to three high slip patches on the fault plane suggested that a complex rupture pattern is detectable for a moderate size earthquake. however, the so-called nucleation phase was invisible in the present analysis. 1. introduction the description of earthquake rupture histories is crucial not only to understand the physical processes of earthquake generation but also to predict strong motions of large earthquakes. detailed studies of earthquake rupture processes revealed that rupture begins with a relatively low moment rate before it propagates dynamically. this idea was introduced by theomailing address: dr. ali k. abdel-fattah, national research institute of astronomy and geophysics, helwan 11421, cairo, egypt; e-mail: ali_kamel@yahoo.com key words empirical green’s function – aftershocks distribution – relative source time function – spatiotemporal slip models retical models (e.g., andrews, 1976; das and scholz, 1981; dieterich, 1986, 1992; shibazaki and matsu’ura, 1998) and laboratory experiments (e.g., dieterich, 1979; ohnaka et al., 1987) to simulate the initial stage of earthquakes. theoretical and experimental results indicated that slip initiates and accelerates gradually within a small finite part of the entire fault before the dynamic rupture occurs. recently, many seismological studies have described the behavior of earthquake initiations. for example, brune (1979) concluded that the initiation of large earthquakes is similar to that of small events. umeda (1990, 1992) reported the existence of a low amplitude initial phase preceding the main p-phase in large earthquakes. iio (1992, 1995) observed this view for micro-earthquakes of nagano prefecture. abercrombie and mori (1994) explained that the beginning of the m 7.3 landers, california earthquake was similar to an 576 ali k. abdel-fattah aftershock of m 4.4. anderson and chen (1995) showed that no systematic difference was observed in the p-wave initiation of m 3.0-m 8.0 events in michoaca, mexico. furthermore, mori and kanamori (1996) supported that earthquakes of all sizes initiate in a similar manner and begin to grow dynamically within a few hundredths of seconds after the rupture initiation. ellsworth and beroza (1995) showed that the hector mine earthquake exhibited rupture complexities similar to large earthquakes. they pointed out that the hector mine earthquake started with about a 1.8 s foreshock or nucleation phase followed by the main rupture. the imaging of earthquake slip histories might enhance our understanding of earthquake physical processes. fletcher and spudich (1998) inverted the source time functions of moderate earthquakes into slip distribution and rupture time to infer fault properties of the initiation zone at parkfield. they analyzed three earthquakes of complex slip histories started at a small spike and then expanded into the largest patch. such studies for small to moderate earthquakes require good quality of near-field observation records. on the 28th january 1999, the earthquake research institute (eri) seismic network stations, tokyo university recorded an earthquake of magnitude 4.9 and its aftershocks that took place in nagano prefecture. this earthquake sequence yielded many good waveform records, which provided an opportunity to determine the slip distribution in space and time. in the present study, the deconvolution technique in the time domain was applied to retrieve rstfs using velocity records of aftershocks as egfs. in a second step, the most appropriate egfs were used to invert the waveform data and recover the spatio-temporal slip distribution on the fault plane. results were interpreted in terms of rupture complexities. all analyses were confined to the first few seconds of the p-wave records. 2. data to investigate the slip history of a moderate earthquake using egfs, a magnitude 4.9 event of the japanese earthquakes was chosen. the respective event was followed by aftershocks with a range of magnitudes 1.5 to 4.0 and all were recorded by nearby seismic stations of the eri seismic network (fig. 1). all instruments are 1 hz velocity sensors with a dynamic range 24-bit digital recording system and a sampling rate of 100 samples per second. in addition to eri permanent seismic stations, four portable seismic stations were installed close to the mainshock to locate aftershocks accurately. the error in aftershock locations is less than 100 m in width and 200 m in depth. figure 2 shows the distribution of aftershock hypocenters determined by the eri. the location of the mainshock and of three egfs is shown in fig. 3. the capital letters m, af1, af2 and af3 correspond respectively to mainshock and aftershocks as listed in table i. appropriate egfs were selected by examining aftershocks with magnitude nearly 2 units smaller than that of the mainshock. the appropriate aftershocks were chosen on the basis of their high signal to noise ratio. 3. method briefly, two steps were used in the present study. the first step was to extract rstfs by deconvolution of egfs records from those of the respective event. in the second step, three appropriate egfs were used to invert the waveform data and obtain the slip history on the fault plane using the method of ide and takeo (1997). 3.1. empirical green’s function detailed investigations of earthquake source processes require good knowledge of the effects of path, site and instrument responses. by considering seismic records of small earthquakes located close to a larger one, the forementioned effects would be available using small events as egfs. for two earthquakes with different sizes but having similar hypocenters and focal mechanisms, one can treat the waveform of the smaller event as an egf and deconvolve it from the larger event to obtain the rstf of the larger earthquake (mueller, 1985). since the pair of events is recorded at the same station by the same 577 source characteristics of a moderate earthquake (m 4.9) using empirical green’s function technique fig. 1. locations of the 28th january 1999 nagano earthquake (solid star) and eri seismic stations used in the deconvolution and waveform inversion processes mainshock fig. 2. focal mechanism of the mainshock, epicentral and hypocentral distribution of mainshock (solid star) and aftershocks (open circles) 578 ali k. abdel-fattah instrument and has almost the same propagation path, the deconvolution process should result in an rstf that is corrected for the path, site and instrument effects. since hartzell (1978) proposed an egf method to estimate synthetic seismograms, many researchers have applied and developed this method (e.g., kanamori, 1979; hadly and helmberger, 1980; irikura and muramatu, 1982; dibona and boatwright, 1989). the egf deconvolution technique has been used to extract rstfs for small earthquakes to estimate the earthquake source parameters (e.g., frankel et al., 1986; li and thurber, 1988; mori and frankel, 1990; xie et al., 1991). rstfs were also used to reveal rupture complexity and directivity (e.g., frankel et al., 1986; li and thurber, 1988; mori and frankel, 1990; abdel fattah and badawy, 2001). in this study, rstfs were retrieved to examine rupture directivity and complexity of a moderate earthquake. rstfs were obtained by applying the deconvolution technique in the time domain with non-negative constraints (lawson and hanson, 1974). the deconvolution was performed as a least square problem (abdel fattah and badawy, 2001). as an example, fig. 4a-e shows the deconvolution procedure for the mainshock using the vertical component record of the ued station. a key assumption of using a small event as an egf is that the source duration of its source time function is short compared to that of the larger event. therefore, this small event can be considered as an impulsive source. thus, the resulting rstf reflects the source characteristics of the larger event. a cross-correlation analysis was performed to distinguish the waveform similarity between the mainshock and its aftershocks. the events having a good correlation to the mainshock were used as green function events. after deconvolution, a butterworth low-pass filter with a corner frequency of 30 hz was applied to reduce the high-frequency noise of the retrieved rstfs. the filtered rstfs exhibited similarity in the pulse shapes for three cases of egfs (fig. 5a-c). in order to confirm the appropriateness of the egfs, three appropriate aftershocks were adopted. in addition, fig. 5a-c displayed that rstfs were complex with narrow pulse widths toward north and south directions. the pulse duration of rstfs was measured from the deconvolution delay time of egf (0.8 s) to the first zero crossing. the pulse widths ranged from 0.27 to 0.6 s. azimuth pulse width variations of the ob id date origin time lat. (°) long. (°) depth (km) mjma* m 99.01.28 10:25:48.40 36.3733 137.9837 09.3 4.9 af1 99.01.28 11:05:38.40 36.3699 137.9920 11.5 2.9 af2 99.01.28 11:41:11.70 36.3730 137.9899 11.3 2.7 af3 99.01.28 11:59:27.90 36.3790 137.9891 11.4 2.8 *m jma denotes the magnitude determined by the japan meteorological agency (jma). table i. the focal parameters of mainshock and egf aftershocks. fig. 3. epicentral locations of egf’s (open squares) used in deconvolution and waveform inversion with respect to the mainshock location (solid star). 579 source characteristics of a moderate earthquake (m 4.9) using empirical green’s function technique tained rstfs were a diagnostic of the rupture directivity. variations of the rstf pulse width with its station azimuth are shown in fig. 6a-c. the azimuthal variations of rstf pulse widths showed that rupture propagates bilaterally. from fig. 6a-c, it was also obvious that the pulse widths were narrow at stations to the north and south. the advantage of the egf deconvolution technique is that it allows investigation of rupture complexity and directivity. 3.2. determination of slip distribution once the appropriate egfs were determined, a simple egf method was used to obtain the spatio-temporal slip distribution on an assumed fault plane. various inversion methods have been developed to determine spatio-temporal distribution of slip on a fault plane from seismic waveform data (e.g., olson and apsel, 1982; hartzell and heaton, 1983, 1986; kikuchi and fukao, 1985; mori and shimazaki, 1985; yoshida, 1986, 1988; kikuchi and kanamori, 1986; takeo and mikami, 1987; satake, 1989; beroza and spudich, 1988; mori and hartzell, 1990; ide et al., 1996). to express the total rupture process as a spatio-temporal slip distribution on a fault plane, the whole fault plane was divided into many subfaults and the slip was estimated at each subfault. the problem here was how to choose the dimensions of subfaults or the number of subdivisions. if the fault is divided into a small number of large subfaults, the solution will have poor resolution. alternatively, if a large number of small subfaults are used, the inversion will be unstable and rapidly vary due to the noise contained in the data. the relation between the synthetic record at a station and the slip rate distribution on the fault plane is repre-˙ ,u x ti ( ) fig. 4a-e. an example of the rstf of the mainshock derived from p-waves by using the record of event af1 as a green’s function at the ued station. a) the mainshock record. b) the convolution result of the rstf with the record of egf. c) the waveform data of the mainshock (solid line) and the result of the convolution of the rstf with the record of egf (dotted line). d) the record of af1. e) the deconvolved rstf. a b c d e 580 ali k. abdel-fattah sented as (3.1) where g ij (x, t, ξ, τ) is an empirical green’s function representing the j t h component of velocity waveforms record at a station when an impulsive slip rate in the i th direction is applied at x = ξ, t = τ. ide and takeo (1997) expanded the spatiotemporal distribution of slip rate as (3.2) where, a ilmn are expansion coefficients and φ1 l (x1), φ2 m (x2) and ψn(t) are the basis function in strike direction, dip direction and time, respectively. in this study, each basis function is an isosceles triangle determined by three nodes, and hence the slip distribution is continuous everywhere spatially and temporally. substituting (3.2) into (3.1), the observed displacement equation can be expressed as (3.3) ˙ ,u x t a x x ti ilmn l m n( ) = ⋅ ( ) ⋅ ( ) ⋅ ( )∑ φ φ ψ1 1 2 2 u x t a g x tj ijlmn ij, , ; ,( ) = ( ) ⋅∫∫∑ ξ τ φ d dl m n⋅ ( ) ⋅ ( ) ⋅ ( )φ ξ φ ξ ψ τ ξ τ1 1 2 2 ˙ , ,u x x ti 1 2( ) u x t g x t u d dj ij i( , ) , ; , ˙ ,= ( ) ⋅ ( )∫∫ ξ τ ξ τ ξ τ fig. 5a-c. the obtained relative source time functions (rstf’s) for the mainshock using empirical green’s functions (egfs) (a) af1, (b) af2 and (c) af3, respectively. a b c 581 source characteristics of a moderate earthquake (m 4.9) using empirical green’s function technique this equation can be written in vector form as . d = g m (3.4) where d is the vector representing observed data, m is the model parameter vector of a ilmn , and g is a n by m convolution matrix obtained by (3.4) with sampling corresponding to d. this linear inversion equation was solved by bayesian modeling described in ide et al. (1996). two smoothing constraints were applied, spatial and temporal constraints, to minimize the difference between the coefficients of spatio-temporally neighbouring basis functions. the weights of the constraints were determined using akaike’s bayesian information criterion (abic, akaike, 1980). the non-negative least-square (nnls) algorithm was employed (lawson and hanson, 1974). the approach of modeling large events with a summation of smaller ones has the advantage of simplicity, but its potential is limited by the fact that suitable empirical records are not always available. 3.3. preparation to inversion the slip is assumed to be distributed on the fault plane whose geometry is determined from the focal mechanism solution in combination with aftershock distributions. figure 2 shows the eri focal mechanism solution determined using the moment tensor inversion. the focal mechanism solution indicated reverse faulting with a strike-slip component. the plane striking n-s and dipping to the east is constrained with the aftershock distribution and selected as the fault plane. following ide and takeo (1997), a seismic source model could be represented as a spatiotemporal distribution of slip which expanded by triangle shaped basis functions arranged along the two dimensional fault plane (strike and dip) and along the time axis. the unknown parameters are coefficients of the basis functions. the numbers of the basis functions are 9, 9 and 7 in the strike, dip and time, respectively. node intervals are 0.25 km in the strike, 0.25 km in the dip and 0.02 s in the time. the total source duration is about 0.16 s. the total number of model parameters used in inversion is 567. the central point on the assumed fault represents the initiation a b c fig. 6a-c. the distribution of rstf (derived from deconvolution technique) pulse widths against station azimuths. a) using the p-wave record of event af1. b) using the p-wave record of event af2. c) using the p-waves record of event af3. 582 ali k. abdel-fattah p-velocity s-velocity depth density (km/s) (km/s) (km) 5.5 3.09 0.0 2.5 6.1 3.43 4.5 2.7 6.7 3.76 15.0 3.0 8.0 4.49 32.0 3.2 point of the rupture. in the linear inversion, we assumed a fixed rupture velocity of 2.7 km/s which determines the start time for a basis function at each point on the fault plane from the hypocenter. the travel times from each subfault to observers are calculated using a layered crustal structure shown in table ii. 4. results and discussion the development of waveform inversion methods revealed that source processes of the large earthquakes are generally complex (e.g., hartzell and heaton, 1983; beroza and spudich, 1988; takeo, 1988; waled and heaton, 1994; ide et al., 1996). for small earthquakes, tsukuda (1980) carefully analyzed the p-waveforms of events with m 1-2 and showed that their sources were represented by a simple fault model. recently, detailed analysis of seismic sources also indicated that source processes of small to moderate earthquakes are apparently similar to those reported for large earthquakes. nishigami (1987) analyzed several earthquake groups and observed that earthquakes with m ≥ 2.5-3.0 showed complicated p-waveforms (multiple shocks composed of two or three subevents). mori and frankel (1990) determined source time functions for small earthquakes (m l = 1.7 to 4.4) using an egf approach and showed that some of them exhibited significance pulse shapes suggesting more complex ruptures. li et al. (1995) determined the stf for m n = 1.2 to 4.4 earthquakes in the charlevoix seismic zone, quebec, canada by employing an empirical green’s approach. they used the stf pulse amplitudes as a function of station azimuths to estimate the rupture directions and the rupture velocities. they showed rupture complexities in an earthquake of magnitude 3.3. the rupture complexities of micro-earthquakes in miramichi, canada (m ~ 3 to 4.1) and in eastern maine (16 september 1994, m = 3.9) were also observed from retrieved rstfs (li et al., 1994, 1995). ide (2001) determined a set of detailed source models of 18 moderate earthquakes (m 4-5), representing a part of the swarm activity beneath the hidamountains in central japan in 1998, using a waveform inversion procedure of the egf. the rupture process of these earthquakes is complex and is apparently similar to the rupture process in large earthquakes. further, abdel-fattah and badawy (2002) retrieved complex source time functions, using empirical green’s function, of the october 11, 1999 earthquake (m l ~ 4.9) occurring in southeast beni-suef, northern egypt. in the present study, a simple egf method was used to reveal the rupture process of a moderate earthquake. the obtained rstfs and the set of spatio-temporal slip distribution models on the fault plane were used to describe the rupture complexity of the respective event. three different appropriate aftershocks were adopted to obtain rstfs and slip models and to evaluate the validity and applicability of the empirical green’s function method. rstfs were obtained by egf deconvolution technique in time domain. on other hand, the slip distribution was expanded by basis functions and the unknown parameters are the expansion coefficients of these functions (ide and takeo, 1997). the model parameters were determined using the non-negative least squares algorithm of lawson and hanson (1974). the difference between the coefficients of the spatio-temporally basis functions is minimized by bayesian modeling constrains described by ide et al. (1996). on the basis of the obtained slip models and rstfs as well as aftershock distribution, the source processes of our respective earthquake were defined. the rupture was roughly propagated bilaterally along the strike and dip of the fault plane. the spatio-temporal distribution of source time functions for each subfault along the fault plane is shown in fig. 7a. the result of the inversion for the set of slip distribution models is shown in fig. 7b. e01, e02 and e03 represent the slip models using af1, af2 and af3, table ii. velocity structure used in this study. 583 source characteristics of a moderate earthquake (m 4.9) using empirical green’s function technique fig. 7a,b. a) the source time function for each subevent using egfs af1, af2 and af3, respectively. b) total slip distribution models e01, e02 and e03 using egfs af1, af2 and af3, respectively. a b -1.00 -0.50 0.00 0.50 1.00 -1.00 -0.50 0.00 0.50 1.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 -1.00 -0.50 0.00 0.50 1.00 -1.00 -0.50 0.00 0.50 1.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 -1.00 -0.50 0.00 0.50 1.00 -1.00 -0.50 0.00 0.50 1.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 dip strike slip(m) slip(m) slip (m) northsouth u p d ip d o w n d ip (e03) (e02) (e01) 0.5km 584 ali k. abdel-fattah fig. 8. observed (solid lines) and synthetic velocity waveforms (dashed lines) of the p-wave records using the obtained rupture models shown in fig. 7a,b. 585 source characteristics of a moderate earthquake (m 4.9) using empirical green’s function technique respectively. in general, the slip patterns are quite similar. the geometry of slip patterns is approximately elliptical. two to three high slip patches were observed on the fault plane and can be interpreted in terms of asperities. finally, fig. 8 shows a good comparison between the observed records and the synthetic seismograms for the obtained rupture models (fig. 7a,b). the total seismic moment is 1.74 e + 16 nm, 1.11 e + + 16 nm and 1.6 e + 16 nm for e01, e02 and e03, respectively. the corresponding moment magnitude is 4.8, 4.6 and 4.7, respectively. the main result of this study indicates a complex rupture history for earthquakes of such moderate size, similar to large earthquakes. 5. conclusions the p-wave records of nagano earthquake were analyzed in an attempt to understand the rupture behavior of moderate earthquakes. three source rupture models of the respective event were investigated using three small aftershocks as empirical green’s functions. the distribution of aftershock and the characteristics of rstfs were consistent with the obtained slip distribution models. the analysis of the respective study reflected that the rupture propagates bilaterally. at the first stage of rupture, rstfs showed growth of a dynamic rupture. the set of slip models distinguished two to three high patches on the fault plane, indicating a complex rupture history. all results indicated that the investigated event occurs with a complex rupture process similar to the rupture process of large earthquakes. it could be emphasized that the detailed analysis, the set of spatio-temporal distribution of slip and the obtained rstfs, do not reflect the so-called nucleation phase. acknowledgements i would like to express my deep appreciation to s. ide for his valuable comments and guidance. also, i thank t. heneda for offering his aftershock hypocentral determination. the calculations carried out at the earthquake research observation, earthquake research institute, university of tokyo, japan. some of the figures have been generated using the genetic mapping tool. i am grateful to two anonymous reviewers whose comments enhancement this study. references abdel-fattah, a.k., and a. badawy (2001): source process of the southeast beni-suff, northern egypt earthquake using empirical green’s function technique, j. seismol., 6, 153-161. abercrombie, r. and j. mori (1994): local observations of the onset of a large earthquake: 28 june 1992 landers, california, bull. seismol. soc. am., 84, 725-734. akaike, h. 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(received january 11, 2002; accepted october 25, 2002) an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 1 study of thin clouds at cn r-imaa atmospheric observatory (ciao) marco rosoldi*, pilar gumà claramun t, fabio madon n a, aldo amodeo, ioan n is bin ietoglou, giuseppe d’ amico, aldo giun ta, lucia mon a, n ikolaos papagian n opoulos, gelsomin a pappalardo con siglio n azion ale d elle ricerch e, istitu to d i metod ologie p er l’ an alisi am bien tale, tito scalo, poten za, italy i. in troduction p ace-based m easu rem en ts allow u s to stu d y atm osp h eric variables at global scale, bu t grou n d -based observation s are n ecessary for calibration an d valid ation [gcos, 2006]. up com in g satellite m ission s aim at sh ow in g th e ben efit com in g from th e availability of tech n ological im p rovem en ts an d in n ovation s. h ow ever, research is n ecessary to im p rove retrieval algorith m s of atm osp h eric variables an d to assess th e real ad van ces in th e kn ow led ge of w eath er an d clim ate. qu ality con trolled vertical p rofiles of atm osp h eric key variables p rovid ed by grou n d -based ad van ced atm osp h eric observatories rep resen t th e op tim al basis for th e satellite cal/ val p rogram s. so far, atm osp h eric observatories are w orkin g tow ard s th e d evelop m en t of n ew observation strategies an d th e fu ll exp loitation of th e sy n ergy am on g active an d p assive p rofilin g se n sors. th is is h igh ly relevan t for th e n ew gen eration satellites equ ip p ed w ith m u ltip le sen sors on board th e sam e observation p latform . grou n d -based observation s p rovid e lon g -term m on itorin g of p aram eters th at, at p resen t, ca n n ot be m on itored from sp ace observation s. for in stan ce, th e stu d y of th in liqu id w ater clou d s as w ell as th e m ech an ism s lead in g to d rop let activation is ch allen gin g for satellite p assive sen sors, w h ereas th ey are im p ortan t for w eath er an d clim ate stu d ies. th e im p act of th in clou d s on clim ate is h igh ly u n certain . th in liqu id w ater clou d s, w h ich are low or m id level su p er -cooled clou d s ch aracterized by a liqu id w ater p ath (lwp) less th an 100 g m -2, are d ifficu lt to be observed accu rat ely an d large d iscrep an cies exist am on g d iffe ren t observation tech n iqu es [tu rn er et a l., 2007]. th e im p ortan ce of th ese clou d s is rela ted to th eir exten sive global m ean coverage: low an d m id level clou d s, often con tain in g liqu id w ater, are ch aracterized by a m ean lwp valu e of 51 an d 60 g m -2, an d cover 27.5% an d 19% of th e global su rface, r esp ectively, w h ile th e global m ean clou d coverage is 68.6% [rossow an d sh iffer, 1999]. grou n d -based p rofilin g tech n iqu es, su ch as ad van ced m u lti-w avelen gth lid ar an d dop p ler rad ar tech n iqu es, allow u s to stu d y aerosol an d clou d m icrop h ysics; th ey can be co n sid ered as th e n eed ed brid ge betw een in -situ an d satellite m easu rem en ts to fill in th e gap s in th e u n d erstan d in g of th e p h ysical m ech an ism s lead in g to clou d form ation . in th is w ork, tw o exam p les of p ossible solu tion s tow ard s th e im p rovem en t of th e accu racy in th e estim ation of th in clou d p rop erties are p rop osed . th e stu d y is based on th e m u ltiw avelen gth lid ar, dop p ler rad ar an d m icrow ave rad iom eter m easu rem en ts p erform ed at * corresp on d in g au th or: marco rosold i, con siglio n azion ale d elle ricerch e, istitu to d i metod ologie p er l’ an alisi am bien tale, c.d a s. loja, i – 85050, tito scalo, poten za, italy; em ail: m arco.rosold i@im aa.cn r.it. s an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 2 th e cn r-imaa atm osp h eric observatory (ciao). th e observatory is located in poten za, sou th ern italy, on th e ap en n in e mou n tain s (40.60n , 15.72e, 760 m a.s.l.), less th an 150 km from th e west, sou th an d east coasts [mad on n a et al., 2011]. th e first exam p le ad d resses th e en h an cem en t of dop p ler rad ar sen sitivity ach ieved ad op tin g a sp ectra avera gin g over th e tim e, w ith th e a im to d etect tin y d rop lets, as th ose form in g fog or th in clou d s. in th e secon d exam p le, p articu lar atten tion h as been p aid to an op tically th in broken strat ocu m u lu s clou d th at gives u s th e op p ortu n ity to in vest igate th e d ifferen ces betw een n on -satu ration an d satu ration con d ition s in an aerosol layer an d th e d rop let activation m ech an ism s, u sin g ram an lid ar m easu r em en ts. ii. th in clouds: radar sen sitivity sen sitivity of m illim eter -w avelen gth dop p ler rad ars allow s to stu d y m icrop h ysical p rop e rties of n on -p recip itatin g clou d s. th ese rad ars p rovid e accu rate reflectivity m easu rem en ts over a d yn am ic ran ge of ap p roxim ately seven ord ers of m agn itu d e, from -50 d bz to +20 d bz th rou gh ou t th e trop osp h ere [kollias et al., 2001]. a large fraction of clou d s is com p osed of h igh con cen tration s of sm all h yd rom eteors w ith d iam eters from a few to ten s of m icrom eters, w ith reflectivity valu es from –60 d bz to – 40 d bz [kollias et al., 2001]. wh en th e reflectivity of th e sm all h yd rom eteors is very w eak (less th an -50 d bz), th ese targets fall ou tsid e th e rad ar d etectable ran ge. th e p rim ary m easu rem en t p rovid ed by do p p ler rad ars is th e dop p ler sp ectru m . typ ically, th e first th ree m om en ts of th e rad ar dop p ler sp ectru m are p rovid ed an d u sed in rad ar d ata an alysis: th e sign al-to-n oise ratio (sn r – zeroth m om en t), th e m ean dop p ler velocity (first m om en t) an d th e dop p ler sp ectru m w id th (s econ d m om en t). for system s op eratin g in zen ith p oin tin g m od e, th e typ ical tim e resolu tion of a sin gle rad ar vertical p r ofile is of th e ord er of secon d s, w h ile th e typ ical vertical re solu tion is of a few ten s of m eters. in ord er to in crease th e rad ar sen sitivity, th e sp ectra p re-p rocessin g of rad ar m easu rem en ts p erform ed at ciao station is p erform ed u sin g tw o d ifferen t op tion s: (1) “rou tin e” p rep rocessin g, based on averagin g 200 sp ectra w ith a vertical resolu tion of 30 m an d a tim e resolu tion of 10 s; (2) “averagin g” p re p rocessin g, based on averagin g 3600 sp ectra (averagin g th e sp ectra over 3 ran ge gates an d 1 m in u te) w ith a vertical resolu tion of 90 m an d a tim e resolu tion of 1 m in u te. th e secon d o p tion allow s u s to en h an ce th e rad ar sen sitivity of 5-10 d bz, u p to abou t -55 d bz. figu re 1 sh ow s th e tem p oral evolu tion of rad ar reflectivity factor ze in th e “rou tin e” p re p rocessin g op tion (p an el a) an d in th e “avera gin g” op tion (p an el b), an d th e lin ear d ep olar ization ratio (ldr) in th e “averagin g” op tion (p an el c), m easu red on 10 october 2011 by r ad ar at ciao. all th ese tim e series are clu tter filtered . th e clu tter filterin g is carried ou t after th e estim ation of th e n oise level in th e rad ar p ow er sp ectru m [bau er-pfu n d stein , 2007]. in figu re 1a, th e black lin e sh ow s th e h eigh t of th e m eltin g layer retrieved u sin g su rface m ea su rem en ts of tem p eratu re an d th e m od eled tem p eratu re p rofiles of a stan d ard atm osp h ere. in both p rocessin g op tion s, ze in d icates th e p resen ce of ice clou d at arou n d 7 km above sea level (a.s.l.) at 13:00 ut an d low -level liqu id w ater clou d s below 3 km a.s.l., altern ated w ith occasion al rain sh ow ers. moreover, liqu id w ater layers are d etected u p abou t 2 km a.s.l. betw een 00:00 an d 04:00 ut an d betw een 17:00 an d 20:00 ut, on ly u sin g th e “averagin g” p re p rocessin g op tion (figu re 1b). th is is d u e to th e tin y size of th e d rop lets of th ese layers, likely fog, w h ose liq u id p h ase is con firm ed by th e absen ce of sign atu re in th e ldr (fig u re 1c). th e d iscu ssed case sh ow s h ow th e averagin g of rad ar sp ectra over larger vertical an d tem p oral d om ain s a llow s th e rad ar to exten d its ow n sen sitivity: th is is h igh ly relevan t in th e stu d y of th in clou d s an d fog. an en h an ced rad ar sen sitivity allow s u s to exten d th e size ran ge of h yd rom eteors d etected by th e rad ar as w ell as to con tribu te to a better exp loitation an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 3 figu re 1: (a) tim e series of vertical p rofiles of clu tter filtered rad ar reflectivity factor (ze) m easu red on 10 october 2011 at ciao u sin g th e “rou tin e” p rocessin g (tim e resolu tion is 10 secon d s an d vertical resolu tion is 30 m eters), black lin e in d icates th e m eltin g layer h eigh t; (b) sam e as rep orted in p an el (a) bu t u sin g th e “avera gin g” p rocessin g (tim e resolu tion is 1 m in u te an d vertical resolu tion is 90 m eters); (c) tim e series of vertical p r ofiles of clu tter filtered rad ar lin ear dep olarization ratio (ldr) for th e sam e d ate, obtain ed u sin g th e “averagin g” p rocessin g. of in stru m en t syn ergies. th e stu d y of aerosol, w ater vap or an d liqu id w ater in th in clou d s can also be con sid ered as a good o p p ortu n ity for th e qu an tification of th e aerosol in d irect effect. th is w ork is cu rren tly on g oin g at ciao alon g w ith th e evalu a tion of d ow n d rafts an d u p d rafts o bserved w ith th e dop p ler rad ar. an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 4 iii. th in clouds: raman lidar an d droplet activation aerosol p articles m ay be activated in to d r op lets w h en su p er -satu ration con d ition s are reach ed in a risin g air m ass. clou d d rop let activation is a h igh ly com p lex an d n on lin ear p rocess: on ly a fraction of p articles can grow beyon d th e critical sizes to form d rop lets, d ep en d in g on several factors su ch as size d istribu tion , ch em ical com p osition an d u p d raft velocity [min g et al., 2006]. from th e grou n d based rem ote sen sin g p oin t of view , th e p ossibility of in vestigatin g th is p rocess is related to th e cap ability of th e in stru m en ts to p rovid e accu rate estim ation of aerosol an d clou d s p rop erties. th in clou d s are on e of th e p ossible targets th at allow u s to stu d y th ese p rocesses. th ou gh th in liqu id w ater clou d s are d ifficu lt to be observed a ccu rately an d large d iscrep an cies exist am on g d ifferen t observation tech n iqu es [tu rn er et al., 2007], th ey are effectively observed th rou gh grou n d -based rem ote sen sin g rath er th an th rou gh p assive satellite rem ote sen sin g. at ciao, tw o m u lti-w avelen gth ram an lid ars, pearl (poten za earlin et ram an lid ar) an d musa (mu lti-w avelen gth system for aerosol), are op eration al [mad on n a et al., 2011]. both system s em it ligh t p u lses at 355, 532 an d 1064 n m an d d etect th e elastically backscattered rad iation by atm osp h eric co n stitu en ts an d th e ram an backscattered rad iation by n itrogen m olecu les at 387 an d 607 n m , an d by w ater vap or m olecu les at 407 n m , th e last on ly for th e pearl syst em . vertical p rofiles of aerosol op tical p rop erties an d w ater vap or m ixin g ratio are retrieved from lid ar sign als acqu ired w ith a raw vertical r esolu tion of 15 m (7.5 m for 1064 n m ) for pearl an d 3.75 m for musa, an d a raw tim e resolu tion of 60 secon d s. figu re 2a sh ow s th e tem p oral evolu tion of th e lid ar ran ge-corrected sign al at 1064 n m m easu red by musa on 4 au gu st 2011 from 19:22 to 21:16 ut. th e p lot sh ow s th e p r esen ce of broken clou d s betw een 3 an d 4.5 km of altitu d e a.s.l. th e observ ation of broken clou d s gives u s th e op p ortu n ity to calcu late th e vertical p rofiles of aerosol op tical p rop e rties an d w ater vap or con ten t in tw ofold m an n er: “w ith clou d s”, by averagin g all lid ar sign als, an d “w ith ou t clou d s”, by skip p in g th e sign als sh ow in g th e p resen ce of clou d s. th e n u m ber of skip p ed sign als in th e p rofiles “w ith ou t clou d s” is exp ressed th rou gh th e so-called skip p ed fraction , d efin ed as th e n u m ber of skip p ed sign als d iv id ed by th e total n u m ber of sign als. th e clou d con tam in ated lid ar retu rn s h ave been id en tified by visu al in sp ection an d assu m in g th at a stron g n egative slop e in th e n itrogen ram an lid ar sign als at 387 n m corr esp on d s to th e p resen ce of liqu id w ater d r op lets. th is clou d d etection tech n iqu e h as been u sed p r elim in arily in ord er to verify th e reliability of all th e p rofiles con tain in g clou d s. h ow ever, th e p roced u re can be au tom ated , an d th is w ill be d on e in th e n ear fu tu re th rou gh th e u se of th e earlin et sin gle ca lcu lu s ch ain (scc) for th e au tom atic retrieval of th e aer osol op tical p rop erties startin g from raw lid ar sign als [d’ am ico et al., 2012]. figu re 2b sh ow s th e vertical p rofiles of aer osol extin ction coefficien t at 355n m “w ith clou d s” (red lin e) an d “w ith ou t clou d s” (black lin e) obtain ed u sin g ram an lid ar tech n iqu e, averagin g lid ar sig n als over th e sam e tim e in terval of 17 m in u tes, from 19:33 to 19:50 ut. besid es, figu re 2c sh ow s th e vertical p rofiles of relative h u m id ity “w ith clou d s” (red lin e) an d “w ith ou t clou d s” (black lin e) estim ated w ith ram an lid ar tech n iqu e, averagin g lid ar sign als from 19:33 to 19:50 ut, an d calibrated w ith w ater vap or m easu rem en ts p erform ed by a co -located an d sim u ltan eou s rad io -sou n d in g. in both figu res 2b an d 2c th e p rofiles h ave an effe ctive vertical resolu tion of 780 m an d a skip p ed fraction of 0.29. fin ally, in figu res 2d an d 2e th e sam e as in figu res 2b an d 2c is r ep orted , bu t averagin g lid ar sign als over a tim e in terval of 103 m in u tes, from 19:33 to 21:16 ut, w ith an effective vertical resolu tion an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 5 figu re 2: (a) tim e series of ran ge-corrected sign al at 1064 n m obtain ed from musa on 4 au gu st 2011 from 19:22 to 21:16 ut u sin g a tim e resolu tion of 1 m in u te an d a vertical r esolu tion of 3.75 m ; (b) vertical p rofiles of 355 n m aerosol extin ction coefficien t retrieved u sin g lid ar sign als in clu d in g clou d s (“w ith clou d s” red lin e) an d skip p in g clou d s (“w ith ou t clou d s” black lin e) an d averagin g th e lid ar sign als over 17 m in u tes, from 19:33 to 19:50 ut; (c) vertical p rofiles of relative h u m i d ity “w ith clou d s” an d “w ith ou t clou d s”, corresp on d in g to th e p rofiles rep orted in p an el (b), calibrated u sin g a co-located an d sim u ltan eou s rad iosou n d in g. in both (b) an d (c) p an els, th e p rofiles h ave an effective ve rtical resolu tion of 780 m (skip p ed fraction = 0.29); (d ) vertical p rofiles of 355 n m aerosol extin ction coefficien t retrieved u sin g lid ar sign als in clu d in g clou d s (“w ith clou d s” red lin e) an d skip p in g clou d s (“w ith ou t clou d s” black lin e) an d averagin g th e lid ar sign als over 103 m in u tes, from 19:33 to 21:16 ut; (e) vertical p rofiles of relative h u m id ity “w ith clou d s” an d “w ith ou t clou d s”, corresp on d in g to th e p rofiles rep orted in p an el (d ), calibrated u sin g a co-located an d sim u ltan eou s rad iosou n d in g. in both (d ) an d (e) all th e p rofiles h ave an effective vertical resolu tion of 660 m (skip p ed fraction = 0.40). an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 6 of 660 m an d a skip p ed fraction of 0.40. th e error bars in th e p rofiles of figu res 2b an d 2c are larger th an th ose of figu res 2d an d 2e, becau se th e sm aller th e tim e in terval over w h ich th e lid ar sign als are averaged , th e low er th e sign al to n oise ra tio. in figu res 2b an d 2d , extin ction coefficien t p rofiles for both “w ith clou d s” an d “w ith ou t clou d s” cases are fou n d to be in good agr eem en t reach in g an altitu d e of abou t 3.2 km a.s.l. th is su ggests th at aerosol p articles d o n ot n u cleate. in th e region a bove, “w ith clou d s” p rofiles (red lin es) rise sign ifican tly com p ared to “w ith ou t clou d s” p rofiles (black lin es) becau se of th e aerosol activation an d th e d rop let form ation . from abou t 4.5 km a.s.l., th e extin ction p rofiles “w ith clou d s” an d “w ith ou t clou d s” are in good agreem en t again . th erefore, th e layer exte n d in g from 3.2 km to 4.5 km a.s.l. can be co n sid ered as th e region w h ere aerosol p a rticles are activated an d d rop lets form . th ese r esu lts seem to be con firm ed by th e relative h u m id ity p rofiles sh ow n in figu res 2c an d 2e. su ch p rofiles sh ow a sign ifican t in crease of relative h u m id ity w ith in th e activation layer an d a satu ration con d ition reach ed in th e m id d le p art of th e activation region w h ere d rop lets form . th e in terp retation of su ch p rofiles n eed s fu rth er in vestigation . th e p resen ted case stu d y reveals th e a d van tage of stu d yin g broken th in liqu id w ater clou d s, as th ey offer th e p ossibility to ch ara cterize th e aerosol activation region an d to sep arate it from th e region n ot affected by th e activation p rocess. iv. summary qu ality-con trolled vertical p rofiles of atm o sp h eric key variables p rovid ed by ad van ced atm osp h eric observatories, like ciao, are h igh ly relevan t for d efin ition an d im p lem en tation of su itable strategies for calibration an d valid ation for both geostation ary (e.g. metop ) an d p olar orbitin g (e.g. omi, te rra/ aqu a, adm-aeolu s an d earth care) satellite p latform s. th ey allow u s to stu d y m acrop h ysical an d m icrop h ysical p rop erties of aerosol an d clou d s over lon g term , in clu d in g p aram eters ch allen gin g to be ad d ressed w ith satellite observation s. th in clou d s rep resen t on e of th e m ajor sou rce of u n certain ty in th e estim ation of global rad iative balan ce an d in th e ch aracte rization of aerosol-clou d in teraction s. tw o exam p les relative to th e u se of dop p ler rad ar an d ram an lid ar m easu rem en ts h ave been d iscu ssed . th e first stresses th e n eed for u sin g a su itable tem p oral an d vertical avera gin g of rad ar sp ectra in ord er to in crease th e sen sitivity of rad ar in th e d etection of sm all d rop lets, as th ose form in g fog or th in clou d s. th e secon d d escribes th e ad van tage of stu d yin g th in broken clou d s, u sin g ram an lid ar m easu rem en ts, to d istin gu ish th e region s w h ere aerosol p articles are activated or n ot. com p lem en tary w ater vap or m ixin g ratio m easu rem en ts w ith ram an lid ar con tribu te to id en tify th e d rop let form ation region . th e p resen ted m eth od ology is cu rren tly u n d er in vestigation over a large d ataset of rad ar an d lid ar m easu rem en ts collected at ciao in th e p eriod 2009-2012. stu d y of th in liqu id w ater clou d s, as w ell a s th e related m ech an ism lead in g to d rop let activation , is ch a llen gin g u sin g sp ace observation s on ly. grou n d based lid ar an d rad ar observation s can stron gly con tribu te to im p rove satellite retrievals an d to ad vise on th e tech n ological requ irem en ts for fu tu r e satellite m ission s. ackn owledgmen ts th e au th ors gratefu lly ackn ow led ge dr. ma tth ias bau er for h is su p p ort in th e rad ar d ata p rocessin g. th e fin an cial su p p ort for earlin et by th e eu rop ean un ion u n d er gran t rica 025991 in th e sixth fram ew ork program m e is gratefu lly ackn ow led ged . sin ce 2011 earlin et is in tegrated in th e actris research in frastru ctu re project su p p orted by th e eu rop ean un ion seven th fram ew ork program m e (fp7/ 2007 2013) an n als of geoph ysics, 56, fast track 1, 2013; 10.4401/ ag 6337 7 u n d er gran t agreem en t n . 262254. referen ces [bau er-pfu n d stein , 2007] bau er-pfu n d stein , m., an d u. gö rsd orf (2007). target sep ar ation an d classification u sin g clou d rad ar d op p ler-sp ectra, in proceed in gs 33rd in tern . con f. on rad ar meteorology, cairn s. 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[tu rn er et al., 2007] tu rn er, d. d., an d coa u th ors (2007). th in liqu id w ater clou d s: th eir im p ortan ce an d ou r ch allen ge, bu ll. am er. meteor. soc., 88, 177–190. l e d i m e n s i o n i d e l l e s o n d e r a d i o a t t i v e e l ' e f f e t t o d i c a r i c a s p a z i a l e renato cialdea i n un r e c e n t e l a v o r o ( ' ) è s t a t a d e t e r m i n a l a t e o r i c a m e n t e l ' i n f l u e n z a d e l l a c a r i c a s p a z i a l e sul p o t e n z i a l e m i s u r a l o da u n a s o n d a r a d i o a t t i v a , p o s t a in u n c a m p o e l e t t r i c o g e n e r a t o da d u e a r m a t u r e p i a n e e p a r a l l e l e ; n e l c a s o l a t e r a l m e n t e i n d e f i n i t o , f u r o n o t r o v a t i v a l o r i d i v e r s i da q u e l l i o t t e n u t i s p e r i m e n t a l m e n t e ( 2 ) c o n u n a c o m u n e sonda a p o l o n i o ; t a l e d i f f e r e n z a p u ò a t t r i b u i r s i al f a t t o e l l e l ' a z i o n e i o n i z z a n t e , e r a l i m i t a l a ad u n r i s t r e t t o v o l u m e e q u i n d i l ' i n f l u e n z a d e l l a c a r i c a s p a z i a l e e r a n o t e v o l m e n t e m i n o r e . l a p r e s e n z a di u n a s o n d a r a d i o a t t i v a dà l u o g o a d u e d i s t i n t e p e r t u r b a z i o n i del c a m p o e l e t t r i c o in c u i è i m m e r s a . p r i m a d i t u t t o , il v o l u m e s o g g e t t o a l l ' a z i o n e i o n i z z a n t e si p u ò c o n s i d e r a r e c o m e u n c o n d u t t o r e , d a t o l ' e l e v a t o v a l o r e d e l l a c o n c e n t r a z i o n e d e g l i i o n i d ' a m bo i segni ivi p r e s e n t i , a l t e r a n d o q u i n d i il c a m p o e l e t t r i c o : q u a l o r a il c o n d u t t o r e stesso p o t e s s e c o n s i d e r a r s i l a t e r a l m e n t e i n d e f i n i t o , d i s p e s s o r e [> m i s u r a t o l u n g o l e l i n e e di f o r z a del c a m p o e c o n c a r i c a s p a z i a l e e s t e r n a n u l l a , il p o t e n z i a l e p r e s e n t e r e b b e l ' a n d a m e n t o d e l l a s p e z z a l a b) d e l l a fig. 1. l a r e t t a (/) d e l l a stessa figura r a p p r e s e n t a ovv i a m e n t e il p o t e n z i a l e m i s u r a t o da u n a sonda d i s p e s s o r e t r a s c u r a b i l e r i s p e t t o a l l a d i s t a n z a d e l l e a r m a t u r e , e m p i e c o n c a r i c a s p a z i a l e n u l l a al di f u o r i d e l l a s o n d a . l a s e c o n d a p e r t u r b a z i o n e c o m i n c i a a f a r s e n t i r e l a sua i n f l u e n z a a l l o r c h é gli i o n i c h e si t r o v a n o a l l ' e s t e r n o d e l v o l u m e s o g g e t t o a l l ' a z i o n e i o n i z z a n t e h a n n o u n a d e n s i t à t a l e c h e la c a r i c a s p a z i a l e n o n p o s s a e s s e r e t r a s c u r a t a ; in q u e s t o c a s o il p o t e n z i a l e a s s u m e l ' a n d a m e n t o r a p p r e s e n t a t o d a l l a c u r v a t e o r i c a e ) . d a q u a n to o r a d e t t o , è p r e v e d i b i l e c h e la d i m e n s i o n e d e l l a s o n d a l u n g o l e l i n e e di f o r z a del c a m p o i n f l u i r à p r i n c i p a l m e n t e l i l l a l u n g h e z z a d e i t r a t t i n i o r i z z o n t a l i d e l l a b) e d e l l a c) ( f i g . 1 ) e sul v a l o r e d e l c a m p o e l e t t r i c o n e l l ' i n t e r n o del c o n d e n s a t o r e ; i n v e c e l e a l t r e d u e d i m e n s i o n i ( ' ) r . c i a i . m i i a . i . o s l ' i m o g . z a n o t e l u . ann. di geofisica. i . 2 . 1 5 5 (19-48.1. ( = ) r . c i a l d e a a . l o s c u d o . ann. di geofisica. i , 1 , 4 9 ( 1 9 4 8 ) . i . e d i m e n s i o n i d e l l e s o n d i ; h a d i o a t t i y e e ( . ' e f f e t t o di i alili v s p a z i a l e 53 m i s u r a l e n o r m a l m e n t e a l l e l i n e e di f o r z a i n f l u i r a n n o p i u t t o s t o s u l l ' e f f e t t o d e l l a c a r i c a s p a z i a l e . i n f a t t i , p o i c h é i v a l o r i s p e r i m e n t a l i o t t e n u t i con una sonda c o m u n e -onn r i s u l t a t i p i ù v i c i n i a l l a spezzata b ) c l i c non a l l a c u r v a c ) , è da a t t e n d e r s i c h e . a u m e n t a n d o l e d i m e n s i o n i t r a s v e r s a l i della s o n d a , c i si d e b b a a v v i c i n a r e al caso l a t e r a l m e n t e i n d e f i n i t o . c o n s i d e r i a m o i n f a t t i la s e z i o n e del v o l u m e soggetto a l l ' a z i o n e i o n i z z a n t e , m i s u r a t a p a r a l l e l a m e n t e a l l e a r m a t i n e del c o n d e n s a t o r e : gli i o n i p r o v e n i e n t i d a l l a sua p e r i f e r i a s e g u o n o le l i n e e di forza soggette a l l e p e r t u r b a z i o ni ai b o r d i e q u i n d i v a n n o d i r a d a n d o s i c o n u n a d e n s i t à c l i c d i m i n u i sce r a p i d a m e n t e m a n m a n o c h e si a l l o n t a n a n o ; gli a l t r i i n v e c e , p r o v e n i e n t i d a l l a sua zona c e n t r a l e , seguono le l i n c e di f o r z a n o n p e r t u r b a t e e q u i n d i ri m a n t e n g o n o p a r a l l e l i c o n u n a d e n s i t à pressocc h è c o s t a n t e . di conseguenza è p r e v e d i b i l e c h c i p r i m i d i a n o a l l ' e f f e t t o d e l l a c a r i c a s p a z i a l e u n c o n t r i b u t o m o l t o più p i c c o l o dei s e c o n d i ; u n a sonda r e a l e c q u i n d i finita p o t r à essere p e r c i ò ass i m i l a t a a l l a sonda t e o r i c a i n d e f i n i t a , a l l o r c h é i p r i m i s i a n o in n u m e ro t r a s c u r a b i l e r i s p e t t o a i s e c o n d i , c i o è a l l o r c h é l e p e r t u r b a z i o n i ai b o r d i del suo v o l u m e soggetto a l l ' a z i o n e i o n i z z a n t e p o s s a n o e s s e r e i r a s c u r a t e . a l l o scopo d u n q u e di s t u d i a r e l ' i n f l u e n z a e f f e t t i v a d e l l e d i m e n s i o n i e d e l l a f o r m a di d e t t o v o l u m e n e i riguardi d e l l ' e f f e t t o d e l l a car i c a s p a z i a l e , h o c o n d o t t o a l t r e e s p e r i e n z e c o n un n i n n e r ò m a g g i o r e di s o n d e , sia p o n e n d o l e t u t t e su u n a sola fila, sia d i s t r i b u e n d o l e mi un t e l a i o q u a d r a l o a m a g l i e l a r g h e . p e r q u e s t a r i c e r c a ho r e a l i z z a t o un m e t o d o di m i s u r a c h e mi ha p e r m e s s o di e l i m i n a r e quasi del tutto gli e r r o r i c a u s a t i da e v e n t u a l i 3 6 6 r e n a t o c i a l d e a 5 r s }/ -| [—wvww— ri r—/avwws i—7vwwv\a—1 i . e d i m e n s i o n i d k i . i . e s o n d e il v i i [ o a t t l \ e e i " ehe i i o iii ( villi \ s i ' v z i u . e p e r d i l e d o v u t e ad i o l a n i e n l o 11011 p e r f e t t o . k" n o l o i n f a l l i ( " ) c h e , e il sostegno i s o l a n t e della sonda ha una r e s i s t e n z a r -, di v a l o r e finito, il p o t e n z i a l e f " m i u r a l o d a l l ' e l e t t r o m e t r o è l e g a l o , i n d i c a n d o con ra la r e s i s t e n z a a p p a r e n t e della s o n d a , al p o t e n z i a l e i del p u n t o in cui essa si trova (fi g. 2 ) d a l l a r e l a z i o n e / ' = /' =1 — ri + r, 1 + e dove -i è posto e -r;/ra s e n e deduci c h e l ' e r r o r e relativo della m i s u r a è a v _ 1 v " i + 7 k n e c e s s a r i o p e r ò t e n e r c o n t o a n c h e d e l l ' e r r o r e o di l e l l u r a d e l l o s t r u m e n t o n e l l a m i s u r a di p o t e n z i a l e c o n s i d e r a t a : uuindi per l ' e r r o r e m a s s i m o c o m p l e s s i v o si h a av __ ì__ v _ f + e ' v nella lig. 5 è r i p o r t a t o ( c u r v a i ) l ' a n d a m e n t o di q u o t o e r r o r e in f u n z i o n e del ( r a p p o r t o £ n e l c a s o : i 1 0 0 v o l t . i r = 1 0 ~ 1 v o l t . 11 m e t o d o a d o p e r a l o n e l l a p r e s e n t e r i c e r c a , o l t r e ad a n n u l l a r e quasi del t u t t o l e p e r d i t e p e r i s o l a m e n t o non p e r f e t t o , dà la p o s s i b i l i t à di m i s u r a r e il p o t e n z i a l e assunto da una sonda r a d i o a t t i v a m e d i a n t e uno s t r u m e n t o e l e t t r o m a g n e t i c o a b a s s a r e s i s t e n z a , l'ale m e t o d o c o n s i s t e nel p o r r e s u l l ' i s o l a n t e c l i c s o s t i e n e la o n d a s u n c o n d u t t o r e m e t a l l i c o di g u a r d i a m . c h e può essere p o r t a l o a p o t e n z i a l i d i v e r s i m e d i a n t e un s i s t e m a p o l c n z i o m c t r i c o /' ( f i g . 3 ) : q u a n d o tale c o n d u t t o r e h a lo slesso p o t e n z i a l e d e l l a sonda, non vi può essere più c o r r e n t e di ' d i s p e r s i o n e v e r s o t e r r a a t t r a v e r s o il sostegno i s o l a n t e posto tra m. ed n e q u i n d i i! s i s t e m a f u n z i o n a c o m e un i s o l a t o r e p e r f e t t o . p e r a c c o r g e r s i clic m ed s a b b i a n o lo l e s s o p o t e n z i a l e , l ' e l e t t r o m e t r o , m o n t a t o in m o d o s i m m e t r i c o . h a c o l l e g a l o l ' a g o ad v e la c u s t o d i a i s o l a l a al c e n t r o d e l l a b a t t e r i a a u s i l i a r i a r ed al c o n d u t t o r e di g u a r d i a a . a n c h e la r e s i s t e n z a di isol a m e n t o d e l l ' e l e t t r o m e t r o , tra l ' a g o e la c u s t o d i a , si p u ò c o n s i d e r a r e c o m e una resistenza rc posta in p a r a l l e l o a q u e l l a ri d e l l ' i s o l a n t e m\ clic sostiene la s o n d a . i n v o l t o m e t r o ( ' c h e p u ò e s s e r e s i a e l c l t r o (-m 1 1 . b k n n d o k f . il ien. lìvr. 1 1 ! ! . 1 1 7 1 ( 1 9 0 9 ) . 56 r e n a t o c i a l d e a s t a t i c o c l i c e l e t t r o m a g n e t i c o , m i s u r a il p o t e n z i a l e r i s p e t t o a t e r r a di m c q u i n d i di s, a l l o r c h é l ' e l e t t r o m e t r o è stato r i p o r t a t o a zero. d a l l o s c h e m a e l e t t r i c o della figura 4 si p u ò d e t e r m i n a r e (piale sia l ' a p p r o s s i m a z i o n e d e l l e m i s u r e , q u a n d o si c o m m e t t a un e r r o r e i| di l e t t u r a i n t o r n o a l l o z e r o d e l l ' e l e t t r o m e t r o . p e r d e t e r m i n a r e la d i f f e r e n z a di p o t e n z i a l e e s i s t e n t e tra m ed n, d o v u t a ad u n a e v e n t u a l e c o r r e n t e di d i p e r s i o n e a t t r a v e r s o l ' i s o l a n t e l / y . o c c o r r e t r o v a r e q u a l e sia il v a l o r e m a s s i m o c h e p u ò a s s u m e r e t a l e c o r r e n t e senza c l i c l ' e l e t t r o m e t r o a c c u s i u n a d i f f e r e n z a di p o t e n z i a l e a p p r e z z a b i l e ai c a p i d e l l ' i s o l a n t e . s i a i] l a d i f f e r e n z a di p o t e n z i a l e m i n i m a a p p r e z z a b i l e ed i la c o r r e n t e di d i s p e r s i o n e , si h a = ; v — v — t ) = i r t ri+r. da q u e s t e si o t t i e n e l ' e r r o r e r e l a t i v o ak __ 11 1 + e v v e 7 ? i rc d o v e si è posto £ = r,(rr\-rc) t e n e n d o i n o l t r e c o n t o d e l l ' e r r o r e o d i l e t t u r a d e l l o s t r u m e n t o c si ott i e n e , p e r l ' e r r o r e t o t a l e a f _ 1 + e v v e j nella fig. 5 è r i p o r t a t o ( c u r v a 2 ) l ' a n d a m e n t o di t a l e e r r o r e i n funz i o n e del r a p p o r t o e e nel caso / itili v o l t , ii = 1 0 ~ 2 v o l i e a — i o 1 v o l t . si v e d e s u b i t o c h e , m e n t r e col p r i m o m e t o d o si p o s s o n o o t t e n e r e d e l l e m i s u r e con u n a a p p r o s s i m a z i o n e d e l l ' i / ó p e r v a l o r i d e l l a rcr i s t e n z a di i s o l a m e n t o c o m p r e s i t r a i n f i n i t o e 1 0 0 v o l t e q u e l l a d e l l a s o n d a , n e l l ' a l t r o l e m i s u r e possono essere eslese a v a l o r i m o l l o più bassi d e l l a r e s i s t e n z a di i s o l a m e n t o e c i o è fino ad u n c e n t e s i m o di q u e l l a d e l l a s o n d a . l e m i s u r e sono s t a t e e s e g u i t e p o n e n d o l e s o n d e i n u n c a m p o e l e t t r i c o u n i f o r m e , g e n e r a l o a r t i f i c i a l m e n t e m e d i a n t e d u e g r a n d i p i a ni m e t a l l i c i , disposti p a r a l l e l a m e n t e tra l o r o a d u n a d i s t a n z a d i 8 8 c m , ira i q u a l i si a p p l i c a v a u n a d i f f e r e n z a di p o t e n z i a l e . l ' e l e t t r o i.e d i m e n s i o n i d e l l e sondi; h a d i o a t t i y e e ( . ' e f f e t t o di i alili v s p a z i a l e 3 6 9 m e t r o a d o p e r a t o e r a a q u a d r a n t i con una s e n s i b i l i t à di 1 0 " ' v o l t / d i v i s i o n e , m e n t r e lo z e r o si p o t e v a a p p r e z z a r e con u n a p r e c i s i o n e del c e n t e s i m o di v o l t ; il p o t e n z i a l e del c o n d u t t o r e di g u a r d i a v e n i v a m i s u r a t o da un v o l t m e t r o e l e t t r o m a g n e t i c o di 2 5 0 . 0 0 0 olmi di r p i s i e n z a i n t e r n a . c o m e è stato già d e t t o , e r a da a t t e n d e r a i c l i c le d i m e n s i o n i d e l l a o n d i m i s u r a t e n o r m a l m e n t e a l l e l i n e e di forza del c a m p o , l'ossero q u e l l e c l i c influissero s u l l ' e f f e t t o d e l l a c a r i c a s p a z i a l e ; p e r q u e s t a r a g i o n e ilo a u m e n t a t o la sezione del v o l u m e soggetto a l l ' a g e n t e ionizz a n t e , p a r a l l e l a a l l e a r m a t u r e del c o n d e n s a t o r e : i r i s u l t a t i -olio stati r i p o r t a t i n e l l a fig. t>. i n q u e s t a figura sono t r a c c i a t e , tra 1 a l t r o , la r e t t a d e l l ' a n d a m e n t o del p o t e n z i a l e n e l l ' i n t e r n o del c o n d e n s a t o r e , in 58 r e n a t o c i a l d e a assenza delia sonda e la c o i v a t e o r i c a di una o n d a l a t e r a l m e n t e ind e f i n i t a ( c u r v a a t r a t t o p i e n o ) . sonda *ingolu. — l a o n d a c o n s i s t e in un d i s c o m e t a l l i c o di 12 min di d i a m e t r o , c o n u n a d e l l e d u e f a c c e p i a n e r i c o p e r t a di p o l o n i o . lo p a z i o o g g e t t o a l l ' a z i o n e i o n i z z a n t e d e l l e p a r t i c e l l e -i p u ò r i t e n e r e clic a b b i a u n a s e z i o n e di c i r c a 2 5 c m " , (piando la s u p e r f i c i e a t t i v a è n o r m a l e a l l e a r m a t u r e del c o n d e n s a t o r e . l ' a n d a m e n t o del p o t e n z i a l e da es-a assunto è r i p o r t a t o n e l l a curva a t r a t t i n i d e l l a fig. 6 . sonda rettilinea. — p e r a u m e n t a r e le d i m e n s i o n i , ilo a d o p e r a t o a n z i t u t t o u n ' a s t a m e t a l l i c a s o t t i l e , lunga 28 c u i , dove p o t e v a n o ess e r e s i s t e m a t e 2 3 sonde uguali alla p r e c e d e n t e , una a f i a n c o dell alt r a . t a l e asta v e n i v a n a t u r a l m e n t e c o l l o c a t a nel c a m p o e l e t t r i c o con il lato più lungo p a r a l l e l o a l l e a r m a t u r e del c o n d e n s a t o r e . l a sua s e z i o n e e r a di c i r c a 125 cm1', c i o è c i n q u e v o l l e q u e l l a s i n g o l a . a l c u n i dei valori o t t e n u t i sono stati r i p o r t a t i con dei c e r c h i e t t i n e l l a fig. 6 : -i v e d e s u b i t o c l i c essi si d i s c o s t a n o di p o c h i s s i m o da q u e l l i trovati c o n la sonda s i n g o l a . sonda (/tuidrata. — e s s a è c o s t i t u i t a da un t e l a i o q u a d r a l o a d u e m a g l i e e n t r o il q u a l e p o t e v a n o e s s e r e s i s t e m a t e o t t o s o n d e . anc h e esso v e n i v a c o l l o c a t o nel c a m p o p a r a l l e l a m e n t e a l l e a r m a t u r e del c o n d e n s a t o r e . l a sua s e z i o n e aveva una a r c a di 125 e n r e le s u p e r ficie a t t i v e e r a n o d i p o s t e in m o d o c l i c n e l l ' i n t e r n o del t e l a i o -i e s t e n d e s s e d a p p e r t u t t o l ' a z i o n e i o n i z z a n t e d e l l e p a r t i c e l l e a . a l c u ni dei v a l o r i o t t e n u t i sono r i p o r t a l i con dei t r i a n g o l i n e l l o -tessa fig. 6 . e i n t e r e s s a n t e c o n f r o n t a r e le m i s u r e e s e g u i t e c o n la sonda r e t t i l i n e a e con q u e l l a q u a d r a t a , p o i c h é a p a r i t à di s e z i o n e esse m o s t r a n o un c o m p o r t a m e n t o n e t t a m e n t e d i v e r s o : l ' e f f e t t o d e l l a c a r i c a s p a z i a l e inf a t t i è p r e s s o c h é n u l l o p e r la p r i m a , m e n t r e p e r la s e c o n d a è già n e t t a m e n t e s e n s i b i l e , p u r non r a g g i u n g e n d o a n c o r a i valori t e o r i c i d e l l a sonda l a t e r a l m e n t e i n d e f i n i t a . grande sonda piana. — in fine ho a u m e n t a l o a n c o r a la s e z i o n e r e a l i z z a n d o u n a sonda di c i r c a 7 2 0 c i n " . a t a l e s c o p o h o a d o p e r a t o un t e l a i o a tre m a g l i e dove p o t e v a n o e e i e s i s t e m a t e 2 7 s o n d e . anc h e per q u e s t o t i p o , le s u p e r f i c i a t t i v e e r a n o d i s p o s t e in m o d o c h e tutto lo s p a z i o i n t e r n o del t e l a i o fosse sede d e l l ' a z i o n e i o n i z z a n t e d e l l e i . e d i m e n s i o n i d e l l e s o n d i ; h a d i o a t t i y e e ( . ' e f f e t t o di i a l i l i v s p a z i a l e 3 7 1 p a r t i c e l l e u. i risultali ollenuli sono slati ri portai i nella fig. <> f o n «lei c e r c h i e l l i p i e n i . con queste dimensioni si h a già un e l ì c i l o di c a r i c a spaziale m o l l o notevole e (piasi prossimo a ipiello teorico d e l l a sonda l a t e r a l m e n t e indefinita. roma — istituto nazionale di geofisica — maggio 1948. r i a s s u n t o nella presente nota sono riportate alcune esperienze per determinare i influenza delle dimensioni di una sonda radioattiva sul! effetto di carica spaziale nella determinazione del potenziale elettrico: si è così verificala una formula teorica che presentava degli scarti rispetto alle misure sperimentali eseguite con sonde comuni a polonio. down-hole geophysical characterization of middle-upper quaternary sequences in the apennine foredeep, mirabello, italy annals of geophysics, 59, 5, 2016, s0543; doi:10.4401/ag-7114 s0543 down-hole geophysical characterization of middle-upper quaternary sequences in the apennine foredeep, mirabello, italy luca minarelli1,*, sara amoroso2, gabriele tarabusi3, marco stefani4, gabriele pulelli5 1 geotema srl, university of ferrara spin-off company, ferrara, italy 2 istituto nazionale di geofisica e vulcanologia, sezione roma 1, l’aquila, italy 3 istituto nazionale di geofisica e vulcanologia, sezione roma 1, bologna, italy 4 università di ferrara, dipartimento di architettura, ferrara, italy 5 progeo srl, forlì, italy abstract the 2012 earthquakes sequence stroke a wide area of the alluvial plain in the emilia-romagna region and triggered a new research interest on the role of the subsurface stratigraphic architecture and petrophysical property distribution in the modulation of the local seismic effects. few direct shear wave velocity vs data were however available below the depth of 50 m. the only available vs measurements were obtained from an anticline area, characterized by a reduced stratigraphic thickness and peculiar sedimentary facies, hardly representative of the majority of the alluvial plain subsurface. the study provides the first vs profile available from middle-upper quaternary successions deposited into a fast subsiding syncline area of the apennine foredeep basin. the p-wave velocity vp and the s-wave velocity vs logs fill in the previous data gap on the geophysical parameters needed for the estimation of the local seismic response. both vp and vs logs were continuously acquired to the depth of 265 m. the log records a velocity increase with depth, punctuated by sharp increases at some stratigraphic discordance surfaces. the value of 800 m/s that characterizes the “seismic bedrock”, as defined by the italian building code [ntc 2008] was never reached at any depth. the investigated succession records a depositional evolution from deltaic-marine to alluvial plain conditions, punctuated by six glacio-eustatic depositional cycles, developed in middle-upper quaternary times. the stratigraphic units described in the syncline log were correlated at a regional scale, with the thinner anticline succession of mirandola. correlatable units deposited into syncline and anticline areas reveal similar shear wave velocity values, supporting the regional extrapolation of the measured values. 1. introduction in may 2012, several medium-large earthquakes impacted on a wide portion of the alluvial plain of northern italy (figure 1), belonging to the lombardy, veneto and emilia-romagna regions. the strongest earthquake (ml 5.9) occurred close to finale emilia, while the second largest one (ml 5.8) took place near mirandola [pondrelli et al. 2012]. the causative faults belong to the ferrara arc thrust system [bonini et al. 2014, diss working group 2015], forming the external portion of the apennines chain (figure 1). the earthquake effects were modulated by the changing seismic amplification, associated with the largely variable pliopleistocene successions [geomol team 2015]. the thickness of the post-miocene units varies from several kilometres in syncline areas, to 200 m, in anticline zones, near mirandola and casaglia (figure 1). large lateral variations in stratigraphic thickness are visible also within the upper quaternary units [geomol team 2015]. the seismic acceleration induced pervasive sand liquefaction phenomena within holocene fluvial bodies [caputo and papathanasiou 2012, papathanassiou et al. 2012, emergeo working group 2013]. the 2012 earthquakes triggered a widespread interest on the seismic hazard assessment in the alluvial plain area of the emilia-romagna region [di manna et al. 2012, malagnini et al. 2012, meletti et al. 2012, milana et al. 2014, papathanassiou et al. 2015, tonni et al. 2015]. the available data on the subsurface were however scanty. the regional administration therefore collected many previously performed shallow logs, such as cone penetration tests, stratigraphic corings and water well stratigraphies, available from the shallow subsurface, and performed several new investigations, reaching an average depth of 30-50 m [martelli and romani 2013]. only few data were however available from deeper levels. a measurement of the shear wave velocarticle history received july 14, 2016; accepted october 6, 2016. subject classification: shear wave velocity, deep geophysical test, 2012 emilia earthquake, emilia-romagna stratigrafic units, active tectonics. ity vs was not normally available at depths exceeding 30-50 m. a crucial parameter for the estimation of the local amplification was thus widely lacking. direct vs measurements exceeding the depth of 50 m were acquired only at two sites, near medolla and mirandola (figure 1), down to the depths of 101 m and 127 m respectively [martelli and romani 2013, paolucci et al. 2015, garofalo et al. 2016]. these two sites are less than 4 km apart and belong to an anticline area [tarabusi and caputo 2016]. the logs therefore show comparatively thin stratigraphic successions, with sedimentary facies quite different from those accumulated into the vast majority of the emilia-romagna region (figure 2). the extrapolation of the aforementioned geophysical data to the thick successions accumulated into the syncline areas was therefore uncertain. direct vs measurements from syncline regions were clearly needed to improve the accuracy of the local seismic response studies. the present research, for the first time, provides a vs distribution acquired from a deep down-hole test carried out in a syncline area. the investigation was performed in the middle-upper quaternary succession accumulated into the fast subsiding syncline zone southwest of ferrara, between the villages of mirabello and san carlo (figure 1). 2. structural and stratigraphic framework the study area belongs to the external part of the apennines chain, buried beneath the recent alluvial plain. this portion of the chain consists of blind thrust and fold structures [pieri and groppi 1981, boccaletti et al. 2004], generated through neogene and quaternary times [ghielmi et al. 2013, vannoli et al. 2015]. the fast regional subsidence of the southern portion of the foredeep basin is superposed to the ongoing fault-fold deformation [carminati et al. 2010]. the syncline areas thus record much larger subsidence values than the anticline ones. the lateral variation of the subsidence rate strongly influenced the stratigraphic thickness (figure 2), the sedimentary facies architecture and the petrophysical parameters distribution [bigi et al. 1992] and thus the seismic wave propagation properties of the plio-pleistocene units. the quaternary of the region records the impact of active tectonic deformation and of massive climatic and eustatic fluctuations. the whole of these factors induced key stratigraphic minarelli et al. 2 figure 1. altimetry map of the study alluvial plain area with the position of the discussed logs. some of the major subsurface overthrusts are schematically illustrated. the line a-a’ depicts the location of the geological profile of figure 2. 3 surfaces that support the subdivision of the sedimentary successions into allostratigraphy units, organized in different hierarchical levels [regione emilia-romagna et al. 1998]. two main synthems are separated by a discordance surface, associated with a structural reorganization of the foredeep basin area. the synthems are further subdivided into a number of subsynthems (figure 2), recording the large glacio-eustatic and climatic fluctuations. the transgressive surfaces induced by the eustatic rises associated to deglaciation were chosen as the subsynthem basis. the base of each subsynthem unit corresponds to the deactivation of the synglacial coarse-grained sedimentation, accumulated into middle alluvial plain environments. syn-transgressive units are capped by organic-rich marsh mud, deposited into lower alluvial plain settings, or, in the deeper portions of the successions, by marine-deltaic sediments. the regressive trend of the alluvial plain successions culminates into a new interval of synglacial lowstand sands. the ongoing transgressive-regressive cycle has been ascribed to the ravenna subsynthem (aes8), the base of which correlates with the stage 1 of the global marine oxygen isotope curve [martinson et al. 1987], corresponding to the base of the holocene (about 14 ka). the lower boundary of the underlying villa verucchio subsynthem (aes7) is assigned to the marine oxygen isotope stage m.o.i.s. 5e, (about 120 ka) whereas the base of the previous bazzano subsynthem (aes6) correlates with m.o.i.s. 7 (about 240 ka). moving downwards, two further depositional cycles are ascribed to the undivided lower portion (aesi) of the upper emiliano-romagnolo synthem and lack a formal stratigraphic terminology. the lower emiliano-romagnolo synthem shows several transgressive-regressive pulses, recording a globally shallowing evolution. 3. data acquisition methodology the mirabello drill (latitude: 44.8100°; longitude: 11.4264° wgs84) crossed a middle-upper quaternary succession, reaching a depth of 372 m. the deep borehole was used to perform a down-hole test, hereafter named dh deep. both compressional (vp) and shear (vs) wave velocities were measured, down to the depth of 265 m. figure 3 shows a schematic plan layout of the seismic sources, surface geophones and the core location. a grout mixture was pumped into the core and a pvc pipe, with an internal diameter of 162.8 mm, was installed, down to 265 m. an additional steel casing, with an internal diameter of 263 mm, was emplaced in the upper 18 m. three different seismic sources were used (figure 4): an impulsive source of 250 kg, hitting vertically a steel square from a height of 3 m, to generate compressional (p) waves; two symmetric pendulum hammers, hitting horizontally a steel rectangular base pressed vertically against the soil, to produce horizontally polarized shear waves (sh). the seismic sources triggered a data recording system measuring the travel time of the wave train from sources to receivers. the deep dh receivers consisted of three uniaxial, one vertical and two horizontal, 10 hz geophones. seven 14 hz geophones, three vertical and four horizontal, were placed at the surface, to support deep down-hole test in syncline area figure 2. geological cross section interpreting the first 400 m of subsurface between cento and occhiobello. the impact of the active tectonic deformation and the associated differential sediment compaction on the stratigraphic geometry is clearly visible. vertical exaggeration × 25 (modified after martelli and romani [2013], paolucci et al. [2015]). figure 3. schematic plan layout of the down hole seismic investigation. the rephasing correction to the recorded dh seismograms. the system recorded 2000 samples, at a sampling time interval of 0.5 m/s. the p and s waves arrival time was interpreted according to astm d7400-14 [2014]. the dh deep data were acquired from the depth of 18 m down to 265 m, because of the steel casing of the uppermost part of core. an estimation of the vs in the uppermost 18 m was derived from the closest down-hole sounding, the dh m1 (figure 1). the latter sounding was performed in 2012 by the regione emilia-romagna, about 2800 m to the north-east of the dh deep site. the two adjacent logs are in good agreement in the shared 18-30 m stratigraphic interval and therefore a reasonable level of confidence can be estimated for the interpolation of the upper 18 m of the study log. figure 5 provides the seismograms acquired at the 265 m depth, produced by the vertical hammer blow (black line “p-wave shot”) and by the horizontal hammer blows striking the anvil on the two opposite sides (red line “s-right wave shot” and blue line “s-left wave shot”). the acquired stratigraphic and geophysical data are illustrated by figure 6. 4. stratigraphy and seismic velocities the stratigraphic succession logged in mirabello was correlated with the vp and vs profiles plotted over depth at the same site (figure 6a). the velocity logs acquired in mirandola [martelli and romani 2013, garofalo et al. 2016] were associated with our stratigraphic interpretation (figure 6b) of the published lithology log [regione emilia-romagna et al. 1998, martelli and romani 2013], based on our direct examination of the sediment cores and the knowledge of the regional stratigraphy framewok. the correlation of successions drilled at mirabello and mirandola were based on published information, mainly deriving from the national geological mapping project (carg) [cibin and segadelli 2009] and from regional geological profiles [martelli and romani 2013]. the logs provided also some dating evidence, particularly through 14c analysis from the younger intervals. several continuously cored logs were studied in proximity of the mirabello drilling site, within the framework of the geological mapping project [cibin and segadelli 2009]. particularly useful was the comparative analysis of the log 203/s9, providing an allostratrigraphic framework of the sedimentary succession to the depth of 220 m and reaching the lower emilianoromagnolo synthem. the succession crossed by the core 203-s9 shows thicker correlative stratigraphic units than that revealed by the mirabello drilling (203/s9 unit bases: aes8 21 m, aes7 68 m, aes6 98 m, aesi 187 m). from the core 203/s9, significant palaeo-environmental and micropalaeontological data were derived [fiorini and colalongo 2009]. the stratigraphic interpretation and dating of the upper portion of the study drilling was improved by the correlation with other two adjacent logs, the 203-s6 and the 203-s13 (figure 1) reaching the depth of 34 and 68 m respectively. the correlation with the section cored at mirandola has been based on our direct sedimentological and stratigraphical examination of the continuous cores, on the published lithological logs [martelli and romani 2013], and on the correlation with published stratigraphic profiles [pao-lucci et al. 2015]. the regional correlation demonstrates that the mirabello drilling crossed the entire upper emiliano-romagnolo synthem and lower emiliano romagnolo synthem and a large portion of the marine quaternary. within the succession, each subsynthem has an average thickness of about 35 m and shows a similar internal organization. the drilled succession records a globally shalminarelli et al. 4 figure 4. the three sources of seismic waves energized at the surface (see text). figure 5. seismograms acquired at 265 m from the dh deep test (figure 6a). 5 lowing evolution, from marine environments to alluvial plain systems, spanning over a time period of roughly 600,000 years. the stratigraphic organization correlates with the measured velocity values. the vp log (figure 6a) confirms the presence of fully water saturated deposits between 18 m and 265 m, providing an average value of 1500-2000 m/s through the investigated depth. the vs increases gradually with depth, from 116-200 m/s, in the ravenna subsynthem, to 310-525 m/s, in the lower deep down-hole test in syncline area figure 6. stratigraphic column and seismic velocity logs from the mirabello (a) and mirandola (b) sites. the map shows the location of the two log sites and a schematic representation of the main syncline and anticline axis in the region. emiliano-romagnolo synthem (table 1). in the mirabello succession, the ravenna subsynthem aes8 frames the first 22 m of subsoil and consists of outcropping reno river sands, soft organic-rich marsh muds, with very low s-wave velocities, and older pedogenized continental silts, showing comparatively higher vs values. fine-grained sediments dominate the villa verrucchio subsynthem aes7 (23-58 m), associated with some fluvial sand intercalations, deposited by apennine rivers. the aes7 unit top is associated with a sharp increase of the s-wave velocities, while at the lower portion of the unit a further vs increase is recorded, probably associated to pedogenized levels enriched in carbonate concretions. thick bodies of fluvial sands deposited by the po river [cibin and segadelli 2009] dominate the bazzano subsynthem aes6 (59-85 m), but finer-grained sediments are nevertheless well developed in the lower portion of the unit. the underlying aesi unit (86-162 m) consists of two transgressive-regressive cycles, separated, at about 127 m, by a transgressive surface. foraminifera, palecypod bioclasts and gastropods (turritella) were sampled from the marine portions of the unit. the lower part of the upper cycle (105-127 m) is associated to a further increase of the s-wave velocity. at about 161 m, the study well crossed the top of the aei synthem. the synthem consists of thick sandy or argillaceous bodies, deposited into marine, deltaic and alluvial plain environments. the fine-grained unit below 210 m correlates with a marine interval preserving infralittoral zone foraminifera [fiorini and colalongo 2009]. the unconformity marking the unit top correlates with a sharp increase of the vs values, almost reaching 500 m/s. few further peaks of vs are associated to underlying thick layers of sand (160-165 m and 255-260 m). the aei unit contains biogenic methane that could have affected the lowering of both vp and vs values, at depths between 235 and 260 m. the correlative anticline succession of mirandola is much thinner than the mirabello one. from the previously described correlation, we can confidently assume that aes unit is about 100 m thick, is enriched in fluvial sands, and lacks any marine influence. the succession nevertheless records s-wave velocity trends and values [garofalo et al. 2016] similar to the mirabello ones (table 1, figure 6a). the correlation between the aei unit in mirabello and the pebbly sands drilled at mirandola (figure 6b) below 100 m, showing clasts of palaeozoic acidic volcanites with porphyric structures, is uncertain. the lowermost portion of the mirandola succession, underlying a sharp unconformity surface, was ascribed to the lower pliocene [paolucci et al. 2015, tarabusi and caputo 2016]. this unit consists of lithified turbiditic sands, clays and marls, deposited into deep marine environments, and shows the highest vs values, above 600 m/s [garofalo et al. 2016]. these are the highest values recorded from the study successions, but they are still lower than the values officially assumed for the “seismic bedrock” [en 1998-5 2004, ntc 2008]. 5. discussions and conclusions the deep down-hole test performed in mirabello, reaching a depth of more than 250 m below surface, has provided a unique deep seismic log from a tectonic active syncline area of the apennine foredeep basin. no direct vs measurements deeper than 50 m were previously available in the syncline areas affected by the 2012 emilia earthquakes. the new log, integrated by the correlation with the previous logs available in the region, supports a well constrained seismo-stratigraphic model of the interval spanning from the lower emiliano-romagnolo synthem to the recent outcropping sediments (figure 6). a good correlation is visible between the shear wave velocity distribution and the stratigraphic organization of the depositional succession. stratigraphic discordance surfaces are related to sharp increases in the vs values, particularly at the boundary between the lower and upper emiliano-romagnolo synthems. the logs from the syncline area of mirabello was correlated with the thinner succession of mirandola, deposited into an anticline area. mirandola was the only site in the emilia-romagna region where direct vs measurements were previously available from a deep level, and where down-hole and cross hole tests were performed and validated by different international research groups [garofalo et al. 2016]. correlatable units in the mirabello and mirandola boreholes show very similar s-wave velocity trends and values (table 1, figure 6a,b). the good agreement confirms the reliability of the deep down-hole interpretation at the mirabello site. the small variations detectable between the two logs can be explained considering both the variation in sedimentary facies and lithologies and the different effective stresses of the mirabello and mirandola subsoils. the stronger compaction affecting the minarelli et al. 6 table 1. vs values (m/s) measured within correlative stratigraphic units in the syncline site of mirabello (figure 6a) and in the anticline succession of mirandola (figure 6b). for each stratigraphic interval, the lowest, highest, and average values are indicated. unit mirabello mirandola aes8 116 – 200 (161) 158 – 213 (174) aes7 207 – 326 (281) 218 – 308 (251) aes6 310 – 356 (334) 284 – 339 (312) aesi 355 – 464 (394) 284 – 460 (375) aei 310 – 525 (434) 430 – 449 (439) 7 deeper argillaceous intervals at mirabello influences the development of vs values similar to those recorded in correlative coarser grained intervals preserved at shallower level in the mirandola succession. the average seismic values recorded in mirabello can be extrapolated with some confidence to the widespread syncline successions, through large portions of the alluvial plain. the new dataset here provided can support the definition of the input parameters for the seismic response analyses in similar emilia-romagna stratigraphic context, characterized by the absence of deep vs profile and the presence of a deep seismic bedrock. neither the mirabello log nor the mirandola one reached a vs value of 800 m/s, which officially defines the “seismic bedrock” into the european building code [en 1998-5 2004] and the italian building code [ntc 2008]. some extra investigation, based on indirect surface approaches, as large 2d passive seismic arrays [e.g. di giulio et al. 2016], can be useful to obtain deeper velocity profiles, which, even if less accurate [garofalo et al. 2016], will integrate the provided direct seismic velocity measures, supporting a better constrained and more reliable ground response analysis. acknowledgements. special thanks to the protezione civile, to the regione emilia-romagna, and to the mirabello municipality, for funding the seismic microzonation of mirabello. we thank also luca martelli and paolo severi (emilia-romagna region) for providing data from the emilia-romagna geological database, and ferruccio laurenti for providing the access to the site of the mirabello deep borehole. references astm d7400-14 (2014). standard test methods for downhole seismic testing, astm international. bigi, g., g. bonardini, r. catalano, d. cosentino, f. lentini, m. parlotto, r. 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sismica emiliana del 2012 su un tratto di argine del canale diversivo di burana (fe), rivista italiana di geotecnica, pàtron editore, bologna, anno xlix, n. 2, 28-38; issn 0557-1405. vannoli, p., p. burrato and g. valensise (2015). the seismotectonics of the po plain (northern italy): tectonic diversity in a blind faulting domain, pure and applied geophysics, 172, 1105-1142; doi:10.1007/s0 0024-014-0873-0. *corresponding author: luca minarelli, geotema srl, university of ferrara spin-off company, ferrara, italy; email: minarelli@geotema.it. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. minarelli et al. 8 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice de-1 versus cluster spectral density observations: the effect on ion outflow above the polar cap annals of geophysics, 58, 5, 2015, g0547; doi:10.4401/ag-6800 g0547 de-1 versus cluster spectral density observations: the effect on ion outflow above the polar cap hamza a. abudayyeh*, imad a. barghouthi, ghadeer al-sarsour, husain alsamamra al-quds university, department of physics, jerusalem, palestine abstract wave-particle interaction is a very important mechanism in describing the outflow of ions at high latitudes and high altitudes. quasi-linear perpendicular velocity diffusion coefficients are used to describe the effect of wave-particle interactions, therefore it is essential to determine the correct diffusion coefficients that must be used to model the outflow of ions. in this study a monte carlo method is used to assess the role of different diffusion coefficients for o+ and h+ ions at high altitudes above the polar cap. two different sets of diffusion coefficients obtained from barghouthi [1997]; barghouthi et al. [1998] and nilsson et al. [2013] are used. barghouthi [1997]; barghouthi et al. [1998] used spectral density measurements from dynamic explorer 1 spacecraft (de-1) observations to calculate the diffusion coefficients, while nilsson et al. [2013] used spectral density measurements from the cluster spacecraft to obtain the diffusion coefficients. it was found that diffusion coefficients from barghouthi [1997]; barghouthi et al. [1998] in the cusp (aurora) and central polar cap (polar wind) respectively, describe well the observations of ion outflow at altitudes lower than 5 re , but yield unreasonably high parallel velocities and temperatures at higher altitudes. also diffusion coefficients from cluster spectral density measurements produce reasonable results for high altitudes and unreasonably low parallel velocities and temperatures for the low altitude region. therefore it is suggested that a combination of these diffusion coefficients is used where the diffusion coefficients given by barghouthi [1997]; barghouthi et al. [1998] are used at low altitudes and the diffusion coefficients obtained from cluster measurements are used at high altitudes. 1. introduction the outflow of plasma from ionospheric origin along open geomagnetic field lines above the polar cap is subject to intense investigation. it is well observed that ions have high perpendicular temperatures at high altitudes despite the presence of perpendicular adiabatic cooling [huddleston et al. 2000, bouhram et al. 2004, nilsson et al. 2004, nilsson et al. 2006]. this is an indication of a perpendicular heating mechanism that acts at high altitudes. arvelius et al. [2005] showed using the cluster cis/codif instrument that at 8-12 re geocentric distance the distribution of o+ outflows cannot be explained by velocity filter dispersion only. this is a clear suggestion of further additional energization processes for outflowing o+ ions at high altitudes and high latitudes in the dayside polar region. observations indicated a significant level of electromagnetic turbulence at high altitudes above the polar cap [gurnett et al. 1984, gurnett and inan 1988, nilsson et al. 2012, waara et al. 2012] which suggested an interaction known as wave-particle interaction (wpi)[chang and coppi 1981, chang et al. 1986, retterer et al. 1987, crew et al. 1990, retterer et al. 1994, barghouthi 1997, barghouthi et al. 1998, bouhram et al. 2004, waara et al. 2010, slapak et al. 2011, waara et al. 2011, waara et al. 2012]. field aligned acceleration mechanisms such as field aligned electric fields [lundin et al. 1995, maggiolo et al. 2006] and centrifugal acceleration [cladis 1986, horwitz et al. 1994, demars et al. 1996, nilsson et al. 2008, nilsson et al. 2010] were also investigated to explain the energization up to kiloelectronvolt energies of o+ ions above the polar cap. the properties of an ion species may be determined from its distribution function fs(r, v, t) where the development of the distribution function is governed by the boltzmann equation [schunk 1977]: where vs, ms are the velocity and mass of the s th species respectively; ∇, ∇vs are the gradient in space and velocity coordinates respectively; f is the total external force acting on the ion species (for example, the ambipolar electric force, mirror force, and gravity); and dfs— dt represents the rate of change of the distribution function in a region of phase space due to collisions and wave-particle interactions. article history received may 25, 2015; accepted july 23, 2015. subject classification: wave-particle interaction, ion outflow, polar wind, central polar cap, cusp, polar cap, diffusion coefficients, cluster, de-1. (1) at altitudes considered in this study the plasma is tenuous enough to neglect collisions. in such case the right hand side of equation (1) reduces to [retterer et al. 1987]: where v⊥s is the component of the velocity of the s th species that is perpendicular to the geomagnetic field and d⊥s is the quasi-linear velocity diffusion coefficient for the sth species. the perpendicular velocity diffusion coefficient may be written as [retterer et al. 1987]: where qs is the charge of the s th ion species and sl is the electric field spectral density at the ion gyrofrequency that is efficient in heating the ions. sl is related to the total spectral density at the ion gyrofrequency (s) by sl=hs, where h is the fraction of spectral density at the gyrofrequency of the ion that is efficient in heating the plasma. at lower altitudes (few re), values for h ranging from a few percent to 10% were used in several studies [bouhram et al. 2004, barghouthi and atout 2006]. nilsson et al. [2013] found that for geocentric distances between 8 and 10 re in the central polar cap (cpc) and cusp, good agreement was attained for a value of h = 1–2 . the relation between perpendicular temperature and wave activity is determined by two competing effects: the sporadic nature of the waves and how dominating the most recent heating is. a good correlation could be established if the recent heating is dominating, and higher sporadic wave activity can be tolerated. if ion heating is not sporadic then recent heating does not necessarily need to dominate in order to obtain a good correlation between ion temperature and wave intensity. therefore sporadic heating can explain a region of high temperature and relatively low wave activity. waara et al. [2012] studied this relation and found that there is indeed a correlation except at the highest perpendicular temperatures which indicates that the strongest heating is most likely sporadic. barghouthi et al. [1998] used data from the plasma wave instrument on board dynamics explorer 1 (de-1) to estimate the perpendicular diffusion coefficients in the polar wind for both o+ and h+ ions. four sets of data were chosen that cover altitude ranges from 1.5 to 4.5 re at polar latitudes and sample both polar caps. using these spectral densities, and assuming a value of h = 1–8 , the perpendicular diffusion coefficients were calculated using equation (3) and were found to be: in the auroral region barghouthi [1997] assumed that the altitude dependence of the spectral density to resemble that in the polar wind [barghouthi et al. 1998]. by using the altitude dependence from barghouthi et al. [1998] and the measured spectral density at auroral latitudes at 2.0 re from retterer et al. [1987], the diffusion coefficients for o+ and h+ ion species were calculated. the diffusion coefficient for the ions in the auroral region may be given by: waara et al. [2011] used data from three instruments on board the cluster spacecraft namely: efw (electric field and wave experiment), fgm (fluxgate magnetometer) and cis (cluster ion spectrometry experiment). the data set consisted of efw and fgm wave data above 5 re when outflowing o + ions were seen in the energy spectrograms of cis. using this data the average spectral density above the polar cap was calculated and estimates for the diffusion coefficients were obtained. nilsson et al. [2012] further subdivided this data set into three regions with respect to their latitude: the central polar cap (cpc), nightside polar cap, and cusp. using these values for the electric field wave activity near the o+ gyrofrequency, nilsson et al. [2013] used equation (3) to calculate the diffusion coefficients in each region. in this study we will use spectral density measurements at the h+ gyrofrequency from nilsson et al. [2012] to calculate the diffusion coefficients for h+ ions based on cluster observations. then we will use a onedimensional kinetic model that includes the effects of ambipolar electric field, wave-particle interaction, gravitational and mirror force to investigate the effect of varying the diffusion coefficients from those based on de-1 spectral density measurements to those based on cluster measurments for altitudes up to 15 re. two flux tubes called central polar cap (cpc) and cusp were taken to be along geomagnetic field lines calculated by using tsyganenko t96 model tsyganenko [1995, 1996]. in such case we will be assuming that the diffusion coefficients from de-1 measurements are valid for all altitudes including those higher than 5 re and the diffusion coefficients from cluster measurements are valid for all altitudes including those less than 5 re which is just an extrapolation for each data set. in performing such a abudayyeh et al. 2 (2) (3) (4) (5) 3 calculation we aim to provide modelers using more sophisticated 3 dimensional models with proper diffusion coefficients for their studies. this paper will be organized as follows: section 2 includes a brief description of the model used. in section 3 we will present our results. discussions and conclusions will be presented in section 4. 2. method this study is an extension to barghouthi [2008] in the aurora and barghouthi et al. [2011] in the polar cap. a monte carlo method was used to study the outflow of o+ and h+ ions for geocentric distances from 1.7 to 14.7 re in the cpc and from 1.2 to 15.2 re in the cusp. the flight trajectories that we call cpc and cusp are defined in nilsson et al. [2012]. aside from the modification of the flight trajectories three other modifications on the barghouthi model were conducted: 1. the radial magnetic field was replaced by a magnetic field calculated by the t96 model [tsyganenko 1995, 1996]. 2. the perpendicular velocity diffusion coefficients were varied such that the set based on de-1 spectral density measurements were first used and then the set based on cluster spectral density measurement were used. 3. the boundary conditions were taken to be as follows: the o+ velocities at the exobase were taken to be 2.0 and 0.5 km/s for the cpc and cusp respectively [nilsson et al. 2013]. for h+ ions the initial velocities were taken to be 18 [nilsson et al. 2013] and 16 km/s [barghouthi 1997] for the cpc and cusp respectively. the densities at the lower altitude boundary were taken to be 10 and 10,000 cm-3 for o+ ions for the cpc and cusp respectively [nilsson et al. 2013] and 34 [nilsson et al. 2013] and 200 cm-3 [barghouthi 1997] for h+ ions in the cpc and cusp respectively. the electron temperature in addition to the temperature of both ions were assumed to be 3000 k at the exobase [barghouthi et al. 1998]. the ions are injected at the lower altitude boundary (r0= 1.7 re for the cpc and r0= 1.2 re for the cusp) with random velocities consistent with a drifting maxwellian distribution. the ion motion is tracked for a small time interval dt as it moves under the effect of the ambipolar electric field and the gravitational and mirror forces. the influence of wave-particle interaction (wpi) was simulated by changing the perpendicular velocity of the ion by a random increment dv⊥ such that dv⊥= 4d⊥dt. the flux tube is divided into bins (130 bins for the cpc and 140 bins for the cusp) which act as recorders. when an ion enters one of these bins its parallel and perpendicular velocities are registered for later use. this procedure is repeated for a large number of ions (105-107 ions) until they exit either from the bottom or the top of the flux tube. finally the registered data are used to calculate the moments and distribution functions of both ions. however, since the ambipolar electric field depends on the number densities of the ions which are outputs of the simulation, an iterative approach is used until convergence is achieved. for a more detailed description on how the moments and distribution functions are calculated see barghouthi et al. [2003]. 3. results the spectral density at the h+ gyrofrequency was obtained from nilsson et al. [2012] and is shown in figure 1 for the central polar cap and cusp. using a power law fit and equation (3) the diffusion coefficients for h+ ions were found to be d⊥(h +) =h40(r/re) 9.77 cm2s-3 for the central polar cap and d⊥(h +) =h2.02 × 106 (r/re) 5.61 cm2s-3 for the cusp. therefore, using the diffusion coefficients for o+ ions obtained from nilsson et al. [2013], the diffusion coefficients based on spectral densities measured on board cluster spacecraft are given by for the central polar cap and de-1 vs. cluster observations figure 1. h+ observed electric field spectral densities [(mv/m)2/hz] at different geocentric distances [re] are shown by circles for the central polar cap (black) and cusp (magneta). power law fits to the data are shown with solid lines. (6) (7) for the cusp with a value of h = 1–2 for both regions. in this study we will present our simulation results for two sets of perpendicular velocity diffusion coefficients namely; 1. diffusion coefficients by barghouthi et al. [1998] in the polar wind and barghouthi [1997] in the aurora given by equations (4) and (5) respectively and the results due to these diffusion coefficients will hereinafter be denoted by barp and bara respectively. 2. diffusion coefficients by nilsson et al. [2013] in the cpc and cusp given by equations (6) and (7) respectively and the results due to these diffusion coefficients will hereinafter be denoted by nilp and nila respectively. figure 2 represents the perpendicular velocity diffusion coefficients for o+ (top panel) and h+ (bottom panel) ion species. the difference between the two sets is more significant for o+ and at higher altitudes. for o+ ions the set calculated from de-1 observations is nearly 5 orders of magnitude larger than those calculated by cluster observations at 5 re and nearly 8 orders of magnitude higher at 15 re. for h + ions the difference is more significant in the sunward part of the polar cap at higher altitudes as the diffusion coefficients given by equation (5) (bara) are higher than those given by equation (7) (nila) by 4 orders of magnitude at 15 re. figures 3 and 4 show the moments of o+ and h+ ion species respectively. the density of o+ ions at lower altitudes is nearly identical for both sets. at higher altitudes however, the density resulting from diffusion coefficients calculated from de-1 spectral density measurements are smaller than those from cluster observations. this is more evident in the barp results since the initial density is three orders of magnitude less. one significant observation is that the profiles for nilp and nila are identical even though the diffusion coefficients and initial densities are different. the profiles of h+ density are only slightly affected by the change of diffusion coefficients. the alternation of diffusion coefficients has a profound effect on the parallel bulk velocity of o+ ions as it increases the bulk velocity by three orders of magnitude at the highest altitudes (from going from nila to bara for example). therefore unrealistically high velocities are experienced when using diffusion coefficients calculated by de-1 observations at high altitudes. on the other hand, at lower altitudes nilp and nila diffusion coefficients produce relatively low bulk velocities at altitudes lower than 5 re, where the parallel bulk velocity of o+ ions is only about 10 km/s. for the same set of diffusion coefficients, it may be noticed that the bulk velocity is higher in the more sunward part of the polar cap (bara and nila). h+ bulk velocity is less affected by interchanging the diffusion coefficients. nonetheless, at high altitudes high bulk velocities are experienced in barp and bara profiles. the profiles for perpendicular and parallel temperatures also experience unusually high values at high altitudes for calculations based on de-1 observations. for example, in the central polar cap region (polar wind) the perpendicular and parallel temperatures for o+ ions are approximately 2 × 1010 and 4 × 108 k at 10 re for barp profiles. while for nilp profiles at the same altitude the perpendicular and parallel temperatures are only about 2 × 106 and 9 × 104 k at the same altitude. the same holds for h+ ions although with a much lesser extent. it may also be noted that the parallel bulk velocity, perpendicular and parallel temperature profiles for h+ ions are decreasing or nearly constant up to altitudes of approximately 5 re and increasing above that. figure 5 shows the contours for o+ and h+ distribution functions. as may be expected from wpi, conics in phase space are formed. in general it may be noticed that conics form at lower altitudes in bara and barp than in nila and nilp. also for the same set of diffusion coefficients conics form at more lower altitudes for o+ abudayyeh et al. 4 figure 2. perpendicular velocity diffusion coefficient profiles for o+ (top panel) and h+ (bottom panel) ion species. barp refers to diffusion coefficients calculated by barghouthi et al. [1998] (equation 4). bara refers to the diffusion coefficients calculated by barghouthi [1997] (equation 5). nilp and nila refer to diffusion coefficients in the central polar cap and cusp respectively calculated by nilsson et al. [2013] (equations 6 and 7 respectively). 5 de-1 vs. cluster observations figure 3. o+ (a) density (b) parallel bulk velocity (c) perpendicular temperature and (d) parallel temperature profiles. figure 4. h+ (a) density (b) parallel bulk velocity (c) perpendicular temperature and (d) parallel temperature profiles. ions in comparison with h+ ions. finally the latitude dependence is clear, since conics form at more lower altitudes in the most sunward regions of the polar cap. 4. discussion and conclusion as we noted in the previous section using diffusion coefficients calculated from de-1 observations for high altitudes produces unreasonably high parallel velocities and parallel and perpendicular temperatures for o+ ions species. the effect is less evident for h+ ion outflows, but nonetheless it is notable for altitudes higher than 10 re. it is also evident that o+ diffusion coefficients calculated based on cluster spacecraft spectral density measurements are unsuitable for altitudes lower than 5 re. to support our claim, table 1 displays observational and simulation results for the cpc and cusp for o+ ions. it is evident that using diffusion coefficients calculated from de-1 spectral density observations yields results (barp,bara) that are near the observations at low altitudes, but that are not at all compatible with observations for higher altitudes. the opposite may be said of results from using diffusion coefficients calculated based on spectral density measurements from cluster data (nilp,nila). these results agree with high altitude observations but are too low to explain low altitude behavior. this also suggests that the early history of ions along their trajectories is irrelevant for the strong heating region and only the most recent heating affects the moments of the ions. in other words most recent heating dominates. therefore it may be anticipated that for the regions considered in this study, where the heating is mostly intense, ions with different initial conditions will have nearly the same velocities and temperatures at higher altitudes. due to the findings of this study we conclude that using one set of diffusion coefficients is not suitable to abudayyeh et al. 6 figure 5. o+ (first four columns) and h+ (columns 5-8) distribution function contours at 1.7, 3.7, 5.7, 7.7, 9.7, 12.7, and 14.7 re, where c̃ = (v − u)/√2kt/m. the contours decrease successively by a factor of e0.5 from the maximum. 7 explain the outflow of o+ and h+ ions for all altitudes (at least those considered in this study). hence some combination of these diffusion coefficients should be used where the diffusion coefficients given by equations (4) and (5) should be used at low altitudes for the cpc and cusp respectively, and diffusion coefficients given by equations (6) and (7) should be used for high altitudes in the cpc and cusp respectively. we cannot put a definite altitude at which the diffusion coefficients should be altered. it is important to note that we cannot suggest a given altitude to change the diffusion coefficient due to the following reasons: 1. observations show that in the mid altitude region (5-7 re) of the cpc perpendicular temperature falls with altitude which suggests that this region experiences low or no wpi [nilsson et al. 2012, 2013]. this would lead us to introduce a third region where either the spectral density is low (which is unlikely) or the portion of wave activity that is efficient in heating the ions is low. 2. it is reasonable to assume that the change in spectral density with altitude is continuous and doesn’t experience a sudden jump as would be the case if we changed the diffusion coefficients suddenly at a given altitude. still, as a first study such an alteration may be assumed. 3. spectral density data from de-1 only cover altitudes up to 4.5 re while spectral density data from cluster only cover altitudes above 7 re, which leaves the mid altitude region with no spectral density data to deduce the correct diffusion coefficients. however, waara et al. [2011]; nilsson et al. [2012, 2013] only used cluster orbits with apogeum towards the dayside polar cap which limited the data to altitudes above 7 re. cluster can however, provide data for lower altitudes which may give us insight into the mid altitude region. it must also be noted that in each set of diffusion coefficients there are relatively high uncertainties which may correspond to different geophysical conditions. this subject in addition to determining the effect of various combinations of diffusion coefficients and the effect of varying boundary conditions is the objective of an ongoing study. in this study we have neglected the effect of centrifugal acceleration which is a very important acceleration mechanism especially at high altitudes. the effect of centrifugal acceleration on the outflow of ions above the polar cap is the subject of abudayyeh et al. [2015, in press]. we have shown that centrifugal acceleration increases the parallel bulk velocity of the ions and decreases the parallel and perpendicular temperature. in this study we have used a steady state 1-d model, which is usually referred to as the barghouthi model. the applicability of the barghouthi model has been discussed in several previous studies such as barghouthi et al. [2012, 2014] and nilsson et al. [2013]. the barghouthi model has previously been shown to produce results that are similar to the observations [barghouthi 2008, barghouthi et al. 2011]. to make the results more reliable we have modified this model by: (a) subdividing the polar cap region into three trajectories to increase the spatial resolution; (b) replacing the radial field by the tsyganenko t96 model which takes into consideration both internal and external sources of the magnetic field; and (c) replacing the previously used boundary conditions by more recent boundary conditions suggested by nilsson et al. [2013]. in this model we have assumed that the ion outflow is along a field line without convection (e × b drift). such a convection would lead to a drift from one region to another therefore mixing the plasmas of the two regions. by using this model we have therefore assumed low convection velocities which occur at quiet geomagnetic activity periods. furthermore this model does not take into account the temporal variations of the outflow. hence, this model is used to study the typical evolution of ion outflows for large de-1 vs. cluster observations geocentric distance region characteristics observations barp/bara nilp/nila 3 re cusp temperature (ev) ~20-1000 bouhram et al. [2004] 740 (bara) 1 (nila) 3 re cusp parallel bulk velocity (km/s) ~25-100 bouhram et al. [2004] 63 (bara) 1.9 (nila) 11.8 re cusp perpendicular temperature (ev) ~870-2280 nilsson et al. [2013] 8.24 × 108 (bara) 1109 (nila) 11.8 re cusp parallel bulk velocity (km/s) ~83 nilsson et al. [2013] 2.9 × 104 (bara) 54 (nila) 2.3 re cpc parallel bulk velocity (km/s) ~1-2 abe et al. [2004] 1.5 (barp) 0.5 (nilp) 10.1 re cpc parallel bulk velocity (km/s) ~56 nilsson et al. [2013] 2000 (barp) 30 (nilp) table 1. comparison between observational and simulation results for o+ ions. distances along the geomagnetic field lines and the spatial and temporal transients are rather smoothed out. for a more accurate model that takes into consideration all these effects, we suggest that a 3-d model such as barakat and schunk [2006] needs to be used. however, due to their complexity, these models consume a great deal of time and resources, which makes conducting a parametric study like this impractical. consequently, we suggest that a simplistic 1-d model such as the barghouthi model be used to supply the more complicated 3-d models with the correct set of parameters for their implementation. references abe, t., a.w. yau, s. watanabe, m. yamada and e. sagawa (2004). long-term variation of the polar wind velocity and its implication for the ion acceleration process: akebono/ suprathermal ion mass spectrometer observations, journal of geophysical research: space physics, 109 (a9). abudayyeh, h.a., i.a. barghouthi, r. slapak and h. nilsson (2015). centrifugal acceleration at high altitudes above the polar cap: a monte carlo simulation, j. geophys. res. space physics, 120, 6409-6426; doi:10.1002/2015ja021325. arvelius, s., et al. 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(1977). mathematical structure of transport equations for multispecies flows, reviews of geophysics, 15 (4), 429; doi:10.1029/rg015i004p00 429. slapak, r., h. nilsson, m. waara, m. andré, g. stenberg and i.a. barghouthi (2011). o+ heating associated with strong wave activity in the high altitude cusp and mantle, annales geophysicae, 29 (5), 931944; doi:10.5194/angeo-29-931-2011. tsyganenko, n.a. (1995). modeling the earth’s magnetospheric magnetic field confined within a realistic magnetopause, journal of geophysical research, 100 (a4), 5599; doi:10.1029/94ja03193. tsyganenko, n.a. 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corresponding author: hamza a. abudayyeh, al-quds university, department of physics, jerusalem, palestine; email: abudayyehhamza@gmail.com. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. de-1 vs. cluster observations << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7558 1 the social sense of geological literacy h éctor luis lacreu dep artam en to d e geología, un iversid ad n acion al d e san lu is, argen tin a lacreu @gm ail.com abstract w ays and methodologies to teach geology are widely debated and are frequent topics in geoscientific discussion, whereas there is much less attention to the subject of geological literacy. it is more and more frequent to hear complaints by un iversity teachers regarding the deficient geological knowledge of incoming university students, and this is especially the case in a rgentina’s universities. teachers simply characterize the problem affirming: “the students do not know an ything about geology” or “in high school nobody taught them geology”. a dditionally, most geologists consider that it is not their problem and consider secondary teachers as uniquely responsible. n evertheless, the matter is more complicated than this and deserves a different approach for resolution. in this contribution i table reflections on the need to address the lack of preparation and the scarcity of geological knowledge in terms of geological literacy, rather than in terms of teaching geology. 1. in trod uction n ad equ ate geological kn ow led ge n ot on ly affects stu d en ts in u n iversities, bu t also th e m ajority of citizen s w h o fin ish th eir secon d ary stu d ies w ith ou t h avin g learn ed an y basic geological n otion s. th e m ost w orrisom e asp ect in th is situ ation is th at th is ign oran ce also ch aracterizes p olitician s, tech n ician s, jou rn alists an d scien ce com m u n icators, w h o act an d give op in ion s on n atu ral resou rces an d en viron m en tal p roblem s, w ith ou t an y com p r eh en sion as to cau ses an d effects of th e issu es, th ereby con tribu tin g to th e in crease in con fu sion in th e p u blic. for too m an y years, th e sam e op in ion s on th is m atter h ave been rep eated , by u sin g tw o key con cep ts, “kn ow led ge” an d “teach in g”, an d by om ittin g th e con cep t of “learn in g”, w h ile th e th ree con cep ts sh ou ld be all con sid ered sim u ltan eou sly w ith in th e fram ew ork of th e real con d ition s in w h ich th ey in teract. with ou t d ou bt th ere are efforts to im p rove th is state of affairs, bu t for th e sake of brevity i w ill m ake som e gen eralization s, th at in som e cases m ay seem im p rop er an d in correct. th erefore, m y ap ologies for th ose w h o w ill feel sligh ted an d m y en cou ragem en t to th em to sp read th eir ach ievem en ts th rou gh p u blication s th at are a ccessible to ed u cators. im p rovin g teach in g of geology requ ires an a d equ ate ch aracterization of th e p roblem , by focu sin g on th e sen se of geological learn in g an d kn ow led ge. th is rep resen ts a great ch allen ge for th e geological com m u n ity, both in term s of u n d erstan d in g th e con cep t of geological liter acy an d in term s of in terven tion w ith con crete action s in p rim ary, secon d ary an d u n iversity stu d ies. an oth er ch allen ge is to im p rove th e ap p roach to th e p roblem , given th e com p lexity to in terven e in th ese d om ain s, th e lack of in d ivid u als w h o are exp ert in th e su bject, th e con flicts of in terests an d th e resistan ce of som e teach ers to in trod u ce ch an ges in th eir u su al p ractices. in th at sen se, w e cou ld list oth er ch allen ges, bu t i w ill d o so later, after a few reflection s on scien tific literacy in gen eral an d geological literacy in p articu lar . 2. some n otion s on scien tific literacy th e term “literacy” w as first u sed in th e late n in eteen th cen tu ry to in d icate th e cap ability to read an d w rite. it is on e of th ose self-d efin in g term s, su ch as “freed om ”, “ju stice”, “h ap p in ess”, w h ich w e in corp orate in ou r cu ltu re for th eir con ten t, n eed ed qu alities an d d esirable attribu tes (braslavsky, 2003). sin ce th en , th e i an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7558 2 term “literacy” h as been rep h rased an d a con sen su s h as been bu ilt on th e m ean in g of scie n tific an d tech n ological literacy (act in sp a n ish ). it “d esign ates a typ e of kn ow led ge, skills or com p eten ces th at in ou r tech n ical-scien tific w orld corresp on d to w h at literacy w as in th e last cen tu ry” (fou rez, 1997: 15). in ad d ition , th e term “literacy” p rop itiates a m ore com p lex cu ltu re th at gives valu e to both th e p ertin en ce an d th e ad equ ate u se of kn ow led ge as an eth ical com m itm en t to reflect on the social an d p olit ical con sequ en ces of its ap p lication . with in th is orien tation , it h as been p oin ted ou t th at “given th e com p lexity of p resen t an d fu tu re global ch allen ges, h igh er ed u cation h as th e social resp on sibility to ad van ce ou r u n d e rstan d in g of m u ltifaceted p roblem s th at im p ly social, econ om ic, scien tific an d cu ltu ral asp ects, as w ell as ou r cap ability of ad d ressin g th em . h igh er ed u cation sh ou ld take on social lead e rsh ip in global kn ow led ge-bu ild in g to ad d ress global ch allen ges, in clu d in g food secu rity, clim ate ch an ge, w ater m an agem en t, in tercu ltu ral d ialogu e, ren ew able en ergy an d p u blic h ealth ” (un esco, 2009). as a resu lt, w e can assu m e th at scien tific liter acy is n ecessary to im p rove th e qu ality of citizen p articip ation in d ecision -m akin g, in econ om ic p olicies d evelop ed in d ifferen t sectors (fo r exam p le: legislative p rojects at m u n icip al, p r ovin cial an d / or n ation al scale), as w ell as in th e p u blic h earin gs con ven ed w h en w orks th at h ave an im p act on th e en viron m en t are carried ou t. su ch tim ely an d relevan t p articip ation r equ ires from “citizen s m ore th an a very h igh level of kn ow led ge, th e lin kin g of a m in im u m of sp ecific kn ow led ge, p erfectly accessible to citizen s, w ith global ap p roach es an d eth ical con sid eration s th at d on ’t requ ire an y sp ecialization .” (gil y vilch es, 2003). 3. d imen sion s of geological literacy geological literacy is a m u ltid im en sion al ch a llen ge an d requ ires n ew cu rricu lar p rop osals overcom in g trad ition al teach in g, restricted o n ly to con cep tu al an d m eth od ological geological con ten ts. in th is sen se, it is recom m en d ed to in corp orate oth er d im en sion s su ch as th e h ist ory of geological id eas, ep istem ology, th e n atu re of geology, sp ecific tech n iqu es an d m eth od s, p rofession al p ractices, an d th eir role in p erso n al an d social life (ad ap ted from bybee, 1997; qu oted in gil-pérez et al., 2005). in th is ap p roach , it can be p oin ted ou t th at ge ological cu rricu la for all th e citizen s sh ou ld h ave th e follow in g com m on elem en ts (ad ap ted from marco, 2000):  practical geological literacy th at allow s u s to m ake u se of geological kn ow led ge in d aily life, in ord er to im p rove liv in g con d ition s, as w ell as featu res, n atu ral resou rces an d risks of ou r en viron m en t.  civic geological literacy, so th at all p eop le can socially in terven e in p olit ical d ecision s w ith d ifferen t criteria, in clu d in g eth ical an d scien tific criteria.  cu ltu ral geological literacy th at d ebates th e m ean in g of scien ce an d tech n ology an d th eir im p act on th e social stru ctu re . in sh ort, geological literacy in a broad sen se sh ou ld in tegrate p ractical literacy, civic literacy an d cu ltu ral literacy for citizen s in gen eral. 4. geological literacy in ed ucation al levels th e ch an ge from “geological teach in g” to “ge ological literacy” im p lies both an exp an sion of con ten t an d a n ew ap p roach , cen tered on lear n in g, in w h ich th e con cep ts red u ce th eir trad ition al role, to sh are it w ith skills an d attitu d es in th e u se of su ch con cep ts. th is ap p roach r equ ires n ew d id actics, ap p rop riate to each ed u cation al level, to th e “geological h orizon ” th at su rrou n d s each in stitu tion , to th e resou rces available, an d so on . all th ese elem en ts con figu re a set of sp ecific n eed s th at are su bjects of stu d y, creation an d research of a n ew d iscip lin ary field called th e “geolod actics” (lacreu , 2012). in th e social im agin ary, in th e gen eral ed u cation al regu lation s an d in th e sp ecific cu rricu lar m od els, it is exp ected th at p re-u n iversity ed u cation sh ou ld be aim ed at train in g citizen s, sim u ltan eou sly train in g for w ork in sertion an d p rop aed eu tic train in g. in argen tin a, w ith regard s to th e p rim ary level of ed u cation , it is p erceived th a t “p ractical geoan n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7558 3 logical literacy” in clu d es su fficien t geological con ten ts. h ow ever, sequ en cin g an d grad u ality are con sid ered in ad equ ate, w h ereas d id actic strategies often ign ore field w ork an d th e cts ap p roach (lacreu , 2015). on th e oth er h an d , in th e argen tin e secon d ary ed u cation al level, th e geological con ten ts are alm ost n u ll w ith in th e n ap (priority learn in g n u cleu s) of n atu ral scien ces for th e com m on cycle of all h igh sch ools. th ey are also absen t in m ost of th e orien tated h igh sch ools, excep t for th e orien tation s in “n atu ral scien ces” an d “en viron m en t”, in w h ich th e n ew sp ecific cu rricu lar sp ace of “earth scien ces” w as created . h ow ever, it is n ot p ossible to d evelop geolog ical con ten ts in d ep th , sin ce th e n ecessary p r erequ isites d o n ot exist (lacreu , 2014). in relation to th e secon d ary ed u cation al level, it is im p ortan t to n ote an im p ortan t con tribu tion on literacy in earth scien ces m ad e by a grou p of sp an ish colleagu es (ped rin aci et al., 2013) w h o m an aged to syn th esize 5 objectives an d 10 key id eas th at con stitu te an in d isp en sable gu id e to review th e argen tin e cu rricu la . with referen ce to th e u n iversity level, geology careers offer an ad equ ate h igh er ed u cation in sp ecific geological asp ects, far su rp assin g th e n eed s of “p ractical geological literacy”. h o w ever, th ere are sh ortcom in gs in “civic geological literacy” an d “cu ltu ral geological literacy”, save som e excep tion s, th at p ersist th rou gh ou t th e form ative stage, w eaken in g th e in tegral train in g an d , th erefore, th e p rofession al p rofile of geologists, as r equ ired by th e n ew regu lation s in force sin ce 2008. precisely for th is re ason , i sh are th e great con cern th at exists in th e in tern ation al aren as w h ere th e p roblem of h igh er ed u cation is d iscu ssed . in m y exp erien ce of m ore th an 30 years in u n iversity teach in g, in d ivu lgation activities an d train in g of teach ers i h ave been able to w itn ess th e in su fficien t literacy of u n iversity asp iran ts. in d eed , th is is m an ifested in th e scarce geolog ical an d m ath em atical kn ow led ge an d in a n otable d eficit in th e cap ability of read in gcom p reh en sion as w ell as in com m u n icative com p eten cies. for th ese reason s, th e n ation al u n iversities ad op ted su p p ort p olicies to rein force su ch in stru m en tal skills an d to red u ce th e n u m ber of stu d en ts th at d rop ou t. in ad d ition , geology cou rses in clu d e su bjects su ch as “in trod u ction to geology” an d cou rses on read in g, com p reh en sion an d w ritin g of scien tific texts, as a w ay of com p en satin g for th e d eficit in th e “p ractical geological literacy” of en tran ts. un fortu n ately, th ere are n o kn ow n m ech an ism s th at attem p t to revert th e in su fficien t cultural literacy, referrin g to th e h istorical, ep ist em ological an d m eth od ological asp ects of geo logy, as w ell as th e d eficit in citizen literacy, w h ich is reflected in th e d ifficu lties to id en tify m ain an d secon d ary cau ses, or to recogn ize eth ical con flicts in p rofession al p ractices, or to exp ress on e's ow n id eas, takin g in to accou n t d ifferen t p oin ts of view . in sh ort, i con sid er th at su ch com p eten cies lim it th e in d ivid u als to fu lly an d actively p articip ate in civ ic life. fin ally, a balan ce cou ld be created : ju st as w e w on d ered abou t th e scien tific geological liter acy of citizen s, w e sh ou ld also w on d er abou t th e scien tific literacy of th e scien tists. th is ap p a ren t p lay on w ord s refers to th e con cep t of lite racy as a m etap h or for th e in tegral train in g of citizen s th at w e h ave alread y con sid ered as in su fficien t. th is is w h y w e can form u late som e rh etorical qu estion s th at cou ld h elp to th in k th e ch allen ges of th e 21st cen tu ry :  w hy do the national universities and the national geology curricula not address the shortcomings in “civic” and “cultural” literacy, with the same dedication they have to overcoming practical literacy?  how much better would be the training of geologists, how much better and more ethical could their professional practices and contributions to sustainable development be, if the dimensions of “civic” and “cultural” literacy were explicitly and formally incorporated in the curricula? th e an sw ers to th e above qu estion s an d th eir p ractical d evelop m en t w ill rem ain p en d in g. h ow ever, som e reflection s an d p rop osals are sh ared on som e of th e ch allen ges th at i believe w ill h ave to be assu m ed w ith in th e d iscip lin ary field of geology, w h ile in teractin g w ith oth er d iscip lin ary field s. th e in teraction betw een “ge olod acts”, teach ers, acad em ic au th orities an d p rovin cial an d n ation al p olitical officers w ill be essen tial to im p rove geological literacy in p rim ary, secon d ary an d h igh er ed u cation , in keep in g w ith th e an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7558 4 com p lexity th at is en visaged for th e 21 st cen tu ry. 5. con clusion : some challen ges 5.1 creating favorable instances for the deve lopment of geolodactics th e scarce geological train in g of p re -u n iversity teach ers an d th e lack of geolod actic resou rces often gen erate a certain d egree of in secu rity an d fear for teach in g geology. th u s, it is im p ossible to m otivate stu d en ts. th is is a seriou s p roblem w h ose ap p roach requ ires a n u m ber of sp ecialists in geolod actics in a greater n u m ber th an cu rren tly exists in argen tin a. con sequ en tly, th e n ation al u n iversities h ave th e social an d p olitical resp on sibility to fav or th e creation an d d evelop m en t of geological research cen ters, as w ell as to in corp orate research ers w ith exclu sive d ed ication , from geology to p ed agogy (lacreu , 2107a). train in g an d u p d atin g of teach ers is im p er ative to aban d on th e u su al w ay to th in k of teach in g geology an d to rep lace it w ith n ew ap p roach es th at p rom ote geolog ically sign ifican t learn in g. in th at sen se, w e m u st w ork on th e d id actic tran sp osition (lacreu , 1996) an d on th e re-sign ification of geology as a h istorical scien ce. 5.2 renew ing the teaching of geology by e mphasizing the character of historical science prim ary an d secon d ary ed u cation sh ou ld be ren ew ed by in corp oratin g ap p roach es d ifferen t th an u su al. on th e on e h an d , m ean in gfu l lear n in g sh ou ld be p rom oted th rou gh sch ool r esearch , w ith an d w ith ou t field trip s, p roblem solvin g or case stu d ies. on th e oth er h an d , it is n ecessary to ren ew th e ep istem ological a p p roach an d to d evelop strategies th at allow on e to u n d erstan d th e h istorical an d in terp retative ch aracter of geology (frod em an , 1995). it h as lon g been p rop osed th at “geology is a h istorical scien ce of n atu re, becau se it stu d ies n atu ral p rocesses th rou gh form s (fossilized in th e origin al) reflected in geological stru ctu res” (pot ap ova, 1968). in th is sen se, som e exp erien ces r elated to th e con stru ction of th e geological h ist ory of lan d scap e can be con sid ered , both th rou gh field w ork (lacreu , 2007) an d virtu al classroom w ork (lacreu , 2012b). in th is w ay, attem p ts h ave been m ad e to d elete con cep tu al errors abou t th e im m u tability of relief, its rocks an d stru ctu res an d to p rom ote aw aren ess of th e h u m an im p act on n atu re. man leaves m arks on th e geological lan d scap e, ju st as n atu ral p r ocesses of th e p ast h ave left m arks th at w e can see tod ay on th e p resen t lan d scap e. th e h u m an m arks are of a d ifferen t n atu re, bu t th eir in te n sity is so stron g as to su ggest th e in trod u ction of a n ew geological ep och : th e an th rop ocen e (cru tzen , 2002). 5.3 promoting epistemological coherence in the teaching of the natural sciences n atu ral scien ces can n ot be p rop erly tau gh t if geology is absen t. th is im p lies th e reform u lation of cu rricu la at th e p re-u n iversity level, u n iversity teach in g form ation , an d train in g cou rses in n atu ral scien ces, so th at geology can be in corp orated as an in h eren t d iscip lin e in th is grou p of scien ces. th is con stitu tes a p olitical d ebt of u n iversity au th orities (an d a ch allen ge for th e geological com m u n ity), th at h as been tolerated by action or om ission . th is p erm issiven ess h ap p en s u n d er th e assu m p tion th at it is an ed u cation al p roblem , alien to geology, w ith ou t realizin g th at its resolu tion in evitably requ ires th e com m itted p articip ation of geologists in gen eral an d geolod acts in p articu lar. in th is sen se, in ord er to ach ieve an in tegrative teach in g of th e n atu ral scien ces, it is essen tial to u se th e h istorical ap p roach , to h igh ligh t th e fact th at m an y of th e biological, p h ysical an d ch e m ical p rocesses occu rred in th e p ast h ave been record ed in geological m aterials, in d ifferen t tim es an d p laces th r ou gh ou t th e h istory of th e earth . th ese m aterials h ave u n d ergon e ch an ges of sh ap e an d p osition an d m an y of th em con tain m in in g resou rces n ecessary for society. th erefore, th e search for n ew resou rces r equ ires th e h istorical recon stru ction of geolog ical lan d scap es. 5.4 promoting epistemological coherence in the teaching of the natural sciences th e lack of access to civic an d cu ltu ral geolog ical literacy, as w ell as th e scarce cap ability of w ritin g an d scien tific com m u n ication , seriou sly an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7558 5 th reaten th e ach iev em en t of th e p rofession al p rofile requ ired by n ation al regu lation s (lacreu , 2017b). con sequ en tly, cu rricu la sh ou ld recogn ize su ch sh ortcom in gs an d e xp licitly in corp orate com p en satory strategies. for th is p u rp ose, th e n ation al u n iversities com m itm en t to society sh ou ld be rem em bered : geology h as been d eclared a d iscip lin e of p u blic in terest, becau se its p rofession al p ractice can in flu en ce p u blic safety, by p u ttin g risk on h ealth , safety, righ ts, p rop erty or train in g of th e in h abitan ts, as in d icated in th e arg en tin e regu lation s. it is n ecessary th at stu d en ts are p r ogressively train ed accord in g to th e n ation al r equ irem en ts of h igh er ed u cation for th e 21st cen tu ry (un esco, 2009), w h ich p articu larly em p h asizes th e n eed for geologists to carry ou t th eir p rofession al activities w ith a sen se of eth ical an d social resp on sibility, an d w ith th e com m itm en t to favou r su stain able d evelo p m en t. 5.5 contributing to sustainable development and to the clarification of its meaning bru n d tlan d , in h er first an d valu able rep ort (un , 1987), p rop oses th at su stain able develop m en t “is cap able of resp on d in g to th e n eed s of th e p resen t, w ith ou t com p rom isin g th e p o ssibility of fu tu re gen eration s to satisfy th eir n eed s”. at th e m om en t, it cou ld seem a sed u ctive bu t som ew h at n aive statem en t . in fact, it d oes n ot sp ecify w h ich social sectors are ben eficiaries of th e “n eed s of th e p resen t” an d ca n n ot an ticip ate w h at th e n eed s of “fu tu re gen e ration s” w ill be. besid es, m ost en viron m en tal p roblem s are th e resu lt of com p an ies th at d efin e th em selves as su stain able, bu t too often th ey on ly favor econ om ic grow th to th e d etr im en t of social equ ity an d ecological balan ce. th ese situ ation s requ ire th at geologists are aw are of th e role (p assive or active) th at th ey can p lay in th e creation / solu tion of th ese p roblem s (lacreu , 2017b). referen ces braslavsky b. 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(2013). alfabetización en cien cias d e la tierra. en señ an za d e las cien cias d e la tierra, 21.2., 117-129. potap ov a m.s. (1968). geologia com o u m a ciên cia h istórica d a n atu reza. terræ did atica, 3(1):86-90. un (1987). rep ort of th e world com m ission on en viron m en t an d dev elop m en t: ou r com m on fu tu re. ch ap ter 2: tow ard s su stain able develop m en t, h ttp :/ / w w w .u n d ocu m en ts.n et/ w ced -ocf.h tm (accessed 21 febru ary 2018). un esco (2009). la n u eva d in ám ica d e la ed u cación su p erior y la in vestigación p ara el cam bio social y el d esarrollo. com u n icad o d e la con feren cia mu n d ial d e la un esco, parís.. [figu ered o an d wolf, 2009] figu ered o, a. j. an d wolf, p. s. a. (2009). assortative p airin g an d life h istory strategy a crosscu ltu ral stu d y. h u m an n atu re, 20:317–330. [figu ered o an d wolf, 2009] figu ered o, a. j. an d wolf, p. s. a. (2009). assortat ive p airin g an d life h istory strategy a crosscu ltu ral stu d y. h u m an n atu re, 20:317–330. bruno 753_768.pdf annals of geophysics, vol. 45, n. 6, december 2002 753 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius offshore (southern italy) pier paolo g. bruno (1), vincenzo di fiore (2) and antonio rapolla (2) (1) osservatorio vesuviano, istituto nazionale di geofisica e vulcanologia, napoli, italy (2) dipartimento di scienze della terra, università degli studi di napoli «federico ii», napoli, italy abstract the campanian volcanism develops near the sea. therefore, the geophysical study of the marine environment is a key to a better understanding of the tectonic evolution and the origin of volcanism in the area. an abundance of high quality seismic data in the marine sector, where little direct information is available, is critical to the study of campanian volcanism. this paper concerns the reprocessing of a seismic reflection dataset acquired in naples bay and processed during 1973. even though the overall data quality was high for that time, of course their acquisition technological limits have been overcome by the new processing. our reprocessing aimed at: 1) reduction of random noise in the data; 2) removal of unwanted coherent events; 3) reduction of spatial aliasing by means of trace interpolation on commod shot point (csp) gathering; 4) improvement of resolution of the seismic wavelet with spiking deconvolution algorithms and finally 5) reposition of reflectors in their correct locations in the spacetwt domain by means of dip moveout and post-stack time migration. a comparison between the new and old data shows that the new sections are characterized by a much higher s/n ratio. diffraction hyperbole has been collapsed. reverberations, ghosts and multiples have been removed or greatly attenuated, especially between the reflectors of interest, allowing us to follow them with more detail and with greater continuity. furthermore, data resolution has been boosted by the reprocessing, allowing the interpreter to evaluate reflector position and continuity in greater detail. the reinterpretation phase of such lines, that is already in an advanced stage, will therefore allow us to gain new insights into the structural setting of the bay, with the aim of exploring the connection between tectonics and volcanism. 1. introduction this paper concerns the reprocessing of a seismic reflection dataset acquired in naples bay (nb) by ogs (istituto nazionale di oceanografia e geofisica sperimentale) during 1973 (finetti and morelli, 1974) in order to acquire new data in a vital area for the understanding of the local active volcanism. this area, which experienced a regional extension during the plio-quaternary is of great importance because it is the site of active volcanism: the presence of volcanoes with geological and historical records of very explosive events (such as ignimbrites and plinian eruptions) in a densely populated territory, implies a high risk. naturally, explosive volcanic activity also includes a strong and very frequent seismic activity, even if limited to localized areas, such as pozzuoli town. although the recent mailing address: prof. antonio rapolla, dipartimento di scienze della terra, università degli studi di napoli «federico ii», largo san marcellino 10, 80138 napoli, italy; e-mail: rapolla@unina.it key words marine geophysics − seismic reflection − naples bay − campi flegrei − somma-vesuvius 754 pier paolo g. bruno, vincenzo di fiore and antonio rapolla seismic crises of 1970-1972 and 1982-1984 in campi flegrei (cf) did not culminate in a volcanic eruption, they caused extensive damage, and the evacuation of an entire district of pozzuoli town (rione terra: de stefano et al., 1988). the campanian volcanism develops near the sea; procida and ischia are volcanic islands. bruno et al. (2002) showed that the remnant of a once larger volcanic complex extended to the western offshore of ischia island. somma vesuvius (sv) complex is located on the coastline, at about 10 km to the sw of the city of naples (fig. 1). probably this volcano started its activity in a shallow water environment whereas cf volcanic field constitutes a promontory located at about 10 km to the west of naples. morphological, geophysical and geological data suggest that the cf caldera continues in the gulf of pozzuoli (gp) (agip, 1987; rosi and sbrana, 1987; orsi et al., 1996; florio et al., 1997). therefore, the geophysical study of the marine environment is a key to better understanding the tectonic evolution and the origin of volcanism in the area with clear spin-off on the seismic and volcanic hazard. an abundance of high quality geophysical data (and among them seismic data, which allow the highest resolution) is critical to the study of marine environments where little direct information is available. unfortunately, high quality geophysical data are missing in the nb. the spatial resolution of the available aeromagnetic and marine gravity data collected in fig. 1. location of the seismic lines in the gulf of naples. 755 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... the bay (agip, 1982) is insufficient. more recent and detailed potential field data acquisition only concerned limited sectors of the bay. at present, the only deep seismic dataset available in the area was collected by ogs (istituto nazionale di oceanografia e geofisica sperimentale) during 1973. deep seismic reflection data allow very high detail, but the acquisition is quite expensive, and often it is not affordable for public research institutions. indeed, more recent seismic data collected in nb are, in most cases, high resolution − single channel sparker and uniboom profiles, which allow the shallow plio-quaternary cover to be studied in detail, but do not penetrate deep. therefore, fault geometry and kinematics, and in some cases the tectonic involvement of the deep geological units are hypothetical. for all these reasons the sea environment geology in the gulf of naples is less known than the onshore areas. to gain more information on the deep part of nb, near the gp, we decided to reprocess the 1973 ogs seismic dataset. reprocessing old seismic data has become a standard in the oil industry since technological advances bring notable improvements to the data in terms of signal-to-noise (s/n) ratio, allowing the interpreter to obtain new information at low cost. 2. geological settings nb is the submerged sector of the campanian plain, a quaternary graben bordered by mesozoic carbonates. the plain and the overlooking tyrrhenian sea originated from stretching and thinning of the continental crust due to the rotation the italian peninsula (patacca et al., 1990). a pliocene distension phase (sgrosso, 1998) created the conditions for magma uprising in the area. the active volcanoes develop along the coast with the presence of two main volcanic centres: cf and sv. cf are a volcanic field developed within a nested caldera (fig. 1) which was created by two large ignimbritic eruptions that produced widespread ash-flow deposits: the «campanian ignimbrite» (ci) at about 34 000 a b.p., and the «neapolitan yellow tuff» (nyt) about 12 000 years ago. after the nyt eruption, the caldera was invaded by the sea (scandone et al., 1991). subsequent volcanic activity emplaced only within the caldera structure, frequently along its rims and sometimes involving intra-calderic collapses (lirer et al., 1987). the erupted products range in composition from k-basalts to alkali-trachyte and phonolite. the complex has been active since at least 47 000 a b.p. an uplift of the caldera floor already occurred between 10 and 5 ky b.p. (cinque et al., 1985). a marine terrace (la starza), raised to height of about 40 m, currently borders the northern shore of the gulf of pozzuoli. the uplift was accompanied and followed by revived volcanic activity. sea-level measurements made in the ruins of a roman market (serapeo), built near the seashore in the town of pozzuoli, have indicated a slow sinking of the area since roman times. (these slow movements of the ground have been called bradyseism from the greek «bradi» slow and «seism» movement). the serapeo floor, had to be upraised only two centuries after its first construction, because of the sea ingression. at the beginning of 1900, early land levelling showed that the maximum value of ground sinking was occurring in the town of pozzuoli. this slow sinking continued until 1968. an inversion of tendency occurred during 1970-1972 and 1982-1984 with two important episodes of inflation (berrino et al., 1984). during these periods an uplift of 170 cm and 182 cm respectively was measured at the point of maximum deformation, located in pozzuoli town. the inflation geometry is the mirror image of the slow sinking observed until 1968; it has a circular symmetry around pozzuoli and regularly decreases toward the margin of the caldera. a partial deflation of about 20 cm occurred after the 19701972 episode. about 70 cm have been recovered since the 1982-84 episode. deformation at cf seems strongly controlled by the caldera structure. luongo et al. (1991) and civetta et al. (1995) explain the uplift with a caldera-resurgence phenomenon, which involves both brittle and ductile deformation (orsi et al., 1996). scandone et al. (1991) suggest instead that the pattern of ground deformation does not seem 756 pier paolo g. bruno, vincenzo di fiore and antonio rapolla compatible with magma migration toward shallow depth, because in such case a decrease of the aerial extent of the inflation would be expected. contrary to the sinking phase, uplift episodes are accompanied by marked seismic activity. earthquakes occur mostly in the coastal region around pozzuoli, fewer deeper events occur within the bay, but they do not extend outside the border of the cf caldera (fig. 2a). hypocenters are located between a few hundred meters and about 5 km depth. the maximum recorded magnitude (4.0) was observed in 1984. aster and meyer (1988) made a tomographic study of the crustal structure of the caldera by a simultaneous tridimensional inversion of velocity and hypocenters of earthquakes. they found that the central part of the pozzuoli caldera has an anomalous high vp /vs ratio and low vp and vs , indicating an incompetent highly fractured medium, saturated with water. according to this study, areas of anomalous low vp /vs ratio occur at the borders of the caldera depression. position of the deepest earthquakes focus delimit an inward-dipping elliptical zone, interpreted as a ring fault. with the exception of three short-duration uplift episodes in 1989, 1994 and 2000, the last sixteen years of ground deformation at cf have been characterised by slowly descending movements. the last uplift started in early march, 2000 and reached its peak value during the following summer (fig. 2b). accompanying the ground uplift, two swarms of low-energy earthquakes occurred on july 7 and august 22, 2000. by analysis of the first swarm, saccorotti et al. (2002) hypothesised a direct involvement of magmatic/hydrothermal fluids in the source process. conversely, the spectra of the august events are typical of shear failure. location techniques applied to the august swarm allowed for the recognition of two parallel alignments trending ne-sw. this direction corresponds with that of the main focal plane obtained from fault plane solutions. the other active volcanic complex located in the study area is the sv. structurally, the complex is positioned at the crossing point of two regional faults with nw and se trends (bruno et al., 1998). sv is a stratovolcano constituted by an old apparatus (mt. somma) that includes an asymmetric caldera. the new apparatus, mt. vesuvius, is grown within this caldera. the more ancient outcropping products date 25 about ky (alessio et al., 1974); these products are related to the oldest known plinian eruption of sv (basal pumice 19 ky b.p.), that is found above the ci deposits. ancient sv lava samples found at a depth of 1125 m, date about 300 ky: this age is possibly indicative of the beginning of sv volcanic activity. a b fig. 2a,b. a) deformation measured along a naplesmiseno line and crosscutting pozzuoli town (berrino et al., 1984); b) ground level fluctuation during 2000 uplift episode deduced from a marigraphic station located at pozzuoli port (vesuvian observatory data, 2000). 757 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... mt. somma edifice is constituted prevalently by lava. somma morphology is the result of different collapse episodes which have determined the westward downfall of part of the mountain (bruno and rapolla, 1999), the sinking of the upper part of the volcano and the formation of a caldera. such caldera records at least five collapse phases linked to plinian eruptions (andronico et al., 1995, 1996). the recent activity occurred prevalently in the caldera ring of the mt. somma. eruptions at sv are often characterised by both effusive and explosive activity. the effusive activity is often accompanied by pyroclastic flows and by lahar running along narrow canyons. the explosive activity, characterised by plinian and sub-plinian eruptions (scandone et al., 1993), usually begins with a violent ejection of large amounts of pyroclastic products, such as pumice and ashes, followed by pyroclastic flow and surges. an example is the plinian eruption of pompei in 79 a.d. volcanic and historical records allowed the reconstruction of the history of the volcano in the last 25 ky. in particular, researchers distinguish three principal eruptive cycles (e.g., andronico et al., 1995, 1996). the first cycle (between 25 and 11 ky b.p.) is mainly characterized by two plinian events. during the second cycle (11ky 79 a.d.), there were three plinian eruption (mercato, 7.9 ky b.p.; avellino, 3.8 ky b.p.; pompei, 79 a.d.) and several sub-plinian eruptions. the third cycle (79 a.d. present) includes two or perhaps three sub-plinian eruptions (pollena, 472 a.d.; 512; 1631), a long strombolian activity, and more recently, mixed style eruptions (e.s. 1906 and 1944). nowadays, vesuvius is in a quiescent phase (scandone et al., 1993). explosive volcanic activity at vesuvius also implies the presence of seismic activity. the seismic activity of vesuvius is characterised by swarm events that include events with large differences in magnitude. for example after the last eruption of 1944, seismic activity at mt. vesuvius has been characterized by about 200 events/year with magnitudes ranging between 0.4 and 3.6 (vilardo et al., 1996). there are periods during which the number of earthquakes increases drastically: during these periods, referred to as crises, 100-150 earthquake events per month can occur (saccorotti et al., 2002). the earthquake hypocentres are localized at depths that range from the top of the crater down to about 6 km b.s.l. clustering occurs between 2 and 3 km b.s.l. because of the shallow depth of the hypocentres, the events of greater magnitude can cause local damage. 3. data reprocessing the ogs seismic dataset consist of 11 profiles (fig. 1). this data set was already processed by ogs in 1973. even though their overall quality was high for that time, they suffer acquisition technological limits that can be mainly condensed into two points: the choice (probably forced at that time) of the field recording parameters and of the energizing source. the low number of geophone groups (only 24 against 120 and more used now) limited the spatial sampling and the maximum offsets of the data, with negative repercussions on the quality of the velocity functions, particularly at high two-way times (twt). furthermore, the use of the flexotir source, largely utilized in the past and now abandoned by the oil industry, has certainly induced bubble effects which contributed to distort the seismic wavelet. the old 1973 processing sequence consisted of deconvolution, band pass filtering and stacking. such processing, especially in the cf marine sector, where the s/n ratio is low, did not yield clear, high quality, seismic sections. indeed, in the old seismic section, multiple reflections and diffraction hyperbolas are very frequent. some lines of the gp are characterized by such a low s/n ratio that it is virtually impossible to indentify any geological structure below the very shallow quaternary drape. our reprocessing therefore aimed at: 1) reduction of random noise in the data; 2) removal of unwanted coherent events; 3) reduction of spatial aliasing by means of trace interpolation on commod shot point (csp) gathers; 4) improvement of resolution of the seismic wavelet with spiking deconvolution algorithms and finally 5) reposition of reflectors in their correct locations in the space-twt domain by means of dip moveout and post-stack time migration. in detail, our initial procedure of reprocessing consisted of data quality check and field 758 pier paolo g. bruno, vincenzo di fiore and antonio rapolla geometry assignment. trace editing aimed at detection and removal of dead or very noisy traces and spikes that may induce problems with forward fast fourier transform (fft). a top muting eliminated the first breaks (directed/ refracted arrivals) from the seismic traces. automatic gain control (agc: sheriff and geldart, 1995) allowed a trace normalisation. the agc operator uses a time window of given length (in our case 500 ms), which is moved down the trace sample by sample, and calculates a scale factor at each location. the scale factor is equal to the inverse of the root mean square (rms) amplitude in the window. this scalar was applied to the sample at the centre sample of the time window. the presence of spatial aliasing is visible as parallel energy bands with a low dip in the negative halfspace in the frequency-wavenumber (fk) domain (bracewell, 1965; yilmaz, 1987) (see fig. 3c) that wraps around and superimposes the useful signal. in such case, a band pass filter is useless. spatial aliasing problems were reduced by means of trace interpolation. the method used transformed the data to the tau-p domain (alam and lasocki, 1981; diebold and stoffa, 1981), weighted each transformed sample based on a coherency measure, and then inverse transformed the data to the new spatial sampling. trace interpolation allowed us to increase the spatial resolution on the common shot point gathers (csp) and reduce the spatial aliasing on csp and on the final sections (fig. 3b,d). an important part of the processing time was spent on data deconvolution. the aim was to compensate for the low resolutive wavelet of the flexotir source and for some other undesirable effects included in the recorded earth response, such as reverberation, multiple arrivals and ghosting. spiking pre-stack and post-stack deconvolution was applied to improve temporal resolution. spiking deconvolution is a leastsquares inverse filter that compresses the seismic source wavelet into a zero lag spike. the filter coefficients are estimated from the auto-correlogram of the seismic trace that contains information on the source signature. predictive deconvolution (peacock and treitel, 1969) attempts to predict and remove repetition in the fig. 3a-d. raw csp #16 relative to seismic line na12 (a) and its fk spectrum (c); shot gather after trace interpolation (b) and its fk spectrum (d). note that the increased number of traces (from 24 to 47) reduces the wrap around effect in the fk space. wrap around starts at about 25 hz on original data and at about 50 hz after trace interpolation. a c d b 759 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... recorded seismograms. the process of multiple energy estimation is controlled by the interpreter again by the analysis of the trace autocorrelation. an example of predictive deconvolution is reported in fig. 4a,b. after deconvolution we also applied a time and space variant bandpass filter (tvf) (telford, 1990) to remove selectively the high and low frequency noise (also boosted by the deconvolution process) along the sections. multiple rejection was an important stage in our work. in addition to pre-stack and post-stack predictive deconvolution, another multiple suppression technique, based on fk dip filtering, was applied. fk multiple attenuation is a process combining: 1) velocity analysis; 2) forward normal move out (nmo) correction; 3) fk dip filtering, and 4) inverse nmo correction. fk multiple attenuation offers a further gain in multiple energy suppression over stacking. the filter performs more quickly than other multiple suppression methods, and it is useful for multiple suppression on pre-stack data. in the fk domain, it is possible to discriminate between primary and multiple events, based on their different velocity. if the velocity analysis is done by picking the primary events, then nmo correction will flatten the primary reflections while the multiple reflections will be undercorrected. in such config. 4a,b. a) part of seismic line na20 with no deconvolution; a multiple of the sea bottom reflection is visible at 0.6 s, twt; b) the same data processed with predictive deconvolution using an operator length of 90 ms and a prediction lag of 40 ms. note that the multiple event at 0.6 s is strongly attenuated. a b 760 pier paolo g. bruno, vincenzo di fiore and antonio rapolla dition, removal of multiple energy in the fk space consists of defining a narrow fan near the frequency axis. this fan will allow all primary reflectors to be passed (with infinite apparent velocity after nmo correction) and will attenuate the multiples. an example of fk multiple attenuation is shown in fig. 5a-d. a qualitatively high velocity analysis is a key for obtaining a good seismic stack. a correct velocity function will align all reflection hyperbole. stacking nmo corrected cdp gathers will increase the s/n ratio of the reflections by (n)1/2, where n is the trace fold. stacking with the correct velocity function will also reduce the amplitude of coherent noise such as refractions, guided waves and multiples. velocity analysis was made using the semblance function, s(v,t) that is defined as a normalized cross-correlation (3.1) where p (x, t, v) is the nmo corrected trace, n is the number of traces in cdp, t is the twt, v is s v t p x t v n p x t v xt dt dt xt dt dt , , , , , ( ) = ( ) ( ) = + = + 2 2 fig. 5a-d. a) cdp gather # 50 of seismic line na08, with nmo correction applied. note the presence of a seabottom multiple (m) reflection at 2.60 s twt, characterized by a residual moveout not entirely removed by the nmo correction (which was finalised to primary events). b) fk spectrum of panel a; the primary reflectors are collapsed near the f axis while the multiple energy has a dip in the k positive halfspace. c,d) cdp gather in the (x, t) and ( f, k) space after filtering with a narrow «pass» fan in the fk space. both graphs show a strong reduction of the energy associated with the 2.60 s twt multiple reflection. b da c 761 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... the velocity and x is cdp spacing. figure 6a shows a graph of s (v, t) that is also referred to as «velocity spectrum». the points where the semblance values are higher generally indicate the best stacking velocity. obviously, the process of picking the velocity function must be completely interactive; in other words the interpreter must have complete control of the process. generally, the semblance graph improves after the application of other processes such as multiple attenuation, fk and bandpass filtering, residual static, etc. in fig. 6a, the velocity spectrum shows the presence of low velocity peaks at high twt. these low velocities are unrealistic (because they are not consistent with respect to depth) and are linked to multiple events and/or to other types of disturbances, such as guided waves, etc. figure 6b shows the same spectrum as fig. 6a after noise attenuation processes. the improvement in the velocity spectrum is evident. our velocity functions were obtained using three or four cycles of velocity analysis and residual static corrections, before and after the application of noise reduction processes. this fig. 6a,b. semblance function for a selected cdp gather of seismic line na05 before (a) and after (b) fk passprimary filtering, predictive deconvolution and residual static correction. note the improved quality of the velocity spectrum after suppression of multiple reflections (leftmost peaks on the semblance graph), and the reduction of the energy associated to the multiple of the sea bottom at 2.4 s twt on the seismic traces. a b 762 pier paolo g. bruno, vincenzo di fiore and antonio rapolla iterative cycle improves the quality of the velocity function. for marine data, it is easy to define a reference datum for static corrections, that generally coincides with the mean sea level (msl). residual static corrections consisted of a cross-correlation procedure that evaluates the time shift due to source-receiver position at surface. cross correlation between adjacent traces was calculated using the following formula: (3.2) where p (t) is the seismic trace, t is the time window length and is the lag time. the process creates time varying residual or trim statics within selected windows along the traces in a cdp gather. these windows must include the reflectors of interest and must exclude coherent noise (such as the multiples). the process is also useful for cleaning up a brute stack which will be used as model traces for external model correlation statics or for removing residual moveout before a brute stack. trim statics are computed for each time window. after the statics are computed they are applied at each sample by interpolating the statics between the window centers above and below the sample. figure 7a shows a nmo corrected cdp gather, relative to line na08 before the application of residual static correction. there is a static shift between the first and the second part of the gather, well visible on the reflector at 1.5 s twt. such static shifts lead to degradation of the stack. after application of residual statics such shift was removed (fig. 7b). as a final pre-stack process, we applied the dip moveout (dmo) correction (yilmaz and claerbout, 1980). dmo correction is a dipdependent, partial migration, applied to nonzero offset seismic data. after dmo correction, the data exhibit the same hyperbolic zero-offset trend for all offsets. this transformation from non hyperbolic offset to hyperbolic (zero) offset yields improved velocity estimates and higher lateral resolution, as well as a few other desirable side effects, such as the attenuation of coherent noise. application of dmo to prestack data and time migration to post-stack data emulates a full pre-stack migration, with the advantage of being computationally faster. after dmo, the stacked data were migrated in time, using two different algorithms: stolt (stolt, 1978) and kirchhoff (yilmaz, 1987) migration. stolt fk migration migrates either common-offset or stacked seismic sections. it is computationally efficient and is very accurate for constant velocities, but has difficulty imaging steep dips in areas where there are large vertical or lateral velocity variations. this algorithm uses stolt’s stretching technique to account for vertical and lateral velocity variations. in more complex areas, where the reflectors have larger dips and complex velocity fields, such as in the gp, we used the kirchhoff algorithm. kirchhoff time migration performs a migration by applying a green’s function to each cdp location using a c p t p t p p i i i n t i i+ = ( ) +( )+ = 1 1 1 fig. 7a,b. a) cdp gather 273 of seismic line na19 after nmo correction; b) the same cdp after residual static correction. a b 763 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... traveltime map. traveltime maps relate the time from each surface location to a region of points in the subsurface. this migration uses a vertically and laterally variant rms (root mean square) velocity field, v rms (x, t), in time. this migration provides good handling of steep dips, up to 90 degrees, and of horizontal variation of velocity along the line. an example of seismic line processed with dmo correction and with kirchhoff migration is reported in fig. 8a,b. migration collapsed the diffraction events in their apex and repositioned the seismic reflectors at their correct locations. this is particularly evident comparing the shape of the channel located between cdp 850-950. the last process applied to the post-stacked data was the frequency-space (fx) deconvolution. fx deconvolution is a reliable multifig. 8a,b. a) part of seismic line na20 (cdp 750-1575); b) the same line after the application of spiking deconvolution and pre-stack dmo and post-stack kirchoff time migration. refer to the text for explanation. a b 764 pier paolo g. bruno, vincenzo di fiore and antonio rapolla channel noise-reduction filter. it preserves the most dominant dipping energy while removing random noise or dips with very low energy (cary and upham, 1993). fx deconvolution produces a very natural looking result, with fewer artifacts than other methods (such as in fk and radon filtering; chase, 1992). for this reason, it is highly favoured for post-stack noise attenuation. although the danger of rejecting weak coherent signal is always there for stacked data, in practice fig. 9a,b. comparison between the reprocessed seismic line and the section of finetti and morelli (1974). a) seismic line na09 (cdp 2590-3210) with the new processing sequence; b) the same line processed in 1974. notice the slot that improved the s/n ratio allows a clearer identification of reflectors a, b and k. a b 765 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... fx deconvolution is surprisingly robust. the fx deconvolution algorithm (gulunay, 1986) applies a fourier transform to each trace of an input ensemble, a complex, wiener, unit prediction filter (robinson and treitel, 1964; treitel, 1974) in distance for each frequency in a specified a b fig. 10a,b. a) seismic line na19 (cdp 590-1250) with the new processing sequence; b) the same line processed in 1974. on the left side of the new section (cdp 1000-1200) reflector b of finetti and morelli (1974) is discontinuous and is probably affected by faults. reflector k is also highly discontinuous and probably is placed at a different depth with respect to that inferred on the basis of the old data. 766 pier paolo g. bruno, vincenzo di fiore and antonio rapolla a b fig. 11a,b. comparison between the reprocessed seismic line and the old section data of 1974. a) seismic line na20 (cdp 50-660) with the new processing sequence; b) the same line processed in 1974. position of reflectors a on the old data does not correspond to any reflector on the new seismic section. reflector k can be followed more easily on the new data and its trend shows discrepancies with that outlined on the old data. 767 seismic reflection data processing in active volcanic areas: an application to campi flegrei and somma vesuvius ... range, and then inverse transforms each resulting in frequency trace back to the time domain. each sample in the transformed data has both real and imaginary components. events with similar dips appear as a sinusoidally complex signal along a given frequency slice. the output trace should have less random noise than the input. 4. discussion of results the study area shows an extremely variable seismic response. on relatively narrow distance, zones with excellent reflectivity and areas of low s/n are found. the main reason of the signal decay is due to the massive presence of volcanic rocks and structures, alternating to the middle and late quaternary and pliocene series and producing a strong scattering of the seismic energy, especially in the gp. for this reason, the gp is characterized by a completely different seismic response, if compared to other parts of nb. on the old seismic sections of 1973 (finetti and morelli, 1974), the lines acquired in the gp were characterized by such a low s/n ratio that it was virtually impossible to identify any geological structure below the very shallow quaternary drape. this s/n decay is due not only to the abundant presence of volcanites but also to a large number of fault fractures and other disturbances, such as volcanic bodies that cause diffuse diffraction and backscattering of the seismic energy thus limiting heavily the interpretability on the old unmigrated seismic sections. the s/n ratio has been greatly improved on this new set of seismic sections allowing us to gain new information on the geological structure of this part of the bay. figures 9a,b to 11a,b report some comparisons between the new seismic sections and the old data. this comparison should not be seen as a criticism of the work done by finetti and morelli (1974) that was very advanced for the time, but as an example of the potential of such data, once reprocessed, of furnishing new geological information at low cost. in particular, comparison between the new and the old data show that the new sections are characterized by a much higher s/n ratio. diffraction hyperbole has been collapsed. reverberations, ghosts and multiples have been removed or greatly attenuated, especially between the reflectors of interest, allowing us to follow them with more detail and with greater continuity. furthermore, data resolution has been boosted by the reprocessing, allowing the interpreter to evaluate reflector position and continuity in more detail. for example, fig. 9a,b reports a part of seismic line na09. the three major seismic reflectors, referred to as «a», «b» and «k» in finetti and morelli (1974) and relative to major geological unconformities of the area, can be followed more easily on the new seismic sections. the same can be seen on line na19 (fig. 10a,b) and on line na20 (fig. 11a,b). the reinterpretation phase of such lines, that is already in a advanced stage, will therefore allow us to gain new hints on the structural setting of the bay, with the aim of exploring the connection between tectonics and volcanism. acknowledgements the authors wish to thank the local scientific organizers of the kyoto and kobe meeting: prof. h. akihama, of nihon university and prof. m. nakashima and his staff at kyoto university for the tremendous effort in organizing the symposium; to dr. a. volpi, scientific attaché at the italian embassy in japan and the italian embassy in japan. thanks are due to prof. i. finetti for the continuous encouragment and to gnv 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(1987): seismic data processing, society of exploration geophysics, tulsa. yilmaz, o. and j.f. claerbout (1980): prestack partial migration, geophysics, 45, 1753-1777. vol49,4_5,2006 961 annals of geophysics, vol. 49, n. 4/5, august/october 2006 key words seismic regime – strong electrical discharges – non-linear dynamics 1. introduction the dynamics of seismic process is far from being clearly understood and modeled. it presents recent several aspects showing that seismicity is certainly not a pure random process under a multidisciplinary approach. magnitude, time and spatial distribution of earthquakes present aspects of self similarity or fractal character evidenced by several authors (de rubeis et al., 1993; turcotte, 1997). on the other hand seismicity cannot be deterministically explained although efforts have been made to show its quasi periodic character. a direct consequence of this situation is the almost complete impossibility to precisely predict earthquakes (geller et al., 1997; main, 1999). in recent years, non-linear dynamics has offered several tools to analyze and characterize seismicity. concepts like phase space reconinfluence of strong electromagnetic discharges on the dynamics of earthquakes time distribution in the bishkek test area (central asia) tamaz chelidze (1), valerio de rubeis (2), teimuraz matcharashvili (1) and patrizia tosi (2) (1) institute of geophysics, georgian academy of sciences, tbilisi, georgia (3) istituto nazionale di geofisica e vulcanologia, roma, italy abstract from 08/01/1983 to 28/03/1990, at the bishkek electromagnetic (em) test site (northern tien shan and chu valley area, central asia), strong currents, up to 2.5 ka, were released at a 4.5 km long electrical (grounded) dipole. this area is seismically active and a catalogue with about 14100 events from 1975 to 1996 has been analyzed. the seismic catalogue was divided into three parts: 1975-1983 first part with no em experiments, 1983-1990 second part during em experiments and 1988-1996 after experiments part. qualitative and quantitative time series nonlinear analysis was applied to waiting times of earthquakes to the above three sub catalogue periods. the qualitative approach includes visual inspection of reconstructed phase space, iterated function systems (ifs) and recurrence quantification analysis (rqa). the quantitative approach followed correlation integral calculation of reconstructed phase space of waiting time distribution, with noise reduction and surrogate testing methods. moreover the lempel-ziv algorithmic complexity measure (lzc) was calculated. general dynamics of earthquakes’ temporal distribution around the test area, reveals properties of low dimensional non linearity. strong em discharges lead to the increase in extent of regularity in earthquakes temporal distribution. after cessation of em experiments the earthquakes’ temporal distribution becomes much more random than before experiments. to avoid non valid conclusions several tests were applied to our data set: differentiation of the time series was applied to check results not affected by non stationarity; the surrogate data approach was followed to reject the hypothesis that dynamics belongs to the colored noise type. small earthquakes, below completeness threshold, were added to the analysis to check results robustness. mailing address: prof. tamaz chelidze, institute of geophysics, georgian academy of sciences, alexidze street 1, 380093 tbilisi, georgia; e-mail: chelidze@ig.acnet.ge 962 tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi struction and fractal dimension of the attractor help to distinguish between a purely random process and a complicated process driven by a finite, limited set of rules. this was allowed by the recognition that non linearity can produce complex dynamic behavior in systems driven by a finite number of factors. the enormous gap between «simple» linear deterministic models and random, complicated and strongly unpredictable processes seems to be filled with these new analytical tools. the aim is to render tractable, in a certain way, phenomena and data otherwise not clearly depicted. the present work investigated the influence of strong em discharges on earthquakes temporal distribution. experiments on the triggering effect of mhd (magneto-hydro-dynamic) soundings on the microseismic activity of the region were performed in 1975-1996 by ivtan (institute of high temperatures of russian academy of sciences) in the central asia test area (tarasov, 1997; tarasov et al., 1999; jones, 2001). during these experiments deep electrical sounding of the crust was carried out at the bishkek test site in 1983 to 1989. the source of electrical energy was the mhd generator, and a load was an electrical dipole of 0.4 ω resistance with electrodes located at a distance of 4.5 km from each other. when the generator was fired, the load current was 0.28-2.8 ka, the sounding pulses had durations of 1.7 to 12.1 s, and the energy generated was mostly in the range of 1.223.1 mj (volykhin, 1993). evidence of some relationships between em discharges and seismic activity have been pointed out under a statistical aspect and in a time range of days after em experiments (tarasov, 1997). here the general dynamical aspect is considered. a good seismic catalogue of the area is available well before, during and well after this period. a simple causal relationship between the two processes is not strongly evident. relations appear to be present but data noise is also relevant. it is useful to investigate if the seismic dynamics, in periods before, during and after em experiments, is influenced by the introduction of strong electric current into the ground. 2. methods the investigation was performed according to the general scheme of time series non-linear analysis (abarbanel et al., 1993; sprott and rowlands, 1995; kantz and schreiber, 1997; goltz, 1998; hegger and kantz, 1999). in general data analysis can be performed firstly under a more qualitative and visual approach and subsequently a more quantitative methodology can be applied. the qualitative approach includes visual inspection of reconstructed phase space. namely, p-dimensional phase space from the scalar time sequences was reconstructed by the method of time delay (packard et al., 1980; takens, 1981). according to takens’ theorem it is possible to catch the essential dynamic properties of a system by a reconstruction of its phase space by only one variable. twoand three-dimensional phase space portraits encapsulating essential dynamic properties of the analyzed complex process were used as qualitative tests. also other qualitative tools have been used such as iterated function systems (ifs) (jeffrey, 1992) and recurrence plots (rp) (eckman et al., 1987). generally recurrence analysis is a graphical method designed to locate hidden recurring patterns and structure in time series. the rp is defined as (2.1) here εi is a cut-off distance (often ε = 0.1σ, σstandard deviation), θ(x) is the heaviside function, θ(x) = 0 if x < 0 and θ(x) = 1 if x ≥ 0. according to eckman the values one and zero in this matrix commonly are visualized as black and white (eckman et al., 1987). the black points indicate the recurrences of the investigated dynamic system revealing their hidden regular and clustering properties. by the definition, rp has a black main diagonal (line of identity) formed by distances in the matrix compared with themselves. in order to understand rp it should be stressed that it visualizes the distance matrix which represents autocorrelation in the series at all possible time (distance) scales. as far as distances are computed for all possible pairs, on the rp plots elements near the diagonal correspond to short range correlation, whereas r x x,i j i i jεθ= − −_ i influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... the long range correlations are revealed by the points distant from the diagonal. hence if the analyzed dynamics (time series) is deterministic (ordered, regular), then the recurrence plot shows short line segments parallel to the main diagonal. qualitative patterns of unknown dynamics presented as fine structure of rp are often are too difficult to be considered in detail. therefore one uses a modern quantitative method of analysis of complex dynamics for rp approach (recurrence quantitative analysis or rqa) (webber and zbilut, 1992). rqa is especially useful to overcome the difficulties often encountered dealing with non-stationary and rather short real data sets. as a quantitative tool of complex dynamics analysis rqa defines several measures mostly based on diagonally oriented lines in the recurrence plots: recurrence rate, determinism, maximal length of diagonal structures, entropy, trend, etc. in the present work recurrence rate − rr(t) and determinism − det(t) measures based on analysis of diagonal oriented lines in the recurrence plot have been calculated (weber and zbilut, 1994; marwan et al., 2002). generally the recurrence rate rr(t) is the ratio of all recurrent states (recurrence points) to all possible states and is therefore the probability of the recurrence of a certain state. stochastic behavior causes very short diagonals, whereas deterministic behavior causes longer diagonals. the ratio of recurrence points forming diagonal structures to all recurrence points is called the determinism det(t). det(t) is the proportion of recurrence points forming long diagonal structures of all recurrence points. again, stochastic and heavily fluctuating data cause none or only short diagonals, whereas deterministic systems cause longer diagonals. an iterated function system (ifs) is a hutchinson operator on kk (finite set of functions moving points around in some space), which maps a set of points to another set of points. if a hutchinson operator is repeatedly applied to a compact set, in the limit it will render the unique attractor of the ifs (peitgen et al., 1992). for time series analysis purpose ifs attractors can be used as qualitative measures of self similarity of analyzed dynamics (more order is in time series more regular are structures in ifs attractor). we use ifs as an additional qualitative tool for detection of hidden structure in analyzed time series (sprott and rowlands, 1995). these tests enables a first qualitative visual inspection of unknown dynamics and help to uncover general properties of analyzed process. qualitative analysis reveals the possible existence of specific attractors, e.g., strange ones which point to the deterministic chaotic behavior. as the main tool for quantitative analysis of earthquakes dynamics, correlation integral calculation of reconstructed phase space of temporal distribution has been performed (abarbanel et al., 1993; kantz and schreiber, 1997; hegger and kantz, 1999). this approach is based on idea of correlation sum. correlation sum c(r) of a set of points in the vector space is defined as the fraction of all possible pairs of points which are closer to each other than a given distance r. the basic formula useful for practical application is (2.2) where again θ(x) is the heaviside step function, ; x i − x j; is the euclidian norm, i = j are excluded. for fractal systems for enough long time series and for a small r, c(r)∝r ν relationship is correct. commonly such dependence is correct only for the restricted range of r values, so called scaling region. correlation dimension ν or d2 is defined as . (2.3) in practice d2 value is found from the slopes of logc(r)/log(r) curves for different phase space dimensions. the correlation dimension of an unknown process is the saturation value of d2, which does not change by increasing of phase space dimension. in order to reduce possible spurious conclusions on considered dynamics, noise reduction and surrogate testing methodologies were used (kantz and schreiber, 1997; hegger and kantz, 1999). besides, as additional quantitative test for relatively short time series, the lempel-ziv algorithmic complexity measure (lzc) was calculated ( ) ( ) lim og log l d r c r r 2 0 ν = = " ( ) ( ) c r n n r x x 1 2 i j j i n i n 11 θ= − − − = += _ i// 963 964 tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi (lempel and ziv, 1976). it is based on the transformation of the original one dimensional time series into a finite symbol sequence and is defined as (2.4) where n is the length of the original time series, l (n) ∼ nw (n)(logbnw (n )+1) is the total length of the encoded sequence, where nw (n ) ≤ n is the total number of code words. being one of the tools for time series non-linear analysis, lzc is ( ) lim supc n l n lz n = "3 especially suitable for relatively short real data sets because it is not so demanding to the time series length as other methods (zhang and thakor, 1999; matcharashvili and janiashvili 2001). 3. data and analysis in the present study non-linear analysis was performed on about 14100 time intervals (in seconds) between earthquakes from 1975 to 1996. in the original catalogue, the energy of fig. 1a,b. results of the completeness analysis of the ivtan site seismic catalogue. a) cumulative number of events versus time for magnitude class step = 0.5 for 0.< m > 3.0. note that cumulative number of events is rescaled among magnitude classes. b) log cumulative number of earthquake versus magnitude (gutenbergrichter relation) for the whole catalogue; values of regression fit equation: y = − 0.83∗x + 5.40. coefficient of determination, r-squared = 0.995. a b influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... the events was expressed as energy class, which we converted to magnitude using the following relation (riznichenko, 1985) (3.1) where m is magnitude and e is the energy class. the completeness of the catalogue was investigated first by considering the realization of the gutenberg-richter relationship at low magnitudes: departure from a straight line was interpreted as a lack of completeness at low magnitudes. as a result the catalogue was considered complete, under the sole magnitude aspect, for m ≥ 1.7. the gutenberg-richter b-value was found to be equal to 0.83 with a reasonably good fit. earthquakes with magnitude higher than 6 seem to show behavior typical of characteristic events. a second test was oriented to check the time completeness. as is well known, a catalogue’s completeness changes with time, usually as a result of improving seismic-network performance (e.g., increasing number of stations), leading to greater magnitude sensitivity. the completeness analysis was performed by employing the method of mulargia et al. (1987). the method consists in separating all events into magnitude classes and plotting separately the cumulative number of events versus time. assuming that during the considered time interval seismicity had a constant rate, the flat behavior at the beginning of the time period may be due to a lack of data; this is normal for low-magnitude ranges. only for magnitudes higher than 2.0 is our catalogue complete over the entire time period (number of earthquakes n = 5297). if a lower magnitude limit is desired, the time period from year 1980 is more appropriate (fig. 1a,b). as a result of the analysis performed, a relatively complete catalogue was obtained with a lower magnitude threshold of 1.7 from the year 1980. for the present study the catalogue was analyzed under the time aspect, specifically on inter-event (waiting) times. a catalogue subset of waiting times was used according to completeness analysis, i.e. all time spans and m > 2.0. then all data used were selected by the same procedure for confirmation of results and to test their robustness. m e 18 4 = − 4. results and discussion figures 2a-f presents the results of qualitative analysis of waiting times sequences above the mentioned threshold. namely in fig. 2a,c,e, ifs clumpiness test (jeffrey, 1992; sprott and rowlands 1995) and in fig. 2b,d,f, recurrence plot analysis (zbilut and webber, 1992) results reveal that after the beginning of the experiments some structure in plots is visible, pointing to the increased amount of functional interdependence in earthquake temporal distribution. as to the quantitative approach, the variation of correlation dimension versus dimension of phase space where reconstructed dynamics is embedded (embedding dimension) is presented in fig. 3. the integral time series (5297 time intervals) for the whole period of observation (1975-1996) containing time intervals sequences between all events above the threshold reveals clear low correlation dimension (d2 = 1.22 ± 0.43, asterisks). shorter time series also were considered. namely 1760 waiting times data before (1975-1983), 1953 waiting times during mhd experiments (1983-1988) and 1584 waiting times of the period after experiments (1988-1992). time series before and especially during mhd experiments also have low correlation dimension (d2 < 5). namely d2=3.83± 0.80 before and d2 = = 1.04 ± 0.35 during experiments. on the other hand, opposite to what was mentioned above, after cessation of experiments (fig. 3, triangles) correlation dimension of waiting times sequences noticeable increases (d2 > 5.0), exceeding low dimensional threshold (d2=5.0). that means that after termination of experiments the extent of regularity or extent of determinism in process of earthquake temporal distribution decreases. the considered process becomes much more random both qualitatively (fig. 2e,f) and quantitatively (fig. 3. triangles). for clarity in fig. 3, results for random number sequence are shown too (diamonds). the found low correlation dimension of waiting interval time series is in good accordance with earlier published results for caucasus (matcharashvili et al., 2000) as well as with goltz’s results (goltz, 1998) for other seismic active regions. this result together with qualitative analysis results shown in fig. 2a-f provide evidence that 965 966 tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi fig. 2a-f. qualitative analysis of temporal distribution of earthquakes (complete catalogue, m ≥ 1.7) before the beginning of em experiments (1975-1983), during experiments (1983-1988) and after accomplishing of experiments (1988-1992). ifs-clumpiness test for inter-event time interval sequences: a) before experiments; c) during experiments; e) after experiments. recurrence plots analysis of waiting times sequences: b) before experiments; d) during experiments; f) after experiments. note diagonal lines in ifs plot and compact structure in rp during experiments. after the beginning of em discharges the temporal distribution of earthquakes around ivtan test area becomes more regular, or events of corresponding time series become functionally much more interdependent. at the same time the absence of typical phase space structure (not shown here), ifs and recurrence plot attractors (fig. 2a-f) do not allow us to consider the process deterministically chaotic. a c e b d f influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... in order to the reduce effects of possible noise we analyzed waiting time series after a noise reduction procedure (shreiber, 1993; kantz and schreiber, 1997). namely, we used the methodology of non-linear noise reduction (which in fact is phase space non-linear filtering) instead of common linear filtering procedures. the latter, as is well known, may lead to destroying the original non-linear structure of analyzed complex processes (hegger and kantz 1999; schreiber and schmitz, 2000). non-linear noise reduction relies on the exploration of a reconstructed phase space of considered dynamic process instead of frequency information of linear filters (schreiber, 1993; kantz and schreiber, 1997; hegger and kantz, 1999). correlation dimension versus embedding space dimension of noise-reduced time series is presented in fig. 4. as follows from the obtained results, correlation dimensions are not essentially affected by unavoidable noises. therefore obtained results assure that the differences found in d2-phase space dimension (p) dependence before, during and after experiments (fig. 3) are indeed related to dynamic changes in temporal distribution of earthquakes after beginning of mhd discharges experiments. when describing unknown dynamics of waiting times fluctuation, the differentiation of original time series can be useful to avoid improper conclusions related to the effects of trends or nonstationarity in data sets, even when those are not clearly visible (as in the case of considered time series) (goltz, 1998). as shown in fig. 5, differentiation of our time series according to goltz (1998) does not lead to significant changes in obtained results (see fig. 3). so our results could not be affected by trends or non-stationarity in used data sets. 967 fig. 3. correlation dimension versus embedding dimension of waiting times sequences (complete catalogue): asterisks – integral time series (1975-1996); circles – before beginning of experiment (1975-1983); squares – during experiments (1983-1988); triangles – after experiments (1988-1992); diamonds correspond to random number sequence. fig. 4. correlation dimension versus embedding dimension of waiting times sequences (complete catalogue) after noise reduction: diamonds – before experiments; squares – during experiments; triangles – after experiments. fig. 5. correlation dimension versus embedding dimension of differenced waiting times sequences (complete catalogue): diamonds – before experiments; squares – during experiments (1983-1988); triangles – after experiments. 968968 analysis of different time series may be important also in the sense of inherent dynamic structure testing (prichard et al., 1994). namely, the test is based on finding that estimated non-linear measure (correlation dimension in our case) for the differentiated series is larger than that estimated for original data, if the structure of their dynamics is caused by the linear stochasticity. at the same time for chaotic (low dimensional) processes these measures are the same. from this point of view analysis of differentiated time series before detailed surrogate testing provides the first additional evidence that variation of waiting times indeed has inherent non-linear structure and that their dynamic properties are not caused by linear relationships between data. indeed, the curves of figs. 3 and 5 are characterized by comparable values of correlation dimension. moreover, in order to have a basis for more reasonable rejection of spurious conclusions caused by possible linear correlations in considered data sets, we have used the surrogate data approach to test the null hypothesis that our time series are generated by a linear stochastic process (theiler et al., 1992; rapp et al., 1993, 1994; kantz and schreiber, 1997). in other words we would like to reject correctly the possibility that revealed dynamics belongs to the colored noise type. namely, random phase (rp) and gaussian scaled random phase (gsrp) surrogates sets for waiting times series were used (matcharashvili et al., 2000). rpsurrogate sets are obtained by destroying the non-linear structure through randomization of phases of fourier transform of original time series and then performing a backward transformation. gsrp surrogate sets were generated in a three-step procedure. at first a gaussian set of random numbers was generated, which has the same rank structure as the original time series. after this phase randomised surrogates of these gaussian sets were constructed. finally the rank structure of the original time series was reordered according to the rank structure of the phase randomized gaussian set (theiler, 1992). figure 6a,b shows the results of surrogate testing of waiting time sequences before and during experiments using d2 as a discriminating metric. for each of our data sequences, we generated 75 of rp and gsrp surrogates. there are several ways to measure the difference between the discriminating metric measure of original (given by morig) and surrogate (given by msurr), time series (rapp, 1994). investigators often use the significance criterion s=⎪〈 msurr〉 − m orig⎪/σsurr, where σsurr is standard deviation of msurr (theiler et al., 1992). the significance criterion s, according to theiler et al. (1992) for analyzed time series before experiments is significant: 22.4 ± 0.2 for rp and 5.1 ± 0.7 for gsrp surrogates. consequently after the beginning of experiments the null hypothesis that original time series is the linearly correlated noise was rejected with significant value of s criterion: 39.7 ± 0.8 for rp and 6.0 ± 0.5 for gsrp surrogates. fig. 6a,b. correlation dimension versus embedding dimension of original (diamonds) and surrogate (squares – gsrp, triangles – rp) waiting time sequences: a) before beginning of experiments, b) during experiments. a b tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi 969 influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... 969 these results can be considered strong enough evidence that analyzed time series are not linear stochastic noise and that corresponding processes of earthquakes temporal distribution before and especially during experiments are characterized by inherent low dimensional non-linear structure. according to the ivtan catalogue, each considered time period contains one large (m ≈ 6.1-6.3) earthquake. therefore in order to refine whether the above results are caused by special properties of a separate large earthquake or reflect total changes in dynamics caused by em discharges, we analyzed waiting time sefig. 7a-f. qualitative analysis of 1000 data waiting times sequences (complete catalogue), after largest events before beginning of em experiments (1975-1983), during experiments (1983-1988) and after accomplishing of experiments (1988-1992). ifs-clumpiness test for inter-event time interval sequences: a) before experiments; c) during experiments; e) after experiments. recurrence plots analysis of waiting times sequences: b) before experiments; d) during experiments; f) after experiments. a b d f c e 970 quences (above appropriate threshold) after each largest event. namely, 1000 consecutive waiting time intervals after 24/03/1978 m=6.1 (k = 15.0), 24/01/1987 m=6.3 (k = 15.3) and 798 time intervals after 30/12/1993 m=6.1 (k = 15.0) events were analyzed. it is important to say, that each of these relatively short time series are localized in corresponding time periods named above as «before», «during» and «after» experiments. it becomes clear from ifs-clumpiness and rqa analysis results (fig. 7a-f) that qualitatively this situation is like that shown in fig. 2a-f, i.e. after the beginning of experiments, considered dynamics become more regular and, after accomplishing experiments, dynamics is most random-like. quantitatively, it is shown in fig. 8 that these short time series generally reveal that after experiments analyzed dynamics also become more random than before. some differences are noticeable in saturation values of correlation dimension (in fig. 8) before (circles, d2 = 3.1 ± 0.4) and during (squares, d2 = 2.1 ± 0.7) experiments. the latter may be caused by too limited data length for proper non-linear analysis of these time series (untypical shape of curve at high fig. 8. correlation dimension versus embedding dimension of 1000 data waiting times sequences (complete catalogue) after largest events: circles – time period before beginning of experiments (19751983); squares – time period during experiments (1983-1988); triangles – time period after accomplishing of experiments (1988-1992). tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi embedding dimensions) as well as by artificially increased fraction of aftershocks in short time series which contains events only after the largest earthquakes. in any case, our main conclusion about low dimensional dynamical structure of earthquake temporal distribution during experiments and increasing randomness after termination remains valid even for periods of separate large earthquakes. the above conclusion on the increase in regularity in earthquakes’ temporal distribution after the beginning of experiments in some degree is confirmed also by the results of lempel-ziv algorithmic complexity (clz) measure calculation (lempel and ziv, 1976). indeed clz is larger when necessary code words are longer i.e. when regular patterns of analyzed time series are minor. indeed, measured values of lempel-ziv complexity before, during and after experiments for original time series above threshold consequently are clz = 0.99 ± 0.07; clz = 0.87± ± 0.05; clz = 1.00 ± 0.08. quantitative rqa results also lead to the same conclusion: namely rr(t) = 9.6, det(t) = = 3.9 before experiments, rr(t) = 25, det(t) = = 18 during and rr(t) = 3, det(t) = 1.5 after experiments. the increase in order in earthquake temporal distribution under em influence is confirmed for short time interval sequences above threshold after the largest earthquakes. indeed lempel-ziv complexity measure values were: clz = 0.98 ± ± 0.08; clz = 0.74 ± 0.05; clz = 1.00 ± 0.09 before, during and after mhd runs consequently (note that clz = 0.04 for periodic and clz = 1 for random processes). also, the increase in order in temporal distribution is documented by rqa results for mentioned short time series; namely rr(t) = 9.8, det(t) = 6.5 before experiments, rr(t) = 19.5, det(t) = 49.3 during and rr(t) = = 7.1, det(t) = 1.6 after experiments. in other words for situations where the shape of d2 (fig. 8), is not informative for finding changes in dynamics possibly due to too short time series, lempel-ziv and rqa analysis reveals the increase in regularity. on the basis of results of previous research it is known that, small earthquakes play a very important role in the general dynamics of earth971 influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... quake temporal distribution (matcharashvili et al., 2000). therefore, additionally we carried out an analysis of time series containing all available from the whole catalogue waiting time sequences, including those between small earthquakes below magnitude threshold. this test is also valid to check results robustness in case of adding a new not necessarily complete set of data to our original set. the total number of events in the whole catalogue increased up to fig. 9a-f. qualitative analysis of temporal distribution of earthquakes including small events (whole catalogue, all events) before beginning of em experiments (1975-1983), during experiments (1983-1988) and after accomplishing of experiments (1988-1992). ifs-clumpiness test for waiting times sequences: a) before experiments; c) during experiments; e) after experiments. recurrence plots analysis of inter-event time interval sequences: b) before experiments; d) during experiments; f) after experiments. a c e b d f 972 tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi 14100, while in the complete catalogue for the three above-mentioned periods (before, during and after mhd experiments) there were about 4000 data in each one. results of ifs and recurrence plots tests analysis of these time series are shown in fig. 9a-f. noticeable qualitative differences in waiting time intervals distribution dynamics during as well as after accomplishment of experiments is obvious. figure 10 presents the results of correlation dimension calculation for these time series. practically there are no differences from results obtained for the case with m > 2.0 (fig. 3). namely, according to fig. 10, the integral time series (14100 time intervals) for whole period of observation (1975-1996) reveals clear low correlation dimension (d2 = 2.40 ± 0.71) (diamonds). the time series before beginning of experiment (squares) is characterized by correlation dimension (d2 = 3.50 ± 0.63) which is still below accepted low dimensional threshold (d2 = 5.0). during experiments (fig. 10, triangles) correlation dimension of time interval sequence, in comparison with the situation before, noticeably decreases (d2 = 1.71 ± 0.09). after termination of experiments the correlation dimension of waiting time interval sequences noticeably increases (d2 > 5.0), exceeding low dimensional threshold (d2 = 5.0). as in the case of the complete catalogue, it means that after termination of experiments the extent of determinism in the process of earthquake temporal distribution decreases. the considered process becomes much more random both qualitatively (fig. 9c,f) and quantitatively (fig. 10 circles). both the complete and whole catalogues of waiting time sequences reveal low dimensional non-linear structure in temporal distribution of earthquakes before and especially during experiments, which was confirmed by 70 surrogate testing analyses (fig. 11a,b). the significance criterion s for analyzed time series before experiments gives: 32.3 ± 0.2 for rp and 5.3 ± 0.6 for gsrp surrogates; consequently after the beginning of experiments the null hypothesis that fig. 10. correlation dimension versus embedding dimension of waiting times sequences of the whole catalogue: diamonds – integral time series (19751996); squares – before beginning of experiment (1975-1983); triangles – during experiments (19831988); circles – after experiments (1988-1992). fig. 11a,b. correlation dimension versus embedding dimension of original waiting time sequences of whole catalogue (triangles) and their surrogate (circles-gsrp, squares-rp): a) before beginning of experiments; b) during experiments. a b 973 influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... the original time series is the linearly correlated noise was rejected with significant value of s criterion: 46.2 ± 0.5 for rp and 6.5 ± 0.7 for gsrp surrogates. the correlation dimension versus embedding space dimension of noise-reduced time series of the whole catalogue is presented in fig. 12. it is clear from this picture that calculated values of correlation dimension are not affected by noises as for the complete catalogue. obtained results show that differences found in the d2-phase space dimension (p) relationship before and during experiments in both catalogues are indeed caused by dynamic changes in temporal distribution of earthquakes during em experiments. we also analyzed waiting time sequences after each of the largest (m ≈ 6.1-6.3) events for the whole catalogue, namely, 1000 consecutive waiting time sequences after 24/03/1978 m = = 6.1 (k=15.0), 24/01/1987 m = 6.3 (k=15.3) and 30/12/1993 m = 6.1 (k=15.0) event. as is shown in fig. 13, these short time series generally reveal dynamic characteristics similar to those of the time series obtained from the complete catalogue. found differences which are noticeable in saturation values of correlation dimension before (circles, d2 = 2.0 ± 1.1 in fig. 13) and during (squares, d2 = 3.2 ± 0.8, fig. 13) experiments may be caused both by shortness of these time series or by influence of increased fraction of aftershocks. thus, conclusions concerning the influence of hot and cold em runs on the general characteristics of earthquakes temporal distribution dynamics remain valid for small earthquakes too. it is interesting to note that on the laboratory scale the effect of triggering and synchronization of acoustic emission during slip imposed by strong em field is well documented in numerous experiments (chelidze et al., 2002, 2005; chelidze and lursmanashvili, 2003). 5. conclusions the question whether electromagnetic experiments on a specific site can influence the dynamics of a seismic region is a complex argument. a complete answer to it, if any could be given, would involve a repeated set of analyses for different seismic regions over large periods of time with and without em experiments. a theoretical explanation showing the cause and effect relationships between the two phenomena is also fundamental. this paper addressed the question under statistical aspect involving nonfig. 12. correlation dimension versus embedding dimension of inter-event time interval sequences of whole catalogue after noise reduction: diamonds – before experiments; squares – during experiments; triangles – after experiments. fig. 13. correlation dimension versus embedding dimension of 1000 data waiting times sequences of the whole catalogue after largest events: circles – time period before beginning of experiments (19751983); squares – time period during experiments (1983-1988); triangles – time period after accomplishing of experiments (1988-1992). 974 tamaz chelidze, valerio de rubeis, teimuraz matcharashvili and patrizia tosi linear dynamics methods. these methods were chosen because there are not trivial, simple and direct relations between the two phenomena: this means that relations are of a complicated order. moreover seismicity is very probably a critical process with a per se complicated evolution: under given conditions possible relations must not be direct and simple. with non-linear methods the time evolution of seismicity was investigated looking at relations with em experiments. waiting times constitute the aspect analyzed. the whole time period was divided into three parts, the middle being the one when em experiments took place. phase space attractor, reconstructed with delay time technique, for the whole time period shows low correlation dimension values; this indicates, at least, the presence of a few processes driving seismicity. the same analysis on the three sub catalogues confirms the result, with the exception for the period after the em experiments: strong em discharges lead to the increase in the extent of regularity in earthquakes’ temporal distribution while, after cessation of em influence, earthquakes’ temporal distribution becomes much more random than before experiments. this is the main result of the analysis and it was confirmed changing the conditions of the analysis itself. non-linear noise reduced time series confirmed such results as did surrogate testing. the middle period contains a large seismic event (january 24, 1987 m = 6.3 derived from energy class k=15.3) this event has certainly a well identified aftershocks activity and this can be a strong factor influencing time dynamics. the root question is: is this event with its related sequence responsible for the change in the dynamics of analyzed data? if the answer is yes we are forced to answer immediately the new question if this earthquake is related to em experiments? but it must be noted that inside the other two periods there are also important events of comparable magnitudes and the analysis was conducted on the three sequences of catalogue after each strong event separately. general results confirmation was shown. the same results were revealed with the use of the whole catalogue regardless of the completeness criteria. this analysis is not certainly exhaustive: the seismic catalogue covers a broad area and all complete data were used, no distinction was made for space location of seismic events. the energy aspect was not fully considered: all events were considered equal regardless of their magnitude. these are strong simplifications and results must be considered under these constraints. however results appear to be consistent: em experiments influence seismic time dynamics to some extent, increasing the regularity of waiting times. after em experiments seismic waiting times increase their random character to a level higher than before experiments. references abarbanel, h.d., r. brown, j.j. sidorovich and l.sh. tsimring (1993): the analysis of observed chaotic data in physical systems, rev. mod. phys., 65 (4), 13311392. chelidze, t. and o. lursmanashvili (2003): electromagnetic and mechanical control of slip: laboratory experiments with slider system, non-linear processes geophys., 20, 1-8. chelidze, t., n. varamashvili, m. devidze, z. chelidze, v. chikladze and t. matcharashvili (2002): laboratory study of electromagnetic initiation of slip, ann. geophysics, 45 (5), 587-598. chelidze, t., t. matcharashvili, j. gogiashvili, o. lursmanashvili and m. devidze (2005): phase synchronization of slip in laboratory slider system, nonlinear processes geophys., 12, 1-8. de rubeis, v., p. dimitriu, e. papadimitriou and p. tosi (1993): recurrent patterns in the spatial behaviour of italian seismicity revealed by the fractal approach, geophys. res. lett., 20, 1911-1914. eckmann, j.p., s. kamphorst and d. ruelle (1987): recurrence plots of dynamical systems, europhys. lett., 4 (9), 973-977. geller, r.j., d.d. jackson, y.y. kagan and f. mulargia (1997): earthquakes cannot be predicted, sciences, 275, 1616-1617. goltz, c. 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(1999): is the reliable prediction of individual earthquakes a realistic scientific goal?, in nature debates (available on line at http://www.nature.com/nature/ debates/earthquake/equake_frameset.html). marwan, n., n. wessel, u. meyerfeldt, a. schirdewan 975 influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution ... and j. kurths (2002): recurrence-plot-based measures of complexity and their application to heart rate variability data, phys. rev. e, 66, 026702.1-026702.8. matcharashvili, t. and m. janiashvili (2001): investigation of variability of indexes of myocardial contractility by complexity measure in patients with arterial hypertension, in non-linear dynamics in life and social sciences, edited by w. sulis and i. trofimova (ios press, amsterdam), 204-214. matcharashvili, t., t. chelidze and z. javakhishvili (2000): non-linear analysis of magnitude and waiting time interval sequences for earthquakes of caucasian region, non-linear processes geophys., 7, 9-19. mulargia, f., p. gasperini and s. tinti (1987): contour mapping of italian seismicity, tectonophysics, 142, 203-216. packard, n.h., j.p. crutchfield, j.d. farmer and r.s. shaw (1980): geometry from a time series, phys. rev. lett., 45, 712-716. peitgen, h.o., h. jurgens and d. saupe (1992): chaos and fractals: new frontiers of science (springer, ny). prichard, d. and j. theiler (1994): generating surrogate data time series with several simultaneously measured variables, phys. rev. lett., 73 (7), 951-1018. rapp, p.e., a.m. albano, t.i. schmah and l.a. farwell (1993): filtered noise can mimic low-dimensional chaotic attractors, phys. rev. e, 47 (4), 2289-2297. rapp, p.e., a.m. albano, i.d. zimmerman and m.a. jumenez-montero (1994): phase-randomized surrogates can produce spurious identification of non-random structure, phys. lett. a, 192 (1), 27-33. riznichenko, yu.v. 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(1992): embeddings and delays as derived from quantification of recurrence plots, phys. lett. a, 171, 199-203. zhang, x. and n.v. thakor (1999): detecting ventricular tachicardia and fibrillation by complexity measure, ieee trans. biomed. eng., 46 (5), 548-555. (received december 10, 2005; accepted march 2, 2006) correction to “moment tensor inversion of early instrumental data: application to the 1917 high tiber valley, monterchi earthquake” by f. bernardi et al. annals of geophysics, 59, 3, 2016, s0324; doi:10.4401/ag-7112 s0320 correction to “moment tensor inversion of early instrumental data: application to the 1917 high tiber valley, monterchi earthquake” by f. bernardi et al. fabrizio bernardi1,*, maria grazia ciaccio2, barbara palombo1, graziano ferrari3 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy 2 istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy 3 istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy this article corrects: f. bernardi, m.g. ciaccio, b. palombo and g. ferrari, moment tensor inversion of early instrumental data: application to the 1917 high tiber valley, monterchi earthquake, annals of geophysics, 59 (3), 2016, s0318; doi:10.4401/ag-6850. corrigenda [1] page 1, abstract, fourth last line: for “slip” read “rake”. [2] page 9, left column, third last and second to last line: for “this method has been tested on some recent earthquakes.” read “this method has been tested on some recent earthquakes and applied in vannoli et al. [2015].” [3] page 10, left column, ninth line from bottom: for “strike/dip/slip” read “strike/dip/rake”. [4] page 13, references: the following reference needs to be added: “vannoli, p., g. vannucci, f. bernardi, b. palombo and g. ferrari (2015). the source of the 30 october 1930, mw 5.8, senigallia (central italy) earthquake: a convergent solution from instrumental, macroseismic and geological data, b. seismol. soc. am., 105 (3), 1548-1561; doi:10. 1785/0120140263”. * corresponding author: fabrizio bernardi, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy; email: fabrizio.bernardi@ingv.it. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. article history received and accepted july 13, 2016. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice carcione_final:layout 6 annals of geophysics, 57, 1, 2014, g0186; doi:10.4401/ag-6324 g0186 mathematical analogies in physics. thin-layer wave theory josé m. carcione1,*, vivian grünhut2, ana osella2 1 istituto nazionale di oceanografia e di geofisica sperimentale (ogs), sgonico (trieste), italy 2 universidad de buenos aires, departamento de física ifiba conicet, buenos aires, argentina abstract field theory applies to elastodynamics, electromagnetism, quantum mechanics, gravitation and other similar fields of physics, where the basic equations describing the phenomenon are based on constitutive relations and balance equations. for instance, in elastodynamics, these are the stress-strain relations and the equations of momentum conservation (euler-newton law). in these cases, the same mathematical theory can be used, by establishing appropriate mathematical equivalences (or analogies) between material properties and field variables. for instance, the wave equation and the related mathematical developments can be used to describe anelastic and electromagnetic wave propagation, and are extensively used in quantum mechanics. in this work, we obtain the mathematical analogy for the reflection/refraction (transmission) problem of a thin layer embedded between dissimilar media, considering the presence of anisotropy and attenuation/viscosity in the viscoelastic case, conductivity in the electromagnetic case and a potential barrier in quantum physics (the tunnel effect). the analogy is mainly illustrated with geophysical examples of propagation of s (shear), p (compressional), tm (transverse-magnetic) and te (transverse-electric) waves. the tunnel effect is obtained as a special case of viscoelastic waves at normal incidence. 1. introduction the 19th century was a period when scientists had a global vision of science, making wide use of analogies between different field of physics, in particular the analogy between light and elastic waves to study the behaviour of light in matter. for instance, fresnel’s formulae and maxwell’s equations were obtained from mathematical analogies with shear wave propagation and hooke’s law, respectively [e.g., carcione and cavallini 1995, carcione and robinson 2002]. this practice dates back to the 17th century. hooke believed light to be a vibratory displacement of a medium (the ether), through which it propagates at finite speed. the role of mathematical analogies has been well illustrated and explained by tonti in a series of papers [e.g., tonti 1972, 1976]. quoting tonti [1972]: “many physical theories show formal similarities due to the existence of a common mathematical structure. this structure is independent of the physical contents of the theory and can be found in classical, relativistic and quantum theories; for discrete and continuous systems”. carcione and cavallini [1995] found analogies between vector wave files. they show that the 2-d maxwell equations describing propagation of the transverse-magnetic mode in anisotropic media is mathematically equivalent to the sh wave equation in an anisotropic-viscoelastic solid where attenuation is described with the maxwell model. later, carcione and robinson [2002] establish the analogy for the reflection-transmission problem at a single interface, showing that contrasts in compressibility yields the reflection coefficient for light polarized perpendicular to the plane of incidence (fresnel’s sine law the electric vector perpendicular to the plane of incidence), and density contrasts yields the reflection coefficient for light polarized in the plane of incidence (fresnel’s tangent law). these two papers provide the time-domain differentials equations and the analysis for a single interface, while in the present paper, we exploit the analogy for a layer embedded between two dissimilar half spaces, showing how the same equation hold for shear, compressional and electromagnetic waves, in addition to the quantum tunnel effect. in 1821, fresnel obtained the wave surface of an optically biaxial crystal, assuming that light waves are vibrations of the ether in which longitudinal vibrations (p waves) do not propagate. an anisotropic elastic medium having orthorhombic symmetry, mathematically analogous to fresnel’s crystal, exists. this medium was found by carcione and helbig [2008], who obtained the differential equations describing the wave motion. in analytical terms, the above mentioned papers deal with a homogeneous medium and a single interface. the next step to exploit the analogy is to study the wave rearticle history received april 8, 2013; accepted october 14, 2013. subject classification: thin layer, electromagnetism, viscoelasticity, reflection and transmission coefficient, geophysics, quantum mechanics. sponse of a layer, namely the reflection and transmission of waves by a thin plane layer embedded in a homogeneous medium. the reflection coefficient for isotropic media is given in equation (5.18) of brekhovskikh [1960] and section 1.6.4. (equation 57) of born and wolf [1964]. these authors report the expressions for the te case. here, we mainly consider the tm case. te waves are equivalent to acoustic (p) waves propagating in liquids, while tm waves are equivalent to shear (sh) waves propagating in solids [carcione and robinson 2002]. moreover, the equation in born and wolf [1964] is given for purely dielectric media but the lossy version can be obtained by replacing the real dielectric constant by the complex permittivity. an equivalent viscoelastic model can be obtained in the elastic case, using the maxwell model. here, we consider the media to be anisotropic with one of the principal axes parallel to the layering. in geophysics, the problem finds application in reflection seismology. widess [1973] and bakke and ursin [1998] consider the normal incidence case of a thin layer, while liu and schmitt [2003] obtain the reflection coefficient as a function of the incidence angle corresponding to p waves. bradford and deeds [2006] and deparis and garambois [2009] consider the electromagnetic case applied to amplitude variations with the incidence angle for surface radar problems. in quantum mechanics, the same mathematical approach can be used to analyze the tunnel effect [anderson 1971]. we choose to present geophysical examples mainly. the analogy is applied to horizontally polarized shear (sh) waves through a fluid-filled fracture and tm waves through thin metal films, which are shown to be similar to the tunnel effect. moreover, other examples consider p and sh seismic waves reflected from a thin and lossy sandstone bed and tm waves reflected from a dyke of quartz. finally, we compare the tm and te cases by using the analogy. 2. reflection and transmission coefficients of a thin layer. viscoelasticity we start from the viscoelastic equations and then apply the analogy to obtain the equivalent electromagnetic and quantum expressions. figure 1 shows the layer embedded between two half spaces with different properties. in the following, we denote particle velocity by v, stress by v, magnetic field by h, electric field by e, density by t, elasticity constant by c, viscosity by h, magnetic permeability by n, dielectric permittivity by e and electrical conductivity by v. moreover, (x, y, z) indicates the spatial variables, ^x a partial derivative with respect to x and a dot above a variable denotes time differentiation. to distinguish between the stress and conductivity components, we use letters and numbers as subindices, respectively, e.g., vxy is a stress component and v11 is a conductivity component. the viscoelastic medium is characterized by the mass density t and elasticity and viscosity matrices (1) respectively [carcione and cavallini 1995, carcione 2007]. it corresponds to a viscoelastic medium described by the maxwell mechanical model. the subindices “44” and “66” refer to the voigt notation of the elasticity and viscosity tensors [e.g., carcione 2007]. the differential equations describing the wave motion of sh (shear) waves are (2) the boundary conditions at the interfaces require continuity of [e.g., carcione 2007]. in the electromagnetic case, continuity of the tangential components of the electric and magnetic fields is required [born and wolf 1964] (see below). let us assume that the incident, reflected and refracted waves are identified by the subscripts and superscripts i, r and t. for a single interface, the particle velocities of the incident, reflected and refracted waves are given by and , c c0 0 0 044 66 44 66 h h c cm m and vyz yv i i , i , ,exp exp exp v v r v t s x s z t s x s z t s x s zt y i y y r t x z x z r x z t ~ ~ ~ = = = ^ ^ ^ h h h 6 6 6 @ @ @ , . , v v c v c x xy z yz y z y yz y xy xyx yz44 1 44 1 66 1 66 1 2 2 2 2 v v t h v v h v v + = == + o o o carcione et al. 2 figure 1. plane wave propagating through a layer. the viscoelastic properties are indicated. (3) (4) 3 respectively, where is the slowness vector, ~ is the angular frequency, r and t are the reflection and refraction (transmission) coefficients, t is the time variable and i = . the equations obtained below hold for an incident inhomogeneous plane waves (non-uniform waves in electromagnetism), i.e., waves for which the wavenumber and attenuation vectors do not point in the same direction. in the special case where these two vector coincide, the wave is termed homogeneous (uniform in electromagnetism), and we have where i is the incidence angle and is the complex velocity, with [carcione and cavallini 1995, carcione 2007]. in the general case, the reflection and transmission coefficients (tm case in electromagnetism) are given by where with and with “pv” denoting the principal value [carcione and robinson 2002, carcione 2007]. to obtain the reflection and transmission coefficients of the layer, we follow the procedure indicated in section 6.4 of carcione [2007]. at depth z in the layer, the particle-velocity field is a superposition of upgoing and downgoing waves of the form where v− and v+ are upgoingand downgoing-wave amplitudes. combining equations (3) and (12), the normal stress component is omitting the phase exp[i~(t − sxx)], the particlevelocity/stress vector can be written as then, the fields at z = 0 and z = h are related by the following equation: where equation (15) plays the role of a boundary condition. note that when h = 0, b is the identity matrix. using equations (2), (4) and (9), the particle-velocity/stress field at z = 0 and z = h can be expressed as where and r and t are here the reflection and transmission coefficients of the layer. substituting equation (17) into (16), we have if h = 0, { = 0 and we obtain r = (zi − z"t)/(zi + z"t) as expected. if, moreover, the two media have the same properties, it is z"t = zi and the reflection coefficient is nil. equation (19) coincides with equation (2) of bradford and deeds [2006] in the absence of wave loss. these authors used the convention exp(−i~t), instead of exp(i~t) used here, so that the sign of the frequency has to be changed in equation (2). if the upper and lower half spaces have the same properties, we get sin cos v p p66 2 2 44 t i i = + i iandp c p c 1 1 1 1 44 44 1 66 66 1 ~h ~h = + = + -44 66 ` `j j , 2 ,r z z z z z z z i t i t i t i x= + = +l l l , ,z p s z p si z t z t 44 44= =l l l snell’s law ,s s s s s sx r x x x r z t i z= = = =-^ h pv ,s p s 1 z t x 44 66 2t t= l l ll i i i , exp exp exp v z v s z v s z t s x y z t z t x ~ ~ ~ = + + + l l ^ ^ ^ ^ h h h h 6 6@ @ i i i i . exp exp exp z p v z v s z v s z t s x yz z y t z t z t x 44 2v ~ ~ ~ ~ = = + l l l l ^ ^ ^ ^ h h h h 6 6@ @ i i i i . t t exp exp exp exp z v s z s z z s z z s z v v z v v y yz z t z t t z t t z t / / v ~ ~ ~ ~ = = = + + l l l l l l ^ c ^ ^ ^ ^e c ^ c h m h h h ho m h m ,t b t h0 $=^ ^h h , , , ,t t t z r z r h 0 1 1 1 t i = = + r r m ^ ^^ ^ ^ h hh h h pv , ,s p s z p s1z t x t z t 44 66 2 44t t= = m m m m mm m , i i . cos sin cos sin r a a a z z z z z z 1 1 t i t t t t { { { { = + = + + l m l l mc m , , ,sin coss s v 1 x z i i= r r^ ^h h 12i 2i , exp exp r r r 1 1 2 { { = ^ ^ h h6 @ ( , )s sx z < i i , b t t cos sin sin cos h z z s h 0 t t z t 1 1 $ { { { { { ~ = = = l l l ^ ^ ^e h h h o thin-layer wave theory (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) where r is given by equation (8). equation (19) is similar to equation (5.22) of born and wolf [1964] if the media are isotropic and lossless. in order to be identical, this equation has to be rewritten as where equation (21) differs from equation (5.22) in the fact that r12 and r23 correspond here to tm waves, while in equation (5.22) these quantities correspond to te waves. another difference is the minus sign in the exponent, which arises from the fact that born and wolf [1964] use the factor exp(−i~t). in the case in which the media above and below the layer have the same properties, i.e., when r23 = −r12 = −r, equation (22) becomes equation (20). on the other hand, the transmission coefficient can be obtained from equation (17). it gives which can be written as this expression coincides in form with that of born and wolf ([1964], equation 58). 3. analogy with a modified acoustic wave equation carcione and robinson [2002] introduced a modified acoustic wave equation with a time-dependent mass density. denoting the particle velocities by v and the pressure f ield by p, the equations can be written as where l is the fluid compressibility, tf is the fluid density and c = 0 yields the standard acoustic equations of motion. equations (26) correspond to a generalized density of the form ci(t) + tf h(t), where h(t) is the heaviside function and i(t) is the integral operator. in the frequency domain, a complex density governs the wave equation and has the expression tf + c/(i~). this additional density term introduces wave loss as that of the biot attenuation mechanism in poroelasticity theory [e.g., carcione 2007]. equations (26) are mathematically analogous to the isotropic-medium sh equations (2) for the following correspondence where c44 = c66 = c and h44 = h66 = h. 4. analogy with electromagnetism 4.1. sh-tm analogy the analogy between sh waves and transversemagnetic (tm) waves states that the equivalent electromagnetic medium has the following permittivity and conductivity tensors respectively, describing a uniaxial absorbing crystal. the tm maxwell’s equations are the mathematical analogy is [carcione and cavallini 1995, carcione 2007], where the electromagnetic tensors are redefined here as 2 × 2 matrices for simplicity. 2i i , exp exp t r r1 12 23 12 23 { x x { = + ^ ^ h h and . z z z2 t t t 12 23x x x= = +l m l , , , v v p p v v p v v z z x x z z f z fx x x 2 2 2 2 l c t c t + == + = + o o o v v v p c xy y x z f yz 1 1 , , , , , , v v h t c t l and ,0 0 0 0 0 0 0 0 0 0 0 0 11 11 11 11 3333 e e e v v vf fp p , , . e e h h e e h e e x z z x y z y x x y zx z 11 11 33 33 2 2 2 2 n v e v e = = + = + o o o v h e e y yz xy y x z ,v v -f fp p c c0 0 0 044 1 1 66 33 11 , e e -c cm m 0 0 0 044 1 66 1 11 33 , h h v v-c cm m ,,t n 1 i 2i , exp exp r r r r r 212 23 12 23 { { = + + ^ ^ h h and .r r r z z z z t t t t 12 23= = + l m l m ,cos sint z i z z z 2 1 i t t t i t 1 { {= + + + z z m l m lc cm m; e carcione et al. 4 (21) (22) (23) (24) (25) (26) (27) (28) (29) (32) (31) (30) (33) 5 4.2. sh-te analogy the te maxwell equations are [carcione 1998, carcione and robinson 2002], where we have considered the lossless case (zero conductivity). as can be seen, these equations are similar to those of an isotropic medium, since only one component of the permittivity tensors appears. this fact indicates that te waves are also equivalent to sound waves. the tm-te analogy has been given by carcione [1998] and carcione and robinson [2002]. the complete mathematical analogy is 5. analogy with quantum mechanics. tunnel effect for simplicity, we consider the time-independent schrödinger equation where = 1.055 × 10−34 j s is planck’s constant, m is the mass of the particle, ] is the wave function, e is the total energy of the particle, v is the height of the potential barrier, and d is the laplacian. on the other hand, in the isotropic case, equation (2) can be written as where p = p44 = p66 and we have assumed an harmonic wave. equations (36) and (37) are mathematically equivalent if ] = vy and outside and inside the barrier, respectively. the tunnel effect occurs when e < v. in this case, the waves within the potential barrier are evanescent with a complex velocity given by where we have expressed the energy as e = . the analogy holds with sh waves at normal incidence (i = 0). of interest is the transmission coefficient through the barrier, given by equation (24), when the media above and below the layer (i.e., the barrier) have the same properties, i.e., when r23 = −r12 = −r, x12 = x and x23 = (z't/zi)x. then where r and x are given by equation (8). at normal incidence, where the density t is uniform in the whole space and can have any arbitrary value. alternatively, the analogy can be established by assuming variable density media and uniform stiffness p. in this case, t = sp and t' = s'p, with zi = ps and z't = ps' with p taking any arbitrary value. equation (40) coincides with equations (5.11) (e > v) and (5.12) (e < v) of anderson [1971], considering that this author reports t2 instead of t. the interpretation of the transmission coefficient |t|2 in this case is the probability of a particle to be tunneling through the barrier. an approximation to equation (40) can be obtained for v > e and { ≫ 1, which is the simplified equation found in many textbooks [feynman et al. 1965]. 6. results we first consider cases equivalent to the tunnel effect. particular cases of interest are fractures in elastic media and thin metal films in electromagnetic media. assuming c'44 = c'66 → ∞ and h'44 = h'66 ≡ h', we obtain a newtonian fluid from equation (7), the complex velocity (6) becomes where the third and fourth expressions correspond to shand p-waves, respectively. tm sh te h e e v c e h h c y x z y yz xy y x z 11 44 1 1 33 66 22 , , , , , , , , , , , , e v v n e n n et ,m e v2 2' ] ]d= -^ h 0 , ,v s v s py y 2 2~ t d + = = ands me s m e v2 2 ' '~ ~ = = l ^ h i ,v s m v2 1 ' ' ~ ~ = =l l^ ^ h h 1 i , i exp exp t r z z 2 t i 2 2 { x { = -l ^ ^ ^ h h h , and ,z s z s s h e s hi t ' t t { ~= = = =l l l l 4 , exp exp exp t v e v e m v e h 1 2 ' . ?{ {= = ` ^ ^ ^c j h h h m i .p p p44 66 ~h= = =l l l l ' '~ , , h h e h e h e z x y z x y x z y x z 222 2 2 2 f n n = = = o o o i i ,v p t t ~h c l ~ = = = l l l l l l thin-layer wave theory (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) on the other hand, an anisotropic metal has with where the last expression holds for an isotropic medium, with v'11 = v'33 = v'. note that in the viscoelastic and electromagnetic cases , while in quantum mechanics (see equation 39), i.e., there is a r/4 phase difference. let us first consider sh waves and a fracture thickness h = 1 cm. the upper and lower medium are the same and defined by c44 = 10 gpa, c66 = 14 gpa and t = 2000 kg/m3, with zero viscosity. figure 2 shows the reflection coefficient of sh waves as a function of viscosity h' (a) (normal incidence, i.e., i = 0) and incidence angle (b) (for h' = 10 kpa s). the frequency is f = ~/(2r) = 60 hz. as can be seen, increasing viscosity and incidence angle implies decreasing reflection amplitude and increasing transmission. for fluid viscosities comparable to water, oil or gas, the reflection coefficient is practically 1 for h = 1 cm. to obtain a significant energy transmission the thickness has to be of the order of tens of angstroms, since the wave has a shear nature whose polarization (particle motion) is parallel to the fracture surface. in real fractures, significant transmission takes place at larger thicknesses, since the surface of the fracture is rough and there are many contact points [e.g., gangi and carlson 1996]. next, consider a thin conductive isotropic layer of thickness h = 1 nm embedded in a lossless medium with permittivity components e11 = 8e0 and e33 = 12e0, where e0 = 8.85 10 −12 f/m. the magnetic permeability is the same for the two media, i.e., n = n0 = 4 r 10 −7 h/m. based on the analogy (30)-(33), figure 3 shows the reflection coefficient of the electromagnetic wave as a function of conductivity (a) (normal incidence) and incidence angle (b) (for v' = 30000 s/m). the frequency is 1 mhz. as can be seen, the film totally rei and i ,p p44 66 11 33v ~ v ~ = =l l l l i i ,sin cosv 33 2 11 2 n ~ v i v i n v ~ = + = l l l l lc m iv ? iv ? carcione et al. 6 figure 2. absolute value of the reflection coefficient of sh waves as a function of viscosity (a) (normal incidence, i.e., i = 0) and incidence angle (b) (for h' = 10 kpa s). the frequency is f0 = 60 hz and the fracture thickness is 1 cm. figure 3. absolute value of the reflection coefficient of the electromagnetic wave as a function of conductivity (a) (normal incidence) and incidence angle (b) (for v' = 30000 s/m). the frequency is f0 = 1 mhz and the layer thickness is h = 1 nm. (45) (46) 7 flects the wave for very high conductivities. this is the case for copper (v' = 6 107 s/m) for instance. total transmission occurs for very resistive materials. we consider now the tunnel effect. let us define the reference wavelength as an example, we consider the tunneling of an electron with mass m = 9.1 × 10−31 kg and energy e = 1.6 × 10−19 j (1 ev), giving m = 1.23 nm. figure 4a and b show the transmission coefficient as a function of h for v = 10e (a), and as a function of v for h = 0.5 nm (b), normalized by m and e, respectively. as expected, the probability of tunneling decreases with increasing h and v. the next example considers a sandstone layer embedded in a shale formation, whose properties are taken from thomsen [1986] (table 1). the sandstone (taylor sandstone) has the properties vp = = 3368 m/s, vs = = 1829 m/s, c = (c66 − c44)/(2c44) = 0.255, and t = 2500 kg/m3, where vp and vs are pand swave velocities and c33 is an elasticity constant. the properties of the shale (mesaverde mudshale) are vp = 4529 m/s, vs = 2703 m/s, c = 0.046, and t = 2520 kg/m3. we obtain c44 = 18.4 gpa, c66 = 20.1 gpa, c'44 = 8.4 gpa, c'66 = 12.6 gpa, and assume h'44 = h'66 = 0.5 gpa s. this choice of the sandstone viscosity yields the c33 t 2 . me2 ' m r = thin-layer wave theory c44 t medium es (e0) ef (e0) vs (s/m) vf (s/m) e (e0) ks (gpa) kf (gpa) z (%) k (gpa) shale 30 4.23 0.01 0.4 (30.15, −11.31) 20 2.25 15 12.76 sandstone 5 2 0 10−5 (3.79, −8.6 × 10−4) 39 0.025 35 5.59 figure 4. absolute value of the transmission coefficient of an electron through a potential as a function of the width h and height v (properly normalized). in (a) v = 10e and in (b) h = 0.5 nm. table 1. properties. a = 3; ~ = 100 mhz. figure 5. sandstone layer in mudshale. absolute value (a) and phase (b) of the sh-wave reflection coefficient as a function of the incidence angle and three values of h. quality factors q44 = ~h'/c'44 = 9 and q66 = ~h'/c'66 = 6 (see carcione [2007]: equation 2.156), which implies high attenuation. in fact, the effective pressure for this sandstone is zero in table 1 of thomsen [1986] meaning that the medium is over-pressured. we consider a frequency f = 60 hz, which implies a wavelength of 30 m based on the s-wave velocity of the layer. figure 5 shows the reflection coefficient of the sh waves, equation (20): absolute value (a) and phase (b), as a function of the incidence angle and three values of h (1 m, 3 m and 6 m). there is a brewster angle at approximately 60°, where the reflection coefficients become almost zero and change phase [carcione 2007]. we may obtain the reflection coefficient of p waves using the analogy (27). then, we can use equation (20) by replacing c44 = c66 by t −1 f , t by c −1 33 , c'44 = c'66 by (t'f ) −1, t' by ( c'33) −1, where tf and t'f are the densities of the shale and sandstone, respectively. the viscosities are equal to ∞, i.e, there is no attenuation in this case. figure 6 shows the absolute value (a) and phase (b) as a function of the incidence angle and three values of h (1 m, 3 m and 6 m). in this case, the behaviour of the curves is quite different compared to s-wave case, with no brewster angle. next, we compute the reflection coefficient of tm waves from a vein of quartz embedded in a limestone.we consider e11 = 9 e0, e33 = 0.8 e11, e'11 = e'33 = 4 e0, n = n0 and zero electrical conductivity for both rocks. the frequency is f = 100 mhz, typical of groundpenetrating radar surveys. the analogy (30)-(33) and equation (20) are used to obtain the reflection coefficient, shown in figure 7 for three different thicknesses of the quartz layer. in this case, there is a brewster angle at 37° approximately. the curves are similar to those of the s wave, since it is known that shear and tm waves are analogous mathematically [e.g., carcione and cavallini 1995]. in the following, we test the sh-te analogy and compare the tm and te reflection coefficients. we consider the last example, but in the isotropic case, with n = n0, e11 = e22 = e33 = 9 e0 and e'11 = e'22 = e'33 = 4 e0. the thickness of the layer is h = 20 cm. the te curve carcione et al. 8 figure 6. sandstone layer in mudshale. absolute value (a) and phase (b) of the p-wave reflection coefficient as a function of the incidence angle and three values of h. figure 7. vein of quartz in limestone. absolute value (a) and phase (b) of the tm-wave reflection coefficient as a function of the incidence angle and three values of h. 9 is in agreement with equation (1) of bradford and deeds [2006]. it also agrees with equation (2) of deparis and garambois [2009] provided that, in this paper, z = 2dk2 cos ii is replaced by z = 2dk2 cos it, which is the correct expression. the te and tm curves are very different, mainly at the brewster angle. finally, we consider a co2-saturated sandstone embedded in brine-saturated shale and use a crossproperty relation to obtain the bulk modulus as a function of the complex permittivity [carcione et al. 2007, mavko et al. 2009]. the gas is in its supercritical state. in the following, k, t, v and e denote bulk modulus, mass density, electrical conductivity and dielectric permittivity, respectively, and the subindices “s” and “f” denote grain and fluid properties. gassmann equation gives the bulk modulus of the saturated rocks where is the dry-rock modulus and z is the porosity, which can be obtained from the crim equation, where e is the rock complex dielectric constant, a is an empirical parameter and “re” takes the real part. on the other hand, the rock density is given by t = (1 − z)ts + ztf. assuming ts = 2650 kg/m 3 for both shale and sandstone, tf = 1000 kg/m 3 for brine and tf = 500 kg/m 3 for co2, we obtain t = 2400 kg/m 3 and t =1897 kg/m3, respectively. moreover, we assume the properties given in table 1, which also shows the calculated gassmann moduli. we compute the p-wave and tm reflection coefficients for a wavelength/thickness ratio r = 10, i.e., ~h = 2rc/r, where c is the p-wave velocity or the electromagnetic tm-wave velocity (100 mhz) of the layer. figure 9 shows the absolute value of the reflection coefficients versus incidence angle. as can be seen, the results indicate that the layer is more detectable with (high-frequency) electromagnetic methods. 7. conclusions theories describing wave phenomena and diffusion in different fields of physics consist in partial differential equations, which have identical or similar mathematical expressions. here, we have considered the reflection/transmission problem through an anisotropic and lossy layer. the equations hold for any layer thickness, not necessarily thin, although their uses are relevant for thin layers, i.e., when the wavelength of the signal is much larger than the thickness. we have shown that the same mathematical equations can be used in elastodynamics, electromagnetism and quantum mechanics by establishing appropriate analogies re i i i s s s f f s .z e v ~ e v ~ e e v ~ -f p 1 ,k k k k k k k k k k 1 m s s f s m m s f z z z z z z = + + ^ ^ ^ ^ h h h h 1k k 1 1m a sz z= z z+ -^ ^ ^ ^h h h h thin-layer wave theory figure 8. absolute value (a) and phase (b) of the teand tm-wave reflection coefficients as a function of the incidence angle at f = 100 mhz and h = 20 cm. figure 9. absolute value of the tm-wave and p-wave reflection coefficients as a function of the incidence angle, corresponding to a sandstone layer saturated with supercritical gas embedded in brinesaturated shale. (47) (48) (49) between the different physical variables and medium properties. in particular, the analogy has been obtained for p and sh elastic waves, te and tm electromagnetic waves and wave mechanics in quantum theory. further research involves the analysis of the present problem in the space-time domain using numerical simulations. in this case, the same computer code, with appropriate input variables can be used to solve the different physical problems. references anderson, e. (1971). modern physics and quantum mechanics, w.b. saunders co. bakke, n.e., and b. ursin (1998). thin-bed avo effects, geophysical prospecting, 46, 571-587. born, m., and e. wolf (1964). principles of optics, oxford: pergamon press. bradford, j.h., and j.c. deeds (2006). ground-penetrating radar theory and application of thin-bed offset-dependent reflectivity, geophysics, 71, k47-k57. brekhovskikh, l.m. (1960). waves in layered media, academic press inc. carcione, j.m., and f. cavallini (1995). on the acousticelectromagnetic analogy, wave motion, 21, 149-162. carcione, j.m. (1998). radiation patterns for gpr forward modeling, geophysics, 63, 424-430. carcione, j.m., and e. robinson (2002). on the acousticelectromagnetic analogy for the reflection-refraction problem, studia geophysica et geodaetica, 46, 321-345. carcione, j.m. (2007). wave fields in real media. theory and numerical simulation of wave propagation in anisotropic, anelastic, porous and electromagnetic media, 2nd edition, elsevier. carcione, j.m., b. ursin and j.i. nordskag (2007). crossproperty relations between electrical conductivity and the seismic velocity of rocks, geophysics, 72, e193-e204. carcione, j.m., and k. helbig (2008). elastic medium equivalent to fresnels double refraction crystal, j. acoust. soc. am., 124 (4), 2053-2060. deparis, j., and s. garambois (2009). on the use of dispersive apvo gpr curves for thin-bed properties estimation: theory and application to fracture characterization, geophysics, 74, j1-j12. feynman, r.p., r.b. leighton and m. sands (1965). the feynman lectures on physics 13, addison-wesley. isbn 0-7382-0008-5. gangi, a.f., and r.l. carlson (1996). an asperity-deformation model for effective pressure, tectonophysics, 256, 241-251. liu, l., and d.r. schmitt (2003). amplitude and avo responses of a single thin bed, geophysics, 68, 1161-1168. mavko, g., t. mukerji and j. dvorkin (2009). the rock physics handbook: tools for seismic analysis in porous media, cambridge univ. press. thomsen, l. (1986). weak elastic anisotropy, geophysics, 51, 1954-1966. tonti, e. (1972). on the mathematical structure of a large class of physical theories, accademia nazionale dei lincei, excerpt from ‘rendiconti della classe di scienze fisiche, matematiche e naturali’, series 8, vol. 52 (1). tonti, e. (1976). the reason for analogies between physical theories, appl. math. modelling, 1, 37-50. widess, m.b. (1973). how thin is a thin bed?, geophysics, 38, 1176-1180. *corresponding author: josé m. carcione, istituto nazionale di oceanografia e di geofisica sperimentale (ogs), sgonico (trieste), italy; email: jcarcione@inogs.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. carcione et al. 10 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals n.6/2003 ok 23/04 1361 annals of geophysics, vol. 46, n. 6, december 2003 key words macroseismic intensity – sicily – earthquake sound – macroseismic effects 1. introduction the principal aim of a macroseismic investigation is to define an intensity field following a specific intensity scale. the definition of a particular scale reflects the effort to mediate heterogeneous damage information into a systematic and structured description. the functionality of an intensity scale resides in its proper effects and damage subdivision. specific damage should fulfill certain requirements: it must be easily recognizable, it must be specific of as narrow as possible a range of energy released, it must be often associated with other effects of the same degree but must maintain a sufficient independence from them northern sicily, september 6, 2002 earthquake: investigation on peculiar macroseismic effects calvino gasparini, patrizia tosi and valerio de rubeis istituto nazionale di geofisica e vulcanologia, roma, italy abstract the northern sicily, september 6, 2002 earthquake (ml = 5.6, mw = 5.9) is investigated under macroseismic aspect: peculiar effects are collected besides standard effects normally used to define mercalli-cancani-sieberg (mcs) intensity. they include sound heard during the quake, fear felt and a simple qualitative description of ground movement felt. spatial coverage of such information is dense enough to be statistically processed, to give an interpolated, smoothed field for each data type. sound heard is compared with theoretical sound field produced considering source geometry and transmission of waves to air, it also confirms the southern sicily amplification disclosed by macroseismic intensity values. fear felt is also in agreement with macroseismic intensity field while type of ground motion is a partly independent aspect. mailing address: prof. calvino gasparini, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: gasparini@ingv.it (this can be defined as the power of representation of a specific intensity degree). a particular macroseismic intensity scale is the set of all damage and effects organized in a hierarchical order. other effects are considered not particularly useful, if not confusing, and they are marginally treated. within this paper some not-standard effects are presented because they received a sufficient spatial completeness of data. these effects are: – hearing of earthquake sound shortly before or during the shaking; – kind of ground movement felt by people during the event; – percentage of people feeling fear. the main shock occurred on september 6, 2002, 01:21 tu, localized by the national earthquake center on the southern tyrrhenian sea (latitude 38.45 n, longitude 13.70 e) at a distance of approximately 40 km from the sicilian town of palermo, with a magnitude ml = 5.6. aftershock events are distributed on an ene-wsw direction in agreement with the focal mechanism (thrust type strike = 255, dip = 49, slip = 121; pondrelli et al., 2003). 1362 calvino gasparini, patrizia tosi and valerio de rubeis the analyzed data came from macroseismic questionnaires that the italian ingv (istituto nazionale di geofisica e vulcanologia) send to every village within a specific distance from the earthquake epicenter. the questionnaire comprises 80 questions mainly based on the effects described by mcs intensity scale. macroseismic effects (fig. 1) were noticed across all over sicily and in part of the calabria region and they were collected as 549 completed questionnaires from 360 different villages (de rubeis et al., 2003). mercalli-cancani-sieberg intensity was defined on each location of the territory following methods described in de rubeis et al. (1992). macroseismic intensities range from not felt to a few data of vii degrees, with the majority of data occurring within iv and v degrees. iii, ii and not felt degree are shown within an almost horizontal central belt, which introduces a peculiar character to the field: an anomalous increment in intensity in the south of sicily. intensity data were filtered and interpolated using a kriging method based on a modeled experimental semivariogram connected to a fractal dimension of intensity data. this choice permitted a separation of regional field from noise (de rubeis et al., 2003). 2. data analysis many people reported having heard a rumble shortly before or during the earthquake. information on this effect was collected in two ways: one was a questionnaire aimed at describing the sound heard that people voluntarily compiled on line; the other was the standard macroseismic questionnaire in which a question was pertinent to the hearing of earthquake sounds during the quake. we received 38 compiled questionnaires fig. 1. macroseismic data of september 6, 2002 event with smoothed and interpolated field reconstruction (black isoseismals). original intensity values are represented by color dots, while epicenters of the sequence are identified by red stars. 1363 northern sicily, september 6, 2002 earthquake: investigation on peculiar macroseismic effects fig. 2. earthquake sound heard. original data with its interpolated and smoothed spatial reconstruction (color shading), compared with the theoretical sound pressure level in relative values (violet contour of equidistance 10 db) obtained using focal mechanism data. of the sound oriented kind. the analysis of these shows that many people in palermo city or nearby were able to identify the source direction, stating that the rumble was coming from the north. in most cases, the sound loudness was compared to that of heavy traffic, often capable to waking people up. a larger data set came from the macroseismic questionnaire. such data are treated statistically in a simple way, to assure the mitigation of anomalies and the spatial continuity of answers. errors may be due to misinterpretation of sounds (buildings or other structures produce sound during the shaking) or, by simply underestimating the process due to attention driven by other more involving effects (shaking, damage, etc.). this binary information is converted by simply counting the percentage of affirmative perception on total answers, weighted by a power 2 of distance, inside a circular area of 30 km. such filtered and interpolated data are compared with a theoretical model of the acoustic pressure transmitted from the p-waves to the air in each point, calculated using the focal mechanism parameters of the event and, consequently, the radiation pattern of p-waves (tosi et al., 2000). results are depicted in fig. 2 where point data (heard, not heard) are plotted with their smoothed interpolation and theoretical sound pressure level. the theoretical field is represented in the figure with a contouring of decibel relative value, as the absolute sound pressure level can vary widely in relation to the soil properties in each site. sound was heard by many people until distances of around 150 km in an ese direction from the epicenter, and until distances of 110 km to the south. then after a belt of practically total absence, sound was heard again on the iblei mountains, within a range of 165 to 220 km from the epicenter. 1364 calvino gasparini, patrizia tosi and valerio de rubeis the second effect investigated concerns the kind of movement felt: the choice is between subsultory and undulatory. the resulting field is represented with data in fig. 3. there is a prevalence of undulatory response over most of the island of sicily and the overall pattern is different from that of the other investigated effects. it is interesting to note that in the southern part of the island (mt. iblei area), corresponding to a relatively high mcs intensity, the kind of movement in prevalence is subsultory. also for earthquake sound this zone is particular, in fact, several locations here reported hearing a quake rumble at more than 160 km of distance from the epicenter. such a coincidence supports the hypothesis of site amplification behavior in the area. similar attention was given to another effect not considered for standard intensity evaluation. complete questionnaires contained information on the degree of fear felt by people: nobody, few, many or all. these categories were ranked and used to build a continuous smoothed and interpolated field. the method followed is similar to that used to build a sound field. fear can be due to several factors such as the mcs intensity level, earthquake sound, people’s excitability and their group influence, etc. results shown in fig. 4 reveal an overall pattern close to both the filtered intensity field and sound. thus, all fields collaborate to confirm a general pattern of effects provided by this seismic event. 3. discussion from the analysis of the earthquake sounds it can be seen that the area where the rumble was heard by most people is in good accordance with the modeled acoustic pressure given by the p-waves. on the contrary, the comparison with the sv radiation pattern showed a minimum in fig. 3. prevailing type of ground movement perceived by people during the event (subsultory or undulatory). original data are represented with their interpolated and smoothed spatial reconstruction (color shading). 1365 northern sicily, september 6, 2002 earthquake: investigation on peculiar macroseismic effects the same zone. it is interesting to note that in the central belt of the island, where the sound was not heard, there is a prevalence of undulatory movement. the explanation can be that here the distance from the epicenter is 100 km. according to suhadolc and chiaruttini (1987) at these distances the largest component of motion is the horizontal one, due to the lg-wave group. in the mt. iblei area, beyond 150 km, people reported having heard the earthquake sound and the ground movement is again, as near the source, subsultory. using the event focal mechanism it was possible to calculate that in the se direction there was a minimum in the radiation pattern of the s-wave, thus lg phase (resulting from the interference of multiple s-wave reflections at the moho discontinuity) was no longer dominant. the sound here could have been due to the critically refracted p-wave traveling in a low attenuation basement, probably in association with site effect. 4. conclusions peculiar macroseismic effects, not strictly related to a particular intensity degree or to a specific energy release, can be of interest if sufficiently collected and interpreted. answers on simple perception of sound heard during the quake, fear felt and description of felt ground movement were systematically collected for the event that struck sicily on september 6, 2002 (magnitude ml = 5.6, mw = 5.9). original data were composed basically of qualitative information: heard sound and ground movement are divided into two categories (heard, not heard and subsultory, undulatory respectively), felt fear was ranked into four levels (nobody, few, many or all). spatial distribution of all this information followed village locations and a simple averaging statistics was implemented to give continuity to data. smoothed and interpolated sound fields showed a correlation with both fear felt and fig. 4. fear felt by people during the event with its interpolated and smoothed spatial reconstruction: compare these results with the macroseismic intensity field (fig. 1). 1366 calvino gasparini, patrizia tosi and valerio de rubeis macroseismic intensity field: the southern amplification of effects was confirmed. the general theoretical behavior of sound sufficiently agreed with experimental data. ground movement was generally undulatory except for the western side of sicily and southern side where it had a remarkable amplification character for most macroseismic effects. finally, the spatial distribution of the fear effect showed that the overall pattern of the mcs intensity field can be reconstructed even using very simple transient effects. references de rubeis, v., c. gasparini and p. tosi (1992): determination of the macroseismic field by means of trend and multivariate analysis of questionnaire data, bull. seismol. soc. am., 82, 1206-1222. de rubeis, v., p. tosi, c. gasparini and a. solipaca (2003): macroseismic analysis of northern sicily, september 6, 2002 earthquake: results of application of kriging technique, bull. seismol. soc. am. (submitted). pondrelli, s., a. morelli, g. ekström, s. mazza, e. boschi and a.m. dziewonski (2003): on line catalog of quick european-mediterranean regional centroid-moment tensors, http://www.ingv.it/seismoglo/rcmt/. suhadolc, p. and c. chiaruttini (1987): a theoretical study of the dependence of the peak ground acceleration on source and structure parameters, in strong ground motion seismology, edited by m.o. erdik and m.o. toksoz (reidel, dordrecht), 143-183. tosi, p., v. de rubeis, a. tertulliani and c. gasparini (2000): spatial patterns of earthquake sounds and seismic source geometry, geophys. res. lett., 27, 2749-2752. (received october 10, 2003; accepted december 15, 2003) adg vol5 n02 papa 401_416.pdf annals of geophysics, vol. 45, n. 2, april 2002 401 acoustic emission (ae) as a diagnostic tool in geophysics gabriele paparo (1), giovanni p. gregori (1), ugo coppa (2), riccardo de ritis (1) and alberto taloni (1) (1) istituto di acustica (idac-cnr), roma, italy (2) osservatorio vesuviano, napoli, italy abstract acoustic emissions (ae) are effective for monitoring ground deformation and temporal variation of its porosity. ae are complementary to seismic information, related to the same area, though ae and earthquakes focus on observational evidence concerned with substantially different spaceand time-scales. ae information is pertinent (i) either for geodynamically stable areas, where it probes the diurnal thermal and/or tidal deformation, (ii) or for seismic areas where it provides some as yet unexploited precursors, (iii) or for volcanic areas, where it appears capable of recognising precursors originated by some hot fluid that penetrates by diffusion into rock pores, from those associated with eventual plutonic magma intrusions, (iv) and also for monitoring periods of time during which a volcano is «inflated» by underground hot fluids compared to others during which it «deflates». upon direct comparison between 6 data sets concerned with different physical settings, it seems to be possible (fig. 3 and table ii) to distinguish a few significantly different behaviours associated either (i) with a mere compression (such as occurs for stromboli, vesuvius, and a sample compressed in the laboratory), or (ii) with a slip strain, such as it typically occurs in association with faulting or with diurnal thermal rock deformation. 1. introduction we address the heuristic potential of acoustic emissions (ae) as a diagnostic tool for studying microscale processes associated with ground deformation, microfracturing, and porosity variation. such application can be concerned (i) with the study of matter exchanges across soil surface, or (ii) with a few different processes that occur within the ground and that are associated either with thermal variations, or with tectonic activity, or with hydrothermal or volcanic phenomena. ae applications are normally almost neglected in geophysics, depending on intrinsic difficulties in ae propagation through disjointed ground, and on their random detection capability. such previous standard defeatist feeling can be overcome considering ae like a yelp, whine, moan or whimper, ... that we can only occasionally catch. you may eventually be capable of hearing it or missing it, whether loud or feeble. however, whenever you detect it, you should afford to recognise its meaning. hence, the focus is on seldom-detected ae messages by the system. observations prove how, in several circumstances, some very useful ae signals are detected, which provide relevant unprecedented information. our purpose is to show the concrete feasibility of such applications, including mailing address: ing. gabriele paparo, istituto di acustica (idac-cnr), via fosso del cavaliere 100, 00133 roma, italy; e-mail: paparo@idac.rm.cnr.it key words acoustic emission – precursors – earthquakes – volcanoes dilatancy 402 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni comparison of field data with ae released by laboratory samples. a few examples referring to different tectonic settings and their potential are discussed, including hints for further developments. the key relies on methodology. we make a simple application of fractal algorithms. the analysis is not concerned with the ae amount, frequency, intensity, or energy, etc. ae are released by an unknown widespread source distribution, and several sources, e.g., of anthropic origin, have no real physical relation with the effect of concern. our method, when considered a posteriori, is very robust with respect to anthropic or natural perturbations. however, it would be presumptuous (either in the negative or in the positive sense) to compare either our records, methods of analysis, or results, with previous investigations. there is no «better» or «worse» method. a given database and/or method of analysis can be suited or not for some given purpose. different approaches must be realistically compared with each other with no preconceived understatement or overstatement. 2. the data table i summarises our data set. two frequency ranges were monitored. the time sequence of ae events at different frequencies appears correlated and time-shifted with respect to each other. every acoustic transducer (e.g., cuomo et al., 2000) gives a signal in terms of an electric potential. in general, unless otherwise stated, for geophysical applications the transducer signal was recorded averaged over ~ 25 s, while in laboratory experiments no averaging was applied. the following definitions were used: i) an ae burst is called a raw record, with some high time resolution, typically similar e.g., to a seismogram, though on different scale. ii) an ae microevent is a record of the sum of the maximum amplitudes of all bursts occurring during every time interval of ~ 10 ms. a microevent is the sum of several bursts. iii) an ae event is similar to a microevent, though in terms of a much longer averaging time interval. an event is the sum of several microevents. the typical time scale required for defining an event depends on the physical system. for instance, table i. the ae data base. site monitored comments frequency gran sasso 200 khz ~ 4 years records available (fig. 1). southern apennines (potenza) 25 khz several records available, including one case history 200 khz on the occasion of a m d = 4.6 earthquake with epicentre ~ 18.2 km from the ae recorder (fig. 4b). stromboli 160 khz a few years of discontinuous data available, with techniques slightly improved versus time, still working. vesuvius 25 khz several discontinuous data monitored since 1995, and 200 khz also recently after october 1999 by means of an improved instrumentation during a crisis period; the recording site is located almost on the top of the volcano, very close to the rim of its crater, still working. laboratory sample 200 khz a cylindrical unconfined sample was compressed 6 times, by a force transversal to its axis, although not implying the final destruction of the sample (fig. 2). 403 acoustic emission (ae) as a diagnostic tool in geophysics an event within a stressed alloy implies phenomena that occur to the micro-crystal structure of the sample within short time scales. a phenomenon occurring within a tectonic setting relies on some prime yielding bonds that are other than within a sample stressed in the laboratory. we characterised an event by means of an amplitude, which is the average amplitude of all microevents falling inside some given pre-chosen acquisition time-lag (for every geophysical application, we currently chose ~ 25 s). such ae time series was suitable for investigating geophysical settings. gran sasso is the main massif (composed mostly of dolomia and limestone) of the apennines in central italy. on the time scale of our records, its tectonic setting is comparatively stable, notwithstanding its surrounding area is prone to seldom occurring strong earthquakes, although with much longer return-times. the records (fig. 1) resulted significant, displaying some very regular daily variation, its day to day difference being likely related to climate (see below). the ae transducers were put both on top of a steel bar inserted into the rocky ground for ~ 12 m, while another was hanging in the air (blank signal). the natural electric field was recorded (in co-operation with ingv) using the same steel bar as a receiving antenna tuned on the same frequency. the ae are conspicuous during the daily cooling of ground (local nighttime). the diurnal cooling of rocks makes their external layer contract over a warmer and temporarily more expanded interior: cleavage a rb it ra ry u n it s junemay days fig. 1. records in the gran sasso area, may 29 through june 5, 1996, showing ae at 200 khz both inside rock (lower black) and in air (blank signal, lower light grey) and the natural electric field (upper plot) recorded by metal rod inserted into ground for ~ 12 m (in co-operation with ingv) and used as a receiving antenna tuned on the same frequency. 404 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni rupture must occur. the record of the natural electric field is associated with the ionospheric tide (contrary to oceanic and solid earth’s tides, the atmospheric tides are well known to be mostly thermal and only at a much lesser extent also gravitational). since marconi’s times, such (eventual) expected natural ae disturbance is well known to display a violent maximum during daytime. in contrast, in our experiment, both ae signals (the one actually recorded, and the one expected being originated by rock cooling) show a maximum during nighttime. that is, for sure there is clear anticorrelation between ionospheric disturbances and the recorded ae signal. hence, no ionospheric effect enters into the ae records. moreover, the blank signal monitors eventual disturbances originated from any kind of external sources (either anthropic, or ionospheric, or other, i.e. generated by sources that in no case can be related to underground phenomena, or within the electronics of the recording system, etc.). such disturbances, if they exist, should cause the same effect both on the sensor within air and on the sensor put on the metal rod. our results very clearly show that no such perturbation affects our ae records (at least as far as our gran sasso monitoring is concerned). therefore, at least at middle latitudes and in the absence of major anthropic disturbances, one can safely put ae transducers either on an insulator bar, or on a metal rod, with no bias on measurement. under such circumstances, every opposite conclusion (e.g., such as diodati et al., 2001; a formal comment to that paper submitted to that journal was not published; no reply was given to our objections) appears mere nonsense. the southern apennines site (giuliano) is close to potenza, within a tectonically active area that during the time spanned by our records suffered an m d = 4.6 earthquake. the two ae transducers (for two frequencies) were put on top of two steel bars inserted into a solid flysch outcrop. stromboli is a volcanic island with an unclear tectonic nature. its isotopic chemism appears more reminiscent of an oceanic island rather than of an island arc (crisci et al., 1991; esperança et al., 1992; esperança and crisci, 1995). for over ~ 3 millennia stromboli has made ~ 100 explosions per day, resulting in an unprecedented natural environmental laboratory (e.g., chiappini et al., 2002; gregori et al., 2001; paparo and gregori, 2001). its volcanic edifice is comparable in size to etna’s, though almost completely immersed within the sea. the ae transducers were located on top of two ~ 1 m bars (one made of steel, one of glass) inserted into a rock dyke. vesuvius is (with etna) the historically bestdocumented volcano. it had its last effusive eruption in 1944. since that time it has been apparently quiescent, although its seismic activity recurrently and irregularly experiences disquieting periods. its historical eruption series display some regularities (gregori et al., 1992; gregori, 1993, 1996), apparently supported by a correlation with the activity versus time of every other historical volcano of the world (gregori et al., 1994). ae monitoring may provide some unprecedented information on new degrees of freedom for diagnosing the state of the system. ae transducers were located on top of ~ 1 m glass bars inserted into a lava dyke, outcropping almost on the very top of the volcano, at an axial distance of a few hundred metres from its crater. gran sasso can be likened to some unique rocky block. at all other sites, ae propagation can be biased by damping while crossing disjointed material. on the other hand, the collected observations guarantee per se as a matter of mere observational fact about such concern. on a speculative basis, three possible ways can be envisaged. first, one can presume that some solid rocks, almost like elongated ores crossing through otherwise disjointed soil, are capable of acting like wave-guides for ae propagation. in principle, such possibility can be checked by some dense array of ae detectors: spatial distribution, crossed correlation, and time shift of ae records within the array should disclose such speculated wave-guides. ae should appear to be a local phenomenon, implying a serious drawback due to the choice of the critical location of the ae transducers. second, speculate that some teleconnection mechanism at much lower frequencies (lf) than ae’s makes the signal to propagate through disjointed material. whenever it stresses some rock, it triggers high frequency (hf) ae. in such case, whenever ae records are carried out within 405 acoustic emission (ae) as a diagnostic tool in geophysics some suitable array and within some wide fan of different frequencies, the lf propagation should be directly detected, including the timeand spaceseries of the hf ae trigger. detector location is critical, although ae should be almost ubiquitous and correlated with local geology. third (and more likely), some entire area, or physical volume, on the regional or maybe even continental scale is deformed, notwithstanding no effect is detected by any standard method. some strain, however, is in progress over the entire area, and it occurs basically everywhere. at some very small local scale, i.e. wherever some geological solid formation is present, the ae are monitored by transducers put right on top of such bodies. the ae results are a strictly local phenomenon (detected only whenever the transducer is put right on top of a suitable solid body). they result from the strain of some very local solid feature, although being the consequence of some large-scale stress. that is, ae monitors a yelp, whine, moan or whimper, ... of some comparatively very small solid body, which is part of some huge geological region that is suffering from some large-scale deformation. differently stated, time (ks) 53.5 54.0 54.5 55.0 55.5 56.0 56.5 57.0 57.5 fig. 2. ae bursts recorded during a laboratory experiment. a solid sample composed of a mix of gran sasso limestone was compressed and decompressed. the five stronger ae sequences correspond to increased stress. the highest peaks denote amplifier saturation level. the sampling rate is 200 ms. redrawn after petri et al. (1994) and vespignani et al. (1995). 406 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni the monitoring apparatus contains, as an essential constituent, also the solid body that extends deep into the ground to some unknown range. such solid body is like a true natural tentacle probing some fraction of a large geological and geophysical system, and capable of monitoring (just as a mere matter of observational fact) its reaction to stress and strain. the second and third such possibilities are ultimately the same, their difference being only mathematical. the second possibility refers to some teleconnection by means of one given lf wave, while the third possibility refers to some large scale deformation, the «signal» of which can be mathematically fourier decomposed into a continuous spectrum of lf waves. on a mere intuitive basis, the first and second ways appear less realistic than the third one. it does not seem possible to envisage any different possible way. hence, the third possibility appears more likely. no array of ae transducers was available to the authors. in every case history, however, ae signals definitely seem to be physically significant and useful for geophysical monitoring. their real heuristic potential is presumably different in different applications, depending on tectonic setting, local morphology and eventual disturbances, etc. in addition, some ae records were also considered for comparison purposes, collected in the laboratory while stressing and straining a solid sample obtained from a mix of gran sasso limestone (fig. 2). seismic data are complementary, suitable for correlation with ae. they reflect a later stage of the evolution of the system. observations seem consistent with the hypothesis that phenomena always start by hf ae (~160-200 khz) (originated when some very small pores yield), followed by lf ae (~ 25 khz) (when pores yield and coalesce into larger microcavities), and finally at some later time by an earthquake (~ 1 hz, when the large scale mechanical structure of the system yields). hence, in principle, one should expect that a hypothetical monitoring carried out by some large set of different frequencies should detect a clear sequence of ae signals of progressively decreasing frequency. such sequence, however, ought to span a limited frequency range, because, according to the aforementioned third explanation, ae is concerned with local geological structures of limited spatial extension, unlike the large seismic energy release that ought to be originated by a tectonic structure other than the ae source, though both sources respond to a common prime deformation and trigger. concerning vesuvius, a careful seismic log has been available since 1972 a.d. seismic data for the potenza area, in lucania, southern apennines, were available from ingv, although spanning some limited period. 3. the physics ground deformation is originated by either one of the following prime physical causes: i) thermal contraction and expansion; ii) tidal fluctuation; iii) tectonic strain; iv) endogenous pressure by a fluid (i.e. water, oil, geogas, etc.); and v) endogenous pressure by plutonic intrusion of magma. every such process implies phenomena that have to be studied beginning from their atomic or molecular scale through their respective large-scale effects. ae are triggered whenever some chemical bond yields. the phenomenon eventually propagates like a chainreaction, eventually leading to strain deformation or to rupture, with morphology, timing, and speed, which depend altogether on the structural environment and on the prime trigger. a mere compression e.g., by a hot fluid that diffuses into the pores can be expected to imply a different behaviour than the case of a lateral displacement, such as occurs for crystal cleavage or tectonic faulting. every such feature must be physically fitted. the temporal evolution of ae series closely depends on the prime energy source, hence on the morphology and geometry of the stress and subsequent strain. this recalls the classical dilatancy hypothesis (e.g., nur, 1972; rikitake, 1976; mogi, 1985) for explaining earthquakes precursors. according to the rationale of the present study, the model here speculated ought to be considered as a peculiar case of some wider and more general conceptual perspective. much as always occurs in every discipline, every understanding is based on some simplifying abstraction. therefore, its consequent model, aimed at explaining some specific ob407 acoustic emission (ae) as a diagnostic tool in geophysics servations in some detail, is eventually physically significant only, and strictly only, within the limits of its own basic approximations, and never outside them. thermal (non-endogenous) effects are originated by a source external to the rock. this occurs in the case of solar diurnal heating and cooling that penetrates only down to several tens of centimetres. whenever such warming reaches some depth, the rock layer can freely expand and the process is smooth. in contrast, whenever a rock layer cools off, its outer layers contract while they are compelled to contain internal layers that are still more expanded. hence, a cooling (unlike a warming) object ought thus to imply some lateral displacement (i.e. it is a 2d slip of the shrinking outer layers). such phenomenon occurs within some shallow layer, and it should occur only on the surface, unless there are some peculiar structures that act like the aforementioned ae wave-guides to deeper layers. tidal deformation uniformly applies to the entire earth. compared to thermal deformation it appears likely to be a smooth phenomenon, originated in 3d space that should emit some lesser ae, or even no ae at all. for instance, every huge building does not necessarily release ae or crunching, notwithstanding it actually experiences conspicuous diurnal tidal oscillations (typically by several tens of centimetres). tectonic strain should imply ae mainly originated by lateral displacement (here called tout court «slip» ae), much as occurs for thermal deformation, though occurring over some much wider surface, i.e. on the slip plane of a fault, which can be either of small or large extension. the analogy between slip ae associated with either thermal or tectonic strain seems to be almost perfect, the difference relying on the fact that the thermal case is concerned with ruptures of atomic bonds within a crystal structure, while tectonic strain is concerned with cohesion forces within the ground, which characterise soil hardness and that under different settings avoid land slides. such bonds are concerned with soil physics (unlike e.g., in an alloy, or in the concrete of a building, where the yielding bonds refer to rupture along micro-crystal cleavage planes). a still different physical environment is concerned with snow-slides, i.e. with snow or ice hardness, provided that it is possible to perform some ae monitoring at the appropriate location. in such case history, the ae are released depending on ice physics. it ought to be emphasised, however, that the slip ae observed on the occasion of a tectonic strain could be either originated over the fault plane, or also by the microfracturing of the rocky body over which the transducer was located. the mathematical rationale and method of analysis may eventually be the same, though applied to physically different systems and case histories. the endogenous pressure by a fluid ought to be a typical feature of every system dominated by hydrothermal or phreatic breeding. ae sources should be distributed into a 3d region, where 3d fluid diffusion occurs, and therefore they should display their own signature, distinct from ae originated by an externally imposed stress, which should recall some more organised, less 3d and comparatively more 2d, spatial distribution. the endogenous pressure by plutonic intrusion of magma has to be likened to a tectonic strain. in fact, fluids such as water, oil, geogas, etc. compared to magma have a very great mobility and compressibility. therefore, they can easily reach the smallest pores, and exert, almost randomly, a pressure that rapidly propagates by 3d diffusion, producing local micro-cracks, and coalescence into progressively larger microcavities, etc. in contrast, a plutonic body has some extremely low mobility, and produces only very slow ground deformation, often undetectable from the ground surface, at least until the phenomenon is not in some comparatively late stage of evolution. such effects are generally reported by standard observations during the few days preceding the opening of a new boca (implying ground upheaval by several tens centimetres). the two case histories are therefore very different. in the case that a plutonic intrusion is also associated with hydrothermal or gas propagation underground, ae will detect both such kinds of event. hence, the leading (though not the unique) effect of a plutonic intrusion, prior to its large-scale late manifestation as ground deformation, ought to be likened to some comparatively more or less large or small tectonic deformation, implying 2d slip along a surface of some small (and undetectable from 408 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni surface) faults. in every such case this is more a 2d than a 3d phenomenon. it should be stressed that in every such case history, the comparatively more 2d than 3d character of the recorded ae is not necessarily associated with some large 2d slip surface or fault plane. since we are interested in the character of the prime temporal sequence of ae associated with a reaction chain while the chemical bonds yield, even phenomena occurring along the slip plane of sub-microscopic crystal size do produce the 2d expected effect, even when no «macroscopic» slip or fault plane etc. can be detected. one very similar occurrence, though in the space (linear) scale of ~ 10 000 km and on the geologic time scale, is concerned with superswells (typically observed in the pacific area where seamounts are a dense array of natural mareographs). according to the evidence discussed by gregori and dong (1996) and based on the output of the hawaii hot spot, the push by the endogenous thermal source appears to be the origin of some generalised large-scale uplift, i.e. the superswell, by which plates start sliding by gravity on its slopes, thus acquiring kinetic energy. such stage often precedes the outburst of a large igneous province (lip). whenever this occurred on emerged land, even the river drainage recorded such large-scale slopes (cox, 1989). the kinetic energy is later transformed into friction heat, and by ~ 50 000-100 000 years it is finally manifested as a large increase in volcanism. consider a laboratory experiment and compress a rock sample, while it is kept laterally confined. it is expected that the ae be similar to the case of an endogenous 3d pressure alone, exerted e.g., by a fluid. however, if the sample is not confined, a smaller percentage of transversal slip occurs and its associated 2d slip ae. summarising, the key-item of such entire discussion deals with the distinction between 2d and 3d distribution of the prime ae sources. it is physically characterised in terms of a different timing between subsequent yields of the bonds (either within crystals, or within ground). this is the rationale for recognising the kind of yelp, whine, moan, whimper, .... the theory of stress and strain of a non-perfectly elastic material was extensively investigated for purposes other than geophysics (e.g., scott, 1991). moreover, as far as an explicit calculation is concerned of the associated formal schrödinger equation, it is a most difficult job even for the computation of a pure crystal, while it is essentially impossible for an alloy or for every unknown texture of different substances such as occurs in natural rocks or ground (see e.g., eberhart, 1999). ae’s are complementary, from the viewpoint of the aforementioned dilatancy hypothesis, to other standard methods (topographic/bathymetric or seismic or gps techniques, or repeated geodetic prospecting, tiltmetry and strainmetry, high sensitivity gravimetry, etc.) in the fact that they reflect the stage of the micropore yielding occurring on strictly local and very partial geological structures, long before the much larger mechanical deformation and final rupture that causes an earthquake. in any case history, every natural system is to be expected to be largely heterogeneous. hence, ae ultimately monitor some structure of limited size, unless it responds to some large-scale teleconnection. in either case, however, it is clearly indicative of some wide and basically unknown, although never random, process of the system, originated at some more or less great depth, and involving some extended volume, or area. owing to this fact, the ae information that can be monitored on the ground surface ought to be better conceived in terms of a monitoring array, rather than as a point-like record. considering the present state-of-the-art outlined in section 1, the main purpose of the present study is a feasibility assessment about the effective heuristic potential of the ae technique. moreover, the analysis here envisaged is focused on time series of ae events, while the possibility of inferring some analogous and more precise information directly derived from the temporal evolution of every single ae «event» is currently in progress. 4. the analysis 4.1. fractal properties consider the timing of the ae events, and infer whether their sources are 2d or 3d distributed. neglect the intensity of every ae 409 acoustic emission (ae) as a diagnostic tool in geophysics fig. 3. statistical distributions of the slopes h computed by the box counting method applied to the 6 case histories listed in table i. the observed distributions seem to partake into two families with d ~ 1. and d ~ 0.9, respectively. three different histograms for every case are plotted, being defined by the identical data set, though by using different elementary intervals on abscissas. the significance of distinguishing such distributions is tested by means of standard algorithms as per table ii. the lowest two lines of histograms are shown with a lighter shading in order to emphasize that their peaks occurr at d ~ 0.9, unlike others that are peaked at d ~ 1.0 410 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni table ii. similarity of the fractal dimensions d of the ae event sequences observed in different environments and case histories. vesuvius vesuvius stromboli laboratory sample gran sasso potenza 200 khz 25 khz 160 khz 200 khz 200 khz 25 khz vesuvius 0.079 0.33 0.70 0.000002 0.0001 200 khz 0.085 0.33 0.62 0.000002 0.0001 0.97929 1.7 10 8 0.39 --0.56 0.0016 ±0.00015 1. 1. 0.22 1. --0.98784 0.17 0.36 --0.000002 0.0000008 ±0.00003 0.036 vesuvius 0.079 0.009 0.0014 1.2 10 12 0.0000017 25 khz 0.085 0.012 0.0491 5.5 10 10 0.0000065 0.99485 1.7 10 8 4.8. 10 10 0.0017 3.7 10 7 1.2 10 14 ±0.000009 1. 1. --1. --0.99588 0.17 0.006 0.0021 1.1 10 8 2.2 10 8 ±0.00001 stromboli 0.33 0.009 0.72 0.0004 0.0012 160 khz 0.33 0.012 0.62 0.0004 0.0011 0.9664 0.39 4.8. 10 10 0.091 0.15 0.0185 ±0.0002 1. 1. --1. --0.8455 0.36 0.006 0.43 0.00004 0.00009 ± --laboratory 0.70 0.0014 0.72 0.0004 0.0202 sample 0.62 0.0491 0.62 0.0002 0.0004 200 khz --0.0017 0.091 0.366 0.0055 0.97320 0.22 -- -- -- --±0.00005 --0.0021 0.43 0.0021 0.0034 0.97297 ±0.00007 gran sasso 0.000002 1.2 10 12 0.0004 0.0004 0.28 200 khz 0.000002 5.5 10 10 0.0004 0.0002 0.27 0.9176 0.56 3.7 10 7 0.15 0.366 0.00025 ±0.0001 1. 1. 1. -- --0.9121 0.000002 1.1 10 8 0.00004 0.0021 0.70 ± --potenza 0.0001 0.0000017 0.0012 0.0202 0.28 25 khz 0.0001 0.0000065 0.0011 0.0004 0.27 0.8970 0.0016 1.2 10 14 0.0185 0.0055 0.00025 ±0.0006 -- -- -- -- --0.8452 0.0000008 2.2 10 8 0.00009 0.0034 0.70 ± --411 acoustic emission (ae) as a diagnostic tool in geophysics event. as a first-trial, apply the box counting method. we considered 21 case histories for every one of the 6 different data sets listed in table i (we used 23 samples of southern apennines records, while only 6 data sets were available for the laboratory experiment). concerning vesuvius and the southern apennines records, also the time variation of the fractal properties were investigated. the box counting method was applied as usual, by least-square fitting on the plotted points in the log-log diagram (richardson’s plot) of number of counts versus sampling interval µ, and after rejecting the ruler intervals biased by limited statistics. the fractal behaviour, however, is missed whenever µ is excessively short, i.e. when it is comparable either with the minimum time resolution of the recorder or, with the shortest time interval elapsing between two consecutive bond yields. similarly, when µ is excessively long compared with the total time lag spanned by the ae records, large statistical fluctuations enter into play. hence, the fit must be limited within a suitable interval µ1 < µ < µ2. in every application the numbers of fitted points were always large and comparable with each other. the (negative) slope h of the line in the richardson plot is the fractal dimension d = – h. refer to turcotte (1992, eqs. (2.1) and (7.12)) for whom a fractal is defined whenever the number n n of objects or fragments composing a system associated with a characteristic linear dimension r n is n n = c/r n d where r n = l/n and c is a constant, d is the fractal dimension, and l is the total span of the database over which the box counting method is applied. hence, comparatively smaller slopes h ought to be displayed by slip ae (which are likely to be comparatively more 2d than 3d) compared to compression ae (which have a 3d space distribution). a uniform time-distribution of the events ought to imply d = 1. moreover, it can also be formally shown that it must always be 0. d 1. (gregori, 1998). just by a matter of definition, every truly random time series of ae events has the same d as a uniform ae series. therefore, every kind of law that makes the observed ae time series deviate from such ideal and mere randomness shall imply some corresponding reduction of d. hence, if the box counting method gives d = 1. we infer that the comments to table ii the result of every respective comparison is here denoted by the probability that every two given observed distributions have either a similar or a significantly different value of some given parameter, or that they be the same or not. for instance, a very low probability means that it is very likely that the two given distributions are significantly different compared with each other. it is customary to consider e.g., a threshold of 0.05, by which, when the probability is < 0.05 the two distributions are considered significantly different. different printing characters are used to give direct visual evidence of a few different ranges of such probabilities. large bold characters denote distributions that appear significantly similar to each other. the 4 or 5 kinds of probabilities that are indicated for every such quantitative comparison are concerned, respectively, with a few different standard assumptions about the nature of the two distributions being compared with each other, as follows: i) student’s t-test for checking the difference of the mean, when the distributions are thought to have the same variance; ii) the same when the two distributions are thought to have significantly different variance; iii) f-test for checking whether the two sample have a different variance; iv) 2-test for checking whether the two distributions are different (such a test can be carried out only whenever the two distributions have the same number of elements); v) the same by means of the kolmogorov-smirnov test. the left column also contains: i) the (non-weighted) mean of d including its standard deviation, plus ii) the weighted mean and its error bar. sometimes some input datum was found having a formal vanishing standard deviation. in such a case, some small and arbitrary value (10 12) was assigned to it, by which the resulting weighted mean is almost completely determined by such small-error value(s), while its error bar results insignificantly small. in such a case, the standard deviation of the non-weighted mean can be used as an indicative error bar. 412 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni ae sources are randomly 3d distributed in space. in contrast, when d < 1 some organised evolution of the system is likely to be in progress, such as e.g., when the ae source distribution slowly evolves from 3d towards some comparatively more 2d space organisation. 4.2. comparing different tectonic settings figure 3 and table ii show the comparison between different settings. the formal statistical comparison is reported in detail between the distributions of d concerned with the 6 different case histories. it is concluded that there are two sets of distributions that appear similar to each other, respectively: i) vesuvius, stromboli, and the laboratory sample, and ii) gran sasso and potenza. such two sets appear significantly different when they are compared with each other (table ii). it is important to stress that the success of our analysis largely resulted from the possibility of comparing with each other analogous applications to physically different case histories, by which the actual heuristic potential of the ae diagnostic tool can be tested, and sensitivity and reliability assessed. figure 3 and table i appear crucial in such respect. according to the tentative intuitive rationale outlined in section 3, stromboli seems to represent a typical ae case history origifig. 4a. time series of h computed every day during a few months before the occurrence of the earthquake reported in fig. 4b, close to the recording station giuliano. the system reveals a clear trend from random ae towards a progressively better organised phenomenon, characterised by rupture along some preferred plane, to be identified either with a fault plane, or maybe more likely (see text) with cleavage of some local solid structure. it is a complex system, originally disordered, which evolves toward an increasing order, until its mechanical structure yields. 413 acoustic emission (ae) as a diagnostic tool in geophysics with slip ae occurring during cooling, i.e. by local nighttime (see section 3). all such conclusions are to be supported by suitable additional laboratory experiments (in progress) capable of significantly simulating several different pore-yielding conditions, and of testing the associated aforementioned tentative inferences. 4.3. time variation of fractal dimension figure 4a shows the temporal variation of the tilt h at giuliano a few months before the occurrence of the earthquake reported in fig. 4b. every point plotted in fig. 4a refers to the ae fig. 4b. time delay between the relative variations of ae at 25 khz (purple), the same at 200 khz in rock (green), and in air (red), plus self-potential (blue). units on ordinate scale are arbitrary (see fig. 1). a ~ 2 days precursor appears in the lf ae, and a lesser one in the hf ae in rock. this is associated with an increased porosity of ground, by which the fluid-flow increases, and one day later also the self-potential. when the self-potential starts recovering, an m d = 4.6 earthquake occurs with epicentre at ~ 18.2 km from the recording site. redrawn after cuomo et al. (2000). nated by phreatic diffusion. vesuvius strongly resembles it. by this, any possibility that the late 1999 crisis of vesuvius was originated by plutonic intrusions can be excluded. in addition, the laboratory sample simulates a dominant ae originated by pressure, with a lesser contribution by ae slip, shown by the lesser peak in its histogram in fig. 3. in contrast, the giuliano site appears associated with an area where the faulting process generates some ae slip (either directly from the fault, or from a local structure, as per section 3). significantly smaller d are detected. the same result is observed on gran sasso, where the ae is consequent to the thermal daily deformation of rocks, by which ae ought to be associated 414 gabriele paparo, giovanni p. gregori, ugo coppa, riccardo de ritis and alberto taloni recorded during 1 day. the system appears progressively becoming more 2d, i.e. towards some slip ae compared to a starting random ae. during earlier times only records of lesser accuracy were available. concerning subsequent times, ae records are currently being analysed. in either case, there is clear evidence that it is always h ~ – 1. figure 5 shows the temporal trend observed for vesuvius during the first half of 2000 a.d. when d ~ 1. vesuvius is during a charging phase, i.e. some hot fluid pressure, increasing versus time, diffuses into the pores producing a random ae and the system is thus «inflated». in contrast, when d < 1. the endogenous fluid pressure is temporarily lowered, vesuvius «deflates», the entire system collapses, much like a dome that is no longer supported from inside. hence, the system eventually breaks along preferred crack or minifault planes (maybe, even almost microscopic) normally undetectable by standard observations. such guess is supported by seismic activity. more shocks occur during inflation than during deflation. deflation shocks are comparatively much stronger than inflation shocks. consider the total energy integrated versus time during an entire period of inflation: it ought to be roughly the same when integrated during the subsequent deflation period. the eventual observed difference ought to reflect the seismic energy associated with shocks that are not detected by the seismic network, due either to instrument sensitivity, or to the damping of feeble signals within the ground (in progress). better multiparametric records are recommended. in any case, the ae technique seems to provide the very first effective method for fig. 5. daily estimates of h versus time of the ae collected on vesuvius during january through june 2000 a.d. see text. 415 acoustic emission (ae) as a diagnostic tool in geophysics monitoring inflation/deflation of a volcano. the present analysis has a time resolution of ± 1 day, although in principle the same method can be applied with a much higher detail (in progress). 5. conclusions according to our inferred evidence, ae seems to be a heuristically effective tool for investigating ground deformation, microfracturing, and the time variation of its porosity. the apparently most reasonable interpretation is in terms of a strictly local effect, released by geologic structure of limited extension and of great rigidity (such as dykes, rocks, lava, etc.) that are stressed and strained following some large-area tectonic or volcanic event. ae appears to be potential and useful complementary evidence in addition to, and in eventual correlation with, seismic and geochemical information. it appears possible to distinguish phenomena triggered by the diffusion of some fluid into rock pores, yielding a temporal sequence of ae events characterised by a slightly higher fractal dimension d (or equivalently a slightly steeper slope h in the richardson plot), compared to the case of ae originated by some strain during either crystal cleavage or fault sliding. a few laboratory experiments seem to support such inferences. additional ones ought to be performed. in general, the ultimate advantage of ae compared to the standard approach in terms of seismic activity relies on its implicit direct capability of probing processes that occur on the atomic or molecular scale, much before (i.e. several up to a few tens hours, or even several months in advance) a comparatively much larger seismic energy release. no matter whether such approach is more or less useful for mitigating volcanic and seismic hazards, its investigation ought to be exploited to assess some observational inference that apparently cannot be investigated by any other kind of observations, and that therefore can open unprecedented diagnostic applications. figures 4a and 5 ought to be considered as a possible indication (respectively) of an unprecedented know-how for monitoring areas where an earthquake is going to be prepared, and of an unprecedented possibility of monitoring «inflation» and «deflation» times of a volcano. it should be stressed, however, that it is possible to monitor the temporal evolution of a system for assessing under what circumstances an eventual hazard is going to evolve towards a risk, although simply by this no alert can be issued. the next steps shall be twofold. on one side, an attempt will be made to improve the temporal resolution while monitoring the «inflation» and «deflation» times of a volcano. a second step shall attempt to recognise the chain-reaction that occurs during the time sequence of the yielding bonds of one single occurrence. the definition of «event» however shall be critical. the concept of microevent (lasting typically ~ 10 ms) still remains the same, being the elementary monitored element. however, some suitable total duration t for defining the concept of «event» shall be chosen, eventually different from ~ 25 s as in section 2. the ultimate target is to search for a substantial improvement of the temporal resolution of ae monitoring, aimed at measuring additional and unprecedented degrees of freedom of the system, focusing as far as possible on its prime process, even on the atomic or molecular scale. references chiappini, m., g.p. gregori, g. paparo, c. bellecci, g.m. crisci, g. de natale, p. favali, i. marson, a. meloni, b. zolesi and e. boschi (2002): stromboli. a natural laboratory of environmental science, j. volcan. geotherm. res., 113, 429-442. cox, k.g. 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(1985): earthquake prediction, academic press (harcourt brace jovanovich, publ.), tokyo etc., pp. 355. nur, a. (1972): dilatancy pore fluids, and premonitory variations of t p /t s travel times, bull. seismol. soc. am., 62, 1217-1222. paparo, g. and g.p. gregori (2001): volcanoes and environment and the natural laboratory of stromboli, in giovannelli (2001), 289-309. petri, a., g. paparo, a. vespignani, a. alippi and m. costantini (1994): experimental evidence for critical dynamics in microfracturing processes, phys. rev. lett., 73 (25), 3423-3426. pi c c i o n e, v. and c. an to n e l l i (editors) (1996): proceedings of the 4th workshop of progetto strategico clima, ambiente e territorio nel mezzogiorno, lecce, november 11-14, 1991, 2 vol., pp. 644, cnr, roma. rikitake, t. (1976): earthquake prediction (elsevier, amsterdam etc.), pp. 357. schröder, w. (editor) (1993): the earth and the universe, festschrift in honour of prof. h.-j. treder, newsletter of idch-iaga, (20), 1-497, science edition, bremenroennebeck. schröder, w. and j.p. legrand (editors) (1992): solar terrestrial variability and global change, pp. 243, idch of iaga, bremen-roennebeck. schröder, w. and m. colacino (editors) (1994): geophysics: past achievements and future challenges, newsletter of idch-iaga, (20), 1-191, science edition / idch of iaga, bremen-roennebeck. scott, i.g. (1991): basic acoustic emission (gordon and breach science publishers, new york, etc.), pp. 246. turcotte, d.l. (1992): fractals and chaos in geology and geophysics (cambridge univ. press), pp. 221. vespignani, a., a. petri, a. alippi, g. paparo and m. costantini (1995): long range correlation properties of aftershock relaxation signals. fractals, 3 (4), 839-847. 737 annals of geophysics, vol. 51, n. 5/6, october/december 2008 key words geoacoustic emission – deep borehole, diurnal variation – earthquake occurrence – natural electric field 1. introduction the concepts of cracking within heterogeneous media are the fundamental to modern models of earthquake preparation and source site formation. as indicators of micro-fracturing processes such as cracking, shifts along blocks boundaries, pore coalescence, changes in interstitial fluid pressure etc., spatio-temporal characteristics of acoustic emission (ae) contribute to validation and/or improvement of such models. this equally involves the results of laboratory-scale experiments on rock specimens to simulate tectonic straining processes, and geoacoustic emission (gae) surveys (seismoacoustics, by other term (belyakov et al., 1996)) aimed to study natural acoustic emission of embedded rocks. the results on gae variations recorded in a tunnel at 100 m depth (matsushiro seismological observatory, prefecture nagano, japan) were represented in (gorbatikov et al., 2001). the measurements were carried out with the help of three-component mag-3s geophone designed at earth physics institute of the russian academy of sciences on the basis of magneto-elastic sensor (belyakov, 2000). the output signal of such sensor is proportional to the third derivative of the ground displacement, and the gain slope of mag-3s geophone is equal to 60 db variations in geoacoustic emissions in a deep borehole and its correlation with seismicity valery gavrilov (1), leonid bogomolov (2), yulia morozova (1) and aleksander storcheus (1) (1) institute of volcanology and seismology, far east branch of the russian academy of science, petropavlovsk-kamchatsky, russia (2) institute of marine geology and geophysics far east branch, of the russian academy of science, yuzhno-sakhalinsk, russia abstract continuous geoacoustic emission (gae) measurements were acquired using a three-component geophone placed in a borehole at a depth of near 1000 m at petropavlovsk-kamchatsky starting in august 2000. using geophones consisting of magneto-elastic crystal ferromagnetic sensors, and installed at such a depth allows measurement of natural geoacoustic background with signal amplitude less than 1×10-4 m/s3 in frequency band from 3 to 1500 hz. according to the data from a 4-year survey period the characteristics of diurnal geoacoustic variations change before every earthquake with mlh≥ 5.0 that occurs at a distance of less than 300 km from the observation point or before each earthquake with mlh≥5.5 occurring at distance r≤550 km from the observation point. the changes in gae regime correlate with the strongest earthquakes that occurred during survey period. measurements of the natural electromagnetic field of the earth were carried out simultaneously with the help of an underground electric antenna. the behavior of gae in aseismic periods appears to be related to the effect of diurnal variations of the natural electromagnetic field. mailing address: dr. leonid bogomolov, institute of marine geology and geophysics of the russian academy of science, nauki str., 1 b, 693022, yuzhno-sakhalinsk, russia 738 v. gavrilov, l. bogomolov, y. morozova and a. storcheus tions of gae level were also recorded (but in frequency range 30-1200 hz) when the geoacoustic surveys were carried out with the help of mag -3s geophones installed in boreholes (belyakov et al., 1999). sensor allocation inside deep enough boreholes (700 m depth or more) where the level of geoacoustic emission is controlled by natural crust (lithospheric) processes is a proper way to provide necessary sensitivity of gae measurements. unfortunately the results represented in (belyakov et al., 1999) were obtained mainly by measurements in seismically inactive regions and therefore do not reveal cause-and-effect relations of characteristics of geoacoustic emissions with stressed-strained state of a terrestrial crust. this paper is devoted to the results of the continuous geoacoustic measurements performed in 2000-2003 in the seismically active region of kamchatka, russia, with the use of mag-3s geophone, installed inside a deep g-1 borehole at the depth of 1035 m. some preliminary results of these measurements were described in (gavrilov et al., 2006; gavrilov, 2007). hereinafter we analyze the results of geoacoustic measurements together with the data of seismicity of southern and central parts of kamchatka. 2. equipment and observation technique the station for gae measurements was built on the base of the g-1 borehole (lat. 53.05 n, long. 158.63 e) located in petropavlovsk-kamchatsky, a zone of deep northwest oriented fault. the depth of the borehole is 2540 m. the borehole is water-filled and cased along the whole length. the structure of the measuring system is presented in fig. 1. the geophone, which is the main measuring unit of the system, was installed at a 1035 m depth. the body of the sensor is crowded against the casing by a spring. the sensitivity of the vertical channel of the geophone (evaluated on preliminary amplifier output) is 0.15 v×s3/m, and the resonant frequency of the sensor is 1250 hz. the sensitivity of the horizontal channels is 0.60 v×s3/m, the resonant frequency being 300 hz. the output signals of geophone pre-amplifiers are transfor a decade of frequency change. such characteristics allow measurement of gae natural background with minimal amplitude of signals (evaluated as equivalent ground displacement) less than 1×10-10 m. according to the results of (gorbatikov et al., 2001) some increase in gae level has been recorded in 5 cases of earthquakes with m=3.7÷5.2 that occurred at epicentral distances up to 110 km. the observed variations occurred nearly 12 hours before the events. these results demonstrate that geoacoustic measurements using high-sensitivity geophones (belyakov et al., 1999) are able to provide new significant information and put some light on the earthquake preparation process. until now most surveys have been conducted in seismically inactive regions. interesting results related to gae measurements with a geophone have been revealed recently in works (cuomo et al., 2000; gregori et al., 2002; paparo et al., 2002), where another approach has been developed with a similar purpose: to distinguish signals originated by stressed-strained crust from the background of extraneous noise. the authors (cuomo et al., 2000; gregori et al., 2002; paparo et al., 2002) argued that ultrasonic acoustic emission measurements of the earth surface can be a diagnostic tool for changes in crust stress in seismic areas. this is possible provided that the sensors are installed on bedrock outcroppings, and acoustic emission signals are recorded in the ultrasonic frequency range (10200 khz) higher than usual frequencies of gae measurements. according to (paparo et al., 2002) bedrocks from top to bottom can play a role of a giant probe, so that ae signals recorded on the outcrop are sensitive to variations of tectonic stress and are relevant to seismic processes. although ultrasonic signals are not able to extend to large distances, anomalies of ae flow before strong earthquakes have been revealed during ae measurements in apennines, italy (cuomo et al., 2000; gregori et al., 2002; paparo et al., 2002). the existence of distinct diurnal variations of gae level in the frequency range 10-200 khz and their interrelations with changes in natural electric field strength follow from data given by (paparo et al., 2002). diurnal varia739 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity data management center of the institute of volcanology and seismology, far east branch of the russian academy of sciences, petropavlovsk-kamchatsky, on request of the operator. 2.1. gae level dependence on depth and borehole log before geophone installation at a permanent depth 1035 m, measurements of gae level were performed on various minor depths (fig. 2). at 200 m depth the level of noise in frequency bands 30 and 160 hz dropped approximately by 17-20 db (7-10 times) compared to that on the original ground. subsequent deepening of the geophone up to 600 m depth was followed by smooth damping of the noise in the noted bands: it decreased by 10-13 db comferred through armored cable to the main processing unit, located at the mouth of the well. the main analog unit provides additional amplification, subsequent filtering of the initial signals of each geophone channel by a third octave band pass filter, distinguishing the bands with the four central frequencies chosen: 30, 160, 560 and 1200 hz, and, finally, the measurement of averaged values of output filtered and rectified signals (from each of 12 filters). the further processing of signals is realized with the help of the microprocessor controller. the functions of the microcontroller involve analog-digital converting of the input analog signals (the sampling rate is 32 hz on the channel), calculation of average values of recorded signals in one minute moving window, and, finally, data saving on the disk memory. the data are transferred via telephone channel to the fig. 1. the structure of measuring complex. 740 v. gavrilov, l. bogomolov, y. morozova and a. storcheus layer of 700-900 m depth and decrease sharply from 900-1035 m may be related to peculiar features of the geologic profile at the borehole site. the g-1 borehole is located in a small valley with rocky edges. according to the geological cross-section, the valley bed is filled up to 80 m depth by sandy-argillaceous deposits with inclusions of pyroclastic strata. the layer from 80 to 110 m depth is formed by siltstones and sandstones stratigraphically alternated by clay interlayers. denser slates of uniform makeup occur at depths more than 110 m. according to the cross-section, the rocks lying at depths from 520 to 840 m are characterized by horizontal schistosity. there are four explicit layers on this interval of depth (fig. 2), each layer being of 50-100 m thick. hydrothermal veins with pyrite and calcite are represented widely at depths from 650 to 740 m. this is dissimilar to only one layer occurring at depth between 100 and 520 m. the rocks lying at 820-1035 m depth pared to that at 200 m depth (in the band nearly 30 hz). a sharp decrease of gae level in low frequency bands at shallow depths was predetermined by contribution of man-made noise produced at the daylight surface. the bands with central frequencies 30 and 160 hz were close to urban frequencies. man-made noise at frequencies 560 and 1200 hz was less, and it did not contribute to gae levels even at shallow depths of 100-200 m. correspondingly, only slight decreases in gae levels were observed at higher frequency bands (560 and 1200 hz). significant increases of gae levels in all frequency bands occurred in the interval of 600 to 900 m depth. the greatest increment of noise (12-13 db) was denoted in the bands 30 and 160 hz. subsequent geophone embedding to 1035 m depth resulted in a decrease in noise levels for all bands. the non-trivial observation that gae amplitudes increase in all frequency bands in the fig. 2. the dependence of mean gae level at several frequency bands on depth of geophone location. gae signals horizontal component has been plotted. 741 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity simultaneously with it or some hours later. the records of horizontal channels were never influenced by raining. snowfalls exerted a stronger influence upon variations of gae level on horizontal channels that has been confirmed by observation of the effect in 11 of 13 cases. the effect of snow falloff appeared as 50-100 % increase in average noise level on horizontal channels, usually starting in 12-15 hours after the beginning of snowfall and ending in 3-12 hours after snowfall stopped. the most apparent effect of precipitations occurred during strong snowfalls when the daily accumulation of snow was 30-60 mm (fig. 3). sometimes, a delayed noise was noted even when the snowfall did not exceed 10-15 mm per day. interestingly, the data from the vertical channel never showed the effect of snowfall on the gae. the effect of precipitations on gae measurements can be explained based on the following model, which appeals to the geological structure of the valley. it was already mentioned that sediments of the uppermost layer (above 110 m depth) are not so consolidated as slate-like rocks situated below 110 m. snow precipitations resuls in additional weight, and this is the reason for some extra compaction of sediments at shallow depths in territory of the valley. simple calculations show that a 30-mm-thick snow layer (the case of observed correlation with gae) may produce additional pressure of nearly 300 pa. the lithostatic pressure at a depth of a few hundred meters is in the order of 2-5 mpa, in which the lowest estimate is given for the 110 m depth, i.e. the bottom of the uppermost layer. the ratio of additional pressure to lithostatic one is equal approximately to 10-4, and the relative change in uppermost layer strain is of the same value. we note that a typical value of young modulus of upper sediments (occurred in this layer) is of the order of 1010 pa, (scheidegger, 1975). using this value we may estimate the increment of absolute strain at 110 m depth, caused by weight of 30-mm-thick snow layer, as 3×10-8. meanwhile, lunar-solar tides are characterized by a 10-8 change in strain components (as noted in work (diakonov et al., 1990) for latitude 40-50 degrees). according to a well known hypothesis (rykunov and smirnov, 1985; diakonov et al., represent the single strata as well. presumably, the rocks of enhanced stratification, with softened inclusions at the bedding interface, cause enhanced dynamic and geoacoustic activity at that depth (the more contacting surfaces, the more microscopic shifts). some results of geoacoustic surveys in other regions (belyakov et al., 1996, 1999) may confirm this reasoning on stratification and heterogeneity (the cause of seeming crust anelasticity). all this may explain, to a certain extent, the maximal value of gae level found at depths in the middle of interval 600-900 m. apart from the schistosity interval (both upwards and downwards) the level of gae drops. certainly, the data on vertical dependence of gae indicate that the layer of depth from 700 to 900 m is the most preferred for permanent measurements with the geophone (rather than its actual location at the 1035 m horizon). however these data (summarized in the fig. 2) were obtained after the geophone installed at 1035 m depth. thereafter, because of technical reasons, we had no opportunity to change the position of the geophone. the main methodical result of gae amplitude vertical scanning (fig. 2) is direct proof that geophone installation inside the borehole at near 1000 m depth greatly reduces low frequency exogenous noise (30 and 160 hz bands). the level of noise reduced by 40 db (two orders of value) at 30 hz frequency, and also reduced by 17 db (nearly order) at 160 hz frequency, both cases in comparison with level of noise on original ground. 2.2. influence of meteorological factors comparisons of gae variations with changes in atmospheric pressure, air temperature and speed of wind revealed no appreciable correlation. a possible influence of rain on measurements on the vertical channel was noted in two of 24 cases. in the first case, the rainfall was about 20 mm in a day, in the second about 50 mm in a day. the effect of precipitations appeared in the form of 20-30 % bay-like increases in average level of noise which were delayed of 12-15 hours after the beginning of rain and terminated 742 v. gavrilov, l. bogomolov, y. morozova and a. storcheus face) is very sensitive to contact state changes. rain precipitations have practically no effect on the results of gae measurements, because the water goes away quickly. hence, minor increment of weight after raining together with the short term action of water weight may induce only weak perturbation in comparison with the case of snow precipitation. so, the change in gae level on horizontal channel is expected to correlate to temporal variations of normally acting force (horizontal pressure). the correlation has a lag of 12-15 hours after accumulation of snow precipitations. actually such a lag is a transition time during which pressure change transforms to plastic strain wave and propagates along the casing tube towards horizon of geophone location. the vertical channel of gae is not affected by this mechanism, since receiving condition for gae shearing component is not so sensitive. according to the mentioned observations and their interpreting, the gae data from horizontal channels have been distorted by exogenous inter1990), the tides have potential to trigger some fault slips, as well as geoacoustic emissions, since the rate of tidal stress is high enough with respect to a tectonic stress. the growth of additional strain during an intensive snowfall is still more rapid than that in the case of tides. actually, a snow accumulation of 30-mm-thick layer may occur in few hours, and the strain rate reaches the value 3×10-12 1/s. the rate of tidal strains was estimated as 1-3 × 10-12 1/s. so, the change in the uppermost layer strain, caused by snow falloff, may have some influence on inelastic processes (similarly to the case of solar-lunar tides). the effect is probably amplified due to a structural factor. edges of the valley are steeply inclined, although not vertical in the cross-section. the compaction of the upper sedimentary layer inside a limited area may cause a portion of the horizontal stress to act normally to the surface of the casing tube. the contact of the casing and the adjacent rock becomes more or less strong. the receiving condition of gae of horizontal component (normal to contact surfig. 3a-c. episodic influence of snow falloff on results of gae measurements: a) daily accumulation of snow, b) temporal dependence of vertical component gae level, 30 hz central frequency; c) the same for horizontal component. 743 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity where the lines above variables or expressions note their averaging over time window. as result of the solving system of the equations (3.2) one can receive: a fg f g g = − ⋅ −1 2 ; b f fg g g = − ⋅ −1 2 ; (3.3) this means that the coefficient a is the averaged amplitude of dgaed, whereas b is the daily average level of gae. coefficients a and b concern to data on entire window (all n counts of function f (ti )). we have selected a two-day window (n = 2880) for computations by the outlined algorithm. transition intervals of 30 min length close to the estimated times of sun rising and a sunset have been excluded from the analysis. the exceeding of the threshold value ac of 500 relative units (ru) by the average amplitude of dgaed has been considered as a criterion for the presence of gae diurnal distribution provided that the maximal value of dgaed amplitude was equal to amax of 2804 ru. the result of computations is that the characteristic diurnal variations have been distinguished in gae time series during approximately 60% of observation time. an example of such variations in temporal intervals during august 13-16, 2001 and september 3-7, 2001 is presented on fig. 4a and 4b (greenwich time is hereafter used). transition times from the minimum values of emission level to the maximal one and vice versa are about 5-10 minutes. the moments of transitions correspond to the times of terminator line crossing (times of sunset and sunrise) at the observation point. the most explicit diurnal variations (day time minimum and nocturnal maximum) have been revealed on gae series recorded by channels for vertical components, with the central frequencies of 30 hz and 160 hz. 3.1. the changes in gae characteristics related to the preparation of earthquakes as a rule, considerable perturbations of the diurnal gae distributions (dgaed) have been observed before earthquakes mlh≥ 5.0 in the ferences to a greater extent than that from the vertical channels. for this reason, only measurements of the vertical component of gae signals (i.e. the data from the vertical channel) have been used below in this paper, as outlined below. 3. results of continuous gae surveys at the 1035 m depth continuous gae measurements started in august 2000. in this section we consider peculiar features of temporal dependences of geoacoustic emission obtained by continuous surveys during august 5, 2000 to july 31, 2003. during primary data analysis we focused on manifestations of characteristic diurnal variations of averaged gae levels. characteristic variations constitute diurnal geoacoustic emission distributions (dgaed) of 24 hours periodicity which usually emerge in aseismic periods (gavrilov et al., 2006). an especially developed pc program has been used to establish the presence or absence of dgaed in gae time series from august 5, 2000 to july 31, 2003. sometimes a simple visualization of these time series (plotted output) can reveal diurnal variations. the program of the gae time series processing is based on «recognition» of the gae variations similar to logistic function of sunrise/sunset g (t), so-called meander (trial rectangular increment). the function g (t) is equal to 1 (unity) when the sun is above the horizon, and it is -1 after a sunset. the algorithm of recognition involves the selection of such coefficients of regression a and b, to minimize the regression function (below) in the given time window r f t ag t bi i i = −∑( ( ) ( )– ) ,2 (3.1) where f (ti) current value of gae level, t – time. coefficients a and b can be determined from system of the equations: ∂ ∂ = − + = r a ag fg bg2 2 2 02 ; ∂ ∂ = − + = r b b f ag2 2 2 0; (3.2) 744 v. gavrilov, l. bogomolov, y. morozova and a. storcheus prior to the seismic swarm in october 2002, which occurred 170 km to the south of petropavlovsk-kamchatsky, the dgaed disappeared seven days prior to the first shock (fig. 6). until this moment, a steady diurnal distribution was observed during about three months. being typical for aseismic periods, gae day night variations restored a day after the strongest (mlh =5.7) earthquake of the swarm. according to the catalog of earthquakes of the geophysical service of the russian academy of science, 56 earthquakes mlh≥ 5.0 inarea of the kamchatkan subduction zone. typical examples of such perturbations prior to the earthquakes are presented in figs. 4c and fig. 5. figure 4c shows the disturbance of dgaed before an mlh=5.2 earthquake, which occurred on september 1, 2001, 120 km to the northeast from the observation point. in this case the diurnal gae distributions, which appear to be normal during august 12-16, 2001, are conspicuously distorted (almost disappeared) during august 17-31, 2001. the dgaed return to the normal form soon after the earthquake. fig. 4a-c. a plot of characteristic diurnal variations of mean geoacoustic emission level (for z component of gae signals in a band with central frequency of 160 hz). the greenwich time is noted on time axis. fig. 5. examples of loss of diurnal gae distributions (dgaed) prior to the occurrence of earthquakes. 745 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity fig. 6. mapping of epicenters of earthquakes occurred during the period january 1, 2001july 31, 2003 in zone r≤300 km, centered at petropavlovsk -kamchatsky (events of mlh≥5.0 magnitude), and in zone r≤550 km (events of mlh≥5.5 magnitude). 746 v. gavrilov, l. bogomolov, y. morozova and a. storcheus of disorder of gae diurnal distribution was followed by an earthquake (as shown in fig. 7). meanwhile, the disturbances of dgaed (full disappearance or degradation) occurred before all 15 earthquakes of magnitude мlh≥5.0 inside circular zone around point of observation, of 300 km radius. also, such disorders occurred before all 21 strong earthquakes, мlh≥5.5 whose epicenters located at distances less than 550 km from the geophone point. the same is valid for the case of the strongest mw =7.3 earthquake that occurred on november 17, 2002 at 1000 km epicentral distance. so, the disorder of diurnal variation may be considered a necessary but not sufficient condition of a forthcoming earthquake. our measurements at the single point give no result on prediction of place of shock. but the most important consequence of the observations is that during a period of regular diurnal variations in gae (lasting 7 days or more) no earthquake is expected as long as a disorder of the dgaed is to occur. below we consider statistical aspects of a relationship between times of strong earthquakes occurrence and cases of the dgaed disorder/restoration. cluding aftershocks of strong earthquakes occurred within 550 km in distance from petropavlovsk-kamchatsky during august 1, 2000 – july 31, 2003. a subsample of 36 seismic events was selected and used for analysis, taking into account 15 earthquakes of magnitude мlh≥5.0, occurred in near-field zone (inside a circle of r≤300 km radius) and 21 earthquakes of major magnitude (мlh≥5.5), with epicenters located in circular zone r≤550 km. parameters of these earthquakes are given in table i and locations of epicenters are shown on fig. 6. the strongest mw =7.3 earthquake during the last five years that occurred on november 17, 2002, at 459 km depth is also presented in table i. the epicenter of the earthquake was located in the okhotsk sea at 1000 km from the point of observation. figure 7 compares periods of dgaed presence or absence with exact times of occurred earthquakes. it is shown that earthquakes mostly occur during periods without characteristic diurnal variations. this includes an example of rather short interval (24 hours) without characteristic diurnal variations, for which earthquake of august 2, 2001 occurred. not every episode fig. 7. the periods of presence/absence of diurnal variations of gae versus times of major earthquakes occurrence: 1presence of characteristic dgaed, 2-data gaps. the earthquakes with мlh≥5.0 occurred at epicentral distances of r≤300 km from the base station and the earthquakes with мlh≥5.5 occurred at epicentral distances of r≤550 km. the earthquake of november 17, 2003, mw = 7.3 occurred at near1000 km epicentral distance. 747 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity table i. earthquake’s parameters, date january 1, 2001july 31, 2003, location r≤300 km, мlh≥5.0; location r≤550km, мlh≥5.5; location r≤1000 km, мw>7.0) note: hearthquake depth; rdistance from epicenter to borehole g-1 earthquake no date mlh lat.n long.e h, km r, km 010207 2001:02:07 6.4 52,28 153,66 476 346 010802 2001:08:02 6.2 56,21 164,05 25 495 010901 2001:09:01 5.2 53,92 159,75 134 120 010917 2001:09:17 5.3 52,84 159,98 41 191 011007 2001:10:07 5.2 52,39 160,67 2 153 011008a 2001:10:08 6,1 52,62 160,46 31 129 011008b 2001:10:08 6.3 52,65 160,49 24 130 011008c 2001:10:08 5.1 52,5 160,59 15 142 011009 2001:10:09 5.0 52,43 160,59 18 146 011010a 2001:10:10 5.5 52,51 160,57 17 141 011010b 2001:10:10 5.2 52,46 160,72 8 152 011103 2001:11:03 5.5 55,93 161,35 104 366 020106 2002:01:06 5.9 48,67 155,09 40 543 020128 2002:01:28 6.0 49,29 155,98 21 454 020215 2002:02:15 5.0 52.12 159,89 18 130 020315 2002:03:15 5.5 49,44 155,96 21 439 020426 2002:04:26 5.9 53,36 160,99 57 160 020503 2002:05:03 5.0 52,49 160,79 20 155 020508a 2002:05:08 5.6 52,22 160,44 32 150 020508b 2002:05:08 5.9 53,73 160,93 35 170 021008 2002:10:08 5.3 52,72 160,30 33 115 021016 2002:10:16 5.7 51,66 157,68 108 165 2002:11:17 мw= 7.3 47.82 146.21 459 1050 021218 2002:12:18 5.0 52,91 159,82 40 79 030120 2003:01:20 5.7 49,06 155,88 54 480 030221 2003:02:21 5.3 55,45 159,79 349 279 030315 2003:03:15 5.8 52,15 160,66 4 166 030317а 2003:03:17 5.0 52,25 160,58 13 155 030317b 2003:03:17 5.1 52,09 160,72 13 174 030317c 2003:03:17 5.5 52,26 160,54 32 153 030318 2003:03:18 5.4 52,23 160,61 15 159 030319 2003:03:19 5.9 52,16 160,85 48 176 030325 2003:03:25 5.0 52,02 160,70 40 178 030423 2003:04:23 6.1 55,98 163,44 20 450 030424 2003:04:24 6.0 48,76 155,21 42 531 030529 2003:05:29 5.7 50,65 157,53 40 274 030616 2003:06:16 6.7 55,3 160,74 190 263 748 v. gavrilov, l. bogomolov, y. morozova and a. storcheus p q n k q qn k( , , ) ,–= ckn k(1− ) (3.4) where ckn is a binomial coefficient which is determined by the formula ckn = n!/ ( k!(n-k)! ) to use (4) we associated the number n of tests for gae data with the number of strong earthquakes which occurred independently of one another during the time of observations. some seismic swarms were recorded in the period october, 2001–march, 2003. since shocks in a swarm were not independent events, we considered each swarm as a test (in the same manner as an ordinary earthquake). the total number of independent seismic events (tests) is n=26. only one earthquake occurred when the characteristic diurnal variations were present (the date is june, 16 2003, see fig. 7). so, k = 1 and the number of successful events is (n – k) = 25. a value of probability q may be estimated as a ratio of time without gae diurnal variations to total time of observations. in the case considered q = 0,4. the binomial distribution gives following numerical estimate of probability for noted value of parameters q,n,k: р = 1.8 ⋅ 10-9. such value of probability is very small compared to unity, and in comparison with the maximum of expression (3.4). it is well known that the expression of binomial distribution (3.4) reach maximum when q = (n-k)/n. in our case the maximum of (3.4) is approximately equal to рmax ≈ 0.18. in a conditional case when all 37 earthquakes are independent, n=37, and k=1 the value of probability р for the same q is of order 10-14. so, the hypothesis is not valid that a disorder of characteristic diurnal variations and final phase of earthquake preparation is a random coincidence. one can deduce from the negligible probability of co-incidental correlation between gae variations mode and times of strong seismic events that onset of gae diurnal variations damping regularly precedes the possible occurrence of an earthquake. 4. discussion of a possible origin to study the correlation between dgaed and variations of averaged strength of natural electromagnetic (em) field we carried out syntemporal intervals from the beginning of the «usual» dgaed perturbation until the time of the earthquake mlh≥ 5.0 are in limited to 1÷25 days. the relationship between noted temporal intervals and parameters of the earthquakes is still debated. this also concerns the recovery time for diurnal distribution after the earthquake occurred. restoration of dgaed following the earthquakes takes several days in about 30 % of all cases, but in some cases the recovery time was much greater. it is worth mentioning that we considered several of limits on magnitude мlh and radius of epicentral zone r. we attempted to separate stronger earthquakes (occurring times to be compared with presence/absence of diurnal variations) and the flow of weak and moderate seismic events. on the other hand, we need a sufficient accumulation of strong events for the analysis. enumeration of possibilities resulted in the optimal choice of above limits of magnitude мlh and radius r. we selected two limiting values of мlh (5.0 and 5.5) and two corresponding values of r (300 and 550 km) for the sake of strong validity of the results. our optimal choice of мlh and r limits allowed us to obtain the above relationship between gae diurnal variations and the seismic process in the studied region. another selection of these limits may modify the result represented in fig. 7. for example, reducing the мlh limit under condition of the same value of radius r entails a growth of the number of earthquakes that occurred on the background of gae diurnal variations. in one case an earthquake occurred during a period of diurnal distribution restoration, when the time of dgaed disappearance ended. this does not contradict the fact that the disappearance of dgaed preceded the occurrence of all 37 earthquakes. but we prefer to consider this debatable case as unsuccessful event. we estimated the probability of coincidence of anomalies of gae observations (disorder of characteristic diurnal variations) and strong earthquake occurrence with the help of probability binomial distribution technique. the binomial distribution describes the probability p(q,n,k) of occurrence of k unsuccessful events in n tests on conditions that the probability of successful result of individual test is (1-q): 749 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity phasizing that we intended to involve just electromagnetic field because of following a priori reason. it is well-known that a temporal dependence of natural electric field (vertical component en, in particular) has diurnal variations with nocturnal maxima and day time minima. this notion is based on studies of characteristics of radio noise and electromagnetic waves propagation in the frequency band 30 hz 30 khz (osinin, 1982; remizov, 1985). lightning discharges in remote world thunderstorms centers are the main sources of electromagnetic waves in noted frequency band at northeastern part of territory of russia (remizov, 1985). chronous measurements of gae and em variations. an underground electric antenna was used for measurements of the vertical component of the natural electric field, en. the perceptibility of the antenna whas reached with wire elements of more than 1 km length: a free conductor of multicore cable of the geophone and borehole casing. the parameters of electromagnetic measuring channels were identical with geoacoustical ones (the only difference is in the source of signals), measurements were carried out in the same frequency bands. the measurements started in may 2003 at the same point (g-1 borehole). it is worth emfig. 8a-c. results of simultaneous measurements of geoacoustic and electromagnetic temporal variations: a) gae level measurements at 160 hz frequency band (z component); b) em field level measurements at the same frequency band; c) cross correlation coefficient ρ. the vertical line indicates earthquake moment. 750 v. gavrilov, l. bogomolov, y. morozova and a. storcheus periods when dgaed were steady. meanwhile such periods coincided with aseismic time gaps. during these periods the cross correlation coefficient ρ was of order of value ρ = 0.80 ÷ 0.99. so, for instance ρ = 0.91 ÷ 0.99 for interval of april 9-12, 2004, fig. 8; ρ = 0.92 ÷ 0.93 for interval of june 15-17, 2003, fig. 9. but, before strong earthquakes (a day or more) and during relaxation periods after the events the variations of gae and of electromagnetic field became dissimilar: ρ = 0.53 ÷ 0.89 for interval of april 13-19, 2004, fig. 8; ρ = 0.001 ÷ 0.34 for interval of may 28 june 09, 2003, fig. 9. it should be noted especially, that the strong correlation between results of gae and natural the day-night variations of ultra low frequency (ulf) electromagnetic field are pre-determined by daylight sharp deterioration of radio wave propagation condition in a waveguide between ionosphere and the earth surface primarily due to lowering of ionosphere height resulted from daylight occurrence of d-layer at near 80 km height. in kamchatka, ulf electromagnetic emission with explicit diurnal variations is caused mainly by lightning discharges in a remote world thunderstorm center, located on azimuth 190-255 degrees. the results of simultaneous geoacoustic and electromagnetic measurements demonstrated that the variations of gae level and electromagnetic ones were practically identical during fig. 9a-c. an example of resumption of day-night variations of mean gae level: a,b,cthe same as on the fig. 8. 751 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity wires of the multicore cable to be sure that output signals of gae depended on sensor only. this differs from features of induced noise which is sensitive to position of conductor (distance from wire to core screen). some closely related results of other works confirm the physical rather than instrumental origin of correlated diurnal changes in gae and em signals. for instance, a high degree of correlation of diurnal variations of gae and electromagnetic field during the aseismic period is correlated with the result of (paparo et al., 2002) where diurnal variations of ae level on bedrock outcroppings with maxima at local nighttime (similarly to gae, see interval of april 9-12, 2004, fig. 8) were described. the synchronization of increase/decrease of ae and of natural telluric electromagnetic field have been noted. both temporal dependences can be characterized with a meander line. the similarity of daily distributions of gae and ae which differ by frequency range and depths of sources speak in favour of the fact that this phenomenon is a manifestation of behavior of the loaded geological medium. no particular factor like bias of electric induction over long cable or thermal stress near bedrocks surface due to nighttime cooling (paparo et al., 2002) is adequate for consistent explanation of those synchronized variations. the comparison of results obtained by borehole gae measurements with the results of laboratory experiments with loaded specimens exposed to electromagnetic field (chelidze et al., 2002; sobolev and ponomarev, 2003; il’ichev et al., 2003; bogomolov et al., 2004; zakupin et al., 2006a) has become the theory of a unified role of electric field for ae and gae variations. according to the experiments referred above electromagnetic action over rock specimens which are tested by press driven compressive loads accompanied by appreciable growth of ae activity. the main feature of the revealed effect of ae electrostimulation is the temporary growth of averaged level of ae activity after short impact or during a session with enhanced electric strength, provided that the main compressive load is constant and its value is in the range from 0.7 up to 0.95 of disruption. this is the so-called ae response to electric em field measurements (in particular, synchronous diurnal variations in both signals) cannot be explained by electromagnetic noise induced by the environment. non-instrumental origin of the observed effect may be verified by presented facts and considerations. usually, electromagnetic noise collected by sensor input and/or transmission line (long cable), particularly the influence of the neighboring measuring line, is a constant, when the location of sensors and cables is fixed. but the results of simultaneous geoacoustic and electromagnetic measurements represented by fig. 8 demonstrate the change in gae record, meanwhile the plot of natural electric field (vertical component) is without change. diurnal variations of gae disappeared rapidly (fig. 8a), and the correlation coefficient between gae and eme series reduced. the dissimilar change in gae was observed shortly before earthquake, but this aspect plays no role for the point of that how to check up the absence of noise induced by neighboring channel. another example is given by fig. 9, where the period from may 28 to june 11, 2003 is without gae diurnal variations, thereafter the variations arise in interval june 12-17, 2003 and their amplitude increases gradually. but diurnal variations in the natural electromagnetic field (fig. 9b) are of the same amplitude during the period of may 28-june 17, 2003. this fact may be considered a solid argument that variations in output signals on gae measuring channels do not interference induced by external electromagnetic field via sensor details or cable connecting the geophone with recording and processing units located at original ground. in the contrary case the amplitude of gae diurnal variations should be steady throughout the period of observations. moreover, the temporal plots of horizontal and vertical component of gae have quite a different form, although the frequency band is the same (gavrilov et al., 2006). the constructive elements of both channels are identical. so the characteristics (such as shunt capacitance and mutual inductance to circuit of electromagnetic channel) are close, and they predetermine noise occurrence. we checked this before geophone installation. also, we reconnected geophone output to 752 v. gavrilov, l. bogomolov, y. morozova and a. storcheus strained geological medium. deepening of the geophone at 1000 m depth in borehole at petropavlovsk-kamchatsky reduced the level of anthropogenic noise by two orders of magnitude in the band of 30 hz central frequency, and reduced it almost by an order of magnitude in the 160 hz band. furthermore, increasing the depth of measurements has practically eliminated the bias of meteorological factors on measurements of the vertical component of gae signals. new gae measurements have revealed diurnal temporal distribution of gae level in bands with central frequencies 30 and 160 hz. diurnal distribution of gae is presumably related to daily variations of the level of electromagnetic field strength in earth’s crust. the loss or damping of geoacoustic day-night variations (provided continued diurnal variations in natural electric field) appears to indicate changes in stressed-strained state of the geological medium which allow a considerable growth of probability of strong earthquake occurrence. a generalized sequence of obtained results is proposed as a precursor of «short-term aseismicity»; an indicator that no strong earthquakes are likely to occur in a circular (300 km radius) area around the observational point, during a period of robust presence of gae diurnal variations, as long as a disorder of gae mode terminates such period. this important principle (precursory statement) may be used for recognition and/or reliable predictions of temporal intervals, when the probability of strong earthquake occurrence in a limited area is very low (at least, for kamchatkan region and north kuriles islands). acknowledgements this research was partially supported by the rfbr (grants 06-06-05-96071 and 07-0500687) and feb ras (the project 06-iii-а-08-327). references belyakov, a.s., a.g. gamburtsev, v.s. lavrov, a.v. nikolaev and n.k. privalovskii (1996): vibration action on rocks and their induced seismic emission, power action, epi, (zakupin et al., 2006a; 2006b). heuristically, such acoustic emission response to epi seems to look similar to the effect of nighttime gae level growth correlated to nocturnal enhancement of electric field level (the more value of natural electric field strength the more level of gae in the same frequency band). our approach to unified consideration of laboratory and full-scale survey results is based on the similarity of stressed state of embedded rocks and laboratory tested ones. actually, ae responses were recorded on specimens when the main compression stress was in the range 20 100 mpa. a tectonic stress at nearly one km depth is of the same order of magnitude (kropotkin et al., 1987). but a difficult point for the hypothesis of electric stimulation of nighttime gae level (resulting in the modulation) arises during quantitative estimates of strength of natural electric field and that applied in laboratories. quantitative estimates to analyze the observed modulation of gae level by natural electromagnetic emission of the earth are as follows. according to reference (remizov, 1985), the maximal value of the electric component of the em field measured on earth surface, over the whole frequency band may be evaluated as 0.45÷1.0 mv/m. the damping of electric component at 1 km depth, calculated for 160 hz and the soil of moderate moisture (the conductivity is about σ = 0.01 (ohm×m)-1) is near 9 db. hence, the amplitude of electric field strength at the depth of geophone location can not exceed 0.16÷0.36 mv/m. measurements in other boreholes and in other regions are required to establish that correlated diurnal changes in gae are a general effect (although the mechanism of electricgeoacoustic conversion is not clear completely). alternatively they may be caused by specific oversensitivity of gae sources located in the vicinity of the geophone at the point of g-1 borehole. 5. summary continuous geoacoustic measurements in deep boreholes with high-sensitivity geophones are an effective method of monitoring stressed753 variations in geoacoustic emissions in a deep borehole and its correlation with seismicity gregori, g.p., g. paparo, u. coppa and i. marson (2002): acoustic emission in geophysics: a reminder about the methods of analysis, bull. geophys. teor. appl., 43 (12), 157-172. il’ichev, p.v., a.v. alad’ev, l.m. bogomolov, v.n. sychev et al. (2003): parameters of acoustic emission signals initiated by electric action over loaded specimens, in geodynamics and geoenvironmental problems of highmountain regions, edited by s.v. goldin and yu.g. leonov (printhouse, moscow-bishkek), pp. 286-303. kropotkin, p.n., v.n. efremov and v.m. makeev (1987): stressed state of earth crust and geodynamics, geotectonics, (1), 3-24. osinin, v.f. (1982): radio noise from natural sources in the east of the ussr (nauka, moscow), pp. 161 (in russian). paparo, g., g.p. gregori, u. coppa, r. de ritis and a. taloni (2002): acoustic emission (ae) as a diagnostics tool in geophysics, annals of geophysics, 45 (2), 401-416. remizov, l.t. (1985): natural radio-interference (nauka, moscow), pp. 198 (in russian). rykunov, a.l. and v.b. smirnov(1985): variations of the seismicity under effect lunar-solar and tidal deformations, fizika zemli (solid earth phys.), 21 (1), 97-103. scheidegger, a.e. (1975): physical aspects of natural catastrophes, (elsevier scientific, amsterdam), pp. 289. sobolev, g.a. and a.v. ponomarev (2003): physics of earthquakes and precursor, (nauka, moscow), pp. 270. zakupin, a.s., a.a. avagimov and l.m. bogomolov (2006a): responses of acoustic emission in geomaterials to the action of electric pulses under various values of the compressive load, physics of the solid earth, (10), 43-50. zakupin, a.s., a.v. alad’ev, l.m. bogomolov, v.n. sychev et al. (2006b): interrelation of electric polarization and acoustic emission of terrestrial materials specimens under conditions of uniaxial compression, volcanology and seismology, (6), 22-33. fizika zemli (solid earth phys.), 32 (2), 68-74. belyakov, a.s., a.g. gamburtsev, v.s. lavrov, a.v. nikolaev and l.l. khudzinskii (1999): underground acoustic noise and its relation to tidal deformations, ibid, 35 (12), 1002-1009. belyakov, a.s.(2000): magnetoelastic acoustic geophones for geophysical research, seismic equipment (uipe ras, moscow), book 33, 27-45. bogomolov, l.m., p.v. il’ichev, v.a. novikov, v.i. okunev, v.n. sychev, a.s. zakupin (2004): acoustic emission response of rocks to electric power action as seismicelectric effect manifestation, annals of geophysics, 47 (1), 65-72. chelidze, t., n. novaramashvili, m. devidze, z. tchelidze, v. chikhladze and t. matcharashvili (2002): laboratory study of electromagnetic initiation of slip, annals of geophysics, 45 (5), 587-598. chelidze, t., t. matcharashvili, j. gogiashvili, o. lursmanashvili and m. devidze (2005): phase synchronization of slip in laboratory slider system, nonlinear processes geophys., 12, 1-8. cuomo, v., v. lapenna, m. macchiato, i. marson, g. paparo, d. patella and s. piscitelli (2000): geolectrical and acoustical anomalous signals compared with seismicity in a test side of southern apennines (italy), phys. chem. earth. (a), 25 (3), 255-261. diakonov, b.p., b. s. karryev, o.b. khavroshkin, a.v. nikolaev, l.n. rykunov, r.r. seroglasov, a.k. trojanov and v.v. tsiplakov (1990): manifestation of earth deformation processes by high-frequency seismic noise characteristics, phys. earth and planet. inter., 63, 151-162. gavrilov, v. a., yu.v. morozova and a.v. storcheus (2006): variations in the level of geoacoustic emission in deep well g-1, kamchatka and their relation to seismicity, volcanology and seismology, (1), 52-67. gavrilov, v. a. (2007): physical causes of diurnal variations in the geoacoustic emission level, doklady ran, earth sciences, 414, (4), 638-641. gorbatikov, a.v., o.a. molchanov, m. hayakava, s. uyeda, et al. (2001): acoustic emission before and after earthquakes, volcanology and seismology, (4), 66-78. layout 6 annals of geophysics, 60, 6, s0663, 2017; doi: 10.4401/ag-7305 temporal analysis of δ13cco2 and co2 efflux in soil gas emissions at mt. etna: a new tool for volcano monitoring salvatore giammanco1,*, bor krajnc2,3, jože kotnik3, nives ogrinc2,3 1 istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy 2 jožef stefan international postgraduate school, ljubljana, slovenia 3 jožef stefan institute, department of environmental sciences, ljubljana, slovenia article history received november 11, 2016; accepted october 2, 2017. subject classification: mt. etna; carbon isotopes; co2 efflux; volcanic activity; hydrothermal systems. s0660 abstract we monitored the soil gas emission of co2 from selected sites of mt. etna volcano during the period february 2009 to december 2010 by measuring periodically the soil co2 efflux together with the associated stable carbon isotope composition of co2. correlation between the two parameters showed distinct behaviors depending on the sites as a reflection of the different interactions between crustal and sub-crustal fluids. where deep co2 interacted with shallow cold ground water and/or with shallow biogenic co2, a positive correlation between soil co2 effluxes and carbon isotopes was evident and it depended strongly on the velocity of gas through the soil. in these cases, the highest co2 effluxes corresponded to δ13cco2 values similar to those of the deep magmatic co2 emitted from the crater and pericrateric gas emissions at the summit. in areas where a shallow hydrothermal system was presumed, then a similar correlation was less evident or even absent, suggesting strong control on c isotopes arising from the interactions between co2 gas and dissolved hco3that occur in aquifers at t>120 °c. marked temporal variations were observed in both parameters at all sites. no significant effect of meteorological parameters was found, so the observed changes were reasonably attributed to variations in volcanic activity of mt. etna. in particular, the variations were attributed to increased degassing of co2 from incoming new magma, possibly coupled with increased hydrothermal activity in at least some of the shallow aquifers of the volcano. the largest anomalies in the monitored parameters preceded the opening of the new southeast crater in late 2009 and therefore they could represent a key to unveiling the dynamics of the volcano. 1. introduction mt. etna volcano, one of the most active volcanoes in the world, is also known as a one of the main sources of magmatic co2. the total emission rate has been recently estimated at about 60,000 t·d-1 [hernández et al. 2015]. most of the release of co2 occurs through the active summit vents of the volcano, but a remarkable fraction of this gas (about 10 % of the total amount according to d’alessandro et al. 1997b and hernández et al. 2015] is released from its flanks. in this case, co2 emissions at the surface occur in diffuse form through volcano-tectonic faults [e.g., giammanco et al. 1998, aiuppa et al. 2004] and they are produced by magma outgassing from reservoirs located at intermediate to great depths beneath the volcano [bruno et al. 2001, aiuppa et al. 2004, giammanco et al. 2013]. studies on soil gas carried out in the last 17 years at mt. etna showed that the central eastern flank and the lower southwestern flank of the volcano are characterised by the strongest anomalies in both soil gas concentration and soil co2 efflux and by the highest content of magmatic co2, which dissolves into local groundwater [anzà et al. 1989, giammanco et al. 1995, 1996, allard et al. 1997, brusca et al. 2001, aiuppa et al. 2004, giammanco and bonfanti 2009]. the aim of the present work was to study the temporal variations of soil co2 efflux and the associated stable carbon isotope composition of co2 over a time span of several months, covering the period between the end of the 2008-2009 flank eruption and the opening and eruption of the new southeast crater (nsec), today the most active summit crater of mt. etna (figure 1). the main objectives of our study are: i) to better define the correlation between the above two parameters in selected high-degassing sites of the volcano; ii) to model the dynamics of geochemical interactions between co2 and the surrounding environment and iii) to recognize possible influences of volcanic activity on soil degassing. 2. eruptive phenomena at mt. etna during the studied period mt. etna is a large quaternary composite volcano, which grew to its present elevation of 3320 m by accumulation of lavas and tephra erupted during the last 200 kyr [gillot et al. 1994]. at present, its activity is represented by summit and/or flank eruptions, the former being mostly short-lived and characterized mainly by production of tephra with minor lavas, the latter being, on average, long-lived and mostly producing lava effusions that may emplace voluminous lava flow fields. our study began during the final stages of the longstanding 2008-2009 flank eruption. details on this eruption are given in bonaccorso et al. [2011] and in corsaro and miraglia [2014]. the eruption started on may 13th from a fissure that developed from the se crater togiammanco et al. 2 figure 1. map of the study area. the red circles indicate the location of soil co2 sampling points. the three main rift zones (ne, w and s) are also shown. vb = valle del bove morphological depression. the blue squares indicate the location of the two sias meteorological stations used for the purposes of this study. the yellow star shows the location both of the se crater and of the nearby nsec. 3 δ 13cco2 and co2 efflux in soil gases of mt. etna wards the valle del bove (vb in figure 1), a morphological depression that occupies a large part of the east flank of mt. etna. lava flows from this fissure expanded inside the vb, reaching a maximum length of about 6 km and a total volume of about 70 ± 20 × 106 m3 [branca s. quoted in corsaro and miraglia 2014]. the eruption ended on july 7th, 2009, 419 days after its onset. just a few months later, on november 6th, 2009, a new vent opened on the lower eastern slopes of the southeast crater, one of the four summit craters of mt. etna and the most active one in the last thirty years [e.g., behncke et al. 2014]. this new vent, named new southeast crater (nsec), appeared as a relatively small open pit emitting hot pressurised gas. its birth marked the end of the eruptive activity at the se crater, thus virtually “replacing” it as a new opening in the conduit of the previous se cone. the vigorous degassing activity of nsec continued until the end of 2010, without appreciable changes. in the meantime, since august 25th, 2010, some isolated explosive episodes occurred at the bocca nuova crater (bn), the westernmost of the four summit craters of etna (for details on this and on the following volcanological information, see http://www.ct.ingv.it/en/rapporti.html). the strong gas and ash explosion of august 25th, 2010 at the bn was followed by several smaller explosive events that produced short-lived emissions of volcanic ash. seven major explosions occurred at bn until august, 29th, although with less intensity than the first one. other significant explosive events took place at bn on october 7th and on november 1st, both producing minor ash plumes. after more than one month of relative quiescence at etna, a new strong explosion occurred at bn on december 22nd, 2010. this explosion was probably less powerful than the one on august 25th, but certainly stronger than the numerous events between late august and november 2010. on the afternoon of december 23rd, 2010, strombolian activity apparently started for the first time inside the nsec since its formation. however, bad weather conditions prevented clear observation of the summit area, so this activity could not be followed in detail. on the late afternoon of january 2nd, 2011 mild strombolian activity appeared again within the pit of nsec, preceded by several days of repeated emissions of hot gas. strombolian explosions, although with variable intensity, continued through january 6th and then ceased. mild explosive activity resumed on january 11th, and its intensity began to increase on january 12th until producing a small lava outflow. finally, a paroxysmal eruption occurred during the night between january 12 and 13, with lava fountains and voluminous lava flows. the event ceased completely after about two hours. this was the first of a long and complex sequence of eruptive events that characterized the evolution and growth of the nsec until 2015 [behncke et al. 2014, de beni et al. 2015]. 3. materials and methods 3.1. sampling sites for the purposes of this study, we selected the sites most representative of soil degassing (figure 1), as they are all characterized by anomalous high emissions of soil co2. in particular, site p39 is located on the southwestern flank at an altitude of 115 m a.s.l., ~2 km sw of the town of paternò, in an area with visible absence of vegetation due to the intense gas emissions from the soil. the other three sites (p78, zaf06 and zaf08) are located on the eastern flank of mt. etna, at altitudes between 320 m and 510 m a.s.l.. apart from co2, the chemical composition of the gas released from all sites, but especially at p39, shows trace amounts of ch4, he and sometimes h2 and co [giammanco et al. 1998]. previous geochemical studies on the gases emitted at sites p39 and p78 indicated that the origin of these gases is dominantly magmatic [d’alessandro et al. 1997a, giammanco et al. 1998, pecoraino and giammanco 2005]. in particular, the chemical and isotope features of gases emanating at site p39 show evidence of their direct origin from the mantle source of mt. etna basalts [giammanco et al. 1998a, caracausi et al. 2003a, 2003b]. this site is located on a ne-sw-directed regional fault that is thought, based mainly on geochemical data, to be part of the deep feeder system of mt. etna [>10 km; caracausi et al. 2003a, 2003b]. sites p78, zaf06 and zaf08 are part of an anomalous degassing zone that is aligned on a wnw-ese fault system [anzà et al. 1993, giammanco et al. 1995]. a recent study on soil co2 emissions in the central eastern flank of mt. etna [giammanco and bonfanti 2009], recognized this line as a major fault characterized by clear anomalous degassing whose changes in time are linked to volcanic activity. gases from this site are assumed to derive from a shallower portion (5 10 km) of the magma feeder system of mt. etna [giammanco et al. 1998, bruno et al. 2001]. 3.2. soil co2 efflux soil co2 effluxes (expressed in g m -2 d-1) were measured at all selected sampling sites using the method of the dynamic accumulation chamber [e.g., parkinson 1981, chiodini et al. 1998, farrar et al. 1995]. co2 effluxes were measured in at least duplicate during each survey, and the arithmetic average of the values was considered for the temporal analysis of data. the sampling frequency was variable but, on average, once every 20-30 days. the data cover the period from february 2nd, 2009 to december 12th, 2010. in order to evaluate the possible influence of the main meteorological parameters on the temporal variations of co2 efflux, values of air temperature, total daily rainfall, minimum and maximum relative humidity and wind speed at a height of 2 m above the ground were also acquired thanks to the monitoring network of the servizio informativo agrometeorologico siciliano (sias). in particular, we chose the station of paternò, representative of the weather conditions of the low sw flank of mt. etna, and the station of riposto, representative of the weather conditions of the low e flank of mt. etna (see figure 1). because the co2 efflux data are log-normally distributed [ahrens 1954, giammanco et al. 2010], all graphic analyses of these data in the present paper have been performed on a log10 scale. 3.3. stable carbon isotope composition of co2 samples for stable isotope analysis were collected in the soil at a depth of 50 cm using a 5 mm id teflon tube connected to a syringe. the gas aliquots were immediately injected into pre-evacuated 12-ml glass serum vials through a double-holed needle. to avoid possible contamination with gas from the sampler tube, the system was purged twice with the soil gas. the septum penetration needle allows direct delivery of the gas sample into the pre-evacuated vial thereby minimizing possible contamination with atmospheric air [torn et al. 2003]. the stable isotope composition of co2 was determined according to the methods of knohl et al. [2004] and spötl [2004]. these methods have been applied successfully in various forest soils in slovenia [čater and ogrinc 2011, krajnc et al. 2016, krajnc et al. 2017]. measurements were made directly from vials using a europa scientific 20-20 continuous flow isotope ratio mass spectrometer (irms) coupled to an anca-tg preparation module for gas samples. stable isotope results are reported using the conventional delta-notation (δ13cco2), in per mil (‰) relative to the vpdb reference standard δ13cco2. the accuracy of the measurements was checked using “co2 iso-top, high” co2 standard with δ 13cco2 value of −4.3 ± 0.2‰. the precision of measurements was ±0.2‰. 4. results and discussion 4.1. geochemical characterization of sites the results of the measurements carried out during our study are shown in table 1. figure 2 shows the correlation plot between the δ13cco2 values and the corresponding log10 values of co2 efflux measured at all sites. site p39 displays the highest levels of co2 efflux (up to about 32,000 g m-2 d-1) and the most positive values of δ13cco2 (between +1.6 and +3.1‰). furthermore, the isotope composition of carbon showed very small variation with time, despite the wide range of co2 effluxes measured. this is in line with previous measurements of both parameters at this site [giammanco et al. 1998, pecoraino and giammanco 2005]. the isotopic shift of carbon observed at this site was explained as being due to strong interaction between deep magmatic fluids and a hydrothermal aquifer at t° > 120 °c because, in this case, the fractionation factor in water between gaseous co2 and dissolved hco3 favors enrichment of the heavier carbon isotope in the residual gas phase that passes through the ground water [mook et al. 1974]. higher water temperature causes a larger positive isotopic shift in δ13cco2 values. in fact, the presence of a geothermal aquifer beneath the area of paternò has already been postulated by chiodini et al. [1996], who calculated a temperature of water of up to about 150 °c, based on liquid and gaseous geothermometers. in our case, the deep co2 that interacts with the hydrothermal aquifer at about 150 °c should undergo an isotopic shift of about +2‰ [mook et al. 1974]. therefore, the estimated δ13cco2 values of the pristine deep co2 before interaction with thermal water would be very similar to those measured in the high-temperature fumaroles near the summit of mt. etna [giammanco et al. 1998, aiuppa et al. 2004], assumed as representative of the c-isotope composition of magmatic co2 of this volcano (grey horizontal band in figure 2). however, the c isotope values found in this study at site p39 are slightly more positive than those found at the same site in previous work [giammanco et al. 1998, pecoraino and giammanco 2005]. if we assume a pristine c-isotope composition in the range from -2.5 to -1‰ as representative of the magmatic co2 emitted from etna [giammanco et al. 1998, pecoraino and giammanco 2005], then the range of values that we observed would be compatible with an isotopic shift of +3 to +4‰. according to mook et al. [1974], such a fractionation factor would be due to interaction between co2 and the hydrothermal system at a water temperature of 180-200 °c. a summary of the above geochemical interactions for site p39 is shown in the graphic model of figure 3a. the other monitored sites showed a distinct and generally coherent behavior. figure 2 highlights that, differently from site p39, δ13cco2 values at sites p78, zaf06 and zaf08 showed a wider range of variations giammanco et al. 4 5 δ 13cco2 and co2 efflux in soil gases of mt. etna with time (from about -18‰ to about +1‰), although with a smaller deviation (from about -6‰ to about +1‰) at site zaf06. the most negative δ13cco2 value clearly points to a marked influence of an organic component in the emitted co2, produced from a shallow biogenic source of gas in the thick and well-structured soil in these sites [e.g., giammanco et al. 1998]. it must be considered, however, that a shift towards more negative values of δ13cco2 may also occur as consequence of fractionation of c-isotopes between gaseous co2 and dissolved hco3 when deep co2 reacts with cold ground water for a long time. the eastern flank of mt. etna is characterized by the ubiquitous presence of large unconfined aquifers. therefore, deep co2 rising at high pressure (>> 100 kpa) through faults inevitably encounters ground water and at least in part reacts with it, both chemically and isotopically. the entity of these reactions is more or less marked as a function of the contact time between gas and water. a long reaction time occurs because of long pathways of co2 gas through a thick aquifer or more generally because of low gas flux conditions. given enough contact time between high-pressure dissolved co2 and water, the fractionation factor ranges from -9.6 to -8.5‰ in the temperature interval between 10° and 25°c [mook et al. 1974]. the above geochemical interactions for sites zaf06, zaf08 and p78 are summarized in the graphic model of figure 3b. the slightly more positive values observed at site zaf06 may be due at least to partial interaction between deep magmatic co2 and thermal ground water, similar to that observed at site p39. it should be noted that this site is located very close to a water well (“petrulli” well) with figure 2. correlation plot between δ13cco2 values and the corresponding log10 values of co2 efflux measured at all sites. the horizontal bands indicate the ranges of δ13cco2 values related to the main sources of co2 found at mt. etna (i.e., biogenic, magmatic and hydrothermal). water temperature constantly a little higher than the average temperature of mt. etna ground water and the highest among the ground waters of the east flank of the volcano [bonfanti et al. 1996, brusca et al. 2001, giammanco et al. 6 date zaf06 zaf08 p78 tair rain rh min rh max wind co2 flux δ 13cco2 co2 flux δ 13cco2 co2 flux δ 13cco2 °c mm % % m/s 02/03/09 3753 -5.10 233 -7.37 514 -12.93 14.70 0.00 55.00 100.00 1.09 02/16/09 12.4 -3.94 9.10 -17.69 5.30 -15.12 7.87 0.00 33.00 72.00 0.88 02/26/09 28.2 -6.17 7.60 -14.98 7.80 -14.43 9.20 0.20 30.00 73.00 1.95 03/12/09 77.5 -4.72 82.3 -16.21 68.5 -14.08 12.06 0.00 31.00 92.00 3.21 03/24/09 115 -4.42 39.1 -17.17 169 -16.18 10.96 0.00 38.00 79.00 0.98 04/09/09 2369 -5.10 62.3 -11.92 47.8 -11.03 13.23 2.80 66.00 100.00 0.53 04/20/09 n.d. n.d. 493 -6.92 406 -10.58 14.38 9.00 78.00 100.00 0.69 04/29/09 798 -5.00 280 -5.83 145 -12.43 15.98 0.00 39.00 98.00 0.75 05/13/09 709 -3.00 1480 -4.13 909 -9.02 18.22 0.00 40.00 92.00 0.87 06/01/09 587 -2.10 882 -3.62 1075 -6.00 21.14 0.00 64.00 100.00 0.80 06/15/09 2534 -2.62 548 -3.34 676 n.d. 23.69 0.00 27.00 69.00 0.89 06/26/09 1212 n.d. 515 -3.81 427 -4.80 21.80 0.00 41.00 79.00 0.81 07/13/09 2017 -2.37 991 -4.41 856 n.d. 24.00 0.00 37.00 84.00 0.98 07/30/09 920 -2.39 968 -3.14 1118 -4.39 26.46 0.00 34.00 78.00 0.97 08/10/09 736 -2.16 1493 -3.44 1188 -4.36 25.95 0.00 52.00 94.00 0.74 08/25/09 603 -2.31 764 -3.16 1104 -4.31 26.07 0.00 55.00 100.00 0.65 09/07/09 667 -1.90 619 -3.22 702 -4.22 23.46 6.40 38.00 100.00 1.17 10/22/09 2604 -1.91 2138 -3.12 1887 -6.52 20.15 8.00 83.00 100.00 0.42 11/01/09 1921 -1.11 232 -2.52 491 -4.95 13.53 0.00 43.00 82.00 0.77 11/16/09 757 -1.04 2104 -2.54 396 -5.06 17.65 0.00 25.00 74.00 0.80 12/10/09 534 -0.55 52.2 -7.06 338 -4.37 13.13 0.00 39.00 76.00 2.88 12/30/09 601 -1.00 2722 -2.35 678 -4.71 15.34 0.00 48.00 89.00 0.64 01/19/10 272 -1.03 37.3 -15.60 198 -5.44 9.99 1.60 42.00 100.00 1.20 01/29/10 3200 -1.76 408 -3.18 727 -6.67 9.51 0.00 59.00 90.00 0.68 02/12/10 193 -0.08 10.8 -16.91 29.0 -4.05 9.19 0.00 16.00 100.00 2.49 03/01/10 1540 -1.31 895 -2.57 740 -4.82 14.53 0.00 52.00 100.00 0.63 03/19/10 220 -0.53 18.8 -13.05 111 -5.90 11.28 0.00 40.00 81.00 1.05 04/09/10 1487 -0.87 336 -2.45 697 -4.09 14.10 0.00 50.00 85.00 0.56 04/26/10 4638 -0.74 670 -2.77 556 -3.31 16.83 0.00 55.00 100.00 0.72 06/08/10 2158 1.16 661 -2.96 949 -1.32 21.25 0.00 42.00 100.00 0.48 06/22/10 1479 1.01 312 -4.52 1223 -0.94 19.87 0.00 43.00 100.00 0.15 07/14/10 608 0.83 685 -2.16 2506 -0.85 25.93 0.00 48.00 100.00 0.93 07/29/10 521 0.72 422 -1.74 1362 -0.92 23.98 0.00 53.00 83.00 0.90 08/26/10 614 0.71 1156 -1.49 2415 -1.08 26.45 0.00 44.00 100.00 0.78 09/09/10 3037 0.55 550 -2.69 2723 -1.02 23.68 104.20 53.00 100.00 1.09 09/23/10 281 0.13 321 -2.29 605 -1.28 20.33 0.40 61.00 100.00 0.57 10/06/10 2777 -0.30 1971 -1.45 4606 -1.79 21.67 0.00 72.00 100.00 0.59 10/22/10 838 -0.30 8.40 -5.91 24.2 -3.85 16.95 0.80 75.00 100.00 0.75 11/19/10 176 -0.08 34.9 -6.92 224 -1.83 14.74 0.00 52.00 95.00 0.76 12/07/10 167 -0.80 281 -2.98 709 -3.04 13.19 0.00 63.00 100.00 0.86 table 1. values of soil co2 efflux (in g m -2 d-1) and δ13cco2 (in ‰ vs. vpdb) measured at sites zaf06, zaf08 and p78 during the studied period. also shown are the corresponding meteorological data measured at the station located at riposto. n.d. = not determined. 7 aiuppa et al. 2004]. a general positive correlation exists between the δ 13cco2 values measured at sites p78, zaf06 and zaf08 and the corresponding values of soil co2 efflux (on a log10 scale) (figure 2). in detail, assuming a log10 fit of data on the plots, the correlation coefficients (r2) between the pairs of parameters for each site were calculated as +0.40 for site p78, +0.07 for site zaf06 and +0.66 for site zaf08. these results also confirm preliminary results based on previous measurements performed in the same area. a similar positive correlation was found at site p78 by giammanco et al. [1998] and by pecoraino and giammanco [2005]. according to these authors, when co2 effluxes from site p78 were the highest, the corresponding values of δ13cco2 became very similar to the pristine magmatic ones assumed for mt. etna. more negative values of δ13cco2 would then indicate a higher degree of mixing between the magmatic component and the biogenic source in the soil gas, particularly evident at low co2 efflux values. this indicates that when co2 efflux becomes higher, magmatic co2 can reach the surface more efficiently, thus surmountδ 13cco2 and co2 efflux in soil gases of mt. etna date p78 tair °c rain mm rh min % rh max % wind m/s co2 flux δ 13cco2 05/13/09 568 2.79 18.59 0.00 13.00 91.00 0.48 06/01/09 31608 2.53 22.09 0.00 15.00 87.00 1.77 06/15/09 11384 n.d. 23.15 0.00 12.00 77.00 1.08 06/26/09 24468 2.50 22.58 0.00 12.00 86.00 1.07 07/13/09 22838 2.30 24.57 0.00 12.00 90.00 1.04 07/30/09 15143 2.33 26.73 0.00 18.00 82.00 1.49 08/10/09 10446 n.d. 26.78 0.00 26.00 82.00 1.49 08/25/09 7213 2.16 26.97 0.00 19.00 94.00 1.26 09/07/09 5288 2.13 24.38 0.00 26.00 94.00 1.24 11/16/09 1378 2.47 15.53 0.00 22.00 98.00 0.50 12/10/09 3076 2.44 13.36 0.00 38.00 80.00 1.90 12/30/09 3054 2.58 15.77 0.00 25.00 95.00 0.63 01/07/10 1777 2.94 12.81 0.00 43.00 96.00 1.64 01/29/10 1160 2.65 9.57 0.00 53.00 90.00 0.69 02/12/10 307 2.90 7.05 1.00 42.00 97.00 0.84 03/01/10 904 3.10 14.88 0.00 41.00 97.00 1.00 03/19/10 733 2.87 10.46 0.00 30.00 97.00 0.88 04/09/10 5124 2.92 13.87 0.00 40.00 93.00 1.72 04/26/10 5027 2.68 16.19 3.00 27.00 97.00 1.04 06/08/10 2374 2.99 22.44 0.00 26.00 91.00 1.83 06/23/10 3297 1.60 22.65 0.00 25.00 66.00 1.46 07/14/10 4227 2.79 27.23 0.00 18.00 79.00 1.80 07/29/10 10535 2.64 24.88 0.00 23.00 93.00 1.23 08/26/10 7871 2.65 27.30 0.00 20.00 87.00 1.74 09/09/10 4362 2.66 24.50 5.60 47.00 94.00 1.64 09/23/10 3699 2.47 19.85 1.20 52.00 95.00 1.38 10/06/10 5419 2.55 21.49 0.00 47.00 94.00 0.50 10/22/10 3044 2.31 16.12 1.40 61.00 93.00 0.95 11/19/10 1991 2.29 13.44 0.00 39.00 99.00 0.40 12/07/10 2339 2.31 13.59 0.00 42.00 99.00 0.45 table 2.values of soil co2 efflux (in g m -2 d-1) and δ13cco2 (in per ml vs. vpdb) measured at site p39 during the studied period. also shown are the corresponding meteorological data measured at the station located at paternò. n.d. = not determined. ing the shallow biogenic component of the gas. this in turn confirms the transport of free gas (mostly co2) along preferential pathways in the crust that correspond to tectonic faults, where the local permeability is much higher than in the surrounding volcanic rocks (values of permeability in etna rocks are in the range from 2.5 × 10-11 to 2.9 × 10-10 m2, according to aureli 1973 and ferrara 1975) and where local ground water quickly becomes saturated with co2. this behavior appears to be valid, more or less, for all three sites of the east flank of mt. etna investigated during this study. interestingly, our findings are also in agreement with those of similar studies carried out at the solfatara crater (phlaegrean fields, italy) by chiodini et al. [2008], who found a similar positive correlation between co2 efflux and the δ 13cco2 values of the corresponding soil co2 efflux (i.e., c isotopes were not measured in soil gas samples, but at the outlet of the co2 efflux measurement device) from fumaroles located inside the crater and from diffuse emissions occurring both inside and outside of the crater. they explained the correlation as being due to a combination of three distinct statistical populations of samples: i) a high-flux population with a mean δ13c value of 2.3±0.9‰, representative of pure hydrothermal degassing of co2 (this value was actually very similar to that of local fumarolic co2, that is -1.48‰); ii) a low flux population with a mean δ13c value of about 19.4±2.1‰, representative of biogenic degassing of co2 (from microbial decomposition of organic matter in the soil, from plant residues or from root respiration); iii) an intermediate efflux population with a mean δ 13c value of about -9.8±3.7‰, representative of a mixture of the two above. 4.2. temporal changes of the monitored parameters the temporal patterns of the soil co2 efflux values for each site monitored, plotted on a log10 scale (figure 4), show some common features between sites. a marked and sharp decrease was observed in midfebruary 2009 simultaneously at sites p78, zaf06 and zaf08, to then give way to a steady increase since the end of march 2009. further decreases were recorded, in giammanco et al. 8 p39 tair rain rh min rh max wind p39 1.00 0.51 -0.13 -0.55 -0.31 0.27 t° air 1.00 0.02 -0.56 -0.48 0.45 rain 1.00 0.34 0.21 0.11 rh min 1.00 0.41 -0.15 rh max 1.00 -0.49 wind 1.00 table 3. correlation matrix between the soil co2 efflux values measured at site p39 and the corresponding meteorological parameters measured at the station of paternò. figure 3. schematic graphic models of the geochemical interactions between magmatic co2 gas and shallow fluids in the monitored sites: a) interaction between magmatic co2 and hydrothermal fluids (depicted as an orange horizontal layer) at site p39, with consequent positive shift of δ13c values due to partition of 12c into the liquid phase as hco3 -; b) interaction between magmatic co2 and cold ground water (depicted as a blue horizontal layer) at sites zaf06, zaf08 and p78, causing partition of 13c into the liquid phase, followed by mixing with shallow biogenic co2 produced in the soil (depicted as a green horizontal layer) and consequent enrichment in 12c in the gas phase. 9 a more scattered way, from november 2009 to march 2010 and finally, following a sudden drop, in late october-december 2010, the latter observed only at sites zaf08 and p78. the highest effluxes were observed during june-july 2009 at most of the sites, more evidently at p39. high effluxes were also recorded in april 2010 at site zaf06, in july 2010 at p39 and, during septemberoctober 2010, at more or less all sites. site zaf06 showed slightly more stable co2 effluxes after the abovementioned period of very low values from january to march 2009. in order to determine whether the above changes were at least in part caused by variations in the main meteorological parameters, we performed a linear cross-correlation analysis of all the data, divided into areas (se flank and e flank). the resulting correlation matrixes (tables 3 and 4 for the area near paternò and for that near riposto, respectively), show no significant correlation between the considered parameters. all correlation coefficients were in the range between -0.56 and +0.7 and, apart from a very weak direct correlation between co2 efflux and air temperature at site p78 (r = +0.61), all other coefficients between soil co2 efflux and meteorological parameters had values between -0.56 and +0.51. this clearly suggests a non-environmental, probably volcanic, cause for the observed temporal changes in soil co2 efflux values (and by reflection also in δ 13cco2 values) at all monitored sites. the temporal patterns of δ13cco2 values (figure 5) show dissimilar behavior between site p39 and the other three sites. as noted above, the δ13cco2 values at site p39 in general varied within a much narrower range (only about 1‰) than at the other sites and showed no appreciable correlation with the co2 efflux values. this points to a marked geochemical stability of the hydrothermal system beneath site p39, likely due to its large volume that makes it less sensitive, at least in terms of isotopic composition of the escaping co2, to changes in the input of high-enthalpy magδ 13cco2 and co2 efflux in soil gases of mt. etna figure 4. temporal pattern of the soil co2 effluxes measured in the monitored sites during the study period: a) p39; b) p78; c) zaf08; d) zaf06. the pink area indicates the period of the 20082009 flank eruption. the vertical red line indicates the moment of the opening of the nsec. figure 5. temporal pattern of the δ13cco2 values measured in the monitored sites during the studied period: a) p39; b) p78; c) zaf08; d) zaf06. the pink area indicates the period of the 2008-2009 flank eruption. the vertical red line indicates the moment of the opening of the nsec. matic fluids coming from depth. the other sites show, instead, a consistent behavior, marked by a more or less sharp positive shift of δ13c values in april-may 2009, followed by a slower trend towards even more positive values. carbon isotope values remained quite stable or decreased slightly from june to october 2010 to show a fairly more marked shift towards negative values in october-november 2010. the overall picture arising from the above data indicates the significant arrival of deep magmatic co2 just before the end of the 2008-2009 flank eruption, reasonably related to emplacement of new gas-rich magma within relatively deep portions of the volcano and along conduits that were not directly connected with those feeding the ongoing flank eruption. this interpretation is strongly supported by other geochemical and geophysical parameters that were measured during the same period by the monitoring network of the istituto nazionale di geofisica e vulcanologia [mattia et al. 2015]. in particular, the behavior observed in the temporal pattern of soil co2 efflux in early 2009 can be interpreted according to geochemical studies made both on soil co2 emissions from mt. etna [giammanco et al. 1995, bruno et al. 2001], and on crater fumarole emissions at vulcano island [nuccio and valenza 1992, 1998]. strong decreases in the co2 output from soils or from fumaroles accompany early stages of magma motion towards the surface. in those cases, magma undergoes sudden pressure decreases, which causes marked exsolution of a separate vapor phase in the volatile-saturated melt and consequent partition of volatiles into the newly formed bubbles according to the different gas solubilities. because of its low solubility in magma, co2 will be highly enriched in the separate vapor phase. the consequent decrease in the concentration of residual dissolved co2 in the uppermost portion of a magma body produces a kinetic diffusion effect that translates into an inward and upward migration of dissolved gas. this process results in an initial decrease of volatile fluxes released from the deep magma towards the surface. subsequently, when co2-rich gas bubbles are able to escape from magma, outward advective fluxes of magmatic volatiles through the main pathways across the structure of the volcano (i.e., volcanic conduits, degassing faults) will markedly increase. the sequence observed in our data in early 2009, therefore, would represent a complete cycle of fresh magma transfer from deeper to shallower levels in the volcano. the simultaneous change in c isotope values would simply reflect the isotopic shift related with the above mentioned changes in the degree of mixing between the magmatic component and the biogenic source as function of co2 efflux. after the end of the 2008-2009 eruption a further slow increase in magmatic co2 degassing was in general observed, suggesting that deep-coming magma moved further upward and accumulated into shallower portions of the volcanic system, thus releasing higher amounts of fluids. this in turn produced a higher input of high-enthalpy fluids into the shallow aquifers of mt. etna, thus raising the temperature of water in the subsurface hydrothermal systems within the volcano. this process would explain the further isotopic shift towards positive δ13c values observed during the summer of 2010. besides, the progressive pressurization of the shallow volcanic system as consequence of magma accumulation also reasonably led to the opening of the nsec in early november 2009 and prepared for its following eruptive activity. the sharp decrease in co2 efflux, combined with the slight negative shift of δ13c values, observed since october 2010 at sites p78, zaf08 and, to a lesser extent, also at zaf06, might be the consequence of a sudden upward migration of magma. marked transient decreases in soil co2 emissions at mt. etna were interpreted by giammanco et al. [1995] as an indication of rapid upward migration of the gas source (i.e., a co2oversaturated magma). shallow magma up-rise produces a greater gas pressure gradient between the giammanco et al. 10 zaf06 zaf08 p78 tair rain rh min rh max wind zaf06 1.00 0.18 0.32 0.16 0.26 0.29 0.19 -0.31 zaf08 1.00 0.51 0.45 0.00 0.18 0.07 -0.34 p78 1.00 0.61 0.33 0.35 0.31 -0.28 t° air 1.00 0.18 0.14 0.18 -0.35 rain 1.00 0.11 0.17 0.02 rh min 1.00 0.56 -0.49 rh max 1.00 -0.24 wind 1.00 table 4. correlation matrix between the soil co2 efflux values measured at sites zaf06, zaf08 and p78 and the corresponding meteorological parameters measured at the station of riposto. 11 source of gas and the summit of the volcano, which greatly overcomes the gradient existing between the gas source and the peripheral areas. this large contrast favors the release of magmatic gas along the direction source-summit of the volcano, thus significantly dampening diffuse flank degassing. this hypothesis is also strongly supported by the following occurrence both of strombolian activity at nsec in late december 2010 and, mostly, of the long sequence of eruptive episodes since january 12, 2011; the latter episodes would testify the final arrival of magma at the surface. 5. concluding remarks the results of this study will help to better understand and define the physico-chemical mechanisms that rule the interactions occurring between deep co2 deriving from magma degassing inside mt. etna feeder conduit and the shallow fluids within the volcano. the observed temporal variations of the studied parameters are explained in terms of variable degree of geochemical interaction between high-enthalpy magmatic fluids and shallow ground water, as a function of the supply of magmatic gas from depth. at high soil co2 effluxes, δ 13cco2 values in the peripheral sites are similar to those found in the fumarole gases emitted close to the active summit craters of mt. etna. when this occurs, we can reasonably hypothesize that new gas-rich magma is accumulating in relatively deep (5-10 km) volumes within the volcano, causing a strong pressure increase in the whole volcanic system. conversely, when flank co2 emission is low, values of δ 13cco2 become more negative, pointing to a greater interaction with shallow non-magmatic fluids, in our case cold ground water and biogenic co2 [giammanco et al. 1998, pecoraino and giammanco 2005]. our results are encouraging for future monitoring of the eruptive activity of mt. etna. the sampling sites chosen are always easy to access under most weather conditions. further, the sampling procedure is simple and the analytical methods are precise and reliable. a higher sampling frequency will certainly improve the temporal resolution on the studied parameters, thus permitting their use also as short-term indicators of the dynamics of mt. etna. acknowledgements. we wish to thank s. consoli, f. calvagna and m. lopez for their help during the field work. we also thank the “regione siciliana sias servizio informativo agrometeorologico siciliano” for kindly providing the meteorological data used in this work. we acknowledge the financial support of slovenian research programme “cycling of substances in the environment, mass balances, modelling of environmental processes and risk assessment” (p1-0143) and bilateral slovenian-italian cooperation “mercury emissions, its influence and correlation with rn on etna area”. special thanks are given to prof roger h. pain for linguistic corrections. references aiuppa, a., allard, p., d’alessandro, w., giammanco, s., parello, f., and m. valenza (2004). magmatic gas leakage at mount etna (sicily, italy): relationships with the volcano-tectonic structures, the hydrological pattern and the eruptive activity, in mt. etna: volcano laboratory, a.g.u. geophysical monograph series 143, 129-145, doi: 10.1029/143gm09. allard, a., jean-baptiste, p., d’alessandro, w., parello, f., parisi, b., and c. flehoc (1997). mantle-derived helium and carbon in groundwaters and gases of mount etna, italy, earth planet, sci. let., 148, 501-516. anzà, s., dongarrà, g., giammanco, s., gottini, v., hauser, s. and m. valenza (1989). geochimica dei fluidi dell'etna: le acque sotterranee, miner. petrogr. acta, 32, 231-251 (in italian). ahrens, l.h. 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(2004). a simple method of soil gas stable carbon isotope analysis, rapid commun. mass spectrom., 18, 1239-1242, doi: 10.1002/rcm.1468. torn, m.s., davis, s., bird, j.a., shaw, m.r. and m.e. conrad (2003). automated analysis of c-13/c-12 ratios in co2 and dissolved inorganic carbon for ecological and environmental applications, rapid commun. mass spectrom., 17, 2675-2682. doi:10.1002/rcm.1246. *corresponding author: salvatore giammanco, istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy; email: salvatore.giammanco@ingv.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. δ 13cco2 and co2 efflux in soil gases of mt. etna 087_095 adg v–5 n01 bilge .pdf annals of geophysics, vol. 45, n. 1, february 2002 87 variation of the feedback coefficient with r12 and the geographic latitude in 1-h ahead forecast of f0 f2 ayşe h. bilge (1), eti mizrahi (1) and yurdanur tulunay (2) (1) department of mathematics, istanbul technical university, maslak, istanbul, turkey (2) faculty of aeronautics and astronautics, istanbul technical university, maslak, istanbul, turkey abstract the «prediction» and «forecast» of the critical frequency of the f 2 layer (f 0 f 2 ) is an important issue for frequency planning in short wave radio communications. in this context, «prediction» is used for the determination of monthly median values of f0 f2 for each hour, while «forecast» denotes the determination of hourly values. in a previous paper we proposed a «sliding window» technique for prediction combined with «feedback» for forecast (bilge and tulunay, 2000). in the present paper we obtain the variation of the feedback coefficient with r12 and geographic latitude. 1. introduction the prediction and forecasting of the ionospheric critical frequency f0 f2 is crucial in planning hf communication and for radar and navigation systems. the monthly median values of f 0 f 2 for each hour can be considered as a first approximation to the data and tabulated values for these medians provide a good guideline for frequency planning. however the monthly medians fail to follow short term irregular variations and forecasting methods are needed. the «feedback» method that is widely used in control engineering and signal processing areas was applied 1-h ahead of forecast of f 0 f 2 in a previous paper (bilge and tulunay, 2000). in the present paper we investigate the dependency of the feedback coefficient on physical parameters such as the 12-month smoothed sunspot number r 12 and the geographic latitude. the monthly median values for each hour depend mainly on solar activity and season. various sophisticated models using different indices for solar activity have been proposed (smith and king, 1981; kane, 1992a,b; alberca et al., 1999) but for practical purposes it is preferable to use r12 as the only physical parameter, because of its availability, reliability and predictability (bradley, 1994). the wellknown hysteresis and saturation effects characterize the dependency of f0 f2 on r12. namely, f0 f2 changes linearly with r12 for low and medium solar activity and then reaches saturation. in addition, various periodic components of the f0 f2 variation are modulated by r12. the saturation effect is usually dealt with by using a parabolic fit to the data. more sophisticated functional fits using square roots mailing address: dr. ayşe h. bilge, department of mathematics, istanbul technical university, 80626 maslak, istanbul, turkey; e-mail: bilge@itu.edu.tr key words f 0 f 2 − critical frequency − forecast − feedback 88 ayşe h. bilge, eti mizrahi and yurdanur tulunay and higher order polynomials (bilge and tulunay, 1998) give better fits but they are not stable for long-term prediction. the character of the dependency of f 0 f 2 on r 12 is different in rising and falling phases of a solar cycle and it also changes from solar cycle to solar cycle. these differences are important for understanding the mechanism underlying ionospheric processes, however for prediction purposes, non-stationarity of the data can be overlooked by using short term past data to build the models for prediction. we have noted that the dependency of f0 f2 on r 12 is more or less linear over a time span of 2 to 4 years and we developed a «sliding window» technique to predict the monthly median f0 f2 using immediately past 48 month data within 3-4% error, compared with 6-7% errors based on 20 year models (baykal, 1998). prediction by «sliding windows» and forecast by «feedback» was proposed in a previous work (bilge and tulunay, 2000), and tested in the framework of cost 251 action stanisl/awska et al., 1999). the feedback coefficient is the key parameter in single step feedback, and it is determined by a one-dimensional optimization. the aim of the present work is to study the dependency of the optimal feedback coefficient on r 12 and geographic latitude. we used those data described in detail in mizrahi et al. (2001), provided in the cost 251 cd-rom, namely data from 48 stations between 1958 and 2000. eliminating those stations with less regular data coverage, we based our study on 13 stations arranged in 3 groups according to their latitudes. the first group includes the stations lycsele, kiruna, arkhangelsk, uppsala and leningrad, which lie nearly above 60n line. the second, mid latitude group includes the stations in the 50n-57n band, i.e. slough, juliusruh, moscow, pruhonice, kiev. the stations tortosa, rome and sofia in the 40n-45n band are arranged in the third group (mizrahi et al., 2001). our preliminary investigations have shown that the feedback coefficients for forecast based on predicted or actual monthly medians differ in general by about 0.1 and the interrelations are the same. thus for simplicity of data processing we based our forecast on actual monthly medians. 2. forecasting with feedback the estimation of the actual hourly values of f 0 f 2 is called «forecasting», and it is dealt with by means of neural network (tulunay et al., 2000) and «feedback» methods (bilge and tulunay, 2000), in addition to more standard autocorrelation techniques (stanisl/awska et al., 1999). the feedback method is a standard tool in control engineering, for maintaining a constant output or equivalently for following or tracking a given signal. at each step, an appropriate multiple or combination of the measurement error is «fed back», to modify the controlling signal. here, the error is the difference between the measurement and the prediction of monthly medians. an appropriate multiple of the error in the i’th step is added with the reverse sign to correct the prediction at the i +1’st step. that is, if the measurement and prediction at the time t i are respectively f0f2(ti) and f0f2 pred(ti), then the error at stage t i is e (t i ) = f0 f2(ti ) – f0 f2 pred(ti ). as the predicted value is available at the stage t i+1, the forecast at the hour ti+1, denoted by f0 f2 * is obtained from f0 f2 *(t i+1 ) = f0 f2 pred(ti+1 ) – k e(ti ) where k is the feedback coefficient. this method has been applied to data from rome, poitier and uppsala ionosonde stations over 1986-1990 (bilge and tulunay, 2000). the comparison of the monthly median (prediction) data, actual f 0 f 2 and predicted f 0 f 2 is shown in fig. 1, for a typical storm time disturbance followed by quiet days, rome, 16-23 april 1958. in one-step feedback, there is a single parameter to be adjusted, namely the feedback constant k. the «best» feedback constant is found by applying feedback with k ranging in a certain interval and computing the error with respect to a certain norm. the value of k corresponding to a local minimum in the chosen norm of the error is called k*. 89 variation of the feedback coefficient with r 12 and the geographic latitude in 1-h ahead forecast of f 0 f 2 we have run the feedback algorithm applied to actual monthly medians for a feedback constant in the range 0.2-1.1. we used the l2 norm of the error as our performance criterion. however preliminary investigations have shown that rms error also leads to same values for optimal feedback constant. 3. results the optimal feedback coefficient was computed by minimizing the l2 norm of the error between the hourly values and monthly medians for each hour, i.e. constant feedback is used during a year. a total of 369 samples were analyzed. the feedback constant ranges all times between 0.5 and 1.0. the stations are arranged into three main groups according to their latitudes. the percentage of occurrences of the k* values in each group is given in table i. the first group includes lycsele, kiruna, arkhangelsk, uppsala and leningrad; the second group includes slough, juliusruh, moscow, pruhonice, kiev and the third group includes tortosa, rome and sofia stations. the number of samples in these groups is respectively 147, 151 and 71. thus the optimal feedback coefficient ranges mostly in the 0.7-0.9 interval, hence the value k = 0.8 used in bilge and tulunay (2000) is typical. there is a tendency of k* to be lower at lower latitudes. the graph of the feedback constants for all stations versus years is given fig. 1. comparison of the monthly medians, hourly data and forecast results. data show a typical storm condition in rome 1958 data for 8 days from april 16 to april 23. the forecast by feedback follows well the depression at the 3rd and 4th days but fails to follow closely the midday and midnight fluctuations. 90 ayşe h. bilge, eti mizrahi and yurdanur tulunay in fig. 2. we note that the extreme value k = 1 occurs during 1958 and 1959 corresponding to an extremely high solar activity. on the other hand, the extreme low value k = 0.5 occurs for low r 12 (with an exception of 1969, for sofia). we can thus conclude that the value of the optimal feedback constant increases with r12. the optimal feedback coefficients and corresponding forecast errors are shown in figs. 3a-f for selected stations. in these graphs, r 12 is drawn to scale, while k* is multiplied by 100 and the errors are multiplied by 10 for convenient display. the variation of k* follows the r12 variation quite regularly for rome, slough, juliusruh, uppsala, leningrad and lycsele stations, oscillating between 0.6-0.8 for rome, and between 0.7-0.9 at higher latitudes. the variation of k* does not follow r 12 as regularly at table i. the percentage of occurrences of the feedback constants for all stations and years. feedback constant k* 0.5 0.6 0.7 0.8 0.9 1.0 latitude:40n-43n % 9.86 15.49 26.76 36.62 9.86 1.41 latitude:50n-55n % 0.00 2.65 11.26 46.36 39.07 0.66 latitude:59n-68n % 0.68 0.68 14.29 53.06 30.61 0.68 all stations % 2.17 4.34 15.45 47.15 30.08 0.81 fig. 2. a plot of the feedback constants versus years for all stations. 91 variation of the feedback coefficient with r 12 and the geographic latitude in 1-h ahead forecast of f 0 f 2 a b fig. 3a,b. values of the feedback constant k (*) versus years for stations rome (a) and slough (b). a graph of r12 is shown for comparison. the corresponding errors are shown with (+). the r 12 is drawn to scale, while k values ranging between 0.5 and 1.0 are multiplied by 100 and percentage errors ranging between 6%-14% are multiplied by 10 for convenient display. these figures are typical examples of regular variations with r 12 at low, mid and high latitudes. 92 ayşe h. bilge, eti mizrahi and yurdanur tulunay c d fig. 3c,d. values of the feedback constant k (*) versus years for stations uppsala (c) and sofia (d). a graph of r12 is shown for comparison. the corresponding errors are shown with (+). the r 12 is drawn to scale, while k values ranging between 0.5 and 1.0 are multiplied by 100 and percentage errors ranging between 6%-14% are multiplied by 10 for convenient display. figure 3c is a typical example of regular variations with r 12 at low, mid and high latitudes. figure 3d shows some irregularities. 93 variation of the feedback coefficient with r 12 and the geographic latitude in 1-h ahead forecast of f 0 f 2 e f fig. 3e-f. values of the feedback constant k (*) versus years for stations moscow (e) and kiruna (f). a graph of r 12 is shown for comparison. the corresponding errors are shown with (+). the r 12 is drawn to scale, while k values ranging between 0.5 and 1.0 are multiplied by 100 and percentage errors ranging between 6%-14% are multiplied by 10 for convenient display. these figures show some irregularities. 94 ayşe h. bilge, eti mizrahi and yurdanur tulunay fig. 4. a plot of the feedback error versus feedback constant. sofia, pruhonice, moscow, arkhangelsk and kiruna, while data for tortosa are scarce. we have shown the graphs for rome, slough, uppsala, in figs. 3a-c, and the graphs for sofia, moscow and kiruna in figs. 3d,f, as typical examples of regular and irregular behavior in each latitude band. an observation of these graphs also reveals that the errors tend to be higher for low feedback constant k. a plot of the forecast errors versus the optimal feedback coefficient k* given in fig. 4 confirms this observation, as the plot has a negative slope. the errors range mostly between 6% and 12%. furthermore, we also observed that the error is not too sensitive to k; in the range [k* – 1, k* + 1]. in order to quantify the dependency of k* on the latitude and on r12, we grouped the yearly samples from all stations, according to the latitudes of the stations as in table i, and the r 12 value of the corresponding years as given in table ii. the average value of k* corresponding to each group of latitude and r12 range is given in table iii. we note that, except for the second column, the average k* in the highest latitude range is slightly lower than the value in the mid latitude range. this result is similar to the behavior of the variation of the upper deciles of the negative deviations with r12, presented in mizrahi et al. (2001) and the «on the day-to-day variation of muf over europe» reported in kouris et al. (2000). these parameters increase with latitude but then decrease slightly after 60n. as the feedback compensates for the disturbances, it is 95 variation of the feedback coefficient with r 12 and the geographic latitude in 1-h ahead forecast of f 0 f 2 plausible that we need higher feedback constants when the fluctuations in the data have larger amplitudes, which also explains the correlated latitude dependency of k* and the upper deciles. references alberca, l.f., g. juchnikowski, s.s. kouris, a.v. mikhailov, v.v. mikhailov, g. miro, b. de la morena, p. muhtarov, d. pancheva, g.j. sole, i. stanisl/ awska and t. xenos (1999): comparison of various f0 f2 single-station models for european area, acta geophys. pol., 47, 42-56. baykal, ş. a. (1998): a model study of ionospheric critical frequencies for telecommunication purposes, m.sc. thesis, middle east technical university, ankara. bilge, a.h. and y. tulunay (1998): semi empirical single station modeling of f 0 f 2 variations: spectral analysis, poster presented at the 23rd general assembly of european geophysical society, nice, france. bilge, a.h. and y. tulunay (2000): a novel on-line method for single station prediction and forecasting of the ionospheric critical frequency f0 f2 1-hour ahead, geophys. res. lett., 27, 1383-1386. bradley, p.a. (1994): further study of f 0 f 2 and m(3000)f 2 in different solar cycles, ann. geofis., 37 (2), 201-208. kane, r.p. (1992a): solar cycle variation of f 0 f 2 , j. atmos. terr. phys., 54, 1201-1205. kane, r.p. (1992b): sunspots, solar radio noise, solar euv and ionospheric f0 f2, j. atmos. terr. phys., 54, 463-466. kouris, s.s., d.n. fotiadis and r. hanbaba (2000): on the day-to-day variation of the muf over europe, phys. chem. earth (c), 25 (4), 319-325. mizrahi, e., a.h. bilge and y. tulunay (2001): statistical properties of the deviations of f0 f2 from monthly medians, poster presented at egs xxvi symposium, nice, france, march 2001. smith, p.a. and j.w. king (1981): long term relationship between sunspots, solar faculae and the ionosphere, j. atmos. terr. phys., 43, 1057-1063. stanisl/ awska, i., lj.r.cander, e. tulunay, y. tulunay, a.h.bilge, g.juchnikowski, m. kadioğlu, c. özkaptan and z. zbyszynski (1999): instantaneous mapping of 1-h ahead f0 f2 forecasting values over europe, in cost 251 4th workshop, madeira, portugal, 22-26 march 1999, 235-241. tulunay, e., c. özkaptan and y. tulunay (2000): temporal and spatial forecasting of the f0 f2 values up to twenty four hours in advance, phys. chem. earth (c), 25, 281-285. table iii. average values of the optimal feedback coefficient in different latitude groups and r 12 ranges. r 12 : 0-29 r 12 : 30-59 r 12 : 60-109 r 12 : 140-189 latitude: 40n-43n % 6.588 6.824 7.680 7.917 latitude: 50n-55n % 7.750 8.111 8.380 8.862 latitude: 59n-68n % 7.543 8.162 8.275 8.708 table ii. the breakdown of years 1958-1998 into r 12 ranges. r 12 years 0-29 (1963,1964,1965,1975,1976,1985,1986,1995,1996,1997) 30-59 (1961,1962,1966,1973,1974,1977,1984,1987,1993,1994) 60-109 (1960,1967,1968,1969,1970,1971,1972,1978,1982,1983,1988,1992,1998) 140-189 (1958,1959,1979,1980,1981,1989,1990,1991) 163_168 adg v–l5 n01 romano.pdf annals of geophysics, vol. 45, n. 1, february 2002 163 magnetic and solar effects on ionospheric absorption at high latitude vincenzo romano, massimiliano cerrone, loredana perrone and marco pietrella istituto nazionale di geofisica e vulcanologia, dept. rm2-uf/faa, roma, italy abstract some periods of intense solar events and of strong magnetic storms have been selected and their effects on the ionospheric d region have been investigated on the basis of ionospheric absorption data derived from riometer measurements made at the italian antarctic base of terra nova bay (geographic coordinates: 74.69 s, 164.12 e; geomagnetic coordinates: 77.34 s, 279.41 e). it was found that sharp increases in ionospheric absorption are mainly due to solar protons emission with an energy greater than 10 mev. moreover, the day to night ratios of the ionospheric absorption are greater than 2 in the case of strong events of energetic protons emitted by the sun, while during magnetic storms, these ratios range between 1 and 2. 1. introduction the electron density below 80 km in the ionosphere is greatly dependent on the chemistry of this region that is in turn affected by direct solar radiation (ranta et al., 1984). under disturbed conditions, one of the principal causes of ionization in the d region is due to the influx of the energetic particles penetrating the atmosphere that increase the electron density and then the ionospheric absorption. therefore the ionospheric absorption is a suitable parameter to disclose the effects on the lower ionosphere due to the magnetic and solar activity. during and after magnetospheric storms two types of ionospheric absorption in the d region can be observed (see, e.g., ranta et al., 1984, and references therein): a) an ionospheric absorption increasing at high latitude during the main phase of the magnetospheric storms. b) an ionospheric absorption increasing during the recovery phase of the magnetospheric storms, clearly separated from the first one, well known as ‘post-storm effects’ (pse) (bremer, 1998) at mid-latitude. the poleward boundary of pse has been found to be a function of storm-time, reaching even l = 4 (wagner et al., 1982). as terra nova bay is a polar station located outside the pse poleward boundary, the first component (a) is expected to be the main effect of magnetic perturbations on the absorption data. another important type of absorption that can occur at high latitudes, is the so-called polar cap absorption (pca). this event is caused by energetic protons emitted by the sun in connection with major solar flares. the main effects of pca absorption are seen at lower altitude (60, 70 m) more than other types of absorption due for example to magnetospheric storms. the chemistry of the ionized constituents is markedly different and this mailing address: dr. vincenzo romano, istituto nazionale di geofisica e vulcanologia, dept. rm2-uf/faa, via di vigna murata 605, 00143 roma, italy; e-mail: romano@ingv.it key words ionospheric absorption – solar protons event – magnetic storm – antarctic region 164 vincenzo romano, massimiliano cerrone, loredana perrone and marco pietrella a b c d f ig . 1a -d . f ro m t h e to p t o t h e b o tt o m :t em p o ra l b eh av io u r o f th e d ai ly a ve ra g e io n o sp h er ic a b so rp ti o n , o f th e d ai ly p ro to n f lu x w it h e n er g ie s > 1 0 m ev a n d o f th e d ai ly a ve ra g e a p ( ) d u ri n g t h e p er io d a p ri l 1 8 3 0 1 9 9 8 ( a) ; a u g u st 2 0 s ep te m b er 3 1 9 9 8 ( b ); s ep te m b er 1 9 -2 8 1 9 9 8 ( c) ; s ep te m b er 2 9 o ct o b er 7 1 9 9 8 ( d ). 165 magnetic and solar effects on ionospheric absorption at high latitude probably gives rise to the diurnal variation of pca. during a pca the radio absorption measured by riometers by day is typically 2 or more times larger than by night (ranta et al., 1984). therefore the day to night absorption ratio can be used to verify that a pca phenomenon is occurring. the day to night ratios are calculated on the basis of the day-time and night-time ionospheric absorption values. the boundary between day (sunrise) and night (sunset) is generally determined by the solar zenith angle χ = 90°. hargreaves (1993) found that at sunset and during a pca, a significant decay of electron density can be observed for χ = 96°, corresponding to a height of about 60 km, i.e. the lower ionospheric region here investigated. so, in this study, the day to night ratios at solar zenith angle ≥ 90° were carried out for some periods of strong solar and magnetic events in 1998 (section 2). the results are presented and discussed in sections 3 and 4. 2. data analysis ionospheric absorption data (ut), the time weighted magnetic index ap (τ) (ut), and the solar protons flux (ut) (monitored by the noaa goes-8 satellite in geo-synchronous orbit) were considered for the period 1st january 31st december 1998. ionospheric absorption data (a2 method) are derived from 1 min cosmic noise measurements recorded at terra nova bay riometer station (30 mhz) (cerrone et al., 1996; de franceschi et al., 1997). the time-weighted accumulation index ap (τ) (wrenn, 1987), based on the history of the geomagnetic index ap, is considered to represent the magnetic activity effects on the ionosphere. in fact, it has been shown that ap (t ≅ 0.8), that corresponds to a 15 h time delay, is better correlated with the ionospheric parameters, at least in the f region (for details see wu and wilkinson, 1995; perrone and de franceschi, 1999; perrone et al., 2001). firstly, strong magnetic storms were selected on the basis of the daily mean of ap (τ) ≥ 30 (de franceschi et al., 1999). some days before and after the onset of the storm were also taken into account to study the behaviour of the ionospheric absorption before and after the storm. this procedure disclosed the following 3 periods: april 18-30 1998, august 20 september 03 1998 and september 19-28 1998. another time-interval was considered from september 29 to october 7 1998, characterized by a strong protons emission event without any important magnetic activity. figure 1a-d plots the daily mean of the ionospheric absorption (calculated from the absorption values at the first minute of each hour of the day), the daily solar protons flux with energies > 10 mev, and the daily mean of ap (τ) for each selected period: high correlation between the absorption and solar protons flux maxima can be recognized. the hourly mean of absorption values (calculated on the basis of 1 min cosmic noise sampling) during sunset transition between χ = 87° and χ = 102° both for the days of solar protons flux and ap (τ) maxima are reported in tables i and ii. moreover, the day to night ratios of the ionospheric absorption obtained by considering χ = 90° as sunrise and χ = 96° as sunset have been calculated (table iii). 3. results and discussion a careful inspection of fig.1a-d leads to the following main considerations: – the period april 18-30 shows a ionospheric absorption peak which occurred on 21st due to the solar proton event (associated with a solar flare at 20 classified as m1), that started on 20th, reached its maximum on 21st and stopped on 24th. during this period the ionospheric absorption remained relatively high. a moderate magnetic storm (april 24, ap (τ) = 30) can be observed probably due to a coronal mass ejection which started on 21st. as on 24th, the proton flux remained relatively high (1.1⋅106 counts ⋅ cm−2 ⋅ day−1 ⋅ sr−1), most likely both the solar proton event and the magnetic storm affected the ionospheric absorption value on 24th, but it is not clear which phenomenon gave the major contribution. – from august 20th to september 3rd the ionospheric absorption that maximized on august 26th was due to the solar proton event that started on 24th (in conjunction to a solar 166 vincenzo romano, massimiliano cerrone, loredana perrone and marco pietrella table i. hourly ionospheric absorption data as function of the solar zenith angles between 87° and 102°; the days where the solar proton flux affects the ionospheric absorption more than the magnetic storm were considered. date solar zenith angle ionospheric absorption (db) 21 april 1998 87° 3.67 90° 2.46 93° 1.59 96° 0.89 99° 0.68 102° 0.58 26 august 1998 87° 3.21 90° 2.46 93° 1.60 96° 1.73 99° 1.73 102° 1.40 27 august 1998 87° 1.08 90° 0.79 93° 0.61 96° 0.60 99° 0.60 102° 0.41 01 october 1998 87° 1.94 90° 0.95 93° 0.95 96° 0.62 99° 0.45 102° 0.45 flare classified as x1/3b), reached its maximum on 26th, and stopped on 29th. the strong magnetic perturbation on august 27th should also be due to the same solar flare. the ionospheric absorption value on 27th depends on both the solar proton event and the magnetic storm, the proton flux being relatively high on 27th (2.8 ⋅ 106 counts ⋅ cm−2 ⋅ day−1 ⋅ sr−1) and the magnetic activity very strong (ap (τ) = 110) but, as in the previous case, it is difficult to assess a priori where the main contribution came from. – the period september 19-28 showed an increase in ionospheric absorption starting from 24th up to 28th with two peaks on 25th and 27th. as a magnetic storm evolved between 24th and 26th with a strong maximum on 25th, just when a solar protons emission occurred (these two events were probably due to the solar flare classified as m7/3b started on 23), both the phenomena concurred to the absorption growth. given the rather low level of proton flux (2.2⋅104 counts ⋅ cm−2 ⋅ day−1 ⋅ sr−1) and magnetic activity (ap (τ) = 13), the absorption peak observed on 27th could be better investigated by using a local geomagnetic index. – a sudden increase in the ionospheric absorption from september 30th with a peak observed on october 1st was due to the solar proton event (associated to a solar flare occurring on 30th september of importance m2/2n.) that reached its maximum the same day. the hourly mean values (tables i and ii) and the day to night ratios (table iii) of the absorption 167 magnetic and solar effects on ionospheric absorption at high latitude were considered as related to the occurring days of proton flux and of magnetic storm maxima in order to identify, as far as possible, which phenomenon causes the ionospheric absorption. table i shows a significant decrease of the ionospheric absorption as a function of the solar zenith angle from = 87° to = 102°. in particular, at = 96°, the absorption is at least twice smaller that calculated at = 87°. with regard to august 27th, the day to night ratio equal to 2.0 (table iii), that is the lower limit indicating a pca phenomenon (ranta et al., 1994), suggests a ionospheric absorption essentially due to the solar proton event (started on 24th) rather than the magnetic storm. this could be supported (despite the strong magnetic activity) by the fact that on august 27th the proton flux remains very high (2.8 106 counts cm 2 day 1 sr 1). april 21st, august 26th and october 1st, for which the day to night ratios are greater than 2.0, confirm the occurrence of pca phenomena. any significant ionospheric absorption decrease from = 87° to = 102° can be assessed for the days reported in table ii. these days show a day to night ratio smaller than 2.0 that is not typical for a pca phenomenon as expected (table iii). this could suggest that on april 24th and september 25th the ionospheric absorption is more affected by the magnetic storm rather than by the solar protons event. in particular, for september 25th, the proton flux, although occurring by day, remained very small (only 3.4 .105 counts cm 2 day 1 sr 1) while the magnetic activity was very strong (ap ( ) = 94). table iii. day to night ratios for proton fluxes and magnetic storm events calculated choosing = 90° (sunrise) and = 96° (sunset). proton fluxes events day to night ratios magnetic storm events day to night ratios 21 april 1998 6.52 24 april 1998 1.64 26 august 1998 2.82 25 september 1998 1.40 27 august 1998 2.00 01 october 1998 15.79 table ii. hourly ionospheric absorption data as function of the solar zenith angles between 87° and 102°; the days where the magnetic storm affects the ionospheric absorption more than the solar proton flux were considered. date solar zenith angle ionospheric absorption (db) 24 april 1998 87° 0.67 90° 0.59 93° 0.49 96° 0.45 99° 0.51 102° 0.51 25 september 1998 87° 0.37 90° 0.35 93° 0.35 96° 0.53 99° 0.53 102° 0.47 168 vincenzo romano, massimiliano cerrone, loredana perrone and marco pietrella 4. conclusions this investigation found that the hourly mean ionospheric absorption calculated at = 96° is at least two times less than that calculated at = 87° during a pca phenomenon. this agrees with what was found by hargreaves et al. (1993), according to which the electron density is down to half its original value at = 96°. the hourly mean ionospheric absorption during a pca phenomenon is always a decreasing function of the solar zenith angle (table i), except for a slight increase observed at = 96° on august 26th, probably due to the magnetic storm effect that started on that day (fig. 1c). a different behaviour can be observed when the ionospheric absorption is more affected by a magnetic storm rather than by a solar proton event (table ii). table iii shows that when the maximum protons flux occurs by night (21st april, 26th and 27th august) the ionospheric absorption day to night ratios are smaller than that occurring by day (october 1st). the ionospheric absorption day to night ratio seems to be a suitable index to distinguish the ionospheric absorption due to a solar proton event from that due to a magnetic storm. the cases analysed confirm that a day to night ratio equal to or greater than 2.0 characterizes a pca phenomenon (ranta et al., 1984). an investigation into the absorption peak observed on september 27th (fig. 1c) could be carried out using a different geomagnetic index: as ap ( ) comes from planetary index ap, and concerns above all the f region, the use of a local polar index such as the pc index (troshichev et al., 1979) could be more suitable to characterize the magnetic storms and hence to better disclose their possible influence on the ionospheric absorption. acknowledgements the authors would like to thank dr. g. de franceschi (ingv-rome, italy) for helpful discussions and comments, the space environment center (noaa, u.s. dept. of commerce, boulder-co, u.s.a.) for supplying proton data from goes satellite, and the italian national program of antarctic researches (pnra) for supporting this study. references bremer, j. (1998): post storm effects in middle and subauroral latitudes, adv. space res., 22,837-840. cerrone, m., c. bianchi, g. de franceschi and b. zolesi (1996): ionospheric observations at terra nova bay, in italian geophysical observatories in antarctica, edited by a. meloni and a. morelli (istituto nazionale di geofisica, roma), 167-179. de franceschi, g., a. de santis and l. perrone (1997): analysis of riometric data recorded at terra nova bay, antarctica, in conference proceedings of solar terrestrial predictions workshop, edited by g. heckman, k. marubashi, m.a. shea, d.f. smart and r. thompson, 431-434. de franceschi, g., t. gulyaeva, l. perrone and b. zolesi (1999): mac: an oriented magnetic activity catalogue for ionospheric applications, ursi, international reference ionosphere news letter, 6 (4), 5-6. hargreaves, j.k., a.v. shirochkov and a.d. farmer (1993): the polar cap absorption event of 19-21 march 1990: recombination coefficients, the twilight transition and midday recovery, j. atmos. terr phys., 55, 857-862. pe r ro n e , l. and g. d e f r a n c e s c h i (1999): a correlation study between time-weighted magnetic indices and the high latitude ionosphere, phys. chem. earth (c), 24 (4), 389-392. perrone, l., g. de franceschi and t. gulyaeva (2001): the time-weighted magnetic indices ap ( ), pc ( ), ae ( ) and their correlation to the southern high latitude ionosphere, phys. chem. earth (c), 26 (5), 331-334. ranta, h., a. ranta and t.j. rosenberg (1984): the day to night ratio in auroral and subauroral zone riometer measurements during auroral absorption, j. atmos. terr. phys., 46, 395-398. t ro s h i c h e v , o.a., n.p. d m i t r i e va and b.m. kutznetsov (1979): polar cap magnetic activity as a signature of substorm development, planet. space sci., 27, 217-221. wagner, c.u., h. ranta and a. ranta (1982): nighttime ionospheric absorption during and after magnetospheric storms from auroral (l = 6.0) to medium (l = 2.5) latitudes, j. atmos. terr. phys, 9, 747-757. wrenn, g.l. (1987): time-weighted accumulations ap ( ) and kp ( ), j. geophys. res., 92, 1763-1770. wu, j. and p.j. wilkinson (1995): time-weighted indices as predictors of ionospheric behaviour, j. atmos. terr. phys., 57, 1763-1770. a climatological morphology of ionospheric disturbances at high and polar latitudes annals of geophysics, 58, 6, 2015, a0659; doi:10.4401/ag-6696 a0659 a climatological morphology of ionospheric disturbances at high and polar latitudes dimitris n. fotiadis1,*, sotiria d. matta2, stamatis s. kouris3 1 hellenic telecommunications and post commission (eett), spectrum management dept., maroussi, greece 2 postgraduate student at aristotle university of thessaloniki, fac. of philosophy, sch. of philology, thessaloniki, greece 3 aristotle university of thessaloniki, electrical and computer engineering dept., thessaloniki, greece abstract after a historical introduction on the first well-documented observations of ionospheric phenomena and a review of the current, state-of-the art polar ionospheric studies, a climatological morphology of the irregular fregion plasma structures at high and polar latitudes is presented, following a feature-aided pattern recognition method. using the available in three solar cycles hourly fof2 data from 18 ionosonde stations, an ionospheric definition of disturbed conditions, independent of any causative mechanism, is being applied and positive/negative disturbances of duration smaller than 24 hours are sorted out. no latitudinal/longitudinal bins or seasons are defined and disturbances are handled separately in each month, station and four local time intervals of storm commencement, according to solar zenith angle. in order to gain statistical significance, a median profile per disturbance is produced and studied only when storms of the respective class (phase and duration) are present for more than 15% of time from year-to-year. non-systematic features are excluded from this analysis by careful selection of the time window under morphological investigation. first, the median profiles of disturbance patterns are fitted to standard distributions; if they fail, median disturbance patterns of each class are grouped according to their major characteristic features and are described by upper and lower limits of the variability area, along with their distribution in space and time. the present model, while being a non-conditional stand-alone model of ionospheric storms at high and polar latitudes offered to radio users, may complement existing empirical models. finally, the present model may ultimately reveal causeeffect relationships with geomagnetic field or interplanetary parameters after further correlation studies undertaken in the future. 1. introduction 1.1. historical background since when is ionosphere known? marconi’s successful trans-atlantic radio communication experiment in 1901 led to the first consideration for the existence of a plasma in the atmosphere, due to the solar ionizing radiation. however, before such experimental evidence, the hypotheses for the existence of electrified layers in the upper atmosphere were based solely on the observation of physical phenomena and limited upon the knowledge of physics of charged particles and magnetic fields at the time. thus, graham [1724] discovered the fluctuations of geomagnetic field and celsius [1741] noticed that auroras are accompanied by enhanced geomagnetic fluctuations. furthermore, dalton [1793] and gauss [1839] proposed that electricity in motion performs a principal part at the aurora borealis phenomenon. therefore, it is the aurora borealis, the luminous aurora, or ‘the northern lights’ [daglis and akasofu 2004], nature’s manifestation of ionosphere usually originated at high latitudes, that permitted the most ancient observations of the ionospheric medium. at the low geomagnetic latitudes of the meditteranean sea a visible aurora is rare but not impossible; an average incidence of one in a decade is expected, hence not far from the maximum of the eleven-year solar activity cycle. thus, researchers compiled catalogues of ancient auroras, from which the one by stothers [1979] stands out for its classification and justification. the presocratic philosophers anaximenes (585-528 b.c.) and xenophanes (570-475 b.c.) provided the first potentially valid descriptions of auroral phenomena, such as: ‘inflammable exhalations from the earth’ and ‘moving accumulations of burning clouds’, respectively [stothers 1979]. however, the merit for the earliest greek account of what is known as luminous aurora nowadays should article history received november 13, 2014; accepted november 23, 2015. subject classification: ionosphere, ionospheric storm, statistical analysis, history of science. in memoriam of professor stamatis s. kouris go to anaxagoras of clazomenai (500-428 b.c.) [bicknell 1968, stothers 1979], who described an event in 467 or 468 b.c. and is preserved by plutarch: “for seventy five days, there was seen in the heavens a fiery body of vast size, as if a flaming cloud, not resting in one place but moving, with intricate and irregular motions so that fiery fragments broken from it by its plunging and erratic course were carried flashing fire in all directions…” yet, for many researchers the first reliable scientific description of an auroral display is attributed to aristotle who mentions at about 330 b.c.: “…appearances can be seen taking shape in the sky, such as ‘chasms’, ‘trenches’ and blood-red colours”. in conclusion, by late 4th century b.c. the basic features of auroras visible at low latitudes had been described by greek natural philosophers. furthermore, although several roman reports on auroras date from even 464 b.c., it is only at first century a.d. that roman writers did describe auroras in a scientific way. characteristic is the following incident in seneca’s naturales quaestiones, after categorizing ‘atmospheric fires’ like aristotle above: “…the sky seems to be on fire. sometimes its glow is so high it appears to be actually among the stars. sometimes it is so low that it gives the illusion of a fire some distance away. in the reign of tiberius caesar (a.d. 14-37) watchmen rushed to the aid of the colony of ostia just as though it were ablaze, since throughout most of the night there had been a glow in the sky, dull, as of a thick smoky fire. concerning these phenomena no one doubts that they have the flame which they show; there is a definite substance to them”. summarizing, the luminous usually red-coloured auroras seen at the ancient peoples around the mediterranean seem to be the first contact of humanity with ionosphere. however, auroral displays occur mainly at the high-latitude ionospheric regions i.e. poleward of 60° magnetic latitude ({m), as seen at their statistical distribution which led to the concept of auroral oval [feldstein 1963]. so, it is the high-latitude and polar ionosphere, a highly dynamic spatial and temporal structure, and especially its large-scale plasma irregularities that are under investigation here. 1.2. high-latitude and polar ionospheric studies: state of the art unlike midor low-latitude ionosphere, the highlatitude ionosphere presents plasma irregularities of various spatial scale sizes which form and propagate in a highly dynamic way, mainly because this region is modified by the interaction between earth’s magnetosphere and the solar wind; the geomagnetic field lines, which run here almost vertical, connect the polar upper atmosphere to the outer magnetosphere causing this region, especially near the polar cusps around ±78°{m, to be readily accessible either to energized particles within the magnetosphere, or, to particles with sun origin via the solar wind. thus, the orientation of the interplanetary magnetic field (imf) plays also an important role to the overall morphology of the polar ionosphere [carlson 1994]. reliable navigation, especially at the arctic region, has become recently of high importance to many nations due to the unexplored natural resources and the possible transport routes from the receding polar ice. therefore, plasma irregularities and instabilities should be temporally and spatially forecasted. scintillation phenomena is produced by ionospheric irregularities, whose scales range from hundreds of kilometers down to centimeters. especially around the auroral oval zone, amplitude and phase scintillations largely degrade the performance of global navigational satellite system (gnss) receivers [aarons 1982, yeh and liu 1982]. the ionospheric scintillations arctic campaign coordinated observation (isacco) project [de franceschi et al. 2006] has brought new insights and monitoring capabilities in scintillation studies. spogli et al. [2009] have developed the ground-based scintillation climatology technique which by selecting imf bz conditions allows for the specification of the cusp and auroral oval boundaries as regions with the highest probability scintillation occurrence. when the above methodology used tec maps obtained by midas (multi-instrument data analysis system; spencer and mitchell [2007]), it showed that amplitude/phase scintillations are present where also steep tec gradients exist, i.e. at the edge of larger scale irregularities (patches) [de franceschi et al. 2008]. in this direction, scintillation climatology studies, newly embarked [alfonsi et al. 2011], attempted to catalogue scintillations according to the amplitude (s4) and phase (v{) scintillation indices, the mean and sigma values of rate of tec changes (rot) and index (roti), thus identifying the scale sizes of irregularities leading to scintillations (from few kilometers down to hundreds of meters). finally, a probabilistic forecasting of gps phase scintillation at high latitudes due to solarwind events has been recently proposed by prikryl at el. [2012], aiming to create a physically based scintillation forecast tool. apart from the small-scale ionospheric structures, the highand polar-latitude ionosphere of the f-region presents large-scale, irregular plasma structures which include polar cap patches and boundary blobs [crowley 1996]. these structures are enhancements of electron density (by a factor of 2-10 above the background), not fotiadis et al. 2 3 produced locally, which may have a duration up to 3-4 hours (patches), or even greater than 12 hours (boundary blobs). their broad characteristics may be also found in table 5.1 in hunsucker and hargreaves [2003]. however, although much is known for the physical processes responsible for the formation of these structures (e.g. see crowley [1996], for their seasonal variation: wood and pryse [2010] and for their diurnal distribution: moen et al. [2007]), their prediction in physical models [e.g. millward et al. 1996, schunk and sojka 1996] is not yet accurate enough due to their variability in the occurrence, location and morphology. statistical modeling may reveal some systematic morphological features, offering a climatology of the irregular f-region plasma structures at high and polar latitudes. of course, as usual in empirical statistical models, their power will always be limited by the distribution and density of the selected data set, in terms of times and locations. for instance, the international reference ionosphere (iri) is such an empirical ionospheric model [biliza 2001], which provides users with monthly averages for magnetically quiet conditions and corrected values for active conditions. the storm model of araujo-pradere et al. [2002] has been incorporated to allow iri coping with ionospheric response to disturbed magnetic conditions. however, this is only one of the factors affecting ionospheric behaviour in the highly dynamic highand polar-latitude f-region ionosphere. furthermore it should be noted that liu at al. [2007] have found that iri tends to predict electron density about 15-20% higher at polar latitudes than champ satellite observations. another approach in the road of statistical modeling, being complementary to existing ionospheric models such as iri, has been proposed by fotiadis and kouris [2006]. its main hypotheses are: a) the monthly median value for a specific hour gives a ‘quiet’ reference and the deviations of hourly data from the respective monthly median separate perturbations from regular behaviour; of course, although the use of monthly median value does introduce some artifacts [pietrella 2012] (mainly due to the seasonal variation at specific seasons e.g. near equinoxes), such differential maps of the f2layer critical frequency ( fof2) [kouris et al. 1998, 1999], or, of the total electron content (tec) [jakowski et al. 1999, foerster and jakowski 2000] have been successfully used to extract general, common features of ionospheric storms; b) the definition of a disturbance (i.e. the selection of a suitable threshold) should ensure the selection of solely disturbed periods; c) the definition of a disturbance should be ‘ionospheric’; that is, it should be based exclusively on amplitude and duration and not initially linked with any of potential causes e.g. geomagnetic field, imf etc., but allow the total amount of variability to be included. following the methodology presented by fotiadis and kouris [2006], scope of the present paper is to reveal the morphology of the most systematic and dominant patterns of shortand medium-duration fof2 disturbances (patches and blobs respectively, if disturbances are positive), which prevail at high and polar latitudes, and provide radio users with upper and lower limits of the variability area and their distribution in time and space. moreover, the present analytical product may complement and improve existing operational algorithms and empirical models. correlations of the extracted patterns with geomagnetic field and solar wind parameters will be undertaken in the future in order to enhance the prediction operability of the present model. 2. data and analysis method an ionospheric definition of disturbed fof2 conditions, based exclusively on amplitude and duration, has been developed by kouris et al. [1998, 1999]. a day is characterized as disturbed when the normalized deviation of hourly fof2 to its corresponding monthly median (d fof2) exceeds 0.30 in absolute value for at least 3 consecutive hours [kouris et al. 1998]. deviations greater (smaller) than the monthly median conditions are considered as positive (negative) disturbances. the disturbance ends when the absolute d fof2 remains less than or equal to 0.20 for more than a 3-hour period [kouris et al. 1999]. the selection of the 0.30 d fof2 threshold ensures that an ionospheric disturbance is present since such deviations are usually observed for less than 10% of the monthly time [kouris and fotiadis 2002]. furthermore, even a 0.20 absolute d fof2 threshold seems, according to buresova and lastovicka [2007], adequate to avoid inclusion of neutral atmospheric origin perturbations, as for example caused by acoustic gravity waves. before stepping into the feature-aided pattern recognition method, in order to extract disturbance morphology, the variables should be selected. any link with solar, geomagnetic field or imf activity is hereby avoided. also, handling/grouping together disturbances of different longitude/latitude, or different months could insert important errors in modeling, and then in predictions. however, the role of the local time (lt) of disturbance commencement as seen by the solar zenith angle at 300 km is important. thus, the 24hour period is divided in four lt windows: day (cos|>0.20), night (cos|= 0) while dawn and dusk fall in between [fotiadis et al. 2004]. on the other hand, each station is handled sepamorphology of ionospheric disturbances rately and so its longitudinal universal time (ut) sector; this is because daily variations of the f-region at these high latitudes depend on ut as well as lt [sojka et al. 1994]. the ut variation depends on the separation of geographic and geomagnetic poles [hunsucker and hargreaves 2003]. significant to note that in the northern hemisphere, where separation between the poles is smaller, the ut effect is less pronounced than in the south. finally, the disturbances extracted are studied in two gross duration classes: a) small-duration disturbances with duration of 3-5 hours, and b) medium-duration disturbances lasting for 6-24 hours. table 1 shows the 18 stations with geomagnetic latitude greater than 60°{m and the period of their daily hourly fof2 considered for the current analysis. first, the aforementioned disturbance definition of kouris et al. [1998, 1999] was applied and the disturbances were separated according to their class (i.e. their phase and duration), and lt commencement for each month and station. while, initially, the potential study cases for each disturbance class could be 4 (lt commencement window) × 12 (months) × 18 (stations), after applying the disturbance definition mentioned above, it was found that disturbances of that depth and duration were not present at all stations and months for the different classes. then, for each set of disturbances of the same class, the hourly d fof2 values of each disturbance were arranged with respect to zero local storm-time, and a large time window, 2 days before and 3 days after this point, was considered for superposition analysis. in order to exclude non-systematic trends, a minimum frequency of occurrence from year-to-year was required to be at least 0.15; that is, only sets of disturbances containing at least five disturbances in the full three 11-year solar cycle period (5/33 > 0.15) were further considered for analysis. followingly, for each storm time hour and set of disturbances, the median d fof2 value was calculated forming thus a new disturbance pattern to be considered in the pattern recognition analysis which will be described below. in order to model only the main phase of a disturbance, a visual inspection of all trends was carried out in order to select a more narrow time window with respect to the duration of the main phase of the disturbances. a rough rule to achieve this is to ensure that absolute d fof2 values remain well within 0.20 (off the median) for at least 3 hours. optimum time windows fotiadis et al. 4 area/station code geographic geomag. periods latitude (°) longitude (°) latitude (°) of data european arkhangelsk ark 64.4 40.5 60.0 1969-93 lycksele lyc 64.7 18.8 61.3 1964-96 sodankyla sod 67.4 26.6 63.6 1964-89 loparskaya lop 68 33 63.9 1964-77, 1981-84, 1991-96 kiruna kir 67.8 20.4 64.4 1964-86, 1991-96 asian/pacific provideniya bay pro 64.4 186.6 60.3 1964-70, 1979-83 norilsk nri 69.4 88.1 63.8 1968-88 tiksibay tik 71.6 128.9 65.2 1964-71, 1987 dikson dik 73.5 80.4 67.7 1982-96 campbell island cll -52.5 169.2 -60.2 1970-85 mawson maw -67.6 62.9 -70.5 1964-87, 1991-93 scott base sco -77.9 166.8 -79.9 1970-83 casey cas -66.3 110.5 -80.7 1964-74, 1990-91 american winnipeg win 49.8 265.6 60.7 1966-76 goosebay goo 53.3 299.2 62.6 1987-96 college col 64.9 212.2 65 1964-67, 1988-89, 1991, 1994 churchill chu 58.8 265.8 69.5 1966-74 resolute bay res 74.7 265.1 83.8 1964-91 table 1. list of stations and years of fof2 data used in the analysis. the geomagnetic latitude is calculated by the international geomagnetic reference field (igrf) for the year 1986 for a 300 km height. 5 for smalland medium-duration disturbances’ modeling were found to be within [−6, 12] and [−6, 20] hours of local storm-time respectively. now, in order to extract the potential disturbance patterns, first the median profiles for each station and month and class (phase, duration, lt commencement window) were checked if they follow the gaussian (g) or the lognormal distribution (l). this happens when a correlation coefficient greater than 0.90 is achieved and, at the same time, an overall fitting standard error smaller than 4%. it is found that a large number of median disturbance profiles do not follow any standard distribution, and the failure in the fitting procedure with the g or l distribution, apart from residual outliers, is attributed to: a) the existence of more than one maxima during the peaking of a disturbance, b) the significantly different ‘noise level’ before and after the disturbance, and c) the assymetric skewness before and after the disturbance. therefore, the following three criteria are applied to the disturbance profiles of each class, which do not follow any standard distribution: the number of maxima and their relative storm-time occurrence, the existence of systematic preor post-storm variability enhancements/depletions and the differences in the skewness before and after the disturbance. so, by visual inspection these patterns, which are as previously arranged with respect to zero local storm-time, were grouped according to the above criteria and upper and lower d fof2 values were calculated for each disturbance class (phase and duration). summarizing, the final product of the present paper includes: a) the characteristic disturbance pattern envelopes (i.e. upper and lower variability limits) for each class (positive/negative phase and small/medium duration), and b) table charts with the distribution of the above variability envelopes, as well as the gaussian/lognormal storm types in time (month and lt window of commencement) and space (station). 3. morphology of negative ionospheric disturbances 3.1. pattern description figures 1 and 2 illustrate the variability envelope patterns for negative disturbances of small (3-5 hours) and medium (6-24 hours) duration, respectively. negative disturbances of a-pattern (figure 1) present a −0.15 variability level for more than 3 hours before their commencement. at main phase they seem quite shallow, being constant for at least 4 hours, remaining at about −0.35 with no eventual peaks. a-type disturbances end after about 6 hours at main phase (recovery phase), having almost identical variability levels at the respective times before commencement. on the other hand, b-pattern negative disturbances (figure 1) present two discrete minima during the first three hours at main phase: a) when the peak at commencement is more powerful for about 5% (b1-pattern) morphology of ionospheric disturbances figure 1. upper and lower variability limits of disturbance patterns for small-duration (3-5 hours) negative fof2 storms. y-axis: deviation from the monthly median (d fof2), x-axis: local storm time after kouris et al. [1998, 1999]. and b) when the 2-hour delayed peak reaches a greater depth than the peak at commencement hour (b2-pattern). the most outstanding difference between these two profiles lies in the recovery phase of b2-pattern which may oscillate even around −0.20 variability level. other negative disturbance patterns may be considered when a single main phase peak delays after storm commencement: for just one hour (c-patterns in figure 1), for 2 hours (d-pattern), or, for 4 hours (e-pattern) (figure 2). furthermore, two types in c-pattern may be identified, when at recovery phase: a) an oscillation at variability between ±0.15 is observed (c1-pattern) and b) only an inverse variability of positive phase is present which exceeds +0.30 level (c2-pattern); thus, the c2-pattern describes the so called bi-phase disturbances. finally, it has to be noted that dand e-pattern negative disturbances last for more than 8 hours (figure 2) while all others described previously last for less than about 6 hours. 3.2. storm space/time distribution fotiadis et al. [2004] have provided the occurrence probability for positive and negative disturbances of small and medium scale depending on their lt onset and month, at all latitudes in different continental regions. now, the space/time distribution of the disturbance patterns found here should be investigated so as to increase their operability from radio users. tables 2 and 3 present the distribution of small and medium scale negative disturbance patterns respectively, in time and space, depending on their lt window of commencement. regarding small scale negative disturbances (table 2), a-pattern storms, with maximum depth around −0.35, break out only at daytime, most commonly around spring and appear in autumn equinox months at about ±70°. b1-pattern disturbances (with onset peak being deeper) commence only during night: at equinox above about 65°{m, but mainly in winter months at polar latitudes (~80°{m). likewise, b2-pattern storms break out only at sunset (i.e. the stronger plasma depletion takes place at early night), at equinox and deep winter months at the polar region. thus, we may conclude that b-patterns, which show a characteristic dual peak distribution, are representative of polar latitudes. the c1-type negative storms may begin mostly at sunset or, at a lesser extent, at night, usually at early/late winter months i.e. november and february, at latitudes below about 65°{m. on the other hand the c2-pattern storms, which are characterized by a +0.30 variability level for several hours at recovery phase, break out only during sunset in deep winter months (december-january), mainly at the european region again below about 65°{m. thus, while at sunset and early night hours a negative phase is present, with the night progressing a reverse -positivephase prevails. depending on the dominant causative mechanisms, negative ionospheric storms at higher/polar latitudes may last longer than 8 hours (table 3). dand fotiadis et al. 6 figure 2. upper and lower variability limits of disturbance patterns for medium-duration (6-24 hours) negative fof2 storms. y-axis: deviation from the monthly median (d fof2), x-axis: local storm time after kouris et al. [1998, 1999]. 7 e-patterns are present at sunset during equinoxes but have in general low frequency of occurrence. however, e-pattern storms may emerge at night during winter and early equinox at the european and asian zone; on the other hand, the d-pattern may be found at the same period mostly at the american zone. table 4 summarizes the above findings, showing the most dominant negative disturbance patterns at difmorphology of ionospheric disturbances °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60.7 win g g sunrise 62.6 goo l l l 83.8 res l l l l l °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark a a a day 61.3 lyc l a 62.6 goo a a 69.5 chu a a a -70.5 maw a a a a a °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark c2 c1 c1 g g l g c1 c1 c2 sunset 61.3 lyc c2 c1 g g g l l g c1 c1 c2 63.6 sod c1 c1 l l g g c1 c1 63.9 lop c2 l g l c1 64.4 kir c2 c2 c1 c1 60.3 pro g l l l g 63.8 nri g g c1 c1 c1 62.6 goo c2 l l c1 83.8 res g b2 b2 -60.2 cll l l l l c1 c1 l l -70.5 maw c1 -79.9 sco b2 c1 b2 b2 -80.7 cas b2 b2 b2 b2 b2 b2 b2 g b2 °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark g g g l c1 g night 61.3 lyc l g c1 g g g 63.6 sod g g l c1 c1 c1 c1 63.9 lop l c1 c1 g g g 64.4 kir g l l c1 63.8 nri g l c1 l c1 g 60.7 win g c1 l g g 62.6 goo l l l g l l l 65 col c1 69.5 chu g l b1 b1 l b1 83.8 res l l g b1 b1 l -60.2 cll g g g l g g -79.9 sco b1 b1 b1 b1 -80.7 cas b1 b1 b1 table 2. monthly distribution of small-scale (3-5 h) negative disturbance patterns per station, depending on the lt window of commencement. from top to bottom: sunrise, day, sunset, night. g/l: gaussian/lognormal distribution, other symbols according to figure 1. dash (-): while ionospheric storms occur, with frequency greater than 15% of the time over the respective periods of table 1, the present study does not model them. ferent high and polar latitudes and seasons, stressing on specific geographical theatres where disturbance patterns present a local character. 4. morphology of positive ionospheric disturbances 4.1. pattern description the variability envelope patterns for positive disturbances of small (3-5 hours) and medium (6-23 hours) duration are shown in figures 3 and 4, respectively. positive disturbances of a-pattern (figure 3) present remarkable plasma depletion (variability levels exceeding −0.20) until 2 hours before their commencement. the outbreak of a-pattern storms is characterized by the maximum variability level which is greater than 0.50. later, at recovery phase i.e. at least 6 hours after commencement time, a new positive disturbance progresses with variabilities even around 0.40 which lasts for more than 5 hours. all in all, a-type positive disturbances could be addressed as bi-phase because of the sudden phase transition in variability at their outbreak and are also characterized by further positive outbursts fotiadis et al. 8 °{m 1 2 3 4 5 6 7 8 9 10 11 12 60 ark e1 . d1 d1 d1 e1 e2 sunset 61.3 lyc . d1 e1 e1 d1 d1 e2 -79.9 sco d1 e2 e2 d2 d2 °{m 1 2 3 4 5 6 7 8 9 10 11 12 60 ark e1 e1 e1 e1 e1 e1 night 61.3 lyc e1 e1 e1 e1 e1 e1 63.6 sod e1 e1 63.9 lop e1 e1 64.4 kir 60.3 pro e1 e1 e1 63.8 nri e1 e1 e1 e1 e1 60.7 win e1 d1 62.6 goo d1 d1 83.8 res d1 -60.2 cll d1 e1 igrf cml (°{m) geographical zones season/ months pattern success da y 60º-65º all 3-5: nh a 6/9 65º-75º all 3-4, 9-10: nh and sh a 7/8 su ns et 60º-65º all 2, 11: nh c1 9/15 all but asia, australia 12, 1: nh c2 7/11 60-62º europe 3-5, 8-10: nh d 6/10 >75º all 10-2: nh 4-8: sh b2 10/15 ni gh t 60º-65º all 3, 11: nh c1 7/15 europe, asia 10-12, 1,2: nh e 21/30 america 10-12, 1-3: nh d 3/6 >65º all 10-12: nh 4-8: sh b1 11/15 table 3. monthly distribution of medium-scale (6-24 h) negative disturbance patterns per station, depending on the lt window of commencement. from top to bottom: sunset, night. symbols according to figure 2. dash (-): while ionospheric storms occur, with frequency greater than 15% of the time over the respective periods of table 1, the present study does not model them. table 4. synoptical distribution of disturbance patterns of negative disturbances in different geographical zones and magnetic latitude bins depending on the lt window of commencement. from top to bottom: day, sunset, night. ‘success’ column indicates the number of pattern hits with respect to the zones and season/months which have some sign, even a (-) dash (i.e. where storms are present for more than a 15% of time from year-to-year). 9 at ‘recovery’ phase. on the other hand, b-pattern positive disturbances (figure 3) present two discrete maxima during the first three hours at main phase and the commencement peak is more powerful for about 10%. the fof2 levels before commencement and at recovery phase oscillate with small amplitude (±10%) around the median value. other positive disturbance patterns may be considered when a single main phase peak delays after storm commencement for 2 hours (c-patterns in figmorphology of ionospheric disturbances figure 3. upper and lower variability limits of disturbance patterns for small-duration (3-5 hours) positive fof2 storms. y-axis: deviation from the monthly median (d fof2), x-axis: local storm time after kouris et al. [1998, 1999]. figure 4. upper and lower variability limits of disturbance patterns for medium-duration (6-24 hours) positive fof2 storms. y-axis: deviation from the monthly median (d fof2), x-axis: local storm time after kouris et al. [1998, 1999]. fotiadis et al. 10 °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark a a l l g l l a sunrise 61.3 lyc a a a l b l l l a a a 63.6 sod a a l l b l a a 63.9 lop a l l a a 64.4 kir a a l l l 60.3 pro a a l l l 63.8 nri a l l l a l 62.6 goo b l 65 col b g g a 69.5 chu l l 83.8 res b l l b l b b -70.5 maw b l g l -79.9 sco l b l b l l -80.7 cas b b b l °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark c1 c1 c1 c1 c2 c1 c1 c1 c1 sunset 61.3 lyc c1 c1 c1 c1 c2 c2 c2 c1 c1 63.6 sod c1 c1 c1 c2 c1 c1 c1 63.9 lop c1 c1 c2 c1 64.4 kir c1 c1 c1 c1 60.3 pro c1 c1 c1 63.8 nri c1 c1 62.6 goo c1 c1 83.8 res b b b b -79.9 sco b b b -80.7 cas b b b b b b °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark c2 c2 c2 l day 61.3 lyc g c2 g -70.5 maw g g l °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark b l g g g l night 61.3 lyc l l l b b l l 63.6 sod l l l l b l 63.9 lop l b l l l 64.4 kir l l b b 60.3 pro g l 63.8 nri b l l 65.2 tik b g 67.7 dik l b b 60.7 win l g g g g g l l 62.6 goo l b b l g l b l l 65 col b b b l 69.5 chu b b l b l g g g 83.8 res b b l b l -60.2 cll g l l l l g g -64.4 mac l l -70.5 maw b b -79.9 sco g -80.7 cas b b b b b table 5. monthly distribution of small-scale (3-5 h) positive disturbance patterns per station, depending on the lt window of commencement. from top to bottom: sunrise, sunset, day, night. g/l: gaussian/lognormal distribution, other symbols according to figure 3. dash (-): while ionospheric storms occur, with frequency greater than 15% of the time over the respective periods of table 1, the present study does not model them. 11 ure 3). two types in c-pattern could be identified: a) the c1-pattern, when the delayed maximum is powerful (around 0.50) and at the recovery phase variability turns negative, around −0.20 for three hours at least but even later, somewhat 10 hours after outbreak, they exhibit +0.20 levels as well, b) when the delayed maxima for 1-2 hours could only reach 40% above monthly median while there is a smooth transition to lower positive variability levels (c2-pattern). the positive disturbances described so far last about 4 hours. however, there are positive disturbances with significant frequency of occurrence presenting a duration of more than 7-8 hours (figure 4). d-pattern storms have also two peaks: the maximum (exceeding 0.60) and a secondary peak of about 0.50. in d1-type the maximum is found 2 hours after storm outbreak, while the secondary peak appears at the 6th hour after commencement. in d2-type a secondary peak is found at the outbreak, while the overall maximum occurs 4 hours after commencement. it is to be noted that a d2pattern disturbance may be followed by outbursts (spikes) of +0.20 variability levels 12 hours after its establishment. finally, there can be cases of a delayed maximum at least 2 hours after the storm outbreak. the disturbance later, e.g. 8-9 hours after commencement, progresses into negative variability levels around −0.20 (quite bi-phase). 4.2. storm space/time distribution in a similar way with negative disturbances, tables 5 and 6 show the time/space distribution of small and medium scale positive disturbance patterns respectively, depending on their lt window of commencement. regarding small scale positive disturbances (table 5), apattern storms may commence only during sunrise from october to march (in northern hemisphere) below about 65°{m. to be noted that a-type storms may be characterized as bi-phase, showing sharp transition from negative to positive variability values at their onset with further deterioration at recovery phase. on the other hand, b-pattern positive disturbances of small duration do not have a quite specific, easy predictable distribution in time and space. in the case of outbreak at sunset such storm patterns are found at polar latitudes above 75°{m almost all year long except summer time. this fact is in fair agreement with the occurrence of b2-patterns of negative small scale storms. furthermore, b-pattern small positive disturbances may begin at night from about october to march at higher and polar latitudes. however, such storm patterns may occasionally emerge during sunrise, mostly at polar latitudes around equinoxes. the c1-pattern positive disturbances break out at sunset from winter solstice, mainly, to equinox months gradually at a lesser extent, at latitudes below 65°{m. similarly, a c2-type storm usually begins at sunset in morphology of ionospheric disturbances °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark l l sunrise 83.8 res d2 d2 l d2 d2 °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark d2 d2 g d2 e sunset 61.3 lyc e g g l g g g l d2 e e 63.6 sod e d2 d2 g e 63.9 lop d2 d2 l g e °{m s/n 1 2 3 4 5 6 7 8 9 10 11 12 60 ark l l l l l l night 61.3 lyc l d1 l l l 63.6 sod l d1 d1 63.9 lop l l 64.4 kir l 60.3 pro l l d1 l d1 63.8 nri l d2 d2 d2 60.7 win d2 d2 d1 l d2 62.6 goo d1 l 83.8 res d2 d2 d2 l -60.2 cll l l l l table 6. monthly distribution of medium-scale (6-24 h) positive disturbance patterns per station, depending on the lt window of commencement. from top to bottom: sunrise, sunset, night. g/l: gaussian/lognormal distribution, other symbols according to figure 4. dash (-): while ionospheric storms occur, with frequency greater than 15% of the time over the respective periods of table 1, the present study does not model them. summer months, at latitudes below about 62°{m, or at equinox months at higher latitudes around about 64°{m. however, these storm patterns may break out during daytime at (late) spring at about 60-61°{m. it is hereby worthy to mention that c2-type disturbances of small duration are found only in the european region, showing their local character, while judging from their overall space/time distribution, they may be characterized as the ‘equinox near summer’ pattern. furthermore, it should be stressed that only storms of b-type are met in the australian zone. positive disturbances of medium scale (table 6), i.e. lasting more than 8 hours, exhibit a distinct locality at times; the e-pattern storms, for example, break out only during sunset at the european high latitudes below 64°{m around winter solstice (november to january). however, most prevalent are the d2-pattern disturbances which begin: a) at sunrise, only at north american polar latitudes, from november to march, b) at sunset, only at the european high latitudes, below 64°{m around equinoxes (february, march and october) and c) at night at the asian/american zone above around 60.5°{m from october to march. furthermore, d1-pattern storms may break out at night below about 63.5°{m, again from october to march. finally it should be stressed than none of the aforementioned patterns of medium scale positive disturbances are met at the australian zone. table 7 summarizes the above findings on the most dominant positive disturbance patterns at specific high and polar latitudes and seasons, stressing also on specific geographical zones and stations where a local character of disturbances is prominent. 5. conclusions scope of the present work is to reveal the systematic and dominant morphological features of smalland medium-duration fof2 disturbances at high and polar latitudes. the avoidance of grouping disturbances in any spatial bins, or seasons, as well as the requirement of a minimum storm frequency of occurrence, in order to extract a median disturbance profile, ensure that non-systematic trends are excluded. each disturbance pattern is presented by upper and lower variability limits and, thus, radio users are provided with dynamic variability bounds which allow them for fine tuning adjustments. the distribution of the disturbance patterns in time and space found is provided for each month and station separately, depending on the local time of storm commencement. overall, the calculated here disturbance patterns may contribute to improve existing storm-time models, such as the one embedded in the iri empirical model, fine-tune the input of neural networks, while being at the same time a non-conditional stand-alone model for radio users. ultimately, the correlation of specific storm patterns with geophysical or interplanetary parameters in the future, such as the imf or solar-wind parameters, may reveal cause-effect relations difficult to extract otherwise and improve significantly short-term forecasting. fotiadis et al. 12 igrf cml (°{m) geographical zones and stations season/ months pattern success su nr is e 60º-65º all but australia 10-2: nh a 21/30 >75º all 2,10,11: nh 8,4,5: sh b 6/9 >80º (resolute bay station) 11-12, 1-3: nh d2 4/4 da y 60º-62º europe 3-5: nh c2 4/5 su ns et 60º-65º all 10-12, 1-3: nh c1 31/38 60-64º europe 4-8: nh c2 6/8 2,3,10: nh d2 7/10 11,12,1: nh e 7/8 >75º all 10-12, 1: nh 4-7: sh b 8/12 ni gh t 60-63.6º europe/asia 10-12,2:nh d1 5/16 >60º america (and norilsk station) 10-12,1-3: nh d2 9/19 >=65º all 11-12, 1-3: nh 5-9: sh b 17/33 table 7. synoptical distribution of disturbance patterns of positive disturbances in 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 1 impacts of ports along the pilbara coast, western australia – a coastline of global geoheritage significance that services a mineral-rich hinterland margaret brocx dep artm en t of en vir on m en tal an d con serv ation scien ce , mu rd och un iversity, au stralia geoh eritage@iin et.n et.au vic semen iuk v & c sem en iu k research grou p , au stralia vcsrg@iin et.n et.au abstract the pilbara region in remote north-western a ustralia is mineral-rich with ores being mined/quarried and exported since the 1960s for the wealth of the a ustralian n ation and exported from a range of ports developed specifically for such pu rposes. however, the pilbara coast is one of few arid coasts around the w orld and the most arid coast in a ustralia it stands unique as the most geomorphologically / geologically diverse arid coast globally and therefore has global coastal geoheritage significance. ports along the coast have been and continue to be developed without, or with little regard to their natural values, with impacts in terms of geoheritage and biological values the parameters for port selection are based on engineering and economic perspectives of coastal proximity and coastal bathymetry in spite of information available for proper management and wise use of this coastal zone. consequently, some significant coasts have been destroyed or markedly modified. w ith the intended growth of the mineral industry, there can be expected further destru ction unless government agencies address the geoconservation issues but, in this context, there seems to be both a widespread lack of understanding on the part of government agencies of the geoheritage values of this coast and a lack of geoethics. this contribution describes the natural heritage significance of the pilbara coast, the ports therein, their impacts and, from a geoethics viewpoint, the notion of centralizing ports rather than to indiscriminately construct facilities dictated by economic and port ownership. the pilbara coast provides a case study of geoethics where natural history assets of global significance conflict with industrial use. 1. in trod uction h e pilbara coast in th e n orth -w estern au stralia (fig. 1) is globally u n iqu e an d sign ifican t in its geoh eritage valu es, coastal p rocesses an d lan d form s, stratigrap h y, m an grove ecology, an d biod iversity. it is on e of eigh t trop ical arid coasts in th e world bu t m arked ly d ifferen t to th e rest; also, com p ared to th e oth er coasts it is th e m ost geologically, geom orp h ologically, an d sed im en tologically d iverse of th e arid coasts (sem en iu k, 1996) an d th u s globally sign ifican t (brocx, 2008; brocx an d sem en iu k, 2015) an d , in its n atu ral state, sh ou ld h ave been recogn ized as a world h eritage site. th e pilbara coast is also th e m ost arid coast in au stralia an d , w ith coastal featu res p resen t n ow h ere else in au str alia, e.g., lim eston e barriers, arid -zon e d eltas, arch ip elago/ ria sh ores, exten sive salt flats (sem en iu k, 1996; sem en iu k, 2012; brocx an d sem en iu k, 2015), it is n ation ally sign ifican t . com p rised of qu atern ary an d tertiary sed im en ts, an d precam brian rock, th e pilbara coast t an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 2 bord ers th e pilbara u p lan d s, w ith a ran ge of rivers d rain in g th e u p lan d s to th e coast. qu atern ary coastal p rocesses h ave p rod u ced d iffe ren t coastal typ es th at rep resen t stages of d eve lop m en t of active to in activ e arid -zon e d eltas, stages of pleistocen e lim eston e barrier islan d d evelop m en t an d erosion , con stru ction of h o locen e d u n e barriers, an d local ou tcrop of pr ecam brian rock (sem en iu k, 1996; an d fig. 1). th e pilbara coast con tain s m an y featu res of geoh eritage sign ifican ce, from th e large scale to th e sm all scale (sem en iu k, 1996; brocx, 2008): lan d form s su ch as d eltas, pleistocen e oolitic lim eston e barriers an d th eir m u d -filled sw ales, arch ip elago/ ria sh ores, d eltas th at m erge in to lin ear d u n e field s, rocky sh ore erosion of pleistocen e lim eston es, qu atern ary stratigrap h y su ch as pleistocen e oolitic sh oalin g lim eston e sequ en ces an d lim eston e rocky sh ores, h olocen e m an grove sequ en ces, tem p estites in beach / d u n e san d s, an d th e com p lexities of beach -rock d evelop m en t. in th e precam brian rock sequ en ces th ere is coastal geom orp h ology an d m icro-geom orp h ology reflective of th e v a riable precam brian rock typ es, an d w ellexp osed precam brian ou tcrop s w ith lith ology figure 1: the pilbara coast in a ustralia, showing simplified geology, the river-dominated grain of the upland terrain, and, in the lower row of insets, the main coastal types in the region (modified after semeniuk, 1996 and brocx, 2008). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 3 an d stru ctu re em p h asized by cliff erosion . prior to th e in ten sified m in eral exp loration , d evelop m en t of m in es, solar salt p rod u ction , n a tu ral gas extraction an d liqu efaction , an d th e exp ort of th ese com m od ities d u rin g th e p ast five d e cad es (brocx, 2008; brocx an d sem en iu k, 2015), th e pilbara region h osted p astoral an d agricu ltu ral activity w h ich h ad im p acts m ain ly on th e h in terlan d (n ot th e coast), an d exp orts d erivin g from th ese latter in d u stries took p lace from local jetties w ith relatively m in im al effects on th e coast region ally. h ow ever, sin ce 1960 w h en th e au stralian govern m en t lifted it s restriction s on th e exp ort of iron ore, en viron m en tally th e pilbara coast h as been slow ly d egrad in g, w ith m in eral, solar salt, an d n atu ral gas d evelo p m en t an d exp ort th at h as resu lted in m assive an d m u ltip le exp an sion of p orts an d in in d u strialization . th is coast p rovid es a case stu d y of geoeth ics w h ere n atu ral h istory assets of global sign ifican ce con flict w ith in d u strial u se. th e pilbara region an d its coast are rem ote (th ou san d s of kilom eters from m ajor cities/ tow n s of au stralia), an d in h abited ove rw h elm in gly w ith m in e w orkers, laborers, m ach in ery d rivers an d m ech an ics, an d ed u cated bu t in d u stry-orien ted p erson n el (geologists, m in in g en gin eers, an d civil en gin eers), all largely w ith an eth os to create w ealth p erson a lly an d for th e n ation . th ere exists little en v iron m en tal/ con servation eth os, or a p h ilosop h y th at can be u sed to focu s on th e geoeth ics of en viron m en t-d am agin g activities. a sim ilar critical an alysis of th e au stralian eth os an d w ay-of-life w as p rovid ed ov er 50 years ago by h orn e (1964) in a book “th e lu cky cou n try”. th is book w as m isin terp reted by m ost au stra lian s, w h o n ever read p ast th e title, as com p lim en tary in fact it w as stin gin g criticism of th e factors m en tion ed above an d so n oth in g h as ch an ged . in ou r in d ivid u al exp erien ce of 50 years each in en viron m en tal m atters in western au stralia, w e h ave con clu d ed th at several factors co n tribu te to im p ed e scru tin y of th e en viron m en tal d am age an d p oor m an agem en t of region s su ch as th e pilbara coast (som e of th ese m atters are d iscu ssed in brocx [2008] an d brocx an d sem en iu k [2015]). firstly, th e pilbara coast is r em ote; it con trasts w ith perth , th e cap ital of western au stralia. th at h as 2 m illion p eop le, five u n iversities, en viron m en tal issu es th at are p roxim al, an d in n u m erable en viron m en tally orien ted com m u n ity grou p s (e.g., frien d s of yellagon ga h ttp :/ / w w w .frien d sofyellagon ga . com .au / , th e con servation cou n cil of western au stralia h ttp :/ / w w w .ccw a.org.au / , th e urban bu sh lan d cou n cil h ttp :/ / w w w .bu sh lan d p erth .org.au / , an d th e wetlan d s con servation society h ttp :/ / w w w .bu sh lan d p erth .org.au / m em ber -grou p s/ 6-statew id e –region al/ 78-w etlan d s-con servation -society, am on gst m an y oth ers). th e en viron m en tal con flicts betw een con servation grou p s an d d eve lop ers an d govern m en t agen cies (su ch as m ain roads w estern a ustralia, th e environmental protection a uthority, an d th e department of planning, am on gst oth ers) associated w ith d eve lop m en t of bu sh lan d , w etlan d s, an d th e coast p rovid e exam p les w h ere th ere is focu sed co n servation -orien ted activity by th e com m u n ity becau se of in form ed grou p s an d a critical m ass of p erson n el. in th e pilbara coast region , a coastal d evelop m en t is com p leted gen erally w ith ou t social con flict becau se th e area is larg ely u n kn ow n , th ere h as been a relaxin g of reg u latory en viron m en tal safegu ard s (in sp ite of th e fact th ere h as been an in crease in kn ow led ge in en viron m en tal m atters an d geoh eritage), an d th ere is n ot th e crit ical m ass of in terested an d p assion ate p eop le th e ease w ith w h ich a p ort w as d evelop ed at cap e preston an d th e facilities at port h ed lan d w ere exp an d ed are exa m p les. secon d ly, th e eth os of th e state govern m en t is d riven by an econ om ic im p erative, an d th e m in eral an d fossil-fu el w ealth h as trad ition ally been th e sou rce of n ation al w ealth . accord in g to govern m en t statistics, th e m in eral an d en ergy in d u stry in 2016-2017 in western au stralia w as valu ed at $105 billion (dep artm en t of min es, in d u stry regu lation an d safety, 2017). iron ore rem ain ed th e state’s h igh est valu e com m od ity w ith $67 billion in sales w ith e xp ort of som e 790 m illion ton n es of ore, an d th e total p rod u ction from th e en ergy sector w as valu ed at $19 billion . th is sh ow s th e sign ifican ce of th e m in in g an d en ergy in d u stries to th e govern m en t an d to th e econ om y of western au stralia. th ird ly, w h ile scien tific literatu re on th e region id eally sh ou ld h ave or cou ld h ave been u sed to an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 4 raise th e en viron m en tal con sciou sn ess of th e scien tific com m u n ity, en gin eerin g com m u n ity, regu latory au th orities, or th e p u blic, it is eith er econ om ically -orien ted (e.g., geological a n d exp loration -orien ted literatu re), or “blu e sky” r esearch (e.g., sem en iu k, 1996), th e latter eith er of little ap p aren t u se to th e broad er scien tific an d en gin eerin g com m u n ity an d lay -p erson , or h as n ot been in terp reted to be of u se for en viro n m en tal m an agem en t or to aid th e regu latory au th orities to m ake in form ed d ecision s. th at is, scien tific literatu re on th e r egion u sefu l for con serv ation an d ecologically -sou n d d evelop m en t is gen erally n ot read or u n d erstood by p rofession al scien tists an d ad m in istrators. acad em ic staff in au stralian u n iversities trad ition ally u sed to be social an d en viron m e n tal w atch d ogs of society bu t in th is p ast h alf ce n tu ry an d m ore sp ecifically in th is p ast qu a rter cen tu ry th is role h as d im in ish ed . th e sign ifican t in flu en ce of in d u stry fu n d in g to u n iversities (w ith atten d an t con sequ en ces of cu rricu lu m con trol in in d u stry -based an d career orien ted cou rses) is on e u n d erlyin g reason for th is. also, govern m en t agen cy officers in m a n agem en t p osition s to m an age/ safegu ard th e pilbara coast, in ou r exp erien ce an d from ou r in terview s (brocx, 2008), are ill-read as to th e global con text of th e western au stralian coast an d d o n ot keep u p w ith th e p u blish ed w ork, d o n ot u n d erstan d it, or d o n ot kn ow h ow to u se it for ration al d evelop m en t – for in stan ce, regu latory au th ority officers w ill focu s on p rod u ctivity of m an groves bu t w ill m iss th e h olistic p ictu re th at th e pilbara coast rep r esen ts. takin g th is a step fu rth er gen erally h avin g n ever ven tu red ou tsid e of western au stralia in a research cap acity (travellin g u su ally is for tou rism ), in ou r exp erien ce sp an n in g d e cad es, m an y regu latory au th ority officers are n ot even aw are of th e u n iqu en ess of th e pilbara coast. as brocx (2008) n oted , n on e of th e var iou s govern m en t agen cies for con servation an d en viron m en t in western au stralia, th ou gh n eed in g to d eal w ith h olistic m atters in en v iron m en tal m an agem en t, h ave h ad a qu alified geologist or geoh eritage p ractition er on staff th is situ ation still exists in 2017. th u s, p u blish ed stu d ies th at ou gh t to h ave resu lted in global recogn ition of th e pilbara coast in term s of geoh eritage, or resu lted in p rop er coastal m an agem en t h ave n ot p rovid ed th e an ticip ated p ositive en viron m en tal ou tcom es. th e m atter of th e effects of m in in g econ om ica lly, sociologically, p olitically an d , to som e exten t, en viron m en tally in western au stralia h as been d iscu ssed an d d ebated by a n u m ber of au th ors (e.g., h arm an an d h ead , 1982; jacobs, 1995). recen tly, bru eckn er et al. (2014) p rovid ed a collection of articles on th e im p acts of con tem p orary resou rce exp loitation in western au stralia as a basis for com p aratively assessin g th eir ben efits an d ou tcom es. an d w h ile th ere is d iscu ssion an d d ebate on th e social, econ om ic, an d p olitical ou tcom es of resou rce d evelo p m en t th ere h as been rela tively less focu s on en viron m en tal con sid eration s th is is w ith in a con text th at western au stralia is view ed as a ‘qu arry’ for m in erals globally, an d h en ce a m ajor in com e earn er. as su ch , en viron m en tal co n cern s gen erally h ave low er p riority. moreover, th e en viron m en tal focu s in d ebates is on biod iversity n ot geoh eritage. th e (geo)eth ics of su ch econ om ically -d riven exp loitation again st an eth os of th e in tern ation al ‘declaration of th e righ ts of th e mem ory of th e earth ’ as form u lated by martin i (1993) h as been d iscu ssed by brocx (2008), albrech t an d ellis (2014), an d brocx an d sem en iu k (2015) w ith a su ggestion th at th ere be an altern ative eth os to th e cu rren t econ om ically -d riven , lan d scap e-d egrad in g, an d geoh eritage-d estroyin g p arad igm . on e a ltern ative to resou rce exp loitation h as been th e su ggestion for u sin g th e m in eral-rich region s in western au stralia for th eir geological v alu es as a focu s for geotou rism (pforr et al., 2014). we argu e th at w h ile geotou rism p rovid es an alte rn ative base for th e econ om y an d , con com itan tly, can raise th e con sciou sn ess of th e p u blic to th e im p ortan ce of geology an d geoh eritage, th ere also are in trin sic valu es of geology th at are w orth y of geocon servation in th eir ow n righ t w ith ou t an y con n otation s for econ om ic ben efit (e.g., th e globally-sign ifican t arch aean zircon crystals of jack h ills, western au stralia, or th e globally-u n iqu e m u lti-d im en sion al geological featu res of sh ark bay, western au stra lia; cf. brocx an d sem en iu k, 2007 an d 2010). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 5 ind igenou s peoples have been in the coast al pilbara for tens of thou sand s of years, bu t their im pact has been m inim al (e.g., shell m id d ens attesting to life styles and d iet, and rock art celebrating their life w ith natu re). in contrast, the a rrival of eu rop ean colonists and ind u strialists m arked the beginning of m ajor environm ental im pacts. and ed u cation seem s to have m ad e little d ifference to app reciating geoheritage and biological aspects of the pilbara coast: m ost geologists, ecologists in ind u strial em ploym ent, and the range of engineers seem not to have been tou ched by the u niqu eness and beau ty of the pilbara coast and have becom e p arty to the ind u strial exploitation and d estru ction of this sp ecial coastal environm ent. in term s of archaeology, ports and ind u strial d evelop m ents in the coastal region also have m ajor im p acts on ind igenou s heritage. they particu larly have had im p act on internationally significant ancient rock engravings (petroglyp hs) and archaeological sites (habitation, artefacts, m id d ens) (h arrison, 2009; mcdonald and veth, 2009; mu lvaney, 2015). for instance, the petroglyp hs on bu rru p peninsu la (in the dam pier region; for location see fig. 2) are consid ered to be one of the m ost significant archaeological sites in the w orld (donald son, 2011; mu lvaney, 2011; black et al., 2017), how ever, the p roxim ity of coastal ind u stry either contribu ted to their d egrad ation (black et al., 2017) or to their d irect d estru ction (brocx, 2008). these m atters of the im p act of ports and ind u stry on coastal ind igenou s heritage, thou gh very im portan t, are ou tsid e the scope of this p aper. figure 2: m ain locations of iron ore mines (some locations represent amalgamations of a series of relatively closedspaced nearby sites) and the generalized pathways by which ore is delivered to the coast; also shown are locations of the ports and their export commodities. location of the main sites for mining of cu, m n, and cr, and the location of solar salt operations also are shown. the main ports are: onslow, cape preston, dampier, burrup peninsula, cape lambert, and port hedland. details of solar salt operations are presented in brocx and semeniuk (2015). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 6 given th e backgrou n d in form ation above, th is con tribu tion exam in es p orts alon g th e pilbara coast, sh ow in g th at th ey are p laced w ith little regard to su rrou n d in g n atu ral valu es, w ith con sequ en t im p acts on th is coast in term s of geoh eritage an d biological valu es in a lack of ap p lication of geoeth ics in best p ractices m a n agem en t. as pep p olon i an d di cap u a (2015a) p oin t ou t, geoscien tists an d geo-en gin eers sh ou ld carry social an d eth ical r esp on sibilities tow ard s society an d th e p lan et, p articu larly (in ou r view ) in th e con serv ation of geological h e ritage an d geod iversity th e case of th e in d u strialization of th e pilbara coast tran sgresses th is eth os. th is p ap er d escribes: th e m in eralrich pilbara h in terlan d an d n eed for p orts for exp ortin g ores/ m in erals; th e typ es of location s for p orts p aram eters for selection ; an d im p acts of p orts an d loss of geoh eritage valu es. 2. the min eral-rich pilbara hin terlan d an d the n eed for ports the pilbara hinterland is m ineral-rich. it consists of several geological provinces (h ickm an, 1983; geological su rvey of western au stralia, 1990): 1. arch aean pilbara craton (gran itoid s an d fold ed green ston es); 2. proterozoic sequ en ces of volcan ic rocks, iron ston es, ch erts, an d sh ale; an d 3. proterozoic sequ en ces of sh ale, d olom ites, an d ch erts. iron m in erals, as h em atite, goeth ite, an d m agn etite com p rise th e d om in an t ores in th e pilbara region occu rrin g as la yered iron ston es in th e arch aean an d prot erozoic sequ en ces, or a m esa -cap p in g tertiary goeth ite. they occu r at a range of localities in the region (fig. 2). in ad d ition, the pilbara hinterland has been m ined for ores of cop p er, chrom iu m , and m anganese and , locally, gold , lead , zinc, and silver (geological su rvey of western au stralia, 1990). 3. location s for port d evelopmen t, an d parameters for selection to exp ort ores from western au stralia, p orts h ave been establish ed an d selected on en g in eerin g an d econ om ic bases of coastal p roxim ity an d bath ym etry in sp ite of oth er in form ation available to gu id e m an agem en t an d w ise u se of coastal zon es. th e m ain p orts alon g th e pilbara coast are: on slow , cap e preston , dam p ier, bu rru p pen in su la, cap e lam bert, an d port h ed lan d . th e old est p ort is dam p ier exp ortin g fe ore sin ce 1963. in term s of size a n d age, port h ed lan d is th e largest an d th e secon d old est p ort in th e pilbara region , com m en cin g exp or tin g fe ores in 1966, origin ally from sh ay gap an d mou n t n ew m an ; tod ay it h an d les ores of fe, cr, mn , an d cu , an d solar salt. cap e la m bert com m en ced exp ortin g fe ore in 1972. th e variou s p orts servicin g th e oil/ gas in d u stry on th e bu rru p pen in su la w ere establish ed in th e 1980s. th e m ost recen t p ort con stru ction w as at cap e preston in 2010. wh ile port h ed lan d , dam p ier, an d cap e lam bert h ave been exp or tin g ores sin ce th e 1960s, m ore recen tly, fe ore h as been m in ed at mard ie an d roy h ill, an d oth er location s exten d ed from an d n ear old er m in e sites (e.g., maran d oo, brockm an , ch a n n ar) (fig. 2), an d n ew ly exp loited fe ores h ave been exp orted from r ecen tly d evelop ed facilities (e.g., cap e preston ) or from existin g e xp an d ed old er p orts. ports requ ire: 1. som e p roxim ity to a m in e site; 2. existin g sh eltered in let, or op p ort u n ity to bu ild load in g jetties; 3. n atu ral d eep en tran ce ch an n el (steep sh ores an d steep n ear -sh ore bath ym etry are id eal), or on e th at can be read ily d red ged ; preca m brian rock arch ip elagos are best; 4. stable terrain lan d w ard of th e jetty or p ort for in frastru ctu res/ facilities (regu larly flood ed d elta p lain s w ou ld n ot qu alify, m obile d u n es m igh t n ot qu alify, bu t precam brian rock rid ges are id eal). th u s, som e terrain s alon g th e coast h ave n ot been selected as p orts. id eal terrain for p ort con stru ction is precam brian rock (e.g., dam p ier, cap e lam bert, cap e preston ) or pleistocen e lim eston e (e.g., port h ed lan d ). on ce a site is selected for a p ort, th e coast an d im m ed iate h in terlan d m ay requ ire m od ification to m axim ize its u se for p ort facilities. with th ese m od ification s arrive th e p roblem s of alterin g ge om orp h ic an d geological featu res an d p ossible an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 7 d estru ction of featu res of geoh eritage sign ifican ce. wh ere a p ort h as been d evelop ed , th ere are tw o p attern s to its h istory. if it is state op erated (su ch as port h ed lan d ), th ere can be fu rth er p ort exp an sion as ad d ition al in lan d m in in g in terests m ay u tilize existin g p orts. th e state-op erated p orts th en exp an d to becom e m u lti-taskin g, exp ortin g w h atever com m od ity is brou gh t to th e coast. if th e p ort is vested in a m in in g com p an y, th ere is n o sh a rin g or leasin g of facilities an d each com p an y establish es its ow n p ort of d isp atch . 4. impacts of existin g ports an d loss of geoheritage values the im pacts of the existing p orts on terrain and the loss of geoheritage valu es are m anifold and range from the large scale (w here there is levelling of terrain at the coast, rem oval of gravel from coastal allu vial fans, and qu arrying of coastal d u ne sand ) to the sm all scale (w ith d estru ction of innu m erable u niqu e and special geological featu res, d iagenetic featu res, and arid -zone-specific m icro-geom orp hic featu res). the larger scale im p acts of the variou s ports and ind u stries on the geoheritage valu es of the pilbara coast are listed below and d escribed in m ore d etail by v & c sem eniu k research grou p (1996) and brocx and sem eniu k (2015), the latter d iscu ssing the geoethical im p lications of loss of geoheritage valu es associated w ith solar salt p rod u ction. in the context that the pilbara coast is globally u niqu e, loss of these sites/ featu res of geoheritage significance is of som e consequ ence (assessed u sing brocx and sem eniu k, 2007):  on slow : barrier d u n e m od ification ;  cap e preston : d estru ction of th e m ost sou th erly exp ression of arch ip elago/ ria sh ores an d a sp ecific ran ge of rock typ es at th e coast; th is w as an in tern ation ally sign ifican t site;  dam p ier an d bu rru p pen in su la: m ajor m od ification of arch ip elago/ ria sh ores an d islan d s; th is w as a in tern ation ally sign ifican t site an d is th e cen tral location of th ree su ch sh ores alon g th e coast; figure 3: a nnotated aerial photograph showing range of industrial impacts in the port hedland area, a limestone barrier coastal setting. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 8  cap e lam bert: m ajor m od ification of arch ip elago/ ria sh ores an d d u n e coast; th is w as an in tern ation ally sign ifican t site an d th e n orth ern location of on ly th ree su ch sh ores alon g th e pilbara coast;  port h ed lan d : m ajor m od ification of (pleistocen e) lim eston e barrier rid ges an d loss of geom orp h ology of tid al-creek-d issected in ter-rid ge m u d -filled sw ales, an d tid al flats (fig. 3); to create th e p ort, large m u d -filled sw ales w ere excavated or in -filled to form fou n d ation s for in frastru ctu res, an d th e in let ch an n el w as d red ged for sh ip access; at sm aller scales, m u ch qu atern ary h istory an d stratigrap h y w as lost; th is w as an in tern ation ally sign ifican t site an d a m ain location of lim eston e barriers alon g th e coast. more d etails on th e im p act of in d u stry sp ecifically on th e m an grove com p on en t of th e coast are p resen ted in sem en iu k an d cressw ell (2018). wh ile th ere are a ran ge of exp ort com m od ities an d in d u strial activities associated w ith p orts, an exam p le of th e im p acts of a p ort on th e coast is p rovid ed by th e activities an d in frastru ctu res for fe ore p rocessin g, refin in g, an d exp ort (v & c sem en iu k research grou p , 1996) bu t th e sam e p rin cip les can be d escribed for solar salt p rod u ction an d oth er exp ort com m od ities. in th e pilbara region , fe ore is m in ed in in lan d qu arries, cru sh ed , an d tran sp orted to th e coast by rail for fu rth er p rocessin g (secon d ary an d tertiary cru sh in g) an d sh ip p in g; m in es an d p orts of d isp atch are sh ow n in fig u re 2. iron ore in d u stries in coastal areas h ave a lim ited ran ge of in frastru ctu res an d activities associa ted w ith th em an d th ey are relatively con tain ed system s; from m in e to p ort th ey are:  rail lin es rail yard s, car d u m p ers to receive ore from rail sy stem s;  cru sh ers;  con veyors;  p elletizin g p lan ts an d sin ter p lan ts;  rescreen in g p lan ts;  fu el an d oil storage tan ks, p ow er gen eration station s;  veh icles, veh icle storage sh ed s, veh icle an d m ach in ery w orksh op s, equ ip m en t stor age sh ed s;  w ater storage tan ks;  p ip e n etw orks for w ater an d fu el;  road s;  ad m in istration facilities, am en ities bu ild in gs, accom m od ation areas;  ore stock p iles;  lagoon s/ p on d s (for coolin g w a ter, slu g catch ers, etc.);  jetties. th ese in frastru ctu res h ave im p acted on th e coast, th ou gh n ot all are d evelop ed at th e on e coastal site (v & c sem en iu k research grou p , 1996). the fe ore ind u stries established on the pilbara coast have requ ired extensive m od ification of coastal land , w ith the typ e and d egree of m od ification d ep end ent on the setting. all su ch coastal sites have involved rail and road constru ction, coast alteration and d red ging for p orts, jetties and harbors, and d red ge spoil d isposal. facilities sited on bed rock archip elago/ ria coasts, and areas of ru gged terrain involved terrain m od ification and land fill to level (flatten) the sites for stockp iles and infrastru ctu re. those facilities on lim estone barriers requ ired constru ction of cau sew ays for rail an d road s (crossing high tid al flats), and tid al land m od ification (land fill throu gh d red ge sp oil d isposal) to create sites for infrastru ctu res. barrier island s are not large enou gh to su p p ort infrastru ctu res and so ad joining tid al land s w ere reclaim ed . 5. d iscussion an d con clusion s th e pilbara coast is sp ecial, in com p arison to oth er coasts in au stralia an d globally (sem en iu k, 1996), w ith d istin ctive coastal form s, geoch em ical p rod u cts an d stratigrap h y, all of w h ich reflect a qu atern ary h istory of com p lex sed im en tation , arid ity, an d cyclon ic storm s. as a resu lt, th e coast is d istin gu ish ed by a ran ge of featu res: con stru ction of arid -zon e d eltas, d elta d estru ction an d sed im en t r ed istribu tion , cyclon e-in d u ced erosion an d sed im en tation , m an grove ecology an d associated d ep osits, evolu tion of coastal grou n d w ater h yp ersalin ity, an d form ation of beach rock, h igh -tid al cru sts an d gyp su m p recip itates, am on gst oth ers. th e pilbara coast th u s p r ovid es a globally im p ortan t m od el, unparalleled elsewhere in the an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 9 world, for th e d evelop m en t of a ran ge of m egascale lan d form s th rou gh to sm aller -scale geom orp h ic, sed im en tary, strat igrap h ic, an d d iagen etic p rod u cts d evelop ed w ith in an arid clim ate, p rovid in g for th e earth scien ces an im p ortan t an d u n iqu e m od el of a rid zon e coastal sed im en tation , d iagen esis, an d stratigrap h ic evolu tion (sem en iu k, 1996). loss of geoheritage valu es and geod iversity along the pilbara coast d enies hu m anity the op p ortu n ity to learn of the globally im portant great stories of coastal history and geological prod u cts that this region has to offer or (w here alread y d estroyed ) had to offer. this is a factor that bohle (2015) highlights by em phasizing the im p ortance of “selling” earth sciences throu gh storylines to raise the consciou sness of the pu blic in regard s to the im p ortance of the geosciences in d aily life, bu t w e ad d the im portance of the geosciences to u nd erstand ing earth history, earth d iversity, and u ltim ately biod iversity. som e of th e in d u strial im p acts alon g th e pilb ara coast took p lace d u rin g th e 1960s to 1980s in ign oran ce of th e sign ifican ce of th is coast bu t recen t p orts at on slow an d cap e preston w ere con stru cted after th e p rin cip les an d con cep ts of geoh eritage an d biological con serv ation w ere establish ed . moreover, th ere are in d u strial a ctivities p resen tly bein g p lan n ed or exp an d ed w ith cu rren t kn ow led ge th at th e pilbara coast is a globally u n iqu e. for exam p le, p lan n in g for coastal in d u stries in th e pilb ara region is based on a rep ort by dover con su ltan ts (1995) w h ich ad vocates a sp ecial em p h asis on “m axim izin g th e n atu ral ad van tages of western au stralia ” by p rom otin g n ew coastal h eavy in d u stry ass ociated w ith d eep w ater p orts in clu d in g fu rth er d evelop m en t at dam p ier, karrath a an d port h ed lan d . in ad d ition , th e dep artm en t of resou rces develop m en t an d th e dep artm en t of min erals an d en ergy, actively p rom ote d eve lop m en t of th e pilbara th rou gh its pilbara d evelop m en t com m ission , based on th e pilbara 21 rep ort (pilbara 21 stu d y grou p , 1992), as w ell as on stu d ies su ch as th e basic raw mat erials su rvey karrath a (martin ick, 1997), an d th e activities of geological su rvey of western au stralia (ru d d u ck, 1999). th ese govern m en t bo d ies h ave d evelop m en t an d econ om ic grow th as an over-rid in g p arad igm , leavin g en viron m e n tal m atters to th e en viron m en tal protection a u th ority (epa). h ow ever, if th e epa reco m m en d s again st a given d ev elop m en t on en v iron m en tal grou n d s, th e h istorical tren d h as been to d rive d evelop m en t by min isterial d ecision s an d u se of th e state agreem en ts act. each state agreem en t is n egotiated ad h oc to be th en ratified as an act of parliam en t to en able th e p roject to p roceed ou tsid e m ost state law s, afford in g p rivileges an d en viron m en tal con cession s to facilitate d eve lop m en t (h illm an , 2006). develop m en t h as ch an ged irreversibly th e ch aracter an d p oten tial of th e pilbara (kerr, 1979) w ith loss of m an y geoh eritage sites an d a cu ltu ral d islocation as a resu lt of u n p lan n ed exp loitation of m in eral resou rces in an en v iron m en tally u n iqu e region . th is ou tcom e is th e p rod u ct of over forty years of ad h oc p lan n in g d ecision s an d resou rce exp loitation for sh ortterm econ om ic ben efit of th e d evelop m en t p artn ers, i.e., th e state govern m en t an d th e d evelop ers, con trad ictin g kerr's op tim istic p red iction of th e social an d econ om ic ben efits th at d evelop m en t w ou ld brin g. clearly, w ith th e sp ecial n atu re of th e pilbara coast in term s of its rich geoh eritage an d th e typ e of econ om ically-d riven d evelop m en ts alread y u n d ertaken , th ere is a n eed for strategic p lan n in g an d p olicy d evelop m en t th at can balan ce th e con flictin g asp ects of geoh eritage, in d igen ou s cu ltu re, an d th e p u rsu it of w ealth by exp loitin g raw m ater ials. to d ate, little h as been carried ou t in th is regard in term s of p olicy. th is case stu d y of p ort d evelop m en t alon g th e pilbara coast h igh ligh ts th e con flictin g an d in variably com p etin g valu es p resen t in th e u se of n atu ral coastal geoh eritage. su bstan tial tracts of th e coast of th is an cien t lan d scap e h ave been an d con tin u e to be overp rin ted by resou rce d evelop m en t based on govern m en t p olicies th at h ave n ot been p u blicly d ebated . decision s h ave been an d con tin u e to be m ad e ad h oc, an d are fou n d ed on cap italist valu es. du e to in d iffe ren ce an d ign oran ce, or both , of th e n atu ral v a lu es of th e pilbara coast, th ese d ecision s h ave resu lted in a loss or m od ification of tribal lan d s, loss of an cien t rock art sites, an d loss of areas rich in geoh eritage valu es. th e pilbara is m in eral rich an d as it is fu rth er exp loited for m in erals, it m ay be exp ected to see m ore p orts. a fu n d am en tal p h ilosop h y th at an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7495 10 w ou ld be en viron m en t-frien d ly w ou ld be state-w id e p lan n in g th at lim its th e n u m ber of p orts in th e region , i.e., rath er th an h avin g a m u ltip licity of p orts, com m en su rate w ith every m in in g com p an y th at is floated , th ere n eed s to be p lan n in g su ch th at th ere is a cen tralized a p p roach . an oth er im p ortan t asp ect to d iscu ss is th at western man , in western au stralia, sees th e n atu ral en viron m en t as a resou rce to exp loit or u tilise. rath er th an th e pilbara coast bein g seen as a u n iqu e coastal “w ild ern ess”, it is view ed as “a place for man an d n atu re”, w ith clearin g of vegetation , levellin g of rocky terrain for bu ild in g sites, in stallin g tran sp ortable bu ild in g, d iggin g tren ch es for p ip es, brin gin g in p lastic ch airs, etc., w h ich em p h asizes th e id ea th at “man is p art of n atu re” regard less th at man brin gs in u n n atu ral an d artificial stru ctu res an d com p on en ts. th is w e reject, an d view th at th e pilbara coast is a globally d istin ct en viron m en t m u ch of w h ich is w ild an d n atu ral an d sh ou ld be left as a w ild ern ess. th e p roblem s h igh ligh ted for th e pilba ra coast are sim ilar to th e p roblem s an d geoeth ics in volved th at are cu rren tly bein g faced in eastern au stralia, e.g., d evelop in g p orts for econ om ic/ em p loym en t op p ortu n ities in th e r egion of th e great barrier reef world h eritage area (un esco, 2014). bu t su ch p roblem s exten d beyon d au stralia th ey are p revalen t th rou g h ou t th e world an d n eed ad d ressin g in p rin cip le to fin d “a w ay-ou t-of-th e-con u n d ru m ”. su ggestion s to h elp resolve su ch p roblem s in clu d e: brin gin g geoh eritage in to th e co n sciou sn ess of con serv ation m an agers so th at it is ad d ressed in d evelop m en t p rop osals an d p lan s; creatin g an in ven tory of coastal featu res an d id en tifyin g th eir sign ifican ce (u sin g th e geoh eritage toolkit of brocx an d sem en iu k, 2009); an d coastal zon es id en tified as of h igh sign ifican ce sh ou ld be treated as h igh geocon servation zon es (th e exten t th at man in tru d es, or m an ages, or h as m u ltip le u se of th e oth er coastal zon es of lesser sign ifican ce sh ou ld be treated in a grad ed r esp on se). geoeth ics, w ith an objective to p rotect th e earth an d featu res of geology, as a n ew em ergin g en d eavor (as d iscu ssed by pep p olon i an d di cap u a, 2015b), n eed s to be ad d ressed by regu latory an d 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(2000) the authors present urban material characterization, while in xiao et al. (1999) vegetation canopies and tree analysis in a so called «urban forest» is addressed. land cover classification and sealed parts detection in urban areas are considered in segl et al. (2000), heiden et al. (2001), roessner et al. (2001a). in all of these papers the need for hyperspectral data for corhysens data exploitation for urban land cover analysis fabio dell’acqua (1), paolo gamba (1), vittorio casella (2), francesco zucca (3), jon atli benediktsson (4), graeme wilkinson (5), andre galli (6), eva savina malinverni (7), graeme jones (8), darrel greenhill (8) and lennart ripke (8) (1) dipartimento di elettronica, facoltà di ingegneria, università degli studi di pavia, italy (2) dipartimento di ingegneria edile e del territorio (diet), facoltà di ingegneria, università degli studi di pavia, italy (3) dipartimento di scienze della terra, facoltà di scienze matematiche fisiche e naturali, università degli studi di pavia, italy (4) department of electrical and computer engineering, university of iceland, reyjkavik, iceland (5) general faculty of technology, lincoln university, lincoln, u.k. (6) facoltà di agraria, università politecnica delle marche, ancona, italy (7) facoltà di ingegneria, università politecnica delle marche, ancona, italy (8) digital imaging research centre, school of computing and information systems, kingston university, surrey, u.k. abstract this paper addresses the use of hysens airborne hyperspectral data for environmental urban monitoring. it is known that hyperspectral data can help to characterize some of the relations between soil composition, vegetation characteristics, and natural/artificial materials in urbanized areas. during the project we collected dais and rosis data over the urban test area of pavia, northern italy, though due to a late delivery of rosis data only dais data was used in this work. here we show results referring to an accurate characterization and classification of land cover/use, using different supervised approaches, exploiting spectral as well as spatial information. we demonstrate the possibility to extract from the hyperspectral data information which is very useful for environmental characterization of urban areas. mailing address: dr. fabio dell’acqua, dipartimento di elettronica, facoltà di ingegneria, università degli studi di pavia, via ferrata 1, 27100 pavia, italy; e-mail: fabio.dellacqua@unipv.it 312 fabio dell’acqua et al. rectly accomplishing these tasks is clear, and the reasons may be summarized as follows: – imaging spectrometry helps to characterize urban sealing and soil composition, providing in a unique way information useful to define the risks of urban runoff and flooding. – hyperspectral imaging may allow identifying and cataloguing potentially dangerous construction materials that have been used for coverings in the past (e.g., asbestos concrete). – hyperspectral data acquired in the thermal infrared are useful in studies of urban heat effects, to establish the correlation between building material composition, heat flux and ground temperature. our project aims at producing detailed land cover maps exploiting the potentials of hyperspectral mapping, since it has been demonstrated (roessner et al., 2001b) that significant improvements in classification of urban areas could be achieved using the spectral surface characteristics in the visible and near-infrared range, with a suitable spectral resolution. more in detail, we want to propose precise and useful algorithms for: – characterization and classification of land cover in urban areas using supervised approaches. – correlation of the spectral characteristics of urban cover materials with their geometry and illumination conditions. – definition of the vegetation status and corresponding risk maps for air pollution. – in summary, medium and high-resolution characterization of urban environment that includes built structures, road networks, etc. finally, it is worth noting that recent technical studies show that more refined results can be obtained by combining hyperspectral measurements with those from different, complementary sensors or data sets (madhok and landgrebe, 1999) or by combining hyperspectral and radar measurements (gamba and houshmand, 2001; see also nevatia et al., 1999). so, to make the study more effective, hyperspectral data were collected over an urban area where a multi-sensor data set is already available (ers, sir-c/x-sar, landsat, ikonos, erosa1, as well as multitemporal aerial data). an integrated system has been designed, and is currently under extensive testing, to manage this rather large data set (costamagna et al., 2001). data used in this work come from the eu funded hysens project, proposed by the dlr (german aerospace agency) to make the access to research infrastructures easier. in this frame, free access to the data from dlr’s airborne imaging spectrometers dais 7915 (digital airborne imaging spectrometer) and rosis (reflective optics system imaging spectrometer) were offered to the european remote sensing community. imaging spectrometer data sets acquired over test areas proposed by the individual user groups were system corrected and calibrated to at-sensor radiance. dais and rosis have a spectral resolution capable of delineating the highly complex land uses, covers and patterns of urban environments, uncovering environmental degradation, such as vegetation stress and soil/artificial materials decomposition. however, environmental urban analysis is a new development in remote sensing (ben-dor et al., 1998), for which suitable analysis tools are required. in the following we will show a more detailed description of the collected data set, the methodology implemented in the project, and some preliminary results obtained with dais data, referring to supervised classification approaches, exploiting spectral as well as spatial information. 2. data set the data used in this work correspond to the records of four flight lines over the urban area of pavia, northern italy, acquired in the summer of 2002. the area was imaged by means of both dais and rosis, with different spatial and spectral resolution. the flight altitude was chosen as the lowest available for the airplane (about 1500 m), which resulted in the finest spatial resolution, i.e. 2.6 and 1.2 m for dais and rosis, respectively. the lines were chosen so that the higher resolution sensor (rosis) covered the whole urban area. therefore, the images from the dais sensor are partially overlapping, which will allow studying the effects of the directional reflectance of urban materials on mapping accuracy. unfortunately, due to unprecedented problems with the recorder of navigation parame313 hysens data exploitation for urban land cover analysis ters, no rosis data was provided so far to our team and thus only dais data were analysed. during the flight campaign, many ancillary data were collected, and in particular a detailed inspection of the materials in portions of the urban area was performed. ground spectrometer and sun spectrometer data were collected as a support for the atmospheric correction procedure (relying on modtran), which requires the availability of both ground and at-sensor spectra for a number of land cover cases. only atmospherically corrected data were then used in this work. 3. methodology accurate georeferencing is required to make maps obtained from high spatial resolution data useful for comparison with other data sources and for integration into a gis. for this reason one of the efforts of this work has been the choice of an algorithm to obtain such a result. the work was then devoted to information extraction from the hysens imagery using different approaches for urban and non-urban areas, whose results were then combined. the main reason for processing the data in two parts is that the characteristics of scenes are very different. for the urban part, spatial structure plays a very important role in discrimination. for classification of the non-urban area, the spectral information is of prime importance. the main features were extracted from both data sets using two well established extraction methods, namely discriminant analysis feature extraction (dafe) and decision boundary feature extraction (dbfe) (landgrebe, 2003). classification algorithms were then applied to both the original data and the extracted features. for urban areas, the neuro-fuzzy spectral + +spatial approach outlined in gamba and dell’acqua (2003) was followed, and results compared with that of classical classification algorithms. for non-urban areas, instead, the data were assumed to be gaussian distributed. a very limited number of labeled training samples is available, as the scene is mainly urban. therefore, methods based on the use of enhanced statistics (landgrebe, 2003) were applied. the enhanced statistics methods use some unlabeled samples to improve the estimates of the parameters used in the classification. the maximum likelihood classifier was used for classification. the methodology applied may be summarized in fig. 1, and corresponds to the structure of this paper, where the next section will be devoted to geocoding issues, while section 5 will provide some examples of classification results exploiting spatial as well as spectral characteristics of the urban environment. for the vegetation, ndvi was calculated over the entire image after averaging red and ir bands; the ndvi image was then processed to extract the weighted mean patch size (wmps) index and the lacunarity index which were then used to characterize the distribution of vegetation inside the considered urban area. 4. geocoding to geocode the dais images, two nonparametric methods were experimented. a polynomial geometric correction was carried out using ground control points (gcps) selected manually from 1:2000 vector topographic maps and a polynomial cubic function. for the fig. 1. the work flow of the research presented in this paper. 314 fabio dell’acqua et al. flight line 2 we used 28 gcps, distributed over the whole area obtaining the average rms error of 1 pixel. the same procedure was followed for the flight line 3 with 36 gcps resulting in an average rms error of 0.91 pixel. to improve the accuracy another method was tested. the software pci orthoengine 8.2 satellite project was used to carry out geometric corrections based on rational functions (see dowman and dolloff, 2000), where the relation between ground points and corresponding image points is defined as a ratio of polynomials. no system data is needed, but a dem is necessary, as well as a minimum number of gcps, related to the degree of the polynomial employed. we performed the geocoding of the image line 3 using 33 gcps and a polynomial of the 2nd order, resulting in an average rms error in x pixel coordinate of about 0.26 and y pixel coordinate of about 0.27. the rectified images (line 2 and line 3) were resampled using nearest neighbour so as to retain the spectral scale values for subsequent multispectral classification. 5. urban mapping after geocoding, urban land cover mapping and classification was considered. as a preliminary step, an analysis of the spectra of materials commonly found in the urban area was performed, using cnr spectral library. as a consequence, after proper radiometric and atmospheric calibration (based on modtran, berk et al., 1989), and taking into account the spatial ground dimension of the dais pixel, nine cover classes were considered, namely water, tree, asphalt, parking (asphalt with some concrete and soil), bitumen, bricks, meadow, bare soil, shadow. more refined classes are expected to be found after proper analysis of the results of this preliminary processing step of the data set. 5.1. spectral and spatial land cover mapping to provide a quantitative evaluation of the classification results, a sample of the data was considered (fig. 2), and ground truth was provided by available high resolution aerial and satellite images and with a ground survey. as already mentioned in the previous section, a complete analysis of the urban land cover cannot be obtained without considering both a spectral and a spatial characterization. therefore, we applied either on the original data set or on the one reduced by means of the dafe or bdfe approaches, the neurofuzzy classification chain introduced in gamba and dell’acqua (2003). this is a two step approach, considering first a spectral analysis of the image with a neural approach (spectral artmap), and followed by a re-processing step taking into account the spatial patterns of the first classification map (spatial artmap). the spectral analysis using dbfe processed data provided an overall accuracy of 94.3%, while the data coming from dafe feature extraction process provided a value of 96.7%. after the second step, these values were enhanced to 95.3% and 97%, respectively. see a visual comparison of spectral and spatial classification results in fig. 3a,b and table i. other supervised classification methods were tested on the same area. results, in terms fig. 2. a sample image of the test area used for classification, near the town center (north is to the bottom); this image was obtained by averaging several channels in the visible range of wavelengths. 315 hysens data exploitation for urban land cover analysis fig. 3a-d. classification maps for a) the spectral artmap and b) spatial artmap classifier applied to the data after a dafe analysis; classification maps for c) the echo and d) maximum likelihood classifier applied to the data after a dafe analysis. a b c d table i. accuracy values for artmap applied to the test area in fig. 2. spectral artmap spatial artmap producer’s accuracy user’s accuracy producer’s accuracy user’s accuracy water 100% 100% 100% 100% trees 93.6% 98.8% 94.2% 98.5% asphalt 98.6% 96.9% 98.8% 96.6% parking 96.9% 99.9% 90.9% 92.6% bitumen 99.5% 99.9% 97.7% 99.7% bricks 92% 87.7% 99.4% 99.9% meadow 99.4% 82.7% 99.5% 82.6% bare soil 86.9% 98.3% 87.6% 98.1% shadow 90.9% 81.1% 87.3% 88.5% 316 fabio dell’acqua et al. of classification accuracy, are shown in table ii. the spectral angle mapper (sam) obtained poor results; this is probably due to the choice of the training set, optimized for statistical methods but unsuitable for sam which works best on pure spectra instead. figure 3c,d and table iii present the results for the maximum likelihood and echo classifiers applied after the dafe feature extraction procedure. table iii reports the best of the two results, i.e. the one given by the maximum likelihood classifier. 5.2. urban spatial structure investigation together with accurate urban land cover maps, among the possible spatial characterizations of an urban environment there is a growing need for quality of life indicators. in particular, the availability and distribution of green areas is a good indicator of both the air quality (which heavily affects the quality of life, especially in urban centers) and the possibility to have openair entertainment areas. the approach taken in this work is to characterize the distribution of patches of vegetation in urban areas. as a first processing step, the pixels in our dais data set were classified into vegetative or non-vegetative pixels using the normalized difference vegetation index (ndvi, tucker and sellers, 1986). the red bands 7 to 12 (0.63 to 0.70 mm) were averaged to make a red image, and the near infrared bands 16 to 21 (0.76 to 0.85 mm) were averaged to make a near infrared image. the ndvi image was then thresholded to create an image that contains patches of vegetative pixels. these patches were then analyzed using two metrics: a weighted mean patch size metric and a lacunarity metric. the weighted mean patch size (wmps) index (li and archer, 1997) calculates the patch area weighted by the number of patches in the scene possessing that area. an image was derived which shows the wmps calculated over a local window within the image. a window is moved across the image in a raster scan fashion, and the wmps is assigned to the central pixel in each window. the wmps image thereby shows the spatial distribution of patches within the image. the lacunarity index (plotnick et al., 1993) shows the variability of size of vegetated patches, and is an indicator of «clumpiness». in a way similar to the wmps, it has been applied to calculate a single value for each point in the scene. the lacunarity index has an additional parameter known as the box size, which results in a lacunarity image for each box size; results are shown in fig. 4a-d. these experiments should help provide new insights into landscape structure, which can be exploited in land use planning and in the creation of heuristics for planning sustainable urban development. thresholding the normalized difference vegetation index (ndvi) pixels are classified into vegetative or non-vegetative pixels. the quality of the lacunarity and weighted mean patch size (wmps) metrics is determined by the robustness of this classification. the upper table iii. accuracy values for maximum likelihood classifier applied to the test area in fig. 2. producer accuracy user accuracy water 100% 100% trees 95.2% 98.3% asphalt 96.2% 98.9% parking 91.6% 82.2% bitumen 98.3% 99.4% bricks 99.8% 99.2% meadow 98.4% 90.1% bare soil 94.3% 97.7% shadow 89.2% 82.7% table ii. overall accuracy values for the test area in fig. 2. original data dbfe dafe maximum likelihood 95.6% 96.3% 97.6% fisher 97% 96.7% 97% minimum distances 73.6% 87% 96.6% echo 95.6% 96.3% 97.6% sam with 0.05 rad 87.4% = = 317 hysens data exploitation for urban land cover analysis and lower areas of the image (fig. 4a-d) contain the most vegetative pixels (displayed in white in fig. 4b). these regions also have the highest lacunarity (fig. 4c) and wmps (fig. 4d). the centre of the region is mostly urban, and has low values for both metrics. a large number of ecological metrics are available and it is important to establish which are the most useful. early results from this work appear to suggest that for environmentally sustainable land use in sub-urban areas local wmps should be maintained at values equivalent to 0.16 km2 and lacunarity at values exceeding 2.0 at box size 28 ×28 m and exceeding 1.4 at box size 204×204 m. these values are intended to ensure that a relatively low density of housing is maintained with a good clustering of local green areas, which would be supportive of recreation and diversification of flora and fauna. 6. conclusions and future work this work shows that it is possible to extract from hyperspectral data very important inforfig. 4a-d. a) the area of interest; b) ndvi image of the same area, thresholded at 0.5; c) lacunarity image (window size 251, box size 9); d) wmps image (window size 251). c d ba 318 fabio dell’acqua et al. mation, useful for the environmental characterization of urban areas. even if in its preliminary stages, this research has shown many potentialities for urban remote sensing. the co-registration of overlapping areas acquired by the dais in parallel flight lines having opposite directions opens the possibility to characterize urban materials considering also directionality effects. this study will be helped by a number of precise ground reflectance measurements, made available thanks to cnr efforts. acknowledgements the authors wish to thank the dlr for performing the campaign and providing the data within the framework of the hysens project and the cnr for collecting the ground reflectance data and the atmospheric calibration data and for their support. references ben-dor, e., n. levin and h. saaroni (1998): utilization of imaging spectrometry in urban areas: a case study over tel-aviv, israel using the casi sensor, in proceedings of the 1st earsel workshop on imaging spectrometry, zurich, switzerland, 473-479. berk, a., l.s. bernstein and d.c. robertson (1989): modtran: a moderate resolution model for lowtran 7 (phillips lab., hanscom afb, ma), tech. rep. gl-tr-89-0122. 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remote sensing of primary production, int. j. remote sensing, 7, 1395-1416. xiao, q., s.l. austin, e.g. mcpherson and p.j. peper (1999): characterization of the structure and species composition of urban trees using high resolution aviris data, in proceedings of the aviris workshop, pasadena, ca. muography of the puy de dôme annals of geophysics, 60, 1, 2017, s0105; doi:10.4401/ag-7380 s0105 muography of the puy de dôme pasquale noli1,*, fabio ambrosino1,2, lorenzo bonechi3, alan bross4, luigi cimmino1, raffaello d’alessandro3,5, vincenzo masone1, nicola mori3,5, giuseppe passeggio1, anna pla-dalmau4, giulio saracino1,2, enrico scarlini3,4, paolo strolin1,2 1 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 2 università di napoli federico ii, dipartimento di fisica, naples, italy 3 istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy 4 fermilab, batavia, illinois, usa 5 università di firenze, dipartimento di fisica e astronomia, sesto fiorentino, florence, italy abstract muon radiography is an imaging technique that relies on the transmission of cosmic muons through matter. it allows the measurement of density maps of large structures such as volcanoes. during the second half of 2013 the muray detector prototype carried out a data taking at the puy de dôme in the framework of the scientific collaboration with the experiment tomuvol in order to compare the results and performance of the two different detectors. both experimental apparatuses measure a muon transmission of some orders of magnitude higher than that expected highlighting a background that perturbs these measures. 1. introduction the puy de dôme is a large lava dome, originating from one of the youngest volcanoes in the region of massif central in south-central france. a campaign of muographic measurements was performed jointly by the muray [ambrosino et al. 2014] and tomuvol [cârloganu et al. 2013] collaborations, using different detector techniques. tomuvol (tomographie muonique des volcans) has been using resistive plate chambers (rpcs). muray (muon radiography) has been using an early prototype of a muon detector of a new generation, based on the use of plastic scintillators. the prototype was still incomplete for optimal background rejection. the purpose of this approach was a better understanding of systematic effects and of backgrounds, by a combined data analysis. the muray detector was installed at the col de ceissat, in a location at 1078 m a.s.l. altitude. the data taking started at the end of june 2013 for about 92 days. we report the detector performances during the data taking and preliminary results, in particular concerning the background. 2. imaging with muons muography is an imaging technique that enables to study the interior of large structures such as volcanoes, mountains or mines through the study of absorption of high energy muons created by the interactions of primary cosmic rays in the upper earth’s atmosphere. the absorption depends on the thickness and the density of the traversed rock. its measurement provides data to construct a map of the average density of the crossed rock. muography can directly image the inner structure of volcanoes in order to show the possible presence of lava conduits, dikes or zones with different densities. conventional measurement methods (gravimetric, seismological or electromagnetic) achieve resolutions of the order of several 100 m. in optimal acquisition conditions, muography can improve the resolution by one order of magnitude, though limited to the volcano edifice. moreover, combined with other techniques it enriches the information with a sheer density measurement that can help in solving ambiguities and disentangling the structural information. the first application of the cosmic-ray muon flux to detect density variations in a large structure dates back almost 40 years and it is due to luis alvarez [alvarez et al. 1970]. he investigated the internal structure of the pyramid of the chefren pharaon, looking for an unknown burial chamber. indeed, unlike in the pyramids article history received july 5, 2016; accepted october 5, 2016. subject classification: detector, muon radiography, volcano, cosmic rays, muon. of his father cheope and of his grandfather sneferu, in the chefren pyramid only one burial chamber has been found suggesting the existence of a hidden chamber. using a muon detector based on the spark chamber technique, alvarez and collaborators measured the directional cosmic ray flux penetrating through the pyramid. the technique was proven to be successful, although no evidence of a hidden chamber was found. new technologies were needed for the development of detectors suitable for more extensive applications of muography. since 2003, muography is being applied to volcanoes in japan, france and italy. the breakthrough occurred in 2007, almost forty years after alvarez, providing an image of internal structure of the summit of mt. asama in japan [tanaka et al. 2007]. so far, muon radiographies have been performed in japan over rock thicknesses of less than 1 km. for a substantial progress in the practical application of muography, the sensitivity must be improved one or two orders of magnitude. to that purpose, the most critical item is background rejection. muography implies a multi-disciplinary effort, as the basic detection techniques come from particle physics. 3. the muray detector the muray muon telescope is based on the use of plastic scintillators bars. volcanic areas are usually characterized by a hard environment and by difficulties of access, requiring a modular structure to ease transportation and installation. the basic “module” consists of a planar array of 32 plastic scintillator bars (1 m long) (figure 1) with triangular cross-section and a central hole to allocate a wave-length shifting (wls) fibre (figure 2). the triangular cross-section allows the construction of very compact, crack-free planar arrays. moreover, the weighted average of the light output produced by two adjacent bars enables improving the spatial resolution. the scintillator bars similar to those used in the d0 [baringer et al. 2001] and minerva (http://minerva. fnal.gov) experiments have been produced by extrusion at the fermilab nicadd facility. the scintillator is doped with blue-emitting fluorescent compounds (ppo 1% and popop 0.03%). the central hole is co-extruded, together with a 0.25 mm thick titanium oxide coating that increases the internal reflectivity and shields from environmental light. the wls shifting fibres carry the light produced by a particle crossing the scintillators to silicon photomultipliers (sipm), a recent solid-state technology that in a number of applications is replacing the photo-multiplier tubes. their low power consumption (less than 1 mw per channel) is an important feature for applications on sites where an electricity supply is not available. on the other hand, the temperature dependence of their gain requires special attention to monitor and control the sipms temperature by a cooling system. at the puy de dôme, the front-end electronics of each module was based on the asic chip spiroc, especially developed for sipms by the omega group at lal, cnrs-in2p3 (http://omega.in2p3.fr). in later developments, the spiroc chip has been upgraded to the easiroc chip. the chip has 32 channels, the correct number to equip one module. it is hosted on a hybrid printed circuit board, where the electro-optical connector and the data acquisition electronics of the module are also located. the front-end board (“slave” boards) of each module generates an or signal from its 32 bars (or32), which triggers local data acquisition and is sent to a “master” board. on the basis of the or32 from all modules, the master board generates a trigger that is sent to the slaves for data transmission to the master itself. a detector “plane” consists of two adjacent modules, with 64 scintillator bars in total that are read out by two independent front-end boards. it has an area of about 1 × 1 m2 area and gives one of the two coordinates (x or y) of the impact point of the muon. two planes with orthogonal bars form an x-y station. the muon telescope at the puy de dôme consisted of three x-y stations, assembled on an aluminium alloy support frame (figure 3). a 3 cm thick iron plate, was inserted near the central station, as a pre-shower to identify and reject electrons. the support frame can be rotated in the horizontal plane, in particular to acquire data with the muon telescope pointing to the free sky for calibration purposes. the muon trajectory is reconstructed by a linear fit of the x-y-z positions of the hits in the three stations, where z is the longitudinal coordinate. the tracking redundancy provided by three stations rejects noli et al. 2 figure 2. the triangular profile of the scintillators. figure 1. a module with 32 scintillator bars. the wavelength shifting fibers and their connector are visible. 3 the background induced by cosmic-ray showers simultaneously hitting the three stations. the three stations also reduce the probability of accidental triggers produced by thermal noise in the sipms. in the future, a further background rejection will come from a timeof-flight measurement to discriminate muons coming from the opposite direction. the background from stray particles of low energy coming from the correct direction is less straightforward to handle. its estimate is among the aims of the measurement campaign at the puy de dôme. 4. muon track reconstruction the first step in muon track reconstruction is performed independently for each detector module and consists in clustering hits above threshold. the aim is the reconstruction of the coordinates corresponding to muon trajectories, as input to the following step that consists in the reconstruction of the muon tracks. among advantages, the use of a solid-state technology as the sipms implies handling the dark rate from thermal noise. for this purpose, a calibration of each channel is periodically performed. all channels are acquired randomly, giving the distribution of their electronic pedestal and the dark noise. the distribution of the pedestals of each channel has a peak with specific width and amplitude, which depend on the entire electronic chain. a signal from a sipm is selected for further analysis if it is at least 5 rms above the pedestal amplitude. adjacent bars produce a cluster with size equal to their number. each cluster is characterized by its position (average of the positions of the corresponding bars), by the number of its bars and by the cluster size, i.e. by the total number of rms from the pedestal amplitude of its strips. the latter is related to the light release in the bars. the clusters are sorted in descending order according to their size. in the following step, the x-z and y-z track projections are reconstructed independently. all possible combinations of clusters in the external planes are considered, starting from the clusters with the largest size. if a cluster in the inner plane is found in the neighbourhood of the predicted track position, a linear fit of three clusters is performed and a track projection candidate is obtained. in case of more than one track, the best track is selected as that with the highest size of the smallest cluster (in rms units). the pair of the best tracks in the x-z and y-z projections forms a three-dimensional track. the efficiency of the reconstruction and tracking algorithm is 92%. further cuts are applied to improve the quality of the selected sample. the values of the quality cuts were obtained, using the punzi estimator [punzi 2003], giving maximum the reconstruction and tracking efficiency on muon sample, and minimum on random triggers sample. as quality cuts, the |2 of the tracks fits in each projection are required to be less than 5, the size of minimum cluster of the track is required to be in the range 20-120 (in rms unit) and the number of bars of clusters of the track is required to be less than 5. a further cut the isolation cut is applied to select muons and reject spurious triggers or other particles, e.g. electrons. the isolation cut requires that no further cluster is present in the detector together with the best track candidate in the event. this sample of “golden tracks” amounts to about 47% of the triggered events. 5. data taking the measurement campaign the puy de dôme lasted 92 days in real time from june to december 2013. accounting for the duty cycle, they correspond to about 80 effective days. about 27.5 million events were acquired. the first part of the measurement campaign was dedicated to the calibration of the detectors on site, integrating approximately 3.5 million events pointing to the free sky. the golden track sub-sample consists of about 11.4 million and 1.7 million respectively. during the data taking the detector was constantly monitored by taking pedestal runs and by measurements of the rates of all channels. the monitoring reduced the duty cycle to about 80%. the trigger rate (the coincidence of the six single view planes) had an average value of 3.6 hz and 4 hz with the telescope pointing towards the puy de dôme and towards the free sky for calibration runs, respectively. the selection criteria reduce the rate to about 2 hz for golden tracks. figure 4 shows that the golden track rate, the trigmuography of the puy de dôme figure 3. the detector frame with the three x-y planes mounted. the frame can be oriented using the rotating platform visible at the bottom. ger rate and their ratio over the effective days of data taking period, corrected for the dead time (20%). some fluctuations are observed in the trigger rate, but the stability of the fraction of golden tracks indicates that shows that the data sample is homogeneous. in spite of the fact that the temperature of the sipms was maintained constant by a cooling system, some fluctuation was observed in the or32 rate. this fluctuation was correlated (figure 5) to the temperature of the laboratory housing the detector, which was monitored and had fluctuations of about 3 °c. indeed, the front-end electronic boards followed the room temperature. however, the fluctuations in the or32 rate were not reflected on the rate of their coincidences required by the trigger (figure 6). 6. results future applications of muography require the capability of carrying out investigations over rock thickness larger of 1 km. the evaluation and the study of the background to be overcome was one of the aims of the measurement campaign at puy de dôme, with the purpose of having indications about possible upgrades. we thus selected a “control region” with a average rock thickness about 1.5 km as seen by the muon telescope (figure 7) and in this region we compared the measured to the expected muon transmission. the muon transmission is defined as the ratio of the muon noli et al. 4 figure 4. golden track rate (red dots), trigger rate (green dots) and their ratio (blue dots and scale) over the data taking period. figure 5. correlation of the front-end board temperature with the or32 rate. figure 6. trigger rate versus fluctuations of the or32 rate. figure 7. puy de dôme rock thickness. the red rectangle indicates the control region. figure 8. measured transmission (a) and expected transmission (b). (a) (b) 5 flux crossing the volcano to the muon flux incident on the volcano itself. the incident flux can be estimated by a monte carlo simulation based on the spectra available from the literature, or can be obtained by taking data with the telescope directed towards the open sky. the latter method has the advantage that the efficiency and the geometrical acceptance of the detector cancel out in the transmission measurement, as they equally affect both the measured and the free sky flux. figure 8 shows the measures and the expected trasmission over the whole mountain. in the control area the measured rate is about 4.5 mhz (figure 9), to be compared to an expected rate of 0.078 mhz. the measured rate is two order of magnitude higher than expected. 7. conclusion the measurement campaign at the puy de dôme showed that the muray detector prototype acquired stably for a period of about 3 months. the measurements indicate that for a rock thickness larger than 1 km a substantial background affects the data. this background has physical origin and, probably, consists of low energy muons that are scattered in the rock deviating from their initial direction or in general stray particles. these muons cross the detector and mimic muons from different parts of the mountain, producing a uniform residual background. additional tools for background rejection are foreseen, starting from a the time-of-flight measurement. the insertion of additional layers of absorbers will be studied and optimized for the rejection of stray particles. references alvarez, l.w., j.a. anderson, f. el bedwei, j. burkhard, a. fakhry, a. girgis, a. goneid, f. hassan, d. iverson, g. lynch, z. miligy, a.h. moussa, mohammedsharkawi and l. yazolino (1970). search for hidden chambers in the pyramids, science, 167, 832-839. ambrosino, f., et al. (2014). the mu-ray project: detector technology and first data from mt. vesuvius, j. instrum., 9, c02029. baringer, p., et al. (2001). cosmic-ray tests of the dø preshower detector, nucl. instrum. meth. a, 469, 295-310. cârloganu, c., et al. (2013). towards a muon radiography of the puy de dôme, geosci. instrum. meth., 2 (1), 55-60. punzi, g. (2003). sensitivity of searches for new signals and its optimization, phystat2003 (slac, stanford, california, september 8-11, 2003), 79-83. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida and k. niwa (2007). development of an emulsion imaging system for cosmic-ray muon radiography to explore the internal structure of a volcano, mt. asama, nucl. instrum. meth. a, 575, 489-497. *corresponding author: pasquale noli, istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy; email: noli@na.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. muography of the puy de dôme figure 9. measured golden track rate per day in the control region. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics,60, 6, s0664, 2017; doi: 10.4401/ag-7450 computation of wave attenuation and dispersion, by using quasi-static finite difference modeling method in frequency domain qazi adnan ahmad1,*, guochen wu2, wu jianlu3 1 china university of petroleum (east china), school of geoscienc, qingdao, shandong, china 2 china university of petroleum (east china), college of geo resources and information, department of geophysics dong ying, shan dong, china 3 china university of petroleum huadong, dongying, shandong, china article history received may 21, 2017; accepted october 9, 2017. subject classification: exploration geophysics; seismic methods; seismology; waves and wave analysis; measurements and monitoring; mathematical geophysics. s0664 abstract in seismology, seismic numerical modeling is regarded as a useful tool to interpret seismic responses. the presence of subsurface heterogeneities at various scales can lead to attenuation and dispersion during seismic wave propagation. in ongoing global research, the study of wave attenuation and velocity dispersion due to wave induced fluid flow (wiff) at mesoscopic scale become the subject of great interest. although, seismic modeling technique is efficient in estimating wave attenuation and velocity dispersion due to wave induced fluid flow (wiff) at mesoscopic scale. it is possible to further improve the efficiency to accurately predict wave attenuation and velocity dispersion at mesoscopic scale. to achieve this goal, a quasi-static finite difference modeling method in frequency domain is implemented to estimate frequency dependent p-wave modulus of mesoscopic heterogeneous porous media. the estimated complex and frequency dependent pwave modulus will assist to estimate frequency dependent wave attenuation and velocity dispersion within a saturated porous media exhibiting mesoscopic heterogeneities. the proposed quasi-static finite difference modeling method is further validated with theoretically predicted high and low-frequency limits and also with the analytical solution of white’s 1-d model which is for rock saturated with two immiscible fluids creating heterogeneity at mesoscopic scale. furthermore, the proposed method is further extended to rock saturated with three phase fluids exhibiting heterogeneity at mesoscopic scale. subsequently, seismic wave attenuation (inverse quality factor q-1) and the effects on p-wave velocity in 1-d models with different patch size under same gas saturation were also computed. our proposed quasi-static method is simple to be implemented by the computing scheme of parallelization and have a potential to extend it for two-dimensional case comparatively in a flexible way. 1. introduction the main challenge we are facing in oil and gas exploration is the estimation of effects on wave characteristics due to the presence of multiscale subsurface heterogeneities. during wave propagation, the heterogeneous nature of earth subsurface results into pressure gradient at different spatial scale. consequently, the resulted pressured gradient will accelerate the fluid to flow (at different scales) from high pressured zone to comparatively low-pressure zone. the wave induced fluid flow (wiff) at wavelength scale is named as macroscopic, whereas, at pore scale, it is termed as microscopic, on the other hand, at a scale much smaller than wavelength but larger than the pores scale it is named as mesoscopic fluid flow [muller et al. 2010]. the influences of fluids on seismic responses remain a key topic among researchers. biot’s [m. a. biot 1956, m. a. biot 1956] classical theory of poroelasticity led the investigation regarding wave attenuation and velocity dispersion in a saturated porous medium. in his theory, macroscopic scale heterogeneities due to presence of single phase fluid were outlined as predicted cause of wave attenuation and velocity dispersion. wave propagation through such macroscopic heterogeneous media creates pressure gradient at wavelength scale, equilibration of this pressure gradient results into a loss in wave energy. two compressional and one shear wave was predicted in biot’s theory, experimental results also validate the presence of slow p-wave [plona 1980, kelder et al. 1997]. meanwhile, another promising feature in biot’s theory is the low-frequency limiting velocities are identical with gassmann predicted velocities. although biot’s research opens new horizons in the field of geophysical prospecting, however, the outcomes of biot’s theory deals significantly at higher frequencies subsequently, it exceedingly under estimate wave attenuation and velocity dispersion within low-frequency limits [vogelaar et al. 2007]. accordingly, extrinsic attenuation (scattering) dominates in biot’s theory, when the wavelengths are of the order of grain scale (microscopic scale). therefore, some new techniques are required to overcome these existing challenges. in order to compute wave attenuation and velocity dispersion at grain (microscopic) scale mavko and nur [mavko et al. 1975, mavko et al. 1979, dvorkin et al. 1995] presented the concept of fluid flow from soft crack to stiff pores, that eventually causes wave attenuation and velocity dispersion. several analytical solutions are also put forward to estimate wave attenuation and velocity dispersion due to wiff at microscopic scale [mavko et al. 1975, mavko et al. 1979, budiansky et al. 1976, palmer et al. 1980, dvorkin et al. 1995, chapman et al. 2002, pride et al. 2003, gurevich et al. 2009]. apart from the ability to precisely predict wave attenuation and velocity dispersion at sonic frequency frequencies, investigations reveal that the microscopic scale theories lack to predict wave attenuation and velocity dispersion within seismic frequency band. this inadequacy motivates the researchers in opening new avenues to estimate the influences of wiff at mesoscopic scale. the study of wave propagation through saturated porous media demands to estimate inevitable wave attenuation and velocity dispersion within seismic frequency band. wave propagation through saturated porous rock exhibiting heterogeneity at mesoscopic scale (which is larger than microscale but much smaller than the macro scale) causes significant attenuation and velocity dispersion within a seismic frequency band [muller et al. 2010]. pressure gradient arises at mesoscopic scale, due to presence of heterogeneity within solid (i.e. between complaint pores/cracks and stiff pores) and fluids (i.e. due to compressibility difference between saturating fluids) at mesoscopic scale. this heterogeneity give rise to fluid flow at mesoscopic scale that eventually leads to wave attenuation and velocity dispersion [pride 2004]. within a porous rock presence of two or more immiscible fluids at mesoscopic scale arises patchy saturation. exploring patchy saturation in a spherical manner, white [white 1975] conducted a significant work and declared that mesoscopic fluid flow plays an influential role in wave attenuation and velocity dispersion. white’s concept was further extended to patchy saturation in layered form [white et al. 1975]. in subsequent studies, dutta and ode [dutta et al. 1979] further investigated gas-water patchy saturation in terms of biot’s poroelastic theory [m. a. biot 1956]. johnson [johnson 2001] analyzed consequences of arbitrary shape patchy saturation on wave characteristics and figured out the low and high-frequency limits for p-wave velocity. additionally, with the growing importance of mesoscopic scale theory, upcoming researchers analyzed wave attenuation and velocity dispersion due to the presence of three-dimensional heterogeneities at mesoscopic scale [müller et al. 2005, toms et al. 2007]. in recent years, there has been a growing interest in analyzing the effects of mesoscopic heterogeneity on wave characteristics. during seismic and acoustic propagation through saturated porous rock with mesoscopic heterogeneity, significant wave attenuation and velocity dispersion can be produced due to wiff [muller et al. 2010] within seismic frequency band. in order to analyze the effects of meso-scopic fluid flow on wave characteristics, seismic modeling is proved as an effective tool which is frequently applied in recent studies concerning estimation of wave attenuation and velocity dispersion [masson et al. 2006, rubino et al. 2011, rubino et al. 2012, quintal et al. 2011, milani et al. 2015]. masson and pride [masson et al. 2007] proposed an efficient method to estimate wave attenuation and velocity dispersion due to wave induced fluid flow (wiff) at mesoscopic scale. according to their proposed quasi-static creep test, they used finite difference method in the time domain for the solution of biot’s quasi-static equations for wave propagation in poroelastic media. furthermore, rubino [rubino et al. 2009] and quintal [quintal et al. 2011] have put forward two different quasi-static numerical strategies, in which they used the finite element method in frequency and time domain, for the computation of seismic attenuation due to wave-induced fluid flow. exploiting the quasi-static finite element method in frequency domain, carcione [carcione et al. 2012] and rubino [quintal et al. 2014] have discussed the effects of fracture connectivity and anisotropy on the seismic attenuation and dispersion. lately, the effects on seismic characteristics of fractured magmatic geothermal reservoirs were numerically modeled [grab et al. 2017] in a simple and efficient way. despite the fact that, the application of seismic modeling to efficiently estimate wave attenuation and velocity dispersion due to wave induced fluid flow ahmad et al. 2 3 analysis of seismic responses from micro structures (wiff) at mesoscopic scale has been improved in recent years. nonetheless, it is possible to further improve the efficiency of the methods by put forwarding more flexible methods for accurate prediction of wave attenuation and velocity dispersion at mesoscopic scale. to achieve this goal, recent work seeks to propose a simple and more effective method to compute p-wave modulus for a wide range of frequencies compared with prior to the finite difference in time domain methods. in this article, a new numerical technique is suggested in which finite difference in frequency domain is used to solve biot’s [biot 1941] quasi-static equations of consolidation based on the finite difference in time domain approach, presented by masson and pride [masson et al. 2007]. our suggested methodology eventually improves the efficiency to estimate seismic wave attenuation and dispersion in a 1d saturated medium exhibiting mesoscopic heterogeneity. we further predicted that seismic wave attenuation is intensively sensitive to variation in patch size in a one-dimensional saturated porous medium. both, the implementation and the computational procedure for measuring the complex p-wave modulus of our suggested methodology is much easier. at the same time, our proposed method can be further extended to two-dimensional case, in a flexible way. the remainder of the paper is organized as follows: the outlines of relationships of 1d biot’s poroelastic theory are given first. afterwards, the presentation of poroelastic relations of biot’s theory and the outlines of quasi-static finite difference in frequency domain are given, which will assist in computing wave attenuation and velocity dispersion in frequency domain. then the implementation of proposed quasi-static numerical method in frequency domain and its comparison with analytical results of 1d white's model for rock saturated with two immiscible fluids is described. then the numerically estimated results for different patch sizes having identical gas saturation are given. after that, the numerical measurements of 1d model saturated with three different kinds of fluids are presented. the end of the paper summarizes the results and draws the conclusions from our work. 2. methodology 2.1 1d biot’s poroelastic relationships we have implemented a quasi-static finite difference modeling method in frequency domain for the estimation of frequency dependent complex p-wave modulus of a mesoscopic heterogeneous porous media. we have used the biot’s [maurice a. biot 1941] quasi-static equations for linear consolidation in a saturated porous media in 1d frequency domain, which can be written as (1) (2) (3) (4) in above relations τzzand p defines total stress and fluid pressure, where usand uf represents the solid and fluid displacement. the bulk modulus and shear modulus of the dry frame is indicated as k and m, where η, ω and k0 represents viscosity, angular frequency and permeability of the saturated porous medium, , α and m are given as (5) (6) in above equations, ks and kf represents the bulk modulus of the solid matrix and the fluid phase respectively φ represents the porosity of the media. 2.2 the outline of quasi-static finite difference in frequency domain the aim of this study is to compute complex and frequency dependent p-wave modulus for a certain frequency band in case of a saturated porous media exhibiting mesoscopic heterogeneity. to achieve this goal, many researchers have proposed various methods, including finite element method in time/frequency domain and the finite difference method in time domain. to estimate wave attenuation and velocity dispersion due to wiff at the mesoscopic scale, we have implemented traditional frequency domain finite difference method to compute complex and frequency dependent p-wave modulus. then the boundary conditions are adjusted in such a manner that the applied stress positions, pressure, and solid/fluid displacement are remained fixed on the same grid point. in order to ensure the undrained condition, we have applied the normal stress p0 on both sides of 1d synthetic sample and kept the fluid displacement as zero at both ends of the sample(figure 1). by doing so, the solid displacement at both ends (us top (ω) and us bottom (ω)) of the sample can ∂τ zz ∂z = 0 zz = k + 4 3 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ∂us ∂z −αp ∂u f ∂z +a ∂us ∂z + p m =0 iωu f + k0 η ∂p ∂z =0 τ m i = −1 α =1− k ks m = ks α −φ +φks kf( ) be measured easily. these measurements will assist in measuring the total strain (ω) in the synthetic sample. after estimating the strain and knowing the applied stress, the frequency dependent complex p-wave modulus m (ω) can be easily computed by using equations (7-8). eventually, seismic wave attenuation q-1 (ω) and p-wave velocity vp (ω) can be measured by using equation [9-10) [masson and pride 2007]. (7) (8) (9) (10) where l indicate the total length of the described model, is the effective density which can be obtained by using the formula, given below (11) where ρs and ρf are the solid and fluid densities. furthermore, the first-space derivative is obtained by applying second order space-operator of differencing approximation is given by (12) 3. numerical results 3.1 1d white’s model we have proposed a quasi-static finite difference numerical method to quantify seismic wave attenuation and dispersion in frequency domain. to verify our quasistatic finite difference method, we implemented our ahmad et al. 4 ρ = (1−φ) ρs + φ f ∂f ∂z z= j*dz = (fj+1 − fj−1) / 2dz ( j=1,2,....n ) !ε ω( ) = us top −us bottom( ) l m ω( ) = p0 !εω ( ) q−1 ω( ) = image m ω( )( ) real mω ( )( ) vp ω( ) = real mω ( )( ) /ρ ρ figure 2. white’s model. figure 3. numerical simulations of white’s model stress (b) fluid pressure (c) solid displacement (d) fluid displacement. (a) (b) (a) (b) (c) (d) figure 1. the outline of quasi-static finite difference modeling in frequency domain. !ε 5 analysis of seismic responses from micro structures proposed method for the calculation of seismic wave attenuation and velocity dispersion of the white’s 1d model (figure 2a) exhibiting mesoscopic heterogeneity due to alternating layers of gas and water within a porous media. the statistical representative volume element (rev) enclosing a pair of alternating layers of saturated fluid, is given in figure 2b. we implemented our proposed method on the described model having petrophysical properties given in table 1, where dz = 0.1cm is the grid size and10cm as thickness of each layer. the results of total stress, pressure, solid and fluid displacement are given in figure 3. the comparison of our numerical results with the white’s [white et al. 1975] 1d layer model’s analytical results of seismic wave attenuation and dispersion is given in figure 4. the results validate the accuracy of our proposed method over a wide frequency range (10-2-105). 3.2 numerical experiments with different patch sizes for the same gas saturation in order to compute seismic wave attenuation and velocity dispersion due to wiff at mesoscopic scale, a novel technique is presented in this manuscript. the supposed caused of wiff is due to the presence of alternating layers of gas and water within a porous media creating heterogeneity at mesoscopic scale. we further investigated the effect of variation in the patch size by keeping constant gas saturation. the detail of our 8 different cases about the variation in patch size is given in figure 5, the description of mesoscopic heterogeneity in first four cases indicate a gradual decrease in patch size where the random patch sizes are selected in other cases. the regions saturated with gas (brighter) and water (darker) are all divided into two parts in accordance with the size of 2 to 1 figure 4. seismic attenuation (a) and velocity dispersion (b) of white’s model. skeleton gas oil water ks = 33.4 (gpa) kf = 9.6x10 -3 (gpa) kf = 1 (gpa) kf = 2.2 (gpa) ρf = 2700 (kg/m3) ρf = 70 (kg/m 3) ρf = 700 (kg/m3) ρf = 1000 (kg/m3) k = 0.05 (d) η = 1.5x10-5 (pa·s) η = 0.04 (pa·s) η = 6x10-4 (pa·s) ms = 1.4 (gpa) φ = 0.3 figure 5. seismic attenuation (a) and velocity dispersion (b) of rock samples for different cases. table 1. properties of the skeleton and fluids. (a) (b) (a) (b) respectively. the low-frequency limit of gassmannwood and high-frequency limit of gassmann-hill [müller et al. 2010] are also calculated by using the physical properties of the solid and the saturating fluids given in table 1. figure 6 shows the visual representation of frequency dependence of wave attenuation and velocity dispersion with a change in the size of the heterogeneity. the results for the case of a decrease in patch sizes shows that the velocity dispersion and attenuation peaks shifted towards higher frequencies with the decrease in size of heterogeneity. from case 5 to 8 patch sizes are randomly selected, there is a little difference in results of seismic wave attenuation and velocity dispersion. infect the results of rock sample of the case 5 and case 6 have almost identical values of wave attenuation and velocity dispersion, also the rock samples having patch size as shown in case 7 and case 8 have the same behavior.it can be concluded from results that, the change in the spatial size of the saturating fluids having lesser compressibility values have fewer effects on seismic wave attenuation and velocity dispersion than the fluids having higher compressibility values. ahmad et al. 6 figure 6. geometries of the partially saturated rock samples with different patchy sizes under the same condition of gas saturation, grey and black phases are saturated with gas and water respectively. figure 7. rock sample saturated with three kinds of fluid. figure 8. numerical simulations of the sample saturated with three kinds of fluid stress (b) fluid pressure (c) solid displacement (d) fluid displacement. (a) (c) (d) (b) 7 3.3 application of quasi-static finite difference in frequency domain, on a 1d model, saturated with alternating layers of three different fluids we designed a new quasi-static finite difference technique for the estimation of seismic wave attenuation and velocity dispersion in frequency domain. although several analytical/numerical studies have been carried out to estimate wave attenuation and velocity dispersion for rock saturated with two distinct fluid, little attention has been paid to predict wave attenuation and velocity dispersion for rock saturated with three different fluids. the problem with the analytical prediction of wave attenuation and velocity dispersion in rock saturated with three different fluids is that it is not easy to predict the complex p-wave modulus analytically. this part of the article demonstrates the feasibility of estimating the complex p-wave modulus and eventually the wave attenuation and velocity dispersion in rock saturated with three different fluids. we implemented the proposed quasi-static finite difference modeling in frequency domain to a 1d model saturated with alternating layers of three different kinds of fluids (figure 7). numerical simulations of the investigated sample are demonstrated in figure 8 and the estimated results of wave attenuation and velocity dispersion are visualized in figure 9. according to our findings, this is the first study in which rock saturated with three different sorts of fluids is investigated and interestingly, there are two peaks in the attenuation curve at seismic frequencies and also the results are stable over a wide frequency range. the attenuation peak appears at the low frequency is because of the reason that at the interface between gas and oil the complex p-wave modulus gets increased whereas the peak appears at high frequency is indicating the increase in complex p-wave modulus at the interface between water and gas. velocity dispersion at the same frequencies where there is high modulus dispersion validates the concept of higher values of complex p-wave modulus at the interfaces between different fluids. because of little difference between modulus of oil and water, attenuation at the interface between oil and water is disappeared in the attenuation curve. 3.4 effects of changing the order of saturating fluids presence of fluids within the subsurface rocks creates heterogeneity at a different scale which significantly influences the characteristics of propagating wave. accurate computation of these influences on wave characteristics is an essential need of oil and gas exploration. we have proposed a quasi-static finite difference method in frequency domain to compute the influence of three different fluids within a porous rock. figure 10 gives the pictorial view of numerical simulation of investigated sample with change in fluid order (i.e. 10a gas, oil, water and 10b gas, water oil) and the results of wave attenuation and velocity dispersion due to the change in the order of saturating fluids are demonstrated in figure 11 (i.e. 11a wave attenuation and 11b velocity dispersion). the black solid line shows the wave attenuation and velocity dispersion when the fluids are in order of gas, oil, and water while blue circle shows the order of gas, water, and oil. it is interesting to notice that, there is a good match between gas, oil, water and gas, water, oil saturation. one possible reason for this situation could be, because of having little difference in compressibilities of water and oil, the pore pressure difference created by the passing wave in both cases is same. so the footprints of pore pressure equilibration at the interface between gas/oil and gas/water occurs at the same frequencies. analysis of seismic responses from micro structures figure 9. seismic attenuation (a) and velocity dispersion (b) of the sample saturated with three kinds of fluid. (a) (b) 3.5 effects of changing the layer thickness of saturating fluids for a visual representation of the dependence of layer thickness of three-phase fluids in a porous rock, the reader is referred to figure 12. from the figure, it can be seen that with decreasing the layer thickness of saturating fluids, the phenomenon of pore pressure equilibration starts to occur at higher frequencies. the possible reason for the shift towards higher frequencies is that at a wavelength higher than layer thickness it’s ahmad et al. 8 figure 10. numerical simulations of the sample saturated with three kinds of fluid in different order (a) gas, oil and water (b) gas, water and oil. (b) (a) 9 easy for the wave to pass through the layer, while it gets trapped (attenuate) at higher frequencies when the layer thickness becomes equal to the wavelength. the result of velocity dispersion shows the same trend of shifting towards higher frequencies while the upper and lower velocity limits remain the same. 3.6 effects on wave attenuation and velocity dispersion due to change in thickness of layer’s having gas saturation we started by investigating the effects of mesoscopic heterogeneity (due to the presence of fluids) on wave propagation. several numerical experiments were carried out to compute the influence of varying fluids properties, on wave propagation. in results of figure 13, we analyzed the influence of a change in thickness of a gas layer on wave characteristics, by keeping the total gas saturation constant. the analysis of numerical findings indicates that variation in thickness of gas layer at top and bottom significantly influences wave characteristics. the increase in thickness of gas layer at the top cause increase in attenuation at low frequencies due to presence of highly compressible fluid (gas), while analysis of seismic responses from micro structures figure 13. seismic attenuation (a) and velocity dispersion (b) of the sample saturated with three kinds of fluid. (a) (b) figure 12. seismic attenuation (a) and velocity dispersion (b) of the sample saturated with three kinds of fluid. (a) (b) figure 11. seismic attenuation (a) and velocity dispersion (b) of the sample saturated with three kinds of fluid in different order. (a) (b) this attenuation peak gradually become flat with a decrease in thickness of gas layer at the top. maximum attenuation obtained when the thickness of gas layer becomes identical at top and bottom of the representative element. 4. conclusions this study is related to estimation of wave attenuation and velocity dispersion in fluid-saturated porous media. our work, suggest a new technique that improves the ability to estimate complex p-wave modulus and eventually wave attenuation and velocity dispersion. in our proposed technique, by solving biot’s quasi-static equations, a 1d finite difference scheme in frequency domain is used for the deduction of complex p-wave modulus of a mesoscopic heterogeneous porous media. in order to investigate the validity and accuracy of our proposed numerical scheme, we have compared our results with the analytical results of white’s 1d model. our results are in good agreement with the analytical results of white's 1d model exhibiting mesoscopic heterogeneity due to saturation of two distinct fluids in a porous media. moreover, we also investigated the effect of, variation in mesoscopic patches, on wave attenuation and velocity dispersion. the findings of our research are quite convincing, and thus the following conclusions can be drawn: the proposed scheme of quasi-static finite difference in frequency domain, assist well in estimating more precise results of frequency dependent p-wave modulus over a wide frequency range (10-2-105). attenuation and velocity dispersion is much more sensitive to change in the patch size of fluid having higher compressibility values. also, attenuation peak shift towards higher frequency with the decrease in patch size. furthermore, variation in patch size shows negligible effect on high and low-frequency limit in case of identical gas saturation. acknowledgements. we would like to acknowledge the sponsorship of the national 973program of china (2013cb228604). references biot, m.a. 1941. general 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(december 2006): p.685-695. white, j.e. 1975. computed seismic speeds and attenuation in rocks with partial gas saturation. geophysics 40(2): p.224-232. white, j.e., mihailova, n., & lyakhovitsky, f. 1975. low-frequency seismic waves in fluid-saturated layered rocks. the journal of the acoustical society of america 57(s1): p.s30. available at: http://scitation.aip.org/content/asa/journal/jasa/57/s1/10.1 121/1.1995164. *corresponding author: qazi adnan ahmad, china university of petroleum (east china), school of geoscience, qingdao, shandong, china; email: qaa.geo@gmail.com. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. analysis of seismic responses from micro structures vol49_2_2006 739 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words earthquake location – ocean bottom seismometers and hydrophones – seismic networks – synthetic tests – sicily 1. introduction the southern tyrrhenian region (fig. 1) is characterised by a fairly rapid transition from a nearly oceanic crustal structure beneath the abyssal plain to a continental-like structure beneath sicily and calabria. fault discontinuities have been mapped on grounds of geological and geophysical investigations: i) major nwtrending fault systems potentially capable to generate magnitude ≥ 6 earthquakes cross the lithosphere underneath the southernmost tyrseismic location improvements from an obs/h temporary network in southern tyrrhenian sea graziella barberi (1), laura beranzoli (2), paolo favali (2) (3), giancarlo neri (4) and tiziana sgroi (2) (1) istituto nazionale di geofisica e vulcanologia, sezione di catania, italy (2) istituto nazionale di geofisica e vulcanologia, roma, italy (3) università degli studi di chieti «g. d’annunzio», chieti scalo, italy (4) dipartimento di scienze della terra, università di messina, italy abstract we present the first investigation performed on the seismicity of southern tyrrhenian sea, off-shore sicily with the contribution of data from broad-band ocean bottom seismometers and hydrophones (obs/h). off-shore data were recorded during the tyrrhenian deep-sea experiment (tyde) from december 2000 to may 2001 in the southern tyrrhenian sea. hypocenter locations of a cluster of 53 seismic events which occurred in march 2001 in north-eastern sicily were estimated by the integration of land (permanent network) and off-shore (temporary network) data and compared with locations estimated from land data only. the scatter of the cluster was evaluated by dispersion parameters. the off-shore data significantly reduced the scatter of the swarm hypocenters also restricting the depth range of the cluster. moreover, space trends of the event distribution originally shown by the land data were only partially confirmed by the land-sea joint data. in order to assess the efficiency of a landsea integrated network with respect to a land-based network in terms of hypocenter mislocations in the subject area, we performed simulations by assuming a grid distribution of earthquakes and a recent local 3d velocity model, computing synthetic arrival times of body waves to the stations of both network configurations (integrated and land-based) perturbing the computed times and relocating earthquakes by inversion. the results of the synthetic tests demonstrated that the presence of sea bottom stations in the tyrrhenian basin can reduce the mislocations of large magnitude and/or superficial earthquakes in the southernmost calabria and messina strait and of low magnitude and/or deep earthquakes in north-eastern sicily. the accuracy of synthetic earthquake locations obtained including obs/h data provides additional support to the interpretation of the cluster occurred in march 2001 and the opportunity of long-term installation of an off-shore network like tyde in the study region. mailing address: dr. graziella barberi, istituto nazionale di geofisica e vulcanologia, sezione di catania, piazza roma 2, 95125 catania, italy; e-mail: barberi@ct.ingv.it 740 graziella barberi, laura beranzoli, paolo favali, giancarlo neri and tiziana sgroi rhenian sea (finetti and del ben, 1986; neri et al., 2003); ii) nneto n-trending normal faults are responsible for earthquakes of magnitude up to 7 in the messina strait and southern calabria (boschi et al., 1997); iii) e-trending reverse faults produce events of magnitude up to 6 seismic in central and western sicily (ben-avraham and grasso, 1990, 1991; neri et al., 2003). these fault systems cross the crust underneath off-shore areas, and this emphasises the importance of off-shore seismometry in the region. an ionian lithospheric slab subducting to wnw beneath the southern tyrrhenian sea (fig. 1) has been suggested on the basis of distribution of intermediate and deep seismicity (e.g., gasparini et al., 1982; anderson and jackson, 1987; giardini and velonà, 1991; selvaggi and chiarabba, 1995), high velocity revealed in the upper mantle by tomographic images (piromallo and morelli, 1997; lucente et al., 1999; cimini, 1999) and calk-alkaline volcanism of the aeolian islands (barberi et al., 1973; argnani and savelli, 1999). according to many investigators (e.g., malinverno and ryan, 1986; faccenna et al., 1996, 2001a; hirn et al., 1997; argnani, 2000; nicolich et al., 2000; doglioni et al., 2001; gvirtzman and nur, 2001) geological and geophysical evidence in this region can be interpreted in the framework of a geodynamic model assuming an approximately nw-se slow convergence between the african and european plates (the average motion direction of the african plate respect to the european plate is shown in fig. 1 by black arrows) and gravity-induced south-eastward roll-back of the fig. 1. structural map of southern italy, from meletti et al. (2000) and neri et al. (2005). in agreement with standard symbols, barbs indicate the downthrown blocks in normal faults, fault arrows show the strike-slip component of the fault mechanism, triangles indicate the sense of dipping of thrust structures. the shaded belt indicates the location of the apennine chain in the calabrian arc. the arrows indicate the motion direction of africa with respect to europe as predicted by large-scale crustal motion models (sella et al., 2002; calais et al., 2003; nocquet and calais, 2004). nw-se plate convergence is considered the main tectonic engine in the study region acting together with gravity-induced southeastward roll-back of the ionian lithosphere subducting beneath the tyrrhenian sea (e.g., faccenna et al., 2001b). the inset displays a map of the italian region showing the main tectonic units and microplates identified between the africa and europe stable margins (scandone and stucchi, 1999). 741 seismic location improvements from an obs/h temporary network in southern tyrrhenian sea ionian subducting slab. roll-back is widely believed to have been the primary tectonic source for: i) tyrrhenian basin opening; ii) southeastward kinematics of the southern tyrrhenian lithosphere, and iii) its thinning and overthrusting onto the ionian lithosphere. the space-time evolution of the roll-back process would have produced a clear structural differentiation between the calabrian arc and the marginal tectonic unit of sicily. north-eastern sicily experienced disastrous earthquakes in the last few centuries, often accompanied by tsuna-mis, producing wide destruction and hundreds of thousands of casualties (boschi et al., 1997; tinti et al., 2004). active volcanoes (e.g., mt. etna, aeolian island arc) represent additional sources of risk. although the regional seismometric network, solely land based, has been largely improved in the last two decades both in type of sensors and distribution, the seismicity of several sectors in the study region is still hard to locate accurately because of the presence of large marine areas. a good opportunity to obtain hypocentre locations with an accuracy higher than usual was given by the deployment of a temporary network of fourteen obs/h stations for around six months in the area between the tyrrhenian abyssal plain and the northern coast of sicily (fig. 2) in the framework of the tyrrhenian deep-sea experiment (tyde; dahm et al., 2002) partially supported by the european commission, coordinated by the istituto nazionale di geofisica e vulcanologia (ingv) with the cooperation of ifmgeomar and the university of hamburg. tyde has been the most important obs/h experiment, in terms of number of sensors installed and duration, ever performed in the italian region. two previous shorter experiments were carried out with the installation of obs in the south-eastern tyrrhenian sea off-shore calabria (soloviev et al. 1990) and around the aeolian islands (beranzoli et al., 1997; favali et al., 2004). data recorded in the framework of tyde allowed an unprecedented joint analysis of on-shore and offshore seismic data in italy (see also sgroi et al., 2006) exploring the advantages of land-sea data integration with respect to local seismicity location. 2. temporary and permanent networks and data the tyde temporary obs/h stations were installed at depths ranging from 1500 m to 3500 m on the sea bottom around the aeolian islands from the northern coast of sicily to marsili basin. the stations were equip-ped with broadband seismometers (pmd sensor: flat response in the range 0.025-32 hz; spharwebb-scripps sensors: flat response 0.02-100 hz) and hydrophones (e-2pd: bandwidth 0.5-50 hz) and 21bit digitisers with sampling rate of 50 hz. further technical details and the description of installation and recovery procedures can be found in the papers by dahm et al. (2002) and sgroi et al. (2006). the network operated in continuous mode and local recording from december 2000 to may 2001. we selected ten obs/h stations, out of the fourteen deployed, the closest ones to the area under investigation. the locations of the ten temporary stations are reported in fig. 2 (full fig. 2. map of on-shore (empty triangles) and offshore stations (full triangles) used in the present study. the on-shore stations belong to the local and national networks of ingv for the mt. etna and italian territory surveillance. the square shows the north-eastern sicily area of the march 2001 swarm. 742 graziella barberi, laura beranzoli, paolo favali, giancarlo neri and tiziana sgroi triangles) together with those of the thirty-one permanent on-shore stations (empty triangles) which also provided data for this study. four stations were also temporarily installed by ingv on land (sicily and aeolian islands) to make the coverage of the area denser. the permanent land stations are managed by ingv in the frame of a regional network addressed to monitoring eastern sicily, where the mt. etna volcanic edifice is located, and in the frame of the italian national seismological network. the land stations selected for this study are equipped with short-period three-component seismometers (eleven stations) and short-period vertical component seismometers (twenty stations). the signals are acquired with a sampling rate of 50 hz (national network) and 160 hz (regional network), and are transmitted via radio or phone cables from the remote stations to the acquisition centres – catania (sicily) for the regional network, rome for the national one – where they are stored and analysed. 3. the seismic swarm of march 2001 during the tyde experiment, a low-energy seismic swarm occurred from 26 to 30 march, 2001 in north-eastern sicily (boxed area in fig. 2). this swarm comprised 53 shocks with duration magnitude 1.4 ≤md ≤2.9, and was originally located by means of the land network data at depths ranging approximately from 25 to 40 km. the characteristics of the events are reported in fig. 3; the upper panel reports the number of events per day with the indication of the maximum magnitude, and the bottom panel shows the number of events per magnitude range. the swarm started close to the locality of santa lucia on 26 march with six shocks and maximum magnitude 2.3. most of the earthquakes are concentrated on 27 and 28 march, with more than twenty shocks per day and maximum magnitude 2.4 and 2.9 respectively. in the last two days of the swarm occurrence, the number of events abruptly decreases to a single shock per day with magnitude 1.7 (29 march) and 2.4 (30 march) to cease completely. the first step of our analysis concerns the integration of the data belonging to the land-based network and the sea-bottom stations. the integration requires, as usual, the correction of the data time tag as the clock signal of each obs/h module progressively delays over time. the internal clock of each obs/h is indeed synchronised before the deployment and after the recovery and, given a linear trend of the clock drift, data can be corrected before processing. after the correction, the p and s arrival times of the swarm events are picked on the obs/h waveforms and integrated with the readings of the land stations in a single database to be used for further steps. the final database resulted in 724 p-phase and 522 s-phase arrival times, 242 and 120 of which from sea-bottom stations for the two types of phases. in order to compare the location results obtainable by the ‘integrated’ data set and the ones obtainable by the land-based network, the «1d minimum velocity model» and the 3d local crustal model of the study area, both estimated by neri et al. (2002), were used to locate the events of the swarm. in fig. 3. upper: histogram of the number of events per day: each histogram column reports the maximum magnitude of the day. bottom: histogram of the number of events per magnitude range. 743 seismic location improvements from an obs/h temporary network in southern tyrrhenian sea particular, velest (kissling et al., 1994) and simulps12 (evans et al., 1994) algorithms were applied to carry out the locations in 1d and 3d respectively. as expected, the locations achieved with the 3d model produced a clearly better fitting of the experimental arrival times (rms=0.16s) compared to the 1d locations (rms= = 0.30s). we therefore decided to use only 3d locations in the subsequent steps of the analysis and to compare the locations of the swarm events obtained using two different network configurations: configuration ls – integrated land-sea network; configuration l – land network only. the epicentral map and hypocenter distribution with depth resulting from the location procedure in the 3d model are displayed in fig. 4 for both ls and l configurations. hypocenter distribution obtained with configuration l shows trends, like for instance a roughly e-w distribution of epicentres, which are not found when using configuration ls; on the other hand, the westward deepening trend of the hypocenters of the swarm is confirmed in both cases. using configuration ls, the hypocenter volume appears mainly restricted to depth ranging from 27 to 37 km. these values of hypocentral depths are quite unusual in general for the italian peninsular region, characterised by a seismicity hardly exceeding 15-20 km. as the events are clustered both in time and space, we assumed that they were generated by a single structure or by different structures very closely linked to each other. according to this assumption, the space dispersion of the swarm can be used as an indicator of the overall quality of locations. starting from these considerations, we defined the dispersion factors dxy and dxyz of the earthquake locations around the swarm barycentre of a given dataset in the horizontal plane and in the 3d space (3.1) == -(lat dxy ( ) ( ) ) ( ) lat lon lon d i i i n xy i n 2 2 1 2 1 + = = = n n i / / fig. 4. epicentre locations and hypocentre depth distribution of the santa lucia swarm obtained using the 3d model by neri et al. (2002) and the data from the integrated network (configuration ls, left panel) and the landbased network (configuration l, right panel). triangles and crosses indicate the stations lying in the swarm area and the barycentres used for estimates of the dxy and dxyz dispersion factors (see eqs. (3.1) and (3.2) in section 3). 744 graziella barberi, laura beranzoli, paolo favali, giancarlo neri and tiziana sgroi (3.2) with (3.3) (3.4) where i identifies the generic event in the dataset including n earthquakes, , and are the average values of epicentre latitudes lati, epicentre longitudes loni and focal depths depi. the terms lat and lon represent linear coordinates. figure 5a-d shows the histograms of the distances and : in the case of configuration ls (panels a and b) most of the events have distances and generally smaller than the ones estimated for configuration l (panels c and d). the estimates of dxy and dxyz are 3.19 km and 3.65 km in the case of network ls and 4.63 km and 6.04 km for l. this result can be considered an element supporting the conclusion that off-shore stations, even far and with an asymmetric distribution around the swarm area, have reduced the uncertainty in the locations of the d xy id xy i d xyz id xy i dep lonlat ( ) ( ) ( ) d lat lat lon lon dep dep xyz i i i i 2 2 2 = + + + + ( ) ( )d lat lat lon lonxy i i 2 2= + -i = (( ) ( ) ( ) ) ( ) d lat lat lon lon n dep dep n d xyz i i i n i xyz i n 2 2 1 2 2 1 + + + = + = = i / / swarm events. it is worth noting that the rms of the swarm locations is higher for network ls (0.16 s) than for network l (0.13 s). this can be explained by the fact that the inversion is better constrained by the experimental data of the network ls. in addition it should be remarked that the sea-bottom stations were sited in a zone of the 3d model where the spread function become larger than 4 (neri et al., 2002), suggesting a reduction of model reliability in the marine sector. 4. synthetic tests for hypocenter location accuracy we performed a series of synthetic tests to assess the accuracy of the location of events with different energy release occurring in a crustal volume wider than the one involved in the march 2001 swarm both ls and l configurations. the area selected for the synthetic tests is shown in fig. 6 (grey rectangle) together with the sets of stations chosen to form ls and l network configurations. we consider a hypo-center distribution coincident with the nodes of a 3d grid extending from the earth surface down to 35 km of depth with a horizontal and vertical node spacing of 1.5 and 5.0 km respectively. the theoretical arrival times of p and s-waves from the grid nodes to the stations of the two networks were computed with the 3d-model of neri et al. (2002). the synthetic arrival times were then perturbed to simulate random reading errors. based on a critical evaluation of the fig. 5a-d. histograms of and (see equations in section 3) of the events of the swarm from the integrated network ls (panels a and b) and the land-based network l (panels c and d). arrows indicate the average values of d. d xyz id xy i a b c d 745 seismic location improvements from an obs/h temporary network in southern tyrrhenian sea reading uncertainties during the investigation of march 2001 earthquakes, we assumed a standard deviation of perturbations of 0.3 s. the perturbed arrival times were inverted using the same 3d velocity model (neri et al., 2002). the locations were performed by placing the starting hypocenter at a distance (∆lat = ∆lon = 6 km; ∆depth = 8 km) from the true hypocenter (grid node) larger than the grid spacing. the dislocation of the initial hypocentres is comparable to the largest errors that we expect from routine hypocenter locations in this area. we performed different trials by changing the starting hypo-center locations toward n, s, e, w, ne, nw, se, sw with ∆lat = ∆lon = ±6 km and increasing depth of ∆depth = +8 km. we found that these dislocations determine mislocations smaller than the grid spacing. two parameters were estimated for each synthetic event (grid node): the epicentral distance d and the focal depth difference h between the synthetic and relocated event. figure 7 shows the horizontal distribution at different depths of the d values obtained using the network configurations ls and l. the comparison between the d values computed from the two different network configurations reveals that, in spite of the uneven geometry of the sea network with respect to the area, the presence of seismological stations in the tyrrhenian basin reduces the horizontal mislocations at all depths, not only in north-eastern sicily but also in the southernmost ionian calabria, south-east of messina strait. this reduction appears to be significant in the depth range 25-35 km also for the latter zone where the red areas (d > 6 km) appear to be less extended. the horizontal distribution of the h values for the ls and l configurations are displayed in fig. 8. it is worth noting that the vertical mislocation of events can be reduced in north-eastern sicily more than in calabria by the presence of sea-bottom stations in the tyrrhenian sea. this provides an additional element in support of the evaluation of the depth range of the swarm of march 2001 and interpretation. an additional synthetic test is performed with a standard deviation 0.05 s of the perturbation introduced into the arrival times. this value accounts for energetic and/or superficial events. figures 9 and 10 show the d and h horizontal distributions at different depths obtained with this smaller value of perturbation. the results of this additional test show a significant reduction of both d and h values in configuration ls in calabria and messina strait in the depth range 5-25 km. 5. conclusions p and s arrival times of local seismicity recorded by a temporary seafloor network of obs/h and ingv land-base network, for a total amount of 1246 data, were used to analyse a moderate energy swarm in march 2001 in north-eastern sicily. moreover, the sea-bottom station distribution was used jointly with the land station distribution fig. 6. network configurations used for tests performed with synthetic earthquakes in order to assess the hypocentral location quality. triangles and circles mark the land and sea-bottom stations. the integrated network (configuration ls) included both sets of stations. the land stations constitute the network configuration l. hypocenters of synthetic earthquakes were positioned on a 3d grid, with horizontal spacing of 1.5 km and vertical spacing 5 km from the earth surface down to 35 km. the gray rectangle indicates the area selected for the synthetic tests. 746 graziella barberi, laura beranzoli, paolo favali, giancarlo neri and tiziana sgroi fig. 7. comparison of d-parameter horizontal distributions at different depths obtained by synthetic tests with the ls integrated network (left panels) and the l land-based network (right panels). the standard deviation of the random noise perturbation of synthetic arrival times is 0.3 s. fig. 8. comparison of h-parameter horizontal distributions at different depths like in fig. 7 with the same standard deviation (0.3 s). 747 seismic location improvements from an obs/h temporary network in southern tyrrhenian sea fig. 9. comparison of d-parameter horizontal distributions at different depths like in fig. 7. the standard deviation of the random noise perturbation of synthetic arrival times is reduced to 0.05 s. fig. 10. comparison of h-parameter horizontal distributions at different depths like in fig. 7. the standard deviation of the random noise perturbation of synthetic arrival times is 0.05 s. 748 graziella barberi, laura beranzoli, paolo favali, giancarlo neri and tiziana sgroi to assess the efficiency of a sea-land integrated monitoring network in terms of hypocenters location accuracy for events eventually occurring in north-eastern sicily and southern calabria. the swarm of 53 events, with maximum magnitude 2.9, was relocated using a 3d velocity model of the region (neri et al., 2002). the locations were performed twice, the first time using land and sea stations jointly (integrated network) and the second time using only land stations. in the case of integrated network configuration the relocated swarm has dispersion factors dxy and dxyz smaller than those found with the land-based network relocation. that could be attributed to a lower parameter variance due to a smaller scatter of the hypocenters. the westward dipping of swarm hypocenters seems the only feature confirmed by both network configurations. the depth range of the swarm turned out between 27 and 37 km, a quite unusual depth for seismogenesis in the italian peninsula. the depth range of the swarm could be interpreted in the frame of the transition nature of the area from the geological and geodynamic point of view. north-eastern sicily belongs to the southernmost sector of the calabrian arc unit and is adjacent to the hyblean foreland. the southern border of the ionian lithospheric slab has been roughly positioned just beneath north-eastern sicily by previous studies (e.g., argnani, 2000). moreover, selvaggi and chiarabba (1995) estimated the depth of the shallower part of the subducting slab in the area to decrease from 50 to 25 km in the nw-se direction. in order to assess the improvement of hypocenter location accuracy achievable by the integration of sea-bottom seismological observations and land-based data, synthetic tests for a larger area, including the swarm, and extended up to southern calabria were performed. the assessment was accomplished through the evaluation of parameters d and h, defined as the 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p l i c a l a , r e c e n t e m e n t e fio i d e a t o e r e a l i z z a t o u n n u o v o m e t o d o p e r l a m i s u r a q u a n t i t a t i v a d e l c o n t e n u t o d i e m a n a z i o n e r a d i o a t t i v a ( r a d o n , t o r o n , a c t i n o n ) d e l l ' a r i a t e l l u r i c a ; i l m e t o d o è e s e n t e d a l l e i n c e r t e z z e c h e g r a v a n o p e r e s e m p i o sul m e t o d o di e l s t e r e g e i t e l p e r il f a t t o c l i c n o n r i m a n e b e n p r e c i s a t e il v o l u m e dal q u a l e si a s p i r a l ' a r i a c l i c p o i si a n a l i z z a . i l n u o v o m e t o d o c o n s i s t e e s s e n z i a l m e n t e n e l l ' a t l i v a r e u n e l e t t r o d o p o s t o i n u n a c a v i t à c i l i n d r i c a v e r t i c a l e d e l t e r r e n o m a n t e n e n d o v e l o p e r u n c e r t o t e m p o ad e l e v a t o p o t e n z i a l e e l e t t r i c o n e g a t i v o e n c l j ' c s a m i n a r e poi i n c a m e r a di i o n i z z a z i o n e l ' a t t i v i t à c o n q u i s t a t a d a e s s o e l e t t r o d o . p e r a u m e n t a r e al m a s s i m o p o s s i b i l e l ' a t t i v i t à r a e e o g l i b i l e d a l l ' e l e t t r o d o , i n u n d e t e r m i n a t o i n t e r v a l l o di t e m p o , ( e c i ò p u ò e s s e r e i n d i s p e n s a b i l e i n t e r r e n i d e b o l m e n t e r a d i o a t t i v i ) c o n v i e n e di e s e g u i r e u n a c h i u s u r a p r e v e n t i v a del f o r o d i e s p e r i e n z a e p e r c i ò l ' a p p a r e c c h i o p e r l a r a c c o l t a d e l l ' a t t i v i t à è c o s ì c o s t i t u i t o : l ' e l e t t r o d o d a a t t i v a r e è c o n t e n u t o i n un a s t u c c i o m e t a l l i c o a ( v e d i fig. 1) s o s t e n u t o v i da u n g a m b o m e t a l l i c o g i s o l a t o e l e t t r i c a m e n t e d a l l e p a r e t i d e l l ' a s t u c c i o . l ' a s t u c c i o p o r t a n e l l a p a r t e i n f e r i o r e u n m a n i c o t t o m di g o m m a a d o p p i a p a r e t e c h e si p u ò g o n f i a r e ; l a p a r e t e del f o r o p r a t i c a t o •nel t e r r e n o è r i v e s t i l a d a u n t u b o , d i g r è s n e l n o s t r o c a s o , b e n a d e r e n t e al t e r r e n o . i n t r o m' a i i / i|p c~ \z " c ; v' 1* —i r i—f i g . i ij n u o v o m e t o d o l ' e l i la m i s u r a d e l c o n t e n u t o r a d i o a t t i v o e c c . d o l i o n e l l'oro l ' a s t u c c i o c o n t e n e n t e l ' e l e t t r o d o e m u n i t o di e o p e r c h i e l t o c , v i e n e gonfialo il m a n i c o t t o di g o m m a ; c o n q u e s t a o p e r a z i o n e , al f o n d o del b u c o , >olto l ' a s t u c c i o r i m a n e l i b e r o u n p o ' m e n o d i m e z z o m e t r o di f o r o ; la c o n e a m c r a z i o n e così f o r m a t a sotto l ' a s t u c c i o r i m a n e c h i u s a e senza c o m u n i c a z i o n e d i r e t t a con l ' a r i a a t m o s f e r i c a . l a c h i u s u r a del f o r o o t t e n u t a n e l m o d o ora d e t t o , si fa d u r a r e p e r un c e r i o n u m e r o di o r e , d o p o di c h e si s p i n g e l ' e l e t t r o d o e f u o r i d a l l ' a s t u c c i o d a l l a p o s i z i o n e 1 a l l a p o s i z i o n e 2 , gli si a p p l i c a i m m e d i a t a m e n t e u n a differenza di p o t e n z i a l e r i s p e t t o a l l ' a s t u c c i o ( c o n n e s s o e l e t t r i c a m e n t e con l a t e r r a ) p e r un t e m p o d e t e r m i n a l o , u n ' o r a p e r e s . , e p o i si r i t i r a l ' e l e t t r o d o n e l l ' a s t u c c i o , si sgonfia il m a n i c o t t o di g o m m a , si toglie l ' a s t u c c i o dal f o r o , si p r e l e v a l ' e l e t t r o d o e lo si p o r l a n e l l a c a m e r a di i o n i z z a z i o n e per m i s u r a r v i l ' a t t i v i t à . projondita mt. 2 oo 0 0 o . °n j§ * — — — js tempo di chiusura 10 20 30 a0 ore 50 fig. 2 c o n il d i s p o s i t i v o ora d e s c r i t t o , u s a t o in d u e pozzetti p r o f o n d i r i s p e t t i v a m e n t e m 2 e m 3 si sono e s e g u i t e n u m e r o s e m i s u r e dal giug n o 1 9 4 7 fino a l l o scorso f e b b r a i o , a l l o scopo di p r e c i s a r e le c a r a t t e r i s t i c h e di f u n z i o n a m e n t o d e l l ' a p p a r e c c h i o . l e m i s u r e sono s t a t e f a t t e a v e n d o di m i r a la s o l u z i o n e dei seg u e n t i p r o b l e m i : 1. — d e t e r m i n a r e l ' i n f l u e n z a d e l l a d u r a l a di c h i u s u r a preventiva del pozzetto sulla q u a n t i t à di e m a n a z i o n e e s i s t e n t e nel pozzetto stesso e c o n f r o n t a r e i d a t i s p e r i m e n t a l i con a l c u n e c o n s i d e r a z i o n i teor i c h e r e l a t i v e al r i f o r n i m e n t o di e m a n a z i o n e dal suolo verso il pozz e t t o . l e figure 2 e 3 r a c c o l g o n o l ' i n s i e m e d e l l e m i s u r e e s e g u i t e n e i d u e p o z z e t t i e in c i a s c u n a di esse è t r a c c i a l a la l i n e a c h e m e g l i o r a p p r e s e n t a i r i s u l t a t i : sulle ascisse =ta la d u r a t a p r e v e n t i v a di c h i u s u r a d e l pozzclto, s u l l e o r d i n a t e l ' a t t i v i là osservata s u l l ' e l e t t r o d o d e d o t t a d a l l ' o s s e r v a z i o n e al m i n u t o 2 0 d e l l a d i s a t t i v a z i o n e . 2 . —d e t e r m i n a r e il t i p o di r a d i o a t t i v i t à c o n t e n u t o n e l l ' a r i a t e l l u r i c a di p a v i a ; a q u e s t o s c o p o è stalo n e c e s s a r i o c a l c o l a r e l e c u r v e t e o r i c h e di d i s a t t i v a z i o n e r e l a t i v e a l l e tre e m a n a z i o n i r a d i o a t t i v e esi371 c. aliveari t e n t i in n a t u r a ; 1 u s o (li tali c u n e n e l l a i n t e r p r e t a z i o n e d e l l a curva s p e r i m e n t a l e h a p e r m e s s o di s t a b i l i r e c h e a p a v i a è p r e s e n t e essenz i a l m e n t e s o l t a n t o il r a d o n . 3 . — r e n d e r e q u a n t i t a t i v o il m e t o d o c o n f r o n t a n d o l o con un a l t r o m e t o d o q u a n t i t a t i v o . \ q u o t o c o p o è s t a l a usata la c a m e r a profondita mt. 3 • tempo di chiusura 20 30 f i g . 3 di r a c c o l t a vliv c r t i l o v era ( 1 ) pei' il p r e l i e v o d e l l ' a r i a t e l l u r i c a e successiva a t t i v a z i o n e di un e l e t t r o d o posto in essa c c a r i c a t o n e g a t i v a m e n t e ( v e d i fig. 1). q u e s t o c o n f r o n t o , u n i t a m e n t e ad a l t r e m i s u r e e s e g u i t e con r a p p a r e c e h i e l t o di fig. i m u n i t o s u c c e s s i v a m e n t e di e l e t trodi di v a r i a l u n g h e z z a , ha p e r m e s s o di s t a b i l i r ò c h e con l ' e l e t t r o d o del n u o v o a p p a r e c c h i o a l i v c r t i . n e l l e c o n d i z i o n i i n d i c a t e p i ù sù, -i d e p a u p e r a di r a a l ' a r i a del t r a t t o c i l i n d r i c o o c c u p a t o d a l l ' e l e t t r o d o . il e o n l r o n t o in q u e s t i o n e e la c o n o s c e n z a d e l l e c u r v e di fig. 2 e fig. 3 h a n n o p e r m e s s o i n f i n e di e l a b o r a r e tulli i dati s p e r i m e n t a l i r e n d e n d o l i o m o g e n e i e q u i n d i atti alla l'orinaz i o n e di valori m e d i : da t a l i v a l o r i m e d i c stato p o s s i b i l e individ u a r e i f a t t o r i m e t e o rologici i n f l u e n z a n t i il c o n t e n u t o r a d i o a t t i v o d e l l ' a r i a t e l l u r i c a . d a l l ' i n s i e m e dei risultati n u m e r i c i racc o l t i nel c o r s o d e l l a ri1 — i — c e r c a si t r a g g o n o le seguenti c o n c i u i o n i : •a u (',. inverti f i g . i g . laverà, r i c . s c i e n t . . à i . nuovo m e t o d o l ' e l i la a l i s i ih del c o n t e n u t o hadioat ti vo ec( a . — il c o n t e n u t o in radon d e l l ' a r i a t e l l u r i c a di p a v i a ( o s s e r v a t o r i o g e o f i s i c o a s . s p i r i t o ) a l l a p r o f o n d i t à fra 2 e 3 ni è c i r c a c o s t a n t e ed e q u i v a l e n t e a una r a d i o a t t i v i t à s u l l ' e l e t t r o d o di 1 . 6 e m a i l : q u e s t a r a d i o a t t i v i t à c o r r i s p o n d e n e l l ' a r i a t e l l u r i c a a 1111 c o n t e n u t o dell ' o r d i n e ili to' a t o m i di radon p e r c e n t i m e t r o c u b o . 1>. — a p r e s s i o n e d i m i n u e n t e il c o n t e n u t o r a d i o a t t i v o d e l l ' a r i a t e l l u r i c a è in m e d i a m a g g i o r e c l i c a prensione c r e s c e n t e : i n f a t t i le m e d i e s o n o : p r o f o n d i t à ni 2 p r o f o n d i t à 111 3 p r e s s i o n e d e c r e s r . 1,72-1 e m a i l 1,681 « creso. 1.415 1,511 c . —e s i s t e una s e n s i b i l e d i f f e r e n z a fra l ' a t t i v i t à c l i c -i trova a t e r r e n o secco e q u e l l a a t e r r e n o b a g n a t o : p r o f o n d i t à 111 2 p r o f o n d i t à 111 3 t e r r e n o a s c i u t t o 1 , 6 7 6 1 , 6 7 5 » b a g n a l o 1,5112 1.592 i). — si è c e r c a t a e non si è trovata una r e l a z i o n e con la vel o c i t à del v e n t o . e . — il nuove, m e t o d o ila p e r m e s s o i n f i n e di s t u d i a r e le cond i z i o n i di d i f f u s i o n e del radon dal t e r r e n o verso l ' a r i a del pozzetto t e n u t o a p e r t o . la p r e s e n t e r i c e r c a e stata eseguila a p a v i a n e l l ' o s s e r v a t o r i o dell ' i s t i t u t o n a z i o n a l e di g c o f i s i c a con un c o n t r i b u t o del c o n s i g l i o naz i o n a l e d e l l e r i c e r c h e . c r e d o d o v e r o s o r i n g r a z i a r e la ditta p i r e l l i c h e . non e s s e n d o p o s s i b i l e t r o v a r e in c o m m e r c i o in conseguenza dogli eventi b e l l i c i i m a t e r i a l i isolanti e ili g o m m a o c c o r r e n t i , h a f a b b r i c a t o e s p r e s s a m e n t e e d o n a t o d e l t i m a t e r i a l i . i.a r i c e r c a ( h e qui ò esposta s o m m a r i a m e n t e v e r r à p u b b l i c a l a per esteso n e l l a r i v i s t a g e o m i n e r a r i a , s t a m p a l a a cur a del! i t i t i l l o di g c o f i s i e a a p p l i c a t a del p o l i t e c n i c o di m i l a n o . istituto nazionale di gcolisii u — osscrv. di l'aria — giugno 1918. 621_628 agopyan 18.pdf annals of geophysics, vol. 45, n. 5, october 2002 621 severe magnetic storm effects in the ionosphere over istanbul: a case study harutyun agopyan istanbul university, vocational school of high technology, istanbul, turkey abstract the present study concentrates on the effects on the ionosphere of an individual severe magnetic storm of the sudden commencement (ssc) type, with ssc taking place in the daytime hours. the storm started on 29 october 1968 and went on to 2 november 1968 with geomagnetic 3-hourly magnetic activity index reaching values of kp ≥ 7. interplanetary magnetic field polarities included a (field polarity away from the solar wind) positive and t (field polarity towards the solar wind) negative polarities. in these conditions, the local response of true height of fregion (hf) ionization fails nonlinearly from fixed plasma densities. the interplanetary b z , the magnetic field hcomponent and k p were examined for the 5 days following the sudden commencement. due to intensive geomagnetic sc effects, the lower and upper limits of the f-regions were unbalanced because of gravity relaxation and solar wind pressure effects, until normal levels were restored. in the f-layer 70% deviations of critical frequencies ( f 0 f ) from median values, as well as hf level variations reaching hundreds of kilometers, were observed and were responsible for the destruction of communication channels. with a plasmapause location value l = 1.6 and with k p ≥ 7 the protonosphere reservoir should take 1.1 days for its replenishment; one day was in fact insufficient for a full plasma recovery. 1. introduction a geomagnetic storm (gs) is the result of strong enhancement of ring currents (rc) in which the earth’s magnetic field (emf) is usually depressed below its normal quiet day value. an ionospheric storm (is) is the ionospheric effect of a gs that increases or decreases the electron density (n e ) and the total electron content (n tec ) of the f-region; please refer to förster and jakowski (2000) and references therein, for a review on these phenomena. there are two kinds of iss known as negative iss and mailing address: associate prof. dr. harutyun agopyan, istanbul university, vocational school of high technology, 34320, avcilar, istanbul, turkey. former address: i.c.t.p., 34100 trieste, italy. e-mail: harutyun@istanbul.edu.tr key words geomagnetic activity − ionospheric positive storm − neutral gas concentration excess − radio reception − ring current positive iss, depending on the local time (lt) of onset of gs, latitude and season. the response of critical frequency of the f-region ( f0f2) depends markedly on season and solar and geomagnetic activity, and also on geomagnetic latitudes (matsushita, 1959). most of the winter storms show enhancements, whereas summer storms and equinox storms show depressions following short-lived enhancements. the interplanetary magnetic field (imf) during spring has an a (field polarity away from the solar wind), anti-sunward polarity, inducing + b z values, and t (field polarity toward the solar wind) during autumn it has a sunward polarity, inducing – b z values. thus, inverse b z signs are most common for october storms. solar flare-induced storms are weak but last a few days longer, because of the time taken for the stream to pass over the earth. several years ago, dungey (1961) and akasofu and chapman (1963) claimed that the energy transfer mechanism from the solar wind to the magnetosphere is a magnetic reconnection between the southward imf – b z and the earth’s 622 harutyun agopyan dipole emf. the non-linearity of the magnetosphere and ionosphere both depend on the rc energy injection rate (akasofu, 1981). mendillo and klobuchar (1975) presented tec storm behavior after the important discovery of a trough producing a negative phase that begins progressively later at sites with lower latitudes. prölss (1987) investigated the temperature response of the upper thermosphere, and gave quantitative results on composition changes. he also published a thermospheric model but suggested that further studies are required for negative iss at middle latitudes. models differ significantly near 200 km where the nitrogen (n 2 ) density is important and oxygen (o) is depleted in the summer. prediction of atomic o density in new solar cycles will require new satellite missions and revival satellite drag analyses, or improved methods of monitoring from the ground (hedin, 1988). rees et al. (1988) found that, at fixed pressure levels, the increase in mean molecular weight spread only a short distance from the auroral oval can be taken as evidence that negative iss at lower latitudes cannot be due to changes in the neutral gas composition. the n2/o density ratio is important together with that of other minor constituents assuming that the pressure scale height is approximately constant above 280 km. hewish and duffet-smith (1987) suggested a means of giving warning of impending high speed solar wind streams at 1au. this would allow predictions of time intervals where there will be higher probabilities of ms occurrence (tsurtani et al., 1988). the present study aims to contribute to satellite communications and as well as to ionospheric model studies for especially high geomagnetic activity conditions. 2. observation observations of the hourly behaviour of the critical frequency of the f2 layer of the ionosphere have been analyzed for their percentage deviations from their monthly median values the above measure of ∆f 0 f 2 , peak height values of hp f2,minimum height values of hminf and virtual height values of h′f for istanbul (geographic coordinates 41°n; 29°e; geomagnetic coordinates 39°n; 108°e; mc ilwain (1961) earth’s magnetic shell parameter l ≅ 1.6) have been analyzed to reveal the behavior of the ionosphere during strong gs disturbances. observations were made during the high solar activity period from 29 october 1968 to 2 november 1968. during this interval, d(γ) ≥ 120 and quite high magnetic activity indices of k p ≥ 7 were observed extensively (uyar, 1964; bulat and agopyan, 1980; agopyan, 1986, 1988 and 1996). figure 1a shows the percentage deviation of f0f2 (mhz) during the severe magnetic storm (ms) period of 5 days. it is seen that the f 0 f 2 values always show clear increases from the monthly medians of about 50% to 70% 8 or 10 h after the beginning of a ssc. however, the wireless short wave (sw) prediction 10 mhz for f 0 f 2 goes to 17 mhz during a severe gs. so it is advisable to tune to the higher frequencies. in other words, it would be better to lower the wavelength for sw radio reception (i.e. instead of noisy reception on the 31 m band, it would be better to tune to the 19 m band). in fig. 1b, hourly values of h p f2 and hminf are plotted. this shows the ionospheric plasma situation more clearly during a strong gs series (29/30/31 october 1/2 november 1968). during the first day of the storm, the height changes are over 250 km. it can be seen that no changes in height occur during the sunlit hours but they are significant during night hours. nocturnal h p f 2 and h min f crossing on november 1 electric current increases and cross generations of emf present ionospheric g condition existence respectively (agopyan, 1988). in fig. 2, maximum electron density (n m f 2 ⋅ 105 e cm–3 ), frequency (f0 f2 ⋅ 1 mhz), monthly median (nmedf2 ⋅ 10 5 e cm–3 ) and k p ranges are plotted against time for 28 october 1968 from 00 h 00 lt to 24 h 00 lt. the following days containing missing values have been omitted. the electron-density versus time curves are not suitable for studying the electron concentration changes at various heights because of strong day/ night differences during the storm. therefore ∆f f f f f f f f 0 2 0 2 0 2 0 2 100= ( ) − ( ) ( ) ×hourly median median . 623 severe magnetic storm effects in the ionosphere over istanbul: a case study fig. 3 shows isoionic contour plots of electron densities derived from true height profiles. here fixed plasma density variations show a decreasing amplitude of plasma wavelike behaviour as isoionic counter plots for the storm that started on 29 october, 1968 and continued for 5 days. the increase in height for various electron density values during the night and their nonlinear behaviour are not only the result of coronal mass ejections (cme) of the sun but also partly due to the effects of tides, wind pressures and gravity relaxation occurred progressively. from the fig. 3, most of the height changes of density values in between 1.5-6.5 ⋅ 105 e/cm3 occur near 20 h 00 lt. the variation is from 500 km to nearly 700 km at the topside window of the ionosphere (agopyan, 1988). here, the electron concentration of o+ ions is too low for a reliable fig. 1 a,b. a) percentage critical frequency deviations from monthly medians [∆f 0 f 2 (mhz)] versus consecutive local time (lt = ut + 2) hours with storm sudden commencement (ssc) occurrences for 29 october to 2 november 1968; b) minimum to maximum height variation of the ionospheric f-layer versus local time (lt ) hours. a b fig. 2. electron density (105 e cm–3 ), n med f 2 (105 e cm–3 ), frequency (mhz), and k p ranges versus local time for 29 october 1968 between 00 h 00 lt to 24 h 00 lt hours. 624 harutyun agopyan determination of the height. the most convenient time of day seems to be in the evening at 17 h 00 lt, when height variations of no more than 50 km are seen (the height remains between 250 and 300 km). the data have been subjected to spectral analysis. in fig. 4, the power spectra including hanning windowing (claerbout, 1985 and press et al., 1988) show the effects of planetary waves having about 12 hourly period. lunar and solar hourly periods with periods around 5 h, 18 h, and 64 h. furthermore, the power spectral density including hanning windowing shows that the similar results are present during the storm period as is seen in fig. 5. besides solar tide that can affect the acceleration of gravity over 280 km where pressure scale height becomes important on n2 /o density ratio equilibrium not only plasma waves, minor constituents but also lunar and tidal local effects should be taken into consideration for further investigations. in fig. 6, k p , imf (bz) and the h-component of the magnetic field are plotted for the 5 days of the sc storms (agopyan, 1986) observed worldwide from 29 october 1968 to 2 november 1968, including a, antisunward positive and t, toward the sun negative polarities. if the imf sector boundary crossings are effective with their a or t polarities, thus the variation of f 0 f 2 during t→a and a→t transitions could be explained by the solar wind speed (fig. 2 and fig. 9; bremer, 1988). there is a positive correlation between amplitudes and phases of sunspot cycle, k-index and emf, but, particularly in equinoctial months, the first harmonic of the h-component varies irregularly with sunspot number. the seasonal variation of amplitudes and solar cycle variation of amplitude harmonics (smoothed) have been investigated for disturbed days together with the yearly variation and sunspot number and k-index, and also for the h-component (işıkara, 1971). the k-index since 1952 from the kandilli fig. 3. isoionic contours derived from n(h) analysis of ionograms for between 29 october to 2 november 1968 show true height variations h (km) of whole storm series throughout the 5 k p ≥ 7 days showing disturbed wavelike behaviour of plasma densities. 625 severe magnetic storm effects in the ionosphere over istanbul: a case study fig. 4. power spectra of the electron density value of the ionospheric storm including hanning windowing. fig. 5. power spectral density of the electron density value of the ionospheric storm including hanning windowing. 626 harutyun agopyan observatory shows a statistically significant relation with the amplitudes of the solar and lunar variations of the emf. all components of the emf and k-index show an increase in amplitude generally with increasing sunspot number as n m f2, f0f2 and subpeak electron content (agopyan and bulat, 1990). 3. discussion of results and conclusions the results presented in the graphs may be summarized as follows: a) the f-layer starts to rise up in a rather normal way near sunset. coincident with this rising of the layer where there are sudden increases in ionization, which reaches a value about 60% to 70% larger than its normal level, for the three sscs. sc1-(29 october 1968 11 h 09 lt, k p = 7), sc2-(31 october 1968 11 h 00 lt, k p = 8) and sc3-(1 november 1968 11 h 17 lt, k p = 7). b) for the 3 cases represented in figures, after reaching a maximum of concentration excess (∆f0f2), the layer and ∆f0 f go down by about 100160 km and then return to normal. c) the negative phase for the third storm is very marked and coincides with a possible expansion of the f-layer. possible dominant mechanisms for the increase in ionization concentration about 8-10 h after the ssc could be: i) intensification of the neutral wind circulation equatorward bringing up ionization to the regions of lower recombination processes; ii) ionospheric feeding by plasma, flowing from the plasmasphere towards the equator due to its compression. this compression could be due to the e × b ions drifting from polar (higher) to equatorial (lower) l-shells causing further motion along magnetic flux tubes. the saturation of the flux tubes is reached after 8-10 h. the reason for the rapid decrease following the peak of concentration could be due to: i) a sudden change in the structure of the fig. 6. comparison of kp, imf (bz ) and h-component magnetic field variations for the 5 days of the sc storms observed on 29, 30, 31 october 1, 2 november 1968, including a (field polarity away from the solar wind) positive and t (field polarity toward the solar wind) negative polarities. 627 severe magnetic storm effects in the ionosphere over istanbul: a case study electric field that causes downward drift to the f-layer as a whole to regions of higher recombination processes; ii) a sudden change in direction of the neutral winds. the source of heating of the thermosphere at lower latitudes is more difficult to understand. it appears that the f-layers oscillate afterwards, as illustrated by its wavelike structure. the return to higher values of f-layer heights could be due to the following: i) the sudden vanishing of the special structure of the electric field; ii) the returning of neutral wind to normal conditions. nevertheless, the positive phase cannot be due to the increase of ionizing radiation from the sun because in that case we would observe a positive peak rather closer to the sc of the storm. from the aeronomic point of view, in general, istanbul (l = 1.6) experiences equatorial storm characteristics because of its location in the same longitudinal network as athens (l = 1.4), rome (l = 1.6) and cape kennedy (l = 1.8) and it also shows phase similarities, parallel morphologic features, and characteristic mendillo peak. for the geomagnetic latitude of istanbul, with a plasmapause location value l = 1.6 for k p ≥ 7, the saturation of the protonospheric reservoir would be expected to take t s = 0.17 (l)4 ≅ 1.1 days (kamide and richmond, 1986). thus one day is insufficient for its replenishment. the case is not similar for stations having similar coordinates but it is for equivalent l values. if the f2 layer was completely solarcontrolled, the lt contour maps of f 0 f 2 would follow lines of geographic latitudes (fox and mcnamara, 1988). it should be kept in mind that when the sun is fast, with a lead-time of about a quarter of an hour (i.e. the equation of time is positive), there is an extreme energy injection into the ionosphere by way of the magnetospheric plasma and vice versa for the high solar geomagnetic activity conditions. during the times with − b z solar wind, the magnetosphere energy transfer function derived by perreault and akasofu (akasofu, 1981) is dominant near 180° due to the term sin4ν/2. here ν is the polar angle of the imf vector in the y-z plane of the solarmagnetosphere coordinate system. it is known that ε = vb2i 0 sin4ν/2 where v is the velocity of the solar wind, b is the magnitude of the imf and i0 = 7 earth radii. the variation of the ε function gives evidence that the energy transfer from the solar wind into the magnetosphere is higher during imf t pro sectors than during a anti sectors. after a cme only the variation of the solar wind speed determines the mean energy input, but the increase during the anti → pro sector transition is steeper than the decrease during the transition from pro → anti sector. plasma-sheet distance depends on cross-tail potential drop, φ t , or cross-polar cap potential, φ cp , and φ t in turn depends on ε via the relation with the correlation coefficient ∝ 0.92 where the power (ε) of solar windmagnetosphere dynamo changes as a function of time (reiff et al., 1977). the sources of temperature increases and their physical explanations give the most reliable information. but magnetic activity variations still remain a critical area needing improvement for all models (hedin, 1988). acknowledgements the author wishes to thank prof. dr. taner bulat for his leading cooperation of ionogram scaling and prof. dr. stuart r.c. malin for his critical review of the manuscripts. i would also like to thank professors abdus salam, luciano bertocchi, giuseppe furlan, neille j. skinner and sandro m. radicella, the international atomic energy agency, unesco for hospitality at the ictp, ics, and icem. i am indebted also to dr. emin demirbag and mr. taner arslan for their computer assistance. references agopyan, h. (1986): ionospheric behaviour on geomagnetic storms, in proceedings of the international symposium on «radio beacon contribution to study of ionization and dynamics of the ionosphere and corrections to geodesy», oulu, finland, edited by antti taurainen, part-1, 11-25. agopyan, h. (1988): severe magnetic storm effects on the ionosphere over istanbul, in proceedings of abstracts on «international beacon satellite symposium (ibss) 88 on the investigation of the ionosphere by means of φ φt pc≅ ∝ ε 628 harutyun agopyan beacon satellite measurement», april 18-21, beijing, china, edited by cao chong, china research institute of radiowave propagation, 2. agopyan, h. (1996): the reason for severe magnetic storm effects in the ionosphere over istanbul (st066), in xxi general assembly of the european geophysical society, 6-10 may 1996, «solar terrestrial events and their effects on the ionosphere and atmosphere (st10)», the hague, holland, ann. geophysicae, suppl. iii, vol. 14, c755. agopyan, h. and t. bulat (1990): the changes of subpeak electron content of the ionosphere depending on seasons and sunspots, pubblication of istituto nazionale di geofisica n. 523, roma, italia. akasofu, s. (1981): energy coupling between solar wind and the magnetosphere, space sci. rev., 28, 121-190. akasofu, s. and s. chapman (1963): the development of the main phase of magnetic storms, j. geophys. res., 68, 125-129. bremer, j. (1988): the influence of the imf structure on the ionospheric f-region, j. atmos. terr. physics, 50, 831-838. bulat, t. and h. agopyan (1980): tepealti elektron yogunlugunun gunes lekelerine ve mevsimlere gore degisimi (the changes of sub-peak electron content of ionosphere depending on seasons and sunspots), in 7th turkish scientific and technical research association congress, tubitak, i.zmir, turkey, 387-392. claerbout, j.f. (1985): imaging the earth’s interior (blackwell, oxford, england), 67-75. dungey, j.w. (1961), interplanetary magnetic field and auroral zones, phys. rev. lett., 6, 47-48. förster, m. and n. jakowski (2000): geomagnetic storm effects on the topside ionosphere and plasmasphere: a compact tutorial and new results, surv. geophys., 21, 47-87. fo x, m.w and l.f. mc n a m a r a (1988): improved worldwide maps of monthly median f 0 f 2 , j. atmos. terr. physics, 50, 1072-1086. hedin, a.e. (1988): atomic oxygen modeling in the upper thermosphere, planet. space sci., 36, 907-920. hewish, a. and p.j. duffet-smith (1987): a new method of forecasting geomagnetic activity and proton showers, planet. space sci., 35, 487-491. işikara, a.m. (1971): the solar and lunar influences on the earth’s magnetic field at istanbul, revue de la faculté des sciences de l’universite d’istanbul, geophysique, ser. c, 36, 117-127. kamide,y. and a.d. richmond (1986): recent advances in studies of magnetosphere ionosphere coupling, j. geomagn. geoelectr., 38, 653-714. matsushita, s. (1959): a study of the morphology of ionospheric storms, j. geophys. res., 64, 305-321. mc ilwain, c.e. (1961): coordinates for mapping the distribution of magnetically trapped particles, j. geophys. res., 66, 3681-3691. mendillo, m. and j.a. klobuchar (1975): investigations of the ionospheric f-region using multistation tec observations, j. geophys. res., 80, 643-650. press, w.h., b.p. flannery, s.a. teukolsky and w.t. wetterling (1988): numerical recipes, the art of scientific computing (cambridge university press), 420-429. prölss, g.w. (1987): storm induced changes in the thermosphere composition at middle latitudes, planet. space sci., 35, 807-811. rees, d., t.j. fuller-rowell and h. risbeth (1988): the use of mass spectrometer measurements to derive thermospheric temperatures and density, planet. space sci., 36, 281-290. reiff, p.h., t.w. hill and j.l. burch (1977): solar wind plasma injection at the dayside magnetospheric cusp, j. geophys. res., 82, 479-491. tsurutani, b.t., w.d. gonzales, f. tang, s.-i. akasofu and e.j. smith (1988): origin of interplanetary southward magnetic fields responsible for major magnetic storms near solar maximum (1978-1979), j. geophys. res., 99, 8519-8531. uyar, m.o. (1964): magnetic storms, republic of turkey, ministry of national education, magnetic service of kandilli observatory istanbul, 1-5. (received april 8, 2002; accepted october 11, 2002) akiyama 791_798.pdf annals of geophysics, vol. 45, n. 6, december 2002 791 collapse modes of structures under strong motions of earthquake hiroshi akiyama real estate science, school of science and technology, nihon university, kanda-surugadai, chiyoda-ku, tokyo, japan 1. objective in the hyogoken-nanbu earthquake, 1995, various types of failure modes and collapse modes were disclosed not only in reinforced concrete structures but also in steel structures. among these modes of failure and collapse, the most fundamental and common cause for the total collapse of structure is the impairment of the strength to support the gravity loading due to the seismic loading. in this paper, the process to the final collapse is described and the effective measure to eliminate the final cause of collapse is made clear. mainly, the collapse modes of the multi-storied frames under shear deformations are discussed. 2. loading effect under earthquakes, structures are subjected to a combined effect of gravity loading and seismic loading. gravity loading has a definite effect expressed by the force proportional to the mass under the field of constant acceleration of g. thus, a structure is characterized by a potential energy proportional to the height of the structure. the ground motions under an earthquake are divided into horizontal and vertical. since the framed structure is primarily designed against gravitational loading with a sufficiently large margin of safety, the loading effect due to the vertical seismic ground motion is of minor importance. therefore, only the loading effect due to the horizontal ground motion is discussed in the paper. the loading effect of a seismic ground motion can be grasped as an energy input. the total mailing address: dr. hiroshi akiyama, real estate science, school of science and technology, nihon university, kanda-surugadai, chiyoda-ku, tokyo, japan. abstract under strong motion earthquakes, structures receive various types of damage. the most fatal damage is the loss of lateral strengths of a structure. the loss of lateral resistance causes a total collapse of the structure due to the p−δ effect associated with the lateral displacements and the gravity loading. to eliminate such a collapse mode, to introduce into the ordinary stiff structure a flexible element which remains elastic is very effective. the flexiblestiff mixed structure can behave preferably in many aspects under strong earthquakes. key words collapse mode − damage concentration − p − δ effect − flexible-stiff mixed structure 792 hiroshi akiyama energy input into a structure by an earthquake is a stable amount influenced by the total mass and the fundamental natural period and is scarcely influenced by the other structural parameters such as the strength distribution, the mass distribution and the stiffness distribution (akiyama, 1985). the balance of energy is described as follows: wr = we + wp = e (2.1) where wr is the resistance of a structure in energy absorption; we is the elastic vibrational energy in a structure; wp is the cumulative inelastic strain energy in a structure, and e is the total energy input by an earthquake. we + wp forms the absorbed energy by a structure. under the horizontal ground motion, the structure develops horizontal displacements. horizontal displacements of structure and the gravity loading form an additional effect of socalled «p − δ effect». the p − δ effect corresponds to the release of potentional energy in the gravitational acceleration field, and causes the reduction of the horizontal resistance of the structure in strength. considering the p − δ effect, the total resistance of a structure against a horizontal ground excitation in terms of energy, twr is described as twr = we + wp − wpδ (2.2) where wpδ is the release of potential energy by the p − δ effect. the total energy input can be expressed by more understandable quantities such as the equivalent velocity and the equivalent height in the gravitational field defined as follows: (2.3) (2.4) where m is the total mass of the structure; v e is the equivalent velocity, and heq is the equivalent height in the gravitational field. even under the strongest level earthquakes which occurred in japan, it is very rare for v e to exceed 300 cm/s. heq for ve = 300 cm/s becomes 46 cm. when one story of a multi-story frame fails to support the gravitational force, the story collapses and the potential energy stored in the upper stories beyond the collapsed story is released. the equivalent height of the released energy is the story height, h. taking an ordinary value of h to be 400 cm, the seismic energy input which corresponds to heq = 46 cm is realized to be far modest compared to the released potential energy in the event of story collapse. 3. collapse modes under earthquakes collapse of a structure starts at the weakest point of the structure. in cases influenced by the p − δ effect, the energy balance shown by eq. (2.1) is modified as w r = w e + r k w pk = e + w pδ (3.1) where wr is the resistance of a structure in inelastic energy absorption; wpk is the resistance of the weakest story in inelastic energy absorption; k is the number of the story which yields the minimum value of r k , and rk = wp /wpk is the damage concentration factor. the damage concentration factor takes unity when the sheer damage concentration takes place in the k th story, generally ranging 1.0 ≤ rk ≤ erk (3.2) where e rk is the energy distribution when the structure remains elastic. the value of rk is influenced by the structural type and the strength distribution. in weakcolumn type, rk becomes smaller compared to the case of weak-beam type. the strength distribution is normalized by dividing with the strength distribution in the elastic case. rk tends to take the smallest value at the weakest story in the normalized strength distribution. collapse occurs when wr can not reach e + wpδ . the energy absorption capacity wpk is determined by the mechanical properties of structure including material properties. e mv v e m e e = = 2 2 2 ( ) e mgh h e mg v geq eq e= = =( ) 2 2 793 collapse modes of structures under strong motions of earthquake in the reinforced concrete structures, an adequate reinforcement and yielding of reinforcing steel bars ensure the energy absorption capacity. crush of concrete and buckling of reinforcing bars limit the capacity. in particular, shear failure accompanied by crush of concrete drastically impedes the development of inelastic deformation. in steel structures, a high energy absorption capacity can be expected as far as the inherently ductile material properties are adequately developed. various modes of buckling impair the energy absorption capacity. another important mode of failure is brittle fracture of steel members which was widely disclosed in the hyogoken-nanbu earthquake, 1995 and the northridge earthquake, 1994. as a result of the intensive researches (nakashima et al., 1998; akiyama, 2000; roeder, 2000), the effective measures to eliminate the brittle fracture were made clear in the following directions. 1) to exclude the material which indicates low toughness against brittle structure. 2) to apply minute structural details to reduce stress concentration. the final cause for collapse is the p − δ effect. the p−δ effect appears as a release of energy by the amount of wpδ in eq. (3.1). and also, the p − δ effect prompts the damage concentration in the multistory frames, making rk smaller. in fig. 1, typical examples of story collapse found in the hyogokennanbu (kobe) earthquake (1995) are shown. these buildings are reinforced concrete frames. fig. 1. story collapse of reinforced concrete frames in kobe earthquake. 794 hiroshi akiyama in the weakest intermediate story, the energy absorption capacity of columns expired. after the energy balance was lost the potential energy was released, resulting in the story collapse. stories other than the collapsed story were equipped with sufficient strength to resist the impulsive forces associated with the release of potential energy, and thus, escaped collapse. when stories other than the initially collapsed story cannot resist the impulsive forces, a successive collapse of other stories can take place, leading to the total release of potential energy, and the total collapse of the entire frame. 4. p effect referring to fig. 2, the p effect in the relationship between the story shear force and the story drift in the multi-storied structure is expressed by a negative spring effect as shown in the next equation (4.1) where w is the total weight which rests on the story; qp is the decrease of shear force resistance due to the p effect; is the story drift; h is the height of story; and kp = w/ is the spring constant characterized by the p effect. the work done by this additional shear force, qp until the maximum deformation m is reached, is written as (4.2) where w is the work done by the p effect. under the maximum story deformation of m, the weight which rests on the story moves downward by as shown in fig. 1. the downward displacement is written as (4.3) the work done by the p effect is just equal to the product of and w. the influence of the p effect on the seismic response of structures can be most explicitly stated in terms of the energy input. through numerical analyses, it was found that the increase in energy input due to the p effect can be related to kp as follows (akiyama, 1984): (4.4) where k 1 is the spring constant of the first story in the elastic range, and kp 1 is kp in the first story. thus, observing the first story, eq. (3.1) is rewritten as (4.5) q w h kp p= = w r w e k k e p p+ = +1 1 1 1 1 10 ( ) . fig. 2. p effect. w w h m= 2 2 = m h 2 2 . w k e kp p= 10 1 1 795 collapse modes of structures under strong motions of earthquake gravity loading and keeping elasticity within the range of the maximum drift anticipated under the seismic loading. such a compound structure consisting of the flexible element and stiff element is recognized to be the flexible-stiff mixed structure (akiyama, 1998). structural performances in the flexible-stiff mixed structure are enhanced as the elasticity in the flexible element is increased. the intensity of elasticity of the flexible element is measured by rq defined by the following equation: (5.4) where rq is the elasticity intensity in the flexiblestiff mixed structure; s q y is the yield strength of the stiff element; f q m is the maximum shear force in the flexible element, and prefix f and s identify quantities of the flexible and stiff elements. the shear force-story deformation relationship of the flexible-stiff mixed structure is shown in fig. 3a-c. the shear force-story deformation relationship of the stiff element is assumed to be the elastic-perfectly plastic type. under the elastic-perfectly plastic q relationship as shown in fig. 4, the energy absorption due to plastic deformation is related to the cumulative plastic deformation as follows: w p = q y p = q y y (5.5) where q y is the yield shear force; p is the total cumulative plastic deformation; p + , p are cumulative plastic deformations in positive and negative loading domains, and = p / y is the cumulative plastic deformation ratio. on the other hand, the maximum deformation in the story, m is another important damage index. the non-dimensionallized maximum deformation is defined as (5.6) where µm is the plastic deformation ratio. when the story shear force resistance is totally nullified due to the combined action of structural collapse modes and the p effect, the static equilibrium of forces in the vertical direction is broken. then, the weakest story takes a path to the total collapse under the release of potential energy in the gravitational acceleration field. the released potential energy can make stories other than the weakest story successively collapse, thus inviting a progressive process to the total collapse of the structure. 5. cancel of gravity loading effect if it is possible to eliminate the p effect, the major cause of collapse of structure can be removed. the most effective measure to eliminate the p effect is to equip a structure with an elastic element which can cancel the negative spring effect shown by eq. (4.1). in the multistoried structure, the elastic element is required to satisfy the following conditions: (5.1) where is the original elastic spring constant of the frame; and m is the predicted maximum story drift under earthquakes. then, the substantial elastic spring constant, kf after cancelling the p effect becomes as k f = f k0 + kp . (5.2) the condition given by eq. (5.1) is same as k f 0 . (5.3) the structure must also be equipped with the energy absorbing element to resist earthquakes. the energy absorbing element must be equipped with high elastic rigidity and large inelastic deformation capacity. the elastic element is characterized by the relatively smaller elastic rigidity and can be categorized by «the flexible element». the energy absorbing element with relatively high elastic rigidity can be categorized by «the stiff element». the flexible element is designed under the condition of supporting the f p mk k w h 0 = r q q k q q f m s y f m s y = = f k0 p p p= + + µm m y y = 796 hiroshi akiyama the efficiency in plastic energy absorption can be measured by η / µm. the hysteretic loop which corresponds to η / µm is shown in fig. 5. when the flexible element does not exist, η / µm was found to be in the following range: (5.7) in this case, rq can be negative due to the p−δ effect. then, the deformation tends to develop one-sidedly. the smallest value of η /µm can be easily realized in the case where the p−δ effect becomes significantly large. as rq increases, η /µm increases and the following values are practicable in the design of the flexible-stiff mixed structure: (5.8) in fig. 5, neq denotes the number of hysteretic loop with the plastic deformation amplitude of µ m δ y . the advantage of the flexible-stiff mixed structure is found in cancellation of the p − δ effect, unification of damage distribution and minimization of drift under a certain amount of energy absorption. these preferable performances are secured as far as the following condition is satisfied: rq ≥ 1.0 . (5.9) the p − δ effect is intensified as the number of story increases. therefore, the importance of introducing the flexible-stiff mixed structure becomes clear in high-rise buildings. the efficiency of the flexible-stiff mixed structures is expressed by the largeness of µ m and η . in order to make µ m large, two methods are effective as shown in eq. (5.6): 1 0 4 0. .≤ ≤η µm . η µm = 8 0 12 0. . . to fig. 4. elastic-perfectly plastic type of q − δ relationship. fig. 3a-c. a) flexible element; b) stiff element; c) mixed system. a b c 797 collapse modes of structures under strong motions of earthquake 1) to maximize δm (maximum elastic deformation). 2) to minimize sδy (elastic limit deformation of the stiff element). in particular, to minimize sδy is a promising approach, resulting also in the reduction of δm. fig. 5. equivalent hysteretic loop associated with η /µm . now in japan, high-rise buildings with high performance are being pursued in a structural type which consists of main structural skeletons remaining elastic and additionally equipped energy absorbing elements with small elastic limit deformations. 798 hiroshi akiyama 6. conclusions structures can withstand earthquakes by absorbing the total energy input, as far as the release of the potential energy in the gravitational acceleration field is prevented. since the total energy input exerted by an earthquake is a very stable amount, the damage concentration occurs inevitably and accelerates the collapse of structures. there can be various modes of collapse inherent to individual structures. when the structural collapse modes are combined with the p − δ effect, the structure loses its lateral strength and the total collapse can occur. the essential measure to eliminate the total collapse is taken by cancelling the p − δ effect by introducing the elastic element. the generalized structural form equipped with the elastic element is identified to be the flexible-stiff mixed structure and can develop higher performances under earthquakes. references akiyama, h. (1984): p−δ effect of energy absorption capacity of steel framed structures subjected to earthquakes, trans. a.i.j., no. 340, 11-16. akiyama, h. (1985): earthquake-resistant limit-state design for buildings (university of tokyo press), 1-372. akiyama, h. (1998): a prospect for future earthquake resistant design, eng. struct., 20 (4-6), 447-451. akiyama, h. (2000): evaluation of fractural mode of failure in steel structures following kobe lessons, j. constr. steel res., 55, 211-227. nakashima, m., k. suita, k. morisako and y. maruoka (1998): tests of welded beam-column subassemblies, j. struct. eng., 124 (11), 1245-1252. roeder, c.w. (2000): sac program to assure ductile connection performance, behaviour of steel structures in seismic areas, in proceedings of the third international conference stessa 2000, 659-666. adg vol5 n02 te+krz 273_278.pdf annals of geophysics, vol. 45, n. 2, april 2002 273 anomalous piezoelectric effects found in the laboratory and reconstructed by numerical simulation krzysztof p. teisseyre institute of geophysics, polish academy of science, warszawa, poland abstract various rocks and minerals, which are not piezoelectric in the common sense, exhibit transient electric polarization in response to sudden changes in stress load. this anomalous piezoelectric effect differs from the regular, static piezoelectric response, in which electric charges appear as a result of crystal lattice deformation. the anomalous piezoelectricity is dynamic decaying in a few seconds or a few tens of seconds. however, in some materials different polarization properties are discovered. to explain certain aspects of the polarization signal increase and decay, some complicated mechanisms of electric charge generation and relaxation need to be assumed in their number – concurrence of two or three relaxation processes. the hypothetical mechanisms are only mentioned, as the purpose of this work is to construct numerical models, behaving like the rocks investigated. examples of experimental plots are shown together with the results of the numerical simulation of these experiments. 1. laboratory experiments with rock samples subjected to a change in load the laboratory research was done by vassilios hadjicontis and claire mavromatou, in the department of physics, university of athens. samples of various rocks held initially under a certain steady load, were subjected to a quick increase in load, in some cases followed by a decrease after some time to almost the initial level. the charge detecting-recording system contains an electrode adjacent to the sample wall (at a distance of 5 mm), the load sensor positioned below the sample and a digital recording apparatus with oscilloscope and printer. either the plot of applied load and curve of electric signal versus time, or the plot of the load time-derivative and again the plot of the generated electric signal were seen on the screen. a detailed description of the experimental setup was published by hadjicontis and mavromatou (1995). these authors suggest that the obtained curves of electric signal are proportional to the time-derivative of load (pressure) acting on the sample. the present work checks this hypothesis by means of numerical analysis and suitable simulating algorithms. mailing address: dr. krzysztof p. teisseyre, institute of geophysics, polish academy of science, ksiȩcia janusza 64, 01-452 warszawa, poland; e-mail: kt@igf.edu.pl key words anomalous piezoelectric effect – transient electric polarization – electric properties of rocks 274 krzysztof p. teisseyre 2. modelling of the signal components generation and relaxation the time derivative of load was, for this modelling, read from experimental curves and written as the input data (for future simulations, use of data in digital form is planned). certain formulae for the simulations were developed and tested, corresponding to some models of electric signal generation and relaxation in the rock. the stimulus – actual value of time derivative of load (dp/dt), multiplied by proportionality coefficient k, contributes to the state of polarization at a given stage, and indirectly also at following s t a g e s . t h e i n d i r e c t e ff e c t d i m i n i s h e s exponentially with time. in other words, the polarization process may create at each stage some element of electric response and this element immediately starts to decay, according to exponential routine. therefore, the electrical signal at a given stage i results from the value of actual stimulus and the sum of previous stages influence where v is the generated electrical signal; k proportionality coefficients; l relaxation coefficient. in order to simulate the experimental results, some complications are added to this method. the most important is: the energy given by each stimulus (dp/dt) is divided into two or three parts: a quickly-decaying electric polarization process; a medium speed process and a long-relaxation one; the differences lie in the values of relaxation coefficient l. a multiple mechanism of relaxation is also proposed by varotsos et al. (2001), who explain the greek limestone and polish peridotite behaviour (in similar tests) by concurrence of three relaxation processes: one fast and two long relaxation ones. they also suggest that the deformation-induced c h a rg e f l ow m e c h a n i s m m i g h t b e t h e appropriate generation mechanism for the seismic electric signals. rock samples, which are at first sight very similar, respond in the experiments slightly differently. to reconstruct the course of their polarization and its decay, different coefficients are assumed. moreover, the same sample may react in various way to the rise of load and to decrease – partial removing of the force. this points to the micro-cracks and dislocation arrays mediation in the process of charge separation and migration. in some cases of simulation, the so-called «opposite components» are added to the signal. these are caused by stopping the load increase and to explain their existence, a kind of rebound processes within the sample is postulated. the decrease of direct response should be abrupt enough to act as a metastimulus on the material, that is to create a component of the electric signal bearing the opposite sign. such phenomena may be explained if one assumes the mediation of dislocations and micro-crack movements in the processes of charge separation and summation. only in a few cases does the opposite components pile give an effect of opposite bay-bay-like flexion of the signal curve below the initial level. one of such cases is included here. such defined opposite components depend on the proportionality coefficient k mentioned previously, and on the additional coefficient kop. although it may be expected that kop should not be greater than 1, for some cases, a greater value of this coefficient must be taken, in order to obtain similarity with the experimental results. opposite bays phenomena, as seen here in fig. 2, are hard to explain in another way. besides, at the time of response generation to any stimulus or meta-stimulus, this direct response is normalized in such a way that it cannot exceed a certain value, common for all considered cases. influence of this normalization is mild. it must be pointed out that physical values of stimulus, in dp/dt and the simulated signal in mv, are calculated ex post; simulation de facto consisted of transforming one synthetic plot into another. nevertheless, the physical values of k-coefficients were ex post calculated. v dp dt k i n l i n i = − − = ∑ ( ) ( ) 1 exp n 275 anomalous piezoelectric effect found in the laboratory and reconstructed by numerical simulation 3. three examples of experiment and simulation results here we compare the laboratory data and those obtained by numerical simulation; in presented figures the upper part shows the load acting on the sample (one division corresponds to 100 kg), below – the recorded electric signals and the load time-derivatives, the lower part represents simulation results. in the experimental diagrams, the horizontal axis shows time in seconds – one division on horizontal axis corresponds respectively in case (1) to 5 s, in other cases to 2 s. 1) volcanic tuff sample (fig. 1). two episodes of increase in the load; the second caused a new signal peak before the first vanished completely. initial load = 330 kg, maximal load ~ 800 kg. on the plot of the observed electric signal, one division in the vertical direction fig. 1. experiment with sample of volcanic tuff: the load, electric signal, load time-derivative and the simulation of electric signal curve. 276 krzysztof p. teisseyre corresponds to 50 mv. the calculated proportionality coefficient k is 7.2 ⋅ 10−7 mv s/pa; in the simulation: k = 0.51. proportion of quickly decreasing part to whole new signal component is at first 70%, of slowly decreasing part – 30%; respective l – coefficients are 8 and 320. 2) jurassic limestone from poland (fig. 2). four consecutive strikes were applied to the sample. initial load = 330 kg, maximum ~ 750 kg. on the plot of observed electric signal, one division in the vertical denotes 100 mv. the calculated proportionality coefficient k is 1 ⋅ 10−7 mv s/pa; in the simulation: k = 1.1; proportion of quickly decreasing part to new signal is at first 53%, for middle quick – 29%, remaining 18% starts the slowest, long-lasting part; coefficients l are 4, 10 and 50. additional proportionality coefficient for opposite components k op is very big: 2.43. 3) another experiment on the jurassic limestone from poland (fig. 3). episode of the load increase, stationary phase and the phase of fig. 2. experiment with sample of limestone: the load, electric signal, load time-derivative and the simulation of electric signal curve. 277 anomalous piezoelectric effect found in the laboratory and reconstructed by numerical simulation load decrease. initial load = 310 kg, maximal ~ 633 kg. on the plot of registered electric signal, one division denotes 20 mv. the calculated proportionality coefficient k is for load increase phase 2.35 ⋅ 10−6 mv s/pa, and for the phase of load decrease: 6.8 ⋅ 10−6 mv s/pa; in the simulation: k = 0.235 and for the load decrease k = 0.68. proportion of quickly decreasing part to whole new signal component is at first 82%, of middle one – 15%, slowest one has initially 3%. respective relaxation coefficients l are 4, 10 and 50. additional proportionality coefficient for opposite components kop is 0.13. 4. discussion we present some examples of the electric responses to stress load on rocks to emphasize their complex response mechanisms. the current theories of piezoelectricity or piezoelectric grafig. 3. another experiment with the limestone: the load, electric signal, load time-derivative and the simulation of electric signal curve. 278 krzysztof p. teisseyre dient, include usually only the direct response and, additionally, its relaxation. numerical simulations undertaken in the presented project show that the induced electric potential response to stress load has a complex structure and that its relaxation includes at least a slow and a rapid mechanism. for the volcanic tuff, a simple model appears to be satisfactory: a twin-component signal summation and relaxation. on the other hand, the response of some rock materials may be satisfactorily modelled only with the assumption of a larger number of polarization processes occurring simultaneously, each one decaying with appropriate schedule, depending on the l coefficient of the presented formula. to achieve a better agreement with the experiments on some rocks including limestone, a hypothesis of opposite components is formulated. in the presented case (2), the opposite components increase is ruled by a greater coefficient of proportionality k than that of the main component. probably some peculiar structure of the sample is responsible for this effect and it cannot be excluded that in other parts of the sample, a little further away from the electrode, the signal behaviour may be different. satisfactory agreement between experiment and its reconstruction was not obtained in some cases (see fig. 2) and this suggests that more components are in fact involved. it cannot be stated that all components necessary to explain the complex behaviour of some rocks are already discovered, experiments as well as modelling are underway. examples of more complex modelling have recently been published by teisseyre et al. (2001). transient electric polarization was also detected in various rocks subjected to high loads, close to brittle fracture (enomoto et al., 1994). in experiments conducted in athens such signals were also observed close to the sample fracturing (hadjicontis and mavromatou, personal communication). production and propagation of electric signals under variable load are important phenomena both in the physics of rocks and in the search for various detectable earthquake precursors (enomoto et al., 1994; varotsos et al., 2001). references enomoto, y., t. shimamoto, a. tsutsumi and h. hashimoto (1994): transient electric signals prior to rock fracturing: potential use as an earthquake precursor, in: electromagnetic phenomena related to earthquake prediction, edited by m. hayakawa and y. fujinawa (terrapub, tokyo), 253-259. hadjicontis, v. and c. mavromatou (1995): electric signals recorded during uniaxial compression of rock samples: their possible correlation with preseismic electric signals, acta geophysica polonica, 43 (1), 49-61. teisseyre, k.p., v. hadjicontis and c. mavromatou (2001): anomalous piezoelectric effect: analysis of experimental data and numerical simulation, acta geophysica polonica, 49 (4), 449-462. varotsos, p., v. hadjicontis and a.s. nowick (2001): the physical mechanism of seismic electric signals, acta geophysica polonica, 49 (4), 415-421. a dynamical study of frictional effect on scaling of earthquake source displacement spectra annals of geophysics, 59, 2, 2016, s0210; doi:10.4401/ag-6974 s0210 a dynamical study of frictional effect on scaling of earthquake source displacement spectra jeen-hwa wang institute of earth sciences, academia sinica, nangang, taipei, taiwan abstract the scaling of earthquake source displacement spectra is analytically studied based on the continuous form of one-dimensional dynamical spring-slider model in the presence of either linearly slip-weakening friction or linearly velocity-weakening friction. the main parameters of the model are the natural angular frequency, ~o, and the (dimensionless) decreasing rate, d, of friction with slip (or the characteristic displacement) for slip-weakening friction as well as the (dimensionless) decreasing rate, y, of friction with velocity (or the characteristic velocity) for velocity-weakening friction. the analytic solution includes the complementary and particular parts. the former shows the travelling wave and the latter denotes vibrations at a site. the complementary solution exhibits ~-1 scaling in the whole range of ~ for both friction laws. for the particular solution, slip-weakening friction results in spectral amplitudes only at three values of ~. for velocity-weakening friction with y> 0.5, the log-log plot of spectral amplitude versus ~ exhibits almost ~0 scaling when ~ is lower than the corner angular frequency, ~c, which is independent on y and increases with ~o. when ~>~c, the spectral amplitude monotonically decreases with ~ following a line with a slope value of −1, which is the scaling exponent. 1. introduction the body-wave seismic spectrum, p(~), where ~= 2rf is the angular frequency and f is the frequency, is controlled by the seismic moment mo and the corner angular frequency ~c, which is associated with the source dimension. the generally accepted earthquake source functions have either ~-2 or ~-3 high-frequency spectral decay, and are commonly referred to as ~-square and ~-cubic models [haskell 1966, aki 1967, brune 1970, aki 1972, kanamori and anderson 1975]. of course, some authors [cf. boatwright 1978, dysart et al. 1988, patanè et al. 1997] claimed that neither of them is appropriate for describing the observations. kim et al. [1997] considered the effect of seismic attenuation on the seismograms from which the source displacement spectrum is retrieved. they also suggested a method to separate the attenuation and source effects. huang and wang [2002] studied the scaling law of the displacement spectra from the seismograms recorded at nine near-fault stations generated by the 1999 chi-chi, taiwan, earthquake. results show that the values of corner frequency fc at the nine stations are almost 0.2 hz. the pattern of displacement spectrum at each station is similar to the theoretical one proposed by aki [1967]. the spectral amplitude is almost constant (or ~0 scaling) when f < 0.2 hz and decays at a higher decreasing rate when f > 3 hz. in the frequency range of 0.2 3 hz, p(~)~~-b, b varies from 1.63 to 3.04 at the nine nearfault stations, and decreases from north to south. such a variation might be mainly due to the source effect. results seem also to suggest that a ~-square model is appropriate for the single-degree-of-freedom rupture processes on the southern fault plane, and a ~-cubic model for the two-degree-of-freedom ones on the northern fault plane. scaling of spectral amplitude with ~ is generally specified with a form of ~-2 or ~-3. this shows a type of f -c signal. the f -2 signal is considered to be a result of the brownian motions. bak et al. [1987, 1988] proposed self-organized criticality to explain f -c signal. frankel [1991] assumed that the high frequency energy of an earthquake is produced by a self-similar distribution of subevents. the number of subevents with radii greater than r is proportional to r -d, d being the fractal dimension. in his model, an earthquake is composed of a hierarchical set of smaller events. the static stress drop is parameterized to rn, and strength is assumed to be proportional to static stress drop. he found that a distribution of subevents with d = 2 and stress drop independent of seismic moment (n = 0) produces an earthquake with an ~-2 falloff, if the areas of subevents fill the rupture area of the earthquake. based on an ideal system under external random forces, koyama and hara [1993] article history received january 26, 2016; accepted april 8, 2016. subject classification: source spectrum, spring-slider model, weakening friction, natural frequency. studied the dynamical process of random activation. they applied the langevin equation to represent the time evolution of the system and took a scaling rule (represented by an auto-correlation function) to describe the random activation for generalizing the system. their model predicts the fractional power spectrum f -c from a white spectrum to a lorentzian. the exponent c is a function of the fractal dimension of the scaling rule. the fractal dimensions of 2, 1, and of about 0.47 indicate a lorentz spectrum, a f -1 spectrum, and a power spectrum of f -1.53 type, respectively. herrero and bernard [1994] and bernard et al. [1996] proposed the lsquare model to approach the theoretical ~-square model. hisada [2000] proposed a model modified from the l-square model proposed by bernard et al. [1996] under three assumptions: (1) the spatial wave-number spectrum of the slip distribution falls off as the inverse of the wave-number square; (2) a kostrov-type slip velocity model is included; and (3) the incoherent rupture time is introduced to model variable rupture velocities. he claimed that his model can produce the ~-square source displacement spectrum. aki [1967] described the scaling of earthquake source displacement spectra using the dislocation model based on empirical assumptions about some model parameters. although this approach is good enough, it is still significant to study such scaling based on a dynamical model. up to date, only shaw [1993] studied this problem using a modified version of the one-dimensional dynamical spring-slider mode (abbreviated by the 1-d bk model hereafter) proposed by burridge and knopoff [1967]. meanwhile, friction controls the rupture processes of an earthquake [e.g., nur 1978, knopoff et al. 1992, rice 1993, shaw 1993, wang 1996, 1997, 2000, 2004, 2006a, 2007, 2008, 2009, 2012, 2016]. shaw [1993] considered velocity-weakening friction, as shown below, suggested by carlson and langer [1989]. he took a homogeneous spatial distribution of static frictional force in his study. his results show a difference in spectra, p(~), between large events and small ones. for large events with mo> moc, where mo is the seismic moment and moc is the characteristic seismic moment, there are different power-law relations in three angular-frequency regions: p(~)~~0 for ~ < 2r/mo; p(~)~~-1 for 2r/mo< ~ <2r/p; and p(~)~~ -c for ~> 2r/p, where p= 2ln(4l2/v)/a. the definitions of a and v can be seen in shaw [1993]. when v is small and a> 1, the exponent c has almost a value of 2.5. obviously, there are two turning points in the source spectrum for large events. at low ~, the theoretical result is similar to that proposed by aki [1967]. at medium ~, the theoretical power spectrum shows the so-called 1/f noise [cf. bak et al. 1987]. at high ~, the theoretical result is somewhat between ~-2 and ~-3 models, because c is about 2.5. this is somewhat different from that proposed by aki [1967]. for large events, shaw’s theoretical source spectra are more complicated than aki’s. for small events with mo< moc, there are two power-law relations in two angular-frequency ranges: p(~)~~0 for ~ < 2r/l and p(~)~~-2 for ~ > 2r/l, where l is the rupture length of an event. there is only a turning point, which is dependent upon l. obviously, shaw’s theoretical result for small events is similar to aki’s ~-square model. shaw [1993] only used a velocity-weakening friction law for numerical simulations. it seems necessary to explore the effect on the source displacement spectrum due to slip-weakening friction. in this work, i will focus on the analytical study of frictional effect on scaling of earthquake source displacement spectra using the continuous form of the 1-d bk model in the presence of two types of friction, i.e., slip-weakening friction and velocity-weakening friction. 2. one-dimensional spring-slider model the 1-d bk model (see figure 1) consists of n sliders of equal mass, m, and springs with one slider being linked by a coil spring of strength, kc, with the other. wang 2 figure 1. an n-degree-of-freedom one-dimensional dynamical spring-slider system. 3 each slider is also pulled by a leaf spring of strength, kl, on a moving plate with a constant velocity, vp. at time t = 0, all the sliders rest in the individual equilibrium states. the i-th slider (i = 1, …, n) is located at position xi, measured from its initial equilibrium position, along the horizontal axis. each slider is subjected to a slipand/or velocity-dependent frictional force. the frictional force is denoted by fi (xi; vi), where vi=∂xi/∂t, with a static frictional force, fsi, at rest. elastic strain of each slider gradually accumulates due to the moving plate. once the elastic force at a slider is greater than fsi, the static frictional force drops to the dynamic frictional force, fdi, and the slider will move subject to fdi. the equation of motion is (1) wang [1995] defined the ratio l= kc/kl to be the stiffness ratio of the system. it is an important parameter representing the level of conservation of energy in the system. larger (smaller) l shows stronger (weaker) coupling between two sliders than between a slider and the moving plate. this results in a smaller (bigger) loss of energy through the kl spring, thus indicating a higher (lower) level of conservation of energy in the system. since the fault system is a dynamically coupling one with dissipation, l must be a non-zero finite value. the plate velocity vp is usually small and in the order of ~10 -12 m/s. it is noted that the 1-d bk model can only generate a longitudinalwave-type rupture [wang 1996]. however, the result obtained from this model is still significant for understanding the p-wave-type source scaling. in crack mechanics, the load is applied remotely on a 3-d extended fault. for the present model, the load is directly linked to each slider. as mentioned below, when the driving force is higher than static frictional force, only the earthquake rupture is taken into account and the driving force and static frictional force will be cancelled out each other and can be ignored. hence, only a single instability is considered and therefore the problem can be treated easily. the present advantage is that the closed-form analytical solutions can be obtained. 3. friction velocity-dependent friction and slip-dependent friction the frictional force between two contact planes is classically considered to drop from static one to dynamic one after the two planes move relatively. in fact, friction is a very complicated physical process. from laboratory experiments, dieterich [1972] first found time-dependent static frictional strength of rocks in laboratory experiments. dieterich [1979] and shimamoto [1986] observed velocity-dependent dynamic frictional strengths. dieterich [1979] and ruina [1983] proposed empirical, velocityand state-dependent friction laws. in fact, a large debate related to the friction laws governing earthquake ruptures has been made for a long time. although this problem is important, it is out of the scope of this article and thus will not be described in details. a detailed description of the generalized velocityand state-dependent friction law and the debates can be found in several articles [e.g., marone 1998, wang 2002, bizzarri and cocco 2006c, wang 2009, bizzari 2011]. for theoretical analyses and numerical simulations of earthquake ruptures, several simple friction laws have been taken into account. burridge and knopoff [1967] first considered a velocity-dependent, weakening-hardening friction law. carlson and langer [1989] and carlson et al. [1991] considered a velocity-weakening friction law: f(v) = fo/(1 + v/vc), where fo is the static frictional force and vc is a characteristic speed to specify the variation in f with velocity. f(v) is fo at v = 0 and decreases monotonically from fo to zero as |v| becomes large. several authors [e.g., carlson and langer 1989, carlson 1991, carlson et al. 1991, beeler et al. 2008] theoretically modeled earthquakes by using this velocity-weakening friction law. cochard and madariaga [1994] and madariaga and cochard [1994] assumed that purely velocity-dependent friction models can lead to unphysical phenomena or mathematically ill-posed problems. hence, those models are very unstable at low values of the fault slip velocity both during the passage of the rupture front and during the possible slip arrest phase. moreover, ohnaka [2000] stressed that purely velocity-dependent friction is in contrast with laboratory evidence that is the friction law is not a one-valued function of velocity. bizzarri [2011a,b] deeply discussed this point. nevertheless, in order to obtain a closed-form solution the single-valued velocity-dependent friction law is taken into account. the present study can be regarded as a marginal analysis of the effect of velocity-dependent friction on the scaling of source displacement spectra of earthquakes. analytic results will help us to understand such an effect. for the first-order approximation, wang [1995, 1996] considered a piece-wise, linearly velocity-dependent weakening-hardening friction which is simplified from the friction law proposed by burridge and knopoff [1967]. the decreasing (weakening) and increasing (hardening) rates of dynamic friction strength with sliding velocity are two main parameters of this friction law. the two rates are denoted, respectively, by rw and rh. the function f(v) is defined only for v ≥ 0 and f(v) is a negative infinity when v < 0. this means that ; . m x x x x x x t k kl vp t fi vi 2i i i i i i vi c kl vp fi 2 2 1 12 2 = + + -r q q q w v v v a dynamical study of earthquake spectra no backward motion is allowed. when v = vc, f(v) reaches the minimum value, i.e., gfo (0<g <1). the smaller g is, the larger the force drop is. thus, smaller g is more capable of generating a larger event than larger g. a drop in the frictional force from fo to gfo behaves like an extra source supplying additional energy to a slider for proceeding sliding. the velocity hardening stage is a fast re-strengthening process associated with some healing of slip. cao and aki [1984/85] took a displacement softening-hardening friction law: f(d) = foexp [−(d−do) 2/c2], where d is the fault slip and fo, do, and c are constants. cao and aki [1986] applied rateand state-dependent friction to controlling the motion of a slider. from a comparison between a slip-dependent friction law and a velocitydependent one through numerical computations, they concluded that the two kinds of friction laws cause different effects on simulation results. friction can also be produced from thermal pressurization [cf. fialko 2004, bizzari and cocco 2006a,b, rice 2006, wang 2009, wang 2013]. rice [2006] proposed two end-members models for thermal pressurization: the adiabatic-undrained-deformation (aud) model and slip-on-a-plane (sop) model. he also obtained the shear stress-slip functions, x(d), caused by thermal pressurization: xaud(d) = f (vn− po)exp(−d/dc), where dc=tch/fk and t, c, h, f, and k are, respectively, the density, heat capacity, thickness, friction coefficient, and undrained pressurization factor of the slip zone, for the aud model; and xsop(d) = f (vn− po) exp (d/l *) erfc (d/l*)1/2, where erfc(z) denotes the complementary error function and l*= 4(tc/k) 2(at 1/2 +ah 1/2)2/f 2v, where at, ah, and v are, respectively, the thermal diffusivity, hydraulic diffusivity, and slip rate of the fault zone, for the sop model. the two parameters dc and l * control the shear stress-slip functions of the individual endmember models. clearly, the two friction laws exhibit exponentially slip-weakening friction for the aud model and slip-weakening friction for the sop model. static frictional force and breaking strengths the distribution of the static frictional force (or the breaking strengths) affects dynamical behavior of a fault system. carlson and her co-workers used an almost uniform distribution of the breaking strengths in their studies. thus, de-localized events, for which all sliders of the model are in an unstable state, can be easily generated in their simulations. nussbaum and ruina [1987] claimed that such a homogeneous fault stress is generally unstable. it is known that the fault zones related to earthquakes are usually quite complicated. seismological and geological observations show that the mechanical properties and geometry of a fault zone are heterogeneous. for example, the breaking strengths on the chelungpu fault, along which the 1999 ms 7.6, chi-chi, taiwan, earthquake ruptured, are heterogeneous [see wang 2006a,b]. the breaking strength is the main mechanical parameter representing the state of stress over the fault zone for generating ruptures and thus influences seismicity and its scaling laws. two phenomenological models have been long accepted to describe the heterogeneous fault strengths: one is the barrier model proposed by das and aki [1977] and aki [1979] and the other the asperity model suggested by kanamori and stewart [1978]. 4. analytical studies and discussion in this study, i will analytically study the conditions of generating scaling of source displacement spectra of earthquakes utilizing the 1-d bk model in the presence of either velocityor slip-weakening friction. as mentioned above, the friction law is quite complicated. in order to perform analytic studies, i take the simplified linear friction laws: f(v) = fo(1−v/vc) (v = dx/dt = the velocity and vc= the characteristic velocity) for velocity-weakening friction and f(x) = fo(1−x/xc) (x = the slip and xc= the characteristic slip distance) for slipweakening friction. the former and the latter are almost the approximations, respectively, of the friction law used by carlson and langer [1989] at low velocities and of that used by cao and aki [1986] or those proposed by rice [2006] at small displacements. it is obvious that fo/vc is the weakening rate with velocity and fo/xc is the weakening rate with slip. hence, in some sense the marginal analyses of generalized friction laws are made in this study. the characteristic velocity and characteristic slip distance are also the slopes of the respective laws, thus representing the respective decreasing rates. a uniform distribution of the breaking strengths is considered in this study. this means that only steady travelling waves are taken into account. letting xi = xi−vpt leads to vi= dxi/dt = dxi/dt − vp= vi+vp, and thus xi= xi+vpt. this makes equation (1) become (2) when a slider slips, vpt and vp are, respectively, much smaller than xi and vi, and thus can be ignored. thus, equation (2) becomes (3) obviously, vp is no longer a parameter of the equation of motion. ; . m x t k x x x kl x fi x vp t v vp 2i c i 1 i i 1 kl i fi i vp i vp 2 22 2 = + + + + -r q qw v v ; .m x t k x x x kl x fi x v2i c i 1 i i 1 kl i fi i i 2 22 2 = + -+ -r q qw v v wang 4 5 dividing equation (3) by a unit area, i.e., dydz, where y and z denote the coordinates, respectively, along and normal to the 1-d system, leads to (4) defining ta= m/dydz, kc= kc/dydz, kl = kl/dydz, and fi = fi/dydz, equation (4) becomes (5) to simplify the mathematical manipulation, the continuous form will be normalized on the basis of some normalization factors. now, the stiffness ratio becomes l = kc/kl. let fi(xi;vi) = foz(xi;vi), and define do= fo/kl(=fo/kl) to be the characteristic slip distance of a slider exerted by a force fo through a spring with strength of kl. larger fo yields longer do when kl is fixed. define ~o= (kl/ta) 1/2 = (kl/m) 1/2 and x = ~ot. the quantity ~o/2r is the predominant frequency of oscillation of a single slider attached to a leaf spring in the absence of friction and thus a characteristic parameter of the model. this gives to= 2r/~o to be the natural period of oscillation. the ratio do/vp is the loading time for a leaf spring to stretch enough for overcoming the static frictional force, and thus vp/do~o is equivalent to the ratio of the slipping time ~o -1 to the loading time. obviously, do and ~o are two basic units to scale, respectively, the spatial coordinates, xi, and time, t. defining ui = xi/do and vi = dui/dt and substituting the above-mentioned normalized factors into equation (5) lead to (6) to further perform the problem easily, like carlson and langer [1989] the equivalent differential equation, i.e., the continuous form, is made from equation (6). the difference terms in the right side of equation (6) are divided by the square of the unit length, a, between two sliders in the equilibrium state and thus equation (6) becomes (7) it is noted that the length ‘a’ is not an explicit one of the original equation. the limits of la2, ui, and vi, are represented, respectively, by h2, u, and v as the value of ‘a’ approaches zero. hence, equation (7) becomes (8) where p is the normalized horizontal coordinate of the model. the trial solution of equation (8) is u = −z(u;v)≈constant when ∂2u/∂x2 = 0 and ∂2u/∂p2 = 0. this solution means that all sliders are moving uniformly. consider to add a small perturbation to u (p,x), that is, u (p,x) = −z(u;v) + uoexp (iqp + xx), where uo is a very small quantity, q is the wave-number, and x = ~/~o is the dimensionless angular frequency. this gives ∂u/∂x = xuoexp(iqp+xx), ∂ 2u/∂x2 = x2uoexp(iqp+xx), and ∂2u/∂p2 = −q2uoexp(iqp+xx). substituting those quantities into equation (8) leads to x2uoexp(iqp+xx) = −h 2q2 uoexp(iqp + xx) − [−z(u;v) +uoexp(iqp + xx)]−z(u;v) . this gives x2uo= −h 2q2uo− uo or (x 2+ h2q2+1) uo= 0. for non-null uo, we have (x 2+ h2q2 +1) = 0. this gives x = i (h2q2 +1)1/2. since h2q2 +1 > 0, x is purely imaginary. hence, the solution is u(p,x)=−z(u;v) +uoexp[i(qp+xx)]. this shows that any small perturbation added to a slider, no matter how long its wavelength is, does not diverge when u = −z(u;v)≈constant. slip-weakening friction as mentioned above, the simplified slip-weakening friction law is z(u)=1−u/(xc/do). define d = xc/do, we have z(u)=1−u/d, where d is the dimensionless characteristic slip distance and also the dimensionless decreasing rate of friction with slip. this makes equation (8) become (9) in order to solve equation (9), we take the laplace transformation (lt, denoted by l), which is described in appendix a. according to appendix a, the lt of equation (9) is (10) this gives (11) to solve equation (11), let u = uc+ up where uc and up are, respectively, the complementary and particular solutions. let } = (s2 + 1 − d-1)1/2. according to the method shown in appendix b, the general solution of equation (11) is (12) where uc= c1e -}p/h + c2e }p/h and up=−1/s} 2. equation (11) consists of two kinds of waves: the first . m yd z yd z x x kl yd z x yd z x t k x fi 2 i i i i c i kl fi 2 2 1 12 2yd d yd d yd d yd d = + + -q r r r qv w w w v .x x x k xt k x fi2i i i i fic i la 2 2 1 12 2t -= + -+ -q v ; .u u u u uu v2i i i i ii i 2 2 1 12 2x l z= + -+ -q qv v ; .u a u u u a u u v2i i i i 1 i i i 2 2 2 1 22 2x l z= + --+q qv v" % ; ,vu u u u vh2 2 2 222 2 2 2x p z= -r qw ,v .u h u u u12 2 2 2 22 2 2 2x p d= -r qw v , , , ,/ . s u s u s u s u s h s1 2 2 2 22 2p p p p p d = + q q q q v v v v / .h su s u 112 2 2 2 12 2p d+ + =-q v , c ec e / ,u s s1hh 21 2 p }= + -}p }p-q v a dynamical study of earthquake spectra one is the travelling wave along the model represented by the first and second terms in its right-handed-side and the second one is the displacement at a site given by the third term. the first and second terms represent the waves travelling along the direction of increasing p and that of decreasing p, respectively. the second term with p< 0 can indeed be re-written as e-}|p|/h. from equation (a2) in appendix a, l-1[e-}|p|/h] with |p|/h > 0 is (13) where c = c1 or c2, c= (1 −d -1)1/2 and h (x−|p|/h ) is the unit step function (h (x) = 0 as x< 0 and h (x) = 1 as x≥ 0) representing a travelling plane wave. the two parameters x =~ot and p= x/do are, respectively, the normalized time and normalized (dimensionless) rupture distance, hence, h is the normalized (dimensionless) rupture velocity and equal to vr/do~o, where vr is the rupture velocity. the arrival time of rupture wave at p or x is xa=|p|/h or ta= x/vr. when the rupture propagates from 0 to pl, which is the normalized (dimensionless) total rupture length and equal to l/do (l = the total rupture length), the normalized total duration time is xd= pl/h, and thus the total duration time is td= xd/~o= pl/~oh, thus giving td= pl/~oh = (l/do)/~o(vr/do~o) = l/vr. based on the above-mentioned parameters, equation (13) can be transferred to (14) hence, the ft of the first part of equation (14), i.e., xc1(y,t) = cdoh (~ot−|y|/doh), is xc1(y,~)= f [xc1(y,t)]=f [cdoh(~ot−|y|/doh)]=cdo[rd(~)− (i/~)exp(|y|/doh)]. when ~ > 0, the spectrum shows ~-1 scaling. the ft of the second part of equation (14), i.e., xc2(y,t) = −cdoc~otaj1[c~o(t 2 − ta 2)1/2]/(t2 − ta 2)1/2} h[~o (t − ta)], is xc2(y,~)=f [xc2(y,t)]=−cdoc~ota∫{ j1[c~o (t2 − ta 2)1/2]/(t2 − ta 2)1/2} h [~o(t − ta)]e -i~tdt, with an integral range from −∞ to +∞. at each site, the rupture occurs just between ta and tr= ta+tr where tr is the rise time of rupture. when t >tr. the rupture stops. we are only interested on the spectrum at a site and thus different values of ta at different sites just lead to phase changes and cannot influence the spectral amplitudes when the wave attenuation is ignored. hence, the value of ta inside the integral can be set to be zero to simplify the problem. this gives xc2(y,~)=−cdoc~ota∫[j1(c~ot)/t]e -i~tdt, with an integral range from 0 to tr. since f [j1(t)/t] = [2(1 − ~2)/r]1/2rect(~/2c~o), where rect(~) is the rectangular function: rect(~) = 1 for |~|< 1/2, 1/2 for |~|= 1/2, and 0 for |~|>1/2, we have f [j1(c~ot)/t]= {2[1−(~/ c~o) 2]/r}1/2rect(~/2c~o). clearly, the fourier transform is equal to 1 only when |~|<c~o. this means that the fourier amplitudes of xc2(y,t) exist only when |~|<c~o. hence, the complementary solution cannot lead to ~-n source scaling at high frequencies. the third term of equation (12), i.e., up(p,s)= −1/2s (s2 + 1 −d-1), represents source behavior of slip at a site. according to appendix a, up(p,x)=l -1[up(p,s)] = [1−cos(cx)]/2c. obviously, the solution does not exist when 1 −d-1= 0 or d =1. when 1 −d-1≠0 or d≠1, we have (15) there are two cases in equation (15): one with 1 −d-1<0 or d <1 and the other with 1 −d-1> 0 or d >1. for the first case, sin [(1 −d-1)1/2 x/2] = i·sinh [(d-1−1)1/2 x/2], and thus sin2 [(1 −d-1)1/2 x/2] = − sinh2 [(d-1−1)1/2 x/2]. the solution displays a negative hyperbolic sinetype function. it cannot represent a commonly-defined wave. for the second case, the solution equation (14) shows vibrations, specified with a positive sine-type function, at a site. replacing up(p,x) by xp(y,t), p by x/do, and x by ~ox into equation (15) leads to (16) obviously, xp(y,t) = 0 when t = 0. the ft of xp(y,t), i.e., xp(y,~) =f [xp(y,t)], is (17) obviously, the values of xp(y,~) exist only at ~ = −c~o, 0, and c~o. thus unable to represent a commonly acceptable earthquake source displacement spectrum. this means that the simplified linearly slipweakening friction law cannot work well for earthquake ruptures based on the 1d bk model. since this friction law has been widely used by others for other models as mentioned above, i assume that a more complicated slip-dependent friction law like that used by cao and aki [1984/85] or those proposed by rice [2006] should be taken into account for the 1d bk model in the future. bizzarri [2012] considered a single spring-slider system and assumed a linear slip-weakening friction law to govern the motion of the slider. he provided a closed-form, analytical solution of the physical system. his solution of slip includes two exponential time functions, like e-ct for t > 0. the function can be represented , c j h ,$ x y t t t t t t t t1c o a o o a a a 2 2 1v 2 1 2 2 1v 2c~ c~ ~= -q q q qv 1v v 1v ! " ,$ %e i , c jv h ,v u h h h h 1c 1 2 2v 1v 2 2 2v 1$ 2 p x c p c x p x p x p = q q q q qv 2 v jv ,v 2v 1v ! " 1$ %e i , .cos sinu 1 22p 2 2 p x cx c cx c-= =q q qv v v! $ , .sinu d t 2p o o 2 2 p x c~ c=q rv w , . xp y d 2 2 2 xp o o o 2 ~ r d ~ d ~ c~ d ~ c~ c = + + q q q qv v v v! $ e i wang 6 7 by e-ct h(t). like the previous complimentary solution, the fourier spectrum of his slip function is 1/(i~ + c), thus showing ~-1 scaling in the entire range of ~. velocity-weakening friction as mentioned above, the simplified velocity-weakening friction law is z(v) = 1− v/(vc/do~o). define the dimensionless characteristic velocity to be y = vc/do~o, we have z(v) = 1− v/y, where y is also the dimensionless decreasing rate of friction with velocity. this makes equation (8) become (18) according to appendix a, the lt of equation (18) is (19) this gives (20) let g= (s2 + 1 − sy-1)1/2. from appendix b, the general solution of equation (20) is (21) equation (21) shows the superposition of two kinds of waves, i.e., the travelling waves along the model represented by the first and second terms in its righthanded-side and the displacement at a site shown by the third term. the first and second terms represent the waves travelling along the direction of increasing p and that of decreasing p, respectively. the second term with p < 0 can indeed be re-written as e-g|p|/h. let v = (1 −1/4 y2)1/2 . according to appendix a, l-1[e-g|p|/h] with |p|/h > 0 is (22) obviously, this expression represents a travelling plane wave. in the right-handed side, the quantities in the front of h (x−|p|/h) show the wave amplitude. unlike equation (13) for slip-weakening friction, equation (22) has an additional term ex/2y, which increases exponentially with time as displayed in figure 2. when the rupture propagates from 0 to pl, the normalized total rupture duration time is xd= pl/h, and thus the total rupture duration time is td= xd/~o= pl/~oh = (l/do)/~oh. substituting uc(p,x) = xc(y,t)/do, p= y/do, x=~ot, ta=y/vr, and s = ~o/2y into equation (22) leads to (23) the first part of equation (23) is (24) its ft is xc1(y,~)=f [xc1(y,t)]=cdo∫e -(i~ s)tdt. at each site, the integration is made from ta to tr= ta+tr, where tr is the rise time of rupture. hence, we have (25) this give |xc1(y,~)|= cdo|exp(str) − exp(sta)|/ (~2+s2)1/2. clearly, the spectral amplitude decays with ~-1 in the entire range of ~, and |xc1(y,0)|= cdo|exp (str) − exp(sta)|/s. the ft of the second part of equation (23), i.e., xc2(y,t) = −cdov~otae st j1[v~o(t 2 − ta 2)1/2] h [~o(t − ta)]/(t 2 − ta 2)1/2}, is xc2(y,~) =f [xc2(y,t)] = −cdov~ota ∫{ j1[v~o(t 2 − ta 2)1/2]/(t2 − ta 2)1/2} est [h [~o(t − ta)] e -i~tdt, with an integral range from −∞ to +∞. at each site, the rupture occurs just between ta and tr=ta+tr, where tr is the rise time of rupture. we are only interested on the spectrum at a site and thus different values of ta at different sites just lead to phase changes and cannot influence the spectral amplitudes when the wave attenuation is ignored. hence, the value of ta inside the integral can be set to be zero to simplify the problem. this gives xc2(y,~) = −cdov~ota∫[ j1(v~ot)/t] e st e-i~tdt, .u h u u v12 2 2 2 22 2 2 2x p y= -r qw v , , , ,/ s u s h u s u s u ss s1 2 2 2 22 2p p p p p y = +q q q qv v v v /h u s u ss 112 2 2 2 12 2p y =+ -q v , c e c e / .u s s1h h1 2 2 p g= + -gp gp-q v , c jv h . eu h h h h 1c 2 1 2 2v 1v 2 2 2v 1 2 p x v p v x p x p x p = x yq q q q q qv 2 v jv v 2v 1v ! " $ %e i , c e jv h . x y t d t t t t t t t 1c o o a o a / a / o a t 1 2 2 1v 2 2 2 1v 2 v~ v~ ~ = sq q q q qv 1v jv v 1v ! " $ %e i , c e h .x y t d t tc o o a t 1 ~= sq qv v! $ , c . exp exp x y i i d i t t c o o o a r1 ~ ~ s ~ s ~ s+ + = + -q q q q v v v v! ! $ $e h a dynamical study of earthquake spectra figure 2. the function of exp(~ot/2y) versus t. the solid curve from ta= y/vr to tr= ta+tr, where vr and tr are the rupture velocity and rise time, respectively, displays the integration range. with an integral range from 0 to tr. since f [ j1(t)/t] = [2(1− ~2)/r]1/2 rect(~/2), where rect(~) is the rectangular function as mentioned above, we have f [ j1(v~ot)/t] ={2[1−(~/v~o) 2]/r}1/2 rect(~/2v~o). obviously, the fourier transform is equal to 1 when |~|<v~o. the fourier transform of e st is f [est ] = ∫est e-i~tdt, with an integral range from 0 to tr, and equal to [exp(s − i~)tr−1]/(s − i~). mathematically, f {[ j1(v~ot)/t] e st }=f [ j1(v~ot)/t]*f [e st ], where the symbol “*” denotes the convolution of f [ j1(v~ot)/t] and f [est ], i.e., (26) with an integral range from −v~o to +v~o. equation (26) is quite complicated and cannot be easily integrated. since only spectral scaling at high frequencies is considered here, the integration is conducted only for high frequencies. since |z|< v~o<~o and s = ~oy/2<~o, s− i(~−z) can be approximated by i~ when ~ >>~o. under this condition, equation (26) can be re-written as (27) with an integral range from −v~o to +v~o. the two integrals in equation (27) are finite and represented by i1 and i2, respectively, for the former and latter. hence, f{[ j1(v~ot)/t] e st } = (1/i~v~o)(2/r) 1/2(i1e -i~− i1). this gives |f {[ j1(v~ot)/t] e st } |=|~v~o| -1(2/r)1/2|[(i1 2 − 1)cos2(~) − 2i1i2cos(~)+i2 2]1/2| although the magnitude of |f {[ j1(v~ot)/t] e st } | is dependent upon ~, but it decreases with increasing ~ in a power-law form of ~-1. this means that the complementary solution shows ~-1 source scaling at high frequencies. of course, the magnitude of |f {[ j1(v~ot)/t] e st } | is also in terms of v and ~o. the third term of equation (21), i.e., up(p,s) = −1/2s(s2+1− sy-1), directly shows source behavior of slip at a site and can be re-written as −(1−4y2)-1/2{−1/(s+ a)+[1−(1−4y2)1/2]/2(s+b)−[1+(1−4y2)1/2]/2(s+c)}, where a = 0, b = − (1/2y)[1+(1−4y2)1/2], and c= − (1/2y) [1−(1−4y2)1/2]. obviously, the solution does not exists when 1−4y2 = 0 or y= 0.5. when 1−4y2≠0 or y≠0.5, the ilt of up(p,s) is in equation (28), there are two cases: one with 1−4y2 >0 or y<0.5 and the other with 1−4y2 <0 or y> 0.5. the respective results are described below. (i) for 1−4y2 >0 or y<0.5: let sinh(a) = (ecx/2y − e-cx/2y)/2 and cosh(a) = (ecx/2y +e-cx/2y)/2. equation (28) is re-written as (29) obviously, the solution cannot represent a commonly known wave and thus is excluded in this study. (ii) for 1−4y2 <0 or y> 0.5: this gives 4y2 −1> 0. let q = (4y2 −1)1/2, consider a right triangle with three sides: the longest side with a length of l = (12 + q2)1/2 = 2y, and the other two with lengths of q and 1, respectively. the angle between the longest side and the side with a length of 1 is set to be i. hence, we have cos(i) = 1/2y, sin(i) = q/2y, and tan(i) = q. the tangent term gives i = tan-1(q). define sin(a) = (eiqx/2y − e-iqx/2y)/2i and cos(a)=(eiqx/2y + e-iqx/2y)/2. replacing up(p,x) by xp(y,t)/do, p by y/do, x by ~ot, and s = ~o/2y into equation (29) leads to (30) obviously, xp(y,t) = 0 at t = 0. this solution shows a sine-function-type standing wave. unlike equation (21), equation (30) has an extra term est, which shows a temporal increase in the amplitude with a constant of s-1 = 2y/~o. since the motion starts at t = 0, xp(y,t) is null when t < 0 and thus equation (30) must have the form of xp(y,t)h(t). the ft of −doh(t) is xp1(y,~)=−do[rd(~)− i/~]. the first term of xp1(y,~) is a delta function of slip at ~= 0, and it does not influence the source displacement spectrum when ~> 0. since the second term of xp1(y,~) exhibits a decay of motion with ~ in a power-law function of ~-1 for the entire range of ~. j" e i i d , f exp t t z z tr z 2 1 1 o o / / t tr 1 1 v 2 2w 1 2 # v~ r v~ s ~ s ~= s q q r q q 1 v v v 2w v " " j" ! % % % $%f f i i# j! e i e i d d , f exp t t z tr z z z z 2 o o / i o / o / t tr 1 1 1v 2 2v 2 1 2 2v 2 1 2 v~ ~v~ r v~ v~ = s ~-q q q q q qv v 1 v v 2v 2v j! ! ! $ % $ e e h h # # , e e e e . eu 2 2p 2 2 2 2 2 p x c c c=+ +cx y cx y cx y cx y cx y q q qv v v" % , e .sinh coshu 2p 2 p x c a c a c=+ -x yq q qv v v! $f i , e .sin sinx y t q td 1p o t s i i+ -=sq q qv v v" % wang 8 figure 3. the plot of ~*/~o versus v.(28) 9 meanwhile, its amplitude is only do, it cannot result in a remarkable effect on the source displacement spectrum. define ~*= q~o= (4y 2 −1)1/2~o be the predominant angular frequency of the wave. figure 3 displays the plot of ~* versus ~o. obviously, ~ * first increases with ~o at low ~o and then slightly increases with ~o at high ~o. due to the existence of est, the ft of the second part of equation (30) cannot be obtained directly from the ft table. hence, we must conduct the integration to get its ft. the integration is made in the range from 0 to tr. the second term of equation (30) includes a sine function with a period of tp= 4ry/q~o. as displayed in figure 4, the solid plus dotted curve represents the velocity waveform, while the solid curve denotes the displacement waveform. when t = tr, the displacement at a site reaches the maximum value. since the wave is considered to propagate from −y to +y, the motion stops at the time instant when the velocity is equal to zero. in figure 4, tp and tr are, respectively, the period of velocity waveform and the rise time of displacement waveform. clearly, tr is roughly equal to tp/2, thus giving ~otr/2y = r/q. the ft of e st sin (qst −i)/sini is made from 0 to tr, and the result is (31) the absolute value of xp2(y,~) is (32) at ~= 0, |xp2(y,~)|= (e 2rq+1)/~oy = r/~oy(4y 2 − 1)1/2, which is a function of both ~o and y. when (~o/y) 2 << 2, {~4+ [(~o/y) 2 − 2]~2+~o 4}1/2≈~2−~o 2. this leads to the existence of singularity when ~ = ~o. numerical tests show that singularity appears when ~o is lower than a certain angular frequency, ~l, which depends on y. hence, in order to obtain a small value of ~l = ~o/y, the choice of ~o must be dependent on y. the larger y is, the higher ~o can be. for example, ~l = 0.4r when y = 0.55. because of ~o= (ll/ta) 1/2= (kl/m) 1/2, weak coupling between the moving plate and the fault and/or a low density of fault-zone materials will lead to low ~o, thus resulting in singularity. figure 5 displays the log-log plot of |xp2(~)| versus ~: (a) for y = 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, and 1.00 (from top to bottom) when ~o= 2r; and (b) for ~o= 0.2r, 0.4r, r, 2r, 4r, 6r, 8r, 10r, 12r, and 14r (from top to bottom) when y = 0.55. to produce figure 5, do is taken to be 1 unit. since a uniform fault is considered here, the spectral amplitude is only a function of ~ and related source parameters and independent on the position. hence, |x (~)|=|xp2(y,~)| is displayed in the figure. the values of |x (~)| are normalized by respective maximum values. the dashed line in each diagram has a slope value of −1. figure 5b shows a bump around ~ = ~o when ~o=0.2r as mentioned previously. essentially, figure 5a is similar to figure 5b. the spectral amplitude decreases with increasing y as well as increasing ~o. higher y as well as larger ~o leads to smaller spectral amplitude. the difference in spectral amplitudes between two consequent values of y decreases with increasing y as well as increasing ~o. this means that at high y an increase in y cannot produce stronger spectral amplitudes. this phenomenon also exists for ~o. in figures 5a and 5b, log(|x (~)|) is almost constant (exhibiting ~0 scaling) at low ~ when ~ is lower than a turning one, i.e., the corner angular frequency, ~c, which is located at the tip of the convex curve and not displayed in figure 5, when ~ > ~c, log(|x (~)|) monotonically decreases with ~ following the respective lines with a slope value of −1, which is the scaling exponent. there are some differences between figure 5a and figure 5b. first, the lines with a slope value of −1 at higher ~o are parallel to one another in figure 5a and merge together at much higher ~o in figure 5b. secondly, ~c is almost independent on y (see figure 5a) and increases with ~o (see figure 5b). a comparison between equation (16) and equation (29) reveals that a major difference between the two equations is the existence of an exponential term in equation (29) and not in equation (16). such an additional effect is frequencydependent. this makes the , e e i i . xp cos sin y d tr tr 1 2 xp o o o t tr t tr 2 2 2 2 ~ ~ ~ ~ y ~ ~ ~ s~ = + + s s q q r q q v v w v v ! $ , e e . xp cos y d tr 1 2 xp o / o / o o / tr q q 2 2 $ 2 2w 2 1 2 4 2w 2 2 1 2 ~ ~ ~ y ~ ~ ~ y ~ ~ = + + + + r r q r q rv 2w v 2w! " "1$ % % f i a dynamical study of earthquake spectra figure 4. the solid plus dotted curve represents the velocity waveform and the solid curve denotes the displacement waveform. tp and tr are, respectively, the predominant period of the velocity waveform and the rise time of displacement waveform. difference in the results between the two friction laws. nonlinearity of slip-weakening friction, as shown in equations (3) (5), could lead to different results. it is significant to examine the effect on source displacement spectrum caused by nonlinear slipweakening friction in the future. figure 5 clearly show that linearly velocity-weakening friction results in |xp2(y,~)|~~ 0 at low ~ and |xp2(y,~)|~~ -1 at high ~. this is different from ~-square source scaling as proposed by aki [1967]. this might be due to a use of 1-d model in this study and a use of 2d model by aki [1967]. the conventional concept is that ~-square scaling is caused by the smoothing effect due to time and fault length and ~-cubic scaling due to time, fault length, and fault width [cf. aki and richards 1980]. however, the present model consists only of time and fault length. it seems that time does not causes the smoothing effect. i assume that ~-square scaling should be caused by the smoothing effect due to both fault length and fault width. this must be examined by using the 2-d model. the present results are somewhat different from those obtained by shaw [1993], even though using the same 1-d bk model in the two studies. unlike his results, in this study a single ~-1 source scaling law at high ~ is operative for both small and large earthquakes. a possible reason to cause the difference is that his velocityweakening law is nonlinear and more complicated than the present one. nonlinearity can produce unexpected results. it is significant to analytically and numerically investigate the effect of different friction laws on source scaling near future. the previous studies as mentioned in the section of “introduction” suggest that fractal and/or heterogeneous spatial distributions of fault strengths, displacements etc. on the fault plane are major factors in affecting scaling of earthquake source displacement spectrum. the present study obviously suggests that linearly velocityweakening friction can also play a significant role on such scaling. the corner angular frequency (~c) is commonly considered to be a ratio of the rupture velocity to the fault length [cf. aki and richards 1980]. figure 5 exhibit that ~c increases with ~o, yet is not a function of y. this indicates that ~o, which depends on the mass of a slider and coupling between a slider and the moving plate, controls ~c, while the characteristic velocity or the decreasing rate of friction with velocity is not a factor in influencing ~c. however, two problems should be resolved in the near future. first, the present study cannot suggest whether the ~-cubic source scaling holds or not, because only the 1-d model is taken into account. secondly, the present result is valid only for the longitudinal-wave-type rupture. included also in earthquake are the shear-wavetype waves. in order to explore the two problems, a similar study on the basis of the 2-d dynamical spring-slider mode extended from the 1-d bk model by wang [2000, 2012] should be performed further. 5. conclusions scaling of earthquake source displacement spectra is analytically studied based on the continuous form of the one-dimensional dynamical spring-slider model proposed by burridge and knopoff [1967] in the preswang 10 figure 5. the log-log plot of |x(~)| versus ~: (a) for y= 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, and 1.00 (from top to bottom) when do= 1 unit and ~o=2r; and (b) for ~o= 0.2r, 0.4r, r, 2r, 4r, 6r, 8r, 10r, 12r, and 14r (from top to bottom) (from top to bottom) when do= 1 unit and y= 0.55. 11 ence of two types of friction, i.e., linearly slip-weakening friction and linearly velocity-weakening friction. the general solution has the complementary and particular parts. the complementary solution exhibits ~-1 source displacement spectra for the two types of friction. for the particular solution, slip-weakening friction cannot produce ~-n source displacement spectra, while velocity-weakening friction results in acceptable spectra. for velocity-weakening friction, the log-log plot of spectral amplitude versus angular frequency is mainly controlled by the natural angular frequency, ~o, of the system and the (dimensionless) decreasing rate, y, of the friction law. essentially, the spectral amplitude exhibits almost ~0 scaling at low ~ and decreases with increasing ~ following a line with a slope value of −1, which is the scaling exponent. however, the plots in the range of medium ~ are clearly different between ~o<~l and ~o> ~l, where ~l depends on y. for example, ~o<0.4r and ~o> 0.4r when y= 0.55. for ~o> 0.4r, the spectral amplitude exhibits almost ~0 scaling when ~ is lower than the corner angular frequency (denoted by ~c), and the spectral amplitude monotonically decreases with ~ and follows a line with a slope value of −1 when ~>~c. the corner angular frequency increases with ~o and independent on y. on the other hand, for ~o<0.4r, the spectral amplitude first exhibits almost ~0 scaling at low ~, then increases with ~ up to a peak value, and finally decreases with increasing ~, following a line with a slope value of −1, which is the scaling exponent. the angular frequency associated with the peak value is almost ~o. consequently, the source displacement spectrum shows ~-1 scaling at high ~ when linearly velocity-weakening friction is taken into account under the condition of y > 0.5. acknowledgements. the author thanks an anonymous reviewer and prof. andrea bizzarri (associated editor of the journal) for useful comments and suggestions to improve the article. this study was financially supported by academia sinica, the ministry of science and technology (grand nos.: most 103-2116-m-001-010 and most 104-2116-m-001-007), and the central weather bureau (grand nos.: motc-cwb-104-e-07 and motc-cwb-105-e-02), roc. references aki, k. 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(2016). studies of earthquake energies in taiwan: a review, terr. atmos. ocean. sci., 27 (1), 119; doi:10.3319/tao.2015.10.13.01(t). * corresponding author: jeen-hwa wang, institute of earth sciences, academia sinica, nangang, taipei, taiwan; email: jhwang@earth.sinica.edu.tw. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. a dynamical study of earthquake spectra appendix a: the laplace transformation the laplace transformation (lt, denoted by l), of a function u, that is, l [u(p,x)] = u(p,s)=∫e-sxu(p,x)dx, in which the integration is made from –∞ to +∞. the laplace transforms of several functions used in this study are: l [d2u(p,x)/dx2] = s2u(p,s), l [du(p,x)/dx] = su(p,s), and l [1] = 1/s. in order to obtain the inverse laplace transform (ilt, l-1) of u(s)=1/(s+a)(s+b)(s+c), which is the main form in this study, u(s) is first transferred to −[(b−c)/(s+a)+(c−a)/(s+b)+(a−b)/(s+c)]/ (a−b)(b−c)(c−a). due to l-1[1/(s+d)] = e-dx, (a1) also used in this study is the following formula: (a2) where j1[y] and h [y] denote the bessel function of the first kind with order 1 and the unit step function, respectively, when a>0, in addition, l-1{u(s −a)} = eaxu(x) and l-1[1/s] =1 are also used in this study. appendix b: the solution of a second-order linear differential equation the method to solve a second-order linear differential equation can be seen in johnson and kiokemeister [1968]. for a second-order inhomogeneous linear differential equation: (b1) where a, b, and c are three constants, the solution is u = uc+ up where uc and up are, respectively, the complementary and particular solutions. uc is the solution of a homogeneous form of equation (b1), i.e., (b2) inserting the trial solution uc= e mp into equation (b2) leads to am2emp+ bmemp+ cemp= 0. this gives am2 + bm + c = 0. thus, the solutions of m are m = −b± (b2−4ac)1/2/2a. hence, the general form of uc is c1e -mp+ c2e mp. let y (p) = emp and z (p) = e-mp. to solve up, we let (b3) the two function y1 and z1 should meet the following conditions: y1’y + z1’z = 0 and y1’y ’+ z1’z’= f (x), where y1’=dy1/dp, y’=dy/dp, z1’=dz1/dp, and z’=dz/dp. hence, y1 and z1 can be solved from the following integral equations: y1= −∫[zf (x)(y1’y ’+ z1’z)/(yz’−y’z)]dp and z1=∫[yf (x)(y1’y ’+ z1’z)/(yz’−y’z)]dp. inserting the related quantities into equation (b3) leads to the particular solution. b c e c a e a b e a b b c c a . l u s a b c t t t 1 =+ + -q q q q q q q v v v v v v v ! !$ $ jb b h ,v l exp s 1 / / / 1 2 2 1v 2 1 2 2 1v 2 2 2 1v 2 a b a jb x a x a x a + = q q q q 1v 1v 1v ,v! 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 1, 2004, 01/07def 157 annals of geophysics, vol. 47, n. 1, february 2004 key words seismic and electromagnetic studies – earth’s crust – baltic and ukranian shields 1. introduction at first the low velocity layers were determined in the crust from seismological data in tectonic active regions. the main indications of the velocity inversion with depth were the «shadow zones» (sharp attenuation of the first arrivals at some epicentre distances) and recording of the channel waves, which travel along the low velocity layer (waveguide) when an earthquake appears in the layer. the first deep seismic sounding (dss) confirmed the existence of the low velocity layers in the crust of tectonic active regions by the observed «shadow zones». these layer were interpreted as a result of high temperatures and partly melting. later the low velocity layers were determined in the cold crust of shield areas at a depth of 10-15 km. for example, they were determined on the ukrainian shield, where detailed seismic observations, so called continuous profiling, were carried out (pavlenkova, 1979). it was the most detailed form of dss: multichannel seismic stations moved along the profile with seismograph spacing of 100 m. explosions from several direct and overlapping shotpoints with interval of 40-60 km were recorded. these low velocity and low electrical resistivity layers in the middle crust nina i. pavlenkova schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia abstract some deep seismic sounding (dss) revealed low velocity layers in the upper and middle crust of old platforms. the layers are often characterised by a lower electrical resistivity. it is not clear, however, how reliable the layers recognized from dss data are, if they are regular or occasional events and how they correlate with other geophysical parameters. to answer these questions the experimental dss data obtained in the baltic and ukranian shields by different institutions were reinterpreted by the author with unified methods. the shield areas are well studied using both the dss and high-frequency magnetotelluric sounding (mts) methods. as a result a marked velocity inversion (waveguide) was observed in a 10 to 20 km depth range in the majority of the dss profiles. an increase in the electrical conductivity is typical for the waveguide. a comparison of the results with the data from other platform regions allow the conclusion that this low velocity and high electrical conductivity layer has a global significance. in the continental crust, the layer is characterised by changes in the reflectivity pattern, earthquakes number and changes in velocity pattern where the block structure is transformed into a subhorizontal layering. these structural features suggest that the layers separate brittle and weak parts of the crust. usually they play the role of detachment zones at crustal block moving. a possible factor responsible for this phenomenon is an increase in porosity and in the salinity of the waveguide pore water compared with the upper crust. this suggestion is confirmed by the kola superdeep borehole data. porosity increasing in the middle crust is explained by the change in rock mechanical properties with depth, by fracturing porosity and by dilatancy effect, at a depth of 10-20 km. mailing address: dr. nina i. pavlenkova, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia. 158 nina i. pavlenkova results, however, were considered unrealistic because it was difficult to explain the nature of this layer: the velocity inversion in the upper crust of the cold shield might not be explained by temperature influence. actually, the temperature at a 10 km depth under the ukrainian shield is not higher than 150-200°c, which can decrease the velocity gradient with depth but cannot form an inversion zone. apparently, the geophysical studies revealed a «cold» type of weakening of rocks, which has another nature than the layers in tectonically active regions. the velocity inversion zones were determined in the upper and middle crust in many platform regions: in the baltic shield (grad and luosto, 1987), on the russian plate (eurobridge seismic working group, 1999) and in the siberian craton (pavlenkova et al., 2002). often these layers are characterised by higher electrical conductivity (kovtun et al., 1994; beljavsky et al., 2001). it became clear that such layers may have a global significance, but up to now the nature of the layer has been debated (jones, 1992; karakin and kambarova, 1997; astapenko and frainberg, 1999; vannjan and pavlenkova, 2001; berzin et al., 2002). the goal of this paper is to answer the following questions. how reliable are the low velocity layers recognized from dss data and are they a regular or occasional feature of the continental crust? how do they correlate with electrical conductivity model? are there any correlations between the layer structure, tectonic features and other geophysical parameters of the crust and what is the origin of these layers? to answer these questions all experimental dss data obtained in the baltic and ukranian shields during the last 40 years were reinterpreted with a unified methodology and the obtained velocity models were compared with the magnetotelluric and other geophysical and geological data. the shields were chosen for this research because they are the most investigated regions with detailed seismic and high frequency electromagnetic methods and their results may be compared with kola superdeep borehole data. then the obtained data on the low velocity and high conductivity layers in the upper and middle crust of the shields were compared with the data on other platform regions. finally problems of the layer origin are discussed. 2. low velocity layers in the crusts of the baltic and ukranian shields more than twenty seismic profiles crossing all tectonic structures of the baltic shield have been processed in this region during the last 40 years (fig. 1c). they were carried out by different institutions with different instrumentations, techniques and interpretation methods. as dss is developed the old data were reinterpreted many times and now there are several velocity models for each profile. the low velocity layers (velocity inversion zones or waveguides) were distinguished in the middle crust at depths of 10-15 km along the seismic profile sveka (grad and luosto, 1987). the inversion is observed not only in the p-wave velocities but in the svelocities as well. the inversion zones, however, were not distinguished along other profiles. it was difficult to understand if the layer is local and does not exist in other regions or they were missed during the data interpretation. in order to obtain comparable data for the whole baltic shield, a comparative analysis of experimental records was made by the author for all dss profiles. the wave fields were analysed and interpreted using unified methods. as a result the low velocity layers were determined in the upper crust along the most profiles. some results of the old seismic data reinterpretation were published in berzin et al. (2001). in this paper, new models are presented for the profiles quartz and fennia. the quartz profile was carried out by centre geon (moscow) as a part of the russian superlong seismic profile net made for the lithosphere studies (egorkin, 1991). the fennia profile was made by the finnish institute of seismology with distances between the seismic stations of 2-3 km and between shots of 80 km. as mentioned above, the main feature indicating a velocity inversion is a regular attenuation of the first arrivals at some distances from the source (appearance of so-called «shadow 159 low velocity and low electrical resistivity layers in the middle crust zone»). figure 1a-c shows the traveltime curves and seismic rays for the inversion zone determined from sp 9 along the quartz profile. seismic rays penetrating the crust and arriving at the surface at source-receiver distances of about 100 km (pg-wave) form a traveltime branch having an apparent velocity of about 6.4. km/s. the rays entering a velocity inversion zone are deflected downward rather than upward, intersect the zone, and come back, being reflected from its base (k1 interface) or refracted in the underlying layer. the related reflection and refraction traveltime branches have velocities not higher than 6.5 km/s. the k1 and pg traveltime plots are nearly parallel and are separated in time by ∆t. the time delay depends on the waveguide thickness (∆h) and a value of the velocity inversion (∆v) and provides major constraints on these parameters. the average velocity in the waveguide and its thickness are not uniquely constrained by refraction traveltime data. a set of the crustal velocity models having different waveguide parameters ∆v and ∆h may be determined. the velocity decrease could not be ambiguously determined even with a sufficiently long traveltime curve of the wave reflected from the waveguide base. for instance, in the case shown in fig. 1a-c, the velocity decrease can vary from 0 to 0.4 km/s, with the base depth varying from 15 to 11 km. to obtain comparable velocity models for all profiles, the data from the kola superdeep borehole were used. the borehole determined the velocity at depths of the waveguide of 7-12 km as 6.1 to 6.2. km/s (kozlovskii, 1984; pavlenkova, 1991). these values were used for the all velocity inversion zones in the baltic shield. other parameters of the waveguide and fig. 1a-c. a) traveltime plots and (b) seismic rays for the crustal velocity model constructed along the quartz profile (baltic shield) with an inversion zone at depths of 7 to 12 km. pg – refracted wave in the upper crust (5.8-6.4 km/s); k1 – wave reflected from the base of the inversion zone (6.4-6.7 km/s); k2 – wave reflected from the top of the lower crust (6.8-7.2 km/s); ∆t is the time delay between pg and k1 branches. thick lines in the cross section are reflectors. c) a schematic map showing the position of dss (solid lines) and mts (dotted line) profiles in the kola peninsula and karelia region used in this work. a b c 160 nina i. pavlenkova underlying layer were determined from 2d mathematical modelling (ray tracing). the analysis has shown that the observed wave pattern involved in the recognition of low velocity layers is not always clear. it is often complicated by the fact that the shadow zone is not observed due to diffraction phenomena, and the waves reflected from the waveguide base are not recorded. wave patterns, which look like the «shadow zones», may also be observed at strong lateral inhomogeneity of the crust. to discriminate them from effects of low velocity layers, it is necessary to analyse the whole system of reversed and overlapping observations. the «shadow zones» connected with a low velocity layer are to be observed at the same distances (offsets) from all shots and it is the main criteria to trace the inversion zone along the profile. for this a special method of the travel-time analysis was applied and they were presented in form of the intercept time cross-sections (pavlenkova, 1982). figure 2a-c presents an example of the time section for the fennia profile. figure 2a gives the system of the traveltime curves in a traditional form. from all shots the time curves show a regular picture. the waves pg are recorded with apparent velocities around 6.06.3 km/s at offsets of 0-100 km. at larger offsets pg attenuate and the k1 reflections became the first arrivals with velocities about 6.3 km/s. a time delay of about 0.5 s is observed between the pgand k1-waves. at offsets of 110-170 km the k2-waves are recorded with velocities of 6.8-7.0 km/s without any time delays regarding the k1-waves. to see how this picture changes along the profile, the observed travel-times are shown in fig. 2a-c. observed travel times and the velocity model of the upper and middle crust along the fennia profile. a) travel times in a traditional form, the reduction velocity of 8.0 km/s; b) a time section: the observed travel times reduced with the reduction velocity 6.5 km/s and plotted at source-receiver midpoints with downwards time axis; the lines d = 10 and 30 km coincide the travel times at the offsets of 10 and 30 km, the dotted lines are intercept time curves for k1 and k2 boundaries; c) velocity model, thick lines are reflectors. a b c 161 low velocity and low electrical resistivity layers in the middle crust fig. 2b in another form: they are reduced with velocity 6.5 km/s (typical velocity at the inversion zone bottom) and are plotted at source-receiver midpoints. the lines coincided the travel times of first arrivals at constant offsets (d = =10 and 30 km) show the uppermost crust structure. the line t (d = 30 km) at which the first arrivals attenuate (1.0-1.2 s) outlines the inversion zone top. an envelope of the reflection k1 times may be used to determine a form of the inversion zone bottom. this line is intercept time (ti) line for the k1 boundary and they may be recalculated in the depths to the boundary (the boundary reflections tough the ti-line in the critical points). the k1 reflection envelope shows strong inclination of the boundary between the shot points (sp) b and c. in contrary the travel-times of refracted waves from the k2 boundary form a near horizontal ti-line at the reduction velocity of 6.5 km/s. the time section was used as a starting model for the following ray tracing. resulting velocity model is given in fig. 2c. the low velocity layer is determined at the same depth as along the quartz profile. but the layer structure is more complicate along the fennia profile. beneath sp-c the layer depth increases from 8 to 14 km and its thickness increases as well. it seams that it is influence of a crustal fault. similar velocity models were obtained for the ukranian shield. in the shield area, several dss profiles were made in the 60-70’s. that was the continuous profiling with analogue multichannel seismic stations. the data were fig. 3a-c. a) time section, b) velocity model along the ukrainian shield and (c) scheme of seismic profiles. numbers are p-wave velocities in km/s, striped zones are «shadow zones» in the time section and the low velocity zone in the velocity model. a b c 162 nina i. pavlenkova fig. 4. record section from dss profile in the ukrainian shield which shows the «shadow zone» (attenuation of the pg-wave) at distances from the source d = 75 km. legend in fig. 1a-c. reinterpreted with the same methodology as the baltic shield data. below the results of the reinterpretation are presented for the profile along the shield (fig. 3a-c). figure 4 presents a record section that is typical for many profiles in the ukrainian shield. it shows a clear «shadow zone» at offsets of 75 km. the observed travel times transformed to the midpoints and the corresponding time section (fig. 3a) show that the «shadow zone» may be traced along the whole profile. the time sections determine an increase in the waveguide thickness in the central part of the profile and a local anomaly in the profile interval of 50-70 km. figure 3a-c also shows that the time section along the ukrainian shield is similar to that in the baltic shield and the low velocity layer was determined at the same depth of 10-15 km. thus, the main result of the time section comparison is that the velocity inversion exists 163 low velocity and low electrical resistivity layers in the middle crust in the shield crust along the most seismic profiles. thickness of the inversion zone varies but there is no doubt that it is a regular feature of the shield crust. 3. magnetotelluric data electrical conductivity constraints on the upper crust of the karelia region and kola peninsula were gained from high-frequency (10− 2-10− 4 s variation period range) magnetotelluric soundings (mts) (kovtun et al., 1994). relatively dense coverage of the baltic shield by dss and mts profiles provides a good basis for examining in detail the upper crust anomalous layers and gaining constraints on their origin. the most important result of these observations was the detection of an abrupt drop in the apparent resistivity in a variation period range of 10− 2-1 s, which is direct evidence of a decrease in the electrical resistivity at depths of 812 km. the apparent resistivity reaches a minimum near a 1 s period and increases by a few times with the further increase in the variation period. such behaviour of the apparent resistivity indicates the presence of a conductive layer in the area studied. as seen from fig. 5, the conductive layer is traced along the ns profile about 1000 km long from the barents sea coast to lake ladoga (fig. 1c). in the ukrainian shield many magneto-telluric soundings were also carried out (beljavsky et al., 2001). for the most of them the high electrical conductivity layers are determined at a depth of 10-20 km. a combined analysis of seismic and electromagnetic data makes their physical and geological interpretation more reliable, because these methods differ in their constraints on crustal properties. the seismic wave velocity is mostly controlled by the rock composition and partly by fluid concentration in pores and fractures. the electrical conductivity of fluid-bearing rocks is nearly independent on the solid phase composition and is determined by the fluid content and its salinity. if the depth and thickness of crustal conductive layers correlate with the parameters of the low p-velocity layers they determine the layers as zones of higher porosity and higher concentration of fluids. the mts data may be used for determination of the porosity in the high conductivity layer which is difficult to retrieve from seismic data. for the baltic shield such determinations show the following (vannjan and pavlenkova, 2002). the upper 10 km of the shield crust are characterised by low porosity. however, even negligible amounts of pore water dramatically decrease the electrical resistivity. this makes it fig. 5. geoelectrical structure of the crust along the combined ns teriberka-loimola (tl) mts profile (kovtun et al., 1994). the electrical resistivity contours in the crust are results of the 1d interpretation. a conductive layer is discovered in 8 to 15 km depth interval (black strip). the position of the profile is shown in fig. 1c. 164 nina i. pavlenkova possible to estimate the porosity ρ using the archie law, connecting the porosity with the electrical resistivities of the water-saturated rock ρ and pore water ρw: a high mineralization of the pore water can be expected in the static zone of the crystalline basement. various direct effects indicate the presence of brines whose resistivity drops to 0.03 ω⋅m at the upper crust temperature. using this value and an average resistivity of the crust of 104 ω⋅m, it was found ρ = 0.1%. this value coincides with the generally accepted porosity of granite observed in samples tested in laboratory. this estimate can somewhat increase, if pores and fractures are filled with a fresher solution. thus, the sensitivity of geoelectric data to the water saturation of rocks is high enough for detecting a porosity of a few tenths of a percent. the depth and thickness of the karelia and kola peninsula crustal conductive layers correlate well with the parameters of the low p-velocity layers. these layers are characterised with lower values of the s-wave velocities as well. the pand s-velocities ratio do not differ from the ratio in other crustal layers. it is natural to conclude that the velocity and electrical resistivity decreases have a common origin. the increase in the porosity to about 1%, determined from electrical resistivity is also correlated with the seismic data. the 0.2-km/s decreases in the p-wave velocity results in the same value porosity (1%). both approaches yield evidence that a 5 km layer containing up to 1% of water is present at a depth of about 10 km. a similar value (1.19%) was obtained from data of the kola overdeep borehole (kozlovskii, 1984). the mts data show another important feature of the layer discussed. the inclined high conductivity zones which correspond to the fault zones, flattened out to the low velocity layer. this example is presented in fig. 6 for the polar profile. the latter was determined as a result of the data reinterpretation at depth of 10-20 km. the same relation between the faults and the inversion zones are also observed from cdp data. most faults, identified from geological data are traced as clearly visible reflectors usually within the upper crust. their shape regularly changes with depth, so that near the surface they are practically vertical and at depth of about 10 km they flatten out. but it does not mean that there are no deep faults that cross the whole crust. the deep faults flatted out to the moho are observed from the mts data along the sveka profile (corja et al., 1993) and from dss and cdp data along the kem-uchta profile, karelia (berzin et al., 2001). the low velocity layers were determined in other regions of the east-european platform as well. their depths are 10-20 km in the south and central parts of the platform (pavlenkova, 1996; baranova and pavlenkova, 2003) and along eufig. 6. a 2d geoelectrical model along the polar profile (korja et al., 1989). the mt sites are indicated by the inverted triangles. the position of the profile is shown in fig. 1c. 165 low velocity and low electrical resistivity layers in the middle crust robridge profile in the western part of the platform (eurobridge seismic working group, 1999). the inversion is observed in the s-wave fields as well. the pand s-velocity ratio is determined as 1.71, the same as in the upper crust. the same results were obtained from the sveka profile in the baltic shield (grad and luosto, 1987). the low velocity and high conductivity layers were also discovered in the middle crust in other platform regions: in the indian and canadian shields, in the american ancient platforms (berdichevsky et al., 1984; jones, 1992; pavlenkova, 1996; padilha et al., 2000). in the young west-european plates the correlation between the low velocity layers and the high conductivity layers is also the same as in east-european platform (aichruth et al., 1992). such correlation allows the conclusion that this low velocity and high electrical conductivity layer in the middle crust has a global significance. what might be the origin of this layer? 4. nature of the low velocity and high conductivity layer in the middle crust the low velocity and high electrical conductivity layer in the middle crust is characterised by some structural peculiarities which are important for understanding the origin of the layer. the data presented above and the results of other data analysis show that in the crust at depths of 10-20 km there are a number of properties which identify the low velocity layer as a principal structural element of the continental crust. among these properties the most typical are the following: – the velocity inversion zones divide the crust in two parts with different structural pattern: the inclined reflectors which may be interpreted as faults, flatten out at the low velocity zone and the steeply dipping structures of the upper crust are replaced by sub-horizontal structures in the lower crust. – a distinct seismic boundary (the reflector k1) often underlies the low velocity layer. – detailed research by seismic reflection method (cdp) shows the greater sub-horizontal stratification of the crust (increasing reflectivity) at depths below 10-15 km. this was also proposed from the detailed dss data (pavlenkova, 1996). – the lower edges of the crustal bodies that give rise to gravity and magnetic anomalies are usually found at depths not lower than 10-15 km. – a good correlation is observed between the velocity inversion and the distribution of the earthquakes: they occurred above the inversion zone. in the baltic shield crustal earthquakes are normally located above depth of 12 km (korhonen and porkka, 1981). it means that the midcrustal layer is a weakened zone. the main structural features of the crust are shown in a schematic form in fig. 7. fig. 7. generalised structural and velocity models of the continental crust. 166 nina i. pavlenkova it should be noted, that these structural characteristics of the crustal waveguide are observed not only in the old platform areas. in the baikal rift zone the low velocity layer, determined in the middle crust, has the same characteristics (fig. 8): higher reflectivity, flattening out of faults and lower earthquake number (krilov et al., 1990). it suggests that the crust of the baikal rift zone has not been transformed by higher temperature regime and up to now maintains the platform crust features. according to the current notions of crust rheology presented over the last fifteen years, the main factor responsible for the weakening of rocks and for the velocity inversion is the temperature increase with depth. the depth of the brittle/plastic transition zone increases as the heat flow in the studied region decreases. this depth exceeds 20-25 km under shields. new studies of the baltic shield rheology (kukkonen and peltonen, 1999; kaikkonen et al., 2000) confirmed that the depth of the brittle/plastic transition zone in the archean crust of karelia is about 30 km. thus, the heating-induced weakening of the crust is expected to occur at a depth three times greater than its value constrained by seismic and geoelectric data. actually, the temperature at a 10 km depth under the baltic shield is not higher than 150°c, which can decrease the velocity gradient with depth but cannot form the inversion zone. apparently, the geophysical studies revealed a «cold» type of weakening of rocks at a depth of about 10 km, which cannot be accounted for by the temperature factor effective in tectonically active regions. to explain the crustal high conductivity zones, the problem of the fluids origin is mainly discussed (jones, 1983). dehydration is considered as a more possible factor favouring the formation of lower velocity and higher electrical conductivity zones in the middle crust. this effect is supported by data from the kola superdeep borehole. here, a 0.2-0.3 km/s decrease in seismic velocities, discovered in a homogeneous sequence at a 4.5 km depth, correlates with a decrease in the bound water concentration and an increase in the amount of free water (kozlovskii, 1984). new studies of the earth’s fluid regime show that the earth degasation is the main source of the mantle and crustal fluids (letnikov, 2000). so the main problem is not the fluid origin but an origin of higher porosity in the middle crust at a depth of 10-20 km that enables concentration of the fluids. the most reasonable explanation of the higher porosity and other properties of the low velocity crustal layer follows from the theory of dilatancy cracking of the crust (nikolaevsky, 1985; karakin and kambarova, 1997). the theory is based on laboratory data on the mechanical properties of the fig. 8. seismic cross-section and velocity model of the baikal rift zone (krilov et al., 1990). vp is seismic velocity, n is earthquake number. 167 low velocity and low electrical resistivity layers in the middle crust rocks under high pressures and temperatures and on theoretical considerations of the rock mechanical properties at the pt condition typical for the cold crust (below 500-700oc). in the upper crust normal vertical faults are formed as a result of horizontal displacement stresses; tilted shear faults are located below, and at a depth of 7-10 km they degenerate into completely sheared rocks. the same characteristics of faults in the uppermost crust have been described in jackson and mckenzie (1983). the development of extremely fine cracking at depths of more than 10 km results in the saturation of rocks by fluids, the appearance of the low-velocity and high-conductivity layers and a corresponding increase in their plasticity. thus, the upper crust is a region of brittle deformations, the features of the low velocity and high conductivity layer appear to indicate an increase in the fracturing porosity in the middle continental crust. in other words, it is a weakened water-saturated layer. this conclusion is supported by the kola supperdeep borehole data. at a depth of 7-12 km the hole reaches a zone of weakened rocks and higher fluid flow (pavlenkova, 1991). the suggestion that weakened zones in the upper and middle crust are developed due to rock fracture and dilatancy effect can also explain all the structural peculiarities of the upper and middle crusts that were pointed out above and that are not considered by other hypotheses. the suggestion agrees with the observed structural peculiarities of the crust and with geological data. according to these data, the weakened layers can be associated with zones of relative horizontal motions of the upper versus lower crust (detachment zones, fig. 7). these are zones of fractured rocks similar to subhorizontal faults. the horizontal displacement of separate layers, the formation of nappes, and plastic flow of the crustal matter can be associated processes causing the formation of a sub-horizontal seismic boundary k1 at the bottom of the crustal waveguide. it is difficult to explain this boundary nature in another way. the weak layers can play an important role in any tectonic processes not only as the detachment zone at crustal block moving. together with deep faults they form a channel system for the mantle fluids and matter transportation. it means that the low velocity and high conductivity crustal layers have a number of uses for identifying zones of mineralization, which are of economic importance for detecting fluids and determining the depth of weak zones. the latter is also important for studying seismicity. 4. conclusions the comprehensive analysis and reinterpretation of the seismic experimental data obtained in the baltic and ukrainian shields show that a layer of lower pand s-wave velocities exists in a depth range of 10-20 km in their crust. the layer is reliably recognised from refraction and wide angle reflection data if systems of reversed and overlapping profiles are analyzed. the low velocities correlate with a lower electrical resistivity. the comparison of the data with other geophysical and geological information from different regions suggests this low velocity and high conductivity layer have a global significance. several structural features are typical for the layer: a change in velocity inhomogeneity where the block structure is transformed into a subhorizontal layering, changes in the reflectivity pattern and earthquake number. these properties of the middle crust layer mean that it can be associated with a weakened zone and it suggests a rheological stratification of the crust. a possible factor responsible for this phenomenon is an increase in porosity and in the salinity of the waveguide pore water as compared with the upper crust. this suggestion is confirmed by the kola superdeep borehole data. porosity increasing in the middle crust is explained by a change in rock mechanical properties with depth and by their fracturing porosity and by delatancy effect at depth of 10-20 km. 168 nina i. pavlenkova acknowledgements the record sections for the seismic profiles carried out in the baltic shield were kindly made available by finnish institute of seismology and by russian ministry of national resourse. i thank u. luosto, yu. yurov and other finish and russian colleagues for the useful discussions of the data reinterpretation results. i am grateful to v.v. spichak for the idea to compare the seismic and electromagnetic data on the crustal structure and to v. lapenna for critical comments to a preliminary version of the paper and to his recomendations how to improve the paper. an anonymous reviewer also gave valuable comments. 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(1996): crust and upper mantle structure in northern eurasia from seismic data, in advances in geophysics, edited by r. dmowska and b. saltzmann (academic press, inc.), 37, 1-135. pavlenkova, g.a., k. priestly and j. cipar (2002): 2d model of the crust and uppermost mantle along rift profile, siberian craton, tectonophysics, 355, 171186. vannyan, l.l. and n.i. pavlenkova (2002): low velocity and low electrical resistivity layer at the base of the upper crust under the baltic shield, izv. phys. solid earth, 1, 1-13. first deep underground observation of rotational signals from an earthquake at teleseismic distance using a large ring laser gyroscope annals of geophysics, 59, fast track 4, 2016 first deep underground observation of rotational signals from an earthquake at teleseismic distance using a large ring laser gyroscope andreino simonelli1,3,6,∗, jacopo belfi1, nicolò beverini1,2, giorgio carelli1,2, angela di virgilio1, enrico maccioni1,2, gaetano de luca5, gilberto saccorotti4 ∗andreino.simonelli@pi.infn.it 1istituto nazionale di fisica nucleare, pisa, italy 2università di pisa, dipartimento di fisica “e. fermi”, pisa, italy 3ludwig-maximilians-universität, munich, germany 4istituto nazionale di geofisica e vulcanologia, pisa, italy 5istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy abstract recent advances in large ring laser gyroscopes (rlg) technologies opened the possibility to observe rotations of the ground with sensitivities up to 10−11 rad/sec over the frequency band of seismological interest (0.01-1hz), thus opening the way to a new geophysical discipline, i.e. rotational seismology. a measure of rotations in seismology is of fundamental interest for (a) the determination of all the six degrees of freedom that characterize a rigid body’s motion, and (b) the quantitative estimate of the rotational motions contaminating ground translation measurements obtained from standard seismometers. within this framework, this paper presents and describes gingerino, a new large observatory-class rlg located in gran sasso underground laboratory (lngs), one national laboratories of the infn (istituto nazionale di fisica nucleare). we also report unprecedented observations and analyses of the roto-translational signals from a tele-seismic event observed in such a deep underground environment. i. introduction thanks to the optical measurement principle, ring laser gyroscopes (rlg) are insensitive to translations, thus representing the best sensors for capturing the tiny rotational motions associated with the transit of seismic waves. the need for a rotational sensor in geophysics was first claimed in a fundamental text about seismology (aki and richards (2009)), where the authors state that ”... note the utility of measuring rotation near a rupturing fault plane (...), but as of this writing, seismology still awaits a suitable instrument for making such measurements”. the search for such a sensor is of actual interest, as shown by many recent studies (kaláb et al. (2013); brokešová and málek 6“regional doctoral school in earth sciences”, xxx cycle, pisa, italy (2010); schreiber et al. (2006a)). so far, rlgs allowed to achieve important results in a variety of disciplines, spanning from geodesy (schreiber et al. (2004)) to both regional and global seismology (igel et al. (2005); pancha et al. (2000); simonelli (2014); schreiber et al. (2006a)). depending on the scope, the size or rlg changes from some centimeters to more than four meters. rlgs for navigation are very small and lightweight; they are produced commercially and are widely adopted on either underwater or airborne platforms. their sensitivity, however, is not sufficient for geophysical applications. sensitivity and accuracy of rlgs increase with size: maximizing the dimensions of the apparatus implies a minimization of those physical effects that provoke departure from an ideal linear regime. scientific results like monitoring of the solid-earth tides or a 1 annals of geophysics, 59, fast track 4, 2016 measure of the length of the day (lod) are only achievable by very-large-frame rlgs. at present, the g-ring deployment in wettzel (germany) represents the reference rlg for geodetic and seismological observations. smaller in size and less expensive are rlgs of the class geosensor, (schreiber et al. (2006b); belfi et al. (2012b,a)). recently, the italian national institute for nuclear physics (infn hereinafter) supported the development of the gingerino rlg. within the context of a larger project of fundamental physics, this apparatus is intended to reach the high sensitivity needed to observe general relativity effects, by the same time providing crucial data for geophysical and seismological applications (for more detail see the url https://web2.infn.it/ginger/ index.php/it/). ii. instrumental apparatus the gingerino is located within the infn’s gran sasso national laboratory (lngs) (fig. 1). figure 1: map of the lngs underground laboratories the equipment of geophysical and seismological interest is constituted by the following instruments: the large he:ne ring laser visible in fig. 2; this is a 3.6 m side square cavity ring laser installed over a granite structure block anchored to the rock of the b knot tunnel of the lngs. this is our rotation sensor, it is able to detect rotations around the symmetry axis (oriented vertically) with a sensitivity better than 10−10 rad/s in the band of interest for global seismology (5hz-300s). a nanometrics trillium 240s seismometer installed at the center of the rlg granite frame fig. 3, providing the ground translation data to be compared to the rlg rotational measurements for shear and surface waves from either micro-seismic or earthquakes sources. figure 2: the gingerino rlg figure 3: the nanometrics trillium 240 s (left) and guralp cmg 3t360s (right) and the lippmann 2-k tilt meter (on top), the red arrow shows the north direction this instrument is also part of the national earthquake monitoring program of the istituto nazionale di geofisica e vulcanologia (ingv hereinafter; see also the url http://iside.rm.ingv.it/iside/standard/ info_stazione.jsp?page=sta&sta=2571. a second broadband seismometer, guralp cmg 3t–360s (fig. 3) is placed in the central block for data redundancy. a lippmann 2-k digital tilt-meter with a resolution better then one nrad is placed beside the seismometer in 2 annals of geophysics, 59, fast track 4, 2016 figure 4: (top) ground rotation and transverse acceleration time histories (black and red lines, respectively), time zero is at 12:40:00 utc. (bottom) zlcc between the above traces. figure 5: zero-lag normalized correlation coefficient (defined between -1 (blue) and 1 (red)) between rotation rate and transverse acceleration, varying the unknown transverse direction from 0◦ to 360◦ . the theoretical propagation azimuth is 202◦. 3 annals of geophysics, 59, fast track 4, 2016 figure 6: superposition of transverse acceleration (black) and rotation rate (blue) and determination of phase velocities as a function of dominant period after narrow-band filtering. each trace has been individually normalised. phase velocities reported at the left side of the plot are obtained from the amplitude ratio of the envelope’s maxima (see text for details) order to monitor the possible slow ground tilt related to either local or wide scale (solid earth tides) effects. iii. method rlgs essentially consist of two laser beams counter-propagating within a closed-loop optical cavity (ring). if the cavity undergoes a rotation with respect to an inertial reference frame, then the optical path of the two beams changes, inducing a slight frequency shift between the clockwise and anti-clockwise propagating waves (sagnac effect). this tiny frequency separation produces a characteristic beat note once the two beams are mixed outside the ring. the beat frequency f , also called the sagnac frequency, is related to the rotation rate around the normal vector n to the surface enclosed by the ring (see fig. 2) through the equation: ω = λhe:ne l cos θ f (1) where λhe:ne is the wavelength of the he:ne laser (632 nm), l is the square side length of the optical cavity and θ is the angle between the versor n̂ and ~ω. we know from theory (aki and richards (2009)) that rotations can be retrieved from ground displacement as the curl of the wave-field. ~ω = 1 2 (∇×~u) (2) for example, the displacement caused by a love wave traveling as a plane wave along the x-direction is expressed through the equation: uy = ae iω(x/cl−t) (3) by applying eq. 3 to eq.2 we obtain the relationship: ωz = üy 2cl (4) 4 annals of geophysics, 59, fast track 4, 2016 which provides a direct estimation of the phase velocity cl from a single-site measurement. from this latter formulation it is also evident that the sensing of ground rotations over the seismic frequency band requires high sensitivity: considering the range for shear wave velocities within the earth, ground rotations are expected to be two to three orders of magnitude smaller than the associated translational movements. for this reason a very sensitive and completely decoupled from translations device is required, and at present large rlgs are the best candidates. iv. first results during the longest run of continuous data acquisition from june 11 to june 19, 2015 our instruments recorded an m=7 earthquake occurred on june 17, 2015 12:51:32 (utc), with epicenter at sea along the southern mid atlantic ridge. though the recordings exhibit a poor signal-to-noise-ratio (snr), their quality is sufficient to perform some analysis of seismological interest. the following processing steps have been performed. the n-s and e-w ground displacement traces are rotated with steps of 1◦ over the 0◦, 360◦ angular range. for each rotation step, the zerolag-cross-correlation (zlcc) between the rotational signal and transverse accelerations is calculated. the maximum correlation is found at a rotation angle of 198◦ n, which compares well with the 202◦ n theoretical azimuth derived from epicenter and station coordinates. such small discrepancy between the observed and theoretical azimuthal values is likely related to possible seismometer misorientation and deviation of surface wave trajectories from the great circle path as a consequence of lateral velocity heterogeneities.the zlcc between translational and rotational traces is calculated using a 200 seconds long window, sliding with 50% overlap. the love-wave arrival is marked by a clear correlation peak (see fig.4). ground rotations and transverse accelerations (respectively blue and black lines in fig. 6) are band-pass filtered over narrow frequency bands using a fir filter with 1-s bandwidth spanning the 1 s 50 s period range. in the frequency bands where zlcc is above a threshold of 0.7, the amplitude ratio between the maxima of the rotational and translational envelopes (evaluated via hilbert transform) provides a direct measure of phase velocity for that particular period (see fig. 6). v. conclusions in this paper we presented the preliminary results from the operation of gingerino, a ring laser gyroscope co-located with a broad-band seismometer within the infn’s gran sasso laboratories. our data constitutes the very first underground observations of earthquakegenerated rotational motions. the source backazimuth inferred from the directional analysis is in good agreement with the theoretical one, suggesting that the joint rlg seismometer deployment can provide precise, singlesite estimation of the propagation direction of seismic wave-fields. for those time intervals in which the translational and rotational signals are significantly correlated, we also obtained estimates of love-wave phase velocities, which span the 3800m/s – 4500m/s range over the 10s-50s period interval. such velocities are in good agreement with those expected for love waves propagating in the prem earth’s model (dziewonski and anderson (1981)). gingerino is presently running in a preliminary test mode: current efforts are aimed at optimizing the experimental settings in order to increase the sensitivity and to achieve continuous acquisition. such improvements will allow extending the seismological analyses to a ranges of magnitude larger than those considered until now (belfi et al. (2012c)). the simultaneous measurement of broad-band ground translation and rotation will thus permit the definition of the dispersive properties of love waves over a broad frequency range, from which a local shear-wave velocity profile can be inferred with resolutions on the order of 100 m and penetration depths up to several tens of kilometers. sensibility improvements will also permit studying the partition of elas5 annals of geophysics, 59, fast track 4, 2016 tic energy in the microseism wavefield (e.g., tanimoto et al. (2015)), whose main spectral peaks at the test site (3s and 10s) are at present only a factor five below our noise floor. references aki, k. and richards, p. g. (2009). quantitative seismology. university science books. belfi, j., beverini, n., bosi, f., carelli, g., di virgilio, a., kolker, d., maccioni, e., ortolan, a., passaquieti, r., and stefani, f. (2012a). performance of ”g-pisa” ring laser gyro at the virgo site. journal of seismology, 16(4):757– 766. belfi, j., beverini, n., bosi, f., carelli, g., di virgilio, a., maccioni, e., ortolan, a., and stefani, f. (2012b). a 1.82 m2 ring laser gyroscope for nano-rotational motion sensing. applied physics b, 106(2):271–281. belfi, j., beverini, n., carelli, g., di virgilio, a., maccioni, e., saccorotti, g., stefani, f., and velikoseltsev, a. (2012c). horizontal rotation signals detected by ”g-pisa” ring laser for the m w = 9.0, march 2011, japan earthquake. journal of seismology, 16(4):767–776. brokešová, j. and málek, j. (2010). new portable sensor system for rotational seismic motion measurements. review of scientific instruments, 81(8):084501. dziewonski, a. m. and anderson, d. l. (1981). preliminary reference earth model. physics of the earth and planetary interiors, 25(4):297 – 356. igel, h., schreiber, u., flaws, a., schuberth, b., velikoseltsev, a., and cochard, a. (2005). rotational motions induced by the m 8.1 tokachi-oki earthquake, september 25, 2003. geophysical research letters, 32(8). kaláb, z., knejzlík, j., and lednická, m. (2013). application of newly developed rotational sensor for monitoring of mining induced seismic events in the karvina region. acta geodyn geomater, 10(2):170. pancha, a., webb, t., stedman, g., mcleod, d., and schreiber, k. (2000). ring laser detection of rotations from teleseismic waves. geophysical research letters, 27(21):3553–3556. schreiber, k., velikoseltsev, a., rothacher, m., klügel, t., stedman, g., and wiltshire, d. (2004). direct measurement of diurnal polar motion by ring laser gyroscopes. journal of geophysical research: solid earth (1978–2012), 109(b6). schreiber, k. u., stedman, g. e., igel, h., and flaws, a. (2006a). ring laser gyroscopes as rotation sensors for seismic wave studies. in earthquake source asymmetry, structural media and rotation effects, pages 377– 390. springer. schreiber, u., igel, h., cochard, a., velikoseltsev, a., flaws, a., schuberth, b., drewitz, w., and müller, f. (2006b). the geosensor project: rotations, a new observable for seismology. in observation of the earth system from space, pages 427–443. springer. simonelli, a. (2014). il giroscopio laser come sensore di rotazioni: simulazioni e attività sperimentale. master’s thesis, university of pisa, https://etd.adm.unipi.it/theses/ available/etd-06202014-163524/. tanimoto, t., hadziioannou, c., igel, h., wasserman, j., schreiber, u., and gebauer, a. (2015). estimate of rayleigh-to-love wave ratio in the secondary microseism by colocated ring laser and seismograph. geophysical research letters, 42(8):2650–2655. 6 annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 1 social responsibility: a new challenge in graduate university education roberto g. lencina facultad de ciencias naturales e iml-universidad nacional de tucumán, argentina robertolencina@yahoo.com.ar abstract the key for including ethics and social responsibility (sr) in the university curricula is the recognition that, in our world, academia has become a relevant place for the production of knowledge and the promotion of values that support the integration of economic, social and environmental aspects, in order to build a better society. universities have become institutions with technical-professional and ethical-social objec-tives, whose purpose is to add value to society and all its stakeholders. at the heart of this assertion there is the conviction that the rs of the university acts as a double bridge: firstly, as a link between ethics and wisdom, then, as a connection between wisdom and commitment to sustainability and social cohesion. the concept of sr, in its broadest sense, is perceived in university classrooms as an understandable abstraction imposed by reality itself. even though it is not yet a formal subject of academic discussions, the truth is that daily professional practice, and the important social questions that practice raises, impose a new agenda in university curriculum design. 1. introduction ustainable development does not imply considering the resources of our planet to be intangible, but to promote a harmonic economic development. any approach to an integral ecology, which by definition includes human beings, needs to take account of the value of labour: “god placed man and woman in the garden he had created (cf. gen 2:15) not only to preserve it (“keep”) but also to make it fruitful (“till”)”. labourers and craftsmen thus “maintain the fabric of the world” (si 38:34). “the lord created medicines out of the earth, and a sensible man will not despise them” (si 38:4) (pope francis, 2015). “to take care of the land is not like taking care of a museum, which is a single place to guard, clean and preserve; keeping its pieces of art without any biological life in the best possible state. to take care of the land means to develop it according to its vital possibilities, taking full advantage of those potentials that god has designed, which is, in turn, a scientist’s task to keep on discovering, developing to the common human welfare” (sanchez sorondo m., 2018). on the other hand, the first decade of the 2000s was characterized by the emergence and proliferation of conflicts and socio-environmental movements in different communities in argentina. among them, those that arose in opposition to large-scale mining projects occupy an important percentage at the national level, with the “esquel case” being the precedent of the local community option (wagner, 2010). 2. current state of affairs of natural resources the permanent and increasing demand for mineral resources has led humanity to discuss the possible social and environmental consequences of the extraction of these products on large scales (figg. 1 and 2). as a recent fact to prove this, we can see the agreements achieved in africa (in congo principally) on deposits of cobalt, copper and molybdenum, metallic minerals, which will constitute the base of the technological advancement. many are the factors that encourage and push extractive companies to invest on exploration and exploitas annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 2 tion of minerals in different countries of our planet. the exponential growth of the need for rareearths, lithium, cobalt, graphite, copper, tin, tungsten and other base metals; the technological change led by electronic mobility; nonconventional energies and the paris climate agreement, which promises support to countries highly affected by global warming (below 2 degrees celsius) are only some of the reasons that entice companies to seek profits from natural resources (meilán, 2018). 3. the role of universities this current situation in natural resources matters demands more and better-prepared professionals, especially geologists and geophysicists who are committed to the specific studies in question. in this vein, higher education curricula for these professionals must include the basic meaning and implications of the notion social responsibility (sr) and of the new practices related to the extractive industries. therefore, university content should be the starting point to begin analyzing, discussing and incorporating all notions related to the traditional practice of exploration and exploitation plus the legal procedures and social responsibility to carry out the industrial activities. the key to understanding the institutionalization of ethics and sr in the university is the recognition that, in our world, academia has become a relevant site for the production of knowledge and the promotion of values that support the integration of economic, social and environmental aspects involved in building a better society. in this way, universities have become institutions with technical-professional and ethical-social objectives, whose purpose is to add value to society and all its stakeholders. at the heart of this assertion is the conviction that the sr of the university acts as a double bridge: first, it is a bridge between ethics and wisdom, and second, it is a bridge between wisdom and commitment to sustainability and social cohesion (guédez, 2012). sr in its broadest sense appears in university classrooms as an understandable abstraction imposed by reality itself. even though it is not yet a formal object of academic discussions, at least not in the “hard sciences”, the truth is that related daily professional practice, and the strong social questioning that exists are multiplying as a result of extractive industry projects figure 1: steel intensity for different power technologies, in tonnes per mw (wellmer, 2012). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 3 and impose a new agenda on university culum design. 4. the beginning of the crisis and the process of developing awareness in social responsibility matters people’s assemblies emerged in argentina at the beginning of the 21st century as a form of social expression that demanded environmental protection and knowledge about the destination of the income gained from productive projects like:  botnia: a conflict about a huge cellulose factory uruguay built on its side of the uruguay river (constanzo, 2009).  esquel: the meridian gold project that had to be cancelled because of strong popular opposition.  famatina: osisko’s gold project in the province of la rioja that was stopped because of popular opposition. this marked the beginning of a process that over time gained strength and forced the state and companies to consider this phenomenon. the consequences of these social movements were delays to investments in some cases and the definitive loss of investments in others. in the face of this scenario the argentine ties could not or did not know how to accompany this process in an organic way; to the contrary, in isolated and spasmodic occurrences universities pronounced themselves in favour of or against natural resource productive initiatives. this information would be of little importance from a purely academic point of view as it would not have an impact on their scientific production or on their research projects, but it is important to the degree it has a large social impact because of bad professional practices in government projects and decisions. it is generally clear that argentinian universities did not have the capacity to evaluate the situation correctly, especially with respect to their role in the development of university professionals. the national university of tucumán is a partner of ymad (yacimientos mineros de agua de dionisio), a mixed consortium that has rights to the exploitation of bajo de la alumbrera and farallón negro mines. in this case, the focus of the discussions since 1997 has been on royalties and on the negative consequences of the mining enterprise. now bajo de la alumbrera is at the final stage and its estimated final closure will be in 2018. here the curriculum appears as an absent actor: in spite of the fact that the universities are figure 2: the increase in the complexity of metal assemblages in generic products (reuter and kojo, 2012). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 4 undertaking a large process of accreditation and curricular renovation, sr does not appear among the priorities in academic education in central areas such as geology, engineering, biology, economy and others. 5. social responsibility as a crosscutting concept in curricular design in the face of this reality, university graduate curricular designs should seriously consider the incorporation of content related to sr. there is a need for a broad academic discussion that starts a process of transformation in the perception of the problems. university-private sector-state interaction has to be the engine that drives the changes; there has to be an understanding that curricular designs flow from permanent analysis of the reality that surrounds us and that structures have to be flexible and dynamic. achieving this will certainly be an enormous challenge in view of the almost total absence of debate on the subject. questions about content and strategies have to result from the different perceptions, and from the democratic agreement of the actors involved (teachers, students, graduates, etc.). the truth is that the proposal has to be born from the certainty that this is now a crying need and demand. the cross-cutting of the proposals, in which each formal and informal curricular space adds to the total, has to be central at the time of final design. this process is necessary to allow graduates to count on at least a minimal vision of real life. in the face of a probable lack of concrete experiences within the universities, we propose internships and paid practice, similar to canadian summer student programs in state organizations and companies in which sr is part of the experience. 6. the lack of formal debate about sr in argentine universities in argentina, especially in geology, discussion on the issue began in 1988 when the department of education and justice of the nation (mejn) summoned representatives of national universities and professional advisory committees to define the particular professional duties required for a geology degree. when an agreement was reached to define the profile of the geologist, resolution 210/88 promulgated these requirements (lacreu, 2005). the formal document defined three elementary aspects that answered to the following questions: what does the geologist know? what can he or she do? what values and basic principles do guide her or him to develop professionally? the last question reflected the challenges that called for discussion of sr issues. nevertheless, little was done in the academic curricula of geology after the reforms. in 2008, the notion of sr was a separate point within the plenary of agreement n. 44 of the university advisory committee. the same meeting set the standards for the process of accreditation following sr standards. once the process of accreditation was properly functioning, sr and ethical values contents were incorporated into the geologist’s profile. even though there has been considerable progress in terms of sr in general, the academic debate was absent until 2017 when it was brought to the fore in the frame of the xx argentine geological congress, held in tucumán. the event made space for re-opening the debate in some of the scientific meetings and symposia. these showed that there is still much work to do on this issue. 7. the ambit of application of sr curricular content this point of reflection appears almost obvious; however, we want to analyze what are the areas in which to apply the notions and commitments that flow from sr. university, as a place of work permanently linked with society, is without doubt the first place to apply sr in its narrow or broad sense. convictions clearly have to form the basis of public education policies. that is to say, “i practice what i teach”. this is probably the most important point of reference in this process. to reach this objective there ia a need for an earlier stage that consists of university actors’ knowledge of reality; this sensitization could then lead to making decisions on curricular annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 5 matters or at least trigger a discussion that results in a strong desire for change. obviously, if sr content is inserted into the universities, a second stage that consists of theoretical-practical preparation of teachers will begin. workshops and seminars will be good tools with particular modalities that can help with the exchange of opinions and the maturation of the central meanings that each university may want to give to the matter according to its perception of reality and especially the social context in which it carries out its tasks. finally, the classroom will be the ambit par excellence within which to work on the proposed concepts. here, the methodologies will vary with the possibilities of each university. it is important that the crosscutting direction of the contents be clear. some possible strategies can be internships with precise objectives, consulting for companies and integration with policy decision making. the final question thus will be “what do we do in the meantime”. in this case, the ambit of natural resources productive projects is a good place to introduce contents and principles. the university can work on the transition, proposing consulting assignments to companies in which social perceptions, theoretical frameworks, etc. can be involved. 8. work proposal with transversal sr content at this point, i will share some years of experience both in the university graduate classroom and in concrete work in mining projects (lencina, 2017). i propose the central axiom: knowing sensitization involvement commitment action  knowing: real case studies will be worked on in the classroom, with a brief orientation guide.  sensitization: it will flow from the analysis of the cases from multiple points of view (company employees, community members etc.)  involvement: it will be the inexorable next step when i propose trials that include different end points each with its connected arguments.  commitment: it is the key in the process. it is the driver that keeps the rest going. at this point i propose concrete actions that can be carried out in the framework of previous agreements that have been signed [internships, coop students, etc.]  action: acting is to make things concrete for learning; it is putting tension in convictions and proposals. it means transposition of ideas to the real world. i propose that students develop their own strategies than can be carried out in their projects (visits to community members, life stories, and informal meetings with company employees to exchange views, etc.). likewise, it is important to consider that in the natural resource projects the training has to include the professionals in a project, and all others from management levels down to employees with minor responsibilities. an additional object for this training task has to be the policy sector and the (actual) extractive productive projects in progress have to provide the opportunity for this experience. other points to for consideration include:  public presentations, formal project dissemination events, meetings with different political groups.  social acceptance of the initiatives has to involve the very professionals and employees of the companies; working based on conviction and not based on convenience will always be more sustainable.  sr in the curriculum is possible, provided those responsible for its design and implementation have been able to previously pass through the logical process of maturation of ideas, perceptions, theoretical frameworks and reigning reality. its transversality will then be unquestionable because the reference to real cases will necessarily lead each curricular space to deal with the issue and assign classroom time to it. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 6 the freedom to think and express oneself, without dogmas or intellectual authoritarianism will be very important as part of the process that we suggest initiating in university classrooms and that certainly has an open and proper final outcome for each professional throughout her or his life. 9. the fiambalá case, catamarca, argentina one of the major challenges related to obtaining of the social license for a lithium project took place in the district of fiambalá, catamarca, northwest argentina. in 2012, inhabitants of fiambalá and a neighboring city, tinogasta, were affected by one of the most relevant events in the mining sector: the announcement of possible uranium exploration projects of. the announcement encouraged a great part of the society and some political groups committed to environmental issues to take action using any means to prevent the projects from reaching state’ approval. at the end of 2016, the international company liex s.a. arrived in argentina. the company, where i work as consultant, functions as a neolithium corp's subsidiary, has also experienced social opposition. in some parts of the country, such as cauchari, jujuy, la carolina, san luis, opposition to mining led to violence that, in some cases led to ceasing all project activities. in view of the possibility of such outcomes, liex s.a. began a complex process of social insertion on the basis of dialogue and transparency of operations. at present, the company is properly functioning in the zone and has been accepted by the community. what is more, the same firm has established high standards of sr and environmental ethics. this fact is not only highly estimated by the local government but also by the community and other companies in the field. to achieve social insertion and acceptance, liex s.a. is working with a special attention to the community's interests and priorities. the following points are very important in their day-to-day operations:  definition of integral policies that include sr among the priorities.  convictions that must be adopted by the ceo of the company and transmitted to the whole staff.  permanent education of the social actors (employees-community-politicians-mass media).  knowledge of the community.  dialogue and negotiation.  responsibility for the industrial activities of the company.  knowledge of the real interests of all actors involved.  transparency in the actions.  developing agreement.  to make state-community-company work as a single entity. moreover, conditions for an effective social insertion policy are the following:  creating work teams with social sensibility and experience.  investing in community relations from the first steps of activities.  performing activities with logical sense.  adopting respectful beahviours and humility.  knowing and respecting times and processes of local communities.  using an ethical approach in every step of the productive process.  respecting those who think differently and learning to coexist.  respecting the freedom of press and mass media. 10. conclusion: a proposal for implementing strategies to incorporate the notion of sr in argentine universities my experience in numerous mining projects in argentina; my analysis and participation in different stages of the process of academic incorporation of sr in the career of geology; and the growth of the number of social conflicts linked to the extractive industries, all lead me to conclude that there is a gap between the contents of the curricula and the actual professional practices that university graduates need. hence, in an attempt to reduce the gap between knowledge and practice, design of the new curricula must consider two dimensions: annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 7 a. political dimension the actual situation of most extractive enterprises and the obstacles that they have to face due to lack of knowledge about sr and all its implications in the process of social insertion have to become common knowledge, especially for state organisms. knowing this state-of-affairs and taking public action on the matter will provide transparency to the affected society and an authorized course of action to industry, according to the standards of sr. to achieve such a possible solution, university hierarchies and committees should begin dealing with the issue in their advisory assemblies and meetings. maybe, discussing the most troubled and violent situations and registering in a formal document all sort of drawbacks posed either for the industrial group or the society will be useful. b. academic dimension university committees in charge of curriculum designs and career contents must take a prominent role. the starting point to change the situation must center in the institutions that form future professionals. then, those in charge of the design of higher education courses should be aware of the negative and positive outcomes of including a comprehensive knowledge of sr or lack thereof within the academic curricula. as a complement of the obligatory subjects, the curricula may include complementary subjects directly related to the issue of sr. a place where students can discuss, analyze, evaluate and understand the issue with the guidance of professionals experienced in related matters. finally, to check the relevance of adopting the proposals or refusing them thorough research is necessary, including surveys and analytical investigations that can prove the socio-economic impact of extractive activities that are socially and environmentally aware. the road is long and complicated, but reality is actually even more complicated. transforming reality starting locally and through small actions without neglecting the construction of more favourable scenarios over time will be the challenge to work on. references constanzo m. (2009). papeleras: un problema de comunicación. propuesta de plan integral de rr pp para mejorar las relaciones entre botnia y gualeguaychú. proyecto de graduación. trabajo final de grado, universidad de palermo, argentina (in spanish). guédez v. (2012). la institucionalización de la ética y de la responsabilidad social en la universidad: hacia una nueva cultura organizacional. en: enfoque y herramientas de formación en responsabilidad social empresarial. isabel licha, compiladora. pnud (in spanish). lencina r. (2017). la responsabilidad social como un nuevo desafío curricular en la formación de grado en las universidades argentinas. actas xx congreso geológico argentino. 7-11 agosto 2017, tucumán, argentina (in spanish). meilán d. (2018). el tan resonado “no a la mineria” ¿adónde nos lleva? artículo revista mining press: http://www.miningpress.com/documento /2585/daiel-meilan-que-afecta-a-la-mineria (in spanish) (accessed 22 february 2018). pope francis (2015). encyclical letter “laudato si’” on care for our common home, chapter one, http://w2.vatican.va/content/francesco/e n/encyclicals/documents/papafrancesco_20150524_enciclica-laudato-si.pdf (accessed 30 january 2018). reuter m., guyonnet d. and bleischwitz r. (2012). vers une utilisation éco-efficace des matières premières minérales. géosciences, 15, pp. 56-63 (in french). reuter m.a. and kojo i.v. (2012). challenges of metals recycling. materia, 2, p.p. 50-56. sanchez sorondo m. (2017). relevancia de la encíclica laudato si en el tema de la educación, http://www.javeriana.edu.co/documents/ 15832/5510469/ponencia+monsen%cc%83 or+sa%cc%81nchez+sorondo.pdf/14594b b6-36ca-4a01-b285-797db39ae3e6 (in spanish) (accessed 22 february 2018). wagner l. (2010). problemas ambientales y conflicto social en argentina. movimientos socioambientales en mendoza. la defensa del agua y el rechazo a la megaminería en los inicios del siglo xxi. argentina. tesis annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7559 8 doctoral, universidad nacional de quilmes (in spanish). wellmer f.-w. (2012). sustainable development and mineral resources. géosciences, 15, pp. 8-15. 201_205 adg vol5 n01 miro.pdf annals of geophysics, vol. 45, n. 1, february 2002 201 short note worst cases for a one-hop high frequency link gloria miró (1), benito a. de la morena (1), sandro maria radicella (2) and miguel herraiz (3) (1) atmospheric sounding station «el arenosillo», national institute of aerospace technology, magazón, huelva, spain (2) aeronomy and radiopropagation laboratory, abdus salam ictp, trieste, italy (3) department of geophysics and meteorology, faculty of physics, complutense university, madrid, spain abstract the characterisation of a hf channel by means of monthly electron density profiles can be complemented with a detailed study of radio propagation «worst cases» on situations with extremes conditions of radiopropagation for a given period. these «worst cases» correspond to conditions that can be identified by means of cumulative distributions of the key parameter f0 f2. this paper analyses the main parameters of the hf channel: time delay, apogee, elevation angle and transmission frequency with mean and extreme conditions. the method used to characterise the ionospheric channel is based on ray-tracing techniques. 1. introduction an hf communication link is characterised by the maximum usable frequency that is the limit above which the signal will not be reflected by the ionosphere under medium conditions. however, it is convenient to study all possible conditions. for this reason, an analysis of worst cases aimed at extending the results obtained by means of monthly median conditions to extreme ionospheric situations. the concept of worst cases was introduced in ray tracing techniques (miró et al., 2000) to extend the channel characterisation to extreme ionospheric situations far from median conditions for hf links. worst cases selection was performed using f0 f2 through hourly cumulative frequency distributions. the 2d ray tracing technique used (called abcray03), is based on the expressions introduced by croft and hoogasian (1968) and croft (1969) and it has been considered in previous ionospheric studies (moorhead and radicella, 1998; miró, 2000). the outputs obtained with this computing technique are the main radio propagation parameters: time delay of the signal travelling from the transmitter to the receiver, reflection height or apogee, elevation angle and transmission frequency. mailing address: dr. gloria miró, atmospheric sounding station «el arenosillo», national institute of aerospace technology, 21130 magazón, huelva, spain; e-mail: miroag@inta.es key words ionosphere − radio propagation − worst cases − ray-tracing 202 gloria miró, benito a. de la morena, sandro maria radicella and miguel herraiz a particular ionospheric condition is introduce in the ray tracing program using model or experimental electron density profiles. 2. data as previously indicated, the parameter f0 f2 was used to perform the worst cases selection through hourly cumulative frequency distributions. this study considered hourly revised f0 f2 values from the digisonde dgs256 situated at el arenosillo atmospheric station and the 19931997 period. an average of 1826 values was available for each 24 h and cut-off frequencies limiting 1% of these values with either the lowest or the highest frequencies have been chosen as worst cases conditions. for example, at 00 ut hours 1% of cases had f 0 f 2 lower than 2.4 mhz and higher than 7.3 mhz. once the worst cases selection has been done, the corresponding electron density profiles are introduced in the ray tracing technique. 3. methodology at each hour, profiles belonging to these upper/lower conditions are grouped to calculate a mean upper/lower extreme profile. this profig. 1. hourly monthly electron density profiles and the corresponding to upper and lower selection at el arenosillo atmospheric sounding station. 203 worst cases for a one-hop high frequency link cedure provides three mean profiles per hour: upper extreme, lower extreme and hourly mean profile for all cases. these mean profiles were obtained following the methodology described in huang and reinisch (1996). figure 1 shows an example of such profiles. as has been noted, the ionospheric conditions in the reflection point necessary to characterise the hf channel are introduced by electron density profiles. it is important to point out that the profiles obtained after the worst case selection belong to one of the link extremes, el arenosillo station. however, the particular link between el arenosillo and ebro observatorio is situated at fig. 2. oblique ionograms obtained by abcray03 program corresponding to upper, lower and mean conditions. midlatitudes. therefore, communications with a range close to 785 km correspond to one ionospheric hop with e (e mode) or f (f mode). moreover, the ionospheric conditions at the reflection point which is located around the middle point in this specific link, can be approximate to data from one of the extremes, transmitter or receiver (miró, 2000). using the three mean profiles at each hour (fig. 1), ionospheric conditions are introduced in the abcray03 program to derive at each 24 h the radio propagation parameters: transmission frequencies, elevation angles, time delays and apogees under extreme and standard situations. 204 gloria miró, benito a. de la morena, sandro maria radicella and miguel herraiz 4. results the distribution of the group path p’ (proportional to time delay) versus the transmission frequency gives what is known as an oblique ionogram. figure 2 displays synthetic oblique ionograms obtained with the ray tracing technique corresponding to some of the 24 h studied. differences between the three conditions (upper extreme, lower extreme and mean) can be clearly seen mainly at f 2 heights. from the hf communication systems point of view, the main problem is to find out the existence of low f0 f2 on low electron density values. this situation is critical because this phenomenon reduces the maximum usable frequency and the communication frequency band becomes narrower than that from mean and upper conditions (fig. 2). for this reason, special attention has been paid to the number of cases in which the signal arrives at the receiver in spite of the narrowing of the frequency range originated by low electron density conditions. results are summarised in table i. as can be seen, only daylights hours (8:00 ut 18:00 ut) provide an acceptable percentage of situations in which a link is established under all conditions. as expected, most of these signals belong to e modes that do not appear during the rest of the hours. in general, if the daily transmission is done with a frequency between 6 and 8 mhz, the signal will reach the receiver under extremes and mean situations. this study indicates that the characterisation obtained using ionospheric mean conditions can be applied in most situations. only some cases table i. number of lower conditions cases which arrive at the receiver. hour (ut) total lower % e mode % e mode f mode % f mode 0:00 4 1 25 0 0 1 100 1:00 3 1 33 0 0 1 100 2:00 4 1 25 0 0 1 100 3:00 4 1 25 0 0 1 100 4:00 4 1 25 0 0 1 100 5:00 7 1 14 0 0 1 100 6:00 7 0 0 0 0 0 0 7:00 16 2 13 1 50 1 50 8:00 23 15 65 11 73 4 27 9:00 24 18 75 13 72 5 28 10:00 26 20 77 17 85 3 15 11:00 28 19 68 17 89 2 11 12:00 30 21 70 16 76 5 24 13:00 28 20 71 17 85 3 15 14:00 27 20 74 16 80 4 20 15:00 27 18 67 15 83 3 17 16:00 28 16 57 11 69 5 31 17:00 27 12 44 5 42 7 58 18:00 24 6 25 2 33 4 67 19:00 19 2 11 0 0 2 100 20:00 15 2 13 0 0 2 100 21:00 11 2 18 0 0 2 100 22:00 6 1 17 0 0 1 100 23:00 6 1 17 0 0 1 100 205 worst cases for a one-hop high frequency link during night hours can present problems if the transmission frequency used is near the maxi-mum usable frequency because in these conditions, the signal will not be reflected by the low ionospheric electron density present in its ray path. 5. conclusions the main conclusion reached in this paper can be summarised as follows: – in general, the characterisation obtained by means of monthly electron density profiles (miró, 2000) could be applied even with extreme ionospheric conditions. the main problem can appear during night time for transmission frequencies close to the maximum usable frequency. acknowledgements the authors are grateful to dr. moorhead of neptune radar ltd., gloucester u.k., for his help in understanding the ray tracing technique, abcray03. this work has been possible thanks to financial support granted by the national institute of aerospace technology (inta). references croft, t.a. (1969): methods and applications of computer raytracing, stanford electronics labs tech., report. susel69-007. cro f t, t.a. and h. h o o g a s i a n (1968): exact ray calculations in a quasiparabolic ionosphere, radio sci., 3, 69-74. huang, x. and b.w. reinisch (1996): vertical electron density profiles from digisonde ionograms the average representative profile, ann. geofis., 39 (4), 751-756. miró, g. (2000): caracterización del canal ionosférico para un enlace hf punto a punto mediante técnicas d e trazado de rayos, ph.d. thesis, universidad complutense de madrid. miró, g., s.m. radicella, m. herraiz, and b.a. de la morena (2000): análisis de situaciones extremas para la caracterización del canal ionosférico en comunicaciones hf, in tendencias actuales en la investigación de la ionosfera, edited by m. herraiz and b.a. de la morena, colección física de la tierra, n. 12, universidad complutense de madrid, 337-351. moorhead, m.d. and s.m. radicella (1998): using 2d raytracing to model the effects of vertical profile variability on oblique path propagation at hf frequencies, in proceedings of the side workhop of cost 251. l ' e r u z i o n e l a t e r \le d e l l ' e t n a del 3 0 6 1 9 4 2 e susseguenti fenomeni esplosivi al c r a t e r e c e n t r a l e g \ i : t \ n o p o m i . la m a l l i i i a (lei .5(1 giugno 19-12 alenili m i l i t a l i s a l e n d o sul c r a t e r e e e n i r a l e deli l i n a p e r il s e n t i e r o a \\" d e l l ' i )s>er\ a l o r i o . v e r s o quota .'5100 s e n t i r o n o t r e m a r e il suolo o t t o i loro p i e d i . p e r q u a n t o p r e o c c u p a l i dell i n s o l i l o fatto v o l l e r o p r o s e g u i r e fino ali orlo ove g i u n s e r o verso le o r e 8 e \i si f e r m a r o n o c i r c a m e z z o r a a v e n d o t r o v a t o il c o n o i n t e r e r a l e r i e o c a l i n o d o p o l ' i n t e n s a a t t i v i t à esplosiva clic d u r a v a da più di un m e s e . a n c h e la hocea s u h l e r m i n a l e di \ l fumava l a c i t a m c n t e . \i r i t o r n o , m i l l o slesso posto ove alla a l i l a a v e v a n o s e n t i t o t r e m a r e il n o l o , a v v e r t i r o n o a n c o r a l'orli c u o i i m e n i i e rumori o l t c r r a n e i c h e ii c o l r i n s c r o ad a l l o n t a n a r s i r a p i d a m e n t e . mie iii . 1 0 " . ora l e g a l e , poco p r i m a c h e giunge—ero al l a c a n t o n i e r a e t n e a ( 1 8 8 0 ili), u d i r o n o un f o r t i s s i m o b o a t o al q u a l e ne s e g u i r o n o a l i l i , a v v e r t i t i anc h e da ( l a l a n i a . l ing. s . p o n t e , t e n e n t e deli v e r o n a u l i e a . il (piale t r o v a v a i ali a l b e r g o l i n a ( l(>.>0 m s m ) per -crv izio m i l i t a r e , c o m u n i c ò u b i l o a l l ' i s t i l l i l o di \ iileaiiologia c l i c a p o n e n t e dei m . f r u m e n t o s u p i n o , a l l e o r e 10 e ìò'" ( i ) -i e r a n o a p e r t e d e l l e b o c c h e e s p l o s i v e , d a l l e q u a l i -i s o l l e v a v a n o m a t e r i a l i i n c a n d e s c e n t i e d e n s e c o l o n n e di f u m o a c c o m p a g n a l e da b o a t i , f u s u b i t o c h i c l a al c o m a n d o d e l i " a e r o n a u t i ca una r i c o g n i z i o n e a e r e a p e r a v e r e la esatta u b i c a z i o n e d e l l e b o c c h e e r u t t i v e e del c o r s o d e l l a c o l a t a lav ica. nei p o m e r i g g i o si a v e v a n o (lidie r i u s c i t i s s i m e l'olografie a e r e e del t e a t r o e r u t t i v o c l i c p e r m i s e r o di •-egnare sulla nuova c a r t a s l c r e o f o l o g r a m m e t r i c a dell i s t i t u t o g e o g r a fico m i l i t a r e ii posto e s a l t o d e l l e b o c c h e e r u t t i v e e il c o r s o d e l l a col a l a . s u ' fianco m e r i d i o n a l e del v u l c a n o , a l l a b a e del c o l i o t e r m i n a l e , ad ovest del m. f r u m e n t o s u p i n o , -i e r a n o a p e r l e diverse s p a c c a t u r e p a r a l l e l e d i r e t t e vei so s\v. duo di e—e o r a n o m o l l o e v i d e n t i . l a più al l a i n c o m i n c i a v a da q u o l a 2 . . ) 0 . un c e n t i n a i o di m e t r i o l i o il c i g l i o n e del p r e i s t o r i c o c r a t e r e del p i a n o del la( j a k t a n o p u n i i ; go e >i e s l e n d e v a c i r c a 8 3 0 in in b a s s o , c i o è (ino a «[itola 2 5 5 0 . nel t r a i l o s u p e r i o r e di questa f e n d i t u r a e r a n o a t t i v i s s i m e v a r i e b o c c i l e e s p l o l i v e , m e n i r e la lava sgorgava d a l l a p a r t e i n f e r i o r e . l a l t r a spacc a t u r a si era a p e r t a poco più in b a s s o , c i r c a 6 0 in ad est della prim a ed era lunga c i r c a 70 m ; si e l e n d c \ a da q u o t a 2 5 4 5 a q u o t a 25.'!!l in d i r e / i o n e ss\\ ed aveva v a r i e b o c c i l e e f f u s i v e v i c i n i s s i m e , d a l l e q u a l i g o r g a v a la m a g g i o r e q u a n t i t à di l a v a . le s p a c c a t u r e f u r o n o suliilo c o p e r t e dai d e t r i t i l a n c i a l i d a l l e e s p l o s i o n i , q u i n d i non si poi è v e d e r e da qual posio e r a n o i n c o m i n c i a l i i t r a b o c c h i di lava d u r a n t e il p r i m o i m p e l o e f f u s i v o . m i e o r e 1 2 , 3 0 ' . «piando f u r o n o f a l l e l e p r i m e l'olografie a e r e e , c i o è c i r c a un ora e mezza d o p o lo s c o p p i o dell e r u z i o n e , la lava aveva p e r c o r r o d u e c h i l o m e t r i in d i r e z i o n e s\\ . \ ioino alla p i c c o l a c o l a t a del ifi7'). su p e n d i o i n c l i n a t o del ."><) e m a l g r a d o il f u m o i n t e n s o , d i e m a s c h e r a v a in gran p a r i e il t e a t r o e r u t t i v o , -i s c o r g e v a n o d i v e r s e col a l e a c c o s t a l e le u n e a l l e a l t r e e in m o l l i posti a e o n t a l l o ira di l o r o . f ' i m p o r t a n t e l a r e r i l e v a r e c l i c le f o t o g r a f i e l'alte d a l l ' a e r o n a u t i c a m i l i t a r e m o s t r a n o in m o d o c h i a r o c h e d u r a n t e 1 e r u z i o n e l a t e r a l e il c r a l e r e c e n t r a l e e la b o c c a di \ k e r a n o p e r f e t t a m e n t e c a l m i . il c u s t o d e d e l l ' i )ssci'\aiori( c i u c o , \ . h a r b a g a l l o . il (piale a l l o s c o p p i o dell e r u z i o n e si era stillilo r e c a l o v i c i n o al t e a t r o e r u t t i v o , com u n i c ò c h e una s e r i e di b o c c h e e s p l o s i v e e r a a t t i v i s s i m a lungo le s p a c c a t u r e ed i m a t e r i a l i p i r o c l a s t i c i l a n c i a l i a v e v a n o f o r m a l o dei c o n c i l i m a m m i l l a r i di s c o r i e alti da .'> a 5 in. le b o c c h e a (piota più a l l a , le p r i m e ad a p r i r s i , ( o r m a r o n o un g r u p p o s t a c c a t o da q u e l l o el'fu-ivo e l a n c i a v a n o p r e v a l e n t e m e n t e i n a i c i iali non c o e v i , p e r c h è in (pici posto la s p a c c a t u r a era più p r o f o n d a . il c u s t o d e c o s t r e t t o a r i m a n e r e sul lato o r i e n t a l e d e l l e b o c c h e e r u t t i v e p e r l ' a b b o n d a n t e f u m o c l i c il v e l i l o r i p i e g a v a verso sud, non p o t è , in q u e l l a p r i m a vigila al t e a t r o e r u t t i v o v e d e r e il c o r s o d e l l e c o l a l e c l i c , p e r ò , l'inolio s u b i l o messe in evid e n z a d a l l e l'olografie a e r e e a v u t e d a l l ' a e r o n a u l i c a . noi p o m e r i g g i o il c u s t o d e r i t o r n ò ai t e a t r o e r u l l i v o e s a l e n d o verso la p a r i e più a l l a r i s c o n t r ò c h e il s u o l o , a p a r t i r e d a l l ' i n i z i o d e l l e s p a c c a t u r e , c i o è da q u o t a 28011 fino a «piota 3 0 0 0 , p r e s e n t a v a n u m e r o s e f e n d i t u r e non c o n t i n u e , d i r e l l c «piasi t u l l e verso ssw , l a r g h e fino a un m e t r o e j i r o f o n d e 3 e più n i e l l i , e s s e m o s t r a v a n o n e l l ' i n t e r n o d e l l a n e v e , q u e l l a sic—a olio il in m a r z o 1940 era s i a l a c o p e r t a dai dell'ili l a n c i a t i dal v u l c a n o d u r a n t e q u e l l ' i n t e n s a a l i ì v i t a p l i n i a n a e ohe, b e n p r o l o l l a dal o l e , si m a n t e n e v a da due a n n i i n quel p o s t o . i . ' k h u z i o . n k i i k i . i . ' k t n a i i k i . .ill-.'i-l12 1113 e v i d e n t e m e n t e il m a g m a e r a p e n e t r a t o l a t e r a l m e n t e a p a r t i r e d a l la p a r i e a l l a del c o n d o t t o e r u t t i v o e -i e r a i n c u n e a t o f r a a n t i c h e c o l a l e d e l l a c o m p a g i n e e l e v a l a del v u l c a n o . d u r a n t e la p e n e t r a z i o n e , l a t e r a l e del c u n e o i g n e o di lava ii t e r r e n o e r a s t a t o f o r t e m e n t e c o m i ed in line q u a — a l o a p a r t i r e d a l l a b a s e del c o n o t e r m i n a l e ove app a r v e r o l e p r i m e s p a c c a t u r e , m e n t r e più giù e n e a p r i r o n o a l t r e l i l l e q u a l i i m p i a n t a r o n o i c o n c i l i e s p l o s i v i e le b o c c h e c l l u i v e . \ ersi» ii c a l a r e del g i o r n o .'sii g i u g n o la p o r t a l a d e l l a lava e r a d i m i i i u i t a n o t e v o l m e n t e . p i ù l a r d i , n e l l a n o t t e a v a n z a l a , e n t r ò in a l t i \ i;ìi e s p l o s i v a ii c r a t e r e c e n t r a l e , c l i c , ali i n i z i o dell e r u z i o n e l a t e r a l e , e r a r i m a s t o c a l m i s s i m o . d a l l a s t a z i o n e c a n t o n i e r a , ove -i t r o v a v a il c u s t o d e l i a r b a g a l l o . e d a l l ' a l b e r g o e t n a , ove s o s t a v a n o dei m i l i t a r i , f u r o n o a v v e r t i t i l'orli b o a t i , m e n l r e c a d e v a una d e n a p i o g g i a di cen e r e v u l c a n i c a l a n c i a l a dal c r a t e r e c e n t r a l e , e n t r a t o in a t t i v i t à e s p l o siva fra vivi b a g l i o r i di f u o c o . d o p o p o c h i m i n u t i le e s p l o s i o n i a n d a r o n o d i m i n u e n d o d i n t e n s i t à con q u a l c h e b r e v e , m a v i v a c e r i p r e s a e d u r a r o n o c i r c a u n ' o r a e m e z z a . p o c o d o p o la m e z z a n o t t e l u t t o e r a e e s s a l o , a n c h e ! e r u z i o n e l a t e r a l e . la m a t t i n a del i" l u g l i o le b o c c h e l a t e r a l i e r a n o t r a n q u i l l e ed a n c h e il c r a t e r e c e n t r a l e . il b r a c c i o d e l l a c o l a l a più a v a n z a l o e più l a r g o a n d ò a f e r m a r s i p o c o a n o r d del m o n t e f o n t a n e l l e s u l l ' a l t o v e r s a n t e s w d e l l ' e t n a , v e r s o q u o t a 1 7 5 0 , (piasi al m a r g i n e d e l l a p i n e t a di h i a n c a v i l l a , d o p o un p e r c o r s o , a p a r l i l e d a l l e b o c c i l e più b a s s e , di c i r c a i r e c h i l o m e t r i c o n una l a r g h e z z a da 150 a 2 0 0 m . il b r a c c i o sud più c o r t o e p i ù s t r e t t o -i f e r m ò a q u o t a 1940. la q u a n t i t à totaledi c i r c a un m i l i o n e e m e z z o di m e t r i di lava ci fa c l a s s i f i c a r e q u e s t a e r u z i o n e l a t e r a l e i r e le più p i c c o l e a v v e n u t e siili f i n a . la sua p o r t a l a non d o v e t t e s u p e r a r e i 3 5 m : al s e c o n d o . i d a n n i a r r e c a l i f u r o n o po( h i . a v e n d o la c o l a l a c o p e r t o t e r r e n i privi di v e g e t a z i o n e a r b o r e a c o n m a g r i p a s c o l i , p r e v a l e n t e m e n t e c o s t i t u i t i da islriiualus aclncnsis. nella n o t t e dal i" al 2 l u g l i o , f u r o n o \ i-li d e b o l i b a g l i o r i i n t e r m i t t e n t i al c r a t e r e c e n t r a l e e duranteil g i o r n o 2 f u r o n o u d i t e d a l l a c a n t o n i e r a e t n e a e d a l l ' a l b e r g o f i n a d e l l e e s p l o s i o n i c h e r i p e t e v a n o a l u n g h i i n t e r v a l l i a c c o m p a g n a l i dal l a n c i o di c e n e r e c h e con i v a p o r i v u l c a n i c i f o r m a vailo d e l l e (leui s « i m c v o l u t e o s c u r e . il 4 l u u i i o |lassò p i u t t o s t o t r a n q u i l l o , m a a l l e 2 2 ' 15" f u r o n o visti ili t u t t a la s i c i l i a vivi b a g l i o r i sul c r a t e r e c e n t r a l e c h e -|ic--u liv c n i v a n o i n t e n s i s s i m i e d u r a v a n o l u l l a la n o t t e . m i e o r e 6 . 5 5 " del g i o r n o 5 f u r o n o uditi da c a t a n i a e dai paesi e t n e i dei f o r t i s s i m i e p r ò illfl i.vki i m i u n i i ; l u n g a l i h o u l i p r o v e n i e n t i dal c r a t e r e c e n t r a l e , a c c o m p a g n a l i da l a n c i o di d e t r i t i . sul v e r s a n t e m e r i d i o n a l e e o r i e n t a l e dell f i n a c a d d e r o m o l t e s c o r i e c o e v e c h e , s p i n t e dal v e n t o , g i u n s e r o l i n o a c a t a n i a o v e in un m e t r o q u a d r a t o n e f u r o n o r a c c o l t e gr 1.70 c i r c a . l a t t i v i t à e s p l o l i v a i l u r ò lino a l l e o r e 13 s e n z a t r e g u a ; p o i a n d ò d i m i n u e n d o e c e s s ò c o m p l e t a m e n t e a l l e 14. l ' a s s i s t e n t e d o l i . v b b r u z z c s e , c l i c q u e l l a m a l l i n a t r o v a v a s i in c a m m i n o v e r s o il c r a t e r e c e n t r a l e a s s i e m e al e l i l o d e b a r b a g a l l o , p o t è m e g l i o d ogni a l t r o o s s e r v a r e da v i c i n o l ' a n d a m e n t o d e l l i m p o n e n t e f e n o m e n o , \ e r o l e o r e 7 v i d e sul c r a t e r e c e n t r a l e f r a i n t e n s i b a g l i o r i , d e l l e c o n t i n u e m a s s e r o v e n t i : i a r i a fu r a p i d a m e n t e o l i e n e b r a l a , m e n t r e u n a g r a g n u o l a di l a p i l l i c a d e v a t u l i a l t e r n o c o n f r a g o r e a s s o r d a n t e . m a l g r a d o il p e r i c o l o di q u e l l a p i o g g i a <ii grò —i l a p i l l i , c h e si f a c e v a s e m p r e p i ù a b b o n d a n t e , i a s s i s t e n t e e il c u s t o d e v o l l e r o p r o s e g u i r e c o p r e n d o s i la testa c o m e m e g l i o p o t e r o n o e g i u n s e r o ali o s s e r v a t o r i o e t n e o (29-12 m s m ) c o n g r a n d e d i f f i c o l t à p e r i m a t e r i a l i v i a v i a s e m p r e p i ù g r o s s i c h e c a d e v a n o . il p r o f . c . c u m i n . d i r e t t o r e d e l l ' i s t i t u t o di g e o g r a f i a , c l i c t r o v a va-i a l l a c a n t o n i e r a e t n e a ivi r e c a t o s i a n c l i ' c g l i p e r lo s t u d i o d e l l ' e r u z i o n e . o s s e r v ò elle la e a d u l a d e l l e s c o r i e e r a i n t e r m i t t e n t e ed ogni ."> 0 (> m i n u t i a v e v a d e l l e r i p r e s e . il r u m o r e c o n t i n u o , i o n i e di un f o r t e i n o l i a r e fu a n c h e a v v e r t i l o da c a t a n i a e d u e v o l l e fu cosi i n t e n s o d a s e m b r a r e d v o l o d ' u n o s t o r m o d ' a e r o p l a n i s u l l a c i t t à . q u e s t a f o r m i d a b i l e a t t i v i t à p l i n i a n a , c l i c si m a n i f e s t ò a c c o m p a g n a l a da m i t r i l i g e l l i di s c o r i e c o e v e p r o v e n i e n t i d a l l o s b r a m i c i l a m e n t o d e l l a g r a n d e m a s s a di lava c h e si s o l l e v a v a a f o n t a n a , d u r ò fino a l b o r e 13 e a l l e 14 s u b e n t r ò 1 la più c o m p l e t a c a l m a . il 7 l u g l i o il c u s t o d e d e l l ' o s s e r v a t o r i o , in s e g u i t o ad u n a v i s i t a b i l i a al c r a t e r e c e n t r a l e , t r o v ò c l i c l e s p a c c a t u r e f o r m a t e s i d u r a n t e 1 e r u z i o n e l a t e r a l e del 3 0 g i u g n o sul f i a n c o s\\ del c o n o t e r m i n a l e , e r a n o s i a t e c o m p l e t a m e n t e c o p e r t e d a l l ' e n o r m e m a t e r i a l e c a d u t o il •"> l u g l i o . sui h a l l o tra l ' o s s e r v a t o r i o e t n e o e l ' o r l o s\y d e l c r a t e r e c e n t r a l e si s c o r g e v a n o , f r a le a b b o n d a n t i s c o r i e c o e v e c a d u t e , d e i m a s s i di c o n s i d e r e v o l e g r o s s e z z a fino a m e z z o m c l r o c u b o . 11 c o n o i n l e r c r a t e r i c o e r a s t a l o c o p e r t o d a l l a g r a n d e m a s s a di m a t e r i a l e c h e si a c c u m u l ò s u l l a lei razza c r a t e r i c a d i n a n t e l ' a t t i v i l a p l i n i a n a . ivi e r a s c o m p a r s o ogni r i l i e v o . si s c o r g e v a s o l o d e l m a t e r i a l e c a o t i c o c o s t i t u i t o da f r a m m e n t i di v a r i e d i m e n s i o n i m i s i i a l a v a c o e v a p i ù o m e n o scor i a c e a e f r a m m e n t a r i a . li m a l e r i a l e c h i s i i i a l i c o l a n c i a l o dal c r a t e r e c e n t r a l e il 5 l u g l i o i . ' e i i u z i o . m : i i e i . i . ' e t v a d e i . 3 0 6 1 9 1 2 1942 s u p e r ò in ( p i a n t i l a q u e l l o l a v i c o d e l l ' e r u z i o n e l a t e r a l e del 'ìli giug n o ; ma n o n è s t a l o p o s s i b i l e l'aie un c a l c o l o a p p r o s s i m a t i v o d e l l a q u a n t i t à t o t a l e l a n c i a t a d a l l e e s p l o s i o n i m a n c a n d o c i i d a t i , sia p u r e a p p r o s s i m a i i \ i di q u e l l o c a d u t o in m a r e p e r vasti —imo h a l l o ed in c a l a b r i a verso i a s p r o m o n t e . dai f e n o m e n i osservati siili e t n a -i p u ò r i t e n e r e c o m e c e r i o elle il c o n d o t t o e r u t t i v o c e n t r a l e d e l l ' e t n a p r i m a d e l l ' e r u z i o n e l a t e r a l e del 3 0 g i u g n o si trovasse a p e r t o a l l e fluttuazioni del m a g m a : d i f a l l i e r a n o s t a l e f r e q u e n t i l e e r u z i o n i i n t e r c r a l c r i c h e di lava e di d e t r i t i , t a l u n e v i o l e n t i s s i m e , c o m e q u e l l a del io m a r z o 19-10. il 3 0 g i u g n o 1932 il m a g m a , a v e n d o t r o v a t o m o d o d ' i n c u n e a r s i ira gli s t r a l i del v ulc a n o a p a r t i r e da 30011 m d ' a l t i t u d i n e e q u i n d i sotto la terrazza c r a t e r i c a c e n t r a l e . produsse con i suoi c o n a t i , que gli s e u o l i m c n l i e q u e l l e s p a c c a l n r e ira q u o t a 3 1 0 0 e 2 8 0 0 sili fianco san del c o n o t e r m i n a l e c h e f u r o n o i soli segni p r e c u r s o r i d e l l ' e r u z i o n e l a t e r a l e . in s e g u i l o il m a g m a c o n t i n u ò la sua p e n e t r a z i o n e d i s c e n d e n t e nel s o t t o s u o l o e venne a s q u a s s a r e il fianco del v u l c a n o verso q u o t a 2 8 0 0 da dove i n c o m i n c i a r o n o ad a p r i r s i le s p a c c a l n r e clic si e s t e s e r o fino a q u o t a 2 5 3 0 . c i o è p e r c i r c a un c h i l o m e t r o . nel p r i m o t r a i l o d e l l a s p a c c a t u r a , tra (piota 28(111 e 27011. i a t t i v i t à fu e s p l o s i v a e di b r e v e d u r a t a , m e n t r e nel t r a t t o s o t t o s t a n t e i n c o m i n c i ò il t r a b o c c a m e n t o d e l l a lava c h e si loc a l i z z ò presto f o r m a n d o p a r e c c h i e b o c c h e e f f u s i v e v i c i n e tallio da form a r e q u a s i unica c o l a l a di lava. e e f f u s i o n e lavica p r o c e d e t t e i n i n i e r r o l t a m e n l e per c i r c a 12 o r e e poi -i affievolì e p o c o d o p o cessò, m e n l r e i n c o m i n c i ò ad e n t r a r e in a t t i v i t à p l i n i a n a il c r a t e r e c e n t r a l e . si p e n s a p e r t a n t o c l i c si sia o s t i n i l o il passaggio l a t e r a l e ed il m a g m a , s p i n t o d a l l e forze e n d o g e n e si sia s o l l e v a l o p i ù in a l l o nel c o n d o t t o e r u d i v o c e n t r a l e fino ad i n v e s t i r e il m a t e r i a l e u m i d o d e l l a t e r r a z z a c r a t e r i c a d a n d o luogo alla c o n s e g u e n t e v i o l e n t a r e a z i o n e e s p l o s i v a l'reat iea. o c c o r r e f a r e r i l e v a r e c h e d u r a n t e la e r u z i o n e l a t e r a l e gli -1111i n e n l i s i s m i c i , i s t a l l a l i dali i s t i l l i l o n a z i o n a l e di g e o f i s i e a n e l l o scant i n a l o d e l l ' i s t i t u t o (li v u l c a n o l o g i a d e l l ' i n i v e r i l à di c a t a n i a , non died e r o a l c u n a r e g i s t r a z i o n e , solo verso la m e z z a n o t t e d e l 3 0 . q u a n d o inc o m i n c i ò l ' a t t i v i t à esplosiva al c r a t e r e c e n t r a l e , e il 5 luglio successivo a p p a r v e r o dei m i c r o s i s m i c h e s a r a n n o oggetto di studio dell i t i luto di g e o fisica p e r s t a b i l i r e se fu ron o in r e l a z i o n e c o n i e r u z i o n e . q u e s t a e r u z i o n e ha l'atto a p p a r i r e s e m p r e p i ù e v i d e n t e il m e c c a n i s m o d e l l e eruzioni c i n c e così c o m e p e r il p r i m o lo c o m p r e s e m a r i o li:: c a l t a m i p o n t e ( i c m m e l l a r o a i p r i n c i p i » d e l s e c o l o s c o r s o e c o m e è s i a l o p i ù a m p i a i n c n l e e s p o s t o i n v a r i e p u l i l d i c a z i o n i ( i ) . (.(iliuiiii istituto i ulcanologieo etneo dell università. hi issu \to i engono riassunti gli as/x'tti essenziali dell' eruzione etnea del i') 12. b l i i l . l o g i ì a l ' i \ l ' i g. i ' o v i e . meccanismo iteli'' oraziani etnee. zciisclirift jiir e. volkanologìe. tinnii i. p. io-i1). in.. tl:e reeent erllplion oj etna. pillare. v o i . 1 1 2 . 2811). p . 5 i s . in., i.o recenti eruzioni dell'etna. s o c . g e o g r a f i c a 11.. s e r . \ l . neil. v i . g e n n a i o k i l l . p. 21-2,". \ . m a d s e n . etna: a m u r e n a verden. 1921. s0220 annals of geophysics, 60, 2, 2017, s0220, doi: 10.4401/ag-7128 a seismic ray tracing method based on fibonacci search luciano onnis1*, josé m. carcione2 1 universidad de buenos aires. consejo nacional de investigaciones científicas y técnicas. instituto de física de buenos aires (ifiba). facultad de ciencias exactas y naturales. departamento de física. buenos aires, argentina 2 istituto nazionale di oceanografia e di geofisica sperimentale (ogs), sgonico, trieste, italy article history received august 5, 2016; accepted december 18, 2017. subject classification: ray tracing, fibonacci search, passive seismics, radiation pattern, attenuation. abstract we design a fast ray tracing technique to simulate the response of seismic sources located at depth such as passive data due to fluid injection, hydraulic fracturing, volcanic tremors or earthquakes, with receivers located at the surface recording the displacement (and particle-velocity) field. the ray tracing is based on the fibonacci search algorithm. we consider interfaces of arbitrary geometry and homogeneous layers defined by the seismic velocity, the quality factor and the mass density. amplitude losses consider geometrical spreading, intrinsic attenuation (related to the quality factor) and transmission losses at the interfaces. the traces corresponding to time spikes are then convolved with a ricker wavelet to obtain band-limited synthetic seismograms. we compare the results with full-wave seismograms computed with a direct algorithm based on the fourier pseudospectral method. the ray tracing algorithm is much faster and from a practical point of view can be used as a forward modeling algorithm for the location of different sources of seismic energy. 1. introduction location algorithms of different energy sources such as micro-seismicity due to fluid injection, hydraulic fracturing, volcanic tremors or earthquakes require a fast and practical forward modeling algorithm (e.g., battaglia and aki, 2003; fischer et al., 2008, kumagai et al., 2009; shapiro, 2015]. in particular battaglia and aki [2003] use a very simple analytical equation to compute the amplitude decay from the source location (a volcanic tremor) to the receiver. this is due to the fact that the medium is assumed homogeneous. however, in more realistic situations, the medium is heterogeneous and a numerical algorithm is required. a ray tracing algorithm including the source radiation pattern, attenuation and transmission losses is sufficiently efficient for this purpose. we propose a ray tracing technique based on the fibonacci search method. the modeling intends to simulate the response of seismic sources located at depth and recorded at the surface. we model the source radiation pattern due to tensile sources, which may approximate fairly well the energy released in cases of micro-cracks due to fluid injection, fracking and magma flow inducing volcanic tremors. we focus on direct arrivals since multiple events have a lower amplitude and are difficult to pick and detect. we compare our results to fullwave synthetic seismograms computed with the fourier pseudospectral method [carcione et al., 1988, carcione, 2014]. 2. methodology we compute synthetic seismograms of a viscoacoustic 2d heterogeneous medium by using a ray tracing method based on a fibonacci search algorithm, where the geological model is described in terms of vertically stacked layers, each uniform and isotropic. we consider a single source, located at depth, and a set of receivers on the surface. 2.1 definition of the model each layer is isotropic, anelastic and homogeneous, and characterized by its acoustic velocity, quality factor and density. the interfaces can have any geometrical shape provided that they do not cross each other (e.g., pinchouts are not allowed) and extend from one side of the model to the other. they are defined by a small number (∼50) of equi-spaced points along the horizontal direction (x-variable). spline interpolation between the starting points is performed at a more densely sampled set of equi-spaced points (∼5000). the larger set of points is connected by line segments. receivers are located on the surface. onnis and carcione 2 2.2 initial value problem for a given source point and starting ray angle, the determination of the ray path constitutes an initial value problem. proper ray tracing requires the solution of a two-point problem, to find the ray (defined by its starting angle) going from the source location to a receiver. we find such rays by performing a fibonacci search on the starting angle, which requires to iteratively solve the initial value problem as follows. with the geological model defined in the (x, z)-plane, we consider a ray departing from source point s, making an angle θ with the vertical z-axis. because each layer is isotropic and homogeneous, the ray follows a straight path until it intersects an interface. each interface is composed of a large number of straight segments. to determine where an intersection between the ray and an interface occurs, the algorithm loops over all the segments of an interface to the left of the source point if θ < 0, and to the right of the source point if θ > 0, checking if an intersection exists. if the loop ends without positive results then there is no intersection within the model boundaries. starting in the layer where the source is located, the algorithm performs three tests: first, that no intersection with the underlying interfaces exists. second, that an intersection with the overlying interface exists within the model boundaries. and third, that no total internal reflection occurs. if any of these checks fails, no ray path is computed. if all three tests are successful, the intersection point and transmission angle are saved, and the three tests are run again on the upper layer, considering the new ray starting point and direction. this procedure continues until an intersection with the surface layer is found, or until one of the tests fails. 2.3 source-receiver ray tracing we define fs,i (θ) as the distance between the i-th receiver and the ray’s intersection point with the surface measured along the x-axis. in order to find the source-receiver ray paths, a fibonacci search (outlined in appendix a) is carried out on θ to minimize fs,i (θ) for each receiver. the method requires the specification of an initial angular interval [a1 , b1] for each receiver, and the desired final interval length l. it then prescribes a fixed number (q) of fs,i (θ) evaluations at sequentially determined θ values. the integer q is chosen in accordance with equation (12), so that bq − aq < l. all fs,i (θ) evaluations are carried out by solving the initial value problem as described in the preceding section. the starting interval for each receiver is obtained by shooting a large number of rays with starting angles uniformly distributed in every direction and selecting the pair of angles which produce the closest arrivals on each side. while keeping the source point fxed, the computational cost of this procedure becomes relatively smaller as the number of receivers increases, because the same set of bracketing intervals is used for each receiver. the method is only feasible if fs,i (θ) is unimodal in the starting angular interval. this is clearly not the general case. for instance, if the receiver lies in a shadow zone, the prescribed bracketing intervals could contain the shadow zone limits on both sides, which would correspond to two different minima. additionally, even if the receiver is not in a shadow zone, it can occur that more than one ray reaches it. in these cases the algorithm will likely fail to converge to the receiver positions. if the starting interval is adequate, the fibonacci method reduces the starting interval length for each receiver by a factor 2fq , where fq is the q -th fibonacci number. let θ*i be the central value of the final interval corresponding to the i-th receiver. rather than having accuracy in the angle parameter we are interested in accurate surface arrival positions. thus, for each receiver we check that fs,i (θ * i) < d with d the arrival distance tolerance. the complete ray tracing results are the ray paths, as defined by the intersection points and the incidence/ transmission angles across each interface, for all receivers where acceptable rays are found. receivers for which the tolerance condition is not satisfied, and those for which a premature termination condition is triggered at any point during the ray tracing procedure are completely discarded from additional computations. 2.4 amplitude losses and travel time once we have determined the ray path from the source to a given receiver, we can compute the amplitude. the intrinsic (physical) attenuation along the ray is described by (1) where r is the distance, a is the attenuation factor, f is the frequency of the signal, c is the wave velocity and q is the quality factor. this equation holds for q >> 1 [carcione, 2014; eq. (2.123)]. velocity dispersion is neglected. energy is also lost at interfaces, where transmission rays do not reach the critical angle. we neglect intrinsic attenuation effects on the transmission coefficient, assumed as a 2nd-order effect. let’s assume that we record the vertical displacement at the surface. a1(r)=exp(−αr),       α = π f cq , fibonacci search ray tracing 3 the displacement transmission coeffi cient is (2) where z = ρc is the impedance, ρ is the mass density and θ is the ray angle with respect to the line perpendicular to the interface. primed quantities correspond to the transmission medium. equation (2) is complemented with snell’s law: sin θ/c = sin θʹ/cʹ. the loss by transmission involves the energy fl ux and is given by (3) [carcione, 2014]. equations (2) and (3) are demonstrated in appendix b. moreover, the signal decays by geometrical spreading. in 2d media, the decay is proportional to √r, i.e., a pulse with amplitude 1 at the source location will decay to 1/√r at a distance r from the source. the total amplitude loss from the source to the surface is given by (4) where n is the number of interfaces and ri is the length of the ray in medium i. in addition to the amplitude loss, we compute the travel times of the ray inside each layer as ri/ci and add them over all the traversed layers to obtain the total travel time t*. a pulse at time t*, with amplitude a given by equation (4), represents the system impulse response if the source has amplitude 1 at time t = 0. only fi rst arrivals are considered. 2.5 source spectrum and radiation pattern to obtain a more realistic seismogram, we perform a convolution between the impulse response and a wavelet representing the source time history. we consider a ricker wavelet of the form (5) where t is the period of the wave and we take ts = 1.4t. the peak frequency is fp = 1/t. the frequency involved in equation (1) is f = fp. the frequency spectrum is (6) the source has a radiation pattern, i.e., it does not emit the same amplitude at every angle. let us denote with δ, λ and φ the dip, rake and strike angles, respectively, and ϕ is the slope angle describing the tensility of the source, such that ϕ = 90° for pure extensive sources, ϕ = 0° for pure shear sources and ϕ = − 90° for pure compressive sources. according to carcione et al. [2015], a tensile source in 2d space is obtained for ϕ = λ = φ = 90° , while shear sources are described by ϕ = 0 and λ = φ = 90° . then, using equation (1) of kwiatek and ben-zion [2013] we get the radiation pattern for p waves due to a tensile source: (7) [see also ou, 2008], where θ is the angle between the ray and the vertical direction and ν is the poisson ratio of the medium. the source strength versus θ is a0 r(θ), where a0 is a constant. 3. examples figure 1 shows a 3-layer subsoil model, and the ray paths traced from a source at (1.5,−1) km to a set of equi-spaced receivers at the surface, in the 0.5-2.5 km range, separated by 50 m. interfaces where initially defi ned by 61 points with 50 m spacing, and then interpolated with 1 m spacing, defi ning 3000 line segments. the initial bracketing angles were equi-spaced with π/20 radians, and a fi nal interval length shorter than 10−5 radians was required. from equation (12) it follows that the total number of evaluations per traced ray is 20. the largest ray-source arrival distance is less than 3 cm. having completed the ray tracing from the source point to the receivers, we compute the amplitude recorded at each receiver. figure 2 shows the results with the surface receivers now separated by 10 m. we consider the radiation pattern of a pure tensile source, as given by equation (7), and compute the amplitudes t = 2 ʹz  cosθ ʹz  cosθ  + z cos ʹθ , a2 = z cos  ʹθ ʹz  cos θ   t a= πi=1 n+1 a1iπj=1 n  a2 j πk=1 n+1  rk ,       a1i =exp(αiri) h(t)= u− 1 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟exp(−u),u = π(t −ts) t ⎡ ⎣⎢ ⎤ ⎦⎥ 2 h(ω)= t π ⎛ ⎝ ⎜ ⎞ ⎠ ⎟aexp(−a−iωts),a = ω ωp ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ 2 ,ωp = 2π t rp = 2v 1−2v +sin2θsin2δ+2(sin2δsin2θ +cos2δcos2θ) figure 1. ray paths traced from the source point s to a set of surface receivers, represented by circles. the smooth horizontal curves indicate the layer interfaces, q is the quality factor, c is the acoustic velocity and ρ is the density. onnis and carcione 4 for different values of the source dip angle δ (ϕ = λ = φ = 90°). the poisson ratio of the lower medium is 0.25. figure 3 shows the seismograms for three different dip angles. the source time history is a ricker wavelet given by equation (5) with fp = 30 hz. as can be seen in figures 2 and 3, the dip angle highly affects the amplitude distribution. figures 4, 5 and 6 compare the results of the ray tracing with full-wave simulations (vertical component of the particle velocity) for horizontal and dipping layers. in all the three cases we consider isotropic sources, with fp = 30 hz. for the ray tracing, the initial bracketing angles, the number of points defi ning each interface, and the fi nal angular interval length were confi gured as in the example given in figure 1. a velocity inversion is considered in figure 5. for the full-wave simulations, the peak frequency of the relaxation mechanism coincided with fp. the dipping interfaces of figures 4 and 5 require a smaller spacing between the grid points of the full-wave simufigure 2. surface amplitudes corresponding to the geological model of figure 1 and a pure tensile source with amplitude 1 at s. each curve corresponds to a different source dip angle δ. figure 3. synthetic seismograms corresponding to the geological model of figure 1. the source time history is a ricker wavelet given by equation (5) with fp =30 hz. the source dip angle δ is different for each example. (a) δ = 0°, (b) δ = 45° and (c) δ = 90°. figure 4. (a) geological model and ray paths determined from source s at (1.498,−1.204) km to geophones on the surface indicated by circles. actual ray tracing is performed from s to 200 geophones equispaced in the 0.5-2.5 km range. (b) ray-tracing amplitudes compared to full-wave simulations (vertical component of the particle velocity) corresponding to the model in (a). the amplitude values are scaled with respect the maximum ones. the full-wave simulation uses a 429×221 mesh with 7 m spacing, and 0.4 ms as time step. figure 5. (a) geological model and ray paths determined from source s at (2.1,−1.204) km to geophones on the surface indicated by circles. actual ray tracing is performed from s to 200 geophones equispaced in the 0.5-2.5 km range. (b) ray-tracing amplitudes compared to full-wave simulations (vertical component of the particle velocity) corresponding to the model in (a). the amplitude values are scaled with respect the maximum ones. the full-wave simulation uses a 429×221 mesh with 7 m spacing, and 0.3 ms as time step. fibonacci search ray tracing 5 lation than those of the horizontal layers of figure 6. in turn, an smaller time step is needed in the runge-kutta method to satisfy the stability condition [carcione, 2014; eq. 9.12]. as a consequence, the run time of the full-wave simulations of figures 4 and 5 is nearly 10 minutes, while the full-wave modelling of figure 6 requires 140 s. the ray tracing is in good agreement with the full-wave simulations, requiring less than 2 s. the reported times correspond to an intel core i5 4590 cpu. 4. conclusions we have developed a seismic ray tracing algorithm based on fibonacci search. the method is fast enough to be applied as a forward modeling kernel to be used in inversion algorithms to locate seismic energy sources and model their seismic responses as well. the algorithm considers the effects of geometrical spreading, transmission losses, intrinsic attenuation and the source radiation pattern, and shows a good agreement with full-wave numerical simulations. references bazaraa, m. s., sherali, h. d. and shetty, c. m. (2006). nonlinear programming theory and algorithms, 3rd edition wiley. battaglia, j., and aki, k. (2003). location of seismic events and eruptive fi ssures on the piton de la fournaise volcano using seismic amplitudes, j. geophys. res., 108(b8), 2364, doi:10.1029/2002jb002193. carcione, j. m. (2014), wave fields in real media. theory and numerical simulation of wave propagation in anisotropic, anelastic, porous and electromagnetic media, 3rd edition elsevier. carcione, j. m., da col f., currenti, g., and cantucci, b. (2015), modeling techniques to study co2-injection induced micro-seismicity, international journal of greenhouse gas control, 42, 246–257. carcione, j. m., kosloff, d., and kosloff, r. (1988), wave propagation simulation in a linear viscoacoustic medium, geophys. j. roy. astr. soc., 93, 393–407. erratum: 1988, 95, 642. fischer t., s. hainzl, l. eisner, s. a. shapiro, and calvez, j. l. (2008), microseismic signatures of hydraulic fracture growth in sediment formations: observations and modeling, journal geophysical research, 113:b02307. kiefer, j. (1953), sequential minmax search for a maximum, proceedings of the american mathematical society, 4, 502–506. kumagai, h., palacios, p., maeda, t., barba castillo, d., and nakano, m. (2009), seismic tracking of lahars using tremor signals, j. volcanol. geotherm. res., 183, 112–121. kwiatek, g. and ben-zion, y. (2013). radiation pattern formulas for shear-tensile source model, downloadable from http://onlinelibrary.wiley.com/ doi/10.1002/jgrb.50274/full -support information ou, g. b., (2008), seismological studies for tensile faults. terr. atmos. ocean. sci., 19(5), 463–471. shapiro, s. (2015), fluid-induced seismicity, cambridge university press. appendix a: fibonacci search algorithm the fibonacci search method [kiefer, 1953] is a sequential search algorithm for minimizing an unimodal function over a closed interval, based on the fibonacci numbers fn defi ned as: (8) unlike other sequential search methods (i.e. uniform, dichotomous, or the closely related golden section search method) fibonacci search requires a predetermined number of iterations. its main advantage is that for the same starting uncertainty interval and number of iterations no other sequential search technique can ref0 = f1 =1,fn = fn−1 +fn−2,n≥ 2 figure 6. (a) geological model and ray paths determined from source s at (1.5,−1.2) km to geophones on the surface indicated by circles. actual ray tracing is performed from s to 200 geophones equispaced in the 0.5-2.5 km range. (b) ray-tracing amplitudes compared to full-wave simulations (vertical component of the particle velocity) corresponding to the model in (a). the amplitude values are scaled with respect the maximum ones. the full-wave simulation uses a 221×221 mesh with 10 m spacing, and 0.8 ms as time step. onnis and carcione 6 sult in a smaller final uncertainty interval. let f(x) be the function to minimize, q the total number of iterations and [am, bm] the uncertainty interval after the m-th iteration. the algorithm pre scribes the evaluation of the function at two interior points, cm and dm given by (9) if f(cm) > f(dm) the uncertainty interval at the next iteration, [am+1, bm+1], will we [cm, bm], otherwise if f(cm) ≤ f(dm), it is [am, dm]. this is due to the function’s unimodality. in both cases, it follows from equation (9) that the new interval length is (10) additionally, it can be shown (e.g., bazaraa et al., 2006) that for the next iteration, either cm+1 = dm or dm+1 = cm. thus every iteration after the first requires only one function evaluation. after q − 1 iterations (and q function evaluations) the interval length is (11) for the very last evaluation, equation (9) yields cq−1 = dq−1, both in the center of interval [aq−1, bq−1]. in order to distinguish them one of them is slightly displaced. taking dq−1 = cq−1 + ε, with ε very small, it is possible to reduce final interval length by approximately 1/2, obtaining (12) the total number of iterations must be selected a priori, taking equation (12) into account to achieve the desired accuracy level. appendix b: transmission losses to prove equations (2) and (3), we perform a mathematical analogy between sh and p waves. the sh wave equation is (13) where µ is the rigidity, ρ is the mass density, σ is stress, v is particle velocity, a dot above a variable denotes time derivative and ∂i is a spatial derivative with respect to the variable xi. the indices 1, 2 and 3 correspond to the spatial variables x, y and z, respectively. on the other hand, the p-wave equation is (14) the analogy is v2 → σ, σ12 → v1, σ23 → v3, µ −1 → ρ and ρ−1 → ρc2. this implies zs = √ρµ → z −1, where zs is the shear impedance. then, equations (2) and (3) results from eqs. (8.158) and (8.184) in carcione (2014) (note the following errata: eq. (8.158): remove the primes in the numerator; eq. (8.815): move the primes to the denominator). appendix c: full-wave modeling method the full-wave synthetic seismograms are computed with a modeling code based on the viscoacoustic stress-strain relation corresponding to a single relaxation mechanism, based on the zener mechanical model. the equations are given in section 2.10.4 of carcione [2014]. the 2d particle velocity-stress formulation is (15) where v is particle velocity, σ is stress, fi are directional forces, s is the source (explosion), e is a memory variable, and τ denotes relaxation times. these are given by and (16) where q0 is the minimum quality factor and τ0 is defined as follows. if fp is the central frequency of the source wavelet, we assume that the relaxation peak is located at ω0 = 1/τ0 = 2πfp. the velocity c in these equations corresponds to the unrelaxed or high-frequency limit velocity. the numerical algorithm is based on the fourier pseudospectral method for computing the spatial derivatives and a 4th-order runge-kutta technique for calculating the wavefield recursively in time [e.g., carcione, 2014]. cm =am + fq−m−1 fq−m+1 (bm −am) dm =am + fq−m fq−m+1 (bm −am) bm+1 −am+1 = fq−m fq−m+1 (bm −am) bq−1 −aq−1 = b1 −a1 fq bq −aq = b1 −a1 2fq ρ!v1 =∂1σ , ρ!v3 =∂3σ , ρc2(∂1v1 +∂3v3)= !σ , !v1 = 1 ρ ∂1σ + f1, !v3 = 1 ρ ∂3σ + f2, !σ = ρc2(ϑ +e)+s, !e = 1 τε − 1 τσ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ϑ − e τσ , ϑ =∂xvx +∂zvz τε = τ0 q0 q0 2 +1+1( ) m=1,...,q−1 τσ = τ0 q0 q0 2 +1−1( ), µ−1 !σ12 =∂1v2, µ−1 !σ23 =∂3v2, ρ−1(∂1σ12 +∂3σ23)= !v2, fibonacci search ray tracing 7 *corresponding author: luciano onnis universidad de buenos aires. consejo nacional de investigaciones científicas y técnicas. instituto de física de buenos aires (ifiba). facultad de ciencias exactas y naturales, departamento de física, buenos aires, argentina; email: leonnis@df.uba.ar 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved vol49,4_5,2006 key words gradiometric gravimeter – accelerometer – whispering gallery cavity 1. introduction the gravimetric methods are widely used in geological structure investigations, in particular in oil and mine exploration industry. this kind of research requires a very high level of instrumental sensitivity and an accuracy of the order of one microgal (1 gal = 1 cm/s2), that is 10−9 g, in order to achieve significant results. at this sensitivity level spurious effects, not connected with underground mass distributions, but that are due to surface topographic irregularities or to anthropic activities or to tidal fluctuations become important. the instrumental drift, unavoidable in mechanical sensors because of creep of the elastic elements, thermal gradients etc., is another source of errors, which are more and more important as the data acquisition time is longer. much more severe constraints rise in implementing a gravimeter on a ship or on a plane, because the instrument cannot distinguish between gravity and kinematic acceleration vectors. gravity gradiometric methods can avoid many of these problems. this technique is based on the measurement of one or more of the five independent components of the tensor a sapphire monolithic differential accelerometer as core sensor for gravity gradiometric geophysical instrumentation alessandro bertolini (1)(3)(∗), nicolò beverini (1) (2), andrea de michele (1), francesco fidecaro (1) and francesco mango (1) (1) dipartimento di fisica «e. fermi», università degli studi di pisa, and istituto nazionale di fisica della materia (infm), cnr, pisa, italy (2) consorzio universitario della spezia, la spezia, italy (3) marwan technology srl pisa, italy abstract gradiometric gravimetry is a survey technique widely used in geological structure investigation. this work demonstrates the feasibility of a new class of low frequency accelerometers for geodynamics studies and space applications. we present the design features of a new low noise single-axis differential accelerometer; the sensor is suitable to be used in a gravity gradiometer (gg) system for land geophysical survey and gravity gradient measurements. a resolution of 1 eötvös (1 eö = 10−9s−2) at one sample per second is achievable in a compact, lightweight (less than 2 kg) portable instrument, operating at room temperature. the basic components of the sensor are two identical rigidly connected accelerometers separated by a 15-cm baseline vector and the useful signal is extracted as the subtraction of the two outputs, by means of an interferometric microwave readout system. the structure will be engraved in a monocrystal of sapphire by means of computer-numerically-controlled (cnc) ultrasonic machining: the material was chosen because of its unique mix of outstanding mechanical and dielectric properties. mailing address: prof. nicolò beverini, dipartimento di fisica «e. fermi», università degli studi di pisa, largo b. pontecorvo 3, 56127 pisa, italy; e-mail: beverini@df.unipi.it (∗) present address: desy, notkestrasse 85, d 22607 hamburg, germany. 1095 annals of geophysics, vol. 49, n. 4/5, august/october 2006 1096 alessandro bertolini, nicolò beverini, andrea de michele, francesco fidecaro and francesco mango γij = ∂ gj /∂ xi (that is the gradient of the gravity vector g), as the difference between the values of the gj components across a fixed distance basis. this technique has demonstrated his significance in the oil deposit signature and in the research and localization of underground cavities. the on-field implementation of the gradiometric principle can be made by reading, with a high sensitivity gravimetric instrument, the vertical gravity values gz in two different point, separated by a known vectorial basis ri, and evaluating the γiz component of the tensor. however, the full exploitation of the peculiarity of the gradiometric surveys could be achieved by using a single instrument with built-in differential readout capability, in order to efficiently reject common mode noise and spurious signal, like kinematic effects due to linear accelerations of the measurement platform. the basic practical implementation consists of two identical accelerometers fixed rigidly together with a common sensitive axis (see fig. 1). below the mechanical resonant frequency ω0 of the accelerometers, the difference in the vertical position of the proof masses is linearly related to the gravity gradient component γiz according to the second term is related to static mechanical mismatch δω between the two accelerometers. being almost impossible to build two perfectly matched accelerometers, a residual sensitivity to kinematic effects arises, polluting the signal channel by an amount proportional to the platform motion acceleration az. the factor ω0 / 2δω is defined as the common-mode-rejection-ratio (cmrr). detecting 1 eö gravity gradient in presence of 10−3 m/s2 linear acceleration, with an instrument with 15-cm baseline, requires a 130 db of cmrr. non-linearities of the accelerometer spring system also affect the cmrr for large environmental accelerations. this imposes suitable vibration isolation platforms, especially in airborne instruments. at present only a few gravity gradiometers have been developed. spaceborne gravity gradiometer devices are capable of outstanding noise levels (10−4 − 10−3 eö) but rely on the very low environmental noise of a low acceleration satellite (at least one million times more silent than a typical light aircraft, for instance). two examples are the 6-axis superconducting differential accelerometer developed by ho jung paik group at university of maryland (chain and paik, 1988a,b), and now being built by oxford instruments, and the goce mission accelerometer (rebhan et al., 2000). laboratory use only apparatus have also been devised by doubling the core sensor of absolute gravimeters, like the superconducting levitation device from gwr inc. (richter and warburton, 1998), the free-fall michelson interferometer from micro-g solutions inc. (http://www.microgsolutions.com/gradiometer.pr ess.htm) and the atom interferometer developed by kasevich group at yale university (snadden et al., 1998). all these instruments claim a resolution ranging from 0.1 to 1 eö at one sample per second, but they are still far away from the portability and the on-field applications. the most popular portable instrumentation is the gravity gradiometer instrument (ggi), de.z z r g a 2 iz i z z 1 2 0 2 0 0 2. ω ω δω ω γ + + b ^ l h fig. 1. example basic configurations of a singlecomponent gravity gradiometer. the geometric arrangement of the two linear accelerometers is shown. a sapphire monolithic differential accelerometer as core sensor for gravity gradiometric geophysical instrumentation veloped primarily by lockheed martin for military application, but recently declassified and presented now on the civil market (goldstein and brett, 1998). they are instruments specifically designed to be mounted on mobile platforms (aircrafts or ships). in the simplest form it consists of two matched pairs of force-balance accelerometers mounted orthogonally on a rotating platform (see fig. 2). linear accelerations perpendicular to the rotation axis are modulated at the spinning frequency, while the gravity gradient effect is modulated at twice spinning frequency. in this way the useful signal is separated in the frequency domain from the instrument bias and vehicle accelerations (e.g., reference ellipsoid, latitude, earth tide and isostatic effects). the signal is extracted by a suitable combination of the difference in readings between opposing pairs of accelerometers. a high cmrr is achieved by a feedback system acting on the accelerometers scale-factor, and by the effect of the rotating platform. the sensitivity declared is of the order of 5 eö. the ggi is the basic element of the most complete instrument, the bell geospace full tensor gradiometer (http://www.bellgeo.com/ tech/system.html). this instrument measures at the same time all the components of the gradient tensor, allowing a complete reconstruction of the gravity map. the performances of this instrument are very high; it is however a high cost cumbersome instrument, of large volume and weight that must be carried on a ship or a plane. more recently the bell technology has been shared for advanced developments, the falcon gravity gradiometer project, with the australian company bhp billiton discovery technologies (lee, 2001). the falcon system has proven to be effective in detecting sources with γzz > 10 eö rms and a full width at half maximum of 500 m of spatial resolution. at this time, the most promising portable instrument seems to be the mobile superconducting beam-balance gravity gradiometer developed by f. van kann (van kann et al., 1994) at university of western australia and now being launched on the market by gedex inc. (for details see matthews, 2002). the instrument is specifically designed for airborne surveys and is capable of unprecedented cmrr (exceeding 190 db) and of a resolution better than 1 eö at one sample per second, even in the presence of 1 m/s2 linear accelerations. however, even in this case, the size of the accessory inertial platform and of the liquid helium cooling system still limit the portability of the instrument. no available device currently fills the gap between laboratory-only apparatus and large size, costly ggs for airborne and shipborne applications. in this work we present the design of a new core sensor for an advanced lightweight portable gravity gradiometer. the sensor is a differential vertical accelerometer using state-of-art dielectric crystals machining techniques and microwave position sensing technologies to achieve 1 eö level resolutions operating even at room temperature. 2. the mechanical apparatus the instrument consists of three main parts: the accelerometer plate, hosting two identical accelerometers sensitive along the z-axis, the refer1097 fig. 2. basic scheme of a ggi (gravity gradiometer instrument): two opposite matched pairs of linear accelerometers sit on a spinning platform. the gravity gradient signal is given by a linear combination of the accelerometer outputs at twice the spinning frequency. 1098 alessandro bertolini, nicolò beverini, andrea de michele, francesco fidecaro and francesco mango ence plate, hosting the motion sensing microwave elements and a conductive shield. both the accelerometer and the reference plates are completely carved out of a high quality singlecrystal sapphire plate 1-cm thick by means of computer numerically controlled (cnc) chisel ultrasonic machining (bertolini et al., 2003). ultrasonic machining yields complex shapes in crystals, ceramics and composite materials without any release of thermal and mechanical stress into the work piece. because of its geometry, in the present apparatus design, the instrument is sensitive to the off-line γyz component of the gravity gradient tensor. each accelerometer is a small torsion pendulum with a 0.04 kg proof mass suspended by two torsion beams. the baseline, defined as the relative position of the centers of mass of the moving bodies (fig. 3), is 15cm. a force fz applied to the proof mass produces a twist θ of the torsion pendulum given by where l, b, t are respectively the length, height and thickness of the torsion beams; g is the rigidity modulus of the material and d is the disbt g f ld3 z 3.ϑ tance between the center-of-mass and the rotation axis. by using sapphire beams 1×1 mm2 in section and 10 mm long, a resonant frequency of 20 hz can be obtained. in this configuration, a 1 eö gravity gradient is equivalent to a differential displacement of the gg test masses of 10−14m. sapphire has been chosen because its excellent mechanical and dielectric properties. it combines high strength, low hysteresis and very low electrical losses; hemex™ grade sapphire single-crystal exhibit mechanical and electrical quality factors greater than 105 even at room temperature (http://www.crystalsystems.com). the monolithic structure obtained by cnc machining allows the accuracy in the relative positioning of the different mechanical parts to be kept within a few micrometers. the monolithic design also allows very low hysteresis, eliminating shear effects at the contact surfaces between separate mechanical parts (stick-andslip), while the use of a single material limits the instrument’s thermal sensitivity. co-machining of both the torsion springs effectively strongly reduces directional cross-talks due to misalignments between the components in the assembly, typical of a composite structure. in this way, a 60 db mechanical common mode rejection factor can be reasonably achieved just by construction. a fine trimming of the cmrr will be achieved by acting on the «flap» electrodes. by applying a high-voltage on both sides of a flap, a tunable negative stiffness effect is realized. the trimming procedure will make use of a suitable tilting jig which eventually should even be integrated in the gg system to allow for an automated balancing routine. tilting is the easiest way to inject a 1-g level common-mode acceleration into the system and to allow in this way for a high signal-to-noise ratio in the balancing operation. we expect to gain an additional 60 db in the instrument’s cmrr. the residual sensitivity to environmental linear accelerations will be cancelled out by using a suitable vertical mechanical active isolation system, driven, with a proper feedback gain, by the common-mode acceleration signal itself. both the elastic constant of the single oscillator and its inertia moment are affected by temperature. such thermal effects are not a fig. 3. top view of the differential accelerometer mechanics. two identical torsion pendula are carved out of a single-crystal sapphire plate by means of cnc ultrasonic impact grinding. the two disks are used as the tuning elements of two sapphire-loaded microwave resonators. two flaps with evaporated gold electrodes are used for electrostatic stiffness fine balancing and cold damping of the accelerometers. 1099 a sapphire monolithic differential accelerometer as core sensor for gravity gradiometric geophysical instrumentation problem, as long as both sensors are at the same temperature. a difference of 1°c between the two sensors can however produce a sensitivity difference of the order of 10−5. in order to achieve 1 eö sensitivity the temperature differences must be controlled better than 10−5 k. it will be necessary an efficient passive isolation of the instrument container. the monolithic structure of the apparatus and the vacuum environment, already necessary in order to avoid viscous damping of the mechanical resonance, can warrant a very high thermal inertia. semiconductor temperature sensors will monitor the eventual thermal gradient. 3. the microwave transducer and readout system the motion of the proof-mass of each accele-rometer is detected by means of a microwave sapphire dielectric resonator transducer (sdrt) (blair et al., 1992). in the simplest form, a sdrt consists of two coaxial disks mounted inside a conducting shell which prevents radiation losses and unwanted coupling with the environment. each disk resonates with a whispering gallery (wg) mode of the electromagnetic field. these modes are generated by total reflection of the electromagnetic field on the external surface of the dielectric cylinder. by coupling two of these resonators we obtain a new resonator whose resonant frequency ƒ is a function of the gap width between the two disks (fig. 4). with a suitable choice of the resonator geometry is possible to optimize the merit factor of the transducer q⋅dƒ/dz, the product between the quality factor of the wg coupled mode and the tuning coefficient, which determines the displacement sensitivity of the transducer itself. there are two types of wg modes: e modes (ez, hρ, hφ) and h modes (hz, eρ, eφ), where z denotes the component along the disk rotation axis of the field vector, ρ its radial component, and φ its azimuthal component. each mode is labeled by three numbers m, n and k, describing the number of field maximums in the azimuth, radial and longitudinal direction respectively. the principal modes (n = k = 1) allow q factors that can be equal or even greater than the inverse of the dielectric loss tangent of the dielectric material. the most sensitive resonances to changes in the gap spacing has been demonstrated to be the wg e modes, with high order azimuthal number (blair et al., 1998). the reason is the e modes have a higher field density in the gap between the disks. because of the continuity condition of the electromagnetic vector at the boundary of a dielectric, the ez component of e mode increases from the value ez (vacuum) in the gap spacing to εzez(vacuum) in the dielectric, whereas the main field components of h mode remain unchanged. the tuning coefficient is also a function of the height of the disks. when the disk thickness decreases, the fraction of the electromagnetic power in the gap increases. as a consequence, the value of dƒ/dz increases. in our design the proof mass of each accelerometer is used as the tuning disk of the sdrt formed with the corresponding reference puck dug into the reference plate (see fig. 5). the dimensions of the disks was chosen in the way that the high order azimuthal whispering gallery e modes (m=7−12) resonate in the xband (between 9 and 12 ghz). we have performed some preliminary tests on the sdrt using two sapphire disks with 42 mm fig. 4. sketch of a double-disk whispering gallery (wg) microwave resonator. 1100 alessandro bertolini, nicolò beverini, andrea de michele, francesco fidecaro and francesco mango in diameter and 5 mm in height. the sapphire crystals were of optical grade quality and without surface polish. the distance between the two disks was changed moving vertically one of them mounted on a micrometer screw. a microwave sweeper was used as tuneable microwave source and a diode as square law detector. table i reports the experimental and theoretical value of the resonance frequencies of some modes when the two disks are in contact (zero distance), and the q factor of the resonances. the theoretical frequencies are calculated using the method described in blair et al. (1998). figure 6 plots experimental and theoretical values of resonance frequency and of the tuning coefficient dƒ/dz, as a function of the distance between the two disks for the e11,1,1 mode. these results demonstrate that a gap spacing of 100 µm ensures for the e11,1,1 mode in our geometry a high tuning coefficient, around 6 mhz/µm, and a quality factor of the order of q ≈ 50 00060 000 even at room temperature. higher q values can be expected with sapphire crystals of better quality. radiation loss shielding is achieved by gold plating the inner surface of the sapphire plates. the readout system will be realized following the fully differential interferometric scheme introduced by blair et al. (1994), which has been demonstrated to be capable of displacement resolutions better than 10−14 m. the idea is to use one of the two resonators to build a loop oscillator and excite the second one. the differential acceleration is detected by measuring the frequency mismatch between the two sdrt with a balanced detector. a practical implementation is sketched in fig. 7. a stable microwave signal is generated by a loop oscillator including one of the gradiometer resonators as the reference cavity, a phase shifter and a low noise high-gain microwave amplifier. a power divider splits the microwave power between the two resonators used in trasmission mode. coupling is achieved by means of two circulators. the power transmitted by the sdrts is compared by means of a 180° power combiner, whose output is demodulated in-phase with respect to the loop oscillator in a low-noise double-balanced mixer (dbm). the low-pass filtable i. experimental and theoretical resonance frequencies of a sapphire dielectric resonator made of two disks of 42 mm diameter and 5 mm height in contact. mode exp (ghz) theo (ghz) q e7,1,1 8.235 8.262 14000 e8,1,1 8.889 8.918 18000 e9,1,1 9.551 9.581 24000 e10,1,1 10.221 10.250 26000 e11,1,1 10.897 10.926 55000 e12,1,1 11.578 11.606 58000 fig. 5. section view of the differential accelerometer. the accelerometer and the reference plates are surrounded by a metallic shielding enclosure. the two pairs of control electrodes allow for fine trimming of the cmrr and cold damping of the accelerometer motion. a sapphire monolithic differential accelerometer as core sensor for gravity gradiometric geophysical instrumentation tered output of the dbm is a voltage proportional to the sensed differential acceleration, in the frequency band dc-20 hz. the balanced interferometric scheme allows for carrier suppression, enhancing the resolution, especially at low frequencies. otherwise the sensitivity would be limited by the loop oscillator phase noise and by the frequency discriminator noise. by considering a reasonable dynamic range of 130 db for the mixer, the frequency mismatch between the two microwave resonators must be limited within 100 khz to allow the gradiometer to operate. a trimming procedure will be then necessary, after the machining, to suppress the bias effect due to shape mismatch in the dielectric resonators, producing a static frequency offset. the high mechanical q level calls for a mechanism that limits the dynamics of the oscillators. this will be done by an active damping feedback control as used in sensitive detectors like the virgo gravitational antenna (acernese et al., 2004). each sensor will be fitted with an actuator consisting in a small magnet fixed on the upper face of the moving arm, driven by a coil fixed to the frame. a laser beam is reflected by the moving arm and sent to a position sensitive detector. a current signal propor1101 fig. 7. schematic of the interferometric microwave readout system. one resonator is used as the reference cavity for a x-band loop-oscillator. its output is used to interrogate the other dielectric resonator and the frequency difference is read by a double-balanced mixer. fig. 6. resonance frequency and tuning coefficient of the e11,1,1 mode in function of the distance between the two disk of 42 mm diameter and 5 mm height (points-experimental values; line-theoretical values). 1102 alessandro bertolini, nicolò beverini, andrea de michele, francesco fidecaro and francesco mango tional to the moving arm velocity is injected. it is possible in this way to reduce the dynamic of the oscillation by orders of magnitude. the minimum detectable gravity gradient per bandwidth unit can be evaluated by comparing the expected signal with the noise. by considering thermal noise due to the brownian motion of the accelerometer test masses and the phase noise of the loop oscillator, we find the equation where ω0 is the mechanical resonant frequency, l is the accelerometer baseline, m is the test mass, q is the mechanical quality factor, d f /dx is the tuning coefficient, sφ is the loop oscillator phase noise spectral density and ε is the balancing factor of the interferometric system. at room temperature typical expected parameters (tobar et al., 1993) are q ≈104, sφ≈10 −3/f 2 rad2/hz and ε ≈ ≈ 0.01, yielding γmin ≈ 1 eö for 1-s of integration time. 4. conclusions we have presented the design features of the core differential accelerometer for new low cost lightweight gravity gradiometer instruments suitable for land geophysical surveys. the combination of the monolithic cnc machined mechanics and of a fine electrostatic stiffness balancing would produce a very high mechanical cmrr, of the order of 120 db, allowing 1-eö level resolutions to be achieved. at present the main components of the first prototype are being developed and fixed. the core mechanics is being built in the ultrasonic machining facility of university of pisa, using a low cost, lower purity grade hemlite™ sapphire. the microwave readout system design is being done by investigating on the wg modes of the single transducer. after the integration of the core sensor with the necessary sub-systems like the temperature control, the gimbal leveling frame, the balancing jig etc., a first labora( ) ( ) d d l x f s mq q k t4 b 0 2 2 2 0 2 2 0 2 0 2 $ $ ω ω π ω ε ω ω ω ωω ω γ = + + φ f ^ _p h i7 a tory prototype of γyz gradiometer will be tested. moreover new γzz sensors, less sensitive to the alignment with respect to the vertical direction, will be designed. acknowledgements this work has been partially supported by «fondazione della cassa di risparmio di pisa». references acernese, f. et al. (2004): a local control system for the test masses of the virgo gravitational wave detector, astroparticle phys., 20, 617-628. bertolini, a., n. beverini, g. dal lago, r. desalvo, f. fidecaro, f. francesconi, m. francesconi and g. omiccioli (2003): a 5-axis cnc ultrasonic cutting machine: design and preliminary test, ligo technical note t020198-00. blair, d.g., e.n. ivanov and h. peng (1992): sapphire dielectric resonator transducers, j. phys. d, 25, 1110. blair, d.g., e.n. ivanov and h. peng (1994): an ultrahigh sensitivity sapphire transducer for vibration measurements, j. phys. d, 27, 1150. blair, d.g., e.n. ivanov, m.e. tobar and b.d. cuthbertson (1998): sensitivity and optimization of a high-q sapphire dielectric motion-sensing transducer, ieee trans. ultrason. ferroelect. freq. contr., 45, 1303-1313. chain, h.a. and h.j. paik (1988a): superconductive gravity gradiometer for sensitive gravity measurements, i. theory, phys. rev. d, 35, 3551. chain, h.a. and h.j. paik (1988b): superconductive gravity gradiometer for sensitive gravity measurements, ii. experiment, phys. rev. d, 35, 3572. goldstein, m.s. and j.j. brett (1998): precision gravity gradiometer/auv system, in proceedings of the 1998 workshop on autonomous underwater vehicles, auv’98, 167-174. lee, j.b. (2001): falcon gravity gradiometer technology, exploration geophys., 32, 247-251 (available on line at http://falcon.bhpbilliton.com/docs/falcon_gravity_gradiometer_technology.pdf). matthews, r. (2002): mobile gravity gradiometry, phd thesis (university of western australia), (unpublished). rebhan, h., m. aguirre and j. johannessen (2000): the gravity field and steady-state ocean explorer mission goce, esa earth observation q., 66, 6-11. richter, b. and r.j. warburton (1998): a new generation of superconducting gravimeters, in proceedings of the 13th international symposium on earth tides, série géophysique, royal observatory of belgium, brussel, 545-556. snadden, m.j., j.m. mcguirk, p. bouyer, k.g. haritos and m.a. kasevich (1998): measurement of the earth’s gravity gradient with an atom interferometer-based gravity gradiometer, phys. rev. lett., 81, 971-974. tobar, m.e., a.j. giles, s. edwards and j. searls (1993): 1103 a sapphire monolithic differential accelerometer as core sensor for gravity gradiometric geophysical instrumentation high q te stabilised sapphire microwave resonators for low noise applications, in proc. ieee frequency control symposium, u.s.a. van kann, f.j., m.j. buckingham, c. edwards and r. matthews (1994): performances of a superconducting gravity gradiometer, physica b, 194 and 196, 61-62. (received january 28, 2006; accepted july 20, 2006) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 soil gas geochemical behaviour across buried and exposed faults during the 24 august 2016 central italy earthquake giancarlo ciotoli1,2*, alessandra sciarra2,1,5, livio ruggiero3, aldo annunziatellis4,1, sabina bigi3 1consiglio nazionale delle ricerche – istituto di geologia ambientale e geoingegneria, rome, italy 2istituto nazionale di geofisica e vulcanologia, rome, italy 3università degli studi di roma la sapienza, rome, italy 4istituto superiore per la protezione e la ricerca ambientale, rome, italy 5università degli studi di ferrara, ferrara, italy *giancarlo.ciotoli@igag.cnr.it abstract following the earthquake (ml=6.0) of 24 august 2016 that affected large part of the central apennine between the municipalities of norcia (pg) and amatrice (ri) (central italy), two soil gas profiles (i.e., 222rn, 220rn, co2 and co2 flux) were carried out across buried and exposed coseismic fault rupture of the mt. vettore fault during the seismic sequence. the objective of the survey was to explore the mechanisms of migration and the spatial behaviour of different gas species near still-degassing active fault. results provide higher gas and co2 flux values (about twice for 222rn and co2 flux) in correspondence of the buried sector of the fault than those measured across the exposed coseismic rupture. anomalous peaks due to advective migration are clearly visible on both side of the buried fault (profile 1), whereas the lower soil gas concentrations measured across the exposed coseimic rupture (profile 2) are mainly caused by shallow and still acting diffusive degassing associated to faulting during the seismic sequence. these results confirm the usefulness of the soil gas survey to spatially recognise the shallow geometry of hidden faults, and to discriminate the geochemical migration mechanisms occurring at buried and exposed faults related to seismic activity. i. introduction oil gas survey has been widely used to trace buried faults and to study the behaviour in the shallow environment of endogenous gases with different origins (i.e., trace gases, i.e., radon and helium, and carrier gases i.e., carbon dioxide, nitrogen, methane, etc.) and [e.g., king, 1985; baubron et al., 2002; ciotoli et al., 2007; fu et al., 2008; walia et al., s 1 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 2009; ciotoli et al., 2014; bigi et al., 2014; sciarra et al., 2014]. furthermore, over the past several years’ soil gases has captured considerable attention as earthquake precursors [wakita et al., 1980; reddy et al., 2004; walia et al., 2009; perez et al., 2007; ghosh et al., 2009; hashemi et al., 2013; petraki et al., 2015], in fact that the stress/strain changes related to seismic activity may force crustal fluid to migrate up, especially along active faults, thereby altering the geochemical characteristics of the fault zone at surface [rice, 1992; sibson, 2000; collettini et al., 2008]. the migration of these gases by diffusion and/or advection along buried active faults can generate shallow anomalies with concentrations significantly higher than background levels; these anomalies can provide reliable information about the location and the geometry of the shallow fracturing zone, as well as about the permeability within the fault zone [king et al., 1996; baubron et al., 2002; ciotoli et al., 2007; annunziatellis et al., 2008; bigi et al., 2014; sciarra et al., 2014]. among the soil gases, 222rn, a radioactive inert gas with a half-life of 3.82 days, is considered a convenient fault tracer in geosciences, because of its ability to migrate to comparatively long distances from host rocks, as well as the efficiency of detecting it at very low levels. while other gases have also been considered as tracer of hidden faults, however, bulk of reports in the scientific literature are focused on radon. local increases in radon emanation along faults could be caused by a number of processes, including precipitation of parent nuclides caused by local radium content in the soil [tanner, 1964; zunic et al., 2007], increase of the exposed area of faulted material by grainsize reduction [holub and brady, 1981; koike et al., 2009; mollo et al., 2011], and carrier gas flow around and within fault zones [e.g., king et al., 1996; annunziatellis et al., 2008]. given the acceptance that the concentrations of radon gas are the result of advective gas flow associated with elevated permeability in fault zones, soil gas surveys are widely regarded to be an effective tool to map buried or blind faults not detected during mapping of the surface geology [king et al., 1996; burton et al., 2004; ciotoli et al., 2007, 2015, 2016]. soil gas migration does not necessarily occur in the same way through all faults, and the mechanism is still disputed. two possible scenarios can be considered: in correspondence of hidden faults, with low permeability core bounded by damage zones, high soil gas concentrations should occur laterally above the fracture zones. in contrast, in correspondence of exposed and fresh faulted zones the open fracture network provides interconnected gas migration pathways, resulting in low gas concentrations and higher gas flux rates [annunziatellis et al., 2008]. this paper is aimed to study the processes that produce gas anomalies of radon isotopes, 220rn and 222rn, and a co2 as carrier gas, in correspondence of the vettore-gorzano active fault system (about 20 km long, with direction nnw-sse, w dipping) that generated the strong earthquake (ml=6.0), and the seismic sequence of the central apennine on 24 august 2016 between the municipalities of norcia (perugia) and amatrice (rieti) (fig. 1a). the movement of this fault caused an extension of the apennine range of about 3-4 cm between the tyrrhenian and adriatic coast [ama_loc working group 2016]. starting from the nucleation point (about 8 km deep in the proximity of the accumoli village), the rupture of the fault propagated both towards no and se. accelerometer and sar data indicated that the fault is not homogeneously displaced along its length, but is characterized by two main areas 2 http://www.sciencedirect.com/science/article/pii/s1350448712000601%23bib40 http://www.sciencedirect.com/science/article/pii/s1350448712000601%23bib11 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 of displacement (maximum about 1m) on the fault plane. concentration of 222rn, 220rn and co2 in soil gas and the co2 flux was measured along two profiles crossing buried and exposed sectors of the fault coseismic rupture that extends for about 5.2 km along the sw flank of the mt. vettore (fig. 1b). in order to explore the possible mechanisms of soil gas transport near active fault, we infer that gas anomalies are originated by both (1) advective flow along the fault zone, and (2) increased diffusive migration in the soil due to surface processes associated with faulting. ii. methods the mt. vettore-mt. bove fault system, together with the mt. castello-mt. cardosa system, and the norcia-mt. fema system [calamita and pizzi, 1993; calamita et al., 1993], is a part of the sibillini regional thrust that bounds the apennine mountain front, separating it from the marche-abruzzi foothills. this thrust was active all along the messinian and controlled the position of the southern basin margin [milli et al., 2007; bigi et al., 2009], superimposing the pelagic meso-cenozoic carbonates successions onto the laga messinian turbidite deposits [centamore et al., 1992; bigi et al., 2011]. the post-orogenic structures in the study area consist on pliocene-quaternary n150° highangle normal, oblique and, subordinately, strike-slip faults [tavani et al., 2012]. these fault systems are formed by fault segments (generally, wsw-dipping) with an en-échelon geometry or connected to each other by transfer faults. the main slip surfaces of the fault crop out along the western slopes of the mt. vettore, forming prominent fault scarps [pierantoni et al., 2013]. the mt. vettore-mt-porche carbonatic structure belongs to the basal hydrogeological complex of the mts. sibillini carbonatic hydrostructure constituted by calcare massiccio, corniola formations and separated by the silicic-calcareous complex from the upper aquifer of the maiolica system. the hydrogeological flow direction of the basal complex is affected by the main tectonic elements of the area, thus resulting in a n-s drainage along the structure [boni & petitta, 2007; nanni & petitta, 2012]. few springs occur on the eastern side of the sibillini mts. along of the structural contact (i.e., sibillini thrust) between the carbonate hydrostructure of the mt. vettore and the laga flysch at about 900 m a.s.l. [boni et al., 1987]. in this area, soil gas measurements were carried out along two parallel profiles (p1 and p2) crossing buried and exposed sectors of the mt. vettore fault system, respectively (fig. 1b). the exposed fault is represented by a coseismic rupture with an offset of about 20 cm visible in the field at 1500 m a.s.l. (fig. 1c). the rupture crosses the road sp34 and at about 1400 m a.s.l. is buried under the sedimentary cover of a small valley; in the valley a morphological scarp of about 4-5 m occurs along the fault direction (fig. 1d). soil-gas and flux measurements were performed in september in a period of stable meteorological conditions (i.e., an average day temperature of 25°c and no precipitation). a total 50 soil gas samples, as well as 69 co2 flux measurements, were collected along two profiles according to a sampling distance ranging from 10 to 30 m. soil gas samples were collected using a 6.4 mm, thick-walled, stainless-steel probe pounded in the soil at a depth of about 0.6-0.8m by using a co-axial hammer [ciotoli et al., 2007; beaubien et al., 2015]. soil gas was pumped from the probe into the portable devices at the velocity of 1 l m-1. the sampling 3 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 depth ensures little influence of meteorological parameters due infiltrating atmospheric air [hinkle, 1994]. soil gas analysis have been conducted by using a portable gas analyser (draeger x-am 7000, accuracy <5%) connected to the probe for simultaneous analyses of carbon dioxide (co2, range 0-100%), oxygen (o2, range 0-21%), methane (ch4, range 0-100% lel), hydrogen (h2, range 0-600 ppm) and hydrogen sulphide (h2s, range 0-1000 ppm). radon and thoron were measured by using durridge rad7 instrument (+/-5% absolute accuracy, and a sensivity of 0.25 cpm/(cci/l), 0.0067 cpm/(bq/m3) and performing three/four measurements of radon and thoron activity each with 5-minute integration time. the measurement is repeated until the difference of the last two integrations was reduced at least below 10-5%. the result was determined by taking the average of the last two integrations. co2 flux measurements were accomplished by using the west system (west systems tm) accumulation-chamber method equipped with beam co2 infrared sensor (licor li8200) with an accuracy of 2%, repeatability ±5 ppmv and full scale range of figure 1. the figure shows: digital terrain model of the area hit by the earthquake (ml=6.0) of 24 august 2016 that affected large part of the central apennine between the municipalities of norcia (pg) and amatrice (ri) (central italy), with different type of field recognised coseismic ruptures (a); the location of the soil gas profiles (b); photos of the investigated sites (c and d). 4 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 2000 ppmv, and wireless data communication to a palm-top computer. the gas fluxes are automatically calculated through a linear regression of the gas concentration build-up in the chamber. iii. results data statistics for co2, o2, 222rn and 220rn are listed in table 1. no detectable concentrations of ch4, h2 and h2s were observed. generally, soil gas samples exhibited high concentrations compared with atmospheric air content (co2: 0.036%, 222rn and 220rn: 0.01 kbq m-3) [ciotoli et al., 2007]. the calculation of the main statistical indexes indicates clear differences of the mean values for 222rn and 220rn between the two profiles: higher mean values are calculated across the buried sector of the fault (fig. 2a, p1), whereas lower mean values occur across the coseismic rupture (fig. 2b, p2). comparable co2 mean concentrations were observed along both profiles. co2 flux values also show a higher mean for the p1, approximately twice than that calculated for the p2, and a high variability being affected by the vegetation, shallow soil characteristics (i.e., porosity, fracturing, moisture content, temperature, etc.), as well as meteorological conditions (i.e., wind speed, air temperature, rainfall, etc.) [metzger et al., 2008]. normal probability plots (npp) were used to statistically select background, anomalous values and extra outliers of the collected data [sinclair, 1991; ciotoli et al., 2007]; in particular values above 10 kbq m-3 for 222rn, 6 kbq m-3 for 220rn, and 1.0% for co2 have been considered as “anomalies”. furthermore, co2 flux values above 20 (g m-2 day-1) may be considered statistically anomalous, as background values can be approximately defined by the vegetation contribution (up to 10 g m-2 day-1) from meadows and pasture typical of this mountain environments in central italy [bahn et al., 2008].. table 1. main statistics of soil gas data collected along the two profiles. ci, confidence interval; gm, geometric mean; min, minimum value; max, maximum value; lq, lower quartile; uq, upper quartile; st.dev. standard deviation. gas concentrations are expressed: carbon dioxide, co2 (%, v/v); oxygen, o2 (%, v/v); radon isotopes, 222rn and 220rn (kbq m-3) and flux of co2, φco2 (g m-2 day-1). n mean + (ci) gm + (ci) min max lq uq st.dev. profile 1 co2 21 1.06 (0.92-1.30) 0.96 (0.85-1.09) 0.30 2.60 0.80 1.20 0.52 o2 21 19.10 (18.57-19.62) 19.06 (18.50-19-63) 15.10 20.30 19.00 19.60 1.15 222rn 21 9.68 (4.57-14.80) 5.31 (3.09-9.11) 0.49 36.90 4.03 8.16 11.24 220rn 21 9.48 (5.95-13.02) 6.53 (4.24-10.05) 0.88 26.30 3.33 13.70 7.766 φco2 21 39.54 (28.81-50.25) 32.80 (24.40-44.08) 10.73 88.30 21.39 53.21 23.55 profile 2 co2 16 0.84 (0.71-0.96) 0.79 (0.63-0.98) 0.20 1.20 0.80 1.00 0.23 o2 16 19.58 (19.38-19.78) 19.58 (19.37-19.78) 18.60 20.10 19.45 19.80 0.37 222rn 16 3.88 (2.56-5.22) 3.17 (2.20-4.57) 0.72 9.15 2.26 5.56 2.49 220rn 16 6.72 (4.08-9.35) 5.01 (3.23-7.79) 1.20 14.80 2.45 10.65 4.94 φco2 16 18.39 (13.44-23.33) 16.31 (12.34-21.54) 4.79 42.71 12.08 25.08 9.28 5 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 lewicki et al., [2000] reported comparable anomalous co2 flux > 18 g m-2 day-1 along san andreas fault. all studied gases show peaks different in magnitude and distances along the profiles. the spatial distribution of 222rn and co2 values along p1 highlights higher values than those measured along p2, but not exclusively, proximal to the coseismic rupture (fig. 2). along p1, 222rn and 220rn sharp peaks occur on both sides of the hidden fault trace at a distance ranging between 210-240 m (up to 35 kbq m-3 and 25 kbq m-3, respectively) on the footwall, and a minor peak occurs at a distance of 130 m (about 15 kbq m-3 and 10 kbq m-3, respectively) on the hanging wall (fig. 2a). the lowest 222rn activity (about 1-2 kbq m-3) occur between the two peaks in correspondence of the base of the fault scarp, and beyond the distance of 310 m where both radon isotopes activity drops to background values (about 6 kbq m-3). co2 concentrations also follow a similar trend to radon isotopes, being generally higher on both side of the fault than within the fault scarp (1.5–2.5 versus 1.0 %). clear major peaks up to 2.5 % occur on the right side of the buried fault at a distance ranging between 210-240 m, and minor peaks (up to 1.5%) occur on the left side of the fault at distances of 40 and 130 m, respectively. along this profile, co2 background values are below 0.6%; therefore, the minor peaks up to 1.5%, coincident with radon peaks, could also be considered significant. at p2, 222rn activity increases from 2 to 10 kbq m-3 up to the shallow coseismic rupture (at a distance of about 200 m) (fig.2b); then radon abruptly decreases up to 1-2 kbq m-3 on the right side of the rupture. co2 concentrations seem to fluctuate around the mean value (0.8 %), and shows a sharp decrease at 160 m, about 40 m from the visible coseismic rupture where the co2 concentration increases up to 1.2 %. however, the difference between the fault (anomalous) and background populations is not statistically significant along this profile. figure 2. radon isotopes (222rn and 220rn) and co2 soil gas profiles carried out across a fault scarp of the mt. vettore fault (profile 1, a-a’) (a), and across a coseismic rupture (profile 2, b-b’). the radon isotope activity along the profile 2 is multiplied by 3 in order to better highlight peaks and for an easy comparison with the higher activities measured along the profile 1. figure 3 shows the comparison of co2 flux measurements vs co2 along the two profiles. in both profiles values are significantly higher (about 4 and 2 times, respectively) than the normal co2 flux produced by the soil vegetation (maximum 10 g m-2 day-1) of the area [bahn et al., 2008]. figure 3. comparison between the co2 flux values and the co2 concentrations measured along the two profiles. 6 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 in particular, p1 shows major peaks more or less coincident (a shifting about 10m toward the origin) with those highlighted by soil gas data. carbon dioxide flux along p2 highlights a good coincidence with the co2 and 220rn peaks (figg. 2b and 4). iv. discussion active faults contribute to gas leaks because they are deep weakened zones composed of highly fractured materials that increase soil permeability at surface. however, the interpretation of a gas anomaly near a fault is a com plex challenge, and involves the evaluation of the type of migration mechanism (i.e., diffusion/advection at different site-specific conditions, including the presence of potential carrier gases), soil composition and thickness, soil permeability, and fracturing. in the present study, this potential scenario is complicated by the fact that sampling was accomplished in the middle of the seismic sequence of the earthquake of 24 august 2016 and that, at that time, two events of magnitude 3.9 with epicentres close to about 4 km from the survey area occurred. in this phase of an earthquake, spike anomalies occur in correspondence of lateral sectors of buried active faults as a result of the opening fractures, as well as diffuse degassing, affected by the shallow conditions (i.e., potential atmospheric dilution), in the exposed fractured areas (fig. 1a). soil-gas profiles crossing the buried (p1) and exposed (p2) sectors of the mt. vettore fault rupture highlight a non-continuous leakage of the studied gases, both in magnitude and spatially. in particular, high concentrations were measured along the p1, with sharp peaks of 222rn, 220rn and co2 on both side of the fault, and background values in correspondence the fault core scarp. in contrast, p2 shows lower concentrations of the studied gases, but both 222rn and 220rn abruptly drop of their activities on the eastern side of the fault (i.e., footwall). obtained results can be resumed by the two hypothesised gas leak scenarios (fig. 4). figure 4. sketch draws of the two inferred tectonic scenarios along the two profiles: hidden fault with impermeable core, high soil thickness and lateral diffuse fracturing which highlights advective 222rnmigration with peaks on both sides of the fault and a peak of 220rn /222rn ratio in correspondence of the fault core (a). exposed coseismic rupture with low soil thickness a diffuse shallow fractures which highlights low concentrations of 222rn and higher 220rn peaks due to a diffusive shallow degassing. 7 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 along p1 coincident peaks of co2 and radon isotopes on either side of the fault plane, hidden below significant thickness of unconsolidated cover material, can be linked to the extensive fractures proximal to the fault core that significant increase gas channelling and surface leaks. in contrast, the fault core constitutes a less permeable zone where the outgassing rate is decreased by the closure of the fractures caused by the strain changes and formed in the plastic soil cover. however, because in correspondence of hidden faults fractures can remain open at depth, they provided a steady, but reduced gas flux to the surface (fig. 4a). on the contrary, along p2 lower radon isotopes and co2 concentrations occur due to the increased permeability caused by the opening of the coseismic fractures during the earthquake, as well as by the low soil thickness that facilitates dilution due to the gas exchange with atmospheric air, therefore masking any deep input. more complex is the interpretation of the co2 flux that in general is affected by many variables such as, vegetation, soil humidity and temperature, as well as meteorological parameters. the anomalous co2 flux measured along p1 and p2 suggests the presence of a deeper contribution to the “typical” soil production in this area, as well as a high permeability of the fault zone where both advective (mostly at p1) and diffusive (mostly at p2) contributions occur. in general, high co2 flux is correlated with high soil gas co2 concentrations in zones characterised by high permeability dominated by advective transport, but the opposite is not always true due to the many parameters that affected co2 flux in the shallow environment (beaubien et al., 2008; west et al., 2015). however, when both co2 flux and concentration are function of soil production rate, a diffusive transport across soil-air interface can be assumed. according to lewicki et al., [2000], we suppose that in correspondence of high permeable fracture zones (profile 1) the enhanced near surface diffusion increases advection mechanisms at depth; whereas, in correspondence of the coseismic rupture (profile 2) a diffusive gas transport provides low co2 concentrations and high co2 flux. considering that samples have been collected during the seismic sequence, the hypothesis that anomalous co2 flux can be linked to the degassing along the fault damage-zone after the 24th august earthquake cannot be excluded. in support of this hypothesis, it is worth to note that measured co2 flux values are comparable with those measured during the emilia 2012 seismic sequence (ranging from 2.3 to 77.3, mean value 16.8 g m-2 day-1) [sciarra et al., 2012]. regarding the origin of the co2, we refer to chiodini et al., 2011 (and references therein) in which an input of co2 derived from metamorphic decarbonation of local carbonates is reported in the regional groundwater aquifers of the apennines. the interpretation of deep origin radon anomalies along the two profiles may be related to the strain changes during preparation of earthquakes [holub and brady, 1981; ghosh et al., 2009; zoran et al., 2012; namvaran and negarestani, 2012]. radon is considered an excellent fault tracer, but due to the short half-life of its isotopes (220rn has a half-life of 55.6 s and 222rn has a half-life of 3.82 days), its migration distance is limited. however, where the advective migration occurs, the maximum migration distance is strongly increased by the “carrier effect” of other major gases (i.e., co2); in this context the velocities and the travel distances can reach respectively 0.1–1 cm/s and about 1m for 220rn and up to few km for 222rn [tanner, 1978; huxol et al., 2012; davidson et al., 2016]. furthermore, the different migration mechanism of each isotope can provide infor 8 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7242 mation about the processes that contribute to their concentration near faults. due to its short migration distances, 220rn provides indication of radon gas production within soil; in contrast, the 222rn could migrate both from the soil or from deeper sources, in some cases kilometres below the ground surface. therefore, the comparison of these two radon isotopes can contribute to distinguish the soil radon emanation from the deep migration with carrier gas. the calculation of 220rn/222rn ratio offers a tool to eliminate the contribution of radon isotopic fraction generated in the shallow soil materials (i.e., 220rn). in the profile 1, the 220rn/222rn ratio shows a clear peak in the area of the fault scarp, and a rapid drop on both side of the fault. this behaviour is in contrast with the 222rn along the profile and supports the hypothesis that in the fault core a radon production occurs due to the high porosity of the shallow cover material that masks the radon gas migration from deep opened fractures (fig. 4a). on the contrary, the 220rn/222rn ratio along the p2 figures out the same behaviour of 220rn, thus demonstrating that the shallow production of this isotope is originated in the soil, and diffuses at surface due to high fracturing that pervades the exposed coseismic rupture (fig. 4b). v. conclusion soil gas and co2 flux anomalous values have been recorded in correspondence of buried and exposed sectors of the active mt. vettore fault system that generated the strong earthquake (ml=6.0) on 24 august 2016. elevated 222rn, 220rn and co2 gas concentrations, as well as co2 flux values were measured along p1 on both side of the buried fault due to gas advection within the fault-related fracture zone. along p2 the lower soil gas concentrations can be partially attributed to local production and to the increased permeability of this area caused by scarce soil thickness and the presence of high fracturing of the exposed fault rupture. although co2 flux measurements could be affected by site-specific conditions, the high measured values, as well as the coincidence with the peaks of the other soil gases, suggest a degassing process along the buried and exposed fault sectors probably linked to the occurring seismic sequence. results confirms that soil gases in general, but especially 222rn, constitute excellent tracers of buried faults, whereas in correspondence of the coseismic rupture degassing processes are demonstrated only by flux measurements probably related to the seismic activity that enhanced gas leakage underground. the obtained results encourage the renewal of the research about the use of radon and other gases as earthquake precursors. at this regard, two multi-parametric geochemical monitoring stations (gaspro) will be deployed proximal to the mt. vettore fault and to the gorzano fault (few kilometer south). this last active fault, activated during the l’aquila seismic sequence in 2009 [bigi et al., 2012] crosses a high exposure zone due to the presence of the dam of rio fucino, campotosto lake (central italy). the continuous monitoring of geochemical parameters in the areas of high seismicity, and along active fault zones, could help to better describe the leakage variation through these structure. in fact, usually measurements are performed after the main earthquakes events and few information are available for gas/fluid activity and circulation during the pre-seismic period of time. this period instead is characterized by physical variation theoretically well known, but not so 9 annals of geophysics, 59, fast track 5, 2016; 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(2007). a campaign of discrete radon concentration measurements in soil of niška banja town, serbia. radiation measurements 42: 1696 – 1702. 13 http://dx.doi.org/10.1016/j.gexplo.2014.08.015 soil gas geochemical behaviour across buried and exposed faults during the 24 august 2016 central italy earthquake giancarlo ciotoli1,2*, alessandra sciarra2,1,5, livio ruggiero3, aldo annunziatellis4,1, sabina bigi3 1consiglio nazionale delle ricerche – istituto di geologia ambientale e geoingegneria, rome, italy 2istituto nazionale di geofisica e vulcanologia, rome, italy 3università degli studi di roma la sapienza, rome, italy 4istituto superiore per la protezione e la ricerca ambientale, rome, italy 5università degli studi di ferrara, ferrara, italy abstract estimate of ulf electromagnetic noise caused by a fluid flow during seismic or volcano activity annals of geophysics, 58, 6, 2015, s0655; doi:10.4401/ag-6767 s0655 estimate of ulf electromagnetic noise caused by a fluid flow during seismic or volcano activity vadim v. surkov1,*, vyacheslav a. pilipenko2 1 national research nuclear university mephi, moscow, russia 2 institute of physics of the earth, russian academy of science, moscow, russia abstract the elaboration of theoretical models, even oversimplified, capable to estimate an expected electromagnetic effect during earthquake preparation process is not less important than the advancement of observational technique to detect seismic-related electromagnetic perturbations. here possible mechanisms of ulf electromagnetic noise associated with seismic or volcanic activity are discussed. the electrokinetic (ek) and magnetohydrodynamic (mhd) effects due to an irregular flow of conducting rock fluid or magma flow are being revised. the conventional theory of ek effect in a water-saturated rock has been advanced by consideration of elliptic-shaped channels. a contribution of both mechanisms to observed ulf signal on the ground is shown to be dependent on the pore channel size/rock permeability. estimates of magnetic and electrotelluric perturbations caused by magma motion along a volcano throat indicate on the important role of the surrounding rock conductivity. these estimates have proven that the mechanisms under consideration are able to generate ulf electromagnetic perturbations which could be detected by modern magnetometers under favorable conditions. 1. introduction at the time being it is clear that the tectonic plate dynamics can provide long-term (tens-hundred years) earthquake prediction, but not short-term (days-weeks) seismic warning. this situation demands the search for alternative techniques for the short-term prediction of impending earthquakes. a special credit has been paid in the last decades to the study of a variety of electromagnetic and other non-seismic phenomena possibly associated with the earthquake preparation process. considerable efforts have been devoted to the study of electromagnetic signals/noise in the ultra-low-frequency (from few mhz to tens of hz) band (extensive list of references can be found in papers collected by hayakawa and molchanov [2002], molchanov and hayakawa [2008], and hayakawa [2013]). these studies were stimulated by effective detection of electromagnetic effects in a wide frequency band accompanying sample fracture in laboratory [e.g., cress et al. 1987, freund 2000, vallianatos et al. 2012]. easy availability of data from world-wide array (~200) of magnetometers favored an extensive search for seismic-related ulf anomalies. however, it was soon realized that ulf perturbations possibly related to the seismic activity are weak as compared with typical magnetosphere/ionosphere pulsations and industrial interference, so they can be directly recorded only under exceptionally favorable conditions: close proximity to an epicenter of impending earthquake and geomagnetically quiet period [e.g., molchanov et al. 1992]. therefore, much effort has been concentrated in search of peculiar features of seismic-related ulf perturbations that would make possible to reveal them even under low signal/noise ratio. there were numerous attempts to find anomalous ulf behavior with a simple measure of their spectral features the slope of averaged power spectrum (“fractal properties”) [e.g., gotoh et al. 2004]. another approach uses the ratio between the ulf vertical z and horizontal g components. it is expected that an underground source produces a signal on the ground with larger z/g ratio than a magnetospheric/ionospheric source does [hayakawa et al. 1996]. attempts to discriminate seismic and magnetospheric ulf sources were made with the use of the gradient observations [krylov and nikiforova 1995, kopytenko et al. 2006]. more advanced technique the principal component analysis, seems promising to identify and suppress magnetospheric pulsations and industrial interference [gotoh et al. 2002]. however, many of seemingly successful results of seismic-related ulf perturbation discovery could not pass simple tests: lack of correlation with geomagnetic activity and absence of claimed features at distant stations [campbell 2009, thomas et al. 2009, masci 2011]. article history received march 25, 2015; accepted july 31, 2015. subject classification: models and forecasts, earthquake source and dynamics, groundwater processes, volcano seismology, magmas. development in a contentious field of earthquake prediction requires an advance not only in monitoring technique, but in a reliable estimate of physical models plausibility. electromagnetic disturbances in the ulf frequency band, with the skin-depth corresponding to crustal earthquake hypocentral depth, are still considered as one of the most promising monitors of earthquake precursors. majority of modern theories predict that the amplitude of the seismic-related ulf signals can be of the order of or greater than background ulf noise at the epicentral distances no more than one hundred km. the generation mechanisms of seismic-related ulf electromagnetic fields considered so far (see the book by surkov and hayakawa [2014] for a complete review) comprise: the electric charge redistribution during microcracking [molchanov and hayakawa 1995]. however, the estimated amplitude of this effect seems to be much lower than the background noise level because of the random orientation of the dipole moments of individual microcracks [surkov and hayakawa 2014]; inductive geomagnetic response to the tension crack openings in a conductive rock [surkov 1997, surkov and hayakawa 2006]. an advantage of this mechanism is that the effective magnetic moments of all cracks are co-directed and anti-parallel to the earth magnetic field; therefore they operate as a coherent amplifier of ulf noise; the stress-induced electric current in the rock caused by the changes in mobility of charged dislocations [tzanis and vallianatos 2002] and/or point defects [freund 2000]. some theories interpreted the occurrence of ulf electromagnetic noise as a result of the crust fluid dynamics. the flow of high-pressure fluid in fault zones has an irregular character (“stop-and-start”) [byerlee 1993]. such non-steady filtration of conductive fluid is to be accompanied by electromagnetic disturbances due to magnetohydrodynamic (mhd) effect [draganov et al. 1991]. however, the importance of this mechanism was overestimated by four orders of magnitude by draganov et al. [1991] because of an unrealistic rock permeability used in this study [surkov and pilipenko 1997, 1999]. the electrokinetic effect (ek) is another promising candidate which can give a plausible interpretation of anomalous ulf electromagnetic disturbances observed before strong earthquakes [e.g., pride 1994]. the ek effect was applied to interpret both the occurrence of precursory ulf perturbation before earthquake [ishido and mizutani 1981, surkov et al. 2002] and a coseismic electric impulse caused by propagating seismic waves [nagao et al. 2000]. in realistic geophysical media a combination of several mechano-electromagnetic mechanisms may occur: for example, a high level of geoacoustic impulses produced by microcracking can enhance the fluid filtration and ek processes [pilipenko and fedorov 2014]. similar ulf electromagnetic effects may accompany volcano activity [johnston 1989, uyeda et al. 2002]. numerous underground chambers in the rock surrounding a volcano are filled with underground fluid whose pressure varies from hydrostatic level up to lithostatic pressure depending on the chamber sizes, rock permeability, and other parameters. the magma movement along the volcano throat and variations of tectonic stresses may cause the destruction of chambers followed by changes in pore fluid pressure, which in turn results in generation of ek currents [johnston 1997, zlotnicki and nishida 2003]. indeed, several days before and after a volcano eruption electromagnetic noise in the band 0.01-0.6 hz was observed by fujinawa et al. [1992]. at the same time, an irregular movement of a highly-conductive magma along a volcano throat can produce magnetic ulf noise by the mhd effect [kopytenko and nikitina 2004a, 2004b]. to evaluate the significance of possible effects and their dependence on crust parameters and to identify favorable locations for electromagnetic monitoring one needs an approximate but handy model. hopefully, on the basis of adequate theoretical models a special technique, but not standard magnetometers, for a search of seismic-related ulf perturbations will be designed. therefore, the development of analytical models, though sometimes oversimplified, is of primary importance for the progress in search of reliable ulf electromagnetic precursors. in this paper we revisit the conventional theory of the ek phenomenon by incorporating of the mhd effect in the description of pore fluid flow. then we compare contribution of the ek and mhd mechanisms for various sizes and shapes of pores/channel cross sections. we model pores/cracks as ellipsoidal channels and apply this model for a qualitative estimate of ulf magnetic perturbations caused by seismic or volcano activity. 2. the electrokinetic effect in a medium with elliptic-shaped channels the ek effect in multiphase porous media builds up as a result of fluid filtration followed by appearance of a contact potential drop at the interfaces. it is usually the case that the groundwater contains the electrolyte solutions including ions and dissociated molecules. the surfaces of cracks and pores can adsorb ions of certain polarity from the solution that results in a charge separation between the crack walls and fluid followed by surkov and pilipenko 2 3 the formation of electric double layer (edl) at the solidfluid interface. as a rule, the solid is negatively charged due to the adsorption of hydroxyl groups originating from acid dissociation [parks 1965]. the edl includes a diffuse mobile layer extending into the fluid phase. the moving fluid drags solvated cations thereby exciting the ek current. we introduce the model of a pore as a cylindrical channel with the elliptic cross-section x2/a2 + y2/b2 = 1, where a and b are the ellipse semi-axes. in this approach, we can analyze the effect of the cross-section shape on the ek phenomenon since this model involves either circular channels (a = b) or plane cracks (a>>b). a viscous fluid is assumed to flow along z axis. the channel is surrounded by an incompressible solid matrix. the crust is immersed in the geomagnetic field b. we assume a laminar fluid flow because of small value of reynolds number [sparnaay 1972]. the fluid velocity v is controlled by the fluid pressure gradient ∇p along a channel, so the velocity distribution over the channel cross-section v (x,y) is given by [landau and lifshitz 1959] (1) where h is the fluid viscosity. due to the ek effect the fluid contains an excess of ions, more frequently cations, while the channel wall adsorbs the opposite electric charges. as a result, the edl is formed in the fluid near the channel walls. the typical size of the edl is of the order of cation debye radius which is much smaller than the characteristic size of the channel [sparnaay 1972]. the electric potential { in the pore fluid satisfies the poisson equation (2) where q denotes the cation charge, dn is the number density of the cation excess, f is the dielectric permeability of the fluid, and f0 is the dielectric permittivity of a free space. the total current density inside the channel is composed from the conduction and hall currents, and the ek current with density jek= qdnv, as follows: (3) here vf is the fluid conductivity, and e = −∇{ is the electric field strength. it is generally accepted that the conduction current vf e is much larger than the hall current vf (v×b). the mean density of the ek current 〈jek〉, averaged over the channel cross-section, is determined from (2) through the following integral (4) where s =rab is the area of the cross-section and ds is the small element of this area. further, for the sake of simplicity we shall omit the symbol 〈〉. substituting equation (1) for v into equation (4) and taking into account the fact that { changes rapidly in the narrow edl near the channel walls, one can simplify the integral in equation (4). in the case of a circular cross-section (a = b), equation (4) is reduced to the known form [e.g., surkov et al. 2002] (5) here g is the potential drop across the edl, or socalled zeta potential. in the case of an elliptic cross-section some mathematical complications can arise due to the potential variations on the surface of channel. this problem is studied in a greater detail in the appendix. the analysis shows that if the potential is assumed to be constant on the channel surface then the ek current density is given by the same equation (5). thus, in the first approximation the ek currents through the circular and elliptic crosssections are described by the same equation. owing to a finite conductivity vr of the dry rock surrounding the channel the surface potential tends to be equal. the above approach holds true if the relaxation time ∝f0/vr is much smaller than the period of variations of pore fluid pressure and velocity, and this requirement is valid in the processes under consideration. in any homogeneous conducting medium with an arbitrary distribution of pore fluid pressure, the total magnetic effect due to ek effect vanishes [fitterman 1979]. that is, on average the magnetic effect of the electric current resulted from the motion of the pore fluid is cancelled by the effect of the backward conduction current. a non-zero magnetic effect occurs only in an inhomogeneous medium, and its magnitude depends on the degree of heterogeneity. 3. mhd effect and the onsager reciprocal relations in this section we ignore the ek effect for a moment and focus on the mhd effect only. the motion of the conducting underground fluid in the geomagnetic field b gives rise to the generation of hall current jh=vf (v×b) pointed normal to the channel axis. taking into account equation (1) for v we obtain ulf electromagnetic noise in the case of non-conductive rock the hall current is closed by the conduction and ek currents in the fluid. in fact, the total current may flow out of the channel due to a finite rock conductivity. the closed system of longitudinal and transverse electric currents excited in the rock and fluid is shown in figure 1. according to de groot and mazur [1962], the mean ek current density in rocks reads 〈jek〉= −lev∇p, where lev stands for the streaming current coupling coefficient. this coefficient can be derived from (4) to yield lev = −ff0gm/(hb 2), where m is the rock porosity, and b is the pore space tortuosity. similarly, generalizing (6) for the hall current density yields, 〈jh〉= −leb(∇p × b), where leb~levvf s/(4rff0g) = vf ms/4rhb 2 is the hall coefficient. in a more complete theory the relationship between leb and lev should depend on the rock permeability rather than on s. thus the total mean current density in a porous rock can be written as follows: (7) where ∑= lee stands for the mean rock conductivity. to summarize, we note that the electric current in porous rocks can result in electroosmotic and other kinetic effects. according to the onsager reciprocal relations the mean flux density of the fluid flow, j, is given by (8) where lev = lve, and leb= − lbe. the first term in the right-hand part of equation (8) describes the electroosmotic effect, the second term describes darcy law in a porous rock, while the last term arises from the magnetic force acting on a moving conductive fluid. the equations (7) and (8) permit the extension of basic thermodynamic principles [e.g., de groot and mazur 1962] to the case of rock immersed in ambient magnetic field. in order to compare the ek and hall current densities, we return to the consideration of individual channels. combining equations (5) and (6), assuming a ~ b and taking into account that the hall current reaches its peak value at the center of the channel, yields (9) where a is the angle between vectors ∇p and b. notice that if b<<a; that is, in the case of plane channel/crack, the above ratio includes the additional small factor b/a <<1. it follows from equation (9) that the ek current dominates over the hall one in narrow channels; that is, under the requirement (10) taking the typical values of the groundwater parameters g=10 − 50 mv [ishido and mizutani 1981], f= 80 , the electrolyte solution conductivity vf = 0.1 − 0.2 s/m, geomagnetic field b = 5 ×10-5 t and a=r/3, we obtain the following estimate of the critical cross-section s * ≈0.1 − 1 m2. it should be emphasized that this is only a rough estimation since the pore fluid may acquire other properties under the influence of high stress and temperature deep in the crust. equation (10) shows that the hall current appears to have insignificant effect in natural rocks because of small sizes of pores and channels. however, it seems likely that the mhd effects can be significant in the broken rock with high pore space and in the case of macroscopic fluid flow such as magma motion along a volcano throat. 4. geomagnetic perturbations due to magma motion in a volcano throat ulf electromagnetic noise occasionally observed during volcano pre-eruption activity is assumed to be due to the motion of a conducting magma along the surkov and pilipenko 4 figure 1. a geometry of the magma flow model and schematic configuration of total currents j flowing across a volcano throat. the magma velocity v is directed along z axis. the effective current moment def f is perpendicular to v and geomagnetic field b. circles with dots and crosses indicate the directions of magnetic disturbance db and magma velocity. (6) 5 throat. the oscillations of the magma surface in volcano cavity, vortical motion due to coriolis force, as well as upward magma lifting along the volcano throat can serve as possible mechanisms for ulf electromagnetic noise [kopytenko and nikitina 2004a, 2004b]. here we estimate the effect caused by magma lifting. in order to build an analytically treatable model, we assume that the magma is confined within an ellipsoidal underground cavity. the origin of the coordinate system is in the center of the ellipsoid and the coordinate axes coincide with axis of ellipsoid symmetry in such a way that the ellipsoid surface is described by the following equation x2/a2 + y2/b2 + z2/c2 = 1, where a, b and c are the ellipsoid semi-axes. this model is a convenient mathematical approximation because in subsequent study parameter c will tend to infinity in order to describe the magma motion along the infinite cylindrical channel. the cross-section of this cylinder is assumed to be greater than the critical value s * in equation (10). this implies that the ek in equation (3) can be ignored. in this notation the movement of conductive magma with velocity v in the geomagnetic field b results in the generation of electric current with density j =vm(e+v×b), where vm is the magma conductivity. at the same time, in the surrounding crust the conduction back current j =vr e flows. usually, the rock conductivity vr =10 -3−10-2 s/m is much smaller than that of magma vm=10 -2 −10 s/m [gaillard and marziano 2005]. in the ulf frequency range, the electromagnetic fields and currents may be considered as quasi-stationary. for mathematical simplicity we suppose that fluid velocity v is steady and directed along z axis. further we shall apply this approach to an infinite cylinder (c→ ∞) where such kind of fluid motion is possible. the problem under consideration is mathematically similar to the known problem of polarization/magnetization of an ellipsoid in a homogeneous electric/magnetic field [landau and lifshitz 1960]. based on this similarity, one can find that electromagnetic perturbation outside the ellipsoid is derived via its effective current moment (11) here mx and my are the components of the hall current moment m = jhw, x̂ and ŷ stands for unit vectors, and w = 4rabc/3 is the ellipsoid volume. the depolarization factors n(x) and n(y) depend on parameters a, b, c. notice that the vectors d and m are not parallel. now we consider a case when the conductive magma flows along an infinite cylinder/throat with a fixed radius a. this geometry corresponds to parameters of the ellipsoid as follows: a = b and c̃ → ∞. for this case one gets n(x) = n(y) = 0.5 [landau and lifshitz 1960]. substituting these parameters into equation (11), we come to the relationship for the effective current moment def f per unit of the cylinder length (12) let axis y be parallel to the vector def f as shown in figure 1, so the vectors v and b are in the plane x, z. due to the hall current jh =vm(v×b) the cylinder is polarized in such a way that the opposite charges accumulate at the cylinder surface as schematically shown in figure 1. the same figure shows the transverse electric field e which is parallel to the conduction current jc =vme, flowing opposite to the hall current. the total current j = jh + jc is closed by the conduction currents, j =vr e, in the surrounding rocks. the system of currents flowing across the channel is schematically shown in figure 1. this current system generates magnetic disturbance db along the cylinder z-axis. the problem under consideration is axially symmetric, therefore we may introduce the polar coordinates (r,i): r is the distance from channel axis and i is the angle measured from positive x-axis direction. the component of magnetic disturbance along z axis is derived via the effective moment (12) as follows: (13) combining (12) and (13), we obtain the magnetic perturbations in the region outside the cylinder (r > a): (14) where bx is the geomagnetic field component along x-axis. the magnetic disturbance decays away from the cylinder axis as ∝r -1. if the magma conductivity is much greater than the rock conductivity, that is vm >>vr , then the relationship (14) can be simplified (15) equation (15) differs from the relationship derived by kopytenko and nikitina [2004a, 2004b] by a small factor vr /vm << 1. this difference is due to the fact that they ignored the conductivity of the crust vr and thus ulf electromagnetic noise overestimated an expected magnitude of magnetic perturbations. the rock conductivity determines the current leakage from a channel into the environment, and thus may greatly affect the magnitude of magnetic perturbations. the transverse conduction current inside magma is directed opposite to the hall current thereby reducing it. as a result, the total current inside the cylinder is smaller than jh and is directed opposite to the electric field e: (16) it should be noted that both j and e are homogeneous inside the cylinder (see figure 1). the electric field induced in the surrounding rock due to the magma movement in the geomagnetic field is estimated to be (17) if vm >>vr , this field only weakly depends on the rock conductivity. the electric field disturbance decays away from the cylinder more rapidly ∝r -2 than the magnetic disturbance does. a typical amplitude of magnetic disturbance estimated from relationship (15) is as follows: dbmax~ n0a 2vr vmax b/r, where vmax is the amplitude of the magma flow velocity variations. for the same parameters vr =10 -3−10-2 s/m, b = 5 · 10-5 t, vmax = 5 m/s, a = 0,1−1 km as used by kopytenko and nikitina [2004a, 2004b] one gets the estimate of magnetic disturbance at distance r = 1 km of dbmax~ 3 · 10 -3− 3 nt. this estimate is compatible with observations of the magnetic perturbation, about several nt, during volcano activity [johnston 1997]. the electric component of disturbance estimated from (17) is as follows: emax~a 2vmax b/r 2. for the same parameters emax~ 2.5 − 250 nv/m. telluric fields of such amplitude can be detected by modern sensors. the apparent impedance of disturbance produced by the magma flow dynamics can be estimated as z =n0de/db ~ (vrr) -1. this value differs considerably from the apparent impedance of magnetospheric waves, and this distinction could be used for their discrimination. the spectrum of the ulf electromagnetic noise observed on the ground is determined by a source spectrum and by attenuation factor due to the skin-effect. the fluctuations of magma velocity along with oscillations of the magma surface or seismic vibrations of the underground cavity can contribute to the source spectrum. fundamental frequency of such vibrations is f ~vs/l, where l = 0.1−10 km is the cavity scale, and vs = 2 − 2.5 km/s is the magma sound velocity. for these typical values, this frequency, f ~ 0.2 − 25 hz, falls into ulf/elf band which is consistent with observations [fujinawa et al. 1992, johnston 1997]. the analytical relationships (14) and (15) show that magnetic perturbation depends mainly on the conductivity of the surrounding rocks rather than on magma conductivity. under low rock conductivity, the generated system of hall currents is nearly completely shortcircuited by the conductivity currents within magma, and the magnetic effect on the ground is to be weak. under high rock conductivity the conduction currents expand far into the rock, and the magnetic effect on the ground is more significant. 5. discussion the fact that some published results on ulf “precursors” were not supported upon a more detailed analysis [thomas et al. 2009, masci 2011] should not rule out the problem of seismo-electromagnetic phenomena entirely. for a search of seismic-related ulf signals just standard magnetic observations could be inappropriate because of a small value of signal-to-noise ratio. an elaboration of specialized detection methods of ulf seismic-related signals/noise, and their discrimination from the magnetospheric waves, are to be based on some models, even oversimplified. the elaboration of theoretical models capable to estimate an expected effect under observational conditions is not less important than the advancement of observational technique. the proposed paper is a step in this direction. the increase of ulf electromagnetic noises associated with enhancement of seismic or volcano activity can be explained in terms of different physical mechanisms. the average current densities and fluid fluxes can be described in terms of onsager reciprocal relations. the rough estimate of the ek and hall current amplitudes has shown that the ek effect plays a key role as the mean cross-section of channels is smaller than a certain critical value. this situation is typical for realistic water-saturated rocks with a weak permeability. the mhd effects dominate a macroscopic flow such as the groundwater migration through a broken rock with a high permeability or magma motion along a volcano throat. in the above consideration we have neglected the atmosphere-ground interface. the account of it may modify the estimates, but not significantly, less than by factor about 2 [fedorov et al. 2001]. our analysis has shown that the ek and hall currents in an individual channel have different structures. surkov and pilipenko 6 7 the ek and back conduction currents are directed along the fluid flow; that is, parallel to the channel walls, whereas the hall currents are predominantly concentrated in the cross-section of the channels. therefore, the resulted magnetic disturbances have different field polarization. the magnetic field perturbation due to the mhd effect is parallel to the axis of a channel (dbz component in figure 1). by contrast, the magnetic perturbation caused by the ek effect (dbr and dbi components) is perpendicular to the channel axis. the averaging of these effects over the rock volume cannot cancel this tendency since there is a predominant direction in ground fluid filtration or magma motion. however, in practice it seems very difficult to distinguish between these two factors only on the basis of signal polarizations. actually the fluid-filled cracks, pores and channels are randomly distributed in the rock. in order to interpret the observations adequately, the hall and ek current densities given by equations (5) and (6) should be averaged over the rock volume. this problem requires further consideration with a numerical modeling. it follows from our model that the ek current densities through the circular and elliptic cross-sections are described by the same equation. so, we may assume that the cross-section shape of pore channels has a minimal effect on the ek phenomena except for the case of very narrow cracks when the distance between the crack surfaces becomes comparable with the edl thickness. the latter case should be studied separately because of the overlap of the adjacent edls inside the crack space. it appears that the crack tortuosity may have a more significant effect on both the rock permeability and ek effect. using a simplified model of steady flow of conducting magma along a cylindrical channel we have estimated the amplitude of magnetic perturbations at small distances from a volcano. in contrast to [kopytenko and nikitina 2004a, 2004b], we have found that the rock conductivity reduces this estimate essentially. however, the upper limit of this estimate (~3 nt and ~250 µv/m) is close to the amplitudes of signals occasionally observed during volcanic eruption and the enhanced seismic activity possibly associated with magma motion. despite uncertainties with factual parameters of magma flows, volcano geometry, and crust parameters, the estimates prove that under favorable conditions ulf magnetic monitoring on the ground of the underground magma flow becomes feasible. the ulf electromagnetic effects possibly associated with enhancement of seismic activity can be explained in terms of different physical mechanisms. in this study we have reanalyzed only two such mechanisms the ek and mhd effects. the ulf magnetic perturbation produced by acoustic noise in conducting layers of the ground is another promising mechanism [surkov 1997, surkov and hayakawa 2006]. what mechanism makes a major contribution to the observed seismic-related signals is the key question to be answered. a progress in observational studies of possible anomalous seismic-related ulf electromagnetic fields would be impossible without elaboration of specialized detection methods, based on adequate models. elaboration of such models may indicate what ulf signatures (e.g., polarization, impedance, gradients, waveforms, etc.) can be used as an indicator of an impending earthquake or volcano eruption. 6. conclusions we have considered the ek and mhd effects due to an irregular flow of the crust fluid or magma as a possible mechanism of ulf electromagnetic noise associated with seismic or volcanic activity. the conventional theory of the ek effect has been advanced by considering elliptic-shaped channels. a contribution of both mechanisms to observed magnetic disturbance is shown to be different depending on the pore/channel permeability. magnitudes of magnetic and electric field perturbations depend on a contrast between fluid/magma and rock conductivities. the suggested model proves the possibility to estimate analytically by order of magnitude the expected electromagnetic effect of the fluid/magma flow under chosen geophysical parameters. such estimates prove a feasibility of the ulf electromagnetic monitoring of magma dynamics in a volcano conduit, supplementary to the observations of volcano tremor. acknowledgements. this study was supported by the grant no. 13-05-12091 from rfbr. we appreciate useful comments of all reviewers. references byerlee, j. 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j.j. love and m.j.s. johnston (2009). on the reported magnetic precursor of the 1989 loma prieta earthquake, phys. earth planet. in., 173, 207215. tzanis, a., and f. vallianatos (2002). a physical model of electrical earthquake precursors due to crack propagation and the motion of charged edge dislocations, in: m. hayakawa and o. molchanov (eds.), seismo electromagnetics: lithosphere atmosphere ionosphere coupling, terrapub, tokyo, 117-130. uyeda, s., m. hayakawa, t. nagao, o.a. molchanov, k. hattori, y. orihara, k. gotoh, y. akinaga and h. tanaka (2002). electric and magnetic phenomena observed before the volcano-seismic activity in 2000 in the izu islands regions, japan, proc. natl. acad. sci. usa, 99, 7352-7355. vallianatos, f., a. nardi, r. carluccio and m. chiappini (2012). experimental evidence of a non-extensive statistical physics behavior of electromagnetic signals emitted from rocks under stress up to fracture. preliminary results, acta geophys., 60, 894-909. zlotnicki, j. and y. nishida (2003). review of morphological insights of self-potential anomalies on volcano, surv. geophys., 24, 291-338. * corresponding author: vadim v. surkov, national research nuclear university mephi, moscow, russia; email: surkovvadim@yandex.ru. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. appendix a: ek current through a channel with elliptic cross-section to perform integration in equation (4) we first introduce the elliptic coordinates n and o according to x = c coshncoso, y = c sinhnsino, where n≥0, 0≤o<2r and c2 = a2 − b2 (a > b). the lines n(x,y) = c1 and o(x,y) = c2 (c1 and c2 are constants) determine the families of confocal ellipses and hyperboles which form an orthogonal grid. taking the notice of lame coefficients hn= ho= c (sinh 2n+ sin2o)½, the unit cross-section is ds = h2ndndo. the laplace equation for the potential { is given by (a1) since inside the edl the electric potential { varies most rapidly along the direction normal to the wall, the derivative with respect to o in equation (a1) can be neglected. taking into account this approximation and substituting equations (1) and (a1) into equation (4) we get: integrating equation (a2) by parts and taking into account that coshnmax= a/c, sin hnmax= b/c, and d{(0)/dn= 0, we come to (a3) considering the fact that { changes rapidly in the narrow edl near the walls and d{/dn tends to zero as n→ 0, we set n=nmax in the factor sin h 2n in the integrand. we can thus perform the integration in equation (a3), arriving at (a4) assuming that g={(nmax) −{(0) is a constant value and performing integration in equation (a4), we arrive at equation (5). ulf electromagnetic noise (a2) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49_1_2006def 219 annals of geophysics, vol. 49, n. 1, february 2006 key words decomposition rates – hyper-spectral image – ndvi – co-kriging 1. introduction environmental assessment at landscape and regional level claims new and efficient technologies and procedures in order to take full account of both structural and functional properties of environmental systems on a broad spatial extent. nowadays, remote sensing is an essential tool for ecosystem and landscape status monitoring. multiand hyper-spectral remotely-sensed data are more and more frequently used for ecological data analysis, landscape unit mapping and spatial analysis of ecosystem structural organization. in the past few years, spatial data analysis has been frequently applied to remotely sensed imageries. only recently has attention been given to the spatial analysis of processes at landscape level (dugan et al., 1994; curran, 2001). yet coupling structures which are remotely detected with functioning still remains one major task of the environmental monitoring activities. litter decomposition is a key process in terrestrial ecosystem functioning because it controls nutrient availability in the soil, thus regulating primary production, improves soil structure and reduces soil erosion. decomposition is a complex process mapping litter decomposition by remote-detected indicators letizia sabetta (1), nicola zaccarelli (2), giorgio mancinelli (1), stefania mandrone (3), rosamaria salvatori (3), maria letizia costantini (4), giovanni zurlini (2) and loreto rossi (4) (1) laboratorio di ecologia, dipartimento di scienze e tecnologie biologiche ed ambientali, università degli studi di leccce, ecotekne, lecce, italy (2) laboratorio di ecologia del paesaggio, dipartimento di scienze e tecnologie biologiche ed ambientali, università degli studi di leccce, ecotekne, lecce, italy (3) istituto sull’inquinamento atmosferico (iia), cnr, roma, italy (4) dipartimento di genetica e biologia molecolare area ecologica, università degli studi di roma «la sapienza», roma, italy abstract leaf litter decomposition is a key process for the functioning of natural ecosystems. an important limiting factor for this process is detritus availability, which we have estimated by remote sensed indices of canopy green biomass (ndvi). here, we describe the use of multivariate geostatistical analysis to couple in situ measures with hyper-spectral and multi-spectral remote-sensed data for producing maps of litter decomposition. a direct relationship between the decomposition rates in four different corine habitats and ndvi, calculated at different scales from landsat etm+ multi-spectral data and mivis hyper-spectral data was found. variogram analysis was used to evaluate the spatial properties of each single variable and their common interaction. co-variogram and co-kriging analysis of the two variables turned out to be an effective approach for decomposition mapping from remote-sensed spatial explicit data. mailing address: dr. letizia sabetta, laboratorio di ecologia, dipartimento di scienze e tecnologie biologiche ed ambientali, università degli studi di leccce, ecotekne, sp lecce-monteroni, 73100 lecce, italy; e-mail: letizia.sabetta@unile.it 220 letizia sabetta et al. that occurs through different phases and depends on many environmental factors. the rate of litter decomposition is regulated by abiotic and biotic factors, such as climate, litter quality and decomposer activity (swift et al., 1979; gallardo and merino, 1993; aerts, 1997; perez-harguindeguy et al., 2000). a variety of litter quality indices based on the initial chemical composition (aber and melillo, 1980; aber et al., 1990) and leaf structure parameters (gallardo and merino, 1993) have been used as decay predictors; moreover, works on the use of near infrared reflectance spectroscopy (nirs) have shown that it is a powerful and rapid method for predicting the biochemical composition of forest foliage (card et al., 1988; martin and aber, 1994) and also litter decomposability (gillon et al., 1999). according to saunders (1976), decomposition rate is a function of both the substrate and decomposer concentrations as bimolecular secondorder reaction. in forested habitats, the substrate concentration strictly depends on litter input from canopies which can be remotely sensed and estimated through the normalized difference vegetation index (ndvi, rouse et al., 1974). in this study we present results from multivariate geostatistical analyses used to couple in situ measures of decomposition rates with hyper-spectral and multi-spectral remotely sensed data in terrestrial ecosystems. relationships between decomposition rates, estimated for four corine habitats, and ndvi values estimated at various scales from multi-spectral landsat etm+ and hyper-spectral mivis data, were investigated. variogram and co-kriging analyses were carried out to evaluate the spatial properties of each ecosystem variable, their common interaction and for mapping them from remotesensed spatially explicit data. 2. materials and methods study area – the study was carried out in the catchment-basin of lake vico, a volcanic area located in central italy about 50 km north of rome (42°19ln, 12°10le). since 1982 this area (about 3200 ha) has been part of a regional reserve, which includes the lake (510 m a.s.l.) and mount venere (851 m a.s.l.). plant communities follow a typical altitudinal gradient from reed thickets (phragmites australis) located along the lake shores and hazelnut cultivated areas to mixed forests. in this study we have considered four plant communities referring to four corine habitats (coordination de l’information sur l’environnament; european union/directorate general xi, 1991): hazelnut areas (dominated by corylus avellana, corine code 83.1), chestnut forests (dominated by castanea sativa, corine code 41.9), turkey oak forests (dominated by quercus cerris, corine code 41.74) and beech forests (dominated by fagus sylvatica, corine code 41.181). decomposition studies – plant litter breakdown was studied using the litterbag technique in 20 sampling sites (5 sampling sites for each corine habitat). sampling sites were georeferenced through a gps system. fifty litterbags (mesh-bags of 0.5 cm mesh size containing 3 dry-g of the dominant species leaves) were randomly placed at each sampling site to simulate the natural heaps of detritus. ten litterbags were retrieved monthly from each site from december to october. samples were carefully and separately placed in polythene boxes and then brought to laboratory, where their ash-free dry masses were measured after leaf cleaning, oven-drying (at 60°c for 72 h) and ignition (at 800°c for 3 h). remote sensing data – the remote sensed imageries came from landsat etm+ and mivis (daedalus aa5000 multispectral infrared & visible imaging spectrometer, italian national research council – lara project) and they were acquired by the end of the vegetative period, before leaves falling, in order to evaluate the detritus input. mivis scanner is a hyper-spectral airborne imaging system with 102 spectral bands covering visible and near infrared (0.43-0.83 nm, 1st sensor), middle infrared (1.15-1.55 nm, 2nd sensor; 1.98-2.50 nm, 3rd sensor) and thermal infrared (8.21-12.70 nm, 4th sensor) regions of the electromagnetic spectrum. mivis images had different spatial resolutions (4, 8, 10 m) corresponding to different fly heights (2000, 4000 and 5000 m a.s.l.). the resolution interval considered in this study thus ranged from 4 to 30 m (from landsat images). mivis data were acquired on 22 september 2000, at 12.00 am local time. all mivis images were geocoded by the 221 mapping litter decomposition by remote-detected indicators delaunay triangulation procedure using a common set of ground referenced points and ortophotos of the area (richards, 1994). maximum root mean square error (rms) was 0.3. landsat data were acquired on 16 august 2000, which was the only available scene with no cloud cover, and it was geocoded by an affine transformation of first order with an rms error of 0.2. landsat data radiometric calibration and conversion to radiance were performed by applying stored parameters derived by the calibration parameter file supplied with the original data and using chander and markham formulas (usgs, 2004). mivis data were acquired as radiance values by the provider. basic atmospheric corrections were applied to all images: landsat data were corrected using a dark object subtraction (dos) approach (chavez, 1988) with lake vico as a target; mivis data were corrected by an «empirical line calibration» procedures based on in situ collected spectra for the vegetation cover of the four corine habitats and bare soil (kruse et al., 1990). data analyses – leaf mass losses were fitted to the simple exponential model of decomposition, mt = m0 e−kt where m0 is the initial leaf afdm (ash free dry mass), mt is remaining leaf afdm at time t (in days) and k is a breakdown coefficient expressed in days−1 (olson, 1963). the half-life, i.e. the time necessary to reduce detritus mass to 50% of its initial afdm, was expressed as ln(2)/k. ndvi values were calculated using band number 3 (0.63-0.69 nm) and 13 (0.67-0.69 nm) as red channels and band number 4 (0.750.90 nm) and 19 (0.79-0.81 nm) as infrared channels from landsat and mivis data, respectively. in plots of 60 × 60 m2, centered on each sampling site, we determined the mean ndvi values at each spatial resolution. plot dimensions were based on the position accuracy of landsat data, which was at least of 30 m, i.e. one pixel, if the geocoding process was performed with rms error lower than 1. mean plot ndvi values were calculated for each pixel of each image of the study area. the relation between the litter half-life time and ndvi was investigated applying both classical and spatial statistics: classical linear regression analysis and ancova were performed to evaluate the scale information decay while a geostatistical approach based on variogram evaluation was carried out to model spatial dependency within scales. spatial continuity among observations of a given set of variables may be characterised by a variogram or a co-variogram, which reveals the random and the structured aspects of the spatial dispersion. the variogram and the co-variogram have been widely used to describe the spatial structure of ecological variables (legendre and fortin, 1989; rossi et al., 1992). the traditional estimator of the co-variogram is defined as (isaaks and srivastava, 1989) where z(xi), z(xi + h) and t(xi), t(xi + h) are measurements of two variables at locations xi and xi + h separated by the vector of directional distance h, and n(h) is the number of pairs of samples considered in the given distance class. this calculation is repeated for different values of h and provides the empirical co-variogram, which is a plot of the values of c(h) as a function of discrete distance h describing the joint spatial cross-covariance of two variables. when only one variable is analysed (i.e. z and t are the same) c(h) is called empirical variogram. to provide a smooth, continuous description of the covariance and cross-covariance spatial structure of the parameters, variogram models were derived by applying admissible functions (wackernagel, 1995) to empirical estimates calculated from the data sets. basic variogram models were added together in a nested structure and fitted to experimental estimates by an ordinary least square procedure (pebesma and wesseling, 1998). modeled variograms and covariograms were used in ordinary block kriging and cokriging techniques for spatial interpolation to produce decomposition maps (isaaks and srivastava, 1989; rossi et al., 1992). kriging is a weighted, moving-average interpolation method where the set of weights to estimate values at unmeasured points is computed as a function of the variogram model and locations of the ( ) ( ) ( ( ) ( )) ( ( ) ( )) n z z t tx x x x h h h h 2 1 i i i i i $ $ ) = + + c / fig. 1. ndvi values for mivis and landsat images at different spatial resolutions (lines) and leaf half-time life (bars) in the four vegetation types. data are mean values from five sampling sites per vegetation type. 222 letizia sabetta et al. samples. kriging is a blue estimator (best linear unbiased estimator, isaaks and srivastava, 1989) attempting to minimize the estimation variance of predicted values. estimation can be done for single points or blocks (i.e. block kriging) and exploiting the spatial properties of just one variable or the joint variation of two (i.e. cokriging). for a detailed description of the procedure refer to isaaks and srivastava (1989) or wackernagel (1995). 3. results litter mass loss showed a significant fit with the negative exponential model (olson, 1963) in all study sites and for each plant litter type. senescing leaves of hazelnut (corylus avellana) had shorter average half-life (199±22 days) showing faster decomposability than, in order, chestnut (castanea sativa, 356 ± 28 days), oak (quercus cerris, 237±17 days) and beech (fagus sylvatica, 461±37 days; table i; fig. 1); differences in the litter decomposition rate among vegetation types were significant (nested ancova, table ii(a)). we also observed significant differences of the decomposition rate among study sites within each vegetation type (nested ancova, table ii(a)). ndvi value differed both among vegetation types and among spatial scales; hazelnut area had lower average ndvi value at each considered spatial scale, showing lower vegetation cover, than chestnut, oak and beech areas (table i; fig. 1). on an average ndvi was lower from landsat than from mivis images, but the differences observed among plant types were scale-invariant (two-way anova, table ii(b)). in the studied area litter decomposition rate (half-life) and vegetation cover (ndvi) were positively related to all considered spatial resolutions (table iii). slopes of this observed relationships did not differ from each other (ancova, variable: litter half-life time, covariable: ndvi); on the contrary, intercepts of the linear relationships differed significantly between landsat and mivis (table iv), the former being lower than the latter (903 versus 1178 ± 85). significant spatial autocorrelations of the half-life and ndvi value were found within the study area (table v). a significant amount of the observed variation in half-life and ndvi was spatially structured; semivariograms showed that table i. litter half-life time and ndvi for mivis (three flight heights) and landsat images in the four vegetation types. mean values from 5 sampling points and standard errors (in brackets) are reported. type half-life (d) mivis ndvi landsat ndvi 5000 m 4000 m 2000 m corylus avellana 199 (22) 0.567 (0.013) 0.593 (0.021) 0.615 (0.015) 0.479 (0.062) castanea sativa 356 (28) 0.683 (0.042) 0.697 (0.049) 0.707 (0.051) 0.619 (0.093) quercus cerris 237 (17) 0.673 (0.012) 0.701 (0.013) 0.725 (0.009) 0.586 (0.013) fagus sylvatica 461 (37) 0.711 (0.014) 0.730 (0.014) 0.754 (0.011) 0.594 (0.030) 223 mapping litter decomposition by remote-detected indicators table ii. (a) nested ancova analysis of remaining litter mass as variable and time as covariable (sampling sites nested in vegetation type as main factor); (b) two-way anova analysis of ndvi as variate (factor a = spatial resolution, factor b = vegetation type) followed by tukey hsd test (main factor: vegetation type on the left and spatial resolution on the right). source df ms effect f p-level (a) nested ancova: litter mass (covariate: time) between vegetation types 3 0.34 8.699 1.22∗10−5 within vegetation types 16 0.076 1.943 0.015 (b) two-way anova: ndvi spatial resolution (a) 3 0.066 22.28 5.4∗10−10 vegetation type (b) 3 0.073 24.37 1.2∗10−10 interaction a×b 9 0.001 0.3 0.97 tukey hsd test corylus avellana quercus cerris fagus sylvatica landsat 2000 m 4000 m quercus cerris 1.5∗10−4 2000 m 1.5∗10−4 fagus sylvatica 1.5∗10−4 0.44 4000 m 1.5∗10−4 0.64 castanea sativa 1.5∗10−4 0.99 0.63 5000 m 1.7∗10−4 1.5∗10−4 0.59 table iii. regression and cross-variogram statistics of half-life time (y) and ndvi (x) from landsat etm+ and mivis images. regression cross-variogram function r p model r p landsat y = 903x−180 0.53 < 0.05 linear 0.98 < 0.001 5000 m y = 1256x−492 0.72 < 0.001 linear 0.95 < 0.001 mivis 4000 m y = 1087x−407 0.65 < 0.01 2000 m y = 1190x−500 0.71 < 0.001 table iv. summary statistics for estimated marginal means (a) and p-values for pairwise comparisons following tukey hsd test (b) from ancova analysis of half-life time as variate and ndvi as covariate after test for parallelism. (a) spatial scale mean (1) standard error 95% confidence interval lower bound upper bound landsat 397.93 26.87 344.34 451.51 2000 m 270.11 24.39 221.47 318.75 4000 m 289.86 23.48 243.04 336.68 5000 m 311.09 23.02 265.18 357.01 (b) landsat 2000 m 4000 m 2000 m 0.002 4000 m 0.005 0.548 5000 m 0.018 0.222 0.519 (1) adjusted means for ndvi equal to 0.646. fig. 2a,b. spatial variation of half-life time in 60 × 60 m-support block size. a) ordinary block kriging map. b) ordinary block co-kriging map based on ndvi from mivis data (5000 m). open circles locate the sampling sites (corylus avellana, no; quercus cerris, ce; castanea sativa, ca; fagus sylvatica, fa). 224 letizia sabetta et al. the proportion of sample variance (c0 + c) accounted for by spatially structured variance (c) was on average 81 ± 12%. the semivariance of ndvi increased regularly with the separation distance, up to 1541 m for landsat data and 2364 m for mivis data, following spherical model, and showed negligible nugget variance (table v). the decomposition rate had an increase of semivariance up to 1627 m and a high nugget variance. table v. variogram model parameters of ndvi from the landsat etm+ and mivis (5000 m) images and the litter half-life time. variable model semivariogram parameters proportion r 2 rss nugget (c0) sill (c0+c) range (m) c/(c0+c) landsat ndvi spherical 0.006 0.02 1541 0.700 0.993 9.2∗10−7 mivis ndvi spherical 0.002 0.01 2364 0.800 0.986 1.3∗10−6 half-life time linear with sill 1230 17870 1627 0.931 0.895 2.05∗10+7 a b 225 mapping litter decomposition by remote-detected indicators litter half-life and ndvi were also spatially co-structured; the cross-variogram analysis showed inter-relationships among the two autocorrelated variates with positive co-regionalization (table iii). the half-life increased northwards (fig. 2) from the hazelnut area to the beech forests. cokriging maps from landsat and mivis data did not differ. 4. discussion and conclusions some major points can be drawn from the results: – decomposition rate is both plant species-dependent and spatially dependent. in fact it varies with the vegetation type and within each forest type; it is significantly heterogeneous because of the heterogeneous decomposer (microorganisms and detritivores) distribution. ndvi, a remote-sensed index reflecting structural (i.e. cover and density of the leaf layer) and functional (i.e. physiological status of the leaf layer) properties of vegetation, is peculiar to the four corine habitats under study and satisfactorily relates with the decomposition rates, measured in situ. – the information content of ndvi varies at each resolution scale. differences between mivis and landsat data are related mainly to spatial and spectral resolution of the sensors, even if a residual atmospheric interference not accounted for dos approach has to be suggested. further investigation is needed to evaluate their relative importance. however, the information decay from mivis to landsat images does not modify the relationship existing with the litter decomposition rate. – using ndvi as co-variable, the co-kriging map of litter decomposition is better shaped than the kriging map. co-kriging offers additional advantages over kriging as it involves covariate that is cross-correlated with the variable of interest and that can be more easily sampled (isaaks and srivastava, 1989). ndvi, as spatially explicit co-variable, can thus sustains the mapping algorithm where decomposition information is scarce or missing. in conclusion, the results show that remote sensing techniques can provide outstanding help in developing the cartography of ecosystem functions, also when availability of the groundtruth data is low. landsat data are as useful as mivis data for the remote sensing analysis and multiple scales mapping of decomposition. this is a clear advantage since the landsat data can be more cheaply and regularly acquired than the mivis data. acknowledgements this research was founded by the joint program «progetto n. 2: conoscenza, conservazione e gestione della biodiversità» of the italian ministry of the environment and the italian national research council. we would like to acknowledge the contributions made by the two anonymous reviewers who helped with valuable comments. references aber, j.d. and j.m. melillo (1980): litter decomposition: measuring relative contribution of organic matter and nitrogen to forest soil, can. j. bot., 58, 416-421. aber, j.d., j.m. melillo and c. mcclaugherty (1990): predicting long-term patterns of mass loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems. can. j. bot., 68, 2201-260. aerts, r. 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(1995): multivariate geostatistics. an introduction with applications (springer-verlag, berlin). 479_495.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 479 precursory scale increase and long-term seismogenesis in california and northern mexico frank evison (1) and david rhoades (2) (1) institute of geophysics, victoria university of wellington, new zealand (2) institute of geological and nuclear sciences, lower hutt, new zealand abstract a sudden increase in the scale of seismicity has occurred as a long-term precursor to twelve major earthquakes in california and northern mexico. these include all earthquakes along the san andreas system during 1960-2000 with magnitude m 6.4. the full list is as follows: colorado delta, 1966, m 6.3; borrego mt., 1968, m 6.5; san fernando, 1971, m 6.6; brawley, 1979, m 6.4; mexicali, 1980, m 6.1; coalinga, 1983, m 6.7; superstition hills, 1987, m 6.6; loma prieta, 1989, m 7.0; joshua tree, 1992, m 6.1; landers, 1992, m 7.3; northridge, 1994, m 6.6; hector mine, 1999, m 7.1. such a precursory scale increase ( ) was inferred from the modelling of longterm seismogenesis as a three-stage faulting process against a background of self-organised criticality. the location, onset-time and level of are predictive of the location, time and magnitude of the future earthquake. precursory swarms, which occur widely in subduction regions, are a special form of ; the more general form is here shown to occur frequently in a region of continental transform. other seismicity precursors, including quiescence and foreshocks, contribute to or modulate the increased seismicity that characterises . the area occupied by is small compared with those occupied by the seismicity precursors known as amr, m 8 and lurr. further work is needed to formulate as a testable hypothesis, and to carry out the appropriate forecasting tests. 1. introduction anomalies of seismicity are intuitively the most plausible of all proposed earthquake precursors. the universal power-law relating earthquake magnitude and frequency shows that each earthquake of magnitude m has associated with it, in a statistical sense, about three hundred earthquakes of magnitude m-2.5 or greater. for a shallow earthquake, aftershocks typically provide about one-tenth of these associated smaller earthquakes. immediate preshocks, commonly known as foreshocks, seldom provide more than a few, and often none at all. what about preshocks occurring over longer time periods, and perhaps larger areas? the m 8 model (kossobokov et al., 1999) and the accelerating moment release (amr) model (varnes, 1989; bowman et al., 1998) are both concerned with preshocks over moderately long time periods and over large areas. shorter time periods and large areas are specified in the load/unload response ratio (lurr) model (yin et al., 2000). in the present study, the time period is the longest so far suggested for a precursory phenomenon (apart from regular recurrence models), while the area is comparatively small. mailing address: prof. frank f. evison, institute of geophysics, victoria university of wellington, p.o. box 600, wellington, new zealand; e-mail: frank.evison@vuw.ac.nz key words precursory seismicity – seismogenesis – california – mexico 480 frank evison and david rhoades seismicity precursors are also the easiest to study, because of the very large database contained in observatory catalogues. many workers consider that it simplifies matters to decluster the catalogue, i.e. to remove aftershocks (reasenberg, 1985), and perhaps other concentrations of small earthquakes as well. in the present study the whole catalogue is used, apart from applying a magnitude threshold in the interest of homogeneity. earthquake concentrations, including aftershocks, are found to be highly relevant to the understanding of seismogenesis. second only to aftershock sequences, the easiest concentration of earthquakes to recognise in many catalogues is the swarm. swarms are common in the shallow seismicity of subduction regions: systematic studies in new zealand, japan and greece show that swarms make a large contribution to long-term preshock activity in subduction regions, where swarms are predictive of mainshocks (evison and rhoades, 2000). this phenomenon is explained by modelling seismogenesis as a three-stage faulting process occurring against a background of self-organised criticality (evison and rhoades, 1998, 2001). an inference from the model, however, is that preshocks need not occur as swarms. this has been confirmed in a region of continental collision (evison and rhoades, 1999a): the precursor is a long-term set of preshocks, irrespective of how they are distributed in time. the present paper extends the study to a region of continental transform, and better defines the precursor as a sudden increase in the scale of long-term minor seismicity in the neighbourhood of the future mainshock. the aim is to show that the scale increase is a seismogenic phenomenon, preparatory to formulating an hypothesis that can be tested by long-range forecasting. 2. the -phenomenon the precursory scale increase, here called the -phenomenon, is a seismicity pattern in space, magnitude and time. the essential features of the phenomenon are shown schematically in the three diagrams in fig. 1a-c. first, the area occupied by the phenomenon indicates the location and space-scale of the seismogenic process (fig. 1a). this is found by adjusting the position, size and shape of a rectangular area containing the main-shock and aftershocks, so as to obtain the highest seismicity rate in the precursory period relative to the rate in the prior period (see fig. 1c). such a procedure could be refined by formal optimisation, which could readily allow for areas to be elliptical, with any orientation, rather than the present n-s, e-w rectangles. for the 12 examples presented below, the median main-shock magnitude is m6.6, and the median area is 4500 km2. all earthquakes that occur in the rectangle (and have magnitudes at or above the threshold) are included in the analysis. secondly, the magnitude level m p of the precursory seismicity (fig. 1b) determines the magnitude-and-time scale of the seismogenic process. a robust measure of magnitude level, adopted here as in previous studies of the swarm precursor, is the average of the three largest magnitudes in the relevant time-period. since much of the precursory seismicity tends to occur early in the precursory period, a good estimate of this magnitude level, which is a crucial seismogenic parameter, is usually available at an early stage. thirdly, the jump in seismicity that marks the onset of seismogenesis is indicated by the sudden change of slope in fig. 1c. this graph is a cumulative magnitude anomaly (cumag), c(t), which is a type of cusum (page, 1954) designed to display the average rate of seismicity between any two points in time. c(t) is defined by (2.1) (2.2) where m i is the magnitude and t i the time of the ith earthquake, m c is the threshold magnitude, and k is the average rate of magnitude accumulation between the starting time t s and the finishing time t f . accordingly, each earthquake c t m m k t tts ti t i c s ( ) = +( ) ( )0 1. k m m t tts ti t f i c f s = +( ) ( )0 1. / 481 precursory scale increase and long-term seismogenesis in california and northern mexico micity rate expressed in units of magnitude per year (mu/yr) for the relevant area. (for example, if the chosen threshold magnitude (m c ) is 4.0, and in a particular interval of a year there occurs a single earthquake, with magnitude 4.9, the seismicity rate for that time interval is 1.0 mu/yr). a major upward jump in rate produces a sharp minimum in the c(t) graph; the low point is taken as the onset of the -anomaly, and the date of the low point marks the start of the precursor time t p . the precursory rate is then given by the gradient of the straight line joining the low point to the zero point immediately before the mainshock, while the prior rate is obtained by joining the initial zero point to the low point. throughout the sequence is represented by an upward jump equal to the amount by which the magnitude exceeds the baseline value, which is 0.1 below the threshold magnitude. the downward slope between successive earthquakes is equal and opposite to the sum of all the upward jumps, divided by the total time; thus, a given plot begins and ends at the value zero. unlike most cusum graphs, the cumag abscissa is a linear scale of time. from eqs. (2.1) and (2.2) it follows that the gradient of the line between any two points on the c(t) curve is a measure of the average rate of earthquake activity during the corresponding time period. gradients are translated into seismicity rates by means of a protractor, with seis. fig. 1a-c. -phenomenon: schematic. a) typical epicentral area of precursory seismicity, mainshock and aftershocks. b) prior magnitude level and jump to precursory level m p . magnitude level is derived from the data: it is the average of the three largest magnitudes in the relevant period of time. m m is the mainshock magnitude. c) prior and precursory seismicity rates, showing sudden change at start of precursor time t p . seismicity rate is derived from the data by means of the cumag (see eqs. (2.1) and (2.2)); it is averaged over the relevant period of time. a protractor is included to indicate the rate corresponding to any given slope; rate is given in units of magnitude above base-level, per year (mu/yr). 482 frank evison and david rhoades 3. the -phenomenon in california and northern mexico the -phenomenon is documented here for twelve major earthquakes in california and northern mexico, including all the earthquakes of magnitude m 6.4 that occurred along the san andreas system in the period 1960-2000. this exploratory study is a basis for the future development of a testable hypothesis, and the conduct of formal tests. for the special case of the precursory swarm phenomenon, such tests are already in progress in new zealand, japan and greece (evison and rhoades, 1997, 1999b, 2000). the earthquakes discussed here are the following: colorado delta, 1966, m 6.3; borrego mt., 1968, m 6.5; san fernando, 1971, m 6.6; brawley, 1979, m 6.4; mexicali, 1980, m 6.1; coalinga, 1983, m 6.7; superstition hills, 1987, m 6.6; loma prieta, 1989, m 7.0; joshua tree, 1992, m 6.1; landers, 1992, m 7.3; northridge, 1994, m 6.6; hector mine, 1999, m 7.1. the database used is the cnss worldwide earthquake catalog, 1954-2000. this catalog is contributed to by member networks of the u.s. council of the national seismic system, and is available from the northern california earthquake data center. since magnitude level is here defined by the three largest earthquakes in the relevant time-period, thresholds are adopted which abundantly satisfy the requirements of homogeneity: m 4.0 for two of the larger earthquakes, and m 3.5 for the rest. the areas within which the phenomenon and associated earthquake occurred are shown in the location map of fig. 2. the observations are shown in figs. 3i-xii. all the catalogued earthquakes within the indicated space and time windows, and above the magnitude thresholds, are included in the analysis and plotted in the diagrams. no processing such as declustering is applied, nor any transformation of the catalogued magnitudes into moment, energy or benioff strain release. the length of time preceding the onset is indeterminate on the view of seismogenesis adopted here, but where possible the period before the onset has been taken as about equal to that after. with this standard format a remarkable similarity is apparent between most of the examples. shorter pre-onset periods sometimes have to be accepted; this applies both at borrego mt. (fig. 3ii), near which a neighbouring earthquake occurred a few years earlier (rabbit peak, 1954.03.19, m 6.4), and at superstition hills (fig. 3vii), which occurred close to the preceding brawley earthquake (fig. 3iv). the essential features of the -phenomenon, as shown schematically in fig. 1a-c, are identified in figs. 3i-xii by means of dashed lines. the areas occupied by the phenomenon are typically much smaller than those occupied by some seismicity precursors, including m 8 (kossobokov et al., 1999), amr (bowman et al., 1998), and lurr (yin et al., 2000). in all twelve examples in figs. 3i-xii the data display the sudden increase in the rate of seismicity and the associated increase in magnitude level (figs. 3i-xii(b)). parameter values are given in table i. the times of the twelve seismicity jumps (i.e. the onset times) are widely fig. 2. areas within which the -precursor and associated major earthquake occurred at: i) colorado delta; ii) borrego mt.; iii) san fernando; iv) brawley; v) mexicali; vi) coalinga; vii) superstition hills; viii) loma prieta; ix) joshua tree; x) landers; xi) northridge; xii) hector mine. 483 precursory scale increase and long-term seismogenesis in california and northern mexico fig. 3i,ii. -phenomenon: data and interpretation for major earthquakes at (i) colorado delta and (ii) borrego mt. interpretation is explained in fig.1a-c. a) epicentres. b) magnitudes versus time, also showing (dashed lines) the derived prior and precursory magnitude levels. c) cumag (eqs. (2.1), (2.2)), also showing (dashed lines) the derived prior and precursory seismicity rates. (the cumag scale on the right-hand side refers to the mainshockaftershock period). 484 frank evison and david rhoades fig. 3iii,iv. -phenomenon: data and interpretation for major earthquakes at (iii) san fernando and (iv) brawley. for explanation see caption fig. 3i,ii. 485 precursory scale increase and long-term seismogenesis in california and northern mexico fig. 3v,vi. -phenomenon: data and interpretation for major earthquakes at (v) mexicali and (vi) coalinga. for explanation see caption fig. 3i,ii. 486 frank evison and david rhoades fig. 3vii,viii. -phenomenon: data and interpretation for major earthquakes at (vii) superstition hills and (viii) loma prieta. for explanation see caption fig. 3i,ii. 487 precursory scale increase and long-term seismogenesis in california and northern mexico fig. 3ix,x. -phenomenon: data and interpretation for major earthquakes at (ix) joshua tree and (x) landers. for explanation see caption fig. 3i,ii. 488 frank evison and david rhoades fig. 3xi,xii. -phenomenon: data and interpretation for major earthquakes at (xi) northridge and (xii) hector mine. for explanation see caption fig. 3i,ii. 489 precursory scale increase and long-term seismogenesis in california and northern mexico nb: m is the prior magnitude level; m p is the precursory magnitude level; t p is the time in days between the onset of seismogenesis and the mainshock; scale increase m is m p minus m ; scale increase rate is precursory rate/prior rate. table i. precursory scale increase ( ) data. locality prior onset precursory scale mainshock increase m rate date m p rate t p (d) m rate date m m colorado d. 4.2 0.82 1964.01.17 5.1 11.57 933 0.9 14.04 1966.08.07 6.3 borrego mt. 4.1 1.38 1957.01.24 5.1 5.42 4093 1.0 3.92 1968.04.09 6.5 s. fernando 3.5 0.03 1964.02.08 4.1 0.56 2558 0.6 19.79 1971.02.09 6.6 brawley 3.6 0.16 1974.12.06 4.8 6.55 1774 1.2 40.37 1979.10.15 6.4 mexicali 4.3 1.12 1976.01.03 5.0 5.89 1619 0.7 5.27 1980.06.09 6.1 coalinga 3.8 0.23 1975.01.06 5.1 2.07 3038 1.3 8.83 1983.05.02 6.7 super. hills 3.6 0.44 1981.04.25 5.6 4.22 2404 2.0 9.70 1987.11.24 6.6 loma prieta 4.7 0.30 1979.05.08 5.9 1.89 3816 1.2 6.34 1989.10.18 7.0 joshua tree 4.1 0.87 1986.07.08 5.0 4.00 2116 0.9 4.61 1992.04.23 6.1 landers 4.7 0.42 1979.03.15 5.7 2.37 4854 1.0 5.59 1992.06.28 7.3 northridge 3.5 0.07 1988.11.21 4.8 1.71 1883 1.3 23.00 1994.01.17 6.6 hector mine 3.5 0.05 1992.06.28 5.0 1.72 2667 1.5 32.29 1999.10.16 7.1 scattered between 1957 and 1992. this makes it unlikely that the -phenomenon in california and northern mexico could be an artificial effect produced by improvements in the seismograph network. the scale increase in magnitude ranges from 0.6 to 2.0 magnitude units, with a median of 1.1. the scale increase in rate ranges from 3.92 to 40.37, with a median of 9.27. these median values correspond rather closely; for two gutenberg-richter sets with b-values of unity, a jump of 1.0 magnitude units would correspond to a scale increase of 10. the wide range of values for each of the measures of scale increase is to be expected, on the present view of seismogenesis, since the level of seismicity in the prior period, like the length of that period, is indeterminate, and has no effect on the seismogenic process. (it should be noted that the rates in table i are not to be compared from one example to another, since they depend on the size of the relevant area, as well as on the adopted threshold magnitude). in many of the examples the mainshock epicentre is near the centre of the rectangle, but this is not always to be expected, since the hypocentre may occur at any point on the fracture. at colorado delta (fig. 3i) and at brawley (fig. 3iv), the mainshock epicentre is near the edge of the rectangle, while aftershocks are located among the precursory seismicity. once the occurrence of a jump in the rate of seismicity indicates that seismogenesis has begun, it is the time of the jump, and the value of the new magnitude level, that allow the parameters of the major earthquake to be estimated, as will be explained below. the precursory rate given in table i is that for the whole precursory period. the rate up to any earlier point can equally be obtained, from the slope of the cumag between the time of the jump and that of the point in question. thus it is easy to see to what extent the occurrence of a -anomaly would become apparent at an early stage, for example if the seismicity were being monitored in real time. the cumags in figs. 3i-xii show that rates that would be observed early in the precursory period are usually higher than the final value; i.e. lines from the low point to intermediate points along the precursory data-plot are usually steeper than the 490 frank evison and david rhoades dashed line. at superstition hills (fig. 3vii), this is so for about the first half of the precursory period, but the -anomaly could hardly have been recognised, because of the short time that elapsed between the nearby brawley earthquake (fig. 3iv) and the change of scale at superstition hills. in addition, many of the precursory earthquakes at superstition hills occurred as immediate foreshocks, thus adding to the unusual appearance of the cumag graph. (the dashed line in fig. 3vii(b) (lower), as in the other cumag graphs in figs. 3i-xii, indicates the average seismicity rate from the time of the scale increase to immediately before the mainshock). despite these complications, the parameter values given in table i for superstition hills are compatible with those for the other examples. the precursory earthquakes, like the aftershocks, contribute directly to the set of minor earthquakes associated with the major earthquake. the two contributions appear to be roughly comparable, since the magnitude levels are about the same (evison and rhoades, 1998). together, therefore, they contribute roughly one-fifth of the minor earthquakes that are statistically associated with the mainshock through the gutenbergrichter relation. the remaining four-fifths evidently occur outside the seismogenic locationtime space; in this larger space the mainshock would not stand out as anomalously large. the following correlations support the view that the -anomaly is a seismogenic phenomenon, and suggest how it might be applied to long-range earthquake forecasting. 4. predictive correlations the -phenomenon is related to the major earthquake in location, magnitude and time. the epicentres of the precursory seismicity are located close to those of the mainshock and aftershocks, as shown in figs. 3i-xii(a). secondly, the relation between mainshock magnitude (m m ) and precursor magnitude (m p ) agrees closely with the regression that has previously been calculated for the swarm phenomenon (fig. 4a). thirdly, agreement is also evident with the relation between precursor time (t p ) and precursor magnitude (fig. 4b). these agreements are to be expected, since precursory swarms are a special form of the phenomenon. the set of precursory earthquakes may occur in a variety of ways. in the shallow subduction regions of greece, japan and new zealand they occur in the highly organized and recognizable form of swarms. in california and northern mexico, on the other hand, many different types of distribution occur, as can be seen in figs. 3i-xii. the agreement with regard to fig. 4a,b. predictive regressions (after evison and rhoades, 2000) obtained from systematic studies in greece, japan and new zealand, with california and northern mexico data superimposed. a) mainshock magnitude versus precursor magnitude. b) precursor time versus precursor magnitude. nb: precursor magnitude and precursor time refer to the special case of the swarm precursor for the greece, japan and new zealand data, and to the more general -precursor for the california and northern mexico data. 491 precursory scale increase and long-term seismogenesis in california and northern mexico precursor time depends in part on the tendency of swarms to occur early in the precursory period, i.e. at or soon after the onset of seismogenesis. taking the swarm and results together, the relations between precursor and mainshock as regards magnitude and time are now supported, as fig. 4a,b shows, by 40 examples of major earthquakes in new zealand, japan, greece, california and northern mexico. the present study has yet to be extended to include a search for instances of the -phenomenon occurring unrelated to a mainshock event, or vice versa. nevertheless, the new results help to sharpen the definition of the -phenomenon, and they augment the empirical basis both for testing the predictive capability of the phenomenon and for modelling seismogenesis. 5. three-stage faulting model a minor variation on the accepted process of faulting crack formation, shear fracture, healing is sufficient to account for the above correlations. this variation is to regard crack formation as a stage that is separable in time from the consequent shear fracture. a major fault such as the san andreas is the result of many such faulting processes through geologic time. as a corollary of the separation of crack formation from shear fracture, it is postulated that a major crack generates a set of minor cracks, in the same way that, in the mainshock/aftershock phenomenon, a major fracture generates a set of minor fractures. modelling the faulting process in three separable stages was first proposed to account for the precursory swarm phenomenon (evison and rhoades, 1998). an inference from the model, however, was that precursory earthquakes need not occur as swarms. this extended the scope of the long-term seismicity precursor to regions where swarms do not usually occur, i.e. to other than subduction regions. in the continental collision region of new zealand, swarms are replaced by more dispersed groups of precursory earthquakes, which have been called quasi-swarms, or quarms (evison and rhoades, 1999a). the present study is the first in which a sudden increase in the scale of seismicity is taken as the precursor; this is the most general form suggested by the model. under the three-stage faulting model, the formation of a major crack starts the process which eventually culminates in a major shear fracture (and the resultant major earthquake). the major crack at once generates a set of minor aftercracks, which fracture over time, and it is these minor fractures that produce the precursory seismicity. healing of the set of minor fractures is a necessary condition for the fracture of the major crack, which generates the mainshock and aftershocks. finally, healing of these fractures restores the medium to the condition it was in before the particular process started. three-stage faulting accounts, then, for the following features of the -phenomenon. the jump increase in seismicity marks the onset of fracturing of the set of aftercracks generated by the major crack. it has been observed that the set of precursory earthquakes has a similar magnitude level to the aftershocks (evison and rhoades, 1998); this follows, in the model, from the major crack and fracture being necessarily of the same size. (in the 12 examples presented here the ratio of aftershock to precursor magnitude level ranges from 0.84 to 1.37, with a median of 0.99). the long duration of the seismogenic process is explained by the need, according to mogi’s (1963) criteria, for the stress field across a fault to become uniform before fracture can occur; uniformity across the major crack is attained by the healing of the set of aftercracks, after they have fractured and generated the precursory earthquakes. finally, the essential independence of major earthquakes from one another is explained by the healing which follows the mainshock and aftershocks. inter-earthquake triggering, while not excluded, occurs as a second-order effect. the failure of experiments based on the regular-recurrence hypothesis, such as the experiment at parkfield, california (roeloffs and langbein, 1994), is explained. the seismogenic process as modelled by three-stage faulting is further elucidated by the proposal that it takes place against a background of self-organised criticality (evison and rhoades, 2001). self-organised criticality (soc) systems display an extreme sensitivity to initial con492 frank evison and david rhoades ditions. thus under the model it is acknowledged that the sudden increase in seismicity may be the earliest recognisable signal of a future major earthquake, just as in meteorology, where soc is the widely accepted background condition, a tropical depression is the earliest signal of a future tropical cyclone. the scaling principle, which is also basic to soc, is exemplified in the model by the role of the major crack in determining the scale of the entire seismogenic process, with regard to space, magnitude and time. thus the term «major» is purely relative: the major crack can be of any absolute size. again, the soc principle of hierarchy is exemplified in the occurrence of smaller seismogenic processes embedded in larger ones, as with the joshua tree process (fig. 3ix), which was entirely embedded in the precursor to the landers earthquake (fig. 3x). more generally, the context of a given major earthquake can be understood in two distinct ways, depending on scale. on the larger scale, as already mentioned above, the earthquake belongs to a gutenberg-richter set, and occurs in a context of self-similarity. on the smaller scale, the earthquake is anomalous: as a mainshock, it is too large to belong to the gutenbergrichter set of its preshocks and aftershocks. this is the scale of the predictive correlations that have been presented above. under self-organized criticality, one can visualise that any of a large number of small earthquakes has the potential to avalanche, thus nucleating a large earthquake. this has been interpreted to mean that individual earthquakes are intrinsically unpredictable. the present model reconciles long-range forecasting with selforganized criticality by accommodating the avalanche-nucleation concept at the crackformation stage of seismogenesis. there is no reason to regard the initial cracking as predictable, but once it has occurred the remaining stages, including the major earthquake, are determined. this is somewhat analogous to the nucleation and development of a tropical cyclone. 6. associated precursors several types of precursor occur as fluctuations which modulate the average seismicity during the precursory period. large swarms mark the onset of seismogenesis at colorado delta (fig. 3i) and borrego mt. (fig. 3ii). foreshocks occur at the end of the precursory period, occasionally in large numbers, as at superstition hills (fig. 3vii) and at landers (fig. 3x). in contrast, quiescence frequently occurs, as at northridge (fig. 3xi) and hector mine (fig. 3xii), in both of which an interval occupying about 40% of the precursory period was devoid of earthquakes. the well-known accelerating moment release precursor will be discussed in detail below. a seismicity parameter frequently studied in the context of precursory phenomena is the b-value in the gutenberg-richter equation log10n(m) = a bm. a related parameter is the mean magnitude of a gutenberg-richter set (aki, 1965). the loma prieta and northridge earthquakes have been studied by smith (1998) in terms of precursory changes in mean magnitude. the areas considered are similar to those in fig. 3viii and fig. 3xi. using two different types of cusum graph, smith found an anomaly before the loma prieta earthquake, beginning at about the same time as the -anomaly reported above. this may be a coincidence, since anomalies are closely related to the a-value in the gutenberg-richter equation, and this is usually held to be independent of the b-value. precursors involving phenomena other than seismicity may in some cases be compatible with the present model. the crack-formation stage of faulting may produce anomalies in acoustic emission (scholz, 1990), and coda q 1 (jin and aki, 1989), due to a form of dilatancy. here, the dilatancy will consist of a fractal set of cracks, rather than pores of more or less uniform size. electromagnetic emissions, too, have been widely reported in association with cracking (e.g., guo et al., 1994). on the present model, all such crackrelated anomalies are to be looked for early in the precursory period. many of these precursors have been observed, but the reporting of examples remains largely anecdotal. this is consistent with the three-stage faulting model. the contributory precursors involving seismicity are possible but not necessary under this model; further, most studies of precursors related to cracking have concentrated on time periods rather close to the mainshock time. 493 precursory scale increase and long-term seismogenesis in california and northern mexico 7. and amr precursors contrasting interpretations of the seismicity preceding major earthquakes are suggested by the y and amr (accelerating moment release) patterns. a ready comparison can be made between these interpretations for california, since, of the earthquakes discussed above, bowman et al. (1998) have presented detailed amr interpretations of borrego mountain, san fernando, coalinga, superstition hills, loma prieta, landers, and northridge. the two studies have different backgrounds. the amr study was a search for empirical support of a theoretical model. the incentive for the study, on the other hand, was an inference from the three-stage faulting model, which was itself developed to explain the precursory swarm phenomenon. widely differing areas around the earthquake source are involved in the two interpretations. in amr theory, an increasing correlation length in the regional stress field reaches a critical point, and this is when the earthquake occurs. the process involves an area very much larger than the earthquake source area. in the case of , by contrast, a three-stage faulting process is postulated in the more immediate neighbourhood of the source area, although still involving faults besides that on which the major earthquake occurs. figure 5 shows the areas of the amr circles in bowman et al. (1998) and those of the rectangles, for the same set of earthquakes. also shown is utsu’s (1961) relation between aftershock area and mainshock magnitude, and parallel lines are fitted to the amr and sets of data. overall, the areas are 10 times larger, and the amr areas 140 times larger, than the aftershock areas. further, the scatter is considerably less for the areas (variance 0.07) than for the amr areas (variance 0.25). one might think that the amr phenomenon would extend into the source region, but according to jaumé and sykes (1999) it occurs primarily outside. that no vestige of accelerating seismicity occurs in most of the plots (figs. 3i-xii) is thus only to be expected. the exceptions are superstition hills (fig. 3vii), loma prieta (fig. 3viii), and landers (fig. 3x). these apparent occurrences of accelerating seismicity were not identified as amr by bowman et al. (1998), who showed (their fig. 5) that for all the earthquakes that they studied, except loma prieta, a linear or decelerating moment release was indicated for areas of intermediate size. widely differing distributions of the precursory seismicity with respect to time are also proposed by the two models. in amr, the increase in seismicity is initially emergent, and accelerates up to the time of the earthquake, while in , the jump in seismicity occurs at the start of the precursory period. this dominant feature of seems to be irrelevant to amr, since in the plots given by bowman et al. (1998, fig. 6) for the borrego mountain, san fernando, coalinga, superstition hills and northridge earthquakes, the starting date was later than the jump in seismicity. little in common can thus be found between the and amr precursors. they may nevertheless offer complementary information on the location, magnitude and time of future major mainshock magnitude a re a (1 0 0 0 sq . km ) 6.0 6.5 7.0 7.5 0 .1 1 .0 1 0 .0 1 0 0 .0 amra fters hoc ks ( uts u, 1 961 ) am r ψ ψ fig. 5. and amr precursory areas versus mainshock magnitudes for the earthquakes at borrego mountain (1968, m 6.5); san fernando (1971, m 6.6); coalinga (1983, m 6.7); superstition hills (1987, m 6.6); loma prieta (1989, m 7.0); landers (1992, m 7.3), and northridge (1994, m 6.6). the utsu (1961) relation between aftershock area and mainshock magnitude is shown for comparison, and parallel lines are fitted to the amr and data. 494 frank evison and david rhoades earthquakes. as regards location, the much larger areas involved in amr are nevertheless centred, in the study by bowman et al. (1998), on the earthquake epicentre. robinson (2000) has shown, however, in a study of three new zealand earthquakes, that the amr centre was best placed at a distance of 50-60 km from the earthquake epicentre. as regards magnitude, amr relates this to area, but as shown in fig. 5 above, there is much scatter in magnitude as a function of area. in the precursor, the mainshock epicentre can be anywhere in the rectangular area, as discussed above and exemplified in figs. 3i-xii(a), while the magnitude is given as a probability distribution, with scatter as shown in fig. 4a. the time of the major earthquake is indicated in amr by an exponentially increasing function of the time to failure. this seems superior to the probability distribution and scatter obtained for the -precursor (fig. 4b). but according to robinson (2000) the earthquake time estimated by amr is in practice only loosely constrained. 8. conclusions the -phenomenon is one of the simplest earthquake precursors so far identified. it is manifested in the origin times, locations and magnitudes routinely listed in earthquake catalogues. the predictive parameters that it supplies consist of a rather closely defined area, a date, and a magnitude, and these give longterm estimates, in the form of probability distributions, of the location, time and magnitude of the major earthquake. as the longest-term of all suggested precursors (unless one includes the hypothesised «seismic gap» as a precursor), it accommodates a variety of shorter-term anomalies. precursory swarms, when they occur, are a special case of the -precursor. thus the above examples from california and northern mexico can be added to the 28 examples previously published from japan, greece and new zealand, making in all 40 large earthquakes that are similarly related to the -precursor. this constitutes a clear description of the -phenomenon as a long-term precursor, and part of the seismogenic process, in some of the main types of seismotectonic environment. formulation of a testable hypothesis can now proceed, and by means of the appropriate methodology (rhoades and evison 1979, 1993) the -precursor in california, northern mexico and elsewhere can be evaluated for possible use in long-range earthquake forecasting. acknowledgements the authors thank e.g.c. smith, w. d. smith and j.j. taber for critical readings of the manuscript, and paolo gasperini and an anonymous reviewer for valuable comments. the work has been supported by the new zealand foundation for research, science and technology, under contract c05x0006. the first author acknowledges facilities provided under an honorary fellowship at victoria university of wellington. references aki, k. 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(received february 4, 2002; accepted july 4, 2002) 169_175 adg v–5 n01 stanisl.pdf annals of geophysics, vol. 45, n. 1, february 2002 169 forecasting of ionospheric characteristics during quiet and disturbed conditions iwona stanisl/awska and zbigniew zbyszyński space research centre, polish academy of sciences, warsaw, poland abstract an autocovariance forecasting procedure for single location ionospheric characteristics is presented. its accuracy is illustrated as a function of the amount of time extrapolation for selected european stations under quiet and disturbed conditions. 1. introduction day-to-day and hour-to-hour ionospheric variations are generally irregular. irregular ionospheric variations are variations that cannot be predicted by any linear prediction method. they are caused by irregular changes in amplitudes of ionospheric parameters as well as their time spread. in order to predict them much interest and efforts have been dedicated in the past. various non-linear techniques have been proposed. the latest publications deal with a statistical approach (muhtaov and kutiev, 1999), or with a modern neural network technique (wintoft and cander, 1999, 2000; tulunay et al., 2000). this paper presents a continuation of this work (stanisl/awska and zbyszyński, 2001) in the application of the autocovariance prediction method for ionospheric purposes. 2. the autocovariance prediction method application the autocovariance prediction method was originally elaborated at the space research centre for prediction of irregular variations in earth rotation (kosek, 1993, 1997). in this method the first prediction point outside the data time interval in the future and in the past is computed and added at the beginning or at the end of data, respectively, so the next prediction point can be computed. the difference between the prediction and data at a particular time in the future computed at different starting prediction epochs reveals unpredictable or irregular variations of the considered ionospheric parameter. along this line the analysis of potential application of this method in the ionosphere was presented in stanisl/awska (1994). the forecasting capabilities of the method for f0 f2 parameter were shown in stanisl/awska and zbyszyński (2001). this paper presents a similar analysis for other ionospheric characteristics as well as the forecast dependence on the time range considered. one of the advantages of the autocovariance method is that it does not require any additional parameters, which describe solar and/or geophysical conditions. it also means that any mailing address: dr. iwona stanisl/awska, space research centre pas, 00-716 warsaw, bartycka 18a, poland; e-mail: stanis@cbk.waw.pl key words forecasting of ionospheric characteristics 170 iwona stanisl/awska and zbigniew zbyszyński additional uncertainty connected with the need to use a prediction of these is avoided. the only information needed is a long enough period of observation. another advantage is that it can be used for forecasting without any knowledge of the morphological and physical processes in a medium, such as the ionosphere. 3. results and conclusions the present study investigates the f0 f2, f 0f1, f 0 e, and m(3000)f 2 parameters. data have been taken from ral-cd-rom, prepared by the rutherford appleton laboratory, united kingdom, for european cooperation in the field of scientific and technical research (cost) action 251 (hanbaba, 1999) and from the ionospheric despatch centre in europe (stanisl/awska et al., 1999) (http://www.cbk.waw.pl/rwc/idce.html), that is a cost 251 initiative. a list of relevant stations is presented in table i. in this method the prediction estimation is computed as a function of an observed variable (different ionospheric characteristic) from several ionospheric stations only. the sampling interval of ionospheric characteristics, in our case, is 1 h. in this paper, 1-, 2-, 4-, 8-, 12-, 24and 48-h-ahead forecast have been obtained. particular purposes of this application are to consider the needs of the forecast obtaining for instantaneous situation by operational service of the ionospheric situation. to deal with a real situation any specific method for gaps filling has been used. any data gaps were replaced by 7-days-smoothed average only. also the requirements of the time period of available data were limited. data from separate periods within september november 1998 were used. the number of data used in the computation for the presented statistics is shown in table ii. for the calculations of the short time forecast (up to 12 h ahead) 73 past values (3 days) hour by hour, have been taken, while for a longer time forecast (24-48 h ahead) 28 values from the previous 28 days, for each hour separately, have been taken. to satisfy the requirements of the method, as input data we used deviations of the measurements from the 7-days-smoothed table i. stations and their geographical coordinates. location station latitude, °n longitude, °e tortosa eb040 40.8 0.5 rome ro041 41.9 12.5 juliusruh jr055 54.6 13.4 sofia sq143 42.7 23.4 warsaw mz152 52.2 21.2 uppsala up158 59.8 17.6 lycksele ly164 64.6 18.8 kiruna ki167 67.8 20.4 table ii. number of considered data points. disturbed quiet total f0 e 1900 3900 5800 f 0 f 1 500 900 1400 f 0 f 2 6100 9200 15300 m(3000)f2 6100 8500 14600 171 forecasting of ionospheric characteristics during quiet and disturbed conditions fig. 1. bar chart created for rms errors for f0 f2 characteristic, for disturbed data (upper panel), quiet data (middle panel), and for all data together (lower panel). autocovariance forecasting method: for1, 1 h ahead for2, 2 h ahead, etc. persistence: p1. fig. 2. the same as fig. 1 for f0 f1 characteristic. fig. 3. the same as fig. 1 for f0 e characteristic. fig. 4. the same as fig. 1 for m(3000)f 2 characteristic. fof2 dist urbed dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.2 0.8 1.4 fof2 quiet dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.2 0.8 1.4 fof2 all dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.2 0.8 1.4 fof1 dist urbed dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.1 0.3 0.5 fof1 quiet dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.1 0.3 0.5 fof1 all dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.1 0.3 0.5 foe dist urbed dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.0 0.2 0.4 foe quiet dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.0 0.2 0.4 foe all dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.0 0.2 0.4 m(3000)f2 dist urbed dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.05 0.15 0.25 m(3000)f2 quiet dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.05 0.15 0.25 m(3000)f2 all dat a f o r1 f o r2 f o r4 f o r8 f o r1 2 f o r2 4 f o r4 8 p 1 it u -r 0.05 0.15 0.25 1 2 3 4 172 iwona stanisl/awska and zbigniew zbyszyński average value calculated for each hour separately. an additional correction factor for 1-hahead forecast is introduced in the following manner: for the forecast at time t, parameters at times t-1 and t-2 are calculated. if the values in t-1 and t-2 differ from the measurements by more than 30%, the current calculated forecast is changed exactly by the last value of the gradient. the forecast was analysed for the disturbed and quiet periods separately. for the distinction between quiet and disturbed conditions, the catalogue of disturbances (kouris et al., 1998) observed at selected ionospheric stations was used. the catalogue is available at the ionospheric despatch centre in europe. the prediction error is presented as an rms error and percentage deviation of the prediction against the measurements. for comparison with the forecast results using the itu-r-predicted median value and the measurement from the previous hours (persistence) have also been given. figures 1 to 4 show the bar charts of the rms error for four considered characteristics for quiet and disturbed conditions separately, as well as for all data together. the method has been used fig. 5. 1and 24-h-ahead forecast, and measurements for f0 f2 characteristic for three 5-day-periods in september 1998 at kiruna station. 173 forecasting of ionospheric characteristics during quiet and disturbed conditions fig. 6. the same as fig. 5 for f 0 e characteristic at uppsala station. for 1-, 2-h ahead, etc. samples of 1-, and 24-hahead forecast for f0 f2, f0 e and m(3000)f2 characteristics for the most quiet, incidentally chosen moderate disturbed and the most disturbed periods are shown in figs. 5 to 7. five day periods in october and september 1998 observations, 1and 24-h ahead, are presented. incidentally chosen, but representative for the numerical experiments performed, stations from midand high latitudes are presented. only the plot for f0 f1 characteristic (fig. 8) is presented for a 5-day-period in september 1998 for a mixed quiet and negatively disturbed period for the mid-latitude warsaw station only. for all the considered data the forecast shows much better results than medians and persistence. except for the most disturbed data for f 0 f 2 characteristic, when the persistence is better than the forecast 2-, 4-, etc. hours ahead. but obviously, while considering the forecast for longer than 1-h-ahead period, the persistence does not exist. it ought to be mentioned that itu-r prediction is never available with such accuracy, as used in this paper, because of the actual solar activity parameters used (not prediction). for m(3000)f 2 characteristic, increasing the time range of the forecast also increases the errors. this increase is not so 174 iwona stanisl/awska and zbigniew zbyszyński fig. 7. the same as fig. 5 for m(3000)f2 characteristic for three 5-day-periods in october 1998 at juliusruh station. fig. 8. the same as fig. 5 for f0f1 characteristic for one 5-day-period in september 1998 at warsaw station. 175 forecasting of ionospheric characteristics during quiet and disturbed conditions pronounced as for f 0 f 2 . for f 0 f 1 and f 0 e characteristics the situation is quite different. the impact of the data from sunrise and sunset hours, particularly for f 0 f 1 data, and night hours for f 0 e, enlarge the errors, because the forecast is sometimes given for non-existing f1 layer, and vice versa. generally, the quiet situation, which is represented by much more smoothed data than for the disturbances, is predicted with higher accuracy. during the disturbance lasting several hours, the method might generate some rapid fluctuations. when the method gives too high, or too low values, the correction by gradients improves the forecast, but only for the second, and higher measured values hours. so the forecast curve for later hours is much smoother and closer to observations. however, for individual disturbances the errors might still be substantial. this effect can be avoided using data with higher resolution sampling, as 15, or 5 min. in such a case 1-h-ahead forecast will follow the observations with higher accuracy after half an hour, or 10 min, respectively. generally, the autocovariance method shows the correctness of this approach for any ionospheric characteristics. the autocovariance method of ionospheric characteristics forecasting provides an acceptable accuracy. this might also be the crucial point for predicting the electron concentration height profiles. its quite reliable results for quiet, as well as for disturbed conditions allow us to conclude that this method can be used in operational services of ionospheric situation, as used to update the limited-area ionospheric forecast at the regional warning centre warsaw of the international space environment service. acknowledgements this research was partly supported by polish committee of scientific research grant 2 p03c 006 17. references hanbaba, r. (1999): cost 251 final report, src print. off., warsaw. kosek, w. (1993): the autocovariance prediction of the earth rotation parameters, in 7th international symposium «geodesy and physics of the earth», iag symposium no. 112, potsdam, germany, october 5-10, 1992, edited by h. montag and ch. reigber (springer verlag), 443-446. kosek, w. (1997): autocovariance prediction method of short period earth rotation parameters, artif. earth satell., 32 (2), 75-85. kouris, s.s., d.n. fotiadis and t.d. xenos (1998): on the day-to-day variation on f 0 f 2 and m(3000)f 2 , adv. space res., 22 (6), 873-876. muhtarov, p. and i. kutiev (1999): autocorrelation method for temporal interpolation and short-term prediction of ionospheric data, radio sci., 34 (2), 459-464. stanisl/ awska, i. (1994): a single-station prediction model as a contribution to instantaneous mapping, ann. geofis., 37 (2), 153-157. stanisl/ awska, i. and z. zbyszyński (2001): forecasting of the ionospheric quiet and disturbed f 0 f 2 values at single location, radio sci., 36 (5), 1065-1071. stanisl/ awska, i., t.l. gulyaeva and r. hanbaba (1999): ionospheric despatch centre in europe, phys. chem. earth (c), 24 (4), 355-357. tulunay, e., c. ozkaptan and y. tulunay (2000): temporal and spatial forecasting of the f0 f2 values up to twenty four hours in advance, phys. chem. earth (c), 25 (4), 281-285. wintoft, p. and lj.r. cander (1999): short-term prediction of f0 f2 using time-delay neural network, phys. chem. earth (c), 24 (4), 342-347. wintoft, p. and lj.r. cander (2000): ionospheric f 0 f 2 storm forecasting using neural networks, phys. chem. earth (c), 25 (4), 267-273. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 comparison of the mipas products obtained by four different level 2 processors piera raspollini∗,a, enrico arnoneb, flavio barbaraa, bruno carlia, elisa castellib, simone ceccherinia, bianca maria dinellib, anu dudhiac, michael kieferd, enzo papandreae, and marco ridolfie aistituto di fisica applicata "n. carrara" (ifac) del consiglio nazionale delle ricerche (cnr), firenze, italy bistituto di scienza dell’atmosfera e del clima (isac) del consiglio nazionale delle ricerche (cnr), bologna, italy catmospheric, oceanic and planetary physics, clarendon laboratory, oxford university, uk dkarlsruhe institute of technology (kit), institute for meteorology and climate research (imk), karlsruhe, germany euniversità di bologna, bologna, italy i. introduction m ipas [fischer et al., 2008] is a midinfrared fourier transform spectrometer that sounded the atmospheric emission at limb onboard the envisat satellite. it was launched on 31 march 2002 in a sun-synchronous polar orbit of about 800 km altitude and operated until 8 april 2012. the limb scanning sequence observed by the instrument is made of spectra that sound the atmosphere at different tangent altitudes (the tangent altitude is the minimum altitude reached by the line of sight and is also the altitude from where most of the observed signal originates). the inversion of the measurements allows the determination of the vertical profiles of the atmospheric quantities of interest in the range of the tangent altitudes covered by the limb scanning sequence, which is 6-70 km for the nominal measurement mode. four different retrieval codes have been developed for the analysis of mipas measurements and have been used for the processing of the entire mipas mission. these are: • the esa processor [raspollini et al., 2013, raspollini et al., 2006, ridolfi et al., 2000]: ml2pp v6 of this processor and the corresponding dataset will henceforth be referred as ml2pp; • the gmtr, developed at bologna university [carlotti et al., 2006, dinelli et al., 2010]: this processor and the corresponding dataset v2.3 will henceforth be referred as bol; • the algorithm obtained by the joint effort of the institut für meteorologie und klimaforschung (imk) at karlsruhe institute of technology and the instituto de astrofísica de andalucía (iaa) [von clarmann et al., 2003a,von clarmann et al., 2009]: this processor and the corresponding dataset (v5r_222 for ch4 and n2o, v5r_220 for the other species) will henceforth be referred as imk; ∗piera raspollini, ifac-cnr, via madonna del piano 10, 50019 sesto fiorentino (fi), italy; p.raspollini@ifac.cnr.it, phone: +390555226307 1 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 • the algorithm morse developed at oxford university [dudhia, 2008]: this processor and the corresponding dataset v1.4 will henceforth be referred as oxf. the four retrieval algorithms use the same level 1b data (calibrated and geolocated spectra) provided by esa. an assessment of the internal consistency of their products, which so far has been proven only by means of a blind test retrieval experiment based on synthetic spectra [von clarmann et al., 2003b], while a comparison using real data has been performed only for ozone products [laeng et al., 2013], is the objective of this paper. a comprehensive data set is now available and it represents a great opportunity to make a statistically significant comparison aiming at the determination of the possible systematic errors and at the identification of possible improvements. ii. differences between the four algorithms all algorithms use the global fit approach, i.e. the spectra of each scan are fitted simultaneously (minimizing the quadratic norm of the noise-weighted residuals between measurements and forward model calculations, with a constraint), but while three of them perform a one-dimensional (1d) retrieval, i.e. each scan is fitted separately, the forth one (bol) performs the simultaneous retrieval of all scans of the orbit (two-dimensional (2d) retrieval). generally the different species are retrieved sequentially, with some exceptions for bol and imk, and retrieved profiles from previous retrievals are used as assumed profiles in subsequent retrievals. the forward models of the four algorithms compute the radiative transfer integral along the line of sight taking into account the atmospheric vertical inhomogeneities, but different assumptions are made by the four algorithms about the horizontal inhomogeneities. the atmosphere is assumed to be in local thermodinamic equilibrium (lte) by all algorithms except imk, which is able to properly handle deviations from lte (non-lte), but it routinely uses only for selected species. scattering is not included in the radiative transfer integral, and the spectra affected by thick clouds, identified by the cloud filtering algorithm [spang et al., 2002, spang et al., 2004], are not included in the analysis. retrievals are performed limiting the fit to selected spectral intervals (called microwindows, mws) containing most information on the target parameters and minimizing the systematic errors also introduced by the assumptions in the forward model. the main differences between the four algorithms (constraints, retrieval grid, microwindows, forward model, cloud filtering thresholds) are summarized in table 1. iii. procedure for the comparison the performances of the four algorithms are compared in terms of seasonal averages of single scan retrieval error and seasonal averages of single scan vertical resolution of their products. possible biases between the products of the four algorithms are searched by computing differences between seasonal averages of each of the three algorithms oxf, bol and imk with respect to ml2pp. seasonal averages, in a three month period, of temperature profiles and volume mixing ratio (vmr) profiles of water vapor, nitric acid, methane, nitrous oxide and nitrogen dioxide are compared for six latitude bands (90◦n – 65◦n, 65◦n – 20◦n, 20◦n – 0◦, 0◦– 20◦s, 20◦s – 65◦s, 65◦s – 90◦s) and for daytime and nighttime observations. daytime (nighttime) profiles are identified with the sun elevation angle at the geolocation of the tangent altitude of the middle sweep of the scan. the analysis is performed considering the nominal mode measurements made in two years (2008 and 2009) that correspond to the second phase of the mipas mission (years 2005-2012). this phase is characterized, with respect to the first phase (years 2002-2004), by a reduced spectral resolution but an improved spatial resolution (optimized resolution – or). independent analyses are performed for two years (2008 and 2009) to 2 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 table 1: main differences between the four algorithms retrieval approach assumptions in the forward model algorithm constraint retrieval grid multitarget 1d/2d retrieval spectral intervals (mws) horizontal homogeneity local thermodinamic equilibrium cloud filtering threshold* ml2pp levenberg-marquardt regularization approach and a posteriori self-adapting tikhonov regularization (for all species but h2 o retrieval). pressure grid corresponding to the 27 tangent altitudes of the nominal measurement mode. no 1d oxf mws yes yes 1.8 oxf optimal estimation. pressure grid corresponding to the 27 tangent altitudes. no 1d oxf mws yes yes 1.8 imk tikhonov regularization against ecmwf temperature for temperature retrieval, and with constant constraint for gas retrievals. altitude grid with spacing of 1 km up to 44 km and of 2 km between 44 and 70 km. ch4 , n2 o 1d imk mws assumed horizontal gradient of temperature along the line of sight. modelling of non-lte condition in no2 retrieval 4 bol optimal estimation and levenberg-marquardt regularization approach altitude grid, with 3 km vertical steps from 6 to 42 km and at 47, 52, 60 and 68 km. t, o3 , h2 o 2d dedicated mws for t, h2 o and o3 , oxf mws for the other species fitted horizontal gradients for all species yes 4 * ci=ratio of the integrated radiances in two mws, with the second mw mostly sensitive to aerosol and cloud emissions. spectra with ci smaller than ci threshold are filtered out. with a larger ci threshold more clouds are filtered out. evaluate if the results are consistent enough to consider the results of one year representative of or measurement performances. comparisons are made on a common pressure grid, corresponding to an altitude grid of 1 km below 56 km and 2 km above 56 km. this fine grid, chosen in order to reduce the resampling error in the comparison, approximately corresponds to the finest of the four retrieval grids. the averages are computed including all the retrieved profiles that passed the filtering procedure. for all datasets, only profile levels not filtered out for the clouds and profiles for which convergence has been reached are included in the average. furthermore, in bol database, all profiles with information gain less than 0.3 are filtered out, as well as all profile levels xi with absolute deviation from the median larger than 6 times the mad. mad is calculated by finding the median of the absolute deviation between observations xi and the median, m, of n data points: madn = median (|xi − mn|). in oxf database, any profile level whose retrieval random error is greater than 70% of the profile value is discarded. in the ml2pp dataset, profiles characterized by a chi-square larger than a species dependent threshold are discarded. in imk dataset, profile levels whose diagonal values of the averaging kernel matrix (akm) [ceccherini et al., 2010] are smaller than 0.03 are discarded. the consequence of the individual filtering procedure is that, for the different algorithms, not exactly the same measurements enter the averages. this has been verified not to introduce a significant difference in the seasonal averages, because the number of the averaged 3 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 profiles for each latitude band and each season is always much larger than the number of different measurements. an exception may be encountered for night polar summer and day polar winter where averages are performed on a statistically small number of samples. however, also these differences are interesting, because the main objective here is to compare seasonal averages obtained by the different mipas algorithms in order to see how all the different choices made in the four algorithms (cloud filtering, microwindows, regularization, filtering and so on) have an impact on them. iv. comparison of diagnostic parameters of the four algorithms the performances of each retrieval are characterized by the trade-off that the retrieval constraints (either regularization or a-priori) determine between the two diagnostic parameters: the retrieval error and the vertical resolution. the retrieval error is the propagation of the measurement noise through the retrieval and its covariance matrix (cm) [ceccherini et al., 2010] is computed by the four algorithms and provided for each scan (oxf and bol products just include the diagonal elements of cm). the retrieval error depends on the sensitivity of the measurements to the target parameters, which, in turn, is driven by the amplitude of the emitted radiance and by the temperature of the atmosphere. given the large seasonal and latitudinal variability of the temperature profile, the retrieval errors are characterized by a large variability. the vertical resolution is estimated using the akm calculated by the four algorithms. the vertical resolution at altitude zi is computed, for each profile, as ∆zi /akmii, where ∆zi is the retrieval step at altitude zi and akmii is the diagonal element of the akm at the altitude zi. ml2pp and imk provide for each scan the full akm and its diagonal element respectively, and for their products it is possible to compute the vertical resolution in a rigorous way. bol provides representative akms for selected season and latitude bands. oxf does not provide routinely the akm of each scan in its products, neither the complete cm, and hence the akmii has been estimated as equal to 1 − (sdx /sda)2, with sdx and sda respectively the retrieval and the a priori error, that is only considering the diagonal terms of the cm. this approach, being rigorous only if the off-diagonal terms of the cm are negligible, and hence if the retrieved points are uncorrelated, overestimates the diagonal terms of the akm when an a priori constraint is used and consequently underestimates the value of the vertical resolution. seasonal averages of the diagnostic parameters are computed after interpolating both retrieval error profile and vertical resolution profile of each scan with the same method used for the temperature and vmr profiles, because the average performances on the native retrieval grid rather than the performances of the average are the objective of our analysis. figure 1 shows, for ozone, the seasonal averages of single scan absolute retrieval error (left plot) and the seasonal averages of single scan vertical resolution profile (right plot) for the four processors, for equatorial and southern hemisphere polar winter conditions. bol has the worst vertical resolution, with a peak of 8 km at high altitudes. this is a consequence of the used retrieval grid, being significantly coarser than the retrieval grid of the other algorithms (see table 1). furthermore, for temperature, ozone and water vapor, that are obtained with a joint retrieval, the retrieval error takes into account also the propagation of the error of the profiles that are jointly retrieved and this leads to a more comprehensive, but larger retrieval error. among the other three algorithms, oxf has the largest retrieval error and the best vertical resolution. the use of a selfadapting regularization strength based on the retrieval error of each scan allows ml2pp to maintain a fairly constant vertical resolution for different atmospheric conditions, while the retrieval error changes significantly. contrarily, imk uses a regularization with a fixed strength, and hence its regularization is stronger (and hence vertical resolution is worst) when the in4 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 figure 1: ozone zonal means of single scan random error profile (left plot) and vertical resolution profile (right plot) of bol (red curves), oxf (green curves), imk (blue curves) and ml2pp (pink curves) for equatorial (curves with circles) and southern hemisphere polar winter (curves with triangles) conditions. formation content of the measurement is lower, while the retrieval error does not change significantly. similar considerations can be done for the other species. the plots of the seasonal averages of single scan absolute retrieval error and of single scan vertical resolution profile for the four processors for temperature, water vapour, nitric acid, methane, nitrous oxide and nitrogen dioxide are reported in the supplementary material (figs. s1-s6). v. differences in mean temperature and vmr profiles the differences between the zonal means of bol, oxf and imk with respect to ml2pp have been computed and reported in three pressure-latitude maps for each season and each year. absolute differences are reported for temperature and relative differences for the vmr of the various species. averages are performed also distinguishing between daytime and nighttime profiles to highlight possible problems due to diurnally varying systematic errors. figure 2 shows the maps for temperature relative to june-july-august 2009 period, for both daytime and nighttime conditions. the maps relative to other seasons and other species are reported in the supplementary material (figs. s7-s61) ), and here below the overall results are summarized. discontinuities in the maps are due to the coarse discretization of the latitude bands on which the comparison is performed. the mean profiles of the four algorithms, as well as the differences between oxf, imk and bol products with respect to ml2pp products, computed for five ’reference’ atmospheres (polar summer, polar winter, equatorial, midlatitude daytime and midlatitude nighttime) are collected in summary plots for each species. mean differences are compared with the systematic error profiles estimated for oxf microwindows for each reference atmosphere [dudhia, 2008], and hence representative of oxf, ml2pp, and, partly, bol systematic errors. imk systematic errors are generally comparable [von clarmann et al., 2009]. figure 3 reports the summary plots for ozone, the plots relative to the other species are contained in the supplementary material (figs. s62-s67). for all species, no significant difference was observed between the independent analyses performed for years 2008 and 2009. 5 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 figure 2: maps of the differences between the temperature zonal means of bol and ml2pp (left plots), oxf and ml2pp (center plots), imk and ml2pp (right plots) for june-july-august 2009 period; upper plot: daytime conditions, lower plot: nighttime conditions. v.1 temperature between 100 hpa and 1 hpa temperatures from the four algorithms are consistent within 1◦k, apart from a few exceptions (see fig.2 and figs. s7-s13 and s62 of the supplementary material): • in the polar winter, between 100 and 30 hpa, ml2pp and oxf are colder than bol up to 3◦k and than imk up to 6◦k. impact of unaccounted polar stratospheric clouds could have a role here. • in midlatitude winter and autumn, mainly bol, but also imk, differ from ml2pp and oxf by about 1-3◦k and this difference changes sign for daytime/nighttime conditions. the cause of this behavior can be ascribed to the geometry of observation of mipas measurements and to the assumption of horizontal homogeneity done in ml2pp and oxf forward models. indeed, mipas looks backwards with respect to the satellite’s flight direction in the nominal observation mode. this means that during the descending part of the orbit (when flying from north to south), which away from the poles corresponds to daytime observations, the instrument looks northward, while during the ascending part (when flying from south to north), which away from the poles corresponds to nighttime observations, it looks southward. as a consequence, in a region with temperature increasing northwards it sees a negative temperature gradient in nighttime observations and a positive temperature gradient in daytime observations [kiefer et al., 2010]. this asymmetry in the observations, in presence of non-linearity effects, leads to a bias in the retrieved temperature when horizontal gradient are not taken into account and this is seen as a bias between the algorithms that 6 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 figure 3: mean ozone profiles of the four algorithms and mean differences with respect to ml2pp, plotted versus ml2pp estimated systematic error profile, computed for five reference atmospheres (polar summer: seasonal mean of jja 2008 period, daytime profiles, latitude band: 90◦n – 65◦n; polar winter: seasonal mean of jja 2008 period, nighttime profiles, latitude band: 65◦s – 90◦s; midlatitudes day: annual mean of 2008, daytime profiles, latitude band: 20◦n – 65◦n; midlatitudes night: annual mean of 2008, nighttime profiles, latitude band: 20◦n – 65◦n; equatorial: annual mean of 2008, daytime profiles, latitude band: 0◦– 20◦s). take into account inhomogeneities and the others. the bias in the retrieved temperature of ml2pp and oxf is only visible when comparing the mean of ascending and descending profiles separately: when averages are performed including measurements from both daytime and nighttime observations, as well as from different seasons, differences compensate each other and the resulting bias is very small (see fig. s62 of the supplementary material). at high altitudes, in particular between 1 and 0.2 hpa, bol and oxf are 2-4◦k warmer than ml2pp in the tropics and in the summer and spring hemisphere for midlatitudes. between 0.2 and 0.08 hpa imk is up to 3◦k colder than bol, ml2pp and oxf almost at all latitudes. at low altitudes, in particular below 10 hpa, oxf and ml2pp are comparable within 1-1.5◦k with the exception of the tropical regions in some seasons, imk is more than 3◦k warmer than oxf and ml2pp at almost all latitudes. further investigations are needed to understand the causes of these differences, but these have to be searched in the use of different microwindows, different cloud filtering thresholds and different interferences due to sequential or multi-target retrievals. 7 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 v.2 water vapor (h2o) between 60 hpa and 0.2 hpa water vapor profiles from the four algorithms are consistent within 10% (figs. s14-s21 and s63 of the supplementary material). in the troposphere, in particular between 200 and 70 hpa, differences between the four algorithms are larger than 40%, with the imk mean profile being the highest one and oxf and bol profiles being the lowest ones. differences between the vertical resolution of the different algorithms and hence difficulties in matching the hygropause can be responsible of some of the observed differences. in the mesosphere, in particular between 0.9 and 0.2 hpa, the bol mean profile is the smallest, while for pressures lower than 0.2 hpa oxf and imk profiles are the largest. error due to non-lte may have a major role here. v.3 ozone (o3) between 40 and 0.5 hpa ozone profiles from the four algorithms are consistent within 5-10% (see fig. 3 and figs. s22-s29 of the supplementary material), with the largest differences around the ozone peak. in particular, in the southern hemisphere polar winter, imk is biased low with respect to the three algorithms by more than 10% at ~3 hpa (near the ozone peak) and is biased high by ~15% at ~20 hpa (see fig. 3 and figs. s26 and s27 of the supplementary material). this could be an effect of the different regularization approaches used by the four algorithms and of the choice of microwindows. in the tropics, between 60 and 40 hpa ml2pp is biased high by about 15% with respect to the other three algorithms, while between 200 and 80 hpa ml2pp is always biased high with respect to imk but it is biased low with respect to bol and oxf. outside the tropics, oxf and ml2pp are consistent within 10% between 500 and 0.1 hpa. ml2pp and oxf are biased high with respect to imk for pressures larger than 100 hpa at all latitudes and with respect to bol for pressures larger than 200 hpa. the positive bias of ml2pp, oxf and bol with respect to imk in the troposphere, also confirmed by validation of mipas products with ground based measurements [laeng et al., 2013], can be attributed to the different microwindows used by the different retrievals. for pressures smaller than 0.08 hpa ml2pp is biased low for more than 30% with respect to all others algorithms. this is also true between 0.2 and 0.08 hpa, but only with respect to imk and partially bol, while oxf is even smaller than ml2pp in this pressure range. from fig. 3 we can also see that the mean differences between the profiles of the four algorithms are generally smaller than the systematic errors of the individual retrievals and this is true also for the other species. indeed, the estimation of the systematic errors includes all errors that are not just propagation of the random measurement error through the retrieval. given the length/time scale of variation of each systematic error, when averaging on a long period and a latitude band, some of these errors may even change sign and compensate each other. the results found for ozone are consistent with those of the comparison of the ozone profile retrieved by the four algorithms made in the frame of ozone climate change initiative [laeng et al., 2013]. v.4 nitric acid (hno3) between 100 and 5 hpa mean differences between the nitric acid profiles from the four algorithms are generally within 5-10% (see figs. s30-s37 of the supplementary material for the different seasons and fig. s64 for the summary plots), apart from in the tropics and in the southern hemisphere winter and spring polar conditions. in the tropics bol is about 10% larger than the other 3 algorithms between 20 and 6 hpa and up to 20% smaller between 60 and 25 hpa. oxf nitric acid value around 100 hpa is significantly larger than the values retrieved by the other three algorithms, probably due to difficulties in resolving the knee of the profile. in the southern hemisphere polar winter, mostly 8 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 ml2pp, but also oxf, are biased low for more than 50% with respect to bol and imk between 80 and 30 hpa (see figs. s34-s35 and s64 of the supplementary material). in midlatitudes, especially in winter and summer (see figs. s30-s31 and s34-s35 of the supplementary material), (bol ml2pp) differences change sign for daytime and nighttime profiles. as in the case of temperature, this behavior can be attributed to the horizontal inhomogeneities that are only accurately handled in bol retrievals. the diurnal change of the differences is significantly less evident in the (imk ml2pp) differences, indicating that the approach used by imk for handling the horizontal inhomogeneities (i.e. modeling the gradient of temperature) only accounts for a part of the problem and only bol corrects for errors due to hno3 gradients. the fact that a similar behavior is not seen for the other species can be explained by the following two considerations. first, the temperature bias caused by horizontal gradients does not cause biases in the minor constituent retrievals because the non-linearities present in the radiative transfer of temperature and minor constituents are similar and the “effective” temperature, retrieved when neglecting the horizontal gradients, is the most suitable for the minor constituent retrieval. second, with the exception of nitric acid, either horizontal concentration gradients or species dependent non-linearities present in minor constituent retrievals are usually small [carlotti et al., 2013]. at high altitudes, i.e. for pressures smaller than 4 hpa, ml2pp is more than 40% smaller than the other three algorithms at all latitudes. at low altitudes, i.e. for pressures larger than 100 hpa, ml2pp is more than 20% larger than the other three algorithms. further investigations are needed to understand this behavior. v.5 methane (ch4) differences between the methane profiles from the four algorithms are within 10% between 50 hpa and 0.1 hpa (figs. s38-s45 and s65), with the exception of the southern hemisphere winter and spring polar conditions, where differences between oxf and imk with respect to ml2pp are larger. for pressures larger than 50 hpa, imk is 10-20% larger than ml2pp and oxf at all latitudes and seasons, even larger with respect to bol, that is about 10% smaller than ml2pp below 150 hpa. around 30 hpa, imk is smaller than the other three algorithms at almost all latitudes. the cause may be ascribed to the different microwindows used by imk. for pressures smaller than 0.2 hpa ml2pp is larger than imk and smaller than oxf. v.6 nitrous oxide (n2o) similar to methane, differences are within ~ 10% in the altitude range 100 hpa – 1 hpa (figs. s46-s53 and s66 of the supplementary material), with the exception of the southern hemisphere polar winter and spring, where oxf and imk nitrous oxide values are significantly larger than ml2pp and bol ones. for almost all latitudes and seasons, imk is about 10-15% larger than the other three algorithms for pressures smaller than 2 hpa, while between 1 hpa and 0.8 hpa ml2pp is more than 20% smaller than the others. v.7 nitrogen dioxide (no2) the nitrogen dioxide profiles have very different retrieval ranges in the four algorithms and also in the common retrieval range consistent results are obtained only in a limited interval: differences between the algorithms are within 10% in the altitude range 10 hpa – 0.3 hpa for nighttime measurements and in the altitude range 10-1 hpa for daytime measurements (figs. s54-s61 and s67 of the supplementary material). impact of non-lte, that is taken into account only in imk retrievals, can be responsible of some of the found differences. in the polar winter, oxf profiles are significantly different from the others. for pressures larger than 10 hpa ml2pp has a positive difference up to 30-40% with respect to the other three algorithms, especially for daytime measurements. this could be explained 9 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 by the error coming from a wrong assumption of the profile below the lowest retrieved altitude. vi. conclusions the consistency of the different retrieval procedures implemented in the four algorithms performing mipas processing was here evaluated with the objective of obtaining information for the refinements of the algorithms themselves and for a better assessment of the systematic errors that affect the mipas products. very similar results are obtained for years 2008 and 2009, and these conclusions can be considered to be representative of mipas optimized resolution measurements. the trade-off between retrieval error and vertical resolution of the mipas products varies for different atmospheric conditions and different algorithms, and these differences are explained by the different retrieval strategies and regularization approaches adopted by the four algorithms. despite the significant differences in the four algorithms, in general in the stratosphere the seasonal averages of their products are consistent within 1◦k for temperature and within 5-10% for the vmr of the analyzed species, with differences smaller than the estimated systematic error of individual retrievals. differences larger than 10% are generally found in the troposphere, where clouds may have a major role, in the mesosphere, where contribution of non-lte may have an impact, and in the southern hemisphere winter polar conditions, where the retrieval error is significantly larger than in other conditions and hence also the impact of the different retrieval constraints may be larger. evidence of the impact of unaccounted horizontal inhomogeneities in ml2pp and oxf is seen, for middle latitude bands, in the temperature (bol ml2pp) and (imk ml2pp) differences and in nitric acid (bol ml2pp) differences. for the other species no evidence of the impact of the horizontal inhomogeneities is found in (bol ml2pp) differences, indicating that the “effective” temperature that is retrieved by 1d retrievals (ml2pp and oxf) is sufficient to compensate for temperature horizontal inhomogeneities in the vmr retrievals. some of the observed differences can be explained by the known differences in the forward model or in the retrieval of the four algorithms (handling of horizontal inhomogeneities, selected microwindows, cloud filtering, regularization strategies), but some differences are still unclear and deserve a deeper analysis. these findings, that however contribute in providing a better assessment of the systematic errors of mipas products, will be used as guidance for further investigations and future improvements in the algorithms. the final assessment of the accuracy of mipas products requires also the comparison of mipas measurements with accurate correlative measurements. acknowledgments this work has been performed under the esa study “support to mipas level 2 product validation”, contract esa-esrin no. 21719/08/iol. references [carlotti et al., 2006] carlotti, m., brizzi, g., papandrea, e., prevedelli, m., ridolfi, m., dinelli, b. m. and magnani, l. (2006). gmtr: two-dimensional geofit multitarget retrieval model for michelson interferometer for passive atmospheric sounding/environmental satellite observations. appl. opt., 45, 716–727. [carlotti et al., 2013] carlotti, m., arnone, e., castelli, e., dinelli, b. m. and papandrea, e. (2013). position error in profiles retrieved from mipas observations with a 1-d algorithm. atmos. meas. tech., 6, 419– 429, doi:10.5194/amt-6-419-2013 [ceccherini et al., 2010] ceccherini, s. and ridolfi, m. (2010). technical note: variancecovariance matrix and averaging kernels 10 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 for the levenberg-marquardt solution of the retrieval of atmospheric vertical profiles. atmospheric chemistry and physics, 10, 3131–3139. [dinelli et al., 2010] dinelli, b. m., arnone, e., brizzi, g., carlotti, m., castelli, e., magnani, l., papandrea, e., prevedelli, m. and ridolfi, m. (2010). the mipas 2d database of mipas/envisat measurements retrieved with a multitarget 2-dimensional tomographic approach. atmos. meas. tech., 3, 355–374, doi:10.5194/amt-3-355-2010. [dudhia, 2008] dudhia, a. (2008). mipasrelated section of the web-site of the oxford university: http://www.atm.ox.ac. uk/group/mipas [kiefer et al., 2010] kiefer, m., arnone, e., dudhia, a., carlotti, m., castelli, e., von clarmann, t., dinelli, b. m., kleinert, a., linden, a., milz, m., papandrea, e. and stiller, g. (2010). impact of temperature field inhomogeneities on the retrieval of atmospheric species from mipas ir limb emission spectra. atmos. meas. tech., 3, 1487–1507. [fischer et al., 2008] fischer, h., birk, m., blom, c., carli, b., carlotti, m., von clarmann, t., delbouille, l., dudhia, a., ehhalt, d., endemann, m., flaud, j. m., gessner, r., kleinert, a., koopman, r., langen, j., lópez-puertas, m., mosner, p., nett, h., oelhaf, h., perron, g., remedios, j., ridolfi, m., stiller, g. and zander, r. (2008). mipas: an instrument for atmospheric and climate research atmos. chem. phys., 8, 2151–2188, doi:10.5194/acp-82151-2008. [laeng et al., 2013] laeng, a., hubert, d., verhoelst, t., von clarmann, t., dinelli, b. m., dudhia, a., raspollini, p., stiller, g., grabowski, u., keppens, a., kiefer, m., sofieva, v., froideveaux, l., walker, k. a., lambert, j.-c. and zehner c. (2013). the ozone climate change initiative: comparison of four level 2 processors for the michelson interferometer for passive atmospheric sounding (mipas), submitted to remote sensing of the enviroment for ozone cci special issue. 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(2000). optimized forward model and retrieval scheme for mipas near-real-time data processing. appl. optics, 39, 1323–1340. [spang et al., 2002] spang, r., eidmann, g., riese, m., preusse, p., offermann, d., pfister, l. and wang, p. h. (2002). crista observations of cirrus clouds around the tropopause. j. geophys. res., 107, 8174, doi:10.1029/2001jd000698. [spang et al., 2004] spang, r., remedios, j. j., and barkley, m. (2004). colour indices for the detection and differentiation of cloud types in infra-red limb emission spectra. adv. space res., 33, 1041–1047. 11 http://www.atm.ox.ac.uk/group/mipas http://www.atm.ox.ac.uk/group/mipas annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6338 [von clarmann et al., 2003a] von clarmann, t., glatthor, n., grabowski, u., höpfner, m., kellmann, s., kiefer, m., linden, a., mengistu tsidu, g., milz, m., steck, t., stiller, g. p., wang, d. y., fischer, h., funke, b., gil-lópez, s. and lópez-puertas, m. (2003). retrieval of temperature and tangent altitude pointing from limb emission spectra recorded from space by the michelson interferometer for passive atmospheric sounding (mipas). j. geophys. res., 108, 4736, doi:10.1029/2003jd003602. [von clarmann et al., 2003b] von clarmann, t., ceccherini, s., doicu, a., dudhia, a.,funke, b., grabowski, u., hilgers, s., jay, v., linden, a., lópez-puertas, m., martin-torres, f.-j., payne, v., reburn, j.,ridolfi, m., schreier, f., schwarz, g., siddans, t. and steck, t. (2003). a blind test retrieval experiment for infrared limb emission spectrometry. j. geophys. res., 108, 4746, doi:10.1029/2003jd003835, 2003 [von clarmann et al., 2009] von clarmann, t., höpfner, m., kellmann, s., linden, a., chauhan, s., funke,b., grabowski, u., glatthor, n., kiefer, m., schieferdecker, t., stiller, g. p. and versick (2009). retrieval of temperature, h2o, o3, hno3, ch4, n2o, clono2 and clo from mipas reduced resolution nominal mode limb emission measurements. atmos. meas. tech., 2, 159-175, doi:10.5194/amt-2-1592009. 12 introduction differences between the four algorithms procedure for the comparison comparison of diagnostic parameters of the four algorithms differences in mean temperature and vmr profiles temperature water vapor (h2o) ozone (o3) nitric acid (hno3) methane (ch4) nitrous oxide (n2o) nitrogen dioxide (no2) conclusions 699_701 adam.pdf annals of geophysics, vol. 45, n. 5, october 2002 699 letters to the editor remote effects in real induction arrows antal ádám geodetic and geophysical research institute (ggri), sopron, hungary in august 2001, annali di geofisica, 44 (4), pp. 659-669, published the paper: «anomalous directional behaviour of the real parts of the induction arrows in the eastern alps: tectonic and palaeogeographic implications» written by marcus gurk and pierre-andré schnegg. concerning this paper we have two brief comments. 1) the authors refer to the induction arrows (wiese vectors) measured in the western part of hungary as an indication of the eastward continuation of the directional behaviour of the real induction arrows hypothetically connected to the areal distribution of the mesozoic sediments of the north penninic bündnerschiefer-facies. as the hungarian results were not shown in their paper, for confirmation of the statement of gurk and schnegg, a map is given here in fig. 1 on all hungarian real induction arrows, emphasizing the difference between the generally southward directed induction arrows measured inside the country and those obtained in its western border where the arrows rotate strongly westward according to the anomaly described in the referred paper. the observed tendency is geologically proved by the outcrops of the penninic rocks in the köszeg-rechnitz mountains. the general southward direction of the induction arrows in hungary, i.e. in the pannonian basin, is best demonstrated by the arrows measured in transdanubia (nw hungary) south of the lake balaton where the supposed weak effect of the carpathian anomaly inside the basin (ádám et al.,1972) is strongly increased by the pinching out of the transdanubian conductivity anomaly (tca) (ádám, 2001) detected between the two great tectonic lines, i.e. the balaton and rába line in the area of the transdanubian central range and partly north of it in the little hungarian plain. 2) arora and ádám (1992) wrote about an other more general directional characteristics of the real induction arrows experienced in many areas in the world at long periods. we cite from this paper: «…it was found, that straddled between oppositedly directed induction arrows which signify the existence of a conductive structure, a narrow belt exists over which real induction arrows align with the strike of the conductor». a typical example of this phenomenon mentioned by arora and ádám is just from the transdanubian conductivity anomaly (tca) shown in fig. 2 where the direction of the arrows changes from north and south to southwest in two conductive stripes (wallner, 1977). in transdanubia, the regional strike is really nesw in the conductor as it was determined by different methods including the groom-bailey decomposition, the phase maps, the em distortions, etc. (ádám, 2001). what is very curious is that inside the conductive stripes, especially mailing address: dr. antal ádám, geodetic and geophysical research institute of the hungarian academy of sciences, csatkai e. u. 6-8, h-9401 pobox 5, sopron, hungary, e-mail: adam@ggki.hu 700 antal ádám in the southern one, the sw direction of the very small induction arrows is very stable, i.e. it does not change, indicating a remote effect, too. if we look at the block diagram of the geological structures of western transdanubia (in fig. 3, horváth, 1993), we can see that the with p marked penninic layer underplates, according to the idea of horváth, not only the austroalpine nappes (la,ua) but partly the transdanubian central range where the conductors are certainly embedded in the paleozoic layers as concluded from the layer sequences and the depth of the conductors (4-7 km). fig. 1. real wiese induction arrows (vectors) measured at a given period in hungary. fig. 2. long period (t > 20 min) wiese induction arrows in the area of the transdanubian conductivity anomaly measured by ggri (wallner, 1977). 701 remote effects in real induction arrows fig. 3. block diagram to illustrate the mechanism of lithospheric extension in the little hungarian plain, western part of the pannonian basin. note that there is no vertical exaggeration and block edges are about 100 km (front) and 80 km (side) long. legend: 1 = hp/lt metamorphic rocks of the penninic (p) nappes; 2 = mediumto highgrade metamorphic rocks of the lower austroalpine nappes represented by the wechsel series in this area; 3 = lowto medium-grade metamorphic rocks of the upper austroalpine nappes represented by the graz paleozoic in this area; 4 = late paleozoic low-grade metamorphic rocks of the tcr; 5 = jurassic and triassic of the transdanubian central range (tcr); 6 = ductile lower crust and the moho discontinuity; 7 = lower lithosphere and asthenosphere; 8 = thrust fault and its reactivation as normal fault and strike slip fault, respectively (horváth, 1993). of course, further geological and geophysical data will help to confirm this supposition. references ádám, a. (2001): relation of the graphite and fluid bearing conducting dikes to the tectonics and seismicity (review on the transdanubian crustal conductivity anomaly), earth planets space, 53, 903-918. ádám, a., j. vero'' and á. wallner (1972): regional properties of geomagnetic induction arrows in europe, acta geod. geophys. mont. hung., 7, 25l-287. arora, b.r. and a. ádám (1992): anomalous directional behaviour of induction arrows above elongated conductive structures and its possible causes, phys. earth planet. inter., 74, 183-190. gurk, m. and p.a. schnegg (2001): anomalous directional behaviour of the real parts of the induction arrows in the eastern alps: tectonic and palaeographic implications, ann. geofis. 44 (4), 659-669. hovárth, f. (1993): towards a mechanical model for the formation of the pannonian basin, tectonophysics, 6, 333-357. wallner, á. (1977): the main features of the induction arrows on the area of the transdanubian conductivity anomaly, acta geod. geophys. mont. hung., 12, 145-150. 702 antal ádám c o n t r i b u t o a l l o s t u d i o d e l l e c a r a t t e r i s t i c h e s i s m i c h e d e l m e d i t e r r a n e o c e n t r o o r i e n t a l e paolo emilio v a l l e premessa. e ' n o t o clic si i n u m o . i n c o r a p o c h e c o n o s c e n z e s u l l e c a r a t t e r i s t i c h e s i s m i c h e del m e d i t e r r a n e o c e n t r o o r i e n t a l e . h o q u i n d i r i t e n u t o o p p o r t u n o e s e g u i r e u n a r i c e r c a s u l l a p r o p a g a z i o n e d e l l e o n d e s p a z i a l i p r o d o t t e d a l l a scossa p r i n c i p a l e del t e r r e m o t o a v v e n u t o in c a l a b r i a i i i m a g g i o 1 9 4 7 . i r i s u l t a l i , s e b b e n e m o d e s t i a c a u s a d e l l a s c a r s e z z a di s t a z i o n i s i s m i c h e e f f i c i e n t i n e l l a s u d d e t t a r e g i o n e , m i s e m b r a p o s s a n o c o s t i t u i r e un c o n t r i b u t o di q u a l c h e i n t e r e s s e . l i n t e n s i t à d e l l a scossa p r i n c i p a l e , l a q u a l e t u p r e c e d u t a e s e g u i t a d a scosse m i n o r i , p u ò v a l u t a r s i d i g r a d o v i i ] n e l l a z o n a p i ù c o l p i t a , c o m e m o s t r a l a c a r t i na d e l l a fig. 1. elici h o c o m p i l a t o in b a s e a l l e n o t i z i e m a c r o s i s m i c h e p e r v e n u t e m i . e ' n o t e v o l e l a i r r e g o l a r i t à d e l l a d i s t r i b u z i o n e d e l l ' i n t e n s i t à d e l l a scossa n e i r i g u a r d i d e g l i effetti m a c r o s i s m i c i . coordinate epieentrali e tempo origine. p e r il c a l c o l o d e l l e c o o r d i n a t e e p i e e n t r a l i e del t e m p o o r i g i n e . h o u s a t o , i n m a n c a n z a di un n u m e ro s u f f i c i e n t e d i osserv a z i o n i s u l l e o n d e d i r e t t e , i t e m p i di a r r i v o d e l l e o n d e di d i l a t a z i o n e r i t r a i l e ( p n ) , r e g i s t r a t i in 9 s t a z i o n i s i l u a t e in un i n t e r v a l l o d e l l a contililllto 11.1.0 sti 1)10 i1ixi.i. c.aiia tti-ll i mici! k sismiche 2(>, d i s i a n z a e p i c e n t r a l e a c o m p r e s o ira ,")00 e 1100 k m e a . in quemo i n t e r v a l l o hi v e l o c i t à a p p a r e n t e d e l l e o n d e l'ii p u ò a n c o r a r i t e n e r i c o s t a n t e con b u o n a a p p r o s s i m a z i o n e . il m e t o d o di c a l c o l o è il s o l i t o . s u p p o s t a n o t a la d r o m o c i o n a , d e l t i rp', a, c // i v a l o r i a p p r o s s i m a t i d e l l e e o o r d i n a t e i ( g e o c e n t r i c h e ) d e l l ' e p i c e n t r o e del t e m p o o r i g i n e , -i c a l c o l a i l t e m p o di a r r i v o in ogni s t a z i o n e : a(cpì',xì; cp',à) [ 1 ] ' i r = r " + 1 1 i (i = 1 . 2 n ) d o v e (( i , /.; sono le c o o r d i n a t e ( g e o c e n t r i c h e ) d e l l a i"'a s t a z i o n e . il t e m p o di a r r i v o o s s e r v a t o è i n v e c e r e l a t i v o al v a l o r e v e r o <|,., h.i d e l l e c o o r d i n a t e e p i e e n t r a i i e del t e m p o o r i g i n e e q u i n d i : a(<| ì'./.ì ; ((•„',>.„) a + 11,. [ 2 ] "io i '« i i s v i l u p p a n d o in e i i e la d i f f e r e n z a (/•„, — /i(.) e t r a s c u r a n d o i le m i n i di o r d i n e u p e r i o r e al p r i m o , -i h a : [ 3 ] p o s t o : li,. 'ir — / i , «to' < ( ' = f. , '>•., à = il //,. // r , 1 i ja i b, , 1 r a r \ s r p ' i i v \ d i [ 4 ] -i o t t i e n e se n > 3 ii s i t e m a li e q u a z i o n i l i n e a l i i n c o m p a t i b i l i : t f b; f + ci i i + l , = l'i [ 5 ] dal q u a l e si r i c a v a n o i v a l o r i p i ù p r o b a b i l i d e l l e i n c o g n i t e r, e, il, m e d i a n t e il m e t o d o d e i m i n i m i q u a d r a t i . p e r s t a b i l i r e un v a l o r e a p p r o s s i m a t o del t e m p o o r i g i n e ho u a l o il m e t o d o di w a d a t i e i t e m p i ili a r r i v o d e l l e o n d e fri ed sri n e l l e stazioni di r o m a . s o f i a , i ' i r e n z e . b e l g r a d o , p r a t o . z a g a b r i a e i r i c s t e . l e ( f i l a l i , t r o v a n d o s i ai m a r g i n i del c o n t i n e n t e , è p r e s u m i b i l e s i a n o s i t u a t e in u n a zona di u n a c e r t a u n i f o r m i t à s i s m i c a . p a o l o k m 1 1 . 1 0 v a l l e k" r i s u l t a t o : ( 1 , 3 3 6 4 4 0 , 0 8 7 0 1 ) ( s „ — p n ) — p „ —07h 32 , n 14,"83 6 , « 8 4 [6] (/li s c a r t i sono c o n t e n u t i n e l l a t a b e l l a v 1. l ' e r r o r e m e d i o d i ii è m o l t o e l e v a t o , -i d e v e p e r ò o s s e r v a r e c l i c il m e t o d o u s a l o n o n c o n s e n te g e n e r a l m e n t e u n a g r a n d e p r e c i s i o n e . ho a s s u n t o c o m e v a l o r i a p p r o s s i m a t i d i cp', x e ii i s e g u e n t i : cp' = + 3 8 ' 2 8 ' , 7 à= + j6" 5 0 ' ii = 07" 32"1 i4 s ,9 a b e l l a n . 1 stazioni: vi s e c . r o m a — 0 . 3 8 s o l i a + 1 , 1 0 f i r e n z e x . — 0 , 9 3 b e l g r a d o + 3 , 4 8 p r a t o — 0 , 6 3 z a g a b r i a 1 . 5 2 t r i e s t e — 1 , 4 4 [v] p e r il t e m p o d i t r a g i t t o d e l l e o n d e l ' n b o r i t e n u t o v a l i d a , p e r p r o f o n d i t à n o r m a l i del e e n t r o s i s m i c o , la r e l a z i o n e ( j ) : /,.„ in sec. a i n k m a 8 + 11 [8] l e d i s t a n z e e p i e e n t r a l i e l e l ' e l a t i v e d e r i v a t e r i p e t t o a rp' e x n e l p u n t o d i coord i n a l e [ 7 ] sono s t a t e c a l c o l a t e con le noie f o r m u l e ("). l a t a b e l l a n . 2 c o n t i e n e i v a l o r i di b\, cj, i,; q u e s t i u l t i m i sono d a t i con t r e dei • i in a 1 i a p u r o scopo di c a l c o l o . t a b e l l a n . 2 stazione /i s e c . l), s e c . / p r i m i e-, s e c . / p r i m i s e c . r o m a + 0 , 5 7 5 — 0 , 1 6 5 + 0 , 1 2 6 + 0 5 1 5 s o f ì a — 0 . 6 0 0 — 0 , 1 5 2 — 0 . 1 3 6 — 0 , 0 3 4 f i r e n z e x . — 0 . 2 2 5 — 0 , 1 8 3 + 0 , 1 1 1 0 2 7 5 p r a t o + 0 , 6 0 0 — 0 , 1 8 3 + 0 . 1 1 1 + 0 . 5 5 0 z a g a b r i a + 0 . 1 2 5 — 0 , 2 3 0 + 0 , 0 1 5 + 0 . 2 3 2 t r i e s t e — 0 . 2 2 5 — 0 . 2 2 1 + 0 0 5 3 — 0 . 1 9 3 p a v i a — 0 , 8 5 0 — 0 , 1 8 0 + 0 , 1 1 4 — 0 9 0 3 c o i r à + 0 , 2 7 5 — 0 . 1 9 8 + 0 . 0 9 3 + 0 , 2 4 6 z u r i g o — 0 , 1 0 0 — 0 , 1 9 6 + 0 , 0 9 6 — 0 1 3 4 c o n t r i b u t o ai.i.o s t i d i o d e l l e c a r a t t e r i s t i c h e s i s m i c h e . ecc il v a l o r e p i ù p r o b a b i l e d e l l e i n c o g n i t e è r i s u l t a t o t = + 0 , s 4 6 1 1 / 4 2 4 e = 4f ' , 3 8 6 7 ' , 5 5 1 [ 9 ] u = 2 ' , 3 2 2 2 ' , 2 5 7 gli e r r o r i m e d i -ono r e l a t i v a m e n t e g r a n d i : c i ò è da a t t r i b u i r s i al1 i n i z i o d e l l a f a s e , c h e è (piasi s e m p r e consistito in u n ' e m e r s i o n e , e a l l a p o c o f a v o r e v o l e d i s p o s i z i o n e d e l l e stazioni r i p e t t o a l l ' e p i c e n t r o . d a l l e p o s i z i o n i [ 4 ] e d a i v a l o r i [ 9 ] , -i o t t i e n e in d e f i n i t i v a : <p'„=-|-38" 3 0 ' , 1 7 ' , 6 ( g r o c c n t r i i u ) cp„ = + 38" 4 1 ' , 4 7 ' , 6 ( g e o g r a f i e a ) noi ),„ = + 16" 4 7 ' , 7 ; 2 ' , 2 l j jefo = 0 7 ' , 3 2 m 1 5 " , 4 : l s , 4 f t m . e . c . ) . non è stato p o s s i b i l e c a l c o l a r e la p r o f o n d i t à i p o c e n t r a l c p e r la m a n c a n z a , c o m e si è g i à d e t t o , di o s s e r v a z i o n i r e l a t i v e a s t a z i o n i p r o s s i m e a l l ' e p i c e n t r o . si p u ò p e r a l t r o r i t e n e r e , a n c h e i n r e l a z i o n e a l l o s v i l u p p o d e l l a fase m a s s i m a , c h e t a l e p r o f o n d i t à non i a s u p e r i o r e a l l a n o r m a l e . onde pn, p*. su. s*. f i c a l c o l o d e l l e d r o m o e r o n e d e l l e o n d e pn ed -s'n è t a t o e s e g u i t o in b a s e ai d a t i d e l l e 7 s t a z i o n i , c l i c si t r o v a n o ai m a r g i n i del c.on1 r i u l l a t i a cui sono p e r v e n u t o o n o : t,.„ = h 9 , 9 2 /,.,„ /.„ in sec. 7 , 9 3 8 0 , 1 7 7 a in km a , „ „ „ /„„ = ^ 1-18.83 4 , 5 7 1 0 , 0 8 5 h o p o t u t o i n d i v i d u a r e l e o n d e /'* s o l t a n t o n e l l e r e g i s t r a z i o n i d i r o m a . s o f i a , f i r e n z e z a g a b r i a e p r a t o . 1/ r i u l t a l o : s c c /,.,. = + 8 , 7 8 5 , 1 3 . [ 1 2 1 \ in km 6 , 9 5 7 0 , 4 0 6 2 tu l ' a d i . < ) m a i i l . i o y a i . i . i ; l e o n d e .s:-: sono s i a t e r e g i s t r a t e con ini/i a b b a s t a n z a n e t t i n e l l e e l l e stazioni c o n s i d e r a t e . i 11 n o t e v o l e e s e m p i o m o s t r a n o i s i s m o g r a m m i di kimia ( f i g . 1). la d r o m o c r o n a è : /,. = a + 1 0 , 2 5 4 , 2 8 . [ 1 3 1 \ in fi in 4 , 0 0 0 0 , 0 9 4 ( i l i scarti t'j c o i i s i s l e n l i n e l l a d i f f e r e n z a tra i t e m p i di t r a g i t t o oss e r v a t i e q u e l l i c a l c o l a t i m e d i a n t e l e [ 1 1 1 . l12|, [ 1 3 ] sono c o n t e n u t i n e l l a t a b e l l a v 3. l e s t a z i o n i c l i c inumo u n a d i s t a n z a c p i e e n t r a l e s u p c r i o r e a t r i e s t e , si t r o v a n o l u l t c sul c o n t i n e n t e e u r o p e o e i t e m p i di t r a g i t t o d e l l e l ' i g . 2 . o n d e l o n g i t u d i n a l i . • m u l e di d i l a t a z i o n e e di d i s t o r s i o n e r i f r a t t e sono in buon a c c o r d o con le d r o m o e r o n e o s s e r v a t e p e r i kuropa c e n t r a l e d a d i v e r s i r i c e r c a t o r i . la sola s t a z i o n e a f r i c a n a d i cui ilo p o t u t o a v e r e i s i s m o g r a m m i è h e l avan. c h e d i s i a d a l l ' e p i c e n t r o p r e s s ' a poco cjuanlo u c c i e . e ' n o t e v o l e il f a l l o e l l e le o n d e -s s e m b r a n o a r r i v a r e ad i f e l w a n con un a n t i c i p o di c i r c a 25 sec. r i s p e t t o ad u c c i e . conthibuto al.1.0 si i dio dia.i.e callvtteitlslk.lll s i s m i c h e . lc( 271 f i g . 3 . o n d e t r a s v e r s a l i . p e r q u a n t o r i g u a r d a le s t a z i o n i di c a t a n i a e t a r a n t o , c h e d o v r e i ) itero trovarsi nel n o r m a l e r a g g i o di r i c e z i o n e d e l l e o n d e d i r e t t e . nota u n s e n s i b i l e a n t i c i p o nei t e m p i di a r r i v o d e l l e o n d e l o n g i t u d i n a l e t r a s v e r s a l i . u n c o n f r o n t o d e i t e m p i di t r a g i t t o o s s e r v a t i e i t e m p c a l c o l a t i m e d i a n t e l e d r o m o e r o n e d e l l e o n d e pg ed sg r e l a t i v e a l i i l a l i a c e n t r a l e ('"') e s e t t e n t r i o n a l e ( ! ) f o r n i s c e i s e g u e n t i r i s u l t a t i . p a o l o e m [ [ . i o v a l l e t a b e l l a x . 3 \ p„ 5 n p* s * stazione k m i i s e e . l'i s e e . s e e . l ' i s e c . r o m a 5 0 9 + 0 . 0 3 — 0 . 5 5 + 0 . 6 6 + 0 . 0 8 s o f i a 7 0 9 + 0 . 3 9 + 1 . 6 9 — 1 . 0 9 + 0 . 0 7 f i r e n z e ( x . i 7 3 1 + 11.62 + 0 . 3 8 — 0 : 2 5 1 . 1 4 b e l g r a d o 7 4 6 1 . 2 7 + 0 . 4 0 — + i i . 3 1 p r a t o 7 4 8 — 0 , 2 3 — 0 . 8 4 — 1.711 + 0 . 3 1 z a g a b r i a 7 9 1 + 0 . 0 8 — 0 , 9 0 + 2 . 3 9 + 2 . 3 1 t r i e s t e 8 1 2 + 0 4 1 — 0 . 2 1 1 . 6 9 c a t a n i a (a = 197,8 k m ) t a r a n t o (a = 202 k m ) i t a l i a m e r i d i o n a l i ( t e m p i o s s e r v a l i i s e c . i t a l i a l ' e n t r a l e ( t e m p i c a l c o l a t i ) onde longitudinali 32.5 37.3 3 3 . 6 38.1 malia s e l l e n l r i o n a l t ( t e m p i c a l c o l a l i ) s e c . 36.1 37.1 c a t a n i a t a r a n t o onde trasversali 5 4 . 5 6 5 , 3 5 5 . 6 6 6 . 7 60.9 62.1 (/li i n i z i , s p e c i e d e l l e o n d e di d i l a t a z i o n e , si p r e s e n t a n o c o s i n e t t i e l l e è p r e s u m i b i l e al m a s s i m o u n e r r o r e d i q u a l c h e d e c i m o d i s e c o n d o n e l l a m i s u r a d e i t e m p i d i a r r i v o . w • r o m a at=+lm39?e sabraamffiisejwsneb 5 1 ™ „ r . p ì j . 4 galitzim . i.m.g s n i a i l tmumba&mnent p n ' il * ln e mhmjpmm^jkbvm&m fin. 4 c o n t k i h u t o a i x o s t i 111» i i e i . i . e c a i i a t t e k i s t l c l i e s i s m i c h e . e c ( t e n e n d o conto clic l ' e r r o r e m e d i o del t e m p o o r i g i n e è 1\4 1 ant i c i p o d e i t e m p i d e l l e o n d e /' ed s è e v i d e n t e . l e v e l o c i t à a p p a r e n t i r e l a l i v e a c a t a n i a c t a r a n t o u n o : vp k m s e c . j „ k m s e c . c a t a n i a 6 , 0 9 : 1 0 . 2 6 3,63 0 , 0 9 t a r a n t o 6,01 0 , 2 5 3,63 0 . 0 9 [ 1 4 ] m e n t r e |)er i i t a l i a c e n t r a l e e s e t t e n t r i o n a l e r i u l t a , con e r r o r e t r a c u . l a b i l e , in m e d i a : v,.„ k m s e c . 1 ( k m s e c . 1 1 e s i t a l i a c e n t r a l e 5,3 3 , 0 3 i t a l i a s e t t e n t r i o n a l e 5 . 7 3 , 3 6 al l i n e di t r a r r e q u a l c h e c o n c i l i i o n e a l m e n o q u a l i t a t i v a , è u t i l e c o n f r o n t a r e la v e l o c i t à d e l l e o n d e pn. p*. sri. s * d e s u n t e d a l l a p r e sente r i c e r c a , con le a n a l o g h e t r o v a l e in a l t r i s t u d i . d a l l ' e s a m e dei d a t i d e l l a t a b e l l a v i e m i n a c h e la v e l o c i t à d e l l e o n d e pn t e n d a a d i m i n u i r e d a l l ' e u r o p a c e n t r a l e al m e d i t e r r a n e o cent r o o r i e n t a l e m e n t r e l e v e l o c i t à d e l l e o n d e s * ed sii crescono s e n s i b i l m e n t e . c a t a n i a v at=+-18,4 3 2 m + vcrficate wlechert80 kg « 2 7 1 i ' m l l . 0 i v m i i . i o \ . \ i . u ; sci r i g u a r d i d e l l e o n d e -i n o i a un a l i m e n t o d e l l a v e l o c i t à d a l i l l a l i a s e t t e n t r i o n a l e e e e n t r a l e a l l ' i t a l i a m e r i d i o n a l e ; ciò p e r a l t r o non è c e l l o a i a i i a del f o r t e e r r o r e m e d i o da c u i è a f f o l l o il v a l o r e i ro\ a l o . t a b e l l a n . 4 itiaaom: p r o f o n d i t à k m vi-11 k m s e e . ' v s n k m s e e . —1 k m s e c . —1 v 8 * k m s e e . — 1 l l a l i a s e t t e n t r i o n a l e c a n s i i i l i o l'i 17 8,1)0 0 . 0 1 1.11 1.02 6 , 6 1 0,0-1 3 , 1 6 i t a l i a c e n t r a l e a p p . i i i l i 7 . 8 6 11.01 1.31 0 . 0 2 ( ì a r l a g n a n a ( ' ' ) 2 6 8 . 1 6 0 . 0 6 1.11 0 . 0 2 6 . 6 3 0 , 0 8 3 , 5 7 : >-04 a l t i » a d r i a t i c o l7i u p . a l l a n o r m a l e 7.77 0 , 0 8 1,80 u-01 — ( ' a l a l i r i a n o r m a l e 7,9-1 11.18 1 -57_ 0 , 0 8 6 , 9 6 l l . l l 1.00 •0.091 1 o l e i o n i e <si — 7 . 6 8 — — | t i r o l o i!l| 36 8 , 3 0 . 2 1.1 6 . 7 11-2 3 . 6 g e r m a n i a m e r i t i . 1 ' " ) normale 8 2 1 7.1 3 , ti) sdnvadorl' ( " l 90 8 . 1 2 1 1.321 6,-17 3 , 5 7 ! l ila r i c e r c a sul t e r r e m o t o d e l basso tirreno ( ' " ) a v v e n u t o il l(> m a r z o 1941 ini messo in e v i d e n z a un n o t e v o l e a n t i c i p o dei t e m p i di a r r i v o d i l l e o n d e di d i l a t a z i o n e e di d i s t o r s i o n e n e l l e s t a z i o n i p i ù v i c i n e a l i e p i c e n t r o . \ a p e r ò t e n u t o c o n i o d e l l a p r o f o n d i t à i p o c e n t r a l e , v a l u t a l a 8.~> k m . a n a l o g h e o s s e r v a z i o n i sono s t a t e f a t t e n e l l o s t u d i o del t e r r e m o t o d e l l e isole l i p a r i del ih a p r i l e 1938. la cui p r o f o n d i t à i p o e e n t r a l e e l a t a c a l c o l a t a 312 k m (™). osservazioni. i. a u m e n t o d e l l a v e l o c i t à d e l l e o n d e /'* ed e l ' a n t i c i p o dei tempi di a r r i v o d e l l e o n d e di d i l a t a z i o n e e di d i s t o r s i o n e i n d i e s t a z i o n i c h e d o v r e b b e r o e s s e r e c o n t e n u t e n e l n o r m a l e r a g g i o di r i c e z i o n e d e l l e o n d e d i r e t t e , mi s e m b r a c h e po—a t r o v a r e s p i e g a z i o n e nel v u l c a n i smo d e ! m e d i t e r r a n e o c e n t r o o r i e n t a l e ( " ) . lo s t r a t o i a l i c o s u p e r i o r e con r i u n i r ò u . i . o s n u r o m a . i r. i u m i l i u m h . h i s i s a i k i i i : . i:< < . d o v r e b b e m a n c a r e o, q u a n t o m e n o , e s e r e e l r e m a m e n l c o t l i l e in q u e s t a z o n a . d i r e t t a m e n t e al d i o t t o d e l l o i r a t o del b a s a l t o d o v r e b b e t r o v a r s i il m a g m a , nei q u a l e s e m b r a elle la v e l o c i t à d e l l e o n d e di d i l a t a z i o n e d i m i n u i s c a l e g g e r m e n t e . s e si s u p p o n e i c s i s t e n z a di un g r a d i e n t e t e r m i c o o r i z z o n t a l e dir e n o d a l c o n t i n e n t e al m e d i t e r r a n e o c e n t r o o r i e n t a l e , la d e n i l à dei m a t e r i a l i -illiati a l l a stessa p r o f o n d i l a d o v r e b b e e s e r e m i n o r e o l i o il m e d i t e r r a n e o e c n l r o o r i e n t a l e clic sotto il c o n t i n e n t e . d ' a l t r a p a r i e -i a v r e b b e una c o n t e m p o r a n e a d i m i n u z i o n e d e l l e c o s t a n t i /. e [t. d a l l e n o i e r e l a z i o n i : /. + 2 l i ., u v; = , iv = — i l o ] t> l» -i trova u b i l o la c o n d i z i o n e a cui d e b b o n o s o d d i s f a r e i r a p p o r t i i d). | /k , i d\l j / li, j dg j /q affinché nel pa--arcd a l c o n t i n e n t e al m e d i t e r r a n e o . -i osservi u n a p r o b a b i l e d i m i n u z i o n e d e l l a v e l o c i t à d e l l e u c c l e a t = 2 5 t < z c . 3 6 m |p" c o m p . e w •ii;. 6 h e l . w a n a i = 0.0 3 5 r n ip comp. e w |s fi". o n d e l'n ed un a u m e n t o di v e l o c i t à d e l l e o n d e sii. c o m e e m i l i a n o m o s t r a r e i r i s u l t a t i d e l l a p r e s e n t e r i c e r c a . d i f f e r e n z i a n d o le [ 1 5 ] -i ila: 2 7 0 p a o l o e m i l i o v a l l e d a t o clic d vp < 0, d va 0, r i s u l t a : < 0 l q da do _ ! l l1 s u p p o n e n d o c h e p e r r i n c r e i n e i i t o d e l l a t e m p e r a t u r a dk, d\i e d(t s i a n o n e g a t i v i , s a r à : d\ ^ dq ), " " e» da d(i ' il q e q u i n d i : ' / ! j • < ' h j < d f [ 1 6 ] ii {) x a p p e n d i c e tempi di arrivo <ielle onde di dilatazione e di distorsione 1. c a t a n i a a = 1 9 7 . 8 k m 5. f i r e n z e ( x ) a = 7 3 1 k m p 07h 32'" 47 s ,9 pn <>7h 33m 58" s 33 (19,9 i !' •• 34 09 2. t a r a n t o a = 2 0 2 k m ! p p * ? 2 9 , 5 p 07h 32m 49".0 1 sri 35 1 4 , 5 s 33 11 s* 27 3 . r o m a . a = 5 0 9 k m 6. b e l g r a d o a = 7 4 6 k m pn (i7!l 33m 29". 15 pn 07h 33m 58" p* 38 rp*? 34 3 2 , 5 iii"--? 55 sn 35 17.8 sn 34 25 s * 3 2 . 5 .s* 33 7. p r a t o a = 7 4 8 k m 4 . s o f i a a = 709 k m pn 07h 33m 59 s ,3 pn 07h 33m 55" p * 34 10 p * 34 05 sn 35 17 r p * ? 2 7 . 5 s * 33 sn 35 11 8. z a g a b r i a a = 7 9 1 k m s * 23 ; pn 07h 34m 0 5 \ 4 co m i t i ih i o a l . 1.0 s t i 1)10 d k i . i . e ( alta i t i i! i s t i c l l k s i s m i c i ! ] : . 1 ( 1 . 13 p * 2 0 , 7 sn 35 27 s * 4 6 , 5 trieste a = 8 1 2 k m pn 07h 34'" 08" sn 35 3 1 , 6 s * 47 p a v i a a 958 k m l ' n 07h 34m 2t c o i r à a = 1083 k m l'n 07h 34 m 41 s ,5 z u r i g o a = 1 i 72 k m l'n 07h 34"' 53s sn 36 54 b a s i l e a a = 1234 k m 1> 07h 35m 00 s ,5 s 37 07 s t o c c a r d a a 1275 k m p 07h 35m 05s s 37 16 p r a g a a 1278 k m /' 07h 35"' 05-.5 16. s t r a s b u r g o a 1317 k m p ' 07h 35'" 09 s .6 s 37 26 17. i ortosa a 1414 k m p 07h 35 m 21s s 37 48 18. j e n a a = 1420 k m p 07" 35'" 22s .s 37 49 19. h e h v a n a 1654 k m p 07h 35'" 47s s 38 17 20. u c c i e a 1662 k m p 071' 35m 51s s 38 4 2 , 5 21. t o l e d o a = 1798 k m p 07" 36m 0 5 s . 4 s 39 1 0 . 6 22. c o p e n a g h e n \ 1917 k m p 071' 36'" 18s s 39 3 5 , 5 roma, istituto nazionale di geofisica, dicembre 1917 r 1 a s s u \ t 0 sj espongono i risultati ili una ricerca sulle onde longitudinali e trasversali prodotte dalla scossa principale del terremoto avvenuto i 11 maggio 19 it in calabria. detcrminate le coordinate dell'epicentro, l' i. ha calcolato, in base ai dati delle stazioni clic si trovano ai margini del continente, le velocità delle onde p n , s n . p * ed s * . da un confronto con i valori delle analoghe velocità trovate da altri autori sembra che la velocità delle onde pn tenda a decrescere dal continente al mediterraneo centro-orientale, mentre la velocità delle onde sn tende a crescere. i n sensibile aumento della velocità risulta per le onde p * ed s : , : . dai sismogrammi delle stazioni di catania e t a r a n t o , che dorrebbero trovarsi nel normale raggio (li ricezione delle onde dirette, si rileva un notevole anticipo dei tempi di arrivo delle onde di dilatazione e di distorsione. l'autore ritiene che i risultati della ricerca trovino spiegazione nel vulcanismo del mediterraneo centro-orientale. lo strato 2 7 8 p a o l o k m 11.10 v a l l e sialico superiore iti ijtieslu zona dovrebbe mancare o, (pianto meno, essere esl remameli te sottile. ciò spiegherebbe l'aumento ili velocità delle onde ili dilatazione e di distorsione riscontralo nelle stazioni di catania e taratilo, e delle onde p * e s * . il magma inoltre jwlrebbe trovarsi direttamente sotto lo strato del basalto e ciò sarebbe in relazione alla diminuzione della velocità delle onde p n e all' aumento della velocità delle onde s u . b i b l i o g r a f i a l ' i c u o i i ' . : temili ili tragitto per terremoti ad origine vicina r i c . s f i n i i . . 5 . 3 8 8 3 9 7 ( 1 9 3 9 ) . -i camiiik !.. .).: tlic geocemric direction cosines o j seismological obscrrutorics • b r i l i l i w . \ < ! v a m . ol' s c i e n c e . ( 1 9 3 8 1 . valle l\ e . : sulla determinazione delle coordinale ipocentrali di un sisma lontano iì<> 11. s o c . s i m . i t a l i a n a . x i . . 7 3 8 1 l l 9 4 2 l . i l caloi i'. rosimi e . : sui tempi ili tragitto delle onde pg ed sg nell'india centrale r i c . s c i m i . . 12. 9 4 7 9 5 1 ( 1 9 4 0 1 . i ! i caloi i ' . : ricerche su terremoti ad origine vicina kit-, s c i e n l . . 7-8. i08-1i6 ( 1 9 3 8 i . i cai.(n i ' . : caraneristiche sismiche dell'appvnnino tosco-romagnolo r i c e r c a s c i e i i t . . i. 2 1 8 2 3 4 ( 1 9 4 0 1 . i"i rosi.m e . : il terremoto delta garfagnana del l j ottobre 1939 r i c e r c a s c i e n l . . 7 8 , 1 9 6 5 1 7 ( 1 9 4 0 ) . (") caloi 1 ' . : il terremoto adriatico del 30-11-1934 b o l l . s o c . s i ^ n i . i t a l i a n a . x x x v . 9 3 1 1 9 1 1 9 3 7 i . (sl agamennone g : il terremoto del 24-1-1912 e sua velocità di propagazioni a l l i v e r . l i n c e i . c i . s e . l i . . 2 1 . 6 1 6 6 5 2 ( 1 9 1 2 ) . i 'i giiai'k. i l . vo.n: dos \ordliroier ih ben i oni ii. october 193(1 z c i i d i r . g e o p h v i k 8 . 1 4 1 1 5 4 (1932.1. l j " i gì'tknhfc.ni; i!.: truvel-lime ciirves al snudi dislances and nave velociiii• > in southern california g e r l a m l s b e i t . g e o p h y i k 3 5 , 6 1 5 11932.1. ljll conha» \ . : l)as schwailorfer beben vnin ii. october 1927 g e r l a m l s u e i t r . g e o p l i v > i k 2 0 . 2 1 0 2 7 7 ( 1 9 2 8 ) . l'-i di i'iliim'0 1).: studio microsismico del terremoto del bassi> tirreno del 16 marzo 1911 b o l l . s o c . s i s m . i t a l i a n a x x x i x . 3 2 5 ( 1 9 4 1 ) . ii ;i demethesci i ) . : sur la delerminiilion des liypocentres par des observation proches b o l l . s o c . s i b i l i . i t a l i a n a x x x y i l i 19391. agamennone g . : sulla profondilii del foco del terremoto culiibro-siciilo del 1.: aprile 19311 b o l l . s o c . s i m . i t a l i a n a x x x v i i 119391. i j , i ciiitlkos x. a . : relazioni ira i fenomeni sismici e le manifestazioni vulcaniche nel mar l',geo e nella grecia orientale " g e o f i s i c a p u r a e d a p p l i c a l a , n i l i > 1 9 4 6 ) . a rate-state model for aftershocks triggered by dislocation on a rectangular fault: a review and new insights rodolfo console and flaminia catalli istituto nazionale di geofisica e vulcanologia, rome, italy e-mail: console@ingv.it, e-mail: catalli@ingv.it november 2005 submitted to annals of geophysics abstract we compute the static displacement, stress, strain and the coulomb failure stress produced in an elastic medium by a finite size rectangular fault after its dislocation with uniform stress drop but a non uniform dislocation on the source. the time-dependent rate of triggered earthquakes is estimated by a rate-state model applied to a uniformly distributed population of faults whose equilibrium is perturbated by a stress change caused only by the first dislocation. the rate of triggered events in our simulations is exponentially proportional to the shear stress change, but the time at which the maximum rate begins to decrease is variable from fractions of hour for positive stress changes of the order of some m p a, up to more than a year for smaller stress changes. as a consequence, the final number of triggered events is proportional to the shear stress change. the model predicts that the total number of events triggered on a plane containing the fault is proportional to the 2/3 power of the seismic moment. indeed, the total number of aftershocks produced on the fault plane scales in magnitude, m , as 10m . including the negative contribution of the stress drop inside the source, we observe that the number of events inhibited on the fault is, at long term, nearly identical to the number of those induced outside, representing a sort of conservative natural rule. considering its behavior in time, our model does not completely match the popular omori law; in fact it has been shown that the seismicity induced closely to the fault edges is intense but of short duration, while that expected at large distances (up to some tens times the fault dimensions) exhibits a much slower decay. key words: aftershocks, earthquake triggering, fault parameters, rate-and-state, omori law. 1 introduction studying the space-time interaction of earthquakes is important not only for the comprehension of earthquakes, but also for its possible application to the assessment of seismic hazard. in this paper we revisit the physical modeling of the interaction between seismic events, seeking a relationship between the source parameters of an earthquake and the rate of all other earthquakes that follow it. the results obtained from the simulations performed with 2 the physical model proposed in this study are not totally innovative; however, they are rarely taken into account by the numerous studies concerning the fault interaction and earthquake triggering. a purely statistical approach to this phenomenon exists with the name of ”epidemic model” (ogata, 1988, 1998; console et al., 2003). in the epidemic model each earthquake source is supposed capable of increasing the probability of new earthquakes according to a spatiotemporal kernel representing the probability distribution, which is decreasing from the causative source. the epidemic model is applicable not only to the aftershock phenomenon but also to the foreshock. however, despite it provides an accurate statistical representation of earthquake interaction, the physical interpretation is neglected. in order to formulate a more physical description of clustered seismicity, we consider that seismic events modify the stress field around the causative fault. our main goal in this work is to observe the spatio-temporal variation of the seismicity rate after an event introducing in the algorithm a rate-state model approach derived by ruina (1983) and dieterich (1986, 1992, 1994). in particular, we seek a physically based relationship between the magnitude of the causative earthquake and the number of its aftershocks. some recent studies put in evidence that sudden stress variations, even of small magnitude, can produce large variations of the seismicity rate. it is recognized as a phenomenon of triggering on faults that are already loaded by tectonic stress and in close-to-failure condition. the seismicity rate increases in general where the stress change (called coulomb stress change) is positive, according to the coulomb model (mendoza and hartzell , 1988; boatwright and cocco, 1996; stein et al., 1997; gomberg et al., 1998, 2000; harris, 1998; king and cocco, 2001; stein, 1999; toda and stein, 2000; kilb et al., 2002; belardinelli et al., 2003; nostro et al., 2005). these studies were able to give a physical interpretation for earthquake interaction observed in specific real cases, but were not suitable for the general application of the model in predictive way that can be tested for example using the information contained in a seismic catalog. in particular, the coulomb stress change criterion does not allow to model the hyperbolic time decay of the aftershock rate after a mainshock, known as the omori law. in this simple model all the impending earthquakes would be clock-advanced by a constant time that depends on the stress change and on the loading 3 rate but does not depend on the physical properties of the fault (gomberg et al., 1998). the experience shows that the earth does not rather behave in this way. the most popular empirical description of the phenomenon is the omori law, stating that the aftershock rate decays in time as t−1. among the various theories modeling the omori law, we have taken in consideration only the one that assumes the rate-and-state dependent constitutive model of faults. we have considered only the static stress changes created by an earthquake. we have not considered, even without rejecting them in principle, other hypotheses that predict a time variation in stress, like viscoelastic relaxation or the diffusion of fluids in the crust. the rate-state model for earthquake nucleation seems capable of substantially explaining all the phenomenology. in this model a promoted failure does not occur instantaneously at the exceeding of a threshold stress as for the coulomb-amonton criterion, but follows a more complex non-linear time history with different phases. this time history depends on the physical properties of the fault too. gomberg et al. (2005) have shown that the rate-state dieterich model may be derived from simple general ideas. the application of the rate-and-state model to the evolution of seismicity is not new; toda et al. (1998) have used this model to predict the spatio-temporal seismicity rate after the kobe earthquake. other examples of application of the same model can also be found in the literature (toda and stein, 2000, 2002, 2003). following the rate-state model it has recently been possible to simulate seismicity quite realistically, accounting for the rate of events triggered by stress changes even at large distance from the inducing earthquake (ziv , 2003; ziv and rubin, 2003). basing our work on the classical theory of the elasticity, we start from the formulation of the stress released by a rectangular fault with uniform stress-drop; then we apply the rate-state model (dieterich, 1994) obtaining the complete time and space distribution of the seismicity induced by a given earthquake. it is important to remark that for present purposes we neglect the interaction between subsequent events, supposing that only the first event perturbs the stress field. we also neglect the effect of the free surface of the earth. in this study we focus our attention on the behavior of induced seismicity in space (on the fault plane) and time and its relationship with the fault’s parameters. the results of simulations are analyzed to find out the scaling relationships existing between the free parameters of the model and the expected 4 seismicity in a way that will allow the validation of the model by real observations. 2 elastic model for a rectangular fault the theoretical approach of this work has been made as simple as possible, still preserving the capability of modeling the stress transfer from a seismic source to another. suppose that a fault is embedded in a homogeneous, isotropic, elastic medium where the stress tensor σ is uniform. at a particular moment, the fault slips generating an earthquake, and suppose that the earthquake fully releases the component of the traction parallel to the slip vector across the fault. we call ∆σ (the stress drop) the uniform negative change of traction on the fault. it is well known that the slip distribution on a fault in similar conditions is not uniform. the slip distribution will satisfy the theory of the elasticity at the new equilibrium, and it will be zero on the edges of the fault. it is also well known that the shear stress on the fault plane outside the edges of the fault increases significantly. the analytical solution for the slip distribution on a fault at the equilibrium does not exist except that for very simple geometries, such as a rectangular fault of constant width and infinite length (knopoff , 1957) and for a circular fault (keilis-borok , 1959; udias, 1999). as mentioned by kostrov and das (1998), the analytical solution is not known even for a geometry as simple as a rectangular shape of width w and length l. in the literature it is possible to find the computation of the stress field generated by an ideal rectangular fault with uniform slip in an infinite homogeneous space or half-space (chinnery, 1961; iwasaki and sato, 1979; okada, 1985, 1992). however, as mentioned above, this is not a realistic model since it implies the com-penetrability of the medium at the edges of the slip region. for this reason, here we make use of a non uniform slip distribution that is approximatively compatible with a uniform stress drop on the fault. to calculate the static displacement, stress and strain, we start from the somigliana tensor and we make use of the double couple point source elastic theory. we compute the modified coulomb failure stress from the relation: σc = ts + µ ′tn, (1) 5 where ts and tn are respectively the traction parallel to the slip vector and the traction normal to the fault plane (negative for compression); µ′ is the effective coefficient of friction incorporating the pore fluid pression that is commonly assumed to be proportional to normal stress change (although this assumption is not general, see cocco and rice (2002)). the effective coefficient of friction in coseismic stress change studies has varied from 0.0 to 0.75, with an average value of µ′ = 0.4 often used. this is the value we have used in our simulations. our model of an extended source is drawn heuristically from that of a rectangular fault infinitely extending in one direction and that of a circular fault, and it is characterized by a slip distribution as: ∆u = ∆σ µ √√√√[( w2 )2 − x2] [( l2 )2 − y2] lw 4 , (2) defined for −w/2 ≤ x ≤ w/2 e −l/2 ≤ y ≤ l/2, where x, y are the coordinates on the fault plane whose origin is coincident with the center of the rectangle, ∆σ is the stress drop and µ the shear modulus (rigidity) of the elastic medium. equation (2) satisfies the condition of zero slip on the edges of the rectangle. the slip distribution is of a form very similar to that obtained numerically by kostrov and das (1998) with the finite differences method. its analytical form is similar to the solution for a circular fault. moreover, for a fault of square shape (w = l) it achieves a seismic moment consistent with those obtained analytically for two circular faults, respectively inscribed and circumscribed to the square. bonafede and neri (2000) have shown that, when imposing a unidirectional traction release in the strike or dip direction, a minor component of slip is present, over the fault plane, even in the direction perpendicular to the released traction. we have neglected this component. for our numerical applications we discretize the continuous slip distribution of equation (2) with a set of point sources densely and uniformly distributed on the fault. the more numerous are the point sources, the more accurate will be the approximation. in our numerical simulations, we arbitrarily, but without loss of generality, suppose that the fault is on the (x1, x3) plane. let us notice that some fault parameters are connected among each other; in particular, the scalar moment m0 is defined through the relation: m0 = µ∆uw l = π2 16 µ∆umaxw l = π2 32 ∆σ(w l)3/2, (3) 6 where ∆u is the average slip on the fault and the maximum slip, ∆umax, is defined as: ∆umax = ∆σ µ √ lw 2 . (4) we also assume that the magnitude m is proportional to the logarithm of the seismic moment m0 (measured in n m) according to the kanamori and anderson (1975) relation: log m0 = 9.1 + 1.5m. (5) the computer code allows the user to chose among several options: the focal mechanism characterizing the causative fault and the induced events constrained by the strike φ, the dip δ and the rake λ angles; dimensions and stress drop, seismic moment, or magnitude of the causative fault; type of output: displacement (3 components), stress or strain (6 independent components), traction parallel to the slip vector and perpendicular to the fault plane (2 component) and coulomb failure stress (1 component). the output values are given on a rectangular grid of points, whose orientation (horizontal, or vertical parallel to one of the co-ordinate axes), dimensions, and spacing are also selected by the user. see figure 1 for the conventions we adopted for the reference system and the angles. in this work we make, for sake of simplicity, the assumption that all the aftershocks have the same focal mechanism of the causative earthquake. figure 2 shows the component of the stress field σ12 parallel to the slip calculated through the parameters listed in table 1. note that this figure shows the stress change and not its absolute value. figure 2 clearly shows that the shear stress σ12 is maximum near the edges of the fault, in particular along the slip direction x1, and decreases with increasing the distance from the fault. inside the fault the stress change is negative and approximately equal to the imposed value of −∆σ. 3 the rate-state model as already stated, we assume that the time-dependent behavior of the seismicity triggered by a shear stress change in a population of faults is described by the rate-state model introduced by 7 dieterich (1992, 1994) where the constitutive model proposed by ruina (1983) and dieterich (1986) is used. according to this model the rate r of earthquakes due to a ∆τ shear stress perturbation is given by: r = r τ̇ τ̇r[ τ̇ τ̇r exp ( −∆τ aσ ) − 1 ] exp ( −t ta ) + 1 τ̇ 6= 0, (6) whose various parameters are defined in table 2. note that equation (6) does not account for the magnitude of the earthquakes, for the distribution of which we assume the classical gutenbergrichter frequency-magnitude relationship. however, both r and r refer to the number of earthquakes in a specific magnitude range. we make the assumption τ̇ = τ̇r. the meaning of this assumption is that the primary earthquake does not change the long-term stress rate due to the tectonic driving forces in the environment. it is easily understandable from (6) that the integral of r over infinite time diverges. this is due to the fact that the limit of r for t → ∞ is the background rate r. we prefer to deal with the net triggered seismicity rate r′ = r−r. the plot of r′ versus time for different values of ∆τ is shown in figure 3 (see also dieterich (1994)). it shows that the initial value of the triggered seismicity rate, immediately after the triggering event, is proportional to the exponential of the stress variation but the time period over which r(t) is maximum and relatively constant decreases exponentially by the same factor for increasing shear stress change. this time period, in substantial agreement with dieterich (1994), can be obtained computing the intersection between r′(t = 0) and the straight line corresponding to the limit of r′ when ∆τ → ∞. the difference with dieterich (1994) is that we have preferred to make use of r′, rather than r, and we ignore the case τ̇ = 0 which we judge not realistic. such time period ranges from few hours for a variation ∆τ = 5m p a to nearly one year for ∆τ = 1m p a. figure 3 finally shows that the total duration of the triggered seismicity is the same but the shape of the decay is different for different ∆τ . the integration of r′ over infinite time leads to the not completely trivial result that the total number of triggered events in an area of constant stress change is proportional to ∆τ : 8 ∫ ∞ 0 r′(t)dt = rta aσ ∆τ = r τ̇ ∆τ. (7) this relationship, predicted by the friction model, had been observed for ”landers aftershocks” by gross and kisslinger (1997). consider the integral over time of the triggered seismicity rate r′, once for a positive ∆τ , and then for a negative ∆τ of the same size, and we sum the two functions. we obtain a quantity that initially has a positive increase and then tends back to zero (figure 4). this result can be interpreted as a sort of conservative law of the seismicity, by saying that the expected total number of aftershocks generated in a place close to a fault can be balanced, at long range, by the total number of those events that are inhibited by the stress drop in a place internal to the fault if the stress drops are equal in absolute value. it seems to us that this circumstance has been ignored, so far, in all previous studies on this subject. for numerical applications it is necessary to define the value of the various parameters appearing in equation (6). table 3 contains a list of values inferred from several geophysical considerations and used in our simulations. the reference seismicity rate, r, is a quantity that should be known experimentally for a given area, and it is related to the average strain rate τ̇ of that area. we use for r a uniform value of four events of magnitude exceeding 3.0 per year per 1000 km2, and for τ̇ a value of 5 kp a per year. these are simplifications of the reality where r and τ̇ are not geographically uniform. these parameters seem however reasonable for an area of moderate seismicity. the reason why r is expressed as a number of events per unit area and time is given below. the parameter a of the constitutive law has a value ranging from 0.005 to 0.015 obtained from laboratory experiments (dieterich, 1995). generally, simulations that most nearly resemble earthquakes in nature were obtained with rather small values of a, near to a ≈ 0.001. the value of aσ has been evaluated for different sequences of earthquakes by several authors (stein, 1999; belardinelli et al., 2003). the parameter σ is the normal stress on the fault (of the order of some tens of m p a). the characteristic time of aftershocks, ta, depending on the other parameters through the relation ta = (aσ)/τ̇ , in this context is of the order of several tens of years. this value is essential to model a long term triggered seismicity, because the effect of the stress change disappears completely after a time of the order of the 9 double of ta. 4 application to a rectangular fault dieterich (1994) discussed in detail the case of a finite circular dislocation with uniform stress drop. in our simulations we have taken into consideration a vertical, rectangular strike-slip fault embedded in an infinite, homogeneous, isotropic, elastic medium. the values assumed for the fault parameters are again those reported in table 1. assuming a constant stress drop ∆σ, the scalar seismic moment m0 is proportional to the quantity (w l) 3/2, that can be considered a sort of equivalent source volume. scaling w and l by a given factor, m0 scales accordingly, and also the shape of the stress distribution in space scales by the same factor. assuming that the background seismicity rate is given as the number of events per unit volume and time, and integrating over the space, we find that the number of events triggered by the earthquake is proportional to the source volume, i.e. to the seismic moment m0, or to 10 3 2 m , where m is the magnitude. in this study we analyze only those aftershocks occurring on the fault plane neglecting off-fault seismicity and assuming that this is the plane of weakness optimally oriented with respect to the tectonic loading stress. the modeling of the space-time distribution of seismicity triggered by an earthquake is straightforward, introducing the expression for the coulomb failure stress σc in equation (6) for ∆τ considering that on the fault plane σc ≡ ∆τ . in fact, on the fault plane, the normal stress changes in a homogeneous and isotropic medium are likely to vanish. in this case, the number of triggered events would be proportional to the fault area of the triggering event and then to m 2/3 0 , or to 10 m . if this scaling is true, one earthquake of magnitude m would have a number of aftershocks ten times larger than one earthquake of one magnitude unit smaller. assuming, then, that the b value of the gutenberg-richter frequencymagnitude relation is equal to 1, it would follow that the earthquakes of magnitude m , all together, produce the same number of aftershocks as the earthquakes of one magnitude unit smaller. this hypothesis is supported by the statistical analysis of real observations (utsu, 1969; yamanaka and shimazaki, 1990; felzer et al., 2001; helmstetter , 2003; helmstetter et al., 2005) and will be used in the following of the paper. 10 a first group of simulations concerning the seismicity triggered by a fault as a whole have been performed to study the variation of the total number of triggered events (in space and time), ntot, versus the main free source parameters: the stress drop ∆σ, the linear dimensions l and w and the magnitude m . tables 4, 5 and 6 show the results obtained assigning a fixed value for one of the parameters at a time, and letting the others change according to equations (3) and (4). these simulations have been carried out by summation of the single contributions of the elementary cells constituting the area surrounding the fault. only the seismicity triggered on the fault plane, but outside the fault edges, has been considered. looking at table 4 one can notice that, under our hypotheses, for a constant stress drop, the total number of triggered events increases about proportionally to the fault area lw . theoretically it should be exactly proportional to the area of the source, but in the calculations there are numerical imprecisions due to the grid size approximations. these numerical simulations show, also, that the number of aftershocks is proportional to the 2/3 power of the scalar seismic moment or to the exponential of the magnitude (10m ), in agreement with the above theoretical arguments. table 5 clearly shows the proportionality existing between the stress drop ∆σ and ntot, having fixed the linear dimensions of the fault. this is a simple consequence of the proportionality between the stress drop, ∆σ, and the shear stress change, ∆τ , at any point of the space around the source; furthermore, it is an example of the proportionality expressed in equation 7 between ∆τ and the total number of triggered events, integrated on the whole space. the results reported in table 6 give credit to the hypothesis that, for a constant magnitude, the earthquakes of smaller linear dimensions generate a larger number of triggered events. indeed, a point source would yield the largest quantity of triggered events for the same magnitude. let us mention, lastly, the results of a simple numerical test. we just have theoretically shown in the previous section that the total number of events triggered by a stress change ∆τ per unit area is, after a time much larger than the characteristic time ta, identical to the number of events inhibited by a negative shear stress change −∆τ on the same area. the numerical simulations about an extended source, with parameter still shown in table 1, reflect this idea in a more general way. in fact, if we take the total number of aftershocks produced in a time 11 interval of 100 years by a fault of 100 km2 on the portion of an area of 104 km2 external to it, we find 353.2 events, while the same computation made including the negative contribution of the area internal to fault itself gives only 15.6 (less than 5%). the idea of an overall null balance is, then, substantially met: the phenomenon of clustering does not change the long term seismicity rate of a large area. this is in contrast with the opinion that trigger zones produce many more shocks than are missing from the shadows (toda and stein, 2003). we nevertheless regard for marsan (2003) that put in evidence the practical difficulty to observe in nature the lack of events (the quiescence) mainly when looking at weakly active regions and short timescales. 5 temporal behavior of the triggered seismicity let us now consider in more detail the dependence on time of the number of triggered events predicted by the present hypothesis. kagan and jackson (1991) and dieterich (1994) found, in their empirical works, that shallow aftershocks, above a given magnitude cutoff, decay globally (for different distance intervals) in about 10 years and this time decay appears to systematically decrease with increasing depth. in our model the behavior in different zones depends on the different values of the stress changes, that decrease with the increasing distance from the edges of the fault. it can be then noted that the plateau of the plot of the seismicity rate versus time (see figure 3) has a duration that increases with the increasing distance from the source. this result depends on our assumption of considering only aftershocks on the mainshock fault plane. it can also be biased by the fact that we consider only the effect of the static stress step ∆τ caused by a mainshock and we neglect the interaction between others events. we also do not account for heterogeneities of frictional or constitutive properties nor for the dynamic stress concentration at the crack tip. in spite of that, we believe that our model can describe a behavior not so far from reality. therefore we keep as our reference the model of fault given in table 1 again. as shown in figure 5a, in the close neighboring of the fault (within 0.5 km from the perimeter of the fault), most of the aftershocks occur within one day after the occurrence of the inducing earthquake, and more than 1/3 of them occur already in the first hour. this is clearly 12 not the case for further distance from the source: in the slice between 0.5 and 8.5 km from the fault, about half of the activity is exploited within about one year. the further we go apart from the source, the longer is the time interval necessary for the exploitment of the triggered activity. in fact, in the distant zone (from 20 to 100 km from the fault) most of the triggered events occur longer than 20 years after the inducing earthquake (figure 5b). the behavior shown in figure 5a and 5b is illustrated also by the snapshots of the spatial distribution of the seismicity rate at different times. figures 6, 7 and 8 clearly show how the distribution of aftershocks resembles that of the stress change (figure 2) and how the time decay is different in the different slices. these also show how shadow zones (where the seismicity is inhibited) have an effect longer in time. it should be noted again that we make the assumption that the aftershocks occur mainly on the fault plane and that these have the same focal mechanism of the causative event. we are now interested to check if the generalized omori law is suitable to describe the temporal behavior of r′(t) predicted by our model in every spatial slice and, in such case, by means of which free parameters. we refer to the generalized omori law as to the formula describing the decay of aftershock rate after a mainshock (utsu et al., 1995) through three free parameters a, c e p: r′(t) = a (c + t)p . (8) for this purpose we make use of an algorithm for the least squares best-fit of the sets of data reported in figure 5 by the integral of (8) over the time: y = ∫ t 0 r′(t′)dt′ = a [ (c + t) 1−p − c1−p 1 − p ] . (9) the results obtained for the best-fit are shown in table 7. it is evident that the values of the parameters obtained are quite different from case to case (for instance, the c parameter is extremely small for the slices closest to the fault edges), and that the standard deviation is larger for the slices that include the wider range of distances from the fault. these results show that the same set of parameters in equation (9) can not fit the synthetic values obtained for different zones. dieterich (1994) reached a similar conclusion for a source model represented by a circular shear crack. however, his analysis was limited to the spatial variation of p parameter letting c = 0. the observation that the omori p parameter varies spatially in real aftershocks 13 sequences has been documented not only by dieterich (1994) but also in a number of papers (wiemer and katsumata, 1999; wiemer , 2000; enescu and ito, 2002; wiemer et al., 2002; wiemer and wyss, 2002). wiemer et al. (2002) reported a case of significantly different c values in the northern and southern hector mine aftershock volume, respectively. enescu and ito (2002) in their study of aftershock activity of the 2000 western tottory earthquake found c-values very close to 0 except for larger magnitude earthquakes, arguing that this feature could result from incompleteness of data, or might also reflect the complexity of the rupture process. the situation is illustrated also by the plots of figures 9 and 10, showing the graphical comparison between the synthetic data and the relative theoretical approximation for different slices. 6 conclusions in the development of this model some approximations have been made. two of the most important are (a) having neglected the dishomogeneity of the stress drop on the fault, and consequently the very complicated pattern of slip (this must certainly have influence on the aftershock pattern: some aftershocks may even occur inside the fault, where the gradient of slip is high); (b) having neglected the interaction between subsequent events. the first step of our modeling, concerning the stress change in the medium following an earthquake, has given results comparable to those of previous papers as, for instance, chinnery (1961) and king and cocco (2001). this substantial agreement, though the simplifications introduced in this specific case of rectangular source, supports the validity of our methodology. neglecting the interaction among seismic events and using the rate-and-state model of dieterich (1994) entails that the total number of triggered events per unit area is proportional to the shear stress change. nevertheless, the time constant by which the rate of events decreases has a large variability, ranging from a fraction of hour for a shear stress change of several m p a, up to tens of years for stress variations smaller than 1 m p a. a first set of simulations carried out with a model of rectangular fault, based on the hypothesis that most of the aftershocks occur on the same plane of the fault, has lead to the relationship between the main source parameters and the number of triggered events. the 14 simulations lead to the conclusion that many earthquakes of small magnitude can produce, in total, a number of aftershocks comparable to that of fewer larger earthquakes. our model predicts, in fact, that, for a constant stress drop, the number of triggered events is roughly proportional to the seismic moment at the power of 2/3, or equivalently to 10m , as observed in various occasions. helmstetter (2003); helmstetter et al. (2005) suggest that this behavior can be related to the fractal structure of the spatial distribution of the seismicity and to the nature of earthquake interactions. for a costant stress drop we also observe that the total number of triggered events is proportional to the area of the fault; on the other way, if the linear dimensions of the fault are kept constant, the number of triggered events is proportional to the stress drop. an important aspect of the model concerns the time-space behavior of the triggered seismicity. the simulations show that the application of the rate-state model and equation (6) introduced by dieterich (1994) to the static model of stress change has as a consequence something more complicate than the conventional omori law for the temporal decay of aftershock rate. in fact, the seismicity is expected to be intense (in accordance to the stress change) and have a maximum constant value, close to the fault edges of the primary earthquake, but the time necessary for the starting of decay is shorter. on the contrary, the seismicity expected at larger distances (up to 10 times the linear dimensions of the fault) is weak, but the time necessary for the decay from the maximum value is longer (even some tens of years). we can conclude that the temporal behavior of aftershock rate varies in space. moreover, the model predicts that the total long term production of aftershocks, including wider areas, is not negligible respect to the short term production. in this respect, the regional background seismicity could be interpreted as a sort of noise, or memory, due to the superposition of the effect of many older earthquakes. the time decay described by the popular omori law could be interpreted as an apparent average result of the contribution from the various areas on the plane containing the primary fault. the rate-state model (dieterich, 1994) used in our algorithm for the time distribution of the seismicity produced by a dislocation on a rectangular fault, has allowed a partial justification of the omori law. we outline the following main conclusions obtained from the simulations: 15 • the seismic activity is very intense during the first few hours or days after the occurrence time of the primary earthquake, in the rectangular slice close to the edges of the fault; • the immediate decay of this intense activity is followed by a period of time during which the activity at larger distance (of the order of the linear dimensions of the fault) is nearly constant. for longer time (several years) the total number of triggered events in the external zone becomes comparable with that of the most internal one. • the number of events triggered over a long time scale at distances larger than the fault linear dimensions is significantly large; • the seismic activity inside the fault drops to negligible values at the occurrence of the primary earthquake and returns to normal values only after a time much longer than the characteristic time (several tens of years in our simulations); for a long time range the number of events inhibited inside the fault is comparable to that of the events triggered outside: the net balance can be considered null; the seismicity rate changes in space and time, but it does not have influence on the rate averaged over very large intervals of space and time. in light of the consequences derived from our model, both positive and negative perturbations of the seismic activities resulting from the interactions among earthquakes would last for a time of the order of decades. in this respect, it seems unrealistic to define and observe what is commonly called ”background seismicity” of an area. we can conclude that the model here analyzed, in spite of its very simple conception, gives a physical justification both to the very popular phenomenon of short term earthquake clustering (aftershocks and foreshocks in strict sense) and to that of the long term induction and quiescence, also observed in various occasions. we are confident that the refinement of the model and its validations with real catalogs will bring to even more interesting consequences. 16 acknowledgments we are grateful to massimo cocco and maria elina belardinelli for the stimulating discussions and their criticism which led to a significantly better version of the paper. we thank tom parsons for many useful suggestions to help make this paper clear. 17 authors r. console, istituto nazionale di geofisica e vulcanologia, via di vigna murata, 605 00142, rome, italy mailto: console@ingv.it phone:+39 06 51860417 fax:+39 06 5041181 f. catalli, istituto nazionale di geofisica e vulcanologia, via di vigna murata, 605 00142, rome, italy mailto: catalli@ingv.it phone:+39 06 51860571 18 tables table 1: source parameters used in numerical applications. parameter value strike, dip and rake 0, 90, 0 dimension of the fault 10 km×6 km spacing on the fault 0.1 km dimension of the grid 30 km×30 km spacing on the grid 0.5 km magnitude 6 table 2: constitutive parameters used in dieterich’s relation. symbol description r reference seismicity rate τ̇ shear stressing rate τ̇r reference stressing rate ∆τ earthquake shear stress change a fault constitutive parameter σ normal stress t time ta characteristic time for seismicity to return to the steady state 19 table 3: values of the parameters used in the numerical applications of the model for induced seismicity. parameter value r 4 events/(y · 1000km2) τ̇ 5 kp a τ̇r 5 kp a a 0.008 σ 30 m p a ta 48 y 20 table 4: relation between fault parameters and aftershock production. the value of ∆σ is fixed at 4 m p a; the grid used in this computation has an area of 104 km2, the spacing on it is 0.5 km. the spacing on the fault is 0.05 km. l · w [km2] m m0 [n m] ntot 4 4.6 9.9 · 1015 13.4 25 5.4 1.5 · 1017 78.2 100 6.0 1.2 · 1018 263.2 225 6.4 4.2 · 1018 532.8 table 5: relation between fault parameters and aftershock production. the area of the fault is fixed at the value of 100 km2, the spacing is of 0.05 km; the grid used in the computation has an area of 17 × 17 km2, the spacing on it is of 0.5 km. ∆σ [m p a] m m0 [n m] ntot 1 5.6 3.1 · 1017 51.8 2 5.4 6.2 · 1017 103.4 3 5.9 9.3 · 1017 155.2 4 6.0 1.2 · 1018 206.8 5 6.1 1.6 · 1018 258.6 table 6: relation between fault parameters and aftershock production. the value of m is fixed at 6.0; the output grid is of 20 km×20 km, the spacing is 0.5 km. the spacing on the fault is 0.05 km. l · w [km2] ∆σ [m p a] ntot 4 510.7 overf low 25 32.7 597.6 100 4.1 223.2 225 1.2 107.8 21 table 7: results obtained for the best-fit of the sets of data reported in figure 5 by the relation (9). the symbol rms indicates the standard deviation. slice a [yp−1] c[y] p rms (0 ÷ 0.5)km 3.24 ± 0.17 0.37 · 10−7 ± 0.2 · 10−6 1.02 ± 0.02 1.86 (0 ÷ 8.5)km 11.0 ± 0.76 0.23 · 10−7 ± 0.2 · 10−6 0.91 ± 0.032 9.16 (0.5 ÷ 8.5)km 83.7 ± 70.0 3.1 ± 1.96 1.44 ± 0.19 3.3 (0 ÷ 20)km 34.3 ± 8.0 0.13 · 10−1 ± 0.019 1.14 ± 0.077 9.22 (0 ÷ 100)km 36.8 ± 9.1 0.15 · 10−1 ± 0.023 1.12 ± 0.08 10.8 (20 ÷ 100)km 12.5 · 103 ± 10.9 · 103 61.0 ± 5.43 2.8 ± 0.96 · 10−3 0.45 22 references belardinelli, m., a. bizzarri, and m. cocco (2003), earthquake triggering by static and dynamic stress change, j. geophys. res., 108 (b3), 2135, doi:10.1029/2002jb001779. boatwright, j., and m. cocco (1996), frictional constraints on crustal faulting, j. geophys. res., 101 (10), 13,895–13,910. bonafede, m., and a. neri (2000), effects induced by an earthquake on its fault plane: a boundary element study, geophys. j. int., 141 (1), 43–56, doi:10.1046/j.1365-246x.2000.00074.x. chinnery, m. 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(2000), introducing probabilistic aftershock hazard mapping, geophys. res. lett., 27 (20), 3405–3408. 26 wiemer, s., and k. katsumata (1999), spatial variability of seismicity parameters in aftershock zones, j. geophys. res., 104 (b6), 13,135–13,152. wiemer, s., and m. wyss (2002), spatial and temporal variability of the b-value in seismogenic volumes, adv. geophysics, 45, 259–302. wiemer, s., m. gerstenberger, and e. hauksson (2002), properties of the aftershock sequence of the 1999 mw 7.1 hector mine earthquake: implications for aftershock hazard, bull. seism. soc. am., 92 (4), 1127–1240. yamanaka, y., and k. shimazaki (1990), scaling relationship between the number of aftershocksand the size of the mainshock, j. phys. earth., 38, 305–324. ziv, a. (2003), foreshocks, aftershocks, and remote triggering in quasi-static fault models, j. of geophys. res. (solid earth), 108, 14,1–14,13. ziv, a., and a. m. rubin (2003), implications of rate-and-state friction for properties of aftershock sequence: quasi-static inherently discrete simulations, j. of geophys. res., 108, 2051, doi:10.1029/2001jb001219. 27 figure captions figure (1): conventions adopted for the reference system and for the parameters of the orientation of a fault: the strike φ, dip δ and rake λ angles and the unit vectors n and l. the x3 axis is positive toward the nadir direction. figure (2): shear stress, σ12, parallel to the slip direction for a strike-slip fault projected on the fault plane (x1x3, x2 = 0.2 km). figure (3): distribution of the triggered seismicity against time for different values of induced shear stress. the values of the parameters used are: r = 2 events/y, ∆τ = 3 m p a, aσ = 0.24 and τ̇ = 5 kp a/y. figure (4): distribution of the number of expected events in four different cases: (a) for a positive value of earthquake shear stress equal to 5 mpa, (b) for a negative value of it equal to -5 mpa, (c) the sum of (a) and (b) and (d) the integral of (c) over time. the values of r and ta are fixed at 1. figure (5): representation of the total number of induced events (on vertical axis) in six different arbitrary slices (red, green and blue columns) around the principal fault after different intervals of time (on horizontal axis). the distances of the limits of the slices are computed from the perimeter of the source and the reference magnitude in the simulation is 6.0. histogram (a) represents the distribution nearest to the source at shorter intervals of time (the time scale is purely qualitative). histogram (b) represents the distribution farther from the fault at longer intervals of time. 28 figure (6): spatial distribution of the density of induced events at t=0.01 year. figure (7): spatial distribution of the density of induced events at t=1 year. figure (8): spatial distribution of the density of induced events at t=100 year. figure (9): comparison between the temporal distribution of the cumulative number of induced events corresponding to synthetic data and the best fits obtained by the omori law for the closer slices from the source. figure (10): comparison between the temporal distribution of the cumulative number of induced events corresponding to synthetic data and the best fits obtained by the omori law for the farther slices from the source. 29 figures figure 1: figure 2: 30 figure 3: figure 4: 31 figure 5: 32 figure 6: figure 7: 33 figure 8: 34 figure 9: figure 10: 35 vol49_1_2006def 127 annals of geophysics, vol. 49, n. 1, february 2006 key words radiative transfer code – water vapor – split windows – mivis – atmospheric correction 1. introduction the parameterization of variables defining the interaction between soil and atmosphere radiances plays an important role to define an operative methodology to characterize soil spectral properties over wide surfaces through mivis (multispectral infrared and visible imaging spectrometer) optical hyperspectral data. assessment of water vapor content from mivis tir data cristiana bassani (1), vincenzo cuomo (1), vito lanorte (1), stefano pignatti (1)(2) and valerio tramutoli (3) (1) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy (2) laboratorio aereo ricerche ambientali (lara), iia-cnr, tor vergata (rm), italy (3) dipartimento di ingegneria e fisica dell’ambiente (difa), università degli studi della basilicata, potenza italy abstract the main objective of land remotely sensed images is to derive biological, chemical and physical parameters by inverting sample sets of spectral data. for the above aim hyperspectral scanners on airborne platform are a powerful remote sensing instrument for both research and environmental applications because of their spectral resolution and the high operability of the platform. fine spectral information by mivis (airborne hyperspectral scanner operating in 102 channels ranging from vis to tir) allows researchers to characterize atmospheric parameters and their effects on measured data which produce undesirable features on surface spectral signatures. these effects can be estimated (and remotely sensed radiances corrected) if atmospheric spectral transmittance is known at each image pixel. usually ground-based punctual observations (atmospheric sounding balloons, sun photometers, etc.) are used to estimate the main physical parameters (like water vapor and temperature profiles) which permit us to estimate atmospheric spectral transmittance by using suitable radiative transfer model and a specific (often too strong) assumption which enable atmospheric properties measured only in very few points to be extended to the whole image. several atmospheric gases produce observable absorption features, but only water vapor strongly varies in time and space. in this work the authors customize a self-sufficient «split-window technique» to derive (at each image pixel) atmospheric total columnar water vapor content (twvc) using only mivis data collected by the fourth mivis spectrometer (thermal infrared band). mivis radiances have been simulated by means of modtran4 radiative transfer code and the coefficients of linear regression to estimate twvc from «split-windows» mivis radiances, based on 450 atmospheric water vapor profiles obtained by radiosonde data provided by noaa\nesdis. the method has been applied to produce maps describing the spatial variability of the water vapor columnar content along a trial scene. the procedure has been validated by means of the mivis data acquired over venice and the contemporary radiosonde data. a discrepancy within 5% has been measured between the estimate of twvc derived from the proposed self-sufficient split-window technique and the coincident radiosonde measurements. if confirmed by further analyses such a result will permit us to fully exploit mivis tir capability to offer a more effective (at image pixel level) and self-sufficient (no ancillary observations required) way to obtain atmospherically corrected mivis radiances. mailing address: dr. cristiana bassani, istituto di metodologie per l’analisi ambientale (imaa), cnr, c.da s. loja, 85050 tito scalo (pz), italy; e-mail: bassani@imaa.cnr.it 128 cristiana bassani, vincenzo cuomo, vito lanorte, stefano pignatti and valerio tramutoli atmospheric absorption and scattering processes by atmospheric constituents (mainly water vapor, carbon dioxide, oxygen and ozone) and aerosols, invalidate the interpretability of data to various degrees because of their interference with the energy at the sensor level. the correction of radiance values at the sensor level has been resolved by applying correction methods based on algorithms (like 6s, tanré et al., 1986; atrem, gao and goetz, 1990; acorn, aig, 2001, etc.), which enable atmospheric profiles (altitude, pressure, temperature, water vapor and ozone) and both standard and user-defined aerosol models to be managed. among all the parameters determining the revision process of the radiance measured at the sensor, water vapor plays the most important part as it is the foremost element for thermodynamics of the atmosphere. its very high spatial and time variability makes the water vapor measurement derived from a single meteorological balloon or ground station inadequate whenever these data are to be used for correcting scenes covering several kilometers. from this point of view, the definition of a spectralbased water vapor columnar content evaluation at image pixel level can assure better prospects. this paper proposes a split-window technique (following the approach proposed by kleespies and mcmillin, 1984) which establishes a linear relation between the radiances measured in two tir mivis channels and the total columnar water vapor content (tcwvc) in the atmosphere. the coefficients of the linear multiple regression are derived from synthetic mivis radiances and an experimental dataset of 450 radiosonde water vapor profiles supplied by noaa-nesdis (national environmental satellite, data and information service, www.nesdis.noaa.gov). the proposed method was validated using real mivis radiances measured during a flight over the venice lagoon to produce a (pixel by pixel) water vapor map over the whole scene and to give a first estimate of its accuracy by comparison with tcwvc achieved by contemporary radiosonde data. 2. measurement of the water vapor columnar content in order to correct data collected by different hyperspectral sensors (in particular aviris), the three channel rationing method proposed by gao and goetz (1990) was implemented in the atrem atmospheric correction code. it enables the tcwvc to be retrieved on a pixel by pixel basis using radiances measured in the water vapor absorption bands around 0.94 nm and 1.14 nm. this technique makes it possible to attain a fig. 1. mivis channel 99 (blue) and 101 (red) response functions versus trasmittance (green). 129 assessment of water vapor content from mivis tir data good assessment of the water vapor columnar content if 0.94 nm and 1.14 nm channels are used. it is then inapplicable to the mivis (which does not have the same spectral capability) and inefficacious when employed in the nir mivis channels sounding in the water vapor absorption bands at 0.72 nm and/or 0.82 nm. the split-window technique described by keesplies and mcmillin (1984), jedlovec (1990), ottle et al. (1997) and menzel and gumley (1998) instead defines the structure of a linear relation between the atmospheric tcwvc and radiances collected in the water vapor absorption bands in the 8-13 nm thermal infra-red (tir) spectral region. this region is well covered by the mivis with 10 channels (within 8.2 and 12.7 nm). figure 1 shows the spectral response of the mivis channels 99 and 101 versus water vapor transmittance. the mivis band 99 centered at 11 nm corresponds to a window region for water vapor, while the band 101 centered at 12 nm intercepts small water vapor absorption peaks. the combination of the 99 and 101 mivis bands allows the water vapor columnar content to be solved by means of a split window technique according to menzel’s theory. in the atmospheric window from 8 to 14 nm the water absorption is so low that the taylor series expansion of the atmospheric transmittance (xw) could be approximate to the first-order (2.1) where u and kw are respectively the total optical atmospheric column absorption path length due to the water vapor and the extinction coefficient at the absorbing water vapor wavelength (w) which is supposed constant from the ground to the sensor altitude. under this assumption, an infinitesimal variation of xw is expressed (2.2) in clear sky conditions the radiance received at the sensor at wavelength w is the sum of the radiance contributions from the earth’s surface (first addendum of eq. (2.3)) and from all levels in the atmosphere (second addendum of eq. (2.3)) until sensor level as follows (schwarzschild’s equation) .d k duw w= -x e k u1w k u w w= = -x (2.3) where us represents the total atmospheric column absorption path length due to water vapor, bw is the planck radiance from all the atmosphere levels from sensor altitude to the surface at the w wavelength, and bws expresses the black body radiance from the surface at the same wavelength. taking into consideration the integral medium value theorem eq. (2.3) could be expressed as follows: (2.4) as a consequence us could be expressed as (2.5) setting w equal both at 11 and 12 nm, respectively in a water vapor window and absorbing region spectrally close to it, it follows that: (2.6) (2.7) taking into consideration the split window approximation which allows b11s=b12s to be set, the eqs. ((2.6) and (2.7)) could be expressed as function of b11s=b12s, therefore 1/us is expressed as follows: (2.8) where is assumed proportional to bws by means of the proportionality coefficient aw (2.9)b a bw w ws= bw u a r r r b r r r1 s 12 11 11 12 11 12= + ( ) .u k b b r b s s s 12 12 12 12 12= ( ) u k b b r b s s s 11 11 11 11 11= ( ) .u k b b r b s w w w s w w s = ( ) .r b k u k b u1w w s w s w w s= + b du b du u w w u 0 0 s s = # # ( )r b k u k b du1w w s w s w w u 0 s = + # 130 cristiana bassani, vincenzo cuomo, vito lanorte, stefano pignatti and valerio tramutoli therefore (2.10a) (2.10b) consequently a and b are the coefficients to be estimated to assess the water vapor columnar content from eq. (2.8). in the case of the mivis, the r11 and r12 radiances, which solve the contrast between the surface and the atmosphere close to it (menzel and gumley, 1998), correspond to the band 99 (centered at 11 nm) and the band 101 (centered at 12 nm). the a and b (eqs. (2.10a) and (2.10b)) coefficients were derived by applying a multiple regression analysis (mra) to the synthetic mivis radiances obtained through be application of the radiative transfer code modtran (berk et al., 1989) to 450 nesdis radiosonde profiles. in order to obtain the expected mivis radiances in the channels 99 and 101, the tir radiances were estimated by modtran (setting time and geographic location variables according to the real mivis scene under examination) and then integrated on the mivis 4th spectrometer’s spectral response functions. the obtained data set simulates the radiances that the sensor would have if measured under the atmospheric condition of each nesdis radio sounding. .b k k a11 11 12= a k a k12 12 12= each nesdis radiosounding was then used to obtain 1/us in eq. (2.8) which determines an overestimated system of 450 linear equations. the radiances r11 and r12 simulated by modtran, for each nesdis profile, create the matrix of independent variables while real us value, derived from the nesdis observation, composes the vector of dependent variables. the reliability of the mra on this system made of 450 equations and two unknowns, a and b (eqs. (2.10a) and (2.10b)), is corroborated by the high value of the correlation index (r = 0.76) and by the relatively low value (0.57 cm) of root mean square (fig. 2). 3. experimental data set the mivis flight performed on july 26, 2001 at 11:30 local time over the venice lagoon at 4000 m a.s.l. corresponding to a pixel ground resolution of 8 m, was chosen to test the proposed method. the area selected for the test corresponds to the golf course of malamocco where ground truth and atmospheric measurement were taken during the mivis image acquisition. the mivis flight line was converted into instrument perceiving radiance with an ad hoc idl code running independently from the usual pre-processing chain that produces brightness temperature in the tir region. atmospheric profiling was achieved by means of a vaisala radiosonde launched according to the airborne survey. the radiosonde atmospheric profiles (from surface up to 10000 m a.s.l.) were acquired in either the ascending and descending phase. the radiances measured by the mivis channels 99 and 101 were employed (together with the a e b coefficients, previously determined) to estimate, on the basis of the eq. (2.8), the tcwvc at each image pixel. the derived water vapor content map stresses how the described procedure does not introduce rough numerical errors, since all the values found over the scene (4000 × 755 pixels) are compatible and comparable with those used by the most common radiative transfer models which adopt atmospheric profiles ranging from 0 to 6 cm of precipitable water. as matter of fact the water vapor spatial histogram of the exfig. 2. correlation index between the columnar content estimated and nesdis. 131 assessment of water vapor content from mivis tir data fig. 3. meteorological parameters profile, temperature and rhu (relative humidity) obtained with radiosounding balloon. fig. 4. selected portion of interest (516 pixels) over radiosounding launched area (malomocco, lat: 45°72ln; long: 12°52lw) into the nadir direction. the polygon is defined to apply the statistics of water vapor estimated by spectral-based method. amined scene has a gaussian shape centered at about 3 cm of water vapor with minimum value equal to 0.03 cm and maximum to 5.5 cm. such average spatial behavior of the tcwvc field is perfectly compatible with the measurement (tcwvc = 3.1 cm) performed by the radiosonde launched at the same time of the mivis flight. selecting an area of interest (aoi) of 516 pixels around the balloon launch area (fig. 3), into the nadir direction, the water vapor columnar content can vary with a range from 1.15 cm up to 3.63 cm with a mean value of 3.28 cm. in this aoi of about 13 000 m2 (comparable to a square with a side of more than 100 m), over which the balloon was supposed to move during the ascending phase (fig. 4), the difference between the tcwvc value estimated by the proposed method and the one measured by the radiosonde is 0.16 cm only (about 5%). 4. conclusions the procedure for water vapor assessment applied to the mivis images is based on a multiple linear regression technique, connected to the application of a split window technique to the instrument’s perceiving tir radiance data. the mra on the radio-sounding values of the nesdis data base and mivis simulated radiances in the tir region (channel 99 centered at 11 nm and channel 101 centered at 12 nm) allowed the production of maps describing the spatial variability of the water vapor columnar content along the studied scene. this procedure applied to the airborne hyperspectral data set acquired over the venice lagoon has given a difference of 0.16 cm (5%) with the tcwvc assessed by contemporary radiosounding. it is clear that just a single mivis image cannot be considered statistically significant to affirm the exactness and validity of this procedure. additional analyses will be carried out by using the mivis data collected during the campaigns in sweden and in sicily in june and july 2003. the several radiosoundings launched during these campaigns will be used to assess the reliability of this method in different landscapes. the data collected during the sweden campaign will be particularly useful for this purpose, since the 3 4 132 cristiana bassani, vincenzo cuomo, vito lanorte, stefano pignatti and valerio tramutoli survey will be conducted within an international context and different hyperspectral airborne sensors will be employed. the trial will include pushbroom sensor covering the vnir region and whiskbroom scanner like the mivis. concomitant data acquisition in the vnir region covering the 0.94 nm water absorption bands (not present in the mivis spectral range) and in the tir by mivis will allow a comparison of the straight line technique in the vnir and the split window technique in the tir. this experimental configuration based on the use of different methodologies to assess the water vapor columnar content will be an opportunity to better evaluate possible effects of surface material’s spectral behaviour on the tcwvc estimate. acknowledgements the authors are grateful to angela mirabelli for helping to improve the paper. references adler-golden, s.m., m.w. matthew, l.s. bernstein, r.y. levine, a. berk, s.c. richtsmeier, p.k. acharya, g.p. anderson, g. felde, j. gardner, m. hike, l.s. jeong, b. pukall, j. mello, a. ratkowski and h.-h. burke (1999): atmospheric correction for shortwave spectral imagery based on modtran4, spie proc., 3753, 61-69. aig (2001): acorn user’s guide, stand alone version, analytical imaging and geophysics (aig), llc, p. 64. berk, a., l.s. bernstein and d.c. robertson (1989): modtran: a moderate resolution model for lowtran7, rep. gl-tr-89-0122 (air force geophys. lab., bedford, ma). cuomo, v., v. tramutoli, n. pergola, c. pietrapertosa and f. romano, (1997): in place merging of satellite based atmospheric water vapor measurements, int. j. remote sensing, 18 (17), 3649-3668. gao, b.-c. and a.f.h. goetz (1990): column atmospheric water vapor and vegetation liquid water retrievals from airborne imaging spectrometer data, j. geophys. res. atmos., 95 (d4), 3549-3564. jedlovec, g.j. (1990): precipitable water estimation from high-resolution split window radiance measurements, j. appl. meteorol. , 29, 851-865. kleespies, t.j. and l.m. mcmillin (1984): physical retrieval of precipitable water using split window technique, in preprints conf. on satellite meteorology/remote sensing and applications, ams, boston, 55-57. menzel, w.p. and l.e. gumley (1998): modis atmospheric profile retrieval algorithm theoretical basis document, atbd-mod-07 (university of wisconsin, madison). ottle, c., s. outalha, c. francois and s. lemaguer, (1997): estimation of total atmospheric water vapor from split-window radiance measurements, remote sensing environ., 61 (3), pp.410-418. tanré, d., c. deroo, p. duhaut, m. herman, j.j. morchrette, j. perbos and p.y. deschamps (1986): simulation of the satellite signal in the solar spectrum (5s), users’s guide (u.s.t. de lille, 59655 villeneuve d’ascq, france: laboratoire d’optique atmospherique). ionospheric trends in southern hemisphere stations due to the increasing greenhouse gases concentration annals of geophysics, 58, 3, 2015, a0332; doi:10.4401/ag-6707 a0332 ionospheric trends in southern hemisphere stations due to the increasing greenhouse gases concentration josé v. venchiarutti1, blas f. de haro1,*, ana g. elías1,2 1 universidad nacional de tucumán, departamento de física, tucumán, argentina 2 consejo nacional de investigaciones científicas y técnicas, conicet, argentina abstract the lower and middle atmosphere present long-term variations in temperature and other parameters linked to anthropogenic sources, such as the increase in greenhouse gases concentration since the start of the industrial era. some examples are the well known temperature increase in the troposphere and stratosphere cooling. upper atmosphere parameters also present long-term variations. while in the case of the middle and lower atmosphere it is debated whether the origin of the trends is primarily anthropogenic or solar, in the upper atmosphere other sources are also able to induce long-term changes, such as long-term variations in geomagnetic activity and secular variations of the earth’s magnetic field. in this paper, trends of the f2 layer critical frequency, fof2, measured at three southern hemisphere stations (brisbane, canberra and christchurch) are analyzed to determine the importance of increasing greenhouse gases concentration effect. according to our results for the period covering solar cycles 18 to 21 (period 1944-1986), it is more important than natural forcings. update estimates including solar cycle 23 are presented although the difficulty due to two of the three stations present big data gaps during cycle 22 and traditional ionospheric filtering is no longer entirely reliable for cycle 23. the aim of this study is to contribute both to an active area of aeronomy as is the study of trends in the upper atmosphere, and to the understanding of climate change. 1. introduction since the beginning of the 1990s there has been a great interest in the study of long-term trends in the upper atmosphere and several papers have been published since then on this topic (see lastovicka et al. [2012] for a very good review). some of these studies associate trends in the ionosphere with cooling in the thermosphere due to increased concentration of greenhouse gases [roble and dickinson 1989, rishbeth 1990]. if the concentration of co2 is doubling, according to theoretical models a cooling of 30-40 k in the thermosphere should be expected together with a decrease in air density, a decrease of 15-20 km of the peak height of the f2 layer, hmf2, and a decrease in the critical frequency of the f2 layer, fof2, less than 0.5 mhz globally. however the overall pattern of hmf2 and fof2 trends estimated with experimental data of several worldwide stations is very complex and would not completely agree with the greenhouse gases hypothesis. another possible cause of the ionospheric trends would be the long-term variation of geomagnetic activity [mikhailov and marin 2000] and the secular variations of the earth’s intrinsic magnetic field [foppiano et al. 1999, elias 2009, cnossen 2014]. in this paper we estimate fof2 trends measured at three southern hemisphere ionospheric stations, which have records since solar cycle 18, and analyze them in terms of the cooling expected from the anthropogenic effects. being able to determine the causes of the observed ionospheric trends, would contribute to the study of variability in the ionosphere for predictive purposes and the problems of global change directly linked to increased concentration of greenhouse gases. 2. data analysis monthly median values of fof2 from damboldt and suessmann database [2012] (ftp://ftp.ips.gov.au/ wdc-data/iondata/medians/damboldt/) were used at 12 lt. three southern hemisphere stations which are listed in table 1 were selected which had long enough records and few data gaps. canberra is the only that presents an almost uninterrupted record for the period 1941-2009. christchurch presents a first almost complete period 1941 to1986, and then returns in 1993-2009, while in the case of brisbane the almost complete data periods are 1943-1986 and 1998-2009. due to the big data gap in solar cycle 22, trends are estimated first for the period 1944-1986 which covers four complete solar cycles, 18 to 21. then article history received december 3, 2014; accepted may 22, 2015. subject classification: ionospheric trend, euv proxies, forecasts, dynamics. they are updated to include solar cycle 23 with special care in the filtering process required in ionospheric trend estimation. 2.1. trends during period covering solar cycles 18 to 21 in order to minimize spurious effects introduced by the filtering process here applied [lastovicka et al. 2012, elias 2014], trends should be assessed for the same period in all the cases analyzed and include complete solar cycles. in our case this period corresponds to 19441986, that is solar cycles 18 to 21. the steps followed to estimate fof2 trend are: 1. seasonal variation was filtered out assessing 12month running mean of each data series analyzed. 2. periods with rz> 160 were eliminated due to saturation effect occurring in the f2 region. 3. solar cycle 23 was not considered due to lack of a close agreement between rz (or f10.7) and solar euv [emmert et al. 2010, chen et al. 2011, tapping and valdes 2011]. its inclusion is analyzed in the following section. 3. solar activity effect was filtered out estimating the residuals from the regression between the experimental fof2 values and the solar activity index rz, that is: fof2res =fof2exp − (a + b rz) (1) where fof2exp is the experimental fof2 value, rz is the sunspot number used as a proxy for the solar euv radiation, and a and b are the regression constants assessed with least squares. 4. the linear trend a (in mhz/year) is calculated from fof2res = a t + b (2) figure 1 shows fof2res together with the fitted linear trend. table 1 shows the values of the trends obtained. trend errors in all the cases are around 8-11% of trend values, that is ~6-9×10-4 mhz/year. all trend values are significantly different from zero at a 95% confivenchiarutti et al. 2 figure 1. fof2 residual for (a) brisbaen (27.5°s, 152.9°e), (b) canberra (35.3°s, 149.0°e) and (c) christchurch (43.6°s, 172.8°e), after filtering seasonal and solar activity effects using rz as solar euv proxy (black dots) and fitted linear trend (solid line). station geographic latitude geographic longitude fof2 trend [mhz/year] period cycles 18-21 period cycles 18-23 rz as proxy f10.7 as proxy rz as proxy f10.7 as proxy brisbaen 27.5°s 152.9°e -0.0086 -0.0120 +0.0004* -0.0102 canberra 35.3°s 149.0°e -0.0074 -0.0107 -0.0008* -0.0089 christchurch 43.6°s 172.8°e -0.0073 -0.0108 +0.0015 -0.0071 table 1. geographic coordinates of the southern hemisphere ionospheric stations here analyzed and linear fof2 trend in mhz/year calculated for four different conditions: filtering seasonal and solar activity effects using rz (4th and 6th columns) and f10.7 (5th and 7th columns), and for periods covering solar cycles 18 to 21 (4th and 5th columns) and 18 to 23 (6th and 7th columns) (values with * mean not significantly different from 0). a b c 3 dence level. the significance was assessed calculating the statistic t as a/v, where (3) v is the standard deviation of trend and r the correlation coefficient of the regression between fof2res and time. rz was chosen as solar euv proxy instead of f10.7, which is the one recommended [lastovicka et al. 2012, elias 2014], since f10.7 is not available since the beginning of cycle 18. in addition, since we are not including here cycle 23, the difference between filtering solar activity using rz or f10.7 should not be large. as a test, we linearly extrapolated f10.7 values to 1944, using a linear regression between rz and f10.7 for the period 1947-1986. trends, listed in table 1, were assessed following the same steps as in rz case. it can be noticed that although the difference is ~40-50% for the three stations, they are all significant decreasing trends. 2.2. extending period to cycle 23 the three stations here analyzed have available data records until year 2009. however two of them, christchurch and brisbane, present a big data gap during solar cycle 22, which cannot be easily interpolated for trend detection purposes. as a first approach, without interpolation, trends for the period covering cycles 18 to 23 were estimated. special care should be taken in fof2 filtering process when solar cycle 23 is included [elias et al. 2014]. the period 2007-2009 that corresponds to minimum activity level, is characterized by lower euv solar radiation than previous solar cycles [solomon et al. 2013]. in addition it is different than that deduced from traditional solar euv proxies as rz and f10.7. emmert et al. [2010] suggested that the long-term relationship between euv irradiance and f10.7 has changed markedly since around 2006 with euv levels decreasing more than expected from f10.7 proxy. rz instead, underestimates euv during the maximum and declining phase of the cycle [lukianova and mursula 2011]. in addition, rz and f10.7, which were used interchangeably as euv proxy, present a significant change in their relationship since solar cycle 23 [tapping and valdes 2011]. taking all this into account, trends were estimated for each station considering rz and f0.7 to filter solar activity effect. trend values are listed in table 1. figure 2 shows fof2res together with the fitted linear trend, considering both rz and f10.7 separately as solar activity filters. it can be clearly noticed that the inclusion of solar cycle 23 marks a significant difference not only trend values, but also in the sign, between both euv proxies. 3. geomagnetic activity effect danilov and mikhailov [1999], mikhailov and marin [2000] and danilov [2002] with a new approach, obtained a dependence of the trend magnitude on geomagnetic latitude (with more negative values at higher latitudes) indicating that f2-layer trends might be related to long-term changes in geomagnetic activity and t n 2 r r 1 r n 2 1 12 2v a a a = = = ionospheric trends in southern hemisphere figure 2. fof2 residual for (a) brisbaen (27.5°s, 152.9°e), (b) canberra (35.3°s, 149.0°e) and (c) christchurch (43.6°s, 172.8°e), after filtering seasonal and solar activity effects using rz (black dots) and f10.7 (red dots) as solar euv proxy and fitted linear trend (corresponding solid lines). a b c f2-layer storm mechanisms. they show those periods with negative and positive fof2 trends exist, which correspond to the periods of long-term increasing or decreasing geomagnetic activity. however, more recently, according to danilov [2012], the assumptions made at the end of 1990’s that the observed trends in ionospheric parameters are a manifestation of secular changes in geomagnetic activity could be proved only for the data before 1980-1990. in our case, the three stations here analyzed are mid-latitude stations, so the geomagnetic activity effect should not be large. to prove this, aa index was added to equation (1) to filter fof2, that is fof2res =fof2exp − (a + b rz + c aa) (4) this index is a global geomagnetic activity index which measures the disturbance level of the earth’s magnetic field. the index, in nt units, is available from http://www.wdcb.ru/stp/data/geomagni.ind/aa/ and from http://isgi.cetp.ipsl.fr/source/indices/. following the same steps described in section 2.2, trends were assessed considering period 1944-1986, and 1944-2009, using rz and f10.7 as solar euv proxies separately. trend values are shown in table 2. as can be noticed, in all the cases trend values are almost the same as those seen in table 1. this can be expected since the correlation coefficient between fof2 and rz or f10.7, is already greater than 0.97 meaning that rz (or f10.7) explain more than 94% of fof2 variance in all the cases. including aa into the linear regression model, only increases the correlation by ~0.005, that is it adds only 1% to the explained variance. taking into account the result of danilov [2012] that there is a different fof2 negative trend after ~19801990, and of lastovicka et al. [2012] that observes a change of dependency of f2 trends after 2000, we made a rough analysis in the case of canberra, which presents complete data set covering the period 19442009. we estimated the aa coefficient, c in equation (4), considering f10.7 as euv proxy, for the period 19441989 and 1990-2009 separately. in the first case it results −0.15±0.03 and in the latter −0.04±0.03, while b results 0.395±0.003 and 0.382±0.003. that is, after 1990 the coefficient of aa is almost null. this could be evidence of a decline in importance of aa, however when we look at trends estimated with and without aa in fof2 filtering, we cannot arrive to a definitive conclusion through this type of analysis. anyway as it has been observed by bremer [1992, 1998] and mikhailov and marin [2000] the inclusion of a geomagnetic index in the regression in fact does not remove the geomagnetic dependence. 4. earth’s magnetic field effect the possibility of ionospheric trends induced by the earth’s magnetic field secular variations was first suggested by foppiano et al. [1999], and followed then by other papers [elias 2009, cnossen 2014]. earth’s magnetic field, generated in the earth’s core, presents long term variations in the field’s strength and orientation. a simple mechanism through which trends in the earth’s magnetic field would affect the ionosphere is through changes in the dip angle (i). the sin(i)cos(i) factor, associated to effects of neutral winds on hmf2 [rishbeth 1998] will also change. the horizontal thermospheric wind u drives ions and electrons, up during the night and down during the day, along the geomagnetic field lines at speed u cos(i). the vertical component u sin(i) cos(i) raises the f2-peak during night time (when u blows from pole to equator) and lowers it during daytime (when u blows from equator to pole), increasing or decreasing the peak electron density. an increase in the sin(i)cos(i) factor would produce an additional lowering of the f-region with a decrease in fof2, during daytime, and an additional raise of the region with an increase in fof2 during the night. a decrease in the sin(i) cos(i) factor would produce the opposite effect. the region of strongest variations of fof2, lies between 10°n and 30°s in latitude and between 20°e and 80°w in longitude, which is also the region of strongest changes in i and sin(i) cos(i) factor [elias 2009, cnossen venchiarutti et al. 4 station fof2 trend [mhz/year] period cycles 18-21 period cycles 18-23 rz + aa f10.7 + aa rz + aa f10.7 + aa brisbaen -0.0083 -0.0118 +0.0001* -0.0101 canberra -0.0068 -0.0105 -0.0007* -0.0089 christchurch -0.0068 -0.0103 +0.0016 -0.0070 table 2. fof2 trend in mhz/year calculated for four different conditions: filtering seasonal and solar activity effects using rz and aa (4th and 6th columns) and f10.7 and aa (5th and 7th columns), and for periods covering solar cycles 18 to 21 (4th and 5th columns) and 18 to 23 (6th and 7th columns) (values with * mean not significantly different from 0). 5 2014]. at the mid-latitude zone, corresponding to the locations of the ionospheric stations here analyzed, the expected trends due to changes in i, should not be statistically significant different from zero. a qualitative analysis can be made through the sin(i) cos(i) factor. figure 3 presents the time variation of this factor calculated from the international geomagnetic reference field (igrf) model (http://ccmc. gsfc.nasa.gov/modelweb/models/igrf_vitmo.php), and for the three stations here analyzed, it can be noticed an increase for the period 1944-1986. this implies that although it is very small, an increasing trend in daytime fof2, is contrary to the values obtained from measured fof2 data. 5. conclusions all stations present a decreasing trend in fof2 during the period 1944-1986, covering complete solar cycles from cycle 18 to 21, consistent with the consequences expected in the ionosphere f2 region due to the increased concentration of greenhouse gases. for a doubling of co2 concentration qian et al. [2009] obtain a maximum decrease of 18% to 20% in fof2. making a linear extrapolation to the actual increase in co2 during the period analyzed in this paper, and assuming an average fof2 value of 10 mhz, a fof2 decrease of −0.005 mhz/year should be expected. according to the values here obtained when rz is used as solar euv proxy (~0.008 mhz/year), the greenhouse effect would account for 60% of these trends. when f10.7 is used as euv proxy, extrapolating to cover the complete period 1944-1986, this account diminish to 50%. with regard to natural forcings, in the case of longterm variation in geomagnetic activity, our results hardly vary when considering the aa index as another linear term in equation (1). probably in a different kind of analysis, and considering the result of danilov [2012] this could be more noticeable. in the case of secular variations of the intrinsic magnetic field of the earth, its ability to induce trends depends on the geographic location. the values of the expected trends are very small (even lower than those expected by anthropogenic effect) except in the region between 10°n and 40°s and 60°w and 10°e [elias 2009, cnossen 2014] . the stations considered are located outside this region. in addition a rough qualitative estimation of the expected effect does not even agree in the sign of the expected trends. when using the complete data sets that go until solar cycle 23, the most reliable trends are those obtained using f10.7 as euv proxy [lastovicka et al. 2012, elias 2014]. trend values are close to those corresponding to period 1944-1986, but less negative, very likely due to a lack of agreement between f10.7 and the solar euv radiation which f10.7 is supposed to represent. the importance of understanding and measuring trends in the upper atmosphere lies in: 1) the importance of understanding and predicting the behavior of the whole atmosphere, which is essential to human life. 2) the effect of the decreasing density on space debris. there are approximately 22,500 objects larger than 10 cm in earth orbit, and 95% of them is classified as “space debris”. these objects represent a significant risk to spacecraft operations due to possible collisions. recent studies [lewis et al. 2011] suggest that the decrease in the density of the upper atmosphere makes the space debris remains in orbit longer than expected. the results obtained here are intended to be a contribution to both topics. references bremer, j. (1992). ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effect, j. atmos. terr. phys., 54, 1505-1511. bremer, j. (1998). trends in the ionospheric e and f regions over europe, ann. geophysicae, 16, 986-996. chen, y., l. liu and w. wan (2011). does the f10.7 index correctly describe solar euv flux during the deep solar minimum of 2007-2009?, j. geophys. res., 116; doi:10.1029/2010ja016301. cnossen, i. (2014). the importance of geomagnetic field changes versus rising co2 levels for long-term change in the upper atmosphere, j. space weather space clim., 4, a18. damboldt, t., and p. suessmann (2012). consolidated database of worldwide measured monthly medians of ionospheric characteristics fof2 and m(3000)f2, inag bulletin on the web, inag-73; www.ips.gov. au/ipshosted/inag/web-73/2012/damboldt_con solidated_database.pdf. ionospheric trends in southern hemisphere figure 3. standardized sin(i) cos(i) factor estimated with international geomagnetic reference field (igrf) model (http://ccmc.gsfc.nasa. gov/modelweb/models/igrf_vitmo.php) for brisbaen (black solid line), canberra (red solid line) and christchurch (dashed black line). danilov, a.d., and a.v. mikhailov (1999). long-term trends in the parameters of the f2region: a new approach, geom. aeron., 39, 473-479. danilov, ad. (2002). overview of the trends in the ionospheric e and f2 regions, phys. chem. earth, 27, 579-588. danilov, a.d. (2012). long-term trends in the upper atmosphere and ionosphere (a review), geom. aeron., 52, 271-291. elias, a.g. (2009). trends in the f2 ionospheric layer due to long-term variations in the earth’s magnetic field, j. atmos. solar terr. phys., 71, 1602-1609. elias, a.g. (2014). filtering ionosphere parameters to detect trends linked to anthropogenic effects, earth, planets and space, 66, 113. elias, a.g., b.f. de haro barbas, s. kiyoto and j.r souza (2014). effect of solar cycle 23 in fof2 trend estimation, earth, planets and space, 66, 111. emmert, j.t., j.l. lean and j.m. picone (2010). recordlow thermospheric density during the 2008 solar minimum, geophys. res. lett., 37; doi:10.1029/2010 gl043671. foppiano, a.j., l. cid and v. jara (1999). ionospheric long-term trends for south american mid-latitudes, j. atmos. solar terr. phys., 61, 717-723. lastovicka, j., s.c. solomon and l. qian (2012). trends in the neutral and ionized upper atmosphere, space sci. rev., 168, 113-145. lewis, h.g., a. saunders, g. swinerd and r.j. newland (2011). effect of thermospheric contraction on remediation of the near earth space debris environment, j. geophys. res., 116. lukianova, r., and k. murusla (2011). changed relation between sunspot numbers, solar uv/euv radiation and tsi during the declining phase of solar cycle 23, j. atmos. solar terr. phys., 73, 235-240. mikhailov, a.v., and d. marin (2000). geomagnetic control of the fof2 long-term trends, ann. geophysicae, 18, 653-665. qian, l., a.g. burns, s.c. solomon and r.g. roble (2009). the effect of carbon dioxide cooling on trends in the f2-layer ionosphere, j. atmos. solar terr. phys., 71, 1592-1601. rishbeth, h. (1990). a greenhouse effect in the ionosphere?, planet. space sci., 38, 945-948. rishbeth h. (1998). how the thermospheric circulation affects the ionospheric f2 layer, j. atmos. solar terr. phys., 60, 1385-1402. roble, r.g., and r.e. dickinson (1989). how will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere?, geophys. res. lett., 16, 1441-1444. solomon, s.c., l. qian and a.g. burns (2013). the anomalous ionosphere between solar cycles 23 and 24, j. geophys. res., 118, 6524-6535. tapping, k.f., and j.j. valdes (2011). did the sun change its behavior during the decline of cycle 23 and into cycle 24?, sol. phys., 272, 337-350. corresponding author: blas f. de haro, universidad nacional de tucumán, departamento de física, tucumán, argentina; email: blasdeharo2000@yahoo.com.ar. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. venchiarutti et al. 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 031_040 adg v–5 n01.pdf annals of geophysics, vol. 45, n. 1, february 2002 31 the response of the ionosphere to faint and bright solar flares as deduced from global gps network data edward l. afraimovich, alexandre t. altynsev, victor v. grechnev and ludmila a. leonovich institute of solar-terrestrial physics sd, russian academy of sciences, irkutsk, russia abstract results derived from analysing the ionosphere response to faint and bright solar flares are presented. the analysis used technology of a global detection of ionospheric effects from solar flares as developed by the authors, on the basis of phase measurements of the total electron content (tec) in the ionosphere using an international gps network. the essence of the method is that use is made of appropriate filtering and a coherent processing of variations in the tec which is determined from gps data, simultaneously for the entire set of visible gps satellites at all stations used in the analysis. this technique is useful for identifying the ionospheric response to faint solar flares (of x-ray class c) when the variation amplitude of the tec response to separate line-on-sight to gps satellite is comparable to the level of background fluctuations. the dependence of the tec variation response amplitude on the bright flares location on the sun is investigated. 1. introduction the enhancement of x-ray and ultraviolet (uv) emission observed during chromospheric flares on the sun immediately causes an increase in electron density in the ionosphere. these density variations differ for different altitudes and are called sudden ionospheric disturbances (sid) (donnelly, 1969; davies, 1990). sids are generally recorded as the short wave fadeout (swf) (stonehocker, 1970); sudden phase anomaly (spa) (jones, 1971; ohshio, 1971); sudden frequency deviation (sfd) (donnelly, 1971; liu et al., 1996); sudden cosmic noise absorption (scna) (deshpande and mitra, 1972); sudden enhancement/decrease of atmospherics (ses) (sao et al., 1970). much research is devoted to sid studies, among them a number of thorough reviews (mitra, 1974; davies, 1990). sfd are caused by an almost time-coincident increase in electron densities in the eand fregion on the dayside of the earth (davies, 1990; donnelly, 1969; liu et al., 1996). a limitation of this method is the uncertainty in the spatial and altitude localization of the uv flux effect, the inadequate number of paths, and the need to use special-purpose equipment. the effect of solar flares on the ionospheric f-region is also manifested as a sudden increase of total electron content (sitec) which was measured previously using continuously operating vhf radio beacons on geostationary satellites (mendillo et al., 1974b; davies, 1980). a serious limitation of methods based on analyzing vhf signals from geostationary mailing address: p r o f . e d wa r d l . a f r a i m ov i c h , institute of solar-terrestrial physics sd, russian academy of sciences, p.o. box 4026, irkutsk, 664033, russia; e-mail: afra@iszf.ru key words sudden ionospheric disturbances (sid) – solar flare – gps 32 edward l. afraimovich, alexandre t. altynsev, victor v. grechnev and ludmila a. leonovich satellites is their small and ever decreasing (with time) number and the nonuniform distribution in longitude. hence it is impossible to make measurements in some geophysically interesting regions of the globe, especially in high latitudes. a further, highly informative, technique is the method of incoherent scatter (is) (thome and wagner, 1971; mendillo et al., 1974a). however, the practical implementation of the is method requires very sophisticated, expensive equipment. the is method’s time resolution is inadequate for the study of ionospheric effects from solar flares. the time of electron density enhancement in the e and f 1 regions during impulsive flares can be 2-3 min. consequently, none of the above-mentioned existing methods can serve as an effective basis for the radio detection system to provide a continuous, global sid monitoring with adequate space-time resolution. furthermore, the creation of these facilities requires developing special purpose equipment, including powerful radio transmitters contaminating the radio environment. it is also significant that when using the existing methods, the inadequate spatial aperture gives no way of deducing the possible spatial inhomogeneity of the x-ray and uv flux. the advent and evolution of a global positioning system (gps) and also the creation on its basis of widely branched networks of gps stations (at least 900 sites at the august of 2001, the data from which are placed on the internet) opened up a new era in remote ionospheric sensing. high-precision measurements of the tec along the line-of-sight (los) between the receiver on the ground and transmitters on the gps system satellites covering the reception zone are made using two-frequency multichannel receivers of the gps system at almost any point of the globe and at any time simultaneously at two coherently coupled frequencies f1 = 1575.42 mhz and f2 = 1227.60 mhz. the sensitivity of phase measurements in the gps system is sufficient for detecting irregularities with an amplitude of up to 10−3-10−4 of the diurnal tec variation. this makes it possible to detect ionospheric disturbances from different sources of artificial and natural origins. the tec unit (tecu) which is equal to 1016m–2 and is commonly accepted in the literature, will be used throughout the text. afraimovich (2000) and afraimovich et al. (2000b, 2001a,b) developed a novel technology of a global detection of ionospheric effects from solar flares and presented data from first gps measurements of global response of the ionosphere to powerful impulsive flares of september 23, 1998, july 29, 1999, and december 28, 1999, were chosen to illustrate the practical implementation of the proposed method. authors found that fluctuations of tec, obtained by removing the linear trend of tec with a time window of about 5 min, are coherent for all stations and los on the dayside of the earth. the time profile of tec responses is similar to the time behavior of hard x-ray emission variations during flares in the energy range 25-35 kev if the relaxation time of electron density disturbances in the ionosphere of order 50-100 s is introduced. no such effect on the nightside of the earth has been detected yet. the objective of this paper is to use this technology for analysing the ionosphere response to faint and bright solar flares. 2. processing of the data from the gps network following is a brief outline of the global monitoring (detection) technique for solar flares. a physical groundwork for the method is formed by the effect of fast change in electron density in the earth’s ionosphere at the time of a flare simultaneously on the entire sunlit surface. essentially, the method implies using appropriate filtering and a coherent processing of tec variations in the ionosphere simultaneously for the entire set of visible (during a given time interval) gps satellites (as many as 5-10 satellites) at all global gps network stations used in the analysis. in detecting solar flares, the ionospheric response is virtually simultaneous for all stations on the dayside of the globe within the time resolution range of the gps receivers (from 30 s to 0.1 s). therefore, a coherent processing of tec variations implies in this case a simple addition of single tec variations. the detection 33 the response of the ionosphere to faint and bright solar flares as deduced from global gps network data sensitivity is determined by the ability to detect typical signals of the ionospheric response to a solar flare (leading edge duration, period, form, length) at the level of tec background fluctuations. ionospheric irregularities are characterized by a power spectrum, so that background fluctuations will always be distinguished in the frequency range of interest. however, the spatial scale on which the averaging is carried out substantially exceeds the correlation scales of background fluctuations. with a typical length of x-ray bursts and uv emission of solar flares of about 5-10 min, the corresponding ionization irregularity size does normally not exceed 30-50 km; hence the condition of a statistical independence of tec fluctuations at spaced beams is almost always satisfied. therefore, coherent summation of responses to a flare on a set of los spaced throughout the dayside of the globe permits the solar flare effect to be detected even when the response amplitude on partial los is markedly smaller than the noise level (background fluctuations). the proposed procedure of coherent accumulation is essentially equivalent to the operation of coincidence schemes which are extensively used in x-ray and gamma-ray telescopes. if the sid response and background fluctuations, respectively, are considered to be the signal and noise, then as a consequence of a statistical independence of background fluctuations the signal/noise ratio when detecting the flare effect is increased through a coherent processing by at least a factor of , where is the number of los. the gps technology provides the means of estimating tec variations on the basis of phase measurements of tec i in each of the spaced two-frequency gps receivers using the formula (hofmann-wellenhof et al., 1992; calais and minster, 1996) (2.1) where l 1 λ 1 and l 2 λ 2 are the increments of the radio signal phase path caused by the phase delay in the ionosphere (m); l 1 and l 2 stand for the number of complete phase rotations, and λ1 and λ2 are the wavelengths (m) for the frequencies f1 and f 2 respectively; const is some unknown initial phase path (m); and nl is the error in determining the phase path (m). phase measurements in the gps system are made with a high degree of accuracy where the error in tec determination for 30-s averaging intervals does not exceed 1014m–2, although the initial value of tec does remain unknown (hofmann-wellenhof et al., 1992). the data analysis was based on using the stations where the local time during the flare varied from 10 to 17 lt. primary data include series of slant values of tec i (t), as well as the corresponding series of elevations θ (t) and azimuths α (t). these data are calculated using our developed convtec program which converts the gps system standard rinex-files on the internet (gurtner, 1993). the determination of sid characteristics involves selecting continuous series of i (t) measurements of at least a one-hour interval in length, which includes the time of the flare. series of elevations θ (t) and azimuths α (t) of the los are used to determine the coordinates of subionospheric points. in the case under consideration, all results were obtained for elevations θ (t) larger than 30°. the method of coherent summation of time derivatives of the series of variations of the «vertical» tec value was employed in studying the ionospheric response to solar flares. our choice of the time derivative of tec was motivated by the fact this derivative permits us to get rid of a constant component in tec variations; furthermore, it reflects electron density variations that are proportional to the flux of ionizing radiation. the coherent summation of time derivatives of the series of variations of the «vertical» tec value was made by the formula (2.2) where n is the number of los. the correction coefficient k i is required for converting the slant i f f f f l l= − −( ) +[1 40 308 1 2 2 2 1 2 2 2 1 1 2 2. λ λ nl+ + ]const n n sd di t dt ki i i n = × = ∑ ( ) / 1 34 edward l. afraimovich, alexandre t. altynsev, victor v. grechnev and ludmila a. leonovich tec to an equivalent «vertical» value (klobuchar, 1986) (2.3) where r z is earth’s radius; and hmax is the height of the ionospheric f 2 -layer maximum. next the trend determined as a polynomial on a corresponding time interval is removed from the result (normalized to the number of los) of the coherent summation of the time derivatives. after that, the calculated time dependence (sd ex (t)) is integrated in order to obtain the mean integral tec increment ∆i(t) on the time interval specified [t1 − t2]. (2.4) this technique is useful for identifying the ionospheric response to faint solar flares (of xray class c) when the variation amplitude of the tec response to separate los is comparable to the level of background fluctuations. 3. ionospheric response to faint solar flare an example of a processing of the data for a faint solar flare july 29, 1999 (c2.5/ sf, 10:11 ut, s16w11) is given in fig. 1a-e. one hundred los were processed for the analysis of this event. panels (a) and (b) present the typical time dependencies of tec variations for separate los, and their time derivatives. the brus (prn14, thick line) and bahr (prn29, thin line) stations are taken as our example. it is apparent from these dependencies that no response to the flare is distinguished in the tec variations and in their time derivatives for the individual los, because the amplitude of the tec response for the individual los is comparable to the level of background fluctuations. a response to the solar flare is clearly seen in the time dependence (fig. 1c) which is a normalized result of a coherent summation sd k r r h i z z i= +         θcos arcsin cos max ( ) i t sd t dtex t t = ∫( ) ( ) . 1 2 ∆ 29.07.99 x-ray flare c2.5 fig. 1a-e. faint solar flare july 29, 1999 (c2.5/sf, 10:11 ut, s16w11). the typical time dependencies of tec variations for separate los (a) and their time derivatives (b) for stations brus (prn14, thick line) and bahr (prn29, thin line); a normalized result of a coherent summation sd of the time derivatives of the tec variations for all los (c); the same curve (c), upon subtracting the trend determined as a polynomial of degree 3 on the time interval 10:0710:19 ut (d); mean integral increment of tec (thick line) and soft x-ray emission flux (goes-10) in the range 1-8 å (thin line) (e). a b c d e brus prn14 bahr prn29 35 the response of the ionosphere to faint and bright solar flares as deduced from global gps network data of the time derivatives of the tec variations for all los. upon subtracting the trend determined as a polynomial of degree 3 on the time interval 10:07-10:39 ut, the same curve (c) is presented in fig. 1d as sd ex (t). next the calculated time dependence was integrated over the time interval 10:07-10:39 ut to give the mean integral increment of tec (fig. 1e, thick line). a comparison of the resulting dependence with the values of the soft x-ray emission flux (goes-10) in the range 1-8 å (fig. 1e, thin line) reveals that it has a more flattened form, both in its rise and fall. a maximum in x-rays is about 6 min ahead of that in tec. examples of the application of our technology for the analysis of the ionospheric response to faint solar flares are given in figs. 2a-h and 3a-h. figure 2a-h gives the data processing results on tec variations for solar fig. 2a-h. faint solar flare july 29, 1999 (c2.2, 11:00 ut) (a-d); soft x-ray emission flux (goes10) in the range 1-8 å (a); normalized result of a coherent summation sd of the time derivatives of the tec variations for all los (b); the same curve (b), upon subtracting the trend determined as a polynomial of degree 3 on the time interval 10:4811:11 ut; mean integral increment of tec (d). analogous dependencies for faint solar flare july 29, 1999 (c6.2/sn, 15:14 ut, n25e53) on panels (e-h). fig. 3a-h. faint solar flare november 17, 1999 (c7.0/ 1n, 09:38 ut, s15w53) (a-d); soft x-ray emission flux (goes-10) in the range 1-8 å (a); normalized result of a coherent summation sd of the time derivatives of the tec variations for all los (b); the same curve (b), upon subtracting the trend determined as a polynomial of degree 3 on the time interval 09:1809:54 ut (c); mean integral increment of tec (d). analogous de pendencies f or faint solar flar e november 14, 1999 (c5.0, 15:40 ut) on panels (e-h). 29.07.99 x-ray flare c2.2 29.07.99 x-ray flare c6.2 a b c d e f g h 17.11.99 x-ray flare c7.0 14.11.99 x-ray flare c5.0 e f g h a b c d 36 edward l. afraimovich, alexandre t. altynsev, victor v. grechnev and ludmila a. leonovich flares: july 29, 1999 (c2.2, 11:00 ut) on panels a, b, c, d, and july 29, 1999 (c6.1/sn, 15:14 ut, n25e53) on panels e, f, g, h. figure 3a-h shows the results of a data processing of tec variations for solar flares of november 17, 1999 (c7.0/1n, 09:38 ut, s15w53) on panels a, b, c, d, and november 14, 1999 (c5.0, 15:40 ut) on panels e, f, g, h. based on the above examples, it is possible to follow a correlation between the peak power of the faint flares in the x-ray range and the tec enhancement in the ionosphere. for class c2 flares (fig. 1a-e, fig. 2a-d), the respective amplitude of tec response is about twice as small as that for class c5-c7 flares (fig. 3a-h, fig. 2e-h). for c2.2 (fig. 2a-d) and c5 flares (fig. 3e-h), information about their localization on the sun is unavailable. 4. ionospheric response to bright solar flares an example of a processing of the data for the bright solar flare of july 14, 1998 (m4.6/ 1b, 12:59 ut, s23e20) is given in fig. 4a-e. fifty los were used in the analysis of this event. figure 4a presents the time dependencies of hard x-ray emission (cgro/batse, 25-50 kev, thick line on panels a) and of the uv line soho/summer 171 å, thin line) in arbitrary units (aschwanden et al., 1999). it should be noted that the time dependence of the uv 171 å line is more flattened, both in the rise and in the fall, when compared with the hard x-ray emission characteristic. the increase in the uv 171 å line starts by about 1.8 min earlier, and the duration of its disturbance exceeds considerably that of the hard xray emission disturbance. panel (b) presents the typical time dependencies of the tec variations for separate los. the aoml (prn24, thick line) and acsi (prn18, thin line) stations are taken as examples. a response to the bright flare is clearly distinguished for separate los. the normalized sum sd of the time derivatives of the tec variations for all los is presented in fig. 4c; panel (d) plots the same curve (c), upon subtracting the trend determined as a polynomial of degree 3 on the time interval fig. 4a-e. bright solar flare july 14, 1998 (12:59 ut, m4.6/1b, s23e20). time dependencies of hard x-ray emission (cgro/batse, 25-50 kev, thick line) and of the uv line (soho/summer 171 å, thin line) in arbitrary units (a); typical time dependencies of the tec variations for separate los for stations aoml (prn24, thick line) and acsi (prn18, thin line) (b); the normalized sum sd of the time derivatives of the tec variations for all los (c); same curve (c), upon subtracting the trend determined as a polynomial of degree 3 on the time interval 12:48-13:12 ut (d); the mean integral increment of tec (e). a b c d e 14.07.98 x-ray flare m4.6 37 the response of the ionosphere to faint and bright solar flares as deduced from global gps network data 12:48-13:12 ut. next the resulting time dependence was integrated in order to obtain the mean integral increment of tec (fig. 4e). it should be pointed out that the time dependence of the mean integral increment of tec has a more flattened form in the rise than the emission flux characteristics; however, the onset time of its increase coincides with that of hard x-ray emission, and is delayed by about 1.8 min with respect to the uv 171 å line. a total of 23 events was processed (see table i). the class of x-ray flares was from m 2.0 to m 5.7. it was found that the mean tec variation response in the ionosphere depends on the flare location on the sun (central meridian distance, cmd) (fig. 5a). 5. discussion our results are consistent with the findings reported by donnelly (1969, 1971, 1976), donnelly et al. (1986a), where a study of extreme uv (euv) flashes of solar flares observed via sfd was made. in the cited references it was shown that the relative strength of impulsive euv emission from flares decreases with increasing cmd and average peak frequency deviation is also significantly table i. bright solar flares. n data flare max time x-ray/optic δtec lat cmd dd.mm.yy ut class tecu degree 1 14.07.98 12:59 m4.6/1b 0.50 s23 e20 2 19.08.98 12:42 m2.3/sf 0.13 n35 e78 3 14.03.99 01:27 m2.1/1n 0.3 n13 e39 4 04.04.99 15:25 m5.4/1f 0.44 n18 e72 5 08.05.99 14:40 m4.6/1f 0.33 n23 w75 6 29.07.99 19:36 m5.1/1n 0.5 n25 e51 7 19.08.99 16:36 m3.0/2n 0.7 s31 e04 8 27.08.99 13:07 m5.5/2n 0.53 s23 w09 9 14.11.99 16:07 m5.6/0.31 n18 e60 10 22.12.99 19:04 m5.3/1b 0.66 n24 e19 11 28.12.99 00:48 m4.5/2b 0.27 n20 w56 12 10.01.00 13:50 m3.3/2n 0.13 s13 e69 13 23.03.00 12:14 m2.0/sf 0.5 n15 w69 14 26.03.00 17:34 m2.3/sf 0.26 s12 w55 15 08.04.00 02:40 m2.0/sf 0.6 s15 e26 16 10.06.00 16:02 m5.2/3b 0.5 n22 w38 17 23.06.00 04:07 m2.6/2b 0.67 n19 w30 18 12.07.00 18:49 m5.7/2f 0.30 n16 w64 19 14.07.00 13:52 m3.7/1n 0.53 n20 w08 20 16.07.00 02:03 m5.5/1n 0.08 n09 e81 21 18.07.00 14:19 m3.0/2n 0.4 s14 e15 22 20.07.00 20:25 m5.0/1b 0.67 s15 w11 23 19.09.00 08:26 m5.1/1n 0.46 n14 w46 38 edward l. afraimovich, alexandre t. altynsev, victor v. grechnev and ludmila a. leonovich lower for sfds associated with hα flares at large cmd. donnelly (1971) is of the opinion that percentage of hα flares with sfds tends to decrease for large cmd of hα flare location (fig. 5b). similar effects at the center and limb were observed in the ratio of euv flux to the concurrent hard x ray flux (kane and donnelly, 1971). using a fourth-order polynomial to fit the results in fig. 5b with cmd in degrees (donnelly, 1976) gives a(cmd) � 1 − 0.0484 ⋅ cmd + + 0.001426 ⋅ cmd2 − 1.79 ⋅ 10−5 ⋅ cmd3 + + 7.43 ⋅ 10−8cmd4. (5.1) equation (5.1) implies that on the average the fig. 6a,b. average central meridian distance dependence based on (mosher, 1979) for x-ray (riddle, 1969) and (aschwanden et al., 1999) for f10, and (samain, 1979) for the uv (a); modeling results on the sfd occurrence probability at the time of the solar flare as a function of the cmd (dashed lines) (donnelly, 1976), values of the mean amplitude of the tec response in the ionosphere to solar flares (dots), and cos(cmd) fitting curve for solar flares in the x-ray range m 2.0 – m 5.7 (solid lines) (b). flares x-ray m2.0 m5.7 a b fig. 5a,b. cmd dependence of the increment of the mean amplitude of the tec response to solar flares (in the range of x-ray class m 2.0 – m 5.7 (dots), cos(cmd) fitting curve for solar flares in the x-ray range m 2.0 – m 5.7 (solid lines) (a); percentage of hα-flares accompanied by sfd’s as a function of the central meridian distance of the hα flare (b). a b 39 the response of the ionosphere to faint and bright solar flares as deduced from global gps network data impulsive euv emission is more than one order of magnitude weaker for flares near the solar limb than for flares at the central meridian. donnelly (1976) has assumed that it results from the low-lying nature of the 104 k-106k flare source region and from absorption of euv emission in the surrounding cool nonflaring atmosphere. this conclusion is consistent with the findings reported by donnelly and puga (1990). in the cited reference it was found the empirical curves of the average dependence of active region emission on its cmd (fig. 6a) for several wavelengths (donnelly et al., 1986a,b). assume a quiet sun plus one average active region that starts at the center of the backside of the sun and rotates across the center of the solar disk with a 28-day period. figure 6b presents the result of a modeling of the sfd occurrence probability at the time of the solar flare as a function of cmd (dashed lines) in arbitrary units, as well as the values amplitude of the tec response in the ionosphere to solar flares (in the range of x-ray class m 2.0-m 5.7, dots) as a function of cmd. the modeling used eq. (5.1). the figure suggests that the results of our measurements do not contradict the conclusions drawn by donnelly (1976) donnelly et al. (1986b) that the relative strength of impulsive euv emission from flares decreases with increasing cmd. it should be noted that in the case of solar flares whose class is similar in x-ray emission, the dependence under study resembles cos(cmd) rather than a polynomial of degree 4. the fitting cos(cmd) curve for solar x-ray m 2.0-m 5.7 flares is plotted in figs. 6b (solid line) and 5a (solid line). 6. summary this paper suggests a new method for investigating the ionospheric response to faint solar flares (of x-ray class c) when the variation amplitude of the tec response to individual los is comparable to the level of background fluctuations. the dependence of the tec variation response amplitude on the flare location on the sun is investigated. in the case of solar flares whose class is similar in x-ray emission, the dependence under study resembles cos(cmd). the high sensitivity of our method permits us to propose the problem of detecting, in the flare x-ray and euv ranges, emissions of non-solar origins which are the result of supernova explosions. for powerful solar flares it is not necessary to invoke a coherent summation, and the ionospheric response can be investigated for each beam. this opens the way to a detailed study of the sid dependence on a great variety of parameters (latitude, longitude, solar zenith angle, spectral characteristics of the emission flux, etc.). with current increasing solar activity, such studies become highly challenging. in addition to solving traditional problems of estimating parameters of ionization processes in the ionosphere and problems of reconstructing emission parameters, the data obtained through the use of our method can be used to estimate the spatial inhomogeneity of emission fluxes at scales of the earth’s radius. acknowledgements authors are grateful to e.a. kosogorov and o.s. lesuta for preparing the input data. thanks are also due to v.g. mikhalkovsky for his assistance in preparing the english version of the manuscript. this work was done with support under rfbr grant of leading scientific schools of the russian federation no. 00-15-98509 and russian foundation for basic research (grants 99-05-64753, 00-05-72026 and 00-02-16819a), gntp «astronomy». references afraimovich, e.l. 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(1970): advanced telecommunication forecasting technique in agy, 5th., ionospheric forecasting, agard conf. proc., 29, 27-31. thome, g.d and l.s.wagner (1971): electron density enhancements in the e and e regions of the ionosphere during solar flares, j. geophys. res., 76, 6883-6895. annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 1 the surface layer observed by a highresolution sodar at dome c, antarctica stefania argentini1, igor petenko1, angelo viola1, giangiuseppe mastrantonio1, ilaria pietroni1, giampietro casasanta1,*, eric aristidi2, christophe ghenton3 1 istituto di scienze dell'atmosfera e del clima, cnr, rome, italy 2laboratoire lagrange, uns/cnrs/oca, nice cedex 2, france 3 laboratoire de glaciologie et géophysique de l’environnement, ujf-grenoble 1/cnrs, grenoble, france abstract a one-year field experiment started on december 2011 at the french italian station of concordia at dome c, east antarctic plateau. the objective of the experiment was the study of the surface layer turbulent processes under stable/very stable stratifications, and the mechanisms leading to the formation of the warming events. a sodar was improved to achieve the vertical/temporal resolution needed to study these processes. the system, named surface layer sodar (sl-sodar), may operate both in high vertical resolution (low range) and low vertical resolution (high range) modes. sl-sodar observations were complemented with in situ turbulence and radiation measurements. a few preliminary results, concerning the standard summer diurnal cycle, a summer warming event, and unusually high frequency boundary layer atmospheric gravity waves are presented. i. introduction t dome c, light wind and clear sky favor weak turbulence and mixing, and strong temperature gradients near the surface. the boundary layer height varies depending on the relative contribution of the mechanical and thermal generation of turbulence. because of the extremely low temperature and humidity, and the high elevation, dome c is a potentially ideal site for astronomical observations. for this reason, the optical turbulence over the antarctic plateau has been a subject of studies by astronomers [lawrence et al. 2004, aristidi et al. 2005, agabi et al. 2006, lascaux et al. 2009]. at dome c, ricaud et al. [2012] made an experiment to monitor the vertical evolution of the planetary boundary layer (pbl) temperature and humidity in the transition from winter to summer, by using a microwave radiometer operating at 60 ghz and 183 ghz. due to the instrument low vertical resolution, the fine structure of the thermal turbulence could not be evidenced. * corresponding author: giampietro casasanta, g.casasanta@isac.cnr.it a annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 2 ghenton et al. [2010] analyzed the dome c 45 m meteo tower measurements (temperature, humidity, wind speed and direction measurements) for a three-week period in summer 2008. the main task of their work was to compare these measurements with the 6hourly european center for medium-range forecasts (ecmwf) analyses and the daily radiosoundings. pietroni et al. [2012], using the temperature profiles measured with a passive microwave radiometer [kadygrov and pick 1998, argentini et al. 2004], characterized the behavior of the surface-based temperature inversions over the course of a year. they found that during the winter and the summer “nights” strong temperature inversions allow for a mixing depth of a few tens of meters with a quiescent layer above, decoupled from the surface layer. during the summer, despite the low surface temperatures, weak convection generates the development of a mixed layer characterized by a maximum depth of 200-400 m [argentini et al. 2005]. the diurnal behavior of this mixed layer, monitored with a sodar, was described by mastrantonio et al. [1999], argentini et al. [2005], and king et al. [2006]. the sodar measurements, because of the membrane ringing just after the tone burst emission, allowed for a first echo recording starting at 20-30 m, depending on the membrane ringing time. because of this limitation, those measurements did not allow to study the surface turbulent layer under stable conditions, neither in summer nor in winter. an advanced high-resolution sodar named surface-layer sodar (hereafter sl-sodar), allowing for the lowest observation height at ≈ 2 m and a vertical resolution of ≈ 2 m, was developed by the isac-cnr [argentini et al. 2011]. the sl-sodar was deployed at concordia station after a preliminary test period at the isac-cnr research centre of rome [argentini et al. 2011]. in this paper, a few preliminary results from the summer season are shown. ii. site and instrumentation concordia is a permanent station located at dome c (75.1° s, 123.3° e, 3233 m a.s.l.), on the east antarctic plateau, at approximately 1000 km from the nearest coast. one-year in situ turbulence and radiation measurements, as well as sl-sodar observations, were carried out at concordia station from december 2011 up to december 2012. table 1. sl-sodar setting parameters. mode 1 mode 2 carrier frequency 2000 hz 4850 hz pulse duration 50 ms 10 ms repetition rate 3 s 2 s maximum range 430 m 280 m lowest height 8 m 2 m vertical resolution 8 m 2 m annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 3 the sl-sodar [argentini et al. 2011] is an improved version of the sodar described by mastrantonio et al. [1999] and argentini and pietroni [2010], with the possibility of zooming in the atmospheric surface-layer thermal turbulent structure. the sl-sodar consists of 3 horn-type antennas, placed symmetrically around a 1.2 m diameter parabolic receiving antenna, emitting simultaneously acoustic pulses at the same frequency. the receiving antenna is noise-protected by a shielding structure of 1.5 m l x 1.5 m w x 2.0 m h in size. the transmitting and receiving circuits are kept separated to minimize the “cross-talk” between channels. the carrier frequency, the pulse duration, and the pulse repetition rate figure 1. sodargram for 28 december 2011 with mixing height estimate (straight line) (a), downwelling longwave radiation (𝐿𝑊 ↓ ) and sonic temperature ( 𝑇𝑠) (b), wind speed and direction (c), momentum (𝑢∗) and heat fluxes (𝐻0) (d). annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 4 can change according to the two modes listed in table 1. “mode 1” allows to monitor the convective mixed layer, while “mode 2”, with higher vertical resolution, is used to investigate the near-surface stable layer. measurements of turbulence were made with a metek usa-1, a three-axes sonic thermoanemometer (sampling frequency of 10 hz) installed on a 3.5 m mast. the heat and momentum fluxes are estimated using the eddy covariance method [lee et al. 2004]. the longwave and shortwave radiation components (up and down) are measured using kipp & zonen cnr1 pyrgeometers and pyranometers, installed at 1.5 m above the snow surface. in this paper, unless told otherwise, the local standard time (lst) is used. iii. results summertime abl diurnal behavior the facsimile recording of the vertical/temporal variation of the acoustic backscattering (sodargram) “depicts” the thermal structure of the atmosphere. the scattering elements producing the change of echo intensity are the small-scale temperature inhomogeneities due to thermal turbulence. temperature fluctuations are usually associated with the convective plumes originating from the surface, or with potential temperature gradients and wind shear usually occurring in the inversion layers. during the summer, the boundary layer at dome c can reach a depth of 200-400 m. therefore, the “mode 1” setting (see table 1) was used to catch the whole vertical evolution during the daily cycle. figure 1a shows the typical boundary layer sodargram during a clear summer day (28 december 2011), with the superimposed estimate of the mixing height (mh, straight line). a stable boundary layer occurs between 0000 and 0900 lst; from 0900 to 1630 lst the mh increases because of the convective activity, and then it drops to 50 m because of the surface radiative cooling. the mh was estimated using a method originally proposed by beyrich and weill [1993], which uses the backscattered range corrected signal (rcs). under stable nocturnal stratification, the mh was determined either from the minimum of the first derivative or from the maximum curvature of the rcs, depending on the stage of the planetary boundary layer evolution, and on the shape of the sodar profile. under convective conditions, the mh was es figure 2. sodargram for 5 february 2012, the full dots represent the mixing height estimate. high amplitude waves are observed between 0800–0930 lst. annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 5 timated as the height at which an elevated secondary maximum occurs (i.e. the height of the turbulent zone characterizing the top of the mixing layer). for the same day, figure 1b shows the downward longwave radiation 𝐿𝑊 ↓ and the sonic temperature 𝑇𝑠, figure 1c the wind speed and direction. the heat turbulent flux 𝐻0 and friction velocity 𝑢∗ are plotted in figure 1d. 𝐿𝑊 ↓ ranges between 100 and 150 w m-2, while 𝑇𝑠 reaches its minimum (35°c) at 0300 lst, and its maximum (-25°c) between 1200 and 1500 lst. the direction indicates a wind from the continent persisting the whole day. the maximum wind speed (6 ms-1) occurs because of the momentum transfer from the free atmosphere to the surface layer, as a consequence of the turbulent mixing (confirmed by the positive and increasing values for 𝐻0 and 𝑢∗) during convective hours. gravity waves in the abl between 2 and 5 february 2012, waves with periods of a few minutes are observed under stable stratification for more than 35% of the time. the resolution achieved by the sl-sodar with the “mode2” setting (table 1) allowed to visualize the fine structure of these wave patterns. at the transition time from the stable to the unstable boundary layer (between 0800 and 1000 lst) the capping inversion layer oscillates with an amplitude that reaches 70 m (figure 2). the apparent period of these oscillating structures was estimated through the spectral analysis of the sonic temperature and the wind components of the sonic anemometers, installed at 3 different levels (7.0, 22.8, and 37.5 m) on a 45-m meteorological tower [genthon et al. 2009] located at ≈ 1 km from the slsodar. in figures 3a and 3b, the sodargram and the power spectral density of the temperature and horizontal wind components measured at 37.5 m (between 0945 and 1000 lst on 5 february) are shown. a peak occurs simultaneously in the temperature and wind components power spectral density (psd) at 0.0098 hz, corresponding to a period of 102 s. the analysis of the sonic anemometers measurements at the three levels, limited to the time interval 0800 figure 3. sodar echogram for february 5, 2012 (0930 to 1000 lst) (a); power spectral density (psd) of the horizontal wind components (ux, uy) and sonic temperature (𝑇𝑠) at 39 m (b) for the indicated 10minutes interval (the scale is given in arbitrary units). annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 6 0930 lst, gives an apparent period ranging between 90 and 120 s. the apparent period remains approximately the same also when the inversion strength and height change. this behavior indicates that the origin of these waves might be a disturbance (probably the wind shear) originating between the inversion layer and the free atmosphere. a similar behavior was observed during other days. a summer warming event warming events of particular intensity were regularly observed at dome c during the winter [argentini et al. 2001, petenko et al. 2007, ghenton et al. 2013]. during these events the surface temperature sometimes has a sharp increase of 20-40 °c [argentini et al. 2001], reaching then the typical summer values. studies carried out at south pole [carroll 1982, stone et al. 1990, stone and kahl 1991, stone 1993] have evidenced that these warming events are generally observed in presence of clouds. neff [1999], analyzing the particles trajectories across antarctica, found that these warming processes are mostly due to warm and moist air intrusion and to the condensation of nuclei originating from the weddell sea, producing a wide variety of cloud types. carroll [1982] suggested two possible mechanisms of this phenomenon: the advection of warm air, and/or the vertical mixing of air from different layers. schwerdtfeger and weller [1977] related the surface warming to the variation of long-wave radiation emitted by the clouds associated to the moist air in the upper part of the atmosphere. figure 4. downwelling longwave radiation (𝐿𝑊 ↓) and sonic temperature (𝑇𝑠) with superimposed the linear trends (a), the wind speed and direction (b), the heat (𝐻0) and momentum (𝑢∗) fluxes (c) for days 814 january 2012. annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 7 the measurements collected during a summer warming event observed between 8 and 17 january 2012 have been analyzed. figure 4 shows the time series of the downwelling longwave radiation 𝐿𝑊 ↓ and the sonic temperature 𝑇𝑠 (figure 4a), the wind speed and direction (figure 4b), the heat flux 𝐻0 and the friction velocity 𝑢∗ (figure 4c) during the selected period. starting from 9 january 2012, wet and warm coastal air masses are advected from the coast toward the dome c area. until the end of january 10 the 𝐿𝑊 ↓ and 𝑇𝑠 (figure 4a) do not show the typical diurnal behavior observed during the previous and the following days. an increasing trend is evident starting on 9 january. the wind direction changes, rotating from s to ne-nw, and the wind speed is low up to 1200 lst of 10 january. due to the presence of the clouds, the downwelling long wave radiation and the surface temperature increase, initiating the convective activity shown in the sodargram of figure 5. the vertical mixing reduces the decoupling between the boundary layer and the free atmosphere. as a consequence, the wind speed increases: at 2400 lst of 11 january the wind speed is ≈ 6 m s-1 (figure 4b). 𝐻0 and 𝑢∗ (figure 4c) confirm this anomalous behavior. at the end of 11 january figure 5. sodargram for 10 january 2012 with the mixing height estimate (dots). annals of geophysics, 56, 5, 2013; 10.4401/ag-6347 8 the behavior is again the typical summer one, with a peak in the wind speed at 1200 lst. the sodargram for 10 january, with the mixing height estimate superimposed (dots), confirms this hypothesis. a clear and intense convective activity is observed during the whole day even in the nighttime between 9 and 10 january. iv. summary the main results of this observational study can be summarized as follows: • during the summer, under steady weather conditions, the atmospheric boundary layer thermal structure is characterized by the alternation of a stable stratified layer with a convective boundary layer, following a behavior similar to that observed at mid-latitudes. • a regular wave activity was observed within the inversion layer. the time period of these waves ranges between 90 s and 120 s, and their origin can be attributed to the wind shear across the inversion layer. • the summer warming events take origin from the presence of clouds advected from the coast toward the dome c area. clouds modify the surface radiation budget by increasing the downward longwave radiation, which in turn produces an increase of surface temperature, leading to convection. due to the decrease of the temperature inversion strength, the vertical mixing, combined with the wind shear, allows the transport of warm air from the upper parts of the atmosphere towards the surface, further contributing to the surface warming. acknowledgements. this research is supported by the italian national research programme (pnra) in the frame of french-italian projects for dome c. the authors thank mr. a. conidi, mr. n. ferrara, and mr. s. ciampichetti for the technical support provided in the realization of the sl-sodar antennas, and the logistics staff of the concordia station during the field work references [agabi et al. 2009] agabi, a., e. aristidi, m. azouit, e. fossat, f. martin, t. sadibekova, j. vernin, and a. ziad (2006). first whole atmosphere nighttime seeing measurements at dome c, antarctica, publ. astron. soc. pac., 118, 840, 344-348. 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(1993). properties of austral winter clouds derived from radiometric profiles at the south pole, j. geophys. res., 98, 12961-1297. the surface layer observed by a high-resolution sodar at dome c, antarctica stefania argentini1, igor petenko1, angelo viola1, giangiuseppe mastrantonio1, ilaria pietroni1, giampietro casasanta1,*, eric aristidi2, christophe ghenton3 1 istituto di scienze dell'atmosfera e del clima, cnr, rome, italy 2laboratoire lagrange, uns/cnrs/oca, nice cedex 2, france 3 laboratoire de glaciologie et géophysique de l’environnement, ujf-grenoble 1/cnrs, grenoble, france abstract table 1. sl-sodar setting parameters. 801 annals of geophysics, vol. 51, n. 5/6, october/december 2008 key words electric properties – complex conductivity – frequency – hematite – water saturation 1. introduction electrical spectra of porous rocks in the low frequency range reflect numerous polarization processes resulting mainly from rock heterogeneity. the first is the maxwell-wagner polarization (mendelson and cohen, 1982) due to differences in the electric bulk properties of rock components. maxwell-wagner theory allows the prediction of electrical spectra of mixtures from bulk partial volumes, the properties of their components and their microstructure. however, the maxwell-wagner theory was unable to explain the anomalously large polarization in water-bearing rocks at low frequencies. the problem is that the mixture theory does not take into account surface conductivity and polarization, as well as clustering of components in disordered mixtures. the surface conductivity (in s) of a mineral grain is defined as the anomalous conductivity existing in the close vicinity of the mineral surface and integrated over the electrical double-layer thickness (leroy and revil, 2004). surface conductivity is frequency dependent. surface conductivity has two contributions; one is associated with the stern layer and one with the diffuse layer in which counter-ion density obeys a boltzmann distribution. none of these contributions can be neglected but the stern layer contribution dominates. this paper studies the a. c. electrical conductivity and dielectric constant of a partially and fully saturated hematitic sandstone sample (aswan area, egypt) in the frequency range from 0.1 hz to 100 khz. this geological materelation between electric properties and water saturation for hematitic sandstone with frequency mohamed mahmoud gomaa national research centre, geophysical sciences dep., el-tahrir st. dokki, 12311, egypt abstract this paper focuses on the effect of water saturation on a. c. electrical conductivity and dielectric constant of fully and partially saturated hematitic sandstone sample (aswan area, egypt). the saturation of the sample was changed from partial to full saturation. complex resistivity measurements at room temperature (~16°c), were performed in the frequency range from 0.1 hz to 100 khz. experimental electrical spectra indicate, generally, that the electrical conductivity and dielectric constant vary strongly with water saturations and frequency. the low frequency electrical conductivity and dielectric constant are mainly controlled by surface conduction and polarization of the electrical double layer. the behaviour of the electrical conductivity and dielectric constant, with increasing water content, were argued to the orientational polarization of bound water for very low saturations, displacement of the excess surface charges for relatively low saturations, and free exchange of excess ions in double layer with the bulk electrolyte and generation of transient diffusion potentials which lag behind the applied field for high saturations. mailing address: dr. mohamed mahmoud gomaa, national research centre, geophysical sciences dep., eltahrir st. dokki, 12311, egypt.; e-mail: mmmsgomaa@ yahoo.com 802 m. m. gomaa 1) adsorption water that can be adsorbed by soil particle from the air. it consists of a monomolecular layer around negatively charged mineral surfaces and additional tightly and loosely bound adsorption water layers (mitchell, 1992). 2) capillary water: it is the moisture in soil which is not bound around grains, but does not respond to gravity. 3) free water: is attracted to the soil solids so loosely that it may respond to the pull of gravity and move downwards. the addition of a small amount of water causes dramatic increases in the values of dielectric constant and conductivity. the adsorbed liquid forms at first monoor polylayer islands. as saturation increases, continuous fluid films are formed on the grain surface. if water molecules form strong bonds with the surface so that they cannot migrate in an electrical field, the most probable polarization mechanism is the orientation polarization of adsorbed water molecules or reversible hopping of h ions along the surface (chelidze and guéguen, 1999; chelidze et al., 1999). 2. background on polarization phenomena of porous media the physical mechanism of the polarization of adsorbed layers can be either the orientational polarization of dipole molecules of water or the transfer of charges (ions in the helmoholtz or diffuse double layer or protons) along hydroxilic groups or chains, so-called surface charge transfer complexes (chelidze and guéguen, 1999; chelidze et al., 1999). pride (1994) referred to the mechanism of surface proton conduction, and he concluded that the hydrogen ions may efficiently conduct through the network of hydrogen-bonded water molecules present in the adsorbed layer, but he ignored this mechanism in his analysis. the mechanism of polarization due to formation of a physically adsorbed multilayer of water is much more effective than the orientational polarization of dipoles and results in high experimental values of dielectric constant; which are larger than the largest dielectric conrial is an example of a multi-component system, a sand/hematite/water/air system. the concentration of the sample is mainly sand (~60%) with hematite (~40 %). the grain size of sand in the sample is in the order of ~60 micrometer and the hematite is in the form of pigment. the porosity of the sample was calculated to be (~23%). the aim of this work is to study the effect of water on the electrical conductivity and dielectric constant of the sample and to try to find out the main conduction mechanisms that are responsible for the measured behaviour. the sample was chosen as an example of the composite material that does not have a chemical reaction at the surface like clay, so the only supposed response may be argued only due to the addition of water. the study of the electrical properties of the sample may be of good importance in the application of spectral induced polarization in the field and in the interpretation of dielectric and conductivity spectroscopic measurements (a. c. electrical conductivity and dielectric constant). the components of the wet hematitic sandstone interact strongly to affect the electrical behavior of the mixture. dry rocks without a metallic component at room temperatures are good dielectrics with conductivity of the order of 10-10 s/m and a relative dielectric constant of the order of several units. for partially and fully saturated rocks the water-rock interfaces give rise to large values of dielectric constant and strong frequency dependence (chelidze and guéguen, 1999; chelidze et al., 1999) which can be explained by surface conduction and polarization processes. in the absence of an applied electric field, ions form a double layer at the water-rock interface. when an oscillating electric field is applied, the ions polarize around the rock grains giving rise to large dipoles and to large apparent dielectric constants (chelidze and guéguen, 1999; gomaa, 2004). as the frequency increases, ions have less time to travel to polarize and contribute more to the conductivity, since they are more in phase with the applied oscillating electric field. saarenketo (1998) suggests that water in soils can be classified according to its electrical properties as: 803 relation between electric properties and water saturation for hematitic sandstone with frequency around the particles by an electric field. the displacement of counter-ions in the double layer is shown to be equivalent to the existence of a surface capacitance displaying a diffusion controlled relaxation. this effect can be expressed by an additional apparent dielectric constant of the particles, exceeding their actual dielectric constant at low frequencies by many orders of magnitude. a distribution of the activation energies of the counter-ion particle movement on the particle surface is introduced to account for the relaxation spectrum of the dispersion curves. chelidze et al. (chelidze and guéguen, 1999; chelidze et al., 1999) discussed the «gigantic low frequency polarization» theory (glfp) caused by polarization of the electrical double layer (edl) at the solid–liquid interface. the glfp theory allows for a free exchange of edl excess charges with the electrolyte (polarization). application of an electric field distorts the edl due to migration and diffusion of charges in it, which results in the appearance of local clouds with the excess of positive charges on one side of the particle and with their deficit on the other side within the bulk electrolyte, so concentration gradients near the particle appear, outside the edl, in addition to the electric polarization field of the deformed edl. however, steady-state polarization due to the induced concentration gradients is established controlled by diffusion over distances of the order of the linear dimension of the particle. the conduction currents lag behind the applied field, which appear as displacement currents and seen experimentally as high values of the dielectric constant. this results due to the large electric charge of a particle, and the large electrolyte conductivity. chew and sen (1982) studied the dielectric constant and conductivity of a dilute immobile ensemble immersed in an electrolytic solution. they concluded that the existence of double layer greatly enhances the counter-ion current in the double layer region in the presence of an external field. this counter ion current piles up charges at the polar ends of the particle, which discharge into the bulk solution through a diffusion process in addition to a conduction process. the diffusion current in the diffusion cloud stant of the mixture components (here hematite, sand, water or air). at low humidity, defect sites on grain surface present a high local charge density and a strong electrostatic field, that promote water dissociation providing protons as charge carriers of the hopping transport mechanism. subsequent layers of water molecules are physically adsorbed. charge transport occurs when h 3o[ ] + releases a proton to a neighboring water molecule which accepts it while releasing another proton, and so forth (grotthuss chain) (garrouch, 2001; garrouch and sharma, 1994; garcia-belmonte et al., 2003). at high humidity, liquid water condenses in the pores, and electrolytic conduction takes place along with protonic transport in the adsorbed layers. the contribution of surface processes to the ohmic conductivity of porous systems is considered by (glover et al., 1994a; pride, 1994; dukhin, 1971; glover et al., 1994b). also, modeling of surface conductivity from the modeling of the electrical double layer combined with complexation of the surface sites is due to (revil and glover, 1998) for silica and leroy and revil (2004) for alumino-silicates. revil and glover (1998) give a value of the specific surface conduction associated with proton transfer in pure water to be 2.4 × 10-9 s and examined the dependence of the surface conduction on salinity. surface migration and diffusion evoke polarization mechanisms, which are often much stronger than the mixture component polarization (chelidze and guéguen, 1999; chelidze et al., 1999). revil and glover (1998) concluded that the contribution of the electrical diffuse layer to the surface conductivity can be neglected for silica and alumino-silicates, and that the main contribution to surface conductivity is probably due to counterions located in the stern layer with surface mobilities approximately ten times smaller than in the bulk electrolyte. another contribution is related to the proton transfer directly on the mineral surface and is salinity independent. schwarz (1962) proposed that at low frequencies the dielectric dispersion (for colloidal particles suspended in electrolytes) is due to the polarization of the counterion atmosphere 804 m. m. gomaa been shown in studies by knight (knight and nur, 1987; knight and endres (1990). in these studies the real part of the dielectric constant of sandstone samples was measured as the level of water saturation was varied. the frequency range of the measurements was from 60 khz to 4 mhz. there was a general increase in the electrical conductivity and dielectric constant of the rock sample with the increase of water. they showed that there may be a critical saturation point for water wet samples, below which the conductivity increases gradually with saturation and above it the conductivity increase more slowly. they observed a power-law dependence of the dielectric constant (e) on the frequency (w). they related the power law exponent to the water saturation and the surface area to volume ratio of grains. the power law response is thought to be due to the random nature of the constituents within the samples (jonscher, 1999; gomaa et al., 2000). chelidze et al. (chelidze and guéguen, 1999; chelidze et al., 1999) gave an explanation for the large low-frequency polarization of the fully saturated rocks that it takes into account the heterogeneity in the distribution of the electrical charges on the mineral surfaces. in this model the relaxation time depends on the sizes of clusters of charges on the surfaces of minerals rather than on the grain size of particles. 3. experimental procedures laboratory measurements were made on a thin disk-shaped hematitic sandstone sample with the following dimensions: thickness 6 mm and diameter 37 mm. a two electrode technique was used with stainless electrodes (of agilant dielectric test fixture 16451b) on the two opposite faces of the sample disk. data were collected in the frequency range from 0.1 hz up to 100 khz using a hioki 352250 lcr hitester impedance analyzer. a voltage of 1v was applied. the current density in the sample was nearly 1×10-3 (microa/cm2) at a frequency of 10 hz for the saturated case and nearly 4×10-6 (microa/cm2) at the same frequency for the dry case. is out of phase with the applied field and has a pack up effect in the double layer causing the current to be out of phase. hence the charges piled up at the ends of the sphere have an out of phase dipole moment. at high frequencies there is no build up of a large diffusion cloud outside the double layer. wong (1979) idealized a disseminated ore by a system of electronically conducting metallic spheres randomly dispersed in an electrolytic ally conducting host medium. with an applied electric field the transport of cations and anions in the inter-phase region near the metal-electrolyte interface will involve both drift and diffusion flux densities. the presence of excess or deficit makes ions to accumulate, which are loosely held to the surface. the cloud of ions surrounding each particle and the diffusion-controlled charge transfer reaction at the interface are responsible for inducing a time or frequency dependent electric dipole moment on each particle. mehaute and crepy (1983) and (glover et al., 1994a; wong, 1987; ruffet et al., 1991a, 1991b) considered the geometric effects resulting from the existence of fractal surfaces. they assumed that the internal surfaces of rocks are rough and examined how the charge layer is built up at the interface. when an electric field is applied along the y-axis normal to the surface, ions have to migrate in this direction. as minerals are mainly dielectrics, ions do not penetrate into them and have to migrate along the rough surface in the x-direction in order to progress further in the y-direction. the main idea is that the exponent a of the cole and cole (1941) response function is closely related to the fractal dimension df of the surface of the solid or, in other words, the distribution of relaxation times corresponds to the distribution of asperities on a rough surface. for these models, the frequency dependent exponent reflects the distribution of relaxation times, which in turn is connected with ion fluxes passing obstacles of various sizes on the rough surface. the experimental data of (ruffet et al., 1991a, 1991b) has been made on the basis of the fractal surface model (mehaute and crepy, 1983; wong, 1987). the effect of rock/water interaction on the dielectric behavior of saturated sandstones has 805 relation between electric properties and water saturation for hematitic sandstone with frequency ws% = (sww-swd)/(sws-swd)×100 the saturation level by volume is given by wv% = [(sww-swd) × wvd × 100]/(sww) where the ws%= relative water saturation percentage by weight, sww = sample weight wet, swd = sample weight dry, sws = is the sample weight saturated, wv% = water volume percentage, and wvd = wet volume density. 4. interpretation and discussion of results the conductivity and dielectric constant increase with the increase of relative saturation level. figures 1 and 2 show the variation of the dielectric constant and conductivity, respectively, with frequency at different levels of relative saturation. if the sample is dry it is supposed that there is no water and the pores are completely filled with air. as the saturation increases the adsorbed water forms the first monolayer around the grain surfaces and as the saturation increases gradually the adsorbed water tends to make another layer and so on until it makes multi-layers of water. the first monolayers are supposed to be bound to the surface of the grains and the others are supposed to be looser. the properties of adsorbed water differ significantly from those of the bulk liquid. this means that the properties of adsorbed molecules are intermediate between bulk water and ice (chelidze and guéguen, 1999; chelidze et al., 1999; knight and nur, 1987; knight and endres, 1990; hoekstra and doyle, 1971). as the water saturation increases the adsorbed water coats completely the surfaces of the grains and begins to contact with each other forming islands of water between grains but those islands are still isolated from each other. the inner layers of water molecules form strong bonds with the surface so that they cannot migrate in an electrical field, and the most probable polarization mechanism is the orientation polarization of adsorbed water molecules or reversible hopping of h ions along the surface (chelidze and guéguen, 1999; chelidze et al., 1999; glover et al., 1994a). as saturation inthe measured parameters are the series and parallel capacitance and the series and parallel resistance at different frequencies. measurement of the electrical properties of a material can be made in either the series or parallel mode. in the series mode, the complex impedance z is measured, z = rs - ixs, where rs is the series resistance (real impedance) and xs = 1/wcs is the reactance, cs is the series capacitance. the complex resistivity is ρ* = z × (a/d), where a is the cross-sectional area of the sample, d is its thickness. the parameters were calculated from the following equations, rez = rs, imz = 1/wcs. for the parallel model σ = 1/ρ, ρ = rp × (a/d). eʹ = cp/co, co = (a/d)e0, where w is angular frequency, rp is the parallel resistance, cp is the parallel capacitance, e0 is the permittivity of free space (8.85 × 10-12 f/m), gp is the parallel conductance, and cp is the parallel capacitance. the effect of water saturation on electrical conductivity is the most important parameter in this study. first, the sample was measured in the relative atmospheric humidity (~ 65%) in an isolated chamber (desiccators). the sample weight dry was 14.1 gm. the sample was fully saturated by initially evacuating it in a pressure vessel, then allowing distilled water to flow into the vessel. the saturated weight was 15.45 gm. measurements on the fully saturated sample were made quickly after it was removed from the pressure vessel. subsequent measurements, with the determination of weight, were made while the sample was let to dry. the bulk volume porosity of the interconnected pore volume of the vp sample was calculated as following: 1) the rock sample is weighed dry (w1); 2) the sample is then saturated with distilled water for 24 hours by using a vacuum pump; and 3) the completely saturated sample is weighed again (w2) and the following equation is then applied: vp = (w2 w1)/σf where; σf is the fluid density in g/cm3; σf = 1.0 g/cm3 for the distilled water. the uncertainty associated with this method is of the order of ± 5%. the relative saturation levels (in weight) were calculated as 806 m. m. gomaa nearly there is only one region. the high frequency region (region 2) has low values of the dielectric constant and nearly has no dispersion in dielectric constant for low saturations (below 11%), while there is a slight dispersion for the dielectric constant with a very minute slope (≈ - 0.1) for high saturations (above 11%). at low saturations (mainly region 2) the physical mechanism of the polarization in adsorbed layers can be either the orientational polarization of dipole molecules of water, which may be effective at higher frequencies, or the transfer of charges (ions in the helmoholtz or diffuse double layer or protons) along hydroxilic groups or chains, so-called surface charge transfer complexes (chelidze and guéguen, 1999; chelidze et al., 1999) or surface polarization in the stern layer (schwan et al., 1962; schwarz, 1962). the gradual increase in the dielectric constant with the increase of humidity may be attributed to the decrease of the air space distances between the wet grains that behave as capacitors (for low saturations). the capacitance gradually increases as the humidity increases, the electrodes become connected with continuous fluid film. figure 1 shows the variation of the dielectric constant with frequency at different levels of humidity and saturations. values of the dielectric constant at low frequency (0.1 hz) vary from 103 for low saturated hematitic sandstone to 2 × 106 for full saturation, while the values at high frequency (105 hz) range from 30 for low saturated hematitic sandstone to 200 for full saturation (nearly the order of water dielectric constant). the dielectric constant dispersion (fig. 1) can be generally divided into two regions with power law dependencies on frequency e∝f -a with different exponents (slopes on the curves), which are not clearly separated at a certain frequency. in the first region at low frequencies the slope is steep (≈ - 0.67) for high saturation while, for relatively lower saturations it reaches a slope of value ≈ - 0.5 for low saturations. that region ends at nearly 2 hz for ~ 1% saturations, and extends for higher saturations to 20 khz (for 50% saturation), while it takes the whole frequency range for higher saturations. for the fully saturated sample fig. 1. the variation of the dielectric constant with frequency at different levels of relative saturations (in volume). 4% 0, 8% 0, 11% 0, 14% 0, 18% 0, 25% 0, 32% 0, 40% 0, 50% 0, 55% 0, 96% 0, 100% 0, respectively. there are many symbols that are not present. fig. 2. the variation of the conductivity with frequency at different levels of relative saturations (in volume). 4% 0, 8% 0, 11% 0, 14% 0, 18% 0, 25% 0, 32% 0, 40% 0, 50% 0, 55% 0, 96% 0, 100% 0, respectively. there are many symbols that are not present. 807 relation between electric properties and water saturation for hematitic sandstone with frequency (0.1 hz) vary from 4 × 10-9 sm-1 for low saturated hematitic sandstone to 3 × 10-5 sm-1 for full saturation, while values for high frequency (105 hz) ranges from 6 × 10-6 sm-1 for low saturated hematitic sandstone to 2 × 10-3 sm-1 for fully saturation (near the distilled water conductivity). the first region is characterized by the presence of continuous paths of water and hematite between the electrodes (above about 10% saturation), thus causing nearly constant conductivity with frequency. below this critical saturation (region 2), the continuous paths of water between the grains increase with increasing water content, thus the conductivity increases. the low frequency conductivity (fig. 2) and dielectric constant change more than five decades with the change of saturation. it should be noted that these curves are not completely identical in behaviour to other samples of the same material. unlike curves may be attributed to the variation of the texture from sample to another (parkhomenko, 1967; efros and shklovskii, 1976; knight, 1983; olhoeft, 1985; sen, 1989; levitskaya and sternberg, 2000). the equivalent circuit shown in fig. 3 is assumed to represent the sample in the humid case. the conducting grains are completely insulated (blocked) from each other by air. here, cg and r∞ is the capacitance and resistance of the sample at high frequencies, respectively, where no space charges accumulate. ci is the interfacial capacitance (frequency independent) and ri is the interfacial resistance (macdonald, 1974). the resistance decreases as the frequency increases, at frequencies lower than the radio frequency range, where space charges are accumulated at interfaces. when continuous paths are formed (due to water and/or hematite) in the sample, the impedance behavior is represented by the equivalent circuit shown in fig. 4. here rd represents a direct frequency-independent conducting path between the two electrodes which is effective in the low frequency range (gomaa, 1996). the additional resistor rd is equivalent to the formed percolation or continuous paths (due to water or hematite) in the sample. creases. when the adsorbed water begins to form a continuous path between the electrodes additional mechanisms are involved to contribute to increase the dielectric constant, namely the gigantic low frequency polarization described in the introduction (chelidze et al., 1999; chelidze, guéguen, ruffet, 1999). the continuous increase of the dielectric constant with decreasing the frequency may be attributed to the presence of a distribution of relaxation times due to the presence of polarization centers of different dimensions on the grains surfaces. the frequency separating regions 1 and 2 increases with increasing the water saturation above the critical value (wilkinson et al., 1983). such critical relative water saturation is low (about 15%) for water coating grains (dukhin, 1971). charges either diffuse along the humidity coating the grains if it is unable to leave the surface, or diffuse through the bulk of the specimen. the data in the region of saturation is a result of the rock/water interaction. charge transport can occur either through the bulk of the solid matrix (hematite) or along the grain boundaries of solid aggregates (water). when soil minerals are exposed to water, exchangeable ions go into solution, forming an ionic halo around the particles. most of the ionic or covalent bonded rock forming minerals such as quartz, mica, and feldspars are nonconductors. when the surfaces of these minerals come into contact with liquid water, electrolytes are formed and ionic drift associated with the electrical field causes electrical conduction (knight and nur, 1987). in the conductivity curves (fig. 2), two nearly corresponding regions may be generally found with power law variation of the conductivity with frequency, which are also clearly identified with two regions with different slopes. in the first region at low frequencies, for low saturations, there is slight dispersion for the conductivity. that region ends at nearly 100 hz for saturations below 10 % and extends for higher saturations to 20 khz (for 25 % saturation), while it takes the whole frequency range for higher saturations. the second region has large slope (~ 0.9) for low saturation (below 10 %). values of the conductivity at low frequency 808 m. m. gomaa creases with the increase of the saturation, i.e. the conductivity increase (grant, 1958). such dominance of the real resistivity over the imaginary resistivity due to the increase of water saturation is representative to the increase in continuous conductor (hematite or water) paths in the sample. figure 9 can be divided into two parts, a high frequency part and a low frequency part. at higher frequencies the experimental points figures 5, 6, 7, 8 and 9 show the complex resistivity plane representation of the sample for partial and fully saturated cases. the data are presented in 5 figures due to the wide range of values on a linear scale. for the low partial saturation case nearly a straight line can be identified fig. 5. as the saturation increases an arc of a depressed semicircle is obtained in figs 6, 7, 8 and 9, which reflects the effect of the resistance rd. the depression of the arc incg r ∝ zi ri ci cg rd r ∝ ri zi ci fig. 3. equivalent circuit that represent the sample in the low saturation case (cg and r∞ are the capacitance and resistance of the sample at the high frequency, respectively, ci is the interfacial capacitance and ri is the interfacial resistance). fig. 4. equivalent circuit that represents the sample in the high and full saturation case (cg, r∝, ci and ri is the same as shown in fig. 3). rd represents a frequency-independent conducting path between the two electrodes. fig. 5. the complex resistivity plane representation of the sample for relative saturations 4% ( ), 8% ( ), 11% ( ), 14% ( ), respectively. fig. 6. the complex resistivity plane representation of the sample for relative saturations 18% ( ), 25% ( ), respectively. 809 relation between electric properties and water saturation for hematitic sandstone with frequency fig. 5. the complex resistivity plane representation of the sample for relative saturations 4% ( ), 8% ( ), 11% ( ), 14% ( ), respectively. tivity or warburg impedance (macdonald, 1974). the warburg impedance behavior at the low frequencies was attributed to the electrode effect by (roberts and lin, 1997); and also by (knight and nur, 1987). the latter authors stated that the frequency range of electrode effect extends to higher frequencies with the increase of water. 5. conclusions effects of water saturation on the electrical conductivity and dielectric constant of humid, partially, and fully saturated hematitic sandstone sample is investigated in the frequency range from 0.1 hz to 100 khz. we change the saturation of the sample from normal relative humidity (~ 50% rh) to partially and fully saturated condition. experimental data indicate that the electrical conductivity and dielectric constant vary strongly with water saturations. a transition from nearly normal to gigantic transport is observed at a certain frequency depending on the water content. the rate of change of dielectric constant with saturation is found to decrease with frequency. lie on an arc of a semicircle. the low frequency part is a straight line that reflects diffusion controlled resistivity. at low frequencies the electrode polarization takes place, which is humidity or saturation dependent. the straight line may represent diffusion controlled resisfig. 7. the complex resistivity plane representation of the sample for relative saturations 32% ( ), 40% ( ), respectively. fig. 8. the complex resistivity plane representation of the sample for relative saturations 50% ( ), 55% ( ), respectively. fig. 9. the complex resistivity plane representation of the sample for relative saturations 96% ( ), 100% ( ), respectively. 810 m. m. gomaa chew, w.c., and p.n. sen (1982): dielectric enhancement due to electrochemical double layer : thin double layer approximation, j. chem. phys., 77 (9), 46834693. cole, k.s. and r.h. cole (1941): dispersion and adsorption in dielectrics (i), j. chem. phys., 137, 341-351. dukhin, s.s. (1971): dielectric properties of disperse systems, in «surface and colloid science», (matijevic, e., editor), wiley and sons, new york, vol. 3, pp. 83166. efros, a.l. and b.i. shklovskii (1976): critical behaviour of conductivity and dielectric constant near the metal– non-metal transition threshold, phys. status sol., 137, 475-489. garcia-belmonte, g., v. kytin, t. dittrich and bisquert (2003): effect of humidity on the ac conductivity of nanoporous tio2, j. appl. phys., 94 (8), 52615264. garrouch, a.a. (2001): effect of wettability and water saturation on the dielectric constant of hydrocarbons rocks, 41st annual logging symp. (spwla), paper nn. garrouch, a.a. and m.m. sharma (1994): the influence of clay content, salinity, stress and wettability on the dielectric properties of brine-saturated rocks: 10 hz to 10 mhz, geophysics, 137, 909-917. glover, p.w.j., p.g. meredith, p.r. sammonds and s.a.f. murrel (1994a): ionic surface electrical conductivity in sandstone, j. geophys. res., 99, b11, 21635-21650. glover, p.w.j., p.g. meredith, p.r. sammonds and s.a.f. murrel (1994b): measurements of complex electrical conductivity and fluid permeabilities in porous rocks at raised confining pressures, in rock mechanics in petroleum engineering, proc. eurorock94, 29-36, balkema, amsterdam. gomaa, m.m. (1996): frequency response study on iron ore bearing rock samples, (m. sc. thesis, cairo university, egypt). gomaa, m.m. (2004): induced polarization study on iron ore bearing rock samples, (ph. d. thesis, cairo university, egypt). gomaa, m.m., s.a. hussain, e.a. eldiwany, a.e. bayoumi and m. ghobashy (2000): modeling of a. c. electrical properties of humid sand and the effect of water content, 70th annual international meeting, society of exploration geophysics (seg) and international exposition, session «rock properties/borehole: rock physics 1», oral rpb6.7, calgary, alberta, canada, 6-11 august, 1850-1853. grant, f.a. (1958): use of complex conductivity in the representation of dielectric phenomena, j. appl. phys., 29, 1, 76-80. hoekstra, p. and w.t. doyle (1971): dielectric relaxation of surface adsorbed water, j. colloid interface sci., 137, 513-521. jonscher, a.k. (1999): dielectric relaxation in solids, j. phys. d: appl. phys., 32, r57–r70. knight, r. (1983): the use of complex plane plots in studying the electrical response of rocks, j. geomag. geoelectr., 137, 767-776. knight, r. j. and a. nur (1987): the dielectric constant of sandstones, 50 khz to 4 mhz, geophysics, 52, 644654. knight, r.j. and a.l. endres (1990): a new concept in modeling the dielectric response of sandstones: definthe changes in the electrical conductivity and dielectric constant due to humidity variation were argued to the mobile surface charges at watergrain interfaces. the simplest surface effect is due to surface migration and diffusion of charges under an applied field limited by a thin surface layer; this can be a conductive film or a closed electrical double layer. film polarization is especially strong at moderate partial saturations, where it can produce strong lf polarization. very strong dielectric effects are caused by the polarization of «open» electric double layers on the solid–liquid interface at large saturations. whereas polarization of bound charges in «closed» double layers produces relatively weak polarization, akin to that of films, diffusion potentials generated by the free exchange of ions between «open» deformed edl and the adjoining electrolyte produce out-of-phase currents, and hence large values of effective dielectric constant. this model is known as glfp; it can also be applied to heterogeneously charged surfaces. the anomalous dielectric properties of partially saturated rocks are also interpreted partially using percolation theory. this theory predicts that, when the conductive fraction (water) increases, clustering of conductive inclusions develops, and the thickness of insulating gaps (air) between conductive clusters decreases, causing a large increment in the capacitance of the sample and its conductivity. the results can be explained by electronic hopping conduction in the dry case, proton conduction in the humid case and low saturations, and electrolytic conduction along with protonic transport in the fully saturated case. as a recommendation for this paper more controlled saturated samples should be measured to study clearly the effect of grain size and grain shape on the electrical properties of rock samples. references chelidze, t. and y. guéguen (1999): electrical spectroscopy of porous rocks: a review i. theoretical models, geophys. j. int., 137, 1-15. chelidze, t., y. guéguen and c. ruffet (1999): electrical spectroscopy of porous rocks: a review -ii. experimental results and interpretation, geophys. j. int., 137, 16-34. fig. 8. the complex resistivity plane representation of the sample for relative saturations 50% ( ), 55% ( ), respectively. 811 relation between electric properties and water saturation for hematitic sandstone with frequency ing a wetted rock and bulk water system, geophysics, 55, 586-594. leroy, p., and a. revil (2004): a triple layer model of the surface electrochemical properties of clay minerals, journal of colloid and interface science, 270, 2, 371380. levitskaya, t.m. and b.k. sternberg (2000): application of lumped-circuit method to studying soils at frequencies from 1 khz to 1 ghz., radio science, 35, 2, 371-383. macdonald, j.r. 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(received may 7, 2008; accepted june 30, 2008) adg vol5 n02 teis 393_399.pdf annals of geophysics, vol. 45, n. 2, april 2002 393 electromagnetic radiation related to dislocation dynamics in a seismic preparation zone roman teisseyre and tomasz ernst institute of geophysics, polish academy of sciences, warszawa, poland abstract electromagnetic emission is observed frequently before earthquakes as high noise level in vlf and ulf bands. we present theoretical considerations on electromagnetic radiation caused by dislocation dynamics in the preseismic micro-sources (micro-crackings) located in an earthquake preparation zone. some of these micro-sources could be located near the ground surface and their electromagnetic signals could be accessible in some recording stations. the examples of the numerically simulated induction and radiation fields are given and one example of the observed radio-noise recording is shown. 1. introduction the preseismic micro-source activity may be followed by a rebound motion resulting in the main break: the earthquake. the dynamics of micro-sources is related to the accelerated motion of dislocations and to coalescences of the opposite dislocation arrays (micro-cracks). the theory of stress and dislocation evolution is based on the equation of motion with the self-stress field (continuum with defect distribution) on the slipfault plane; this equation is supplemented with the source/sink function entering in a similar way as the body forces in the equation of motion for displacements. this theory of stress evolution in a pre-seismic zone combined with the physics of the charge dislocations leads to precursory electromagnetic emission. 2. evolution of stresses most breaks start along the pre-existing faults or micro-faults. considering a plane problem containing a fault, we may assume that the rigidity on a fault µ* is smaller than the bulk value µ of the surrounding rocks: µ* < µ. for the boundary between the fault zone and the intact rocks y = 0, we can take the stressdislocations relation (see further on) which leads us to the condition *µ* = µ. this is due to the continuity of tangent stress derivatives. we obtain a jump of dislocation density on a fault: áá * > . (2.1) the preseismic micro-source activity may be mailing address: prof. roman teisseyre, polish academy of sciences, institute of geophysics, ul. ks. janusza 64, 01-402 warszawa, poland; e-mail: rt@igf.edu.pl key words stress evolution – instabilities – charge dislocations – electromagnetic emission µ = 1 s x 394 roman teisseyre and tomasz ernst followed by a rebound motion resulting in a main break: earthquake. the dynamic processes on a fault, or on micro-faults, can be described by the equation of motion for stresses including the interaction with the self-stress fields (dislocation stresses). we follow the results obtained in the papers by teisseyre (1996, 2001a,b) and teisseyre and yamashita (1999). according to kossecka and dewitt (1977), the motion equation is expressed by the balance of divergence of elastic stresses and elastic acceleration. elastic acceleration is introduced as a difference between the total acceleration and self (plastic) acceleration (the total acceleration can be expressed by time derivatives of displacements). we obtain in this way the motion equation for the elastic continuum with defects (2.2) where s are the elastic stresses, is the elastic velocity, u is the total displacement, s is the self/ plastic velocity (total velocity: u . = + s), is the source/sink function ( . corresponds to a body force). in 1d, a dislocation density is related to stresses s on a fault and also to a slip as follows: (2.3) where s f is the resistance stress (friction stress in the fracture phase) related to slip progress on fault, is here a certain reference thickness: when computing dislocation density related to slip process we count the dislocations comprised in this thickness. the commonly used stick-slip model, together with the elastodynamic equation, eliminates the self-stress fields from the objects on a fault plane. in our approach the self stresses in interaction with elastic stresses govern a slip evolution process. on a fault (x, t) the dynamic processes cause a rapid increase in the self-stress fields expressed by formation of dislocations and dislocation arrays (cracks – in the fracture phase). the total stresses can be split into elastic stress field and self-stress field, which for this case correspond to wave field and dislocation stress field: s t = s + s s . for dislocation density , dislocation velocity v and also for the selfstresses, we may assume, according to condition (2.1), a rapid decrease away from the fault plane (2.4) where a is a constant. the dislocation velocity can be also related to the local stress field (mataga et al., 1987). finally, for the near fault zone we come to the following equations for dislocation density (the 1d case {x, t}; teisseyre and yamashita, 1999; teisseyre, 2001a): (2.5) where c is shear wave velocity. the dynamic evolution is related to the dislocation fields and it depends mainly on the source/sink function . this function describes the nucleation of the new dislocations and the coalescence processes (mutual annihilation of dislocations of the opposite signs – the process equivalent to coalescence of two neighboring dislocated elements or formation of a crack). the source/sink function shall first of all describe the coalescence processes between the dislocation arrays of the opposite signs (the coalescence of the arrays well describes a nucleation process of a crack material fracturing). we may relate this function to a stress = + t t div d d d d s µ = 1 s, µ = ( )s s f x y t ay x t v x y t ay v x t s x y t ay s x ys , , , , , , , , , ( ) = [ ] ( ) ( ) = [ ] ( ) ( ) = [ ] ( ) exp exp exp 4 4 4 r r r = + 2 2t t t sdiv d d d s u d = 1 , 1 c t x = + 395 electromagnetic radiation related to dislocation dynamics in a seismic preparation zone surface curvature, or a gradient of dislocation density (inverse of a mean distance between the opposite dislocation groups). attraction forces between the opposite dislocations can effectively produce a weakening effect, and in this sense the function π acts equivalently to the friction weakening laws. for a review related to source/sink function π, see the paper by teisseyre and nagahama (1998). here, we use the modified form of this function (teisseyre, 2001b) in terms of dislocation density (2.6) the first term describes the formation process of new small elements of dislocations (stresses decrease as 1/r2 ) and proportional to dislocation density α and to square of stress curvature, while the second term describes coalescence process of the opposite dislocation lines (stresses decrease as 1/r) and proportional to square of dislocation density α 2 and to stress curvature; ε is a constant. 3. numerical simulations of dislocation evolution following the paper by teisseyre (2001b) we briefly present the results of the numerical simulations of dislocation evolution, assuming that a micro-source is represented by a microdomain {x}, {t} of the dimensions {0,2π .10−3[m]}, {0,1.6 . 10−5[s]}, and the initial conditions chosen as follows: according to eq. (2.5) and definition (2.6), we obtain the equation the numerical simulations show some instabilities in time which might be identified with the microseismic events acting in a premonitory domain. the computed fields remain much smoother along the x-axis. 4. electric current and electromagnetic field at micro-source the electric polarization induced by dislocation processes depends on the square of dislocation density, because both the electric charge and the coalescence break distance between the dislocations are proportional to dislocation density α. for polarization density and its derivatives we obtain while, according to ogawa et al. (1985), for electromagnetic exitations we have the following formulae expressing the near or electrostatic field, as well as the induction and radiation electric fields related to the dipoles activated in rocks (4.2) π π= ∂ ∂ − ∂ ∂         0 2α α εα α( ) x x . ( ) = ( )0 2 0α α πt, , . p = 1 2 2κα , p = καα˙ ˙ , p = +( )2κ αα α˙̇ ˙̇ ˙ (4.1) e p r near = 1 4 0 3πε , e p c r ind = 1 4 0 0 2πε ˙ , e p c r rad = 1 4 0 0 2πε ˙̇ α πx x, .0 0 05 10 16( ) = − −( )[ ] ⋅     0.2 sin m ∂α ∂ x t t ,( ) + α ∂ α ∂ x t x t x . . , , + − ⋅ ( )( ) ( ) +− −10 0 41 0 1 102 6 α αx t x t. , ,+ ⋅ ( ) ( ) =−1 5 10 11 α ∂α ∂ x t x t x . . , , − ⋅ ( ) ( ) ⋅−4 63951 5 10 13 α ∂α ∂ x t x t x , , .⋅ ( ) − ( ) −10 3( )2 396 roman teisseyre and tomasz ernst where 0 is the dielectric constant; the time derivatives are related to the source {x i 0} and related activity time t0; c0 is light velocity. of course, we are aware of the fact that the different processes related to excitation of the electromagnetic field take place in a precursory time domain and during an earthquake event (shevtsova, 1984; varotsos and alexopoulos, 1984, 1986; gokhberg et al., 1985; varotsos et al., 1992; park, 1993; teisseyre, 1992, 1995). for review on the dislocation processes and formation of electric charges, see whitworth (1975). here, we try to include also the electromagnetic emission from the pre-seismic microsources and to explain this emission by dislocation dynamic processes. 5. numerical estimations for electric current and electromagnetic radiation we refer to estimations presented by teisseyre (2001b). an order of magnitude of dislocation density at the time of micro-fracturing may be compared to density at the tip of dislocation array. for the dislocation density treated as a product of line density and the burgers vector (the definition used in theoretical papers) and referred here to the band width and the n dislocations with the burgers vector 0 (lattice constant), densely packed in an element d0 = m 0 , we obtain (5.1) while for the density of dislocation lines (the definition of line density used in papers on laboratory experiments, as a reference value we may cite its order for the elastic regime of a crystal body: 1011[m 2]) (5.2) the maximum of this density at local microfraturing (n m) may theoretically approach max (corresponding to the line density at a crack tip). the dislocation dynamics, as discussed in our model, can cause the appearance and activation of the electromagnetic dipoles in preseismic sources located in an earthquake preparation zone, but not exactly at the future main seismic spot. based on numerical simulations for the evolution dislocation fields, we can present here the numerical simulation of the dipole intensities in the pre-seismic sources; the dipole source intensities as a function of time for the induction and radiation terms (4.1) are presented in figs. 1 and 2 in the following scales: in the scale ; for x i = 1,2,3,4 . 10 3[m] and {t} in the scale [10 5 s], and in the scale for x i = 1,2,3,4 . 10 3[m] and {t} in the scale[10 5 s]. these electromagnetic phenomena evidently precede a seismic event; the sharp increases of the electric current source intensities both for the induction and radiation fields are observed. for density electric current, or induction field in a microfracture source we obtain the estimate (teisseyre, 2001b): while for the source intensity of radiation field x t x ti i, ˙ ,( ) ( ) 10 117 2m s x t x t x ti i i˙ , , ˙̇ ,( ) + ( ) ( )2 10 122 2 2 ; m s j ṗ ˙= = [ ] < [ ]10 100 9 2 6 2a m a m = n d 0 0 , = n m 0 0 1 , max [ ]106 1m = 0 = 0 0 1n m max = [ ]1016 2m . << << 397 electromagnetic radiation related to dislocation dynamics in a seismic preparation zone fig. 1. source intensities of electromagnetic induction field (x,t) . (x,t): the plots of (x i ,t) . (x i ,t) in the scale 1017 [1/m2s]; for x i =1,2,3,4 .10 3[m] and {t} in the scale [10 5s]. fig. 2. source intensities of electromagnetic radiation field . 2(x,t)+ (x,t) .. (x,t) : the plots of . 2(x i ,t)+ (x i ,t) .. (x i ,t) in the scale 1022 [1/m2s2]; for x i =1,2,3,4 .10 3[m] and {t} in the scale [10 5s]. 398 roman teisseyre and tomasz ernst we obtain such intensity peaks may appear at the isolated small areas of the precursory microfracturings; activation of such local peaks of the and . fields causes the electromagnetic emission. 6. example of the observed electromagnetic excitations there exist numerous examples of radio-noise excitations before earthquake events; there, we present one example. at l’aquila geophysical observatory (central apennines) a radio-noise recording system operating at frequencies of 10, 20 and 40 khz has been installed (cooperation between the institute of geophysics, polish academy of sciences, warszawa, and the istituto nazionale di geofisica e vulcanologia, roma). several seismic events with magnitudes over 4 and in some cases more than 5 were felt at the end of 1997 at distances of 60-80 km from the observatory. in fig. 3 (meloni et al., 2001; ernst et al., 2001) we present the radio-noise recording (here for 20 khz; similar records have been obtained for other frequencies) for the time when these seismic events occurred (marked on the plots). we shall note that for several years since the station was installed no such signals have appeared in the radio-noise observations; throughout this time, the seismicity was very low, with earthquakes reaching magnitudes of only m � 3. fig. 3. vlf/recording (20 khz) of radio-noise at l’aquila observatory (after meloni et al., 2001, and ernst et al., 2001): september and october, 1997; two greater seismic events with m 5 are marked by the black circles, straight intervals correspond to the gaps of recording. t j =0 d d ˙̇̇ ˙ ˙̇ .p = +( ) [ ]< [ ]10 109 2 2 11 2a m a sm 399 electromagnetic radiation related to dislocation dynamics in a seismic preparation zone references ernst, t., j. jankowski, r. teisseyre, a. meloni and p. palangio (2001): observations of electromagnetic radiation in the central apennines seismic region, acta geophys. pol., 49, 43-53. gokhberg, m.b., i.l. gufeld, n.i. gershenzon and v.a. pilipenko (1985): electromagnetic effects during rupture of the earth’s crust, izv. akad. nauk sssr, fiz. zemli, 1, 72-87 (in russian); izv. earth phys., 21 (1), 52-63. kossecka, e. and r. dewitt (1977): disclination kinematic, arch. mech., 29, 633-651. mataga, p.a., l.b. freund and j.w. hutchinson (1987): crack tip plasticity in dynamic fracture, j. phys. chem. solid, 48, 985-1005. meloni, a., d. di mauro, g. mele, p. palangio, t. ernst, and r. teisseyre (2001): evolution of magnetotelluric, total magnetic field, and vlf field parameters in central italy: relations to local seismic activity, ann. geofis., 44 (2), 383-394. ogawa, t., k. oike and t.miura (1985): electromagnetic radiations from rocks, j. geophys. res., 98 (d4), 6245-6249. park, s.k., m.s. johnston, t.r. madden, f.d. morgan and f.f. morrison (1993): electromagnetic precursors to earthquakes in the ulf band; a review of observations and mechanisms, rev. geophys., 31 (2), 117-132. shevtsova, i.n. (1984): charging of dislocations during deformation of crystals with ionic-type bonds, izv. earth phys., 20 (8), 643-648. teisseyre, r. (1992): earthquake premonitory processes: evolution of stresses and electric current generation, terra nova, 4, 509-513. teisseyre, r. (1995): electric field generation in earthquake premonitory process, in theory of earthquake premonitory and fracture processes, edited by r. teisseyre, 282-292, pwn, warszawa. teisseyre, r. (1996): motion and flow equation for stresses, acta geophys. pol., 44, 19-29. teisseyre, r. (2001a): evolution, propagation and diffusion of dislocation fields, in earthquake thermodynamics and phase transformations in the earth’s interior, edited by r. teisseyre and e. m a j e w s k i (academic press, san diego, san francisco, newyork, boston, london, sydney, tokyo), international geophysical series, vol. 76, 167-198. teisseyre, r. (2001b): dislocation dynamics and related electromagnetic excitation, acta geophys. pol., 49, 55-73. teisseyre, r. and h. nagahama (1998): dislocation field evolution and dislocation source/sink function, acta geophys. pol., 46, 13-33. teisseyre, r. and t. yamashita (1999): splitting stress motion equation into seismic wave and fault-related fields, acta geophys. pol., 47, 135-147. varotsos, p. and a. alexopoulos (1984): physical properties of the variations of the electric field of the earth preceding earthquakes. ii determination of epicenter and magnitude, tectonophysics, 110, 99-125. varotsos, p. and a. alexopoulos (1986): thermodynamics of point defects and their relation with bulk properties (north-holland publ. comp., amsterdam new york), pp. 474. varotsos, p., n.g. bogris and a. kyritsis (1992): comments on the depolarization currents stimulated by variations of temperature or pressure, j. phys. chem. solids, 53 (8), 1007-1011. whitworth, r.w. (1975): charged dislocations in ionic crystals, adv. phys., 24 (2), 203-304. ag_56.05.13_omotosho_finalonline:layout 6 annals of geophysics, 56, 5, 2013, a0568; doi:10.4401/ag-6342 a0568 cloud attenuation studies of the six major climatic zones of africa for ka and v satellite system design temidayo victor omotosho1,*, jit singh mandeep2, mardina abdullah2 1 covenant university, department of physics, ota, ogun state, nigeria 2 universiti kebangsaan malaysia, department of electrical, electronic and system engineering, bangi, malaysia abstract cloud cover statistics, cloud base and top height, cloud temperature, frequency of precipitation, freezing height, total cloud liquid water content (tclwc) and cloud attenuation data have been obtained for the six major climatic zones of africa. the present results reveal a strong positive correlation between the monthly distribution of low cloud cover, cloud top height, cloud temperature, and frequency of precipitation in the six zones. the cumulative distribution of the tclwc derived from radiosonde measurement in each climatic zone shows a departure from the tclwc recommended by the itu study group 3 data, with an exceedance percentage difference of 32% to 90% occurring 0.01% to 10% of the time. the underestimation of the tclwc is greatest in the tropical rain forest. a comparison of the cloud attenuation cumulative distribution in the ka and v bands reveals that the international telecommunication union – region (itu-r) is an intergovernmental organization that develops rain model based on collected data around the world. this model underestimates the cloud attenuation in all of the six climatic zones by 2.0 db and 4.7 db for the arid sahara desert, 1.3 db and 3.0 db in semi-arid north africa, 1.3 db and 1.5 db in savannah north africa, 2.0 db and 3.6 db in the tropical rain forest, 1.3 db and 2.9 db in savannah south africa and 0.9 db and 2.6 db in semi-arid south africa, respectively, at 30 and 50 ghz. overall, the cloud attenuation in the tropical rain-forest zone is very high because of the high annual total cloud cover (98%), high annual frequency of precipitation (4.5), low annual clear sky amount (8%), high cloud depth (10,937 m), high 0°c isotherm height (4.7 km), high tclwc (4.0 kg/m2 at 0.01%) and low seasonal cloud base height (356 m). 1. introduction raincis considered to be most important attenuator for high-availability satellite links. impairment caused by clouds is often ignored, but the frequent presence of clouds may cause persistent attenuation for low-margin modern systems operating in the ka (30/20 ghz) and v (50/40 ghz) bands [omotosho et al. 2011]. rain can be traced to the formation of clouds; therefore, for low-availability satellite services such as very small aperture terminals (vsat) and ultra small aperture terminals (usat), rain effects may only contribute a relatively small part of the total propagation link margin [omotosho and babtude 2010]. deep fades may occur because of the higher probability of the occurrence of clouds [salonen and uppala 1991, mattioli et al. 2009]. many experimental studies have been conducted on millimeter wave atmospheric attenuation. based on these studies, researchers can estimate with some confidence the losses arising from cloud attenuation up to approximately 200 ghz [slobin 1982, salonen and uppala 1991, al-ansari et al. 2003, sarkar et al. 2005, mandeep and hassan 2008, mattioli et al. 2009]. some models that use several effective input parameters have been developed [altshuler and marr 1989, dintelmann and ortgies 1989, dissanyake et al. 1997, wrench et al. 1999, itu-r 2009a]. technological advances in remote sensing, the increasing number of satellite missions (including nasa (a-train) atmospheric infrared sounder (airs), moderate resolution imaging spectroradiometer (modis) key instrument aboard the terra (eos am) and aqua (eos pm) satellites that collects complimentary observations of earth’s surface and atmosphere, cloud cover statistical analysis program (cloudsat) is an experimental satellite that will use radar to study clouds and precipitation from space, cloud-aerosol lidar and infrared pathfinder satellite observations (calipso) is a lidar that provides high-resolution vertical profiles of aerosols and clouds, tropical rain measurement mission (trmm) microwave imager is a passive microwave sensor designed to provide quantitative rainfall information over a wide swath under the trmm satellite, and the cloud and earth radiation sensor (ceres), direct measurements and recorded observations by weather staarticle history received april 22, 2013; accepted june 24, 2013. subject classification: cloud attenuation, ka band, satellite, v band, climate. tions, and the nasa student cloud observation online (scool) have contributed greatly to the wealth of cloud data available to the scientific community for indepth radio wave propagation studies. the most important properties of clouds for the cloud attenuation modeling of earth-space communication links in the ka and v bands are the statistics of cloud cover, cloud base and top height, cloud thickness, total cloud liquid water content, cloud temperature, horizontal extent and variability. cloud climatologies have recently been developed from two types of data: (1) radiance measured by satellites in geostationary and polar orbits and (2) visual observation of clouds from the earth’s surface, as coded in weather reports from stations on land and ships on the ocean. satellites principally detect clouds using the visible and thermal-infrared wavelengths. during the day, clouds can be detected in both wavelength regions, but at night they can be detected only in the thermal infrared. satellite observations of clouds are obtained more frequently than surface observations and cover more land and ocean area, but ground observation offers a useful adjunct to satellite observation. the surface observer views clouds from below; thus, they observe low clouds that are often hidden from the view of satellites by higher clouds. multiple cloud layers often occur together; consequently, the views from above and below are complementary [warren and hahn 2003]. in this work, we report the annual clear sky cloud cover statistics, cloud base and top height, cloud temperature, and frequency of precipitation based on cloud climatology data from ground and satellite observations from 1952 to 2008. additionally, we report the integrated cloud liquid water content (kg/m2) obtained from daily radiosonde data measurements from 1953 to 2011 for the six major african climatic zones and compare it with the integrated cloud liquid water content data of the recent itu-r study group 3 (sg3) [itu-sg3 website]. the above data sets were used as input climatic data to investigate the impact of clouds on the ka (30 ghz) and v (50 ghz) bands for the six major climatic zones of africa (the arid sahara desert, semi-arid north africa, the north african savannah, the tropical rain forest, the south african savannah and semi-arid south africa. table 1 presents the geographical coordinates and the climate summary for the six zones, including the rainfall type, annual rainfall accumulation, annual temperature, and vegetation type. figure 1 presents the map of africa and the boundary omotosho et al. 2 climatic zones latitude (degrees) longitude (degrees) rainfall type climate type annual rainfall accumulation (mm) vegetation type (1) arid sahara desert 15-32.5n 11.5w-32.5e sparse rainfall desert uniformly dry, no moisture, temperature is -4 to 54c less than 130 mm few oases exist (2) semi arid north africa 12-17.5n 11.5w-32.5e low rainfall steppe climate 100 to 200 mm confine to june and september relatively sparse grass, shrubs predominate (3) savannah north africa 7.5-12.5n 11.5w-32.5e moderate rainfall hot dry season (winter) wet season (summer) occupy one-fifth of africa land 100 to 400 mm vary vegetation open canopied forest, grassy under story dominate (4) tropic rain forest 7.5s-7.5n 11.5w-32.5e high-all-year rainfall wet and warm climate, tropical wet, no month is rainless 1800 to 3000 mm evergreen trees, oil palms, tropical hard woods, dense surface shrubs and ferns moses (5) savannah south africa 17.5s-7.5s 14-42.5e moderate rainfall hot dry season (winter) wet season (summer) occupy one-fifth of africa land 100 to 400 mm vary vegetation open canopied forest, grassy under story dominate (6) semi arid south africa 22.5s-17.5s 17-32.5e low rainfall steppe climate 100 to 200 mm relatively sparse grass, shrubs predominate table 1. climate summary and geographical coordinates of the of the six climatic zones. 3 of each climate zone [adeyewa and nakamura 2003]. finally, out of eight cloud attenuation models available in the literature, the two best estimates of cloud attenuation based on recent experimental data were used to estimate cloud attenuation, and the two results are compared for the african climatic zones. 2. cloud and radiosonde data sources cloud-cover data were collected for six major african climatic zones, and the average properties of the different cloud types from daily visual observations of cloud climatology at land stations worldwide between 1952 and 1996, developed by hahn [hahn and warren 1999], were used for their analysis. cloud-cover percentages were determined for different cloud types over land areas with a resolution of 5° × 5° in longitude and latitude. the data grid resolution is commensurate with the geographical distribution of the data collection centers, and this level of resolution is deemed reasonable for the calculation of cloud attenuation in the six major climate zones of africa. five cloud types were selected in this work for the study of cloud-cover statistics: stratus (st), stratocumulus (sc), cumulus (cu), cumulonimbus (cb) and nimbostratus (ns). the seasonal cloud base height data from 1970 to 1996 were obtained from hahn [hahn and warren 1999]. the mean cloud temperature and cloud top height were obtained from the international satellite cloud climatology project (isccp, 1983-2008) and the cloud-aerosol lidar and infrared pathfinder satellite observations (calipso, 2006-2011) data, respectively, for the six climate zones. the retrieved cloud data were processed and averaged on monthly, annual, and seasonal bases. radiosonde data were retrieved from the integrated global radiosonde archive (igra) [durre et al. 2006] from 1953 to 2011. six stations were chosen to represent each climatic zone: sebha, libya, representing the arid sahara desert; bamako-senou, mali, representing semi-arid north africa; bouake, côte d’ivoire, representing the north african savannah; lagos oshodi, nigeria, representing the tropical rain forest; nampula, mozambique representing the south african savannah; and bulawayo, zimbabwe, representing semi-arid south africa. table 2 presents detailed information about the six stations used in this study. consistent radiosonde (sonde model i1 vaisala rs80) measurements, obtained twice daily at 00 and 12 gmt from 1953 to 2011, contain data for pressure, temperature, relative humidity, and dew point temperature at different heights as well as 0°c isothermal height (freezing height) with a resolution varying from a few tens of meters to a few hundreds of meters to a height of 37 km. the best daily radiosonde data were selected for each station and used in this work, covering approximately 124 to 15,396 radiosonde launches. these data were used to estimate the cloud liquid water statistics for each of the six stations. the radiosonde data can indicate the presence of cloud liquid water if the relative humidity exceeds the critical humidity, as defined by [geleyn 1980]. the phase of the water within the cloud is determined by the temperature. if the temperature is greater than 0°c, the water is completely in the liquid phase; if the temperature is less than −20°c, only ice exists. the range of temperatures between 0°c and −20°c yields a mixture of liquid water and ice. salonen [salonen and uppala 1991] proposed a model for cloud water density in terms of the temperature and height from the cloud base that can be used to obtain the liquid water density profile within a cloud. the water density within a cloud can vary significantly. pure ice does not cause the attenuation of radio signals, but super-cooled water can cause significant attenuation [animesh and chakraborty 2009]. 3. theoretical considerations several models are available in the literature for the prediction of cloud attenuation [gunn and east 1954, staelin 1966, slobin 1982, altshuler and marr 1989, liebe 1989, salonen and uppala 1991, dissanyake et al. 1997, itu-r 2009a]. the salonen and uppala and itu-r models have been reported to produce the best estimates of cloud attenuation, based on experimental data in the equatorial and tropical climates [mandeep and hassan 2008]. these models are particularly effective because the cloud attenuation component is obtained from the liquid water content, which is calculated cloud attenuation for ka and v bands figure 1. major climatic zones of africa used in this study. their geographical coordinates are specified in table 1. omotosho et al. 4 c li m at ic zo n es w m o id c o u n tr y n am e st at io n n am e st at io n la ti tu d e st at io n lo n gi tu d e st at io n el ev at io n (m ) a ve ra ge su rf ac e re la ti ve h u m id it y (% ) a ve ra ge su rf ac e te m p er at u re (k ) a ve ra ge su rf ac e p re ss u re (h p a) r ad io so n d e d at a st ar t r ad io so n d e d at a en d t o ta l n u m b er o f go o d d ai ly ra d io so n d e d at a u se d in t h e st u d y (1 ) a ri d sa ha ra de se rt 62 12 4 l ib ya se b h a 27 .0 2 14 .4 3 44 0 29 30 0 96 7 19 74 20 11 2, 56 6 (2 ) se m i a ri d n or th a fr ic a 61 29 1 m a l i b a m a k o / se n o u 12 .5 3 -7 .9 5 38 1 51 30 1 97 0 19 68 20 11 15 ,3 96 (3 ) sa va nn ah n or th a fr ic a 65 55 5 c o t e d ’i v o r e b o u a k e 7. 73 -5 .0 7 37 6 70 29 9 97 0 19 88 19 97 12 4 (4 ) t ro pi c ra in fo re st 65 20 2 n ig e r ia l a g o s/ o sh o d i 6. 55 3. 35 38 73 29 8 10 06 19 53 20 09 3, 49 0 (5 ) sa va nn ah so u th a fr ic a 67 23 7 m o z a m b iq u e n a m p u l a -1 5. 1 39 .2 8 44 1 60 29 9 96 5 19 71 20 05 3, 00 0 (6 ) se m i a ri d so u th a fr ic a 67 96 4 z im b a b w e b u l a w a y o -2 0. 15 28 .6 2 13 44 62 28 8 87 1 19 69 20 05 5, 58 1 t ab le 2 .s u rf ac e m et eo ro lo g ic al d at a an d ge og ra ph ic al c oo rd in at es fo r ea ch o f th e re pr es en ta ti ve s ta ti on s fo r th e cl im at ic z on es . 5 for each radio-sounding profile. the procedures for the two models used in this work are given by [salonen, and uppala 1991] and itu-r model [itu-r 2009a, itu-r 2009b, p.840-4] and this method has been used in previous work for nigeria and tropical wet location yaounde, cameroon [omotosho and babtude 2010, omotosho et al. 2011] respectively. 3.1. slobin using the expression provided by slobin, the specific attenuation of radio waves by clouds at different wavelengths is given by the following expression: (1) where v is the specific attenuation caused by clouds (db/km), m is the cloud water particle density (g/m3), t is the cloud temperature (°k), and m is the wavelength of the radio wave (cm). the cloud particle temperature is the temperature of the region in which the cloud exists, measured at the height of the cloud. the mean cloud temperature and cloud top height were retrieved from the international satellite cloud climatology project (isccp). 3.2. cloud liquid water content (lwc) at different height level the formation of cloud is related to the high relative humidity whereby the radiosonde data indicate the presence of lwc depending on whether the relative humidity has exceeded its critical value. cloud is formed as the relative humidity exceeds 95% and the phase is determined by the temperature profile. the lwc for each height level can be calculated by the following expression: (2) where: t(z) = air density; cp = specific heat at constant pressure; l = latent heat of vaporization; cd = dry adiabatic lapse rate; cs = moist adiabatic lapse. the adiabatic condition produces a maximum value of the lwc that is given by equation (3) due to the circulation of air mass accompanied by precipitation and freezing temperatures. (3) where dh = height above the cloud base. lwc is then calculated at each pressure level at a particular radiosonde ascent. integrating the lwc profile over height, the total value of lwc is obtained at each ascent [chakraborty and animesh 2012] 4. results and discussion 4.1. low cloud cover and frequency of precipitation in this section, we present and discuss the annual low cloud type, monthly variation of low cloud amount, and monthly variation of precipitation frequency average results from 1970 to 1996 for the six climatic zones. these quantities are important for the basic understanding of the cloud impairment of radio wave signals in the six zones. figure 2 presents the annual distribution of the five low cloud types: stratus (st), stratocumulus (sc), cumulus (cu), cumulonimbus (cb) and nimbostratus (ns). the amount of all low cloud types was highest in the tropical rain forest (36.3%) because of the higher incidences of sc, cb and cu clouds, followed in decreasing order by the south african savannah (29%), semi-arid south africa (21.5%), the north african savannah (18.6%), the arid-sahara desert (11%) and semi-arid north africa (5.1%). stratocumulus clouds are well-known to produce drizzle or light precipitation, and their presence may precede or follow more severe weather. cumulus clouds are the product of free convective air mass instability, and they can become more storm-like, towering and vertical, thereby building into cumulonimbus, whose continued upward growth suggests showers and other precipitation. cumulonimbus clouds are violent; when they produce rain, it is often associated with lightning and thunderstorms. cumulus and cumulonimbus clouds have been reported by slobin to contain substantial cloud liquid water: up to approximately 0.67 g/m and 1 g/m3, respectively [slobin 1982]. figures 3 and 4 show the monthly variation of precipitation frequency and low cloud cover for the period 1970 to 1996 in the six climatic zones. figures 3 and 4 have exactly the same shape, which means that the frequency of precipitation and low cloud cover are positively correlated. the monthly low cloud cover reaches its maximum in august in the four climatic zones in northern africa (the arid sahara desert, semiarid north africa, north african savannah, and tropical rain forest), whereas it reaches its minimum in august in the southern climatic zones (south african savannah and semi-arid south africa maximum in january). the monthly precipitation frequency reaches its minimum in august in the south, but it peaks in the north, except for two zones: the arid sahara desert, which reaches its maximum in january, and the tropical rain forest, which has two maxima in april and october. these maxima correspond to the rainy season in the tropical rain forest. the period of higher cloud cover in the climatic zones corresponds to the rainy season for all six zones. cloud attenuation for ka and v bands . . ,m4 343 10 1 16 . ( )t 2 0 0122 291 1# # # v m = lwc ( ) ( ) ( )h z l c dzad p d st c c= -# lwc lwc (1.239 0.145 )ln had d= 4.2. percentage of clear sky and total cloud cover in this section, we present the monthly clear sky variation and monthly total cloud cover variation (low + middle + high cloud cover) from 1970 to 1996 for the six major climatic zones of africa. figures 5 and 6 present the clear sky and total cloud cover results. the two graphs are negatively correlated, with the same shape in the opposite direction. this result was expected because the month with the greatest amount of clear sky often corresponds to a period of little or no rainfall because rainfall is often preceded by the formation of certain types of cloud. the highest monthly amount of clear sky occurred in semi-arid south africa (67.8%), followed in decreasing order by the arid sahara desert (48%), the south african savannah (43.2%), the north african savannah (31.9%), semi-arid north africa (29%) and the tropical rain forest (17.5%). these patterns of monthly clear sky in the six zones were expected; dust storms in the arid sahara desert zone reduce the monthly values, but these events are rare in semi-arid south africa. the tropical rain forest zone has the least clear sky because of its high all-year rainfall pattern and highest rainfall accumulation (between 1,800 and 3,000 mm), as shown in table 1. the maximum total cloud cover for the south african savannah and semi-arid south africa zones occurs in january, with values of 75.4% and 62.9%, respectively. similarly, the northern semi-arid north africa and north african savannah zones experience their maximum total cloud cover in august, with values omotosho et al. 6 figure 2. annual low cloud distribution in the six major climatic zones of africa. 7 of 68.9% and 81.4%, respectively. the maximum total cloud cover in the tropical rain forest differs from that in the other zones, with two peak values of 72.2% and 74.2% in april and october, respectively. the arid sahara desert has its maximum total cloud cover in december (34.8%), which is the lowest among the six climatic zones (this result is shown in figure 3). the sahara zone also has the lowest frequency of precipitation. it is interesting to note that the months of maximum total cloud cover correspond to the month of maximum rainfall in the south african savannah and semi-arid south africa ( january), semi-arid north africa and the north african savannah (august), the tropical rain forest (april and october) and the arid sahara desert (december) [adeyewa and nakamura 2003]. table 3 presents the annual amount of clear sky, frequency of precipitation and cloud cover statistics for the six climatic zones. the annual total cloud cover is highest in the tropical rain forest (97.9%), followed in decreasing order by the north african savannah (85.4%), the south african savannah (72.7%), semi-arid north africa (62.2%), semi-arid south africa (49.9%) and the arid sahara desert (37.7%). the annual frequency of precipitation is highest in the tropical rain forest and the south african savannah cloud attenuation for ka and v bands figure 3. monthly distribution of precipitation frequency in the six climatic zones. (4.5%), followed in decreasing order by semi-arid south africa (3.5%), the north african savannah (2.5%), the arid sahara desert (1%) and semi-arid north africa (0.9%). the annual amount of clear sky is highest in the arid sahara desert (40.4%), followed in descending order by semi-arid south africa (32.1%), semi-arid north africa (19.8%), the south african savannah (17.5%), the north african savannah (12.7) and the tropical rain forest (8%). 4.3. average cloud temperature, seasonal and annual cloud base, top height and cloud depth table 4 presents the cloud temperature results for the six climatic zones. this value is an important input parameter for calculating the specific attenuation of radio waves within the cloud at different wavelengths (see equation 13) [slobin 1982]. the cloud temperature is higher in the arid sahara desert (281 k) and lower in semiarid south africa (245 k). the lowest seasonal cloud base height is found in the tropical rain forest (356 m), with an average cloud temperature of 261 k. the lowest annual cloud top height and cloud depth occur in the arid sahara desert, whereas the highest cloud top height and cloud depth are found in the north african savannah and the tropical rain forest, followed in descending order by semiarid north africa, the south african savannah and semiarid south africa. the results in tables 3 and 4 reveal that a radio wave signal passing through clouds over the tropical rain forest or the north african savannah will travel a longer distance because these two zones have the greatest cloud depth and total cloud cover. omotosho et al. 8 figure 4. monthly distribution of low cloud cover in the six major climatic zones. 9 4.4. monthly variation of freezing height figure 7 presents the monthly variation of freezing (the 0°c isothermal height) derived from daily radiosonde data for the six major climatic zones. the monthly distribution of the 0°c isothermal height in the south african savannah and semi-arid south africa follows exactly the same curve shape as the total cloud cover. the freezing height maximum for the two zones occurred in february at 4.61 km and 3.57 km, respectively, which corresponds to the same period as the maximum total cloud cover. however, for the arid sahara desert zone, the pattern follows a normal distribution, and the maximum freezing height occurs in august at 4.46 km. the freezing heights in semi-arid north africa and the north african savannah have three different peaks, and their maxima occur in november and june at 4.43 km and 4.54 km, respectively. the tropical rain forest freezing height distribution reaches its minimum in july at 4.53 km. the tropical rainforest zone has the highest 0°c isothermal height of approximately 4.8 km, and it reaches its minimum in july during the period of maximum low cloud cover (figure 4), which corresponds to the rainy season in the zone. 4.5. total cloud liquid water content derived from radiosonde data figures 8 and 9 present the daily tclwc and cumulative distribution of the total cloud liquid water content from 0.01% to 100% exceedance for the six climatic zones. in figure 9, the cumulative distribution of cloud attenuation for ka and v bands figure 5. monthly clear sky distribution in the six major climatic zones. the tclwc was plotted on the same graph with the data of the itu radiocommunication study group 3 [2011], as described in section 3.2.2, for comparison. the daily tclwc could be as high as 4.5 kg/m2 in the tropical rain forest, and it could exceed 4.0 kg/m2 0.01% of the time. figure 9 shows that the values of the tclwc recommended by itu underestimate the values of the tclwc in all six climatic zones at all percentages of exceedance. between 0.01% and 10% exceedance, the underestimation is as follows: the arid sahara desert, 37% to 86%; semi-arid north africa, 37% to 88%; the north african savannah, 32% to 86%; the tropical rain forest, 46% to 90%; the south african savannah, 37% to 82%; and semi-arid south africa, 34% to 81%. the highest underestimation occurs in the tropical rain forest climatic zone. 4.6. cloud attenuation in conjunction with the expression provided by slobin, the average cloud temperature shown in table 4 was used to compute the specific cloud attenuation of radio waves from 12 ghz to 100 ghz for the six climatic zones. the results are presented in table 5. the specific cloud attenuation decreases with increasing cloud temperature, which is highest for the arid sahara desert (281 k); therefore, it has the lowest specific cloud attenuation of 7.4 db/km at 100 ghz. this result was expected for a desert climatic zone. the north african savannah has the lowest cloud temperature (238 k); consequently, it has the highest specific cloud attenuation of 25.1 db/km at 100 ghz, followed in decreasing order by semi-arid south africa (245 k) at 20.6 db/km, the south african savannah omotosho et al. 10 figure 6. monthly distribution of total cloud cover in the six major climatic zones. 11 cloud attenuation for ka and v bands a fr ic an cl im at ic z o n es m o n th w it h m ax im u m cl ea rsk y m o n th w it h m ax im u m to ta l c lo u d c ov er m o n th w it h m ax im u m fr eq u en cy o f p re ci p it at io n a n n u al lo w c lo u d co ve r (% ) a n n u al m id d le c lo u d co ve r (% ) a n n u al h ig h c lo u d co ve r (% ) a n n u al to ta l c lo u d co ve r (% ) a n n u al fr eq u en cy o f p re ci p it at io n a n n u al cl ea r sk y am o u n t (% ) (1 ) a ri d sa ha ra d es er t ju ne d ec em be r fe br u ar y 11 .0 11 .5 15 .2 37 .7 1. 0 40 .4 (2 ) se m i a ri d n or th a fr ic a ja nu ar y a u gu st a u gu st 5. 1 17 .0 40 .1 62 .2 0. 9 19 .8 (3 ) sa va nn ah n or th a fr ic a ja nu ar y a u gu st a u gu st 18 .6 26 .4 40 .4 85 .4 2. 5 12 .7 (4 ) t ro pi c ra in fo re st ja nu ar y o ct ob er o ct ob er 36 .3 35 .2 26 .4 97 .9 4. 5 8. 0 (5 ) sa va nn ah s ou th a fr ic a ju ly ja nu ar y ja nu ar y 29 .0 26 .9 16 .8 72 .7 4. 5 17 .5 (6 ) se m i a ri d so u th a fr ic a a u gu st ja nu ar y fe br u ar y 21 .5 18 .9 9. 5 49 .9 3. 5 32 .1 t ab le 3 .a nn u al c lo u d co ve r st at is ti cs , c le ar s ky , a nd fr eq u en cy o f pr ec ip it at io n. omotosho et al. 12 g ro u n d o b se rv at io n o f se as o n al cl o u d b as e h ei gh t (m ) sa te ll it e o b se rv at io n ( c a l ip so ) o f se as o n al cl o u d t o p h ei gh t (m ) se as o n al c lo u d d ep th ( m ) a n n u al cl o u d b as e h ei gh t (m ) a n n u al cl o u d b as e h ei gh t (m ) a n n u al cl o u d b as e h ei gh t (m ) a fr ic an cl im at ic zo n es a n n u al cl o u d te m p er at u re fr o m i sc c p sa te ll it e (k ) d jf m a m jj a so n d jf m a m jj a so n d jf m a m jj a (1 ) a ri d sa ha ra de se rt 28 1 10 33 95 8 75 8 86 2 56 00 57 37 55 18 57 60 45 67 47 79 47 60 48 98 90 3 56 54 (2 ) se m i a ri d n or th a fr ic a 26 0 18 63 13 53 97 5 10 64 76 10 10 23 7 11 28 3 96 53 57 47 88 84 10 30 8 85 89 13 14 96 96 (3 ) sa va nn ah n or th a fr ic a 23 8 69 4 77 1 59 7 59 8 98 20 13 35 8 12 06 0 11 56 5 91 26 12 58 7 11 46 3 10 96 7 66 5 11 70 1 (4 ) t ro pi c ra in fo re st 26 1 41 0 43 6 41 2 35 6 12 05 0 13 72 5 91 85 10 40 0 11 64 0 13 28 9 87 73 10 04 4 40 4 11 34 0 (5 ) sa va nn ah so u th a fr ic a 25 1 42 3 48 4 46 6 46 9 13 41 0 10 29 5 55 42 91 92 12 98 7 98 11 50 76 87 23 46 1 96 10 (6 ) se m i a ri d so u th a fr ic a 24 5 50 8 48 6 45 2 60 5 12 60 0 90 75 49 78 87 40 12 09 2 85 89 45 26 81 35 51 3 88 48 t ab le 4 .a nn u al a ve ra ge c lo u d te m pe ra tu re a nd s ea so na l c lo u d ba se , t op h ei gh t an d cl ou d de pt h. 13 (251 k) at 17.1 db/km, semi-arid south africa (260 k) at 13.4 db/km and the tropical rain forest (260 k) at 12.9 db/km. figures 10 and 11 present the results of the cumulative distribution of cloud attenuation at 30 and 50 ghz obtained from radiosonde measurements. the same figure also shows the data obtained from the itu-r model for the six climatic zones for comparison. the itu-r model underestimates the cloud attenuation at all six zones because of the lower values of tclwc predicted by its database. between 0.01% and 10% exceedance, the difference at 30 ghz and 50 ghz is as high as 2 db and 4.7 db in the arid sahara desert, 1.3 db and 3.0 db in semi-arid north africa, 1.3 db and 1.5 db in the north african savannah, 2 db and 3.6 db in the tropical rain forest, 1.3 db and 2.9 db in the south african savannah, and 0.9 db and 2.6 db in semiarid south africa, respectively. the underestimation is highest in the arid sahara desert. the highest similarity in the results between the radiosonde measurements and the itu-r model was obtained in the north african savannah, with a difference of 1.3 db and 1.5 db at 30 and 50 ghz, respectively. overall, the maximal cloud attenuation was observed for the tropical rain forest at all percentages of time and at both frequencies as a result of the high annual precipitation frequency (4.5), low annual amount of clear sky (8%), high cloud depth (10,937 m), high 0°c isotherm height cloud attenuation for ka and v bands figure 7. monthly variation of 0°c isothermal height derived from daily radiosonde data. (4.7 km), high tclwc (4.0 kg/m2 at 0.01%) and low seasonal cloud base height (356 m). 5. conclusion based on long-term data measurements (19532011), the cloud cover statistics, cloud base height, top height, cloud temperature, frequency of precipitation, freezing height, tclwc and cloud attenuation have been determined for the six major climatic zones of africa. the present results reveal a strong positive correlation between the monthly distribution of low cloud cover, cloud top height, cloud temperature, and frequency of precipitation at the six zones. the cumulative distribution of the tclwc derived from radiosonde measurement at each climatic zone shows a departure from the tclwc predicted by the itu study group 3 data, with a difference of 32 to 90% for exceedance 0.01% to 10% of the time. the underestimation of the tclwc was highest in the tropical rain-forest. a comparison of the cloud attenuation cumulative distribution in the ka and v bands reveals that the itu-r model underestimates the cloud attenuation at 30 and 50 ghz by 2.0 db to 4.7 db in the arid sahara desert, 1.3 db to 3.0 db in semi-arid north africa, 1.3 db to 1.5 db in the north african savannah, 2.0 db to 3.6 db in the tropical rain forest, 1.3 db to 2.9 db the south african savannah and 0.9 db to 2.6 db in semi-arid south africa, respectively. overall, the cloud attenuation is very high in the tropical rain-forest zone because of the high annual total cloud cover (98%), high annual frequency of precipitation (4.5), low annual amount of clear sky (8%), high cloud depth (10,937 m), high 0°c isotherm height (4.7 km), high tclwc (4.0 kg/m2 at 0.01%) and low seasonal cloud base height (356 m). omotosho et al. 14 figure 8. daily distribution of total cloud liquid water content (kg/m2) obtained from radiosonde measurements in the six climatic zones. 15 acknowledgements. the support of stephen g. warren, and carole j. hahn, department of atmospheric sciences university of washington, seattle, washington 98195-1640 and imke durre, national oceanic and atmospheric administration (noaa) is gratefully acknowledged for download access to surface-based cloud climatology over land and ocean and the integrated global radiosonde archive data from their websites, respectively. the authors would also like to thank universiti kebangsaan malaysia for the support of this work. references adeyewa, z.d., and k. nakamura (2003). validation of trmm radar rainfall data over major climatic regions in africa, j. appl. meteor., 42, 331-347. al-ansari, k., p. garcia, j.m. riera and a. benarroch (2003). one-year cloud attenuation results at 50 ghz, electron. lett., 39 (1), 136-137. altshuler, e., and r. marr (1989). cloud attenuation at millimetre wavelengths, ieee trans. antennas propag., 37 (11), 1473-1479. animesh, m., and s. chakraborty (2009). cloud liquid water content and cloud attenuation studies with radiosonde data at a tropical location, j. infrared milli terahz waves, 30, 367-373. cloud attenuation for ka and v bands figure 9. cumulative distribution of total cloud liquid water content (kg/m2) obtained from radiosonde measurements; also shown is the data obtained from the itu-r model for the six major climatic zones of africa. chakraborty, s., and m. animesh (2012). a comparative study of cloud liquid water content from radiosonde data at a tropical location, int. j. geosci., 3, 44-49. dintelmann, f., and g. ortgies (1989). semiempirical model for cloud attenuation prediction, electron. lett., 25 (22), 1487-1488. dissanyake, a., j. allnut and f. haidara (1997). a prediction model that combines rainattenuation and other propagation impairments along earth satellite paths, ieee trans. antennas propag., 45, 1546-1558. durre, i., r.s. vose and d.b. wuertz (2006). overview of the integrated global radiosonde archive, j. climate, 19, 53-68. geleyn, i.f. (1980). some diagnostics of the cloud/radiation interaction in ecmwf forecasting model, in: ecmwf workshop on radiation and cloud-radiation interaction in numerical modelling (october 15-17, ecmwf), 133-162. gunn, k.l., and t.w. east (1954). the microwave properties of precipitation particles, quart. 1. royal meteor. suc., 80, 522-545. hahn, c.j., and s.g. warren (1999). extended edited cloud reports from ships and land stations over the globe, 1952-1996. numerical data package ndp-026c, carbon dioxide information analysis center (cdiac), department of energy, oak ridge, tennessee documentation. itu-r (2009a). attenuation due to clouds and fog, itur recomm. p.840-4, geneva, switzerland. itu-r (2009b). guide to the application of the propagation methods of radiocommunication study group 3, itu-r recomm. p.1144-5, geneva, switzerland. itu-r study group 3 (sg 3) (2011). software for ionospheric and tropospheric propagation and radio noise data; http://www.itu.int/oth/r0a04000065/en. omotosho et al. 16 frequency (ghz) band name (1) arid sahara desert specific attenuation (db/km) (2) semi arid north africa specific attenuation (db/km) (3) savannah north africa specific attenuation (db/km) (4) tropic rain forest specific attenuation (db/km) (5) savannah south africa specific attenuation (db/km) (6) semi arid south africa specific attenuation (db/km) 12 ku 0.11 0.19 0.36 0.19 0.25 0.30 14 ku 0.14 0.26 0.49 0.25 0.33 0.40 20 ka 0.29 0.53 1.00 0.51 0.68 0.83 30 k 0.66 1.20 2.26 1.16 1.54 1.86 40 v 1.18 2.14 4.01 2.06 2.73 3.30 50 v 1.84 3.34 6.27 3.21 4.27 5.16 75 w 4.15 7.51 14.10 7.23 9.60 11.61 100 w 7.37 13.36 25.07 12.85 17.06 20.64 table 5. specific cloud attenuation at various frequencies for the six climatic zones. 17 liebe, h.j. (1989). mpm, an atmospheric millimeter wave propagation model, int. j. infrared millimeter waves, 10 (6), 631-650. mandeep, j.s., and s.i.s. hassan (2008). cloud attenuation in millimeter wave and microwave frequencies for satellite applications over equatorial climate, int. j. infrared millimeter waves, 29, 201-206. mattioli, e., p. basili, s. bonafoni, p. ciotti and e.r. westwater (2009). analysis and improvements of cloud models for propagation studies, radio science, 44, rs2005; doi:10.1029/2008rs003876. omotosho,t.v, and e.b. babatunde (2010). study of cloud impact on fixed satellite communication link at ku, ka and v bands in nigeria, australian journal of basic and applied sciences, 4 (8), 3287-3298. omotosho, t.v., j.s.mandeep and a. mardina (2011). cloud-cover statistics and cloud attenuation at kaand v-bands for satellite systems design in tropical wet climate, ieee antennas and wireless propagation letters, 10, 1194-1196. salonen, e., and w. uppala (1991). new prediction method of cloud attenuation, electron. lett., 27 (12), 1106-1108. sarkar, s.k., i. ahmad, j. das and a.k. de (2005). cloud height, cloud temperature and cloud attenuation in microwave and millimeter wave frequency bands cloud attenuation for ka and v bands figure 10. cumulative distribution of cloud attenuation at 30 ghz obtained from radiosonde measurements. also shown is the data obtained from the itu-r model for the six major climatic zones of africa. over indian tropical east cost, int. j. infrared millimeter waves, 26, 329-340. slobin, s.d. (1982). microwave noise temperature and attenuation of clouds: statistics of these effects at various sites in the united states, alaska and hawaii, radio sci., 17, 1443-1454. staelin. d.h. (1966). measurements and interpretation of the microwave spectrum of the terrestrial atmosphere near i-centimeter wavelength, j. geophys. res., 71 (12), 2875-2881. warren, s.g., and c.j. hahn (2003). clouds/climatology, encyclopedia of atmospheric sciences, 476-483. wrench, c.l., p.g. davies and j. ramsden (1999). global prediction of slant path attenuation on earth space links at ehf, int. j. satell. commun., 17, 177-186. *corresponding author: temidayo victor omotosho, covenant university, department of physics, ota, ogun state, nigeria; email: temidayo.omotosho@yahoo.com. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. omotosho et al. 18 figure 11. cumulative distribution of cloud attenuation at 50 ghz obtained from radiosonde measurements. also shown is the data obtained from the itu-r model for the six major climatic zones of africa. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 60, 6, s0667, 2017; doi: 10.4401/ag-7333 a mixed automatic-manual seismic catalog for central-eastern italy: analysis of homogeneity marco cattaneo1,*, massimo frapiccini1, chiara ladina1, simone marzorati1, giancarlo monachesi1 1 istituto nazionale di geofisica e vulcanologia, ancona, italy article history received november 22, 2016; accepted october 20, 2017. subject classification: automatic picking; seismicity; seismic catalog; central-eastern italy. s0667 abstract a comparison between pickings and locations obtained by automatic and manual procedures in the analysis of the seismicity of central-eastern italy is presented. in a first step we compared automatic and manual pickings, demonstrating that in many cases the adopted algorithm, after some tuning, is able to reproduce both the timing and the weight assignment of a human operator. the comparison of automatic and manual locations allowed to demonstrate that, when the automatic procedure is able to reach a solution stable from the statistical point of view, these locations are comparable with the manual ones within the estimated error limits. once established these reliability criteria, we began to produce a mixed automatic-manual catalog: the events located by the automatic procedure with estimated errors below the selected thresholds (2 km in horizontal and 3 km in vertical) were directly introduced in the catalog, other events were revised by a human operator. in this way more than 64% of the events did not need human intervention, allowing to correctly manage also a period of increased seismicity, characterized by more than 4000 events per month: in total 121894 events were located with good accuracy in a time period of less than 7 years (august 2009 april 2016). in a last step, a further control of the reliability of the whole procedure was performed, by manually analyzing all the events occurred in the last month of the analyzed period and classified as reliable by the automatic procedure: two expert seismologists interpreted these events, and the comparison demonstrated that the differences between the automatic and manual pickings and locations are slightly larger, but comparable with the differences between two human operators. as further checks, an analysis of the distribution of the depth estimates on the whole catalog demonstrated that data from the manual or the automatic part are nearly indistinguishable for the central, better monitored area; furthermore the automatic system demonstrated to be able to correctly locate also quarry blasts, with a reasonable estimate of the depth of these very critical events. finally, a quick look at the geographical and depth distribution of the seismicity summarized in the catalog is presented; also in this case the main result is the good overlap of automatic and manual locations, at least for the well-monitored areas. 1. introduction reasons for a mixed catalog the routine work of analyzing the seismicity recorded by a seismic network by means of manual pickings performed by expert analysts can become heavy and time-consuming, particularly when dealing with dense networks and areas characterized by a rather high rate of seismicity. also to overcome these difficulties, in the last years many algorithms of automatic pickings both for p and for s phases have been developed [e.g. spallarossa et al. 2014, ross et al. 2016] and relevant quotations). these techniques allow to obtain reliable automatic pickings, and thus automatic locations, for many events, but can fail in particular circumstances: low signal-to-noise ratio in some stations, superposition of closely located nearly simultaneous events, stations characterized by non-stationary noise producing many false pickings. in these cases, the intervention of a human operator can allow the correct location of these events. this mixed approach has been adopted for the routine analysis of the seismicity in central-eastern italy; in this work we want to analyze the possible dis-homogeneity in the locations introduced by this technique. it is worth noting that a mixed approach (automatic first location and successive manual review) is a quite common procedure, and is adopted e.g. by the ingv national monitoring system [mazza et al. 2011]; the originality of our approach is that, when the automatic analysis obtains a stable result, the manual revision is completely skipped. a possible inconvenience of the automatic processing procedures is the mis-location inside the network of external events. this is particularly true for networks of limited extension, and dealing with p phases only: in these cases, sometimes the fake inner location could also be characterized by low error estimates. in our case, the extension of the network, and the systematic search of marzorati.qxp_layout 6 13/12/17 12:01 pagina 1 s waves, limits the risk of such behavior. in our experience, the external events, when not correctly recognized, are located nevertheless outside the network, at the distance limit of the pn waves from the edges of the network itself, and are usually associated with high estimated location errors, so that they are sent to the manual revision phase. the seismic monitoring of central-eastern italy has been gradually improved during the last 20 years. after the 1997 colfiorito seismic sequence the ingv national seismic network (rsn) changed its configuration in the area, anticipating the process of re-design of the network geometry that interested the whole network starting on 2002 [amato et al. 2006]. in the same years, the regional networks of marche, umbria and abruzzo improved their instrumentation too, and, more important still, started a process of more efficient data sharing both between each other and with the rsn, under the coordination of ingv. some results of this process of data integration are presented in de luca et al. [2009]. starting in 2009, the network geometry and the procedures of data managements changed again: the whole regional network of the marches, and part of those of umbria and abruzzo, where merged in the rsn [d’alema et al. 2011, monachesi et al. 2013]; moreover, the beginning of the taboo experiment led to the installation of a dense multi-parametric network in the altotiberina area [chiaraluce et al. 2014]. obviously the continuous improvement of the network led to a progressive reduction of the detection threshold, so that the obtained catalog cannot be considered as completely homogeneous. the aim of this paper is just to analyze if the adoption of a mixed approach introduced a further, artificial level of dis-homogeneity. at the beginning of 2016, about 100 stations in the area 42.4-44.2;11.0-14.0 are centralized in the ingv office in ancona and managed in an homogeneous way. most of these data are shared between the acquisition systems of ancona and the central monitoring system of rome, where they are fully integrated in the rsn, allowing the real-time control of italian seismicity [mazza et al. 2011]. figure 1 shows the position of the stations, superimposed to the map of recent seismicity (2009-2016) and to the main geographical features of the area. a description of the tectonic framework is beyond the scope of this paper [see e.g. carannante et al. 2013, for a description of the same area]. besides a real time control of the seismicity, adopting the same tools utilized by rsn in rome, the data centralized in ancona are subjected to a more detailed off-line analysis procedure. in particular, the continuously registered waveforms are analyzed by means of a sta/lta algorithm cattaneo et al. 2 figure 1. map of the study area, reporting the located seismicity in the period 2009-2016 (size of the circles proportional to the magnitude), the stations position (green triangles) and the main geographical elements quoted in the text. marzorati.qxp_layout 6 13/12/17 12:01 pagina 2 3 a mixed seismic catalog for c-e italy on the band-pass filtered signals, joined to a coincidence control based on the definition of sub-networks of key stations. more in detail, the signal is filtered between 2 and 15 hz, sta and lta windows are 1 and 100 s long respectively; the trigger is declared for ratio great then 5 with duration at least of 3 s; the duration is stopped when ratio is reduced below 2. between the 100+ stations, just 41, characterized by low background noise and low occurrence of transient noise, are selected, and divided in 10 groups of closely located stations (less than about 60 km). the coincidence control is thus performed on these groups of stations on 10 s long windows overlapped by 50 %. the crossing of the coincidence threshold is controlled by weights assigned at each single component of seismic stations, based on their level of noise, to enhance the contribution of better stations: this allows to maintain low sta/lta ratios (and thus high sensitivity) without increasing too much the probability of false events declaration due to coincidences happening by chance. in this way, even detecting events down to magnitude around -1, the number of false detections is very low (usually smaller then 5%, see figure 2 below). obviously, magnitude -1 is not the catalog completeness limit, neither the average detection threshold of the network: marzorati and cattaneo [2016] demonstrated that the detection threshold of this network is quite strongly spaceand time-dependent, spanning from values around -1 for the best monitored areas to values around 1.5 for the most marginal ones. up to may 2013, each detected earthquake was manually treated by human operators: all the steps of phase picking, earthquake location and magnitude computation were preformed by means of interactive programs. nevertheless, in the meantime automatic procedures were developed, mainly devoted to furnish nearly-real time results for the civil protection purposes. the continuous improvement of the detection capability of the seismic network, connected with a significant increase of the seismicity of the area during 2013, led to over-charge the manpower devoted to the seismic analysis. figure 2 shows the number of detected triggers and of located events for each month in the period august 2009 april 2016. it can be observed that, following a gradual increase of the mean number, with fluctuations linked to peculiar sequences, from april 2013 onwards we observed a strong increase of seismicity, reaching values between 6000 and 7000 events/month in the period december 2013 april 2014. it is also evident the small difference between the number of detected triggers and located events, confirming that the mean number of false events is very low. at the same time, the improvement of the automatic picking and location procedures led to obtain locations and magnitude estimates that were often comparable with the manual ones. obviously the automatic procedure cannot always reach the same results as a human expert; for this reason we decided to adopt a mixed approach, in which the human operator intervenes whenever the automatic procedure fails to reach a satisfying result. figure 2. number of triggers produced by the detection procedure, and of the located events, in the analyzed period (august 2009 april 2016). note the strong increase of seismicity from the end of 2013 to the beginning of 2015. marzorati.qxp_layout 6 13/12/17 12:01 pagina 3 2. comparison between manual and automatic results before attempting this approach, we had to be sure that the automatic procedure was able to mimic the output of an human operator. the core of the automatic system is the rsni-picker [spallarossa et al. 2014, scafidi et al. 2016]. this picker is based on the akaike information criterion [aic, akaike 1974], but this technique is inserted in an iterative procedure, that allows to strongly reduce the occurrence of false pickings. moreover, a user-calibrated procedure allows to assign to each picking a weight that should mimic the weight assignment of a human operator. the adopted weight code is based on the so-called “hypo-family” code [lee and lahr 1972]: the best quality data have assigned weight 0 (full weight), while increasing uncertainty is represented by increasing code (from 1 to 4), meaning decreasing weight (¾, ½, ¼, no weight in the original hypo71 code). in our application, 0 code means uncertainty of the order of 0.01-0.03 s, 1 between 0.03 and 0.06 s, 2 between 0.06 and 0.2 s, 3 between 0.2 and 0.5 s, 4 larger than 0.5 s. in order to compare the picking performances of the automatic system with those of a human operator, we analyzed 2 months’ worth of data (january and february 2013, 1578 events) both with the automatic procedure and with the usual manual procedure. for the earthquakes for which the automatic procedure was able to furnish a stable location, we compared the p and s pickings with those obtained by a human operator; data are divided on the basis of the automatically assigned weight (figures 3 a-b). it is quite evident that for p phases most of the automatic pickings fall in the 0 weight class (or, better, most of the other pickings were discarded by the procedure), and that most of them differ from the manual pickings for very small amounts. more in detail, 87.5% of the automatic p picking with 0 weight show differences below 0.03 s with respect to the manual ones (the nominal upper limit of the 0 weight class); taking into account that also the manual pickings are affected by some uncertainty, we could consider a reading potentially correct if the difference lies below 0.05 s, in which case the percentage is 93.9%.while the percentage grows to 96.6% for differences below 0.1 s, i.e. readings not clearly wrong. the results of these comparisons between automatic and manual pickings and weights were used to re-calibrate the weight assignment procedure, adopted in the routine procedure starting in june 2013. on the basis of these pickings, earthquakes are located by using the hypoellipse code [lahr 1999]. the adopted propagation model is based on the 1-d model obtained by de luca et al. [2009], with station corrections re-computed for the stations installed after the publication of that paper. it is worth noting that the code allows to use the so-called jeffreys weighting [jeffreys 1973]: data producing residuals that fall in the non-gaussian tail of the error distribution are progressively cattaneo et al. 4 figure 3. differences between automatic and manual pickings in the period january february 2013. a): p pickings; b): s pickings. weights are related to hypo71 quality classes. (a) (b) marzorati.qxp_layout 6 13/12/17 12:01 pagina 4 5 a mixed seismic catalog for c-e italy down-weighted during the iterations, and often completely cut off at the end. this is particularly important when dealing with automatic pickings, in which a small percentage of data could show errors completely outside the mean statistical distribution, and that could strongly influence the location procedure. as already stated, locations are computed using both pickings from the automatic procedures and pickings obtained by an expert seismologist revision of waveforms. a comparison of the whole dataset of automatic location (1226 earthquakes, figure 4) shows that more than 90% of the locations show horizontal differences below 3 km and vertical differences below 4 km. it can be noted that the good coherency of the automatically estimated depths with respect to the reference ones is mainly due to the reliability of the s-wave pickings. indeed, if we compare the results obtained by using only the p-wave pickings (figure 5), the distribution of horizontal errors is just slightly enlarged towards higher values, while the vertical error is strongly affected: in this case less than 80% of the locations show a vertical difference below 4 km. the location program (hypoellipse) furnishes also an estimate of the location error; the complete defigure 4. comparison between automatic and manual locations, period january-february 2013, all data. figure 5. comparison between automatic and manual locations, period january-february 2013, using just p pickings. marzorati.qxp_layout 6 13/12/17 12:01 pagina 5 scription is represented by the 68% joint confidence ellipsoid, but this is often synthesized in the horizontal error (erh), related to the maximum elongation of the projection of the ellipsoid on the surface, and the vertical error (erz), related the maximum vertical elongation of the ellipsoid [lahr 1999]. obviously these values cannot represent the “true” location error, but only a statistical estimate, based on the estimated standard error of arrival times, the weight code assigned to each arrival time, and, for each station, the partial derivatives of travel time with respect to latitude, longitude and depth. we can thus select, from the automatic locations, only the events showing estimated errors below a particular threshold; adopting 2 km as erh threshold and 3 km as erz threshold, we obtain a sub-set of 759 more stable locations (48% of the analyzed triggers). the comparison of these locations with those obtained by the human revision is shown in figure 6; it is quite evident that the difference distribution is more concentrated towards low values; more than 90% of such events show horizontal differences below 2 km, and depth differences below 3 km. obviously the human expert location cannot be assumed as the “true” location; the most interesting result at this stage is that the automatic procedure is able to mimic the behavior of the human procedure, as regards the location accuracy, within the estimated error in most cases. 3. the mixed catalog and a-posteriori control on the basis of the previously shown results, we decided to assume as reliable the automatic locations showing estimated horizontal error below 2 km and vertical error below 3 km. as an exception, events showing an automatic local magnitude above 2.5 were in any case revised by a human operator, also to pick polarities for focal mechanism computation. from june 2013 till april 2016 this choice allowed to promote as reliable (and thus insert directly in the catalog) 51783 automatic locations, out of a total of 80296 events. this means that more than 64% of the events were located with the requested accuracy by the automatic procedure. this rate is significantly higher than that observed in the first two months of analysis (48%). there can be two reasons for this result: on one side, the first analysis has been used to re-calibrate the weight assignment scheme of the procedure, so that it is likely that the subsequent application of this scheme improved the location capability of the system; on the other side, starting in august 2013, a rich seismic sequence interested the altotiberina valley area, where the seismic network is more dense, and thus the probability of a good quality location is higher. in order to further verify the reliability of the whole automatic procedure, for the last month of analysis (april 2016) the data that were expected to be taken from the automatic procedure, on the basis of the selection criteria, were also analyzed by two human operators (the two seismologists that usually perform the control of the low-quality data). a first comparison is referred to the pickings: in figure 7 the difference of the automatic pickings with respect to those of one of the two human operators is represented, both for p and s waves. it is quite evident that for p pickings (3581 readings) the distribution of differences is quite symmetrical with respect to cattaneo et al. 6 figure 6. comparison between automatic and manual locations, period january-february 2013, selection with erh < 2 km and erz < 3 km. marzorati.qxp_layout 6 13/12/17 12:01 pagina 6 7 the 0 line, and that about 90% of the pickings show differences below 0.1 s; on the other side, for s phases (4844 readings) the distribution has an asymmetrical tail (prevailing positive differences), and just about 83% of the pickings show differences below 0.2 s, confirming the higher criticality of the s picking. but, if we restrict the analysis to just the clearer readings (assigned weights, in the hypo code, of 0 or 1 both from the human and the automatic operators, figure 8), results are quite different: for p phases (3161 readings) the distribution is still narrower, and about 94% of the picking show differences below 0.1 s. for s phases (2771 readings) the asymmetry appears strongly reduced, and about 92% of the differences lie below 0.2 s. if on the contrary we compare the pickings of the two human operators, results are rather different: looking at the whole dataset (5695 p and 5689 s readings, figure 9), the distribution of differences is rather symmetrical with respect to the 0 line, with a scatter higher for s phases, as expected: about 92% of p phases show differences below 0.1 s and 97% below 0.2s, while for s phases we have to enlarge the interval to 0.2 and 0.4s in order to obtain 93% and 98%, respectively. if we restrict the analysis to the best quality data (weights 0 and 1, 3969 p and 3379 s readings, figure 10), the trend is the same observed for the automatic data, with a stronger narrowing of the distribution towards low values: more than 98% of the pickings show differences below 0.1 s for p phases and below 0.2 s for s phases. in summary, the automatic picker behaves quite difa mixed seismic catalog for c-e italy figure 7. comparison between automatic and manual pickings, period april 2016, all data. marzorati.qxp_layout 6 13/12/17 12:01 pagina 7 ferently from a human operator mainly for low-quality s readings, while for p readings and for high-quality s readings differences from a human operator are just slightly higher than differences between two expert human operators. as the following step, we can see how these picking differences influence the location procedure: figure 11a shows the distances between the automatic locations and those obtained by one of the human operators; for the depth differences, both the absolute values of the depth differences and the difference itself are shown, in order to verify the asymmetry of the distribution. it can be observed that about 94% of the locations show distances below 2 km in the epicentral position, and about 95% below 3 km in depth difference (2 and 3 km were the adopted thresholds of estimated error for the selection of the automatic locations). if we restrict the analysis to the really best located events, mainly on the basis of the number of phases (> 12), of the maximum azimuthal gap of the recording stations (< 200°), of the rms of the residuals (< 0.3 s) and of the distance of the closest station (< 20 km), the results are just slightly better (figure 11b): about 95% of events lie within 2 km as horizontal distance, and 97% within 3 km in depth. this means that just in a very few cases the location, even with good statistical parameters, is somehow intrinsically unstable, also beyond the estimated errors. the same comparison can be made between the locations by the two human operators (figure 12a for the whole dataset and 12b for the dataset selected on the bacattaneo et al. 8 figure 8. comparison between automatic and manual pickings, period april 2016, hypo weights 0 and 1. marzorati.qxp_layout 6 13/12/17 12:01 pagina 8 9 sis of the same quality criteria): in this cases very few events of the whole dataset show differences above the maximum estimated error, while in the restricted dataset only one event shows an horizontal distance slightly above 2 km and a depth distance above 4 km. 4. stability of the depth estimate as already stated, the most critical parameter of the location procedure is the focal depth; in particular, in areas where the station inter-distance is larger, the depth estimate becomes more critical, and more strictly dependent on the availability of good-quality s wave readings also for stations rather far from the epicenter. in these cases, the limits of the automatic picker can reduce the capability of a correct depth estimate. on the contrary, in areas where the station inter-distance is smaller, it is more probable that the automatic procedure can be able to compute depth estimates with good precision. in order to verify this a-priori hypothesis, we performed a statistical analysis of the geographical depth distribution in our catalog. at first, we analyzed the whole catalog: we selected the more stable solution (erh < 2 km, erz < 3 km), we subdivided the analyzed area in a grid sized 0.05° both in latitude and in longitude, and we computed the depth interval containing at least 80% of the events located within each cell. figure 13a shows a map view of the depth of the top and bottom of this layer. it can be observed that for a quite large subset of the analyzed area a rather narrow definition of the main a mixed seismic catalog for c-e italy figure 9. comparison between two manual pickings, period april 2016, all data. marzorati.qxp_layout 6 13/12/17 12:01 pagina 9 seismogenic layer is possible; in particular some specific trends, already described e.g. in de luca et al. [2009] and in carannante et al. [2013] can be recognized. a detailed discussion of this result is beyond the scope of this paper; here we just want to compare this result with the analogous obtained by using just the automatic locations present in the catalog (figure 13b). it is evident that the well-resolved area is more limited, as expected: only the region monitored by the denser part of the seismic network can produce stable automatic locations. within this area, the main features of the depth distribution appear well reproduced by the automatic catalog with respect to the complete one. the main difference appears in the area around the point 43°18’ 12°20’: in this area, the complete catalog shows a patch of maximum depth around 16-20 km, while the automatic catalog does not show anything similar. in this case, a possible explanation can be found in the different time period covered by the two catalogs: the complete one spans a time interval of nearly 7 years (august 2009 april 2016), the automatic one covers about 3 years only (june 2013april 2016). the anomalously deep seismicity observed in this area (beneath the tiber valley close to umbertide) is rather episodic: it was recognized during a temporary experiment with a dense seismic network in this area during 2000-2001 [piccinini et al. 2003], and has been again observed in the following years, but without any regularity. the complete catalog shows 47 such events, enough in order to influence the depth statistics; on the contrary the automatic catalog shows just 14 such events, not sufficient to influence the seismogenic layer estimate. cattaneo et al. 10 figure 10. comparison between two manual pickings, period april 2016, hypo weights 0 and 1. marzorati.qxp_layout 6 13/12/17 12:01 pagina 10 11 as a general issue, the comparison of the depth distribution in different time spans could be quite critical, being not assured the stationarity of the phenomenon. in this case, dealing mainly with lowlevel, probably background seismicity, we are quite confident of the correctness of this assumption, and our results seem to confirm it. 5. further control of automatic location quality: quarry blasts location the above presented analysis is based on data relevant to the so-called “tectonic earthquakes”. however, in this area different sources of non-tectonic earthquakes have been also recognized [cattaneo et al. 2014]. the routine procedure is able to identify these events, on the basis of preliminary locations and waveforms similarity. the events can thus be subtracted from the seismic catalog, while their data are stored, so that it is possible to use them for a further quality control. the use of quarry blasts for verifying the reliability of the whole location procedure (phase picking, propagation models, location programs, …) has been already presented [see e.g. cattaneo et al. 1999, or more recently viganò et al. 2015]. a mixed seismic catalog for c-e italy figure 11. comparison between automatic and manual locations, period april 2016; a): all data; b) selected dataset (see text for details). (a) (b) marzorati.qxp_layout 6 13/12/17 12:01 pagina 11 it can be assumed that the location of quarry blasts is more critical than that of an earthquake of the same equivalent magnitude: on one hand, a shot does not generate strong direct s waves, but what we record in the far field is mainly composed by converted waves; moreover it is more critical to verify the location of a very shallow event is than of a deeper one, both for geometrical reasons and on account of the higher level of heterogeneity that we should expect in the shallowest layers, that for a deeper earthquake are crossed by the rays just close to the stations (and the possible inconsistency with respect to the propagation model is usually included in a station correction term), while for a quarry blast are crossed also near the source. in this sense, we can assume a quarry blast location as a “worst case scenario” for our location procedures. on the cattaneo et al. 12 figure 12. comparison between two manual locations, period april 2016; a): all data; b) selected dataset (see text for details). (a) (b) marzorati.qxp_layout 6 13/12/17 12:01 pagina 12 13 other side, for quarry blasts we know with good precision the location, so that they represent an interesting tool in order to verify the reliability of our estimates. we thus selected 491 quarry blasts located by the automatic procedure in the period june 2013-april 2016 with the quality criteria already discussed in the previous paragraphs, and compared their locations with the location of the closest active quarry. the difference in horizontal location and in depth are presented in figure 14a; it can be noticed that about 90% of the events show horizontal and vertical errors below the adopted error thresholds (2 km and 3 km, respectively). still better results are obtained if we reduce the analysis to the quarries within the best monitored area (we just excluded quarries south of 43.00n and north of 43.55n), and we select the locations characterized by at least one station within 15 km; in this case (figure 14b), about 95% of the locations lie within 2 km from the quarry, and 99% within 3 km. as regards depth, about 93% show depth estimates shallower than 3 km, and more than 99% shallower than 4 km. taking into account the already discussed criticality of the quarry blast locations, we can thus be rather confident that the automatic procedure is able to correctly locate even low-magnitude very shallow events, with a good accuracy on the horizontal coordinates and with the capability at least to restrict the depth estimate to the shallowest layers. 9. preliminary analysis of the mixed catalog in summary, the analysis procedure above presented allowed to locate, in the period august 2009april 2016, 117430 tectonic earthquakes and 4464 a mixed seismic catalog for c-e italy figure 13. map of top and bottom of the main seismogenic layer (containing 80% of the seismicity for each cell of a grid); a) all data; b) just automatically located data. (b)(a) (d)(c) marzorati.qxp_layout 6 13/12/17 12:01 pagina 13 non-tectonic earthquakes in the selected area. these locations are generated by the use of 1373646 p phases (824470 manual readings and 549176 automatic pickings) and 1668915 s phases (797326 manual readings and 871589 automatic pickings). the excess of s phases with respect to p phases, mainly for the automatic readings, could appear surprising, but it must be taken into account that, for low magnitude events recorded at distances larger than the focal depth, often the signal to noise ratio is higher for s waves than for p waves, and thus the automatic picker finds an easier job for s phases. this number of events can be considered rather high, taking into account the moderate seismicity of this part of italy and the lack of events with magnitude higher than 4.9 in the analyzed area and time period, that could generate strong sequences (see e.g. the mw 6.1 l’aquila earthquake of april 2009, that generated at least 64000 aftershocks in few months, valoroso et al., 2013). in our opinion, this catalog could represent an interesting tool in order to analyze the so-called “background seismicity”. the magnitude distribution of the events in the catalog is presented in figure 15: 87.4% of the located events shows a local magnitude below 1.0, and 40.8% below 0.0. a quite similar distribution is obtained by analyzing just the automatic locations (top histogram): 92.2% below 1.0 and 47.1% below 0.0. it is thus demonstrated the capability of the automatic picker to correctly manage events of even very low magnitude, if they are located in an area with a dense enough network. a quick look to the main characteristics of the seismicity depicted by this catalog can be furnished by some cross-sections perpendicular to the main apennines trend (figure 16). both in the map view and in the cross-sections, the manual locations are reported as black dots and the automatic ones as red dots. these sections can be compared with those presented in de luca et al. [2009] and in carannante et al. [2013]; the main difference is that in this case a very simplified location model is adopted and automatic locations are cattaneo et al. 14 figure 14. statistics of the distances of automatically located quarry blasts and the barycenter of the closest quarry, and of the estimated quarry blast depth; a) all data; b) data from quarries located in the best resolved area. (a) (b) marzorati.qxp_layout 6 13/12/17 12:01 pagina 14 15 also included, but on the other side the number of located events is significantly higher. the main purpose of this analysis at this stage is to check the coherency of the data of this mixed catalog with the main results of the previous, more accurate works. starting from nw, two main trends are recognizable as regards the depth distribution: the shallower part shows a mean deepening from sw to ne, while the deeper part shows an opposite trend. the ne-deepening trend of the shallow part is very evident from section 5 to section 8; here the socalled altotiberina fault represents a quite sharp limit to the distribution of seismicity in depth [piccinini et al. 2003, chiaraluce et al. 2007], with few exceptions: the most evident is the already quoted seismicity of the umbertide area, recognizable in sections 6 and 7 at depths between 15 and 20 km. as anticipated, the deeper part shows an opposite trend, going from about 20 km beneath the adriatic coast down to about 60 km beneath the apennines chain. this trend is very evident for sections 1-11, less evident but present till section 15, and hardly recognizable in the se-most sections. this seismicity can be associated with the well-known sinking of the adria lithosphere beneath the apennines chain [selvaggi and amato 1992]. as already described in carannante et al. [2013], the most complex depth distribution is related to the central sections (from 11 to 15). here the ne-ward deepening of the shallow seismicity appears less evident, and a deeper seismogenic layer is present starting at the ne bound of this seismicity, concentrated between 15 and 25 km. a detailed analysis of this seismicity is beyond the scope of this paper: here we just wanted to be confirmed in the fact that our routine locations show a very good coherence with the previous results, and that the automatic part of our catalog completely overlaps the manual part. indeed, the main trends of the depth distribution are well represented also by the automatic locations (red dots) mainly in the central part of the central sections, as expected. we already noticed that the automatic system performs at its best where the seismic network is denser. on the contrary, for the more external locations (sw limits of section 15-18 and sections 19-20), the contribution of automatic locations is strongly reduced. another exception can be noticed for the seismicity at sea, very evident at the ne limit of sections 11-12, mainly related to the seismic sequence of july-august 2013 following a mw 4.9 earthquake: in this case, given the criticality of locations, characterized by a large azimuthal gap of stations, we decided to manually interpret all the events. 7. conclusions this study was aimed to convince first of all ourselves that a mixed automatic-manual approach can produce an homogeneous catalog of events. indeed, automatic procedures have already demonstrated their ability to achieve very accurate pickings and locations also for very large datasets [see e.g. valoroso et al. 2013 or scafidi et al. 2016], but usually in these cases no effort is made to maintain the whole completeness of the analyzed catalog: events not fulfilling the quality criteria are simply discarded from the analysis. on the contrary, when dealing with the long-term monitoring of micro-seismicity in an area, every effort should be made to maintain the completeness and the a mixed seismic catalog for c-e italy figure 15. magnitude distribution of the events in the whole catalog (august 2009 april 2016). top histogram: just automatic locations. bottom histogram: all data. marzorati.qxp_layout 6 13/12/17 12:01 pagina 15 homogeneity of the location quality for all the events fulfilling some pre-defined detection criteria. in our opinion, the most efficient tool to achieve these requirements is the mixed approach, in which human intervention is required when the automatic procedure is unable to reach the target quality thresholds.. in a first step, pickings from the adopted automatic procedure were compared with pickings derived from the manual revision activity; as a by-product, this comparison allowed to re-calibrate the weight assignment of the automatic procedure, in order to better mimic the human assignment, following the procedure described in spallarossa et al [2014]. the comparison of the locations obtained from these pickings allowed to cattaneo et al. 16 figure 16. map view and anti-apennines cross-sections of the seismicity synthesized by the catalog (august 2009april 2016). black dots: manual locations; red dots: automatic locations. marzorati.qxp_layout 6 13/12/17 12:01 pagina 16 17 verify the correctness of the statistical errors associated to these locations; in particular it was possible to verify that, when the automatic procedure is able to obtain a location stable from a statistical point of view, and thus with estimated error below reasonable thresholds (that we arbitrarily fixed in 2 km for the horizontal error, and 3 km for the vertical one), the difference with respect to the human location is nearly always smaller than the estimated error. based on these criteria, the mixed catalog started to be populated from june 2013, and is continuously updated. for a better characterization of the automatic pickings and locations, a further comparison has been carried out for data recorded in the last month of the analyzed time window (april 2016); the same data were picked by two different expert seismologists, in order to verify also the somehow intrinsic instability of our estimates. this analysis demonstrated that the automatic system produced differences slightly larger than, but comparable to the differences between the two human operators, thus confirming the reliability of the procedure. also the comparison of the geographical depth distribution, and the analysis of the automatically located quarry blasts, confirmed the overall stability of our locations. this procedure is now routinely applied to the seismic monitoring of central-eastern italy; in our opinion the produced mixed dataset (both pickings and locations) can be used as a whole for any seismological analysis in this area. data and sharing resources some figures were created using the gmt software package [wessel et al. 2013]. the catalog of tectonic events in central-eastern italy (august 2009 april 2016) is available in electronic supplements of the paper. record format: 1-12: event id in the rsm database 14-92: event location, hypo71 format 14-30: origin time 32-39: latitude 41-49: longitude 51-56: depth (km) 60-63: local magnitude 4-66: number of phases with non-zero weight in the location 68-70: maximum azimuthal gap 71-75: distance of the 3-rd closest station 76-80: root mean square of residuals of the final location 82-85: erh, max. horizontal projection of the error ellipsoid as computed by hypoellipse (km) 87-90: erz, max. vertical projection of the error e lipsoid as computed by hypoellipse (km) 92: summary location quality, as assigned by hypoellipse (a: best, d: worst) 94: flag for the adopted location; a: automatic, m: manual acknowledgements. this study was carried out thanks to the collaboration between the department of civil protection of the marche region and ingv for the seismometric monitoring in central italy. we thank viviana castelli for her revision of a preliminary draft, and two anonymous reviewers for their constructive comments, which helped to improve this paper. references akaike h. 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(( o o, r2 o o r () \ i, y . f = 3 o . i a, oa\ — h, o r --• /. 1 . m = 1 v i ! . r i f e r i a m o la curva n o r m a l e cosi s c h e m a t i z z a t a a t r e sistemi ili --i c a r t e s i a n i o r t o g o n a l i . l ' o r i g i n e del p r i m o si i l e n i a sia in o,: l a s s e d i r e t t o p o s i t i v a m e n t e verso a , , l ' a s e \ p o s i l i v a m e n l e verso m. ouelo p r i m o sistema r a p p r e s e n t a la p a r t e di curva n o r m a l e f r a a.j e a , , ia o l ' o r i g i n e del s e c o n d o sistema c a r t e s i a n o . l e c o o r d i n a t e di o.. i s p e t t o al p r i m o s i s t e m a sono ( — ' " . . o ) : x è i n t e s o p o s i t i v o da o, a, q aoj 0, 0 2 a, p a0 h, / / p i e t r o c a i x j i \erso ( ) , . y 2 p o s i t i v o d a o., v e r s o n , . il t e r z o s i s t e m a h a o r i g i n e ii (),, x:, e s s e n d o p o s i t i v o e o n l a l o v e r s o o , , y 3 p o s i t i v o d a 0:l v e r s o -v h e c o o r d i n a t e di 0 3 r i s p e t t o al p r i m o s i s t e m a s o n o (—r.... o> l i c e o le e q u a z i o n i d e l l e r e t t e i e 2 . r i f e r i t e al p r i m o s i s t e m a : hi{x)=hl 11 — j l j , h.jx) = jil 1 1 f . [ 3 ] l ' a n a l o g a d e l l a 3 r i f e r i t a al s e c o n d o s i s t e m a »:,(*) = »* (i—-] r-n e q u e l l a d e l l a i r i f e r i t a al t e r z o s i s t e m a ft1(*) = 7 l . , ( l + — j [ 5 ] v e n g o n o i n o l t r e l e r e l a z i o n i <i.—r. ; a.. = r„ — . h, li.. " _ hl h. p o n i a m o , r i c o r d a n d o la [ 2 ] , x ; « , 2«, fi i 1 r, 1 a'/', 1 2nn„ . l «. ; « , — , > 1 "/ì 1 tv 1 i 1 a, 1 a'/', 1 2nn„ . i x : « . , 2r;2 « 1 1 v 1 g'>. 1 2na„ . : « . , 1 •~ — —r— 1 1 1 a., 1 g'>. 1 2na„ . 1 + :v ; f ' : i 2(i.. 2 ( 7 . . . f1 r„ 1 g'h 1 2 n n i 1 + a.. ; f ' : i 1 g^i ' 1 f1 a. 1 g'h 1 2 n n i ' 1 + x ; « i fi " <1 1 1 ' 1 + ; « i 1 a, [ 6 ] 5 e r le [ 1 ] a v r e m o ? , » ' , = } a j a i v j + b ^ , ( „ , ) | s i n ti (t t| -l = t " s a m « ' i ) + b i y m ! s i n n ( £ r ) ^ 2 o i t ) ? s « s = l . / 1 ( « r 2 ) i / i 2 y 1 ( , r 2 ) j sin „ ( / — t ) s u i p e 1 u 0 u i di o s c i l i . a z i o n e l i b e r a d e l \ e r r a n o l ' i = * ) aj0(h\j) + b2yu(ic2) j «in f i ( f — t ) 2(i„ i \ : , " • » = j ajxuj + b.y^w,) | « i n n ( f — t ) l,j,,(«•,)+ b:j„(u-,) { «in r i ( f — t ) r : i 2a.. i ' aj^irj + bj^ir,) j s i n r i ( f — t ) f ' ] j 1 j „ ( i o + £ f 4 y 0 ( i f 4 ) j « i n n ( t — t) -1 4 ' ' d o v e l e p r i m e d u e . la ó:l e la 6 a sono r i f e r i t e al p r i m o s i s t e m a , m e n t r e la 3 a e la f s o n o r i f e r i t e al s e c o n d o s i s t e m a e l e u l t i m e d u e a. terzo s i s t e m a . n e l l e [ 7 ] s o n o da d e t e r m i n a r e le n o v e e o s t a n t i i,. b,. l . b ... b,, n, v a l e n d o c i d e l l e c o n d i z i o n i ai l i m i t i . c o n s i d e r i a m o d e l l e p r i m e o t t o il r a p p o r t o ii/bi. l e c o n d i z i o n i a i l i m i t i « o l i o : a g l i e s t r e m i p e q del l a g o d e v ' e s s e r e c = 0 : x,p , ?. = 0 ; .v:; —q , ?< = () . [8] i n o l t r e , p e r * t = o ; , . [ 9 ] p e r il p u n t o f l . .r. = + r , , a\, = 0 ; = , [ 1 0 7 p e r iì p u n t o 0 •v1 ~~'ri , *i: = 0 i = u , = [ 1 1 ] d a l l e [ 7 ] , r i c o r d a n d o le [ 6 ] e ]>er l e 1 8 ] , si h a + = 0 , bja.,=—ji{nìk'] , [ 1 2 ] 1 t ( n |l) o p p u r e b„ l [ 1 3 ] p e r . t , = 0 è — = — ; p e r ;vt= + r , , = 0 «ì h a ir, ir.,— 3, u._ ir. a. p e r xl — —r... x = 0 è a n c o r a n\ i c , = p e r c u i , r i c o r d a n d o l e [ 9 j [ 1 0 ] e [ 1 1 ] e p o s t o p j t f l , " ; ) •"= l ' i ( n | l ) / , ( « « „ ) — j l [ n \ i i ) y 1 (rido) p 0 ( p s , a j = y , ( r i [ g / „ ( « « „ ) — / 1 ( n p 2 ) y „ f f i « ; ) n r f,!!'1), , (/1) = v 1 ( n p 4 ) ./,(nu.,)—,/,(ri|i,)l',(ri(i 4 ) pou'i 7 «i» = /clnit,) j . ( n ( 3 4 ) l ' 0 ( » « 4 ) , l'i p i e t r o calol h a .-/j/jinuj) i^y.jrmj = /(./-.lix.i : « , ! ' „ ( ; m , ) = a..j{(n<i...) lì ) .(/!•' ' •l,./„(««..) lì yjn «..i • t j ^ i l i ^ m ^ ll _ f (|-5.„<(.,) / 1 / 0 ( ' j p 1 ) r b i y i l ( n p l j ) = ' ^ l f0(|j.,,((.,) </., /(j ) , (fi p„) 1, 1\in |i..) + 113y, (n £ , ) = -^2p,(ii,,</..,) ; ,/: ;ì0i«ì-!3i b31'0(«po)=~~.. j q ".1 /'1 » i ( « p 4 ) ii d e t e r m i n a n t e dei c o e f f i c i e n t i d e l l e i n c o g n i t e .-(,, b, b . _/, u g u a g l i a t o a z e r o , dà l ' e q u a z i o n e dei p e r i o d i : [ 1 5 ] 1 , ( m i , ) (j "1 . 0 — / „ ( « « . . ) •ai « p i ) _ i l p i ( a g 1 , p a . " s ) (np„> () / 0 ! n p i ) "l 1*2 a : //, 1' 0 ( p . . « 8 ) 0 0 (1 0 0 0 0 r «.. 0 • " i l " : 0 0 0 (1 0 v (n p,3 ps p i i p a " , = 0 [ 1 6 j v . („,3..| " 4 l ' i ( " p j ) s u i p e r i o d i di o s c i l l a z i o n e l i b e r a d e l y e r b a n o l a [ 1 6 ] v a l e n e l ! i p o l e s i c l i c gli e s t r e m i del l a g o s i a n o s m u s s a t i . in r e a l t à , in .-/,,a„ il la<ro t e r m i n a a d a n g o l o a c u t o . c o n s i d e r a n d o , q u e s t o c o m e un c a s o l i m i t e d e l l a p r e c e d e n t e r a p p r e s e n t a z i o n e , p o s i a m o p o r r e j>~(i-,, q = al. n e l l e [ 4 ] , [ 5 ] a v r e m o p e r t a n t o e ' i n o l t r e , 1 , = 0 , p 4 = o . hm'i±wà=0 : l u n ^ ^ u 0 l ' i o ^ , ) j34 = 0 ì'i("ìjj.|1 p e r cui d a l l e [ 1 4 ] p . ( ? . ((..i , . . p01,3., , (/„) , hm 1 1 ' = , / , ( / ? « . , ) ; /im r — -./0(mh„) i m i ' r ' ^ = . / 1 ( n « j | ; /mi — ! = . / „ ( « « . , ) j34 = 0 , 3 4 = 0 5 ' i c " ^ ) l a [ 1 6 ] , o s s e r v a n d o c l i c j3( a , a., <7.. /;., «„ a, « 4 « 4 /(, (7, d i v i e n e a l l o r a , m o l t i p l i c a n d o la 5 a e la 6 1 r i g a p e r — , a.. ./,(/)((,) y , ( / u t , ) o — j j i m t . , ) y , ( r i a . , ) 0 / 0 ( n « , ) y 0 ( m v 0 •/„(««..) y o ( " < t : . ) <> « j x k , •/,(«?,) y i ( n ^ ) 0 0 0 ,) «i:-,,../,!., / . ( " p i ) w i > «a><«"«> 0 0 0 , 0 0 0 ^ ( n ^ , ) -/,(«'(.,) 0 0 » .7„(»p..) y „ ( " h , | , / 0 { n a t ) [ ! " ] 2 . — p a s s i a m o a l l a s o l u z i o n e d e l l a [ 1 7 ] . d a i dati r e l a t i v i a l l a c u r v a n o r m a l e , p e r i q u a l i r i m a n d o al lav o r o c i t a t o ( ' ) si o t t i e n e , n e l c a s o d e l l a s c h e m a t i z z a z i o n e p r e s c e l t a . /k = 16.0 x 10 o r ' , «, = 1.3025x 10'ni" . ir,= 1-1.8 x 10" m1 a , = 71* , 175 « , = •2920 x 10' ni r, = -256 x ] 0 " in" <t,=164s . 2935 p3 = ll 977 a 2 = -481 x i o hir, = -0825x10 n r |3,= 57 . (>88 «,. = 1212 . 6 « , = •1265 x 1 0 ' n r h, = 128,8 x 10" ni2 re, = 772 645 . ì l = . 0 7 4 6 6 <1., —i= 2 . 3 0 8 3 = . 0 3 3 7 8 p i e t r o caloi l e q u a z i o n e (lei p e r i o d i d i v i e n e p e r l a n l o i ) ( n ) j,( 164,29») ì^116l,29/i i 0 — j, • 71.175» i 1',(71.175»i n 70(164,29h) !'„( 164,29«.) il j,j 71.175//) v„i 71.175» i ii 7,157,688») 1,(57,688/n 7„l 772,645/n 0 ii ii () 7,(57,688») v*0(57,688»,l 7,(772.645/n i) 0 ii i) 0 ii 7,(41,977/11 1^(41,977/1) 7,(1242,6») 0 ii n 7„t41,977»i }"0(41,977//i 7„(1242.6»> [18] f.a r i s o l u z i o n e della [ 1 8 ] è s i a l a p r o p o s t a a l l ' i s t i l u t o p e r le a p p l i c a z i o n i del c a l c o l o , t a n t o b e n e m e r i t o p e r l ' a u s i l i o v a l i d i s s i m o c h e t o r n i s c e a l l a r i c e r c a s c i e n t i f i c a . f u r o n o d e s u n t i p e r le p r i m e sei r a d i c i p o s i t i v e di i ) ( n ) i seguenti v a l o r i = 0 . 0 0 2 3 1 , ri i = 0 . 0 0 6 8 2 71, = 0 . 0 0 3 5 2 ?i, = 0 . 0 0 7 7 8 7i, = 0 . 0 0 5 0 5 7i ,, = 0 . 0 0 9 6 3 . cui c o r r i s p o n d o n o , a m e n o d i 5\ i p e r i o d i t , = 15'",35 tv, = 2 0 " ' . 7 5 t , = 13"'.5 t-, = 29"', 75 j , = 1 5 " , 3 5 t , = i0"\85 p e r le p r i m e sei sesse del \ c i b a n o . 3 . — nel l a v o r o p r e c e d e n t e sulle sesse del l a g o m a g g i o r e si s o n o o t t e n u t i i s e g u e n t i v a l o r i p e r le sesse u n i b i e l r i n o d a l i : ri\ v , col m e t o d o d e f a n i 48"\7 3 7 " ' . 4 2 3 m , 4 » » h i d a k a 4 7 m , 4 3 3 " ' , 7 2 ] " ' , 0 . l a c c o r d o tra i v a l o r i o t t e n u t i c o n i ti c m e t o d i , s o d d i s f a c e n t e p e r la scssa u n i n o d a l e , cessa di e s s e r l o p e r la b i n o d a l e . m e n t r e è b u o n i s s i m o p e r la i r i n o d a l e . q u e s t e d i v e r s i t à , p a r t i c o l a r m e n t e sensibili p e r la b i n o d a l e , v a n n o a t t r i b u i t e a l l e n o t e v o l i v a r i a z i o n i c l i c il b a c i n o del iago p r e s e n t a in l a r g h e z z a e n e l l ' a n d a m e n t o d e l l a l i n e a dì v a l l e ; v a r i a z i o n i c h e lo d i s c o s t a n o s e n s i b i l m e n t e d a l l e i p o l e s i su ni p o g g i a n o le v a r i e t e o r i e . c o m u n q u e , e i n t e r e s s a n t e o s s e r v a r e e l l e , p r e s c i n d e n d o d a l l a b i n o d a l e . i p e r i o d i d e l l e scsse u n i n o d a l e ci t r i n o d a l e c a l c o l a t i con ire m e t o d i d i f f e r i s c o n o m o l t o p o c o e la d i f f e r e n z a è p a r t i c o l a r m e n t e p i c c o l a p e r la i r i n o d a l e . p e r la q u a l e i v a l o r i o t tenuti c o n i m e l o d i di h i d a k a e di c h r y s t a l p r a t i c a m e n t e c o i n c i d o n o . s u i p e r i o d i di o s c i l l a z i o n e l 1 i ì e r a d e l v e r s a n o 383 e " n o l o c l i c , q u a l u n q u e «ia il m e t o d o usato, i v a l o r i c a l c o l a t i p e r i pei iodi d e l l e sesse di m a g g i o r e n o d a l i t à d i n e r i s c o n o s e m p r e m e n o col c r e s c e r e d e l l a n o d a l i t à . p e r t a n t o i v a l o r i dei p e r i o d i d e l l e sess3 a q u a t t r o , c i n q u e e sei n o d i c a l c o l a t i in q u e s t o l a v o r o , possono r i t e n e r c i m o l t o p r o s s i m i a q u e l l i r e a l i . a n c o r a non è stalo p o s s i b i l e o t t e n e r e b u o n e r e g i s t r a z i o n i d e l l e se«-e del \ o r b a n o . c o m u n q u e dai v a l o r i finora o s s e r v a l i , s e m b r a c l i c i p e r i o d i f o r n i t i dal m e t o d o di h i d a k a s i a n o i p i ù a t t e n d i b i l i . roma — istituto nazionale (li geolìsica — aprile 1918. riassunto partendo dalle busi della teoria di clirystal, si sviluppa un metodo analitico per la determinazione dei periodi di oscillazione libera delle acque del lago maggiore. risolta vequazione dei periodi — costituita da un determinante di sesto ordine — . .si sono calcolati i ndori dei periodi corrispondenti alle oscillazioni libere di uno. due, tre. ... sei nodi. b i b l i o g r a f i a i ) c a l o i i ' . . d e p a n f i l i » m . . g i o i i c i m . . p e h o n a c i f . , le sesse del l.m>o maggiore. « a n n a l i di g c o f i s i e a » . i , 2 ( 1 9 4 8 ) . ( ) c a l o i /'.. le sesse del lago di garda. p a r t e i i « a n n . di g e o f . » i , 2 ( 1 9 4 8 1 . annals 47, 1, 2004, 01/07def 171 annals of geophysics, vol. 47, n. 1, february 2004 key words earth crust – tolman-stewart effect 1. introduction the excitation of magnetic oscillations by seismic waves has attracted considerable attention for a long time (e.g., breiner, 1964; eleman, 1966; belov et al., 1974; iyemory et al., 1996; tsegmed et al., 2000). the interest in seismomagnetic phenomena is motivated by the hope for advances in understanding mechano-magnetic transformations in the earth’s crust. the challenging task on this way is the detection of the rather small seismomagnetic signals against a noise background. strong interferences hamper the collection of experimental data. recently tsegmed et al. (2000) proposed the polarization method for recognition of signals. the method is based on the fact that the magnetic signal has circular polarization in the vertical plane regardless of the particular mechanism of magnetic field excitation by the seismic wave. the polarization plane is perpendicular to the seismic wave front, and the rotation direction of the magnetic vector is controlled by the direction of seismic wave propagation. in the present paper we describe the polarization method and apply it to the detection of the magnetic oscillations accompanying the propagation of surface love wave after a strong earthquake. the selection of this type of seismic waves is not accidental. it is motivated by some special physical reasons. the love wave is of immediate interest to the experimentalist because theoretically this wave induces the tolman-stewart effect in the earth’s crust, which is responsible for the magnetic field that is observable over the earth’s surface (guglielmi, 1992, 1999). on the excitation of magnetic signals by love waves anatol v. guglielmi (1), alexander s. potapov (2) and battuulai tsegmed (2) (3) (1) schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia (2) institute of solar-terrestrial physics, siberian branch of the russian academy of sciences, irkutsk, russia (3) astronomy and geophysics research center, mongolian academy of sciences, ulaanbaatar, mongolia abstract the polarization method for recognition of seismomagnetic waves against a noise background is presented. the method is applied to detection of magnetic oscillations accompanying the propagation of surface love wave after a strong earthquake. a specific property of the love waves is that theoretically the tolman-stewart effect is alone responsible for the magnetic field that penetrates into the earth’s surface. data from the mondy magnetic observatory and the talaya seismic station suggest that the arrival time, duration, period, and polarization of magnetic signals conform with the idea of generation of alternating electric currents due to fluid vibrations in pores and fractures of rocks under the action of the inertial force associated with the love wave propagation. mailing address: dr. alexander s. potapov, institute of solar-terrestrial physics, siberian branch of the russian academy of sciences, p.o. box 4026, 664033 irkutsk, russia; e-mail: potapov@iszf.irk.ru 172 anatol v. guglielmi, alexander s. potapov and battuulai tsegmed 2. mechano-magnetic transformations the equation of the mechano-magnetic transformations in the earth’s crust makes it easy to understand our interest into the love waves. indeed, let us put b = ∇ × a. the equation for the vector-potential a has the form da s=t (2.1) where d t 2 2 2 d= at (2.2) is the evolution operator, and the term s m e u b v a0 0# $ #d d= + + -a b c (2.3) describes the underground sources of the magnetic field b (x, t) (guglielmi, 1999; tsegmed et al., 2000). we have used the following designations: c2 , , , ., lnc m e m p p p c u c u u x u x t t b u v u a v 2 2 i ll ij ij j ij t ij l t ij ij i j j i ef 2 0 1 2 0 2 2 2 $ 2 2 2 2 2 2 d 2 2 2 2 = = = = + = + = + = = = a f v b v i c m d m t t id i _ ^ _ i h i here the mechanical values a, v, and u are the acceleration, velocity and displacement of the rocks, pij and uij are the stress and strain tensors, e 0 is the electric field associated with the terrestrial currents, b0 is the geomagnetic field, ct and cl are the velocities of transverse and longitudinal elastic waves, c is the light velocity, ε 0 is the permeability of free space, ρ and σ are the density and conductivity of rocks, e and mef are the charge and effective mass of the conduction ion in the porous fluid. the four terms in the right-hand side of eq. (2.3) have the following interpretation: the first accounts for the piezomagnetic effect, the second describes the modulation of the terrestrial currents by the seismic vibrations, and the two last terms describe the induction and inertial effects. the five phenomenological parameters of the model have the following interpretation: α is the diffusion coefficient, and β, γ, λ1, λ2 are the coefficients describing the conversion of mechanical energy into the energy of magnetic field. we see that generally at least four mechanisms operate in the earth’s crust simultaneously and independently of each other. so we have a rather cumbersome superposition of fields. however, it is always interesting to select and clarify the limiting case when only one generation mechanism concerned prevails. and in this respect, the love wave is especially interesting. a specific property of this wave is that, theoretically, the inertial mechanism alone is responsible for the magnetic field which penetrates into the earth surface (guglielmi et al., 1996). moreover, the love wave makes it possible to switch on the least understood generation mechanism. the most striking feature of it is its universality. it operates in any conductive body and, therefore, it operates in the earth’s crust too. this simple and universal mechanism was brought into classical physics in 1936 by darwin in his attempt to explain the tolman-stewart effect. darwin’s theory predicts that the coefficient of mechano-magnetic transformation γ is proportional to the mass of electron (e.g., landau and lifshitz, 1984). this is true in the case of a metal. but in the case of the porous moist body, we expect that γ is proportional to some effective mass mef, which is many orders above the mass of electron (guglielmi, 1992). it should be noted that the tolman-stewart effect was never observed in the laboratory using rock samples and we assume that it is practically impossible because the typical skin-length is many orders higher than the feasible size of a rock sample. below we present our attempt to crack a problem by the full-scale observations. 3. polarization method it is common knowledge that observations of the seismomagnetic signals are impeded by the noise of magnetospheric origin. to suppress the magnetospheric interference, we have elaborated a special method based on the tapping of a priori information on the polarization state of seismomagnetic field (tsegmed et al., 2000). in this section, we present a general idea. the application of the polarization method to record 173 on the excitation of magnetic signals by love waves magnetic wave that accompany the love wave will be described in the next section. we introduce the cartesian coordinate system (x, y, z) such that the earth’s surface coincides with the (x, y) plane and the z-axis is directed upward (fig. 1). let a plane (∂ / ∂y = 0) monochromatic elastic wave travel in the positive direction of the x-axis in the lower halfspace (z < 0) supposed to be horizontally homogeneous. we are interested in the magnetic field b (x, z, t) in the upper half-space (z ≥ 0), where it obeys the laplace law ∇2b = 0 and the solenoidality condition ∇∴ b = 0. we suppose that the field b is excited by the elastic wave, and it is linearly related to the mechanical variables. then b ∝ exp (ikx − iωt), the solenoidality condition takes the form b k i z b x z 2 2 = (3.1) and the laplace equation yields z b k b 0 z z2 2 2 2 2 = (3.2) where k = ω /cl, ω is the frequency and cl is the horizontal velocity of elastic wave (in the next sections cl will be considered the velocity of love wave). the magnetic field tends to zero with increasing the height above the earth’s surface because the field sources are located beneath the surface. hence, the eq. (3.2) gives bz ∝ exp ⋅ ⋅ (− kz). substitution of this expression in (3.1) yields bz = ibx. as regards by component, it is negligible in comparison with bx and bz (in the particular case of the love wave by = 0) therefore we have , , .exp b i k ix z i tb 2 1 0= ~^ ]h g6 @ (3.3) this means that on the earth’s surface and above it the modulus bremains constant. the magnetic wave has circular polarization with counterclockwise rotation. the vector b rotates with the frequency ω in the vertical plane parallel to the direction of seismic wave propagation. these polarization properties are quite general. besides, these properties are so specific, that one can try to use them for the detection of seismomagnetic signals. 4. observations an earthquake with magnitude m = 7.9 occurred on june 18, 2000, at a depth of 10 km with epicenter in the indian ocean (see the site of the iris consortium at www.iris.washington.edu). to search for a seismomagnetic signal, we used data from the mondy magnetic observatory (51.6ºn, 100.8ºe) and the talaya seismic station (51.7ºn, 103.6ºe). the mondy observatory is equipped with a high-frequency digital three-component induction magnetometer developed at the university of tokyo and kindly provided by prof. k. hayashi. the main parameters of the magnetometer are as follows: frequency range, 0.001-3.0 hz; sampling rate, 10 hz; gps time synchronization; data storage on magneto-optical disks. the talaya seismic station 200 km east of mondy is equipped with a highly sensitive digital seismometer with a sampling rate of 1 hz. talaya seismic wave records were taken from the internet (www.fdsn.org/station_book/ii/tly/tly_3.html). the epicentral distances to the mondy observatory fig. 1. polarization of seismomagnetic oscillations (see the text). 174 anatol v. guglielmi, alexander s. potapov and battuulai tsegmed and talaya station are, respectively, 7280 and 7269 km. the azimuthal aperture between these two points is 4.2º. first, the seismogram was used to visually assess the arrival time (15:17:45 ut), characteristic period (t = 23 s), and amplitude (about 300 µm/s) of surface love waves. the onset of waves corresponds to an average velocity of 3.6 km/s. thus, the love wave delay at mondy is 3 s, i.e. it is small compared to the period of seismic waves. the distance between the observation points of magnetic and seismic waves is also small compared to the epicentral distance. therefore, for simplicity, we will ignore the effects due to the difference between locations of the observation points. for subsequent analysis, we transformed the initial numerical dataset to a coordinate system rotated about the vertical axis z in such a way as to bring the x-axis into coincidence with the tangent to the arc of the great circle passing through the epicenter and to direct it away from the epicenter. moreover, the initial data were subjected to broadband filtering with a passband of 5-200 mhz in order to eliminate high-frequency noise and long-period trends. finally, using the results of the preliminary visual analysis, we chose a frequency-time window containing love waves and constructed the wave amplitude spectrum, after which we determined more accurately the carrier frequency (f = 43 mhz) and the time interval (15:13:20-15:25:00 ut) suitable for detecting seismomagnetic oscillations. figure 2 shows oscillograms of mechanical and magnetic oscillations after the rotation of the coordinate axes and broadband filtering. here, vy is the transverse component of the velocity, and bx : and by : are time derivatives of the horizontal and vertical components of the magnetic field, respectively. the seismogram dis. b x, n t /s v y, n m /s -0.025 0 0.025 15:13:20 15:20:00 15:26:40 ut -0.025 0 0.025 -300 300 . b z, n t /s fig. 2. seismic waves and magnetic oscillations recorded at the talaya station and the mondy observatory, respectively, on june 18, 2000. here, vy is the transverse component of the displacement velocity, and bx : and by : are the horizontal and vertical components of the magnetic field, where dots mean the differentiation with respect to time. the coordinate system is oriented in such a way as to make the x-axis parallel to the arc of the great circle that passes through the earthquake epicenter (see text). 175 on the excitation of magnetic signals by love waves plays quasi-harmonic oscillations typical of love waves in the far-field zone. the record of the magnetic field is severely complicated by noise. against the background of this noise, the seismomagnetic signal cannot be detected by simple comparison of the oscillograms. we attempted to identify the seismomagnetic signal by the spectral method. figure 3 presents the spectra of mechanical and magnetic oscillations in the time interval 15:16:40-15:25:00 ut. the mechanical spectrum exhibits a distinct peak at a frequency of 43 mhz. however, this effect is not observed in the spectra of magnetic field components. therefore, we applied the method of spectral polarization filtering based on the theoretical idea that the electromagnetic field of harmonic love wave has the structure of an hwave with a left-hand circular polarization in the vertical plane. the state of polarization can be conveniently described by the ellipticity parameter varying from − 1 to + 1 and chosen such that its value − 1 corresponds to the lefthand polarization. the variation in the ellipticity of magnetic oscillations in the vertical plane at the frequency of 43 mhz is shown in the middle part of fig. 4. the lower plot shows the variation in the amplitude b of the circular component of oscillations with a left-hand rotation of the magnetic vector at the same frequency. as expected, the lowest values of the ellipticity and the highest amplitudes of magnetic oscillations with the left-hand circular polarization are observed during the passage of the 43 mhz love wavetrain. this leads to the assumption that we have detected seismomagnetic oscillations with the amplitude b = 0.02 nt. 5. discussion the period, polarization, onset time, and duration of the detected magnetic oscillations are consistent with the concept according to which these oscillations are excited by love wave. it is quite clear that the seismic waves are no more than a causa instrumentalis; i.e. they are only an external cause of the field generation, whereas the immediate cause is the current, and our task is to identify the current-generating mechanism. in the general case, this is a difficult problem because several mechanisms operate in the crust concurrently and independently of each other during seismic wave propagation. we chose the love waves because the theory (guglielmi et al., 1996) leaves no other alternative and unambiguously indicates an inertial generation mechanism in this particular case. this universal mechanism, describing by the equation t b b 2 2 2 d+ =c aω_ i (5.1) operates in the terrestrial and celestial conductive bodies which are in motion with time-dependent vorticity ω = ∇ × v. (the values v, α, u . b z( f) u, n t 0 0.4 0.8 1.2 1.6 0 0.2 0.4 0.6 0.8 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 f, mhz u . b x ( f) u, n t u v y( f) u, n t fig. 3. spectra of oscillations shown in fig. 2. 176 anatol v. guglielmi, alexander s. potapov and battuulai tsegmed and γ are defined above in the section 2). for example, sedrakian (1974) proposed that the magnetic fields of the neutron stars could be explained in part by the inertia effect. in his model the pulsar consists of the rotating superfluid of neutrons and protons, and the normal fluid of electrons. because the angular velocity of neutron star is not constant, the protons move relative to the electrons creating the toroidal electric currents that produce extremely strong magnetic field. this is a sort of tolman-stewart effect. we must realize that the tolman-stewart effect provides the generation of magnetic field in any conductive body, which moves with nonzero vorticity ω. in this regard there is no question that the love wave generates the magnetic field oscillations because ω ≠ 0 during the passage of this wave. the real problem is the amplitude of magnetic oscillations, which is determined by the coefficient of mechano-magnetic transformation γ = me f /e. in a metallic conductor mef = me, where me is the mass of electron. then γ = − 5.68⋅10− 12 kg/s⋅a, and it is likely that eleman (1966) expected just this case when he rejected a priori the inertial mechanism as a possible cause of seismomagnetic signals. the electronic mechanism of conductivity operates in the earth’s core and lower mantle, and here the tolman-stewart effect is truly negligible. in the upper mantle the ionic conductivity takes place, i.e. mef ≈ mi. however, the tolman-stewart effect is also negligible in the upper mantle. the earth’s crust may be considered as an ionic conductor too, but mef > > mi because the inertial force sets in motion the porous fluid, and not just the conduction ion (guglielmi, 1992). a rough estimate gives γ ≈ ρ f k, where k is the electrokinetic coefficient, and ρ f ≈103 kg/m3 is the density of porous fluid. laboratory experiments with the rock samples indicate that k varies from 10 mv/mpa to 10 v/mpa dependv y, n m /s 15:13:20 15:20:00 15:26:40 ut 0 0.01 0.02 -1 -0.5 0 0.5 1 -300 0 300 el li pt ic it y b , n t fig. 4. comparison of seismic oscillations vy (top panel) with variations in the ellipticity of magnetic oscillations (middle panel) and the amplitude of magnetic oscillations with a left-hand circular polarization in the tangent plane (bottom panel). 177 on the excitation of magnetic signals by love waves ing on the temperature and salinity of water and on the structure and saturation of the porous space. the coefficient of mechano-magnetic transformations γ varies correspondingly from 10−5 to 10−2 kg/s. let us show that our observations do not contradict these estimates. one can see from eq. (5.1) that γ ∼ β / kv, where k = ω / cl. according to our measurements b ∼ 2⋅10− 2 nt, v∼ ∼ 3⋅10− 4 m/s, cl ∼ 3.6 km/s, so that γ =10−3 kg/s⋅a. 6. summary we have presented the polarization method for recognition of seismomagnetic waves against a noise background. the method is applied to detection of magnetic oscillations accompanying the propagation of surface love wave after the strong earthquake. we have focused our attention on the love wave because theoretically the tolman-stewart effect in the earth’s crust is responsible for the magnetic field that penetrates to the earth’s surface. data from the mondy magnetic observatory and the talaya seismic station suggest that the arrival time, duration, period, and polarization of magnetic signal conform with the idea of generation of alternating electric currents due to fluid vibrations in pores and fractures of rocks under the action of the inertial force associated with the love wave propagation. the estimated ratio of the amplitude of magnetic oscillations to the amplitude of love waves is indirect evidence that the inertial mechanism of converting the seismic wave energy into the electromagnetic field energy is reasonably effective. we believe that the inferred result provides an additional basis for the interpretation of seismoelectromagnetic phenomena. our conclusion is as follows. in his ‘foreword’ to the recent monograph seismo-electromagnetics, prof. uyeda (2002) said that «the earthquakes are nothing but physical phenomena». these are remarkable and optimistic words in the context of the earthquake prediction research. the specifics of such research is that the laboratory modeling of the pre-seismic evolution of the geophysical fields is practically impossible. this resembles in some respects the situation with the inertial mechanism of magnetic field generation. it is rather difficult to carry out laboratory measurements because the skin-length is many orders higher than the feasible size of a rock sample. we hope that our modest experience in an effort to detect the magnetic signal due to the love wave may be useful for the prediction research as an example to crack a problem of the sort by full-scale measurements. acknowledgements we are grateful to prof. k. hayashi, who provided us with digital magnetometer data. this work was supported by the russian foundation for basic research, project no. 04-05-64265. references belov, s.v., n.i. migunov and g.a. sobolev (1974): magnetic effect of strong earthquakes in the kamchatka region, geomagn. aeron., 14, 380-382. breiner, s. (1964): piezomagnetic effect at the time of local earthquakes, nature, 202, 790-791. eleman, f. (1966): the response of magnetic instruments to earthquake waves, j. geomagn. geoelectr., 18, 43-72. guglielmi, a. (1992): elastomagnetic waves in a porous medium, phys. scr., 46, 433-434. guglielmi, a. (1999): some problems in seismoelectrodynamics: a review, in atmospheric and ionospheric electromagnetic phenomena associated with earthquakes, edited by m. hayakawa (terra scientific publishing co., tokyo), 931-938. guglielmi, a.v., v.t. levshenko and d.o. pokhotelov (1996): comparative analysis of the dynamic and quasi-static of seismomagnetic signals generation mechanisms, fiz. zemli, 12, 18-25. iyemori, t., t. kamei, y. tanaka, m. takeda, t. hashimoto, t. araki, t. okamoto, k. watanabe, n. sumitomo and n. oshiman (1996): co-seismic geomagnetic variations observed at the 1995 hyogokennanbu earthquake, j. geomagn. geoelectr., 48, 10591070. landau, l.d. and e.m. lifshitz (1984): electrodynamics of continuous media (pergamon, oxford), pp. 460. sedrakian, d.m. (1974): the magnetic fields of pulsars, in gravitational radiation and gravitational collapse, edited by c. dewitt-morette (d. reidel publ. co., dordrecht-holland/boston-u.s.a.), pp. 187. tsegmed, b., a.v. guglielmi and a.s. potapov (2000): application of the polarization method to the observation of seismomagnetic waves produced by strong earthquakes, fiz. zemli, 12, 41-50. uyeda, s. (2002): foreword, in seismo electromagnetics, edited by m. hayakawa and o.a. molchanov (terra scientific publishing co., tokyo), p. ix. microsoft word mipas-val_v3_shortref.doc annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6329 1 long-term intercomparison of mipas additional species clono2, n2o5, cfc-11, and cfc-12 with mipas-b measurements gerald wetzel, hermann oelhaf, felix friedlvallon, anne kleinert, guido maucher, hans nordmeyer, johannes orphal karlsruhe institute of technology, institute for meteorology and climate research, karlsruhe, germany gerald.wetzel@kit.edu i. introduction he michelson interferometer for passive atmospheric sounding (mipas) aboard the environmental satellite envisat is a limb-viewing fourier-transform emission spectrometer working in the mid-infrared spectral region between 685 cm-1 and 2410 cm-1 [fischer et al. 2008]. during the ten years of operation, mipas was able to observe temperature and a large number of climate-relevant trace gases from the upper troposphere up to the mesosphere (nominal observation mode) and even the thermosphere (special modes). for about two years after the launch of the satellite on 1 march 2002, mipas recorded spectra with full spectral resolution of 0.025 cm-1. after anomalies in the velocity of the interferometer drive unit and a subsequent interruption of the measurements (march 2004), a lower (so-called optimized) spectral resolution of 0.0625 cm-1 together with a higher spatial resolution was chosen such that operational observations could restart in january 2005. following the loss of communications with the satellite platform, there are no mipas data available since 8 april 2012. the description and characterization of the operational european space agency (esa) processor products have been presented by raspollini et al. [2006, 2013] and references therein. while previous operational product processing was limited to temperature and the molecules h2o, o3, hno3, ch4, n2o, and no2, the new ml2pp v6 data includes also the minor “additional” trace gases cfc-11 (ccl3f), cfc-12 (ccl2f2), n2o5, and clono2. in this paper we focus on the comparison of these additional species. several flights with mipas-b, the balloon version of mipas, have been carried out during the operational period of envisat. table 1 gives an overview of mipas-b flights where mipas-b observations coincide in space and time with mipas on envisat (mipas-e) measurements. mipas-b limb sequences recorded during these flights are compared to the corresponding mipas-e overpasses. mipas-b was strongly involved in the envisat validation activities and the high accuracy of the mipas-b data has been assessed in numerous mipas-e related validation studies [see, e.g., cortesi et al. 2007, höpfner et al. 2007, milz et al. 2009, payan et al. 2009, ridolfi et al. 2007, steck et al. 2007, wang et al. 2007a, 2007b, wetzel et al., 2007, 2013, zhang et al. 2010a, 2010b]. t annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6329 2 ii. mipas-b data analysis and intercomparison method mipas-b can be regarded as a precursor of the satellite instrument [see friedl-vallon et al., 2004 and references therein]. therefore, a number of specifications are quite similar, such as spectral resolution and spectral coverage. however, for some essential parameters, the mipas-b performance is superior, e.g. in terms of the nesr (noise equivalent spectral radiance), and in the case of the pointing accuracy. further improvement of the nesr is achieved by averaging multiple spectra taken at the same pointing angle. mipas-b measures all atmospheric parameters covered by mipas-e. essential for the balloon instrument is the sophisticated line of sight stabilization system, which is based on an inertial navigation system and supplemented with an additional star reference system leading to an after all knowledge of the tangent altitude in the order of 90 m (3 σ). the mipas-b data processing from interferograms to calibrated spectra including instrument characterization is described in friedl-vallon et al. [2004] and references therein. the measurements were done typically at a 1.5 km vertical grid. retrieval calculations of atmospheric target parameters were performed at a 1 km grid with a least squares fitting algorithm using analytical derivative spectra calculated by the karlsruhe optimized and precise radiative transfer algorithm [kopra; stiller et al. 2002; höpfner et al. 2002]. a tikhonov-phillips regularization approach [phillips, 1962, tikhonov, 1963] constraining with respect to a first derivative a priori profile was adopted. the resulting vertical resolution is typically between 2 and 5 km for the analyzed species and is therefore comparable to or slightly better than the vertical resolution of mipas-e. the species clono2, cfc-11, cfc-12, and n2o5 are analyzed in the spectral windows 779.7 780.7 cm-1, 840 860 cm-1, 918 924 cm-1, and 1220 1270 cm-1, respectively. different spectral windows within these molecular bands were used for the mipas-e data analysis [raspollini et al. 2013]. spectroscopic parameters for the calculation of limb emission spectra originate from the highresolution transmission (hitran) molecular absorption database [rothman et al. 2005]. the mipas-b error estimation includes random noise as well as the mutual influence (covariance) of the fitted parameters, temperature errors, pointing inaccuracies, errors of nonsimultaneously fitted interfering gases, and spectroscopic data errors (1 σ). for details on the mipas-b data analysis and error estimation, see wetzel et al. [2012] and references therein. since pressure is the primary vertical coordinate of mipas-e, vertical profiles of the satellite sensor have been interpolated to the mipas-b pressure-altitude grid. differences between measured volume mixing ratios (vmrs) of mipas-e and the validation instrument mipas-b are expressed in absolute units and relative differences. the mean difference δxmean for n profile pairs of compared observations is given as: )( 1 , 1 , nb n n nemean xx n x    (1) where xe and xb are vmr values of mipas-e and mipas-b at one altitude level. the mean relative difference δxmean,rel of a number of profile pairs is calculated by dividing the mean absolute difference by the mean profile value of the validation instrument mipas-b: annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6329 3 %100 1 1 , ,      n n nb mean relmean x n x x . (2) differences are displayed together with the combined errors σcomb of both instruments which are defined as: 22 becomb   , (3) where σe and σb are the precision, systematic or total errors of mipas-e and the validation instrument, respectively. precision errors characterize the reproducibility of a measurement and correspond, in general, to random noise errors. systematic errors of the v6 processor used for mipas-e data have been assessed in a corresponding esa study [raspollini et al. 2013]. the uncertainty of the calculated mean difference (standard error of the mean, sem) is given by σ/n0.5 where σ is the standard deviation (sd). a bias between both instruments is considered significant if the sem is smaller than the bias itself. the comparison between the vmr difference and the combined systematic error (for statistical comparisons) or total error (for single comparisons) is appropriate to identify unexplained biases in the mipas-e observations when they exceed these combined error limits. since the vertical resolution of the used vmr profiles of both instruments is of comparable magnitude, no smoothing by averaging kernels has been applied for the intercomparison of the observed profiles. iii. intercomparison results table 1: overview of mipas-b flights used for intercomparison with mipas-e. distances and times between coincident trace gas profile pairs observed by mipas-e and the validation instrument refer to an altitude of 20 km (kiruna) and 30 km (aire-sur-l’adour and teresina). location date distance (km) time difference (min) 20 mar 2003 16/546 14/15 11 mar 2009 187/248 5/6 kiruna (sweden, 68°n) 24 jan 2010 109/302 5/6 aire-surl’adour (france, 44°n) 24 sep 2002 21/588/4 10/146 12/13/15 /16 14 jun 2005 109/497/ 184/338 228/229/ 268/269 teresina (brazil, 5°s) 06 jun 2008 224/284/ 600/194 157/158/ 169/170 in this section, esa operationally processed (ml2pp v6) mipas-e vmr profiles of the molecules clono2, n2o5, cfc-11, and cfc-12 are compared to coincident mipas-b observations. table 1 summarizes corresponding balloon flights in terms of date, time, locations, and differences in time and space to the satellite measurements. mean deviations of all coincident clono2 profiles measured by mipas-b and mipas-e between 2002 and 2010 are shown in figure 1. up to 18 collocations are available per altitude. the agreement between the measured profiles appears to be quite good with a mean deviation over all altitudes of 7.2 %. some biases are visible in limited altitude regions but they are not significant with regard to the combined error limits. figure 2 displays the differences concerning the molecule n2o5. this molecule exhibits a pronounced diurnal variation in its mixing raannals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6329 4 tio. hence, a photochemical correction using a box model [wetzel et al. 1995], which takes into account the nighttime variation of n2o5, has been applied to the tropical mipas-e measurements where both, diurnal variation and the time difference of the observations (see table 1), are quite large. apart from the lowermost altitude region between 15 and 17 km (where only 2 collocations are available) there is no significant bias visible between the balloon and satellite measurements of mipas. the overall agreement between both sensors is fairly good with a mean deviation of -6.7 %. the comparison for the species cfc-11 is shown in figure 3. in contrast to the previously discussed molecules, significantly higher values are visible in the mipas satellite data in the complete altitude region under comparison. while below 19 km this positive bias is still within the combined systematic and total errors, it clearly exceeds these error limits above and therefore remains unexplained. figure 4 displays the observed differences for the molecule cfc-12. unlike the case of cfc11, the agreement of measured cfc-12 vmr is quite good with a mean difference of 11.5 %. no significant biases are visible at all. iv. conclusions vertical profiles of six mipas balloon flights between 2002 and 2010 have been used for an intercomparison study of the species clono2, n2o5, cfc-11, and cfc-12, which have been processed for the first time by the operational esa processor (ml2pp v6). for the molecules clono2, n2o5, and cfc-12 the agreement between both mipas instruments is generally good. no clear biases in the observed mixing ratios are visible. furthermore, no seasonal or latitudinal dependence of deviations could be recognized. however, standard deviations are slightly larger than the combined precision errors. concerning the species cfc-11 a significant positive bias is evident at all altitudes under comparison. above 19 km, this bias clearly exceeds the combined systematic and total errors and remains unexplained. hence, this molecule cannot be recommended for scientific users at the present state. investigations are ongoing within the esa processor development team to understand the reason of the bias. it should also be mentioned that some single mipas-e profiles of these additional species exhibit retrieval oscillations which show up in vmr differences (in limited altitude regions) to mipas-b observations but which may in turn be smoothed out in comparisons of mean differences. scientific users should therefore be careful in their interpretation when using single mipas-e profiles of these trace gases. 40 35 30 25 20 15 10 -0.2 -0.1 0.0 0.1 0.2 -0.2 -0.1 0.0 0.1 0.2 -40 -20 0 20 40 40 35 30 25 20 15 10 -40 -20 0 20 40 100 10 2 2 6 6 6 8 10 10 10 10 12 12 14 16 16 16 18 18 18 18 18 18 18 18 18 18 18 2 2 6 6 6 8 10 10 10 10 12 12 14 16 16 16 18 18 18 18 18 18 18 18 18 18 18 a lt itu d e ( km ) a lt itu d e ( km ) mipas-e mipas-b mean diff.: 0.02 ± 0.01 ppbv 7.2 ± 4.0 % difference ( sd) prec. systematic tot. mean comb. err. clono 2 vmr difference (ppbv) clono 2 vmr difference (%) p re ss u re ( h p a ) figure 1: mean absolute and relative clono2 vmr differences of all collocations between mipas-e and mipas-b (red solid lines) including standard deviation (red dotted lines) and the standard error of the mean, plotted as error bars around the mean deviation together with precision (blue dotted lines), systematic (blue dashdotted lines) and total (blue dashed lines) mean combined errors. red values indicate the number of collocations used for the statistical analysis. differences of clono2 mixing ratios are well within the combined error limits. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6329 5 40 35 30 25 20 15 10 -0.4 -0.2 0.0 0.2 0.4 -0.4 -0.2 0.0 0.2 0.4 -60 -40 -20 0 20 40 60 40 35 30 25 20 15 10 -60 -40 -20 0 20 40 60 100 10 2 2 2 2 4 8 10 10 10 10 12 14 14 14 16 16 18 18 18 18 18 18 18 18 18 2 2 2 2 4 8 10 10 10 10 12 14 14 14 16 16 18 18 18 18 18 18 18 18 18 a lt itu d e ( km ) a lt itu d e ( km ) mipas-e mipas-b mean diff.: -0.02 ± 0.02 ppbv -6.7 ± 16.7 % difference ( sd) prec. systematic tot. mean comb. err. n 2 o 5 vmr difference (ppbv) n 2 o 5 vmr difference (%) p re ss u re ( h p a ) figure 2: same as figure 1, but for the molecule n2o5. the overall agreement of both sensors is fairly good. 40 35 30 25 20 15 10 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -40 -20 0 20 40 40 35 30 25 20 15 10 -40 -20 0 20 40 100 10 2 2 4 6 7 9 9 10 10 10 10 10 12 14 15 16 17 17 18 18 18 18 2 2 4 6 7 9 9 10 10 10 10 10 12 14 15 16 17 17 18 18 18 18 a lti tu d e ( km ) a lti tu d e ( km ) mipas-e mipas-b mean diff.: 0.032 ± 0.003 ppbv 134.8 ± 66.2 % difference ( sd) prec. systematic tot. mean comb. err. cfc-11 vmr difference (ppbv) cfc-11 vmr difference (%) p re ss u re ( h p a ) figure 3: same as figure 1, but for the species cfc-11. a positive bias is visible in the mipas-e data, especially above 19 km. 40 35 30 25 20 15 10 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -40 -20 0 20 40 40 35 30 25 20 15 10 -40 -20 0 20 40 100 10 2 2 4 7 9 9 10 10 10 12 12 12 14 14 15 16 16 16 17 17 18 18 18 18 18 18 18 18 18 18 18 2 2 4 7 9 9 10 10 10 12 12 12 14 14 15 16 16 16 17 17 18 18 18 18 18 18 18 18 18 18 18 a lt itu d e ( k m ) a lt itu d e ( k m ) mipas-e mipas-b mean diff.: 0.001 ± 0.003 ppbv 11.5 ± 9.4 % difference ( sd) prec. systematic tot. mean comb. err. cfc-12 vmr difference (ppbv) cfc-12 vmr difference (%) p re ss u re ( h p a ) figure 4: same as figure 1, but for the molecule cfc12. no significant bias is obvious between the measured profiles of both mipas instruments. acknowledgements financial support by the national aeronautics and space research centre dlr (project 50ee0020) and esa for the mipas-b balloon flights is gratefully acknowledged. we thank the centre national d'etudes spatiales (cnes) balloon launching team and the swedish space corporation (ssc) esrange people for excellent balloon operations and the free university of berlin (k. grunow and b. naujokat) for meteorological support. references cortesi, u., j. c. lambert, c. de clercq, et al. 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(2010b). validation of atmospheric chemistry measurements from mipas, sciamachy, gomos onboard envisat by observations of balloon-borne mipas-b, sci. china earth sci., 2010, 53, 1533–1541. vol51,1,2008_delnegro 67 annals of geophysics, vol. 51, n. 1, february 2008 key words time gravity changes – gravity record – volcanic processes – air pressure admittance 1. introduction a wide set of dynamic phenomena (i.e. geodynamics, seismicity, volcanic activity) can produce temporal gravity changes, with a spectrum varying from short (1-10 s) to longer (more than 1 year) periods. an impending eruption, for instance, is generally associated with the ascent of magma producing changes in the density distribution at depth, and leading to ground deformation and gravity changes observed at surface. the amplitude of such gravity variations is often quite small, on the order of 10−9-10-8 g (10-102 nms−2; 1-10 µgal), so their detection requires high quality data and a rigorous procedure to split up from the records those weak gravity signals coming from different sources. what exactly would time-variable gravity (tvg) tell us about mass redistribution below a volcano? the detected tvg is the sum strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius umberto riccardi (1), giovanna berrino (2), gennaro corrado (1) and jacques hinderer (3) (1) dipartimento di scienze della terra, università degli studi di napoli «federico ii», napoli, italy (2) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli, italy (3) ecole et observatoire des sciences de la terre (eost), institut de physique du globe de strasbourg, université louis pasteur (umr 7516 cnrs-ulp), strasbourg (france) abstract this research is intended to describe new strategies in the processing and analysis of continuous gravity records collected in active volcanic areas and to assess how permanent gravity stations can improve the geophysical monitoring of a volcano. the experience of 15 years in continuous gravity monitoring on mt. vesuvius is discussed. several geodynamic phenomena can produce temporal gravity changes. an eruption, for instance, is associated with the ascent of magma producing changes in the density distribution at depth, and leading to ground deformation and gravity changes the amplitude of such gravity variations is often quite small, in the order of 10-102 nms-2, so their detection requires high quality data and a rigorous procedure to isolate from the records those weak gravity signals coming from different sources. ideally we need gravity signals free of all effects which are not of volcanic origin. therefore solid earth tide, ocean and atmospheric loading, instrumental drift or any kind of disturbances other than due to the volcano dynamics have to be removed. the state of the art on the modelling of the solid earth tide is reviewed. the atmospheric dynamics is one of the main sources precluding the detection of small gravity signals. the most advanced methods to reduce the atmospheric effects on gravity are presented. as the variations of the calibration factors can prevent the repeatability of high-precision measurements, new approaches to model the instrumental response of mechanical gravimeters are proposed too. moreover, a strategy for an accurate modelling of the instrumental drift and to distinguish it from longterm gravity changes is suggested. mailing address: dr. umberto riccardi, dipartimento di scienze della terra, università degli studi di napoli «federico ii», largo s. marcellino 10, 80138 napoli, italy; email: umberto.riccardi@unina.it vol51,1,2008_delnegro 16-02-2009 21:27 pagina 67 68 u. riccardi, g. berrino, g. corrado and j. hinderer could be useful to characterize the deformational behaviour in some geodynamic contexts. several investigations (e.g., melchior and ducarme, 1991; melchior, 1995; robinson, 1989, 1991) carried out to date show for instance that a correlation exists at «regional» scale between heat flow and the gravity tide. at very local scale arnoso et al. (2001) suggest that the tidal response can be strongly influenced by the structure and mechanical properties of the crust. those anomalies are associated, respectively, with areas of thin crust, high heat flow values, and recent basaltic-type volcanic activity, and with stable structures that have a deeper moho discontinuity and lower heat flow. robinson (1989, 1991) relates the correlation found in his studies to features in the upper crust, suggesting a measurable upper crustal tidal response. arnoso et al. (2001) obtained interesting results from the analysis of the gravity tide collected in two continuous stations in lanzarote island (canary islandsspain). after a suitable reduction of the otl effect by means of global ocean charts complemented with regional and local ones, they obtained anomalous m2 and o1 delta factors and phases consistent with a body tide effect. these results were interpreted as the response of a porous or cavity-filled, local, upper crust under the influence of tidal strain. moreover, knowledge of the specific tidal parameters for an area is required to calculate the luni-solar effect, which has to be removed from the gravity record to obtain gravity residuals. as we are interested in modelling the transfer function between the observed gravity and the underground mass redistribution due to volcanic activity, ideally we need residual gravity signals free of all effects which are not of volcanic origin. in fact, natural (mainly body tides), man-made and instrumental sources affect the signal to noise ratio and hide the subtle volcanic signals. therefore solid earth tide, ocean and atmospheric loading, instrumental drift, hydrological effects or any kind of disturbances other than due to the volcano dynamics have to be modelled to be reduced in the gravity signal. the atmospheric dynamics is one of the main sources precluding the detection of of the gravitational signals originating from all geophysical sources at work at any given time. sorting out different geophysical signals in the data is a challenge, but in principle can be facilitated by recognizing the different temporal and spatial characteristics of different geophysical phenomena (e.g., chao, 1994). unlike the repeated relative gravity measurements on network, there are limited data on continuous gravity observations at active volcanoes (e.g., imbò et al., 1965a; davis, 1981; vieira et al., 1991; goodkind and young, 1991; berrino et al., 1997; budetta and carbone, 1997; bonvalot et al., 1998; arnoso et al., 2001, carbone et al., 2003, 2006). two different approaches may be adopted to extract from the gravity records some insights related with the volcano dynamics, i.e., the analysis of the tidal gravimetric factor (delta: δ) and the analysis of gravity residuals. according to the recommendations of the working group on the theoretical tidal model (ssg of the earth tide commission sec. v of the iag), the delta factor (δ) is defined as the earth’s transfer function between the body tide signal (∆gn(r)) measured at the station by a gravimeter and the amplitude of the vertical component of the gradient of the external tidal potential (vn) at the station. where r is the radius of the earth and , ,are volume love numbers of degree n (complex value), which characterize the spherical elasticity of the earth. thus delta factor is the ratio between the observed gravity tide and the luni-solar gravitational attraction. as it defines the earth transfer function of the external tidal potential, the delta factor is frequency-dependent and is related to the elastic property of the earth. because of the viscoelastic behaviour of the earth, its reaction to the external perturbation due to the luni-solar gravitational attraction is characterized by a certain phase shift. so the study of the tidal parameters (delta factor and phase) for the main tidal waves and eventually their time evolution knuhnu n h n n k1 2 1 n n nδ = + +u u u ( ) n r g r vn n nδ∆=u u vol51,1,2008_delnegro 16-02-2009 21:27 pagina 68 69 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius small amplitude gravity signals. pressure changes can reach several tens of hecto-pascal (say 50 hpa) in specific locations, so the amplitude of the atmospheric contribution to gravity is as large as 200 nms-2, then it could be higher than volcanic signal. this is why a large part of this paper have been devoted to illustrating the most advanced developments in that field of research and the experience of the authors is presented. the goal of this paper is to describe new strategies in the processing and analyses of continuous gravity record collected in active volcanic areas. the experience of about 15 years at mt. vesuvius (southern italy) is reported. the time dependent behaviour of the tidal gravimetric factors is compared with the results from relative and absolute gravity surveys and seismic activity. the results are interpreted in the framework of the present-day dynamics of mt. vesuvius. mt. vesuvius is a quiescent volcano whose last eruption occurred in march 1944. currently, its activity consists of a low level of seismicity, sometimes increasing in numbers of quakes and energy (hereafter called seismic crises), small ground deformation, gravity changes and moderate gas emission. 2. the mt. vesuvius permanent gravity station the mt. vesuvius recording gravity station (fig. 1) is located at the osservatorio vesuviano (fig. 1), where a recording gravity station has been operating since 1987 (berrino et al., 1993b) and where a first experiment of continuous gravity measurements dates back to 1960s (imbò et al., 1964, 1965a). the permanent station is assembled on a concrete pillar located in an artificial cave, 20 m deep (ϕ: 40.828n, λ: 14.408e; h: 608 m) (berrino et al., 1997), where the daily temperature variations are about 0.1°c and the annual ones are within 2°c. the gravity sensor is the lacoste and romberg model d, number 126 (lr-d126), equipped with a feedback system (van ruymbeke, 1991), with a range equivalent to 3⋅104 nm/s2 (implemented at the rob, royal observatory of belgium in brussels and upgraded in 1994). the data acquisition is provided by das or mdas systems developed at the rob (van ruymbeke et al., 1995) at a sampling rate of 1 data/min (0.01667 hz). here we focus on the results of gravity records since 1994 (fig. 2), when the instrument and siting of the station were improved. the station belongs to a relative gravity network, spanning the vesuvian area, periodically surveyed since 1982. it is close to an absolute gravity station established on the volcano in 1986. the absolute value of g was measured in 1994, 1996, 1998 and 2003 (berrino, 1995; berrino, 2000). in order to check the reliability of the gravity signals, the instrumentation is periodically calibrated and the background noise level at the station is analyzed. in fact, instrumental sensitivity can change, not always linearly, as a consequence of mechanical perturbations and the noise level at the gravity station. to characterize the background noise level, which could affect the instrumental response, the 1 min sampled residual gravity was analyzed to detect any fig. 1. location of the recording gravity station and gravity network on mt. vesuvius. vol51,1,2008_delnegro 16-02-2009 21:27 pagina 69 70 u. riccardi, g. berrino, g. corrado and j. hinderer fig. 3. power spectra of the background noise level computed in each seasons at the mt. vesuvius gravity station, with the indication of the standard new low noise model (nlnm) as reference. fig. 2a,b. hourly values of gravity records (a), drift corrected gravity residuals (b). anomalous record with abnormal drift and very large residuals are highlighted in circles. possible seasonal dependence or the presence of spectral components which could hide or mask geophysical signals. several time windows lasting about 1 week were selected in each season. the amplitude and spectral content of the noise (berrino and riccardi, 2004) show a flat trend in the analyzed spectral band (fig. 3), according to the standard new low a b vol51,1,2008_delnegro 16-02-2009 21:27 pagina 70 71 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius noise model [nlnm] (peterson, 1993). the high noise level during the autumn is a consequence of the meteorological condition (mainly wind) at mt. vesuvius during that season. changes through time of the calibration factors for different kinds of mechanical gravimeters have been detected by several authors (e.g., bonvalot et al., 1998; budetta and carbone, 1997; riccardi et al., 2002). however, a complete understanding of the physical processes affecting the instrumental sensitivity is still far from being achieved. as changes in instrumental sensitivity can prevent the repeatability of measurements and affect the phase and amplitude of the recorded gravity signals, the accurate calibration of gravimeters plays a key role in high precision gravity measurements (riccardi et al., 2002). the calibration of a gravimeter at an accuracy level of 10−8 to 10−9 g, is difficult to attain because of the many problems in pursuing a known gravity change («standard») at such a level of accuracy. the stability of the calibration factors of lr-d126 has been periodically investigated on site. this kind of calibration is obtained by inducing changes in the spring length through a known «dial» turning and fitting this, by least-squares, against the instrumental output. this is the most frequently table i. comparison between lr-d126 and superconducting sg-tt70-t015 meters: results (delta factor and phase) of the tidal analysis for the main tidal waves. in the last column the ratio (sg/d) of delta factors obtained by the records from the superconducting and d meters is listed. the tidal waves nomenclature is: o1-diurnal lunar; p1-diurnal lunar; k1-diurnal luni-solar; s1-diurnal solar; m2 semi-diurnal lunar; s2 semi-diurnal solar. lr-d126 sg-tt70-t015 wave delta phase (°) delta phase (°) lr/sg o1 1.147 ± 0.003 -0.2 ± 0.1 1.14 ± 0.01 -0.1 ± 0.6 1.006 ± 0.009 p1s1k1 1.137 ± 0.002 0.0 ± 0.09 1.130 ± 0.009 0.1 ± 0.6 1.006 ± 0.007 m2 1.184 ± 0.001 1.14 ± 0.06 1.176 ± 0.003 0.9 ± 0.2 1.007 ± 0.004 fig. 4. calibration factors obtained with on-site and absolute calibration for lr-d126 gravity meter; units of vertical axis (ndiv) are number of scale divisions. vol51,1,2008_delnegro 16-02-2009 21:27 pagina 71 72 u. riccardi, g. berrino, g. corrado and j. hinderer adopted calibration procedure for continuous gravity station equipped with relative mechanical instruments. various schemes of dial turning have been tested at mt. vesuvius station (riccardi et al., 2002). hereafter, such calibrations are referred to as «on-site» calibrations. moreover, two additional calibrations of the feedback were carried out in june 1994 and november 1997 in sèvres, at the bureau international des poids et mesures (bipm) during the international comparison of absolute gravimeters (becker et al., 1995, 2000). a calibration of the instrumentation was also obtained in 1997 by means of a joint intercomparison with the superconducting gravimeter sgtt70-t015 (table i) and the absolute fg5-206 gravimeter (riccardi et al., 2002). the intercomparison between spring and superconducting gravimeters is the most suitable way to determine the transfer function of spring gravimeters in the tidal band. detailed information on the different calibrations and operational procedures carried out at this station are given in riccardi et al. (2002). the time distribution of the calibration factor is shown in fig. 4. in this plot the value from the intercomparison with the fg5-206 absolute gravimeter is shown and the suitable range of repeatability of the calibration factor at 1% level is also drawn. a large scattering of the calibration factor occurs from 1999 to 2001, when some anomalous signals were detected. the anomalous signals (highlighted in fig. 2a) were characterized by an abnormal drift and then very large gravity residuals. as a consequence, the tidal analysis on the 1998-2000 data furnished a sharp decrease of the δ factors at the beginning of 1999 (berrino and riccardi, 2001). a theoretical value of the instrumental sensitivity was computed and compared with the calibration factors monitored «on site» to evaluate whether the calibration factor truly reflects changes in the instrumental response or is merely due to the adopted «on site» calibration procedures. the theoretical instrumental sensitivity was determined by a regression analysis between the meter’s output signal and the synthetic gravity tide. thus, a set of weekly theoretical values of calibration factor was obtained and compared with the results from the repeated calibrations (fig. 5). a good agreement between the temporal evolution of the theoretical factors and those obtained through the «on site» calibration was detected (riccardi et al., 2002). moreover, the set of the «on site» and theoretical calibration factors has been plotted against the time occurrence of certain large worldwide earthquakes (ml > 5) that shook the meter strong enough to send it out of range (fig. 5). a time correlation between the larger changes of instrumental sensitivity and the occurrence of seismic events can be observed. more detailed discussion concerning the instrumental sensitivity changes on the occasion of large earthquakes is given in fig. 5. on-site and theoretical calibration for lr-d126 against the occurrence of some large earthquakes (ml >5); «shot» on vertical axis is the arbitrary unit for the feedback frequency output. vol51,1,2008_delnegro 16-02-2009 21:27 pagina 72 73 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius riccardi et al. (2002) and berrino and riccardi (2004). they suggest a mechanical perturbation of the sensor, due to some dominant frequencies of the noise at the station on the occasion of large earthquakes. in fact the higher frequency of the seismic free oscillation excited by large earthquakes includes the fundamental mode of oscillation (t0: 15 to 20 s) of the lacoste and romberg spring gravimeters (torge, 1989). these instrumental disturbances due to large earthquakes can last several weeks. 3. ocean loading and atmospheric reduction the state of the art of the modelling of the ocean loading effect on local gravity data is hereafter reviewed and the most advanced methods in pursuing the reductions of the atmospheric effects are presented. to focus on the methodological upgrades, we present our attempt to model the barometric «local» effect on gravity data in a stable non-volcanic area by means of a barometric array. however, by the way of the methodological approaches, these results can be fruitfully applied to fix the problem of air pressure effect on local tvg in volcanic areas. in a very general sense, ocean tide loading (otl) is the deformation of the earth due to the weight of the ocean tides. the ocean tides induces water mass redistributions causing periodic loading of the ocean bottom. the earth’s deformation (vertical and horizontal displacement, tvg, tilt and strain) under this load is called ocean tide loading. the ocean tides as well as the body tides have more than one periodicity, so they can be described as the sum of several harmonic components having their own period. problem areas are mostly islands and shallow seas with large tidal amplitude and fast varying phase lag. these include among the others in europe: the mediterranean sea and the north sea. to compute the ocean tide loading the ocean tides are integrated with a weighting function g (3.1) here l is the loading phenomenon (displacement, gravity, tilt or strain) at the station located at distance r. the ocean tide at r’ is given in its complex form z=aeiϕ, where ϕ is the phase; ρ is the mean density of sea water and g is green’s function for the distance |r−rl|. the integral is taken over all global water masses a. ( ) ( )l r z r g r r da a ρ= -l l# table ii. amplitude (l; in nm s-2) and phase (ϕ in degrees) of the main harmonics of the otl computed for mt. vesuvius station by means of different models. scw80 csr 3.0 fes 95.2 tpxo 6.2 tpxo 7.0 wave l ϕ l ϕ l ϕ l ϕ l ϕ m2 11.5 -85.4 9.2 -68.0 10.6 -69.1 9.5 -67.8 9.6 -66.9 s2 4.0 -58.8 3.1 -58.6 4.0 -58.1 4.6 -57.7 3.3 -49.6 n2 2.3 -100.3 1.7 -82.4 2.2 -84.9 2.1 -66.7 2.2 -80.5 k2 1.1 -58.9 0.8 -59.1 1.1 -63.4 0.9 -33.9 1.1 -37.6 k1 1.1 -131.8 2.0 -84.1 2.8 -76.6 1.5 -83.9 1.5 -81.1 o1 1.2 169.0 1.3 -131.7 2.0 -145.3 1.3 -147.4 1.3 -153.0 p1 0.4 -136.8 0.7 -90.0 0.9 -84.4 0.6 -100.3 0.4 -121.8 q1 0.3 116.2 0.1 167.7 0.3 168.2 0.4 129.9 0.4 131.4 mf 0.4 -16.8 0.5 27.1 0.5 27.1 0.4 33.8 0.3 32.2 mm 0.2 -109.7 0.3 23.2 0.1 22.9 0.3 30.3 0.2 21.0 ssa 0.3 -93.1 0.3 4.5 0.2 3.9 0.3 4.5 0.3 4.5 vol51,1,2008_delnegro 16-02-2009 21:27 pagina 73 74 u. riccardi, g. berrino, g. corrado and j. hinderer green’s function determines how much the earth deforms due to the point load (a general solution of this problem is given in farrel, 1972). the next step is to replace the convolution integral by a summation. most ocean models are given on a 0.5° by 0.5° grid, which justifies direct summation over these ocean grid cells if the station is more than 10 km from the coast. otherwise some re-gridding is necessary. some local solutions (models) are obtained by means of a re-gridding the model gradually towards the station. table ii lists the amplitude (l) and phase (ϕ) of the main 11 harmonics of the otl computed for mt. vesuvius station. these 11 harmonics are the largest in amplitude and represent most of the total tidal signal. these have been computed by means of a free otl provider developed at the onsala space observatory and maintained by m.s. bos and h.-g. scherneck (http://www.oso.chalmers.se/~loading/). solutions coming from classical (scw80) (schwiderski, 1980) and most recent models (csr 3.0; fes95.2, tpxo6.2, tpxo7.0) (eanes, 1994; le provost et al., 1998; egbert and erofeeva, 2002) have been obtained. if the stations are close to the coast, like the mt. vesuvius one, an automatic interpolation is applied using a mask having a coastline resolution of 0.6 km. schwiderski’s model (scw80) is one of the oldest and it has been considered the standard for many years. it is a hydrodynamic model, given on a 1° by 1° grid and uses an interpolation scheme to fit the tide gauges; scw80 model does not account for the mediterranean sea tide. fes95.2 is an upgrade of the fes94.1 model, a pure hydrodynamic tide model tuned to fit tide gauges globally, which includes the mediterranean sea tide. in fes95.2 the tides in the arctic were improved and topex/poseidon satellite altimeter data has been used to adjust the long wavelength behaviour of fes94.1. it has been calculated on a finite element grid with very fine resolution near the coast. the version used at mt. vesuvius station is given on a 0.5° by 0.5° grid. the csr3.0 models are nothing other than a long wavelength adjustment of fes94.1 model by using topex/poseidon data and are given on a 0.5° by 0.5° grid. tpxo.6.2 and 7.0 have been computed using inverse theory using tide gauge and topex/poseidon data. these models have a resolution of a 0.25° by 0.25° grid. the results lead to a maximum otl effect at mt. vesuvius of about 10 nm/s2 (1 mgal) with a slightly lower amplitude obtained by means of models accounting for mediterranean sea tides. the amplitude and phase of such effect has to be accounted for to avoid a tidal modulation (diurnal and semi-diurnal) in the residual gravity signals. besides solid earth and ocean tides, atmospheric pressure variations are one of the major sources of surface gravity perturbations preventing a highly accurate detection of small amplitude gravity signals (see e.g., hinderer and crossley, 2000, 2004). the continual redistribution of air mass in the earth’s atmosphere causes periodic variations in local gravity at the solar tidal frequencies as well as random variations (warburton and goodkind, 1977; spratt, 1982). gravity (measured positive down) and local atmospheric pressure correlate with an admittance of about −3.0÷−3.5 nms−2/hpa (e.g., warburton and goodkind, 1977; müller and zürn, 1983; merriam, 1992). knowing that pressure changes can reach 50 hpa in specific locations, the amplitude of atmospheric contribution to gravity may be as large as 200 nms−2, which is typically only 10 times less than the solid earth tides. moreover, because this effect varies both in time and with frequency (richter et al., 1995), the contribution is spread over a wide spectral domain and may inhibit the observation of small signals of non-tidal origin. the global atmosphere acts on surface gravity through two competing effects: a direct «newtonian» attraction by air masses and an elastic contribution due to the earth’s surface loading. the amplitude and polarity of these two effects vary with distance, so the net contribution of the atmosphere coming from different distances from the gravity station is variable (e.g., spratt 1982; merriam, 1992; mukai et al. 1995; boy et al. 1998). the coherence scale of pressure fluctuations and some considerations on the hydrostatic approximation of the atmosphere, led some authors to suggest a division of the globe into «local» (within 50 km), «regional» (501000 km) and «global» zones (> 1000 km) vol51,1,2008_delnegro 16-02-2009 21:27 pagina 74 75 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius (atkinson, 1981; merriam, 1992). in the local zone (< 50 km) pressure can change rapidly in time, but is spatially coherent, so that pressure observations collected at the gravity site are sufficient to obtain an accurate reduction within a few tenths of nm/s2 except when a front is passing through the local zone (rabbel and zschau, 1985). when a pressure front moves through or larger horizontal gradients affect the local zone, the band from 1 to 10 km from the gravity station becomes a critical area for which more detailed pressure data are needed. atmospheric effects on gravity are routinely reduced using a barometric admittance, which is a simple transfer function adjusted by least square fitting between pressure and gravity, both measured locally. the use of a single scalar admittance has been well established (e.g., warburton and goodkind, 1977; crossley et al. 1995). when atmospheric pressure p (hpa) is recorded jointly with gravity g (nms−2) at a single station, the gravity can be reduced (gr) by using the relation (3.2) where pn is a reference pressure at the station and α is either a nominal value of −3.0 nms-2/hpa or determined by a least squares fit of p to g. the effectiveness and simplicity of this method has led to its widespread use in gravity studies for many purposes. this reduction typically accounts for some 90% of the total atmospheric effect. the drawback of this method is that the admittance shows some variation with time (e.g., richter, 1987; van dam and francis, 1998), usually on seasonal time scales, whereas the atmosphere is certainly variable on short time scales and local weather systems can move rapidly over a station in a few hours (müller and zürn, 1983; rabbel and zschau, 1985). so there is no guarantee that the correlation implied by eq. (3.2) is satisfied over all length and time scales. furthermore crossley et al. (2002) found that the admittance is sensitive to the time averaging windows applied on data, namely a higher admittance is found for shorter windowing. moreover, the simple reduction using only the local pressure measurements cannot take into account either the global scale or re( )g g p pr nα= gional (1000 km around the gravimeter) atmospheric effects. several approaches using the local pressure more effectively have been attempted, particularly with a frequency dependent admittance (e.g., warburton and goodkind, 1977; crossley et al. 1995; neumeyer, 1995; neumeyer et al., 1998; kroner and jentzsch, 1998; van dam and francis, 1998). the method represents a transformation of the eq. (3.2) from the time domain to the frequency (ω) domain and allowing the admittance (α) to be frequency dependent (3.3) minimising ⎮gr(w)⎮2 over the whole frequency range leads to (3.4) which is equivalent to the complex admittance defined by warburton and goodkind (1977). crossley et al. (1995) demonstrated that the complex admittance, as expressed in eq. (3.4), is a powerful and versatile tool to model both local atmospheric effect and contribution due to the solar harmonics sn and allows to select the frequency ranges of the air pressure reduction. 3.1. atmospheric reduction by means of a barometric array riccardi et al. (2007) investigated the efficiency of a barometric array (fig. 6) to improve the reduction of the «local» atmospheric effects on gravity data in normal weather conditions and also under extreme weather conditions. this research has been developed by using superconducting gravity (sg) data collected in strasbourg and barometric records in five sites around the sg station at distances ranging between 10 and 60 km (fig. 6). six months of gravity and air pressure records (fig. 7) have been analyzed both in the time and frequency domains. some further analyses have been addressed on three time intervals (highlighted in fig. 7) characterized by large and fast air pressure changes. ( ) ( ) ( ) ( ) p g p 2α ω ω ω ω = / / ( ) ( ) ( ) ( )g g pr ω ω α ω ω= vol51,1,2008_delnegro 16-02-2009 21:27 pagina 75 76 u. riccardi, g. berrino, g. corrado and j. hinderer riccardi et al. (2007) used the multi-linear regression (mlr) method of van camp and vauterin (2005) to express an objective signal sobj (gravity residuals) as a linear combination of a set of m component signals sc (air pressure record of each station) (3.5)drift residuals a sobj lm cm n m = + +/ fig. 6. location map of the superconducting gravimeter (filled circle) and the stations of the barometric array. fig. 7a-c. the strasbourg hourly data sets: a) gravity record; b) gravity residuals; c) air pressure. the time spans highlighted in gray are characterized by large and fast pressure changes and are the target of further analyses. b c a vol51,1,2008_delnegro 16-02-2009 21:27 pagina 76 77 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius in this study we set n=1 (a linear regression). the drift term, which is used to model the longterm instrumental behaviour in the gravity record consists of a polynomial of degree k (t is the time) (3.6) the parameters a1 ... an, b1 ... bn, were estimated by least squares adjustment. b t$+fb b t b t b t$ $ $+ + + +=( )drift a p k0 1 2 2 3 3 fig. 8a-f. residual gravity obtained for a time span 11 days long during december 2000 (ref. fig. 7a-c). comparison of different kinds of reduction: a) by means of: a single admittance coefficient; b) a fit to pressure data from strasbourg only; c) a fit to pressure data from the whole array; d) a frequency dependent admittance; e) air loading computed with the hybrid model; f) fourier spectrum of the reduced residual gravity. a b c d e f vol51,1,2008_delnegro 16-02-2009 21:27 pagina 77 78 u. riccardi, g. berrino, g. corrado and j. hinderer the mlr approach allows us to jointly account for the atmospheric effect as it can be probed trough the barometric array. a similar methodology has also been applied by dittfeld (1995) and kroner and jentzsch (1998). the results obtained on the whole data set (6 months) demonstrated that negligible improvement in the «local» atmospheric reduction derives from the use of an array, as also shown by dittfeld (1995). moreover the short length of our study did not permit us to investigate seasonal variations in the pressure reduction to gravity. we further compared the efficiency of the atmospheric reduction by means of the barometric array with the one performed through global loading computation (boy et al., 2002; petrov and boy, 2004). we use ecmwf (european centre for medium-range weather forecasts) surface pressure fields from the 4dvar model as they have the highest temporal (3 h) and spatial (0.5°) resolution. a more detailed description of the global loading computation is given in riccardi et al. (2007) and petrov and boy (2004). the results showed that gravity residuals reduced by means of the atmospheric loading using the 4dvar surface pressure are slightly worse than those obtained by applying a single admittance coefficient. this is likely due to the low resolution of the loading model, which is unable to reconstruct high frequency local barometric changes. these considerations could be extended when the mean atmospheric conditions are not far from the hydrostatic equilibrium. so we decided to analyze data during shorter time intervals, mainly characterized by abnormal weather conditions like when an atmospheric front is passing. here the results for 11 days during december 2000 are shown (figs. 7 and 8). in order to improve the time and spatial resolution of the computed global atmospheric loading, we tested a hybrid method which consists in the following steps: – a global loading reduction using the 3 h and 0.5°×0.5° grided surface pressure fields everywhere except in the local zone. the global contribution has been recomputed from the 4dvar model and oversampled to 60 s with spline interpolating functions; – a local loading reduction obtained by dividing the local zone into a smaller grid using interpolated data and the 60 s pressure samples from the closest stations of the barometric array, except the central zone; – a central zone reduction using the j9 station pressure. all three contributions have been added leading to a hybrid time series of the atmospheric loading. the effect of the hybrid model in the load computation is clear, the increase in the model resolution has led to improve the efficiency of the global loading reduction giving results similar to those obtained through the barometric array (fig. 8c,e). to evaluate the potentiality of each aforesaid method of reduction riccardi et al. (2007) considered the standard deviation (σ) of the reduced gravity residuals as an estimator of the efficiency of the applied air pressure reduction. the standard deviations of gravity residuals reduced by means of the barometric array have been better than to those reduced by using an air pressure record collected in a single station. the use of the barometer array lowers the standard deviation of the gravity residuals by about 30%. the improvement is essentially due to a removal of an almost quadratic background trend by using the array (see fig. 8a-c). the trend could be related to some coherent features of the barometric field at local scale sensed by the array. moreover an improvement in the atmospheric reduction has been achieved with a frequency dependent admittance (fig. 8d); as demonstrated by several authors (crossley et al., 1995), the reduction is significantly better mainly at high frequency (>2 cpd), because large-scale pressure fluctuations are less correlated with gravity than are local pressure fluctuations. in only one of the 3 periods of rapid changes we investigated did the array improve on standard methods, and even in that case the improvement was noticeable only in the low frequencies, but actually worse at high frequencies. the spectra of the unreduced and reduced gravity residuals according to all the reduction methods are also drawn (fig. 8f). they clearly show that a single admittance coefficient is enough to reduce the energy in all the spectral bands. data from the barometer network improve the reduction at low frequenvol51,1,2008_delnegro 16-02-2009 21:27 pagina 78 79 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius cies (<2.5 cpd) while at higher frequencies the results are worse. in fact, comparing the gravity residuals reduced by a single admittance coefficient with those reduced through data from the barometric array (fig. 8a,c), an increase in high-frequency (>3 cpd) noise is quite evident. these features could be due to the summation of correlated high-frequency noise in the pressure data series. hence the use of pressure data acquired by an array to improve the gravity reduction requires special care, because they could introduce an artificial high-frequency noise. the application of the hybrid method improves the air pressure reduction in almost the entire spectral band except for the 2.0 cpd band (fig. 8e). this would be the result of two model defaults i.e. an inefficient tidal fitting in the residual gravity computation and an inadequate modelling of the air loading due to the thermal s2 component (see ponte and ray, 2002). finally we note that during normal atmospheric conditions, when the atmosphere is in apparent hydrostatic equilibrium, the use of our local array of barometers gave no improvement over the use of the pressure at the station itself. accounting for the geometry of the available barometric array and the typical amplitude (10−2 hpa) of the pressure signal in the «local» zone, we could expect some improvement with a more dense array of higher quality sensors using the methods described in this paper. a more general consideration arises from this experience: the highest level of development in air pressure reduction of local gravity data make sense only for gravity signals collected by superconducting gravimeters. otherwise a single barometer can be enough to account for the main part of the pressure effect originating in the local zone. however it is noteworthy that a significant progress in modelling atmospheric effects, as demonstrated by several authors, can be pursued by using a frequency dependent admittance, which allows us to model the weather contribution at different frequency ranges. the reduction is significantly improved mainly at high frequency (>2 cpd) and consequently the reduced gravity residuals are much smoother than the others obtained by applying different kind of reductions. 4. analysis of gravity record at mt. vesuvius and results this section reports the most remarkable results coming from the experience of about 15 years of gravity recording at mt. vesuvius (southern italy). the time dependent behaviour fig. 9. calibration functions (dotted and continuous lines) interpolating the factors obtained with the on-site calibrations (points with error bars) and polynomial fitting (dashed line). the thickest line is the calibration function adopted to convert gravity records in nm/s2. vol51,1,2008_delnegro 16-02-2009 21:27 pagina 79 80 u. riccardi, g. berrino, g. corrado and j. hinderer of the tidal gravimetric factors is compared with the results from relative and absolute gravity surveys and seismic activity. the results are interpreted in the framework of the present-day dynamics of mt. vesuvius. to reduce the instrumental effect on tidal parameters (δ factors and phases) computed at mt. vesvius gravity station, a calibration function has been computed to convert the recorded signal into nm/s2 (fig. 9); this function derives from the available data-set of the calibration factors periodically checked at the gravity station. in detail, two calibration functions have been computed respectively by including or excluding the highest outlier of the calibration factors data-set obtained in 1999. the harmonic tidal analyses were repeated on the gravity record calibrated by means of the two functions to rule out any dependence of δ factors and phases from instrumental effects, namely the temporal changes of the calibration factors. thus, taking into account these results, the first calibration function (dotted line in fig. 9) was rejected and we adopted the second function (bold continuous line in fig. 9) to calibrate the gravity record (fig. 2) spanning 19992001 interval. all of the gravity records were analysed to obtain tidal parameters and gravity residuals (fig. 2). the latter have been computed by subtracting the luni-solar effect (body tide), according to tamura’s gravity potential catalogue (tamura, 1987) from the gravity record, as well as a first order correction for the atmospheric effect and instrumental drift. the mean coefficient, −3.5 nms−2/hpa, has been adopted to reduce the atmospheric effect in gravity record (berrino et al., 1997, 2000). the wahr-dehantzschau (wdz) earth model (wahr, 1981; dehant, 1987; zschau and wang, 1987) has been adopted to compute tidal parameters, while for the computation of gravity residuals a synthetic tide was calculated using tidal parameters computed from the local gravity records since 1994. as regards the reduction of the drift, accurate modelling is necessary to remove the instrumental drift and to distinguish it from longterm gravity changes due to volcanic sources. this is mainly required in quiescent volcanic areas, where «slow» and small temporal gravity changes are expected. otherwise in the case of large short-lasting (few hours or days) gravity variations, as observed in open-conduit volcanoes (carbone et al., 2006), the instrumental drift can be easily modelled. here, drift has been constrained by taking into account the temporal gravity changes obtained by both relative and absolute measurements periodically performed at mt. vesuvius. the latter show a negligible contribution on the trend observable in fig. 2b; thus, the long term component of the gravity record can be considered instrumental drift. the drift corrected gravity residuals are shown in fig. 2b. the data set has been analysed by means of an algorithm for tidal analysis: «eterna 3.3» (wenzel, 1996). the results for the main tidal waves are summarized in table iii. the analyses have been performed on the gravity record rearranged in some temporal subsets to check the time stability of the solutions and investigate the temporal changes of δ factors with a better resolution. the results of these tidal analyses have also been compared with the previous ones from 1987-1991 and the 1960 (table iii, fig. 10c). table iii. comparison among tidal gravimetric factors determined during the 1960s, 1987-91 and 1994-2000 (for tidal waves nomenclature refer to table i). wave 1959-1961 1961-1965 1987-1991 1994-1998 1999-2000 imbò et al. (1965a) imbò et al. (1965a) berrino et al. (1993b) o1 1.156 1.038 1.08±0.02 1.126±0.002 1.143±0.004 k1 p1s1k1:0.928 1.083 1.05±0.01 1.117±0.001 1.123v0.003 m2 1.154 1.068 1.11±0.01 1.1488±0.0007 1.155±0.001 s2 1.147 1.107 1.03±0.02 1.144±0.004 1.155±0.003 vol51,1,2008_delnegro 16-02-2009 21:27 pagina 80 81 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius although a calibration function has been adopted, aimed at eliminating or at least reducing the instrumental effects, the results of the tidal analyses (table iii) show an increase in the δ factor in the period 1999-2000. anyway the assumption of a calibration function and all the efforts aimed at achieve a rigorous calibration of the lcr d126 do not rule out bona fide some instrumental effects on the observed delta temporal changes at mt. vesuvius gravity station. it is noteworthy that these variations are well correlated with some changes in the activity of mt. vesuvius. in fact a seismic crisis began in october 1999 (iannaccone et al., 2001) and a significant inversion of the trend of the gravity changes occurred in 1994 as deduced by both relative and absolute measurements. in order to better understand the relationship, if any, between the results of tidal analyses and volcano dynamics, δ factor and gravity changes have been reconstructed by the available data for the last forty years and interpreted in the context of the activity of mt. vesuvius (fig. 10a-c). fig. 10a-d. time behaviour of vesuvius dynamics from 1959 to 2001: a-b) seismic activity; c) tidal gravity factor for m2 tidal wave; d) gravity changes (µgal) at the osservatorio vesuviano station plus gravity and elevation changes at torre del greco. a b c d vol51,1,2008_delnegro 16-02-2009 21:27 pagina 81 82 u. riccardi, g. berrino, g. corrado and j. hinderer figure 10d shows the observed tvg at the osservatorio vesuviano station. the reliability of this gravity change may be strongly constrained by taking into account data from others stations of the mt. vesuvius relative gravity network. as an example, fig. 10d shows the gravity changes at torre del greco, about 5 km sw of the mt. vesuvius crater, and the vertical ground movement continuously obtained by tide gauge data. tide gauge data were collected very close to the torre del greco gravity station. an inversion of the trend, detected in 1993-1994, is also evident in the ground movement. a high degree of similarity in the changes observed at both stations is clear. the results of these tidal analyses have also been compared with the previous ones from 1987-1991 and the 1960s (table iii, fig. 10c). an increasing trend from 1961 to the present in the amplitude of the tidal waves is clearly detectable. taking into account the logistic and instrumental differences between the 1959-1965 (askania meter gs9, gs11) and 1987-2001 recording stations, a rough comparison among the different data can be made. from 19611965 to 1987-1991, changes in the tidal parameters cannot be considered significant, while an increase can be noted from 1991 to 1994 and, as previously discussed, in 1999-2000. the latter shows tidal parameters similar to the values determined in the 1959-61 time interval. it is noteworthy that the main changes in tidal parameters again seem well correlated with the temporal behaviour of the activity of mt. vesuvius (berrino et al., 2006). at least three crises (1989-1991, 1996, 1999-2000) (figs. 10a-b) are reported (berrino et al., 1993a; vilardo et al., 1996; iannaccone et al., 2001). moreover, an increase in seismicity was also well documented in 1963-1964 (imbò et al., 1965b). concerning gravity changes, here we focus on the results of the episodic gravity measurements since 1982 collected on mt. vesuvius relative gravity network (berrino, 2000; berrino and riccardi, 2000). the observed temporal gravity change at the osservatorio vesuviano station (fig. 10d) shows an increase (more then 20 µgal/year) in the 19591983 time interval (berrino and riccardi, 2000; 2001), as inferred by reviewing data collected during 1959-1960 (tribalto and maino, 1962) and 1965 (bonasia, unpublished data) gravity surveys. this trend fits the gravity data collected during the 1980s, when a continuous gravity increase affected the whole area (berrino, 2000). focusing on the most recent data (fig. 10), it is interesting to note that the increase of the d factor from 1991 to 1994 (fig. 10c) occurred during or soon after the 1989-91 seismic crisis. a gravity decrease of about 60 µgal (fig. 10d) (berrino et al., 1993a) was also detected between 1989 and 1991 at the osservatorio vesuviano gravity station by both relative and absolute gravity measurements. the tidal response of the investigated area could indicate a variation in the deformational behaviour probably due to the change in the mean mechanical properties of mt. vesuvius, as already suggested by berrino et al. (1997). up to now any additional information on volcanic sources may be inferred by the gravity residuals. although their time distribution clearly shows an increase in amplitude and scattering during 1998-2001, coinciding with the increasing seismicity, there are not enough clear gravity signals to detect or hypothesize the presence of volcanic input. 5. conclusions the above described results show how the continuous gravity record on active volcanoes could be a useful investigative tool to detect volcanic inputs, but much care must be taken to remove from the recorded signals the effects due to the instrumental response and non-volcanic sources. as changes in instrument sensitivity can reduce the repeatability of measurements and affect the phase and amplitude of recorded gravity signals, the accurate calibration of gravimeters in high-precision gravimetry is topical. the stability of the calibration factors has to be deeply investigated through different calibration methods (e.g., inter-comparison with ag and sg). concerning the modelling of the non-volcanic contribution to tvg, currently the tide generating potential is at a suitable level of acvol51,1,2008_delnegro 16-02-2009 21:27 pagina 82 83 strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of mt. vesuvius curacy (1 nm/s2), so highly accurate catalogues of tide potential are available. even the otl modelling is highly accurate. the topex/poseidon satellite altimeter data deeply improved the studies on the otl effects. nevertheless some local solutions are needed for instance for volcanic islands. moreover the state of the art demonstrates that the gravity record could be able to characterize the deformational behaviour of the volcano through the time evolution of the δ factor. however, as mentioned by several authors (e.g., arnoso et al., 2001; robinson, 1991; melchior, 1995), this points out the existing necessity of theoretical studies and observations of the highest quality to answer the different questions regarding the significance of the tidal gravity anomaly and how it relates to mechanical properties of the upper crust. on the other hand, the capability of gravity residuals at least in the volcanic area characterized by a low level dynamics, requires a significant improvement in modelling mainly instrumental drift. however, the joint application of relative, absolute and continuous gravimetry is strongly recommended, to better remove the longterm instrumental drift. thus the recognition of real gravity changes from apparent ones, due to instrumental behaviour, becomes more reliable. currently, no additional information on volcanic sources may be inferred from the gravity residuals at mt. vesuvius permanent station. an improvement in the study of the mass redistribution due to volcanic processes by means of gravity residuals could derive from the use of at least a reference station outside the volcano, which would make it possible to model and exclude long-term and non-volcanic «regional» effects (berrino et al., 1997). in quiescent volcanic areas, undertaken by «slow» and small temporal gravity changes, it is hard to recover signals (residual gravity) related to volcanic activity by means of mechanical gravimeters. this is mainly due to the strong and non-linear instrumental drift affecting the signals acquired by such sensors. the availability of sgs in active volcanic areas is hoped for because of their very small instrumental drift, high and stable sensitivity. sgs would allow to detect very slow and small tvgs often related with re-filling process of magma chamber. the reduction of the air pressure effects on local gravity through the most advanced methods, such as global loading computation or array application, is redundant for tvg collected by means of mechanical gravimeters. acknowledgements the authors are very grateful to j.-p. boy from eost for the computation of the global loading air pressure effect on local gravity. the authors are deeply indebted to the anonymous referee who greatly improved the paper with his/her comments and suggestions. for the final form of this manuscript the comments of d. carbone have also been considered. the authors thank c. del negro from ingv for his editorial work. references arnoso, j., j. fernandez and r. vieira (2001): interpretation of tidal gravity anomalies in lanzarote, canary islands, j. geodyn., 31, 341-354. atkinson, b. w. 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(1996): the nanogal software: earth tide data processing package eterna 3.30, bull. inf. marées terrestres, bruxelles, 9425-9438. zschau, j. and r. wang (1987): imperfect elasticity in the earth’s mantle. implication for earth tides and long period deformation, in proceedings of the 9th international symposium on earth tides, new york, 605-629. vol51,1,2008_delnegro 16-02-2009 21:27 pagina 85 an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 1 geological services tow ards a sustain able use and management of the subsu rface: a geoethical imperative serge f. van gessel geological su rvey of th e n eth erlan d s (tn o), utrech t, th e n eth erlan d s serge.van gessel@tn o.n l klaus h in sby geological su rvey of den m ark an d green lan d (geus), cop en h agen , den m ark kh i@geu s.d k gerry stan ley geological su rvey of irelan d (gsi), du blin , irelan d gerry.stan ley@gsi.ie jø rgen tulstrup geological su rvey of den m ark an d green lan d (geus), cop en h agen , den m ark jtu @geu s.d k yvon n e sch avemaker geological su rvey of th e n eth erlan d s (tn o), utrech t, th e n eth erlan d s yvon n e.sch avem aker@tn o.n l kris piessen s royal belgian in stitu te of n atu r al scien ces – geological su rvey of belgiu m , bru ssels, belgiu m kp iessen s@n atu ralscien ces.be paul j.f. bogaard geological su rvey of th e n eth erlan d s (tn o), utrech t, th e n eth erlan d s p au l.bogaard @tn o.n l abstract a geological service for europe provides users with tailored and transnational consistent information on subsurface resources, capacities and processes within their surrounding environment. such information underpins the responsible management of subsurface space and its resources, which is often administered by different authorities. n ational and regional geological surveys have come together to develop much-needed innovations and improvements that will int egrate information and knowledge across different geological and geographical settings in europe. this cooperative framework aims to meet societal challenges and protect valuable resources for future generat ions while ensuring that geoethical principles are honored. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 2 1. backgroun d h e earth , alon g w ith en ergy from th e su n , is th e sou rce of all th at is n ecessary to su stain life an d p rovid e th e raw m at erials th at su p p ort h u m an kin d , society an d ou r w ay of life. wh en w e con sid er w h at su stain s life, w e th in k of food , p lan ts, an im als an d w ater. wh en w e con sid er w h at su p p orts ou r lifestyle, w e con sid er ou r sh elter (h om es an d cloth in g); sou rces of en ergy – in clu d in g h yd r ocarbon s, n u clear, an d ren ew ables su ch as solar, w in d an d geoth erm al; raw m aterials for ever yd ay livin g; m od ern tech n ologies in clu d in g th e m an u factu re of good s, d evices an d tools; tran sp ort (lan d , sea an d air); an d en ergy gen e ration , tran sm ission an d storage. altogeth er th e su bsu rface is a vital elem en t for th ese com m od ities, be it as p rovid er of cru cial resou rces (w ater, en ergy, m in erals), a reservoir for tem p orary bu fferin g of resou rces, a sin k for p erm an en t storage of w aste, a footin g for su rface an d su bsu rface con stru ction s an d in frastru ctu re in clu d in g tu n n els an d bu ild in gs, or th e fou n d ation for p reservin g clean en viron m en ts an d u n iqu e n atu re reserves. th e un forecasts th at in 2050 th e global p op u lation w ill reach 9.77bn from 7.6bn tod ay (un , 2017). with in creasin g n u m bers of p eop le com es in creasin g p ressu re for th e secu re su p p ly of raw m aterials, w ater an d en ergy to su p p ort th e p op u lation . in 1983 th e un ited n ation s establish ed th e bru n d tlan d com m ission to e xam in e issu es critical to th e en viron m en t an d d evelop m en t. th e com m ission focu sed its a tten tion on th e areas of p op u lation , food secu r ity, th e loss of sp ecies an d gen etic resou rces, e n ergy, in d u stry, an d h u m an settlem en ts realizin g th at all of th ese are con n ected an d can n ot be treated in isolation on e from an oth er. th e fin al rep ort w as p u blish ed in 1987 an d p r op osed th e con cep t of “su stain able d evelo p m en t” w h ich w as d efin ed as: “sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” it h as been w id ely ad op ted arou n d th e w orld an d led th e un ited n ation s to d evelop th e 2030 agen d a for su stain able develop m en t an d its 17 su stain able develop m en t goals (sdg) (un ited n ation s, 2015a an d 2015b). in p articu lar sdgs n o. 6 (clean w ater an d san itation ); 7 (afford able an d clean en ergy); 9 (in d u stry, in n ovation an d in frastru ctu re); 12 (resp on sible con su m p tion an d p r od u ction ), 13 (clim ate action ), 14 (life below w ater) an d 15 (life on lan d ) ap p ly to th e issu es u n d er d iscu ssion in th is p ap er – w ater, en ergy an d raw m aterials. sim ilarly, th e eu rop ean un ion focu sed its efforts on a total of 7 societal ch allen ges (scs) th rou gh th e h 2020 research an d in n ovation p rogram . asp ects of th ese ch allen ges are eith er d irectly (e.g. w ater, en ergy, raw m in erals) or in d irectly (e.g. food , h ealth , clim ate, en viron m en t an d biod iversity) related to th e u se or p reserv ation of su bsu rface sp ace an d resou rces an d th e w ater -food -en ergy n exu s (bazilian et al., 2011). th e far-reach in g im p act an d in flu en ce of scs an d sdgs exten d beyon d th e n ation al scale, an d th e n eed for tran sn ation al in form ation an d coop eration is illu strated by th e follow in g e xam p les: th e m arkets for som e m in erals (in p articu lar m etals) an d en ergy resou rces are in creasin gly globalized . in tern ation al d ep en d en cies w ith regard s to th eir su p p ly can stron gly in flu en ce econ om ies an d societies at a n ation al scale. secu re su p p ly of r esou rces is an im p ortan t factor in creatin g or m ain tain in g su ch stron g econ om ies an d a lso in p reservin g p olitical stability (a n d rew s-sp eed et al., 2017; ec, 2011). tran sn ation al collaboration on m an agem en t of resou rces is cru cial in th is con text, con sid e rin g th e fact th at m ost su bsu rface resou rces are d eterm in ed by location -sp ecific geological con d ition s an d th erefore u n equ ally d istribu ted by d efin ition . em ission red u ction an d tran sition from fo ssil fu el gen eration system s to ren ew able e n ergy gen eration system s p resen t u s w ith a sign ifican t ch allen ge to secu re an d balan ce fu tu re en ergy su p p ly strategies. th is r equ ires in tern ation al coop eration an d th e su ccessfu l m an agem en t of d ifferen t en ergy gen eration an d storage solu tion s in clu d in g th e exp loration an d safe extraction of var iou s typ es of su bsu rface en ergy resou rces. t an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 3 the m itigation and red u ction of hu m an im p acts on clim ate change is a m ajor societal challenge for eu rop e and m any other cou ntries across the globe (ec, 2014) as clim ate change w ill affect the entire planet. the pr ed icted changes in clim ate and rise in sea level w ill have a profou nd im pact on grou nd w ater and su rface w ater system s (treid el et al., 2012). the ability to ad apt to these changes and the options to accom m od ate to im p acts w ill vary from region to region. yet ou r su ccess in p rotecting ecosystem s, keeping areas habitable and secu ring the safety of citizens and su p p lies of clean d rinking w ater (h insby et al., 2008), w ill d ep end on how w e cooperate in d eveloping joint solu tions. it has been argu ed that the hu m an im p act on earth has severely affected the environm ent and crossed planetary bou nd aries and threshold s e.g. biogeochem ical flow s of nitrogen and phosph oru s in the anthrop ocene (steffen et al., 2007 and 2015). th e su bsu rface is an im p ortan t p art of th e sy stem s in tegration n eed ed for global su stain ability in th e fu tu re (liu et al., 2015), an d ou r sele ction of op tion s m u st be based on (geo)eth ica l con sid eration s (martín ez-frías, et al., 2011; pep p olon i an d di cap u a, 2017). 2. a parad igm for a geological service for europe geoscien tists an d geological su rvey organ iz ation s (gsos) h ave a key resp on sibility to su p p ort th e realization of sdgs an d societal ch allen ges w ith reliable an d u n biased su bsu rface in form ation an d ad vice. th e con tin u ou s im p rovem en t, m ain ten an ce an d p u blic d issem in ation of th is in form ation an d ad vice, as w ell as th e scien tific tools an d m eth od s n eed ed for su ch p rovision s, are togeth er d escribed in th is p ap er as “geological services”. geological services p rovid e th e in form ation for evid en ce-based d ecision -m akin g on issu es w ith both sh ort an d lon g -term p ersp ectives. geoscien tific in form ation is critical to th e p rovision of evid en ce to back w h atever cou rse of action is p rop osed for an y sp ecific situ ation . it is largely collected an d con stru cted for p u blic good by gsos – eith er n ation al or region al – th rou gh ou t eu rop e. as geology is su ch a w id e scien tific en d eavor, each gso m ay focu s on a p articu lar su b-d iscip lin e (ran gin g from geoh azard s to w ater resou rces an d th e en viron m en t; from h yd rocarbon s to geoth erm al; an d from con stru ction m aterials to m etal raw m aterials) d ep en d in g on n ation al p riorities. as a resu lt th e state of kn ow led ge in an y on e su b-d iscip lin e v aries from on e gso to an oth er. in form ation collected by on e gso m ay be d ifferen t from an oth er an d it is often n ot easy to in tegrate d ata an d in form ation across ju risd iction s. geology, h ow ever, d oes n ot recogn ize th e bord ers b etw een cou n tries – geological form ation s tran scen d th ese bou n d aries an d w ater an d oth er flu id s flow (freely) across th e bord ers above an d below grou n d . th e fore-m en tion ed tran sn ation al con text of societal ch allen ges an d con t in u ity of geological featu res an d resou r ces across bord ers sh ou ld th erefore be reflected by th e geological services as w ell. th e key scien tific ch allen ge for gsos an d associated scien ce in stitu tes is th u s to overcom e th e fu n d am en tal d ifferen ces in th e w ay geological services are establish ed an d com p osed at n ation al an d region al level. th ese d ifferen ces e xist for variou s reason s, e.g.: differen t scop e an d level of su bsu rface e xp loration an d exp loitation resu ltin g in co n trastin g d ata p attern s an d qu alities. differen t ap p roach es, form ats an d stan d ard s for m ap p in g, m od elin g an d an alyzin g su bsu rface d ata (e.g. 2d vs. 3d, d efin ition of geological u n its, scale an d accu racy). differen t ju risd iction s m ay h ave reg u lation s coverin g w h at d ata can or can n ot be sh ared an d u sed . in ord er to serve en d -u sers effectively, th e geoscien tist m u st tran sform d ata an d kn ow led ge in to solu tion s th at d irectly relate to th e d ecision or altern atives u n d er ev alu ation . for exam p le: “wh ere can co 2 be safely stored an d in w h at qu an tities?”, “sh ou ld a certain m in eral d ep osit be con sid ered for d evelop m en t?”, or “wh at m easu res m u st be im p lem en ted to m itigate an d m on itor im p acts on a grou n d w ater bod y?” geological su rveys p rovid e su ch an sw ers by m easu rin g an d m od elin g su bsu rface stru ctu re an d p rop erties in fou r d im en sion s, an d u sin g an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 4 resu ltin g d ata an d geo -m od els to assess th e d istribu tion an d exp loitability of resou rces an d cap acities, as w ell as th e p ossible effects in d u ced by su bsu rface activities. th e in tegration of th ese ou tcom es in th e m u lti-d im en sion al geo-m od el sp ace h elp s th e d ecision -m aker s to ap p reciate th eir sp atial-tem p oral relation sh ip s an d en ables fu rth er ap p raisal of th e viability an d p ossible con sequ en ces of d evelop m en t scen arios. th is ap p roach is reflected by th e g en erically ap p licable w orkflow p rin cip les in fig. 1. th e key to establish in g an effective an d su stain able geological service at eu rop ean level lies in a p rop er tran sn ation al h arm on ization of each step in th is w orkflow , w h ile resp ectin g th e u n iqu e an d h igh ly variable geological ch a racteristics an d societal asp ects w ith in in d ivid u al region s. in th e follow in g section s th e key scien tific ch allen ges tow ard s establish in g th is in a eu rop ean con text are briefly d escribed for each of th e m ain w orkflow stages. as w ill be d em on strated later in th is p ap er, t h e id ea to realize a eu rop ean geological service is n ot m erely d riven by p ragm atic reason s, alth ou gh am p le ben efits are associated w ith reach in g across n ation al an d geological bo rd ers. at th e h earth of th is lies th e realization th at geoeth ical p rin cip les requ ire an accep ted an d d u rable eu rop ean stru ctu re for sh arin g of d ata, m eth od s, exp ertise, view s an d even resp on sibilities. 2.1 geo -models and data su bsu rface d ata an d m easu rem en ts are typ ica lly con verted in to 2d, 3d or 4d geo -m od els rep resen tin g th e sp atial layou t of geological u n its an d stru ctu res, rock an d flu id p rop erties, in te ractin g su bsu rface p rocesses (van d er meu len et al., 2013) at variou s sp atial an d tem p oral scales. by extrap olatin g d ata an d in form ation in to a h igh er d im en sion al sp ace (e.g. 1d w ells or d rill h oles in to 2d cross-section s or 3d layer m od els) or con vertin g in form ation from on e typ e in to an oth er (e.g. 3d seism ic in to 3d stru ctu re an d p rop erties), p red iction s can be m ad e on figure 1: generic workflow underpinning the presented geological service for europe. in this concept, subsurface management and resource assessments are linked to the consistent 3d geomodels from which they are derived. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 5 w h eth er certain geological con d ition s are likely or p ossible at p articu lar location s. movin g in form ation in to h igh er d im en sion s also m akes it p ossible to assess v ariou s sp atial an d tem p oral relation sh ip s betw een d ifferen t rock u n its an d th e d yn am ic p rocesses w ith in th ese u n its. th e con sisten t correlation an d lin ka ge of m easu rem en ts w ith in geologically m od eled u n its en ables th eir an alysis in an ap p rop riate gen etic con text. th e effectiven ess of tran sn ation al su bsu rface m an agem en t stron gly relies on a co n sisten t collation an d correlation of geo -m od el in form ation across bord ers. th is brin gs a m ajor ch allen ge w ith regard s to h ow th e variety of m od els an d m easu rem en ts from d ifferen t r egion s an d geological settin gs can be m erged or in tegrated . in ad d ition to th e geological asp ects (e.g. stratigrap h ic d efin ition s), th is task also requ ires solu tion s in in form ation m an agem en t (e.g. d ealin g w ith d ifferen t scales, geom etries an d m od el stan d ard s). 2.2 resources and utilization: energy, groundw ater and minerals su bsu rface resou rces an d cap acities are eith er id en tified d irectly from observation s (e.g. in w ells or d rill h oles) or p red icted from a com b in ation of in d icative p rop erties an d favorable geological con d ition s rep resen ted in th e geo m od els. a h arm on ized eu rop ean overview of geological resou rces an d p oten tial su bsu rface u ses w ill of cou rse rely on th e qu ality an d co n sisten cy of u n d erp in n in g d ata (e.g. obtain ed from su rface or airborn e geop h ysics). th e a p p lication of com p arable an d u n iform asses sm en t m eth od ologies an d criteria is of equ al im p ortan ce, esp ecially w h en th e en d re su lts are u sed in in tern ation al resou rces an d reserves classification sch em es. makin g su re th at su ch m eth od ologies are able to d eal w ith th e sp ecifics of in d ivid u al region s (geological settin g, available d ata, exp loration level, etc.), an d th at th e variou s geological u n certain ties are p rop erly accom m od ated in th e en d resu lts, are key scien tific ch allen ges in th is resp ect. 2.3 effects, impacts and protection th e con sequ en ces of su bsu rface d evelop m en t are in m ost cases restricted to th e sp ecific location w h ere th e activities take p lace, yet th eir m an ifestation often h as a far -reach in g in flu en ce on society. th e evalu ation an d u n d erstan d in g of th e su bsu rface p rocesses an d con d ition s th at lead to in d u ced h azard s an d en viron m en tal im p acts p resen ts sign ifican t scien tific ch allen ges. relevan t on -site in form ation m ay often be sp arse an d ap p rop riate an alogu es d ifficu lt to fin d . geological su rveys can brin g m u ch -n eed ed in n ovation to th is field of research by sh arin g e xp erien ces, m ergin g an d in tegratin g cru cial d ata sets an d join tly d evelop in g best p ractice s an d in n ovative m eth od ologies. tran sn ation al h arm on ized 2d/ 3d/ 4d geo m od els w ill greatly assist in a p rop er exch an ge an d com p arison of an alogu e system s an d th ereby en able n ew in sigh ts in to cau ses, effects an d tren d s. th e sam e is tru e for th e efficien t d evelop m en t of in n ov ative ap p roach es th at r ely on h arm on ized in p u t d ata. it is th is broad collaboration th at w ill p ave th e w ay for im p roved n ation al an d eu gu id an ce on th e safe d evelop m en t of th e su bsu rface an d an im p roved an d com m on u n d erstan d in g of th e h azard s an d im p acts, both w ith in an d b etw een mem ber states. 2.4 subsurface management and decision support information th e establish m en t of in tegrated an d coh eren t m an agem en t of su bsu rface resou rces an d u ses in eu rop e is on e of th e im p ortan t goals of a eu rop ean geological service. th e step s ou tlin ed above w ill facilitate d ecision -m akin g th at is u n d erp in n ed by con sisten t an d com p arable d ata, in form ation an d kn ow led ge. su bsu rface m an agem en t an d d ecision su p p ort in volves th e d evelop m en t of com m on p rin cip les an d strat egies on h ow to tran sform an d in corp orate th is d ata an d in fo rm ation in to com m on su bsu rface d ecision an d m an agem en t p rotocols. again , th e 2d/ 3d/ 4d geological fram ew ork w ill be cru cial for th e ap p rop riate evalu a tion an d rep resen tation of sp atial-tem p oral in teraction s an d d ep en d en cies, an d th e an ticip ation of con flicts an d syn ergies arisin g from d evelo p m en t scen arios of d ifferen t resou rces by d iffe ren t mem ber states. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 6 3. in formation stan d ard s an d d issemin ation : a fun d amen tal pre-requisite for geological services a p rop er eu rop ean geological service can on ly fu n ction an d be relevan t if it can p rovid e co m p lete, u p -to-d ate an d in terop erable d ata an d in form ation on an on -goin g basis. th e eu rop ean un ion , as a p arty to t h e aarh u s con ven tion , recogn izes th e follow in g righ ts in th is con text : 1. provid e access to en viron m en tal in fo rm ation ; 2. provid e th e p u blic th e righ t to p articip ate in en viron m en tal d ecision m akin g; an d 3. provid e a m ech an ism to rev iew p roced u res on d ecision s m ad e w ith resp ect to th e en v iron m en t, w h ich h ave n ot resp ected th e tw o p reviou s righ ts or en viron m en tal law in gen eral. to th at en d a com m on in form ation p latform m u st be d evelop ed th at sh ares an d exch an ges d ata an d resu lts betw een all cou n tries an d d iscip lin es. n ot on ly w ill th e p latform be im p ortan t for th e d issem in ation of in form ation an d resu lts to en d -u sers. it w ill also d rive th e p rop er im p lem en tation of th e m easu rem en t -tod ecision w orkflow s by d eliverin g th e critical m od el arch itectu res, fu n ction alities, stan d ard s an d m etad ata stru ctu res. su ch stru ctu res an d fu n ction alities d efin e th e basis for ad d -on w ebservices d evelop ed by th ird p arties or w ith in pu blic-private-partn ersh ip s betw een th e gso’s an d relevan t stakeh old ers e.g. for th e eu rop ean en viron m en t agen cy. th e eu rop ean un ion in spire directive (ec, 2007) establish in g an in frastru ctu re for sp atial in form ation in th e eu rop ean com m u n ity aim s to facilitate th e h arm on ization an d stan d ard iz ation of sp atial d ata an d m ake th e d ata fin d able, accessible, in terop erable an d reu sable accor d in g to th e “fair” p rin cip les (wilkin son et al., 2016). geoscien ce d ata is on e of th e sp atial d ata th em es w ith in th e rem it of in spire. as su ch it requ ires th e con stru ction of a com m on term in ology. to th is p u rp ose gsos, am on g oth ers, h ave p articip ated in d evelop in g cod e-lists, e.g. for m in erals w ith in th e min erals4eu p roject. h ow ever, th e d evelop m en t an d im p lem en t ation of cod e-lists for oth er asp ects is a task th at sh ou ld be ach ieved by th e en vision ed geolog ical service for eu rop e. 4. societal challen ges an d geological services for the sustain able d evelopmen t: a geoethical imperative th e con cep t of geological services w ith in th e eu rop ean con text is in tertw in ed w ith th e fu n d am en tal p rin cip les of geoeth ics. th e d efin ition of geoeth ics, as p rop osed by th e in tern ation al association for prom otin g ge oeth ics (h ttp :/ / w w w .geoeth ics.org , accessed 15 may 2018), is: “geoethics consists of research and reflection on the values which underpin appropriate behaviors and practices, wherever human activities interact with the earth system. geoethics deals with the ethical, social and cultural implications of geoscience education, research and practice, and with the social role and responsibility of geoscientists in conducting their activities.” th e geoscien ce com m u n ity n eed s to p rom ote an eth ical ap p roach tow ard s scien ce an d p ra ctice of research . th is w ill be a core p rin cip al at th e h eart of d eliverin g a geological service for eu rop e. th e term “geoeth ics” is u sed from ’90 (pep p olon i an d di cap u a, 2015), an d th e grow in g aw aren ess in th e geological com m u n ity for th e n eed to take eth ics in to con sid eration in its w ork p reced es th e cu rren t d efin ition (wyss & pep p olon i, 2015). th e “cap e tow n statem en t on geoeth ics” (di cap u a et al., 2017) d escribes th e fu n d am en tal valu es of geoeth ics: en su rin g su stain ability of econ om ic an d social activities in ord er to assu re fu tu re ge n eration s’ su p p ly of en ergy an d oth er n atu ral resou rces. sh arin g kn ow led ge at all levels as a valu able activity, w h ich im p lies com m u n icatin g scien ce an d resu lts, w h ile takin g in to a ccou n t in trin sic lim itation s su ch as p robabilities an d u n certain ties. verifyin g th e sou rces of in form ation an d d ata, an d ap p lyin g objective, u n biased an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7500 7 p eer-review p rocesses to tech n ical an d scien tific p u blication s. th ese statem en ts p rovid e clear referen ces to th e em in en t p osition of geoeth ics as an u n d e rp in n in g p rin cip le for geoscien tists an d geoscien tific in form ation an d kn ow led ge to follow w h ile ad d ressin g societal ch allen ges an d th e sdgs. as su ch , geological services in clu d e g eoeth ics valu es as th ey are aim ed at safegu ar d in g th e exp loration an d safe d evelop m en t of n atu ral resou rces an d su bsu rface cap acities based on im p artial an d scien tifically v alid ated in form ation an d kn ow led ge, alw ays h avin g th e in terests of society as a core valu e. 6. con clud in g remarks an d outlook in tern ation al econ om ic d evelop m en ts, resou rce d ep en d en cies, clim ate ch an ge im p acts, en ergy tran sition , grow in g societal aw aren ess, an d th e n eed for n ew tech n ologies an d ad van ced u n d erstan d in g, are all im p ortan t d rivers for a u n iform an d in terop erable eu rop ean geological service. th is service su p p orts th e collaborative m an agem en t an d p rotection of su bsu rface r esou rces an d cap acities an d can on ly be relevan t if it can con tin u ou sly p rovid e com p lete, u p -tod ate, an d h arm on ized su bsu rface in form ation . to th at p u rp ose th e n ation al an d region al ge ological su rveys of eu rop e are takin g a m ajor step by establish in g th e p recu rsor of a eu rop ean geological service th rou gh close collabor ation w ith in th e d ed icated cross-th em atic eran et for ap p lied geoscien ces (geoera, 2017). th rou gh th is eran et th e m an y scien tific ch allen ges th at sep arate u s from a tru ly h arm on ized ap p roach , w ill be ad d ressed . ge ological an d tech n ical solu tion s at d ifferen t le vels of th e w orkflow w ill be d evelop ed an d tested for en ergy, grou n d w ater an d m in erals in variou s geological settin gs an d region s of eu rop e. th e resu lts from th is eran et w ill be in corp orated an d sh ared th rou gh a com m on in form ation p latform (egdi, 2016), w h ich w ill con n ect region al an d n ation al geological d at abases in a sin gle access p oin t. th e con tin u ed in volvem en t of local an d in tern ation al stak eh old ers w ill assist th e d elivery of th e geolog ical service so th at it w ill becom e an effective in stru m en t for n ation al an d eu rop ean en d u sers, an d a reliable in form ation sou rce for th e p u blic. it is on ly th rou gh this eu rop ean coo p eration th at th e n ation al an d region al gsos can con tin u e to p lay th eir cen tral role for society an d econ om y, w h ile gu aran teein g th at th e ge oeth ical p rin cip les of su stain ability, tran sp are n cy, verification , an d objectivity are fu lly r esp ected . ackn owledgemen ts th e con ten t of th is p ap er w as sh ap ed th rou gh n u m erou s d iscu ssion s w ith m an y colleagu es from gsos an d oth er in stitu tes, both eu rop ean an d n on -eu rop ean , as w ell as by exp erien cin g th e ben efits of in ten se coop eration in cross bord er p rojects. th e au th ors w ou ld like to th an k everyon e th at w as to som e d egree in volved , as w ell as th e region al, n ation al an d in tern ation al fu n d in g organ ization s th a t allow ed bu ild in g u p to th e geoera p roject. th e geoera p roject (establish in g th e eu rop ean geological su rveys research area to d eliver a geological service for eu rop e) h as received fu n d in g from th e eu rop ean un ion 's h orizon 2020 research an d in n ovation p rog ram u n d er gran t agreem en t n o. 731166. referen ces and rew s-speed p., bleischw itz r., boersm a t., johnson c., kem p g., vandeveer s.d. 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(2015). geoeth ics: eth ical ch allen ges an d case stu d ies in earth scien ces. elsevier, walth am , ma, isbn 978‐0‐12‐799935‐7. 073_086 adg v–5 n01 belehak.pdf annals of geophysics, vol. 45, n. 1, february 2002 73 investigation of the relative bottomside/topside contribution to the total electron content estimates anna belehaki and ioanna tsagouri institute for space applications and remote sensing, national observatory of athens, palaia penteli, greece abstract high-resolution vertical electron density true height profiles from athens digisonde are used to calculate the electron content up to the peak of the f2 layer (bottomside electron content) and also the topside electron content up to 1000 km, using the huang-reinisch method, to investigate the relative behavior of these two parameters during storm events. it was shown that the topside electron content represents roughly the 2/3 of the total electron content of the ionosphere and it is strongly affected by the geomagnetic activity, much more than the bottomside electron content. during daylight hours the values of the topside electron content exhibit intense fluctuations, while they become smoother in nature during events of night-time f 2 layer uplifting caused by the auroral activity. all these facts demonstrate that the huang-reinisch method provides a realistic tool for monitoring the variations of the ionospheric ionisation at a given location and it could be used in the frames of a world wide effort for the development of realistic models to accurately predict the electron content and to support effectively earth-space communications including navigation systems. 1. introduction so far, dual frequency transmissions from global positioning systems (gps) have been used to derive the total electron content (tec) of the ionosphere and to study aspects of the thermospheric/ionospheric storm event. as former studies have shown (e.g., titheridge and buonsanto, 1980; jakowski et al., 1990, 1999; ho et al., 1996), tec data can help to understand the complex behaviour of competing interactive processes in the course of ionospheric storms. a useful add-in to the study of ionospheric storms in the light of tec variations would be the study of the variation of electron content in various ionospheric layers, separately. the operation of a lowell dps4 in athens (38°n, 23.5°e) since september 2000, gave us the possibility to obtain a large amount of high quality ionospheric data with 15 min time resolution. among them, three parameters were selected to be used in this study to determine the structure of the f2 layer: the critical frequency f0 f2; the true height of the f2 peak, hmf2; and finally the lower boundary of the f 2 layer height, h m f 2 -b0, where b0 is the iri thickness parameter. three more parameters were derived to determine the electron content in various ionospheric layers, that is the vertical total electron content from ionograms, itec, the bottomside electron mailing address: dr. anna belehaki, institute for space applications and remote sensing, national observatory of athens, metaxa and vas. pavlou str., palaia penteli 15236, greece; e-mail: belehaki@space.noa.gr key words bottomside electron content – topside electron content – ionospheric storm effects 74 anna belehaki and ioanna tsagouri content btec and the topside electron content ttec. the method of derivation of these three parameters will be presented in detail in the following section. the aim of this contribution is to study the behaviour of the topside electron content, derived from ground ionograms, under various levels of geomagnetic activity and to investigate the contribution of bottomside versus the topside electron content to the total electron content estimates. 2. ionograms processing and electron content calculation athens digisonde portable sounder auto scales the ionograms and provides radio channel information in real time. data and ionograms are available in real time on the world wide web (http://www.iono.noa.gr). polarisation switching and multibeam forming are the important characteristics for successful auto scale under disturbed and undisturbed conditions. auto scaling algorithms find the leading edge of the o echo traces, i.e. h′( f ), and determine the standard ionosphere characteristics. artist software provides reliable vertical electron density profiles, using the chebyshev polynomial fitting technique (reinisch and huang, 1983; huang and reinisch, 1996). regarding the determination of the topside vertical electron density profile, the new technique of huang and reinisch (2001) is applied. this method uses information from ground based ionosonde measurements. according to this technique, the vertical total electron content from ground-based ionograms (itec) is given by where, n is the vertical electron density profile, n b is the vertical electron density profile up to f2 layer peak (bottom profile) calculated using the inversion program proposed by huang and reinisch (1996) and n t is vertical electron density profile above the f 2 layer peak (topside profile). n b (h) is calculated from the measured h′(f) traces in the ionogram (information directly provided from the ionograms). n t (h) is approximated by an a-chapman function with a constant scale height h t that is derived from the profile shape at the f2 peak, where and the characteristic parameters that define the shape of the bottomside electron density profile determine also the value of itec parameter. a preliminary itec validation was performed by huang and reinisch (2001) by comparing itec with tec values derived from incoherent scatter radar and geostationary satellite beacon measurements at middle latitudes, and with topex measurements at the equator. they concluded that since itec is obtained from vertical measurements it does not suffer from the uncertainties associated with converting slant tec to vertical tec. all ionograms used in this study to derive the itec values and also various parameters which characterize the state of the f2 layer, were further edited to ensure data integrity and minimize uncertainties especially during periods of high disturbances. 3. validation of itec values used in this analysis to validate of itec values extracted according to the huang and reinisch (2001) method using parameters from the vertical ground ionograms taken with athens digisonde, we compared the 15 min itec estimates with the tec values measured from the gps network. the gps-tec estimates were provided by luigi ciraolo of the istituto di ricerca sulle onde elettromagnetiche in florence, italy, using itec = = + ∞∞ ∫∫∫ n h dh n h dh n h dhb t h f h f m m ( ) ( ) ( ) 2 2 00 n n f z et m z= ⋅ − −( )      −exp2 1 2 1 z h h f h m t = − .2 75 investigation of the relative bottomside/topside contribution to the total electron content estimates a b fig. 1a,b. the gps-tec and the itec estimates for two time periods: a) a quiet one, from 11 to 15 december 2000, consisting of five consecutive days of geomagnetic and ionospheric conditions over athens; b) a disturbed period 21-26 december 2000, during which a geomagnetic storm caused positive storm effects in the ionosphere over athens. 76 anna belehaki and ioanna tsagouri slanting tec from matera and ankara gps receivers. an assumption of a mapping function and an approximation of the vertical tec by a linear development in latitude and longitude are involved to estimate gps-tec over athens (ciraolo and spalla, 1997). the comparison between the two data sets was concentrated on two time periods, a quiet one, from 11 to 15 december 2000, consisting of five consecutive days of quiet geomagnetic and ionospheric conditions over athens, and a disturbed period during which a geomagnetic storm caused positive storm effects in the ionosphere over athens. the time plots of the two data sets are presented in fig. 1a,b respectively for the quiet and the disturbed period. the main points that have to be discussed from the comparison between the two data sets are: a) the two parameters are generally in good agreement during either quiet or disturbed geomagnetic conditions. b) systematic bias during night-hours is observed. the gps-tec values are higher by 1-2 tec units in comparison to itec estimates. this bias might be caused by errors involved in the procedure followed to derive the gps-tec (l. ciraolo, personal communication), that are uncertainties in mapping function and in the vertical tec model, possible influence of multipath signal propagation and possible presence of gradients. also possible cause for the observed bias might be the different estimation of plasmaspheric fluxes contribution assumed by the two methods. c) systematic appearance of a discontinuity in gps-tec time series between the end of the day and the beginning of the next day, especially in presence of disturbances. this is probably due to the lack of observations at the processed local time, since the analysis is a single day analysis (l. ciraolo, personal communication). d) the gps-tec measurements appear to be smooth in nature in contrary to the highly fluctuated itec estimates, especially during day hours. the gps-tec estimates are resulted from the conversion of slant tec to vertical tec using values from matera and ankara averaged in space. in contrary, itec estimates are based on the athens digisonde measurements, which looks directly to the local zenith. in summary, the comparison shows a reliable behaviour of itec variation, which is consistent to the ionospheric storm effects over athens, as these are described with the gps-tec values. 4. variation of bottomside/topside electron content during specific storm events several storm events were selected to study the evolution of ionospheric disturbances over athens. 29 september 1 october 2000 – during this storm the dst index reached a minimum value of – 78 nt at 1400 ut on september 30. according to the imf observations from ace spacecraft (not shown here) the z component of the imf turned southward at the start of the initial phase of the storm. at the same time the solar wind velocity and density remain in their undisturbed values. the weak values of the imf have as a result a low level of solar wind magnetospheric coupling giving this storm event of moderate intensity that fully recovers on october 2 at 0600 ut. the f region characteristics, the critical frequency f 0 f 2 and the height boundaries of the f 2 layer (h m f2 and hmf2-b0) are presented in fig. 2, together with the itec, btec and ttec estimates over athens. the solid lines represent the observed parameters. the dotted lines represent the diurnal quiet time behaviour of athens station formed by the average of the quiet days october 20 (– 2 nt < dst < 7 nt and kp < 1) and october 21 (0 nt < dst < 18 nt and kp < 2 –), when no substorm activity was detected by the ae indices. also during the day before, october 19, 2000, only weak isolate substorm activity was recorded by the ae index at auroral latitudes, when – 13 nt < dst < 3 nt and k p < 3. the vertical dotted lines mark to local midnight. during this storm, negative effects were observed during day-time. true height disturbances resulting to the uplifting of the f 2 layer can be clearly seen, especially during the second night of the storm. itec variations in general follow the f0 f2 disturbances, presenting a noticeable fluctuation during daytime hours. the btec and ttec parameters are following 77 investigation of the relative bottomside/topside contribution to the total electron content estimates fig. 2. from the top to the bottom, the dst index in hourly resolution is presented to have an overall view of the magnetospheric disturbances recorded during the storm occurred between september 29 and october 1, 2000, the f region characteristics, the critical frequency f0 f2 and the height boundaries of the f2 layer (hm f2 and hm f2-b0) and finally the tec, btec and ttec estimates over athens. the solid lines represent the observed parameters. the dotted lines represent the diurnal quiet time behaviour of athens station. the vertical dotted lines mark to local midnight. 78 anna belehaki and ioanna tsagouri fig. 3. same as fig. 2 for the storm interval october 12-16, 2000. 79 investigation of the relative bottomside/topside contribution to the total electron content estimates the itec variations. the contribution of the bottomside electron content to the itec remains constant throughout the course of this event, as can be seen in the bottom panel of fig. 2. 12-16 october 2000 – the ionospheric response recorded over athens during this storm interval is presented in fig. 3 in the same format as in fig. 2. the diurnal quiet time behaviour of athens station, overplotted as dotted curve, is formed by the average of the quiet days october 20 and 21. at the beginning of this interval the magnetosphere-ionosphere system was in a moderately disturbed state as it is evident from the negative values of the magnetospheric index dst, but also from the ionisation depletion observed on october 12, 2000. the start of this storm can be determined as the time of occurrence of a storm sudden commencement (ssc) detected at 2228 ut on october 12. the ssc signify that the earth’s magnetic field has been compressed by a shock front from a flare. this results to a rapid ring current development. the dst index reached a minimum value of – 63 nt at 0500 ut on october 13. the storm recovery phase lasted 9 h. immediately afterwards the dst index started to decrease again, with a slow rate this time indicating that the large-scale magnetospheric convection caused a slow development of the ring current that gave the second storm observed during this period with the dst index reaching its minimum value of – 110 nt at 1600 ut on october 14. the main observations regarding the f2 layer response may be summarized as follows: – negative effects are observed at night from 12 to 13 october over athens. this is the ionospheric response to the first storm event caused by an ssc. the bottomside, topside and total electron content shows also a negative response, following the f0 f2 variations. during the second night of this time interval (13 to 14 october), that corresponds to the initial and main phase of the second storm, positive nighttime effects were seen in athens. the same pattern was observed during the next two nights (14 to 15 and 15 to 16 october) during which the magnetic storm was in its recovery phase. the electron content in the bottomside and topside ionosphere increases also in respect to their quiet values. the contribution of btec to the total electron content remains constantly equal to approximately the 1/3 as can be seen in the bottom panel of fig. 3. – a noticeable up lifting of the f2 layer is observed at night. the estimated h m f2 parameter is smoother in nature during these night-time uplifting events rather during daylight, where the height boundaries of the f 2 layer remain in their undisturbed values. this is reflected in the itec values also. these daytime fluctuations in itec, if really exist, are of great importance for navigation and positioning systems. 2-8 october 2000 – the second storm is more intense and its development consists of two distinct steps. the start of the dst decrease (~ 0700 ut on october 2) coincides with a weak southward turning of the imf, according to ace data (not shown here). a ssc, occurred at 0054 ut on october 3, marks the start of the main phase of the storm development. this storm seems to recover with the dst reaching a minimum of – 23 nt at 0200 ut on october 4. right afterwards, dst index starts to decrease again with a high rate of change, marking the initial phase of the second step of this storm. a well-defined ssc event occurred at 0326 ut on october 5 caused new storm intensification and a reactivation of the main phase of the storm, with the dst index reaching its maximum of – 192 nt at 1400 ut on october 5. this was also the start of the recovery phase of the storm. this storm can be classified as a type b storm (kamide et al., 1998) as the ring current presents a two-step development. this is initially the result of large-scale convection in the magnetosphere, and eventually substorm associated injection of ionospheric oxygen ions into the inner magnetosphere. the time variation of the f2 layer parameters is presented in fig. 4 in the same format as fig. 2. the diurnal variation of the quiet time is formed by the average of the quiet days october 20 and october 21. the variations of the f 0 f 2 critical frequency parameter are well correlated to the itec variations and the btec and ttec parameters are following. during day-time the itec over athens increases or decreases fol80 anna belehaki and ioanna tsagouri fig. 4. same as fig. 2 for the storm interval october 2-8, 2000. 81 investigation of the relative bottomside/topside contribution to the total electron content estimates fig. 5. same as fig. 2 for the storm interval september 16-21, 2000. 82 anna belehaki and ioanna tsagouri lowing the positive and negative storm phases, respectively. during night-time, ionisation enhancements recorded over athens (e.g., the nights between 2-3 and 4-5 october) are coincided with itec enhancements. the uplifting of the f2 layer that was detected during the first four nights of the storm development, caused probably to waves launched from the auroral oval at each substorm activation, are correlated neither to ionisation variations extracted by f0 f2, nor to itec variations. it is also noticeable that during night-time uplifting events of the f 2 layer, the electron content appears again to be smoother in nature compared to other intervals where no height variations are observed in f2 layer for instance during the last two days of this storm interval. 16-21 september 2000 – an interplanetary coronal mass ejection (icme) occurred during this interval caused a gradual increase of the solar wind velocity whose values reached the 850 km/s and large increase in solar wind density (from ace data, not presented here). the dst index was already depressed prior to the arrival of the shock, caused by two storm sudden commencements (ssc) occurred at 0450 ut and 1912 ut on september 15, 2000, according to noaa archives (http://www.ngdc.noaa.gov/ stp/geomag/geomag1.html). a fast forward shock arrives at 1900 ut on september 17 causing rapid decrease in the dst index, reaching its minimum value equal to – 172 nt at 2300 ut on september 17. while the system recovered, at 1444 ut on september 18 a new ssc caused a deceleration in dst recovery. this time dst reached the minimum value of – 80 nt on september 19, which recovered slowly until the end of the next day. the behaviour of the ionosphere over athens is presented in fig. 5, showing the time plots of the critical f0 f2 frequency and of the height boundaries of the f2 layer together with the dst index. the variation of itec, ttec and btec parameters is presented at the bottom of fig. 5. the dotted lines represent the diurnal quiet time behaviour formed by the corresponding observations during the quiet day september 11 (9 nt < dst < 24 nt and k p < 1), when no substorm activity was detected by the ae indices. also during the day before, september 10, 2000, no substorm activity was recorded by the ae index at auroral latitudes, when 0 nt < dst < 20 nt and k p < 1. night-time ionisation depletion was observed over athens during this disturbed interval, which is deduced from f 0 f 2 and btec and ttec time variations. noticeable uplifting of the f2 layer is observed during the first two nights of this interval (16 to 17 and 17 to 18 september) with the maximum increase in height observed the second night of the major storm occurrence. large amplitude fluctuations are observed in electron content estimates during daytime whereas during nighttime f2 layer uplifting events these values are smoother in nature. all quantities btec, ttec and itec present the same behaviour and correlate perfectly with f0 f2 variations. the relative contribution of btec to the itec remains constantly equal to ~ 0.3. table i. the main characteristics of the five geomagnetic storms analysed in this paper. storm event dst-min ∂dst/∂t standard deviation standard deviation ∆ttec%−∆itec% ∆btec%−∆itec% 29 sept.-1 oct. 2000 −78 nt 2.8 nt/h 3.4 8.5 12-13 oct. 2000 −63 nt 5.3 nt/h 3.5 8.5 13-16 oct. 2000 −110 nt 4.1 nt/h 3.8 9.5 2-8 oct. 2000 −192 nt 1st step: 2.9 nt/h 4.1 10.4 2nd step: 4.8 nt/h 16-21 sept. 2000 −175 nt 51 nt/h 11.1 nt/h 4.9 11.1 83 investigation of the relative bottomside/topside contribution to the total electron content estimates 5. statistical analysis of observations in accordance with the above analysis, five geomagnetic storms were determined during the selected time intervals. their characteristics are summarized in table i. their intensity is expressed in terms of the minimum dst value at the end of the main phase and also in terms of the dst decrease rate. to distinguish between the behaviour of bottomside and topside electron content during storm intervals, we estimated the standard deviation of the relative disturbance of ttec and btec in respect to their quiet values (∆ttec% and ∆btec% respectively), from the corresponding itec relative deviations (∆itec%), for each storm separately. the results are plotted in fig. 6a-e and the numerical values are presented in the last two columns of table i. there are two groups of events: the first three storms are of rather moderate intensity, according to the minimum dst value; the last two are intense storms. it is concluded that as geomagnetic activity increases, the ttec is more drastically perturbed rather than btec. it follows that ttec is more sensitive to geomagnetic activity. as a next step, we studied the variations of the time derivative of ttec and btec in absolute values, with the h m f 2 time derivative. the results are presented in fig. 7a-e for the five storm intervals separately. it is obvious that the topside electron content varies much more drastically with the variations of the true height of the f 2 layer, giving one more evidence that ttec is more sensitive to geomagnetic activity, since height variations are mostly caused by geomagnetic disturbances, at least for the time intervals considered in this study. fig. 6a-e. the standard deviation of the relative disturbance of ttec and btec in respect to their quiet values (∆ttec% and ∆btec% respectively), from the corresponding itec relative deviations (∆itec%), for each storm separately. a b c d e 84 anna belehaki and ioanna tsagouri 6. discussion and conclusions in this work we analyzed ionospheric observations from athens digisonde (the f 0 f 2 critical frequency), and also various estimates that determine the structure of the ionosphere over athens such as: the true height of the peak of the f 2 layer, h m f 2 , extracted by the true height bottomside electron density profile according to the huang and reinisch (1996) inversion technique; the bottomside electron content (btec) estimated as the integral of the electron concentration up to the peak height of the f 2 layer; the topside electron content (ttec) according to the newly proposed technique of huang and reinisch (2001), estimated as the integral of the topside electron density profile up to the 1000 km; the total electron content (itec) estimated as the sum of the bottomside and topside electron content. it should be noted that btec and ttec estimates are two independent parameters, since two different techniques are followed for their derivation. the aim of this paper was to investigate the behaviour of the two parameters btec and ttec during storm events. to approach this goal, first we presented a comparison between the itec estimates over athens, and the tec estimates from the ground gps network provided by l. ciraolo (fig. 1). the two curves have a surprising good agreement during either quiet or disturbed periods. nevertheless both methods (the huang and reinisch, 2001 method for itec estimation from ground ionograms only, and the well known technique of tec estimation from ground gps networks) have fig. 7a-e. the variations of the time derivative of ttec and btec in absolute values, with the h m f 2 time derivative for the three storm intervals separately. a b c d e 85 investigation of the relative bottomside/topside contribution to the total electron content estimates some limitations. to obtain the optimum results using the huang and reinisch method, especially at times of ionospheric disturbances, the automatic ionogram scaling results should be checked for their correctness because faulty echo traces will produce wrong itec values. on the other hand, the method of deriving tec from gps measurements introduces significant errors during night-hours, which have to be taken under consideration. the next step of this investigation was to analyze the variation of ttec and btec parameters during storm events of different intensity. the first and the second storm (29 september 1 october and 12-13 october 2000) were of moderate intensity. the third one (13-16 october 2000) was a rather intense storm and the last two storms were very intense events (2-8 october and 16-21 september 2000). two of these storms were characterized by the occurrence of a ssc at the beginning of the initial phase (12-13 october 2000, 16-21 september 2000). the other three storms were the result of the large-scale convection system of the magnetosphere, and are characterized as «gradual driven» storms. the two different types of storms caused different ionospheric effects over athens (belehaki and tsagouri, 2002). the storms with ssc produced negative effects while the gradual driven storms caused ionisation depletion during the day and ionisation enhancements during the night, according to the f 0 f 2 variations. the itec variations are in very good correlation with the f0 f2 variations in both types of storms. the main characteristics of the topside electron content calculated according to the huang and reinisch (2001) method, as raised from this analysis, may be summarized as follows: the topside electron content represents roughly the 2/3 of the total electron content of the ionosphere and it is strongly affected by the geomagnetic activity, much more than the bottomside electron content as it is extracted from the analysis presented in fig. 6a-e. during daylight hours the ttec values exhibit intense fluctuations. ttec becomes smoother in nature during events of night-time f2 layer uplifting caused by the auroral activity. all these facts demonstrate that the itec proved to be a realistic tool for monitoring the variations of ionospheric ionisation. indeed, the variations of ttec and btec parameters reflect the effects of ionospheric disturbances at a given location. the great importance of the huang and reinisch (2001) method for a more comprehensive discussion of ionospheric storm phenomena, in particular if simultaneously measured gps derived tec data are available, is the most important conclusion of this study. the cooperation of as many as possible ground ionosondes having the capability to compute the itec parameter in association with the gps-tec measurements could lead to the development of realistic models for the accurate prediction of electron content during either disturbed or undisturbed intervals, to support effectively earth-space communications including navigation systems. acknowledgements we thank luigi ciraolo, istituto di ricerca sulle onde elettromagnetiche in florence, italy, for providing us with the gps-tec values and for the constructive comments; and bruno zolesi, istituto nazionale di geofisica e vulcanologia, for helpful discussions and useful suggestions. references belehaki, a. and i. tsagouri (2002): on the occurrence of storm induced nighttime ionization enhancements at ionospheric middle latitudes, j. geophys. res. (in press). ciraolo, l. and p. spalla (1997): comparison of ionospheric total electron content from the navy navigation satellite system and the gps, radio sci., 32 (3), 1071-1080. ho, m.c., a.j. mannucci, u.j. lindqwister, x. pi and t.t. ts u ru ta n i (1996): global ionosphere perturbations monitored by the worldwide gps network, geophys. res. lett., 23, 3219-3222. huang, x. and b.w. reinisch (1996): vertical electron density profiles from the digisonde network, adv. space res., 18 (6), 121-129. huang, x. and b.w. reinisch (2001): vertical total electron content from ionograms in real time, radio sci. (special issue for ies meeting), 36 (2), 335-342. jakowski, n., e. putz and p. spalla (1990): ionospheric storm characteristics deduced from satellite radio beacon observations at three european stations, ann. geophys., 8, 343-352. 86 anna belehaki and ioanna tsagouri jakowski, n., s. schluter and e. sardon (1999): total electron content of the ionosphere during the geomagnetic storm on 10 january 1997, j. atmos. solarterr. phys., 61, 299-307. kamide, y., n. yokoyama, w. gonzalez, b.t. tsurutani, i.a. daglis, a. brekke and s. masuda (1998): twostep development of geomagnetic storms, j. geophys. res., 103, 6917. reinisch, b.w. and x. huang (1983): automatic calculation of electron profiles from digital ionograms, 3, processing of bottomside ionograms, radio sci., 18, 477. reinisch, b.w., x. huang, a. belehaki and j.-c. jodogne (2001): bottom and topside ionospheric t e c o b t a i n e d f r o m g r o u n d b a s e d i o n o s o n d e measurements, in proceedings of the international beacon satellite symposium, boston college, ma. t i t h e r i d g e, j.e. and m.j. b u o n s a n to (1980): a comparison of northern and southern hemisphere tec storm behaviour, j. atmos. terr. phys., 50, 763-780. vol49_1_2006def 235 annals of geophysics, vol. 49, n. 1, february 2006 key words hyperspectral data – surface temperature – spectral emissivity – solfatara (phlegraean fields) – dais 1. introduction surface temperature and spectral emissivity of materials are valuable parameters in volcanology since they provide information on several aspects of the current activity and of past events. mapping surface temperature allows us to estimate the heath flux from the surface and therefore to control activity inside vents and fractures. spectral emissivity in the thermal infrared is a physical parameter necessary to calculate temperature, and moreover it enables recognition of surface materials on the basis of their spectral pattern (salisbury and d’aria, 1992); it is particularly suitable to study volcanic materials because the main absorption bands of silicates are located in the thermal infrared (gillespie, 1985). remote separation and identification of surface deposits in volcanic areas is useful to map lava flows on the basis of their composition, to characterize surface alteration due to hydrothermal phenomena and to identify newly formed deposits related to fumarolic activity. surface kinetic temperature and emissivity properties of materials can be estimated with high accuracy by remote sensing if images collected on several channels in the thermal infrared are available. thermal infrared multi-channel sensors have been operating for a long time at low spatial resolution (less than one km, mainly for meteorologispectral emissivity and temperature maps of the solfatara crater from dais hyperspectral images luca merucci (1), maria paola bogliolo (2), maria fabrizia buongiorno (1) and sergio teggi (3) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto superiore per la prevenzione e la sicurezza del lavoro, monteporzio catone (rm), italy (3) università di modena e reggio emilia, modena, italy abstract quantitative maps of surface temperature and spectral emissivity have been retrieved on the solfatara crater at pozzuoli (naples) from remote sensing hyperspectral data. the present study relies on thermal infrared images collected on july 27, 1997 by the dais hyperspectral sensor owned by the german aerospace center (dlr). the emissivity spectrum normalization method was used to make temperature and emissivity estimates. raw data were previously transformed in radiance and corrected for the atmospheric contributions using the modtran radiative transfer code and the sensor response functions. during the dais flight a radiosonde was launched to collect the atmospheric profiles of pressure, temperature and humidity used as input to the code. retrieved temperature values are in good agreement with temperature measurements performed in situ during the campaign. the spectral emissivity map was used to classify the image in different geo-mineralogical units with the spectral angle mapper method. areas of geologic interest were previously selected using a mask obtained from an ndvi image calculated with two channels of the visible (red) and the near infrared respectively. mailing address: dr. luca merucci, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: merucci@ingv.it 236 luca merucci, maria paola bogliolo, maria fabrizia buongiorno and sergio teggi table i. dais 7915 main technical and spectral characteristics. spectrometer spectral region no. of channels wavelength 1 0.4-1.0 nm 32 15-30 nm 2 1.5-1.8 nm 8 45 nm 3 2.0-2.5 nm 32 20 nm 3.0-5.0 nm 1 2.0 nm 4 8.0-12.6 nm 6 0.9 nm dynamic range 15 bit ground resolution 5-20 m (function of the flight altitude) scan line dimension 512 pixels fig. 1. topographic map of the solfatara crater. gray squares indicate the surface temperature measurement sites; the black circle (rs) locates the radiosonde launch site. 237 spectral emissivity and temperature maps of the solfatara crater from dais hyperspectral images cal or oceanographic use), and only few instruments provide multi-channel images at high spatial resolution in this spectral range. the airborne hyperspectral sensors tims (nasa), mivis (cnr-lara project) and dais (german aerospace center dlr: http://www.op.dlr.de/dais/) and the spaceborne sensor aster onboard terra and aqua satellites (nasa) are examples of this kind of instruments. with the aim of understanding the capabilities of the dais (digital airborne imaging spectrometer) hyperspectral sensor to provide information of volcanological interest, we analyzed the thermal infrared dais data of an active volcanic area, according to three work phases: 1) retrieval of the radiance emitted by the surface, through atmospheric modeling; 2) estimation of surface temperature and spectral emissivity; 3) application of spectral mapping techniques to produce a thematic map of the main surface units. 2. data set remote sensing data used in this study were provided by the dais 7915 spectrometer owned by the german aerospace center (dlr). the main spectral and technical characteristics of dais are summarized in table i. the image under investigation was collected on july 27, 1997 at 10:27 (local time) on the solfatara crater at pozzuoli (naples, italy) in the framework of the e.c. large scale facilities project, and under request and supervision of the remote sensing laboratory of the italian national institute of geophysics and volcanology (ingv) of rome. dais was flown on a do 228 aircraft at an altitude of 1600 m a.s.l., and the collected images ground resolution is about 5 m. the data spectral subset used here corresponds to the six dais infrared thermal channels. simultaneously with the dais flight, a ground measurement campaign was carried out to measure the parameters necessary to atmospheric correction and validation of the retrieved maps. vertical atmospheric profiles of pressure, temperature and relative humidity at about 10 m vertical resolution were measured by means of a radiosonde launch performed near the solfatara (fig. 1). surface brightness temperature was measured with an infrared thermometer (everest inters. inc.) at different sites (fig. 1), selected on homogeneous and horizontal surfaces easy to locate on the images. in order to compensate for the spatial scale difference between the ground measures and the remote sensing data, 10 measurements were acquired on different points at each site: the average value was considered to be representative of the site brightness temperature. temperature inside fumaroles was also measured with a thermocouple. 3. geological setting solfatara is a volcanic crater located in the central area of the phlegraean fields caldera complex, west of the city of naples (italy) (figs. 2, 3). it represents the most active zone of phlegraean fields, and sits inside the sprawling urban area of pozzuoli. activity in the phlegraean area has been dominated by two eruptions that produced widespread ash-flow deposits: the campanian ignimbrite and the smaller neapolitan yellow tuff (lirer et al., 1987; scandone et al., 1991). the eruption producing the campanian ignimbrite occurred 37 000 years bp and resulted in the collapse of a large area including campi flegrei and part of the gulf of naples. the eruption of neapolitan yellow tuff occurred 12 000 years bp, had a very complex history that led to the formation of a caldera of smaller dimensions inside that of the campanian ignimbrite. in the last 12 000 years, the bottom of the neapolitan yellow tuff caldera has been the site of intense volcanic activity and ground deformation (di vito et al., 1999). volcanism was concentrated in three periods: 12 000 and 9500 years bp, 8600 and 8200 years bp, and 4800 and 3800 years bp (e.g., at cigliano, agnano-monte spina, astroni, averno, solfatara), followed by the last eruption in 1538 that led to the formation of the monte nuovo (di vito et al., 1987). since 1800, sea-level measurements made at ancient roman ruins have indicated a slow sinking of the area. this slow sinking of the ground continued until 1968. in the periods 1970-1972 and 1982-1984 two important bradyseismic 238 luca merucci, maria paola bogliolo, maria fabrizia buongiorno and sergio teggi events (ground uplift) occurred in the pozzuoli area (corrado et al., 1976; berrino et al., 1984), (maximum uplift of 1.7 and 1.8 m respectively) accompanied by shallow seismicity. more recently, two minor sudden ground uplifts and seismic swarms were recorded in 1989-1990 and in 1994. the solfatara volcano is one of the most recent (about 4000 years bp) of the phlegraean fields caldera. it measures 0.5 × 0.6 km, with steep walls on the north, east and south sides. to the west the crater wall is missing. its rectangular shape is mostly due to the presence of faults at nw-se and sw-ne. the volcanic cone is made of pyroclastic rocks, with the exception of mt. olibano that is a dome of trachytic lava. the flat-floored crater (piano sterile) is characterized by strong fumarolic activity which fig. 2. schematic geological map of the phlegraean fields (scandone, 1997). 239 spectral emissivity and temperature maps of the solfatara crater from dais hyperspectral images causes both single vent emissions, with temperature up to 160°c, and diffuse degassing. the gases emitted are mostly composed by h2o, co2, h2s and smaller quantities of h2, ch4, he, hcl, ar (valentino et al., 1999). one of the most recent hydrothermal models (chiodini et al., 1997) describes a system divided into three parts: 1) a heat source which is made up of a relatively shallow magmatic chamber; 2) one or more aquifers located above the chamber; the degassing magma supplies fluids and heat to them; 3) an intensely fractured zone, sited above the uppermost aquifer and occupied by a pure vapor phase, which is produced through the boiling of the underlying aquifers. the intense fumarolic activity has given origin to strong hydrothermal alteration of the original rocks (de gennaro et al., 1980) and to secondary deposits. in fact, the trachytic rocks of the floor and of the flanks of the volcano are bleached and corroded by the effluent vapours, with formation of gypsum, alum, kaolin and alunite (valentino et al., 1999). moreover, sublimation of the emitted gas causes deposition of sulphur, arsenic sulphide (realgar), ammonium chloride (sal ammoniac), mercury sulphide and antimony sulphide. 4. atmospheric correction the spectral radiance ld received by a remote sensor in the thermal infrared can be expressed as (4.1) where lu is the upward atmospheric radiance, ld is the downward atmospheric radiance, ts is the surface kinetic temperature, f is the surface emissivity, x is the atmospheric transmittance between the surface and the sensor, and b(ts) is the plank function. in order to calculate the radiance emitted by the surface (l(m, ts) =f(m) ⋅ b (m, ts) ), remote sensing data must be corrected for atmospheric effects. to estimate the atmospheric radiative contributions (lu, ld and x), we used the modtran 3.5 radiative transfer code (berk et al., 1989), giving as input the atmospheric profiles measured during the field campaign. the obtained values were convolved with the sensor response functions of the six thermal infrared channels (fig. 4). the atmospheric terms were estimated for 20 different optical paths corresponding to view angle increments of ≈ 3° with respect to the nadir. lower angle increments result in negligi$=( ) ( ) ( , ) ( ( )) ( ) ( ) ( ) l b t l l 1d s d u $ $ $ + + m f m m f m m x m m 6 @ fig. 4. response functions of the dais thermal infrared channels. fig. 3. solfatara crater at pozzuoli (naples, italy). picture from «pozzuoli dal cielo» by aeromap data. 240 luca merucci, maria paola bogliolo, maria fabrizia buongiorno and sergio teggi ble variations. the different sets of atmospheric terms were used to apply separate corrections on image stripes corresponding to the view angle increments and parallel to the image axis. 5. evaluation of surface temperature and emissivity the spectral radiance emitted by a body having a kinetic temperature ts can be expressed as a function of the spectral emissivity f of its surface and of the plank function b (5.1) if the radiance l is measured at n wavelengths m, the relation is expressed by a system of n equations with n + 1 unknowns, i.e. the n values of spectral emissivity and the temperature. this makes it difficult to estimate surface temperature and emissivity from remote sensing data because the equation system is not closed. in the literature, different «non-exact» solutions are given to this problem, called «temperature and emissivity separation» (gillespie et al., 1998; li et al., 1999). in our work we applied the well established method of the emissivity spectrum normalization (esn) (gillespie, 1985; realmuto, 1990). according to this method, emissivity is normalized to a value fmax corresponding to the maximum value expected in the scene. this method demonstrated good capability to maintain the spectrum shape, that is the most important feature in applications of spectral analysis and mapping. assuming f=fmax for each value of m, the inversion of eq. (5.1) gives n values of temperature; the highest of them is assumed to be the estimate of the kinetic temperature ts. with this value, and using the plank equation, the black body spectral radiance of the pixel is calculated. finally, the emissivity is obtained dividing the surface radiance measured by the sensor by the calculated black body radiance, at each wavelength. repeating this procedure for every pixel, temperature and spectral emissivity maps are obtained. in this study we assumed a value fmax = 0.97 on the basis of the spectral libraries data (salisbury et al., ( , ) ( ) ( , ) .l t b ts s$=m f m m 1991; salisbury and d’aria, 1992) available for the typical minerals of the solfatara crater. 6. selection of the areas of interest the atmospheric correction and the temperature and emissivity estimation were performed on an image window chosen in the solfatara crater. urban areas adjacent to the solfatara were excluded applying a suitable mask cropped on the original image. on these areas reliable estimates of surface temperature and spectral emissivity cannot be obtained by the esn method due to the urban covers extreme heterogeneity that makes it impossible to define a value of fmax valid for the whole image. moreover, the estimated surface temperature can be considered validated only inside the crater, where the in situ temperature was measured during the ground campaign. in the images showing the elaboration results (figs. 5, 6 and 8), non-processed areas were masked with an image of a thermal channel to preserve topographic reference. a further selection was applied to identify the areas of interest for geo-mineralogical interpretation: the vegetation cover was excluded with a mask based on the normalized difference vegetation index (ndvi). the data of two channels of visible (red) and near infrared are transformed by ndvi in a new image related to the green biomass using a spectral property of green vegetation: the sharp increase in reflectance between 0.65 and 0.80 nm (red edge). the ndvi image was obtained using the dais channels 10 (0.659 nm) and 18 (0.802 nm); it was then converted in a mask by selecting a threshold value between vegetated and non vegetated pixels. 7. results figure 5 shows the quantitative map of surface kinetic temperature. values retrieved in the ground measurement sites (fig. 1) were extracted from this map and compared with ground temperatures: the comparison revealed a good agreement, as shown in table ii. table ii. comparison between ground measured temperature and image retrieved temperature at the sites shown in fig. 1. site measurement ground temperature image retrieved temperature start/end (local time) (°c) (°c) fumarole; dark grey soil (1ff) 11.10/11.15 45.18 ± 4.31 44.2 central area (1ca) 11.20/11.25 44.91 ± 2.60 42.7 white tuffs (1wt) 11.30/11.31 37.18 ± 0.24 36.9 campground (2cg) 10.40/10.43 44.54 ± 1.87 42.7 dry grass (2dg) 10.45/10.50 54.71 ± 3.31 50.7 tuffs (2t) 10.55/11.00 42.32 ± 2.32 40.0 fig. 5. surface temperature map of the solfatara crater retrieved by dais data. 241 spectral emissivity and temperature maps of the solfatara crater from dais hyperspectral images the six images of spectral emissivity allowed us to discriminate different surface covers on the basis of their spectral characteristics. an example is reported in fig. 6, where an rgb composition of spectral emissivity calculated in channels 78, 76 and 74 is reported. by analyzing the emissivity images and the corresponding spectra, we could discriminate 6 main units, whose spectra differ strongly from each other. fumarolic fields are well separable since they have a very distinctive spectral pattern, and materials with different emissive prop242 luca merucci, maria paola bogliolo, maria fabrizia buongiorno and sergio teggi fig. 6. spectral emissivity image in dais channels 78-76-74 (rgb). fig. 7. emissivity spectra retrieved from the dais data. average values were calculated on regions of interest selected on the main surface units. 243 spectral emissivity and temperature maps of the solfatara crater from dais hyperspectral images erties are recognizable, probably corresponding to different degrees of hydrothermal alteration of the surface and to depositions from fumarolic gases. an example of the emissivity spectra of the separated units is reported in fig. 7. spectral information retrieved from the emissivity images was used to produce a map of the main geo-mineralogical units outcropping at the solfatara crater, applying the spectral angle mapper (sam) technique (kruse et al., 1993) (fig. 8). sam is a physically-based spectral classification that uses an n-dimensional angle to match pixels to reference spectra (end-members). the algorithm determines the spectral similarity between two spectra by calculating the angle between the spectra, treating them as vectors in a space with dimensions equal to the number of bands (rsi, 2002). nonprocessed areas and zones covered by vegetation were previously masked as already described. pure spectra required by this technique were obtained averaging spectra of small groups of pixels, selected on the image on the basis of information drawn from field survey and spectral analysis (fig. 7). the six units detected were only separated but not yet identified at this stage of the work: to do this, representative emissivity measurements are needed to be compared with image spectra. emissivity spectra of surface materials are strongly influenced by various factors (salisbury and d’aria, 1992) such as granulometry, temperature, compaction level, water content. in a remote sensing scene, pixel spectra are also influenced by local un-homogeneities of the deposits. for these reasons, a direct comparison between image spectra and reference spectra of minerals from public spectral libraries is not feasible; field measurements of spectral emissivity are required to perform reliable comparisons. fig. 8. map of the main geo-mineralogical units outcropping at the solfatara crater. 244 luca merucci, maria paola bogliolo, maria fabrizia buongiorno and sergio teggi 8. conclusions and future work dais thermal infrared data were used to retrieve surface temperature ad spectral emissivity maps of the solfatara crater. our results confirm dais capability to provide valuable volcanological information based on the thermal infrared properties of materials. the quantitative temperature map of the area we obtained allows us to locate thermal anomalies corresponding to active fumaroles and to characterize their thermal properties. the spectral emissivity image allows the mapping of the main geo-mineralogical units on the basis of their spectral properties. this thematic map points out that six thermal infrared channels are sufficient to detect and separate not only different surface covers but also similar materials whose differences are mainly related to their alteration degree. in order to complete the image interpretation and perform the automatic identification of the classes, field acquisition of emissivity spectra is planned. spectral emissivity will be measured with a portable ftir (fourier transform infrared) spectrometer (design & prototypes ltd) working in the 2-16 nm spectral range with a spectral resolution between 2 and 16 cm−1. sampled materials will be analyzed in the laboratory to retrieve chemical and mineralogical features corresponding to the field collected spectra. references berk, a., l.s. bernstein and d.c. robertson (1989): modtran: a moderate resolution model for lowtran-7, af-gl technical report gl-tr-89-0122 (air force, hanscom, ma, u.s.a.). berrino, g., g. corrado, g. luongo and b. toro (1984): ground deformation and gravity changes accompanying the 1982 pozzuoli uplift, bull. volcanol., 47 (2), 187-200. chiodini, g., r. cioni, g. magro, i. marini, c. panichi, b. raco and m. russo (1997): chemical and isotopic variations of bocca grande fumarole (solfatara volcano, phlegraean fields), acta vulcanol., 8 (2), 228-232. corrado, g., i. guerra, a. lo bascio, g. luongo and r. rampoldi (1976): inflation and microearthquake activity of phlegraean fields, italy, bull. volcanol., 40 (3), 169-188. de gennaro, m., e. franco and d. stanzione (1980): le alterazioni ad opera di fluidi termali alla solfatara di pozzuoli (napoli), mineral. geochim., 49, 5-22. di vito, m., l. lirer, g. mastrolorenzo, g. rolandi and r. scandone (1985): volcanological map of campi flegrei (min. prot. civ., università degli studi di napoli). di vito, m., l. lirer, g. mastrolorenzo and g. rolandi (1987): the 1538 monte nuovo eruption (campi flegrei, italy), bull. volcanol., 49, 608-615. di vito, m.a., r. isaia, g. orsi, j. southon, s. de vita, m. d’antonio, l. pappalardo and m. piochi (1999): volcanism and deformation in the past 12 ka at the campi flegrei caldera (italy), j. volcanol. geotherm. res., 91, 221-246. gillespie, a.r. (1985): lithologic mapping of silicate rocks using tims, in the tims data user’s workshop, jet propulsion laboratory publ. 86-38, 29-44. gillespie, a., s. rokugawa, t. matsunaga, j.s. cothern, s. hook and a. kahle (1998): a temperature and emissivity separation algorithm for advanced spaceborne thermal emission and reflection radiometer (aster) images, ieee trans. geosci. remote sensing, 36 (4),1113-1126. kruse, f. a., a.b. lefkoff, j.b. boardman, k.b. heidebrecht, a.t. shapiro, p.j. barloon and a.f.h. goetz (1993): the spectral image processing system (sips) – interactive visualization and analysis of imaging spectrometer data, remote sensing environ., 44, 145-163. li, z., f. becker, m. stoll and z. wan (1999): evaluation of six methods for extracting relative emissivity spectra from thermal infrared images, remote sensing environ., 69, 197-214. lirer, l., g. luongo and r. scandone (1987): on the volcanological evolution of campi flegrei, eos, trans. am. geophys. un., 68, 226-234. realmuto, v.j. (1990): separating the effects of temperature and emissivity: emissivity spectrum normalization, in proceedings of the 2nd tims workshop, jpl publ. 90-55. rsi (2002): envi 3.6 user’s guide (research systems, inc.). salisbury, j.w. and d.m. d’aria (1992): emissivity of terrestrial materials in the 8-14 mm atmospheric window, remote sensing environ., 65, 83-106. salisbury, j.w., l.s. walter, n. vergo and d.m. d’aria (1991): infrared (2.1-25 µm) spectra of minerals (johns hopkins university press), pp. 294. scandone, r. (1997): campi flegrei (on line: http://vulcan.fis.uniroma3.it/campi_flegrei/geol.gif/, modified after di vito et al., 1985). scandone, r., f. bellucci, l. lirer and g. rolandi (1991): the structure of the campanian plain and the activity of neapolitan volcanoes, j. volcanol. geotherm. res., 48 (1-2), 1-31. valentino, g.m., g. cortecci, e. franco and d. stanzione (1999): chemical and isotopic compositions of minerals and waters from the campi flegrei volcanic system, naples, italy, j. volcanol. geotherm. res., 91, 329-344. vol50,2,2007 165 annals of geophysics, vol. 50, n. 2, april 2007 key words fourier transform – diffraction tomography – inverse scattering 1. introduction the reconstruction of a compact-supported function starting from a finite number of samples of its spectrum is a problem potentially of interest in various fields. in particular, inverse scattering problems in a loss-less background medium can be often recast as the reconstruction of some compact supported object function from a limited portion of its spectrum (lesselier and duchene, 1996; persico and soldovieri, 2004a). due to the regularity of the spectrum of a compactsupported (square-integrable) function, in some cases there is a theoretical possibility to prolong this spectrum outside the limited region directly related to the data (slepian and pollack, 1961) (apart from ill-posedness problems, not explicitly considered here). this rationale has given rise to various uniqueness theorems for the solution of several inverse problems (colton and paivarinta, 1992; sheen and shepelsky, 2000). however, uniqueness theorems always require a continuous set of data to be available and often assume some regularity of the object function, which is not achieved in many practical cases (ramm, 1990; colton and paivarinta, 1992). moreover, and above all, this regularity cannot usually be supposed a priori because, let us remember, the object function is the unknown of the problem. finally, even if the object function were regular (i.e. continuous with its spatial derivatives of the first order) and a continuous (multi-experiment) set of data were available, still the uniqueness might not be guaranteed in some cases (devaney, 1978; fisher and langenberg, 1984). if the number of data is finite, instead, it is just impossible to retrieve a compact-supported function from a finite number of samples of its fourier transform uniquely. it is opinion of the author that this fact is worth outlining at least once, because in any practical case one has at disposal necessarily only a finite number of data. the above mentioned impossibility stems from the fact that a compact-supported object function is isomorphic to a countable set of ex«paralipomena» on uniqueness in inverse scattering from a finite number of data raffaele persico istituto per i beni archeologici e monumentali (ibam), cnr, lecce, italy abstract this paper shows new proof of non-uniqueness of the solution for the retrieving of a compact-supported function starting from a finite number of samples of its spectrum. as will be shown, this is relevant for linear inverse scattering problems, that in many cases can be recast as the reconstruction of a compact supported function from a finite set of samples of its spectrum. since this reconstruction is not unique, from a practical point of view, any linear inverse scattering algorithm that can be recast in terms of a fourier relationship between unknowns and data necessarily «trusts» on the absence of invisible objects in the particular situation at hand. mailing address: dr. raffaele persico, istituto per i beni archeologici e monumentali (ibam), cnr, via monteroni, campus universitario, 73100 lecce, italy; e-mail: r.persico@ibam.cnr.it 166 raffaele persico pansion coefficients along some functional basis. this means that the object function is characterised by an infinite number of independent parameters, that cannot correspond in a biunique fashion to a finite number of data. the only way to solve this kind of inverse problem uniquely is to choose a suitable finite dimensional space wherein to look for the solution. in this way, a whole class of somewhat «improbable» solutions is discarded a priori. in spite of the simplicity of these considerations, however, it is not immediate to show explicitly (i.e. by providing an example) the nonuniqueness of the solution of the problem at hand, so that one could forget the fact that, when considering a finite number of data, the solution to any inverse scattering problem is actually only one of the possible solutions. from a practical point of view this means, for example, that when one performs the reconstruction of the inner of a subsurface region by elaborating gpr (alberti et al., 2002, 2003; daniels, 2004) data, they can never be mathematically sure of their reconstruction results. they can say that those results are reasonable and probable but, from a mathematical point of view, they cannot exclude the possibility that the actual buried objects are something strange and complicated, even very different from the foreseen objects but compatible with the same gathered data. finally, they should not ascribe this possibility only to the presence of some (even possible) non-radiating density of current or to the lack of conditions for the validity of some complicated uniqueness theorem. rather, they should recognize the much more immediate fact that they necessarily have only a finite number of data available. this paper is aimed to show this fact explicitly. more precisely, the pursued goal is to show, by means of a counterexample, the fact that the reconstruction of a compact supported function from a finite number of samples of its fourier transform is never unique. actually, the result demonstrated here is already known (levinson, 1940; levin, 1980). however, to show a systematic way to obtain counterexamples is, to the best of the author’s knowledge, a new result. moreover, apart from the «novelty» of the approach to this «old» problem, the effort presented here seemed worth making because the new proof is quite simple, in the sense that it does not require a deep knowledge of the properties of the entire functions (levin, 1980) in order to be understood. in particular, it is plainly accessible for people that have a non-trivial knowledge of mathematics (as, e.g., geophysicists and engineers), but nevertheless are not mathematicians. the next section provides some examples of physical inverse problems that can be recast as the reconstruction of a compact supported function from a finite number of spectral data. in this way, the physical sense of the mathematical problem dealt with here will be more precisely circumstantiated. then, in section 3 a method to construct the sought counterexample is exposed. an example is provided in section 4. conclusions follow. 2. spectral algebraic relationships in inverse problems this section provides some algebraic relationships connecting the available scattered field data to the object function to be reconstructed in microwave inverse problems. these relationships are picked up from that branch of electromagnetic inverse problems commonly referred to as diffraction tomography (dt) (lesselier and duchene, 1996), and the aim of this exposition is to give a physical sense to the (otherwise merely mathematical) dealing of the two next sections. in this paper, dt will be somewhat prompted and essentially referred. of course, this brief report does not claim to be a review and is, necessarily, far from complete. after this premise, in dt the problem is to retrieve buried objects, or more in general buried inhomogeneities, from scattered field data. the general three dimensional geometry of the problem is given in fig. 1, wherein the data are constituted by samples of the scattered field gathered within the observation domain γ at several frequencies within a fixed band ω, and the buried objects are described by means of the so called contrast function, defined as (2.1) elsewhere0 ( ) ( ) p p p d1 bkg r !χ ε ε = −* 167 «paralipomena» on uniqueness in inverse scattering from a finite number of data where εr ( p) is the unknown relative dielectric permittivity in the investigation domain d and εbkg is the relative background permittivity, i.e. the relative dielectric permittivity of the soil (or of the layered medium) where the object is buried. from eq. (2.1), we can see that the contrast expresses the relative difference between the dielectric permittivity of the buried inhomogeneities and that of the host medium surrounding them. in general, the contrast is also a function of the frequency, but this fact is often neglected in the adopted model, unless a dispersion law is a priori assumed (catapano et al., 2006). under a linear born model (chew, 1995), and with reference to fig. 1, the relationship between the contrast and the scattered field data is given by (lesselier and duchene, 1996) (2.2) where g is green’s function of the problem, einc is the incident field (i.e. the field that would pd)(p) χ, ω( ,e p p), ωk ( ,g p p=)( , , , , e p p p p incs o s s o s d s o 2 ! ! ω ω ω γ l l l l# be present in the investigation domain d in absence of any buried object), ks= is the wavenumber in the background medium (being c0 the propagation speed of the light in free space), po is the observation point and ps is the source point, both supposed variable within the observation domain γ and, finally, pl is the integration point variable in the investigation domain d. if losses can be neglected, several spectral relationships can be obtained by taking into consideration particular cases of eq. (2.2). some of them are provided in the following. 2.1. the case of a one dimensional half-space the case of a one dimensional half-space can be of interest (e.g.) when a humidity profile or any stratification profile only varying versus the depth is looked for. if the source is constituted by a plane wave impinging from the upper half-space, the inverse problem can be recast as a one dimensional scalar problem wherein a contrast χ(z) is looked for. in this case, it can be shown that the only independent observation parameter is the frequency, whereas some furcbkg 0ω ε fig. 1. geometry of the problem. 168 raffaele persico ther independent source parameters can be provided by diversity in incidence angle. for more details the reader is referred to pierri et al. (1999), persico and soldovieri (2004a) and, soldovieri and persico (2004). here, we limit to the case of transverse electromagnetic (tem) incidence. in this case, the substitution of the specific values of green’s function and of the incident field in eq. (2.2) provides, after some passages (soldovieri and persico, 2004) (2.3) where , being eo the amplitude of the impinging plane wave and k0 = ω/c0 the wavenumber in free space. in eq. (2.3), and also in the following equations, the hat symbols stand for «fourier transform». from eq. (2.3), it is clear that gathering scattered field data at n frequencies ω1...ωn corresponds to retrieving the spectrum of the contrast function in a finite set of points 2ks1...2ksn. moreover the contrast is compact supported because the investigation domain necessarily extends only up to a certain finite depth. 2.2. the case of a two dimensional half-space the case of a two dimensional half-space is of scientific interest for retrieving elongated objects (e.g., buried pipes or buried walls). moreover, a two dimensional model is of applicative interest in many three dimensional situations, because it often offers acceptable results in a much shorter time with respect to a three dimensional algorithm. in a two dimensional half-space, the object function is variable along the depth z and along a horizontal abscissa x (but it is constant along the other horizontal direction y). if the incident field is provided by a filamentary current parallel to the y-axis, the inverse problem can be recast as a two dimensional scalar problem. in particular, limiting to the case of two antennas that move on the soil along the x-axis with a fixed offset ∆ between each other (which corresponds to the so called common offset configuration, customarily adopted in gpr prospecting; daniels, 2004), substitution of the proper values of green’s function and of the incident field in eq. (2.2) provides, ( ) [( ) / ( ) ]g j k k e k k k2 o s o o s s 2ω = − + ( ) ( ) ( )e g k2s s !ω ω χ ω ω= t after some passages, the relationship (2.4) where (2.5) and where (2.6) in eqs. ((2.4)-(2.6)), p is the conjugate variable of the source position xs (the observation position is xs + ∆), whereas η and ζ are the conjugate variables of the horizontal abscissa x and of the depth z in the investigation domain, respectively. the object function is the quantities in eq. (2.4) also depend on the frequency, that appears both in the spectral weighting function g(p) and (above all) in the arguments of the spectrum of the contrast (see eq. (2.6)). at the right-hand side of eq. (2.5), f is the frequency, µo is the magnetic permeability of the free space and io is the level of the current. unlike the previously shown one dimensional situation, in this case we do not have a direct proportionality between the data and the spectrum of the object function, but rather we have proportionality between the spectrum of the data and the spectrum of the object function. this is due to the fact that in two dimensional problems we can extract information from spatial diversity of the observation point, which could not been done in one dimensional cases. from a practical point of view, however, this does not change the problem too much. in fact, on condition that the error ( , ) ( , ) / .x z x z z1χ χ= ( )p pη = p k p 2 4s s 2 2−w2= = .( )pς c m z [ \ ]] ]] ( )w p = ( ) k p w p s k p o s s 0 2 2 2 2 − = − * ( ) exp g p w p w p jk fi j p p w p w p 4 2 4 2 2 4 2 o s s o o 2 2 2 2 2 3 2 $ $ µ π π ∆ = + + + b cc cc c cc bc l mm mm m mm lm ( ) ( ) ( ( ), ( ))e p g p p ps 1χ η ς=t tt 169 «paralipomena» on uniqueness in inverse scattering from a finite number of data due to the truncation of the scattered field along xs can be neglected (which is reasonable when the source moves far from the buried objects), from a finite number of samples of the scattered field one can work out a finite number of independent samples of the spectrum of the scattered field by means of an fft algorithm. at this point, eq. (2.4) relates these samples of the spectrum of the scattered field to the corresponding samples of the spectrum of an object function linked to the contrast. in conclusion, also in this case, what we can retrieve from a finite number of data is essentially a finite number of samples of the spectrum of a proper object function. for more details, the interested reader is referred to persico et al. (2005). 2.3. the case of a three dimensional half-space in the case of a fully three dimensional halfspace the mathematics is more involved, and it is somewhat foreseeable. essentially, the point is that in three dimensions we cannot avoid the intrinsic vector aspects of the problem. in particular, green’s function of eq. (2.2) has to be considered a dyadic one, and we should understand the product between green’s function and the incident field as a matrix-vector product. in formulas we have . (2.7) equation (2.7) means that each of the components of the incident field influences all the components of the scattered field. in the following, the attention is focussed on hertzian dipole antennas placed on a soil parallel to each other along the y axis and moved in common offset along several lines parallel to the x axis. this is a case that describes quite well many gpr prospecting operations performed with linearly polarized antennas (daniels, 2004). in this case, we essentially gather a signal proportional to the y component of the scattered field and therefore, one more time, we can refer to a scalar inverse problem recast as g g g g g g g g g g e e e e inc inc inc incxx yx zx xy yy zy xz yz zz y z x = = j l k k k j l k k k n p o o o n p o o o (2.8) where the kernel of the integral operator is defined as (2.9) a substitution of the proper values of the incident field and of the green’s function in eq. (2.8) leads to a spectral relationship of the following kind: . (2.10) where (2.11) in eqs. ((2.10)-(2.11)), u is the conjugate variable of the first coordinative of the source position xs, v is the conjugate variable of the second coordinative of the source position ys (the observation position in this case is given by (xs+∆, ys)), η and ξ are the conjugate variables of the horizontal co-ordinates x and y and ζ is the conjugate variable of the depth z in the investigation domain, respectively. the object function is defined as χ2(x, y, z)= χ (x, y, z)/z. it can be seen that eq. (2.10) is a quite «natural» extension of eq. (2.4). with respect to eq. (2.4), we have a further spectral variable with regard to the scattered field (because the observation domain is a surface rather than a line) and a further spectral variable with regard to the contrast (because the investigation domain is a buried volume rather than a buried area). for more details, and in particular for the explicit expression of the spectral weighting function g(u, v), the interested reader is referred to hansen and johansen (2000). what is relevant here is to outline that, by means of a two dimensional fft, also in this case a finite number of sam( , ) ( , ) u v u u v v η ξ = = k u v4 s 2 2 2− −( , )u v =ζ . z [ \ ]] ]] ( , ) ( , ) ( ( , ), ( , ), ( , ))e u v g u v u v u v u vsy 2χ η ξ ζ=t t tt t ( , , , ) .kr p p p g e g e g einc inc inco s yx x yy y yz zω = + +l pd)(pχ)k , ω= ( , ,kr p p p)( , , , , e p p p p sy o s s o s d s o 2 ! ! ω ω ω γ l l l# 170 raffaele persico ples of the scattered field essentially provides a finite number of samples of the spectrum of the object function. therefore, one more time the problem can be recast as the reconstruction of the object function χ2, compact supported in the investigation domain d, from a finite number of samples of its spectrum. 2.4. prompts on further relevant cases several further possible cases of interest can be recast as the retrieving of some object function from a finite number of samples of its spectrum. the essential reason for this fact is that, in eq. (2.2), both green’s function and the incident field can be expressed by means of their plane wave spectrum (psw) (clemmow, 1996). due to this fact, the problem of retrieving inhomogeneities buried in any lossless layered medium (i.e. not only half-spaces) can be dealt with by making use of suitable dt spectral algebraic relationships. in particular, the case of a one dimensional slab (i.e. a one dimensional medium with three layers) has been dealt in persico and soldovieri (2004a,b), whereas the case of a two dimensional slab has been dealt with in crocco and soldovieri (2003). furthermore, also the radiation characteristics of the transmitting and receiving antennas can be included in a dt formulation. essentially, the radiation characteristics of the antennas affect the spectral weighting function g. a two dimensional dealing of this problem can be found in soldovieri et al. (2005a), whereas a three dimensional dealing can be found in meincke (2001). still, different configurations (other than common offset) can also be dealt with by means of a dt formalism. essentially, in these cases the arguments of the spectrum of the object function will be somewhat varied. for more details, the interested reader is firstly referred to persico et al. (2005), and then also to lesselier and duchene (1996) and soldovieri et al. (2005a,b). finally, even some aspects relative to the consequences of the truncation of the observation domain can be understood in terms of a dt formulation. details can be found in leone and soldovieri (2003) and persico et al. (2006). 3. a method to construct a counterexample in this section we come back to the mathematical problem presented in the introduction. in particular, this section is devoted to finding a (non null) function with a compact support such as its spectrum is equal to zero in a finite set of points k1, k2, ..., kn. in particular, the problem is afforded in the one dimensional case for the sake of simplicity in the exposition, but extensions to more complicated cases (two and three dimensional) are straightforward. of course, such a function is an element of the null space of the linear operator that transforms a compact supported function into the column vector of the values of its spectrum in the points k1, k2, ..., kn. therefore, to find such a function means to prove that the (semi-discrete) operator at hand can never be univocally inverted (bertero and boccacci, 1998). the searched function can be found as the limit of a succession of functions, i.e. it can be built. in order to initialize the algorithm, let χ1(z) be an arbitrary square integrable function supported in prefixed interval [−a, a], i.e. be χ1∈ l2(−a, a). let f1(k) be the fourier transform of χ1(z). as it is well known, f1(k) is a square integrable function (f1∈ l2(r)) and has relevant regularity properties (paley and wiener, 1934). for the purposes of this paper, at any rate, it is sufficient to outline the only fact that f1(k) is continuous (which is well known and, even if it were not known, it could be plainly proved directly). let us label a1, a2, ..., an the vector of the values assumed by f1(k) in the points k1, k2, ..., kn respectively. at this point, let us define a «corrected version of f1» as follows: (3.1) λ(k) being the well known roof function defined as (3.2) 0 elsewhere ( ) 1 ( , ) k k k 1 1! λ = − − * ( ) ( ) ... ... f k f k a m k k a m k k a m k k n n 2 1 1 1 2 2λ λ λ = − − − − − − − b b b l l l 171 «paralipomena» on uniqueness in inverse scattering from a finite number of data f2 is continuous because it is a linear combination of continuous functions. moreover, on condition that m is chosen small enough, we can impose that f2(k) = 0 in each of the points k1, k2, ..., kn. moreover, for any chosen positive k, on condition that m is chosen small enough we can also guarantee that . (3.3) in particular, we require here that k>1. in the end, f2 is a slight modification of f1 (as close as we like to f1) that shows the required roots. however, in general f2 is not the spectrum of a compact supported function, and therefore the searched result has not been reached yet. therefore, let χ2(z) be the inverse fourier transform of f2(k), and χ3(z) be the truncated version of χ2(z) in the interval [−a, a], i.e. χ3 be defined as (3.4) let us now outline two inequalities. first, since the fourier transform is an isometry (kantorovic and akilov, 1982), we have . (3.5) second, as an easy consequence of the definition of χ3(z) we have (3.6) in fact, reminding that both χ1 and χ3 have compact support included in [−a, a], it results (3.7) therefore, due to the first triangular inequality (kantorovic and akilov, 1982) = =$ + . + d d d d d z z z z z [ , ] [ , ] a a z a a a a z a a a a 1 2 2 1 2 2 1 3 2 1 3 1 3 2 χ χ χ χ χ χ χ χ χ χ χ χ − = − − − − g g − − − − − 2 2 2 2 2 # # # # # . k 1 1 3 1 2 ##χ χ χ χ− − f f k 1 1 2 1 2 #χ χ− = − ( ) ( ) [ , ] elsewhere z z z a a 0 3 2 ! χ χ = − .) f f k 1 1 2 #− . (3.8) at this point, let f3(k) be the fourier transform of χ3(k). f3(k) is a continuous function because it is the spectrum of a compact supported square integrable function. moreover, since the fourier transform is isometric, we have . (3.9) let f4(k) be a «corrected version of f3(k)» constructed in the same fashion in which f2(k) had been built starting from f1(k), so that f4(k) is equal to zero in the points k1, k2, ..., kn. moreover, this time we require a new value for m, such as it is guaranteed . (3.10) of course, we can do this because the choice of m is arbirtary. going on, χ4(k) is the inverse fourier transform of f4(k) and the cycle begins again. the procedure is iterated by subsequent truncations in the space domain and subsequent «corrections» in the spatial frequency domain. for each even value of the index w, a correction in the «k-domain» occurs, and it is imposed that . (3.11) consequently, for each even value of w, it results . (3.12) for each odd value of w, a truncation in the «zdomain» occurs. the truncation guarantees that, for each odd value of w, it results (3.13) and, consequently, it also results (3.14) in this way, two sequences of functions χ1, χ2, ..., χn, ... and f1, f2, ..., fn, ... are identified. .f f k 2 . ( )w w w1 0 5 1#− − − k 2 . ( )w w w1 0 5 1#χ χ− − − k 1 .w w w1 0 5#χχ − − f f k 1 .w w w1 0 5#− − f f k 1 4 23 #− f f k 2 2 3 2 3 #χ χ− = − k 2 32 13 1 2 ##χ χ χ χ χ χ− − −+ 172 raffaele persico from eqs. ((3.11)-(3.14)), both these successions verify the cauchy condition (let us remind that we have chosen k >1), i.e. the condition that the quantity = vanishes versus the index w. therefore, due to the completeness of the space l2, two limit functions χl and fl exist for the two successions χ1, χ2, ..., χn, ... and f1, f2, ..., fn, ... respectively (kantorovic and akilov, 1982). moreover, fl is the fourier transform of χl because of the continuity of the fourier operator. the sought function is χl. in fact, χl is a function supported within the interval [−a, a] because it is definitively closer and closer to any one of the truncated (compact supported) functions χ2w−1, on condition that w is high enough. moreover, the spectrum of χl assumes the value zero in the points k1, k2, ..., kn. in order to show this fact, let us outline that fl is a regular (in particular continuous) function, because it is the fourier transform of a compact supported function. therefore, we have that f1, f2, ..., fn, ... is a succession of continuous functions that converges towards a continuous limit. in these conditions, it is easy to recognize that the convergence in l2 also guarantees the punctual convergence, even if it does not in general (kantorovic and akilov, 1982). in order to complete the proof, however, we still have to guarantee that χl is not a null function, otherwise only a trivial result would have been found. in order to show this fact, let us note that, due to the first triangular inequality (3.15) in this last passage we have exploited again the hypothesis k > 1, which guarantees the convergence of the geometrical series of ratio 1/k. inequality (3.15) essentially means that the limit function can be found as close as we like to the initial function χ1 (in other words, the null space of the semi-discrete operator at hand is dense in l2(−a, a)). due to the arbitrary choice of χ1, therefore, it is easy to guarantee that χl is = .3=f+f= ++ +f f+ ...+ + + =++ + +# +# = k k k k k k k k k k k k 1 2 1 2 1 2 3 3 3 1 1 1 3 l w w n n n 1 1 2 2 3 1 2 2 2 f χ χ χ χ χ χ χ χ− − − − + − − − − b l f fw w 1− −w w 1χ χ− − a non null function. for example, one can require that (3.16) a being some positive constant. consequently, by virtue of the second triangular inequality (kantorovic and akilov, 1982), we have (3.17) and this completes the proof. 4. an example this section illustrates the method for the construction of a compact supported function whose spectrum shows some (imposed) roots, exposed in the previous section, in a particular case. therefore, let us suppose that we have to identify a function supported in the interval [−1, 1] on the z-axis, and let us suppose that we require that the spectrum of this function has to assume the value zero in the points −0.7π (≅−2.199) and 0.7π (≅+2.199) on the k-axis. the initial point is chosen equal to the function (4.1) therefore, the «first function» in the spectral domain is given by (4.2) which shows the first roots at k = ± π (here, sinc(k) is meant as sin(k)/k). according to the procedure explained in the previous section, let us choose k=1.2, and let us specify a corrected version of f1 which shows the required roots. based on eq. 12 and on the parity of the sinc function, f2 can be chosen as (4.3)+ ( ) ( ) ( . ) . . sincf k f k m k m k 2 0 7 0 7 0 7 2 1 1 1 π π π λ λ = − − + + b b l l ; e ( ) 2 ( )sincf k k2 = ( ) 1 [ 1, 1] 0 elsewhere z z z 21 ! χ π= = − b l ) a 0 l l l l 1 1 1 1 1 1 2 $ $ χ χ χ χ χ χ χ χ χ χ + − − − = − − = a k 1 3 1 $χ + − 173 «paralipomena» on uniqueness in inverse scattering from a finite number of data according to the previous section, we have to choose m1 such as (4.4) on condition that the supports of the correcting roof functions do not superpose to each other, it can be seen that the norm at the left-hand side of inequality (4.4) is equal to , and therefore relationship (4.4), joined to the condition that the supports of the roof functions do not superpose to each other, provides the condition (4.5) in the case at hand, the second member of (4.5) is equal to 0.7 π, so we can choose m1 = 0.7π. at this point, the inverse fourier transform of f2 provides ( . ) , . .min sinc m k4 0 7 3 0 71 2 2# π πc m 2 / (0.7 )sincm 31 π ( . ) . . . sincf f m k m k k 2 0 7 0 7 0 7 1 2 1 1 1 # π π π λ λ − = − + + + b b l l ; e (4.6) and the truncation in spatial domain provides (4.7) the fourier transform of χ3 provides (4.8) where the symbol ⊗ stands for convolution product. at this point, f4 is given by (4.9) ( ) ( ) ( . ) . . . f k f k f m k m k 0 7 0 7 0 7 4 3 3 2 2 π π π λ λ = − − + + + b b l l ; e ( ) 2 ( ) 4 ( ) (0.7 ) . . sinc sinc sincf k k k m k m k0 7 0 7 3 1 1 7 $ $ π π π λ λ = − − + + b bl l; e $sinc4 (0.7 )sincm ππ−π= z z m z 2 2 22 1 2 1 $ χ (0.7 )zπcos b b bl l l .( . )z0 7πcossinc( . )sincm4 0 7π−π= z m z 2 22 1 2 1χ b bl l fig. 2. graphs of χ2, χ4, χ6 and χ8, dimensionless units. 174 raffaele persico and m2 has to guarantee that (4.10) inequality (4.10) can be solved essentially in the same way of inequality (4.4). therefore, the condition on m2 is still given by relationship (4.5) on condition that we change k in k2 and 4sinc2(0.7 π) in ⎟f3 (0.7 π)⎥. for this numerical example, the «compatible» value m2 = 0.4 π has been chosen. the procedure, which at this point is straightforward, has been iterated up to χ8 and f8 (the subsequent compatible values m3 = 0.2 π and m4 = 0.1 π have been chosen). figure 2 represents the first four χn functions with even index (those with odd index are truncated, therefore they automatically verify the required condition about the support of the searched function). it can be seen that χ4 and χ6 are indistinguishable from a function supported in the interval (−1, 1). figure 3 represents the first four fn functions with odd index (those with even index are ( . ) . . f f f m k m k k 0 7 0 7 0 7 1 4 3 3 2 2 21 π π π λ λ − = − + + + . b b l l ; e «corrected», therefore they automatically show the two required roots ±0.7 π). it can be seen that the first roots of f5 and f7 occur very near to the required points ±0.7 π ≅ ± 2.199. 5. conclusions this paper has shown a new proof of the non-uniqueness of the solution of the problem of the reconstruction of a compact supported function from a finite number of samples of its spectrum. this problem is, in particular, related to diffraction tomography, as shown in section 2. the problem has been formally afforded in 1d cases, but the extension to 2d or 3d cases is straightforward. in fact, the isometric properties of the fourier transform hold also in n-dimensional cases (for any integer n), as well as the triangular inequalities and all the mathematical statements exploited in section 3. the only difference is that in an n-dimensional case we should perform the corrections in the spectral domain by making use of some n-dimensional version of the roof function (which can be provided by the product between n one-dimenfig. 3. graphs of f1, f3, f5 and f7, dimensionless units. 175 «paralipomena» on uniqueness in inverse scattering from a finite number of data sional roof functions versus each of the spectral variables). a further development thought of is the extension of the proof presented here to the case of a finite number of samples of the laplace transform rather than the fourier transform. this extension is not straightforward for several reasons (e.g., some isometric properties of the fourier transform do not hold any longer for the laplace transform). however, such an effort seems worth trying, because it would also allow lossy cases to be considered in a rigorous way (lesselier and duchene, 1996). references alberti, g., l. ciofaniello, m. della noce, s. esposito, g. galiero, r. persico, m. sacchettino and s. vetrella (2002): a stepped frequency gpr system for underground prospecting, ann. geophysics, 45 (2), 375391. alberti, g., l. ciofaniello, g. galiero, r. persico, m. sacchettino, g.m. signore and s. vetrella (2003): experimental results from a stepped frequency gpr, ann. geophysics, 46 (4), 707-717. bertero, m. and p. boccacci (1998): introduction to inverse problems in imaging (taylor & francis), pp. 352. catapano, i., l. crocco, r. persico, m. pieraccini and f. soldovieri (2006): linear and non-linear microwave tomography approaches for subsurface prospecting: validation on real data, ieee trans. antennas wireless propag. lett., 5, 49-53. chew, w.c. 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(received february 2, 2006; accepted september 22, 2006) vol50,4,2007 547 annals of geophysics, vol. 50, n. 4, august 2007 key words radon – hydrogen – seismicity – geodeformation wave – earthquake 1. introduction for half a century work on soil radon recording was carried out in different geological structures using different equipment and showed the benefits of the seismic-emanation method (dubinchuk, 1991; king, 1991; rudakov, 2003). therefore, further development and perfection of this method, both in methodical (steinitz et al., 1999, 2003; utkin, 2000) and technical (kim et al., 1997) aspects, is an essential goal for earthquake forecasting purposes. a network for subsoil radon monitoring was created at the petropavlovsk-kamchatsky geodynamic proving ground in 1997-2000 with the purpose to search for the precursors of the avacha gulf strong earthquakes (firstov, 1999; firstov and rudakov, 2003). the network was extended and modernized in the following years. this report analyses the features of the soil radon mass transfer response to changes in stressed-deformed state of the lithosphere on the northern flank of the kuril-kamchatka subduction zone over the period july-august, 2004 the nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the kuril-kamchatka subduction zone pavel p. firstov (1), valentina s. yakovleva (2), vladimir a. shirokov (1), oleg p. rulenko (1), yury a. filippov (1) and olga p. malysheva (1) (1) institute of volcanology and seismology feb ras, petropavlovsk-kamchatsky, russia (2) tomsk polytechnic university, tomsk, russia abstract the comparison of kinematics and dynamic parameters of radon and molecular hydrogen concentration in subsoil air on the stations network at the petropavlovsk-kamchatsky geodynamic proving ground with seismicity of the northern flank of the kuril-kamchatka subduction zone was fulfilled in the period from july till august 2004. on the basis of correlation analysis of the regional seismicity and variations of radon flux density calculated using the data of gas-discharge counters of sts-6 type and ssntds it was shown that the radon mass transfer abnormal variations are conditioned by both regional seismicity in total and the subduction zone of proving ground. the azimuths of «geodeformation waves» coming to the registration points are calculated during clearly expressed anomaly beginnings, which coincide with directions to earthquake epicenters taking place at the same time. the geochemical anomalies recorded are presumptively deformative by nature and can be conditioned by processes of «quasi-viscous» flow of the lithosphere during rearrangement of tectonic stress fields of the subduction zone. the short-term (predicted time τ< 14 days) precursor of the earthquakes swarm was revealed in hydrogen dynamics on august, 4-5 (four earthquakes had m≥5.3 and epicentral distance about 130 km from the paratunka base station). mailing address: dr. pavel p. firstov, institute of volcanology and seismology feb ras, b. piip blvrd 9, petropavlovsk-kamchatsky 683006, russia; e-mail: firstov@kscnet.ru 548 pavel p. firstov, valentina s. yakovleva, vladimir a. shirokov, oleg p. rulenko, yury a. filippov and olga p. malysheva for the purpose of revealing short-term strong earthquake precursors. 2. space-time seismic characteristics of research areas and observation techniques the network consisting of 4 monitoring stations functioned in the examination period, its coordinates and conditions of detector placement are given in table i. the scheme of location of the monitoring stations is shown in fig. 1. two stations were located in the northern part of paratunka hydrothermal system, which structurally refers to the graben of the southern part of nachikinskaya fold-blocky zone. a structural feature of the examined area is blocky tectonics (serezhnikov and zimin, 1976). the block mosaic is caused by a combination of motions along long-standing ruptures. the majority of faults are reflected on a relief. some faults located at the valley of paratunka river under loose sediments were traced by geophysical methods. the faults divide the whole structure into four main blocks. the base station «paratunka» (prt) is located on the terrace of the korkina stream, which traces a sublatitude fault of the paratunka graben. a well developed network of hydrothermal groundwater reservoirs refers to paratunka graben. shows of thermal waters with the radon concentration up to 1500 bq m−3 were found in 700 m away from the prt station downstream. the annual radon-hydrogen measurements were conducted across the valley of korkina stream. figure 2 shows the schematic profile of table i. coordinates of soil radon monitoring stations of petropavlovsk-kamchatsky geodynamic proving ground (july-august, 2004). name of work station abbreviation coordinates short description of locality and measurement conditions lat (n) long (e) paratunka prt 52.90 158.26 hydrothermal system, zone of dynamic influencing of (base station) the fault, which is traced by the korkina stream. radon measurements were made by radiometer «revar» with 2 gas-filled counter tubes (sts-6 type). two depths of radon measurements (point 1 zone of aeration; point 2 zone of the full water saturation). institute of ikr terrace of the paratunka river (2 km from a bank). cosmophysical two depths of radon measurements (point 1 0.8 m and researches point 2 1.35 m). and radio wave propagation left avacha lvch 53.50 158.19 left avacha river fluvial terrace (150 m from bank). a fissile fault, two variable-depth points in a zone of aeration (point 1 0.9 and point 2 1.9 m). institute of ins 53.07 158.61 geology: sediments of old pyroclastic streams. volcanology radon measurements were made in: and seismology point 1 inside the basement of the institute building. point 2 inside a bunker of drilled hole at point 1, zone of aeration. point 3 inside a casing pipe of a drilled hole on depth of 5 m. point 4 measurement of radon flux from the earth surface in a bunker. 549 the nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the kuril-kamchatka subduction zone the stream valley, radon-hydrogenous monitoring results and placement of measure points on the prt station. the concentration of molecular hydrogen in soil air was measured by a hydrogen geophysical signaling device model vsg-2, in relative units (mv), and radon concentration was measured by a radiometer model «rga-1». high values of radon concentration and hydrogen concentration were recorded at two sites of the profile: on the left-hand slope and at the bottom of the valley. these sites should be attributed to disjunctive ruptures characterized by high permeability. the gas-filled counter tubes for continuous radon measurement were placed on those sites of the profile where the background level was observed, which proved to be evidence of the quasi-homogeneity of the medium. the second station (ikr) is located in the territory of the institute of cosmophysical researches and radio wave propagation. another two stations are located in the region of petropavlovsky horst. the station «left avacha» (lvch) is located at the bank of the left avacha river within the fault zone, traced by a river-bed. the fourth station (ins) is located in the territory of institute of volcanology and seismology. all stations are equipped with revar radiometers. passive measurements will be used on the basis of a chamber with an electrostatic field, in which the sensor is the gas-discharge geiger counter registering β-radiation decomposition product 222rn. it allows autonomous long-time observations to be performed with duration of a time period of accumulation from 10 s to 99 h with a step of 1 s. the stored data are saved in a plug-in module of a storage and are then transferred in the pc through a serial port rs-232. at the prt station the rn concentration was measured in alluvial-sandy sediments at a depth of one meter from the surface at two points: aeration zone (point 1, fig. 2) and total water saturation zone (point 2, fig. 2). two gas-filled counter tubes of the revar radiometer were put in alluvial-clay sediments on 2 depths (table i). in addition to standard radon measurements by instantaneous method, radon measurements by an integrated method with ssntds at point 3 (fig. 2) were carried out at different depths for the period from july 12 till august 26, 2004. two holes spaced 0.5 m apart were drilled by a customized soil auger; one hole was 50 cm deep, and the other was 100 cm deep. a plastic pipe casing with diameter 5.5 cm was inserted into the hole. radon radiometers with tracketch detectors of lr-115 type iii-b (nikolaev and iliç, 1999) were placed in the holes. then, the holes were covered to be air-tight and left for 96 h. the soil gas radon concentration was determined for the aist-tral complex by the fig. 1. the scheme of location of radon monitoring stations over the period of july-august 2004 and earthquake’s epicenters for the period of july 12-september 1, 2004, with energy class (k): 1 – 11; 2 – 12; 3 – 13; 4 – 14; and a focal depth: 5 – 0-19; 6 – 20-39; 7 – 4059; 8 – 60-100 km; 9 – monitoring stations; 10 – azimuth to the epicenter. 550 pavel p. firstov, valentina s. yakovleva, vladimir a. shirokov, oleg p. rulenko, yury a. filippov and olga p. malysheva method described elsewhere (yakovleva, 2005). the etching and track counting methodology are described elsewhere (nikolaev et al., 1993). the space-time characteristics of seismicity were studied using the operative catalogue of kamchatka experimental-methodical seismological department of geophysical service, ras. 3. results of observation for radon concentration and hydrogen concentration in subsoil air the results of radon monitoring for the period from july 12 to september 1, 2004 are given in fig. 3a-c. solid lines mark the moments of strong earthquakes which occurred on south kamchatka (4th and 30th august) and the time of stepwise radon anomaly observed on august 16. the temporal dynamics of air temperature, pressure and precipitations are given in fig. 3d. figure 3a shows the dynamics of radon concentration at the prt base station at two points. triangles mark the phase correlation between the signals of 4th and 16th august. firstov and rudakov (2003) found that synchronous increasing of radon concentration at both zones of full water saturation and the aeration zone (like that on august 4; fig. 3a) testifies the increase in convective soil gas velocity resulting from changes in stressed-deformed state of the geological medium. it should be noted that the positive anomaly of august 4-6 was also observed at both points of the station ikr (fig. 3b) located within paratunsky graben. at two other stations (ins and lvch) the anomalies were not observed with an abrupt front (fig. 3b). the weaker anomalies, which were qualitatively similar to previous ones in their forms, were recorded at all stations on august 12. the anomaly of 16th august represents a synchronous decrease of radon concentration at both points of station prt and nearly stepwise decrease at other stations (fig. 3b). anomalies of 4th and 16th august were also observed in the data obtained by ssntds (fig. 3c). figure 4a,b shows detailed dynamics of radon concentration at the stations ikr, ins and lvch, forming a triangle with an aperture of about 70 km for the periods of august 3-6 (a) and 15-17 (b). it is conditionally possible to separate the moments of the beginning anomalies and to determine the time of their delay comparatively to the station ins. assuming that the front of the incoming wave is plane, the azimuths fig. 2. schematic structural map of the prt station’s area and results of radon-hydrogen measurements across the valley of korkina stream. 551 the nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the kuril-kamchatka subduction zone f ig . 3a -d . r ad on c on ce nt ra ti on m ea su re d by r e v a r r ad io m et er s at t he s ta ti on s a) p r t a nd b ) ik r , in s , l v c h; c ) by s s n t d s at d if fe re nt d ep th s (i ) an d ca lc ul at ed r ad on f lu x de ns it y (i i) ; d) m et eo ro lo gi ca l pa ra m et er s. a b c d 552 pavel p. firstov, valentina s. yakovleva, vladimir a. shirokov, oleg p. rulenko, yury a. filippov and olga p. malysheva f ig . 4a ,b . r ad on d yn am ic s at t he s ta ti on s ik r , in s a nd l v c h fo r th e pe ri od s of a ug us t a) 3 -6 a nd b ) 15 -1 7. t ri an gl es m ar k th e ti m e of a no m al y co m in g to e ac h st at io n. f ig . 5a ,b . a ) a gg re ga te d si gn al s of r ad on c on ce nt ra ti on c on st ru ct ed o n th e ba si s of 4 m ea su re m en t po in ts o f s ta ti on s p r t a nd i k r ; b) h yd ro ge n co nc en tr at io n dy na m ic s. a b b a 553 the nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the kuril-kamchatka subduction zone of its arrival were calculated accounting for the differences in the coming delay for the stations ikr and lvch and it was 105°±15° in the first case and 72°±13° in the second (fig. 1). for those stations where the radon concentration was observed at two different depths, the values of radon flux density from the earth’s surface were calculated by the formula (yakovleva, 2005): (3.1) where a1 and a2 are the radon concentrations at depths h1 and h2 = 2h1; de is the radon diffusion coefficient; η is the soil porosity. for poorly humidified alluvial sediments with a porosity of about 23% the diffusion coefficient 0.0055 cm2 s−1 was considered (novikov, 1989). the values of radon flux density were calculated with the use of eq. (3.1) for the ikr and lvch stations (fig. 5a) and the prt station (fig. 3c). to reveal the common tendencies in behavior of radon concentration within paratunka graben the aggregated signals were constructed on the basis of a four-dimensional data sequence (2 measure points of stations prt and ikr) with the use of ljubushin’s methods (ljubushin, 1998). aggregated signals clearly reveal the anomaly of 4th august and weaker disturbances of 11th and 16th august that are marked by triangles (fig. 5a). figure 5b shows the temporal dynamics of molecular hydrogen concentration in subsoil air (dashed line). a clearly defined positive correlation between hydrogen concentration and air temperature is observed (the correlation coefficient, r = 0.4). the hydrogen concentration compensated for temperature by ljubushin’s methods and averaged by moving mean on 24 points is marked by a full line. two months’ average value was found to be 64±40 mv. in the period from july 27th till august 4th the measured values exceed 100 mv (the average value plus standard deviation), therefore, as a first approximation, this period can be counted as abnormal. due to the deep origin the response to changes in the stressed-deformed state of the ( ) lnq z d a a a h a a 2 1 1 1 0z e 1 2 1 1 1 2 $ $ $η= − − = f p lithosphere would be observed in hydrogen dynamics much earlier than in soil radon dynamics. therefore, it is possible to consider the marked excess of hydrogen concentration as a short-term precursor of the earthquake swarm of august 4-5 with mmax = 5.6 and epicentral distance of 130 km from the registration point. 4. seismicity of the northern flank of the kuril-kamchatka subduction zone the earthquakes which occurred in the kuril-kamchatka region reflect the time reorganization of seismotectonic stress fields connected with interaction processes between pacific and eurasian tectonic plates. from 10th july till 31st august 2004 an investigation of space-time seismicity characteristics of the northern flank of the kuril-kamchatka subduction zone stretching from commodore islands in the north up to paramushir island in the south was carried out. figure 1 shows the map of earthquake epicenters constructed on the basis of operative data of kamchatkan experimental and methodical seismological department, geophysical service, ras. the map indicates the earthquakes with energy class k≥10.5 (k = loge, j) for cross waves and hypocenter depth h≤100 km occurred on the northern flank of the kurilkamchatka subduction zone. the time distribution of these earthquakes is shown in fig. 6b. the value of seismotectonic movement velocity depends mainly on the number of seismic events (n) per unit of time and much less on other parameters of a seismic mode. at first, the value of seismotectonic movement velocity can be estimated by a density of events with k≥ ≥10.5 per unit of time (n=n/t) within an examined seismoactive volume (riznichenko, 1977). the velocity determines a slope of the earthquakes cumulative number plot. the size of seismotectonic movement velocity determines four phases of seismic activity on (fig. 6a): i – july, 12-august, 3; ii – the earthquake swarm in avachinsky gulf on august, 4-6; iii – august 7-15; iv – august, 16-30. in the first phase, the value n was close to the background value. in phases ii and iv the 554 pavel p. firstov, valentina s. yakovleva, vladimir a. shirokov, oleg p. rulenko, yury a. filippov and olga p. malysheva value n considerably exceeds the background value. however, if in phase ii the seismic process was located in a small area of avachinsky gulf, the earthquakes of phase iv occurred within all examined territory covering the komandor islands in the north up to paramushir island in the south (fig. 1). the last circumstance denotes the regional effect of increase in seismotectonic movement velocity, which may possibly be caused by an increase in the speed of the moving pacific plate. 4.1. results analysis let us compare the data of radon-hydrogen monitoring with the denoted phases of seismic activity. phase i is apparently connected with relaxation of tectonic stress field after last year’s strongest earthquake in kamchatka on 10th june, 2004. this earthquake had k=14.0 (m = 6.4), h = 200 km and occurred at the kronotsky cape area. due to relaxation processes which developed 16 days after this earthquake, a swarm of shallow-focus earthquakes (kmax= =11.6) took place in the same area on june 2627th and the following more powerful swarm of earthquakes (kmax=13.0) took place on august 4-6 in the adjacent avachinskaya «key». apparently increasing seismotectonic deformations preceded this swarm in the avachinskaja «key» area which triggered an appropriate decrease of radon flux density at the paratunka graben area (stations prt and ikr) and an 8 day anomaly in a hydrogen concentration at the prt station (fig. 5a,b). in recent years many researchers have focused on the rheological properties of a geological material and have devised a model of earthquake preparation process. morgunov (2001) in his work showed that in a focal zone a stage of quasi-viscous rock flow results from creep preceding a brittle rupture or an earthquake. the minimal time of this stage is estimated at not less than 10 h before a seismic event. let us consider the radon anomaly of august 4-5 in more detail. we will presumably consider the anomaly as a reaction of a soil radon field on the rock deformation resulting from quasi-viscous flow of geological material (in other words «a geodeformation wave»). this assumption is supported by the fact that the azimuth of the anomaly incoming to the radon monitoring stations is close to the direction towards the epicenter area of the earthquakes swarm of august 4 (fig. 1). this anomaly had started to shape (in aggregated signal) 11 h before the beginning of the earthquake swarm (fig. 5a) which allows it to be considered a short-term precursor. a b fig. 6a,b. events of earthquakes a) with energy class k≥10.5 per a day; b) maximal energy class. 555 the nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the kuril-kamchatka subduction zone as shown in ljubushin’s experimental work (ljubushin, 1993) the «geodeformation waves» influencing radon field and anomalies have various shapes depending on a location of measurement points relative to borders of different blocks. in phase iii a speed of seismotectonic movement was much lower than the background value which, in our opinion, reflects a stage of quasi-viscous creep within the whole seismic zone. at the final stage of this phase (august 15-16) a speed of plastic deformations caused by quasiviscous flow was increased and resulted in generation of a «geodeformation wave» owing to dislocation of blocks. it was reflected in a radon field as an anomaly. azimuth of its incoming is close to a direction on epicenter of the first earthquake (k = 10.9) of phase iv which occurred on august 16 in the south of kronotsky gulf (fig. 1). researchers at the institute of volcanology and seismology at kamchatka also revealed anomalies on august 15-16 in other geophysical fields. 5. conclusions on the basis of our analysis the following conclusions were derived: 1) the network of radon monitoring stations of petropavlovsk-kamchatsky geodynamic proving ground during the period june-august recorded abnormal behavior on august 4-5 and 16, which was interpreted as the reaction of a soil radon field to «geodeformation waves» caused by plastic deformations in a subduction zone. 2) on the basis of the analysis of spacetime seismic characteristics the four phases of seismic activity were defined on the northern flank of kuril-kamchatka islands line. 3) a satisfactory correlation was found between temporal dynamics of radon concentration and space-time characteristics of seismicity was found. moreover, the abrupt increase in the amplitude of the aggregated signal 11 h before a swarm of earthquakes on august 4-5 can be considered a short-term precursor. 4) the increase in molecular hydrogen concentration in subsoil air recorded at station prt before a swarm of earthquakes (mmax=5.6 and h=130 km) on august 4-5 exceeds the average value by two, which could be considered a short-term precursor. acknowledgements the work was undertaken with financial support of rfbr no. 02-05-64556, the grant of the president of the russian federation no. mk-3295.2004.5 and governmental scientific program «the development of scientific potential of the higher schools» under the section 1.2 «universities of russian federation» no. ur.09.01.416. references dubinchuk, v.t. (1991): radon as a precursor of earthquakes, in isotopic geochemical precursors of earthquakes and volcanic eruption, vienna, 37-42. firstov, p.p. (1999): monitoring of subsoil radon volumetric activity on paratunka geothermal system in 1997 1998 with the purpose to search for the precursors of strong earthquakes of kamchatka, volcanol. seismol., 6, 1-11 (in russian). firstov, p.p. and v.p. rudakov (2003): results of subsoil radon registration in 1997-2000 on the petropavlovsk kamchatsky geodynamic polygon, volcanol. seismol., 1, 26-41 (in russian). kim, i.s., a. ahhleby and g.h. sigel jr. (1997): observation of the trapping of radioactive inert gas radon on oxide glass surfaces: macroporous scintillating-glassfiber bundle alpha detector, nucl. instrum. methods phys. res., a390, 419-422. king, c.-y. (1991): gas-geochemical approaches to earthquake prediction, in isotopic geochemical precursors of earthquakes and volcanic eruption, vienna, 22-36. ljubushin, a.a. jr. (1993): the multivariate analysis of time series of systems of geophysical monitoring, phys. earth, 1, 103-108 (in russian). ljubushin, a.a. jr. (1998): the aggregated signal of systems of low-frequency geophysical monitoring, izvestiya (physics of the solid earth), 1, 69-74 (in russian). morgunov, v.a. (2001): the creep of the rocks at a finishing stage of preparation of earthquakes, izvestiya (physics of the solid earth), 4, 3-11 (in russian). nikolaev, v.a. and r. iliç (1999): etched track radiometers in radon measurements: a review, radiat. meas., 30, 1-13. nikolaev, v.a., m.g. buzynniy, i.b.vorobyev, a.v. gromov, a.s. krivokhatskiy, i.p. los, a.v. zelenskiy and yu.a. tomilin (1993): application of the track method for radon measurements in ukraine, nucl. tracks radiat. meas., 21 (3), 433-436. novikov, g.f. (1989): the radiometric exploration, lenin556 pavel p. firstov, valentina s. yakovleva, vladimir a. shirokov, oleg p. rulenko, yury a. filippov and olga p. malysheva grad, pp. 406 (in russian). riznichenko, yu.v. (1977): calculation of speed of deformations at seismic current of mountain weights, izvestiya (physics of the solid earth), 54-65 (in russian). rudakov, v.p. (2003): the seismoemanation effects of geological structures, in problems of geophysics of xxi century. the book 2 (znanije publ., moscow), 95-113 (in russian). serezhnikov, a.i. and v.m. zimin (1976): geological structure of paratunka geothermal area, influence of separate geological factors on modern geothermal activity, in hydrothermal systems and thermal fields on kamchatka, vladivostok, 115-142 (in russian). steinitz, g., u. vulkan and b. lang (1999): radon flux at the northwestern segment of the dead sea (dead sea rift) and its relation to earthquakes, isr. j. earth sci., 48, 283-299. steinitz, g., z.b. begin and n. gazit-yaari (2003): statistically significant relation between radon flux and weak earthquakes in the dead sea rift valley, geology, 6, 505-508. utkin, v.i. (2000): radon and problem of tectonic earthquakes, sozh, 6 (12), 64-70 (in russian). yakovleva, v.s. (2005): a theoretical method for estimating the characteristics of radon transport in homogeneous soil, ann. geophysics, 48 (1), 195-198. waymark-ace-fts-val-ag-30jan2014_ok annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   1   ace-fts version 3.0 data set: validation and data processing update claire waymark 1, kaley a. walker1, 2,*, chris d. boone2, and peter f. bernath3, 4 1department of physics, university of toronto, toronto, canada 2department of chemistry, university of waterloo, waterloo, canada 3department of chemistry and biochemistry, old dominion university, norfolk, virginia, usa 4department of chemistry, university of york, heslington, uk   i. introduction n 12 august 2003, the canadian-led atmospheric chemistry experiment (ace) was launched into a 74° inclination orbit at 650 km with the mission objective to measure atmospheric composition using infrared and uv-visible spectroscopy (bernath et al., 2005). the ace mission consists of two main instruments, ace-fts and maestro (mcelroy et al., 2007), which are being used to investigate the chemistry and dynamics of the earth’s atmosphere. here, we focus on the high resolution (0.02 cm-1) infrared fourier transform spectrometer, ace-fts, that measures in the 750-4400 cm-1 (2.2 to 13.3 µm) spectral region. this instrument has been making regular solar occultation observations for more than nine years. the current ace-fts data version (version 3.0) provides profiles of temperature and volume mixing ratios (vmrs) of more than 30 atmospheric trace gas species, as well as 20 subsidiary isotopologues of the most abundant trace atmospheric constituents over a latitude range of ~85°n to ~85°s. this letter describes the current data version and recent validation comparisons and provides a description of our planned updates for the acefts data set.1                                                                                                                 * corresponding author: kaley a. walker, kwalker@atmosp.physics.utoronto.ca ii. validation results an extensive validation exercise was undertaken for the ace-fts baseline species (vmr profiles of o3, h2o, ch4, n2o, no2, no, hno3, hcl, hf, co, ccl3f, ccl2f2, n2o5, and clono2) and temperature for the version 2.2 (+updates for o3, n2o5 and hdo) data set. the retrievals for this version have been described in detail in boone et al. (2005). the validation results were reported in a special issue of atmos. chem. phys. (http://www.atmoschem-phys.net/special_issue114.html). by building on these comparison results, a newer version of the ace-fts data set (version 3.0) was produced that incorporated a new set of microwindows and updated spectroscopic parameters. it addressed the unphysical oscillations that were found in the mesospheric temperature profiles and an artefact (or “glitch”) that occurred in the temperature profiles near 23 km. in addition, the altitude ranges for nearly all species have been extended. these increases range from a few km to as much as 35 km for n2o. these improvements have been briefly summarized in table 1 and are documented in boone et al. (2013). for the version 3.0 data set, work has been undertaken to characterize the results by performing both comparisons between versions and comparisons with measurements by other satellite instruments. herein, we focus on describing the direct version comparisons for the o annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   2   ace-fts baseline species. these comparisons have been carried out in order to identify the changes to the ace-fts retrievals between the new version 3.0 and the previous wellvalidated (version 2.2 +updates) data set. thethese results allow users to understand the data quality of the version 3.0 data set in relation to the earlier validation studies for the version 2.2+updates data set. table 1: summary of the processing differences between ace-fts v2.2 and v3.0. retrieval parameter version 3.0 (v3.0) versus version 2.2+updates (v2.2) altitude range ranges increased for most species by a few km up to ~35 km for n2o. upper altitude limits that varied with latitude were employed for some molecules in v3.0. microwindows updated to new set for all species, typically using more microwindows in v3.0 to improve the information content on the target constituent. vmr retrieval isotopologues can be treated as separate molecules, i.e., each isotopologue serving as an interferer in a retrieval has an independent vmr profile. in v2.2, different isotopologues of the same molecule were assumed to have the same vmr profile. spectral line list updated line list using hitran 2004 with several updates routinely processed species number of species routinely processed increased to include coclf, cocl2, o2, h2co, ch3oh and hcfc-141b, as well as a number of additional subsidiary isotopologues: 18o12c16o, 17o12c16o, 18o13c16o, 18o16o16o, 16o18o16o, 16o17o16o, n218o, oc34s, and o13cs empirical function in ils calculation a new empirical function was used to characterize the ace-fts’s self apodization, one which yielded improved residuals compared to v2.2. temperature interpolation in p/t retrieval altitude interpolation approach for temperature was changed to match what was used for vmr retrieval. this fixed the unphysical oscillations observed in v2.2 temperature profiles. issue at ~23 km no empirical function is used in the retrieval of pressure below 23 km in v3.0; pressure at each analysed measurement is used as a fitting parameter altitude lower limit in p/t retrieval changed from 12 km (v2.2) to 15 km (v3.0) high altitude retrieval in p/t retrieval the retrieved co2 vmr profile at high altitudes was forced to match fixed co2 vmr at the interface (near 60 km) tangent height separation in p/t retrieval the tangent height separation is calculated in a way that improves the stability of the retrieval compared to v2.2 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   3   the comparison approach taken for this work follows the method described by dupuy et al. (2009). rather than finding pairs of “coincident” measurements by different instruments, the pairs used here are all ace-fts occultations for which profiles are available for both data versions. for each trace gas species (or subset of these data), the mean profiles and 1-σ standard deviation of the mean are calculated for each data version. the absolute and relative differences are calculated from individual pairs of profiles and then the means of these differences are calculated (see eqs. 3 and 4, respectively, in dupuy et al., 2009). to calculate the relative differences, the mean of the version 2.2+updates and version 3.0 vmr is used as the denominator. as was done for most of the version 2.2+updates validation studies, the standard deviation of the bias-corrected differences (which will be referred to as the “debiased standard deviation”) is calculated for these comparisons (e.g. eq. 5 in dupuy et al., 2009). this gives a measure of the combined precision of the two ace-fts data versions (von clarmann 2006). for the results shown here, no data screening has been applied in order to examine all of the profiles produced for the two ace-fts data versions. the ace-fts measurements made between 21 february 2004 (beginning of routine operations) and 30 september 2010 have been used in this work. this analysis has been performed for the full latitude range as well as for selected latitude bands (typically using 30º bins) for all fourteen of the ace-fts baseline species. an example of the direct version comparisons for o3 is shown in figure 1 for the latitude band between 30º s and 60º s. the results shown here are consistent with those seen in all of the different latitude bands. figure 2 shows two additional examples of these direct version comparisons for ch4 and h2o. in addition to subdividing the data into various latitude bands, the data have also been compared separately for different time periods. figure 3 shows an example for o3 where the relative differences for each year are calculated separately to examine the year-to-year 0 1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 vmr [ppmv] a lti tu de [k m ] ftsv3 ftsv2 0 1 2 3 standard deviation [ppmv] 1.5 0.5 0.5 1.5 difference [ppmv] 40 20 0 20 40 difference [%]   figure 1: an example of the ace-fts direct version comparisons for o3 for the 30-60º s latitude range. this compares the version 2.2 o3 update (blue) product with the version 3.0 (red) product. the mean vmr profiles (solid lines) and the 1-σ standard deviations (dot-dashed lines) are shown in the left panel, the absolute differences are shown in the centre panel and the relative differences are shown in the right panel. the dashed lines in the centre and right panels indicate the de-biased standard deviation of the mean differences.   changes. it can be seen that these differences are quite consistent for most years, with all years seeing quite large variability below ~20 km. however, above ~20 km, 2008 has larger variability than the other years as shown in the de-biased standard deviation of the mean difference. this is primarily due to increased numbers of outlier profiles, which we are currently in the process of characterizing to understand their origin and to provide guidance to users to allow these to be filtered out as needed. a list of occulations with known issues is provided by the ace science operations centre (https://databace.scisat.ca/validation/data_is sues.php). in addition to consulting this list, users are encouraged to submit reports the ace team outlining any issues they find. a summary of the results obtained for the direct version comparisons of the ace-fts baseline species is shown in table 2. for each species, the reference describing the version 2.2+updates validation study and the altitude range for the version 3.0 retrievals are listed. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   4     figure 2: two further examples of version 2.2 versus version 3.0 comparisons (mean profiles and absolute differences) for ch4 (left panels) and h2o (right panels). the legend for each pair of panels is the same as given for the corresponding panel in fig. 1. measurements from all latitudes are included in these comparisons.   in addition, brief summaries of the version 2.2+updates validation results are given in jones et al. (2012). the differences described for the version 3.0 data set, compared to version 2.2+updates, are consistent with the need to reduce the bias seen in the version 2.2+updates validation studies. a more detailed validation paper is in preparation in which the ace-fts v3.0 dataset is compared to the ace-fts v2.2+updates dataset as well as several other satellites (sage ii, sage iii, poam iii, haloe, osiris and mls). figure 3: the mean relative differences for the direct version comparisons for o3 are shown for each year from 2004 (far left) to 2010 (far right).   annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   5   table 2: summary of the version 2.2+updates and version 3.0 direct version comparisons. molecule altitude range (km) comparison result (behavior of version 3.0 relative to version 2.2+updates) previous validation results (version 2.2) o3 5-95 a reduction of ~5% is seen above the peak, at ~35 km and higher altitudes. dupuy et al. (2009) (version 2.2 o3 update) ch4 5-62 a reduction of ~10% is seen near ~35-40 km and slight reduction is seen at ~23 km. de mazière et al. (2008) h2o 5-89 small differences of ~±2% are seen over the altitude range between ~20 and ~55 km carleer et al. (2008) no 12-105 a slight increase of ~2 % is seen between ~25 and ~40 km and reduction of ~2% is seen above ~40 km. kerzenmacher et al. (2008) n2o 5-60 a reduction of ~10% is seen above ~35 km strong et al. (2008) no2 13-45 a reduction of ~10% is seen in the altitude range of ~40-45 km. kerzenmacher et al. (2008) n2o5 15-40 above ~20 km, an increase is seen that is up to ~0.04 ppbv at ~26-30 km. below ~20 km, a reduction is seen that is up to ~0.035 ppbv at ~15 km. wolff et al. (2008) (version 2.2 n2o5 update) hf 10-50 a reduction of ~5% is seen over profile. mahieu et al. (2008) hcl 8-57 a reduction of ~5% is seen over profile mahieu et al. (2008) ccl3f 2-22 a slight increase is seen around ~15 km. mahieu et al. (2008) ccl2f2 6-28 an increase of ~2-5% is seen over the ~6-22 km altitude range. mahieu et al. (2008) clono2 12-35 from ~17-22 km, a ~20-30% reduction is seen. a ~7% reduction was also seen above the peak, at ~30-32 km. wolff et al. (2008) co 5-105 below ~10 km and between ~35 and ~45 km, a small decrease is seen. between ~14 and ~18 km, a small increase is seen. clerbaux et al. (2008) hno3 5-37 above ~25 km, an increase of ~5% is seen. wolff et al. (2008) annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   6   iii. ace-fts processing plans the next processing version (version 4.0) is now in development (boone et al., 2013). the primary motivation for this update is to develop a data set that is more appropriate for studying longer-term changes and investigating trends. in ace-fts processing versions 3.0 and earlier, the assumed rate of change in co2 as a function of time is too low and will therefore be changed to match better with observations. in version 4.0, the shape of the co2 vmr profiles at low altitudes will vary with latitude, and a seasonal cycle will be included, features that were not present in previous processing versions. an issue was identified in the input a priori temperature/pressure profiles for low altitudes that has affected all ace-fts retrievals beginning in october 2010 (boone et al., 2013). because of this, data from both version 2.2+updates and version 3.0 should not be used after 30 september 2010. new processing versions (2.5 and 3.5) are being produced to provide corrected results for the affected time period (october 2010 onward). these new versions do not include any changes in the retrieval process other than employing more appropriate a priori pressure and temperature information (boone et al., 2013). iv. conclusion ace-fts and the scisat satellite continue to perform well in their tenth year in orbit and produce a valuable data set for investigating the composition of the earth’s atmosphere. through direct profile comparisons, the current ace-fts v3.0 data set is generally seen to improve on the v2.2+updates data set. comparisons with other satellite data sets are in progress and these results are being used to provide feedback for future ace-fts data versions. acknowledgements funding for ace is provided mainly by the canadian space agency (csa) and the natural sciences and engineering research council of canada. this work was supported by the csa. references [bernath et al., 2005] bernath, p.f., et al. (2005). atmospheric chemistry experiment (ace): mission overview, geophys. res. lett., 32:l15s01. [boone et al., 2008] boone, c. d. et al. (2008). retrievals for the atmospheric chemistry experiment fourier transform spectrometer, appl. optics, 44:7218–7231. [boone at al., 1013] boone, c. d. et al. (2013). version 3 retrievals for the atmospheric chemistry experiment fourier transform spectrometer (ace-fts) in the atmospheric chemistry experiment ace at 10: a solar occultation anthology, peter f. bernath (editor), a. deepak publishing, hampton, virginia. [carleer et al., 2008] carleer, m.r., et al. (2008). validation of water vapour profiles from the atmospheric chemistry experiment (ace), atmos. chem. phys. discuss., 8:4499–4559. [clerbaux et al., 2008] clerbaux, c., et al. (2008). co measurements from the ace-fts satellite instrument: data analysis and validation using ground-based, airborne and spaceborne observations, atmos. chem. phys., 8:2569–2594. [de mazière et al., 2008] de mazière, m., et al. (2008). validation of ace-fts v2.2 methane profiles from the upper troposphere to lower mesosphere, atmos. chem. phys., 8:2421–2435. [dupuy et al., 2009] dupuy, e., et al. (2009). validation of ozone measurements from the atmospheric chemistry experiment (ace), atmos. chem. phys., 9:287–343. [jones et al., 2012] jones, a. et al. (2012). technical note: a trace gas climatology deannals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6339   7   rived from the atmospheric chemistry experiment fourier transform spectrometer dataset, atmos. chem. phys., 12:5207–5220. [kerzenmacher et al., 2008] kerzenmacher, t., et al. (2008). validation of no2 and no from the atmospheric chemistry experiment (ace), atmos. chem. phys., 8:5801– 5841. [mahieu et al., 2008] mahieu, e., et al. (2008). validation of ace-fts v2.2 measurements of hcl, hf, ccl3f, and ccl2f2 using space-, balloonand ground-based instrument observations. atmos chem phys, 8:6199–6221. [mcelroy et al., 2007] mcelroy, c.t., et al. (2007). the ace-maestro instrument on scisat: description, performance and preliminary results, appl. opt., 46:4341–4356 [strong et al., 2008] strong, k., et al. (2008). validation of ace-fts n2o measurements. atmos. chem. phys., 8:4759–4786. [von clarmann et al., 2006] von clarmann, t. (2006). validation of remotely sensed profiles of atmospheric state variables: strategies and terminology, atmos. chem. phys., 6:4311–4320. [wolff et al., 2008] wolff, m.a., et al. (2008). validation of hno3, clono2, and n2o5 from the atmospheric chemistry experiment fourier transform spectrometer (acefts), atmos. chem. phys., 8:3529–3562. vol49,6,2006 1157 annals of geophysics, vol. 49, n. 6, december 2006 key words hydrophones – bragg fiber-laser – pressure sensors 1. introduction in the field of acoustic detection, piezoelectric sensors are the most widely used devices. this well established technology offers high reliability and relatively low cost production. sensors based on optical fibers have well known advantages over conventional electro-mechanical sensors. they offer electrically passive operation and immunity from electromagnetic fields, since the fiber is realized entirely with dielectric materials (glass and plastic). they have very small dimensions (outer diameter ∼125 µm for a standard fiber), and have multiplexing capabilities for a quasi-distributed measurement configuration, by using a single opto-electronic control unit. remote measurement is also possible. indeed, the very low signal attenuation (∼ 0.3 db/km) of the fibers in the region around 1.55 µm makes it possible to place the opto-electronic control unit several km far from the measurement point. moreover, high sensitivity and wide dynamic measurement range can usually be achieved. 2. fiber bragg grating sensors a fiber bragg grating (fbg) consists of a periodic perturbation of the optical fiber core refractive index. this grating acts on the radiation traveling in the fiber as a wavelength selective mirror. if the grating pitch is λ, the reflection band is peaked at a wavelength (2.1) where λbragg is the so-called bragg wavelength and neff is the effective refractive index. the width of the reflection band can be regulated by controlling the reflectivity of the mirror, that is by controlling the length of the grating (its total number of lines) and the depth of the reflection index modulation. typical reflection bandwidth values are around 0.2 nm. radiation outside the bragg resonance condition will propagate in the fiber without perturbations (fig. 1). 2nbragg effλ λ= developing fiber lasers with bragg reflectors as deep sea hydrophones nicolò beverini (1)(3), riccardo falciai (2), enrico maccioni (1)(3), mauro morganti (1)(3), fiodor sorrentino (1) and cosimo trono (1)(2) (1) dipartimento di fisica «e. fermi», università degli studi di pisa, italy (2) istituto di fisica applicata «nello carrara» (ifac), cnr, firenze, italy (3) istituto nazionale di fisica nucleare (infn), sezione di pisa, italy abstract the present paper will discuss the work in progress at the department of physics of the university of pisa in collaboration with the ifac laboratory of cnr in florence to develop pressure sensors with outstanding sensitivity in the acoustic and ultrasonic ranges. these devices are based on optically-pumped fiber lasers, where the mirrors are bragg gratings written into the fiber core. mailing address: dr. nicolò beverini, dipartimento di fisica «e. fermi», università degli studi di pisa, largo pontecorvo 3, 56127 pisa, italy; e-mail: beverini@df.unipi.it 1158 nicolò beverini, riccardo falciai, enrico maccioni, mauro morganti, fiodor sorrentino and cosimo trono fbgs are fabricated using the phase-mask technique on special ge-doped fibers (hill and meltz, 1997). a phase-mask is a diffractive optical element which spatially modulates the uv writing beam (typically at λ = 248 nm, where the photosensitivity of the fiber is at its best). the near-field fringe pattern, which is produced behind the mask, photo-imprints a refractive index modulation on the core of the photosensitive fiber. these gratings are highly sensitive to external perturbations that affect the fiber. with regard to eq. (2.1), every strain and temperature or pressure variation on the fiber changes both the grating pitch and the refractive index, producing a shift in the bragg wavelength (fig. 2). the information is encoded on the wavelength, with the considerable advantage that a number of different gratings, with different bragg wavelengths, can be inscribed on the same fiber and interrogated by the same opto-electronic unit, thus enabling a quasi-distributed measurement. the typical sensitivities of a fiber bragg grating to strain, temperature, and pressure (which acts as an isotropic strain) are (2.2) (: )strain n p p p1 2bragg bragg eff 2 12 11 12λ λ ν ε ∆ − − −= 6 @& 0 fig. 1. fiber bragg grating reflection and transmission properties. fig. 2. strain, temperature and pressure effects on a bragg grating reflection spectrum. 1159 developing fiber lasers with bragg reflectors as deep sea hydrophones (2.3) (2.4) where neff is the fiber effective refractive index, ν is poisson’s ratio, ε=∆l/l is the strain (bragg grating length l), pij are pockel’s coefficients of the stress-optic tensor, αλ is the thermal expansion coefficient, αn represents the thermo-optic coefficient, and e is young’s modulus. for silica fibers, at 1550 nm the strain sensitivity is ∼1.2 pm/µε, the temperature sensitivity is ∼10 pm/°c, and the pressure sensitivity is ∼ −3.6 pm/mpa. a typical fbg sensor device includes a broad-spectrum source (i.e. a superluminescent led with ∼ 40 nm spectral emission width). its radiation is coupled into the fiber, and interacts with the grating. the wavelength shift of the reflection peak is then detected by means of spectro-photometric methods. fbgs sensors are robust, can have a wide dynamic range, and can be easily multiplexed in order to realize multipoint detection at a low cost. their ultimate sensitivity is limited, however, by the spectral bandwidth of the bragg grating which at best can be of the order of 0.2 nm. a typical application for fbg sensors is the control of deformations in structures to which the gratings can be attached or within which p∆ ( ) ( )pressure: e e n1 2 2 1 2 bragg bragg 2 12 11 $ λ λ ν ν ∆ − − + −= $ )p p2 +( : f ( )temperature: t bragg bragg λ λ α α ∆ ∆+ νλ= they can be embedded. the following is a summary of fbg sensor applications made by the ifac group: – applications for the cultural heritage (castelli et al., 2003): in situ measurement and continuous monitoring of deformations in painted wood panels. − automotive applications: monitoring of deformations in car windshields (falciai et al., 2004). − structural health monitoring: fbgs embedded in carbon fibers in composite strips for concrete beam reinforcement and monitoring (falciai et al., 2005). fbgs show only a limited responsivity to variations in pressure (the typical wavelength resolution of fbg interrogation systems is ∼1 pm, which means approximately 0.3 mpa, or 3 atm). therefore, their typical application is for high pressure sensing (xu et al.,1993). 3. fiber lasers a distributed bragg reflector fiber laser (dbr-fl) consists of two bragg gratings with identical reflection wavelengths (ball and glenn, 1992), which are directly inscribed in a singlemode erbium-doped optical fiber (fig. 3). when pumped with 980 nm radiation, this structure acts as an active medium inserted in a fabry-perot laser cavity, with an emission peak around 1530 nm. the power emitted is a function of the broadband source pumping power, the length of the laser cavity and the bragg gratings reflectivity, fig. 3. fiber laser pumping scheme. 1160 nicolò beverini, riccardo falciai, enrico maccioni, mauro morganti, fiodor sorrentino and cosimo trono but it is in any case some orders of magnitude higher than the power available on the reflected radiation from a passive fbg sensors. cavity length is an important laser parameter. the emitted laser power increases with the cavity length, but a shorter length means a larger separation between the resonant longitudinal modes (λ= =λ2/2l). with a cavity length of a few cm, only one cavity mode lies inside the optical bandwidth of the bragg gratings, and it becomes possible to operate with the laser in a stable single-frequency emission mode. in these conditions the laser linewidth is very narrow (<5 khz, equivalent to a coherence length of over 30 km). the comparison between the performances of devices using the fbg and the dbr is provided in table i. the very narrow line-width means that the dbr laser sensor is intrinsically more sensitive than the fbg sensor; for this reason, it seems particularly suitable for very low-pressure measurements. dbr fiber lasers were fabricated in the ifac laboratories. an excimer (krf) uv laser, with emission at 248 nm, was used for bragg gratings writing. we obtained a bragg grating with λbragg ≅ 1532 nm by a phase mask with a nominal spatial period of 1059.9 nm on an erbium doped fiber having an absorption coefficient of 14 db/m at 980 nm. the length of bragg gratings was 1 cm, and we chose the distance between the two fbg reflectors and the fbgs reflectivity in order to obtain the maximum power emission in stable, single longitudinal mode regime. typical values are: output fbg reflectivity: > 90%; back fbg reflectivity: > 99%; distance between the fbgs: 2 mm-2 cm; emission power: 10 µw-1 mw with about 200 mw of 980 nm pumping power. the laser line-width was measured by heterodyning on a fast photodiode the beams from two different lasers with slightly different emission wavelengths, and then observing the resulting beat note on a radio-frequency spectrum analyzer. we could estimate a line-width narrower than 5 khz. it is possible to inscribe several laser structures on a single doped fiber with the aim of multiplexing a few sensors, by working at slightly different wavelengths on the same fiber with a common pump laser. in order to test multiplexing operation, we wrote two lasers on the same optical fiber with a different grating pitch. figure 4 fig. 4. emission spectrum of an array of two fiber lasers. table i. comparison of fbg and dbr performances. power line-width line-width (khz) (nm) fbg (passive) 1-100 nw ∼ 107 ∼ 0.1 dbr (active) 100 µw-1 mw ~ 5 ∼ 5⋅10−8 1161 developing fiber lasers with bragg reflectors as deep sea hydrophones reports the emission spectrum of this two-laser array, acquired by means of an optical spectrum analyzer (wavelength resolution: 0.1 nm). 4. fiber laser hydrophone in developing highly sensitive hydrophones for deep sea applications, the ultimate goal is to achieve a sensitivity at the level of the acoustic background noise of the quiet ocean, which is conventionally represented by the so-called deep sea state zero (dss0) (wenz, 1962). at 1 khz, the dss0 level is 100 µpa/hz1/2, which should correspond (eq. (2.4)) to a wavelength shift of approximately 10−12 nm, 11 orders of magnitude less than the typical fbg bandwidth. the dbr fiber laser emits monochromatic radiation with a line-width of approximately 4⋅10−8 nm, and an interferometric detection technique provides the possibility of pushing the sensitivity a factor 103-104 below the laser line-width, thus approaching the sensitivity required by the dss0 level. figure 5 shows the scheme of an unbalanced mach-zender interferometer (mzi), which we used in our detection scheme. the interferometer converts the pressure-induced wavelength shift of the radiation emitted by the dbr fiber laser into a phase delay. the phase difference ∆ϕmz at the mzi output is dependent on the fiber laser output wavelength shift ∆λ and on the optical path difference (opd= = nl), which is the length difference l of the two interferometer arms times the fiber core refractive index n. the following relationship holds: (4.1) with λ of the order of 10−12 nm (dss0 conditions), an opd of 300 m gives a value of ϕmz ≈ 1 µrad, which is a resolution achievable with the present technology (kersey et al., 1992). 5. experimental tests we made a preliminary characterization of the sensor in our laboratory and tested the responsivity of our lasers by using a mzi with opds of 150 m and 450 m. according to eq. (4.1), the mzi responsivity increases by increasing the unbalance, up to the limit given by the coherence length of the laser radiation, which for these lasers consists of many km. the monotonic operative range and the dynamic range are, however, reduced proportionally. indeed, the phase detector signal is not univocally related to the wavelength shift, when the phase difference grows over 2π radians, and the low frequency noise of the laser exceeds this limit, yet with a few meter of length unbalance. we were then obliged to lock the mzi phase to the laser frequency at low fourier frequencies, with a cut-off of the order of several khz, by using a servo loop that acts on the length of one arm of the interferometer by stretching the fiber through a piezoelectric actuator. the interferometer was used in the condition usually defined as «quadrature» detection (kersey et al., 1992), with the mzi locked at one side of a fringe in the middle point, where the sinusoidal function is proportional to the phase and the responsivity has its maximum value. the experimental apparatus is sketched in fig. 6. the fiber laser was sink in a 1 m3 water tank, close to a conventional piezo-transducer (pzt) hydrophone for calibration and validation; a sdr hs/150 model was used with a nominal responsivity of 1.778 mv/pa (29 db . opd2 mz 2 $ ϕ λ π λ∆ ∆= fig. 5. the mach-zender interferometer. fig. 7. fiber laser spectral response to a 65 khz test tone (opd = 150 m). top (a): the fiber laser response (output of the mzi) to a 65 khz loud-speaker acoustic test note. bottom (b): the laser output spectrum (without the mzi). 1162 nicolò beverini, riccardo falciai, enrico maccioni, mauro morganti, fiodor sorrentino and cosimo trono amplification). a pzt loud-speaker provided a cw sinusoidal uncalibrated acoustic signal. the laser emission wavelength modulation, induced by the acoustic signal, was converted into phase modulation by the mach-zender interferometer, and analyzed by means of an electronic spectrum analyzer (125 hz resolution bandwidth). fig. 6. experimental setup. 1163 developing fiber lasers with bragg reflectors as deep sea hydrophones figure 7 compares the mzi output with the laser output spectrum in the absence of the interferometer when the fiber laser was stimulated with a 65 khz acoustic wave. from these two spectra we deduced a limit for the performance of a particular fl sensor, referred to a device of a specific optical power (100 µw), and a fixed orientation with respect to the acoustic source. in fact the upper base line −80 dbv/hz1/2 level, corresponding to about 56 mpa/hz1/2, was produced by the actual environmental noise level. the bottom −118 dbv/hz1/2 level, which corresponds to about 0.7 mpa/hz1/2, represented the opto-electronic noise level, and thus the lowest fig. 8. fl output signal versus pressure (measured with the calibrated reference hydrophone). fl optical power 10 µw, test note frequency 40 khz; opd ∼ 450 m. error bars represent the reading uncertainty. fig. 9. fl responsivity for different test note frequencies; opd ~ 450m; fl optical power 10 µw. 1164 nicolò beverini, riccardo falciai, enrico maccioni, mauro morganti, fiodor sorrentino and cosimo trono attainable limit for the minimum detectable signal of the fiber laser. the acoustic level in mpa was estimated by means of a comparison with the readings of a calibrated pzt hydrophone. we measured the response of the device to in-water acoustic notes at several fixed frequencies in the region between about 15 and 100 khz, for different intensities of loud-speaker excitation. by making a comparison with the calibrated pzt hydrophone, we observed a highly linear trend of the fl output signal versus pressure (fig. 8). in this particular experimental condition, we measured a responsivity of 76 mv/pa at 40 khz, where the voltage value refers to the photodiode signal at the mzi output. the reading error bars (± 0.5 db) on the single measurement are reported. however, the responsivity is changing significantly as a function of the acoustic wave frequency. figure 9 reports measured responsivities for a same fl and a same mach-zender opd. the large spread for the responsivity values depends strongly on the particular set of experimental parameters. in fact, it can be affected at some specific frequencies by the acoustic reflections between the walls of the water tank, due to its reduced dimensions. also mechanical resonances of the fiber laser holder, and of the fiber itself, may have caused a dependence of the frequency response. moreover, the acoustic perturbation in water may act differently on the two bragg gratings of the laser, because they are spatially separated. all these factors dictate that a true characterization of the sensors should be made only under better control of experimental conditions, operating in a large pool and in pulsed excitation regime, in order to avoid reflection effects. in particular, it will be important to study the dependence of the response on the orientation of the sensor. in any case, we can say that the pressure dynamic range for the fl sensor set-up shown in figs. 8 and 9 is roughly of four decades. in fact, the minimum detectable pressure in this experimental condition is of the order of 1 mpa/ hz1/2 at every frequency; this is, for example, the measured value of the first point on fig. 8. the output voltage of the device is linear with pressure up to about 1 v, that is, up to 10 pa with minimum responsivity in the 15-100 khz range of approximately 100 mv/pa (fig. 9). 6. conclusions the results of the study, development and experimental validation of a dbr fiber laser for acoustic sensing in marine environment, have been reported. single mode lasers were fabricated by writing two bragg gratings on an erbium-doped fiber core. the very narrow linewidth (< 5 khz), combined with an interferometric detection, could make possible a wavelength resolution of ∼ 10−12 nm. the comparison with a calibrated pzt hydrophone (15-100 khz frequency range) showed a highly linear trend of the fiber laser output signal versus pressure. on the other hand, fl responsivity was strongly frequency dependent. this fact was probably connected to the particular experimental set-up, such as the acoustic reflections between the walls of the water tank and the mechanical resonance of the fl holder and of the fiber itself. the fbg hydrophones offer a wide range of applications, ranging from the marine environmental acoustic monitoring to the deep-sea study and the survey of dolphins and whales. in general, a network of multiplexed fbg hydrophones could provide much information on what happens in the «under-the-sea world» in the acoustic and ultrasound regimes, whichever could be the origin of the propagating pressure wave. an interesting field of application could be the acoustic detection of particle showers produced in water by very high-energy neutrinos, as an alternative method to the more conventional photo-detection. acknowledgements this research was supported by the italian university ministry (miur), by the university of pisa, by national institute of nuclear physics (infn), and by the italian national research council (cnr). references ball, g.a. and w.h. glenn (1992): design of a singlemode linear-cavity erbium fiber laser utilizing bragg reflectors, j. lightwave tech., 10, 1338-1343. castelli, c., g. lanterna, r. falciai and c. trono 1165 developing fiber lasers with bragg reflectors as deep sea hydrophones (2003): continuous monitoring of wooden works of art using fiber bragg-grating sensors, j. cultural heritage, 4, 285-290. falciai, r., c. trono, p. castelli, r. galli, n. mattiucci and n. pallaro (2004): automotive applications of fiber bragg grating sensors, in proceedings of the 8th conference sensors and microsystems «aisem 2003», edited by c. di natale, a. d’amico, g. soncini, l. ferrario and m. zen (world scientific publisher), p. 545. falciai, r., j.m. kenny, a. terenzi, c. trono and r. mezzacasa (2005): reinforcing and monitoring of concrete structures with composites and fiber optic sensors, in proceedings of the 9th conference sensors and microsystems «aisem 2004», edited by c. di natale, a. d’amico, g. martinelli, m.c. carotta and v. guidi (world scientific publisher), 329-336 hill, k.o. and g. meltz (1997): fiber bragg grating technology fundamentals and overview, j. lightwave technol., 15 (8), 1263 kersey, a.d., t.a. berkoff and w.w. morey (1992): high-resolution fibre-grating based strain sensor with interferometric wavelength-shift detection, electron. lett., 28 (3), 236-238. wenz, g.m. (1962): acoustic ambient noise in the ocean: spectra and sources, j. acoust. soc. am., 34, 1936-1956. xu, m.g., l. reekie, y.t. chow and j.p. dakin (1993): optical in-fibre grating high pressure sensor, electron. lett., 29, 398-399. (received september 6, 2005; accepted may 17, 2006) 125_130 adg v5 n01 huang.pdf annals of geophysics, vol. 45, n. 1, february 2002 125 electron density profiles of the topside ionosphere xueqin huang (1), bodo w. reinisch (1), dieter bilitza (2) and robert f. benson (3) (1) center for atmospheric research, university of massachusetts lowell, ma, u.s.a. (2) raytheon itss, gsfc, code 632, greenbelt, md, u.s.a. (3) gsfc, code 692, greenbelt, md, u.s.a. abstract the existing uncertainties about the electron density profiles in the topside ionosphere, i.e., in the height region from h m f 2 to ~ 2000 km, require the search for new data sources. the isis and alouette topside sounder satellites from the sixties to the eighties recorded millions of ionograms but most were not analyzed in terms of electron density profiles. in recent years an effort started to digitize the analog recordings to prepare the ionograms for computerized analysis. as of november 2001 about 350 000 ionograms have been digitized from the original 7-track analog tapes. these data are available in binary and cdf format from the anonymous ftp site of the national space science data center. a search site and browse capabilities on cdaweb assist the scientific usage of these data. all information and access links can be found at http://nssdc.gsfc.nasa.gov/space/isis/isisstatus.html. this paper describes the isis data restoration effort and shows how the digital ionograms are automatically processed into electron density profiles from satellite orbit altitude (1400 km for isis-2) down to the f peak. because of the large volume of data an automated processing algorithm is imperative. the topside ionogram scaler with true height algorithm topist software developed for this task is successfully scaling ~ 70% of the ionograms. an «editing process» is available to manually scale the more difficult ionograms. the automated processing of the digitized isis ionograms is now underway, producing a much-needed database of topside electron density profiles for ionospheric modeling covering more than one solar cycle. 1. introduction modeling of the topside ionosphere (from the f peak to about 2000 km) suffers from a scarcity of data because ground-based ionosondes only probe up to the f-peak. satellite-borne sounders provided information about the topside ionosphere. but only a small percentage of these mailing address: prof. bodo w. reinisch, center for atmospheric research, university of massachusetts lowell, 600 suffolk st., lowell, ma 01854, u.s.a.; e-mail: bodo_reinisch@uml.edu data have been processed into electron density profiles, which is the parameter of greatest interest for topside modeling. an ionosonde transmits signals sweeping through a typical frequency range and records the time delay it takes the signal to be reflected back to the receiver. the ionogram established in this way usually consists of an ordinary and extraordinary trace (oand x-trace). ionogram analysis includes finding these traces and then inverting them into an electron density profile. the problem of the topside sounder missions in the sixties and seventies and the reason for the low percentage of obtained electron density profiles is the fact that the scaling had to be done manually. in the eighties and nineties, automated scaling and inversion algorithms were developed key words electron density profile topside ionosphere ionogram processing 126 xueqin huang, bodo w. reinisch, dieter bilitza and robert f. benson and perfected (huang and reinisch, 1982) and are now well-established tools of space weather observations with ground-based ionosondes (reinisch et al., 2001; huang and reinisch, 2001). this paper describes an effort to apply these automated scaling and inversion routines to the large database established by the u.s./ canadian alouette and isis topside sounders. the alouette-1, -2 and isis-1, -2 satellites were the first satellites that were equipped with topside sounder instruments to monitor the topside ionosphere from the satellite orbit altitude down to the f-peak. the launch dates and orbit parameters are listed in table i. isis 1 launched in 1969 and isis 2 launched in 1971 were operated by nasa until 1979, then by crc (canada) until 1984, and finally by rrl (japan) until 1990. additional instruments carried by these satellites included vlfreceivers, energetic particle detectors (epd), ion mass spectrometers (ims), retarding potential analyzers (rpa) and photometers. these satellites either had limited onboard recording capabilities (isis) or none at all (alouette). data were therefore primarily recorded within the viewing area of several telemetry down-link stations. because of the great interest in these data many nations participated in the alouette/isis program and provided dedicated telemetry stations. table ii lists 24 of these stations illustrating their global distribution. a number of additional stations operated at various times, but these 24 are the ones considered in our data restoration effort. several institutions were also involved in data reduction efforts to obtain electron density profiles from manually scaled ionograms. of the close to a million ionograms recorded by the alouette/isis satellites several tens of percent are stored on more than 12 000 rolls of 35 mm microfilm at the national space science data center (nssdc). only a few percent of the ionograms were processed into electron density profiles. about 150 000 profiles were submitted to nssdc’s archives and are now available online from nssdc’s anonymous ftp site at ftp://nssdcftp.gsfc.nasa.gov/. this is also the story of a successful data restoration effort that saved a considerable portion of an irreplaceable data set from the brink of extermination. the alouette and isis telemetry data were stored on more than 100 000 7-track tapes in the canadian public archives (cpa). in the early nineties the cpa indicated its intent to discard these tapes because of storage space and cost limitations and the dormant state of these data. with help from g. james (crc, ottawa) and with funding from nasa/oss/ aisrp, r. benson managed to save about 18 000 of these tapes (specifically selected for time and location; see next section) and ship about 14 000 to the goddard space flight center (gsfc) for processing and analysis. the rest of the telemetry tapes was discarded and the information contained on them is now lost. 2. digitization of isis ionograms at gsfc the analog topside sounder recordings from the 7-track tapes were digitized and made available online through nssdc’s anonymous ftp archive. before this, however, the important first step in this data restoration effort was the selection of desirable tapes, because it became clear that it was feasible to only save about 10-20% of the tape total. the tapes were selected in order to obtain global coverage and to accommodate special requests that address subjects and time periods of particular interest. table i. isis-1 and -2 orbit parameters. satellite launch date height range/km inclination/degree alouette 1 1962-09-29 1000 80 alouette 2 1965-11-29 500-3000 80 isis 1 1969-01-30 550-3500 88 isis 2 1971-04-01 1400 88 electron density profiles of the topside ionosphere 127 the selection included data from 24 telemetry stations from the years 1972-1984. table ii provides some of the specifics in terms of the number of tapes from a specific satellite and stations and the years covered. in selecting these tapes we also considered the time periods already covered by nssdc’s data holdings. typically, 80-100 tapes/year were specified centered on each of the equinoxes and solstices. about 8000 tapes were selected with special emphasis on time periods coinciding with the de satellites and from stations close to the magnetic equator, an area not so well covered by the existing data sets. the digitization at gsfc’s data evaluation laboratory (del) was performed using an a/d converter and software device driver compatible with the os/2 operating system used by a 486based programmable telemetry processor (ptp). the digitized sounder data consist of 8-bit receiver-amplitude values collected at a 40 khz rate and contain the time and the frequency location station id lat. long. al1 al-2 isis-1 isis-2 a resolute bay, canada res 75 265 327 (76) 504 (73-79) a tromso, norway tro, trm 70 19 320 141 (73-76) a sodankyla, finland sod 67 27 63 (77-79) a fairbanks, alaska ula 65 212 1 (62) 244 (73-79) 439 (73-79) a winkfield, u.k. wnk 51 359 2 (66) 319 405 (73-79) a ottawa, canada ott 45 284 1187 (69-83) 991 (73-83) a kashima, japan ksh 36 141 103 (78-81) 879 (73-79) a las palmas canary island, spain can 28 345 106 (74-75) ahmedabad, india ame 23 73 265 (73-77) a ouagadougou, burkina faso odg 14 359 745 (73-?) 214 (73-75) a kwajalein, marshall islands kwa 9 168 140 a a kourou, french guyana kru 5 307 212 (74-77) a quito, equador qui 1 281 1 (62) 700 483 (69-72) 366 (73-79) a brazzaville, congo brz, bzv 4 15 34 (73-74) a ascension island, u.k. can 8 346 174 (75-77) a lima, peru lim 12 283 11 a johannesburg, south africa bur, job 26 28 192 (73-75) a santiago, chile snt, ago 33 298 428 209 (69) 240 (73-76) a orroral, australia orr 36 149 66 (72-?) 232 (73-78) a lauder, new zealand lau 45 170 604 (73-80) a kerguelen islands, france ker 49 70 98 (81-83) 464 (77-83) a falkland island, u.k. sol 52 302 421 45 (72) a terre adelie, antarctica adl 67 140 54 (82-83) 738 (73-83) a syowa base, antarctica syo 69 40 241 (78-82) a table ii. the 24 ground stations for which tapes were selected; for each of the four satellites the number of tapes is shown and in brackets the years covered. 128 xueqin huang, bodo w. reinisch, dieter bilitza and robert f. benson associated with each sounder pulse. the data are provided in full and average (4 consecutive range bins averaged to yield one amplitude sample) in os2 binary format; the average resolution data were also converted to cdf format at nssdc. the virtual range resolution is 3.75 km for the full resolution ionograms and 15 km for the average resolution data. as of november 2001 about 350 000 ionograms have been digitized. a small selection of these data has also been made available for plotting and browsing on cdaweb (http:// cdaweb.gsfc.nasa.gov/). other services available from the isis homepage include a search page for locating ionograms for specific times, locations, and other search criteria and software for an interactive idl interface for plotting, scaling, and inverting the ionograms based on the inversion program of jackson (1969). 3. topist description our goal was to automatically deduce the electron density profiles from the digitized ionograms. this is the logical next step after the digitization process since topside electron densities are one of the most important data products derived from ionograms. this requires locating and identifying the echo traces on the ionogram and then applying an inversion algorithm that inverts the echo traces into an electron density profile. because of the large volume of ionograms this has to be done automatically in order to provide the maximum input to ionospheric models. our first project step was therefore the development of a computer program for the automated tracing and inversion of topside ionograms. a topside ionogram scaler with true height algorithm (topist) program has been developed that is fig. 1. flow chart of topist. table iii. processing functions in topist. process reading ionogram file noise filter search for resonance frequencies consult model f 2 peak parameters trace scaling electron density profile calculation method full or average ionogram/os2 binary. individual threshold for each frequency. most probable amplitude at low ranges and at large ranges is determined. smaller value is selected as nominal threshold. 5-frequency comb with f n as free variable slides along frequency axis in search for maximum amplitude. f0f2 and hmf2 values are calculated from ursi and ccir coefficients and used as guide for the auto-scaling. if ground echo traces are observed, they also are used in the f0f2 search. find potential trace points for each frequency line storing their amplitude and connection length to neighbors. find optimal combination of resonance frequencies and oand x-traces using a family of generic functions. introduce a reduced frequency variable and represent profile with chebyshev polynomials. inversion can work with o-trace only, or x-trace only, or both traces and possibly the z-trace. electron density profiles of the topside ionosphere 129 fig. 2. topist opening window. successfully scaling ~70 % of the ionograms. the topist software also includes an «editing option» (called manual scaling in fig. 1) for the manual scaling of the more difficult ionograms, which could not be scaled during the automated topist run. the most difficult part of the task is the automatic scaling of the echo traces. unlike the ionograms from modern ionosondes (reinisch, 1996) the isis ionograms do not identify the wave polarization of the different echo traces, so physical logic must be applied to identify the o and/or x traces, and this is not always successful. characteristic resonance features seen in the topside ionograms include those at the gyro and plasma frequencies. an elaborate scheme was developed to automatically identify these resonance frequencies in order to determine the local plasma and gyro frequencies. this information helps in the identification of the o and x traces, and it provides the starting density of the electron density profile. the inversion of the echo traces into electron density profiles uses the same modified chebyshev polynomial fitting technique that is successfully applied in the analysis of topside ionograms and ground-based digisonde ionograms (huang and reinisch, 1982; reinisch and huang, 1983). 4. processing procedures the flowchart in fig. 1 and table iii describe the individual steps that topist takes to analyze the ionograms. the topist opening window (fig. 2) allows one to specify the processing parameters. the performance characteristics of topist are summarized in table iv. 130 xueqin huang, bodo w. reinisch, dieter bilitza and robert f. benson 5. summary and discussion a considerable amount of a very valuable data source for the topside ionosphere has been saved in a last minute effort. close to 340 000 topside ionograms have so far been digitized and this process continues at a rate of about 6000 per month. a software algorithm (topist) was developed for automated scaling and inversion of ionograms and is now successfully scaling about 70% of the digitized ionograms. the resulting topside electron density profiles are being made available online from nssdc’s anonymous ftp archive (nssdcftp.gsfc.nasa.gov). this unique new data source for modeling the topside electron density will more than triple the amount of electron density data previously available from the alouette and isis satellites, and it will greatly extend the solar cycle coverage of the combined alouette/isis database. a better representation of conditions during very high solar activity is especially important because the largest topside electron densities are found during these time periods and, as a result, the strongest space weather effects occur during such intervals. for real time space weather monitoring future topside sounders should provide for automatic scaling of the ionograms as proposed by reinisch et al. (2001). the data set will also help special investigation that were considered in selecting the original telemetry tapes, e.g., combining isis data with coincident de data, the investigation of sounder-stimulated plasma resonances, and studies of high latitude and low latitude plasma processes. the importance of these new digital data was recently demonstrated by benson and grebowsky (2001). they produced several orbitplane electron-density contours through the winter, nighttime polar cap ionosphere during solar minimum. their observations, combined with other data, suggest that an absence of an f-layer ionization peak may be a frequent occurrence at high latitudes. acknowledgements this work was supported through nasa/ oss applied systems and information research program (asirp) grant nag5-8145. we are grateful to w.b. schar of emergent/gsfc for his key role in producing the isis digital ionograms and transferring them to the nssdc under aisrp rtop grant # 370-03-00-04 at gsfc. references benson, r.f. and j.m. grebowsky (2001): extremely low ionospheric peak altitudes in the polar hole region, radio sci., 36, 277-285. hua n g, x. and b.w. re i n i s c h (1982): automatic calculation of electron density profiles from digital ionograms. 2. true height inversion of topside ionograms with the profile-fitting method, radio sci., 17 (4), 837-844. huang, x. and b.w. reinisch (2001): vertical electron content from ionograms in real time, radio sci., 36 (2), 335-342. jackson, j.e. (1969): the reduction of topside ionograms to electron-density profiles, proc. ieee, 57 (6), 960-976. re i n i s c h, b.w. and x. h ua n g (1983): automatic calculation of electron density profiles from digital ionograms. 3. processing of bottomside ionograms, radio sci., 18 (3), 477-492. reinisch, b.w. (1996): modern ionosondes, modern ionospheric science, edited by h. kohl, r. ruster and k. schlegel, european geophysical society, 37191 katlenburg-lindau, germany, 440-458. reinisch, b.w., d.m. haines, g.s. sales, r.f. benson, j.l. green and w.w.l. taylor (2001): radio sounding in space: magnetosphere and topside ionosphere, j. atmos. sol.-terr. phys., 63, 87-98. table iv. topist performance. function test database success rate auto-scaling failures description ~ 800 digitized isis-2 ionograms 70% of manually scalable ionograms are successfully auto-scaled scaling errors occur during severe spread; when data are missing in a large frequency interval; in the presence of unidentified traces (oblique?); when resonance frequencies are incorrectly identified harmonizing and comparing single-type natural hazard risk estimations annals of geophysics, 59, 2, 2016, s0216; doi:10.4401/ag-6987 s0216 harmonizing and comparing single-type natural hazard risk estimations kevin fleming1,*, stefano parolai1, alexander garcia-aristizabal2, sergey tyagunov3, sergiy vorogushyn4, heidi kreibich4, holger mahlke5 1 gfz german research centre for geosciences, centre for early warning, potsdam, germany 2 amra center for the analysis and monitoring of environmental risk, naples, italy 3 institute for civil engineering, technical university of berlin, berlin, germany 4 gfz german research centre for geosciences, section 5.4 hydrology, potsdam, germany 5 institute for meteorology and climate research, karlsruhe institute of technology, eggenstein-leopoldshafen, germany abstract single-type hazard and risk assessment is the usual framework followed by disaster risk reduction (drr) practitioners. there is therefore a need to present and compare the results arising from different hazard and risk types. here we describe a simple method for combining risk curves arising from different hazard types in order to gain a first impression of the total risk. we show how the resulting total (and individual) risk estimates can be examined and compared using so-called risk matrices, a format preferred by some drr practitioners. we apply this approach to cologne, germany, which is subject to floods, windstorms and earthquakes. we then use a new series of risk calculations that consider epistemic uncertainty. the mann-whitney test is applied to determine if the losses arising from pairs of hazards are comparable for a given return period. this benefits decision makers as it allows a ranking of hazards with respect to expected damage. such a comparison would assist planners in the allocation of resources towards the most efficient mitigation actions. however, the results are dependent upon the distribution of estimates (i.e., level of uncertainty), which is in turn a function of our state of knowledge. 1. introduction although there is an increasing awareness of the importance of the potential interactions that arise between different hazard types and their associated risks, there is still a need for robust and meaningful comparisons between the risks arising from individual hazard types occurring in the same area [kappes et al. 2012, marzocchi et al. 2012]. the reason for this is partly due to the importance of single-type risks to stakeholders and decision makers owing to their relative ease of use, the different competencies in risk management (e.g., water boards are often only responsible for floods, not other hazards) and the (single-risk) mandate that authorities are usually assigned [e.g., komendantova et al. 2014]. therefore, while the single-type approach for natural hazard and risk assessment (i.e., individual types are treated separately, without considering how different hazards may interact with each other at the various levels of the risk assessment chain) is still the general one followed by the disaster risk reduction (drr) community [e.g., kreibich et al. 2014], it often lacks a common framework allowing for the comparability of quantitative risk assessments, both in terms of comparable spatial and temporal scales, and with respect to the metrics being employed. in addition, while several attempts have been made to provide a more holistic view in terms of comparing natural hazard risk curves [e.g., grünthal et al. 2006, schmidt-thomé et al. 2006, garcia-aristizabal et al. 2015], the issue of uncertainties has not usually been considered. these points therefore lead to a number of concerns. consider first the issue of what should be employed as the most appropriate risk metric (a matter of “comparing apples with apples”). such a metric must allow losses from different types of disaster to be meaningfully compared, assisting both responders to disasters and those involved in longer-term planning. for example, in the case of germany, although the summer 2003 heat wave resulted in the highest number of deaths from an extreme natural event for the period 1980-2010 (9,355 people), the associated economic losses were relatively low (€ 1.65 billion) compared to the floods of 2002 (€11.6 billion) which caused the deaths article history received february 10, 2016; accepted may 6, 2016. subject classification: natural hazards, risk, risk curves, risk matrices, mann-whitney. of 27 people (preventionweb website, www.prevention web.net/english/countries/statistics/?cid=66). similarly, comparing numbers of casualties arising from, for example, an earthquake and a hurricane, neglects how there is a greater capacity for early warning (hence the undertaking of mitigating actions) for the latter than for the former, leading to the suggestion that the numbers of people lacking shelter as a result of the event may be a more useful metric [monfort and lecacheux 2013]. the next question deals with the spatial and/or temporal scales being dealt with, each of which depends upon the hazard of concern. considering spatial scales, different hazards have their own spatial pattern, for example, direct losses from floods are generally restricted to lower-lying areas close to water bodies, and so a flood may be rather localized, although in larger events, the extent of direct damage may be considerable (e.g., germany, 2002, 2013, thailand, 2011). by contrast, the area affected by an earthquake is not limited by such factors and primarily depends upon the event’s magnitude, although again, depending upon geological conditions, there may be considerable spatial variability in the resulting ground shaking [e.g., parolai et al. 2007]. with regards to temporal scales, certain hazards display a degree of regularity, for example, seasonal winter storms or hurricanes, while others, such as earthquakes and volcanoes, follow less regular patterns or exhibit longer return periods and thus, must be considered over much longer times. this in turn leads to the difficulties in dealing with how to prioritize high probability/lower damage versus low probability/high damage events. however, a serious associated problem is that historical records may not be adequate to gain a proper understanding of what can be expected over a given time period, let alone potential extreme events. this may lead to the case where more familiar events (e.g., hurricanes) are accommodated, while rarer ones (e.g., earthquakes) are neglected, as was the case for older buildings in kobe, japan, whose heavy roofs were suitable for seasonal typhoons, but not for less-frequent earthquakes [otani 1999]. as mentioned above, individual hazards and risks are generally treated separately, leading to the possibility of underestimating the total risk an area may be threatened by. hence, there is the need to be able to combine the risk estimates associated with each hazard, while presenting uncertainties in a meaningful manner. this capacity is essential in that it allows an understanding of the relative importance of different hazards and risks in order to assist decision makers in their prioritizing of mitigation activities, especially given the frequent lack of resources available. associated with these issues is the need for a robust understanding of the associated uncertainties themselves, not only their actual magnitudes, but their nature when one considers them as being either aleatory, i.e., inherent in the system under consideration, or epistemic, i.e., related to our lack of knowledge about the processes involved. in this paper, we propose a framework that allows the estimation of the total risk arising from multiple independent hazards affecting an area that also allows for risk comparability, while considering uncertainties. it is worth noting that the approach presented in this work does not take into account hazard and risk interactions, which would require a much more in-depth multi-risk analyses [see e.g., garcia-aristizabal et al. 2015, liu et al. 2015]. we generally consider risk in the form of loss estimates from damaged/destroyed residential buildings over annual time scales and urban spatial scales, expressed as expected loss per annum (in euros) versus probability of exceedance. the test case is the city of cologne, the fourth-largest city in germany (1,036 million inhabitants, 2011; statistisches jahrbuch [2012]) and a major industrial, financial, and cultural center. it is also an important transport hub, including one of the largest inland ports in europe and a critical segment in east-west transport across europe. although the level of natural hazard risk in cologne is relatively low compared to many parts of the world, it is still under threat from three major hazard types: earthquakes, windstorms and floods, which are the most important hazards in germany [grünthal et al. 2006, kreibich et al. 2014]. there is also the potential for interactions at various levels of the risk assessment chain, in particular, earthquakes affecting flood defenses and hence increasing flood risk [fleming et al. 2014]. cologne has been a test case for a number of research programs dealing with hazard and risk (e.g., dfnk german research network natural disasters, matrix new multi-hazard and multi-risk assessment methods for europe, sensum framework to integrate space-based and in-situ sensing for dynamic vulnerability and recovery monitoring). specifically, the work presented here is part of the matrix project, which focused on multi-type hazard and risk assessment, including interactions at all levels of the disaster risk chain [e.g., garcia-aristizabal et al. 2013] and different forms of loss, i.e., direct versus indirect, and tangible versus intangible losses, as well as issues related to personal and institutional biases in decision-making within multi-risk environments. however, it by no means ignored the importance of single-type risk assessment and the issues just discussed [parolai et al. 2014]. in the following section we present a method for combining risk estimates leading to the harmonized comparison of natural hazard risk curves. such results fleming et al. 2 3 may be presented using the so-called risk matrix, a table where one dimension represents frequency, probability, etc. of an event, while the other dimension categorizes the event’s consequences, impact, etc. [e.g., cook 2008, cox 2008]. risk matrices are a commonly used form for quantitative and qualitative representation of risks (e.g., the german federal office of civil protection and disaster assistance; bbk [2011]) and were included in the european commission’s guidelines for risk mapping [ec 2010]. this is followed by a discussion that describes the use of the mann-whitney test (also known as wilcoxon-mann-whitney, or u-test; barlow [1989]). the mann-whitney test is a non-parametric test of the null hypothesis that two samples come from the same population (in our case, the samples are from different sources of loss estimates) against an alternative hypothesis that one population tends to have larger values than the other. we employ it to compare the range of risk estimates from different hazards for specific return periods; these ranges in risk estimates arise when considering uncertainties in the input parameters and models. we conclude with a discussion of the steps still required and the consequences such efforts may have on drr decision making. 2. combining single-type risks decision makers often have to deal with the effects of more than one type of risk. for this reason, providing them with information about the likelihood of a given loss value independent of its cause may be an important piece of data for planning purposes. therefore, considering the results of independent, non-interacting single-type risk assessments, we are interested in calculating a “total risk” curve relating the exceedance probability of a given loss value, independent of the risk source (or sources) causing it. if pi(lj) is the probability of exceedance of the jth loss per annum (lj) for the i th risk source (e.g., earthquakes, floods, landslides, etc.), then the total annual exceedance probability curve can be calculated as: p(lj)tot = 1 ∏ (1 − pi(lj)) (1) which is valid for i independent single-type risk sources (i.e., neglecting possible risk interactions). we apply this approach to the risk curves obtained by grünthal et al. [2006], which represent the annual probability of the exceedance of direct loss for each of the three hazards mentioned in the introduction. the exposed elements considered are buildings and contents in the sectors of private housing, commerce and industry, relative to the year 2000. the curves were shown only for the most probable estimates, meaning that uncertainties were not taken into account in the risk comparisons, nor the potential interactions between the different hazards, e.g., the flood estimates do not consider the failure of river dikes. figure 1 shows the three risk curves of grünthal et al. [2006] for earthquakes, floods and windstorms, along with each of the possible combinations among them single-type hazard and risk harmonization figure 1. the individual risk curves for the three main hazards (earthquakes eq, floods fl, windstorms ws) that affect cologne, as presented by grünthal et al. [2006], and their various combinations derived using equation (1). according to equation (1). note the original values from grünthal et al. [2006] were losses found for a series of probabilities. we took these and, from the loss estimates themselves, interpolated where necessary in order to find the resulting probability for all three hazards together exceeding a given loss. it is notable, although expected, that for the loss-range over which all hazards have results, the resulting combination of the three curves differs little from combining only floods and windstorms, which are the dominant risks for higher probability/low loss events. however, if, for example, we consider all risk-types for the cases of losses of the order of €100 million and combine them, the resulting curve indicates a greatly enhanced probability, from 15 to 35% in 50 years for the individual hazards, to over 50% in 50 years when combined, all the more important when one considers that potential interactions are still not considered. now we present such changes in risk by means of a risk matrix. figure 2 shows an example of a risk matrix for cologne using some of the estimates arising from the three hazards discussed in figure 1. the ranges of impacts (here economic losses, but such a format can be used for other forms of losses such as casualties) and likelihoods (annual probabilities of exceedance) have been divided into five categories over a logarithmic scale. for the likelihood, we employed the ranges presented in bbk [2011] and shown in table 1, where the probabilities range from ≤0.1/year for the upper range of very likely to ≤0.00001/year for the upper limit of very unlikely. for the impact, we adopted a similar form, with insignificant having a lower bound of ≥€1 million and disastrous being ≥€10 billion (see figure 1 of bbk [2011]). it should be pointed out, however, that we use these bounding values for only illustrative reasons and that different limits may be set based on more extensive expert judgment. similarly, we use the same color ranking of risk (expressed as likelihood x impact) as presented in bbk [2011], although we have modified slightly the actual distribution of the colors, recognizing, however, that according to some authors [e.g., cox 2008], the color distribution itself should be modified (again, this exercise is for illustrative purposes). for this case, considering earthquakes, it is easily recognizable how the impact is significant for events with recurrence periods of 10,000, bordering between significant and moderate for 1,000 years, and minor for those with return periods of the order of 100’s years. to show how the situation changes when we consider the combining of risk estimates, we present in fleming et al. 4 figure 2. risk matrix (exploiting the values presented in figure 1) showing how combining the risk associated with individual perils (eq earthquake, fl flood, ws windstorm) can lead to a significantly higher probability of exceeding a given level of loss (eq+fl+ws). the individual and combined risk estimates outlined by the ellipse correspond to the annual exceedance of losses of ⇔100 million, hence why they are all along the same impact row. the ranges for the different classifications are presented in table 1. the color scheme is derived from that used by bbk [2011]. 5 figure 2 the combination of the three risks probabilities that give an approximate loss of €100 million/annum. these examples are outlined by the ellipse, where the result of combining the windstorm (triangle), earthquake (diamond) and flood (square) is shown by the circle. as we are looking at the likelihood of occurrence here, all values have the same impact, i.e., aligned along the same y-axis value, hence they only vary in their location along the likelihood or x-axis. it can be seen how the total risk has moved towards the right from “likely” to “very likely”. one therefore can see how this form of presentation allows changes in risk to be identified, at a qualitative level, when considering different factors. in other words, it allows a ready means of presenting how the consideration (or otherwise) of different hazard combinations, as well as their interactions, may be seen and understood [e.g., mignan et al. 2014]. one can also imagine how, based on expert opinion, the relative distribution of the risk color scheme and boundaries between the impact and likelihood classifications may be altered to better reflect the case at hand. 3. prioritization of risk considering uncertainties in this section, we make use of a series of risk estimates that take into account uncertainties for the natural hazards relevant to cologne. for each peril, the same metric is used, that is direct damage to residential buildings, and the same total costs (which differ from those of grünthal et al. [2006], who also considered commercial buildings and other items). the value of the exposed assets was derived by considering commercial geomarketing data provided by infas geodat [infas 2010]. note that we did not use the results outlined below for the previous exercise, since for flood risk, estimates for only three return periods were available. we also note that the following are not intended to replace the previous results, but are meant to demonstrate our framework for allowing a quantitative comparison between hazard and risk types. for the seismic risk, the estimates were obtained using a logic tree approach that considers a range of input parameters for the hazard [e.g., grünthal et al. 2010, crowley et al. 2011a, 2011b], exposure, vulnerability [e.g., tyagunov et al. 2006, infas 2010, wieland et al. 2012] and loss components (e.g., ems-98; hwang et al. [1994]), representing different models and sources of information (see for details tyagunov et al. [2014]). in the end we obtained a family of 180 seismic risk curves, representing the risk in terms of probable monetary losses ranging from millions to billions of euros. what is important to note is that the calculated risk estimates show considerable uncertainties. the observed large spread can be explained by our assigning of equal weights to the different (both the regional and adapted) models used in the risk calculations. furthermore, comparing the uncertainty distribution for different return periods, one can observe the bimodal shape of the distribution, which is due to the fact that some of the used models neglected the contribution of small earthquake magnitudes. that is why the bimodality is especially distinct at lower hazard levels/shorter return periods, while for larger earthquakes/longer return periods the models give closer risk estimates and therefore the spread appears to lessen. the flood risk was derived using a coupled hybrid probabilistic-deterministic dike breach-hydrodynamic model iham [vorogushyn et al. 2010]. the model was setup for the rhine reach between river gauges andernach and düsseldorf, including a total of 123 river kilometers with the target area of cologne located in the middle of the reach. the iham model was run in a monte carlo simulation for scenarios with return periods of 200, 500 and 1,000 years, resulting in a set of about 1,000 inundation patterns for each return period. based on these inundation scenarios, economic damages to residential buildings were estimated using seven flood damage models to reflect the uncertainty associated with different damage model structures. these are four stage-damage curves [murl 2000, hydrotec 2001, 2002, icpr 2001, merz et al. 2013], multi-parameter flood loss estimation models (flemops, thieken et al. [2008]; flemops+r , elmer et al. [2010]) and the decision tree-based model rt2 [merz et al. 2013]. besides the damage model structure uncertainty, uncertainty single-type hazard and risk harmonization value likelihood classification annual probability expected return period impact classification lower value (× 106 euros) 5 very likely ≤ 0.1 10 disastrous 10,000 4 likely ≤ 0.01 100 significant 1,000 3 conditionally likely ≤ 0.001 1,000 moderate 100 2 unlikely ≤ 0.0001 10,000 minor 10 1 very unlikely ≤ 0.00001 100,000 insignificant 1 table 1. the bounding limits for the risk matrix presented in figure 2 (for the likelihood, see table 3 in bbk [2011], for the terms of likelihood and impact, see figure 1, bbk [2011]). associated with flood hydrograph and dike breach processes was also taken into account. the windstorm risk estimates employed the vienna enhanced resolution analysis or vera tool [steinacker et al. 2006]. the vera analysis provided gridded data of the 10 minutes mean wind for the area of cologne for a time period of 35 years (1971 to 2005). to estimate statistical uncertainty, a scheme was used that includes monte carlo simulation methods. as a result, a number of hazard and risk curves were produced (some 9,000,000 in this case), allowing a range in the overall uncertainty to be derived. the building damage estimation method of heneka and ruck [2008] was employed, where the monetary damage to buildings is proportional to gust wind speed, and the amount of damage is a function of the relative wind (i.e., the ratio of maximum wind gust speed during the event and the wind gust speed of a 50 year return period event). the losses are based on the reconstruction costs of residential buildings using the disaggregated data developed for the flood risk and the same totals as in the other hazards (see also heneka and hofherr [2011]). the objective of this comparative risk assessment is to assess if the losses arising from two independent typologies of hazards for a specific return period are significantly different. for this, we use the distributionfree ranking mann-whitney test mentioned above [e.g., barlow 1989]. this involves testing a null hypothesis asserting that two variables have the same probability distribution against an alternative hypothesis that one population tends to have larger values than the other [e.g., hollander and wolfe 1999]. this test has been used in a variety of hazard and risk related studies, for example, temporal changes in wildfires [salis et al. 2014] and inter-regional comparisons with respect to meteorological hazards [bommer and senkbeil 2010]. since in our cases the population distributions might not be normal for the different hazard types, we decided to repeat the test 10,000 times by resampling randomly each population (the number of tests was found by trial and error, in fact, the results differ little if only 1,000 tests were used). this is to reduce the consequence of situations where the random selections of samples are clustered in some way. the results are analyzed in term of the frequency of rejecting the null-hypothesis. we consider each pair of hazards (earthquake flood, earthquake windstorm, flood windstorm) for the return periods 200, 500 and 1,000 years when comparing earthquakes and floods, and 200 and 500 years for floods and windstorms, and windstorms and earthquakes (figure 3). fleming et al. 6 figure 3. comparing the distribution of results for each pair of risks. (ac) floods (green, fl) and earthquakes (red, eq) for (a) 200, (b) 500 and (c) 1,000 year return periods, (d-e) floods and windstorms (blue, ws) for (d) 200 and (e) 500 years, (f-g) windstorms and earthquakes for (f ) 200 and (g) 500 years. the vertical lines of the same colors are the respective medians. 7 considering first the earthquake loss distribution, we immediately note its tendency to have a bimodal character, especially for shorter return periods (see above and tyagunov et al. [2014]). this is an example of how an unreliable comparison may arise if we simply considered the resulting medians or averages of two distributions of risks. this is the added value of propagating uncertainties arising in the risk assessment process up to the resulting risk curve, as done in this work. to objectively compare risk curves obtained for different phenomena taking into account uncertainties, we confront the variability of these risk curves at different return periods (figure 3). in practice, we extract “slices” at different return periods and compare the resulting distributions in order to define if they are significantly different or not. from the results of the mann-whitney test comparing earthquake and flood loss distributions at different return periods, we observe that for the 200 year return period (figure 3a), the null-hypothesis is rejected, i.e., the two distributions are different. for the 500 and 1,000 years return periods (figure 3b and 3c, respectively), the null-hypothesis cannot be rejected and therefore these distributions can be considered to be comparable. these results indicate that when considering loss levels expected with a 200 return period, the losses tend to be higher for floods with respect to earthquakes; conversely, for longer return periods (i.e., 500 and 1,000 years), the losses from earthquakes and floods in this area are of the same order of magnitude. on the other hand, comparing the losses caused by windstorms and floods for both the 200 (figure 3d) and 500 (figure 3e) years return periods, the distributions can be considered as being significantly different, with floods of greater concern in both cases. finally, comparing losses caused by earthquakes and windstorms (figure 3f-g), these appear for the 200 year return period (figure 3f ) to be comparable, while for 500 years (figure 3g), this does not appear to be the case (with higher losses expected from earthquakes). 4. conclusions a simple method for combining different independent single-risk curves to derive a total exceedance probability loss curve, along with a means of graphically showing how total risk changes as one combines the individual components (namely the risk matrix) is demonstrated for the case of cologne. as other authors have demonstrated [e.g., komendantova et al. 2014, mignan et al. 2014], risk matrices are also useful in showing how total risk varies when interactions are considered, providing a bridge between the qualitative analysis commonly used by the drr practitioners and the quantitative assessment provided by the scientific community. however, some authors have expressed reservations as to the accuracy of the value of such schemes as risk matrices [e.g., cox 2008] and so care must be made when employing this method of presentation of results. a means of assessing the significance in the differences (or similarities) between pairs of risk types when considering a range of plausible values for a given return period was examined. in a rigorous quantitative context, the approach considers the uncertainties taken into account in the assessment of the single risks. such an exercise may be of benefit to the decision making process, whereby if the risks associated with two types of hazard are statistically significantly comparable (i.e., they have a similar likelihood of inflicting the same amount of damage), then the required risk management schemes may need to consider both of them for cost/benefit analyses, or at least help decision makers when deciding on how to allocate resources. for example, while the losses for two different hazards may be of the same order of magnitude for a given return period, it is possible that implementing mitigation actions for one may be much more expensive than for the other. it also shows that one needs to accommodate uncertainties, since as commented above (and suggested by the earthquake risk results’ distribution) simply using, for example, average curves, may yield misleading conclusions about the relative importance of a given combination of hazard types. however, it is also important to note that the actual results for the test case would vary as the range of employed input models and parameters are updated (i.e., the epistemic uncertainty is refined as our knowledge improves). there is also the point that the extent of the uncertainties may (or rather should) influence decision making, as a wide uncertainty range may suggest both serious and negligible consequences. still, the presented approach potentially allows the combination and comparison of updated risk estimates in an effective way using the proposed framework. acknowledgements. the work leading to this document has received funding from the european community’s seventh framework programme [fp7/2007-2013] under grant agreement no. 265138 (“new multi-hazard and multi-risk assessment methods for europe” or matrix). the documents referred to as project deliverables may be obtained from the project website (http://matrix. gpi.kit.edu/deliverables.php) where further details as to the computation of the risk curves may be found. references barlow, r.j. 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6 (4), 573-586; doi:10.5194/nhess-6-5732006. tyagunov, s., m. pittore, m. wieland, s. parolai, d. bindi, k. fleming and j. zschau (2014). uncertainty and sensitivity analyses in seismic risk assessments on the example of cologne, germany, natural hazards earth system science, 14, 1625-1640; doi:10.51 94/nhess-14-1625-2014. vorogushyn, s., b. merz, k.-e. lindenschmidt and h. apel (2010), a new methodology for flood hazard assessment considering dike breaches, water resource research, 46 (w08541); doi:10.1029/2009 wr008475. wieland, m., m. pittore, s. parolai, j. zschau, b. moldobekov and u. begaliev (2012). estimating building inventory for rapid seismic vulnerability assessment: towards an integrated approach based on multi-source imaging, soil dynamics and earthquake engineering, 36, 70-83. * corresponding author: kevin fleming, gfz german research centre for geosciences, centre for early warning, potsdam, germany; email: kevin@gfz-potsdam.de. © 2016 by the istituto nazionale di 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 497_511.pdf 497 annals of geophysics, vol. 45, n. 3/4, june/august 2002 nonparametric analysis of the time structure of seismicity in a geographic region graciela estévez-pérez (1), henrique lorenzo-cimadevila (2) and alejandro quintela-del-río (3) (1) departamento de matemáticas, facultad de ciencias, universidad de a coruña, spain (2) departamento de ingeniería de los recursos naturales y medio ambiente, escuela de ingeniería técnica industrial, universidad de vigo, spain (3) departamento de matemáticas, facultad de informática, universidad de a coruña, spain abstract as an alternative to traditional parametric approaches, we suggest nonparametric methods for analyzing temporal data on earthquake occurrences. in particular, the kernel method for estimating the hazard function and the intensity function are presented. one novelty of our approaches is that we take into account the possible dependence of the data to estimate the distribution of time intervals between earthquakes, which has not been considered in most statistics studies on seismicity. kernel estimation of hazard function has been used to study the occurrence process of cluster centers (main shocks). kernel intensity estimation, on the other hand, has helped to describe the occurrence process of cluster members (aftershocks). similar studies in two geographic areas of spain (granada and galicia) have been carried out to illustrate the estimation methods suggested. 1. introduction the problem of searching for stochastic models to describe the sequence of occurrence times of earthquakes from some geographic region is of great interest to seismologists. in effect, a detailed analysis of such process might reveal new aspects of the pattern of occurrence of earthquakes, and suggest important ideas on the mechanism of earthquakes. the development of detailed stochastic models to describe the list of origin times or equivalently that of time intervals between consecutive earthquakes is quite recent. vere-jones (1970) surveys some of the stochastic models (clustering models and stochastic models for aftershock sequences) proposed in the literature and describes their behavior in several data sets. more specifically, udias and rice (1975) propose the gamma distribution to describe the series of time intervals between consecutive shocks. these authors also deal with hazard and intensity functions. other more recent models include the trigger models (lomnitz and nava, 1983), the epidemic-type aftershock sequence (etas) model (ogata, 1988), whose extensions can be seen in ogata (1998), or refinements of hawkes’ (1971) self-exciting point process model, which describes spatial-temporal patterns in a catalog. mailing address: dr. graciela estévez-pérez, departamento de matemáticas, facultad de ciencias, universidad de a coruña, rua de maestranza s/n, 15003 a coruña, spain; e-mail: graci@udc.es. key words nonparametric estimation hazard function intensity function clustering dependent data 498 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río however, standard models applied to seismic data do not always fit the data well. in part, this is because parametric models are usually only well suited for a sequence of seismic events that have similar causes. moreover, parametric models can be insensitive to poorly-fitting events, which often are at least as interesting as wellfitting ones (see ogata, 1989). in this article, we suggest nonparametric methods for analyzing seismic data. these methods do not require formulation of structural models, so they are not affected by the deficiencies noted above. they involve several different approaches to nonparametric estimation of the hazard and intensity functions of point processes that evolve with time. this enables us to split up and analyze the occurrence of temporal processes of earthquakes within a region without constraining them to having predetermined properties. we argue that nonparametric methods for the analysis of earthquake data are valuable supplements to more conventional parametric approaches, especially as tools for exploratory data analysis. the objective of our analysis is to show two statistical tools (hazard and intensity functions) which could help to describe the whole cycle of seismic activity in a region without imposing predetermined conditions on this activity. that is, our analysis is based on the information provided by the data and on the universally accepted assumption of temporal grouping of earthquakes. the hazard function is used to confirm this grouping and characterize the occurrence process of main shocks. on the other hand, the aftershock sequences (clusters) have been studied by means of the intensity function. the paper is organized as follows: in section 2 we describe the occurrence process of earthquakes in terms of its evolution in time. section 3 introduces the nonparametric estimator of hazard function and section 4 contains the analysis of seismic activity of two spanish geographic regions using the nonparametric methods beforehand mentioned. 2. the occurrence process of earthquakes earthquakes can be represented by point events in a five-dimensional space-time-energy continuum ( i , i , h i , t i , m i ) where i and i are the latitude and longitude of the epicenter, h i the depth of the focus, t i the origin time and m i the magnitude. a complete statistical analysis of earthquakes must consider the distributions and correlations of these five parameters. this would involve handling a five-dimensional series, which generally constitutes a very complex problem. the starting point of such problem is the consideration of the onedimensional series of occurrence times {t i }. to give a precise meaning to this time series, its space, time and magnitude boundaries must be specified. obviously, these boundaries will be chosen according to the objectives of the study: to characterize different seismic areas in the same time period, to analyze several seismic series in a particular region, etc. space specifications define the volume from which the population of earthquakes, represented by the time series {ti } is taken. this may be done in practice by specifying an area a and a lower limit in depth h. since detectability is limited, a lower limit of magnitude m0 must also be specified. this limit is a function of the station distribution and sensitivity, and defines the lowest magnitude for which all events from anywhere in the bounded region can be detected. a bounded set of earthquakes will then be a series {t i } defined for a certain area a, depth h and lower limit of magnitude m 0 , such that, it would include all shocks originating from inside the defined volume of magnitude larger than m0 for a particular time interval from t0 to tn. once a bounded set of time occurrence {t i } is established, a series can be constructed with the time intervals between consecutive earthquakes { t i }, such that t i = t i t i 1 for i = 1, ..., n. the distribution of the values of these intervals is of great interest to specify the time structure of the seismicity of a region. the simplest statistical model to fit a series of occurrence times of earthquakes is the poisson process, under which the time intervals between consecutive events are exponentially distributed. this model presupposes independence of the events, so that the occurrence of one earthquake is not influenced by that of previous ones, which is very far from reality. in effect, several authors (knopoff, 1964; lomnitz, 499 nonparametric analysis of the time structure of seismicity in a geographic region 3. kernel estimation of hazard function the distribution of time intervals between consecutive earthquakes can be characterized using the hazard function, which is defined by where t is the random variable that measures the time between consecutive shocks and p (. .) indicates the conditional probability. this function can also be written as with f (.) and f(.) being the density and distribution functions of t , respectively. thus, r(t) �t can be interpreted as the approximate probability of a shock in the time interval (t, t +�t) given that the immediately preceding one happened at time 0. in other words, it is considered the instantaneous hazard earthquake occurrence at time t. nonparametric estimation of hazard function started with watson and leadbetter (1964 a,b) who introduced the kernel estimator (see (3.1)), and from that time on many papers on this topic have appeared in the literature (see e.g., hassani et al., 1986 for a survey). most of this literature is based on the assumption of independence of the data. however, in the case of microearthquake studies, for instance, the hypothesis of dependence, that is, the existence of causality or interaction between the occurrence times of shocks, is more suitable (rice and rosenblatt, 1976). papers on dependent hazard estimation include sarda and vieu (1989), vieu (1991), estévez and quintela (1999, 2002) and many others. throughout this paper we will use the kernel estimator of the hazard function (watson and leadbetter, 1964 a,b) because it has been studied in great detail in dependence contexts. it is defined as (3.1) 1966; vere-jones, 1970; udias and rice, 1975) have found, for different populations of earthquakes, that the poisson fit to the time series of microearthquakes is very poor, especially for active periods. the deviation from the model is principally due to the existence of a much larger number of small intervals than expected. the reason for such a large number of small intervals is that the earthquakes happen forming clusters, that is, one main shock is followed and/or preceded by a stream of smaller shocks, called aftershocks and/or precursors (lomnitz and hax, 1967; vere-jones and davies, 1966) produced in the same general focal region. therefore, some authors (e.g., vere-jones, 1970; hawkes, 1971 and many others) have defined the occurrence process of earthquakes in terms of two components: i) a process of cluster centers; and ii) a subsidiary process defining the configuration of the members within a cluster. the final process is taken as the superposition of all the clusters. several possible models for these processes can be found in the literature, for example the compound poisson processes (vere-jones and davis, 1966), trigger models (vere-jones and davies, 1966; lomnitz and nava, 1983) or epidemic-type models (hawkes, 1971; lomnitz, 1974; ogata and akaike, 1982; ogata, 1988 and their references). all these models assume structural conditions on the occurrence of earthquakes, as for example, that the process of cluster centers is stationary and poissonian. in section 4.1, we will show that this hypothesis is not very likely either for particular cases. a drawback of these approaches is that they depend very much on the models, and so are subject to the instability and goodness of fit problems noted in section 1. because the standard parametric models do not always fit the earthquake time series well, we suggest making use of nonparametric analysis techniques. as mentioned in the previous section, nonparametric methods for analyzing the distribution of time intervals between consecutive earthquakes will be considered, obtaining another attempt to describe temporal behavior of an earthquake series in a geographic region. r t p t t t t t tt ( ) = < +( ) lim � � �0 r t f t f t ( ) = ( ) ( )1 r t f t f t h h h ( ) = ( ) ( )1 500 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río estévez and quintela (1999, 2002) proposed two versions of the cross-validation method to select the bandwidth h when the data are dependent: the global and the local choice. the first one selects only one h, searching the lowest global estimation error. that is, if the global estimation error is the value of cross-validation bandwidth used by these authors is h cv , that satisfies , with (3.2) where and are the kernel estimators of f (t) and f(t), respectively; k(.) is a kernel function, , and h = h(n) r+ the smoothing parameter or bandwidth. this parameter is crucial in the estimation method since the shape of the resulting estimator varies greatly according to its value. f t nh k t t hh i i n ( ) = = 1 1 ( ) f t f x dx n h t t h h h i n it( ) = ( ) = = 1 1 0 ( ) h t k x dx t ( ) = ( ) mise h e r x r x dxh( ) = ( ) ( )( ) 2 h cv hcv h = ( )argmin cv h r x dxh( ) = ( ) 2 x n f x f x f x h i i h i i n ii n ( ) ( )( ) ( )( )=1 2 1 1 fig. 1. seismicity of granada in the period 1983-1999. 501 nonparametric analysis of the time structure of seismicity in a geographic region an estimation of global error mise(h). the functions f h i(.), f h i(.) in (3.2) are kernel estimations of density and distribution functions modified to take into account the dependence of the data and fn(.) is the empirical distribution function. note that mise(h) depends on r (.) that is an unknown function and hence it is necessary to estimate it. on the other hand, the local cross-validation method takes a different h cv, x for each estimation point x, obtaining the lowest estimation error for each x. in this case, with cv x (h) an estimator of local error mise x (h) = e(r h (x) r(x))2. we will illustrate these two approaches in the sections that follow. 4. examples in this section we try to study and compare the seismic activity of two spanish geographic regions: granada in the southeast and galicia in the northwest of spain. 4.1. granada earthquakes data the data-set to study is a group of microearthquakes recorded during the period 19831999 with epicenters between 36.5° to 37.7°n and 3.5° to 4.5°w and magnitude larger than or equal to 2.5 on the richter scale. the scatter plot of epicenters (fig. 1) for the n = 1117 shallow shocks (focal depths of less than 30 km) illustrates the seismicity of the region studied. basic descriptive plots of the event magnitudes are given in fig. 2a,b. the first one presents the cumulative number of shocks with time with the associated magnitudes. the second one represents the distribution of magnitudes in this region. note in particular the straight line fit in (b), which indicates that the gutenberg-richter h cv hcv x h x, = argmin ( ) fig. 2a,b. basic descriptive plots of the seismic data: a) cumulative number of shocks and plot of the magnitudes versus the occurrence times; b) cumulative distribution of magnitudes. a b 502 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río time intervals between consecutive earthquakes { t i } i =1 1116 (fig. 4) show a prominent trend towards clustering, particularly clear in april, 1998 with more than 100 events per 5 days. note that large time intervals ( t i ) indicate slack periods and times close to 0 show strong clustering. arrows pointing downward at the upper part of the histogram mark the occurrence of one or more earthquakes (the number of bars indicates the number of shocks) with magnitude equal to or larger than 4 on the richter scale. the average number of earthquakes per day is 0.18 but the activity, as is shown by figs. 2a, 3 and 4, is not uniform with time. figure 2a shows five time periods whose seismic activity seems fairly different. combining the information provided by this graph and fig. 4, we can distinguish: a quite active time span from 1985 to 1989, a slack span from 1990 to 1995, and finally, strong seismic activity in april 1998. thus, it is obvious that the process { t i } i = 1 1116 is not stationary. a possible cause of the lack for stationarity could be that the studied time period is quite short because, as ogata (1988) points out, «stationary models are considered here in the prior belief that such geophysical activity for a long time span should be stationary». concerning the hypothesis of dependence of the data, the theory of runs and the analysis of autocorrelations establish that this hypothesis is verified. in effect, the run test up and down gives a p-value of 0.039, the run test above and below the median produces 2.81.10 18and the autocorrelation function in fig. 5 gives autocorrelations significantly different from zero. the previously mentioned non-stationarity makes the estimation of hazard function difficult, however, the transformed process y i = ln tj + 1 ln tj ; j =1,..., n, which is stationary as shown in fig. 6, solves the problem. in effect, as and the random variable ln t n+1 conditioned to has the same distribution as , that is (1944) law of magnitude frequency holds for these data. this relation suggests that the catalog includes essentially all the events of this magnitude that occurred. the histogram for the origin times in hours {t i } i = 1 1117 (fig. 3) and the sequence graph for the fig. 3. time history of seismic activity during the period 1983-1999 given in number of shocks per 208 days. arrows indicate the occurrence of one or more earthquakes with magnitude m 4. y t tn n n= +ln ln1 t t t t tn n n1 1 1,..., ,...,ln +( ) ln lnt t t y t t tn n d n n n+ +( )1 1 1,..., ,..., y t t t n n n +( )ln 1,..., fig. 4. sequence of time intervals between consecutive earthquakes { t i } i =1 1116. 503 nonparametric analysis of the time structure of seismicity in a geographic region so, first we compute with k(u) = the epanechnikov kernel and h calculated in the same way as in estévez and quintela (1999). then, by means of a translation and a change of variable, we estimate the hazard function of time intervals between earthquakes, given that the last one has happened at time t n 17/12/1999 at 22:31:59.06 h and t n = e 7 . 5= 1807.65 h, obtaining the curve in fig. 7. the shape of this curve (high hazard for small times and low hazard in other cases) suggests a prominent clustering. moreover, the last time interval, t n = e 7 . 5= 1807.65, plays an important role in the estimation process: if, for example, t n was equal to e3.9 5= 51.93 h or e5.25= 190.57 h (the two last shocks could be closer), the instantaneous hazard earthquake occurrence at time t n (dotted line and dashed line, respectively) would be higher. the main drawback of this method is that we can only estimate the hazard for time tn. now, let us suppose that the time series { t i } is stationary, that is, that we have not detected the lack of stationarity. in such case, we would estimate the hazard function of { t i } using for t > 0 and h the cross-validation bandwidth (estévez and quintela, 1999). by comparing this curve (fig. 8) with the previous ones, we can conclude that both methods engender very the hazard function of time intervals between consecutive earthquakes, given that the last registered time interval was t n , is estimated using the hazard function estimation of y n (r h,1 (.)). fig. 5. sample autocorrelation function for { t i } i =1 1116. fig. 6. sequence graph of transformed process { y j }. r y f y f y nh k y y h n h y y h h h h i i n i i n , , , 1 1 1 1 1 1 1 1 1 ( ) = ( ) ( ) = = = ( ) ( ) { 34 1 2( )u 0 r t f t f t h h h ( ) = ( ) ( )1 if u [ 1, 1] if u [ 1, 1] 504 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río fig. 7. conditional hazard at t n (solid line). hazard estimation for ln t n = 3.95 (dashed line) and for ln t n = 5.25 (dotted line). fig. 8. hazard estimation under stationarity. similar estimations when ln t n is not an outlier. so, if we are interested in estimating the hazard at a particular time tn, the first procedure, which takes into account the information provided by tn, should be used. however, to estimate the hazard function globally, the stationary could be ignored and the second procedure, which is easier, would be used. in both cases the estimated hazard function suggests the natural strong clustering of the occurrence times of shocks. by defining a cluster as a set of earthquakes originating from a relatively small volume and separated in time by intervals smaller than a fixed duration («cluster length»), and cluster center as a representative of the cluster (the first shock, for instance), the occurrence process of earthquakes is taken as the superposition of all the clusters. in this problem we have considered clusters with «cluster length» of less than 120 h, obtaining a set of independent cluster centers. that is, after removing aftershocks and precursors, the remaining shocks can be taken as independent events. a length of 120 h was chosen because it was the smallest value producing independent shocks, that is, removing the effect of the aftershocks. figure 9a,b presents the sequence graphs of the cluster sizes (fig. 9a) and the magnitudes of the centers (fig. 9b). by comparing fig. 9a,b with fig. 3, we observe that the stronger earthquakes occur at the start of or within small clusters. in other words, the great clusters are formed by events of small magnitude. the largest cluster will be studied later. we begin by analyzing the occurrence process of cluster centers by means of the distribution of time intervals between them { t i �} i=1 345. these events happen independently (the p-values of runs test above and below the median, and up and down are 0.553 and 0.522, respectively) but they do not have exponential distribution (the p-value of the kolmogorov-smirnov test for goodness of fit is less than 0.01). thus, such centers do not form a stationary poisson process contrary to what vere-jones and davies (1966), vere-jones (1970) and udias and rice (1975) used. since several tests for goodness of fit show that any known model of distribution seems suitable to describe the distribution of { t i �} i=1 345, we resort to nonparametric methods to estimate this distribution. in particular, we use the kernel estimator of hazard function (3.1). figure 10 presents this estimation for { t i �} i=1 345 when the bandwidths have been globally selected (see estévez and quintela, 1999) (solid line) and locally selected 505 nonparametric analysis of the time structure of seismicity in a geographic region fig. 10. hazard estimation of time intervals between cluster centers with global bandwidth (solid line) and local bandwidths (dashed line). fig. 9a,b. a) sequence graph of cluster sizes (a total of 346); b) sequence graph of magnitudes of cluster centers. (see estévez and quintela, 2002) (dashed line). the low values of hazard function and the small fluctuations throughout indicate that clusters happen with a low occurrence rate and are almost constant with time. the occurrence process of the members within the largest cluster is formed by 133 events which happened from april 11 to april 30, 1998. their epicenters are concentrated in a small area close to loja, southwest of granada (spain). although the number of shocks per day was large (6.65), none exceeded magnitude 3.9 on the richter scale. the sequence graph of time intervals between consecutive cluster members { t i ��} i=1 132, shown in fig. 11, indicates that such process is not stationary and the last span t132 �� is large compared to the other ones. therefore, it will not be possible to estimate the distribution of time intervals between cluster members. the behavior of the cluster will be studied using the process of origin times {t i ��} i=1 133 and its intensity function. this function gives, for each time t , (t) the average number of earthquakes per unit of time, t hours after the main shock (cluster center). the advantage of using the intensity function is two-fold: firstly place is not affected by the lack of stationarity of the time intervals between shocks. secondly, it clearly shows the evolution of the cluster with time, and therefore, it is suitable for comparing the seismic activity of several geographic regions. in this work, we propose to estimate the intensity function by means of a kernel estimator, which a b 506 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río is defined as (wand and jones, 1995; choi and hall, 1999 and their references), where k(.) is the kernel function and h the bandwidth, as in (3.1). for this example, the estimated curve appears in fig. 12, showing a clear increase in the number of shocks at the beginning of the cluster until the time with highest intensity (approximately at t = 30 h) and then a decrease. 4.2. galicia earthquakes data seismic activity increased in this region during the 1990’s. the data analyzed correspond to the 978 earthquakes occurring from january fig. 11. sequence graph of time intervals between consecutive cluster members. fig. 12. intensity estimation of the largest cluster. fig. 13. seismicity of galicia in the period 19872000. ˆ , '' t h k t t h t r i i n ( ) = = + 1 1 ( ) fig. 14. cumulative distribution of magnitudes. 507 nonparametric analysis of the time structure of seismicity in a geographic region 15, 1987 to may 3, 2000. their epicenters, scattered throughout galicia (fig. 13), show small spatial groupings at high risk areas: sarria, monforte and celanova. the magnitude distribution of this region is shown in fig. 14, which supports the gutenbergrichter law of magnitude frequency. the histogram of origin times {t i } i =1 978 , the sequence of time intervals between consecutive shocks { t i } i=1 977 and the graph of the cumulative number of shocks with their magnitudes are shown in fig. 15a-c. these graphs show a quiet period until c fig. 15a-c. a) histogram of origin times {t i } i =1 978, number of shocks per 208 days; b) sequence graph of time intervals between consecutive earthquakes { t i } i =1 977; c) cumulative number of shocks with sequence graph of magnitudes. a b 508 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río september 1995 and three temporal groupings in december 1995, may 1997 and may-june 1998, which are related to the strongest earthquakes. the hazard function estimation (fig. 16), which was constructed as in the previous example, presents a common shape: it decreases suddenly in the first hours and then it fluctuates around a small risk. this shape confirms the well known fact that earthquakes form clusters. similarly, as in the granada case, 206 clusters with «cluster length» less than 144 h have been formed. the sequence of sizes and magnitudes of cluster centers (fig. 17a,b) show three important clusters, the first one beginning with a big shock (4.7 on the richter scale). figures 17a,b and 15c also suggest that the strongest earthquakes are related to small clusters, that is, such events are not followed or preceded by many earthquakes. the hazard estimation of time intervals between cluster centers (fig. 18) suggests small and stable risk of clusters in time. however, as in the granada case, the distribution of time intervals is not exponential either (the p-value of the kolmogorov-smirnov test for goodness of fit is 0.0006). finally, the occurrence of cluster members in each grouping was analyzed using the intensity function. the first cluster (fig. 19a) is composed of 147 shocks, whose epicenters are contained in a small area close to sarria (lugo). the intensity estimation indicates that: i) the cluster begins with a high number of shocks (the first of magnitude 4.6); ii) there is another grouping of fig. 16. hazard estimation of time intervals between consecutive shocks. fig. 17a,b. a) sequence graph of cluster sizes (a total of 206); b) sequence graph of magnitudes of cluster centers. a b 509 nonparametric analysis of the time structure of seismicity in a geographic region events coinciding with another earthquake of magnitude 4.6, and iii) the intensity function is low in the remainder of the range. the second cluster (fig. 19b) consists of 190 earthquakes, whose epicenters are also near sarria (lugo). the shape of the intensity function reflects the behavior of the cluster members: a small group of precursors warns that important shocks are to arrive (one of magnitude 5.1 and another of 4.9) and then there is a sequence of aftershocks. finally, the third cluster (fig. 19c) involves 79 events, with epicenters in celanova (orense). this is the weakest cluster (few shocks of low magnitude). its estimated intensity function shows fluctuations around quite small values the whole time. 5. conclusions in this work we show how the hazard and intensity functions represent another way of describing the temporal structure of seismic activity in a geographic region. kernel estimation of hazard function has confirmed what verejones (1970), udias and rice (1975) and many others have noted: earthquakes have the tendency fig. 18. hazard estimation of time intervals between center clusters with global bandwidth (solid line) and local bandwidths (dashed line). fig. 19a-c. intensity estimations for important clusters: a) december 1995; b) may 1997, and c) mayjune 1998. c a b 510 graciela estévez-pérez, henrique lorenzo-cimadevila and alejandro quintela-del-río to group. the occurrence process of these groups was also studied by means of the hazard function and each important cluster has been described using the intensity function. as we argued, a major advantage of nonparametric methods is that they do not require formulation of structural models, which are often well suited only for data that have closely related seismic causes. another novelty is that we take into account the possible dependence of the data to estimate the distribution of time intervals between earthquakes, which has not been considered in most statistics studies on seismicity. in reference to the results obtained on the seismic activity of granada and galicia, we may point out that: due to the effect of aftershocks, the hazard function of time intervals between consecutive shocks in an earthquake catalog will present a shape as that shown in fig. 8 and 16. its greater or smaller above zero is an indicator of the intensity of seismic activity of each region. in granada and galicia the maximum values of hazard are similar when shocks with magnitude from 2 to 5 are considered. next, to analyze the seismic activity of the granada and galicia region we formed clusters lasting less than or equal to 120 and 144 h, respectively. in both cases, a sequence of main shocks mutually independent with low rate of occurrence and almost constant with time has been obtained. thus, consecutive earthquakes far away from 120 or 144 h do not present interaction. the processes of main shocks seem very similar for both regions. concerning the greatest clusters, although the intensities take different shapes and values (see fig. 12 and 19a-c), there is a clear predominance of shocks at the beginning of clusters. however, the granada cluster is shorter and intense and in the galicia region the aftershocks are more aloof in time. moreover, the clusters of galician earthquakes present intensity functions with different shapes. this characteristic suggests the use of intensity function, of its shape in particular, to characterize and compare different seismic regions. note this characterization is possible thanks to the use of nonparametric methods, which do not predetermine the shape of such curves. it is important to point out that although our conclusions contradict the belief that granada has more seismic activity than galicia, seismicity in the latter has grown much in recent years. acknowledgements we would like to express our gratitude to j. ibáñez and j.a. esquivel (instituto andaluz de geofísica, universidad de granada, spain) for their generosity in providing the data used throughout the work and for their comments, which have helped prepare a better version of the paper. this research was financed by the xunta de galicia (spain) under research project pgidt01pxi10504pr and by the dges under research project pb98-0182-c02-01. references choi, e. and p. hall (1999): nonparametric approach to analysis of space-time data on earthquake occurrences, j. comput. graphic. stat., 8 (4), 733-748. estévez, g. and a. quintela (1999): nonparametric estimation of the hazard function under dependence conditions, commun. stat. theory methods, 28 (10), 2297-2331. estévez, g. and a. quintela (2002) : estimación no paramétrica de la función de riesgo: aplicaciones a sismología, questiió, 25 (3), 437-477. gutenberg, r. and c.f richter (1944): frequency of earthquakes in california, bull. seismol. soc. am., 34, 185-188. hassani, s., p. sarda and p. vieu (1986): approche non paramétrique en théorie de la fiabilité, rev. stat. appl., 35 (4), 653-676. hawkes, a.g. 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(received february 15, 2002; accepted july 29, 2002) the first month of the 2016 central italy seismic sequence: fast determination of time domain moment tensors and finite fault model analysis of the ml 5.4 aftershock annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 the first month of the 2016 central italy seismic sequence: fast determination of time domain moment tensors and finite fault model analysis of the ml 5.4 aftershock laura scognamiglio*, elisa tinti, matteo quintiliani istituto nazionale di geofisica e vulcanologia *laura.scognamiglio@ingv.it abstract we present the revised time domain moment tensor (tdmt) catalogue for earthquakes with ml larger than 3.6 of the first month of the ongoing amatrice seismic sequence (august 24thseptember 25th). most of the retrieved focal mechanisms show nnw–sse striking normal faults in agreement with the main ne-sw extensional deformation of central apennines. we also report a preliminary finite fault model analysis performed on the larger aftershock of this period of the sequence (mw 5.4) and discuss the obtained results in the framework of aftershocks distribution. i. introduction he ml 6.0 amatrice earthquake, which occurred at 01:36:32 utc august 24th 2016 is the first mainshock of the amatrice seismic sequence. it is the largest earthquake to strike this portion of central apennines since the m 6.2, 1639 october 7th monti della laga earthquake archived as an i=ix-x (mcs) and m=6.2, [cpti15, rovida et al., 2016]. the earthquake caused 299 fatalities and partially destroyed the towns of amatrice, accumoli and several surrounding small towns. the hypocenter is located at 42.70° n, 13.23° e and at depth of 8km, only 1 km far from the accumoli village and just 9 km from the amatrice town [marchetti et al., this issue]. almost one hour after the mainshock, an aftershock of ml 5.4 occurred; it is located 12 km nw of the mainshock close to the norcia town. no significant foreshocks were recorded. soon after the mainshock evidence for 5.2 km of surface ruptures, preliminarily interpreted as a direct expression of coseismic fault rupture, was observed along the northwest striking mt. vettore fault trace by emergeo working group geologists [emergeo w. g., this issue] (figure 1). the average surface offsets found is in the order of 15-20 cm for both heave and throw components. the kinematic finite fault model, retrieved starting from the tdmt mainshock solution, shows that the amatrice earthquake ruptured a fault dimension 26 km long and 16 km width [tinti et al., 2016]. the most relevant features of this model are (i) clear bilateral rupture, (ii) relat annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 tively fast rupture velocity, (iii) heterogeneity of the slip distribution characterized by two main slip patches and iv) quite different value of the rake on the two patches [tinti et al., 2016]. the rupture from this normal fault earthquake propagates bilaterally to se and to n-nw directing the strongest shaking toward the city of amatrice, where peak ground accelerations (pgas) between 19%g and 43%g is recorded, and toward the town of norcia, where pgas ranges between 22%g and 37%g [faenza et al., this issue]. the preliminary aftershocks distribution defines a main fault plane sw-dipping with an average dip angle of ~50° in agreement with the computed moment tensor solution. the main fault plane is simpler to the south while it becomes more complex to the northernmost portion where it is evident the activation of a shallower and antithetic splay. the aftershocks distribution suggests an along strike fault length of about 35 km [michele et al., this issue]. here, we report on source geometries of the, still active, amatrice seismic sequence relatively to the period of time spanning from august 24thto september 25th. we present the revised moment tensor solutions of all the events with ml > 3.3. due to the complex aftershocks distribution on the northern part of the seismic sequence, we decided to try to understand the larger aftershock of the sequence with the aim of identifying which is the activated fault plane and the main characteristic of the rupture. we believe that understanding the main features of this first period of the amatrice seismic sequence could contribute to explain the complexity of the seismogenic processes active in the central apennines that has been marked by three mainshocks in two months of moment magnitude 6.0, 5.9 and 6.5 respectively (http://cnt.rm.ingv.it/en/tdmt). ii. seismic sequence moment tensor solutions moment tensor solution presented in this study are computed by following the full-waveform tdmt technique originally proposed by dreger & helmberger (1993) and implemented in an automatic-way at ingv by scognamiglio et al. (2009). starting from a given hypocentral location, the algorithm inverts local to regional three component broad-band velocity waveforms to estimate moment tensor in a point-source approximation. we adopt the pre-calculated and stored green’s functions obtained using the cia (central italian apennines) velocity model [herrmann et al., 2011] that has been inferred for the central apennines during the 2009 l’aquila sequence. quality and reliability of moment tensors are based on the goodness of fit between synthetic and observed waveforms, which is quantified through the variance reduction (vr) parameter, that represents an l2-like norm [scognamiglio et al., 2010]. for events in the magnitude ml 3.8 and larger range we have found that an appropriate filter is low-pass at 0.05 hz followed by a high-pass filter of 0.02 hz. lower magnitude earthquakes were inverted in the frequency band of 0.02-0.1hz. we have revised tdmt solutions for 64 events of the first month of the amatrice seismic sequence including all the events with ml larger than 3.2 for which we were able to obtain a well-constrained solution (figure 1 and table 1). comparing definitive to automatic solutions, computed immediately after the earthquakes, we found really smaller adjustments to quick determinations. this highlights the robustness of the methodology adopted and the appropriate choice of the velocity model, that allows us to reach for this sequence a magnitude lower threshold equal to mw 3.2. most of these solutions have ‘aa’ quality flag, that means very good fit between data and synthetics and high double-couple value (http://cnt.rm.ingv.it/en/help#tdmt). centroid depth of all solutions is between 1 and 8 km, in most cases shallower than the ingv released locations [marchetti et al., this issue]. the 01:36 mainshock moment tensor solution, obtained inverting 50 stations in a distance range is of ~65-260 km, shows normal faulting with annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 figure 1. map view of tdmt solutions of the first month of the amatrice 2016 seismic sequence. beach balls are colored as a function of magnitude that is also reported on top of each mechanism. white stars are mainshock and biggest aftershock location. red stars are the focal mechanism location. black dots are the michele et al. [this issue] relocated aftershocks. green triangles are the strong motion stations used for the finite fault analysis. nodal planes striking along the apenninic direction equal to strike 155°/331°, dip 49°/41° and rake −85°/-93°. the estimated scalar seismic moment is 1.07 10+18 n·m for a preferred centroid depth of 5 km, corresponding to a moment magnitude of mw 5.96. the ml 5.4 biggest aftershock, which occurred at 02:33:29 utc 24 august 2016, also features a normal fault mechanism, the obtained focal parameters are strike 135°/327°, dip 47°/43°, and rake –98°/-81°. the solution is calculated using 62 stations in the distance range of 51-130 km. the preferred centroid depth is 5 km, while the seismic moment is 1.33 10+17 n·m resulting in a mw 5.4. with the exception of the mainshock, this is the only earthquake of the first month of the sequence with mw larger than 5. the remaining 62 moment tensor solutions shows that nnw–sse striking normal faults dominate, which is in overall agreement with the trends of structures of this sector of central apennines. only in a few cases, a strike-slip kinematic characterizes the source geometry of the obtained mt, as for the august 31, 11:52 and 18:12 (utc) aftershocks, with magnitude mw 3.4 and 3.5 respectively, and for the september 3, 10:19 (utc) mw 4.3. these events are all located in annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 the northern portion of the area activated by this first month of the sequence, the same area that would have been affected by the other two mainshocks of 26th october, mw 5.9, and 30th october, mw 6.5. the complete information and the fit to recorded waveforms are available on the dedicated webpage (http://cnt.rm.ingv.it/en/tdmt), where automatic good quality solutions of smaller events are also available. date origin time longitude latitude depth strike 1 dip 1 rake 1 strike 2 dip 2 rake 2 mw 2016-08-24 01:36:32 13.23 42.7 5 155 49 -87 331 41 -93 5.96 2016-08-24 01:56:00 13.28 42.6 2 120 60 -123 353 43 -47 4.34 2016-08-24 02:33:28 13.15 42.79 5 135 47 -98 327 43 -81 5.35 2016-08-24 02:59:35 13.13 42.8 5 332 58 -105 179 35 -67 3.89 2016-08-24 03:08:10 13.25 42.62 5 331 64 -85 139 26 -101 3.69 2016-08-24 03:17:59 13.14 42.76 5 351 62 -104 198 31 -66 3.64 2016-08-24 03:40:10 13.24 42.61 6 329 59 -90 150 31 -89 4.12 2016-08-24 04:06:50 13.12 42.77 3 149 47 -97 340 44 -82 4.40 2016-08-24 04:25:58 13.24 42.64 6 345 59 -87 159 31 -96 3.44 2016-08-24 04:33:09 13.21 42.62 5 325 49 -93 149 41 -87 3.42 2016-08-24 04:38:09 13.22 42.63 6 334 67 -96 169 23 -76 3.39 2016-08-24 04:44:38 13.18 42.73 6 155 54 -71 305 40 -114 3.46 2016-08-24 04:57:37 13.04 42.85 2 105 48 -109 311 45 -71 3.48 2016-08-24 05:02:24 13.29 42.46 7 139 47 -107 343 45 -72 3.35 2016-08-24 05:31:32 13.19 42.66 6 175 59 -69 318 37 -122 3.33 2016-08-24 05:36:19 13.14 42.8 7 338 81 -117 231 29 -19 3.27 2016-08-24 06:54:54 13.19 42.8 6 320 58 -134 201 52 -42 3.21 2016-08-24 07:10:55 13.16 42.78 5 340 70 -71 116 27 -131 3.2 2016-08-24 09:31:43 13.19 42.81 12 53 89 24 323 66 179 3.38 2016-08-24 11:50:30 13.16 42.82 6 321 59 -88 137 31 -94 4.53 2016-08-24 14:02:20 13.24 42.8 3 207 56 -55 336 47 -130 3.77 2016-08-24 17:46:09 13.21 42.66 7 340 57 -89 158 33 -91 4.24 2016-08-24 20:21:36 13.15 42.78 5 317 56 -117 180 43 -56 3.32 2016-08-24 23:22:05 13.21 42.65 6 338 63 -89 156 27 -91 4.00 2016-08-25 03:17:16 13.19 42.75 6 345 61 -105 193 32 -65 4.34 2016-08-25 04:12:11 13.23 42.69 5 180 47 -84 350 44 -97 3.25 2016-08-25 04:51:40 13.33 42.63 2 143 60 -91 325 30 -88 3.76 2016-08-25 12:36:05 13.28 42.6 5 129 55 -115 347 42 -60 4.42 2016-08-25 19:40:44 13.29 42.59 5 119 59 -132 360 50 -41 3.43 2016-08-26 00:04:09 13.28 42.66 3 149 58 -95 338 33 -82 3.56 2016-08-26 04:28:25 13.29 42.6 5 128 53 -106 333 40 -70 4.76 2016-08-26 05:17:05 13.21 42.75 7 125 69 98 283 22 70 3.20 2016-08-26 05:32:52 13.15 42.77 6 342 71 -119 222 34 -35 3.30 2016-08-26 16:05:29 13.16 42.69 6 319 62 -92 143 28 -86 3.47 2016-08-27 01:26:39 13.24 42.84 2 162 54 -79 324 37 -105 3.73 2016-08-27 02:50:59 13.24 42.84 3 158 53 -83 327 38 -99 4.00 2016-08-27 06:20:30 13.31 42.55 3 334 50 -65 118 46 -117 3.23 2016-08-27 10:40:14 13.24 42.85 2 173 65 -60 299 39 -137 3.52 2016-08-28 06:37:19 13.2 42.72 7 326 56 -102 166 35 -73 3.33 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 2016-08-28 13:07:32 13.29 42.6 6 156 61 -100 356 30 -72 3.52 2016-08-28 15:37:38 13.12 42.77 6 342 52 -83 150 39 -99 3.45 2016-08-28 15:55:35 13.23 42.82 5 206 55 -87 22 35 -94 4.19 2016-08-28 16:42:01 13.14 42.82 5 340 51 -86 154 39 -95 3.73 2016-08-29 01:44:25 13.19 42.76 7 129 50 -113 342 45 -66 3.36 2016-08-30 00:35:55 13.14 42.8 6 347 60 -51 108 48 -137 3.27 2016-08-31 11:26:01 13.13 42.83 5 345 56 -80 147 35 -105 3.91 2016-08-31 11:52:31 13.22 42.85 4 99 89 177 189 87 1 3.41 2016-08-31 13:23:04 13.23 42.75 6 161 71 -67 288 29 -140 3.32 2016-08-31 18:12:52 13.26 42.82 7 318 83 -164 225 74 -8 3.48 2016-09-01 03:53:04 13.31 42.62 5 139 57 -88 315 33 -93 3.58 2016-09-01 11:35:57 13.3 42.56 5 290 81 -124 186 35 -17 3.27 2016-09-03 01:34:12 13.13 42.77 4 345 51 -92 169 39 -87 4.22 2016-09-03 10:18:51 13.22 42.86 8 199 88 -7 289 83 -178 4.30 2016-09-07 18:13:26 13.24 42.8 1 134 49 -91 315 41 -89 3.27 2016-09-15 14:40:52 13.13 42.77 2 356 54 -102 196 37 -74 3.67 2016-09-19 23:34:25 13.28 42.67 4 322 61 -52 83 46 -138 3.65 2016-09-20 01:20:53 13.29 42.68 6 323 87 -68 59 23 -173 3.23 2016-09-22 20:03:55 13.19 42.76 5 337 58 -71 123 37 -118 3.42 2016-09-30 19:22:28 13.25 42.9 5 177 57 -67 318 39 -122 3.34 2016-09-30 19:38:37 13.25 42.89 5 182 59 -61 315 41 -129 3.44 2016-10-02 23:47:07 13.23 42.79 2 133 53 -88 309 37 -93 3.21 2016-10-04 12:41:35 13.12 42.85 5 103 57 -136 345 54 -42 3.37 2016-10-08 12:19:03 13.17 42.74 3 334 47 -86 148 43 -94 3.53 2016-10-08 18:11:09 13.19 42.74 2 156 51 -91 338 39 -88 3.91 table 1. moment tensor solutions obtained using the tdmt technique. the complete information and the fit to recorded waveforms are available on the dedicated web-page (http://cnt.rm.ingv.it/en/tdmt). iii. analysis of the mw 5.4 aftershock fault plane one hour after the 24th august mainshock, a mw 5.4 event occurred almost 12 km nw from the mainshock. tdmt procedure reveals a normal faulting moment tensor solution having strike directions diverging from the mainshock fault rupture of ~20° for the west-dipping plane and ~4° for the east-dipping plane. this is the biggest aftershock of the sequence, the only one with magnitude larger than 5. it is located in the northern portion of the region activated by the amatrice seismic sequence, ~5 km far from norcia town. to unravel the ambiguity on what fault plane actually ruptured, we perform a series of inversions using waveform data recorded by 15 strong-motion ingv [michelini et al., 2016] and ran (http://ran.protezionecivile.it/et/index.php) stations, and adopting the tdmt source geometry (strike and dip) on an overly-large-dimension fault plane centered on the nll relocated hypocenter: latitude 42.793° n, longitude 13.162° e and depth 6.835 km [michele et al., this issue]. the epicentral distances of the selected recording sites are less than 45 km we use the inversion code based on the method of hartzell and heaton [1983], and implemented by dreger et al. [2005] consisting in a non-negative, leastsquares inversion method with simultaneous smoothing and damping, the same used to infer the source parameters of the mainshock. this approach assumes a constant rupture velocity and rise time and the best fitting values have been selected iteratively by performing annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 inversions with different values of these parameters and quantitatively measuring the fit based on a variance reduction, as defined above for tdmt solutions [scognamiglio et al., 2010]. rake parameter can be heterogeneous on the fault plane or assumed constant all over the fault and chosen iteratively as the rupture velocity. the slip velocity is modeled by imposing a simple box-car source-time function. the green’s functions are computed on a regular grid sampling the focal volume every 1 km horizontally and 1 km vertically and filtered between 0.02 and 0.5 hz, the same as for the recorded data. the velocity structure is cia model [herrmann et al., 2011], the same adopted for tdmt solutions. we are aware that the maximum selected frequency could prevent us to identify the details of the rupture history for a m 5 earthquake, but it allows us to reduce the impact of site effects like those reported for norcia station (nrc) for higher frequencies [bindi et al., 2011]. we run more than 1000 inversions for both planes by setting rise time and rupture velocity ranging between 0.3 ÷ 1 s and 2.0 ÷ 4 km/s respectively, while rake is allowed to vary in a 30°-long range respect to the tdmt values. the faults are parameterized using subfaults having a 2 x 2 km2 area. the overly-large modeled fault planes are 10 km x 10km. figure 2(a, b) shows the resulting preferred slip models for both the inverted planes and the corresponding waveforms fit. the east-dipping plane displays a main slip concentration located ~4 km southeastward from the hypocenter at similar depth. this patch accounts for a maximum slip of about 20 cm. the picked rise time is 0.3 s, the rupture velocity is 2.7 km/s, and the rake is -91°. the total inferred seismic moment is 9.78 1016 nm corresponding to mw = 5.29. the west-dipping plane features two slip patches both located 2 km deeper than the hypocenter. the first asperity is located just below the nucleation and has average slip equal to 16 cm, the second one is located ~ 4 km se and reports a maximum slip of 22 cm. the retrieved rise time is 0.3 s, while the rupture velocity is 3.7 km/s and the rake -115°. the total inferred seismic moment is 9.53 1016 nm corresponding to mw = 5.29. both the inverted rupture planes prefer the shortest rise time that, for the inverted frequencies, means to consider a delta-like slip velocity function. otherwise, while the preferred rupture velocity for the n327° plane finds a clear maximum within the explored rupture velocities, the high value found for the n135° plane cannot be considered well-constrained due to a very similar variance reduction (~30%) we obtain for rupture velocities between 3.0 and 3.9 km/s (figure 3a). although we adopt an overlylarge fault plane to image the model parameters, significant slip occurs only on an area of 8 x 5 km2. the comparison between recorded and synthetic data retrieved from the two rupture models is presented in figure 2c. the synthetic ground velocities generated by the n327° slip model match fairly well the main body wave pulses of the majority of the recorded seismograms. on the contrary, the rupture model obtained for the west-dipping plane poorly fits the main features of real data except for the three farthest stations and nrc. both models show discrepancies at some sites most likely resulting from 3d variations of velocity structure in this area not included in our adopted 1d velocity model [casarotti et al., this issue]. as an example, stations located in the northern side show a poor fit (e.g. fema) and would require a faster velocity structure to allow a better alignment between real and synthetic phases. figure 3b shows the evolution of the vr values for the performed inversions with rise time equal to 0.3s. among all the performed inversions, we have found more than 60% of the solutions adopting the east-dipping plane having vr larger than 30% with an overall vrmax= 35%, while, for the alternative west-dipping plane, only 13% feature vr larger than 30% with an overall vrmax= 31%. the finite fault inversion result opts for the activation of the strike 327° fault plane. however, the small difannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 figure 2. rupture models imaged by inverting ground velocity time histories for the (a) east dipping plane (n327°) and (b) west dipping plane (n135°) respectively. the black arrows indicate the slip direction (rake angle) for slip larger than 15 cm. (c) fit to the data: synthetic ground velocity filtered between 0.02 and 0.5 hz (green and red lines are for the east and west dipping planes respectively) and recorded strong motions (black lines). numbers in box represent the amplitude range in cm/s. numbers in percentage represent the variance reduction for each station. ference between the resulting vrmax values do not allow us to exclude a priori the activation of the strike 135° fault. to solve the ambiguity of the kinematic result we decide to analyze the aftershocks distribution and verify the existence of earthquakes alignnrc rqt csc amt fema mnf lss sno spm guma spo1 tre trl tero ant 4 km 13 km 14 km 21 km 21 km 30 km 30 km 30 km 34 km 34 km 35 km 35 km 41 km 41 km 42 km yl im [1. 2 1. 2] c m /s yl im [0. 5 0. 5] c m /s 50.3% 31.4% 43.4% 16.3% -16.1% 36.6% 40.0% 47.5% 15.3% 33.7% 17.8% -8.4% 26.9% 17.3% 14.0% 62.5% 7.9% 41.0% -39.1% 12.9% -44.4% 16.5% -22.6% -1.6% -1.4% -2.1% -12.6% 34.6% 32.3% 22.6% 30 seconds 30 seconds 0. 5 0 .5 1 1 1 1.5 1. 5 1.5 2 2 2 0. 5 0 .5 1 1 1 1.5 1. 5 1.5 2 2 2 0. 5 0 .5 1 1 1 1.5 1. 5 1.5 2 2 2 2.5 2 .5 2 .5 3 327° strike, east dipping plane along strike (km) 0 1 2 3 4 5 6 7 8 9 10 a lo n g d ip ( km ) 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 0 .5 0 .5 1 1 1 1 1 1. 5 1.5 1.5 1. 5 0 .5 0 .5 1 1 1 1 1 1. 5 1.5 1.5 1. 5 135° strike, west dipping plane along strike (km) 0 1 2 3 4 5 6 7 8 9 10 a lo n g d ip ( km ) 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 se nw nw se (cm) (cm) east north vertical east north vertical annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 figure 3 (above). resulting variance reduction for a subset of 336 inversions performed with rise-time equal to 0.3s. rupture velocity and rake angle vary within the explored range. green dots are for the striking n327° plane while red dots are for the striking n135°plane. (a) vr is shown as a function of rupture velocity; (b) vr is sorted for increasing values. the variability of vr for each rupture velocity (panel a) reflects the variability in rake. figure 4 (right column). (a) map view of the relocated aftershocks [michele et al., this issue] with traces of the active mapped faults [emergeo w. g., this issue]. white stars represent the location of the largest events of the studied sequence. earthquakes are colored as a function of date of occurrence. yellow and green boxes are the map projection of the inverted fault while the yellow and green lines correspond to the vertical sections reported in panels (b) and (c) showing the earthquakes occurring within 8km from the vertical line. ment on the investigated rupture planes. we plot the first 8 days of the aftershocks sequence relocated by michele et al. [this issue] along two vertical sections oriented n45°e and n57°e, perpendicular to the strike of the tdmt planes (figure 4). due to the not completely 100% double couple component of the moment tensor solution, the profiles are rotated 12° from each other. aftershocks are shown with dots colored as function of date of occurrence. each section reports earthquakes occurring in the 8 x 15 km2 boxes mapped in figure 4a and the trace of the inverted fault planes, as well as the mapped main normal fault systems [emergeo w. g., this issue]. at first glance, both profiles show a cross-like events distribution that reveals the activation in this area of both synthetic and antithetic annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 structures. in the n45°e oriented a-a’ profile, the seismicity does not clearly image a continuous single west-dipping plane, but it is quite diffuse and probably related to the nw termination of the fault activated during the mainshock [tinti et al., 2016, michele et al., this issue]. on the contrary, along the b-b’ section, the distribution of the aftershocks along the dip of the inverted n327° rupture plane is irregular but in some way more evident. for this plane we also find a quite good geometrical correspondence with the mapped norcia antithetic fault. both the vertical sections also image a separated hypocenters cluster below the mt. vettore fault system. iv. conclusion in this paper we have presented the revised tdmt catalogue for all the events with ml larger than 3.2 belonging to the first month if the 2016 amatrice (central italy) seismic sequence. the moment tensor solutions immediately pointed up a prevalent normal faulting mechanism for the occurring earthquakes, in agreement with the present-day active stress field in this sector of the apennines. only in a few cases, mainly located in the northern portion of the area activated by the seismic sequence, a strike-slip kinematic characterizes the source geometry. centroid depths are between 1 and 8 km, typically shallower than the ingv released location [marchetti et al., this issue]. we have found a really good agreement between revised and automatic solutions, computed immediately after the earthquakes occurrence, confirming once again the implemented method as a robust and practical tool for real-time determinations of the point-source focal parameters and magnitude. we have analyzed the largest aftershock of the first month of the sequence (mw 5.4) in terms of finite rupture model with the aim of identifying the activated fault plane. we have performed more than 1000 inversions to model both the fault plane geometries identified by tdmt solution and we have found a quite satisfactory fit by using strong motion data at epicentral distance less than 45 km and frequencies up to 0.5 hz. the finite fault inversion results opt for the activation of the strike n327° fault plane. the small difference of variance reduction between the two best solutions does not allows us to exclude a priori the activation of the strike 135° fault. in any case, both the models reveal, as a common feature, a main asperity located south-east from the hypocenter characterized by a maximum slip of ~ 20 cm. to give an additional constraint to the kinematic result we have decided to analyze the first 8 days of aftershocks distribution and verify the existence of earthquakes alignment on the investigated rupture planes. the preliminary analysis of finite fault model and aftershocks distribution suggests that, for the mw5.4 aftershock, the most probably activated fault plane is the one striking n327°and dipping 43° toward ne. for this plane we find a quite good geometrical correspondence with the mapped norcia antithetic fault. references [bindi et al., 2011] bindi, d., l. luzi, s. parolai, d. di giacomo, and g. monachesi (2011). site effects observed in alluvial basins: the case of norcia (central italy), bulletin of earthquake engineering, 9(6), 1941-1959, doi:10.1007/s1051 8-011-9273-3. [dreger et al., 2005] dreger, d. s., l. gee, p. lombard, m. h. murray, and b. romanowicz (2005). rapid finite-source analysis and nearfault strong ground motions: application to the 2003 mw 6.5 san simeon and 2004 mw 6.0 parkfield earthquakes, seismological research letters 76 (1), 40-48. [emergeo w. g., this issue] emergeo working group (2016). coseismic effects of the 2016 amatrice seismic sequence: first geological results. annals of geophysics, this issue. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7246 [faenza et al., this issue] faenza, l., v. lauciani and a. michelini (2016). the shakemaps of the amatrice, m6, earthquake. annals of geophysics, this issue. [hartzell and heaton, 1983] hartzell, s.h., and t.h. heaton (1983). inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley, california, earthquake, bull. seis. soc. am 73 (6, part a),1553-1583. [herrmann et al., 2011] herrmann, r.b., l. malagnini, and i. munafò (2011). regional moment tensor of the 2009 l’aquila earthquake sequence, bull.seism.soc.am.,101(3), 975-993. [marchetti et al., this issue] marchetti a., m.g. ciaccio, a. nardi, a. bono, f.m. mele, l. margheriti, a. rossi, p. battelli, c. melorio, b. castello, v. lauciani, m. berardi, c. castellano, l. arcoraci, g. lozzi, a. battelli, c. thermes, n. pagliuca, g. modica, a. lisi, l. pizzino, p. baccheschi, s. pintore, m. quintiliani, a. mandiello, c. marcocci, m. fares, d. cheloni, a. frepoli, d. latorre, a.m. lombardi, m. moretti, m. pastori, m. vallocchia, a. govoni, l. scognamiglio, a. basili. a. michelini and s. mazza (2016). the italian seismic bulletin: strategies, revised pickings and locations of the amatrice seismic sequence, annals of geophysics, this issue. [michele et al., this issue] michele m., di stefano r., chiaraluce l., cattaneo m., de gori p., monachesi g., latorre d., marzorati s., valoroso l., ladina c., chiarabba c., lauciani v. and m. fares (2016). the amatrice 2016 seismic sequence: a preliminary look to the mainshock and aftershocks distribution, annals of geophysics, this issue. [michelini et al., 2016] michelini, a., margheriti, l., delladio, a., pintore, s., cecere, g., g. d'anna, g., moretti, m., cattaneo, m., marchetti, a., demartin, m., mazza, s., mele, f., lau ciani, v., bono, a., marcocci, c., quintiliani, m., scognamiglio, l., faenza, l., amato, a., basili, a., selvaggi, g., nardi, a., danecek, p., pignone, p., nostro, c., casale, p., mandiel-lo, a. and s. rao (2016). the italian national seismic network and the earthquake and tsunami monitoring and surveillance systems, (submitted to advances in geosciences). [rovida et al., 2016] rovida a., locati m., camassi r., lolli b., gasperini p., eds. (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes, istituto na zionale di geofisica e vulcanologia; doi:http:// doi.org/10.6092/ingv.it-cpti15. [scognamiglio et al., 2009] scognamiglio, l., tinti, e. and michelini, a. (2009). real-time determination of seismic moment tensor for italian region, bull. seism. soc. am., 99(4), 22232242. [scognamiglio et al., 2010] scognamiglio, l., e. tinti, a. michelini, d. s. dreger, a. cirella, m. cocco, s. mazza, and a. piatanesi (2010). fast determination of moment tensors and rupture history: what has been learned from the 6 april 2009 l’aquila earthquake sequence, seism. res. lett., 81(6), 892-906; doi:10.1785/gssrl.81.6.892. [tinti et al., 2016] tinti, e., l. scognamiglio, a. michelini, and m. cocco (2016). slip heterogeneity and directivity of the ml 6.0, 2016, amatrice earthquake estimated with rapid finitefault inversion, geophys. res. lett., 43, 10,74510,752; doi:10.1002/2016gl071263. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 177_189 adg v–5 n01 w.pdf annals of geophysics, vol. 45, n. 1, february 2002 177 mapping plasma structures in the high-latitude ionosphere using beacon satellite, incoherent scatter radar and ground-based magnetometer observations jürgen watermann (1), gary s. bust (2), jeffrey p. thayer (3), torsten neubert (4) and clayton coker (2) (1) danish meteorological institute, copenhagen, denmark (2) applied research laboratories, the university of texas at austin, tx, u.s.a. (3) sri international, menlo park, ca, u.s.a. (4) danish space research institute, copenhagen, denmark abstract in the autumn of the year 2000, four radio receivers capable of tracking various beacon satellites were set up along the southwestern coast of greenland. they are used to reconstruct images of the ionospheric plasma density distribution via the tomographic method. in order to test and validate tomographic imaging under the highly variable conditions often prevailing in the high-latitude ionosphere, a time interval was selected when the sondrestrom incoherent scatter radar conducted measurements of the ionospheric plasma density while the radio receivers tracked a number of beacon satellites. a comparison between two-dimensional images of the plasma density distribution obtained from the radar and the satellite receivers revealed generally good agreement between radar measurements and tomographic images. observed discrepancies can be attributed to f region plasma patches moving through the field of view with a speed of several hundred meters per second, thereby smearing out the tomographic image. a notable mismatch occurred around local magnetic midnight when a magnetospheric substorm breakup occurred in the vicinity of southwest greenland (identified from ground-based magnetometer observations). the breakup was associated with a sudden intensification of the westward auroral electrojet which was centered at about 69° and extended up to some 73° corrected geomagnetic latitude. ground-based magnetometer data may thus have the potential of indicating when the tomographic method is at risk and may fail. we finally outline the application of tomographic imaging, when combined with magnetic field data, to estimate ionospheric joule heating rates. key words high-latitude ionosphere – ionospheric tomography – magnetospheric substorm – auroral electrojet – joule heating 1. introduction the high-latitude ionosphere is strongly coupled to the magnetosphere and influenced by its dynamic variations. the magnetosphere responds to variations of the solar wind (a stream of ionized particles, predominantly protons and electrons) and the embedded interplanetary magnetic field and thus ultimately to solar activity. the most important solar wind parameters which have significant bearing on the state of the mailing address: dr. jürgen watermann, danish meteorological institute, solar-terrestrial physics division, lyngbyvej 100, dk-2100 copenhagen ø, denmark; e-mail: jfw@dmi.dk 178 jürgen watermann, gary s. bust, jeffrey p. thayer, torsten neubert and clayton coker magnetosphere are plasma density, temperature, bulk speed, and the magnitude and orientation of the interplanetary magnetic field. variations in these parameters lead to variations of the state of the magnetosphere through electrodynamic interaction. they are eventually propagated to the high-latitude ionosphere via electric fields, fieldaligned currents and energetic particle precipitation. the coupled solar wind-magnetosphereionosphere system is generally very dynamic during and just after solar maximum (the most recent of which occurred in the years 2000-2001). since solar wind-magnetosphere-ionosphere interaction is most direct in the auroral zone and polar cap in a broad sense (i.e. at geomagnetic latitudes exceeding some ± 60°) the high-latitude ionosphere can be highly variable on time scales of solar wind changes, which may be less than a few minutes. ionospheric effects at high latitudes are particularly dramatic when the interplanetary magnetic field is oriented southward (i.e. opposite to the main geomagnetic field), in which case field line merging can occur, or when its orientation fluctuates a great deal. in recent years, with the advent of satellites equipped with stable multi-frequency radio transmitters (beacon satellites) the method of ionospheric tomography has become an important tool for imaging the ionospheric plasma density distribution. various measurement techniques, based on satellite-to-ground or satellite-to-satellite observations of the total electron content (tec), and different inversion schemes have been developed. initially, a differential doppler technique was applied simultaneously to a meridian chain of receivers in order to determine the latitudinal variation of tec profiles (leitinger et al., 1984). subsequently, computer tomography was developed to produce 2d images of the ionospheric plasma density (austen et al., 1988). the image reconstruction algorithm was further developed in various ways (e.g., vasicek and kronschnabl, 1995) to improve its accuracy. the tomographic method has successfully been applied to the mid-latitude ionosphere (e.g., bust et al., 1997) where the ionosphere tends to be stationary over the time interval needed for tomographic inversion (typically up to 20 min, depending on the satellites tracked). application of the method to high geomagnetic latitudes (auroral zone and polar cap) has also been reported (e.g., mitchell et al., 1995; pryse et al., 1997) though little experience has yet been gained. bernhardt et al. (1998) present tomographic images of the ionospheric trough and of narrow auroral arcs obtained at high latitudes under stable ionospheric conditions. since the ionosphere tends to be very variable at high geomagnetic latitudes, where its state may change on time scales shorter than those needed to collect reasonably complete satellite observations for reconstructing tomographic images, satisfactory performance of the tomographic method is not yet confirmed. in order to address some of the problems of tomographic imaging at high latitudes, a chain consisting of four satellite receivers was set up along the southwestern coast of greenland, colocated with several ground-based magne-tometer stations and the sondrestrom incoherent scatter radar (isr) facility. the ionospheric plasma density distribution, among other parameters, can be derived from isr measurements on relatively short time scales and be compared with tomographic images obtained from beacon satellite observations. we thus have a procedure at hand to test the tomographic method at high latitudes, validate it if possible, and try to determine under which conditions it fails. similar verification concepts were employed by foster et al. (1994) and mitchell et al. (1995). the former examined storm-time observations of the mid-latitude ionosphere and showed that largescale discrete f region ionization enhancements and a deep ionospheric trough were recognized in both, incoherent scatter radar data and tomographic images. the latter demonstrated, by comparing tomographic images with eiscat observations of the ionospheric plasma density, that the tomographic method can render accurate results in the auroral zone if the ionosphere remains stable. next we give a description of the experiment, specifically of the instruments used. this is followed by an evaluation of the performance of the tomographic method. we then outline the estimation of joule heating as a science application well suited for the particular situation in greenland. we finally summarize the results and elaborate on future work. 179 imaging high-latitude ionospheric plasma structures 2. experiment description a map of greenland is reproduced in fig. 1. it shows the magnetometer sites of the danish meteorological institute (dmi), the satellite receiver stations of the applied research laboratories at the university of texas, austin (arl:ut), and the sondrestrom incoherent scatter fig. 1. greenland with the magnetometer sites of the danish meteorological institute, dmi (full black circles with three-letter station codes), the satellite receiver stations of the applied research laboratories at the university of texas, austin, arl:ut (full gray diamonds with numbers), and the sondrestrom (stf) incoherent scatter radar facility operated by sri international (open black diamond). geomagnetic latitude contours (dashed) are plotted in addition to geographic coordinates (dotted), and the isr antenna scan trace at 350 km altitude is marked by a heavy line. radar facility operated by sri international. those instruments were used for this study and are described below. the southern part of greenland is most often located in the auroral zone and the central and northern parts in the polar cap. the boundary between auroral zone and polar cap is often found in the vicinity of sondrestrom but can also shift considerably in latitude, depending on the local time and interplanetary medium. we are thus in a situation where we apply tomographic imaging to the latitudes which cover both, auroral zone and polar cap. 2.1. satellite radio receivers the coherent ionospheric doppler receiver (cidr) was developed at arl:ut. it is capable of tracking the coherent beacon from the navy ionospheric measurements satellites (nims) as well as other satellites including radcal, argos and gfo, and can receive three different satellites simultaneously at data rates up to 1 khz. the measurements from an array of receivers are converted into relative tec profiles and ingested into an inversion algorithm which renders a threedimensional image of the ionospheric plasma density distribution. for an outline of the algorithm consult a recent paper by bust et al. (2001) and references therein. for our paper, twodimensional maps in a specific vertical plane were extracted from the 3d inversion results in order to facilitate comparison with radar and magnetometer measurements. the tomography algorithm requires, in its present form, an ionospheric plasma density pattern which is stationary over the time of the radio reception, typically between 10 and 20 min. 2.2. incoherent scatter radar the sondrestrom incoherent scatter radar (kelly, 1983) utilizes techniques which allow the direct measurement of basic ionospheric plasma parameters along the radar beam, including electron number density, electron and ion temperatures, and ion line-of-sight velocity. the 32-m parabolic antenna is fully steerable which 180 jürgen watermann, gary s. bust, jeffrey p. thayer, torsten neubert and clayton coker allows, for instance, to determine the plasma density distribution in a vertical plane oriented along the magnetic meridian through sondrestrom. we operated the radar in a mode in which the antenna performs a sequence of elevation scans between geomagnetic north (at a declination of 27° from geographic north) and geomagnetic south. each scan started at an elevation of 30° toward geomagnetic north, went through the zenith to geomagnetic south, stopped at an elevation of 30°, and returned the same way but in opposite sense, with a one-way scan taking about four minutes to complete. the antenna speed changed with elevation, using a lower velocity at lower elevation and higher velocity overhead, in order to provide equal ground distance increments irrespective of elevation angle. this also compensates for the greater signal power loss for lower elevation angles (longer distance from ionosphere). we used a signal integration time of 20 s which overhead is equivalent to about 100 km integration width at 350 km. the 1.3 ghz two-channel radar transmitter was operated using an alternating code and a 320-µs long pulse the latter of which gives a range resolution of about 48 km with sufficient return signal strength from the f region. the alternating code provides 3-km range resolution but f region backscatter is usually too weak to render reliable results. tomographic images in the antenna scan plane were then compared with twodimensional electron density maps produced from individual 4-min isr scans centered on the time interval used for tomographic reconstruction. 2.3. magnetometer chain the greenland west coast magnetometer chain (friis-christensen et al., 1985) comprises 12 variometer stations along the west coast of greenland, complemented by three geomagnetic observatories (colocated with variometer stations). the stations are approximately lined up along the same corrected geomagnetic (cgm) meridian so that they order quite naturally according to cgm latitude. the magnetometers sense the magnetic field of electric currents in the ionospheric e region (basically hall currents, also known as auroral electrojets). the magnetograms can be numerically inverted to infer the temporal and spatial distribution of the equivalent ionospheric current (thinsheet ionospheric hall current assumed to flow at a mean e region height of between 110 and 115 km) crossing the magnetometer chain (popov et al., 2001). the current density depends linearly on the product of electric field and hall conductance (height-integrated conductivity) the latter of which depends primarily on the electron density profile, c.f. section 4. significant variations in the current strength indicate significant variations of the electric field or the electron density or both. the magnetometer chain can thus serve as an indicator of a changing ionosphere which potentially poses problems to the tomographic method by rendering its assumption of stationary plasma density distribution invalid. on the other hand, can the magnetometers serve to complement tomographic images by providing a means to estimate the ionospheric electric field and joule heating rate. 3. performance evaluation in order to test the reliability of the tomographic method at high magnetic latitudes under various ionospheric conditions, we compared tomographic images obtained over southwest greenland with measurements from the sondrestrom isr. the time interval chosen for comparison covers the night from september 29, 2000, 2145 ut through september 30, 2000, 0500 ut during which the radar operated in the mode described above. this interval compares to 1820-0135 solar local time (slt) and 1930-0245 magnetic local time (mlt). the ionosphere was practically absent at e region altitudes, and only f region measurements above some 170 km are compared. it appears that the f region is sufficiently dynamic to pose problems so that it can well serve to test the tomographic method. figure 2 shows the f region plasma density obtained from isr measurements at a reference altitude of 350 km in a latitude-time diagram. the plasma density appears to vary significantly over latitude and time. at 0400 ut, for instance, we find 1.5 1011 m 3 around 72° and more than 1012 m 3 poleward of 75° geomagnetic latitude. more critical 181 imaging high-latitude ionospheric plasma structures to the tomographic method is the temporal variation which becomes apparent through a sequence of high-density ( 1012 m 3) plasma patches about two degrees wide in latitude which move through the isr scan plane with an equatorward velocity component between 200 and 600 m/s. seven 3d tomographic images were obtained during this time period, and cross sections along the radar scan plane were extracted. the satellite reception time intervals which were used for image reconstruction are marked by rectangles along the top and bottom abscissae. five out of the seven tomographic images show good agreement with isr electron density maps inferred from scans centered on the reconstruction intervals. the 0247 ut case is shown as an example of good agreement in fig. 3a. the tomographic image taken around 0247 ut (contour lines) is laid over the plasma density map obtained from the 0247-0251 ut isr scan (color-coded). the display is centered on sondrestrom, with ground distance projected from range and elevation on a spherical earth, such that «0» refers to the sondrestrom isr location. the two density peaks at about 350 km height, one above the radar and the other some 500550 km to the north, are well reproduced in the tomographic image as is the total extent of the plasma structure. the fit would be even better if the tomographic image were shifted down in altitude by 20-30 km. fig. 2. f region (350 km altitude) plasma density in a latitude-time diagram, obtained from isr measurements. each rectangle represents one 20-s integration cell expanded to the 8-min duration of the respective back-and-forth scan. the full rectangles along the top and bottom abscissae represent the time intervals from which satellite radio signals were used to reconstruct tomographic images. 182 jürgen watermann, gary s. bust, jeffrey p. thayer, torsten neubert and clayton coker fig. 3a,b. a) electron density map from isr measurements in the vertical plane of the antenna scan which lasted from 024649 ut through 025052 ut, and tomographic image reconstructed in the same reference plane as the antenna scan, centered on 0247 ut. 0 km ground distance and 0 km range refer to the sondrestrom isr site (stf). b) as (a) but for the 010348-010751 ut radar scan and the 0106 ut tomographic image, respectively. a b 183 imaging high-latitude ionospheric plasma structures fig. 4. low-pass filtered magnetic field variations along the greenland west coast (5 min cutoff), separated into geomagnetic north, east and vertical components. since all stations approximately line up along the same corrected geomagnetic (cgm) meridian the magnetic field amplitude is plotted in a cgm latitude versus time diagram. the stations are listed along the right-hand ordinate at their respective cgm latitude. the magnetic field intensity is colorcoded and white areas denote missing data. 184 jürgen watermann, gary s. bust, jeffrey p. thayer, torsten neubert and clayton coker the 0106 ut tomographic image shows partial agreement with simultaneous isr observations (fig. 3b). the enhanced plasma density structures at the poleward and equatorward edges of the isr antenna scan field, again at about 350 km height, are well reproduced in the tomographic image. the latter, however, shows an additional 350-km altitude peak 200-250 km south of stf, i.e. around 72° geomagnetic latitude, which does not appear in the 0104-0108 ut radar scan, and a minor peak at 500 km altitude on a field line just north of the radar. the major peak does appear, though, in the preceding isr scan, a few minutes earlier. that scan falls within the satellite reception interval (cf. fig. 2) so that the structure seen in that scan but not later is likely to be representative for the extra peak resolved in the tomographic image. the 0145 ut image (not shown) agrees poorly with isr measurements. this, too, can possibly be explained by the particularly disturbed state of the high-latitude ionosphere. the isr scan sequence coincident with the tomographic reconstruction interval (0135-0153 ut) reveals first the development and decay of mixed plasma density enhancements lacking uniform propagation characteristics, and then, from 0147 ut on, a new well defined equatorward moving plasma patch. the latter interval (after 0147 ut) is largely (though not entirely satisfactorily) reproduced in the tomographic image while the former, more turbulent interval, is not. the poorly matching 0145 ut case is interesting because of the geophysical conditions prevailing as inferred from ground-based magnetic field observations. figure 4 reveals that the horizontal magnetic field in the equatorward fig. 5. stacked high-pass filtered magnetic field variations along the greenland west coast (15 min cutoff ), separated into geomagnetic north, east and vertical components. 185 imaging high-latitude ionospheric plasma structures fig. 6. e region (130 km altitude) plasma density distribution in a format similar to fig. 2. section of the magnetometer chain was positive (northward) in the beginning and decreased after 23 ut to values around zero. at about 0145 ut it decreased suddenly to become strongly negative (southward). the vertical component was negative (upward) in the beginning, approached zero after 23 ut and at 0145 ut increased suddenly, simultaneously with the horizontal field, thereby exhibiting a steep latitudinal gradient from negative (upward) below 70° to positive (downward) above 70° cgm latitude. this is equivalent to a westward ionospheric current centered at about 70° and extending up to some 73° cgm latitude. the electrojet intensification is accompanied by the start of magnetic pi2 pulsations at 0145 ut with highest amplitudes between 66° and 70° cgm latitude and lasting for more than half an hour (fig. 5). taken together these observations are convincing substorm onset indicators. isr measurements of the plasma density at e region height confirm our view. figure 6 shows, in a format similar to fig. 2, that the plasma is tenuous at 130 km altitude prior to 0145 ut but increases to some 3 1010 m 3 after 0145 ut (in the middle of the night, at 2220 slt and 2330 mlt). this is most likely a consequence of auroral electron precipitation (electrons with a characteristic energy of several kev). it may turn out that the onset of a magnetospheric substorm will frequently pose a problem to the tomographic method, but a firm conclusion can only be drawn once more cases have been investigated. for the six cases of good and partial agreement between isr and tomography images we attempted to determine whether the match is improved by shifting the images against each other, in altitude as well as horizontally. this was done in a semi186 jürgen watermann, gary s. bust, jeffrey p. thayer, torsten neubert and clayton coker quantitative way by fitting the peaks and certain contours visually. it appeared that no substantial improvement can be gained by shifting the images. figure 7 shows the shifts applied to the tomographic images against the radar maps in order to obtain the best match. they can amount to some 40 km in horizontal and 30 km in vertical direction which is not more than the inherent resolution of the radar and radio beacon measurements in this experiment. the figure further reveals that the shift is probably randomly distributed. we are thus confident that tomographic reconstruction was not systematically biased with respect to isr observations. 4. joule heating inference inference of ionospheric joule heating rates constitutes one of our research objectives and one reason for operating satellite receivers colocated with magnetometers. joule heating is among the important high-latitude processes of transferring energy from the magnetosphere to the upper atmosphere, which ultimately means energy from the sun via the solar wind into the atmosphere. joule heating is thus a process which governs an important part of the effect the dynamic sun exercises on the terrestrial environment. while incoherent scatter radar measurements remain a superb way to determine joule heating rates, they suffer from limited radar operation time, a constrained radar field-of-view, and a multitude of antenna and transmitter modes not all of which are suitable for that purpose. a combination of ground-based magnetometer observations (basically measurements of the ionospheric hall current) and tomographic plasma density images yields, under certain conditions, estimates of the joule heating rate on a fairly regular basis. magnetometer measurements are performed continuously, and radio satellite reception occurs about 20 times per day, though at irregular time intervals. since most of the energy transfer from the magnetosphere into the upper atmosphere occurs in the lower ionosphere (below some 200 km altitude), sufficient resolution of the ionospheric e region is necessary for our method to work. the capability of the tomographic method to resolve an e region plasma density enhancement was demonstrated by mitchell et al. (1995) who used almost simultaneous eiscat incoherent scatter radar and beacon satellite observations above northern scandinavia. our tomography versus isr comparison discussed in the previous section is based on night time observations when e region ionisation was practically absent. however, prior to the start of the isr operation, tomographic images were reconstructed which indeed show the presence of plasma in the e region, see fig. 8. note that the plasma density increases to more than 10 · 1010 m 3 just below a 5 · 1010 m 3 valley observed at 150 km altitude in the center, and also in a blob located at – 800 km ground range and 100 km altitude. we lack, however, quantitative confirmation from radar observations. if we impose the condition of zero neutral wind speed in the ionosphere as a first approximation, we can derive joule heating rates 40 20 0 20 40 40 20 0 20 40 plasma density image shift tomography vs. isr < d o w n [k m ] u p > < south [km] north > fig. 7. horizontal and vertical shifts applied to the six tomographic images which agreed well or partially with sondrestrom isr observations, in order to achieve best agreement with the isr plasma density maps. 187 imaging high-latitude ionospheric plasma structures in the following way. the ionospheric pedersen and hall current densities, j p and j h , read (4.1) where p and h denote the pedersen and hall conductivities, respectively, and e the (horizontal) electric field. since the ionospheric electric field does not change along the magnetic field line because of the prevailing high parallel conductivity which exceeds the hall and pedersen conductivities by orders of magnitude, height-integrated conductivities and current densities can be used and are in the following represented by capital letters. from eq. (4.1) we obtain (4.2) and further (4.3) which leads to an estimate of the height-integrated joule heating rate, w, in the ionosphere (4.4) w = j e = jp p h h .2 2 in order to determine electric field and joule heating in the ionosphere we require information about the height-integrated hall current density and the pedersen and hall conductances. the heightintegrated hall current, j h , can be approximated by the equivalent ionospheric current (basically a thin-sheet hall current) which is derived from magnetometer measurements. the technique is well developed, and the algorithm which is applied to the greenland west coast magnetometer chain is described in detail by popov et al. (2001). the ionospheric conductivities depend primarily on theelectron number density, n e , and read (e.g., watermann et al., 1993) (4.5) here, the subscripts «e» and «i» stand for electrons and various ion species with partial pressure (relative abundance) p i , respectively. the ion population consists primarily of atomic and molecular oxygen and nitric oxide. the electron and ion gyro frequencies, e and i , are determined by the geomagnetic field, b0, which is accurately modeled by the international geomagnetic reference field (igrf), c.f. mandea and macmillan (2000). the electron and ion collision rates, e and i , reflect mainly collisions with neutral atoms and molecules since the neutral number density in the lower ionosphere is orders of magnitude higher than the ion number density. the neutral number density is well represented by the msis-86 model (hedin, 1987) and much less variable than the electron number density. the main unknown variable in these equations is therefore the electron density, n e . once the plasma density fig. 8. tomographic image reconstructed from satellite passes around 2000 ut, prior to the start of the incoherent scatter radar operation. p, h p, hj = e h h h h = e e = j j p p p h hj = e = j p e e e e e = e b n + + 0 ( 2 2 i i i i i+ + p ions 2 2 ) h e e e e = e b n + 0 ( 2 2 2 i i i i + p ions 2 2 2 ) . 188 jürgen watermann, gary s. bust, jeffrey p. thayer, torsten neubert and clayton coker profile along the magnetic field line is known, e.g., from tomographic images, the height-integrated conductivities can be modeled via eq. (4.5), and the joule heating rate can be inferred with the help of the equivalent ionospheric current via eq. (4.4). work on this topic is currently in progress. 5. summary and outlook we compared seven cases of simultaneous tomographic images and isr electron density maps from the high-latitude ionosphere and obtained in five cases good and in one case partial agreement. the lack of agreement could be traced back to the presence of plasma patches moving with a few hundred meters per second through the field-of-view. the tomographic method employed tends to reproduce the integrated effect of the plasma structures moving through the imaging plane during the satellite reception interval. but the method did not appear to produce results confused by the plasma motion and entirely misleading except for observations during the onset of a moderate substorm which constituted the worst agreement. since a substorm can easily be detected in ground magnetic field measurements, such observations may possibly serve as a means to predict the performance of the tomographic method in specific cases. however, this needs to be further investigated using a larger observational data base. the result has potentially significant bearing on the application of the tomographic method at high latitudes not only to ionospheric imaging but also to investigations of the lower atmosphere. we have further outlined a method to combine tomographic imaging with ground-based magnetometer observations to derive the ionospheric electric field and joule heating rate. this requires accurate mapping of the ionospheric e region, and it has yet to be confirmed that high-latitude tomography can reliably resolve the e region plasma density distribution. a test and validation of e region imaging will be performed in a way similar to the one described in this paper, namely by comparing tomographic images with isr maps of e region plasma density, electric field strength and joule heating rates. the assumption of negligible neutral wind speed is not always justified. from an analysis of sondrestrom isr measurements, thayer (2000) deduced that neutral winds can reduce the transfer of electrical energy from the magnetosphere to the upper atmosphere by some 20% for moderate energy deposition rates (< 10 mw/m) and between 30% and 50% for higher deposition rates. refinements to the method outlined here are needed in order to obtain a quantitatively more realistic assessment of the joule heating rate. acknowledgements the tomographic receivers on the greenland west coast are operated by the applied research laboratories, the university of texas at austin (arl:ut), with financial support from the national science foundation (nsf) through grant atm-9813864, and in collaboration with the danish meteorological institute (dmi). the sondrestrom incoherent scatter radar is operated by sri international under the nsf cooperative agreement atm-9813556 and in cooperation with dmi. the greenland ground-based magnetometers are operated by dmi. references austen, j.r., s.j. franke and c.h. liu (1988): ionospheric imaging using computerized tomography, radio sci., 23, 299-307. bernhardt, p.a., r.p. mccoy, k.f. dymond, j.m. picone, r.r. meier, f. kamalabadi, d.m. cotton, s. charkrabarti, t.a. cook, j.s. vickers, a.w. stephan, l. kersley, s.e. pryse, i.k. walker, c.n. mitchell, p.r. straus, h. na, c. biswas, g.s. bust, g.r. kronschnabel and t.d. raymund (1998): twodimensional mapping of the plasma density in the upper atmosphere with computerized ionospheric tomography (cit), phys. plasmas, 5, 2010-2021. bust, g.s., t.l. gaussiran ii and d.s. coco (1997): ionospheric observations of the november 1993 storm, j. geophys. res., 102, 14,293-14,304. bust, g.s., c. coker, d.s. coco, t.l. gaussiran ii and t. lauderdale (2001): iri data ingestion and ionospheric tomography, adv. space res., 27(1), 157-165. foster , j.c., m.j. buonsanto, j.m. holt, j.a. klobuchar, p. fougere, w. pakula, t.d. raymund, v.e. kunitsyn, e.s. andreeva, e.d. tereshchenko and b.z. khudukon (1994): russianamerican tomography experiment, int. j. imag. syst. tech., 5, 148-159. friis-christensen, e., y. kamide, a.d. richmond and s. matsushita (1985): interplanetary magnetic field control of high-latitude electric fields and 189 imaging high-latitude ionospheric plasma structures currents determined from greenland magnetometer data, j. geophys. res., 90, 1325-1338. hedin, a.e. (1987): msis-86 thermospheric model, j. geophys. res., 92, 4649-4662. kelly, j.d. (1983): sondrestrom radar initial results, geophys. res. lett., 10, 1112-1115. leitinger, r., g.k. hartmann, f.-j. lohmar and e. putz (1984): electron content measurements with geodetic doppler receivers, radio sci., 19, 789-797. mandea, m. and s. macmillan (2000): international geomagnetic reference field the eight generations, earth planets space, 52, 1119-1124. mitchell, c.n., d.g. jones, l. kersley, s.e. pryse and i.k. walker (1995): imaging of field-aligned structures in the auroral ionosphere, ann. geophys., 13, 1311-1319. popov, v.a., v.o. papitashvili and j.f. watermann (2001): modeling of equivalent ionospheric currents from meridian magnetometer chain data, earth planets space, 53, 129-137. pryse, s.e., l. kersley, m.j. williams, i.k. walker and c.a. willson (1997): tomographic imaging of the polar-cap ionosphere over svalbard, j. atmos. terr. phys., 59, 1953-1959. thayer, j.p. (2000): high-latitude currents and their energy exchange with the ionosphere-thermosphere system, j. geophys. res., 105, 23,015-23,024. vasicek, c.j. and g.r. kronschnabl (1995): ionospheric tomography: an algorithm enhancement, j. atmos. terr. phys., 57, 875-888. watermann, j., o. de la beaujardiére and f.j. rich (1993): comparison of ionospheric electrical conductances inferred from coincident radar and spacecraft measurements and photoionization models, j. atmos. terr. phys., 55, 1513-1520. miscellanea 553 annals of geophysics, vol. 51, n. 4 august 2008 key words geomagnetism – magnetic dating – relocation 1. introduction the geomagnetic field is commonly described through its potential, which can be represented by a spherical harmonic series. a series of coefficients (that are referred to as gauss coefficients) of the spherical functions are enough to accurately depict the features of the field for a given time. the international association of geomagnetism and aeronomy (iaga) publishes regularly estimates of the gauss coefficients at five-yearly intervals (maus et al., 2005) these constitute the so-called igrf models that cover the evolution of the geomagnetic field from 1900 up to present. another set of time-dependent gauss coefficients, covering the interval from 1590 to 1990, compose the gufm1 model by jackson et al. (2000). the historical part of the model is essentially based on ship log books. recently, both geomagnetic models (igrf9 and gufm1) have been used to compute the distribution of errors due to relocation of geomagnetic data (casas and incoronato, 2007). relocation is routinely carried out by archaeomagnetists and consists on reducing data to a central location. this procedure permits to compare geomagnetic data from different locaapplication of the relocation-error distribution on geomagnetic databases. analyses on the «historical italian geomagnetic data catalogue» lluís casas (1) and alberto incoronato (2) (1) departament de geologia, universitat autònoma de barcelona, spain (2) dipartimento di scienze della terra, università degli studi di napoli, italy abstract the reliability of the historical italian geomagnetic data catalogue, comprising 536 directions and 393 intensities, has been assessed by comparing the historical geomagnetic measurements with the gufm1 model predictions. such measurements were assessed at three selected relocation centres. for all the data contained in the catalogue it has been calculated the discrepancy between the relocated data and the gufm1-model prediction at the relocation centres. there is a correlation between relocation distance and the mean discrepancy. the upper limit of discrepancy assumable as relocation error has been selected using error distributions previously calculated using geomagnetic field models. angular and intensity threshold lines have been slightly shifted upwards to account for the estimated error of gufm1 model itself at the considered region, mainly due to the crustal field. the italian database proved to contain reliable data, as only a very low percentage of data (namely 14 directions and 20 intensities) can be considered anomalous. possible explanations for such questionable data are suggested. all the remaining data of this catalogue could thus be added to the databases used to produce regional or global geomagnetic models. mailing address: dr. lluís casas, departament de geologia, universitat autònoma de barcelona, edifici c, campus de la uab, 08193 bellaterra (cerdanyola del vallès), spain; e-mail: lluis.casas@uab.cat miscellanea 9-03-2009 14:41 pagina 553 554 l. casas and a. incoronato tions. the relocation process is usually performed using the «conversion via pole» method (cvp) and assumes a purely dipolar field within the area where the relocation is done. as is stated in casas and incoronato (2007) the distribution of relocation errors can be used by archaeomagnetists as an indication of the maximum error induced by such a practice. however, this conclusion is restricted to periods with similar or lower values of the ratio between dipole to non-dipole contributions than in the present geomagnetic field; this could cover at least the last 7000 years (korte and constable, 2005). fig. 1. map of italy and neighbouring areas with indication of several data locations: solid dots (•) indicate declination and inclination data at a given time from the historical italian geomagnetic data catalogue (cafarella et al., 1992a); open dots (�) indicate declination and inclination data at a given time from the database used to compute gufm1 model; small dots (·) indicate single data values (declination or inclination) from the database used to compute gufm1 model and finally open diamonds (�) indicate the three locations selected as relocation centres. miscellanea 9-03-2009 14:41 pagina 554 555 relocation-error distribution to analyse the «historical italian geomagnetic data catalogue» another possible application of the distribution of relocation errors can be found on observatory data from historical periods (last 400 years). the error distribution can be used to detect spurious data within historical geomagnetic databases. the historical italian geomagnetic data catalogue (cafarella et al., 1992a) has been used as a demonstrative example. this database, fully available at the cited reference (cafarella et al., 1992a) is an excellent compilation of declination, inclination and intensity data (both isolate and simultaneous determinations essentially from the 19th century) from many locations in italy obtained from libraries of old institutes and universities, as well as data from italian observatories (cafarella et al., 1992b). the limited resolution of historical (and older) models that often results in an imperfect representation of the non-dipole field structure, brings a worthy motive to apply the relocationerror distribution, which checks the self-consistency of data assemblies, instead of founding decisions on data quality solely on a geomagnetic field model. moreover, the methodology used and presented in this paper allows a quick survey on long lists of geomagnetic data without the need of computing the «true» values (from a global model) at every single location but only at a few selected relocation centres. 2. analysis on the historical italian geomagnetic data catalogue 2.1. methodology the geomagnetic field for the time period of the data from the italian catalogue is well described by the gufm1 model (jackson et al., fig. 2. comparison between the neapolitan geomagnetic data, circles, from the historical italian geomagnetic data catalogue (cafarella et al., 1992a) and the gufm1 (jackson et al., 2000) field model, grey line, in naples. inset: angular mismatch between data from the catalogue and the corresponding values computed using gufm1 model. miscellanea 9-03-2009 14:41 pagina 555 556 l. casas and a. incoronato 2000). it is worth to note that data from the italian catalogue were not used to build the gufm1 model (see fig. 1), one of the authors of this model (a. jackson) kindly provided us with the entire database used in his model and he also confirmed in a personal communication with the authors that the italian catalogue was not used to develop gufm1. however, the consistency between the italian catalogue and the model can be assessed by comparing the historical geomagnetic measurements made in naples (which is the most represented location within the italian catalogue) with the model predictions for this location (fig. 2), coefficient r2 is 0.995. the calculation of the relocation error for any given geomagnetic datum requires the knowledge of the «true» value of the magnitude considered (direction or intensity) at the relocation centre. relocation error distributions presented in casas and incoronato (2007) were calculated considering a grid of relocation centres. for each of them, geomagnetic data from neighbouring sites (within a radius of 1200 km) obtained from a model (igrf or gufm1) were relocated. the relocation error was calculated as the difference between the relocated data and the data predicted by the model at the relocation centre. relocation error distributions calculated using the historical database were computed similarly. the «true» values at the relocation centres were calculated using the gufm1 geomagnetic field model. data containing both declination and inclination of a given site and time were selected from the italian catalogue to carry out the directional analysis. for the intensity analysis, data containing both inclination and the horizontal fig. 3. angular discrepancy between the relocated italian-catalogue data and gufm1 values at viterbo as a function of relocation distance. correlation is apparent. miscellanea 9-03-2009 14:42 pagina 556 557 relocation-error distribution to analyse the «historical italian geomagnetic data catalogue» projection (h) of the magnetic vector were selected. all the selected data (536 directions and 393 intensity values) were relocated to each one of three central locations distributed along the italian latitude range (la spezia, viterbo and naples) and their alleged relocation error was evaluated as the discrepancy between the relocated values and the ones predicted according to the gufm1 model. the correlation between relocation distance and the mean «discrepancy» is the same found in casas and incoronato (2007). as an example the angular discrepancy between relocated data from the italian catalogue and gufm1 values at viterbo as a function of relocation distance is plotted in fig. 3. the correlation is apparent, r2 parameter is 0.167. to cancel out the dispersion effect the correlation parameter can be computed averaging the data at 10 km intervals, then r2 raises to 0.454 and using only data with a relocation distance lower than 450 km is already 0.815. however some data exhibit discrepancy values that cannot be explained as solely due to relocation error. the upper limit of discrepancy assumable as relocation error has been selected using error distributions presented in casas and incoronato (2007). figure 4 shows the evolution of the maximum values of relocation errors (both angular and intensity errors) through time according to the models. three types of maximum values have been plotted: the absolute global maxima, the maxima for the range of latitudes of italy and finally the actual italian maxima. the absolute global errors have kept increasing for the last 200 years, this is due to the decline of the dipole to non-dipole field ratio although it might be also partially due to the increasing resolution of the model for the most modern years. in any case, the local maximum errors (those across the italian latitude or simply those of the italian area) are similar and relatively constant through the time period analyzed. therefore, the upper limits of assumable discrepancy for fig. 4. evolution of the maximum values of (a) angular and (b) intensity relocation errors through time according to the gufm1 and igrf-9 models (dashed and continuous lines respectively). bold (italian maxima), dark grey (italian latitude maxima), light grey (global maxima). miscellanea 9-03-2009 14:42 pagina 557 558 l. casas and a. incoronato the italian region have been selected as 0.35º and 300 nt per every relocated hundred of kilometres. threshold lines have been shifted upwards 0.5º and 300 nt to account for the estimated error of gufm1 model itself at the considered region, which is mainly due to the crustal field (jackson, et al., 2000). this methodology allows a quick survey on long lists of geomagnetic data without the need of computing the «true» values (from a global model) at every single location but only at some relocation centres. 2.2. results figure 5 shows the discrepancy between relocated and predicted values at viterbo as a function of relocation distance. the shifted and unshifted upper limits have been plotted to reveal the outliers, though they show discrepancy values well above the average ones. the shift of the threshold lines reduces the number of outliers and can be considered as a cautious filter to avoid the exclusion of appropriate data. tables i and ii show the assembly of data that being relocated fail to lie below the line of assumable relocation error for at least one of the three relocation centres. these are 14 directions and 20 intensity data that could be regarded as anomalous. they only represent the 2.6% and the 5.1% of the total amount of directions and intensity values respectively. 3. discussion although some data are revealed as erroneous for any relocation centre considered, it is noticeable that every relocation centre has a higher sensitivity to detect outliers from nearby locations. for instance, spurious data from northern italy (milano, moncalieri…) are especially detected from la spezia, whereas spurious data from sicily (palermo, caltanissetta…) are particularly detected from naples. this indicates that the defined threshold could actually be logarithmic instead of linear. however, systematic evaluations described in casas and incoronato (2007) point to a linear dependence of the maximum errors up to relocation distances of 1200km. anyhow, this non-uniform sensitivity advises to perform this kind of analysis from a grid of relocation centres instead of a single central point. the anomalous values could be due to locally important crustal disturbances, for instance 50% of the spurious directional data comes from moncalieri and lanzo torinese (two locations from the piedmont only 30 km away from each other). these data were published by a single author (boddaert, 1907). however, most fig. 5. angular (a) and intensity (b) discrepancy between relocated data from the italian catalogue and gufm1 values at viterbo as a function of relocation distance. shifted line of maximum assumable relocation error has been plotted to identify anomalous data. miscellanea 9-03-2009 14:42 pagina 558 559 relocation-error distribution to analyse the «historical italian geomagnetic data catalogue» of the data in the italian database from these two locations and from superga, spelled as soperga in the database, (~10 km away from moncalieri) published by several authors lie just below the defined threshold. in fact, situating moncalieri and its surrounding locations on an aeromagnetic anomaly map of italy (caratori-tontini et al., 2004) one notices that they lie in a zone with important positive and negative magnetic anomalies and thus with important anomaly gradients which can result in high relocation errors. concerning the anomalous directional data from other locations, we could presume instrumental errors for one datum from naples (1829.5) and two data from rome (1640.5 and 1833.1) as they are the oldest measurements (comprising both declination and inclination) from these two locations. the correlation between time and angular mismatch observed in fig. 2 (inset) supports the signs of lower quality for the oldest data. although not being that old, possibly instrumental errors are also affecting the anomalous values registered in bormio and castelnuovo because they are detected for any of the three selected relocation centres and their associated relocation errors are several times the average relocation error values and thus the discrepancy cannot be merely attributed to the combined effect of relocation procedure and crustal field. anomalous intensity values can be partly explained by the low quality of their associated directional data which is fundamental to compute the relocation of intensity. for a significant part of the anomalous intensity data (40%) the associated declination values were not available and thus, for the relocation of the intensity, the position of the virtual pole obtained from a table i. directional data from the historical italian geomagnetic data catalogue (cafarella et al., 1992a) that fail to lie below the line of assumable relocation error for at least one of the three relocation centres: la spezia (s), viterbo (v) or naples (n). failure is indicated by ✓ sign. references are labelled according to cafarella et al. (1992a). location time reference s v n roma 1640.5 kircher*(1643) ✓ ✓ ✓ napoli 1829.5 fisher1 ✓ roma 1833.1 pianciani1 ✓ milano 1863.8 buzzetti*(1864) ✓ bormio 1887.0 chistoni*(1888) ✓ ✓ ✓ castel piano 1889.6 palazzo10 ✓ castelnuovo 1890.0 kesslitz10 ✓ ✓ ✓ moncalieri 1905.5 boddaert*(1907) ✓ moncalieri 1905.5 boddaert*(1907) ✓ lanzo torinese 1906.0 boddaert*(1907) ✓ moncalieri 1906.5 boddaert*(1907) ✓ moncalieri 1906.5 boddaert*(1907) ✓ moncalieri 1906.5 boddaert*(1907) ✓ lanzo torinese 1907.0 boddaert*(1907) ✓ ✓ miscellanea 9-03-2009 14:42 pagina 559 560 l. casas and a. incoronato table ii. intensity data from the historical italian geomagnetic data catalogue (cafarella et al., 1992a) that fail to lie below the line of assumable relocation error for at least one of the three relocation centres: la spezia (s), viterbo (v) or naples (n). failure is indicated by ✓ sign. values in italics indicate that the corresponding directional data (table i) also failed. references are labelled according to cafarella et al. (1992a). location time reference s v n napoli 1805.0 humboldt33 & humboldt1 ✓ ✓ ✓ a napoli 1829.5 fisher1 ✓ milano 1834.9 von waltershansen1 ✓ ✓ ✓ a firenze 1835.0 listing1 ✓ ✓ ✓ a palermo 1836.0 listing1 ✓ ✓ roma 1838.4 bache1 ✓ a mantova 1846.6 kreil1 ✓ ✓ ✓ napoli 1859.5 fox1 ✓ a ruta 1859.5 fox1 ✓ a spoleto 1859.5 fox1 ✓ a napoli 1860.3 quetelet1 ✓ ✓ a milano 1863.8 buzzetti*(1864) ✓ roma 1870.6 braun1 ✓ napoli 1871.0 secchi1 ✓ trapani 1881.0 chistoni*(1881) ✓ palermo 1881.9 chistoni*(1881) ✓ catania 1882.0 chistoni10 ✓ ✓ ✓ catania 1882.0 chistoni10 ✓ ✓ ✓ arenzano 1885.9 chistoni10 ✓ caltanisetta 1890.6 chistoni10 ✓ adeclination values were not available. nearby location datum of similar age was used. the higher incidence of such kind of values compared with their occurrence (9%) within the general database of intensity values is evidence for the additional source of error due to the lack of knowledge of declination. two more data have directional values already detected as anomalous and thus the corresponding relocated intensity could lack of meaning. it is worth noting that in these cases the original (non-relocated) intensity values could be in fact not erroneous at all. finally, from the 9 remaining anomalous intensity values it is remarkable the high rate of data from sicily (6 values from 5 miscellanea 9-03-2009 14:42 pagina 560 561 relocation-error distribution to analyse the «historical italian geomagnetic data catalogue» different locations) although it is difficult to explain this geographical concentration in terms of crustal anomalies as the sicilian directional data does not appear as doubtful. 4. conclusions the distribution of relocation errors calculated using igrf-9 and gufm1 models has been applied to a historical geomagnetic database (the historical italian geomagnetic data catalogue) to detect abnormal data. specific conclusions (on the italian database) and general conclusions (on the approach used) can be drawn. the italian database has proven to contain reliable data, only a very low percentage of data (14 directions and 20 intensities) can be considered anomalous. this catalogue could thus be added to the databases used to produce regional or global geomagnetic models. some reasons can be hypothesized to explain some of the outliers: i) poor quality of the measurements, this appears to correlate with the oldest data, ii) intensity data can appear anomalous as a consequence of the poor quality of their associated directional data, iii) local crustal disturbances of the internal field, grounds for this effect are found for several data from the piedmont area. the latter reason implies that not all anomalous values are actually erroneous; discrepancies can arise from the fact that gufm1 model was devised as a model for the core-mantle boundary and its extrapolation to the earth’s surface disregards crustal contributors. the described methodology takes advantage of the higher resolution of the geomagnetic models that describe the present field to define the limits to decide whether a value is consistent or requires additional explanation. simple comparison between the historical geomagnetic database values and its corresponding values computed from a model would evade an overview on the global structure of the field and it would be difficult to establish acceptance limits as there are no uncertainty indications related to the values predicted by the model. a secondary advantage of the described methodology over the simple comparison between every single data and its value computed from a model is precisely the fact that it is not necessary to compute all the data through the model. this could be important whenever the model is difficult to compute or simply not computable for some areas (e.g. when using a non-global model to evaluate data that lie outside the application limits of the model, the data could be relocated to a centre within the application area). finally, even without any model at all, the approach presented here could be used as a way to check the self-consistency of any assembly of data by relocating contemporaneous data to every neighbouring available data location. this should be helpful in areas where the existing models predict the field based on only a few historical records and specially when dealing with assemblies of archaeomagnetic data from a given area. acknowledgments financial support from aarch – research training network (ue, contract n. prnct2002-00219) and eme2005-42 projects is acknowledged. the paper benefited from constructive reviews by two anonymous reviewers. references boodaert, d. (1907): misure magnetiche nei dintorni di torino. declinazione e inclinazione, memoria della r. accademia delle scienze di torino, 58, series 2, 397-450. cafarella, l., a. de santis and a. meloni (1992a): the historical italian geomagnetic data catalogue, ing, rome, pp. 160. cafarella, l., a. de santis and a. meloni (1992b): secular variation in italy from historical geomagnetic field measurements, phys. of the earth and planet. int., 73 (3-4), 206-221. caratori-tontini, f., p. stefanelli, i. giori, o. faggioni and c. carmisciano (2004): the revised aeromagnetic anomaly map of italy, annals of geophysics, 47 (5), 1547-1555. casas, ll. and a. incoronato, (2007): distribution analysis of errors due to relocation of geomagnetic data using the «conversion via pole» (cvp) method: implications on archaeomagnetic data, geophys. j. int., 169, 448-454. jackson, a., a jonkers, and m. walker (2000): four centuries of geomagnetic secular variation from historical records, phil. trans. r. soc. lond., a. 358, 957-990. korte, m. and c.g. constable, (2005): the geomagnetic miscellanea 9-03-2009 14:42 pagina 561 562 l. casas and a. incoronato dipole moment over the last 7000 years–new results from a global model, earth and planet. sc. letters, 236, 348–358. maus, s., s. macmillan, t. chernova, s. choi, d. dater, v. golovkov, v. lesur, f. lowes, h. lühr, w. mai, s. mclean, n. olsen, m. rother, t. sabaka, a. thomson and t. zvereva (2005): the 10th generation international geomagnetic reference field, phys. of the earth and planet. int. 151 (3-4), 320-322. (received march 26, 2008; accepted april 23, 2008) miscellanea 9-03-2009 14:42 pagina 562 annals of geophysics, 60, 2, 2017; s0216; doi:10.4401/ag-7077 seda a software package for the statistical earthquake data analysis: a tutorial application to the 2009 l’aquila and the 2012 emilia (italia) sequences anna maria lombardi 1 1 istituto nazionale di geofisica e vulcanologia, rome, italy article history received september 6, 2016; accepted november 30, 2016. subject classification: earthquake interactions and probability, statistical analysis, algorithms and implementation. abstract the main purpose of this paper is to provide a tutorial application of sedav1.0, the first version of a software package, recently designed for the statistical analysis of earthquake data. sedav1.0 consists of a user-friendly matlab-based interface, to facilitate the interaction with the application, and of a computational core of fortran codes, to guarantee fast running times. the main part of sedav1.0 is devoted to the etas modeling. for the first time, an almost complete set of consistent tools based on etas models is collected in a single, free software. moreover, seda guarantees the research reproducibility, which is becoming an increasingly major concern among scientists. the peculiarities of some routines of sedav1.0 are discussed in this paper, by the application to two important recent seismic sequences occurred in italy. specifically, the paper illustrates how using sedav1.0, to estimate the completeness magnitude and the b-value, to set and test the etas model and, finally, to identify the earthquakes sequences, basing on causal connections. 1. introduction this paper describes the use of the first version of seda (statistical earthquake data analysis, sedav1.0), a new software designed for the statistical analysis of earthquake data, by the application to two recent sequences occurred in italy. the tools collected in sedav1.0 are classified in two main topics (lombardi, 2016). the first class, called catalog analysis, allows the descriptive analysis of an earthquake catalog, the selection of its subsets and the estimation of the magnitude distribution. this set of tools includes original, but not innovative, codes and supports the user, in managing the database and in evaluating its homogeneity and magnitude completeness. the second group of tools, called etas model, is designed for the analysis of an earthquake database by the etas (epidemic type aftershock sequence) modeling (ogata, 1988; 1998). it is the core of sedav1.0 and contains original and partially innovative fortran codes. the design of software as sedav1.0 is requested by the code share policy, which is a main point for the reproducibility of published research results and highly recommended by the most important scientific journals (nature editors, 2014a). sharing a code means that the source or the executable code is freely accessible to the public. this allows replication of results, which is a key concept in science, and ensures that the scientific community can apply the methodology to their own data, without the need of re-implementing the algorithms. moreover, the free distribution of a code allows the evaluation of its performance and helps the comparison of different methodologies. sedav1.0 is freely provided via the zenodo open access platform (https://zenodo.org/), a service that allows deposit and doi assignment to software, besides of ensuring an easy and stable access. please refer to https://zenodo.org/record/55277 to download the first version of seda for the mac operating system (a windows version will be available soon), including the user manual. some technical details about sedav1.0 are discussed also in lombardi (2016). this paper illustrates the use of sedav1.0, by mean of an application to two important sequences, occurred in italy in the last years, following the l’aquila (april 6, 2009, ml5.9) and the emilia (may 20, 2012, ml5.9) earthquakes (see figure 1). firstly, i estimate the completeness and the b-value for the magnitude distribution. second, the etas model, implemented in sedav1.0, is applied and tested on both sequences. finally, the procedure for sequences identification is presented and discussed. s0216 lombardi 2. cases study: the 2009 ml5.9 l’aquila and the 2012 ml5.9 emilia sequences the l’aquila and the emilia sequences are located in areas with different tectonic structure and have specific peculiarities. the l’aquila region is inside the central apennines belt, with prevalent normal faulting, whereas the emilia area covers alluvial lowland, with thrust faulting. the l’aquila mainshock was preceded by at least 3 months of moderate-size seismicity and struck an highly hazardous seismic zone of italy (stucchi et al., 2010). the emilia sequence, occurred in a relatively low seismic hazard area, was characterized by a migration of the seismicity towards the e and ne and its strongest aftershock (may 29, 2012, ml5.8) has a magnitude close to the mainshock. the data of the italian seismicity are downloaded from the site of the official ingv bulletin, www.iside.rm.ingv.it. i select the events occurred from april 16, 2005 up to april 30 2016. the starting data marks the start-up of a new seismic network, causing a significant improvement of the earthquakes detection (bono and badiali, 2005; schorlemmer et al., 2010). the areas interested by the l’aquila and the emilia sequences are [13.15-13.65e, 42.10-42.70n] and [10.60-11.80e, 44.70-45.10n], respectively (see figure 1). the ingv bulletin collects 29,787 events (ml from 0.1 to 5.9 and depth above 28km), inside the first area, and 3,014 earthquakes (ml from 1.2 to 5.9 and depth above 37km), in the emilia region. the lower minimum magnitude of the l’aquila dataset is due to 5˚ 5˚ 10˚ 15˚ 15˚ 20˚ 20˚ 35˚ 35˚ 40˚ 40˚ 45˚ 45˚ 0 4 8 12 16 20 24 28 32 36 40 13˚30' 42˚30' 11˚00' 11˚30' 45˚00' de pt h l’aquila emilia figure 1. map of italian seismicity, occurred from 04/16/2005 to 01/05/2016, with magnitude above ml2.5 and depth above 40kms. the blue rectangles identify the areas of the two most important sequences of the last years: the april 6, 2009 l’aquila (ml5.9) and the may 20, 2012 emilia (ml5.9) sequences, of which seismicity is zoomed in the smaller panels. blue stars mark the events with magnitude above ml5.0. 2 a tutorial application of software seda the dense seismic network in this zone, whereas the deeper events of the emilia region are generated from the deep thrusting structures, well-recognized in this zone (vannoli et al., 2015). in the following sections i present and discuss the data analysis. the title of each of them contains, in the brackets, the reference to the sedav1.0 tools, used for the specific application discussed inside. all the results refer to a retrospective analysis: the parameters of the etas model are set on relocated earthquake data, including those of the l’aquila and emilia sequences. this strongly differs from the purely prospective, real-time earthquake forecast experiments, made during the l’aquila and the emilia seismic emergencies, using provisional data and models independently set (marzocchi and lombardi, 2009; marzocchi et al., 2012). 3. completeness magnitude and b-value estimation (catalog analysis tool “b-value analysis”) this first part of sedav1.0 is devoted to a quick descriptive analysis of an earthquake catalog, including the subsets selection and the magnitude completeness and b-value analysis (lombardi, 2016). in sedav1.0 the gutenberg-richer law is adopted for magnitudes; it has the following probability density function (1) where β=b⋅ln(10) is a parameter and mc is the completeness magnitude of the database. further magnitude distributions will be added in future versions. sedav1.0 assumes a magnitude step of 0.1 and uses two methods to estimate b and mc: the mc and b-value stability method (mbs; cao and gao, 2002; woessner and wiemer, 2005) and the goodness of fit test method (gft; wiemer and wyss, 2000). this last is performed in sedav1.0 both at 90% and at 95% confidence levels. moreover, sedav1.0 fixes a magnitude range equal to 0.5 to calculate the b-value means for the mbs method (see woessner and wiemer, 2005, for details). i run the sedav1.0 “b-value analysis” tool on the l’aquila and emilia databases. previous studies, based on the national seismic network detection, defined mc = 2.5 as a reasonable completeness magnitude for most of the italian territory, starting from april 16 2005 (schorlemmer et al., 2010). by applying the sedav1.0 “b-value analysis” tool on the l’aquila region, i find a completeness local magnitude mc=1.2, 1.5 and 1.8 by mean of the gft(90%), gft(95%) and mbs methods, respectively (table 1). the related b-values are b=0.85, 1.0 and 1.1. i find larger, more consistent, values of mc=2.1, 2.2, 2.2 for the emilia area, while the related b-values are b=0.9, 0.97, 0.97. the larger completeness magnitude of the emilia region, respect to l’aquila area, is due to the larger depth of the events and to the lower detection of the national seismic network. all the above results are consistent with schorlemmer et al. (2010). previous estimation does not take into account any possible temporal variations of the completeness magnitude. figure 2 shows the time/magnitude plots of the seismicity, soon after the occurrence of the strongest events of both sequences (april 06 2009, ml5.9, for the l’aquila sequence; may 20 2012, ml5.9, and may 29 2012, ml 5.8, for the emilia sequence). they reveal an increase of the minimum detection magnitude up to ml2.5. the values of mc and b, estimated on the seismicity of the first 12 hours after the strongest events, are reported in table1. for both sequences there is a significant increase of mc, up to ml2.5, respect to the overall sequence. moreover, the b-values decrease down to 0.7, in the emilia region. for all above said, i select the events above mc=2.5 for both sequences. the inferred b-values are 1.1 and 1.0 for the l’aquila (1153 events) and the emilia (964 events) regions, respectively (see table1 and figure 3). f (m)= β ⋅exp −β ⋅ m−mc( )⎡⎣ ⎤⎦ 2012.3825 2012.383 2012.3835 2012.384 2012.3845 2012.385 2012.3855 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 2009.26 2009.2605 2009.261 2009.2615 2009.262 2009.2625 2009.263 2009.2635 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 time (year) m ag ni tu de l’aquila emilia 2012.407 2012.4075 2012.408 2012.4085 2012.409 2012.4095 2012.41 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 emiliaml5.8 29 may 2012 ml5.9 20 may 2012 ml5.9 06 april 2009 figure 2. time-magnitude plot for the events occurred in the first day, after the strongest events, of the l’aquila and the emilia sequences. the plot shows an increase of the detection in the first hours after their occurrence. 3 lombardi magnitude 0 1 2 3 4 5 6 lo g1 0 of n um be r of e ar th qu ak es 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 fixed mc cumulative number histogram gr fit b fix mc = 0.31 (0.00) mc: 2.5 b fix mc = 1.08 (0.03) magnitude 1 2 3 4 5 6 lo g1 0 of n um be r of e ar th qu ak es 0 0.5 1 1.5 2 2.5 3 3.5 fixed mc cumulative number histogram gr fit b fix mc = 0.37 (0.00) mc: 2.5 b fix mc = 0.99 (0.04) a) b) figure 3. b-value estimation for the l’aquila and the emilia databases, obtained by sedav1.0, fixing mc=2.5. 4. etas model the core of sedav1.0 is a set of tools related to the temporal-magnitude (tm) and the time-magnitude-space (tms) etas modeling. a so comprehensive set of tools, based on etas, is collected in a single, free software, for the first time. some packages are been developed and made available in the past, but they refer to different versions of the etas model and allow only a partial analysis of an earthquake catalog (see lombardi, 2016, for a comprehensive list). sedav1.0 collects two classes of tools, based on the etas model. specifically, the etas basic tools are the most important algorithms concerning an etas model. they allow the estimation of the model (parameters and background), the log-likelihood calculation, the declustering of the catalog, the testing of an etas model on data, forecasting calculations and the simulations of earthquakes databases. the etas additional tools are designed to deep the investigation of a catalog. they allow the calculation of the background/triggering probabilities for all events, the selection of sequences and the computation of retrospective forecasts. the conditional intensities of the tm and tms etas models, implemented in seda are, respectively: (2) where f(m) = β ⋅exp −β ⋅(m−mc)[ ] 1−exp −β ⋅(mmax −mc)[ ] is the magnitude probability density function and mmax is the maximum magnitude allowed; ht is the history of the process up the time t; λtm t,m|ht( )= µ+ k⋅exp α ⋅ mi −mc( )⎡⎣ ⎤⎦ (t −ti +c) p ti<t ∑ ⎧ ⎨ ⎪ ⎩⎪ ⎫ ⎬ ⎪ ⎭⎪ f (m) λtms(t,m,x,y|ht )= µ ⋅u(x,y)+ k⋅exp α ⋅(mi −mc)⎡⎣ ⎤⎦ (t−ti +c) p ti<t ∑ ⎧ ⎨ ⎪ ⎩⎪ l’aquila emilia overall april 06 2009 ml 5.9 firts 12h (454 events) overall may 20 2012 ml 5.9 firts 12h (220 events) may 29 2012 ml 5.8 firts 12h (239 events) mc b mc b mc b mc b mc b gft 90% 1.2 0.85 (0.005) 2.3 0.97 (0.04) 2.1 0.91 (0.02) 2.1 0.59 (0.03) 2.3 0.64 (0.04) gft 95% 1.5 1.00 (0.008) 2.5 1.10 (0.07) 2.2 0.97 (0.02) nan nan nan nan mbs 1.8 1.09 (0.01) 2.5 1.10 (0.07) 2.2 0.97 (0.02) 2.3 0.69 (0.05) 2.5 0.71 (0.05) fixed magnitude (2.5) 2.5 1.08 (0.03) 2.5 1.10 (0.07) 2.5 0.99 (0.04) 2.5 0.75 (0.06) 2.5 0.71 (0.05) table 1. estimation of the gutenberg-richter distribution for the l’aquila and emilia overall seismicity and for the first hours after the occurrence of the strongest events. ⋅ cd,q,y ri 2 +d2 ⋅e2γ(mi−mc)⎡⎣ ⎤⎦ q ⎫ ⎬ ⎪ ⎭⎪ f (m) 4 {u(x,y),(x,y)er} is the spatial density function of the background events in the region r of interest. this is assumed uniform in each of the nc cells cj (of area aj) of a regular grid (in degrees), recovering r, so that with (3) {μ,k,p,c,α} and {μ,k,p,c,α,d,q,γ} are the 5/8 parameters for the tm/tms model to be estimated; ri is the distance (in kms) between the location (x,y) and the epicenter of the i-th event (xi, yi); c(d,q,γ ) is a normalization constant so that seda gives the opportunity of including a precursory period (an interval time before the target period, on which the model is estimated or applied) into calculations. in this way, there is a better evaluation of the interactions, since the triggering effect of precursory earthquakes on the target period is taken into account. the following subsections describe and discuss the application of some of the sedav1.0 “etas model” tools on the l’aquila and emilia sequences. 4.1 etas parameters estimation (basic etas tool “estimation of parameters”) one of the most innovative codes in sedav1.0 is a simulated annealing (sa) algorithm for the maximum likelihood estimation of the etas model (lombardi, 2015). unlike the newton methods, largely used by the other published codes (harte, 2010; ogata, 2006), the method implemented here allows an exhaustive evaluation of the model uncertainties, by mean of multiple runs. in this way, sedav1.0 both provides a probability distribution for each parameter (including the background rate) and identifies possible correlations among the parameters, causing a multimodal distribution of the maximum log-likelihood (lombardi, 2016). finally, this new algorithm rules out any dependence on the starting values of parameters, with the consequent risk of finding local maxima of the log-likelihood (lombardi, 2015). sa algorithms are random iterative procedures that generate a candidate point, that optimizes a function, and move to this point or stay at the current one, based on a stochastic mechanism. this latter is controlled by a parameter t, called temperature, which decreases during the algorithm, according to a “cooling” scheduled. the temperature and the cooling schedule are of critical importance to the success of sa. when t decreases, the search of new points becomes more directive; therefore, a too low t might cause an overly restricted search, whereas a high t or a slow cooling schedule might cause an overly high computational time and a possible unsuccessful search. finally, a too fast cooling schedule can trap the algorithm in a local maximum. in sedav1.0 a new cooling schedule for the temperature is implemented, respect to what proposed by lombardi (2015). specifically, sedav1.0 adopts the schedule proposed by ingber (1996) (4) where: a) tj is the temperature at the j-th iteration and t0 is the initial temperature; b) d is the number of parameters; c) l has the form l =-δ⋅exp(-ν/d). if one imposes that after j=exp(ν) iterations the values of the temperature is t − , then one can estimate the parameters δ and ν by the formulas (5) some applications of the algorithm on simulated data, with j varying from 10 to 100 and t − varying from 10-8 to 10-4, indicate as best parameters j = 30 and t − = 10-5, giving δ = 3.4 and ν = 13.8. therefore, the cooling schedule adopted in sedav1.0 is (6) i estimate the parameters of the tms etas model on the l’aquila and emilia regions, with the learning period april 16 2005, 00:00:00 january 01 a tutorial application of software seda u(x, y)= uj ajj=1 nc ∑ 1 x,y( )εcj{ } uj j=1 nc ∑ =1. cd,q,γ ri 2 +d2 ⋅e2γ(mi−mc)⎡⎣ ⎤⎦ q∫∫ =1∀i. tj =t0 exp l ⋅ j 1 d ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ v = ln(j) t =tj =t0 exp(−σ)=−ln t t0 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎧ ⎨ ⎪ ⎩ ⎪ tj =t0 exp −13.8⋅exp − 3.4 d ⎛ ⎝ ⎜ ⎞ ⎠ ⎟⋅ j 1 d ⎡ ⎣ ⎢ ⎤ ⎦ ⎥⋅ l’aquila emilia μ 0.024 (0.022, 0.27) 0.01 (0.009, 0.011) k 0.047 (0.042, 0.052) 0.050 (0.047, 0.055) p 1.29 (1.24, 1.31) 1.33 (1.31, 1.36) c 0.04 (0.03, 0.05) 0.028 (0.023, 0.037) α 1.45 (1.36, 1.53) 1.14 (1.08, 1.19) d 1.0 (0.9, 1.2) 1.0 (0.9, 1.2) q 2.7 (2.4, 2.9) 2.5 (2.2, 2.7) γ 0.50 (0.46, 0.53) 0.51 (0.47, 0.55) table 2. tms etas parameters for the l’aquila and the emilia sequences, estimated by sedav1.0. the 95% confidence bounds are reported in the brackets. 5 lombardi 2007, 00:00:00 and the testing period january 01 2007, 00:00:00 may 01 2016, 00:00:00. the step of the background grid is equal to 0.01°. table 2 lists the values of parameters, together with the 95% confidence bounds, obtained by 100 runs of the simulated annealing algorithm (lombardi, 2015), implemented in sedav1.0. the two sets of parameters are surprisingly very similar, considering the different regional tectonic settings of two sequences. the significantly different background rates μ are due to the different seismic potential of two areas. the smaller α value of the emilia sequence is likely due to the presence of stronger aftershocks, respect to the l’aquila sequence (see figure 3). figures 4 and 5 show the distribution of the 8 parameters {μ,k,p,c,α,d,q,γ} and of the maximum log-likelihood for both sequences. the maximum log-likelihoods given in all runs are very close and mostly recover a range of 2 values, showing a good performance of the estimation procedure. this result is due also to correlation of some parameters, so that different combifigure 4. distribution of the 8 parameters {μ,k,p,c,α,d,q,γ} estimated for the l’aquila sequence, by running 100 times the sedav1.0 tool. the last panel shows the distribution of the maximum log-likelihood. 0.02 0.025 0.03 0 100 200 300 400 0.04 0.045 0.05 0.055 20 40 60 80 100 120 1.2 1.25 1.3 1.35 0 5 10 15 20 0.02 0.04 0.06 10 20 30 40 50 1.3 1.4 1.5 1.6 0 2 4 6 8 0.8 1 1.2 1 2 3 4 2 2.5 3 0.6 0.8 1 1.2 1.4 1.6 0.45 0.5 0.55 0 5 10 15 20 25 -4700 -4695 -4690 0 0.1 0.2 0.3 0.4 l’aquila μ k p c α d q γ loglik figure 5. the same of figure 4, but for the emilia region. 0.008 0.01 0.012 100 200 300 400 500 600 0.045 0.05 0.055 0.06 0 50 100 150 1.25 1.3 1.35 1.4 0 5 10 15 20 25 0.02 0.03 0.04 0 50 100 150 1 1.1 1.2 1.3 0 2 4 6 8 10 0.8 1 1.2 1.4 0 1 2 3 4 5 2.2 2.4 2.6 2.8 0.5 1 1.5 2 2.5 0.45 0.5 0.55 0.6 0 5 10 15 20 -3080 -3078 -3076 -3074 0 0.2 0.4 0.6 μ k p c α d q γ loglik emilia 6 a tutorial application of software seda nations of them give very close log-likelihood values and, therefore, indistinguishable models (see figures 6 and 7). specifically, the sets of parameters {k,p,c,α} and {d,q,γ} are clearly correlated, whereas the background rate μ seems to be uncorrelated from any other parameter, revealing an univocal way to distinguish the background and the triggered activity. 4.2 model testing (basic etas tools “testing the model/residual analysis” and “testing the model/number of events test”) a further novelty of sedav1.0 concerns the testing of the etas model. actually, seda has three test options: the residuals analysis, the number of events test and the log-likelihood test. whereas the residuals analysis is a well-known procedure (ogata, 1988), the other two tests are a revised version of the analogue tests adopted by the csep laboratories (schorlemmer et al., 2007). specifically, the number of events and the log-likelihood tests consist in comparing the size and the log-likelihood of a catalog with the related theoretical distributions, inferred by the etas model. these last are obtained computing the size and the log-likelihood on a certain number of etas simulated cata mu k mu p mu c mu al ph a mu d mu q mu ga m m a k p k c k al ph a k d k q k ga m m a p c p al ph a p d p q p ga m m a c al ph a c d c q c ga m m a alpha d alpha q alpha ga m m a d q d ga m m a q ga m m a l’aquila mu k mu p mu c mu al ph a mu d mu q mu ga m m a k p k c k al ph a k d k q k ga m m a p c p al ph a p d p q p ga m m a c al ph a c d c q c ga m m a alpha d alpha q alpha ga m m a d q d ga m m a q ga m m a emilia figure 6. plot of all possible couples of tms etas parameters, estimated by the 100 runs of the simulated annealing algorithm, implemented in sedav1.0, for the l’aquila sequence. figure 7. the same of figure 7, but for the emilia sequence. 7 lombardi logs (see lombardi, 2014, for details). more detailed tests, concerning also the spatial distribution of the events, will be implemented in sedav1.0, in the future. firstly, i test the inferred etas models on both l’aquila and emilia regions, by mean of the residual analysis (ogata, 1988). specifically, sedav1.0 checks the hypothesis of a poissonian distribution of residuals, by mean of two tests: the runs test, which examines whether there is a temporal trend in the inter-event transformed times, and the one-sample kolmogorov-smirnov (ks1) test, which determines if the inter-event transformed times follow an exponential distribution (lombardi and marzocchi, 2007). sedav1.0 provides a p-value equal to 0.1 and 0.6, for the runs and the ks1 tests, respectively, for the l’aquila sequence. for the emilia region the correspondent p-values are 0.3 (runs) and 0.001 (ks1). the inconsistences between model and data, mainly limited to the first hours after the occurrence of the stronger shocks, are not significant for the l’aquila sequence. the difference between the observed and the expected number of events (see figure 8) becomes significant for the emilia sequence, after the occurrence of the strongest aftershock (may 29 ml5.8). i perform a daily number of events test, which tacks into account the aleatoric uncertainty of the model (lombardi, 2014), to better evaluate the consistency between model and data. specifically, i compare the observed daily number of events with the distribution obtained from 1000 etas simulations. the daily forecasting windows are updated each 3 hours, together with the history of events actually occurred before. figure 9 shows a good agreement between forecasts and observations, since these last are inside the 95% or the 99% confidence intervals of forecasts. some inconsistencies concern the day before the occurrence of the strongest events (april 06 2009, ml5.9, for the l’aquila sequence; may 20 2012, ml5.9, and may 29 2012, ml 5.8, for the emilia sequence); this reveals the unpredictability of their occurrence and, therefore, of the following significant increases of the seismic rate. the sensible agreement between model and data shows that the inconsistencies founded during the real-time forecasting experiments (marzocchi and lombardi, 2009; marzocchi et al., 2012) are likely due to the provisional nature of the earthquake data and of the etas model. there is, therefore, no need to turn to nonstationary versions of the etas models, for describing these sequences. 4.3 sequences identification (etas additional tool “identify sequences”) one of the most important etas additional tools of sedav1.0 is designed for the identification of setime 2008 2010 2012 2014 2016 n um be r of e ve nt s 0 200 400 600 800 1000 1200 events model 07/04/09 17/04/09 200 400 600 time 2008 2010 2012 2014 2016 n um be r of e ve nt s 0 200 400 600 800 1000 events model 21/05/12 31/05/12 200 400 600 a) b) figure 8. residuals analysis for the l’aquila and the emilia sequences. a) cumulative plot of observed events (blue points) in the l’aquila region, together with the expected cumulative distributions (red line). the small panel shows a zoom around the occurrence of the mainshock. b) the same of panel a), but for the emilia sequence. time 30/03/09 01/04/09 03/04/09 05/04/09 07/04/09 09/04/09 11/04/09 13/04/09 n um be r of e ve nt s 0 50 100 150 200 250 300 350 time 17/05/12 19/05/12 21/05/12 23/05/12 25/05/12 27/05/12 29/05/12 31/05/12 n um be r of e ve nt s 0 50 100 150 200 250 300 l’aquila emilia figure 9. number of events test for the l’aquila and the emilia regions. the expected and observed daily numbers of events are compared. the forecast times are updated each 3 hours. the red and the blue lines mark the observed and expected number of events, respectively. the forecasts are obtained as the median of 1000 etas simulations. the light and dark gray shaded areas mark the 95% and 99% confidence interval of forecasts, respectively. the vertical black dashed lines mark the first forecast made with included the strongest events in the history (april 06 2009 ml5.9 for the l’aquila sequence; may 20 2012 ml5.9 and may 29 2012 ml 5.8 for the emilia sequence). 8 quences and is based on the stochastic reconstruction, proposed by zhuang et al. (2004). specifically, the tool “identify sequences” applies the random method of zhuang et al. (2004) nrun times and assigns a set of integers {isi,1,…,isi,nrun} to each event ei, where isi,k is a number, that identifies the sequence of ei, for the k-th run. then, the user may choose a target event e − j and select all the events belonging to the same sequence of e − j , with a probability above a chosen probability level pl. the probability pi(e − j ) that ei and e − j , belong to the same sequence is computed as (8) the tool is run, by choosing the mainshocks as target event e − j , for both sequences. sedav1.0 identifies a sequence of 817 events, from 30/03/2009 to 12/08/2009, for 2009 l’aquila sequence, by mean of nrun=1,000 stochastic reconstructions, with a threshold probability pl=0.99. for pl=0.95, sedav1.0 identifies 855 events, from 17/02/2009 to 14/06/2010 (see figure 10). for the emilia sequence, sedav1.0 selects 843 events, from 19/05/2012 to 05/09/2012, for pl0.99 and 875 events, from 19/05/2012 to 26/10/2012, for pl0.95. if one runs the tool more times, then the number of events and the temporal limits of the sequences may slightly change, due to the stochastic nature of the method. figure 11 shows the temporal limits of both sequences, together with the number of selected events, as function of pl. for both sequences, the temporal limits significantly vary for pl>95% and are more stable for smaller values of pl, down to about 75% for l’aquila and 60% for emilia. the remaining values of pl are probably too small for an unequivocal assignment of the events to the sequences, given also the limited size of both catalogs, due to the relatively high value of mc. 4.4 retrospective forecast (etas additional tool “retrospective forecast”) the retrospective forecast gives a further possibility to test a model. the “retrospective forecast” sedav1.0 tool was implemented in a module apart from the other tests, among the etas additional tools, since it is not still a formalized test. it currently consists in comparing the overall expected number of events above a magnitude mf≥mc, given by the catalog, with the analogue values, given by a certain number of simulated catalogs. specifically, sedav1.0 computes the forecasts fri tm (tm model) and frij tms (tms model) (i.e. the number of expected events, conditional on the past) for the i-th simulated catalog, by the formulae (7) where ht i is the history up time t, collected in the i-th simulated catalog. these quantities represent a probability distribution for the etas forecasts and give the probabilities to have larger values of the “observed” expected values frtm or frj tms, obtained by replacing the actual observed history ht, in formulas (7). the retrospective forecast can provide an impora tutorial application of software seda pi ej( )= 1 isi ,k=isj ,k{ }k=1 nrun ∑ nrun figure 10. screenshot from sedav1.0 showing the results of the l’aquila sequence identification for pl=0.95, choosing as target event the mainshock (april 6 2009, ml5.9; see text for details). the figure shows the time-magnitude plot of the identified events. figure 11. temporal limits (blue line) and number of events (red line) as functions of the probability threshold pl for both the l’aquila and the emilia sequences. the target events are the mainshocks (april 06 2009 ml 5.9 for the l’aquila and may 20 2012 ml 5.9 for the emilia). the probability threshold pl goes from 0 to 1, with a step of 0.01 (see text for details). pl 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ti m e 01/01/09 01/01/10 01/01/11 01/01/12 01/01/13 01/01/14 01/01/15 01/01/16 01/01/17 l’aquila n e v 400 600 800 1000 1200 pl 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ti m e 01/01/12 01/01/13 01/01/14 01/01/15 01/01/16 01/01/17 emilia n e v 600 800 1000 fri tm = λtmmf mmax ∫t1 t 2 ∫ (t,m|hti)dtdm frij tms = λ(t,x,y,m|ht i )dtdxdydm mf mmax ∫c j∫∫t1 t2 ∫ 9 lombardi tant feedback, regarding the performance of the etas model and may be applied in different way, by varying the forecast time period or the threshold forecast magnitude mf. two specific applications are shown in the following. the first is an evaluation of the probability to have an event above ml5.0, in the l’aquila area, during the day april 06 2009, when the mainshock actually occurred. in the second example, i compute the probability to have an event above ml5.5, during the day may 29 2012, in which the strongest aftershock occurred (see figure 12). in the first example, i set as learning period april 16 2005, 00:00:00 april 06 2009, 00:00:00 and i computes the forecasts frij tms by mean of 10,000 simulations. the value of mf is 5.0 and the forecast period is april 06 2009, 00:00:00 april 07 2009, 00:00:00. the overall median expected rate median{ } is equal to 0.0039 (the 95% confidence interval is [0.0029 0.015]). the observed value is equal to 0.32 that is outside the inferred distribution. 3 on 10,000 simulated catalogs have 2 events above ml5.0, as actually occurred in the forecast day. in the second example, related to the emilia sequence, the forecast period is may 29 2012, 00:00:00 – may 30 2012, 00:00:00 and mf=5.5. sedav1.0 gives an overall median expected rate equal to 0.0071 and a 95% confidence interval [0.0042 0.021]. the observed value is 0.16 that is outside the inferred distribution. 90 simulated catalogs on 10,000 have 1 event above ml5.5, at least, as actually observed. the comparison of these last results with those obtained by the model testing (see subsection 4.2) shows a good ability of the etas model to reproduce the overall seismic rate, but also the “unpredictability” of the time occurrence of specific strong shocks. 5. conclusions: the future of seda the program seda, presented in this paper, has been developed with the main aim of allowing an easy use of the tools. specifically, the matlab gui facilitates the user control and allows the display of the results. moreover the modular design of the gui allows an easy upgrade of the actual version and the inclusion of new modules. for the first time, the most important operations concerning the etas model are collected in a single, free, user-friendly, software. from a scientific point of view, the most important novelties are the estimation method: this ensures the reaching of the overall best solution and gives an effective evaluation of the model uncertainties. particular care has been taken in developing the algorithms for the model testing and for the identification of sequences. seda has been developed in order to guarantee the reproducibility of research. the computational methods used in many published papers are often not fully explained or described. this is due both to confrij tms j=1 nc ∑ ;i =1,...,10000 a) b) figure 12. screenshots from sedav1.0 showing the results of the retrospective forecasting for the l’aquila and the emilia sequences. a) retrospective forecasting of the seismicity occurred from april 06 2009, 00:00:00 to april 07 2009, 00:00:00, in the l’aquila area, with mf=5.0 and 10,000 simulations. the histogram shows the distribution of the number of events above 5.0 for the simulations. the observed number is 2. b) retrospective forecasting of the seismicity occurred from may 29 2012, 00:00:00 to may 30 2012, 00:00:00, in the emilia area, with mf=5.5 and 10,000 simulations. the map shows the spatial distribution of the expected median number of events (median { }) above 5.5 for each cell cj (of size 0.01°x0.01°). frjj=1 nc ∑ tms frij tms j=1 nc ∑ ;i =1,...,10000 10 straints imposed by the traditional research papers and to the reluctance to share intellectual property. as a consequence, it is difficult to reproduce research results, to verify their correctness and to build on them in future research and applications. reproducible research has become a prominent issue in several academic fields (see, for example, liu et al., 2015; irving, 2015, among many others published papers). journals as science and nature have published numerous articles about reproducibility and editorials in favor of policies that promote open science. finally the code sharing should at least lead to better quality of codes themselves (nature editors, 2014a; 2014b; mcnutt, 2014; geoscientific model development editors, 2013). the main aim of this paper is to provide a tutorial example for the use of sedav1.0, but it gives also important information on the etas modeling. the analysis reported here clearly shows that the two sets of etas parameters, estimated on the l’aquila and the emilia regions, are surprisingly very similar, considering the different regional tectonic settings of two sequences. significant differences are found only for the μ and α parameters (see table 2). the first parameter is a proxy of the seismic potential of the zone and the estimated values reveal the lower hazard of the emilia region. the α parameter is related to the magnitude distribution and to the presence of large aftershocks in major sequences. the magnitude distribution of the emilia dataset shows a clear bump for magnitudes above ml4.5. the l’aquila sequence has 5 aftershocks above ml5.0, with a magnitude ranging from ml5.0 to ml5.4, whereas the emilia sequence has 8 aftershocks above ml5.0, having a maximum magnitude equal to ml5.8. this may be the reason of the lower α value estimated for the emilia dataset. the tests of the etas models and the retrospective forecast calculations reveal a good agreement with observations (see figures 8 and 9). specifically, the inferred models are able to predict the temporal evolution of the overall rate, without the inclusion of time-varying parameters. nevertheless, they do not have good skill in forecasting specific large earthquakes and the following sudden increase of the seismic rate (figure 12). the future of seda is somewhat unclear and will be driven by research interests and by the collaborations, besides of suggestions and criticism. anyway, some advances and improvements are already scheduled (lombardi, 2016). great effort has been devoted to test all the tools of sedav1.0, nevertheless some undetected errors may exist and some features may remain cryptic to many users. please, send an e-mail at the address annamaria.lombardi@ingv.it, for questions, suggestions or bugs to report. references bono, a. and l. badiali (2005). pwl personal wavelab 1.0, an object-oriented workbench for seismogram analysis on windows system, comput. geosci., 31, 55-64. cao, a.m. and s.s. gao (2002). temporal variation of seismic b-values beneath northeastern japan island arc, geoph. res. lett., 29(9), doi:10.1029/ 2001gl013775. geoscientific model development editors (2013). editorial: the publication of geoscientific model developments v1.0, geosci. model dev., 6, 1233– 1242, doi:10.5194/gmd-6-1233-2013. harte, d. 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(2006). ism computer science monographs, no. 33, the institute of statistical mathematics, tokyo, japan. schorlemmer, d., m.c. gerstenberger, s. wiemer, d.d. jackson and d.a. rhoades (2007). earthquake likelihood model testing, seism. res. lett., vol. 78, no. 1, 17-29. schorlemmer, d., f. mele and w. marzocchi (2010). a completeness analysis of the national seismic network of italy, j. geophy. res., 115, b04308; doi: 10.1029/2008jb006097. stucchi, m., c. meletti, a. rovida, v. d’amico and a.a. gomez capera (2010). historical earthquake and seismic hazard of the l’aquila area, progettazione sismica, 3, 23-34. vannoli, p., p. burrato, and g. valensise (2015). the seismotectonics of the po plain (northern italy): tectonic diversity in a blind faulting domain, pure appl. geophys., 172, 1105-1142. wiemer, s. and m. wyss (2000). minimum magnitude of complete reporting in earthquake catalogs: examples from alaska, the western united states, and japan, bull. seism. soc. am., 90, 859–869. woessner, j. and s. wiemer (2005). assessing the quality of earthquake catalogues: estimating the magnitude of completeness and its uncertainty, bull. seism. soc. am., 95 (2), 684-698. zhuang, j., y. ogata and d. vere-jones (2004). analyzing earthquake clustering features by using stochastic reconstruction, j. geophy. res., 109. *corresponding author: anna maria lombardi, istituto nazionale di geofisica e vulcanologia, rome, italy email: annamaria.lombardi@ingv.it 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved 12 overview on the ‘atmospheric emissions from volcanoes’ special issue annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6753 1 overview on the ‘atmospheric emissions from volcanoes’ special issue shona mackie1*, stefano corradini2, simona scollo3 and i. mattew watson1 1 school of earth sciences, university of bristol, bristol, u.k., 2 istituto nazionale di geofisica e vulcanologia, cnt, rome, italy 3 istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy * shona.mackie@bristol.ac.uk i. introduction he session ‘atmospheric emissions from volcanoes’ formed part of the 2014 general assembly of the european geosciences union (egu), held in vienna from 27 april to 2 may. this special issue presents some of the work that was discussed during the session. volcanoes are a pathway between the lithosphere and atmosphere and are prodigious sources of fine ash < 63 micron diameters) and gases. it is estimated that they emit 1200 (+/500) mt of h2o and 600 (+/-200) mt co2 into the atmosphere annually [burton et al., 2013], as well as a range of trace gases, including 9 (+/ 1.5) mt so2 and h2s [halmer et al., 2002], and less well constrained amounts of halogen-bearing species, including hcl, hf, hbr and other gases [symonds et al., 1994]. volcanic plumes are complex, multi-phase physico-chemical environments and there are many reasons why it is important that we understand, monitor and predict their evolution. they provide an opportunity to investigate subsurface processes [e.g. aiuppa et al., 2007; holland et al., 2011]; they can have a profound impact on the local environment, affecting human, animal and crop health [e.g. delmelle, 2003; hansell et al., 2006]; they can affect both local and regional climate [e.g. robock, 2000; oppenheimer, 2003; gao et al., 2008] and they can present a significant hazard to aviation [e.g. casadevall, 1994; prata and tupper, 2009; thomas and prata, 2011]. satellite remote sensing has been used to detect [prata, 1989] and quantify [wen and rose, 1994] the presence of volcanic ash and so2 for decades, facilitating the creation of global inventories such as that compiled by carn et al. [2003]. more recently, the eruption of eyjafjallajokull in april and may 2010 highlighted the need to incorporate interpretations from satellite remote sensing data into operational procedures. the eruption also demonstrated some of the risks in using so2 as a proxy for volcanic ash, a practice sometimes followed because so2 is typically more straightforward (relative to ash) to detect in infrared observations, which are readily available day and night [thomas and prata, 2011]. in fact, when ash and so2 are co-erupted it can be challenging to separate their individual signals [prata and kerkmann, 2007; corradini et al., 2009; kearney and watson 2009]. recently launched hyperspectral imagers such as airs and iasi provide new opportunities for more advanced retrievals of ash, so2 and other species associated with volcanic eruptions t annals of geophysics, fast track 2, 2014 2 [carn et al., 2009; carboni et al., 2012; mackie and watson, 2014]. ground based instrumentation is also used for monitoring volcanic ash, both near and far from the volcanic source. weather radar has historically provided data that can be used to infer volcanic plume properties [e.g. harris and rose, 1983; marzano et al., 2010]. lidar and sun-photometer networks covering large geographical areas provide valuable measurements that can often provide information on parameters that are useful to the interpretation of satellite observations, and can well-constrained estimates of plume concentration and altitude [e.g. ansmann et al., 2010; gasteiger et al., 2011, scollo et al., 2012]. in addition, ground based instruments have the advantage of providing a different perspective in the case of thick volcanic plumes, particularly when these are at higher altitudes [scollo et al., 2014], since they are generally most sensitive to the base and lower layers of a volcanic plume, while satellite-borne instruments are generally more sensitive to the uppermost part of the plume. observations from ground-based instrumentation have been used to validate dispersion models and satellite-derived estimates of plume characteristics [e.g. devenish et al., 2012]. dispersion models are used to predict the evolution of a volcanic plume in space and time and form the basis of advice issued by volcanic ash advisory centres (vaacs). different vaacs use different models [witham et al., 2007], and there is a growing effort to incorporate observation data from satellite and/or ground-based systems in order to constrain the model predictions [stohl et al., 2010]. ii. special issue content in this special issue, koukouli et al. report on a satellite-based observation system for monitoring volcanic emissions, which is validated using observations from airand ground-based instrumentation. corradini et al. present a comparison between inversion procedures for the volcanic ash and so2 retrievals using synthetic multispectral satellite-based measurements. a technique for exploiting the output from dispersion models to constrain the ash properties retrieved from satellite data is presented by steensen et al., and spinetti et al. presents a study examining estimates of so2 flux from lava fountains on mount etna that are inferred from observations made by different satellite sensors, and by a ground-based observation network. advancements in the exploitation of radar measurements of volcanic plumes are proposed and demonstrated by marzano et al., and aranzulla et al. present a demonstration of how the global positioning system can be used to monitor volcanic emissions. trace elements present in volcanic plumes are investigated through soil analysis in a study by daskalopoulou et al., and through biomonitoring by calabrese et al. four studies focusing on dispersion modelling are also presented. wilkins et al. demonstrate a method for the assimilation of satellite-derived ash properties into a dispersion model, while egan et al. present a case study focusing on so2 dispersion, and pattantyus et al. and businger et al. use an ensemble approach to model so2 and sulphate aerosol dispersion. references [aiuppa et al., 2007] aiuppa, a., r. moretti, c. federico, g. giudice, s. gurrieri, m. liuzzo, p. papale, h. shinohara and valenza m. 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(2007). comparison of the vaac atmospheric dispersion models using the 1 november 2004 grimsvotn eruption, meteorol. appl., 14, 27-38. vol49_1_2006def 133 annals of geophysics, vol. 49, n. 1, february 2006 key words doas applications in urban areas – atmospheric pollutants tomography – aircraft measurements of atmospheric trace gases 1. introduction the growth in human activity over the last century has led to an enormous increase in energy consumption with the consequent emission of substances in quantities greater than those which the atmosphere can absorb. in metropolitan areas wide-gridded networks of in situ analysers are normally used for the continuous recording of gas pollutant concentrations. these networks are, however, expensive and the single point measurements are often insufficient to characterise the photochemical and transport phenomena even over small areas. remote sensing systems can play an important role, representing a good compromise between spatial resolution and sensitivity of the measurements, providing more extensive coverage. in particular, doas (differential perspectives of 2d and 3d mapping of atmospheric pollutants over urban areas by means of airborne doas spectrometers giorgio giovanelli (1), elisa palazzi (1), andrea petritoli (1), daniele bortoli (1)(2), ivan kostadinov (1)(3), federico margelli (1), simonetta pagnutti (1), margherita premuda (4), fabrizio ravegnani (1) and giuliano trivellone (1) (1) istituto di scienze dell’atmosfera e del clima (isac), cnr, bologna, italy (2) geophysics centre of évora university of évora (cge-ue), évora, portugal (3) solar-terrestrial influences laboratory (stil), bulgarian academy of science, stara zagora, bulgaria (4) enea, divisione fisica applicata, centro ricerche bologna, italy abstract in the field of air quality control, optical remote sensing systems can measure the spatial distribution of gas pollutants, offering numerous advantages over conventional networks of in situ analysers. we propose some innovative solutions in the field of doas (differential optical absorption spectroscopy) remote systems, utilizing diffuse solar light as the radiation source. we examine the numerous potentialities of minor gas slant column calculations, applying the «off-axis» methodology for collecting the diffuse solar radiation. one of these particular approaches, using measurements along horizontal paths, has already been tested with the spectrometer installed on board the geophysica aircraft during stratospheric flights up to altitudes of 20 km. the theoretical basis of these new measurement techniques using doas remote sensing systems are delineated to assess whether low altitude flights can provide 2d and 3d pollution tomography over metropolitan areas. the 2d or 3d trace gas total column mapping could be used to investigate: i) transport and dispersion phenomena of air pollution, ii) photochemical process rates, iii) gas plume tomography, iv) minor gas vertical profiles into the planetary boundary layer and v) minor gas flux divergence over a large area. mailing address. dr. giorgio giovanelli, istituto di scienze dell’atmosfera e del clima (isac), cnr, area della ricerca cnr, via gobetti 101, 40129 bologna, italy; e-mail: g.giovanelli@isac.cnr.it 134 giorgio giovanelli et al. optical absorption spectroscopy) remote sensors open up new perspectives in the field of pollutant analysis and control such as, for example, the monitoring of a large number of gases simultaneously with a single instrument and a better characterisation of the area examined through the integration of gas concentrations over the entire optical path of the measurements. in the field of air quality control and monitoring, one of the current, most important problems is how to measure the three-dimensional distributions of airborne pollutants in the lower atmospheric layers within the planetary boundary layer (pbl) over urban and industrial areas, and in a systematic way. over the past few decades diffusion and chemical modeling techniques have been developed which allow calculation of 3d trace gas distributions with a spatial resolution of less than 1000 m and the detailed control of air quality in urban and industrial areas has become a fundamental aim for keeping environmental health within acceptable limits (von kuhlmann et al., 2003). passive remote sensing systems, such as spectrometers used in conventional modes, can at best calculate 2d maps of columnar abundances of the gaseous species under investigation integrated over the given measurement path (for example, from the ground to the flight altitude with a spectrometer, on board the aircraft, making «nadir» measurements). even gas profiles obtained using inversion methods (petritoli et al., 2002a) from a series of zenithsky doas observations during twilight cannot supply the vertical and temporal resolution necessary for chemical transport models (ctms) validation (finzi et al., 2001). airborne differential-lidar (browell et al., 1998) could, theoretically, provide 3d mapping of gases but, besides the «eye-safe» power limits of the technique (several tens of mw), there are obvious economic and technical difficulties in reaching a payload compatible with the requirements of a local service carrying out atmospheric analyses and air quality control. thus, it would seem that there are, at present, no economically accessible remote sensing techniques available for systematic 3d mapping of pollutants in the lower atmosphere which could be utilized in widescale national air quality programs and surveys. on the other hand, there is a field to be explored with great attention: that of diffuse solar light measurements, with uv-vis or ir spectrometers utilised in non-conventional configurations, defined as «off-axis» methods, to distinguish them from the classic nadir and zenith observations. in this work we will examine the response of a spectrometric system with zenithal movement of the input optics and estimate the diffuse radiation flux values, collected by the input optics of the spectrometer, as a function of the angle from the zenith. the final purpose of these efforts is to formulate a new method for the quasi-real-time reconstruction of the 2d and 3d trace gas distributions in the atmospheric layers closest to ground level. for example, by flying at an altitude between 2000 and 4000 m, it will be possible to measure the vertical distribution of gaseous pollutants, such as no2, so2, o3, hno2, benzene, formaldehyde, toluene and xylene within the pbl. 2. doas methodology for pollutant gas measurements in urban areas the doas methodology was first introduced by noxon (1975) in the early 70’s for measurements of total atmospheric gas columns, using diffuse solar light along the vertical path as the source of radiation (passive mode). the use of this technique rapidly became widespread in ground-based climatic stations and in airborne (giovanelli et al., 2000) and satellite systems (geophysica, er2 stratospheric aircraft, gome and sciamashy) to study the climatic distribution of minor stratospheric gases and ozone loss phenomena. later, platt and perner (1980) began to use the method to measure optical depths of trace gases along horizontal trajectories, using a uv lamp as an artificial radiation source (active mode). more recently, the technique has also offered a method for environmental monitoring in urban and industrial areas (brocco et al., 1992; edner et al., 1993; evangelisti et al., 1995; ravegnani et al., 1997). in fact, the doas methodology permits the simultaneous measurement with 135 perspectives of 2d and 3d mapping of atmospheric pollutants over urban areas by means of airborne doas spectrometers a single instrument of the concentrations of various gases with distinct absorption bands in the uv/vis spectral range (conventional analysers are gas-specific) such as o3, no2, so2, formaldehyde ch2o (platt and perner, 1980), nitrous acid hno2 (platt et al., 1980), the nitrate radical no3 (platt et al., 1981), benzene c6h6, toluene c7h8 and other hydrocarbons of general formula cxhy. with the doas methodology, gas concentrations are retrieved from their absorption characteristics in the uv-vis spectrum. the method is based on the lambert-beer law in its differential form (the low frequency spectra are subtracted from the original ones) (2.1) where i0(m) is the reference spectrum, i.e. the radiance outside the atmosphere, i(m) is the radiation intensity after passing through a layer of thickness l, ∆vg is the differential absorption cross section (dcs) of the gth gas at the wavelength m, obtained by subtracting the low frequency spectrum from its absolute absorption cross section (acs). the summation is over all absorbers in the examined spectral range. the result of the equation gives the integral of the gth gas concentration (cg) along the measurement path (l). a more detailed description of doas methodology is available in solomon et al. (1987), giovanelli et al. (1990). at the isac-cnr institute various types of instruments, known as gascod (gas absorption spectrometers correlating optical differences), have been developed for the estimation of gas column quantities and profiles from zenith diffuse solar radiation measurements at groundbased stations. an airborne multi-input version (gascod/a4r) has been developed for measurement of both gas total columns and actinic flux on board the geophysica-m55 stratospheric aircraft, together with other remote sensing and in situ equipment. the gascod/a4r operated successfully during the ape-theseo (seychelles) and ape-gaia (ushuaia, argentina) campaigns in 1999 (petritoli et al., 1999) and, more recently, in a validation program of the envisat satellite (kostadinov et al., 2006). ( ) ( ) )(ln ln i i i i c lgg g n 0 0 1 = m m m m mv∆ = ] d ] d g n g n / a modified version of this spectrometer has also been used to measure pollution at ground level in urban areas along horizontal paths, employing a xenon lamp as radiation source. starting from this latest prototype, an italian company has developed a commercial version for installation in networks of in situ analysers for atmospheric analyses and air quality control in urban and industrial areas. in conventional ground-based networks active doas systems can be configured as bistatic, where the measurement path length l is defined by the instrument (receiver) and by the projector creating the beam (emitter), or monostatic (fig. 1), where the emitter and the receiver are deployed at the same site with the use of retroreflectors for probing of the same optical path examined in bistatic mode, but doubled. although the most immediate use of doas instrumentation is air quality control in an environmental monitoring network, its potential can be fully exploited when used in less conventional ways. there are innovations both in the measurement techniques and in the subsequent data analysis. often these two aspects become so intertwined that it becomes difficult to draw a distinction. as mentioned above, the doas methodology was first used to obtain slant columns of minor gases in the stratosphere by processing zenith diffuse solar radiation measurements from ground-based stations. the subsequent «off-axis» approach was a logical extension deriving from the need to detect the presence of gases, in the stratosphere, with low absorption coefficients and low concentrations (e.g., total o3 column, nitrofig. 1. monostatic configuration of a doas spectrometer for air pollution measurement. 136 giorgio giovanelli et al. gen oxides, chlorine and bromine during ozone depletion processes) (giovanelli et al., 2001). the most recent application of the off-axis method is the «horizontal view looking» technique, which allows quasi in situ trace gas concentration measurements (petritoli et al., 2002b). its application in «horizontal doas tomography» represents an important methodological innovation in the interpretation of doas data to obtain 2d and 3d pollutant distributions over a limited area (hashmonay et al., 1999; vogel et al., 2000). however, doas tomography in the groundbased configuration, recently proposed by the iup (institut für umweltphysik) of the university of heidelberg, requires the use of a significant number of spectrometers and retro-reflectors to obtain 2d and 3d pollutant mapping. in the 1980s a similar approach was attempted to obtain the bidimensional structure of a stack plume in terms of the optical depth of so2 using off-axis measurements with a mask correlation remote sensor (giovanelli et al., 1979). the reconstruction of plume structure was obtained by making a series of azimuthal scans of diffuse solar radiation downwind from the stack, and for each of these zenithal scans were also made. various european research groups, besides our own, are beginning to show increasing interest in off-axis measurements both in ground-based and airborne configurations, derived from doas methods, to obtain 2d or 3d air pollutant mapping inside the pbl in urban and industrial areas. 3. doas measurements with vertical view looking configurations usually, estimation of the columnar abundances of minor atmospheric gases from measurements of solar diffuse radiation (at zenith or in other directions) is quite complex, since it requires the derivation of a suitable atmospheric transfer equation (ate). processing of doas measurements in the vertical view looking configuration, includes the following steps: i) derivation of the ate, which provides the best description of the atmospheric transfer for diffuse solar radiance measurements along the vertical axis. ii) measurements and retrieval of the slant column of the given gas, using multiple linear regression or single value decomposition (svd). the slant column is the integral of the gas config. 2. schematic diagram of path lengths in the atmospheric transfer model of doas zenith solar light measurement. 137 perspectives of 2d and 3d mapping of atmospheric pollutants over urban areas by means of airborne doas spectrometers centration along the atmospheric slant paths defined with respect to the solar zenith angle (sza) i (fig. 2). iii) calculation of the intensity weighted optical path (iwop), which correlates the optical trajectory of the diffuse radiation with the radiation intensity along that trajectory. iv) calculation of the «air mass» or «enhancement» factor (amf), defined as the ratio between the slant column and the vertical column, defined as the integral of the gas concentration along the vertical path. in the ate model developed in our group (giovanelli et al., 1990), the atmosphere is subdivided into l = 120 spherical layers, each ∆z = 1 km thick (fig. 2) and summation is used instead of the integral. the irradiance scattered along the vertical path and collected on the ground can be represented by the following equation: (3.1) where l=1, 2, …, l denotes the lth scattering level and each il, s(m) is the contribution of the radiation flux towards the ground, produced by scat( ) ( )i i, ,ve s l s l l 1 =m m = / tering in the lth layer and defined by the equation (3.2) where i0(m) has the same meaning defined in (2.1), v0(m) represents the scattering cross section of air molecules, f(i) is the molecular scattering function, derived from the rayleigh scattering formula, which defines the fraction of the scattered radiation flux in the ground direction, versus the diurnal variation of szai, n0 is the average air molecule density and ∆z is 1 km. the bracketed terms [ave] and [asl] are, respectively, the absorption along the vertical path and along the slant path. [ave] is defined as follows: (3.3) where i = 1, 2,…, l is the ith vertical layer, vg(m) is the absorption cross section of the gth absorber with g = 0, 1, 2, …, n (the 0 subscript indicates the molecular cross section for rayleigh scattering), ni,g(m) is the average molecular -exp=[ ] ( ) ( ) ( )a n z,ve g i g g n i i l 01 $ $ $v m m ∆ == d n= g// ( ) ( ) ( ) ( ) [ ] [ ]i i f n z a a,l s ve sl0 0 0$ $ $ $ $ $=m m v m i ∆ fig. 3. computed intensity of downward diffuse solar radiation as function of the scattering point along a vertical path. four curves are plotted for different sza, considering m= 330 nm and vis = 5 km. the accumulated probability trends, respectively due to the i= 70° irradiance curve and to the i= 90° irradiance curve are also shown. 138 giorgio giovanelli et al. density of the gth absorber in the ith layer. [asl] is defined by the equation (3.4) where j = (z+1), (z+2), …, l is the jth slant layer with z the scattering level, vg(m) and nj,g(m) have the same meaning defined in (3.3), ng(i) is the gth absorber’s air mass factor, including mie, rayleigh and ring amfs, computed for the most probable slant path obtained through computation of the iwop. by applying this atmospheric transfer equation it is possible to compute, for different wavelengths, the downward scattered radiation received by the spectrometer input optics, as a function of both the height at which the scattering occurs along the vertical direction and the varying sza. in fig. 3 the flux of zenith diffuse solar radiation is plotted for different i values and for m= 330 nm. it is interesting to observe how, in these irradiance curves, the level of the maximum scattering value increases as a function of higher -exp=[ ] ( ) ( ) ( ) ( ) a n s , ( ) sl g j g g g n j z l j 01 $ $ $ $v m m n i i∆ == + $ d ^ n h = g // szas. this can also be observed in the two accumulated probability curves of irradiance. 4. doas off-axis measurements in order to increase the gas optical depth to measure compounds that do not show strong absorptions in the uv-vis range, measurements are performed along oblique atmospheric paths, generically called off-axis, by rotating the optical input of the spectrometer from the zenith. the off-axis measurements are divided into several configurations according to the field of application and the various ates. the latter have to be developed for an adequate interpretation of the flux of diffuse solar radiation along the particular off-axis direction being observed. the off-axis measurements are named according to the particular angle being examined: if the line of sight (los) of the input optics ({) changes from 90° towards 0°, the measurement method is known as oblique-up view looking, whose ate is briefly described below. there are two other configurations: «horizontal view looking» and «oblique-down view looking». the former is briefly described in the next parafig. 4. schematic diagram of path lengths in the atmospheric transfer model of doas solar light measurement at oblique-up view looking. 139 perspectives of 2d and 3d mapping of atmospheric pollutants over urban areas by means of airborne doas spectrometers graph and the latter is still the subject of study and experimental verification. the oblique-up view looking method is used to detect thin optical depth stratospheric minor gases, such as o3 and no2 in polar region during ozone depletion and denitrification periods and halogen compounds (bro and oclo), (giovanelli et al., 2001). by analogy with equations presented in the previous section, the ate can be derived for oblique paths. the irradiance scattered along the oblique path and collected at ground level can be represented by the following equation: (4.1) where the «ob» subscript defines the paths of the light collected by the input optics, which should not be confused with the slant paths of the solar radiation before scattering which are defined by the subscript «sl» (fig. 4) and each iob,s,l(m) is the contribution of the radiation flux towards the ground, produced by scattering in the lth layer and is defined by the equation (4.2) where i0(m), s0(m), f(i−{), n0 have the same meaning defined in the previous equations, { indicates the spectrometer’s los, ∆ob({) is the thickness of the layer in the los direction and the bracketed terms [aob] and [asl] are, respectively, the absorption along the oblique path and the slant path. as for the case of vertical view looking configuration, it is possible to evaluate the trends of the scattered radiation received by the spectrometer input optics as a function of both the height at which the scattering occurs along the oblique direction and the varying sza. 5. doas measurements with horizontal view looking configuration measurements with doas systems using the horizontal view looking configuration present $( )ob {∆$n$( )f -i {$( )v m$( )i m=( ) [ ] [ ] i a a , ,ob s l ob sl 0 0 0 $ $ m ( ) ( )i i, , ,ob s ob s l l l 1 =m m = / some interesting fields of application both in climatic studies as well as in air pollution control. regarding climatic studies in the stratosphere, our group has recently developed a novel application of horizontal view looking, using a uv-vis spectrometer on board the geophysica aircraft (petritoli et al., 2002b). the measurement geometry is very similar to the limb-scanning mode used so far by satellite borne instruments to measure atmospheric radiance (i.e., in the ir spectral region by mipas-e and in the uv-vis region by sciamachy for the doas methodology, and both together on board the envisat satellite). the scattered solar radiation collected along a horizontal trajectory is compared, in terms of the lambert-beer law, with the scattered solar radiation collected along a zenithal path (fig. 5). the differential slant column signal is composed of two contributions from the v and h0,4 path (fig. 5). radiative transfer calculations show that v is negligible with respect to h0,4 so that by calculating h0,4 with the model it is possible to obtain the average concentration of the trace gas along the horizontal direction at flight level. this method, which allows quasi in situ gas concentration estimates, combines vertical and horizontal measurements and is known as acila (average concentration inside the layer near the aircraft). h0,4 is a function of the spectral interval we are measuring and of the trace gas we are interested in. for ozone at 330 nm at an average flight altitude of 20 km, h0,4 is about 50-70 km. the method has already been tested with measurements carried out in the frame of ape-gaia campaign (15 september to 14 october 1999), during which the m55 geophysica aircraft flew over the antarctic peninsula (carli et al., 2000). in the field of air pollution control the question is whether a source of artificial radiation is indispensable for horizontal measurements or whether it is possible to operate with diffuse solar radiation, thus allowing a greater degree of freedom in the measurements. the latter hypothesis is innovative and opens up fields of application previously considered impossible, or extremely onerous due to the nature of the equipment required. the enormous potential of these near ground-level horizontal measurements of dif140 giorgio giovanelli et al. fuse solar radiation using doas lies, especially, in the field of air pollution control and monitoring. it will also allow a more rapid use of doas instruments, operating in the uv/vis region, in the new and expanding field of air pollution tomography. the analytical methodology is quite similar to that employed in horizontal measurements in fig. 5. measurement geometry used to retrieve acila. when spectra from the zenith are used as reference in the doas calculation the sv path and s0,4 paths cancel out and what remain in the signal is due to the absorption along v and h0,4. model calculations give h0,4 > > v so that v can be neglected. fig. 6. computed intensity of diffuse solar radiation collected along a horizontal path, as function of the scattering distance from the instrument. four curves are plotted for different sza and visibility values, considering m= 330 nm. the accumulated probability trends, respectively due to vis= 23 km and to vis= 5 km are also shown. 141 perspectives of 2d and 3d mapping of atmospheric pollutants over urban areas by means of airborne doas spectrometers the stratosphere: the value of the given gas slant column is found from the difference between measurements of diffuse solar radiation along a «vertical» (or equivalent) path and those along a «horizontal» path. however, compared to acila, this method requires the derivation of an ate which includes simulation of multiple scattering (palazzi, 2003) and a knowledge, during measurement, of additional parameters such as, for example the spectral extinction coefficient of the atmosphere along the horizontal measurement path, which is related to the visibility during the measurement time. figure 6 shows the results of calculations along a near-ground-level horizontal path for two different values both of sza (50° and 70°) and visibility (5 and 23 km) for a wavelength of 330 nm. it is interesting to observe that, when the visibility is known, one can obtain the integral of the given gas concentration along the horizontal measurement path. 6. conclusions the use of doas for the measurement of trace gases in the atmosphere is currently passing through a stage of rapid development. doas, as opposed to other remote sensing techniques, does not require absolute irradiance measurements since it exploits the differences in normalised radiation fluxes over the observed spectral region, thus allowing great flexibility and economy in making such measurements. doas measures the columnar abundances of trace gases such as: no2, so2, o3, no3, bro, oclo, hno2, ch2o, benzene, toluene and xylene. doas can also measure concentrations of the nitrate radical, no3 and oclo, which are not accessible using other techniques, as well as providing more accurate and reliable measurements of oh radicals, one of the main initiators of the chain of photochemical processes. the development of inversion algorithms, applied to sets of doas measurements for the determination of concentration profiles of trace gases, increases the information content as well as its range of applications, including the important area of the validation of satellite data. for these reasons, the interest in doas is shifting from climatic studies in the stratosphere to air quality control and monitoring. in fact, the high information content of doas measurements allows its use not only in air quality surveys or networks, but also in special investigations such as the interpretation of transport and dispersion processes, and the chemical transformations taking place within the pbl. because the installation of the spectrometer is relatively easy on board an aircraft, this system can be used to map atmospheric pollutants over large metropolitan areas. by flying at altitudes between 2000 and 4000 m, doas can detect the distribution of gas pollutant concentrations within the pbl. to fully exploit the multiplicity and flexibility of doas methodology for air quality control and monitoring, the development of a set of measurement procedures, including atmospheric transmission models, to be applied according to the particular application, will be crucial. this paper indicates the main areas of study necessary to bring on line the proposed methodologies, such as horizontal view looking or oblique-down view looking, which are both deduced from the classical configuration of measurement, i.e., the zenith-sky or vertical view looking approach. these new methodologies and their combined use under opportune conditions 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schweitzer et al., 2002). compared to classical seismic networks, the main advantage of seismic arrays consists in their ability to detect weak or emergent signals, and to allow for an effective noise reduction through multichannel waveform stacking. moreover, once combined with data from adjacent networks, the wave azimuths determined at one or several arrays provide strong constraints on the location of low-magnitude events detected by a small number of stations (e.g., bratt and bache, 1985). such reduction in the detection threshold represents a critical step toward a successful assessment of the location, geometry and kinematics of active fault systems. following these arguments, the university of salerno, the italian national institute for nuclear physics and national institute for geoperformances of the underground seismic array for the analysis of seismicity in central italy gilberto saccorotti (1), bellina di lieto (2), fabrizio tronca (1), costantino fischione (1), roberto scarpa (2) and rosanna muscente (3) (1) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli, italy (2) dipartimento di fisica «e.r. caianello», università degli studi di salerno, baronissi (sa), italy (3) parco scientifico e tecnologico d’abruzzo, l’aquila, italy abstract this paper presents the first results from the operation of a dense seismic array deployed in the underground physics laboratories at gran sasso (central italy). the array consists of 13 short-period, three-component seismometers with an aperture of about 550 m and average sensor spacing of 90 m. the reduced sensor spacing, joined to the spatially-white character of the background noise allows for quick and reliable detection of coherent wavefront arrivals even under very poor snr conditions. we apply high-resolution frequency-slowness and polarization analyses to a set of 27 earthquakes recorded between november, 2002, and september, 2003, at epicentral distances spanning the 20-140 km interval. we locate these events using inversion of pand s-wave backazimuths and s-p delay times, and compare the results with data from the centralized national seismic network catalog. for the case of s-wave, the discrepancies among the two set of locations never exceed 10 km; the largest errors are instead observed for the case of p-waves. this observation may be due to the fact that the small array aperture does not allow for robust assessment of waves propagating at high apparent velocities. this information is discussed with special reference to the directions of future studies aimed at elucidating the location of seismogenetic structures in central italy from extended analysis of the micro-seismicity. mailing address: dr. gilberto saccorotti, istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, via diocleziano 328, 80124 napoli, italy; e-mail: gilberto@ov.ingv.it 1042 gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente physics and volcanology (hereinafter referred to as infn and ingv, respectively) conducted a joint effort for the deployment of a smallaperture, dense array of short period seismometers in the gran sasso (central italy) underground physics laboratories. designed and tested during the late 90’s, the underground array (underseis) became fully operative by early may 2002, providing low-noise, high-dynamic continuous recordings of ground shaking signals (scarpa et al., 2004). in italy, multichannel deployments have already been successfully tested for their detection capabilities on local seismicity (e.g., braun et al., 2004); however, underseis represents the first example of a seismic antenna deployed in an underground setting. this paper presents the first results from the operation of the underseis array. we first describe the array configuration and setting, and discuss its resolving capabilities in detecting and measuring plane-wave signals. we then describe the techniques routinely used for retrieving the propagation and polarization parameters from the multichannel recordings. examples of application are presented for a set of low magnitude (md < 3) earthquakes recorded at local and regional distances. s-p delay times and backazimuths of these events are then used to obtain epicentral estimates, the robustness of which is demonstrated by comparison with the catalogue locations from the ingv’s centralised national seismic network (cnsn). these results will be discussed further with special reference to the directions for future works aimed at a) improving the array performance, and b) investigating the details of wave propagation and the location of low-magnitude events in the investigated area. 2. the underseis arrey located in central italy, some 120 km ene of rome, the gran sasso is a limestone massif fig. 1. map of central italy with location of the gran sasso infn laboratories (square) and epicenters of the analysed earthquakes (black dots). sites mentioned in the text are labeled. the inset at the upper rigth shows the configuration of the underseis seismic array. performances of the underground seismic array for the analysis of seismicity in central italy constituting the highest peak (2914 m) of the apennines chain (fig. 1). since 1986, subnuclear particle physics laboratories have been hosted in underground tunnel systems located at a mean elevation of 1500 m, being therefore naturally shielded by a 1400-m-thick limestone cap. the area is affected by major seismicity associated with the mainly distensive tectonics affecting the apennines since the late pliocene. the major historical event was the 1915, ms = 6.8 avezzano earthquake, which caused about 32 000 casualties (amoruso et al., 1998). in addition, low-to moderate-size earthquakes occur quite frequently, the most recent in 1992, 1994 and 1996 (de luca et al., 2000; chiarabba et al., 2005). in the same area a temporary, large-scale seismic deployment was installed aimed at defining the location and geometry of seismogenic structures from microseismicity data (chiarabba et al., 2004). the low noise associated with the underground setting (de luca et al., 1998), and the quite considerable seismicity make the gran sasso labs an ideal site for high resolution seismic observations. the design of the underseis hardware and software components began on the late 90’s; the array became fully operative by may, 2002. underseis intersects a main seismogenetic fault where the occurrence of slow earthquakes has recently been detected through two wide-band geodetic laser interferometers (crescentini et al., 1999; amoruso et al., 2002). in its present configuration, the array consists of 13 elements, each equipped with a mark product l4c-3d, 1 hz 3-component seismometer, whose signals are digitized at a sampling interval of 0.01 s and a dynamic range of 24 bits. further details on the design and working principles of the array are reported in scarpa et al. (2004). in its present configuration, underseis depicts an average sensor spacing of about 90 m and the widest distance among sensors of about 550 m (fig. 1). location of the array elements was constrained by the geometry of the tunnel system, thus hindering the achievement of uniform station spacing and relative azimuths distribution which are warranted for multichannel analyses (fig. 2). recently, however, new galleries have been made available, and installation of additional array elements is already in progress with the specific goal of improving array aperture and geometry. 3. array performance in this section we investigate the array’s capabilities and limitations, in turn deriving useful hints for the selection of parameters to be adopted for the subsequent frequency-slowness measurements. 3.1. array response patterns the resolving capabilities of a multichannel system are conveniently represented by the «beam pattern» function (capon, 1969) 1043 fig. 2. histograms of station spacing (left) and relative orientation (right) for all the indepedent station pairs of the array. 1044 gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente (3.1) where ⎢k⎪= 2 π/ λ is the wave number corresponding to wavelength λ, n is the number of stations, the x r i s are the station coordinates, and x r 0 is the vector position of the reference sensor. this function describes the shape of the broad-band wave number spectrum for a vertically-incident plane wave. in practice, it represents the ’pulse response’ of the array in the wavenumber domain, and is particularly useful for investigating the effects of spatial aliasing which manifest as spurious peaks located at wave-numbers other than the origin. the underseis wavenumber spectrum is shown in fig. 3. as a consequence of the enewsw elongation of the array, the main spectral peak depicts a marked nnw-sse trend, this implying low resolving capabilites once measuring waves propagating along that particular direction. aliasing peaks appear at wavenumbers of abouts 20 cycles/km, corresponding to wavelengths in the order of 300 m. on the other side, b n e 1 s i x x i n k i 0= $− −2 2 ^ ^h h/ the lowest bounds for body waves apparent velocities correspond to the seismic velocities of the shallowest crust, here taken equal to 4 km/s (pwaves) and 2.3 km/s (s-waves). consequently, the nyquist frequencies for the two wavetypes are in the order of ≈ 13 hz and ≈ 6 hz, respectively. 3.2. signal coherency all multichannel processing methods are based upon the common waveform model of the signal: in other words, it is assumed that the signal recorded by each array element is the same except for a phase factor associated with the propagation across the deployment. in order to fullfil such assumption, it is crucial for the signal to maintain significant coherency throughout the different sensors. using a number of pand swave arrivals recorded at different snrs, we calculated the array-averaged signal coherency as a function of frequency for both type of waves using the formula (saccorotti and del pezzo, 2000): (3.2) where f is the frequency, np = n (n−1)/2 is the number of independent station pairs in the array, and czi,j is the fisher’s z-transform of the correlation coefficient among the band-pass filtered signals from stations i and j. signal coherency is expected to vary from event to event as a function of a number of factors, including snr and the frequency content of the signal and noise. however, we observed that in general p-waves maintain significant correlations over the 1-10 hz frequency range, while s-wave are significantly coherent only over the 1-6 hz frequency range. combined with the nyquist frequency limits for spatial aliasing mentioned above, these considerations provided constraints on the frequency bands to adopt for the analysis of earthquake wavefields described throughout the next sections. similar correlation analyses were performed over different noise windows collected at dayand nigh-times. for the whole set of interstation distances, significant noise coherency was only observed in association with marine microseisms, which typically manifest at frequencies c f a n cz i jtanh 1 , , p i j i j n !=^ eh o/ fig. 3. broad-band beam pattern spectrum (see eq. (3.1) in text) representing the response of the array to a monochromatic, vertically-inciding plane wave. kx and ky are the two components of the wavenumber vector with respect to a cartesian system in which the xand y-axes are oriented w-e and s-n, respectively. performances of the underground seismic array for the analysis of seismicity in central italy lower than ≈ 1 hz (fig. 4). at higher frequencies, the correlation plots reported in fig. 4 fully support the assumption of spatially-uncorrelated seismic noise required by the multichannel processing schemes presented in the following. 4. methods of analysis we analysed underseis data using upto-date techniques aimed at fully exploiting the resolving capabilities of a multi-channel system toward measurement of the propagation and polarization parameters of the incident wavefields. 4.1. polarization for polarization analysis, we adopt the technique proposed by jurkevics (1988), which consists of a multichannel extension of the original covariance method of kanasewich (1981). for a given signal window, the polarization parameters (azimuth and incidence angles, degree of linearity) are derived from the eigenvectors and relationships among eigenvalues of an array-averaged covariance matrix of the three-components of ground motion. in theory, the array-averaging procedure allows the contribution of noise (here assumed to be spatially uncorrelated) to be reduced by a factor to the covariance estimates. for each window of analysis, signals from individual stations should be aligned according to the slowness measured for that window. in our situation, however, alignment is not necessary considering that the inter-station time delays are much smaller than the dominant period of the signal. 4.2. frequency-slowness analysis for extracting the propagation parameters of underseis data we use music, the multiple signal classification technique (schmidt, n 1045 fig. 4. noise correlation as a function of frequency for different distance ranges. the correlation functions have been obtained from averaging the frequency-dependent correlation estimates associated with station pairs whose spacing d is included into the bounds reported at the top of each plot. 1046 gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente 1986; goldstein and archuleta, 1987, 1991). once compared to more classical methods (e.g., beamforming or capon’s (1969) high resolution), music has demonstrated superior performances for both the cases of low snr signals and more plane waves simultaneously impinging on the array (e.g., goldstein and archuleta, 1987, 1991; chiou and bolt, 1993). for a given window of analysis, the music slowness spectrum is expressed as (chiou and bolt, 1993) (4.1) where m is the number of signals impinging at the array, whose number is estimated applying akaike’s information criterium (aic; wang and kaveh, 1985) to the ordered set of eigenvalues of the spatial cross-spectral matrix (csm); vi is the i-th eigenvector of the csm, and a(s r )h is the hermitian of a column vector containing the expected inter-station phase delays for a monochromatic plane-wave of frequency ω0 propagating with slowness s r . note that the eleq s sa v1 1 h i i m 2 1 $ = − = ^ ^ h h/ ments of vector a correspond to the terms in the summation of eq. (3.1) above. maximizing eq. (3.2) means finding those slowness vector(s) for which the array response has a maximum projection onto the signal subspace, which is spanned by the i=1, ..., m eigenvectors vi. peaks in the q(s r ) function above are thus associated with the horizontal slownesses of individual plane-wave components crossing the array. the amplitude of these peaks gives a measure of the extent to which the observed inter-station phase delays fit the model of a plane-wave propagating at the particular slowness s r , and is completely independe of the actual amplitude of the signal. 4.3. uncertainties estimation we obtain an empirical estimate of the uncertainties associated with the measurement of propagation parameters by considering the range of horizontal slownesses and propagation azimuths associated with points of the slowness spectrum whose power is greater than 90% of fig. 5. sketch of the procedures used for retrieving uncertainties in the estimates of the propagation (left) and polarization (right) parameters. on the left, a sample slowness spectrum is displayed, with evidenced the contour line encompassing that region for which the power is greater than 90% of the main peak’s power. this region is used to define errors in azimuth and ray parameter estimates. on the right plot, the shaded area marks the projection onto the horizontal plane of the polarization ellipsoid. el are the horizontal projections of the three eigenvectors; ϕ is the estimated polarization azimuths, whose uncertainties are defined through the angles ∆ϕ. performances of the underground seismic array for the analysis of seismicity in central italy 1047 fig. 6a,b. a) raw velocity seismograms and amplitude spectra for the pand s-wave arrivals as recorded by the vertical-and rotated transverse-component, respectively, for a md = 2.4 earthquake on august 26, 2003, at 08:43:56 gmt. the epicentral range is about 25 km. seismograms and spectra are normalized to the largest-amplitude trace. thin lines in the spectral plots refer to noise spectra calculated over 2-s-long windows preceeding the onset of the event. b) the same as in a), but for a md = 3.3 earthquake on december 1, 2002, at 23:29:23 gmt. the epicentral range is about 124 km. the peak’s power (fig. 5, left). for the polarization azimuth, angular uncertainties are derived from the ratio of the horizontal projection of the principal and secondary eigenvectors (fig. 5, right). this procedure represents a first attempt toward accounting for the large azimuthal uncertainties which arise for small incidence angles, even when the motion is very rectilinear. a similar procedure is followed for obtaining the uncertainties on the incidence angle. 5. analysis of local and regional seismicity figure 6a,b shows sample seismograms and amplitude spectra for the vertical-and transversea b 10481048 component of ground velocity associated with two earthquakes recorded at local and regional distances. for both cases, the figure also reports the amplitude spectra associated with noise windows preceeding the onset of the events. despite the low magnitude of these earthquakes, seismograms depict an excellent signal-to-noise ratio, and significant wave coherence is observed among the different array elements. in the following, we proceed by describing application of the techniques illustrated above to an heterogenous set of earthquakes as recorded by underseis. the first step in calculating music slowness spectra is to obtain robust estimates of the narrow-band spatial covariance of the signal. therefore, we first band-pass-filter the array recordings over a narrow frequency band using a zero-phaseshift, 2-pole butterworth filter. the filtered signals are then hilbert-transformed, and converted to analytic signals which eventually serve to obtain the complex-valued estimates of the spatial covariance. eigenvalues and eigenvectors of the covariance matrix are derived using singular value decomposition. in order to account for the variable frequency content associated with different wavetypes, we conduct the analysis over consecutive, 2-hz-wide frequency bands spanning the 1-10 hz and 1-6 hz frequency range for pand s-waves, respectively. slowness spectra evaluated over individual frequency bands are then fig. 7a-e. a) vertical-component seismograms for an event occurred at 10:32:30 gmt on august 31, 2003. b) time interval used for p-wave slowness analysis. c) multichannel signal coherency (eq. (3.2) in text) over the frequency band selected for the analysis. the different lines refer to the five consecutive time position of the window of analysis. d) the ordered set of normalized eigenvalues derived from diagonalization of the spatial covariance matrix. e) time-frequency stacked slowness spectrum. propagation azimuth φ and ray parameter ⎢p⎪ associated with the peak value are reported at the bottom left of the plot. the arrow points to the direction of wave propagation. a b c d e gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente 1049 performances of the underground seismic array for the analysis of seismicity in central italy 1049 performances of the underground seismic array for the analysis of seismicity in central italy stacked to obtain a broad-band spectrum in which the contribution of body-waves is enhanced and the effects of the frequency-dependent peaks due to either spatial aliasing or surface-wave contaminations is instead attenuated (spudich and oppenheimer, 1986). this procedure is repeated over five consecutive time windows encompassing the selected wave arrival, and a final slowness spectrum is derived from the stack of these timevarying estimates. we first apply slowness analysis to p-wave arrivals recorded at the vertical components. then, we rotate the horizontal components into the transverse direction to the backazimuth obtained from the p-wave, and evaluate s-wave slowness spectra over the rotated components. examples of application of music slowness measurements are displayed in figs. 7a-e and 8a-e, which report pand s-waves slowness spectra obtained for a md = 2.4 earthquake recorded on august 31, 2003, at an epicentral distance of about 25 km. figure 9a-f depicts the particle motion patterns for the pand s-wave arrivals of the same event. in addition to the slowness spectra and coherence plots associated with the selected time windows, figs. 7a-e and 8a-e also report the ordered set of eigenvalues derived from diagonalization of the spatial covariance matrix. the absolute dominance of the main eigenvalue over the remaining ones indicates that a single signal is impinging on the array, in turn confirming the marked separation between the signal and noise subspaces previously postulated on the basis of noise correlation analyses. the performances of underseis toward detection and location of local seismicity become even more evident for the case of low fig. 8a-e. the same as in fig. 7a-e, but for the s-wave arrival measured on the transverse components. a b c ed 1050 gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente snr signals. an example is given by a md = 2 earthquake which occurred on december 27, 2002, and located by the rsnc at coordinates 41.938°n, 14.007°e, ese of the town of sulmona. this event was recorded by underseis with a very low amplitude, thus making impossible any visual estimate of the p-wave onset (fig. 10). however, the multichannel analyses provide consistent slowness estimates, which eventually permit a robust location of this event. for the sample earthquake reported in fig. 10, both the power of the stacked slowness spectrum and the multichannel coherency before the signal’s onset are very low, thus confirming that the background noise is essentially uncorrelated among the different array elements. based on this property, the power of the slowness spectra may be used as a powerful tool for automatic signal discrimination applied to real-time, routine analysis. the above procedures are extended to a further 25 earthquakes between november 2002 and september 2003. magnitudes and epicentral distances of these events span the 2.3 ≤ m ≤ 3.5 and 20-140 km ranges, respectively (table i). figure 11 illustrates the relationship among the measured pand s-waves ray parameters and the epicentral range, here presented in terms of s-p delay time. for the two farest events (∆ ≈ 140 km), p-wave ray parameters range between 0.1 s/km and 0.15 s/km, corresponding to apparent velocities spanning the 6.5-10 km/s range. once accounting for the measurement a b c d e f fig. 9a-f. a) three-component, array-stacked waveforms for the same event displayed in figs. 7a-e and 8a-e. b, c) time windows respectively used for pand s-wave polarization estimates. d, e) p-wave particle motion orbits over the horizontal and radial-vertical plane, respectively. f) s-wave particle motion over the horizontal plane. p-wave polarization azimuth and incidence angles obtained from the covariance analysis are 35° and 12°, respectively. the s-wave polarization azimuth is 110°. 1051 performances of the underground seismic array for the analysis of seismicity in central italy fig. 10. time behaviour of the propagation parameters for a md = 2 event at 12:42 gmt on december 27, 2002, at an epicentral distance of about 80 km. from top to bottom, sample vertical-component seismogram, propagation azimuth, ray parameter, multichannel coherency and power of the frequency-stacked slowness spectra. the p-wave arrival is marked by the sudden increase in both the slowness spectral power and coherency values about 4 s into the event. table i. list of analysed earthquakes. event id year month day ot ts-tp (s) md rsnc lat (°) rsnc long (°) 8 2002 11 20 02:31:51 3.1 2.4 42.480 13.240 9 2002 12 1 23:29:23 16.5 3.3 41.660 14.790 10 2002 12 2 20:52:59 18 2.7 41.670 14.860 12 2002 12 23 04:17:40 5.5 2.3 42.688 13.027 14 2002 12 24 07:14:05 3.3 2.7 42.490 13.240 15 2002 12 27 12:42:37 7.3 3 41.940 14.010 1 2003 1 1 20:23:40 7.5 2.6 41.900 13.690 3 2003 1 5 12:41:22 2.7 3 42.500 13.300 4 2003 4 7 15:01:15 2.8 2.5 42.320 13.450 5 2003 4 10 05:50:34 7.5 2.5 41.920 13.870 6 2003 4 11 11:49:10 3 2.5 42.510 13.270 7 2003 5 2 20:14:09 2.8 2.6 42.450 13.260 181 2003 6 30 22:32:20 3.5 2.5 42.300 13.240 186 2003 7 5 05:07:43 3.3 3.1 42.310 13.270 187 2003 7 6 09:13:40 3.4 2.2 42.310 13.230 fig. 11. pand s-waves ray parameters as a function of s-p delay times for all the events listed in table i. 1052 table i (continued). event id year month day ot ts-tp (s) md rsnc lat (°) rsnc long (°) 188 2003 7 7 12:37:12 3.8 2.2 42.340 13.230 192 2003 7 11 02:05:33 3.9 2.2 42.290 13.270 197 2003 7 16 05:09:26 3.2 2.6 42.330 13.280 238 2003 8 26 08:43:56 3.4 2.4 42.300 13.240 243 2003 8 31 10:33:00 3.6 2.4 42.370 13.310 239 2003 8 27 17:18:13 3.6 2.4 42.520 13.27 244 2003 9 1 00:07:40 3.9 2.4 42.310 13.260 251 2003 9 8 23:03:50 3 2.7 42.290 13.260 261 2003 9 18 21:00:40 3.8 2.4 42.300 13.230 267 2003 9 24 09:03:00 3.2 2.8 42.480 13.230 270 2003 9 27 17:35:25 4.35 3.5 42.220 13.250 errors, these values are compatible with those expected for pn waves. events located at smaller epicentral distances depict however similar, or even greater apparent velocities, indicating the steep incidence of pg waves from sources whose depth is comparable to the epicentral distance (braun et al., 2004). as previously mentioned by biondi and kostov (1989), ray parameter estimates are much more unstable than the measurement of azimuths. the comgilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente 1053 performances of the underground seismic array for the analysis of seismicity in central italy parison of pand s-wave ray parameters shows a scattered pattern; however, the estimate of their mutual relationships provides a meaningful value (1.78 ± 0.24) for the p-to-s velocity ratio (fig. 12). within the error bounds of individual measurements, a general consistency is also observed among the pand s-waves wave propagation azimuths (fig. 13), as well as among the p-waves polarization and propagation azimuths (fig. 14). the large uncertainties of slowness estimates observed in figs. 11 and 12 for both pand s-waves result from the large width of the main peak in the array response pattern, which is in turn controlled by the array aperture. thus, major resolution improvements are expected following the enlargement of the array presently under completion. 6. location at the beginning, we had hoped to obtain consistent estimates on the locations of underseis data from back-tracing the seismic wave-vectors evaluated from frequency-slowness analysis (e.g., almendros et al., 2001). however, due to the large errors associated with ray parameter measurements, and the intrinsic fig. 12. comparison of the ray parameters derived from slowness analyses of pand s-waves. fig. 13. comparison of the propagation azimuths derived from slowness analyses of pand s-waves. width and height of the boxes correspond to the uncertainty on slowness estimates. fig. 14. comparison of the propagation and polarization azimuths derived from analyses of p-waves arrivals. width and height of the boxes correspond to the uncertainty on slowness and polarization estimates. 1054 gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente uncertainties in wave types for events located at ranges comparable to the cross-over distance, this procedure didn’t provide satisfactory results. we thus applied a much simpler procedure, deriving epicentral locations from propagation azimuths and using a factor γ of 7.5 to convert s-p time delays to epicentral ranges. error bounds on such locations are conveniently derived from the uncertainties in azimuths estimates, and considering a variation on γ of ± 0.5. such uncertainty accounts for errors in sp time delays estimates, and for the fact that the same s-p delay time may be associated with events having different depths and hence different epicentral distance. for all the cases, the epicenters given by the cnsn catalogue are included into the confidence bounds of our estimates. figure 15 reports the discrepancies among underseis and cnsn locations as a function of epicentral distance (top) and ray parameter (bottom) for both pand s-waves. we observe that locations from s-waves depict discrepancies which rarely exceed 10 km, and are much more consistent than those retrieved from p-wave data. this is quite surprising: one would expect slowness analysis on the s-wave to provide less robust estimates, caused by path and recording site effects and by inaccurate sensor orientation. on the other side, however, the amplitude of s-waves is generally larger than that of p-waves, thus implying significant snr improvements. moreover, we have to consider that our slowness spectra were calculated over a cartesian slowness grid of regularlyspaced grid nodes: therefore, slowness measurements become progressively less accurate as one moves toward the origin of the grid, i.e. as the apparent velocity increases. under such conditions, loss of precision is also induced by round-off errors once deriving very small interstation phase shifts from the eigen-structure of the spatial cross-covariance matrix. examination of the relationships among location errors and ray parameters shown in fig. 15 confirms these two latter observations. the largest location errors are in fact associated with p-wave propagating at low ray parameters, thus suggesting that wave amplitude and velocity are the main factors controlling the precision of slowness measurements. keeping in mind these limitations, the above location results may be considered rather satisfactorily, thus indicating that future operations of underseis will significantly help to fig. 15. location discrepancies as a function of epicentral distance (top) and ray parameter (bottom) from both pand s-waves data. 1055 performances of the underground seismic array for the analysis of seismicity in central italy better delineate the distribution of micro-seismicity in the central apennines. 7. discussion and conclusions 7.1. limitations of the array the two key factors controlling the accuracy of array-based location procedures are: 1) the precision of slowness measurements, and 2) the ability to correctly predict ray-paths through the region encompassing the array and the epicenter. as for the first factor, in the previous section we observed that the largest location errors were associated with steeply-inciding pwaves. this is in agreement with theoretical prediction of slowness uncertainties (e.g., goldstein and archuleta, 1991), in which for a given array configuration the robustness of slowness estimates are mostly controlled by snr and apparent velocity. thus, slowness measurements derived from the faster-propagating, lower-amplitude p-waves are expected to be less accurate than those retrieved from the corresponding swaves arrivals. at the time of revising this paper (early january 2006), 6 additional elements were added to underseis, which may now count on a total aperture of about 800 m. major technological upgrades are also under development in order to allow for sampling rates higher than actually available. from such improvements, we expect a significant increase in measurement precision: on one side, the larger array aperture will improve the resolving capabilities toward fast waves, while on the other the higher number of available stations will reduce the contribution of noise, whose effects are seen on the measurement accuracy scale as . regarding the second factor mentioned above, the accurate prediction of ray paths would require knowledge of the velocity structure down to scale lengths in the order of a few hundreds of meters, a task which is obviously beyond the purpose of this study. however, the currently missing assessment of source depths may be attained using gradient velocity models. such procedure, although producing biased estimates for individual event locations, would preserve the alignments depicted by clusters of n1 adjacent sources; final constraints on absolute locations of these clusters will eventually be gained by joint inversion of array wave-vectors and cnsn arrival-time data associated with the most energetic events. 7.2. future perspectives in the present study, we were mostly interested in evaluating the location performances of the array: therefore, we selected and analyzed only a subset of the array catalog for which cnsn locations were available. the present efforts are mostly aimed at developing procedures for the automatic detection of earthquake signals, and a prototype on-line processor based on the algorithms presented above is already under testing. following these efforts, the most immediate goal of underseis is to improve the completeness of the seismic catalogue for the central apennines. at present, the detection magnitude threshold of the cnsn varies between 1.5 and 2.6 throughout the national territory, and is about 2.2-2.4 for the central apennines (cattaneo et al., 2002). however, from the arguments presented above, we do expect underseis to contribute significantly to the detection and location of micro-earthquakes under poor snr conditions, thus opening the way to the precise definition of seismicity patterns in the surroundings of the array. in this work, we selected the relevant parameters for slowness measurements (frequency band and length of the time windows) based on sample estimates of the signal’s frequency content and coherency. these properties, however, vary significantly from event to event. any automatic array processing scheme should therefore include the possibility of performing the analysis only over those time-frequency windows for which the signal has maximum energy and coherency (cornou et al., 2003), in turn adjusting the length of the window of analysis to the frequency band being analysed. this process could be made faster without loss of resolution by adopting sophisticated time-frequency decompositions of the signal for the calculation of the frequency-dependent spatial covariance matrices. for this purpose, an extension of the polarization and slow1056 gilberto saccorotti, bellina di lieto, fabrizio tronca, costantino fischione, roberto scarpa and rosanna muscente ness measurements to the wavelet-transformed signals (e.g., bear and pavlis, 1997) is currently under development. such improvements would significantly simplify the computational tasks, thus permitting an easier implementation of automatic, on-line detection procedures. once systematically analysing underseis data, a further factor which should be taken into account regards the quantitative definition of the properties of seismic noise, and their variation over time. although we observed that the noise was spatially uncorrelated, there may be periods of particular meterological condition or intense human activity for which coherent seismic signals may propagate across the array. future studies must therefore address the problem of a systematic analysis of the characteristics of seismic noise throughout extended time spans, in order to fully elucidate the regions of the slowness space which may be affected by the propagation of coherent signals other than those associated with earthquake sources. acknowledgements editor a. rovelli and reviewers t. braun and g. milana provided many thoughtful comments which greatly helped to improving the quality of the manuscript. the financial support from consorzio gran sasso and laboratori nazionali del gran sasso made possibile the realization of the seismic array. we acknowledge e. bellotti and e. boschi for support and stimulation. this work has been conducted in the framework of the program agreement between ingv-osservatorio vesuviano and istituto nazionale di fisica nucleare. we also acknowledge financial support from miur-prin2005 project «analisi e modellistica dei processi sismici e deformativi nell’appennino centrale». references almendros, j., b.a. chouet and p.b. dawson (2001): spatial extent of a hydrothermal system at kilauea volcano, hawaii, determined from array analyses of shallow long-period 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(received january 19, 2006; accepted october 20, 2006) l a d e t e r m i n a z i o n e d e l l " a c c e l e r a z i o n e m a s s i c i v n e i f e n o m e n i m a c r o s i s m i c i p . caloi a . l o s u r . n o f . peronaci l ' a c c e l e r a z i o n e s i s m i c a n e l l e z o n e e p i e e n l i a l i -i d e t e r m i n a g e n e r a l m e n l c c o l m e t o d o s u g g e r i t o d a o m o r i , c h e c o n s i s t e n e l c a l c o l a r e l ' a c c e l e r a z i o n e n e c e s s a r i a p e r f a r c a d e r e a l c u n i c o r p i di f o r m a s e m p l i c e , c o m e c o l o n n e , p i l a s t r i e c c . c o n q u e s t o m e t o d o , q u a n d o n e l l a z o n a c o l p i t a d a l t e r r e m o t o si t r o v a n o o g g e t t i di f o r m a e d i m e n s i o n i a d a t t e , si p u ò d e t e r m i n a l e u n i n t e r v a l l o , p i ù o m e n o a m p i o , i n cui è c o m p r e s a , p e r c i a s c u n a c o m p o n e n t e o r i z z o n t a l e . i a c c e l e r a z i o n e m a s s i m a : i c o r p i c a d u t i f o r n i s c o n o il l i m i t e i n f e r i o r e , q u e l l i r i m a s t i in p i e d i il l i m i t e s u p e r i o r e . i l m e t o d o di o m o r i si presla a g r a v i c r i t i c h e . d i f a l l i i c o r p i a p p o g g i a l i su un p i a n o o r i z z o n t a l e «i t r o v a n o i n e q u i l i b r i o s t a b i l e e. p r e s e n t a n o q u i n d i p e r e f f e t t o d e l l a g r a v i l a , e n t r o c e r t i l i m i t i , u n a r e a z i o n e a l l o s p o s t a m e n t o c h e l e n d e a r i c o n d u r l i n e l l a p o s i z i o n e i n i z i a l e : si p o s s o n o p e r t a n t o c o n s i d e r a r e c o m e d e i s i s t e m i o s c i l l a n t i a v e n ti p e r i o d i p r o p r i di o s c i l l a z i o n e . la c a d u t a di q u e s t i c o r p i n o n d i p e n d o p e r c i ò s o l t a n t o d a l l a m a s s i m a a c c e l e r a z i o n e o r i z z o n t a l e r a g g i u n t a d a l t e r r e m o t o , m a a n c h e d a l l a l e g g e di v a r i a z i o n e d e l l ' a c c e l e r a z i o n e sis m i c a , c h e n o n si c o n o s c e : c o n i m p u l s i s u c c e s s i v i s i n c r o n i il c o r p o p u ò c a d e r e a n c h e se t a l e a c c e l e r a z i o n e è m o l t o p i ù p i c c o l a di q u e l l a c a l c o l a t a ; c o n i m p u l s i c o n t r a s t a n t i , i n v e c e , il c o r p o p u ò r i m a n e r e in p i e d i a n c h e p e r a c c e l e r a z i o n i m o l t o p i ù g r a n d i . s o l o n e l c a s o i n cui q u e s t a legge fosse c o n o s c i u t a e p a r t i c o l a r m e n t e s e m p l i c e , p e r e s e m p i o s i n u s o i d a l e , si p o t r e b b e r i s a l i r e al v a l o r e d e l l a m a s s i m a a c c e l e r a z i o n e : m a s i c c o m e n o n è p o s s i b i l e f a r e a l c u n a i p o t e s i s u l l a l e g g e tlcl m o l o n e l l e r e g i o n i e p i e e n l i a l i , o i n q u e l l e ad esse v i c i n e , q u e s t o m e t o d o m a n c a d i s i c u r o f o n d a m e n t o . a l f i n e di p o t e r o t t e n e r e d a t i a t t e n d i b i l i sul v a l o r e m a s i i n o ragg i u n t o d a l l ' a c c e l e r a z i o n e s i s m i c a d u r a n t e un t e r r e m o t o n e l l e r e g i o n i e p i c e n t r a l i , e r a s t a l o s u g g e r i t o ( ' ) l ' i m p i e g o di a p p a r e c c h i i d e a t i a p p o ( ' ) a . l o sl'kbo, rendiconti lecitili mia ilei lincei. v o i . n i x . 5" s e r i e , i>. 1 9 . r . c a l o ! a . lo s i lì do f . p e d o n a c i s i t a m e n t e p e r r e n d e r e p o s s i b i l e la d e t e r m i n a z i o n e del v a l o r e m a s s i m o d e l l ' a c c e l e r a z i o n e s i s m i c a i n d i p e n d e n t e m e n t e , e n t r o a m p i l i m i l i , d a l l ' i n f l u e n z a d e l l a legge di v a r i a z i o n e . e d a l l o s c o p o di p o t e r r a c c o g l i e r e a m p i e n o t i z i e sul v a l o r e d e l l ' a c c e l e r a z i o n e stessa n e l l e r e g i o n i e p i c e n t r a l i , era ~[uto p r o s p e t t a t o l ' i m p i e g o di a p p a r e c c h i assai s e m p l i c i , m e d i a n t e i q u a l i la d e t e r m i n a z i o n e potesse esser f a l l a s u b i t o d o p o il t e r r e m o t o , s e m p l i c e m e n t e e senza r i c h i e d e r e c o g n i z i o n i s p e c i a l i . a p p a r e c c h i così f a t t i p o s s o n o essere d i s t r i b u i t i l a r g a m e n t e n e l l e r e gioni s i s m i c h e ed affidati a p e r s o n e n o n d o t a t e di p a r t i c o l a r e c o m p e tenza t e c n i c a , c o m e si usa f a r e da t e m p o p e r i t e r m o m e t r i ed i p l u v i o m e t r i c h e s e r v o n o a l l o s t u d i o dei b a c i n i m o n t a n i e del r e g i m e d e l le a c q u e n e i c a n a l i e n e i fiumi. f i . 1 l ' i s t i t u t o n a z i o n a l e di g e o f i s i c a p r o v v e d e a t t u a l m e n t e a d o t a r e l a r e l è s i s m i c a i t a l i a n a di s p e c i a l i a c c e l e r o m e t r i i d e a t i a l l o s c o p o di ris p o n d e r e a d e g u a t a m e n te a l l e esigenze s o p r a i n d i c a t e . i p r i n c i p i f o n d a m e n t a l i su cui tal i s t r u m e n t i sono b a s a ti f u r o n o già e n u n c i a ti da l o s u r d o ( " ) . il (piale s u g g e r ì a l c u n i a c c o r g i m e n t i p e r la l o r o p r a t i c a r e a l i z z a z i o n e . ogni a p p a r e c c h i o è in sostanza c o s t i t u i t o da una m a s a s p o s t a b i le in una sola d i r e z i o n e , t r a t t e n u t a c o n t r o un o s t a c o l o da u n a forza c o s t a n t e c h e a g i s c e ( 2 ) a n n u a r i o del r . o s s e r v a t o r i o del m u s e o ili f i r e n z e . 1 9 1 1 , p . 3 6 . la d e t e r m i n a z i o n e d e l l ' a c c e l e r a z i o n e m a s s i m a nei f e n o m e n i m a c r o s i . s m i c i 3 1 7 n e l l a sles-a d i r e z i o n e : p e r c h é avvenga il d i s t a c c o d a l l ' o s t a c o l o o c c o r r e c h e la m a s s a >ia soggetta ad una f o r z a di i n t e n s i t à m a g g i o r e di q u e l l a c h e v e la t r a t t i e n e . q u e s t e c o n d i z i o n i possono e s s e r e r e a l i z z a t e , ad e s e m p i o , c o n l ' a p p a r e c c h i o r a p p r e s e n t a t o s c h e m a t i c a m e n t e n e l l a fìg. 1 c h e s e r v e pell e c o m p o n e n t i o r i z z o n t a l i . i n c o r p o o m o g e n e o m a v e n t e la f o r m a di un p a r a l l e l e p i p e d o r e t t a n g o l o è sospeso in m o d o c h e n e l l a p o s i z i o n e di r i p o s o le q u a t t r o f a c c e l a t e r a l i l i m i t i n o v e r t i c a l i , ed u n a d i esse c o i n c i d a col p i a n o 0 0 eli una p a r e t e v e r t i c a l e : la f o r z a c o s t a n t e c h e t t fig. 2 xigisee n e l l a stessa d i r e z i o n e in cui p u ò m u o v e r s i il c o r p o m è f o r n i t a da u n a m o l l a s . p o i c h é n e l l a p o s i z i o n e di r i p o s o d e l l a m a s s a sospesa n o n vi è a l c u n a c o m p o n e n t e d e l l a g r a v i t à n e l l a d i r e z i o n e in cui il c o r p o p u ò m u o v e r s i , e s o è t r a t t e n u t o c o n t r o il p i a n o o o . p e r p e n d i c o l a r e a q u e s t a d i r e z i o n e , s o l t a n t o p e r e f f e t t o d e l l a forza e s e r c i t a t a d a l l a m o l l a . i n d i c a n d o con v la c o m p o n e n t e d e l l o p o s t a m e n t o del n o l o , o q u i n d i a n c h e del sostegno di m e del p i a n o < >< >, n e l l a d i r e z i o n e stessa s e c o n d o cui p u ò m u o v e r s i il c o r p o m al d i s t a c c o dal p i a n o o o . avrem o c h e il c o r p o r e s t e r à s e m p r e a c o n t a t t o col p i a n o fino a t a n t o c l i c d-x dif dove / i n d i c a la l'orza e s e r c i t a t a d a l l a m o l l a in d i r e z i o n e n o r m a l e al p i a n o o o , ed m la m a s s a del c o r p o m . q u i n d i il d i s t a c c o di m dal ' i r a c h e l ' a c c e l e r a z i o n e s i s m i c a , nel verso in cui i a p p a r e r ìio è d e s t i n a t o a f u n z i o n a r e , ha s u p e r a l o il v a l o r e ' p i a n o 3 1 3 i ' . c a l c i " a . lo s u r do i \ p e k o n a c i vegli a c c e l e r o m e t r i p e r la c o m p o n e n t e v e r t i c a l e , fìg. 2 , il c o r p o m ( a n c h ' e s s o di l'orma s e m p l i c e , p e r es. p a r a l l e l e p i p e d o r e t t a n g o l o o c i l i n d r i c o ) è soggetto al suo p e s o mg, e ad u n a l'orza v e r t i c a l e v e r s o l ' a l t o e s e r c i t a t a d a l l a m o l l a s . le m a s e v e n g o n o s p ì n t e c o n t r o un p i a n o o r i z z o n t a l e : p o s a l e su di esso (piando si fa p r e v a l e r e il p e s o s u l l ' a z i o n e d e l l a m o l l a , m g > / , o s p i n t e in a l t o , c o n t r o il p i a n o i n f e r i o r e d e l l a b a s e , q u a n d o si fa p r e v a l e r e la f o r z a d e l l a m o l l a sul p e s o , mg<ij, n e l m o d o i n d i c a l o r i s p e t t i v a m e n t e a s i n i s t r a e a d e s t r a n e l l a fig. 2 . i l d i s t a c c o dal p i a n o a v v i e n e , n e l l a d i r e z i o n e n o r m a l e z. ( p i a n d o l ' a c c e l e r a z i o n e , n e l vcr»o in cui l ' a p p a r e c c h i o è d e s t i n a l o a f u n z i o n a r e , s u p e r a il v a l o r e i he risulta d a l l a r e l a z i o n e d~z "mg— — ) = f . dt~ c i a s c u n a p p a r e c c h i o c m u n i t o di u n ' a s t i c e l l a a , figg. 3 4 , c h e , s p i n t a d a ! suo stesso peso o da una p i c c o l a m o l l a , r i s p e t t i v a m e n t e n e g l i a c c e l e r o m e t r i p e r l e c o m p o n e n t i o r i z z o n t a l i e in q u e l l i p e r l e c o m p o n e n t i v e r t i c a l i , a p p e n a a v v i e n e il d i s t a c c o d e l l a m a s s a d a l r i s p e t t i v o p i a n o si s p o s t a , e r i m a n e s p o s t a t a in m o d o c h e r i m a n g a tracr i a d e l l ' a v v e n u t o d i s l a c c o : c i ò i n d i c a c h e l ' a c c e l e r a z i o n e s i s m i c a h a s u p e r a t o il v a l o r e per il q u a l e è g r a d u a t o l ' a p p a r e c c h i o . l ' i n d i c a z i o n e p e r n i a n e f i n c h é d e t t a a s t i c e l l a n o n v i e n e r i p o r t a l a n e l l a p o s i z i o n e i n i z i a l e . d i s p o n e n d o q u i n d i di un c e r t o n u m e r o di q u e s t i a p p a r e c c h i , p e r ogni c o m p o n e n t e e p e r i d u e v e r s i , si p u ò s t a b i l i r e d o p o u n terr e m o t o un i n t e r v a l l o e n t r o il q u a l e è c o m p r e s o il v a l o r e e s t r e m o ragg i u n t o d a l l ' a c c e l e r a z i o n e . 1 d e t t a g l i di c o s t r u z i o n e di a l c u n i a c c e l e r o m e t r i di q u e s l o t i p o , c o s t r u i t i p e r la r e t e s i s m i c a d e l l ' i s t i l l i l o n a z i o n a l e di g e o f i s i c a , sì possono r i l e v a r e d a l l e figure 3 e 4 . negli a p p a r e c c h i p e r l e c o m p o n e n t i o r i z z o n t a l i ( f i g . 3 ) l ' a p p o g gio del p a r a l l e l e p i p e d o sul j i i a n o v e r t i c a l e n o n a v v i e n e m e d i a n t e c o n tatto di t u t t a la f a c c i a p i a n a l a t e r a l e , m a , p e r l i m i t a r e la s u p e r f i c i e di c o n t a t t o , s o l t a n t o m e d i a n t e a l c u n e l i s i e r e l l e so t t ili fissate sui b o r d i di essa. n e l l a p a r e t e d ' a p p o g g i o è p r a t i c a t o u n l'oro a t t r a v e r s o il q u a l e passa un filo f c h e c o l l e g a la m o l l a a s p i r a l e s al c o r p o m : la trazione della m o l l a , c h e v i e n e e s e r c i t a l a in d i r e z i o n e del c e n t r o la d e t e r m i n a z i o n e d e i . i . " a c c e l e r a z i o n e m a s s i m a n e i f e n o m e n i m a c r o s i s m 1ci 3 1 9 d i m a s s a , è r e g o l a b i l e m e d i a n t e u n ' a s t a f i l e t t a t a , a l l a q u a l e si agganc i a u n e s t r e m o d e l l a m o l l a . nel pezzo >ul q u a l e a p p o g g i a il c o r p o è p r a t i c a t o u n f o r o v e r t i c a l e e n t r o c u i p a s s a , c o n a t t r i t o t r a s c u r a b i l e , l ' a s t i c e l l a a. i l p e r m e t t o fissato sul c o r p o m passa a t t r a v e r s o un f o r e l l i n o orizz o n t a l e e t r a t t i e n e in a l t o l ' a s t i c e l l a finché il c o r p o m è a c o n t a t t o c o n l a p a r e t e v e r t i c a l e : b a s t a u n o s p o s t a m e n t o d e l l a m a s s a di q u a l c h e d e c i m o di m i l l i m e t r o p e r c h é l ' a s t i c e l l a sia svincolat a , e q u i n d i c a d a . i d e t t a g l i di c o s t r u z i o n e d e g l i a c c e l e r o m e t r i p e r la c o m p o n e n t e v e r t i c a l e si r i l e v a n o d a l l a fig. 4 , dove sono r a p p r e s e n t a t i a s i n i s t r a e a d e s t r a r i s p e t t i v a m e n t e gli a p p a r e c c h i c h e s e r v o n o p e l l e a c c e l e r a z i o n i di segni ug u a l e ed o p p o s t o a q u e l l o d e l l a g r a v i t à . i n q u e s t i a p p a r e c c h i il c o r p o m è c i l i n d r i c o , l e as l i c e l l e a , c h e f a n n o da ind i c e . sono d i s p o s t e orizzont a l m e n t e e v e n g o n o s p i n t e v e r s o l ' i n t e r n o da u n a p i c c o l a m o l l a . l a t e n s i o n e d e l l a m o l l a s v i e n e e s e r c i t a t a v e r t i c a l m e n t e ed in d i r e z i o n e d e l c e n t r o di m a s s a ; u n p e r m e t t o v e r t i c a l e fissato sulla m a s s a m i m p e d i s c e lo s p o s t a m e n t o d e l l ' i n d i c e fin t a n t o c h e il c o r p o è a c o n t a t t o c o l p i a n o o r i z z o n t a l e . n e l l a fig. 5 si v e d e un c o m p l e s s o di a c c e l e r o m e t r i p e r le d u e c o m p o n e n t i o r i z z o n t a l i , e p e r ogni c o m p o n e n t e n e i d u e v e r s i , c h e s e r v e p e r la d e t e r m i n a z i o n e di sei g r a d i di u n a s c a l a sismica a s s o l u t a . ogni s t r u m e n t o v i e n e t a r a t o p e r u n a a c c e l e r a z i o n e l i m i t e . q u e s t a t a r a t u r a si fa c o m e si può v e d e r e n e l l a fìg. 6 p e r l ' a c c e l e r o m e t r o c e n t r a l e . s u l b o r d o d e l l a b a s e è fissata u n a s p e c i a l e p i c c o l a b i l a n c i a il cui giogo, e q u i l i b r a t o o p p o r t u n a m e n t e , ha d u e b r a c c i di e g u a l e l u n g h e z z a , u n o o r i z z o n t a l e , e l ' a l t r o v e r t i c a l e : a l p r i m o è a t t a c c a t o un p i a t t e l l o sul q u a l e si c o l l o c a n o i pesi o c c o r r e n t i p e r il d i s t a c c o ; f i n . 3 p . caloi " a . lo s l i 1 d o f . p e r o n ' a c i al secondo è a t t a c c a t o un filo c l i c l a c a p o ad u n g a n c e t t o fissalo nel c e n t r o d e l l a f a c c i a e s t e r n a del c o r p o m , p e r m o d o c l i c la f o r z a di t r a z i o n e agisca o r i z z o n t a l m e n t e ed in d i r e z i o n e del c e n t r o di m a s s a . s e si v u o l e c h e i a c c e l e r o m e t r o f u n z i o n i p e r u n a a c c e l e r a z i o n e lim i t e yi o c c o r r e c h e il d i s t a c c o di m c o r r i s p o n d a ad un peso c o m p l e s sivo del p i a l l i n o e dei pesi a g g i u n t i rn g t a l e c l i c 8 p e r c i ò si r e g o l a la t e n s i o n e / d e l l a m o l l a in m o d o c h e r i s u l t i e g u a l e d-x alla l'orza r n ' g . e a l l o r a se l ' a c c e l e r a z i o n e s i s m i c a dl~ s u p e r a y , , e n t r o i l i m i t i d e l l ' a p p r o s s i m a z i o n e c o n s e n t i t a d a l l ' a p p a r e c c h i o , si h a il d i s t a c c o del c o r p o dal p i a n o v e r t i c a l e e l a r e l a t i v a s e g n a l a z i o n e d e l l ' a s t i c e l l a c l i c fa da i n d i c e . v .s*s gg s ! 1 1 a j ; [vjyfyj~v:— n e l l a fig. 7 si v e d e il c o m p l e s s o degli acc e l e r o m e t r i p e r la c o m p o n e n t e v e r t i c a l e a n a l o g o a q u e l l o p e r l e c o m p o n e n t i orizzont a l i , fig. 5 . l a t a r a t u r a di q u e sti a c c e l e r o m e t r i si fa in m o d o a n a l o g o a q u e l l o avanti i n d i c a to. p e r le a c c e l e r a z i o n i n e l verso c o n t r a r i o a q u e l l o d e l l a g r a v i t à , al peso del c o r p o m si a g g i u n g e q u e l l o del p i a t t i n o e dei pesetti o c c o r r e n t i al d i s t a c c o , di m a s s a c o m p l e s s i v a m2 ( f i g . 8 , a sinistra) e si e q u i l i b r a il peso ( m -; m.,)g con la t e n s i o n e / d e l l a m o l l a : allora l ' a p p a r e c c h i o e r e g o l a l o p e r l ' a c c e l e r a z i o n e l i m i t e y 2 t a l e c h e d~z m„e=m =111-,'., dtfig. i i.a d e t e r m i n a z i o n e d e l l ' a c c i e l e r a z i o n e m a s s i m a n e i f e n o m e n i m a c r o s i s m i c i 3 2 1 f i g . 6 f i j r . 8 la d e t e r m i n a z i o n e d e l l a c c e l e r a z i o n e m a s s i m a n e i f e n o m e n i m a c i i o s i s m i l i p e r l e a c c e l e r a z i o n i n e l l o slesso v e r s o d e l l a g r a v i t à (fig. 8 , a des t r a ) , l a t a r a t u r a si fa p o r t a n d o a l l ' e q u i l i b r i o la t e n s i o n e d e l l a m o l l a con il peso del c o r p o m , d i m i n u i t o di m-,g, p e s o c o m p l e s s i v o del p i a t t e l l o e d e i p e s c t t i a g g i u n t i . l a verifica degli a c c e l e r o m e t r i qui d e s c r i t t i , f a t t a m e d i a n t e a p p o site p i a t t a f o r m e o s c i l l a n t i , ha c o n f e r m a t o c h e e>-i possono r i s p o n d e r e a g l i scopi cui sono d e s t i n a t i . 1 r i s u l t a t i d e l l ' a p p l i c a z i o n e di q u e s t o m e t o d o di r i l e v a m e n t o dell ' a c c e l e r a z i o n e n e i f e n o m e n i m a c r o s i s m i c i , a t t u a l m e n t e in c o r s o di org a n i z z a z i o n e p e r o p e r a d e l l ' i s t i l l i l o ^ a z i o n a l e di g e o f i l i c a , v e r r a n n o a suo t e m p o p u b b l i c a t i . roma — istillilo nazionale dì geofìlica — giugno 19-18. riassunto gli autori descrivono alcuni speciali apparecchi destinali alla determinazione del calore massimo dell' accelerazione sismica raggiunta durante un terremoto nelle regioni epicentrali. le indicazioni da essi fornite non di pendono, entro ampi limiti, dalla legge di variazione del moto, in quanto essi non costituiscono dei sistemi oscillanti. questi apparecchi, per la semplicità di costruzione e di uso. possono essere largamente distribuiti nelle regioni sismiche ed affidati a persone non dotale di particolare competenza tecnica. early warning of tsunami from seismo-ionospheric fluctuation after japan’s march 11, 2011, m=9.0 tohoku earthquake using two-dimensional principal component analysis annals of geophysics, 58, 4, 2015, a0442; doi:10.4401/ag-6710 a0442 early warning of tsunami from seismo-ionospheric fluctuation after japan’s march 11, 2011, m=9.0 tohoku earthquake using two-dimensional principal component analysis jyh-woei lin national cheng kung university, department of earth science, tainan, taiwan abstract two-dimensional principal component analysis (2dpca) is implemented to analyze the total electron content (tec) anomalies after japan’s tohoku earthquake that occurred at 05:46 on march 11, 2011 (utc) (mw=9). 2dpca and tec data processing were conducted just after this devastating earthquake. analysis results show an earthquakeassociated tec anomaly near the epicenter that began at 05:47. this may represent an extension of the precursor of the earthquake, to the precursor of china’s wenchuan earthquake on may 12, 2008, detected by the study of lin [2012], for which the data were obtained at a height of 150200 km by the formosat-3 satellite system. it is impossible that such precursor caused by the acoustic shock waves. another tec anomaly near the epicenter occurred at 05:53, and this initiated the propagation of the tsunami effect related to the ionosphere through the acoustic shock waves from the epicenter. however, the tec anomalies did not appear to be affected by a contemporaneous geomagnetic storm and other non-earthquake effects. the propagation of anomalous fluctuation could be an early warning of the tsunami for the regions far from the epicenter as it began to propagate with the higher speed of 3960-4950 km/h than the tsunami speed (720-800 km/h). 1. introduction research into total electron content (tec) anomalies associated with earthquakes has been greatly facilitated by the advent of global positioning system (gps) technology, especially the number of gps satellites and density of ground receivers [heki and ping 2005, otsuka et al. 2006, kakinami et al. 2010, liu et al. 2011, matsumura et al. 2011, saito et al. 2011a, saito et al. 2011b, tsugawa et al. 2011, khegai et al. 2013, jin et al. 2014]. a possible cause of earthquake-related tec anomalies is acoustic gravity waves (agws). agws are caused by vibrations at the solid-earth’s surface and at sea, resulting from earthquakes, meteor strikes and even anthropogenic activity [woulff and mcgetchin 1976, rothkaehl et al. 2004, shinagawa et al. 2007]. the data of global ionospheric maps (gims) analyzed with two-dimensional principal component analysis (2dpca) will be used to detect such anomalies. tsugawa et al. [2011] and saito et al. [2011b] have investigate tec maps over japan for this earthquake event and shown that the tec variation appeared approximately 7 minutes after the earthquake occurrence. this scenario is reproduced by computer simulations by matsumura et al. [2011]. effects of tsunami on the ionosphere propagate from the epicenter have been shown by makela et al. [2011]. occhipinti et al. [2011] describes the tsunami following mw=9 tohoku earthquake that occurred at 05:46 ut on march 11, 2011, produced internal gravity waves into the neutral atmosphere and large disturbances in the overlying ionospheric plasma while propagating through the pacific ocean. therefore the remote sensing of atmosphere and ionosphere can give new tools for tsunami monitoring and detection. liu et al. [2011] describe fast propagating plasma disturbances as a type of rayleigh waves [yuen et al. 1969] quickly travel away above the epicenter along the main island of japan with a speed of 2.3-3.3 km/s, followed by circular ripples and a tsunami speed of about is 720-800 km/h. gps users with single-frequency receivers need ionospheric electron content information in order to achieve positioning accuracy similar to dual-frequency receivers. the global differential gps (gdgps) system provides a global real-time map of ionospheric electron content. these maps are also of value in monitoring the effect of the ionosphere on radio signals, power grids, and on space weather. the maps are derived using data from the ~100 real-time gdgps tracking sites. the integrated electron density data along each receiver-gps satellite link is processed through a kalman filter to proarticle history received december 19, 2014; accepted july 22, 2015. subject classification: two-dimensional principal component analysis, total electron content, tohoku earthquake, geomagnetic storm, tsunami. duce the global ionospheric tec maps (gim). the maps are available from multiple gdgps operation centers (gocs) as images, as text files containing the gridded tec values, and tec is an integration of the plasma density from the height 150-450 km, or as a binary data stream containing the gridded tec values. tec measured errors (biases), and their correction using the kalman filter, are described in the following references: wu and bar-sever [2005], kechine et al. [2004], ouyang et al. [2008]. the gdgps system is a complete, highly accurate, and extremely robust realtime gps monitoring and augmentation system. employing a large ground network of real-time reference receivers, innovative network architecture, and awardwinning real-time data processing software, the gdgps system provides decimetre (10 cm)-scale positioning accuracy and subnanosecond-scale time transfer accuracy on ground, in air, and in space, independent of local infrastructure. a complete array of real-time gps state information, environmental data, and ancillary products are available in support of the most demanding gps augmentation operations assisted gps (a-gps) services, situational assessment, and environmental monitoring globally, uniformly, accurately, and reliably. the largest real-time tracking network on the ground is shown in figure 1. the global tec data were real-time measured by the sensors in the gdgps satellites, and then they were sent back to this network, therefore the spatial resolutions of the original tec data would have 5.0 and 2.5 degrees in longitude and latitude, respectively [hernández-pajares et al. 2009]. the estimated tec data have been corrected for biases during measurements of dual-frequency delays of gps signals e.g. carrier phase biases, satellite state corrections, ionospheric delay and troposphere, which need to be removed using ground-based post-processing software [mannucci et al. 1998, raman and garin 2005, wu and bar-sever 2005]. the gims contain vertical (vtec), which has been converted from the slant (stec) at the ionospheric pierce points as stec=vtec · me + b + r, where me = 1/cos(i) is the mapping function, i is the zenith angle of gps satellite at the single layer height of the ionosphere, b and r are the instrument biases of the satellites and receivers, respectively. then vtec and the instrument biases b and r are obtained by combining interpolation and least-square fitting procedure. for details of the method used to derive the vtec from gps measurements, please refer to mao [2007] and mao et al. [2008]. the estimated vtec data (after processing as stated above and later is named tec in this study) at 05:47, 05:53 and 06:47 (utc) on march 11, 2011, are examined just after tohoku earthquake that occurred 05:46 utc on march 11, 2011, at (38.322°n, 142.369°e) with a depth of 24.4 km during a contemporaneous geomagnetic storm. these estimated tec data are not interpolations and gims are used only for observation. in this study, the estimated tec data are used to process and plot the eigenvalues shown figures 4-6. for the reasons stated previously, building gims via interpolation is beyond the scope of this study and therefore, artificial effects due to interpolation have been avoided. 2. method 2.1. 2dpca for 2dpca, signals are represented by a matrix a (the dimension of n × m). linear projection of the form is considered as followed [sanguansat 2012], y = ax (1) here x is an n dimensional project axis and y is the projected feature of signals on x called principal component vector. sx = e(y − ey)(y − ey) t (2) here sx is the covariance matrix of the project feature vector. the trace of sx is defined; j(x) = tr(sx) (3) tr(sx)=tr{x t gx}, where g=e[(a − ea)t (a − ea)]. (4) the matrix g is called signal covariance matrix. the vector x maximizing equation (4) corresponds to the largest (principal) eigenvalue of g and is the largest eigenvalue of the most dominant component of the data and therefore represents the main characteristics and thus the inhomogeneity of the data is detected in this study. 2dpca can remove computing or artificial errors when dealing with small sample signal size (sss) jyh-woei lin 2 figure 1. the largest real-time tracking network on the ground (the global differential gps (gdgps) system; from www.gdgps.net). 3 (low-dimensional data) problem. non-linear pca and pca convert the one-dimensional data prior to the covariance matrix calculation. the covariance matrix of the non-linear pca and pca is based on an input matrix with dimensions of m × n, which is reshaped from one-dimensional data (length of m multiplied by n). reshaping the data will cause computing errors because non-linear pca and pca are tools for handling one-dimensional data. therefore, lin’s study [2012], for which the data were obtained at a height of 150-200 km by the formosat-3 satellite system and the international gnss service (igs) had computing or artificial errors. the formosat-3 satellite system has dense tec data and therefore, the computing or artificial errors should be suppressed by the tec anomalies related to the earthquake. the tec data used in this study were not dense; however, 2dpca can process such tec data to examine tec anomalies related to the earthquake and tsunami effects after the removal of computing or artificial errors. now backing to discuss the concept of 2dpca, for 2dpcd, the spatial structure and information cannot be well preserved due to some original information loss when inverting to original dimension under the condition of the matrix being small sample size (sss). such information loss is called sss problem. however, the covariance matrix in 2dpca is full rank for a matrix of low dimension. the curse of dimensionality and sss problem (low-dimensional data problem) can be avoided by 2dpca. therefore 2dpca will be performed to detect tec anomaly related to tohoku earthquake due to the non-dense tec data set. 2.2. data processing the estimated tec data (after processing stated in section 1) are processed at 05:47, 05:53 and 06:47 (utc) in figures 2-4 on march 11, 2011 (utc). this global region is divided into 600 smaller areas, 12° in longitude and 9° in latitude to detect more detailed tec information because the resolution of the original tec data for this gps system is 5.0 and 2.5 degrees in longitude and latitude, respectively [hernández-pajares et al. 2009], and therefore in each area at least 4 tec data points are taken (12/5=2.4, 9/2.5=3.6, the 2 and 2 tec data are taken in longitude and latitude, respectively. if 6 points are taken, which are 2 and 3 points in longitude and latitude, then more computing time is needed, however the results are the same as using 4 tec points). in any place of each smaller area (a grid) includes 4 tec data and thus 4 tec data are taken to analyze. the 4 tec data must not have uniform distribution in each smaller area (a grid). the 4 tec data form a matrix a of dimensions 2 × 2 in equation (1) in section 2.1. the matrix belongs to the sss data. therefore the 2dpca is more suitable for sss data. the tec data are not uniformly distributed in each grid. it means that the tec data are not uniformly distributed in the geographical coordinate. four of the tec data in each grid are sss data (low-dimensional data), and 2dpca is therefore able to manage such data. results showed an eigenvalue in any place within each grid (each grid is filled with a uniform color for convenience). this study therefore shows large eigenvalues near the epicenter at 05:47, 05:53, which spread out until at 06:47 (utc) on march 11, 2011. 3. results the first earthquake-related tec anomaly with a large eigenvalue near the epicenter began at 05:47 on march 11, 2011 (utc) shown in figure 5. this anomaly may be an extension of precursor just before this earthquake that was the same as the precursor of china’ wenchuan earthquake on may 12, 2008, detected by early warning of tsunami of tohoku earthquake figure 2. gim at 05:47 utc on march 11, 2011. figure 3. gim at 05:53 utc on march 11, 2011. figure 4. gim at 06:47 utc on march 11, 2011. lin’s study [2012] because the acoustic waves did not generate on the seasurface propagate into the ionosphere at this time [saito et al. 2011b, tsugawa et al. 2011]. after 05:47, the precursor disappeared quickly because it was slighter weaker than the tec anomalous fluctuation caused by the tsunami effect (i.e., in figure 5, the smaller magnitude of the eigenvalue near the epicenter compared with the later eigenvalue figures). this tec disturbance was caused by a different mechanism to that responsible for the tec anomaly, which will be discussed later, i.e., they represent different tec effects. therefore, these two tec disturbances (05:47, 05:53) were caused by different mechanisms for which it is possible to have a short time lag. moreover, in lin’s study [2012], the tec data were obtained from an integration of the plasma density from the height 150-200 km by the formosat-3 satellite system, and the p-type effects disappeared quickly. the p-type effects should result in lower temporal and spatial tec variations caused by slow rock-cracking under stress as compared to those caused by acoustic shock waves [freund 2003, liu et al. 2011], and therefore, it could be detected from the tec data of the formosat-3 satellite system with low temporal and spatial resolution [lin 2012]. the tec variations of the gdgps system would result in higher variations caused by acoustic shock waves as indicated by the findings of liu et al. [2011]. however, in this case, such tec variations may have included some low temporal and spatial tec variations at 05:47 (ut) on march 11, 2011, and therefore, the p-type effects could be found by 2dpca at this time. the tec data used in this study were obtained from the gdgps system and therefore, both tec data anomalies are different. both tec anomalies, separated by 1-7 minutes, should be considered as isolated events for which having a short time lag is reasonable. for comparing with other background noises, the global ionospheric tec data were examined using 2dpca. through this examining, other tec anomalies of other places (e.g. strong magnetic storm) and strong background noises resulted in small eigenvalues and thus a lager eigenvalue of 2dpca over the epicenter should be a pattern of earthquake-related tec anomaly. it means that the eigenvalues of non-earthquake effects have been suppressed by the eigenvalues of the earthquake and tsunami effects. therefore 2dpca method can distinguish an earthquake-related tec anomaly with a large eigenvalue from other tec anomalies during the examined time period. the reason to distinguish pattern of earthquake-related tec anomaly may be from the technology of 2dpca, which is suitable to deal with low dimension tec data, thereby pattern of earthquake-related tec anomaly was accidentally caught. the tec data after 05:47 were examined by 2dpca. another earthquake-associated tec anomaly (occurring simultaneously with the tsunami effects on the ionosphere at 05:53 initiated the propagation of an anomalous fluctuation through the acoustic shock waves from the epicenter (figure 6). after about 1 hour, this anomalous fluctuation spread out in a similar way to the propagated effects of the tsunami on the ionosphere (from the epicenter at 36° longitude and 45° latitude, as shown in figure 7). the types of tec variations represented by the principal (largest) eigenvalues were tec large disturbances, such as those in the study of liu et al. [2011]. another interesting aspect of this study is that the dst index (figure 8) indicates a geomagnetic storm for this time period [hamilton et al. 1988, mukherjee 1999, elsner and kavlakov 2001, plotnikov and barkova 2007]. studies of earthquake related tec anomalies often completely discount days when geomagnetic storms occur because such storms are believed to distort tec analysis profiles. however, in this case the tec anomalies were detected regardless of the geomagnetic storm and other background noises, which resulted in small eigenvalues. the principle eigenvalues shown in this study could represent the inhomogeneity of the tec within an area of 12° longitude and 9° latitude, which is why tec variations with a spatial scale size of several hundred kilometres are picked up. tec variations caused by geomagnetic disturbances and other non-earthquake effects could be smaller than the tec anomalies related to both earthquake and tsunami effects on the ionosphere. a large eigenvalue indicates the patterns of the tsunami effect jyh-woei lin 4 figure 5. the figure gives a color-coded scale of the magnitudes of principal eigenvalues corresponding to figure 2. the color within a grid denotes the magnitude of assigned principal eigenvalues, respectively. note it resulted in an eigenvalue in any place of each grid. however, for convenience to show, each grid was filled with a uniform color. this study has shown large eigenvalue near the epicenter. 5 and earthquake-related tec anomalies, and these patterns may represent the types of certain particular spatial tec frequencies or characteristics that were detected by the 2dpca. it is worth mentioning that if artificial effects caused by 2dpca really occurred and then the computing errors of the 600 eigenvalues (figures 5, 6 and 7) will have almost the same magnitudes, thus the larger eigenvalues could also show a relation to the earthquake and tsunami effects. 4. discussion 2dpca was able to detect a tec anomaly related to the earthquake that began at 05:47 near the epicenter, which is considered to possibly be an extension of the precursor of this earthquake which may be the same as stark p-type semiconductor effects [lin 2012]. after this tec anomaly disappeared, a tec anomaly at 05:53 initiated the propagation of an anomalous fluctuation and propagated the effects of the tsunami on the ionosphere through the acoustic shock waves from the epicenter. as stated previously, one reason should be acoustic shock waves. this anomaly resembled what one would expect from rising acoustic shock waves because of strong motion. as discussed in the introduction, earth’s atmosphere could act as a natural amplifier due to declining atmospheric density with height. a large earthquake, such as this earthquake, was characterized by many fine vibrations at the earth’s surface which could produce a vertical stark acoustic pressure wave of great amplitude by the time it reached the ionosphere. such a description could possibly represent the concentrated energy of an acoustic shock wave with fast speed being formed in the lower atmosphere after the earthquake traveling up into the ionosphere to cause the anomalous fluctuation near the epicenter. this anomalous fluctuation could be an early warning for the regions far from the epicenter when this anomalous fluctuation beginning to propagate as a type of rayleigh waves. as stated in section 3, this anomalous fluctuation spread out around the region of tec anomaly after 1 h in figure 4. when 1° is about 110 km and then the propagating speed of this anomalous fluctuation is about 3960-4950 km/h (1.1-1.3 km/s) during the examined time period. chen et al.’s study [2011] described the anomalous fluctuation propagate with the speed of 3600-6120 km/h (1-1.7 km/s) for the same early warning of tsunami of tohoku earthquake figure 6. the figure gives a color-coded scale of the magnitudes of principal eigenvalues corresponding to figure 3. figure 7. the figure gives a color-coded scale of the magnitudes of principal eigenvalues corresponding to figure 4. figure 8. this figure shows the dst indices in march 2011 (world data center for geomagnetism, kyoto). earthquake. therefore the result in this study should be reasonable. moreover, some studies had different results that the propagation velocity of the tec disturbances from the epicenter was quite larger e.g. liu et al. [2011] describe fast propagating plasma disturbances as a type of rayleigh waves [yuen et al. 1969] quickly travel away above the epicenter along the main island of japan with a speed of 2.3-3.3 km/s. however, they used another data source. the tsunami arrived with the speed being about 720-800 km/h at the far regions very slower than such anomalous fluctuation propagated. through this study, the tec fluctuation propagation could be as an early warning of the tsunami caused by this earthquake. please note that it resulted in an eigenvalue at any place within each grid (the largest eigenvalue locations were surely near the epicenter and spread out from the epicenter). however, for convenience, each grid was filled with uniform color. this study has shown many square regions that are presented only for convenience. the aim of this paper was to compute the propagation of the anomalous fluctuation with its speed of 3600-6120 km/h, and it was not focused on the shape of the propagating anomalous fluctuation. 5. conclusion 2dpca had the advantage to detect the tec anomalies related to the march 11, 2011, tohoku earthquake. results have shown that a local ranging tec anomaly may be an extension of the precursor of this earthquake and that another tec anomaly at 05:53 began the propagation of the effects of the tsunami on the ionosphere through the acoustic shock waves from the epicenter. the tec anomaly could be a tec anomalous fluctuation with the speed being 3960-4950 km/h. the anomalous fluctuation could be an early warning of the tsunami for far regions because the anomalous fluctuation propagating speed was fast than the speed of tsunami with about 720-800 km/h. acknowledgements. the author is grateful to nasa’s global differential gps (gdgps) system for the useful references data. the author is gratefully mourning the death of the japanese in this earthquake on march 11, 2011. references chen, c.h., a. saito, c.h. lin, j.y. liu, h.f. tsai, t. tsugawa, y. otsuka, m. nishioka and m. matsumura (2011). long-distance propagation of ionospheric disturbance generated by the 2011 off the pacific coast of tohoku earthquake, earth planets space, 63, 881-884. elsner, j.b., and s.p. kavlakov (2001). hurricane intensity changes associated with geomagnetic variation, atmos. sci. lett.; doi:10.1006/asle.2001.0040. freund, f.t. 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continuous, traveling coupling between seismic waves and the ionosphere evident in may 1968 japan earthquake data, j. geophys. res., 74 (9), 2256-2264. corresponding author: jyh-woei lin, national cheng kung university, department of earth science, tainan, taiwan; email: pgjwl1966@gmail.com. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. early warning of tsunami of tohoku earthquake << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice microsoft word 7358-18223-1-ed_bohle et al.-rev daniela.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7358 1 a concept of society-earth-centric narratives martin bohle* directorate general research and innovation, european commission martin.bohle@ec.europa.eu anna sibilla* easme anna.sibilla@ec.europa.eu robert casals i graells* council of the european union robert.casalsgraells@consilium.europa.eu abstract this essay presents a concept of narratives as powerful means for people to associate themselves with anthropogenic global change. the starting point of our reflection is that people’s awareness is related to their story-telling practices to communicate insights and to induce or inspire behaviour. by analysing traditional and modern settings of "earthcentric" narratives and their respective societal context, one can sketch the necessary features of society-earth-centric narratives. these features seem suitable to raise public awareness, in particular of urban people, of the interaction of human activities and the earth system, and of anthropogenic global change. 1. introduction uman-driven change of natural environment has been a concern since the onset of the industrial revolution [fressoz, 2012; lewis and maslin, 2015; waters et al., 2016]. full understanding of earth system dynamics requires insight into people-people interactions and people-earth interactions. sound humanearth system interactions need reliable science and engineering expertise as well as the valueloaded considerations of the ethical, social and cultural implications of human activities [press, 2008; biermann, 2012; wilson, 2014; ellis, 2015; ellis et al., 2016; veland and lynch, 2016]. as anthropologists attest, humans are storytellers [pagel, 2012; wilson, 2012]. throughout the history of humankind, narratives were developed to share insights among people and to guide their behaviour. this essay sketches how the weaving of several concerns into common threads should lead to compelling narratives that can raise public awareness of the intersections of human activities with the earth system; that is, of anthropogenic global change. 2. earth-centric narratives: past and present traditional communities refer to the marvellous to ascribe meaning to natural phenomena and to maintain social cohesion. by the same means, these communities control the exploitation of the environment on which they depend. as an example arising from the western tradition, some rural alpine cultures in the aosta valley in the italian alps have maintained these approaches into contemporary times [sibilla, 2012]. their particular narration is about how the rugged mountains around the valley protects its greenest and most fertile grounds from human exploitation. these types of traditional narratives were earth-centric to inspire a behaviour that might sustain stable and lasting human-earth system interactions. the behaviour was justified with h annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7358 2 faith-based reasoning referring to the supernatural, which were common and shared views in the respective communities [durand, 1960; whitehouse and mccauley, 2005; botero et al., 2014]. although based mainly on faith and beliefs, the traditional thinking was functional because it had encoded practical experiences into a solid frame of reference values. when this traditional basis is disrupted by explanations that use insights into natural processes and technologies, the traditional balance in the community between "earth-centric" and “society-centric” goals can be altered. this alteration can disrupt the equilibrium of people's relationships with the earth system. to encourage an earth-centric behaviour, modern societies that dwell on science and technology, industrialisation, and global urbanisation will need to incorporate other narratives, as the traditional societies did. with a greater awareness of earth processes and the health of ecosystems, public concerns about pollution, hazard mitigation, demographics, and use of resources become familiar, in particular for urban populations [fressoz, 2012]. contemporary narratives of human-earth system interactions have to use natural science, humanities and history, anthropology, philosophy, and politics to strike a knowledge-based balance between "earthcentric" and “society-centric” contents [kleinhans et al., 2010; philipps, 2012]. 3. illustrating modern geoscience narratives landscapes, history, or art can furnish ample opportunities for narratives about geoscience matters and societal concerns. the following is an example to illustrate what we mean by "opportunities" [bohle, 2015]. crater lake is situated in a caldera in south-central oregon (usa). this unique lake was a sacred site for the native klamath tribe. their legends tell of battling gods, of sky and underworld, and establish crater lake as a profoundly spiritual place. the deep lake was formed around 7,700 years ago by the collapse of a volcanic caldera [zdanowicz et al., 1999]. it has neither surfacewater inflow nor outlet, and is known for the clarity and dark blue colour of its water, which reflects the sky and backscatters blue light. evidently, bringing the legend and modern science in context with the beauty and uniqueness of the site offers an attractive story. modern society-earth-centric narratives draw on scientific knowledge from humanities and natural sciences. it is postulated that additional features are needed to make the narrative operational that is, to render it society-earth-centric, namely: it must hold relevance for mundane matters; it must address value creation; and it must reflect urban lifestyle. • relevance for mundane matters. daily weather and, over longer time intervals, climate can exert significant impact on humans' lives. weather reports for specific professional audiences have been produced since the mid-19th century. regular weather forecast became feasible since the early 1950s [bauer et al., 2015]. today, modern media combine in a single narrative the weather forecast, information on meteorological phenomena, and impacts on economic activities. these meteorologist's narratives about mundane matters provide a benchmark. • addressing value creation. the application of geosciences allows modern societies to function. craftsman, technicians, architects, and engineers use geoscience when they create artificial environments, extract minerals, determine the stability of foundations, or in myriad other activities in which they create engineered works in a geological context. beyond offering economic value, geoscience provides cultural value it provides reliable knowledge about the lifesupporting functions of the earth's systems [hazen, 2012]. narratives about these cases of applied geosciences help us understand the development of human-geosphere interactions and the perception of history [latour, 2013]. • reflecting urban lifestyle. the gilgamesh poem and the story of uruk (a town built 5000 years ago in mesopotamia) are fascinating cultural artefacts through which ancient city-dwellers recorded their obvious connections with the geosphere [george, 1999]. in the modern urban environment, the experiential connections of people with the geosphere are constrained. experiential connections of urban people with the geoannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7358 3 sphere are biased towards events that disrupt the proper functioning of the engineered structures that form their dwellings and support their lifestyles. when modern city dwellers are isolated from nature through engineered environments, their lives are dominated by social experiences. an urban lifestyle resonates in dense people-people interactions. any earth-centric narrative must link to these familiar societycentric narratives in order to be fully understood. modern society-earth-centric narratives are told in the context of anthropogenic global change, although the effect of people's activity on earth has been noticeable for several thousand years [foley et al., 2013]. today, human impact on earth's environment goes beyond regional industrial pollution, urbanisation of rural areas, or replacing pristine wilderness by rural landscapes [ellis et al., 2013], regardless of whether these impacts are endogenous or even understood by society. non-scientists perceive contemporary anthropogenic global change in their own ways, and public perception of the widespread effects of economic activity on the earth system is generally limited. moreover, it is not yet fully evident how these interactions depend on culture and the individual or shared values of people [bohle, 2016]. still, to handle these interactions efficiently and effectively, society will have to be aware of them. as most people live in towns, narratives for urban dwellers are particularly needed and must be focused on familiar societal perceptions and values in order to be useful. effective narratives should engage a wide cross section of society, including those who have expert knowledge, are practitioners, represent traditional knowledge, or share views of lobbies or political representatives [hulme, 2011]. 4. a framework for society-earthcentric narratives we can draw upon what we know to suggest what sorts of society-earth-centric narratives are needed. first, acknowledge that people shape their environment in response to their needs as these are perceived through the filter of their world-views and preferences. in that context, people use narratives to develop abstract mental concepts, compare them with observations, assess their cultural and social background, or influence others through making value statements. second, acknowledge that people's narratives about the intersections of their activities with the earth system have evolved throughout history. learn from that history, that narratives can have richer features than is provided by science alone, and should include a focus on people and social interactions. to achieve awareness of the public, narratives have to be both earth-centric and society-centric [gluckman, 2014]. third, acknowledge that everybody needs to understand the interactions of human economic activity and the earth system, and therefore narratives have to stay within the average person's ordinary value-loaded frame of reference [lidström et al., 2015]. society-earth-centric narratives benefit from geoscience expertise that is relevant for both economy and value setting, and is interwoven with arts, linguistic and cultural histories. 5. summary the historical and political discourse about the path of human development during recent centuries [malanima, 2010] shows how people entered the anthropocene: “ces histories nous invitent à reprendre politiquement la main sur des institutions, des élites sociales, des systèmes symboliques et matériels puissantes qui nous ont fait basculer dans l'anthropocène" [bonneuil and fressoz, 2013; page 27: "these stories invite us to take control of political institutions, social elites, powerful symbolic and material systems that have tipped us into the anthropocene."]. global anthropogenic change is now part of the public perception of ‘the state of the globe.’ many people in western culture perceive this scenario as a threat to their lifestyle and well-being [ehrlich et al., 2012; biermann et al., 2012; walton and shaw, 2015]. facing anthropogenic global change requires a public debate that is anchored in experiences [moore, 2016] and supported by society-earthannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7358 4 centric narratives. rich narratives can be spun on a broad range of perspectives: (i) beauty or particularity of ordinary or unusual phenomena, (ii) evaluating hazards for or from mundane 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(1999). mount mazama eruption; calendrical age verified and atmospheric impact assessed. geology 277: 621–624. * disclaimer: all views expressed herein are entirely of the author, do not reflect the position of the european institutions or bodies and do not, in any way, engage any of them. acknowledgement: the authors are grateful to their employers for the consent to work on these matters outside their profession al duties. the authors declare no conflict of interest. this paper is the extended version of the author's contribution to the 35 international geological congress, 2016 cape town. the affiliation of third author was at the research executive agency (rea) when the work was performed. finally, the authors like to thank the anonymous reviewer for their help. 1 annex (doi: 10.4401/ag-6762) volcanic ash cloud observation using ground-based ka-band radar and nearinfrared lidar ceilometer during the eyjafjallajökull eruption marzano, f. s., et al. volcanic ash plumes are formed during explosive volcanic eruptions. after advection over several thousands of kilometers, volcanic ash particles are highly fragmented, dispersed and aged with micronsized sorting. this annex describes the ash microphysical modeling and the simulated radar and lidar signatures. a.1. volcanic ash microphysical modeling in this work all particles are assumed to be spheroids, i.e. ellipsoids with circular symmetry. this shape is, on the one hand, quite general and, on the other hand, simplifies the numerical treatment of scattering. similarly to what described in marzano et al. (2012), we set up a microphysical model of ash particle distributions. this means to specify: a) particle size distribution (psd); b) particle ellipsoidal axial ratio (ar); c) particle orientation distribution (pod); d) particle density and dielectric constant. ash psds are typically modeled through a normalized gamma or weibull size distribution [e.g., marzano et al., 2006; 2010]. in case of a multimode size distribution, it is always possible to suppose more than one analytical psd, characterized by different mean sizes and total number of particles. in this study, we have adopted the scaled gamma (sg) psd as a general model for all particles. if de is the diameter of a spherical volume-equivalent particle, the scaled-gamma psd np [mm-1m-3] for a generic class of ash particles p can be written as:                   np e np p d d np e npep e d d ndn  )( (1) where the “intercept” parameter nnp and the “slope” parameter np in a logarithmic plane are related to the “shape” parameter p and to the particle density p through: 1 4 6 10 4 4 6      pnp pnpp npp np d c n p    ; )( (2) with cp the mass concentration [g/m3] and dnp [mm] the number-weighted mean diameter. psd in (1) is completely specified by the three parameters p, dnp and cp (with a particle density p between 1 and 2.5 g/cm3). if particles are sphere equivalent, their mass is mp=p(/6)de3. the number of ash classes with respect to their average size is set to 5 as follows: i) very fine ash (va) with mean equivalent diameters uniformly distributed between 2-3-23 m (0.125-8 m); ii) fine ash (fa) between 23-26 m (8-64 m); iii) coarse ash (ca) between 26-29 m (64-512 m); iv) small lapilli (sl) between 29-212 m (0.512-4.096 mm); and finally v) large lapilli (ll) between 212-215 m (4.096-32.768 mm). with respect to previous studies [marzano et al., 2012], we have added the class of large lapilli and very fine ash to take into account ballistic fallout as well as dispersed small particles in order to deal with a fairly general observation scenario (closer to and farther from the volcanic vent). moreover, uniform distribution around the mean size has been assumed instead of gaussian probability. each diameter class may be subdivided with respect to other main parameters, e.g. the ash concentration and orientation angle [marzano et al., 2012]. the following sub-sets have been introduced: i) 4 classes of ash concentration, i.e.: very small (vc), small (sc), moderate (mc), and intense (ic) concentrations. these sub-sets are eventually compacted into 1 class uniformly going from 10-6 g/m3 up to 10 g/m3; 2 ii) 5 classes of particle orientation with gaussian pod characterized by a mean canting angle m, i.e.: tumbling with m=30° (to.1), tumbling with m=45° (to.2), tumbling with m=60° (to.3), oblate m=0° (oo); prolate m=90° (po). these 5 sub-sets are reducible to 3; iii) 2 classes of ar models, i.e.: ratio basalticandesitic (rb) and ratio rhyolitic (rr), even though we have here selected only the rb case; iv) 3 classes of va, fa, ca spherical particles (sp) with all 5 concentrations. particle dielectric constant models have been derived from literature; in particular, their dependence on sio2 weight wsio2 and relative humidity fraction (rh) is derived from the available data [e.g., adams et al., 1996; shettle and fenn, 1971]. ash classes can exhibit a mixed-phased permittivity due to liquid and ice water in the atmosphere. particle classes with a mixed refractive index have been also introduced [marzano et al., 2010]. in such a case, we have limited our attention to particles smaller than lapilli, so that 3 new mixed-phase classes have been labeled by vam, fam, cam for all 5 concentrations. liquid and ice water particle classes (e.g., cloud droplets, ice crystals, snowflakes) can be added to generalize the scenario. considering all combinations, the nominal number of classes is 233 (5·4·5·2+15+15+3) which can be effectively reduced to 20 (5·1·3·1+3+2), as we will discuss later on in this work. a.2. volcanic ash particle scattering and extinction the conditions allowing for the rayleigh scattering regime, depending on particle refraction index and the ratio between the particle diameter and the incident wavelength , are not always satisfied by the characteristics of the problem under discussion. simulated polarimetric radar observables for spheroidal particles can be obtained by using the t-matrix numerical method [mishchenko et al., 1996], implemented through the hydrometeor ash particle ensemble scattering simulator (hapess) [marzano et al., 2012]. in order to simulate the optical polarimetric signatures of volcanic particles we have included within hapess the geometrical optical limits which cannot be evaluated by t-matrix code due to numerical convergence problems [min et al, 2003; gasteiger et al., 2011]. in the next sub-sections we will introduce observables related to backscattering amplitude since those related to forward scattering phase are beyond the scopes of this paper [see marzano et al., 2012]. polarimetric radar observables using horizontal (h) and vertical (v) polarization states, copolar (zhh, zv) and crosspolar (zvh, zvh) radar reflectivity factors can be expressed in terms of ash psd. if  is the wavelength associated to a given frequency f, kp is the ash particle radar dielectric factor and  is the particle canting angle, sxy(b) are the backscattering components at x (x=h,v for the receiving mode) and y (y=h,v for the transmitting mode) polarization of the complex scattering matrix s, we can define:     ),(sin)( )(),,()( )( )(         e b xy p ep epe b xy p xy ds k dddp dnfds k fz 4 8 25 4 2 0 2 025 4 (3) in (3) angle brackets stands for average over psd np and pod pp, assuming a uniform azimuthally symmetric pod [marzano et al, 2012]. note that: i) zhv=zvh for cross-polarization reciprocity; ii) the canting angle  is defined in the polarization plane with respect to its vertical polarization unit-vector; iii) if zxy is expressed in [mm6m-3], then zxy=10log10(zxy) is conventionally expressed in dbz. 3 the derived differential reflectivity zdr, which is the ratio of reflectivity at the two polarization states, and the linear depolarization ratio ldr are defined as: )( )( )( fz fz fz vv hh dr  ; )( )( )( fz fz fl hh hv dr  (4) where zdr and ldr are unitless and usually expressed in db. the specific power attenuation hh (vv) at polarization h (v) can be obtained from:   ),,(im)( )( fdsf efxyxy  42 (5) the apex “f” of s indicates its forward components. note that, if xy is in [km-1], the above quantities are usually defined in db within the radar community xy=4.343xy. note that, if the ellipsoidal axial ratio shows an intrinsic variability, the particle non-sphericity can significantly affect the differential reflectivity zdr. the ldr is not dependent on the particle concentration and it is governed by the inverse of reflectivityweigthed axial ratio within the rayleigh scattering. the ldr signature will then increase with increasing canting angle and axial ratio variability. polarimetric lidar observables the lidar backscattering coefficients hh, vv and vh at horizontal (h) and vertical (v) polarization states can be expressed similarly to the corresponding radar coefficients. indeed, from (4) their definition can be written as:     ),(sin)( )(),,()( )( )(    e b xyep epe b xyxy dsdddp dnds 4 4 2 00 (6) where x=h,v again stands for the receiving mode and y=h,v for the transmitting mode polarization. note that xy is usually expressed in [km-1sr-1] so that xy=10log10(xy) is here conventionally expressed in db (in analogy to dbz for zxy). the same applies to the specific attenuation or extinction coefficient xy, which similarly to (5) is expressed in [km-1] and is defined as:   ),,(4im2)( )(  efxyxy ds (7) analogously to (7), if xy is in [km-1], xy=4.343xy is conventionally expressed in db. similarly to zdr and ldr in (4), the unitless lidar linear co-polarization ratio co and cross-polarization ratio cr can be defined by: )()( )()( )(    hhvv hhvv co    ; )( )( )(    hh vh cr  (8) radar and lidar polarimetric signatures in order to discuss combined polarimetric signatures, we have considered existing radar and lidar instrument setups, e.g. zenith-pointing kaband radars and colocated lidars operating at 355 nm, 532 nm and 1024 nm wavelength [e.g., madonna et al., 2010]. hapess outputs have been validated for volcanic ash with numerical simulations available in literature [e.g. wiegner et al., 2009]. figure 1a: correlation between zenith ka-band polarimetric radar observables in terms of copolar reflectivity zhh(36ghz), linear depolarization ratio ldr(36ghz), and specific attenuation hh(36ghz) for fa size class, using all ash concentrations classes (vc, sc, mc and ic) and for different particle orientations (oo, po, sp and to.2) with basaltic axial ratio model. -80 -70 -60 -50 -40 -30 -20 -10 0 10 -10 3 -10 2 -10 1 ka: fine ash z hh [dbz] l d r h v [ d b ] -30 -20 -10 0 10 20 30 40 50 60 -10 3 -10 2 -10 1 ka: small lapilli z hh [dbz] l d r h v [ d b ] -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 10 -8 10 -6 10 -4 10 -2 ka: fine ash zhh [dbz]  h h [ d b /k m ] fa-oo fa-po fa-sp fa-to -40 -30 -20 -10 0 10 20 30 40 50 60 10 -6 10 -4 10 -2 10 0 ka:small lapilli zhh [dbz]  h h [ d b /k m ] sl-oo sl-po sl-sp sl-to 4 this section illustrates examples of ash sub-classes, limited here for brevity to fa signatures in terms of ka-band and near infrared (nir) polarimetric observables (which are used for the combined system introduced later on). figure 1a shows how zhh(36ghz) correlates to linear depolarization ldr(36ghz) and specific power attenuation ahh(36ghz). note that the particle orientation has an impact on ldr(36ghz), with the oo (mainly horizontal with respect to the ground) providing values of about -40 db. specific attenuation is pretty low for fa (lower than 0.15 db/km), but larger of 1 db/km for larger particles. in figure 1a we have also added the behavior of spherical particles (sp) where we expect theoretically zhv=0 and ldr=0. figure 1b: same as in the figure 1a panels, but for the zenith lidar copolar backscatter coefficient hh(1064nm), linear crosspolarization ratio δcr(1064nm), and extinction coefficient hh(1064nm) with relative humidity ranging between 30 and 70%. in figure 1b panels show the correlation between zenith-pointing nir backscatter coefficient hh(λ), linear cross-polarization ratio cr(λ), and extinction coefficient ahh(λ) for fine ash. note that hh(1064nm) ranges from -80 todfor fa-po and -40 todfor fa-sp, whereas the sensitivity to orientation is no more evident for the larger particles.the extinction hh(1064nm) ranges from -80 to 40 db/km for fa, whereas hh (1064nm) from -110 to -10db/km for larger particles. it is worth noting that if hh(λ) increases with increasing ash concentration cp, it decreases with increasing particle mean diameter dn, a behavior which is explained by the optical limit regime of scattering taking place at nir for particles larger than few microns [wiegner et al., 2009; min et al., 2003]. on the opposite, the radar reflectivity zhh(f) increases with the increase of both cp and dn. references [adam et al. 1996] adams, r., et al. 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(2009). numerical simulations of optical properties of saharan dust aerosols with emphasis on lidar applications, tellus, 61b, 180-194. vol49_2_2006 643 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words high energy neutrino telescopes – deepsea cabled laboratories 1. introduction ocean depths represent today a new frontier for the exploration of the earth. the study of these vast regions is a scientific and technological challenge that has been undertaken by scientists of various disciplines. the main problem in the exploration of deepsea regions is represented by the very harsh environment (high pressure, corrosion, etc.). up to now studies have been limited by the possibility to gather data only from surface vessels or by mooring instruments for limited time periods on the sea bottom. recent developments in the field of communication technology, robotics and sensors may now allow a completely different strategy: realising permanent underwater infrastructures that can allow in situ continuous monitoring of deep-sea environments in real time. the possibilities that this new approach can open are of utmost importance in many fields of research: oceanography, geophysics, seismology and deep-sea biology. ocean depths have recently also attracted the interest of astrophysicists and high-energy physicists. some years ago it was proposed to use deep waters as a detector of high energy cosmic particles. the proposed detector could be realized by setting up a lattice of optical sensors able to detect the faint light produced by the passage of these particles through the water. these apparatuses could detect the most elusive known particle, the neutrino, and open the new field of neutrino astronomy. 2. astronomy with neutrinos almost everything we know about the universe comes from its observation by means of underwater laboratories for astroparticle physics and deep-sea science emilio migneco (1) (2), antonio capone (3) and paolo piattelli (1) (1) laboratori nazionali del sud, catania, italy (2) dipartimento di fisica, università degli studi di catania, italy (3) istituto nazionale di fisica nucleare (infn), sezione di roma c/o dipartimento di fisica, università degli studi di roma «la sapienza», roma, italy abstract the exploration of deep-sea environments is currently at the dawn of a new era: underwater laboratories, permanently installed on the seafloor and offering power and on-line data transmission links to the shore, will allow continuous monitoring of oceanographical properties. an important boost in this direction has been provided by the high energy physics scientific community, that aims at the realization of an underwater detector for cosmic high energy neutrinos. neutrinos are considered a very promising probe for high energy astrophysics and many indications suggest that some of the most energetic sources known in the universe could also be high energy neutrino sources. the expected neutrino fluxes indicate that a km3-scale detector must be realised to achieve this ambitious aim. the quest for the realization of such a detector in the mediterranean sea has already started. mailing address: dr. paolo piattelli, laboratori nazionali del sud, via sofia 62, 95123 catania, italy; e-mail: paolo.piattelli@lns.infn.it 644 emilio migneco, antonio capone and paolo piattelli electromagnetic waves. from the observational point of view, electromagnetic waves offer many advantages: they are copiously produced, they are stable, electrically neutral and therefore insensitive to magnetic fields, and they are easy to detect over a wide energy range that spans from low energy radio waves to infrared, visible, ultraviolet, x-rays up to high energy gamma rays. the disadvantage is that the internal regions of the astrophysical objects, where energy production takes place, are completely opaque to photons and therefore inaccessible to direct observation. for example the light that comes from the sun comes from its «photosphere». properties of its internal core, where nuclear reactions take place, can only be inferred indirectly. nevertheless, the observation of the sky by means of electromagnetic radiation has allowed the discovery of extremely powerful sources, which are probably powered by massive black holes, located in the most remote regions of the observable universe. unfortunately, the observation of these sources through their gamma ray emission is strongly unfavoured since highenergy gamma rays are absorbed by the cosmic infrared and microwave background radiation. this limits the observable horizon for high energy gamma rays. in fact, this horizon is restricted to less than one million light years for gamma rays of more than 1015 ev (1 pev). additional information comes from the observation of cosmic rays: protons or heavier nuclei. unfortunately, due to their charge, these particles are deflected by the galactic and intergalactic magnetic fields, thus preventing the identification of their sources. moreover, the extremely high-energy charged particles interact with photons of the infrared radiation background and with the cosmic microwave background. this is known as the greisen-zatsepinkuz’min (gzk) effect (greisen, 1966; zatsepin and kuz’min, 1966). in order to observe the inner workings of the astrophysical objects and to obtain a description of the universe over a larger range of energies, we need a probe which is electrically neutral, such that its trajectory will not be affected by magnetic fields, stable, such that it will reach us from distant sources, and weakly interacting so that it will penetrate regions which are opaque to photons. the only candidate particle currently known to have these properties is the neutrino. several low energy neutrino detectors are currently active around the world (boger et al., 2000; fukuda et al., 2003). these detectors have allowed us to study the neutrino fluxes from the sun, resulting from the nuclear reactions that take place in its inside, which is not accessible to direct observation. these «telescopes» are also sensitive to neutrinos produced in supernova explosions. two detectors, kamiokande (hirata et al., 1987) and imb (bionta et al., 1987), have in fact recorded a short burst of neutrinos in coincidence with the explosion of supernova 1987a, which was the latest supernova explosion taking place at a relatively close distance from the earth. neutrinos interact so weakly with matter (gandhi et al., 1996) that only huge detectors can collect a sizeable sample of their interactions. thousands of tons are needed to detect the low energy neutrinos coming from the sun core, but these volumes are still not sufficient to detect low energy neutrino sources at cosmological distances. high energy neutrinos offer a better detection opportunity since their interaction cross section increases with energy and since the products of their interactions can be more easily detected as their energy increases. an instrument able to detect high-energy cosmic neutrinos and identify their direction of arrival, a «neutrino telescope», would enlarge the observation horizon allowing the study of objects at the limit of the universe. 2.1. high energy neutrino sources astrophysical sources of high-energy neutrinos have not yet been observed directly, but their existence can be inferred from the properties of cosmic rays. primary cosmic rays are protons, with some admixture of heavier nuclei. the energy spectrum follows a power law that extends up to extremely high energies: values exceeding 1020 ev have been observed in recent years. protons themselves have limited use as astrophysical messengers, because they are charged and therefore subject to deflection by cosmic magnetic 645 underwater laboratories for astroparticle physics and deep-sea science fields: only the very highest-energy protons are likely to retain any memory of their source direction but, as already mentioned, they suffer from interaction with the background radiation. the production mechanism of the highestenergy cosmic rays is currently unknown, although supernova remnants and active galactic nuclei have been proposed as their sources. whatever the source, it is clear that accelerating protons to such high energies implies also the production of a large flux of pions, originated in the interaction with the photon background. pions subsequently decay to yield gamma rays and neutrinos. for these reasons it is generally assumed that the existence of very high-energy protons in the cosmic rays implies the existence of a flux of high-energy neutrinos. extra-galactic objects, such as the sources of gamma-ray bursts (grbs) and active galactic nuclei (agn), plausibly generate cosmic rays up to the maximum observed energies, and are therefore likely sources of neutrinos in the tev (1012 ev) to pev (1015 ev) energy range. grbs are transient flashes of gamma-rays, lasting typically for 1÷100 s, that are observed from sources at cosmological distances. although we do not yet understand in detail the internal mechanisms that generate these powerful explosion, some evidences suggest that they are cataclysmic processes associated with the collapse of massive stars to a black hole. agn consist of both persistent and flaring sources with apparent luminosities reaching about 1048 erg/s. they are tought to be powered by mass accretion onto supermassive (106÷ 109 solar-mass) black holes residing at the centre of galaxies. in both grbs and agn, the mechanism of mass accretion is believed to drive a relativistic plasma outflow that results in the acceleration of high-energy particles. both agn and grbs have been proposed as high energy neutrino sources, and neutrino observations will provide unique information on the physics of the underlying engine, which is not well understood despite many years of research. other theorized neutrino sources are associated with compact astrophysical objects (supernova remnants, x-ray binaries and microquasars) or with the annihilation of the yet unobserved particles which may constitute the dark matter. a comprehensive review of candidate neutrino sources and flux model predictions can be found in learned and mannheim (2000) and in halzen and hooper (2002). neutrino observations are specifically interesting because the detection of high-energy (>tev) neutrinos will provide unambiguous evidence for cosmic acceleration of protons and nuclei, and their arrival direction will point to the location of the accelerators. neutrino telescopes will permit us not only to look into the engines driving powerful sources such as distant agn and grbs that cannot be explored directly with photon observations, but also to look far into the universe. cosmological sources cannot be observed at photon energies exceeding 100 gev because of attenuation by γγ pair production on the diffuse intergalactic infrared background radiation. by contrast, highenergy neutrinos will propagate unhindered directly to us from their sources. thus, neutrinos can provide a new window to explore the highenergy phenomena in the distant universe. a question still remains on how large a high energy neutrino telescope must be to allow for an unambiguous detection of some sources. as we have seen the strongest case is based on the existence of extreme high energy cosmic rays. from this observed high energy charged particle flux we can derive a neutrino flux assuming that a cosmic accelerator produces equal energies in cosmic rays, gamma rays and neutrinos. these flux estimates set the needed scale for a high energy neutrino telescope: a 1 km2 effective area telescope will be able to detect a few tens of events per year. 2.2. high energy neutrino detection the most promising method to achieve the ambitious goal of realizing a km2 scale neutrino telescope is the tracking of secondary muons produced in the interaction of neutrinos in the volume close to the telescope. high energy muons are extremely penetrating charged particles since they lose energy, interacting with matter, mainly through coulomb scattering. processes like the emission of high energy photons (bremsstrahlung), that is typical of high 646 emilio migneco, antonio capone and paolo piattelli energy electrons, is strongly reduced since muons are heavier (≈ 200 times) than electrons. when a high-energy muon, having a velocity υ close to the speed of light in vacuum (c), propagates in a transparent medium with index of refraction n > 1, its velocity will be greater than the speed of light in the medium that is equal to c/n. in these conditions visible light is radiated along the track with an angle θc, with respect to the muon direction, defined by the relation cos(θc)=1/(βn), where β= υ/c. this process, called cherenkov effect, is similar to the shock wave, the sonic boom, produced when an airplane exceeds the speed of sound in air. in the water, that has a refraction index n ≈ 1.35 in the blue light region, θc is approximately 42°. the light radiated by cherenkov effect forms a conical wavefront that propagates inside the detector, leading to a relation between the muon direction and the arrival time of photons in different points of the space (fig. 1). the energy loss of a muon via cherenkov radiation is only a negligible fraction of the total energy loss and the number of cherenkov photons emitted in water is roughly 300 per centimeter of track. nevertheless, numerical simulations show that equipping a large volume of a natural transparent medium (like the oceans or the antarctic ice) with optical sensors capable to detect even single photon signals one can identify a muon track and reconstruct its direction. water optical properties will determine the detector granularity (i.e. the optical sensor density). water is transparent only in a narrow range of wavelengths (350 ≤ ≤ µ ≤ 550 nm) in the blue light region of the spectrum. in this region the absorption length in clear ocean waters is about 70 m. this number roughly sets the spacing distance between the optical sensors of the detector. muons carry, on average, a significant fraction of the neutrino energy and propagate with nearly the same direction of the incident neutrino. for neutrino energies larger than 10 tev the angle between the originating neutrino and the emerging muon is on the average less than 0.2°. therefore the reconstruction of the muon, if performed with an accuracy of the order of 0.2° or better, can infer the direction of the incoming neutrino. a significant excess of neutrino events coming from the same direction would yield the identification of an astrophysical source. a high energy muon can travel up to several km in water. this implies that the cherenkov neutrino telescope will be sensitive to neutrino interactions occurring even far outside the detector instrumented volume. muons originated by astrophysical neutrinos have to be selected out of the more intense flux of muons (the background for our signal) produced in atmospheric showers resulting from the interactions of primary cosmic rays (protons and heavier nuclei) in the atmosphere. the atmospheric muon flux at sea level is about times 1011 times more intense than the flux of muons produced by astrophysical neutrinos but is typically less energetic, therefore these atmospheric muons cannot penetrate large thicknesses of matter. this property can be used to reduce the background by installing the detectors in an underground laboratory (like the gran sasso laboratory, near l’aquila in italy). obviously a km3 detector, like the one needed for high energy neutrino astronomy, cannot be hosted in a tunnel or in a natural cave, but one can obtain the same result by deploying it in the ocean depths where the overlaying water would reach the same effect. at 3000 m the flux of atmospheric muons would be reduced by a a factor 106 making it feasible the search for astrophysical neutrinos. fig. 1. detection principle of an underwater neutrino telescope. astrophysical neutrinos that interact close to the detector generate secondary muons that can be detected by means of their cherenkov light emission. the measure of arrival times of the cherenkov light on the optical modules allows to reconstruct the muon direction. 647 underwater laboratories for astroparticle physics and deep-sea science the role played by seawater is therefore threefold: it constitutes the target where astrophysical neutrinos interact to produce the secondary muons; it acts as a transparent radiator where relativistic muons induce the cherenkov light; it acts as a shield against the cosmic muon background. seawater above the detector can reduce the atmospheric muon background but not totally eliminate it. for each astrophysical neutrino about 105 down-going atmospheric muons will cross the detector. but none of these atmospheric muons can cross the earth, unlike neutrinos that can traverse the whole thickness of the earth and interact in the vicinity of the detector originating an up-going muon (fig. 2). this fact completely eliminates the atmospheric muon background: a track reconstructed as crossing the detector with up-going direction will be a clear signature of a neutrino-induced muon. the neutrino telescope will then be sensitive to neutrinos, originated in astrophysical sources or in atmospheric showers, coming from the southern hemisphere, from the «bottom» and not from the «sky» as all the other astronomical instruments we are used to. however, a remaining background exists, due to neutrinos that are produced in the interactions of charged particles in the atmosphere and are therefore called «atmospheric neutrinos». this background is isotropic and becomes negligible at very high energy (above 10 tev). point like sources of cosmic neutrinos can be identified if they can emerge from this background. 3. high-energy neutrino telescopes in recent years the water cherenkov technique has been successfully used to detect solar and atmospheric neutrinos in 103-104 m3 scale detectors, like sno (boger et al., 2000) and superkamiokande (fukuda et al., 2003). as we have seen, the expected number of neutrino events from astrophysical sources in the 1012÷ 1015 ev range is of the order of 10 ÷ 100 events per km2 per year, much less than the fig. 2. detection principle of an underwater neutrino telescope. this instrument will use the whole thickness of the earth to shield the charged particles cosmic flux and therefore its field of view will be restricted to the hemisphere below the horizon. nevertheless, a background of atmospheric neutrinos, originated by the interaction of charged particle in the atmosphere in the opposte hemisphere, will always be present. 648 emilio migneco, antonio capone and paolo piattelli number of events expected in the low energy range. therefore, only 109 m3 scale detectors could allow the source identification. as we have seen, conceptually the idea at the base of a cherenkov high energy neutrino telescope is simple. a high-energy muon neutrino interacts with a nucleus in the water and produces a muon travelling in nearly the same direction as the neutrino. the high-energy muon track can range from several hundreds metres up to few km, depending on its energy. the cherenkov light emitted along the muon track, with a well defined angle with the track itself, can be detected by a sparse array of optical sensors, deployed in deep-sea (3000-4000 m depth) and arranged in a geometry that allows the reconstruction of the track direction. the average distance in between the sensors will be a function of the light propagation properties in the water: the lower the light absorption, the higher can be the distance between sensors. also the detector cost is a function of the water transparency: the longer is the absorption length of deep-sea water the less is the number of light sensors needed to equip the same instrumented volume. light sensors, also called optical modules (om), constitute the active part of the detector. they are realised with large area (8 to 13 inches) photomultiplier tubes (pmt) hosted inside pressure resistant glass spheres. they have to provide the information of the intensity of the light that hits their surface and of the photons arrival time. these quantities are transformed locally into digital information that can be transmitted, by means of optical fibres, to a shore laboratory. on shore all the data received can be stored and analysed. the idea to construct a high energy neutrino detector by instrumenting large volumes of natural media was proposed long time ago by markov (1960) and markov and zheleznykh (1961), but only recently the technological developments in mechanics, electronics and data transmission system seems to be sufficient to transform the project in reality. the payoff for this choice is that one has to face several technological problems for the construction, deployment and maintenance of the instrument. the construction of km3 scale neutrino telescopes requires, in fact, detailed preliminary studies and intense r&d efforts. the layout of the detector must be optimized to achieve an effective detection area close to 1 km2 together with a pointing accuracy close to the intrinsic uncertainty due to the neutrino interaction and an energy resolution of the order of some tens percent. the choice of the underwater installation site must be carefully investigated, since water properties strongly influence the detector performance. the electronics design must limit power consumption and allow the transmission of high data flows from the detector to the shore. the mechanical design must allow easy deployment (and possibly maintenance and recovery) operations and the deployed structures must be reliable over a period of more than 10 years. the positioning system must be realised to determine the position of optical sensors with 10 cm accuracy. the pioneering effort to develop a neutrino telescope was carried out by the dumand collaboration (babson et al., 1990), starting more than twentyfive years ago in the ocean waters offshore hawaii island. after several years the project was abandoned, mainly because of the problems encountered in the deployment of the equipment in the sea (roberts, 1992). at present two small scale neutrino telescopes are in operation, one at lake baikal and the other, amanda, under the ice of the south pole. two other projects, antares and nestor, are aiming at the realization of deep-sea prototypal detectors in the mediterranean sea. studies and r&d activities towards the realization of a deep-sea km3 scale neutrino detector in the mediterranean have been up to now conducted by the italian nemo collaboration. these activities are expected to converge in the future in one large international collaboration to realize the mediterranean km3 detector. 3.1. the running neutrino telescopes 3.1.1. the lake baikal experiment baikal was the first collaboration to install an underwater neutrino telescope, which, after more than ten years of operation, is still the only one located in the northern hemisphere. the 649 underwater laboratories for astroparticle physics and deep-sea science lake baikal neutrino telescope (belolaptikov et al., 1997) exploits the deep fresh water of the great siberian lake as a detection medium for high energy neutrinos. its lifetime spans almost two decades from the small initial nt-36 detector, that has proven the capability of the experiment to search for neutrinos by the detection of first neutrino candidates (balkanov et al., 1999, 2000), to the present neutrino telescope nt200, which was put into operation in 1998. the experiment is located in the southern part of lake baikal, 3.6 km from the shore. the nt200 detector is an array of 200 optical modules moored between a depth of 1000 and 1100 m. the deployment and recovery operations are carried out at the end of the winter season, when a thick ice cap (about 1 m) is still present over the lake. the limited depth and the qualities of lake water (light transmission length of 15 ÷ 20 m, high sedimentation and bio-fouling rate, optical background due to bioluminescence) limit the detector performances as a neutrino telescope. 3.1.2. amanda the amanda (antarctic muon and neutrino detector array) detector (andres et al., 2000) is currently the largest neutrino telescope installed. it is located close to the amundsen-scott research station at the south pole and uses the deep antarctic ice as detection medium (more information on the amanda detector can be found on the experiment’s web site at http:// amanda.uci.edu). in the present stage, named amanda-ii, the detector consists of 677 optical modules all downward oriented. optical modules are arranged in 19 vertical strings deployed in holes drilled in the ice between 1.3 and 2.4 km depth, where the ice optical properties are suitable for cherenkov detection. thanks to the high absorption length in ice (about 100 m), amanda is a good calorimeter for astrophysical events. on the contrary, due to the small light scattering length in ice (tens of cm), the detector angular resolution is worse than the one expected for underwater neutrino telescopes. the main advantage of the amanda location is the absence of optical noise sources, like 40k and bioluminescent organisms in the bulk ice. the amanda data have permitted to measure for the first time the upgoing atmospheric neutrino spectrum in the energy range from few tev to 300 tev, proving the capabilities of the cherenkov detection technique and allowing to set what is up to now the most restrictive experimental bound on the diffuse high energy neutrino flux (ahrens et al., 2003; ackermann et al., 2005a). a search for cosmic point-like sources has also been attempted using a targeted search strategy focusing on known objects known to be candidate neutrino sources. however, among the collected sample of neutrino induced events, no significant excess due to astrophysical point sources has been observed (ackermann et al., 2005b). 3.2. small scale deep-sea demonstrator detectors 3.2.1. nestor nestor (neutrino extended submarine telescope with oceanographic research) was the first collaboration to propose the installation of a neutrino telescope in the mediterranean sea (tzamarias, 2003). the goal was to deploy a modular detector at about 4000 m in the ionian sea (more information on the nestor experiment can be found on the experiment’s web site at http://www.nestor.org.gr). the nestor site is located 20 km sw offshore methoni, in the peloponnese (greece). the proposed array should comprise a series of semi rigid structures called «towers». each tower would be 360 m high and 32 m in diameter and should be equipped with about 170 pmts looking both in upward and downward directions. in march 2003, after a long r&d activity, nestor has successfully deployed 12 optical modules at a depth of 3800 m acquiring, onshore, underwater optical noise data and cosmic muon signals (745 events reconstructed) for about one month (tzamarias, 2005). 3.2.2. antares the construction of the proposed antares (astronomy with a neutrino telescope and abyss environmental research) detector (an650 emilio migneco, antonio capone and paolo piattelli tares collaboration, 1999) is currently in a well advanced stage (more information on the antares experiment can be found on the experiment’s web site at http://antares.in2p3.fr). antares will be a demonstrator with a detection area of 0.1 km2 for high energy muons generated by astrophysical neutrinos, and will be a fundamental step towards the km3 telescope in deep-sea. the antares site is located at 2400 m depth, 40 km se off-shore the city of toulon (france). in the present design antares will be a high granularity detector consisting of 12 strings, each one equipped with 75 optical modules, placed at an average distance of 60 m. this configuration is optimized to reduce the muon detection threshold down to about 10 gev allowing the investigation of lower energy phenomena such as atmospheric neutrino oscillations and search for dark matter. with respect to amanda and baikal, strongly affected by light scattering in the medium, the dense antares detector is expected to reach a pointing accuracy which will be close to 0.1°. in december 2002 the detector installation started with the deployment of the so called junction box, which will interconnect electro-optical cables from the strings to the shore. after a first operation of two prototypal lines in spring 2003 a new and improved version of a short line equipped with oceanographic instruments and optical modules has been put in operation in april 2005 and is taking data since then. data recorded by the optical modules show an unexpectedly high optical background, strongly fluctuating, as a function of time, from a level of 50 khz to a level of more than 250 khz, with a strong contribution of bioluminescence bursts (anton, 2005). 3.3. future km3 neutrino telescopes the simultaneous observation of the full sky is an extremely important issue, since the distribution of some neutrino sources is expected not to be isotropic. to achieve this goal two km3 scale neutrino telescopes, one in each earth hemisphere, are needed. in the southern hemisphere, and therefore looking at the northern sky, the icecube telescope (botner, 2005) will be the natural extension of amanda to km3 size. its construction started in january 2005 with the deployment of the first string. when completed in 2010 it will be an array of 4800 pmt arranged in 80 strings. all the optical modules will be downward looking as in amanda and simulations show that an average spacing of 125 m between the strings is an optimal compromise between the two requirements of angular resolution of better than 1° and a high energy muon detection area of approximately 1 km2. it is worth mentioning that under-ice detectors are not affected by radioactive and biological optical noise. this makes them suitable for the search of low energy neutrino fluxes from galactic supernova explosions (more information on the icecube detector can be found on the experiment’s web site at http://icecube.wisc.edu). in the northern hemisphere the mediterranean sea offers the most favourable conditions for the construction and maintenance of an underwater km3 cherenkov neutrino detector: optimal water characteristics, deep-sea sites at close distance from shore, proximity to scientific and industrial infrastructures, good weather and sea conditions for a large fraction of the year. an underwater detector offers, compared to icecube, the possibility to be recovered, maintained and reconfigured. the long light scattering length of the mediterranean abyssal seawater preserves the cherenkov photon directionality and will permit excellent pointing accuracy (of the order of 0.1° for 10 tev muons). unlike deep polar ice, the sea is a biologically active environment where organisms produce background light. the selection of a marine site with optimal oceanographic and optical characteristics is therefore a major task for the collaborations involved in the km3 project. at present, the effort towards the construction of a large area underwater detector is focused on the development of submarine technologies. deep-sea is an extremely hostile environment where pressure (100 bars per 1000 m depth), together with salinity, reduces the lifetime of most of metals and alloys used in surface and shallow water applications. the efforts presently conducted by the three mediterranean collaborations (antares, nemo and nestor) will represent a valuable experience in the construction of 651 underwater laboratories for astroparticle physics and deep-sea science the underwater km3 detector, which should take place in the coming years. 4. research and development for the km3 detector the construction of an underwater km3 scale neutrino telescopes requires detailed preliminary studies: the choice of the underwater installation site must be carefully investigated to optimise detector performance; the readout electronics must have a very low power consumption; the data transmission system must allow data flow transmission, as high as 100 gbps, to shore; the mechanical design must allow easy detector deployment and recovery operations, moreover the deployed structures must be reliable over more than 10 years; the position monitoring system has to determine the position of om within ≈10 cm accuracy. in order to propose feasible and reliable solutions for the km3 installation the nemo (neutrino mediterranean observatory) collaboration has been conducting an intense r&d activity on all the above subjects since 1998 (migneco et al., 2004a), which will be briefly described in this section. 4.1. site selection and characterization the needs of the underwater neutrino telescope impose a series of requirements that the site must fulfill. depth – thickness of the overlaying water has to be enough to filter out the down-going atmospheric muon background to allow the selection capability of the up-going tracks originated from neutrino interactions in the earth and/or the water near the detector. distance from shore –the data transmission to the on-shore laboratory, as well as the transmission of power from the laboratory to the offshore detector, will be performed via an electrooptical multi-fibre cable. at distances closer than 100 km from the coast, commercial systems allow data and power transmission without special hardware requirements (e.g., amplifiers) that would increase the cost and reduce the reliability of the project. moreover, the proximity to the coast and to shore infrastructures simplifies the access to the site for deployment and maintenance operations. site geology – the seabed has to be flat and stable to allow the mooring of the telescope structures. for the detector safety, it should also be located at some tens of km far from the shelf break and submarine canyons. water transparency – the detector performance is not only directly determined by the extension of the instrumented volume but is also strongly affected by the light transmission properties of the water. mainly two microscopic processes affect the propagation of light in the water: absorption and scattering. light absorption directly reduces the effective area of the detector, the scattering spreads the photon arrival times and therefore worsens the detector angular resolution. optical background – optical background in seawater comes from two natural causes: the decay of 40k, which is present in seawater, and the so called bioluminescence that is the light produced by biological organisms. the first one shows up as a constant rate background noise on the optical modules (of the order of 30 khz for a single 10m pmt). the second one, when present, may induce large fluctuations (both in the baseline and as presence of high rate spikes) in the noise rate. the background directly affects the detector performances, in particular the quality of muon track reconstruction. in a high background environment severe cuts to the photon detector data must be applied, hence reducing the detector effective area. downward sediment flux – the presence of sediments in the water can seriously affect the performances of the detector. sediments increase the light scattering, therefore worsening the track reconstruction angular resolution. moreover, a deposit on the sensitive part of photon detectors, i.e. large surface photomultipliers, reduces the global detector efficiency. a direct consequence is that in a high sedimentation rate environment the operation of upward looking optical modules will not be possible. deep-sea currents – these must have low intensity and a stable direction. this is important for several reasons: 652 emilio migneco, antonio capone and paolo piattelli – it does not imply special requirements on the mechanical structures; – the detector deployment and positioning is easier if the water current is limited; – the optical noise due to bioluminescence, mainly excited by variation of the water currents, is reduced. the mediterranean sea offers optimal conditions, on a worldwide scale, to locate an underwater neutrino telescope. the nemo collaboration has performed a long-term research program to characterize deep-sea sites that could be appropriate for the installation of a deep-sea high-energy neutrino detector. studies of deep-sea water optical properties (absorption and diffusion) and the sites environmental properties (water temperature, salinity, biological activity, optical background, water currents and sedimentation) that will be briefly described in the following have been carried out. this activity has demonstrated that the abyssal plateau in the ionian sea (fig. 3) close to the southernmost cape of the coast of sicily (capo passero) shows excellent characteristics to host the km3 underwater neutrino detector. the site was selected after a series of measurements in the region at varying distances from the coast. its location at about 50 km from the shelf break about 80 km from the coast was chosen to ensure the best condition of stability in time of the water parameters and avoid any perturbation coming from the presence of the shelf break. a geological survey of the area verified the flatness and the absence of any evidence of recent turbidity events. deep-sea currents were measured to be, in the average, as low as 3 cm/s, never exceeding 15 cm/s. the study of optical properties in the selected site is extremely important and must be carried out with a long-term program of characterisation carried out in all different seasons. seawater, indeed, absorbs and scatters photons as a function of water temperature, salinity and concentration, dimension and refraction index of dissolved and suspended, organic/inorganic particulate. these parameters are different in different marine sites and change as a function of time. seawater oceanographic parameters (temperature and salinity) and inherent optical properties (light absorption and attenuation) were measured as a function of depth, showing that, while at shallow depths water properties change as a function of season, at large depth (>1500) the water column has stable characteristics. the average value of blue (λ = = 440 nm) light absorption length is of the order of 70 m, comparable with that of pure salt water. the optical background noise was measured at 3000 m depth in capo passero. data collected in spring 2002 and 2003, for several months, show that optical background induces on the optical modules a constant rate of 20-30 khz (compatible with the one expected from 40k decay), with negligible contribution of bioluminescence bursts. these results were confirmed by biological analysis that show, at depth larger than 2500, extremely small concentration of dissolved bioluminescent organisms. 4.2. the underwater km3 detector concept the underwater neutrino detector design must be optimised in order to reach the needed fig. 3. the south ionian sea, showing the location of the site selected and characterized by the nemo collaboration for the installation of the km3 neutrino detector. 653 underwater laboratories for astroparticle physics and deep-sea science effective area of ≈1 km2, a pointing accuracy better than 0.1° for 10 tev muons, an energy resolution of the order of some tens percent in log(e) and an energy threshold close to 100 gev. 4.2.1. detector architecture it is clear that the architecture of the km3 detector should stem from a compromise between performances and technical feasibility of the detector. as a first approach one can consider filling up a volume of about one km3 with a lattice of equally spaced sensors, with a spacing of the same order of the absorption length of light in water. taking the horizontal and vertical distance between down looking sensors equal to 60 m turns out in a total number of 5600 sensors arranged in 400 strings. although the solution of arranging the sensors along a string presents some advantages in terms of simplicity, this architecture can be difficult to realise due to the close distance between strings (60 m) compared with their total height (780 m). moreover, the large number of structures to be moored and connected raises the cost of the detector. therefore, alternative solutions should be studied for the architecture, in which the number of structures is reduced by gathering a relatively large number of sensors on each of them. in particular, constraints on the distance between structures (larger than about 120 m) and on their height (smaller than 1 km) were suggested by a preliminary feasibility study in terms of construction, deployment and maintenance operations of the detector within reasonable costs. following these suggestions a structure composed by a square array of «nemo towers», shown in fig. 4, was proposed. the proposed architecture is «modular», in the sense that it is expandable with the addition of extra towers, and configurable with different seafloor layouts. the performances of this architecture in terms of effective area and angular resolution were evaluated by means of computer simulations, which have shown that a detector built with this architecture can meet the required goal of a detection area larger than 1 km2 for muon energies larger than 10 tev with the required angular resolution. 4.2.2. mechanical structures the nemo collaboration has proposed an innovative structure, alternative to the string concept, to host optical modules: the nemo tower. it is a three dimensional flexible structure composed by a sequence of stories (that host the instrumentation) interlinked by a system of cables and anchored on the seabed. the structure is kept vertical by appropriate buoyancy on the top. the final features of the tower (number and length of stories, number of optical modules per storey, distance between the stories) can be optimized following the results of numerical simulations. however, the modular structure of the fig. 4. the proposed layout of the km3 detector. a hierarchical arrangement of junction boxes (jbs), a main one connected to the main electro-optical cable and a number of secondary jbs, will allow to concentrate the data flow coming from the detector structures and, at the same time, distribute the power. 654 emilio migneco, antonio capone and paolo piattelli tower will permit us to adjust these parameters to the experimental needs. in the preliminary design a 16-storey tower, where each storey is made with a 20 m long structure hosting two optical modules at each end (4 om per storey), has been considered. the vertical separation between storeys is fixed at 40 m, giving a total active height of 680 m. an additional spacing of 150 m is added at the base of the tower, between the anchor and the lowermost storey to allow for a sufficient water volume below the detector. in its working position each storey will be rotated by 90°, with respect to the up and down adjacent ones, around the vertical axis of the tower. the tower will also be equipped with an acoustic triangulation system for precise position reconstruction purposes and with environmental sensors. one of the advantages of this structure is the fact that it can be compacted, by piling each storey upon the other, to simplify transport and deployment. the structure is unfurled, reaching its operating configuration, only after its deployment on the seabed. 4.3. technological challenges for the km3 detector in recent years many innovations have been applied to underwater technology: dwdm (dense wavelength division multiplexing) is permitting a large increase in speed and bandwidth of the optical fibres data transmission; newly developed materials (synthetic fibres, alloys, plastics, ...) can improve long term underwater reliability of complex deployed structures; deep-sea (≈3000 m depth) operations with remotely operated vehicles (rov) or autonomous underwater vehicles (auv) have been developed and standardised. the design of the mediterranean km3 will directly profit of these ultimate technologies. concerning the km3 data transmission system, it is considered a wise approach to transmit all pmt signals, acquired at low threshold level, to shore. in such a way all filtering and event reconstruction procedures can be applied by on-shore data acquisition systems. this approach results in a very high data transmission rate (order of 100 gbps) that has to be transmitted about 100 km offshore. the above considerations impose the use of a fibre optics transmission system based on dwdm telecommunication technique and composed of mainly passive optical components to reduce power consumption and increase reliability. the proposed nemo architecture is designed to collect data hierarchically: each storey (hosting 4 oms) transmits data to a tower main module using an assigned wavelength (or colour). each tower module collects data from the stories and sends all data, multiplexed in one single fibre, to a collector. this collector can receive data from a number of towers, multiplexes them and sends them to the shore. a mirror system on shore de-multiplexes the signals, recovering data from each storey, then from each very single om. to cope with the problem of pressure and corrosion resistance of the underwater containers that have to host the electronics, a new concept has been proposed by the nemo collaboration. this innovative vessel decouples the two problems by placing an inner pressure resistant steel vessel inside an outer glass epoxy vessel filled with non-corrosive oil. another crucial issue for deep-sea detector installation is the feasibility and reliability of underwater connections. however, the technologies of underwater connections have considerably advanced in recent years and reliable connection systems exist that can be operated underwater by means of rovs. these connectors and their operability have been tested both by the antares and nemo collaborations. 4.4. the nemo phase 1 project: a multidisciplinary underwater laboratory at 2000 m the technological solutions proposed for the km3 detector require an adequate process of validation. for this reason the realization of a technological demonstrator is underway. it will consist of a subset of the proposed km3 detector, including some critical elements like a junction box, a tower complete with data transmission, power and calibration systems. this project is called nemo phase 1 (fig. 5) and will be installed at the 655 underwater laboratories for astroparticle physics and deep-sea science underwater test site of the laboratori nazionali del sud in catania at a depth of 2000 m. the test site of the laboratori nazionali del sud consists of an electro-optical submarine cable, that connects the underwater installation to shore, and a shore station. the cable system is composed of a 25 km main electro-optical cable, split in two branches, each one 5 km long. one branch dedicated to the nemo phase 1 experiment (migneco et al., 2004b; piattelli, 2005), and the other one to the sn-1 underwater seismic monitoring station realized by the istituto nazionale di geofisica e vulcanologia (ingv) (favali et al., 2003). a shore station, located inside the port of catania, will host the energy power system of the laboratory, the instrumentation control system, the landing station of the data transmission system and the data acquisition, as well as a mechanics and electronics laboratory for the assembly of the components. in january 2005 the project achieved a major milestone with the installation of the two cable termination frames (that host the wet mateable underwater plugs) and the deployment and connection of a small acoustic detection station on the first branch and the sn-1 station on the second branch. both these stations have been operational ever since, continuously transmitting data to shore. the project will be completed in 2006 with the installation of the junction box and a prototype four storey tower. in its final configuration this project will not only represent a fundamental test of the technologies for the km3 detector but it will also be a fully functioning multidisciplinary laboratory at 2000 m depth, offering the possibility of on line connection to a variety of experiments. 4.5. the nemo phase 2 project: a deep-sea infrastructure at 3500 m although the phase 1 project will provide a fundamental test of the technologies proposed for the realization and installation of the detector, these must be finally validated at the depths needed for the km3 detector. for these motivations the realization of an infrastructure on the site of capo passero has been started. this infrastructure will consist of a 100 km deep-sea cable, linking the 3500 m deep-sea site to the shore, a shore station, located inside the harbour area of portopalo di capo passero, and the underwater infrastructures needed to connect prototypes of the km3 detector. the construction details of the cable have still to be defined, but it will have characteristics of power load (greater than 50 kw) and number of fig. 5. schematic layout of the nemo phase 1 project. 656 emilio migneco, antonio capone and paolo piattelli optical fibres (20 or more) sufficient to serve a detector such as the proposed nemo neutrino telescope. construction work to restore an already existing building that will become the shore station is going to start soon. the station will host the power system, data acquisition and control facilities, together with a large assembling area. the completion of this project is foreseen by the end of 2007. at this time it will be possible to connect one or more prototypes of detector structures, allowing a full test at 3500 m of the deployment and connection procedures. this project will also allow a continuous long term on-line monitoring of the site properties (light transparency, optical background, water currents, …) whose knowledge is essential for the installation of the full detector. the possibility of installing multidisciplinary observatories on the site is also foreseen. 5. multidisciplinary researches in a km3 scale underwater laboratory the core discipline served by an underwater neutrino telescope is astroparticle physics, but it also has potential to attract significant interest from other scientific communities. in fact, the possibility to monitor continuously over long periods and in real time the sea bottom at depths of the order of 3000 m is of utmost interest to a large number of sea-related disciplines like geophysics, biology, chemistry and environmental sciences. the underwater world has not yet been extensively explored. current technology allows autonomous vehicles or remotely operated vehicles (rovs) to carry out scientific experiments at great depths only for relatively short periods and with accompanying support ships. a deep-sea neutrino observatory will provide the community of deep-sea scientists with a continuous supply of power and a high bandwidth data channel, enabling them to make local real-time studies. moreover, the information gathered by the optical sensors and other oceanographic monitoring equipment needed for running the neutrino telescope can be of use to oceanographers and marine biologists. more generally, other types of instrumentation can be added to the observatory array. for example, ocean bottom seismometers can transmit their recordings in real time, thus permitting the localization of the epicenters of seismic events with greater accuracy. the effectiveness of this approach is shown by the realization of the test site laboratory of the nemo project, realized with the primary goal of testing technologies for the km3 project at 2000 m, but also hosting the sn-1 deep-sea seismic and environmental observatory of the istituto nazionale di geofisica e vulcanologia (favali et al., 2003), which constitutes the first active node of the european seafloor observatory network – esonet (more information about the esonet network can be found on the web site at http://www.oceanlab.abdn.ac.uk/research/ esonet.shtml) (priede et al., 2004). 6. conclusions the realization of a km3 telescope for highenergy astrophysical neutrinos is a challenging task at the frontiers of astroparticle science. several collaborations in europe are already working on the realization of first generation demonstrators. more efforts are needed to develop a project for the km3 detector. in its five years of activity the nemo collaboration has contributed in this direction by performing an intense r&d activity. an extensive study on a site close to the coast of sicily has demonstrated that it has optimal characteristics for telescope installation, in particular as concerns the water optical properties and its proximity to already existing infrastructures like the lns in catania. a complete study has been performed to analyse all the detector components both in term of their technical feasibility and their installation. this study, which has produced as a result a preliminary design of the detector, has shown that a detector with effective area over 1 km2 is realizable at an affordable cost. the realization of a demonstrator of some of the key technological solutions proposed for the km3 detector has been started at the underwater test site in catania. this project foresees 657 underwater laboratories for astroparticle physics and deep-sea science the realization of an underwater laboratory including prototypes of the proposed structures. the completion of this project is foreseen by the end of 2006. the design, construction and operation of a km3 neutrino telescope is a great challenge that must be pursued by a large international collaboration. recently, the scientific and technical experiences gathered by the antares, nemo and nestor collaborations have come together in the km3net consortium (more information can be found on the km3net web site at http://www.km3net.org), formed around the european institutes currently involved in the neutrino astronomy projects (thompson, 2005), with the aim of carrying out a design study for the km3 neutrino telescope. based on the leading expertise of these research groups, the development of the km3 telescope is envisaged to be achieved within a period of three years for preparatory r&d work plus five years for construction and deployment. the underwater km3 neutrino telescope will finally open a new observational window on the universe, but will also represent a multidisciplinary facility by construction, since it will offer the availability of deep-sea infrastructures (like connection systems, power distribution, wide band data transmission system, etc.) to scientists of 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[1998] used a multi-methodological approach (ground penetrating radar (gpr), electrical resistivity tomography (ert), self potential and magnetometric methods) in the sabine necropolis in rome, italy, to evidence structures through the study of different parameters. some years later cardarelli et al. [2008] investigated a new area in the same necropolis using ert, fluxgate and gpr methods. furthermore, quesnel et al. [2011] used ert and magnetic measurements to evidence a roman early medieval necropolis in provence. in the recent years local authorities focused their interests in valorizing the necropolis of vulci (figure 1). four necropolis areas dating from the 8th century b.c. have been found around the city of vulci, with the osteria necropolis towards the north. this was the main necropolis of vulci, with tombs dating from the villanovan period [eutizi et al. 2010]. tombs from the 6th and 5th centuries b.c. are generally of the hypogeal type. the habit of placing statues of mythical creatures to guard the tombs is characteristic of vulci. immensely valuable grave goods have been found in these tombs, in particular a large number of ceramics of greek production, and bronze objects of local production [moretti sgubini 1997]. the most important tomb located at the necropolis of ponte rotto is the françois tomb, famous for its paintings (now at villa albani in rome) portraying, as well as the deceased, episodes from greek mythology together with characters from etruscan myths and history [tamburini 1987]. the tombs of the osteria necropolis are mostly article history received may 29, 2013; accepted jannuary 20, 2014. subject classification: vulci archeological park, geological survey, electrical resistivity tomography, magnetic susceptibility. chamber tomb type with an open air vestibule. the sloping corridor led to an antechamber open to the sky. several side chambers completely hewn out of the rock with a ceiling opened on to this antechamber [banti 1973]. considering that the geological formations and structures present in the vulci park have never being studied so far, we retained necessary as first step to elaborate a geological map of the study area with a high resolution scale of 1:10000. the geology of vulci area was then interpreted in the context of vulsini volcanic district (vvd) which is well described in literature [washington 1907, nappi et al. 1986, nappi et al. 1998] and provides crucial information in order to understand the genesis and characteristics of the geological deposits in the vulci site. the geophysical prospections were used to define the electrical and magnetic characteristics of the volcanic lithologies outcropping in the necropolis of vulci. these analyses were conducted to find out if it might be possible, through the use of these methods, to identify other locations where etruscans excavated their tombs. therefore, magnetic susceptibility has been measured in situ and in a laboratory to evaluate magnetic contrasts between different lithostratigraphic units. ert were also performed along two profiles and made it possible to define the areal extension and thickness of the lithotype on which the osteria necropolis was excavated. 2. methods and results 2.1. geology and geological map the area of vulci park is part of the coastal plain of maremma which is characterized by the presence of clastic sedimentary rocks, deposited on pleistocene marine terraces. along the coastal area, the bedrock is mainly constituted by pliocene clays and silts, whereas in the hinterland it consists of cretaceous-oligocenic tuscan and liguride facies sediments [capelli et al. 1994]. in the hinterland, after the building of the apennine chain, extensional tectonics produced intense volcanism in the roman comagmatic province [washington 1907] of which the vvd is part [nappi et al. 1998]. the geological structure of vulci developed in this framework. the fiora river, characterized by huge flow in a ns direction separates the area into two main plateaus. in the past the river flowed along a more easterly course; later deviated by the emplacement of the latera pyroclastic deposits [fabbri and villa 1987]. the two main plateaus are laterally interested by river drainage with up to 30 meter high slopes, exposing outcrops of different lithotypes. rock falls occur along the slopes, mainly comprising tephrite-lava clasts and the bed of the fiora river is occupied by a huge volume of debris. the valley bedrock is a 35 m thick dark-grey colored lava deposit, with a compact fine-grained structure, named tefrite del castellaccio dei vulci [alberti et al. 1970], and characterized by a porphyric isotropic structure. the lava has a teprithic-phonolitic composition with plagioclase, pyroxene, and few sanidine phenocrysts (figure 2). in the fiora gorge this lithotype exhibits a well-developed columnar jointing, possibly due to the fast cooling of the lava in contact with the river water. the limited extension of the lava flow into the present fiora valley led the assumption [sposato et al. 1993] that the lava flow filled a proto fiora valley. the lava flow was probably erupted by local regional fractures now buried under the travertine plateau, on the eastern side of fiora river. the fractures were probably connected to the regional fault systems controlling the evolution of the marchetti et al. 2 figure 1. area location. figure 2. tephritic lava travertines contact. 3 siena-radicofani graben [vezzoli et al. 1987]. the age of this lava flow was investigated by fabbri and villa [1987] and it is dated to the medium pleistocene. in the vulci park area, the prevalent lithology is the sorano formation [palladino et al. 2010] erupted by the latera volcano between 194 ± 5 ka and 187 ± 8 ka ago [turbeville 1992]. this formation is subdivided in two units, characterized by different matrix colors, abundance of clasts and lithification rates. in this study, the units are named sorano unit 1 (the lower unit) and sorano unit 2 (the upper unit). the sorano unit 1 outcrops only near the fiora river, or in minor valleys excavated by secondary streams. the unit is a medium-fine grey-colored ash deposit, about 5 m thick, enriched with light-colored lapillus-sized pumices. the clasts/matrix ratio is about 30 %, this means that about 30% of the lithology is represented by clasts. this unit is poorly lithified and exhibits a pseudo-stratified base due to the presence of clay levels enriched with floating pumices (figure 3), as in sorano unit 2. the pseudostratification evidenced in the lower part of both the sorano units suggests that the volcanic flow entered locally in the water of a low-energy aqueous depositional environment characterized by the presence of several pools, like a shallow marsh. by this reconstruction it is plausible that both the sorano units entered the water of the marshy environment, following different paths. in fact, in distal areas pyroclastic flows might form finger-like lobes [jessop et al. 2012]. the sorano unit 2 is a fine-ash red-colored deposit, 8 m thick. the clasts/matrix ratio is lower than the geology and geophysics at vulci park figure 3. pseudo-stratified sorano unit i. figure 4. tephra lithics scattered in sorano unit 2. marchetti et al. 4 figure 5. sorano unit 2. figure 6. carved ceilings tombs, excavated in sorano unit 2. 5 basal unit (15 %). inside the deposit there are many cmsized lithics (figure 4), ripped up from the underlying tephritic lava. lithics show imbrication, with a ne-sw direction. a process of zeolitization of the glassy matrix could have caused the lithification of this lithology. the lithification and the extent of the sorano unit 2 explain the major number of tombs excavated in this unit. (figure 5). it is our opinion that the progressive cooling of the pyroclastic flow, facilitated by the interaction with the water of the marshy environment present in the area during the unit’s emplacement, have caused the lithification of this lithology. in fact this process in pyroclastic units increases the lithification of the tuffs without compromising their malleability [cerri et al. 2013] and it likely results in higher electrical resistivity values. these characteristics were known by etruscans that chose these lithotypes to excavate their necropolis. thin sections of the sorano units show the same characteristics (clasts/matrix ratios, matrix color) observed in the field. both of them include sanidine and monocline amphibole minerals. public access to tombs with carved ceilings enabled the observation of a few meters of sorano unit 2 (figure 6). the upper part of this unit (figure 7) mostly comprises a homogeneous medium-fine ashy matrix while the lower part is pseudo-stratified and includes a lava-conglomerate bank and three clay levels. above the sorano formation there are few exposures of the grotte di castro formation [palladino et al. 2010] near the vulci ruins. it has been hypothesized that this formation originally had greater extension than now, and that it has been progressively eroded. in the 1.5 m thick deposit, a 30 cm interval is recognizable, characterized by a succession of levels of dark and light pumices with sedimentary ripple structures (figure 8). the sorano formation in the western sector of the study area (figure 9) is based on sand sediments by an unconformity limit. these sediments have been ascribed to the pleistocene and correlated with the circumlittoral pleistocene sand formation known in literature [alberti et al. 1970]. in correspondence with the sands, the morphology of the area exhibits steeper slopes, a dendritic drainage network (figure 9), and a yellow-colored soil enriched with quartz clasts, unlike the brown-colored soil formed by the oxidation of the ferric minerals of the volcanic deposits. geology and geophysics at vulci park figure 7. lithostratigraphic column and magnetic susceptibility log of a section of the sorano unit 2. in the eastern part of the area, lying above the units already described, there is a 8 m thick travertine bank originated by hydro-thermalism (figure 2). travertine shows horizontal pseudo-stratification and contain volcanic clasts. the values of the isotope ratio u234/u238 indicate mixing of deep and shallow groundwater. th230 data suggest the travertine formation is 8.2 ± 1.6 ky old [taddeucci and voltaggio 1987]. 2.2. magnetic susceptibility magnetic susceptibility measurements are often used in archeological prospection in addition to other geophysical surveys both for investigating buried features and their characteristics and for mapping human habitations [schmidt 2007, 2009]. low-field magnetic susceptibility per unit volume (κ) of the vulci deposits was measured using a bartington ms2 susceptibility meter equipped with a d loop sensor [lecoanet et al. 1999]. in particular, measurements performed on a surficial trench revealed that brown-colored soils, derived from the alteration of the volcanic units, exhibit an average susceptibility of 500 x 10-5 si, with the exception of soil derived from alteration of the upper part of the sorano unit 2 which exhibits an average susceptibility of 200-300 x 10-5 si. this finding evidence a contrast between the two kind of soils derived by different parent material. thus, the measure of the magnetic susceptibility of the soil in this area could help in identifying buried tombs in the cases in which the upper part of the sorano unit 2 is near to the surface enough to significantly contribute to the formation of the overlaying top-soil. developing maps of magnetic susceptibility could be taken into account for future prospections. in this case the measurements were carried out on a trench but the effectiveness of the method could be tested also before the beginning of excavations. lava blocks, scattered within the sorano unit 2, present variable grain size and have an average susceptibility of 500 x 10-5 si (a punctual f sensor was employed in this case). a few lava blocks were measured at the laboratory of paleomagnetism of the istituto nazionale di geofisica e vulcanologia using a kappabridge system agico (kly-2 model) and they exhibited a mass specific susceptibility of 850 x 10-8 m3/kg. near the tombs with carved ceilings, a 3.4 m deep stratigraphic section of the sorano unit 2 was selected for detailed geophysical investigation. in this section, the sorano unit 2 has vertical facies variations as the basal part of the pyroclastic flow likely emplaced underwater. the section is therefore ideal for investigating variations in susceptibility derived from different lithological characteristics of the unit. susceptibility marchetti et al. 6 figure 8. sorano unit 2 (lower unit) and grotte di castro formation (upper unit) contact. 7 was measured using an f sensor with sampling resolution of 10 cm. figure 7 shows the relative lithological and susceptibility log. from the bottom up to 1.8 m, the average susceptibility is 393 x 10-5 si and it is characterized by a peak that reaches the highest value of 600 x 10-5 si matching with a gravel bank made of volcanic clasts, and by two peaks corresponding to clay intervals. toward the top of the section, susceptibility decreases and becomes quite constant (average value of 132 x 10-5 si) while the lithology change by pseudostratified to a homogeneous medium-fine ashy matrix. the clasts occurring in the gravel bank have a major grain size compared to those contained in the ashy matrix and may contribute to increase the average susceptibility value. magnetic susceptibility is a concentration parameter and depends on the content of magnetic minerals in rocks and sediments [thompson and oldfield 1986]. therefore, peaks observed for the clay intervals are not surprising and indicate increases in concentration of magnetic minerals. moreover, a major concentration of magnetic minerals in the lower part of the unit (below 1.8 m) could be related to both different depositional conditions of the pyroclastic flow and energy of the system. a low energy system may have favored the deposition of high density magnetic minerals. similarly, in the albano maar in italy, it was recorded a major concentration of magnetic grains in the valley pond facies respect to the overlying ignimbrite veneer facies and it was explained with a lower density and higher turbulence of the flow during the deposition of the upper more diluted deposits [porreca et al. 2003]. in this framework it is likely that a lacustrine-marshy environment was present in the study area at the time of the pyroclastic flow emplacement as suggested by stratigraphic observations. 2.3. electrical resistivity tomography ert is a geophysical technique employed for imaging the sub-surface in many geological [zhou et al. 2000, chambers et al. 2006, rucker et al. 2011, omosanya et al. 2012] and archaeological applications [urbini et al. 2007, tsokas et al. 2007, bermejo et al. 2010, berge et al. 2011, alashloo et al. 2011]. the ert method consists of a multiple electrode string placed on the surface; by mean a computer controlled data acquisition, each electrode can serve both as a source and as a receiver, so at the same time a large amount of electrical data can be collected quickly. electrical resistivity data were collected using a georesistivimeter syscal r2 (iris instruments) equipped with 64 electrodes. it was used the wenner electrode array to investigate the resistivity contrasts of the subgeology and geophysics at vulci park figure 9. geological map (1:10000 scale) of the archeological park of vulci. soil up to a depth of 20 m. the profile length has been established based on the known geological layers depth informations. since the depth of investigation is roughly 1/5 and or 1/6 of the total length of the electrical profile [loke 1999], we collected electrical data by means of 64 electrodes, 2 m spaced, that fitted the requested depth of investigation. raw data have been manually pre-processed using prosys ii from geotomo software. the processing consisted to remove outliers from the measured data before the inversion is run. in our specific case, the collected data were not affected by high resistance contact at the electrode, and therefore no datum has been culled from our dataset. we used the algorithm proposed by loke and barker [1996] for the automatic 2d inversion of apparent resistivity data. the inversion routine is based on the smoothness-constrained leastsquares inversion [sasaki et al. 1992] implemented by a quasi-newton optimization technique. the optimization adjusts the 2d resistivity model trying to iteratively reduce the difference between the calculated and the measured apparent resistivity values (rms). the root mean squared (rms) error measures directly this difference, which in our case is lower than 2 %. the first profile tomography was realized over an outcrop consisting in the sorano formation and grotte di castro formation, in order to verify if the units could be distinguished by their electrical resistivity values. the outcrop, from the top to the bottom, consists of a 2 m thick grotte di castro formation, followed 7 m thick sorano unit 2, which in turn overlay the sorano unit 1, already known to be 5 m thick [conticelli et al. 1987]. the resulting tomography regards the first 20 m, including all the units recognizable in the outcrop. at the limit of the geophysical method resolution, electrostrata and lithological units show an excellent correspondence (figure 10). the upper electrostratum, visible in the first 2 m of the tomography, is characterized by resistivity values from 80 to 130 ω·m. it was associated to the grotte di castro formation and it records changing lateral resistivity. beneath the grotte di castro formation, the electrostratum recognized as the sorano unit 2 is defined by high resistivity values (200 – 400 ω·m). the resistive electrostratum thickness matches this unit, 6-8 m thick, and it decreases on the left of the tomography, because of a paleomorphology hidden on field by alluvial sediments and soil. we interpreted the underlying electrostratum to be the response of the sorano unit 1. this unit shows lower resistivity values (60-100 ω·m) compared to sorano unit 2, likely due to a higher water content or changes in physical characteristics. the resulting tomography shows an excellent match between lithological units and electrostrata produced by the inversion process, so a range of electrical resistivity can be assigned to each volcanic lithomarchetti et al. 8 figure 10. eletric resistivity tomography performed on a outcrop. 9 type (grotte di castro formation: 80-130 ω·m, sorano unit 2: 200-400 ω·m, sorano unit 1: 60-100 ω·m). the sorano unit 2 is geophysically recognizable because of its higher resistivity values compared to those of the underlying and overlying units. this peculiarity consents to identify in an easy way the sorano unit 2 by resistivity tomography. the second tomography was performed in the central part of the osteria necropolis, using 64 electrodes with 2 m spacing, in order to investigate the contact between the two sorano units (figure 11). the upper resistive layer (8 m thick) has resistivity values of 200-400 ω·m and is separated from a lower conductive layer (ca. 80 ω·m) by a limit 10 m deep. referring these resistivity values to those measured on the outcrop described above, the limit can be recognized as the stratigraphic contact between the two sorano units. considering the thickness of the sorano unit 2 (8 m) the presence of other tombs disposed on more levels could be possible in other sectors of the osteria necropolis. 3. conclusions in this work, the lithostratigraphical units present in the vulci park has been identified and interpreted within the context of the vvd. a geological map at 1:10000 scale has been developed, which shows the areal extension and thickness of the volcanic deposits outcropping in the area. magnetic susceptibility and electrical resistivity of the volcanic deposits record variations related to changes in the lithology. the basal part of both the sorano units were likely emplaced in a marshy environment as suggested by lithological observations and supported by magnetic susceptibility data. ert clearly indicates an average thickness of the sorano unit 2 of 7 m corroborated by high resistivity values (200-400 ω·m) and reveals a close match between the geophysical electrostrata and the existing geological units. these results are useful for defining the areal extension and thickness of the lithotype where etruscan necropolis were excavated and furnish new elements to archeologists for better planning new field 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(1987). i titoli funerari della tomba françois e il sepolcreto di vel saties, la tomba di vulci, catalogo della mostra (musei vaticani, 20 marzo-7 maggio 1987). marchetti et al. 10 11 thompson, r. and f. oldfield (1986). environmental magnetism, allen and unwin, london. tsokas, g.n., p.i. tsourlos, g. vargemezis and m. novack (2007). non-destructive electrical resistivity tomography for indoor investigation: the case of kapnikarea church in athens, archaeological prospection, 15 (1), 47-61. turbeville, b.n. (1992). ar40/ar39 ages and stratigraphy of the latera caldera, italy, bullettin of volcanology, 55, 110-118. urbini, s., l. cafarella, m. marchetti, p. chiarucci and d. bonini (2007). fast geophysical prospecting applied to archaeology: results at «villa ai cavallacci» (albano laziale, rome) site, annals of geophysics, 50 (3), 291-299. washington, h.s. (1907). the roman comagmatic region, nabu press. zhou, w., b.f. beck and j.b. stephenson (2000). reliability of dipolo-dipole electrical resistivity tomography for defining depth to bedrock in covered karst terranes, environmental geology 39 (7). vezzoli, l., s. conticelli, f. innocenti, p. landi, p. manetti, d.m. palladino and r. trigila (1987). stratigraphy of the latera volcanic complex: proposals for a new nomenclature, periodico di mineralogia, 56, 89-110. *corresponding author: marco marchetti, istituto nazionale di geofisica e vulcanologia, sezione roma 2, rome, italy; email: marco.marchetti@ingv.it © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geology and geophysics at vulci park acve_letter_v2_formatted_ok annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6330     1   13 years of mopitt operations: lessons from mopitt retrieval algorithm development h. m. worden1, m. n. deeter1, d. p. edwards1, j. gille1, j. drummond2, l. k. emmons1, g. francis1, s. martínez-alonso1 1national center for atmospheric research (ncar), boulder, co. 2dept. of physics and atmos. sci., dalhousie university, halifax, nova scotia, canada hmw@ucar.edu abstract the measurements of pollution in the troposphere (mopitt) instrument on the nasa terra platform has now acquired over thirteen years of global tropospheric carbon monoxide (co) observations, forming the longest satellite record for an important pollutant. mopitt products are routinely exploited for characterizing co sources and for analyzing air quality. for retrieving co concentrations in the lower troposphere, mopitt is equipped with both thermal-infrared and near-infrared channels. i. introduction measurements of carbon monoxide (co) from space have been critical for understanding global atmospheric chemistry and emissions. sources of co are incomplete combustion (both fires and fossil fuels) and hydrocarbon oxidation. co is destroyed primarily by oh oxidation and is a precursor of greenhouse gases carbon dioxide (co2) and tropospheric ozone (o3), giving co emissions an indirect radiative forcing of 0.2 w/m2 [ipcc, ar4: forster et al., 2007]. due to its medium-scale lifetime (weeks to months) co is a useful tracer of pollution since it is transported globally, but is not evenly mixed in the atmosphere, allowing measurements of elevated co that are easily distinguished above background levels. while development of the algorithms used to process mopitt measurements began several years before launch, these activities continue to be important. the mopitt retrieval algorithm has been steadily refined since launch because of the following experience: (1) measurements 'on-orbit' were somewhat different from pre-launch simulations; (2) quantifying and resolving radiance errors has required innovative methods and new analytical tools; (3) errors due to both instrumental and geophysical sources have been identified; (4) little was known about the global variability of co concentrations before launch; (5) ancillary datasets required by the retrieval algorithm have improved during the mission; (6) instrumental changes (both sudden and gradual) have occurred since launch, requiring adaptive algorithm changes; and (7) users have provided feedback with respect to product deficiencies. validation defines the accuracy of the retrieval product and thereby guides retrieval algorithm development. the goal of validation is to quantify and analyze retrieval errors through systematic comparisons with independent and trusted datasets. like retrieval algorithm development, validation is an ongoing long-term process. mopitt validation relies principally on co vertical profiles from aircraft in-situ measurements that represent diverse geophysical conditions and pollution levels. this letter discusses how mopitt retrieval products have improved, as compared to validation data, during the mission as the result of retrieval algorithm changes. ii. the mopitt instrument the mopitt instrument is a multi-channel thermal infrared (tir) and near infrared (nir) instrument on board the eos-terra satellite launched in december, 1999 into a sunsynchronous polar orbit with ~10:30 am local time descending node equator crossing. mopitt has horizontal spatial resolution around 22 km x 22 km and a swath width around 640 km, which allows global coverage every 3 days. mopitt uses gas-cell correlation radiometry to detect atmospheric co absorption at 4.6 µm (tir channels) and 2.3 µm (nir channels), [drummond et al., 2010 and references therein]. retrievals using the methane channels at 2.2 µm have not been possible due to the issues described in pfister et al. [2005]. the mopitt data record covers march 2000 to present. figure 1 shows a map of co total column measurements indicating mopitt daily coverage. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6330     2   figure 1. mopitt co column observations (in 1017molecules/cm2) for jan 10, 2013. iii. mopitt algorithm history mopitt versions v1 & v2 products did not produce quantitative co measurements. these regression-based retrievals produced qualitative maps that were useful for demonstrating mopitt instrument functionality, but did not produce co profiles with error characterization that could be used in further analysis, such as an optimal estimation (oe) product. mopitt v3 products were from oe retrievals that used a single global a priori profile with an a priori covariance (sa) created from in situ profiles [deeter et al., 2003, 2004, 2007b; edwards et al., 2004; emmons et al., 2004, 2007, 2009]. v3, v4 and v5 retrievals all use the ncep (national centers for environmental prediction) global data assimilation system (gdas) product for temperature and water vapor fields, with cloudy scenes discarded following cloud screening using both mopitt radiances and collocated modis cloud detection. v3 processing also accounted for the loss of optical bench 1 (channels 1-4) in may 2001 with phase i data produced from mar. 2000 may 2001 and phase ii data starting aug. 2001 and thereafter. mopitt v4 was developed to address deficiencies in v3 that included a bias in southern hemisphere (sh) co due to the use of a single global apriori; an apriori covariance that over estimated the correlation length; a limited training set of co profiles in the fast radiative transfer (rt) algorithm that caused retrieval failures in extreme co events; and co retrieval parameters in volume mixing ratio (vmr) that did not represent the log-normal co error statistics resulting in retrieval errors for low and high co values [deeter et al., 2007a]. the v4 algorithms addressed these issues by using apriori (profiles and covariances) from a mozart monthly climatology [emmons et al, 2010], an expanded training set for the forward model rt and log10(vmr) co parameters in oe retrievals. the v4 algorithm and validation are described in deeter et al., [2010]. mopitt v5 was developed mainly to provide new products that exploit the mopitt nir radiance measurements. these include the nironly [deeter et al., 2009] and the multispectral tir+nir [worden et al., 2010] products as well as the standard tir-only product. in order to use the information from the nir channel, it was necessary to fully understand all sources of measurement noise. this led to an improved measurement noise characterization described as “geophysical noise” in deeter et al., [2011], that was implemented for all retrieval types. the v5 algorithm also includes a time-dependent radiative transfer model, which accounts for the slow, long-term instrumental degradation shown in figure 2 by using time-interpolated gas cell parameters (rather than static values, as used in previous versions). another change in v5 was a simplification in mapping the retrieval levels, which resulted in the co vertical profile product defined over pressure layers rather than on levels. finally, v5 has an improved co-alignment of modis data in the cloud detection algorithm. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6330     3   v5 validation results are presented in deeter et al., [2012; 2013]. this improved dependence with time for the validation results has made it possible to compute decadal trends for co using mopitt data and compare these to other satellite co measurements [worden et al., 2013]. mopitt v6 (to be delivered in 2013) algorithm changes are more incremental than v5, but address some important deficiencies of the previous versions. v6 uses the merra reanalysis [rienecker et al., 2011] for water vapor, atmospheric temperature and surface temperature (h2o, t, t_surf) to improve the spatial resolution (0.66° x 0.5° vs. 1° x1°), the time resolution for t_surf (1hr. vs. 6 hr.) and problems with missing values as compared to the ncep fields. merra also has more accurate values (actual skin temperatures) for the t_surf initial guess used in mopitt retrievals. in addition, v6 will correct a small geolocation error identified in the previous versions that resulted from a misalignment in mopitt pointing angles with respect to the terra frame of reference. finally, v6 will use a new climatology based on 2000-2009 simulations from cam-chem [lamarque et al., 2012] that is more representative of the mopitt data record than the previous v4 & v5 climatology simulations that covered 1997-2004. iv. algorithm responses to instrument anomalies figure 2 shows a time series since launch for the pressure in lmc3 (length modulated cell #3) indicating sudden anomalies as well as a slow drift due mostly to long term instrument cooling [drummond et al., 2010]. the first cooler failure in may 2001 resulted in the loss of optical bench #1, however, mopitt was able to continue measuring co using the remaining 4 channels on optical bench #2, with adjustments to the algorithms. the l1 and l2 algorithms also required adjustments after the sieve heater anomaly in oct. 2009, and reverted to the preanomaly versions once the sieve heater began working again. no discontinuities in the data values or quality were detected once the algorithms were implemented. figure 2. mopitt pressure vs. time since launch for lmc3. the pressure change is slow (~ 5% in 10 years). phase i represents mopitt data with both optical benches operational; phase ii, after the cooler anomaly in may, 2001 uses only 1 optical bench. a transient molecular sieve heater fault in 2009 lowered the sieve temperature and pressure dropped ~6%, but corrections in l1 and l2 algorithms maintained data continuity and quality until normal operations were resumed on the restart in 2011. the spikes correspond to decontamination and calibration operations. the current trend is about 1% pressure loss per year. (courtesy of f. nichitiu, u. toronto). annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6330     4   v. conclusions on-orbit performance for satellite instruments will always present challenges to algorithm developers compared to the expectations from simulations and ground calibration activities. although it has operated successfully since 2000, the mopitt instrument has had instrument anomalies as well as very slow degradation that required adaptive algorithms. better characterization of instrument performance, such as understanding geophysical noise, has allowed the mopitt team to exploit both tir and nir channels, providing unprecedented sensitivity to near-surface co. improvements to the mopitt retrieval methods and ancillary datasets have also improved accuracy as compared to longterm validation data. given the current state of mopitt health, the outlook for tracking co trends for > 13 years is very promising. acknowledgements the mopitt project is supported by the nasa earth observing system (eos) program. the mopitt team also acknowledges support from the canadian space agency (csa), the natural sciences and engineering research council (nserc) and environment canada, along with the contributions of comdev (the prime contractor) and abb bomem. ncar is sponsored by the national science foundation. references [deeter et al., 2003] deeter, m. n., l. k. emmons, g. l. francis, d. p. edwards, j. c. gille, j. x. warner, b. khattatov, d. ziskin, j.-f. lamarque, s.-p. ho, v. yudin, j.-l. attié, d. packman, j. chen, d. mao, and j. r. drummond (2003). operational carbon monoxide retrieval algorithm and selected results for the mopitt instrument. j. geophys. res. (atmospheres), 108:4399. [deeter et al., 2004a] deeter, m. n., l. k. emmons, d. p. edwards, j. c. gille, and j. r. drummond (2004a). vertical resolution and information content of co profiles retrieved by mopitt. geophys. res. lett., 31:15112. [deeter et al., 2004b] deeter, m. n., et al. (2004b). evaluation of operational radiances for the measurements of pollution in the troposphere (mopitt) instrument co thermal band channels, j. geophys. res., 109, d03308, doi:10.1029/2003jd003970. [deeter et al., 2007a] deeter, m. n., d. p. edwards, and j. c. gille (2007a). retrievals of carbon monoxide profiles from mopitt observations using lognormal a priori statistics. j. geophys. res. (atmospheres), 112:11311. [deeter et al., 2007b] deeter, m. n., d. p. edwards, j. c. gille, and j. r. drummond (2007b). sensitivity of mopitt observations to carbon monoxide in the lower troposphere. j. geophys. res., 112:24306. [deeter et al., 2009] deeter, m. n., d. p. edwards, j. c. gille, and j. r. drummond (2009). co retrievals based on mopitt nearinfrared observations, j. geophys. res., 114, d04303, doi:10.1029/2008jd010872. [deeter et al., 2010] deeter, m.n., d. p. edwards, j. c. gille, l. k. emmons, g. francis, s.-p. ho, d. mao, d. masters, h. worden, j. r. drummond and p. novelli (2010). the mopitt version 4 co product: algorithm enhancements, validation, and long-term stability, j. geophys. res., 115, d07306, doi:10.1029/2009jd013005. [deeter et al., 2011] deeter, m. n., h. m. worden, j. c. gille, d. p. edwards, d. mao, and j. r. drummond (2011). mopitt multispectral co retrievals: origins and effects of geophysical radiance errors, j. geophys. res., 116, d15303, doi:10.1029/2011jd015703. [deeter et al., 2012] deeter, m. n., h. m. worden, d. p. edwards, j. c. gille, and a. e. andrews (2012). evaluation of mopitt retrievals of lower-tropospheric carbon monoxide over the united states, j. geophys. res., doi:10.1029/2012jd017553. [deeter et al., 2013] deeter, m. n., s. martınezalonso, d. p. edwards, l. k. emmons, j. c. gille, h. m. worden, j. v. pittman, b. c. daube and s. c. wofsy (2013).validation of mopitt version 5 thermal-infrared, near-infrared, and multispectral carbon monoxide profile retrievals for 2000-2011, j. geophys. res., 118, 1– 16, doi:10.1002/jgrd.50272. [drummond et al., 2010] drummond, j. r., j. zou, f. nichitiu, j. kar, r. deschambaut, j. hackett (2010). a review of 9-year performance and operation of the mopitt instrument, j. adv. space res., doi:10.1016/j.asr.2009.11.019. [edwards et al., 2004] edwards, d.p., l. k. emmons, d. a. hauglustaine, d. a. chu, j. c. gille, y. j. kaufman, g. pétron, l. n. yurganov, l. giglio, m. n. deeter, v. yudin, d. c. ziskin, j. warner, j.-f. lamarque, g. l. francis, s. p. ho, d. mao, j. chen, e. i. grechko, and j. r. drummond (2004), observations of carbon monoxide and aerosols from the terra satellite: northern hemisphere variability. j. geophys. res. (atmospheres), 109:24202. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6330     5   [emmons et al., 2004] emmons, l. k., m. n. deeter, j. c. gille, d. p. edwards, j.-l. attié, j. warner, d. ziskin, g. francis, b. khattatov, v. yudin, j.-f. lamarque, s.-p. ho, d. mao, j. s. chen, j. drummond, p. novelli, g. sachse, m. t. coffey, j. w. hannigan, c. gerbig, s. kawakami, y. kondo, n. takegawa, h. schlager, j. baehr, and h. ziereis (2004). validation of measurements of pollution in the troposphere (mopitt) co retrievals with aircraft in situ profiles. j. geophys. res. (atmospheres), 109:3309. [emmons et al., 2007] emmons, l. k., g. g. pfister, d. p. edwards, j. c. gille, g. sachse, d. blake, s. wofsy, c. gerbig, d. matross, and p. nédélec (2007). measurements of pollution in the troposphere (mopitt) validation exercises during summer 2004 field campaigns over north america. j. geophys. res. (atmospheres), 112:12. [emmons et al., 2009] emmons, l. k., edwards, d. p., deeter, m. n., gille, j. c., campos, t., nédélec, p., novelli, and g. sachse, (2009). measurements of pollution in the troposphere (mopitt) validation through 2006, atmos. chem. phys., 9, 1795-1803, doi:10.5194/acp-91795-2009. [emmons et al, 2010] emmons, l. k., s. walters, p. g. hess, j.-f. lamarque, g. g. pfister, d. fillmore, c. granier, a. guenther, d. kinnison, t. laepple, j. orlando, x. tie, g. tyndall, c. wiedinmyer, s. l. baughcum, and s. kloster (2010). description and evaluation of the model for ozone and related chemical tracers, version 4 (mozart-4) geosci. model dev., 3, 43-67. [forster et al., 2007] forster, p., v. ramaswamy, p. artaxo, t. berntsen, r. betts, d.w. fahey, j. haywood, j. lean, d.c. lowe, g. myhre, j. nganga, r. prinn, g. raga, m. schulz and r. van dorland (2007). changes in atmospheric constituents and in radiative forcing. in: climate change 2007: the physical science basis. contribution of working group i to the fourth assessment report of the intergovernmental panel on climate change [solomon, s., d. qin, m. manning, z. chen, m. marquis, k.b. averyt, m.tignor and h.l. miller (eds.)]. cambridge university press, cambridge, united kingdom and new york, ny, usa. (ipcc ar4) [lamarque et al., 2012] lamarque, j.-f., et al., (2012). cam-chem: description and evaluation of interactive atmospheric chemistry in the community earth system model, geosci. model dev., 5, 369-411, 2012, doi:10.5194/gmd-5-3692012 [pfister et al., 2005] pfister, g., j. c. gille, d. ziskin, g. francis, d. p. edwards, m. n. deeter, and e. abbott (2005), effects of a spectral surface reflectance on measurements of backscattered solar radiation: application to the mopitt methane retrieval, j. atmos. oceanic technol., 22, 566, doi:10.1175/jtech1721.1 [rienecker et al., 2011] rienecker, m.m., m.j. suarez, r. gelaro, r. todling, j. bacmeister, e. liu, m.g. bosilovich, s.d. schubert, l. takacs, g.-k. kim, s. bloom, j. chen, d. collins, a. conaty, a. da silva, et al., (2011). merra: nasa's modern-era retrospective analysis for research and applications. j. climate, 24, 36243648, doi:10.1175/jcli-d-11-00015.1 [worden et al., 2010] worden, h. m., m. n. deeter, d. p. edwards, j. c. gille, j. r. drummond, and p. p. nédélec (2010). observations of near-surface carbon monoxide from space using mopitt multispectral retrievals, j. geophys. res., 115, d18314, doi:10.1029/2010jd014242. [worden et al., 2013] worden, h. m., et al., (2013). decadal record of satellite carbon monoxide observations, atmos. chem. phys., 13, 837–850, 2013 doi:10.5194/acp-13-837-2013. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7564 1 the marine environment: hazards, resources and the application of geoethics principles roberto a. violan te servicio d e h id rogr afía n aval, división geología y geofísica marin a, bu en os aires, argen tin a gevas red argen tin a asociación civ il iapg – in tern ation al association for prom otin g geoeth ics, argen tin a violan te@h id r o.gov.ar graziella bozzan o servicio d e h id rogr afía n aval, división geología y geofísica marin a, bu en os aires, argen tin a con sejo n acion al d e in vestigacion es cien tíficas y técn icas (con icet), bu en os aires, argen tin a gbozzan o@h id ro.gov.ar elizabeth i. rovere servicio geológico min ero argen tin o (segemar), in stitu to d e geología y recu rsos min er ales, dir. geología am bien tal y ap licad a, bu en os aires, argen tin a gevas red argen tin a asociación civ il iapg – in tern ation al association for prom otin g geoeth ics, argen tin a elizabeth .r overe@segem ar.gov.ar abstract oceans cover three quarters of the earth surface and represent a fundamental component of the global climate system. life on earth is closely tied to the climate system and thus to the oceans. m arine regions are subjected to numerous submarine natural hazards such as earthquakes, volcanic eruptions and landslides, in many cases producing tsunamis that threaten coastal areas and many onshore and offshore man-made facilities. on the other hand, as society and technological needs progressively increase, the impact of human activities on coastal and deep waters become more severe, with consequences that include global warming and sea-level rise, coastal erosion, pollution, ocean acidification, damage to marine resources and ecosystem degradation. n evertheless, humankind seems not to be adequately conscious about the different kind of hazards threatening the marine environment. the challenge for marine geoscientists is to be conscious of the geoethical compromise in order to alert society, industries and policy makers about the needs to minimize the risks of natural and human impacts in the ocean system. 1. in trod uction cean s cover 71% of th e ear th su rface an d accou n t for 96.4% of th e total w ater volu m e, p rovid in g th e largest livin g sp ace on th e en tire p lan et an d p r od u cin g abou t h alf of th e global biosp h eric n et p rim ary p rod u ction (gregg et al., 2003). ocean s are p art of th e com p lex ocean -atm osp h ere cou p lin g th at regu lates global clim ate, as th ey store m ost of th e su n ’s en ergy, d istribu te h eat arou n d th e p lan et an d beh ave as a co 2 reservoir (bigg et o an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7564 2 al., 2003). th erefore, life on earth basically d ep en d s on ocean s. as a fragile en viron m en t, u n d er a d elicate balan ce betw een clim ate an d m arin e p h ysicalch em ical p rocesses, ocean s are esp ecially vu ln erable to both n atu ral an d m an -in d u ced h azard s. n atu ral h azard s in clu d e su bm arin e earth qu akes an d volcan ic eru p tion s, lan d slid es an d oth er u n d erw ater gravity -d riven p rocesses, tsu n am is an d extrem e ocean ograp h ic an d clim atic even ts like storm s, h u rrican es an d w atersp ou ts, am on g oth ers. man -in d u ced th reaten in g factors, resp on d in g e ith er to m isu se or n egligen ce, en com p ass: a) coastal m a n agem en t an d h an d lin g of offsh ore stru ctu res, su ch as in d u strial an d tou ristic h arbou r op eration , p ort-access ch an n els d red gin g, coastal p rotection , d ep loym en t of oil rigs, su bm arin e d u cts an d cables, an d im p lem en tation of off-sh ore ren ew able m arin e en ergy farm s; b) over exp loitation of liv in g an d n on -livin g resou rces, w ith in ten se bottom traw lin g fish in g activities an d oil sp ills as th e m ost d an gerou s; an d c) ocean p ollu tion , iron fertilization an d ch em ical an d solid w aste d u m p in g. oth er h a zard s su ch as green h ou se gas em ission s, global w arm in g, sea-level rise an d coastal erosion m ay h ave both n atu ral an d an th rop ic cau ses. ocean s r esp on d to h u m an im p acts at d ifferen t rates. for exam p le, sh ort/ m ed iu m -term resp on ses in clu d e m od ification of littoral p r ocesses, coastal erosion , coastal flood in g, alteration of h igh ly bio-p rod u ctive n earsh ore areas (d eltas, estu a ries, w etlan d s) an d th reats to m arin e life. lon gterm resp on ses in clu d e sea -level rise, ocean acid ification an d alteration of seaw ater p rop e rties (tem p eratu re, salin ity, oxygen ) (pörtn er et al., 2014). in gen eral, h azard s at coasts an d sea are n ot ad equ ately con sid ered by th e scien tific co m m u n ity u n d er a social p ersp ective, an d et h ical p rin cip les are far from bein g ap p lied in m ost of th e cases. th u s, m arin e geoscien ces p rovid e a vast field for in tegratin g scien tific kn ow led ge an d eth ical p rin cip les, w ith th e objective of d evelop in g a scien ce-based , resp on sible an d su stain able m an agem en t strategies for su ch a large an d fu n d am en tal en viron m en t. th e p u rp ose of th e p resen t con tribu tion is to syn th esize som e of th e m u ltip le issu es related to n atu ral an d an th rop ic h azard s at sea, as w ell as u n d erlin in g th e n eed for a geoeth ical a p p roach for ad equ ately ev alu atin g en viron m e n tal an d social im p acts on ocean s. th ese are critical n eed s, con sid erin g th at t h e u se of th e m arin e sp ace, resou rces, an d en ergy w ill be esse n tial for fu tu re gen eration s (un closs, 1982). 2. geoethics, risk, hazard an d vuln erability at sea geoeth ical p rin cip les p oin t to m ake geoscie n tists m ore aw are of th eir resp on sibilities in an y situ ation w h ere scien tists in teract w ith society (e.g.: pep p olon i an d di cap u a, 2015; 2017). am on g th e fu n d am en tal v alu es of geoeth ics listed in th e cap e tow n statem en t on geoet h ics (di cap u a et al., 2017), th ree are of sp ecific in terest for m arin e geoscien ce: (1) resp ectin g n atu ral p rocesses an d p h en om en a, w h ere p o ssible, w h en p lan n in g an d im p lem en tin g in te rven tion s in th e m arin e en viron m en t; (2) en su rin g su stain ability of econ om ic an d social activ ities in ord er to assu re fu tu re gen eration s’ su p p ly of en ergy an d oth er n atu ral resou rces, an d (3) p rom otin g geo-ed u cation an d ou treach for all, to fu rth er su stain able econ om ic d evelo p m en t, geoh azard p reven tion an d m itigation , en viron m en tal p rotection an d in creased societal resilien ce an d w ell-bein g. th e ap p lication s of geoeth ical p rin cip les on th e ocean en viron m en t h ave been alread y d iscu ssed by several au th ors (e.g.: safin a, 2003, dallm eyer, 2005, zach arias an d gregr, 2005, au ster et al., 2008, pau ly an d stergiou , 2014, maron e et al., 2015). zach arias an d gregr (2005) stated th at all m arin e featu res (biotic an d abiotic) h ave been form ed an d evolved w ith in a certain ran ge of en viron m en tal con d ition s, an d th erefore th ey are sen sitive to d iverse d egrees of stress accord in g to th e in teraction am on g h azard , vu ln erability an d exp osu re, term s alread y d efin ed by tau ben böck et al. (2008) an d di cap u a an d pep p olon i (2014). con sid erin g th at th e con cep t of risk d ep en d s u p on th e d egree to w h ich th e h azard s th reaten vu ln erable an d exp osed h u m an facilities or r esou rces, for m arin e en viron m en ts it seem s obviou s th at th e h igh est risks are closest to in h a bited coasts (e.g.: dafforn et al., 2015). h ow ever, risk can also be h igh in d eep -sea en viron m en ts, p articu larly w h ere fragile ocean ecosystem s an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7564 3 terms causes processes r is k h a z a rd n atural earth qu akes volcan ic eru p tion s gravity-d riven p rocesses-lan d slid es ocean ograp h ic ch an ges large w aves (in clu d in g tsu n am is) n atural and/or anthropic clim ate ch an ge sea level rise coastal erosion coastal flood s sea-w ater p rop erties iron fertilization ocean acid ification meth an e em ission s anthropic coastal m an agem en t urban an d tou ristic facilities in sh ore an d offsh ore m an -m ad e stru ctu res dred gin g facilities for m arin e ren ew able en ergies livin g an d n on -livin g resou rces exp loitation min in g fish in g p ractices im p actin g sea -floor pollu tion (w aste, p lastic, oil d isp osal) v u ln e r a b il it y + e x p o s u re physical, social, economic, ecological geograp h y an d top ograp h y location pop u lation social stru ctu re prod u ctive activities cu ltu ral h eritage en gin eerin g stru ctu res an d in frastru ctu res en viron m en t livin g an d n on -livin g resou rces an d livin g resou rces h ave to coexist w ith offsh ore in frastru ctu res an d oth er m an -in d u ced im p acts (for exam p le p ollu tion an d bottom traw lin g fish ery). 3. marin e n atural an d an thropic hazard s ocean s are so vast th at in p rin cip le it w ou ld seem u n likely th at a locally gen erated h azard in a coastal area cou ld affect far d eep -sea location s. h ow ever, th e ocean circu lation system tran sp orts w ater th rou gh all th e ocean basin s by both su rface an d d eep -sea cu rren ts, so th at d isp ersal of an y com p on en t existin g in th e w ater m ass (ch em icals, p ollu tan ts) far aw ay from th e sou rce can occu r. on th e oth er h an d , th e en ergy gen erated by d eep -sea geological p r ocesses (e.g. su bm arin e earth qu akes, volcan ic eru p tion s or u n d erw ater slid es an d slop es in stabilities) can be tran sferred to su rface w aves, w h ich can p oten tially be tran sform ed in tsu n am is th at rap id ly p rop agate across th e seas. so, th e m arin e en viron m en t can be co n sid ered as a system of com m u n icatin g vessels tran sp ortin g m aterials an d en ergy all arou n d th e w orld . som e of th e n atu ral an d an th rop ic p rocesse s th at m ay p oten tially rep r esen t a risk to ocean s are listed in tab. 1. am on g th em , earth qu akes, table 1: general framework conceptualizing risk, hazard and vulnerability in the marine environment (modified after taubenböck et al., 2008) an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7564 4 volcan ic eru p tion s an d grav ity -d riven p rocesses are n atu ral h azard s th at can n ot be p reven ted , bu t th eir im p acts on society can be m in im ized if geoeth ical p rin cip les are ap p lied at d ifferen t stages of th e risk m an agem en t, like in th e early w arn in g, p reven tion an d m itig ation . after th ese p rocesses, several h u m an -in d u ced h azard s are d iscu ssed , an d h ow th ese can be avoid ed if societies w ill reach th e aw aren ess of th e d am age th ey m ean . 3.1 submarine earthquakes th ey are som e of th e m ost d an gerou s n atu ral p rocesses at sea, becau se th ey are p oten tial gen erators, am on g oth er im p acts, of large w aves in d u cin g tsu n am is. as th e m ost strikin g exam p le, th e 9.1 m agn itu d e su bm arin e earth qu ake of su m atra (2004) gen erated a com p lex series of tsu n am i w aves th at p rop agated at a sp eed of 640 km / h p rod u cin g a sign ifican t loss of lives, th ou san d s of d isp laced or m issin g p eop le an d severe d am ages to th e in frastru ctu res, affectin g th e coasts of 12 n ation s arou n d th e in d ian ocean . waves also travelled ou tsid e th e in d ian ocean reach in g areas very far aw ay in th e atlan tic an d pacific ocean s (e.g.: titov et al., 2005). in 2011, th e 9.0 m agn itu d e eart h qu ake in n orth eastern jap an also in d u ced a tsu n am i w ave th at h it an d d ram atically affected th e region , in clu d in g th e d estru ction of th e fu ku sh im a pow er plan t. th e earth qu akes of ch ile of 2010 an d 2015 also origin ated big tsu n am is w ith w aves as h igh as 11 m h ittin g n ot on ly th e local sh ores bu t also th e coasts of sou th am erica, californ ia, n ew zealan d , h aw aii, jap an an d m an y pacific islan d s, w ith h u n d red s of d eath s an d th ou san d s of d estroyed h ou ses. th ese exam p les sh ow h ow th e geoeth ic p rin cip les ap p lied to th e m arin e en v iron m en t are im p ortan t, esp ecially w h en geoh azard s affect rem ote coastal areas an d islan d s w h ere em ergen cies requ ire sp ecific action s, w h ich in clu d e social an d p olitical aw aren ess of th e p rean d p ost-crisis m an agem en t stages. 3.2 volcanism abou t 80% of th e volcan ic eru p tion s on earth take p lace u n d erw ater, p articu larly in volcan ic arcs associated w ith su bd u ction zon es an d ot h er typ es of su bm arin e volcan oes. h ow ever, volcan oes located in coastal region s an d in h a bited volcan ic islan d s are also very im p ortan t as th ey affect coastal com m u n ities. volcan ic eru p tion s close to th e sh ore are able to trigger ts u n am is (egorov, 2007) th rou gh d ifferen t m ech an ism s th at in clu d e su bm arin e lah ars, cald era collap ses, p h reatom agm atic p rocesses, am on g oth ers. in th is w ay, th e volcan ic h azard ch an ges in to a tsu n am i h azard , w ith th e p oten tial to im p act region s far from th e volcan ic sou rce. for exam p le, th e exp losive eru p tion of krak atoa in 1883 su p p lied several cu bic kilom eters of m aterial th at en tered th e sea d isp lacin g an equ ally h u ge volu m e of seaw at er, an d p rod u cin g a 45 m h igh tsu n am i w ave th at rea ch ed th e coasts of sou th africa (egorov, 2007). th e su bm erged volcan o protector sh oal in sou th sh etlan d islan d s (1962) gen erated a su bm arin e lah ar th at d isp laced 620x10 6 m 3 of tep h ras in d u cin g a big tsu n am i w av e (violan te et al., 2014), w ith n o ad verse effects as it reach ed th e u n in h abited sh ores of an tarctica. th e su bm arin e eru p tion at el h ierro islan d (can arian a rch ip elago, sp ain , 2011), occu rred w h en th e op en in g of a su bm arin e ven t an d th e on set of a fissu re eru p tion at 900 m w ater d ep th an d 5 km offsh ore th e tow n of la restin ga, gave p lace to th e ejection of volcan ic bom bs, ash an d p yr oclasts. in th at occasion , th e civil p rotection p r oceed ed to a m assive evacu ation of th e coastal p op u lation s th at afterw ard resu lted u n n ecessary w ith a n egative im p act on local econ om y (carraced o et al., 2012). th is is an exam p le th at sh ow s th e n eed of ad equ ate early w arn in gs an d solid scien ce-based recom m en d ation s for a p rop er crisis m an agem en t. for exp losive eru p tion s, th e volcan ic exp losivity in d ex (n ew h all et al., 2018) sh ou ld be establish ed by volcan o logists in ord er to evalu ate th e m agn itu d e of th e eru p tion s, foresee th e p ossible im p acts, an d ad vice au th orities on th e m ost reason able r esp on ses an d action s to im p lem en t. 3.3 gravity-driven mass transport un d erw ater tu rbid itic flu xes, slid es an d m ass tran sp ort p rocesses on steep slop es an d in su bm arin e can yon s n orm ally resu lt from slop e in stabilities an d collap ses, alth ou gh th ey can also be triggered by earth qu akes an d v olcan ic an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7564 5 eru p tion s in m an y cases. th ese p rocesses can d am age u n d erw ater m an -m ad e stru ctu res, su ch as oil rigs, d u cts an d cables. in fact, tu rbid ity cu rren ts w ere d iscovered after an eart h qu ake offsh ore th e coast of terran ova, can ad a (1929) broke telegrap h ic cables in th e ocean . an old er w ell-kn ow n su bm arin e slid e is th at of storegga, in th e n orw egian sea, w h ich o ccu rred at 8,200 years bp as a con sequ en ce of a h u ge earth qu ake. th is slid e in d u ced a series of tsu n am is w aves affectin g n u m erou s coastal a reas in th e n orth sea, record in g a m axim u m w ave h eigh t of 20-30 m in th e sh etlan d islan d s (bon d evik et al., 2003). like storegga, m an y su bm arin e slid es an d th eir associated tsu n am is w aves are ackn ow led ged as im p ortan t sou rces of geoh azard w ith a v ast im p act on th e safety both of p eop les livin g on th e coastal areas an d of th e offsh ore an d on sh ore in frastru ctu res (masson et al., 2006). 3.4 sea-level rise and coastal retreat presen t-d ay sea-level rise an d coastal retreat m ay h ave both n atu ral an d h u m an -in d u ced cau ses. th ey are a con cern as th ey stron gly a ffect m ost p op u lated coastal region s th reaten in g both th e littoral en viron m en t an d h u m an facilities. observed m ean sea level h as risen b etw een 17 an d 21 cm sin ce 1901 (ch u rch et al., 2013) d u e to global w arm in g, ocean th erm al exp an sion an d glaciers m elt. th ese valu es re p resen t an average an n u al in crease of 1.7 m m , alth ou gh estim ation s for th e last 25 years are m u ch h igh er (3.2 m m / year). accord in g to n ich olls et al. (2007), lon g -term global sh or elin e ch an ge rates are 50 to 200 tim es h igh er th an sea-level rise, w h ich m ean s th at 10 cm of sea-level in crease w ou ld im p ly 5 to 20 m of coastal retreat. of cou rse, n ot on ly sea -level rise bu t also clim ate (e.g. th rou gh th e en ergy of storm s su rges), u rban ization , an d h u m an in d u ced m isu se of th e sh ores p rom ote coastal erosion , by alterin g th e n atu ral alon g -sh ore sed im en t tran sp ort an d th e equ ilibriu m in th e beach con d ition s. th erefore, th e su stain able u se of th e littoral sp ace is on e of th e im p ortan t issu es w h ere geoeth ical p rin cip les sh ou ld be ap p lied , m ain ly th rou gh an ad equ ate social aw aren ess of its vu ln erability an d th e im p lem en tation of sh ore p rotection m an agem en t, w h ich sh ou ld be su stain ed by lon g -term p olicies aim ed at to p reven t or at least m in im ize th e d ram atic im p act of clim ate ch an ge an d m an -m ad e in frastru ctu res u p on coastal erosion an d recession . 3.5 pollution ocean p ollu tion is an oth er seriou s con cern th at h u m an kin d is facin g tod ay. th e largest am ou n t of p ollu tan ts at sea com es from activities on lan d , p articu larly d u m p in g trash an d litter, oil sp ills, ch em icals an d fertilizers. prod u cts r eleased from sh ip s also con tribu te sign ifican tly. accord in g to th e in form ation p rovid ed by n oaa (h ttp :/ / w w w .n oaa.gov/ resou rce collection s/ ocean -p ollu tion ), 1.4 billion p ou n d s of trash p er year en ter in to th e ocean . plastic d ebris accu m u lates alon g an d off th e coasts in garbage p atch es as a resu lt of ocean cu rren ts an d local ed d ies, an d m icrop lastics h ave been fou n d in d eep -sea sed im en ts (wod d al et al., 2014). its p ervasive p resen ce in a ll ocean s is a big issu e as th e im p act on m arin e ecosystem s an d u ltim ately u p on h u m an h ealth h as n ot been yet fu lly establish ed . th e tim e h as com e to better m on itor th e p ollu tan ts d isp ersal an d d isp osal in th e m arin e en viron m en t an d d eep sed im en ts, to red u ce lan d -based m arin e p ollu tion by im p rovin g sew age an d w astew ater treatm en t, to ed u cate you th an d in crease th e p u blic aw aren ess th rou gh th e m ed ia, an d to in flu en ce th e d ecision -m akin g p rocesses. th rou gh th ese action s, ou r u n su stain able co n su m p tion an d p rod u ction p attern s m ay even tu ally ch an ge in th e fu tu re (un esco, 2012). 3.6 ocean fertilization, acidification and eutrophication ocean fertilization (n atu ral or m an -in d u ced ) gen erally refers to su p p lyin g iron to th e sea w ith sign ifican t im p act on livin g resou rces (wallace et al., 2010). artificial fertilization u sed to in crease p h ytop lan kton to su p p ort fish eries, as w ell as fertilization tech n iqu es u sed to d raw -d ow n atm osp h eric co 2 (jin et al., 2008) are clear exam p les of h ow eth ical p rin cip les sh ou ld be ap p lied to p reven t certain sp ecies from bein g h arm ed w h en oth ers are in ten d ed to be ben efited . n atu ral fertilization an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7564 6 w ith iron sou rced in rivers an d airborn e vo lcan ic ash is an im p ortan t feed er of ocean ic su rface w aters (du ggen et al., 2010). th is is a com m on p rocess in th e sou th w estern atlan tic, w h ere large am ou n ts of iron -rich volcan ic ash es from th e an d ean volcan oes are tran sp orted eastw ard s by stron g w esterly w in d s, p rod u cin g abn orm al p h ytop lan kton an d algal bloom s (sign orin i et al., 2009) th at in som e cases m igh t be h arm fu l to m arin e fau n as. an oth er p roblem is ocean acid ification , w h ich is th e red u ction of seaw ater p h d u e to in creased atm osp h eric co 2. ocean s absorb m ore th an 26 % of th e co 2 released to th e atm osp h ere by green h ou se em ission , an d accord in g to un esco (2012) ocean s cou ld becom e 150 % m ore acid ic th an tod ay by 2100, w h ich w ou ld cau se d ram atic acceleration in loss of biod iversity an d irr eversible ecological tran sform ation s. eu trop h ication is th e en rich m en t of seaw ater w ith e xcessive d issolved n u trien ts, w h ich p rod u ces algal an d p h ytop lan kton bloom s th at m ay d ep lete an d con su m e oxygen , w ith con sequ en t d am age to m arin e life (an sari et al., 2011). a ccord in g to zach arias an d gregr (2005), ocean s h ave su ffered from a lack of qu an titative m et h od s aim ed at d elin eatin g areas w ith d ifferen t d egrees of vu ln erability. th u s, geoscien tists sh ou ld id en tify th e region s w h ere fragile ecosystem s are esp ecially u n d er th reat, m on itor th e occu rren ce of h arm fu l bloom s th at m igh t alter th e m arin e fau n a an d im p ly a risk for h u m an -h ealth an d foster th e p rotection of th e ocean an d its livin g resou rces. 3.7 n on-living resources exploitation as h yd rocarbon exp loration is exp an d in g to p reviou sly in accessible d eep w aters, p oten tial w ater p ollu tion an d d am age to ecosystem s d u e to offsh ore exp loration an d d rillin g n eed s to be evalu ated . deep -sea seism ic exp loration u su a lly em p loys airgu n s as a sou rce of en ergy th at p rod u ces p ressu re w aves th rou gh th e w ater colu m n , w h ich p oten tially d istu rbs m arin e fau n as. oth er, less-kn ow n bu t con tin u ou s con tam in ation effects of offsh ore d rillin g op er ation s con sist in th e d isp osal of d rillin g m u d , brin e w astes, d eck ru n off w ater an d flow lin e an d p ip elin e leaks. catastrop h ic sp ills rep r esen t a seriou s th reat for m arin e ecosystem s. alth ou gh m ore care an d resp on sibility cou ld lim it th e frequ en cy of m ajor accid en ts affectin g offsh ore rigs, p ip elin es an d oil storage, sh i p p in g rou tin es an d n atu ral d isasters p u t oil an d gas exp loitation s at u n p red ictable risk. an oth er p oten tial h azard is d eep -sea m in in g, w h ich con sists of u sin g m assive m ach in es to d red ge an d grin d th ou san d s of ton s of rock at th e sea floor to extract p reciou s m in erals (gold , cop p er, zin c, cobalt, m an gan ese, titan iu m ) from ore, h yd roth erm al an d p olym etallic n od u le d ep o sits. it rep resen ts an extrem ely exp en sive u n d ertakin g d u e to th e tech n ical ch allen ges in volved in retrievin g large am ou n ts of m aterials from great w ater d ep th s. from a geoeth ical p ersp e ctive, th e balan ce betw een econ om ic ben efits an d en viron m en tal an d social im p acts sh ou ld be accu rately estim ated in ord er to avoid vast an d irrem ed iable d am ages to fr agile an d u n iqu e d eep -sea life. ocean scien tists, w ater exp erts an d local com m u n ities sh ou ld coop erate w ith govern m en t, p rivate sector an d p olicy m akers to d evelop p lan s for su st ain able en viron m en tal m an ag em en t as w ell as to p r om ote th e gen eration of legal an d p olicy fram ew orks regu latin g th e a ccess to d eep seabed r esou rces (un esco, 2012). for th e fu tu re, an in creasin g d em an d for n on en ergy m in erals is exp ected to su p p ort even th e ren ew able green en ergies. th e p oin t is w h eth er a resp on sible d eep -sea m in in g is p ossible or n ot. local ben th ic co m m u n ities, w h ich are often extrem ely sp ecia lized , cou ld p erm an en tly be d am aged , an d p lu m es of sed im en t d riven by th e d red gin g m ach in es m igh t p oten tially im p act th e w h ole w ater colu m n by in trod u cin g ch em ical, n oise an d ligh t p ollu tion far aw ay from th e m in in g site. moreover, d eep -sea m in in g in clu d es ad d ed d ifficu lties w h en com p ared to lan d -based extraction : restoration of sea-bottom after m in in g can be slow er th an lan d soil recovery, in th e case th at th ere is an y ch an ce to restore sea -bottom to th e p reviou s u n d istu rbed con d ition s; th e h igh costs to op erate at great d ep th s m ake com p an y’s good p ractice m ore exp en sive an d th u s less p robable; fin ally, d eep -sea ecosystem s associated to h yd roth erm al ven ts are p oorly kn ow n , an d p olicies to p reserve an d restore th ese h abitats are so far in ad equ ate. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7564 7 3.8 living resources exploitation du rin g th e last 50 years, th e ecosystem s h ave been stron gly m od ified by th e in creased h u m an d em an d for food an d en ergy. th is is th e first con clu sion reach ed by “th e millen n iu m ecosystem assessm en t” (h ttp s:/ / w w w .m illen n iu m assessm en t.org/ en / syn th esis.h tm l, accessed 1 ju n e 2018) th at p rovid es a state-of-th e-art abou t th e p resen t -d ay con d ition s an d d eterioration of ecosystem s w orld w id e. th e in d u strial exp loitation of th e ocean ’s livin g resou rces is carried m ain ly by bottom traw lin g, w h ich is a n on -selective com m ercial fish in g tech n iqu e by w h ich h eavy n ets an d gear are p u lled alon g th e sea-floor w ith d irect n egative im p acts n ot on ly on fish p op u lation s an d ben th ic com m u n ities, bu t also on th e sea-floor, as th is p ractice affects an d som etim es d estroys th e su bstratu m w h ere be n th ic an d d em ersal sp ecies liv e an d feed (pu ig et al., 2012). bottom traw lin g is a global, regu lar an d h arm fu l rou tin e an d th u s th e n eed s for a p p lyin g eth ical p rin cip les to m in im ize its im p act an d d am age u p on th e en viron m en t are u rgen t. th ese m igh t in clu d e: red u ction of fish ery effort by lim itin g th e fleet m otor p ow er an d p rom otin g a m ore trad ition al w ay of fish in g (tra m m els, little lon g lin es an d tra p s); ed u cation ad d ressed to th e society for a m ore r esp on sible an d su stain able con su m e of th e livin g resou rces; resp ect of th e n atu ral p rocesses (co n su m e of season al sp ecies); im p lem en tation of p olicies th at p rom ote th e p rotection of th e m arin e en viron m en t in gen eral an d of sp ecific vu ln erable areas in p articu lar (e.g. creatin g m arin e p rotected areas for fish in g); an d fin ally, lau n ch of co-m an agem en t p rocesses, w h ere fish erm en , scien tists an d ad m in istration coo p erate to m ain tain an h ealth y statu s of th e stock. 3.9 marine energies en ergy con tain ed in w aves, tid es, cu rren ts an d offsh ore w in d s, as w ell as th at com in g from h igh grad ien ts of w ater p rop erties (th erm al an d osm otic en ergy) can be feasibly con verted in to electricity. th eir facilities in clu d e floatin g d evices or an ch ored bu oys th at catch th e en e rgy of th e w aves, tid al tu rbin es associated to d am s or barrages bu ilt at estu aries or at th e p assages betw een islan d s a n d arch ip elagos, an d w in d farm s w ith tu rbin e tow ers an ch ored to th e ocean floor. alth ou gh m an y cou n tries are in vestin g tod ay in ren ew able tech n ologies (un esco, 2017; yan g an d cop p in g, 2017) th e offsh ore stru ctu res th at th ese tech n ologies n eed are con sid erably exp en sive. in ad d ition , th ey often sign ifican tly im p act th e local en viro n m en t. for exam p le, w in d m ills m ay alter bird s m igratory corrid ors, tid al tu rbin es m ay h ave a n egative effect on m arin e fau n as, an d large w ave en ergy stru ctu res m ay lim it th e m ixin g of th e u p p er layers of sea-w aters w h ich cou ld ad versely im p act m arin e life an d fish eries, lead in g to th e d isp lacem en t an d alteration of ecosystem s. on th e oth er h an d , id eal sp ots for th eir in stallation are n ot as n u m erou s as on e can exp ect, for exam p le m ost m arin e region s can n ot be exp loited becau se th ey are too far rem ote an d th e costs of con stru ction an d cablin g th e p ow er to th e grid w ou ld be p roh ib itive. alth ou gh ocean ic ren ew able en ergy is t od ay a feasible tech n ology, en viron m en tal im p act assessm en ts, facilities lifetim e p red iction , stru ctu res ru p tu re an d costs-ben efits evalu ation s n eed to be estim ated u n d er a geoeth ical p ersp ective to d evelop a su stain able p oten tial for th eir im p lem en tation . 4. con clusion s th e ap p lication of geoeth ical p rin cip les at s ea in th e con text of n atu ral an d h u m an -in d u ced h azard s is a n ovel con cep t. th ese p rin cip les h ave been trad ition ally ap p lied to lan d -based p roblem s, p erh ap s becau se con tin en ts are w h ere p eop le live an d w h ere w ell-kn ow n geoh azard s (earth qu akes, volcan ic eru p tion s, flood in gs, d rou gh ts, etc.) m ost com m on ly a ffect every-d ay life of th ou san d s of p eop le. on th e oth er h an d , th ere is a gen eral p ercep tion abou t th e ocean s as a “w aterw orld ” con tain in g n oth in g bu t w ater, or at m ost en d less fish eries, w ith n o evid en t h azard s. both coastal an d d eep w aters are su bjected to in creasin g p ressu re from h u m an in frastru ctu res an d activities w ith ou t a clear aw aren ess of th e p oten tial risks. tsu n am is m ay be th e best -kn ow n n atu ral h a zard related to th e sea, bu t oth er sou rces of p oten tial d am ages com e from su bm arin e eart h qu akes, volcan ic eru p tion s an d gravity an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7564 8 in d u ced sed im en tary p rocesses, w h ich can a ffect n ot on ly th e coastal region s bu t also m an y offsh ore h u m an facilities like oil rigs, d u cts an d su bm arin e cables. man -in d u ced p ollu tion an d overexp loitation of livin g an d n on -livin g resou rces are oth er h azard s th at th reaten th e fragile ocean ic ecosystem . marin e cu rren ts are effective agen ts th at tran sp ort sed im en ts, p ollu tan ts an d en ergy across th e ocean basin s. with th e search in g of n ew resou rces an d tech n ologies, th e sea is con sid ered as th e m ain su p p lier of food an d en ergy for th e n ear fu tu re. th e com p reh en sion of th e m arin e en viron m en t is essen tial for p rotectin g h u m an s from ocean sou rced h azard s, an d geoeth ical p rin cip les can be su itable ap p lied on a w id e ran ge of cases. in ord er to p reven t th e im p act of m an -m ad e facilities an d in frastru ctu res on th e m arin e en v iron m en t, h u m an in terven tion s sh ou ld be d on e w ith in th e resp ect of th e n atu ral p rocesses an d p h en om en a of th e ocean ; to p reven t th e m arin e en viron m en t d eterioration , an d for th e fu tu re gen eration s’ ben efit, en ergy an d n atu ral r esou rces of th e ocean s sh ou ld be u sed in a su stain able w ay, avoid in g p ollu tion an d overe xp loitation ; to m in im ize th e risks an d to save lives, geoh azard p reven tion an d m itigation , en viron m en tal p rotection an d in creased societal resilien ce an d w ell-bein g cou ld be ach ieved th rou gh a correct early w arn in g m ech an ism s an d a geo-ed u cation ad d ressed to society an d p olicy m akers, so as to ap p ly th e m ost reason able resp on ses an d action s w h en a geoh azard is ap p roach in g. ackn owledgemen ts au th ors th an k giu sep p e di cap u a an d silvia pep p olon i for in vitin g an d en cou ragin g u s to w rite th is con tribu tion . we are in d ebted to th ree an on ym ou s review ers for th eir valu able correction s, com m en ts, su ggestion s an d recom m en d ation s th at h elp ed u s to su bstan tially im p rove th e qu ality of th e p ap er. an n a bozz an o con tribu ted to th e section “livin g resou rces exp loitation ”. referen ces an sari a.a., gill s.s., lan za g.r. an d rast w. 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(2005). sensitivity and vu lnerability in marine environm ents: an ap proach to id entifying vu lnerable marine areas. conservation biology 19 (1): 86-97. microsoft word 7506-18680-1-ed_pölzler-ortner.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7506 1 ethical but upsetting geoscience research: a case study thomas pölzler institute of philosophy, university of graz, austria thomas.poelzler@uni-graz.at florian ortner department of geography and regional science, university of graz, austria fwf-dk climate change, university of graz, austria florian.ortner@uni-graz.at abstract geoscience research may upset people even though it is ethically acceptable. in this paper we attempt to explore three questions about such research. it will turn out that (1) under most circumstances ethical but upsetting geoscience research is morally permissible, (2) revising this research in response to upset-induced external interference is morally impermissible in the absence of strong countervailing pragmatic reasons and attempts to reduce upset, and (3) potentially upsetting geoscience research ought to be communicated truthfully and tailored to each individual situation. these general propositions are applied to a case of ethical but upsetting research that we ourselves are currently involved. 1. introduction eoscience research occasionally upsets people. in some instances, this is because it violates ethical norms. however, upset may also be caused by research that is perfectly ethically acceptable (abstracting from the fact that it upsets people, which may itself be ethically relevant). examples of ethical but upsetting geoscience research include hydrocarbon explorations near populated areas (e.g., vidic et al., 2013), seismological studies that indicate the possibility of major earthquakes (e.g., moernaut et al., 2018) and interviews with the colleagues of mine workers who died during mining activities or strikes (e.g. alexander et al., 2013). in this paper we attempt to explore the following three questions: (1) under what conditions, if any, is it ethically permissible to conduct ethical but upsetting geoscience research? (2) under what conditions, if any, is it ethically permissible to revise one’s research in response to interference by people outside the research who are upset by it? (3) how ought one to communicate ethical but upsetting research? we begin by explaining a case of ethical but upsetting geoscience research that led us to think about these questions. referring to this case, each of the questions is then explored in turn. our aim is not to develop fully comprehensive answers (i.e., to identify every factor that is relevant to answering the above questions with regard to every kind of upsetting geoscience research). rather, we will specify several factors that in our view are important to the assessment of the particular case with which we are concerned. these considerations can hopefully serve as a useful starting point for assessing and dealing with other cases of ethical but upsetting geoscience research as well. 2. the ee-con project: an example of ethical but upsetting research both authors of this paper are involved in the austrian academy of sciences project economic g annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7506 2 and ethical consequences of natural hazards in alpine valleys (ee-con). due to steep topographical relief, high population density and the economic importance of summer and winter tourism, the alps and their population are particularly vulnerable to geomorphological and hydrological hazards. this problem might be amplified by rising temperatures and more pronounced precipitation events due to climate change. focusing on two alpine valley regions in austria, the johnsbachtal and the sölktäler, ee-con addresses natural hazards by means of interdisciplinary cooperation between geographers, economists and philosophers. for example, we enquire into the causes, frequency and intensity of these hazards, as well as residents’ perception and knowledge, the costs of protection from and adaptation to the hazards, compensation for losses, and the potential resettlement of (parts of) the valleys (e.g. ortner et al., 2017). there are good reasons for believing that the research involved in ee-con is ethical (not the least of which is that the austrian academy of sciences approved it). however, the project has caused significant upset. shortly after ee-con started, it was covered by austria’s largest daily newspaper. following reports of past natural hazards in one of our study areas, as well as the possible impact of climate change, the last paragraph of the article addressed the theoretical option of abandoning parts of these areas. a member of the project team was quoted as follows: “from a purely economic perspective it may one day be reasonable to abandon villages (which are declining in population anyway). but of course, such a decision can never be exclusively grounded in economic considerations” (kronen zeitung, 2016). this is quite a weak statement. in fact, it only states what may be reasonable at some unspecified point in future if one considers the matter from only one relevant perspective. nevertheless, the above-mentioned article significantly upset residents of our study areas. only one day after its publication, the mayor of one of the areas called the university and threatened to withdraw his cooperation, which was important for the project to be properly accomplished. as he explained to us, residents of the valleys simply did not want the option of relocation to be examined in any way. it was, so to speak, a taboo issue for them. in the days that followed, we prepared for an emergency meeting with the mayor and other local representatives. this led us to reflect on the ethical questions raised in the introduction, in particular: was it permissible for us to pursue this project at all? if the mayor requested significant revisions to our original research plan, to what extent ought we to comply with these requests? and how can research such as ours be communicated more appropriately? 3. the permissibility of conducting ethical but upsetting research our first question asks about the conditions under which ethical but upsetting geoscience research is ethically permissible. at first sight, the answer to this question may appear to be trivial: if the research at issue is ethical how could it ever be ethically impermissible to engage in it? recall, however, that by “ethical” we here mean “ethical, abstracting from the fact that the research causes upset”. this means that it is possible that the research at issue, even though ethical in all other respects, is impermissible precisely for the reason that it causes upset. since the geoscience research addressed in this paper is not unethical in itself, any upset caused by this research cannot be a response to it being unethical. the upset must rather be caused by non-moral facts related to the research (facts that make people interpret this research in a certain way, facts about people’s specific nonbasic interests, etc.). in our view, this implication grounds a strong presumption for believing that ethical but upsetting geoscience research is generally permissible (compare mill, 1998 on offence). it is possible for this presumption to be defeated. in practice, however, this will only rarely happen, as it would require very strong reasons for the research to be impermissible, and typically these reasons do not hold. expected research output one fact that may constitute a strong reason against the permissibility of ethical but upsetting geoscience research in our view the most annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7506 3 important one concerns the expected output of the research. suppose experts unequivocally agree that a study will not yield any significantly valuable output, either in terms of expanding our geoscience knowledge or in terms of informing action. then the fact that this research upsets people may imply that the research ought not to be conducted. after all, it seems wrong to upset people for no good reason. reasons for the upset even though ex hypothesi the research addressed here cannot upset people because it is unethical, there are still good and bad reasons for people getting upset. consider our research about relocating communities. while this research may upset one person because he or she is genuinely concerned about having to leave and thus becoming unable to realize his or her life plans (a perfectly understandable reason), another person may be equally upset because of an insubstantial decrease in a financial investment that he or she made in the relevant area (a worse reason). in our view, upset can only possibly constitute a valid for not conducting ethical research to the extent that it is based on good reasons. number of upset people suppose the above two conditions (low expected scientific value and good reasons for upset) are fulfilled. then one among several further relevant factors that determine the permissibility of ethical but upsetting geoscience research is the number of people that this research upsets. the more people who are upset, the stronger one’s reason for not conducting the research. degree of upset in related terms, the extent to which people are upset also matters. the more the geoscience research at issue upsets them, the stronger the reason not to engage in it. distribution of research benefits finally, assuming that the geoscience research at issue is at least somewhat valuable, it also matters who will benefit from this research. when the upset people themselves are not among the recipients of benefits either directly or indirectly (for example, by having access to additional geoscience knowledge) it is more difficult to justify the research than when they are among these recipients. in most cases, geoscience research can be expected to yield valuable results. we accordingly would like to re-emphasize that the fact that a study provokes upset only very rarely implies that this study is impermissible. such an implication does not hold with regard to eecon either. admittedly, the upset caused by this project has to some extent been based on good reasons and has been strong. but it only affected a small number of people and, most importantly, the expected results of our project are valuable both in expanding our knowledge about natural hazards and in scientifically informing actions regarding the protection and potential resettlement of our study areas (thus benefitting the very people that the project upsets). 4. the permissibility of revising ethical but upsetting research let us suppose that an instance of ethical but upsetting geoscience research is permissible according to the above criteria. occasionally, such as with ee-con, research of this kind will nevertheless upset people to such a high degree that they demand revisions of this research. our second geoethical question is under what conditions, if any, is it permissible for researchers to comply with such demands, i.e., to revise their research in response to upsetinduced external influences? in answering this question, we assume the value of the autonomy (or, more broadly, the integrity) of science. admittedly, the notion “autonomy of science” is quite vague. there is agreement, however, that the value entails that science as a social system should be selfgoverning. within the domain of their expertise, and given certain practical limits, those who are part of this system should be free from external constraints (resnik, 2008). in our view, the importance of the autonomy of science creates a presumption against revising one’s research in response to external pressure caused by people who are upset. this presumption can annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7506 4 only be defeated by strong reasons in favor of revisions. persistence of the upset/ interference for it to become possibly permissible to revise one’s research in response to upset-induced interference, one would first have to attempt to alleviate this upset. depending on the details of the case one might have for example, to inform the media (more clearly) about the aims of this research, initiate a public discussion, or schedule a private meeting. only if serious attempts at mediation fail can it possibly become permissible to comply with requests for revision. scientific value of revised research a further relevant factor in determining the permissibility of requested revisions is the nature and extent of these revisions. sometimes, revisions can be implemented without significantly decreasing the expected value of the research. however, to the extent that this value is threatened, researchers should be much more reluctant to give in to external demands. costs of non-compliance for research in some cases, not complying with the upsetinduced request to revise one’s research may lead to a complication or partial or complete termination of the research. for example, this may happen when revisions were requested by one’s funding institution or by cooperation partners, or when subjects refuse to fill in questionnaires. obviously, the higher the risk of complications and some degree of termination of the project the more permissible is it to make concessions to the people who are upset (within ethical boundaries). costs of non-compliance for researchers finally, non-compliance may also involve serious costs for the researchers, who may be harassed or threatened, may lose the ability to acquire funding for future research, or lose their employment. the risk of such costs again renders it proportionately more permissible to conform to the upset people’s requests for revisions (within ethical boundaries). because of the value of the autonomy of science, ideally, researchers would resist all upsetinduced external requests for altering their research. however, we acknowledge that in reality pragmatic considerations may sometimes outweigh this value by calling for compromises. in ee-con, we concluded that, if requested, we would go at least some (but not all) way in accounting for the mayor’s hypothesized request not to study the potential resettlement of communities. this is because with revisions, the part of the project that deals with resettlement would still have delivered valuable results, and so would the parts that do no deal with resettlement. losing the support of the mayor would have involved a high risk of complicating aspects of our research and diminishing its value. 5. the communication of ethical but upsetting research the extent to which potentially upsetting geoscience research actually upsets people depends significantly on how this research is communicated. if it is based on a well-thoughtout communication strategy, such research will typically provoke far less upset than if it is based on a bad or non-existing strategy (stewart and lewis, 2017). information content one of the main determinants of whether research will upset people is what one communicates about it. in the case of potentially upsetting research, it might be tempting either to not inform people and stakeholders about potentially upsetting aspects, to downplay the importance of these aspects, or perhaps even to provide false information. but only in very rare cases would either of the first two options be morally permissible, and the third one certainly is not (see charlton 2009; di capua et al. 2017). to begin with, truth is an intrinsic value that is of primary importance within science. secondly, scientists have a social responsibility to adequately report their research to the public (especially when they are publicly funded). thirdly, omissions, trivializations and lies may have all sorts of bad consequences, ranging from lawsuits and public distrust (in specific projects annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7506 5 and in science in general) to harm to one’s academic reputation. clarity of information while potentially upsetting research must generally be communicated truthfully, there are more and less appropriate ways of doing so. in many cases information about research upsets people because they misunderstand it. it is thus important that researchers state objectives, methods, results, uncertainties, and probabilities as clearly and intelligibly as possible. in particular, they should minimize both ambiguity (the existence of multiple meanings), vagueness (the existence of borderline cases in meaning) and the complexity of their explanations. type of communication bultitude (2010) distinguishes three ways of communicating scientific research: traditional journalism (e.g., newspapers, radio and television), live or face-to-face events (e.g., public debates and lectures), and online interactions (e.g., websites and blogs). each of these types has advantages and disadvantages. for example, while traditional journalism reaches large audiences and is generally perceived as trustworthy, it typically does not allow scientists to report their research in detail, does not give them much control over how it is presented, and prevents audiences from responding by comments or questions (bultitude 2010; see also foresta martin and peppoloni 2017). the appropriateness of researchers’ communication strategies also significantly depends on how they choose the type of communication that best fits their respective situation. timing of communication suppose a person were informed that starting tomorrow, geologists will perform hydrocarbon explorations in the vicinity of his or her property. no doubt the person would become terribly upset and rightly so. by not informing the person earlier, the researchers engaged in this project would have prevented him or her from gathering more information, publicly objecting to the explorations, coming to terms with the project, etc. hence, the timing of the communication of potentially upsetting research matters as well. in particular, such research ought to be communicated as early as possible. recipients of information finally, the way in which potentially upsetting research is communicated must also be tailored to the recipients of the information. a fact sheet for experts of the department of agriculture, for example, requires a substantially different content and form (in terms of comprehensiveness, complexity, etc.) than does a fact sheet for the local population (alexander, 2007; marone et al., 2015; stewart and lewis, 2017). these considerations suggest that developing an appropriate strategy for communicating potentially upsetting research is complex and highly dependent on its context. in retrospect, it seems that in the ee-con project, we did not pay sufficient attention to this task. there probably would have been no need for us to address the resettlement aspect of our research in the newspaper interview mentioned above, as this is only one of several aspects of our project and the article did not aim for comprehensiveness. rather, in communicating this aspect, we should have employed selective, live or face-to-face types of communication, and we should have done so at a much earlier point in time. 6. conclusions in this paper, we have arrived at three conclusions about ethical but upsetting geoscience research. first, under most circumstances such research is morally permissible. secondly, revising this research in response to upsetinduced external interference is morally impermissible in the absence of strong countervailing pragmatic reasons and attempts to reduce upset. and thirdly, potentially upsetting research ought to be communicated truthfully and tailored to each individual situation. thinking about the above issues allowed us to draw valuable lessons with regard to ee-con. in particular, we entered the next meeting with stakeholders with a clear strategy that included a plan of communication and a plan about how much to comply with potential requests for revision. following constructive dialogue, the annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7506 6 mayor and other representatives of the study areas agreed to the project’s resettlement aspect after all and our research is well on the way. acknowledgements this work was funded by the austrian academy of sciences (oeaw) project economic and ethical consequences of natural hazards in alpine valleys (eecon) and the austrian science fund (fwf) under research grant w 1256-g15 (doctoral programme climate change – uncertainties, thresholds and coping strategies). for helpful comments we would like to thank david alexander and the three reviewers of this paper. references alexander d. 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(2013). impact of shale gas development on regional water quality. science, 340, 826-835, doi.org/10.1126/science.1235009. annex 1 to: trace elements mobility in soils from the hydrothermal area of nisyros (greece) trace hydr   map  of princip de geo e elements m rothermal a daskalo f  lakki  plain pe, c., vanderk logie (lausan annex 1   mobility in so area of nisyr opoulou, k., et n  and  samplin kluysen, l., h ne), 44].  enrichm oils from th ros (greece t al.  ng  sites.  loca hunziker, j.c.,  ment factors w he  e).            al  background  2005. geolog with respect to d  is  out  of  th gical map of n o the backgrou he  map  borde nisyros volcan und soil.  er.  [volentik,  no (greece). m a.c.m.,  memoires  francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text sica 799_815.pdf annals of geophysics, vol. 45, n. 6, december 2002 799 seismic behaviour of geotechnical structures stefania sica, filippo santucci de magistris and filippo vinale dipartimento di ingegneria geotecnica, polo delle scienze e delle tecnologie, università degli studi di napoli «federico ii», napoli, italy abstract this paper deals with some fundamental considerations regarding the behaviour of geotechnical structures under seismic loading. first a complete definition of the earthquake disaster risk is provided, followed by the importance of performing site-specific hazard analysis. then some suggestions are provided in regard to adequate assessment of soil parameters, a crucial point to properly analyze the seismic behaviour of geotechnical structures. the core of the paper is centered on a critical review of the analysis methods available for studying geotechnical structures under seismic loadings. all of the available methods can be classified into three main classes, including the pseudo-static, pseudo-dynamic and dynamic approaches, each of which is reviewed for applicability. a more advanced analysis procedure, suitable for a so-called performance-based design approach, is also described in the paper. finally, the seismic behaviour of the el infiernillo dam was investigated. it was shown that coupled elastoplastic dynamic analyses disclose some of the important features of dam behaviour under seismic loading, confirmed by comparing analytical computation and experimental measurements on the dam body during and after a past earthquake. 1. introduction on december 3rd and 4th 2001, the «earth sciences and natural disaster prevention. a japan-italy joint meeting in year 2001» was held in kyoto and kobe. session iii of the meeting was devoted to «civil engineering aspects of seismic and volcanic risk prevention». this paper summarizes the contribution from the department mailing address: dr. filippo santucci de magistris, dipartimento di ingegneria geotecnica, polo delle scienze e delle tecnologie, università degli studi di napoli «federico ii», via claudio 21, 80125 napoli, italy; e-mail: filsantu@unina.it key words case histories − dams − f.e.m. − seismic design − soil dynamics of geotechnical engineering, university of naples (italy), at this session of the conference. after an introduction to the problem of seismic risk evaluation, the paper focuses on a few considerations of the geotechnical site characterization, including some notes on soil behaviour under cyclic loading and measurement of soil parameters. then, some analysis methods that are currently available in literature to study the seismic performance of earth structures will be assessed. finally the dynamic behaviour of an earth dam will be presented as a typical, though complex, case history. since the «seismic behaviour of geotechnical structures» theme is extremely broad, only some of the previously mentioned topics will be extensively treated in the next pages. more extensive details can be found in the references and in the technical literature for the specific topics. 800 stefania sica, filippo santucci de magistris and filippo vinale 2. risk analysis for over forty years, since the devastating 1964 alaska (u.s.) and niigata (japan) earthquakes occurred, considerable progress has been made in the field of soil dynamics and geotechnical earthquake engineering. significant development has been achieved in forecasting the strong ground motion after appropriate seismic hazard analysis, in understanding the mechanical behaviour of geomaterials, in evaluating the ground response during an earthquake and in modeling structures response under seismic loading. these studies provide an important contribution to the general field of seismic risk analysis. seismic risk studies are conventionally based on the analysis of a single component of risk, often directed at a particular region. more complete analyses require the definition of all factors on which an area’s earthquake disaster risk depends. furthermore, the risk level in a particular region should be periodically updated. in general, disaster could be considered not only a function of the expected physical impact of future earthquakes, but also of the capacity of the affected area to sustain that impact, as well as the implications on local economic and social affairs (davidson, 1997). as a matter of fact, while people may not be able to stop the occurrence of earthquakes, they might be able to regulate some of the other contributing factors (e.g., resources available for emergency response and recovery, vulnerability of the infrastructure), thereby reducing the frequency and severity of earthquake-induced damage. it should be considered also that worldwide earthquake disaster risk has grown significantly over the past decades due to exploding populations in seismic regions, unimpeded urbanization, and the increasingly interconnected and internally complex urban plan of most existing cities. therefore, if in a conventional perspective, any risk, including seismic risk, is a convolution of exposure, hazard, and vulnerability, a more extended definition might also include other factors such as external context (political and economic) and emergency response and recovery capability. figure 1 summarizes a proposed definition of earthquake disaster risk after davidson (1997). in this figure, hazard represents the geological phenomena that act as initiating events of an earthquake disaster. earthquake hazards include not only ground shaking but also collateral damage such as liquefaction, landslides, tsunamis, ground rupture and subsidence. exposure includes list of everything that is subject to the physical demands imposed by the hazard. vulnerability describes how easily and how severely physical infrastructures, population, economy, and social-political system can be affected by an earthquake. external context describes how damage to a city affects people and activities outside the city. it incorporates the reality that, depending on a city’s prominence with respect to economics, transportation, politics, and culture, damage to certain cities may have more far-reaching effects than damage to fig. 1. factors contributing to the definition of earthquake disaster risk (after davidson, 1997). 801 seismic behaviour of geotechnical structures others. emergency response and recovery capability describe how effectively and efficiently a community can recover from shortand long-term impact through formal, organized activities. while we have little control over seismic hazard, full awareness of structural and environmental seismic vulnerability, will help seismic engineers concentrate their efforts on reducing structural and environmental weaknesses. to help reach this objective, the seismic behaviour of structures, including geotechnical structures, should be fully understood. 3. the role of geotechnical engineers from the previous paragraph it clearly emerges that seismic risk analysis is a typical multidisciplinary study. while the obvious role of a seismic geotechnical engineer is that of detecting and examining the performance of geotechnical structures under dynamic loading, his «cultural» responsibility in the evaluation of free-field ground response (*) is essential too. here, it is worth remembering that ground response analyses are used to predict ground surface motion for the development of design response spectra, to evaluate liquefaction hazards or landslide occurrences and to determine the earthquake-induced forces on civil structures. thus, an essential responsibility of an earthquake geotechnical engineer consists of detecting ground surface motion that is strongly influenced by the soil strata that might lie above the bedrock. this influence, as well as the general behaviour of earth structures, cannot be fully understood without: 1) proper knowledge of the subsoil stratigraphy; 2) mechanical behaviour of any subsoil strata and of the materials constituting the earth structure, including a proper knowledge of experimental techniques, and 3) proper use of numerical models able to simulate the subsoil response, the geotechnical structure response, as well as their interaction. points (2) and (3) have some peculiarities that will be briefly emphasized in the following paragraphs. point (3) will be developed with the aim of detecting the seismic behaviour of geotechnical structures. 4. soil behaviour under cyclic loading the effect of earthquake loads on a soil element can be represented by a complex shear stress time history τ (t), acting after a previous loading history. depending on the level of the considered earthquake motion and the dynamic properties of the soil-structure system, the soil shear strain level induced by the seismic event can vary. consequently, the soil should be characterized by models of different complexity. typical gross distinctions can be made between soil behaviour at pre-failure and at failure conditions. in the first case, further distinctions are made among the so-called «small-strain region», the «medium strain region» and the «large strain region». distinction can be easily understood by considering the schematic soil behaviour as reported in fig. 2, which shows typical relationships existing between shear stiffness or damping ratio and the shear strain level. at small strains, soil stiffness and damping ratio attained their maximum and minimum values, respectively. soil response can be adequately represented by a linear model. at medium strains, soil shows a clear nonlinear behaviour but the response under cyclic loading is stable (i.e. no plastic volumetric strains or pore water pressure is detected). in this strain range soil behaviour can be represented by linearly equivalent models. finally, at large strains shear-volumetric coupling is apparent and the effect of the number of cyclic loadings cannot be neglected. in this case, elastoplastic effective stress models could be opportunely used to simulate soil behaviour. several parameters affect both initial shear strain and damping ratio and their strain dependency. readers can refer to, for instance, d’onofrio and silvestri (2001) for some information of this topic. (*) detailed explanations on the evaluation of ground motion during an earthquake can be found in kramer (1996). 802 stefania sica, filippo santucci de magistris and filippo vinale 5. measurement of soil parameters in geotechnical engineering design practice, field and laboratory investigation have advantages and disadvantages. therefore, it might be useful to employ both in situ and laboratory measurement procedures to obtain a proper soil characterization. as shear modulus g0 and damping ratio d0 at small strain levels might be affected by sample disturbance, less so in the case of soil non-linearity, the strain dependence soil stiffness might be evaluated through (5.1) and the strain dependency of soil damping by (5.2) in the previous equations (g 0 ) field and (d 0 ) field are preferably evaluated by measurement of in situ shear wave velocity; g( )/g 0 should be determined by laboratory tests, as should d _ ( ). the latter represents the measured damping ratio scaled to the relevant small strain value d0. as the scope here is not to detail experimental techniques and procedures, a list of the most common techniques is reported in table i. some other special apparatuses not listed in table i have been developed at some universities and research centers (refer to, for instance, http:/ /www.iis.u-tokyo.ac.jp/~hayanodex2.html; http:/ /geotle.t.u-tokyo.ac.jp/; http://www.entpe.fr/ prive/index-recherche.htm). however, such machines are still prototypes and not widely available. in situ soil tests for earthquake geotechnical engineering are typically geophysical, including surface tests such as spectral analysis surface waves (sasw), and borehole tests such as downhole (dh) test and cross-hole (ch) test. laboratory tests can be categorized as either: 1) static tests, in which stress-strain properties derive from the direct measure of stress and strain; or fig. 2. typical variation of the soil response under cyclic loading at increasing strain level in terms of shear modulus and damping and stress-strain loops. g g g g ( ) = ( ) ( )0 0 field lab d d d( ) = ( ) + ( )[ ]0 field lab . 803 seismic behaviour of geotechnical structures 2) dynamic tests, in which the soil properties are derived from a dynamic equilibrium analysis of a soil element. static tests usually include cyclic triaxial tests (ctx), cyclic simple shear tests (css) and cyclic torsional shear tests (cts). resonant column tests (rc) and bender element tests (be) are typically considered dynamic tests. choice of experimental technique for soil characterization is a matter of compromise between technical and economic factors. as far as in situ tests are concerned, it is well known that the ch test is the most reliable, albeit most expensive, system for evaluating initial soil stiffness. the multi-receiver ch test also allows for the measurement of field small strain damping ratio d 0 . on the other hand, multistation testing configuration for analyzing surface waves appears to be a promising, although complex, in situ experimental technique that provides not only the stiffness profile but also the damping ratio profile (foti, 1999). as for laboratory tests, sound laboratory practice in itself might be able to mitigate the influence of sample disturbance. a suitable reconsolidation technique up to the in situ stress state (that includes appropriate anisotropic reconsolidation stress-path and ageing time) can be, for instance, of some benefit to obtain more realistic subsequent stress-strain behaviour of soils (vinale et al., 2000). for the resonant column tests, conventional interpretation criteria are based on the assumption that soil can be modeled as a linear visco-elastic medium, an idealization typically made in solving some boundary value problems in seismic geotechnical engineering. it is worth noting that linear viscoelasticity is insufficient for understanding some important features regarding time-dependent behaviour of geomaterials (see tatsuoka et al., 2000), while new non-linear three-component rheology models have been proposed (di benedetto et al., 2002; tatsuoka et al., 2002). in any case, time-dependency in the stress-strain betable i. in situ and laboratory experimental techniques to obtain some relevant properties for analyzing the seismic behaviour of geotechnical structures (after international navigation association, 2001). in geotechnical earthquake engineering cpt and spt should be properly used only providing empirical correlation with relevant soil properties. v s = shear wave velocity; v r = rayleigh wave velocity; f r = resonant frequency; h.p. = half-power method; r.f. = resonance factor method; n = spt blow count; q c = cpt tip resistance; φ ′= friction angle in effective stresses; c u = undrained shear strength; c = coarse grained; f = fine grained soils; q /σ ′ r = deviator/radial stress ratio; τ /σ ′ v = shear/vertical stress ratio; n c = number of cycles. test class test type consolidation shear strain frequency stiffness damping strength stress state γ [%] f [hz] c f spt n→v s →g0 φ ′ penetration cpt q c →v s →g0 φ ′ cu field down-hole lithostatic v s →g0 − geophysical cross-hole < 10−3 10-100 v s →g0 possible sasw v r →v s →g0 − triaxial axisymmetric > 10−2 0.01-1 q:ε a →ε→g q /σ ′ r : n c cyclic simple shear axisymmetric > 10−2 0.01-1 τ :γ → g w d / w s →d τ /σ ′ v : n c torsional axisymmetric 10−4-1 0.01-1 τ :γ → g 0 ,g laboratory shear or true triaxial resonant axisymmetric 10−4-1 > 10 fr→ g0,g h.p., r.f.→d dynamic column or true triaxial bender axisymmetric < 10−3 > 100 v s → g 0 elements 804 stefania sica, filippo santucci de magistris and filippo vinale haviour of soil should be strongly considered in designing any experiment involving geomaterials. recent developments in triaxial apparatuses allow stress-strain properties of soil to be obtained under a wide range of strain and strain rates (tatsuoka et al., 1994; santucci de magistris et al., 1999). it must be emphasized, however, that only young’s soil modulus can be directly obtained from the triaxial apparatus, unless reliable radial strain measurements and a proper soil model are available. a relationship between young’s modulus and shear modulus is not easy to acquire, even in the case of undrained tests (vinale et al., 2001). in general, while the resonant column test (rc) is relatively common in italy (cavallaro et al., 2001), a recent survey shows that this apparatus has become less popular in japan (kuwano and katagiri, 2001). 6. analysis methods many approaches have been developed to analyze the seismic behaviour of geotechnical structures. they can be classified on several bases: geometry assigned to the soil-structure system (i.e. one-dimensional, two-dimensional, three-dimensional), constitutive model describing the material stress-strain response (i.e. linear elastic, non linear elastic, elastoplastic with or without hardening), the modeling of the interaction phenomena among different phases present in a soil (soil, water and air), the way in which the seismic loads are described (i.e. peak ground acceleration, response spectrum, acceleration time history). for the sake of simplicity, all the methods available in literature can be grouped into three main classes, including: 1) simplified analyses, which allow for evaluating the safety factor of a soil-structure mass by simple global force equilibrium. 2) simplified dynamic analyses, which provide the earthquake-induced displacement on the basis of an assumed failure mode of the soilstructure system. 3) full dynamic analyses, in order to evaluate both magnitude of displacements and failure modes. until the sixties, among the simplified analysis methods the pseudo-static approach represented the main tool used to assess the seismic safety of most geotechnical structures (retaining walls, embankments, earth dams, port structures). nowadays this method is still widely used since it is imposed by several national regulations (in italy, the d.m. 24/3/82) and because engineers have confidence in its use. both the commercial codes that implement it and the quantitative definition of the required parameters are very simple. in the pseudo-static approach, the dynamic inertia forces induced by an earthquake are considered static actions proportional to the soil mass by a seismic coefficient (fig. 3). the value of such a coefficient is determined on the basis of the expected peak ground acceleration in the area where the structure is located. national standards generally prescribe the seismic coefficient value according to the seismic category of the site. in the case of retaining walls (gravity and sheet pile walls), the pseudo-static approach requires determination of both active and passive earth pressures. they are usually estimated using the mononobe-okabe equation (mononobe, 1924; okabe, 1924) obtained by modifying the coulomb classical earth pressure solution to account for inertia forces. one of the main limits of the pseudostatic method consists of considering the failure along a well-defined sliding surface as the only type of damage that an earth structure could suffer during an earthquake. this assumption could be reasonable in the case of retaining walls, but it might be misleading for other geotechnical structures. in the case of earth dams, for instance, fig. 3. pseudostatic loads acting on a sliding mass. 805 seismic behaviour of geotechnical structures additional information is required about freeboard loss and safety against liquefaction or fracture of dam elements (core or filter) that assure water tightness. literature documents many case histories of earth dams that failed either partially or totally during earthquakes, even if the pseudo-static approach had given a positive result about their seismic safety. lower san fernando dam (seed, 1979) and hebgen dam (steinbrugge and cloud, 1962) are two classic examples. the simplified dynamic approach, first introduced by newmark, still idealizes the soilstructure system as rigid blocks of soil and structural masses. moreover, the method allows computation of the displacement of the sliding masses by integrating the acceleration time history overcoming the so-called critical acceleration, i.e. the threshold acceleration that causes the block to slide (fig. 4). the seismic safety of the earth structure is evaluated in terms of an acceptable permanent displacement rather than a factor of safety against global instability, as is typical in the pseudo-static approach. simplified dynamic analyses, including the original newmark method (newmark, 1965) and the numerous newmark-derived methods (i.e. makdisi and seed (1978) for embankments and earth dams; yegian et al. (1991) for embankments; franklin and chang (1977) for dikes; richards and elms (1979) for gravity walls; towahata and islam (1987) for sheet pile walls; steedman (1998) for gravity and sheet pile walls) can prove very useful when a performance-based design philosophy is likely to be adopted to overcome the limitations of conventional seismic design. full dynamic analysis directly models the overall soil-structure interaction problem, assuming that both soil and structure behave as continuum and deformable media. the spatial domain is discretized using a numerical technique (generally, finite element or finite difference methods) and the differential equation governing the boundary value problem is solved over the time or frequency domain. since the solution in the frequency domain is admissible only when the material linearity hypothesis persists, it is worth noting that the solution over the time domain is mandatory when more sophisticated constitutive models (inelastic or elastoplastic) are assumed for the soil-structure system. as discussed previously, the choice between a linear or non-linear material model depends on the expected level of earthquake motion relative to the elastic limit of the soilstructure system. inside the class of full dynamic analysis methods, a more detailed distinction can be made in describing the interaction among the different phases − air, water and soil skeleton − of the soil constituting the structure and its foundation. according to this point of view, the dynamic methods can be further classified as: – one-phase or total stress approach; – simplified two-phase or simplified effective stress; – coupled twoor three-phase approaches. in the one-phase approach the interaction among the soil phases is actually neglected, and the soil is assumed to be an equivalent monophase continuum governed by dynamic equilibrium equations, continuity equations and fig. 4. newmark sliding block approach. 806 stefania sica, filippo santucci de magistris and filippo vinale constitutive law. in particular, the constitutive law refers to the behavior of the whole soil skeletonpore fluid system. inside this category, the equivalent linear procedure has been the most widely used technique for computing the dynamic response of soil deposits, embankments and soil-structure interaction. this approach is implemented in very popular codes, such as shake (schnabel et al., 1972), flush (lysmer et al., 1975) or quad-4 (idriss et al., 1973). notwithstanding the simplicity and popularity of the equivalent linear procedure, its main limitation lies in not providing any prediction of earthquake induced permanent displacements. as a matter of fact, the method is meaningless when higher strain levels are involved and permanent displacements should be forecast. many attempts in literature to overcome this limitation of the equivalent linear method are documented (seed, 1979). in the simplified two-phase approaches, a semi-empirical law (excess pore water model) is introduced to evaluate the pore water pressure induced by cyclic shear stresses. this law links the excess pore water pressure to the number and amplitude of the applied deviatoric cyclic stresses (martin et al., 1975; finn and bathia, 1981; dobry et al., 1985; matasovic and vucetic, 1992, 1995). the effect of the excess pore water pressure is incorporated into the equivalent linear analysis or into a direct non-linear analysis by reducing the actual stiffness of the soil. such an approach is implemented in computer codes as flush-l (ozutsumi et al., 2000) and tara-3 (finn et al., 1986). in the coupled two-phase approaches the constitutive law refers only to the soil skeleton. to account for the presence of the pore fluid, the continuity equation of the water is incorporated as well. the stress-strain behaviour of the soil can be described by different constitutive laws according to the level of sophistication required: linear elastic, hypoelastic, elastoplastic with kinematic or combined hardening. in this category, the effective stress methods implementing elastoplastic models based on the concept of combined hardening (isotropic and kinematic) have proven very powerful in computing residual displacements and/or evaluating the structure response beyond the elastic strain level (prevost, 1985; muraleetharan et al., 1988; iai et al., 1998; benzenati, 1993; sica et al., 2001). in the case of earth dams, for instance, these methods allow evaluation of the structure’s safety against most types of earthquake-induced damage: freeboard loss, liquefaction due to excess pore water pressures and post seismic effects. the choice of analysis approach should be based on the principle that structures requiring higher performance (structures with high exposure) should be designed/verified using more sophisticated methods. in light of this, simpler approaches may be used for preliminary design, screening purposes or to evaluate earth structure response in case of low seismic excitation levels. since earth dams can be quite complex geotechnical structures, the next section will focus on seismic response evaluation by applying some of the outlined analysis procedures to a real case history. 7. some issues regarding earth dams the structural complexity of earth dams and the high risk associated with them due to major social, economic and environmental consequences of failure, require very reliable analysis tools to study their performance, especially when carrying out a safety assessment involving seismic loads. principal damage that earth dams may suffer during an earthquake consists of (seed, 1979): – settlements and fractures of the dam body; – freeboard loss up to the limit of overtopping; – global instability of upstream and downstream slopes; – reduction of shear strength up to liquefaction of construction and/or of foundation soils; – differential displacements between embank ment, abutments and spillway; – failure of outlet works crossing the dam body; – disruption of dam by major fault movement in foundation; – overtopping of the dam produced by soil masses sliding into reservoir. permanent displacements induced by earthquakes can be considered the integral effect of volumetric and deviatoric plastic strains devel807 seismic behaviour of geotechnical structures oped within the dam embankment. deviatoric strains produce, as an isolated effect, the so called «lateral spreading» of the dam body. significant crest settlements produce freeboard loss up to overtopping (fig. 5). even if this limit is not reached, differential settlements may induce other critical types of damage such as fractures within the dam body, which could then jeopardize the water tightness of the structure. global failure induced by earthquakes might occur if the shells are so steep that even static conditions are close to instability. seismic forces always produce an increase in instability loads and for some soils a decrease of shear strength as well. the failure mechanism may involve either the dam body and/or the foundation soils. other types of damage could derive from strength reduction of soils induced by excess pore water pressure. the latter might cause two different phenomena: liquefaction and cyclic mobility. liquefaction consists of a cyclic shear stress process that induces a reduction in strength not compatible with static stability conditions acting at the end of the cyclic process itself. cyclic mobility refers instead to a similar process where strength reduction is consistent with static stability. liquefaction induces high deformation in the earth structure even up to collapse either during or after the seismic event. rapid collapse occurred, for instance, in the lower san fernando dam (1971) and in the hebgen dam (1959) several seconds after the end of the earthquake. liquefaction phenomena occurred also in more than ten small irrigation dams in akiba (japan) during the oga earthquake (1939). in most cases failure occurred some hours after the seismic event. cyclic mobility, on the other hand, produces permanent deformations only during the seismic event. additional plastic strains develop only when shear stresses (static + seismic induced) exceed the soil shear strength. plastic deformations and related permanent settlements depend on cyclic number and amplitude of shear stresses. vulnerability to liquefaction of soils used as construction materials depends on the intrinsic soil properties and on the adopted construction technique. it has been observed (seed, 1979) that hydraulic fill dams are more prone to liquefaction than dams constructed using compaction. differential movements between dam body, abutments and spillway could cause internal fractures where water could find less resistance to seepage during post-seismic stages, giving rise to so-called «piping». these effects are very difficult to diagnose and are dangerous because piping could progress until abrupt failure of the dam. 8. the case history of el infernillo dam the el infiernillo dam (fig. 6) has been selected in this study as a case-history sample because it is a high zoned dam (h max = 145 m) located in a highly seismic area (i.e. the balsas river zone around 300 km sw of mexico city). the region is characterized by significant seismicity fed by the subduction mechanism of the pacific plate under the american continent. since its construction, the el infiernillo dam has experienced several earthquakes, among which two strong-motion events (on 14/03/1979 and 19/09/1985). with reference to the earthquakes of 14/03/1979 (richter magnitude equal to 7.6 and epicentral distance of 134 km) and 19/09/1985 (richter magnitude of 8.1 and epicentral distance of 68 km), accelerograms recorded both on the rock abutments of the dam and at reference points along the dam maximum cross section (crest and downstream shell) are available, together with some measurements of earthquake-induced permanent displacements. these field observations have been used in this study to verify the reliability of the selected analysis methods to evaluate the seismic performance of the dam. fig. 5. typical earthquake-induced damages to earth dams. 808 stefania sica, filippo santucci de magistris and filippo vinale 8.1. simplified analysis the stability analysis has been performed using the modified bishop method (bishop, 1955). the method is based on searching for the safety factor along pre-defined potential circular slip surfaces by a limit equilibrium analysis. a trial and error procedure was adopted to identify the location of the slip surface having the minimum safety factor, both in the upstream and in the downstream rockfill shells of the dam. the pseudostatic inertial forces were assumed to act in an outward direction to sliding (i.e. in the upstream and downstream directions, respectively, for the upstream and downstream shell) in order to maximize the effect of the active forces that induce sliding. the analysis was performed under the hypothesis of steady seepage inside the dam with the reservoir level fixed at its operational maximum. as a consequence, the buoyance unit weight was adopted in the computation regarding the upstream shell, while the actual unit weight of the construction soils was considered in searching for the slip surfaces within the downstream side. the seismic coefficient was assumed equal to 0.1, the peak ground acceleration during the 14/03/1979 earthquake at the rock base of the dam. from the two sets of analyses, it appears that higher instability is located in the upstream shell of the structure. potential sliding surfaces are concentrated in the upper part of the dam close to the external boundary. the minimum safety factor was 1.7, an acceptable value for assuring global stability of the dam. 8.2. simplified dynamic analysis among the simplified dynamic methods available in literature, the newmark approach was selected to evaluate the permanent displacement of the el infiernillo dam induced by the 14/03/1979 earthquake. as indicated before, this method requires the determination of the critical acceleration associated with an assumed sliding surface. the selected surface in this case had the lowest safety factor in the pseudostatic analysis and involved a dangerous failure mechanism involving the overtopping of the reservoir water. figure 7 shows the adopted sliding surface for the upstream side of the dam. figure 8 shows the reduction of the safety factor with increasing seismic coefficient. this plot was obtained by computing the pseudostatic safety factor (s f ) for different values of the seismic coefficient. in correspondence to the unit safety factor (s f = 1), a critical seismic coefficient (k c ) of 0.32 was evaluated. this means that if the average acceleration along the slip surface overtakes the fig. 6. main cross section of el infiernillo dam. 809 seismic behaviour of geotechnical structures critical value of 0.32 g (acrit = k c⋅ g), the sliding mass will start moving as a rigid block. the left hand side of fig. 7 shows the variation of the peak horizontal acceleration along the dam axis. this plot was obtained by assuming linear variation of acceleration from the measured value of 0.1 g at the dam base up to the value of 0.36 g that was recorded at the dam crest during the 14/03/1979 seismic event. therefore, an average value of 0.31 g could be reasonably attributed as the unique peak acceleration along the whole sliding surface. since the driving acceleration is lower than the computed critical acceleration, the newmark method does not predict any irreversible displacement within the dam. fig. 7. sliding surface adopted in the newmark analysis for the upstream shell of the el infiernillo dam. fig. 8. pseudostatic safety factor variation with increasing seismic coefficient along the sliding surface. 810 stefania sica, filippo santucci de magistris and filippo vinale 8.3. full dynamic analysis the dynamic analysis was performed using a two-phase coupled approach implemented in the finite element program gefdyn (aubry and modaressi, 1996). the approach is based on the u-p formulation of the biot generalized consolidation theory (zienkiewicz and shiomi, 1984). in particular, the overall dynamic equilibrium and the pore water flow equations were combined with hujeux’s elastoplastic kinematic hardening constitutive law (hujeux, 1985). this law describes some relevant features of the soil response under monotonic and cyclic loading conditions (hysteresis, liquefaction, cyclic mobility and ratcheting). since in a coupled effective stress method the predictions of the dynamic analysis are strongly affected by the pre-seismic conditions of stress and pore water pressure distribution within the structure, it was necessary to simulate all stages of the dam’s history up until the actual seismic event: construction, first impounding, service operations. in this study, the model parameters characterizing the materials of the el infiernillo dam were evaluated from the laboratory tests performed at the time of dam construction (conventional drained and undrained monotonic triaxial tests and oedometer tests) and on a back-analysis procedure of dam observed behaviour (sica, 2001). the dam geometry was discretized in about 400 2d quadratic elements (fig. 9). the acceleration time-history used as input motion at the base of the finite element model is the horizontal accelerogram (upstream-downstream direction) recorded on the outcropping rock during the 14/03/1979 earthquake. at the end of the seismic excitation, the effective stress analysis gives a deformed shape of the dam like that shown in fig. 10. the maximum vertical settlement computed at the end of the earthquake is around 9 cm at the dam crest; the maximum horizontal displacement in the upstream direction is about 8 cm and 4 cm in the downstream direction. these results are consistent with the observed deformed mode of the dam after the same earthquake (resendiz et al., 1982). as far as the excess pore water pressures are concerned, the analysis gives a maximum pore fig. 9. mesh geometry of the el infiernillo dam adopted in the f.e.m. analysis. fig. 10. deformed mesh geometry of the el infiernillo dam after the 14/03/1979 earthquake simulation. 811 seismic behaviour of geotechnical structures water pressure ratio inside the dam body of about 20%, a value far enough from the threshold limit of 100% at which soil liquefaction may occur. in conclusion, the effective stress analysis provided the required information about the dam safety in terms of: 1) overtopping: the computed freeboard loss, equal to the seismic induced vertical displacement at the dam top, is only 1% of the available freeboard ( 10m); 2) liquefaction: the computed pore water pressure ratio r u is less than 100%; 3) shell stability: the computed horizontal and vertical displacements at the end of the earthquake are very small (less than 10 cm) compared to the transverse dimension of the dam. 9. conclusions to properly analyze the seismic behaviour of geotechnical structures, a complete methodology typically employs several steps. first, a detailed evaluation of the regional seismicity should be performed in order to define the earthquake motion at the base formation where the structure is located. next, the specific hazard of the site should be assessed by considering either local amplification of ground motion and other problems like soil liquefaction, ground rupture or landslides. finally, the soil-structure interaction analysis should be performed. since the whole methodology cannot be described in a single paper, this report focuses only on some relevant aspects of the final step. the first point highlighted in the paper is a close interconnection between various technical branches (from seismology to geology, engineering and land planning) and government policy in seismic risk analysis. this means that the seismic response of geotechnical structures should be considered just one component of the wider topic of earthquake disaster risk protection and prevention. the second aspect emphasized in this paper is the importance of proper knowledge of the mechanical properties of the materials constituting the structure and its subsoil. this is a crucial problem for geotechnical engineers because it requires both high technical skill and economic resources for suitable in situ and laboratory investigation. it was further pointed out that for geotechnical structures, determination of soil properties should be adapted to the analysis method chosen to study seismic response. this choice is not simple, especially if one considers that national standards, except for some specific and recent releases, are usually not exhaustive on the safety evaluation of structures under seismic actions. in general, they account for the amplification of the ground motion due to site conditions in a very rough manner, and suggest only simplified analysis methods. conventional approaches provide information on the structure’s capacity to resist a design seismic force, but they are not able to predict structural performance during and after the earthquake. the performance-based design approach, first developed for buildings, is nowadays proposed for geotechnical structures as well. a comprehensive report on this topic, oriented towards port structures, can be found in international navigation association (2001). extracted from that report are the following points: 1) deformations in ground and foundation soils, together with corresponding structural deformation and stress states, are key design parameters; 2) conventional limit equilibrium-based methods are not well-suited to evaluate these parameters, and 3) some residual deformation may be acceptable. in performance-based design, appropriate levels of design earthquake motions must be defined along with acceptable levels of structural damage. for design purposes, two levels of earthquake motions are typically used: – level l1: earthquake motion that is likely to occur during the life-span of the structure (probability of exceeding is 50 %). – level l2: earthquake motion associated with rare events that typically involve very strong ground shaking (probability of exceeding is 10 %). the acceptable level of damage is set according to some specific structural (predicted amount of damage) and operational (level of loss ( )r uu = / vo ' 812 stefania sica, filippo santucci de magistris and filippo vinale of serviceability, time and cost for restoration) factors. once the design earthquake levels and acceptable damage levels have been properly defined, the required performance of a structure may be represented by a performance grade determined by the importance of the structure (critical and key structures, primary structures, ordinary structures and small, easily restorable structures). the principal steps taken in performancebased design are summarized in the flowchart of fig. 11. it is apparent that for geotechnical structures, appropriate damage criteria should be related to the deformation mode of the structure itself. as a consequence, sophisticated analyses should be adopted to model structure performance adequately. in this paper, the seismic behaviour of an earth dam was analyzed as a sample geotechnical structure. even if this case history was examined from a verification rather than design viewpoint, the study is very explanatory of the capability of a coupled dynamic analysis in predicting structure performance during and after a selected earthquake. in particular, this type of analysis method allows forecasting of seismically induced displacements in the dam body with good accuracy, as verified by comparing analytical results with monitoring data. figure 12 reports the computed vertical displacements profiles at three locations along the dam axis. on the right side hand of the figure the residual displacements are plotted together with the measured ones, showing good agreement between the analysis prediction and observed behaviour. fig. 11. steps required for the seismic behaviour analysis of a geotechnical structure using a performance-design based approach (modified after international navigation association, 2001). 813 seismic behaviour of geotechnical structures it is worth emphasizing that while the pseudostatic method does not directly predict displacements, no seismically-induced movements were forecast by the simplified dynamic analyses using the newmark method. in any case, the critical slip surface given by the pseudostatic approach, found by trial and error, lies very close to the area in which maximum plastic shear deformations were detected by the full dynamic analysis (fig. 13). fig. 13. contour plot of plastic shear strains at the end of the 14/03/1979 earthquake. fig. 12. time histories of vertical displacement at three points along the dam axis. 814 stefania sica, filippo santucci de magistris and filippo vinale acknowledgements the authors wish to give thanks to prof. h. akihama, nihon university, and prof. a. rapolla, university of naples, scientific organizers of the kyoto and kobe meeting, for offering us the opportunity to contribute our findings in a high level meeting that was useful for exchanging opinions with several experts in the area of natural disaster mitigation. a vote of thanks should be given to prof. m. nakashima and his staff at kyoto university for the tremendous effort in organizing the symposium; to dr. a. volpi, scientific attaché at the italian embassy in japan and the italian embassy in japan, for the care and the support received; to mr. m. di gianni, honorary consul of japan in naples, and to all participants in the symposium for their friendship and frank exchange of ideas. references aubry, d. and a. modaressi (1996): gefdyn, manuel scientifique, ecole centrale de paris (in french). 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the generalised biot formulation and its numerical solution, int. j. num. anal. methods geomech., 8, 71-96. baskaradas_final:layout 6 annals of geophysics, 57, 1, 2014, a0187; doi:10.4401/ag-6345 a0187 description of ionospheric disturbances observed by vertical ionospheric sounding at 3 mhz james a. baskaradas1, silvio bianchi2, marco pietrella1, michael pezzopane1, umberto sciacca1,*, enrico zuccheretti1 1 istituto nazionale di geofisica e vulcanologia, rome, italy 2 università “la sapienza”, dipartimento di fisica, rome, italy abstract high frequency radio waves reflected by the ionosphere can provide a relevant amount of information within the composite received signal. the ionosphere is indeed a frequency dispersive, bi-refractive, absorbing medium, in which multipath propagation occurs due to disturbance on a varied time-spatial scale. on the time-spatial level of small scale disturbances (ssd) the ionosphere dynamics, detectable by vertical ionospheric sounding (vis), is mainly dependent on wrinkled layers acting as multi-reflectors. the present paper discusses different aspects of the effects of multipath fading suffered by the wave along the propagation path and potentially associated with ssd. to achieve these objectives, a vis campaign at a fixed frequency of 3.0 mhz was conducted at the ionospheric observatory in rome (latitude 41.8 n; longitude 12.5 e), by collecting a series of measurements of the power variations in received echo signals recorded between two consecutive ionograms whose sounding repetition rate was set to 15 min. the obtained results show that: 1) the fading suffered by the wave follows either a rayleigh trend or a nakagami-rice trend, or a mix of them, the mixed case being the most frequent (about 65 % of the analysed cases); 2) the predominant periodicities characterizing the power variation are less than 25 s; such values are compatible with the small scale ionospheric disturbances; 3) for all the 24 hours of the day the ionospheric reflector is pretty stable and for time intervals of 10-30 s the periods of stability occur with a percentage of occurrence ranging between 55% and 95 %; for time intervals of 190210 s the periods of stability occur instead with a percentage of occurrence ranging between 5% and 54 %. 1. introduction vertical ionospheric sounding (vis) is a consolidated technique that was first applied nearly a century ago to unveil the existence of the ionosphere, i.e. the density and altitude of the ionospheric layers. it is still widely applied and generates the largest ionospheric parameter database [reinisch and galkin 2011]. this technique relies on reflection of radio waves from ionospheric plasma exhibiting plasma frequencies equal or higher to the radio frequencies employed for radio sounding. the vis technique uses radio waves propagated in the high frequency (hf) range and reflected by the ionospheric layers acting like mirrors, with the corresponding echo delay related to the reflection virtual height of the reflecting layers. nowadays advanced digital sounders are used in the vis technique for observations that provide detailed information about the structure and dynamics of the bottom side ionosphere. the underlying concepts of the technique remain the same, but modern sounders measure more than just the travel time and amplitude of the echoes. in a measurement campaign, the ais-ingv (advanced ionospheric sounder istituto nazionale di geofisica e vulcanologia) ionosonde, with minor modifications and calibrations, was adapted to measure only power in dbm and virtual height in km (pvh), at a fixed frequency, assuming that the critical frequency and reflection altitude of the ionospheric layers to be investigated were known from routine vis measurements [bianchi et al. 2013a]. the latter is important because only starting from this knowledge the pvh measurements can be usefully conducted. consequently, it is possible to measure the level of received power in nearly controlled conditions, evaluating only certain echo signal characteristics. in the vis technique the emitted pulses are reflected back in a mirror-like way if the frequency of the radio waves is lower than the critical frequency, simplifying the propagation path and other relevant parameters. when operating at a fixed frequency, the vis technique highlights variations in the electron density and virtual heights of the iso-density surfaces, based on the amplitude and article history received may 13, 2013; accepted december 20, 2013. subject classification: fading fluctuation, ionospheric irregularities, multipath fading, ionograms. phase analyses of the echo signal. however, single mirror-like reflections are seldom observed in vis measurements, while dominant multi-reflectors are more probable, following either rayleigh or ricean trends, characterized by a single dominant reflector with other less relevant contributions [bianchi et al. 2013b]. this happens because ionospheric disturbances on a variable time-spatial scale can tilt the ionosphere and corrugate the bottom layer, producing peculiar reflecting characteristics. moreover, these irregularities travel at different horizontal and vertical velocities, as reported in literature [leitinger and rieger 2005, crowley and rodrigues 2012]. the moving reflectors introduce a further complication, making the propagation path time variant. echo signals reflected from the ionosphere and measured by the vis technique are subject to a degree of variability, according to the traveling irregularities on the small, medium, and large scale [blanc 1985, krasnov et al. 2006]. the presence of traveling irregularities in the ionosphere causes the phase difference between different paths vary over time, with consequent time variable signal fading, from a few seconds to tens of seconds. the power signal variations at the receiver input match the time-spatial scale of the irregularities. due to multipath, different paths can interfere constructively or destructively at the receiving point depending on the phase of the wave components. among the periodicity times, this paper addresses the ones related to small scale disturbances (ssd), which are characterized by random processes, by exploiting pvh measurements. the narrow band signal propagating in the ionosphere here analyzed was modelled by rice [1944], nakagami [1960], and watterson et al. [1970], and the corresponding temporal power variations have recently been investigated by bianchi et al. [2013a] by using pvh measurements. depending on the position and the shape of the irregularities, the ionosphere acts like convex-concave mirror for hf waves. when radio waves are reflected from a concave iso-density surface, the waves will undergo a focusing gain; conversely when the surface is convex there is a loss due to the defocusing effect. the latter phenomenon, together with multipath fading, defines the behavior of the ionospheric irregularities considered in this paper, where a series of measurements of radio echo power were performed to study and monitor some dynamic characteristics of the ionospheric propagation path and related reflecting layers potentially associated with ssd. 2. experimental setup and measurement campaign the vertical ionospheric sounding system used for this study was derived from a phase coded hf radar [bianchi et al. 2003] with the purpose of studying the ionospheric fading under definite controlled conditions. a single fixed frequency pvh sounding, lasting a few minutes, was carried out between two consecutive vertical ionospheric soundings. the inspection of these two ionograms, whose sounding repetition rate was set to 15 minutes, is important to be sure that the investigated ionospheric layer is present and falling in the flat part of the ionogram trace corresponding to the base of the f region (see figures 1 and 2). the pvh sounding was performed using the aisingv ionosonde [zuccheretti et al. 2003], employing complementary phase code of 16 chips, which is a low power hf radar based on the pulse compression technique with a mathematical processing gain of 25 db; about 13 db derive from the correlation process while the remainders are related to coherent integration [bianchi et al. 2013a]. the system records pairs of data: power in dbm and virtual height in km are given for the highest signal peak returning from the ionosphere, and previous calibration allows determination of the correct values. the system employs low gain, long wire, cross coupled delta antennas with a beam angle of about 60 degrees. this seems a limiting factor, but by applying a range gating function, a limited portion of the reflecting layer can be selected, making it possible to study a return echo signal from a known ionospheric region. range gating can filter off the unwanted off vertical echoes and noise out of the selected time interval when the receiver is active. the environmental noise always present is instead characterized by an average level of 30 db lower than the strongest signal peak. for this purpose a session of pvh measurements lasting 512 s were performed in alternation with the normal soundings, lasting approximately 180 s. the integration time used to increase the signal to noise ratio should be relatively small so that the ionosphere can be assumed stable within the measurement interval. therefore in order to obtain a good quality signal from the ionosphere, pulses at a rate of 60 hz were considered, providing after 30 integrations a pvh measurement every 0.5 s. figure 1 shows how the power peak (figure 1b) of the received echo at the frequency f = 3.0 mhz is related to the routine vertical ionograms recorded before (figure 1a) and after (figure 1c) the pvh measurement session. in the illustrated case the virtual height of the layer is at 200 km (figure 1a,c), and the corresponding position of the power peak recorded during the pvh measurement is shown in figure 1b. as the pvh measurement session lasts 512 s and the integration time chosen is 0.5 s, the correlation process generates 1024 narrow peaks (see figure 1b) clearly emerging baskaradas et al. 2 3 from the noise level and corresponding to the ionospheric reflectors with an accuracy of about ±0.5 db for the power and ±5 km for the virtual height . figure 2 shows instead two cases for which the inspection of the two consecutive ionograms helped the data analyzer in discarding the corresponding pvh data segment. figure 2a shows that the ionogram recorded just before the pvh measurement shows a maximum reflected frequency below the value of 3.0 mhz, which is right the pvh operating frequency. figure 2b shows instead that the pvh operating frequency would deeply penetrate the plasma, up to heights of the f region critical frequency; these pvh data segments were discarded because in these conditions the polarization losses could be no more neglected (see section 3). pvh measurements were performed at the ionospheric observatory of rome (italy, 41.8 n, 12.5 e), collecting a series of data during the period from january 3 to 22, 2008. over the measurement campaign 1920 sequences of pvh 512 s data segments were acquired. the environmental noise level was also analyzed during all the hours of the day in the campaign period, considering that the band of the receiving section is about 66 khz. the noise resulted approximately around −94 dbm, with the signal ranging from this level up to −64 dbm. as it will be seen in the next paragraph, it is possiionospheric dynamics observed at 3 mhz figure 1. ionograms recorded by vis (a) before and (c) after the (b) pvh measurements of the received echo every 0.5 s at the frequency f = 3.0 mhz highlighted by an open circle in (a) and (c). the ionogram sounding repetition rate is 15 minutes. the pvh measurement session lasts 512 s, which corresponds to 1024 samples of pvh. figure 2. cases for which the inspection of the two consecutive ionograms helped the data analyzer in discarding the corresponding pvh data segment because: (a) the maximum reflected frequency of the first ionogram is below the value of 3.0 mhz (highlighted by an open circle and corresponding to the pvh operating frequency); (b) the pvh operating frequency propagates up to the highest ionospheric layers and this fact prevents from neglecting the polarization losses (see section 3). ble to ignore the term of the frequency shift in the analytic description. this fact is due to the peculiar condition of the near vertical line of sight (los), as inferred from figure 3 [crowley and rodrigues 2012]: only the paths received off-vertical could be affected by a doppler effect due to the horizontal drift of the layer, but they can be easily cut-off by means of a proper range gating. for example, applying a gating of 10 km, if the height is 100 km, the area of the ionospheric layer actually investigated is some hundreds of square km. 3. analytic signal characterization in a multipath time-varying propagation path the signal at the receiving point is the sum of the multipath components generated by the irregularities in the ionosphere. each path is subject to several attenuating factors as shown in table 1. considering table 1, partially derived from [mcnamara 1991], it is important to establish which terms are approximately constant and which terms vary in the observed excursion range under the experimental conditions of pvh measurements. this makes it possible to measure the level of received power in nearly controlled conditions and evaluate specific contributions. the complete mathematical expression of the received echo signal is expressed by the following relation: (1) where t is the time, r(t) is the sum of time delayed attenuated frequency shifted replica of the emitted signal with envelope u(t) containing the bi-phase code, m is the number of paths, dependent on time, ai(t) is the attenuating factor of the i-th path (it assumes values from 0 to 1, taking into account all the loss contributions given in table 1), u(t-xi(t)) is the received waveform, xi(t) is the time dependent delay, f0 is the carrier frequency, {i(t) is the phase, dependent on the time delay xi(t), and n(t) is the noise [bianchi et al. 2013a]. when the layers move the signal can experience a doppler frequency shift that changes the phase according to 2rf0[dxi(t)/dt]·t, approximating to the first order of the taylor series expansion. this latter term is ignored in the present paper together with the term {i(t) because of their irrelevant contributions to the power variation of the signal. given that in this context only variations in the time interval of 512 s are important, that is those related to ssd, the impulsive noise is mitigated by the integration process and has less effect on the overall signal power. with these assumptions, the echo signal is then down converted to the baseband as described in the following relation: ,cos r t t u t t f t t t n t2 i i m i i i 0 0 $ $ $ a x r x { = + + = ^ ^ ^^ ^ ^ ^ h h hh h h h6 @" , / baskaradas et al. 4 loss in the ionospheric propagation path in pvh measurement typical values (db unit) free space loss (lg) ≈ constant 50-150 ionospheric absorption (la) ≈ constant 0-20 ionospheric deviative absorption (ld) ≈ constant <1 scattering (ls) ≈ constant <1 differential doppler (ld) ≈ constant 0.5 blanketing sporadic e (le) ≈ constant 0-1 polarization losses (lp) ≈ constant 3-6 focusing losses (lf) (negative = gain) not constant ±0-1 multipath losses (lm) not constant 0-25 figure 3. multi-reflectors produced by traveling irregularities. signals off vertical represented by dashed line are filtered out by the range gating threshold of the receiver. table 1. loss contributions in the pvh ionospheric propagation path. 5 this is the attenuated composite envelope signal whose components have different times of arrival (multipath components), with different phases interfering in constructive or destructive ways. the attenuating terms in table 1 are carefully evaluated individually in the limited condition of vis operating at a fixed frequency. the free space loss lg as well as the ionospheric absorption la are practically constant because of the range gating that excludes longer or shorter paths, and the 3.0 mhz frequency wave does not penetrate deeply into the plasma, being it often reflected by the flat part of the ionogram trace corresponding to the base of the f region. the terms ld, ls, ld, have the values indicated in table 1, assumed as constants. concerning the le loss, the presence of the es layer is rare, or at least during the measurements. polarization losses lp [james et al. 2006] are not considered since in the considered pvh measurements the low frequency employed does not deeply penetrate the plasma (see figures 1 and 2). for the same reason faraday rotation is ignored while in general this fading can produce a loss from 0 to a few db units. considering the irregular time changing contours of the iso-density surfaces, statistically it is rare to have only one flat reflector. on the contrary, reflectors with a convex-up or concave-up shape and producing respectively further gain or loss to the signal can be expected [bianchi et al. 2003]. these are the focusing losses lf that in the selected experimental conditions do not exceed ± 1 db. in the constraints given by vis the experimental multipath losses lm is the only term that really produces deep fading on the echo signal. 4. data analysis and results this paragraph presents different ways of expressing the results in terms of radio electric parameters from different physical points of view of the effects of the ssd. the ssd undulate the reflecting iso-density surfaces, producing multipath echoes and making different reflection sources possible. from the power variation of the echo signal potentially associated to ssd and observed in the vis technique, three main phenomena have been studied and described in the following paragraphs. in table 2 the main features of the ssd and the other types of tids are summarized, so that it is possible to compare the corresponding horizontal scales (l) of the irregularities, the period (t), the phase velocity and the characteristics of the associated fading. 4.1. trends of the signal the ionospheric propagation path exhibits different trends, according to whether there is a dominant single reflector or multiple reflectors, inferred by the power variation in time of the echo signal. the received power ranges approximately from −64 to −97 dbm, with an excursion of ≈ 33 db over the various hours of the day. the same peculiar trends, rayleigh or nakagami-rice, within the same hour or from hour to hour, were observed in the acquired 512 s time-sequences of the signal. as an example, figure 4 shows one of the time sequences recorded showing a well defined rayleigh trend. in this case a large number of reflecting sources are contributing to the composite signal. the two trends have different peak amplitudes (note the different scale between figures 4 and 5) even if the time periodicity does not differ to a significant degree. in general, the most common trend we obtained (65% of the considered pvh time sequences) is the mixed case nakagami-rice and rayleigh as in figure 6. in general, it is possible to state that ssd cause multi-reflections showing a trend, except in some particular ionospheric conditions. figure 7a,b shows the results of the fft analysis of the temporal sequences shown in figures 4 and 5 respectively for rayleigh and nakagami-rice trends. they show the typical power density behavior and the spectral content of the two sequences. it can be observed that the rayleigh behavior .y t t u t ti i m i 0 $a x= = ^ ^ ^^h h hh/ ionospheric dynamics observed at 3 mhz scale-irregularities l (km) t (min) phase velocity (m/s) associated fading ssd: small-scale disturbances (infrasonic variability) < 100 < 10 < 50 multipath fading less relevant focusing /defocusing phenomena mstid: medium-scale traveling ionospheric disturbances 100 300 10 30 50 300 amplitude fading focusing / defocusing phenomena lstid: large-scale traveling ionospheric disturbances; acoustic gravity waves 300 3000 30 300 300 1000 amplitude fading focusing / defocusing deep fading due to ionospheric tilt table 2. tid classes according to their horizontal scales derived from crowley and rodrigues [2012]. (2) has a higher spectral content at lower frequencies with respect to nakagami-rice. figure 7c highlights the two trends as a result of a 100-values running mean performed on the spectral density sequences of figure 7a,b. baskaradas et al. 6 figure 4. (a) power observations of the radio echo reflected from the ionosphere on january 7, 2008, at 12:25 ut; (b) corresponding rayleigh probability density distribution. figure 5. (a) power observations of the radio echo reflected from the ionosphere on january 13, 2008, at 07:40 ut; (b) corresponding nakagami-rice probability density distribution. figure 6. (a) power observations of the radio echo reflected from the ionosphere on january 7, 2008, at 19:55 ut; (b) corresponding mixed behavior rayleigh -nakagami-rice probability density distribution. 7 4.2. effects of ssd on signal fading as regards the excursion of power variation, it was often observed that a few tens of dbm is not rare, especially when ionospheric tilts or large scale disturbance are present. as an example in this context, considering only the ssd, two almost consecutive 210 s sequences of signal, are shown in figure 8 (power vs. time). it is possible to note completely different fading characteristics. either periodicities or power excursion values can change rapidly during the observation time periods. the power variation imposed by the ionospheric disturbances causes multipath fading in an unpredictable way at different time intervals. it was however observed that short time periodicities are much more frequent compared to longer periodicities (figure 9). nevertheless, this does not exclude longer time periods compatible with mstid and lstid, but these were not considered in the present analysis. however, by and large, these results suggest that the observed power and periodicity excursions are potentially caused by small time-spatial scale disturbances compatible with ssd, as illustrated in table 2. finally, it is worth noting that this phenomenology is a characteristic of the normal ionosphere, that is it is not caused by high levels of the geomagnetic activity. 4.3. effects of ssd on coherence time the time variant propagation path affects the performances of systems that have to rely on stability of the ionospheric reflector for at least some tenths of a second. these include over the horizon radar (othr) and other remote sensing applications that exploit ionospheric reflections. some hf communication systems employing broader band techniques can also be affected by time propagation path variation. in other terms, these systems need to exploit the stability of the propagation path to establish the coherent integration time (cit). the vis technique also relies on this mathematical process, integrating the received echo signal for the necessary time under stable conditions [yau et al. 2006, bianchi et al. 2013a]. in order to determine stability periods, a software program [pietrella and zuccheretti 2010], capable to elaborate the data of the whole measurement campaign, was employed. this software uses a statistical parameter to identify time periods of maximum signal stability over time. the term “maximum ionospheric stability intervals” is used here to refer to periods of time during which the ionosphere does not exhibit significant dynamic changes, which means that the signal power will range within the values to be established according to the criteria described below. standard deviation proved to be an appropriate parameter for quantifying the dispersion of power values and altitude, since it is an index of dispersion, i.e. a measurement of variability of a random variable around a central value. therefore, standard deviation of power (vdbm) and of virtual height (vh') identify periods in which the observed variation data are not significant; in such cases the ionosphere can be considered stable. for each pvh data segments, subsets of different duration, from 10 to 210 s, can be selected and the standard deviation vdbm and vh' can be calculated. reduced vdbm and vh' values ionospheric dynamics observed at 3 mhz figure 7. fft spectral analysis (log of power density vs. frequency) of the (a) rayleigh and (b) nakagami-rice behavior shown in figures 4 and 5 respectively. (c) fft comparison between the rayleigh (continuous curve) and nakagami-rice (dashed curve) behavior obtained by doing a 100-values running mean of the values plotted respectively in (a) and (b). result in greatly reduced power and virtual height variations for the period considered. consequently, choosing the threshold values tdbm and th' , the conditions: (3) can be assumed as the criteria to identify the stability through time of the ionospheric propagation path. applying criterion (3) the maximum duration of stability were calculated [bianchi et al. 2013a]. in order to obtain an overall view, representative of the stability through time of the reflected ionospheric echoes, all the 24 hours of the entire measurement campaign were analyzed. as expected, it was found that the number of time periods of stability are generally less frequent at dusk and dawn compared to night and day. some relevant percentages of occurrence calculated by means of (3) are reported as a bar histogram for each hour in figure 10, which shows the results for stability over the time interval 10-30 s using a threshold of tdbm = 1 dbm and th' = 5 km. in this case the periods of stability occur with a percentage of occurrence ranged between 55-95 %. the analysis results carried out to identify longer stability time periods from 190 to 210 s are reported in figures 11 and 12. when this longer time period is required, the ionospheric layers rarely reveal a stability within the restrictive threshold of 1 dbm and 5 km (figure 11). by relaxing the threshold values to 5 dbm and 10 km, an higher percentage of occurrences ranged between 5-54 % is found (figure 12). it is worth noting that these kind of analyses are often considered for othr applications aimed to target detection. 5. conclusions the study described in this paper had three main objectives: to study the effect of small time-spatial scale irregularities on the ionospheric propagation path, multipath fading, and determination of the time stability characterizing the coherence time of the ionospheric reflectors. these three issues derive from the same unpredictable phenomena, which are the small scale disturbances propagating in the ionosphere. a first aspect that emerges from the analysis is the behavior of the ionospheric channel, which can show a rayleigh or a nakagami-rice trend, or a combination of them. the latter could be applicable for characterizing models for ionospheric propagation path simulators. another aspect is the analysis of the deep fading fluctuations due to ionospheric time-variant propagation path behavior. this permits the identification of a series of ionospheric characteristics varying in time and potentially related to the ssd phenomena. a third asandt tdbm dbm h h1 1v v l l^ ^h h baskaradas et al. 8 figure 8. power observations of two almost consecutive data segments of 210 s showing different fading levels and different periodicities recorded on january 14, 2008, at (a) 06:55 ut and (b) 07:55 ut respectively. figure 9. percentage of occurrence of the periodicities at the various time intervals. 9 pect is the determination of coherence time periods useful for remote sensing, radio, and oth radar in order to maximize the coherent integration time. the described results suggest that the propagation path trend, the multipath fading characteristics, and the time coherence of the ionospheric reflectors, are ionospheric dynamics observed at 3 mhz figure 10. percentages of occurrence of coherence for periods of 10-30 s calculated with the threshold values tdbm = 1 dbm and th' = 5 km. figure 11. percentage of occurrence of coherence for periods of 190-210 s calculated with the threshold values tdbm = 1 dbm and th' = 5 km. figure 12. percentage of occurrence of coherence for periods of 190-210 s calculated with the threshold values tdbm = 5 dbm and th' = 10 km. mainly associated with ssd, which drive all these phenomena. the ssd are not predictable at least in the time-spatial scale considered in table 2. the periodic analysis, limited to 512 s, showed that at this time scale of periodicity no pre-determined trend was observed and the fading follows a rayleigh or a nakagami-rice trend, or a combination of them, with a predominance of the mixed case. it has been found that the irregularities do not maintain a coherent motion for periods in the order of tens of seconds, while the prevalent periodicities are less than 25 s compatible with the ssd characteristics as illustrated in table 2, and that the ionospheric reflector is characterized by periods of stability of 10-30 s for all the 24 hours. references bianchi, c., u. sciacca, e. tabacco, a. zirizzotti and e. zuccheretti (2003). on the shape of reflecting surfaces investigated by a 60 mhz radar, int. j. remote sens., 24 (15), 3049-3058. bianchi, c., j.a. baskaradas, m. pietrella, u. sciacca and e. zuccheretti (2013a). power variation analysis of echo signals from ionospheric reflectors, adv. space res., 51 (5), 722-729; http://dx.doi.org/10.1016/j. asr.2012.09.045. bianchi, c., j.a. baskaradas, m. pezzopane, m. pietrella, u. sciacca and e. zuccheretti (2013b). fading in the hf ionospheric channel and the role of irregularities, adv. space res., 52 (3), 403-411; http:// dx.doi.org/10.1016/j.asr.2013.03.035. blanc, e. (1985). observations in the upper atmosphere of infrasonic waves from natural or artificial sources: a summary, annales geophysicae, 3, 673-688. crowley, g., and f.s. rodrigues (2012). characteristics of travelling ionospheric disturbances observed by the tiddbit sounder, radio science, 47, rs0l22; doi:10.1029/2011rs004959. james, h.g., r.g. gillies, g.c. hussey and p. prikryl (2006). hf fades caused by multiple wave fronts detected by a dipole antenna in the ionosphere, radio science, 41, rs4018; doi:10.1029/2005rs003385. krasnov, v.m., y.v. drobzheva and j. lastovicka (2006). recent advances and difficulties of infrasonic wave investigation in the ionosphere, surv. geophys., 27, 169-209; doi:10.1007/s10712-005-6203-4. leitinger, r., and m. rieger (2005). the tid model for modulation of large scale electron density models, annals of geophysics, 48 (3), 515-523. mcnamara, l.f. (1991). the ionosphere: communications, surveillance, and direction finding, krieger pub. co., 237 pp. nakagami, m. (1960). the m-distribution, a general formula of intensity distribution of rapid fading, in: w.c. hoffman (ed.), statistical methods in radio wave propagation, new york, pergamon press, 3-36. pietrella, m., and e. zuccheretti (2010). coerenza: a software tool for computing the maximum coherence times of the ionosphere, computers & geosciences, 36 (12), 1504-1511; doi:10.1016/j.cageo.20 10.03.020. reinisch, b.w., and i. galkin (2011). a global ionospheric radio observatory (giro), earth planets space, 63, 377-381. rice, s.o. (1944). mathematical analysis of random noise, bell systems technological journal, 23, 282-332. watterson, c.c., j.r. juroshek and w.d. bensema (1970). experimental confirmation of an hf channel model, ieee transaction communication technology, com-18, 792-803. yau, k.s.b., c.j. coleman and m.a. cervera (2006). investigation on fading of high frequency radio signals propagating in the ionosphere results from a jindalee radar experiment, in: 10th iet international conference on ionospheric radio systems and techniques, iet conference, publication issue cp517, 7-11. zuccheretti, e., g. tutone, u. sciacca, c. bianchi and j.b. arokiasamy (2003). the new ais-ingv digital ionosonde, annals of geophysics, 46 (4), 647-659. *corresponding author: umberto sciacca, istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy; email: umberto.sciacca@ingv.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. baskaradas et al. 10 << /ascii85encodepages 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice adg vol5 n02 toz 279_287.pdf annals of geophysics, vol. 45, n. 2, april 2002 279 detecting geomagnetic field nonlinearities by bispectral analysis and a phase coupling nonlinear technique roberta tozzi and angelo de santis istituto nazionale di geofisica e vulcanologia, roma, italy abstract the earth’s magnetic field varies over a wide range of characteristic times, say, from years to centuries, and more. in order to detect some nonlinear features of the geomagnetic field evolution we first apply a nonlinear spectral technique, i.e. bispectral analysis, to the secular variation of hartland geomagnetic observatory (u.k.). then, due to difficulties of bispectral analysis inherent in the shortness of data, we introduce a simpler, but more efficient, technique called spectral phase analysis for quadratic coupling estimation (spaqce). both nonlinear spectral methods here applied are based on the presence of a phase (sum and difference) coupling in case of quadratic interactions between two constituent components of the physical system underlying the generation of the geomagnetic field. the application of spaqce to annual means of a u.k. combined geomagnetic time series allows us to discriminate nonlinear interactions between couples of characteristic times (specifically 5-6, 5-8 and 2-26 years) of the field generated in the outer core. 1. introduction it is well known that to study a gaussian process and to completely describe its statistical properties, it is sufficient to use simple mathematical tools such as the autocorrelation function or the power spectrum. however, these methods are phase blind so, a general random additive process (for instance a coloured noise) cannot be completely discriminated from a random multiplicative process (such as nonlinear cascade process) or a deterministic nonlinear process (e.g., a chaotic phenomenon) throughout second order statistics or conventional fourier power spectrum. in the past, this lack of appropriate analytical tools has led scientists to treat non-gaussian pro-cesses (i.e. the greatest number of natural processes), as if they were gaussian. nevertheless, in recent decades, some powerful techniques have been developed to study nonlinear and non-gaussian phenomena. among them, an important role is played by the higher order spectral analysis (hosa) or, equivalently, higher order statistics (hos) (e.g., mendel, 1991). hosa is an extension of second order spectral analysis (i.e. conventional fourier analysis) to higher orders and is a technique to estimate higher order power spectra, the so-called polyspectra (subba rao and gabr, 1984). polyspectra discriminate nonlinear processes (i.e. processes characterised mailing address: dr. roberta tozzi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: tozzir@ingv.it key words bispectral analysis – nonlinearity – geomagnetic field secular variation 280 roberta tozzi and angelo de santis by a nonlinear relation between any perturbing input and the corresponding output of the system) because of their aptitude to reveal not only amplitude information, but also phase information. for instance, two random processes such as a gaussian and a non-gaussian process can be distinguished only by evaluating their third order spectrum (i.e. the bispectrum). in practice, considering that the bispectrum of a signal can be viewed in terms of a decomposition of the third statistical moment over frequency, it is easy to understand that the bispectrum of a gaussian process is identically zero (e.g., fig. 1b). on the contrary, that related to a non-gaussian process contains very interesting information such as the presence of some quadratic coupling. in this paper we introduce the basic concepts of bispectral analysis which, evidently, is the simpler case of hosa together with its attractive application to a situation of geophysical interest, namely that of the geomagnetic field secular variation (sv). due to the shortness of available geomagnetic data, the results from the application of the hosa are not always reliable. in order to give more robustness to our results we will introduce a simpler, but more efficient technique that allows a sharper and even more consistent discrimination of any quadratic coupling. in the light of these considerations, the intention of this work is to open a new way to verify and even to give a quantitative description of the geomagnetic field sv nonlinear properties. the main interest in this kind of analysis lies in the importance it would have for a better comprehension of the underlying dynamo processes that generate and sustain the field. of course, nothing excludes the possibility to apply the new technique to other kinds of geophysical data, since typically most of them are very short time series. the next sections of this paper are arranged in the following way. the second section defines bispectral analysis and describes the main aspects of the theory. the third section illustrates the application of this methodology to synthetic as well as to experimental geomagnetic data, namely the sv data deduced as annual means first differences; then it shows a simpler alternative to bispectral analysis in the case of short time series, as is the case of sv data. finally, in the last section we discuss the results provided by this technique. 2. bispectral analysis bispectral analysis is a powerful mathematical tool to reveal the presence of quadratic couplings between two harmonics (different or not) each associated with a given frequency. from introductory considerations it follows that the bispectrum provides interesting information only if the third moment (or skewness) of a time series is non-zero. recal-ling that skewness is connected to the asymmetry of the probability density function, we deduce that the bispectrum is a helpful tool to analyse only systems with asymmetric nonlinearities. if this condition is not satisfied, it is necessary to verify successive higher order spectra (i.e. the trispectrum); but higher order spectra are not here considered as the error of spectral estimation increases with the order of spectral analysis. auto-bispectrum b(f i , f j ) of a stationary time series (e.g., nayfeh and balachandran, 1995) defined in time interval t corresponds to a sort of two dimensional fourier spectrum and it is used to check out interactions between pairs of harmonics within a single time series; it is expressed as follows (2.1) where x(f) represents the fourier amplitude spectrum of the time series x(t) at frequency f and x * (f) is the complex conjugate; the < > brackets stand for an ensemble averaging over many statistically similar realisations of the process under scrutiny; in general, the bispectrum (from this point onwards we will omit the preceding term «auto») defined in eq. (2.1) is a complex quantity. evidently, b f fi j ,( ) = t x f f x f x f t i j i j * *= < +( ) ( ) ( ) >lim 1 281 detecting geomagnetic field nonlinearities by bispectral analysis and a phase coupling nonlinear technique evaluation of x( f ) makes use of the finite fourier transform algorithm, which takes into account the narrowness of the analysed series; in fact, when working with data coming from experimental measurements, we usually know x(t) only for a finite time t. in practical applications, it is customary to work only with the magnitude of the bispectrum that gives the strength of quadratic interaction, so graphic representation of the bispectrum is essentially a two-dimensional function in terms of an f i -f j diagram. it is quite evident that the corresponding contours contain a high degree of symmetry owing to symmetrical properties of the bispectrum defined in (2.1). it is possible to demonstrate (chandran and elgar, 1994) that the knowledge of the bispectrum in the principal domain (or non-redundant area), corresponding to the triangular region {f i , f j > 0}, is enough for its complete description. one of the methods to evaluate b(f i , f j ) is the socalled direct method based on a segment averaging approach. some studies (dalle molle and hinich, 1989) have shown that to calculate the nth order spectrum, the length of each segment should be about the (n-1)th root of the time series size. collis et al. (1998) showed that the variance associated with the bispectrum is strictly related to the power spectrum properties of the signal; for this reason instead of the bispectrum of eq. (2.1), another spectral function is used to detect more punctually quadratic interactions, the so-called auto-bicoherence or simply bicoherence. this function, analogous to the bispectrum from which it derives, is rated (normalised) to the power at each contributing frequency; thus, the bicoherence varies between zero and one, just like the square of the correlation coefficient in linear analysis. the main difference is that bicoherence is useful to detect quadratic phase coupling (qpc); the latter phenomenon occurs when two distinct harmonic components in a signal are nonlinearly related so that there is a phase correlation between components at frequencies f i and f j (macdonald, 1989). let us define the auto-bicoherence b2(f i , f j ) as the function that measures the fraction of power at frequency values of f = f i ± f j that is (2.2) from eq. (2.2), it is quite evident that, as already mentioned, bicoherence is a sort of normalised bispectrum and, in practice, it expresses frequency dependence of correlation. if b2( f i , f j ) = 1, there is a complete quadratic coupling between frequency components at fi and fj; if b 2( f i , f j ) = 0 there is no coupling at all while intermediate values of bicoherence correspond to partial quadratic coupling. 2.1. nonlinear coupling in synthetic data in order to verify the potentiality of bispectral analysis, we estimated the bicoherence for some synthetic data, properly generated with known linear and/or nonlinear quadratic couplings. consequently, we studied the time series representing white noise, nonlinear and chaotic sequences (i.e. lorentz, henon and logistic maps) and, of course, also periodic signals. these tests (here not completely presented) disclosed that this technique is really a good method not only to discriminate linear from nonlinear processes but also to estimate the periods of possible quadratically interacting modes. bicoherences obtained from white noise (fig. 1a) are practically flat (fig. 1b). one of these experiments involved a very simple function of time t in terms of a linear combination of two sine functions i(t) = a1sin(2 f1t) + a2sin(2 f2t) (2.3) where a 1 and a 2 are two constant coefficients and f 1 and f 2 are two different given frequencies (we chose 0.64 and 1.11 hz respectively). the function defined in (2.3) can be viewed as an input to a given system. if this system operates nonlinearly, its output contains nonlinear terms; b f fi j,( ) =2 b f f x f f x f x f i j i j i j , = ( ) < +( ) >< ( ) ( ) > 2 2 2 282 roberta tozzi and angelo de santis -0.4 -0.2 0.0 0.2 0.4 -0.4 -0.2 0.0 0.2 0.4 white noise f 1 (hz) f 2 ( h z ) 0.1750 - 0.2000 0.1500 - 0.1750 0.1250 - 0.1500 0.1000 - 0.1250 0.0750 - 0.1000 0.0500 - 0.0750 0.02500 -0.0500 0 - 0.02500 0 25 50 75 100 125 150 175 200 -3 -2 -1 0 1 2 3 w hi te n oi se t (s) fig. 1a,b. a) white noise (arbitrary amplitudes); b) the bicoherence of a white noise is a practically flat twodimensional function. a b fig. 2a,b. a) a period of the function i(t)2; b) bicoherence of the function i(t)2 obtained from i(t) defined in eq. (2.3) with a 1 = a 2 = 1, f 1 = 0.64 hz and f 2 = 1.11 hz. this is the case of a short data set (128 values); it shows that bicoherence gives a good estimation of the actual coupling frequencies although it is characterised by a large variance. -4 -2 0 2 4 -4 -2 0 2 4 (0.77,1.15)hz bicoherence (128 data) f 1 (hz) f 2 ( h z ) 0.1225 - 0.1400 0.1050 - 0.1225 0.0875 - 0.1050 0.0700 - 0.0875 0.0525 - 0.0700 0.0350 - 0.0525 0.01750 - 0.0350 0 - 0.01750 0 10 20 30 40 50 60 0 1 2 3 4 i ( t )2 t (s)a b 283 detecting geomagnetic field nonlinearities by bispectral analysis and a phase coupling nonlinear technique for example if the system squares the input, then the output, o(t) = i 2(t), includes three quadratic terms. a conventional (i.e. second order) power spectrum of the function i 2(t) shows four peaks (corresponding to the two interacting frequencies f1, f2 together with their sum and difference frequencies f 1 ± f 2 ), while the plot of bicoherence displays clearly the quadratic coupling between the two modes with frequencies f 1 and f 2 appearing in eq. (2.3); this result is illustrated in figs. 2b and 3 for the cases of two data sets of 128 and 1024 values, respectively. the comparison of the results from the two analyses shows that bicoherence indeed reveals nonlinear couplings but that the length of the time series is a very critical point. in fact, in the former case the relative error is around 20%, while using more data it decreases to about 2%. this is the reason why in the next paragraph we will propose a method that is particularly suitable for scanty series. of course, this is only a simple example to demonstrate that through the observation of bicoherence, quadratic phase couplings can be discriminated. the application of the method to synthetic data of different origin has shown that the bicoherence of signals associated with linear processes is always flat, whereas signals characterising nonlinear processes may produce both flat and non-flat bicoherence depending only on the presence of quadratic couplings. this means that a flat bicoherence may come from the signal analysis of both linear and nonlinear processes, but if bicoherence is not flat then this is necessarily due to the presence of quadratic phase couplings in the signal, typical of a nonlinear process. -4 -2 0 2 4 -4 -2 0 2 4 (0.63, 1.12) hz bicoherence (1024 data) f 1 (hz) f 2 ( h z ) 0.1225 - 0.1400 0.1050 - 0.1225 0.0875 - 0.1050 0.0700 - 0.0875 0.0525 - 0.0700 0.0350 - 0.0525 0.01750 - 0.0350 0 - 0.01750 fig. 3. bicoherence of the function i(t)2 obtained from i(t) defined in eq. (2.3) with a1 = a2 = 1, f1 = 0.64 hz and f2 = 1.11 hz. this is the case of a longer data set (1024 values); it shows that in this situation bicoherence gives a better estimation of the quadratic frequency coupling with a smaller variance than the case of fig. 2b. 284 roberta tozzi and angelo de santis 2.2. spectral phase analysis for quadratic coupling estimation synthetic tests showed that bispectral analysis might become inaccurate when analysing short time series, therefore to gain in faithfulness we developed a different spectral method. in the following, we will call it spectral phase analysis for quadratic coupling estimation or briefly spaqce, and use it essentially to verify information provided by hosa. this is a nonlinear technique in the common sense that it is a technique suitable to detect nonlinear properties of time series. spaqce is a very direct and, in some ways, crude method that mainly applies the basic concept underlying qpc. we know that a qpc occurs when two characteristic processes are coupled by a nonlinear interaction producing a new signal characterised by a phase that is simply the sum and the difference of the phases of the original signals. in practice, we implemented a fortran algorithm able to compare, for each pair of frequencies, the sum and the difference of their phases with the phases corresponding to the sum and difference of these two starting frequencies, this algorithm can also keep note of the corresponding spectral amplitudes. an example of a quadratic coupling in a short synthetic time series (composed of 128 data) produced by multiplying two sines with slightly different periods (5.5 and 8.5 years) is illustrated in fig. 4. our software returns two series of data: one for the sum of the phases (white circles) and one for the difference (black squares). only points common to both groups of data correspond to a quadratic coupling. an appropriate choice of a threshold in the spectral amplitudes discloses only the most significant phase coincidences. practically this threshold is an appropriate percentage of all spectral amplitudes as appears in figs. 4 and 7. as fig. 4 shows, this method gives good results even for 0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 synthetic signal spectral amplitude threshold = 10% (9.2,5.5) year sum difference f 2 ( y e a r1 ) f 1 (year -1 ) fig. 4. the method of spaqce applied to a short synthetic signal composed of 128 data and consisting of a product of two sines with periods of 5.5 and 8.5 years respectively. this figure evidences the quadratic phase coupling in terms of overlapping between black squares (representing couplings in the differences of couples of frequencies) and white circles (representing couplings in the sums of couples of frequencies). this diagram is based on the largest 10% spectral amplitudes. 285 detecting geomagnetic field nonlinearities by bispectral analysis and a phase coupling nonlinear technique short time series and the relative error associated with this kind of procedure decreases to around 8% against 20% of hosa. 3. the case of geomagnetic field from magnetohydrodynamic equations, it follows that the geomagnetic field enjoys deterministic and nonlinear properties. as a matter of fact, some recent studies (e.g., barraclough and de santis, 1997; hongre et al., 1999) have shown some evidence that geomagnetic data contain clear nonlinearities. in the preceding section, we observed that hosa is a very robust method to detect nonlinearities but it needs many data, which is not the case of annual mean datasets. nevertheless, we can apply the technique of spaqce; with this kind of analysis we are, in principle, able to observe nonlinear couplings between the different harmonics components into which a signal could be split. therefore, we have estimated the bicoherence and applied the spaqce technique as a confirmation to hosa results to geomagnetic data. analysed data consists of annual means of the y component of the secular variation of the geomagnetic field measured in the u.k.; they were formed by a combination of greenwich, abinger and hartland annual means obtained by comparing the annual means from three successive relocations of a british observatory (the most recent at around 51°n, 355°e), properly connected to coincide at their short overlapping running periods. hereafter we will refer to this combination of time series simply as hartland observatory data, i.e. with the name of the most recent observatory. hartland observatory is characterised by a long first-rate tradition in monitoring the geomagnetic field. therefore, the selection of this observatory was made on the basis of the quality of data; in particular we have taken into account of the apparent low degree of noise (statistically determined) and of the significant length of time series if compared with most observatories. figure 5 shows the firstdifferences of y component annual means, it is quite evident the potential presence of larger experimental errors for years preceding 1870; this is the reason why we have analysed only the last 128 years (1871.5 to 1999.5) of the combined time series. considering that the aim of this work is to study eventual nonlinear properties of the earth’s magnetic field of internal origin in terms of its secular variation, we have given particular relief to the y component alone because, among the three geomagnetic components, it is the least dependent on the variations of the field of external origin. this fact can be explained expressing the y component in terms of spherical harmonics (sh). it is a matter of fact that the magnetic effect of the solar activity perturbation can be represented as a combination of zonal harmonics and they are not contained in the sh expression of y. before starting with the bicoherence estimation, we checked the value of time series skewness verifying that is effectively non-zero. equation (2.2) contains averaging over many statistical realisations of the same process. since we have only one record of data, we replaced this average by the average on shorter segments of the time series (i.e. using the direct method). therefore, after preliminary operations to obtain reliable data to be analysed with bispectral analysis we performed bicoherence for sv time series. bicoherences of y component shows very interesting results; a sharp peak corresponding to high values of bicoherence are clearly 1825 1850 1875 1900 1925 1950 1975 2000 -4 -3 -2 -1 0 1 2 hartland sv y [ n t /y e a r] year fig. 5. secular variation of the y component of the geomagnetic field measured at greenwich-abingerhartland evaluated by first-differentiation. 286 roberta tozzi and angelo de santis -0.4 -0.2 0.0 0.2 0.4 -0.4 -0.2 0.0 0.2 0.4 (7,5) year 0.2625 - 0.3000 0.2250 - 0.2625 0.1875 - 0.2250 0.1500 - 0.1875 0.1125 - 0.1500 0.0750 - 0.1125 0.0375 - 0.0750 0 - 0.0375 f 1 (year -1 ) f 2 ( y e a r1 ) bicoherence hartland observatory fig. 6. estimation of bicoherence for the y component of geomagnetic field sv. it returns a quadratic coupling between 7 and 5 years. fig. 7. the method of spaqce applied to the y component of geomagnetic field sv from the combination of u.k. observatories reduced at hartland location: three quadratic phase couplings are quite evident. qpc emerges from the superimposition of black squares and of white circles representing evidences of quadratic couplings in terms of differences and sums of couples of frequencies, respectively. the diagram is based on the 25% largest spectral amplitudes. 0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 (2.4,25.6) year (5.5,8.5) years (5.1,5.8) years hartland observatory spectral amplitude threshold = 25% sum difference f 2 ( y e a r1 ) f 1 (year -1 ) 287 detecting geomagnetic field nonlinearities by bispectral analysis and a phase coupling nonlinear technique observed in fig. 6. according to the interpretation of bispectral analysis, this peak corresponds to a quadratic coupling between two periodic signals with periods of about 5 and 7 years. results very similar to hosa are obtained considering spaqce technique that returns some quadratic couplings (fig. 7), one of which is between two components with periods of 5.5 and 8.5 years. we consider the latter results more reliable on the basis of the estimated relative errors. 4. conclusions the importance of this work lies in the consideration that bispectral analysis is a formal method that discloses out the presence of nonlinearities in a time series. it is frequently applied in physics with successful results (e.g., hidalgo et al., 1995), but it has rarely been used to study geophysical phenomena (e.g., clark and bergin, 1997), especially those related to the geomagnetic field. the most important fact we deduce from our work is that bispectral analysis is a powerful method to detect nonlinearities in a sufficiently long time series. however, it may suffer, like other nonlinear techniques, from the shortness of time series such as the geomagnetic datasets. to overcome this problem we proposed a simple but more efficient nonlinear technique such as spaqce that gives results with a lower relative error. in fact, from the application of the method to synthetic data, quadratic couplings were always detected, thus confirming the nonlinearity of the investigated sv time series. in particular, the application of spaqce to annual means of a u.k. combined geomagnetic time series shows quadratic interactions between pairs of characteristic times, specifically 5-6, 5-8 and 2-26 years, providing important clues for the physical mechanisms generating the magnetic field in the outer core. certainly to reach more accurate results, more data and analyses are needed, and this will be the next step of this work, together with the search for the physical origin of the quadratic coupling in y found here. at the moment, we can only make a suggestive hypothesis: for example it may be related to the variation of the length of the day, whose decadal variation is in turn probably correlated to electromagnetic and mechanic couplings between core and mantle. another interesting hypothesis that would require more investigation is that of possible effects due to the differential rotation of the inner core. acknowledgements we thank drs. g.p. gregori, v. korepanov and prof. a. rivola for their valuable suggestions and comments, as well as drs. g. consolini and p. de michelis for some discussion on the nonlinear characteristics of the geomagnetic field. references barraclough, d.r. and a. de santis (1997): some possible evidence for a chaotic geomagnetic field from observational data, phys. earth planet. inter., 99, 207-220. chandran, v. and s. elgar (1994): a general procedure for the derivation of principal domains of higher-order spectra, ieee trans. sign. proc., 42, 229-233. clark, r.r. and j.s. bergin (1997): bispectral analysis of mesosphere winds, j. atmos. sol.-terr. phys., 59, 629-639. collis, w.b., p.r. white and j.k. hammond (1998): higher-order spectra: the bispectrum and trispectrum, mech. sys. sign. proc., 12, 375-394. da l l e mo l l e, j.w. and m.j. hi n i c h (1989): the trispectrum, in proceedings of the workshop of higher order spectral analysis, 68-72. hidalgo, c., r. balbín, b. brañas, t. estrada, i. garcía-cortés, m.a. pedrosa, e. sánchez and b. van milligen (1995): nonlinear phenomena and plasma turbulence in fusion plasmas, phys. scr., 51, 624-626. hongre, l., p. sailhac, m. alexandrescu and j. dubois (1999): nonlinear and multifractal approaches of the geomagnetic field, phys. earth planet. inter., 110, 157-190. nayfeh, a.h. and b. balachandran (1995): applied nonlinear dynamics (john wiley & sons inc.), pp. 685. macdonald, g.j. (1989): spectral analysis of time series generated by nonlinear processes, rev. geophys., 27, 449-469. mendel, j.m. (1991): tutorial on higher-order statistics (spectra) in signal processing and system theory: theoretical results and some applications, proc. ieee, 79, 278-305. subba rao, t. and m.m. gabr (1984): an introduction to bispectral analysis and bilinear time series models, in lecture notes in statistics, edited by d. brillinger , s. fienberg, j. gani, j. hartigan and k. krickeberg (springer-verlag), pp. 280. g0218 annals of geophysics, 60, 2, 2017, g0218; doi: 10.4401/ag-7129 global nonlinear optimization for the interpretation of source parameters from total gradient of gravity and magnetic anomalies caused by thin dyke arkoprovo biswas1,*, mahesh prasad parija1, sushil kumar1 1 wadia institute of himalayan geology (wihg), dehradun, uttarakhand, india article history received august 5, 2016; accepted december 12, 2016. subject classification: gravity and magnetic anomaly, dyke-type structure, vfsa, uncertainty estimation, ore exploration. abstract an efficient approach to estimate model parameters from total gradient of gravity and magnetic data based on very fast simulated annealing (vfsa) is presented. this is the first time vfsa has been applied in interpreting total gradient of potential field data with a new formulation estimation caused due to isolated causative sources embedded in the subsurface. the model parameters interpreted here are the amplitude coefficient (k), exact origin of causative source (x0) depth (z0) and the shape factors (q). the results of vfsa optimization show that it can uniquely determine all the model parameters when shape factor is controlled to its actual value. the model parameters estimated by the present method, mostly the shape and depth of the buried structures were found to be in excellent agreement with the actual parameters. the method has also the proficiency of evading highly noisy data points and improves the interpretation results. study of histogram and cross-plot analysis also suggests the interpretation within the estimated uncertainty. inversion of noise-free and noisy synthetic data for single structures as well as field data demonstrates the efficacy of the approach. the technique is warily and effectively applied to real data examples (leona anomaly, senegal for gravity, pima copper deposit, usa and matheson area, northern ontario, canada for magnetic/aeromagnetic data) with the presence of ore bodies. the present method can be extremely applicable for mineral exploration or ore bodies of dyke-like structure embedded in the shallow and deeper subsurface. the computation time for the whole process is very small. 1. introduction gravity and magnetic surveying has been extensively used over the years to map regional geological structures, basin researches, especially through the reconnaissance and analysis of anomalies. moreover, it has been widely used in different branches of earth science studies such as mineral exploration, hydrogeological, environmental studies, geodesic, seismological studies, isostatic compensation, subsurface cavity detection, archaeo-geophysics, glacier thicknesses, subsurface modelling and engineering applications as well [telford et al. 1990, reynolds 1997, kearey et al. 2002, jacoby and smilde 2009, hinze et al. 2013]. the idea is based on measuring the variations in the earth’s gravitational as well as magnetic field due to the effects of anomalous density and magnetic susceptibility differences between the subsurface rocks [al-garni 2013, ekinci et al. 2013, pallero et al. 2015, ekinci and yiğitbaş 2015]. within those investigations cited above, mineral or ore explorations take a significant place because of the economic reasons. evaluation of the depth of such anomalous buried bodies from the gravity, magnetic and other geophysical or multi-parametric data has drawn significant consideration in exploration of minerals [mandal et al. 2015, 2013, biswas et al. 2014a, b]. the isolated gravity and magnetic anomaly due to single ore body is commonly interpreted in terms of few model parameters such as location, source geometry, depth and shape [roy et al. 2000, essa 2007, biswas 2015, biswas 2016b]. to interpret the different parameters of the estimated structure, numerous interpretation methods was developed. assuming a fixed simple geometry, various methods was developed for determining some model parameters of the gravity and magnetic sources. the techniques include graphical methods [nettleton 1962, 1976], curves matching standardized techniques [gay 1963, 1965, mcgrath and hood 1970], monograms [prakasa rao et al. 1986], characteristic points and distance approaches [grant and west 1965, abdelrahman 1994], ratio methods [bowin et al. 1986, abdelrahman et al. 1989], neural network [elawadi et g0218 biswas et al. 2 al. 2001], fourier transform [odegard and berg 1965, bhattacharyya 1965, sharma and geldart 1968], euler deconvolution [thompson 1982], mellin transform [mohan et al. 1986], hilbert transforms [mohan et al. 1982], least squares minimization approaches [gupta 1983, silva 1989, mcgrath and hood 1973, lines and treitel 1984, abdelrahman 1990, abdelrahman et al. 1991, abdelrahman and el-araby 1993, abdelrahman and sharafeldin 1995a], werner deconvolution [hartmann et al. 1971, jain 1976, kilty 1983]; walsh transformation [shaw and agarwal 1990], continual least-squares methods [abdelrahman and sharafeldin 1995b, abdelrahman et al. 2001a, b, essa 2012, 2013], euler deconvolution method [salem and ravat 2003], fair function minimization procedure [tlas and andasfahani 2011a, asfahani and tlas 2012], dexp method [fedi 2007], deconvolution technique [tlas and asfahani 2011b]; regularised inversion [mehanee 2014, mehanee and essa 2015]; simplex algorithm [tlas and asfahani 2015], simulated annealing methods [gokturkler and balkaya 2012], very fast simulated annealing [biswas and acharya 2016, biswas and sharma 2016a, b; biswas 2015, biswas and sharma 2015, biswas and sharma 2014a, b, sharma and biswas 2013a], particle swarm optimization [singh and biswas 2016] and differential evolution [ekinci et al. 2016] have been used to solve similar kind of non-linear inversion problems for different types of subsurface structures. also, there are different interpretation methods for gravity and magnetic data that can be found in different literatures [abdelrahman et al. 2015, 2012, 2009, 2007, 2006, 2005, 2003, 1996, 1994, asfahani and tlas 2007, 2004, tlas et al. 2005]. amongst several interpretation methods mentioned above, mostly inverse modelling processes aim to best appraisal of the model parameters whose responses are similar to the measured data. in this way, the fittings between the observed and modeled anomalies can be investigated. however, the well-known non-unique, non-linear problem and ill-posed nature of the potential field data inversion makes the processing and interpretation rather difficult. hence, the inverse modelling problem of potential field anomalies strongly require some constraints in order to recover interpretable and realistic model solutions [last and kubik 1983, li and oldenburg 1996, 1998, ekinci 2008, zhdanov 2009, feng et al. 2014, biswas 2015, biswas 2016, ekinci et al. 2016]. however, in most of the cases, the measured potential field anomaly was interpreted for residual anomalies. the objective of the present work is to develop an integrated approach for quantitative interpretation of gravity and magnetic fields over dyke like structure fulfilling laplace’s condition. this procedure is based on the calculation of first order horizontal and vertical derivatives of the observed gravity and magnetic anomaly. the square root of the sum of the squares of these derivatives [horizontal and vertical] is called as total gradient [tg] and is identical with the amplitude of the analytical signals [nettleton 1971, nabighian 1972, nabighian et al. 2005a, b]. a detailed explanation about the tg is explained in appendix i. further, to develop the method, a variant of simulated annealing [sa], called as very fast simulated annealing [vfsa] is used to determine the various model parameters related to thin dyke type structures for tg of gravity and magnetic anomalies. this algorithm has a competence to escape local minima by performing a stochastic search within the model space and does not require well-constructed initial model providing a robust and versatile search processes without negotiating the resolution [sen and stoffa 2013, sharma and kaikkonen 1998, 1999a, b, sharma and biswas 2011, sharma 2012, sharma and biswas 2013a, b, biswas and sharma 2015, biswas and sharma 2016a, b, biswas, 2016a] and is used in interpreting the tg of gravity and magnetic anomaly data. the application of the proposed technique is performed with the help of synthetic data and two examples from leona anomaly, south saint-louis, western coastline, senegal and pima copper deposit, arizona, usa from ground based survey and one magnetic anomaly from matheson area, northern ontario, canada from aeromagnetic survey. the method can be used to interpret the gravity and magnetic anomalies occurred due to thin dyke-type mineralized bodies. 2. methodology 2.1. mathematical formulation for forward modeling the general expression of a tg of gravity and magnetic anomaly v(x) for thin dyke at any point on the surface (figure 1) is given by the equations [after abdelrahman et al. 2001a, b, srivastava et al. 2014]: (1) where, k is the amplitude coefficient, z is the depth from the surface to the top of the body (thin dyke), x0 (i = 1,…,n) is the horizontal position coordinate, q is the shape factor. the q value for gravity and magnetic anomaly is 0.5 and 1.0 respectively. the detailed derivation of mathematical formulations can be found in nettleton [1971], nabighian [1972], srivastava and agarwal [2010], srivastava et al. [2014]. for brevity, v(x) =k[ 1 [(x x0 ) 2 +(z)2 ]q ] vfsa in gravity and magnetic anomaly 3 the derivation is not discussed here and is shown in the appendix ii. for multiple structures, the equation can be written as [biswas and sharma, 2014a]: (2) where vj (xi) is the gravity or magnetic anomaly at xi location for jth body and m is the number of bodies. 2.2. inversion method: very fast simulated annealing global optimization different conventional least-squares approaches are mainly used for potential field inverse problems. however, in present days, the problems in least-square approaches were overcome by metaheuristic algorithms which do not require good initial estimates to reach the global minimum. such metaheuristic algorithm or now-a-days the global optimization methods such as simulated annealing, genetic algorithms, artificial neural networks, particle swarm optimization and differential evolution have been used in various geophysical data sets [e. g., rothman 1985, 1986, dosso and oldenburg 1991, sen and stoffa 2013, sharma and kaikkonen 1998, 1999a, b, zhao et al. 1996, juan et al. 2010, sharma and biswas 2011, sharma 2012, sharma and biswas 2013a, b, biswas and sharma 2014a, biswas and sharma 2014b, biswas and sharma 2015, biswas 2015, singh and biswas 2016, ekinci et al. 2016]. the basic idea of very fast simulated annealing (vfsa) is a global optimization method; the process of chemical thermodynamics where heating a solid in a heat bath and then slowly allowing it to cool down and anneal into a state of minimum energy. the main advantage of vfsa over other methods is its flexibility and its ability to approach global optimality. it has the ability to avoid becoming trapped in local minima, high resolution, and fast computation as well as less memory [ingber and rosen, 1992]. the main difference between sa and vfsa is the faster cooling schedule in vfsa due to a sharper cauchy probability distribution for the random selection of model parameters. further, sa takes samples at the predefined interval that limits the model resolution, while vfsa can take any value in the model space and increase resolution. further, vfsa does not remember all models in the optimization process, and hence needs very small memory. further detailed explanation can be found in various literatures [sharma and biswas 2011, sen and stoffa 2013, sharma and biswas 2013a]. the same principal is used in geophysical inversion which aims to minimize an objective function called error function or the misfit. the error function is analogous to the energy function in a way that error function is directly proportional to the degree of misfit between the observed data and the modeled data. in the present study, the misfit (φ) between the observed and model response is used for potential field data interpretation [after sharma and biswas 2013a]. (3) where n is number of data point, vi 0 and vi c are the ith observed and model responses and v0max and v0min are the maximum and minimum values of the observed response respectively. the details of the inversion process can be found in different literatures such as sen and stoffa [2013], sharma [2012] and sharma and biswas [2013], biswas [2015], biswas [2013]. in the present vfsa optimization process, parameters such as initial temperature 1.0, cooling schedule 0.4, number of iterations 2000 and number of moves per temperature 50 is used in the present study. next, to find the global model, probability density function (pdf) and uncertainty analysis, it has been done based on the procedures established by mosegaard and tarantola [1995], sen and stoffa [1996]. this code is developed in window 7 environment using ms fortran developer studio on a simple v(xi)= vjj=1 m ∑ (xi) ϕ = 1 n vi 0 −vi c |vi 0 |+(vmax 0 −vmin 0 )/2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ i=1 n ∑ 2 figure 1. a diagram showing cross-sectional views, geometries and parameters for thin dyke-type structure. biswas et al. 4 figure 2. convergence pattern for various model parameters and misfit for (a) gravity data and (b) magnetic data. figure 3. gravity data: (a) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is uncontrolled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (10% random) when q is uncontrolled for thin dyke-model 1, (c) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is uncontrolled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (20% gaussian) when q is uncontrolled for thin dyke-model 2. figure 4. gravity data: (a) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is controlled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (10% random) when q is controlled for thin dyke-model 1, (c) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is controlled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (20% gaussian) when q is controlled for thin dyke-model 2. vfsa in gravity and magnetic anomaly 5 desktop pc with intel core i7 processor. for each step of optimization, a total of 106 forward computations (2000 iteration×50 number of moves×10 vfsa runs) are performed and accepted models stored in memory. the total time taken to complete a single inversion is 35 seconds. 3. results and discussion 3.1 synthetic example the vfsa global optimization is used considering noise-free and noisy synthetic data (10% random noise and 20% gaussian noise) for gravity and magnetic anomaly over a thin dyke-type model. initially, all model parameters are optimized for each data set. the general interpretation for both gravity and magnetic method using vfsa is applied for all synthetic and field examples. at first, synthetic data is generated using eq. (1) for a dyke-model and 10% random and 20% gaussian noise is added to the synthetic data. vfsa inversion is employed using noise-free and noisy synthetic data to recover the actual model parameters and study the effect of noise on the interpreted model parameters. principally, a suitable search range for each model parameter is selected and a single vfsa optimization is executed. afterward the proper convergence of each model parameter is studied (k, x0, z, and q) and misfit by adjusting vfsa parameters (such as initial temperature, cooling schedule, number of moved per temperature and number of iterations). next, to access the reliability of the method and to get mean model, 10 vfsa runs are performed. then, histograms are prepared using accepted models whose misfit is lower than10-4. next, a statistical mean model was computed using models that have misfit lower than 10-4 and lie within one standard deviation. moreover, cross-plots are also studied to check whether the model parameters arewithin the high pdf region (one standard deviation). also, comparison between the observed and model data is shown for each model. this method is followed for every synthetic and field example. 3.1.1 model 1 (gravity model with 10% random noise) inversion of the gravity data is implemented as mentioned above using noise free and noisy synthetic data. figure 2a shows the convergence pattern for all model parameters. figure 3a shows the histogram for all model parameters (k, x0, z, and q). the histogram reveals that the location of the body can be well resolved after inversion. however, there is a slight wide range in the other parameters. in the next step, since the shape factor q shows near its actual value, it is set to its actual value and the inversion procedure is repeated again. figure 4a shows that the histogram shows a definite peak at the actual value and all the parameters are well resolved. the cross-plots analysis (figure 5a) also shows that there is a wide range in the other model parameter. after controlling q to its actual value, the model parameters are very close to its actual value (figure 5b). the fittings between the observed and model data are shown in figure 6a. the interpreted parameters and mean model is shown in table 1. next, 10% random noise is added to the data and the procedure is repeated to check the effect of noise. figure 3b shows the histogram when q is uncontrolled and figure 4b shows the histogram when q is controlled. analysis of cross-plots (figure 5c and d) also suggests the effect of noise added in the data however, the estimated model parameters are within the uncertainty limits and within high pdf. table 1 shows the interpreted mean model for noisy data. a fitting between the observed and model data for noisy model is shown in figure 6b. 3.1.2 model 2 (gravity model with 20% gaussian noise) additional synthetic data for a dyke model (table 2) and 20% gaussian noise is also added to the synthetic data to check the effect of more noise. inversion is implemented using noise-free and noisy synthetic data to retrieve the actual model parameters and study the effect of higher noise on the interpreted model parameters. figure 3c and d shows the histogram of noise free synthetic and noisy data model parameters actual value search range mean model (noise-free) mean model (noisy data) q uncontrolled q controlled q uncontrolled q controlled k(mgalxm) 1000 0-2000 1001.6±10.1 1000.3±2.3 1002.9±34.4 982.5±9.7 x0 (m) 200 0-500 200.0±0.0 200.0±0.0 199.9±0.2 199.9±0.3 z (m) 15 0-30 15.0±0.1 15.0±0.0 14.7±0.3 14.6±0.3 q 0.5 0-2 0.50±0.0 0.50 (fixed) 0.50±0.0 0.50 (fixed) misfit 4.2x10-8 2.6x10-8 1.3x10-3 1.3x10-3 table 1 actual model parameters, search range and interpreted mean model for noise free, 10% random noise with uncertainty-gravity data (model 1). biswas et al. 6 when q is uncontrolled. figure 4c and d shows the histogram of noise free synthetic and noisy data when q is controlled. cross-plots also suggest the same as shown in model 1 and for brevity, it is not presented here. fittings between the observed and model response for this noise free and noisy model is shown in figure 6c and d. 3.1.3 model 1 (magnetic model with 10% random noise) inversion of the magnetic data is executed as mentioned above using noise free and noisy synthetic data. figure 2b shows the convergence pattern for all model parameters. figure 7a shows the histogram for all model parameters (k, x0, z, and q). the histogram reveals that the location of the body can be well resolved after vfsa inversion. however, there is a slight wide range in the other parameters such as k. hence, in the following step, the shape factor q is controlled to its actual value and the inversion procedure is repeated again. figure 8a shows that the histogram shows a definite peak at the actual value and all the parameters are well resolved. the cross-plots analysis (figure 9a) also shows that there is a wide range in the other model parameter. after constraining q the model parameters are very close to its actual value (figure 9b). the fittings between the observed and model data are shown in figure 10a. the interpreted parameters and mean model is shown in table 1. next, 10% random noise is added to the data and the procedure is repeated to check the effect of noise in magnetic data as well. figure 7b shows the histogram when q is free and figure 8b shows the histogram when q is controlled. cross-plots also suggest the same as shown in model 1 (gravity data) and for brevity, it is not presented here, and however, it is also within the uncertainty model parameters actual value search range mean model (noise-free) mean model (noisy data) q uncontrolled q controlled q uncontrolled q controlled k(mgalxm) 5000 0-8000 4986.5±96.4 4996.4±12.6 3740.9±162.3 4765.3±37.8 x0 (m) 250 0-500 250.0±0.1 250.0±0.0 250.0±0.2 250.0±0.4 z (m) 25 0-50 24.9±0.2 25.0±0.1 22.2±0.4 24.5±0.4 q 0.5 0-2 0.50±0.0 0.50 (fixed) 0.47±0.0 0.50 (fixed) misfit 5.3x10-8 2.8x10-8 5.5x10-3 5.5x10-3 table 2. actual model parameters, search range and interpreted mean model for noise free, 20% gaussian noise with uncertainty-gravity data (model 2). model parameters actual value search range mean model (noise-free) mean model (noisy data) q uncontrolled q controlled q uncontrolled q controlled k(nt) 800 0-1000 786.4±29.4 800.0±3.6 964.0±56.8 795.5±16.6 x0 (m) 200 0-500 200.0±0.0 200.0±0.0 199.9±0.2 199.9±0.2 z (m) 10 0-20 9.9±0.1 10.0±0.0 10.3±0.2 9.9±0.2 q 1.0 0-2 1.0±0.0 1.0 (fixed) 1.0±0.0 1.0 (fixed) misfit 4.1x10-8 3.9x10-10 1.7x10-4 1.6x10-4 table 3. actual model parameters, search range and interpreted mean model for noise free, 10% random noise with uncertainty-magnetic data (model 3). model parameters actual value search range mean model (noise-free) mean model (noisy data) q uncontrolled q controlled q uncontrolled q controlled k(nt) 400 0-800 409.5±30.6 399.9±1.4 635.26±60.2 385.1±7.1 x0 (m) 250 0-500 250.0±0.1 250.0±0.0 250.1±0.2 250.1±0.3 z (m) 30 0-50 30.1±0.3 30.0±0.1 31.4±0.4 29.5±0.4 q 0.5 0-2 1.0±0.0 1.0 (fixed) 1.1±0.0 1.0 (fixed) misfit 4.7x10-7 1.3x10-8 3.6x10-3 3.6x10-3 table4. actual model parameters, search range and interpreted mean model for noise free, 20% gaussian noise with uncertainty-magnetic data (model 4). vfsa in gravity and magnetic anomaly 7 limits and within high pdf. table 3 shows the interpreted mean model for noisy data. a fitting between the observed and model data for noisy model is shown in figure 10b. figure 6. gravity data: fittings between the observed and model data for thin dyke: model 1(a) noise-free synthetic data and (b) 10% random noisy synthetic data, and model 2(c) noise-free synthetic data and (d) 20% gaussian noisy synthetic data. figure 5. gravity data: (a) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-4 for noise-free data) (blue), and models with pdf >60.65% (red) for noise free data when q is uncontrolled; (b) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-4 for noise-free data) (blue), and models with pdf >60.65% (red) for noise free data when q is controlled; (c) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for noisy data) (blue), and models with pdf >60.65% (red) for noisy data when q is uncontrolled; (d) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for noisy data) (blue), and models with pdf >60.65% (red) for noisy data when q is controlled. figure 7. magnetic data: (a) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is uncontrolled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (10% random) when q free for thin dyke-model 1, (c)histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is uncontrolled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (20% gaussian) when q is uncontrolled for thin dyke-model 2. figure 8. magnetic data: (a) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q is controlled (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (10% random) when q fixed for thin dyke-model 1, (c) histograms of all accepted models having misfit<10-4 for noise-free synthetic data when q fixed (b) histograms of all accepted models having misfit<10-2 for noisy synthetic data (20% gaussian) when q is controlled for thin dyke-model 2. biswas et al. 8 3.1.4 model 2 (magnetic model with 20% gaussian noise) another synthetic data for a dyke model (table 4) and 20% gaussian noise is also added to the synthetic data to check the effect of more noise. inversion is implemented using noise-free and noisy synthetic data to retrieve the actual model parameters and study the effect of higher noise on the interpreted model parameters. figure 7c and d shows the histogram of noise free synthetic and noisy data when q is uncontrolled. figure 8c and d shows the histogram of noise free synthetic and noisy data when q is controlled. cross-plots also suggest the same as shown in model 1 and are shown in figure 9c and d to check the effect of higher amount of noise in the data. the estimated model parameters for 20% gaussian noisy data also reveals that the interpreted parameters are within the estimated uncertainty limits and high pdf. fittings between the observed and model response for this noise free and noisy model is shown in figure 10c and d. 3.2 field example to show the efficacy of the approach three field examples of gravity and magnetic anomaly were presented. it is worth to make a note that the field data is often corrupted with noise and in common, exact shape of the subsurface structure cannot be found in geological nature. hence, field data cannot be fitted accurately well with the model response from the dyke like structure. moreover, it is important to note that in nature, real structures might not have the standard geometrical shape and structure. along these lines, modeling and inversion of real field information utilizing the specified standard geometrical definition may not yield the genuine subsurface structure. any, deviation of the real structure from the displayed structure can be comprehended as systematic erraticism from the demonstrated curves brought on by the distinction from dyke like structures. under such conditions, the multi-dimensional objective function will be to a great degree of unpredictable and straightforward inversion methodology may neglect to show the subsurface structure. henceforth, global optimization is much more important to manage such conditions. besides, it ought to be highlighted that unpredictable modeled bodies can’t be resolved correctly utilizing any interpretation strategy unless and until numerous bore-hole data are accessible. hence, the primary objectives is to find out the near probable shape, depth at where the body is located and the exact location of the body from the surface, which can be successfully utilized for drilling purposes. figure 9. gravity data: (a) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-4 for noise-free data) (blue), and models with pdf >60.65% (red) for noise free data when q is uncontrolled; (b) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-4 for noise-free data) (blue), and models with pdf >60.65% (red) for noise free data when q is controlled; (c) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for noisy data) (blue), and models with pdf >60.65% (red) for noisy data when q is uncontrolled; (d) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for noisy data) (blue), and models with pdf >60.65% (red) for noisy data when q is controlled. figure 10. magnetic data: fittings between the observed and model data for thin dyke: model 1(a) noise-free synthetic data and (b) 10% random noisy synthetic data, and model 2(c) noise-free synthetic data and (d) 20% gaussian noisy synthetic data. vfsa in gravity and magnetic anomaly 9 3.2.1 leona anomaly, south saint-louis, western coastline, senegal a residual gravity anomaly over an area (30 km length) on the west coast of senegal in west africa [nettleton, 1976] is shown in figure 13. this anomaly was interpreted by several authors as spherical structure [tlas et al. 2005, asfahani and tlas 2012, mehanee 2014]. vfsa optimization is repeated for this profile as mentioned for synthetic data. table 5 depicts the interpreted model parameters and comparison with other published results. histogram shows that the model parameters are well resolved when q is controlled (figure 11a) and cross plots also suggest that the estimated parameters are within the uncertainty limits (figure 12a) and high pdf. the depth of the body estimated in the present study is 4.6 km. the depth obtained by tlas et al. 2005 (z = 9.17 km), asfahani and tlas, 2012 (z = 9.13 km), mehanee, 2014 (z = 12.2 km) are presented as interpreted as sphere. moreover, mehanee, 2014 and biswas, 2015 also interpreted the same anomaly as vertical cylinder as well where the depth is estimated at 4.59 and 4.6 km respectively. in the present study, it is found that the shape factor is pointing towards a thin dyke and interpreted the same. comparison of interpretation results by various methods also reveal that present approach is in good agreement with other interpretation methods. a comparison between the field data and modeled data is shown in figure 13. 3.2.2 pima copper deposit, arizona, usa a 750 m-long magnetic anomaly profile caused due to a thin dike over the pima copper mine, arizona, united states (gay 1963) is taken (figure 14). this anomaly was interpreted by several authors (tlas and asfahani, 2015, abdelrahman and essa, 2015, asfahani and tlas, 2007, asfahani and tlas, 2004, abdelrahman and sharafeldin, 1996, gay, 1963) assuming a thin dyke model. the anomaly is interpreted using vfsa to obtain the different parameters. the vfsa process is applied in this magnetic field anomaly keeping q free and fixed as discussed in synthetic model data. the histogram shows that all the model parameters are well determined when q is controlled (figure 11b) and cross-plots also advocate that the estimated parameters are within the uncertainty limits (figure 12b) with high pdf. the interpreted results are shown in table 6. the depth of the body estimated in the present study is 68 m. the depth obtained by other workers such as gay, 1963 (z = 70 m), abdelrahman and sharafeldin, 1996 figure 11. (a) histograms of all accepted models having misfit<10-2 for field data when q fixed for gravity anomaly, (b) histograms of all accepted models having misfit<10-2 for field data when q is controlled for magnetic anomaly, (c) histograms of all accepted models having misfit<10-2 for field data when q is controlled for aeromagnetic anomaly. figure 12. (a) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for field data) (blue), and models with pdf >60.65% (red) for field data when q is controlled for gravity data; (b) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for field data) (blue), and models with pdf >60.65% (red) for field data when q is controlled for magnetic data, (c) scatter-plots between amplitude coefficient (k), depth (z), shape factor (q) for all models having misfit<threshold (10-2 for field data) (blue), and models with pdf >60.65% (red) for field data when q is controlled for aeromagnetic data. biswas et al. 10 (z = 66 m), asfahani and tlas, 2004 (z = 71.50 m), asfahani and tlas, 2007 (z = 71.50 m), abdelrahman and essa, 2015 (z = 60 m), tlas and asfahani, 2015 (z = 64.1 m), ekinci, 2016 (z = 67.9 m using derivative method and 68.3 m using pso) and abo-ezz and essa, 2016 (z = 61.5 m) are in good agreement with the other published literatures as shown in table 6. figure 14 depicts the fitting between the observed and interpreted mean model. the other results are also in respectable agreement. 3.2.3 matheson area, northern ontario, canada another field example is taken from the total field aeromagnetic anomaly (srivastava and agarwal, 2010) over a magnetic body associated with a mapped bedrock diabase dyke in the matheson area of northern ontario, canada (figure 15). the amplitude of 2-d analytic signal, from the measured magnetic field anomaly for the same example is taken from srivastava and agarwal, 2010. the aeromagnetic data was taken over a flight height of 12 m (salem et al. 2005). the anomaly is interpreted using the same approach as discussed earlier. the histogram and cross-plots are shown in figure 11c and 12c. the interpreted results are shown in table 7. the depth of the body estimated in the present study is 133.4 m. the depth obtained by srivastava and agarwal, 2010 (z= 135.2 m), vallee et al., 2004 (z= 145), salem et al., 2005 (z= 139.6 m) and agarwal and srivastava, 2008 (z= 142.6 m) are in good agreement. figure 14 depicts the fitting between the measured amplitude of 2-d analytical signal and model amplitude. a borehole taken at that location intersects the bedrock at 41 m (vallee et al. 2004). 4. conclusions in the present work, an attempt is being made to test the applicability and effectiveness of vfsa on the parameter estimations from potential field anomalies using total gradient method. as far as this work is concerned, this is the first attempt of applying vfsa for model parameter estimations using total gradient of gravity and magnetic anomaly. in the present algorithm, the test studies are performed using theoretically produced data and field data sets. the determination of the appropriate amplitude coefficient, location, depth and shape, of a buried structure from total gradient anomaly profile can be well resolved using the present method. synthetic data experiments are performed using both noise-free and noisy gravity data sets due to simple-shaped causative bodies. the present study reveals, while optimizing all model parameters (amplitude coefficient, location, depth, shape) together, the vfsa method yields very good results. the resulting histogram and cross-plots analysis suggests that figure 13. fittings between the observed and model data for leona anomaly, south saint-louis, western coastline, senegal. figure 14. fittings between the observed and model data for pima copper deposit, arizona, usa. figure 15. fittings between the measured amplitude of 2d analytical signal and model amplitude for matheson area, northern ontario, canada. vfsa in gravity and magnetic anomaly 11 the obtained parameters are within the high probability areas. the efficacy of this approach has been successfully proved, established and validated using noise-free and noisy synthetic data. the applicability of this method for practical application in mineral exploration is effectively illustrated on three field examples. the method can also be used to interpret multiple structures from the anomaly data. the estimated inverse parameters for the field data are found to be in excellent agreement with the other methods as well as from the geological results. acknowledgements. we thank the editor prof. paola de michelis and an anonymous reviewer whose comments and suggestion has improved the quality of the manuscript. the authors would like to thank prof. a. k. gupta, director, wadia institute of himalayan geology for necessary facilities to complete this work. appendix i we present here the detail explanation of tg. it is well known what is currently normally called the 3d analytical signal (and same for 2d case additionally) ought to accurately be known as the total gradient. it might be called attention to here that the analytical signal in 3d for total magnetic anomaly does not comply with the state of being analytic unless the anomaly is reduced to pole [haney et al. 2003]. all in all, tg has been approximated by a bell-shaped function [nabighian 1972, green 1976, stanley and green 1976, srivastava and agarwal 2009, 2010, srivastava et al. 2014] for 2d source geometries (or profile information) and it is additionally valid for 2d circularly symmetrical anomalies (in perception plane) created by 3d circularly symmetrical sources, to be specific, a sphere or a vertical cylinder delivering different potential field. moremodel parameters search range present method (vfsa) thin dyke tlas et al. [2012] ashfahani and tlas [2012] mehanee [2014] (sphere) mehanee [2014] (vertical cylinder) biswas [2015] (vertical cylinder) k(mgalxkm) 10-1000 433.6±2.94 6971.83 mgalxkm2 6931.78 mgalxkm2 13026.03 mgalxkm2 436.31 94.7±0.7 x0 (km) -5-5 -0.4±0.0 0.22 -0.4±0.0 z (km) 0-20 4.6±0.0 9.17 9.13 12.2 4.59 4.6±0.0 q 0.5 0.5 1.499 1.499 1.5 0.5 0.5 misfit 3.8x10-4 3.8x10-4 table 5. search range and interpreted mean model for leona anomaly, south saint-louis, western coastline, senegal. table 6. search range and interpreted mean model for pima copper deposit, arizona, usa. model parameters search range present method (vfsa) thin dyke abo-ezz and essa [2016] ekinci [2016] pso tlas and asfahani [2015] abdelrahman and essa [2015] asfahani and tlas [2007] asfahani and tlas [2004] abdelrahman and sharafeldin [1996] gay [1963] k(nt) 0-1000 613.0±2.2 1219 39267.31 42700 577.6 577.61 596.5 x0 (km) -50-50 -4.3±0.2 z (km) 0-100 68.0±1.7 61.5 68.29 64.1 60 71.50 71.50 66 70 θ (°) -90-90 -66.4 -50.76 -44.7 -50.50 -50.46 -53 -50 q 1.0 1.0 1.0 0.95 misfit 8.3x10-4 model parameters search range present method (vfsa) srivastava and agrawal, 2010 agrawal and srivastava, 2008 salem et al., 2005 vallee et al., 2004 k(nt) 100-106 586830.2±6333.87 4286 x0 (km) 400-700 561.9±0.7 563.6 567 752.2 z (m) 0-200 133.4±0.9 135.2 142.6 139.6 145 q 1.0 1.0 1.0 1.16 1.13 1.2 misfit 8.1×10-4 table 7. search range and interpreted mean model for matheson area, northern ontario, canada. biswas et al. 12 over, the total magnetic field anomaly (in 2d) must be reduced to pole (rtp) before tg examination [haney et al. 2003]. the anomaly constriction rate [ravat 1996] or decay rate (β) – see appendix ii) of tg is represented by a power law which is identified with source geometry. the analytical signal, albeit broadly utilized as a part of magnetic, is utilized little as a part of gravity systems, fundamentally due to the sparser way of gravity information, which makes the count of subordinates less dependable. for magnetic profile information the level and vertical derivatives fit actually into the genuine and nonexistent parts of analytic signals [nabighian 1972]. in two dimensions [nabighian 1972], the amplitude of the analytical signal is the same as the tg. in three dimensions, roest et al. [1992] presented the tg of magnetic anomaly data as an augmentation to the 2d case. the outcomes acquired for magnetic anomaly data can be reached out to gravity information too. assist, the surmised horizontal area of the causative source corresponds to the pinnacle of tg. amplitude of the 2-d analytic signal of the magnetic anomaly profile is autonomous of the bearings of the earth’s magnetic field vector and leftover polarization of the causative source. it shows crests relating to the areas of the sides of a causative source, demonstrated by say a polygon. it likewise shows a pinnacle comparing to various source geometries identified with the structural indices. this amplitude is figured from the first order horizontal and vertical derivatives of the field magnetic anomaly and is moderately less eccentricity than second order derivatives. appendix ii we show the detailed derivation of the tg. following srivastava et al. [2014], let us assume that d (x) correspond to the potential field function satisfying the laplace’s equation in 2-d. this means that a profile perpendicular to the strike length of the contributing source from any structure. the first order horizontal (x) and vertical (z) derivatives, computed via the wave number domain computation or any other appropriate method is represented as the amplitude of the total gradient, d(x) is defined by (1) the total gradient (tg) field over several idealized source geometries can be approximated by (2) where r2=|(x-x0) 2+z2| (3) x0 and z0 are the horizontal location and depth of causative source, β – a positive integer called ‘source geometry factor’ (sgf), and b is a constant governing the amplitude of tg. the first order horizontal and vertical derivatives of the magnetic anomaly over a corner formed by two infinite extending edges of a horizontal faulted slab are related through hilbert transform pair as shown by nabighian [1972]. the concept of 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(2004). estimating depth and model type using the continuous wavelet transform of the magnetic data, geophysics. 69, 191–199. zhao, l. s., sen, m. k., stoffa, p. l., frohlich, c., (1996). application of very fast simulated annealing to the determination of the crustal structure beneath tibet, geophysical prospecting, 125, 355–370. zhdanov, m. s., (2009). new advances in regularized inversion of gravity and electromagnetic data. geophysical prospecting, 57, 463–478. *corresponding author: arkoprovo biswas wadia institute of himalayan geology (wihg), general mahadev singh road, dehradun, uttarakhand, india.; email: arkoprovo@gmail.com 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved microsoft word 7411-18628-1-ce_keane-boland.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7411 1 dilemmas of promoting geoscience workforce growth in a dynamically changing economy christopher m. keane and maeve boland american geosciences institute keane@americangeosciences.org, mboland@americangeosciences.org abstract the geosciences as an occupation are experiencing substantial change, with a combination of economic cycles, mass retirements, and rapid technological innovation. for the next generation of geoscientists, flexibility and well-defined competencies will be key to employment resilience. these required core competencies reflect the new economy that the geosciences function in, yet most educational programs reflect the economy of the past. we examine the human capital dynamics of the geosciences in the united states as an example, and the critical challenges faced in recruiting the right next generation workforce best prepared for the rapid changes in the market including the likely new phase of the long-term cyclicity of geoscience labor demand. mismatching educational goals and workforce needs negatively impacts individuals’ career prospects without their knowledge or recognition. historical trends inform us on likely changes with the emerging political, economic, and demographic realities that will affect the geosciences. some of the competing interests within the workforce development process are examined for their ethical challenges, such as the need to maximize enrollments while challenging students to be ready for the workforce. this has profound implications regarding how we promote the science to students, so that we are not leading people down paths that will not yield productive careers and in turn not foster a healthy profession. 1. introduction eoscience-related employment in the united states is cyclic. historically the center of gravity has shifted as the fortunes of various industries such as petroleum, mining, and environmental consulting expand and contract. like many technical fields in the developed world, the geosciences also face the challenge posed by the retirement of the baby boom generation, which is warping the dynamics of the employment cycle (carnevale et al., 2014). together these forces necessitate providing geoscience students and early-career employees with the skills and awareness to cope with the current and expected future dynamic change in the discipline. students and junior employees look to educators and employers for advice to make informed career choices. it is incumbent on educators and employers to fulfill their ethical duty to act responsibly, honestly, and with integrity (american geosciences institute, 2015) even as they face their own employment and professional pressures. understanding the context of geoscience employment will help mentors provide accurate information and training to the next generation of geoscientists who must be adaptable and flexible.. 2. a brief history of u.s. geoscience human capital geoscience employment in the united states was first measured by the agi manpower committee in 1955, as the post-war economic expansion shifted in response to the cold war. demonstrable growth in the demand for geoscientists in industry and government occurred in the late 1950s and early 1960s as the united states aggressively implemented petroleum and mineral strategic reserve programs and became a major buyer of commodities. however, demand decreased following this initial buildg annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7411 2 out. through the 1960s and into the 1970s employment levels remained flat, but university geoscience enrollments grew in concert with general increases in college enrollments. with the oil embargos of the 1970s and the rapid rise in oil prices, enrollments and employment in the geosciences rose dramatically. at its highest levels, demand for geoscientists was so strong that energy companies recruited “at the endpoints” – hiring both the best faculty and lower caliber graduates (milling, 2002). oil prices fell dramatically in 1986 and so did employment for geoscientists (keane et al., 2008). as the petroleum industry shed positions, many talented individuals left the field. a number migrated, along with new geoscience graduates, to employment opportunities related to superfund and other projects in the environmental industry. employment demand in this new sector was met quickly, however, and created a relatively slow hiring environment for geoscience graduates until the 2010s. genuine structural growth in geoscience employment resumed with the broad use of hydraulic fracturing that rejuvenated the u.s. onshore oil and gas industry. this growth yielded dramatic demand for geoscientists not only working directly in the oil and gas industry but also in support services and the environmental and engineering consulting industry. the decline in oil prices in 2014 once again saw a slowing of geoscience hiring in the energy sector. however, demand has remained strong in the environmental and geoengineering industries (wilson, 2016). geoscience enrollment, and especially the number of degrees awarded, most closely tracks demand from the petroleum industry. this trend is particularly marked for male students. data from agi’s annual enrollment surveys demonstrate that between 2010 and 2014 most enrollment growth was male and, as hiring in the energy sector slowed, those individuals exited geoscience degree programs and the gender balance returned to traditional levels of approximately 44 percent female (keane, 2016). a recent phenomenon is the rapid increase in the number of master’s degrees awarded, with a one-year increase of 46 percent in 2012. in many industrial geoscience fields, a master’s is regarded as an appropriate terminal degree (wilson, 2016). perhaps not surprisingly, some geoscience departments report that occasionally doctoral students have transferred to the master’s degree to enable them to enter the industry workforce more rapidly (wilson, 2016). 3. current predictions for the geoscience workforce currently there are approximately 324,000 individuals employed in geoscience-related occupations in the united states (wilson, 2016). this number is derived from u.s. bureau of labor statistics data using several key relationships between occupations and geoscience activities. based on an estimate of general economic growth of 1.5 percent per year and u.s. government projections (u.s. bureau of labor statistics, 2017) for additional growth in the energy and in environmental and engineering industries, aggregate demand of 355,00 geoscience full-time equivalents (ftes) is predicted by 2024 (wilson, 2016). this prediction already accommodated expected substantial decreases in government employment. with current graduation rates there would be a deficit of 90,000 geoscience ftes by 2024. open markets respond to labor shortages through several mechanisms. first, labor shortages may stifle projects especially in the resource development and infrastructure sectors. second, individuals from other science, technology, engineering, and math (stem) fields, especially physics, mathematics, and civil engineering, may substitute for geoscientists. even actuarial professionals may substitute for geoscientists who work on risk assessment in sectors such as hazards and the extractive industries. the increasing use of “big data” for statistical assessments in place of technical assessments may lead to more computational experts filling traditional geoscience positions in both the private sector and government. finally, innovation and developments in technology may reduce the demand for geoscientists. given these potential scenarios, it will be increasingly important for geoscientists to develop a broad portfolio of skills. 4. challenges for academia annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7411 3 geoscience, in general, has been a countercycle labor market, where high commodity prices slow down the general economy but yield rapid expansion in geoscience employment. likewise, given the inherent upstream nature of much geoscience work, low commodity prices lead to softening geoscience employment. young adults entering the university or the workforce should understand this phenomenon and be equipped with skills to cope with cyclicity and its likely impacts on their careers. university students choosing a major are making a profound life choice, one that may take nearly a decade to realize success. in the geosciences, a master’s degree is the degree required for many positions, especially in the private sector, so for many students, the total time from enrolling in college to employment is approximately six years. students may choose geoscience at a time when employment prospects are high but by the time they graduate demand for geoscientists, which is often on a 5–10-year cycle, may have changed. completing a doctoral degree may open additional options for employment, but this imposes additional time and financial costs. educators and mentors should help students manage expectations with some fundamental precepts. students need to understand that economic cycles are real. resilience depends on the student’s flexibility; the geoscience workforce and profession is global and mobile so there are fewer opportunities for people who are unwilling to move (keane and gonzales, 2010). it is also important to impart to students that while getting a degree doesn’t guarantee a job, not getting a degree will certainly reduce earnings (carnevale et al., 2014). however, it is true that even in the worst of times the best students still get hired (mobil oil company, 1999). the mismatch between cycles of hiring and the production of students creates dilemmas for universities. academic departments are under pressure to increase revenue from tuition fees and grants as well as graduate more students within six years of enrollment. this pressure is due, in part, to the u.s. department of education’s college scorecard, which rates colleges and universities on three metrics: average actual out-of-pocket annual cost, six-year graduation rate, and the median income of graduates 10 years after entering the school. these metrics motivate schools to reduce costs, increase completions, and seek ways to improve employability for the majority of their students. they also, in some cases, encourage schools to close down high-cost programs with low student numbers (heads and chairs meeting of the american geophysical union in 2015). the success rate for geoscience grant applications at the national science foundation is currently approximately 25 percent (wilson, 2016). increasing enrollments is seen in some departments as an alternative way to boost income. geoscience enrollments have been at record levels since 2013 and many graduate programs report that they are at capacity (wilson, 2016). with larger class sizes, faculty-intensive comprehensive field classes have been reduced, and the large pool of students has not countered the trend that geoscience majors have the lowest mean sat scores of all stem fields. employers report that advanced math courses are a good predictor of graduate success in the workplace (keane, 2016). nevertheless, there are continued deficiencies in quantitative coursework in the geosciences, in part because rigorous math requirements tend to reduce enrollments and retention (malcolm and feder, 2016). consequently, u.s. universities are producing more geoscience graduate than ever, but they are not necessarily meeting industry requirements for high-quality workers available on demand. efforts to diversify the geoscience profession by attracting specific target populations have not yielded substantive changes in the diversity of the geoscience student body and workforce (wilson, 2016). efforts to engage underrepresented populations are well-motivated and can be transformative (houlton et al., 2012), but the lack of progress raises questions of their efficacy. other areas that may need additional attention are the lack of problemsolving skills by incoming students, the assumption that exposure to earth science in secondary schools will lead to increased geoscience enrollments, and the need to support the “middle third” of secondary students – those students who are academically capable of uniannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7411 4 versity study but face critical social, spatial, temporal, and economic barriers that prevent them from pursuing additional formal education (smith, 2010). 5. conclusion: systemic approaches to lifetime career management given employment cycles, students and early career geoscientists must be prepared to maximize their employment resilience (keane, 2015). the geoscience profession needs to develop a common perception of the geosciences that will facilitate movement between technical sectors within the profession. systemic structures are needed to help individuals selfmanage their education and careers. faculty and academic advisors need to separate themselves from the perspective that the preparation pathway for an academic career is applicable across all geoscience occupations, or that the skill sets and experiences which have made them successful apply uniformly to their students and advisees. the skillset of a successful mid or late career professional is not the same of an early career individual, and nor can assumptions be made that what constitutes necessary skills for later in a career will remain the same as the science evolves and technology changes processes. by utilizing an objective competency-based approach, the variations of skill-needs to career-desires and phases can be identified, which has been utilized in the energy sector already. competency-based career planning is used in parts of the energy industry (gelling, 2013), and is central to the geospatial community’s education, training and employment strategies (johnson and davis, 2010). rather than focusing on the degree as the key qualification for employment, the emphasis shifts to a portfolio of competencies. developing a competency matrix for the geosciences, in which individuals can map the learning outcomes from their degree programs and identify skills needed for future pathways, would help geoscientists identify beneficial educational opportunities that strategically enhance their career path. it would also provide a common language for employers and educators to discuss geoscience education that supports employment. the u.s. department of labor has promoted the idea of competency pyramids that show the progression from basic functionality to higherlevel critical thinking as new learning outcomes or skills develop. competency pyramids highlight the need for lifelong learning and ongoing professional development. lifelong learning experiences, however, are only required in selected licensure jurisdictions, so the onus is on individuals to take responsibility for continuously developing their competencies. the geoscience community needs to come together to create structures and expectations that support resilient career pathways for geoscientists. scientific and professional societies, in addition to universities, can play a major role in providing the educational opportunities, career advice, and institutional structures needed by geoscientist throughout their careers. references american geosciences institute (2015). agi guidelines for ethical professional conduct. alexandria, va, american geosciences institute. carnevale a.p., smith n., and stohl j. 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(2002). trends in teh geosciences; education, employment, and careers. abstracts with program geological society of america, boulder, geological society of america. mobil oil company (1999). the quest for energy rewarding careers in petroleum exploration. agi academic associates conference, houston, tx, american geological institute. smith p. (2010). harnassing america's wasted talent: a new ecology on learning, san francisco, jossey-bass. u.s. bureau of labor statistics (2017). occupational outlook handbook. from u.s. bureau of labor statistics: https://w w w .bls.gov/ooh/lifephysical-and-social-science, accessed 30 october 2017. wilson c. (2016). status of the geoscience workforce. alexandria, va, american geosciences institute. a new md-ml relationship for mt. etna earthquakes (italy) annals of geophysics, 58, 6, 2015, s0657; doi:10.4401/ag-6830 s0657 a new md-ml relationship for mt. etna earthquakes (italy) tiziana tuvè*, salvatore d’amico, elisabetta giampiccolo istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy abstract a homogenous database of magnitude observations is a basic requirement for seismic hazard estimation and other seismic studies. unfortunately, the magnitude reported in the seismic catalogue of mt. etna is not homogenous. only the duration magnitude (md) is available up to 2005, while since then the more stable local magnitude (ml) has also been calculated. to have a uniform dataset, earthquake data recorded at mt. etna during the period 2005-2014 were used to derive a new relationship between local and duration magnitude, by applying the general orthogonal regression (gor), which is an alternative to least squares when the ratio between errors on the independent and the dependent variables can be estimated. the relationship obtained is: ml = 1.164 (±0.011) * md − 0.337 (±0.020) the new relationship allows to back-extend the local magnitude dataset to cover a period of about 15 years. 1. introduction mt. etna, in eastern sicily, italy, is a basaltic stratovolcano with persistent volcanic activity. it is located in a seismically active region astride the complex tectonic zone marking the boundary between the african and european plates [mckenzie 1970, barberi et al. 1973, lentini 1982, montalto et al. 1996], where major regional structural lineaments play a key role in the dynamic processes of the volcano [bonaccorso et al. 1996]. volcano-tectonic earthquakes mainly occur at mt. etna in the form of swarms, whereas foremainaftershocks sequences are rarely recorded and seldom exceed magnitude 4.0 [e.g., azzaro et al. 2011, alparone et al. 2015]. the strongest historically recorded events, albeit of low magnitude (m≤5.2 according to cmte [2014]), produced severe damage or even destruction in the epicentral area where the observed intensity reached up to ix-x ems-98 [grünthal 1998]. most of the seismicity is located in the eastern flank of the volcano and is characterized by shallow volcano tectonic earthquakes with focal depth less than 7 km b.s.l. (figure 1). it shows mediumto low-frequency seismic signals and complex signatures at stations just a few kilometers away from the epicentral area [patanè and giampiccolo 2004]. earthquakes characterized by high frequency content and sharp arrivals, typical of tectonic areas, are instead mainly located below the volcano between 5 and 20 km. despite the low values of magnitude, studies on seismicity are extremely important for the high seismic hazard given the strong effects observed in the epicentral areas; hence, the estimation of magnitude is a crucial parameter to be routinely measured with minimum uncertainty. among the different ways of estimating magnitude, the richter [1935] “local magnitude” (ml) represents a reference milestone because of its simple definition and widespread usage in different parts of the world [gasperini 2002]. the local magnitude (ml) of an earthquake, as defined by richter [1935], is the logarithm of half the peak-to-peak amplitude measured in microns, recorded by a wood–anderson seismograph at a distance of 100 km from the epicenter of that earthquake. the wood– anderson (wa) is a standard torsion seismograph measuring a high-pass filtered displacement with a frequency domain response. a correction factor describes the variation of maximum amplitude taking into account the distance from source to receivers. since standard wa seismometers are often not available, local magnitudes are generally computed from amplitude recordings of other types of seismometers, digitally simulating a wa seismometer. at mt. etna, seismic signal recordings have been available since the early 1990s, although at that time seismic stations were mostly equipped with vertical rather than horizontal seismometers, which are required by the richter [1935] definition. therefore, the earthquake magnitude at mt. etna has been estimated by the duration (md) of the seismogram coda calibrated for article history received july 16, 2015; accepted november 6, 2015. subject classification: mt. etna, duration magnitude, local magnitude, general orthogonal regression. mt. etna by the caltabiano et al. [1986] relationship: md = −1.367 + 2.068 log x + 0.212 log d (1) where x is the duration time of the event in seconds and d is the hypocenter distance in km from the reference station. the advantage of the above method is that the duration, defined as the time interval between the onset of the first pulse and the time when the amplitude of the seismogram coda decreases below the noise level, is influenced minimally by inaccuracies in the instrumental response function and the hypocentral location [gasperini et al. 2013]. however, in volcanic areas the decay of the earthquake coda may be masked by the presence of noise, volcanic tremor or other shocks [del pezzo and petrosino 2001, d’amico and maiolino 2005], so that calculation of the local magnitude, based on the less ambiguous signal amplitude, is desirable to estimate magnitude properly. a first attempt to estimate the local magnitude ml was performed by d’amico and maiolino [2005], who used a dataset of 288 earthquakes, recorded by the istituto nazionale di geofisica e vulcanologia, osservatorio etneo (ingv-oe) during the 2002-2003 eruption, and the lahr [1999] relationship: ml = log a + a log d − b (2) where a is maximum half-amplitude of the horizontal component of the seismic recording, measured in mm, and the term «+a log d − b» takes the place of the term «− log(a0)» of richter’s [1935] relationship with a = 0.15, b = 0.16 for d< 200 km). beginning in 2005, thanks to the improvement of the ingv-oe seismic network of mt. etna and the installation of digital stations equipped with broadband three-component sensors, local magnitude estimation became a routine procedure in monitoring the seismicity of the volcano. today, ml is estimated as the mean value from several stations simulating a wa seismometer, as described in d’amico and maiolino [2005]. to ensure the continuity of a long dataset of coherent magnitude observations, the duration magnitude is still evaluated. magnitude estimation in md and ml, though mutually related, do not produce the same results. for this reason, it is mandatory to adopt an empirical conversion to produce a homogeneous catalogue for the mt. etna region. the use of earthquake data compiled at different tuvè et al. 2 figure 1. map of mt. etna volcano and spatial distribution of the earthquakes used in this study. 3 times, subjected to different magnitude scales or from different networks, means dealing with the problem of magnitude homogeneity [e.g. habermann 1991, zùniga and wyss 1995, wiemer and wyss 2000]. for reliable estimates of seismicity and hazard it is also necessary to make use of as much data as possible. in order to overcome this difficulty, many studies have relied on magnitude conversions based on linear regressions without regard for the time and method of the original observations on which they are based [zùniga and figueroa-soto 2012]. for mt. etna volcano-tectonic earthquakes, some authors have proposed relationships between md and ml through linear regression using selected datasets related to the large seismic sequences of 2001 and 2002-03 eruptions [d’amico and maiolino 2005, giampiccolo et al. 2007], or using algorithms that take into account errors in both md and ml with the same weight as in murru et al. [2007]. the purpose of this study is to determine the md-ml relationship by applying the general orthogonal regression (gor) [e.g. carrol and ruppert 1996] procedure, which has been widely used in the recent literature, to a dataset of 3921 ml and md available for earthquakes recorded at mt. etna from 2005 to 2014 [gruppo analisi dati sismici 2015]. the relationship obtained is then used to produce a homogeneous magnitude for the whole mt. etna ingv-oe catalogue from 2000 to present, by calibrating the older md with the more recent and reliable ml. 2. instruments and dataset at mt. etna, spatial density and quality of the stations of the permanent seismic network run by ingvoe have been improved over time. in particular, starting from 2005, the technology of several stations has been upgraded from analogical short period 1-component to digital broadband 3-component. at present, the seismic monitoring network comprises about 50 stations, mainly equipped with broadband (40-second periods), 3-component, nanometrics trillium seismometers and distributed all over the volcano. the dataset used in this study consists of 3921 earthquakes (figure 1) occurring from 2005 to 2014 and selected from the “earthquakes catalogue of eastern sicily and southern calabria from 1999 to 2015” of ingvoe [gruppo analisi dati sismici 2015]. iterative linear earthquakes location was performed by using the hypoellipse algorithm [lahr 1999] and the 1d crustal velocity model proposed for mt. etna area by hirn et al. [1991]. both duration and local magnitudes are available for the whole dataset. the accuracy of the ml estimates performed by ingv-oe, has been tested through the comparison with the ml estimates furnished, at national scale, by ingv seismic network in the “italian seismic bulletin” [bsi 2012] and in the “italian seismic instrumental and parametric data-base” [iside 2015]. in figure 2, the solid line represents the best-fit curve defining the relationship between the local magnitude computed with data from the two networks. coefficients of the relationship show that ml values of ingva new md-ml relationship for mt. etna earthquakes figure 2. plot of ml values of earthquakes occurring at mt. etna from 2005 to 2014 calculated by both local and national seismic network. y-coordinate data from mt. etna permanent seismic network [gruppo analisi dati sismici 2015]; x-coordinate data from the italian seismic bulletin [bsi 2012] and italian seismological instrumental and parametrical database [iside 2015]. figure 3. plot of md vs. ml values of the earthquakes occurring at mt. etna from 2005 to 2014. the grey scale indicates the number of earthquakes for each md-ml bins. black squares indicate the md value that occurs most often (mode) for each ml bin (0.1 unit). oe are in good agreement with those estimated using the national seismic network stations of ingv. it is known in literature that a degree of bias is present when comparing md vs. ml estimates [e.g. gasperini 2002, d’amico and maiolino 2005, castello et al. 2007]. in particular, for the our dataset md values of small earthquakes are overestimated with respect to ml values and, conversely, they are underestimated for the strongest events. in figure 3, the magnitude values are plotted in terms of density of number of couples of md-ml, classified by grey scale. mode of md distribution for each ml bin (0.1 unit) is plotted with black squares. the plot shows a wide scatter of the data points that can be attributed to the uncertainties in the magnitude estimate. in fact, both duration and local magnitudes are calculated as the mean value from several stations. moreover, in a volcanic area like mt. etna, where the noise is mainly related to the volcanic activity recorded by the instruments, uncertainties of coda duration have to be taken into account. on the other hand, uncertainty in the estimate of the ml depends on the measurement of the maximum amplitude, which may be influenced by site effects due to soil amplification [di grazia et al. 2001]. the modes of md distributions for each ml bin show a good linear trend for ml > 1.0. below this threshold, the same md value is calculated for several ml bins. in particular, earthquakes with ml < 1.0 are often estimated in md as 1.1-1.2. similar behavior may be observed for earthquakes with ml > 3.0 that are often estimated as md= 3.0. moreover, it is noteworthy that two strong earthquakes (ml≥4.5) had much smaller md estimations. since duration magnitude estimation of smaller earthquakes appears to be unreliable, we removed 352 md-ml data having ml< 1.0 from the dataset used to calculate the gor. the absence of duration magnitudes outside the range 1.0-4.0 may clearly be observed also by computing the gutenberg-richter (g-r) frequency magnitude distribution of the catalogue shown in figure 4a, for both duration and local magnitude. small magnitude values are well-represented by ml but not by md. the distribution seems very similar from 1.0 to 3.0. in addition, the slope (b-value) of the cumulative curve is higher for md than for ml. this must be taken into account in carrying out studies using the b-value or seismicity rate, since it may have implications for both source processes and hazard estimation. similarly, the cumulative plot of the seismic energy released in time (figure 4b) enhances this md-ml behavior. the overall difference in terms of cumulative energy strain, released between that calculated in md and that calculated in ml is 3.61 × 10 6 j0.5. since the dataset is the same for both magnitudes, the observed difference has to be ascribed to the different estimation of the two magnitude scales, and it is particularly evident during the most powerful seismic sequences, such as those of may 2008 and december 2009. 3. regression analysis and results the standard linear regression (slr) is the simplest and most commonly used regression procedure applied in literature [e.g. gasperini and ferrari 2000, gasperini 2002, bindi et al. 2005, braunmiller et al. 2005]. however, its application implies that error on the independent variable is negligible compared with the error on the dependent variable. therefore, the use of slr, without checking whether its basic requirements are satisfied, may lead to erroneous results [castellaro et al. 2006]. another approach is the simple orthogonal regression (sor), which assumes that both varituvè et al. 4 figure 4. (a) frequency magnitude distributions, in normal and cumulative scale, and (b) cumulative seismic strain release calculated using the richter [1958] relationship for the earthquakes used in this study. blue for md dataset and red for ml dataset. 5 ables have the same uncertainty [ambraseys 1990, gutdeutsch et al. 2002]. as the above conditions are not fully appropriate for magnitude data sets, the use of the general orthogonal regression (gor), which is designed to account for the effects of measurement errors on both linearly related variables [castellaro and bormann 2007, das et al. 2011], is generally preferable [castellaro et al. 2006, gasperini et al. 2013]. gor basic procedure is described in detail in literature [madansky 1959, kendall and stuart 1979, fuller 1987, carroll and ruppert 1996, castellaro et al. 2006, das et al. 2011] and only a brief description of the main aspects involving magnitude is therefore given below. the principle of gor involves the minimization of (3) in the unknowns a, b and xi. each measurement is sampled from a two-dimensional normal distribution centered at the value represented by the couple (xi, yi) and with major and minor axes equal to vx and vy, which represents the standard deviation error of x and y, respectively. the paths from the observed (xi, yi) to the “true” (xi, yi) have slopes which depend on the errors affecting the two magnitudes. equation (3) represents the weighted orthogonal distance when h ≠ 1 and the squared euclidean distance when h = 1. one drawback of the gor application is that knowing the variance ratio (4) between the two variables (md and ml) is mandatory. the variance (v2) needs to be known for both magnitude scales. this requires that several independent estimations, based on data of different stations for the same earthquake, must be available. when the square root of the error variance ratio h is in the range between 0.7 and 1.8, gor relation is qualitatively better than slr relation [e.g. castellaro and bormann 2007]. to perform the regression analyses, a great deal of effort was made to assess the magnitude uncertainties that are required for the application of the gor method. the local magnitude, for each earthquake in the catalogue, is calculated as the average generally by using a number of stations between 5 and 18. the standard deviation for each ml value is known. for the whole dataset, the standard deviation of the estimated magnitudes ranges between 0.14 and 0.67, with average vml= 0.27. conversely, notwithstanding that the duration magnitude is estimated as an average value, no information about its uncertainty is known. to overcome this problem and obtain a reliable value of uncertainty on the estimation, for a subset of 210 events the duration of the seismogram coda was re-estimated for all available recording stations (generally from 2 to 12). we were thus able to estimate mean md and its standard deviation v for each of the 210 events. for this subset of events, v of the estimated magnitudes ranges between 0.11 and 0.64 with average vmd= 0.22. given that neither the procedure to measure the coda duration nor the seismic instruments used have changed since 2005, we may assume that this subset is representative of the whole dataset. since the square root of h= 1.23, we are confident that the application of gor gives better results than slr [castellaro and bormann 2007]. the gor relationship obtained is: ml = 1.164 (±0.011) * md − 0.337 (±0.020) (5) where the rms between calculated ml and observed ml is 0.25. the relationship (figure 5, black line) is in good agreement with the trend of the mode of the distribution of md per ml bins (see figure 3). dashed segments indicate the magnitude values outside the range used for the regression. figure 5 also shows slr, inverse-sr (isr) and other relations known in literature [murru et al. 2007, d’amico and maiolino 2005]. in particular, the murru et al. [2007] relationship, although similar to gor, calculates smaller values of magnitude ml m 2 2 h v v= md 2 y x x xi i i ii n 2v 2v1 h a b+ = q q 2v 2v# &| a new md-ml relationship for mt. etna earthquakes figure 5. plot of the obtained gor relationship between ml and md (black line). grey boxes indicate md-ml couples. for comparison, slr, isr, murru et al. [2007] and d’amico and maiolino [2005], are also plotted. dashed lines indicate data outside the range of definition of the relationships. (0.2 unit difference for md= 1.0) for the less powerful events. conversely, slr and isr show good agreement only for small earthquakes. to verify the goodness of the new relation, a t-test was performed between the observed ml and calculated ml by gor. in particular, the null hypothesis that the means of the two populations are equal has been tested. the test indicates that there is a probability lower than 6.48% that the observed and calculated magnitude are different. in figure 6 the difference between observed and calculated ml versus md is plotted; black circles indicate the most frequent values for each md bin. they fall within the range defined by the standard error of the relationship (±0.25) for md values between 1.0 and 3.2, although there is some disagreement over such range. 4. conclusions for seismological applications, including homogenization of earthquake catalogues, it is important to know how different magnitude determinations compare as well as the associated measurement errors. mt. etna catalogue covers earthquakes occurring in the area since the 90s and, as in most cases, changes in instrumentation and the network involved in the monitoring, may have introduced changes in magnitude with time. in this study, a new relation to convert md magnitude to ml of mt. etna earthquakes has been derived using a dataset of earthquakes recorded in the period 2005-2014 and applying the most commonly used gor approach. a retrospective analysis of the cumulative energy strain release of the last 14 years (figure 7), calculated by ml (observed or estimated, red line), shows that, overall, the energy strain release is increased by about 1×107 units with respect to the estimated md (blue line). this behavior is particularly noticeable during the seismic crisis of 2001 and 2002 related to the volcanic eruptions and it is coherent with that observed in 2008 and in 2009 (see also figure 4b for a detailed magnification of this time span). the grey areas in background indicate when a calculated ml was used. the calibration of various magnitude data sets described above has been used to build a homogenized catalogue in ml from 2005 to 2014. the new relationship is an effective way to back extend the more reliable local magnitude to the whole mt. etna catalogue and allows obtaining a homogeneous dataset of magnitudes. moreover, taking into account that ml calculated with the seismic stations of the mt. etna permanent tuvè et al. 6 figure 6. residuals between observed ml and calculated ml with respect to observed md. thin horizontal lines indicate the standard error threshold of the md-ml relationship. figure 7. cumulative seismic strain release of mt. etna volcano from 2000 to 2014 calculated using the richter [1958] relationship. dotted blue line for md dataset and red line for observed or calculated ml dataset. grey areas indicate when a calculated ml was used. 7 network are in good agreement with those calculated by the national seismic network (figure 2), the relationship may be used to integrate ingv-oe data with those of different networks. references alparone, s., v. maiolino, a. mostaccio, a. scaltrito, a. ursino, g. barberi, s. d’amico, g. di grazia, e. giampiccolo, c. musumeci, l. scarfì and l. zuccarello (2015). instrumental seismic catalogue of mt. etna earthquakes (sicily, italy): ten years (2000-2010) of instrumental recordings, annals of geophysics, 58,4, 1-11. ambraseys, n.n. 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(1935). an instrumental earthquake magnitude scale, b. seismol. soc. am., 25 (1), 1-32. richter, c.f. (1958). elementary seismology, w.h. freeman and co., san francisco. wiemer, s., and m. wyss (2000). minimum magnitude of completeness in earthquake catalogs: examples from alaska, the western us and japan, b. seismol. soc. am., 90, 859-869. zùniga, f.r., and m. wyss (1995). inadvertent changes in magnitude reported in earthquake catalogs: influence on b-value estimates, b. seismol. soc. am., 85 (6), 1858-1866. zùniga, f.r., and a. figueroa-soto (2012). converting magnitudes based on the temporal stability of aand b-values in the gutenberg-richter law, b. seismol. soc. am., 102 (5), 2116-2127. * corresponding author: tiziana tuvè, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy; email: tiziana.tuve@ingv.it. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. tuvè et al. 8 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol. 48, 01, 05ok.qxd 1 annals of geophysics, vol. 48, n. 1, february 2005 key words methane – lithosphere degassing – mud volcanoes – greenhouse gas – geodynamics 1. introduction geodynamics and geophysical processes of lithosphere degassing are generally neglected in contemporary global climate change research. nevertheless, recent studies have suggested that lithosphere carbon dioxide (co2) and methane (ch4) outgassing is an important component of the natural greenhouse gas sources (etiope and klusman, 2002; morner and etiope, 2002). this is particularly evident for methane, whose geological sources have been object of detailed investigations during recent years. methane is one of the main greenhouse gases playing a significant role in global climate changes, on geological, quaternary and contemporary time scales. natural sources of methane include wetlands (> 100 mt yr–1), termites (20 mt yr–1) and oceans (10 mt yr–1). the intergovernmental panel on climate change (ipcc, 2001) does not include in its official tables any geological source of methane, apart from hydrates (5-10 mt yr–1). only recently, it has been suggested that several geologic processes may lead to the release of significant amounts of methane into the atmosphere, mainly from submarine seepage, mud volcanoes and microseepage (etiope and klusman, 2002; etiope et al., 2003, 2004a; milkov et al., 2003; etiope and milkov, 2004). today’s global estimates available for methane flux from these sources are probably underestimated and have a great potential of being increased. this work aims at evaluating this potential for mud volcanoes and microseepage, discussing present limits and introducing new data. the global microseepage estimate is re-calculated on the basis of an upgraded experimental data set and on a new evaluation of the global microseepage area. mailing address: dr. giuseppe etiope, istituto nazionale di geofisica e vulcanologia, via vigna murata, 605, 00143 roma, italy; e-mail: etiope@ingv.it mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget giuseppe etiope istituto nazionale di geofisica e vulcanologia, roma, italy abstract mud volcanoes and microseepage are two important natural sources of atmospheric methane, controlled by neotectonics and seismicity. petroleum and gas reservoirs are the deep sources, and faults and fractured rocks serve as main pathways of degassing to the atmosphere. violent gas emissions or eruptions are generally related to seismic activity. the global emission of methane from onshore mud volcanoes has recently been improved thanks to new experimental data sets acquired in europe and azerbaijan. the global estimate of microseepage can be now improved on the basis of new flux data and a more precise assessment of the global area in which microseepage may occur. despite the uncertainty of the various source strengths, the global geological methane flux is clearly comparable to or higher than other sources or sinks considered in the tables of the intergovernmental panel on climate change. 2 giuseppe etiope 2. mud volcano emissions mud volcanoes (mvs) are the largest surface expression of migration of hydrocarbon fluids through neotectonic faults in petroleumbearing sedimentary basins (fig. 1). geology and formation mechanisms are described in a wide literature (e.g., milkov, 2000; dimitrov, 2002; revil, 2002). methane flux from mvs is object of detailed studies only starting from 2001, when the main terrestrial mvs of europe, located in romania and italy were investigated (etiope et al., 2002, 2003, 2004a). more recently, gas flux has been measured in azerbaijan, which hosts the world’s biggest mvs and densest mv population (etiope et al., 2004b). thanks to these studies, it has been possible to elaborate a first estimate of global emission of methane from mvs to the atmosphere, that is at least 6-9 mt yr –1 (etiope and milkov, 2004). this is the same level of the estimates today considered for ocean and hydrates sources. methane emission from mvs (fig. 2) includes not only the gas flux from localised vents (craters, gryphons, bubbling pools and salses) but also the diffuse exhalation from soil, known as microseepage, whose mechanisms are discussed in the next section. the data collected from 2001 to 2003 in europe and azerbaijan (etiope et al., 2002, 2003, 2004a,b) refer to the quiescent degassing. it is known however that many mvs, especially those in azerbaijan, can erupt violently, generally in relation to seismic activity, injecting huge amounts of gas into the atmosphere within a few hours. so far, however, only some rough estimates of the eruptive flux of mvs in azerbaijan have been reported, generally based on subjective visual observations. for example, it has been reported that during the eruption of the touragai mud volcano (azerbaijan) in 1946, about 0.36 mt of ch4 were emitted, and more than 40 000 t of ch4 emitted from the duvannyi island volcano in 1961. bolshoi maraza erupted for three days in 1902 injecting more than 80 000 t of ch4 into the atmosphere (guliyev and feyzullayev, 1997). from 1810 until the present, about 250 eruptions of 60 mud volcanoes have been observed in azerbaijan. sokolov et al. (1969) described violent eruptions of mud-volcanoes in the southern caspian basin, which have released hundreds of millions of cubic meters of gas and estimated that mud volcanoes in azerbaijan have produced 106 mt of gas in the last million years. most of these eruptions followed large earthquakes. in their global estimation of gas flux from mud volcanoes, milkov et al. (2003) concluded that the global eruptive degassing may be approximately equal to the global quiescent degassing. in contrast, dimitrov (2002) suggests that gas flux from quiescent periods is significantly (by a factor of up to 30) less than the gas flux during eruptions. direct measurements of methane flux from submarine mvs have rarely been performed (linke et al., 2005), and only in a few active points. some rough estimates, generally based on the volumes of mud extruded, are available as reviewed by kopf (2002). on the basis of available data, including mvs dimensions, depth and gas dissolution models, etiope and milkov (2004) have estimated that at least 0.5 mt of methane are injected into the atmosphere from mvs occurring at depths less than 200 m (shelf mvs). however, recent discoveries (e.g., holland et al., 2003) suggest that shelf mvs are fig. 1. sketch of methane origin and emission in hydrocarbon-prone basins. 3 mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget more abundant than previously assumed and that many of them release significant amounts of gas bubble plumes, which may easily cross the water column and enter the atmosphere. therefore detailed studies and measurements of gas flux during eruption, and direct measurements of gas flux from submarine mud volcanoes appear to be critical to further constrain the global gas flux from mvs. 3. microseepage etiope and klusman (2002) defined microseepage as the slow, continual loss of ch4 and light alkanes from depths of 2-5 km in sedimentary basins where thermal degradation of indigenous organic matter is occurring. microseepage is basically a pervasive, diffuse exhalation of methane from soil resulting from natural gas migration from underground hydrocarbon reservoirs. it is assumed that microseepage is a general phenomenon driven by buoyancy of the gas phase relative to connate waters (price, 1986; klusman, 1993; klusman and saeed, 1996; matthews, 1996); frequently, gas migration can be considered in terms of microbubbles, bubbles and slug flows along faults and fractured rocks (etiope and martinelli, 2002). it is evident that microseepage is enhanced along faults, especially those produced by neotectonics (klusman, 1993; etiope, 1999). in dry lands, methane flux is generally negative, from the atmosphere to the soil, due to methanotrophic oxidation by ch4-consuming fig. 2. typical mud volcano morphology and methane emission structures: (top) single crater mv, trinidad (from the geological society of trinidad and tobago); (bottom) multi-crater (gryphons) mv, paclele (eastern romania). 4 giuseppe etiope bacteria in the soil. due to this biological activity, dry lands are considered a net sink of atmospheric methane, on global scale (around 30 mt yr–1), with fluxes generally in the order of –5 to –1 mgm–2d–1 (dong et al., 1998). microseepage is instead responsible for less negative or positive fluxes of methane, indicating that soil consumption can be lower than the input from underground sources. the positive fluxes are typically of a few units or tens of mgm–2d–1, but may be at the hundreds level over wide tectonised and faulted areas in the most active microseeping regions. these values are comparable with the ch4 emission in wet, anaerobic ecosystems, which are typically in the range 1500 mgm–2d–1 (batjes and bridges, 1994). in mv areas microseepage may easily reach fluxes in the order of 103-105 mgm–2d–1. the highest microseepage flux ever reported has been found close to the fire of yanardag, in azerbaijan: > 560 000 mgm–2d–1 (etiope et al., 2004b). a review is made by etiope and klusman (2002), and data on microseepage linked to mvs are in etiope et al. (2002, 2003, 2004a,b). the global coverage of microseepage is unknown. potentially, microseeping areas are all the sedimentary basins in a dry climate, with petroleum and gas generation processes at depth: this area has been estimated to be around 43 366 000 km2 (klusman et al., 1998). preliminary models suggest that this area can produce a mean microseepage flux of 4.42 mg ch4 m–2d–1 (klusman et al., 1998, 2000) and 90% of methanotrophic consumption leading to a global emission of methane of about 7 mt yr–1. this is only a first, rough estimate, very likely quite conservative. today it is possible to suggest another estimate, based directly on experimental values and on the area of the tectonic zones (faulted) actually hosting gas reservoirs. we have first to distinguish microseepage close to mvs (mv microseepage) and microseepage far from mvs or in sedimentary basins without mvs (simply microseepage). global emission of mv microseepage has already been estimated by etiope and milkov (2004), who considered the diffuse flux occurring within the mv morphologic structure (hill, muddy cover, and external bound of 250 m); this mv microseepage is at least 1-2.4 mt yr–1. 3.1. upgraded microseepage data-set in order to estimate the global non-mv microseepage it is possible to refer to an upgraded data-set, including microseepage from united states (klusman et al., 2000), former soviet union (voitov, 1975; balakin et al., 1981) and new data from reconnaissance surveys, carried out in 2002, in non-mv zones of transylvania, central romania and along the adriatic coast of central italy. these are two of the most important gas producing areas of europe (schlumberger, 1987; cranganu and deming, 1996). in these areas, 40 soil-atmosphere flux measurements were carried out in soils hosting wheat and grass communities, typical of temperate climates, by closed-chamber method; gas was analysed in duplicate by portable micro-gc (etiope et al., 2002). the flux values ranged from – 5 to 142 mgm–2d–1, with a mean of 20 mgm–2d–1. only 6 flux values were negative (from –5 to –1.5 mgm–2d–1); the highest values (from 90 to 142 mgm–2d–1) were measured in the «cupello» gas reservoir (vasto) on the italian adriatic coast. here biogenic gas is exploited from sandy reservoirs at depths between 800 and 1100 m and thermogenic gas occurs in deeper carbonate reservoirs (schlumberger, 1987). the average microseepage value derived from the surveys cited in table i (excluding the higher values of great caucasus and azerbaijan) is around 10 mgm–2d–1. 3.2. new estimate of global microseepage area the flux data available today suggest that microseepage corresponds closely to the spatial distribution of underground petroleum reservoirs. instead of considering the whole area covered by sedimentary basins, as made by klusman et al. (2000), it is today possible to estimate the global area of the onshore petroleum reservoirs. this has been made elaborating the data from the last us geological survey world petroleum assessment (usgs, 2000). this work named and mapped 159 of the largest total petroleum systems (tps’s) in the world using geographic information system. the tps’s are the hydrocarbon-fluid systems in the lithosphere including the essential elements and processes table i. microseepage in hydrocarbon-prone (no mud volcanos) areas. reference no. of sites flux range (mean) mgm–2d–1 denver-julesburg basin (colorado) klusman et al. (2000) 84 – 41 to 43.1 (0.57) piceance (colorado) klusman et al. (2000) 60 – 6.0 to 3.1 (– 1.1) powder river (wyoming) klusman et al. (2000) 78 – 14.9 to 19.1 (0.02) railroad valley (nevada) klusman et al. (2000) 120 – 6.1 to 4.8 (– 0.2) great caucasus balakin et al. (1981) unknown 430 lesser caucasus balakin et al. (1981) unknown 12 kura depression balakin et al. (1981) unknown 8 azerbaijan voitov (1975) unknown 28-200 transylvania (central romania) tarnaveni-bazna this work 5 2 to 64 (24) abruzzo adriatic coast (central italy) vasto this work 30 – 5 to 142 (22) pescara this work 5 – 4 to 13 (3.5) 5 mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget needed for oil and gas accumulations, migration and seeps. it is assumed, therefore, that microseepage occurs throughout the onshore tps areas. based on a careful analysis of tps map and gis data-sets, the global microseepage area can be estimated in the order of 8 × 106 km2. assuming conservatively a mean microseepage in the range 5-10 mgm–2d–1, a simple scalingup would give a global emission of 14-28 mt yr–1. 4. conclusions mud volcanoes and microseepage are closely related to neotectonic and seismic processes, and represent two important natural sources of atmospheric methane. the estimate of global emission of methane from onshore mud volcanoes has recently been refined thanks to new experimental data sets acquired in europe and azerbaijan. global microseepage has been estimated with less accuracy due to the few measurements available. a refinement is here proposed considering new data from hydrocarbon areas in u.s.a., former soviet union, romania, italy, and a more accurate assessment of the global area in which microseepage may occur. potentially, the resulting global microseepage output can be in order of 14-28 mt yr–1. this is a provisional estimate based on the assumption «microseepage area = tps area». a large number of data over wide areas, from different tps, and more accurate scaling-up procedures are necessary to reach a more constrained estimate. given these uncertainties, the global emission of methane from geological sources, including mvs (6-9 mt yr–1), marine seepage (20 mt yr–1), geothermal flux (2.5-6.3 mt yr–1) and microseepage in petroliferous basins (14-28 mt yr–1) would amount at least to 40-60 mt yr–1. the previous estimate was 35-45 mt yr–1 (etiope and milkov, 2004). these numbers are of the same level of or higher than other sources or sinks considered in the tables of the intergovernmental panel on climate change (ipcc, 2001), such as biomass burning (40 mt yr–1), termites (20 mt yr–1), oceans (10 mt yr–1) and soil uptake (30 mt yr–1). these results show clearly that geologic methane sources, strictly controlled by geodynamic and tectonic processes, have a primary role in the atmospheric greenhouse gas budget. acknowledgements thanks are due to feliks persits (usgs, central energy resources team) for providing the gis data-set for tps. 6 giuseppe etiope microseepage data of transylvania are from preliminary surveys carried out with calin l. baciu (babes-bolyay university of cluj-napoca), franco italiano and antonio caracausi (ingv, palermo section), in the framework of a nato project (contract est.clg.977422). references balakin, v.a, g.a. gabrielants, i.s. guliyev, f.g. dadashev, v.m. kolobashkin, a.i. popov and a.a. feyzullayev (1981): test of experimental study of hydrocarbon degassing of lithosphere of south caspian basin and adjacent mountains systems, using laser gasanalyzer «iskatel-2», dokl. akad. nauk sssr, 260 (1), 154-156 (in russian). batjes, n.h. and e.m. bridges (1994): potential emissions of radiatively active gases from soil to atmosphere with special reference to methane: development of a global database (wise), j. geophys. res., 99 (d8), 16,479-16,489. cranganu, c. and d. deming (1996): heat flow and hydrocarbon generation in the transylvanian basin, romania, aapg bull., 80 (10), 1641-1653. dimitrov, l. (2002): mud volcanoes – the most important pathway for degassing deeply buried sediments, earthsci. rev., 59, 49-76. dong, y., d. scharffe, j.m. lobert, p.j. crutzen and e. sanhueza (1998): fluxes of co2, ch4 and n2o from temperate forest soil: the effect of leaves and humus layers, tellus, 50b, 243-252. etiope, g. (1999): subsoil co2 and ch4 and their advective transfer from faulted grassland to the atmosphere, j. geophys. res., 104 (d14), 16,889-16,895. etiope, g. and r.w. klusman (2002): geologic emissions of methane to the atmosphere, chemosphere, 49, 777-789. etiope, g. and g. martinelli (2002): migration of carrier and trace gases in the geosphere: an overview, phys. earth planet. int., 129, 3-4, 185-204. etiope, g. and a.v. milkov (2004): a new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere, environ. geol., 46 (8), 997-1002. etiope, g., a. caracausi, r. favara, f. italiano and c. baciu (2002): methane emission from the mud volcanoes of sicily (italy), geophys. res. lett., 29 (8), doi:10.1029/2001gl014340. etiope, g., a. caracausi, r. favara, f. italiano and c. baciu (2003): reply to comment by a. kopf on «methane emission from the mud volcanoes of sicily (italy)», and notice on ch4 flux data from european mud volcanoes, geophys. res. lett., 30 (2), 1094, doi:10.1029/2002gl016287. etiope, g., c. baciu, a. caracausi, f. italiano and c. cosma (2004a): gas flux to the atmosphere from mud volcanoes in eastern romania, terra nova, 16 (4), 179-184. etiope, g., a. feyzullayev, c. baciu and a.v. milkov (2004b): methane emission from mud volcanoes in the eastern azerbaijan, geology, 32 (6), 465-468. guliyev, i.s. and a. feyzullayev (1997): all about mud volcanoes (baku pub. house, nafta-press), pp. 120. holland, c.w., g. etiope, a.v. milkov, e. michelozzi and p. favali (2003): mud volcanos discovered offshore sicily, mar. geol., 199, 1-6. ipcc (intergovernmental panel on climate change) (2001): climate change 2001: the scientific basis, edited by j.t. houghton, y. ding, d.j. griggs, m. noguer, p.j. van der linden, x. dai, k. maskell and c.a. johnson (cambridge univ. press., cambridge, u.k.), pp. 881. klusman, r.w. (1993): soil gas and related methods for natural resource exploration (john wiley and sons, chichester, u.k.), p. 483. klusman, r.w. and m.a. saeed (1996): comparison of light hydrocarbon microseepage mechanisms, in hydrocarbon migration and its near-surface expression, edited by d. schumacher and m.a. abrams, am. assoc. petrol. geol. mem. 66 (tulsa, ok), 157-168. klusman, r.w., m.e. jakel and m.p. leroy (1998): does microseepage of methane and light hydrocarbons contribute to the atmospheric budget of methane and to global climate change?, assoc. petrol. geochem. explor. bull., 11, 1-55. klusman, r.w., m.e. leopold and m.p. leroy (2000): seasonal variation in methane fluxes from sedimentary basins to the atmosphere: results from chamber measurements and modeling of transport from deep sources, j. geophys. res., 105 (d20), 24,661-24,670. kopf, a.j. (2002): significance of mud volcanism, rev. geophys., 40 (2), 1005, doi:10.1029/2000rg000093. linke, p., k. wallmann, e. suess, c. hensen and g. rehder (2005): in-situ benthic fluxes from an intermittently active mud volcano at the costa rica convergent margin, earth planet. sci. lett. (in press). matthews, m.d. (1996): hydrocarbon migration-a view from the top, in hydrocarbon migration and its nearsurface expression, edited by d. schumacher and m.a. abrams, am. assoc. petrol. geol. mem. 66, (tulsa, ok), 139-155. milkov, a.v. (2000): worldwide distribution of submarine mud volcanoes and associated gas hydrates, mar. geol., 167 (1-2), 29-42. milkov, a.v., r. sassen, t.v. apanasovich and f.g. dadashev (2003): global gas flux from mud volcanoes: a significant source of fossil methane in the atmosphere and the ocean, geophys. res. lett., 30 (2), 1037, doi:10.1029/2002gl016358. morner, n.a. and g. etiope (2002): carbon degassing from the lithosphere, global planet. change, 33 (1/2), 185-203. price, l.c. (1986): a critical overview and proposed working model of surface geochemical exploration, in unconventional methods in exploration for petroleum and natural gas-iv, edited by m.j. davidson (southern methodist university, dallas, tx), 245-309. revil, a. (2002): genesis of mud volcanoes in sedimentary basins: a solitary wave-based mechanism, geophys. res. lett., 29 (12), 1574, doi:10.1029/2001gl014465. schlumberger (1987): well evaluation conference, wec 1987, schlumberger. 7 mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget sokolov, v.a., z.a. buniat-zade, a.a. geodekian and f.d. dadashev (1969): the origin of gases and mud volcanoes and the regularities of their powerful eruptions, in advances in organic geochemistry edited by p.a. schenck and i. havenaar (oxford, pergamon press), 473-484. usgs world energy assessment team (2000): us geological survey world petroleum assessment 2000, description and results, digital data series – dds-60 (us dept. of the interior, usgs). voitov, g.i. (1975): gas breath of earth, nature, 3, 9198 (in russian). 191_200 adg vol5 n02 xenos.pdf annals of geophysics, vol. 45, n. 1, february 2002 191 the effects of f0 f2 variability on tec prediction accuracy thomas d. xenos department of electrical engineering, aristotelian university of thessaloniki, greece abstract in this paper hourly daily f2-layer critical frequency data recorded at rome and one minute daily tec data recorded at florence were used and the relevant variabiles were calculated. it was concluded that there was no clear evidence as to how they correlated. in order to obtain a measure of the f0f2 and tec variability, the normalised differences df 0 f 2 and d tec from the relevant monthly median values were also considered. since no clear evidence could be obtained as of how df0f2 and d tec correlate, a new parameter, the ∆ ap/∆ r ratio was tried. ∆ ap was taken as the difference between the maximum value of a p measured at the relevant disturbance and that corresponding at the beginning of the disturbance. ∆ r corresponded to the two above mentioned values of a p . this parameter was compared to the differences of the corresponding df 0 f 2 values called ∆ df and d tec values called ∆ dt. in wintertime, when ∆ a p /∆ r was negative, for the vast majority of the occurrences either ∆ df or ∆ dt was negative; ∆df and ∆dt were never observed to be negative at the same time whereas they were both positive in fewer than 10% of the observations. when ∆ a p /∆ r was positive then either ∆ df or ∆ dt were negative. in summertime when ∆ a p /∆ r was negative both ∆ df and ∆ dt were negative. when ∆ a p /∆ r was positive, while a positive ∆ df corresponded almost always to a positive ∆ dt, a negative ∆ df would equiprobably indicate either a positive or a negative ∆ dt. 1. introduction the prediction of ionospheric total electron content (tec) is a complex problem. the greatest contribution to the tec is from the ionospheric f-layer, which in turn is a very variable ionised region of the higher atmosphere, whose electron concentration and distribution are governed (kouris et al., 1998, 1999) mainly by solar and geomagnetic phenomena. the introduction of f0 f2 in neural network based, one-hour ahead, one-day ahead, two-days ahead and seven-days ahead tec forecasting models has been recently investigated (xenos, 1999) and proved very successful. in fact these models are far more accurate than the well known and widely used physical or empirical models that incorporate statistical or numerical methods. however, the tec variability is not governed exactly by the same factors as f 0 f 2 variability, since the topside ionosphere and influences from the plasmasphere above the f-region are important contributors to tec. although recently, the f 0 f 2 was used successfully as an index in tec prediction models (xenos et al., 2000), due to its strong variability (kouris, 1999), it is reasonable to investigate the correlation between the f0 f2 and tec variability. the present work, investigated the problem of the correlation between the f 0 f 2 and tec variability. therefore, f2-layer critical frequency data recorded at rome and tec data recorded at florence have been used. mailing address: dr. thomas d. xenos, aristotle university of thessaloniki, faculty of technology, department of electrical engineering, telecommunications division, 54006 thessaloniki, greece; e-mail: tdxenos@vergina.eng.auth.gr key words ionosphere – ionospheric modelling – ionospheric variability – neural networks 192 thomas d. xenos 2. data and analysis hourly-daily tec values produced from one minute faraday-rotation measurements, from geostationary satellites, recorded at florence (spalla et al., 1987) from the years 1975-1982 and 1989-1991 were correlated to f0 f2 hourly-daily data measured at rome. the daily a p and r indices were used to define whether the ionosphere was quiet or disturbed. therefore, f 0 f 2 , tec, a p and r graphs were compiled. when a p exceeded 40 the ionosphere was characterised disturbed and the consequences of the disturbance on f 0 f 2 and tec were studied. for a more detailed analysis the time span of the study preceded and followed the disturbance occurrence by 24 h. in order to obtain a measure of the f0 f2 and tec variability, the normalised differences df 0 f 2 and d tec from the relevant monthly median values were also considered. the formulas used for these calculations were (2.1) (2.2) where f0 f2obs the observed hourly daily f0 f2 values; tecobs the observed hourly daily tec values; f 0 f 2med the hourly monthly median f 0 f 2 values; tec med the hourly monthly median tec values. since no clear evidence could be obtained as to how df 0 f 2 and d tec correlated, a new parameter, the ∆ a p /∆ r ratio was tried. ∆ a p was taken as the difference between the maximum value of a p measured at the relevant disturbance and that corresponding at the beginning of the disturbance i.e. as soon as a p exceeded 40. ∆ r corresponded to the two above mentioned values of ap. this new parameter was compared to the differences of the corresponding df0f2 values called ∆ df and d tec values called ∆ dt. 3. results and discussion from figs. 1a-c it can be seen that when a p increased and exceeded 40, i.e. when the ionosphere could be considered as disturbed, f 0 f 2 showed a steep increasing trend whereas tec usually, though not always, had an increasing one with respect to what was shown before the disturbance occurrence. a cross correlation analysis using a variable correlation period showed that the response time difference between the f0 f2 and the tec was of the order of 3-5 h, the f0 f2 leading. the gradients measured between the f 0 f 2 and tec values corresponding to the start of the phenomenon and their maximum or minimum values, depending on the trend, were almost always proportional to the a p values, more specifically to the a p increase rate, and were stronger at high solar activity periods. it is worth mentioning that after the end of the disturbance, the f0 f2 value reached a minimum, which almost always coincided with the minimum value of the month for the specific hour (kouris and fotiadis, 2000). since no clear evidence of the behavioural differences between f0 f2 and tec values could be obtained, a comparison between their variability was attempted. using eqs. (2.1) and (2.2), the normalised differences df 0 f 2 and d tec for the above data set were obtained. figures 2a-d show several characteristic cases. again, no clear evidence could be obtained as to how df 0 f 2 and d tec correlate, since a positive df0 f2 may be accompanied by a positive or negative d tec and vice versa. therefore, the ∆ a p /∆ r ratio was compared to ∆ df and ∆ dt. it can be observed (fig. 3a) that in winter and when the ionosphere is characterised as disturbed, the ∆ a p /∆ r ratio is usually negative, whereas this ratio takes positive values for over 60% of the occurrences in summer (fig. 3b). in wintertime, when ∆ a p /∆ r was negative (fig. 4a), for the vast majority of the occurrences either ∆ df or ∆ dt was negative; ∆ df and ∆ dt were never observed to be negative at the same time whereas they were both positive in fewer than 10% of the observations. when ∆ a p /∆ r was positive then either ∆ df or ∆ dt were negative. df f f f f f f f 0 2 0 2 0 2 0 2 = − obs med med dtec tec tec tec = −obs med med 193 the effects of f 0 f 2 variability on tec prediction accuracy 0 40 80 120 160 200 a p aug 1982 3 6 9 12 15 18 f0 f 2 [ m h z] 0 20 40 60 80 100 t e c [ t e c u ] 0 50 100 150 200 250 300 1 101 201 301 401 501 601 701 hours r fig. 1a. characteristic month showing the f0 f2 (solid line) and the tec (dashed line), ap and r values. 194 thomas d. xenos 0 20 40 60 80 100 t e c [ t e c u ] 3 6 9 12 15 18 fo f 2 [ m h z] oct 1989 0 40 80 120 160 200 a p 0 50 100 150 200 250 300 r 1 101 201 301 401 501 601 701 hours fig. 1b. characteristic month showing the f 0 f 2 (solid line) and the tec (dashed line), a p and r values. 195 the effects of f 0 f 2 variability on tec prediction accuracy 0 20 40 60 80 100 t e c [ t e c u ] nov 1989 3 6 9 12 15 18 fo f 2 [ m h z] 0 40 80 120 160 200 a p 0 50 100 150 200 250 300 1 101 201 301 401 501 601 701 hours r fig. 1c. characteristic month showing the f0 f2 (solid line) and the tec (dashed line), ap and r values. 196 thomas d. xenos fig. 2a. presentation of df0 f2 (solid line), d tec (dashed line), ap and r. -2 0 2 4 6 8 d t e c -0.5 0 0.5 1 1.5 2.5 d fo f 2 feb 1978 0 40 80 120 160 200 a p 0 50 100 150 200 250 300 1 101 201 301 401 501 601 hours r 197 the effects of f 0 f 2 variability on tec prediction accuracy fig. 2b. presentation of df0 f2, d tec, ap and r. jun 1978 -1 -0.5 0 0.5 1 1.5 2 d fo f 2 -4 -2 0 2 4 6 8 d t e c 0 40 80 120 160 200 a p 0 50 100 150 200 250 300 1 101 201 301 401 501 601 701 hours r 198 thomas d. xenos -1 -0.5 0 0.5 1 1.5 2 d fo f 2 -4 -2 0 2 4 6 8 d t e c 0 40 80 120 160 200 a p 0 50 100 150 200 250 300 1 101 201 301 401 501 601 701 hours r jul 1991 fig. 2c. presentation of df 0 f 2 , d tec, a p and r. 199 the effects of f 0 f 2 variability on tec prediction accuracy fig. 2d. presentation of df 0 f 2 , d tec, a p and r. jun 1990 -1 -0.5 0 0.5 1 1.5 2 d fo f 2 -4 -2 0 2 4 8 d t e c 0 40 80 120 160 200 a p 0 50 100 150 200 250 300 1 101 201 301 401 501 601 701 hours r 200 thomas d. xenos fig. 3a. ∆ a p /∆ r behaviour in winter. fig. 3b. ∆ a p /∆ r behaviour in summer. fig. 4a. ∆ df versus ∆ dt behaviour in wintertime and when ∆ a p /∆ r is negative. fig. 4b. ∆ df versus ∆ dt behaviour in summertime and when ∆ a p /∆r is positive. in summertime, when ∆ a p /∆ r was negative both ∆ df and ∆ dt were negative. on the other hand, when ∆ a p /∆ r was positive (fig. 4b), while a positive ∆ df corresponded almost always to a positive ∆ dt, a negative ∆ df would equiprobably indicate either a positive or a negative ∆ dt. acknowledgements the author thanks dr. paolo spalla for providing the tec data together with the necessary interpretation and valuable explanations. references kouris, s.s. and d.n. fotiadis (2000): a study on the variability of some ionospheric characteristics, paper 1252 presented at ap-2000 meeting at davos, switzerland kouris, s.s., d.n. fotiadis and t.d. xenos (1998): on the day-to-day variation of f0f2 and m(3000)f2, adv. space res., 22 (6), 873-876. kouris, s.s., d.n. fotiadis and b. zolesi (1999): specifications of the f-region variations for quiet and disturbed conditions, phys. chem. earth, 24 (4), 321-327. spalla, p., e. capannini and l. ciraolo (1987): sirio: a good chance for eight years of ionospheric research, alta freq., lvi (1-2), 167. xenos, th.d. (1999): neural network based single station models of the f 0 f 2 and m(3000)f 2 ionospheric characteristics, ursi 99, xxvi general assembly. xenos, th.d., p. spalla and c. mitchell (2000): neural network based tec forecasting models, paper 0904 presented at ap-2000 meeting at davos, switzerland. winter 0 20 40 60 80 (-) (+) ∆ a p /∆ r winter ∆ap / ∆r negative -100 -80 -40 -20 0 20 40 60 80 100 (−) ∆df (+) ∆ d t -60 -80 -60 -40 -20 0 20 40 60 80 summer ∆ap/∆r -100 100 (-) ∆df (+) ∆ d t summer 0 20 40 60 80 (-) (+) ∆ a p /∆ r adg vol5 n02 fili 233_245.pdf annals of geophysics, vol. 45, n. 2, april 2002 233 aerial remote sensing hyperspectral techniques for rocky outcrops mapping carolina filizzola (1), nicola pergola (1), stefano pignatti (1) and valerio tramutoli (2) (1) istituto di metodologie per l’analisi ambientale, cnr, tito scalo (pz), italy (2) dipartimento di ingegneria e fisica dell’ambiente, università della basilicata, potenza, italy abstract in this work the mivis (multispectral infrared and visible imaging spectrometer) hyperspectral data, acquired during aerial campaigns made in 1998 over the pollino national park in the framework of the «progetto pollino», have been used to set up a supervised technique devoted to identify the presence of selected rocky outcrops. tests have been performed over an extended area characterised by a complex orography. within this area, serpentinite was chosen as a test-rock because it is present in isolated outcrops, distributed all over the test-area, besides subtending important problems of environmental nature as it contains asbestos. geological information, coming from field observations or geological maps, was used to trigger the algorithms and as ground truth for its validation. two spectral analysis techniques, sam (spectral angle mapper) and lsu (linear spectral unmixing), have been applied and their results combined to automatically identify serpentinite outcrops and, in some cases, to mark their boundaries. the approach used in this work is characterised by simplicity (no atmosphere and illumination corrections were performed on mivis data), robustness (material of interest is identified for certainty) and intrinsic exportability (the method proposed can be applied on different geographic areas and, in theory, to identify any kind of material because no datum about atmospheric and illumination conditions is required). 1. introduction it is well-known that any material reflects or emits radiation with different intensity at different wavelength, according to its own physicalchemical properties. theoretically, it is possible to identify different materials by analysing their characteristic spectral response (spectral signature, in absorption, emission or reflection) on the basis of suitable multi-spectral radiometric measures (fig. 1). that has been easily done in controlled laboratory conditions and attested by a number of published works reporting laboratory spectral signatures of almost any material (e.g., clark et al., 1990; salisbury and d’aria, 1992). a number of additional problems arise when surface material identification is attempted by airborne multispectral sensors. the e.m. signal leaving the investigated surface, before reaching the airborne remote sensor, is affected by spectrally selective atmospheric absorption and mailing address: dr. valerio tramutoli, dipartimento di ingegneria e fisica dell’ambiente, università della basilicata, campus di macchia romana, 85100 potenza, italy; e-mail: tramutoli@unibas.it key words hyperspectral sensor – airborne remote sensing – robust techniques – geological mapping – lsu – sam – mivis – serpentinite – pollino 234 carolina filizzola, nicola pergola, stefano pignatti and valerio tramutoli scattering that corrupt the original spectral behaviour of the signal making the identification of surface properties by simple comparison with laboratory spectra misleading. other known variable factors, not present during laboratory observations, which could affect remotely sensed spectra are mainly related to the illumination conditions (in the solar spectral range) the angles of view and surface homogeneity within the ground resolution cell. in particular, even considering the same relative sun position and ground target characteristics, atmospheric conditions and topography greatly influence the electromagnetic signal which, leaving the ground, achieves the airborne sensor. the presence of atmosphere, as a transmission medium through which radiation travels, modifies e.m. signals according to three main physical processes (atmosphere-radiation interaction): absorption, scattering and emission. absorption is a spectrally selective process (mainly due to atmospheric molecules of water, carbon dioxide, ozone and oxygen) that selectively reduces the incoming e.m. signal, whereas scattering is the result of diffused multiple reflections of e.m. radiation by atmospheric gases and suspended particles which, similarly, affect the intensity (but also the spatial distribution) of the signal received by the sensor. moreover, the atmosphere acts as a source of e.m. radiation due to its thermal state, according to planck’s law. topographic effects are particularly important for observations in the solar spectral range. the local landscape orientation with respect to the sensor view angle and the sun controls both the incoming sun radiation and the signal reflected toward the airborne sensor. the effects on the reflected radiances can be severe, especially in areas of high and variable relief. figure 2a,b shows, as an example, how the reflectance spectra of the same materials (limer e fl e c ta n c e [ % ] wavelength [ µm] 0.5 1.0 1.5 2.0 2.5 0.0 0.1 0.2 0.3 0.4 0.5 0.6 limestone serpentinite vegetation fig. 1. laboratory reflectance spectra of limestone, vegetation and serpentinite samples collected in the test-area using an asd, fieldspec fr(350-2500 nm) spectroradiometer. r e fl e c ta n c e [ % ] wavelength [µm] 0.5 1.0 1.5 2.0 2.5 0.0 0.1 0.2 0.3 0.4 0.5 0.6 limestone serpentinite vegetation r a d ia n c e [ n w /s tr / n m /c m 2 ] wavelength [µm] 0.5 1.0 1.5 2.0 2.5 0 1500 3000 4500 6000 7500 9000 10500 limestone serpentinite vegetation a b fig. 2a,b. mivis spectra (not atmospherically corrected) for limestones, vegetation and serpentinites present in the test-area: a) reflectances, b) radiances at the sensor: they were used as reference for the sam and as end-members for lsu processing steps (see text). 235 aerial remote sensing hyperspectral techniques for rocky outcrops mapping stone, serpentinite and vegetation) represented in fig. 1 change when retrieved from multispectral radiances acquired by an airborne sensor (mivis in this case) instead of controlled laboratory conditions. comparing fig. 1 and fig. 2a, the differences between mivis reflectance spectra and the laboratory ones (due mainly to the presence of the atmosphere, different observational conditions and possible soil heterogeneity within the ground resolution cell) are quite evident. 2. mivis nowadays, many airborne hyperspectral sensors (aviris, hydice, dais, hymap, etc.) are active. they are able to acquire data in a great number of narrow spectral bands which, in theory, obtain from airborne or spaceborne platforms, spectral response curves comparable with those obtained in the laboratory. among the other hyperspectral sensors, mivis (bianchi et al., 1994) offers a wide range of wavelengths (from the visible to the thermal-ir) allowing the collection of accurate spectra of any exposed material on the whole optical range. mivis, operational since 1994 in the framework of the «cnr-lara (airborne laboratory for environmental studies) project», is a multispectral imaging system with a spectral resolution ranging between 8 and 540 nm (moving from the vis to the tir range) over 102 spectral bands. its main features are reported in table i. mivis data were acquired during two flights (in june and november 1998) carried out in the framework of the «pollino project» (cuomo et al., 2001) over the whole lucan area of the pollino national park (fig. 3a,b). during each flight (made at around local noon), 15 data strips (about nnw-sse flight direction) were collected at a (relative) altitude of 3000 m and with a scan rate of 17 scans/s. according to orography and angular view, the dimensions of ground resolution cells range between 5.8 and 13.5 m. figure 3c points out the test-area among mivis strips. 3. methodology to identify a material by remotely sensed radiances usually they are compared with laboratory spectra (fig. 4a), after a preliminary correction step to clear atmospheric, topographic and illumination effects out of the measured signal (mcardle et al., 1992; ben-dor et al., 1994; farrand et al., 1994; clark et al., 1995; smith and milton, 1999). digital elevation models are needed to correct topographic effects, while the atmospheric ones are usually evaluated by applying well-tested radiative transfer models (rtm) like 6s, modtran, etc. such models simulate atmosphere-radiation interactions on the basis of information to be given on atmospheric (chemical-physical) and observational conditions at the time of the observations. contemporary table i. main characteristic of the mivis sensor. fov (field of view) 71.059° ifov (instantaneous fov) 2 mrad spatial resolution 6 m at 3000 m of (relative) flight height swath width 4.2 km at 3000 m of (relative) flight height 102 bands simultaneously sampled and recorded visible (0.43-0.83 µm) 20 bands near-ir (1.15-1.55 µm) 8 bands mid-ir (2.0-2.5 µm) 64 bands thermal-ir (8.2-12.7 µm) 10 bands 236 carolina filizzola, nicola pergola, stefano pignatti and valerio tramutoli fig. 3a-c. a) position of lucan area of the pollino national park in southern italy; b) flight plan of mivis airborne hyperspectral sensor; c) mivis strips (the test-area is indicated by a black box). b c a 237 aerial remote sensing hyperspectral techniques for rocky outcrops mapping ground-based measurements of atmospheric parameters (like vertical temperature and water vapour profiles, aerosol dimensional distribution and optical thickness, etc.) if available, otherwise standard atmospheric models, are required, to give the appropriate inputs to rtm codes. whether based on punctual ground-based measurements or on standard atmospheric models, rtm codes assume an horizontally homogeneous atmosphere (physically and chemically) to perform those corrections on the whole scene. but because of the high spatial dynamic of atmospheric chemical-physical properties, the radiometric correction process will suffer errors which still remain high. so the use of traditional radiometric correction methods can reveal poorly reliable results (large errors due to the use of not appropriate atmospheric models) and difficulties of application (requiring contemporary ground observations not always available). in this paper, a different method (fig. 4b) is proposed to map surface materials using hyperspectral radiances at the airborne sensor and a limited set of field observations. it starts from the experimental observation that raw (i.e. not corrected for atmospheric effects) reflectance spectra (fig. 2a), notwithstanding the atmospheric effects that make them different from laboratory spectra, still distinguish different materials. this is still evident as far as radiances at the sensor are considered (fig. 2b). in this case, moreover, it is possible to see (fig. 5) how the same material, under different illumination conditions, presents traditional method proposed method mivis hyperspectral radiances at the sensor ground measures atmospheric models dem radiometric and geometric corrections mivis corrected data laboratory spectra identification ground observations on a small area reference spectral curves identification mivis hyperspectral radiances at the sensor fig. 4a,b. material identification by hyperspectral data: flow diagrams for traditional (a) and proposed (b) methods. a b 238 carolina filizzola, nicola pergola, stefano pignatti and valerio tramutoli different spectra but still with the same shape (excluding noising bands). starting from such considerations, spectral shapes of mivis radiances at the sensor (instead of mivis reflectance spectra obtained after correction for atmospheric effects) were considered for materials identification. to this aim, the sam (spectral angle mapper, kruse et al., 1993) supervised classification technique was implemented: it identifies surface materials not by comparison with reference laboratory spectra but using, as reference, spectral shapes exhibited by selected pixels on the scene, whose actual composition is independently (for instance by field observations) known (fig. 4b). 4. application of the proposed method on a test-area the proposed method was applied to identify serpentinite outcrops in a topographically complex area of the pollino national park. the area is geologically characterised by ophiolitic rocks, meso-cenozoic platform units and plioquaternary sediments (monaco et al., 1995; schiattarella, 1996). for this area a detailed geological survey together with a 1:50 000-scale map were available. among ophiolitic rocks, serpentinite was chosen as a test-rock because it is present in isolated outcrops in the test-area, besides subtending important environmental problems as it contains asbestos. before describing, step by step, the application procedure, some aspects must be pointed out: – the e.m. radiation, detected by airborne sensors operating in the optical range, transports information related only to the earth surface skin (from 50 µm up to few centimetres of depth moving from the vis to the tir spectral range). this means that rocks covered by vegetation cannot be detected and identified. for this reason, a vegetation mask was generated to restrict the fig. 5. mivis spectra (in the range 0.4-2.5 µm) for limestones and serpentinites present in the test-area under different illumination conditions. 239 aerial remote sensing hyperspectral techniques for rocky outcrops mapping following image processing steps only on areas with well outcropping rocks. – rocks to be identified can be under different illumination conditions (fig. 5): this possibility was considered choosing supervised classification algorithms less affected by these effects (as the above mentioned sam working on the shapes of the spectra). – each ground pixel can be composed by mixing distinct materials (different kinds of rocks, soil, vegetation, etc.): this circumstance was considered when the possibility to estimate rocky outcrops boundaries was verified. all 102 mivis channels, except noisy bands (at 2.234 µm, 2.265 µm, 2.411 to 2.474 µm), were processed and used in the following steps. 4.1. step 1: vegetation mask generation vegetated areas were excluded from the following processing steps by generating a vegetation mask on the basis of spectral signatures, specific to the vegetated surfaces, in the available vis and tir mivis channels: the mivis bands ratio, channel 6/channel 13 (i.e. 0.542 µm/0.682 µm), is known to be generally higher whereas the radiant temperature in channel 93 (8.37 µm) is expected systematically lower for vegetated pixels compared with non-vegetated areas. to discriminate non-vegetated areas and, notwithstanding substantial agreement (fig. 6a,b) between the two different tests, the most restrictive solution (radiant temperature test) was chosen and applied to the whole scene to detect fig. 6a,b. vegetation masks (s. severino area) obtained considering: a) mivis radiant temperature in channel 93 (8.37 µm) not greater than 303 k; b) bands ratio 6/13 (0.542 µm/0.682 µm) greater than 1.3. vegetated areas are black, the non-vegetated ones are white (see text). a b 240 carolina filizzola, nicola pergola, stefano pignatti and valerio tramutoli «well exposed areas» to be retained for the following steps. 4.2. step 2: reference radiance spectra selection supervised processing techniques, used in the following steps for rocky outcrops identification as well as for the evaluation of outcrops boundaries, require spectral curves to be used as reference for the materials to be identified on the scene. for this purpose, on the basis of geological maps and aerial photos, limited areas, representative of different materials, were identified on the scene and the corresponding mean spectral radiances (obtained by averaging mivis radiances at the sensor for each area) were used, instead of laboratory spectra, as reference (in fig. 7a, for instance, the region of interest chosen for serpentinite is indicated). 4.3. step 3: serpentinite identification to identify serpentinite outcrops, a supervised classification was used. such a technique identifies in a scene pixels which have radiance spectra the most similar to the ones exhibited by selected pixels assumed as reference. identification is made by rules which differ depending on the chosen method. in particular, to identify serpentinite outcrops in fig. 7a,b. sam results corresponding to some zones of the test-area: s. severino area (a) and the torno outcrop (b). serpentinite identified by sam is indicated in green. small circles, indicated in different colors, point out isolated pixels corresponding to unmapped serpentinite identified by sam. the blue circle indicates the site where the photo shown in fig. 9a was taken during the field check. s.severino lucano region of interest of serpentinites a b torno 241 aerial remote sensing hyperspectral techniques for rocky outcrops mapping where r i denotes the measured radiance of the mixed spectrum in band i, r ij is the radiance in the same band expected for the end member j, f j is the fraction of material j in the mixture and ε i is the residual error on r i . lsu was applied to define boundaries of serpentinite outcrops, where serpentinite presence was already identified by sam. we will discuss here, in particular, the case of the torno outcrop because its outcropping conditions (vegetated hillside without visible reference points) made it difficult to accurately delimit its boundaries even by direct field observations. in the selected area dominant materials were limestones, serpentinites and vegetation. so their reference spectra were considered as end members to be used in lsu. they have been defined in step 2 and are represented (in the range 0.4-2.5 µm) in fig. 2b. 5. results in order to verify sam results, mivis imagery and image processing products were preliminarily co-registered upon available digital geological maps of the investigated area. in situ observations were moreover performed where serpentinite outcrops identified by sam were not documented elsewhere. figure 7a,b shows sam results in two zones, s. severino area (a) and the torno outcrop (b), of the test-area: pixels recognized as serpentinitemade are green. all the small circles, indicated in different colors, emphasize isolated pixels (otherwise not easily visible on the image) identified as serpentinite.they were all checked in field and a photo was taken in the site indicated by the blue circle in fig. 7a and reported in fig. 9a. sam results were overlaid on geological maps: fig. 8 shows two examples of that. pixels recognized as serpentinites by sam are red coloured. the overlaying of sam results over geological maps shows that wherever sam identifies serpentinite it is really present. in particular, the identification takes place where: • serpentinite outcrops as bodies which are also pointed out in geological maps (90% of observational conditions which can be variable across the scene, the sam method (kruse et al., 1993) was chosen. working on spectral shapes (instead than on absolute values) the sam classifier is expected to be less affected by the variable contributions of illumination, topographic and atmospheric effects across the scene. in particular, sam considers the angle ϑ between the n-dimensional vector r ≡ (r 1 , r 2 , r 3 , ..., r n ) associated with the reference radiance spectra (being r i the radiance measured in the band ∆λ i and n the total number of bands) and the one, r ≡ ( r 1 , r 2 , r 3 , ..., r n ), measured at the pixel on the scene to be identified the smaller are the θ angles, the closer the resemblance to the reference spectrum. pixels further away than a specified maximum angle threshold θ max are identified as not belonging to the class of materials identified by the specific reference spectral curve r. in our case, the mivis reference spectrum r for serpentinite (plotted in fig. 2b) was chosen in the well exposed zone of timpa della guardia (shown in fig. 7a) and the sam classifier applied to the whole study area using a strong cut with θ max = 0.1 rad. 4.4. step 4: rocky outcrops boundaries evaluation the possibility to estimate rocky outcrops boundaries was verified, combining sam and lsu (linear spectral unmixing; adams et al., 1989). the latter is a technique which attempts to model the observed spectral radiance r i as a mixture of representative «prototype» spectra (end-members) as given below r i = f j . r i j + ε i with ∑ j f j = 1 ∂ = ⋅ ⋅ =arccos( )r r r r = + + + + + +( ) + + +( )           arccos r r r r r r r r r r r r n n n n 1 1 2 2 1 2 2 2 2 1 2 1 2 2 2 2 1 2 ... ... ... 242 carolina filizzola, nicola pergola, stefano pignatti and valerio tramutoli 1 km0 present-day and recent alluvium landslide bodies and detritus covers frido unit: shales frido unit: calc-shists metabasites serpentinites granitoides gneisses calabro-lucano flysch unit saraceno fm castronuovo conglomerates 1 km0 fig. 8. serpentinite identified by sam (red), laid on geological map of s. severino area (left) and frido stream (right). a b fig. 9a,b. some photos showing unmapped serpentinite grounds identified by the proposed method. left: serpentinite ground along a country road crossing geologically different grounds; right: serpentinite used as road-bed. «well exposed» outcrops in the test-area were identified by sam); • it is present as serpentinite soils which are not mapped elsewhere. field check (fig. 9a,b) has shown that such serpentinite soils: – correspond to landslide bodies or alluvial grounds; – are present, because of man’s action, along country roads crossing geologically different grounds; 243 aerial remote sensing hyperspectral techniques for rocky outcrops mapping fig. 10a,b. outcrop boundaries definition by combined use of sam and lsu (see text): a) geological map of the area around torno with serpentinites outcrop boundaries as recognized during field geological survey; b) result of lsu analysis. grey tones represent different levels, from low (dark) to higher (bright), of serpentinite contributions. pixels already recognized as serpentinite-made by sam are coloured in red. the new identified boundaries are indicated in light-blue by comparison with the previously ones (pink). 1 km0 serpentinites bifurto and cerchiara fms. cretaceous platform limestones calabro-lucano flysch unit frido unit: shales legend of the geological map a b 244 carolina filizzola, nicola pergola, stefano pignatti and valerio tramutoli – are present, because of gravity action, along roads below serpentinite outcrops. sam was able to locate serpentinite outcrops, but not to determine their true extension. this happens because of vegetation (which masks the signal leaving the underlying soil), but also because of the presence of mixed pixels whose spectral response is, as already pointed out, the result of the mixing of the contribution of many materials (not exclusively serpentinite). the less is the serpentinite contribution in each mixed pixel, the more the similarity between the pure serpentinite spectral end member and the observed spectra decreases. as a result, sam does not identify that such pixels also contain serpentinite. the combined use of sam and lsu estimated selected outcrop boundaries. figure 10a,b shows the combined use of sam and lsu analysis in the case of the torno serpentinite outcrop. figure 10a shows the geological map of the area around torno with serpentinites outcrop boundaries as recognized, or simply inferred, during field geological survey. such boundaries are reported, as a pink transparency, also in fig. 10b where pixels recognized by sam as serpentinite-made are coloured in red. the underlying grey-tone image in fig. 10b is the result of lsu analysis performed using only three end-members corresponding to the dominant surface material (vegetation, limestones and serpentinites) present in the torno area. grey tones represent different levels, from low (dark) to higher (bright), of serpentinite contributions. such interpretation remains valid wherever the mixing of surface materials includes the ones considered for the torno area and not where the presence of other materials (not included as end-members in the lsu processing) can make such an interpretation misleading (see for example as even asphalt roads appear bright in fig. 10b). starting from this consideration, it is correct to suppose that at least white pixels around the red ones (already recognized as serpentinite by sam) have a really high probability of being dominated by the presence of serpentinite in concurrence of lower percentages of vegetation and limestones. in this way, considering the combination between sam and lsu results, it is possible to give evidence of an underestimation of the outcrop extension whose northern boundary has to be moved to nw with respect to boundaries derived from field geological survey (fig. 10b). in conclusion: • test-rock identification by sam shows: – starting from field information relative to small areas (reference outcrops), it is possible to locate the same rock within a rather wide area, pointing out hyperspectral data possibility in searching any material within a wide area of distribution; – possibility to identify test-rock along riverbeds, which can be important in detecting a particular material within alluvial ground; – the proposed technique can be used to locate materials containing polluting substances (serpentinite contains asbestos). • the combined use of lsu and sam shows that such a methodology could be helpful in geological mapping when ground conditions do not allow an easy survey. • the proposed methodology can be exported to any kind of material, any geographical area (even with complex orography) and independently on the availability of coincident information on atmospheric parameters, as no atmospheric or topographic correction are required. acknowledgements this work was carried out within the framework of the «progetto pollino» (pop-fesr 1994-96), funded by european economic community and regione basilicata. references adams, j.b., m.o. smith and a.r. gillespie (1989): simple models for complex natural surfaces: a strategy for the hyperspectral era of remote sensing, in proceeding of the igarss ’89 symposium, july 10-14, vancouver, canada, 16-21. ben-dor, e., f.a. kruse, a.b. lefkoff and a. banin (1994): comparison of 3 calibration techniques for utilisation of ger 63-channel aircraft scanner data of makhtesh-ramon, negev, israel, photogramm. eng. remote sensing, 60, 1339-1354. bianchi, r., c.m. marino and s. pignatti (1994): airborne hyperspectral remote sensing in italy, in proceedings europto series «recent advances in 245 aerial remote sensing hyperspectral techniques for rocky outcrops mapping remote sensing and hyperspectral remote sensing» spie roma, 27-29 september 1994, 2318, 29-37. clark, r.n., t.v.v. king, m. klejwa, g.a. swayze and n. vergo (1990): high spectral resolution reflectance spectroscopy of minerals, j. geophys. res., 95, 12,653-12,680. clark, r.n., g.a. swayze, k. heidebrecht, r.o. green and a.f.h. goetz (1995): calibration to ground reflectance of terrestrial imaging spectroscopy data: comparison of methods, in proceeding of the 5th annual jpl airborne geoscience workshop, jpl publication 95-1 (pasadena, ca: jet propulsion laboratory), 41-42. cuomo, v., n. afflitto, m. blumetti, a. bonfiglio, o. candela, t. carone, g. di bello, c. filizzola, t. lacava, a. lanorte, v. lanorte, r. lasaponara, m. macchiato, l. minervini, f. mundo, n. pergola, c. pietrapertosa, s. pignatti, e. romano, t. simoniello, v. tramutoli and a. zaccagnino (2001): pollino project action d: a multi-scale approach, in the space-time domain, to environmental risk monitoring, in remote sensing for environmental monitoring, gis applications and geology, edited by m. eh l e r s, e. zi l i o l i and h.j. ka u f m a n n, proceedings of spie 4545-02. farrand, w.h., r.b. singer and e. merenyi (1994): retrieval of apparent surface reflectance from aviris data – a comparison of empirical line, radiative-transfer and spectral mixture methods. remote sensing environ., 47, 311-321. kruse, f.a., a.b. lefkoff, j.b. boardman, k.b. heidebrecht, a.t. shapiro, p.j. barloon and a.f.h. goetz (1993): the spectral image processing system (sips)-interactive visualization and analysis of imaging spectrometer data, remote sensing environ., 44, 145-163. mcardle, s.s., j.r. miller and j.r. freemantle (1992): airborne image acquisition under cloud: preliminary comparisons with clear-sky scene radiance and reflectance imagery, in proceedings of the 15th canadian symposium on remote sensing (toronto: canadian remote sensing society & canadian aeronautics and space institute), 446-449. monaco, c., l. tortorici, l. morten, s. critelli and c. tansi (1995): geologia del versante nord-orientale del massiccio del pollino (confine calabro-lucano): nota illustrativa sintetica della carta geologica alla scala 1:50.000. boll. soc. geol. it., 114, 277-291. salisbury, j.w. and d.m. d’aria (1992): emissivity of terrestrial materials in the 8-14 µm atmospheric window. remote sensing environ., 42, 83-106. schiattarella, m. (1996): tettonica della catena del pollino (confine calabro-lucano). mem. soc. geol. it., 51, 543-566. smith, g.m. and e.j. milton (1999): the use of the empirical line method to calibrate remotely sensed data to reflectance, int. j. remote sensing, 20, 2653-2662. ac and dc global electric circuit properties and the height profile of atmospheric conductivity annals of geophysics, 59, 5, 2016, a0545; doi:10.4401/ag-6870 a0545 ac and dc global electric circuit properties and the height profile of atmospheric conductivity irina g. kudintseva1, alexander p. nickolaenko2,*, michael j. rycroft3,4, anna odzimek5 1 v.n. karazin kharkov state university, kharkov, ukraine 2 a.ya. usikov institute for radio-physics and electronics, national academy of sciences of the ukraine, kharkov, ukraine 3 caesar consultancy, cambridge, united kingdom 4 university of bath, department of electronic and electrical engineering, bath, united kingdom 5 institute of geophysics, polish academy of sciences, warsaw, poland abstract an apparent discrepancy is pointed out at all heights, and by up to an order of magnitude between the height profiles of atmospheric conductivity derived at ac using elf propagation studies, especially from information on schumann resonance of the earth-ionosphere cavity, and using a model of the dc global atmospheric electric circuit. this serious issue is resolved by creating a hybrid profile of these two mid-latitude profiles, the first of which refers to conditions by day and the second by night. this hybrid profile is thus a first order attempt to represent globally averaged conditions. close to the earth’s surface, where the resistance of the atmosphere is largest, the properties of the dc global model exert the greatest influence, whereas in the middle atmosphere, at heights between 40 and 100 km, full wave computations show that the ac results are the more crucial. the globally averaged hybrid profile presented here has some limitations, and the physical reasons for these are addressed. they are due to the presence of aerosol particles of ice and/or of meteoric material which reduce the ionospheric d-region conductivity by an order of magnitude over only ~2 km of height, thereby causing ledges of ionisation. in the context of the globally averaged profile, published observations of the ionospheric effects of the giant gamma-ray flare from sgr 1806-20 (a neutron star having an enormously large magnetic field) occurring at 21:30 u.t. on december 27, 2004, are briefly discussed. 1. introduction the atmospheric electrical conductivity is defined as v = e (µ+n+ + µ−n−), where e is the magnitude of the charge on the electron, µ+ and µ− are, respectively, the mean positive and negative ion mobilities (where the ion mobility is the stationary velocity acquired by an ion in a unit electric field) and n+ and n− are the concentrations of small positive and negative ions present in the atmosphere [rycroft et al. 2008]. the conductivity thus depends mainly on the positive and negative ion densities. knowledge of the vertical profile of the atmospheric electrical conductivity is important when solving many problems in geophysics. for example, the conductivity within the boundary layer essentially determines the strength of the fair weather electric field at the earth’s surface. the conductivity of the air controls the leakage current in the global earth–ionosphere electric circuit. the conductivity at 70 90 km altitude determines the crucial properties of the earth– ionosphere waveguide, namely the phase velocity and the attenuation rate of radio waves from vlf transmitters. at lower frequencies, natural electromagnetic oscillations of the earth–ionosphere cavity, termed schumann resonances, were predicted theoretically by schumann [1952]. they have been observed at frequencies between a few hz and several tens of hz and they have been interpreted to give conductivity information; see, e.g., balser and wagner [1960], chapman and jones [1964], galejs [1965], rycroft [1965], jones [1976], sao et al. [1973], bliokh et al. [1977], tran and polk [1979a,b], polk [1983], sentman [1990, 1995, 1996], fullekrug [2000, 2005], fullekrug et al. [2002], nickolaenko and hayakawa [2002, 2014], kulak et al. [2003], rakov and uman [2003, section 13.4], sato and fukunishi [2003], roldugin et al. [2004], williams et al. [2006], sátori et al. [2013] and dyrda et al. [2015] and references therein. for their interpretation, the air conductivity has to be considered across the entire range article history received september 15, 2015; accepted september 1, 2016. subject classification: vertical conductivity profile of atmosphere, global electric circuit, elf radio wave propagation, schumann resonances, characteristic heights, d-region ionosphere, effects of aerosols, fair weather field. of altitudes from 0 to 100 km. having a realistic conductivity profile is mandatory for successful modelling of the schumann resonances using modern direct techniques of computation such as the finite difference time domain (fdtd) method; see, e.g., simpson and taflove [2002], otsuyama et al. [2003], soriano et al. [2005], yang and pasko [2005], yang et al. [2006], simpson [2009], and zhou and qiao [2015]. unfortunately, experimental data on middle atmosphere conductivity are rather scarce. one of the reasons for this is that for radio physical remote sensing methods in the elf frequency range ( f < 3 khz) there are no man-made sources available. direct measurements made from aircraft can only be undertaken at up to about 15 km altitude or from balloons up to about 35 km [e.g., holzworth 1991, curto et al. 2001]. only geophysical rockets are able to provide information on the conductivity in the altitude range from 35 100 km, but such launches are relatively rare and very expensive. thus, the observational base of directly measured conductivity profiles is rather limited. much information on the electron and ion concentrations in the lower ionosphere is obtained by different methods including various probes aboard sounding rockets, and radio wave propagation experiments to receivers aboard rockets (see, e.g., hale et al. [1981], hale [1984], friedrich and torkar [2001]) and also different ground-based radar systems. hale [1984] shows a sharp electron density ledge at the bottom of the d-layer. using such data sets, mechtly et al. [1972], park and dejnakarintra [1973], widdel et al. [1976], swider [1988] and croskey et al. [1990] published models of the height profile of conductivity. a fairly recent and very valuable review by friedrich and rapp [2009] presented rocket observations, which typically show sharp gradients of electron density (termed “ledges”) and also a variation of electron density at any particular height above 70 km by up to ± one order of magnitude, particularly at auroral latitudes. friedrich and rapp [2009] considered the importance of nanometer-sized particles as aerosols in the lower ionosphere. electrons are captured by these aerosols, causing a marked reduction of the electron density and hence of the electrical conductivity. aerosol ions are heavier than atmospheric molecular ions and this effect also contributes to reducing the conductivity. tiny ice crystals (especially at high latitudes during summer) and ablated meteoric material [plane 2003] form these aerosols. mcneil et al. [1998] showed that the bottoms of the sodium, calcium and magnesium ion layers were found at altitudes between 83 and 90 km. williams et al. [2015] have emphasized the role played by meteoric ions in forming a sharp ledge in the conductivity at ~85 km altitude. their calculations show that “the conduction current and the displacement current are matched at the height of the observed ledge over a wide range of vlf frequencies, pinning this altitude as the sharp boundary for the global vlf waveguide”. friedrich et al. [2013] presented high latitude rocket observations obtained by several techniques simultaneously which show a very sharp ledge at night at 80 km on one night and at 79 km on two other occasions. the electron density (and hence also the conductivity) increases by two orders of magnitude from 80 to 90 km altitude. schlegel and fullekrug [2002] evaluated the conductivity profile, v(h), from 70 to 100 km using eiscat measurements at high latitude. they showed that the conductivity was between 3.10-5 and 3.10-4 s/m at 84 km, and with the scale height for the variation of log10 v with altitude lying between 6 and 8 km. in their figure 2d, kero et al. [2014] have compared eiscat (high latitude) observations made at altitudes down to 60 km with results derived from their new spectral riometer technique. han et al. [2011] have interpreted observed daytime mid-latitude vlf sferics produced by lightning discharges to find the electron density profile from 50 to 90 km, with the electron density typically increasing from 107 to 1010 m-3. they compared this with values derived from other observations of vlf transmitter signals, from the ionospheric reference ionosphere (iri) model and from the firi model of friedrich and torkar [2001]. a general, rather than near perfect, agreement was found. sferics observed at night, known as tweeks, exhibit a fundamental cut-off frequency near 1.8 khz and higher harmonics (see han et al. [2011], and references therein). these are interpreted by tan et al. [2015] to show that at low latitudes the average electron density increases from 3·107 m-3 at 82 km to 2·108 m-3 at 87 km, i.e. by nearly an order of magnitude over a height range of 5 km. their electron density values were slightly less than those given by the iri-2012 model. however, these observations do not show a sharp ledge in the electron density. the relative lack of observations is compensated in part by modern numerical simulations. equations for the ionisation balance are used for the multi-component plasma in the framework of a more or less realistic model of the density and composition of the atmosphere, and accounting for the ionising electromagnetic radiation. for example, pavlov [2014, 2016] has investigated the photochemistry of the ionospheric d-region in detail. in such a way the altitude profiles for the concentration of the conducting particles (heavy ions from several different sources, ion complexes and various aerosols and dust particles) and free electrons kudintseva et al. 2 3 may be calculated and the resulting conductivity of the medium may be determined. the system of balance equations comprises up to 200 equations, which are solved by modern computers using the monte carlo method, e.g., inan et al. [2007]. it is found that the conductivity v(h) increases rapidly at altitudes above about 55 km, where the concentration of electrons exceeds the negative ion density. therefore, the v(h) profile bends sharply at around 55 km; it exhibits a “knee” at this altitude. the objective of the present paper is simply to demonstrate the applicability of schumann resonance phenomena as a probe for testing the conductivity profiles found by other techniques, including for models of the dc global circuit, many important contributions to which have been discussed by wilson [1929], roble and tzur [1986], williams [2003, 2009], markson [2007], rycroft et al. [2008, 2012], rycroft and harrison [2012], and williams and mareev [2014]. for these demonstrations we shall use the particular profiles suggested by cole and pierce [1965], which is a model daytime profile, and by rycroft et al. [2007], a model night-time profile designed to apply to conditions when sprites are observed (see also huang et al. [1999], and fullekrug et al. [2006]). it is noteworthy, but not at all surprising, that the values of conductivity in the first model exceed these of the second one at all heights by about an order of magnitude. 2. conductivity profiles using dc and ac global circuit data in models of the dc global atmospheric electric circuit (see, e.g., rycroft et al. [2000]), various different empirical or semi-empirical height profiles of the conductivity of the air have been used; see, for example, hays and roble [1979], roble and hays [1979], makino and ogawa [1984, 1985], ogawa [1985], roble and tzur [1986], sapkota and varshneya [1990], tinsley and zhou [2006], rycroft et al. [2007, 2008], tinsley [2008], rycroft and odzimek [2010], odzimek et al. [2010], rycroft and harrison [2012] and rycroft et al. [2012]. in all such models the conductivity of the air is considered to be produced near the earth’s surface by radioactive elements escaping from the surface and, higher up, by galactic cosmic rays colliding with air molecules [harrison et al. 2014]. in some models, spatial variations due to orography and temporal variations due to changing solar/geomagnetic activity and to volcanic aerosols are included. electrons become detached and then rapidly attach themselves to molecules of air to form relatively heavy negative ions. the peak ion production rate occurs at heights near 15 km [harrison et al. 2014]. thus the conductivity of the air rises rapidly from values ~10-14 s/m just above the surface of the earth to values about a thousand times greater at 30 km altitude, in the stratosphere. the conductivity continues to increase, reaching values exceeding 10-7 s/m in the lowest ionosphere. figure 1 depicts the average conductivity profiles of the atmosphere considered in this work. the original smooth red line is the night-time profile used for dc global electric circuit studies [rycroft et al. 2007]. for the dc circuit, the most crucial region is below 15 km altitude; as shown by rycroft et al. [2008], more than 95% of the resistance of the global circuit lies here. this profile was defined up to 80 km altitude, and we have extended it above 75 km with an exponential function having a scale height of 2.57 km. it may be noted that in this night-time model the conductivity increases by two orders of magnitude from 80 km up to 91 km, but there is no 2 km thin ledge. the black line with dots shows the classical profile introduced by cole and pierce [1965]. this daytime profile was derived by considering theoretically the production and loss (but not transport) of ionisation throughout the atmosphere due to radioactivity from the ground, cosmic rays and solar radiation. ions heavier than normal atmospheric ions were, however, not considered; thus, their conductivity value at a particular height may be an overestimate. ionic conductivity dominates below 50 km and electron conductivity above 60 km. the knee in this profile near 55 km altitude is crucial for the ac circuit properties owing to the fact that the so-called “electric” characteristic height is found here. this is the height where the conduction current density and the displacement current density become equal at the fundamental schumann resonance frequency of 8 hz. physically, this is the height above which the electric field of this schumann resonance starts to decrease rapidly with altitude [greifinger and greifinger 1978, nickolaenko and rabinowicz 1982, kirillov 1993, sentman 1995, kirillov 1996, sentman ac and dc gec properties and air conductivity figure 1. comparison of height profiles of air conductivity v, given in units of s/m. 1996, fullekrug 2000, mushtak and williams 2002, pechony and price 2004, williams et al. 2006, greifinger et al. 2007, pechony 2007, galuk et al. 2015]. the other important characteristic (“magnetic”) height is ~95 km: the wave magnetic field penetrates to this altitude. unfortunately, the cole and pierce [1965] profile does not match either the dc or ac observational data. the function v(h) must be transformed to somewhat lower values in the troposphere and to higher conductivities above 40 km altitude, thus forming the hybrid day/night profile, which is thus a global profile, averaging over all local times (blue line with stars in figure 1). this modification allows us to fit the schumann resonance data well [nickolaenko et al. 2016]. the hybrid profile is coincident with the dc red profile in the troposphere and it matches the classical cole and pierce [1965] height dependence around 25 30 km altitude, remaining rather close to this profile above 40 km. as we shall see below, deviations from this classical profile lead to realistic values of the elf radio propagation constant and to reasonable schumann resonance patterns. the values relevant to this global hybrid profile are listed in table 1 in the appendix. the table contains the altitude above the ground, the relevant conductivity of the air column of 1 m2 cross-section, and the logarithm of air conductivity, in units of s/m. 3. full wave solution of the schumann resonance problem and standard elf propagation constant a realistic model of the earth–ionosphere cavity incorporates the perfectly conducting spherical earth surrounded by a poorly conducting air shell bounded by the horizontally stratified ionospheric plasma. the relative dielectric constant of the medium above the ground is given by f(r) =1+ i v(r)— ~f0 , where r is the radius vector in a spherical coordinate system (r, i, {) with the origin at the earth’s centre. the electromagnetic problem in an isotropic inhomogeneous resonator is solved via the following steps (see, e.g., bliokh et al. [1980], nickolaenko and hayakawa [2002]). after writing maxwell’s equations for the electric and magnetic field components and introducing scalar (or debye) potentials, we choose the potential and the helmholtz equation corresponding to the transverse magnetic wave (e wave), as only this wave propagates at elf. the earth–ionosphere cavity is taken to be uniform in the angular coordinates i and {, while its electric properties vary along the radius; the relative permittivity of the atmosphere is f=f(r, ~). this allows us to separate variables in the usual way, to obtain the standard tesseral functions, and to arrive at the following equation for the radial function [bliokh et al. 1980]: (1) the solutions to equation (1) are expressed as the spherical hankel functions h(1)n (kr√f) and h (2) n (kr√f). the relevant details can be found in the books by wait [1962, 1970], galejs [1972], bliokh et al. [1980] and nickolaenko and hayakawa [2002]. the electromagnetic problem is solved for a system of thin horizontal layers of constant f for both upward going and down going radio waves. the continuity condition holds at each interface for the tangential fields. thus, a system of linear algebraic equations is obtained for the amplitudes of waves within the layers. if one applies n layers, the linear system contains 2n equations. such a treatment is regarded as the full wave solution, e.g., wait [1970]. it was used in the schumann resonance band by knott [1998] and jones and knott [1999, 2003]. a more convenient variant of the full wave solution exploits the ratio of horizontal electric to horizontal magnetic field at each boundary, d(r), i.e. the surface impedance. the problem is reduced to a nonlinear differential equation of the first order (riccati equation), see, e.g., wait [1970]. its solution is constructed numerically using an iterative procedure [e.g., hynninen and galuk 1972, bliokh et al. 1977, galuk and ivanov 1978, bliokh et al. 1980, galuk et al. 2015]. for the time dependence exp[+i~t], we obtain the following equation: (2) here, d(r) is the spherical surface impedance at the strata interface (see nickolaenko and hayakawa [2002], chapter 2), o(~) is the sought complex propagation constant, ~ = 2rf is the circular wave frequency, k =~/c is the wave number in free space, c is the velocity of light in free space, and r is the radius vector of the spherical coordinate system (r, i, {); f(r) is the complex dielectric constant of the medium varying with height, and f0 is the permittivity of free space. the operator of the problem is defined in the height interval from zero to infinity, and its eigen-value is m=o(o+1). at the perfectly conducting ground, the spherical surface impedance d(a) = 0, where a denotes the earth’s radius. this is the first boundary condition. an elf radio wave of frequency f penetrates into the ionosphere and rapidly attenuates there, so that the plasma properties above some virtual altitude given by dr d k r r n n r dr d r rrrr r 1 1 0 2 2 2 2 2 2 f f f + + = q q q q q v v v v v# & g j .dr d r r r ik r ik ikr 1 02 2d f d f o o + + + =q q q q qv v v v v kudintseva et al. 4 5 r1 have only a very minor impact on the sub-ionospheric radio propagation. this altitude varies with frequency. however, it does not exceed the 100 km level: r1 = a + 100 km. we may suggest that the ionosphere is vertically uniform above this 100 km altitude, and the surface impedance at r1 is equal to d(r1) = [f(r1)] -½, where |f(r1)|= const >> 1. this is the second boundary condition for equation (2). the problem of finding the complex eigen-value is reduced to solving the equation (3) with respect to the parameter m. the d(a; m) function is found by the numerical integration of equation (2) from the top r = r1 to the bottom r = a. the roots of the equation are found by iterations starting from o= ka. let mm be the m-th iteration to the eigen-value m; then the next, m + 1, iteration is found by following newton’s procedure: after obtaining the (m + 1)-th iteration, we repeat the downward integration of equation (2) along the radius with the new eigen-value, and thus we obtain the surface impedance at the ground for the next iteration. the initial value is: m0 = ka (ka + 1). the iterations stop when the relative difference between the new and the previous eigen-value is smaller than 10-7. the derivative —̂̂ m d(a; m) is found concurrently with the d(r) function by integrating the differential equation for the function d1(r) = —̂̂m d(r,m). the relevant equation and the condition at r = r1 are obtained by differentiation of equation (2) with respect to the parameter m: the upper height r1 was established by an exhaustive search when the solutions were constructed for a set of r1 values. finally, the height r1 = 100 km was chosen since the relative change of the output eigenvalue did not exceed 10-7 when the starting height exceeded 100 km. it is only above this height that the anisotropy of the ionosphere, which is due to the presence of the earth’s magnetic field [madden and thompson 1965, rishbeth and garriott 1969], becomes very important in determining the tensor value of the conductivity. it is interesting to note that the approach described allows us to obtain not only the eigen-value of the zeroth mode in the earth–ionosphere waveguide, but also the roots of the “trapped” higher order modes. newton’s method converges toward the eigen-value closest to the initial one. to treat the higher order modes, we have to exclude the roots of lower modes that have already been found. for this purpose we use bézout’s theorem [korn and korn 1968, section 1.7.2]: the function is used in the process instead of the initial function {(m) = 0, where m are the roots of the {(m) function which have already been found, and n denotes the number of these already established roots. prior to computing the elf field components, we have to compute the so-called normalizing integrals. these are found automatically in the above scheme, as the following relation is valid: the normalizing integral n0 allows us to obtain the lower characteristic height of the earth–ionosphere cavity [kirillov 1993, kirillov 1996, kirillov et al. 1997, kirillov and kopeykin 2002]: (8) (heights which are complex quantities are shown as capital (upper case) letters, to emphasize this fact). this characteristic height is regarded as the “capacitance”, or the “electric”, height of the earth–ionosphere system: (9) it is used when obtaining the capacitance elements of the artificial two-dimension rlc circuit of the earth–ionosphere cavity resolved with the help of the 2d telegraph equations [madden and thompson 1965, kirillov 1993, kirillov 1996, kirillov et al. 1997, kirillov and kopeykin 2002, pechony and price 2004, pechony 2007] or in the approximate formulas for the elf propagation constant [greifinger and greifinger 1978, nickolaenko and rabinowicz 1982, mushtak and williams 2002]. the upper characteristic height, the “inductance” (or the “magnetic”) height m m1 m m= -+ ; ; . a a m m 2 2 m d m d mq q v v . . dr d r ik r r r ikr r r 2 1 0 0 21 1 1 d f d d f d + = = q q q q q qv v v v v v n i n 1 { { { = = q% imv ; .n ika a ika a0 2 2 12 2 m d m d= =q qv v .h f i h dh ika a 1c 00 2 1, v ~f d = 3 q q qv v v# , , .h e a e a dr1c r r a i i i= 3 q q qv v v# ;a 0d m =q v ac and dc gec properties and air conductivity (4) (5) (6) (7) is introduced by using the relation: (10) heights hc and hl are physically equivalent to the now popular electric and magnetic characteristic heights he and hm. these are the altitudes above which the relevant fields start to rapidly decrease. the distinction is that heights hc and hl are found from the rigorous full wave solution, which exploits the continuous conductivity profile, while the characteristic heights he and hm are introduced by the approximate heuristic relations [e.g., mushtak and williams 2002, williams et al. 2006]. to stress this distinction, we use different notations for the heights. the standard description for sub-ionospheric radio wave propagation in the extremely low frequency band (elf: 3 hz 3 khz) uses the propagation constant o( f ), the source–observer distance i, and the current moment of the dipole source mc( f ). the relevant equations can be found in nickolaenko and hayakawa [2002, 2014]. we use here the commonly accepted standard frequency dependence o( f ) suggested by ishaq and jones [1977], which incorporated the vast collection of experimental material from the global array of the schumann resonance observatories available at that time. according to this model, the complex propagation constant o( f ) is found from the following heuristic equations presented by ishaq and jones [1977]: where o is the dimensionless elf propagation constant, s is the complex sine function relevant to brillouin waves in the earth–ionosphere cavity [e.g., wait 1970], v and c are the phase velocity of radio wave and velocity of light, respectively, the parameter a which accounts for the wave attenuation is in napier/radian, and f is measured in hz. figure 3 shows frequency variations of the imaginary part of the propagation constant, or the losses in uniform cavity. we must remind the reader here that we take into account only the vertical (radial) non-uniformity of ionosphere. the real ionosphere is characterized by the day-night boundary and the polar non-uniformity, and the plasma becomes an anisotropic medium above ~80 km altitude. all these features are ignored at present, as our goal is only to present an average (on the global scale) v(h) profile, which is in reasonable agreement with both dc and ac observations and models. accounting for the above mentioned details is a possible area of demanding future work. it is interesting to note in this regard that recent satellite observations have demonstrated the presence of schumann resonance signals in the topside ionosphere [simoes et al. 2011, dudkin et al. 2015], indicating that energy is being lost from the cavity, even though this is inconsistent with considerations of r1 discussed earlier. this indicates that an explanation of the space-borne observations of sub-ionospheric schumann resonances should be based on the possible in, ,h h a a r h r dr1l 0 i i i= 3 { { q q qv v v# hl1 hlo o+ =q v .hl c . . , . ,f . . . , . ln ln f kas s c v i f c v f f f 0 25 0 5 5 49 1 64 0 1759 0 01791 0 063 / . 2v 1 2 2$ 0 64 o a a = + = = + = q q q q v 2v v v ! ! $ 2$ kudintseva et al. 6 figure 2. real parts of the elf propagation constant obtained with the full wave solution for the cole and pierce [1965] profile (black line with dots); profile based on the global circuit data (red smooth line); the hybrid profile (blue curve with stars). these are compared with the reference model (green curve with diamonds). figure 3. imaginary part of the elf propagation constant obtained with the full wave solution for the cole and pierce [1965] profile (black line with dots); profile based on the global circuit data (red smooth line); the hybrid profile (blue curve with stars). the elf standard reference model (equations 10 to 13) is shown by the green curve with diamonds. (11) (12) (13) (14) 7 fluence of ionospheric anisotropy on elf propagation through the plasma. consider a sub-ionospheric transverse electromagnetic (tem) wave entering the ionospheric plasma; its magnetic field reaches the altitude of 80 90 km where the plasma anisotropy becomes important. here, the electromagnetic wave transforms into an elf hydromagnetic wave that penetrates up to satellite altitudes, as was mentioned by madden and thompson [1965] in their seminal paper which contains rigorous numerical computations for both day and night conditions. the theoretical treatment of madden and thompson [1965] estimated that one third of the schumann resonance energy escapes from the cavity at night, that the “ionisation levels at around 40 50 km are very important for the schumann resonances” and that most damping occurs near these heights. they also stated that “conduction currents dominate over the displacement currents above about 60 km”. further, they mentioned that dust from meteor showers could influence schumann resonances. the full wave solutions of madden and thompson [1965] are obtained only at altitudes above about 25 km. below this altitude, of great relevance for the global circuit, their system of linear algebraic equations cannot obtain the reflection and transmission coefficients at the interfaces between different slabs. that is why we have used the riccati equation for the surface impedance [bliokh et al. 1977], which works below 25 km. the models of the imaginary part of the propagation constant provide different results here. the values deduced from parameters of the night-time dc global circuit (red curve) result in the smallest radio wave attenuation. the black line with dots based on the cole and pierce [1965] daytime model shows the highest attenuation rate. the elf standard reference model (green line with diamonds) is rather close to the blue line with stars computed for the hybrid (global average, over all local times) conductivity profile, which is introduced in this paper. the plots presented in figure 2 and figure 3 indicate that the red profile describing the leakage current between the earth and the ionosphere (the dc circuit) overestimates the phase velocity of radio wave and underestimates the elf attenuation rate. in contrast, the cole and pierce [1965] profile underestimates the phase velocity and the schumann resonance frequencies correspondingly, and it overestimates the elf attenuation rate. physically this means that the real conductivity at altitudes ~50 km and above is somewhat higher than dc model predicts and lower than that of classical conductivity profile. however, such a detail could not be sensed in the static electric field within the troposphere where the major part of the ionosphere-toground potential difference exists; only schumann resonance parameters are sensitive to the conductivity at this altitude. the very small modifications of the dc electric field at ground level are within the measurement accuracy and within the variations with universal time, known as the carnegie curve [harrison 2013]. if we turn to the earth–ionosphere leakage current, changes in the air conductivity around 50 km altitude definitely will not alter the fair weather field at the ground. it is only at the “electrosphere”, the height where “horizontal spreading” of the current occurs, that will be reduced from, say, 70 to 60 km. the classical conductivity profile presented by cole and pierce [1965] overestimates the losses in the cavity. to match the elf standard propagation model, we have to slightly increase the air conductivity starting from the stratosphere. in this case the blue curve is obtained as being very close to the elf standard model. 4. power spectra for globally uniform distribution of lightning strokes the dispersion curves of figure 2 and figure 3 allow us to compute the power spectral density of the vertical electric field in the schumann resonance band, which was first observed experimentally by balser and wagner [1960]. the resonance pattern depends on the source–observer distance. to eliminate any influence of this factor, we apply a globally uniform spatial distribution of lightning strokes [bliokh et al. 1977, bliokh et al. 1980, sentman 1995, sentman 1996, nickolaenko and hayakawa 2002, williams et al. 2006, nickolaenko and hayakawa 2014]. we suggest that independent random lightning strokes occur with the same probability at any point of the globe, i.e., the probability density of the source coordinates is the constant: electromagnetic radiation from global thunderstorms forms a random succession of independent elf pulses at the radio receiver, and the delay of the pulse arrival times is supposed to have an exponential distribution. in this case we speak of a poisson random process [middleton 1960, korn and korn 1968 (section 18.11.5), tikhonov 1982]. the mutual interference of pulses disappears in the averaged power spectrum of such a process, and “individual power spectra of pulses are summed”. the resulting power spectrum of the vertical electric field is found from the following equation: ,w 4 1 i { r =q v ac and dc gec properties and air conductivity (15) where n = 0, 1, 2, 3... is the schumann resonance mode number. the series in equation (16) was obtained by integrating the probability density (15) multiplied by the product of two zonal harmonic series for the electric field (see bliokh et al. [1980], nickolaenko and hayakawa [2002], nickolaenko and hayakawa [2014], for details). the series (16) converges rather fast, and the computations are simple. figure 4 shows the power spectra given by (16) computed for the three propagation constants o( f ), except the one relevant to the cole and pierce [1965] profile. the abscissa here shows the frequency ranging from 4 to 50 hz, and the ordinate depicts the power spectral density in arbitrary units. we may note that the schumann resonance pattern is clearly observed in all graphs. as was expected, the maxima of the red curve occur at higher frequencies, and this is explained by the higher values of re[o( f )] noted in figure 2. owing to the smaller attenuation factor (figure 3), the peaks of the red resonance curve (based on the dc global circuit model) have smaller widths in comparison with those for the standard black spectrum and for the global hybrid profile (blue line). the green and blue lines in figure 4 are close to each other in the vicinity of the first three resonance modes (i.e. for n = 0, 1, 2, 3). this is explained by the proximity of the relevant lines in figure 2 and figure 3. however, we notice that the resonance pattern is more sensitive to the slight deviations in the dispersion curves. indeed, the green and blue lines of re[o( f )] functions (phase velocities) are almost coincident in figure 2, while the positions of the spectral peaks are obviously different in figure 4. deviations in the phase velocity and in the wave attenuation cause the evident differences between the green and blue spectra, especially at the higher modes n ≥ 4. this indicates that the hybrid profile does not exactly match the standard elf propagation model. such deviations are quite acceptable from the practical point of view, as only three schumann resonance modes are clearly observed in the experimental records and are used in interpretations. of course, this does not mean that a better conductivity profile might be constructed that fits the standard spectrum (and observations) in a better way. when comparing the red (the dc data) and blue (ac data) spectra in figure 4, we note obvious deviations. these are due to differences between the profiles within the middle atmosphere. the global electromagnetic resonance is sensitive to the conductivity in the mesosphere and thus it can be used for selecting the models based on the dc ground-based observations. 5. discussion the validity of the vertical profiles of atmospheric conductivity at different altitudes might be checked by measurements of the low frequency electromagnetic fields and of the fair weather field observed in the troposphere and at ground level [e.g., harth 1982, holzworth and chiu 1982, holzworth 1991, 1995]. the elf/ vlf radio waves are particularly sensitive to the properties of the ionosphere over narrow (~ few km) height ranges between 50 and 120 km [galejs 1961, wait 1962, madden and thompson 1965, wait 1970, galejs 1972], whereas the fair weather field mainly depends on the conductivity of the troposphere [e.g., ogawa 1985, rycroft et al. 2007, rycroft et al. 2008]. schumann resonance signals “sense” the conductivity in the range of heights from ~40 to ~100 km and therefore their interpretation can contribute to the information derived from the sparse data derived from different observations. the applicability of the blue conductivity profile can be verified by comparing the wave attenuation with direct measurements of monochromatic radio signals from elf transmitters [bannister 1999, nickolaenko 2008]. data used in the paper by bannister [1999] exploit the amplitude monitoring of elf signals from the us navy transmitter, regarded as the wisconsin test facility (wtf). the average attenuation at the 76 hz frequency was equal to 1.08 db/mm, and the relative standard deviation due to seasonal variations was ±25%. the imaginary part of the propagation constant at this frequency for the hybrid profile is equal to im[o( f )] f =76 hz = 0.86 napier/radian, and this value corresponds to the attenuation factor c(76 hz) = 1.17 db/mm. this attenuation is practically coincident with observations, and this fact is certainly in favour of the model. ,e fq fr n nq n 2 1 1 1 2 1 n 2 2 2 0 \1 2 fr o oq o oq + + + + 3 = fq oq nq oqv v v v| kudintseva et al. 8 figure 4. power spectra of vertical electric field in arbitrary units (see text for a full explanation). (15) 9 the wave attenuation measured at 82 hz was also published by nickolaenko [2008]. it was equal to im[o( f )] f =82 hz = 0.92 napier/radian, which corresponds to the attenuation factor c(82 hz) = 1.25 db/mm. this attenuation rate was inferred from the distance dependence of the signal amplitude detected in the vertical electric field of the radio wave emitted by the soviet kola peninsula transmitter [velikhov 2014]. the model attenuation is exactly equal to the value measured experimentally. thus, the comparison with observations of man-made elf radio transmissions supports the validity of the hybrid conductivity profile very well. we now evaluate the value of the fair weather field for the three conductivity profiles. the altitude variations of atmosphere conductivity v(h) shown in figure 1 were transformed into the air resistance t(h) = 1 /v(h) for this purpose. the relevant plots are shown in the upper frame of figure 5 for all three model profiles. the smooth red curve shows the global circuit model of rycroft et al. [2007], and the blue and black lines depict profiles pertinent to the hybrid model and to the classical cole and pierce [1965] profile, respectively. the upper plot in figure 5 indicates that the resistance of the air column having 1 m2 cross-section varies from 1014-1013 ohm m at the ground to about 1010 ohm m at around 40 km altitude. the values obtained for the different models vary by about one order of magnitude. plots in the lower frame of figure 5 depict the voltage drop across a 1 m gap in the air (the strength of the fair weather field at different altitudes) corresponding to the three conductivity profiles. these values are readily obtained using ohm’s law, by multiplying the air resistance t(h) = 1 /v(h) by the leakage current of global circuit of j = 1.3 pa/m2 [rycroft et al. 2000]. the relevant fair weather field at ground level becomes equal to 86 v/m in the hybrid and the global circuit models and 12 v/m in the cole and pierce [1965] model. if we integrate the voltage drop with altitude, we find that the ionosphere-to-ground potential difference, vi, is equal to 247 kv for the hybrid model, 263 kv for the rycroft et al. [2007] model, but only 61 kv for the cole and pierce [1965] profile. that value is unrealistically low, due to the large daytime conductivity values of that profile. the resistance profile based on the conductivity of the global circuit model of rycroft et al. [2007] proved to be useful in elaborating the source of a parametric q-burst [nickolaenko 2011] caused by the giant galactic gamma-ray flare that arrived from the magnetar sgr 1806−20 at earth at 21:30:26 u.t. on december 27, 2004. the illuminated hemisphere was practically coincident with the dayside of the globe: the sub-flare point was only 450 km away from the centre of the daytime hemisphere. the gamma-flare also transformed the schumann resonance spectra for a short time [nickolaenko et al. 2010, tanaka et al. 2011]. feasible “parametric” pulsed excitation of the earth–ionosphere cavity was predicted [nickolaenko 2011] caused by modification of the global electric circuit. such a source deviates from the ordinary lightning strokes, which excite schumann resonances, and the major features of elf transient are conditioned by the great horizontal size of the modification to the cavity, covering almost a hemisphere. such a coherent source drives only the lowest modes of schumann resonance, and this was confirmed experimentally [nickolaenko et al. 2010, schekotov et al. 2013, nickolaenko et al. 2014]. in modeling the parametric elf source, vlf information was also used, being relevant to the particular gamma-flare; the lower edge of the dayside ionosphere fell suddenly from 60 to 40 km as a result of ionisation produced in the mesosphere and upper stratosphere [inan et al. 2007]. the modification covered the whole ac and dc gec properties and air conductivity figure 5. altitude profiles of the air resistance (upper plot, in ohm m) and fair weather field amplitude (in v/m, lower plot). the corresponding value of the ionospheric potential vi is shown in the legend for the lower plot found in the rycroft et al. [2007] model, for the hybrid model presented here, and for the cole and pierce [1965] model (cp). the value derived for the latter model is certainly far too low, whereas the small discrepancy between the red and blue values is within the range of variations with universal time on a particular day and within the observed day-to-day variability. of the pacific ocean, and it was recorded at the antarctic palmer station for different vlf paths up to 60° from the sub-flare point. the modification of the air resistance was estimated by using the conductivity profile of rycroft et al. [2007] as a starting model for several modifications [nickolaenko 2011]. computations indicated that changes below the tropopause were definitely important for the source current of the parametric elf transient. the final model provided a sudden increase of the earth–ionosphere leakage current from 1 pa/m2 to 6 pa/m2 associated with the two-fold reduction of the air resistance at ground level (the modification of air conductivity increased linearly with altitude). we do not know whether there are any experimental observations, which could confirm or negate this prediction of a much increased fair weather conduction current density during this most unusual event. 6. conclusions we have shown here that a model for the globally averaged height profile of air conductivity which is a hybrid (blue curve) between the early cole and pierce [1965] daytime profile (black curve) and the empirical night-time profile (red curve) used in a model of the dc global electric circuit by rycroft et al. [2007] satisfies both observations of schumann resonance (ac global circuit properties) and fair weather observations of the electric field throughout the atmosphere (dc global circuit properties) very well. the crucial height regions for the dc part are the lowest 15 km, i.e. the troposphere; around the knee in the profile at ~55 km altitude is most crucial for the ac part. this new hybrid model leads to a realistic variation of the electric field strength through the fair weather regions of the atmosphere and to a realistic value of the potential of the ionosphere with respect to the earth’s surface, namely vi = 247 kv. this value fits within the range observed by markson [2007]. we suggest that this hybrid profile, a global profile which harmonises ac and dc profiles, which is in overall agreement with many different types of observations but not those associated with aerosols and showing a sharp conductivity ledge from 84 to 86 km altitude at some locations and at some times should be useful in future studies of both ac and dc global circuit phenomena. acknowledgements. the authors are most grateful to many colleagues in different countries for sharing their ideas on the topics discussed here, and especially to the referee for raising several points which necessitated further consideration and clarification before publication. references balser, m., and c. wagner (1960). observations of earth-ionosphere cavity resonances, nature, 188, 638-641. bannister, p.r. 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(1929). some thundercloud problems, j. franklin institute, 208, 1-12. yang, h., and v.p. pasko (2005). three-dimensional finite difference time domain modeling of the earth– ionosphere cavity resonances, geophys. res. lett., 32, l03114; doi:10.1029/2004gl021343. yang, h., v.p. pasko and y. yair (2006). three-dimensional finite difference time domain modeling of the schumann resonance parameters on titan, venus, and mars, radio sci., 41, rs2s03; doi:10.1029/2005 rs003431. zhou, h., and x. qiao (2015). studies on the variations of the first schumann resonance frequency during the solar flare on 7 march 2012, j. geophys. res., 120, 4600-4612; doi:10.1002/2014jd022696. *corresponding author: alexander p. nickolaenko, a.ya. usikov institute for radio-physics and electronics, national academy of sciences of the ukraine, kharkov, ukraine; email: sasha@ire.kharkov.ua; sashanickolaenko@gmail.com. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. kudintseva et al. 14 15 appendix 1. here we show the data describing the hybrid profile. ac and dc gec properties and air conductivity h, km v(h), s/m lg(v) h, km v(h), s/m lg(v) 0 1.512e-14 -13.820 50 3.380e-10 -9.482 1 2.160e-14 -13.665 51 3.514e-10 -9.465 2 4.090e-14 -13.403 52 3.710e-10 -9.441 3 6.962e-14 -13.172 53 3.998e-10 -9.409 4 1.049e-13 -12.994 54 4.299e-10 -9.377 5 1.482e-13 -12.844 55 5.229e-10 -9.292 6 2.019e-13 -12.709 56 6.160e-10 -9.221 7 2.712e-13 -12.581 57 8.044e-10 -9.105 8 3.549e-13 -12.464 58 9.929e-10 -9.013 9 4.651e-13 -12.346 59 1.398e-9 -8.864 10 5.943e-13 -12.240 60 1.803e-9 -8.754 11 7.629e-13 -12.131 61 2.731e-9 -8.573 12 9.578e-13 -12.032 62 3.659e-9 -8.446 13 1.209e-12 -11.931 63 5.896e-9 -8.239 14 1.499e-12 -11.837 64 8.133e-9 -8.099 15 1.867e-12 -11.742 65 1.365e-8 -7.874 16 2.285e-12 -11.654 66 1.916e-8 -7.726 17 2.796e-12 -11.567 67 3.249e-8 -7.497 18 3.375e-12 -11.485 68 4.582e-8 -7.347 19 4.093e-12 -11.401 69 6.887e-8 -7.170 20 4.891e-12 -11.323 70 9.636e-8 -7.024 21 5.864e-12 -11.244 71 1.419e-7 -6.848 22 6.930e-12 -11.172 72 1.893e-7 -6.723 23 8.227e-12 -11.097 73 2.839e-7 -6.547 24 9.634e-12 -11.029 74 4.258e-7 -6.371 25 1.135e-11 -10.957 75 5.678e-7 -6.246 26 1.320e-11 -10.892 76 7.570e-7 -6.121 27 1.545e-11 -10.823 77 9.463e-7 -6.024 28 1.788e-11 -10.760 78 1.183e-6 -5.927 29 2.085e-11 -10.693 79 1.467e-6 -5.834 30 2.416e-11 -10.629 80 1.751e-6 -5.757 31 2.774e-11 -10.569 81 2.176e-6 -5.662 32 3.164e-11 -10.512 82 2.602e-6 -5.585 33 3.626e-11 -10.452 83 3.265e-6 -5.486 34 4.207e-11 -10.388 84 3.927e-6 -5.406 35 4.925e-11 -10.319 85 5.157e-6 -5.288 36 5.762e-11 -10.251 86 6.388e-6 -5.195 37 6.821e-11 -10.178 87 8.895e-6 -5.051 38 7.947e-11 -10.111 88 1.145e-5 -4.941 39 9.359e-11 -10.040 89 1.713e-5 -4.766 40 1.101e-10 -9.970 90 2.285e-5 -4.641 41 1.3461e-10 -9.882 91 3.686e-5 -4.433 42 1.555e-10 -9.819 92 5.091e-5 -4.293 43 1.840e-10 -9.746 93 9.042e-5 -4.044 44 2.097e-10 -9.689 94 1.230e-4 -3.886 45 2.410e-10 -9.629 95 2.652e-4 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49,6,2006 1189 annals of geophysics, vol. 49, n. 6, december 2006 key words ionosphere-atmosphere interaction – mid-latitude ionosphere – ionospheric disturbances – atmospheric waves 1. introduction various processes in the lower-lying layers of the atmosphere, particularly in the troposphere, summarized for simplicity under the term ‘meteorological processes’, can affect the ionosphere basically through two channels: i) electrical and electromagnetic phenomena, and ii) upward propagating waves in the neutral atmosphere. we treat the latter category, with upward propagating waves in the neutral atmosphere, which are more important from the point of view of energy deposition and atmospheric modification than the phenomena under (i). namely we focus on the effects of planetary waves on the ionosphere. various tropospheric and to a limited extent stratospheric and mesospheric «meteorological» processes and periodic solar heating and cooling excite waves in the neutral atmosphere. upward propagating waves in the neutral atmosphere and their modifications, interactions and modulations affect the ionosphere, when and if they reach it. those waves are planetary waves, tidal waves, gravity waves, and infrasonic waves. most of the ‘meteorological influences’ on the ionosphere arise from the upward propagating gravity, tidal and planetary waves (e.g., kazimirovsky et al., 2003). the meteorological influences play an important role in the overall ionospheric variability (e.g., forbes et al., 2000; rishbeth and mendillo, 2001). laštopersistence of planetary wave type oscillations in the mid-latitude ionosphere jan laštovička, petra šauli and peter križan institute of atmospheric physics, academy of sciences of czech republic, prague, czech republic abstract planetary wave type oscillations have been observed in the lower and middle atmosphere but also in the ionosphere, including the ionospheric f2 layer. here we deal with the oscillations in fof2 analysed for two japanese and two us stations over a solar cycle (1979-1989) with the use of the morlet and paul wavelet transforms. waves with periods near 5, 10 and 16 days are studied. only events of duration of three wave-cycles and more are considered. the results are compared with the results of a similar analysis made for fof2 and the lower ionosphere over europe (laštovička et al., 2003a,b). the 5-day period wave events display a typical duration of 4 cycles, while the 10and 16-day wave events are less persistent with typical duration of about 3.5 cycles and rather 3 cycles, respectively, in all three geographic regions. the persistence pattern in terms of number of cycles and in terms of number of days is different. in terms of number of cycles, the typical persistence of oscillations decreases with increasing period. on the other hand, in terms of number of days the typical persistence evidently increases with increasing period. the spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration. thus the predictability of the planetary wave type oscillations in fof2 seems to be very questionable. the longitudinal size of the planetary wave type events increases with increasing wave period. the persistence of the planetary wave type events in fof2 and the lower ionosphere is similar in europe, but the similarity in occurrence of individual events in fof2 and the lower ionosphere is rather poor. mailing address: dr. jan laštovička, institute of atmospheric physics, academy of sciences of czech republic, bocni ii, 14131 prague, czech republic; e-mail: jla@ufa.cas.cz 1190 jan laštovička, petra šauli and peter križan vička (2006) briefly reviewed effects of atmospheric waves (planetary, tidal, gravity and infrasonic) on the ionosphere. planetary waves (periods of about 2-30 days) are very predominantly of tropospheric origin and can penetrate directly to heights slightly above 100 km. they have to propagate upwards into the f-region ionosphere via an indirect way. their effects were observed in the lower ionosphere (e.g., laštovička et al., 1994), in the ionospheric e region in h’e (e.g., cavalieri, 1976) and sporadic-e layer (e.g., haldoupis et al., 2004), and in the f2 region (e.g., forbes and zhang, 1997; altadill and apostolov, 2001, 2003; laštovička et al., 2003b; altadill et al., 2004). typical planetary wave periods are broad spectral peaks around 2, 5, 10 and 16 (very broad spectral peak) days, but the planetary wave spectrum is very variable and on individual days it can be much different. they roughly correspond to eigenfrequencies of the atmosphere, which slightly differ for various modes; for instance for the wave with the zonal wave number 1 they attain values of 1.2, 5, 8 and 12 days (these periods are doppler shifted by the prevailing wind). all planetary wave periods are quasi-periods with the exact period varying within a period range. amplitudes of planetary waves are unstable, as well; planetary waves typically occur in bursts of a couple of waves. the duration of such wave bursts in the critical frequency of the f2 region, fof2, in other words the persistence of the planetary wave type events, is the main topic of the paper. laštovička et al. (2003b) made such investigations for four representative european stations listed in table i. the primary aim of this paper is to make such investigations for representative stations from the mid-latitude u.s.a. and japan in comparison with our previous results on the persistence of planetary wave events over europe. it should be mentioned that in the f region it is correct to use the term planetary wave type oscillations, because not all oscillations in the planetary wave period range are related to planetary waves in the neutral atmosphere; some of them are caused by periodic variability of geomagnetic activity (e.g., altadill and apostolov, 2003). planetary waves cannot propagate directly to the f2 region heights. they have to propagate indirectly via modulation of other agents like upward propagating tides or gravity waves, vertical drifts, or the turbopause height and its properties. laštovička et al. (2003a) studied the persistence of planetary wave events in the lower ionosphere over europe. data and methods used by laštovička et al. (2003a,b) allow examine the possible similarity of planetary wave type oscillations in the lower ionosphere and fof2, which is the second topic of the paper. the third topic is to use similarity and/or dissimilarity of planetary wave events in fof2 over europe, japan and the u.s.a. for rough estimate of the longitudinal size of planetary wave type events. 2. data and methods as the basic ionospheric parameter we use the main characteristics of the f2 region, the table i. co-ordinates of ionospheric stations and missing intervals from period 1979-1989. station geographic latitude geographic longitude missing intervals boulder 40.0°n 105.3°w 1982/1983 wallops island 37.9°n 75.5°w 1979, 1987/1988, 1988, 1988/89, 1989 akita 39.7°n 140.1°e 1984/1985, 1988/1989, 1989 wakkanai 45.4°n 141.7°e 1988/1989 juliusruh 54.6°n 13.4°e slough 51.5°n 0.6°w pru° honice 50.0°n 14.6°e 1988/1989, 1989 rome 41.8°n 12.5°e 1979 1191 persistence of planetary wave type oscillations in the mid-latitude ionosphere widely available critical frequency fof2. data from four representative stations in europe, two in the u.s.a. and two in japan are used over the period 1979-1989 (from maximum to maximum of the solar cycle). the co-ordinates of those stations are listed in table i. noontime (10-14 lt) average values of fof2 are used. sometimes there are problems with quality and availability of fof2 data with some stations for some periods (e.g., burešová, 1997). the above stations and the analyzed period were selected with respect to reduction such problems to minimum. therefore good quality data with minimum gaps are used in the paper. single data gaps were interpolated with the use of data of the same station at other local times and/or from neighboring stations at the same time, or combination of both, based on availability of data. nevertheless, some intervals have not been taken into analysis due to more data gaps. the selection of the optimum period was made for european stations, therefore the number of missing data intervals for the us and japanese stations are larger. for the «vertical» comparison of the planetary wave type oscillation events in europe we need data representing the lower ionosphere. the radio wave absorption measurements obtained by the a3 method utilizing the oblique wave propagation are used. data from two radio paths from central europe are used. the path luxembourg-panská ves has parameters as follows: f = 6.09 mhz, feq = 2.1-2.2 mhz (equivalent vertical frequency), reflection point 50°04ln, 10°18le, transmitter-receiver distance 610 km. the path deutschlandfunk-panská ves has different parameters: f = 1539 khz, feq = 650700 khz, reflection point 50°16ln, 11°47le, transmitter-receiver distance 390 km. the absorption along the former radio paths is formed very predominantly at altitudes of about 90-100 km, whereas the latter absorption is formed mostly at altitudes of about 85-90 km. as a mathematical tool for the description of the persistence of planetary wave type oscillations within time series of fof we use wavelet analysis (torence and compo, 1998; vidakovic, 1999). the continuous wavelet transform (cwt) is an excellent tool for analysing changing properties of non-stationary signals. in analyses of signals, the wavelet representations allow us to view a time-domain evolution in terms of scale/period components. the cwt is a convolution of the data sequence with scaled and translated version of the mother wavelet. varying the wavelet scale and translating along the localised time is possible to construct a picture showing both the amplitude of any feature versus the scale/period and how this amplitude varies with time. for a thorough introduction to wavelet transforms, the reader is referred, e.g., to mallat (1998). the wavelet transform of the data x(t, z) is defined as (2.1) where ψ0(t) is a mother-wavelet. in the present work, we used morlet and paul (complex) mother-wavelets, defined as (2.2) (2.3) where param counts the number of oscillations of the (real part of the) wavelet and provides the user with a degree of freedom that can be easily tuned to a given purpose. wavelets, as building blocs of models, are well localized in both time and scale (period, frequency). as the paul wavelet decays more quickly in the time-domain then morlet, it enables better time localization. the greater difference is in frequency/ period domain. for the given count of evident oscillations in the wavelet the paul wavelet is much less efficient. as a source matlab code the wavelet software for morlet wavelet computations provided by torrence and compo was used. matlab codes are available at url: http://paos.colorado.edu/research/wavelets/. since laštovička et al. (2003a) used only less suitable meyer wavelet in analysis of the planetary wave activity in the lower ionosphere, we use for the «vertical» comparison over eun ( ) ( ) ! ! ( )paul: param t n i it n 2 2 1, ( ) n n n n 0 1ψ π = − = − + ( 2 )exp exp i tπν=morlet: ( ) param t t 2 2 , / n0 2 1 4 2 2 0 0 ψ πσ σ πσν − = − ] bg l z!, z0>aduψ( , )x u z=( , , )t a t z a a u t1 x r 0 − b l# 1192 jan laštovička, petra šauli and peter križan rope only the results of the mayer wavelet transform as published by laštovička et al. (2003a,b). the mayer (e.g., daubechies, 1994) and morlet wavelet transforms yielded the same statistical characteristics of the planetary wave type event persistence for fof2 over europe (laštovička et al., 2003b). commercial matlab-wavelet software has been used to compute the meyer wavelet transform. the wavelet analysis is applied to consecutive 1-year long intervals, shifted by half a year (january-december 1979, july 1979-june 1980, january-december 1980, etc.). thus for slough and juliusruh we have 21 partly overlapping intervals for the period 1979-1989, for other stations a bit less due to data gaps. only wave events with duration of at least three wave cycles are taken into account. 3. persistence of planetary wave type events in fof2 figure 1 presents an example of the normalized wavelet power spectrum for the morlet wavelet transform. the morlet wavelet transform results are evaluated for the normalized absolute power, i.e. we search for intervals with values larger than a fixed value. in terms of colours in fig. 1 it means that we consider only intervals with yellow and red colour of the duration of at least three wave cycles. only results inside of the conus-of-influence (black thin curve) are taken into account. the border effects affect the information out of the thin curve. even though the values shown in fig. 1 are normalized, the way of their interpretation with respect to a fixed level is hereafter called «abfig. 1. planetary wave type activity inferred from fof2 for akita, 1 july 1981-30 june 1982, morlet wavelet transform. top panel: time series of raw fof2 data. bottom panel: wavelet power spectrum of the planetary wave activity changing by colour from white and black-blue (minimum values) through green to red and black-red (maximum values). wavelet power spectrum is normalized to 1. 1193 persistence of planetary wave type oscillations in the mid-latitude ionosphere solute» power. however, the morlet wavelet transform results are also evaluated in terms of «relative» power, when the seasonally variable level of the background fof2, shown in the top panel, is taken into account. we consider relative intensifications with respect to the vicinity of the events with duration of at least three wave cycles to be planetary wave type events. this allows us to compare the results of the relative and absolute power approach to interpretation of the results as it was done for european data by laštovička et al. (2003b). figure 2 shows an example of the results of the paul wavelet transform. the results of the paul wavelet transform are evaluated in terms of the «absolute» power in the same way as morlet results. the most pronounced feature of figs. 1 and 2 is a large temporal and partly spectral variability of the planetary wave activity. the migration of periods of planetary wave activity is also well visible. the large temporal variability of planetary wave activity has to result in a limited persistence of the individual planetary wave events. the statistics of duration of individual events of the enhanced planetary wave activity for all yearly intervals and period bands centred at 5, 10 and 16 days is summarized in tables ii, iii and iv for paul transform, «absolute» evaluation, morlet transform, «absolute» evaluation and morlet transform, «relative» evaluation, respectively. the tables present the number of events together with their mean, median and the most often occurring number of cycles for individual stations and the average values for all four stations. tables ii-iv reveal for the 5-day wave the typical persistence of well-developed wave events to be of 4 cycles consistently for all three ways of analysis. for the 10-day wave, both the fig. 2. the same as fig. 1 but for paul wavelet. 1194 table ii. statistics of persistence of planetary wave type oscillations in fof2 for stations from the northern america and japan based on the paul wavelet transform («absolute» evaluation). the average values for medians and the most frequent values are presented with a step of 0.5. station period (days) number of events median value mean value most frequent value boulder 5 24 4 4.3 4 10 15 4 4.0 3.5 16 20 3.5 3.9 3 wallops island 5 20 4 4.2 4 10 15 3.5 3.6 3.5 16 16 3.5 3.5 3 wakkanai 5 15 4 3.9 4 10 12 3.5 4.0 3.5 16 13 3.5 3.5 3 akita 5 18 4 4.0 4 10 14 3.5 3.8 3 16 15 3.5 3.5 3 average values 5 19 4 4.1 4 10 14 3.5 3.7 3.5 16 16 3.5 3.6 3 table iii. statistics of persistence of planetary wave type oscillations in fof2 for stations from the northern america and japan based on the morlet wavelet transform (absolute values). the average values for medians and the most frequent values are presented with a step of 0.5. station period (days) number of events median value mean value most frequent value boulder 5 45 4 4.4 3 10 33 3.5 4.2 3 16 32 3.5-4 4.9 3 wallops island 5 49 4 4.3 3.5 10 31 4 4.5 3.5-4 16 21 5 5.1 3 wakkanai 5 43 4 4.4 4 10 34 3.5-4 4.7 3.5 16 34 3.5 4.1 3 akita 5 52 4 4.5 4 10 38 4 4.0 3.5 16 33 4 4.8 3 average values 5 47 4 4.4 3.5 10 34 4 4.4 3.5 16 30 4 4.7 3 table iv. statistics of persistence of planetary wave type oscillations in fof2 for stations from the northern america and japan based on the morlet wavelet transform (relative values). the average values for medians and the most frequent values are presented with a step of 0.5. station period (days) number of events median value mean value most frequent value boulder 5 85 4 4.2 4 10 59 3.5 4 3 jan laštovička, petra šauli and peter križan 1195 persistence of planetary wave type oscillations in the mid-latitude ionosphere median values and the most frequent occurrences provide the typical persistence of 3.5 cycles. for the 16-day wave, medians point to a typical persistence of 3.5 cycles and the most frequent values reveal a typical persistence of just 3 cycles. thus the persistence in terms of the number of cycles slightly decreases with increasing period. in terms of days the typical duration is 20 days for t = 5 days, about 35 days for t = 10 days and about 55-60 days for t = 16 days. in other words, the duration of the wave events in terms of days is longer, not shorter for longer periods. the median, mean and most frequent values are mostly very similar for all four stations and in tables ii-iv. this means that american and japanese stations reveal essentially the same statistical characteristics of persistence of the planetary wave type events in fof2, and that the «absolute» evaluation of the paul wavelet transforms and the «absolute» and «relative» evaluation of table v. statistics of persistence of planetary wave type oscillations in fof2 over europe based on the morlet wavelet transform (relative values). the average values for medians and the most frequent values are presented with a step of 0.5. after laštovička et al. (2003a). station period (days) number of events median value mean value most frequent value juliusruh 5 45 4 4.5 4 10 39 4 4.0 3+4 16 35 3.5 3.8 3-3.5 slough 5 37 4 4.3 4 10 29 3.5 3.6 3.5 16 34 3.5 3.6 3-3.5 pru° honice 5 49 4.5 4.7 4 10 42 4 4.1 3 16 33 3.5 3.8 3 rome 5 31 4 4.0 3.5-4 10 33 3.5 3.7 3.5 16 25 3 3.5 3 average values 5 40 4 4.3 4 10 36 3.5-4 3.9 3.5 16 32 3.5 3.7 3 table iv (continued). station period (days) number of events median value mean value most frequent value 16 42 4 4.8 3 wallops island 5 89 4 4.4 4 10 45 3.5 3.9 3.5 16 35 3.5 4.2 3 wakkanai 5 98 4 4.4 4 10 61 3.5 4.1 3.5 16 35 3.5 3.8 3 akita 5 106 4 4.5 4 10 55 4 4.1 3.5 16 40 3.5-4 4 3 average values 5 95 4 4.4 4 10 55 3.5 4 3.5 16 38 3.5 4.2 3 1196 jan laštovička, petra šauli and peter križan the morlet wavelet transforms yield very similar statistical characteristics of the planetary wave type events. the only substantial difference is in the number of events derived by different methods, as tables ii-iv clearly demonstrate. however, it does not affect the statistical characteristics of the persistence. one source of differences between the «absolute» and «relative» evaluation is the existence of relatively long intervals of high but variable activity. such an interval is evaluated as one event in «absolute» way, but may provide two or even three events in «relative» way due to that internal structure of the interval. the different number of identified events for the morlet wavelet transform compared with the paul wavelet transform is probably influenced by different mother wavelets. tables v and vi show statistical characteristics of the persistence of the planetary wave type events over europe obtained with the morlet wavelet transform. they are either identical with or very slightly larger than such values from the u.s.a. and japan for all three periods of waves. this means that the statistical characteristics of planetary wave type event persistence are essentially the same in all three geographic regions and, thus, may be considered to be representative for middle latitudes of the northern hemisphere. 4. planetary wave type events in fof2 and the lower ionosphere over europe as for the «vertical» comparison, we can compare the similarity of statistical characteristics of persistence of events, and of occurrence of individual planetary wave type events in fof2 and the lower ionosphere in europe using data and results of laštovička et al. (2003a,b). the typical persistence of the 5-day wave in fof2 is four wave cycles (laštovička et al., 2003b). this is slightly less than the typical persistence of 5 cycles for the 5-day wave in the lower ionosphere (laštovička et al., 2003a). for the 10-day and 16-day waves, the typical persistence in fof2 and the lower ionosphere is identical. thus the typical persistence of planetary wave type oscillations in the lower ionosphere and fof2 appears to be almost identical. a quite different pattern is provided by comparison of individual events. the similarity of table vi. statistics of persistence of planetary wave type oscillations in fof2 over europe based on the morlet wavelet transform (absolute values). the average values for medians and the most frequent values are presented with a step of 0.5. after laštovička et al. (2003a). station period (days) number of events median value mean value most frequent value juliusruh 5 55 4 4.8 4 10 39 4.5 4.6 4.5 16 30+1* 4 4.8 3 slough 5 52 4.5 4.8 3-4 10 38 4-4.5 4.7 3.5 16 35 4 4.3 3 pru° honice 5 33 4 4.6 4 10 42 4 4.4 3 16 31 4.5 4.7 4 rome 5 55 4.5 4.9 3.5-4 10 52 4 4.2 4.5 16 38 4 4.6 4 average values 5 49 4-4.5 4.8 4 10 43 4 4.4 4 16 34 4 4.6 3.5 1* = ∼220 days long period of persistent occurrence in 1980. 1197 persistence of planetary wave type oscillations in the mid-latitude ionosphere occurrence of individual events in fof2 and the lower ionosphere is mostly poor, worse than expected from the results of some studies, which compared neutral wind oscillations in the mlt region with oscillations in fof2 or in the radio wave absorption in the lower ionosphere (e.g., pancheva et al., 1994). the quasi-simultaneous occurrence of events in fof2 and the lower ionosphere is rare. the reason might be that the absorption fluctuations are related to fluctuations in the prevailing wind (e.g., pancheva et al., 1989), while those in fof2 seem to be related more to fluctuations in the tidal winds (e.g., laštovička and šauli, 1999). the planetary wave type fluctuations in the prevailing wind and the tidal wind differ to some extent. why and to what degree these fluctuations differ should be examined. another reason of the poor similarity might be the geomagnetic origin of a significant part of planetary wave type fluctuations in fof2 (altadill and apostolov, 2003), whereas the geomagnetic activity does not play a role in planetary wave type oscillations in the lower ionosphere (pancheva et al., 1989). on the other hand, probably not all planetary wave type oscillations from the lower ionosphere have chance to be (indirectly) transported to heights of fof2. 5. longitudinal size of planetary wave events we analysed the planetary wave events for eight stations, which represented europe, japan, and the mid-latitude u.s.a. similarity of occurrence (in the sense of simultaneous occurrence) of individual planetary wave type events observed in these three separated regions makes possible to estimate the longitudinal size of events or at least to put some constraints on that size. altadill and apostolov (2003) used another approach and found a typical longitudinal size of the planetary wave type events in fof2 as follows: t = 5-6 days, 80° t = 10 days, 100° t = 16 days,180° with individual events covering up to the whole globe. our analysis reveals that there is almost no simultaneous occurrence of the t = 5 days events observed in europe, japan and us, even though statistical characteristics of their persistence are closely similar. table vii shows an example of such a t = 5 days event. the event first occurs in us (wallops), immediately after the end of the event in us it appears in europe, and a few days after the end of the event in europe it appears in japan, which looks like an eastward propagation of an event of limited longitudinal size. however, if we consider the longitudinal difference between the regions (stations) and the typical longitudinal size of 80° found by altadill and apostolov (2003), the observed absence of simultaneous occurrence of wave events coincides with the typical longitutable vii. a t = 5 days event and a t = 10 days event, both observed in the second half of 1986, morlet wavelet transform. too short means shorter than 3.0 cycles; short bursts means only short, separated bursts occur. period 5 days 10 days station days cycles duration days cycles duration juliusruh 300-316 3.2 16 days 245-279 3.4 34 days slough too short 240-283 4.3 43 days pru°honice 300-316 3.2 16 days 240-281 4.1 41 days roma short weak too short boulder too short no effect wallops 266-299 6.6 33 days short bursts wakkanai 320-336 3.2 16 days no rffect akita 320-335 3.0 15 days no effect 1198 jan laštovička, petra šauli and peter križan dinal size of such events established by altadill and apostolov (2003). we analysed several t = 7 days events. such events observed at european, japanese and us stations showed again almost no simultaneous occurrence of individual events. the t = 10 days events also mostly displayed absence of simultaneous occurrence, as the event shown in table vii, in correspondence to their typical longitudinal size of 100° as found by altadill and apostolov (2003). on the other hand, a few t = 16 days events, which were studied, revealed much often simultaneous or partly simultaneous occurrence of wave events between european, us and japanese stations. one such event happened in 1984 with beginning around day 60 and persistence of four wave cycles in europe (at all four stations), and beginning slightly later and persisting again about four wave cycles for wakkanai, boulder and wallops island (at southernmost akita persistence shorter than three wave cycles). this is consistent with their much larger typical longitudinal size found by altadill and apostolov (2003). 6. discussion we investigate the persistence of planetary wave type oscillations over europe among others in order to clarify possible predictability of such oscillations with applications to the predictions of the radio wave propagation conditions. for the sake of predictions, not only the typical values of the persistence of planetary wave type events, but also the spectral distribution of persistence of individual events (= event duration) is required. an example of the spectral distribution of event duration is shown in fig. 3, which presents for each region one station. the spectral distribution of event duration is too broad to allow a reasonable prediction of event duration from fof2 measurements themselves. moreover, figs. 1 and 2 display many events of duration shorter than three cycles, which is the lower limit of studied events. thus the predictability of the planetary wave type oscillations in fof2 from the measurements of fof 2 itself seems to be very questionable. we have to look for other predictors, which is complicated due to different physical origin of different planetary wave type events. unfortunately, we will probably be unable to separate predictable planetary wave type events, which mean that they remain a part of the prediction noise. improvement of the quality of radio wave propagation predictions in such a way seems to be rather impossible, or at least very difficult. some planetary wave events have sharp onset and end. then the determination of planetary wave persistence depends on the time/period resolution of the used wavelet. shape of selected wavelet influences how the wavelet captures/detects each wave-like event. however, the onset and end of most events is not so sharp and the accuracy of the determination of onset duration in such cases may be up to a few days, particularly for the «relative» evaluation. therefore we can consider the accuracy of determination of the typical persistence of planetary wave events to be rather around half a cycle for 5-day waves, and better for longer periods in terms of wave cycles. fig. 3. spectra of event duration (number of wave cycles) for the 5-day oscillations, morlet wavelet transform for boulder («absolute» evaluation, solid curve), akita («relative» evaluation, long-dash curve), and juliusruh («absolute» evaluation, short-dash curve). 1199 persistence of planetary wave type oscillations in the mid-latitude ionosphere 7. conclusions the basic characteristics of the f2 region, the critical frequency fof2, were analyzed for two us stations, boulder and wallops island, two japanese stations, akita and wakkanai, and the results were compared with the results of laštovička et al. (2003b) for four european stations juliusruh, slough, pru° honice and rome, and laštovička et al. (2003a) for the lower ionosphere above central europe. all data were analyzed over the period 1979-1989 (one solar cycle). noontime average values (10-14 ut) of fof2 were used. the persistence of planetary wave type oscillations at periods near 5, 10 and 16 days and sometimes 7 days was studied with the use of the paul, morlet and meyer wavelet transforms. only events of duration of three cycles and more were considered. the main results are as follows: 1) there is a large temporal and partly spectral variability of planetary wave type activity. the migration of periods of planetary wave activity is also well visible. the large temporal variability of the planetary wave activity results in a limited persistence of the individual planetary wave type events. 2) for the 5-day wave, a typical persistence of well-developed wave events in fof2 is 4 wave cycles. for the 10-day wave, it is rather 3.5 wave cycles. for 16 days, the most frequent values provide typical persistence no more than 3 wave cycles. 3) in terms of the number of wave cycles in the planetary wave type events, the persistence decreases towards longer periods. however, the persistence of wave events in terms of days increases towards longer periods. 4) the planetary wave type wave persistence characteristics for europe are either identical with or very slightly larger than corresponding values from the u.s.a. and japan. 5) the spectrum of event duration is very broad. the character of the spectrum does not allow us to predict the duration of an event when we observe its beginning or, say, first 2-3 wave cycles. 6) the longitudinal size of the planetary wave type events increases with increasing period, making the 5-day and 10-day period events in europe, america and japan essentially dissimilar, and the 16-day oscillations much more similar among the three regions. 7) while the typical persistence of planetary wave type oscillations in fof2 and the lower ionosphere over europe is similar, the correspondence of occurrence of individual events is rather poor. acknowledgements this work has been supported by the academy of sciences of the czech republic under project avoz30420517. references altadill, d. and e.m. apostolov (2001): vertical propagating signatures of wave type oscillations (2 and 6.5 days) in the ionosphere obtained from electron density profiles, j. atmos. solar-terr. phys., 63, 823-834. altadill, d. and e.m. apostolov (2003): time and scale size of planetary wave signatures in the ionospheric fregion. role of the geomagnetic activity and mesosphere/lower thermosphere winds, j. geophys. res., 108 (a11), 1403, doi: 10.129/2003ja010015. altadill, d., e.m. apostolov, j. boška, j. laštovi čka and p. šauli (2004): planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability, ann. geophysics, 47 (suppl. to n. 2/3), 1109-1119. burešová, d. (1997): results of fof2 data testing with the undiv program, studia geophys. geod., 41, 82-87. cavalieri, d.j. (1976): travelling planetary-scale waves in the e-region ionosphere, j. atmos. terr. phys., 38, 965974. daubechies, i. (1994): ten lectures on wavelets, cbms, siam, 61, 117-119. forbes, j.m. and x. zhang (1997): quasi 2-day oscillation of the ionosphere: a statistical study, j. atmos. solarterr. phys., 59, 1025-1034. forbes, j.m., s. palo and x. zhang (2000): variability of the ionosphere, j. atmos. solar-terr. phys., 62, 685-693. haldoupis, c., d. pancheva and n.j. mitchell (2004): a study of tidal and planetary wave periodicities present in mid-latitude sporadic e layers, j. geophys. res., 109, a02302, doi: 10.1029/2003ja010253. kazimirovsky, e.s., m. herraiz and b.a. de la morena (2003): effects on the ionosphere due to phenomena occurring below it, surv. geophys., 24, 139-184. laštovička, j. (2006): forcing of the ionosphere by waves from below, j. atmos. solar-terr. phys., 68, 479-497. laštovička, j. and p. šauli (1999): are planetary wave type oscillations in the f2 region caused by planetary wave modulation of upward propagating tides?, adv. space res., 24, 1473-1476. laštovička, j., v. fišer and d. pancheva (1994): long1200 jan laštovička, petra šauli and peter križan term trends in planetary wave activity (2-15 days) at 80-100 km inferred from radio wave absorption, j. atmos. terr. phys., 56, 893-899. laštovička, j., p. križan and d. novotná (2003a): persistence of planetary waves in the lower ionosphere, studia geophys. geod., 47, 161-172. laštovička, j., p. križan, p. šauli and d. novotná (2003b): persistence of the planetary wave type oscillations in fof2 over europe, ann. geophysicae, 21, 1543-1552. mallat, s. (1998): a wavelet tour of signal processing (academic press, san diego), pp. 577. pancheva, d., e. apostolov, j. laštovička and j. boška (1989): long-period fluctuations of meteorological origin observed in the lower ionosphere, j. atmos. terr. phys., 51, 381-388. pancheva, d., l.f. alberca and b.a. de la morena (1994): simultaneous observation of the quasi-two day variations in the lower and upper ionosphere, j. atmos. terr. phys., 56, 43-50. rishbeth, h. and m. mendillo (2001): patterns of ionospheric variability, j. atmos. solar-terr. phys., 63, 1661-1680. torrence, c. and g.p. compo (1998): a practical guide to wavelet analysis, bull. amer. met. soc., 79 (1), 61-78. vidakovic, b. (1999): statistical modelling by wavelets (a wiley-interscience publication, john wiley & sons), pp. 408. (received october 14, 2005; accepted may 17, 2006) vol50,2,2007 191 annals of geophysics, vol. 50, n. 2, april 2007 key words permo-triassic – pangaea – absolute reconstruction – earth magnetic field – polarity transition 1. introduction different rock types are capable of recording the directions of the earth’s magnetic field (emf). through these records, we know that the dipolar component of the geomagnetic field periodically inverts its polarity. our knowledge on what happens as the field reverses comes from the evaluation of transition palaeomagnetic records. the earliest analysis of geomagnetic polarity reversals was that determined for the jurassic stormberg volcanic rocks of lesotho in southern africa (van zijl et al., 1962). however, most studies of this intriguing process of the emf have used cainozoic data (see coe and glen, 2004), because transitional records are difficult to determine in older lithologies; as these rocks have probably undergone more geological processes that resulted in remagnetizations resetting total or partially the original data. analysis of a permo-triassic polarity transition in different absolute reconstructions of pangaea, considering a model with features of the present earth magnetic field haroldo vizán and maría andrea van zele consejo nacional de investigaciones científicas y técnicas (conicet) and departamento de ciencias geológicas, facultad de ciencias exactas y naturales, universidad de buenos aires, argentina abstract the main objective of this paper is to show that the distribution of transitional palaeomagnetic data recorded at 250 ma are in agreement with simulated data that depend on the sampling site, using a model that considers features of the present earth magnetic field. the analysis was performed comparing simulated reversals with the permo-triassic polarity transition recorded in the siberian trap basalts. the palaeomagnetic data were corrected according to the palaeo-latitude and palaeo-longitude of siberia (absolute reconstruction) at 250 ma using hotspot tracks. to obtain the motion of siberia relative to hotspots from the present time back to 250 ma, three different pangaea models were considered (pangaea a, pangaea a2, pangaea b). in spite of the uncertainties associated with the use of hotspot frameworks and pangaea configurations, both the modelled and recorded data show a remarkable fit when absolute reconstructions of pangaea a and a2 configurations are performed. the agreement between both simulated and recorded data suggests that similar features to that of the present earth magnetic field could have been involved in reversals since the permo-triassic. mailing address: dr. haroldo vizán, consejo nacional de investigaciones científicas y técnicas and departamento de ciencias geológicas, facultad de ciencias exactas y naturales, universidad de buenos aires, intendente güiraldes 2160, ciudad universitaria, pabellón ii, 1er piso, c1428ega buenos aires, argentina; e-mail: haroldo@gl.fcen.uba.ar 192 haroldo vizán and maría andrea van zele on the other hand, for all of geological time (including the cainozoic) transition records depend on the capabilities of rocks to act as natural magnetic recorders. lava flows are considered to record the most accurate spot readings of the geomagnetic field (hoffman, 1992) but due to the episodic nature of eruptions, their transition records can be discontinuous or multiple lava flows can be emplaced over a shorter time span compared with the rate of variation of the emf, in which case the flows will preserve redundant records of this field (prévot and camps, 1993). sedimentary rock sequences can be quasi-continuous records but they become magnetized by physical and chemical processes of relatively long duration that occur from the deposition of the sediment till the diagenesis. magnetic directions of sedimentary rocks considered as transitional records could be, indeed, artefacts and not real readings of the emf (langereis et al., 1992). in spite of these problems, longitudinal confinements of virtual geomagnetic poles (vgps) of cainozoic transitional records were interpreted as representative of geomagnetic behaviours (clement, 1991; laj et al., 1991; hoffman, 1992; love, 1998) and different geodynamo models have been suggested to explain cainozoic polarity transitions. hoffman (1992) has suggested that inclined dipolar reversal states of the emf have dominated cainozoic reversals. gubbins and coe (1993) and gubbins (1994) provided an explanation to link longitude-confined vgp paths to variations in the magnetic flux on the earth’s core surface based on a model proposed by gubbins (1987). glatzmaier et al. (1999) simulated a suite of reversals which examples were presented by coe et al. (2000); the most interesting of these simulated reversals is that produced by the tomographic geodynamo model where the heat-flux at the core-mantle boundary is patterned on the large-scale variation of seismic velocity from tomographic studies of the lowermost mantle. as indicated above, transition records for reversals older than the cainozoic are much more difficult to determine and there are no specific models to explain the polarity transitions of the emf during the mesozoic or palaeozoic times. however, reliable transition records of those times can be analyzed considering the models proposed for the cainozoic and then, speculate about the mechanisms that manage mesozoic or palaeozoic reversals. a recent study by heunemann et al. (2004) provided reliable directions and intensities for a reversal of the emf recorded in the permo-triassic (ca. 250 ma) siberian trap basalts. heunemann et al. (2004) corrected the corresponding vgps of the recorded directions for a permo-triassic palaeo-geography of siberia and invoked two alternatives, based on the models presented by coe et al. (2000) and hoffman (1992), to account for the characteristic features observed in their data. it is noteworthy that the euler rotation pole used by heunemann et al. (2004) is just to determine the palaeo-latitude of siberia at 250 ma (smethurst et al., 1998) but not the palaeolongitude of this continental block at that age. this paper compares simulated reversals determined by the model proposed by gubbins (1994) with the permo-triassic polarity transition recorded in the siberian trap basalts. palaeomagnetic data were corrected in accordance with the palaeo-latitude and palaeo-longitude of siberia at 250 ma using hotspot tracks as has been done for the analysis of the jurassic transition recorded in the stormberg lavas (vizán and van zele, 1995; prévot et al., 2003). to obtain the motion of siberia relative to hotspots from the present back to 250 ma, several relative reconstructions from a variety of sources were used assuming that three different models of pangaea are valid for 250 ma (pangaea a, pangaea a2, pangaea b). remarkable agreements between the simulated and recorded transitional data were obtained after latitude and longitude palaeo-geographic corrections, especially if pangaea a and a2 models were used. 2. discussion on the data and methodologies the analyzed palaeomagnetic data were compiled from table 2 of heunemann et al. (2004). the extremely high lava productivity makes the siberian trap basalts a possible continuous record for the palaeo-directions of the emf. according to heunemann et al. (2004) its stratigraphically lower section contains three 193 analysis of a permo-triassic polarity transition successive flows of reversed polarity followed by flows that record a transitional state of the emf. the upper section shows normal polarity and a group of directions that is tentatively interpreted by heunemann et al. (2004) as an excursion. analysis in this paper concentrates on data that record the transitional state (considering also those three data of reverse polarity in the lower section and some of normal polarity of the upper section). data belonging to the possible excursion as interpreted by the same authors were not analyzed. the transitional record shows a clustering of data that may be explained as a rapid succession of lava flows; however the palaeo-intensity results indicate that this phenomenon has a geomagnetic cause as several independent field states are recorded (huenemann et al., 2004). this paper uses the geomagnetic polarity reversals mechanism proposed by gubbins (1994) to analyze the transitional record of the siberian trap basalts, because it is based on the configuration of emf at the core surface derived from historical records (gubbins, 1987) and has the interesting feature that for the same reversal, different paths of vgps are predicted at different measurement sites (see gubbins, 1994; his figs. 12 and 13). the present work analyses whether the vgps simulated by gubbins’ model are comparable to the cluster of vgps recorded in the location of the siberian trap basalts at about 250 ma. hotspot models were used to determine palaeo-latitudinal and palaeo-longitudinal corrections in our analysis. whether hotspots move slowly enough with respect to one another to provide a useful reference frame to make absolute reconstructions has been the subject of discussion (i.e. torsvik et al., 2002). for some workers hotspot models are not very reliable, but others use them in tectonics and geomagnetic models. absolute reconstructions, for the last 120 ma, for africa based on a moving hotspot reference frame, have been recently suggested by o’neill et al. (2005) many tectonics and geomagnetism studies (i.e. engebretson et al., 1985; besse and courtillot, 2002; prévot et al., 2003) have used an earlier model that considered that hotspots are reasonably fixed and form a convenient reference frame to determine absolute motions of africa for the last 200 ma (morgan, 1983). hotspot tracks have been recognized by zonenshain et al. (1985) to calculate a clockwise rotation of siberia from 130 to 280 ma. this paper used all three of these hotspot models and also determined if an absolute reconstruction of the sampling site based on these models, gives a better agreement between simulated and recorded data. in order to determine the motion of the sampling site with respect to the hotspots it is necessary to perform a relative reconstruction of eurasia with africa at present geographic coordinates using a reconstruction of pangaea at 250 ma. however, after more than 25 years of discussions among palaeomagnetists, there is no general consensus about the configuration of pangaea during the late palaeozoic-early mesozoic. it is generally agreed that the pangaea configuration proposed by wegener (1922), named pangaea a and quantified by bullard et al. (1965), was the starting point for the break up of this super continent in the middle jurassic. however, palaeomagnetic poles (pps) of gondwana and laurasia for the late carboniferous to middle triassic cannot be reconciled with this reconstruction and show a systematic discrepancy that could be due to different factors such as inclination shallowing in sediments, unrecognized overprints that have not been removed during demagnetization, underestimation of rock ages (rochette and vandamme, 2001; mcelhinny, 2004; van der voo and torsvik, 2004). this systematic misfit between pps has also been interpreted in geodynamic or geomagnetic terms. briden et al. (1971), van der voo and torsvik (2001) and torsvik and van der voo (2002) propose that a possible explanation for this disagreement is the existence of persistent non-dipolar (octupolar) fields. many other workers have invoked different reconstructions of pangaea called pangaea b (i.e. morel and irving, 1981; torcq et al., 1997; muttoni et al., 2003; rakotosolofo et al., 2005) and pangaea a2 (i.e. van der voo and french, 1974; van der voo et al., 1976; smith and livermore, 1991) to produce an agreement between the laurasia and gondwana pps. pangaea a2 is not very different from pangaea a and the transition of one to another, involves a small dextral mega shear of about 500 km but the transforma194 haroldo vizán and maría andrea van zele tion of pangaea b to pangaea a involves a large dextral mega shear (of 4000-3500 km). muttoni et al. (1996) proposed the evolution of pangaea from pangaea b to a2 during the mid-permian and from a2 to a during the late triassic. the polarity transition analysis presented here also determined which pangaea model gives the better agreement between the transition data produced using the gubbins (1994) model and the data recorded from the siberian trap basalts. 3. palaeoreconstructions of the palaeomagnetic data recorded in siberian trap basalts the sections studied by heunemann et al. (2004) are located in the listvjanka (69°28ln, 88°43le) and icon/abagalakh river valleys (70°22ln, 90°04le). for the present study the geographic localities were averaged and the palaeomagnetic data were normalised to 69°55ln, 89°23le. the geographic coordinates of this locality in the permo-triassic were determined from a mean siberian trap pp using the different models of pangaea and hotspot frameworks mentioned above. we considered a pp that is the mean of four entries as a representative pole position for the siberian trap basalts. the four entries are 1) a mean siberian trap pole at 54.7°n/140.5°e given in torsvik and andersen (2002); 2) global palaeomagnetic database (gpmdb)-refno 2832 (59°n/150°e), see also gurevitch et al. (1995); 3) a pole from gurevitch et al. (2004: 54.6°n/146°e); 4) a pole from heunemann et al. (2004: 57.1°n/148.2°e). the geographic coordinates and the fisherian statistical parameters of the mean pp are: lat = 56.4°n, long = =146.0°e, a95 = 3.5°, k = 677.9. this mean pp is quite similar to that obtained with 3 entries by van der voo and torsvik (2004) for the siberian traps (lat = 56°n, long = 150°e, a95 = 6°). in our case the mean pp includes the pole yielded by the stable reverse and normal directions of the sections that recorded the transition (heunemann et al., 2004) and then the a95 is reduced from 6° to 3.5°. recently van der voo and torsvik (2004) compared their mean siberian trap pole with the mean gondwana pp for 250±10 ma of torsvik and van der voo (2002) in a pangea a configuration assuming that siberia formed part of eurasia at ca. 250 ma (smethurst et al., 1998; fig. 1a-c. comparison of gondwana mean palaeopole (gp) for 250 ± 10 ma and siberian trap mean palaeopole (stp) in different pangaea reconstructions with central africa held fixed at present geographic coordinates. a) pangaea a model, notice the discrepancy between the mean palaeopoles. b) pangaea a2 model, the mean palaeopoles are indistinguishable at 95% of confidence level, although there is a great circle distance of about 9° between them. c) pangaea b model, the mean palaeopoles get the better fit. a supposed mega-shear of about 3700 km is required to pass from this pangaea configuration to a pangaea a model (the starting point of the break up of this super-continent during middle jurassic time). a b c table i. rotation poles for the sampling site in pangaea a model. euler pole latitude (°n) longitude (°e) rotation (degree) reference 1) eurasia relative 88.5 27.7 −38 bullard et al. (1965) to north america 2) north america relative 66.95 −12.02 75.55 klitgord and schouten (1986) to nw africa 3) nw africa relative 12.23 19.01 3.44 schettino and scotese (2005) to central africa 4) africa relative to hotpots 17.03 −27 −29.72 o’neill et al. (2005) (0 to 120 ma) 5) africa relative to hotpots 31.38 3.4 32.18 morgan (1983) (*) (120 to 200 ma) 6) eurasia relative to hotspots 85.03 252.46 25 zonenshain et al. (1985) (**) (200 to 250 ma) 7) true polar wander 0 187.46 28.15 (***) (*) stage pole calculated using the euler poles of 120 ma and 200 ma of morgan (1983). (**) the geographic coordinates of this euler pole (now at 54°n, 104°e) were calculated for 200 ma in pangaea a model. (***) rotation about an equatorial euler pole to account for the displacement of the rotation (palaeomagnetic) axis with respect to core/mantle boundary (cox and hart, 1986). 195 analysis of a permo-triassic polarity transition torsvik et al., 2001). van der voo and torsvik (2004) noted that using these poles there is still a continental overlap of 1000 km if pangea a is used. this paper also compare the new siberian trap mean pp with a gondwana pp for 250 ma ± 10 ma in three models of pangea (a, a2 and b, see fig. 1a-c). thirteen poles for the stable parts of gondwana continents compiled by torsvik and van der voo (2002) were combined with the reconstruction parameters for gondwana continents recently proposed by schettino and scotese (2005). these new reconstruction parameters were used noting that torsvik and van der voo (2002) were not entirely satisfied with published gondwana fits. the resulting mean gondwana pp in present central africa geographic coordinates is based on a conventional fisherian calculation; its geographic coordinates and statistical parameters are: lat = = 48.4°n, long = 249.3°e, a95 = 8.4°, k = 25.6. the comparison was made with central africa held fixed and transferring the siberian trap pole to the geographic coordinates of this continental block through a plate circuit (euler poles 1, 2 and 3 in table i). the first rotation of the siberian trap pole was to reconstruct the continent of laurasia «closing» the north atlantic through the movement of eurasia towards north america. for this reconstruction the bullard et al. (1965) euler pole was used because van der voo (1990) and torsvik et al. (2001) have demonstrated that this reconstruction gives best agreement between late carboniferous-late triassic european and north american pps. the second rotation of the siberian trap pole used the reconstruction parameter of klitgord and schouten (1986) from north america to northwest africa, which belongs to a model of pangaea a. finally the pole was transferred from northwest africa to central africa using the reconstruction parameters of schettino and scotese (2005). figure 1a shows gondwana and siberian trap pps in central africa geographic coordinates using the pangaea a configuration. the pps do not fit, they are distinguishable at 95 % of confidence level and the great circle distance between them is of 15.2°. to compare both mean pps with a model of pangaea a2, the siberian trap pole was rotated using also the finite pole of rotation of van der voo and french (1974) «closing the gulf of 196 haroldo vizán and maría andrea van zele mexico». figure 1b shows the pps of gondwana and siberian traps transferred to central africa geographic coordinates (euler poles 1, 2, 3 and 4 in table ii) in a pangaea a2 configuration. the great circle distance between the mean pps is of 9.1°; however there is an overlap between the 95% confidence level intervals and the 4 poles of siberian trap basalts share a common mean at 95% confidence (mcfadden and lowes, 1981) with gondwana pps. to compare gondwana and siberian trap mean pps with a model of pangaea b, a finite pole of rotation to obtain the motion of north america relative to central africa was calculated using the analysis of morel and irving (1981) and the reconstruction parameters of klitgord and schouten (1986) and schettino and scotese (2005). this rotation parameter has its pole at lat = 36.2°n, long = 04.0° w, and an angle of rotation = 77.35° (counter clockwise). the relative position between the atlantic bordering continents obtained with this reconstruction parameter is shown in fig. 1c together with gondwana and the siberian trap mean pps transferred to central africa geographic coordinates (euler poles 1, 2, 3 and 4 in table iii). the great circle distance between these pps is 2.5° and, obviously, they are undistinguishable at 95% of confidence level. to perform our transition record analysis, absolute reconstructions of the sampling site were made by considering the siberian trap mean pp (as representative of the palaeomagnetic spin axis at 250 ma), different models of pangaea and the hotspot frameworks already mentioned. firstly we transferred the siberian trap mean pp to present central africa coordinates using the reconstruction parameters to build the different models of pangaea. after that, we rotated the plates and the pole position according to the euler pole of o’neill et al. (2005) for 120 ma. then again we rotated the plates and the pole potable ii. rotation poles for the sampling site in pangaea a2 model. euler pole latitude (°n) longitude (°e) rotation (degree) reference 1) eurasia relative 88.5 27.7 −38 bullard et al. (1965) to north america 2)north america relative 66.95 −12.02 75.55 klitgord and schouten (1986) to nw africa 3) nw africa relative 12.23 19.01 3.44 schettino and scotese (2005) to central africa 4) rotation to close 17.38 −8.91 16.13 van der voo and french (1974) (*) the gulf of mexico 5) africa relative to hotpots 17.03 −27 −29.72 o’neill et al. (2005) (0 to 120 ma) 6) africa relative to hotpots 31.38 3.4 32.18 morgan (1983) (**) (120 to 200 ma) 7) eurasia relative to hotspots 69.17 268.5 25 zonenshain et al. (1985) (***) (200 to 250 ma) 8) true polar wander 0 187.03 44.17 (****) (*) re-calculated considering the reconstruction parameter of klitgord and schouten (1986) for north america relative to nw africa and the rotation parameter of schettino and scotese (2005) for nw africa relative to central africa. (**) stage pole calculated using the euler poles of 120 ma and 200 ma of morgan (1983). (***) the geographic coordinates of this euler pole (now at 54°n, 104°e) were calculated for 200 ma in pangaea a2 model. (****) rotation about an equatorial euler pole to account for the displacement of the rotation (palaeomagnetic) axis with respect to core/mantle boundary (cox and hart, 1986). 197 analysis of a permo-triassic polarity transition ative to hotspots from 200 to 250 ma, we used mongolian hotspot tracks that indicate that siberia rotated 75° in a clock wise direction from 280 to 130 ma about an euler pole that is now located at 54°n, 104°e (zonenshain et al., 1985). we transferred this pole to its geographic position at 200 ma in accordance with the different pangaea models and using the hotspot frameworks of o’neill et al. (2005) and morgan (1983). assuming siberia rotated at a constant velocity, we proposed a rotation of 25° of this continent between 200 and 250 ma. the mean siberian trap pp and the plates were then rotated using the corresponding euler poles for each pangaea model accounting for the movement of siberia relative to the hotspot track between 200 and 250 ma. finally we calculated the displacement of the rotation (palaeomagntic) axis with respect to the hotspot frameworks for every pangaea model and rotated all the plates about the equatorial table iii. rotation poles for the sampling site in pangaea b model. euler pole latitude (°n) longitude (°e) rotation (degree) reference 1) eurasia relative 88.5 27.7 −38 bullard et al. (1965) to north america 2) north america relative 66.95 −12.02 75.55 klitgord and schouten (1986) to nw africa 3) nw africa relative 12.23 19.01 3.44 schettino and scotese (2005) to central africa 4) transition from pangaea −25.79 47.49 35.79 morel and irving (1981) (*) a to b 5) africa relative to hotpots 17.03 −27 −29.72 o’neill et al. (2005) (0 to 120 ma) 6) africa relative to hotpots 31.38 3.4 32.18 morgan (1983) (**) (120 to 200 ma) 7) eurasia relative to hotspots 59.77 314.62 25 zonenshain et al. (1985) (***) (200 to 250 ma) 8) true polar wander 0 198.77 47.9 (****) (*) re-calculated considering the reconstruction parameter of klitgord and schouten (1986) for north america relative to nw africa and the rotation parameter of schettino and scotese (2005) for nw africa relative to central africa. the motion of north america relative to central africa in a pangaea b model can be determined through a cumulative rotation including the reconstruction parameters 2), 3) and 4). (**) stage pole calculated using the euler poles of 120 ma and 200 ma of morgan (1983). (***) the geographic coordinates of this euler pole (now at 54°n, 104°e) were calculated for 200 ma in pangaea b model. (****) rotation about an equatorial euler pole to account for the displacement of the rotation (palaeomagnetic) axis with respect to core/mantle boundary (cox and hart, 1986). table iv. absolute reconstructions of eurasia in different pangaea types and geographic coordinates of the sampling site at 250 ma. pangaea absolute reconstruction sampling site type euler poles lat °n long °e angle (°) lat °n long °e a 58.37 85.38 66.89 62.46 63.08 a2 59.09 86.34 68.49 62.46 64.97 b 42.15 71.89 45.89 62.46 36.10 sition, using a stage euler pole for the time span between 120 and 200 ma (lat = 31.38° n, long = = 3.4° e, and an angle of counter clockwise rotation = 32.18°) calculated from the reconstruction parameters proposed for africa by morgan (1983). to perform the rotation of the plates rel198 haroldo vizán and maría andrea van zele euler pole that belong to each case. the rotation poles to obtain the absolute reconstructions of the sampling site at 250 ma in different versions of pangaea are listed in tables i, ii and iii. these poles are numbered in the sequential order that should be followed to perform the reconstructions. the cumulative rotation for all the movements described before has different reconstruction parameters for the different pangaea models. in table iv are listed the euler poles for the absolute reconstruction of siberia in pangaea a, a2 and b and the corresponding palaeogeographic coordinates of the sampling site at 250 ma. 4. analysis of the transitional record and discussion the analysis was carried out comparing calculated and recorded data. the behaviour of the emf is currently analyzed by plotting the vgps as computed by palaeomagnetic directions. however, a vgp is calculated assuming a dipole model, which is at variance with the strong non-dipolar field expected during a reversal (i.e. gubbins and coe, 1993). for this reason we have analyzed not only the computed vgps but also their directions. the simulated data were determined according to the geographic coordinates of the measured site at 250 ma for the different absolute reconstructions of pangaea configurations using one of the models of gubbins (1994). gubbins (1994) developed two models (e and w) assuming the structure of the transitional field is substantially non-dipolar. since the reversal recorded in the siberian trap basalts is from reverse to normal and according to the path of its transitional vgps, the model w was adopted to simulate the transitional record. figures 2, 3, 4 and 5 compare modelled and recorded data; declinations, inclinations and palaeointensities are represented versus the latitude of the corresponding vgps. notice that unfortufig. 2. modelled and measured data of the polarity transition recorded in siberian trap basalts. the data have just a palaeo-latitudinal correction according to the reconstruction parameter of smethusrt et al. (1998) also used by heunemann et al. (2004). the transitional modelled and recorded data form similar vgps clusters however they do not agree. 199 analysis of a permo-triassic polarity transition fig. 3. modelled and measured data of the polarity transition recorded in siberian trap basalts in an absolute reconstruction of pangaea type a configuration. the palaeo-latitudinal and palaeo-longitudinal corrections were done on the basis of hotspots frameworks and the mean siberian trap palaeopole. notice the remarkable fit between both the modelled and recorded data. fig. 4. modelled and measured data of the polarity transition recorded in siberian trap basalts in an absolute reconstruction of pangaea type a2 configuration. the palaeo-latitudinal and palaeo-longitudinal corrections were done on the basis of hotspots frameworks and the mean siberian trap palaeopole. notice the remarkable fit between both the modelled and recorded data as in fig. 3. 200 haroldo vizán and maría andrea van zele nately heunemann et al. (2004) could not determine the corresponding palaeointensites for all the recorded palaeomagnetic directions. figure 2 shows the modelled and recorded data after the palaeo-latitudinal reconstruction of smethrust et al. (1998) that was also used by heunemann et al. (2004). figures 3, 4 and 5 show the data in the different pangaea models after the palaeo-latitudinal and palaeo-longitudinal reconstructions based on the mean siberian trap pp and the hotspot frameworks. in all analyzed reconstructions there is a good fit between the modelled and recorded inclinations meanwhile the fit between the declinations is better for data which vgps are in the north hemisphere. in all the cases the modelled transitional data show a cluster of vgps in the north hemisphere as that represented by the recorded transitional data. both transitional clusters are better grouped in the models of pangaea with palaeo-latitudinal and palaeo-longitudinal (absolute) reconstructions (compare fig. 2 with figs. 3, 4 and 5). pangaea a and a2 models show better fits between the transitional vgps clusters than does the pangaea b model (compare fig. 3 and 4 with fig. 5). 5. conclusions an analysis of the reversal recorded in the siberian trap basalts suggests that similar features to those considered by gubbins (1994) could have driven even reversals in the permotriassic. in spite of the uncertainties associated with the hotspot frameworks and pangaea configurations, the modelled and recorded data show a better fit when both palaeo-latitudinal and palaeolongitudinal reconstructions are considered. pangaea a and a2 show remarkable fits between the recorded and modelled clusters of transitional vgps. pangaea b, shows the best fit between the mean siberian trap pp and a permo-triassic gondwana pp but has fewer geofig. 5. modelled and measured data of the polarity transition recorded in siberian trap basalts in an absolute reconstruction of pangaea type b configuration. the palaeo-latitudinal and palaeo-longitudinal corrections were done on the basis of hotspots frameworks and the mean siberian trap palaeopole. the modelled and recorded data are similar but they do not get as good fits as in a or a2 pangaea models (figs. 3 and 4). 201 analysis of a permo-triassic polarity transition logical arguments in favour for 250 ma (hallam, 1983; irving, 2004) and does not show as good fit between the recorded and our modelled transitional clusters of vgps. acknowledgements we would like to thanks to conicet (consejo nacional de investigaciones científicas y técnicas, argentina) and universidad de buenos aires (argentina) for supporting our investigations. thanks also to dr. robert ixer (univeristy of birmingham) for his suggestions after reading an early version of this paper. comments of the anonymous referee are greatly appreciated. references besse, j. and v. courtillot (2002): apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr, j. geophys. res., 107 (b11), 2300, doi: 10.1029/2000, jb 000050. briden, j.c., a.g. smith and j.t. sallomy (1971): the geomagnetic field in permo-triassic time, geophys. j. r. astron. soc., 23, 101-118. bullard, e.c, j.e. everitt and a.g. smith (1965): the fit of continents around the atlantic, philos. trans. r. soc. london ser. a, 258, 41-51. clement, b.m. 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(received april 3, 2006; accepted june 1, 2007) adg vol5 n02 albe 375_391.pdf annals of geophysics, vol. 45, n. 2, april 2002 375 a stepped frequency gpr system for underground prospecting giovanni alberti, luca ciofaniello, marco della noce, salvatore esposito, giovanni galiero, raffaele persico, marco sacchettino and sergio vetrella co.ri.s.t.a., consortium for research on advanced remote sensing systems, napoli, italy abstract in the framework of archeo, a national research project funded by the italian ministry for universities and scientific and technological research (murst), a new ground penetrating radar (gpr) has been developed by the italian consortium for research on advanced remote sensing systems (co.ri.s.t.a.). the system has been specially designed to satisfy archaeological requirements and it will be used to identify and characterise buried objects. the system is a stepped frequency gpr. it can work within a wide band of frequencies both in gated and ungated mode, and it has been already widely described in alberti et al. (2000). this paper describes the results of calibration tests in ungated mode performed in the co.ri.s.t.a. laboratory. in particular, it is shown that the system can guarantee a dynamic range of about 96 db. 1. introduction it is well known that electromagnetic waves propagate beyond the physical discontinuities that are between different media, and this makes it possible to investigate the internal features of dielectric (possibly lossy) bodies. an important aspect of this property is the possibility to investigate the presence of objects buried in the soil. with regard to this aspect, a number of experiments are reported, focused on hardware of gpr systems (daniels, 1996; stickley et al., 1999; alberti et al., 2000) or possible kinds of processing of gpr data (kooij et al., 1996; bucci et al., 2000; pierri et al., 2000; wright et al., 2000). there are, moreover, many papers focused mostly on the results of experimental campaigns performed (colla et al., 1995; ferrucho da rocha et al., 2000; meglich, 2000). in this paper, we focused on the aspect of the hardware of a gpr system developed by the consortium for research on advanced remote sensing systems (co.ri.s.t.a.). this system was conceived within the framework of the scientific project archeo, financed by italian murst and aimed to develop advanced devices and techniques to be used for detection and recovery of archaeological zones (alberti et al., 2000). before entering the subject of this paper, however, let us outline that, within the project archeo, the data processing has been taken into account. the present system is equipped with a positioning system (alberti et al., 2000) that collects data in a multiview and multistatic configuration (and at several frequencies) in order to use an inverse scattering algorithm exploiting this measurement configuration (persico et al., 2000). mailing address: ing. giovanni alberti, co.ri.s.t.a., consorzio di ricerca su sistemi di telesensori avanzati, piazzale tecchio 80, 80125 napoli, italy; e-mail: alberti@unina.it key words stepped frequency – gpr 376 giovanni alberti et al. within archeo, some instruments other than the gpr have also been designed (see the two papers by fiorani et al., 2000a,b), in order to have a more complete set of tools useful for archaeological purposes. before choosing to develop a stepped frequency gpr system, an investigation on some further non-destructive techniques available was conducted. the possibility to exploit seismic methods, inductive electromagnetic methods, magnetic methods, resistive methods and methods based on gravity strength were first studied (alberti et al., 2000) and the gpr technique was the most suitable for the purposes of archeo. the final choice was a stepped frequency system because it ensures, with respect to the traditional impulsive gpr systems, a larger dynamic range and a greater signal to noise ratio, due to the larger amount of processed energy and to the narrow if band (noon, 1996). moreover, a stepped frequency system is the most suitable instrument to gather data for a multifrequency inverse scattering algorithm (persico et al., 2000; pierri et al., 2000). however, the choice of a stepped frequency system can be considered innovative because, although the advantages of the stepped frequency have been known since the seventies (robinson et al., 1974), most of the gpr systems currently adopted are still impulsive. to give a rough (but not meaningless) quantification of this statement, in the proceedings of the international gpr congress of the years 1998 and 2000, there are two archaeological sections reporting only experiments performed with impulsive systems. even the stepped frequency was deemed unsuitable for archaeological applications by conyers and goodman (1997), but without a precise justification. undoubtedly, the technology of impulsive systems is more assessed and therefore there are more manufacturers of impulsive systems (daniels, 1996). however, we think that it is worth trying to exploit the theoretically better performances offered by the stepped frequency technology, that may only need some more efforts. the gpr system at hand has been conceived to match some requirements specified by interacting with archaeologists (festinese, 2000; signore, 2000). so, it was established that the radiation has to penetrate the soil to a depth of about three meters (in a dry soil), with a vertical resolution of 15 cm and a horizontal resolution of 50 cm. the system is conceived to look for dielectric targets like walls, floors, foundations, wells, ovens, temples, graves, roads and waterworks. the system presented here can work both in ungated and gated mode, in order to exploit the greater dynamic range obtainable in ungated mode but also (in gated mode) to avoid saturation drawbacks due to the leakage or to strong reflections from very shallow objects (noon, 1996; stickley et al., 1999). however, at the moment only laboratory tests in ungated mode are available. the present paper is organised as follows: section 2 describes the architecture of the system and section 3 outlines some tests performed on the system in ungated mode, followed by conclusions and future developments. 2. architecture of the system in this section we show and explain the block diagram of the system. the scheme of the system is shown in fig. 1. to simplify the exposition, we deal with the ungated and the gated modes separately. 2.1. ungated mode in ungated mode, the two frequency synthesisers tx and rx (see fig. 1) generate two tones at distance 1 mhz on the frequency axis. the coherent oscillator (coho) is not enabled and, consequently, the mixers r1 and s1 and the filters r1 and s1 are not enabled either (the signal bypasses them). a small part of the signal generated by the tx transmitter is picked up by a directional coupler and is combined by the mixer s1 with half of the signal generated by the rx synthesiser, picked up by a power splitter. analogously, the signal received by the rx antenna is combined, by the mixer r2, with the other half of the signal generated by the rx synthesiser. the filters s2 and r2 have the task to reject the harmonic components at frequency 377 a stepped frequency gpr system for underground prospecting switch rx timing adc lna coho mixer r1 switch s1 switch r1 filter r1 power splitter switch r2 mixer s2 mixer s1 filter s1 switch s2 lo lo rfrf switch driver rx antenna lo lo rf rf directional coupler tx synthesiser tx antenna digital attenuator tx switch power amplifier power splitter rx synthesiser amplifier mixer r2 filter r2filter s2 computer digital attenuator switch driver switch r1, r2, s1, s2 sy nc hr o ni s m ch ai n fig. 1. block diagram of the gpr system. 378 giovanni alberti et al. sum. so, the signals in output of the two filters (when a steady condition is reached) are two tones at 1 mhz, whose phase difference is related to the electrical path of the signal into the soil (once the electrical path inside the system is excluded by calibration). the bandwidths of the filters s2 and r2 are different (see table i). in particular, the bandwidth of s2 is the larger one, so to make the filter faster in reaching the steady condition at each of the swept frequencies. in fact, this output has to provide, by passing through the timing system, the clock signal to sample the 1 mhz tone at the output of the filter r2. the sampling clock is implemented by a frequency triplication followed by a threshold device, both included in the timing system. the wider bandwidth of the filter s2 with respect to the filter r2 also implicates the theoretical possibility that a stronger noise affects the output of s2. this is not a severe problem, because s2 is on the synchronism chain, and therefore the signal at its input only has propagated inside the system. the signal at the input of the filter r2, instead, can be corrupted by external kinds of noise or interference or clutter gathered by the receiving antenna. this is the reason why the two filters have different bandwidths. apart from these specifications, the overall result provided by the system is a tone at 1 mhz (at the output of the filter r2) sampled with three samples out of a period, and this for each of the radiated frequencies. the discrete data so obtained can be averaged at each frequency and then stored. after this storage, an inverse fft can provide the response to the radiated synthetic pulse (daniels, 1996; noon, 1996). 2.2. gated mode in the gated mode, the reception is inhibited while radiating the signal and the radiating is inhibited while receiving. this is done by a switch signal provided by the computer syntable i. some of the main characteristics of the system. frequency range of the radiated waves 100-900 mhz frequency range of the synthesizers 10 khz-1 ghz frequency resolution of the synthesizers 0.2 hz length of a single frequency step from 10 µs up to 10 s intermediate frequency for both gated and ungated modes 1 mhz coho frequency for the gated mode 100 mhz resolution of the adc 23 bit band of the filter r1 2 mhz band of the filter r2 25 khz band of the filter s1 12 mhz band of the filter s2 140 khz range of the output power level of the synthesizers from – 137 dbm up to 13 dbm power level resolution of the synthesizers 0.1 db maximum switching time of the synthesizers 15 s maximum number of steps of the synthesizers 8192 nominal dynamic range of the adc more than 100 db maximum sampling ratio of the adc 20 mhz power at the antenna feed point up to 10 w size of the positioning system 320 × 320 × 100 cm3 resolution of the positioning system 1 cm 379 a stepped frequency gpr system for underground prospecting chronised with the coho at 100 mhz, in the sense that the switch happens after a certain (integer) number of periods of the coho, and this number is established via computer. in gated mode, the two synthesisers produce two tones at distance 101 mhz on the frequency axis. therefore, after the first mixing and filtering there are two signals centred on the frequency 101 mhz, one on the receiving chain (at the output of the filter r1) and the other on the synchronism chain (at the output of the filter s1). in particular, r1 is an antialiasing filter aimed not only to attenuate the component at frequency sum (which is obvious) but also to reject the possible image band centred about the frequency 99 mhz. therefore, the bandwidth of the filters r1 is 2 mhz (centred about 101 mhz). again, the first filter on the synchronism chain is allowed to have a larger bandwidth (and therefore to be faster in reaching the steady condition), because the signal does not pass through an external path, and so one can reasonably suppose that there is no image band to be rejected. after the first mixing and filtering, the signal on the receiving chain and the signal on the synchronism chain are mixed again with the tone at 100 mhz provided by the coho. so, after the second filtering (which is performed by the same filters exploited in the ungated mode) there are two tones at 1 mhz, likewise the ungated mode. the processing on these two tones is the same as that for the ungated mode. 3. tests performed in ungated mode as a first test, we measured the noise introduced by the clock that we have implemented within the timing system. in fact, this clock is a possible bottleneck for the dynamic range of the overall system (sheer, 1993). incidentally, other components were also tested, although specialised manufacturers guaranteed them. in particular, the adc is by hp and is equipped with a high quality internal clock at 20 mhz. however, the internal clock of the adc cannot be adopted within the overall system because it is needed to synchronise the sampling of the received signal with the transmitted signal, which requires a dedicated device. in particular, the frequency switches are associated with some random phase shift of the transmitted signal, which needs the exploitation of a synchronism signal that «follows» the same shifts. this is why the acquisition is driven by the synchronism chain. in order to measure the noise introduced by the timing system, the output of the filter r2 was disconnected from the adc (see fig. 1). in these conditions there is no signal coming from the receiving chain and the signal delivered to the computer is the noise introduced by the clock. instead, if the clock of the timing system is not enabled, whereas the internal clock of the adc is enabled, the output delivered to the computer is the noise that would introduce the internal clock of the adc if it could be adopted. as said, the clock internal to the adc cannot be adopted, but the measure of its noise is interesting for comparison purpose. the noise introduced by the clock of the timing system is shown in fig. 2, and the noise introduced by the internal clock of the adc is shown in fig. 3. in figs. 2 and 3 the average power of the noise is measured versus the frequency all over the band of the system. for the tests shown in figs. 2 and 3, a full scale of – 6dbm was adopted for the input of the adc. this is a trial value: in general the full scale adopted is a compromise between the need to retrieve signals as weak as possible and to reduce the probability of truncation of too strong signals. as can be seen, the noise introduced by the timing system is of the same order as the noise of the adc internal clock (just a few db stronger). in particular, this noise level guarantees a dynamic range of at least 93 db (corresponding about to 16 bits). this noise level can be further decreased if some averaging is performed on the data. figure 4 shows the noise signal of the timing system, after an average performed on 32 samples (at each frequency) whereas fig. 5 shows the homologous quantity referred to internal clock of the adc. we are speaking of an average performed via software. actually, the adc already performs via hardware some averaging which we do not dwell upon, but that is to be quoted for sake of completeness. in particular, it can reliably be assumed that each of the samples that reaches the computer (to be stored and/or 380 giovanni alberti et al. 100 200 300 400 500 600 700 800 900 1000 -145 -140 -135 -130 -125 -120 -115 -110 -105 -100 -95 gpr clock (3 mhz) frequency [mhz] p o w e r [d b m ] 100 200 300 400 500 600 700 800 900 1000 -135 -130 -125 -120 -115 -110 -105 -100 -95 adc internal clock (20 mhz) frequency [mhz] p o w e r [d b m ] fig. 2. noise generated by the timing system. fig. 3. noise generated by the clock internal to the adc. 381 a stepped frequency gpr system for underground prospecting 100 200 300 400 500 600 700 800 900 1000 -135 -130 -125 -120 -115 -110 -105 gpr clock (3 mhz) frequency [mhz] p o w e r [d b m ] 100 200 300 400 500 600 700 800 900 1000 -145 -140 -135 -130 -125 -120 -115 -110 adc internal clock (20 mhz) frequency [mhz] p o w e r [d b m ] fig. 4. result of an averaging performed on the noise generated by the timing system. fig. 5. result of an averaging performed on the noise generated by the clock internal to the adc. 382 giovanni alberti et al. processed) represents a sample of the steady condition harmonic response of the system at the current frequency. in other words, we can be confident that there is no meaningful transient effect in the samples in input at the computer. it can be seen that the noise introduced by our clock guarantees a dynamic range of about 103 db, and that of the internal clock of the adc would guarantee a dynamic range only slightly larger (let say about 108 db). on the other hand, this test also shows that a sort of spurious modulation appears superimposed on the noise signal generated by the clock of the gpr. this indicates that the behaviour of the averaged noise is going to reach some steady condition. therefore, it is hopeless to think that an average on many more samples would provide further dramatic improvements: only a very few further db might be obtained, at the cost of making the acquisition time twice longer if not even more. let us now show some tests performed on the coupling. the term «coupling» is meant to indicate all the spurious signals that enter the adc from the receiving chain and that are due to the electromagnetic interaction by the different electronic components of the system. to quantify this coupling, for a fixed value of the power of the signal generated by the tx synthesiser (precisely 5 watt, corresponding to 37 dbm), the receiving antenna was removed and the relative coaxial cable connected to a matched load. moreover, the output of the filter r2 was again connected to the adc (see fig. 1). in these conditions, the signal that enters the adc from the receiving chain is substantially due to spurious electromagnetic couplings internal to the system itself. figure 6 shows the coupling signal recorded versus the frequency, with a full scale of – 6 dbm (which is again a trial value). in fig. 6 no average on the output samples is performed: the shape itself of the signal makes us understand that there is a very little «degree of randomness», and therefore any averaging is substantially useless with regard to the coupling. as an explicit verification, fig. 7 shows the coupling signal after an averaging performed on 32 samples. the reason for this uselessness of the averaging is that the coupling signal reaches the adc by passing through the narrow-band filter r2. therefore, the coupling is by its own nature a someway-averaged signal. there is an important difference between the noise introduced by the clock and the coupling: the former is meaningfully dependent on the quantization introduced by the adc, because it substantially results in a sampled quantization noise. instead, the coupling is related to the power level of the signal produced by the tx synthesiser (that, let us remember, has been fixed at 5 watt). consequently, the noise of the clock is expected to depend on the full scale adopted, whereas the coupling does not. in other words, with regard to the clock noise, what can be considered as «absolute» is the dynamic range with respect to the full scale adopted whereas, with regard to the coupling, what is to be regarded as «absolute» is the noise level in dbm. based on these considerations, a suitable choice for the full scale of the adc might be one that «shifts» the clock noise on the same level of the coupling. in these conditions, there will be the maximum dynamic range. in fact, a lower full scale would decrease the dynamic range available, because the coupling would remain unchanged. on the other hand, a larger full scale would not enlarge the dynamic range because the clock noise would follow the increasing of the full scale. actually, it might be useful to increase the full scale only if the truncation probability were greater than the probability to receive a signal weaker than the adc resolution. in the situation at hand, the full scale that makes equal to each other the power level of the clock noise and that of the coupling is 24 dbm, corresponding to 0.25 watt. received signals stronger than this level are extremely improbable in our applications, because the power radiated is less than 5 watt whereas the attenuation encountered can be expected of the order of several tens of db even if a reflection at less than one meter occurs (noon, 1996). to be more precise, the overall attenuation and amplification between the tx synthesiser and the transmitting antenna compensate each other, so the incident power at the input terminal is nominally 5 watt. therefore, the radiated power can only be lower than this value, due to some unavoidable mismatch and losses in the antenna. with the choice of a full scale of 24 dbm, fig. 8 shows the comprehensive 383 a stepped frequency gpr system for underground prospecting 100 200 300 400 500 600 700 800 900 1000 -100 -95 -90 -85 -80 -75 hardware coupling frequency [mhz] p o w e r [d b m ] 100 200 300 400 500 600 700 800 900 1000 -95 -90 -85 -80 -75 hardware coupling frequency [mhz] p o w e r [d b m ] fig. 6. electromagnetic coupling internal to the gpr system. fig. 7. result of an averaging performed on the electromagnetic coupling. 384 giovanni alberti et al. noise (coupling plus clock) recorded at the output of the adc. in fig. 8, no average is performed on the data: the weakest signal that can be recovered is at level of – 66 dbm, and therefore the dynamic range can be quantified at 90 db. however, if an average on 32 samples is performed, then the dynamic range grows to 94 db, as can be understood by glancing at fig. 9. for most of the radiated frequencies, the dynamic range can be considered equal to 96 db or even more, therefore it is not unfair to assess that that dynamic range is indeed about 96 db. again, an average performed on many more samples is not expected to provide dramatic improvements of the dynamic range because, as said, the randomness of the clock noise has already been reduced and, above all, the averaging does not influence the coupling meaningfully. let us now show some tests on the phase and amplitude error at each frequency. these tests are relative both to the noise and to the possible distortion phenomena arising inside the system, that could deform the shape and position of the expected returned synthetic pulse. in particular, in the performed tests the nominal harmonic signals generated have all been of the same amplitude, and therefore the theoretical received synthetic pulse is expected to be sinc-like. in order to test it, the two antennas were removed (see fig. 1) and the input of the transmitting antenna connected to the output of the receiving antenna with a coaxial cable that only introduces a known delay. in this manner the delay introduced by the overall gpr system was also measured, for calibration purposes. by this insertion, the theoretical signal to be received is known, based on the knowledge of the generated signal and on the knowledge of the components of the system. therefore we can compare the received signal with the theoretical one. figure 10 shows the relative amplitude error versus the frequency, whereas fig. 11 shows the phase error. since the phase error is not a relative error but an absolute error, its effective entity cannot be evaluated by simply glancing at its behaviour. rather, some criterion is to be introduced to evaluate the weight of this phase error. to do this, the synthetic pulse received and the theoretical one can be compared: the discrepancy between them provides a quantification of the effects of the phase (and amplitude) error. figure 12 reports these two pulses. in particular, the test pulse in fig. 12 is synthesised by switching the frequency of the tx synthesiser from 100 mhz up to 902 mhz with a step of 2 mhz. moreover, 64 samples were collected at each frequency, and the average of the last 20 samples were stored as the response of the system to the current frequency. to retrieve the synthetic pulse from the amplitude and the phase at each frequency, an ifft on 8192 points was performed (daniels, 1996; noon, 1996). the ideal pulse is depicted as a solid line, whereas the actual recorded one is depicted in a dotted line: they are almost indistinguishable. to provide a worst case result, we show the ideal and the recorded synthetic pulse that we have (for the same experimental data) if only the frequencies from 802 up to 902 mhz are processed in retrieving the synthetic pulse. in fact, while the percentage amplitude error is quite low throughout the band (fig. 10), the higher frequencies of the swept band are those with a higher phase error. the result is shown in fig. 13 (again, the ideal pulse is in solid line whereas the actual pulse is in dotted line). the difference between them is slightly more evident on the secondary lobes, but substantially the main lobe remains unperturbed. finally, we show a test on the whole system working in ungated mode, i.e. a system with the overall system (including the antennas) mounted and working. we have looked for the reflection from the bottom of the floor of the laboratory in which the system (including the positioning system) is mounted. in fact, since there is another room under the laboratory, if the system is made to work in such a situation (with the two antennas positioned on the floor) the main reflection is expected from the ceiling of the room below. to synthesise the pulse, a band of 512 mhz was exploited, swept with a step of 2 mhz starting from the frequency of 100 mhz. the laboratory is a small (a surface of 5.7 × 3.6 m2 and a tallness of 3.3 m) non anechoic environment, with the possibility of several multiple reflections from the walls, floor, ceiling, and the many metallic and dielectric objects present 385 a stepped frequency gpr system for underground prospecting 100 200 300 400 500 600 700 800 900 1000 -100 -95 -90 -85 -80 -75 -70 -65 gpr clock (3 mhz) plus hardware coupling frequency [mhz] p o w e r [d b m ] 100 200 300 400 500 600 700 800 900 1000 -90 -88 -86 -84 -82 -80 -78 -76 -74 -72 -70 gpr clock (3 mhz) plus hardware coupling frequency [mhz] p o w e r [d b m ] fig. 8. electromagnetic coupling plus timing system’s noise. fig. 9. result of an averaging performed on the sum of the electromagnetic coupling and the noise generated by the timing system. 386 giovanni alberti et al. 100 200 300 400 500 600 700 800 900 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 relative amplitude error frequency [mhz] a m p li t u d e 100 200 300 400 500 600 700 800 900 -6 -4 -2 0 2 4 6 8 10 12 phase error frequency [mhz] p h a s e ( ° ) fig. 10. relative error (db) between the nominal and the registered amplitude of a test signal versus frequency. fig. 11. absolute error (degrees) between the nominal and the registered phase of a test signal versus frequency. 387 a stepped frequency gpr system for underground prospecting synthetic pulses for the ideal and real received signals time [ns] a m p li t u d e synthetic pulses for the ideal and real received signals time [ns] a m p li t u d e fig. 12. comparison of theoretical and received synthetic pulses making use of the stepped frequency signal on the overall band. fig. 13. comparison of theoretical and received synthetic pulses making use of the stepped frequency signal on the «worst band» between 802 and 902 mhz. 388 giovanni alberti et al. in the laboratory. therefore, the conditions for this kind of measurement are not favourable at all. on the other hand, there is the a priori information that a reflection is expected from a plane surface parallel to the floor plane. the configuration of the measurement was in common source, and the receiving antenna was placed in eight different positions. the first four positions were arranged as follows: the receiving antenna was placed on one side of the transmitting antenna and the distance between the gaps was 40, 50, 60 and 70 cm respectively. the other four positions were chosen so that the receiving antenna was placed on the opposite site with respect to the transmitting antenna at the same distances. due to the particular case at hand, the a priori supposition that the main reflection always occurs at the same depth was adopted. therefore, even if the measurement configuration is a common source, the data were processed as if they were in common depth point, according to the scheme illustrated in the appendix. figure 14 reports one of the 8 synthetic time responses (the others show all a quite similar behaviour). there is a strong reflection at the time of 18 ns, but there is also a meaningful reflection at about 420 ns, probably due to the floor of the underlying room. the depths worked out for all the distances (see appendix) were averaged, and so the final result of 48 cm was worked out. the time values at which the various maxima occurred were exploited in working out the thickness of the floor. actually, the external wall of the building in which co.ri.s.t.a. is located has zones where the plaster has fallen down, and therefore we were able to measure directly the thickness of the floor: it is 38.5 cm. due to the bandwidth of the signal exploited, the uncertainty is expected of the order of 20 cm (see appendix), and therefore the obtained result can be considered satisfactory. fig. 14. one of the received synthetic time responses gathered in the laboratory of co.ri.s.t.a. the function is normalised and expressed as a function of time in ns. 389 a stepped frequency gpr system for underground prospecting appendix this appendix outlines the processing performed to work out the thickness of the floor of the laboratory. as said, the processing was performed as if the data were gathered in a common depth point: therefore we refer to fig. 1a in order to explain the processing. the procedure is simple and conceptually similar to that exposed by daniels (1996) for the case of a multimonostatic (common offset) configuration. with respect to fig. 1a, we have (a.1) (a.2) for the pythagorus theorem. moreover (a.3) (a.4) where c is the (unknown) propagation velocity of the electromagnetic radiation in the medium under consideration, assumed homogeneous. 4. conclusions and future developments a stepped frequency gpr system devised by the research consortium co.ri.s.t.a. within the financed project archeo has been described. the system can work both in ungated and gated mode and is equipped with a positioning system to allow measurements in a multistatic multiview configuration. laboratory tests performed in the co.ri.s.t.a. laboratory have been described. they show that the system is able to guarantee a dynamic range of the order of 96 db in ungated mode. following developments of the work, within the project archeo, experimental tests will be performed in a pool filled with sand or clay soil in which known buried targets will be buried (alberti et al., 2000). during the outdoor tests, innovative inverse scattering algorithms of the gpr data will also be tested (persico et al., 2000). afterwards, the system is to be tested in an archaeological site in southern italy. acknowledgements this study has been funded by the italian ministry for universities and scientific and technological research under research contract 179201-1325/458. p d l1 2 1 2= + p d l2 2 2 2= + t p c 1 12 = t p c 2 22 = 390 giovanni alberti et al. therefore, by substitution of (a.1) in (a.3) and (a.2) in (a.4), we obtain (a.5) the inversion of (a.5) with respect to d provides (a.6) once d is retrieved, these passages allow retrieving the propagation speed c, by substitution of the known value of p1 in (a.3) or p2 in (a.4). in the case at hand, by averaging the 8 velocities obtained along the 8 electrical path considered, we have worked out c = 10 cm/ns. once the propagation speed has been retrieved, one can estimate the order of magnitude of the spatial uncertainty as the product of this velocity times the temporal resolution. the temporal resolution can be estimated as 1/b, where b is the bandwidth of the synthetic pulse (daniels, 1996; noon, 1996). in the case at hand, as said, the band is 512 mhz and so the temporal resolution is about 2 ns. therefore the order of magnitude of uncertainty on the evaluation of the depth is 2 ns times 10 cm/ns, i.e. 20 cm. to be more precise, this calculation provides the uncertainty 2 p on the length 2p of some average path of the signal within the floor (see eqs. (a.1) and (a.2) and fig. 1a), which is not d. by differentiating eq. (a.1) or (a.2) one might say that the uncertainty on the depth would be d = (p/2d) p, expected lower than 2 p but of the same order of magnitude. on the other hand, this calculation neglects the uncertainty on the propagation velocity itself and the fact that, actually, the floor is not really homogeneous. d l t l t t t 1 2 2 2 2 2 1 2 1 2 2 2 = t t d l d l 1 2 2 1 2 2 2 2 = + + fig. 1a. geometrical scheme to retrieve the depth of a reflection from several measurements in common depth point. p1 p2 d l1 l 2 391 a stepped frequency gpr system for underground prospecting references alberti, g., l. ciofaniello, m. della noce, s. esposito, g. galiero, r. persico and s. vetrella (2000): advanced stepped frequency gpr development, in proceedings of the conference on subsurface sensing technologies and applications ii, at spie’s annual meeting in july/august 2000, san diego, u.s.a., 484-492. bucci, o.m., l. crocco, t. isernia and v. pascazio (2000): inverse scattering problems with multifrequency data: reconstruction capabilities and solution strategies, ieee trans. geosci. remote sensing, 38 (4), 1749-1756. colla, c., p. das, d. mccann and m. forde (1995): investigation of stone masonry bridges using sonics, electromagnetics and impulse radar, in proceedings of international symposium on non-destructive testing civil engineering (ndt-ce), september 1995, 629-636. conyers, l.b. and d. goodman (1997): ground penetrating radar – an introduction for archaeologists (alta mira press, a division of sage publications). daniels, d.j. 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(2000): progetto archeo: cales, un’applicazione, in «rischio archeologico: se lo conosci lo eviti, edited by m.p. guermandi. proceedings of convegno su cartografia archeologica e tutela del territorio, ferrara, march 24-25 2000», ibc, documenti 31, firenze 2001, 384-390. stickley, g.f., d.a. noon, m. cherniakov and i.d. longstaff (1999): gated stepped-frequency ground penetrating radar, j. appl. geophys., 43 (2000), 259-269. wright, d.l., d.v. smith, j.d. abraham, r.s. hutton, e.k. bond, t.j. cui, a.a. aydiner and w.c. chew (2000): imaging and modelling new vetem data, in proceedings of the 8th international conference on ground-penetrating radar, queensland, australia, 146-150. t e m p i d i t r a g i t t o d e l l e o n d e « ivi » p e r l ' i t a l i a c e n t r a l e m . giorgi p . e . v a l l e i n una n o s t r a p r e c e d e n t e r i c e r c a a b b i a m o o s s e r v a t o c l i c l t e m p i di e m e r s i o n e d e l l e o n d e « m » del t e r r e m o t o del m o n t e a m i a t a ( ' ) p o s sono e s s e r e c a l c o l a t i p e r l ' i t a l i a c e n t r a l e m e d i a n t e la r e l a z i o n e ( " ) : ' 5 , m s e c /, = + 0 . 4 5 + jt, [ 1 ] a i n k m 2 , 6 0 0 , 0 8 11 d e n o m i n a t o r e c l i c c o m p a r e n e l l a [ 1 ] r a p p r e s e n t a la v e l o c i t à a p p a r e n t e d e l l ' i n i z i o d e l l a l a s c , la q u a l e , nel n o s t r o c a s o , r i s u l t a c o s t a n t e . a l m e n o p e r le d i s t a n z e c o m p r e s e e n t r o i 4 0 0 k m . d a t o c h e l e o n d e « m » p r e s e n t a n o c o s p i c u i e f f e t t i di d i s p e r s i o n e ed a s s o r b i m e n t o c o m b i n a t i , il t e m p o di e m e r s i o n e , a l l e v a r i e d i t a n z e e p i c e n t r a l i . v a r i a ili r e l a z i o n e a l l o s p e t t r o d e l l a p e r t u r b a z i o n e i n i z i a l e e q u i n d i , a r i g o r e , l a ! 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(•!•, \ ai. le i'. e . . sul coefficienze di assorbimento delle onde sismiche su per hfiali. r i e . s c i e n l . e r i c o s t r u z . 11 ( 1 9 4 6 ) . layout 6 1 annals of geophysics, 64, 1, pa109, 2021; doi:10.4401/ag-8528 a relationship between temperature, oxygen dissolved in blood and viral infections dario camuffo national research council of italy institute of atmospheric sciences and climate, corso stati uniti 4, 35127 padua, italy article history: received july 21, 2020; accepted january 5, 2021 abstract an investigation is made on the environmental factors that may determine the seasonal cycle of respiratory affections. the driving role of temperature is examined, for its inverse synergism with the dissolution of oxygen in human plasma. two best-fit equations are discussed to interpolate the experimental data about the oxygen solubility and the saturation levels reached at various temperatures, referring to the value of the basic alveolar temperature. a vulnerable condition is when the airways temperature is lowered, e.g. breathing cold air, or increasing the breathing frequency. in winter, the upper airways reach lower temperatures and greater oxygen concentrations; the opposite occurs in summer. as low temperatures increase the dissolution of oxygen in plasma, and blood oxidation favours viral activity, an explanation is given to the seasonality of infections affecting the respiratory system. keywords: temperature; oxygen solubility; body temperature; respiratory disease; viral infection; covid-19. 1. introduction since the period of the earliest meteorological observations, attention was paid to investigate the relationships between climate, agricultural production and human diseases. among the most famous scientists who gave the most significant contributions to this field, specific mention should be made to bernardino ramazzini for his yearly reports about agricultural and human epidemics related to the seasonal climate variability [ramazzini, 1718] and his book on occupational illnesses and injuries [ramazzini, 1700] in which he considered the impact of the environmental conditions on health. james jurin coordinated an international network of the royal society, london, concerning indoor observations useful for climate and health studies [jurin, 1723], especially useful considering the conditions of the sick buildings in the 18th century. this network was followed by a similar one organized by felix vicq d’azyr and father louis cotte. they set up a series of national and international records on behalf of the newly founded societé royale de médecine (royal society of medicine), paris, [cotte 1774; 1788]. giuseppe toaldo took and analyzed meteorological records studying the relevance of seasonal and astronomical cycles. he published a famous meteorological essay to relate the influence of the moon and other celestial bodies, the seasonal cycles, the dry or rainy weather, the extreme weather events and the frequency in human mortality [toaldo, 1770]. this book was translated in french, english and german and was reprinted several times. today it is known that certain viral forms, but not all, have a seasonal character. the relationship with temperature is clear for sars-cov-2 [chin et al., 2020]. as far as covid-19 is concerned, briz-redón and serrano-aroca [2020] found no evidence. on the other hand, roy [2020] found that global temperature played an important role and a moderately cool environment was the most favourable state for virus transmission. the risk from the virus was reduced significantly for warm places and countries. the seasonal risk has been confirmed by the european centre for disease prevention and control [ecd, 2020]: the number of covid-19 deaths in europe, as of 29 december 2020, shows a marked peak in april, a flat minimum over summer and then a sharp rise starting from october. however, even in cases where seasonality is evident, the complete mechanism has not been clarified, especially for diseases transmissible person-to-person [shek and lee, 2003; lofgren et al., 2007; chan et al., 2011; fisman, 2012; alvarez-ramirez and meraz, 2020; chen et al., 2020; gutierrez-hernandez and garcia, 2020; moriyama et al., 2020]. explanations have been based on a series of environmental, physical, chemical, biological, medical, social and other factors, that may be grouped into three categories: • environment and climate-related factors, e.g. temperature, humidity, solar radiation and in particular uv radiation acting as a disinfectant for contaminated nonporous materials or because it favours the synthesis of vitamin d in the body and the body defence [sloan et al., 2011; azziz baumgartner et al., 2012; beck et al., 2016; isaia and medico, 2020; isaia et al., 2020; sagripanti and lytle, 2020; schuit et al., 2020; ratnesar-shumate et al., 2020]; air pollution [lebowitz, 1996; cui et al., 2003] and transport of viruses (e.g. arrival of migratory birds) [fao, 2007]. • social activities and human behaviour that may increase transmission (e.g. longer residence time inside poorly ventilated rooms, crowding, contagion opportunities, droplet transmission) [willem et al., 2011; zayas et al., 2012; han et al., 2013; van doremalen et al., 2020; liu et al.; 2020]. besides, low relative humidity levels, either caused by heating or air conditioning, enhance indoor dustiness and the suspension time of airborne particulate matter [chan et al., 2011; camuffo, 2019] thus increasing the probability of inhaling viruses. • human body defence (e.g. response of the mucosal surface of the respiratory tract), physiological risk factors (e.g. age, obesity) and other medical items [fokkens and scheeren, 2000; iwasaki et al., 2017; li et al., 2011; moriyama et al., 2020]. the above factors, alone or in synergism, are supported by reasonable explanations, correlations and statistical calculations. some researchers [fisman, 2012; gutierrez-hernandez and garcia, 2020] agree that cool and dry environments are most frequently related to the virus diffusion, but their feeling is that there are only indications, rather than scientific evidence, i.e. atmospheric conditions may explain a limited part of the dynamics of viral affections. this paper is not intended to analyse the virus life, viral concentration in air, diffusion in indoor and outdoor environments, transmission from person to person etc. that are widely studied in scientific literature. this paper investigates other possible linkages to relate viruses to environmental conditions, in particular the potential effect of some small changes in temperature of certain organs inside the body. the inhaled air, with its physical characteristics, constitutes the starting point, regardless of where it is breathed, either indoors or outdoors. a three-step mechanism is considered, i.e. the relationship between specific physical characteristics of the inhaled air (e.g. temperature, relative humidity, ventilation rate) and the oxygen dissolved in blood; how the temperature of the various parts of the body is subject to change; why the consequences of this fact may be relevant for viral infections. the work is organized in three sections. the first section calculates the amount of oxygen dissolved in blood as a function of the ambient temperature. the second deals with the temperature of the respiratory system, that is neither constant nor homogeneous, because the temperature of the upper airways is driven by the temperature of the inhaled air. the third considers that different temperatures will cause different saturation levels of the oxygen in the blood and that some viral affections may take advantage from higher oxygen concentrations. 2. air temperature and the oxygen dissolved in blood the oxygenation of blood is a mechanism that may explain a significant synergism between environmental and physiological factors. oxygenation is highly relevant because oxygen acts as terminal electron acceptor at the end of the electron transport chain whereby oxidative phosphorylation results in the synthesis of adenosine triphosphate (atp). this coenzyme supplies energy to all active metabolic processes [dunn et al., 2016], including viral activity. in addition, it has been demonstrated that antioxidants display relevant antiviral activity [silva et al., 2011; wangkheirakpam, 2018]. therefore, an elevated oxygen concentration in blood constitutes a vulnerable situation because it favours viral activity. gaseous oxygen is first dissolved in plasma and then most of it is chemically combined with the haemoglobin. dario camuffo 2 oxygen dissolution constitutes the input of the complex system that distributes oxygen to all parts of the body. oxygen delivery in the human respiratory system depends on several factors including the partial pressure of oxygen, the efficiency of gas exchange, the concentration and affinity of haemoglobin to oxygen and cardiac output [law and bukwirwa, 1999]. the highest oxygen concentration is typically found in the respiratory tract, from where it is distributed throughout the body. the oxygen content of arterial blood is the sum of the oxygen bound to haemoglobin and oxygen dissolved in plasma [dunn et al., 2016]. the solubility of oxygen in an aqueous solution is regulated by the henry’s law [pauling, 1947] that states that, at a constant temperature, the concentration of oxygen in the aqueous phase is proportional to the partial pressure of oxygen in the gaseous phase in equilibrium with the liquid. however, the coefficient of proportionality is an inverse function of temperature, i.e. decreasing when temperature increases and may be represented with a single or a combination of exponential functions. this has been established for pure water, ponds, aquacultures, marine environments [millero et al., 2002; geng and duan, 2010; karbowiak et al., 2010; valderrama et al., 2016; eze and ajmal, 2020], but also for physiological aqueous solutions of nacl and human plasma [christoforides et al., 1969; christmas and bassingthwaighte, 2017]. christoforides et al. [1969] measured the concentration of oxygen xo2 [ml(gas)/l(fluid)] dissolved in human plasma at different temperatures t (°c) in the interval from 10 to 60° c, at standard atmospheric pressure. the observed data may be interpolated with two best-fit equations (figure 1). the logarithmic best-fit is 𝑋�₂ = ‒9.401 ln(𝑇)+55.684 (1a) with excellent determination coefficient r2 = 0.997 and, over the whole range, the observed values depart less than 1 [ml(gas)/l(fluid)] from the logarithmic best-fit. the exponential best-fit is 𝑋�₂ = 36.616 exp(‒0.014 𝑇) (1b) with slightly smaller r2, i.e. r2 = 0.971. the exponential approximation is equally good within the 20-40°c interval 3 temperature, oxygen and viral infections figure 1. solubility of oxygen in human plasma xo2 [ml(gas)/l(fluid)] at standard atmospheric pressure. circles: experimental data by christoforides et al. [1969]; continuous line: logarithmic interpolation; dashed line: exponential interpolation. dario camuffo 4 but, externally to it, the observed values depart from the best-fit, reaching 2 [ml(gas)/l(fluid)] at the lower extreme (i.e. 10° c), and a similarly at the upper one (i.e. 60° c). however, in the 20-40° c interval, that is the most relevant for the human body, both approximations are satisfactory. 3. uneven distribution of temperature in the respiratory system the temperature of the body of a healthy person has an uneven distribution. under normal conditions, the internal organs benefit of thermoregulation and keep a fairly constant temperature, i.e. 37°c that is the basic situation for the blood oxygenation in the lung alveoli [d’amato et al., 2018]. as opposed, the temperature of the peripheral areas of the body, the skin and the upper airways may be variable. this temperature is determined by complex exchanges of heat and moisture (i.e. sensible heat and latent heat) between the body and the environment. the environmental factors that should be considered for this complex heat balance are: air temperature, infrared radiation and, secondarily, relative humidity and ventilation [fanger, 1982; iso-7730, 2005]. the airways represent an internal interface between the body and the environment. the epidermis is the external interface and constitutes a very small secondary respiratory system, i.e. 5% of the total. over the year, the various parts of the body undergo a temperature cycle whose relevance is determined by their position and function. the exhaled breath temperature can be easily measured [popov et al., 2012]. however, with the use of catheters with thermistors, fiberoptic bronchoscopes and other devices, also the temperature inside the respiratory ways has been monitored. it has been found to be determined by ambient temperature and ventilation, i.e. exchanged air volume [afonso et al., 1962; mcfadden et al., 1985]. in winter, when cold air is inhaled, the upper airways from nose to trachea are severely affected by cooling and the cooling decreases progressing in the lower airway inside lungs. the expiration too is affected: the greater the cooling during inspiration, the lower the temperature during expiration. for instance, experiments made with very cold air (i.e. -18.6° c) at various ventilations (ve) per minute shown that during inspiration the glottis temperature falls from 28° c at ve = 15 min-1 to 20.5° c at ve =100 min-1 and during expiration from 29.5° c at ve 15 min-1 to 22.5° c at ve=100 min-1 [mcfadden et al., 1985]. each cold inhalation is followed by a milder, humid exhalation, determining a sinusoidal trend in which the solubility of oxygen is favoured during the colder inhalation phase. in winter, under normal conditions, each breath brings in cold, dry air, that exchanges sensible and latent heat at a frequency of 12 to 20 inhalations per minute [flenady et al., 2017]. when cold air is inhaled, the airway tissues are cooled and dried, with negative effects on the lungs for people with respiratory diseases and in particular asthma [d’amato et al., 2018]. 4. consequences of the different saturation levels reached by oxygen in blood the studies concerning the gas solubility in plasma and the temperature changes of the respiratory airways may be combined to explain some respiratory morbidity related to the seasonal climate cycle. this means to calculate how the dissolved concentration of oxygen in blood changes in relations to the reference value xo2 = 21.4 [ml(gas)/l(fluid)] found by christoforides et al. [1969] at normal lung temperature, i.e. 37° c, and at standard atmospheric pressure. in this paper, the amount of oxygen dissolved in the human plasma has been calculated for temperatures from 20 to 45° c and the result has been expressed in percent (%) of the typical saturation value at 37° c, assumed to be 100%. (figure 2). the result is a comparison between the various oxygen saturation levels (slo2); the percent representation is similar to the output of a saturimeter, also called oximeter, i.e. the medical instrument to monitor the blood oxygenation level. the selected range is representative of the upper airways [mcfadden et al., 1985]. this variable has been calculated with the equation (2) in the 20 to 45°c range, the logarithmic and the exponential interpolations 𝑆𝐿�₂ (𝑇) = 100 +100 𝑋�₂ (𝑇)‐𝑋�₂(37° 𝐶) 𝑋�₂(37° 𝐶) 𝑆𝐿�₂ = ‒48.28 ln(𝑇) + 275 (3a) 𝑆𝐿�₂ = 171.8 exp(‒0.014 𝑇) (3b) are characterized by similar determination coefficients, i.e. r2 = 0.998 for the logarithmic interpolation and r2 = 0.994 the exponential one. both equations can be used to evaluate how much the oxygen solubility and the reached saturation level are increased when the blood cools, or depressed when the blood warms. to illustrate the mechanism, some hypothetical examples of oxygen dissolved in blood at selected temperatures are reported in table 1. the first row includes the headings with selected temperatures. the next two rows report the percentage difference compared to the normal alveoli temperature assumed to be 100%. table 1. comparison between the levels of oxygen dissolved at saturation in human plasma at selected temperatures, making reference to the basic value at normal alveoli temperature, i.e. 37° c. the table shows the example that, if the temperature drops to 30° c or 20° c, the oxygen saturation level will increase by +11% or +29% respectively, compared to the reference value at 37° c. conversely, it decreases if the temperature rises. in particular, in the interval around the normal alveolar temperature, the change rate is 1.4 %/° c. in winter, the respiratory system lowers its temperature, oxygenation increases efficiency and tends to hyperoxemia, like the effect of higher ventilation. airborne viruses find more oxygenated cells that constitute the very first hosting site and determine the prerequisite for their development and harmfulness. generally, viruses temperature 20° c 25° c 30° c 35° c 40° c 45° c saturation value 129 % 120 % 111 % 103 % 97 % 92 % difference from 37° c +29 % +20 % +11 % +3 % -3 % -8 % 5 temperature, oxygen and viral infections figure 2. saturation level of the oxygen dissolved in human plasma at various temperatures, referring to the value at 37° c, assumed to be 100%. circles: saturation of experimental data; continuous line: logarithmic interpolation; dashed line: exponential interpolation. horizontal lines pinpoint the 90, 100 and 110% levels. that naturally infect well-oxygenated organs (i.e. in the cold season) are less able to infect cells under hypoxic conditions (i.e. in the warm season) and vice-versa [gan and ooi, 2020]. some viral affections, typical of the cold season, take advantage from the higher oxygen concentration in blood. this suggests that the greatest vulnerability is in winter, when one enters a crowded building, because the temperature of the upper airways is lower, the oxygenation level higher, and the virus transmission easier. as opposed, any temperature increase, such as the febrile response, would decrease the oxygen dissolution and increase the resistance to viruses [ogoina, 2011]. fever generates a temporary increase in body temperature to fight infections, reducing the availability of oxygen and reproduces the mechanism determined by the warm season. in this respect, besides to filter air, protective face masks provide, although to the minimum extent, another unexpected advantage. they reduce the free exchange of air and the co2 dissipation, which implies fewer airways cooling and lower oxygenation rate and both these factors contribute to reduce viral infections. the inhalation of hot summer air raises the airway temperature and oxygenation will decrease tending to hypoxemia, with an effect similar to lower ventilation. it is known that summer heat waves may lead to hypoxia, as it has been observed in human communities, animals and marine environments [frölicher and laufkötter, 2018; stillman, 2019; mcarley et al., 2020]. it is also known that cardiovascular affections dramatically increase in hot days [petralli et al., 2012; grasso et al., 2017] and in urban heat islands [paravantis et al., 2017] for the effort of combining thermoregulation with intense heart activity needed to compensate for the lower oxygen concentration. last but not least, if high summer temperatures represent an adverse situation for viruses living in cold environments, rooms overcooled with air conditioning systems may break the physiological protective cycle that the warm season offers to the upper airways, altering the degree of oxygenation with not easily predictable implications for respiratory morbidity. the saturation level of peripheral oxygen, as well as the respiratory frequency, are considered the first and second most sensitive vital signs, used as predictors of in-hospital mortality [barfod et al., 2012]. 4. conclusions this paper has considered that the ambient air temperature, its influence on the upper airways, the dissolution of oxygen in blood and the viral activity generate a sequential mechanism and each step of it is in accordance with findings reported in the literature. this mechanism may offer the key to understanding some relationships between environmental and physiological factors, including the seasonality of viral infections. of course, this study does not include the contribution of human factors and transmission opportunities. the applications to human health are of potential interest but need a thorough multidisciplinary analysis and verification before being applied. the upper airways are exposed to direct contact with inhaled air, airborne aerosols and viruses and follow a dynamic equilibrium with the external temperature and, secondarily, relative humidity. the seasonal cycle of environmental variables (i.e. the temperature and, secondarily, relative humidity) affects the temperature and the moisture balance of the upper tract of the respiratory system. it has been found that the best-fit of the oxygen solubility may be represented either by a logarithmic or an exponential function, but the logarithmic one is supported by a slightly higher determination coefficient. this investigation has shown that the seasonal cycle of temperature determines a cycle of the oxygen concentration in blood and the saturation level changes at a rate of 1.4 %/° c. the temperature-oxygenation relationship and related hypothermia and hyperthermia, are physical mechanisms and the human body can take advantage of them (e.g. the fever as a form of defence of the organism against viral infections) or disadvantage (e.g. heat waves and hypoxia; 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hooper and shoemaker, 1986). the understanding of hydrological flow and river flow generation has gained an increasing importance in a wide range of studies for water resource, contaminant transport and geochemical and even biochemical issues. in the last 40 years, many publications have tried to elucidate the mechanisms by which water is collected in catchments and how it moves down hillslopes into small rivers (hoehn et al., 1989). among the many methodologies, those involving naturally occurring tracers have become important. tracer studies generally lead to the separation of river flow into two or more components and eventually, through the radioactive tracers like anthropogenic tritium, natural radon and radium isotopes, give an estimation of the groundwater residence times. numerous studies demonmailing address: dr. antoine kies, laboratoire physique des radiations, centre universitaire de luxembourg, 162a avenue de la faïencerie, l-1511 luxembour; e-mail: kies@cu.lu using 222rn for hydrograph separation in a micro basin (luxembourg) antoine kies (1), harald hofmann (1), zornitza tosheva (1), lucien hoffmann (2) and laurent pfister (2) (1) laboratoire physique des radiations, centre universitaire de luxembourg, luxembourg (2) centre de recherche public – gabriel lippmann (crebs), luxembourg abstract in order to obtain information on the hydrological signature of rivers during and after heavy rain events, small catchment areas are selected as experimental sites. hydrograph separations based on environmental tracers are performed. natural isotopic tracers such as 18o, 2h and particularly 222rn may help to distinguish the components dominating the outflow, particularly of ‘pre-event waters’, ‘event waters’ and ‘post-event waters’. even with moderate concentrations in groundwater, radon can be a very sensitive indicator of groundwater input into rivers. the selected microbasin under investigation is situated in the western part of luxembourg and belongs to the attert river catchment. at chosen points at the basin’s outflow radon detectors continuously measure radon activity in water. the radon monitors are installed together with high precision thermometers, conductivity meters, flow meters and automatic water samplers for chemical analysis. besides the continuous measurements, grab water samples are taken at different locations along the stream, most of them during periods of heavy rain events. presented are the results of a one year measurement campaign. during the dry season i.e. during more or less continuous discharge conditions, the observed mean values do not show substantial variations and can be used as reference values. fluctuations of the measured data during rain events are discussed and the interplay between the different parameters analysed. 102 antoine kies, harald hofmann, zornitza tosheva, lucien hoffmann and laurent pfister strate the utility of the stable hydrogen isotope deuterium 2h and the stable oxygen isotope 18o. studies involve other tracers like calcium (genereux et al., 1993), silicium and various other major ions, depending on the ionic strength of the waters and their possibility to acquire substantial amounts of those elements from the soil and rocks. many of common tracer studies focus on the effect of storm flow, the most common type of analyses being a separation of hydrographs into ‘old’ water present in the watershed before the start of the rain event and the ‘new’ water mixing in during the event of interest. for the study of river flow generation it is unrealistic to expect that whatever tracer methods are applied, all the answers to the important questions concerning hydrological flow paths on watersheds and information on the water movement in a catchment during a rainfall-runoff event are given. it is in applying multiple field techniques that an optimal approach can be achieved. in the cycleau project the aim is to use a multifunctional approach. in the present paper only the involvement of the tracer element radon together with temperature and electrical conductivity are presented and discussed. differences in radon content of subsurface waters arise from differences in radon emanation by porous media (bedrock or soil) and from the differences in the degree of subsurface degassing. the factors influencing the behavior of 222rn in the subsurface have been the subject of numerous investigations (e.g., clements and wilkening, 1974; schery et al., 1984 for the elder references) and are not part of the present paper. here we expect only markedly different radon concentrations in the natural waters, especially corresponding to zones of unsaturated and saturated soils. simple three-end-member mixing models may provide a useful framework for riverflow generation, the three different waters being superficial water (vadose zone water), soil ground water and bedrock water (genereux et al., 1993). merot et al. (1995) suppose a four-component hydrograph separation in studying the complex variations of groundwater, riparian zone seepage, hillslope subsurface flow and event water. 222rn can be useful in distinguishing between saturated zone water and unsaturated superficial water; the former usually has a much higher radon content than the latter. water in unsaturated zones may lose much of its radon to the atmosphere by degassing to soil air, whereas water in saturated zones generally retains most of its radon. superficial water that enters a saturated zone begins to accumulate 222rn, acquires the 222rn signature of the rock and soil underground at a rate controlled by the retention time and the radioactive decay. superficial water having been in a saturated zone for several days can be considered groundwater. radon can reach relatively high concentrations in groundwater but, because of its low solubility, it degasses rapidly in surface waters. therefore it can be a very sensitive indicator of groundwater input to streams. the 222rn content of river water is strongly affected by volatilization to the atmosphere, and this must be accounted for in using radon data to estimate a possible groundwater influx from subsurface water sources, important in river flow generation. in the following we consider a given length ∆x of a river and two sampling sites at the upstream and downstream ends. a simple one-dimensional model equation can be proposed as q q q x kqrn rn rn rn q avg avg2 2 1 1 = + x∆5 5 5 5? ? ? ? where q (m3/s) is the river discharge, the subscripts 1 and 2 designate the upstream and downstream ends, q (m3s–1m–1) the inflow rate per unit of river length, τ the travel time, k a radon-degassing rate constant; qavg and [rn]avg are the mean river discharge and the mean radon volumetric concentration in the stream. we assume that the length is sufficiently small allowing the same radon concentration for lateral inflowing water. as the travel time is very short, a loss of radon due to decay (t = 3.8 d) is negligible. due to the turbulent runoff of the small rivers, the primary mechanism of radon removal is gas exchange. 2. material and methods to obtain information on the hydrological signature of small rivers during and after heavy 103 using 222rn for hydrograph separation in a micro basin (luxembourg) rain events, four small catchment areas of the same river were selected as experimental sites. ephemeral or first-order streams drain those watersheds of 10-100 ha. the selected sub-catchments under investigation are situated in the western part of luxembourg and belong to the attert river basin, the latter being integrated in the european network of experimental research basins (erb). the catchment area of the river basin is 318 km2. the altitudes vary from 540 m to 210 m, with slopes presenting a maximum of 16°. the geological underground is formed of devonian schist in the highest northern part, triassic marls und mudstones for the middle part and liassic sandstones for the southern part. the present paper concerns the micro basin h (fig. 1), located in the southern part of the attert basin. it is covered to 93% by forest and 7% by pasture; the underlying rocks and soils are over 81% sandstone, 13% marls, 3% marls/mudstones and 3% alluvium deposits. the topography stratifies the watershed into two subunits, hilltops and valleys, with altitudes between 380 and 320 m. 2.1. radon in water measurements liquid scintillation counting (lsc) is used for the determination of α-emitters in environmental samples. presently, this technique is used for radon and radium in water measurements. for radon measurements, normally 14 ml of water are mixed to 7 ml of scintillation cocktail betaplate, out of which 5 ml are counted in the triathler portable liquid scintillation device. this device provides efficient alpha-beta discrimination. for the very low concentrations of river water, the sample volume is increased to 250 ml, thus a limit of detection down to 50 mbq/l can be obtained. samples are prepared by carefully introducing the water under the cocktail prepared in a glass vial. measurements are performed after a waiting time of at least 3 h, permitting equilibrium between radon and the decay products. concerning continuous radon-in-water measurements, for radioprotection purposes, a detection limit of 10 bq/l is needed, but for hydrogeological studies detection limits lower than 1 bq/l are required. when deciding which fig. 1. map of the catchment areas of the attert river, shown are the four micro basins under study. the present work concerns micro basin h. 104 antoine kies, harald hofmann, zornitza tosheva, lucien hoffmann and laurent pfister method to use, an important point is how much measuring time is necessary to obtain an acceptable precision and give a signal that significantly differs from the background. radon concentrations in a spring can vary considerably with time. the sampling frequency has to be adapted to the dynamics of the corresponding aquifer. however, one rarely knows how fast an aquifer will react to changing environmental conditions, e.g., to heavy precipitations. extreme cases are karst springs where discharges are reported to increase by two orders of magnitude within hours after a storm (eisenlohr and surbeck, 1995). even springs emerging from an aquifer containing old water may react quickly to precipitation because of a rapid change in hydraulic pressures. in order not to miss important features, our continuous radonin-water monitors have a temporal resolution of half an hour. there are several possibilities to monitor radon continuously in water. we decided to use the principle of radon gas measurements in a closed circuit coupled to the water. the coupling consists either in bubbling air through the water, or replacing the bubbling facility with a diffusion tube (fig. 2). in combining the degassing unit or the diffusion tube with a radon detector based on 222rn and 218po, it is possible to have a resolution below 1 bqm–3. this is necessary if one wants to monitor continuously radon concentrations in river water far away from the sources where nearly all of the radon has degassed. if one puts the diffusion tube into the water it is essential to have a high water flow as a depleted or enriched zone exists around the diffusion tube, not having the same radon concentrations as the water flowing in. in order to avoid this drawback we let water circulate in the tubing, measuring the radon concentration in the vessel, either by putting a small radon monitor (doseman, sarad company, dresden) into the exchange vessel, or pumping the air from the exchange vessel to the measuring chamber of the radon monitor. as the semiconductor devices normally have to work under dry conditions, air passes a drierite column prior to counting. another drawback with the diffusion tube is the change in the oswald coefficient with temperature. water temperature has to be measured and a correction performed. preferably we use the diffusion-tube method to monitor the water at springs where temperature variations are small. temperature and conductivity measurements are performed each time a grab sample is taken for radon measurements. continuous radon measurements are always coupled with continuously working thermometers and conductivity meters. a locally installed weather station informs on atmospheric parameters, namely rainfall. fig. 2. principle of the use of an accurel diffusion tube for radon-in-water measurements (surbeck, 1996). fig. 3. variations with time of the grab-samples taken at different sources (with concentrations higher than 10 bq/l) and at 2 points along the river (h1 and h6). table i. summary of results of radon measurements in 8 sources of the catchment area under study. location no. of meas mean min max (bq/l) (bq/l) (bq/l) h4 14 13 9.6 15 h5 9 23 22 25 h6 14 4.8 2.8 8.4 h8 14 10 6.8 12 h10 12 16 9.5 19 h11 14 12 11.0 14 h17 7 13 10.6 14 h18 6 14 12.0 17 water. nevertheless, if the water output is on a hillside, often prior to the physical output, radon degassing may occur due to underground turbulent flow and small water cascades. figure 3 shows the temporal evolution of radon concentrations measured at 8 sources. during this period no major rain event was observed and the radon concentrations do not experience wide variations. nevertheless a rainy period at the end of august and at the end of november induced a slight increase in radon concentrations. figure 4 shows radon concentrations measured during a december rainy period at the outlet of the microbasin. among the data, most interesting are those documenting continuous measurements of radon, electric conductivity and temperature; barometric pressure did not influence the measured data. the data of fig. 4 were collected at the outflow point h1 of the microbasin under study. for the period under investigation, every major rain event induced a rapid decrease of the electric conductivity and an increase in radon concentrations. often a time lag between conductivity and radon is observed. the drop in conductivity is due to the input of superficial water to river water, whereas increased radon concentrations are due to groundwater. infiltrating water to the aquifer has a piston effect on residential groundwater in the fracture systems close to the main underground pathways and canals. radon-rich water is pressed out of the fracture system and enters the river, sometimes with a time delay to the conductivity, the lat105 using 222rn for hydrograph separation in a micro basin (luxembourg) 3. results and discussions to quantify the influx of groundwater to surface discharge, it is necessary to define a typical 222rn value for the groundwaters of the basin. this value was established by measuring the 222rn concentration of a number of springs in the investigated microbasin (table i). radon concentrations in the springs entering the river range between 10 and 25 bq/l, which is about a factor 10 higher than river radon concentrations. spring water is generally collected less than 1 m from the point where it first emerged from the ground. degassing over a short distance can be admitted as insignificant, and no volatilization correction has to be applied to 222rn spring 106 antoine kies, harald hofmann, zornitza tosheva, lucien hoffmann and laurent pfister ter tracing the direct superficial flow. radon peaks, consecutive to a rain event, last much longer as do conductivity lows thereby documenting a difference in their contribution of hillslope subsurface and groundwater or superficial runoff water. there exists an apparent contradiction between the hydrometric evidence of rapid flow along surface and near-surface pathways and the radon evidence of a high portion of old water contributing to storm flow. storm flow generation results in a rapid increase in groundwater level increasing the hydrostatic pressure and thus the underground flow in near channel areas (mulholland, 1990). one can expect higher radon concentrations at the output as water normally retained in underground fracture zones close to the main flow channels is pushed into the flow channels. 4. conclusions radon occurs naturally in all groundwater with varying concentrations depending on lithology and geological structure. here we describe a methodology that uses 222rn to provide information on river inflows by admitting that water from different pools contributing to riverflow differs in 222rn concentration. superficial water has a markedly different radon content from groundwater, the latter differing in radon concentrations if originating from saturated soils or from fractures in the bedrock. levels of 222rn found in rivers are at least one order of magnitude lower than the associated groundwater concentrations, thus radon is a sensitive way of detecting groundwater inflow. 222rn data can be used in a simple mass-balance equation in conjunction with river discharge data to quantify groundwater inputs to surface flow. in addition, in order to allow the location and quantification of groundwater and surface water deliveries, independent estimates of groundwater discharge and recharge or aquifer storage capabilities are possible. the response of a forested watershed to advective rain events is the main purpose of the present study. the results, even if preliminary, are very promising and partly illustrate the given objectives. acknowledgements the research was undertaken in the framework of the «cycleau» national project based on the study of the interactions between different parameters in the water cycle, at several scales. cycleau (fnr/01/03/02) is funded by the luxembourg national research fund. fig. 4. river water radon and conductivity versus time measured at h1, the outflow of the microbasin area. very high water levels, consecutive to a very strong rain event on 2nd january 2003, upset the radon measuring device. radon concentrations measured after are those of outside air. 107 using 222rn for hydrograph separation in a micro basin (luxembourg) references clements, w.e. and m.h. wilkening (1974): atmospheric pressure effects on 222rn transport across the earthair interface, j. geophys. res., 79 (33), 5025-5029. eisenlohr, l. and h. surbeck (1995): radon as a natural tracer to study transport processes in a karst system. an example in the swiss jura, c.r. acad. sci. paris 321, ser. iia, 761-767. freyer, k., h.-ch. treutler, j. dehnert and w. nestler (1997): sampling and measurement of radon-222 in water, j. environ. radioactivity, 37, 327-337. genereux, d.p. and h.f. hemond (1990): naturally occurring radon 222 as a tracer for streamflow generation: steady state methodology and field example, water resour. res., 26 (12), 3065-3075. genereux, d.p., h.f. hemond and j.p. mulhoplland (1990): use of radon-222 and calcium as tracers in a three-end-member mixing model for streamflow generation on the west fork of walter branch watershed, j. hydrol., 142, 167-211. hoehn, e. and h.r. von grunten (1989): radon in groundwater: a tool to assess infiltration from surface groundwater to aquifers, water resour. res., 25, 1795-1803. hooper, r.p. and c.a. shoemaker (1986): a comparison of chemical and isotopic streamflow separation, water resour. res., 22 (10), 1444-1454. merot, p., p. durand and c. morisson (1995): four-component hydrograph separation using isotopic and chemical determinations in an agricultural catchment in western france, phys. chem. earth, 20 (3-4), 415425. mulholland, p.j., g.v. wilson and p.j. jardine (1990): hydrogeochemical response of a watershed to storms: effects of preferential flow along shallow and deep pathways, water resour. res., 26 (12), 3021-3036. schery, s.d., d.h. gaeddert and m.h. wilkening (1984): factors affecting exhalation of radon from a gravelly sand loam, j. geophys. res., 89, 7299-7309. sklash, m.g., r.n. farworden and p. fritz (1976): a conceptual model of watershed response to rainfall, developed through the use of oxygen-18 as a natural tracer, can. j. earth sci., 13, 271-283. surbeck, h. (1996): a radon-in-water monitor based on fast gas transfer membranes, in proceedings of the international conference on technologically enhanced natural radioactivity (tenr) caused by non-uranium mining, szczyrk, poland, 177-192. 513_526.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 513 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 christoforos benetatos (1), zafeiria roumelioti (1), anastasia kiratzi (1) and nikos melis (2) (1) department of geophysics, aristotle university of thessaloniki, greece (2) institute of geodynamics, national observatory of athens, greece abstract teleseismic body wave modelling, time domain moment tensor inversion of regional waveforms and spectral analysis of the far-field p-wave pulses are used to derive the source parameters of the july 26, 2001 skyros earthquake (m 6.5). its epicentre is located south of the sporades islands in the north aegean sea (greece). previous focal mechanism solutions indicate motion on strike-slip faults. the time domain moment tensor inversion is applied for the first time to the regional waveforms of the recently established broadband network in greece. its application gave results which are highly consistent with teleseismic waveform modelling. the results of this study, in combination with the distribution of aftershocks, indicate left-lateral strike slip motion on a nw-se striking fault with parameters: fault plane (strike = 151°, dip = 83°, rake = 7°) and auxiliary plane (strike = 60°, dip = 84°, rake = 173°), depth 12 km and m0 = 5.98e18 n m. moreover, the time domain moment tensor inversion technique yielded a pure double couple source with negligible clvd. the spectral analysis of the far-field p-wave pulses resulted in a fault length l ~ 32 km, stress drop ~ 9 bars and average displacement u ~ 30 cm. these values are in very good agreement with those estimated from empirical scaling relations applicable to the aegean area. 1. introduction on july 26, 2001 an earthquake of m 6.5 occurred in the aegean sea, a few kilometres nw of the island of skyros (fig. 1). despite its large magnitude and the fact that it was felt in a wide region, the earthquake did not cause extensive damage as its epicentre was located at sea (38.995°n, 24.382°e; geophysical laboratory, aristotle university of thessaloniki). limited damage was only observed in the island of skyros, concerning mainly old residential buildings, a 1000-year-old monastery located inside the island’s castle and limited landslides and rock falls. the aim of this paper is to investigate in detail the source parameters of the skyros mainshock. we use teleseismic data from the global seismograph network (gsn) as well as regional broadband waveforms recorded by the recently installed broadband network operated by the national observatory of athens (noa). the focal mechanism and related parameters are determined through a comparative application of two different methodologies: teleseismic waveform modelling (nábelek, 1984) and time mailing address: prof. anastasia a. kiratzi, department of geophysics, aristotle university of thessaloniki, gr 54124, thessaloniki, greece; e-mail: kiratzi@geo.auth.gr key words source parameters skyros earthquake aegean greece 514 christoforos benetatos, zafeiria roumelioti, anastasia kiratzi and nikos melis domain moment tensor inversion (dreger, 2000). the latter methodology is applied for the first time in the study of aegean earthquakes. furthermore, the dimensions of the causative fault, the stress drop and the average displacement across the ruptured area, are determined from the far-field displacement spectra of the event. 2. teleseismic waveform modelling 2.1. methodology and application the technique of body-waveform modelling, described in detail by nábelek (1984) and mccaffrey et al. (1991), was used to calculate the focal mechanism of the skyros mainno rth ae ge an tro ug ht sporades isl. m fig. 1. regional map showing the sites of the broadband stations, operated by the geodynamic institute of the national observatory of athens, and the epicentre location (asterisk) of the skyros july 26, 2001 earthquake. 515 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 .fig. 2. teleseismic waveform modelling results: comparison between observed (solid line) and synthetic (dashed line) p and sh long-period waveforms. the estimated strike, dip and rake (in degrees), the depth (in km) and the seismic moment (in n m) are shown at the top of the figure. stations are ordered clockwise by azimuth and the station code of each waveform is accompanied by a letter corresponding to its position within the focal sphere. vertical bars show the inversion window. the employed source time function and the waveform time scale are depicted at the centre of the figure. waveform amplitude scales (in microns) are shown to the left of the focal sphere. the insets show the first motion polarities at stations with epicentral distances lower than 30°, not included in the inversion, but complimentary used to constrain the nodal planes. 516 christoforos benetatos, zafeiria roumelioti, anastasia kiratzi and nikos melis shock. the employed data consist of p and sh waveforms from stations of the global seismograph network (gsn). all waveforms have a sampling frequency of 1 hz and were recorded at epicentral distances ranging from 30° to 90°. the nodal planes of the focal mechanism were constrained by the use of p-wave polarities from stations located at distances less than 30°. the applied methodology has been extensively described in past papers (e.g., kiratzi and louvari, 2001; louvari and kiratzi, 2001) and therefore we will only present a brief description of it. a fir (finite impulse response) band pass filter was first applied to the data, in order to remove the high frequency noise. the corner frequencies of the filter were determined visually, by comparing the fft amplitude spectrum of the signal with the corresponding spectrum of the noise. in most cases, the frequencies of intertable i. source parameters of the july 26, 2001 skyros earthquake derived from the two applied methodologies. in the case of the teleseismic waveform modelling, the numbers in the brackets indicate the lower and upper bounds of the uncertainties. method time lat. long. m m0×10 18 depth 1st plane 2nd plane h:m:s (°n) (°e) (n m) (km) strike (°) dip (°) rake (°) strike (°) dip (°) rake (°) teleseismic 00:21:38 38.99 24.38 6.4 4.46 12 145 80 8 54 82 170 waveform modelling (+1/-3) (+5/-2) (+3/-3) (+4/-2) time domain moment 00:21:38 38.99 24.38 6.5 5.43 14 156 86 5 66 85 176 tensor inversion fig. 3. comparison of the «minimum misfit solution» obtained in this study (upper part) to the harvard cmt solution (lower part). the solutions are not very different but our solution gives a considerably better fitting (hia and aak for instance). 517 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 fig. 4. presentation of the tests performed in order to define the uncertainties in strike, dip, rake and depth (as discussed in the text and listed in table i). 518 christoforos benetatos, zafeiria roumelioti, anastasia kiratzi and nikos melis est in the long-period waveforms covered the range from f 1 = 0.01 hz to f 2 = 0.1 hz. the aforementioned frequency limits were used as corner frequencies of the filter. all waveforms were integrated to displacement prior to the inversion. we used the mt5 version of mccaffrey and ambers (1988) algorithm to invert p and sh waveform data in order to obtain the strike, dip, rake, centroid depth, seismic moment and source time function of the examined event. the methodology assumes that the source can be represented as a point (the centroid) in space, although not in time. the time history of the displacement on the fault is represented by a source time function made up of a series of overlapping isosceles triangles, whose number and duration are defined by the user. the inversion routine yields amplitudes for each triangular shape. amplitudes were corrected for the geometrical spreading, the epicentral distance (langston and helmberger, 1975), and the attenuation using futterman’s (1962) operator with t* = 1 s for pand t* = 4 s for sh-waves. the inversion adjusts the relative amplitudes of the source time function elements, the centroid depth, the seismic moment and the source orientation (strike, dip, rake) to minimize the misfit between observed and synthetic seismograms. we refer to this solution as the «minimum misfit solution». the covariance matrix associated with the «minimum misfit solution» usually underestimates the true uncertainties associated with the source parameters. to find more realistic uncertainties, we followed the methodology of mccaffrey and nábelek (1987) and molnar and lyon-caen (1989) by fixing some of the source parameters at values close to, but different from those of the «minimum misfit solution» and allowing all the other parameters to vary during the inversion. the errors are determined by visual examination when the match of the observed to synthetic seismograms significantly deteriorates. synthetics were generated for a point source buried in a half-space. we used a p-wave velocity of 6.5 km/s, s-wave velocity 3.7 km/s and density 2.75 g/cm3. we also used a 300 m water layer overlying the half-space. 2.2. inversion results the focal mechanism of the 26 july 2001 earthquake was computed by inverting 24 p and 22 sh long-period waves with good azimuthal coverage (fig. 2). the results are shown in table i. first-motion polarities of pwaves recorded at 8 stations located at epicentral distances 0°-30° are compatible with the inversion solution. the inversion yielded a source time function of boxcar shape and duration of ~ 8 s. our solution is in good agreement with the solution published in the harvard cmt catalogue, although the synthetics produced by the cmt solution (strike, dip and rake fixed, all other parameters free) indicate poor amplitude fitting for some stations like hia, aak and kdak as shown in fig. 3. figure 4 summarizes some of the tests that were carried out, in order to investigate the errors of our minimum misfit solution. the fixed values for strike, dip, rake and depth tests are 135° and 170°, 70° and 90°, 20° and 0° and 16 km and 7 km, respectively. 3. time domain moment tensor inversion 3.1. methodology in addition to the teleseismic waveform modelling, we applied the time domain moment tensor inversion technique, which uses digital waveform data recorded at regional distances. the employed inversion code has the capability of revealing the seismic moment tensor using a single three-component station, although the use of a larger number of stations provides greater stability (dreger and helmberger, 1991). generally, the methodology can be applied successfully if the following conditions are met: 1) the regional crustal model is sufficiently well known to explain the lowfrequency part of the recorded waveforms; 2) the event location can be well represented by the high-frequency hypocentral location, and 3) the source time history is synchronous for all of the moment tensor elements and may be approximated by a delta function (dreger, 519 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 2000). furthermore, the inversion procedure accounts only for the deviatoric tensor, neglecting any volumetric changes in the source area. the applied methodology is based on a simplified general representation of the seismic source by considering a point source in both space and time (3.1) where u n is the nth observed component of displacement, g ni, j is the nth component of the theoretical green’s function for specific forcecouple orientations, x corresponds to the sourcestation distance, z is the source depth and m ij is the scalar seismic moment tensor. the general force-couples for the deviatoric moment tensor are represented by the three fundamental faults, namely a vertical strike-slip, a vertical dip-slip and a 45° dip-slip (jost and herrmann, 1989). equation (3.1) is solved using linear least squares, assuming a constant source depth for each inversion. the estimated scalar seismic moment tensor, m ij , is then decomposed into the scalar seismic moment, a double-couple moment tensor and a compensated linear vector dipole moment tensor. the procedure of the decomposition is described in detail in jost and herrmann (1989). the optimum hypocentral depth is found iteratively through an examination of both an objective function and the variance reduction. the objective function, f, depends upon the rms of the difference between the observed waveforms (d ) and the synthetic waveforms (s), modulated by the percent double couple ( pdc): (3.2) while the variance reduction (vr), is estimated through the relation (3.3) u x t m g x z tn ij ni j, , ,,( ) = ( ) f d s = ( )rms pdc successful applications of the methodology result in small values of the objective function and large values of the variance reduction, indicating that both the waveform fit and the percent double couple are large. 3.2. application and inversion results in order to apply the time domain moment tensor inversion technique for estimation of the skyros mainshock focal mechanism, an appropriate 1d velocity model had to be chosen. unfortunately, due to the geological and seismotectonic complexity of the aegean area, one single 1d model is not adequate to explain the low-frequency wave propagation along the different wave paths (see fig. 1 and http:// www.gein.noa.gr for more information on the noa stations). recently, zahradnik (2001) tested the model proposed by novotny et al. (2001), through a forward simulation of the same waveforms used in the present study (regional broadband data of noa) and found that the examined 1d model can satisfactorily explain the frequency range 0.05-0.08 hz of the waveforms recorded at stations ape, prk and ath (fig. 1). these stations are among the closest to the mainshock epicentre (their epicentral distances range from 135 to 245 km) and present the larger signal to noise ratios. we also employed the model of novotny et al. (2001) and used it as input in the frequencywavenumber integration code (fkrprog) developed by saikia (1994) in order to compute synthetic green’s functions for the examined epicentre-station distances. the synthetic green’s functions were then used in the time domain moment tensor inversion code to derive the focal mechanism of the examined event. we found that the employed velocity model works very well for stations ath, prk and ape (variance reduction is larger than 90% for all three stations), although it is inadequate for the rest of the epicentre-station paths. nevertheless, the applied methodology gives reliable results by using one single station, thus the combination of three stations provides high levels of accuracy in the derived focal mechanism. vr d s dt d dt = [ ] 1 0 2 . . 2 520 christoforos benetatos, zafeiria roumelioti, anastasia kiratzi and nikos melis the results from the time domain moment tensor technique are presented in fig. 5 and table i. the observed tangential, radial and vertical components at stations ape, prk and ath (solid lines) are compared to the synthetics (dashed lines). table i shows that the two applied methodologies gave converging results. 4. source parameters from far-field displacement spectra 4.1. methodology source parameters such as moment (m0), fault length (l), average displacement (u _ ) across the fault and static stress drop ( ), were determined for the skyros mainshock, using the far-field amplitude displacement spectra. the data consist of long period p-waves (sampling frequency 1 hz) recorded at teleseismic distances (30°90°) from the gsn stations. we used a time window starting at the p-arrival and ending before the s-wave arrival. the displacement waveforms were corrected for the instrument response, attenuation and radiation pattern. for the latter correction we used an average radiation pattern over the focal sphere, which in the case of the pwaves is assigned the value r = 0.51 (fletcher, 1980). the far-field amplitude displacement spectra are characterized by 3 parameters: i) the low frequency level 0, which is proportional to seismic moment; ii) the corner frequency, f c , and iii) the power of the high frequency asymptote. following brune (1970, 1971), we shall define corner frequency at the intersection fig. 5. results of the time domain moment tensor inversion. in the left part the comparison between observed (solid line) and synthetic (dashed line) regional broadband waveforms is shown. in the right part the beach-ball, of strike, dip and rake for the two nodal planes, the scalar seismic moment, m 0 , and the moment magnitude, m are also given. the time scale is shown below the vertical component, while information on the station name, examined frequency range, maximum observed amplitude (in cm) and variance reduction is shown below waveforms. tangential radial vertical 30.00s 30.00s 30.00s strike = 156; 66 rake = 5; 176 dip = 86; 85 mo = 5.43e+25 = 6.5 percent dc = 99 percent clvd = 1 m station: ath frequency_interval: 0.05-0.08 hz max_ampl= 2.92e-01cm vr=91.7% station:prk frequency_interval: 0.05-0.08 hz max_ampl= 4.03e-01cm vr=94.3% station:ape frequency_interval: 0.05-0.08 hz max_ampl= 4.53e-01cm vr=92.0% 521 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 0.01 0.1 1e-6 1e-5 1e-4 = 1.7 e-04 f c = 0.07a m p li tu d e (m *s ) frequency (hz) brvk 0.01 0.1 1e-6 1e-5 1e-4 1e-3 = 1.92 e-04 f c = 0.069a m p li tu d e (m *s ) frequency (hz ) cmla 1e-3 0.01 0.1 1e-7 1e-6 1e-5 1e-4 = 7.24 e-05 f c = 0.067a m p li tu d e ( m *s ) frequency (hz) dwpf 1e-3 0.01 0.1 1e-7 1e-6 1e-5 1e-4 = 9.03 e-05 f c = 0.065a m p li tu d e (m *s ) frequency (hz) hrv 0.01 0.1 1e-7 1e-6 1e-5 1e-4 = 9.93 e-05 f c = 0.064a m p li tu d e (m *s ) frequency (hz) sfj 0.01 0.1 1e-7 1e-6 1e-5 1e-4 1e-3 = 1.14 e-04 f c = 0.077a m p li tu d e (m *s ) frequency (hz ) tly 0 0 0 0 0 0 fig. 6. far-field amplitude displacement spectra used to estimate the fault dimensions and other parameters of the skyros mainshock. the low and high frequency asymptotes (fitted by eye) are depicted as straight lines. the corresponding values of the low-frequency part of the spectrum, 0, and the corner frequency, fc, are also shown for each spectrum. 522 christoforos benetatos, zafeiria roumelioti, anastasia kiratzi and nikos melis of lowand high-frequency asymptotes in the spectrum. almost all far-field displacement spectra were characterized by a constant low-frequency level, 0 , and a fall-off above a corner frequency f c , at a rate proportional to f y. spectra that did not show such a shape were not analysed. determination of the spectral parameters ( 0, f c ) was performed be eye fitting lowand highfrequency asymptotes to the observed spectra. figure 6 shows indicatively the displacement amplitude spectra for 6 stations together with their best fitting results. the scalar seismic moment was calculated from the relation (keilis-borok, 1959) (4.1) where 0(p) denotes the low-frequency asymptote of the spectrum, the density, r the radiation pattern coefficient for p-waves from a double couple point source, r the epicentral distance and the p-wave velocity at the source. in order to calculate the source dimensions and the stress drop, a circular fault of radius r was assumed. following madariaga (1976), the radius of the source can be estimated from relation (4.2) where f cp is the corner frequency of the p-wave spectra and is the velocity of the s-waves. we have also assumed that the diameter of the circular fault area is equal to the observed fault length (hanks and wyss, 1972). stress drop was calculated from the relation of keilis-borok (1959) (4.3) and the average displacement, u, was calculated from the relation of aki (1966) (4.4) where µ is the shear modulus (taken equal to 3.3 * 10 10 n/m2) and a is the fault surface in km2. average values < x > were computed for each parameter (stress drop, fault length, average displacement) following archuleta et al. (1982): (4.5) where n is the number of stations used. the basic reason for using this relation is that in the case of simple arithmetic average, the mean values are biased towards the larger values. the corresponding standard deviation of the logarithm sd(log< x >) and the multiplicative error factor, e x , were also calculated from the relations of archuleta et al. (1982): (4.6) (4.7) 4.2. source parameters of the mainshock from the spectral analysis the distance, azimuth, the radiation pattern of the p-waves, r (p), the low frequency asymptote, 0 , and the corner frequency, f c , of the stations used are listed in table ii. table iii lists the average values and the multiplicative error factor of the stress drop, the radius of the circular fault and the average displacement across the ruptured area. in our calculations, we used 6.5 km/s for the p-wave velocity, 3.7 km/s for the s-wave velocity, 2.6 g/cm3 for density and 3.3 * 10 10 nt/m2 for the shear modulus, µ. m p p r p r0 0 34( ) = ( ) ( ) = 7 16 0 3 m r m au0 = µ < > = =x n xii n anti log ( )1 1 log sd log log log< >( ) = < >( ) = x n x x i i n 1 1 2 1 1 2 e sd x x = < >( )( )anti log log . r p fcp ( ) = 0 32. 523 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 station epicentral azimuth take-off angle omega corner frequency code distance (°) (°) (m.s) (hz) (km) ma2 5232 27 21 1.45e-04 0.064 majo 7241 48 18 1.02e-04 0.071 tly 4233 49 27 1.14e-04 0.077 brvk 2618 50 32 1.70e-04 0.070 bjt 5734 56 23 1.13e-04 0.070 wmq 3855 63 29 1.47e-05 0.075 mdj 6006 64 21 9.00e-05 0.071 xan 5802 65 23 1.85e-04 0.074 chm 3130 67 31 1.59e-04 0.080 tkm2 3184 67 31 1.46e-04 0.083 usp 3104 67 31 1.23e-04 0.092 enh 6147 68 24 1.30e-04 0.072 aak 3128 68 31 1.59e-04 0.087 eks2 3084 68 31 1.86e-04 0.081 kbk 3155 68 31 1.82e-04 0.079 ulhl 3250 68 31 1.63e-04 0.069 uch 3146 69 31 1.67e-04 0.079 aml 3099 69 31 1.59e-04 0.081 chto 6356 85 23 1.35e-04 0.065 msey 5370 140 28 1.02e-04 0.089 bosa 7196 180 23 1.18e-04 0.078 ascn 6167 227 25 1.05e-04 0.088 bdfb 9182 247 17 5.29e-05 0.075 sacv 4737 254 28 1.24e-04 0.070 sjg 7648 285 19 8.16e-05 0.070 cmla 3382 285 31 1.92e-04 0.069 dwpf 7715 302 18 7.24e-05 0.067 hrv 5786 308 22 9.03e-05 0.065 wvt 7245 311 18 6.21e-05 0.075 borg 2545 329 31 1.00e-04 0.087 sfj 3292 330 28 9.93e-05 0.064 cola 5232 357 20 9.73e-05 0.063 table ii. station parameters and spectral parameters obtained from the far-field displacement spectra of p-waves for the july 26, 2001 skyros earthquake. 524 christoforos benetatos, zafeiria roumelioti, anastasia kiratzi and nikos melis table iii. mean values and multiplicative error factors of the seismic moment, m0, the fault radius, r, the stress drop, and the average displacement, u, as defined by means of the far-field p-wave spectral analysis. m 0 r (km) (bars) u (cm) (1018 n m) mean value 8.06 15.9 8.7 31 multiplicative error 1.94 1.11 1.97 1.92 standard deviation ± 5.34 ± 1.7 ± 5.9 ± 20.0 fig. 7. regional map showing the focal mechanism of the skyros mainshock, the distribution of the aftershocks, the orientation of the fault that moved during the earthquake, and other focal mechanisms previously determined. the activation of an nw-se striking fault is evident. 525 source parameters of the m 6.5 skyros island (north aegean sea) earthquake of july 26, 2001 the scalar moment, based on eq. (4.1), is m0 = 8.06.10 18 n m. this value is larger compared to the values obtained by the teleseismic body waveform modelling and the time domain moment tensor inversion. nevertheless, the previously determined values for the seismic moment of the examined event lie within the range of ± 1 standard deviation of the present estimation from the spectral analysis. 5. conclusions we investigated the source parameters of the skyros m 6.5 earthquake that occurred in the northern aegean sea using body wave teleseismic waveform modelling, time domain moment tensor inversion using regional waveforms, as well as spectral analysis of the farfield displacement p-wave spectra. we used the first two methodologies, in parallel, both to ensure reliable estimation of the focal mechanism, for further use in more detailed studies of the source process during the skyros event (e.g., slip distribution studies), and to validate the results of the time domain moment tensor inversion, which is applied for the first time in the aegean area. the converging results of both methodologies were quite encouraging for further application of the time domain moment tensor inversion scheme to study moderate size aegean earthquakes. the parameters of the july 26, 2001 skyros earthquake, averaging the values obtained, are: fault plane (strike = 151°, dip = 83°, rake = 7°) and auxiliary plane (strike = 60°, dip = 84°, rake = 173°), depth 12 km and m 0 = 5.98e18 n.m, fault length l ~ 32 km, stress drop ~ 9 bars and average displacement u ~ 30 cm. figure 7 shows the focal mechanism of the mainshock together with previously determined focal mechanisms of earthquakes of the area (papazachos et al., 1998 and references therein). in the same figure, the aftershock distribution is also shown for comparison (melis et al., 2001). it is seen that the nw-se striking plane is the fault plane which implies sinistral strike-slip motion contrary to what is expected from the strands of the north anatolian fault zone that terminates in this area, which is well known for its dextral strike-slip character. this observation of sinistral strike-slip motion in the northern aegean sea is documented for the first time from the study of an earthquake recorded by the modern global and regional networks. the significance of the nw-se striking structures, inherited from the old compressional phase of the aegean tectonics will be further discussed in another paper. however, this event gave strong evidence that these structures can be activated from the modern stress field presently acting in the aegean area. acknowledgements green’s functions were computed using the fkrprog software developed by chandan saikia with urs granger, woodward clyde federal services. the time domain moment tensor inversion was based on the code developed by douglas dreger of the berkeley seismological laboratory, and kindly provided to us. this work was partially supported by the earthquake planning and protection organisation (oasp) of greece through project 20246/2000 and by projects 972342 (seis albania) and 1868/99 gsrt. references aki, k. (1966): generation and propagation of g-waves from the niigata earthquake of june 16, 1964. part 2. estimation of earthquake moment, released energy, and stress-strain drop from the g-waves spectrum, bull. earthquake res. inst., tokyo univ., 44, 73-88. archuleta, r.j., e. cranswinck, ch. mueller and p. spudich 1982): source parameters of the 1980 mammoth lakes, california, earthquakes sequence, j. geophys. res., 87, 4595-4607. brune, j.n. (1970): tectonic stress and the spectra of seismic shear waves from earthquakes, j. geophys. res., 75, 4997-5009. brune, j.n. (1971): correction (to brune, 1970), j. geophys. res., 76, 5002. dreger, d. (2000): manual of the berkeley automatic seismic moment tensor code, release 1.0, berkeley seismological laboratory, pp. 21. dreger, d.s. and d.v. helmberger (1991): complex faulting deduced from broadband modelling of the 28 february 1990 upland earthquake (m l = 5.2), bull. seismol. soc. am., 81, 1129-1144. fletcher, j.b. 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(received february 21, 2002; accepted may 27, 2002) accelerated ice-sheet mass loss in antarctica from 18-year satellite laser ranging measurements annals of geophysics, 59, 1, 2016, s0101; doi:10.4401/ag-6782 s0101 accelerated ice-sheet mass loss in antarctica from 18-year satellite laser ranging measurements shuanggen jin1,2,*, mosta abd-elbaky1,3, guping feng1 1 shanghai astronomical observatory, chinese academy of sciences, shanghai, china 2 bulent ecevit university, department of geomatics engineering, zonguldak, turkey 3 university of chinese academy of sciences, beijing, china abstract accurate estimation of the ice-sheet mass balance in antarctic is very difficult due to complex ice sheet condition and sparse in-situ measurements. in this paper, the low-degree gravity field coefficients of up to degree and order 5 derived from satellite laser ranging (slr) measurements are the first used to determine the ice mass variations in antarctica for the period 1993-2011, which are compared to gravity recovery and climate experiment (grace). results show that the ice mass is is decreasing at the rate of −36±13 gt/y in antarctica, −42±11 gt/y in west antarctica and increasing at 6±10 gt/y in east antarctica from 1993 to 2011. the ice mass variations from the slr 5×5 have a good agreement with the grace 5×5, grace 5×5 (1&2) and grace (60×60) for the entire continent since 2003, but slr solution of 5×5 is not sufficient to quantify ice losses in west and east antarctica, respectively. the rate of ice loss in antarctica is −28±17 gt/y for 1993-2002 and −55±17 gt/y for 2003-2011 within one sigma, indicating significant accelerated ice mass losses since 2003. furthermore, the results from slr are comparable with grace measurements. 1. introduction the antarctic ice sheet is the largest ice sheet on earth, which contains 26.5 million km3 of ice [steig et al. 2009]. accurate estimates of its mass variability would greatly contribute to understand global climate change and sea-level change. however, measuring and monitoring antarctic mass variability is difficult because of the ice sheet’s size, thickness and complexity as well as sparse in-situ measurements. larger differences in estimating the glacier mass balance were found with different time spanning observations [shepherd and wingham 2007]. recent geodetic techniques provided mass balance estimates, such as satellite radar altimetry, interferometric synthetic aperture radar (insar) and recent gravity recovery and climate experiment (grace) [wahr et al. 1998, tapley et al. 2004, jin et al. 2011, shepherd et al. 2012, jin et al. 2014]. the satellite laser altimetry showed mass gain in east antarctica at 45±7 gt/yr in 1992-2003 [davis et al. 2005], –31±12 gt/yr in 1992-2002 [zwally et al. 2005], +27±29 gt/yr in 1992-2003 [wingham et al. 2006] and about [–84,–103] gt/yr in 2003-2007 from ice, clouds, and land elevation satellite (icesat) [gunter et al. 2009]. satellite observations showed large ice loss from –48 to –132 gt/yr in west antarctica and from –4 to +22 gt/yr in east antarctica within one sigma [thomas et al. 2004, shepherd et al. 2012]. the significant loss of ice mass in antarctica was observed from grace measurements [ramillien et al. 2006, velicogna and wahr 2006, chen et al. 2009, wu et al. 2010, barletta et al. 2012, king et al. 2012]. however, larger differences of ice-mass change in antarctica are found due to some uncertainties and errors, such as technique-specific systematic errors, different glacial isostatic adjustment (gia)models, smoothing and filtering effects and leakage errors [velicogna and wahr 2013]. in particular, the gia has been recognized as the major difference in quantifying the mass change in antarctica from grace [velicogna and wahr 2006, barletta et al. 2008]. furthermore, the observation time span is relatively short. satellite laser ranging (slr) technique measures the round trip time of flight of ultrashort pulses of light to determine the distance from ground-based stations to satellites equipped with retroreflectors. currently the slr provides millimeter level precision to determine satellite’s orbits, which are influenced by temporal variations in the earth’s gravity field. therefore, slr can accurately determine the temporal variations of the earth’s low-degree gravity field [cheng et al. 2013], which can be used to estimate the large-scale temporal article history received april 21, 2015; accepted january 8, 2016. subject classification: gravity variations, ice mass balance, antarctica, slr, grace. mass variations, including ice volume change. in this paper, the ice-sheet mass variations in antarctica with monthly resolution are the first derived from approximate 18 years of monthly low-degree stokes coefficients of the earth’s gravity field with degree and order of up to 5 based on slr measurements ( january 1993 may 2011). the new gia model (w12a) is used from whitehouse et al. [2012b]. in order to evaluate our results, we compare with ice-sheet mass variations in antarctica from grace observations since 2003. section 2 shows the observation data and methods. section 3 presents the results of ice-sheet mass variations in antarctica and comparison with grace observations. conclusions are given in section 4. 2. observation data and methods the weekly low-degree stokes coefficients of the earth’s gravity field with up to degree and order 5 are determined from slr measurements with the same strategies as grace solution by cheng et al. [2011]. these products are derived from five geodetic satellites measurements (lageos 1 and 2, starlette, ajisai, and stella). the coefficients are given as variations relative to the mean gravity field ggm02c. the slr-derived weekly gravity field coefficients are used to estimate the mass lose in antarctica. the effects of land-ocean leakage are corrected using the forward modelling technique [e.g., jin and zou 2015]. in order to evaluate our results, we compare with results from the monthly grace data (csr level-2 rl05) with harmonic degree and order of up to 5 from january 2003 to may 2011. the degree 1 gravity field coefficients (c10, c11 and s11) are used from swenson et al. [2008]. on one hand, the degree 2 gravity field coefficients are not replaced by slr in order to keep the data independent between grace and slr (grace 5×5). on the other hand, the degree 2 gravity field coefficients are replaced by slr [cheng and tapley 2004] as grace 5×5 (1&2). in addition, the 300 km width of gaussian filter and a special de-striping filter are used [swenson and wahr 2006]. in order to compare with slr solutions, the monthly grace data set with harmonic degree and order of up to 60 is also used to obtain the ice-sheet mass variations in antarctica (grace 60×60). the ice-sheet mass variations in antarctica in water equivalent thickness can be directly determined by gravity coefficient anomalies (dclm, dslm) of slr and grace observations as [jin and feng 2013, hassan and jin 2014]: where tave is the average density of the earth, tw is the density of fresh water, a is the equatorial radius of the earth, ~plm is the fully-normalized associated legendre polynomials of degree l and order m, kl is load-deformation coefficient of degree l [han and wahr 1995], i is the co-latitude, and z is the longitude. in order to estimate the accurate secular variations of ice-mass change, the gia influence should be removed. the gia models describe the ongoing viscoelastic response of the solid earth to past changes in surface loading by ice and water. here, we use the recent gia model w12a for antarctica, which was driven by a glaciologically-consistent antarctic deglacial history and constrained to fit observations of past ice extent [whitehouse et al. 2012a, 2012b]. therefore, the monthly ice-sheets mass changes in antarctica can be obtained from slr (1993.01-2011.05, about 18 years) and grace (2003.01-2011.05, about 8 years). 3. results and discussions since the ice-sheet mass variation time series have strong seasonal signals, seasonal signals are removed from the slr and grace time series of the ice mass variations at each grid point. the secular and quadratic signals of ice mass variations are estimated in antarctica after removing the seasonal signals. 3.1. secular variations of ice-sheets the trends of ice-sheet mass variations in antarctica from january 2003 to may 2011 are estimated from , , cos cos sin t a pl kl l c m s m 3 1 2 1 land w ave plmm l l kl lm lm 00 hd i z t t i z z hd d d+ = = + + +3 == q q q q q v v v v v || jin et al. 2 figure 1. the trends of ice-sheet mass variations over antarctica from (a) slr 5×5, (b) grace 60×60, (c) grace 5×5, and (d) grace 5×5 (1&2) with replacement of degree 1 and c20. unit is mm/y of equivalent water thickness change. the gia effect is removed. ~ (1) 3 slr and grace solutions. figure 1 shows the linear term of ice-sheet mass variations over antarctica from (a) slr 5×5, (b) grace 60×60, (c) grace 5×5, and (d) grace 5×5 (1&2) with replacement of degree 1 and c20. unit is millimeter of equivalent water thickness change per year, mm/y. the gia effect is removed. the linear trend of ice-sheet mass variations from slr almost agrees with the grace results, indicating that the low-degree gravitational coefficients obtained by slr are sensitive and efficient to correctly capture the ice mass change in antarctica. the ice mass is significantly losing in west antarctica and gaining in east antarctica, which almost agree each other. the linear trend of ice sheets variations in antarctica is –55±17 gt/y based on slr 5×5, –45±18 gt/y based on grace 60×60, –36±19 gt/y based on grace 5×5, and –31±17 gt/y based on grace 5×5 (1&2). for west antarctica, the linear trend is –66±13 gt/y based on slr 5×5, –105±12 gt/y based on grace 60×60, –51±18 gt/y based on grace 5×5, and –61±15 gt/y based on grace 5×5 (1&2) within one sigma. for east antarctica, the linear trend is 11.58±15.05 gt/y based on slr 5×5, 60±15 gt/y based on grace 60×60, 16±13 gt/y based on grace 5×5, and 30±16 gt/y based on grace 5×5 (1&2) (table 1). it indicates that the mass losses from slr in west antarctica are the dominant contributor to ice mass loss from antarctica. the values with grace 60×60 are twice bigger than grace 5×5. in contrast, there seems to be a better agreement for the entire continent. when we process grace data using 300 km width of gaussian filter, the noise is attenuated, but the geophysical signal too. it may apply a restoration factor to grace data after filtering [landerer and swenson 2012]. in addition, we also estimate the ice-sheet mass variations over antarctica from january 1993 to may 2011 based on slr 5×5. the ice-sheet mass is significantly losing at –36±13 gt/y in antarctica, –42±11 gt/y in west antarctica, and 6±10 gt/y in east antarctica, which are close to the combined results by shepherd et al. [2012]. 3.2. accelerated variations of ice-sheets the time series of ice mass-loss from both slr and grace clearly shows a accelerating variation in ice mass. for example, figure 2 shows the time series of ice mass variations from 1993-2011 at (74.5° s, 144.5° w). a quadratic trend is estimated from the monthly time series of ice-sheet mass variations in antarctica. figure 3 shows the quadratic term of ice-sheet mass variations over antarctica from (a) slr 5×5, (b) grace 60×60, (c) grace 5×5, and (d) grace 5×5 (1&2). the acceleration in ice sheet mass loss for the observed period is –18±9 gt/y2 based on slr 5×5, –5.8±9 gt/y2 based on grace 60×60, –15±7 gt/y2 based on grace 5×5, and –5.8±8 gt/y2 based on grace 5×5 (1&2). these are consistent with grace and satellite altimetry observations, e.g., the ice depletion of –4.4±16 gt/y2 from grace by king et al. [2012], while velicogna [2009] showed larger ice-loss acceleration of –26±14 gt/y2 based on grace data over the period 2002 to 2009. furthermore, the quadratic term of the ice-sheet mass variations over antarctica is estimated from january 1993 to may 2011 based on slr 5×5. the acceleration of ice sheets variations in antarctica is –2.6±5 gt/y2, ice-sheet mass loss in antarctica from slr models january 2003 may 2011 west antarctica (gt/y) east antarctica (gt/y) antarctica (gt/y) slr 5×5 –66±13 12±15 –55±17 grace 60×60 –105±12 60±15 –45±18 grace 5×5 –51±18 16±13 –36±19 grace 5×5 (1&2) –61±15 30±16 –31±17 january 1993 may 2011 slr 5×5 –42±11 gt/y 5.9±10 gt/y –36±13 gt/y table 1. the trends of ice mass variations in antarctica based on four gravity models. figure 2. time series of ice-sheets mass variations in (74.5° s, 144.5° w) from slr 5×5, grace 5×5 and grace 60×60. which is smaller than rignot et al. [2011]’s solution of –14.5±2 gt/y2. 3.3. interannual variations the ice mass-loss from both slr and grace clearly has a significant interannual variation in antarctica. in order to analyze the interannual variability of the ice sheets in antarctica, the trends of ice mass variations are obtained every 3-year (in mm of equivalent water thickness change) in antarctica from january 1993 to january 2011 based on slr 5×5 (figure 4). here the 3-year rate is estimated for matching same time spanning for 1993-2011(slr) and 2003-2011 (grace). significant melting of ice mass in west antarctica was starting since 2005 and before it has almost less ice mass loses. we furthermore compare with grace solutions from january 2003 to january 2011 (figure 5). excluding the different trends with short observations of two years from grace, the other two trends for 2005-2008 and 2008-2011 are similar, which confirm significant decrease in east antarctica since 2005 from grace. even though the rate of ice mass change from slr 5×5 is smaller than those from grace 60×60, the spatial pattern and trend agree with each other. we also compare with the in-situ global positioning system (gps) observations, which measures vertical crustal displacement that reflects ice mass loading variations. figure 6 shows the time series of vertical displacement estimated from slr 5×5, gps, grace 60×60 and grace 5×5 at syog station (69.5° s, 39.5° e) from 2003 to 2011. they show quite similar secular trend, especially gps, slr 5×5 and grace 5×5. the linear trend is –0.37 mm/y from slr 5×5, jin et al. 4 figure 3. the quadratic term of ice-sheet mass variations over antarctica from (a) slr 5×5, (b) grace 60×60, (c) grace 5×5, and (d) grace 5×5 (1&2). figure 4. the trends of ice mass variations every 3-year (in mm of equivalent water thickness change) in antarctica from january 1993 to december 2010 based on slr 5×5. 5 –0.16 mm/y from gps, –0.29 mm/y from grace 5×5 and –0.77 mm/y from grace 60×60. the difference between slr 5×5 and grace 5×5 is only –0.08 mm/y. the difference between slr 5×5 and gps is –0.21 mm/y, mainly because the gps observations just reflect deformation only in the specific locations, while the slr 5×5 results reflect the mass variations averaged over a large spatial scale. the grace 60×60 result is larger than the other three results, mainly due to the influence of higher spherical harmonics coefficients. in addition, we evaluate the ice-sheet mass variations over antarctica from january 1993 to may 2011 based on slr 5×5. figure 7 shows the linear and quadratic term of ice-sheet mass variations over antarctica from 1993 to 2011 based on slr 5×5. the gia effect is removed. in recent 19 years, the ice-sheet mass is substantially losing in west antarctica, the ice-sheet is slowly gaining in east antarctica. the linear trend of ice sheets variations is –36.43±13.41 gt/y in antarctica, –42.11±11.42 gt/y in west antarctica, and 5.68±10.19 gt/y in east antarctica with one sigma. the quadratic term of ice sheets variations is –2.63±4.78 gt/y2 in antarctica, –3.75±4.17 gt/y2 in west antarctica and 1.70± 3.07 gt/y2 in east antarctica with one sigma. 4. conclusions in this paper, the monthly ice-sheets mass variations in antarctica are the first obtained and investigated from low-degree time-variable gravity field of up to degree and order 5 based on slr data from 1993 to 2011. the trend of ice mass variations is –36±13 gt/y in antarctica, –42±11 gt/y in west antarctica and ice-sheet mass loss in antarctica from slr figure 5. the trends of ice mass variations every 3-year (in mm of equivalent water thickness change) in antarctica from january 2003 to december 2010 based on grace 60×60. figure 6. time series of vertical displacement estimated from slr 5×5 (green), gps (red), grace 60×60 (blue) and grace 5×5 (black) at syog station (69.5° s, 39.5° e). figure 7. the linear and quadratic term of ice-sheet mass variations over antarctica from 1993 to 2011 based on slr 5×5. 5.7±10 gt/y in east antarctica from 1993 to 2011. the ice mass variations from the slr 5×5 have a good agreement with the grace 5×5, grace 5×5 (1&2) and grace (60×60) for the entire continent.since 2003, but slr solution of 5×5 is not sufficient to quantify ice losses in west and east antarctica, respectively. in addition, significant accelerated ice-sheet mass losses are found since 2003 with the rate of –28±17 gt/y for 1993-2002 and –55±17 gt/y for 2003-2011. the acceleration of ice sheets variations is –2.6±5 gt/y2 from 1993 to 2011 and –7.5±9 gt/y2 from 2003 to 2011 in antarctica. therefore, ice mass in antarctica has less loses during 1993-2002 and fast loses from 2003-2011, particularly since 2005. these results indicate that slr observations can provide insights into antarctic ice mass variations from 1993 to 2011 and allow us to monitor the monthly antarctic ice mass variations prior to the launch of grace mission in 2002. in addition, some gaps after grace may be expected 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e vulcanologia. all rights reserved. ice-sheet mass loss in antarctica from slr << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true /maxsubsetpct 100 /optimize false /opm 1 /parsedsccomments true /parsedsccommentsfordocinfo true /preservecopypage true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 1 the ingv real time strong motion data sharing during the 2016 amatrice (central italy) seismic sequence marco massa*, ezio d'alema*, claudia mascandola*, sara lovati*, davide scafidi **, gianlorenzo franceschina*, antonio gomez*, simona carannante*, davide piccarreda*, santi mirenna*, paolo augliera* *istituto nazionale di geofisica e vulcanologia, milano, via alfonso corti 12, 20133, milano, italy ** università degli studi di genova, distav, viale benedetto xv, 5, 16132, genova, italy marco.massa@ingv.it abstract during the august-september 2016 amatrice, mw 6.0, seismic sequence, the real time ingv strong motion data sharing was assured by the ingv strong motion database (ismd). starting on august 24th, the main task of the web portal was to archive, process and distribute the strong-motion waveforms recorded by the permanent and temporary ingv accelerometric stations for the earthquakes with magnitude ≥ 3.0, occurring in the amatrice area and surroundings. at present (i.e. september 30th, 2016), ismd freely provides more than 21.000 strong motion waveforms to all users. in particular, about 2.200 strong motion waveforms were recorded by the temporary network installed for the earthquake sequence monitoring in the epicentral area by sismiko and emersito working groups. in addition, for each permanent and temporary recording site, the web portal provides a comprehensive description of the necessary information to properly use the strong motion data. i. introduction n august 24th, 2016, at 01:36:32 utc, a mw 6.0 (http://cnt.rm.ingv.it/) earthquake struck a wide area of the central apennines (italy), located between the towns of norcia and amatrice (figure 1, top panel). the mainshock resulted in diffuse building collapses and about 290 casualties. between august 24th and september 30th, the seismic sequence produced thousands of earthquakes, 16 of which with magnitude ≥ 4.0 (http://cnt.rm.ingv.it/). the strongest aftershock (mw 5.4) was recorded on 201608-24 at 02:33:29 utc. during important seismic sequence the scientific research in the seismological and engineering fields requires rapid acquisition of strong motion data for several purposes, such as the evaluation of the available ground motion prediction equations (gmpes) and the verification of ground o annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 2 shaking scenarios and probabilistic seismic hazard maps. nowadays the great demand of strong motion data is satisfied by several strong motion databases, each one with different aim and philosophy. at italian scale, data and metadata are freely available both from the ingv strong motion database (ismd, http://ismd.mi.ingv.it) and the italian accelerometric archieve (itaca, http://itaca.mi.ingv.it). while ismd publishes the real time strong motion data recorded by the permanent and temporary stations of the national seismic network (rsn, http://cnt.rm.ingv.it/instruments/network /iv), itaca provides once a year the manually post processed data recorded by the italian accelerometric network (ran, http://www.protezionecivile.gov.it/jcms/it /ran.wp), even if, since 2014 also a set of ingv stations are included in the database. at european scale, the related counterparts are the rapid raw strong motion database (rrsm, www.orfeus-eu.org/rrsm/) and the engineering strong motion database, http://esm.mi.ingv.it/), respectively. during the amatrice seismic sequence (i.e. from august 24 to september 30, 2016) ismd gave freely available strong motion data (and related metadata) in sac raw and ascii corrected formats of 118 events with 3.0 ≤ m ≤ 6.0. a subset of 71 events (not including the temporary stations for emergency) with m ≥3.5 were also available at rrsm web site, where data are downloadable in miniseed raw format by consulting the european integrated data archive (eida, www.orfeuseu.org/eida). moreover, a manual revision of the 16 events with mw ≥ 4.0 occurred during the sequence was published some days after the earthquakes occurrence on esm web site. this paper describes in detail, the ismd real time strong motion data set made available to the community through the website http://ismd.mi.ingv.it since the early morning of the 24th august 2016. ii. ismd 2.0: database and website ismd 2.0 is based on postgresql (www.postgresql.org), an advanced and reliable open source object-relational database management system. the earthquakes parametric data and the ground motion parameters, as resulting from the automatic analyses, are stored in the postgresql database in order to have a complete integrated archive, easily accessible and ready for use. waveform data and images are linked to the database and stored as separate files, organized by year and event-id, in order to ensure a direct data access for different purposes. data and metadata are freely available after the user registration. through the unique event-id, each earthquake is directly linked to the related web-pages of shakemaps (http://shakemap.rm.ingv.it/) and time domain moment tensor (http://cnt.rm.ingv.it/tdmt/) databases. both permanent and temporary seismic stations are characterized in terms of instrumentation and features of the recording site (i.e. geology, morphology, passive seismic analyzes and seismic code classifications). in case of earthquakes with magnitude ≥ 4.0, ismd 2.0 provides a revised version of the published data in order to avoid false ground motion parameters calculated on recordings with http://ismd.mi.ingv.it/ http://itaca.mi.ingv.it/ http://www.orfeus-eu.org/rrsm/ http://esm.mi.ingv.it/ http://www.orfeus-eu.org/eida http://www.orfeus-eu.org/eida http://ismd.mi.ingv.it/ http://shakemap.rm.ingv.it/ http://cnt.rm.ingv.it/tdmt/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 3 low signal to noise ratio or biased by spurious spikes, malfunctioning in data transmission or event superimposition. the revision is operated by an expert seismologist in the first 48 hours after the event origin time (see man in the last column of the earthquake-list web page). figure 1. top: ingv strong motion stations analyzed during the 2016 amatrice sequence. bottom: pga map for the 24th august, mw 6.0, mainshock. the yellow star indicates the epicenter of the mainshock. the website was written in html 5 (https://www.w3.org/tr/html5/) to meet the most recent validation standards of the w3c (world wide web consortium) and in php (http://www.php.net/). all published materials are licensed under a creative commons license. iii. amatrice 2016 data set on august 24th, 2016, at 05:57:52 (utc) ismd published the first automatic report of the mw 6.0 mainshock. in this case the delay of about 4 hours with respect to the event origin time (i.e. 01:36:32 utc) depended on the time required to revise both the event location (operated by ingv-cnt) and the ground motion parameters calculated from the ismd automatic data processing. moreover, due to the high density of stations installed in the epicentral area, the automatic report required almost 1 hour to make online waveforms available, together with the related metadata and all the automatic analyses (massa et al., 2014). in order to publish data as fast as possible, only the accelerometric stations with epicentral distance less than 200 km were considered in the automatic analyses. for the mainshock, ismd contains a total of 126 accelerometric waveforms related to 42 permanent rsn strong motion stations (figure 1, top panel). further 7 stations were discarded after the manual revision. the two stations closest to the mainshock (figure 1, bottom panel) were rm33 (montereale) and tero (teramo), located 22 km south and 31 km east of the epicenter, respectively. rm33 and tero recorded maximum pgas of 91 https://www.w3.org/tr/html5/ http://www.php.net/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 4 and 84 cm/s2, respectively, both on the n-s horizontal components. however the highest value of acceleration, 241 cm/s2, was recorded at fema (monte fema), located 32 km nw of the epicenter (figure 2). the total dataset collected between august 24th and september 30th, 2016, consists of ~ 21.000 strong motion waveforms recorded by the real-time permanent and temporary ingv accelerometric stations. data are shown in figure 3, considering the soil categories as indicated in eurocode8 (cen, 2003). the data set includes 433 waveforms with pga from 10 cm/s2 to 50 cm/s2, 64 waveforms with pga from 50 cm/s2 to 100 cm/s2 and 8 waveforms with pga > 100 cm/s2 (figure 3, top panel). concerning the pgv, 152 waveforms are characterized by pgv ranging from 1 cm/s to 5 cm/s, 19 with pgv ranging from 5 cm/s to 10 cm/s and 1 with pgv > 10 cm/s (14.5 cm/s recorded at fema during the mainshock). figure 2. top: acceleration (left), velocity (center) and displacement (right) recorded at fema during the mw 6.0 event. bottom: recorded and predicted (bindi et al., 2011) elastic acceleration response spectra. figure 3. top: pga recorded from ingv strong motion stations during the sequence. data are shown for different ec8 soil categories. in red the 24th august mainshock. bottom: data recorded by the permanent (black) and the temporary (red) ingv stations. in particular, the installation of the temporary seismic network allowed us to collect a high quality near field strong motion data set including ~2,200 recordings with the epicentral distance < 30 km (figure 3, bottom panel). between the temporary stations, the highest pga (96 cm/s2) was recorded by t1201 during the mw 4.8 event (2016/08/26 04:28:25 utc). in particular, ismd analyzed in quasi real time ~1,600 strong motion data recorded by sismiko (moretti et al., 2016) and ~600 recorded at amandola by emersito (cultrera et al., 2016). all the strong motion data analyzed during the sequence are downloadable both in raw sac and in processed ascii formats. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 5 iv. discussion figure 4 shows the pga and pgv values recorded during the main italian seismic sequences that occurred in the past 10 years (l’aquila, mw 6.1; mirandola mw 5.8; lunigiana mw 5.1; amatrice mw 6.0; http://cnt.rm.ingv.it/tdmt/). the apparent higher values of pga and pgv observed during the l’aquila (ameri et al., 2009) and the mirandola sequences (luzi et al., 2013) are due to the presence of strong motion stations with epicentral distance < 10 km. indeed, if we consider a range of distances from 20 to 70 km, it is possible to verify that ground accelerations and velocities recorded during the amatrice and l’aquila sequences are fully comparable. the pga distribution observed during the amatrice mainshock (figure 1, bottom panel) shows the highest values of ground motion in n-ne direction. this distribution is also recognizable in the seven aftershocks with mw ≥ 4.5, and can be due to the rupture directivity involving the normal fault (spagnuolo et al., 2016) and/or to the seismic waves propagation effects. the latter property was already observed during the 2009 l’aquila sequence by ameri et al. (2009). the collection of strong motion data published on ismd after the manual revision allows to evaluate the gmpe currently available for italy (bindi et al., 2011). concerning the 24th august mainshock, figure 5 shows that the current models (at least in terms of pga and pgv) tend to slight underestimate the near source and near field data, while at distances > 50 km it is possible to note as the recorded data decay faster with respect to the current predictions, independently from the ec8 sites classification. figure 4. pga (top) and pgv (bottom) recorded from ingv strong motion stations during the main italian sequences occurred in the last 10 years. the same comparison were made considering the 16 events with magnitude ≥ 4.0 (~ 3.200 accelerograms) recorded in the analyzed period. the results are presented in figure 6 in terms of bias, defined as the residual between observed and predicted ground motion, evaluated using the maximum likelihood formalism proposed by spudich et al. (1999). negative and positive bias indicate that the predicted model overestimates and underestimates the recorded ground motion, respectively. specifically, the slope of the straight line that annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 6 best fits the residuals as a function of logdistance (fig. 6, left) or magnitude (fig. 6, right) is indicative of the dependence of the residuals on the independent variable considered. figure 5. pga (top) and pgv (bottom) for the 24th august mainshock (mw 6.0) compared to the bindi et al. (2011) italian gmpes. stars indicate data at distance < 30 km recorded by ran. colors indicate different ec8 (cen, 2003) site categories. in general, both pga and pgv show a good agreement between observed and predicted data. in both cases (fig. 6), the bias indicates a slight overestimation of the predictions. however, figure 6 highlights a clear increasing overestimation of the predictions with increasing distance (more evident for pga, top left panel) and an increasing underestimation of the models with increasing magnitude (more evident for pgv, bottom right panel). in particular, a reason for the pga overestimation at large distance might be the contribution of the anelastic attenuation. in general, it is worth mentioning that the preferential propagation pattern in north direction (fig. 1, bottom) combined to the not uniform distribution of stations around the epicenters might slightly influence the final results. even if this preliminary consideration has to be carefully corroborate with further tests, the results might however represent a warning on the necessity to revise the current models for the central italian apennines in order to assure a more accurate evaluation of the regional seismic hazard. references [ameri, 2009] ameri g., massa m., bindi d., d’alemae., gorini a., luzi l., marzorati m., pacor f., paolucci r., puglia r. and smerzini c. (2009). the 6 april 2009, mw 6.3, l’aquila (central italy) earthquake: strong-motion observations, seism. res. lett., 80, n 6, 951966. [bindi, 2011] bindi, d., pacor, f., luzi, l., puglia, r., massa, m., ameri, g. and r. paolucci (2011). ground motion prediction equations derived from the italian strong motion data base, bull. earth. eng. 9, n. 6, 1899-1920. [cen, 2003] cen (2003). pren 1998-1eurocode 8: design of structures for earthquake resistance. p1: general rules, seismic actions annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7193 7 and rules for buildings. draft 6, doc cen/tc250/sc8/n335, jan 2003, brussels. [cultrera, 2016] cultrera g., d’alema e. and emersito working group (2016). emersito task force activities after the mw 6.0 amatrice earthquake (august 24th, 2016, central italy). ann. geoph., this issue. [luzi, 2013] luzi l., pacor f., ameri g., puglia r., burrato p., massa m., augliera p., castro r., franceschina g., lovati s. (2013). overview on the strong motion data recorded during the may-june 2012 emilia seismic sequence, seism. res. lett., vol. 84, n. 4. [massa, 2014] massa m., lovati s., franceschina g., d’alema e., marzorati s., mazza s., cattaneo m., selvaggi g., amato a., michelini a., augliera p. (2014). ismd, a web portal for the real time processing and dissemination of ingv strong-motion data, seism. res. lett., vol. 85, n. 4, 727-734. [moretti, 2016] moretti m., pondrelli s., margheriti l. and sismiko working group (2016). sismiko: emergency network deployment and data sharing in the 2016 amatrice seismic sequence. ann. geoph., this issue. [spagnuolo, 2016] spagnuolo e., cirella a., akinci a. (2016). investigating the effective ness of rupture directivity during the august 24, 2016 mw 6.0 central italy earthquake. ann. geoph., this issue. [spudich, 1999]. spudich, p., w. b. joyner, a. g. lindh, d. m. boore, b. m. margaris, and j. b. fletcher (1999). sea99: a revised ground motion prediction relation for use in extensional tectonic regimes, bull. seism. soc. am., 89, 1156-1170. figure 6. residual analysis of pga (top) and pgv (bottom), for 16 events with mw≥4.0 recorded in the amatrice area from august 24 to september 30, 2016 with respect to the bindi et al. (2011) gmpes. left and right panels indicate the dependence of the residuals on epicentral distance and magnitude respectively. effects of march 20, 2015, partial (~50%) solar eclipse on meteorological parameters in the urban area of naples (italy) annals of geophysics, 59, 1, 2016, a0106; doi:10.4401/ag-6899 a0106 effects of march 20, 2015, partial (~50%) solar eclipse on meteorological parameters in the urban area of naples (italy) nicola scafetta, adriano mazzarella* osservatorio meteorologico, università di napoli federico ii, distar, naples, italy abstract during the partial (~50%) solar eclipse occurred on march 20, 2015, morning, various meteorological parameters were monitored to study their evolution above the urban area of naples, central italy. the experimental conditions were optimal because of the clear sky situation all over italy, and in naples in particular. the eclipse last about 2 hours between 9:25:06 (ut+1, local italian time) and 11:43:09 (local italian time, ut+1). from the observation site, the incoming solar radiation diminished by about 50% at the eclipse peak at 10:32:18 (local italian time, ut+1), as expected. on the contrary, the uv radiation diminished significantly less, about 25%. this frequency response was likely due to rayleigh scattering. it suggests that about 50% of the uv radiation reaching the surface was direct light and 50% scattered light. during the eclipse, the urban surface temperature and humidity values stayed almost constant instead of increasing and decreasing, respectively, as predicted by their daily cycle. this result was used to estimate that the average emissivity of the city of naples is about f = 0.86. the wind speed decreased significantly during the event while the atmospheric pressure stayed constant to decrease only after the eclipse. finally, we propose a simple empirical method to approximately estimate the cooling effect of an eclipse, which meteorologists could use to correct the temperature model forecast that normally ignores the occurrence of an eclipse. consistency of these results with the literature and its importance is briefly discussed. 1. introduction meteorologists have found solar eclipses attractive for studying the response of the atmosphere under the peculiar condition of an abrupt change of the incident solar radiation [zerefos et al. 2007]. numerous studies have reported abrupt changes in meteorological parameters such as surface temperature, humidity, wind speed intensity and direction and atmospheric pressure [anbar 2006, founda et al. 2007, zerefos et al. 2007, muraleedharan et al. 2010, and references therein]. also direct effects of a solar eclipse on solar and uv surface radiation have been typically studied for better understanding the interaction between the solar irradiance spectrum and the atmosphere [kazadzis et al. 2007, kazantzidis et al. 2007, tzanis et al. 2008]. the response of the atmosphere to an eclipse event strongly depends on the local coordinates, geography and meteorological conditions. studying total and partial eclipses is equally important for a better understanding of the phenomenon. on march 20, 2015, an equinoctial solar eclipse occurred from 7:40 ut to 11:50 ut. the eclipse peak occurred around 9:46 ut on the ocean between iceland and norway (lat. 64°25'54" n, lon. 6°38'48" w). figure 1 depicts the eclipse path map and summarizes its timing and other relevant geographical and astronomical information. the eclipse was mostly visible from the north sea, the faeroe islands and svalbard, and it was partially visible throughout europe. in italy, the sun was obscured by 39% to 67% depending on the latitude and longitude of the observation: see figure 2. herein, we are interested in studying the meteorological changes in the urban area of naples, on the west coast of central italy, during the march 20, 2015, partial eclipse where a nearly 50% solar eclipse was observed, and to use this information to deduce a general meteorological response to an eclipse. 2. data analysis and physical implications table 1 reports the timing and obscuration percent of the eclipse as seen in major italian cities. during march 20, 2015, most italy benefited of meteorological conditions favorable to study the meteorological consequences of the eclipse because of the sunny and cloud-free sky weather. figure 2 shows the weather forecast for the day and the result is confirmed in figure 3 by the high pressure (1020-1025 hpa) and temperature of the day that were higher than the seasonal average present throughout the italian peninsula and throughout most of west europe. this favorable condition could be used to deduce how an eclipse could inarticle history received november 3, 2015; accepted january 11, 2016. subject classification: radiation, solar-terrestrial interaction, meteorological effects of an eclipse. fluence meteorological parameters. figure 4 shows a series of photographs of the various phases of the eclipse as observed in pescara (lat. 42°27'51"n, long. 14°12'51"e), which is located in central italy on the adriatic sea. as reported in table 1, in this location the eclipse started at 9:26:50 (ut+1, local time) and ended at 11:45:56 (ut+1, local time). in pescara, the sun’s maximum obscuration was about 53%, like in rome, at 10:34:38 (ut+1, local time). thus, central italy was an optimal location for studying the meteorological response under the specific condition of an abrupt decrease of incident solar radiation by about 50%. however, the specific 50% condition was better met by the city of naples, where the eclipse screening was 49.40% (table 1). in the following we study what happened in naples. data were collected at the meteorological observatory of the university of naples federico ii [palumbo and mazzarella 1984, di cristo et al. 2007, mazzarella and giuliacci 2011].(1) meteorological data are currently colscafetta and mazzarella 2 figure 1. map of the march 20, 2015, eclipse path on earth. total eclipse is visible within the dark blue areas; partial eclipse is visible within the light blue areas. the eclipse occurs at sunrise or sunset in the pink areas. light blue lines represent a constant fixed fraction of coverage. green lines show the time for greatest coverage. figure from http://eclipse.gsfc.nasa.gov/seplot/seplot2001/se2015mar20t.gif. 3 meteorological effects of the 2015 solar eclipse in naples city coordinates beginning (ut+1) maximum (ut+1) end (ut+1) sun’s dimminglatitude longitude aosta 45°44'14"n 7°19'14"e 9:22:32 10:30:11 11:41:58 67.32% turin 45°04'00"n 7°42'00"e 9:21:59 10:29:36 11:41:24 65.62% milan 45°27'51"n 9°11'25"e 9:24:13 10:32:09 11:44:03 64.88% trento 46°04'00"n 11°07'00"e 9:27:19 10:35:38 11:47:33 64.09% genoa 44°24'40"n 8°55'58"e 9:22:28 10:30:12 11:42:01 62.99% venice 45°26'23"n 12°19'55"e 9:28:02 10:36:25 11:48:17 61.44% bologna 44°29'38"n 11°20'34"e 9:25:30 10:33:35 11:45:26 60.59% trieste 45°38'10"n 13°48'15"e 9:30:05 10:38:37 11:50:26 60.35% florence 43°46'17"n 11°15'15"e 9:24:27 10:32:22 11:44:08 59.15% perugia 43°06'44"n 12°23'20"e 9:25:05 10:32:57 11:44:35 56.50% ancona 43°37'00"n 13°31'00"e 9:27:13 10:35:19 11:46:56 56.31% rome 41°53'35"n 12°28'58"e 9:23:45 10:31:16 11:42:38 53.76% pescara 42°27'51"n 14°12'51"e 9:26:50 10:34:38 11:45:56 53.01% cagliari 39°13'00"n 9°07'00"e 9:16:06 10:22:15 11:32:58 51.65% campobasso 41°33'40"n 14°40'06"e 9:26:27 10:33:57 11:44:57 50.51% naples 40°50'50"n 14°15'29"e 9:25:06 10:32:18 11:43:09 49.40% potenza 40°38'00"n 15°48'00"e 9:27:13 10:34:19 11:44:48 47.12% bari 41°07'31"n 16°52'00"e 9:29:24 10:36:40 11:47:07 46.93% palermo 38°06'56"n 13°21'41"e 9:47:00 10:26:41 11:36:30 44.36% reggio calabria 38°06'52"n 15°39'00"e 9:24:30 10:30:14 11:39:33 41.62% table 1. timing and sun’s dimming of the march 20, 2015, eclipse in major italian cities. the reported timing is in ut+1 to fit the local italian time. figure 2. (left) percent of the sun’s dimming (see also table 1), and (right) meteorological conditions in italy on march 20, 2015. figure from http://www.meteoweb.eu. lected from three automatic stations located on the turret of the building of san marcellino (lat. 40°50'50.2"n, long. 14°15'28.7"e, altitude 50 m). the stations are run by the following weather sensors: temperature (°c), atmospheric pressure (hpa), relative humidity (%), wind speed (m/s), wind direction (°north), precipitation (mm), global and direct solar radiation (w/m2), uv radiation (w/m2). figure 5 shows the total solar irradiance and the uv irradiance records measured at the observatory on the eclipse day. the data are 10-minute averages. the colored area highlights the period of the eclipse’s occurrence in naples from 9:25 to 11:43 local time: see table 1. the dimming of the surface solar and uv radiation records during the eclipse is evident. until noon the meteorological solar radiation record is very smooth, scafetta and mazzarella 4 figure 3. (a) 500 hpa geopotential (gpdm), temperature (°c) and sea-level pressure (hpa), and (b) 850 geopotential (gpdm) and temperature (°c) in europe on march 20, 2015. figure from http://www.wetterzentrale.de. a b 5 a fact that confirms that the sky was clean and cloudfree during the entire eclipse period. we also note that the solar irradiance daily peak last about one hour from about 11:50 to 12:50, while the uv peak last only about 15 min from 11:50 to 12:05, as expected by the geographical position on naples. this behavior could be explained by the fact that the uv band is very narrow (280-400 nm) compared with the band of the solar irradiance. this covers a far larger interval peaking in correspondence of the visible light at 380750 nm making the irradiance measure more variable. at the eclipse’s beginning (9:25 local time) the total solar irradiance was 552 w/m2. it reached a minimum of 334 w/m2 at the eclipse’s apex at 10:32 local time and it increased to 760 w/m2 when the eclipse ended at 11:43 local time. at the eclipse’s maximum in naples, the sun was covered by 49.40%: see table 1. without the eclipse the total solar irradiance at the surface had to be about 660 w/m2, which is slightly larger than the average between the values at the beginning and at the end of the eclipse, that is, 656 w/m2. note that the observation occurred during the late morning when these indexes are expected to rise approximately linearly as indicated in the figure with dash lines. figure 5 shows also the uv record. at the eclipse’s beginning (9:25 local time), the uv irradiance was 0.39 w/m2 and it increased to 0.52 w/m2 when the eclipse ended (11:43 local time). without the eclipse’s obscuration, at 10:32 the uv irradiance had to be about the average between the two values, that is 0.46 w/m2. however, at the eclipse’s apex at 10:32 local time the meteorological effects of the 2015 solar eclipse in naples figure 4. set of photographs of various phases of the march 20, 2015, eclipse as observed in pescara (central italy). the reported timing is in ut+1 to fit the local italian time. photo from http://www.zazoom.it/newsnotizia/post/305603/eclissi-solare-del-2032015-osservata-daterra-e-dallo-spazio. figure 5. total solar irradiance and uv irradiance measured in naples on march 20, 2015. the colored area highlights the period from 9:25 to 11:43 (ut+1, local time) when the eclipse occurred in naples: see table 1. the blue and red broken lines show approximately the expected normal behavior of the uv and solar irradiance at the same day without eclipse. measured uv irradiance was 0.34 w/m2, that is about 75% of the above no-eclipse expected value despite about 50% of the sun was covered by the moon. an uv dimming by only 25% suggests that about 50% of the uv reaching the surface in naples was direct light while the leftover 50% of the uv surface radiation was scattered light.(2) this result could be explained as follows. an eclipse can only block the direct uv light (duv) because only this component is screened by the solar eclipse, whose percent in naples was about se = 50%. because the observed uv radiation (ouv) at the surface was 75% of the expected noeclipse value, 50% of the uv surface radiation has to be direct uv light (duv) and 50% scattered uv light (suv), which is not blocked by the eclipse. in fact, we have that the observed uv radiation during the eclipse must be made of two components as follows: ouv = duv · se + suv = 50% · 50% + 50% = 75%. (1) figure 6 summarizes a number of meteorological variables (temperature, relative humidity, wind speed and atmospheric pressure) measured in naples on march 20, 2015. the meteorological effects of the eclipse, occurred from 9:25 to 11:43 local time, are quite evident in these records. figure 6a shows that during the eclipse, from 9:25 to about 11:00 local time the temperature first slightly increased from about 14.7 °c to 14.9 °c and then decreased again to about 14.8 °c. thus, during the first 1.5 hour of the eclipse the temperature stayed approximately fixed at about 14.8 °c. this temperature standstill is indirectly confirmed by the relative humidity that also halted its morning natural decrease and stayed fixed at about 42.5%. in fact, under moderate wind condition the temperature and the relative humidity are inversely related. after the eclipse, at about 12:00, the temperature raised to about 16.0 °c. using a simple linear interpolation it is possible to calculate that without the eclipse at 10:30 the temperature had to be 0.6-0.7 °c warmer than at 9:30, that is about 15.3-15.4°c. thus, in this occasion the 49.4% eclipse observed in naples has caused a cooling of about 0.50.6 °c relative to the expected temperature. the stationary climatic condition observed during the eclipse from 9:25 to 11:00 can be used to roughly calculate the average emissivity e of the city of naples by taking into consideration that the meteorological observatory of san marcellino is located nearly in the center of the city. we use the stefan–boltzmann law: j = fvt4. (2) here, the stefan’s constant is v= 5.67 * 10-8 wm-2k-4 and the temperature is t=14.8°c + 273.15°c = 287.95 k. scafetta and mazzarella 6 figure 6. meteorological variables versus solar irradiance values measured in naples on march 20, 2015. the colored area highlights the period from 9:25 to 11:43 (ut+1, local time) when the eclipse occurred: see table 1. (a) temperature; (b) humidity; (c) wind speed; (d) atmospheric pressure. 7 the calculation is based on the assumption that the infrared average radiation j emitted by the city had to be equal to the incoming solar radiation because during the eclipse the temperature staid nearly constant, which indicates a balance between the two radiations. thus, we have j = 334 w/m2. using equation (2) and under the above hypothesis we found that the average emissivity of the city of naples is about f = 0.86. during the eclipse, the wind speed decreased from about 3.0 m/s to about 1.5 m/s, which could also be a consequence of the missing increase of the temperature. probably, as the incoming radiation decreased, a moderate temperature inversion between the surface and higher atmospheric layers occurred and partially blocked the lower layer air circulation. to test this hypothesis, we compared the temperature values measured at the observatory of san marcellino versus those measured at the observatory located at monte sant’elmo, which is located at 250 m asl about on the vertical of san marcellino: see table 2. we observed that during the eclipse, between 10:00 and 11:00, the potential temperature difference between sant’elmo and san marcellino was 0.2 °c higher than before and after the eclipse. this indicates a moderate reduction of the thermodynamic instability that could be consistent with the reduced wind stress. the wind was coming mostly from north and no direction change was observed during the eclipse. the wind direction switched to south since 13:30 local time. finally, the atmospheric pressure remained relatively constant at 1023.3 hpa from 8:00 to 11:30 local time: it decreased after 11:30 local time since the eclipse ended. we also suggest a simple methodology to be used by meteorologists to improve the weather forecast in case of an eclipse. in fact, the models used to forecast weather do not take into account the occurrence of an eclipse and meteorologists may be interested in a simple empirical method to correct the theoretical meteorological temperature forecast by a certain eclipse cooling effect dteclipse. let us suppose that: (1) the city of naples is indicative of other cities and locations relatively similar and close to the coast such as most italian cities; (2) in such locations, an eclipse-related reduction of the total solar irradiance by about 326 w/m2 from the expected 660 w/m2 to 334 w/m2 in one hour causes a cooling respect to the expected temperature by about 0.55 °c as observed in naples. the empirical equation must be calibrated against the expected temperature values at the times of the our observed eclipse in naples, when the temperature would have increased by about 0.65 °c from 9:30 to 10:30 if no eclipse had occurred. thus, if texpected is the forecast temperature at the apex of the eclipse in a given location, t9:30 is the expected temperature at that location at 9:30 local time, t10:30 is the expected temperature at that location at 10:30 local time, ssi is the expected solar irradiance at the surface in absence of the eclipse at the time of the eclipse and se is the obscuration percent of the eclipse at the same location, then the temperature teclipse at the apex of the eclipse at that location should be about teclipse = texpected − dteclipse. (3) in fact, if se = 0% (that is an eclipse does not occur) then t ≈ texpected; if se = 49.4% = 0.494, as observed in naples in this occasion, then using the expected temperature values t9:30 = 14.7 °c and texpected = t10:30 = 15.35 °c and the expected solar irradiance at the surface ssi = 660 w/m2 reported above, equation (3) gives t ≈ texpected − 0.55 ≈ 14.8 °c, as observed. the eclipse cooling effect dteclipse in equation (3) should be corrected by a given factor c measuring the average daily variation of ssi in presence of clouds. for example, if during the eclipse the sky remains as cloudy as during the times of reference at 9:30 and 10:30 local time c≈1; if the cloudiness of the sky during the eclipse was lower than during the times of reference at 9:30 and 10:30 local time c would be larger than 1, and vice versa. 3. discussion and conclusion meteorological parameters and solar and uv surface irradiance indices were measured in naples, central italy, during the partial (~50%) equinoctial solar eclipse occurred on march 20, 2015. the observations were optimal for the purposes of this study because the sky was clear and cloud free. during the eclipse, which last about two hours from 9:25 to 11:43 local italian time, the surface solar irradiance dropped by about 50% in about a hour, at 10:32 local time in correspondence of apex of the eclipse. this abrupt irradiance decrease caused a halt in both the urban temperature and relative humidity. without . . . . t t t9 se ssi t t t9 sese ssi 0 55 0 65 0 494 600 0 0026 : : : : exexpxpected exexpxpected 10 30 t9 30 10 30 t9 30 . . c c -q v meteorological effects of the 2015 solar eclipse in naples time interval sant’elmo san marcellino temperature gradient 8:30-9:30 11.9 °c 14.3 °c -2.4 °c 10:00-11:00 12.6 °c 14.8 °c -2.2 °c 11:30-12:30 13.5 °c 15.9 °c -2.4 °c table 2. air temperature gradient between sant’elmo and san marcellino meteorological observatories. the data from sant’elmo were taken from http://www.campanialive.it. · · · · · · the eclipse, these indices had to increase and decrease, respectively, due to their daily cycle. the stationary temperature and relative humidity were likely also favored by naples’ surrounding environment: mainly by the thermal damping effect of the sea-water influence. in fact, a similar response occurred in a number of greek coastal sites during the total solar eclipse of march 29, 2006 [founda et al. 2007]. the sea thermal damping effect is indirectly evident in figure 6a showing that in naples the temperature daily peak occurs 2-3 hours after the peak of the surface irradiance, that is at 14:00-15:00 while in naples the theoretical surface irradiance peak occurs about 10 minutes after noon. on the contrary, the meteorological effects of an eclipse are more significant in arid lands far from the sea coast, where water thermal damping effect is less significant. for example, during a partial (~60%) eclipse in 2005 in the makkah region, a location far from the coast in south arabia, the temperature decreased slightly by about 1 °c and the relative humidity increased [anbar 2006]. significant eclipse-modified boundary layers were also observed in new mexico, u.s.a., during the may 10, 1994, partial (94%) solar eclipse over the desert [eaton et al. 1997]. the stationary temperature and relative humidity occurred during the eclipse could also be used to estimate that the average emissivity of the city of naples is f = 0.86. we observe that this result is realistic because naples is densely built and concrete has an emissivity of about f= 0.85. typically, general constructions materials and asphalts have an emissivity between 0.80 and 0.95 [sobrino et al. 2012, tab. 7 and fig. 4]. as a further confirmation of such an approach, we repeated the analysis using the data from the observatory of casamicciola in ischia known as the green island. we found an emissivity equal to about 0.96 that corresponds to the value for a mostly green surface with few constructions. we have also observed that surface wind-speed, whichwas relativelyweak, decreased during the eclipse from about 3.0 m/s to about 1.5 m/s. this effect was also observed in most sites in greece during the total solar eclipse of march 29, 2006, as a result of the partial cooling and/or stabilization of the atmospheric boundary layers [founda et al. 2007]. thus, likely during the eclipse a moderate thermal inversion occurred between the surface and higher atmospheric boundary layers and this was observed also in naples by comparing the temperature records of the observatories at sant’elmo and san marcellino, located about on the same vertical 200 m apart. we have compared the surface total solar irradiance record against the surface uv record. uv radiation at the surface depends on the total ozone air column. on naples the total ozone integrated vertical profile column density was about 350 dobson units according to an estimate based on the ozone measurement instrument (omi) (http://www.temis.nl/protocols/o3total. html). while at the eclipse peak the surface total solar irradiance value dropped by about 50%, which is a percentage consistent with the moon screening of the sun, the uv record dropped only by 25%, that is equal to the 75% of the expected value if the eclipse had not occurred. the result confirms a strong wavelength dependency of the atmospheric response to surface solar irradiance spectra [rayleigh 1899] where high frequency light is scattered more and, therefore, should be less screened by an eclipse than low frequency light. in particular, we have determined that our observation implies that about 50% of the uv radiation reaching the surface in naples is direct light and 50% was scattered light. our result could be typical of the orography of naples. future studies could determine whether and how this property could be generalized. for example, it could be investigated the influence of the tropospheric ozone, which with clear sky and in metropolitan areas could have play an important uv diffusion. finally, we have proposed a simple methodology, equation (3), to be used by meteorologists to improve the weather forecast in case of an eclipse. the equation attempts to evaluate the cooling associated to a generic eclipse as a proportion relative to the relevant parameters measured in naples in this occasion. the fact that themarch 20, 2015, partial eclipse obscured the sun in naples by about 50% and had its apex at about 10:30, that is during the middle of the warming phase of the temperature diurnal cycle from about 7:00 to 14:00, could make equation (3) a mean estimate that could work in most cases. according to equation (3), if on march 20, 2015, the eclipse was total, in naples the temperature would have dropped by about 1.1 °c respect to the expected 15.3-15.4 °c. thus, we expect that in italy and in other comparable locations an eclipse could cause a cooling of the order of 1 °c respect to the expected temperature in absence of an eclipse. according to equation (3), the eclipse effect should be greater in locations where the diurnal temperature cycle is greater than in naples on march 20, and vice versa. an extensive testing of equation (3) is left to further study. however, herein we can approximately test this empirical equation against some of the results already published in the literature. for example, founda et al. [2007] studied the effect of the total solar eclipse (from 100% to 75%) of march 29, 2006, on meteorological variables in greece. in the city of kastelorizo (lat. 36°9' n, long. 29°35' e, a small greek island near the coast of southern turkey) the eclipse obscuration was scafetta and mazzarella 8 9 nearly 100% and, at the time of the eclipse (about 10:53 ut, 12:53 local time), the solar irradiance at the surface dropped from the expected value ssi ≈ 900 w/m2 to nearly zero. from 9:30 to 10:30 local time (7:30 to 8:30 ut) the temperature increased by about 1.4 °c (cf. founda et al. [2007], fig. 3). thus, equation (3) predicts an eclipse related cooling of about 3.3 °c relative to the expected temperature in absence of the eclipse: by visual interpolation of figure 3 in founda et al. [2007] this is what appears to have occurred. similarly, in the city of thessaloniki (lat. 40°38' n, long. 22°57' e, a city in the north of greece) the eclipse obscuration was nearly 75%. at the time of the eclipse (about 10:49 ut, 12:49 local time), the solar irradiance at the surface dropped significantly from the expected value ssi ≈ 800 w/m2. from 9:30 to 10:30 local time (7:30 to 8:30 ut), the temperature increased by about 3.0 °c (cf. founda et al. [2007], fig. 3). thus, equation (3) predicts an eclipse related cooling of about 4.7 °c relative to the expected temperature in absence of the eclipse, which is again about what appears to have occurred (cf. founda et al. [2007], fig. 3). although approximate, we believe that these results are remarkable given the simplicity of equation (3) that further study could improve and better calibrate. notes (1). the meteorological observatory is attached to the department of earth sciences, environment and resources and is located in the center of naples, italy, in largo s. marcellino. it was established by dictatorial decree of giuseppe garibaldi, on october 29, 1860, at the request of prof. luigi palmieri, holder of the first chair of meteorology after the unification of italy in 1860. the meteorological observatory of san marcellino is, therefore, the highest institution regarding the knowledge of meteorological events of the city of naples and one of the major meteorological centers in italy. (2). rayleigh [1899] found that the particle scattering of electromagnetic radiation is proportional to the fourth power of the frequency so that in air high frequency radiation is more scattered than the low frequency one. references anbar, o.m.y. (2006). solar eclipse effects on the air layer near the surface over makkah region, journal of king abdulaziz university-meteorology, environment and arid land agriculture sciences, 18 (2), 47-59. di cristo, r., a. mazzarella and r. viola (2007). an analysis of heat index over naples (southern italy) in the context of european heat wave of 2003, nat. hazards, 40, 373-379; doi:10.107/s11069-006-0033-7. eaton, f., j. hines, w. hatch, r. cionco, j. byers, d. garvey and d. miller (1997). solar eclipse effects observed in the planetary boundary layer over a desert, bound.-lay. meteorol., 83, 331-346. founda, d., d. melas, s. lykoudis, i. lisaridis, e. gerasopoulos, g. kouvarakis, m. petrakis and c. zerefos (2007). the effect of the total solar eclipse of 29 march 2006 on meteorological variables in greece, atmos. chem. phys., 7, 5543-5553. kazadzis, s., a.f. bais, m. blumthaler, a. webb, n. kouremeti, r. kift, b. schallhart and a. kazantzidis (2007). effects of total solar eclipse of 29 march 2006 on surface radiation, atmos. chem. phys., 7, 5775-5783. kazantzidis, a., a.f. bais, c. emde, s. kazadzis and c.s. zerefos (2007). attenuation of global ultraviolet and visible irradiance over greece during the total solar eclipse of 29 march 2006, atmos. chem. phys., 7, 5959-5969. mazzarella, a., and a. giuliacci (2011). urbanizationinduced variations in air temperature and rainfall in naples southern italy, romanian journal of climatology, 2, 67-77. muraleedharan, p.m., p.g. nisha and k. mohankumar (2010). effect of january 15, 2010 annular solar eclipse on meteorological parameters over goa, india, j. atmos. sol.-terr. phy., 73, 1988-1998. palumbo, a., and a. mazzarella (1984). local recent changes in extreme air temperatures, clim. change, 6, 303-309. sobrino, j.a., r. oltra-carrió, j.c. jiménez-munoz, y. julien, g. sòria, b. franch and c. mattar (2012). emissivity mapping over urban areas using a classification-based approach: application to the dualuse european security ir experiment (desirex), int. j. appl. earth obs., 18, 141-147. strutt, j. (lord rayleigh) (1899). on the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky, philosophical magazine, series 5, 47, 375-394. tzanis, c. ,c. varotsos and l. viras (2008). impacts of the solar eclipse of 29 march 2006 on the surface ozone concentration, the solar ultraviolet radiation and themeteorological parameters at athens, greece, atmos. chem. phys., 8, 425-430. zerefos, c., n. mihalopoulos and p. monks, eds. (2007). the total solar eclipse of 2006 and its effects on the environment, atmos. chem. phys., special issue. * corresponding author: adriano mazzarella, osservatorio meteorologico, università di napoli federico ii, dip. di scienze della terra, dell’ambiente e delle risorse, naples, italy; email: adriano.mazzarella@unina.it. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. meteorological effects of the 2015 solar eclipse in naples << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice profiles of relative permittivity and electrical conductivity from unsaturated soil water content models annals of geophysics, 59, 3, 2016, g0320; doi:10.4401/ag-6990 g0320 profiles of relative permittivity and electrical conductivity from unsaturated soil water content models roberta porretta*, fabio bianchi istituto nazionale di geofisica e vulcanologia, rome, italy abstract a mathematical model of water diffusion in the vadose zone has been implemented for different types of soil textures in order to determine the soil water content (swc) profiles in dependence of depth and time. from these profiles, obtained for different soils, we derived the characteristic electrical parameters, such as relative permittivity (fr ) and electrical conductivity (v), and their variation in time, employing empirical relations available in the scientific literature. the simulation through mathematical models has been performed taking into account different types of soils characterized by the percentage composition of sand, clay and silt in the textural triangle, which provides some physical and chemical properties that affect the water retention in the soil. the resulting simulated profiles of swc and consequently permittivity and conductivity profiles, span over a certain range of values suggesting the best techniques and the limits in geophysical investigation. moreover this a-prior knowledge helps in the elaboration and interpretation of permittivity and conductivity data obtained by the measurements. permittivity and conductivity profiles are particularly useful in some environmental applications when the soil textures are (or supposed to be) known as in the typical case of landfill leachate dispersion. since the soil textures beneath or nearby a disposal waste are characterized by a swc, the values of fr and vat various depth can be directly inferred. 1. introduction electrical conductivity (v) and relative dielectric permittivity (fr ) are two independent physical properties that characterize the behavior of soil when excited by electric currents or electromagnetic fields [settimi 2011]. the electrical conductivity and dielectric permittivity of the constituent minerals are the main controlling factors of the electrical properties of soils [kriaa et al. 2014]. the derivation of these quantities related to the water content provides useful information regarding the practical use of geophysical investigation (gpr, tdr, etc.), applied to unsaturated subsurface vadose zone of medium. this zone has relevant importance in different ambits and especially in geophysical prospection [huisman et al. 2003]. the unsaturated portion of soil concerns the first meters beneath the ground level down to the water table, where the saturated zone begins. in this section hydrogeological properties (physical and chemical) are important since they dominate the electric properties of soil, described by the two quantities v and fr . these last quantities can be directly measured through several techniques or theoretically deduced (constitutive relations) in a homogeneous medium. in this paper these parameters are estimated through soil hydrologic models [huisman et al. 2003]. in the latter case the swc in vadose zone plays an important role to determine both relative permittivity and electrical conductivity since they are strongly dependent on it. the different soil textures can retain different percentage of water, due to its porosity, under the action of two agents: the gravity potential and the matrix potential. the diffusive equation is driven by these quantities and cannot be solved without models. these models need in input physical parameters that quantify the textures of soil. in the past decades authors proposed different numerical parameters [mualem 1976, van genuchten 1980] to solve the so-called richards equation that well describes the water diffusion in the vadose zone. once the input parameters for the particular type of soil are established it is possible to numerically calculate the swc, which is function of time and depth. swc is the most important hydro-geological parameters that jointly to salinity and porosity affects the electrical properties in the unsaturated soils. this paper deals with simulations of soil model in order to evaluate the swc and consequently through topp and archie empirical relationships to determine the electrical parameters of soil. in this specific case, we obtained 3d profiles that show temporal and spatial variation of the electrical properties. the study focuses on soil investigation theoretical approach, that could be a useful tool in some environmental and geophysiarticle history received february 19, 2016; accepted may 17, 2016. subject classification: soil texture, diffusion, water content, conductivity/permittivity profiles. cal applications. furthermore it could represent a preliminary phase which precedes the measurements and facilitates the interpretation of the results (gpr, tdr, ert and other similar geophysical investigation techniques e.g. after raining survey or in landfill control). 2. water content models water flows in saturated porous medium and water content retention in different kind of soils has been subject of many studies. to describe this phenomenon in the vadose zone richards proposed an equation which is a combination between the flux equation and mass conservation, and it characterizes the swc trend over time [richards 1931]. the software developed to derivate swc presented in this paper it’s based on well-known literature models such as van genuchten, mualem, brooks and corey. the introduction of these models was necessary, due to the impossibility to analytically solve the richards’ equation. the water diffusion equation into unsaturated soil in one-dimensional terms can assume the following form: where i is the swc [m3/m3], h represents the hydraulic head: h = } + z , being } the matrix potential and z the elevation above a reference plan (level). k is hydraulic conductivity in (m/s). the above is the socalled richard’s equation that has no closed form solution because k is depend dependent on i. the substitution of the hydraulic head h with }+ z , leads to the so-called richard’s mixed form: (2) here please note that k is the hydraulic conductivity in [cm/day] and i is the soil water content [cm3/cm3]. it describes the movement of the fluid (fluid motion) in a non-saturated and porous medium. it is a non linear partial differential equation and, as such, it has no analytical closed form solution and must be integrated numerically. furthermore, k(i) is not a constant in the strict sense, as it depends on the same i, hence the problem need an appropriate model to be treated. here it is presented in one-dimensional term along the vertical z. equations (2) and (3) have a two dependent variables i and h (or }), so the resolution of the two forms requires the definition of a constitutive equation that binds i and } as in figure 1 [mualem 1976]. from usda (united states departement of agriculture) triangle textures shown in figure 2 typical van genuchten parameters are reported in table 1 [leij et al. 1996]. as mentioned earlier k is not a constant, because of its dependence on i and for this reasons, a model for each different soil textures is required. this demands a constitutive equation that takes into account the water retention curve. such a curve depends on the soil pososity, or, even better depends on the effective saturation se [leij et al. 1997]. the latter can be expressed as: (4) where: is is the water content in saturated conditions and ir is the residual water content. it means that if se = 0 the fluid can be found only in gaseous phase, t kz2 2 2 2 2 2i = s hz2 2 x t z z z z z k zk 12 2 2 2 2 2 2 2 2 2 2 2i w w = ++ =s sx x" "% % t z k2 2 2 2 i i = q" z 12 2 i w +sv x% se ri i= -q v rsi i-q v porretta and bianchi 2 (1) (3) figure 1. water retention curve that links the two variables i or } depending on the type of soil [tuller and or 2005]. figure 2. usda (united states departement of agriculture) textures triangle. 3 while if se = 1 we are in saturated conditions. because the hydraulic conductivity k depends on se, therefore the introduction of models [brooks and corey 1964, mualem 1976] that take into account such a dependence k→k (se) is necessary. in this work the following model proposed by van genuchten has been adopted: (5) such a model [van genuchten 1980] is widely employed in this field. in this work was also introduced the dependence on the matrix potential like: (6) with tabulated parameters a, n = 1 − 1/m (7) equation (3) after some few arrangements becomes equation (8) is numerically solved in matlab program through the van genuchten’s model, given in relations (5), (6) and (7). this allows to obtain both i(z,t) and }(z,t) variables in function of time and depth. since the implemented model it’s based on equations having validity in vadose zone the simulated depth can only reach the water table. limits on the depth parameter are intrinsic in the matrix potential that cannot act on more than 10 m above the water table in ideal conditions, which reflects in a 2-5 m action in real conditions depending on soil texture. also the heterogeneity of the soil has been strongly simplified and the missing data interpolated with a statistical algorithm. 3. empirical relations to derive relative permittivity of the soil many physical and empirical models have been suggested for the evaluation of the i−fr relationship in the literature. the relative permittivity is the main electric quantity employed to define swc in the soil since it can be easily measured through various techniques. the relation that ties these two quantities was empirically established [topp et al. 1980]. this equation is valid for a wide range of mineral soils and independent from soil bulk density, ambient temperature, and salt content. this led many authors to the use of the term “universal” for this equation with appropriate caveat that “in organic soils or heavy clay soils problems arise which may require site-specific calibration” [cosenza et al. 2003, and the reference therein]. in this specific study the topp’s formula [topp et al. 1980] has been used . from well-known water content profiles, obtained by a modeling software [bianchi et al. 2015], topp’s relation (9) allows to derivate the relative permittivity values of soil from the swc. this empirical model was generated using time-domain reflectometry (tdr) at a frequency between 1 mhz and 1 ghz to measure fr for several mineral soils. the estimated error in this model is 0.013. general topp’s model can be expressed: (9) where fr is the relative permittivity and i is the soil volumetric water content. they also provided another inverse relation as follows: z k zt 12 2 2 2 2 2 2 2 i i w w w = +q sv x" % nm h 1 n mv s r nv 1 1 2 2i a} i i a a w + = + -q q qv mv nv se 1 1= +q n mva} mv k s k se e=q v s1 /e m mw1 2%r mw" 2% . . .3 03 9 3 146 76 7r 2 3 f i i i= + + electrical profiles from swc models (8) textural class n ir [cm3/cm3] is [cm3/cm3] a [1/cm] n sand 126 0.058 0.37 0.035 3.19 loamy sand 51 0.074 0.39 0.035 2.39 sandy loam 78 0.067 0.37 0.021 1.61 loam 61 0.083 0.46 0.025 1.31 silt 3 0.123 0.48 0.006 1.53 silt loam 101 0.061 0.43 0.012 1.39 sandy clay loam 37 0.086 0.40 0.033 1.49 clay loam 23 0.129 0.47 0.030 1.37 silty clay loam 20 0.098 0.55 0.027 1.41 silty clay 12 0.163 0.47 0.023 1.39 clay 25 0.102 0.51 0.021 1.20 table 1. typical van genuchten model with parameters (a, n) including residual (ir) and saturated (is). (10) a specific calibration is needed for soils with higher water content or organic matter: (11) for organic soil; (12) for 450 µm glass beds [mukhlisin and saputra 2013]. the model relationships works better for frequencies around 100 mhz. at higher frequencies and moisture contents near to saturation (i~0.4) the topp-model over-predicts relative permittivity by up to 20%. at very low water contents the topp-model doesn’t perform well, particularly for soils with a large clay content [van dam et al. 2005]. however, later studies have shown the dependency of the i−fr relationship on clay content [persson et al. 2000, bouksila et al. 2008] and mineralogy [cosenza and tabbagh 2004], organic matter and porosity or soil density [malicki et al. 1996 and persson et al. 2002], and soluble salt content [dalton 1992, nadler et al. 1999, persson et al. 2000]. ledieu et al. [1986] have shown another form of empirical equations to describe the i−fr relationship, that can be used to expand the topp model for higher water content [van dam et al. 2005] of the form: (13) where b0 and b1 are two empirical parameters depending on soil type. in the same paper ledieu et al. [1986] found the following relationship: (14) this relationship appears to work better for most mineral non-magnetic soils over a range of frequencies between 1 mhz and 10 ghz [hamed et al. 2006]. roth et al. [1990] for mineral soil proposed this empirical relationship: (15) while for organic soil and material is: (16) the error estimations of these equations for mineral soil and organic soil are 0.015 and 0.035 cm3/cm3, respectively [mukhlisin and saputra 2013]. although a comparison of all the above relationships showed a similar trend (figure 3), topp’s formula (blue curve) resulted the relationship that best fitted the input data, since is calibrated for soils that can hold a water content up to 50%. indeed in unsaturated zone the pores in the soil can be filled not only by water but also by air. 4. empirical relations to derive conductivity of the soil another major controlling factor, of the soil electrical proprieties estimated in this study, is electrical . . . . w 5 3 10 2 92 10 5 5 10 4 3 10 r r r r 2 2 4 6 # # # # i f f f f =+ + q v . .1 74 0 34 135 55r 2 3 f i i i= + .. .13 57 31 7 1 68 2r 2 3 f i i i= + bbr 0 1f = + i .0 1138 .r 0 5 i f= .0 17585 . . . .0 0728 0 044 0 00195 0 0000361r r r 2 3 i f f f=+ + . . . .0 0233 0 0285 0 000431 0 00000304r r r 2 3 i f f f=+ + porretta and bianchi 4 figure 3. empirical relations comparison. 5 conductivity. although usually geoelectric investigations are focused on the measurement of soil resistivity, in this paper conductivity is preferred to resistivity (i.e. the inverse quantity), since prevalently we refer to rain water data given in terms of conductivity entering in the following equations. the empirical relation to estimate the electrical conductivity both for saturated and unsaturated soil is the archie’s law [shah and singh 2005] also spread in the literature. this last one connect electrical conductivity of saturated rocks (vo ) to the conductivity of the electrolyte pore of soils (vw ) , it can be expressed as: where vqo is the conductivity contribution which characterizes high clay percentage soils. it can only be neglected for high saline water content filling high porosity soils. f is the formation factor which is equal to: (18) the parameter z is the porosity, and m is the cementation exponent which increases with compaction, cementation and consolidation; it varies between 1.3 and 2.5. unconsolidated sands have values in the range between 1.3 and 1.5 [dannowski and yaramanci 1999, and the reference therein]. generally, the resistivity value is greatly influenced by basic soil characteristics variation such as fraction of solid, air and water. according to [griffiths and king 1981, telford et al. 1990], resistivity value was highly influenced by pore fluid and grain matrix of geomaterials. in compact condition, it was found that at high compaction values corresponds lower resistivity value (or higher electrical conductivity). the volumes of pore in compact condition were reduced and cause the current to easily propagate especially during the existing of water [abidin et al. 2013]. another form has been proposed, known as archie’s second law: which has been developed as an extension for unsaturated rocks and soils including the i factor (20) the saturation index i depends on the degree of saturation sw and the saturation exponent n. this latter was observed to be about 2 for consolidated rocks and to range from 1.3 to 2 for unconsolidated sands [lesmes 2005, and the reference therein]. archie’s second law (19) has been chosen and implemented in the modeling software to obtain profiles of electrical conductibility trend over time which characterizes the vadose zone. this physical quantity depends on a number of parameters as soil texture, water holding capacity, organic matter, salinity and ions exchange capacity. the conductivities of water filling pores is an important factor in the process of electric current flow through the soil; especially when a quantity of salts are dissolved in it. 5. simulations and results the software estimation of the effective water content, in a porous soil is determined by richard’s equation (equation 3). the latter is a partial differential equation that can only be solved with the implementation of the van genuchten soil models. hence water content is strongly affected by physical property of soil material (porosity, capillarity, etc.) and fluid properties (viscosity). indeed at equal initial conditions, retention water content only depends on matrix potential in opposition to the effect of gravity, preventing leaching through the ground till the water table. in the performed software simulation, specific initial condition (infiltration velocity), boundary conditions (water table depth, ground level and saturated zone hydraulic head) and simulation parameters as time and depth have been taken into account. infiltration is the volume of water passing into the soil per unit of area per unit of time. simplifying it has the dimensions of velocity [m3 m-2 s-1]. depth is intended not only as the depth of the water table, or line of saturation, but also the points in the modeled spacef i 1 1 o wv v= w qov+ i sw n= f 1 o wv v= + oqv f mz= electrical profiles from swc models (17) (19) soil type porosity [cm³/cm³] infiltration velocity [cm/day] vw [ms/m] cementation exponent saturation exponent water table depth [cm] time [day] sand 0.375 4.010 5 1.4 2 200 4 silt 0.489 1.688 5 1.4 2 200 4 clay 0.459 1.603 5 2.5 2 200 4 table 2. simulation parameters. where the solution is calculated. hydraulic head is an equivalent measure of the pressure expressed in height of water column, and it is the sum of pressure head (fluid internal pressure) and elevation head (pressure due to gravity). following a summarizing table 2 of the main simulation coefficients, parameters and physical quantities that were used to determinate the electrical conductivity, permittivity and water content relative to the soil textures object of this paper. the simulation through mathematical models of the wc profiles has been reported, respectively for the most common texture soils types (figures 4, 5, 6). the picture obtained by the modeling software, shows range of water content up to 50%, and the relative permittivity and electrical conductivity values linked to it, plotted in a 3d space. a time of four days has been chosen because sand soils after that amount of time lose all the water retained in it, and resulted completely dry. due to this quite fast change in swc, the electrical properties are also subject to a rather remarkable variation. so the results of the simulation show how taking a measure on a certain soil texture (after-rain survey) could be misleading if the soil retention is not taken in consideration. from figure 4 it is possible to see how, in a typical clay soil, water content in the topsoil layer changes very little due to its high degree of retention. in the other hand, sand soil (figure 5) with mediumsize texture have a low moisture holding capacity, which results in a seep through the ground downwards. silt take up an intermediate behavior between sand and clay soils (figure 6). topp’s derivated relative permittivity as shown below for clay, sand and silt (figures 7, 8, 9) and table 3 shown an average of relative permittivity values (fr ). porretta and bianchi 6 figure 4. water content section in clay soil textures. figure 5. water content section in sand soil textures. 7 electrical profiles from swc models figure 6. water content section in silt soil textures. figure 7. relative permittivity section in clay soil textures. figure 8. relative permittivity section in sand soil textures. since relative permittivity is an electrical physical characteristic which exclusively depends on inner water content, for high degree of moisture content, relative permittivity in simulation test increase. indeed, clay soils texture are characterized by higher relative permittivity values than sand soils, which generally shows much lower relative permittivity. silt soils, show a range of values between 0 and 24. profiles for the electrical conductivity have been present in according to archie’s second law (equation 19). also in this case, the profiles were obtained simulating a time of four days as shown in figures 10, 11 and 12. since electrical conductivity is linked to total dissolved solids (tds) and increase considerably in function of this quantity. to simplify the model, a fixed total solute concentration has been considered, which results porretta and bianchi 8 figure 9. relative permittivity section in silt soil textures. i [cm3/cm3] fr 0.10 5.343 0.15 7.451 0.20 10.116 0.25 13.281 0.30 16.889 0.35 20.881 0.40 25.201 0.45 29.791 0.50 34.592 figure 10. electrical conductivity section in clay soil textures. table 3. relative permittivity and corresponding swc values according to topp relation. 9 in a typical rain water electrical conductivity value of about 5 ms/m. furthermore, in the specific case of clay model, the contribution of the surface grains or particle conductivity (of about 90.94 ms/m) has been taken into consideration. clay surface’s mineral particles influence considerably the electrical conductivity of this specific texture which results in an increase of the total current flow through soil as shown in figure 10 and table 4. 6. conclusions this paper dealt with the estimation of relative permittivity and electrical conductivity profiles inferred from swc models for different soil textures. as well known the different percentages of the soil components, namely sand, clay and silt, described trough a numerical model allows to solve the richards diffusion equation and determine the swc in dependence of depth and time. such quantity is mainly responsible for the values assumed by the relative permittivity and conductivity along depth and time. because of seldom geophysicists have a prior knowledge of the subsoil electrical parameters even in the case of homogeneous soil’s composition these simulation yields of swc along the vadose zone once recognized some characteristic properties of soil textures. conversely, more commonly, this information can be used in the opposite sense inferring the swc from the relative permittivity and conductivity. the prior knowledge of the permittivity and conductivity modelled profiles could help geophysicists and operators to better interpret the field measurement’s results as in application of landfill monitoring where the soil textures are known. the modelled swc and corresponding electric parameters are useful to choice the suitable techelectrical profiles from swc models figure 11. electrical conductivity section in sand soil textures. figure 12. electrical conductivity section in silt soil textures. niques to be employed and the optimal mathematical inversion algorithm to retrieve the electric profiles. the results of these simulations as reported in tables 3 and 4, and figures 7, 8 and 9 show the strong dependence of the permittivity from the swc. other soil parameters have negligible influence on this quantity. different is the case in which we consider the conductivity. in fact swc dependence is important even if salinity, temperature, etc. are less relevant but still important for soil conductivity. the estimation of this two profiles have relevance especially when the soil textures are well characterised. in such a case this allows to simulate swc through the above discussed model and consequently estimate these two electric quantities in function of depth and time in unsaturated soil. a comparison with others studies carried out previously [zhou et al. 2001, michot et al. 2003, schwartz et al. 2008], showed the validity of the inverse method to convert soil moisture content to electrical resistivity using ert with a modified form of the archie’s law. starting from a given water content, electrical resistivity trend in time can be traced. application in landfill monitoring is one the fruitful application since leachate dispersion can be profitably inferred through the measurements of the soil electric parameters fr and v. references 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(1931). capillary conduction of liquids through porous mediums, physics, 1 (5), 318-333. roth, k., r. schulin, h. fluhler and w. attinger (1990). calibration of time domain reflectometry for water content measurement using a composite dielectric approach, water resources research, 26, 2267-2273. schwartz, b.f., m.e. schreiber, p.s. pooler and j.d. rimstidt (2008). calibrating access-tube time domain reflectometry soil water measurements in deep heterogeneous soils, soil science society of america journal, 72 (4), 917-930. settimi, a. (2011). performance of electrical spectroscopy using a resper probe to measure salinity and water content of concrete and terrestrial soil, annals of geophysics, 54 (4), 400-413. shah, p.h., and d.n. singh (2005). generalized archie’s law for estimation of soil electrical conductivity, journal of astm international, 2 (5), 1-20. telford, w.m., l.p. geldart and r.e. sheriff (1990). applied geophysics, 2nd ed., cambridge university press, cambridge, 792 p. topp, g.c., j.l. davis and a.p. annan (1980). electromagnetic determination of soil water content: measurements in coaxial transmission lines, water resources research, 16, 574-582. tuller, m., and d. or (2005). retention of water in soil and the soil water characteristic curve, in: d. hillel (ed.), encyclopedia if soils in the environment, elsevier. van dam, r.l., b. borchers and j.m.h. hendrickx (2005). methods for prediction of soil dielectric properties: a review, in: proceedings of spie the international society for optical engineering 5794, 188-197. van genuchten, m.th. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 57, 1, 2014, s0189; doi:10.4401/ag-6363 s0189 the seismic microzonation of level 3 of sant’agata fossili (northern italy) based on a multidisciplinary approach giuseppe di capua1,*, massimo compagnoni2, giuseppe di giulio3, marco marchetti1, giuliano milana1, silvia peppoloni1, floriana pergalani2, vincenzo sapia1 1 istituto nazionale di geofisica e vulcanologia, rome, italy 2 politecnico di milano, dipartimento di ingegneria civile e ambientale, milan, italy 3 istituto nazionale di geofisica e vulcanologia, l’aquila, italy abstract in this paper the results of a detailed seismic microzonation, performed at sant’agata fossili (piemonte region, northern italy) are presented. we study the local seismic response of this small village using a level 3, that is the most accurate level following the italian code of seismic microzonation. the activity steps consist in a gradual widening of knowledge of the different aspects of the amplification phenomena. a multidisciplinary approach has been performed to obtain the local seismic response: including a study of local geology, geophysical and geotechnical characterization of the lithologies, and numerical and experimental analyses. we finally compare the obtained elastic response spectra to the prescribed spectra of the italian building code (in italian: norme tecniche per le costruzioni). our results show the geologic and geophysical differences of the subsoil, that produce different local seismic response in terms of amplification factors and acceleration response spectra. 1. introduction the seismic hazard is usually connected to a high building vulnerability and to significant historic values of the urban centres, especially in italy. in the activities finalized to the assessment of the local seismic hazard and the seismic risk, the geological and geophysical studies can be considered as fundamental to perform activities of prevention and reconstruction. the aim of a study of seismic microzonation is the evaluation of the expected local seismic hazard, related to the expected seismic input and to geologic and geomorphologic properties of the subsoil. on the basis of these considerations the piemonte region (northern italy) has performed, in a strategic european project risknat (http://webgis.arpa.piemonte.it/risknat/), a study of seismic microzonation for the small municipality of sant’agata fossili (approximately 500 citizens). this municipality was affected by the low magnitude seismic event of april 11, 2003 (mw=4.9), and has suffered the most damage in the epicentral area, reaching a macroseismic intensity of imcs=vi-vii. starting from the recent “indirizzi e criteri per la microzonazione sismica” (guidelines and criteria for the seismic microzonation) (icms) [gdl ms 2008], a study of level 3 has been carried out. the level 3 consists of an accurate analysis of seismic microzonation based on geophysical surveys and numerical simulation. a multidisciplinary group has been created and the fundamental steps of the project can be summarized as follows: (1) study of local geology; (2) geophysical and geotechnical characterization of the lithologies; (3) numerical and experimental analyses to evaluate the local seismic response. in our study, the geological and geophysical heterogeneities of the subsoil properties have been defined. these heterogeneities can be considered as responsible for differences in the local seismic response, in terms of amplification factors and acceleration response spectra. furthermore, the resulting acceleration response spectra have been compared to those proposed by the italian seismic code “norme tecniche per le costruzioni” (technical standards for construction) [ntc 2008] in order to have a correct article history received may 30, 2013; accepted november 21, 2013. subject classification: seismic microzoning, amplification factors, elastic acceleration response spectra, seismic refraction, electrical resistivity tomography, hvsr, masw, down-hole, italian building code. level of seismic protection in the project design phases. finally, our study gives some results in term of comparison among different geophysical techniques (noise measurements, down-hole tests dh, multichannel analysis of surface waves method-masw, 2d array, electrical tomography). 2. historical and recent seismicity the information on the historical seismicity for sant’agata fossili is scarce. this could be due to the small dimension of the municipality and therefore its little political and economical importance, that can lead to poor consideration in the historical bibliography. an alternative explanation could be the low local seismicity and consequently the absence of damages; probably all the two causes are present. considering the seismic events in the “catalogo parametrico dei terremoti italiani” (parametric catalogue of italian earthquakes: gdl cpti [2004] and rovida et al. [2011]), the area has been hit by low energy earthquakes. particularly the strongest and nearest events are: 1541 (imcsmax=8), 1680 (imcsmax=7), 1828 (imcsmax=8) and 1913 (imcsmax=5) earthquakes, with magnitude between 4.9 and 5.8, producing minor damage in the municipality (figure 1). the recent seismicity shows events characterized by magnitude lesser than 4 and only the event of april 11, 2003 has reached magnitude producing a damage equal to imcs=vi-vii. 3. experimental setting to investigate the subsoil, 19 pre-existent boreholes (with thicknesses between 15 and 20 m) have been collected, 2 new boreholes (s1 and s2, with thicknesses between 40 and 30 m) and 2 standard penetration test (spt) have been performed, with the collection of 6 undisturbed samples to carry out the dynamic and static laboratory tests. different geophysical investigations, as 2 down-hole surveys (dh), 27 noise measurements, 1 passive 2d array, 1 masw, 1 seismic refraction line, 2 electrical tomography, have been conducted (figure 2). the investigations have pointed out a homogeneous lithological sequence, characterized by different thickness of the deposits. the sequence is generally formed by anthropic filling material or eluvial-colluvial deposits approximately 2 m thick over-imposed to silts, sands and clays. below these deposits, marls can be considered as a geologic bedrock. in the northern-western area, gravels and cemented gravels have been found below the anthropic filling deposits or silts. in the central area an artificially stabilized landslide has been recognized. on the 6 samples, collected in the silts sands and clays, the following geotechnical analyses have been performed: classification tests, oedometric tests, shear and resonant column tests. a series of seismic noise measurements have been carried out (figure 2), finalized to obtain information on the resonant frequencies (f0). di capua et al. 2 figure 1. location of the strongest and closest earthquakes to sant’agata fossili. the orange points show the most important towns in the area closest to sant’agata fossili, the squares indicate the epicentres of historical and recent earthquakes, the stars show the epicentre of the april 11, 2003 earthquake. figure 2. location of the existing and new geotechnical and geophysical investigations. 3 the average spectral ratios h/v have been calculated [hvnsr; nakamura 1989], and the curves are represented in figure 3. the results show clear resonance peaks, characterized by a f0 values varying between 2.1 hz and 7.4 hz in the northern and western sector of sant’agata fossili, whereas, in the eastern sector, the hvnsr curves show the absence of clear resonance peaks, indicating likely the absence of a strong vs contrast in the subsoil [ibs-von seht and wohlenberg 1999, delgado et al. 2000a,b, parolai et al. 2002, bonnefoyclaudet et al. 2006, gosar and lenart 2010, benjumea et al. 2011]. in these two identified sectors, two dh (s1 and s2) have been performed and the results in term of vp and vs velocity profiles are reported in figure 4 (location in figure 2) together with the borehole stratigraphy. the vs velocity profiles show some differences: s1 is characterized by a gradually increment of the vs with depth, and the seismic bedrock (vs > 800 m/s) is reached at a depth of about 40 m (eastern sector), whereas s2 shows an high increment of vs at the contact between the silts and marls (depth 11 m), a second velocity contrast is present in the lower part of the profile at a depth of approximately 30 m (western sector). in the area (blue polygon in figure 2) between the sectors characterized by the different behavior of vs profile, some integrative investigations have been performed, finalized to the study of the limit between the two sectors; particularly a 2d small-aperture array recording seismic noise and a 1d array of 72 geophones aimed to perform a masw analysis [park et al. 2005], using both active (minigun-like) and passive source to derive surface-wave dispersion curves. the hvnsr curves of the 11 stations of the 2d array indicate a f0 varying spatially, in a short distance, from 4 hz to 6 hz. the inversion of surface-wave dispersion curves, measured with the 2d and with the 1d arrays, allows to derive a vs profile. however, the surface wave analysis the seismic microzonation of sant'agata fossili figure 3. location of the seismic noise measurements. the h/v curves and the resonant frequencies are reported. in the gray polygon, in the northern area, the passive measures in array configuration have been performed. indicate a heterogeneity of the vs between the western and eastern sectors of the array area (figure 5), confirming also the variability of the hvnsr curves. summarizing, it is possible to notice from the two vs profiles of figure 5 a first thin layer with a vs of about 100 m/s; then a second layer characterized by different vs and thickness values in the two sectors: western sector of the array area with a thickness of about 10 m and vs of about 340 m/s, eastern sector with a thickness of about 15 m and vs of about 480 m/s and a third layer characterized by a vs greater than 800 m/s. in the studied area 2 electrical tomography lines (figure 6, location in figure 2) have been conducted: the line x-x’ shows the presence of 3 intervals of resistivity values: 5-15 ohm*m, 15-25 ohm*m and 25-45 ohm*m and the line y-y’, performed on the stabilized landslide, shows a large resistivity contrast between surface layers (50-80 ohm*m) and the bottom layers (40 ohm*m) and indicates that the landslide thickness is about 8-10 m. summarizing, on the basis of borehole stratigraphies and results of geophysical measurements, we have identified 4 sectors for the studied area (figure 7). sector 1 in the eastern side of the village shows nearly flat hvnsr curves. this agrees with the results of the dh-s1, which show increasing shear velocity with depth. the vs profile does not indicate a strong velocity contrast between the anthropic filling material and silt deposits and the underlying clayey marl. the lack of a high contrast can be noticed, considering the results of the electrical tomography (thickness of 10-12 m). in sector 2, the noise measurements indicated a clear resonance peak with f0 varying from 3.6 to 4.9 hz. the dh-s2 in this sector shows a thickness of 12 m for a low-velocity deposit (vs < 200 m/s) overlaying a stiffer basement and the electrical tomography shows that this deposit can be correlated to a low resistivity layer (5-45 ohm*m). sector 3 is characterized by the presence of conglomerates outcropping in the northwest portion of di capua et al. 4 figure 4. vp and vs velocity profiles obtained by down-holes in the s1 and s2 boreholes. the different colours identify the man-made deposits and the eluvial-colluvial deposits, the silts and clays as well as the marls. figure 5. inversion of surface-wave dispersion curves, measured with the 2d and with the 1d arrays. 5 the village with a resonance frequency between 2.1 and 3.6 hz. sector 4 is in the central part of the village where a stabilized landslide is present. in sector 4 the two noise measurements show f0 of 7.4 and 5.2 hz, considering the vs values of 200 m/s as characteristic of the landslide material, these values allow to define the thickness of the landslide to about 8 m. the landslide thickness is consistent with the electrical tomography data (figure 6). in conclusion, the comparison of the results shows a good agreement among the different investigations, allowing a valid reconstruction of the subsoil. 4. geolithological models and sections the elaboration of all data (geologic, geophysical and geotechnical) give, as mentioned, 4 main geolithological sectors (figure 7) and different lithological models with the individuation of the geophysical characteristics of each layer. we derive several geo-lithological sections (figure 8), used in the 1d numerical analyses of the local seismic response. the layers consist in man-made and eluvial-colluvial deposits (1-2 m) over-imposed to silts and clays and locally gravels, while the bedrock is formed by marls and, in one area (sector 3), conglomerates. the silt and clay deposits show different characteristics in the 4 sectors: (1) in sector 1 the silt and clay deposits have vs of 250 m/s (figure 4) and a unit weight of 20 kn/m3. in this sector the h/v curves are characterized by a flat behaviour (figure 3); (2) in sectors 2, 3 and 4 the vs value of the deposits is 200 m/s (figure 4) and the unit weight is 19 kn/m3. the contrast of the vs between the deposits and the bedrock is significant, as shown by the clear resonance peaks in the h/v curves in the range 4-5 hz (figure 3). the marls present a different behaviour of the vs, as deduced by the results of the dh, considering the the seismic microzonation of sant'agata fossili figure 6. results of the electric tomography for the 2 analyzed sections. location in figure 2. in the section y-y’, it is traced the possible sliding surface of the landslide and is reported the stratigraphy of the closest borehole. figure 7. areas of the municipality where groups of geophysical investigation, using different techniques, have been performed. two sectors 1 and 2 (figure 7): (1) in sector 1 the s1 survey shows that marls are characterized by a gradual increase of the vs values; the unit weight is about 20-21 kn/m3. the bedrock depth is reached at about 35 m; (2) in sector 2 and 4 the marls are compact with a thickness of 17 m and vs equal to 700 m/s and a unit weight of 21 kn/m3 in the first layer, then the marls reach the vs values of 800 m/s. in sector 3 the sequence is characterized by silts and clays with a thickness of 3 m, over-imposed to cemented gravels and conglomerates characterized by vs values of 600 m/s and unit weights about 21 kn/m3, over-imposed to bedrock (conglomerate). the dynamic geotechnical tests have given the curves of the behaviour of the normalized shear modulus (g/g0) and the damping ratio (d) relating to the shear stain (γ%) for the silt and clay deposits (figure 9). the curves relating to the sample of depth 4.0-4.5 m are applied to the silts and clays of the sector 1, the curves relating to the sample of depth 9.0-9.5 m are applied to the silts and clays of the sectors 2, 3, 4 and to marls. the curves for gravels are taken from the scientific literature [rollins 1998]. in the sector 4, characterized by a stabilized landslide, the subsoil model takes into account the lithological and geophysical characteristics discussed in the previous paragraph. di capua et al. 6 figure 8. geolithological sections (location in figure 2): note the man-made and the eluvial-colluvial deposits, the silts and clays, and the geologic bedrock formed by marls. figure 9. behaviour of the g/g0 and d correlating to γ. 7 5. numerical analysis the numerical and experimental analyses are finalized to the evaluation of the expected seismic response in term of elastic acceleration response spectra and amplification factors, in order to derive a seismic microzonation map of level 3 [gdl ms 2008]. the numerical analyses, as prescribed by level 3 of microzoning, require realistic accelerograms and acceleration response spectra as seismic inputs. based on the seismic hazard of italy [gdl mps 2004] the expected maximum acceleration values (amax) and the values of the spectral ordinates of the acceleration response spectra are calculated for different return periods within the national territory. the return period chosen is 475 years, considered as conventional for the local seismic response analysis. the values of amax for the piemonte region are between 0.036 g and 0.150 g; for sant’agata fossili the closer 4 points of the grid to the coordinates (in latitude and longitude) of the municipality have been selected: the obtained amax is the average of the 4 points with the inverse of the distance between the municipality point and the grid points and it is 0.096 g. [gdl mps 2004]. recorded accelerograms have been extracted from the database itaca [luzi and sabetta 2006]: following the rules prescribed by the ntc [2008]. in particular for the selection of the seismic input we consider the: (1) historical seismicity and seismogenetic characteristic (strike fault) [diss working group 2010]; (2) couple magnitude-distance (from the data in gdl mps 2004, the characteristic event has a magnitude between 4.5 and 5.0 and a distance between 0-10 km); (3) maximum expected acceleration (0.096 g). only the accelerograms recorded on bedrock have been selected (subsoil category “a”; ntc [2008]). the selected accelerograms have been scaled in order to have their acceleration peak similar to the expected amax. the characteristics of the 5 accelerograms are reported in table 1, and are namely: the code of the station, the latitude and longitude of the station, the distance of the station from the epicentre, the name of the event, the name of the station, the component of the motion, the lithology of the station, and the peak ground acceleration (pga). the accelerograms are plotted in figure 10. the geologic and geomorphologic analyses of the area show the presence of horizontal layers and the presence of a ridge, so we use a mono-dimensional (1d) code [idriss et al. 1992] for the study of the lithological amplifications and a two-dimensional (2d) code [callerio et al. 2000] for the study of the topographic amplifications. in the 1d code the soil profile is idealized as a system of homogeneous, visco-elastic sub-layers of infinite horizontal extent. the response of this system is calculated considering vertically propagating shear waves. the bedrock is considered deformable, to avoid the reflection of waves into the model, in fact a rigid layer reflects all the reflected waves from the surface, instead in the case of the deformable layer, the waves are spread into the bedrock. the code adopts the equivalent linear analysis using an iterative procedure to obtain the characteristics of the soil compatible with the effective strain in each layer in each iteration. therefore, the process is iterative and the code works in the frequency domain, using the fourier analysis. on the basis of the total amount of the geological, geotechnical and geophysical data and of the 5 geo-lithological sections (figure 8), 18 1d geolithological columns have been pointed out (figure 11). the local seismic response has been computed for each column, using the 1d code and applying the 5 input accelerograms, g/g0 and d curves. the results are expressed as the average of the 5 analyses in terms of elastic acceleration response spectra (using a 5% of the critical damping) and in terms of amplification factors fa0.1-0.5 and fa0.5-1.5. these parameters were calculated considering the ratio of the elastic velocity the seismic microzonation of sant'agata fossili code lat. (°) long. (°) epicentral distance (km) earthquake station comp. lithology pga (g) pnr 44.877 7.344 16.0 val chisone pinerolo n-s rock 0.044 nzz 44.782 8.357 6.0 zona alessandria nizza monferrato w-e rock 0.090 trt 44.892 8.882 17.0 oltrepo pavese tortona n-s w-e rock rock 0.042 0.086 figure 8. geolithological sections (location in figure 2): note the man-made and the eluvial-colluvial deposits, the silts and clays, and the geologic bedrock formed by marls. response spectra between the integral of output and input in the period ranges between 0.1-0.5 s and 0.51.5 s [pergalani et al. 1999]. the obtained average acceleration response spectra have been compared with the response spectra prescribed by the national code [ntc 2008], assuming the correspondent category of subsoil for each analysed column (using the vs30 value). to perform this comparison, the calculated spectra have been adapted to the behaviour of the code spectra (using di capua et al. 8 figure 10. accelerograms used in the numerical analyses of the local seismic response. figure 11. location of the 18 lithostratigraphic columns used for the analyses of local seismic response 1d. 9 the procedure in gdl ms [2008]) and characterized by a segment of constant spectral acceleration, a segment of constant spectral velocity and a segment of constant spectral displacement. in figure 12 the graphs are reported. some analysed columns show the same response spectra, so they have been joined into one graph. as shown, some national code spectra are lower than the 1d calculated spectra, particularly for low periods. the results in terms of average amplification factors are reported in table 2. the fa0.1-0.5 values are included between the range of 1.5-2.2 and the fa0.5-1.5 values are included between the range of 1.1-1.6, typical values of a medium-high amplification. the 2d analyses were performed using a boundary elements method. the method considers, with linear segments, only the boundary of the real structure, reducing the computational time, using the green function and a linear elastic analysis. the structure is characterized by an elastic and homogeneous material. the code considers a continuous displacement field and works in the frequency domain applying fourier’s transformation to the motion equation. the topographic amplifications due to the presence of a ridge have been calculated by the application of a 2d code. the profile of the ridge is characterized by a width (l) of 1700 m and height (h) of 170 m. this morphology can be classified as a pointed ridge (ratio h/l equal to 0.1). in the numerical analysis a homogeneous, elastic bedrock characterized by a vs of 800 m/s and a unit weight of 22 kn/m3 has been considered. the average response spectra and the average amplification factors do not show morphological effects. 6. conclusions the level of seismic hazard of sant’agata fossili was computed for homogeneous sectors of the municipality, based on geological and geophysical investigations. the values of the amplification factors can be used in planning, for identifying the areas in which the mitigation actions are necessary and the areas suitable for new constructions. the acceleration response spectra can be used during the project design for new buildings the seismic microzonation of sant'agata fossili figure 12. acceleration response spectra of the analyzed columns (location in figure 11) and relative national code spectra. and for the evaluation of the safety of existent buildings. on the basis of the numerical and experimental analyses of the local seismic response, the seismic microzonation map of level 3 has been performed, in which the municipality has been subdivided in 4 main sectors (figure 13). in particular, for each sector the different response spectra are reported and the different sectors show different behaviour. in general, the results of the numerical analyses are higher than the relative national code spectra. only in the sector 3, in presence of the cemented gravels, the amplification phenomena are negligible. the values of the average amplification factors (fa0.1-0.5 and fa0.5-1.5), associated to each sector, are reported in table 3; also in this case, the variability of the amplification factors is evident. only in sector 3, the values of the amplification factors are equal to 1. the hvnsr results show clear resonance peaks, characterized by f0 values between 2.1 hz and 7.4 hz in the northern and western sector of sant’agata fossili, whereas, in the eastern sector, the hvnsr curves show the absence of clear resonance peaks. in any case, the interaction between soil resonance and building resonance effects needs further investigation, considering the height of reinforced-concrete or masonry buildings and of 2 prevailing storeys, that have likely frequencies > 7 hz [ntc 2008]. in conclusion, an integrated approach based on geological and geophysical data allows investigating the heterogeneities in the subsoil properties, also considerdi capua et al. 10 pa1 pa2 pa3 pa4 pb2 pd1 pb3 pc3 pc2 pd2 pe1 pe2 dh1 pc1 pe3 pe4 dh2 pb1 fa0.1-0.5 2.12 1.04 1.98 2.19 2.06 1.55 1.46 1.94 2.15 1.75 2.10 fa0.5-1.5 1.26 1.01 1.55 1.19 1.40 1.12 1.10 1.13 1.27 1.16 1.36 table 2. average amplification factors of the analyzed columns, considering the ratio of the elastic velocity response spectra between the integral of output and input in the period ranges between 0.1-0.5 s and 0.5-1.5 s. figure 13. seismic microzonation map of level 3 of sant’agata fossili. for each sector, similar response spectra as presented in figure 12 have been jointed in one graph. 11 ing a small spatial scale of investigation. this study shows that, in urban contexts, the seismic microzonation analyses are valid and that they offer an instrument of knowledge of the territory, used for the seismic prevention of buildings, if the seismic vulnerability is known. in fact a realistic evaluation of the expected amplifications is necessary to perform the reduction of the seismic risk and the safety of the buildings. acknowledgements. this project has been developed by the financial contribution of regione piemonte and by the technical contribution of mr. riccardo conte. references benjumea b., a. macau, a. gabàs, f. bellmunt, s. figueras and j. cirés (2011). integrated geophysical profiles and h/v microtremor measurements for subsoil characterization. near surface geophysics, 9. bonnefoy-claudet s., f. cotton and p.y. bard (2006). the nature of noise wavefield and its applications for site effects studies. a literature review. earth-science reviews, 79, 205-227. callerio a., v. petrini and f. pergalani (2000). elco, a program for two dimensional analyses using boundary element method, technical report, irrs, milano. delgado j., c. lòpez casado, a. estévez, j. giner, a. cuenca and s. molina (2000a). mapping soft soils in the segura river valley (se spain): a case studi of microtremors as an exploration tool. journal of applied geophysics (45), 19-32. delgado j., c. lòpez casado, j. giner, a. estévez, a. cuenca and s. molina (2000b). microtremors as a geophysical exploration tool: application and limitation. pure and applied geophysics (157), 1445-1462. diss working group (2010). database of individual seismogenic sources (diss), version 3.1.1: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas, http://diss.rm.ingv.it/diss/. gdl cpti (2004). catalogo parametrico dei terremoti italiani, versione 2004 (cpti04). ingv, bologna, http://emidius.mi.ingv.it/cpti04/index.html. gdl mps (2004). redazione della mappa di pericolosità sismica prevista dall’ordinanza pcm 3274 del 20 marzo 2003. rapporto conclusivo per il dipartimento della protezione civile, ingv, milano-roma, aprile 2004, 65 pp.+5 appendici, http://zonesismiche.mi.ingv.it/ (in italian). gdl ms (2008). indirizzi e criteri per la microzonazione sismica. conferenza delle regioni e delle province autonome-dipartimento della protezione civile, roma, 3 vol. e dvd (in italian). gosar a. and a.lenart (2010). mapping the thickness of sediments in the ljubljana moor basin (slovenia) using microtremors. bulletin of earthquake engeneering (8), 501-518. ibs-von seht m., and j. wohlenberg (1999). microtremor measurements used to map thickness of soft sediments. bulletin of the seismological society of america , 89 (1). idriss i.m. and j.i. sun (1992). user’s manual for shake91, a computer program for conducting equivalent linear seismic response analyses of horizontally layered soil deposits, report of dip. of civil and environmental engineering, university of california, davis. luzi l.and f. sabetta (2006). data base dei dati accelerometrici italiani relative al periodo 1972-2004, istituto nazionale di geofisica e vulcanologia 2006. nakamura y. (1989). a method for dynamic characteristics estimation of subsurface using microtremor on the ground surface: quarterly report of the railway technical research institute of japan, 30, no. 1, 25-33. ntc (2008). norme tecniche per le costruzioni, dm 14 gennaio 2008, gazzetta ufficiale n. 29 del 4 febbraio 2008, supplemento ordinario n. 30. istituto poligrafico e zecca dello stato, roma (www.cslp.it) (in italian). park c.b., r.d. miller, n. ryden, j. xia and j. ivanov (2005). combined use of active and passive surface waves. journal of engineering and environmental geophysics (jeeg), 10, (3), 323-334. parolai s., p. bormann and c. milkereit (2002). new relationships between vs , thickness of sediments, and resonance frequency calculated by the h/v ratio of seismic noise for the cologne area (germany). bulletin of the seismological society of the seismic microzonation of sant'agata fossili sector 1 sector 2 sector 3 sector 4 fa0.1-0.5 1.7 2.1 2.1–1.0 2.2 fa0.5-1.5 1.2 1.4 1.3–1.0 1.2 table 3. average amplification factors for each geolithological sector. america, 92 (6), 2521-2527. pergalani f., r. romeo, l. luzi, v. petrini, a. pugliese and t. sanò (1999). seismic microzoning of the area struck by umbria-marche (central italy) ms 5.9 earthquake of the 26 september 1997. soil dynamics and earthquake engineering, elsevier science, vol. 18, 4, 279-296. regione piemonte, progetto risknat, http://www.risknat-alcotra.org/rna/index.cfm/b6.html (in italian). rollins k.m. (1998). shear modulus and damping relationship for gravels. journal of geotechnical and geoenvironmental engineering. rovida a., r. camassi, p. gasperini and m. stucchi (eds.) (2011). cpti11, the 2011 version of the parametric catalogue of italian earthquakes. milano, bologna, http://emidius.mi.ingv.it/cpti, doi: 10.6092/ingv.it-cpti11. *corresponding author: giuseppe di capua, istituto nazionale di geofisica e vulcanologia, rome, italy; email: giuseppe.dicapua@ingv.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. di capua et al. 12 vol49_1_2006def 209 annals of geophysics, vol. 49, n. 1, february 2006 key words lai – inversion – polder – hymap – multi-angular 1. introduction the lsa saf project is part of the ground segment for the eumetsat missions meteosat second generation (msg) and european polar system (eps), developed by the esa. our aim is to develop robust and operational algorithms for retrieving vegetation parameters from the synergistic use of seviri/msg and avhrr-3/eps. these instruments will offer innovative angular capabilities for determining vegetation products over europe and africa thanks to concomitant multiple viewing and illumination geometries (van-leeuwen and roujean, 2002). remotely sensed brf data are the only way to monitor the vegetation on a global scale. traditional approaches rely on the exploitation of the reflectance variations in the spatial, temporal and spectral domains, in lack of directional information. however, observations demonstrate that the anisotropic behaviour of the surface is an important property and the absence of angular information inevitably produces a bias in the parameter evaluation (roujean et al., 1992; leblanc et al., 1997). our main concern is to retrieve fractional leaf area index (lai) corrected from the effects of the sun-target-sensor geometry. linear mixture is the basis hypothesis in some simple brdf (bidirectional reflectance distribution function) models (e.g., geometricaloptical or kernel-driven models) for simulation of scene reflectance or in remote sensing algorithms, e.g., spectral mixture analysis (sma). however, this technique is ineffective to address the presence of multiple scattering, surface anisotropy and the scaling processes (qin and retrieving leaf area index from multi-angular airborne data francisco javier garcía-haro, fernando camacho-de coca and joaquín meliá department of thermodynamics, university of valencia, spain abstract this work is aimed to demonstrate the feasibility of a methodology for retrieving bio-geophysical variables whilst at the same time fully accounting for additional information on directional anisotropy. a model-based approach has been developed to deconvolve the angular reflectance into single landcovers reflectances, attempting to solve the inconsistencies of 1d models and linear mixture approaches. the model combines the geometric optics of large scale canopy structure with principles of radiative transfer for volume scattering within individual crowns. the reliability of the model approach to retrieve lai has been demonstrated using data from daisex99 campaign at barrax, spain. airborne data include polder and hymap data in which various field plots were observed under varying viewing/illumination angles. nearly simultaneously, a comprehensive field data set was acquired on specific crop plots. the inversions provided accurate lai values, revealing the model potential to combine spectral and directional information to increase the likely accuracy of the retrievals. in addition, the sensitivity of retrievals with the angular and spectral subset of observations was analysed, showing a high consistency between results. this study has contributed to assess the uncertainties with products derived from satellite data like seviri/msg. mailing address: dr. francisco javier garcía-haro, department of thermodynamics, university of valencia, c/dr. moliner 50, 46100 burjassot, valencia, spain; e-mail: j.garcia.haro@uv.es 210 francisco javier garcía-haro, fernando camacho-de coca and joaquín meliá gerstl, 2000). most brdf one-dimensional models imply surface homogeneity within an image pixel and are, therefore, limited to address mixed landcovers pixels, which are common in coarse resolution satellites. different hybrid models have been developed in recent years to describe the radiation regime in forest canopies (li and strahler, 1992; chen and leblanc, 1997). these models assume a medium consisting of gaps and regions idealized by a turbid volume with a foliage density of small leaves. in the geometric-optical approach, an overlap calculated from the shape of the crowns allows the estimation of the proportion of shadows cast as a function of view direction relative to the hot spot direction. more recently, the ghost model (lacaze and roujean, 2001b) was developed to address the local scale angular structure of the hot spot, which can be judged relevant for patch and regional scales. although it was specifically designed to simulate the brdf of boreal forests, it is also suitable to study simpler vegetation canopies like crops and pasture (lacaze et al., 2002). this study proposes an operational approach, namely disma (directional sma), to deconvolve the angular reflectance into single landcovers reflectances, attempting to solve the inconsistencies of 1d models and linear mixture approaches. the model formalism and the volume scattering formulation are similar to the ghost model. however, the between-crown gap probability is formulated in terms of the fractional vegetation cover ( f cover) and a geometric variable (h) associated with the shape of plants. a random spatial distribution of plants is assumed to simplify its computation. the invertibility of the model to retrieve lai was demonstrated using airborne polder and hymap multi-angular measurements corresponding to cropland. the next section describes the model formulation. the inversion algorithm is presented in section 3. the model is validated in section 4. finally, the conclusions and future prospects are presented in section 5. 2. model formulation the scene consists of subcanopies idealized by geometric elements superposed on a flat background. the reflectance of an individual pixel is assumed to consist of an area weighted linear combination of the soil and vegetation contributions. the vegetation reflectance is expressed as the sum of single scattering (ss) and multiple scattering (ms) reflectances (hapke, 1981; lacaze and roujean, 2001b) (2.1) where leaf albedo ~ is the sum of the leaf reflectance (t) and transmittance (x), ns, v= cosis, v, p is the phase angle (i.e. the angle between the sun and the viewer), and the factors iss and ims model the proportion of radiation flux which is single/multiple scattered by foliage elements on the downgoing and outgoing optical pathways as a whole, h(ns, v) is the chandrasekhar function for multiple scattering (hapke, 1981) and p(p) is a turbid medium phase function (ross, 1981; lacaze and roujean, 2001b). the ghost model relies on the coupling between a simple hot spot formula (roujean, 2000) and the gfunction that describes the canopy geometry. in our model, g controls only the volume component depending on the within-crown element distribution, whereas the external geometric component depending on the crown shape and dimension is evaluated using an average theory of the gap probability. we assume that the geometrical component and the volume component of the radiation fluxes can be decoupled (2.2) the volume single/multiple scattered component can be described using beers’ law (2.3) where ωe is the clumping index of the shoots, which quantifies the level of foliage aggregation within the tree crown and is generally dependent on the view angle (kucharik et al., ( / ) ( / ) exp lai exp lai i g i g 1 1 1 1 , vol , vol ss e c ms e c = = c c ∆ ∆ ω ∆ ∆ ω l l 6 6 @ @ . i i i i i i , vol , geo , vol , geo ss ss ss ms ms ms $ $ = = $= ( ) ( )r p i h h i 4 1 1v s v ss s v ms+ ~ n n p n n_ i 6 @# 211 retrieving leaf area index from multi-angular airborne data 1997); ωe = 1 means random foliage distribution and ωe < 1 means clumped foliage; lai is the total canopy overstorey leaf area index; gc denotes the f cover and c is a band-specific factor, which was assumed to be dependent on the leaf transmittance (e.g., bégué, 1993). assuming plants with similar foliar density in the scene, lai/gc represents a mean value of the lai of individual plants. this term is more pertinent to describe crown trees transparency than total lai. ∆ denotes the (bidirectional) normalised extinction coefficient for singly scattered radiance. the model adopts the analytical expression derived by roujean (2000) to the hot spot effect, i.e. coupling the downgoing and outgoing optical pathways (2.4) where g is the well-known ross function (ross, 1981). for the multiple scattered component the hot spot phenomenon is ignored (qin and goel, 1995). the geometric component of the single scattering iss, geo is determined by the between-crown light penetration and the visibility of illuminated objects. this component is particularly relevant for discontinuous canopies. the downgoing and outgoing terms of the intercepted flux were formulated in terms of horizontal projection of the crown. let p0(i) denote the (between-crown) monodirectional gap fraction, which corresponds to the fraction of soil seen in the direction i (nilson, 1971). for homogeneous poisson distribution, the probability of observing the ground under the tree crowns in any given pixel approaches to p0(is) = (1−gc)(hs+1) (jasinski and eagleson, 1989), where h is defined as the ratio of ground projected shadow to plant area. it absorbs all the geometric factors that relate canopy area to shadowing area into only one variable. its analytical expression for the most common geometrical bodies is provided in jasinski and eagleson (1990). the probability of having sunlit ground/ crown (pig, c) and viewed ground/crown (pvg, c) can be expressed as follows: cos g g g g g g 2 s s v v s s v v s s v v 2 2 2 2 = + = + n n n n p n n ∆ ∆l (2.5) a functional relationship can also be found between subpixel shaded ground psg and fractional vegetation cover (2.6) this equation is applicable at large sampling scales, when imaging stands resolutions greater than the size of the tree crowns, as confirmed in a preliminary analysis using simulated data (garcía-haro et al., 2002). the probability of observing sunlit crown when pic and pvc are not correlated is simply the product of both probabilities pic pvc, where pic controls the amount of light intercepted by crowns and pvc controls the contribution of visible crowns to the scene radiance. however, hot spot kernels are necessary to account for the correlation between the two gap probabilities along sun and view directions (chen and leblanc, 1997; qin and goel, 1995). we assume that the hot spot has a minor influence on the multiple scattered interception at a crown level, i.e. ims, geo = pic pvc, but introduce a hot spot kernel fc to modulate the dependence between the optical paths for the single scattered radiation (2.7) where fc is usually obtained from the overlap function between viewing and illuminated shadows as projected on the background (li and strahler, 1992; schaff et al., 1994). finally, the soil contribution rs is expressed as the product of the background bidirectional reflectance cs and the vegetation transmittance t. the vegetation transmittance is the sum of the probability that a solar ray beam will reach the ground without intercepting any crown tgeo plus the probability of intercepting a crown without hitting any foliage element (1–tgeo)tvol, i.e. t = tgeo + (1 – tgeo)tvol. the following expressions were considered for the transmissions: i p p p p p f, geoss ic vc ic ic vc c= + -6 @ ( ) .p g g1 1 ( )sg c c 1s= +h ( ) ( ) ( ) ( ) . p p p p p p p p 1 1 vg v ig s vc v ic s 0 0 0 0 = = = = i i i i 212 francisco javier garcía-haro, fernando camacho-de coca and joaquín meliá (2.8) where fg is a hot spot kernel. it tends to zero as the sun and view directions are far apart (p>> 0o), i.e. when the viewer sees the sunlit ground through a gap different from that of illumination. the hot spot gradient function also decreases as the view zenith angle approaches a horizontal view perspective (iv → 90o) and as the leaf density decreases. in this work we have used hot spot kernels fg and fc similar to the hot spot function proposed by white et al. (2001), which match the hot spot region measured in many boreas sites (leblanc et al., 1997). our model neglects side scattered diffuse radiation incident on ground surface after multiple scattering with leaves of neighbouring plants, which can be significant over a bright background. the model is thus less accurate at near-infrared wavelengths at which multiple scattering in plant canopies is the strongest within the solar spectrum. considering also the presence of skylight irradiance, the reflectance can be expressed as a sum of the direct and isotropic diffuse components, where the component of diffuse reflectance is calculated as the average of the measurements in the principal plane. in addition, the model allows us to derive other parameters like the entire brdf distribution, albedo or absorptance, including the relative contribution of vegetation and soil. 3. model inversion the inversions are achieved simultaneously for all spectral bands, i.e. by coupling the spectral and directional data available. the variables to be retrieved are lai and f cover. without a priori information on these variables, the inverse problem typically consists in determining the optimal set of variables that minimizes the distance between observations and modelled values. given brdf values ri (i = 1,..., n), representing the conditions of observation (i.e. wavelengths and view and illumination geometries), the retrievals are performed by comparing observed and modelled yi (i = 1,..., n) ( / ) ( ) exp lait g t p p p p p f vol geo e c ig vg ig ig vg g = = + + c∆ ω brdf’s. the comparisons are evaluated for a full set of prescribed canopy realisations (fcover, lai) that cover a range of expected natural conditions. those pairs for which the canopy model generates inputs comparable with measured data within the limits of accuracy, i.e. (3.1) are considered acceptable solutions, where w is the covariance matrix of the measurements, which accounts for both data and model uncertainties. the distribution of solutions defines a domain for f cover and lai around the «true» values. weighted mean values of f cover and lai averaged over the set of acceptable solutions are taken as solutions. this procedure not only increases the numerical stability of the inversion but also enables us to derive the uncertainty and correlation of derived parameters. although secondary model parameters (lad, clumping, plant dimensions) can be optimised based on prior expected values for each different cover type, in this study they were assumed constant in order to represent a common situation found in global studies (i.e. absence of accurate land cover classification, presence of mixed pixels, etc.). we must note, however, that prior information about the most probable solution values, their uncertainty and their correlation may significantly improve the accuracy of the retrievals, especially for ill posed inversion problems (combal et al., 2002). in order to reduce the computations, we used an automatic method to select the solution bounds. firstly, spectral mixture analysis was applied to obtain f cover from anisotropically corrected reflectance using as input endmembers representing soil and vegetation. secondly, an empirical relationship was used to derive lai from estimated f cover (lacaze and roujean, 2001a). the estimated values for lai and f cover were finally used to define the bidimensional bounds in the solutions domain. the model requires the optical properties of the underlying soil as cs an input. we employed the rpv model (rahman et al., 1993) to extend the brf measured at a few angles to the entire brdf distribution. a variable configuration using two different soil brdfs was used to represent more realistically the influence on reflec( ) ( ) nr y w r yt 1 #-213 retrieving leaf area index from multi-angular airborne data tance of soil moisture and roughness (see fig. 1). although leaf reflectance and transmittance are not equal in many plants and vary greatly between species, it was assumed for simplicity that they are similar. we have proposed a method to estimate the leaf albedo ~ from the data themselves. the method relies on the assumption that pixels presenting a negligible contribution of the soil background (i.e. f cover = 1 and high lai values) can be found in the scene. in these pixels eq. (2.1) is inverted in order to retrieve ~ using an iterative method. results have indicated the adequate convergence of the method, providing reasonable results irrespectively of the data set considered (fig. 1). 4. validation the model invertibility was evaluated using data acquired during the 1999 digital airborne spectrometer experiment (daisex) campaign by the sensors hymap onboard the dlr do-228 aircraft and polder onboard an arat plane. the experiment site selected by esa for the daisex campaigns is a 4 km by 4 km area centred at 39º3ln, 2º5lw, which is located 28 km from albacete (spain), (camachode coca et al., 2002). the polder instrument allows a measurement of surface reflectance directional effects at nine spectral bands (centred at 443, 500, 550, 590, 670, 700, 720, 800 and 864 nm wavelength) in the visible and near infrared. the ccd matrix permits collection of bidimensional images in one shot. the along track and cross track fov is of ± 43º and ± 51º respectively. four flights were undertaken during 3-5 june 1999 at a typical airborne altitude of 3000 m, with a spatial resolution of 20 m. each flight recorded around 140 spectral images. they provide around 50 values of angular reflectance for every pixel, irregularly distribfig. 1. spectral inputs introduced to disma, which correspond to leaf albedo of green (solid) and dry vegetation (dotted), and reflectance of two different soils. 214 francisco javier garcía-haro, fernando camacho-de coca and joaquín meliá uted in the viewing hemisphere, and different for every pixel. the images were calibrated, geo-coded and corrected for atmospheric effects as it is specified in leroy et al. (2001). in this work the brdf was interpolated for the full range of view angles for every pixel. the brdf was then retrieved considering uniform sites of 3 × 3 pixels (60 × 60 m2). the model was inverted against a set of bidirectional reflectance factors taken along the principal plane (i.e. the region with the strongest anisotropy) and along the orthogonal plane. figures 2a,b and 3a,b show a few examples of measured brf’s along with the values predicted by disma. the results demonstrate the potential of the model to accurately explain the spectral and angular variations of the data. disma captures the essential brdf features such as bowl shape, backscattering increase in reflectance and broad hot spot, as well as the spectral contrast between soil and vegetation. the model works better in the visible regions, when the first order scattering effects predominate but in the nir region (band 8) the discrepancies are higher. in this region directional effects are less apparent due to the reduction of contrast between canopy components and the prevalence of multiple scattering. another source of error is attributable to unquantified influences of foliage clumping. the hymap instrument (http://www.hyvista.com/) has 128 spectral bands and a high fig. 2a,b. comparison of directional signatures in the principal plane measured by polder (symbols) and simulated by disma (dashed line) at five different bands for alfalfa. a) noon image (iv = 17°); b) 14:30 utc (iv = 40°). fig. 3a,b. idem to fig. 2a,b but considering another angular sampling (the orthogonal plane) and another vegetation type (sugar beet). the numbers (2, 3, 6, 7 and 8) refer to the considered polder bands. a b a b fig. 5a,b. angular effects in canopy reflectance of sugar beet associated to the six angular configurations considered in fig. 4a: a) measured hymap data; b) predicted by the model. a b 215 retrieving leaf area index from multi-angular airborne data signal-to-noise ratio. to be able to extract the brdf a complex flight scenario was chosen with data acquisition in two orthogonal flight lines (n-s and e-w) at three different sun geometries: early morning (8:00 utc), solar noon and late afternoon (15:00 utc), thus revealing angular reflectance changes also with illumination angle. hymap data were degraded to polder resolution and convolved with the spectral filters of landsat-5 tm and seviri sensors. figure 4a,b shows two examples of measured brf’s along with the values predicted by disma. we can observe that the model addresses the main directional variations found in the data. figure 5a,b shows another example of the influence of the angular effect on canopy reflectance of discontinuous canopies. we can observe strong differences in the spectral dependence for the different configurations (fig. 5a), which are mainly controlled by the directional changes in the proportion of illuminated components, as it is predicted by the proposed model (fig. 5b). these results are indicative of the ability of disma not only to extract useful properties of the vegetation but also to reduce the uncertainty of the derived products. another point of interest in this analysis was to emphasize the usefulness of the diurnal sampling as an angular signature of the surfaces related with the structure of the vegetation cover. fig. 4a,b. measured reflectance (dashed lines + symbols) and simulated (solid lines) at five different spectral channels over sugar beet (a) and alfalfa (b). configurations 1-3 correspond to the n-s flight and 4-6 to the e-w flight. within each flight line, consecutive numbers correspond to 8:00, 12:00 and 15:00 utc, respectively. the figures show several examples of tm-like wavebands derived from real hymap data. a b 216 francisco javier garcía-haro, fernando camacho-de coca and joaquín meliá vegetation in situ measurements corresponding to major agricultural units were taken during the campaign. the measured properties included lai, f cover, canopy height, biomass and chlorophyll content. the licor lai-2000 instrument was used to measure lai. sampling took place at several times along parallel transects inside the fields (garcía et al., 2001). the comparison between in situ lai measurements and the retrieved values using different spectroangular datasets is shown in fig. 6. a linear relationship was found in all cases, irrespective of the data set considered. table i shows the coefficients of the linear fit, revealing a significantly high correlation, with r2 values higher than 0.92 in all cases. a chi-square (|2) test was performed assuming a lai uncertainty of 0.4. it produced values below the chi-square critical value of 92 in all cases (for 71 degrees of freedom and a probability of 0.05), confirming thus the predicted linear relationship. moreover, results indicate a good correspondence between field measurements and retrievals, since the linear relationship is close to the 1:1 line, and the rmse is relatively low (0.35-0.48). another fig. 6. relationship between field-measured lai and airborne-derived lai using different data sets. the figure at the top corresponds to polder data in the principal (left) and orthogonal (right) plane. different symbols are used to represent corn ( ), sugar beet ( ) and alfalfa (∗).4z table i. statistics of the correlation between in situ measurements and retrievals of lai. data set offset slope rmse r 2 | 2 prob. polder 0.007 1.05 0.406 0.933 0.97 (principal) polder –0.03 1.12 0.481 0.926 0.06 (orthogonal) hymap 0.19 0.89 0.355 0.957 0.05 hymap 0.102 1.02 0.425 0.927 0.40 (tm bands) seviri 0.06 1.04 0.425 0.927 0.29 (tm bands) 217 retrieving leaf area index from multi-angular airborne data important result is that disma is able to produce reasonable results under sub-optimal conditions either in the angular sampling (i.e. taking the orthogonal plane) or with a small number of spectral channels, e.g., seviri wavebands. we must also note that even though the retrieved lai reproduces the between crops lai variability observed in the field measurements very well, it fails to address the within crop variability (e.g., lai retrievals tend to saturate within each individual crop). one reason for this is that pixel-by-pixel comparison is strongly hampered by the inaccuracies in the georegistration of field measurements and imagery. another reason is the averaging process performed to the images. one important aspect is the consistency of the retrievals, i.e. its sensitvity with respect to the set of spectro-angular measurements taken in the inversion. the crosscomparison between results from different data sets showed a high consistency between them, with rmse values typically lower than 0.30 (see table ii). finally, we must indicate that although in situ measurements included only a limited number of f cover sampling, the retrieved values were highly coherent with the available field information. 5. conclusions and prospects this study aimed to develop an operational approach to deconvolve the angular reflectance into single landcovers reflectances, attempting to solve the inconsistencies of 1d models and linear mixture approaches. the model relates the spectral and angular variation with the main optical and structural parameters of discontinuous canopies, like lai. the inversion revealed the model potential to combine spectral and directional information to increase the likely accuracy of the retrievals. results also indicate the effectiveness of the algorithm using only seviri channels and information of the diurnal sampling as an angular signature of the surfaces. we preferred to simplify the selection of secondary parameters and the inversion algorithm with specific intent that it not to be canopy dependent. however, for the application of disma to complex scenarios as in global studies the stratification of scene is convenient in order to optimise the model inputs to the knowledge of ecosystem characteristics, reducing misidentification and saving computations (garcía-haro et al., 2003). one important aspect is the sensitivity of retrieved variables to the information used to parameterise vegetation canopy radiative transfer. although the inversion algorithm was satisfactory, difficulties still arise. for example the estimation of leaf albedo is known to be impaired by canopy-level variables like lai. future research is also needed to test the model on brdf datasets comprising different vegetation types (shrubland, forest). acknowledgements this work was supported by the projects daisex and lsa saf. j. garcía-haro has currently a research position (ramon y cajal) from mcyt, spain. special thanks are due to the j. moreno team for providing us with field measurements. references bégué, a. (1993): leaf area index, intercepted photosynthetically active radiation, and spectral vegetation indices: a sensitivity analysis for regular-clumped canopies, remote sensing environ., 45, 45-59. camacho-de coca, f., f.j. garcía-haro and j. meliá (2002): quantitative analysis of cropland’s brdf anisotropy using airborne polder data, in proceedtable ii. statistics of the cross-comparison of lai as retrieved using different data sets. data set offset slope rmse r 2 | 2 prob. polder 0.05 0.90 0.175 0.994 1 principal versus orthogonal hymap –0.18 1.15 0.308 0.970 0.999 versus polder hymap –0.10 1.14 0.197 0.994 1 versus tm hymap –0.18 1.18 0.223 0.992 1 versus seviri 218 francisco javier garcía-haro, fernando camacho-de coca and joaquín meliá ings of the 1st international symposium on recent advances in quantitative remote sensing, 16-20 september 2002, torrent, spain (universitat de valencia, valencia), 303-308. chen, j.m. and s.g. leblanc (1997): a four-scale bidirectional reflectance model based on canopy architecture, ieee trans. geosci. remote sensing, 35, 1316¯1337. combal, b., f. baret, m. weiss, a. trubuil, a. macé, a. pragnère, r. myneni, y. knyazihin and l. wang (2002): retrieval of biophysical variables from bidirectional reflectance using prior information to solve illposed inversion problems, remote sensing environ., 84, 1-15. garcía, j.c., c. cuñat, f. montero, a. brasa, l. alonso, m.c. gonzález, j.r. ruiz, c. martínez, a. palacios and j. moreno (2001): vegetation soil measurements at barrax (esa publication division sp-499, estec, the netherlands), 79-87. garcía-haro, f.j., f. camacho-de coca and j. meliá (2002): retrieval of biophysical parameters using directional spectral mixture analysis, in proceedings of the 1st international symposium on recent advances in quantitative remote sensing, 16-20 september 2002, torrent, spain (universitat de valencia, valencia), 963-970. garcía-haro, f.j, s. sommer and t. kemper (2003): variable multiple endmember spectral mixture analysis (vmesma), int. j. remote sensing (submitted). hapke, b. (1981): bidirectional reflectance spectroscopy, 1. theory, j. geophys. res., 86, 3039-3054. jasinski, m.f. and p.s. eagleson (1989): the structure of red-infrared scattergrams of semivegetated landscapes, ieee trans. geosci. remote sensing, 27, 441¯451. jasinski, m.f. and p.s. eagleson (1990): estimation of subpixel vegetation cover using red-infrared scattergrams, ieee trans. geosci. remote sensing, 28, 253¯267. kucharik, c.j., j.m. norman, l.m. murdock and s.t. gower (1997): characterizing canopy nonrandomness with a multiband vegetation imager (mvi), j. geophys. res., 102 (d24), 29455-29473. lacaze, r and j.l. roujean (2001a): retrieval of biophysical parameters over land based on polder directional and hot spot measurements, in proceedings of the 8th symposium on physical measurements and signatures in remote sensing, aussois, 487-492. lacaze, r. and j.l. roujean (2001b): g-function and hot spot (ghost) reflectance model. application to multi-scale airborne polder measurements, remote sensing environ., 76, 67-80. lacaze, r., j.m. chen, j.l. roujean and s.g. leblanc (2002): retrieval of clumping index using the hot spot signatures measured by polder instrument, remote sensing environ., 79, 84-95. leblanc, s.g., p. bicheron, j.m. chen, m. leroy and j. cihlar (1997): investigation of radiative transfer in boreal forests with an improved 4-scale model and airborne polder data, ieee trans. geosci. remote sensing, 27, 1396¯1414. leroy, m., o. hatecoeur, f. ponchaut, l. alonso-chorda and j. moreno (2001): the airborne polder data in the daisex’99 campaign (esa publication division sp-499, estec, the netherlands), 13-22. li, x. and r.a. strahler (1992): geometrical-optical modeling of the discrete-crown vegetation canopy: effect of crown shape and mutual shadowing, ieee trans. geosci. remote sensing, ge-30, 276¯292. nilson, t. (1971): a theoretical analysis of the frequency of gaps in plant stands, agric. meteorol., 8, 25-38. qin, w. and s.a.w. gerstl (2000): 3d scene modeling of semidesert vegetation cover and its radiation regime, remote sensing environ., 71, 197-206. qin, w. and n.s. goel (1995): an evaluation hotspot models for vegetation canopies, remote sensing rev., 13, 121-159. rahman, h., m.m. verstraete and b. pinty (1993): coupled surface-atmosphere reflectance (csar) model, part 1. model description and inversion on synthetic data, j. geophys. res., 98, 20,779-20,789. ross, j.k. (1981): the radiation regime and architecture of plants stands (dr. w. junk publishers, norwell, ma), pp. 391. roujean, j.l. (2000): a parametric hot spot model for optical remote sensing application, remote sensing environ., 71, 197-206. roujean, j.l., m. leroy, p.y. deschamps and a. podaire (1992): evidence of surface reflectance bidirectional effects form a noaa/avhrr multitemporal data set, int. j. remote sensing, 13, 685-698. schaff, c.b., x. li and r.a. strahler (1994): topographic effects on bidirectional and hemispherical reflectances calculated with a geometrical-optical canopy model, ieee trans. geosci. remote sensing, ge-32, 1186¯1193. van-leeuwen, w.j.d. and j.l. roujean (2002): land surface albedo from the synergistic use of polar (eps) and geo-stationary (msg) observing systems. an assessment of physical uncertainties, remote sensing environ., 81, 273-289. white, h.p., j.r. miller and j.m. chen (2001): four scale linear model for anisotropic reflectance (flair) for plant canopies, part i. model description and partial validation, ieee trans. geosci. remote sensing, 39 (5), 1072¯1083. microsoft word papandrea_accepted annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332   1   validation of minor species of the mipas2d database enzo papandrea1,*, stefano casadio2, enrico arnone3, massimo carlotti1, elisa castelli3, marta de laurentis2, bianca maria dinelli3 1dipartimento di chimica industriale "toso montanari", università di bologna, italy 2ideas serco s.p.a., frascati (roma), italy 3istituto scienze dell’atmosfera e del clima, isac-cnr, bologna, italy enzo.papandrea@unibo.it i. introduction he mipas2d [dinelli et al., 2010] database has been developed applying the tomographic analysis technique gmtr. (geo-fit multitarget retrieval) [carlotti et al., 2006] to measurements acquired in the nominal observation mode of the complete (2002–2012) mipas (michelson interferometer for passive atmosphere sounding) [fischer et al., 2008] mission. the mipas nominal observation mode is the reference and most commonly adopted mode of observation used in the first (2002–2004) and second (2005–2012) parts of the mipas mission (see sect. ii): although different limb patterns have been adopted in the two periods the database has been developed on a single vertical-horizontal grid. the gmtr algorithm enables the vertical distribution of a variety of atmospheric trace gases to be obtained. mipas2d version 2.3 adopted in this work contains 2-d atmospheric fields of pressure, temperature and vmrs (volume mixing ratios) of h2o, o3, hno3, ch4, n2o, no2, n2o5, clono2, cfc-11, and cfc-12 on a fixed altitude/latitude grid for the full mipas mission. the complete validation exercise of the database is in progress. here we report the comparison of mipas2d data for cfc-11, cfc-12 and clono2 with ace (atmospheric chemistry experiment)-fts (fourier transform spectrometer) [bernath et al., 2005] data1. for the purpose we used ace level 2 data version 2.2+updates. the ace data consists of altitude profiles of temperature, atmospheric extinction and 14 atmospheric trace gas species. this work focuses on chlorine-related products of the mipas2d database because they have outstanding importance in the understanding of key atmospheric processes and were previously considered by papandrea et al. [2010], kiefer et al. [2010] and by arnone et al. [2013]. man-made chlorofluorocarbons (cfcs) have been widely used in the past decades and then replaced with substitutes such as hydrochlorofluorocarbons (hcfcs). transport of these long-lived compounds to the stratosphere leads to their photo-dissociation, with release of chlorine atoms that are strongly reactive and affect the stratospheric chemical                                                                                                                 1 this work has been performed in the frame of the mipas quality working group activities. t annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 2 [2010], kiefer et al. [2010] and by arnone et al. [2013]. man-made chlorofluorocarbons (cfcs) have been widely used in the past decades and then replaced with substitutes such as hydrochlorofluorocarbons (hcfcs). transport of these long-lived compounds to the stratosphere leads to their photo-dissociation, with release of chlorine atoms that are strongly reactive and affect the stratospheric chemical composition. among all the cfcs, the most used ones are ccl3f (cfc-11) and ccl2f2 (cfc-12), with the 2008 mean tropospheric concentrations of 245 and 535 pptv respectively [wmo report nr. 52, 2011]. the annual mean mixing ratio of cfc-11 decreased at a rate of 2.0 ± 0.6 ppt/yr from 2007 to 2008. the global atmospheric mixing ratio of cfc12, which accounts for about one-third of the current atmospheric chlorine load, decreased for the first time in the time span from 2005 to 2008 and by mid-2008 had declined by 1.3 % (7.1 ± 0.2 ppt) from the peak levels observed during 2000–2004. chlorine nitrate (clono2) is a major temporary reservoir gas of chlorine in the stratosphere. it plays an important role in the processes of ozone depletion [brasseur and solomon, 2005, and references therein]. the partitioning between active (ozone destroying) chlorine species like cl and clo and their ozone inactive reservoir gases clono2 and hcl controls the amount of polar ozone depletion through chlorine catalytic cycles. heterogeneous chemistry on polar stratospheric clouds particles converts the chlorine reservoirs clono2 and hcl into cl2. in the sunlit antarctic spring, chlorine molecules are converted into active radicals such as cl and clo, which destroy most of the vortex ozone at 14–20 km. cfc-11 and cfc-12 are diurnally stable molecules, and therefore they are easily comparable with correlative measurements. on the contrary, the species clono2 shows diurnal variability. this means that, when comparing measurements coming from different instruments, the different acquisition time must be kept into consideration. preliminary results are reported here. ii. envisat-mipas data mipas [fischer et al., 2008] was a limbscanning spectrometer developed by the european space agency (esa) for the study of the atmospheric composition deployed on board the envisat satellite on a nearly polar orbit. it measured the infra-red atmospheric emission with limb viewing geometry from 4.1 to 14.5 μm (685–2410 cm−1) mostly over the altitude range 6–70 km. mipas acquired atmospheric spectra from july 2002 to april 2012 with different spectral and spatial resolutions: at 0.025 cm−1 (full resolution measurements) at the beginning of the mission to march 2004 and since january 2005 at 0.0625 cm−1 (optimized resolution measurements). mipas measured both daytime and nighttime with a global coverage of the earth, into a sunsynchronous polar orbit with equatorial local crossing time of approximately 10:00 p.m. (ascending node). in a nominal (nom) orbit of the optimized resolution period, mipas limbscans covered the atmosphere from about 6 to 70 km in 27 views (every 1.5 km from 6 to 21 km, every 2 km from 21 to 31 km, every 3 km from 31 to 46 km, and every 4 km from 46 to 70 km), while in the full resolution period the limb scans were made of 17 views (from 6 to 42 km at 3 km steps then at 47, 52, 60 and 68 km). esa provides level 2 retrievals of the nom mode measurements as described in [ridolfi et annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 3 al., 2000; raspollini et al., 2013]. the esa retrieval code makes use of the global-fit approach [carlotti, 1988]. a collaboration between the university of bologna and the isaccnr of bologna led to the development of an innovative retrieval code for mipas observations, called gmtr [carlotti et al., 2006], using both the geo-fit approach [carlotti et al., 2001] and the multi-target retrieval (mtr) functionality [dinelli et al., 2003]. gmtr performs tomographic retrievals with the possibility to simultaneously retrieve interfering species. the tomographic approach enables the observations collected along a whole orbit to be analysed simultaneously. gmtr operates a 2-d discretization of the atmosphere, therefore enabling the horizontal atmospheric structures to be modelled. [carlotti et al., 2001]. in gmtr each limb observation contributes to determine the unknown quantity at a number of different locations among those spanned by its line of sight rather than only at the tangent points as in standard 1-d retrievals. using gmtr, a level 2 database has been obtained (mipas2d [dinelli et al., 2010]) of 2-d fields of pressure, temperature and vmr profiles of atmospheric constituents. each target has been retrieved at fixed latitudes on an altitude grid that corresponds to the nominal sampling of the mipas full resolution mission (from 6 to 68 km, with 3 km steps up to 42 km and then at the altitudes of 47, 52, 60 and 68 km). the retrievals make use of a priori information that, for the species considered in this paper, is derived from climatological data [remedios et al., 2007] and are operated on selected microwindows, i.e. narrow spectral interval (less than 3 cm-1 wide) containing major information on the target parameters to be retrieved. iii. ace-fts data ace [bernath et al., 2005] is a canadian satellite mission aimed at the study of the earth’s atmosphere using the limb scanning technique. it was launched into a high inclination (74°) circular low orbit (650 km) on 12 august 2003. primarily ace operates in solar occultation providing altitude profiles (typically 10–100 km) for temperature, pressure, and the vmrs for several atmospheric species. the ace instrument is a high spectral resolution (0.02 cm−1) limbscanning fts that measures between 2.2 and 13.3 μm (750–4400 cm−1), with high signal-tonoise ratios, in the altitude range from 5 (or cloud top) to 150 km. the ace orbit gives global earth coverage, useful to study a wide range of atmospheric processes. the ace-fts measures limb sequences of atmospheric absorption spectra during sunrise and sunset. level 2 data are retrieved by a non-linear least squares global fitting technique [carlotti, 1988] that operates on selected microwindows (generally 0.3–1 cm−1) [boone et al., 2005]. the analysis approach does not employ constraints from a priori information (i.e. optimal estimation). first pressure and temperature, as a function of altitude, are determined through the analysis of co2 lines, then vmrs are retrieved. the ace level 2 database used here is version 2.2+updates and is available at http://www.ace.uwaterloo.ca. iv. collocation and filtering criteria the mipas2d and ace-fts datasets obtained in the analysis of the measurements from 2005 to 2009 have been used for this validation exercise. only collocated data have been annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 4 figure 1. mipas2d (red) and ace-fts (blue) geolocations of the coincidences for the polar (830 collocations, left panel) and the trp-midlat (260 collocations, right panel) regions. compared: coincidence criteria were determined on the basis of spatial (by distance) and time limits. for all the considered species spatial collocation was assumed when the maximum distance between mipas2d and ace-fts measurements was ≤ 500 km. the latitude domain has been divided into two regions characterized by different time windows: polar: latitudes < -60° and > 60°, 2 hours time window tropical-mid latitudes (trp-midlat): -60° < latitude < 60°, 6 hours time window the reason for increasing the time window in the trp-midlat is to have a statistically suitable number of coincidences. in total we found 830 collocations for the polar regions and 260 for the trp-midlat regions. we then applied a filter to both mipas2d and ace-fts data in order to rejects single vmr values having an estimated standard deviation greater than 30%. this threshold has been chosen with dedicated tests on the basis of a trade-off between the amount of values kept in the statistics and the quality of the data. in case of multiple coincidence (more than one mipas2d match every ace-fts profile), we averaged the mipas2d results in order to obtain only one profile. the datasets have been compared in the pressure domain instead than in the altitude domain. this choice has been done in order to avoid possible additional mipas2d errors introduced by the different assumptions made by the analysis systems regarding the pressure to altitude transformation. therefore mipas2d profiles were logarithmically interpolated on the pressure levels of ace-fts before the comparison. in the comparison we did not smooth the profiles taking into account the averaging kernels. this is an acceptable approximation because the difference in the vertical resolution of the two datasets is small (about 4 km for ace-fts, 4-5 km for mipas2d), leading to similar smoothing errors. the two panels of figure 1 show the geolocations of the data used in the comparison: the red dots represents the geolocation of mipas2d data and the blue dots the geolocations of ace data. v. results annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 5 figure 2. from left to right: cfc-11, cfc-12, clono2 mipas2d-ace differences computed as (1) for polar (top) and trp-midlat (bottom) regions. in each panel the distribution of the differences is indicated using white circles (mean), star symbols (median), and black dots (mode). the horizontal black lines indicate the 80% percentile. the colour (arbitrary) scale indicates the number density of samples having same difference values for each pressure layer from blue to red in increasing order (grey = no data). fractional differences between the vmrs from mipas2d and ace-fts have been computed using the following expression: diff = 2 (1) and divided into fixed pressure bins for both the polar and the trp-midlat regions and for all the considered species. in the analysis the polar regions have been subdivided into acefts sunrise (sr) and sunset (ss) cases as some differences were observed between them. figure 2 shows the distribution of the frequency of the differences (mean, mode and median) in each pressure interval, for cfc-11, cfc-12, and clono2, computed with expression (1) for both the polar (top) and the trp-midlat (bottom) regions. in order to simplify the interpretation of the plots in table 1 we report the median of the differences (along with the number of samples) at each level of pressure. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 6 table 1: percentage median of the differences between mipas2d and ace-fts. the parenthetical values are the number of coincidences. species cfc-11 cfc-12 clono2 pressure (hpa) polar trp-midlat polar trp-midlat polar trp-midlat 3 12 (10) 6 12 (405) 62 (119) 10 7 (968) 22 (557) 20 7 (2) -95 (2) -19 (443) -7 (528) 0 (1333) 0 (670) 40 45 (141) -1 (170) -13 (913) 0 (595) -2 (1128) -12 (356) 60 33 (463) 1 (349) -5 (864) 5 (446) -2 (743) 0 (117) 80 9 (586) 1 (285) -5 (759) 1 (360) 7 (350) 32 (28) 100 3 (557) 0 (214) -7 (693) -1 (269) 12 (192) 37 (10) 120 1 (653) 3 (212) -5 (780) 0 (268) 37 (68) 27 (7) 150 3 (846) 3 (276) -3 (1012) 0 (359) 42 (44) 12 (4) 200 5 (791) 0 (216) -3 (977) -3 (264) 92 (1) 250 5 (469) 1 (160) -1 (582) -3 (193) 300 5 (353) 5 (111) -3 (420) -5 (128) 400 3 (129) 11 (68) -5 (140) -9 (67) 500 23 (8) 7 (17) 37 (1) -7 (12) table 2: range of the bias (considered as percentage median of the differences in a statistically meaningful pressure range) between mipas2d and ace-fts. species polar (sr) [%] polar (ss) [%] polar (all) [%] trp-midlat (all) [%] cfc-11 3–5 (+) 0–9 (+) 1–5 (+) 0–5 (+) cfc-12 1–5 (-) 3–9 (-) 1–7 (-) 1–9 (-) clono2 (60-20 hpa range) ±2 0–2 (-) 0–2 (-) 0–12 (-) we notice in figure 2 that cfc-12 has a double distribution: one close to zero and the other close to 30-40% (polar) or 40-50% (trp-midlat). this non-physical behaviour is caused by some “anomalous” ace-fts profiles. these vmrs are indeed too low and cause the second distribution around 30-50% when computing the difference between mipas2d and acefts profiles. the occurrence of this problem and the dissimilarity between “normal” and “anomalous” ace-fts values are much higher in case of cfc-12 than in the other species. however, even if this problem causes distortions to the arithmetic average, the median is only marginally affected and the mode is totally unaffected. we point out that hereafter we discuss the biases only in terms of median of the differences therefore our conclusions are almost independent from the side distribution. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 7 we summarize the biases that have been found in each atmospheric region for each target in table 2. in general, considering both sunrise and sunset ace-fts measurements, mipas2d cfc-11 shows a small positive bias (1-5%) between 400 and 100 hpa (8-18 km) in the polar regions and a similar bias (0 to 5%) between 300 and 40 hpa (10-24 km) in the trpmidlat. mipas2d cfc-12 generally shows a negative bias (1-7%) from 400 to 60 hpa (8-21 km) in the polar regions, and a similar one (19%) in the trp-midlat from 400 to 20 hpa (8-29 km). for these molecules the variability is lower than 15 % in the polar regions and lower than 20% in the trp-midlat. ace-fts cfc-11 and cfc-12 have been previously compared with a single firs-2 flight and with zonal mean mkiv balloon data [mahieu et al., 2008]. these comparisons show that the ace-fts vertical distributions are reasonably good for both the species, although they generally seem to be lower in most of the altitude range, i.e. between 12 and 20 km (230–70 hpa). however, this analysis was performed with a limited number of coincidences. in particular, for cfc11, below 16 km (130 hpa) differences were smaller than 10% while above they increased with increasing altitude, up to 87% at 19 km (85 hpa) w.r.t. firs-2, below 12 km (230 hpa) differences were about 20% and above of the order of 10% w.r.t mkiv. for cfc-12, they found below 20 km (73 hpa) a positive bias (up to 50%) when comparing ace-fts with firs2 and a negative bias (up to 160% at 24 km or 40 hpa) above. in the inter-comparison with mkiv the findings were similar to cfc-11 but the entity of the bias was smaller (up to 10%). mipas2d clono2 shows a very small negative bias in the trp-midlat (0-2%) between 60 and 20 hpa (21-29 km) and a larger one in the polar regions (0-12%) between 60 and 20 hpa (21-29 km). clono2 comparison has a larger (> 30%) variability. this is due to its diurnal variability that makes the measured vmr highly dependent from the acquisition time. wolff et al. [2008] reported a good agreement between ace-fts and mipas imk-iaa retrievals. the mean absolute differences they found were typically within ±0.01 ppbv and reached not more than −0.04 ppbv (±1%) for 130–27 hpa (16–27 km). the clono2 differences we are reporting can be mostly explained by the time mismatch. above 20 hpa (29 km), the differences tend to increase with altitude (even if is not possible to draw clear conclusions because of the large variability). a bias at high altitudes has been also observed by höpfner et al. [2007] when comparing mipas imk-iaa retrievals with ace-fts. they successfully removed the bias using a chemical transport model to correct mipas data for the time mismatch demonstrating that the reported behaviour is induced by photochemistry. v. conclusions mipas2d 2-d fields of clono2, cfc-11, and cfc-12, obtained applying the tomographic code gmtr to measurements acquired in the nominal observation mode of the complete mipas mission have been compared with ace-fts data. the comparison was restricted to data measured from 2005 to 2009. mipas2d cfc-11 shows a generally small positive bias ranging from 1 to 5%, cfc-12 a negative bias ranging from 1 to 9% and clono2 a very small negative bias in the tropical-mid latitude (0-2%). this bias becomes larger in the polar regions (0-12%). these annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6332 8 general conclusion are valid within the pressure range 300-100 hpa (10-18 km) for the two chlorofluorocarbons and within the more restricted pressure interval of 60-20 hpa (21-29 km) for the chlorine nitrate. in addition of the reasons that are usually cause of biases, some of the differences may be here explained by the two different retrieval approaches, ace-fts uses a 1-d approach while the mipas2d data have been obtained using a 2-d code. the 1-d retrievals may be indeed affected by systematic errors especially when the horizontal gradients of the atmosphere are strong [e.g. kiefer et al., 2010; puķīte et al., 2008]. we are planning to extend the analysis over a longer time period and to increase the number of species in the comparison. references [arnone et al., 2013] arnone, e., e. castelli, e. papandrea, m. carlotti, and b.m. dinelli (2012). extreme ozone depletion in the 2010– 2011 arctic winter stratosphere as observed by mipas/envisat using a 2-d tomographic approach, atmos. chem. phys., 12, 9149–9165. 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(2011). scientific asses2sment of ozone depletion: 2010. global ozone research and monitoring project–report no. 52, 516 pp., geneva, switzerland. *corresponding author: enzo papandrea, dipartimento di chimica industriale "toso montanari", università di bologna, italy, email: enzo.papandrea@unibo.it. microsoft word 7507-18242-1-rv_pölzler_revised and edited.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7507 1 on the contribution of philosophical and geoscientific inquiry to geoethics (qua applied ethics) thomas pölzler institute of philosophy, university of graz, austria thomas.poelzler@uni-graz.at abstract this paper is about the methodology of geoethics qua applied ethics. in particular, i investigate the contributions of philosophical and geoscientific inquiry. my investigation is based on a general model of geoethical research. for each stage of this model i explain the expected contribution of “the philosopher” and “the geoscientist” (assuming that they are different persons). these general considerations are illustrated by the example of a particular geoethical research question that is currently addressed in the austrian academy of sciences project ee-con. it turns out that geoethical research is a complex multi-step process that is highly contingent on philosophical assumptions. in advancing this research it will be helpful for philosophers and geoscientists to work together more closely than they have done so far. 1. introduction eoethics addresses questions at the intersection of geoscience and philosophy. one kind of geoethical questions, among many others, can be understood as a sub-category of what philosophers refer to as “applied ethics” (beauchamp, 2003). researchers in this area investigate controversial geoscience-related moral issues that people face in their everyday lives or that are relevant to political decision-making. in what follows i will be exclusively concerned with this particular area of geoethical research. geoethics is a growing field of research (see, e.g., bobrowsky et al., 2017; hourdequin, 2015; peppoloni and di capua, 2012; peppoloni et al., 2017; philips 2012; wyss and peppoloni, 2015). compared to bioethics, neuroethics and other areas of applied ethics, there has so far been little reflection on its methodology. in this paper i attempt to advance geoethics qua applied ethics by addressing a particularly important methodological issue, namely the contribution of philosophical and geoscientific inquiry. i hence conform to recent suggestions according to which philosophers engaged in geoethics are to “offer a synopsis of how the various disciplines relate within a given problem” (frodeman, 2003: 20), and are “to step into a […] kind of collaboration that investigates relationships between the disciplines […]” (stefanovic, 2015: 21). in clarifying the relation of philosophical and geoscientific inquiry in the above-mentioned area of geoethics i will refer to a general model of geoethical research in this area. for each stage of this model i will explain the expected contribution of “the philosopher” and “the geoscientist” (assuming that they are different persons). these general considerations will be illustrated by the example of a particular geoethical research question that is currently addressed in the austrian academy of sciences project ee-con (ortner et al., 2017). 2. developing a geoethical research question the first step in geoethical research is to develop appropriate research questions. research questions in this area need to conform to the same general requirements as research questions in other areas. most importantly, they must be feasible (in terms of the available methods, time, money, etc.), interesting (in adg annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7507 2 vancing knowledge or being practically relevant), and novel (in going beyond past findings) (see hulley et al., 2007). philosophers will often be able to contribute substantially to developing geoethical research questions. given their background in moral theory, they can identify potential moral issues reliably, and can formulate interesting and precise questions about these issues. at the same time, geoscientists are indispensable to this process as well. they can yield information about relevant facts, the results of related geoscientific studies, the expected feasibility and range of outcomes of future studies, etc. in ee-con geographers found that many residents of three austrian alpine valleys expect that despite their proper awareness of increasing natural hazards they will be able to continue to live in these areas. this expectation is likely to have shaped the residents’ plans. so if at some point in the future the state of austria decided to relocate these residents, it would thereby make it difficult or impossible for them to realize these plans. one important ethical question raised by this potential frustration of the residents’ plans — the question that we address in ee-con — looks as follows: is the harm caused by the frustration of residents’ expectation morally significant, i.e., does it ground a moral claim against relocation or for compensation? 3. adopting a general approach to applied ethics applied ethical questions can be approached in three main ways: top-down, bottom-up, and intermediate (see beauchamp, 2003; hourdequin, 2015). according to the top-down approach (e.g., singer, 1979), answers to such questions must be deduced from moral principles (i.e., general moral statements that apply to all or particular kinds of cases) and the relevant facts. on the most abstract level, for example, the question of where to build a new landfill might be addressed as follows: “we ought to act in such a way as to maximize overall happiness (utilitarian moral principle). building the landfill at place x will maximize overall happiness (relevant fact). ergo, the landfill ought to be built at place x.” according to the bottom-up approach (e.g., toulmin and jonsen, 1988), applied ethical questions are to be answered by paying close attention to the specific details of cases rather than by deduction from principles. proponents of casuistry, for example, argue that moral judgements about particular cases are to be justified by comparing these cases to similar cases, in particular to less controversial ones (e.g., comparing the planned landfill with a landfill at place z that caused severe groundwater pollution). the intermediate approach combines both topdown and bottom-up features. according to its most prominent variant (e.g., daniels, 1979), one’s general moral principles and considered moral judgements about particular cases are justified to the extent that they cohere. this state of “reflective equilibrium” can be approached by continued mutual adjustments at both levels. for example, if the above utilitarian principle conflicts with one’s considered moral judgement about where to build the new landfill one may refine/reject either or both this principle and one’s judgement. whether geoethical questions are approached in a top-down, bottom-up or intermediate manner is the most important determinant of this research’s methodology. here i will proceed under the assumption that justifying moral judgements in geoethics at some point and in some sense requires deductions from moral principles. this assumption is not only consistent with the top-down approach, but also with the reflective equilibrium model (proponents of this model just insist that principles are open to refinement or rejection) and even with some bottom-up approaches (see beauchamp, 2003: 9-10). my considerations will hence have very broad appeal. concerning the relation of philosophical and geoscientific inquiry, the adoption and potential adjustment of any of the above approaches requires considerable philosophical expertise. researchers need to fully understand how moral judgements are justified according to their preferred approach. but geoscientists are essential to this general methodological discussion as well. most importantly, they will stress annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7507 3 and contribute to implementing a problemsolving — as opposed to primarily theoretical — perspective. 4. developing, adopting and/or specifying a relevant moral principle given that geoethical research requires deductions from moral principles, the next step in this research consists in developing, adopting and/or specifying such principles. some moral principles are claimed to apply only to particular kinds of cases. geoscientists are necessary in determining whether the case at issue falls under the scope of such principles. for example, does a particular case of groundwater pollution only raise intragenerational or also intergenerational justice issues? in many other respects, developing, adopting and/or specifying appropriate moral principles is primarily the responsibility of philosophers (since they are familiar with existing moral principles, arguments for and against these principles, and methods of specifying principles). in ee-con we adopted two principles about the moral significance of expectations that were developed by meyer and sanklecha (2011, 2014). first, in order for an expectation to be morally significant it must be epistemically sound, i.e., based on good reasons, as specified by a plausible philosophical theory of justification (e.g., when a person for no good reason expects that she will be given €50 million by the state of austria this expectation’s frustration clearly does not ground a claim to compensation, apology, etc.). and second, the expectation must also be just, i.e., meet substantive constraints of pure procedural justice (e.g., when a person for good reasons expects that she will not be caught cheating on her income taxes this expectation’s frustration does not ground a claim to compensation, apology, etc. either). in order to be applicable to controversial cases moral principles must often be specified. drawing on meyer and sanklecha’s work (2011, 2014), we specified the epistemic soundness and justice principles roughly as follows. a person’s expectation is epistemically sound if (e1) the person has reason to believe that the state has ensured the expectation’s fulfillment in the past and will not discontinue to ensure this fulfillment in the future, or (e2) the person has reason to believe that the state expresses that it will ensure the expectation’s fulfillment. a person’s expectation is just if and only if (j1) the expectation is not based on impartial considerations, and (j2) the expectation is consistent with her general views about justice. 5. deriving a testable geoscientific hypothesis from the moral principle from their preferred moral principle(s) geoethicists must next derive testable geoscientific hypotheses. this will often be a two-step process. first, one must identify the kind(s) of facts about the particular case at issue that, according to the principle, are morally relevant (this step requires detailed knowledge of the principle, and is thus mainly within the domain of philosophy). and second, one must develop testable geoscientific hypotheses that allow for conclusions about the facts identified in step one (this step requires detailed knowledge about the particular case and about geoscientific methods, and is thus mainly within the domain of geoscience). recall the above principle according to which expectations can only be morally significant if they can be inferred from states’ past behavior (e1) or states’ expressions (e2). from this principle it can be inferred that determining the moral significance of residents’ expectation that they will be able to continue to live in our study areas requires investigating whether this expectation is grounded in past behavior or expressions by the state of austria (step 1). these questions may then be explored by testing various kinds of geoscientific hypotheses; for example, that help from disaster relief funds has not decreased over the last decades or that the state still issues building permits for our study areas (step 2). 6. identifying the geoscientific methods most appropriate for testing the hypothesis depending on the moral principle and the particular case, the geoscientific hypotheses at issue may be such that testing them requires annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7507 4 studies in both or either physical geoscience and human geoscience. many of the following considerations apply to studies of both kinds. my primary object of investigation, however, will be geoethical research that is based on studies in human geoscience (such as in eecon). human geoscientists use a variety of different methods. abstracting from any practical constraints, the method(s) that geoethicists should adopt in testing their geoscientific hypotheses is that (are those) method(s) that can be expected to provide the strongest evidence with regard to the hypothesis. in the ee-con project, for example, we investigated residents’ beliefs, emotions and behaviors regarding natural hazards and their potential resettlement both by questionnaires (to obtain more objective, unbiased data) and by structured interviews (to gain deeper insight into the underlying causes of these beliefs, emotions and behaviors). determining which method(s) can be expected to provide the strongest evidence with regard to the geoscientific hypothesis at issue requires both theoretical knowledge about geoscientific methods and experience in employing them. thus, it will typically fall in the domain of the geoscientist. 7. developing a specific research design based on their general choice of methods, researchers in geoethics next need to develop (a) specific design(s) for their geoscientific study (studies). questionnaire studies, for example, raise the following issues: what is the required sample size? how ought one to formulate instructions, scenarios, questions, and answer choices? should one use open-ended or closeended questions (or both)? obviously, the geoscientific studies employed in geoethical research should have as much internal and external validity as possible. ensuring this goal is again mainly the responsibility of the geoscientist (who is familiar with the relevant methods and with potential problems in employing them). in addition, philosophical expertise may be helpful in designing certain kinds of geoscientific studies in geoethics too. this is especially true for studies that attempt to test subjects’ philosophical views. such studies will only be internally valid if the philosophical concepts figuring in their instructions, scenarios, questions, and answer choices are used properly — and philosophers are typically more familiar with these concepts. recall j1, the first condition of ee-con’s justice principle (according to which a person’s expectation can only be morally significant if it is consistent with her general views about justice). in order to test whether residents’ expectation to be able to continue to live in our study areas fulfills this condition one’s study must include answer choices that, if residents chose them, would indicate that their expectation is not consistent with their general views about justice. a view that might be relevant here is libertarianism, which claims that states’ only legitimate function is to protect citizens from criminal behavior (e.g., nozick, 1974). suppose a resident’s study responses indicate that s/he is drawn to libertarianism. as this general view about justice in some sense conflicts with the expectation that the state will offer protection, aid and relief with regard to natural hazards, such a finding would support that the resident’s expectation lacks in moral significance. 8. conducting the geoscientific study with a complete research design at hand, researchers in geoethics can finally go on to test their morally relevant geoscientific hypothesis. several issues in the philosophy of science might be relevant to this task, e.g., the theorydependence and value-ladenness of scientific observations. apart from these general issues, however, testing the morally relevant geoscientific hypothesis is obviously the task of the geoscientist. hence, i will not explain this step any further. 9. assessing the study’s implications for the geoscientific hypothesis in order to be able to support or undermine a particular hypothesis the data resulting from one’s geoscientific studies must be interpreted. given their training and experience, geoscienannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7507 5 tists are better suited to performing this task than philosophers. they identify trends in the data more rapidly and reliably, are more aware of common cognitive biases, etc. again, however, the philosopher’s conceptual knowledge may prove helpful as well — in particular with studies that address subjects’ philosophical views. in the study that led to ee-con’s geoethical research question, for example, many subjects in both the questionnaires and interviews strongly agreed that “[t]he society is responsible for the protection against natural hazards”. at first sight this finding might be thought to support the study’s hypothesis that most residents expect the society to protect them against natural hazards. but this interpretation would be flawed. to expect that an agent performs an action means to believe that the agent will perform that action (meyer and sanklecha, 2014: 2). by saying that an agent is responsible for performing an action, however, we express that the agent ought to perform rather than that s/he actually will perform that action — and it is possible and common to believe that agents ought to perform actions that they actually will not perform. hence, the finding that residents of our study areas agreed that the society is responsible for protecting them against natural hazards does not by itself entail that they expect the society to do so. 10. assessing the study’s implications for the geoethical research question given that all previous tasks were performed successfully, the interpretation of one’s geoscientific data will either support or undermine one’s geoscientific hypothesis. the final step in geoethical research qua applied ethical research is to assess the implications of this result for one’s geoethical research question. one important determinant of this assessment is one’s general approach to applied ethics. suppose one’s moral principle(s) (e.g., eecon’s justice principle) and geoscientific findings (e.g., our interview data) entail a moral judgement that conflicts with one’s considered judgement about the case at issue (e.g., that residents’ expectation that they will be able to continue to live in our study areas is morally significant). on the top-down approach one would then have to give up one’s considered moral judgement. the reflective equilibrium model, in contrast, also allows the rejection or refinement of the principle, and thus requires further philosophical (and perhaps geoscientific) research. when one assumes more than one moral principle, answering geoethical research questions may also require weighing the judgements entailed by one principle against the judgements entailed by others. moreover, issues of feasibility may have to be considered as well. suppose, for example, it turned out that residents’ above expectation grounds a claim to not being resettled. accounting for this expectation (i.e., protecting residents from increasing natural hazards) may be costly — so costly that the state of austria may become unable to (fully) meet other morally significant expectations of its citizens (such as students’ expectations to receive scholarships). to arrive at an all-thingsconsidered judgement, answering ee-con’s geoethical research question would thus require weighing this expectation against other morally significant expectations. in any case, as the above considerations demonstrate, assessing one’s study’s implications for the geoethical research question is primarily the task of the philosopher (as s/he is more familiar with approaches to applied ethics, the weighing of principle, the implications of infeasibility, etc.). the geoscientist will mainly be important when it comes to translating one’s findings into concrete practical solutions. 11. conclusion this paper focused on geoethics qua applied ethics. in particular, i asked about the contributions of philosophical and geoscientific inquiry to geoethics in this sense. it turned out that even on a simplified and idealized representation geoethical research is a complex multi-step process that is highly contingent on philosophical assumptions. moreover, there is no clearcut division of labor between philosophers and geoscientists. at various stages geoethical research requires intensive exchange and collaboration. in advancing geoethics it will thus be annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7507 6 helpful for philosophers and geoscientists to work together more closely than they have done so far. acknowledgements this work was funded by the austrian academy of sciences (oeaw) project economic and ethical consequences of natural hazards in alpine valleys (eecon). for helpful comments i would like to thank florian ortner, giuseppe di capua, and the two reviewers of this paper. references beauchamp, t. l. 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(2015). geoethics: ethical challenges and case studies in earth sciences. amsterdam: elsevier. vol49_2_2006 783 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words submarine hydrothermal activity – fluids geochemistry – sulphur deposits – microbial communities 1. introduction the volcanic arc of the eolian archipelago consists of seven volcanic islands and some sea-mounts formed and developed during the quaternary. two islands are still active volcanoes: stromboli that erupts with regular low energy blasts and vulcano that is now in a state of solphataric activity. low temperature vents are at lipari and panarea islands; no in-land hydrothermal manifestations are recognisable on the other islands. recent investigations highlight the existence of both shallow and deep submarine hydrothermal exhalations off the coasts of all the islands (italiano and nuccio, 1991; francofonte et al., 1996; inguaggiato and italiano, 1998). the researches developed off the eastern coasts of panarea showed the presence of the most active submarine hydrothermal system at shallow depths of the eolian arc. the island of panarea (fig. 1) consists of a volcanic structure that has evolved in recent gethe submarine hydrothermal system of panarea (southern italy): biogeochemical processes at the thermal fluids-sea bottom interface concetta gugliandolo (1), francesco italiano (2) and teresa l. maugeri (1) (1) dipartimento di biologia animale ed ecologia marina, università degli studi di messina, italy (2) istituto nazionale di geofisica e vulcanologia, sezione di palermo, italy abstract among the submarine hydrothermal systems located off-shore the volcanic archipelago of the eolian islands (southern italy), the most active is located off the coasts of panarea island. thermal waters, gases and sulphur deposits coexist at the sea bottom where hydrothermal fluids are released from both shallow and deep vents. the chemical and isotopic composition of the fluid phase shows the presence of a significant magmatic component and the physico-chemical conditions of the geothermal reservoir allow the release of reduced chemical species that are microbially mediated towards the production of organic carbon as a form of biochemical energy. microorganisms inhabiting this environment possess nutritional requirements and overall metabolic pathways ideally suited to such ecosystem that represents a clear example of the close connection between geosphere and biosphere. microscopic examination of the white mat attached to rock surfaces showed the presence of thiothrix-like filamentous bacteria. moderately thermophilic heterotrophic isolates were identified as strains of the genus bacillus. although the hydrothermal system of panarea has to be considered a «shallow» system, it shows many characteristics that make it similar to the «deep» oceanic systems, giving a unique opportunity for improving our knowledge on such an unexplored world by working at this easily accessible site. mailing address: prof. teresa l. maugeri, dipartimento di biologia animale ed ecologia marina, università degli studi di messina, salita sperone 31, 98166 messina, italy; e-mail: tmaugeri@unime.it 784 concetta gugliandolo, francesco italiano and teresa l. maugeri ological times following different stages of activity: firstly, the central apparatus developed with the island of panarea, then the volcanic structure enlarged to the east by a fault system ne-sw oriented. a group of islets, located to the east of the main island, are recognised as the remnants of a crater rim (gabbianelli et al., 1986; rossi et al., 1986). as reported by italiano and nuccio (1991), several gaseous emissions spread over the sea-bottom around panarea area up to a depth of 150 m. the area surrounded by the islets (about 4 km2 wide) was investigated up to a depth of 30 m by scuba-diving, and a great number of underwater exhalative vents were identified and clustered as submarine fields (italiano and nuccio, 1991). their distribution is regulated by the ne-sw and nw-se fault systems, which were recognised as having been active in recent times. the released fluids consist of both gases and thermal waters whose temperatures, detected at the emission points, are in the range of 40÷95°c. generally the fluids come out from open fractures, but several areas with diffuse permeation of warm waters and gases through the sand on the seafloor were identified. a white mat surrounds all the areas where thermal waters discharge. the mat, sometimes, forms massive soft and white deposits (fig. 2) where rocky walls or any hard support is available. fig. 1. location of the hydrothermal area of panarea island and sampling sites. fig. 2. mat deposit at the c7 sampling site. the presence of a rocky wall as support allowed the large formation of thick, soft and white mat deposits mainly composed by bacteria. 785 the hydrothermal system of panarea: biogeochemical processes at the thermal fluids-sea bottom interface the input of energy due to the release of thermal fluids is considered to allow the production of biomass in the absence of light at deep-sea oceanic hydrothermal systems where the presence of chemosynthetic sulphur-oxidizing bacteria supports the hypothesis that chemosynthesis provides a substantial primary food source for the populations of invertebrates living close to the deep vents (karl et al., 1980; jannasch and wirsen, 1981; ruby et al., 1981; tuttle et al., 1983; jannasch, 1985). microbial mats which cover the substratum on the seafloor near the vents opening represent a common feature of various submarine hydrothermal environments (jannasch, 1985; jacq et al., 1989; taylor and wirsen, 1997). at shallow marine vent environments both photoand chemosynthetic organisms occupy the lowest levels of the food chain. in addition to chemical compounds such as metabolic energy for chemosynthesizers, solar radiation represents the energy source for phototrophic planktonic organisms. the shallow marine hydrothermal vents of the eolian islands provide easily accessible sampling locations for studying microorganisms inhabiting extreme marine ecosystems. the thermal springs in the baia di levante of vulcano (eolian islands) host dozens of aerobic and anaerobic, thermophilic, and hyperthermophilic microorganisms belonging to the eubacteria and archaea domain (stetter et al., 1983; fiala and stetter, 1986; huber et al., 1986; huber and stetter, 1989; hafenbradl et al., 1996; caccamo et al., 2000; maugeri et al., 2001). this paper focuses on the geosphere/biosphere relationships for which the submarine hydrothermal system of panarea represents a clear example. geochemical and microbiological considerations allowed to constrain the physicochemical characteristics of the deep-originated hydrothermal fluids feeding the vents where several microbial communities thrive. 2. sampling and analytical methods water samples were collected at three stations (fig. 1) by scuba divers. the thermal waters were collected by a syringe through a special stainless-steel probe which was inserted in the emission orifice in order to minimize the potential contamination by seawater (fig. 3). the sampling procedures for biological determinations followed those reported by gugliandolo et al. (1999), while the samples collection for geochemical analyses followed the procedures described in italiano and nuccio (1991). temperature was recorded in situ. some preliminary analyses were performed on board of the research vessel, where chemical and biological laboratories were installed. the bubbling gases were collected at the sea bottom by means of a stainless-steel funnel config. 3. schematic representation of the sampling system (gugliandolo et al., 1999). 786 concetta gugliandolo, francesco italiano and teresa l. maugeri nected to two-way sampling bottles. the gas composition was determined by gas-chromatography (perkin elmer 8500) using argon as carrier gas and molecular sieve 5a column for gas separation. in order to evaluate the concentration of h2, he, o2, n2, co2 a tcd detector was used while the ch4 and co concentrations were determined by a fid detector associated with a methanizer. the detection limits (ppm by vol) were about 3 for he, 2 for h2 and 0.5 for co and ch4. the h2s content was determined on board the research vessel using reactive tubes (drager) calibrated for both high and low sensitivities in the range 0.01-10 vol%. the 3he/4he ratios were measured by a vg 5400 tft mass spectrometer (resolving power of 600 at the 5% peak height; error <1%) and the 4he/20ne ratio by a quadrupole mass spectrometer vg mass-torr fx (accuracy ±5%). the ph and hco3values of water samples were determined immediately after sampling on board of the vessel. the cations and anions concentrations were determined in the laboratory by ion chromatography (dionex 2000i) with a reproducibility within ±2%. the microbial community was enumerated by direct counts of picoplankton (cells ranging from 0.2 to 2 µm in diameter) following the method of sieburth et al. (1978). water samples were prefiltered through 2-µm pore-size polycarbonate membrane filters, filtered onto 0.2-µm polycarbonate (black) filters and fixed with formaldehyde (2%). picoplanktonic cells were stained with 4l6diamidino-2-phenylindole (dapi) fluoro-chrome on 0.2-µm membrane filters (porter and feig, 1980) using epifluorescence microscopy (olympus bx-60m) at 1000x magnification. the microscope was equipped with an halogen (hg 100) light. g 330-385 exciter filter, ft 400 chromatic beam splitter and lp 420-barrier filter were used for picoplanktonic direct counts according to maugeri et al. (1990). to estimate the total number of autofluorescent cells, phototrophic picoplankton or picophytoplankton, filtered water samples were fixed with glutaraldehyde (1%), and microscopically counted according to the procedure described by maugeri et al. (1990). blue light excitation, g470-490 exciter filter, ft 510 chromatic beam splitter, and lp 520-barrier filter were used. picoplanktonic cell counts were converted to biomass values, according to the method of lee and fuhrman (1987), by multiplying the biovolume values by the conversion factor proposed by fry (1990). biovolume was determined by measuring cells under the microscope and calculating the mean diameter of 100 cells observed for each field. 3. results and discussion 3.1. hydrothermal fluids the analytical results of the sampled gases are reported in table i. both the bubbling and dissolved gases show the typical characters of the geothermal fluids. all the sampled gases were co2-dominated with o2/n2 ratios well below the atmosphere. h2s was present with concentrations up to 2.5%; other reactive gases (h2, co and ch4) were detected in concentrations up to 2000 ppm by vol; he was in the range of a few ppm by volume. the deep physical characteristics of the hydrothermal reservoir off-shore the island of panarea have been constrained by a geochemical table i. chemical composition of the gases sampled during august-september 1996 and estimated equilibration temperature and pressure. the analytical results are expressed in vol% for co2, h2s, n2 and o2; in ppm by vol for he, co, h2 and ch4. r/ra = helium isotopic ratio normalized to the atmosphere (atmospheric 3he/4he= =1.39×10−6). na = not analysed. the estimated equilibrium pressures are in bars. sample he h2 o2 n2 co ch4 co2 h2s 4he/20ne r/ra la calcara 9 5 0.040 2.2 3.8 2158 97.5 0.3 12 4.01 c5 12 15 0.045 1.1 3.4 1 96.5 2.3 26.7 4.1 c7 9 5 0.005 0.4 0.5 160 93.0 1.8 37.5 4.3 787 the hydrothermal system of panarea: biogeochemical processes at the thermal fluids-sea bottom interface method based on the reactive gases (co, ch4, co2 and h2) of the submarine hydrothermal vents (italiano and nuccio, 1991). the deep temperatures of the thermal fluids are calculated to be in the range 190-315°c by the application of the gas-geothermobarometry considerations to the system h2o-co2-co-ch4, and assuming the existence of biphase boiling system at depth. the computed ph2o values are relatively high, ranging between 13 and 105 bars. the calculated values of logpo2 range between −32÷−38 showing a reducing environment (table ii). table iii lists the analytical results of the water samples. the chemical composition of c5 and c7 samples, shows a depletion in na+, cl−, mg2+ and so4 2− compared to the seawater, while k+ and ca2+ are more concentrated. the sample taken at la calcara site seems to be diluted by continental water or, alternatively, by steam condensation. taking into considerations the calculated equilibrium temperatures, the recorded thermal water compositions agreed with high temperature water-rock interactions inside the deep geothermal body. italiano and nuccio (1991) considered these waters as the result of cold seawater mixed with waters of marine origin modified by high-temperature interaction with magmatic rocks (samples c5 and c7) or with low-salinity waters circulating beneath the main island (sample la calcara). the chemical characters of the submarine sampled thermal waters are similar to those reported for the hydrothermal solutions from the deep oceanic vents (edmond et al., 1982; von damm et al., 1985) showing that similar processes occur at these submarine sites whatever was their location at depth. 3.2. microbial abundances and biomasses table iv shows the results obtained by direct counts of microbial abundances. picoplankton table ii. estimated equilibrium temperature and pressures for the sampled gases. water pressure and oxygen fugacity expressed in bars. sample t °c ph2o log f o2 la calcara 236 30 −37.7 c5 314 105 −30.1 c7 192 13 −41.4 table iii. chemical composition of the thermal water samples in comparison with local seawater (sw). the depth of the sampling site is reported in meters, conductivity in ms/cm and concentrations in meq/l. nd = below the detection limits. sample depth ph t °c cond hco−3 li− na+ k+ ca2+ mg2+ cl− br− so4 2− la calcara 19.8 5.1 95.0 36.2 3.0 1.2 341.6 9.3 19.6 82.2 415.5 0.7 38.6 c5 18.4 4.7 48.0 41.1 1.5 nd 457.2 19.1 70.2 108.3 606.8 1.18 42.5 c7 21.3 5.4 54.0 47.0 3.0 0.1 479.0 12.3 31.2 119.3 600.1 0.92 44.6 sw 8.1 15.0 54.0 2.5 nd 552.3 12.1 22.5 120.5 654.5 0.99 58.9 table iv. picoplankton and picophytoplankton abundances and calculated picoplanktonic biomasses in thermal waters sampled at the sea-bottom of panarea. the local seawater (sw) is reported for comparison. nd = not determined. sample picoplankton picophytoplankton picoplanktonic (107cells/l) (106cells/l) biomass (µg c/l) la calcara 3.62 3.04 1.79 c5 5.83 3.12 2.88 c7 2.51 4.74 1.24 sw 100 30 nd 788 concetta gugliandolo, francesco italiano and teresa l. maugeri and picophytoplankton (autofluorescent cells) ranged from 2.51×107 to 5.83×107 cells/l and from 3.04×106 to 4.74×106 cells/l respectively. observations under blue light, showed that yellow-orange autofluorescent prokaryotic organisms, belonging to cyanobacteria, were more abundant than red autofluorescent ones, belonging to the eukaryotic domain. picoplanktonic biomasses were in the range of 1.24 and 2.88 µg c/l (table iv). heterotrophic thermophilic bacteria – isolation of thermophilic aerobic bacteria from samples of la calcara and their identification were carried out as described by maugeri et al. (2001). strains isolated from water and sediment samples were ascribed to the genus bacillus. the phenotypic and genotypic characterization of the 7 s strain has been reported in maugeri et al. (2001). microbial mat – from c7 vent site a sample of a white, filamentous mat was scraped from rocks adjacent to the hydrothermal vents and stored in a corning plastic tube (50 ml). on board, a subsample of this was fixed with formaldehyde (2%) for subsequent microscopic investigation, and the rest was available for culturing experiments. microscopic observation of a subsample from the whitish mat revealed a variety of morphologically distinct filamentous forms resembling to thiothrix genus (fig. 4). the adopted culture techniques did not allowed their cultivation. sulphur-oxidizing bacteria – all strains obtained from water samples after enrichment and isolation on tb medium (tuttle and jannasch, 1972) were small, gram-negative, motile, rodshaped microorganisms. the isolates deposited sulphur externally onto agarised tb medium (fig. 5) and produced sulphate (low final ph) in liquid tb medium. none of the organisms was able to grow on these media at 55 or 60°c, nor to grow on marine agar at any temperature investigated. based on growth characteristics and cellular morphology, the isolates were ascribed to the genus thiobacillus (kelly and harrison, 1989). 3.6. thermal fluids-sea bottom interface interactions oxidation reactions are thought to be the major phenomenon occurring at the benthic boundary layer (bbl) representing the geosphere/biosphere interface. it represents the interface between two natural systems: the oxidising (seawater) and the reducing (hydrothermal fluids) one. the oxygen content of the venting gases ranging from 50 to 500 ppm (0.005-0.05 vol%) (table i) cannot come from the deep magmatic source as also shown by the negligible oxygen fugacity calculated for the fluids rising from the deep hydrothermal reservoir. oxygen should fig. 4. thiothrix-like filamentous bacteria (800x) from the whitish mat sampled at c7 site. it is possible to see the presence of sulphur connected to the bacteria. fig. 5. scanning electron microscope picture (2340x) of short rod bacteria and sulphur crystals grown from the c7 thermal water sample. 789 the hydrothermal system of panarea: biogeochemical processes at the thermal fluids-sea bottom interface originate from an atmospheric component that mixes with the deep-originated hydrothermal fluids at shallow levels. since the investigated hydrothermal system is located in a submarine environment, we argue that air-saturated seawater is the carrier for the atmospheric-derived oxygen. the n2 concentration from 0.4 to 2.2 vol% (table i), implies that the oxygen was possibly consumed by redox reactions and also that n2 has also a different origin than from air. this agrees with the n2 excess always recorded in deep (volcanic and geothermal) gases. the h2 content in the sampled gases is seldom very low and not consistent with the values normally found in gas manifestations linked with geothermal fields. the possibility that h2 is lost because of oxidation processes due to the presence of atmospheric contribution to the hydrothermal fluids was taken into consideration, but the undetectable oxygen concentrations and the high 4he/20ne ratios (about 100 times higher than the atmosphere: ratm = 0.318) ruled out this possibility. alternatively this lost can be due to chemical reactions affecting the h2 content during a slow rising of the fluid from the reservoir to the seafloor which allows a partial equilibration in a chemical reaction involving h2s and so2 species. the reaction (3.1) involves most of the gaseous species of the submarine vents, and it allows expressing the ph2 as reported in the eq. (3.2) (3.1) (3.2) where «g» and «v» denote gas and vapour respectively. since from 300 to 100°c (the estimated deep temperature range for the submarine exhalations) the value of the 1/4logk declines from −6.67 to −8.35, while log ph2o declines from 1.94 to 0, we can expect a quite large h2 partial pressure decline in the fluid up to about 4 orders of magnitude during the fluid rising. it is worth of noticing how the gas flow rate is the main parameter governing the final composi/ / / / log logk log log log p ph p p 1 4 1 2 1 4 1 4 h so h s h o a 2 4 2 2 = + + + h s h o so h h4 2 4( ) ( )g v2 2 4 2 2+ = + + + tion of the venting gases; the proposed observations highlight the importance of the gas-water interactions occurring from the deep geothermal reservoir to the sea-bottom interface. 3.7. sulphur deposits the interaction of the gaseous h2s with marine sulphates and with the dissolved oxygen in the seawater, can be considered the inorganic origin of the white mat, due to the following reactions (housain and krouse, 1978): (3.3) . (3.4) the process schematized in (3.4) gives another explanation to the h2s oxidation to sulphur. however, even though the presence of the sulphur deposits (white mat) can be the consequence of chemical reactions at the sea-bottom interface of the hydrothermal fluids, the formation of elemental sulphur following the reaction (3.4) is considered unlike to occur due to energetic reasons. the origin of the white mat may also be microbially mediated. when brought into contact with the oxygenated waters, hydrogen sulphide becomes as energy yielding substrate for chemosynthetic colorless sulphur-oxidizing bacteria. from the oxidation of hydrogen sulphide different oxidation products may accumulate in the environment, most notably elemental sulphur in form of spherical globules or filaments of white precipitate (taylor and wirsen, 1997). 3.8. organic and inorganic mass output the total mass output was estimated on the basis of the measured co2 output (which is above the 90% of the release fluids) and considering the concentration ratios of other gaseous components (table i). the co2 output in the whole area has already been evaluated to be close to 107 litres per day (italiano and nuccio, 1991). most of the researches were concentrated on the submarine vent c7: at this site a daih s o h o s2 2 22 2 2+ = + h o so h h o s3 2 4 42 4 2 2+ + = + + 790 concetta gugliandolo, francesco italiano and teresa l. maugeri ly release of about 2×105 l of co2 was measured and the output of the ch4 and h2s gaseous components was computed as to be 0.15 and 16.5 kg per day respectively. the organic carbon content in the thermal water samples at the c7 site, was estimated on the basis of the picoplanktonic biomass to be about 1.2 µg/l (table iv). as the thermal water flux at that site has been measured to be 10 l/min, a daily organic carbon flux of about 18 mg is calculated. although the uncertainty of our estimations, it has to be noted that scaling this amount with the measured co2 output, the organic carbon represents an infinitesimal percentage to the respect of the inorganic one (3.9×105 g/day). the very high co2 flux and the reaction kinetics seem to control the organic carbon production from the inorganic co2 at these submarine vents. this preliminary hypothesis seems to find an agreement with the organic carbon estimations at the c5 site, which is three times higher than that of c7 site (about 3.9 µg/l), while the co2 output is only 20% respect of the c7 one (about 6.4×104 l/day). 4. conclusions the hydrothermal system of panarea releases geothermal fluids composed of thermal waters of marine origin modified by high-temperature water-rock interactions, and co2-dominated gases separated from a two-phase boiling system at an estimated temperature in the range of 190-314°c. the gases containing magmatic-type helium reveal a direct contribution of a magmatic source, either related to a cooling magma body or to a deeper connection with an active magmatic source that are both able to provide the thermal energy feeding the local geothermal reservoir. the released geothermal energy allows the life of a variety of microorganisms that possess different nutritional requirements and overall metabolic pathways ideally suited to such ecosystem. the picoplanktonic densities estimated in samples collected were in the order of 107 cells/l, values which were lower than those of coastal seawater samples. these values however were similar to the lower counts reported for the galapagos rift (karl et al., 1980; jannasch, 1985). the picophytoplanktonic densities were also lower than those observed in the local coastal seawater. chemolithotrophic, sulphur-oxidizing bacteria were isolated from thermal water samples collected at the site la calcara. these isolates exhibited uniform morphology of short rods, with extracellular deposits of sulphur, and were tentatively identified as thiobacillus-like bacteria. bacterial mats appear as a white film of sulphur deposits containing numerous filaments of colorless sulphur-oxidizing bacteria of the genus thiothrix. these formations could be a strategy for retainment in environments characterized by active fluid motion (taylor and wirsen, 1997). the whitish mat would also favor the growth of other microbes, since it stabilized the gradient between the sulfidic brine and the oxygenated waters. thermophilic, heterotrophic strains were isolated from panarea island and identified as bacillus spp. previous papers have described new species of bacillus and geobacillus spp. from different hot springs of the eolian hydrothermal vents (caccamo et al., 2000; maugeri et al., 2001; maugeri et al., 2002a). some new species are able to produce novel biomolecules of biotechnological interest (nicolaus et al., 2000; maugeri et al., 2002b). the close link between the geosphere (inorganic) and the biosphere (organic environment) is shown by the relationship between the deeporiginated hydrothermal fluids and the microbial communities displaying a mixed origin of the primary productivity (phototrophic and chemotrophic). this feature appears to be the main characteristic of the shallow hydrothermal vents when compared with other shallow-water regions (only phototrophic productivity) and with deep-sea hot vent areas (only chemotrophic primary productivity). references caccamo, d., c. gugliandolo, e. stackebrandt and t.l. maugeri (2000): bacillus vulcani sp. nov., a novel thermophilic species isolated from a shallow marine hydrothermal vent, int. j. system. evol. microbiol., 50, 2009-2012. 791 the hydrothermal system of panarea: biogeochemical processes at the thermal fluids-sea bottom interface edmond, j.m., c.i. measures, r.e. mc duff, l.h. chan, r. collier, b. grant, l.i. gordon and j.b. corliss (1982): ridge crest hydrothermal activity and the balance of the major and minor elements in the ocean; 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qualitative analysis of the territory (traditionally conducted by photointerpretation of stereoscopic models), but for a quantitative assessment as well. appropriate comparison of photogrammetric surveys of a zone realised in different years allows identification of geometric changes occurring during the time interval in question (differential photogrammetry, anzidei et al., 2000; baldi et al., 2002, 2005). this technique has many fields of application: for example, the study of changes in urban areas, farming activities and evolution of coastlines (dolan et al., 1980; overton and fisher, 1996), checking glaciers (kääb and funk, 1999) and monitoring unstable slopes (kääb, 2000). stereoscopic interpretation of two or more sets of aerial photographs taken over a sufficientqualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis antonio zanutta (1), paolo baldi (2), gabriele bitelli (1), mauro cardinali (3) and alberto carrara (4) (1) dipartimento di ingegneria delle strutture, dei trasporti, delle acque, del rilevamento, del territorio (distart), università degli studi di bologna, italy (2) dipartimento di fisica, università di studi di bologna, italy (3) istituto di elettronica e di ingegneria dell’informazione e delle telecomunicazioni (ieiit), cnr, bologna, italy (4) istituto di ricerca per la protezione idrogeologica (irpi), cnr, perugia, italy abstract the results of two survey methods, geological photointerpretation and historical photogrammetry, are compared in order to evaluate the temporal evolution of a unstable slope located in the tuscan-emilian apennines (italy). historical aerial photos of the area, derived from photogrammetric surveys conducted in 1954 (scale 1:60 000), in 1971 (scale 1:20 000), and in 1976 (scale 1:17 000) were available. a photogrammetric flight was further conducted in 2000, at a scale of 1:4400, with a traditional gps ground survey support. first, the results of photographic analysis with the photointerpretation method are presented: the landslides are described from a geological point of view, showing its temporal evolution. to quantitatively assess the landslide movements, digital terrain models were generated by means of an analytical plotter and a digital photogrammetric workstation, with semi-automatic and automatic procedures. to generate these products, it was necessary to solve problems related to a lack of data concerning the aerial cameras used for the historical flights (internal orientation) and the difficulty identifying control points on the photos in order to define the external orientation. an unconventional photogrammetric methodology, based on identification of homologous points in zones considered outside the landslide area, has been there developed and tested to insert the various surveys into a single reference system. mailing address: dr. antonio zanutta, dipartimento di ingegneria delle strutture, dei trasporti, delle acque, del rilevamento, del territorio (distart), università degli studi di bologna, viale risorgimento 2, 40136 bologna, italy; e-mail: antonio.zanutta@mail.ing.unibo.it. 1068 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara ly long time span (20-30 years) is the most effective and popular technique for the identification and morphological characterisation of landslides (multi-temporal landslide mapping) (van westen and getahun, 2003). this procedure assumes particular theoretical and applicative importance due to the fact that most landslide motion tends to take place within or very close to pre-existing landslide deposits. therefore, knowledge of the location, type and distribution of landslides occurring over time in a territory, is an essential tool for forecasting future events. it must nevertheless be pointed out that the reliability of stereoscopic interpretation strongly depends on the experience of the photo-interpreter. thus, when studying landslide phenomena, it becomes important to utilise more quantitative techniques, such as analytic or digital photogrammetry, in association with photo-interpretation, which is intrinsically subjective and qualitative (brunsden and chandler, 1996; lane et al., 1998). applicability of the two methods obviously depends on the availability of aerial photos of the territory, covering a sufficiently long time span. in this regard, it should be remembered that the first italian national scale photogrammetric coverage, with classic pseudo-vertical photos (23×23 cm), was conducted in 19541955 by the istituto geografico militare (igm). these photos, even though at a scale (1:33 0001:60 000), may be a fundamental source of knowledge for the study of the temporal evolution of slope instability (there are also other old igm surveys, at different scales, taken of individual portions of the territory). starting from 1970, in order to produce large scale maps (technical regional maps) and also in the context of specific projects (for example, monitoring of river valleys, glacier faces, slope landslides, etc.), many public administrations managed the realization of local photogrammetric surveys. to assess the potential offered by the integration of qualitative and quantitative photogrammetric techniques in the study of landslides, we identified as test area an unstable slope located in the territory of the municipality of vergato, near the city of bologna (tuscan-emilian apennines, italy) where in recent years a warning system was implemented installing inclinometers and piezometers, measuring pore water pressure and ground displacements by means of wire extensometers, gps surveys, laser scanning and digital photogrammetric techniques (mora et al., 2003; bitelli et al., 2004; pesci et al., 2004; simoni et al., 2004; dubbini and zanutta, 2005). this work presents the results of photo-interpretive analysis of four sets of aerial photos. such analysis characterises the slope geo-morphologically, pointing out its temporal evolution based on morphological changes identified in the available stereoscopic models. a quantitative analysis of the landslide movements was performed comparing multitemporal dtms produced with an analytical plotter (digicart 40, officine galileo), and a digital photogrammetric workstation (helava system). 2. photo interpretation investigations of the landslide phenomena the territory of the municipality of vergato contains numerous slopes affected by landslides, many of which are currently active. due to its geological-geomorphologic structure and types of land use and settlement, this area may be considered highly representative of the mountainous territory of the emilia-romagna region, for which recent studies claim that of more than 32 000 landslides identified and mapped, approximately 26% are to be considered «active» (bertolini and pellegrini, 2001). table i. main characteristics of aerial photos utilised for production of multi-temporal map of landslides in instudy area. agency photo scale type year month gai, igmi 1:60 000 b&w 1954 june emilia-romagna 1:20 000 b&w 1971 region emilia-romagna 1:17 000 colour 1976 may region alifoto srl 1:4400 b&w 2000 april for enel hydro qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis a multi-temporal landslide map was produced for this area by means of a detailed interpretive study of photos taken during four photogrammetric flights, from 1954 to 2000, with photographic scale ranging from 1:60 000 to approximately 1:4400 (table i; fig. 1a-d). the multi-temporal landslide map was produced by means of careful photo-interpretive examination of all the available aerial photos, 1069 fig. 1a-d. multitemporal aerial photos details concerning landslide bodies (municipality of vergato, bologna, tuscan-emilian apennines, italy; φ = 44°17l25mn, ω = 11°07l16me): a) 1954; b) 1971; c) 1976; d) 2000. this work examines the instability phenomena affecting the slope on the right bank of the reno river, below the vergato-piandisetta road in the vicinity of cà di malta and cà del bosco (bologna). this slope is composed of terrains pertaining to the scaly clay complex: their poor mechanical properties make it one of the most unstable geological complexes in the entire apennine territory. a b c d 1070 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara using a stereoscope with continuous 2-20x zoom. interpretation began with an analysis of the 1954 aerial photos, in which it was possible to identify both large-scale landslides (mostly old and relicts) and smaller landslides, generally more recent. the aerial photos from the other flights were then analysed, separately and in combination with those from 1954, comparing similarities and differences in morphology in each portion of the slope under study. the landslides were classified on the basis of estimated depth, type of movement (slip-flow and flow) and relative age (old or recent). this last parameter was deduced from the degree of morphological «freshness» that the instability presented in each set of photos. the photo-interpreted landslides were first redrawn on map at 1:4400 scale and then digitised and stored in a gis for data display and analysis. figure 2 shows the distribution of the landslides identifiable on the 1954 photos. the oldest landslides may be classified as slip and slipflow within which (as described below) numerous landslides identified in the flights conducted from 1954 to 2000 have developed. the main landslide is a deep slip, mostly relict, with a surface area of about 160 000 m2 and average slope of 15°, extending from 320 m a.s.l, below the vergato-piandisetta road, until the reno river to 185 m a.s.l. over time, this landslide underwent partial reactivations with fig. 2. distribution of old or very old landslide bodies identified in 1954 aerial photos, classifiable as slip and slip-flow, with elements of activity. qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis movements of various magnitude that masked or cancelled some of the features of the main landslide, in some cases making it difficult to identify. the alimentation area, shown by a marked concavity on the slope, is characterized by a main scarp whose arched shape is recognisable at various points, and continues for more than a kilometre. locally, the slope’s concavity is more complex, marked by later landslides that created sudden changes in dip. the accumulation zone shows general longitudinal and transverse convexity to the slope, which becomes more accentuated at the foot, where the reno river was slightly deviated toward the west for more than 350 m. two old slip-flows, with a characteristic narrow, elongated accumulation shape which then fans out at the foot of the landslide, have developed at the sides of this relict landslide. the phenomenon that has developed on the right side of the main slip (hereafter called «cà del bosco») presents a complex and deep scarp area that reduces almost to a funnel around 225 m of high. on the other hand, the deposit presents a sharply lobed convex shape on which several generations or pulsations of landslides may be noted, superimposed on the accumulation of the old main slip. the landslide that developed on the left side of the main slip (hereafter called «cà di malta») presents less evident morphological characteristics. its scarp area is not very accentuated and is highlighted by a large concavity, while the deposit presents a marked lobe at the foot of the landslide that further reduces the reno river channel by a few meters. figure 2 also shows distribution of the landslides classified as slip and slip-flow, recent and/or active, traceable to around 1954. in the photos, these landslides show clear evidence of morphological «freshness», demonstrating movements in evolution. these movements are of low or medium magnitude, with extensions ranging between 1000 and 15000 m2. they are located almost exclusively in the scarp areas of the cà del bosco and cà di malta landslides, without ever pushing down to the base of the slope. the landslides which occurred during the period 1954-1971, are essentially two deep slipflows extending from 40 000 m2 to 70 000 m2, classifiable as reactivations of the previous cà del bosco and cà di malta landslides, within and at the borders of which small landslides no larger than 5000 m2 may be seen. in the period 1954-1971, at least two generations of movement may be seen for the cà del bosco landslide. the first involved the uppermiddle part of the previous instability, without reaching the base of the slope, whose foot appears changed at several points, the left side more convex downhill and advanced by about 20 m compared to its appearance in the 1954 photo. the second was a reactivation of the previous landslide. the scarp has receded by about 100 m, reaching almost near the vergatopiandisetta road, while the deposit has reached the base of the slope, very probably blocking the valley road. the activity of this movement is demonstrated by an irregular surface with mounds, landslips troughs, and numerous tension fractures open both longitudinally and transversely. the medium-high section of the flow (from high 250 to 220 m) is winding, with a sharply depressed and concave transverse profile developing within the centre of the valley. as evidence of the rapid flow of material, along the sides of this section of the flow one may clearly see levees that form crests a few meters high, extending for over 200 m in length. the foot of the flow is clearly convex, with an irregular surface lobed by pronounced bumps. in the cà di malta landslide area, the instabilities occurring in the period 1954-1971 reactivated the entire old body with slip-flow movements, considerably reducing the scarp and advancing its foot by about 20 m to within the reno river bed. specifically, the border has receded by over 50 m, touching the head of the cà del bosco landslide. the main scarp has steps of more than 10 m and extends continuously, mainly along the left side of the landslide for about 200 m. the deposit has an irregular surface, with a generally convex shape that has been remodelled and regularised in the centre section as the result of drainage works with channels and surface drains running rectilinear or transverse to the slope. as shown in the 1971 photos, the foot of the landslide is limited by blocks aligned in the reno river bed, seen for the entire width of the landslide front at about 20 m from the bank, which precisely in this sec1071 1072 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara tion is eroded and receded due to lateral undermining. in addition, the road’s path has been altered along this section of the landslide, making it straighter than it was in 1954. the movements which occurred in the period 1971-1976, identifiable in the colour aerial photos taken in 1976, are mainly due to small flows or slip-flows occurring in the centre channel of the cà del bosco flow or in the scarp area of the cà di malta landslide. on the border of this latter landslide, it is interesting to note the presence of an anthropic intervention that caused about 20 m of stripping and the creation of a rise of more than 1000 m2 halfway up the hill. it is important to point out that the most extensive reactivation began precisely at the site of such work, as seen on the 2000 photos. lastly, the landslides occurring between 1976 and 2000 were essentially a total or partial reactivations of previous cà del bosco and cà di malta landslides, with deep slip-flows measuring between 0.5 and 4.0 ha and small flows with surface area not exceeding 0.5 ha. the most evident reactivations might be traceable to movements occurring in october 1996 following long, heavy precipitation, or in november 1998 in the absence of exceptional rains. these photos show the cà del bosco landslide reactivated at its top, with the formation of a few slip-flows, the most evident of which is the one formed from the right side of the scarp at 290 m high near a few farm annexes of cà di sotto homes. the alimentation area of this landslide is characterised by a sharp depression, bounded by a main scarp of about 20 m and by a series of secondary scarps and tiers. the deposit is channelled in the medium-high section of the old 1971 flow, which is topographically depressed and concave, filling it with more than 10 m of sediment (estimated thickness). the fig. 3. three-dimensional perspective view of southern portion of slope under study, obtained by projecting the 2000 digital orthophoto onto the corresponding 2 × 2 m dtm (heigh exaltation: 30%). the image represents a vector/raster result which has metric characteristics, coming from photogrammetric restitution. the border of the cà di malta landslide is indicated with a yellow line; red lines show local roads. qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis flow stops at 225 high, where it forms a topographically pronounced and prominent lobe. in the area of the cà di malta landslide (fig. 3), the instabilities occurring in the period 19762000 reactivated most of the old 1971 body, with slip-flow and flow movements, considerably pulling back the scarp precisely at the site of the anthropic intervention observed during the 1976 flight. at this point, the border has receded by about 100 m, pushed back as far as the vergatopiandisetta road and creating, once again, a sharp depression with a main scarp of over 20 m. the other movements noted in the scarp area or at the borders of the cà di malta landslide also show a distribution of retrogressive activity, but with more limited retrograding of about 20-30 m. these movements are clearly seen in the medium-high part of the slope, where they all meet at around 200 m high. on the other hand, the medium-low sector of the slope has been heavily remodelled by work done to reinforce the landslide, with drainage and reprofiling of the slope. in these cases, man’s cancellation of shapes makes it very difficult, if not impossible, to define and map the real extent of the movement. nevertheless, careful examination and comparison of the last two aerial photo flights has provided elements useful for defining and extending the landslide up to the reno river. the hypothesis that the landslide may have reached the valley is supported by bibliographic information (mora et al., 2003) reporting partial obstruction of the waterway following the reactivation taking place in october 1996. 3. photogrammetric investigation of the landslide phenomena in order to define the shape and position of objects or portions of land by means of photogrammetry, it is necessary to reconstruct the geometric relationships on which the images were taken. the cameras used in photogrammetry produce photos, which, with sufficient approximation, may be considered central perspectives of the photographed object, that is all the rays going from the image to the object pass through the perspective centre. the basic mathematical model used to convert from a two-dimensional system (photo) to a three-dimensional system (object) is expressed by well-known colinearity equations (kraus, 1997) expressing that the perspective centre, the object points, and the image points are lying on the same projective straight lines. to transform the image coordinates in the corresponding object coordinates, at least two photos of the same object and the nine orientation parameters internal (iop), and external (eop) are needed. as known, in traditional photogrammetry, iops are determined in laboratory by means of camera calibration procedures; eops can be determined by spatial resection by using known ground control points (gcps), visible on the photos, whose coordinates are measured in the image reference systems. orientation parameters defining the camera station coordinates and the orientation angles are often unavailable for historical aerial photos. therefore, to produce the 3d model of the area, it is necessary to adopt an unconventional approach that gives results less precise than the rigorous one. the aerial photos can be considered taken by non metric cameras, and a selfcalibration procedure can then be performed (self-calibrating bundle block adjustment), based on the availability of a sufficient number of gcps. generally this method is not applicable because it is difficult to find enough gcps measured at the same time of the flight. since large-scale maps of the zone are available, one solution may be to detect the gcps from them (chandler et al., 1988a,b; chandler and brunsden, 1995), even if a variety of problems may be encountered: the maps may have been drawn from aerial photogrammetric surveys prior or subsequent to the one being analysed; it is difficult to assess the real error to be assigned to the coordinates of the points measured on the map (for example, altitudes derived by interpolation from level curves); it is often very difficult to identify equivalent points, i.e. the search for map gcps on stereoscopic models. the method adopted in this paper to solve the orientation problem and to co-register the multi-temporal three-dimensional models, is based on identification of a large number of 1073 1074 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara points, located outside the landslide bodies, and visible on multi-temporal stereoscopic models. as mentioned above, three sets of historical aerial photos (from 1954, 1971 and 1976) plus the large-scale photos obtained in 2000 with a calibrated aerial camera, are available for the area under study. simultaneously with the 2000 flight, a large number of ground points (gcps), well distributed in the zone, were measured in the frame of a gps campaign (mora et al., 2003). the procedure consisted of four steps: i) orientation of the 2000 photos. ii) identification of natural points on the 2000 stereoscopic model and editing of precise monographs. iii) photogrammetric measurement of 3d coordinates of such points in the 2000 reference system and identification of the same points on the old photos. iv) computation of eops of various historical models using gcps derived from the 2000 survey. the gcps were selected outside the landslide bodies, giving the preference to artificial elements (for example, buildings), homogeneously distributed in the area. the gcps selection procedure was based on assessment of space resection program for the historical images, rejecting, with an iterative approach, points showing high external orientation residuals respect to a theoretical value coming from the scale of the images. obviously, the ability to correctly identify points unusable for support the orientation is greater if a large number of points is examined. at the end of the procedure, all of the «historical» stereoscopic models were oriented into an absolute reference system derived from the 2000 survey, making it possible to perform comparisons between the dtms, as later discussed. the orientation and plotting steps for the stereoscopic models were first performed with a digicart 40 (officine galileo) analytical plotter. because no calibration certificates were available for the cameras used on the historical flights (1954, 1971, 1976), the internal orientation and image coordinates refinement procedure was applied using the focal length pressed on the films, ignoring effects due to lens distortion, non-planarity of the film, and non alignment of the principal point with its theoretical position. correction was made for the earth curvature. relative orientation was performed using a very large number of points distributed uniformly through the entire area of the models covering the landslide zone. absolute orientation was performed with the procedure described above. once the orientation parameters were known, it was possible to acquire three-dimensional data for the slope under study. dtms of the zone with grids of different size (2 × 2 m, 4 × 4 m, 8 × 8 m) were created from the four models in order to reveal any mass movements and to simultaneously determine the best cell size for describing morphological details. in assessing the degree of uncertainty of the results, it has to be considered that an unconventional method was used to solve the orientation problem, and that the available images are characterized by different scale, ranging from 1:60 000 to 1:4400. this fact created many problems: 1) errors of a 1:60 000 scale model are greater than those of a 1:4400 scale model. 2) it is difficult to identify homologous points to use as reference (especially between 1954 and 2000), due to the different resolution of the photos and to the lack of useful elements in the territory. the search procedure was conducted by dividing the common zone into subareas in which a large number of homologous points were then catalogued. analysis of residuals deriving from the external orientation procedure revealed, at any step, the points that could be considered useful for this purpose, because assumed not subject to movements. 3) in the 1:4400 photos points cover the entire observed area, whereas in the lower-scale images they are concentrated in a small zone of the model. in the absence of data from other sources, simple approaches such as computation of variance propagation and empirical assessments of the quality of results were utilised to estimate the precision of dtms acquired in stereoscopy and therefore to evaluate the significance of the measured deformations. assuming the so called normal case, in order to simplify the colinearity equation (the camera axes are considered parallel to each othqualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis er and perpendicular to the base line) and applying the law of variance propagation, considering both the error assigned to measurement of horizontal parallax (assuming σparallax=±10 µm) and, only for the pre-2000 surveys, the camera focal length (assuming σcamera=±10 µm), we derived a theoretical mean square deviation value expected for z coordinate (elevation); if b is the distance between the camera’s centres relative to two stereo pairs, as known, for usual base-ratios (b/z > 0.5) this is generally greater than the planimetric ones. table ii shows the main flight data and the value of theoretical σz. table ii shows a clear correspondence between the mean standard deviations and the scale factors of the photos: the 1954 survey shows theoretical errors of a higher magnitude than those from 2000. moreover 1971 presents theoretical errors that are slightly lower than those from 1976 data; this is due to the base-ratios values. obviously, the error related to uncertainty in collimation of natural points must be added to the σz theoretical one. a measurement repeatability test was performed: dtms on sample area were generated in different periods, by the same operator, with the same analytical instrument, in semi-automatic mode. the comparison of such results (see table iii) shows good repeatability of the measurements and confirms the close correlation between image scale factor and errors. considering the above observations as a whole and the values listed in table ii, and in order to adopt a simple and homogeneous criterion, we have chosen 1 m as the significant range of movement in dtms comparisons, from 1971 up to 2000. the point differences lying within such interval have therefore been considered non-indicative of deformation due to landslide movement. if the 1954 dtm is used for comparisons, the significant range assumed is 2 m. to guarantee the quality of data to be used in this research, the above-described photogrammetric method was performed using both an analytical plotter and semi-automatic or manual vector procedures, with a significant amount of work of an expert operator, and the digital photogrammetry technique for the automatic generation of terrain models and orthophotos. in particular, in addition to the photogrammetric process performed by analytical stereoplotter, the 2000 survey images were used to generate a dtm with a high-level digital photogrammetric workstation (helava system), adopting semi-automatic and automatic procedures. the films were scanned with a rastermaster (wehrli & associates inc., ny, u.s.a.) photogrammetric scanner at a 1000 dpi resolution, 1075 table ii. characteristics of available photogrammetric surveys and theoretical assessment of level error, assuming σpξ = standard deviation (sd) in measurement of horizontal parallax = ±10 µm and, for pre-2000 surveys, σc = sd of focal length of camera = ±10 µm. values expressed in meters. year 1954 1971 1976 2000 mean flight 9509.9 3376.8 2973.9 1011.3 heigh (m) nominal 0.154 0.152 0.153 0.152 focal length base line 5729.2 1818.5 1137.7 365.6 mean topographic 300.00 300.00 300.00 300.00 heigh (m) mean 60000 20000 17500 4700 scale factor mean b / z 0.62 0.59 0.43 0.51 σz ± 1.45 ± 0.58 ± 0.72 ± 0.18 table iii. measurement repeatability test. 1971 1976 2000 no. points 214 214 850 standard deviation (m) 0.68 0.59 0.36 table iv. characteristics of images acquired with photogrammetric scanner. year of the survey 2000 photo scale 1:4.400 panchromatic film colour image resolution ( µm) 24 ground resolution (cm) 12 file image dimension (mb) 240 1076 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara fig. 4. comparison between the reference dtm (digicart 2 × 2 m) and dtm automatically generated with helava system; colour classified differences are shown overlapped the shaded relief model. fig. 5. high differences obtained from 1976 and 1954 surveys. differences (in meters) superimposed on 1976 ortho-photo are indicated by different colours. qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis sufficient to guarantee a high level of detail (ground pixel size about 12 cm; table iv). the dtm was firstly generated automatically, with post-editing by the operator to correct errors deriving from the correlation procedure; editing consisted mainly of manual insertion of breaklines into incorrect zones, with consequent local recalculation of the surface area. the dtm automatically generated by digital photogrammetry was then compared to the one generated by the analytical plotter; fig. 4 shows the differences between the two models. it may be seen that for 96.5% of the points, the difference between the two models is within an interval of ± 1 m, and that the largest differences are localized mainly in zones with complex morphology and shadowed areas. in addition, together with dtm and classic vector products, a digital orthophoto has been produced, to give a photographic representation, with metric characteristics, of the situation in a landslide area at a defined time. modern digital photogrammetry and gis programs can also generate highly effective three-dimensional views that combine the expressive potentials of ortho-photos and three-dimensional data of dtm. figure 3, generated with ermapper software, refers to the 2000 orthophoto. the comparison of multitemporal dtms, performed without the use of interpolators, allowed us to quantify the geometric variations occurring as a result of evolution of the landslide. the two following figs. 5 and 6 graphically show the results of comparisons of the 1954, 1976 and 1077 fig. 6. high differences between the 8 × 8 m dtms obtained from 2000 and 1976 surveys. differences (in meters) superimposed on 2000 orthophoto are indicated by different colours. 1078 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara 2000 surveys; the movement determined for individual cells is highlighted in colour. for better legibility of the figures, the digital orthophotos generated for the 1976 flight and the 2000 flight, respectively, have been used. figure 5 shows the result of the comparison between dtms from 1954 and 1976, with 8×8 m grid resolution. figure 6 refers to differences relative to the period 1976-2000, for which a histogram of heigh differences is shown in fig. 7. other comparisons, for example those involving the 1971 model, are not discussed in this paper, due to the lack of further information with respect to the presented data. 4. landslide map and differential dtm comparison comparative analysis of the multi temporal dtms indicate that the landslide reactivated in the time interval 1954-1971, while in the following period (1971-1976) the slope was almost inactive. the movements involved both the cà di bosco and cà di malta landslide bodies (fig. 5). the alimentation area is characterized by a main scarp with arched shape, locally complex, that is recognisable at various points. the movements observed in this area confirm a distribution of a moderate retrogressive activity along the left side of the landslide area, as detected by means of the photo interpretation investigations. the amount of material moved is less than assumed by qualitative inspection. the time interval 1976-2000 was characterized by a more important reactivation of the landslide in 1996, after a long period of relative inactivity (mora et al., 2003). subsequently, in 1998, a small rotational slide involved the upper part of the cà di malta main body. part of the effects of these movements, principally at the foot of the landslide, were remodelled by human activity in the frame of consolidation and drainage works. the results of natural and anthropic activities are shown in fig. 6. the largest mass movements occurred at the scarp areas of the cà di malta and cà del bosco landslides, where negative values ranging between 2 and 8 m are present in fairly limited areas. on the other hand, the most significant uplifts are also localized in very restricted areas, mainly at the foot of the landslides, and they do not exceed 4.5 m. over an area of about 15 × 104 m2 the volumetric analysis gives a negative variation of 75×10 3 m3 and an accumulation of about 25×103 m3. observing the statistic of the residual concerning areas assumed to be stable (differences <1.00 m) and assuming systematic errors due to model orientations of about 20 cm for the 1997fig. 7. statistic of differences between 2000 and 1976 dtms. 1079 qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis 2000 data, the estimation of the volumes is given with the precision of 20%. the difference between the depletion and accumulation volumes is principally due to the removal of material along the river and above the road at the foot of the slope. for more precise analysis of the relationships between the high residuals and the 1976 and 2000 landslides map, fig. 6 shows (with coloured rectangles and letters, points a to e) the areas in which the most significant surface lowering or uplifts occurred. the significant topographic changes may be seen at the slipflow that develops along the left side of the cà di malta landslide. point a corresponds to an area of about 4000 m2, with the largest negative variations, and coincides with the landslide alimentation area, quantitatively defining the extent of the depression and the distribution of the slopes of the scarp area. degradation values increase rapidly toward the centre of the area. this depressed area has arched borders located precisely at the main scarp and at the head of the landslide. at point b, the scarp is clearly shown in an area of the differentials map in which major level degradations may be observed. the values presenting the most significant variations (between 2 and 4 m) correspond to the alimentation area of the 2000 slip-flow, which caused a sharp depression bordered by a distinct main scarp and repeated secondary scarps and terracings. point d shows local uplifts. this zone coincides with the foot of the landslide, where the accumulation is elongated and slightly lobed. other significant topographic changes may be seen at points b and e, along the slip-flow that develops starting from the right side of the cà del bosco landslide. point e shows an area in which the differential map presents a distinct uplift of about 4 m. this area corresponds to the point where the landslide stopped, generating a lobe-shaped accumulation at the foot. significant topographic changes may also be seen at point c where the slope was remodelled by man for reinforcement and drainage works. lastly in the area at the foot of the landslide the topographic changes shown on the residuals map are not traceable to landslides, but rather to anthropic activity. in particular, the degradations are due to construction of a short section of road leading to the reno river, while the uplifts are caused by levelling of the slope by man to reclaim the foot of the cà di malta landslide. 5. conclusions stereoscopic analysis of four sets of aerial photos, taken over a time span of almost 50 years, allowed reconstruction of the complex sequence of landslide movements occurring on the slope under study. the 1954 photos, despite the low-scale (1:60 000), provided significant evidence of a slip-flow-type landslide body affecting the entire slope. this event may be considered the primary cause of all of the subsequent movements, which are therefore to be understood as partial reactivations of such landslide. of these, the cà di bosco and cà di malta landslides are the most extensive phenomena. the event cannot be dated on the basis of available data. nevertheless, studies conducted in the apennine area have demonstrated that most of the large landslides are hundreds or even thousands of years old (bertolini and pellegrini, 2001). a quantitative analysis of the territory has been obtained with an appropriate comparison of photogrammetric surveys of the zone realised in different years, allowing identification of geometric changes occurring during the time interval in question. the absence of fundamental information on geometric parameters of the images has been overcome by adopting an unconventional method to co-register all the datasets in the same reference frame, based on the detection of homologous points in the multi temporal models. the areas and the amount of masses involved in the reactivations of the landslides have been estimated, giving a contribution to the morphological interpretation of the landslide deformation phenomena in the last years. the availability of historical stereoscopic images assumes particular importance in the study of long time span phenomena, which produce surface deformations of the earth. furthermore the possibility of adopting the digital photogrammetry approaches will yield faster and higher 1080 antonio zanutta, paolo baldi, gabriele bitelli, mauro cardinali and alberto carrara resolution results, at lower costs with respect to the analytical analysis. the availability of highresolution satellite images which may become competitive with small-medium scale aerial photos will lead to the low-cost creation of image archives with repetition intervals that depend on the temporal resolution of the satellite. acknowledgements the work was partially financed by the prin2003 italian national research project «tecnologie innovative per la previsione, il controllo e la mitigazione dell’impatto delle emergenze ambientali» (nat. resp. prof. sergio dequal). the authors wish to thank to prof. luca vittuari and dr. francesco mancini for their support in data processing. references anzidei, m., p. baldi, f. chiocci, m. marsella, e. martorelli and a. zanutta (2000): integrazione tra un rilievo aerofotogrammetrico e batimetria swath: il dtmm del 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(1997): photogrammetry, advanced methods and applications (dümmler-bonn), vol. 2, pp. 466. lane, s.n., k.s. richards and j.h. chandler (1998): landform monitoring, modelling and analysis (john wiley & sons), pp. 454. mora, p., p. baldi, g. casula, m. fabris, m. ghirotti, e. mazzini and a. pesci (2003): global positioning systems and digital photogrammetry for the monitoring of mass movements: application to the cà di malta landslide (northern apennines, italy), eng. geol., 68, 103-121. overton, m.f. and j.s. fisher (1996): application of 3d computer modeling using digital photogrammetry to measure shoreline change, in proceedings of the 25th international conference on coastal engineering, asce, 3, 3750-3761. pesci, a., p. baldi, a. bedin, g. casula, n. cenni, m. fabris, f. loddo, p. mora and m. bacchetti (2004): digital elevation models for landslide evolution monitoring: application on two areas located in the reno river valley (italy), ann. geophys., 47 (4), 1339-1353. simoni, a., m. berti, m. generali, c. elmi and m. ghiotti (2004): preliminary result from pore pressure monitoring on an unstable clay slope, eng. geol., 73, 117-128. van westen, c.j. and f.l. getahun (2003): analyzing the evolution of the tessina landslide using aerial photographs and digital elevation models, geomorphology, 54, 77-89. (received january 22, 2006; accepted july 17, 2006) miscellanea 727 annals of geophysics, vol. 51, n. 4 august 2008 key words seismicity model – information gain – gutenberg-richter relation – kanto – japan 1. introduction a number of researchers (utsu, 1977, 1982; rhoades and evison, 1979; aki, 1981; hamada, 1983; grandori, et al., 1988) have formulated expressions of earthquake probabilities based on precursory anomalies detected by multidisciplinary observation. their formulas assume that different precursory phenomena occur independently of each other. in such a case, the probability expected from detecting multiple precursory phenomena is given by the product of probability gains for respective observations and the probability estimated from secular seismicity. imoto (2006, 2007) extended their formulas to cases in which precursory anomalies are observed as continuous measurements. he further considered the effects originating from mutual correlations between two precursory anomalies, where he assumed two distributions for each precursory parameter: those associated with only space-time volumes in the vicinity of target events (conditional density distributions) and those associated with space-time volumes excluding target events within a short distance (background density distributions). assuming normal distributions for the conditional and background densities from each discipline, he obtained analytical solutions for the general case in which mutual correlations exist among precursory anomalies in both distributions. in this paper we attempt to build statistic models for earthquake probability in kanto, central japan, based on three parameters: the aperformance of a seismicity model based on three parameters for earthquakes (m ≥≥ 5.0) in kanto, central japan masajiro imoto national research institute for earth science and disaster prevention, ibaraki-ken, japan abstract we constructed a model of earthquakes (m ≥ 5.0) in kanto, central japan, based on three parameters: the a and b values of the gutenberg-richter relation, and the νparameter of changes in mean event size. in our method, two empirical probability densities for each parameter, those associated with target events (conditional density distributions) and those not associated with them (background density distributions), are defined and assumed to have a normal distribution. therefore, three parameters are transformed by appropriate relations so that new parameters are normally distributed. the retrospective analysis in the learning period and the prospective test of testing period demonstrated that the proposed model performs better by about 0.1 units in terms of the information gain per event than the value summed up with those of the three parameters. the results are confirmed by a simulation with randomly selected model parameters. mailing address: dr. masajiro imoto, national research institute for earth science and disaster prevention, tennodai 3-1, tsukaba-shi, ibaraki-ken, 305-0006, japan; tel: +81-29-863-7594; fax: +81-29-863-7876; e-mail: imoto@bosai.go.jp miscellanea 9-03-2009 14:42 pagina 727 728 m. imoto and bvalues of the gutenberg-richter relation and a parameter representing changes in mean event size. we evaluate model performance in terms of information gain per event (igpe). we estimate igpe for the parameters and combinations of the parameters, which are compared with values estimated from data in both the learning period and a testing period. 2. method the hazard function is expressed as the expectation of the number of earthquakes in a space-time volume dx (daley and vere-jones, 2003). we consider unconditional and conditional probabilities of observing a parameter value of θ, which are represented by g(θ)dθ (the «background» density) and f(θ)dθ (the «conditional» density) and are empirically determined with random samples in the whole study volume and samples conditioned on occurrences of earthquakes. the hazard function at a space-time point, x, conditioned on a value of θ , is given by (2.1) where m0 is the number of target events and v0 is the space-time volume for study. taking the poisson model as the baseline, the information gain per event (igpe : daley and vere-jones, 2003; imoto, 2004) for a large number of target events is given by (2.2) where the integral is performed within the whole space of θ defined, r. the above equation represents the fact that igpe is equivalent to the kullback-leibler quantity of information expressing the distance between two probability distributions. assuming that f(θ) and g(θ) are normal distributions of multi-variables, imoto (2007) derived an analytical form to estimate the igpe value. hereafter referring to imoto (2007), the main results of previous works will be introduced for the sake of convenience in the later ( ) ( ) ( ) ,lnigpe f g f d r θ θ θ θ= # ( | ) g ( ) f ( ) ,x x xh d v m d 0 0θ θ θ = application of the present study. for a single parameter θ1, the igpe value for θ1, igpe(θ1), can be represented as follows: (2.3) where, µ1 and σ12 are the mean and variance of f(θ1) and those of g(θ1) have been fixed at 0. and 1. 2.1. correlations in neither the background nor the conditional distributions here, we consider n variables θ1, θ2, ...θn possessing joint density distributions f(θ1,θ2, ...θn) and g(θ1,θ2, ...θn) and their marginal distributions of θi are noted as fi(θi) and gi (θi) for the conditional and background distributions. if variables θ1, θ2, ...θn are mutually independent in both distributions and normally distributed, that is, (2.4) (2.5) the following equation can be obtained, where n(µi, σi2) refers to the normal distribution of the mean µi, and variance σi2 . (2.6) 2.2. correlated conditional distribution we assume here that the correlation among the n variables θ1,θ2, ...θn can be observed only in the conditional density distribution f(θ1,θ2, ..θn) and that the covariance matrix c can be expressed as (2.7) ( , , , ) 2 ( ) 1 2 1 ( ) ( ) c c det exp f 1 2 1 n n t gθ θ θ π θ µ θ µ = --& 0 ( , , ... ) lnigpe n 2 1 1 2 2 2 1 2 n n n n 1 2 2 2 2 1 2 2 2 2 1 2 2 2 2 $ $$ $$$ $$$ θ θ θ σ σ σ σ σ σ µ µ µ = + + + + + + + + + ( , , ... ) ( ) ( , )g g n 0 11 2 n i i i n i n 1 1 θ θ θ θ= = = = % % ( , , ... ) ( ) ( , ),f f n1 2 n i i i n i i i n 1 2 1 θ θ θ θ µ σ= = = = % % ( ) ,lnigpe 2 1 1 2 1 1 1 2 1 2 1 2 θ σ σ µ = + + miscellanea 9-03-2009 14:42 pagina 728 729 performance of a seismicity model based on three parameters for earthquakes (m ≥ 5.0) in kanto, central japan and (2.8) where the superscript -1 refers to the inverse of a matrix, ρij the correlation coefficient between θi and θj. by introducing an appropriate transformation of the coordinate system with an orthogonal matrix s, the covariance matrix can be expressed in a diagonal matrix. at the same time, the vector µ is transformed into µ′ with the same matrix. referring to the previous case, the igpe value is represented by (2.9) where trace denotes the sum of the diagonal elements and is an invariant parameter for a unitary transformation and λi2 (i=1,2,..n) eigen values. 2.3. correlations in both distributions now we consider a general case in which some correlations among parameters are observed in both distributions. it is assumed that the marginal distribution of each variable θi can be expressed in the form of a normal distribution and that correlations among variables are observed in the background distribution. let the correlation coefficient between θi and θj be γij, thus the joint density distribution for the background distribution, g(θ1,θ2,...θn), is given as follows: (2.10) ( , , , ) ( )b b det exp g 2 1 2 1 n n t 1 2 1 gθ θ θ π θ θ = -& 0 ( , , ... ) ( ) ( ) ' ln ln det igpe n trace n c c 2 1 1 2 2 2 2 1 1 2 2 2 1 2 ' ' ' n n n n 1 2 2 2 2 1 2 2 2 2 1 2 2 2 2 2 $ $$ $$$ $$$ θ θ θ λ λ λ λ λ λ µ µ µ µ = + + + + + + + + + = = + + [ ]c 1 2 2 2 2 ij i j ij i j n j σ σ ρ σ σ ρ σ σ σ = j l k k k kk n p o o o oo where matrix b is the covariance matrix (equivalent to the correlation matrix in this case), represented by (2.11) by operating successively appropriate transformations of the coordinate system with an orthogonal matrix and a diagonal matrix, the covariance matrix can be expressed in a unit matrix. the compatible transformations are applied to covariance matrix c, and vector µ. after these transformations, the igpe value can be estimated by eq. (2.9). once we have estimated the means and variances of the parameters together with the correlation matrices for both the conditional and background distributions, we can represent the hazard function of the combined model by substituting eqs. (2.7) and (2.10) into eq. (2.1). this function estimates the hazard rate at any point of interest conditioned on the parameter values observed at the respective point. 3. seismicity model based on three parameters as an application of the above formula, we attempt to build a seismicity model for moderate and larger earthquakes (target events m≥5.0) in kanto, central japan, based on three parameters: the aand bvalues in the gutenberg-richter relation (gr relation) and a parameter representing temporal change in mean event size, νvalue (imoto 2003). the latter parameter is obtained from the difference between two mean event sizes, long-term and short-term mean. the long-term mean is defined as a simple mean size of earthquakes (m≥2.0) within 20km of the point over a period of 960 days prior to the assessment. the short-term mean is calculated in the same way as the long-term one except that an exponentially decaying weight with a time constant of 400 days is used. these space and time windows are reasonably select1 1 1 b , ij ij j c c = j l k k k kk n p o o o oo miscellanea 9-03-2009 14:42 pagina 729 730 m. imoto ed with careful consideration of characteristic features of seismic activity in kanto, and optimization of respective parameters (imoto, 2003). the ν-value could be considered a sort of b-value short-term variation since the mean event size is inversely proportional to the b-value. in this study, we use the hypocenter parameters for the period from 1980 to 2006 located by the kanto-tokai network operated by the national research institute for earth science and disaster prevention (nied). taking a balance between the duration of the catalogue and stable estimation, a longer time window, 3650 days is selected for the aand bvalues. the same spatial window and cutoff magnitude as that of the νvalue are used so as to simplify conditions. the cut-off magnitude is selected at 2.0 since the magnitude-frequency relation of the earthquakes in the present study volume exhibits a linear relationship between size and the cumulative number of earthquakes down to a magnitude of 2.0 (imoto and yamamoto, 2006). we surveyed the a-, the band the νvalues within the study volume (200 x 200 x 90km3; imoto and yamamoto, 2006) from january 1990 to december 1999 for the region. in this specified space-time domain, clustering features of target earthquakes are not observed. the assessment was made at 2km-spaced points and at 10-day intervals. to ensure a reliable distribution, we selected only those points of assessment with both 100 or more earthquakes for 3650 days and 20 and more for 960 days. we classified the selected points into two categories, points of conditional distribution and points of background. a point of conditional distribution was defined as the grid point of assessment nearest to a target in space and immediately before it in time. this definition of a point belonging to the conditional distribution may be restrictive and other definitions could be explored. although a study from this viewpoint would be important, it is beyond the scope of the present study. only the present definition is considered in the study. the other grid points are classified as background. we thus obtained two distributions, the conditional distribution from 33 samples and the background distribution from about 6.6x107, for each of the a-, band νvalues. figures 1a, 1b and 1c illustrate the empirical background (solid line) and conditional (gray line) distributions for aband νvalues. we have already proposed two types of seismicity model, one based on the νvalue (imoto, 2003) and the other on the aand bvalues (imoto, 2006). the hazard functions, hab(x| a,b) and hν(x| ν) are represented as (3.1) and (3.2) where b denotes the beta function, and a1, b1, b2, c0, c1 and c2 are model parameters that have been optimized by the maximum likelihood method in the point process analysis. the hazard function of eq. (3.1) is not monotonic and takes its maximum at b=b1. equation (3.2) is similar to this. in transforming the band nparameters into new ones with a normal distribution, we make the hazard function be monotonically increasing with a transformed parameter. we assumed that the background distributions of the a-, band νvalues are respectively represented by an exponential function (dashed line in fig. 1a), a normal function (dashed line in fig. 1b), and a beta function (dashed line in fig. 1c), which are fitted by the maximum likelihood method. therefore, aband νvalues are transformed into the a’, b’ and ν’ values as follows: (3.3) (3.4) where b0 and s denote the mean and the standard deviation of the b-value in the background . s e db s e e dx 2 1 2 1 2 1 ( ) ( ) ' s b b b b s b b b x 2 2 2 2 0 2 2 0 2 2 π π π + = = 3 3 3 + # # # ( ) . exp a a a da e dx 2 1 2 1 2 2 1 0 0 ' a a x 2 2 2 π = = 3& 0# # ( | ) ( ( ) | , ) h x c x c c0 1 2ν νβ=o ( | , ) 10h x a b a1 ( ) ab a b b b1 2 2 $= miscellanea 9-03-2009 14:42 pagina 730 731 performance of a seismicity model based on three parameters for earthquakes (m ≥ 5.0) in kanto, central japan 0 0.2 0.4 0.6 0.8 1 2 2.5 3 3.5 4 background conditional a-value 0 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 b-value background conditional 0 0.2 0.4 0.6 0.8 1 0.4 6.05.0 ν-value conditional background fig. 1a-c. cumulative background and conditional distributions for the parameters. (a) empirical background distributions for the avalue are plotted as a black solid curve, and conditional distributions as a gray step line. an exponential distribution is fitted to the background distribution (dashed curve). (b) same as fig. 1a but for the bvalue. a normal distribution is fitted to the background distribution (dashed curve). (c) same as fig. 1a but for changes in mean event size. beta function is fitted to the background distribution (dashed curve). a b c miscellanea 9-03-2009 14:42 pagina 731 732 m. imoto distribution, and b+ and bare related to the bvalue by the relation: , (3.5) (3.6) where c3 and c4 are the maximum likelihood estimates of the model parameters for the b function fitted to the background distribution of the νvalue. the upper limit of the integral of the first term on the left side in eq. (3.6), νand the lower limit of the second term, ν’+ satisfy the relations as follow: (3.7) figures 2a, 2b and 2c illustrate the background distribution in black and the conditional distribution in gray for a’, b’ and ν’. normal distributions fitted to the conditional and background distributions are indicated with dashed lines in gray and black. these figures indicate that two distributions in each set are more or less appropriately approximated with normal distributions. the parameters of these normal | , | , | , , , c c c c c c 1 2 1 2 1 2 – –$ ν ν ν ν ν β β β = = = + + ^ ^ ^ h h h ( | , ) ( | , ) , y c c dy y c c dy e dx 2 1 x 3 4 0 3 4 1 2 2 π β β+ = = 3 o o o + # # # ,b b b b b b b b2 2 2 2 2 2 – –$= = -+ +] ] ]g g g distributions are summarized in table i. the standard deviations of these estimates for the conditional distributions are given below in parentheses since those of the background distributions are much smaller. this could be justified as follows. the sample size of the background distributions is by far larger than that of the conditional distributions. the transformations have been performed so the background distributions follow a normal distribution with mean 0 and variance 1. only relative values of the conditional distribution to the background distribution are involved in calculating igpe. weak correlations among the three parameters are observed for the background distributions. tables iia and iib summarize the correlation matrices in the background and conditional distributions. in the latter table, the standard deviations of the correlation coefficients in the conditional distributions, which are estimated from fisher transformation (imoto, 2007), are given in parentheses. it can be concluded that at least the correlation between a′and ν′values is significant. this suggests that the formula of aki and others is not applicable to the present case. 4. results and discussion the last column in table i indicates igpe for each parameter, where both distributions are astable i. terms of normal distributions for each parameter and its igpe value. the standard deviations for the estimates in the conditional distributions are given in parentheses. background conditional igpe av std av std a′ 0 1 0.964 1.063 0.47 (0.180) (0.136) b′ 0 1 0.669 0.986 0.22 (0.172) (0.128) ν′ 0 1 0.283 0.649 0.18 (0.115) (0.088) miscellanea 9-03-2009 14:42 pagina 732 733 performance of a seismicity model based on three parameters for earthquakes (m ≥ 5.0) in kanto, central japan 0 0.2 0.4 0.6 0.8 1 -4 -3 -2 -1 0 1 2 3 4 background conditional a´ -value 0 0.2 0.4 0.6 0.8 1 -4 -3 -2 -1 0 1 2 3 4 background conditional b´-value 0 0.2 0.4 0.6 0.8 1 -4 -3 -2 -1 0 1 2 3 4 background conditional ν´ -value fig. 2a-c. cumulative background and conditional distributions for the transformed parameters. (a). empirical background distribution for the transformed avalue is plotted as a black solid curve, and the conditional distribution as a gray step line. their fitted normal distributions are plotted as dashed lines in black and gray. (b) same as fig. 1a but for the bvalue. (c) same as fig. 1a but for changes in mean event size. a b c miscellanea 9-03-2009 14:42 pagina 733 734 m. imoto sumed to be normally distributed with the parameters in the respective columns. if the three parameters are not correlated in the two distributions, the resultant igpe value, which is the case of aki (1981) and others, is given by the summation of each igpe value in the second to the last column. in the actual case, some correlation is observed in both distributions, as indicated in table iia (background distribution) and table iib (conditional distribution). following the procedure given in the previous sections, we can calculate an igpe value expected from a model combining three parameters. the first row of table iii lists the igpe values thus obtained. in order to confirm the validity of igpe estimates, we estimate the probability gain of each target event directly from the hazard function given in eq. (2.1) for each parameter. we can obtain the hazard function of the combined model by substituting eqs. (2.7) and (2.10) into eq. (2.1) with the correlation matrices. hereafter, the model with the parameters fixed is referred to as the combined model. using this hazard function, the retrospective analysis has been performed for data from 1990 to 1999 (learning period) and the forward test done for that from 2000 to 2006 (testing period). the results obtained are listed in the second and third rows in table iii. comparing each igpe value in the second row to the corresponding value in the first row, we can conclude that the parameter estimation and the model construction are consistent. results obtained from data independent of model construction (testing period) indicate some gaps between expectation and observation. a gain of 0.51 units and a loss of 0.09 units in igpe value from expectation are observed for a′and b′values. these gaps result in gains of 0.48 units for the simple sumtable iia,b. correlation matrix (a) observed in background distribution (b) observed in conditional distribution. the standard deviations are given in parentheses. a′ b′ ν′ a′ 1.000 0.049 0.108 b′ 0.049 1.000 0.079 ν′ 0.108 0.079 1.000 a′ b′ ν′ a′ 1.000 -0.215 -0.402 (0.225) (0.201) b′ -0.215 1.000 -0.022 (0.225) (0.236) ν′ -0.402 -0.022 1.000 (0.201) (0.236) table iii. summary of the igpe values. those expected from the formula are listed in the first row. those calculated as average values of probability gains for each target event are listed in the second row for the learning period and third row for the testing period. those obtained by simulation are listed in the last row, where the standard deviations of these estimates are given in parentheses. the sum in the third to last column indicates the simple summation of the three igpe values. the igpe value of the combined model is given in the second to last column. the difference between these two values gained by an effect of correlations is given in the last column. events a′ b′ ν′ sum_up combined diff. estimation 0.47 0.22 0.18 0.88 0.98 0.10 learning period 33 0.43 0.22 0.17 0.82 0.98 0.16 testing period 37 0.98 0.13 0.25 1.36 1.41 0.05 simulation 0.49 0.25 0.21 0.95 1.16 0.21 (0.17) (0.12) (0.10) (0.23) (0.30) (0.20) miscellanea 9-03-2009 14:42 pagina 734 735 performance of a seismicity model based on three parameters for earthquakes (m ≥ 5.0) in kanto, central japan mation and 0.43 units for our proposed combined model. even in this case, it is notable that the combined model performs better than simple summation. to confirm the above results, we estimated each igpe value and its standard deviation using 1000 sets of randomly generated samples. in generating a set of samples, we only consider the variations of parameter values in the conditional distributions, which are given in tables ii and iib. for each set of samples, we calculate values corresponding to each igpe in table iii, in just the same way as that of the first row. the means and standard deviations are estimated from the 1000 acquired sets of igpe values and listed. the values in the first row are more or less similar to the respective values in the last row. a relatively large gap is observed in the combined case, the second to the last column, but it is still in the range of the standard deviation. the difference in the igpe value in the last column suggests that the estimate by the simulation is no smaller than its standard deviation, and means that the igpe value produced by the combined model is probably larger than that estimated from the formula by aki and others. it should be noted again that the application of the latter formula is no longer appropriate in the present case with a significant correlation in the conditional distribution between two parameters. accordingly, it can be concluded that the combined model proposed in the present paper is more appropriate than that formulated by aki and others, from the viewpoints of its performance and assumption of independency. 5. conclusions this paper introduces a way to combine multi-disciplinary observations into one hazard function and demonstrates its superior performance to that expected from the well known formula of aki and others. the combined model was confirmed using actual data taken from both the learning period and the subsequent testing period. the quantitative matching between the igpe values predicted by our formula and those observed for the learning period is reasonable and demonstrates only the logical consistency in our derivations. however, this is not the case for the data taken from the testing period. the combined model performs only 0.05 units better than aki’s formula, but by 0.43 units better than that of the learning period. this gap is primarily because the igpe value of the a′ parameter for the testing period is 0.51 units greater than that observed for the learning period. this corresponds to an increase of about 1.6 times the probability gain. this change could be interpreted by an increase of this factor in the number of sample earthquakes since such increases raise the hazard rate by the same factor through eq. (3.1) with the baseline (the poisson rate) resuming the same value as before. the average number of earthquakes conditioned on a target event in the testing period exceeds that observed in the learning period by a factor of 1.5. this evidence is consistent with the fact that the seismicity rate of target events in the testing period, 37 events in 7 years, is 1.5 times larger than that in the learning period, 33 events in 10 years. in summary, we attempted to build up a seismicity model based on three parameters, the aand bvalues of the gutenberg-richter magnitude frequency relation and a parameter of changes in mean event size. applying the formula derived by imoto (2007) to the data observed by the kanto-tokai network (nied), we estimated that the igpe value of the model becomes about 0.1 units greater than that of the simple summation formulated by aki and others. the model performs consistently with this estimation both in the learning period and the testing period. acknowlegments this manuscript was greatly improved by the comments of two anonymous reviewers and editor massimo cocco. references aki, k. (1981): a probabilistic synthesis of precursory phenomena, in earthquake prediction, edited by d.w. simpson and p.g. richards, 566-574, agu. daley, d. j. and d. vere-jones (2003): an introduction to the miscellanea 9-03-2009 14:42 pagina 735 736 m. imoto theory of point processes, vol. 1, elementary theory and methods, second edition, (springer, new york), pp. 469. grandori, g., e. guagenti and f. perotti (1988): alarm systems based on a pair of short-term earthquake precursors, bull. seism. soc. am, 78, 1538-1549. hamada, k. (1983): a probability model for earthquake prediction, earthquake prediction res., 2, 227-234. imoto, m. (2003): a testable model of earthquake probability based on changes in mean event size, j. geophys. res., 108, ese 7.1-12 no. b2, 2082, doi:10.1029/ 2002jb001774. imoto, m. (2004): probability gains expected for renewal process models, earth planets space, 56, 563-571. imoto, m. (2006): statistical models based on the gutenberg-richter a and b values for estimating probabilities of moderate earthquakes in kanto, japan, in proceedings of the 4th international workshop on statistical seismology, january 9-13, 2006, ism report on research and education, ism, tokyo, japan, 23, 116-119. imoto, m. (2006): earthquake probability based on multidisciplinary observations with correlations, earth planets space, 57, 1447-1454. imoto, m. (2007): information gain of a model based on multidisciplinary observations with correlations, j. geophys. res., 112, b05306, doi: 10.1029/ 2006jb004662. imoto, m. and n. yamamoto (2006): verification test of the mean event size model for moderate earthquakes in the kanto region, central japan, tectonophysics, 417, 131-140. rhoades, d. and f. evison (1979): long-range earthquake forecasting based on a single predictor, geophys. j. r.astr. soc., 59, 43-56. utsu, t. (1977): probalities in earthquake prediction, zisin ii, 30, 179-185, (in japanese). utsu, t. (1982): probabilities in earthquake prediction (the second paper), bull. earthq. res. inst., 57, 499-524, (in japanese). (received july 6, 2007; accepted july 2, 2008) miscellanea 9-03-2009 14:42 pagina 736 adg vol5 n02 troyn 339_349.pdf annals of geophysics, vol. 45, n. 2, april 2002 339 mathematical modeling to reconstruct elastic and geoelectrical parameters vladimir n. troyan and yurii v. kiselev institute of physics, st. petersburg state university, st. petersburg, russia abstract the monitoring of the underground medium requires estimation of the accuracy of the methods used. numerical simulation of the solution of 2d inverse problem on the reconstruction of seismic and electrical parameters of local (comparable in size with the wavelength) inhomogeneities by the diffraction tomography method based upon the first order born approximation is considered. the direct problems for the lame and maxwell equations are solved by the finite difference method that allows us to take correctly into account the diffraction phenomenon produced by the target inhomogeneities with simple and complex geometry. for reconstruction of the local inhomogeneities the algebraic methods and the optimizing procedures are used. the investigation includes a parametric representation of inhomogeneities by the simple and complex functions. the results of estimation of the accuracy of the reconstruction of elastic inhomogeneities and inhomogeneities of electrical conductivity by the diffraction tomography method are represented. 1. introduction diffraction tomography is an imaging technique that makes use of a large volume of input data (recorded traces) to produce the image of underground medium parameters with high spatial resolution. in contrast to ray tomography (travel time tomography), for which the resolution is connected with the fresnel zone and the large number of the source-receiver pairs is required, diffraction tomography (devaney, 1984; devaney and zhang, 1991; zhou et al., 1993; ryzhikov and troyan, 1994; alumbaugh and morrison, 1995; kiselev and troyan, 1997) provides information on the medium parameters with subwavelength resolution. in our study (a complete review of development of the diffraction tomography is introduced in (devaney and zhang, 1991)) most attention is given to an estimation of the accuracy of multiparametric reconstruction of elastic parameters and reconstruction of an electrical conductivity with the use of sounding by elastic wave and electromagnetic wave correspondingly. as a tool for our investigation, we use numerical simulation in 2d space domain. the direct problem is solved by the finite difference method that allows us correctly to take into account the diffraction phenomenon produced by the target inhomogeneities. the linearized inverse problem is solved by the diffraction tomography method with the use of the first order born approximation (keller, 1969). mailing address: dr. vladimir n. troyan, universitetskaya nab. 7/9, st. petersburg, 199034 russia; e-mail: troyan@hq.pu.ru key words diffraction tomography – mathematical modeling – seismic tomography – electromagnetic tomography 340 vladimir n. troyan and yurii v. kiselev the applicability of the first order born approximation in diffraction tomography is shown (slaney et al., 1984) by numerical simulation conducted on a single cylinder using analytical expressions for the exact scattering field. beylkin and burridge (1990) describe the multiparametric inversion in the time domain in the cases of acoustic and elasticity on the basis of the generalized radon transform. this approach requires a large number of the sourcereceiver pairs. we also implement multiparametric reconstruction in the time domain, but our numerical simulation is realized with not more than three sources and three receivers (nine source-receiver pairs). for the multiparametric reconstruction in the elastic case (reconstruction of the lame parameters, mass density and as a corollary – shear and longitudinal velocities) the optimizing procedures are used. the multiparametric reconstruction makes use of amplitude information connected to the scattering characteristic (wu and aki, 1985; beylkin and burridge, 1990) of the elementary disturbances of the reconstructed parameters. under tomography experiment, these scattering characteristics should be taken into account by the use of the relevant observation schemes. scattering by the elementary disturbances of the parameters can be described with the use of tomography functionals (ryzhikov and troyan, 1994; troyan and ryzhikov, 1994), which in a form of the ray series are represented for elastic and electromagnetic cases. 2. basic equations and algorithms for elastic case the numerical simulation to reconstruct the parameters of local inhomogeneities with a smooth change of the elastic parameters , µ and mass density is carried out for two-dimensional model of the elastic medium, containing the free surface and plane-parallel welded interfaces. the source f f (x, t), located in the point (x = x s , z = z s ) of the cartesian system of coordinates (x, y ,z; e 1 , e 2 , e 3 ), produces the wave field u u (x, z, t) u (x, t) which satisfies the equation (2.1) boundary condition at the free surface (z = 0) is and at welded interfaces (z = z i ) we assume that (2.3) the field u will be produced by the sources f , f 1, f 3 (2.4) with the time dependences f (t) and (t), which are located at the source point x = (x s , z s ) and at the observation point x = (x r , z r ). the velocities of longitudinal ( p ) and shear ( s ) waves, expressed through the quantities , µ, , read as (2.5) we introduce the differences = rf , µ = µ µ rf and = rf of the values , µ, , for the unknown medium, which are connected with µ µ2u u u+ + × + ( ) . 0 0tz z == f f x z t x x z zs s s s( ) = ( ) ( )ˆ ˆ , , ff t( )e3 x z t x x z z tr r r r( ) = ( ) ( ) ( )1 1 1ˆ ˆ , ,f f e x z t x x z z tr r r r( ) = ( ) ( ) ( )3 3 3ˆ ˆ , ,f f e µ µ p s= + = 2 , . (2.2) µ µ 2 2 u f u u t = + +( ) + +ˆ u u z z z zi i= = +=0 0 , t tz z z z z zi i= = +=0 0 . 341 mathematical modeling to reconstruct elastic and geoelectrical parameters the wave field u(x, t) (2.6) and the values λrf (x), µrf (x), ρrf (x) for the known, reference (rf) medium for which the wave field is u rf (x,t) (2.7) assuming δλ, δµ, and δρ are small we can write (2.8) where δu = u − urf is the difference field. the right hand side of (2.8) (2.9) can be considered as a source of this field. we shall represent the components of the difference field δu i from (2.8) at the observation point of x = x r , as following: (2.10) where s is the region of reconstruction; and , are the solutions of the eqs. (with sources from (2.4)) (2.11) and (2.12) respectively. after introducing the tomography functionals (troyan and ryzhikov, 1994) (2.13) the components of the difference field δu i (2.10) can be written down as (2.14) using the linear relations between δλ (x), δρ (x) and δµ (x) δλ (x) = cλδµ (x), δρ (x) = cρδµ (x), (2.15) (cλ = const, cρ = const) λ µ µl l= − ≡ +( ) ∇∇ ⋅ + +ˆu f, u u u∆ λ µ µl ≡ +( )∇∇ ⋅ + +rf rf rf rf rf rf rfu u u∆ u urf rf≈ −δ δl l u u erf rf rf= ∇ × ∇ ×( ) + ∇ ⋅ ∇   + = ∑δ δµ δµ 1 3 2l j j j , u δu ti s rx x, ,( ) = tru u x, x ,˜ ˜≡ ( )1 1 tru u x, x ,˜ ˜≡ ( )3 3 tsu u x, x ,≡ ( ) l u f̂= −rf l i iu f˜ ˆ= −rf p ti r s ρ x x x, , ,( ) = t di r sτ ∂ ∂τ τ τu x x u x x˜ , , , ,= − −( ) ⋅ ( ) ∞ ∫ 2 20 p ti s r λ x x x, , ,( ) = t t di r sτ τ τ u x x u x x˜ , , , ,= − ∇ ⋅ −( )∇ ⋅ −( ) ∞ ∫0 p ti s r µ x x x, , ,( ) = = + ∇ × ˜̃ , , , ,u x x u x xi r st −( ) ⋅ ∇ × ( ) −     ∞ ∫ τ τ0 δ δλ λu t p ti s r i s r s x x x x x x, , , , ,( ) = ( ) ( ) +[∫ δµ δρµ ρp t p t di s r i s rx x x x x x x x x, , , , , ,+ ( ) ( ) + ( ) ( )] . (i = 1, 3) l = − ˆrf rfu f , ˜ , , , , , x x x xij r j j st d) − ∇ −( ) ⋅ ∇ ( )     = ∑ τ τ τ2 1 3 u u u u uu u t + ∇ ∇ ⋅ + ∇ × + ∇ ⋅ ∇( ) −λ µ µ ρ ∂ ∂ 2 2 2 t rf rf rf rf rf rf rf u u u u + ∇ ∇ ⋅ + ∇ × + ∇ ⋅ ∇( ) −λ µ µ ρ ∂ ∂ rf 2 2 2 . δλ δρ ∂ ∂t u u rf rf+ ∇ ∇ ⋅( ) − 2 2 τ δ τ τt l d di s r ru x x u x x x˜ , , , ,) = −( ) ⋅ ( ) ∞ ∫∫ 0 342 vladimir n. troyan and yurii v. kiselev the eq. (2.14) can be rewritten as (2.16) after the digitization of the eq. (2.16), the system of equations for determination of δµ (vector dµ ), cλ and cρ can be written as p(cλ ,cρ )dµ = du (2.17) where d u are the samples of the scattered field. the final version of this system after introducing regularizing terms reads as (2.18) where α1, α2, α3 are the regularizing coefficients; matrices b x and b z are the finite difference images of second partial derivatives with respect to x and z correspondingly; c and d are penalty matrices for non-zero values of δµ at boundary and near boundary points of the reconstructed region s. we find δµ, cλ and cρ by using an iterative procedure. at the first step the system of linear eqs. (2.18) is solved with some initial values cλ (0) and cρ (0). by minimizing the sum of squared differences of the left-hand side and the right-hand side of (2.17), we find c λ (1) and cρ (1), which are the corrected values of cλ (0) and cρ (0). at the second step, the values c λ (1) and cρ (1) are used for solution of the system of linear eqs. (2.18). the convergence of this procedure is based on the distinctions of the scattering diagrams (wu and aki, 1985) created by the elementary disturbances of λ, µ, ρ. similar distinctions can be studied, for example, using formulas (2.21) given below. 2.1. ray representation of the tomography functionals the components of the wave field δu i (x s ,x r ,t) (i = 1, 3) scattered by local inhomogeneity (δλ, δµ, δρ) for incident wave field u (x, x s , t) are given by relations (2.14), (2.13). using the ray method the fields u (x, x s ,t), u~ i (x,x r ,t) (2.11)-(2.13) can be represented as following: (2.19) where τ~ q ≡ τ~ q (x,x r ) (τ q ≡ τ q (x,x s )) is the time of propagation of a wave from point x r , (x s ) to a point x. substituting of the relations (2.19) in (2.13) we write down the quantities in the form of the ray series (2.20) thus the coefficients read as (2.21) x x z zc c p c c b b b b, , ' '( ) ( ) + +( ) +[ λ ρ λ ρ αp' 1 uc c d d p c c' ' ' ,+ + ] = ( )µ λ ρα α d d2 3 f t f tj j( ) ,= ′ ( )+1 ˜ ( ) ˜f t f tj j= ′ ( )+1 p p pi i i, , ρ λ µ +( ) = ′ ( )j jt t1φ φ , ∫ ∞ ( ) = −( ) ( )t d dt f t f d0 2 2 00 0φ τ τ τ˜ p p piqq j iqq j iqq j′ ′ ′ λ µ ρ, , p p p iqq iq q iqq iq q q q iqq q iq q q ′ ′ ′ ′ ′ ′ ′ ′ = − ⋅( ) = − ⋅ ∇( ) ⋅ ∇( ) = ∇ ×[ ]⋅ ∇ ×[ ] − ρ λ µ τ τ τ τ ˜ ˜ ˜ ˜ ˜ 0 0 0 0 0 0 0 0 0 a a a a a a iq q′− ⋅( )˜2 0 0a a ∇∇ ⋅ ∇( )′˜ .τ τq q j =( )0 p p ti iqq j j q q q p sq p sj ρ ρ φ τ τ= − −( )′ ′ ′ === ∞ ∑∑∑ ˜ ,,0 p p ti iqq j j q q q p sq p sj λ λ φ τ τ= − −( )′ ′ ′=== ∞ ∑∑∑ ˜ ,,0 p p ti iqq j j q q q p sq p sj µ µ φ τ τ= − −( )′ ′ ′=== ∞ ∑∑∑ ˜ ,,0 δ λ λ u t c p ti s r i s rx x x x x, , , , ,( ) ≈ ( ) + s [∫ µpi sx x, ,+ xx x x x x xr i s rt c p t d, , , ,( ) + ( )] ( )ρ ρ δµ . ˜ , , ˜ , , ˜ ˜ , , , , , , u x x a x x u x x a x x i r iqj q p sj r j q s qj q p sj s j q t t f t t t f t ( ) = ( ) −( ) ( ) = ( ) −( ) == ∞ == ∞ ∑∑ ∑∑ 0 0 τ τ 343 mathematical modeling to reconstruct elastic and geoelectrical parameters 3. basic equations and algorithms for electromagnetic case numerical simulation to reconstruct the local inhomogeneities of electrical conductivity σ = σ (x), located in the uniform space (electrical conductivity σ = const, electrical permittivity ε′ = εε0 = const, magnetic permittivity µ′ = µµ0 = const) is implemented for the 2d problem. electrical (e = e(x,t)) and magnetic (h = h(x, t)) fields excited by a current density j ex = j ex (x, t) satisfy to the maxwell equations (3.1) electrical field e = e(x,t) in the medium, containing the local inhomogeneity (σ = σ (x), ε′ = ε′(x), µ′ = µ′(x)) (1) is given by a solution of the equation (3.2) the reference medium (rf ) is supposed to be known (σrf , εrf , µrf ) and electrical field erf satisfies to the equation (3.3) as in the case of scattering by elastic inhomogeneities discussed earlier, we assume that magnitudes of the values δσ = σ − σ rf , δε = ε′ − ε rf and δµ = µ′ − µ rf make it possible to write an approximate equality (3.4) where δe = e − erf is the difference field. thus, the value (3.5) can be considered as a source of this field. the components of the difference field δe i is possible to write down as (3.6) that coinciding to within notations with (2.10). wave fields e(x, x s , τ ) and e ~ i (x, x r , t − τ ) satisfy, respectively, the following equations: (3.7) where in a two-dimensional case the sources from (2.4) are used. introducing the tomography functionals (3.8) ∇ × = + + ∇ ⋅ = = ′ t ex ∂ ∂ ρ µ, ,h j j d d b h. = −l t ex ∂ ∂ e j = ′ ∇ × ∇ × + ′ + −l t tµ ε ∂ ∂ σ ∂ ∂ 1 2 2 e e e e − ′ ∇ ′ × ∇ ×[ ] µ µ . 1 2 e l t exrf rfe j= − ∂ ∂ l t rf= ∇ × ∇ × + + µ ε ∂ ∂ rf rf rf rf rfe e e1 2 2 t + − ∇ × ∇ ×[ ]σ ∂ ∂ µ µ 1 2 rf rf rf rf rf e e . l l≈ −δ δrf rfe e δ δε ∂ ∂ δσ ∂ ∂ δµ µ l t t e e e erf rf rf rf rf= + − ∇ × ∇ × 2 2 2 ( ) δ ti s r( ) =e x x, , l = − ˆrf e f l i i= −˜ ˆrf e f ( = 1, 3)i (1) the main relations considered below will be written down in general case, i.e. we will assume an existance of anomalies of electrical permittivity and magnetic permittivity. ∇ × = − ∇ ⋅ = = ′ =e b b d e j e ∂ ∂ ε σ t , , ,0 τ δ τ τe x x e x x xi s r st l d d ˜ , , , ,= −( ) ⋅ ( ) ∞ ∫∫ 0 p t t d p t t d p t i r s i r s i r s i r s i r s i ε σ µ τ ∂ ∂τ τ τ τ ∂ ∂τ τ τ x x x e x x e x x x x x e x x e x x x x x e , , , ˜ , , , , , , , ˜ , , , , , , , ˜ ( ) = −( ) ⋅ ( ) ( ) = −( ) ⋅ ( ) ( ) = − ∇ × ∞ ∞ ∫ ∫ 0 2 2 0 00 ∞ ∫ −( ) ⋅ ∇ × ( )x x e x x, , , ,r st dτ τ τ and 344 vladimir n. troyan and yurii v. kiselev the components of the difference field can be written as (3.9) we will consider the numerical experiments on reconstruction of the anomalies of electrical conductivity ( = 0, µ = 0). in this case, after the digitization, the integral eq. (3.9) can write as the system of linear equations pd = d e (3.10) with respect to vector d , which is sought for the value (x), where d e is the time samples of the components of the wave field scattered by inhomogeneity. the final version of these equations, after introducing the regularization terms, is coincident with the system of linear equations similar to (2.18). 3.1 ray representation of the tomography functionals in section 2.1 the expressions for the tomography functionals (2.13) were written out in the case of the ray description of the wave fields u and u~ i , which are included into the integrands of the right-hand sides of (2.13). by assuming that the value of electrical conductivity for the reference medium is equal to zero, we can represent the wave fields e and e ~ i from (3.8) as (3.11) and can write the tomography functionals (3.8) in a form of the ray series (3.12) in a case of n = 0 the values , and are represented by the amplitude factors of the zero approximation a 0i , a and eikonals ~, (from (3.11)) (3.13) the values (from (3.13)) describe the space characteristics of the scattered fields, which are produced by the elementary inhomogeneities ( , , µ) at the far-field region. the directivity diagrams in the cases of scattering by perturbation of electrical conductivity and electrical permittivity coincide. the wave field scattered by perturbation of electrical permittivity contains higher frequencies than the wave field scattered by perturbation of electrical conductivity. it should be noted that formula (3.13) for the values and coincides with the formula for scattering the elastic wave field by the elementary perturbation of mass density (2). f ti ni n n ˜ ˜ ˜ ˜ ,= ( ) = 0 e a f tn n n ,= ( ) = 0 e a f t f tn n ,( ) = ( )+1 ˜ ˜f t f tn n( ) = ( )+1 0 2 2 0 0 0t d dt f t f d( ) = ( ) ( )˜ , 1t tn n( ) = ( )+ . p i0 p pi i i0 0 0 0= = ( )ã a p i0 µ, ,0 0p pi i p i0 p i0 p i0 p i0 µ p i i0 0 0 µ = ×[ ] ×[ ]( )˜ ˜ .a a (2) graphic representation of directivity diagrams for the elastic and electromagnetic cases can be found in wu and aki (1985) and saintenoy and tarantola (2001). ~ p p ti ni n n= ( ) =0 ˜ p p ti ni n n= ( ) = + 0 1 ˜ p p ti ni n n µ µ= ( ) =0 ˜ e t ei s r ix x, ,( ) = µ µµp p p di i i s = + +( )rf .2 x 345 mathematical modeling to reconstruct elastic and geoelectrical parameters 4. numerical simulation 4.1. elastic case as the result of numerical simulation, we obtain the values , µ and . with formula from (2.5) we get value of p . the observation scheme and the location of inhomogeneity are represented in fig. 1. the sources and observation points (9 pairs) are located at the free surface. the results of reconstruction of p are represented in figs. 2a-d and 3a-d. figure 2a and fig. 3a show the models of two inhomogeneities with 20% contrast relatively to the reference medium. these inhomogeneities are located inside the layer with intermediate velocity. figure 2b,c and fig. 3b show the results of reconstruction obtained by the solution of the system of eqs. (2.18) with the different values of the regularizing coefficients. the relative error of the reconstruction in these cases is 20-25% (with the use of two components of the wave field). for the cases of more contrast inhomogeneities (20-40%) the accuracy of reconstruction can be ~ 50%. figure 2d and fig. 3c,d show the results of reconstruction using one (fig. 2d and fig. 3d) or three (fig. 3c) parametric functions. in these cases, we solve the system of the eqs. (2.17) by minimization of the sum of squared differences between the left-hand side and the right handside of (2.17). the reconstruction with the use of just one simple parametric function is very stable. from numerical simulation, we conclude that the realization of the diffraction tomography method offered in ryzhikov and troyan (1994), under appropriate observation scheme, allows satisfactory accuracy for velocity parameter reconstruction in the case of not very contrast inhomogeneities of size of ~ p at small number of source-receiver pairs. 4.2. electromagnetic case the model of inhomogeneity of electrical conductivity together with the results of reconstruction are represented in fig. 4a-c. the inhomogeneity, comparable in size with the wavelength of the sounding signal, is located inside the uniform space (reference medium) with parameters: = 10, µ = 1, = 0. the maximum . fig. 1. model of medium and observation scheme. s is the region for reconstruction; location of inhomogeneity black color; 1, 2, 3 are the source and the observation points locations; s ; s / p = 1/ .3 346 vladimir n. troyan and yurii v. kiselev a b c d fig. 2a-d. reconstruction of p for symmetric inhomogeneity. a) the model; b,d) results of reconstruction; b,c) solution of the system (2.18) with 3 = 0 and 3 0 respectively; d) reconstruction in case of representation of reconstructed inhomogeneity by simple parametric function. value of electrical conductivity is 10 3 s/m. an apparent frequency of the sounding signal is 5 × 106 hz. we use the observation scheme which is similar to the observation scheme represented in fig. 1. distance between the observation line and the center of inhomogeneity is 10 ( = 20 m). the system of the linear eqs. (3.10) is solved under introducing the minimum magnitudes of 347 mathematical modeling to reconstruct elastic and geoelectrical parameters a b c d fig. 3a-d. reconstruction of p for asymmetric inhomogeneity. a) the model; b,d) results of reconstruction; b) solution of the system of eq. (2.18) with 3 = 0; c,d) representation of reconstructed inhomogeneity by three and one simple parametric function correspondingly. the regularizing coefficients 1 , 2 , 3 (2.18). the errors of reconstruction in considered numerical example are 50% (fig. 4b) and 30% (fig. 4c). the accuracy of reconstruction of inhomogeneities of electrical conductivity has a stronger dependence on the values of the regularizing coefficients in comparison with the elastic case. this difference can be explained by greater 348 vladimir n. troyan and yurii v. kiselev fig. 4a-c. reconstruction of electrical conductivity. a) the model; b,c) results of reconstruction; in the case (b) the regularizing coefficients are ten times greater than in the case (c). a b c 349 mathematical modeling to reconstruct elastic and geoelectrical parameters distortion of a shape of the scattered signal in the case of reconstruction of electrical conductivity. acknowledgements this investigation was supported by intas99-1102, russian foundation of basic research (grant 02-05-65081), ministry of education (grant e00-9.0-82) and «intergeofizika». references alumbaugh, d.l. and h.f. morrison (1995): theoretical and practical consideration for crosswell electromagnetic tomography assuming a cylindrical geometry, geophysics, 60, 846-870. beylkin, g. and r. burridge (1990): linearized inverse scattering problems in acoustics and elasticity, wave motion, 12 (1), 15-52. devaney, a.j. (1984): geophysical diffraction tomography, ieee trans. geosci. remote sensing, ge-22 (1), 3-13. devaney, a.j. and d. zhang (1991): geophysical diffraction tomography in a layered background, wave motion, 14 (3), 243-265. keller, j.b. (1969): accuracy and validity of the born and rytov approximations, j. opt. soc. am., 59, 1003-1004. kiselev, yu.v. and v.n. troyan (1997): estimation of elastic parameters in diffraction tomography, in proceedings of the international seminar «day on diffraction-97», st. petersburg, russia, 3-5 june 1997, 135-139. ryzhikov, g.a. and v.n. troyan (1994): tomography and inverse sounding problems (st. petersburg university, st. petersburg), pp. 220. saintenoy, a.c. and a. tarantola (2001): groundpenetrating radar: analysis of point diffractors for modeling and inversion, geophysics, 66, 540-550. slaney, m., a.c. kak and l.e. larsen (1984): limitation of imaging with first-order diffraction tomography, ieee trans. microwave theory tech., mtt-32 (8), 860-874. troyan, v.n. and g.a. ryzhikov (1994): threedimensional diffraction tomography functionals, in proceedings of 2nd geophysics congress, thessaloniki, 340-348. wu, r. and k. aki (1985): scattering characteristics of elastic waves by an elastic heterogeneity, geophysics, 50, 585-592. zhou, q., a. becker and h.f. morrison (1993): audiofrequency electromagnetic tomography, geophysics, 58, 482-495. microsoft word 7413-18297-1-ed_lucchesi.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 1 geosciences at the service of society: the path traced by antonio stoppani stefania lucchesi phd geologist, fiano romano (rm) italy srstefanialucchesi@libero.it abstract sometime ago, the italian geologist antonio stoppani (1824-1891) highlighted the importance of earth sciences for education, cultural awareness, development and progress of society. in his introduction of the concept “anthropozoic era”, he stressed the importance of human beings as a real “geological agent”, but also emphasized that mankind has the capacity and the responsibility to contribute to an ethical management of georesources. currently many consider him one of the “fathers of geoethics”, since he introduced some of the geoethical criteria that should guide researchers, scholars, professionals and decision makers in the field of earth sciences. among these ideas, he enhanced the dimensions of beauty and harmony of nature as foundations of geo-conservation and geodiversity and the necessity for interdisciplinary collaboration between physical and human sciences, including ethics, to manage the complexity of the ecosystem, for the assessment and mitigation of risk, and for a more integrated growth of human beings while in synergy with the environment. 1. who was antonio stoppani? toppani was born in lecco (lombardy, northern italy) in 1824 in a modest patriarchal family. the historical context in which he lived is characterized by strong political and social struggles in relation to the establishment of the unification of italy, events that deeply affected also his scientific career. after studying philosophy and theology, in 1848 he became a priest and began teaching latin, literature and music. he openly expressed his liberal ideas and participated actively in the independence wars of italy in 1848-49 against the austro-hungarian empire, but unfortunately, given his ideas and active political participation, he was discharged from his teaching position and considered as dangerous by the austrian chancellery. despite these events, his reputation as a scholar allowed him to find employment as a teacher for a family in milan, where he could also devote himself more assiduously to geology and paleontology. in 1856 he published his first great scientific work studi geologici e paleontologici sulla lombardia (geological and paleontological studies of lombardy), a true "revelation to scientists and to himself" (nangheroni, 1975) that earned him the nickname as the “founder of italian geology” by his contemporaries. in 1860, he obtained the high school teaching qualification in natural history from the government for his talents and abilities in the field of the natural sciences, even though he had never received a degree. in 1861, he was named "outstanding teacher" of geology at the university of pavia, in 1863 he taught geognosy (a field comprehensive of lithology, stratigraphy and paleontology) and applied mineralogy at the regio istituto tecnico of milan, the future politecnico, and from 1865 to 1868 he became curator of the civic museum of natural history in milan. in 1874 he was appointed president of the italian society of natural sciences, then in 1875 he was elected honorary member of the pontifical academy of nuovi lincei and in 1877 the ministry of education awarded him the title of officer of the order of the crown of italy for "the s annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 2 high services he presents in public education" (redondi, 2009). from 1877 to 1882 he was the chair of geology at the istituto di studi superiori pratici e di perfezionamento in florence, in 1882 he became director of the museum of natural sciences and philosophy in milan and lastly he taught geology at the istituto tecnico superiore until his death in 1891. the results of his studies are contained in many scientific works, but also in more informal writings, both entertaining and didactic works. among the major examples: studi geologici e paleontologici sulla lombardia (geological and paelontological studies on lombardy) (1856), paleontologie lombarde (paleontology of lombardy) (1856-1881), rivista geologica della lombardia in rapporto con la carta geologica della regione (geological review of lombardy in relation with the geological map of the region) (1859), corso di geologia (course of geology) (1871-1873), a treaty in three volumes on geology, petrography and geomorphology, l’ambra nella storia e nella geologia (the amber in history and geology) (1886), but also some more popular works such as il bel paese (the beautifull contry) (1875) and acqua ed aria, la purezza del mare e dell'atmosfera (water and air, the purity of the sea and of the atmosphere) (1875). 2. stoppani’s contribution to geoeducation and geoethics stoppani was an eclectic man with many interests and passions that blended with a great commitment to the different areas that affected his life: science, literature, education, social and political commitment, and faith. he is regarded by his contemporaries and scholars of all time as "a great scientist, a great naturalist, a great geologist" (nangheroni, 1975). his passion for the earth sciences began as a child as a spontaneous inclination and from 1850 onwards he devoted himself with more assiduity to the study of geology, paleontology, geomorphology and metamorphic petrography of northern italy, but also of the middle east and northern africa. in 1865 he was appointed chairman of the special section on prehistory at the international congress of prehistoric anthropology in la spezia (italy). later, he started to build the museum of natural sciences in milan with the purpose to make available to the general public the knowledge gained by science, and in particular the "treasures of nature", such as those kept in museum collections. the museum opened in 1892, one year after his death. stoppani is perhaps best known to the public as a great lecturer, conversationalist and disseminator for earth sciences. he was considered "the poet of geology" by his contemporaries, thanks to his outgoing character and the enthusiasm used in his lessons and expositions. his passion communicated what he believed, that "every mountain, every rock, every stone, every grain of earth, has a record to tell us" (stoppani, 1971-1973). even pope pius xi (1857-1939) said that “he had the incredibly beautiful gift of an enviable disclosure [...] so great was in him the art to exhibit and to make easy and understandable the scientific thought. dressing with beauty any scientific truth was his constant concern” (leucensis, 1921). his artistic temperament and his popular science vein were highlighted with il mare glaciale a’ piedi delle alpi (the glacial sea at the base of the alps) (1874) and he reached his most fame with il bel paese (1875) that made him one of the first italian writers who started the art of educating the scientific knowledge through storytelling. il bel paese, commonly used as schoolbook, consists of thirty “conversations” where an uncle, the author himself, in a colloquial form, but figure 1: stoppani teaching sciences of italy (il bel paese) to his grandchildren and friends (drawing by e. magistretti preserved at the natural history museum of lecco, italy). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 3 with scientific rigor, describes the natural beauty, geology and physical geography of italy to an audience represented by grandchildren and their friends (fig. 1). this work had a dual purpose: first, it made accessible to italian citizens the knowledge of the beauties of their country, convinced that "we should start with the ‘know yourself’, by knowing that the physical and natural history of their own country" (stoppani, 1875); next, his work aimed to include the literary genre of science fiction, like that of jules verne (1828-1905), as well as to affirm the search of truth from a scientific point of view. 3. the bases of geoethics and the importance of earth sciences in cultural and social growth stoppani is one of the first scientists who understood and communicated the significance of earth sciences as a means for cultural and human growth of society. this may be appreciated as a precursor to geoethical thinking. in particular, he realized that human beings, with the use of reason, are able to change nature and to actively shape the earth surface, and thus define humans as a real "geological agent." having understood the importance of human impact on nature, he felt the need to introduce in the corso di geologia (course of geology” (1871-1873) the concept “anthropozoic era” for defining the latest geological period in which human action began to be significant and decisive in the evolution dynamic of the earth. this intuition, that somehow anticipates the modern concept of anthropocene (crutzen, 2002; steffen et al., 2004), can be considered a preamble of the thought which would deepen later, in nineteenth century, with the birth of geoethics (peppoloni and di capua, 2012; bobrowsky et al., 2017). figure 2: urban fossil on a sidewalk of the city of rome (italy): the travertine sedimentary structures are juxtaposed to the human footprints in cement. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 4 the intuition of an “anthropozoic era”, as well as other concepts developed by stoppani, highlights the topicality and foresight of his conception of science. below are some of the main pillars on which he developed his thinking and scientific approach, and that can be considered as the basis of geoethical thinking. 3.1 human beings as a geological agent stoppani, in the chronological description of the events that have characterized the different geological times, following the neozoic age, describes the “anthropozoic era” and divides it into stone age, bronze age and iron age. in this context, human beings are defined as "an agent, perhaps new, certainly still inexperienced of his power, ... called upon to put the last touches to the work of the ages" (stoppani, 1871-1873). in his vision it is therefore clear that “man” is the last of the agents involved in the evolution of the earth, a "new agent" able to "impose itself on nature" (stoppani, 1871-1873), to interact with it and to modify it with its action (fig. 2). from the awareness that human beings have of their action and ability to alter the dynamics of nature comes the responsibility that one has in respect to the earth system (bobrowsky et al., 2017): through knowledge and progress, human beings can actively promote geoconservation and sustainable development respectful of nature and society (peppoloni and di capua, 2016). these issues have been included in the “geoethical promise” (matteucci et al., 2014), intended as an oath for geoscientists in relation to their responsibilities towards the earth, society and future generations (di capua et al., 2016). 3.2 science as a service to the truth through the analyticalexperimental method in the exemeron (1894), stoppani examines the validity of the experimental method of analysis in scientific research, which consists mainly of a collection of research and experiential data, using reasoned comparison of phenomena and researching their causes according to an inductive process (stoppani, 1873 and 1894). he believes that only by using this approach could geology take on the role of a science and give a systematic approach to scientific research. at the same time stoppani asserts that intellectual speculation must also deal with its own limits as the history of science demonstrates: scientific research is performed through a continuous process of partial achievements and mistakes that allow it to reach, a bit at a time, some truth (stoppani, 1894). this method, although now accepted as part of scientific research, is relevant not only for its methodological meaning, but also for its values. this method commits us to look for and then communicate the objectivity seen and known by experimentation. only later, after a phase of data collection, can a series of elaborations, interpretative models and evolutionary scenarios be drawn, but remembering that they are still in the context of interpretations and one cannot claim to be absolutely certain. as a consequence, the objectivity and limits of the results of scientific research are to be declared also in the dissemination phase for the principle of intellectual honesty (peppoloni et al., 2015). stoppani claims that “sciences are called to give a service to the truth” (stoppani, 1894) thus highlighting the value that earth sciences have in overcoming ignorance, searching the common good for society, promoting geoconservation and developing a geoethical approach in environmental management and introducing geoethics in scientific curricula (peppoloni and di capua, 2016; vasconcelos, 2016; bobrowsky et al., 2017; stewart and gill, 2017). 3.3 the earth as a global heritage to be preserved in its uniqueness and geodiversity the great value attributed by stoppani to the natural sciences is linked to his conception of planet earth, perceived as a reality that has in itself its own sacredness. therefore for its preciousness and uniqueness is an asset to be preserved (stoppani, 1875 and 1881). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 5 he underlines how often the "image of nature" unconsciously reminds all of a sort of "sacred" value that can be found in nature. the recognition of this value is linked to the dimensions of beauty and harmony of nature that are at the basis of geoconservation and the protection of geodiversity (lucchesi and giardino, 2012). in this context, stoppani promoted the educational role of the scientific museum for a positive “globalization” and popularization of knowledge (greco, 2007). he was also one of the first admirers of "geosites", of those places that for their scientific, environment and cultural value are to be considered true monuments of nature and part of the geological and geomorphological heritage of an area (reynard, 2003) (fig. 3). 3.4 holistic view of science and mankind for stoppani science was always at the service of mankind, and therefore also scientific progress, as it is part of human progress, should be respectful of every person (stoppani, 18711873). however, he believed as a critical point in the separation between scientific knowledge and human sciences and highlighted the need for a unified vision of knowledge in order to avoid a segmentation due to a partial or sectoral approach to the truth. he argued that the fragmentation of knowledge is a cause of error and cannot lead to a global response in a complex world ((stoppani, 1873). today more than ever, we are facing a strong fragmentation of knowledge that often does not allow one to have an integrated vision of human beings. that is why stoppani affirmed the need to grow within an interdisciplinary dialogue, marked by unity and distinction between scientific research and human sciences, between physical and metaphysical sciences, between science and ethics. a unified vision of science is therefore critical to the search for the integral good of human beings in the various dimensions that characterize their life and society, for the growth of the common good, the development of peoples and a responsible citizenship. such an integrated approach to science, including its economic, ethical, social, and cultural impact, is presently fundamental to manage the complexity of the figure 3: the toce fall (verbania, italy) in a drawing of 1875 from il bel paese (stoppani, 1875) (left) and in a recent photo (right). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 6 earth system (mejlgaard and bloch, 2012; bobrowsky et al., 2017). 3.5 the earth system one of the most innovative and also modern aspects of stoppani’s thoughts is the vision of the earth that he conceived as a “great machine [...] governed by a supreme principle” that exists through a continuous and simultaneous competition including antagonism from endogenous agents, physical forces, chemical and mechanical properties, that “are called to entertain what you can call globe life", but which also contribute to "the biological forces [...] order to maintain that wonderful circle in motion, that noble balance, so there is variety together with units "(stoppani, 1871-1973 and 1873). for stoppani the force that maintains the balance in the world, considering the complexity and the relational system that characterizes the earth, is the continuous antagonism of natural agents. in this sense, stoppani approaches the more recent gaia hypothesis by james lovelock and lynn margulis, that conceives the earth as a single organism in which living and non-living things are connected to each other by negative retroactive processes that tend to maintain the stability of the main parameters that allow life on the planet (lovelock, 1979). stoppani, albeit in the simplicity of his language and based on the knowledge of his time, seems to have launched the foundations for a modern approach to the study of the earth system from a perspective of dynamic relationships between the various components of the ecosystem. this vision is now indispensable for environmental protection, proper management of georesources, evaluation and risk mitigation, all closely related with geoethical issues and common good (fig. 4). 7. conclusions stoppani as a passionate scholar of earth sciences was able to use his knowledge and his professional skills for training, education and human promotion of the society of his time. the contribution of stoppani is therefore of great significance in establishing the criteria that should guide, also nowadays, researchers, scholars, educators, professionals and decision makers in their work and activities in the field of earth sciences according to a geoethical approach. his importance is also due to his commitment to earth sciences education as mean to improve a proactive citizenship among the italian people, a great challenge that today has become a worldwide necessity. rightfully he should be considered as one of the fathers of geoethics and geoeducation. references bobrowsky p., cronin v.s., di capua g., kieffer s.w. and peppoloni s. (2017). the emerging field of geoethics. in: scientific integrity and ethics with applications to the geosciences, edited by gundersen l.c. figure 4: sketch of the dynamic relations inside the earth system and their interconnection and relationship with human’s responsibility. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7413 7 special publication american geophysical union, john wiley and sons, inc. crutzen p.j. (2002). geology of mankind. nature, 415: 23. di capua g., peppoloni s. and bobrowsky p. (2016). the cape town statement on geoethics. with the contributions of bilham n., bohle m., clay a., lopera e.h., mogk d. iapg international association for promoting geoethics. greco p. (2007). science museums in a knowledge-based society, journal of science communication, 6(2): 1-3. leucensis (1921). antonio stoppani, stab. tipografico l'italia, soc. anonima ed., milano. lovelock j. (1979). gaia, a new look at life on earth, oxford university. lucchesi s. and m. giardino (2012). the role of geoscientists in human progress, annals of geophysics, 55(3): 355-359. matteucci r., gosso g., peppoloni s., piacente s. and wasowski j. (2014). the geoethical promise: a proposal, episodes, 37(3): 190191. mejlgaard n. and bloch c. (2012). science in society in europe, science and public policy, 39(6): 695–700. nangheroni g. (1975). antonio stoppani (18241891): natura, patria, religione, atti soc. it. sc. nat. museo civ. st. nat., milano, 116(34): 161-182. peppoloni s., bobrowsky p. and di capua g. (2015). geoethics: a challenge for the research integrity in geosciences. integrity in the global research arena, pp. 287-294, in: steneck n., anderson m., kleinert s., and mayer t., eds. integrity in the global research arena, world scientific publishing co. peppoloni s. and di capua g. (2012). geoethics and geological culture: awareness, responsibility and challenges. in: peppoloni, s. and di capua, g. (eds.). geoethics and geological culture. reflections from the geoitalia conference 2011, annals of geophysics, special issue, 55(3): 335-341. peppoloni s. and di capua g. (2016). geoethics: ethical, social, and cultural values in geosciences research, practice, and education. pp. 17-21. in: wessel, g.r., and greenberg, j.k., eds. geoscience for the public good and global development: toward a sustainable future: geological society of america special paper 520, 2016. redondi p. (2009). stoppani in cattedra, nuovi documenti sull’insegnamento della geologia nel secondo ottocento, istituzioni, attori e ideali di un secolo di cultura scientifica a milano 1863 – 1963, lettera del ministero della pubblica istruzione a francesco brioschi al direttore del r. istituto tecnico superiore di milano, roma, 27 luglio 1877. www.milanocittadellescienze.it reynard e. (2003). “geosites” definition. in: routledge encyclopedia of geomorphology, ed. goudie a., school of geography and the environment, university of oxford. steffen w., sanderson a., tyson p.d., jäger j., matson p.a., moore b. iii, oldfield f., richardson k., schellnhuber h.j., turner b.l. and wasson r.j. (2004). the anthropocene era: how humans are changing the earth system. in: global change and the earth system: a planet under pressure, springerverlag berlin heidelberg new york, 81-141. stewart i.s. and gill j.c. (2017). social geology integrating sustainability concepts into earth sciences. proc. geol. assoc., 128 (2): 165–172. stoppani a. (1871-1873). corso di geologia, 3 voll., bernardoni, milano. stoppani a. (1873). acqua ed aria ossia la purezza del mare e dell'atmosfera fin dai primordi del mondo animato. conferenze. società editrice internazionale, tip. salesiana, torino. stoppani a. 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(2016). sustainable development and its connection to teaching geoethics. episodes, international journal of geosciences, 39(3): 509–517. vol51,1,2008_delnegro 147 annals of geophysics, vol. 51, n. 1, february 2008 key words ground penetrating radar – magnetic anomalies – electromagnetic scattering – microwave tomography – linear inverse scattering 1. introduction historically, electromagnetic inverse scattering has been focused much more on dielectric anomalies than on magnetic anomalies (colton and kress, 1992; chew, 1995). this is because in the microwave range «to deal with» a dielectric contrast is certainly more common than «to deal with» a magnetic contrast. however, in some cases magnetic anomalies can occur too. this can happen, for example, in the case of buried ceramic pipes or buried brick walls. also, the study presented in this paper is of interest in the diagnostics of new materials being invented nowadays, some of which show magnetic properties in the microwave range (jarvis et al., 2004; chen et al., 2005). moreover, in some situations the soil itself exhibits magnetic more than electric properties. this occurs in presence of some magnetic minerals (nabighian, 1987) such as magnetite, hematite, maghemite and/or iron in its free state, or when the soil is strongly polluted by industrial contaminants. finally, the very recent development of the mars exploration programmes, such as marsis and sharad, has aroused a significant interest in the influence of magnetic minerals on the loss and propagation characteristics of electromagnetic waves in the gpr frequency range (stillman and olhoeft, 2004, 2006). for this reason, the scattering from electric and magnetic anomalies in an electric and magnetic soil is also worth studying. some recent works have dealt with inverse scattering from magnetic anomalies. in particular, gustafsson and he (2000) tackled the two dimensional inverse scattering problem related to anomalies that show both dielectric and magnetic contrast starting from both the measurement of the electric and magnetic scattered field. in addition, abubakar and van der berg (2004) addressed the homologous three dimensional problem. computational problems related to the forward model were dealt with in nie et al. (2006). however, gustafsson and he (2000) and nie et al. (2006) two dimensional inverse scattering from buried magnetic anomalies raffaele persico (1) and francesco soldovieri (2) (1) istituto per i beni archeologici e monumentali (ibam-cnr), lecce, italy (2) istituto per il rilevamento elettromagnetico dell’ambiente (irea-cnr), napoli, italy abstract this paper deals with the problem of the electromagnetic linear inverse scattering from magnetic anomalies buried in a lossy half space, for a scalar and two dimensional case. first, the formulation of the exact model of the electromagnetic scattering is given. then, the linear inverse problem is solved by resorting to the well assessed singular value decomposition tool. the reliability of the solution procedure is tested with synthetic data achieved by a fdtd code. mailing address: dr. raffaele persico, istituto per i beni archeologici e monumentali (ibam-cnr), via per monteroni, campus universitario, 73100 lecce, italy; email: r.persico@ibam.cnr.it vol51,1,2008_delnegro 16-02-2009 21:28 pagina 147 148 r. persico and f. soldovieri refer to the case of objects embedded in a homogeneous medium, and gustafsson and he (2000) and, abubakar and van den berg (2004) cast the inverse problem by assuming as datum both the electrical and magnetic scattered fields. unlike from the papers mentioned above, here we consider a half space two-dimensional geometry and assume as datum of the problem only the scattered electric field. this choice is justified because in gpr prospecting one essentially gathers a quantity roughly proportional to the electric field in the observation point. the paper is organised as follows. section ii introduces the scattering equations relative to the «magnetic» two dimensional scalar scattering operator (providing all the relevant calculations in appendix). the same section also proposes an inversion algorithm based on a linear model of electromagnetic scattering. such a simplified model is similar to the well known born model widely exploited for dielectric objects (crocco and soldovieri, 2003; soldovieri et al., 2007). section iii presents numerical examples where the linear inversion is exploited to process exact scattered field data provided by means of an fdtd code. finally, conclusions follow. 2. formulation of the problem the 2d reference scenario is composed of two homogeneous half spaces, separated by a planar interface at z=0 (fig. 1). the upper half space is free space, whereas the lower half space is characterized by a relative dielectric permittivity εs and by a relative magnetic permeability µs. both εs and µs can be complex to account for losses. we consider the forward and inverse scattering problems in frequency domain. the source is assumed to be infinitely long and invariant along the y-axis. the considered measurement configuration is multi-bistatic (i.e. we consider a bscan prospecting with a fixed offset between the transmitting and the receiving antennas) within a prefixed band of frequencies ω = [ωmin,ωmax]. the source-observation point ranges within the observation domain σ=[−xm, xm] located at the air/soil interface (z=0). the objects are assumed infinitely long along the y-axis, embedded in the lower half space and they are assumed to be located in the investigation domain d=[−a, a]×[zmin, zmin + 2b]. our aim is to obtain a spatial map of the magnetic properties of the investigation domain d, and on the basis of this map to infer the presence, location and geometry of the magnetic anomalies. thus, the quantity of interest is the relative magnetic permeability µr(x, z) in the domain d; this drives to assume, as actual unknown of the problem, the magnetic contrast function . (2.1) it is assumed that the magnetic contrast is different from zero only inside the investigation domain d. in general, the magnetic contrast depends on the frequency. here, such a dependence will be neglected in the inversion model. it is supposed that the buried objects do not show a meaningful electric contrast with the surrounding soil. the equations of the exact electromagnetic scattering from buried magnetic anomalies are fully derived in the appendix. the final result is given as (2.2) ( , ) ( , ) ( , , , ) ( , ) ( , ) ( , ) ( , ) e x z e x z k g x z x z e x z x z x z dx dz x z d 1 inc s im d m m 2 4 d χ χ = + + l l l l l l l l l l ## ( , ) ( , ) x z x z m s r s χ µ µ µ = fig. 1. geometry of the problem for the two dimensional case and for the adopted multi-bistatic configuration vol51,1,2008_delnegro 16-02-2009 21:28 pagina 148 149 two dimensional inverse scattering from buried magnetic anomalies (2.3) where ks is the wave-number in the soil (lower half-space). equation (2.2) accounts for the total electric field inside the investigation domain d and is given as the sum of two contributions: the incident field (i.e., the field in the investigation domain d in absence of the magnetic anomalies) and the field due to the presence of scattering objects in the investigation domain. thus denotes the internal magnetic dyadic green’s function (row vector of two elements) where (2.4) equation (2.3) accounts for the scattered electric field on the measurement domain σ, that represents the datum of an inverse scattering problem. the scattered field can be regarded as the electric field radiated by a magnetic current density. this magnetic current is related to the product of the gradient of the electric field inside the investigation domain times the function involving the unknown contrast. the denotes the external magnetic green’s function (row vector of two elements) where [ , ]g g g imz eim emx e= ( , ) ( ( , ))x z x z1m mχ χ+l l l l = ( ) ( ( ) ) ( ) ( ) ( ) ( ) ( exp exp g k sign z z jk u z z k u k u k u k u jk 4 1 1 imz s zs s zo o zs s zo o zs zs 2π µ µ µ µ + + 3 3 + ( ( ))exp ju x x du( ) ( ))u z z+ l l l l d n # ( ( ) ) ( ) ( ) ( ) ( ) ( ( )) exp exp g k k u jk u z z k u k u k u k u ju x x du 4 1 imx zs s zs o zs s zo o zs s zo zs 2π µ µ µ µ = + + 3 3 + ( ( ) ( ))exp jk u z z+ + l l l d n # [ , ]g g im imx imz=g ( , ) ( , , , ) ( , ) ( , ) ( , ) ( , ) e x z k g x z x z e x z x z x z dx dz x z 1 s s em d m m 2 4 d / χ χ = + l l l l l l l l l l ## (2.5) the gradient vector of the total field is defined in terms of the unique component of the electric total field. in formulas, this gradient is defined as (2.6) similarly to the more widely studied case of dielectric buried anomalies (crocco and soldovieri, 2003; soldovieri et al., 2007), the scattering eqs. (2.2) and (2.3) make the inverse scattering problem non-linear and ill-posed. in order to cope with this difficulty, we adopt a simplified model of the scattering that allows us to deal with a linear inverse scattering problem. the linearization will prevent false solutions, intrinsically related to nonlinearity, and will ensure the stability of the solution by adopting well assessed regularization schemes based on the singular value decomposition (bertero and boccacci, 1998). conversely, the adoption of the linear model of the electromagnetic scattering, analogously to the case of reconstruction of dielectric anomalies, does not achieve a quantitative reconstruction of the buried objects but only retrieves information about their location, size and (approximately) shape. in order to achieve a linear model for the problem at hand, we assume the hypothesis of low contrast levels ⎜χm⎜<< 1. in this case we have (2.7) and (2.8) i.e. the total field inside the investigation domain can be approximated with the incident ( , ) ( , )e x z e x zinc, ( , ) ( , ) ( , ) x z x z x z 1 m m m,χ χ χ + l l l l l l e z e x e 4 2 2 2 2 = l l j l k k kk n p o o oo -= ( ( ) ( )) ( ( ) ) ( ( ) ) ( ) ( ( )) exp exp exp g k k u k u jk u z jk u z k u ju x x du 2 1 emz s s o zs s zo o o zo zs zs 2π ε µ µ ε µ + 3 3 + l l # -= ( ( ) ( )) ( ( ) ) ( ( ) ) ( ( )) exp exp exp g k k u k u jk u z jk u z u ju x x du 2 1 emx s s o zs s zo o o zo zs 2π ε µ µ ε µ + 3 3 + l l # vol51,1,2008_delnegro 16-02-2009 21:28 pagina 149 150 r. persico and f. soldovieri field. the approximation (2.8) can be «physically» justified by stating that the objects are small perturbations with respect to the host medium. at this point, we make the further assumption that (2.9) let us state that eq. (2.9) cannot be straightforwardly inferred from eq. (2.8). however, in the framework of low level contrast, eq. (2.9) essentially amounts to assuming some smoothness properties of the incident field. such an assumption is an increasingly reasonable hypothesis as far as the point in the investigation domain is farther and farther from the transmitting antenna. now, by substituting eq. (2.7) and eq. (2.9) in eq. (2.3), we obtain the linear inverse scattering model given by (2.10) thus, the problem at hand is reduced to the inversion of the linear integral relation (2.10), where es represents the datum of the problem while χm is its unknown. in order to completely specify the integral relationship (2.10), we still have to specify the incident field. since the primary source is a filamentary current, the incident field can be calculated in a fashion analogous to the calculation of the green’s functions provided in the appendix. the result is (2.11) where f is the frequency and io is the level of the current. thus, the gradient vector of the incident field is given by ( , , , ) ( ( ) ( )) ( ( ) ) ( ( ) ) ( ( )) exp exp exp e x z x z f i k k jk z jk z jv x x dv inc s o s o s zo o zs zs zo s µ µ µ ν µ ν ν ν = + 3 3 + l l l l # ( , ) ( , ) ( , ) e x z k g e x z x z dx dz z s s em inc d m 2 4 d /χ = l l l l l l ## ( , ) ( , )e x z e x zinc4 4,l l l l . (2.12) 3. numerical results this section shows some numerical results to back-up the previous formulation. the synthetic scattered field data have been obtained by means of the fdtd code gprmax (giannopulos, 2003). therefore, the code for the data is totally independent from the inversion code. in particular, gprmax provides total field data in time domain, therefore we have to preprocess these data to obtain scattered field data in the frequency domain. the pre-processing essentially consists of muting the first part of the traces, relative to the answer to the air-soil interface and to fourier transform the traces after this muting (soldovieri et al., 2006). the zero time is chosen in the first maximum of the simulated traces. equation (2.10) has been discretized by exploiting methods of moments (collin, 1985). in particular a point matching is considered in data space, while the magnetic contrast function has been represented thanks to a functional basis made up of fourier harmonics along the horizontal direction (x-direction) and pulse function along the depth (z-direction). the inversion of the linear system obtained from the discretization of eq. (2.10) has been performed thanks to the truncated svd (tsvd) (bertero and boccacci, 1998) that achieves a stable solution of the problem. in a first example, we considered a square object with sides of 0.2 m, buried in a soil at the depth of 1.7 m (referred to the upper side). the data are gathered in air at 0.01 m from the interface with a spatial step of 0.02 m and an offset between the transmitting and receiving antennas of 0.1 m. the observation line from the first to the last source point is 1.88 m long, so that we have 95 gpr traces. the investigation domain is 1.98m large and 2m deep, and starts ( ( ) ( )) ( ( ) ) ( ( ) ) ( ) ( ( )) exp exp exp e f i k k jk z jk z jv jk v jv x x dv inc o s o o zs s zo zs zo zs s µ µ µ ν µ ν ν ν = + 3 3 + l ld n # vol51,1,2008_delnegro 16-02-2009 21:28 pagina 150 151 two dimensional inverse scattering from buried magnetic anomalies from the air-soil interface. the work frequency band ranges from 200 mhz to 1 ghz with a frequency step of 25 mhz. the soil exhibits a relative dielectric permittivity equal to 9 and a conductivity equal to 0.01 s/m. which corresponds to a complex equivalent permittivity equal to , with εo=8.854× ×10−12 f/m being the dielectric permittivity of the free space. the relative magnetic permeability of the soil is µs=1. the homogeneous buried object shows the same equivalent permittivity of the soil, whereas its relative magnetic permeability is µr=5. we added a white gaussian noise to the synthetic data, so that the signal to noise ratio for the total field was 20 db. for the inversion scheme, we chose to retain in the tsvd expansion only the terms for which the singular values were larger than 0.01 times the maximum singular value. figure 2 depicts the amplitude of the retrieved contrast function normalized with respect to its maximum; the reconstruction is compared with the actual object depicted with a solid line. the tomographic reconstruction ac( . )j f9 0 01 2r oε π ε= curately locates the upper side of the object and determines its horizontal extent. in order to show possible effects of the shape and mutual interactions between the objects, we propose a further example where the parameters are unchanged with respect to the previous one except that we have two circular buried objects instead of one square target. the two circular objects are buried at 0.7 m (with respect to their centres), their radius is 0.1 cm and the distance between the centres is 0.25 m. the reconstruction is now obtained by considering in tsvd expansion the singular values larger than 0.1 the maximum singular value. the tomographic reconstruction is shown in fig. 3 and compared to the actual objects. since the objects are shallower than the previous case, this time we reconstruct both the upper and lower parts of them. however, the reconstruction of the lower parts is deeper than the actual bottom of the objects because the probing wave propagates in the object more slowly than in the surrounding soil, and this is not accounted for fig. 2. modulus of the retrieved contrast function for a square buried magnetic object at the depth of 1.7 m. the reconstruction essentially images the upper side of the object. fig. 3. modulus of the retrieved contrast function for two circular buried magnetic objects at the depth of 0.7 m. the reconstruction essentially images the upper side of the objects and the spot at the dept of 1 m arises for the mutual interactions between the two objects. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 151 152 r. persico and f. soldovieri within a linear model (crocco and soldovieri, 2003; soldovieri et al., 2007). moreover, the mutual electromagnetic interferences (not accounted by a linear model) between the two objects add constructively in a point at the depth of about one meter thereby creating an artefact. finally, let us stress that the adoption of the two different thresholds in tsvd (0.01 for the first case and 0.1 for the second case) makes it possible to investigate regions at different depth in the investigation domain. in particular, by lowering the tsvd threshold, we have the possibility to exploit in the reconstruction a larger number of singular functions. as a matter of fact, this allows us to image deeper objects because the «support» of higher order singular functions are located at progressively increasing depth. 4. conclusions this paper has dealt with the scattering from magnetic buried anomalies. this topic has been rarely addressed in literature, where in most cases it is assumed that both the soil and the buried objects do not have any magnetic property. some times, however, one can meet a situation where the soil or the buried objects or both can have meaningful magnetic properties. as future developments we are working on a complete model of the 2d scattering from buried objects that show both dielectric and magnetic properties. moreover, we will propose an analysis of the errors related to the fact that one may meet a magnetic object while looking for dielectric objects or, conversely, a dielectric object while looking for magnetic ones. as further future developments, we would also like to address the question of the choice of the type of basis functions exploited to represent the unknowns. in fact, this choice affects both the numerical efficiency of the solution algorithm and the accuracy in the representation of the singular functions characterizing the linear operator to be inverted. finally, let us stress that this paper is only the first step towards the application of the approach also in realistic situations: experimental validations are in order. appendix this appendix works out the mathematical formulation of the 2d scattering equations for magnetic anomalies, the source being constituted by a filamentary electrical current (the reference geometry is in fig. 1). let us start from maxwell’s equations (a.1) and let us write the total fields as the superposition of the incident (unperturbed) fields and of the scattered fields. (a.2) . (a.3) due to the two-dimensional geometry and to the kind of source, we know a priori that the electrical h h h inc s = + e e e inc s = + j h h j e j h e e 0 0 0 4# 4# 4$ 4$ ωµ ωε ε ρ µ = = + = = z [ \ ] ]] ] ]] vol51,1,2008_delnegro 16-02-2009 21:28 pagina 152 153 two dimensional inverse scattering from buried magnetic anomalies field is directed along the invariance direction, whereas the magnetic field is orthogonal to it. with respect to the reference system of fig. 1, therefore, we have . (a.4) by substitution of eq. (a.2) and eq. (a.3) in eq. (a.1), after some straightforward passages, we obtain maxwell equations for the scattered fields (a.5) being (a.6) where ∆µ is the difference between the object and the background magnetic permeability. from the third equation in (a.5), we can rewrite the scattered electric field by means of a potential vector (also reported as fitzgerald vector; collin, 1985) f as (a.7) this equation also means that, in general, we can write the potential vector as (a.8) substituting (a.7) in the second of maxwell eq. (a.5) we have (a.9) therefore, we can express the vector (hs−jωf) as the gradient of a scalar potential function φ, and so we have (a.10) by substitution of this equation in the first of maxwell eq. (a.5), we obtain. (a.11) from which, by means of a well known vector identity (collin, 1985) we have (a.12) kb being the wavenumber of the background medium, equal to (a.13) ( ( ) )im k k z k z d k 0 b b b s s s s 0 0 0 d d # / ω µ ε ω µ ε ω µ ε = = = = * ( )f f k f j jb b b b meq 224 4 4: 4 ωε µ εφ= -( )f j j f j 1 b b meq 4# 4# 4ε ωµ ω φ= + h j f h j f s s &4 4ω ωφ φ= = + ( )h j f 0 s 4# ω= ( , ) ( , )f f x z i f x z ix x z z= + e f 1 s b 4#ε= j j h meq ω µ∆= e j h j h j e e h j j 0 1 s b s meq s b s s s b meq 4# 4# 4$ 4$ 4$ ωµ ωε ωµ = = = = z [ \ ] ] ] ] ] ] ( , ) ( , ) ( , ) e e x z i h h x z i h x z i s sy y s sx x sz z = = + vol51,1,2008_delnegro 16-02-2009 21:28 pagina 153 154 r. persico and f. soldovieri where ko and ks are the wave number in the free space and in the lower half space, respectively. from eq. (a.12) we have (a.14) at this point, by exploiting the gauge of lorentz (collin, 1985), we can impose (a.15) so that eq. (a.14) evolves in . (a.16) moreover, eq. (a.15) substituted in eq. (a.10) arises the expression of the magnetic scattered field vs. the only potential vector . (a.17) at this point, the problem is recast as the resolution of helmotz eq. (a.16) with radiating boundary conditions at infinity. in order to solve it, let us begin by writing eq. (a.16) along its components. (a.18) in order to solve the problem, as is customary, we consider the fourier transform of all quantities along the x-axis. the adopted convention is (a.19) therefore, in the transformed domain (u, z) eqs. (a.18) are rewritten as (a.20) being . (a.21) the imaginary part of the square roots in eqs. (a.21) is meant to be negative to ensure that the amplitude of the fields vanishes far from the sources. since eqs. (a.20) are linear, we can solve them by adding the solutions of suitable impulsive answers, as it is well known, therefore, let us begin to consider impulsive sources placed in some generic (buried) point (x', z'): (a.22) ( , ) ( ) ( ) ( , ) ( ) ( ) j x z i x x z z j x z i x x z z meqx meqx meqz meqz δ δ δ δ = = l l l l k k u k k u z k k u z d zb b z zs s 2 2 0 0 2 2 2 2 d d / = = = = * ( ) ( ) . z f k u f j z f k u f j x zb x b meqx z zb z b meqz 2 2 2 2 2 2 2 2 2 2 ε ε + = + = t t t t t t ( , ) ( , ) ( ) ,expf u z f x z jux dx i x zi i= = 3 3 + t # . x f z f k f j x f z f k f j x x b x b meqx z z b z b meqz 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 ε ε + + = + + = ( ) h j f j f s b b 4 4$ ω ωε µ= f k f jb b meq 2 24 ε+ = f j 0b b4$ ωµ ε φ+ = ( )f k f f j jb b b b meq 2 24 4 4$ ωµ ε εφ+ = + + vol51,1,2008_delnegro 16-02-2009 21:28 pagina 154 155 two dimensional inverse scattering from buried magnetic anomalies which in the transformed domain becomes (a.23) with this sources, eqs. (a.20) are recast as (a.24) because of the radiation condition at infinity (collin, 1985), both (a.24) equations have a general solution that can be written as (a.25) in order to find the four functions a, b, c and d we have to impose four conditions. these are given as: 1) continuity of the tangential component of the electric field at the air-soil interface. 2) continuity of the tangential component of the magnetic field at the air-soil interface. 3) continuity of the potential vector at the depth zl of the source 4) integrability of the source and of the potential vector (i.e. of eqs. (a.24)) about the depth of the source. an important point is the fact that we can look for a solution parallel to the source both with respect to the x-component and to the z-component of the magnetic current density, i.e. we can solve for two uncoupled problems synthesized as follows: (a.26) thanks to this (not trivial) fact, we can retrieve the eight quantities ax, bx, cx, dx, az, bz, cz, dz, from two uncoupled linear systems the calculations are long but straightforward, and the procedure is quite known. therefore, we can limit to provide the final results of these passages, that are (a.27) ( ( ) ( )) ( ) ( ) ( ( ) ) ( ( ) ) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ) ( ) ( ( ) ( )) exp exp exp exp exp exp exp exp exp f k k u k u j k u i jux jk u z jk u z z k u j i jux jk u z z k u k u k u k u jk u z z z z k u j i jux jk u z z k u k u k u k u jk u z z z z 0 2 0 2 < < < < x zo o zs s zo o o zs meqx zo zs zs s meqx zs s zo o zs s zo o zs zs zs s meqx zs s zo o zs s zo o zs zs µ µ ε µ ε µ µ µ µ ε µ µ µ µ = + + + + + + + l l l l l l l l l l t d d n n = = g g z [ \ ] ] ]] ] ] ]] f f i j j i f f i j j i x x meq meqx x z z meq meqz z , , = = = = t t t t ( ) ( ( ) ) ( ) ( ( ) ) ( ) ( ( ) ) ( ) ( ( ) ) ( , ) exp exp exp exp f a u jk u z z b u jk u z c u jk u z z z d u jk u z z z i x z 0 0 < < < < i i z i zs i zs i zs 0 = + =l l t z [ \ ]] ]] ( ) ( ) ( ) ( ) ( ) ( ) exp exp z f k u f i jux z z z f k u f i jux z z x zb x b meqx z zb z b meqz 2 2 2 2 2 2 2 2 2 2 ε δ ε δ + = + = l l l l t t t t ( , ) ( ) ( ) ( , ) ( ) ( ) exp exp j u z i jux z z j u z i jux z z meqx meqx meqz meqz δ δ = = l l l l t t vol51,1,2008_delnegro 16-02-2009 21:28 pagina 155 156 r. persico and f. soldovieri and (a.28) from the potential vector, we can express the scattered electric field thanks to eq. (a.7), being the curl meant in the transformed domain. the result is (after some further straightforward passages) (a.29) this is the solution in the transformed domain and for an impulsive source. in order to pass to the distributed solution relevant to the case at hand, we express eq. (a.6) versus the scattered field and along its components (a.30) therefore, the elementary magnetic currents to be substituted and then integrated in eq. (a.29) are given by (a.31) after substitution of eq. (a.31) in eq. (a.29) and after integration in the variables and in the inves( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) i x z j x z dx dz x z x z z e x z dx dz i x z j x z dx dz x z x z x e x z dx dz 1 1 meq meq m m y meqz meq m m y x x z 2 2 2 2 χ χ χ χ = = + = = + l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) j x z j h x z j j z e x z z e x z x z x z z e x z j x z j h x z j j x e x z x e x z x z x z x e x z 1 1 1 1 meq x y b b b y m m y meq z y b b b y m m y x z 2 2 2 2 2 2 2 2 2 2 2 2 ω µ ω µ ωµ µ µ µ µ µ χ χ ω µ ω µ ωµ µ µ µ µ µ χ χ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ = = = + = = + = = = + = = + l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l ( ( ) ( ))exp jk u z z++ ( ( ) ( )) ( ) ( ) ( ( ) ) ( ( ) ) ( ( ) ( )) ( ) ( ( ) ) ( ) ( ) ( ) ( ( ) ) ( ) ( ) ( ) ( ) ( ) ( ) ( ( ) ) ( ) ( ) ( ) ( ) ( ( ) ( )) exp exp exp exp exp exp exp exp exp exp exp e k u k u k u i jux jk u z jk u z k u k u u i jux jk u z jk z z i jux sign z z jk u z z k u k u k u k u k u ui jux jk u z z k u k u k u k u jk u z z z d 2 2 sy s o zs s zo o o z meq zs zo s o zs s zo o o meqz zs zo meqz zs s zo o zs s zo o zs zs zs meqz zs o zs s zo o zs s zo zs s x d d / ε µ µ ε µ ε µ µ ε µ µ µ µ µ µ µ µ µ = + + + + + + + + + + l l l l l l l l l l l t d d n n = = g g z [ \ ] ] ] ] ] ] ] ] ]] ( ) ( ) ( ) ( ( ) ) ( ( ) ) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ) ( ) ( ( ) ( )) exp exp exp exp exp exp exp exp exp f k u k u j i jux jk u z jk u z z k u j i jux jk u z z k u k u k u k u jk u z z z z k u j i jux jk u z z k u k u k u k u jk u z z z z 0 2 0 2 < < < < z o zs s zo o o meqz zs zo zs s meqz zs o zs s zo o zs s zo zs zs s meqz zs o zs s zo o zs s zo zs µ µ ε µ ε µ µ µ µ ε µ µ µ µ = + + + + + + + l l l l l l l l l l t d d n n = = g g z [ \ ] ] ]] ] ] ]] vol51,1,2008_delnegro 16-02-2009 21:28 pagina 156 157 two dimensional inverse scattering from buried magnetic anomalies tigation domain d, the scattered field in the domain is obtained. in order to achieve the scattered field in the spatial domain an inverse fourier transform is still required. again, the passages are long but straightforward, therefore we only provide the final result of them a.32 in air and a.33 in the soil. at this point, since the total field in the investigation domain is given by the sum of the incident and the scattered field, eqs. ((a.31)-(a.33)) provide quite immediately eqs. (2.1) and (2.2) in the section 2. ( , ) ( ) ( ( ) ) ( ) ( ) ( ) ( ) ( ( )) ( ( )) ( , ) ( , ) ( , ) ( ( ) ) ( ) ( ) ( ) ( ) ( ( )) ( ( )) ( , ) ( , ) ( , ) exp exp exp exp exp exp e x z k k sign z z jk u z z k u k u k u k u jk z z ju x x du x z x z z e x z dx dz k k k u jk u z z k u k u k u k u jk z z ju x x du x z x z x e x z dx dz z 4 1 1 4 1 1 0> sy s s zs s zo o zs s zo o zs d zs m m s s zs zs o zs s zo o zs s zo d zs m m 2 2 2 2 2 2 2 2 π µ µ µ µ χ χ π µ µ µ µ χ χ = + + + + + + + + + + 3 3 3 3 + + l l l l l l l l l l l l l l l l l l l l l l l l l df df n n n o = = f f ### ### ( , ) ( ( ) ( )) ( ) ( ( ) ) ( ( ) ) ( ( )) ( , ) ( , ) ( , ) ( ( ) ( )) ( ) ( ( ) ) ( ( ) ) ( ( )) ( , ) ( , ) ( , ) exp exp exp exp exp exp e x z k k k u k u k u jk u z jk u z ju x x du x z x z z e x z dx dz k k k u k u u u jk u z jk u z ju x x du x z x z x e x z dx dz z 2 1 1 2 1 1 0 sy s s s o zs s zo o o zs zo zs d m m s s s o zs s zo o o zo zs d m m 2 2 2 2 2 2 2 2 1 π ε µ µ ε µ χ χ π ε µ µ ε µ χ χ = + + + + + + 3 3 3 3 + + l l l l l l l l l l l l l l l l l l l l l l f f p p ### ### references abubakar, a. and p.m. van den berg (2004): iterative forward and inverse algorithms based on domain integral equations for three-dimensional electric and magnetic objects, j. comput. phys., 195, 236-262. bertero, m. and p. boccacci (1998): introduction to inverse problems in imaging (iop, bristol, u.k.). chew, w.c. (1995): waves and fields in inhomogeneous media (ieee ppress, piscataway, n.j.). chen, p., r.x. wu, t. zhao, f. yang and j.q. xiao (2005): complex permittivity and permeability of metallic magnetic granular composites at microwave frequencies, j. phys. d appl. phys., 38, 2302-2305. collin, r.e. (1985): antennas and radiowave propagation (mcgraw-hill). colton, d. and r. kress (1992): inverse acoustic and electromagnetic scattering theory (springer verlag). crocco, l. and f. soldovieri (2003): a microwave tomographic approach for imaging targets buried in a layered medium, ann. geophys., 46 (3), 559-572. dural, d. and m.i. aksun (1995): closed-form green’s functions for general sources and stratified media, ieee trans. microwave theory techniques, 43 (7), 1545-1552. giannopulos, a. (2003): gprmax2d v 1.5 (electromagnetic simulator for ground probing radar). gustafsson, m. and s. he (2000): an optimization approach to two-dimensional time domain electromagnetic inverse problems, radio sci., 35 (2), 525-536. jarvis, j.b., m.d. janezic, b.f. riddle, r.t. johnk, p. kabos, c.l. holloway, r.g. geyer and c.a. grosvenover (2004): measuring the permittivity and permeability of lossy materials: solids, liquids, metals, buildings materials, and negative-index materials, nist technical note 1536. nabighian, m. (1987): electromagnetic methods in applied geophysics-theory (society of exploration geophysics), vol. 1. nie, x.c., n. yuan, l.w. li, y.b. gan and t.s. yeo (2006): a fast combined field volume integral equation solution to em scattering by 3d dielectric objects of vol51,1,2008_delnegro 16-02-2009 21:28 pagina 157 158 r. persico and f. soldovieri tic gpr applications, near surface geophys., 5 (1), 29-42. stillman, d.e. and g.r. olhoeft (2004): gpr and magnetic minerals at mars temperatures, in tenth international conference on ground penetrating radar, 735738. stillman, d.e. and g.r. olhoeft (2006): electromagnetic properties of martian analog minerals at radar frequencies and martian temperatures, in proceedings of the 37th lunar and planetary science conference. young, j.l. and c.m. johnson (2004): a compact recursive trans-impedance green’s function for the inhomogeneous ferrite microwave circulator, ieee trans. microwave theory techniques, 52 (7), 1751-1759. arbitrary permittivity and permeability, ieee trans. antennas prop., 54 (3), 961-969. piscitelli, s., e. rizzo, f. cristallo, v. lapenna, l. crocco, r. persico and f. soldovieri (2007): gpr and microwave tomography for detecting shallow cavities in the historical area of sassi of matera (southern italy), near surface geophysics (accepted). soldovieri, f., r. persico and g. leone (2006): a microwave tomographic imaging approach for multibistatic configuration: the choice of frequency step, ieee trans. instrumentation measurements, 55 (6), 19261934. soldovieri, f., j. hugenschmidt, r. persico and g. leone (2007): a linear inverse scattering algorithm for realisvol51,1,2008_delnegro 16-02-2009 21:28 pagina 158 bais 779_790.pdf annals of geophysics, vol. 45, n. 6, december 2002 779 earth modeling and estimation of the local seismic ground motion due to site geology in complex volcanoclastic areas antonio rapolla (1), giovanni bais (1), pier paolo g. bruno (2) and vincenzo di fiore (1) (1) dipartimento di scienze della terra, università degli studi di napoli «federico ii», napoli, italy (2) osservatorio vesuviano, istituto nazionale di geofisica e vulcanologia, napoli, italy abstract volcanic areas often show complex behaviour as far as seismic waves propagation and seismic motion at surface are concerned. in fact, the finite lateral extent of surface layers such as lava flows, blocks, differential welding and/or zeolitization within pyroclastic deposits, introduces in the propagation of seismic waves effects such as the generation of surface waves at the edge, resonance in lateral direction, diffractions and scattering of energy, which tend to modify the amplitude as well as the duration of the ground motion. the irregular topographic surface, typical of volcanic areas, also strongly influences the seismic site response. despite this heterogeneity, it is unfortunately a common geophysical and engineering practice to evaluate even in volcanic environments the subsurface velocity field with monodimensional investigation method (i.e. geognostic soundings, refraction survey, down-hole, etc.) prior to the seismic site response computation which in a such cases is obviously also made with 1d algorithms. this approach often leads to highly inaccurate results. in this paper we use a different approach, i.e. a fully 2d p-wave «turning ray» tomographic survey followed by 2d seismic site response modeling. we report here the results of this approach in three sites located at short distance from mt. vesuvius and campi flegrei and characterized by overburdens constituted by volcanoclastic deposits with large lateral and vertical variations of their elastic properties. comparison between 1d and 2d dynamic amplification factor shows in all reported cases entirely different results, both in terms of peak period and spectral contents, as expected from the clear bidimensionality of the geological section. therefore, these studies suggest evaluating carefully the subsoil geological structures in areas characterized by possible large lateral and vertical variations of the elastic properties in order to reach correct seismic site response curves to be used for engineering projects. 1. introduction volcanic areas are often strongly heterogeneous. they may present large vertical and horizontal variations both in geometry and in their physical characteristics due to their peculiar sedimentation mechanism and to sin-depositional and post-depositional chemical processes. in particular, surge or pyroclastic flow deposits may show strong horizontal and vertical heterogeneities due to both variable granulometry and variable ambient deposition, temperature and chemistry. variations of these parameters cause lithification processes to proceed differently even at a very small scale ( few or tens of meters). despite this heterogeneity, it is common practice, even in a geologically complex environment, like a volcanic area, to evaluate the seismic site response by means of the seismic dynamic amplification factor (or daf, see mailing address: prof. antonio rapolla, dipartimento di scienze della terra, università degli studi di napoli «federico ii», largo san marcellino 10, 80138 napoli, italy; e-mail: rapolla@unina.it key words dynamic amplification factor − seismic tomography − pyroclastic rocks − neapolitan area − finite element method 780 antonio rapolla, giovanni bais, pier paolo g. bruno and vincenzo di fiore later) with monodimensional investigation methods. this approach may lead to highly inaccurate results. a correct (2d or even 3d) daf estimation requires in fact an accurate reconstruction of the subsurface velocity field with a detail that is impossible to obtain in complex areas using conventional seismic methods. such seismic surveyings (i.e. downhole, seismic refraction or even seismic reflection) are specifically directed towards obtaining layered models with sharp interfaces and constant material properties. on the other hand, pyroclastic deposits may show heterogeneities at all scales as demonstrated by geology or welllogs. each ‘layer’ may contains numerous small scale variations of the material properties and these inhomogeneities are so irregularly distributed that they can hardly be imaged with traditional techniques. we have utilized a 2d tomographic technique to correctly define the subsurface structures. then, by means of a 2d daf methodology, the seismic site response was evaluated in such geological complex environments. 2. brief review of pyroclastic deposits properties a pyroclastic deposit is the result of the sedimentation of clastic material produced by one or more pyroclastic eruptions. pyroclastic products can be classified according to the mechanism of emplacement in: a) pyroclastic flow deposits; b) surges deposits; c) fallout deposits. pyroclastic flows are heavier-than-air cloudflows that travel across the ground at velocities ranging from 10 m/s to 300 m/s. they can attain high temperatures and range from high density flows that move down valleys to dilute flows that extend over mountains (fisher, 1966; fisher and smincke,1984). pyroclastic flows are gravity flows and some properties (density and viscosity, for example) can change during transport and consequently influence the depositional properties. pyroclastic surges are relatively low-concentration and highly expanded density currents with particles supported mainly by turbulence (fisher, 1971) and particle concentration increasing within the lower part of the surge (valentine, 1987). pyroclastic surges may be frequently considered equal to «dilute pyroclastic flow». lateral facies transitions in pyroclastic flow deposits show that pyroclastic flows and surges commonly evolve one from the other. flow and surge, hence, may move as turbulent flows, in which the fragments with the higher settling velocities tend to migrate toward the base of the flow, gradually increasing fragment concentration and causing a vertical gradual variation in bulk density (fisher, 1966, 1971, 1983). lateral variation may also be found for example in the ignimbritic deposits (i.e. rich pumice and glass pyroclastic flow deposits) depending on emplacement temperature; ignimbrites range in fact from unconsolidated to welded ignimbrites (fisher and smincke, 1984). on the other hand, pyroclastic fallout consists of fragments that have been ejected from vents and have travelled through the atmosphere before falling (frequently almost cold) down to the earth. due to their depositional mechanism and the consequent granulometric sorting with distance and homogenous deposition temperature, fallout deposits are less chaotic than surge and flow deposits at the scale of the engineering aimed seismic prospecting. the interaction of depositional mechanism with topographic irregularities is also an important factor in the development of the pyroclastic properties. whereas fall out falling down to the earth, drapes over the landscape and pyroclastic surges, due to their dilute characters can override the sides of a valley and may mantle topography similarly to fallout, flow deposits tend to fill depressions (fisher, 1966). therefore, topography strongly affects geometry and thickness of pyroclastic deposits. 3. seismic imaging with traveltime tomography seismic refraction traveltime data for imaging the subsurface is a standard, easy and lowcost technique. however, conventional processing of refraction data generally uses simplified modeling. on the contrary, the information on the geometry and mechanical characteristics of the shallow subsurface is essential for seismic site response when in the 781 earth modeling and estimation of the local seismic ground motion due to site geology in complex volcanoclastic areas presence of heterogeneous subsurfaces like pyroclastic flow and surge deposit. traveltime tomography may furnish a better detailed subsurface velocity model from picked traveltimes measured from seismic data associated with a variety of source-receiver configurations. traveltimes are then used to infer the velocity distribution of the medium through which the seismic energy has propagated. it should be noted that the «traveltime tomography» reconstructs earth velocity models with lower resolution than the other well know technique called «waveform tomography» (lo and inderwiesen, 1994), but, on the other hand, traveltime tomography is, in general, easier to apply and computationally faster. the tomographic method employed in this study (see stefani,1995) is the turning ray tomography (trt). trt is based on first arrivals and this fact is appealing because first arrival times can be generally clearly identified and represent the best data available for near-surface velocity estimation. as said, trt is useful for estimating near surface velocity structure in areas where conventional refraction techniques fail because of the lack of sharp velocity discontinuity and works well in those sites where the overburden is heterogeneous. the method utilizes first-arrival turning rays (continuously refracted direct rays) and head waves from conventional acquisition geometries to iteratively solve for p wave velocity in the near surface between sources and receivers. therefore, trt parametrization includes only velocity gradients without sharp surface discontinuities and utilizes an inversion algorithm based on the «backprojection method» (langan et al., 1984; stork and clayton, 1991). traveltime tomography and particularly trt, have been used mainly for near-surface velocity determination, e.g., for the purpose of «statics» solutions for seismic reflections (e.g., in the areas with large lateral velocity variations; see for example the paper by bell et al. (1994) relative to the mississippi delta area characterized by mudlumps), with the aim of improving the stacking and migration of the deep seismic reflection image, to image salt domes or to check the area of dams (yilmaz, 1988; hale et al., 1992; marsden, 1993; frasheri et al., 1999). other applications can be found in zhu and mcmechan (1989), zhu et al. (2000). these studies suggest great caution when assuming a simple and regular stratification and homogeneity in the overburden as could be in a first approximation assumed for sake of simplification of the computation and mainly for surveying cost reduction. this approximation can only be assumed for a very limited range of divergence from the 1d modeling. 4. dynamical amplification factor: monodimensional versus bidimensional evaluation it is well known that the correct evaluation of the seismic site amplification is fundamental in seismic hazard areas (e.g., italy and japan), for the prevention of damage. such evaluation may be complicated by the complexity of the subsoil which can strongly influence the path, the amplitude and the spectral content of the seismic waves at the surface. many efforts have been made up to now to derive algorithms that take into account all the parameters linked with the seismic wave propagation in the overburden volume of engineering interest (aki et al., 1988; bravo et al., 1988; lee et al., 1989a,b, 1990; fishman and ahmad, 1995; rassem et al., 1995; sherif et al., 1996; bard, 1998; field et al., 2000). the ground motion (g (t )), can be expressed as the convolution operation between the overburden transfer function (t (t )) and the input motion at the basement (i (t )). t (t ) is related to the dynamical and geometrical parameters of the overburden while i (t ) depends on the earthquake source, the path from the epicenter and the elastic parameters of all the crossed rock bodies. for a continuous system g (t ) is given by the following relation: (4.1) where t is the time and τ is the lag time of the convolution operator. in frequency terms, the relation (4.1) can be written as (4.2) where ω is the angular frequency. g t t i t dt() = ( ) ⋅ −( ) ⋅ −∞ +∞ ∫ τ τ g t iω ω ω( ) = ( ) ⋅ ( ) 782 antonio rapolla, giovanni bais, pier paolo g. bruno and vincenzo di fiore to evaluate the seismic site response, we compute the dynamic amplification factor (daf) which represents the spectral ratio between the above said ground motion and input motion (4.3) when the overburden is homogeneous and geometrically regularly layered, the estimation of i(ω) is fairly simple and the daf can be computed by means of 1d modeling (see, for example, idriss and seed, 1968; rapolla and mele, 1996). one of the well known computing techniques is based on a generalized, non linear, viscoelastic model (idriss et al., 1992). this 1d method solves the propagation of harmonic vertical shear waves in a one-dimensional system in the frequency domain. the input physical model for the 1d code can consist of n infinite horizontal layers lying on the bedrock. each layer, considered as homogeneous and isotropic, is characterised by given values of density (ρ), thickness, and shear modulus (g ). the equation utilized in 1d computing is the kelvin-voigt linearized visco-elastic equation (4.4) where u is the displacement, ξ is the viscosity coefficient, t is the time and x is the position. daf computation becomes instead much more complicated when we deal with irregular structures and/or heterogeneous material. many authors have studied the influence of irregular subsoil conditions and/or topography on seismic site response. aki et al. (1988) saw that the finite lateral extent of surface layers and/or their irregular geometry can introduce additional effects such as resonance and focusing of the surface and body waves. bravo et al. (1988) applied the boundary analysis method to study the response of a two dimensional horizontal stratified deposit of arbitrary shape under the incidence of sh waves. the response of a basin with two layers was analysed and the comparison with results from one-dimensional analysis shows significant differences. lee et al. (1989a,b, 1990) published several studies on the scattering and diffraction of sv waves, plane sh waves, and p waves by circular cylindrical canyons with variable depth/width ratios. this study showed that focusing and resonance effects are greater on the valley flanks. recently, other studies have investigated problems of the dynamic amplification factor (daf) evaluation on geological structure with finite lateral extension. fishman and ahmad (1995) modelled alluvial valleys subjected to sh, sv and p waves, and demonstrated the influence on the seismic response of key parameters, such as valley depth, seismic impedance ratio, frequency and angle of incidence on surface ground motion. rassem et al. (1995) reviewed the simple one-dimensional (1d) and the twodimensional (2d) finite element model concluding that the 1d model approach provides a poor approximation of the free-field motion in a valley with limited width, particularly near the valley edge. sherif and lee (1996) studied the diffraction due to plane sh waves around a circular alluvial valley in an elastic wedge-shaped medium. they demonstrated the influence on the surface displacement profile of the angle of the wedge, the frequency of the incident wave, the material properties of the media and the waves angle of incidence. bruno et al. (1999) showed that in the case of a dipping bedrock if the dip angle is higher than about 10°, the results of daf spectra computation by 1d and 2d algorithms start to diverge, both in terms of amplitude and peak periods. 2d modeling methods are based on finite differences (vidale and helmberger, 1988), or on boundary elements analysis (kawase, 1988) or also on finite elements algorithms. as regards the latter (more used) methods, beikae et al. (1994) solved the motion equations at the nodal points of a discrete grid being able to predict a strong generation of rayleigh waves at dipping bedrock when sv waves are incident. their 2d program is based on a dynamic, time domain equivalent linear algorithm and each nodal point is characterised by a value of the stiffness (k ), density (ρ), thickness (h) shear modulus (g) and daf .= ( ) ( ) g i ω ω ρ δ δ δ δ ξ δ δ δ 2 2 2 2 2 2 u t g u x u x t = + 783 earth modeling and estimation of the local seismic ground motion due to site geology in complex volcanoclastic areas damping modulus (d). in addition, their code, incorporates a wave transmitting basement so that the half-space beneath the mesh can be modelled and therefore the need to assume a rigid boundary can be eliminated. in order to compute the seismic site response, the 2d algorithm solves the following set of equations: (4.5) where [m ] is the mass matrix; [d ] is the damping matrix; [k ] is the stiffness matrix; u is the nodal displacement vector and i(t) is the earthquake load vector (input motion). in this non linear system, once the input motion is known, the ground motion estimation is linked to the knowledge of the subsoil geometry and of the elastic parameters. obviously, the more complex the subsoil is, the more detailed the knowledge of its properties needs to be. 5. results the 2d daf evaluation approach described previously was applied to several sites in the neapolitan area, italy, covered by different pyroclastic deposits in the neighbourhood of somma vesuvius and campi flegrei volcanoes. the sites are located are within highly urbanized areas and are often close to high risk buildings (schools, city halls, hospitals, etc.). the vp tomographic models were already described by bais et al. (2002). the other input parameters necessary for daf estimation, specifically density, poisson’s ratio, and damping ratio were obtained from bibliographic data (pellegrino, 1967; nicotera and lucini, 1967; carrara et al., 1987; guadagno et al., 1988; rapolla and mele, 1996) and are summarised in table i. in order to avoid lateral reflective effects during 2d modeling, we extended the length of the model of 50 m at both ends. the reference accelerogram for the sites close to somma vesuvius (fig. 2) was calculated by deriving, with respect to the time, a digitally recorded vesuvian earthquake of local magitude equal to 4.2. on the contrary, the reference accelerogram used for 1d and 2d daf modeling analysis for the campi flegrei site (fig. 3), is a synthetic input motion obtained from the fourier antitransform of the spectral shape of the seismic motion at the rigid bedrock, given by italian seismic law. the maximum peak value of the spectral acceleration was set according to italian seismic law to be 0.04 g and in the fourier domain the signal was recomposed by considering minimum phase shift for all frequencies of interest. in this paper, only three typical examples are reported. the first site is lagno maddalena, near the town of s. anastasia (fig. 1). the tomographic model (fig. 4a) shows an irregular morphology with a central low velocity structure that might probably reflect an erosion feature (fig. 4a), possibly related to a paleochannel. lagno maddalena is indeed located over an alluvial fan derived from the reworking of pyroclastic soils. cross sections cutting the fan (cioni et al., 1999) show old anastomosing drainage patterns withm u t d u t k u i t[ ]       + [ ]      + [ ] {} = (){ }∂ ∂ ∂ ∂ 2 2 table i. average seismic of main lithotypes that characterize the subsoil of the neapolitan volcanic areas. γ is the density, vp the p-wave velocity, vs the s-wave velocity, g the dynamic shear modulus, ν the poisson’s ratio, d the damping ratio (pellegrino, 1967; nicotera and lucini, 1967; carrara et al., 1987; guadagno et al., 1988; rapolla and mele, 1996). lithotype γ vp vs g ν d t/m3 m/s m/s mpa vegetal soil 1.05 450 200 400 0.40 0.05 pumices 0.85 600 250 500 0.39 0.05 pozzolana 1.30 900 400 2000 0.37 0.05 tuff 1.50 1600 800 9400 0.33 0.01 784 antonio rapolla, giovanni bais, pier paolo g. bruno and vincenzo di fiore in the lower pyroclastic units that were subsequently buried by more recent pyroclastic flows and by reworked volcanoclastic sediments. figure 4a also shows the 2d finite element mesh modeling the subsoil of lagno maddalena. the model uses 235 nodal points and 199 cells. figure 4b plots the 1d (black line) and 2d (red line) daf (for both horizontal components) calculated at three surface nodal points. comparison between 1d and 2d daf shows, in all reported cases, different results both in terms of peak period and spectral contents, as was expected. generally speaking, we can see that on 2d curves referred to all tested nodal points, the energy content shifts toward high frequencies with respect to the monodimensional curves. ns and ew daf curves calculated at the nodal point #74, show a main peak at about 0.28 s on the 2d spectrum that is not visible on the 1d spectrum. at the central nodal point #130, the main peak does not differ substantially from the 2d main peak. both spectra have a main period at 0.18 s. this result is probably due to the fact that the nodal point #130 is in the middle of a symmetrical paleochannel. symmetry possibly may cause reciprocal destructive interference of the 2d effects. at nodal point #155 the 1d and 2d daf curves again become different. in fact, the 2d daf curve has several peaks while the 1d daf curve has only one peak at 0.1 s. node 155 is located above the right flank of the paleochannel and the difference between the 1d fig. 1. map of the investigated neapolitan urban and sub-urban areas. the study areas were located in sant’anastasia and giugliano. 785 earth modeling and estimation of the local seismic ground motion due to site geology in complex volcanoclastic areas and 2d response may again here be due to the fact that the monodimensional model does not take into account focusing effects. the second example refers to beneduce site, a place also located near sant’anastasia town (fig. 1). generally speaking, this site is characterized by a gradual lateral variation of the velocity field without strong contrasts (fig. 5a). the final tomographic model (fig. 5a) is characterized by a high velocity structure located between position 0 and 40 m. figure 5a shows the 2d finite element mesh modeling which uses 154 nodal points and 130 cells. daf computations were made on the right side of the trt investigated section (fig. 5d). daf curves are shown at nodal points 21, 71 and 111. at nodal point #21 a peak at 0.26 s on the 2d spectra is not visible on the 1d curve. focusing effects may be again invoked to explain this difference. at nodal point #71, the main peak of the 2d daf curves is at 0.3 s while that of the 1d daf curves is at 0.18 s. at nodal point 111, away from the flanks of the high velocity structure, the 1d and 2d daf curves instead become almost similar. the third example refers to a site near giugliano (fig. 1). the trt model (fig. 6a) shows an articulate bedrock morphology that is fig. 3. the input motion used for 1d and 2d daf modeling in the test sites of giugliano (below). its spectral contents are reported in the upper part of the figure. it is a synthetic accelerogram obtained from the fourier reconstruction of the spectral response of the italian seismic low. fig. 2. input motion utilised for 1d and 2d daf calculation in s. anastasia sites. (courtesy of the vesuvian observatory, naples, italy). 786 antonio rapolla, giovanni bais, pier paolo g. bruno and vincenzo di fiore probably due to erosion of the basement during the emplacement of the upper pyroclastic unit. cole and scarpati (1993) report that in this area during the deposition of member b of the so called neapolitan yellow tuff, representing the overburden of this site, the top floor of the lithic member a (the basement) was strongly eroded. the 2d finite element mesh modeling is shown in fig. 6a. the trt output model was parametrized using 208 nodal points and 182 cells. dafs were computed (fig. 6d) at nodal points 29, 109 and 136. fig. 4a-c. a) finite element model related to lagno-maddalena site; b) 2d (red lines) and 1d (black lines) daf curves (2d daf. the daf was calculated in three nodal points: 74, 130, 155; c) final tomographic model (bais et al., 2002). a b c 787 earth modeling and estimation of the local seismic ground motion due to site geology in complex volcanoclastic areas fig. 5a-d. a) finite element model related to beneduce site; b) 2d (red lines) and 1d (black lines) daf curves (2d daf. the daf was calculated in three nodal points: 21, 71, 111; c) stratigraphic sequence near to the site test: 1) vegetail soil; 2) incoherent pyroclastic products; 3) volcanic sand; d) final tomographic model (bais et al., 2002). a b c d 788 antonio rapolla, giovanni bais, pier paolo g. bruno and vincenzo di fiore comparison between 1d and 2d daf modeling related to this site (fig. 6b) shows significant differences at the nodal points 109 and 136, that is in correspondence of the maximum complexity of the geometry of bedrock high velocity structure while at the nodal point #29, where the subsurface morphology only slightly deviates from the monodimensional case, the 1d and 2d daf curves are more similar. 6. conclusions the problem of the evaluation of real surface seismic motions by daf analysis in volcanic fig. 6a-d. a) finite element model related to pantano site; b) 2d (red lines) and 1d (black lines) daf curves (2d daf. the daf was calculated in three nodal points: 29, 109, 136; c) stratigraphic sequence near the site test: 1) incoherent pyroclastic deposits; 2) volcanic and marine sand; 3) lithoid element «brecce basali»; d) final tomographic model (bais et al., 2002). a b c d 789 earth modeling and estimation of the local seismic ground motion due to site geology in complex volcanoclastic areas areas characterized by a thick heterogeneous overburden have been discussed in the present paper. 2d trt surveys followed by a 2d non linear, viscoelastic daf modeling allowed us to obtain a bidimensional seismic site response in many urban and suburban sites located at short distance from mt. vesuvius and campi flegrei. for all sites, daf was also estimated with 1d non linear viscoelastic monodimensional algorithms. comparison between 2d and 1d daf estimations shows, as can be seen in all reported cases, results significantly different both in terms of peak period and spectral contents, as could have been expected from the clear bidimensionality of the geological profile as deduced from the 2d trt survey. as a matter of fact, many authors have shown that the simplification of the geological problems for daf estimation may lead to unacceptable results. we have confirmed this statement showing that estimation of a 2d structure with 1d algorithms leads to gross mistakes. obviously better results could be obtained by using 3d modeling, where the geometrical boundary conditions require it. in volcanic areas, where large lithological heterogeneities may be present, it is of fundamental importance for engineering purposes to account for subsoil complexity. our results suggest careful evaluation of the subsoil geological structures in these areas characterized by possible large lateral and vertical variations of the elastic properties, to reach correct seismic site response curves to be used for engineering projects. acknowledgements the authors wish to thank the japanese scientific organizers of the kyoto and kobe meeting: prof. h. akihama, of nihon university and prof. m. nakashima and his staff at kyoto university for the tremendous effort in organizing the symposium; to dr. a. volpi, scientific attaché at the italian embassy in japan and the italian embassy in japan. thanks are due to region campania for financial support (grant l.r. 49/85). references aki, k. 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(1988): time-domain response of a semi-circular canyon for incident sv, p, and rayleigh waves calculated by the discrete wavenumber boundary element method, bssa, 78, 1415-1437. langan, r.t., i. lerche and r.t. cutler (1984): tracing rays through an heterogeneous medium: an accurate and efficient procedure, geophysics, 50, 1456-1465. lee, v.w. (1990): on the scattering of plane sh waves by a semi-parabolic cylindrical canyons in an elastic halfspace, geophys. j., 100, 79-86. lee, v.w. and h. cao (1989a): diffraction of sv waves by circular cylindrical canyons of various depth, asce, engineering mechanics division, 115, 2035-2056. lee, v.w. and h. cao (1989b): scattering of plane sh waves by circular cylindrical canyons with variable depth-towidth ratios, eur. j. earthquake eng., 3, 29-37. lo, t. and p. inderwiesen (1994): fundamentals of seismic tomography, geophys. monogr. ser., soc. expl. geophys. (fiterman editor), pp. 178. marsden, d. (1993): static corrections − a review, part 2, the leading edge, 12 (2), 115-120. nicotera, p. and p. lucini (1967): la costituzione geologica del sottosuolo di napoli nei riguardi dei problemi tecnici, in agi, vii convegno di geotecnica, cagliari, 45-104. pellegrino, a. (1967): proprietà fisico-meccaniche dei terreni vulcanici del napoletano, in agi, vii convegno di geotecnica, cagliari, 113-148. rapolla, a. and r. mele (1996): geophysics and the safe city: microzonation of a territory and seismic site response, in the safe city, proceedings italian-japanese symposium on earthquake, eruption and civil defense, napoli messina (italy), edited by a. rapolla and a. nazzaro, 121-123. rassem, m., a.c. heidebrecht and a. ghobarah (1995): a simple engineering model for the seismic site response of alluvial valleys, soil dyn. earthquake eng., 14, 199-210. sherif, r.i. and v.w. lee (1996): diffraction around a circular alluvial valley in an elastic wedge-shaped medium due to plane sh waves, eur. earthquake eng., 3, 21-28. stefani, j.p. (1995): turning-ray tomography, geophysics, 63 (4), 1339-1347. stork, c. and e.w. clayton (1991): linear aspects of tomographic velocity analysis, geophysics, 56, 483-495. valentine, g.a. (1987): stratified flow in pyroclastic surges, bull. volcanol., 49, 616-630. vidale, j.e. and d.v. helmberger (1988): elastic finitedifference modeling of the 1971 san fernando, california earthquake, bssa, 78, 122-141. yilmaz, o. (1988): seismic data processing, society of exploration geophys. zhu,t., s. cheadle, a. petrella and s. gray (2000): firstarrival tomography: method and application, in 69th annual international meeting, society of exploration geophys., expanded abstracts. zhu, x. and g.a. mcmechan (1989): 2d tomographic imaging of velocities in the wichita uplift-anadarko basin region of southwestern oklahoma, bull. seismol. soc. am., 79, 873-887. recovering external contribution from the monthly mean series of a given geomagnetic observatory annals of geophysics, 59, 3, 2016, g0321; doi:10.4401/ag-6971 g0321 recovering external contribution from the monthly mean series of a given geomagnetic observatory bejo duka1,*, eni duka2, klaudio peqini1 1 university of tirana, faculty of natural sciences, department of physics, tirana, albania 2 university of zurich, department of informatics, zurich, switzerland abstract the differences between monthly mean values of the observed geomagnetic field and monthly values predicted by different models of the internal geomagnetic field (named “model biases”) for the time period 2000-2015 at several geomagnetic observatories are analyzed. we notice that increasing the maximum degree of the model is not always followed by the decrease of such “model bias”. the time series of these “model biases” reduced by their average resulted to be approximately the same for all models and should represent the external (non-modeled) contribution to the observed geomagnetic field. these time series for different observatories (close or away to each other) are compared and their power spectra are analyzed. such spectra have common features like the annual and semi-annual variation with some possible sporadic cases of seasonal variation. 1. introduction the magnetic field measured at any geomagnetic observatory is the result of several magnetic contributions generated by various sources. more than 90% of the measured field is of internal origin and is mostly generated in the earth’s outer fluid core. also, of internal origin is the magnetic field generated by the remanent magnetization of the crust and by the induced magnetization of the crust produced from the core and external fields. the known contributions of external origin are magnetospheric and ionospheric fields, fields of ring currents and aligned field currents. the global models of the geomagnetic field and its secular variation (sv), like igrf, wmm, emm, pomme, cm, gufm, mf, etc., are based on spherical harmonic analysis (sha). the magnetic field potential (w) is expressed as the sum of spherical harmonic functions. the internal sources contribution to the radial variation of these functions is quite different from the external sources contribution: respectively (a/r)l+1 and (r/a)l, where a is the reference radius of the earth, and l the spherical harmonic degree. the internal source magnetic field is presented by br, bi, b{ orthogonal components in the geocentric reference frame or x (north), y (east), z (downwards) as: using different data sets of ground or/and satellite measurements in disposal, different models define the gauss coefficients (gml ; h m l ) of spherical harmonic expansion up to a maximum number of degrees l and their time derivative up to a maximum number of degrees l’ < l. generally, the denser and numerous the data used by the model, the greater is the maximum degree of harmonics (l) and the shorter is the minimum wavelength of harmonics i.e. more detailed is the model description of the geomagnetic field, according to the formula [backus et al. 1996]: (2) the separation of internal contributions into the core and the crustal (actually lithospheric) origin is . cos sin cos cos sin cos sin cos sin cos b l r a g m h m p b r a gq m h m p b r a mgmg m mh m p 1 r m l l l l w l m l m l m l w m l l l l m l m l m l w m l l l l m l m l m 01 2 2 01 2 01 2 2 { { i { { i i { { i i = + + = + = + { i == + + == + == q r r r gq q q q q q l w l w v l w v v v v v v|| || || a l2.m r +q / .1 2+ v article history received january 22, 2016; accepted june 24, 2016. subject classification: geomagnetic field model, geomagnetic observatory, internal geomagnetic field, power spectrum, spherical harmonics. (1) based on the spherical harmonics spatial power spectrum [lowes 1974, langel and estes 1982]. it is known that the spherical harmonics up to maximum degree l = 13 represent the core field, while the spherical harmonics of degree greater than 13 represent the lithosphere field and there is an overlap of contributions in between 12 and 14 [cain et al. 1984]. in fact, lithosphere sources have some small contributions to the spherical harmonics of degrees < 13 and core sources have also some small contribution to the spherical harmonics of degrees > 13. some models aim to describe the earth’s magnetic field with high spatial and temporal resolution, for example chaos-5 model robustly determined spherical harmonics up to degree l = 90 for the lithosphere field, up to degree l = 20 for the core field and up to degree l’ = 16 for the time-varying core field [olsen et al. 2014]. temporal changes of the field of internal origin up to degree 16 are usually attributed to changes in the core field itself and temporal changes of the field of internal origin produced by the induced part of the lithosphere magnetization could dominate the core field signal beyond degree 22-24 [hulot et al. 2009]. the time-varying lithosphere field dominates and conceals the time-varying core field beyond that critical degree (22-24), just as the permanent lithosphere field dominates and conceals the permanent of the core field beyond degree 14 [hulot et al. 2009]. it is more difficult to separate the crustal contribution into parts coming from remanent magnetization and from induced magnetization. [lesur and gubbins 2000] developed a new method for such separation, but its application for several observatories was unsuccessful. the so named “crustal bias” [mandea and langlais 2002] in an observatory is estimated by comparing the magnetic components measured in the observatory with those predicted by a geomagnetic model truncated to its nuclear part (i.e., up to degree and order of 13). these models are based only on the satellite data sets [bloxham and gubbins 1986], where the crustal sources contributions are negligible, therefore the differences between observed values and model values are considered as the signature of the crustal field of induced or remanent magnetization origin. such “crustal biases” of annual means over 42 years period are well investigated [verbanac et al. 2007a] for 46 european geomagnetic observatories by using models based on magsat, ørsted, champ and sac-c satellites. also, the temporal evolution of the observatory monthly means (over 9 years period) “crustal biases” are analyzed by verbanac et al. [2015] using the g-field model [lesur et al. 2015]. even in this analysis, “crustal biases” are considered the differences of the magnetic components measured at the observatory with the values predicted by a model obtained from satellite data only. verbanac et al. [2007b] attempted to separate, interpret and explain the external field signals obtained by subtracting from the annual means the core field predicted by models truncated at the maximum degree 14. from the obtained external signal they removed magnetospheric contributions of the external field predicted by the pomme-2.5 model and a sq variation predicted by the cm4 model. averaging these residuals over representative european observatories, they found interesting results regarding the correlations of the residuals with external sources such as solar cycles. here we will study a kind of “bias”, which is estimated as the difference of the monthly mean values of the geomagnetic field components registered at a given observatory with the respective values predicted by the geomagnetic field models based on ground and satellite data. the models are truncated at different maximum degrees providing different “biases” that have mostly signatures of non-modeled local sources of the crust and of external sources. different models provide different “biases” (named “model bias”) at the same observatory. we will compare the results of three different models for different observatories that are geographically close to each other (wik: latitude 48.265°n, longitude 16.318°e, altitude 0.4 km; ngk: latitude 52.072°n, longitude 12.675°e, altitude 0.078 km; fur: latitude 48.17°n, longitude 11.28°e, altitude 0.572 km) and two other observatories located far away from them (her: latitude 34.43°s, longitude 19.23°e, altitude 0.026 m) (kak: 36.23°n, longitude 140.18°e, altitude 0.036 km). we expect that the difference (“model bias”) between the observed value and predicted value given by a model should decrease as the maximum degree of spherical harmonics of the model increases. but this does not happen always for all components at every place on earth, as it will be shown in section 2. the global geomagnetic models supply gauss coefficients and their secular variation for every 0.5 year, 1 year, 2.5 years, etc. then the geomagnetic field at a given time is calculated from such models by interpolating the gauss coefficient values in between time intervals, i.e. each model has limitations on time distinction. when the monthly values of the internal geomagnetic field are predicted by a model, the time distinction of one month is considered. on the other side, the monthly values series of observatories are calculated by averaging the observatory minute registrations for all days of a month. therefore the geomagnetic field variations with period less than a month (pulsations, quite daily variations sq, substorms/storms effects) duka et al. 2 3 are principally removed. by removing the averages of the differences between the predicted monthly series and the respective monthly mean of observed value series, we can recover the un-modeled external source contributions to the geomagnetic field at the given observatory (see section 3). in section 4, we will compare the time series of un-modeled external contributions for different observatories (ngk, fur, wik, kak, her) that are of the same length (2000-2015) and will try to find some common features from their spectral analysis (section 5). 2. differences of different degree models we considered three different models that have a large maximum degree: cm4 (l = 65)/cm5 (l = 85), pomme-9 (l = 133), emm2015 (l = 720). the comprehensive model, cm4 [sabaka et al. 2004], covers the whole time interval from 1960 to 2002. it has been derived from quiet-time pogo, magsat, ørsted and champ satellite data in combination with observatory hourly means. the internal field is expanded to different number of maximum degree of spherical harmonics up to 65, resolving magnetic anomalies down to 611 km. the model also calculates primary and induced magnetospheric field, primary and induced ionospheric field. it offers calculation of the geomagnetic field by two steps. in each step, the user inputs the first and second maximum number of degrees. using codes from (http:// core2.gsfc.nasa.gov/cm/codes.html), the monthly values series of core field (l = 14) and lithospheric field (with different values of maximum degree, from l =15 to l = 65) at different observatory locations are generated. the new generation of cm models, cm5 model [sabaka et al. 2015] used an extended data set and defined the gauss coefficients of the core field up to maximum degree of 20 for the period 2000-2015 and the gauss coefficients of the crust field up to maximum degree of 100. using cubic b-spline, the gauss coefficients of sv with 6 month knot spacing are saved. using matlab code from (http://www.spacecenter.dk/files/magnetic-models/ chaos-5), the monthly series (2000-2015) of core and lithospheric field are generated. pomme-9 (http://geomag.org/models/pomme9.html) is an internal field model representing the geomagnetic field in the region from the earth’s surface to an altitude of a couple of thousand kilometers [maus et al. 2004, 2006, 2010]. the time variations of the internal field are given by a piece-wise linear representation of the spherical harmonic (gauss) coefficients of the magnetic potential. pomme-9 was produced from champ satellite vector magnetic measurements from july 2000 up to september 2010, ørsted satellite total field measurements from january 2010 to june 2014 and swarm satellite vector magnetic measurements from december 2013 to january 2015. it has the same parameterization of the magnetospheric field as pomme-6, pomme-7 and pomme-8. pomme-9 extends the maximum degree of spherical harmonics to 133, resolving magnetic anomalies down to 300 km. the gauss coefficients of every year (from 2000 to 2015) are embedded into the header (.h) files for the core and crust field. the program calculates the core geomagnetic field every day from the starting year 2000, while the crust field coefficients are static from degree 16 to 133. enhanced magnetic model (emm2015) based on the ellipsoidal harmonic representation of earth’s lithospheric magnetic field [maus 2010] represents the main field with maximum degree l =15 and the lithospheric field up to l = 720. it was compiled using a vast amount of data from satellite, marine, aeromagnetic and ground magnetic surveys. it also includes data from the european space agency’s swarm satellite mission. this model (https://www.ngdc.noaa.gov/geomag/emm/emm surveysph.shtml) covers the 2010-2020 period resolving magnetic anomalies down to 56 km. non-modeled external contribution figure 1. comparison of short monthly series of geomagnetic field observed at ngk observatory and predicted by different models (cm4, cm5, pomme-9, emm2015). we tried to find a common time interval for all considered models. all considered models have a short common time interval 2000-2002. we firstly compare the monthly mean series of the ngk geomagnetic observatory with respective monthly values series predicted by these models for that short time period (figure 1). one can see that series predicted by cm4, cm5, emm2015 represent a smooth variation, while the series predicted by pomme-9 model have some irregular variations much like the series of observations. even the monthly values series received by averaging the daily values series generated by pomme-9 show the same behavior. it seems that this model fits better to the observed monthly values series than other models. the cm5 model represents a greater baseline jumps from the observed monthly values series. in figure 2, the “model biases” are presented, i.e. the differences of monthly means of x, y, z components observed at niemegk observatory (ngk) during the common time interval 2000-2015 with respective values predicted by: a) emm2015 model of maximum degree of 720 (dashed line), b) pomme-9 model of maximum degree of 133 (solid line) and c) cm5 model of maximum degree of 85 (dotted line). comparing the results of different models, one can see that for the x component, the pomme-9 model fits the observations better (see also table 1, where the values of averaged “bias” and their errors for different models and different maximum degree are presented). while for the y and z components the emm2015 model (respective averaged “bias”: −1.9 nt and −7.2 nt) fits better with the observations than the pomme-9 model (respective averaged “bias”: 4.9 nt and −56.1 nt). the cm5 model fits the observations worse than the two other models for all three components. it seems that cm5 model gives worse results than cm4 model. see for example figure 4, where the results of cm5 at wik observatory are presented, and table 2, where the averaged “biases” of cm4 model at ngk for a long period (1960-2002) are presented. the same comparison for two observatories, furstenfeldbruck (fur) and wien kobenzl (wik) observatories are presented respectively in figure 3 and figure 4. as it is seen in these figures, the pomme-9 and emm2015 models fit the observed data better than cm5 model. also all models provide the same time variations. in order to study the impact of the maximum degree number of the model on the fitting quality between duka et al. 4 figure 2. differences between monthly mean values observed at niemegk observatory and respective values predicted by emm2015 model (dashed line), pomme-9 model (solid line) and cm5 model (dotted line). ngk 2000-2015 emm2015 model averaged “bias” (in nt) pomme-9 model averaged “bias” (in nt) cm5 model averaged “bias” (in nt) l = 720 l = 520 l = 320 l = 120 l = 133 l = 95 l = 65 l = 45 l = 100 l = 85 l = 60 l = 40 x -24.5± 0.6 -21.7±0.6 -20.5±0.6 -3.6±0.6 1.5±0.7 -20.2±0.7 -43.5±0.7 -43.8±0.7 -24.5±0.6 -21.7±0.6 -20.5±0.6 -3.6±0.6 y -1.9±0.2 12.1±0.2 5.6±0.2 28.1±0.2 4.9±0.3 25.9±0.3 4.0±0.3 -3.5±0.3 -1.9±0.2 12.1±0.2 5.6±0.2 28.1±0.2 z -7.2±0.5 17.2±0.5 7.1±0.5 -32.9±0.5 -56.1±0.6 -34.9±0.6 1.4±0.6 -33.8±0.6 -7.2±0.5 17.3±0.5 7.1±0.5 -32.9±0.5 table 1. averaged “bias” of different models at ngk observatory (2000-2015 period). 5 the observed and predicted series, the monthly values series are generated by emm2015, pomme-9, cm4 and cm5 models with different numbers of maximum degrees at different observatories. some results are presented in the plots of figures 5, 6, 7, and in table 1. in figure 5, one can see that decreasing the maximum number of degrees from l = 133 to l = 95, or l = 65 for the pomme-9 model worsened the fitting quality for x component, but not for z component. in figure 6 (emm2015 model), one can see that changing the maximum number of degrees from 720 to 520 gives better fitting for the x component (see also table 1), while for y and z components the fitting quality is poorer. when the maximum number of degrees is decreased to 120 we see the fitting is worsening for all components. in figure 7 (cm5 model), one can see that fitting behaves differently for different components when the maximum number of degrees (l) is changed. the difference series of y component is always decreased when l is changed from 100 to 40, while difference series of non-modeled external contribution ngk 1960-2002 cm4 model averaged “bias” (in nt) l = 65 l = 45 l = 35 l = 25 x -62.25 ± 0.43 -58.83 ± 0.43 -64.1 ± 0.43 -50.6 ± 0.43 y 17.1 ± 0.15 4.84 ± 0.15 -8.75 ± 0.15 1.53 ± 0.15 z 8.57 ± 0.25 -16.7 ± 0.25 -36.55 ± 0.25 -54.9 ± 0.25 figure 3. differences between monthly mean values observed at fur observatory and respective values predicted by emm2015 model (dashed line), pomme-9 model (solid line) and cm5 model (dotted line). table 2. averaged “bias” of cm4 model at ngk observatory (19602002 period). figure 4. differences between monthly mean values observed at wik observatory and respective values predicted by emm2015 model (dashed line), pomme-9 model (solid line) and cm5 model (dotted line). duka et al. 6 figure 5. x, y, z component differences between monthly mean values at ngk observatory and respective values predicted by pomme-9 model with maximum degrees l: a) 133 (dash-dotted line), b) 95 (solid line), c) 65 (dotted line), d) 45 (dashed line). figure 6. x, y, z component differences between monthly mean values at ngk observatory and respective values predicted by emm2015 model with maximum degrees l: a) 720 (dash-dotted line), b) 520 (solid line), c) 320 (dotted line), d) 120 (dashed line). figure 7. x, y, z component differences between monthly mean values at ngk observatory and respective values predicted by cm5 model with maximum degrees l: a) 100 (dash-dotted line), b) 85 (solid line), c) 60 (dotted line), d) 40 (dashed line). 7 x and z components are considerable increased when l changes from 100 to 85 then shift a little when l changes from 85 to 60 and from 60 to 40. regarding the cm4 model, as its validity period is 1960-2002 we have applied this model for the period 1987-2002 to wik observatory and for the period 19602002 to ngk observatory. as can be seen in figure 8 and in table 2, there is a tendency of decreasing of the average “bias” by increasing of the maximum number of degrees from 25 to 65 for the z component at ngk observatory, that is not so clear for x and y components almost the same happened for wik observatory (the results are not shown here). 3. recovering external contribution we have considered the averaged “biases” as contributions of the crustal sources: remanent magnetization and induced magnetization from the internal sources that are not modeled by the given model. therefore, the residuals of subtracting such average from the model “bias” should represent the contribution of non modeled external sources at a given place (observatory). the time series of differences between the geomagnetic field values observed at an observatory (that are the superposition of magnetic fields from internal and external sources) and respective values predicted by a model of the internal geomagnetic field, are: where i = 1, 2 …n indexes the series of months, k indexes the different models. the averages of these series are: where the first term is the averaged of the internal part not modeled by the k-th model and the second one is the average of external contribution to the observations. subtracting these averages from the series of differences dxi, we have: considering the un-modeled part of internal contribution is almost unchanging during the considered time interval (16 years), then: and these series of residuals do not depend on the kindex, i.e. the series are principally the same for all models and should represent the non-constant external contribution to the given observatory. these series are calculated as residuals of average subtraction from differences between monthly mean values and respective values predicted by different models. these series for three different models emm2015 (l = 720), pomme-9 (l = 133) and cm5 (l = 85), at ngk observatory are plotted in figure 9. the graphs are almost the same for all models and they coincide for each component x, y, z. that means such series have the time variation contributions of the external sources at the ngk observatory for the period 2000. int int int x xd averagage k x x exext x x x exext ( ) ( ) ( ) ( ) mod mod mod mod i k i k i obs i obs i k un k obs dl xd = + -= + + dl r rq q q q q qv v v v v v int int intx x x( ) ( )mod modi obs i k un k, + rq q qv v v .x x exext x exext( )modi k i obs obsd = -l rq qv v ,v int int int x n x exext n x x exext averaragage k n x 1 1 1 ( ) ( )mod mod i obs i obs i i k i obs i un k = + = +r r q q q q q q v v v v v ,v | | | ,vint int intxd x x x x xexext ( ) ( ) ( ) mod mod mod i k i obs i k i obs i obs i k xd = = = + -q q q qv v v ,v non-modeled external contribution figure 8. x, y, z component differences between monthly mean values at ngk observatory and respective values predicted by cm4 model with maximum degrees l: a) 65 (solid line), b) 45 (dashed line), c) 35 (dash-dotted line), d) 25 (dotted line). (2) (3) 2015. while the averages x– obs(ext) provide the averaged “bias” at ngk observatory that is not modeled by the given model. their values are presented in table 1. the same calculations are carried out for different observatories. when plotted for the same observatory, the series of residuals calculated by different models should collapse together. here we present the results for fur nearby ngk observatory (see figure 10) and two other observatories far away from ngk: her (see figure 11) and kak (see figure 12). in all figures one can see that after removing the averages, the “model bias” is the same for all models. again, we noticed that averaged “bias” (non modeled static contribution) for cm5 model is greater than for other models (see table 3, where the averaged “bias” for different models and different components are presented). 4. comparison of external contributions from different geomagnetic observatories plotting the external contributions, calculated as described in section 3, at three nearby observatories ngk, fur, and wik, one can see (figure 13) that the external contributions in these observatories are almost the same. this can be seen in all components. the external contribution to the eastward component (y), which is supposed to be the least affected by the external fields [mandea et al. 2010], looks like noise. in figure 14, the external contributions at three observatories (ngk, kak and her), located far away each other, are plotted. it can be seen that the differences between such contributions, at the three observatories that are more than 100° in latitude or longitude apart, are small ones, especially for the x component. duka et al. 8 figure 9. the external contribution to the ngk observatory, calculated as shown in the text for three models: pomme-9 (solid line), emm2015 (dashed line) and cm5 (dotted line). figure 10. the external contribution to the fur observatory, calculated as shown in the text for three models: pomme-9 (solid line), emm2015 (dashed line) and cm5 (dotted line). 9 while the time series for the z components are similar, but the series of north observatories (ngk, kak) are inverted in phase from the series of the south observatory (her). this reflects the sign change of the z component from north to south hemisphere. regarding the y component, one can notice the expressed noise-like character of these series and the phase inversion can be noticed more between eastern observatories (kak) and less eastern observatories (ngk and her). 5. spectral analyses of the external contribution the external contributions to the monthly mean series of the geomagnetic field observed at different observatories should contain signatures from the same external sources. in order to investigate any common contributors in the series plotted in figures 9 to 14, we have applied the same spectral analysis method (fft) on all of these series. the frequency contents (until the nyquist frequency) of power spectrum of non-modeled external contribution observatory 2000-2015 pomme-9 model averaged “bias” (in nt) emm2015 model averaged “bias” (in nt) cm5 model averaged “bias” (in nt) x y z x y z x y z her -15.67±0.76 -8.58±0.35 -34.68±0.37 -19.51±0.77 -18.23±0.33 -24.97±0.35 83.75±0.80 31.78±0.22 -42.29±0.33 kak 16.9±0.85 9.81±0.27 -79.01±0.31 -1.81±0.87 -1.24±0.14 -44.52±0.27 300.67±0.86 37.68±0.18 -503.78±0.33 table 3. averaged “bias” at her and kak observatories. figure 11. the external contribution to the her observatory, calculated as shown in the text for three models: pomme-9 (solid line), emm2015 (dashed line) and cm5 (dotted line). figure 12. the external contribution to the kak observatory, calculated as shown in the text for three models: pomme-9 (solid line), emm2015 (dashed line) and cm5 (dotted line). these time series (16 years long) with 1 month time sampling are shown in figures 15, 16 and 17, respectively: figure 15 for pomme-9 model, figure 16 for emm2015 model and figure 17 for cm5 model. the resolution of these power spectra, i.e. the ability to discriminate the smaller spectral frequency feature of the signal, is about 0.0052 month-1. in these plots, one can see several peaks that are common for all series, lightly shifted for different components, different observatories and different models. the first one being in the frequency interval 0.0039 ÷ 0.0055 month-1 (period interval 21.37 ÷ 15.15 years), should belong to the time series length (16 years). the second peak is in frequency interval 0.02 ÷ 0.025 month-1 (period interval 4.16 ÷ 3.2 years) and the third one is in the frequency interval 0.049 ÷ 0.051 month-1 (period interval 1.7 ÷ 1.6 years). these two last peaks are not so clear. however there are two distinct and clear peaks in the intervals: 0.082 ÷ 0.086 month-1 (period interval 1.01 ÷ 0.97 years) and 0.165 ÷ 0.168 month-1 (period interval 6.06 ÷ 5.96 months). the amplitudes of these peaks are different for different components and different observatories. in case of pomme-9 model, the greatest peak is reached at kak observatory for x and y components and at ngk observatory for the z component at the first frequency and the feeble maximum is reached at her observatory for the z component. the reason is that the absolute values of x and y components at kak observatory are greater than those at other observatories. it can be noticed that the frequency content of the y component for the pomme-9 model (only for this model) represents a series of small peaks that correspond to the frequencies: 0.3555 month-1 (period 2.82 months); duka et al. 10 figure 13. the external contribution (by pomme-9 model) to: a) ngk observatory (solid line), b) fur observatory (dashed line), c) wik observatory (dotted line). figure 14. the external contribution (by pomme-9 model) to: a) ngk observatory (solid line), b) her observatory (dashed line), c) kak observatory (dotted line). 11 non-modeled external contribution figure 15. power spectrum of x. y, z components of external contribution (pomme-9 model) at different observatories: ngk (dotted line), fur (dashed line), kak (dash-dotted line) and her (solid line). figure 16. power spectrum of x, y, z components of external contribution (emm model) at different observatories: ngk (dotted line), fur (dashed line) kak (dash-dotted line) and her (solid line). figure 17. power spectrum of x, y, z components of external contribution (cm5 model) at different observatories: ngk (dotted line), fur (dashed line) kak (dash-dotted line) and her (solid line). 0.4375 month-1 (period 2.3 months); 0.4766 month-1 (period 2 months). maybe these frequencies can indicate some external contributions with seasonal origin. we think that such distinction of the y component of pomme-9 model can be explained by the way the time series of this model is generated. the model is more accurate if the present state of the magnetosphere is provided as an input in the form of magnetic indices [lühr and maus 2010]. unfortunately, these indices are presently not available in real-time, so during real-time evaluation of the model we have to set these indices to zero. then the time series (especially of y component) of this model should bear some additional magnetosphere contribution. similar results are found in the cases of the emm2015 and cm5 models. the most visible peaks are located in the following frequency intervals: 0.0039 ÷ 0.0059 month-1 (period 21.37 ÷ 14.12 years), 0.0215 ÷ 0.0234 month-1 (period 3.88 ÷ 3.56 years), 0.0332 ÷ 0.0352 month-1 (2.51 ÷ 2.37 years), 0.0488 ÷ 0.0508 month-1 (1.71 ÷ 1.64 years), 0.0820 ÷ 0.0840 month-1 (1.02 ÷ 0.99 years) and 0.1602 ÷ 0.1621 month-1 (6.24 ÷ 6.17 months) for the x component; 0.0039 ÷ 0.0059 month-1 (21.37 ÷ 14.12 years), 0.0332 ÷ 0.0352 month-1 (2.51 ÷ 2.37 years), 0.0488÷ 0.0508 month-1 (1.71 ÷ 1.64 years), 0.0781 ÷ 0.0801 month-1 (1.07 ÷ 1.04 years) and 0.1602 ÷ 0.1621 month-1 (6.24 ÷ 6.17 months) for the y component; 0.0039 ÷ 0.0059 month-1 (period 21.37 ÷ 14.12 years), 0.0215 ÷ 0.0234 month-1 (period 3.88 ÷ 3.56 years) and 0.0820 ÷ 0.0840 month-1 (1.02 ÷ 0.99 years) for the z component (figures 16, 17). interestingly none of the contributions with seasonal origin are visible in the power spectrum of the y component in both emm2015 and cm5 models. however the y component in the case of the cm5 model has a spiky power spectrum, especially in the high frequency part, although there are not any distinguishable peaks. this fact indicates the contributions from external sources of different time scales. a major problem in the spectra of finite length time series is that they suffer from “spectral leakage”. while the infinite-length signal has its power concentrated exactly at the discrete frequencies, the truncated signal has a continuum of power “leaked” around these discrete frequencies. this “spectral leakage” is more evident when data series are short as our monthly time series are (192 month long). thompson’s multitaper method (mtm) provides an improved psd estimate [thomson 1982] by using a bank of optimal bandpass filters. these optimal filters, i.e. with minimal leakage, are derived from a set of sequences known as discrete prolate spheroidal sequences (dpsss, also known as slepian sequences) [percival et al. 1993]. another problem when working with truncated time series is variance. it is shown that while reducing spectral leakage one can increase variance and viceversa. thus an important issue is the appropriate choice of parameters so that a trade-off between spectral leakage and variance is achieved [percival et al. 1993]. wardinski and mandea [2006] used this method in order to investigate the geographic dependence of the annual and semi-annual variations. they use the notation of previous workers, defining k, the number of slepian tapers, and p which is basically the number of points in the bandwidth w of the time series in the frequency domain. the relation between the two parameters is k = 2p − 1. it is proven that only 2p − 1 tapers are good enough for reducing spectral leakage, while averaging their results [percival et al. 1993] also helps reducing variance. for further details we refer the reader to formulas (1)-(3) of wardinski and mandea [2006]. the mtm method with parameters described above is provided by “pmtm” matlab function. for large values of time series length n, one can construct the sequence of slepian tapers for all the values of p usually used (1, 2, 2.5, 3, 3.5, 4). in our case (n = 192), we have used p = 3.5, that means k = 6. so we use 6 slepian tapers which is considered accurate enough for short time series like what we use in this study [park et al. 1987]. apart from the power spectrum density function of frequency: psd ( f ), the “pmtm” matlab function calculates the confidence interval. according to our calculations, for a 95% confidence level, the true spectral density function lies within the interval from 0.52 to 2.3 of calculated psd ( f ) function. decreasing the confidence level one can reduce this interval, but the form of psd ( f ) function does not change. we present here the results for the time series of three different observatories (pomme-9 model; see figure 18). similar plots are obtained for emm and cm5 models that are not shown here. beside the greatest peak corresponding to the series length (192 months), there are two clear and wide peaks centered in the frequencies: 0.08 and 0.16 month-1 corresponding to annual (12.5 months) and semiannual (6.25 months) variations. in case of the y component psd (only for pomme-9 model) there appear to be three peaks centered in the frequencies: 0.36, 0. 44 and 0.48 month-1, corresponding respectively to the periods: 2.78, 2.27 and 2.08 months. 6. conclusions different models of the geomagnetic field of internal origin use different maximum degrees of the spherical harmonic expansion of the geomagnetic field components. comparing the differences between monthly mean values of the geomagnetic field at a given place on duka et al. 12 13 the earth and the respective values predicted by different models, we noticed that these differences have almost the same time dependency for the period considered (20002015). these time dependencies are shifted from each other and this shift can be removed by subtracting from these differences their averages. the resulting residuals representing the external contributions for the different models (emm2015, pomme-9, cm5) at different observatories (wik, ngk, kak, her) show some common features. they are almost in the same amplitude at all observatories (order of 20 nt for the x and z components, order of 5 nt for the y component) are almost the same at the nearly observatories (ngk, fur, wik). in particular the x component varies almost the same in all observatories. the time series of the z component at observatories located in different hemispheres show inversion of the phase differences, whilst the time series of the y component show much more noisy-like behavior. the spectral analyses of these time series for different models, different observatories and different components evidenced some common frequency contents, with maximum of power spectrum for periods of 16 years, 1 year, and 0.5 year. these latest variations show the presence of an annual non-ionospheric and a seasonal modulation of sq [wardinski and mandea 2006]. by varying the maximum degree for a given model of the internal field, the changes on the predictions at given place are seen. we noticed that increasing the maximum degree of the model does not go along with the best fitting of predicted values to the observed values of any geomagnetic field component at any place, i.e. the un-modeled internal or external contributions are not always decreased by increasing the maximum degree. however, we noticed that the shape of the plotted time series is not affected by the change of maximum degree as it is always shifted a little bit up or down according to the considered component, model and maximum degree. by removing this shift, we can easily detect the external contribution in a given place that is not modeled by the models of internal field with different maximum degree. it is easy to notice that the time dependence is practically the same for all series. we have not studied the correlation of these unmodeled external signals with dst-index or for other indices, such as f10.7, suggested by wardinski and holme [2011], that would help in specifying their external sources. removal of these signals enhances the resolution of fine-scale detail in secular variation; this is useful in considering the phenomenology of geomagnetic jerks [wardinski and holme 2011]. acknowledgements. the results presented in this paper rely on the data collected at magnetic observatories: koblenz (wik) austria, niemegk (ngk) and furstenfeldbruck germany, kakioka (kak) japan and hermanus (her) south africa. we thank: zentralanstalt für meteorologie und geodynamik (vienna) for supplying data to us, the national institutes that supported ngk, kak and her observatories and intermagnet for promoting high standards of magnetic observatory practice (www.intermagnet.org). references backus, g., r.l. parker and c. constable (1996). foundations of geomagnetism, cambridge university press, cambridge, uk. bloxham, j., and d. gubbins (1986). geomagnetic field analysis-iv. testing the frozen-flux hypothesis, geophys. j. r. astr. soc., 84, 139-152; doi:10.1111/j.1365246x.1986.tb04349.x. cain, j., d.r. schmitz and l. muth (1984). small-scale features in the earth’s magnetic field observed by magsat, j. geophys. res., 89 (b2), 1070-1076. hulot, g., n. olsen, e. thebault and k. hemant (2009). non-modeled external contribution figure 18. power spectrum density (psd in nt2/frequency units) calculated by pmtm method for external contributions series of pomme9 model at different observatories: ngk (solid line), fur (dashed line), her 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(1982). spectrum estimation and harmonic analysis, ieee proc., 70 (9), 1055-1096. verbanac, g., m. korte and m. mandea (2007a). on long-term trends in european geomagnetic observatory biases, earth planets space, 59, 685-695. verbanac, g., h. lühr, m. rother, m. korte and m. mandea (2007b). contributions of the external field to the observatory annual means and a proposal for their corrections, earth planets space, 59, 251-257. verbanac, g., m. mandea, m. bandi and s. subaši (2015). magnetic observatories: biases over champ satellite mission, solid earth, 6, 775-781; doi:10.519 4/se-6-775-2015. wardinski, i., and m. mandea (2006). annual and semiannual variations of the geomagnetic field components analysed by the multi-taper method, earth planets space, 58, 785-791. wardinski, i., and r. holme (2011). signal from noise in geomagnetic field modelling: denoising data for secular variation studies, geophys. j. int., 185, 653-662; doi:10.1111/j.1365-246x.2011.04988.x. * corresponding author: bejo duka, university of tirana, faculty of natural sciences, department of physics, tirana, albania; email: bejo.duka@fshn.edu.al. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. duka et al. 14 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ag.58.01.15_cover+front+index+preface_layout 6 [special issue_58_01_2015] earthquake model central asia: seismic hazard and risk assessment in central asia. edited by stefano parolai, jochen zschau, ulugbek begaliev. annals of geophysics [special issue_58_1_2015] earthquake model central asia: seismic hazard and risk assessment in central asia preface stefano parolai, jochen zschau, ulugbek begaliev s0101 articles central asia earthquake catalogue from ancient time to 2009 natalya n. mikhailova, aidyn s. mukambayev, irina l. aristova, galina kulikova, shahid ullah, marco pilz, dino bindi s0102 probabilistic seismic hazard assessment for central asia shahid ullah, dino bindi, marco pilz, laurentiu danciu, graeme weatherill, elisa zuccolo, anatoly ischuk, natalya n. mikhailova, kanat abdrakhmatov, stefano parolai s0103 an overview on the seismic microzonation and site effect studies in central asia marco pilz, tanatkan abakanov, kanatbek abdrakhmatov, dino bindi, tobias boxberger, bolot moldobekov, sagynbek orunbaev, natalya silacheva, shahid ullah, sheyshenaly usupaev, pulat yasunov, stefano parolai s0104 probabilistic seismic hazard assessment of bishkek, kyrgyzstan, considering empirically estimated site effects shahid ullah, dino bindi, marco pilz, stefano parolai s0105 towards a cross-border exposure model for the earthquake model central asia marc wieland, massimiliano pittore, stefano parolai, ulugbek begaliev, pulat yasunov, jafar niyazov, sergey tyagunov, bolot moldobekov, saidislom saidiy, indalip ilyasov, tanatkan abakanov s0106 focus maps: a means of prioritizing data collection for efficient geo-risk assessment massimiliano pittore s0107 a summary of hazard datasets and guidelines supported by the global earthquake model during the first implementation phase marco pagani, julio garcia, damiano monelli, graeme weatherill, anselm smolka s0108 global seismic hazard assessment program gshap legacy laurentiu danciu, domenico giardini s0109 building monitoring in bishkek and dushanbe by the use of ambient vibration analysis bojana petrovic, dino bindi, marco pilz, matteo serio, sagynbek orunbaev, jafar niyazov, farhod hakimov, pulat yasunov, ulugbek t. begaliev, stefano parolai s0110 toward a cross-border early-warning system for central asia jacek stankiewicz, dino bindi, adrien oth, stefano parolai s0111 on-site early-warning system for bishkek (kyrgyzstan) dino bindi, tobias boxberger, sagynbek orunbaev, marco pilz, jacek stankiewicz, massimiliano pittore, iunio iervolino, enrico ellguth, stefano parolai s0112 the contribution of emca to landslide susceptibility mapping in central asia annamaria saponaro, marco pilz, dino bindi, stefano parolai s0113 annals of geophysics annals of geophysics preface earthquake model central asia: seismic hazard and risk assessment in central asia central asia is one of the regions of the world with the highest seismic hazard. a number of large events have occurred in this region between the end of the 19th and the beginning of the 20th century, when urbanization was still limited. however, the recent increase in population and, in particular, the expansion of urban areas after the collapse of the soviet union has greatly increased the seismic risk of central asian countries. within the framework of the earthquake model central asia (emca), the regional partnership of the global earthquake model (gem) for central asia, new probabilistic seismic hazard models have been recently derived, along with site effects studies in several cities, and new models for exposure and vulnerability generated based on newly acquired data sets. this volume consist of 12 papers providing both an overview of the activities undertaken and a general description of the main results of the first phase of the emca initiative and of other projects that directly benefited from their cooperation with emca. mikhailova et al. presents the earthquake catalog compiled for the emca project, consisting of 33,620 historical and instrumental events, where the main sources and procedures used to compile the catalogue are discussed. ullah et al. (a) show the area source and the different approaches used for the probabilistic seismic hazard assessment (psha) for central asia, providing a comprehensive comparison of the results obtained by different methods. pilz et al. provide an overview of the experiments carried out for the assessment of site effects in several urban areas in central asia, while ullah et al. (b) presents a first attempt to take them into account for the psha of bishkek, where the importance of site effects in modifying the ground motion is highlighted. wieland et al. provide insights into the development of the first harmonized exposure model for central asia. this model combines commonly used data sources and acquisition techniques with novel rapid assessment approaches (e.g., satellite remote sensing and omnidirectional imaging). pittore introduces the concept of focus maps, and illustrates its application in central asia for an efficient georisk assessment. in the paper of pagani et al., a description of the main outcomes of the gem global initiatives (e.g., global instrumental catalogue, global database of active faults) is provided and some of their initial applications illustrated. danciu and giardini, present a short retrospective overview of the achievements as well as the pitfalls of the gshap. also, they focus their attention to the next generation of seismic hazard models, as elaborated within the global earthquake model regional programs and partnerships: the 2013 european seismic hazard model, the 2014 earthquake model for the middle east, and the 2015 earthquake model for central asia. petrovic et al. report on the results from experiments aiming at assessing the dynamic characteristics of buildings in central asia, while stankiewicz et al. and bindi et al. show how the data collected can be useful for developing regional and on site earthquake early warning (eew)/rapid response systems, allowing real time risk assessment. finally, the paper of saponaro et al. presents results dealing with earthquake-triggered landslide susceptibility for the whole central asian region. the editors are thankful to g. ameri, o. ozel, d. di giacomo, c. geiss, m. garcia-fernandez, d. ehrlich, r. ditommaso, c. milkereit and o. polat who reviewed the submitted manuscripts. stefano parolai jochen zschau ulugbek begaliev helmholtz center potsdam helmholtz center potsdam international university german research center german research center of innovation technologies, for geosciences, for geosciences, bishkek, kyrgyzstan potsdam, germany potsdam, germany annals of geophysics, 58, 1, 2015, s0101; doi:10.4401/ag-6784. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol. 48, 01, 05ok.qxd 9 annals of geophysics, vol. 48, n. 1, february 2005 key words heat flow – helium isotope ratio – geothermal energy – mass transfer 1. heat flow-age dependence a long time ago, regional research showed that the background (regionally averaged) values of conductive heat flow density, q, are inversely correlated with the age, t, of tectono-magmatic activity in both continental and oceanic crust (polyak and smirnov, 1966, 1968; hamza and verma, 1969; sclater and francheteau, 1970). this «heat flow-age dependence» was repeatedly checked, confirmed and refined (cermak, 1976; chapman and pollack, 1976; kutas et al., 1976; chapman and furlong, 1977; vitorello and pollack, 1980; etc.). some of the q versus t plots are shown in fig. 1a-d. the accumulation of the data resulted in the growing scatter of the observable q-values, i.e. the values measured at different sites or in different depth intervals. this scatter came to shade a general trend, so that some researchers began to think that this trend was «a fantasy» (rao et al., 1982). this assumption was wrong as already shown by sclater et al. (1980, 1981), who emphasised the difficulties in the geological dating of q-values (fig. 1c). the scatter of the individual q-values observed in the continental tectonic units of the same age results from many reasons. some of them are the factors disturbing the conductive heat flow distribution near the earth’s surface. these disturbing factors are the following: i) the topography of this surface and the contrasts in thermal conductivity of rocks (thermal remailing address: dr. boris g. polyak, geological institute, russian academy of sciences, pyzhevskii per. 7, 119017 moscow, russia; e-mail: polyak@ginras.ru heat and mass transfer from the mantle: heat flow and he-isotope constraints boris g. polyak geological institute, russian academy of sciences, moscow, russia abstract terrestrial heat flow density, q, is inversely correlated with the age, t, of tectono-magmatic activity in the earth’s crust (polyak and smirnov, 1966; etc.). «heat flow-age dependence» indicates unknown temporal heat sources in the interior considered a priori as the mantle-derived diapirs. the validity of this hypothesis is demonstrated by studying the helium isotope ratio, 3he/4he = r, in subsurface fluids. this study discovered the positive correlation between the regionally averaged (background) estimations of rand q-values (polyak et al., 1979a). such a correlation manifests itself in both pan-regional scales (norhtern eurasia) and separate regions, e.g., japan (sano et al., 1982), eger graben (polyak et al., 1985) eastern china rifts (du, 1992), southern italy (italiano et al., 2000), and elsewhere. the r-q relation indicates a coupled heat and mass transfer from the mantle into the crust. from considerations of heat-mass budget this transfer can be provided by the flux consisting of silicate matter rather than he or other volatiles. this conclusion is confirmed by the correlation between 3he/ 4he and 87sr/86sr ratios in the products of the volcanic and hydrothermal activity in italy (polyak et al., 1979b; parello et al., 2000) and other places. migration of any substance through geotemperature field transports thermal energy accumulated within this substance, i.e. represents heat and mass transfer. therefore, only the coupled analysis of both material and energy aspects of this transfer makes it possible to characterise the process adequately and to decipher an origin of terrestrial heat flow observed in upper parts of the earth crust. an attempt of such kind is made in this paper. 10 boris g. polyak fraction), producing quasi-stationary anomalies mainly of local scales; ii) the changes in climate, i.e. temperature at the upper boundary of the solid earth responsible for non-stationary anomalies; and iii) underground fluid circulation disturbing conductive heat transfer. besides, the abyssal heat flow can be disturbed by iv) tectonic displacements of rock masses. heat flow anomalies of tectonic origin are similar to those generated by fluid circulation in the formation mechanism which represents advective transfer of thermal energy. these anomalies are non-stationary and, in some cases, can exist tens and even hundreds myr (khutorskoi, 1996). when the individual q-values are averaged within a huge tectonic province such as hercynides, alpides, etc., antipodal effects of each factor can be mutually compensated to some extent. therefore, the regionally averaged, i.e. background q-values approach the undisturbed values of the abyssal heat flow density. theoretically, thermal effects of the factors mentioned above can be quantified. however, the full set of the data needed for such calculations at each measurement site is usually unavailable. when these data are known with sufficient accuracy, the measured q-value can be corrected. these corrected individual q-values fig. 1a-d. «heat flow-age dependence». the plots of conductive heat flow density, q, versus age of tectonomagmatic activity: a) from polyak and smirnov (1968); b) dots from polyak and smirnov (1968), triangles from cermak et al. (1976), diamonds from kutas et al. (1976); c) reproduced from data in sclater et al. (1980, 1981), the mean q-values in four age-specific provinces of continents and the relevant data scatters are shown as the lengths of straight line and shaded band, respectively; d) from vitorello and pollack (1980, fig.1), the data points, the standard errors of the mean and the standard deviations are shown for cenozoic, mesozoic, late and early paleozoic, late and early proterozoic an archean provinces, the heat flow components correspond to radiogenic heat from the crust (i), effect of «transient thermal perturbation associated with tecto-genesis» (ii) and background heat flow from deeper sources. hfu (heat flow unit) = 1 ×10 – 6 cal·s–1·cm–2 = 41.868 mw·m–2. a b c d 11 heat and mass transfer from the mantle: heat flow and he-isotope constraints in the same region are more uniform and close to the background (abyssal) value. there is one more reason for the local q-value scatter, namely, v) the lateral variations in radiogenic heat generation (a) in the near-surface rocks. as was shown in roy et al. (1968), lachenbruch (1970) and elsewhere, the values of q and a correlate positively with each other. at the same time, one can see some evidence for increasing avalues in the younger tectonic units (see, e.g., fig. 1a-d in pollack, 1980). the traditional concept on the downward decreasing of radioactive element abundance in the crust forces to consider the nearsurface a-variations and the relevant q-variations as a consequence of the regional (local?) differences in erosion scale. other processes occurring within the interior (metamorphism, seismicity, etc.) are also able to distort the abyssal heat flow and, therefore, to cause a scatter in the measured q-values as well. but energy effects of these processes are difficult, if at all, to quantify because of the uncertainty of the initial data. as a result, the relation between qand tvalues, or «heat flow-age dependence» manifests itself only when the background (average) estimations of the heat flow density in huge tectonic provinces are compared. a cause of the dependence cannot be elucidated on the base of the heat flow data alone. the observed q-t trend discloses the presence of the unknown abyssal heat sources, which exist temporally in more or less limited fragments of the interior. the solution of inverse problems, that is a traditional approach in geophysics, cannot determine unequivocally shape, size, depth, thermal potential and residence time of such sources, allowing only to fit one or another combination of these parameters of source with the observed q-t trend. the material essence of such sources, i.e. their geological nature, remains a mystery. a priori, the sources were postulated to be mantle-derived (asthenospheric) diapirs. the validity of this hypothesis can be proven only if any material evidence of mantle diapirism is found in zones of the highest heat flow. this was done by studying helium isotopes in the fluids freely circulating in the earth crust. this study discovered (i) the large difference between the values of the 3he/ 4he = r in the mantle, where this ratio amounts to ∼1.2·10–5 in the derivatives of the morb reservoir and up to ∼ 5·10–5 in those of mantle plumes (hot spots) like hawaii, iceland, etc., and in the helium generated in the continental crust owing to radioactive decay of u and th, where the «canonic radiogenic» rvalue ≈ 2 · 10–8 (see, mamyrin and tolstikyin, 1984, and references herein), and (ii) the positive correlation between the regionally averaged (background) estimations of rand q-values, i.e., the triple relationship between r, q and t (polyak et al., 1979a). 2. heat flow-3he /4he dependence coupled analysis of the terrestrial heat flow density, q, and the 3he / 4he ratio in geologic objects (polyak et al., 1979a; o’nions and oxburgh, 1983; polyak and tolstikhin, 1985; oxburgh and o’nions, 1987; polyak, 1988; etc.) is very fruitful and fully justified due to the immanent common feature of these parameters: both qand 3he / 4he-values are changed in time due to cooling of the interior (heat sources) and radiogenic generation of 4he, respectively. this feature cardinally discriminates q and 3he/4he from any other quantitative geological, geophysical, and geochemical parameters (except for those referring to other radiogenic isotopes). just for this reason the synthesis of heat flow and he isotope data takes on crucial importance in studying the earth evolution. the 3he/4he ratio values in rocks and minerals vary in a very wide range because of their genetic, structural and compositional features. however, he eventually goes away from rocks into surrounding free-circulating fluids (gerling, 1957). there isotopic composition of he is naturally averaged in proportion to contributions from all the he-sources, and the resulting r-value represents a background characteristic of a given tectonic unit. in itself, the presence of mantle-derived helium in underground fluids does not suggest the same origin for their other components. however, as is known, these origins can be refined due to the juxtaposition of the contents of these components with the concentration of attendant 3he. fig. 3a,b. heat flow3he/4he dependence, inter-regional juxtapositions. a) the plot of individual 3he/4he values versus background heat flow density in the fsu area from (polyak et al., 1979a); b) the relation of background 3he/4he and heat flow density values in different tectonic units of the northern eurasia from (polyak, 1988). a b 12 boris g. polyak this approach was offered by i.n. tolstikhin (polyak et al., 1976) and developed by h. craig, r.k. o’nions and other researchers in application to c-bearing gases (see, e.g., jenden et al., 1993). another fruitful approach in gas geochemistry is the juxtaposition of the r-values with the total helium content, (he), in gas phase of various fluids. such analysis shows, that the variations in the 3he / 4he ratio reflect the mixing of crustal and mantle-derived end-members, whereas the dispersion of the [he]-values results from i) solubility-related fractionation of he and other gases in gas-water system and ii) generation or consumption of other gases (co2, ch4, n2) serving as he-carriers in the earth crust. the studies of gas phase in various underground fluids (waters, hydrocarbons, volcanic emanations, etc.) have revealed i) an extremely wide spectrum both of the contents of he isotopes and the r-values and ii) tectonic ordering of the r-variations (fig. 2). the similar spatial variability of he isotopic composition in underground fluids and background conductive heat flow density stimulated to study the q-r relation. indeed, the q-r dependence was found out firstly as an inter-regional direct correlation on the territory of the fsu and northern eurasia as a whole (fig. 3a,b). then the regional manifestafig. 2. the relation between the contents of [3he] and [4he] in subsurface fluids freely circulated in rocks (from mamyrin and tolstikhin (1984), added). 1 – pre-ryphean eastern european platform, eep; 2 – the eep parts re-activated in ryphean-paleozoic; 3 – baikalides (timan-pechora province); 4 – hercynides (scythian plate, sp); 5 – the sp parts re-activated in cenozoic; 6 – hercynian central french massif re-activated in cenozoic; 7 – italian volcanic areas; 8 – iceland hot spot. 13 heat and mass transfer from the mantle: heat flow and he-isotope constraints tions of this dependence were demonstrated for japan (sano et al., 1982), the ohrje and baikal rifts, eastern carpathians, northern caucasus (polyak et al., 1982, 1992, 1999, 2000), the eastern china rift system (du, 1992) and italy (italiano et al., 2000). this dependence is a typical paragenetic relation reflecting geophysical and geochemical consequences of the same cause: discharge of the ascending heat-mass flux transferring the mantle-derived helium and an excess of thermal energy to the earth surface. what is a mechanism of this process? what is an agent of this coupled transfer? although helium is second only to hydrogen in thermal capacity, such an agent cannot consist of he alone, since its output from the interior is negligible in mass: the total flux of terrestrial 4he to atmosphere is estimated as low as ∼1010 atom·m–2·s−1 (ozima and podosek, 1983) or even lower (o’nions and oxburgh, 1988). heat output by ascending mass flux can be quantified as excess of thermal losses, q, in the flux discharge zones distinguished by higher magmatic (volcanic) and tectonic activity in comparison with stable ancient (pre-riphean) units of continents. according to heat flow study, this excess is 40-50 mw·m–2 or ≥ 0.42·1012 w in total (polyak, 1988). could this q really be provided by a flux of other volatiles? q ≥ 0.42·1012 w could be provided by the h2o flux with flow rate of ≥ 3 km3/year at ρ = 1 g/cm3 and enthalpy of 4180 kj/kg ∼ 103cal/g (similar to supercritical fluid with t ∼ 900°c). this flux would fill the world ocean (1.37·109 km3) for a period no longer than 430 myr, i.e. during the post-silurian time only. therefore, a coupled ascending flux of abyssal heat and mass marked by mantle-derived he cannot be identified as an autonomous flux of volatiles. it leads to the conclusion that such a flux consists of silicate substance. this deductive inference has empirical confirmation. 3. 3he/4he-87sr/86sr dependence such a confirmation resulted from the comparison of isotopic compositions of volatile (atmophile) he and lithophile sr in products of the recent volcanic and hydrothermal activity. the expedience of this juxtaposition of these elefig. 4. 3he/4he-87sr/86sr dependence in the products of volcanic and hydrothermal activity in italy (polyak et al., 1979b). fig. 5. the q-t-3he/4he triple relationship. see explanation in the text. 14 boris g. polyak ments sharply distinguished in their properties and behaviour in the nature was already noted a long time ago by tolstikhin et al. (1976). shortly later, this approach was used in our researches of geothermal activity in italy (polyak et al., 1979b). the comparison of the values of the 3he / 4he ratio in gases issued from thermal and mineral springs in tuscany, latium, campania, and sicily with those of the 87sr / 86sr ratio in volcanics of the same areas revealed the close negative correlation of these parameters. the plot shown in fig. 4 reflects the merging of the crustal and mantle-derived components. these observations were confirmed by the following researches (hooker et al., 1985; parello et al., 2000). the inverse correlation between the 3he/4he and 87sr / 86sr values in products of volcanic and hydrothermal activity is seen in other places as well: in the east sunda and banda island arcs (hilton and craig, 1989), along the pacific margin of south america (hilton et al., 1993) and in the caucasian segment of the alpine fold belt (polyak et al., 2000). it may be inferred that the 3he/4he-87sr/86sr correlation indicates the coupled transfer of these elements from the mantle into the crust. the only agent capable of transporting both atmophile he and lithophile sr is silicate matter. so, this conclusion supports the above mentioned postulate on relation of q-variations with intrusions of mantle-derived substance into the crust. 4. the coupled q-3he/4he evolution the mantle-derived intrusions create the excess thermal potential in activated zones of the crust (fig. 5). this potential provides both enhanced heat losses observed in these zones and structural and metamorphic transformations of the interior. subsequently, an unspent remnant of thermal energy is evacuated from the interior through overlying sequences by weakening conductive heat flow. at the same time, the mantle signature of he, issued from cooling intrusive bodies into circumambient fluids, becomes erased gradually by generation of 4he within the crust at constant dissipation of earth helium in the atmosphere and then in cosmic space. eventually the mantle-derived matter «forgets» its origin. 5. geothermal «age» of the continental crust the triple r-q-t relationship supporting the above mentioned a priori postulate on asthenospheric diapirs, showed that the q-values measured near the earth’s surface could partially relate to the mantle-derived thermal impulses manifested in tectono-magmatic activity as well. geological observations show that this activity manifested itself repeatedly in the history of almost any block of the continental crust. the recurrence of this activity in sepa15 heat and mass transfer from the mantle: heat flow and he-isotope constraints rate geoblocks indicates several subsequent mantle-derived impulses. each of these impulses would influence the thermal state of the interior. the age of one or another tectonic phase dates the relevant impulse, whereas the impulse magnitude, i.e. rock mass and thermal energy accumulated in them, remains obscure. it is known, however, that the intensities of synchronous tectono-magmatic activity in the different regions are usually different, as well as those of its subsequent manifestations in the same region. this fact indicates the different magnitudes of the relevant mantle-derived impulses and, hence, the different duration of their relaxation. the different intensities of both subsequent impulses in the same geological province and synchronous impulses in the different provinces seem to be the main cause for concealment of the q-t relationship. the new impulses in particular parts of the same province would change the q-values there. hence, these impulses can be considered a primary cause of the q-value dispersion that is naturally influenced by all of the disturbing factors enumerated above. if the magmatic activity manifests itself only once in the geological history of the given geoblock, the heat flow evolution should be described by an expression like qab → f (m1/t1), where m denotes a mass of the mantle-derived melt intruded into the crust at the time t, and qab designates a value of the abyssal heat flow density at the given site nowadays. the q-values corrected for influence of the disturbing factors must be equal to each other in all geological units formed by synchronous impulses equivalent in mass (e.g., by mass m1 in the moment t1). if a geoblock was reactivated by several impulses with the same enthalpy, the qab magnitude at the given site should be described by an expression like , , , , ,q f t m t m t m t m t mab n n n n 1 1 2 2 3 3 1 1 " f b l. if m1, m2, m3 … were known, it would be possible to calculate the «average» age of tectonomagmatic activity t av in the observation site as .t t m m 1 i i av = ! ! this expression is merely a convention. such t avvalues could present only the formal estimations of «geothermal age» which do not correspond to any real dates of thermal (geological) events taken into consideration. but using the t av-values (if they were possible to be calculated) would decrease the dispersion of the q-values assigned to the same times of the geologic history. fig. 6. heat flow and thermal events in the earth crust. 1 – crustal block formed by the impulse m1 during the epoch t1; 2 – its fragments re-activated by m2 during t2; 3 – by m3 during t3. (*) english versions of russian journals: doklady earth science section russian academy of science reports; geotectonics geotektonika; international geological review transactions of the russian academy of science, geological series; geochemistry international geokhimiya. 16 boris g. polyak in the light of this approach, one can explain a specific relation of q-values determined in the geoblock formed by the impulse m1 at the times t1, if two parts of this block were reactivated by the impulses m2 and m3 at the times t2 and t3, respectively (fig. 6). in this case q2>q1<q3. when m2/m1≤m3/m1, q3>q2, but if m2/m1>m3/m1, q3 can be both higher and lower than q2. unfortunately, the accurate estimates of the m-values are incredible. such estimates are impossible to make without a quantification of the hidden (plutonic) discharge of the mantle-derived melts into the crust. the scales of this discharge may be much more than the traces of volcanic activity, which are more or less removed by the following erosion. but the above consideration rules out the doubts about the existence of the q-t trend in the continental crust. 6. conclusions mantle-derived helium is transferred from the interior toward the earth’s surface by silicate flux along with abyssal heat. this flux is evident in the mors, island arcs, continental rifts, etc., and supposedly exists in mature continental crust as «magmatic underplating» (lachenbruch and morgan, 1990). this concept agrees with the triple q-t-3he/4he relation observed in the post-ryphean continental units. however, i) the data on the antiquity of the continental crust (mcculloch and wasserburg, 1978; o’nions et al., 1983; allegre and rousseau, 1984; etc.), and ii) the ideas on intracrustal thermal activation of collisional origin (leonov, 1993; rozen and fedorovsky, 2001; yakovlev, 2002; etc.) show that the problem needs further study. acknowledgements this work was supported by the russian foundation for basic researh, project no. 03-0564869. the author is grateful to sächsische akademie der wissenschaften, technische universität bergakademie freiberg and copernicus gmbh for the invitation to attend and to present this work at 7th international conference on gas geochemistry in freiberg, 22-26 september 2003. references (*) allegre, c.j. and d. rousseau (1984): the growth of the continents through geological time studies by nd isotopic analysis of shales, earth planet. sci. lett., 67, 19-34. cermak, v. 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(2002): the earth crust hydration of and generation of chemogenic heat, geokhimiya, 12, 13141322 (in russian). vol49_2_2006 515 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words multidisciplinary seafloor observatories – marine science and technology 1. introduction much of what we know about the oceans is the result of ship-based expeditionary science dating back to the late 19th century. but, it is now clear that to answer many important questions in the ocean and earth sciences, a co-ordinated research effort of long-term investigations is required. experiments and research programmes, from the 1980s to the present, reflect the progressive enhancement of monitoring systems in the ocean basins. during this time we have witnessed the achievement and strengthening of the concept of «seafloor observatories» and the integration of earlier, quite simple, stand-alone seafloor mono-parameter monitoring modules inside more complex multi-parameter platforms with extended lifetime and performance. simple stand-alone modules like obss (acronyms and abbreviations are listed before the references), already in use, although with a power autonomy up to 1 year, are characterised by local data storage and a very limited set of sensors, while deployment and reseafloor observatory science: a review paolo favali (1) (2) and laura beranzoli (1) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) università degli studi di roma «la sapienza», roma, italy abstract the ocean exerts a pervasive influence on earth’s environment. it is therefore important that we learn how this system operates (nrc, 1998b; 1999). for example, the ocean is an important regulator of climate change (e.g., ipcc, 1995). understanding the link between natural and anthropogenic climate change and ocean circulation is essential for predicting the magnitude and impact of future changes in earth’s climate. understanding the ocean, and the complex physical, biological, chemical, and geological systems operating within it, should be an important goal for the opening decades of the 21st century. another fundamental reason for increasing our understanding of ocean systems is that the global economy is highly dependent on the ocean (e.g., for tourism, fisheries, hydrocarbons, and mineral resources) (summerhayes, 1996). the establishment of a global network of seafloor observatories will help to provide the means to accomplish this goal. these observatories will have power and communication capabilities and will provide support for spatially distributed sensing systems and mobile platforms. sensors and instruments will potentially collect data from above the air-sea interface to below the seafloor. seafloor observatories will also be a powerful complement to satellite measurement systems by providing the ability to collect vertically distributed measurements within the water column for use with the spatial measurements acquired by satellites while also providing the capability to calibrate remotely sensed satellite measurements (nrc, 2000). ocean observatory science has already had major successes. for example the tao array has enabled the detection, understanding and prediction of el niño events (e.g., fujimoto et al., 2003). this paper is a world-wide review of the new emerging «seafloor observatory science», and describes both the scientific motivations for seafloor observatories and the technical solutions applied to their architecture. a description of world-wide past and ongoing experiments, as well as concepts presently under study, is also given, with particular attention to european projects and to the italian contribution. finally, there is a discussion on «seafloor observatory science» perspectives. mailing address: dr. paolo favali, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: paolofa@ingv.it 516 paolo favali and laura beranzoli covery are performed either by means of rovs or by free-fall landing and pop-up procedures. more complex systems with better performance, namely seafloor observatories, have been developed to overcome the limitations of the stand-alone modules with respect to multidisciplinary monitoring and (near)-real-time communications and integration. much of seafloor observatory research is interdisciplinary in nature and has the potential to greatly advance the relevant sciences. to obtain further advances, long time-series measurements of critical parameters, such as those collected using seafloor observatories, are needed to supplement traditional seagoing investigations (nrc, 1998a, 1999, 2003c). in 1991 the ec promoted feasibility studies aimed at identifying both the scientific requirements (thiel et al., 1994) and the possible technological solutions for the development of seafloor observatories, the abel concept (berta et al., 1995). the «symposium on seafloor observatories» (islamorada, florida, 2000) was an opportunity to discuss the scientific potential and technical needs associated with the establishment of a network of seafloor observatories. this meeting was followed by the nrc report «illuminating the hidden planet. the future of seafloor observatory science». in this report a complete definition of the term «seafloor observatories» was given for the first time: «[…] an unmanned system, at a fixed site, of instruments, sensors, and command modules connected to land either acoustically or via a seafloor junction box to a surface buoy or a fibre-optic cable […]» (nrc, 2000). in the fall of 1999, nsf asked nrc to investigate the scientific merit, technical requirements, and overall feasibility of establishing the infrastructure needed for a network of unmanned seafloor observatories. the nrc committee on seafloor observatories was appointed to carry out this task and concluded that seafloor observatories present a promising and in some cases essentially new approach for advancing basic research in the oceans. nsf was thus encouraged to move ahead with plans for a seafloor observatory programme (nrc, 2000). the scientific benefit of seafloor observatory investigations has been recognised for many years, and, as such, numerous independent national and international efforts have been proposed or are underway. many of these challenging initiatives are described here (see section 4). although seafloor observatories have primarily research goals, the data collected by them will also provide an important contribution to operational observing systems such as goos (nrc, 1997; goos, 1999). this paper reviews the efforts made world-wide in a new emerging science, «seafloor observatory science» and its perspectives, focusing on european and italian contributions. 2. scientific motivation for seafloor observatories the question we have to answer is: «why establish seafloor observatories?». in recent decades oceans, earth and planetary sciences have been shifting from a discontinuous, expeditionary mode toward a mode of sustained in situ observations. this change in the mode of investigation stems from the realisation that earth and its oceans are not static, but are dynamic on many time and space scales, not just the short time scales involved in catastrophic events. as examples of catastrophic events we may note the influence on global weather of the el niño events of 1982-1983 and 1997-1998 or earthquakes, with associated tsunami waves, like the catastrophic earthquake (mw 9.5 main shock) and related tsunami that occurred at the end of december 2004 in the indian ocean (lomnitz and nilsen-hofseth, 2005; merrifield et al., 2005). understanding earth and its oceans means investigating processes as they occur. a scientifically powerful component of the observatory concept is the long time-series collection of multiple variables at a single location. these multidisciplinary data sets will enable the enhancement of more traditional methods, giving strong benefits to many disciplines, like geophysics (favali et al., 2002), physical oceanography (millot, 2002) and biology (thiel et al., 1994). seafloor observatories could offer earth and ocean scientists new opportunities to study multiple, interrelated processes over time scales ranging from seconds to decades. scientific processes with various time scales should benefit from data collected by seafloor observatories. these include: a) episodic processes; b) process517 seafloor observatory science: a review es with periods from months to several years; c) global and long-term processes. episodic processes include, for instance, eruptions at midocean ridges, deep-ocean convection at high latitudes, earthquakes, and biological, chemical and physical impacts of storm events. the category «b» includes processes like hydrothermal activity and biomass variability in vent communities. the establishment of an observatory network will be essential to investigate global processes, such as the dynamics of the oceanic lithosphere and thermohaline circulation. 2.1. role of the ocean in climate climate variations have widespread societal, economic, and environmental impact (nrc, 2003b). as a result, vigorous research efforts are currently aimed at improving our understanding of the spectrum of climate system variations. the ocean is a component of earth’s climate system, playing an increasingly important role in determining the nature of climate variability, as time scales expand. the goal is ultimately to predict climate variability and change. another important issue is to separate natural interannual-tocentennial climate variations from anthropogenically induced climate change. this aspect is critical for predicting future variations and magnitudes of climatic changes. therefore, we rely increasingly on models of the climate systems. even though present ocean circulation models are much improved, our knowledge of ocean physics is not comprehensive enough. a substantially improved observational basis for determining the necessary model enhancements is required. oceanographic variability has a significant influence on climate. the deep-sea has also recently been warming significantly (e.g., fuda et al., 2002) and it is very important to understand the causes and its role in a climate change context. it is essential to fully resolve many scales of variability and this requires nested, complementary observing systems. moreover, such observing systems provide the physical oceanographic context for interpreting biological and chemical distribution. to further our understanding of the role of the ocean in climate, seafloor observatories should be long-term facilities. a fundamental change achieved by pursuing the observatory concept would be the maintenance of existing sites and establishment of new sites. this may be the key to moving from the current focus on long-term science projects, such as jgofs or clivar, to the implementation of a sustained global ocean-observing system (nrc, 1998a,b; 2004). the main scientific goals achievable by using seafloor observatories are listed in table i. for instance, reliable samtable i. list of scientific goals where the seafloor observatory science is very useful (nrc, 2000). sectors scientific goals role of the ocean – to test and improve ocean circulation models. in climate – to understand the physics of the exchange processes between the ocean and atmosphere. – to observe the ocean climate anomalies from generation to destruction. – to predict climate variability and change. – to monitor, understand and predict: ° the sequestration of carbon dioxide in the ocean; ° productivity and biomass variability, including factors controlling chemistry; ° the full temporal and vertical evolution of thermohaline structure; ° rapid episodic changes of the ocean; ° changes in water mass transformation processes; ° air-sea exchanges (e.g., heat and gases); ° vertical exchanges of heat, salt, nutrients and carbon; ° thermohaline variability in the arctic and antarctic; ° the pathways of ocean transports; ° the role of eddies in transport and mixing. 518 paolo favali and laura beranzoli table i (continued). sectors scientific goals – to provide reference sites for calibration or verification of: ° air-sea fluxes for numerical weather prediction models, satellites; ° absolute interior and eckman layer velocities; ° remote sensed variables (e.g., sea surface temperature, sea level, wind); ° model statistics, physics and parameterisations. fluids and life – to investigate the chemical and biological response to episodic volcanic in the ocean crust and hydrothermal events. – to inquire into marine food webs on the seafloor. – to understand the linkages between geological, biological and chemical processes in ocean crust. – to assess the extent of sub-seafloor biosphere and determine its biological and chemical character. – to assess the impact of fluid and gas flow and related processes on crustal structure and composition, ocean chemistry, and biological productivity. – to determine the fluid flow patterns on ridge crests and flanks, and in convergent margins through space and time. – to directly observe the changes in heat, chemical fluxes, and biological diversity produced by ridge-crest magmatic and tectonic events. – to determine how hydrothermal-event plumes form and assess their global importance. – to directly observe how biological productivity and diversity change in response to fluctuations in fluid and chemical fluxes at vents and seeps on ridge crests and flanks, and at convergent margins. – to assess the extent to which sediment cover and spreading or subduction rates affect fluid chemistry and biological diversity at ridge crests and flanks, and subduction zones. – to quantify the importance of chemosynthetic productivity on the seafloor. – to understand the relations between tectonic and fluid processes in subduction zones. – to determine rates of gas hydrate formation and dissociation in response to perturbations of pressure, temperature, and fluid chemistry and flow rate and determine the influence on ocean chemistry, biology and climate. dynamics of oceanic – to investigate: lithosphere and ° global earth structure; imaging earth’s ° core-mantle dynamics; interior ° seismogenesis at subduction zone megathrusts; ° seismogenesis at convergent margins; ° ridge-crest processes and creation of oceanic crust; ° marine volcanism; ° upper mantle dynamics; ° oceanic plate kinematics, plate deformation and faulting; ° geo-hazard mitigation. coastal ocean – to investigate: processes ° sediment transport; ° coastal eutrophication; ° the impacts of global environmental change on the coastal environment; ° fishery; ° the structure and function of coastal ecosystems. turbulent mixing – to observe and understand processes that modulate vertical turbulence statistics. and biophysical – to generalise turbulence flux parameterisations. interactions – to determine the relationships between temporal and spatial distribution of turbulence in the ocean. – to map sub-surface distribution of mesoscale and sub-mesoscale horizontal turbulence. – to determine the impacts of turbulent mixing on biochemical distribution. 519 seafloor observatory science: a review pling of the deep-sea environment (over 50% of the earth’s surface) is important for reconstructing episodes linked to past climatic variability; to do this appropriate tools for sediment sampling and for deep-sea geotechnics are necessary (e.g., oebius and gerber, 2002). 2.2. fluids and life in the ocean crust although ocean chemistry is greatly influenced by the movement of fluids through oceanic crust, the processes controlling this flow are poorly understood. four different environments are important for research on fluids and life in the oceanic crust: ridge crests, ridge flanks, convergent margins, and coastal areas on passive margins. within each of these environments, it is critical to determine the nature and the linkages among tectonic, thermal, chemical, and biological processes at different temporal and spatial scales. previous observations of fluids related to the ocean crust have been made mainly by deploying single, short-duration experiments that stored data rather than transmitted information in real time. to make significant advances, it is essential to observe co-varying processes by making large-scale simultaneous collections of measurements over a variety of time scales. furthermore, real-time data collection through seafloor observatories is essential, as it will allow scientists to respond to unusual events or modify experiments if necessary. the chemistry and biology of fluids within the oceanic crust is a cutting edge research field for which seafloor observatories are an essential investigative approach (table i). one of the most exciting scientific problems that can be addressed using observatory science concerns the nature of the sub-surface biosphere. this is thought to contain a population of dormant microbes that are periodically driven into a population explosion by input of heat and volatiles into the crust during magma emplacements. eruptive events on the seafloor can release great volumes of hydrothermal fluid that affect the chemistry and biology of the overlying water and generate a unique type of hydrothermal plume, called an event plume. although it is not known how event plumes are formed, it is clear that they are produced by a sudden catastrophic release of large quantities of hot water. eruptions also extrude lavas on the seafloor, creating new habitats for endemic vent faunas and increasing production and export of deep-living microbial populations (delaney et al., 1998; summit and baross, 1998). initial changes in the water column and the seafloor after an eruption are very difficult to study using expeditionary approaches; a seafloor observatory system near a volcanically active site would provide an important platform for characterising the early stages of an event while also monitoring longer-term changes. a good example is the nemo observatory (see section 4.2). another exciting and closely related scientific problem concerns the general response of the hydrothermal system and associated biota to seafloor spreading events in which magma is injected into the crust. this research would include the response of seafloor biological communities at convergent margin seepage sites to abrupt changes in fluid and chemical fluxes caused by seismic activity. table i (continued). sectors scientific goals ecosystem dynamics – to detect and follow episodic ecological events (e.g., faunal responses to volcanic and biodiversity eruptions or hydrothermal fluid events). – to characterise and understand long-term (annual to decadal) ecological cycles. – to characterise and understand shorter-term (diel, tidal to seasonal) biological cycles. – to detect and monitor ecosystem responses to anthropogenic perturbations (e.g., influences of climate change on nutrients, trace metals and trace gases). – to forecast population and community changes (e.g., forecasting changes in fisheries stocks). 520 paolo favali and laura beranzoli similarly, the dynamics of gas hydrate formation and dissociation, especially in response to perturbations produced by tectonic cycles or global warming, is a problem of current interest that could be addressed by observatory science. for future studies of fluids and biota in the oceanic crust, access to the sub-surface is a critical requirement. one possibility for sampling and observing fluids and biota in the crust is by drilling, and some seafloor observatory experiments would be conducted in conjunction with boreholes drilled into the crust, like the corks observatories (see section 4.1). thus, odp, its continuation iodp and the development of new drilling capabilities will be an essential adjunct to seafloor observatory studies. 2.3. dynamics of oceanic lithosphere and imaging earth’s interior geoscience research in the oceans is moving beyond the exploration and mapping of the seafloor and is focusing on the dynamics of the solid earth system and the interaction of geological, chemical and biological processes through time. many of earth’s dynamic tectonic systems will be difficult to understand fully without continuous observations provided by the establishment of seafloor observatories. these include the complex magmatic and tectonic systems at ridge crests and submarine volcanoes; the genesis of destructive earthquakes and tsunamis and their relationships to largescale plate motions, strain accumulation, fault evolution, and sub-surface fluid flow; the geodynamics of earth’s interior and the origin of earth’s magnetic field; and the motion and internal deformation of lithospheric plates. seafloor observatories also have the potential to play a key role in the assessment and monitoring of geo-hazards, as many of earth’s most seismogenic zones and most active volcanoes occur along continental margins. the short-term expeditionary approach, common in the ocean sciences in the past, is poorly suited to detect or understand longertime changes. continuous measurements are required with the ability to react quickly to episodic events, such as earthquakes and volcanic eruptions. geophysical observatories have long been an integral component of earth science research on land; advances in technology and our understanding of the oceans now make it feasible to establish long-term observatories on the seafloor, as many experiments and projects have demonstrated (see section 4 for details). many areas of earth science would be advanced through the collection of time-series observations on the seafloor (table i), these include: a) global earth structure and core-mantle dynamics; b) seismogenesis at subduction zone megathrusts, and at convergent margins; c) ridge-crest processes and oceanic volcanism; d) oceanic plate kinematics, plate deformation, and faulting; e) geo-hazard mitigation. the scientific objectives that can be addressed particularly with geophysical data from long-term ocean-bottom observatories include two broad subject areas: earth structure and natural hazards. these two areas can each be divided into sub-areas according to the global, regional, and local spatial scale under investigation: a) global scale: mantle dynamics, core studies, moment tensor inversion; b) regional scale (between 500 and 5000 km): oceanic upper mantle dynamics, lithosphere evolution, and tsunami warning and monitoring; c) local scale (< 500 km): oceanic crustal structure, sources of noise, and detailed earthquake source studies (tomography of the source, temporal variations). for the global earth structure, there are fundamental scientific questions concerning the dynamics of earth’s mantle and core; for instance, the spatial scale of convection and the existence of distinct mantle geochemical reservoirs. the nature and origin of hotspots and their interaction with the lithosphere are other important questions in mantle dynamics. new paleomagnetic data are inconsistent with the idea of «fixed» hotspots (e.g., torsvik et al., 2002) and geochemical data appear to be compatible with different origins for hotspots, including the 670km discontinuity, a boundary in the lower mantle, or the core-mantle boundary (e.g., zhao, 2001; de paolo and manga, 2003). four centuries after the demonstration by gilbert that earth’s magnetic field is largely of internal origin, our understanding of the dynamo process responsible for generating the field remains incom521 seafloor observatory science: a review plete. improved spatial sampling provided by long-term seismic and geomagnetic observations in the ocean promises great gains in understanding the geodynamics of earth’s interior and the origin of earth’s magnetic field. large gaps exist in the global network of seismic and geomagnetic stations, since the oceans cover 7/10 of the whole earth surface. these gaps can not be filled with island stations only, particularly in the eastern pacific and southern oceans. many authors have recognised the importance of filling these gaps by using seafloor observatories (e.g., purdy and dziewonski, 1988; montagner and lancelot, 1995; lowes, 2002). the osn, first proposed over a decade ago, is envisioned as part of a larger ion, and would include about eight seafloor magnetic observatory sites as necessary for an improved global characterisation of the shortterm behaviour of earth’s magnetic field. for seismogenesis, plate tectonic theory provides a quantitative framework within which lithospheric deformation and faulting can be understood as well as a first-order explanation of the global distribution of seismicity. however, investigations are focusing on the deformation process itself and the still poorly understood interplay among tectonic stress, rock rheology, fluid distribution and faulting (e.g., monna et al., 2003 and references therein). subduction zone megathrusts produce the largest and potentially most destructive earthquakes and tsunamis on earth. despite the societal impact of these great earthquakes, little is known about the seismogenic zones. understanding the origin of major earthquakes at subduction zones, such as offjapan, central america and cascadia, is the key focus of the seize international initiative (moore and moore, 1998). the scientific strategy employed by seize involves a combination of geophysical imaging, drilling, and long-term monitoring over an earthquake cycle at a few representative subduction zones. these observations, in concert with laboratory experiments on the behaviour of material and theoretical modelling, offer the potential for major advances in our understanding of earthquake processes. a seafloor observatory programme would complement land-based studies by providing the capability to monitor deformation and faulting of offshore active fault systems. for ridge-crest processes and oceanic volcanism, the volcanism controls the heat flux, mass and volatiles from earth’s interior. the circulation of hydrothermal fluids transfers heat from the crust to the overlying ocean, and modifies ocean chemistry. hydrothermal processes affect the chemical, thermal and biological balance of oceanic environments. as a result of successful interdisciplinary international programmes, such as ridge and interridge, the importance of understanding the links among geological, physical, chemical, and biological processes in submarine volcanic systems is well established. current ship-based studies allow only periodic visits, at intervals of months to years. thus, hydrothermal and biogeochemical processes that occur at and immediately after the time of an eruption have never been observed. the links, mentioned above, can be studied by installing long-term observatory nodes at sites along the global mid-ocean ridge system and at a few oceanic volcanoes. for oceanic plate kinematics, deformation, and faulting, our picture of plate tectonics is largely based on historical data sets, such as the geomagnetic reversal record averaged over million of years, geomorphologic estimates of transform-fault azimuths, and present-day earthquake slip vectors. these data have provided input to a plate tectonic model. but this model lacks detail because most of the plate boundaries are underwater. seafloor observatories provide an opportunity to advance the embryonic science of monitoring geodetic motions on the seafloor. it will finally be possible to continuously observe motion near plate boundaries. for geo-hazard mitigation, as the human population continues to grow, the potential social and economic dislocation provoked by natural hazards, such as earthquakes, volcanoes, submarine landslides and tsunamis, has increased. these impacts are especially detrimental to developing nations. the destructive earthquakes and related tsunamis that occurred at the end of 2004 in the indian ocean, and that strongly affected sumatra, malaysia, indonesia, the andaman islands, thailand, myan mar, bangla desh, sri lanka, india and the maldives in terms of lives and economic impact, are only the most recent examples. another cause of hazard is the 522 paolo favali and laura beranzoli natural leakage of gaseous hydrocarbons at the seafloor, gas bubbles in the sediment, leaking hydrates, mud volcanoes on land and on the seafloor, and submarine gas seepage in pockmark fields (etiope and favali, 2004). new submarine gas sources are being continuously discovered (e.g., holland et al., 2003). off-shore gas leakage has not been adequately explored, but is an important hazard for off-shore oil exploration, exploitation and construction of related infrastructures (etiope et al., 2002). autonomous multidisciplinary benthic observatories can be used for reliable characterisation and monitoring of specific sites, like pockmarks (marinaro et al., 2004). the main advantage of long-term monitoring is to assess the temporal variability of the phenomena. 2.4. coastal ocean processes an important factor limiting coastal ocean research is the inability to quantify vertical and horizontal transport of water, elements and energy through the coastal ocean system. long time-series measurements of critical parameters, such as temperature, salinity, nutrients and trace elements, will help to provide the necessary data to remedy this deficiency. the coastal ocean includes estuaries, and the continental shelf and slope, and incorporates a great deal of environmental diversity. furthermore, the coastal ocean is where terrestrial influences encounter the broader ocean. it is the area of the ocean most strongly affected by anthropogenic impacts, it displays a strong geographic diversity, and it is the most biologically productive part of the ocean and hence is most heavily fished. some major problems in coastal ocean science are the following: a) coastal ecosystems are extremely productive, but not very well understood in terms of how their structures and functions respond to variations in environmental conditions; b) there are concerns both about how global change might affect littoral areas, and about how the resulting variations within the coastal zone might affect the rest of the ocean; separating natural from anthropogenic changes is essential; c) coastal management issues will require long, high-resolution time series of coastal ocean processes. the need to understand oceanographic change then implies a need to document environmental changes and their potential forcing agents. because the oceanographic variability can change with time over weeks to years, there is a great need to collect long-time series in order to document important variations in critical parameters (table i). one way of making long-term measurements in the coastal ocean would be to deploy arrays of moorings equipped with a wide spectrum of sensors. a low-cost model for a moored-buoy observatory has been proposed by frye et al. (1999). 2.5. turbulent mixing and biophysical interactions successful modelling of the distribution of organisms and chemical compounds in the ocean depends directly on the predictive quality of circulation models, which are limited by our ability to model turbulent mixing in the ocean. because turbulent motions result from highly non-linear dynamics acting across a range of time and space scales, progress in understanding these motions is difficult. advances depend on systematically collecting long-term measurements and resolving small vertical and horizontal scales throughout the range of turbulent regimes that are controlled by extremes of mechanical forcing, buoyancy forcing and topography. turbulent mixing occurs over a broad spectrum of time and space scales, strongly affecting the distribution of momentum, heat, chemical compounds and living organisms in the ocean. turbulent mixing at the sea surface mediates airsea exchange of biologically reactive compounds, such as dimethyl sulfide and carbon dioxide. turbulence in the bottom boundary layer plays a role in benthos-pelagic coupling of nutrients, and affects chemical signalling, habitat choice and genetic exchange in benthic communities. the grand objective is a parameterisation of turbulence statistics as a function of larger scale, more deterministic flows. the physical oceanography community has identified sub-regions of general circulation models that are greatly in need of improvement, including deep 523 seafloor observatory science: a review convection, boundary currents and benthic boundary layers, the dynamics and thermohaline variability of the upper mixed layer, fluxes across the air-sea interface, diapycnal mixing, and topographic effects. there is a strong connection between the understanding of coastal ocean processes and diapycnal mixing. because of the bottom slopes, flow across depth contours implies a vertical density transport that can determine transport in the boundary layer. flow near the bottom is important because of its role in the transportation of materials, such as sediments and benthic biota, between shallow and deep water. these bottom flows govern the behaviour of stronger alongshore flows in the overlying water column. as with the diapycnal turbulent fluxes, there is a clear need to improve our knowledge of horizontal turbulence. this research is a new frontier for physical oceanography. furthermore, in this case also effective tools for sampling the benthic boundary layer can greatly help (e.g., oebius and gerber, 2002). the main challenge for the parameterisation of turbulent mixing is to obtain high-quality turbulence statistics and their variation. to develop a universal parameterisation it is necessary to conduct observational studies in a broad range of environmental conditions (table i). in this context, a seafloor observatory programme would be of great benefit to advance our understanding and parameterisation of mixing in different dynamic regimes. some of these dynamic regimes that need to be observed include: a) a smooth bottom dominated by steady geostrophic flows; b) rough-bottom topography; c) dense water formation; d) internal tidal solitons; e) sill overflows; f) extreme atmospheric forcing at the near-surface boundary layer; g) double diffusive regime; h) hydrothermal vent fields; i) the continental shelf; j) vortex shedding; k) baroclinic instability; l) sediment gravity flows. 2.6. ecosystem dynamics and biodiversity the biological, ecological and biogeochemical questions likely to benefit most from sustained ocean time-series observations are those involving time-dependent processes or episodically triggered events, and those requiring longterm data sets. seafloor observatories are crucial for addressing many of the major scientific problems (table i). it is also important to note that the observatory approach, while necessary for solving many problems in marine ecology and biological oceanography, is not sufficient alone, and must be used in concert with other approaches. oceanic ecological observatories will extend into deep water the concept of the lter network. the mission of lter is similar to that proposed for seafloor observatories in that it aims to understand ecological phenomena occurring over long temporal and broad spatial scales and to increase our understanding of major natural and anthropogenic environmental perturbations at selected sites. just as restricting a seismic network only to land limits the ability of geophysicists to understand the dynamics of the earth, restricting ecological observatories only to land limits the ability of ecologists to fully understand the dynamics of the biosphere. leo-15 constitutes a good example of a long-term ecosystem underwater observatory (schofield et al., 2002). in this case, a seafloor observatory is considered in the broadest sense as a system supporting measurements from the seafloor to the ocean surface. an observatory might consist of a series of stationary observatory nodes to monitor the seafloor and the water column and auvs dispatched to provide broader spatial and temporal coverage. this definition does not include lagrangian drifters or floats, but their use will greatly complement an array of fixed observatories. the exploitation of deep-sea resources, begun more than 50 years ago, is expected to increase in the next future. in these activities, impact assessments must be conducted to avoid any misuse of the environment, by using the deep-sea either for waste deposition or for ore mining and hydrocarbon extraction (thiel, 2002 and references therein). 3. technical solutions for seafloor observatory architecture the principal characteristic of a seafloor observatory is a two-way communication between platforms and instruments and shore. during the 524 paolo favali and laura beranzoli last 30 years deep-sea investigations moved from scarce observations to continuous measurements of a wide set of parameters in selected areas. until the 1980s, deep-sea investigations relied on autonomous bottom landers, in the middle of the 1980s they were based on deep manned submersibles (table ii). starting from the 1990s, the concept of benthic stations and (relocatable and long-term) seafloor observatories appeared (person et al., 2006). seafloor observatories are characterised by the following basic elements: a) multiple payload; b) autonomy; c) capability to communicate; d) possibility to be remotely reconfigured; e) accurate positioning; f) data acquisition procedures compatible with those of shore observatories. it is also useful to introduce some definitions: 1) seafloor observatory is an unmanned station, capable of operating in the long-term on the seafloor, supporting the operation of a number of instrumented packages related to various disciplines. seafloor observatories can have as possible configurations: autonomous, acoustic linked and cabled. autonomous – observatory in stand-alone configuration for power, using battery packs, and with limited capacity of connection, using, for instance, capsules or an acoustic link from the surface, which can transfer either status parameters or a very limited quantity of data. table ii. deep-rated rovs (r) and manned submersibles (s) for research purposes (operation depth greater than 2000 m) (e.g., nrc, 2000; romanowicz et al., 2001; sagalevitch, 2004). vehicle type depth rating (m) support research vessel operating institution pisces iv s 2000 ka‘imikai-o-kanaloa noaa pisces v s 2000 ka‘imikai-o-kanaloa noaa shinkai 2000 s 2000 natsushima jamstec aglantha r 2000 opportunity imr cyana s 3000 opportunity ifremer dolphin 3k r 3000 natsushima jamstec hyper-dolphin r 3000 kaiyo jamstec little hercules r 4000 opportunity ife hercules r 4000 opportunity ife quests r 4000 opportunity marum modus(*) r 4000 opportunity tub tiburon r 4000 western flyer mbari alvin s 4500 atlantis whoi ropos ii r 5000 opportunity cssf nautile s 6000 nadir/la thalassa ifremer victor 6000 r 6000 l’atalante/la thalassa ifremer mir-1 s 6100 akademik keldysh shirshov institute mir-2 s 6100 akademik keldysh shirshov institute jason ii r 6500 atlantis/opportunity whoi shinkai 6500 s 6500 yokosuka jamstec isis r 6500 opportunity noc urov7k r 7000 kairei jamstec kaiko (**) r 11000 kairei jamstec (*) primarily designed for geostar; (**) lost in 2003. 525 seafloor observatory science: a review acoustically linked – observatory able to communicate by acoustics to an infrastructure, such as a moored buoy or another observatory. cabled – observatory having as infrastructure a submarine cable (retired cables, dedicated cables or shared cables devoted to other scientific activities, such as neutrino experiments). 2) infrastructure is any system providing power and/or communications capacity to an observatory (e.g., a submarine cable, a moored buoy, another observatory); an infrastructure may also serve as support for other instrumented packages. 3) instrumented package is a sensor or instrument devoted to a specific observation task; it may be hosted inside the observatory, be operated autonomously, be directly connected to an infrastructure or be placed near an observatory and interfaced to it (thus having the observatory as its infrastructure). the technological issues regarding longterm observatories versus temporary stations are quite similar. however, the technological developments are largely dependent on the period of operation of the station; a long-term observatory is much more difficult to maintain than a temporary ocean bottom station, due to the problems of power supply, failures in data retrieval and transmission, and corrosion and bio-fouling. long-term observatories can be used as reference stations and as nodes for short-term experiments with a dense coverage of stations (montagner et al., 2002). these temporary arrays, for instance of obs/h, are needed to study seismicity in particular areas. many experiments have already demonstrated the importance of integrating obs/obh with landbased seismic stations or of using a dense array of obs/obh, thereby greatly improving the quality of results (e.g., hino et al., 1996; dahm et al., 2002; montuori, 2004; shinohara et al., 2004; barberi et al., 2006; sgroi et al., 2006). this approach is completely different from the long-term seafloor observatories: it is a sort of mobile network for specific experiments similar to terrestrial ones. the obs/obhs are normally deployed in a free-falling mode, and this introduces an uncertainty in the positioning, in the orientation of the three components, and in the coupling of sensors with the seabed. depending on the sampling rate, they also have a limited autonomy. moreover, there is no control of the instrument while it is on the seafloor. only a posteriori, after recovery with a pop-up system, is it possible to discover if the sensor has worked correctly and to recover the collected data. for studying local seismicity deployments of a few months are sufficient. on the contrary for global tomographic studies 1-2 year deployments are preferable (bialas et al., 2002). at present, there are two ways to provide two-way connection: using either a cabled or an acoustic link (e.g., stojanovic, 1996; sozer et al., 2000) from the seafloor to a surface buoy that communicates via satellite and/or radio to shore, or using submarine electro-optic cables linked directly to a shore station. the links through cables can be made using either new cable deployments or decommissioned cables (e.g., nagumo and walker, 1989). on the other hand, individual nodes established for various scientific purposes can differ in size, complexity, scientific instrumentation, and technical capabilities in order to balance overall network objectives and cost. the actual designs must be driven by science needs. standardisation is one of the key issues. another important issue is to have proper marine logistics for the management of fixed seafloor observatories (ships and rovs) and joint use of auvs able to extend the capabilities of mapping and sampling. other important requirements for establishing a seafloor observatory network are sensors, power and data telemetry. although many sensors are currently available for underwater use, there is a lack of new sensors, especially for long-term use in the deep-sea environment. one of the most crucial aspects of the seafloor observatory is to furnish and to maintain the necessary power, especially in the autonomous mode. finally, there are concerns related to data telemetry and management. the energy and telecommunications industries can be involved in many aspects of ocean observatories, from supplying the cables, buoys, and instruments for observatory infrastructure to the ships, rovs, and support services needed to maintain and operate this infrastructure over the long term (nrc, 2003a). 526 paolo favali and laura beranzoli 3.1. autonomous observatories this type of observatory is characterised by a stand-alone configuration. therefore they are powered by battery packs and their autonomy depends on the battery capacity, but lifetimes are typically at least one year. the connection between the seafloor autonomous observatory and the surface is provided through releasable data capsules able to send information on the status of the observatory and a limited quantity of data. acoustic systems constitute another possibility to provisionally link the seafloor observatory to the surface. these systems are composed of an underwater transducer and acoustic modem, and a surface transducer and surface acoustic unit. examples are, for instance, the japanese «mobile seafloor observatory» equipped with data capsules (momma et al., 2001), geostar when using the messengers (beranzoli et al., 1998, 2000a,b; marvaldi et al., 2002), or sn-1 and mabel in experiments where they can be interrogated acoustically from the surface (see section 4.5). 3.2. acoustically linked observatories these observatories can have long-term acoustic links either with a moored buoy or another observatory as a node of an underwater network, as in the orion-geostar-3 project. in the first option the observatories transmit data to a surface buoy acting as a central communication node, with a satellite and/or direct radio link to shore. this surface buoy is anchored to the seafloor and communicates with the sub-sea nodes acoustically or via an electrical or fibre-optic cable. in contrast to cabled observatories, moored-buoy systems are generally less expensive to install, but the trade-off is a greatly diminished data bandwidth and reduced power availability. data transfer rate, power consumption, and system stabilisation requirements are all interdependent, but it is possible to obtain high data transfer rates in balance with power and buoy stability. some satellite systems, because of the need to use a directional antenna, impose strict stability requirements on the surface buoy (< 10 degrees per second in pitch, roll and yaw). satellite communications can also consume substantial power. data transfer requirements range from less than 1 to up to 100 kb/s. in order to prevent disruption to the time-series data sets, a back-up communication system should be included in the design. furthermore, the system must have the capacity to store data. moored-buoy systems will need to be engineered for deployment in different environments, including those at high latitudes. deployment location will greatly impact mooring design due to variations of sea state, wind velocity, ocean and air temperatures, water depth and satellite coverage. furthermore, other factors have to be taken into consideration, such as the suitability of solar panels at high latitudes, potential for vandalism and visibility of the mooring in shipping lanes. it is estimated that, at present, the average maintenance interval for a moored-buoy observatory will be about 12 months. 3.3. cabled observatories cabled seafloor observatories use undersea communications cables to supply power, communications, and command and control capabilities to scientific monitoring equipment at nodes along the cabled system (e.g., chave et al., 2004, 2006). each node can support a range of devices that may include an auv docking station. cabled systems will be the preferred approach when power and data telemetry requirements are great. early generation commercial optical undersea cables that are soon to be retired will have the communications capacity, but will possibly have insufficient power capability. if these cables are suitably located for seafloor observatory research, their use could reduce the need for expensive new cables. more than 35000 km of electro-optical telecommunications cables on the ocean floor will be retired in-place by the industry within the next few years, and more are likely to follow during ensuing years (nrc, 2003a). there are many good examples of the re-use of retired cables, such as h2o, geo-toc and venus (jp) (see sections 4.2 and 4.3 for details). 527 seafloor observatory science: a review the major components of a cabled observatory are: a) shore station (containing high-power and -voltage, direct-current generation, network and science experiment management); b) undersea cable (containing optical fibres and power conductors); c) undersea observatory nodes (containing power conditioning, network and science experiment management, and standardised interfaces); d) network and science experiment command and control, and communications systems; e) specific sensors and auvs. the current generation of commercial optical cables can satisfy all the seafloor observatory data requirements for long (greater than approximately 300 km) and short systems. these cables can provide data transfer rates on the order of 500 gb/s per fibre pair. undersea mateable connectors are currently available. but substantial engineering development will be required for the design and packaging of the power conditioning, network and science experiment management to be placed at the observatory nodes. in order to meet the necessary specification for high system-operational time, low repair costs, and overall equipment lifetime, significant trade-offs will have to be considered between the use of commercially available and custom-built equipments. 3.4. ships, rovs and auvs ships must have suitable characteristics to execute sea operations related to seafloor observatories, particularly dgps, navigation system and dp, and adequate a-frame and handling systems. for instance, medium size vessels, like the italian r/v urania owned by cnr (fig. 1), have already demonstrated the feasibility of managing seafloor observatories such as geostar-class observatories in the mediterranean and in the deep-sea (over 3000 m w.d.). the use of vehicles, like rovs and auvs, is complementary to seafloor observatories. rovs are likely to play an important role in installing, servicing, and repairing seafloor observatories, fig. 1. r/v urania, a medium-size vessel (overall length: 61 m; gross tonnage: 1100 t) owned by cnr and managed by sopromar. the top-left corner shows the ship during the launch of a geostar-class observatory. 528 paolo favali and laura beranzoli even if the payload is limited to a few hundred kilos. rov technology is available with a wide range of capabilities and has been advancing, both within the oceanographic community and industry. the largest pool of rovs is in the offshore industry, but their depth rating normally does not exceed 2000 m (3000 m in a few cases). presently only rovs specifically developed for scientific applications have depth ratings exceeding 3000 m (see table ii). in fig. 2 the french rov victor 6000 is shown as an example. some common uses for rovs include: a) high-resolution site mapping (seafloor maps are important for site preparation prior to the seafloor observatory installation; towed vehicles are routinely used for these surveys, but rovs and auvs capable of flying precision tracks produce the highest quality maps); b) installation of instrumentation; c) servicing installed instrumentation (instrumentation positioned at depth can experience biofouling or short-term failures); d) servicing node components; e) plugging and unplugging platforms and instruments; f) burying cables and sensors. the ocefig. 2. the french deep-rated rov victor 6000 built and managed by ifremer (see also table ii). a detailed view of the rov is shown in the bottom-left corner. 529 seafloor observatory science: a review anographic community has already demonstrated the use of rovs in aspects of observatory work, such as scripps at osn-1, whoi for h2o and mbari for work in monterey bay (see section 4.2). auvs have the potential to undertake a variety of mapping and sampling missions while using fixed observatory installations to recharge batteries, off-load data and receive new instructions. a major use for auvs will be to map seafloor and water-column properties and to document horizontal variability. another use will be to extend the spatial observational capability of seafloor observatories. scenarios for employing auvs as elements of seafloor observatories envision small vehicles, weighing at most a few hundred kilos, with docking capabilities. vehicle endurance is dictated by survey speeds (typically about 5 km/h) and power consumption by onboard computer and sensor payload. general goals of auv missions include: a) seafloor mapping (auvs operate very close to the seafloor and they can collect high-resolution, high-accuracy mapping data, in addition to other geophysical parameters, such as bathymetry and magnetics); b) water-column mapping (auvs provide the capability to map physical and chemical parameters, horizontally, vertically or in three-dimensions); c) measuring fluxes (auvs provide this capability at specific locations); d) initialising and constraining models (auvs are capable of obtaining the physical parameters needed). recently, the oil and gas industry has demonstrated a growing interest in auvs for deep-water surveys driven by the combination of the move of off-shore drilling to greater water depths and the increasing maturity of auv capabilities. 3.5. scientific instrumentation a wide variety of sensors are already available for undersea research, and there are many instruments that are being deployed for long durations (e.g., thermistors, seismometers, hydrophones and current meters). there are also several types of samplers available for the collection of fluids and biological samples that require shore-based analysis. many of these sensors and samplers are not suitable for long-term deployments for a variety of reasons (e.g., instability of chemical reagents, sensitivity to biofouling, or loss of calibration). it is clear that development of new sensors will be critical in order for seafloor observatories to be fully effective. sensor technology (e.g., in chemistry and biology), especially for long-term use in the deep-sea, is not sufficiently advanced to take advantage of the seafloor observatory infrastructure. in addition, enhancements of existing sensors are necessary for unattended operations for a long time. the general requirements for new sensors or for enhanced types are: low power consumption, longterm stability, standard interface and adequate choice of materials. if an ocean observatory infrastructure is to be established, it will be necessary to make substantial parallel investments in sensor technology (e.g., brewer et al., 2002, 2004) or packages, for example seabass developed as a vlf system (2-50 hz) to study ambient noise at and below the seafloor (stephen et al., 1994), sappi an autonomous pore-pressure instrument (kaul et al., 2004), hydrophone arrays for measuring ocean temperature (atoc project; dushaw et al., 1999) or geodetic measurements using both gps and acoustics (spiess et al., 1998; gagnon et al., 2005a,b). much effort has been made to develop prototypes in parallel with the activities of the ec geostar and related italian projects (see sections 4.4 and 4.5). each sensor has its own requirements for correct installation and data acquisition in terms of sampling rate (see favali et al., 2006). an important issue is the synchronisation of all the sensors. it can greatly help to understand better the mutual relationship among different processes when one is comparing data sets acquired by accurately synchronised sensors. this is particularly true for seismometers, where the required clock stability is <10−9 s. another important issue for seismometers is how to minimise the level of noise recorded in different frequency bands (depending on the noise sources), so as to keep within the high and low background noise reference models (peterson, 1993). different modes of sensor installation and of «cleaning» the seismological data using other types of instruments sampled at high rates (like current meters and/or dpgs) are the tools used for minimising noise (e.g., stakes et al., 530 paolo favali and laura beranzoli 1998a; webb, 1998; webb and crawford, 1999; crawford and webb, 2002; monna et al., 2005; shinohara et al., 2006). a challenge for any observatory data management structure will be the processing, distributing and archiving of the large data sets produced (e.g., momma et al., 2001). a fully integrated plan for data handling should be developed in the early stages for any seafloor observatory programme. redundancy must always be built in. 3.6. technology for power supply power supply is one of the most critical aspects in seafloor observatories, especially those in stand-alone configurations. at present, for these applications primary lithium batteries are normally used, as in geostar-class observatories (see section 4.4). this type of battery easily supports multidisciplinary experiments for more than one year. efforts must be made to develop new sources of energy (e.g., fuel cells) and to design systems that are re-chargeable from the surface using the surface buoy as primary source, or substituting battery packs on the seafloor systems by rovs. for a cabled observatory, the power aspects are less critical, but significant engineering development will be necessary to provide sufficient power and to adapt the power transmission schemes used in the sub-sea telecommunications industry. the following developments in seafloor power hardware will be required: a) design of specific direct-current conversion hardware with the required reliability; b) provision of a workable thermal environment for the power conditioning, network and science experiment management (the physical design of the thermal paths from the electronics to seawater need to be carefully engineered); c) the physical power path to the observatory will need to be designed so as to minimise the probability and effects of corona discharge; d) the configuration and hardware for power surge protection need to be designed to provide a fault-tolerant observatory network. these scenarios could be quite complex for an observatory with many nodes and more than one shore station. 4. world-wide past and ongoing experiments the scientific benefits of establishing a seafloor observatory network for geophysical, oceanographic, climatological and meteorological investigations have been recognised for many years. following the pioneering efforts in the 1960s (sutton et al., 1965), the importance of filling the existing gaps in the global landbased networks (particularly in the network of seismic and geomagnetic stations) by using seafloor observatories has been recognised (e.g., purdy and dziewonski, 1988; montagner and lancelot, 1995; lowes, 2002). since the 1990s, many important workshops have been organised outlining long-term scientific strategies for the use of observatories: for instance, the three «international workshops on the scientific use of submarine cables» organised in 1990 in honolulu (chave et al., 1990), in 1997 in okinawa (utada et al., 1997) and in 2003 in tokyo (kasahara and chave, 2003), the international conference «science-technology synergy for research in marine environment: challenges for the xxi century» held in erice (sicily) in 1999 (beranzoli et al., 2002), the «symposium on seafloor observatories» held in 2000 in islamorada (florida), the ohp/ion joint symposium «long-term observations in the oceans» in yamanashi (japan) (romanowicz et al., 2001) and oceans’04 in kobe (yada et al., 2004). this section aims at reviewing all the relevant past and ongoing experiments, projects and programmes on seafloor observatories in a world-wide context. table iii lists the twentyeight seafloor observatories validated through long-term missions at sea. 4.1. international 4.1.1. experiments and projects corks – from 1991 to 1997 odp installed 13 long-term hydrogeological observatories (corks) to study fluid flow processes in situ in two flow regimes: sedimented young oceanic crust and accretionary prisms (e.g., davis et al., 1992; becker et al., 1998; becker and malone, 531 seafloor observatory science: a review 2001). these observatories involve sealing a borehole at the throat of the re-entry cone with a sensor string suspended in the hole. a longterm data logger is positioned so as to be accessible by a submersible on the seafloor for periodic data transfer and re-programming. international initiatives like seize, funded within the nsf programme margins, can greatly benefit from permanent observatories including seafloor seismic and fluid flow, and borehole monitoring devices. corks can provide a realtime record of sub-surface transient events in temperature, pressure and water chemical data. seize was developed to study the shallow subduction zone interface that is locked and accumulates elastic strain which is periodically released in large or great earthquakes, often tsunamigenic (margins, 2003). dsdp legs 52, 88 and 91, and odp leg 128 – borehole seismology was introduced to ocean drilling on dsdp leg 52 in 1976-1977 at site 417 in the atlantic ocean (stephen, 1978; stephen et al., 1980a,b); then on dsdp leg 88 table iii. twenty-eight seafloor observatories validated at sea; a: autonomous; al: acoustic linked; c: cabled. name type starting year country jma omaezaki c 1978 japan jma off-boso c 1985 japan jamstec off-hatsushima c 1993 japan eri off-ito city c 1994 japan eri off-sanriku c 1995 japan nied hiratsuka c 1995 japan leo-15 c 1996 u.s.a. geo-toc c (re-used) 1997 japan hugo c 1997 u.s.a. jamstec off-muroto peninsula c 1997 japan moise a 1997 u.s.a. nemo al (buoy) 1997 u.s.a. h2o c (re-used) 1998 u.s.a. mobile seafloor observatory a 1998 japan jamstec off-kushiro-tokachi c 1999 japan venus (jp) c 1999 japan geostar al (buoy) 2000 eu mvco c 2000 u.s.a. nereid-1 a 2000 japan nereid-2 a 2000 japan wp-2 a 2000 japan wp-1 a 2001 japan mobb a 2002 u.s.a. orion-geostar-3 (node 3) al (observatory) 2003 eu orion-geostar-3 (node 4) al (observatory) 2003 eu assem (4 nodes) al (buoy) 2004 eu gmm c 2004 eu sn-1 nemo(*) c 2005 italy-eu (*) sn-1 was used in autonomous configuration in 2002. 532 paolo favali and laura beranzoli in 1982 at site 581 in the pacific ocean with the emplacement of a seismometer built by the hawaii institute of geophysics (duennebier et al., 1987); and finally on dsdp leg 91 in 1983 at site 595 in the pacific ocean, during the ngendie programme (adair et al., 1987; jordan et al., 1987). in all cases the glomar challenger vessel drilled a cased hole. in september 1989, a feedback-type accelerometer capsule was installed in hole 794d in the japan sea during odp leg 128 (ingle et al., 1989; suyehiro et al., 1995). the instrument recorded a teleseismic event (mb 5.4 at about 4000-km epicentral distance) that clearly showed a surface wave dispersion train (tamaki et al., 1992). technologies for re-entering boreholes with observatory style sensors have played a vital role in the evolution of the seafloor observatory concept. these include the ifremer nadia systems (versions 1 and 2) (legrand et al., 1989; gable, 1992) and the sio wireline re-entry system (spiess et al., 1992; stephen et al., 1994; becker et al., 2004). nero (odp leg 179) – odp leg 179 in april-june 1998 set out with two primary objectives (pettigrew et al., 1999). one of these was to prepare the nero site where researchers could establish a gobo as part of the future network of seafloor observatories proposed in the ion programme, drilling a cased re-entry hole into basaltic basement on the ninety east ridge. this objective specifically included drilling a single hole as deep as possible into the basement, and installing a re-entry cone and casing beyond basement to prepare for the future installation of the observatory. the depth of penetration below the seafloor (493.8 m) in a water depth of 1648 m as well as the firm attachment of casing to the basement, should isolate the instrument from noise. geophysical observatories currently operating world-wide share a common attribute and shortcoming in that they are only emplaced on continents or islands. since the world’s oceans cover more than two-thirds of the planet’s surface, the coverage given by observatories on oceanic islands is incomplete. installation of a downhole observatory at this location in the east indian ocean will fill one of the major gaps in global seismic monitoring coverage. the nero project is a joint initiative among jamstec, ifremer and ipgp. odp leg 186 – two borehole geophysical observatories were installed (about 1100 m below the seafloor) in august 1999 on the deepsea terrace of the japan trench during odp leg 186 at site 1150 (nereid-1; 39°11ln, 143°20le) and at site 1151 (nereid-2; 38°45ln, 143°20le) (sacks et al., 2000). the sites are located in areas with contrasting seismic characteristics. the northern site is within a seismically active zone where microearthquakes are frequent and m 7 earthquakes recur. the southern site is within an aseismic zone where no microearthquakes are observed. these features coexist within the seismogenic zone of the japan trench plate subduction zone, where the > 100-ma portion of the pacific plate is subducting at a fast rate (∼ 9 cm/yr) beneath northern japan causing major earthquakes along the trench. leg 186 is the first scientific venture to succeed in installing state-of-the-art strain, tilt, and seismic sensors for long-term operation in seafloor boreholes. the systems started collecting data in september 1999. these stations make invaluable additions to the existing geophysical network over the western pacific. this type of multiple-sensor seismo-geodetic observatory can now be emplaced by the joides resolution drilling vessel in many other areas where active processes wait to be monitored. although not always the case, normal coring objectives and observatory objectives often overlap and are interrelated as in this leg or recent cork legs. once an observatory is established, ways and means to recover the data and to keep the station running become necessary. such tasks are not easily undertaken even if a site only needs servicing once a year. a new fibre-optic cable owned by the university of tokyo already exists and currently terminates near site 1150. once site 1150 proves to be functioning, connections will be made to supply power, send commands, and retrieve data in real time on land. furthermore, a 50-km cable extension is planned to connect site 1151 as well. the borehole geophysical observatories at 533 seafloor observatory science: a review sites 1150 and 1151 greatly improve source location (particularly depth) and focal mechanism and rupture process determinations for earthquakes near the japan trench (nishizawa et al., 1990, 1992; suyehiro and nishizawa, 1994; hino et al., 1996). near-field data, obtained from these observatories with the aid of ocean bottom seismographs, will particularly improve the resolution of source mechanisms for very slow rupture events such as tsunami earthquakes (hino et al., 2001; hirata et al., 2003), or tsunamis generated by submarine landslides (e.g., driscoll et al., 2000), contributing to better constraint simulation (e.g., ohmachi et al., 2001). odp leg 191 – odp leg 191 (july-september 2000) had two main goals: to drill and case a borehole at a site in the north-west pacific ocean between japan and the shatsky rise and to install therein a borehole seismic observatory (kanazawa et al., 2001). the seismic observatory was successfully installed at site 1179 (called wp-2) and left ready for activation by a future rov cruise. a high priority for ion has been to install a station beneath the deep seafloor of the north-west pacific to gain a better understanding of regional earthquake patterns and to enhance tomographic images of the earth’s interior. the seismometer augments a regional network consisting of land stations in eastern asia, japan, and the western pacific islands and borehole seismometers installed during odp leg 186 (sacks et al., 2000) and planned at that time for odp leg 195 (see below). owing to its location, the site 1179 seismometer will provide critical seismic observations from the seaward side of the japan trench. the objective is to understand the processes driving earth’s dynamic systems from a regional to a global scale by imaging the earth’s interior with seismic waves. unfortunately, few seismometers are located on the 71% of the earth’s surface covered by oceans. the asymmetry and non-uniformity of seismic station distribution makes high-resolution imaging of some parts of the mantle nearly impossible. ocean-bottom seismometers are needed to accomplish the goals of international geoscience programmes that use earthquake data. downhole instruments for these western pacific borehole observatories have been developed under the ongoing japanese ohp programme (see section 4.3). aside from plugging an important gap in the global seismic array, the site 1179 observatory produces high-quality digital seismic data (shinohara et al., 2006). tests with other borehole seismometers show that the background noise level for oceanic borehole instruments is much less than most of their counterparts on land (e.g., stephen et al., 1999). odp leg 195 – odp leg 195 (march-may 2001), among three distinct objectives, dedicated the first segment to coring and setting a longterm geochemical observatory (cork) at the summit of south chamorro seamount (site 1200). the second segment was devoted to coring and casing a hole in the philippines sea abyssal seafloor (site 1201) and the installation of broad-band seismometers for a long-term sub-seafloor borehole observatory (salisbury et al., 2002). the drilling and observatory installation at chamorro was designed to: a) examine the processes of mass transport and geochemical cycling in the subduction zones and forearcs of non-accretionary convergent margins; b) ascertain the spatial variability of slabrelated fluids in the forearc environment; c) study the metamorphic and tectonic history of non-accretionary forearc regions; d) investigate the physical properties of the subduction zones as controls over dehydration reactions and seismicity; e) investigate biological activity associated with subduction zones. a high-quality digital seismic observatory was therefore installed in the west philippine basin as an important component of the ion seismometer net. the observatory (called wp-1) allows more precise study of the seismic structure of the crust and upper mantle in the philippine plate, as well as better resolution of earthquake locations and mechanisms in the north-west pacific subduction zone. the observatory is designed as a stand-alone system with its own battery pack and recorder on the seafloor so that it can be serviced and interrogated by a rov, like the borehole observatories installed at sites 1150 and 1151. however, there is a coaxial tpc-2 cable near site 1201 (shinohara et al., 2006) 534 paolo favali and laura beranzoli and there are plans to connect data, control and power lines to this cable. this will also be done under the auspices of ohp (see section 4.3). odp leg 196 – leg 196 was the second of a two-leg program of coring, logging, and installing acork long-term subseafloor hydrogeological observatories in the nankai trough, a convergent margin accreting a thick section of clastic sediments (mikada et al., 2002). the nankai subduction zone off southwest japan forms an «end-member» sediment-dominated accretionary prism. the philippine sea plate underthrusts the margin at a rate of about 4 cm/yr along an azimuth of 310°-315° (seno et al., 1993) down an interface dipping 3°-7° (kodaira et al., 2000), causing repeated great earthquakes (magnitudes > 8) with an average recurrence interval of about 180 years (ando, 1975). although the role of fluid is thought to be a key factor in the study of seismogenic zones (hickman et al., 1995), the magnitude and location of active fluid flow in this accretionary prism and the potential linkage to the nankai seismogenic zone are not clearly defined. the investigation of this system motivated the deployment of long-term hydrogeological and geochemical monitoring systems. leg 196 (may-july 2001) focused on logging while drilling and installing acorks at two sites (808 and 1173) near the toe of the nankai prism. odp leg 203 – the odp leg 203 science programme is part of a multidisciplinary project that primarily represents the interests of the nsf’s component of the international deos(b) planning effort, and ion (orcutt et al., 2003). the leg (may-july 2002) drilled a cased re-entry hole (hole 1243a, osn-2, 3882 m w.d. at 5°18ln, 110°4lw) in the eastern equatorial pacific, the location of a future deos multidisciplinary observatory. the drill site was located in 10to 12-ma lithosphere. the hole was drilled to a total depth of 224 m, which included 121 m of sediment and 103 m of basement penetration. hole 1243a will subsequently be used to install an observatory-quality broadband three-component seismometer (0.001-5 hz) as well as a high-frequency three-component seismometer (1-20 hz) to ensure high-fidelity recording over the range of frequencies normally recorded by the terrestrial gsn. the seismic system, as well as other instrumentation associated with the observatory, will be connected to a deos(b) mooring, for both power and high-speed data telemetry, to a land station and to the internet. the equatorial site satisfies two scientific objectives of crustal drilling: a) it is located in one of the high-priority regions for the osn and deos(b); b) it is in oceanic crust created by fast seafloor spreading, providing a rare opportunity to examine crustal genesis, evolution, and crust/mantle interaction for a seafloor-spreading end-member responsible for generating the majority of the oceanic lithosphere. the drilling and establishment of a cased legacy hole at the remote equatorial pacific ion multidisciplinary observatory site provides an ideal location for the initial installation of a moored observatory. this site is one of the high-priority prototype observatories for the osn (purdy, 1995) (see section 4.2). odp leg 205 – costa rica is an important area for studies of the seismogenic zone and subduction factory for several reasons. science objectives for leg 205 (september-november 2002) had two primary foci, both related to seismogenic zone and subduction factory questions. the first was to determine the igneous and alteration history of the uppermost part of the downgoing plate at reference site 1253. the second was to characterise and monitor two of the three hydrological systems: in basement at site 1253 and along the décollement (or upper fault zone) at sites 1254 and 1255 (morris et al., 2003). these goals were accomplished by 1) targeted coring at selected intervals, 2) downhole temperature and pressure measurements, 3) logging at site 1253, and (4) installation of long-term observatories (cork-iis) to monitor temperature and pressure and to sample fluids and gases in each of the hydrologic systems. installation of long-term seafloor observatories was a major focus of leg 205. two boreholes were drilled on the costa rica subduction zone to study the geochemical fluxes and related processes. the holes (1253a and 1255a) were outfitted with modified corks (cork-iis) that include instruments capable of fluid sam535 seafloor observatory science: a review pling and measuring flow rates, temperature, and pressure (jannasch et al., 2003). 4.1.2. programmes ion – the ion committee was established in june 1993 and the iaspei executive committee granted it commission status. it is an international association affiliated to iugg (romanowicz and suyehiro, 2001). ion was formed to foster synergies among different disciplines, and to promote international co-operation in the development of critical elements of the seafloor observing systems, harmonisation of those elements of the system that would allow shared maintenance of the observatories and development of common plans for the use of international resources (e.g., goos, iodp). ion was originally created for the purposes of the seismological community; the seismic data from a wwssn, established in the early 1960s, accelerated advances in seismology and were a great resource for new discoveries till the 1970s. during the past 15 years, our knowledge of the processes of the deep earth has been greatly improved by the development of new generations of global monitoring networks in seismology and geodesy and by the continuation of long-term observations in geomagnetism (e.g., geoscope, iris, geofon, and mednet). improvements in the observatory locations for seismology, geodesy, and geomagnetism, particularly in the oceans, can greatly enhance our understanding of the earth’s interior. ion emphasises that «oceans are seismic deserts» (this also applies to geomagnetic observatories). except for a few stations on oceanic islands, very large zones are unmonitored, particularly in the pacific, south atlantic, and east indian oceans. in 1995 participation in ion was enlarged to include the geoscience community, and in 2001 to include the oceanographic community. during its meetings ion has formulated recommendations, among the most important of which are: a multidisciplinary approach to ocean observatories is essential; extension of observing systems to the oceans is essential not only for solid earth geosciences, but also in other disciplines such as biology. at the end of 2004 ion gained an inter-association status. under its umbrella several experiments have been performed, among them moise and nero, and many observatories installed in boreholes have been supported. 4.2. u.s.a./canada 4.2.1. experiments and projects leo-15 – the leo-15 real-time cabled underwater observatory was established in august 1996 in 15 m of water 7 km off-shore of great bay (new jersey). one of the major goals of leo-15 is to develop a real-time capability for rapid environmental assessment and physical/ biological forecasting in coastal waters (glenn et al., 2000). it provides communications and power for different instruments at many sub-sea nodes via a cable, and shore facilities. continuous measurements made at each node are used to model and to aid biologists in their research into benthic communities and phytoplankton ecology (traykovski et al., 1999; howe et al., 2002; schofield et al., 2002). mvco – another coastal observatory (mvco) has recently been installed starting in 2000 by whoi off the south coast of martha’s vineyard (massachusetts) to monitor coastal atmospheric and oceanic conditions (edson et al., 2000; austin et al., 2002). the observatory is able to provide scientists with direct access to the coastal environment and to allow continuous measurements of environmental parameters. based on a telemetry system, the observatory has been designed to be in operation for a minimum of 25 years with minimal maintenance. spare power conductors and optical fibres in the main cable provide for significant expansion capability for future nodes, auv docking stations and other special experiments. hugo the university of hawaii developed the hugo project, a seafloor observation system cabled to an on-shore station by means of a 47-km fibre-optic cable (duennebier et al., 2002a). hugo was installed in 1997 and aimed at creating a permanent multidiscipli536 paolo favali and laura beranzoli nary ocean-floor laboratory at the top of the loihi submarine active volcano through the integration of marine electro-optical cables with existing sensor technologies (fig. 3). hugo has the potential of supporting experiments from many disciplines, including volcanology, biology and geochemistry. the main components of hugo are the shore station, supplying power to the observatory and recording data; the electro-optical cable; the junction box; the power distribution and data collection centre on lahoi; multiplexing nodes and secondary distribution points. hugo could potentially supply electrical power, command capability and real-time data services to more than 100 instruments connected to and removed from the ocean floor by submersible or rov. unfortunately in april 1998 the main hugo cable developed a short circuit to sea water and a new cable must be obtained and installed. despite the failure, this experiment accomplished several important tasks, among them the transmission of high-rate, high-fidelity data from the top of loihi. fig. 3. a multibeam map of the summit of the submarine volcano loihi (hawaii) where the hugo experiment was performed (rectangle). the route of the 47-km fibre-optic cable is shown on the right (duennebier et al., 2002a). 537 seafloor observatory science: a review h2o – in mid 1998 a similar experiment was undertaken for the development of h2o based on a retired commercial submarine telephone cable (burns, 1999; butler et al., 2000; chave et al., 2002; petitt et al., 2002). it consists of a cable termination and a junction box in 4979 m of water placed in the eastern pacific, roughly half-way between california and hawaii (141.6°w, 28.5°n). in fig. 4 a photograph of the junction box is shown, together with a map of its location and the route of the decommissioned cable used, an at&t telephone cable system that originally connected san luis obispo, california, and makaha, on oahu island, hawaii. instruments are connected by means of a rov to the junction box, which provides two-way digital communication at variable data rates and a total of about 500 w for both junction box systems and user equipment. there are two shallow buried seismometers (∼ 1 m below the seafloor), consisting of a modified guralp cmg-3t broad-band seismometer and a conventional 4.5 hz three-component geophone (duennebier et al., 2002b). this sensor has been transmitting seismic data to shore (oahu) continuously and in real time for >2 years. the seismic data are forwarded to the iris data management center in seattle and are included in the gsn database for use in global and regional earthquake studies. to reduce the noise level significantly (collins et al., 2001), it was decided in future to place the broad-band seismometer in a borehole. during odp leg 200 (december 2001-january 2002) a re-entry hole was drilled and cased into basement near the existing hawaii-2 cable and the h2o junction box in order to establish a longterm borehole geophysical observatory for continuous real-time seismic monitoring, as well as for other geophysical experiments. the longterm h2o site satisfies three scientific objectives of crustal drilling: 1) it is located in one of the high-priority regions for the osn; 2) its proximity to the hawaii-2 cable and the h2o junction box makes it a unique site for realtime, continuous monitoring of geophysical, microbiological, and geochemical experiments in the crust; and 3) it is on fast-spreading pacific crust (71 mm/yr half-rate) (stephen et al., 2003a). moise and mobb – as part of the mbari margin seafloor experiment, conducted from 1996 through 1999, the moise experiment deployed a suite of geophysical and oceanographic instruments by means of the rov ventana (< 2000 m operative depth) on the western side of the st. andreas fault system off-shore of fig. 4. the h2o junction box (on the left). a map of the h2o location and the route of the retired cable used is shown on the right (chave et al., 2002). 538 paolo favali and laura beranzoli central california. the experiment ran in autonomous mode at around 1000 m w.d. and collected three months of data in the summer of 1997, having developed a corehole seismometer for low-noise seismic data (stakes et al., 1998a) to constrain continental margin seismicity by extending the seismic network to offshore sites (begnaud and stakes, 2000). periodical rov reconnection to moise’s logger permitted data download from the sensor package (romanowicz et al., 1998; stakes et al., 1998b, 2002; stutzmann et al., 2001). in april 2002, mbari also installed mobb at 1000 m w.d., 40 km off-shore, using a rov, as a direct follow-up of moise. this observatory was buried and besides a broad-band three-component seismometer comprises a current meter and a dpg. the ultimate goal is to link mobb by continuous telemetry to the shore, so that it becomes part of bdsn. this awaits the installation of the mars cable (romanowicz et al., 2003, 2006). in the next iodp leg 312 (october-november 2005) an engineering leg is planned for the development of a borehole observatory as part of the mars activity. mbari is also testing several high-risk technologies associated with a cable-linked moored buoy observatory at the moos test mooring site in 1860 m of water (nrc, 2003a). deos – the deos(a) seafloor observatory initiative, concentrating on geophysics, started in 1997. initially the focus was on deep-water geo-observatories, but this was subsequently expanded to include near-shore observatories and water-column studies. to reflect this effort to engage the wider oceanographic community, the acronym was changed in 1999 to deos(b). the deos(b) steering committee includes representatives of the major u.s.a. geo-science observatory programmes and additional members from the microbiological community. the deos(b) planning initiative in the u.s.a. and the u.k. (supported by nsf and nerc respectively), in co-ordination with partners in several member states of the eu and japan, represented by iaspei/ion, has identified a network of sites for multidisciplinary observatories focused on the atmosphere, ocean, and the earth beneath it. whereas for centuries observatories have been commonly used on land for many purposes, long-term continuous observations of natural phenomena in the oceans represent a new frontier for the sciences. a component of deos(b) seeks to establish a global network of 20 moored-buoy ocean observatories that could comprise the global network component of the ooi (fig. 5; nrc, 2003a). in other locations, generally those closer to land, deos(b) calls for fig. 5. location of proposed ooi global observatory network sites (nrc, 2003a). 539 seafloor observatory science: a review the use of direct submarine cable connections to shore. the deos(b) ocean observatory planning initiative was launched to foster a long-term continuous observational presence at the air-sea interface, throughout the water column, on the seafloor, and below. deos(b) transitioned into ooi/orion (see below). another major international ocean observatory programme with close ties to the nsf’s ooi is the b-deos programme (nrc, 2003a). the b-deos programme plan calls for the establishment of multidisciplinary moored ocean observatories at 3 sites: south of the azores on the mid-atlantic ridge, on the reykjanes ridge south of iceland, and in the drake passage-east scotia risesouth sandwich islands area. neptune – one major component of the deos(b) and ooi/orion planning effort is neptune, a project to establish a lithospheric-plate scale multidisciplinary observatory network on the juan de fuca plate, located a few hundred kilometres off the us-canadian west coast, and connected to two land-based research laboratories (shore stations) using highspeed, fibre-optic submarine communications cables (fig. 6; delaney et al., 2000; neptune, 2000). the project started in june 1998 with the u.s.a. neptune feasibility study, completed and published in june 2000. the canadian neptune feasibility study started in june 1999 and was completed and published in october 2000. the deployment of a 3000-km cable, largely along the margins of the juan de fuca plate is planned, with approximately 30 nodes spaced roughly at 100-km intervals and «extension cords» to permit the location of instruments 50 km or more from a node (massion et al., 2004). the multidisciplinary approach covers as preliminary examples: a) cross-margin particulate fluxes; b) seismology and geodynamics, seafloor hydrogeology and biogeochemistry; c) ridge-crest and subduction zone processes (fluid venting and gas hydrates); d) deep-sea ecology; e) water-column processes; f) fisheries and marine mammals. the system has been designed to provide real-time data transmission, interactive control and power to instruments and vehicles. the planned lifetime is 30 years. in 2002-2003 both cabled-observatory test beds, venus (can) in the strait of georgia, the strait of juan de fuca and saanich inlet, british columbia (3 different cables), and mars in monterey bay (a 62-km cable down to 1220 m w.d.) were funded (nrc, 2003a). nemo – the nemo observatory was established in autumn 1997 at the axial seamount (1520 m w.d.) on the juan de fuca ridge (weiland et al., 2000), and will form part of the future neptune network. nemo is acoustically linked to a surface buoy and has the objective to monitor and sample geophysical, geochemical, and microbial variability on an active segment of a mid-ocean ridge system to determine of the relationships between sub-seafloor magma movement, faulting, and changes in the biologic, chemical and physical properties of the subsurface biosphere (e.g., daughney et al., 2004). fig. 6. picture of the 3000-km cable planned in the neptune project to establish a lithospheric-plate scale multidisciplinary observatory network on the juan de fuca plate (delaney et al., 2000; neptune, 2000). 540 paolo favali and laura beranzoli osn – another major component of deos(b), ooi/orion and ion programmes is the establishment of a permanent osn consisting of about 20 sites throughout the world’s oceans for improved geophysical imaging of the internal structure of earth, as part of the iris/gsn networks (orcutt and stephen, 1993). the intention is to deploy sensor packages reentering in boreholes, such as odp holes, using a special thrustered tool. these planetary-scale fixed ocean observatories may also serve as long-term measurement sites for other types of oceanographic and climate studies, such as those envisioned by the geo system of moorings, now called oceansites. the oceansites goal is to establish oceanographic observatories at selected sites around the world’s oceans for the collection of time-series measurements of surface meteorology; air-sea exchanges of heat, fresh water and momentum; and full-depth profiles of water properties, including temperature and salinity, and ocean velocity. time-series measurements will be an essential element of the strategy developed to build accurate fields of air-sea fluxes. these observation stations have been proposed as an important component of goos. as part of the osn activity, between february and june 1998 osnpe collected data over 115 days from a broad-band borehole seismometer at odp site 843b in 4407 m w.d. offoahu about 200 km south-west of hawaii (called osn-1) (stephen, 1998; stephen et al., 1999, 2003b; collins et al., 2001, 2002; sutherland et al., 2004). 4.2.2. programmes orion-ooi – the nsf’s division of ocean science established the orion in 2003 programme to operate and manage existing and future ocean observing sites funded by nsf some of which will be constructed using funds for ooi. the total 5-year construction costs are budgeted at 208 m usd for the period 20062010 (nrc, 2003a). the ooi infrastructure is an integrated observatory with three elements: 1) a regional cabled network consisting of interconnected sites on the seafloor spanning several geological and oceanographic features and processes, such as neptune; 2) relocatable deep-sea buoys that could also be deployed in harsh environments, such as the southern ocean; 3) new construction or enhancements to existing facilities leading to an expanded network of coastal observatories (nrc, 2003a; isern, 2005). the research-focused observatories enabled by the ooi will be networked, becoming an integral part of the proposed ioos, that is an operationally-focused national system and in turn will be a key to enable the u.s.a. contribution to the international goos and the geoss (summerhayes, 2002). the orion programme will also co-ordinate the science driving the construction of this research observing network as well as the operation and maintenance of the infrastructure; development of instrumentation and mobile platforms and their incorporation into the observatory network; and planning, co-ordination, and implementation of educational and public outreach activities. the orion programme will be the most complex initiative that ocean scientists have undertaken within the u.s.a. and it will revolutionise the way that oceanographers study the sea. the nsf-supported ooi will have close ties to ocean and earth observing systems supported by other agencies including noaa, nasa and usgs (isern, 2005). in addition to the programmes listed above, there are other international research programmes that include significant observatory related research initiatives which ooi infrastructure could facilitate. examples include: 1) the interridge initiative for long-term monitoring of the northern midatlantic ridge (momar); 2) the intermargins international consortium to understand seismogenic zones in subduction settings; 3) ion activities to co-ordinate global network observatory efforts; 4) the international oceansites for studying climate and ecosystems in the world’s oceans; and 5) iodp. 4.3. japan in 1978, japan started to manage cabled seafloor observatories for scientific use, and also for real-time monitoring for seismic and tsunami warning. submarine cable-based seismic moni541 seafloor observatory science: a review toring began in five sea areas as part of the national earthquake preparedness efforts (asakawa et al., 2003) following the recommendations of the headquarters for earthquake research promotion. for instance, in 1993, off-hatsushima island a cable connected multidisciplinary observatory was installed by jamstec (momma et al., 1998). the observatory was equipped with geophysical and oceanographic sensors and video-cameras. continuous observations were made for about six years. in july 1999 the observatory stopped working because of a short circuit of the submarine cable. since then the replacement of the primary observatory and the design of a new one, enhanced to acquire additional measurements (e.g., tsunami pressure gauge, γray spectrometer) and based on new technologies, has been completed. in 2000 a new observatory was deployed with a new cable. at present, eight cabled experiments are running (table iii and fig. 7) and all these experiments will be part of a larger project based on cable technology, called arena (massion et al., 2004) and proposed in 2002 by ieee-oceanic engineering society of japan. arena plans to deploy electro-optical cables all around japan including in this network all the eight existing experiments. a feasibility study is presently ongoing for the development of the arena project. arena will be used for many scientific fields such as seismology, geodynamics, oceanography, marine environmental sciences, ecology, biology and exploitation of mineral resources. the arena architecture will be based on a mesh-like network, connecting underwater observatories and terrestrial stations, and covering the vast research area with 3600 km of cable powering the observation nodes. to satisfy the power requirements a new current-to-current converter has been proposed (asakawa et al., 2005). when fully operational, the system will be equipped with 320 observation nodes (one every 20-50 km) over a total cable length of 16000 km (shirasaki et al., 2003). a wide-band optical system to transmit high-definition images will also be developed. finally, expandable and mobile real-time monitoring systems for deep-sea use have also been developed (kawaguchi et al., 2001, 2002). geo-toc and venus (jp) – although long-term (a few months) seafloor experiments have been carried out with mobile stations (e.g., momma et al., 2001), the main trend of the scientific and technological research has been toward the use of underwater cables. while all the submarine cables described above have been specifically deployed for scientific purposes, the utilisation of decommissioned underwater telecommunications cables for scientific observation has also been considered. the use of these latter cables makes possible a low-cost implementation of scientific submarine cable systems. typical examples of the utilisation of a submarine communications cable are the geo-toc project using the japan-u.s.a. cable tpc-1, which connects ninomiya and guam, and venus (jp) which uses the tpc-2 cable running between okinawa and guam islands. a map of the two cables is shown in fig. 8 (kasahara et al., 1995, fig. 7. map of the cable-connected ocean bottom observatories in japan (shirasaki et al., 2003): a) jma – omaezaki; b) jma – off-boso; c) eri – offito city; d) eri – off-sanriku; e) nied – hiratsuka; a) jamstec – off-hatsushima; b) jamstec – off-muroto peninsula; c) jamstec – off-kushirotokachi. a map of the planned arena cable is shown in the top-left corner. 542 paolo favali and laura beranzoli 1998a,b, 2000; hirata et al., 2002; kasahara, 2002). in january 1997, as part of the geo-toc project, a multidisciplinary observatory was deployed on the forearc slope of the izu-bonin trench at 2708 m w.d. the observatory included a hydrophone, accelerometers and a quartz pressure and temperature sensor. under the venus (jp) project, in august-september 1999 a multidisciplinary observatory was installed at a depth of 2170 m on the slope of the ryukyu trench, and attached to the okinawa-guam cable. the observatory was equip-ped with a triaxial broadband seismometer, a tsunami pressure sensor, a hydrophone array, a precision range meter for crustal movement observations, an ad hoc observation system, a potentiometer, a magnetometer and a video-camera. these instruments were connected to the junction box via an underwater mateable connector using a deep-towed unit, rov and manned submersible (fig. 8; kasahara et al., 2006). this arrangement allows the maintenance of instruments by means of a rov and future up-grading of sensors to those with new functions. in addition, it is planned to insert sensors into the geo-toc system. the venus (jp) system enabled the continuous acquisition of observational data from sensors installed on the ocean floor until a break down in communications occurred after several months caused by the failure of an underwater connector on the junction box. ohp – the ohp project plans to cover the east asia-west pacific area by a permanent geophysical network. ohp is a long-term multidisciplinary project funded by the ministry of education, culture, sports, science and technology and started in april 1996. the ohp aims to study and to build a new concept of the structure and dynamics of the earth’s deep interior by constructing a global network of multidisciplinary geophysical observations on the hemisphere including the whole of the pacific ocean. this network consists of three components: seismic, electromagnetic and geodetic. each network is further composed of observations using different fig. 8. map of the tpc-1 and tpc-2 cables. the junction box used in the venus (jp) experiment is shown on the right (kasahara et al., 2006). 543 seafloor observatory science: a review technologies and/or methods. ohp will improve and expand the broad-band seismic network in the north-western pacific region, unifying the observation system, and also deploying semibroad-band obss, such as the experiment in the philippines sea (nine months in 2000-2001). another goal of the project has been the installation of four seafloor borehole geophysical stations (nereid-1, nereid-2, wp-1 and wp-2; see previous section 4.1.), established by odp and managed by jamstec (suyehiro et al., 2002; shinohara et al., 2006). this type of installation of a long-term observatory has shown that a properly cemented ocean-floor borehole in bedrock can also be very quiet at long periods (araki et al., 2004). moreover, under ohp three types of long-term obs have been developed for seafloor observations: 1) lt-vbb (360-0.05 s); lt-bb (30-0.05 s); lt-obs (1-0.05 s). the geoelectromagnetic network aims to study the electrical conductivity structure of the earth’s mantle, earth’s main magnetic field variations and core-mantle dynamics, using the same type of magnetometer. the distribution of permanent geomagnetic observatories is not dense enough. therefore to improve the spatial resolution three types of temporary stations were established: obem, mt and sfems (toh et al., 1998; shinohara et al., 2006). in the last case ohp uses trans-continent submarine retired coaxial cable for measuring geoelectrical potentials, in collaboration with the iris consortium (iris, 1994). finally, geodetic measurements were performed by a permanent network of 10 gps stations, called wing (kato et al., 1998), and by seafloor instruments. as part of ohp, instruments for seafloor geodesy have been developed and tested (fujimoto et al., 1998; osada et al., 2005) jointly with sio, which was the first to develop precision acoustic transponders with a sub-cm resolution in 4-5 km (spiess et al., 1998; gagnon et al., 2005a,b). 4.4. europe ecord-jeodi – esf has created a european research structure, ecord that will serve as the basis for all aspects of scientific ocean drilling research in europe as part of the iodp programme. recently (from 2001) the ec has funded a thematic network (jeodi) to implement ecord and move towards a single research and operational funding structure for science in europe enabled by ocean drilling. ofm – as part of the french efforts to contribute to the establishment of «permanent» seafloor observatories (montagner et al., 1998), the technical goal of the french pilot experiment ofm conducted in april and may 1992 (montagner et al., 1994a-c) was to show the feasibility of installing and recovering two sets of threecomponent broad-band seismometers, one inside an odp borehole and another inside a benthosphere in the vicinity of the hole. all the logistic support for the sea operations was provided by ifremer; the oceanographic vessel nadir, the submersible nautile and the re-entry logging system nadia-2 were used. secondary goals were to obtain the seismic noise level to conduct a comparative study of broad-band noise on the seafloor, downhole, and in continental regions, and to determine the detection threshold of seismic events on the seafloor. after the installation of both sets of seismometers, seismic signals were recorded continuously for 10 days. it was observed that the noise level tends to decrease as time goes by for both ocean-bottom and borehole seismometers, which means that the equilibrium stage had not yet been attained by the end of the experiment (beauduin et al., 1996a,b). the characteristics of microseismic noise in oceanic and continental areas are completely different. the background microseismic noise is shifted toward shorter periods for ocean-bottom and borehole seismometers compared to a continental station. this might be related to the difference in the crustal structure between oceans and continents. the experiment was a technical and scientific success and demonstrated that the installation of a permanent broad-band seismic and geophysical observatory on the seafloor as well as in a borehole is possible and can provide the scientific community with high-quality seismic data. ec feasibility studies – as already mentioned (see introduction), since the early 1990s the ec has promoted feasibility studies aimed at 544 paolo favali and laura beranzoli identifying the scientific requirements (thiel et al., 1994) and the possible technological solutions for the development of seafloor observatories (abel; berta et al., 1995). in particular, abel proved the feasibility of the concept of a benthic laboratory managed by a surface deployer. the configuration proposed by tecnomare (a member of the eni group) was a network of co-operating fixed nodes associated with an auv that can interact with the fixed observing station nodes and extend their capabilities by surveying the area investigated (berta et al., 1995; gasparoni et al., 2002). such a configuration has inspired the ec geostar, geostar-2 and orion-geostar-3 projects (see below). in parallel, other studies and activities, such as desibel (rigaud et al., 1998), have been carried out at ec level, aimed at defining the needs and expectations for longterm investigations at abyssal depths. within desibel four concepts have been investigated: a) an active docking system with a mobile hook; b) an active docking system with a special rov; c) a light scientific rov; d) a free swimming vehicle. the main outcomes of the feasibility studies abel and desibel were the victor 6000 rov of ifremer, and a special rov able to deploy and recovery heavy payloads on the deep seafloor. this rov, called modus, was built as part of the geostar projects (clauss and hoog, 2002; clauss et al., 2004). geostar and geostar-2 – starting in 1995, after the feasibility studies described above, the ec supported the geostar and geostar-2 projects aimed at the development and testing in actual deep-sea conditions of a single-frame seafloor autonomous observatory for long-term (up to one year) multidisciplinary monitoring at abyssal depths (at present, the maximum operative depth is 4000 m). the power supply is provided by a primary lithium battery. beranzoli et al. (1998) describe the main sub-systems: a) bottom station; b) modus; c) the communications systems. the bottom station (i.e. the seafloor observatory) manages all the scientific payload, provides the power supply, and hosts the underwater part of the communications system. the data acquisition is driven and controlled by a central unit called dacs (gasparoni et al., 2002) using a single clock signal to provide a common reference and quickly compare the different time series. modus is the deployment/recovery vehicle, which is lighter and smaller than traditional rovs. although lacking manipulation devices, modus takes care of the observatory from the sea surface to the sea bottom and during the deployment/recovery operations is also the primary communications link with the observatory. the geostar communications are based on different parallel systems: data capsules (messengers) releasable upon surface command or automatically in case of emergency, that can transmit their position on the sea surface and small quantities of data via argos; acoustic seafloor-sea surface links with a buoy or a portable surface station on a ship of opportunity (marvaldi et al., 2002). near-realtime communication with the observatory on the seafloor is assured by a surface buoy able to link by acoustics with the bottom station, and by radio/satellite links with an on-shore station. details of the geostar sub-systems and characteristics of the sensors used can be found in clauss and hoog (2002), gasparoni et al. (2002), marvaldi et al. (2002), clauss et al. (2004) and favali et al. (2006). figure 9a-d shows all the geostar sub-systems. in 1998, a demonstration mission was carried out in the shallow waters of the northern adriatic (42 m; jourdain, 1999; beranzoli et al., 2000a) collecting multiparameter data (beranzoli et al., 2000b). the analysis of the data demonstrated the complete reliability of the whole system, including modus functionality, and in particular demonstrated the scientific potentiality of unique time-referenced multiparameter data (beranzoli et al., 2003). also the effect of the observatory structure on the reliability of the measurements was considered (fuda et al., 2006). in 2000-2001 the first long-term deep-sea mission (geostar-2 project) took place at about 2000 m w.d. in the southern tyrrhenian sea for seven months. a surface buoy was deployed in the vicinity of the mission site to test the near-real-time communication with the observatory on the seafloor. the long-term mission validated the ability of the system to work properly in actual deep-sea conditions. the quality of 545 seafloor observatory science: a review the data collected was also high, as shown, for instance, by magnetic (de santis et al., 2006), gravimetric (iafolla et al., 2006), and geochemical and oceanographic data that evidenced ocean-lithosphere interactions in the benthic boundary layer (etiope et al., 2006). all the marine operations, either for the adriatic shallowwater or for the tyrrhenian deep-sea missions, were managed by the r/v urania. also during the geostar-2 deep-sea mission fourteen obss and obhs were deployed in an area between ustica island and the aeolian islands as far as stromboli to better monitor the seismicity of that area. the experiment, called tyde, lasted six months from the end of november 2000 to the middle of may 2001. details can be found in dahm et al. (2002). some scientific results of tyde are presented and discussed in montuori (2004), sgroi et al. (2006) and barberi et al. (2006). a significant example of technological spin-off from the geostar projects is the use of modus for the management of an instrumented frame, called scipack, for exploratory surveys (casts and profiles) as part of the ec biodeep project. in particular, the exploration of the deep (> 3000 m w.d.) hypersaline anoxic basins of the eastern mediterranean sea has been conducted by acquiring real-time video images, measurements and accurate videoguided samplings (malinverno et al., 2006). the geostar projects represent the starting point of a set of european and italian initiatives, which have led in around 10 years to the development and operation of other geostarclass observatories. the next step, promoted in fig. 9a-d. the complete geostar sub-systems: a) bottom station, which includes dacs, power supply, underwater acoustic communications and scientific payload, with modus on the top; b) modus ship-based remote control system; c) surface communication buoy (geostar-2 configuration); d) five messengers within the bottom station, the orange argos antennas are visible (see also favali et al., 2006). a c d b 546 paolo favali and laura beranzoli 2002 by the ec, was to move from a single benthic observatory to seafloor observatory networks. the two ec projects assem and orion-geostar-3 have such a goal. assem – assem is a project funded by the ec in the period 2002-2004 which developed a seafloor network for the long-term monitoring of geotechnical, geodetic and chemical parameters over a seabed area with maximum extent of 1 km2 (blandin et al., 2003). the network consists of light nodes able to communicate with a surface buoy and providing power supply and data logging resources to a set of sensors placed inside or around the node frame. one of the monitoring nodes also acts as a gateway for the data transmission to the buoy and land stations for processing and access through fig. 10. bathymetric map of the gulf of corinth where one of the assem pilot experiments was carried out. the dots represent the locations of the underwater nodes, the two red circles indicate the m1 node (a, underwater photograph on the right); and the orion-geostar-3 node 4 (b, underwater photograph on the right) (blandin et al., 2003). 547 seafloor observatory science: a review the internet. the network deployment and recovery requires the use of traditional rovs with manipulation functionalities to assist the operations on the seafloor. assem nodes were deployed in 2004 down to 400 m in the gulf of corinth, down to 40 m in the patras gulf (both sites in greece) and down to 50 m at finneidfjord (off-shore norway). the experiments lasted in total seven months. in the gulf of corinth the array included three nodes equipped with geodetic sensors based on acoustic distance meterzzs and tiltmeters, ctd methane and oxygen sensors. the array was integrated with a guest station (node 4) developed as part of the orion-geostar-3 project, to demonstrate the full compatibility between the two networks, assem and orion-geostar-3 (see below). figure 10 shows the bathymetric map of the gulf of corinth in which the experiment was carried out and the location of the nodes. an additional node, gmm, was developed and deployed within a methane-bearing pockmark in the patras gulf at 40 m w.d. this module was equipped with three methane sensors, a h2s sensor and ctd and was linked to shore by submarine cable for real-time data transmission (marinaro et al., 2004). orion-geostar-3 – the oriongeostar-3 ec project (2002-2005) has the objective to make a significant step forward in ocean networking starting from the results of fig. 11. swath bathymetry of the southern tyrrhenian sea, on the right a 3d picture viewed from the north of the marsili underwater volcanic seamount (central tyrrhenian bathyal plain) (marani et al., 2004). the yellow star indicates the site of the orion-geostar-3 experiment at the nw base of the seamount (over 3300 m w.d.). photographs of geostar (right) and node 3 (left) are shown at the bottom of the figure. 548 paolo favali and laura beranzoli the geostar projects (beranzoli et al., 2004). as part of orion-geostar-3, geostar was upgraded to operate as the gateway node of a seafloor network, two satellite observatories were developed (named orion nodes 3 and 4) and horizontal acoustic telemetry was developed to provide a bi-directional communications link between geostar and the satellite nodes. communication between the seafloor network and land is once again performed by the geostar surface buoy via radio/satellite link to a shore station. the deployment/recovery of each network node is made by modus. this opens a large perspective in the standardisation of observatory management. the long-term pilot experiment ran from december 2003 at over 3300 m w.d. nw of the marsili volcanic seamount (southern tyrrhenian sea) (fig. 11; marani et al., 2004) involving geostar, an additional node placed 1 km away (orion node 3), the improved geostar buoy, and a shore-station, sited in the ingv observatory of gibilmanna (northern sicily). the surface buoy allows the automatic transmission to the shore station of periodic messages coming from node 3 to the gateway (geostar node) via horizontal acoustics and then via vertical acoustics from the gateway. the buoy-shore link is by radio through the relay station on salina island (aeolian islands) and/or by satellite (iridium). it is also possible to retrieve pieces of waveforms collected by the seismometer by interrogating either the geostar node or orion node 3 from the shore. although the experiment was ongoing at the time of preparation of this paper, the full set of multiparameter data related to the period december 2003-april 2004 was recovered during a maintenance intervention. the r/v urania has been used for all the sea operations in this project. as part of the assem experiment in the gulf of corinth, one of the orion-geostar-3 nodes (orion node 4) was deployed and integrated as far as the communications were concerned. the demonstration of the compatibility of the two networks was required by the ec to favour the synergy between similar projects and to enhance the results of individual projects. this can assess the capability of european research and technology to provide advanced systems for marine monitoring in (near)-real-time from coast to deep-sea. the results of the orion-geostar-3 project open a new perspective for safe and relatively inexpensive management of seafloor observatories and for the potentiality of a multiparameter approach to the improvement of our knowledge of processes in the benthic boundary layer. esonet – the esonet project promotes activities to develop the multidisciplinary «seafloor segment» of the esa-ec gmes profig. 12. map of the «key-sites» of the european ocean margins proposed by esonet (priede et al., 2003, 2004). 549 seafloor observatory science: a review gramme. these activities are particularly aimed at contributing to research into geo-hazards, global change and biodiversity. esonet has produced guidelines for the establishment of a european seafloor network of observatories from the baltic sea to the black sea. in this context one of the most important outcomes has been the definition of «key-sites» (fig. 12) at which cabled multidisciplinary observatories should be deployed (priede et al., 2003, 2004). at several sites several scientific activities have been running for a long time in preparation for the introduction of cabled observatories. among these sites, the one in eastern sicily is already operative in real-time, having, at the end of january 2005, connected the sn-1 observatory to an electro-optical cable and acquired data at the shore station (see section 4.5 for details). at other sites, such as porcupine (off-shore western ireland), actions to acquire and deploy an electro-optical cable are being developed (waterworth, 2004). 4.5. italian initiatives the italian coasts have in some cases been affected by tsunamis (tinti et al., 2004), most of them directly linked to earthquakes or to volcanic eruptions and consequent submarine landslides (e.g., tinti et al., 1995; piatanesi and tinti, 1998; de martini et al., 2003), as at the end of 2002 when an unusual eruption of stromboli (aeolian islands, southern tyrrhenian sea) occurred together with the collapse of a flank of the volcano. this collapse caused submarine slumping associated with tsunami waves. ingv has recently established a permanent laboratory on stromboli, which is an ideal natural laboratory, and an extensive interdisciplinary programme is being planned (chiappini et al., 2002). one of the most exposed areas to tsunami hazard is eastern sicily, where the biggest italian earthquakes have occurred, often associated with tsunamis (1693, m 7.4; 1908, m 7.1; boschi et al., 1997; tinti et al., 2004). therefore, the first pilot station of the italian tsunami warning system has been installed there, as a result of the ec gitec-two project (19961998) (maramai et al., 2002). the seafloor observatory sn-1 has also been installed and connected in real time to the shore in that area (see below). sn-1 – in the period 2000-2004 the italian gndt funded a project proposed by ingv to set up and operate off-shore eastern sicily in the western ionian sea (central mediterranean) a multiparameter deep-sea observatory, sn-1, mainly devoted to seismological and oceanographic measurements. sn-1 is a geostarclass observatory with reduced size compared with geostar and represents the effort of italian marine research towards the realisation of a seafloor network in the seas surrounding the peninsula (fig. 13). the site of sn-1 was chosen as the first one, because it is the most fig. 13. map of the nodes of the future italian «permanent» multidisciplinary seafloor network. the numbers indicate the sites of geophysical (particularly seismological and/or volcanological) interest in order of priority from 1 to 10, while the letter «a» indicates sites of environmental interest. the number 1 represents the sn-1 location, the first existing realtime connected node. 550 paolo favali and laura beranzoli exposed in italy to destructive earthquakes and the tsunamis that are often connected with them (boschi et al., 1997; tinti et al., 2004). sn-1 has basically the same features of geostar: it is deployed/recovered by modus, it hosts a set of sensors, and is equipped with a dacs providing data streams with a unique time reference. in contrast to geostar, sn-1 has a standard vertical acoustic system enabling the surface unit to control its status and to retrieve portions of data and it is not supported by a surface moored buoy. from october 2002 to may 2003 sn-1 successfully completed its first longterm mission at 2105 m w.d. off-shore catania. during this mission, sn-1 acquired, in autonomous mode, around 10 gb of data, 7.65 gb of which were collected by a 100 hz sampling rate broad-band seismometer (favali et al., 2003). sn-1 has demonstrated the improvement of seismic event detection obtainable with an observation site at sea. in fact, it acquired hundreds of events not recorded on land, fully demonstrating that seismicity in marine areas is poorly monitored and localised by the on-shore network only (favali et al., 2004b). moreover, the relatively simple procedure, already used in previous geostar experiments, of de-coupling the seismometer housing from the frame, protecting the housing with another external bell, and coupling the instrument with the seabed has been definitively validated and has resulted in the collection of high-quality seismological data (monna et al., 2005). at the end of january 2005, the sn-1 observatory was again deployed at the same site (about 25 km east of etna volcano at 2060 m w.d.) and connected to a submarine cable owned by infn (fig. 14). the sea operations have been carried out by the c/v pertinacia, owned by elettra tlc; the sn-1 connection to the cable termination was made through a rov mateable connector managed by the deep-rated rov available on board. in this way sn-1 receives power from shore and is able to communicate in real-time with a shore station. this makes possible the integration of sn-1 with the existing italian land-based network, providing a significant contribution to our knowledge of a fig. 14. sketch of the infn cable from the shore laboratory in catania harbour to a plateau of the malta escarpment located about 25 km east of etna volcano (> 2000 m w.d.). a photograph of sn-1 is also shown in the top-right corner (favali et al., 2003). the geographic location of the observatory is indicated by a red dot on the map in the bottom-left corner (2060 m w.d.). 551 seafloor observatory science: a review key sector of central mediterranean geodynamics. the infn cable will also carry out a scientific experiment to detect natural neutrinos in deep-sea waters (nemo pilot experiment, see section 4.6). these scientific achievements have been achieved through joint activities between ingv and infn, regulated by a specific mou. moreover, sn-1 is also one of the keysites defined by the esonet project around the european ocean margins as sites for the development of seafloor observatories connected in real time to the shore (priede et al., 2003, 2004). sn-1 is the first operative cabled observatory in europe. mabel – mabel is a deep-sea multiparameter observatory for polar seas under development as part of pnra (calcara et al., 2001). it is designed to acquire geophysical, geochemical, oceanographic and environmental time series, and to operate autonomously for one year. because mabel is a geostar-class observatory with the same dimensions of sn-1, the launch and recovery of the observatory will be managed by modus. the first long-term mission has started at the end of 2005 in the weddell sea, using the r/v polarstern. the use of this vessel is regulated by a mou between ingv and awi. the mechanical and electronic behaviour of the whole system at low temperatures has been tested in the hsva basin, where it is possible to simulate polar water and air conditions and therefore to indicate the weak points that must be overcome (cenedese et al., 2004). sensor prototypes – an intense parallel activity has been planned and carried out to develop sensor prototypes suitable for long-term use in deep-sea single-frame multiparameter platforms, such as geostar-class observatories. in particular, a triaxial fluxgate magnetometer (de santis et al., 2006), a very sensitive gravity meter (iafolla and nozzoli, 2002) and an automatic chemical electrode analyser have been developed, tested and used in long-term missions down to over 3300 m w.d. the main characteristics of the scientific payload, the dimensions and weights of the available geostar-class platforms are summarised in favali et al. (2004a, 2006). 4.6. neutrino telescopes in the last few years high energy particle physicists have become very interested in carrying out experimentsto detect neutrinos in the deep oceans. solar and supenova physics have indicated the specific role of neutrinos in the processes involved in stellar evolution, opening the way to so-called «low energy neutrino astrophysics». the underwater/ice čerenkov technique is widely considered to be the most promising experimental approach to building high energy neutrino detectors. the first generation of underwater/ice neutrino telescopes, baikal (belolaptikov et al., 1997) and amanda (andres et al., 2001), despite their limited sizes, have already set the first constraints on tev neutrino astrophysical models. the construction of km2 size detectors has already started: at the south pole the icecube neutrino telescope is under construction (ahrens et al., 2004); the antares (katz et al., 2004), nemo (capone et al., 2002) and nestor (zhukov et al., 2004) collaborations are working towards the installation of a neutrino telescope in the mediterranean sea. neutrino detectors must identify faint astrophysical neutrino fluxes against a diffuse atmospheric background. the cosmic muon flux, which at sea surface is about 10 orders of magnitude higher than the number of neutrino-induced up-going muons, strongly decreases below the sea surface as a function of depth and of zenith angle, falling to zero near the horizon and below. for this reason astrophysical neutrino signals are searched for among upward-going muons. at 3000 m depth, an underwater neutrino telescope is hit by a cosmic muon flux that is about 106 times greater than the up-going atmospheric neutrino signal. another background source is the optical noise in seawater. this background is due to the presence of bioluminescent organisms and radioactive isotopes. radioactive elements in water (mainly 40k) produce electrons above the čerenkov threshold. the expected number of detectable high energy astrophysical neutrino events is of the order of 10-100 per km2 per year (gaisser et al., 1995; bahcall and waxman, 2001) and only detectors with an effective area (aeff ) of order 1 km2 would allow the identification of their 552 paolo favali and laura beranzoli sources. after the pioneering work carried out by the underwater dumand collaboration offshore hawaii island (roberts, 1992), starting in the second half of the 1990s, two small-scale neutrino telescopes, the underwater baikal (aeff =104 m2; belolaptikov et al., 1997) and the under-ice amanda (aeff = 0.1 km2; andres et al., 2001), have demonstrated the possibility of using the underwater/ice čerenkov technique to track eν >100 gev neutrinos and thus measuring the atmospheric neutrino spectrum at high energies. baikal was the first underwater neutrino telescope and, after more than ten years of operation, it is still the only neutrino telescope located in the northern hemisphere. baikal is an array of 200 photomultipliers, moored between 1000 and 1100 m depth in lake baikal (russia). amanda is currently the largest neutrino telescope. in the present stage, amanda ii, the detector consists of 677 optical modules of pressure-resistant glass hosting downward-oriented photomultipliers and readout electronics. the optical modules are arranged in 19 vertical strings, deployed in holes drilled in the ice between 1.3 and 2.4 km depth. the success of these two experiments has opened the way to the construction of km3 underwater neutrino telescopes. icecube (ahrens et al., 2004), which will extend the amanda detectors to km3 size at the south pole, will be deployed starting in austral summer 2004-2005, to be completed by 2010. when completed it will consist of 4800 photomultipliers deployed in 80 strings. in the northern hemisphere three collaborations, antares, nemo and nestor, are building demonstration detectors and prototypes, aimed at the construction of a km3 detector in the mediterranean sea. the simultaneous observation of the whole sky with at least two neutrino telescopes in opposite hemispheres of the earth is essential for the study of transient phenomena. nestor (zhukov et al., 2004) the first collaboration to operate in the mediterranean sea, will deploy a modular detector at 3800 m depth in the ionian sea, near the peloponnese coast (greece). antares (katz et al., 2004) will be a demonstration neutrino telescope with an effective area of 0.1 km2 for astrophysical neutrinos. it is located at a marine site near toulon (france), at 2400 m depth. between december 2002 and march 2003 the collaboration has deployed a junction box and two prototype lines equipped with oceanographic instruments and optical modules. the construction of km3 scale neutrino telescopes requires detailed preliminary studies: the choice of the underwater installation site must be carefully investigated to optimise detector performance; the readout electronics must have a very low power consumption; the data transmission system must allow data flow as high as 100 gb/s to shore; the mechanical design must allow easy detector deployment and recovery, and the deployed structures must be reliable over more than 10 years. in order to propose feasible and reliable solutions for the km3 installation the nemo collaboration has been conducting intense research and development on all the above subjects since 1998 (capone et al., 2002; migneco et al., 2004). nemo has intensively studied the oceanographic and optical properties at several deep-sea sites (depth ∼ 3000 m), close to the italian coast. results indicate that a large region located 80 km se of capo passero (sicily) is excellent for the installation of the km3 detector. in recent years many innovations have been applied to underwater technology. dwdm is permitting a large increase in the speed and bandwidth of optical fibre data transmission; newly developed materials can improve the long-term underwater reliability of complex deployed structures; deep-sea operations with rovs or auvs have been standardised. the design of the mediterranean km3 telescope will directly profit from these advanced technologies. in order to test the technical solution for the km3 installation, the infn installed a deep-sea test site at over 2000 m depth deploying an electrooptical cable, 25 km e of the harbour of catania (sicily) supported by the infrastructure of lns (fig. 14). an underwater station consisting of a basic nemo module (junction boxes, towers and data acquisition system) will be installed by 2007. the neutrino telescopes can provide infrastructures that support other experiments and seafloor observatories, as the nemo test site has already demonstrated. in fact it also hosts other experiments such as geophysical ones through the deployment and real-time connec553 seafloor observatory science: a review tion to the cable of the sn-1 multidisciplinary observatory (favali et al., 2003) made at the end of january 2005 (see section 4.5). the research and development activities conducted by the three mediterranean collaborations have provided valuable experience in the construction of the underwater km3 detector, which will be the result of their efforts. a step in this direction is represented by their joint effort in the ec km3net project for a design study of the km3 mediterranean neutrino telescope. further details on neutrino telescopes can be found in this volume (migneco et al., 2006). 5. conclusions and perspectives the establishment of a network of seafloor observatories will represent a new direction in ocean science research, and it will require a major investment of human and economic resources over many decades. potential benefits and risks are envisaged (nrc, 2000). the potential risks include: – installation of poorly designed and unreliable observatory systems. – potential for interference between experiments. – lack of standardisation and inter-operability. – inefficient use of resources if important technological questions are not adequately resolved. – possible compromise in system performance if critical technologies are not available when needed. the potential benefits include: – establishment of a basis for new discoveries and major advances in the ocean sciences. – advances in relevant areas of research, such as marine biotechnology, the ocean’s role in climate change, and the assessment and mitigation of natural hazards (like earthquakes and/or tsunamis). – improved access to oceanographic and geophysical data, enabling researchers to study the ocean and earth in real-time or near-realtime by providing multidisciplinary observatory infrastructures. – establishment of permanent observation sites over the 70% of earth’s surface covered by oceans to provide truly global geophysical and oceanographic coverage. – enhancement of interdisciplinary research for improving the understanding of interactions between physical, chemical and biological processes in the oceans. – increased public awareness of the oceans by proving new educational opportunities for students at all levels using seafloor observatories as a platform for public participation in real-time experiments. carefully evaluating the potential risks and benefits, seafloor observatories represent a promising approach for advancing basic research in the earth and ocean sciences. however, adequate resources are required, estimated at several tens of million euros per year. a comprehensive sea-floor observatory programme should include both cabled and moored-buoy systems to adapt to the diverse possible applications. because of the scientific need to study transient events, it is also important to develop rapidly deployable observatory systems. applications demanding high telemetry bandwidth and large amounts of power will preferably use submarine communication cables. retired cables may become available from time to time in areas of scientific interest. alternatively, new cables may be deployed as part of a seafloor observatory programme. moored-buoy observatories will be the preferred approach at remote sites, when bandwidth and telemetry requirements are modest, or when the duration of experiment does not justify the cost of a fibre-optic cable. scientific benefit should be the leading factor influencing the future technological developments in marine instrumentation. needs for the future may be summarised: a) extend the number and type of platforms, with particular reference to landing systems, benthic observatories and rovs; b) extend in time the acquisition of time series by more traditional stations (like buoys or moorings); c) collect in real time, increase the number and type of instrumented packages. a promising perspective is the linking of fixed observatories with auvs, which have the potential of being more cost effective than survey ships. the need for more technological developments related to long-term 554 paolo favali and laura beranzoli multidisciplinary deep-sea observatories promises a more fruitful relationship between scientific institutions and industries that are moving into deeper waters. standardisation is one of the key issues for globality, modularity, common installation and recovery tools, and could provide easier maintenance procedures (ollier et al., 2002). the first step in establishing a seafloor observatory system should be the development of a detailed programme and project implementation plan. this includes a management structure to ensure access to observatory infrastructure. this last requirement is likely to be similar to other large, co-ordinated programmes in the earth, ocean and planetary sciences (e.g., joides, iris and na-sa). a phased implementation strategy should be developed, with adequate prototyping and testing, before deployment of sea-floor observatories on a large scale. a seafloor observatory programme should include funding for three key elements: observatory infrastructure, new sensors and auv technology. it is essential that this programme be only one component of a much broader ocean research strategy. new mechanisms should be developed for the evaluation of highly interdisciplinary proposals requiring long time-series observations. an open data policy should be agreed upon to support information centres for archiving observatory data and disseminating data products and information. also an active public outreach and education programme should be a high-priority component of a seafloor observatory programme. the seafloor observatory programme must include co-operation within international initiatives, like ion. acknowledgements first of all, we are indebted to the ec which has supported «seafloor observatory science» in europe by funding many projects as part of the mast and environment programmes (fp 4, 5 and 6). the authors also wish to thank their partners in the european and italian projects: geostar (ec): ingv (co-ordinator; paolo favali, giuseppe smriglio); ismar-cnr (michael p. marani); tecnomare-eni spa (francesco gasparoni); tub (günther clauss); tfh (hans w. gerber); ifremer (jean marvaldi); lob-cnrs (claude millot); orca instrumentation, now sercel-underwater acoustic division (jean-michel coudeville). geostar-2 (ec): ingv (co-ordinator; paolo favali, giuseppe smriglio); ismarcnr (michael p. marani); tecnomare-eni spa (francesco gasparoni); tub (günther clauss); tfh (hans w. gerber); ifremer (jean marvaldi); lob-cnrs (claude millot); orca instrumentation, now sercel-underwater acoustic division (david fellmann); ipgp (jean-paul montagner). assem (ec): ifremer (co-ordinator; jean-francois rolin, jerome blandin); ipgp (pierre briole); ingv (giuseppe etiope); hcmr (vasilios lykousis); patras university (george ferentinos); capsum technologie gmbh (michel masson); ngi (james strout); fugro engineers (david cathie). orion-geostar-3 (ec): ingv (co-ordinator; paolo favali, laura beranzoli); ismarcnr (fabiano gamberi); tecnomare-eni spa (francesco gasparoni); tub (günther clauss); tfh (hans w. gerber); ifremer (jean marvaldi); orca instrumentation, now sercelunderwater acoustic division (michel nicot); ifm-geomar (ernst r. flueh). tyde (ec): ingv (co-ordinator; paolo favali); ismar-cnr (michael p. marani); ifm-geomar (ernst r. flueh); university of hamburg (torsten dahm). esonet (ec): university of aberdeen oceanlab (coordinator; monty priede, martin solan), ingv (paolo favali), tecnomare-eni spa (francesco gasparoni); ismar-cnr (nevio zitellini); university of tromsø (jürgen mienert, stephanie guidard); ifremer (roland person); royal nioz (tjeerd c.e. van weering); ifm-geomar (olaf pfannkuche, peter linke); csa group ltd. (nick o’neill, ed slowey, martin davies, eamonn kelly); imbc (anastasios tselepides); iub (laurenz thomsen); awi (michael klages, thomas soltwedel); marum (christoph waldmann); lob-cnrs (claude millot); tfh (hans w. gerber); cgul (jorge m. a. de miranda). sn-1 (gndt): ingv (laura beranzoli); tecnomare-eni spa (francesco gasparoni); 555 seafloor observatory science: a review tub (günther clauss); tfh (hans w. gerber); university of roma-3 (claudio faccenna); university of catania (stefano gresta); university of messina (giancarlo neri); university of palermo (dario luzio); ogs (marino russi); ismar-cnr (fabiano gamberi, michael p. marani); ifsi-inaf (valerio iafolla). mabel (pnra): ingv (paolo favali, massimo calcara); tecnomare-eni spa (francesco gasparoni); ogs (renzo mosetti); tub (günther clauss); tfh (hans w. gerber); collaborations: awi (wilfried jokat); dna-iaa (josé febrer). special thanks to claudio viezzoli and marcantonio lagalante (marine logistics); capts. emanuele gentile and vincenzo lubrano and the crew of r/v urania, vessel owned by cnr and managed by sopromar; capt. alfio di giacomo and the crew of m/p mazzarò, vessel owned by gestione pontoni spa; capt. vincenzo primo and the crew of c/v pertinacia, owned by elettra tlc spa (giuseppe mauge-ri, chief of mission). the authors are grateful to reviewers for their help to improve the quality of the paper and to david barraclough for his comments and revision of the text. this paper is dedicated to the memory of giuseppe smriglio and luc floury, who pioneered this science and clearly recognised its novelty and its great potential many years ago. list of acronyms and abbreviations used in the text abel: abyssal benthic laboratory. acorks: advanced circulation obviation retrofit kits. agu: american geophysical union (www site: http:// www.agu.org). amanda: antarctic muon and neutrino detector array (www site: http://amanda.uci.edu). antares: astronomy with a neutrino telescope and abyss environmental research (www site: http:// antares.in2p3.fr). arena: advanced real-time earth monitoring network in the area (www site: http://homepage.mac.home/ ieee_oes_japan/ arena). argos: advanced research and global observation satellite (www site: http://www.cls.fr/html/argos/ welcome_en.html). assem: array of sensors for long-term seabed monitoring of geo-hazards (www site: http:// www.ifremer.fr/ assem). atoc: acoustic thermometry of ocean climate (www site: http://atoc.ucsd.edu). at&t: american telephone and telegraph (www site: http://www.att.com) auv: autonomous underwater vehicle (www site: http:// www.cacs.lo-uisiana.edu/~kimon/auv). awi: alfred-wegener-institut für polarund meeresforschung (www site: http://www.awi-bremerhaven.de). b-deos: british-dynamics of earth and ocean systems (www site: http://www.deos.org). bdsn: berkeley digital seismic network (www site: http://quake.geo.berkeley.edu/bdsn). biodeep: biotechnologies for the deep (www site: http:// www.geo.uni-mib.it/biodeep). cgul: centro de geofìsica da universidade de lisboa (www site: http://www.cgul.ul.pt). clivar: climate variability and predictability programme (www site: http://www.clivar.org). cnr: consiglio nazionale delle ricerche (www site: http://www.cnr.it) cnrs: centre national de la recherche scientifique (www site: http://www.cnrs.fr). corks: circulation obviation retrofit kits (www site: http://www.rbr-global.com/cork.htm). cssf: canadian scientific submersible facility (www site: http://www.ropos.com/cssf/cssf.htm). ctd: conductivity, temperature and depth. c/v: cable vessel. dacs: data acquisition and control system. deos(a): deep earth observatories on the seafloor (www site: http:// www.coreocean/deos). deos(b): dynamics of earth and ocean systems (www site: http:// www.deos.org). desibel: deep-sea intervention on future benthic laboratory (www site: http://dbs.cordis.lu/cordis-cgi/ srchidadb?caller=projadvancedsrch&srch&qf_ep_rcn_ a=27267&action=d). dgps: differential global positioning system (www site: http://chartmaker.ncd.noaa.gov/staff/dgps.htm). dna-iaa: direccion nacional del antartico-istituto antartico argentino (www site: http://www.dna.gov.ar). dp: dynamic positioning. dpg: differential pressure gauge. dsdp: deep-sea drilling project (www site: http:// www-odp.tamu.edu) dumand: deep underwater muon and neutrino detection (www site: http:// www.phys.hawaii.edu/dmnd/ dumand.html). dwdm: dense wavelength division multiplexing (www site: http://www.iec.org/online/tutorials/dwdm). ec: european commission (http://europa.eu.int/comm). ecord: european consortium for ocean research drilling (www site: http://www.ecord.org). eni: ente nazionale idrocarburi (www site: http:// www.eni.it). eri: earthquake research institute (tokyo) (www site: http://www.eri.u-tokyo.ac.jp). esa: european space agency (www site: http:// www.esa.int). esf: european science foundation (www site: http:// www.esf.org). esonet: european seafloor observatory network (www site: http://www.abdn.ac.uk/ecosystem/esonet). 556 paolo favali and laura beranzoli eu: european union (www site: http://europa.eu.int). fp: framework programme (www site: http:// europa.eu.int/ comm/research). geo: global eulerian observatories (www site: http:// www.oceantimeseries.org/oceansites). geofon: geoforschungsnetz (www site: http://www.gfzpotsdam.de). geoscope: réseau geoscope de sismologie globale (ipgp programme) (www site: http://geoscope.ipgp.jussieu.fr). geoss: global earth observing system of systems (www site: http://earthobservations.org). geostar: geophysical and oceanographic station for abyssal research (www site: http://www.ingv.it/ geostar/geost.htm). geostar-2: geostar 2nd phase: deep-sea mission (www site: http://www.ingv.it/geostar/geost2.htm). geo-toc: geophysical and oceanographic trans ocean cable (www site: http://wwweprc.eri.u-tokyo.ac.jp/ kaiiki/geo_eng.html). gev: giga electronvolt. gitec-two: genesis and impact of tsunamis on the european coasts-tsunami warning and observations. gmm: gas monitoring module (www site: http:// www.ifremer.fr/assem/corinth/photo_gallery/photo_ gallery.htm). gmes: global monitoring for environment and security (www site: http://www.gmes.info). gndt: gruppo nazionale per la difesa dai terremoti (www site: http://gndt.ingv.it). goals: global ocean-atmosphere-land system (www site: http://www.nap.edu/books/0309051800/html). gobo: geophysical ocean-bottom observatory. goos: global ocean observing system (www site: http://ioc.u-nesco.org/goos). gps: global positioning system (www site: http:// gps.losangeles.af.mil) gsn: global seismographic network (www site: http:// www.iris.edu/ about/gsn). hcmr: hellenic centre for marine research (www site: http://www.hcmr.gr). hsva: hamburgische schiffbau-versuchsanstalt gmbh (www site: http://www.hsva.de). hugo: hawaii undersea geo-observatory (www site: http://www.soest.hawaii.edu/hugo). h2o: hawaii-2 observatory (www site: http:// www.soest.hawaii.edu/h2o). iaspei: international association for seismology and physics of the earth interior (www site: http:// www.iaspei.org). ieee: institute of electrical and electronics engineers (www site: http://www.ieee.org). ife: institute for exploration (www site: http:// www.mystica-quarium.org/). ifm-geomar: leibniz-institut für meereswissenschaften an der universität kiel (www site: http://www.ifmgeomar.de). ifremer: institut français de recherche pour l’exploitation de la mer (www site: http://www.ifremer.fr). ifsi-inaf: istituto di fisica dello spazio interplanetarioistituto nazionale di astrofisica (www site: http:// www.inaf.it). imbc: institute of marine biology of crete (www site: http://www.imbc.gr). imr: institute of marine research (norway) (www site: http://www.imr.no). infn: istituto nazionale di fisica nucleare (www site: http://www.infn.it). ingv: istituto nazionale di geofisica e vulcanologia (www site: http://www.ingv.it). insu: institut national des sciences de l’univers (www site: http:// www.insu.cnrs.fr). interridge: international ridge interdisciplinary global experiments (www site: http://interridge.org). iodp: integrated ocean drilling programme (www site: http://www.iodp.org). ion: international ocean network (www site: http:// www.iaspei.org/commissions/ion.html). ioos: integrated and sustained ocean observing system (www site: http://www.ocean.us). ipcc: intergovernmental panel on climate change (www site: http://www.ipcc.ch). ipgp: institut de physique du globe de paris (www site: http://www.ipgp.jussieu.fr). iris: incorporated research institutions for seismology (www site: http://www.iris.washington.edu). ismar: istituto di scienze marine-cnr, sezione di geologia marina di bologna (www site: http:// www.bo.ismar.cnr.it). iub: international university bremen (www site: http:// www.iu-bremen.de). iugg: international union of geodesy and geophysics (www site: http://www.iugg.org). jamstec: japan marine science and technology centre (www site: http://www.jamstec.go.jp). jeodi: joint european ocean drilling initiative (www site: http://www.ecord.org/about/j/jeodi1.html). jgofs: joint global ocean flux study (www site: http:// usj-gofs.whoi.edu) jma: japan meteorological agency (www site: http:// www.jma.go.jp). joi: joint oceanographic institutions (www site: http:// www.joi-science.org). joides: joint oceanographic institutions for deep earth sampling (www site: http://poseidon.palaeoz.geomar.de). km3net: km3 network (www site: http://www.km3net. org). ldeo: lamont-doherty earth observatory (www site: http://www.ldeo.columbia.edu). leo-15: long-term ecosystem underwater observatory, 15 m b.s.l. (www site: http://marine.rutgers.edu/nurp/ factech.html). lns: laboratori nazionali del sud (www site: http:// www.lns.infn.it). lob: laboratoire de océanologie et de biogeochemie (www site: http://www.com.univ-mrs.fr/lob). lter: long-term ecological research network (www site: http://lternet.edu). lt-bb: long-term broad band. lt-obs: long-term ocean bottom seismometer. lt-vbb: long-term very broad band. mabel: multidisciplinary antarctic benthic laboratory (www site: http://www.ingv.it/geostar/mabel.html). margins: margins programme (www site: http:// www.mar gins.wustl.edu). mars: monterey accelerated research system (www 557 seafloor observatory science: a review site: http://www.mbari.org/mars). marum: zentrum für marine umweltwissenschaften (www site: http://www.marum.de). mast: marine science and technology (www site: http://www.cor-dis.lu/mast). mbari: monterey bay aquarium research institute (www site: http://www.mbari.org). mednet: mediterranean network (www site: http:// mednet.ingv.it). mobb: monterey bay ocean broad-band observatory (www site: http://quake.geo.berkeley.edu/bdsn/ mobb.overview.htm). modus: mobile docker for underwater sciences. moise: monterey bay ocean bottom international seismic experiment (www site: http://www.mbari.org/ shake/moise). momar: monitoring the mid-atlantic ridge (www site: http://www.momar.org). moos: monterey bay ocean-observing system (www site: http:// www.mbari.org/moos). mou: memorandum of understanding. m/p: moto pontoon. mt: magneto telluric (array). mvco: martha’s vineyard coastal observatory (www site: http://www.whoi.edu/mvco). nasa: national aeronautic and space administration (www site: http:// www.nasa.gov). nemo: new millennium observatory (www site: http:// www.pmel.noaa.gov/vents/nemo). nemo: neutrino mediterranean observatory (www site: http://nemoweb.lns.infn). neptune: north east pacific time-series undersea networked experiments (www site: http://www.neptune.washington.edu). nerc: natural environment research council (www site: http://www.nerc.ac.uk). nereid: neath seafloor equipment for recording earth’s internal deformation nero: ninety east ridge observatory (www site: http://www.ifre-mer.fr/dtmsi/programmes/nero.htm). nestor: neutrino experimental submarine telescope with oceanographic research (www site: http:// www.nestor.org.gr). ngi: norges geotekniske institutt (www site: http:// www.ngi.no). nied: national research institute for earth science and disaster prevention (www site: http://www.bosai.go.jp). noaa: national oceanic and atmospheric administration (www site: http://www.noaa.gov). noc: national oceanography centre, southampton (www site: http://www.noc.soton.ac.uk). nrc: national research council (www site: http:// www.nationalacademies.org/nrc). nsf: national science foundation (www site: http:// www.nsf.gov). obem: ocean bottom electro magnetometer. obh: ocean bottom hydrophone. obs: ocean bottom seismometer. oceansites: ocean sustained 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platform for ocean science (www site: http://www.ropos.com). rov: remote operated vehicle (www site: http:// my.fit.edu/~swood/ rov_pg2.html). royal nioz: royal netherlands institute for sea research (www site: http://www.nioz.nl). r/v: research vessel. sappi: satellite-linked autonomous pore pressure instrument. seabass: seafloor borehole array seismic network. seize: seismogenic zone experiment (www site: http:// www.margins.wustl.edu/seize/seize.html#webinfo). sfems: seafloor electro magnetic station. sio: scripps institution of oceanography (www site: http://sio.ucsd.edu). sn-1: submarine network-1 (www site: http://www.ingv.it/ geostar/sn.htm). tao: tropical atmosphere-ocean array (www site: http://www.p-mel.noaa.gov/tao). tev: tera electronvolt. tfh: techniche fachhochschule berlin (www site: http:// www.tfh-berlin.de). tpc: transoceanic phone cable. tub: technische universität berlin (www site: http:// www.tu-berlin.de). tyde: tyrrhenian deep-sea experiment (www site: 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doi: 10.4401/ag-7582 1 “plasma acceleration on multiscale temporal variations of electric and magnetic fields during substorm dipolarization in the earth’s magnetotail„ elena igorevna parkhomenko1, helmi vitalevna malova2,1,*, elena evgenevna grigorenko1, victor yurevich popov4,3,1, anatolii alekseevich petrukovich1, dominique c. delcourt5, elena aleksandrovna kronberg6,7, patric w. daly6, lev matveevich zelenyi1 (1) space research institute, russian academy of sciences, moscow, russia (2) nuclear physics institute, moscow state university, moscow, russia (3) physics department of lomonosov moscow state university, moscow, russia (4) national research university “higher school of economics”, moscow, russia (5) laboratoire de physique des plasmas, ecole polytechnique, cnrs, france; institut des sciences de la terre d'orleans, université d'orléans, orléans, cnrs, france (6) max planck institute for solar system research, göttingen, germany (7) ludwig-maximilian university of munich, germany 1. introduction a number of observational and theoretical investigations have revealed the importance of accelerated particle flows in the earth’s magnetotail [sharma et al., 2008; retino et al., 2008; yamada et al., 2010; fu et al., 2011; runov et al., 2011a; birn et al., 2012, 2013; ashour-abdalla et al., 2015; grigorenko et al., 2015], that conversely point to specific regions for energy conversion during substorms [zelenyi et al., 2008; zelenyi et al., 2011 and references therein; angelopoulos et al., 2013]. thanks to geotail, cluster, themis, mms and other spacecraft observations in the earth’s environment, significant information on the acceleration mechanisms has been collected [e.g., artemyev et al., 2012; lui, 2014 , runov et al., 2014; grigorenko, 2015; kronberg et al., 2017; liang et al., 2017] but details of these mechanisms are not completely understood up to the present day. some satellite missions demonstrated the presence of particles with energies up to hundreds of kev in the earth’s magnetotail [e.g., zhou et al., 2010; ashour-abdalla et al., 2015; kronberg et al., 2017] but their origin and the processes at work remain unclear, although studies during the last decades [baker et al., 1985, zelenyi et al., 2011; grigorenko et al., 2011; angelopoulos et al., 2013, and references there in] have made clearer the role of magnetotail current sheet evolution in the energy conversion during substorms [birn article history receveid november 9, 2017; accepted march 15, 2018. subject classification: magnetotail; dipolarization; electric field fluctuations; particle acceleration; numerical modeling. abstract magnetic field dipolarizations are often observed in the magnetotail during substorms. these generally include three temporal scales: (1) actual dipolarization when the normal magnetic field changes during several minutes from minimum to maximum level; (2) sharp bz bursts (pulses) interpreted as the passage of multiple dipolarization fronts with characteristic time scales < 1 min, and (3) bursts of electric and magnetic fluctuations with frequencies up to electron gyrofrequency occurring at the smallest time scales (≤ 1 s). we present a numerical model where the contributions of the above processes (1)-(3) in particle acceleration are analyzed. it is shown that these processes have a resonant character at different temporal scales. while o+ ions are more likely accelerated due to the mechanism (1), h+ ions (and to some extent electrons) are effectively accelerated due to the second mechanism. high-frequency electric and magnetic fluctuations accompanying magnetic dipolarization as in (3) are also found to efficiently accelerate electrons. et al., 2012; lui, 2014]. during the growth phase of substorms, magnetic flux in the magnetospheric lobes is increased, leading to thinning of the magnetotail current sheet from a thickness of 1 to 2 re down to a thickness of a few ion gyroradii (250-2000 km) [sergeev et al., 1993; sergeev et al., 1996; runov et al., 2008, nakamura et al., 2008]. such a thin current sheet (tcs) configuration has been shown to be metastable and can be a reservoir of free magnetic energy that is explosively released during current sheet destruction [galeev, 1979; baker et al., 1985; zelenyi et al., 1990; zelenyi et al., 2008]. later phases of substorms are characterized by abrupt changes of magnetotail magnetic field and earthward propagation of dipolarization fronts (df) [runov et al., 2009, 2011, 2012; yao et al., 2016] that are step-like structures with sharp enhancements of the normal magnetic field bz [e.g., nakamura et al., 2002; runov et al., 2009; sergeev et al., 2009] (note that gsm coordinate system is used in the present study). these fronts are often associated with an enhancement of plasma and magnetic pressure as well as bursty bulk flows (bbf) that are accelerated earthward [e.g. angelopoulos et al., 1992; sergeev et al., 2011; runov et al., 2011; fu et al., 2011]. dfs typically appear in the midtail region after substorm onsets [lui, 2014] and have a tendency to slow down and to thicken during their motion toward the earth [hamrin et al., 2013 ]. several mechanisms have been proposed to explain the generation of dfs: (1) bbf-type flux ropes [slavin et al., 2003], (2) nightside flux transfer events [sergeev et al., 1992], (3) generation of burstlike magnetic structures by impulsive magnetic reconnection in the magnetotail [heyn and semenov, 1996; semenov et al., 2005; longcope and priest, 2007, sitnov et al., 2009; sitnov and swisdak, 2011]. evidences of such dfs have been found in a variety of spacecraft observations [e.g., nakamura et al., 2002; sharma et al., 2008; runov et al., 2009] and their relation with reconnection processes is now clearly established. acceleration of charged particles as a result of magnetic reconnection is now considered as one of the most effective mechanisms to accelerate particles to high energies [yamada et al., 2010]. furthermore, in a number of instances, the passage of dfs is accompanied by strong plasma turbulence or electrostatic fluctuations [ono et al., 2009; el-alaoui et al., 2013; lui, 2014; grigorenko et al., 2016] and by plasma acceleration [zhou et al., 2010; fu et al., 2011; artemyev et al., 2012; birn et al., 2012, 2013; grigorenko et al., 2011, 2015, 2017]. ions with energies about a few hundreds of kev have been frequently observed during substorm dipolarization in the near-earth tail [e.g., ipavich et al., 1984; nosé et al., 2000; ono et al., 2009; keika et al., 2010] or in the presence of turbulent electromagnetic field in the plasma sheet [cattell and mozer, 1982; hoshino et al., 1994; bauer at al., 1995, ono et al., 2009]. because dfs travel over large distances from the middle (or deep) magnetotail toward the planet without significant evolution [runov et al., 2009], particle acceleration in the course of the interaction with dfs can be effective due to the long time dynamics of such fronts. generally several mechanisms responsible for particle acceleration or heating by dfs have been proposed: 1) nonadiabatic (in the sense of violation of the first adiabatic invariant) plasma acceleration by the inductive electric field that results from the magnetic field reconfiguration [delcourt et al., 1990; veltri et al., 1998; delcourt, 2002; ono et al., 2009; greco et al., 2015 ]. works by [perri et al., 2009; greco et al., 2009; ono et al., 2009] also demonstrated the importance of nonadiabatic acceleration of h+ ions in the course of their resonant interaction with the low-frequency magnetic fluctuations in the region behind the dipolarization front. 2) adiabatic energy gain of magnetized electrons and ions due to local increase of the magnetic field [delcourt et al., 1990; birn et al., 2004; apatenkov et al., 2007; fu et al., 2011; birn et al., 2012]; 3) particle acceleration near the magnetic reconnection site by the cross-tail electrostatic field ey [e.g., hoshino, 2005; retino et al., 2008]; 4) reflection from fronts [zhou et al., 2010, 2012] and resonant capture by fronts [artemyev et al., 2012b; ukhorskiy et al., 2017] can also lead to nonadiabatic gain of energy because df thickness is comparable to the gyroradii of hot ions. 5) particle pick-up in outflows near reconnection regions and consequent heating up to thermal velocities of the order of the alfvén speed [drake et al., 2009]. from a theoretical viewpoint, most of these mechanisms have been investigated with help of numerical simulations in model magnetotail current sheets, and estimates of their efficiency have been provided in different studies [e.g., zelenyi et al., 1990; veltri et al., 1998; zelenyi et al., 2008; greco et al., 2009; artemyev et al., 2012, greco et al., 2015]. acceleration by the dawn-dusk electrostatic potential drop is clearly limited in efficiency and can reach maximum energies about 30-90 kev in the potential drop of about ~50 70 kv across the magnetotail [grigorenko et al., 2009]. on the other hand, electromagnetic turbulence can provide sevparkhomenko et al. 2 eral orders of magnitude in particle energy increase, but this effect was only investigated in the distant (about 100 re) magnetotail where the normal magnetic field component becomes weak and stochastic [perri et al., 2008; zelenyi et al., 2008, 2011]. acceleration of heavy ions due to plasma turbulence has also been investigated in [greco et al., 2015; grigorenko et al., 2015; catapano et al., 2016; liang et al., 2017]. all the above mentioned works put forward a spatial resonance character between particles and fields, that depends upon the larmor radius and the nonstationary structures of the magnetic field. the purpose of the present work is to put into perspective the combination of the different processes occurring during magnetic dipolarization, from (1) local large-scale dipolarization including (2) multiple front passages and (3) subsequent high-frequency electromagnetic oscillations with f ≤ fce (fce being the electron cyclotron frequency). these latter oscillations that are observed during the late stage of dipolarization have been shown to contribute to electron energization [grigorenko et al., 2016]. in the following, we will emphasize the multiscale temporal character of the overall dipolarization process (1)-(3) and the distinct resonant contributions to particle acceleration. this acceleration will be investigated using a simple numerical model taking into account different time scales, a temporal analog of the well known russian “matreshka” consisting of many dolls embedded one into the other as it is shown in figure 1. more specifically, to reconstruct the above mentioned (1)-(3) processes during magnetic dipolarization, three different time scales were considered in the present study. we will compare the numerical results obtained with in situ measurements, and will discuss the role of the acceleration mechanisms on different temporal scales. 2. numerical model test particle simulations performed to examine the particle dynamics in the magnetotail configuration b0 in the presence of dipolarization processes δb(t) followed by high frequency fluctuating electric field δe(r,t) and related induced magnetic field δb(r,t) . the time varying magnetic field is considered to be a superposition of the following components: (1) where b0 (z) is the initial field in the magnetotail current sheet before the dipolarization onset, δb(t) = δ{bd(t) + bdf(t)} is the time-dependent magnetic field, which comprises both the general dipolarization δbd(t) and the multiple dfs δbdf(t) arriving to the observer at distinct times t; δb(r,t) is the amplitude of the induced fluctuating magnetic component. the magnetotail field is taken in the form of the harris solution [harris, 1962; lembege and pellat, 1982] for a tangential magnetic component bx(z) supplemented by a nonzero normal component of the magnetic field bz (t). at t = 0, its value is equal to bz0: (2) the electric field is taken as: (3) where the large-scale dawn to dusk electric field e0 = ey ey is considered constant in the modeling region, the components of electric field δe(r,t) were found by means the maxwell equations: (4) while δe(r,t) is the fluctuating electric field component. as for the second and third terms in (1), they were taken from adhoc cluster observations of |b|, bx, by, bz on july 20, 2013 (from 01:33:08 to 01:48:11 ut). as for the 3 substorm particle acceleration figure 1. illustration of multiscale embedded property of time scales during geomagnetic dipolarization and the resonance character of particle acceleration, where the maximum energization is achieved when the scales of magnetic disturbances are about particles gyroperiods. b(r,t) = b0 (z)+ �b(t)+δb(r,t) bx = bx0 ⋅ tanh z l ⎛ ⎝ ⎜ ⎞ ⎠ ⎟,by = 0,bz = bz0 e(r,t) = e0 + δe(r,t)+δe(r,t) ∇ × δe=-1 c ∂δb ∂t ∇ ⋅ δe = 0 parkhomenko et al. 4 third term in (3), it was considered as a set of plane waves δe(r,t) as proposed earlier in [veltri et al., 1998; greco et al., 2009; perri et al., 2009 ]: (5) here gk = cos(kr + ϕ 1 κ tωκ)hk = sin(kr + ϕ 2 κ tωκ); . initial phases ϕ1κ, ϕ 2 κ are chosen randomly with uniform distribution over the interval [0,2π]. in the present simulations, five hundred harmonics were launched into the system. also, we chose the frequency ωk and amplitude δe(k) of each harmonic consistently with cluster data (recorded by c4) on july 20, 2013. the components of the fluctuating magnetic fields δbx, δby, δbz can be obtained from maxwell equations (assuming quasi-neutrality of plasma and the absence of free charges): (6) in our simulations, 5·105 particles were injected near the neutral plane and their equations of motion were numerically integrated inside a box having the following 3-d boundaries: z∈[-lz, lz] (lz = 2·103 km); x∈[-lx, lx] and y∈[-ly, ly], where lx=7.5·104 km; ly =2·lx = 15·104 km. the thickness lz of the current sheet was set equal to lz = 2000 km, which is comparable to the proton larmor radius (note that all quantities are normalized to the proton characteristics). when test particles leave the simulation box, their final energy is recorded to reconstruct energy distributions of plasma in the investigated region. the initial particle velocity distribution has the form of a shifted kappa distribution that is typical for the plasma sheet of the earth's magnetotail, viz., (7) here n0 is the plasma density; νκε = νt is the thermal velocity; κε = 3 was chosen to make the distribution (6) in accordance with the real one; parameter νd=1400 km/s is the macroscopic characteristic of the initial distribution (6), taken constant within the simulation box. the average thermal energies e of e-,h+ and o+ were chosen to be consistent with cluster data on july 20, 2013 (from 01:33:08 to 01:48:11 ut) and equal to e =1 kev, 6 kev and 12 kev, respectively. typical values of the electric and magnetic field in the earth magnetotail were taken as: bz0 = 4 nt, b0 = 20 nt and ey = 0.2 mv/m. the frequencies ωk and waves amplitude δe(k) are taken for each harmonic according to the data of cluster observations; k are distributed in the range 2π/l·[0.05,4]. the combined profile of magnetic field perturbations considered in the model is shown in figure 2. here two time intervals from t = 0 to t1 = 64s and from t1 = 64s to t2 = 220s describe correspondingly the quiet period and period of multiple dipolarizations during the time interval td = 2.6min in the magnetotail observed by clusters on july 20, 2013; the later time interval demonstrates telf > 220s magnetic field fluctuations on electron gyroperiod scale, calculated accordingly equation (4). in this figure, the onset of strong bz increase (t0 = 60 s) is marked by a vertical dashed line. during ~2.6 min (until t1 ≈ 220s), about 10 short intervals of large bz variations are observed with the average duration of about 20-30 s. at the end of dipolarization, the high-frequency electric and magnetic fluctuations with f ≤ fce are observed. in the following, we examine the maximum particle energies emax and average energies e gained during this event, and we analyze the evolution of the spectral index γ to characterize the acceleration mechanisms. 3. simulation results figure 3 shows selected trajectories of a test proton (in blue), a test oxygen ion (in red), and a test electron (in grey) during the magnetic field dipolarization shown ∂ex=∑k ∂e(k) k⊥ k g k(r,t) ∂ey =∑k ∂e(k) kykx k⊥ k g k(r,t) + kz k⊥ hk(r,t) ∂ez=∑k ∂e(k) −kzkx k⊥ k g k(r,t) ky k⊥ hk(r,t) ∇ × ∂b = 1 c ∂ e ∂t ∇ ⋅∂b = 0 f(v)= n0ak ε2( π kvkε ) 3 ⋅ 1+ v⊥ 2 +(v ii -(-1)s vd) 2 κ ε ⋅ υ 2κ ε s=1,2 ∑ −(κ +1) ε figure 2. combined profile of dipolarization δb over time interval δtd = 2.6 min, which includes multiple dfs observed at time scale tdfs. the dipolarization is followed by the high-frequency magnetic fluctuations observed during the interval telf = 6.3 min. k⊥ = kz 2 + ky 2 , k = kz 2 + ky 2 , k = kx 2 + ky 2 + kz 2 5 substorm particle acceleration in figure 2. it is apparent from figure 3b that, during the time interval preceding t = 220s, protons and oxygen ions can be accelerated by dfs from initial energies of 6 kev and 12 kev up to about 250 kev and 350 kev, respectively. during the same time interval electrons experience a strong acceleration from 1 kev and up to ~100 kev. note also that the particle acceleration consists of sequences of energy gains and losses during the successive dipolarization fronts. after t = 220s, when the high-frequency electromagnetic turbulence started to be observed. figure 3a,b displays different particle behaviors. at this time protons and oxygen ions do not experience any significant acceleration, while electrons are effectively accelerated from ~100 kev up to the 200-250 kev. this clearly demonstrates that different plasma particles may be effectively accelerated during different periods and on multiple temporal scales. while protons and oxygen ions are essentially affected by the induced electric field in the course of dfs, the most prominent electron acceleration occurs after dfs in conjunction with the electric and magnetic fluctuations. figure 4 presents the energy spectra obtained for the different plasma populations, h+, o+ and e, and for three distinct cases of acceleration, viz., passage of a single dipolarization only (case a), passage of a general dipolarization with small-scaled dfs (case b), and passage of a general dipolarization with dfs accompanied by electromagnetic turbulence (case c). particle acceleration manifests in itself as the decrease in the spectral indices. the dependence of particle spectra upon the various time scales of the acceleration processes is also clearly noticeable in this figure. for instance, figure 4a shows that acceleration by a single dipolarization mechfigure 3. (a) model trajectories in xy plane and (b) energy variations as a function of time for test electron (coded in grey), proton (coded in blue) and oxygen ion (coded in red) in the model magnetic field variation shown in figure 1. figure 4. energy spectra of e-, h+, o+ in the case of: (a) single dipolarization; (b) dipolarization with df bursts; (c) dipolarization with df bursts followed by electric and magnetic fluctuations. (a) (b) (c) parkhomenko et al. 6 anism (case a) can lead to the energy gain by one order of magnitude or slightly more for all types of particles. for protons and oxygen ions, the passage of successive dfs (case b) with duration less than 1 min leads to the resonant interactions and energy transfer from the field to the particles (figure 4b). here, electrons are less affected because of their small gyroperiods. in contrast, high-frequency electric and magnetic fluctuations following the passage of dfs (case c) can effectively lead to resonant energization of electrons (figure 4c) with gain of energies about two orders of magnitude. in other words, combination of dfs passage and electric and magnetic fluctuations appears as the most effective mechanism of particle acceleration. table 1 summarizes energetic characteristics obtained for protons, oxygen ions and electrons, viz., initial average energy e 0; final average energy e , maximum energy emax, net average energy gain δe, computed spectral index and spectral index obtained from in situ measurements. note that the spectral index γ is here calculated according to equation (6) of [kronberg and daly, 2013] for the following energy ranges: 70-95 kev for e-, 90-160 kev for h+, and 274-498 kev for o+. in table 1, the smaller the value of γ is, the larger the net particle energization. more specifically, in case b (i.e., dipolarization with df bursts), the smallest γ value is obtained for protons (γh+ = 2.0) while the largest value is observed for electrons γe= 4.7, which reflects their less efficient energization as described above. in case c (dipolarization with dfs and electromagnetic fluctuations), a hardening (i.e., γ decrease) of the electron energy spectra is obtained with γe= 1.2. no significant energization of h+ and o+ ions is noticeable in this case with γh+ = 1.6 and γo+=4.5 compared to the case b. the right column of table 1 shows spectral indexes measured by the rapid instrument onboard cluster-4 on july 20, 2013 [wilken et al., 2001]. specifically, before the dipolarization onset (around 01:37:00 ut) the following spectral indices are observed for electrons (γ0e), protons (γ0p) and o + ions (γ0o+): γ0e = 3.3; γ0p = 4.0; γ0o+ = 5.0. unfortunately, there are many gaps in the rapid observations of energetic oxygen fluxes and the γ index for oxygen ions at dipolarization onset cannot be calculated. as for protons and electrons, rapid observations display a decrease of γ near dipolarization onset (around 01:37:40 ut, case a). during the development of dipolarization, when the multiple dfs are observed a (around 01:39:50 ut, case b) protons and oxygen ions experience resonant acceleration and γ indices accordingly decrease to minimum values. on the contrary, γe does not decrease during this period. equivalently, nonadiabatic electron energization does not operate during this time. at a later stage of dipolarization, when bz stops increasing and highfrequency fluctuations are observed (after 01:47 ut, case c) the γe decreases to its minimum value. at this time, no significant changes of γp and γo are noticeable. in summary, although γevalues measured by cluster and those derived from our numerical simulations cannot be compared one to one, they exhibit very similar time evolutions. the viewpoint developed in the present study is that these time evolutions closely reflect different sequences of resonant energization in the course of dipolarization. acceleration process particles e _ 0, [ kev] e _ , [ kev] e max, [ kev] δe=efin-eini, [kev] |γ| |γcluster| d (a) e1 11 60 10 3.0 h+ 6 7 100 1 3.5 o+ 12 13 200 1 no data d+dfs (b) e1 12 150 11 4.7 3.5 h+ 6 8 300 2 2.0 2.5 o+ 12 15 450 3 3.2 2.8 d+dfs+elf (c) e1 20 250 19 1.2 2.2 h+ 6 8 320 2 1.6 5.0 o+ 12 15 500 3 4.5 4.0 table 1. energetic characteristics of plasma particles before and after acceleration by (from top to bottom) : (a) a single dipolarization (labeled d), (b) dipolarization with df bursts (labeled d+dfs), (c) like case b but followed by electric and magnetic fluctuations (labeled d+dfs+elf). the right column shows spectral indexes measured by cluster on july 20, 2013 (01:33:08 01:48:11 ut). 7 substorm particle acceleration 4. conclusion in this work, we focused on the effectiveness of particle energization during dipolarization events that have a complex multiscale character, being composed of successive dipolarization fronts followed by pile-up and consequent excitation of electric and magnetic fluctuations. we have shown that the multiscale changes of magnetic fields lead to the generation of induced electric fields that interact with plasma particles in a resonant manner, that is, the closer the time scale of the field variation to the particle gyroperiod, the more effective the transfer of energy from fields to particles. the three main time scales considered here are: (1) overall magnetic field dipolarization (of the order of a few minutes), (2) dipolarization fronts observed at smaller 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diamagnetic wings of the magnetotail current sheet, ann. geophys., 22, 2541–2546, doi: https://hal.archives-ouvertes.fr/hal-00317515. zhou, x.z., v. angelopoulos, v. a. sergeev, and a. runov (2010). accelerated ions ahead of earthward propagating dipolarization fronts, j. geophys. res., 115, a00103, doi:10.1029/2010ja015481. zhou, x.z., y. s. ge, v. angelopoulos, a. runov, j. liang, x. xing, j. raeder, q.g. zong (2012). dipolarization fronts and associated auroral activities: 2. acceleration of ions and their subsequent behavior, j. geophys. res., 117, a10227, doi:10.1029/2012ja017677. *corresponding author: h. v. malova, nuclear physics institute, moscow state university, moscow, russia email: hmalova@yandex.ru © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. parkhomenko et al. 10 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49,6,2006 1201 annals of geophysics, vol. 49, n. 6, december 2006 key words forced neural network – gravity anomaly – modeling – synthetic model – gulf of mexico 1. introduction a classical problem in gravity and magnetic exploration is the computation of theoretical anomalies caused by idealized models of known shapes. many workers have published different methods for carrying out such computation, and textbooks on potential theory, e.g. routh (1908), provided various formulas for these models. early publications like barton (1929) dealt with the computation of the gradients of the gravity field. hubbert (1948) used line-integral approach for the computation of gravitational attraction of two-dimensional masses. bhattacharyya (1964), nagy (1966), and plouff (1976) presented closed form of analytical solutions for prism shaped bodies. talwani and ewing (1960), talwani (1965) used numerical integration techniques for the computation of the fields due to models of arbitrary shape by dividing them into polygonal prisms or laminas. parker (1974) tried to find depth and density values using gravity data. green (1975) studied an inverse solution of gravity profiles. last and kubik (1983) estimated underground density distribution with recursive inverse solution techniques. lines and treitel (1984) applied a singular value decomposition (svd) approach for problems in evaluation of gravity and seismic projections. mareschal (1985) used fourier transform for inverse solution of gravity density distributions. murty et al. (1990) focused on density differences of 2d and 3d gravity models. murty and rao (1993a,b) calculated inverse solution of gravity and magnetic a new approach for residual gravity anomaly profile interpretations: forced neural network (fnn) onur osman (1), a. muhittin albora (2) and osman nuri ucan (3) (1) istanbul commerce university, eminonu, istanbul, turkey (2) istanbul university, engineering faculty, geophysical department, avcilar, istanbul, turkey (3) istanbul university, engineering faculty, electrical & electronics dept, avcilar, istanbul, turkey abstract this paper presents a new approach for interpretation of residual gravity anomaly profiles, assuming horizontal cylinders as source. the new method, called forced neural network (fnn), is introduced to determine the underground structure parameters which cause the anomalies. new technologies are improved to detect the borders of geological bodies in a reliable way. in a first phase one neuron is used to model the system and a back propagation algorithm is applied to find the density difference. in a second phase, density differences are quantified and a mean square error is computed. this process is iterated until the mean square error is small enough. after obtaining reliable results in the case of synthetic data, to simulate real data, the real case of the gulf of mexico gravity anomaly map, which has the form of anticline structure, is examined. gravity anomaly values from a cross section of this real case, result to be very close to those obtained with the proposed method. mailing address: dr. onur osman, istanbul commerce university, ragip gumuspala cad. no. 84 eminonu, 34378 istanbul, turkey; e-mail: oosman@iticu.edu.tr 1202 onur osman, a. muhittin albora and osman nuri ucan anomalies of polygonal structures using marquart algorithm. murthy and rao (1993b) proposed some methods in inverse solution of gravity anomalies for circular, cylindrical, and vertical discs. mosegaard and tarantola (1995) applied monte carlo method. tsokas and hansen (1997) studied on crustal thickness with gravity anomalies in greece. artificial neural networks are part of a much wider field called artificial intelligence, which can be defined as the study of mental facilities through the use of computational models (charniak and mcdermott, 1985). they encompass computer algorithms that solve classification, parameter estimation, parameter prediction, pattern recognition, completion association, filtering, and optimization problems (brown and poulton, 1996). they have gained popularity in geophysics during the last decade because these tools can approximate any continuous function with an arbitrary precision (van der baan and jutten, 2000). the location of the buried steel drums is estimated from magnetic dipole source using supervised artificial neural network (salem et al., 2001). neural networks are used to speed up the detection of ferro-metalic objects (selam and ushijima, 2001). depth and radius of subsurface cavities are determined from microgravity data using back propagation neural networks (eslam et al., 2001). neural networks are studied to solve 1d and 2d resistivity inverse problems (el-qady and ushijima, 2001). for 2d modeling cnn (cellular neural networks) is applied to the separation of regional/ residual potential sources in geophysics by albora et al. (2001a,b). artificial neural networks can be divided into two main categories: unsupervised recurrent and supervised feed-forward networks. in the unsupervised recurrent type, the networks allow information to flow in both directions. these modals are called unsupervised because there is no teacher to set the input-output mapping relation during the learning phase. in the supervised because through a set of correct input-output pairs, called the training set, the network learns the relation between the input-output pairs. in this paper, a new algorithm, denoted «forced neural network (fnn)» is proposed. the aim of fnn is to estimate the physical parameters of buried objects. it is first applied to synthetic examples and then real data. we have found satisfactory results for both cases. 2. forced neural network the artificial neural network is composed of many simple processing elements, which are massively interconnected and operate in parallel. the processing elements commonly known as neurons, receive the input from previous elements and send the output to other elements through synaptic connections. these connections have different weights. in order to find the effective values of inputs and outputs, these values are multiplied by these weights. the main purpose of neural networks is to compute such weights giving the best output. to obtain the eligible values for weights, back propagation method being the most popular learning algorithm for neural networks, is used in this study. 2.1. back propagation algorithm the error signal at the output of neuron j at iteration n, is defined by (2.1) where neuron j is an output node, dj(n) is desired output and yj(n) is actual output of neural networks (nn). the instantaneous value of the error energy for neuron j can be defined as . correspondingly, the instantaneous value e(n) of the total error energy is obtained by summing over all neurons in the output layer; only «visible» neurons are the ones for which error signals can be calculated directly. we may thus write, (2.2) where, the set c includes all the neurons in the output layer of the network (haykin, 1999). let n denote the total number of patterns (examples) contained in the training set. the average squared error energy is obtained by summing ( ) / ( )e n e n1 2 j j c 2= ! / / ( )e n1 2 j 2 / ( )e n1 2 j 2 ( ) ( ) ( )e n d n y nj j j= − 1203 a new approach for residual gravity anomaly profile interpretations: forced neural network (fnn) e(n) over all n and then normalizing with respect to set size n, as shown by, . (2.3) the instantaneous error energy e(n), and therefore the average error energy eav, is a function of all the free parameters (i.e. synaptic weights and bias levels) of the network. for a given training set, eav represents the cost function as a measure of learning performance. the objective of the learning process is to adjust the free parameters of the network to minimize eav. to do this minimization, we use an approximation similar in rational to that used for the derivation of the least mean square (lms) algorithm. we consider a simple method of training in which the weights are updated on a pattern-by-pattern basis until one epoch, that is, one complete presentation of the entire training set has been dealt with (2.4) where δj(n) is the local gradient and η is learning speed (haykin, 1999). local gradient points are required changes in synaptic weights and we obtain back-propagation (bp) formula for the local gradient δj(n) as ( ) ( ) ( )w n n y nij j iηδ∆ = / ( )e e nn1av n n 1 = = / (2.5) neuron j is hidden. figure 1 shows the signal-flow graph representation of eq. (2.5), assuming that the output layer consists of ml neurons. the factor involved in the computation of the local gradient δj(n) in eq. (2.5) depends solely on the activation function associated with hidden neuron j. the remaining factor involved in this computation, namely the summation over k, depends on two sets of terms. the first set of terms, δk(n), requires knowledge of the error signals ek(n), for all neurons that lie in the layer to the immediate right of hidden neuron j, and that are directly connected to neuron j which is shown in fig. 1. the second set of terms, wkj(n), consists of the synaptic weights associated with these connections. we may redefine the local gradient δj(n) for hidden neuron j as (2.6) (2.7) neuron j is hidden. ( ( ))v nj( ) ( ) y n e n j j2 2 ϕ= − l ( ) ( ) ( ) ( ) ( ) n y n e n n y n vj j j j 2 2 2 2 δ = − ( ( ))v nj jϕl ( ( ))v n( ) ( ) ( )n n w nj j j k kj k δ ϕ δ= l / fig. 1. signal flow graph of a part of the adjoint system pertaining to back-propagation of error signals. 1204 onur osman, a. muhittin albora and osman nuri ucan the local field parameter vj(n) produced at the input of the activation function associated with neuron j is therefore (2.8) where m is the total number of inputs (excluding the bias) applied to neuron j (haykin, 1999). the synaptic weight wj0 (corresponding to the fixed input y0 = +1) equals the bias bj applied to neuron j. hence the function signal yj(n) appearing at the output of neuron j at iteration n is . (2.9) next differentiating eq. (2.9) with respect to vj(n), we get (2.10) where the use of prime (the right-hand side) signifies differentiation with respect to the argument haykin (1999). 2.2. forced neural network for gravity anomaly this method could be used in modeling arbitrary subsurface body geometry and density contrasts. we begin with a horizontal cylindrical structure, whose gravity anomaly function is shown below, (2.11) ∆ρ is density difference, h and x are the depth and the total length of the cross section respectively, i and j are the levels of the depth and the distance of the cylinder from the starting point, and finally xref is the concerned distance point where the anomaly value is observed. we use as an input of the neuron, which is shown in fig. 2, and there should be (h × x) inputs and these inputs are constant for every a(xref). in fig. 2, ϕ(.) is an activation function. we use partially linear activa( ( )k i i j xref 2 2$ + −7 a ( ) ( ( ) ) a x k i j x i ,ref ref i j j x i h 0 1 1 2 2$ $ρ∆= + − = − = // ( ( ))v n ( ) ( ) v n y n j j j j2 2 ϕ= l ( ) ( ( ))y n v nj j jϕ= ( ) ( ) ( )v n w n y nj ij i m i 0 = = / tion function (haykin, 1999), which gives linear output values between zero and ∆ρ depending on its input. the neuron can be modeled as below: in the method, weights of the neuron are assigned as ∆ρi,j for each pixel and linear function is assumed as an activation function. after using the back propagation, ∆ρi,j are updated and the output of the neuron gives the gravity anomaly. although the density differences are found, the results of this system are not sufficient because of non-uniqueness and horizontal locations that are constrained. therefore, the value of ∆ρ is set to zero if its value is very close to the zero according to the density difference which is obtained form geological features of the region. otherwise, the value of ∆ρ is set to the density difference of the geological region after back propagation. forced neural network means that after sufficient epoch is applied, fixed values are assigned to the output of the neuron according to the density difference ∆ρ, and this process is continued until the mean square error of the output, a(xref) which is shown in fig. 2, becomes sufficiently small. 3. performance of the algorithm in synthetic data our synthetic data are obtained from a cylindrical structure of having a depth of 1 m and a rafig. 2. forced neural network (fnn) design for gravity anomaly. 1205 a new approach for residual gravity anomaly profile interpretations: forced neural network (fnn) dius of 2 m for ∆ρ= 1 mgal as shown in fig. 3. the anomalies of this model are considered as the input data provided to the fnn. in synthetic examples, every learning cycle is comprised of 350 epochs, and two-level quantization (∆ρ or zero) is applied after every 10 learning cycles, which is found to be optimum through experiments. the estimated geological structure obtained via fnn application results in an anomaly profile (dashed line) that is similar to the observed anomaly (solid line), as shown in fig. 3. for a second synthetic model, we choose ttype prismatic structure with ∆ρ = 1mgal. we use the talwani and ewing (1960) 2d method. the estimated geological structure obtained via fnn application results in an anomaly profile (dashed line) that is similar to the observed anomaly (solid line), as shown in fig. 4. in both examples, satisfactory results are obtained. 4. example of application on real data as an example for application of real data in fnn, we use the bouguer anomaly reported by nettleton (1943), whose reproduction is shown in fig. 5. the anomaly was recorded in the gulf of mexico about 241 km away from galveston and at a small distance inside the edge of the continental shelf. the importance of basement architecture to the hydrocarbon exploration in the gulf of mexico basin has been debated on for years. alexander (1999) studied on tectonic and stratigraphic in gulf basin. the origin of the topographic feature was not established until the gravity survey indicated a large closed minimum coincident with the contours of the elevated mound that could be accounted for only by the assumption of a salt dome. the survey was not extensive enough to fig. 3. results of fnn for synthetic horizontal cylinder. fig. 4. performance of fnn for synthetic t-type prismatic structure. 3 4 1206 onur osman, a. muhittin albora and osman nuri ucan define the gravity anomaly, but judicious extrapolation indicated the maximum negative anomaly to be about 9 mgal. the gravity anomaly map given in fig. 5 is obtained from dobrin and savit (1988). figure 6 is composed from the ab cross section of this map and demonstrates nettleton’s interpretation of the salt structure giving rise to the anomaly. the solid line shows the observed anomaly and dotted line shows the anomaly, which is derived from fnn. the results of the proposed method are very close to the observed one. 5. conclusions the forced neural networks (fnn) presented in this paper shows that the gravity field at any point due to a solid body with uniform volume density can be computed as the field due to a fictitious distribution of surface mass-density on the same body. first of all, we applied the fnn technique to two synthetic data. these tests provide successful results in fitting the calculated to observed data. as a real data application, a salt dome gravity anomaly map taken fig. 5. gravity map observed over inferred salt dome causing anomaly in water-bottom tomography in gulf of mexico (contour interval is 5 mgal) (modified form dobrin and savit, 1988). 1207 a new approach for residual gravity anomaly profile interpretations: forced neural network (fnn) from the nw part of the gulf of mexico is considered. this anomaly shows a negative closure from 1060 mgal to 990 mgal. the reason for this negative closure is mostly because of the geological properties of the salt dome. the density contrast in salt dome of the gulf is lower than those in the surrounding rock formations. the anomaly of ab cross-section is modeled using fnn and the anomaly of this model is very close to the observed one. to make a comparison between the methods of nettleton and fnn, we can see that by using fnn the model better fits the observed anomaly. the determination of the depth of a buried body from the gravity anomaly has been solved using forced neural network. our model has approximately the upper surface at about 1 km depth, while the lower one is at 9 km, and its approximate width is 15 km. the advantage of the proposed algorithm, fnn over the classical inversion techniques is that there is no need for initial information on the parameters of the buried structure such as depth and width. acknowledgements this project is supported by research fund of istanbul university. fund no. 28/03092002. references albora, a.m., o.n. ucan, a. ozmen and t. ozkan (2001a): evaluation of sivas-divrigi region akdag iron ore deposits using 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(received november 18, 2005; accepted july 14, 2006) i n v e r n i f r e d d i . r i g i d i . r i g i d i s s i m i e i n v e r n i c a l d i . m i t i . .mit i s s i m ! g i u s e p p i n a a t j y k r t i q u a n d o in m e t e o r o l o g i a -i parl a di inverno i n t e n d e il p e r i o d o e o i n p r e n d e n l e i mesi di d i c e m b r e , g e n n a i o e f e b b r a i o , e s s e n d o in media questi t r e m e s i i p i ù f r e d d i di tutto l ' a n n o . non e i t e p e r ò a n c o r a un c r i t e r i o c o n v e n u t o dai m e t e o r o l o g i per la d e f i n i z i o n e por (•••empio di « i n v e r n o r i g i d o », c q u e s t a l o c u z i o n e -i a d o p e r a f r e q u e n t e m e n t e ; d i s p o n e n d o di n u m e r o s e a n n a t e eli osservazioni m e t e o r o l o g i c h e è p o s s i b i l e t u t t a v i a s t a b i l i r n e uno e q u i n d i i d e n t i f i c a r e con p r e c i s i o n e gli i n v e r n i e c c e z i o n a l i sia rigidi s i a m i t i . p e r f a r e c i ò è n a t u r a l m e n t e o p p o r t u n o ohe le osservazioni di cui -i fa l ' e l a b o r a z i o n e s t a t i s t i c a p r e s e n t i n o un a l l o grado di o m o g e n e i t à : a l l o s c o p o di sceg l i e r e l ' a g g e t t i v o a p p r o p r i a l o da a p p l i c a r e a l l ' i n v e r n o 1 9 1 6 1 7 . ver a m e n t e e c c e z i o n a l e , h o p r e s o q u i n d i in c o n s i d e r a z i o n e i dati m e t e o r o l o g i c i o s s e r v a l i a p a v i a d u r a n t e 53 i n v e r n i e p r e c i s a m e n t e a p a i l i r e dal 189-1-95 fino a q u e l l o ora dei o r s o . i dati in q u e s t i o n e -olio «lati lutti r a c c o l t i india c a p a n n a m e t e o r i c a della sede a t t u a l e d e l l ' o s s e r v a t o r i o g e o f i s i c o (a s . s p i r i l o ) e press a poco s e m p r e con le s l e s e r e g o l e . i risultati d e l l a e l a b o r a z i o n e c o m p i l i l a p r e s e n t a n o n o t e v o l e i n t e resse c l i m a l o l o u i e o . si d e d u c e da c a c h e il valore normale ( c i o è la inedia d e l l e 53 m e d i e s i n g o l e ) d e l l a temperatura malia invernale (') a p a v i a è di 1".52, v a l o r e del m i t o e o n c o r d a n ' i c con q u e l l o d e d o l i o dal p r o f . f . f r c d i a d a l l e m i s u r e e s e g u i l e nel p e r i o d o 1 8 6 6 1 9 0 6 alla s p e c o l a ì n i v e r s i t a r i a in c i t t à . c a l c o l a l o lo s c a r t o di c i a s c u n a m e d i a dal v a l o r e n o r m a l e p r e d e t t o si o l t i c n e uno scarto medio di 1 . 1 2 . o r a si p o t r e b b e r o d e f i n i r e freddi tutti gli i n v e r n i la cui t e m p e r a t u r a m e d i a s c a r t a in m e n o c o m u n q u e d a l l a n o r m a l e : ma e v i d e n t e m e n t e gli inverni con t e m p e r a t u r a m e d i a a p i c c o l o s c a r t o non i n t e r e s s a n o ai fini ( r ) media d i l l o leiiiperalnre medio giornaliere dei mesi di dieemlire, gennaio e 1 ('libraio. m i m : i t i . \ a m . r v i a i t i della c l a s s i ( n a z i o n e c l i m a t o l o g i c a e a l l o r a c o n v i e n e d e f i n i r e freddi tulli gli inverni la cui t e m p e r a t u r a m e d i a s c a r t a in m e n o dal v a l o r e n o r m a l e più d e l l o «cario m e d i o , per p a v i a piti di 1 . 1 2 . d o n una t a l e d e f i n i z i o n e , dei i n v e r n i , r i s u l t a n o freddi ii e p r e c i s a m e n t e : 189-1-95. 1900.1) 1. 1 9 0 1 0 5 . 1 9 0 6 0 7 , 1 9 2 8 2 9 . 1 9 3 1 3 2 . 1933-3 1. 1 9 3 9 4 0 , 1 9 1 0 4 1 . 1 9 4 1 4 2 . 19-16-17: ira c-si ve n e s o n o d i v e r s i c l i c non sono p a r s i tali a l l a p o p o l a z i o n e e d u r a n t e i (piali la t e m p e r a t u r a m e d i a g i o r n a l i e r a ila avuto m a g a r i ell'eltiv a u l e n t e un m i n i m o m a r c a l o m a s t r e t t o : co-i p e r e s e m p i o dur a n l e i i n v e r n o 1900-01 in -oli d i c i o l l o giorni c o n s e c u t i v i di f e b b r a i o sono c o m p r e s i l u l l i i d i e c i inorili con t e m p e r a t u r a m i n i m a al di sotto di i(>\ p u r c o n s e r v a n d o la c l a s s i f i c a z i o n e g e n e r i c a di « fr e ddi » p e r gli i n v e r n i ora e l e n c a l i , c o n v i e n e q u i n d i i n t r o d u r r e la d i c i t u r a inverno rigido per q u e l l i di essi p e r i q u a l i lo s c a r t o d e l l a t e m p e r a t u r a m e d i a i n v e r n a l e dal v a l o r e n o r m a l e valga due o tre volte ( c i o è por pavia r i s p e t t i v a m e n t e 2 . 2 4 o p p u r e 3 . 3 6 ) lo s c a r t o m e d i o . nel p r i m o ea»n r i s u l t a n o rigidi i s egu ent i i n v e r n i : 1894-95 1928-29 1 9 4 6 4 7 : nel s e c o n d o caso s o l t a n t o il 1 9 4 6 4 7 . ci -ì p u ò a l l e n e r ò alla p r i m a d e f i n i z i o n e elio c o r r i s p o n d e b e n e a n c h e a l l e i m p r e s s i o n i u m a n e ; inl a t i i tulli r i c o r d a n o c o n i o e c c e z i o n a l e l ' i n v e r n o 1 9 2 8 2 9 : ed e v e n t u a l m e n t e d e f i n i r e rigidissimi gli inver ni per i q u a l i lo s c a r t o v a l e ire v o l l e lo s c a r t o m e d i o . la s e g u e n t e t a b e l l a c o n t i e n e in una c o l o n n a la t e m p e r a t u r a m e d i a i n v e r n a l e dei c i n q u e anni c h e l ' h a n n o a v u t a p i ù bassa liei p e r i o d o c o n s i d e r a l o ( 1 8 9 4 1 9 4 7 ) e in una seconda c o l o n n a i c o r r i s p o n d e n t i scarti dal v a l o r e n o r m a l e c i t a l o più -u : t e m p e r a t u r a s c a r t o dal i n v e r n o m e d i a i n v e r n a l e v a l o r e n o r m a l e 1 8 9 1 9 5 — 1 . 7 — 3 , 1 8 1 9 0 6 0 7 — 0 . 2 — 1 , 7 6 ] 9 2 8 2 9 1 . 1 — 2 . 6 7 1 9 4 0 4 1 0 . 3 — 1 . 8 4 1 9 4 6 1 7 1 2 3 , 7 1 inverni freddi. rigidi. i m i . m l l s s n n e inverni caldi. miti. a1itissiaii p e r c i a s c u n o dei c i n q u e i n v e r n i ora c o n s i d e r a l i d ò n e l l a seguenti /alleila il n u m e r o dei g i o r n i con t e m p e r a t u r a m i n i m a s o t t o /ero i con m i n i m a i n f e r i o r e a — 1 0 ' ; i n o l i l e il n u m e r o dei g i o r n i con l e m p e l a t u r a m e d i a s o t t o z e r o e con t e m p e r a t u r a m a s s i m a p u r e s o m io z e r o : inverni) n" giorni a min. sotto 0° n° giorni a min. sotto —10<> n" giorni con media sotto ii" n giorni a mass, sullo ii" 1894-95 87 l 62 17 1906-07 7 4 l 15 hi 1 9 2 8 2 9 7 0 ] o 5 3 14 1940-41 i .'> 1 47 15 1 9 4 6 4 7 8 8 16 02 3 0 i n f i n e , a n c o r a nei r i g u a r d i n i m i assoluti : inverno 1 8 9 4 9 5 1 9 0 6 0 7 1 9 2 8 2 9 1 9 4 0 4 1 1 9 4 6 1 7 ila t e m p e r a t u r a , i n t e r e s s a n o i mim i n i m a assillala — 1 5 . 7 — 1 3 , 6 — 1 5 , 7 — l ì . 3 il 1946-47 non lui la m i n i m a assoluta |)iù bassa m a ila avut o due v o l l e la t e m p e r a t u r a m i n i m a o l i o 1 5 ' : una volta il 7 gennai ( " 1 3 , 1 ) e un a l t r a il 27 g e n n a i o ( — 1 5 . . 2 ) . la t e m p e r a t u r a è n a t u r a l m e n t e 1 e l e m e n t o p r e m i n e n t e per la de f i n i z i o n e di i n v e r n o r i g i d o , m a non i u n i c o . h a n n o i m p o r t a n z a h n e v e e la n e b u l o s i t à e p e r questi d u e e l e m e n t i e p e r gli inver ni con s i d e r a l i si h a n n o i seguenti n u m e r i : inverno neve radula (in il" giorni ii" giorni sereni 1 8 9 4 9 5 1 3 0 41 28 1906-07 7 0 3 0 2 8 1 9 2 8 2 9 39 4 1 21 1 9 4 0 4 1 74 4 8 1 6 19-16-47 164 4 8 10 (.11 m j ' i ' l . n a u . i v e h t t dall e s a m e c o m p a r a t i v o ilei (lati r i p o r t a l i n e l l e p r e c e d e n t i t a b e l l e e m e r g e d u n q u e con e v i d e n z a c h e l ' i n v e r n o testé p a s s a l o è il più f r e d d o v e r i f i c a t o s i nel p e r i o d o 189-1-1947; lui i n f a t t i la p i ù b a s s a t e m p e r a t u r a m e d i a i n v e r n a l e ( 2 . 2 ) , ini il n u m e r o più g r a n d e di g i o r n i con t e m p e r a t u r a m e d i a sotto z e r o ( 6 2 ) . ha il n u m e r o p i ù g r a n d e di g i o r n i con t e m p e r a t u r a m i n i m a e m a s i m a sotto z e r o ( r i s p e t t i v a m e n t e 88 e 3 0 ) , e con t e m p e r a t u r a m i n i m a s o t t o — 1 0 ' ( 1 6 ) , il più a l t o spess o r e di n e v e ( ( i n 161), il più gran n u m e r o di g i o r n i c o p e r t i ( 4 8 ) e il più p i c c o l o n u m e r o di giorni s e r e n i ( 1 0 ) . i n f i n e , p o i c h é lo s c a r t o d e l l a sua t e m p e r a t u r a m e d i a i n v e r n a l e dal v a l o r e n o r m a l e v a l e più di tre v o l t e lo s c a r t o m e d i o , a l l ' i n v e r n o 1 9 4 6 4 7 -i p u ò a p p l i c a r e la q u a l i f i c a di « r i g i d i s s i m o >•. inverno 19-17 calabrosa sopra una r i n g h i e r a . i a l e q u a l i f i c a è g i u s t i f i c a l a a n c h e da a l t r i f a l l i c h e h a n n o reso v e r a m e n t e m o l t o p e s a n t i o l t r e c h e i i n v e r i l o a n c h e a l c u n i mesi p r i m a e d o p o . i n f a t t i l ' i n v e r n o è s t a t o p r e c e d u t o e s e g u i l o da m e s i c o n t e m p e r a t u r e i n f e r i o r i a l l e n o r m a l i : i n o l t r e i g i o r n i con m i n i m a s o l t o z e r o sono stati in t o t a l e 105 p e r c h è se ne e b b e r o o l t r e c h e in d i c e m b r e , g e n n a i o e f e b b r a i o a n c h e in o t t o b r e , in n o v e m b r e e p e r s i n o in m a r z o (il g i o r n o 2 di m a r z o -i e b b e una m i n i m a di — 6 , 6 ) ; e il c i c l o c o p e r t o da n o v e m b r e a m a r z o si e b b e p e r 89 g i o r n i . inverni freddi, hicidl, r i u n i s s i vii e inverni cai.di. m i t i . m i t l s s i m l \ p r o p o s i l o dei ire i n v ern i più freddi or o r a s e g n a l a l i e da n o t a r e elle essi sono stati rigidi per tpiasi tutta la p i a n u r a p a d a n a . -dai d a t i p u b b l i c a l i dal p r o f . k r c d i a l'elativi al q u a r a n t e n n i o 1 8 6 6 1 9 0 6 si ]>iiò i n o l t r e d e d u r r e elle l ' i n v e r n o r i g i d o , a n z i r i g i d i s s i m o . p r e c e d e n t e il 1 8 9 1 9 5 è l a l o p e r la p i a n u r a p a d a n a il 1 8 7 9 8 0 . a p a v i a -i e b b e in q u e l l ' i n v e r n o u n a t e m p e r a t u r a m e d i a di — 2 ',(> con uno s c a r t o dal v a l o r e n o r m a l e di 'quel p e r i o d o di — 1 . 2 , il più e l e v a t o q u i n d i di q u e l l i c o n s i d e r a t i . p e r ò si p u ò s e g n a l a r e elle a n c h e n e i r i g u a r d i di q u e l l ' i n v e r n o r i g i d i s s i m o il 1946-47 p r e s e n t a un c a r a t t e r e di p r e m i n e n z a ; i n f a t t i m e n i l e nel 1 8 7 9 8 0 la t e m p e r a t u r a m e d i a d e c a d i c a fu sotto z e r o sei d e c a d i c o n s e c u t i v e ( d i c e m b r e e genn a i o ) , n e l l o scorso i n v e r n o lo fu per o t t o d e c a d i c o n s e c u t i v e ( d u e di d i c e m b r e , g e n n a i o , f e b b r a i o ) . in c o n c l u s i o n e l ' a b b a s s a m e n t o di temp e r a t u r a n e l l o scorso i n v e r n o o l t r e c h e m o l l o p r o n u n c i a t o è s l a t o a n c l i e m o l l o a m p i o , il più a m p i o del p e r i o d o 1 8 7 9 1 9 4 7 . f a t t a i a n a l i s i ora s u n t e g g i a t a per gli i n v e r n i f r e d d i , rigidi e r i g i d i s s i m i m i è vernilo in n i e n t e di c o n s e g u e n z a di e s a m i n a r e la dis t r i b u z i o n e degli inverili c h e s c a r t a n o d a l l e c o n d i z i o n i n o r m a l i in senso o p p o s t o a q u e l l i f r e d d i . ili c o n t r a p p o s t o al l a d e f i n i z i o n e di i n v e r n o f r e d d o , r i g i d o e rigid i s s i m o -i può d a r e ora q u e l l a di i n v e r n o rullio, mite e rattissimo a p p l i c a n d o l a agli i n v e r n i n e i quali la t e m p e r a t u r a m e d i a scarta in j>iù dal v a l o r e n o r m a l e r i s p e t t i v a m e n t e di una, di iluc e di tre volte lo s c a r t o m e d i o . d a l l ' e s a m e dei d a t i c a l c o l a l i r i s u l t a n o c a l d i : 1 8 9 6 9 7 , 1 8 9 7 9 8 , 1 8 9 8 . 9 9 , 1 9 0 3 0 4 , 1 9 0 7 0 8 , 1 9 0 9 1 0 , 1 9 1 1 1 2 , 1 9 1 9 2 0 , 1 9 2 0 2 1 . 1 9 2 1 2 5 . 1 9 2 7 2 8 , 1 9 3 5 3 6 , 1 9 1 3 4 4 , c i o è t r e d i c i i n v e r n i , dei q u a l i s o l t a n t o d u e m e r i t a n o poi la classifica di m i t i : 1 8 9 8 9 9 , 1 9 2 0 2 1 ; nessuno r i s u l t a u n t i s s i m o , p o i c h é nessuno i n f a t t i d e g l i i n v e r n i c o m presi f r a i 53 a n n i c o n s i d e r a l i ha una t e m p e r a t u r a m e d i a i n v e r n a l e c h e s c a r t i dai v a l o r e n o r m a l e t r e v o l l e lo s c a r t o m e d i o . q u i n d i -i p u ò d i r e c h e in quel p e r i o d o non c ' è s t a l o 1 o p p o s t o dell i n v e r n o rig i d i s s i m o 1 9 4 6 4 7 . il m i t e dei d u e i n v e r n i 1 8 9 8 9 9 e 1920-21 è a p p e n a p a r a g o n a b i l e in v a l o r e a s s o l u t o al r i g i d o del 1 9 2 8 2 9 . e c c o i dali r e l a t i v i ai q u a t t r o i n v e r n i c h e h a n n o la t e m p e r a t u r a m e d i a p i ù e l e v a l a : cll'seppl-na al.iyehti inverno!) temperatura media invernale scarto dal valore normale 1 8 9 8 9 9 4 , 1 2 , 5 6 1 9 1 1 1 2 3 , 4 1 , 9 1 1 9 1 9 2 0 3 , 5 2 , 0 0 1 9 2 0 2 1 3 , 9 2 , 3 5 inverno n" giorni con min. sotto 0° n° giorni con media sotto 0° n" giorni ma-i. sullo ii" minima assoluta 1 8 9 8 9 9 37 7 0 — 7 , 2 1 9 1 1 1 2 4 7 8 2 — 6 , 7 1 9 1 9 2 0 4 9 1 3 0 — 6 , 2 1 9 2 0 2 1 3 6 0 0 — 3 . 8 inverno neve eadula cm n° giorni coperti il" giorni 1 8 9 8 9 9 4 , 5 4 1 17 1 9 1 1 1 2 3 5 , 0 4 9 1 0 1 9 1 9 2 0 0 , 0 2 3 3 0 1 9 2 0 2 1 0 , 5 3 6 18 p e r c o n c l u d e r e si p u ò e s a m i n a r e q u a l c u n a d e l l e c a r a t t e r i s l i c l i e stat i s t i c h e d e l l a d i s t r i b u z i o n e d e g l i i n v e r n i c o n s i d e r a t i ; è n o l o c l i e p e r u n a d i s t r i b u z i o n e gaussiana f r a s c a r t o m e d i o e s c a r t o m e d i o q u a d r a t i c o v a l e a p p r o s s i m a t i v a m e n t e la r e l a z i o n e a = 1 , 2 5 3 3 i l ; o r a p e r il n o s t r o c a s o lo s c a r t o m e d i o q u a d r a t i c o fi r i s u l t a 1 , 3 6 6 e q u i n d i la c o s t a n t e di p r o p o r z i o n a l i t à fra i d u e s c a r t i è 1 , 2 1 5 e f f e t t i v a m e n t e p o c o d i f f e r e n t e da q u e l l a n o r m a l e . si p u ò i n o l t r e c a l c o l a r e la p r o b a b i l i t à del v e r i f i c a r s i di i n v e r n i l i g i d i o m i t i , c o m e d e f i n i t i p i ù su, e si t r o v a c l i c essa è del io',1 ù c i r c a n e l l a d i s t r i b u z i o n e g a u s s i a n a ; n e i c i n q u a n t a t r e a n n i c o n s i d e r a t i si s o n o p r e s e n t a t i in t o t a l e 5 i n v e r n i c h e r i s p o n d o n o alla d e f i n i z i o n e d a t a , t r e r i g i d i e d u e m i t i . i n f i n e se si c a l c o l a la p r o b a b i l i t à del v e r i f i c a r s i di i n v e r n i r i g i d i s s i m i e u n t i s s i m i si trova c h e essa c o r r i s p o n d e . s e m p r e n e l l a d i s t r i b u z i o n e g a u s s i a n a , a p o c o p i ù d e l l ' 1 % ; inverni freddi, rigidi, r i g i d i s s i m i e inverni cvi.di. m i t i . m i t l s s i m l 1 1 5 n e i c i n q i i a n t a t r e i n v e r n i c o n s i d e r a t i da noi s o l t a n t o u n o . il 1 9 4 6 1 7 , r i s u l t a r i g i d i s s i m o e ne—uno i n i t i s s i m o . n a t u r a l m e n t e q u a l u n q u e c o n s i d e r a z i o n e s t a t i s t i c a s a r e b b e m o l t o p i ù i n t e r e s s a n t e se il p e r i o d o di a n n i c o n s i d e r a t o fo^sc p i ù l u n g o . pavia osservatorio geojisico dell'i.a.g. aprile 1917. riassunto si propone lina classificazione e definizione di inverni freddi, rigidi, rigidissimi e ili inverni caldi, miti, utilissimi. in base alla elaborazione <lci dali di 53 anni ili osservazioni meteorologiche a pavia (osservatorio geofisico) si segnalano le caraneristiche statistiche relative ai vari tipi di inverni verificatisi a pavia nel periodo clic va dal dicembre 1891 al marzo 1917. vol50,2,2007 259 annals of geophysics, vol. 50, n. 2, april 2007 key words damping – fundamental mode – singledegree-of-freedom – ambient vibration 1. introduction the need for a simple and reliable method to estimate building damping arose during the project nato science for peace 980857 (assasbv) whose aim is to mitigate seismic risk in the cities of skopje, zagreb and ljubljana. one of the tasks of the project is to perform estimates of the fundamental frequency of buildings and of underlying soils, in order to identify possible resonance phenomena. this resonance approach was recently introduced for microzonation studies of large cities (e.g., granada, spain; navarro et al., 2004). obtaining fundamental frequency and damping is the first step to validate vulnerability models of buildings. standard and detailed approaches can be used only for a limited number of selected buildings: the challenge faced in the assasbv project was to obtain simple but reliable estimates of dynamic parameters for thousands of building in a two-year time. this has to be achieved with the same instrumentation used on soils and without induced forcing. the proposed methodology does not require filtering, pre-selection or assumptions on signal non-parametric analysis of a single seismometric recording to obtain building dynamic parameters marco mucciarelli (1) and maria rosaria gallipoli (1)(2) (1) dipartimento di strutture, geotecnica, geologia applicata all’ingegneria (disgg), facoltà di ingegneria, università degli studi della basilicata, potenza, italy (2) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy abstract we propose a simple time-domain, non-parametric method to estimate the damping at the fundamental frequency of a building. the method aims at obtaining quick-and-dirty data on large sets of buildings, at the expenses of the accuracy provided by other, more complex and resource-demanding techniques. the analysis of a 10 min recording of ambient vibration with a single high-resolution seismometer atop the building can provide a good estimate of the required parameters for the first flexural modes on orthogonal components. the proposed methodology does not require complex filtering and assumptions on signal structure, nor multiple measurement points or clear single transients induced by shakers, shocks or release tests. we checked the stability of the proposed method in terms of duration and characteristics of the signal, and compared the results obtained by others with standard techniques. then, we tested the ability of the proposed technique to identify damping and frequency variations due to large displacements, damage or changes in the structural characteristics. the proposed methodology provides a satisfactory agreement when compared with other techniques. even if it is not always possible to obtain higher modes, the advantage is that it is possible to study with limited resources the fundamental parameters for a large number of buildings. this is useful to include experimental data on building behaviour in microzonation studies. mailing address: dr. marco mucciarelli, dipartimento di strutture, geotecnica, geologia applicata all’ingegneria (disgg), facoltà di ingegneria, università degli studi della basilicata, viale dell’ateneo lucano 10, 85100 potenza italy; e-mail: mucciarelli@unibas.it 260 marco mucciarelli and maria rosaria gallipoli structure, nor multiple measurement points or clear single transients induced by shakers, shocks or release test. this overcomes some of the operational disadvantages of well established approaches like randomdec (vandiver et al., 1982), half-power method (clough and penzien, 1975), cross-correlation or wavelet-logarithmic decrement (lamarque et al., 2000). for a detailed review of pros and cons of different ambient and forced vibration tests see huang et al. (1999, and references therein), that also present a method applicable to multiple measurement points. a generally accepted assumption is that the first approximation of the dynamic behaviour of a building is provided by the single-degreeof-freedom damped oscillator (sdof) (1.1) where γ is the damping and ω0 is the fundamental frequency, in its turn related to the mass m and stiffness k of the system as . (1.2) even with this simple formulation, the system identification becomes a problem when the forcing term f(t) is unknown. with ambient vibration excitation, a further warning comes from the difficulty to separate the noise filtered from the structure from the instrumental noise that is unavoidable during the measurements. the same problem is well known from microtremor measurements on soils, see e.g., mucciarelli (1998) and mucciarelli et al. (2005). the solution of eq. (1.1) is given by (1.3) where x0 is the initial amplitude, ϕ the phase and ε a perturbation term. the damping is commonly derived as the logarithmic decrement of the amplitude over a pseudo-period t according to the following equation: . (1.4) now we face the problem of numerical estima( ) / ( )ln t x t t tx 1 γ = + ( ) ( )cosx t x e tt0 ω ϕ ε= + +γ− m k 0ω = ( )x x x f t2 0 2γ ω+ + =p o tion of the above parameters from a discrete time series. if some form of excitation is applied, the damping can be straightforwardly estimated picking the amplitudes of subsequent maxima soon after the excitation is stopped and the system returns to its equilibrium position. for ambient vibration recordings, the randomdec approach solves this problem using a stacking of pre-selected segments exceeding a fixed amplitude threshold xl with common initial conditions 2. proposed method and numerical simulations assuming that the ground ambient vibration is close to white noise, a simple idealisation of the ambient vibration recording atop a building is composed by three factors (2.1) where xω0(ω) is the oscillation associated to the fundamental period (or x (t) in eq. (1.3)), that cannot be observed in its full development because of the superposition of xεg(ω) that is the perturbation induced by the unknown, continuous forcing induced by ground ambient vibration (or ε in eq. (1.3)). the term xεi(ω) represents the effect of instrumental noise during the measurement, as well as other sources of noise unfiltered by the building (e.g., wind impacting on unprotected sensors on a terrace roof). if we suppose that xω0(ω) > xεg(ω) and xω0(ω) >> >>xεi (ω), then we can try to retrieve γ directly from the recordings following this procedure: i) use a high resolution seismometer for taking measurements of ambient noise time histories. accelerometers provide insufficient resolving power, as shown for measurements on soil by mucciarelli (1998) and confirmed recently by the sesame project (duval et al., 2004). ii) after a standard correction procedure (baseline correction and high-pass filtering), integrate the time-history to obtain displacements. iii) select only positive values that are local maxima. build a 2 × m matrix: amplitudes x(ti) of maximum in one column and time of occurrence ti in the other. ( ) ( ) ( ) ( )x x x xg i0ω ω ω ω= + +ω ε ε ( ( ) ; ( ) ( ) ) .x t x x t t x t 0>∆= + −l 261 non-parametric analysis of a single seismometric recording to obtain building dynamic parameters iv) for each subsequent row in the above matrix, verify if x(ti)>x(ti+1); if yes, calculate piecewise pseudo-frequency and damping as (2.2) . (2.3) v) build a 2-histogram, binning piecewise pseudo-frequencies and damping. vi) produce a graphic output to visualise the maximum of the empirical distribution, that is the main result of this non-parametric damping analysis (hereafter nonpadan), providing the fundamental frequency and its associated damping. we implemented the above procedure using matlab: the relevant m-file is freely available for download at the address <http:\\nato.gfz.hr>. to verify the above procedure and hypothesis, we built a numerical model using simulink. figure 1 shows the relevant block diagram. the input is the sum of repeated, large pulses and ( / ) ( ( ) / ( ))x t x t1 2i i i 1:γ π= + / ( )t t1 i i1 1υ = −+ gaussian white noise with variable amplitude. the mass and stiffness of the building model are tuned to obtain a fundamental frequency equal to 3 hz. the damping is set to 3%. before the recording, more gaussian white noise is added to simulate the presence of instrumental noise. figure 2 reports the output of different simulations with increasing noise, both at the base and at the top of the building. it is possible to see how the nonpadan method retrieves correctly both damping and frequency if the noise is absent (maximum is the black dot inside the red area). the increase of input noise leaves the frequency unchanged but makes the damping peak broader. nevertheless, the maximum in the 2d histograms shows the correct values for damping and fundamental frequency. on the contrary, sparse, spurious pseudo-frequencies arise when we add instrumental noise, and the damping is not correctly preserved. this simulation shows that the initial hypothesis is correct: to obtain a reliable estimate it is necessary to keep at a minimum the instrumental noise. fig. 1. simulink block model implemented to verify the assumptions underlying the nonpadan method. 262 fig. 2. effects of increasing input and instrumental noise on the simulated recording atop a sdof building. marco mucciarelli and maria rosaria gallipoli fig. 3. nonpadan estimate of frequency and damping as a function of recording length (left: 30 min; right: 10 min). 263 non-parametric analysis of a single seismometric recording to obtain building dynamic parameters 3. empirical data analysis the first check performed on the reliability of nonpadan is relevant to the duration of the signal. if using randomdec, a minimum of 30 min of recording is required to obtain a stable estimate, according to dunand et al. (2002). a 30 min recording at the 5th floor of the engineering faculty building at the university of basilicata was performed using a digital tromometer (micromed tromino). then, the damping was estimated using both the whole record and a section of 10 min. figure 3 shows the comparison: the fundamental frequency is the same and the damping remains centred at around 3%. in this case it is also possible to identify a higher mode, but one can see in the following that it is not always so. the second check was about the comparison with published results obtained using different methods to estimate frequency and damping. di giulio et al. (2005) used the horizontal to vertical spectral ratio (hvsr) and the horizontal to horizontal spectral ratio (hhsr) technique to estimate the fundamental frequency of some buildings in the town of palermo, italy. the recordings were collected with a lennartz 5 s seismometer and a marslite digitiser. we analysed 30 min of ambient vibration they recorded at the 5th floor of the r.c. building code-named pal8. figure 4 shows the result, with a clear estimate of fundamental frequency (3.2 hz) and damping (3.5%). the estimation of frequency is in a good agreement with the result given by di giulio et al. (2005). during 2005 a series of tests were conducted on a r.c. scale model on the enea shaking table at the casaccia (rome) facility to study retrofit systems (cardone et al., 2003). the input ranged from random noise to real strong motion. cardone et al. (2005) estimated from clear transients a damping around 3% and a fundamental frequency of about 2 hz. figure 5 reports the results obtained with nonpadan, in good agreement with the estimate from cardone et al. (2005). fig. 4. nonpadan estimate of frequency and damping for a r.c. building (pal8 in di giulio et al., 2004). fig. 5. nonpadan estimate of frequency and damping for a scale model tested with random vibration at the enea casaccia shaking table (rome). 4 5 264 marco mucciarelli and maria rosaria gallipoli the last check is relevant to the ability of nonpadan to estimate the variations of frequency and damping due to large displacements, damage or changes in the structural characteristics. for the first two items, the recordings obtained during the 2002 molise earthquake sequence inside a building in bonefro, italy are re-analysed. the record comprises 1 min of ambient vibration, the 1st november shock (mw = = 5.6) that damaged the building and whose vibration lasted 2.5 min and finally 1 min of ambient vibration. mucciarelli et al. (2004) analysed this records using different techniques (gabor transform, wavelets, moving window hvsr). the building has an initial frequency of 2.5 hz. during the large oscillations and subsequent damage the frequency lowers to 1.5 hz, to reach a final value for the damaged building at about 2 hz. figure 6 reports the result of nonpadan obtained for three 1 min windows: the first minute of ambient vibration, the coda of the shock and the last minute of ambient vibration. the frequency shift estimated with nonpadan agrees with the result of mucciarelli et al. (2004). moreover, it is worth noting that during the damaging large oscillations, a secondary, isolated damping emerges with a value of 10%. this is fig. 7. nonpadan estimate of frequency and damping for a 4 stories r.c. building in potenza, italy with the bare frame only (left) and completed with infills (right). fig. 6. nonpadan estimate of frequency and damping for a r.c. building in bonefro, italy, during the damage induced by the 1st november 2002 molise earthquake. 265 non-parametric analysis of a single seismometric recording to obtain building dynamic parameters 2.5 times the starting damping, in agreement with observation on damage in laboratory tests (see for example cardone et al., 2004; and dolce et al., 2004). the estimated damping factors are confirmed by a numerical modelling of the building, as described in dolce et al. (2007). the reliability of nonpadan to estimate the damping variation due to different building characteristics was checked taking advantage of a 4 storey, r.c. building under construction in potenza, italy. the first measurement was taken when the bare frame was completed and the bricks for the infills were loaded at each floor. the second measurement took place after the infills were completed. in this way, a variation of the stiffness occurred while the total mass of the building remained constant. in both case 10 min of ambient vibration were sampled with a micromed digital tromometer. figure 7 shows that nonpadan is able to capture the increase of frequency and the decrease of damping that was expected as a consequence of the increase in stiffness. using this last example, we take the proposed method a step further, suggesting a possible parameterisation of the model and thus the possibility of performing quantitative statistical tests. the best way of observing the true damping of a structure is after forcing, when the free oscillations’ amplitude decreases. the ambient noise is a particular case of output-only, unknown-input system: the forcing is random in amplitude and phase, so the structure cannot fully develop its damped free oscillation. nevertheless, an improved strategy is to look for damping on more than two relative maxima separated by the same pseudo-period. these consecutive, decreasing local maxima are the best approximation of damped free oscillation that ambient noise measurement can produce. of course a compromise is needed, between the search for long sequences (that would provide the best, more stable result) and the fact that the number of available sequences decrease exponentially when increasing their length. if we select only the damping values calculated on three consecutive maxima, the result is less dispersed and the size of the sample still permits to estimate their statistical distribution, which is log-normal. for the r.c. building with and without infills the plot of the two distributions (fig. 8) reveals that there is a constant shift of the damping by a factor 1.3. moreover, a kolmogorov-smirnov test allows rejecting the hypothesis that the two cdfs derive from the same distribution with a 97.5% confidence level. this approach could be useful for measurements perfomed before and after an earthquake to establish if there is a significant frequency and damping shift due to damage: the ratio between cdfs appears to be more stable than the comparison between two couples of values. 4. conclusions we proposed a simple method for the nonparametric analysis of damping of buildings (nonpadan). it is able to capture the fundamental mode and its associated damping with a short, single measurement of ambient vibration atop a building. the resolving power is less than that provided by more advanced techniques and the higher modes and associated damping are rarely visible. however, the advantage of the proposed method is related to the limited resources it requires in terms of time, instrumentation and manpower. this candidates nonpadan as a tool to be used for dense measurement campaigns, when it is more important fig. 8. comparison of cdfs of damping derived by more than two subsequent maxima for the same building of fig. 7. 266 marco mucciarelli and maria rosaria gallipoli to know the fundamental dynamic parameters of large sets of buildings rather than focusing on the detailed behaviour of a single item. acknowledgements this work was funded by nato grant sfp 980857. thanks are due to antonio rovelli and giuseppe di giulio for providing recordings of pal8, felice carlo ponzo for providing recordings of enea test and mauro dolce for comments and suggestions. references cardone d., g. de canio, m. dolce, r. marnetto, c. moroni, m. nicoletti, d. nigro, a. pizzari, f.c. ponzo, e. renzi, g. santarsiero and d. spina (2003): comparison of different passive control techniques through shaking table tests, in proceedings of the 8th world seminar on seismic isolation, energy dissipation and active vibration control of structures, october 6-10, 2003, yerevan, armenia. cardone, d., e. coelho, m. dolce and f. ponzo (2004): experimental behaviour of r/c frames retrofitted with dissipating and re-centring braces, j. earthq. eng., 8 (3), 361-396. cardone, d., a. di cesare, m. dolce, c. moroni, d. nigro, f.c. ponzo and m. nicoletti (2005): comparison of shaking table and pseudodynamic test on a base isolated r/c structure, in proceedings of the 9th world seminar on seismic isolation, energy dissipation and active vibration control of structures, june 13-16, 2005, kobe, japan. clough, r.w. and j. penzien (1975): dynamics of structures (mac graw-hill, new york), 2nd edition, pp. 576. di giulio, g., r m. azzara, g. cultrera, m.s. giammarinaro, p. vallone and a. rovelli (2005): effect of local geology on ground motion in the city of palermo, italy, as inferred from aftershocks of the 6 september 2002 mw 5.9 earthquake, bull. seismol. soc. am., 95, 2328-2341. dolce, m., d. cardone, f.c. ponzo and c. valente (2004): shaking table tests on reinforced concrete frames without and with passive control systems, earthq. eng. struct. dyn., 34, 1687-1717. dolce, m., f.c. ponzo, a. masi, m. mucciarelli, m.r. gallipoli, a. di cesare, m. tetta and m. vona (2007): identification of the structural model and analysis of the global seismic behaviour of the iacp bonefro building, bull. earthq. eng. (submitted). dunand, f., p.-y. bard, j. l. chatelain, ph. guéguen, t. vassail and m.n. farsi (2002): damping and frequency from randomdec method applied to in situ measurement of ambient vibrations: evidence for effective soil structure interaction, in proceedings of the 12th european conference on earth engineering, london, u.k., cd-rom edition, pap. no. 869. duval, a.-m., j.-l. chatelain, b. guillier and sesame project wp02 team (2004): influence of experimental conditions on h/v determination using ambient vibrations (noise), in proceedings of the 13th world conference on earthquake engineering, august 1-6, 2004, vancouver, b.c., canada, pap. no. 306. huang, c.s., y.b. yang, l.y. lu and c.h. chen (1999): dynamic testing and system identification of a multi-span highway bridge, earthq. eng. struct. dyn., 28, 857-878. lamarque, c.-h., s. pernot and a. cuer (2000): damping identification in multi-degree-of-freedom systems via a wavelet-logarithmic decrement, part 1. theory, j. sound vib., 235, 361-374. mucciarelli, m. (1998): reliability and applicability range of the nakamura’s technique, j. earthq. eng., 2 (4), 625638. mucciarelli, m., a. masi, m.r. gallipoli, p. harabaglia, m. vona, f. ponzo and m. dolce (2004): analysis of a r.c. building dynamic response and soil-building resonance based on data recorded during a damaging earthquake (molise, italy 2002), bull. seismol. soc. am., 94 (5), 1943-1953. mucciarelli, m., m. r. gallipoli, d. di giacomo, f. di nota and e. nino (2005): wind influence on seismic noise measurements, geophys. j. int., 161, 303-308. navarro, m., f. vidal, m. feriche, t. enomoto, f.j. sánchez and i. matsuda (2004): expected ground-rc building structures resonance phenomena in granada city (southern spain), in proceedings of the 13th world conference on earthquake engineering, august 1-6, 2004, vancouver, b.c., canada, pap. no. 3308. vandiver, j.k., a.b. dunwoody, r.b. campbell and m f. cook (1982): a mathematical basis for the random decrement vibration signature analysis technique, j. mech. des., 104, 307-313. (received october 27, 2006; accepted april 4, 2007) vol. 48, 01, 05ok.qxd 109 annals of geophysics, vol. 48, n. 1, february 2005 key words radon-indoor-bedrock relationship – radon mapping 1. introduction the czech geological survey (cgs) has been participating in solving the problems of the inhabitants’ irradiation caused by natural radionuclides since 1990 (barnet, 1996). the radon programme of the czech republic (cr) started within the scope of the governmental decision no. 538 in 1999. present research on radionuclides in dwellings has revealed that the cr is one of the countries with the highest level of average 222rn concentration in buildings. therefore great attention is paid to indoor rn measurements. a number of houses above the guidance level of 200 bqm–3 of equilibrium equivalent concentration being detected in the last five years is given in table i. the main target of research work was to process all data and information from available databases and approve the existing relationship between increased indoor radon values and rock types in bedrock considering that a significant part of the territory of the cr is formed by the bohemian massif, which belongs to the european variscan belt represented by the proterozoic and pre-variscan paleozoic crystalline basement. (mísař et al., 1983). similar studies were performed by kies et al. (1996), kemski et al. (2000), popit and vaupotic (2002). 2. data sources geologically based radon data originates from cgs measurements and also from more than 100 private companies of the association mailing address: dr. jitka mikšová, czech geological survey, prague 1, klárov 3, 118 21 czech republic; email: miksova@cgu.cz radon data processing and outputs for the needs of the state office for nuclear safety (according to the czech radon programme) jitka mikšová and ivan barnet czech geological survey, prague 1, czech republic abstract much of the population living in the czech republic is exposed to radiation from natural sources, especially to the radon effect. the aim of geological research defined by the state office for nuclear safety (sons) was to detect areas with estimated high radon concentration in soil gas. a uniform method of measurements and uniform methodology of radon risk category assessment of geological units and a centralized radon database was established. radon risk classification was based on statistical evaluation of soil gas radon concentration and permeability in investigated geological units. prognostic radon risk maps in various scales were the main outputs of this research. with the help of gis tools spatial analyses were found a correlation between soil gas radon values in selected geological units and indoor measurements in dwellings. after verification of the efficiency of track etch detectors placed in dwellings with the help of prognostic maps 75% reliability of these maps was proven. this reliability of analyses induced the sons to widely use radon risk maps to determine areas with predicted high radon risk category. 110 jitka mikšová and ivan barnet radon risk. data sources used for gis applications come mostly from state organizations. – czech geological survey (cgs): soil gas radon database (about 9000 test sites in different rock units), vectorized geological maps (214 map sheets 1 : 50 000). – czech office for surveying, mapping and cadastre: raster topography. – ministry for regional development of the czech republic: database – uir (special data register of regional identification), contours of cadastres, database of residential units and other details of measured dwellings. – national radiation protection institute: indoor radon database (indoor radon measurement – 130 000 points, geometric mean in cadastre – 6299 cadastres). – czech statistical office: database of geographic position (of measured dwellings) and character of dwellings. the field method of soil gas radon measurements and methodology of radon risk category assessments are standardized. a uniform method for soil gas radon measurements is used by all organization dealing with building site assessment. the soil gas sample is taken from a depth of 0.8 m using rods with a «lost tip». at least 15 measurements are performed at each test site. the 75% quantile of radon data set serves as an input parameter for radon risk classification. the permeability is derived from grain-size analysis or from permeametric measurements in situ. all subjects dealing with soil gas radon measurements pass the calibration of devices in radon chamber and field-testing at the reference sites to ensure the comparability and reliability of results. in the czech geological survey the portable radonmeter rda 200 (scintrex) with exchangeable lucas cells is used for soil gas radon measurements. 3. data processing all data are placed in a centralized database administrated by cgs. a sufficient number of measurements (nearly 9000 measured test site with 15 measurements and 29 items concerning localization, radon, geological and classification data for every test site) makes a statistically reliable data set to determine radon risk from bedrock in particular geological units and rock types (mikšová and barnet, 2002). the construction of predictive radon risk maps is based on contours of geological units. the division of rock types into prevailing radon risk categories is done with the help of statistical methods using soil gas radon concentration and permeability. the predictive radon risk maps are not based on the uranium data from the territory of the czech republic but solely on the soil gas radon and permeability measurements (mikšová and barnet, 2002). the rock units are classified after prevailing radon risk category: low risk (mostly younger sediment formations from cretaceous to neogene), interstage risk (mostly inhomogeneous quaternary sediments), medium risk (mostly paleozoic sediments and crystalline gneisses), high risk (granitoids). table i. number of indoor rn measurements in the czech republic in 1998-2002. year number of measured dwellings number of dwellings with range of measured eec (bqm–3) > 200 200-299 300-600 >600 1998 5634 2014 925 773 316 1999 5257 1171 533 455 183 2000 6760 1570 668 684 218 2001 11546 2150 1107 802 178 2002 10841 1749 850 722 177 111 radon data processing and outputs for the needs of the state office for nuclear safety (according to the czech radon programme) fig. 1. the selection of cadastres performed by simple intersection of cadastres and contours of durbachite bodies (bold outlined). the building projects of newly built houses are modified after the radon risk category measured on the building ground (czech technical norm 730601, 1996). the spatial analysis was made to find significant information on the relationship between soil gas radon concentration and indoor measurements. three different approaches for spatial comparison of cadastre’s polygons and contours of the geological units were tested. the way combining the demand for geographical preciseness and the demand for statistical reliability for further analyses was chosen. this method comprised the selection of cadastres with centroidal points situated inside the contour of rock unit, but the border of the cadastre was partially intersecting the contour of the geological unit (barnet et al., 2002). an example of resulting selection performed by three methods is given for cadastres situated in the durbachite bodies of třebíč massif in moravia (figs. 1, 2 and 3). the indoor radon means calculated for different rock types on the basis of spatial analysis are given in table ii. the «indoor intersection» values represent the results of the first method of analysis where polygons of cadastres are intersecting the polygons of geological units with no respect to ratio of aerial intersection. the «indoor inside» values are calculated for polygons of cadastres, which are fully situated inside the contours of the geological unit. the «indoor centroid» values correspond to geographical centroids of cadastres, which lie inside the contour of the geological unit, but the polygon of cadastre can intersect partially the contour of the geological unit. the values placed in shaded columns in table ii approve the relationship of mean soil gas radon values in rock types and mean indoor radon values. 112 jitka mikšová and ivan barnet fig. 3. the selection of cadastres based on placing the centroids of cadastres inside the rock contours plus intersecting the contours of durbachite bodies (bold outlined). this selection represents the shape of rock body most precisely. fig. 2. the selection of cadastres based on placing the polygons of cadastres inside contours of durbachite bodies (bold outlined) without intersecting. table ii. the values of indoor radon geometric means in major rock types calculated by three different methods. major rock types indoor rn (means bqm–3) soil gas rn (means kbqm–3) indoor no. indoor no. indoor no. mean no. intersection int inside ins centroid cent rock rock moldanubian paragneisses 99.24 570 116.96 79 100.53 275 28.08 528 – monotonous series moldanubian crystalline 104.2 464 91.27 42 95.19 236 31.25 465 rocks – variagated series proterozoic – phyllites, 95.81 1062 101.76 54 94.69 470 26.61 345 metamorphosed shales orthogneisses, granulites, 106.01 583 109.49 23 110.62 181 32.05 383 migmatites – moldanubian ultrabasic rocks 96.03 51 0 0 139.93 3 17.2 4 – moldanubian granitoids – cadomian 89.89 192 80.6 3 95.74 59 17.2 155 palaeozoic 91.1 201 103.75 2 96.18 87 39.25 154 – metamorphosed palaeozoic 93.7 552 102.65 121 99.51 320 27.92 402 – unmetamorphosed volcanites – proterozoic, 91.12 606 64.4 2 97.13 99 37.11 45 palaeozoic granites – variscan 104.32 547 128.64 36 105.98 209 58.8 424 granodiorites – variscan 152.38 390 203.09 39 177.86 197 66.3 307 durbachites, syenites 205.36 165 334.91 39 272.3 77 56.78 180 diorites, gabbros – cadomian 107.67 133 0 0 87.23 20 22.63 55 mesozoic – alpine folding 54.39 128 47.53 4 52.74 53 17.35 1788 tertiary – alpine folding 65.22 429 71.76 53 67.15 273 20.28 759 permocarboniferous 74.46 515 72.36 20 75.29 234 33.72 326 mesozoic – sediments 62.23 1224 60.46 67 60.72 590 17.53 1788 tertiary – sediments 78.35 820 66.99 9 72.57 192 20.28 759 neovolcanites – tertiary 65.47 306 69.2 3 60.64 52 21.55 35 quaternary 73.06 2274 73.25 199 68.57 1003 25.37 525 113 radon data processing and outputs for the needs of the state office for nuclear safety (according to the czech radon programme) 4. software platform the source soil radon gas database was based on visual foxpro, for predictive radon risk map production converted to oracle 8i. the data model for geological maps was originally created in arcinfo (esri corp.). later this model was converted into microstation (bentley systems corp.) – mge (intergraph corp.) – oracle (oracle corp.). this model was also used for radon risk maps formation. the spatial analyses were done in esri software arc gis. 5. outputs in 1998 the cgs issued the digital atlas geocr500 – the geological, radiometric and radon risk maps on the scale 1 : 500 000 together with eight maps with geoscientific topic. this radon risk map was based on the vectorized contours of geological units and on the results of gamma dose rate measurements. this atlas was published on cd-rom in gis project for arcview 3.0 (barnet and mikšová, 2001). 114 jitka mikšová and ivan barnet in 1998 the cgs finished the vectorization of geological maps at the scale 1 : 50 000. these maps covered the whole state territory. this fact allowed the formation of more detailed radon risk maps based on geological maps. since 1999 about 154 map sheets of radon risk maps at the scale 1:50 000 (from total number of 214 sheets) have been published up to 2003 (fig. 4). these maps are available in digital and printed form. the digital outputs are published on cd-rom. the selection from the mosaic map enables to open the full view of the map and detailed view of four single quadrants. the points of the test sites are linked to the selected items from the radon database in a separate window. 6. conclusions by comparing the values of radon concentration in dwellings and the measured values on the test sites in geological units, the close correlation between the radon concentration and geological bedrock was established, especially in the areas where igneous and metamorphosed rocks were found. the data processing based on vectorized geological maps was proven as a highly efficient and relevant tool for determination areas where increased indoor radon values caused by radon exhalation from bedrock can be detected. after verification of the efficiency of track etch detectors placed in dwellings with the help of prognostic radon risk maps at the scale of 1:50 000, 75% reliability of these maps was proven. the outputs of geological research are used by regional centers of sons and municipal authorities to set the priority for distributing the track-etch detectors in dwellings. in the case of randomly distributed track-etch detectors only 2% of measured dwellings exceeded the guidance level 200 bq m–3 eec. using radon risk maps on a scale 1:50 000 for setting of detectors into areas with assumed medium or high radon risk category in bedrock the number of affected dwellings increased to 20%. fig. 4. czech republic – completed radon risk map sheets 1:50 000 (bold framed). the priority of map sheets processing was oriented to cover dominantly the extent of granitoid bodies within the czech republic. 115 radon data processing and outputs for the needs of the state office for nuclear safety (according to the czech radon programme) predictive radon risk maps were issued by cgs in the form of printed maps or on interactive cd-rom in gis projects. presentation of radon research work, especially radon risk mapping, are accessible to the wider public on the portal of cgs – www.geology.cz – radon mapping – in czech and english versions. references barnet, i. (1996): the research project no. 3310 radon risk in the czech republic – the activities of the czech geological survey, in third international workshop on the geological aspects of radon risk mapping, edited by i. barnet and m. neznal, prague, 1-9. barnet, i. and j. mikšová (2001): the gis approach to radon risk mapping in the czech republic, in proceedings of the «5th international conference on rare gas geochemistry», debrecen, hungary, 189-196. barnet, i., j. mikšová and i. fojtikova (2002): the gis analysis of indoor radon and soil gas radon in major rock types of the czech republic, in radon investigations in czech republic ix and the sixth international workshop on the geological aspects of radon risk mapping (czech geol. surv. and radon comp.), prague, 5-11. czech technical norm 730601 (1996): protection of building against radon from soil (cni, prague), pp. 27. kemski, j., a. siehl, r. stegemann and m. valdiviamanchego (2000): mapping the geogenic radon potential in germany, in proceeding workshop radon in the living environment, athens, greece, 645-663. kies, a., a. biell and l. rowlinson (1996): radon survey in the grand-duchy of luxembourg – indoor measurements related to house features, soil, geology and environment, environ. int., 22, 805-808. mikšová, j. and i. barnet (2002): geological support to the national radon programme (czech republic), bull. czech geol. surv., prague, 77 (1), 13-22. mísař, z., a. dudek, v. havlena and j. weiss (1983): geology of cssr, i. bohemian massif (spn, prague), pp. 333 (in czech). popit, a. and j. vaupotic (2002): indoor radon concentrations in relation to geology in slovenia, environ. geol., 42, 330-337. 473_478.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 473 the exploding-reflector concept for ground-penetrating-radar modeling josé m. carcione (1), laura piñero feliciangeli (2) and michela zamparo (1) (1) istituto nazionale di oceanografia e di geofisica sperimentale (ogs), sgonico, trieste, italy (2) instituto de ciencias de la tierra, departamento de geofísica, universidad central de venezuela, caracas, venezuela abstract the simulation of a stacked radargram requires the calculation of a set of common-source experiments and application of the standard processing sequence. to reduce computing time, a zero-offset stacked section can be obtained with a single simulation, by using the exploding-reflector concept and the so-called non-reflecting wave equation. this non-physical modification of the wave equation implies a constant impedance model to avoid multiple reflections, which are, in principle, absent from stacked sections and constitute unwanted artifacts in migration processes. magnetic permeability is used as a free parameter to obtain a constant impedance model and avoid multiple reflections. the reflection strength is then implicit in the source strength. moreover, the method generates normal-incidence reflections, i.e. those having identical downgoing and upgoing wave paths. exploding reflector experiments provide correct travel times of diffraction and reflection events, in contrast to the plane-wave method. mailing address: dr. josé m. carcione, istituto nazionale di oceanografia e di geofisica sperimentale (ogs), borgo grotta gigante 42c, 34010 sgonico, trieste, italy; e-mail: jcarcione@ogs.trieste.it 1. introduction the computation of synthetic seismograms for simulating zero-offset (stacked) seismic sections, and the reverse time migration of these sections, requires the use of the explodingreflector concept (loewenthal et al., 1976; carcione et al., 1994) and the so-called nonreflecting wave equation (baysal et al., 1984). this non-physical modification of the wave equation implies a constant impedance model to avoid multiple reflections, which are, in principle, absent from stacked sections and constitute unwanted artifacts in migration processes (baysal et al., 1983). the increasing use of ground-penetrating radar (gpr) for solving a wide range of engineering and environmental problems has been facilitated by the application of standard seismic techniques, such as multi-fold coverage and processing (fisher et al., 1992; pipan et al., 1996). modeling, as an interpretation tool, and migration methods are two of the most important processing techniques. in particular, it is necessary to migrate events generated by nearsurface dipping reflectors and buried elements, such as boulders and localized inhomogeneities. in many cases, the radargrams can also be contaminated by surface events generated by trees and metallic objects. therefore, it is important to develop the concepts of exploding reflector and non-reflecting wave equations for gpr applications. there are two efficient ways for simulating a zero-offset synthetic survey avoiding calculation of common-shot records: the planekey words exploding reflector – zero-offset section – gpr modeling 474 josé m. carcione, laura piñero feliciangeli and michela zamparo wave method and the exploding reflector method. both approaches involve a single calculation with a time-domain modeling algorithm. the first consists in sending a horizontal localized plane-wave front down from the surface and recording the response of the subsurface model back at the surface. the travel times obtained with this method are not those obtained with the standard processing sequence. a simple example is given by a single diffraction point at (0, z). if v is the phase velocity of the medium, the travel time at offset (x, 0) is while a source and a receiver at (x, 0) imply a travel time the method is also approximate for dipping layers. in the exploding-reflector method (baysal et al., 1984; carcione et al., 1994), each reflection point in the subsurface explodes at t = 0 with a magnitude proportional to the normalincidence reflection coefficient. the equation is a modification of the wave equation, where the impedance is constant over the whole model space. in this way, non-physical multiple reflections are avoided and the recorded events are primary reflections. moreover, the method generates normal-incidence reflections, i.e. those having identical downgoing and upgoing wave paths. in order to obtain the two-way travel time, the phase velocities are halved. due to sampling constraints, halving the velocities implies doubling the number of grid points. therefore, the method is less efficient than the plane-wave approach. in the seismic case, the density is used as a free parameter to obtain a constant impedance model and avoid multiple reflections. from the analogy between acoustic and electromagnetic waves (carcione and cavallini, 1995), the density is mathematically analogous to the magnetic permeability. we scale this property by a factor depending on the permittivity, to obtain a model where all the media have the same electromagnetic impedance. in addition, the condition that the phase velocity remains unchanged also requires the scaling of the permittivity and the conductivity. in an example, we consider the transverse-magnetic (tm) wave equations and compare synthetic radargrams computed with both the plane-wave and the exploding-reflector methods. the numerical solver used here consists of the pseudospectral fourier method for computing the spatial derivatives, and a runge-kutta method for time integration (carcione, 1996). however, any other full-wave equations method, finitedifferences, pseudospectral, or finite-elements, can be used. 2. maxwell’s equations maxwell’s equations for isotropic media and time harmonic fields with time dependence exp (i t ), where is the angular frequency, read × = +e h mi µ0 (2.1) and (2.2) where the symbol × denotes the vector product, e is the electric field, h is the magnetic field, j s is the electric source, m is the magnetic source, µ0 is the magnetic permeability of vacuum (appropriate for gpr applications), is the dielectric permittivity, is the con-ductivity, and (2.3) is the complex permittivity (chew, 1990). 3. the exploding reflector implementation of the exploding reflector method in gpr modeling implies the following: 1) a source is placed at every point on the subsurface interface of interest. 2) the phase velocity of each medium is halved, to obtain the correct two-way travel time t v x z v= +/ + /z 2 2 t x z v= + 2 / .2 2 × = + + = +h e e j e ji is s* * ( ) = i the exploding-reflector concept for ground-penetrating-radar modeling 475 given by (3.5) where (3.6) is the electromagnetic impedance (chew, 1990, p. 48), with µ = µ 0 here. equation (3.5) is the reflection coefficient for the electric field of the te or tm waves. the corresponding normalincidence reflection coefficient for the magnetic field of the te or tm waves has the opposite sign. the source strength should be proportional to | r |. since r depends on the frequency , this requirement cannot be satisfied for all frequencies when using a time-domain solver. in this case, we consider the source strength to be proportional to | r ( d ) |, where d is the dominant frequency of the source. 4) we can avoid normal-incidence multiple reflections if all the media have the same electromagnetic impedance. this is evident from eq. (3.5). in order to do this we must scale the material properties accordingly, that is, substitute µ 0 , and with new properties µ, and . the complex impedance can be made constant by using the perfectly matched layer (pml) method introduced by berenguer (1994); i.e. using a complex magnetic permeability. however, this approach gives a different complex velocity compared to eq. (3.2). we may impose that the «instantaneous» impedance is the same for all the media. the instantaneous impedance is defined as (3.7) which corresponds to the unrelaxed response of the medium (t = 0). in this limit / 0, in general. then, we assume (3.8) with k a real constant. moreover, we must impose that the complex velocity (3.2) remains unchanged after introduction of the scaled proand amplitude decay for every diffraction and reflection event. 3) the source strength is proportional to the normal-incidence reflection coefficient at each point on the interface (a zero-offset raypath is normal to the reflecting interface). 4) we require that the electromagnetic impedance be the same for all media. because the algorithm generates non-physical events (the downgoing waves) and approximates a zero-offset stacked section, we must avoid multiple reflections. we propose the following approximations to meet requirements (2.1) to (3.1): 1) the algorithm used here solves the electromagnetic equations in the time domain, and it is based on a grid method for computing the spatial derivatives. this implies a discretization of the model space. then, at every grid point of each interface a source with the same time history is initiated at time t = 0. 2) the phase velocity is given by (3.1) where is the real-part operator, and (3.2) is the complex velocity (carcione, 1996). halving the phase velocity can be achieved by the following substitution: (3.3) since the attenuation factor is (3.4) (carcione, 1996), where is the imaginary-part operator, eq. (3.3) ensures that the amplitude decay corresponds to that of the two-way travel path. 3) the normal-incidence reflection coefficient between medium 1 and medium 2 is v v p = ( )1 1 v = 1 0 µ µ µ0 04 . = ( ) 1 v r i i i i = + 2 1 2 1 i v= µ i i= =( ) i k= = ˜ ˜ µ 476 josé m. carcione, laura piñero feliciangeli and michela zamparo are computed in the space-time domain. let us denote by e and h and j and m the corresponding time-domain electric and magnetic fields and sources, and for convenience, the medium properties are indicated by the same symbols, in both, the time and the frequency domains. under these conditions, ex, ez and hy are decoupled from ey, hx and hz, and the first three fields obey the tm wave differential equations (4.1) (4.2) (4.3) the set of properties (µ, , ) is equal to (µ 0 , , ) for the plane-wave method, and to (µ 0, , ) for the exploding-reflector method. let us consider the model shown in fig. 1. the ray-paths represented with solid lines are perties, i.e. (3.9) the choice (3.10) (the impedance of vacuum) implies (3.11) where (3.12) then, i = i 0 and eq. (3.9) is satisfied. 4. example we assume an isotropic medium, propagation in the (x, z)-plane, and that the material properties and source characteristics are constant with respect to the y-coordinate. the radargrams fig. 1. model and electromagnetic properties. the ray-paths represented with solid lines are the direct arrivals. the dotted line is a physical multiple. the dashed lines represent non-physical multiple events due to the presence of down-going waves in the exploding-reflector experiment (carcione et al., 1994). µ µ0 * ˜ ˜ ˜= ( ).i k i= =0 0 0 µ a = 0 . µ � �z x y yx z t = + h m = + + h j y x x sxz t � � h j y z z szx t = + +� � . ˜ , ˜ ˜µ µ = = = 0 a a a and the exploding-reflector concept for ground-penetrating-radar modeling 477 the synthetic radargrams because the medium is electromagnetically «transparent» due to the requirement (3.1). the quality factor is given by (4.4) (carcione and cavallini, 1995). using 0 = 8.85 10 12 f/m, the quality factors at 100 mhz of the media in fig. 1 are q 1 = , q 2 = 28 and q 3 = 11. the numerical mesh for the plane-wave experiment has n x = n z = 125 grid points, with a grid spacing d x = dz = 15 cm. the source is a horizontal electric current (j sx ), whose time history is a ricker-type wavelet with a central frequency f c = 100 mhz and a cut-off frequency of 200 mhz. the nyquist criterion implies (4.5) where dmax is the maximum allowed grid size, and v pmin is the minimum phase velocity. the minimum phase velocity is that of medium 3 (see fig. 1), with an unrelaxed (optical) value of 6.7 cm/ns. note that a maximum spacing of 16.77 cm is allowed. because the velocities are halved in the exploding-reflector method, we use d x = d z = 7.5 cm, and n x = n z = 243. the algorithm, similar to that developed by carcione (1996), is based on the fourier method for computing the spatial derivatives and a rungekutta algorithm for the time evolution of the wave field. we use a time step of 0.05 ns. figure 2a,b shows the synthetic radargrams of the magnetic-field component, obtained with the plane-wave method and the exploding-reflector method, respectively (the pictures have been scaled to obtain comparable amplitudes for the step-interface response). the differences in travel times of the diffraction hyperbolae are evident. the exploding-reflector travel times are greater than the plane-wave travel times, as indicated in the section 1. moreover, there is a change of polarity in the plane-wave response of the step. the weak event that peaks at 162 ns in fig. 2a is the multiple indicated by a dotted line in fig. 1. note that the non-physical multiples (dashed lines in fig. 1) are absent in fig. 2b. the direct arrivals. the dotted line is a physical multiple, present in the plane-wave response (at 150 ns). the dashed lines represent non-physical multiple events due to the presence of downgoing waves in the exploding-reflector experiment (with arrival times of 148 and 194 ns, respectively). the latter events do not appear in fig. 2a,b. synthetic radargrams of magnetic field component, computed with the plane-wave method (a) and the exploding-reflector method (b) (carcione et al., 1994). q v v = ( ) ( ) = re im 2 2 d v f p c max min = 4 a b 478 josé m. carcione, laura piñero feliciangeli and michela zamparo 5. conclusions we have developed the exploding-reflector method for gpr applications. it provides the correct travel times of diffraction and reflection events, in contrast to the plane-wave method. moreover, unlike one-way equations, the method can simulate ray-paths which turn around via refraction in the presence of large velocity gradients. the use of a nearly constant impedance condition avoids the non-physical multiple events caused by the presence of down-going waves. the method can be used to approximate stacked radargrams with a single calculation, and in migration algorithms, where it is necessary to avoid interlayer reverberations. acknowledgements l. piñero feliciangeli thanks the «consejo de desarrollo científico y humanístico» of ucv for partially funding the research. references baysal, e., d.d. kosloff and j.w.c. sherwood (1983): reverse time migration, geophysics, 48, 1514-1524. baysal, e., d.d. kosloff and j.w.c. sherwood (1984): a two-way nonreflecting wave equation, geophysics, 49, 132-141. berenguer, j.p. (1994): a perfectly matched layer for the absorption of electromagnetic waves, j. comput. phys., 114, 185-200. carcione, j.m. (1996): ground radar simulation for archaeological applications, geophys. prospect., 44, 871-888. carcione, j.m. and f. cavallini (1995): on the acousticelectromagnetic analogy, wave motion, 21, 149-162. carcione, j.m., g. böhm and a. marchetti (1994): simulation of a cmp seismic section, j. seism. explor., 3, 381-396. chew, w.c. (1990): waves and fields in inhomogeneous media (van nostrand reinhold, new york), pp. 637. fisher, e., g.a. mcmechan and p. annan (1992): acquisition and processing of wide-aperture ground-penetrating radar data, geophysics, 57, 495-504. loewenthal, d., l. lu, r. roberson and j.w.c. sherwood (1976): the wave equation applied to migration, geophys. prospect., 24, 380-399. pipan, m., i. finetti and f. ferigo (1996): multi-fold gpr techniques with applications to high-resolution studies: two case histories, j. environ. eng. geophys., 1, 83-103. (received january 10, 2002; accepted march 13, 2002) n u o v o s m o r z a t o r e p e r i s i s m o g r a f i t i p o w i e c h e r t p. caj.oi a. l o surdo l noto c l i c in t u t t i i t i p i d i s i s m o g r a f i \\ i e c l i e r t r a z i o n e fren a n t e v i e n e o t t e n u t a con s m o r z a t o r i ad a r i a : u n p i s t o n e c i l i n d r i c o c a v o , a o s p e o a ( i n a l i l o s o t t i l i fili di b r o n z o fosforoso, p u ò o s c i l l a r e e n t r o un c i l i n d r o di m e t a l l o , di d i a m e t r o l i e v e m e n t e m a g g i o r e , c o m u n i c a n t e con l ' e s t e r n o m e d i a n t e d u e f o r i , l a cui a p e r t u r a p u ò e s c r e r e g o l a l a a p i a c e r e , cosi d a p o t e r p o r t a l e , v o l e n d o , lo s m o r z a m e n t o d e l i m o g i a l o a l l ' a p e r i o d i c i t à . q u e s t o s m o r z a t o r e p r e s e n t a v a r i i n c o n v e n i e n t i di cui i p i ù notevoli sono la d i f f i c o l t à di m e s s a a p u n t o e l ' i n c o s t a n z a di f u n z i o n a m e n t o d o v u t e al s i s t e m a di s o s p e n s i o n e del p i s t o n e e l a r i d u z i o n e del p e r i o d o . d o v u t a al f a l l o e l l e lo s m o r z a t o r e ad a r i a , così coni è s t a l o r e a l i z z a l o d a w i e c b e r t , c o s t i t u i s c e u n » i s t e r n a o s c i l l a n t e di p i c colo p e r i o d o , l a cui m a s s a è r i g i d a m e n t e c o l l e g a t a c o n il s i s t e m i p e n d o l a r e d e l s i s m o g r a f o . i n o l t r e è d a osserv a r e c l i c nei m o d e l l i p i c c o l i (2(10 e 8 0 k g ) il p i s t o n e m o b i l e d e l l o s m o r z a t o r e è c o l l e g a t o con l ' e s t r e m i t à d e l l a p r i m a l e v a d ' i n g r a n d i m e n t o . i n m o d o elle ... , r ìi;1 q u e s t a f o r m a a n g o l o r e t t o con l'as.-e del p i s t o n e . i v a r i p u n t i d e l l a l e v a d i n g r a n d i m e n t o e d e l l ' a s s e d e l l o s m o r z a t o r e si m u o v o n o q u i n d i s e c o n d o t r a i e t t o r i e c i r c o l a r i d i p o s l e in p i a n i f r a l o r o p e r p e n d i c o l a r i ( o r i z z o n t a l e q u e l l o d e l l a l e v a , v e l l i c a l e q u e l l o d e l l ' a s s e ; d a l c o l l e g a m e n t o f r a i d u e s i s t e m i , c h e d o v r e b b e e s s e r e r i g i d o , n e r i s u l l a p e r c i ò u n a s e n s i b i l e a z i o n e p e r t u r b a t r i c e . in q u e s t a n o t a d e s c r i v i a m o un n u o v o di l i p o di s m o r z a t o r e a d a r i a , g i à a d o t l a t o d a l l ' i s t i t u t o n a z i o n a l e di c.eofisica p e r i s i s m o g r a f i nuovo smorzatore per i sismografi tipo wiechert • a b d e g l i osservatori d e l l a rete s i s m i c a i t a l i a n a ; s m o r z a t o l e i d e a t o a p p o s i t a m e n t e p e r o v v i a r e a g l i i n c o n v e n i e n t i a v a n t i a c c e n n a t i . i n q u e s t o n u o v o s m o r z a t o r e ( f i g . 3 ) . il p i s t o n e è p o r t a t o d a l l a p r i m a l e v a di t r a s m i s s i o n e : e s s o r o i n o n f u n z i o n a p i ù c o m e m a — a d i 1111 s i s t e m a p e n d o l a r e , e q u i n d i n o n i n t r o d u c e u n a r e a z i o n e a l l o s p o s t a m e n t o d e l s i s m o g r a f o . c o n q u e s t a d i s p o s i z i o n e si l u i a n c h e il v a n t a g g i o d i e l i m i n a l e i fili d i s o s p e n s i o n e c h e r a p p r e s e n t a n o , c o m e è n o t o , u n a c a u s a d i i n c e r t e z z a n e l f u n z i o n a m e n t o d e l s i s m o g r a f o . al pisto n e o b b l i g a l o a s e g u i r e il mov i m e n t o d e l l a m a s s a del s i m o g i a l o . g i à a m p l i f i c a t o d a l l a p r i m a l e v a , e q u i n d i ad o s c i l l a r e i n t o r n o a l l ' a s s e di q u e s t a , è s t a l a d a t a la f o r m a di un s e t t o r e di toro. p e r s e m p l i c i t à d i c o s t r u z i o n e si è c e l l o il loro g e n e r a l o d a un q u a d r a t o r u o t a t i l e i n t o r n o a l l ' a s s e d e l l a l e v a , d i s p o s t o c o n 2 l a i i p a l n . l . r a l l e l i a l l ' a s s e st-e-so. k s o q u i n d i l i t t l l t l i m i t a l o ( f i g . i ) o r i z z o n t a l m e n t e d a d u e f a c c e m a n e p a r a l l l c , e v e r t i c a l m e n t e d a d u e s u p e r f i c i e c i l i n d r i c h e c o n c e n t r i c h e e d a d u e f a c c e su p i a n i v e r t i c a l i i n t e r s e c a n t i i s u l l ' a s s e d i r o t a z i o n e d e l l a l e v a p o r |.j„ | l a u t e . q u e s t o p i s t o n e è c o n t e n u t o i n u n a c a s s e t t a , il c u i i n t e r n o c o s t i t u i s c e u n o p a z i o d i f o r m a l o r i c a , l a c u i s e z i o n e v e l l i c a l e è q u e l l a s l e s s a d e l p i i o n e , l i e v e m e n t e p i ù g r a n d e o n d e r e n d e r e p o s s i b i l e l o s p o s t a m e n t o c i r c o l a r e d e l p i i o n e s e n z a c o n l a l l o . m a c o n s u f f i c i e n t e t e n u t a . l a c a s s e t a c ( f i g . 2 ) è fissala u l s u p p o r t o d e l l e l e v e m e d i a n t e l e i r e \ i l i i , , i e ' 3 c h e s e r v o n o p e r f a r l e a u m e r e l a p o s i z i o n e n e c e s s a r i a e r e n d e r e l i b e r o il m o v i m e n t o d e l p i s t o n e . l a v i t e o r i z z o n t a l e 1 „ s e r v e p e r r e g o l a r e l ' a p e r t u r a d e l f o r o d i e o m u n i c a z i o n • f r a l e d u e c a r n e i e d ' a r i a l a t e r a l i : l ' a p p o s i ta l e v a ! , c o n s e n t e d i l i m i t a r e o p p o r t u n a m e n t e la c o m u n i c a z i o n e d i d e t t e c a m e r e c o n r e s t e r n o . l a l i g . 3 m o s t r a u n s i s m o g r a f o m u n i t o d e l n u o v o s m o r z a t o r e . 1 p a r s e z i o n e mn l i e o l a r i d i c o s t r u z i o n e d e l l o s m o r z a t o r e s o ! j n o r i l e v a b i l i d a l l e s e z i o n i r i p r o d o t t e n e l l e li figlile 4 e •">. l ' i , 5 1 1 1 ,n : i f •f ' ^ j f m ' . b p. cai.01 " m) sl1iiio k s a m i n i a m o ora l ' i n f l u e n z a d e l l a r e a z i o n e del s i s t e m a p e n d o l a r e c o l i t u i t o d a l l o s m o r z a t o r e o r i g i n a l e d i w i e e h e r l sul p e r i o d o d e l i s m o g r a f o . i n d i c h i a m o con / il m o m e n t o d ' i n e r z i a del s i s t e m a r i s p e l l o a l l ' a s s e o r i z z o n t a l e o, a t t o r n o a cui a v v i e n e il m o v i m e n t o a n g o l a r e cp d e l l a m a s s a : con a la d i s t a n z a del putito di c o n t a t t o i \ d e l l a p r i m a l e v a d i n g r a n d i m c n t o , sul s u p p o r l o d e l l a m a s s a , d a l l ' a s s e ( ) ; con y r i n g r a n d i m e n l o elle il moto d i l \ s u b i s c e p e r effetto d e l l a p r i m a l e v a nel p u n t o di c o n n e s s i o n e p* fra q u e s t a l e v a e l ' a s s e del p i s t o n e : con , la r e a z i o n e p e r u n i t à d i s p o s t a m e n t o d e l l a m a s s a del s i s t e m a , i n / ' , , e con li., la r e a z i o n e p e r u n i t à di s p o s t a m e n t o d e l l a m a s s a del p i s t o n e , ili 1'.,. il moto non s m o r z a t o del i s t e i n a d e v e s o d d i s f a r e q u i n d i a l l ' e q u a z i o n e 1 — +à-/<|+/uy<() = 0 . ch'il p e r i o d o ila p e r t a n t o i e s p r e s s i o n e t0 = 2x\ 1 il t e r m i n e y k-,, c h e si trova nel d e n o m i n a t o l e d e l l e s p r e s s i o n e sotto r a d i c e , è (ptello clic dà u n a r i d u z i o n e d e l p e r i o d o del s i s m o g r a f o p e r effetto d e l l a r e a z i o n e d e l l o s m o r z a t o r e . nel nostro s m o r z a t o r e , es s end o k., — \), r ' il p e r i o d o è q u i n d i m a g g i o r e a p a r i t à di i n g r a n d i m e n t o . i l f u n z i o n a m e n t o dei s i s m o g r a f i d e l l ' i s t i l l i l o n a z i o n a l e di geofisica, n e i q u a l i è stato a d o t t a t o lo s m o r z a t o r e d e s c r i t t o , ha c o n f e r m a t o p i e n a m e n t e la p r a t i c i t à e l ' i m p o r t a n z a del p e r f e z i o n a m e n t o con esso r e a l i z z a t o . roma — istillilo ."vazionale di geo fisica — marzo 19-18. riassumo in t/uosla noia gli autori descrivono un nuovo tipo ili smorzatori' ad aria, già adottato dall' istituto nazionale di (.ivo fisica per i sismografi tipo ii iecliert (irgli osservatori della rete sismica italiana. con (jucslo nuovo smorzatore vengono eliminali alcuni inconvenienti riscontrati con i uso degli smorzato! i originali di ii irclicrt ed il funzionamento dei sismografi asiatici viene resa più seni/ilice, sicuro ed e f ficiente. special issue “the tomo-etna experiment” prologue [special issue_59_04_2016] the tomo-etna experiment. edited by josé morales and giuseppe puglisi. annals of geophysics issn 2037-416x [special issue_59_4_2016] the tomo-etna experiment prologue josé morales, giuseppe puglisi s0425 articles the tomo-etna experiment: an imaging active campaign at mt. etna volcano. context, main objectives, working-plans and involved research project jesús m. ibáñez, janire prudencio, alejandro díaz-moreno, domenico patanè, giuseppe puglisi, birger-gottfried lühr, francisco carrión, juan josé dañobeitia, mauro coltelli, francesca bianco, edoardo del pezzo, torsten dahm, veronica willmott, valérie mazauric s0426 tomo-etna experiment at etna volcano: activities on land jesús m. ibáñez, alejandro díaz-moreno, janire prudencio, domenico patanè, luciano zuccarello, ornella cocina, birger-gottfried lühr, francisco carrión, mauro coltelli, pier paolo g. bruno, francesca bianco, margaret hellweg, rafael abreu, gerardo alguacil, isaac álvarez, carolina aranda, carmen benítez, luisa buontempo, mercedes feriche, luz garcía, daniel garcía-quiroga, josé b. martín, josé morales, inmaculada serrano, manuel titos, lucia urbano, gianpiero aiesi, raffaele azzaro, graziella barberi, massimo cantarero, pasqualino cappuccio, danilo cavallaro, danilo contrafatto, sergio di prima, susanna falsaperla, marco firetto carlino, elisabetta giampiccolo, graziano larocca, carla musumeci, mario paratore, daniele pellegrino, mario pulvirenti, salvatore rapisarda, marco sassano, luciano scarfì, luciano scuderi, antonino sicali, giuseppina tusa, tiziana tuvè, edoardo del pezzo, stefano fiore, danilo galluzzo, mario la rocca, mariantonietta longobardi, luciano nocerino, solange scognamiglio, carla bottari, fabio criscuoli, pasquale de gori, lucian giovani, alfio messina, marcello silvestri, simone salimbeni, torsten dahm, araceli garcía-yeguas, alfonso ontiveros, enrique coello, maría cordero, cayetano guillén, m. carmen romero, hannah mccann, mauricio bretón, sierra boyd, ivan koulakov, sergey abramenkov s0427 the marine activities performed within the tomo-etna experiment mauro coltelli, danilo cavallaro, marco firetto carlino, luca cocchi, filippo muccini, antonino d’alessandro, maria eloisa claude, clara monaco, jesús m. ibáñez, fabrizio zgur, domenico patanè, cosmo carmisciano, giuseppe d’anna, maria teresa pedrosa gonzales, teresa teixidó, roberto d’anna, gioacchino fertitta, giuseppe passafiume, stefano speciale, fausto grassa, aristomenis p. karageorgis, lorenzo sormani, lorenzo facchin, giampaolo visnovic, diego cotterle, rita blanos, paolo mansutti, attilio sulli, fabrizio cultrera, francisco carrión, salvatore rapisarda s0428 seismic and volcanic activity during 2014 in the region involved by tomo-etna seismic active experiment graziella barberi, elisabetta giampiccolo, carla musumeci, luciano scarfì, valentina bruno, ornella cocina, alejandro díaz-moreno, simona sicali, giuseppina tusa, tiziana tuvè, luciano zuccarello, jesús m. ibáñez, domenico patanè s0429 acquisition and preliminary analysis of multi-channel seismic reflection data, acquired during the oceanographic cruises of the tomo-etna experiment marco firetto carlino, fabrizio zgur, pier paolo g. bruno, mauro coltelli, lorenzo sormani, danilo cavallaro, jesús m. ibáñez, domenico patanè s0430 acquisition procedures, processing methodologies and preliminary results of magnetic and rov data collected during the tomo-etna experiment danilo cavallaro, luca cocchi, mauro coltelli, filippo muccini, cosmo carmisciano, marco firetto carlino, jesús m. ibáñez, domenico patanè, marco filippone, erika buttaro s0431 p-wave scattering and the distribution of heterogeneity around etna volcano toni zieger, christoph sens-schönfelder, joachim r.r. ritter, birger-gottfried lühr, torsten dahm s0432 shallow velocity model in the area of pozzo pitarrone, mt. etna, from single station, array methods and borehole data luciano zuccarello, mario paratore, mario la rocca, ferruccio ferrari, alfio messina, stefano branca, danilo contrafatto, danilo galluzzo, salvatore rapisarda, luz garcía s0433 annals of geophysics advances on the automatic estimation of the p-wave onset time luz garcía, isaac álvarez, carmen benítez, manuel titos, ángel bueno, sonia mota, ángel de la torre, josé c. segura, gerardo alguacil, alejandro díaz-moreno, janire prudencio, araceli garcía-yeguas, jesús m. ibáñez, luciano zuccarello, ornella cocina, domenico patanè s0434 partos passive and active ray tomography software: description and preliminary analysis using tomo-etna experiment’s dataset alejandro díaz-moreno, ivan koulakov, araceli garcía-yeguas, andrey jakovlev, graziella barberi, ornella cocina, luciano zuccarello, luciano scarfì, domenico patanè, isaac álvarez, luz garcía, carmen benítez, janire prudencio, jesús m. ibáñez s0435 cetacean behavioral responses to noise exposure generated by seismic surveys: how to mitigate better? clara monaco, jesús m. ibáñez, francisco carrión, l. mario tringali s0436 annals of geophysics prologue in 1989 the first bases of what is now a strong and consolidated spanish-italian scientific relationship in seismology and in volcano-seismology were established. the first stage took place at the university of catania, in the former department of earth sciences, placed in the old science faculty, when a predoctoral student from the university of granada, cartuja observatory (now called andalusian institute of geophysics), arrived to complete his formation in seismology. during this stage, in addition to the training process, new contacts with researchers and mainly with pre-doctoral students of the former international institute of volcanology of the italian national research council (iiv-cnr) were stablished. these contacts have endured, grown and created a strong international scientific network with consolidated and credible research production. in a short time new working groups were integrated such as the osservatorio vesuviano in naples or the department of physics of the university of salerno from the italian side, and the department of applied physics of the university of almería or the volcanology department of the spanish research council (csic) in spain. at the present this non-official (but very active) network comprises many italian research centers belonging to the national institute of geophysics and volcanology (ingv) such as catania, naples, rome, palermo, pisa or bologna; italian universities as salerno, naples, l’aquila or calabrian, several spanish universities such as granada, almería, jaen, complutense, cádiz or la laguna, but also from other countries such as usa, russia, ireland, portugal, méxico, argentina, germany, france, norway or uk among others. this collaboration includes a continuous pre-doctoral students training protocols in which the exchange of fellows among the different institutions is fluent. a great achievement of this network has been the joint participation in many national, international and eu research projects, and the collaboration in several field experiments in different volcanoes around the world. each action included innovative ideas in term of instrumentation, data analysis and models, presenting a large number of high quality research papers in high impact journals. these works follow the philosophy of joint collaborative publications, in which in many cases more than 3 or 4 different institutions appear as participant of them. nowadays this policy is worldwide well considered, but in the 90s of last century this idea was something innovative being the work of ibáñez et al. [1990] the pioneer work of this consortium in which three institutions were involved. this idea of a joint work was also applied to doctoral thesis, being the first that done by ibáñez [1990] in which data and joint direction was established, but at the present more than 20 doctoral thesis have been done under this collaborative philosophy. one of the key advances of this teamwork was the use of seismic antennas in different volcanic environments. the first experiment was performed in teide volcano [del pezzo et al. 1997; almendros et al. 2000] using self-developed technology both in hardware and software [havskov and alguacil 2016]. the development of own products have permitted to the consortium to address more experiment with additional autonomy. this experience was exported primary to the antarctica, deception island volcano, producing an intense and high quality scientific production always under the joint collaborative relationship [e.g. almendros et al. 1997; ibáñez et al. 1997, 2000; alguacil et al. 1999; saccorotti et al. 2001, among others]. contemporaneously, different joint experiment were performed in several volcanoes around the world such as stromboli island [e.g. del pezzo et al. 1998; la rocca et al. 2000, 2004], mt. etna volcano [e.g. del pezzo et al. 2000; saccorotti et al. 2004a; ibáñez et al. 2009], vesuvius and campi flegri [e.g. del pezzo et al. 1999; la rocca et al. 2001; bianco et al. 2005], sao miguel in azores islands [e.g. saccorotti et al. 2004b; martini et al. 2009], copahue volcano in argentina [e.g. ibáñez et al. 2008a], volcán de fuego de colima in mexico [e.g. palo et al. 2009; petrosino et al. 2011; de lauro et al. 2012] or arenal volcano in costa rica [e.g. almendros et al. 2012, 2014] among other places. seismic attenuation analysis is probably the most productive research line of this consortium, both in tectonic and volcanic environment, and it is always present in whatever analysis or experiment performed by this team. the list of high impact papers published by this group is large and comprises different techniques [e.g. ibáñez et al. 1993; akinci et al. 1995; prudencio et al. 2013a, 2013b; del pezzo et al. 2016], places [e.g. del pezzo et al. 1995; bianco et al. 1999, martínez-arévalo et al. 2003; prudencio et al. 2015a] or focal depths [e.g. badi et al. 2009; mancilla et al. 2012]. some of the most remarkable results of these analyses are: a) the importance of scattering processes in volcanic environments [e.g. del pezzo et al. 1996; prudencio et al. 2015b]; b) how high intrinsic attenuation is not always associate with the presence of magma [e.g. de siena et al. 2010]; c) the possibility to perform separate seismic attenuation tomography to better constrain the inner structure of volcanoes [e.g. patanè et al. 2002; martínez-arévalo et al. 2005; prudencio et al. 2015c]. it is noteworthy that other research lines associated to volcano seismology have specific dedication inside of the consortium such as: precise location [e.g. saccorotti et al. 2002; carmona et al. 2010; díaz-moreno et al. 2015], moment tensor inversion [e.g. lockmer et al. 2007], shear-waves splitting and coda waves interferometry [e.g. martínez-arévalo et al. 2003; del pezzo et al. 2004; bianco and zaccarelli 2009; zaccarelli et al. 2009], among others. the necessity to know the inner structure of the studied volcanoes induced this consortium to focus their effort in the analysis of seismic tomography, mainly in velocity producing a set of high quality research products [e.g. chiarabba et al. 2004; patanè et al. 2006; zandomeneghi et al. 2008; alparone et al. 2012; garcía-yeaguas et al. 2014]. contemporaneously, these works showed the necessity of homogeneous distribution of high quality data and seismic stations, not always available in volcanic environments. the success of two important seismic active experiment performed in vesuvius volcano [e.g. gasparini 1998; auger et al. 2001] and campi flegrei [e.g. zollo et al. 2003] aimed this consortium to prepare similar active seismic experiments in other active volcanoes. the tomo-dec experiment carried out in january of 2005 in deception island caldera in antarctica was their first great success [e.g. barclay et al. 2009; ben-zvi et al. 2009; zandomenghi et al. 2009; garcía-yeguas et al. 2011]. the quality of the obtained data, and the exceptional international consortium created under the umbrella of this experiment, allowed this team to face a new challenge: to analyze a larger and complex region as the volcanic island of tenerife in the canary archipelago. thus the so called tom-teidevs experiment was performed in 2007 [e.g. ibáñez et al. 2008b; de barros et al. 2012; garcía-yeguas et al. 2012]. at the same time other active seismic experiment was performed around the stromboli island [castellano et al. 2008; prudencio et al. 2015a]. figure 1. two pictures showing the eruptive activity occurred in mt. etna volcano in the period june-november 2014. pictures kindly provided by (left) dr. jesús ibáñez from andalusian institute of geophysics; (right) by dr. boris benke from ingvsection of catania osservatorio etneo. the obtained results remarked the importance to integrate active and passive seismic sources and to use dense temporal seismic networks to study volcanic structures. these new data provide additional advantages that complement the information given by the earthquakes, mainly covering areas with lack of natural seismicity and eliminating the uncertainty of the position of the focal source. thus, when the eu project med-suv started to be conceived, one of the potential actions to be developed was to perform an ambitious seismic active experiment focused in the study of etna volcano, but also associated region, comprising aeolian island. in this framework the tomo-etna experiment was conceived with the idea of complementing the already available information of the largest active volcano in europe. taking into account the acquired experience of this consortium in previous experiments, and the potential high impact in research of the international collaborations, the tomoetna experiment was conceived. this experiment was designed to integrate marine and terrestrial activities and performing multidisciplinary approaches including wide angle seismic refraction (was), multi-channel seismic (mcs) reflection surveys, magnetic surveys and rov (remotely operated vehicle) dives. the complexity of this experiment, performed between june and november 2014, is reflected by the integration of different research projects from the eu, spain and germany. during this period an important volcanic activity took place in the volcano (figure 1). the experiment used several research vessels as: the spanish oceanographic research vessel (r/v) “sarmiento de gamboa”, the italian hydrooceanographic vessel (h/v) “galatea” and the greek oceanographic research vessel (r/v) “aegaeo” and two support vessels from the italian navy. in total 26 research and academic institutions from italy, spain, germany, ireland, usa, russia, france, greece and mexico were involved and including participation of more than 120 researchers and technicians. it also demonstrated the powerful capacity of the integration between different european funding schemes to support the research, namely the collaborative project (med-suv) and the coordination and support actions for integrating activities (eurofleet). in this special volume we present a set of representative works describing: the nature of the experiment; the activities developed on-land and offshore; the seismic and volcanic activity occurred during the experiment; some preliminary marine analysis; array studies; the analysis of the scattering properties of the wave-field; the preliminary signal processing analysis; the joint inversion tomography software; the impact of this experiment in the marine mammals life. according to previous experiences described above, scientific results will continue along next years (at least 10 years), expecting a high impact scientific productivity. josé morales giuseppe puglisi director coordinator eu med-suv project instituto andaluz de geofísica, istituto nazionale di geofisica e vulcanologia, universidad de granada, sezione di catania, osservatorio etneo, granada, spain catania, italy references akinci, a., del pezzo, e., and ibáñez, j.m. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ag_56.02.13_cover+front+index+intro+ackn_corrected_okok:layout 6 the editors thank the scientific committee for antarctic research for supporting the publication of this volume. [special issue_56_02_2013] grape, gnss research and application for polar environment, expert group of scar. edited by giorgiana de franceschi and maurizio candidi. annals of geophysics special thanks: the editors gratefully acknowledge the help of the following scientists who took some of their time to review the papers in this issue. battacharyya archana, indian institute of geomagnetism, mumbai, india cander ljiljiana, rutherford appleton laboratory, didcot, uk candidi maurizio, istituto nazionale di astrofisica, rome, italy cilliers pierre, south african national space agency, hermanus, south africa correia emília, instituto nacional de pesquisas espaciais, são josé dos campos, sp, brazil delladio alberto, istituto nazionale di geofisica e vulcanologia, rome, italy di mauro domenico, istituto nazionale di geofisica e vulcanologia, rome, italy materassi massimo, istituto dei sistemi complessi, cnr, florence, italy notarpietro riccardo, politecnico di torino, turin, italy sato natsuo, university of tokyo, japan spogli luca, istituto nazionale di geofisica e vulcanologia, rome, italy sreeja veettil, university of nottingham, uk terzo olivier, istituto superiore mario boella, turin, italy wang junhong, national center for atmospheric research, boulder, usa wernik andrzey, space research centre, warsaw, poland zuccheretti enrico, istituto nazionale di geofisica e vulcanologia, rome, italy special thanks go to scar for supporting the activities of the grape expert group. [special issue_56_2_2013] grape, gnss research and application for polar environment, expert group of scar preface grape, solar terrestrial physics in an operational environment giorgiana de franceschi, maurizio candidi p0215 articles an interhemispheric comparison of gps phase scintillation with auroral emission observed at the south pole and from the dmsp satellite paul prikryl, yongliang zhang, yusuke ebihara, reza ghoddousi-fard, periyadan t. jayachandran, joe kinrade, cathryn n. mitchell, allan t. weatherwax, gary bust, pierre j. cilliers, luca spogli, lucilla alfonsi, vincenzo romano, baiqi ning, guozhu li, martin j. jarvis, donald w. danskin, emma spanswick, eric donovan, mike terkildsen r0216 characterization of gps total electron content (gps-tec) in antarctica from 2004 to 2011 emília correia, amanda junqueira paz, mauricio a. gende r0217 electron precipitation events in the lower ionosphere and the geospace conditions josé henrique fernandez, emília correia r0218 measuring gnss ionospheric total electron content at concordia, and application to l-band radiometers vincenzo romano, giovanni macelloni, luca spogli, marco brogioni, giuditta marinaro, cathryn n. mitchell r0219 gps scintillations and total electron content climatology in the southern low, middle and high latitude regions luca spogli, lucilla alfonsi, pierre cilliers, emilia correia, giorgiana de franceschi, cathryn n. mitchell, vincenzo romano, joe kinrade, miguel angel cabrera r0220 thirteen years of integrated precipitable water derived by gps at mario zucchelli station, antarctica pierguido sarti, monia negusini, claudio tomasi, boyan h. petkov, alessandro capra r0221 probabilistic forecasting of ionospheric scintillation and gnss receiver signal tracking performance at high latitudes paul prikryl, veettil sreeja, marcio aquino, periyadan t. jayachandran r0222 eswua: a tool to manage and access gnss ionospheric data from mid-to-high latitudes vincenzo romano, silvia pau, michael pezzopane, luca spogli, enrico zuccheretti, marcio aquino, craig m. hancock r0223 the idipos project: is a multidisciplinary data infrastructure for weather and space weather feasible? vincenzo romano, luca spogli, alberto salvati, claudio rafanelli, lili cafarella r0224 annals of geophysics annals of geophysics preface grape, solar terrestrial physics in an operational environment brief historical outline the scientific committee for antarctic research (scar) action group gps for weather and space weather forecast (gwswf) was established in saint petersburg in july 2008, as a joint group that links the standing scientific groups of physical sciences (ssg/ps) and geosciences (ssg/gs). this happened in response to the invitation of the scar secretariat that initiatives should be encouraged to create cross-disciplinary groups to address new advanced scientific objectives. as detailed on the original webpage for the group, http://www.gwswf.scar.org/, the scientific framework for the operation of gwswf was designed to address the issue of lack of global navigation satellite system (gnss) high-rate sampling receiver coverage over the polar regions, and particularly in antarctica. the aims of the action group are to contribute to answering the questions that are still open within the sun-earth interactions studies. some of the issues of particular interest in the scientific international debate were: 1. characterization of the cause-effect mechanisms driving the formation and evolution of ionospheric irregularities; 2. distribution and evolution of precipitable water vapor in the polar regions that has a key role in the characterization and evolution of the global climate of the earth. the group is to address the following scientific objectives: 1. encourage the establishment of a permanent network of gnss receivers for multi-purposes investigations over the arctic and antarctica. 2. stimulate international collaboration in bi-polar investigations, particularly in conjugate regions. the objectives that were indicated as cross-linking the activities of ssg/ps and ssg/gs included: 1. ionospheric imaging over antarctica. 2. exchange of data and expertise for the application of tomography to other fields of interest for both communities (e.g., three-dimensional water vapor reconstruction). 3. exchange of technologies to install and manage remote global positioning system (gps) stations. 4. possibility to host instruments in the polar stations that the two communities manage. the xxxi scar delegates meeting in buenos aires, argentina (august 9-11, 2010) encouraged the action group gwswf to explore synergies with the geodetic infrastructure for antarctica (giant) and international polar earth observation network (polenet) groups of ssg-gs. the interhemispheric conjugacy effects in solar-terrestrial and aeronomy research (icestar) was reported to be considering ways to interact with gwswf, generating joint scientific objectives. at the scar 4th cross linkages meeting, may 5-6, 2011, in ottawa, canada, the chief officer of ssggs, reported that the action group gwswf intended to upgrade to an expert group, with revised terms of reference. it was noted that the new terms of reference overlapped with those of the icestar expert group, such that the two groups should discuss the best way forward. an action was established with ssg-ps and the chairs of the icestar eg to determine the way forward with regards to the new terms of reference of the gwswf, and to address the question whether they should be merged into a single expert group. in portland, usa, from july 23-25, 2012, at the xxxii scar ssg/ps and ssg/gs meeting, the delegates approved the conversion of the action group gwswf to the expert group gnss research and application for polar environment and weather and space weather forecast (grape/wswf). on that occasion, grape was also included in the framework of the new ssg/gs scientific research program solid earth response and influence on cryosphere evolution (serce) (http://www.scar.org/researchgroups/progplanning/serce_proposal_apr2012.pdf ), that states: the serce and grape (www.grape.scar.org) groups will work collaboratively to apply gnss data to better understand the cause-effect mechanisms driving the formation and evolution of ionospheric irregularities, to improve mapping of the distribution and evolution of precipitable water vapour in polar regions, and to encourage both use of current gnss data for multidisciplinary studies and to encourage establishment of a co-located network of high-rate gnss receivers specifically for research on climate and space weather phenomena. the current terms of reference for the expert group can be found at www.grape.scar.org. research framework according to the statement in the scar strategic plan (http://www.scar.org/strategicplan2011/ scar_strat_plan_2011-16.pdf ): antarctica also serves as a unique vantage point to look outwards from our planet to observe near-earth space, our solar system and beyond. in this statement, scar defines the motivation for research from antarctica concerning solar-terrestrial effects, and for the operation of the grape group. the physics of polar ionospheric phenomena was studied classically in the xx century, with passive remote observations made by dedicated instrumentation, both ground-based and satellite-borne auroral imagers, and riometer stations. active instrumentation was introduced with the ionospheric sounders. sporadic flights of sounding rockets have provided sparse in-situ data in this region that is not accessible to satellite in-situ observations. more recently, extended arrays of ionospheric radars (superdarn) have been established, and these are being deployed to cover the southern polar region in full. although maybe designed for other uses that are generally of commercial interest, a new tool has been recently made available with the development of satellite services, providing data as a side product of the gps infrastructure (gnss in perspective). gps signals propagate from the gps satellite sources to the receivers, most of which are ground based. the signals traverse the ionospheric layers situated along the geometrical path between source and receiver. the ionospheric plasma is highly inhomogeneous and dynamical, it can show the presence of small-scale structures or irregularities embedded in the large-scale ambient plasma. these irregularities can produce short-term phase and amplitude fluctuations in the carrier frequency of the radio waves which pass through them, which are commonly called ionospheric phase and amplitude scintillations. these fluctuations can be analyzed to investigate the physical processes causing them, and conversely, to understand how and when the operational capabilities of gnss receivers are affected, in order to develop countermeasures to improve their performance against the ionospheric threats. the special issue the collection of papers that forms this special issue represents the whole amplitude of research that is being conducted in the framework of grape, while also connecting to other initiatives that address the same objectives in regions outside the polar regions, and worldwide, such as the training research and applications network to support the mitigation of ionospheric threats (transmit; www.transmitionosphere.net), a seventh framework programme (fp7) marie curie initial training network that is focused on the study of ionospheric phenomena and their effects on systems embedded in our daily life, near-earth space data infrastructure for e-science (espas), an fp7-funded project that aims to provide the e-infrastructure necessary to support the access to observations, for the modeling and prediction of the near-earth space environment, concept for ionospheric scintillation mitigation for professional gnss in latin america (cigala) and its follow-up and extension countering gnss high-accuracy applications limitations due to ionospheric disturbances in brazil (calibra), both of which are funded by the european commission in the frame of fp7, for facing the equatorial ionosphere and its impact on gnss. the main objective of the present special issue of annals of geophysics is to collect recent reports on work performed in the polar regions and on the datasets collected in time by the instrumentation deployed across various countries. this collection will set the starting point for further research in the field, especially in the perspective of the new and very advanced space system that will be available in the next few years. in the following, papers will be found that describe the initiatives to deploy instrumental arrays to observe the ionospheric scintillation phenomenon (romano et al., measuring gnss...); to build hardware/ software structures to store the relevant data and to make it available in appropriate formats (romano et al., eswua: a tool...; and romano et al., the idipos project..., in the national context of the italian antarctic programme). other papers deal with more proper scientific analyses of the available data, ranging from the analysis of the relation between scintillation and conditions in the interplanetary medium, and originally on the sun (prykril et al.), also in their bipolar conjugate manifestation (prykril et al.), to the evaluation of the effects taking place in the near earth regions, in the inner magnetosphere (fernandez et al.), and in the statistical representation of ionospheric conditions (correia et al.); a climatological description of a scintillation scenario is given both for the polar regions and for the mid-latitudes (spogli et al.); a different, but no less relevant, analysis is given with respect to the water vapor content and its effects at tropospheric levels (sarti et al.). according to the content of the various papers, they belong to the grape work packages, as follows: work packages 1. s-t interactions and ionospheric effects in the current solar-cycle multi-instruments investigation of the upper atmosphere plasma dynamics and scintillation generation (superdarn, gnss, ionosondes, vlf, etc.). – paul prikryl, yongliang zhang, yusuke ebihara, reza ghoddousi-fard, periyadan t. jayachandran, joe kinrade, cathryn n. mitchell, allan t. weatherwax, gary bust, pierre j. cilliers, luca spogli, lucilla alfonsi, vincenzo romano, baiqi ning, guozhu li, martin j. jarvis, donald w. danskin, emma spanswick, eric donovan, mike terkildsen, an interhemispheric comparison of gps phase scintillation with auroral emission observed at the south pole and from the dmsp satellite. – emília correia, amanda junqueira paz, mauricio a. gende, characterization of gps total electron content (gps-tec) in antarctica from 2004 to 2011. – josé henrique fernandez, emília correia, electron precipitation events in the lower ionosphere and the geospace conditions. – vincenzo romano, giovanni macelloni, luca spogli, marco brogioni, giuditta marinaro, cathryn n. mitchell, measuring gnss ionospheric total electron content at concordia, and application to l-band radiometers. scintillation climatology, tec fluctuations, structure scale, c/n statistics, etc. – luca spogli, lucilla alfonsi, pierre cilliers, emilia correia, giorgiana de franceschi, cathryn n. mitchell, vincenzo romano, joe kinrade, miguel angel cabrera, gps scintillations and total electron content climatology in the southern low, middle and high latitude regions. 2. lower atmosphere delay in gnss-based systems (water vapor reconstruction, etc.) – pierguido sarti, monia negusini, claudio tomasi, boyan h. petkov, alessandro capra, thirteen years of integrated precipitable water derived by gps at mario zucchelli station, antarctica. 3. modelling and models testing – paul prikryl, veettil sreeja, marcio aquino, periyadan t. jayachandran, probabilistic forecasting of ionospheric scintillation and gnss receiver signal tracking performance at high latitudes. 4. data management strategy – vincenzo romano, silvia pau, michael pezzopane, luca spogli, enrico zuccheretti, marcio aquino, craig m. hancock, eswua: a tool to manage and access gnss ionospheric data from mid-to-high latitudes. – vincenzo romano, luca spogli, alberto salvati, claudio rafanelli, lili cafarella, the idipos project: is a multidisciplinary data infrastructure for weather and space weather feasible? 5. coordination with other programmes inside and outside scar (e.g. ursi, cawses ii, superdarn, eiscat 3d) this very introduction to the special issue details the framework of international cooperation in which grape is to be considered. giorgiana de franceschi maurizio candidi istituto nazionale istituto nazionale di geofisica e vulcanologia, rome, italy di astrofisica, rome, italy annals of geophysics, 56, 2, 2013, p0215; doi:10.4401/ag-6366. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. acknowledgements we acknowledge pnra (programma nazionale di ricerche in antartide) and cnr-dsstta (consiglio nazionale delle ricerche, dipartimento scienze del sistema terra e tecnologie per l'ambiente) for supporting the italian activities in the arctic and antarctica. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 1 strong-motion observations recorded in strategic public buildings during the 24 august 2016 mw 6.0 amatrice (central italy) earthquake chiara ladina*, simone marzorati, giancarlo monachesi, marco cattaneo, massimo frapiccini, viviana castelli istituto nazionale di geofisica e vulcanologia, italy *chiara.ladina@ingv.it abstract the marche region, in collaboration with ingv, has promoted a project to monitoring public strategic buildings with permanent accelerometer installed at the base of the structures. public infrastructures play a primary role to maintain the functionality of a local community. information about vibratory characteristics of the building and subsoil, in addition to the seismic instrumental history that describe the seismic shaking at the base of the structure are collected for each buildings. the real-time acquisition of seismic data allows to obtain accelerometric time history soon after the occurrence of an earthquake. the event of 24 august 2016 in central italy was an opportunity to test the functionality of this implemented system. in this work the parameters obtained from strong motion data recorded at the base of the structures were analyzed and the values obtained were inserted with some empirical relationships used to provide intensity microseismic values and damage indices. i. introduction he real-time acquisition of seismic data allows to define quickly the location and focal parameters of an earthquake, through the inversion of the data recorded directly on the surface by a seismic network. one of the immediate benefits is to be able to inform the civil protection authorities about the location and size of the earthquake, and its potential effects, in order that the procedure of intervention and relief are started as soon as possible. so, it is important to determine in a short time the seismic event impact on the territory, assuming mainly the areas with damage to civil and public infrastructures. t annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 2 public infrastructures, such as municipal buildings, hospitals, schools, police stations, firehouses, play a primary role in maintaining the functionality of a local community. for this reason, a project to monitor strategic public buildings, in the framework of the european holistic project (n. 1 st / 0001 project code, cofunded by european instrument for preaccession assistance, ipa adriatic cross-border cooperation) started in the year 2014 by the functional centre of the integrated security policies and civil protection department marche region (dpispc) and the ancona branch of national institute of geophysics and volcanology (ingv). currently in the marche region 14 testbuildings are monitored, through accelerometers installed at the base of the structures; in some cases the project made use of already installed low-cost accelerometers, in other cases new installations were required. seismic data are transmitted in real time and integrated into the seismic network resiico and then used for standard procedures of seismic monitoring [ladina et al., 2015]. this type of monitoring is not exhaustive for knowing the real dynamic behavior of structures during the seismic event and the damage of the structure, obtainable only by means of instruments installed in all parts of the building, as in the standard for the observatory of structures [dolce et al., 2015]. on the contrary, in this project, the aim is to deduce the likely effect suffered by the structure from the ground shaking at the site. the objective is monitoring considerable number of structures spread all over the territory, in order to identify, in a short time after the event, its impact on the territory and the possible disaster areas. the earthquake of 24 august 2016 in central italy was an opportunity to test the functionality of this implemented system by analyzing the strongmotion parameters obtained from strong motion data recorded at the base of the structures. in this work we make use of some empirical relationships to provide intensity macro seismics values and damage indices. ii.building monitoring strategic public buildings are important to maintain the administrative and social functions of a community. after catastrophic events such as earthquakes, it is necessary to estimate in a short time which is the impact and the effect on the territory, identifying the critical issues. the damage of strategic public buildings aggravates the emergency situation resulting from a destructive seismic event. being able to estimate as soon as possible how many strategic public buildings may have been damaged without directly detect the effects on the territory, contributes to program the civil protection assistance through a rapid response. the marche region, in collaboration with ingv, has promoted a project to enhance the accelerometer monitoring to get a better estimate of the seismic events in the region. some accelerometers have been installed at the base of public buildings in order to take advantage of wifi transmission networks and internet inside the structures and so integrate accelerometers in the seismic monitoring network [monachesi et al., annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 3 2013]. later, these accelerometers have been used, in addition to others, to develop seismic monitoring of strategic buildings in the european project holistic. on each of these buildings, environmental seismic noise measurements were performed to identify the fundamental periods of oscillation of the structures [ladina et al., 2015]. at present 14 accelerometers are installed at the base of strategic public buildings (figure 1 and table 1). the acquisition system is gaia2 and sampling rate 200 sps for all stations. in 12 cases, the strong motion stations are equipped with memes accelerometers (colibrys sf3000), cheaper than the standard force-balance accelerometers but anyway assuring a dynamic range adequate to record moderate and strong seismic events. in 2 cases (mntp and mrsc) the strong motion stations are equipped with accelerometers episensor fba es-t. then, in table 1 the eurocode 8 subsoil classes (ec8geo) [cen, 2004] and topographic index (ec8topo) is assigned to the stations. ec8geo is based on lithological and surface geology and classes are assigned according to eurocode8 part1 (class a for rock or other rock-like geological formation (vs30> 800m/s); other classes (b to e) for deposits with vs30 descending). ec8topo is defined in the italian technical norms [ntc, 2008], a simplified classification of landforms divided into four categories (t1 flat surface with i ≤ 15°; t2 slopes with average inclination i > 15°; then, reliefs with ridge top width much smaller than the base, and average inclination 15° ≤ i ≤ 30° for t3 and i > 30° for t4). stations have been classified for these parameters in the project holistic collecting the details of fundamental geological information and geotechnical investigations. the continuous data are transmitted and centralized to contribute to the monitoring of seismicity and then to locate and determine the parameters of earthquakes. continuous recording is useful to collect an instrumental database of ground motion accelerations suffered the structure, thus constructing a seismic history of all events undergone by the building. potentially, the data may be available after the identification of the seismic event as a result of the following procedures: detection, windowing, fine picking of arrival times [cattaneo et al., 2016] and calculating parameters. these procedures, already active at the headquarter of ingv-ancona, would make the data available after about 20 minutes. in particular, the aim is to store up the memory of all events that generated an acceleration greater than 0.001 g at the base of the structure; the value is the threshold beyond which the earthquake can be perceived by people, as shown by some empirical relationships [faenza and michelini, 2010; wald et al., 1999]. it is possible to estimate the impact sustained by a structure at its base, even if it is not possible to know the damage, and then hypothesize possible damage or immediately recommend a check of the structure. the evaluation takes place either by empirical relationships that provide an intensity degree, or by fragility curves that provide probability values for different levels of damage from strong-motion parameters. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 4 figure 1. map of the stations at the base of the strategic buildings (red triangles). epicenter of 24 august 2016 (red star). mcs intensity map modified by shakemap available online (http://shakemap.rm.ingv.it). the shakemap available does not cover the northern part of marche region. iii. data analysis of the 24 august, 2016 amatrice earthquake during the event of august 24, 2016 01:36 utc of magnitude mw 6.0 that struck central italy, 13 of the 14 strong motion stations were active. the distances of the stations from the epicentral area range from 25 to 135 km. in figure 2 the waveforms of the 6 accelerometric stations that registered the greater acceleration peaks are shown. the data transmission of three seismic stations (msm4, mtl1 and mcif) is not complete because of a power supply fault. for the station msm4 the recording lasts up to few seconds after the arrival of the s phase, but stop before the end of the most energetic part of the event. for the station mtl1 the recording includes the most energetic part of the event and stop 10 seconds after the s phase where the amplitude are already decaying. we are confident that the pga calculated value is reliable for mtl1 station. for all accelerometric data some strong-motion parameters were calculated, including peak ground acceleration (pga), peack ground velocity (pgv), housner intensity (hi). the accelerometric waveforms were filtered with bandpass filter between 0.2 20 hz. table 2 shows the results obtained for each station. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 5 code mun depi az buil typol n_f ec8geo ec8topo mmo1 monte monaco 24 19 town hall stone and bricks 3 a t2 msm4 monte san martino 41 25 school reinforced concrete 3 b t2 mntp montappone 52 21 town hall solid bricks and lime mortar masonry 3 b t2 crm1 castelraimondo 58 346 town hall solid bricks and lime mortar masonry 3 c t1 mrsc moresco 59 43 town hall stone and bricks 3 c t2 ssm1 san severino marche 59 355 town hall stone masonry and solid bricks 4 c t1 fiu1 fiuminata 60 335 town hall partially dressed stone masonry with good bonding 2 a t2 mtl1 matelica 64 343 town hall solid bricks and lime mortar masonry 3 c t1 tre1 treia 68 5 town hall solid bricks and lime mortar masonry 3 a t3 apec apecchio 115 325 town hall stone masonry 3 c t1 saiv sant’angelo in vado 126 328 town hall stone masonry and bricks 4 a t1 fano fano 128 352 town hall reinforced concrete 4 c t1 mcif montecalvo in foglia 133 338 town hall partially dressed stone masonry with good bonding 2 b t2 sscv sassocorvaro 133 333 town hall reinforced concrete 2 a t2 table 1: abbreviations, coordinates and technical characteristics of sites. station code (code), municipality (mun), epicentral distance (depi, km), epicenter/station azimuth (az, degrees from n), building type (buil), typology of building (typol), number of floors (n_f), soil (ec8geol) and morphological (ec8topo) categories. information about buildings from holistic project. figure 2. waveforms with greatest acceleration peaks. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 6 code hypod pgamax pgamean pgvmax pgvmean himax mmo1 24 138.48 127.233 6.669 5.880 20.552 msm4 41 66.732 64.245 3.284 3.260 7.417 mntp 53 80.575 75.939 4.309 3.626 12.793 mrsc 60 24.97 21.133 2.743 2.449 8.584 ssm1 60 59.684 35.175 2.589 1.761 7.363 fiu1 61 40.386 34.350 1.652 1.526 4.467 mtl1 65 56.316 52.710 6.048 4.593 21.786 tre1 69 68.233 64.952 5.743 4.457 16.855 apec 117 8.38 7.206 0.897 0.795 3.166 saiv 127 6.587 6.153 0.554 0.503 2.049 fano 130 22.764 20.770 2.769 2.750 11.015 mcif 134 3.270 3.151 0.257 0.227 0.864 sscv 135 7.787 7.717 0.889 0.741 3.332 table 2: results obtained for seismic stations. station code (code), hypocentral distance (hypod, km), maximum peak ground acceleration of horizontal components (pgamax, cm/s^2), geometric mean of peak ground acceleration of horizontal components (pgamean, cm/s^2); maximum peak ground velocity of horizontal components (pgvmax, cm/s), geometric mean of peak ground velocity of horizontal components (pgvmean, cm/s), maximum value of horizontal components of housner intensity (himax, cm). iv. intensity estimates starting from strongmotion parameters it is possible to apply some empirical relationships to estimate the intensity at the site and hypothesize a possible damage scenario. this information can be used as indicator to decide where to focus the investigation and assessment of damage, knowing the strong-motion parameters collected to the base of the structures. as a first test, the relationships of faenza and michelini [2010] were chosen, which are currently used to provide intensity maps from the production of shake maps (shakemap.rm.ingv.it). the reports use pga, pgv to indicate an intensity degree according to the mercalli-cancani-sieberg (mcs) scale. the maximum value of the horizontal component and horizontal geometric mean were used for the pga and pgv estimation. several authors [masi et al., 2010; pergalani et al., 1999; decanini et al., 2002; marcellini et al., 2004] indicate the housner intensity (hi) as more meaningful as damage indicator than pga and pgv. the hi estimates were included in the chiauzzi et al. [2012] relation obtained from the accelerometer data to the base of the buildings. the report use different coefficients for the threshold hi = 0.18 m and refers to the european macroseismic scale (ems98) intensity scale. the table 3 presents the results of intensity estimates according to various relations. although conscious that this is maybe an oversimplification to translate in ordinal values the numeric values resulting from the relations, we have used the annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 7 following procedure: if the value obtained is greater than half a degree and less than the lower and upper half-degree (for example, vi degree is assigned to values from 5.5 to 6.5). the intensities estimates are not always consistent between stations, even referring to different macroseismic scales, but indicate a trend. this trend is of course dependent on the distance of the stations from the epicentral area but do not respect it exactly. on average, the intensity resulting from the faenza and michelini [2010] regression are greater than those obtained by chiauzzi et al. [2012]. the building that may have been affected to a degree ≥ vi, and therefore potentially damaged for all relationships, is the nearest station (monte monaco, station code mmo1), about 20 km from the epicenter. this building was evacuated because it was damaged, by order of the mayor (26 of 31.8.2016, prot.3849 / 2016). the next-to-last building (monte san martino, station code msm4), whose distance from the epicenter is double than that of the mmo1, has an uncertain situation. code iems (hi) imcs (pgamh) imcs (pgagm) imcs (pgvmh) imcs (pgvgm) imcs (pgash) mmo1 6 7 7 7 7 7 msm4 5 6 6 6 6 8 mntp 5 7 7 7 6 7 mrsc 5 5 5 6 6 6 ssm1 5 6 6 6 6 7 fiu1 5 6 6 6 6 6 mtl1 6 6 6 7 7 6 tre1 6 6 6 7 7 7 apec 5 4 4 5 5 saiv 5 4 4 5 4 fano 5 5 5 6 6 mcif 5 3 3 4 4 sscv 5 4 4 5 5 table 3: results of intensity estimates according to various relations. code: station code. iems (hi): intensity from hi [chiauzzi et al., 2012]. imcs (pgamh): intensity from maximum pga of horizontal components [faenza & michelini, 2010]. imcs (pgagm): intensity from geometric mean of the pga of horizontal components [faenza & michelini, 2010]. imcs (pgvmh): intensity from maximum pgv of horizontal components [faenza & michelini, 2010]. imcs (pgvgm): intensity from geometric mean of the pgv of horizontal components [faenza & michelini, 2010]. imcs (pgash) punctual values of intensity obtained from shakemap in figure 1 using faenza & michelini [2010]. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 8 for this building are planned seismic upgrading work although during this earthquake it had no serious damage but only cracks at the seismic joint. its seismic data are incomplete and the peak of the events could not be transmitted. hi is then reduced by the lack of a part of the waveform; the indicated intensities may be considered a lower limit. for the group of stations between 50 and 70 km the intensity response range from vi to vii degree according to faenza and michelini [2010] and from v to vi degree according to chiauzzi et al. [2012]. this range of distances probably indicates a wider area of mild but widespread damage. finally, the group of stations situated more than 100 km from the epicenter, although still in areas where the earthquake was felt, are not indicated as potential damaged sites, having degree < vi according to all relationships. following a request to the municipalities involved, it was confirmed that there was no damage to these buildings. starting from the characteristics of the buildings and with an estimate of vulnerability, the strongmotion parameters pga and hi were compared with fragility curves [rota et al., 2008a; rota et al, 2008b; rota et al, 2008c]. these curves indicate the probability for a particular typology of building to overcome a certain state of damage. even if the application of the fragility curves are not directly applicable, they represent a mean vulnerability for this type of building [rota et al., 2008a]. figure 3 shows the results for the building of mmo1 station. the building has the following characteristics: 3 floors, the type of construction is masonry, irregular and flexible plans and without tie-rods. according to the classification of rota et al. [2008a], the building belongs to the category ima6. the results in figure 3 show that considering the pga, the building has 91% chance of passing the state of mild damage (ds1) and 58% to have exceeded the level of moderate damage (ds2). the estimates considering the hi are respectively 76% for the state ds1 and 36% for the ds2. in both cases, the probability of passing a ds1 damage remains high. with the aim to deduce the likely effect suffered by the single structure, the example of mmo1 station shows that the approach used is more informative compared to methods that averaged / interpolated on a surface (figure 1 and table 3) although it can give similar results. this result is related to a precise acceleration point measurement to the ground. in fact, the municipal building was damaged, but it was an isolated case inside the residential area as shown by the v degree assigned by the macroseismic survey [quest w.g., 2016]. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 9 figure 3. results for the building of mmo1 station modified by rota et al [2008]. a) fragility curve in pga for the typology ima6. b) fragility curve in hi for the typology ima6. v. conclusions the monitoring of strategic public buildings is important to check the status of the structures and collect instrumental seismic history and helps to evaluate the damage scenario following a destructive earthquake in the area. a monitoring approach is in testing phase, using a single accelerometer placed at the base of a structure with low-cost instrumentation, with the dual purpose of improving the estimates of shaking on the territory increasing the points at which accelerometer data are recorded and evaluating the potential damage of public buildings to contribute to the rapid response of civil protection. during the earthquake that struck central italy on 24 august 2016 (mw 6.0), ground accelerations were recorded at the base of public structures with seismic instrumentation. starting from the strong-motion parameters calculated from the waveforms, pga, pgv and hi have been applied to the empirical relationships to estimate a degree of macroseismic intensity and a damage index through fragility curves. for different buildings, placed in a range of 25 to 135 km from the mainshock epicentral area, the results of the intensity relations indicate that the only building with a high probability of being damaged is the closest one (mmo1, i ≥ vi), for the buildings located at the distance between 50 and 70 km, our results indicate lower degrees of damage though in some buildings minor damage cannot be excluded. for the building which has undergone the largest impact, fragility curves corresponding to the type of construction have been applied to estimate a damage index based on an average vulnerability. for the mmo1 station, the estimates indicate a high probability of passing the state of mild damage, both for the pga and for the hi. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 10 the results show that an extension of this kind of seismic monitoring throughout the region would be a useful tool to define, remotely and quickly, an approximate scenario of the impact of future earthquake on strategic public buildings. this type of monitoring, although unsuitable to determine with certainty the degree of damage suffered by each structure, can be a useful tool for civil protection management and decision-making in emergency. acknowledgments the authors thank emanuela ercolani for helpful conversation about macroseismic survey. the authors thank francesca sini and gianni scamuffa for information about buildings from holistic project. figure 1 was created using arcgis®. references [cattaneo et al., 2016] cattaneo, m., frapiccini m., ladina c., marzorati s., monachesi g., a mixed automatic-manual seismic catalog for centraleastern italy: analysis of homogeneity, 2016, annals of geophysics, submitted. [chiauzzi et al., 2012] chiauzzi, l., masi, a., mucciarelli, m., vona, m., pacor, f., cultrera, g., gallovič, f. and emolo, a. (2012). 10: 517. bull earthquake eng doi:10.1007/s10518-011-9309-8. [cen 2004] comité européen de normalisation pren 1998-1eurocode 8: design of structures for earthquake resistance. part 1: general rules, seismic actions and rules for resistance. part 1: general rules, seismic actions and rules for buildings, 2004, brussels. [decanini et al., 2002] decanini, l., mollaioli, f. and oliveto, g. (2002). structural and seismological implications of the 1997 seismic sequence in umbria and marche, italy. in innovative approaches to earthquake engineering, g. oliveto (editor), southampton, wit press, 229-323. [dolce et al., 2015] dolce, m., nicoletti, m., de sortis, a., marchesini, s., spina, d. and talanas, f. (2015). osservatorio sismico delle strutture: the italian structural seismic monitoring network, bull. earthquake eng., 1-21, doi:10.1007/s10518015-9738-x. [faenza and michelini, 2010] faenza, l. and michelini, a. (2010). regression analysis of mcs intensity and ground motion parameters in italy and its application in shakemap, geophys. j. int.,180,1138–1152. [ladina et al., 2015] ladina, c., marzorati, s., frapiccini, m., monachesi, g., cattaneo, p., paratore, m., sini, f. and ferretti, m. (2015). attività sperimentali per l’implementazione del monitoraggio sismico di edifici pubblici nella regione marche. quaderni di geofisica ingv, 127, issn 1590-2595 (in italian). [marcellini and pagani, 2004] marcellini, a. and pagani, m. (2004). seismic zonation methodologies with particular reference to the italian situation. recent advances in earthquake annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7194 11 geotechnical engineering and microzonation, a. ansal (editor), kluwer academic publisher, printed in the netherlands. [masi et al, 2010] masi, a., vona, m. and mucciarelli, m. (2010). selection of natural and synthetic accelerograms for seismic vulnerability studies on rc frames. journal of structural engineering, asce special issue devoted to “earthquake ground motion selection and modification for nonlinear dynamic analysis of structures”. doi: 10.1061/(asce)st.1943541x.209. [monachesi et al., 2013] monachesi, g., cattaneo, m., ladina, c., marzorati, s., frapiccini, m., carannante, s., ferretti, m., sebastianelli, m., delladio, a. and selvaggi, g. (2013). esperienze di monitoraggio integrato: il caso della rete sismometrica dell’italia centro orientale, di quella accelerometrica marchigiana e dei suoi servizi, quaderni di geofisica, issn: 1590-2595, 106, pp.30. [ntc, 2008] nuove norme tecniche per le costruzioni. part 3: categorie di sottosuolo e condizioni topografiche, gazzetta ufficiale n. 29 del 4 febbraio 2008. [pergalani et al., 1999] pergalani, f., romeo, r, luzi, l., petrini, v., pugliese, a. and sanò, t. (1999). seismic microzoning of the area struck by umbria–marche (central italy) ms 5.9 earthquake of 26 september 1997. soil dynamics and earthquake engineering, 18, 4, 279-296. [quest w. g. 2016] quest w.g, the 24 august 2016 amatrice earthquake: macroseismic survey in the damage area and ems intensity assessment, annals of geophysics, vol. 59, fast track 5, 2016. [rota et al., 2008a] rota, m., penna, a. and strobbia, c. l. (2008a). processing italian damage data to derive typological fragility curves. soil dynamics and earthquake engineering, 28(10-11), 933-947. [rota et al., 2008b] rota, m., penna, a., strobbia, c. l. and magenes, g. (2008b). direct derivation of fragility curves from italian post-earthquake survey data, the 14th world conference on earthquake engineering, october 12-17, beijing, china. [rota et al., 2008c] rota, m., penna, a. strobbia, c. l. and magenes, g. (2008c). derivation of empirical fragility curves from italian damage data, iuss press, research report rose 2008/08. [wald et al., 1999] wald, d.j., quitoriano, v., heaton, t.h. and kanamori, h. (1999). relationships between peak ground acceleration, peak ground velocity, and modified mercalli intensity in california, earthq. spectra, 15(3), 557– 564. s0435 annals of geophysics, 60, 4, 2017, s0435, doi: 10.4401/ag-7140 analysis of the seismic site effects along the ancient via laurentina (rome) francesca bozzano1, anna buccellato2, fulvio coletti2, salvatore martino1, fabrizio marra3, stefano rivellino1, chiara varone1,* 1 research centre for geological risks (ceri), university of rome “la sapienza”, rome, italy 2 soprintendenza speciale per il colosseo, museo nazionale romano e area archeologica di roma, rome, italy 3 istituto nazionale di geofisica e vulcanologia, rome, italy article history received september 1, 2016; accepted may 10, 2017. subject classification: seismic site effects, numerical modelling, ancient via laurentina, rome abstract this paper presents an evaluation of the local seismic response (lsr) along the route of the ancient roman road via laurentina, which has been exposed in several areas of southwest rome over the last decade during the construction of new buildings and infrastructures. it is an example of lsr analysis applied to ancient and archaeological sites located in alluvial valleys with some methodological inferences for the design of infrastructure and urban planning. since the ancient road does not cross the alluvial valley (namely the fosso di vallerano valley) normal to its sides, it was not possible to directly perform 2d numerical modelling to evaluate the lsr along the road route. therefore, outputs of 2d numerical models, obtained along three cross sections that were normal oriented respect to the valley, were projected along the route of the via laurentina within a reliable buffer attributed according to an available high-resolution geological model of the local subsoil. the modelled amplification functions consider physical effects due to both the 2d shape of the valley and the heterogeneities of the alluvial deposits. the 1d and 2d amplification functions were compared to output that non-negligible effects are related to the narrow shape of the fluvial valley and the lateral contacts between the lithotecnical units composing the alluvial fill. the here experienced methodology is suitable for applications to the numerical modelling of seismic response in case of linear infrastructures (i.e., roads, bridges, railways) that do not cross the natural system along physically characteristic directions (i.e. longitudinally or transversally). 1. introduction several studies, focused on local seismic response in rome [bozzano et al. 2008, bozzano et al. 2012, bozzano et al. 2016, caserta et al. 2012, martino et al. 2015]; [rovelli et al. 1994, 1995], have already highlighted that the urban areas can be affected by amplification of seismic shaking. the heterogeneous composition of the alluvial deposits of the tiber river and its tributaries is responsible for both 1d amplification effects, which are mainly related to soil stratigraphy and 2d amplification effects, which can be related to wave refraction at the edge of the valley as well as lateral heterogeneities [martino et al. 2015]. while 1d amplification can be evaluated by simple 1d numerical simulations, to evaluate the 2d amplification, the numerical models should take into account the effects related to the shape of the valley and the lateral heterogeneities of the alluvial deposits. this study focuses on the local seismic response (lsr) along the ancient roman road via laurentina (figure 1), which was recently excavated in the fosso di vallerano valley, a left tributary valley of the tiber river. in the literature, the identification of the via laurentina has long been considered secondary to the question of the existence of laurentum and has been linked to the hypothesis of two roads leading to the coast, both of which branch off from the via ostiense road. the western road is thought to correspond to the line formed by the via pontina vecchia road (which replaced the via di decima road in the 1930s) and the via di pratica road. the eastern road initially corresponds to the modern via laurentina road, then follows the via di trigoria road and finally joins the via di pratica road in the direction of lavinio. the identification of laurentum with lavinium and castagnoli’s identification of the town as the vicus augustanus laurentium of later written sources has favoured the recognition of the via laurentina as the road that leads to the town of the laurentes, which is lavinium, the former route being that followed by the via pontina vecchia road. the idenbozzano et al. 2 tification of the town was later confirmed by evidence from excavations conducted by the “soprintendenza speciale per il colosseo il museo nazionale romano e l’area archeologica di roma” (sscol), mnr (museo nazionale romano) and “area archeologica di roma”, between 2007 and 2016, which documented the typical characteristics of the viae publicae. in particular, virgilian records state that the route crosses the valley of the fosso di vallerano basin, ascends a tufa ridge in the tor de’ cenci estate and descends through the valley of the malafede river to the coast; it runs nne-ssw and cuts all the other valleys at a significant angle with respect to the river courses [ascani et al. 2008 and references within]. the choice of this route for the road, which implies in-depth knowledge of the territory during the roman period and even in the proto-historic period, was based on linking routes from castel di decima, probably tellenae, which was one of the autonomous centres of latium vetus that, according to pliny, were destroyed by ancus marcius during rome’s expansion toward the coast. in the fosso di vallerano valley, the road is interesting in terms of the lsr because the ancient route does not cross the fluvial valley normal to the valley sides, which is the expected orientation of 2d effects due to the interaction between the main resonance and lateral seismic waves [bard and bouchon 1985]. as a result, a 2d numerical model of a representative geological cross section of the ancient via laurentina route cannot represent the real 2d response that can occur along a section that is oriented normal to the sides of the valley. to provide a suitable solution to this problem, several cross sections that were oriented normal to the valley were analysed to determine the “physically-based” seismic response of the natural system, and the results were applied to the route of the ancient road by projecting them based on an engineering-geological model of the site. the aim of this study is to propose and demonstrate a reliable methodology that can be applied to seismic response evaluations of linear infrastructure (i.e., roads, bridges, railways) that do not cross the natural system along physically characteristic directions (i.e. longitudinally or transversally). in the case of archaeological structures that are mainly linear (e.g., aqueducts, bridges or walls), such an approach can contribute to their preservation and resilience to expected seismic events. 2. archeological setting of the ancient via laurentina the excavations that have been carried out by sscol have exposed more than one kilometre of the road and have documented the characteristics of the major roman roads and the interventions that were necessary for their management, including: 1. land reclamation, excavation of trenches and dumps of earth and gravel; 2. a road bed width of at least 4 m to allow the simultaneous transit of carts in both directions; 3. construction on foundation layers of large stones (rudus) and a core of crushed stone and paved with perfectly abutting polygonal basalt blocks that form a crowned surface. the roadbed is bordered by edge stones (umbones) that are placed with the flat face on the interior and the wedged side facing outwards, which contained the sidewalks of beaten gravel. gonphi, which were taller stones that prevented the carts from mounting the sidewalks and helped travellers to get on and off the carts, were spaced along the sidewalks. sections paved with earth with a gravel surface (glareata) or basalt, which can be dated to the first century of the empire based on the associated funerary figure 1. views (from s) of the via laurentina road (mile 6). the two towers of the “europarco business park” can be seen in the background. seismic response in ancient via laurentina 3 contexts, are present in several areas. starting from the south between miles 8 and 9 (figure 2) below the via pontina road outside the highway grande raccordo anulare (near the junction with the via degli eroi di cefalonia), two levels of paving were documented at 52.55 m a.s.l. for a length of c. 150 m, which show evidence of intense traffic and maintenance work. in both phases, the road, which was between 4 and 6 m wide, was composed of a bed of stone chips mixed with gravel that was bordered by two rows of basoli and was contained within two opus reticulatum structures, which were built to protect the road from the effects of ground wash (figure 3, frame 1) [buccellato 2005]. the section between miles 5 and 6 was crucial for the identification and recognition of the structural and infrastructural characteristics (figure 3, frame 2). the segment that was discovered on a rise, near the point where the modern via decima and via pontina roads meet, represents the earliest evidence of the road from the late archaic period. in the southern sector, which runs along a ridge, the earliest phase of the road surface that can be identified from cart tracks that are directly on a layer of pyroclastic material can be dated to the late archaic early republican periods based on the stratigraphic sequence and the typology of two chamber tombs in the eastern wall of the road cut. the third level, which had an earth and gravel surface and was dated to the 2nd century b.c. based on the discovery of a coin, revealed restructuring work, including the rectification of the axis and the use of more complex technical solutions, to which the inscription probably refers. further numismatic evidence dates the re-laying of the road surface to after the 2nd century b.c. the new surface was composed of gravel bonded with mortar and was raised by the use of a substantial layer of leucitite stones and was partially supported with tufa blocks in a depression. the northern sector of the final level, which includes basalt paving that dates to the julio/claudian period, represented an important structural work that indicates that geometric and geological criteria were used to calculate the line of the road. the saddle within which the road formed a sharp bend was artificially transformed into a large cut that was 15 to 20 m wide at the summit and 9 m wide at the base. the walls are vertical or terraced in layers of pozzolan sediments that alternate with paleo-soils, while in the lower levels, the clays are up to 5 m high. this allowed the route from the hill at 35.38 m a.s.l. to connect to the plain that leads to the castellaccio valley at 21.38 m a.s.l. with a gradient of 13%. the northern part passed through a ravine that showed substantial anthropic activity. the road showed several signs of use. the cart tracks have an average inter-axis width of 1.4 m. the width of the road increased from 4.1 m to 6 m in the steeper segments to allow vehicles to pass each other and at stopping points near funerary areas. inside the cut, the road was flanked on the east by segments with leucitite paving, which was mainly for pedestrian use. at mile 5, which is below the present via di decima and via dell’acqua acetosa ostiense roads, the first stretch was 4.20 m wide, paved in perfectly adhering basalt polygons, and bordered by a row of basoli placed on edge (umbones, including a gomphus) and a 1.30-m-wide sidewalk made from stone chips. the second segment, which is 20 m long, showed evidence of a later restoration, was up to 6 m wide, and indicates the importance of the route even in a period of general abandonment that can be correlated with the shift of the economic epicentre away from the ostiense area by constantine. descending into the fosso di vallerano basin in the alluvial plain of the ciuccio, acqua acetosa and vallerano creeks, which are tributaries of the tiber, a straight stretch of the road that is approximately 500 m long was exposed at castellaccio [buccellato 2007, buccellato and coletti 2013 and references within]. it showed several levels of construction and maintenance from the archaic to late antique periods, which documented the organisational and technical efforts of the roman state, particularly regarding the need to keep the roadbed above the water table (figure 3, frame 3). figure 2. map of the common “agro romano”: route of the ancient via laurentina, with roman milestones (black line); the red numbers indicate the sites in figure 3. bozzano et al. 4 stratigraphic relationships indicate that the first level, which had a paved of earth and gravel surface, was in use in the 2nd century b.c., but it was probably in use as early as the late 4th century b.c., as was suggested by the first phase of an important necropolis and the rural building located at the junction with a branch road. during the 1st century b.c., a substantial new, raised road surface of earth and gravel, which was supported by an opus incertum wall, was built. a more technically complex intervention occurred in approximately the middle of the 1st century a.d.; the road surface, which was an average of 6 m wide, was raised and paved with basalt polygons, which were contained by opus reticulatum walls that were buttressed at regular intervals. in addition, infrastructure that was probably made of timber made up figure 3. (1) intersection of the via pontina road with the via degli eroi di cefalonia road: photo and section of street levels; (2) intersection of the via pontina road with the via di decima road: photo and section of street levels; (3) basin of the fosso di vallerano (loc. castellaccio): photo, planimetry and section of street levels; 4. basin of the fosso di vallerano: photo and section of street levels. seismic response in ancient via laurentina 5 a land drainage system for the catchment area of the three tributaries of the tiber. this regimentation of the acqua acetosa stream, which has been identified across the entire plain, was intended to reclaim the areas that were subject to flooding and to drain the floodwaters of the tiber. the infrastructure consisted of the improvement of the riverbanks with a lining of opus reticulatum that was c. 2 m high. the road, which ran north-south, crossed the river in the stretch that is presently buried by an arched bridge, of which only the brick parapets survive, that was presumably built as part of the restoration in 177 a.d. based on an inscription that refers to marcus aurelius or commodus. the strict spatial and functional correlation with the acqua acetosa river, which is identified with the border of the ager romanus antiquens, denotes cult interest from proto-historic times at the site. based on the milestones that begin at the servian walls, this site is located within the 6th mile, where sources record the presence of the ancient sanctuary of the god terminus, which was the site of the celebration of the terminalia or lustratio of the borders. this was one of the cults and sanctuaries that were located along the roads that led into the city and delimited the urban territory as early as the late 8th or early 7th centuries b.c. the interventions by the roman state, which had been defined by maintenance and rebuilding, now turned to the creation of infrastructure. this is indicated by the presence of a necropolis along the road, which was dated to the republican and imperial periods, by a rural building that dates to between the 4th and 2nd centuries b.c. at the location where the via laurentina met a secondary road that headed east along the original line of the acqua acetosa ostiense ditch, and by a large architectural complex that covers c. 1380 m2 along the stretch where the basalt paving is preserved. finally, like all of the consular roads, the laurentina was flanked by a necropolis that covered c. 1000 m2 and included more than 250 tombs, mainly inhumation burials and some cremations. the stratigraphy of the burial levels and analyses of the grave contents indicate at least three phases of use between the late 4th and early 3rd centuries b.c. and during the first two centuries of the empire. the road was rebuilt in the late antique period, between the 3rd and 4th centuries, by reusing the surface from the 2nd century. this temporary intervention, which was limited to a few stretches, narrowed the road to 4.50 m wide, removed its borders and shifted the central axis with respect to the original line. during subsequent periods until the middle ages, parts of the road earthwork fell into disuse and were radically altered and heavily robbed for construction materials (figure 3, frame 4). a final 50-m-long stretch was uncovered on the edge of the urbanised area of eur quarter, which has compromised the preservation of the ancient road. this section documents the interventions that were performed on the earliest levels of the road that date to the early republican period, including an increase in the level of the road surface to 8.97 m a.s.l. with rows of tufa blocks up to one metre high that rest directly on sterile soil. through various reconstructions, the third earth and gravel road, which dates to the early imperial period, was 6 m wide and was supported by the canonical structure, contained by rows of stones and flanked by sidewalks. 3. geological setting the city of rome is located on the coast of the tyrrhenian sea in central italy, sw of the apennine mountain chain, and is crossed by the tiber river. after the main development of the central apennine thrust and fold belt, crustal extension has affected the area of rome since the late messinian due to the development of the tyrrhenian back-arc basin [parotto and praturlon 1975]; [sani et al. 2004]. neo-autochthonous marine sedimentation occurred from the pliocene to the early pleistocene, which led to the deposition of the two transgressive cycles of monte vaticano and monte mario formations [bonadonna 1968, conato et al. 1980, marra 1993, marra and rosa 1995]; [cosentino et al. 2009]. since the late pliocene, continuous regional uplift has caused the progressive shift to continental sedimentation that is provided by a primitive drainage network controlled by the paleo-tiber river [marra and florindo 2014]. the area around rome has hosted the delta of the paleo-tiber river since the middle pleistocene [milli et al. 2013], and its geologic evolution is the result of complex geological processes, including tectonics, volcanism and glacio-eustatic fluctuations [karner et al. 2001, and references therein]. the continental to coastal sedimentary successions of the paleo-tiber river have been deposited since 800 ka in response to sea-level rise during the pleistocene glacial terminations, which have been demonstrated by a series of recent studies that used the 40ar/39ar ages of tephra intercalated within the sedimentary deposits to provide geochronologic constraints that link these aggradational successions to the marine isotopic stages (mis) [karner and renne 1998]; [karner and marra 1998]; [marra et al. 1998]; [florindo et al. 2007]; [marra and florindo 2014]; [marra et al. 2016]. the aggradational successions near rome therefore represent a discontinuous stratigraphic record that is composed of a succession of ten major aggradational bozzano et al. 6 units that were deposited during mis 22-21 thorough mis 2-1 and several minor successions that correspond to the more pronounced sub-stages and represent the physical remnants of the glacio-eustatic sea-level cycles during this time period. these successions fill the fluvial valleys and the coastal plain incisions that were excavated during the sea-level lowstands and interfinger with the pyroclastic products of the colli albani and monti sabatini volcanic districts, whose activity spanned 600-20 ka [marra et al. 2009, 2014]; [gaeta et al. 2016 and references therein]. a thick succession of pyroclastic flow deposits and subordinate air-fall deposits interfingers with the continental sediments that were deposited within palaeovalleys during periods of sea-level rise, which caused significant lateral and vertical facies variability. since the middle pleistocene, the volcanic activity has been mainly responsible for the present landforms in terms of the production of rocks that shape the natural landscape, particularly on the left side of the tiber river [del monte et al. 2013]. however, after the most recent volcanic activity in the upper pleistocene, the volcanic plateau was deeply incised during the last glacial maximum, and the palaeovalleys were then filled by thick packages of fluvial deposits [bozzano et al. 2000, mancini et al. 2013, marra et al. 2013]. according to bozzano et al. [2016], 4 main lithostratigraphic groups were identified in the fosso di vallerano valley area: figure 4. geological cross section along the route of the ancient via laurentina (shown in figure 7). legend: recent alluvial deposits: 1) anthropic filling material (al-af); 2) sandy clays characterized by a significant volcanic component (al-vsc); 3) peaty clays (al-pc); 4) clays and silts (al-cs); 5) sands and silty sands (al-sd); 6) polygenic and heterometric gravels (al-gr). pre-wurmian deposits (valle giulia san paolo aurelia vitinia formations): 7) fluvio-palustrine deposits composed of loose gravels, sands and silts (fl). volcanic deposits from the alban hills district (vc): 8) pozzolanelle; 9) pozzolane nere; 10) conglomerato giallo; 11) pozzolane rosse; 12) vallerano lava; 13) tufo terroso con pomici bianche; 14) tufo del palatino; 15) tufo pisolitico di trigoria. paleo-tiber 4 deposits (santa cecilia formation): landslide deposits: 16) clay and silty clay (pt-ls, cl) 17) loose gravels (pt-ls (gr)); 18) sands and silty sands. (pt-sd); 19) clays and silts with peaty layers (pt-cl); 20) loose gravels (pt-gr). plio-pleistocene bedrock (marne vaticane monte mario monte delle piche formation): 21) marine clays and silty clays; 22) marine sands and silty sands. 23) fault. 24) borehole. the black marker indicates the urbanized portion of the cross section, and the red marker indicates the discovered portion of the ancient via laurentina. red numbers indicate the locations of the photos shown in figure 3. the locations of the noise measurement points and seismic station v are also indicated. figure 5. photo of the decima archaeological site. the outcropping volcanic formations are labelled, and the stratigraphc limits are shown by the dashed white lines. seismic response in ancient via laurentina 7 plio-pleistocene marine sediments (pp) (marne vaticane, monte mario and monte ciocci/delle piche formations) that represent the geological bedrock; pleistocene alluvial sediments deposited by the paleo-tiber river (pt) (santa cecilia formation; 650-600 ka; marra and florindo, [2014]; volcanic deposits that erupted from the alban hills and the monti sabatini volcanic district (vc) (561-365 ka); recent alluvial deposits that have filled the incised valley from the end of the würmian regression to the present (al) (18 ka-present); a geological cross section (figure 4) was derived just along the route of the ancient via laurentina as reconstructed by sscol (figure 7). this section is therefore not geometrically cut respect to the valley and its direction continuously changes depending on the route of the ancient roman road. the southern part of the section corresponds to an inhabited area (torrino hill) that is characterized by the presence of vc deposits. to the north, the section crosses the alluvial plain of the fosso di vallerano valley. this portion is characterized by al deposits that overlap the pt deposits, which results in a horst and graben structural setting due to the synsedimentary pleistocene tectonic activity. along the track of the ancient via laurentina that corresponds to the decima archaeological site, a portion of the alban hill volcanic sequence outcrops because of the excavation of the hill during the construction of the ancient via laurentina (figure 5). 4. geophysical data as reported by bozzano et al. [2016], from june until july 2009, a free-field seismic array in short time average to long time average (sta/lta) acquisition mode was installed in the fosso di vallerano valley to record weak-motion events during the tail of the l’aquila seismic sequence. the array was composed of two stations (figure 7) that were instrumented by three single component 1 hz velocimeters (ss1 kinemetrics), which were triaxially arranged and connected to a 24 bit data logger (k2 kinemetrics) and a gps device for absolute timing. one station (v) was located on the alluvial deposits in the ne part of the valley (figure 7), while a reference station (r) [sensu borcherdt 1994] was located on seismic bedrock in the volcanic hills that border the valley (figure 7). the seismic array recorded approximately 30 earthquakes with magnitudes from 2.6 to 4.6. a 5% cosine-taper window starting 1 s before the p-phase onset was applied to the earthquake records to obtain signals with durations of 100 s. the signals were converted to the frequency domain and pass-band filtered in the frequency range of 0.2-10 hz. one of the earthquakes figure 6. time-history (a) and fourier spectrum (b) of the earthquake recorded at the fosso di vallerano valley on 23 june 2009 here used for the visco-elastic numerical modelling. figure 7. location of the ancient via laurentina road in the southern part of rome and sites of noise measurements along the route. the shaded areas correspond to outcrops of seismic bedrock. v and r indicate the seismic stations on alluvial deposits and seismic bedrock, respectively. bozzano et al. 8 that were recorded at the r station was used for the numerical modelling of the lsr along the route of the ancient via laurentina (figure 6). to quantify the local seismic amplification along the route of the via laurentina, seismic noise measurements from previous studies were collected [bozzano et al. 2016], and a new campaign of geophysical measurements was performed, including 13 measfigure 8. results of h/v measurements at 10 sites along the ancient via laurentina. red line: average h/v spectral ratio; dashed black lines: standard deviation of the h/v spectral ratio; dotted zone: unreliable frequency range of the h/v spectral ratio. seismic response in ancient via laurentina 9 urements at excavated sites along the via laurentina as well as locations where the presence of the road has been hypothesized (figure 7). the investigations were performed using a reftek 130 (24 bit) data logger with a lennartz 5s seismometer. the records were obtained for approximately 1 hour at a sampling rate of 250 hz and processed according to the technique of nakamura [1989] and the sesame [2004] standard using the open source software geopsy and the following analysis procedure: subdivision of each time series in 40 s windows with overlaps of 25%; correction, tapering to 5%, calculation of the fast fourier transform (fft) and smoothing of the spectrum for each window using the algorithm of konno and omachi [1998] with the b parameter equal to 40; calculation of the amplitudes of the three spectral components (n-s, w-e, up-down (v)), the geometric mean (h) between the two horizontal components and the spectral ratio (h/v) in each time window; calculation of the average spectral amplitudes for the three components and the average spectral ratio (h/v) in all of the selected time windows; directional analysis of the site response through the graphical representation of the h/v values as functions of frequency and azimuth. a total of 27 noise measurements (see examples in figure 8) were available along the analysed route of the via laurentina across the fosso di vallerano valley. the results of the new campaign well fit the geological model of the valley: in the southern portion of the route, the processed data (lv_27, lv_26, lv_25 and lv_24 in figure 8) show no amplification. this result is related to the outcrops of the volcanic units and the presence of shallow seismic bedrock (approximately 30 m below the soil level) without a significant impedance contrast; in the middle part, no clear peaks are present on figure 9. results of noise measurements (fourier spectra and h/v) performed on the north eurosky tower. t1: roof; t2: middle floor; t3: ground floor. note the attenuation of the peak amplitude from the top of the tower to the bottom and the shift of the peak frequency from the vibration mode of the tower (t1 and t2) to the site main vibration (t3). red line: average h/v spectral ratio; dashed black lines: standard deviation of the h/v spectral ratio; dotted zone: unreliable frequency range of the h/v spectral ratio. bozzano et al. 10 the average h/v spectral ratio (lv_23 in figure 8) except for site lv_22, which has an amplification peak at 4.5 hz related to a shallow impedance contrast. in the northern portion of the route, all of the processed data show a clear stratigraphic peak at 0.8 hz (lv_3 and lv_10 in figure 8) or between 0.7 and 0.9 hz (lv_11, and lv_13 in figure 8) due to the strong impedance contrast between the alluvial deposits that fill the valley (up to 60 meters thick) and the basal gravels. such a contrast, already observed and discussed by bozzano et al. [2016] for the fosso di vallerano valley as well as by martino et al. [2015] for the tiber river valley in the historical centre of rome, justifies that the seismic bedrock generally does not coincide with the geological one in the alluvial context of the rome urban area. in the northern part of the route, the only exception to the above described local seismic response consists in the spectral ratio (h/v) resulting at station lv_13, which includes a weak peak at approximately 4 hz that is likely due to a local and shallow horst in the seismic bedrock. it is worth noting that the significant resonance peak at lv_10 derives from a strong site energization at frequencies of 0.6 to 0.8 hz, which is likely induced by the vibration mode of the nearby towers (figure 9). to verify this effect, h/v seismic noise measurements were performed on the north eurosky tower (top (t1), middle (t2) and base (t3) in figure 9). the processed data clearly show that the main vibration mode of the building is very close to 0.6 hz; this peak is particularly intense on the roof of the tower (t1), but it is also visible with a lower amplitude on the middle floor (t2). the same peak is strongly attenuated at the ground floor, while a main peak (t3) is visible at approximately 1 hz, which is closer to the resonance mode of the alluvial fill measured at stations lv_10 and lv_11. a double seismic resonance effect [chen and scawthorn 2003] is therefore recognizable between the free-field site and the eurosky towers because the soil resonance frequency and the building’s first vibration mode are very similar. 5. numerical modelling 1d and 2d numerical simulations were performed to define the lsr in terms of amplification function along the via laurentina, considering the geological setting of the site in terms of both the valley shape and the heterogeneity of the alluvial fill. the ancient via laurentina does not cross the alluvial valley in the direction in which the main 2d physical effect (i.e., the interaction between the soil resonance and the lateral waves from the sides of the valley) is expected [bard and bouchon 1985]. therefore, 2d numerical modelling along the via laurentina cross section cannot be considered representative of the real 2d effects due to the physical interaction between the seismic waves and the valley system. figure 10 shows the here proposed procedure for extrapolating the lsr from a series of physically-based transverse cross sections to a non-transverse one. considering a generic alluvial valley and a line of interest (lin) which is not normal to the valley’s axis, it is possible to derive the (a(f )x) distribution of the 2d amplification functions along the lin by modelling the a(f ) along a series of figure 10. sketch illustrating the solution used to extrapolate the amplification functions (a(f )) from 2d numerical simulations along several cross sections that are oriented transverse to the alluvial valley to a line of interest (lin) that crosses the valley in a generic direction. the a(f )x distribution along the line is the transposition of several punctual a(f ) functions with physical meanings. figure 11. route of the ancient via laurentina and reliable geological buffers, which include the three cross sections that were used for the 2d numerical modelling. seismic response in ancient via laurentina 11 cross sections that are normal to the axis of the valley. the a(f ) functions that are modelled along each cross section can be transposed (through an orthogonal projection) to the lin if the corresponding projections along lin is inside the representative buffer of the cross section; otherwise, a different cross section is used to perform the transposition. the width of the representative buffer, and thus the number of physical transverse sections required, depends on the local geological setting. the resulting a(f )x function is formed along the lin even though it derives from 2d models that are developed along different cross sections; in this way, the final solution merges the physical effects due to the seismic response of the natural system (valley) with the engineering-geological model of the subsoil. following this approach, in our geological reconstruction of the geological setting of the valley, the route of the ancient via laurentina has to be included in three geological buffers respectively including a transversal section (figure 11). the three geological sections are representative of the lithological and stratigraphic setting of the ancient via laurentina and highlight the geological peculiarities of the valley. section aa’ (figure 12, top) represents the geological setting of the western portion of the ancient via laurentina and is used to simulate the presence of a landslide in the western portion of the fosso di vallerano valley. section bb’ (figure 12, middle) crosses the valley in the middle portion and is representative of the structural setting of the area; i.e., it highlights the presence of the buried horst in the central portion of the valley [bozzano et al. 2016]. finally, the geological section cc’ (figure 12 bottom) characterizes the southern part of the ancient via laurentina and is used to simulate the area that is characterized by volcanic deposits and the transition towards the valley. the 1d and 2d modelling figure 12. geological cross sections along aa’, bb’, cc’ in figure 11. see figure 4 for the key to the legend. bozzano et al. 12 were performed by attributing the parameter values proposed by bozzano et al. [2016] to the lithotecnical units and by assuming visco-elastic conditions with the damping distribution shown in table 1. as reported by bozzano et al. [2016] the attribution of dynamic properties to the subsoils of the fosso di vallerano valley was derived at a single location and seismometric records were used to calibrate the local seismo-stratigraphy taking into account the available high-resolution engineering-geological model of the local subsoil. a total of 27 soil profiles, representative for the different geological conditions along the ancient via laurentina were selected for the 1d numerical models, and the respective amplification functions were calculated using the free software strata [kottke and rathje 2008]. the main resonance frequencies resulting by the modelling along this section are in good agreement with the ones measured. the highest amplification values (i.e., up to 6) of the modelled a(f ) functions are located in the central portion of the ancient via laurentina, which corresponds to the deepest part of the fosso di vallerano valley in the frequency range of 0.7-1.4 hz. this is due to the impedance contrast between the alluvial deposits and the basal gravels (unit al-gr; table 1), which represents the local seismic bedrock [bozzano et al. 2016]. in areas of shallow seismic bedrock (i.e., in the central part of the route; figure 4) as well as near the external portions of the buried sides of the valley, the main amplification values are in the higher frequency range (2-4.6 hz). in the southern part of the ancient via laurentina (near the pontina archaeological site), the outcrops of volcanic rocks that overlie the basal gravels explain the absence of amplification effects. to obtain a a(f )x distribution of the 1d functions that were modelled along the route (figure 13), these functions were interpolated using kriging considering 52 functions (27 simulated and 25 extrapolated assuming constant geological conditions) with an average spacing of 40 m. the 2d models were performed using the cesar-lcpc code [humbert et al. 2005] based on the finite element method (fem) and assuming the same conditions that were used for the 1d simulations. the geological cross sections were discretized using a triangular mesh with linear interpolation; the size of the elements was chosen according to the equation (1): (1) where λ = the wavelength, and δh = the element size. considering the lowest s wave velocity of the alluvial deposits (118 m/s) and the highest s wave velocity of the seismic bedrock (1100 m/s), element sizes of 1 m for the alluvial deposits and 10 m for the seismic bedrock were chosen to allow for reliable numerical solutions up to 10 hz. the same weak motion that was used in the 1d simulation was applied as a shear wave (sh) for the displacements at the bedrock level. fixed boundary conditions were applied to the model boundaries; i.e., the degrees of freedom of each node in the mesh are equal to zero, which means that no movement (horizontal or vertical) is allowed. the numerical a(f ) functions were obtained by considering the spectral ratio between the signals in the valley and the input recorded at the outcropping seismic bedrock (point r in figure 7) [borcherdt 1994]. the resulting amplificaδh= λ 12 litotechnical unit γ (kn/m3) vs (m/s) d (%) al-af 17.0 118 1 al-vsc 16.5 225 1 al-cl 18.3 235 2 al-pc 17.2 150 3 al-gr 21.0 713 1 al/pt-sd 19.2 417 1 pt-cl 18.3 650-357 3 pt-ls(cl) 18.3 250 3 pt-ls(gr) 21.0 550 3 vc 17.5 550 2 la 27.0 850 1 fl 19.2 450 1 seismic bedrock 22.0 750-1100 1 table 1. parameter values attributed to the lithotecnical units reported in figure 4 according to bozzano et al. [2016]. (γ: weight per unit volume; vs: s wave velocity; d: damping). seismic response in ancient via laurentina 13 tion functions were interpolated with kriging to obtain the a(f )x distribution, which can be compared with the distribution obtained from the 1d modelling (figure 14). the modelling results show a significant resonance peak at 0.7 to 1.4 hz where the ancient via laurentina crosses the deepest part of the vallerano valley, while the frequency of the first resonance mode increases to 2.5 hz where the seismic bedrock is shallower (i.e., corresponding to the horst in the middle portion of the route). it is worth noting that the a(f ) values are greater in the 2d results than in the 1d results, and the frequencies that correspond to the higher resonance modes are different. these results demonstrate the non-negligible role of 2d effects on the lsr in the fosso di vallerano valley. to quantify the differences between the 2d and 1d outputs and highlight where the former are better than the latter, an aggravation factor aag(f )x was derived according to the equation (2): (2) the aag(f )x distribution along the route of the via laurentina (figure 15) in this case was also obtained using a kriging interpolator. the analysis of the aag(f )x distribution shows that the 2d effects cannot be neglected along the entire route of the via laurentina (figure 15). in particular, several aag(f ) values up to 2 indicate that the 2d a(f ) value is twice that of the 1d value (figure 15). nevertheless, it is worth noting that this evidence does not include the first resonance frequency (0.8 hz) but only aag ( f )x = a( f )x 2d a( f )x 1d figure 13. 1d a(f )x distribution (top) along the route of via laurentina (bottom). see figure 4 for the key to the legend. location of noise measurements along the route (and related first h/v peak) is also reported. bozzano et al. 14 higher resonance modes, (i.e., 3.5, 6, and 8.5 hz), which may be of interest for buildings or infrastructure. the comparison between the normalized acceleration response spectra (elastic condition at a 5% of damping), which result from 1d and 2d modelling (figure 16), outputs a general good fit in terms of spectral shape even if some specific difference can pointed out: 1d response spectrum are generally more conservative than the 2d ones (b, c and e in figure 16); 1d stratigraphic effects are prevalent out of the valley (a and f in figure 16); 2d amplifications are prevalent close to the sides of the valley (d in figure 16) and they are responsible for higher response spectrum values respect to the 1d models, especially at the lowest periods (i.e. < 0.5 s). 6. conclusions this study presents 2d modelling of the lsr along a reference line that is generically oriented with respect to a natural valley system. the ancient roman via laurentina road was considered because it has been excavated over the last decade in the fosso di vallerano valley, which is located south of the historical centre of rome, during the construction of the “europarco business park”. many specific directives have been issued regarding the insertion of the ancient road into the present urban fabric (excluding the remains that were found beneath the modern road network) to preserve its structures and to make it visible at the surface. in particular, the urban plan in the fosso di vallerano valley has been modified, including changes to the locations of the construction sites and the alignfigure 14. 2d a(f )x distribution (top) along the ancient route of via laurentina (bottom). see figure 4 for the key to the legend. location of noise measurements along the route (and related first h/v peak) is also reported. seismic response in ancient via laurentina 15 ment of the eurosky towers with the ancient road. the remains could not be preserved at their original levels due to the substantial increases in the surface water system and the water table and the presence of modern structures. therefore, the road and monumental structures were adequately protected and covered with soil, and their presence was marked at the surface by lawns that reproduce their plans and are visible within the new central pedestrian square. a route with information about the topography of the area and the use of the ancient structures was also created. the urban plan of the pontina site was also modified to include provisions for preserving the road within the original cut in a private garden with a solution that allows the visualisation of the castellaccio tower’s skyline, which reproduces the line of the ancient road. to account for the physical interaction of the valley system with the seismic waves (i.e., to consider the effects related to the valley shape and the heterogeneity of the alluvial deposits), three 2d cross sections were modelled, and the results were transposed to the route of the ancient via laurentina by projecting the obtained amplification functions (a(f )). an engineering-geological criterion was applied to verify the reliability of the performed projection within a representative buffer. the derived a(f )x distribution is physically based and geologically constrained. a comparison with the results of 1d modelling demonstrates that the 2d effects are not negligible in the study area. moreover, the obtained a(f )x distribution is reliable for evaluating the expected seismic amplification at the foundations of new buildings (i.e., the eurosky towers) that have been designed also to preserve the archaeological site figure 15. aag(f )x distribution (top) along the ancient via laurentina (bottom). see figure 4 for the key to the legend. bozzano et al. 16 figure 16. comparison between 1d and 2d normalized response spectra in different position along the ancient route of via laurentina (bottom). see figure 4 for the key to the legend. seismic response in ancient via laurentina 17 and are therefore aligned with the route of the ancient roman road. 1d and/or 2d numerical models can be regarded as suitable for designing linear structures or infrastructure (e.g., bridges, aqueducts or viaducts) as well as for analysing the lsr of similar archaeological remnants. acknowledgements. this paper reports preliminary results of ph.d. research at the department of earth sciences of the “sapienza” university of rome (chiara varone, phd student). the authors wish to thank l. lenti, ph.d., the university paris-est lcpc and ifsttar (french institute of science and technology for transport, development and networks), who allowed the use of the cesar lcpc software for the phd research. references ascani f., bozzano f., buccellato a., del monte m., matteucci r., vergari f. 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email: chiara.varone@uniroma1.it 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved s0440 annals of geophysics, 60, 4, 2017, s0440, doi: 10.4401/ag-7154 seismic hazard assessment for the protection of cultural heritage in greece: methodological approaches for national and local scale assessment (pilot areas of aighio, kalamata and heraklion) sotiris sboras1,*, john andrew dourakopoulos1, evaggelos mouzakiotis1, pavlos dafnis1, theodoros palantzas1, vassilios k. karastathis1, nikolaos voulgaris2, gerasimos-akis tselentis1 1 national observatory of athens, institute of geodynamics, lofos nymfon, athens, greece 2 university of athens, faculty of geology and geoenviromment, athens, greece article history received september 15, 2016; accepted june, 16 2017. subject classification: historical monuments, sha, structural assessment, geodatabase, greece. abstract the rich greek cultural heritage has been always threatened by the intense seismic activity in the broader aegean region. the objective of the presented project is to develop an integrated tool for the engineers in order to protect the greek monuments, museums and archaeological sites against strong earthquakes. in order to achieve this goal a gis-based database was developed with a bidirectional purpose: to collect and combine all necessary data about the monuments and their regional geological and seismotectonic conditions and to assess seismic hazard for each and every monument using the most modern techniques. in this paper we present the structure and development of our database, we propose a methodological procedure for estimating seismic hazard in greece which will be the basis for the structural assessment of historical structures. the preliminary results show that the estimated values of maximum ground acceleration are quite high for areas in close proximity with large faults, especially when combined with loose ground conditions. therefore an update of the protection code would be necessary. subsequently the estimated values of maximum ground acceleration have been applied to three cases of monuments for the determination of the most vulnerable parts of the structure and the verification of the observed pathology. 1. introduction greece is the most seismologically active region in europe. earthquakes are not only a threat for human lives and infrastructure, but also for monuments that have survived during the centuries and are spread all over the greek territory. for this reason, the earthquake protection policy in greece should be updated by incorporating modern seismic hazard assessment techniques. work package 3 (wp3) of the “aspida” project, which was recently completed, focuses on historical monuments protection. one of the primary targets of “aspida” project is to create a valuable gis-based database providing much of the information required for seismic hazard assessment. the development of such a database delivers multiple benefits: i) it provides a multilevel data management, ii) the input and output data are fully updatable and adjustable, iii) analyses and calculations between data and/or datasets are significantly facilitated, iv) results can be presented in multithematic maps at the scales of interest, and v) the final structure offers ease of use by any end-user. to meet this specific goal, a fourstep procedure was followed: i) development of a gis based geospatial database of sites of cultural interest (monuments, museums, archaeological sites, etc.) summarized by the term ‘monuments’, integrated with geological and seismotectonic data, ii) application of state-of-the-art methodologies for seismic hazard assessment at national scale, i.e. for all monuments inside the greek borders, iii) application of methodologies for seismic hazard and structural assessment of monuments to three case studies in different localities (aighio, kalamata, heraklion), iv) proposal of a frame of instructions and regulations regarding earthquake protection for monuments. in this paper the procedure and the results for both national (steps i and ii) and local (step iii) scale seismic hazard assessment adapted for the historic monuments are presented. the frame of instructions and regulations (step iv) will be presented in a future publication. the final data and sboras et al. 2 results of wp3 are published on a dedicated website (http://aspida3.gein.noa.gr/). for the scope of seismic hazard assessment, a stochastic methodology [exsim algorithm; motazedian and atkinson 2005] was implemented taking advantage of the numerous information regarding the seismic source and the local site conditions. using this technique allowed the determination not only of the distribution of the peak ground acceleration, velocity and displacement, but also the pseudospectral acceleration, velocity and displacement spectra for each monument site. during the presented project, one monument for each one of the three pilot areas were examined: agios nikolaos in platani (aighio pilot area), agioi theodoroi in kampos of avia (kalamata pilot area) and agios minas and pantanassa in crete (heraklion pilot area) are churches built more than six hundred years ago and more specifically in middle byzantine and post byzantine period. throughout their life, earthquakes, either caused from near or far active faults, constitute one of the major reason for various partial or total collapses and extended damages on the structure and therefore, a particular analysis based on seismic hazard assessment should be elaborated. this analysis required a detailed site investigation, highlighting the different approaches when working in such contrasting scales (national and local). 2. seismic hazard assessment at national scale 2.1 the “aspida” geodatabase the “aspida” database is designed and developed using arcgis software in discrete layers in order to serve two purposes: to form a well-organised databank with geographic reference and to comprise a tool for facilitating sha calculations. data can be categorized into two groups: i) the input data, which correspond, but are not constrained, to all available information needed for the sha calculations, and ii) the output data including all calculated strong motion parameters (e.g. pga, pgv, etc.). within the database, strong ground motion values calculated with probabilistic methodologies [papaioannou et al. 2008], as well as the greek seismic code regulation – eak 2000 [eppo 2001] seismic hazard values are included only for reference and for the informative character of the database. three are the important datasets (or layers): the main dataset of monuments and the datasets of the seismic sources and the geotechnical soil conditions. all datasets are referenced to the geographic (longitude/latitude) projection system and wgs84 datum. the main dataset of monuments is updated with the results of the seismic hazard assessment. additional material, such as topographic-morphological data (e.g. hypsometric contours, triangulation points, drainage network) and infrastructural-administrative data (e.g. transportation, municipalities) are also available for producing thematic maps. the structure and data can be accessed in the dedicated website hosted in the servers of noa (the included data are in greek language until the english version is ready): http://aspida3.gein.noa.gr/. 2.2 the monuments dataset/layer the ‘master’ dataset of the database is the monuments dataset. according to the greek legislation (3028/02), ‘immovable monuments’ are “monuments which are attached to and remain on the ground, the seabed or on the bed of lakes and rivers, as well as monuments which are found on the ground, the seabed or on the bed of lakes and rivers and cannot be removed without damage to their values as testimonies. immovable monuments also include installations, structures decorative and other elements, which form an integral part of monuments as well as their surroundings. ancient immovable monuments are the monuments that are dated up to 1830”. the only officially published geographic database of cultural and historic monuments in greece is hosted in the website of “odysseus” (http://odysseus. culture.gr/) under the aegis of the hellenic ministry of culture and sports (hmcs). the database counts over 2000 entries which are categorized in three main groups: museums (279 entries), monuments (1573 entries) and archaeological sites (463 entries). the website version consists of an interactive map and supporting general historic information (descriptive and not parameterized) for every monument. since the beginning of wp3, the data of “odysseus” database were not available due to maintenance. for this reason, “odysseus” data were retrieved manually, directly from the interactive map of the official website, they were crosschecked for the accuracy of position and information, then processed and finally parameterized. nevertheless, further available sources were tracked down and used in order to complement the monuments that miss from “odysseus”. these are: i) the database of voulgaris et al. [2006], an updated database partially based on “odysseus”, ii) the “ongoing catalogue of the listed archaeological sites and monuments of greece”, published since 1993 by the directorate of the national archive of monuments (hmcs), which contains only immoveable monuments, archaeological sites and historic places that seismic hazard assessment for the cultural heritage of greece 3 require a specific legal act of designation, demarcation and protection, and iii) various public data collections and information. all aforementioned data were manually compared for duplicate entries, filtered, individually processed and combined in a single dataset. the compilation of the above mentioned catalogue resulted in a database containing more than 2900 entries (533 museums, 2004 monuments and 953 archaeological sites; figure 1a), containing fields of general information (e.g. type of monument, geographic and administrational location), which was enriched with geological information (e.g. geological formation under foundations) and sha information from literature (e.g. pga from the greek seismic code regulation). at the end of the project, our sha results from both national and local scale investigations were imported in the dataset. 2.3 the seismic sources dataset/layer the most important input for seismic hazard estimation is the dataset of the seismic sources. it was only until recently when such databases started to be developed for the aegean region. “gredass” (greek database of seismogenic sources; pavlides et al. 2010; sboras 2011; caputo et al. 2012) is an updatable openfile gis-based database of the seismic sources in the broader aegean region that uses multi-layer fault/source modelling according to the needs of the end-user. since the prime purpose of gredass is seismic hazard assessment, the fault/source models are geometrically simplified, but fully parameterized with explicitly defined seismotectonic parameters. thus, only seismic sources with magnitude potential greater than 5.5 are included. consequently, gredass prefers the completeness of the parametric information rather than the completeness of the seismic sources. another active fault database in greece of different philosophy is “noafaults” [ganas et al. 2013] which is a single-layered database aiming at the most accurate geomorphological representation of the tectonic structures. thus, this database prefers the completeness of the sources rather than the completeness of the parametric information. both gredass and noafaults are mainly based on published data (articles, maps, technical reports, etc.). as complementary information, the catalogue of “seismic sources and faults” compiled by karakaisis et al. [2010] was also considered. this catalogue explicitly contains earthquake-associated seismic sources of the broader aegean region with their basic seismotectonic parameters obtained from seismological – and only – data. although gredass demonstrates higher compatibility towards the parametric information needed for seismic hazard assessment calculations, the contents and parameters of all available sources were reviewed, compared, complemented and homogenised in a new dataset called “aspida faults” (figure 1b). more specifically, all seismic sources were qualitatively reviewed and filtered one-by-one. in case of data conflict on specific seismic sources, a study of the original source was carried out, but also more recent literature was taken into account for deciding the reliability of the seismic source occurrence and/or its appropriate parameters. there were also few cases for which seismic sources had to be reassessed and parameterized from scratch. within the three pilot areas and close to the monuments, smaller sources capable of producing damage to the monuments were imported in the final dataset after local field mapping. all collected information had to be parameterized (when necessary) and homogenised according to the standards of the new “aspida faults” dataset. the new dataset of seismic sources “aspida faults” (figure 1b) is strictly based on the needs of sha calculations. the total 231 sources are modelled as straight lines, representing the upper part of the fault plane, according to the strike and length of the fault plane. each seismic source contains all the necessary identification and seismotectonic parameters that describe the full geometry of the fault plane (e.g. length, down-dip width, minimum depth of the upper part, strike and dip), as well as its kinematic character (normal, reverse and strike-slip motion). it should be mentioned that fault modelling for the needs of sha is simplified and does not represent the complexity of the faults found in nature; thus, detailed fault traces completely lack from this dataset. 2.4 the seismological dataset/layer the seismological dataset comprises the compilation of two earthquake catalogues (figure 1c), one of the seismological station of the aristotle university of thessaloniki [papazachos et al. 2000, 2010; papazachos and papazachou 2003] for the time period between 550 bc and ad 1963 (included) and the catalogue of the institute of geodynamics (national observatory of athens) for the time period of ad 1964 and afterwards. 2.5 the geological dataset/layer the local soil conditions and their attributes have a major impact in the amplification or mitigation of strong ground motion. this makes local soil conditions an important parameter for seismic hazard essboras et al. 4 timation. the available datasets for the national-scale investigation are obtained as raster data from the basic 1:500,000 scale geological and geotechnical maps of the greek institute of geology and mineral exploration [igme 1983; 1993]. these data were further processed and vectorised. subsequently, geological formations were grouped into categories according to the descriptions of the first four categories (a-d) of eurocode 8 for ground types (figure 1d). each category was assigned an average vs30 value based on the eurocode 8 ground types. an amplification spectrum was attributed for each vs30 value, based on existing literature [margaris and boore 1998, klimis et al. 1999]. it should be noted that due to the national-scale of the project, no specific site amplification function could be derived for each monument site, however, in order to partially account to the site geology the above scheme was utilized in combination with the geotechnical maps of igme. the local-scale categorization in the three study cases involved a different approach, following more detailed investigations (e.g. 1:5,000 scale geological mapping around each monument, geophysical investigations, etc.) in order to classify the foundations soil more accurately according to eurocode 8. 3. methodology strong ground motion parameters were calculated using the exsim algorithm [motazedian and atkinson 2005]. the algorithm utilizes the stochastic methodology of ground motion simulation, as proposed by boore [2003] and expanded by motazedian and atkinson [2005] for finite sources. each seismic source is represented by a number of point sources distributed over a plane which represents the fault. for each point source the acceleration spectrum is defined by a “ω2” spectrum. the synthetic accelerograms for each source are then convolved, after shifting them appropriately in time with regards to the rupture speed figure 1. the basic datasets of the “aspida” geodatabase: (a) monuments categorised, (b) seismic sources, (c) combined seismicity from the catalogues of papazachos et al. [2000], for the period between 550 bc and ad 1963 (included), and noa, since ad 1964, (d) ground geotechnical characterization. seismic hazard assessment for the cultural heritage of greece 5 and source-to-site travel time differences, thus obtaining the complete acceleration time series for the fault. the use of such finite fault models is particularly advantageous for calculating ground motions near large faults, since it takes into account finite-fault effects such as directivity, attenuation and rupture geometry. it should be mentioned, though, that the directivity effect is only partially confronted, since we cannot predict the rupture initiation location on the fault for a possible future event. the use of exsim algorithm also allows the incorporation of a time dependent corner frequency which eliminates the dependence of the results from the number of subsources. when using a static corner frequency, low frequency energy is found to be proportional to the subfault dimensions, whereas high frequency energy is inversely proportional. total energy is inversely proportional to the subfault size [motazedian and atkinson 2005]. the above described procedure was used for the scope of seismic hazard assessment for the whole greek region. for each seismic source (shown in figure 1b), a staggered grid was constructed around the fault trace, with minimum node distance of 1 km near the fault. the grid was rotated towards the direction of the fault trace and for each node, the synthetic acceleration time series was calculated. for each case, several points on the fault were considered as enucleation points and for each one, 20 simulations were performed. the final synthetic time series were an average of the above simulations. based on these synthetic waveforms, the values of pga, pgv and pgd were obtained. calculations were performed for all modelled faults of the database. as a final step, the calculated pga, pgv and pgd grids for every fault were combined into a composite grid, covering the whole greek territory, containing the maximum values for these parameters. in order to produce this composite grid, all the initial grids were interpolated to finer grids with the same node locations. for the construction of the final grid, the highest value of pga for each node was selected. apart from the fault geometry and the site amplification, the main input parameters used in the stochastic simulation are the stress drop, the pulsing percentage, the anelastic attenuation model, the geometrical spreading and the duration model. the stress drop and pulsing percentage parameters were selected on the basis that for near fault simulations, most of the energy is attributed to higher frequencies. for the stress drop parameter a value of 140 bars was used, as suggested by atkinson and boore [2006]. in their work, this average value was found to provide the best fit between the synthetic and the observed accelerogramms for the higher frequency part of the acceleration spectrum (5-10 hz) for 8 well recorded earthquakes. this value was also tested for some well recorded earthquakes on the broader study region and was found to provide sufficient fit between the observed acceleration spectrum and the synthetic one. the pulsing percentage parameter has a much lower effect on the higher frequency part of the synthetic acceleration spectra [motazedian and atkinson 2005, atkinson and boore 2006]. the mean value of 50% suggested by atkinson and boore [2006] was used. anelastic attenuation (q), geometrical spreading and duration models, are related to the propagation medium. for this study, we used q model proposed by hatzidimitriou [1994] for the area of northern greece. with regards to the geometrical spreading model, we used the one proposed by atkinson [2004]. finally, we incorporated the duration model proposed by atkninson and boore [1995]. the above models are presented in table 1. site amplification functions were derived from the microtremor data. more specifically the h/v spectral ratios were calculated on the recorded signals, after applying the instrument transfer function and after filtering the signal. for the survey, broadband sensors were used with a flat response curve above 0.01 hz. for the calculation of the spectral ratios, a 0.1 – 20 hz band pass filter was applied. the signal portions were selected so that frequencies from 0.1 hz and above could be resolved. additionally an anti-triggering window was applied in order to avoid adding sudden energy fluctuations to the calculations. it should be noted that more detailed data and information are required in order to accurately calculate the above parameters for each monument in greece. this was the aim of the three case studies. on the one hand, the national-scale investigation provides an initial, uniform approach of sha for all monuments in greece based on data and information model type model anelastic attenuation [q(f )] 60f0.79 geometrical spreading [g(r)] 1/r1.3 (r < 70km), 1/r (70km < r < 140km), 1/r0.5 (140km < r) duration [d(r)] t0 (r < 10 km), t0 +0.16r (10km < r < 67km), t0 + 0.03r (67km < r < 130km), t0 +0.04r (130km < r) table 1. models used in the simulations. sboras et al. 6 mainly deriving from the literature; on the other hand, new focused investigations were conducted for the local-scale sha, briefly described in the next sections. the earthquake magnitude that was used for each simulation procedure was the maximum credible earthquake (mce) of each seismic source separately, related to the source dimensions as proposed by wells and coppersmith [1994] and papazachos et al. [2004] for the area of greece. although this approach does not result in a probability of exceedance for each earthquake scenario, like the typical probabilistic seismic hazard assessment techniques, it was selected on the basis that historical monuments have survived for hundreds of years and should be preserved for as long as possible. therefore the time aspect of the hazard assessment becomes inf inite and so we use the worst case earthquake scenario with regards to the magnitude and fault distance. the results of the application of the above methodolog y for the whole reg ion of greece (national-scale investigation) can be seen in figure 2. as expected, highest values of pga were calculated for areas in close proximity with large faults, especially when combined with loose ground conditions (e.g. ground type d). usually, such areas represent tectonically controlled basins and valleys, where both active faults and sediments accumulation are met. on the other hand, in reg ions with no signif icant seismic sources nearby, very low values of maximum expected pga were calculated. three large areas of very low values (< 0.1 g) can be distinguished i) along the pindos mountain-chain (western greece), ii) in the mountainous central peloponnesus, and iii) in cyclades. all these areas lack of both signif icant active faults and sedimentary basins. however, as mentioned before, for more accurate results more detailed investigation in a local scale is needed. such investigation was implemented for the following three pilot areas. figure 2. map showing the maximum pga distribution that was calculated in the scopes of this project. the monuments are also shown as black dots. seismic hazard assessment for the cultural heritage of greece 7 4. seismic hazard assessment at local scale: three case studies the “aspida” project required the detailed investigation of three monuments, each within three highly seismically active pilot areas of greece: aighio, kalamata and heraklion areas. the detailed investigations had to be constrained in non-destructive methods only, such as a microtremor survey and further geophysical investigations (unpublished results), while near-field morphotectonic and geological investigations were carried out. as for the needs of the monument final selection procedure, a five-grade scale was applied according to four basic factors: i) seismic activity of the area, ii) selection of a representative historical structure (as for the architectural form, the construction materials, etc.) of the considered period, iii) existence of different discontinuities and cracks or/and construction phases, and iv) cultural heritage importance. in contrast with the national scale sha, the active faults within the pilot areas (15 km radius around each monument – near field) had to be reassessed in order to fill in possible missing sources of lower potential magnitude, but capable of producing a destructive earthquake, and to provide a more detailed parameterisation to the seismic sources. for this reason we also adopted the active faults that were investigated in more detail in wp1 of the same project (e.g. kazantzidou-firtinidou et al. 2016, zygouri et al. 2016]. these faults had to be parameterised in order to provide the necessary data inputs for sha calculations. the seismic sources of the three pilot areas are shown in figure 3 (aighio pilot area), figure 4 (kalamata pilot area) and figure 5 (heraklion pilot area) and in table 2. it should be mentioned that both near filed and far field seismic sources where used in local-scale sha. 4.1 monument #1 (aighio pilot area): agios nikolaos, platani (achaia, n. peloponnesus) the byzantine church of agios nikolaos, is located in platani village in n. peloponnesus, greece (figure 3). the structure which is dated back to the 12th century a.d., is a single-aisle triconch type church with a narthex on the west side. the main part of the church, called ‘naos’ is covered by a dome while the narthex is covered by three cross vaults. the inner faces of the walls of the naos are of rubble masonry contrary to the greater part of the external façades which are made of a combination of cloisonne masonry (local limestone and bricks). the church of agios nikolaos was chosen due to its very special architectural form single-aisle triconch type with a narthex typology). it’s one of the very few still existing characteristic examples of the helladic school (middle byzantine period). the three construction phases in combination with the absence of coating therefore making possible the observance of its pathology, were some of the key points for this selection. the monument is located on a gentle slope at the northern front of the panachaikon mt, in a small distance ne of xylokera river. the geological basement in the narrow area corresponds to plio-pleistocene post-alpine sediments (marls overlain by conglomerates). the underlying marls are cohesive, thinbedded, consisting of clays with some sandy layers. at the upper layers, marls are usually weathered. their total thickness is at the order of 300 m. the overlaying conglomerates, consist of well cemented pebbles in a sandstone matrix with local marl intercalations. their total thickness is at the order of 300 m. plio-pleistocene formations are covered by recent sediments of alluvial fans consisting of clayey sand and gravel, and river and terrace deposits consisting of loose material (pebbles, sand). the broader area belongs to one of the most tectonically active regions worldwide: the gulf of figure 3. (a) small-scale map of the broader aighio pilot area where the monument of agios nikolaos is located. the near field seismic sources are also shown (see text and table 2 for details). seismicity derives from the catalogues of papazachos et al. [2000], for the period between 550 bc and ad 1963 (included), and noa, since ad 1964; the occurrence year and magnitude are also shown for events with m ≥ 5.5. (b) large-scale aerial photograph mosaic (national cadastre and mapping agency s.a.) with the location of the monument. (c) structural plots of agios nikolaos (floor plan and longitudinal cross-section). (d) the structural model and photos of the monument. sboras et al. 8 corinth. there are two basic interpretations for the structure of the gulf. the first one [e.g. king et al. 1985, rietbrock et al. 1996, rigo et al. 1996, flotté and sorel 2001, lyon-caen et al. 2004, gautier et al. 2006], which is the most accepted, is the occurrence of a n-dipping, low-angle, normal detachment fault that branches toward the surface through a system of parallel listric faults that control the morphology of the broader northern peloponnesian coast. the second interpretation considers the gulf as an asymmetric half-graben [e.g. jackson et al. 1982, taylor et al. 2011] or a symmetric graben [moretti et al. 2003, mcneill et al. 2005, bell et al. 2008], depending on the given significance of the north bounding, s-dipping faults. chéry [2001] and jolivet et al. [2010] consider the western part of the gulf as an initial stage of a metamorphic core complex. whichever the case, the northern coast of peloponnesus is occupied by both offshore and onland n-dipping normal faults with directions that range between nesw [mainly near the gulf of patras, e.g. palyvos et al. 2007] and se-nw [mainly east of rion, e.g. doutsos and poulimenos 1992]. numerous greater than mw 6 events have occurred in the broader area in historical and instrumental times. the most recent strong event occurred on june 15, 1995 (mw 6.3) in aighio (ca. 20 km ese of the monument) due to the reactivation of the homonymous fault. the near-field seismic sources in this area are shown in table 2 and figure 3. 4.2 monument #2 (kalamata pilot area): agioi theodoroi, kampos (avia, s. peloponnesus) the post-byzantine church of agioi theodoroi [bouras 2006] is located in kampos village, s. peloponnesus (figure 4). the construction of the monument is dated back to the turkish dominance period (15th – 16th century ad). with respect to the architectural form, it is a slight variant of the cross-in-square typology (with two columns instead of four), topped by a dome, while the construction of the triple layered masonry was made with the use of local limestone. the church of agioi theodoroi was chosen among other monuments, because of its architectural form (cross-in-square typology), which is actually the most popular of the middle and late byzantine period. its small size and its location (non-adjacent to other buildings or monuments) gives the possibility for a further and deeper research (in situ measurements etc.), without having any seismic interference with the othfigure 4. (a) small-scale map of the broader kalamata pilot area where the monument of agioi theodoroi is located. the near field seismic sources are also shown (see text and table 2 for details). seismicity derives from the catalogues of papazachos et al. [2000], for the period between 550 bc and ad 1963 (included), and noa, since ad 1964; the occur-rence year and magnitude are also shown for events with m ≥ 5.5. (b) large-scale aerial photograph mosaic (national cadastre and mapping agency s.a.) with the location of the monument. (c) structural plots of agioi theodoroi (floor plan and longitudinal cross-section). (d) the structural model and photos of the monument. figure 5. (a) small-scale map of the broader heraklion pilot area where the monument of agios minas and pantanassa is located. the near field seismic sources are also shown (see text and table 2 for details). seismicity derives from the catalogues of papazachos et al. [2000], for the period between 550 bc and ad 1963 (included), and noa, since ad 1964; the occur-rence year and magnitude are also shown for events with m ≥ 5.5. (b) large scale aerial photograph mosaic (national cadastre and mapping agency s.a.) with the location of the monument. (c) structural plots of agios minas and panta-nassa (floor plan and longitudinal cross-section). (d) the structural model and photos of the monument. seismic hazard assessment for the cultural heritage of greece 9 er constructions. the different types of cracks (some of them hairlike and some of them more severe), the traces of a prior to the existing foundation, the realization of some non-compatible restoration works (1967-1968) and also the fact that the construction is still functional until today as a religious monument by the local community, were the basic criteria for the final selection in this pilot area. the morphological relief of the surrounding area is smooth with low-height hills. alpine bedrock is represented by flysch and limestone of the tripoli unit. alpine formations are covered by pliocene marine sediments (marly limestone, sandstone, marl, conglomerates). pleistocene terrace deposits (red clays, clayey sand with conglomerate intercalations) are met at the surface covering all the above mentioned formations. terrace deposit thickness varies from a few meters to 50 m. recent alluvial deposits are met locally, consisting of pebbles-gravel and sandy clay. sw peloponnesus lies at the back-arc of the western hellenic subduction, where the quasi-oceanic crust is subducted under the aegean due to the collision between europe and africa [e.g. caputo et al. 1970, le pichon and angelier 1979, 1981; hatzfeld et al. 1989, jolivet et al. 2013]. in contrast with the n-s extension at the central part of the hellenic subduction’s backarc, the western part exhibits a roughly e-w direction of extension [e.g. lyon-caen et al. 1988, papanikolaou et al. 1988, papazachos et al. 1988, hatzfeld et al. 1990, jolivet et al. 1999, 2013] due to the arc’s curvature. this results in the occurrence of n-sto sse-nnw-trending normal faults in sw peloponnesus. strong earthquakes in this region are quite often since historical times. two are the most famous events: the 464 bc sparta [m 6.8 according to papazachos and papazachou 2003] earthquake, caused by the at least 48 km-long, sse-nnw-trending taygetos fault at the western front of the homonymous mountain, and the september 13, 1986 kalamata (mw 5.8) earthquake, caused by the homonymous, ca. 18 km-long, w-dipping normal fault just few kilometres east from the town of kalamata. the seismic sources for the agioi theodoroi monument are shown in table 2 and figure 4. 4.3 monument #3 (heraklion pilot area): agios minas and pantanassa (heraklion, crete) the byzantine church of agios minas and pantanassa is located in the old town of heraklion city, in crete, greece (figure 5). although the initial structure, which was actually a single space topped by a vaulted roof, is dated back to the venetian period (1211-1669), it has two sequential construction phases: the first one took place during the turkish dominance period (1669-1777) with the construction of a south wing covered by a vaulted ceiling and a second one, during the late years (1900-1901) with the construction of a narthex-like (pseudonarthex) structure on the west side [detorakis 1995]. as for the triple layered masonry, it was made with the use of local limestone. the church of agios minas and pantanassa was chosen because of its initial characteristic architectural form (single space topped by a vaulted roof typology) which is actually very close to the one of the greek island churches and at the same time, differentiates from the other study cases that have been selected. the presence of three sequential construction phases coupled with the fact that this monument used to be the first orthodox metropolis of the city of heraklion, comprised two of the most significant criteria from this selection. the broader heraklion area is characterized by a smooth morphological relief on a gentle slope towards the sea. pliocene, marls of the foinikia formation are considered as the geological bedrock of the area. they consist mainly of marls with sandstone beds of 2-5 m thickness. the upper layers of the formation are weathered. thickness of the weather layer varies from 5 m to 15 m. at the monument area, marls are covered by manmade deposits of 2-5 m of thickness. the neotectonic regime of crete is quite complex: the uppermost 10-20 km of the crust are characterized by extension and mainly normal faulting of various directions as it is revealed from kinematic indicators and focal mechanisms [e.g. le pichon and angelier 1979, 1981; taymaz et al. 1990, fassoulas 2001, jost et al. 2002, benetatos et al. 2014, caputo et al. 2010, mountrakis et al. 2012, gallen et al. 2014, mason et al. 2016], whereas the deeper parts, i.e. the subducting lithosphere, are characterized by compression due to the hellenic subduction zone. the shallow extension is ascribed to the southward slab-rollback of the hellenic margin, the southwestward expulsion of the aegean microplate and the anti-clockwise rotation of the african lithosphere relative to eurasia [e.g. ring et al. 2010, jolivet et al. 2013]. extension occurs orientated both arc-perpendicular and arc-parallel, which has led to a complex pattern of normal faulting throughout the region. crustal scale normal faults affect both offshore [e.g. leite and mascle 1982, alves et al. 2007] and on land areas [fortuin and peters 1984, postma and drinia 1993, ten veen and meijer 1998, sboras et al. 10 ten veen and postma 1999, fassoulas 2001, kokkalas and doutsos 2001, monaco and tortorici 2004, peterek and schwarze 2004, caputo et al. 2010, wiatr et al. 2013, mouslopoulou et al. 2014, mason et al. 2016]. the crustal seismic sources near the agios minas and pantanassa monument are shown in table 2 and figure 5. name/code length (km) width (km) strike (°) dip direction dip (°) kinematics mce magnitude aighio pilot area (agios nikolaos) ag01 4.7 4.7 55 s 60 n 5.3 ag02 4.2 4.2 240 n 60 n 5.2 ag03 11.2 11.2 273 n 60 n 6.1 ag04 9.5 9.45 81 s 60 n 5.9 ag07 9.8 9.8 268 n 60 n 6.0 ag08 9.8 9.8 288 n 60 n 6.0 ag09 9.3 9.3 306 n 60 n 5.9 ag10 3.8 3.8 291 n 60 n 5.1 ag11 9.7 9.7 286 n 60 n 5.9 ag14 24.8 24.8 69 s 60 n 6.8 ag19 21.3 10.65 229 w 90 s 6.4 ag28 15.1 15.1 225 n 40 n 6.3 ag29 11.8 11.8 317 n 60 n 6.1 ag30 12.5 12.5 309 n 60 n 6.2 ag34 6.9 6.9 241 w 40 n 5.6 kalamata pilot area (agioi theodoroi) m12 4.8 4.8 169 w 60 n 5.3 kitries 10.8 10.8 177 w 60 n 6.0 m2 6.2 6.2 199 w 60 n 5.5 doli 10.0 10.0 356 e 60 n 6.0 kouris 10.4 10.4 341 e 60 n 6.0 kardamili 12.5 12.5 154 w 60 n 6.2 m4 10.9 10.9 151 sw 60 n 6.0 lefktro 6.0 6.0 141 sw 60 n 5.5 m1 7.4 7.4 195 w 60 n 5.7 m3 7.5 7.5 108 s 60 n 5.7 kourtissa 21.0 21.0 127 sw 60 n 6.6 asprochoma 7.1 7.1 156 w 70 n 5.7 verga 11.2 11.2 193 w 70 n 6.1 heraklion pilot area (agios minas & pantanassa) hr2 3.3 3.3 22 se 60 n 5.0 hr1 3.8 3.8 149 sw 60 n 5.1 hr4 3.4 3.4 175 w 60 n 5.0 gazi 7.3 7.3 354 e 70 n 5.7 kroussonas 11.0 11.0 22 e 65 n 6.1 tilissos 10.8 10.8 4 e 65 n 6.0 yuchtas west 7.8 7.8 174 w 60 n 5.8 yuchta east 6.0 6.0 13 e 50 n 5.5 episkopi 4.4 4.4 212 nw 60 n 5.2 vasilies 6.1 6.1 195 w 70 n 5.5 kounavoi 5.4 5.4 386 e 60 n 5.4 agios syllas 3.8 3.8 189 w 60 n 5.1 table 2. the seismic sources of the three pilot areas with their basic parameters. for kinematics r = reverse, n = normal and s = strike-slip. mce = maximum credible earthquake, calculated from m versus rupture area (ra = length × width) after wells and coppersmith [1994]. seismic hazard assessment for the cultural heritage of greece 11 5. method of structural assessment and results in order to obtain information on the level of seismic noise in the monuments, but also for the natural frequency of the monument, microtremor measurements were performed by placing a special sensor both on the ground and various positions on the monument (roof, floor and window arches; figure 6). from the hvsr analysis results that are shown in figure 6, it can be clearly seen that the resonant frequencies of the monuments range between 5 and 8.2 hz (8.5 hz for agios nikolaos in aighio, 5 hz for agioi theodoroi in kalamata and 8.2 hz for agios minas and pantanassa in herakleion). the fact that the dominant frequencies measured on the ground in the near vicinity of the monuments are much lower (2.3 hz for aighio, 0.65 hz for kalamata and 2.2 hz for herakleion) than those measured over the monument gives further proof regarding the validity of the values that were calculated for each monument. the results were used, after processing, as input for the monument analysis. georadar measurements were also performed in order to define the monument structure (construction materials, thickness etc.) and to detect any non-visible elements in the monument structure. gpr survey was implemented in coordination with the civil engineers in order to cover all areas of interest of the monument. for the analysis of the three churches, solid finite element models have been developed and subjected to scenarios of seismic time-history loading. the data collected from microtremor measurements were used for the calibration of the developed numerical model and the verification of its validity. strong ground motion has been modeled using stochastic simulation methodologies for point sources, as well as finite sources, attributed to local active faults. the aim was to accurately simulate the seismic motion by taking into account the specific source, path and site characteristics, for each earthquake scenario. for each case, source parameters relative to each fault were used in the simulations. regarding path parameters, we used the duration model proposed by atkinson and boore [1995] and a geometrical spreading model of 1/r [atkinson and boore 1995] for the first 50 km of distance (since all scenarios were calculated for small epicentral distances). the q models that we used were the ones proposed in greek literature. more specifically we used the model proposed by tselentis et al. [1988] for the kalamata monument case, since it figure 6. h/v spectral rations resulting from microtremor measurements for the agios nikolaos (aighio pilot area), agioi theodoroi (kalamata pilot area) and agios minas and pantanassa (heraklion pilot area) monuments (left, central and right column, respectively), on the top of the monuments (top row) and on the ground (bottom row). sboras et al. 12 is attributed to this specific region and the model proposed by hatzidimitriou [1994], for the rest, as it has been found to be applicable in previous greek studies. as for site parameters, these were different and relative to each site. more specifically a vs30 value was assigned for each site relative to its soil conditions according to the eurocode 8. for this value the empirical attenuation parameters and kappa values that were proposed by margaris and boore [1998] for soil category b and by klimis et al. [1999] for soil categories c and d were used. more specifically, according to eurocode 8 (design of structures for earthquake resistance), the earthquake motion at a given point on the surface can be represented by an elastic ground acceleration response spectrum, henceforth called an “elastic response spectrum”. moreover, the seismic motion may also be represented in terms of ground acceleration time histories and related quantities (velocity and displacement). when a spatial model is required, the seismic motion consists of three simultaneously acting accelerograms without using the same accelerogram simultaneously along both horizontal directions. depending on the nature of the application and on the information actually available, the description of the seismic motion may be made by using artificial accelerograms and recorded or simulated accelerograms. in the cases of the examined monuments however, where the “life” of the structures is more than 600 years, the use of the maximum expected magnitude was selected over specific magnitudes with specific return periods, in order to simulate the effects of a worst case scenario that could occur during the large lifespan of a monument. it should be noted here that synthetic acceleration time histories for all possible seismic scenarios and local active faults were calculated using the stochastic methodology described in a previous session. the synthetic accelerograms for the three case studies were generated so as the duration ts of the stationary part of the accelerograms that exceeded the minimum of eurocode 8 namely 10 sec. moreover, the mean of the zero period spectral response acceleration values (calculated from the individual time histories) was not smaller than the value of ag·s (where ag is the design ground acceleration on type a ground for the aforementioned examined cases and s is the soil factor) for the site in question. finally, in the range of periods between 0.2t1 and 2t1, where t1 is the fundamental period of the structure in the direction where the accelerogram was applied, no value of the mean 5% damping elastic spectrum, calculated from all time histories, was less than 90% of the corresponding value of the 5% damping elastic response spectrum. in the model for the entire structure, the seismic actions were imposed at the foundation level of the model, by applying the artificial time histories of the examined figure 7. (a) geometry of a modelled byzantine cross vault. (b) the first predominant eigen mode of the entire models of agios nikolaos and agioi theodoroi monuments. seismic hazard assessment for the cultural heritage of greece 13 scenarios. as already mentioned, the entire model of the churches was developed using three-dimensional, tetrahedral finite elements. all geometric details of the structure, including the imperfections in the alignment of the walls, were introduced into the numerical model. the thickness of the walls and the roof varies following the actual geometry in detail. the various parts of the entire model were considered as statically dependent. this is a realistic assumption, since the interlocking of cornerstones is evident across the structure, typical of the byzantine construction method of masonry. the models consist of cross vaults, numerous arches, squinches, pendentives, hemispherical vaults representing the domes, and quarterspherical vaults. in figure 7a, a typical byzantine cross vault is pictured, and simulated with the abaqus code. the boundary conditions of the model (considerably stiff springs) were imported. the mechanical properties of the materials assigned to the entire model were set according to the bibliography concerning the characteristics of the masonry of each monument and the data collected from microtremor tests (table 1 and table 3). with the aforementioned procedure, a reliable calibration of the numerical model representing the structure of the katholikon was ensured. masonry rubblework brickwork marble young’s modulus (mpa) 1600 1100 1000 56000 density (kg/m³) 2240 2200 1800 3000 poisson ratio 0.20 0.20 0.20 0.20 table 3. example of the mechanical properties for the materials of agios nikolaos, platani (aighio pilot area). figure 8. (a) maximum pga distribution for the aighio pilot area that was calculated in the scopes of this project. (b) the calculated synthetic accelerogram at the site of the agios nikolaos monument produced by the ag11 seismic source. (c) the calculated spectrum for the same site. (d) and (e) the monument’s structure response to ground motion after the hypothetical rupture of ag11 seismic source. sboras et al. 14 the first eigenmode for the first and second case study monuments, namely agios nikolaos and agioi theodoroi, computed from the frequency analysis, correspond to deformation of the entire structure (figure 7b) along s-n direction while for the case of agios minas and pantanassa, the first eigenmode corresponds to the motion of the belfry. therefore, the data from the microtremor test was compared to the third eigenmode obtained from the frequency analysis which corresponds to the motion of the monument along s-n direction. after the frequency analysis, the dynamic analysis of the structures subjected to the time-history scenarios was established. in the case of agios nikolaos the network of cracks on the walls due to previous earthquakes was visible contrary to the other two cases of monuments where the recent interventions or existing coating of walls impeded the observation of the masonry pathology. therefore, the results of the numerical analysis concerning time-history analyses for one of the examined scenarios are presented herein, verifying that linear analysis can interpret the existing crack pattern. to this aim, the model regions in which principal tensile (or compressive) stresses significantly outrun the strength of the masonry were compared with the surveyed crack pattern of the monument. in figures 8, 9 and 10 the principal tensile stresses are presented as obtained from the numerical analysis for a specific moment of one of the time histories of the seismic motion examined. after applying all the possible scenarios of the seismic motion in the numerical analysis, it was feasible to determine the most vulnerable parts of each structure. in the case of agios nikolaos, for example, the figure 9. maximum pga distribution for the kalamata pilot area that was calculated in the scopes of this project. (b) the calculated synthetic accelerogram at the site of the agioi theodoroi monument produced by the doli seismic source. (c) the calculated spectrum for the same site. (d) and (e) the monument’s structure response to ground motion after the hy-pothetical rupture of doli seismic source. seismic hazard assessment for the cultural heritage of greece 15 seismic analysis showed that the maximum values of the principal tensile stresses appeared at the upper part of the three conches where nowadays several cracks are visible and at the base of the cross vaults where permanent deformation has been observed. similarly, the analysis of agioi theodoroi showed that the stresses at various parts such as the upper part of the perimetric walls, the vaulted roof and the dome could not be ignored and in the case of agios minas and pantanassa, significant stresses were noticed in several parts such as the base of the belfry and the upperpart of the arches of the northern aisle. 6. conclusions for the needs of “aspida” project, all available data for the historical monuments of greece were collected, assessed, processed and compiled in a gis database arranged in layers. the monuments’ dataset/layer counts more than 2900 entries (monuments) along with their relative data, such as administrative, geological and sha information. pga values were calculated for each and every monument in the greek territory and integrated in the database, providing for the first time an initial, but uniform perception of the seismic hazard for all monuments in greece. the geodatabase is scale-free and can host data of various scales according to the detail of the data. it is designed to be updateable and adaptable in order to process and integrate new data and receive new outputs. the future purpose of the database is to gradually replace the national-scale inputs and outputs with more accurate data and results that will derive from focused investigations such the ones followed in the three pilot areas. the end-user is able to retrieve information easily for the items of each dataset (monuments, seismic sources, etc.), while data can be visualised using the backfigure 10. maximum pga distribution for the heraklion pilot area that was calculated in the scopes of this project. (b) the calculated synthetic accelerogram at the site of the agios minas and pantanassa monument produced by the vasilies seismic source. (c) the calculated spectrum for the same site. (d) and (e) the monument’s structure response to ground motion after the hypothetical rupture of vasilies seismic source. sboras et al. 16 ground layers of google (map or satellite). determination of seismic sources and geological conditions for the whole greek territory was carried out by assessing mostly bibliographic data. these data were then associated with morphological, seismological and other data. note that this determination resulted from small-scale data (1:500,000) that do not have the required accuracy for the detailed assessment of seismic risk. the scope of the national-scale seismic hazard map, based on deterministic approach, is the initial and uniform seismic hazard assessment on every position. in case of specific buildings for which detailed assessment of seismic risk is required, detailed investigation of both the geological conditions around the monument and the detailed mapping and investigation of fault zones in the broader area are necessary. such an approach was followed for the three pilot areas of aighio, kalamata and heraklion. as seen in figure 2, calculated pga values greatly exceed those proposed by the greek earthquake protection law and in many cases reach values close to 1 g. this is mainly due to the fact that the values proposed in the greek seismic code regulation [eppo 2001] are based on probabilistic methodologies and are linked to specific return periods. this approach however is not optimal for the case of monuments, where the life expectancy is expected to be as much as possible and therefore their seismic behaviour should be investigated on the basis of the worst case scenario. additionally, it should be noted that the distribution of the seismic hazard values in the greek earthquake protection law does not explicitly take into account the detailed seismotectonic as well as the specific geotechnical conditions in greece. on the other hand, the greek seismic code regulation [eppo 2001] incorporates the site effect, as well as other design motion multiplying factor differently. furthermore, large pga values, calculated with the previously described stochastic methodology, usually correspond to singular pulses and are not representative of the level of energy released by the shock. subsequently, it is obvious that the earthquake protection code of greece should be updated, using all available seismotectonic and local site condition data in conjunction with modern methodologies which can lead to more accurately calculated seismic hazard. subsequently, based on the calculated seismic hazard, a structural analysis can be implemented so as to determine the most vulnerable parts of the structures and propose the appropriate strengthening measures. the geodatabase of the “aspida” project contains a number of important strong motion parameters for the whole region of greece, being thus a valuable tool to the engineers for the scope of fortifying any important building in the region. the geodatabase also organizes various information from the fields of active tectonics, geology, seismology, engineering and others, in a practical gis environment accessible to all interested parties. in this particular study, the geodatabase was utilized in the scope of sha for a wide region. the database applications however are not limited only to this field of natural hazards. as it keeps being developed, it could also be utilized for landslide hazard assessment, liquefaction potential, tsunami hazard and other fields regarding public safety. acknowledgements. in the frame of the research action “development proposals of research institutions-kripis” of the general secretariat of research and technology, the institute of geodynamics has undertaken the responsibility of conducting the project “upgrading infrastructure for earthquake protection in greece and enhancing services through actions of excellence” (aspida). aspida was a three components project. one of these components was the “measures for cultural heritage protection against strong earthquakes”. the present work was concluded in the context of this component and was supported by a large working group. we also like to thank prof. k. makropoulos (the former director of the institute of geodynamics) and dr d. papanastasiou for their contribution to the project design. thanks to all members of the working group of the project, all referred in the website http://www.gein.noa.gr/aspida3. we also thank an anonymous reviewer and the editors for their fruitful comments that helped improving the article. references alves, t.m., lykousis, v., sakellariou, d., alexandri, s. and nomikou, p. 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(2016). palaeoseismological analysis of the east giouchtas fault, heraklion basin, crete (prelimenary results), bull. geol. soc. greece, 50, paper 120. *corresponding author: sotiris sboras national observatory of athens, institute of geodynamics, lofos nymfon, thiseio, athens, greece; email: sboras@noa.gr. 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved vol49_1_2006def 245 annals of geophysics, vol. 49, n. 1, february 2006 key words hyperspectral – iron bearing minerals – paleoclimate 1. introduction the las tablas de daimiel natural park frames a lake at the head of the river guadiana draining to the atlantic, settled in the large plain of la mancha in central spain (fig. 1). this continental dry plain is subject to a mediterranean dry, semi-arid climate with dry summer and winter and relatively short rainy seasons in fall and spring. periods of drought are recurrent within mapping of semi-arid iron bearing red sands on emerged areas around lake marshes (tablas de daimiel, spain) using hyperspectral dais 7915 spectrometer data asunción riaza (1), eduardo garcia-melendez (2), mercedes suárez (3), andrea hausold (4), ulrich beisl (4) and harald van der werff (5) (1) instituto geológico y minero de españa (igme), madrid, spain (2) área de geodinámica externa, facultad de ciencias ambientales, universidad de león, spain (3) departamento de geología, universidad de salamanca, spain (4) dlr_german aerospace research establishment, remote sensing data centre, oberpfaffenhofen, wessling, deutschland (5) international institute for aerospace survey and earth sciences (itc), enschede, the netherlands abstract wetlands are particularly sensitive environments receiving attention from the natural sciences community due to their wealth of both flora and fauna, and often considered as natural parks. in the tablas de daimiel (la mancha, central spain), digital airborne imaging spectrometer data (dais 7915) have been analyzed to map geological processes on areas around the receding wetland which have never been flooded by water in the past. sediments permanently exposed to the atmosphere dehydrate and oxide, developing different mineralogical associations arranged on planation surfaces. such planation surfaces are key in the geological knowledge of recent climate change and landscape evolution. progressive iron oxide/hydroxide rate and decarbonation can be spectrally followed on the holocene sands framing the current marshy area. such mineralogical changes are geologically registered on flat surfaces at different heights over the receding shore of the paleolake. interacting erosion and sedimentation processes are responsible for the development of the flat morphological surfaces with increasing dryness. maps are built for four different morphological units consisting of planation surfaces following chronologically the receding marsh during the last 2000 years before the present. interactive spectral responses of mineralogical associations are described on the imagery, field and laboratory spectra. mailing address: dr. asunción riaza, instituto geológico y minero de españa (igme), rios rosas 23, 28003 madrid, spain; e-mail: a.riaza@igme.es 246 asunción riaza, eduardo garcia-melendez, mercedes suárez, andrea hausold, ulrich beisl and harald van der werff the lapse of five years. it is a wetland with a linear morphology associated with alluvial river flats and hollows, resulting from the overflooding of the rivers cigüela and guadiana, joining and developing the tablas, and the natural discharge of the underlying aquifer, through upwelling water at locations termed «ojos» (eyes). the study area is located at the northern las tablas within the floodplain of the cigüela river, carrying silty and evaporitic sediments from tertiary and triassic gypsum materials underlying the upstream river course. the floodplain is formed by silty and clay sediments with organic matter and gypsum. the recent sedimentological record of the quaternary deposits in the area points to recent harsh paleoenvironmental conditions (2500-2300 years before present) with diminishing humidity and lowering of groundwater levels (garcía antón et al., 1986). intensive use of groundwater for cereal crop irrigation has produced a serious descent on the groundwater level and coverage, an already irreversible environmental problem. the spectral behaviour of iron bearing minerals has long been studied because of their widescale presence on lunar rocks (hunt et al., 1971). limonitic alteration was one of the first features attracting the attention of geological remote sensing (rowan et al., 1977; hunt and ashley, 1979; segal, 1983; townsend, 1987). weathering processes produce the same minerals as hydrothermal alteration processes (buckingham and sommer, 1983), and mask the spectral response of underlying rocks with coatings and internal mineralogical transformations (lyon, 1997). space imagery is able to depict subtle variations in the reflectance properties of desert surfaces, which are indicative of changes in mineralogical composition. the intensity of the red colour of sands in namibia (logan, 1960) has been used as an indication of age in the dunes. in australia, the intensity of dune reddening resulted in information on the distance from the source (el baz and prestel, 1980). recent use of spectral response of iron bearing minerals in fluvial sediments is addressed to map the contamination of mine waste disposals (farrand, 1997) using hyperspectral imagery. traditional studies on laterite soils are conducted using the relative amount of iron oxides and hydroxides (madeira et al., 1997) in humid rainforest climate. weathering of rock and soil surfaces responsible for the exterior mineral mixtures mainly fig. 1. geological map composed from dais images indicating the main morphological units not previously mapped on emerged areas overprinting the holocene underlying sands. geographical location of the tablas de daimiel wetland and natural park in central spain and the northeastern area within dais data selected for mapping. 247 mapping of semi-arid iron bearing red sands around lake marshes using hyperspectral dais 7915 data contributing to the spectral response detected by remote sensors has gained attention among the remote sensing geological community (pontual, 1987; amos and greenbaum, 1989; riaza et al., 1995, 2000, 2001; lyon, 1997; younis et al., 1997). different stages on weathering products and textures have been used for relative dating of lava flows of the same chemical and mineralogical composition with thermal infrared data (kahle et al., 1988; abrams et al., 1991). the mixtures of iron and clay minerals produced by weathering alteration associated with posthercynian erosion paleosurfaces in the duero basin (central spain) developed under different paleoclimate have been mapped helping to model sediment patterns on various stages of the evolution of the sedimentary basin (riaza et al., 2000). climate-dependent saline soils, carbonate, organic matter and iron oxide surfaces have been mapped along different stages of flooding and emersion in the past 2000 years using hyperspectral data on prior work (riaza et al., 2002). emphasis is focused here towards the areas which have always been over the shoreline. lakes are environments particularly sensitive to climate changes, both actual and paleoclimate. open system lakes fed by groundwater and seasonal runoff show a fluctuating shoreline. the most recent sediments hosting the tablas de daimiel wetland are holocene sands arranged on several morphological units descending towards the marshy area. on a semiarid climate, the well ventilated and open sand deposits which are emerged, easily lose water. the iron bearing minerals decrease in hydroxides (goethite) to gain anhydrous ferric oxide (hematite) (duchaufour, 1984) with the time of exposure to the atmosphere. as the wetland recedes, the whole sedimentological system on the riversides is demantled down towards the river. four geomorphologic units not previously mapped can be traced through the imagery on the everemerged sands surrounding the marsh, based on their different height from the coastline, confirmed by fieldwork and aerial photography interpretation. these units are related to terraces, eolian deposits and desiccated areas. the earlier units are topographically high, and show a bright strong brown colour to the eye because of the comparatively higher hematite contents. further loss of material by erosion and sedimentation produced by rain, runoff, and wind break the planation surfaces, which appear as both continuous and remanent sparse encased surfaces showing different stages of landscape development. 2. data set dais 7915 (digital airborne imaging spectrometer, vis-nir-tir) hyperspectral spectrometer data were recorded in july 2000, aiming at minimum vegetation vigour and maximum soil exposure. flight altitude was decided to ensure the better sensor performance minimizing noise. non-coherent noise was corrected after inflight calibration for dais (strobl et al., 1996) to remove the sensor sensitivity effects on the 79 dais channels. dais data have a spatial resolution of 5 m. two overlapping flight lines were recorded and georeferenced (schläpfer and richter, 2002). different spectral imaging processing tools were tested on atmospherically corrected data with a radiative transfer code using a midlatitude summer profile (richter, 1996; richter et al., 2002) and field spectra from dark and light targets. black body radiance surface and temperature were calculated using thermal infrared channels from field temperature measurements on a water body (richter, 1996; richter and coll, 2002). field reflectance spectra were collected with a ger spectrometer for thematic analysis on geologically representative targets, to be used for interpretative image processing. nonconsolidated soil samples were collected at different stages of the study of the imagery lead by image processing suggestions on sedimentology and time-dependent geomorphological processes resulting in mineralogical changes. laboratory reflectance spectra were acquired on these samples using a perkin-elmer lambda 6 spectrometer (400-2400 nm) provided with an integrating sphere using a standard of barium sulphate. non-consolidated rock samples were dry-sieved before measurement. munsell soil colour indexes (munsell, 1990) were assigned in the laboratory to soil samples under a constant illumination source. 248 asunción riaza, eduardo garcia-melendez, mercedes suárez, andrea hausold, ulrich beisl and harald van der werff x-ray diffraction (xrd) analysis was performed on selected non-consolidated rock samples to confirm field observations regarding mineralogical contents. further spectral measurements were made on powdered samples after xrd aiming to ascertain spectral features which might appear on fine grained samples. image processing was conducted using envi (rsi, 2000) hyperspectral modules and reference spectral libraries. the spectral interpretation of imagery, field and laboratory spectra was led by geological knowledge of the area and conventional geological field and laboratory survey. interaction of all data and multidisciplinary analysis have been used throughout the study at every stage of progress. 3. digital image processing dais images were processed aiming to separate different sediments and minerals indicative of geological processes of palustrine, fluvial and eolian environments (fig. 1). mosaics were composed with the two flight lines for synoptic view of the area, and for selection of areas of interest (fig. 1). masks were built for densely vegetated areas using channels 17 (0.7780 nm) and 13 (0.7070 nm) for a red/near infrared ratio. masks for water were also built using digital number 26 on channel 1 (0.4900 nm) as threshold. a false colour composite with channels 19 (0.8130 nm), 6 (0.5850 nm) and 53 (2.1330 nm) (brg) was selected gathering information on wavelength ranges related to iron bearing minerals, and carbonate, for a preliminary estimation of the spectral variability in the area. the six thermal infrared channels were used for an early evaluation of lithological variety (riaza et al., 1998). both principal components and minimum noise fraction transforms were useful to identify the main lithological regions indicating further hyperspectral image processing. following field sedimentological observations suggested by previous image processing, masks were built using thresholds on selected minimum noise fraction transforms from thermal infrared dais channels to isolate geological units within the sands (riaza et al., 2002). within the areas assigned to each unit, pixel purity index and n-dimensional analysis were run on the 72 vnir dais channels, helping to understand the geological spectral meaning of the various statistical populations among the sands (boardman and kruse, 1994). 4. laboratory spectra the holocene red sands surrounding the tablas de daimiel wetland are sediments composed of sand with quartz as a main component. comparison with usgs mineral spectral library on individual isolated minerals present on soil was made to identify the spectral features on the laboratory spectra possibly related to the minerals present on the mixture (fig. 2a). x-ray diffraction analysis showed no presence of clays on the red sands, but rather widespread carbonate. spectra acquired on clay size powder from soil samples showed the presence of carbonate on most red sands. strong brown colour (munsell, 1990) occurs on the earlier and topographically highest early morphological units rich in hematite resulting in deeper iron absorption in the visible and higher overall reflectance (fig. 2b). dark brown colour on soil samples from recent units with more abundant iron hydroxide in the mineral mixture is associated with spectral lower overall reflectance. the early topographically higher morphological units rich in hematite display a smooth absorption at 850 nm because of the presence of ferric cation (hunt et al., 1971). hematite is also shown by the inflexion point at 548 nm in the visible (fig. 2b). the recent units display a less steep straight line response in the visible and lower overall reflectance, due to the more abundant iron hydroxide. recent units also show a relatively emphasized shape at 2300 nm due to carbonate (hunt and salisbury, 1971). 5. dais spectral response spectral profiles are extracted from the areas finally mapped on dais imagery (fig. 1). the spectra shown in this chapter are highly fig. 3a,b. a) vnir spectral profiles from dais images from areas representative of morphological units on the map in fig. 1. b) tir spectral profiles from dais images from areas representative of morphological units on the map in fig. 1. 249 mapping of semi-arid iron bearing red sands around lake marshes using hyperspectral dais 7915 data mixed areas both in geological and land use terms. the four morphological units are sands containing mixtures of iron bearing minerals which can be spectrally traced by relative transitional mineralogical abundances produced by increasing dehydration and decarbonation with time. the sands are cultivated with vineyard crops, regularly spaced plants which cannot be spectrally removed from data with a spatial resolution of 5 m. crops diminish near the natural fig. 2a,b. a) spectra of the main minerals present in the sediments of the area of study related to dehydration and oxidation of sands exposed to the atmosphere (united states geological survey spectral library). b) laboratory spectra from field non-consolidated rock samples measured with a perkin elmer lambda 6 spectrometer on representative morphological units. a b a b 250 asunción riaza, eduardo garcia-melendez, mercedes suárez, andrea hausold, ulrich beisl and harald van der werff park and the land use changes to an intermittent ploughed and natural vegetation pattern, where the recent and most recent units occur. the following spectral observations must be considered trends of spectral behaviour. no single mineral influence is expected to be identified. dais images present a distinguishing response between once flooded and everemerged areas surrounding the wetland (riaza et al., 2002). spectra of all the emerged areas, whether sand deposits or continental aprons or fluvial deposits, show depressions in the visible mainly due to the presence of iron bearing minerals (fig. 3a). the wide depression in the visible related to iron bearing minerals on red everemerged sands is more pronounced, consistent with their strong brown colour (fig. 3a). in all of the mapped units, early emerged sands display the highest overall reflectance. the areas which were once flooded are less expressive in the visible (riaza et al., 2002) because of the presence of organic matter. the areas emerged display a shoulder between 2.2-2.3 nm non-existent on the flooded areas. this might suggest abundance of clays, which is not confirmed by xrd mineralogical analysis or by sedimentological expectations. widespread carbonate is more likely to contribute to this spectral feature. four systems of reddish sands arranged on four morphological units have been spectrally identified based on different iron minerals related to variable time of exposure to the atmosphere and weather conditions. such morphological units can be distinguished in the field and spectra by intensity of reddening that is directly related to age (figs. 2a and 3a). the intensity of redness in semi-arid environments is related to the increasing hematite contents (duchaufour, 1984) by loss of water in the environment. goethite decreases in the iron bearing mineral mixtures with increasing dryness. such mineralogical changes are spectrally recorded by the imagery. the topographically upper and early morphological units show a shoulder on 0.85 nm which would be related to the ferric ion from hematite (hunt et al., 1971). this is not present on the lower and recent morphological units. the two recent morphological units display a smooth depression between 0.70-0.74 nm due to the iron hydroxide goethite (hunt et al., 1971). vegetation has been discarded as contributing to this feature, since vineyard crops are homogeneously distributed. carbonate is also suggested on the two recent morphological units by a subtle 2.3 nm absorption or weakened shape in comparison with spectra from hematitic surfaces. this absorption is not present on spectral profiles of the two earlier units, which have been exposed long enough to be decarbonated. carbonate is widespread throughout the area both in emerged and flooded areas. imagery is too noisy on the near infrared to be able to show carbonate-related absorption, which is relatively weak on the laboratory spectra. spectral profiles on dais imagery do not depict a clear spectral response due to the low energy available for the fourth detector on the 2000-2500 nm wavelength range, but tendencies of more abundant carbonate are suggested on the early more hydrated units. the thermal infrared channels from dais images display a varied spectral response for the four morphological units (fig. 3b). the recent units rich in hydroxides show a higher radiance surface at 8.58 nm decreasing comparatively to the older units at longer wavelength ranges. further laboratory spectral analysis is necessary to explain diversity on the spectral response. 6. conclusions morphological units developed over sediments not previously mapped have been outlined. such morphological units are planation surfaces which developed during a certain period of time. the four morphological units have been mapped using dais images, and their spectral properties described in terms of geological processes resulting in different mineral mixtures. image processing has helped to lead the identification by conventional geological survey. further image processing produced the final map. hyperspectral imagery has been able to distinguish four morphological units descending towards a receding wetland due to different iron bearing mineral contents associated with variable times of exposure to the atmosphere and 251 mapping of semi-arid iron bearing red sands around lake marshes using hyperspectral dais 7915 data increasingly dry weather conditions. mineralogical variations on iron oxides and hydroxides developed when sediments are emerged are easily recorded in the visible wavelength range. the older units rich in hematite and poor in carbonate depict a distinguishing convex shape in the visible with a shoulder at 850 nm due to the ferric ion from hematite. on the contrary, the younger less dehydrated units, display a smooth depression between 700-740 nm likely due to goethite, and a subtle absorption or flattened response at 2300 nm because of the presence of carbonate. both features are displayed by imagery and laboratory spectra. dais thermal infrared, particularly sensitive to topography, orientated the first image processing to depict spectrally variable areas. a more detailed map of the emerged morphological units has been drawn from the imagery, with further hyperspectral image processing in the 72 vnir channels on selectively masked areas for wide geological areas outlined from thermal infrared. acknowledgements the fifth framework program, improvement of human potential, access to research infrastructures (contr. hpri-ct-1999-00075, ref hs2000-es1) and the science and technology commission in spain (cycit ren 00204433-c02-01) funded this work. luis pascual assisted with the use of the laboratory spectrometer. thanks are due to s. martin alfageme and iñigo martin for software and hardware assistance (stig, university of salamanca). references abrams, m., e. abbott and a. kahle (1991): combined use of visible, reflected 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(1987): discrimination of iron alteration minerals in visible and near-infrared reflectance data, j. geophys. res., 92 (b2), 1441-1454. younis, m.t., a.a., gilabert and j. meliá (1997): weathering process effects on spectral reflectance of rocks in a semi-arid environment, int. j. remote sensing, 18 (16), 3361-3377. 001_029 adg v–5 n01.pdf annals of geophysics, vol. 45, n. 1, february 2002 1 coupling from below as a source of ionospheric variability: a review edward s. kazimirovsky institute of solar-terrestrial physics, russian academy of sciences, siberian division, irkutsk, russia abstract the purpose of this paper is to review shortly some observational backgrounds for the suggestion of a genuine link between processes in the lower atmosphere and ionospheric response. attention is concentrated on the waves which are thought to couple the lower atmosphere with the thermosphere/ionosphere system. mailing address: prof. edward s. kazimirovsky, institute of solar-terrestrial physics, russian academy of sciences, siberian division, post box 4026, 664033 irkutsk 33, russia; e-mail: ekazimirovsky@hotmail.com 1. introduction the study of the earth’s upper atmosphere is of interest to scientists from many disciplines and present day knowledge and understanding of the upper atmosphere to the highest levels is the result of the combined contributions of m e t e o r o l o g i s t s , p h y s i c i s t s , c h e m i s t s , a s t r o n o m e r s , g e o m a g n e t i c i a n s , r a d i o engineers and space scientists. this range of scientific disciplines has, not unexpectedly, been matched by an equally extensive variety of experimental approach from standard meteorological instrumentation to the modern optical, radio and space-vehicle techniques yielding data from distances out to and far beyond, the limits of the atmosphere. these experimental studies have, over the years, been supplemented by much theoretical work to give the picture we now have of the physical state of the earth’s upper atmosphere. the terrestrial ionosphere is a part of the upper atmosphere, it is a cold magnetized plasma environment enveloping the earth whose behaviour is often described as ionospheric aeronomy. the chambers dictionary of science and technology denotes the term «aeronomy» as «the branch of science dealing with the atmosphere of the earth and other planets with reference to their chemical composition, physical properties, relative motion and reaction to radiation from outer space». but the international meteorological glossary (1991) defines it more precisely: «a term sometimes used to denote that branch of the earth’s atmospheric physics which is concerned with those regions, upwards of about 50 km, where dissociation and ionization are fundamental processes». key words planetary waves − gravity waves − middle-upper atmosphere interaction «every truth is fated to enjoy only one moment of triumph between the infinity when it is considered misleading, and the infinity when it is considered trivial...» h. poincare 2 edward s. kazimirovsky the principal physical, chemical and electrical properties of the earth’s upper atmosphere are very largely the result of its interaction with solar wave and particle radiation. the ultraviolet and x-radiations are absorbed at various levels between about 40 and 200 km. the selective absorption of the ultraviolet and x-radiation by particular atmospheric constituents gives rise to the unique electrical properties of the upper atmosphere by providing a series of ionized strata (layers) collectively known as the ionosphere. other ultraviolet radiation is very effectively absorbed lower down in the atmosphere by ozone. owing to the pervasive influence of gravity, the atmosphere and ionosphere are to first order horizontally stratified and conventionally divided into layers based on the vertical structure of different parameters. the atmospheric structure can be neatly organized by a representative temperature profile, while the ionosphere is more sensibly organised by the number density of plasma. a schematic representation of atmospheric regions is shown in fig. 1 (whitten and poppoff, 1971). in the following text we use the terms «upper atmosphere» and «middle atmosphere», which are absent in fig. 1. in the past, meteorologists often designated the entire region above the tropopause as the «upper atmosphere». but rather recently the term «middle atmosphere» became popular in referring to the region from the tropopause to the turbopause and even to the lower thermosphere. in addition to electromagnetic (wave) radiations, the sun also emits continuously streams of energetic electrically charged particles – mainly protons and electrons. this is the so-called «solar wind» – an outward flow of charged particles moving at velocities of a few hundred kilometers per second. they interact with the geomagnetic field in a complicated way, and with the gases of the upper atmosphere. as a result, we can observe the geomagnetic storms, the disturbances to long-distance radio-wave communication via the ionosphere and the visible auroral displays at high latitudes. all these events are closely associated with the occurrence of sunspots and show characteristic time variations over 11 years and 27 days, respectively, related to the sunspot cycle and the period of rotation of the sun. the upper atmosphere/ionosphere acts as the intermediary between the plasma-dominated magnetosphere and the bulk of the neutral atmosphere below. this region is highly complex. interacting dynamic, chemical, radiative and electrical variations occur there. to understand how these coupled elements interact to produce the great variability characteristic of the system is one of the major problems in solar-terrestrial relations. for example, the three-dimensional circulation of the thermosphere changes during and following geomagnetic storms; yet the consequences of the change in circulation on the temperature, density, composition and electric currents of the region are poorly understood. energetic solar particles penetrate the middle atmosphere and produce chemical changes in radiatively important species such as ozone, but their global consequences are not fully appreciated. deeper in the atmosphere, solar induced variations in the flux of cosmic rays may produce variations in the electrical structure of the lower atmosphere, but the effects of these variations on the earth’s global electrical circuit (including the ionosphere) are not fully understood. the earth’s ionosphere is a partially ionized gas that envelops the earth and in some sense forms the interface between the atmosphere and space. since the gas is ionized it cannot be fully described by the equations of the neutral fluid dynamics. on the other hand, the number density of the neutral gas exceeds that of the ionospheric plasma and certainly neutral particles cannot be ignored. therefore the knowledge only of two «pure» branches of physics: classical fluid dynamics and plasma physics is not sufficient. in addition to atmospheric dynamics, space physics, ion chemistry and photochemistry are necessary to understand how the ionosphere is formed and buffeted by sources from above and below and to deal with production and loss processes. until recently, the ionosphere was studied as a merely magneto-active plasma without consideration of the general properties of the atmosphere. it was known that the neutral atmosphere (thermosphere) and ionosphere are coupling from below as a source of ionospheric variability: a review 3 fig. 1. schematic representation of atmospheric regions. classification of atmospheric layers in accordance with temperature (right) and with electron concentration (left) (whitten and popoff, 1971). 4 edward s. kazimirovsky linked by electrodynamic and momentum transfer. nevertheless, almost nobody believed that the terms «meteorology» and «climatology» can be applied to the ionosphere, it was thought that no connection existed between events occurring in the troposphere/ stratosphere system on one hand, and in the ionosphere on the other. meanwhile, however, it has become apparent that the composition, chemistry, energetics, dynamics and resulting structure of the lower and upper atmosphere, which are functions of location and time, are so intricately interrelated that it is not really possible to discuss each of them in isolation. gradually, the viewpoint that the lower and upper atmosphere are substantially uncoupled was rejected. many discoveries and experimental results have confirmed the existence of detailed correlations between the parameters of the lower and upper atmosphere (kazimirovsky and kokourov, 1991). but what is the reason for the correlations? as part of a physical mechanism, the influence of internal atmospheric waves may be considered. the upward propagation of internal atmospheric waves (planetary waves, tides and gravity waves) from the troposphere and stratosphere is an essential source of energy and momentum for the thermosphere and ionosphere. of course, the study of internal waves is the province of meteorology, a discipline that has enjoyed a long and independent development of its own and has its own complicated problems, sufficiently different from ionospheric physics that the two are regarded as separate but neighbouring disciplines. however, the internal waves launched by weather fronts or any other sources in the troposphere and stratosphere sometimes appear to be capable of penetrating into the ionosphere, where they dissipate their energy. the leakage of wave energy from the troposphere and stratosphere at least up to 100115 km was introduced as «coupling from below» (bowhill, 1969) and is considered as a mechanism of the meteorological influence on the ionosphere. this influence has been assumed and is presented by various models. the meteorology of the thermosphere differs considerably from that associated with the familiar weather patterns we experience at the earth surface, although the fluid motions are governed by the same equations as those used by meteorologists studying the weather systems. in the thermosphere, temperature increases with altitude, making for a dynamic system that is less dominated by instabilities than the troposphere, where the temperature gradient is in the opposite direction. also, the viscous and ion drag forces are very important in the thermosphere, with the former tending to transfer momentum between various altitudes and the latter acting to strongly couple the neutral thermosphere to the ionosphere and thereby, to the magnetosphere. it is now established beyond doubt that the atmosphere extends from the ground to the thermosphere, behaving as a complex system coupling fairly closely over wide height ranges. for the aeronomical models, the major questions now posed relate to the interactions or coupling between regions, interactions that are currently only crudely parameterized within the separate models. this is particularly true for the upper atmosphere, where a rich set of physical processes have been identified that couple the various regions together, as well as to the stratosphere and troposphere below and to the magnetosphere above. therefore, the principal scientific challenge facing us today is to understand the coupled system as a whole, including the effects of energy, momentum and compositional interchange between regions. the analysis of «traditional» meteorological and ionospheric data is not only of interest for the exploration of purely theoretical aspects of the stochastic atmosphere system, but also of relevance for modern climatological research. for instance, it is a still unsolved question as to how much the general state of the middle and thus the lower atmosphere and its circulation systems are influenced by changes with various timescales at the upper boundary thermosphere/ ionosphere. such changes may even be left in the troposphere. since the upper atmosphere is generally a good indicator of solar activity, one might assume that the correlation between tropospheric and ionospheric parameters indicates such a solar-atmosphere or solarweather effect. although it is easier to picture coupling from below as a source of ionospheric variability: a review 5 upward dynamic coupling because of the much greater energy density that resides in the lower atmosphere relative to that of the upper atmosphere, it is possible that changes in the upper atmosphere can give rise to significant changes in the lower atmosphere. the subject of solar activity effects on tropospheric weather is still controversial. less controversial, however, is the possibility of solar activity effects on climate. but the very interesting problem «the role of the sun in climate change» requires special consideration elsewhere. our main concept is that the science of the whole atmosphere-ionosphere system becomes greater than the sum of its component parts. the scientific focus is placed on the interactive processes among the various physical regimes. this concept is in the base of international solarterrestrial energy program (step) and poststep activity, comprehensive study of the mutual linkages between the various regions of space in addition to the traditional study of the individual regions themselves. many of the ionospheric phenomena cannot be explained only in terms of the ionizing radiation variations, photochemical processes, solar particle injections or solar flare effect. it seems likely that there are some events due to the effect of atmospheric oscillations, such as the winter anomaly of radiowave absorption, e-sporadic layer occurrence and structure, thermospheric wind regime, the variations of atmospheric emissions, travelling ionospheric disturbances, day-to-day midlatitude ionospheric variability, f-spread event, etc. finally, the atmosphere and ionosphere are linked dynamically, radiatively and chemically. it is why in the following paragraphes we discuss the atmospheric dynamics and ionosphere from the meteorological positions. aeronomy is now the study of the upper atmosphere physical and chemical processes including ionospheric processes. electrical engineers are interested in the propagation of radiowaves; physicists in plasma processes; chemists in the dissociation and diffusion of atmospheric molecules and atoms; meteorologists in dynamics; and so on. as the body of knowledge grew, it became apparent that all aspects are important and that, in fact all processes are interdependent. in upper atmosphere research, as in no other field, the disciplines of electrical engineering, meteorology, chemistry and physics are intertwined (whitten and poppoff, 1971; danilov et al., 1987; hargreaves, 1992; johnson and killeen, 1995; wickwar and carlson, 1999; solomon, 2000; forbes et al., 2000 ). the purpose of this paper is to review briefly some observational backgrounds for the suggestion of a genuine link between processes in the lower atmosphere and ionospheric response. attention is concentrated on the waves which are thought to couple the lower atmosphere with the thermosphere/ionosphere system. detailed explanations of the observations and theories are not provided. of course, the survey presented here will be affected by the personal biases of the author and the limitations of space. since the discussion is intended to be «tutorial» in nature, only a few key references to the literature will be given, and the interested reader can consult these for more detailed discussion and for links to the more extensive literature. 2. internal atmospheric waves one exciting aspect of modern ionospheric research is the significant role of the lower atmosphere in upper-atmospheric variability. the forcing could come from internal atmospheric waves (planetary waves, tides, internal gravity waves). planetary waves are a well-documented group of atmospheric waves, a class of zonally travelling structures of global scale, which have periods of a few days (typically 2-30 days). such waves appear for example in surface pressure data, in standard analyses of upper tropospheric radiosonde data and in satellite stratospheric data. a significant part of the energy of the planetary wave disturbances of the troposphere may propagate into the upper atmosphere and the effective index of refraction for the planetary waves depends primarily on the distribution of the mean zonal wind with height. the international reference atmospheric model (cira-1990) includes the description of planetary waves up to 85 km with the following general features: 6 edward s. kazimirovsky 1) wave amplitudes are small in the tropics throughout the year, slightly larger in the summer season at mid and high latitudes and much larger during the winter season. 2) maximum amplitudes occur at about 6070° north or south latitude during local winter. 3) amplitudes are generally largest in the stratosphere and lower mesosphere but they remain relatively large up to the top level lower thermosphere. there is one important peculiarity in the theory of planetary wave propagation, associated with the existence of critical surfaces or layers, where the basic zonal wind matches the zonal phase speed of wave. the dissipation and nonlinearity become very important in this critical layer. it may be expected to be an enhanced dissipative region for planetary waves or under some conditions the critical layer remains a perfect reflector of planetary waves. the most spectacular manifestations of planetary waves activity in the middle atmosphere, associated with strong coupling between the stratosphere and lower atmosphere, occur during stratospheric sudden warming events. the zonal-mean climatological temperature and zonal wind configuration is dramatically disrupted, with polar stratospheric temperatures increasing rapidly with time, leading on occasion to reversals of zonal-mean winds from westerlies to easterlies (naujokat and labitzke, 1993). current theories suggest that a major sudden warming is initiated by an anomalous growth of a planetary-wave disturbance (mainly comprising wave-number 1 and 2 components) that propagates from the troposphere into the stratosphere and interacts strongly with the preexisting circulation there. the stationary planetary waves that propagate energy upward also transport heat southward (andrews et al., 1987). our understanding of all the observed details of these events, the necessary conditions for their occurrence, interannual variability between one stratospheric winter and another is still by no means complete. much further work will need to be done before a full understanding of the phenomenon is attained. in general, the implication of the theory for the modelling of planetary waves and for interpretation of atmospheric observations are not yet absolutely clear and new experimental information concerning the behaviour and effects of planetary waves, especially in the lower and upper thermosphere is very important. since the seasonal and year-to-year variability of planetary wave activities cannot be simply explained by the steady-state propagation theory, the variations of forcing in the lower atmosphere would be important. atmospheric tides are global-scale oscillations, which are primarily forced by variations of heating due to absorption of solar ultraviolet radiations by atmospheric water vapour in the troposphere, ozone in the middle atmosphere and molecular oxygen o2 in the lower thermosphere. the solar and lunar gravitational forcing that produces ocean tides is much less important for the atmosphere. the migrating tides (diurnal, semi-diurnal, etc.) can propagate through great depths of the atmosphere and can attain large amplitudes at some heights especially in the thermosphere. the semidiurnal tide plays a particularly important role in the lower thermosphere, where the global temperature and density variations are dominated by this mode. at higher altitudes the semi-diurnal tide is dissipated by viscosity and ion drag. in the upper thermosphere, at 300 km, the amplitude of the semi-diurnal tide decreases, and thermospheric density variations are dominated by the diurnal tide that has been forced by the thermospheric absorption of euv solar radiation. in the modern theoretical calculations the semidiurnal tide in the thermosphere is considered as a result of propagation from below. the non-migrating tides (associated, for example, with orography and geographically fixed tropospheric heat sources) would give rise to longitudinal differences in tidal structure. in the classical tidal theory (in viscid atmosphere, background temperature is independent of latitude) the governing equation is separable, giving rise to vertical and latitude structure equations. but in the real atmosphere with winds and meridional gradients of temperature the governing equation may be solved only numerically. coupling from below as a source of ionospheric variability: a review 7 now the modellers try to include in the models the seasonal, latitudinal and longitudinal variations, realistic temperature and wind structures, molecular and eddy diffusion, acceleration and heating of the mean flow by tides, the effect of tides on minor constituent concentration, hydromagnetic coupling all for viscid, rotating, spherical atmosphere (e.g., forbes, 1991). internal gravity waves (igw) are disturbances which are allowed to propagate as a consequence of buoyant forces present in the atmosphere. the temperature and wind structures determine the wave’s propagation characteristics. many seasonally and latitudinally varying sources for middle atmosphere gravity waves have been identified. these include airflow over orography, severe weather fronts, cyclones, instabilities in the planetary boundary layer and in jet stream shears, turbulent motions of different scales, thunderstorms ( e.g., vincent, 1990; gavrilov, 1992). at heights above 85-90 km, the internal gravity waves may be «saturated» and even be broken with deposition of energy and momentum. the «trapping» of igw is also possible, so that one would expect igw to be ducted in the region near the mesopause. it is now believed that the level of gravity wave activity determines the mean state of the mesosphere. moreover, in the lower thermosphere the waves manifest themselves in wind, temperature, density, pressure, ionization, vertical profiles of na, measured by lidars, and airflow fluctuations in the 80-120 km height range and the amplitudes are so large that they can dominate at these altitudes (e.g., krassovsky, 1977; vincent, 1990; gavrilov, 1992). it is now well appreciated that internal gravity waves play an important role in determining the mean circulation and thermal structure of the middle and upper atmosphere. there are theoretical models of the interaction between a gravity wave and the ionization oscillations of ionic velocity and temperature, influence on the electron density, effect on the electron temperature, generation of ionospheric irregularities, etc. 3. the lower thermosphere/ionosphere an atmospheric layer located at about 60 to 100 km altitude can be defined as a transition region, where different fundamental physical mechanisms, dominant in the lower and upper height regions, coexist, showing complicated atmospheric characteristics. for instance, the d-region, the ionosphere at 60-90 km, weakly ionized (mainly in daytime), is an interface between the neutral and ionized atmospheric layers. the d-region is the lowest lying ionospheric region and hence is produced by the most penetrating of the ionizing radiation (galactic cosmic radiation, x-rays, the intense solar hydrogen lyman-alpha emission line and extra-ultra-violet); it is a region of weaklyionized plasma and large neutral species numberdensity as well as complex ion-interchange and electron attachment and detachment reactions (the latter processes are the most distinguishing feature of the d-region). the perturbations of this region greatly affect the absorption of high frequency radio waves and the reflection of the low-frequency signals. various techniques are available for the exploration of the plasma density in the d-region. much observational evidence has accumulated showing that its behaviour is far less simple and straightforward than the regular «chapman-layerlike» behaviour controlled only by solar energy influx and the geometry of its penetration into the atmosphere. the diurnal and especially seasonal variations sometimes demonstrate the significant abnormal peculiarities especially in winter. at the same time groundbased, rocket and satellite-board soundings of temperatures and winds in the upper mesosphere-lower thermosphere (mlt-region) exhibit variation patterns with temporal and spatial scales which are to be described by the term «meteorology». thus, nowadays it is possible to introduce into practice the «meteorological control» of the d-region for the explanations of some events (taubenheim, 1983; danilov et al.,1987). it means that the lower ionosphere exhibits not only a solar, but also a strong non-solar control which is partly of a meteorological nature. the most suitable for the study of meteorological effects upon the d-region are groundbased 8 edward s. kazimirovsky radio propagation observations, because they provide data records continuous in time in addition to the singular rocket launches. the experimental basement for the intensive investigations of the coupling so far are the ionospheric radiowave absorption and dynamic regime (prevailing winds and wave-like fluctuations) measurements in the mlt region. the radiosignals reflected from or propagated through the ionosphere depend on the effect known as absorption, or attenuation of electromagnetic wave energy due to collision of ionospheric electrons with neutral atoms. without considering the details of magnetoionic theory it could be summarized that absorption has a direct relationship with the product of plasma density and the collision frequency and an inverse relationship with the emitted radiowave frequency. the most investigated d-region absorption data are produced by the measurements known as a3 method described in all details by schwentek (1976). that consists in measuring the attenuation of mf radiowaves radiated from a distant transmitter with constant power. the best known manifestation of a meteorological influence upon the d-region is the winter anomaly, i.e. excessive enhancement of both the average level and especially day-to-day variability («spikes») of radio-wave absorption in winter, noted by many investigators even in the first years of ionospheric science. the example of winter anomaly in terms of absorption at a constant solar zenith angle during 19671969 is shown in fig. 2 (taubenheim, 1983). there are two components: 1) «normal» winter anomaly with a rather slow increase and decrease, i.e. in average daytime absorption for usual winter day is higher than for usual summer day; 2) superimposed spikes, «excessive» winter anomaly. although enhancements of absorption may be connected with increasing electron density and/or collision frequency in ionospheric plasma, it is nowadays assumed that the winter anomaly is connected primarily with enhanced electron densities at the mesopause level and above. why do we assume that the winter enhancement of electron density has an internal («meteorological») nature? 1) it occurs on both the northern and the southern hemispheres during the respective winter months. 2) the correlation between the time variations of d-region ionization and solar l-alpha fluxes (measured by ae-e satellite), which is weak but detectable during summer months, is completely missing during winter. 3) the amplitude of interdiurnal variations of absorption in winter months cannot be explained by solar l-alpha variations. 4) the time and spatial scales of anomalous absorption, wave-like structures, are compatible with planetary waves pattern (e.g., de la morena and kazimirovsky, 1996). there are some «scenarios», concerning the d-region meteorological control (e.g., taubenheim, 1983; offerman et al., 1982): 1) «concerted» scenario. – the general enhancement of d-region electron densities is caused by enhanced downward eddy diffusion of no, accumulated in the polar night thermosphere. – in addition, at heights below 85 km, the electron rate is reduced by inhibition of cluster ion formation because of warm mesopause temperatures. – independently, «patchy» (with respect to time and longitude) downward transports of excess no into the d-region are effected by diffusion and/or bulk motions connected with planetary wave patterns. displacement of these patches by enhanced horizontal winds, possibly launched by transient perturbations from below, causes rapid changes in the no distribution within 1-2 days. 2) «unitary» scenario. – winter meteorology at d-region heights is predominantly controlled by the circumpolar vortex of zonal westerly winds, reaching from the stratosphere up into the lower thermosphere. this cyclonic vortex is always tied with: 1) warm mesopause temperatures, inhibiting cluster ion formation. 2) downward vertical wind components (bulk motion). 3) low pressure near the mesopause, decreasing optical depth for solar uv radiation coupling from below as a source of ionospheric variability: a review 9 and hence increasing the ionization rate. all three of these conditions act in the same direction – to increase the d-region electron density. in this «unitary» scenario the enhanced eddy diffusion in winter raises the average background level of d-region ionization and all global or local perturbations of the zonal vortex flow (for instance induced by planetary wave energy transfer from below) cause a prompt decrease or even «breakdown» of the winter anomaly in d-region. both scenarios have their own merits and demerits. it seems still necessary to improve the empirical base to define coupling processes throughout the atmosphere during the winter anomaly. the numerous experimental case studies on the stratosphere-lower ionosphere coupling in middle latitudes analyze the stratospheric temperature variations and lower ionosphere plasma density (radiowave absorption). the correlation between the temperature increment fig. 2. winter anomaly of ionospheric radio wave absorption in terms of the absorption at a constant zenith angle during 1967-1969. the dashed line extrapolates the trends from summer (taubenheim, 1983). a b so rp ti o n ( d b ) at c o s χ = 0 .1 10 edward s. kazimirovsky in the stratosphere and ionospheric absorption during the winter sudden warmings were noted for both hemispheres long ago. figure 3 (shapley and beynon, 1965) demonstrates the supe-rimposed epoch analysis comparing 10 mb temperatures over berlin and radiowave absorption over lindau (germany). the increase in absorption for the day of 10 mb warming is evident. d-region electron density changes accompanying a winter stratospheric warming at 25-30 mb in june and july for new zealand (christchurch, 43°s,173°e) are fig. 3. superimposed epoch analysis comparing 10 mbar temperature over berlin, and radiowave absorption over lindau (germany) (shapley and beynon, 1965). coupling from below as a source of ionospheric variability: a review 11 shown in fig. 4a,b (gregory and manson, 1975). it is interesting that for rather low latitudes the response of absorption to the stratospheric temperature rise cannot exist for all types of warmings, but only for the strong final warmings connected with a radical change in atmospheric circulation, as demonstrated in fig. 5 for el arenosillo, spain (37°6′n, 6°44′w) (de la morena and kazimirovsky, 1996). it is noteworthy that in principle we can observe the inverse correlation between both parameters, in such a way that the appearance of stratospheric warming (i.e. temperature rise at the fixed stratospheric level) coincides with a significant decrease in absorption in the dregion (e.g., lastovicka, 1983). possibly, the effect on ionospheric variability could depend on the region (lastovicka and de la morena, 1987), the meridional wind variability in the d-region (lastovicka et al., 1990) and even on the height of stratospheric level with increasing temperature. fig. 4a,b. ionospheric d-region changes accompanying a winter stratospheric warming in june and july 1963, at christchurch, new zealand, 43°s, 173°e. a) noon electron concentration n; b) stratospheric temperature at 20-30 mbar level (about 25 km altitude) (gregory and manson, 1975). a b 12 edward s. kazimirovsky the height dependence was found recently (pietrella et al., 2001) for antarctic, terra nova bay station, located close or even inside the polar cusp region, where the geomagnetic field lines provide a direct connection between the ionosphere and magnetosphere. it means that in this area the ionospheric variability could be predominantly controlled by the interaction with the magnetosphere. so, it would be difficult to distinguish between the ionospheric variability due to meteorological effects, coming «from below» (if they exist), from the principal cause due to the influx of the energetic particles that increase the electron density and then the ionospheric absorption. nevertheless, during the period with rather low solar activity (f10,7 < 100 ), we found by multiregression analysis (with statistical significance) that at the stratospheric levels between 15 and 22 km the influence of stratospheric temperature on ionospheric absorption seems to be more important than those due to the solar and geomagnetic activity. fig. 5. effect of stratospheric winter warming on daytime absorption of radio waves. a3 method. transmitted frequency f = 2830 khz, 1976-1993, el arenosillo, spain (37°06′n, 6°44′w). key day: day of maximum stratospheric temperature on 30 mbar level (de la morena and kazimirovsky, 1996). coupling from below as a source of ionospheric variability: a review 13 moreover, the superimposed epoch analysis showed that the behaviour of ionospheric absorption response to the rising of the stratospheric temperature is different for the lower and upper stratosphere. on the average, for the range 13-18 km the ionospheric absorption increases after the stratospheric temperature maximum (so called «zero day»), but in the range 19-26 km the ionospheric absorption decreases after «zero day». during the last 15-20 years, a significant number of papers have appeared with observations of regular or quasi-regular fluctuations, observed both in the neutral as well as in the ionized component of the middle atmosphere and lower ionosphere. in some of them, an attempt was made to connect the fluctuations in the d-region or e-region of the ionosphere with similar fluctuations of the meteorological parameters in the lower parts of the atmosphere. we suggest that the wellcorrelating fluctuations in the ionized and neutral component of the d-region, mesosphere and stratosphere are probably due to the large-scale upward propagating planetary disturbances generated in the troposphere (see numerous references in de la morena and kazimirovsky, 1996). the seasonal variation in the amplitudes of quasi-periodical fluctuations of ionospheric parameters with periods of planetary waves, tides, internal gravity waves show that the amplitudes usually are maximal in winter, when conditions of upward leakage of internal waves (depending on the wind and temperature profiles between the troposphere and thermosphere) are preferable (e.g., taubenheim, 1983). it was shown that the quasi-periodical fluctuations observed in absorption are caused not by fluctuations in the solar ionizing flux, but probably by planetary waves in the stratosphere/troposphere (e.g., pancheva et al., 1987). later the model of the transformation of planetary waves of tropospheric origin into waves in absorption in the lower ionosphere was developed (lastovicka et al., 1993). the example of the typical 7-8 and 12-13 days absorption fluctuations is presented in fig. 6 (pancheva et al., 1991). the main features of amplitude variations are very similar at all 3 radio-frequencies (the measurements were provided by a3 method for 3 radiopaths – at bulgaria, czechia and spain) for 7-8 days fluctuations while for 12-13 day fluctuations such a good similarity is observed only in december-january. h 1,2 in fig. 6 are the amplitudes of the 30 hpa height wave 1 (full line) and 2 (dashed line) at 60°n. the intervals of amplification of the fluctuations coincide well with the stratospheric warmings during the winter under consideration. the first «minor» warming developed with two phases: around 10 january, 1986 (h 2 peak) and 20 january, 1986 (h 1 peak), the second «minor» warming takes place around 18 february, 1986 and the major «final» warming occurs after march 20, 1986 both as a consequence of h2 intensification. this cold winter 1985/1986 had an enhanced tendency for the development of an elongated polar vortex, that is a pronounced planetaryscale height wave h2 (naujokat and labitzke, 1993). it can be clearly seen in fig. 6 that almost each amplification of the quasi-stationary planetary waves h2 in the stratosphere leads to the activation of the 7-8 days variations in the ionospheric absorption. the behaviour of the longer-period fluctuations in absorption shows a response to the simultaneous amplification of both height waves (h1, h2 ) in the stratosphere. therefore, we can conclude that the quasiperiodical fluctuations observed in the ionospheric absorption are really connected with the large-scale planetary disturbances generated in the lower atmosphere, which cause the stratospheric warmings. the ionospheric dynamics certainly must be sensitive to the coupling from below. the close connection between the circulation in the lower thermosphere, mesosphere and stratosphere, e.g., nearby radar and rocket measurements demonstrate the consistent crosssections from 20 to 110 km (e.g., meek and manson, 1985). it is generally accepted that while the international reference models of the zonally averaged upper mesosphere/lower thermosphere wind field are still useful for many purposes, significant discrepancies exist between them and new experimental data. the latter demonstrate 14 edward s. kazimirovsky f ig . 6 . t he t im e va ri at io ns o f am pl it ud es o f ra di ow av e ab so rp ti on f lu ct ua ti on s (t = 7 -8 a nd 1 213 d ay s) f or 1 53 9 kh z, 1 41 2 kh z an d 28 30 k h z a n d fo r th e 3 0 h p a h ei g h t w av e 1 ( fu ll l in e) a n d h ei g h t w av e 2 ( d as h ed l in e) a t 6 0 °n , w in te r p er io d 1 9 8 5 /1 9 8 6 ( p an ch ev a et a l. , 1 9 9 1 ). coupling from below as a source of ionospheric variability: a review 15 in addition to seasonal and latitudinal variations also longitude dependent variations; in addition to tides also planetary waves and intraseasonal quasiperiodical fluctuations. it is evident that sometimes the specific regional features of wind regime are very important. if, as has been demonstrated, there is meteorological control of the lower ionosphere, we may expect the existence of a longitudinal effect on the dynamic regime due to well known longitudinal inhomogeneity of the lower atmosphere processes and due to longitudinal differences in the conditions for upward propagation of internal atmospheric waves from the lower atmosphere. the longitudinal effect has really been revealed on the basis of the simultaneous upper mesosphere/lower thermosphere wind measurements (d-region) along one latitude circle at two or more sites (e.g., kazimirovsky et al., 1988). it has been well known since the first analyses of wind variations at the mid-latitude lower thermosphere that diurnal velocity variation could be well described as the sum of prevailing wind, diurnal wave and semidiurnal wave (e.g., sprenger and schminder, 1967). the existence of diurnal and semidiurnal tides at mlt-region was also demonstrated in numerous theoretical models (e.g., forbes, 1991). figure 7a demonstrates prevailing wind (v0) monthly averaged variations for observatory badary (east siberia, 52°n) and observatory collm (central europe, 52°n). the systematic climatological distinctions are evident, especially for the zonal circulation (v0x). during winter, the averaged wind over eastern siberia is about twice as strong as that over central europe. the seasonal variation of the zonal circulation depends on longitude as well: the autumn minimum over siberia occurs earlier than that over europe, and the spring minimum is accompanied by a reversal of wind only over europe, but not over siberia. the observed longitudinal effect may be partly interpreted as resulting from large scale stationary planetary waves formed at the lower thermosphere. in this case, the longitudinal variation of prevailing wind is due to the existence of such waves. figure 7b shows that the seasonal variation of the monthly averaged semidiurnal zonal tide amplitudes (v 2x ) at both observatories are very similar with small discrepancies mainly in the summer months. the systematic climatological distinctions (buth with similar character of seasonal variations) are evident for semidiurnal meridional tide amplitudes (v 2y ), which are systematically larger at east siberia than at central europe. the longitudinal variation of semidiurnal tidal amplitude is a consequence of the longitudinal variation of zonal flow (kazimirovsky et al., 1988, 1999). the zonal and meridional prevailing winds at the lower thermosphere reversed westwards and southwards in a period of less than a week during stratospheric warmings. there is an fig. 7a,b. the non-zonality in the wind field in the lower ionosphere (kazimirovsky et al., 1999). observatory badary, east siberia, 52°n, 1975-1995. observatory collm, central europe, 52°n, 19791997. data are monthly averaged. v ox prevailing zonal wind. v oy prevailing meridional wind. positive direction eastward and northward. v2x amplitude of the zonal semidiurnal tide. v2y amplitude of the meridional semidiurnal tide. a b 16 edward s. kazimirovsky increase in the semidiurnal tide amplitude and phase shift comparatively with undisturbed winter conditions. the response could depend on longitude as well, on the intensity and location of the warming area (e.g., kazimirovsky et al., 1988). this response of the prevailing zonal wind for 3 observatories located on the same geographical latitude (in canada, central europe and east siberia) is demonstrated in fig. 8. planetary waves contribute significantly to the variability of mlt winds. in the mesosphere and lower thermosphere the wave fluctuations are sufficiently large to often mask the prevailing or mean state of the atmosphere. tropospherically forced planetary waves with periods between 2 and 30 days have been observed under favourable conditions to penetrate up to heights near 110 km. a range of wave periods has been identified but the most commonly reported periods fall into four well defined intervals which are 14-20 days, 9-12 days , 4-7 days and 1.6-2.2 days. these are often referred to as «16-day», «10day»,«5-day» and «2-day» oscillations respectively, although a precise determination of the periods involved is often not possible. the observed periods vary around these values in an apparently random way possibly due to doppler shifting by the mean winds. thus, they form «period bands» (lastovicka, 1997). it should be noted that waves of other periods have also been reported. it is usually assumed that these transient oscillations of the wind field in the lower thermosphere are caused by rossby-gravity normal modes generated in the lower atmosphere (vincent, 1990). who may be a preferable carrier of upward influence (especially for planetary waves) across the mesopause to the lower thermosphere and ionosphere? the atmospheric tides are the most probable candidate.the non-zonal modulation of the stratospheric and mesospheric tides penetrating to the lower thermosphere appears to generate a quasi-stationary planetary wave by demodulating the tides by energy dissipation at the lower thermosphere (e.g., forbes, 1991). the modulation of the prevailing wind and semidiurnal tide with periods of planetary waves are shown for two midlatitude observatories (badary and collm) in figs. 9 and 10. the dynamic spectra were calculated for two-year interval with a data «window» of one month. really, the spectra of fluctuations are very wide and variable and differ for different longitudes. the general concept of significant longitudinal structure in the mlt winds was supported and developed during the international mltcs campaigns (manson et al., 1991), and the international dyana project (singer et al., 1994; portnyagin et al., 1994). clear longitudinal effects were observed either for prevailing wind or semidiurnal tides. the recent cooperative fig. 8. the response of the prevailing zonal wind (u m/s) in the lower ionosphere on the stratospheric warmings during winter 1984-1985 (rather low solar activity). positive direction eastward. arrows days of stratospheric warmings (kazimirovsky et al., 1988). 1 observatory badary, east siberia, 52°n; 2 observatory collm, central europe, 52°n; 3 observatory sakatoon, canada, 52°n, height 97 km; 4 observatory saskatoon, 89 km; 5 observatory saskatoon, 80 km. coupling from below as a source of ionospheric variability: a review 17 fig. 9. lower thermosphere zonal prevailing wind dynamic spectra at east siberia and central europe. fig. 10. lower thermosphere zonal wind (semidiurnal tidal amplitude variations) dynamic spectra at east siberia and central europe. «crista/mahrsi campaign» also dealt partly with upper mesosphere/lower thermosphere wind measurements and revealed some regional distinctions (e.g., kazimirovsky et al., 1997). to determine the sources of planetary scale perturbations of the mesosphere and lower thermosphere is one of the topics in the ongoing international program psmos (planetary scale observing system). nevertheless, these processes operating in the mlt-region are still insufficiently understood. probably they include quasistationary planetary waves, psw (jacobi et al., 1999; forbes et al., 1999), but our current understanding of the nonzonal dynamic processes contains numerous unresolved contradictions. therefore, further experimental and theoretical investigations in this field will greatly help to extend our knowledge of the mlt-region spatial structure. coordinated program of the simultaneous measurements of winds might be carried out at a carefully chosen series of sites comprising an east-west chain 18 edward s. kazimirovsky spanning a large sector of longitude. such measurements in conjunction with results of the long-term zonal-mean modeling could be used to determine psw parameters and to estimate the relative contribution of non-migrating tides. it would improve our understanding of the vertical coupling between different atmospheric regions and the coupling mechanisms between tropospheric/stratospheric non-zonal circulation systems and those which may exist in the mltregion. the next ionospheric region e, intermediate between d-region and f-region, is rather less studied in the processes of coupling from below. nevertheless, we have some evidence concerning intensive gravity waves in the variations of the electron density and ion velocities at heights 100-140 km (e.g., nygren et al., 1990), measured by incoherent scatter radars. the sporadic ionization in the d-region, so-called e s (sporadic e-layer), on the meteorologically frontal days sometimes tends to occur more frequently and with higher electron concentration than for preceding and following days. the e s generation is usually connected with neutral wind shear. it is why the dynamics of the e-region could be crucial for e s . the daily averaged departures of the sporadic e parameters f b e s and h′e s from the corresponding monthly medians have been found to decrease during winter-time circulation disturbances connected with stratospheric warmings. the computed neutral wind shear shows decreased values during these disturbances. the effect may be due to a decrease of attenuation of agw, due to which the vertical wavelength increases and the e s producing wind shear decreases with growth of vertical energy flux (numerous references in kazimirovsky and kokourov, 1991). 4. upper thermosphere/ionosphere this topic is rather more speculative than the lower thermosphere/ionosphere. nevertheless, the ideas on dynamic coupling between weather at ground level and f-region behaviour appeared long ago (beynon and brown, 1951; martyn, 1952). of course, the attempts to forecast the surface weather from ionospheric data were too optimistic to be true. but it is evident that planetary, tidal and gravity waves launched by various sources in the troposphere and stratosphere really penetrate into the f2 region. there are short-term correlations between meteorological and ionospheric parameters, and a number of statistically significant correlations, that may be regarded as circumstantial evidence on a longterm basis. the ionospheric response to the forcing from below should be anisotropic and subject to diurnal and seasonal variations, and should vary with geomagnetic and geographic latitude and longitude. in fact, we know that both seasonal and daily, global and regional, composition changes at thermosphere/ionosphere heights can occur even during geomagnetically quiet periods. it is assumed that corresponding changes in the turbopause heights are responsible for the respective observations. but the turbopause heights variations could be sensitive to the meteorological dynamic influence from below. of course, it is generally agreed that there is significant turbulence in the region 80-120 km, although there is still some debate as to its temporal and spatial morphology. the main sources of turbulence are probably gravity waves and tides (including ones, propagating from below), and these generate turbulence by processes such as non-linear breaking, shear instabilities and critical level interactions. we believe that planetary waves sign could be recognized in the global distribution of electron density (n e ) in the f 2 region. the regional structure (so called continental effect) of n e which can be interpreted in terms of the manifestation of the climatic properties of the underlying atmosphere has been discussed on many occasions (danilov et al., 1987). the longitude-dependent distribution of noon critical frequency f0 f2 values (corresponding to maximal electron concentration) is reasonably well approximated by the sum of planetary waves with zonal wave-numbers n = 1 and n = 2. moreover, it was shown that the seasonal variations of planetary wave amplitudes in f0 f2 and in geopotential at 10 mb level are closely correlated but with changeable time lag. coupling from below as a source of ionospheric variability: a review 19 studies of coupling between ionospheric and stratospheric parameters sometimes yield ambiguous conclusions due to the masking effect on the ionosphere from the various geophysical factors, such as the ionizing radiation of the sun, energetic particles fluxes, cosmic rays, etc. thus it is desirable to make the analysis in such a manner that variations caused by these factors are removed. we proposed and successfully realized the following technique. two stations are selected which have close geographic or geomagnetic latitudes but are well spaced in longitude so that they are likely to have substantial differences in meteorological characteristics of the lower atmosphere. for each day, the differences are determined in the ionospheric characteristics (e.g., f0 f2, fmin, absorption of radiowaves, etc.) between these two stations and the same differences in meteorological characteristics (e.g., stratospheric temperature, height of isobaric surface, etc.). from the comparison of the time variations of these differences, one may suppose that the effect of geophysical factors, which are almost the same for the two stations, is eliminated to a considerable extent. on the contrary, the meteorological effect due to different climatology for these two spaced locations will be stressed and thus can be identified. if meteorological factors do indeed have an influence upon the variations of ionospheric parameters, we should have a close correlation between variations of the differences described above. the example can be demonstrated for f0 f2 and stratospheric dynamics. it is well known that the winter cyclonic vortex in the stratosphere is highly dynamic and changeable. the subtropical areas of high pressure when moving northward, may deform and sometimes even split it into two independent cells. this circumstance leads to a substantial difference in the values of stratospheric pressure at two sites, located at the same latitude but at different longitudes. does the behaviour of the f2 ionospheric layer reflect these phenomena? figure 11 presents the seasonal variation of daily values of differences in noon f0 f2 (∆f0 f2) between two sites near 55 on spaced in longitude by 2000 km and the seasonal variation of daily values of the corresponding differences in height of 30 hpa surface (∆ h 30 ). we can see that both during winter and equinoxes the values of ∆ h30 and ∆ f0 f2 are greater than those during summer. by the method of «superposed epochs» it was found that negative extrema of ∆ f 0 f 2 correspond to positive extrema of ∆ h30 and vice versa (kazimirovsky and kokourov, 1991). for some time, the network of ionosonde stations in europe had been dense enough to study the meteorological behavior of the upper ionosphere (bibl, 1989). even the average behaviour of the f-region ionization shows substantial differences with location. in europe, the variations of the local gradients in ionization can differ by a factor of two over two locations separated by 1000 km. this is important for understanding the meteorology of the ionosphere and for precise radio predictions. the existence of quasi-periodic oscillations in the ionospheric f-region ( 2-35 days) which may be connected with planetary wave activity in the lower atmosphere is statistically evident in f 0 f 2 variations. figure 12 (apostolov et al., 1998) demonstrates the averaged periodograms of f0f2 in the periodic range from 30 h to 40 days for the whole solar cycle 21 calculated as a mean of 126 spectra of successive 4-month intervals shifted by 1 month.the 4 stations considered lie latitudinally between 42°n and 60°n. there are well expressed maxima close to 6 (4-7); 9 (7-11);14 (1116) and 29 (11-35) days. the relatively minor peaks occur near 2.5 (1.25-4) and 19 (16-21) days. well studied quasi-periodic oscillations with periods from 2 to 6.5 days have the following characteristics: the probability of existence and the occurrence frequency are maximum in local summer; the oscillation amplitude has maximum near equinoxes and the dominant zonal structure of the oscillations at middle latitudes is a westward traveling wave with zonal wave-number n = 1 and n = 2. these quasi-periodic oscillations contribute significantly to the day-to-day variability of f0 f2 and their contributions depend on the solar cycle, season and latitude (altadill, 2000). as far as the physical cause of planetary scale waves in f0 f2 is concerned, it seems possible that this is the sequence of planetary waves in the neutral atmosphere (rice and sharp, 1977; forbes et al., 1999), in thermospheric density. 20 edward s. kazimirovsky traveling ionospheric disturbances (tids) which are the quasi-periodical spatially large-scale and medium-scale electron density perturbations possess a horizontal component of propagation velocity. it is generally accepted now that they are caused by the passage of internal gravity waves. the sources of these waves could be diverse, but they must be below the f-region since the wave energy (i.e. group velocity) invariably was found to be directed upwards. from the earliest days of hf communications the presence of travelling ionospheric disturbances (tids) and ionospheric irregularities (including fspread event) were revealed and now we know that both are closely connected with internal gravity waves. at the same time, the solar cycle and seasonal variations of oscillations at different heights (e.g., lastovicka and mlch, 1996), the similarities in the f 2 -region and lower ionosphere are such that a common source of waves in the lower and upper ionosphere of tropospheric and stratospheric origin can be postulated. in addition, planetary wave type oscillations in the f2 region can be caused by modulation of upward propagating tides and gravity waves, so even the possibility of prediction of these oscillations from wind measurements in the lower thermosphere/ionosphere could be discussed (lastovicka, 1997, 1998). the sources of these waves have been identified which include severe weather disturbances, earthquakes, nuclear and industrial explosions. in the experiments provided by some investigators, for the majority of the waves the reverse group path can be followed down to the tropopause level, and comparison with meteorological data has shown that many of the possible source regions of the observed waves appear to lie in proximity to the jet stream, or close to regions of typhoons, thunderstorms, tornado, hurricanes, the convectively unstable cold polar air, subtropical heavy rainfalls, tropospheric mesoscale fig. 11. stratospheric effects on the ionospheric f-region. daily values of the difference in noon values of f 0 f 2 and in heights of an isobaric surface at 30 hpa for two stations: moscow (east europe) and ekaterinburg (ural mountains), located almost at 55° n. i, ii, iii...xii months, 1969 (kazimirovsky and kokourov, 1991). coupling from below as a source of ionospheric variability: a review 21 kokourov, 1991; hung et al., 1991; manzano et al., 1998; hocke and tsuda, 2001). tid can be also related to tropospheric vortexes. recently such observations were provided in china where tids were statistically analyzed under the base of observation of an hf doppler array. the backward ray tracing shows that the sources of tids are located in the edges of qinghai-tibet plateau, i.e. in the lee sides of the bulging terrain of the plateau where the vortexes are most probably produced (wan et. al., 1998). f-spread event on ionograms is often connected with wave-like structures of f-region irregularities induced by upward propagating agw’s of meteorological origin. it is especially true for equatorial f-spread. for instance, spread-f occurrence in the african area was investigated by means of non great-circle radiowave propagation on the transequatorial path. the distribution of the horizontal velocity, the horizontal wavelength of the quasi-periodical structures, and the tilt of the irregularity patches led to the assumption that these structures of the equatorial f-spread are connected with agw in the equatorial region (numerous references in gershman et al., 1984). the small-scale ionospheric plasma events associated with severe weather-related gravity waves were also observed in situ (kelley, 1997). the variations of the total electron content (tec) over low-latitude stations sometimes (e.g., shoedel et al., 1973) also show wave-like structures at the time of unusual ground pressure variations with the same period and an amplitude much greater than the amplitude of the normally observed short-period changes in tec. the strong correlation between the oscillations of the ground pressure and the tec gives evidence that the agw were generated by a singular tropospheric event and propagated to ionospheric heights without significant changes in period as seen in fig. 13 for 3 african stations. now it is impossible to deny the possibility of an association between atmospheric disturbances at ground level and in the ionosphere (goodwin, 1980). recently, the effect of gravity waves was investigated successfully to estimate their contribution in the within-an-hour and hour-toconvective complexes (mcc) events, intertropical convergence zone in africa, etc. the possible generation mechanism is the non-linear interaction of shear flow instabilities in the jet stream and penetrative convection (waldock and jones, 1987; numerous references in kazimirovsky and fig. 12. averaged periodograms of f0f2 in the periodic range from 30 h to 40 days for the whole solar cycle 21, calculated as a mean of 126 spectra of successive 4-month intervals shifted by 1 month (apostolov et al., 1998). 22 edward s. kazimirovsky hour variability of f 0 f 2 (boska and lastovicka, 1996). figure 14 (boska et al., 1999) shows the agw spectra for el arenosillo, spain, for one day. in principle, the fluctuations for electron density on the fixed heights can be caused either by fluctuations in ionized matter (i.e. gravity waves) or by fluctuations in ionizing radiation, high-energy particles related geomagnetic/ magnetospheric activity, etc. but we can believe that at middle latitudes and for rather quiet geomagnetic conditions the fluctuations in the period range 10 min-3 h are not of solar or magnetospheric origin. in summary, to predict the effect of planetary and gravity waves on the variability of the ionospheric f-region it is necessary to know at least the scenario of indirect upward propagation of their effects. so far we have no complete modeling in this sense. fig. 13. atmospheric ground pressure variation at tsumeb (lower part), and total electron content variation at tsumeb (ts), outjo (ot), and okanhandja (ok) (upper part) on july 1, 1973 (schoedel et al., 1973). fig. 14. atmospheric gravity wave spectra in the variations of electron concentration on the fixed levels. digisonde-256, el arenosillo, spain (37.1°n, 6.7°w), december 14, 1997, 06ut-18 ut (boska et al., 1999). coupling from below as a source of ionospheric variability: a review 23 5. thunderstorms and upper atmosphere/ ionosphere the widely accepted classical ideas of atmospheric electricity were: 1) thunderstorms are the main generators of the global electric circuit, causing an electric potential between earth and the ionosphere of about 200-500 kv. 2) the influence of thunderstorms and electrical events near the ground on the atmospheric layers well above the perfect conductor («electrosphere») near 60-70 km is forbidden. we know that the classical hypothesis has not yet been proved beyond doubt. moreover, it is now widely recognized that the current systems driven by global thunderstorms and by magnetospheric plasma phenomena coexist in the middle atmosphere and above. some recent models of thunderstorm current systems and experiments show that most of the return current from a thunderstorm generator penetrating the tropopause flows globally through the ionosphere and along plasmaspheric magnetic field lines. the lightning generates broad electromagnetic frequency spectra and some of the wave energy propagates into the ionosphere/ magnetosphere system, where it interacts with ambient plasma particles. we have evidence that thunderstorms can be important in the global ionospheric energy budget. the thundercloud electric fields influence the e-region and fregion of the ionosphere (suess and tsurutani, 1998). recent observations have shown that intense lightning produces a number of interesting and unexpected effects in the middle and upper atmosphere above thunderstorms. some new and diverse classes of energetic electrical effects of thunderstorms have been documented over the past 5-10 years. two of these classes called red sprites and blue jets are large-scale optical emissions excited by lightning. together they span the entire distance between tops of some thunderstorms and the ionosphere. these newly discovered classes of natural electric phenomena provide evidence that thunderstorms are both more energetic and capable of electrically interacting with the upper atmosphere and ionosphere to a far greater degree than has been appreciated in the past. sprites are very large luminous flashes that appear within the mesospheric d-region directly over active thunderstorm systems coincident with cloud-to-ground or intracloud lightning strokes. triangulation of their locations and physical dimensions using simultaneous images captured from widely spaced aircrafts has shown that their terminal altitude extends to the ionosphere. the brightest region of a sprite is red and lies in the altitude range 65-75 km. above this there is often a faint red glow or wispy structure extending upward to about 90 km, to the nighttime eregion ledge. jets are sporadic optical ejections, deep blue in color, that appear to erupt from the vicinity of the overshoot. following their emergence from the tops of the thundercloud, blue jets propagate upward in narrow cones, fanning out and disappearing by about 50 km over a lifetime of about 300 ms. theories proposed to date concerning only red sprites all involve lightning discharges acting either as a causative agent, or as a simultaneous but non-causative consequence of electrical breakdown triggered by cosmic rays. intensive efforts, both experimental and theoretical, are underway to determine the physical mechanisms at work to produce thunderstorm ↔ ionosphere effects. it is unclear whether the absorption within the ionosphere magnetosphere of the energy flowing upward from the lower atmosphere is capable of producing effects that are dynamically significant enough to qualify as a strong link between these layers. however, several longlived secondary effects within the neutral upper atmosphere may occur by way of joule heating, photoexcitation, or electron impact excitation or ionization. understanding where these new electrical processes fit in the solar-terrestrial system and earth’s global electrical circuit is a challenging, and inherently multidisciplinary, problem that spans traditional discipline boundaries separating the lower and upper atmosphere (sentman and wescott, 1995). the evidence for direct interactions between phenomena associated with thunderstorms and 24 edward s. kazimirovsky ionosphere includes perturbations of electron density and temperature (increase of ionization at e-region, e-sporadic occurrence, increase of temperature and electron density in f-region), excitation of electrostatic wave turbulence and enhanced optical emissions. there are some approximate hypotheses about the physical mechanisms – e.g., upward acceleration of electrons at the moment of lightning discharge, ionization by particles, etc. (e.g., pasko et al., 1997). there is theoretical and experimental evidence of magnetospheric electron precipitation simulated by lightning via radio waves, particularly in the elf-vlf range (less than 30 khz). because the precipitating particles may be the reason for dissociation and ionization processes, it is the next channel of thunderstorm influence on the ionosphere. we have vlf signatures of ionospheric disturbances associated with «sprites» (e.g., rycroft, 1994). recently, the numerical model of the interaction created in the bottom of the ionosphere above a strong horizontal (cloud-tocloud) lightning discharge was published (cho and rycroft, 2001). this simulation sheds light on the formation of spatially structured sprites, depending upon height, via the ratio of the elf/ lf wave electric field (radiated by the current in the discharge) to the atmospheric neutral density. it is interesting that the ulf-vlf electric fields were detected up to the external ionosphere over powerful pacific ocean typhoons (mikhailova et al., 2000). thunderstorms can generate agw as well. reverse group ray tracing computations of agw observed by ionospheric doppler sounder array, show that the wave sources are in the neighborhood of storm systems. agw at ionospheric heights are observed when severe thunderstorms are within a radius of several hundred kilometers of the ionospheric reflection points, the convective regions may be embedded in the stratiformanvils of thunderstorms. it is also known that the zones of maximal occurrence of f-spread over west africa and south america coincide well with the zones of high thunderstorm activity (e.g., references in kazimirovsky, 1983). 6. long-term ionosphere variations as a possible consequence of the greenhouse effect releases of trace gases from human activity have a potential to cause a major change in the climate of the earth. but there is no doubt that the subject of global climate warming due to the so called «greenhouse effect» has led to controversy, speculation and confusion. despite the many uncertainties that remain, the consensus of most scientists knowledgeable about these matters is that global warming will occur. some questions remain concerning the timing and the magnitude of change but there are few, if any, who dissent from this general conclusion. the troposphere is expected to warm, the stratosphere to cool, the ozone content to decrease: the consequences of these processes on the atmosphere above 60 km are considered at present and overall results indicate that global change resulting from trace gas variations (e.g., co2 and ch4 doubling) is not confined only to the lower atmosphere but also extends well into the mesosphere, thermosphere and ionosphere regions (golytsin et al., 1996). the projected changes should also lead to some alterations in global circulation, latitudinal distributions of temperature and composition and the response of the atmospheric system to solar and auroral variability (e.g., thomas, 1996). changes in a thin layer of sodium vapour, about 90 km above the earth, could reveal the far-reaching effects of greenhouse gases (from 1972 to 1987 its mean height had fallen by nearly a kilometre). although it is not possible to state with absolute certainty that the decrease in the altitude of the sodium layer is an indication of global cooling in the middle atmosphere, the behaviour of the layer clearly merits systematic surveillance in years to come (e.g., clemesha et al., 1997). the changes in occurrence frequency (a considerable increase) in noctilucent clouds caused either by changes in water vapour concentration or by changes in temperature, are possibly an indication of long-term anthropogenic changes. we should also consider the possible effect of anthropogenic changes in the coupling from below as a source of ionospheric variability: a review 25 mesosphere that could result from aerosols and trace gases diffusing upward into the mesosphere, where they can change the aeronomy. polar mesosphere summer echoes (incoherent scatter radar) might prove to be sensitive tracers for such anthropogenic changes (e.g., thomas, 1996). long-term trends in planetary wave activity at altitudes of about 80-100 km, which are of possibly anthropogenic origin, have been extensively studied with the use of almost 30 years of absorption measurements along various radiopaths in europe. trends are more pronounced in daytime than night time (e.g., lastovicka, 1997). using different radar observations, bremer et al. (1997) detected trends in the prevailing wind components and the amplitudes of the diurnal and semidiurnal tidal wind components in the mlt-region, at heights between about 85 and 100 km. these trends, however, cannot be explained directly by an increasing greenhouse effect. but the well known decrease of the stratospheric ozone content may play an important role for the trends of the tidal characteristics. according to model predictions (e.g., rishbeth and roble, 1992), doubling of the greenhouse gases concentration should lead to a significant cooling of the upper atmosphere and resulting changes in f 2 -region parameters – a decrease of the height of maximal electron concentration (h m f2) by about 15 km with only small changes in f0 f2. the longterm changes in h m f 2 and f 0 f 2 which were in accordance with model predictions (at least qualitatively) were reported for some ionospheric stations in the northern and southern hemispheres, but the results do not present a consistent pattern. figure 15 shows an example of seasonal and yearly negative trends of h m f 2 determined from ionosonde data of the midlatitude ionospheric station juliusruh (germany), 1957-1990, after fig. 15. seasonal and yearly trends of h m f 2 at juliusruh (germany) after elimination of solar and geomagnetic influences (bremer, 1998). 26 edward s. kazimirovsky elimination of solar cycle and geomagnetic influences. trends for winter and the whole year are statistically significant with a confidence level of more than 99% (bremer, 1992). meanwhile rather recently (marin et al., 2000) analysed the long-term trends of the f0 f2 and h m f2 on northern hemisphere 30 ionosonde stations over the time period 1965-1991. whereas the majority of the observed f 0 f 2 trends were seen to be negative, the h m f 2 ones were positive. the opinion of these authors that a global warming in the lower atmosphere, accompanied by cooling in the upper atmosphere, due to the greenhouse effect cannot be the cause of the detected trends. but such longterm trends could be explained by an increase in f2-layer storm activity as a result of the increasing geomagnetic activity observed during the analysed time period. further studies are necessary testing data homogeneity (e.g., rishbeth, 1997; ulich and turunen, 1997; bremer, 1998; jarvis et al., 1998; danilov, 1998). 7. summary the ionosphere, embedded in and tightly coupled to the thermosphere, is strongly influenced by couplings to other geophysical regions. in addition to ionizing energetic solar irradiation, both the magnetosphere and plasmasphere greatly affect the ionosphere by the precipitation of soft and energetic particles, by heat conduction, and by fluxes of thermal particles. below, the middle atmosphere affects it with upwardly propagating waves (gravity waves, tides and planetary waves). exploring these couplings effectively furthers our understanding of at least the dominant processes and interactions that play such an important role in determining the character of this part of the earth’s environment. perhaps the major advance in recent years has been the acceptance of meteorological processes as at least a potential cause of ionospheric variability, and not something in the realm of science fiction. hopefully, progress will be even more rapid in the decades to come. studies of the meteorological effects in the ionosphere are actively underway and the main aim of this review was to stimulate these investigations. whilst some details about coupling between the middle atmosphere and ionosphere are moderately well understood, there are large gaps and deficiencies in our current understanding. further measurements and modeling are undoubtedly required. measurements of quantities associated with predominant neutral particles in the ionosphere are particularly important if this coupling is to be better understood. but due to the current paucity of the efficient experimental methods of measuring neutral parameters in the ionosphere, the importance of modeling studies in parallel with the new observations also must be recognized. we must look forward to a further breakdown of the traditional isolation of the ionosphere from the lower atmosphere. since the upper atmosphere is generally a good indicator of solar activity, one might assume that the correlation between tropospheric and ionospheric parameters may indicate such a solar-weather effect. the question of lower/upper atmosphere coupling is a major challenge for ionospheric physics to which the growing science of aeronomy and meteorology may make an important contribution. the only appropriate way to place these relationships on firm ground is via the study of causal mechanisms. because of the complexity of possible relationships, it is clear that a multidisciplinary approach is required in future investigations, i.e. contributions in the fields of meteorology, climatology, aeronomy, ionospheric physics, atmospheric electricity and plasma physics. further regional, national and international cooperative efforts are needed to organize a global monitoring system of atmospheric oscillations with various new techniques. continuous observations from these networks permit a resolution of planetary waves, atmospheric tides and gravity waves and the hypothesis that such motions propagate upward from the lower atmosphere or are generated in situ could be examined critically with observational data. ionospheric weather (hour-to-hour and coupling from below as a source of ionospheric variability: a review 27 day-to-day variability of ionospheric parameters) a waits an explanation and predictions in the framework of climatological, solar cycle and seasonal variations. if we are to understand and possibly forecast ionospheric variability, then all factors, including coupling from below need to be considered. what is desired is a quantitative appreciation of all the significant couplings, trigger mechanisms and feedback processes. of practical concern are chains of processes that have final results important to life processes, effects on communications and technological activities, and impacts on the scientific observation of natural phenomena. acknowledgements ek is grateful to prof. bruno zolesi for inviting him to present the review to the xxvi european geophysical society general assembly in nice, to department for geophysics and meteorology of the university complutense of madrid for its hospitality during his stay in spain (when this paper was prepared ) and to the nato science program for financial support. references altadill, d. 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earthquake angela petruta constantin*, iren adelina moldovan, andreea craiu, mircea radulian, constantin ionescu national institute for earth physics, bucharest-magurele, romania abstract on november 22, 2014, at 21:14:17 local time (19:14:17 gmt) an earthquake of local magnitude ml= 5.7 occurred in the proximity of marasesti city in vrancea county (romania) in the lower crust (39 km depth). it was the largest event recorded since 120 years in this area (the maximum magnitude was estimated at mw 5.9 for an event occurred on march 1, 1894). the main shock was followed by more than 200 aftershocks detected and located by the romanian seismic network during two months. immediately after the main shock was recorded, the national institute for earth physics (niep) sent macroseismic questionnaires to all affected areas, in order to define the macroseismic field of ground shaking. according to the macroseismic questionnaires survey, the intensity reached vi msk in the epicentral area, and the seismic event was felt up to a distance of 600 km from the epicenter (practically in all the extracarpathian area). also, this intensity value was estimated at 25 localities. the earthquake caused general panic, but the building damage was minor to moderate only. according to macroseismic observations, the damage area was extended mostly to the east and north from epicenter. after the collection of the macroseismic data and the evaluation of the macroseismic effects of this earthquake, an intensity dataset has been obtained for 680 sites. the main purpose of this paper is to investigate the macroseismic effects associated to this earthquake using the msk-64 intensity scale in order to evaluate the impact on the regional seismic hazard assessment. 1. introduction macroseismic intensity obtained through the quantification of the earthquakes effects is an important parameter for seismological and seismic engineering research, government officials/institutions and earthquake insurance programs. this type of data is also useful for the development of the macroseismic intensity prediction equation for specific seismic zones and for the comparison and correlation between the observed intensities and peak ground accelerations and velocities. macroseismic effects in romania are dominated by earthquakes generated in the vrancea region. more than two thirds of the country’s territory are affected by seismicity generated in this region. vrancea is a complex seismic area situated at the triple junction of three tectonic plates: the east european plate, the intra-alpine micro-plate and the moesian micro-plate [constantinescu et al. 1976, airinei 1977, sandulescu 1984, knapp et al. 2005]. this area is well-known for the concentration of seismicity at intermediate depths within a welldefined lithospheric body descending into the mantle [bokelmann and rodler 2014, radulian 2014]. a rate of 3-4 destructive events (magnitude above 7) per century characterizes the activity of this source as pointed out in the romanian earthquake catalog [oncescu et al. 1999]. the seismic activity in the overlying crust is significantly lower in terms of frequency and moment release (mmax < 6.0). this type of seismicity is developed mainly east of the vrancea intermediate-depth source. the major earthquakes produced in the vrancea subcrustal source are destructive over vast areas, extending significantly beyond the country’s border [i.e., pantea and constantin 2011, kronrod et al. 2013, pantea and constantin 2013, constantin 2015]. for this reason, the database of associated macroseismic maps is relatively rich for these earthquakes, including instrumentally recorded and historical events. this is not the case when considering the earthquakes of the vrancea crustal domain. however, although at smaller scale, the effects of the earthquakes located in the vrancea crust may represent an important component in the regional seismic hazard assessment [constantin et al. 2016]. the goal of the present work is to analyze the macroseismic effects of a recent earthquake which occurred on november 22, 2014, in the vrancea crustal range (ml= 5.7, h = 39 km). it was the largest earthquake located in the vrancea crust since the end of 19th century when an earthquake with a magnitude estimated mw= ~5.9 article history received march 3, 2016; accepted october 2, 2016. subject classification: vrancea seismogenic zone, crustal earthquake, intensity, macroseismic questionnaires, intensity map. was reported. the event of 2014 was followed by a sequence of aftershocks lasting for more than two months: more than 200 events recorded by the permanent seismological network operated by the national institute for earth physics (romania) and located in the lower crust (16 to 50 km depth) (see figure 1). the largest aftershocks occurred on december 7, 2014 (ml= 4.5), and january 12, 2015 (ml= 4.2). soon after the earthquake occurrence, the national institute for earth physics (niep) organized a survey in the felt areas using macroseismic questionnaires, in order to assign macroseismic intensities according to the medvedev-sponheuer-karnik scale (msk-64) [medvedev et al. 1967]. in this paper, we evaluate the results of the macroseismic survey and draw conclusions on the implications for seismic hazard assessment in romania. 2. geological and seismotectonic settings of the area the seismic sequence that occurred in 2014 in the proximity of marasesti belongs to the seismicity characterizing the foredeep region of the south-eastern carpathians. this is divided into an internal folded foredeep, and an external unfolded one, with a maximum development in the south-eastern carpathians bending area. the crustal seismicity in the carpathians foredeep region is clustered near the se carpathians bending zone, and along the major faults such as the peceneaga–camena fault, separating the moesian platform from the north-dobrogea–scythian block, and the trotus fault separating the north-dobrogea orogen and scythian platform from the east european platform [mucuta et al. 2006, radulian et al. 2007]. the carpathian foredeep includes the focşani basin which is characterized on the western flank by a thick sequence of shallow lacustrine sediments, dipping towards the east [raileanu and diaconescu 1998, leever et al. 2006]. the focşani basin formed just in front of the se carpathian arc bend, between the intramoesian fault at the south and the trotus fault at the north (see figure 2). this basin has a neo-tectonic activity [matenco et al. 2003, tărăpoancă et al. 2003] evidenced by a large number of active normal faults with a nwse to nnwsse trend that seems to be related to the two major crustal faults existing in the area [mucuta et al. 2006]. the western limit of the basin is not so faulted. the eastern flank of the focşani basin is characterized by eastward floor shallowing affected also by a regional system of active normal faults [leever et al. 2006]. the most large-scale feature of the normal fault system along the eastern focşani flank is the siret fault. this system is juxtaposed with the peceneaga-camena fault [matenco et al. 2007]. according to raileanu et al. [2007] under the focşani basin the crust thickness reaches 46 km, but the crystalline crust does not exceed a thickness of 25 km, being covered by up to 15 km of sedimentary rocks. the crustal thickness is greater than in the surrounding foreland areas due to the major accumulation of neogene-quaternary sediments in the focşani basin [tărăpoancă et al. 2003, mucuta et al. 2006]. crustal epicenters seem to focus along the focşani basin flanks, especially along the eastern flank, highlighting the active tectonics structures that exist at the contact between the focşani depression and the north-dobrogea orogen [matenco et al. 2007, radulian et al. 2007] (figures 2 and 3). based on the historical data from the romanian earthquake catalogue [oncescu et al. 1999] six earthquakes with mw > 5 occurred in the vrancea crustal area in the last 300 years (figure 1). for two of them (march 1784 and march 1894) the maximum macroseismic intensity was estimated at vii on msk scale. the 1894 crustal earthquake had maximum intensities in panciu, focşani and adjud cities (all of them situated in vrancea county). in the epicentral area, especially in focşani, the walls of well-built buildings suffered cracks [hepites 1893, florinesco 1958]. constantin et al. 2 figure 1. mărăşeşti seismic sequence from november 22, 2014, to january 30, 2015. red star refers to the main shock of november 22 (ml= 5.7), the blue stars represent the aftershocks having magnitude greater than 3.0 and the pink dots are all the aftershocks of the sequence. yellow triangles represent niep seismic stations (plus accelerometers sensors) installed in the area and black squares are the cities. the epicenters of crustal earthquakes with m≥5 which occurred in the vrancea seismogenic area in historical time are also displayed in this figure with red dots. macroseismic intensity investigation of the vrancea crustal earthquake figure 2. the main tectonic units and crustal faults in front of the se carpathians (modified after badescu [2005]). the yellow star corresponds to the epicenter of the november 22, 2014, crustal earthquake. the ab line corresponds to the tectonic cross section presented in figure 3. figure 3. tectonic cross section across the se carpathians (after matenco et al. [2007]) along the line ab in figure 2. pcf is peceneaga-camena fault. earthquakes from vrancea zone were projected into the cross section as a function of depth and magnitude. empty circles represent the crustal earthquakes and black dots are intermediate depth earthquakes. green circle is the 2014 vrancea crustal earthquake focus. in the last 40 years several sequences of small to moderate magnitude earthquakes were recorded in the focşani basin area [popescu 2007]. they show a systematic orientation in ne sw direction, which is parallel to the carpathian orogen [bala et al. 2015]. in many cases the hypocenters are located in the lower crustal domain [tugui et al. 2009, popescu et al. 2011]. the most recent sequence of november 22, 2014, is the best ever recorded sequence in the area (82 strong-motion stations installed on the romanian territory, belonging to the niep strong-motion network). at the same time, the felt area was relatively wide, through the extra-carpathian area, and as far as ukraine, rep. of moldova, southern bulgaria and south-eastern turkey (www.emsc-csem.org). 3. macroseismic effects of the main shock and intensity map the macroseismic survey using questionnaires performed by the national institute for earth physics (niep) in the felt areas and the estimation of intensities followed the recommendations on the macroseismic scale application suggested in the msk-64 seismic intensity scale: the assessment of the intensity is based on the damage observed to the building stock and on the human perception of the effects caused by the earthquake. in other words the evaluation was done by identifying which of the descriptions for the various intensity degrees best fit all data collected for each analyzed location [e.g. midzi et al. 2015]. it means that the correct estimation is the one that best expresses the generality of the macroseismic observations [musson and cecić 2002, cecić and musson 2004]. for the assessment of the intensities the msk intensity scale was used because this scale is still employed in romania (see stas 3684-71). the type of macroseismic questionnaire used for this earthquake contains a number of questions related to construction types, nature and degree of damage, effects on the surroundings, and human and animal reactions. the first part of this questionnaire includes the administrative data (e.g., locality, owner, street), the typologies and vulnerability classes of buildings (i.e., a, b, and c structures), and the classification of damage to buildings (all five grades) according to the msk-64 macroseismic scale [pantea and constantin 2013]. each observer thus has the possibility to select the type of building and the level of damage caused by the earthquake to the building in question. another 54 questions are related to effects felt by the people, and the effects on objects and on the environment, and they cover the intensity degrees from ii to x of the msk-64 macroseismic scale [pantea and constantin 2013]. the histogram in figure 4 shows the number of the questions related to each intensity degree according to this type of questionnaire which was used for the macroseismic study of the last four major vrancea subcrustal earthquakes (1977, 1986 and two of 1990) [constantin 2015]. as can be seen in figure 4, 25 questions refer to the effects associated with the medium-low degrees of the msk-64 macroseismic scale. each question is directly related to a specific intensity degree, which means that every degree of the msk-64 scale is represented in the questionnaire by various questions [constantin and pantea 2013]. as mentioned above, the marasesti crustal earthquake was felt in all the extra-carpathian area, so questionnaires were sent to all the local authorities from that area. about 30 questionnaires were sent to each locality. the filled questionnaires were sent back to niep by the local authorities via e-mail, fax, and mail. the average number of questionnaires received for each locality is about 20, except bucharest with 159 forms received. also, soon after the earthquake, people submitted reports to the national institute for earth physics through dedicated online questionnaires, while others reported effects of the seismic event on social networks and various forums (or online newspapers). the online information was considered mainly as additional information to the macroseismic questionnaires. unfortunately, the number of online questionnaires submitted by individuals who had felt the shaking was small (224 questionnaires for 35 localities). also, analyzing the online questionnaires, we noticed that for 19 localities there were only one or two observations (questionnaires), thus reducing confidence constantin et al. 4 figure 4. number of questions related to each degree of the msk64 seismic intensity scale, according to the macroseismic questionnaire used for the november 22, 2014, earthquake. 5 in the evaluation of the effects for those localities. the total number of localities from which positive macroseismic questionnaires forms were received was 680. the answers to the questionnaires account for intensities between ii and vi msk degrees. for the neighboring areas (rep. of moldova, bulgaria) we used the information (testimonies) provided by the european-mediterranean seismological centre site (emsc). the degree of damage was reduced in its severity, but had stronger psychological effects on people. since the effects were not strong enough to cause important damage to the building stock, the assessment of the intensity for many sites was based only on the perception of the effects noticed or experienced by people during the earthquake [constantin and pantea 2013]. the most common building typologies existing in the localities where the earthquake was felt (questionnaire surveyed area) are old adobe dwellings and traditional brick houses, and also brick buildings with reinforced concrete. from the questionnaires it resulted that minor to moderate damage to buildings were reported, such as the appearance of small and sometimes open cracks in the walls, the fall of fragments of plaster in houses, and also large and extensive cracks in the walls of a few old buildings in the localities situated in the epicentral area (see figures 5 and 6), as well as east of the epicentral area. damages included also cracking of chimneys and ceilings. for the november 22, 2014, vrancea earthquake, the maximum intensity value, vi msk, was estimated for 25 localities (figures 7 and 8). the highest intensity values were assigned to localities where many cases of cracking and falling of fragments of plaster in the walls were observed in buildings from classes a and b (adobe and brick houses). in localities from vrancea county some old schools and buildings were seriously affected by the earthquake (deep and open cracks in the walls, macroseismic intensity investigation of the vrancea crustal earthquake figure 5. (a) earthquake damage in focşani (photo: georgiana loredana carab ; digi24); (b) photo with bottles thrown down from supermarket shelves in focşani (source: the vrancea newspaper; http://www.ziaruldevrancea.ro). figure 6. cracks in the school walls in focşani (source: the vrancea newspaper; http://www.ziaruldevrancea.ro). a b constantin et al. 6 figure 7. macroseismic map of the main shock of november, 2014, vrancea seismic sequence. an enlarged view of the rectangular area corresponding to the intensity distribution in the proximity of the epicenter is represented in figure 8. figure 8. intensity distribution of the november 22, 2014, vrancea crustal earthquake in the surroundings of the epicenter. 7 the fall of large fragments of plaster, see figure 6). according to news reports (digi24 television channel), the masonry facade of a very old abandoned house collapsed in tulcea. the earthquake caused power blackout in localities belonging to vrancea and galati counties. also, disruption and damage to water supply systems were noticed (according to inspectorate for emergency situations of galati). mobile phone networks were blocked during the earthquake because of the heavily increased number of telephone calls. table 1 gives the evaluated macroseismic intensities for some of the most important cities in the felt area, and for some localities with highest intensities, together with the epicentral distances. we present below a few examples of cities and localities where maximum effects were reported and highest intensities were estimated. for instance, in the settlement of umbraresti (a commune in galati county of about 6628 people), which is located 29 km southeast from the epicenter, the following effects were observed: small cracks in the walls and falling of fragments of plaster (damage of grade 2) of houses of type a, and in a few of type b; many people from inside were frightened and ran outdoors; in some cases objects were thrown down, including dishes and other glassware, and some of these objects were broken. taking into account these effects, the intensity in commune of umbraresti was estimated at vi msk. similar effects were observed in onesti (bacau), city situated at 50 km nnw from epicenter, and in tecuci town (galati county) (21 km east): many people were scared and ran outside, in different buildings falling of light objects was observed, few glass objects and dishes were broken, some unfixed objects (books and others) were tilted or/and felt from shelves, and of course, minor to moderate damage to buildings was observed. in focşani city, vrancea county (with approx. 79,000 inhabitants) located 18 km south from epicenter, the same effects as in the case of sites mentioned above were reported: small cracks in the walls of buildings with vulnerability classes a and b, the objects were thrown down and broken (dishes and other glassware), books thrown from shelves, etc. as peculiar phenomena observed in the area, loud noise accompanying the earthquake were reported. the intensity distribution in the felt areas of romania is somehow irregular and does not show a regular attenuation law of the intensity with the epicentral distance (see figure 7). for instance, the localities to the south had intensities of v degrees, while the others felt the earthquake with intensity iii or iv. similarly, anomalous high intensities were reported for some localities in the northern part of the country. these anomalies in several localities from south and north can be explained by the influence of the local soil conditions (due to some alluvial deposits in these areas) [pantea and constantin 2013]. also, these high values of intensity at great distances (>150 -200 km) could be due to a considerably long duration of the seismic waves. the distribution of intensity in degrees obtained after the evaluation of macroseismic effects is presented in figure 9. as it can be seen in this figure, 240 intensity data points (idps) have an intensity value of iv msk degrees. 4. conclusions the ml 5.7 earthquake occurred on november 22, 2014, close to mărăşeşti city was the strongest crustal earthquake that occurred in the vrancea seismic region in the last century, both regarding magnitude and macroseismic effects. the analysis of macroseismic observations performed in this study allows us to make the following remarks: the macroseismic investigation of the effects produced by this earthquake was based on information data coming from 680 sites, most of them situated within the extra-carpathian area. the maximum macroseismic intensity assessed after the questionnaires survey for the main shock of the seismic sequence was vi msk. this value was estimated at 25 locations, including one in braila, 90 km from the epicenter. the intensities were assigned on the basis of cumulative effects in the localities where the damages indicate these values. in general, in the affected area the building stock suffered minor to moderate damage; significant damages were observed only in a few old houses. according to macroseismic observations and macroseismic intensity investigation of the vrancea crustal earthquake figure 9. number of intensity data points (idps) obtained for each intensity degree. constantin et al. 8 no. county city/locality lat. n long. e i (msk) d (km) 1 arges mioveni 44.96 24.94 iii-iv 201 2 bacau bacau 46.57 26.90 v 81 3 bacau buhusi 46.72 26.70 v 99 4 bacau moinesti 46.48 26.48 iv 83 5 bacau onesti 46.25 26.77 vi 50 6 bacau racauti 46.22 26.79 vi 49 7 bacau tg. ocna 46.28 26.61 iv-v 61 8 botosani botosani 47.75 26.67 iii 212 9 botosani dorohoi 47.95 26.40 iii-iv 238 10 braila braila 45.28 27.96 vi 90 11 braila faurei 45.11 27.25 v 85 12 braila ianca 45.14 27.46 v-vi 84.5 13 braila insuratei 44.92 27.59 v 111 14 braila maxineni 45.40 27.62 vi 63 15 braila racovita 45.30 27.46 vi 68 16 brasov brasov 45.66 25.59 iii 124 17 brasov zarnesti 45.56 25.35 iii 145 18 bucharest bucharest 44.41 26.11 iv 181 19 buzau buzau 45.16 26.81 iv-v 81 20 buzau pogoanele 44.92 26.98 v-vi 106 21 buzau ramnicu sarat 45.39 27.03 v 56 22 calarasi calarasi 44.20 27.31 iv-v 186 23 calarasi oltenita 44.06 26.61 iv 206 24 constanta cernavoda 44.34 28.03 iv 184 25 constanta constanta 44.17 28.63 iv-v 222 26 constanta medgidia 44.26 28.27 iv-v 199 27 constanta techirghiol 44.06 28.59 iv-v 231 28 covasna sf. gheorghe 45.88 25.80 iii-iv 106 29 covasna tg secuiesc 46.01 26.13 iv 80 30 dambovita moreni 45.00 25.64 iv 153 31 dambovita pucioasa 45.09 25.43 iii 160 32 galati costache negri 45.70 27.73 vi 48 33 galati cudalbi 45.77 27.68 vi 42 34 galati frumusita 45.65 28.07 vi 75 35 galati galati 45.44 28.04 v 86 36 galati jorasti 46.00 27.87 vi 57 37 galati liesti 45.63 27.53 vi 39 38 galati pechea 45.62 27.81 vi 58 39 galati smulti 45.93 27.72 vi 44 40 galati tecuci 45.86 27.41 vi 21 41 galati tg. bujor 45.87 27.94 v 60 42 galati umbraresti 45.71 27.45 vi 29 43 giurgiu giurgiu 43.89 25.96 iv 240 44 ialomita slobozia 44.57 27.37 iv-v 145 45 ialomita urziceni 44.72 26.64 iv-v 134 46 iasi iasi 47.17 27.58 iii-iv 148 47 iasi targu frumos 47.20 27.01 iv 148 48 ilfov otopeni 44.57 26.07 iv 168 49 ilfov pantelimon 44.46 26.24 iv 172 50 neamt piatra neamt 46.93 26.37 iv 133 51 neamt roman 46.92 26.93 iv 118 52 neamt tg. neamt 47.19 26.36 iv 159 53 prahova mizil 45.00 26.43 iv 112 54 prahova ploiesti 44.95 26.02 iii-iv 136 55 suceava falticeni 47.46 26.31 iii 188 56 suceava suceava 47.65 26.26 iii-iv 209 57 teleorman turnu magurele 43.76 24.85 iii-iv 297 58 vaslui barlad 46.23 27.67 iv-v 56 59 vaslui vaslui 46.63 27.72 v 95 60 vrancea adjud 46.10 27.17 v-vi 26 61 vrancea focsani 45.70 27.17 vi 18 62 vrancea panciu 45.90 27.08 v 6.5 63 vrancea urechesti 45.58 27.05 vi 33 64 vrancea marasesti 45.88 27.23 v-vi 5.5 table 1. seismic intensities for the important cities in the felt area, and for some localities with maximum effects. 9 mapping, the damage area was extended mostly to the east and north from epicenter. the damage suffered by building stock and especially by the old (historical) buildings observed in several localities highlights the importance of buildings consolidation and for the future, the compliance with seismic design codes and specifications. the macroseismic study performed in the presented paper provides a valuable database of idps useful for the development of the seismic intensity prediction equation for the earthquakes of the vrancea crustal domain and, also for the evaluation of the impact on the regional seismic hazard. acknowledgements. we want to thank to public authorities for helping us in the collection of the macroseismic information, and especially to the general inspectorate for emergency situations (igsu). we also thank the two anonymous reviewers for helpful comments and recommendations which led to the improvement of the manuscript. this paper was partially carried out within nucleu program, supported by ancsi, projects no. pn 16 35 01 06, pn 16 35 03 01 and pn 16 35 03 05 and the partnership in priority areas program pnii, under men-uefiscdi, daring project no. 69/2014. references airinei, st. 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(in romanian). stas 3684-71. seismic intensity scale. romanian institute for standardization, irs, bucharest (in romanian). tărăpoancă, m., g. bertotti, l. matenco, c. dinu and s.a.p.l. cloetingh (2003). architecture of the focşani depression: a 13km deep basin in the carpathians bend zone (romania), tectonics, 22 (6), 1074; doi:10.1029/2002tc001486. tugui, a., m. craiu, m. rogozea, m. popa and m. radulian (2009). seismotectonics of vrancea (romania) zone: the case of crustal seismicity in the foredeep area, rom. rep. phys., 61 (2), 325-334. *corresponding author: angela petruta constantin, national institute for earth physics, bucharest-magurele, romania; email: angela@infp.ro. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. constantin et al. 10 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice adg vol5 n02 del ne 361_374.pdf annals of geophysics, vol. 45, n. 2, april 2002 361 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau ciro del negro (1), fabrizio ferrucci (2) and rosalba napoli (1) (1) istituto nazionale di geofisica e vulcanologia, sezione di catania, italy (2) dipartimento scienze della terra, università della calabria, arcavacata di rende (cs), italy abstract two marine magnetic surveys were carried out during 1997 and 1999 in the ionian sea off the eastern coast of sicily to investigate the magnetic structures of the eastern base of mt. etna and the hyblean plateau. the investigated area is approximately 85 km long and 15 km wide, running from north to south, in the western ionian sea. models along two profiles parallel to the coast and over the entire area provide a possible distribution of volcanic bodies and volcaniclastic deposits off the eastern coast of sicily and their relations with the sedimentary substratum. 3d modeling suggests the presence of magnetized bodies, inserted in the sedimentary substratum, plausibly related to hyblean plateau volcanism in the south sector and to mt. etna activity in the north. we speculate that the malta escarpment could have produced preferential ways for magma ascents off the hyblean plateau. the spatial continuity of the volcanism affecting the entire investigated area could testify spatial transition between hyblean and etnean volcanism supporting the hypothesis that the magma process migrated with time from south-east to north-west. 1. introduction the geological and structural frame of sicily is the result of complex motions and interactions between the african and european plates, which have produced, in the mediterranean region, the calabro-sicilian subduction process (barberi et al., 1974; lentini, 1982). in this context, eastern sicily represents the connection area between the maghrebian-calabrian thrust belt (european plate), which occupies the northern part of sicily (fig. 1), and the african foreland, whose leading edge, the hyblean plateau, crops out in the southeastern corner of the island (grasso et al., 1983; grasso and ben-avraham, 1992; hirn et al., 1997). the hyblean plateau is cut off along the eastern coast by a regional fault system, the malta escarpment, which splits it from the thinned oceanic crust of the ionian basin (grasso et al., 1983; monaco et al., 1995). within this general compressional tectonic framework, an intense basaltic volcanic activity has intermittently affected eastern sicily for 200 ma (patacca et al., 1979; hirn et al., 1997). this activity, related mailing address: dr. rosalba napoli, istituto nazionale di geofisica e vulcanologia, sezione di catania, piazza roma 2, 95123 catania, italy; e-mail: napoli@ct.ingv.it key words magnetic survey – magnetic modeling – mt. etna – hyblean plateau 362 ciro del negro, fabrizio ferrucci and rosalba napoli 37° 25' 37° 15' 15°00' 15°10' 15°20' 15°30' 15°40' 15°50' 2000 catania plain riposto ridge m alta e scarpm ent simeto river augusta etna tyrrhenian sea ionian sea hp me 100 km ra c l mag h ca37° 35' messina rise 10 00 catania canyon 37° 35' 37° 45' 37° 55' catania aci castello capo mulini n 0 10 km taormina capo schisò pozzillo stazzo riposto mt. etna 15°00' 15°10' 15°20' n 0 10 km mt. etna etnean volcanic rocks main faults thrust 37° 45' alcantara river capo s. croce 37° 25' 37° 15' 1999 survey1997 survey hy ble an pla tea u pcr pdn fig. 1. simplified tectonic map of the eastern coast of sicily. the insets at the right show: at the top the relative position of the hyblean plateau (hp), the malta escarpment (me) and the maghrebian-calabrian thrust belt (magh-cal arc); at the bottom the investigated area in 1997 and 1999. lines represent measurement tracks. 363 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau to different episodes of local extensional tectonics (barberi et al., 1974; grasso and benavraham, 1992), is responsible for the volcanism of both the hyblean plateau and etna. they have been connected to extensional processes, with wnw-ese trend, orientated along a rifting zone, which develops from the hyblean plateau to northern calabria (tapponnier, 1977; tortorici et al., 1986; monaco et al., 1995). in particular, marine seismic profiles executed by hirn et al. (1997) reveal how etna appears structurally linked to an extensional domain reaching offshore from catania and augusta. considering that the structures held accountable for the hyblean and etnean volcanism develop partly offshore along the border of a continental scarp, it is likely that most of the products representing their geological and volcanological evolution are submerged along the eastern ionian coast of sicily. marine geological and geophysical investigations, therefore, may represent a useful tool to support and integrate data gathered inland to improve the knowledge of both the hyblean plateau and etna evolution. since the early 1990s, the area off the eastern coast of sicily has been studied intensively, including two oceanographic campaigns promoted by the italian consiglio nazionale delle ricerche in 1997 and 1999. magnetic data as well as seismic and bathymetric data, integrated by drags and cores to investigate seafloors to a depth of about 2000 m, were collected during these survey cruises. this paper focuses on the analysis of magnetic data gathered along a more than 500 km line. their interpretation, in combination with previous geological and geophysical studies (e.g., gabbianelli et al., 1995), evidences the offshore prosecution of the etnean edifice and the area affected by magmatism off the hyblean coast. the presence of volcanic bodies, and especially their prosecution southwards to the main faults of the malta escarpment, could represent the spatial transition between hyblean and etnean magmatism and highlights the major role played by the regional discontinuities in the spatial and temporal evolution of the pliopleistocene volcanism of eastern sicily. 2. morphological and structural framework the submerged area off the eastern coast of sicily (fig. 1) is characterized by a 900 m wide continental platform related to the last glacialeustatic cycle which reached its maximum regression about 18 ka bp (gabbianelli et al., 1995). it develops parallel to the coast reaching variable depths (160-80 m) and joins the emerged zone and continental scarp. the latter, whose mean width is 17 km, is characterized by average inclinations of 4-6° and extends offshore to a depth of 1800-2000 m (bousquet et al., 1998) where it is replaced by the messina rise, a large sector of the ionian basin with a nnw-sse direction which represents a fragment of the hyblean plateau which collapsed after the middle pleistocene (sartori et al., 1991). the northern part of the investigated area (fig. 1) is dominated by the riposto ridge, a morphological lineament about 20 km long and 4 km wide, which reaches a depth of about 1500 m with an e-w trend. the riposto ridge, made of sedimentary substratum and affected by compressive structures, could represent the offshore prosecution of the units of the maghrebian-calabrian chain (gabbianelli et al., 1995; del negro and napoli, 2002). it is bounded to the north by a canyon while to the south the seafloor shows an irregular morphology up to the canyon of catania, a large depression extended off acicastello (bousquet et al., 1998) and bounded southwards by the catania plain (rossi and sartori, 1981). the area off the etnean edifice is also affected by the presence of ns and nnw-sse striking normal faults, which could be the offshore prosecution of the timpe system (sartori et al., 1991; lanzafame and bousquet, 1997). these faults appear as segments at least 20-30 km long stretching from etna southwards and reaching offshore from catania and augusta (hirn et al., 1997). these long faults define some pleistocene extensional basins which are isolated from each other by shorter faults oriented ene-wsw and ne-sw. some of them seem to represent the continuation of the main faults, which affect the hyblean inland area (sartori et al., 1991). moreover, the offshore area of augusta is characterized by the presence of the northern part of the malta escarpment whose morphologic drop is greater than 2000 m (sartori et al., 1991). 364 ciro del negro, fabrizio ferrucci and rosalba napoli 3. magnetic survey digital magnetic data were gathered simultaneously with bathymetric data during two cruises of r/v urania in 1997 and 1999. firstly, the area between taormina and catania along the eastern coastal side of mt. etna was surveyed, and subsequently the area between catania and capo s. croce off the hyblean coast (fig. 1). the data acquisition array was irregular, with the mesh ranging between 1 and 3 km. the whole area is placed in the western ionian sea between 37°07 0 n-37°53 00 n and 15°08 00 15°26 00 and is approximately 85 km long and 15 km wide. a geometrics g-811 proton precession magnetometer, with 1 nt resolution, was used to make measurements of the total intensity of the earth’s magnetic field. the sensor was dragged at 23 m below sea level by a steel cable about 300 m long to reduce the magnetic effects of the ship. we connected the sensor tip to a glass fiber ring to reduce possible sharp movements, oscillations or rotations produced by waves and currents. we collected about 100 000 readings at 5 s intervals, which is equivalent to a distance of approximately 10 m at the cruising speed of about 4 knots. positioning relied upon the gps (global positioning system) differential system, with base station located on shore at catania. moreover, a gyrocompass defining the direction of navigation allowed to correct the offset error related to the distance between the magnetic sensor and the ship. the magnetic surveys were carried out nighttime to minimize the effect of the solar activity. moreover, surveying was usually done during magnetically quiet days when the values of the magnetic activity k index were generally less than 1 (from j.d. 129:1997 to j.d. 133:1997, j.d. 21:1999, from j.d. 25:1999 to j.d. 26:1999 and from j.d. 31:1999 to j.d. 33:1999). strict control of the level temporal variations was obtained using data continuously recorded at stations of the permanent network for magnetic monitoring of mt. etna (del negro et al., 1997). during the marine cruises, only these stations were working. pcr and pdn stations were the most suitable for use as reference stations since they are located a few kilometers apart westward from the center of the investigated area (fig. 1). measurements of the total intensity field, collected every minute at pcr (in 1997) and pdn (in 1999), have thus been used to correct data for the diurnal variation. to compute the magnetic anomalies and apply secular variation correction, data reduction process included removal of the earth’s magnetic field using the igrf reference fields of the corresponding periods (iaga division v, working group 8, 1996). once the anomaly values were obtained, the magnetic data of each survey were adjusted to minimize discontinuities in the overlap regions (ishihara et al., 1999). the standard deviation of crossover differences was 83 nt for the uncorrected data, but is reduced to 35 nt by applying a constant bias to each cruise. finally, the data from these two surveys were merged to provide only one data set for the entire area. in order to prepare a contour map, the corrected data were gridded at a spacing of 0.5 km using a minimum curvature algorithm. the anomaly magnetic field, which is shown in fig. 2, appears particularly complex close to the coast where several anomalies with different intensity and extension were detected. taking advantage of the very close sampling step adopted (10 m) with respect to the real resolution of the survey (some kilometers) a low-pass filter, with a cutoff wavelength of 3 km was applied to reduce high frequency noise and to enhance the effects of deepseated bodies or broad shallow sources. the cut off frequency was established using a one-dimensional spectral procedure, less representative of the fields but more stable than the radial spectral procedure which is often affected by a migration phenomenon of minimum amplitude spectral points (faggioni et al., 2001). the resulting filtered magnetic map is shown in fig. 3. the general trend is not well correlated with the published magnetic anomaly map of italy and surrounding seas (chiappini et al., 2000). high anomalies with short wavelength and large amplitude, which do not appear on the published map, were observed. the northern part of the area is characterized by a wide anomaly located off the coast between stazzo and capo schisò. southwards, two more intense and well defined anomalies were detected respectively in correspondence of the capo mulini and aci castello lava flows and south of catania very close to augusta. each of these anomalies disappears offshore at 10-20 km from the coast. 365 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau simeto river alcantara river catania aci castello capo mulini taormina capo schisò riposto pozzillo stazzo augusta io ni an s ea 37°45' 37°35' 37°25' 37°15' 15°00' 15°10' 15°20' n 0 10 km -400 -300 -200 -100 0 100 200 300 400 nt hy bl ea n pl ate au fig. 2. map of the total intensity magnetic field. dotted lines represent magnetic measurements. 4. modeling to obtain a picture of the sources of the magnetic anomalies we calculated models along two profiles parallel to the coast (aa′ and bb′) and then over the whole surveyed area (fig. 3). the modeling procedure was carried out by a 3d program (potent v3.06 produced by pc potentials, australia), which adjusts the model using a combination of manual (forward modeling) or automatic (inversion modeling) commands until it achieves a satisfactory fit. 366 ciro del negro, fabrizio ferrucci and rosalba napoli potent calculates magnetic anomalies by assuming that the source bodies are 3d prisms, both rectangular and polygonal, with uniform magnetization. in particular, the magnetic effect of a 3d rectangular prism is calculated using a formula derived along lines similar to those of bhattacharyya (1964). the calculation for the magnetic effect of a 3d polygonal prism is programmed in two stages. firstly, two orthogonal field components normal to the axis of -400 -300 -200 -100 0 100 200 300 400 nt simeto river catania aci castello capo mulini capo schisò riposto pozzillo stazzo augusta taormina a b 37°45' 37°35' 37°25' 37°15' 15°00' 15°10' 15°20' n 0 10 km a' alcantara river b' io ni an s ea hy ble an pl ate au fig. 3. low-pass filtered magnetic map of the investigated area. the cutoff wavelength is 3 km. black lines (aa′ and bb′) represent modeled profiles. 367 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau the prism are calculated using formulas derived along the same lines as those given by cady (1980) for the gravity case. subsequently, the component along the third axis of the prism is evaluated using a numerical integration. this increases the calculation time considerably, but the field calculation is exact at any point external to the 3d polygonal prism. 4.1. rock magnetic properties no direct measurements of remanent magnetization (jnrm) and susceptibility ( ) are available for rock samples offshore eastern sicily. therefore, we decided to infer most of j nrm and values by comparison with petrographically similar rocks, even though petrographic similarity does not constrain complete magnetic similarity (the latter relates also to other factors including the history of each rock). however, unless severe geo-statistical rules are observed, laboratory data obtained from a few samples within a single outcrop would also undergo the same error (fleming and tréhu, 1999). since the chemical composition of dragged rocks matches quite well with the compositional range known for etnean and hyblean products outcropping along the ionian coast (pompilio, personal communication), we used the results of laboratory tests carried out on several samples of lava flows, volcaniclastic deposits and alluvium collected in the south eastern sicily (roberti and scandone, 1975; tabacco, 1998; del negro and napoli, 2002). these measurements reveal a magnetization mean value of 7 am 1 and a susceptibility mean value of 0.034 s.i. for lava flows. instead, volcaniclastic deposits and alluvium display lower values of jnrm (between 2 and 3 am 1) and (between 0.01 and 0.02 s.i.). therefore, we assumed the volcaniclastic deposits and volcanic rocks to have values of jnrm and ranging respectively between 2 and 7 am 1 and between 0.01 and 0.034 s.i., whereas we have assumed the sedimentary basement to have no magnetization. these initial model values required little subsequent modification to provide a reasonable match to the magnetic data. inclination (i) and declination (d) of remanent magnetization were set equal to that of the present-day earth’s field. this assumption is justified by the observation that the area northwards of the simeto river is mainly characterized by etnean products (about 0.5 ma; gillot et al., 1994) which show a remanent magnetization generally much greater than induced magnetization (königsberger ratio is > 1; see e.g., rolph, 1992; tric et al., 1994) and which were emplaced after the last field reversal (0.78 ma; tauxe et al., 1992). on the other hand, also for the hyblean area, which is characterized by volcanic rocks emplaced before the last field reversal (200 ma ago; patacca et al., 1979; hirn et al., 1997), susceptibility values calculated by arisi rota and fichera (1987) tend to exclude the remanent magnetization oriented in a direction not parallel to the present magnetic field, even in the opposite direction. in particular, we obtain the best result assigning to inclination and declination the values obtained by iaga (1996) for the igrf 1995 field (approximately i = 52°n, d = 1°w). since we have few constraints on the detailed crustal structure of the investigated area, the geometry and position of each body were estimated from the isolated magnetic anomaly patterns and varied until the rms error between the observed and calculated data was less than 20 nt for profiles and 100 nt for whole area. the bathymetric data collected simultaneously with the magnetic measurements were used to define sea bottom topography. 4.2. profile aa the first profile (aa’), about 80 km long, runs near the coast from taormina to augusta and consist of two parts nne-ssw and nnw-sse orientation, respectively. the geological outline obtained by the modeling is described below and illustrated in fig. 4. the wide anomaly located between stazzo and capo schisò off the main outcrop inland of the chiancone deposit (del negro and napoli, 2002) is modeled as a single body (a1) about 12-14 km long and outlined up to a depth of about 800 m under the sea level. the mean thickness of a1 is about 200 m and its magnetic properties jr = 3 am 1 and = 0.01 s.i. intermediate between lava products and 368 ciro del negro, fabrizio ferrucci and rosalba napoli sedimentary basement are referable to the chiancone outcropping with a fan morphology spreading from the outlet of the valle del bove to the ionic coast (calvari and groppelli, 1996). northwards a1 is replaced by the sedimentary basement which is overlaid at taormina by a thin lava coverage (a2) referable to the alcantara lava flows. the more intense anomaly located in the middle of the profile at capo mulini is well modeled as a 100-150 m thick lava body, jr = 5 am 1 = 0.034 s.i. (a3). it underlies the sedimentary basement and stretches, growing thinner, both northwards up to the chiancone, and southwards beyond catania. a3 is bounded southwards by a volcaniclastic deposit (a4), which spreads for about 8 km south of catania to the mouth of the simeto river. considering its location and its magnetic properties jr = 3 am 1 and = 0.01 s.i., the same as a1, it is reasonable to assume that a4 0 20 40 60 80 km nt 480 320 160 0 -160 0 -800 -1600 -2400 magnetic properties d ep th (m ) observed field calculated field taorminasimeto riveraugusta pozzilloacicastello a1 a2 a3 a4 a5 a6 aa' sedimentary substratum 0.00 s.i j = 0 a/m = nrm etnean lava (a3 and a2) 0.034 s.i. j = 5 a/m = nrm chiancone deposit (a1) 0.01 s.i. j = 3 a/m = nrm volcaniclastic deposit (a4) 0.01 s.i. j = 3 a/m = nrm hyblean volcanic rock (a5 and a6) 0.03 s.i. j = 4 a/m = nrm . fig. 4. magnetic modeling of profile aa . at the top observed data compared to the modeled anomalies; at the bottom the geological models from which the modeled anomalies were computed. 369 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau is formed by incoherent volcanic material sprung from dismantling of the volcanic outcrops on the western flank of etna, and subsequently carried away and settled at sea by the simeto river. at the southernmost tip of the profile very near to augusta two 100 m thick lava coverages (a5 and a6) 15 and 6 km long respectively were discovered. a susceptibility contrast of 0.03 s.i. between the volcanic products and the underlying sedimentary basement is assumed. 4.3. profile bb another profile (bb ), parallel to the former and about 5 km away from it, was selected and modeled. also this profile, like the aa profile, consists of two parts (fig. 3) and its schematic geological cross-section is shown in fig. 5. a considerable decrease in both extension and in thickness of the volcanic structures described in profile aa is evident, probably due to the great nt 160 0 -160 0 -800 -1600 -2400 -3200 observed field calculated field 80 -80 d ep th (m ) taorminapozzilloaugusta simeto river acicastello km0 20 40 60 80 a1 b2 b3 b1a3 a5 b' b magnetic properties sedimentary substratum 0.00 s.i j = 0 a/m = nrm etnean lava (a3 ) 0.034 s.i. j = 5 a/m = nrm chiancone deposit (a1) 0.01 s.i. j = 3 a/m = nrm volcaniclastic deposit (b1, b2 and b3) 0.01 s.i. j = 2 a/m = nrm hyblean volcanic rock (a5) 0.03 s.i. j = 4 a/m = nrm . fig. 5. magnetic modeling of profile bb . at the top observed data compared to the modeled anomalies; at the bottom the geological models from which the modeled anomalies were computed. 370 ciro del negro, fabrizio ferrucci and rosalba napoli simeto river catania aci castello capo mulini taormina capo schisò riposto pozzillo stazzo augusta io ni an s ea 37°45' 37°35' 37°25' 37°15' 15°00' 15°10' 15°20' n 0 10 km a1 a2 a3 a4 a5 b1 b2 b3 a6 alcantara river hy bl ea n pl ate au fig. 6. 3d interpretation of the entire area. for description of bodies see text. distance from the origin and/or derivation area. in particular, the volcanic bodies (a3 and a5) identified at capo mulini and augusta preserve their magnetic characteristics unaltered and show a mean thickness less than 100 m and an extension of about 12 and 8 km respectively. the thin lava coverage (a2) related to the alcantara lava flows at the northernmost end of the profile is replaced by a deposit (b1) whose magnetic properties jr = 2 am 1 and = 0.01 s.i. are similar to those of the chiancone but is not ascribable to it. as for the chiancone (a1), its extension is reduced to 8 km with a minimum thickness. other volcaniclastic deposits (b2 and b3) are observed to the south and north of augusta probably originated by accumulation driven by marine currents of volcanic products belonging to lava outcrops, which are present at the most proximal areas. 4.4. entire area on the basis of the schematic geological cross-sections described above, we produced a 3d forward model of the entire area. since the magnetic anomaly pattern is quite complicated, the 3d model can only be a semi-quantitative one. the first step was to create a starting model, constrained by the results obtained along profiles aa and bb , which approximately represent the magnetic survey data. afterwards we adjusted the model (fig. 6) by forward and inversion modeling using a combination of contoured plan and cross-sectional profiles until it provided a satisfactory fit to the data (fig. 7). a three dimensional multi-body inversion option supplements the interactive model fitting process. the basis of the method used for the inversion (of selected model parameters) is singular value decomposition of the jacobian matrix, as described by jupp and vozoff (1975). the residual anomalies are modeled using 3d polygonal prisms whose positions were inferred from the magnetic anomalies distribution. in particular, length, width, thickness, top and bottom depth of the prisms (table i) were controlled to find the best match to the waveform of the observed magnetic anomaly. among threedimensional models obtained we choose the simplest (fig. 6) with the minimum rms error (93 nt) between the observed (fig. 3) and calculated values (fig. 7). 5. discussion forward models of the magnetization distribution are not unique, and imply a degree of speculation when strong constraints such as rock magnetic properties, and complete knowledge of surface geology are unavailable. however, we produced geologically meaningful 371 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau models taking advantage of the simple shapes of the magnetic anomalies, their amplitudes, and by restricting source bodies to the upper crust. the models provide a good correlation (rms less than 20 nt for profiles and 100 nt for the entire area) between observed and calculated anomalies, and also reveal a very similar geological outline to that outcropping inland. the intense anomaly detected off the volcanic outcrops of augusta can be interpreted as magnt capo schisò 37°45' 37°35' 37°25' 37°15' 15°00' 15°10' 15°20' -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 simeto river catania aci castello capo mulini riposto pozzillo stazzo augusta n 0 10 km alcantara river taormina capo schisò hy ble an pl ate au io ni an s ea fig. 7. calculated anomalies corresponding to the 3d model shown in fig. 6. for magnetic properties and geometrical features of the bodies see table i. 372 ciro del negro, fabrizio ferrucci and rosalba napoli table i. magnetic properties and geometrical features of the bodies making up the 3d model shown in fig. 6. body remanent susceptibility strike dip maximum maximum mean magnetization ( ) length width thickness (j nrm ) a1 3 am 1 0.01 s.i. n 15.2°e 90° 13 km 14 km 200 m a2 5 am 1 0.034 s.i. n 15.2°e 90° 9 km 6 km 80 m a3 5 am 1 0.034 s.i. n 15.2°e 90° 20 km 16 km 150 m a4 3 am 1 0.01 s.i. n 163°e 90° 8 km 2.5 km 100 m a5 4 am 1 0.030 s.i. n 3.7°e 90° 15 km 13 km 100 m a6 4 am 1 0.030 s.i. n 10°e 90° 6 km 3 km 100 m b1 2 am 1 0.01 s.i. n 20°e 90° 13 km 2 km 100 m b2 2 am 1 0.01 s.i. n 2°e 90° 10 km 4 km 150 m b3 2 am 1 0.01 s.i. n 178°e 90° 20 km 4 km 200 m matic intrusions (a5 and a6), which according to zanolla et al. (1998) could have reached the seafloor surface and various sub-bottom levels by means of faults related to distension phases. in particular, a5 and a6 can be related to one or more stages of hyblean plateau volcanism. in this area, indeed, the volcanic activity continued intermittently up to the pliocene and submarine and subaerial volcanic products were found along the ionian coast from north of augusta to the vicinity of syracuse (outside the investigated area) both in outcrop and underground overlaid by sedimentary deposits (carbone et al., 1982; grasso et al., 1983). moreover, considering that they overlap the northern faults belonging to the malta escarpment it is likely that these magmatic intrusions were injected along this regional discontinuity system. further north, two volcanic bodies, a3 and a2, stretch normally to the coast and are ascribable to mt. etna volcanism. the former was discovered off the lava flows outcropping at capo mulini. it is consistent with the presence of etnean volcanic products (aa. vv., 1979) along the coast from pozzillo to catania and of volcanic bodies related to the submarine or pre-etnean hypo-abyssal activity which are found either offshore or inland inside the pleistocene clays forming the etnean basement (gabbianelli et al., 1995). the latter is the offshore prolongation of the alcantara lava flows, which starts from the northern flank of mt. etna and extends up to 4-5 km from the coast. it represents one of the longest etnean lava flows. modeling, according to drags executed in the same oceanographic cruises (corsaro and pompilio, personal communication), also reveals different deposits of incoherent volcanic material, whose origin, except for the chiancone deposit (a1) which was likely produced by a catastrophic event (del negro and napoli, 2002), can be attributed to transport and settling processes offshore driven by the simeto river (a4) or sea currents (b2 and b3). it is worth noting that far away from the coast (10-15 km), volcanic bodies get thinner and volcaniclastic deposits (b1, b2 and b3), characterized by a lower magnetization and susceptibility (table i) and probably sprung from dismantling of volcanic bodies, increase. here the magnetization contrast, essential for data inversion, diminishes because of the increase in prevalently sedimentary products whose magnetic properties are almost negligible. therefore, modeling reliability is strongly compromised especially concerning the incoherent volcanic deposits dipped in the sedimentary basement. 373 marine magnetic investigation of the submarine base of mt. etna and hyblean plateau 6. conclusions the magnetic anomalies observed off the eastern coast of sicily contribute to shed light on the spatial distribution of the magnetized bodies and provide pieces of information allowing the peculiar development and geodynamic significance of magmatism in this area to be better understood. in particular, the presence of the main magnetic anomalies close to the coast reveals the strong relation between the causative bodies (basaltic rocks and volcaniclastic deposits) and the geological features encountered inland. the modeling of the investigated area, indeed, reveals a sedimentary substratum where the first volcanic products of the hyblean and etnean activity are inserted. the presence of magma intrusions offshore the hyblean plateau in correspondence of the malta escarpment, besides enlarging the hyblean area affected by volcanic activity, points out the role played by this regional discontinuity system which, producing a zone of weak disrupting crust, could have created preferential ways for magma ascents. on the other hand, the volcanic bodies detected off the etnean edifice, and interpreted as the first products of etnean activity, show the spatial continuity of the volcanism affecting this area and could represent spatial transition between hyblean and etnean volcanism supporting the hypothesis that magma process migrated with time from south-east to north-west. acknowledgements the authors are grateful to s. tabacco, a. di bella, e. spadafora for their help during the oceanographic cruises. particular thanks are due to a. sicali who developed the software for magnetometer management and to r. comito for preliminary data analysis. the authors wish to thank cnr for the oceanographic ship urania, the captain v. lubrano and the crew which provided a valid support during data acquisition. references aa. vv. 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(1980): calculation of gravity and magnetic anomalies of finite length right polygonal prisms, geophysics, 45, 1507-1512. calvari, s. and g. groppelli (1996): relevance of the chiancone volcaniclastic deposit in the recent history of etna volcano (italy), j. volcanol. geotherm. res., 72, 239-258. ca r b o n e, s., m. gr a s s o and f. le n t i n i (1982): considerazioni sull’evoluzione geodinamica della sicilia sud-orientale dal cretaceo al quaternario, mem. soc. geol. it., 24, 367-386. chiappini, m., a. meloni, e. boschi, o. faggioni, n. beverini, c. carmisciano and i. marson (2000): shaded relief magnetic anomaly map of italy and surrounding marine seas. ann. geofis., 43 (5), 983-989. del negro, c. and r. napoli (2002): ground and marine magnetic surveys of the lower eastern flank of etna volcano (italy), j. volcanol. geotherm. res., 114 (3/4), 357-372. del negro, c., f. ferrucci and r. napoli (1997): the permanent network for magnetic surveillance of mt. etna: changes in the geomagnetic total intensity observed in 1995, acta vulcanol., 9 (1), 67-72. faggioni, o., n. beverini, c. carmisciano and i. giori (2001): a metrologic method of anomaly field amplitude reduction in undersampled geomagnetic marine surveys, mar. geophys. res., 22, 63-79. fleming, s.w. and a.m. trehu (1999): crustal structure beneath the central oregon convergent margin from potential-field modeling: evidence for a buried basement ridge in local contact with a seaward dipping backstop. j. geophys. res., 104, 20431-20447. gabbianelli, g., g. lanzafame, f. lucchini, m. pompilio and r. sartori (1995): indagini di geologia marina alla base orientale dell’etna, in progetto-etna 1993-1995, edited by f. ferrucci and f. innocenti, cnr gruppo nazionale per la vulcanologia, 79-84. gillot, p.y., g. kieffer and r. romano (1994): the evolution of mount etna in the light of potassium-argon dating, acta vulcanol., 5, 81-87. 374 ciro del negro, fabrizio ferrucci and rosalba napoli grasso, m., f. lentini, a.e.m. nairn and l. vigliotti (1983): a geological and paleomagnetic study of the hyblean volcanic rocks, sicily, tectonophysics, 98, 271-295. grasso, m. and z. ben-avraham (1992): magnetic study of the northern margin of the hyblean plateau, southeastern sicily: structural implications, ann. tectonicae, 6 (2), 202-213. hirn, a., r. nicolich, j. gallart, m. laigle, l. cernobori and etnaseis scientific group (1997): roots of etna volcano in faults of great earthquakes, earth planet. sci. lett., 148, 171-191. iaga wo r k i n g gro u p v-8 (1996): international geomagnetic reference field, 1995 revision, geophys. j. int., 125, 318-321. ishihara, t., g.l. leitchenkov, a.v. golynsky, s. alyavdin and p.e. o’brein (1999): compilation of shipborne magnetic and gravity images crustal structure of prydz bay (east antarctica). ann. geofis., 42 (2), 229-248. jupp, d.l.b and k. vozof, (1975): stable iterative methods for the inversion of geophysical data, geophys. j. r. astron. soc., 42, 957-976. lanzafame, g. and j.c. bousquet (1997): the maltese escarpment and its extension from mt. etna to the aeolian islands (sicily): importance and evolution of a lithosphere discontinuity, acta vulcanol., 9, 113-120. lentini, f. (1982): the geology of the mt. etna basement, mem. soc. geol. it., 23, 7-25. monaco, c., l. petronio and m. romanelli (1995): tettonica estensionale nel settore orientale del monte etna (sicilia): dati morfotettonici e sismici, studi geologici camerti, special issue, 2, 363-374. patacca, e., p. scandone, g. giunta and v. liguori (1979): mesozoic paleotectonic evolution of the ragusa zone (southeastern sicily), geol. romana, 18, 331-369. roberti, n. and r. scandone (1975): thermomagnetic study of basalts from south eastern sicily. boll. geofis. teor. appl., 67, 259-264. rolph, t.c. (1992): high field intensity results from recent and historic lavas, phys. earth planet. inter., 70, 224-230. rossi, s. and r. sartori (1981): a seismic reflection study of the external calabrian arc in the northern ionian sea (eastern mediterranean), mar. geophys. res., 4, 403-426. sartori, r., m.l. colalongo, g. gabbianelli, c. bonazzi, s. carbone, p.v. curzi, d. evangelisti, m. grasso, f. lentini, s. rossi and l. selli (1991): note stratigrafiche e tettoniche sul «rise di messina» (ionio nord occidentale), g. geol., 53 (2), 49-64. tabacco, s. (1998): definizione del basamento etneo sottostante il deposito vulcanoclastico del chiancone (settore orientale dell’etna) attraverso metodiche geologico-strutturali e geofisiche, ph.d. thesis, università degli studi di catania, pp. 105. tapponnier, p. (1977): evolution tectonique du systeme alpin en mediterranée: poinonnement et encresement rigide-plastique, bull. soc. geol. fr., 19, 437-460. tauxe, l., a.d. deino, a.k. behrensmeyer and r. potts (1992): pinning down the brunhes/matuyama and upper jaramillo boundaries: as reconciliation of orbital and isotopic time scales, earth planet. sci. lett., 109, 561-572. tortorici, l., p. tapponier and t. winter (1986): faulting during the 1783 calabria earthquakes and tectonics of the messina strait, agu, eos, 67, 1188. tric, e., j.p. valet, p.y. gillot and i. lemeur (1994): absolute paleointensities between 60 and 160 kyear bp from mount etna (sicily), phys. earth planet. inter., 85, 113-129. zanolla, c., c. morelli, and i. marson (1998): the magnetic anomalies of the mediterranean sea (ibcmm), boll. geofis. teor. appl., 39, 1-36. layout 6 1 annals of geophysics, 64, 1, se105, 2021, doi:10.4401/ag-8544 pgs1, a new low cost and low power portable geophysical station “all in one”. design and test antonio costanza1, gioacchino fertitta1, william yang1, giuseppe d’anna1, claudio chiarabba2 (1) istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo (2) istituto nazionale di geofisica e vulcanologia, osservatorio nazionale terremoti, rome, italy article history: received august 3, 2020; accepted november 11, 2020 abstract pgs1 is a new compact portable seismic station, designed at ingv obs and earth lab, that is specifically intended for the deployment of dense arrays of seismographs on-shore. with its low cost, compact design, high data-quality and long battery life, pgs1 is a perfect solution for seismic monitoring networks. pgs1 design is based on a solid polypropylene suitcase, containing a complete data acquisition system, two battery packs and a photovoltaic panel. the new earth lab 5s medium-period seismic sensor is included. the whole system meets the ip67 standard requirements both in transport and in acquisition configuration. pgs1 is normally equipped with one battery pack, one more pack can be added inside the suitcase achieving 40 days of battery life. the station is equipped with a photovoltaic panel, useful to extend the deployment length. inside the suitcase, there are compartments where to store the seismic sensor, the photovoltaic panel and all the cables. therefore, the station is very easy to transport. keywords: portable seismic station; seismic monitoring network; pgs1; etl3d/5s; obs and earth lab. 1. introduction in the last three decades, the use of mobile digital seismic stations has become increasingly widespread both for active and passive seismological studies. some manufacturers already offer small-size and low-power portable seismic stations. being ready-to-use is probably their greatest advantage. with no development time to wait for, the project for a new seismic array can quickly move into the phase of deployment. of course, this benefit should be weighed against unit price, because portable stations can be quite expensive. in order to reduce costs, researchers often build custom portable stations by assembling off-the-shelf parts, but seldom this approach leads to compact and power efficient solutions; in other cases, some obsolete portable seismic stations were updated introducing new commercial data acquisition systems [castellano et al., 2007; cappello et al., 2007]. here the main benefit comes from reusing components, which helps cut down on cost at the expense of performance. however, portable stations should be suitable for working in forests, snow, deserts and other extreme environments where size, weight and power consumptions considerably matter. the considerations above suggest the existence of a gap between commercial stations, technologically advanced but expensive, and custom stations, cheap but suboptimal. filling the gap means starting new designs, with low production costs, aimed at maximizing ease of transport and power efficiency. in this regard, some attempts have already been made [jing et al. 2015; chiu et al., 1991]. the main purpose of this work is to inform the scientific community of the new low cost and low power portable geophysical station pgs1 “all in one”. pgs1 is the result of an effort to develop an effective solution for dense seismic arrays. one of the project goals was to simplify the logistics of deployment, by designing a small size and low weight station. remarkably, the whole station weighs only 11 kg and occupies a volume of just 28 l. the project, including the production of a first batch of 10 units, was funded by the ministero per lo sviluppo economico (mise). the development and assembly was carried out by the obs and earth lab (formerly obs lab) of the geophysical observatory of gibilmanna. in order to reduce the development time and production costs, some devices and parts were chosen among products available on the market and customized, some others reused from other projects and others designed from scratch. all new parts were made in outsourcing production. the power supply system consists of a photovoltaic panel, a charge regulator, a battery pack (or two if needed) and a step down switching converter. all these parts were found on the market. the photovoltaic panel was customized so that it can be secured to the station during transport and acquisition. the acquisition system consists of the earth lab 5s medium period sensor [fertitta et al., 2020] and the ingv seismolog recorder; both have been entirely designed at the obs and earth lab. the sensor was developed within the mise project, integrating low power electronics, designed for marine applications, with a new housing. the recorder, produced with emso medit funds, was originally intended for ocean bottom seismometers (obs). in 2014, a fleet of obs’, equipped with seismolog recorders, took part to the tomo-etna experiment [coltelli et al., 2016]. as the electronics was originally intended for marine applications, the station has low power consumptions, 500mw on average. the battery life is 40 days with two battery packs. all 10 pgs1 units except one reflect the original design, the same described above. the remaining one is reserved for testing new ideas. to give an example, this unit is currently equipped with a single board computer (sbc), a raspberry pi3. the sbc allows downloading real time data in miniseed format, through a seedlink server over an ethernet link. the sbc is powered off when the user interface is inactive, so that its power consumption, which is quite high, does not affect battery life noticeably. within the ewas project, the catania urban seismic observatory (osu-ct) work group has contributed to this testing by writing and updating the software running on the sbc. in transport mode, all the parts of the station fit inside a commercial polypropylene suitcase. the suitcase was customized by opening holes for cables and connectors, and adding supports for the photovoltaic panel. a metal box was designed to support the electronics and the sensor inside the suitcase. pgs1 is specifically intended for the deployment of dense arrays of seismographs. it can perform different monitoring tasks, detecting local and regional earthquakes, either natural or induced by industrial activities [mise, 2014]. pgs1 produces high quality data and exhibits a long battery life. with its low cost, compact design and high power efficiency, pgs1 is a perfect solution for building seismic monitoring networks. it is intended for harsh natural or artificial environments, including zones damaged by earthquakes. 2. 3d-modeling the design of pgs1 is based on a solid polypropylene suitcase, explorer cases model 3818, and includes the new earth lab 5s medium period sensor, a recorder, one or two rechargeable li-ion battery packs, a photovoltaic panel, a battery charge regulator and other power supply circuits. the whole system meets the ip67 standard requirements, both in transport and in acquisition configuration. on one side of the suitcase, are two bayonet-type connectors, one for the seismic sensor and one for the photovoltaic panel. each one has a rubber o-ring at the interface with the suitcase. the o-ring guarantees water tightness, in compliance with the ip67 standard. in the engineering process, the pgs1 was entirely 3d modeled both in transport (figure 1a) and in acquisition configuration (figure 1b); the 3d modeling allowed an easy choice of commercial components, an effective customization of some of them and a correct design of the original parts. the pgs1 was 3d modeled also with the purpose of simplifying the production and the assembly process [costanza et al., 2017]. 3d software tools were useful in different ways. one of the project goals was making sure that a user would feel antonio costanza et al. 2 comfortable carrying the suitcase. this would mean walking to a distant place, while holding the station by its handle. in this scenario, the suitcase would be in the so-called transport configuration, with the sensor, photovoltaic panel and cables stored inside, as shown in figure 2. achieving the goal above not only means keeping weight low but also balancing. in order to balance the suitcase, the center of gravity must be directly below the handle. otherwise, a neat moment of force would arise. as a result, the suitcase would tend to turn and the user would have to exert an opposite moment to keep the suitcase flat, which could cause premature fatigue in the hand and arm. in figure 2, the green dots represent the projections of the center of gravity on the principal views. figures 2a-c show that the center of gravity is located directly below the suitcase handle. achieving such an accurate placement is all but a trivial task, because the location of the center of mass depends on the placement of all the components inside the suitcase. in addition, every time a component is moved to a different position, the center of gravity must be updated. the use of 3d tools made it possible to try different configurations, focusing on the solution and letting the software do the hard work. the photovoltaic panel, used to extend the deployment length beyond 40 days, is visible in figure 1a-b (in the two configurations). the 3d-modeling allowed choosing a commercial panel that could be adapted to the two different configurations. 3 portable geophysical station pgs1 figure 1. 3d assembly model of pgs1; a) transport configuration (with open suitcase and without pocket), b) acquisition configuration. figure 2. position of the centre of gravity (green dot) and dimensions in transport configuration; a) top view; b) front view; c) lateral view. antonio costanza et al. 4 during transport, the panel is stored inside a pocket, attached to the inside of the suitcase lid. during data acquisition, the panel stays outside the suitcase, gathering solar energy. in order to provide a continuous power income, the panel should maintain a stable tilt and orientation, even resisting moderate winds. a custom frame, fixed to the panel’s original frame, helps secure the latter to the suitcase. on the suitcase shell, are four anchoring points, where it is possible to fasten the custom frame. this system holds the panel at a 52° angle of inclination to the horizontal. as the custom frame is foldable, the panel and frame can still fit inside the internal pocket. noteworthy, the panel installation requires no tools (figure 3). the photovoltaic panel (1) is connected, by means of two hinge-screws, to two foldable brackets (2). by means of an auxiliary frame and two hinge-screws, it is also connected to two foldable pegs (3). to assembly the panel on the suitcase, the operator must simply open the pegs (3), insert them in the appropriate holes of the suitcase, open the brackets (2) and fit them into the relative pins fixed to the case. the mechanical constraints ensure a stable connection and a fixed angle of inclination. 3d-modeling was essential also to define a convenient distribution of the electronic devices inside the suitcase. one of the main goals was to preserve the data-quality, while keeping the assembly procedures as simple as possible. figure 4 shows a photo of the real pgs1. the plastic panel, that normally hides from view the electronic devices, was removed to show internal elements. an aluminum box (1), open on the top side, provides support for the electronics. the box base is fitted with a set of threaded holes, making it easy to fasten each device with screws. another set of screws, evenly distributed along the upper edges, fastens the box to the suitcase. the sensor compartment (2) occupies the box upper-right corner, leaving an l-shaped space available inside the box. the recorder (3) is in the upper left corner of the box, and the frequency reference (4) is installed on top of it. the concerns about dataquality came from the analog inputs of the recorder and relative cables, possibly picking-up noise from other electronic sources. the power supply circuits, on the other hand, are perfect candidates to generate electromagnetic interference (emi). the recorder is in the upper part of the “l” vertical branch, oriented with the analog inputs facing upwards. the analogue cables run for the most part in between the suitcase and the external walls of the box, except for the short segment that goes to the recorder inputs. the switching regulator (5) is directly below the recorder, followed in counterclockwise order by the charge regulator (6) and batteries (7), the farthest object from the analogue inputs. as the battery voltage is quasi-static in normal use, the process underlying noise generation depends on the applied load. lithium batteries are close to ideal voltage sources, thanks to an extraordinary low electronic series resistance (esr). when connected to a switching circuit, they will be able to source very large figure 3. fixing system of the photovoltaic panel to the suitcase, during the acquisition configuration. current pulses. the magnetic fields, associated with the pulses, could be coupled to external circuits producing noise. the considerations above apply to the presented station too, because the charge regulator is actually a switching circuit. however, with the battery and its cables far from the analogue inputs, the risk of picking-up induced noise is very low. the design of the box posed the problem of providing adequate support for the sensor, the most sensitive part to mechanical shocks, during transport. for this purpose, the sensor compartment features a shockproof padded seat that can hold the sensor securely. the angle profile (8), constrained to the sensor compartment, holds the serial port connector, sd card slot, ethernet connector, on/off switch and a set of four leds. 3. the electronics the following sections will briefly describe the pgs1 subsets and the user interface. 3.1 seismolog recorder pgs1 is equipped with seismolog, a geophysical recorder developed by the obs and earth lab, ingv. before seismolog, ingv had already produced two recorders, gaia [cardinale et al., 2010] and gilda [giudicepietro et al., 2015; orazi et al., 2006; orazi et al., 2008]. both projects met their goals, releasing reliable instruments, which served and are still serving ingv’s needs in different fields. however, more than a decade ago, the obs and earth lab, undertaken the construction of a new class of marine instruments for geophysical monitoring, the ocean bottom seismometers (obs). these instruments, with the potential to bring a sensible improvement in the study of marine geophysics, were battery powered and had to work autonomously for long time. the new requirements on power consumption led to rule out both gaia and gilda as viable solutions. 5 portable geophysical station pgs1 figure 4. pgs1 without closing panel. from the beginning, the obs and earth lab has acquired different off-the-shelf solutions. while this strategy worked well in most cases, some limitations, related to the use of commercial instruments, became also evident. often, data acquisition systems are required to implement custom functions, which seldom the manufacturer can provide for an affordable price. in 2014, the obs and earth lab released the first version of seismolog. the new recorder consumed only 120 mw and was intended for passive geophones. in the subsequent years, a new version followed, whose analogue inputs were suitable for active sensors, including broadband seismometers. however, the overall consumption increased to 200 mw. therefore, the two versions coexisted for different applications. a simplified diagram of seismolog, broadband version, is shown in figure 5. the input noise level is one of the most important parameters for seismic recorders. broadband sensors exhibit excellent dynamic range values, over outstandingly large frequency bands, extending to periods higher than 100 s. however, low frequencies are especially problematic for most electronic devices. both active (transistors, operational amplifiers, adcs) and passive electronic devices (resistors) produce higher noise densities at lower frequency. the physical phenomena involved are different for each device, but the effects are similar for most of them: at low frequency the noise power spectral density is proportional to the inverse of frequency, tending to infinity when frequency approaches zero. this kind of noise density is commonly referred as 1/f noise. an effective way to reduce 1/f noise is to use a special family of amplifiers, designed to cancel their own 1/f noise, commonly referred as chopper amplifiers. some adcs implement this technology too. the analogue inputs of seismolog broadband version are designed with chopper amplifiers and chopping-input adcs. as the pgs1 does not include a broadband sensor, the use of seismolog broadband version would not be strictly necessary. however, the electrical characteristics of the 5s sensor, in particular the output-voltage swing, would not be compatible with antonio costanza et al. 6 figure 5. simplified system diagram for the seismolog recorder. the geophone version. therefore, pgs1 is equipped with seismolog broadband version. for this version, the dynamic range, i.e. the ratio of maximum input signal to short-circuit noise power, is greater than 122 db when the sampling rate is 200 hz. this dynamic range translates to an equivalent resolution of 20-bit. seismolog uses a high stability frequency reference to generate the sampling frequency. the reference can be either a state-of-the-art temperature compensated crystal oscillator (tcxo) or a global positioning system (gps) disciplined clock. the latter is the preferred choice for terrestrial use, because achieves the same long-term stability as the gps system. the diagram in figure 6 shows how the recorder synchronizes the data samples with the gps signal. the gps disciplined clock produces two reference frequencies, 1 hz and 4.096 mhz respectively. the latter is an integer multiple of the former, and both are phase-locked to the 1 pps coming from the gps receiver. the residual phase error is about ± 200 ns, when the gps antenna has a clear view of the sky. the 4.096 mhz signal is routed to both the adcs and the microcontroller unit (mcu) inside the recorder. the mcu uses this signal to run the system clock, which exhibits a resolution of 2-16 sec (about 15 μs). clocking the adcs through the reference frequency ensures that the number of samples per second is a constant integer value. the method used to assign a time stamp to each sample is to read time from the system clock, as soon as a sample arrives. this process occurs inside an interrupt service routine (isr), with a latency of a few microseconds. storing the time stamp for every sample would waste memory, because the sampling interval is constant. in fact, the recorder uses the system clock only once in a recording session, when it reads the time of the very first sample. when sample time is to be determined, it is calculated, based on the time of the first sample and the number of samples acquired from start. this is just a conceptual description though, leaving out implementation details and optimizations. for marine 7 portable geophysical station pgs1 figure 6. clock distribution network of the seismolog recorder. applications, the recorder works in a similar way. the tcxo generates the same reference frequencies as the gps disciplined clock. however, the lesser stability of the tcxo affects sample times, limiting accuracy in the long term. 3.2 earth lab 5s (etl3d/5s) seismic sensor pgs1 includes the new earth lab 5s (etl3d/5s) seismic sensor [fertitta et al., 2020], a three-component velocimeter with 5 second period and 100 hz high cut-off frequency. this sensor was designed for monitoring of medium-range earthquakes and can be used in areas where industrial activities may induce seismicity. the sensor has very small size (97 mm diameter, 117 mm height) and low weight (1.7 kg). its low power electronics consumes only 75 mw of quiescent power. the housing, made of aisi 316l stainless steel, is resistant to atmospheric agents and is ip67 waterproof. the sensor is based on a set of three geophones, combined with a low power and low noise amplifier. a revised version of lippmann’s method [romeo and braun, 2007] is used to extend geophone natural period up to 5 seconds. the transfer function of the lippmann’s circuit depends on the geophone’s coil resistance. temperature affects the resistance value, making the transfer function vary over time [mikhail and boulaenko, 2002]. the earth lab 5s implements a temperature compensation scheme that mitigates the effect of temperature variations, keeping period and gain stable. 3.3 power supply system pgs1 uses 14.8 v-17.5 ah lithium battery packs. one or two packs can be housed in the suitcase depending on user needs. if the installation site offers some clear view of the sky, the photovoltaic panel can be used to further extend the acquisition length. the incoming solar energy is transferred to the battery packs by a genasun gv5 charge regulator, a high efficiency controller with maximum power point tracking (mppt) function [xiao et al., 2011]. the gv5 provides also over discharge protection, disconnecting the load when the battery state of charge (soc) drops below a fixed threshold. northern italy average insolation data were used to design the power supply system, whose theoretical energy production exceeds the station’s needs over the entire year. these data can be downloaded from atlante italiano della radiazione solare, a website provided by agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile (formerly ente per le nuove tecnologie, l’energia e l’ambiente, enea). the site uses a mathematical model that takes into account the orientation of the panel, which is defined by a couple of quantities, the tilt and azimuth angles. as already said, the panel tilt is fixed at 52°. the azimuth was assumed 0°, which is the orientation of a panel facing south. with these input values, the model yielded satisfactory insolation values, which guarantee continuous operation throughout the year. in january, for example, it is estimated that the energy production will exceed energy consumption by 20%. remarkably, even if the energy balance were negative, for example by -20%, it would take more than 3 months for one battery pack to be completely depleted. a dcdc switching converter supplies power to the recorder at 9 vdc. the dcdc input is connected to the gv5 power output. when the battery voltage becomes too low, the gv5 suddenly disconnects the load from the battery. in order to prevent data losses, the dcdc switching converter is designed to hold the output voltage for a few seconds after the disconnection. during this time, the dcdc output voltage is not perfectly stable, but decreases slowly. when the voltage drops below a certain threshold, the recorder closes the acquisition files and enters the sleep-mode. as a result, the battery disconnection, operated by the gv5, does not produce any data losses. pgs1 schematic diagram, in figure 7, shows how pgs1 holds the recorder supply voltage when the charge regulator shuts the battery load off. a 10 mf capacitor, connected to the recorder power input, stores electric charge during normal operations. this charge can be delivered to the recorder, as a supply current, when the dcdc is switched off. at the same time, the diode across the dcdc prevents the dcdc output from being at higher voltage than the dcdc input, a destructive condition known as voltage inversion. antonio costanza et al. 8 3.4 user interface pgs1 can be controlled through a computer, connected to the rs232 port. any terminal-type software can be used to send commands and monitor acquisition parameters. the command set can be divided into a few categories: channel-configuration, acquisition-control, synchronization, interface control, special commands. channel-configuration commands let user set up the analog inputs. for example, these functions include turning inputs on and off, or assigning a sampling frequency to each input. the recorder has more inputs than pgs1 requires. actually, the etl3d/5s seismometer is the only sensor connected to the recorder, and the unused inputs are free for future developments. therefore, turning inputs on and off is generally not necessary, as the default configuration enables the seismometer inputs only. for these inputs, selectable frequencies are 1, 10, 20, 25, 50, 100 and 200 hz. acquisition-control commands allow starting and stopping data recording. the recorder responds to each command in this class with a message containing the current configuration and status of input channels, storage memory, and battery voltage. synchronization commands affect the system clock and real time clock (rtc). the former is a mixed hardware and software unit that produces a stable sampling frequency during data acquisition; the latter is a hardware module, which runs on a back-up battery when the recorder is powered off. at boot, the system clock is set to rtc time. upon reception of a specific command, the recorder will try to align both clocks with the utc time, provided by the gps disciplined clock. after a successful synchronization, the time error is very low, actually negligible. the system clock will be continuously aligned with the disciplined clock, even without user’s intervention. therefore, as long as there is a clear view of the sky and the disciplined clock tracks an adequate number of satellites, the time error will stay low. among the synchronization commands is the delay command, which can be used for measuring the time error without altering the system clock. the interface-control commands determine how the user interface work. one of the most important function is to make the interface switch to data-dump mode. in this mode, the recorder sends data over the serial link, in miniseed or in a proprietary format. however, the recorder keeps listening to user commands, so that it can switch back to user mode, upon reception of a specific request. the special commands let the user perform different actions, not required by ordinary operations, such as formatting the data storage memory or updating the firmware of the recorder. a new user interface, based on an embedded web server is currently under development. in the meanwhile, with the help of osu-ct team, a prototype has been developed which can be accessed over an ethernet link, thanks to the introduction of a single board computer (sbc). the pgs1 can now be configured via secure shell (ssh) and can run a seedlink server. however, the sbc cannot operate continuously because of its high power consumption. to overcome this problem, pgs1 powers on the sbc only when a device is connected to the ethernet port. once the ethernet device is disconnected, the sbc is turned off immediately, thus saving energy. once the ssh connection to the sbc is established, the user launches a phyton script stored in the sbc memory. the logic of the script makes the sbc transparent to the user, allowing direct communication with the recorder. 9 portable geophysical station pgs1 figure 7. pgs1 schematic diagram. 3.5 reading data pgs1 stores its data to an sd or sdhc card. binary data, like analog input samples, are written to miniseed files, organized in monthly created folders. data can be easily copied to a computer by means of a flash card reader. table 1 summarizes the most important technical specifications. table 1. electrical and physical specifications. sensor type three-component velocimeter bandwidth 3db points at 5 s and 100 hz sensitivity 360 v/(m/s) +/5% after calibration clip level 12.5 mm/s dimensions diam. 97 mm, height 117 mm cable length 5 m recorder sampling rate 1, 5, 10, 20, 50 ,100 and 200 hz dynamic range 122 db equivalent resolution 20 bit conversion factor 3.32258•107 (counts / v) time accuracy 15 μs (with gps) interface control port type rs232 control port connector db9 female data memory sd or sdhc file format miniseed memory usage up to 4 months with a 16 gb sdhc, three channels at 200 hz synchronization internal gps receiver battery configuration 20 cells, lithium inr18650-35e, 4 series 5 parallel nominal voltage 14.4 v capacity 16.7 ah protection low voltage disconnection (without data loss) battery life 20 days with 1 battery pack 40 days with 2 battery packs endless with photovoltaic panel pv panel peak power 10 w open circuit voltage 21.96 v short circuit current 0.63 a power average consumption 500 mw physical suitcase explorer cases model 3818 transport dimensions 415 x 340 x 200 mm operative dimensions 415 x 480 x 200 mm weight 11 kg (with two battery packs) environmental operating temperature -10 to 50 °c (by design, not tested) ip degree ip67 antonio costanza et al. 10 11 portable geophysical station pgs1 4. testing this paragraph describes different tests conducted on pgs1. the first is a power consumption test aimed at measuring battery life without solar energy. battery life is defined as the time difference between two events. the first is acquisition start. the second is the disconnection of any load from battery, operated by the charge regulator when the low voltage condition (voltage < 12.4 v) results true. this time value is available in a log file, automatically stored to data memory by pgs1. the same file contains battery voltage samples as swell, taken at least once a day. figure 8 shows the test results, with just one battery pack installed. the graph on the left is the battery discharge curve. this goes from the initial 16.7 v to 12.8 v, in about 20 days. the log file tells that battery voltage dropped to 12.4 v a few hours later, triggering a shutdown, as expected. the test did not end at that point, in fact the photovoltaic panel was connected. the goal was to verify if pgs1 could restart with solar energy only, from the state of having a completely depleted battery. the graph on the right of figure 8 shows what happened. the charge regulator kept the acquisition system off until the battery voltage rose above 14 v, which occurred after about two days. then battery voltage increased slowly from this point on, because the acquisition system started to consume power. the plot on the right of figure 8 shows a steady increase in battery voltage over time. this is a desirable result, but the operating conditions needs to be described in more detail, because the test took place in sicily, thus at a relatively low latitude, and in august. in addition, the sky was mostly clear throughout the test. with all the favorable conditions above, pgs1 would have exhibited a very high and optimistic charge rate, if solar energy had not been artificially limited. this was accomplished by placing pgs1 in a site hit by sunlight between 08:30 and 10:30 a.m. only (a few large buildings surrounds the spot, shading it the rest of the day). the second test presented was conducted in 2017. six pgs1s, installed a few kilometers away from one another, acquired about three days of seismological data. the location chosen for this test was the sulcis, a large area in the southwestern part of sardinia, italy. figure 9 shows two pictures from this test, taken during data acquisition (a) and setup (b) respectively. the recordings contain ambient noise and no seismic events, which is consistent with ingv official data for the same dates. data show a strong microseismic activity, probably generated by the nearby sea. this phenomenon produced large peaks of power spectral density, in a frequency range between 0.35 and 0.40 hz for the figure 8. the graph on the left shows the voltage across the lithium battery pack during a 20-days uninterrupted recording, with one battery pack and without photovoltaic panel. the graph on the right shows what happened when the photovoltaic panel was connected. the horizontal line labeled soc 50% shows the point where the battery charge (state of charge, or soc) is 50%. three components, see figure 10. all the spectra compare well at low frequency, which is a predictable result considering the mutual distances between the stations. at high frequency, the spectra diverge instead, but this too is in line with the scale of the experiment, and is ascribable to natural differences in local ambient noise among the sites. despite these differences, for each component all the traces look similar in the time domain. this is because the microseismic peak, which is common to all, represents the most part of the total noise power. the microseismic activity manifests itself as a slow fluctuation, with a series of maxima (and minima) spaced by 2.5 to 2.8 s. as an example, figure 11 shows a comparison among the e/w time series. the two other components are not included because they look similar. in an attempt to measure the self-noise of pgs1, one of the stations was brought to a low noise environment, where it recorded 30 minutes of ambient noise. in the same area is the station of castelbuono, a node of the italian seismic network, run by istituto nazionale di geofisica e vulcanologia (ingv), italy. figure 12 shows the spectra from this recording. the dotted curve is the sensor’s self-noise, as resulted from simulations; the dashed curves are the peterson models, high and low; the continuous curve are the measured spectra. the curves stay within peterson model high and low and above theoretical self-noise. below 0.2 hz, selfnoise becomes higher than ambient noise. as the three components share the same hardware architecture, they also show comparable self-noise levels. this is the reason why the spectra almost coincide at very low frequencies. antonio costanza et al. 12 figure 10. power spectral densities from the recordings made in the sulcis, in 2017. the horizontal components from station pgs1 9 are not included, because the corresponding time series are affected by strong noise, in the form of a train of periodic pulses, which prevents the correct evaluation of the spectra. the cause of this noise was a faulty electrical connection, inside pgs1 9. figure 9. a) pgs1 in acquisition; b) in configuration mode. 13 portable geophysical station pgs1 the last test presented in this paragraph is a comparison with another node of the italian national seismic network, the node gib, located at osservatorio geofisico di gibilmanna, italy. the two seismic stations operated in parallel for a few weeks, and on october 1st 2020 both recorded an earthquake of magnitude 5.2, occurred in greece at 11:05:37 utc time. figure 13 shows the complete waves on the left, and the phase arrival on the right. the node gib has a higher period than pgs1 and is capable of achieving a much higher dynamic range, especially at low frequencies, thanks to the seismic sensor nanometrics trillium 120 s. for this event though, the two wave sets compare well without additional filtering, as shown in figure 13. in the same figure, mean values were cancelled from every wave. this test, once more, confirmed the correct behavior of the analog circuits implemented in both the sensor and recorder. furthermore, the plots show no time shift between the waves, meaning that pgs1 clock was synchronized to gib when the earthquake was recorded. figure 11. ambient noise recorded in sulcis in 2017. only the e/w axis is shown. the time series produced by pgs1 9 are affected by strong noise, in the form of a train of periodic pulses, due to a faulty electrical connection. the data segment above was chosen so that the pulses are not visible. antonio costanza et al. 14 figure 12. component spectra of a data segment, recorded by pgs1 n° 06. the dotted curve is the sensor’s self-noise, as resulted from a simulation; the dashed curves are the peterson models, high and low; the continuous curve is the measured spectrum. 15 portable geophysical station pgs1 5. discussion and conclusion in this work the new low cost and low power portable geophysical station “all in one” pgs1 has been presented. this station was designed and assembled at ingv obs and earth lab of gibilmanna, italy. 3d modeling played a key role; through virtual modeling it was indeed possible to select commercial components, to design new parts, to customize existing ones, and finally, to choose the most suitable distribution of the internal components in order to obtain a convenient position of the centre of gravity. furthermore, 3d modeling allowed selecting a proper configuration of the electronic components, which could minimize the risk of emi problems. the pgs1 is equipped with a photovoltaic panel that extends the battery life. in the natural environment, many agents, like snow or growth of vegetation, can cause complete or partial obscuration of photovoltaic panels, reducing the energy production rate and causing discontinuities in the data acquisition. pgs1 has very low power consumption, because its electronics is derived from battery-powered marine instruments. as a result, the station can operate for long periods without solar energy production. pgs1 is equipped with a compact sensor and a high-resolution acquisition system, which combined produce high quality data, as confirmed by various recordings of ambient noise and seismic waves. figure 13. earthquake of magnitude 5.2, occurred in greece on oct 1st 2020, utc time 11:05:37, as recorded by pgs1 n°8 and by the seismic station gibilmanna (code gib). the final cost of the station, including an estimate for the sensor calibration and pgs1 assembly, is of about 3500 euros. however, a significant margin for cost reduction could come from scale manufacturing and process engineering. future developments include the release of a new user interface, based on a web-server. acknowledgments. we gratefully acknowledge domenico patanè as responsible of the ewas project and of the osu-ct working group. this work has been partially supported by the pon-ars01_00926 project ewas “an early warning system for cultural heritage”. references capello m., m. castellano and p. ricciolino (2007). commercial digital audio recorders: a new life for portable lennartz pcm 5800 seismic stations, orfeus newsletter, 7(2), 1-7. cardinale v., a. castagnozzi, c. d’ambrosio, l. falco, a. memmolo, f. minichiello (2010). wi-fi mesh network: integrazione dell’infrastruttura telematica della rete sismica e geodetica nazionale, rapporti tecnici ingv, 141. castellano m., c. buonocunto, m. capello and p. ricciolino (2007). stazioni sismiche obsolete: il recupero del sistema lennartz pcm 5800, quaderni di geofisica, 48. chiu j.m., g. steiner, r. smalley and a. c. johnston (1991). panda: a simple, portable seismic array for local to regional-scale seismic experiments, bull. seismol. soc. 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(2008). a multiparametric low power digitizer: project and results. in w. marzocchi and a. zollo eds., conception, verification and application of innovative techniques to study active volcanoes, doppia voce, 498. romeo, g. and t. braun. (2007). appunti di sismometria, quaderni di geofisica, 46. xiao w., a. elnosh, v. khadkikar and h. zeineldin (2011). overview of maximum power point tracking technologies for photovoltaic power systems, iecon 2011 37th annual conference of the ieee industrial electronics society, melbourne, vic, 3900-3905, doi:10.1109/iecon.2011.6119946 antonio costanza et al. 16 17 portable geophysical station pgs1 *co r r e s po n d i n g a u t h o r: antonio co s ta n za, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, italy, e-mail: obslab@ingv.it © 2021 the istituto nazionale di geofisica e vulcanologia. all rights reserved adg vol5 n02 vall 313_320.pdf annals of geophysics, vol. 45, n. 2, april 2002 313 a note on the topographic distortion of magnetotelluric impedance filippos vallianatos technological educational institute of crete, branch of chania, crete, greece abstract magnetotelluric surveys are prone to interpretation errors in the presence of rough topography, which may need to be compensated for. in the present work we assume that the surface of the earth is simulated by a single-valued, twice differentiable function f (x, y). by appropriately expanding the surface magnetic field, we obtain the distorted magnetotelluric impedance tensor in terms of an expansion depending on the external radii of curvature of f (x, y) at the observation point and on skin depth. based on first principles, an analytic estimation of the topographic corrections of the magnetotelluric impedance tensor is obtained. 1. introduction a problem commonly appearing in magnetotelluric (mt) surveys is topographic distortion (schnegg et al., 1983, 1986; chouteau and bouchard, 1988; fischer, 1989), especially in the case of measurements carried out in mountainous areas (e.g., draget et al., 1980; kurtz et al., 1986; vallianatos, 1995). interpretation errors may occur if the distortion is not taken into account, or compensated for. telluric currents flowing parallel to the surface of the earth, converge beneath valleys and diverge beneath hills. therefore the current density j and the associated mt field tend to increase under a valley and decrease under a hill. the reduction of this distortion is therefore an important step toward the correct detection and interpretation of subsurface conductivity distributions. several studies exist on the effect of topography on mt measurements, most of which adopt a numerical modeling approach (jiracek, 1973; harinarayana and sarma, 1982; wannamaker et al., 1985, 1986; jiracek et al., 1986; xu and zhou, 1997). under static conditions, the topographic effects have also been calculated analytically, using schwarzchristoffel transformations (thayer, 1975; harinarayana and sarma, 1982; harinarayana, 1986). in the present short work, we present a formulation to analytically estimate the topographic effect, to first and second order correction terms. the proposed approach is based on the simulation of the earth surface by a singlevalued, twice differentiable function and an appropriate expansion of the magnetic field. the estimated corrections depend on frequency and the external radii of curvature of the earth surface at the observation point. mailing address: dr. filippos vallianatos, technological educational institute of crete, department of natural resources engineering, laboratory of geophysics and natural hazards, 3 romanou str., chalepa, gr-73133 chania, crete, greece; e-mail: fvallian@chania.teiher.gr key words magnetotellurics topography 314 filippos vallianatos normal to the surface is (lipschutz, 1989; morse and feshbach, 1963) where in order to generalize eq. (2.1), we note that and (2.2) combining eqs. (2.1) and (2.2), gives or equivalently (2.3) where the sum over j is implied (einstein notation). in the case of a simple one dimensional (1d) structure c gij = zo cij where cij are the elements of a «normalized» surface impedance tensor. it is straightforward to see that for a flat surface c ij = ij . we note that e// and have only components in the tangent plane, so that the normalized surface impedance tensor c, being a surface tensor in curvilinear coordinates, is represented by a 2 × 2 matrix. hereafter, in order to simplify the mathematical formulation, we assume one dimensional structure with a surface profile function f (x, y). 2. mathematical formulation as physical basis for the analysis of any magnetotelluric problem, consider maxwell’s equations for an isotropic, source-free region. in the frequency domain, with the constituent relations d = e, b = µ h, j = e incorporated, these are (see for example rokityansky, 1982): × h = ( i )e = e × e = i µ h h = 0 and e = 0. by combining the curl equations and using the vanishing divergences, it can be seen (jackson, 1975) that both fields satisfy the homogeneous wave equation (helmholtz equation) (wait, 1987) where the propagation constant is k 2= i µ . the mt impedance, can be expressed as a rank 2 tensor. if x and y are orthogonal measurement axes on the earth surface, the tangential to the surface electric field variations e// = (ex, ey) t is directly related to the variations of the two magnetic fields h// = (hx, hy) t by the frequency domain relation or (2.1) where zij are the elements of the impedance tensor (kaufman and keller, 1981). next, consider an earth interface (airearth interface) described by a profile function f (x, y), which is assumed to be single valued and twice differentiable. the vector + =2 2 0 e h e h k = ex ey zxx zxy zyx zyy hx hy =e z h// // ˆ , ˆ ,n z= ( ) ( )[ ]x y f x y , , .( ) = + ( )( )[ ]x y f x y1 2 1 2 n̂ h= ×( )hx y ˆ .n h= ×( )hy x ex ey zxy zxx zyy zyx x y = ×( ) ×( ) ˆ ˆ n h n h e c i ij g j//, ˆ= ×( )n h n̂ h× 315 a note on the topographic distortion of magnetotelluric impedance the equation for the next term with a solution that vanishes at re(w)→−∞ (bronson, 1973; morse and feshbach, 1963) where continuing in way, we expand the magnetic field in terms that include the expression the arbitrariness of the latter term is partially removed by the remaining maxwell equation ∇⋅h = 0. after extensive algebra and differential geometry, we can write for z approaching f (x, y) from below, the component of e tangential to the surface is we introduce the coordinate variables u = x, υ = y, w = [z – f (x, y)]/d where d(ω) = i/k (ω), in which the helmholtz vector equation for h is expanding h in powers of d(ω) (see appendix) we obtain the recursive relation for n = 0, the solution that vanishes at re (w)→−∞ is h (0) = a (0) (u,υ) exp(φw). inserting this in the above recursive relation, we obtain e e n n e// ˆ ˆ= − ⋅( ) 0 h d hi= ( ) = ∞ ∑ i i 62v a= − ∇ ⋅∇∇ ⋅∇( )φ f f f (0) 3 2 22w = ∇ − ∇ ⋅∇∇ ⋅∇ +[φ φf f f f 2+ ∇ ⋅∇( )f ]a(0). ∑ di i iα( ). n a a n+ × + ∇ × + ×km ˆ ˆ(0) (0) φ f a n n a× ∇ ⋅∇( ) − ⋅∇ ×( )  ˆ ˆ(0) (0) ( ) ∂ ∂ 2 2 2 w − = +( ) ( )φ φh w v(1) expw w . h a w v v (1) (1) exp= + − +         ( )( ) 2 4 42 2 φ φ φ φw w w i = − ∇ × − ⋅ ∇ ×( )[ ]{ }ˆ ˆ .h n n h1 σ ωµ ∂ ∂ ∂ ∂ 2 2 2 2 2 2 w d f f w − − ∇ + ∇ ⋅∇ +     φ φ ( ) ∂ ∂ ∂ ∂υ 2 2 2 2 2 2 0d u h+ +    =φ ( ) ( )∂ ∂ 2 2 2 w h− =φ ( )n ∂ ∂ 2 2 2f f w h n= ∇ + ∇ ⋅∇( ) −( )φ 1 ( )∂ ∂ ∂ ∂υ 2 2 2 2 2u h n-− + ( )φ .2 e n a n a// ˆ ˆ= −( ) × + × +     −σ ωεi 1 d (0) (1) 316 filippos vallianatos where the mean surface curvature and s is the extrinsic curvature tensor of the surface (lipschutz, 1989). finally, by applying vector calculus we obtain at w = 0 (2.4) where is the projection operator in the tangent plane, which in matrix terms is given as since the operator p acts in the same way as the identity applied on × h, eq. (2.4) could be written as (2.5) where s = p f p is the extrinsic curvature tensor of the surface (lipschutz, 1989). a comparison of eqs. (2.3) and (2.5) leads to the conclusion that the normalized impedance tensor c = (c ij ) has the form c = i + dc (1), where c (1) is simply the traceless part of the extrinsic curvature tensor of the surface s, given by the expression c (1) = s – k m p. we note that the eigenvalues of c (1) are 0 and ± where since the aforementioned first order correction is expressed by terms that include the second spatial derivatives of f (x,y), it is apparent that the topographic profile with linear slope introduces a topographic correction given by a second order term. following the same procedure and keeping second order terms in our approximations, we reach an expression for the normalized impedance tensor where according to the aforementioned mathematical formulation, it is obvious that a crucial parameter in our expansion is the dimensionless product i n̂ e// = i p p p n h * ˆ= + ( )[ ] ×( ) = 1 d d f km i s p n hˆ= + ( ) ×( )[ ]0z d km = 1 2 1 1 r rmin max ( ). c i c i= + ( )[ ]d d tr c(1) (1) ( ) 2 2 4 ( )[ ] = =tr c(1) 2 22 = ( )[ ] ( ) +tr f f f2 2 4 22 + ( )f f f km 6 2 22 . ( )d f x xi j 2 k r r tr sm = + = ( ) = 1 2 1 1 1 2 ( ) min max e n h// ˆ= ( ) × +i d km 1 1( ) p i n n= ˆ ˆ f f f f= ( )1 2 2 2 p n hˆ+ ×( )f p = ( )1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 ( ) ( ) f x f x f y f x f x f y f y f y f x f y 2 2 317 a note on the topographic distortion of magnetotelluric impedance the external radii of curvature of the earth’s surface at the observation point, leading to accurate results, as the dimensionless product is small. the formulation does not make any assumption about the type of topographic profile (i.e. slopes, height). applying the presented first principles approach, we estimate the correction terms as functions of the lateral second derivatives of the topographic profile. in this way we have obtained a general expression for the topographic correction of the surface mt impedance tensor up to first and second order correction terms. the first order correction c (1) term is given by the traceless part of the extrinsic surface curvature tensor s, while the second order correction is just a scalar given in terms of trace of c (1). where x 1 = x and x 2 = y. it is expected that when they are small the retention of only the first few terms in the expansion should yield inaccurate results. alternatively, we can say that our approximation should be valid as long as k t d( ) is much smaller than unity, where k t is the wavenumber that characterizes the roughness of our terrain. 3. concluding remarks in the present preliminary work we present a formulation to analytically approximate the topographic distortion of the magnetotelluric impedance tensor up to first and second order correction terms. the proposed approach is based on the simulation of the earth’s surface with a singlevalued, twice differentiable function and an appropriate expansion for the magnetic field. the estimated corrections depend on frequency and on ( )d f x xi j 2 appendix consider a time-harmonic plane wave incident from free space into a homogeneous earth surface described by a terrain function f(x,y). in formulating the problem one assumes that the topography acts as a scatterer. since the currents flowing parallel to the earth’s surface converge beneath valleys and diverge beneath hills, it is straightforward to describe the scattering from the topography by a volume distribution of currents induced inside the «effective topographic scatterer». maxwell’s equations corresponding to this representation of the electric field is (verdin and bostick, 1992; habashy et al., 1993) where the wavenumber k2 b = i µ b and the topography factor q t (r) are the terms describing the scattering of currents due to the presence of topography. the solution to eq. (a.1) can be expressed with a dyadic green’s function which is governed by the appropriate boundary conditions and satisfies the equation where i is the identity dyadic. for a homogeneous background, the solution to eq. (a.2) is (kong, 1986) , where g (r, r ) is the scalar, green’s function which satisfies and is given by × × =e r e r r e r( ) ( ) ( ) ( )k qb t 2 × × = ( )g r g r i r r( ) ( ) 'kb 2 ( ) + ( ) = ( )r r, ' , ' 'g r r k g r rb2 2 (r r, 'g )) = = e eik db r r r r r r r r ' '4 4 ' ' . (a.1) (a.2) g r r i r r, ,' '( ) = ( )+ 1 2k g b 318 filippos vallianatos thus the solution to eq. (a.1) is given by (yaghjlan, 1980; habashy et al., 1993; chave and smith, 1994; kong, 1986) (a.3) where vt is the effective volume related to the topography and supports the appearance of q(r). applying ampere’s law to (a.3) gives an analogous integral equation for the magnetic field (a.4) we seek to obtain h as an expansion of d. in order to accomplish this we make the following change in variables x = x + dq1, y = y + dq2. introducing the dimensionless variables q1 and q2 the scalar green’s functions where possesses the expansion (a.5) where (see morse and feshbach, 1963; nietro-vesperinas, 1991) ( ) = ( ) + × ( ) ( ) i d g qth r h r r' r r r, ' ' µ 1 b ( ee r'( ) vt ). ( ) = d g e r r r r r r , ' ' ' 4 = ( ) + ( ) + ( ) ( )( )[ ]r x x y y f x y f x y, , / 2 2 2 2 1 2 ' ' ' ' , ' ,( ) = + ( ) r g r r d e r d r r f q q 0 0 0 2 3 1 2 1 1 2 1 10 ( ) ++ 2 0 0 2 4 1 2 0 2 0 3 0 4 3 2 1 2 0 3 0 4 4 1 2 3 1 2 1 1 1 8 1 1 1 1 8 1 1 + + ( ) + + + ( ) + + ( ) + ( ) =d r r g q q r r r f q q r r h q q o d( ) ( ) ( ), , , == = 1 0 0 0 d e r k d r n n n ( ) r q q q q f x y q f x y q0 1 2 1 2 2 2 1 1 2 2 2 , , ,( ) = + + ( ) + ( )( )[ ] 1 2 b b t vt i k d g qe r e r r r r r e r( ) = ( ) + + ( ) ( ) ( )' , ' ' '( )( )1 2 319 a note on the topographic distortion of magnetotelluric impedance when the latter expansion is substituted into eq. (a.5) we obtain (a.6) from the preceding equation we see that the magnetic field can be expanded in powers of the parameter d = i/k according to . f x y f x1 , /( ) = f q q f q f q qi i ij i j3 1 2,( ) = ( )( ) g q q f q f qi i ijk i4 1 2 1 3 ,( ) = ( ) qq q f q qj k ij i j( ) + ( ) 1 4 2 h q q f q f q qi i ij i j( ) = ( ) ( )4 1 2 2 , . d e r k d qb r n n n t vt ( ) = ( ) + × ( ) ( )+ = 0 1 0 0 'h r h r r r' e r'( ( ) ) . d n n n ( ) = 0 h = h( )) f x y f y2 ,( ) = references bronson, r. (1973): introductory differential equations (mcgraw-hill), pp. 130. chave, a.d. and j.t. smith (1994): on electric and magnetic galvanic distortion tensor decompositions, j. geophys. res., 99, 4669-4682. chouteau, m. and k. bouchard (1988): two dimensional terrain correction in magnetotelluric surveys, geophys., 53, 854-862. dragert, h., l.k. law and p.o. sule (1980): magnetotelluric soundings across the pemberton volcanic belt, british columbia, can. j. earth sci., 17, 161-167. fischer, g. (1989): a strong topographic valley effect in amt and vlf-r measurements, geophys. j., 96, 469-475. habashy, t.m., r .w. groom and b.r. spies (1993): beyond the born and rytov approximations: a nonlinear approach to electromagnetic scattering, j. geophys. res., 98, 1759-1775. harinarayana, t. (1986): distortion of telluric field measurements near a hill, in 8th workshop on em induction in the earth and moon, neuchatel, switzerland, pp. 87. harinarayana, t. and s.v. sarma (1982): topographic effects on telluric field measurements, pageoph, 120, 778-783. jackson, j.d. (1975): classical electrodynamics (wiley), pp. 450. jiracek, g.r. (1973): numerical comparisons of a modified rayleigh approach with other rough surface em scattering solutions, ieee trans. antennas propagat., ap-21, 393-396. jiracek, g.r., r.p. reddig and r.k. kojima (1986): application of the rayleigh-fft technique to magnetotelluric modeling and correction, in 8th workshop on em induction in the earth and moon, neuchatel, switzerland, pp. 114. kaufman, a. a. and g. keller (1981): the magnetotelluric sounding (elsevier), pp. 470. kong, j.a. (1986): electromagnetic wave theory (wiley), pp. 696. kurtz, r.d., j.m. delaurier and j.c. gupta (1986): a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate, nature, 321, 596-599. lipschutz, m.m. (1989): theory and problems in differential geometry (mcgraw-hill), pp. 263. mo r s e, p.m. and h. fe s h bac h (1963): methods of theoretical physics, part ii (mcgraw-hill), pp. 1978. nietro-vesperinas, m. (1991): scattering and diffraction in physical optics (john wiley), pp. 397. rokityanski, i.i. (1982): geoelectromagnetic investigation of the earth’s crust and mantle (springer-verlag), pp. 381. schnegg, p.a., b.v. le quang, g. fiscer and j.t. weaver (1983): audio-magnetotelluric study of a structure with a reverse fault, j. geomagn. geoelectr., 35, 653-671. schnegg, p.a., g. fischer, m. burkhard and i. muller (1986): vlf studie eines engen alpentales, protokoll kolloquium elektromagnetische tiefenforschung, 320 filippos vallianatos bergisch-glacbach, marz, 131-136. thayer r.e. (1975): topographic distortion of telluric currents: a simple calculation, geophysics, 40, 91-95. vallianatos, f. (1995): galvanic distortion of the mtimpedance tensor: application to ioannina van-station (greece), acta geophys. pol., 42 (2), 115-127. verdin, c.t. and f.x. bostick (1992): implications of the born approximation for the magnetotelluric problem in threedimensional environments, geophysics, 57, 587-602. wait, j.r. (1987): electromagnetic wave theory (john wiley & sons), pp. 250. wannamaker, p.e., j.a. stodt and l. rijo (1985): pw2dfinite element program for solution of magnetotelluric responses of two-dimensional earth resistivity structure; user documentation, univ. utah res. inst. rep. esl-158, pp. 40. wannamaker, p.e., j.a. stodt and l. rijo (1986): twodimensional topographic responses in magnetotellurics modeled using finite elements, geophysics, 51, 2131-2144. wannamaker, p.e., j.a. stodt and l. rijo (1987): a stable finite element solution for two-dimensional magnetotelluric modeling, geophys. j. r. astron. soc., 88, 277-296. xu, s. and h. zhou (1997): modelling the 2d terrain effect on mt by the boundary-element method, geophys. prospect., 45, 931-943. yaghjian, a.d. (1980): electric dyadic green’s functions in the source region, proc. ieee, 68, 248-263. vol49_2_2006 819 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words m and g time-variations – satellite geodesy – orbit perturbations – expanding earth 1. introduction for many years unequivocal evidence has accumulated in favour of an expanding earth (scalera, 1993a,b, 1994, 1997, 1998, 2001, 2002, 2003c, 2006). i have tried to test this promising new global tectonics scenario, and in particular the branch that envisages the possibility that earth expansion is directly linked to still unknown properties of the gravitational field – an old idea with roots in the works of yarkovsky (1888), beekman (2006) and hilgenberg (1929, 1931, 1933), but with more ancient background in the mechanical explanation of gravity due to georges-louis le sage (1724-1803) (edwards, 2002). a first significant result has been the possibility to link the actual polar motion (pm) to the expected – and observed – extrusion of new crust with maximum rate in the nazca triple point (scalera, 2002, 2003a,c, 2006). a second result has been to become aware that many geophysical, geodetic and geological data and reasoning lead to the possibility that the earth’s local gravity can increase through geological time (scalera, 2003b). in this short paper my purpose is to discuss the possibility that the anomalies detected in the orbits of the artificial satellites could be linked not only to the mechanical causes of the earth’s shape variations – namely tidal deceleration and glacial isostatic rebound – but can be caused by other ongoing terrestrial physical processes, which are customarily neglected, but which can also play a role in explaining fine details in the detected secular trend of the involved observational quantities. 2. neglected quantities in the j2 variation estimations let us examine the further possibility that the observed anomalous higher value of j2 variation could be a still unrecognized combination of gradients of different quantities. initially, it can be recalled that the j2 variation is usually detected are artificial satellites orbits influenced by an expanding earth? giancarlo scalera istituto nazionale di geofisica e vulcanologia, roma, italy abstract the possible role played by parameters linked to the expanding earth on the effects we observe in the orbital motion of the artificial satellites is discussed here. the important result in this short note is the discrimination between the reality of the glacial rebound process and/or the relaxation of the 100 m excess of equatorial bulge testified by the high rate of , and the improbable role that glacial rebound can have in driving pm and tpw. it is recommended that the new technology of drag-free satellites be used (gravity-probe b is the first step) to reveal possible residual orbital parameter variations ascribable to formerly unrecognized fictitious drag terms due to earth radial increase. j2o mailing address: dr. giancarlo scalera, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: scalera@ingv.it 820 giancarlo scalera from the precession of the artificial satellites’ orbits. if an equatorial bulge is present, the satellites experience a perturbing force that acts trying to transform the orbits to equatorial ones. but the reaction of the orbit is to precess and a retrogradation of the ascending node is observed. the retrogradation rate is not constant but because the earth is moving toward a more spherical shape (due at least to the tidal deceleration and glacial rebound), the distance between two successive nodes is a series of values converging to zero – in the limit of a perfect spherical earth and without considering the apparent drift of the node due to the tidal slowing down of the earth and all the other earth spin irregularities. measuring this deceleration of the node, , it is possible to evaluate using the following formula (kaula, 1966; caputo, 1967), neglecting higher order small terms and recalling that jl = = (with the harmonic expansion coefficients) (2.1) with and as = satellite’s semimajor axis; e = orbit eccentricity; i = orbit inclination; n0 = mean motion; r= =earth radius. with the value of i=109.9° for lageos the node velocity is greater than zero and directed eastward. in this first order approximation the satellite’s semimajor axis is assumed constant and in the following considerations its variations will be assumed as caused only by the several types of drag affecting the satellites’ orbits. if we define , and then we can write (2.2) and then – in the classical way of evaluating – the j2 variation is and also finally holds. ==c c j j20 20 2 2 ω ωo o p o ω$k$p=j=2 2j t2 2 o p ( ) cos j i n e r a p g m r a p k 3 2 1 s s 2 0 2 2 2 2 2 1 2 1 2 7 $ $ $ $ $ $ $ $ $ = = = = ω ω ωo o o b l k g m r as 2 2 1 2 1 2 7 $ $ $= cos p i3 2 2 = ( )e2 1 n t a gm2 s 0 3/ = π ( ) cos t e i n a r j 2 1 3 s 2 2 0 2 2 $ 2 2 = = ω ωo b l cl0l c2 1 l0$+ j2o ωp while in (2.2) the parameter p can be considered constant, the value of k is affected at least by satellite altitude decay due to different types of drag – atmospheric drag, solar radiation drag, albedo, etc. – which are properly taken into account in the computations of j2 variation that are commonly found in literature (milani et al., 1987). it is evident in the above classic elaboration (2.1) and (2.2) that no possibility is envisaged for secular variation of m, g, and r – all contained in k. but this is not the case if an expanding earth with increasing mass and/or g is assumed. if the time variations of g, m, and r are not neglected, it is easy to see that differentiating eq. (2.2), the expression for can be written or (2.3a) which can also be rearranged . (2.3b) from this last expression (2.3b) it is more clear that the astronomically observable quantity is not determined only by – apart from the satellite decay – but it is also under the influence of variations of g, m and r. other factors can produce the total amount of and then there exists the possibility that the value of is in some amount incorrectly estimated because quotas of the observed are not distributed to the other addends of the right side of (2.3b). 3. discussion let us examine the typical values that can be assigned to and to the five addends which appear in the righthand side of the expression (2.3b), listed in table i. now it can be considered: i) among the mechanical causes of the decreasing polar flatness of the earth the isostatic glacial rebound is today considered a main /ω ωp o ωp j/j2 2o ωp /aas so j/j2 2o ωp j j g g m m r r a a 2 1 2 1 2 2 7 s s 2 2= + + + ω ω o p o o o o o j j g g m m r r a a 2 1 2 1 2 2 7 s s 2 2 = + ω ωo o p o o o o $ω$k$p=j g g m m r r a a 2 1 2 1 2 2 7 s s 2 + ω ωo o o p o o o o d n j2o 821 are artificial satellites orbits influenced by an expanding earth? cause, but recently some doubts have been raised – on geologic and geomorphologic basis – about the real cause of the uplift of the fennoscandian mountains (not due to deglaciation) (lidmarbergström et al., 2000). ii) different causes – linked to expanding earth – can be envisaged on the origin of the polar motion (scalera, 2002, 2003a,c, 2006). polar motion was formerly explained as being due to the deglaciation of an irregularly distributed ice cap (peltier, 1976, 1981; sabadini et al., 1982, 1983; peltier and jiang, 1996) (also this irregularity has been criticized: the main difficulty in this view is the estimation of the extensions and thickness of the paleo-icecap on canadian and siberian shields respectively. see clark et al., 2001), but this phenomenon is incapable of explaining pm during the deep geological time (true polar table i. typical values that can be assigned to and to the five addends which appear in the right hand side of the expression (2.3b). kind quantity value comment observed 3.3⋅10–8 if = − 15 marcs/yr2 (based on llr modelling of ut1) (other elaborations of observational data span from − 10 marcs/yr2 by rubincam to − 18 marcs/yr2 based on bih modelling of ut1; see yoder et al., 1983). considering orbital parameters for lageos: – period = 3.758 h; – semimajor axis = a = 12270 km; – eccentricity = e = 0.004; – inclination = i = 109.94°; – equatorial node period = 1046 days. actual value j2 0.00108263 estimated value −2.8⋅10–11 two order of magnitude higher than the expected value. expected value –5.5⋅10–13 expected value taking into account only the decreasing flatness due to the increasing lod (varga, 2002). estimated value –2.6⋅10–8 expected value –5.1⋅10–10 taking only the value coming from the slowing down of the earth spin. neglecting the isostatic glacial rebound contribution. upper limit –10–9÷ –10–10 while precise orbital estimates provide a value of −10–11 (wesson, 1978; müller, 1991), these estimates do not consider the possibility of a variation of the involved masses and consequent compensations in the searched effects. upper limit 10–9 from geological records of heap of uncemented materials (mann and kanagy, 1990; scalera, 2003b), and from palaeogeography linked to secular polar motion (scalera, 2003a,c, 2006). reasonable value 0.8⋅10–10 if =0.5 mm/yr from considerations on an actual low tectonic activity in the recent, following the map of the spreading rates in the geologic time (müller et al., 1997; mcelhinny and mcfadden, 2000). mean value 2.4⋅10–9 if =1.5 cm/yr from palaeogeography on a geologic time window of 220 myr from triassic to recent (scalera, 2001, 2003c). typical value −3.8⋅10–8 if = −1.3 mm/day ≅ − 47 cm/yr.aso/a as so ro/r ro ro/r ro /m mo /g go /j j2 2o /j j2 2o j2o j2o ωp/ω ωp o /ω ωp o 822 giancarlo scalera wander lasts hundreds of myr and cannot be sustained by a glacial rebound that lasts a few myr or a fraction of a myr). iii) the absurd situation is that the same phenomenon, pm and tpw (tpw should be considered the prolongation of pm in the geological past), is explained today in two different ways, in the recent as due to glacial rebound and before recent attributed to the convective motions in the mantle. keeping in mind that the equalities i will write below cannot be perfect – because this is a first order analysis and serves only to make general considerations – i attempt heuristically to neglect completely the glacial rebound contribute with the aim to make evident other possible contributions to the anomalous high value of node deceleration, writing in (3.1b) only the expected value of the addend , and assigning arbitrarily a rate of radius increase of 0.5 mm/yr (3.1a) (3.1b) or, in the case of a rate of radius increase of 1.5 cm/yr (the average rate determined by paleogeography on a triassic-recent time window; scalera, 2001, 2003c) (3.1c) from (3.1c) it is possible to become aware that the dominant term is the last one, the drag term, which produces an acceleration of the node, and that the m and r – if both increasing – reinforce this accelerating effect. it is to be noted that what is really observed is acceleration, because of the dominant effect of the spiralling of the satellites down to lower altitudes where the effect of the equatorial bulge is stronger. the observed value of the nodal deceleration has to be corrected by adding the term of acceleration coming from drag effects. +10$ 10. .10 3 8+ + $ $$ 10 .2 4 10 $= $ $ 2 . j j 3 3 2 1 2 1 2 7 8 2 2 9 9 9 8 -+ o + 10 10 .3 8+ $ $ $ 10 + $ 10$.0 8 =10 $ 10 $ 2 j j 2 1 2 1 2 7 8 2 2 9 9 8 .3 3 + o j j g g m m r r a a 2 1 2 1 2 2 7 2 2= + + + ω ω o p o o o o o j/j2 2o 4. conclusions from the above considerations and from an examination of the magnitude orders of the quantities in (3.1b) and (3.1c) it can be concluded: i) only several centimetres of annual radius increase can have a non-negligible effect on the total of the summation (3.1a). indeed, from the global palaeogeography, the mean rate on the geological time window triassic-recent, nearly 220 myr, is ≅1.5 cm/yr (scalera, 2001, 2003c), and this value does not seem sufficient to have important effects on the (3.1a) balance, remaining the relative addendum one order of magnitude below the required 10–8. due to the low rate of the present global tectonic activity (map of the oceans expansion rate in: müller et al. 1997; mcelhinny and mcfadden, 2000) we should expect a rate in the recent in the order of a few millimetres. moreover the effect of an increasing radius can only be an increase of the velocity of the node (acceleration), while a deceleration is the reality. consequently, a role of increasing radius in forming the observed value of the node deceleration can be excluded. ii) the glacial rebound seems to be the real phenomenon capable of causing the ongoing higher than expected decreasing of the earth’s flatness, but additional contributions to j2 variations, impossible at present to be separated, can come from the relaxation of the well known excess of equatorial bulge (caputo, 1965, 1967; alessandrini and papi, 1987; alessandrini, 1989), and from variations in the distribution of the earth’s waters – also underground water – (dickey et al., 1999; rodell and famiglietti, 1999; grace project). iii) we can continue to consider polar motion (and true polar wander) and decreasing earth’s flatness as independent phenomena. indeed both the following statements can hold: 1) pm-tpw can be driven – with a common explanation – by causes linked to an expansion of the earth (scalera, 2002, 2003c, 2006); 2) ice cap melting can easily explain the decreasing flatness of the earth, but only using complicated modelling can it provide an explanation of pm – that however cannot be prolonged into the deep geological time to explain tpw. thus polar ice caps melting is a suspicious explanation 823 are artificial satellites orbits influenced by an expanding earth? of pm because it produces a lack of unitary cause for clearly identical phenomena. iv) ice cap melting is also a suspicious cause of uplift of mountains because geological and geomorphological clues indicate different causes, eventually analogous to the causes of uplift of mountains at latitude far from the glacial circle parallel (lidmar-bergström et al., 2000; ollier, 2006). v) a decreasing g gives an addend in ((3.1a) to (3.1c)) that can contribute only a minute term – in the order of 10–9 or less (olive and qian, 2004) – to the summation. the same holds for an increasing planetary mass, but with the same unfavourable trend (node acceleration instead of deceleration) as the increasing radius. then g and m play an irrelevant role in this problem and an expansion of the earth, whether due to inner changes of phases or to an increase in mass by an unknown cosmological process, cannot be distinguished by present artificial satellite technology. vi) finally, we may consider the improbable possibility that the increasing earth radius could episodically reach a rate able to produce an acceleration of the node nearly equal to the mean deceleration – a situation of absence of deceleration really observed in the twenty-five years of data (1979-2003) in the four years time window 19972001 and still unexplained (cazenave and nerem, 2002; cox and chao, 2002). to produce a similar total compensation the radius time gradient should be of several cm/yr, which i think may be resolved without ambiguity from astrogeodetic global baseline measurement techniques (vlbi, gps, ...). moreover an episodic radius variation should not be compensated, as that concerns consequences on global angular moment, by deep earth material ongoing differentiation, which is a different and independent process. then we would have to expect a strong effect on lod that has not been observed. vii) the problem of the difficulty in separating the global expansion, with its effect of an increasing radius on the equator, and the glacial isostatic rebound and/or relaxation of the ≅100m excess of equatorial bulge (caputo, 1965, 1967; alessandrini and papi, 1987; alessandrini, 1989) with their effect of a decreasing equatorial radius, is still open (scalera, 2003c). viii) the problem of a precise separation of the drag driven orbital decay from the global uplift of the earth surface (global expansion) is not resolvable at present. the annual rate of radius increase is only from 1/10 (a few centimetres, too optimistic) to 1/100 (a few millimetres, more realistic) of the annual orbit decay (several tens of centimetres). thus variation of the radius, of g and of m can be easily contained in the decay as a small additional unrecognized term. the only way to detect the non-classic parameters variation rate in the satellites’ orbits is to eliminate all sources of drag. in this light, the new experimental development of gravity probe b (anonymous, 2005), which – besides the investigation of minute relativistic effects – may be the first example of an orbiting mass shielded from the causes of drag, promising to open a new epoch in global positioning satellite geodesy. acknowledgements my gratitude and thanks to dr. matt edwards and prof. michele caputo, who provided invaluable suggestions for the improvement of this short note. references alessandrini, b. 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(1888): hypotèse cinétique de la gravitation universelle en connexion avec la formation des éléments chimiques (kinetic hypothesis of the universal gravitation and its connection with the formation of chemical elements), (chez l’auteur, moscou), pp.139 (in french). microsoft word 7420-17766-1-ed_nurmi_ag-rev daniela.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7420 1 green mining a holistic concept for sustainable and acceptable mineral production pekka a. nurmi geological survey of finland, espoo, finland pekka.nurmi@gtk.fi abstract mining has become increasingly difficult for societal and environmental reasons all over the world. there is an increasing competition with other land uses, and water and energy are scarce in many important mining regions. people are not ready to radically reduce the use of mineral-based products, but increasingly oppose mining. the industry faces major challenges to improve its performance and image. the green mining concept (gm) was developed as a major tool to make finland the forerunner in sustainable mining. it promotes material, water, and energy efficiency to reduce the environmental footprint of mineral-based product life cycles. gm allows the recovery of all useful minerals and minimises mining waste. gm aims to ensure the availability of mineral resources for future generations, which requires long-term investment in mineral exploration supported by geoscientific mapping, mineral systems research and the development of exploration techniques. an important goal of gm is to minimise adverse environmental and social impacts in all stages of the operations, and to maximise local benefits. gm helps to organise the operations in such a way that they are safe and meaningful to employees, and harmless for local residents and the environment. after mine closure, gm helps to restore the mining areas to make them safe and preferably to allow other types of land use. the broadbased participation of local residents and other stakeholders is crucial throughout the mining life cycle, from early exploration to mine closure. societies, governments, and investors should not tolerate unsustainable mining companies in the future. the mining industry has to solve increasing social, ecological and technical problems of the future by applying holistic concepts, such as the green mining concept, if they hope to earn the social license to operate. 1. introduction lobalization, the growth of the middle class in most developing economies, and the rapid development of technologies have created growing interest in mineral resources. according to the united nations, the world’s population will rise to 9 billion and 3 billion new people will move to cities by 2050 (united nations, 2012). the total consumption of metals will definitely be higher in the future due to the increasing global population and numbers of middle-class people, although consumption per capita will be lower because of improved resource efficiency, recycling, better product design, and substitution of traditional metals by new materials (world economic forum, 2015). economic mineral deposits are very special rock types that are rare and typically small occurrences within earth’s crust. the formation of economic mineral deposits is governed by complex processes, and their occurrences are typically controlled by large-scale structures and characteristic geological formations. the mining of minerals and their use in downstream industries or as final products largely take place in different continents and countries. for example, eu member countries consume 20-30% of the metals produced globally, but metal mine production within the eu accounts only for a few per cent of global production and many metals are not produced at all (brown et al., 2016). on the other hand, minerals-based industries are of crucial importance for the eu. the combined annual economic effect of the construction, chemical, automobile, aeroplane, machinery and equipment-manufacturing industries is about 1,300 billion euros, and they provide employment for 30 million people (tiess, 2010). g annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7420 2 resource efficiency will reduce dependency on primary minerals, but eu industries will remain vulnerable to disruptions in the metal supply and to market volatility for many decades to come. thus, one of the challenges for the sustainability of the raw material supply chain in europe is to enhance domestic production of not only the critical metals but also the traditional ferrous and base metals (nurmi and mólnar, 2014). the eu has reacted to the situation by launching the raw materials initiative (european commission, 2008) and by listing the critical minerals (european commission, 2014), which are particularly important for the industries and have potentially uncertain availability due to their concentrated production in certain countries, such as rare earth elements in china or cobalt in the democratic republic of congo (nansai et al., 2014). most people are actively seeking a better standard of living and a modern life style, and they are not therefore ready to radically reduce their consumption. despite this, many are increasingly opposed to mining activities in or near their communities, or within environmentally vulnerable areas, such as the arctic region. many people seem to have little or no understanding of the role of minerals in their urban-style everyday life, or that mining is the only way to obtain the minerals necessary for making all the infrastructure and wanted goods. anti-mining groups can be globally organised or locally active. they are typically driven by ideological enthusiasm. they are well organized and have good visibility, partly due to their good social media skills. people also increasingly oppose mineral exploration, and it is very difficult to make the difference understood between exploration projects, which can operate over large areas, cause very low impacts and which seldom lead to mining operations, and mining itself, which may cause high impacts but in small areas. the mining industry has a poor image. it is based mainly on past mistakes and neglecting the voice of local communities, or on various more recent conflicts and severe accidents that have gained global visibility. although most mines operate in a sustainable way and the industry has improved, in general, the failures can be actively used as examples to hinder new mine development in any area. the mining industry as a worldwide entity has also failed to eliminate ongoing problems with pollution and human-rights abuses (e.g., conflict minerals, working conditions in artisanal mines, etc.), or to mitigate the problems of past mining activities that remain with us today. public scepticism of mining is rooted in this reality. 2. challenges for the mining industry the mining industry is facing multiple problems (pricewaterhousecoopers international, 2013), which makes the long-term planning of sustainable mineral production and prediction of the availability of various commodities a difficult task for individual companies, nations or globally (ali et al., 2017). geology defines the occurrence of ore deposits. most traditional mining regions have been explored and mined for decades, and the remaining ore bodies are located deeper in the earth’s crust, making exploration expensive and discoveries much more difficult. geologists have to go to new areas, which are located in remote regions with a less developed infrastructure and more difficult working conditions. new deposits, moreover, tend to be of lower grade and/or more difficult to process. decision making in connection with exploration investments is never based solely on the geological potential, but takes into account numerous other issues. land access and the legal framework are critical factors for the minerals industry. competition with other land use purposes (such as agriculture, the conservation of natural or historical landscapes, infrastructural building) are causing increasing hurdles for mineral exploration. there is a threat that only limited areas will be available for mineral exploration surveys in the future. mine development is also becoming increasingly difficult for societal and environmental reasons. there is increasing competition with other interests in society, such as biological diversity, tourism, and recreation. tightening laws and regulations will make future mining more difficult in many countries. in some regions, it is impossible to find enough energy and water, and it may be difficult to find feasible means of transport. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7420 3 the mining industry seems to have lost much of its attractiveness among younger generation. it has an image of an old fashioned, polluting and dangerous business. for example, many universities ended their mining-related curricula in the 1990s, and now it is difficult to rebuild them due to the lack of competent teaching personnel and reduced appeal among young students. the mining industry needs long-term planning, requires adequate capital and includes high risks. successful mineral exploration is the basis for sustainable mining. exploration needs high-risk investment in projects that only seldom lead to economic discoveries. the time needed to bring a new deposit into production from the start of the exploration project can easily be 10 to 15 years. commodity prices are highly volatile, and during the operation time of a major mine there are many ups and downs. one of the difficulties is the timing of the initial investment to be able to turn the cash flow positive within the first few years. few companies are willing to invest during a low commodity price cycle. mine building typically takes several years, and investment during the peak may mean that at the start of production the good times could be over for multiple years. the use of many critical minerals changes with time and technological development, which makes long-term investments risky in the hi-tech commodities sector. investors also feel a threat from the strengthening demands of resource nationalism. mining is a global business, and in many countries foreign companies are involved in exploring and developing mines. people are asking about the benefits to their country and local communities. operating rules and conditions in an area might change in unpredictable ways because of a new legal framework, taxation regime or, in the worst case, due to a national takeover. 3. the green mining concept finland’s green mining concept was developed in 2011 as a major tool to make finland the forerunner in sustainable mining (nurmi and wiklund, 2012). this concept is based on five pillars (fig. 1). promoting materials and energy efficiency green mining promotes materials and energy efficiency, which reduces the environmental footprint of mineral-based product life cycles. methods that save energy and materials in mining and the enrichment of minerals have to be developed. the purpose of these new solutions is to allow the recovery of all useful minerals and byproducts, and to minimise the amount of waste. solutions for reducing water and energy consumption are being developed. in order to achieve a result that is best for the entire mining operation, there needs to be a reliable way of measuring the material and energy efficiency and the environmental footprint during the life cycle. the lifespan of many mineral-based products is long and commodities are recyclable in most applications. therefore, once produced, metals and minerals remain available for future generations, and sustainable societies will create effective mechanisms for recycling and reducing the growing need for primary resources. ensuring the availability of mineral resources for the future mineral raw materials are unevenly distributed across the earth and concentrated in small volumes of the crust through distinct geological processes. mineral deposits as such are nonrenewable and ore reserves at existing metal mines are finite. continuously increasing demand for mineral resources will exhaust most existing mines within the next few decades, although many mines will have a much longer lifetime than can be estimated according to present ore reserves. green mining aims to ensure the availability of figure 1: the green mining concept of finland. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7420 4 mineral resources for the future. sustainable development requires that our current use of mineral resources does not endanger the ability of future generations to satisfy their needs. in order to ensure the availability of mineral resources for future needs and to fulfil the so-called “mineral debt”, we must continue geoscientific mapping and research, and invest in mineral exploration. the development of exploration, mining and processing techniques is also needed to be able to discover and use new types of deposits. exploration surveys for economic minerals should be performed in such a way that their impacts on the environment and surrounding societies remain minimal. societal judgment of contrasting land use purposes and a decision on possible mine development cannot be made in a balanced way without detailed knowledge of the reserves in an area or the feasibility of the mineral deposit. minimising adverse environmental and social impacts mining operations always impact on the natural environment, economy and social structure of the region. the goal of green mining is to minimise the adverse environmental and social impacts in all stages of the operations. at the same time, the operations strive to maximise social and local benefits. minimising the adverse environmental impacts requires the development of better control and measurement methods that take into consideration the special characteristics of mining operations and the local natural conditions. maximising the societal, economic and cultural impacts in a sustainable way requires research, communication and methods that allow broadbased community participation. participation is especially important at the regional level, because this allows the corporate social responsibility of the mines to be executed in the best possible way. an important question concerns the sharing of benefits. green mining aims at the fair sharing of benefits. all stakeholders must benefit from mining, and the industry should create a long-term positive impact on regional development. improving work and organisational practises mining takes place in harsh conditions, often underground, employing heavy machinery and a variety of chemicals and explosives, which always involves potential safety hazards. work must be organised in such a way that it is safe and meaningful to employees. this can be achieved by automating processes and making them more efficient, as well as by developing new practices and working methods in cooperation with the entire staff. the need for strict health and safety standards and practices is self-evident, as well as education aiming at a good working culture. occupational safety aiming at zero accidents is an important starting point in all development. operations must also be safe for local residents and the environment. increasing automation and the development of technologies helps to reduce the need for a workforce and will improve safety. the mining organisation will become lighter and most operations will be executed in mines and enrichment plants using remote control. ensuring sustainable land use following mine closure the operating time for individual mines can be over a hundred years, but is always limited. after this, the mining areas must be restored to make them safe and allow other kinds of land use. planning of the controlled ending of mining operations and the proper measures for achieving this is started well before the commencement of mining operations, and is developed throughout the project’s life cycle with the broad-based participation of local residents and other stakeholders. closure of a mine also requires functional and tested technical and scientific methods, so that the quarries, waste areas and other infrastructure can be restored in a way that allows further sustainable use of the area according to the plans. the costs of reclamation must be factored into the commodity cost, and funds to accomplish reclamation must be set aside. finland’s green mining programme many companies, nations and international organizations plan, finance and systematically develop technologies and practices for resource efficiency. a good example is the circular annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7420 5 economy initiative by the eu, and the various project calls in its horizon 2020 programme. in finland, following the suggestions of finland's mineral strategy (anonymous, 2010), the finnish fund for innovation launched the green mining r&d&i program (gmp) in 2011. the gmp was a comprehensive approach to green mining. it targeted technologies and service innovations and new businesses that bring together value networks of small and medium-sized companies, research institutes and universities in finland and globally. following the slogan “finland, a leader in green mining 2020,” this five-year gmp turned out to be a trigger for development in all green mining areas. gmp employed international networking and drew on the best international practices. gmp (operated in 2011–2016) had a budget of €115 million and included 98 projects performed by 175 partners from large and sme sector industry and service companies, research institutes and universities in finland and globally (tekes, 2016). the projects aimed at creating improvements in all areas of the green mining concept. the results and new innovations include resource efficient processing, water issues, waste problems, exploration techniques, social issues, improved measurement techniques, and new service and organizational practices (tekes, 2016). the full impact of the results will be seen in the future. 4. discussion mining of primary minerals will grow for at least several decades to come. a powerful circular economy is inevitable, but it cannot alone solve the growing mineral needs of billions of new urban people, and new technologies. the economy, society, energy, infrastructure, transportation and materials will look very different in the latter half of the 21st century compared to the present. metal consumption per capita will decrease, but global consumption will increase due to the growing population and numbers of middle-class people (world economic forum, 2015). therefore, the mining and metals industry will have a significant role to play in future societies (ali et al., 2017). future manufacturing will pay special attention to product design, which will allow effective recycling of all commodities. technological development will allow the substitution of some critical metals by other compounds. on the other hand, rapid technological development and new innovations will require new raw materials for the next generation low-carbon, hitech society, which makes it very difficult to forecast the future critical elements (vidal et al., 2013). therefore, it is impossible to estimate in detail the global needs for various mineral raw materials of the next generations. the availability of commodities is controlled not only by geological accessibility, but increasingly by the availability of water and energy, and by social constraints, politics, legislation and environmental regulations. gamechanging technological innovations for exploration and extraction are needed. many of the future intelligent mines will be based on safe robotics, digital technologies and safe automatic processes; they will use less energy and water, and will employ the concepts of zero waste and zero accidents. thus far, the majority of mining operations have used resources at or near the surface. a major challenge for future mineral production is the increasing difficulty of ore discovery. deep-seated deposits remain largely unknown and it is likely that many of the remaining undiscovered deposits are located deep in the crust, under thick overburden or water, or in remote or sensitive places, such as the arctic regions. better geological knowledge, including mapping, geophysics and mineral systems understanding, will have crucial importance for sustainable raw materials management. long-term investment in geoscientific mapping and research is the basis for mineral exploration, and is essential to ensure a sufficient supply of economic minerals for the future. some studies demonstrate that considerable resources of economic minerals remain undiscovered (herrington, 2013; singer, 2017). future mining will increasingly involve deep underground operations (e.g., sahu et al., 2015), but it will be difficult to find and economically extract all the commodities needed by future communities from deep in the crust, and large, low-grade, open-pit deposits cannot annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7420 6 be avoided. in the far future, the global resources base may be supplemented by resources from new frontiers, such as the oceans, seafloor and/or extra-terrestrial sources. companies need to improve stakeholder relationships and partnerships with a commitment to delivering shared value to industry, governments and communities. challenges in converting natural resource wealth into sustainable economic growth and the long-term development of mining regions have to be solved. in various countries, voices of resource nationalism are strengthening, demanding special mining taxes or domestic, often public involvement in the mining business. people also want limits for foreign ownership, and mandated beneficiation and export levies. the mining industry has largely failed to convince people of its necessity for future development. societies, governments and investors will not tolerate unsustainable mining companies in the future. the mining industry has to solve the increasing social, ecological and technical problems by applying holistic concepts, such as the green mining concept, to earn the social license to operate. acknowledgments i thank my numerous colleagues at the geological survey of finland, other institutes, universities and companies for successful co-operation in developing the green mining concept, and implementing it in numerous projects. i also thank vince cronin and giuseppe di capua for their constructive comments and helpful suggestions which helped me to improve the manuscript. references ali s.h., giurco d., arndt n., nickless e., brown g., demetriades a., durrheim r., enriquez m.a., kinnaird j., littleboy a., meinert l.d., oberhänsli r., salem j., schodde r., schneider g., vidal o. and yakovleva n. 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(2013). metals for a low-carbon society. nature geoscience 6, 894–896. world economic forum (2015). mining and metals in a sustainable world. available online at http://www3.weforum.org/docs/wef_m m_sustainable_world_2050_report_2015.pd faccessed 15 march 2017 , last accessed 15 february 2017. miscellanea 609 annals of geophysics, vol. 51, n. 4 august 2008 key words solar cycle variation – single station model – geomagnetic index – monthly median fof2 1. introduction among ionospheric characteristic parameters, the critical frequency of the f region, fof2 is one of the most important ones. over decades, great efforts have been made to study the statistical model of fof2. an excellent review of many available models has been presented recently by bilitza (2002) and zolesi and cander (2000). among these models, the international reference ionosphere (iri) model, a global statistic model, is most widely used, and is recognized as the standard specification of ionospheric parameters by the committee on space research (cospar) and the international union of radio science (ursi). however, many research workers have developed the single-station model (ssm) for a particular station (moraitis et al., 1991; sizun, 1991; dick and bradley, 1992; kouris et al., 1993; pancheva and mukhtarov, 1998; liu et al., 2004). the basic advantages of the ssm are: i) more accurate results for a particular ionosonde station than the global one, a single-station spectral model of the monthly median fof2 over chongqing, china tong xu (1), zhensen wu (1) , jian wu (2), guohui wei (3) and jian feng (4) (1) school of science, xidian university, xi’an, china (2) national key laboratory of electromagnetic environment, china research institute of radiowave propagation, beijing, china (3) school of science, southwest petroleum university, chengdu, china (4) china research institute of radiowave propagation, qingdao, china abstract hourly values of the critical frequency of the ionospheric f region, fof2, obtained at chongqing ionospheric observatory (geographic 29.50n, 106.40e), china, during the interval of 1977 to 1997 (solar cycle 21 and 22) have been used to investigate the dependence of the monthly median fof2 on solar activity and geomagnetic activity, and to construct single-station model (ssm) using fourier expansion. the results of the present analysis show that there is a significant nonlinear relationship between monthly median fof2 and sunspot number r especially at daytime in each month and nighttime in summer. furthermore, introducing geomagnetic index ap further improves the description of variation of monthly median fof2, and the standard deviations decrease also dependently on time and month. thus the complex influence of solar activity and geomagnetic activity may be approximately expressed by a general multiple nonlinear function at chongqing station, china. the basic aim of this paper is to show that considering the nonlinear influence both of solar activity and geomagnetic activity improves the monthly median model. the ssm is in good agreement with observations, with standard deviation 0.65mhz, while iri model has a lower accuracy with standard deviation 0.96mhz. mailing address: dr. xu tong, school of science, xidian university, box 48, xi’an, shaanxi, 710071, china; email: xutong1104@126.com miscellanea 9-03-2009 14:42 pagina 609 610 t. xu, z. wu, j. wu, g. wei and j. feng and ii) the current updating of a ssm is usually a very easy process (pancheva and mukhtarov, 1996). it is well known that the fof2 depends strongly on solar activity. at middle and low latitudes, the primary source of ionization in the f region is the solar extreme ultraviolet (euv) irradiances. thus ionosphere empirical models depend on how to describe the relationship between fof2 and thesolar activity index. however, measurements of euv have been rather meager, especially in the past. traditionally sunspot number r is considered as an index of solar activity, although other indices, solar or ionospherically derived (liu et al., 1983; caruana, 1990; mikhailov and mikhailov, 1995), are also used. the sunspot number is convenient for use because of its long series of observations and its characteristics of reliability and predictability. the monthly median values of the ionospheric characteristic are frequently presented with linear dependence on r for every hour in the day and every month in the year, e.g. the international reference ionosphere (bilitza, 2001), ionospheric regional model (zolesi et al., 1993; de franceschi and desantis, 1994) as well as single-station model (pancheva and mukhtarov, 1996; holt et al., 2002). for low and medium sunspot numbers, the relationship is reasonably linear, nevertheless at large sunspot numbers, fof2 seems to show saturation effect (huang, 1963; kane, 1992). to take into account this behavior, a second-degree relationship between fof2 and solar activity indices is used frequently (sizun, 1992; xenos et al., 1996; pancheva and mukhtarov, 1998). this «hyseresis» effect depends on latitude (sethi et al., 2002) and historical solar activity (rao and rao, 1969; liu et al., 2004). in fact liu et al. (2004) and sethi et al. (2002) claimed that for low-latitudes, a second-degree relationship made much improvement, while, for european stations, kouris and nissopoulos (1994) have demonstrated that a second-degree equation is needed especially during evening and night hours in the winter and summer-time. moreover the variations of fof2 are complex and cannot be described considering only sunspot number. the ionosphere variations are due to the influence of the solar and geomagnetic activities as well as other sources (forbes et al., 2000; zhang et al., 2004). for the same r, fof2 may show different values during the ascending and descending of the cycle (rao and rao, 1969) and is attributed to possible geomagnetic storm effect (kane, 1992), a view also shared by apostolov et al. (1994). pancheva and mukhtarov (1996) developed a single-station spectral model over sofia with r and kr and demonstrated that in many cases the increase in the monthly standard deviation coincided with the increase in the geomagnetic activity index aa. kane (1992) argued that longterm prediction models need to take into account not just r but some solar index and geomagnetic index as two key parameters to which not much attention has been paid to date. one objective of this work is to assess both the solar activity and geomagnetic activity dependence of the monthly median fof2 over chongqing. because of the advantages of ssm and lack of ssm for fof2 yet at chongqing or in the chinese subcontinent, we develop a spectral model both taking sunspot number r and geomagnetic index ap into account based on the data from 1977 to 1997 over chongqing. the data description is presented in section 2; the relationship between fof2 and solar activity and geomagnetic activity are considered in section 3; the model and results are presented in section 4. the last section contains some discussions and conclusions. 2. data set the time series of monthly median hourly values of fof2 for the time interval 1977-1997 (solar cycle 21 and 22), or 21years, was processed at chongqing station (geographic 29.50n, 106.40e), which, located in central china, is away from the northern crest of equatorial anomaly in east asia. it has significant values for studying ionospheric dynamics in the equatorial anomaly region. because this observatory is in the transition area of middle latitudes to low latitudes, the fof2 variation is expected to be dependent both on nonlinear solar activity and geomagnetic activity. miscellanea 9-03-2009 14:42 pagina 610 611 a single-station spectral model of the monthly median fof2 over chongqing, china 3. solar activity and geomagnetic activity dependence of fof2 we first consider the statistical relationship between fof2 and sunspot number r and geomagnetic ap before constructing spectral model. ionospheric electron density is mainly due to the ionization of neutral atmosphere by solar radiation, thus electron density of f region increases with the increasing of solar activities (balan et al., 1994; kouris et al., 1998; richards, 2001; sethi et al., 2002), whereas the monthly median fof2 increase with solar activities in a rather complicated way (kouris et al., 1998; richards, 2001). monthly median fof2 linearly increases with the long-term solar activity, but saturates at extremely high solar epochs (kane, 1992; liu et al., 2003). many researchers have shown that the nonlinear relationship between fof2 and solar activity. we use regression methods to study the solar activity dependence of fof2 over chongqing. fist regression model is a linear approximation to describe the relationship between sunspot number r and fof2 (3.1) h and m are the hour and the month of interest while a0 and a1 are two matrices of 24×12=288 coefficients for each hour of day and each month of the year, and r is twelve-month running mean values of sunspot number. the linear model has been applied largely (dominici and zolesi, 1987; holt et al. 2002).the regression employing the method of least squares was performed between the measured fof2 data of each hour of each month, and the results at local time noon (12 h) and midnight (0 h) are presented in fig. 1 and fig. 2 with dotted lines. the second-degree regression is expected to make improvement, and the quadratic relationship between r and fof2 is described as (3.2) where b0, b1 and b2 also are the coefficients at specified time h and month m. sample fit results are also represented with solid lines in fig. 1 and fig. 2. 2f f b b r b r0 , 0 , 1 , 2 , 2 h m h m h m h m$ $= + + 2f f a a r0 , 0 , 1 ,h m h m h m $= + as shown in fig. 1, we find that the fof2 is almost entirely dependent on nonlinear solar activity, while in fig. 2 an evident non linear dependence is observed only in summer (may, june and july) which is nearly consistent with the conclusions of kouris and nissopoulos (1994). furthermore at noon in each month, b2<0, i.e. when sunspot number r reaches high enough, the monthly median fof2 decreases with the increasing solar activity. but at midnight in winter, b2>0, indicating that fof2 increases with the increasing solar activity, i.e. the saturation effect is unobvious. liu et al. (2004) firstly reported this phenomena based on the data from 1957 to 1991 at wuhan station (geographic 30.60n, 114.40e), china, which indicates that the phenomena may exist in low/middle-latitude in china but needs more ionosonde observations data to validate. it is well known that ionosphere is principally determined by the level of both solar activity and geomagnetic perturbations, but similar studies for the geomagnetic index dependence of fof2 have been more limited, but some exist (solé, 1988). further significant improvement is expected from a multiple regression model taking geomagnetic activity into account. therefore the third regression model can be expressed as (3.3) where ap is twelve-month running mean values. c0 to c5 are coefficients at given local time h for different month m, in which c3 represents of coaction of solar and geomagnetic activity, while c4 and c5 are the geomagnetic activity amplitudes. figure 3 illustrates the diurnal variations of standard deviations for these three regression fitted models from observed values, which shows that, in general, a second-degree regression of r gives a much lower standard deviations than the linear fit. the significant decreases in standard deviation occur in may from 0.9 to 0.4 and in october from 1.2 to 0.7. over chongqing, the solar cycle dependence of fof2 is obviously nonlinear, especially at 2f f c c r c r c ap r c ap c ap 0 , 0 , 1 , 2 , 2 3 , 4 , 5 , 2 h m h m h m h m h m h m h m $ $ $ $ $ $ $ $ = + + + + + miscellanea 9-03-2009 14:42 pagina 611 612 t. xu, z. wu, j. wu, g. wei and j. feng daytime in each month and at nighttime in summer, which supports the conclusion of kouris and nissopoulos (1994). moreover, liu et al. (2004) obtained similar conclusions using the solar activity index f10.7. on the other hand, when we take geomagnetic activity into account, the standard deviations are smaller than only considering the nonlinear influence of solar activity, but its effect depends on time and month. generally, a decrease about 0.1mhz is obtained at daytime. at nighttime, the two model standard deviation is nearlythe same except in summer, in which the standard deviations decrease about 0.2 mhz using equation (3.3). over chongqing, the influence of magnetic activity on fof2 is evident at daytime in each season, and at nighttime in summer. fig. 1. the responses of the monthly median fof2 to solar activities represented by smoothed monthly mean sunspot number r at local time 12 h during the whole interval of 1977-1997 over chongqing. observed data are plotted with cross. dotted and solid lines represent the linear and quadratic fits, respectively. miscellanea 9-03-2009 14:42 pagina 612 613 a single-station spectral model of the monthly median fof2 over chongqing, china 4. model and results since there are diurnal periodicities inherent in all the geophysical data, fourier expansion generally has the highest priority in constructing empirical models. the diurnal variations can be well expressed by a fourier expansion of cosine and sine functions with periods of 24 hours and higher harmonics, and many ionospheric empirical models are established with fourier expansion (e.g. zolesi et al., 1993; de franceschi and desantis, 1994; pancheva and mukhtarov, 1996; liu et al., 2004). in this way, the diurnal variation of fof2 can be expressed as (4.1) where h is local time and n=6 is the harmonic number (24-, 12-,8-,6-,4.8and 4-hour) and t is equal to 24 hours. these spectral coefficients are a function of r and ap. in this model, these coefficients are approximately expressed as equation (3.3), and can be estimated with a least squares regression analysis for the data set in a specified month m. 2 ( , ) ( 24 2 24 2 ) cos sin fof h m c a ih b ih 0, , 1 , m i m i n i m π π = + + + = / fig. 2. same as fig. 1, but for local time 0 h. miscellanea 9-03-2009 14:42 pagina 613 614 t. xu, z. wu, j. wu, g. wei and j. feng the time series of monthly median hourly values of fof2 during the interval 1977-1997 was processed. figure 4 demonstrates the comparison between our model and the observed data. six years in which solar activity was high (1980 and 1991), in the ascending part of the solar cycles 21 and 22 (1978 and 1988) and in the descending part of the two cycles (1983 and 1994) are chosen. furthermore the years of high activity (2000) and descending part of the solar cycle 23 (2003) are also plotted in fig. 4 to illustrate the model ability in long-term prediction. it is obvious that the model describes the diurnal variation of fof2 successfully. the ssm is compared with iri by statistic errors shown in fig. 5 and fig. 6. figure 5 shows the yearly and diurnal variations of the standard deviations of the model and iri from the observed values for the whole interval 1977-1997. evidently this model is better than iri, and the standard deviations almost decrease about 0.3mhz for both yearly and diurnal variations with our model compared with iri except the yearly variations in 1981 and 1988. figure 6 shows the probability density histogram of fof2 errors with relative deviations from the obserfig. 3. diurnal variation of standard deviations of the values of three regression fits form observed fof2 for the whole interval of 1977-1997 over chongqing. miscellanea 9-03-2009 14:42 pagina 614 615 a single-station spectral model of the monthly median fof2 over chongqing, china vations. the error distribution is evidently normal which is reasonable. the solid thin line is the gaussian curve through least squares fit. as pancheva and mukhtarov (1996) processed, the relative deviations are not lager than 12% at the confidence level of 95% for the ssm, while the errors are about 22% at the same confidence for iri model. moreover, the standard deviation of the model from observations for the whole data set is 0.65 while is 0.96 for iri model. thus our new ssm provides a higher accuracy than iri model. figures 4-6 have basically demonstrated the advantage of our model. 5. discussions and conclusions because global models may smear out features unique to a particular region, single-station models are very useful and widely viewed as the milestone of any ionopheric services (holt et al., 2002, pancheva and mukhtarov, 1998; liu et al, 2004), and there is no stationspecific model over chongqing, we developed a ssm of monthly median fof2 based on the observations from 1977 to 1997. in our model, we chose twelve-month running mean values of sunspot number r and geomagnetic index ap fig. 4. comparison between observed (open circles) and modeled (lines) fof2 values for high solar activity (1980, 1991), the descending part of the solar cycle (1983,1994), the ascending part of solar cycle (1978, 1988), high solar activity (2003) and descending part of solar cycle 23 (2003) for long-term prediction. miscellanea 9-03-2009 14:42 pagina 615 616 t. xu, z. wu, j. wu, g. wei and j. feng rather than monthly average or monthly median indices. in practice, the latter may present the influence of solar or geomagnetic activity on ionopheric average action more effectively in specific month, and some empirical models chosen these indices (e.g. zolesi et al., 1993; fig. 5. the standard deviations (σ) and the distributions of relative deviation of the new ssm values and iri model from observed fof2 over chongqing during the whole interval of 1977-1997. the thin lines are gaussian fitting curves. fig. 6a,b. the (a) yearly and (b) diurnal variations of standard deviations of the ssm and iri model values form observed fof2 for the whole interval of 1977-1997 over chongqing. miscellanea 9-03-2009 14:42 pagina 616 617 a single-station spectral model of the monthly median fof2 over chongqing, china liu et al, 2004), nevertheless the indices are difficult to predict with a long lead time because of their severe variations. but the former are relatively smoother and have the advantages of availability, reliability and predictability. the results of this paper show that there is a nonlinear relationship between monthly median fof2 and solar activity. a second-degree fit gives a much better correlation than the linear fit dependent on hour and month. the standard deviations decrease significantly mostly during daytime in each season and nighttime in summer. moreover, introducing geomagnetic index ap further improves the description of variation of monthly median fof2. we conclude that the variation of fof2 is nonlinearly dependent both on solar activity and geomagnetic activity, at chongqing station, china. considering both nonlinear influence of solar activity and geomagnetic activity, our model are successfully constructed using fourier expansion, which is verified with standard deviation and relative deviations distributions compared with iri model. the ssm has a low standard deviation is 0.65mhz and its relative deviations are not lager than 12% at the confidence level of 95%, while standard deviation is 0.96mhz and the errors are about 22% at the same confidence for iri model. furthermore, the approach of our model could be used for the short-term prediction and disturbance. acknowledgements the authors are grateful to the anonymous referees for helpful comments. this work is supported by nsfc (national nature science foundation of china) under grant no.40310223, the national key laboratory of electromagnetic environment (leme) under grant no.9140c0801060803 and by the national key technology r&d program under grant no.2006bab18b06. references apostolov, e., l. alberca and d. pancheva (1994): longterm prediction of the fof2 on the rising and falling parts of the solar cycle, adv. space. res. 14, 47-50. balan, n., g.j. bailey, b. jenkins, p.b. rao and r.j. moffett (1994): variations of ionospheric ionization and related solar fluxes during an intense solar cycle, j. geophys. res., 99, 2243-2253. bilitza, d. 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(received november 21, 2007; accepted may 9, 2008) miscellanea 9-03-2009 14:42 pagina 618 s i l a c a m e r a di i o n i z z a z i o n e e i l s u o u s o l \ m i s u r e q u a n t i t a t i v e d i r a d i o a t t i v i t a ' a t m o s f e r i c a 0 . a l i v e i i i i e ' n o t o c l i c i! e o n l e i m l o in o l a n / e r a d i o a t t i v e dell a r i a a l i n o l ' e r i e a -i p u ò d e t e r m i n a r e o e s e g u e n d o la m i s u r a dirciluriicnlc siili a r i a a t t i v a |ier e m a n a z i o n e o indirettamente sui e o s i d e t t i d e p o s i t i a t t i v i l a e e o l t i da essa ( c i o è sii ha i rub e liac e r i s p e t t i v a m e n t e tlìa ti, lì e thc). in e n t r a m b i i casi -i t r a t t a di m i s u r a r e l ' a z i o n e i o n i z z a n t e degli a t o m i a i t i v i in q u e s t i o n e p o s t i in un r e c i p i e n t e , la c a m e r a di i o n i z z a z i o n e . si l i e n c o n t o e s s e n z i a l m e n t e d e l l ' a z i o n e i o n i z z a n t e d e l l e p a r t i c e l l e a l f a , e p e r f a r e u n a m i s u r a a s s o l u t a si d e v e c o n s e n t i r e a ogni p a r t i c e l l a a l f a e m e s s a di e s a u r i r e il « p e r c o r s o » e i n o l i l e o c c o r r e s t a b i l i r e n e l l a c a m e r a di i o n i z z a z i o n e 1111 l a m p o e l e t t r i c o ' a l e da av e r e la c o r r e n t e di sai in a z i o n e . n e l l e m i s u r e d i r e t t e n o n è p o s s i b i l e s o d d i s f a r e la c o n d i z i o n e di e s a u r i r e il p e r c o r s o p e r c h é gli a t o m i d e l l e e m a n a z i o n i p r e s e n t i n e l l a c a m e r a vi si t r o v a n o c o m e gas e ([iiindi sparsi o v u n q u e a n c h e i n p r o s s i m i t à d e l l e p a r e t i e d e l i e l e t t r o d o : p e r q u e s t a r a g i o n e il m e t o d o d i r e t t o r i c h i e d e t a r a t u r a d e l l a c a m e r a . c i ò j o t t i e n e o con s o l u z i o n i n o r m a l i di r a d i o o c o n f i a l e t t e t a r a t e di e m a n a z i o n e . n e i m e t o d i i n d i r e t t i di c e r d i c n e di a l i v e r l i è p o s s i b i l e i n v e c e p r o v v e d e r e a l l ' e s a u r i m e n t o del p e r c o r s o d e l l e p a r t i c e l l e a l f a p e r c h é gli a t o m i a i t i v i s o n o fissati «opra l ' e l e t t r o d o d e l l a c a m e r a di i o n i z z a z i o n e : p i ù p r e c i s a m e n t e di u n a m e t à d e l l e p a r l i e e l l c . a l t r e difficoltà c o m u n i a t u l l i i m e l o d i di m i s u r a qui c o n s i d e r a t i r i s i e d o n o poi nel f a t t o c l i c le q u a n t i t à di s o s t a n z a r a d i o a t t i v a da m i s u r a r e s o n o in g e n e r a l e p i c c o l e e c h e lo s p a z i o d o v e sia l a c a m e r a di i o n i z z a z i o n e p i u t t o s t o g r a n d e , è p e r c o r s o in ogni s e n s o da r a d i a z i o n i b e l a e g a m m a e p e n e t r a n t i p r o v e n i e n t i d a l l e s o s t a n z e r a d i o a t t i v e del s u o l o e d e l l ' a r i a c i r c o s t a n t e e d a l l a r a d i a z i o n e c o s m i c a l e (piali i o n i z z a n o p e r c o n t o l o r o l ' a r i a e o n l e n u l a n e l l a c a m e r a di i o n i z z a z i o n e p r o v o c a n d o un a u m e n t o d e l l a c o r r e n t e c h e -i m i s u r a : di ( m e sta a z i o n e c o n s i d e r e v o l e o c c o r r e c o n o s c e r e l ' e n t i t à e s e g u i r n e l e v a . 5 1 l a c a m l l i a i i i i o m z z a / . i o n l k i i . l o l s o r i a z i o n i v e r i l i c a n l c i d u r a m e la m i s u r a . e ciò per p o t e r r i s a l i r e a l l e l l c t t o p u r o di i o n i z z a z i o n e dovuto al l a r a d i o a t t i v i t à elle -i vuol mis u r a r e . o c c o r r e p e r c i ò v a l u t a r e la e o s i d e l t a prillila naturale della e liniera di ionizzazione. nella m i t i r a d i r e t t a di p i c c o l e r a d i o a t t i v i t à e q u a n d o non -i c o n i , p r i m e 1 a r i a n e l l a c a m e r a di i o n i z z a z i o n e -i raggiunge lo scopo u s a n d o il cosidcllo metodo della doppia camera: al r e c i p i e n t e nel q u a l e -i trova 1 a r i a da e s a m i n a r e si c o l l e g a m i secondo r e c i p i e n t e di f o r m a i d e n t i c a m a c o n t e n e n t e a r i a p u l i t a , c i o è non a t t i v a ; le p a r e l i dei due r e c i p i e n t i sono e l e t t r i c a m e n t e i s o l a t e e c a r i c a t e con p o t e n z i a l i di segno o p p o s t o : gli e l e t t r o d i d e l l e due c a m e r e sono in c o n n e s s i o n e met a l l i c a t r a loro e con i e q u i p a g g i o d e l l ' e l e t t r o m e t r o . la limita 1 r a p p r e s e n t a si l i e m a l i c a m e n t e il d i s p o s i t i v o . a n c h e n e l l e m i u r e i n d i r e t t e ( m e t o d i di g e r d i c n e di v i n c i t i ) serve un e l e t t r o m e t r o di piccola c a p a c i t à e a c a m p o a u s i l i a r i o , p e r e s e m p i o q u e l l o a filo di l u t z l i d c l m a n n , e la c a m e r a di i o n i z z a z i o n e h a la p a r e t e i s o l a t a e m a n t e n u t a a p o t e n z i a l e e l e v a t o ( 3 0 0 4 0 0 \ ). l ' e l e t t r o d o a t t i v a l o e da e s a m i n a r e è in c o n n e s s i o n e m e t a l l i c a col filo d e l l ' e l e t t r o m e t r o (\. fig. 2 ) e q u e s t o è a b i t u a l m e n t e t e n u t o in c o n n e s s i o n e m e t a l l i c a col s u o l o ; s o l t a n t o d u r a n t e la m i s u r a il filo v i e n e i s o l a t o da t e r r a e p e r c i ò la v a r i a z i o n e di p o t e n z i a l e ali e l e t t r o m e t r o si m i s u r a s e m p r e a p a r t i l o dal p o t e n z i a l e .zero. l a d e t e r m i n a z i o n e d e l l a p e r d i t a n a t u r a l e d e l l a c a m e r a ili i o n i z z a z i o n e assume i m p o r t a n z a p a r t i c o l a r e nel m e t o d o a l i v c r l i p e r c h é con t a l e m e t o d o occ o r r e d e t e r m i n a r e la curva di d i s a t t i v a z i o n e dell e l e t t r o d o c n o n solt a n t o un p u n t o di essa c o m e con il m e t o d o di g e r d i c n . l e o p e r a zioni c h e -i d e v o n o c o m p i e r e p e r g i u n g e r e alla d e t e r m i n a z i o n e ili q u e l l a curva sono le s e g u e n t i : i) \ eri fica d e l l ' a t t i v i t à p r o p r i a d e l l ' e l e t t r o d o sul q u a l e -i racc o g l i e r à il d e p o s i t o a t t i v o : più p r o p r i a m e n t e si può d i r e v e r i f i c a d e l l a p u l i z i a d e l l ' e l e t t r o d o in q u a n t o esso è c o s t i t u i t o da m a t e r i a l e non a t t i v o o p o c h i s s i m o a t t i v o ; c o n t e m p o r a n e a verifica di un e l e t t r o d o c l i c d i r e m o « t e s t i m o n e » e c h e resterà i m m i l l a t o d u r a n t e l u l l a la m i s u r a . e v e n t u a l m e n t e verifica di un terzo e l e t t r o d o per la t a r a t u r a dell app a r e c c h i o di r a c c o l t a . 21 \lli\az.ione d e l l ' e l e t t r o d o noli a p p a r e c c h i o di r a c c o l t a . 3 ) e s a m e d e l l ' e l e t t r o d o a t t i v a l o e c o n t e m p o r a n e o c o n t r o l l o dcll ' a t l i v i t à n a t u r a l o d e l l a c a m e r a con l ' e l e t t r o d o t e s t i m o n e . p e r i l l u s t r a r e p i ù c h i a r a m e n t e il procedimeli!;> r i p o r t o il proto4 8 0 ( ; . a m v l h t i c o l l o c o m p l e t o di una d e l l e m i s u r e di r a d i o a t t i v i l a e s e g u i l e r e c e n t e m e n t e presso i o s s e r v a t o r i o g e o f i s i c o dell i . x . g . in p a v i a con 1 a p p a r e c c h i o a l i v e r t i . e p e r i e n z a n. io. p a v i a . 2 6 s c l l e m b r e 1946 — v e r i f i c a l a la t e n s i o n e a p p l i c a l a a l l a p a r e t e d e l l a c a m e r a ( 3 3 0 \ ) ; — v e r i f i c a l a la s e n s i b i l i t à d e l l ' e l e t t r o m e t r o : 1 d i v . = 0 . 1 \ : — p u l i t o gli e l e t t r o d i 3 e 4 con c a r i a s m e r i g l i o 0 0 : — temila d e l l a c a m e r a di i o n i z z a z i o n e : 1 o r e d i v . e l e t t r o d o c a d u t a di p o t e n z i a l e ] \ /2min 16 0 8 0 9 10 1 . 0 3 . 9 6 . 7 n . 6 0 , 5 7 12 13 1.1 4 . 3 7 . 3 11. 3 0 , 6 2 1 4 15 16 1 , 0 4 . 0 6 , 8 n. 4 0 . 5 8 17 38 1 9 0 . 6 3 . 4 6 . 5 n . 6 0 . 5 9 2 0 21 2 2 lo 4 . 0 7 . 0 11. 3 0 . 6 0 1 23 2 4 2:; 1.0 4 . 1 7 . 2 11. 4 | 0 . 6 2 s 2 8 0 . 7 3 , 6 6 , 5 11. 6 0 . 5 8 l 1 1 -— r i a s s u n t o delia m i s u r a di t e m i l a : 1 e l e t t r o d o n. 6 ( t e s t i m o n e ) p e r d e 0 . 5 8 v ogni d u e m i n u t i . » n. 3 ( d a a t t i v a r e ) » 0 . 6 1 » » » » » n . 1 ( d a a t t i v a r e ) » 0 . 6 0 » « » n s u l a c a m e r a d i i o n i z z a z i o n e e i l s u o i s o 4 1 5 1 — raccolla degli a t o m i a t t i v i : i n i z i o : 16h 39'" l e t t u r a i n i z i a l e a l l ' a n e m o m e t r o di\. n„ = 0 , ó fine: 17 04 » » )> » n,, = 3 5 0 d u r a t a d e l l a r a c c o l t a : 2 5 m v e l o c i t à d e l l ' a r i a e f f l u i t a d u r a n t e l ' e s p e r i e n z a : 3 4 9 5 = 0 , 3 6 + 0 , 0 8 8 ^ = 1 . 2 2 8 litri/sec — esame degli elettrodi attivati: ora d i v . l e t t e e l e t t r o d o \ /2min \ /min 17 0 6 07 0 8 2 , 1 1 0 , 0 1 7 , 5 n . 3 1 . 5 4 1 , 5 4 — 0 , 6 2 0 . 9 2 0 . 4 6 0 9 10 1 1 1 . 8 6 . 3 i 1,2 n . 4 0 . 9 4 0 , 9 1 — 0 . 6 1 0 . 3 3 0 . 1 6 5 12 13 1 4 15 16 17 1.2 4 , 2 7 . 1 n . 6 0 , 5 9 12 13 1 4 15 16 17 2 , 7 9 , 7 17,0 n . 3 1 , 4 3 1 . 4 3 — 0 , 5 8 0 . 8 5 0 . 4 2 5 18 1 9 2 0 4 . 0 8 , 2 12.9 n. 4 0 . 8 9 0 . 8 9 — 0 . 5 7 0 . 3 2 0 . 1 6 0 2 1 22 2 3 3 , 0 5 . 4 8 . 2 n . 6 0 , 5 2 1 2 4 2 5 2 6 4 , 7 1 1 , 5 18.0 n . 3 1 , 3 3 1.33 — 0 . 5 6 0 . 7 7 0 . 3 8 5 27 3 , 5 2 8 8 . 2 2 9 12,5 1 0 . 9 0 n . 4 0 , 9 0 — 0 , 5 5 0 , 3 5 0 . 1 7 5 4 8 2 g . a l i v e r t i ora d i v . l e t t e e l e t t r o d o \ —m i n \ m i n 3 0 3 . 2 0 , 5 5 3 l 6 . 0 n . 6 3 2 8 . 7 i 33 4 , 3 1.27 1 . 2 7 — 0 . 3 3 5 34 1 0 . 6 1). 0 0 , 6 0 3.") 1 7 . 0 0 , 6 7 3 6 3 . 5 0 , 8 5 0 . 8 5 — 0 . 1 3 0 37 7 . 8 n . 4 0 . 5 9 3 8 12,0 0 . 2 6 3 9 4 0 3 , 1 6 . 0 0 , 5 9 n . 6 4 1 9 , 0 42 3 . 5 1 . 1 6 1 , 1 6 — 0 . 2 8 0 4 3 9 , 5 1 5 . 1 n . 3 0 . 6 0 0 . 5 6 4 5 3 . 0 0 , 8 0 0 . 8 0 — 0 , 1 0 5 4 6 6 . 9 n . 4 0 . 5 9 4 7 1 1 . 0 0 , 2 1 4 8 2 , 8 0 . 5 5 4 9 5 , 4 a . (i 50 8 , 3 51 3 . 9 1 . 1 2 1 . 1 2 — 0 . 2 6 5 52 9 . 5 n . 3 0 . 5 9 5 3 1 5 , 1 0 . 5 3 5 4 3 . 0 0 . 8 2 0 . 8 2 — 0 . 1 2 0 55 7 . 1 ii. 4 0 . 5 8 5 6 1 i 1 0 . 2 4 •> i 2 . 8 0 , 5 8 5 8 5 , 5 n. 6 5 9 8 . 6 s r l.a c a m m a di i o n i z z a z i o n e i", 11. s [ 0 l " < 0 •18,5 ora d i v . l e t t e elei i rodo v 2inin \ min 18 0 0 3.1 1 . 0 1 0 . 5 8 01 8 . 8 n . 3 1.01 — 02 13.2 0.-13 0 3 3 . 0 0 . 6 8 0 . 6 8 — 0 . 2 1 5 0-1 6,4 n . 1 0 . 5 7 0 5 9 . 8 0.11 0 6 o 0 . 5 2 0 . 0 5 5 07 5 . 2 n . 6 0 8 7 . 9 0 9 3.1 0 . 9 2 0 . 9 2 — 0 . 1 7 0 10 8 , 0 12.3 n . 3 0 . 5 8 0 , 3 4 12 13 14 3 . 0 7.1 10,-1 n . 1 0 . 7 4 0 . 7 1 0 . 5 7 0 . 1 7 0 , 0 8 5 15 2 . 5 0 . 5 8 16 n. 6 i 7 8 . 3 fi e. i f i l o d e l l ' e l e t t r o m e t r o s u l l a s c a l a ( ( l e n t e c o m e n e l l a t a b e l l a p r e c e d e i o s c r v a r e e d a s c r i v e r e , c o i p u r e al m i m i l o la m i s u r a ali e l e t t r o m e t r o a v v i e n e c o i : prefissato il m i n u t o in cui -i c o m i n c e r à ad oss e r v a r e si i n t r o d u c e l ' e l e t t r o d o nella c a m e ra e 15-20 secondi prim a del m i n u t o si isola il c o n t a t t o di t e r r a : al m i n u t o c s a t l o . per es. ifi"08'" ( v . t e n u t a ) i osserva la p o i z i o n c del i -i scrive a c c a n t o a l l ' o r a e o r r i p o n : al m i n u t o stieccs-ivo 0 9 si torna ad e a (picsio p u n t o , scritto 4 8 4 c . a l i \ e l r t ' l il n u m e r o sj r i m e t l e il c o n t a t t o eli l e n a , si a p r e la c a m e r a , -i l o g l i o l ' e l e t t r o d o 11. 6 e lo -i sostituisce con il 11. 3 c si p r o c e d e c o m e p r i m a , i s o l a n d o l ' e q u i p a g g i o e l c t t r o m e t r i c o i.">-20 s e c o n d i p r i m a de! m i n u t o i i ed e s e g u e n d o le l e t t u r e ad ogni m i n u t o 11. 12, 1 3 : al m i n u t o 13, eseguita la l e t t u r a si i n s e r i s c e il c o n t a t t o di t e r r a , si sos t i t u i s c e l ' e l e t t r o d o n . 3 c o n il n . 4 e si r i p e t e l ' o p e r a z i o n e ora des c r i t t a . si c o n t i n u a , c o m e è i n d i c a t o n e l l a t a b e l l a , a l t e r n a n d o gli e l e t t r o d i . c o n s t a t a t o clic la p e r d i l a dei vari e l e t t r o d i è r e g o l a r e si eseguo l ' e s p e r i e n z a di r a c c o l t a c o m e i n d i c a t o nel p r e c e d e n t e p r o t o c o l l o a p a g i n a 4 usando d u e tubi c o l l e t t o r i c o n s e c u t i v i ; finita l ' a t t i v a z i o n e si p r e l e v a n o s i i e l e t t r o d i e se n e i n c o m i n c i a i e s a m e n e l l a c a m e r a di i o n i z z a z i o n e a l t e r n a n d o v e l i fra loro e con l ' e l e t t r o d o t e s t i m o n e . g l i e l e t t r o d i a t t i v a t i v a n n o m a n e g g i a t i c o n d e l i c a t e z z a p e r evitarti c l i c si staccili da essi p a r t e del d e p o s i t o a t t i v o e q u i n d i , (piando non s t a n n o n e l l a c a m e r a di i o n i z z a z i o n e , si a p p o g g i a n o a un s o s t e g n o a p p o sito di legno a v e n t e dei fori s u f f i c i e n t e m e n t e d i s t a n z i a t i f r a l o r o nei q u a l i si i n f i l a n o i g a m b i degli e l e t t r o d i . l e p r i m e i r e c o l o n n e ( i e l l e p r e c e d e n t i t a b e l l e c o n t e n g o n o i d a t i c l i c si o t t e n g o n o d u r a n t e l a m i s u r a d i t e n u t a e d u r a n t e q u e l l a d i d i s a t t i v a z i o n e : i n u m e r i d e l l e a l t r e c o l o n n e si d e d u c o n o d a q u e l l i o s s e r v a t i ; l ' u l t i m a c o l o n n a c o n t i e n e i d \ d i n e c e s s a r i p e r il c a l c o l o d e l n u m e r o d i a t o m i d i e m a n a z i o n e d i r a d i o ( e d i t o r i o , s<» d e l c a s o ) c o n t e n u t i p e r c m 1 n e l l ' a r i a e s a m i n a t a . a p r o p o s i t o d e l l a c o r r e z i o n e dovuta a l l a p e r d i t a n a t u r a l e b i s o g n a n o t a r e elle si lien c o n t o dei v a l o r i r e l a t i v i di p e r d i l a o t t e n u t i p e r i ire e l e t t r o d i nella m i s u r a p r e v e n t i v a di t e n u t a e d e l l a p e r d i l a d e l l ' e l e t t r o d o t e s t i m o n e d u r a n t e la s e r i e di m i s u r e con gli e l e t t r o d i a t t i v a t i . e c c e t t u a l o il p u n t o i n i z i a l e , p e r i successivi -i p r o c e d e c o s ì : -i fa la m e d i a d e l l e p e r d i l e n a t u r a l i d e l l ' e l e t t r o d o t e s t i m o n e c l i c c o m p r e n d o n o il pillilo in q u e s t i o n e , p e r es. a l l ' i s t a n t e i 7 h l 3 " ' l ' e l e t t r o d o n . fi p e r d e 0 . 5 9 \ in d u e m i n u t i , a l l ' i s t a n t e 17"22"' p e r d e 0 . 5 2 ; n e l l ' i n t e r v a l l o c o m p r e s o fra il ! 3 i n o e il 2 2 m o m i n u t o p e r d e in m e d i a 0 . 5 5 e a l l o r a al n. 3 -i a p p l i c a la c o r r e z i o n e — ( 0 . 5 5 40 . 0 3 ) — — 0 , 5 8 : al li. 4 a n a l o g a m e n t e la c o r r e z i o n e — 0 , 5 7 ; e c i ò p e r e b é r i s p e t t i v a m e n t e d u r a n t e la tenuta l ' e l e t t r o d o 3 p e r d e v a 0 , 0 3 in più del n . fi e il n. i ne p e r d e v a 0 . 0 2 p u r e in p i ù . s e g u e n d o il c r i l e r i o o r a illus t r a l o si c o r r e g g o n o tutte l e d e t e r m i n a z i o n i e s e g u i t e e -i o t t e n g o n o a p p u n l o i n u m e r i s c r i n i n e l l ' u l t i m a c o l o n n a elle si usali > por d e s c r i v e r e la curva di d i s a t t i v a z i o n e dei d u e e l e t t r o d i ( f i g . 3). s u l . a c a m m a d i i o n i z z a z i o n i : k 11. s i o i s o la p e r d i t a n a t u r a l e d e l l a cam e r a di i o n i z z a z i o n e d i p e n d e , c o m e s'è d e l l o , da diversi fai l o r i : in m o d o p a r t i c o l a r m e n t e e n i b i l e d a l l a r a d i o a t t i v i t à d c l i a r i a a m b i e n t e , poi d a l l a rad i o a t t i v i t à delle p a r e l i d e l l a stanza ove è i n s t a l l a t a la c a m e r a . d a l l a r a d i a z i o n e c o s m i c a e da ogni a l t r a causa i o n i z z a n t e e v e n t u a l m e n t e a g e n t e in prossim i t à d e l l a c a m e r a . q u i n d i essa p u ò essere d i v e r s a da u n a giornata a l i a l t r a e co-ì a n c h e da un p o t o ali a l t r o dove si f a n n o le m i s u r e . c i ò si vede b e n e c o n s i d e r a n d o i v a l o r i d e l l a pei dita n a t u r a l e osservati d u r a n t e serie di m i s u r e e s e g u i t e da m e s e m p r e con lo stesso d i p o s i t i vo. n e r e s e m p i o a p a v i a , a cortina d ' a m p e z z o , a f o r i n o : essa o«cilla c u l l o i seguenti lim i l i : infilili f i g . 2 li pila p e r t a r a t u r a e l e t t r o n i c i ™ : i' eletti-min: ir e b a n i t e : /> a l l i b r a : r = r o n t a l t o ili l e n a : /;= p i a l l o m e t a l l i c o : s c l i e r m o m e t a l l i c o a t e r r a . l a via ( ' o r l i m i f o r i n o da 0 , 3 6 lino a 0 , 6 8 \ ogni due m i n u t i :» 0 , 1 7 » » 0 . 0 2 » » a » » 0 . 8 0 » » 1.20 ii » ii » d ' a l t r a p a r i e il c o n t e n u t o m e d i o in l'a l o c a l i t à c o n s i d e r a l e risulta p e r i periodi sudile ;>er 1 aria pavia c o r t i n a t o r i n o 1.0 a t o m i di radon per cm ire l a p e r d i l a n a t u r a l e è q u i n d i d a v v e r o in m o d o s e n s i b i l e d e t e r m i n a l a d a l l a r a d i o a t t i v i t à d e l l ' a r i a e. c o m e q u e s t a , m u l e v o l e con le v i c e n d e m e t e o r o l o g i c h e e le c o n d i z i o n i di a m b i e n t e . la p e r d i l a n a t u r a l e varia a n c h e d u r a n t e una -ingola m i s u r a , co•136 ( . . a u v e h t i m e i v e d e d a l p r o t o c o l l o r i p o r t a t o p r i m a : q u e s t e v a r i a z i o n i , s e t u t t o v a i t e n e , n o n s o n o g r a n d i , d e l l ' o r d i n e d e l ó ' q u a n d o q u a l c u n a d e l l e p i l e c h e d a n n o l a t e n s i o n e a l l a p a r e t e o il c a m p o d e h ' e l e l l r o m e t r o n o n è p i ù i n b u o n e c o n d i z i o n i , il f i l o d e l l ' e l e t t r o m e t r o d i v e n t a i n s t a b i l e e a l l o r a o c c o r r e i n d i v i d u a l e e d e l i m i n a r e la p i l a g u a s t a ; p e r q u e s t a r a g i o n e n o n si p o s s o n o u s a r e b l o c c h i d i p i l e a l t r i m e n t i si s a r e b b e c o s t r e t t i a s o s t i t u i r e t u l i o il b l o c c o . : f i n . 3 i n b u o n e c o n d i z i o n i d e v o n o e m p i e e s s e r e a n c h e i c o n t a l l i d e g l i i n t e r r u t t o r i a m e r c u r i o . a n c h e la p r e s e n z a di q u a l c h e p e l u c e o i n p r o s i m i l à d e g l i i s o l a n t i p u ò d a r e d i s t u r b o ; p e r t a n t o è n e c e s s a r i a l a m a — i m a p u l i z i a e b i s o g n a a n c h e f a r e a t t e n z i o n e c l i c n o n i i n s t a l l i n o r a g n i i n p r o s s i m i t à d e l l a c a m e r a d i i o n i z z a z i o n e . e n e c e s s a r i o i n f i n e n o n f u m a r e n e l l a s t a n z a o v e si t r o v a il d i s p o s i t i v o d i m i s u r a . l a r a p p r e s e n t a z i o n e g r a f i c a d e i r i s u l t a l i e l a d e s c r i z i o n e d e l l a c u r v a c l i c m e g l i o li r a p p r e s e n t a s e r v e b e n e a d e l i m i n a r e l ' i n f l u e n z a d e l l e o s c i l l a z i o n i di s c h w e i d l c r c o n l e q u a l i -i h a s e m p r e a c h e f a r e i n q u o . s i c m i s u r e . d a l l a c u r v a si d e d u c o n o i v a l o r i f i n a l i d a i n t r o d u r r e n e l c a l c o l o d e l n u m e r o d i a t o m i d e l l e e m a n a z i o n i p r e s e n t i n e l l ' a r i a e s a m i n a l a : q u e s t o c a l c o l o è g i à l a t o i l l u s t r a t o n e i s u o i p a r t i c o l a r i i n p u b b l i c a z i o n i p r e c e d e n t i e p e r c i ò n o n è il c a s o di s o f f e r m a r v i s i q u i . s e l a t a r a t u r a d e l l ' a p p a r e c c h i o è g i à s t a t a e s e g u i t a b a s t a l ' a t t i v a z i o n e di un s o l o e l e t t r o d o e a l l o r a l e m i s u r e d i tenuta e di d i s a t t i v a z i o n e d i v e n t a n o p i ù s e m p l i c i p e r c h é -i a l t e r n a n o n e l l a c a m e r a s o l s u l.a c a m e r a 1)1 i o n i z z a z i o n i . ic 11. s u o u s o 4 8 7 l a u t o il t e t i m o n e e i e l e t t r o d o n . 5 : i j u i n l i d e l l a c u r v a d i d i s a t t i v a z i o n e r i s u l t a n o p i ù f i l l i e l a c u r v a i t r a c c i a p i ù f a c i l m e n t e . n e l m e t o d o di g e r d i c n n o n -i u s a d e s c r i v e r e u n l i m i l o t r a t t o d e l l a c u r v a d i d i s a t t i v a z i o n e , m a s o l t a n t o a l c u n i p u n t i p e r t r o v a r e , e s t r a p o l a n d o . l ' a t t i v i t à a l l ' i s t a n t e z e r o d e l l a d i a t t i v a z i o n e . l a c a m e r a d i i o n i z z a z i o n e è d e l t i p o d i q u e l l a o r a d c c i i t t a p e r il m i o m e t o d o . istituto \ azionale eli geo fìsica — osseti-. di pavia — ni aggio 7 9 / 7 . ri ass l ' s t o si descrive del tagliatamente la trinici/ delle misure indirette di contenuto di radon dell'aria atmosferica e si dà un protocollo completo di una di tali misure eseguita con il metodo diverti. vol51,1,2008_delnegro 159 annals of geophysics, vol. 51, n. 1, february 2008 key words induced polarization – electrical dispersion spectra – geophysical applications 1. introduction electric dispersion in rocks is the phenomenology on which the frequency-domain (fd) induced polarization (ip) geophysical survey method is based. fd ip effects are mostly investigated in mining, environmental and geothermal exploration. ip is the evidence that a complex mechanism of electrical conduction occurs in earth materials, necessitating an extension of the concept of resistivity, which in the fd can no longer be retained independent of frequency. many empirical models have been proposed to explain ip effects in rocks. extended treatments can be found in the review books by wait (1959), bertin and loeb (1976), sumner (1976) and fink et al. (1990). however, as pointed out by wait (1982), the characterization of ip by empirical laws is vague and confusing, since, fundamentally, non-physical descriptions have been employed. to overcome this conceptual drawback, a generalized physical model has been examined in a very recent paper (patella, 2003) by solving in the fd the following electrodynamic equation of a charge carrier subject to the action of an external electrical field e(ω) (1.1) where ω is the angular frequency, i is the imaginary unit, q and m2 represent the electric charge and the mass of the carrier, m0 is an elastic-like coefficient accounting for recall effects, m1 is a friction-like coefficient accounting for dissipative effects due, e.g., to collisions, and r(ω) is the ft of the trajectory of the charge carrier. assuming for simplicity that the dispersive material contains only one species of charge carriers, indicating with k the number of charge carriers per unit of volume, the following expression for the dispersive conductivity function σ(w), called admittivity (stoyer, 1976), has ( ) ( ) ( )m i qr ep p p 0 2 ω ω ω= = / modeling electrical dispersion phenomena in earth materials domenico patella dipartimento di fisica, università degli studi di napoli «federico ii», napoli, italy abstract it is illustrated that ip phenomena in rocks can be described using conductivity dispersion models deduced as solutions to a 2nd-order linear differential equation describing the motion of a charged particle immersed in an external electrical field. five dispersion laws are discussed, namely: the non-resonant positive ip model, which leads to the classical debye-type dispersion law and by extension to the cole-cole model, largely used in current practice; the non-resonant negative ip model, which allows negative chargeability values, known in metals at high frequencies, to be explained as an intrinsic physical property of earth materials in specific field cases; the resonant flat, positive or negative ip models, which can explain the presence of peak effects at specific frequencies superimposed on flat, positive or negative dispersion spectra. mailing address: prof. domenico patella, dipartimento di fisica, università degli studi di napoli «federico ii», napoli, italy; e-mail: patella@na.infn.it vol51,1,2008_delnegro 16-02-2009 21:28 pagina 159 160 d. patella been derived (patella, 2003) (1.2) equation (1.2) is a simple ip model, which describes the behavior of a circuit-like cell made of an rcl series combination. it is equivalent to the lorentz dispersion formula obtained as solution to the 2nd-order ordinary differential equation of harmonic oscillation (balanis, 1989). 2. non-resonant positive ip model let us consider now a system with two different species of charge carriers and put with kj, qj and mp,j (p=0,1,2) the number per unit of volume, the electric charge and the passive coefficients, respectively, of the carriers of the j-th species (j=1,2). such a pair of ionic species can be the result of ionic dissociation of a salt dissolved in pore water. an interesting case to discuss is when one species (j=1) is characterized by negligible recall and inertia terms, and the other species (j=2) by only negligible inertia. this compound can, in fact, be used to explain the well-known ip electrode effect, caused by the presence of metallic or electronic conducting mineral particles occluding pore paths in rocks (madden and cantwell, 1967). considering, for simplicity, a single-salt solution, only one ionic component, namely species 1, is assumed to undergo redox reactions at the two opposite contact faces, in order for the electric current to flow from the solution through the metallic mineral. this is equivalent to claiming that only the passage of the reacting species 1 is allowed. the non-reactant ionic species 2, instead, after running short distances under the influence of the impressed field, must remain blocked along the frontal metallic face, thus giving rise to the ip effect. this simple compound can also explain the ip membrane effect in porous rocks containing dispersed clay particles in contact with an electrolytic solution (madden and cantwell, 1967), generally less strong than the ip electrode effect. considering again a single-salt solution, species 1 ( ) m i m m i kq 0 1 2 2 2 σ ω ω ω ω = + and 2 would represent, respectively, the cations, carrying the electrical current fed by the external source, and anions, traveling, instead, short distances due to the blocking action by the negatively charged membranes (clay particles). referring to eq. (1.2), it is thus assumed that an elementary cell of a dispersive rock can behave like a two-branch parallel circuit, with a branch consisting of a single resistance and the other branch of a rc series sequence, i.e. (2.1) putting (2.2a) (2.2b) (2.2c) from eq. (2.1) we obtain σ(+)(ω) as (2.3) equation (2.3) represents a simple non-resonant positive ip model, whose real and imaginary parts are qualitatively drawn versus frequency in fig. 1. the real part of the admittivity is always positive and its high-frequency (hf) asymptote is located at a finite level higher than the low-frequency (lf) asymptote. the imaginary part, which is always positive, vanishes at both limits and presents a maximum in the inflexion point of the real part. the dispersion law expressed by eq. (2.3) is equivalent to the well known debye model (debye, 1928). generalizing, for each elementary cell either a parallel or a series combination of n twobranch circuits can be considered. for a parallel combination, the admittivity takes the form (2.4) and for a series combination the dispersive resistivity function ρ(+)(ω), called impedivity ( ) ( ) i i ( ) n n n n n n n 2 1 2 1 2 1 c c σ ω ω σ ω σ σ = + + ++ = / ( ) ( ) i i ( ) 2 1 2 1 2 c c σ ω ω σ ω σ σ = + + ++ m m , , 2 1 2 0 2 c = m k q , 2 1 2 2 2 2 σ = m k q , 1 1 1 1 1 2 σ = ( ) m k q m i m i k q( ) , , ,1 1 1 1 2 0 2 1 2 2 2 2 σ ω ω ω = + + + vol51,1,2008_delnegro 16-02-2009 21:28 pagina 160 161 modeling electrical dispersion phenomena in earth materials (patella, 1987), takes the form (2.5) putting (2.6a) (2.6b) (2.6c) equation (2.5) can be written in the following best known form (patella, 2003) . (2.7) however, it is the cole-cole (cc) model (cole and cole, 1941) that has reached the greatest popularity in ip analysis since the pioneering work by pelton et al. (1978), who reported the cc impedivity function ρcc(ω) in the form (2.8) where τ ≥ 0 and c∈[0,1] are heuristic parameters required to adapt eq. (2.8) to experimental data, and m∈[0,1] is the chargeability parameter introduced by seigel (1959). ( ) 1 ( ) ( ) m i i 1 cc c c 0ρ ω ρ ωτ ωτ = + < f ( ) i i 1 1 ( ) n n n n 0 1 ρ ω ρ ωτ ωβ = + + = d n/ n n n n 0 2 1 2 2 c β ρ σ σ= n n n n n 2 1 1 2 cτ σ σ σ= + ( )lim 1 n n n 0 0 1 1 ρ ρ ω σ= ="w + = b l/ ( ) ( )i i ( ) n n n n n n n 1 2 1 2 2 1 c c ρ ω σ ω σ σ ω = + + ++ = / the cc model, though considered an empirical law (wait, 1982), can physically be interpreted as a continuous distribution of debye dispersion terms (pelton et al., 1983), and hence approximated by an expression like eq. 2.7 (patella and di maio, 2003). following the theory developed by patella (1987, 1993), the cc model has been included in the magnetotelluric (mt) method to study the distortions provoked by ip on 1d (di maio et al., 1991) and 2d (mauriello et al., 1996) synthetic responses. ip effects in mt have been experimentally recognized in volcanic and geothermal areas (patella et al., 1991; coppola et al., 1993; giammetti et al., 1996; di maio et al., 1997, 2000; mauriello et al., 2000, 2004). 3. non-resonant negative ip model negative ip effects in earth materials are observed when |σ(ω)|<|σ0| for ω>0. in literature, negative ip is considered a geometric effect (bertin and loeb, 1976) and it was modeled for a polarizable sphere immersed in a uniform non-polarizable half-space (sumner, 1967; wait, 1982) and for certain layered media (nabighian and elliot, 1976). sumner (1976) maintains that positive ip effects are usually associated with negative ip in definite patterns, and that for most surveys, positive values are larger than negative. he warns that if these conditions are not seen in the data, there may well be equipment or coupling problems. madden and cantwell (1967) hint that negative ip effects can be caused by leakages between transmitting and receiving circuits, and bertin and loeb (1976) also mention inductive coupling, which is sometimes added to and sometimes subtracted from the ip. in conclusion, negative ip in the geophysical literature is not considered a physical effect, i.e. an intrinsic physical property of earth materials. this is rather surprising if one considers, for instance, that negative dispersion is an intrinsic physical property of electrons in metals at wavelengths sensibly less than about 10-2 cm (stratton, 1941). it is shown now that a non-resonant negative ip model can be easily derived in the frame of the theory exposed in section 1. fig. 1. sketch diagram of the non-resonant positive ip model in fd. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 161 162 d. patella let us consider again a system with two different species of charge carriers and discuss the case when both species consist of unbound charges and the first species (j=1) has negligible inertia. referring to eq. (1.2), it is thus assumed that an elementary cell of a dispersive rock can now behave like a two-branch parallel circuit, with a branch made of a single resistance and the other branch of a rl series combination, i.e. (3.1) using eq. (2.2a) and eq. (2.2b) and putting (3.2) eq. (3.1) is written as (3.3) equation (3.3) represents a simple non-resonant negative ip model. the similarity with eq. (2.3) allows σ(−)(ω) to be considered a reverse debye model. its real and imaginary parts are qualitatively drawn versus frequency in fig. 2. again the real part of the admittivity is always positive, but now its lf asymptote is placed at a fi( ) ( ) i i 1 ( ) 2 1 2 2 1σ ω ωλ σ σ ωλ σ = + + +m m , , 2 1 2 2 2λ = ( ) m k q m i m k q( ) , , ,1 1 1 1 2 1 2 2 2 2 2 2 σ ω ω= + + nite level higher than the hf asymptote. the imaginary part is now always negative, vanishes again for ω→ 0, ∞ and presents a minimum in correspondence with the inflexion point of the real part. generalizing, for each elementary cell either a parallel or a series combination of n twobranch circuits can again be considered. for a parallel combination, the admittivity takes the form (3.4) while, for a series combination, the impedivity becomes . (3.5) 4. resonant ip models resonant ip effects are indeed included in the general solution given by eq. (1.2), hence a system with two different species of charge carriers is again considered by discussing the case when only the first species (j=1) is characterized by negligible recall and inertia terms. it is thus assumed that an elementary cell of a dispersive rock now behaves like a two-branch parallel circuit, with a branch made of a single resistance and the other branch of a rlc series combination, i.e. (4.1) which, recalling eqs.(2.2a) to (2.2c)) and eq. (3.2) can be rewritten as (4.2) equation (4.2) represents a simple non-resonant flat ip model. its real and imaginary parts are qualitatively drawn versus frequency in fig. 3. the real part of the admittivity is again always positive, but its lf and hf asymptotes are now placed at the same finite level (flat asymptotic ( ) ( ) ( ) i i ( )rf 2 2 2 1 2 2 2 1 2 c c σ ω ω ω λ σ ω λ ω σ σ = + + + ( ) m k q m i m m i k q( ) , , , , rf 1 1 1 1 2 0 2 1 2 2 2 2 2 2 2 σ ω ω ω ω = + + ( ) ( ) i i1( ) n n n n n n n 1 2 2 1 2 1 ρ ω σ σ ωλ σ ωλ= + + += / ( ) ( ) i i 1 ( ) n n n n n n n 2 1 2 2 1 1 σ ω ωλ σ σ ωλ σ = + + += / fig. 2. sketch diagram of the non-resonant negative ip model in fd. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 162 163 modeling electrical dispersion phenomena in earth materials line) and a bump appears with a maximum in correspondence with the resonance frequency ω=(γ2/λ2)1/2. the imaginary part goes to zero for ω→ 0, ∞ and shows an undulation with the positive peak followed by the negative peak and crossing the ω-axis in the resonance frequency. it is worth pointing out that a resonant effect can be considered a sequence of a positive and a negative ip effect. generalizing, for each elementary cell either a parallel or a series combination of n twobranch circuits can as before be considered. with no claim to be exhaustive, this survey on the ip physical modeling theory can now be concluded by considering resonance an additional phenomenon superimposed on either a positive or negative ip effect. it is thus assumed that an elementary cell of a dispersive rock can now contain three different ionic species (j=1,2,3) and behave like a three-branch parallel circuit, with a branch made of a single resistance (j=1), the second branch of either a rc or a rl series combination (j=2), and the third branch by a rcl series combination (j=3), i.e. (4.3) for the resonant positive ip, and ( ) m k q m i m i k q m i m m i k q ( ) , , , , , , r 1 1 1 1 2 0 2 1 2 2 2 2 0 3 1 3 2 2 3 3 3 2 σ ω ω ω ω ω ω = + + + + + + (4.4) for the resonant negative ip. using the same symbolism as above, eq. (4.3) and eq. (4.4) can be given, respectively, as (4.5) (4.6) figures 4 and 5 show qualitatively the behavior of the real and imaginary parts of the resonant positive ip model given by eq. (4.5) and the resonant negative ip model given by eq. (4.6). the result is the sum of the diagrams of the real and imaginary parts depicted in fig. 1 and fig. 2, respectively, plus the homologous diagrams in fig. 3, provided that a shift to the zero-level is applied to the common lf and hf asymptote of the real part. the influence of the resonant positive ip ( ) ( ) . i i i i 1 ( )r 2 1 2 2 1 3 2 3 3 c σ ω ωλ σ σ ωλ σ ω ω λ ωσ= + + + + + ( ) ( ) i i i i( )r 2 1 2 1 2 3 2 3 3 c c c σ ω ω σ ω σ σ ω ω λ ωσ= + + + + + + ( ) m k q m i m k q m i m m i k q ( ) , , , , , , r 1 1 1 1 2 1 2 2 2 2 2 2 0 3 1 3 2 2 3 3 3 2 σ ω ω ω ω ω = + + + + + fig. 3. sketch diagram of the resonant flat ip model in fd. fig. 4. sketch diagram of the resonant positive ip model in fd. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 163 164 d. patella phenomenon on transient em methods has been studied in detail by ageev and svetov (1999) and svetov and ageev (1999). they adopted the cc model given in eq. (2.8), by arguing that for small chargeability values its validity can be extended to values of c∈]1,2[ and that within this interval of c resonance effects can be modeled. 5. conclusions it has been shown that ip phenomena in rocks can be described using conductivity dispersion laws derived as solutions to a linear 2nd-order differential equation defining the motion of a charged particle immersed in an external electrical field. five dispersion laws have been discussed, namely: 1) the non-resonant positive ip model, which leads to the classical debye dispersion law and by extension to the cole-cole law, largely used in current practice; 2) the non-resonant negative ip model, which introduces the possibility of explaining negative chargeability values, known in metals at high frequencies, also as an intrinsic physical effect in some earth materials; 3) the resonant flat, positive or negative ip models, which explain the presence of peak effects at specific frequencies superimposed on flat, positive or negative dispersion spectra. though all the derived dispersion models are related to the physical parameters regulating the motion of charge carriers under the influence of an external electric field, it must once again be remarked that distinguishing ionic species in earth materials by ip spectra still remains a difficult task. however, decrypting field measurements by one of the derived ip models may be very useful in some exploration problems. to conclude, it is worth recalling that nonresonant positive ip responses constitute by far the largest majority of effects observed directly by the ip method in mineral, oil and groundwater investigations, and, indirectly by the mt method, also in geothermal areas. a wealth of literature exists on this topic, covering the last six decades almost continuously, since the pioneering intensive works started in the late 40’s of the last century. less known in literature are resonant ip responses, mostly studied by russian researchers, of which good examples are reported in safonov et al. (1996). finally, regarding the negative ip effect as a true physical property of rocks, there is not yet any documentation firmly attesting such a possibility. nevertheless, it is suspected that in environmental applications, for instance, the observation of nearsurface negative ip effects may be a useful indicator of the presence of massive ionic contaminants in conductive sediments. references ageev, v.v. and b.s. svetov (1999): the influence of rock polarizability on electromagnetic soundings, izvestia ras, physics of the solid earth, 35, 16-24. balanis, c.a. (1989): advanced engineering electromagnetics (j. wiley & sons, new york). bertin, j. and j. loeb (1976): experimental and theoretical aspects of induced polarization (gebrüder-bornträger, berlin), 2 vols. cole, k.s. and r.h. cole (1941): dispersion and absorption in dielectrics, j. chem. phys., 9, 341-351. coppola, b., r. di maio, i. marini, a. merla, d. patella, g. pulelli, f.m. rossi and a. siniscalchi (1993): study of the simplon area geothermal anomaly in the fig. 5. sketch diagram of the resonant negative ip model in fd. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 164 165 modeling electrical dispersion phenomena in earth materials frame of a transalpine deep railway tunnel feasibility projectin, in underground transportation infrastructures, proc. of an intern. conf. of the association française des travaux en souterrain, edited by j.l. reith (balkema, rotterdam), toulon, 1993, 93-102. debye , p. (1928): polar molecules (chemical catalogue company, new york). di maio, r., d. patella and a. siniscalchi (1991: sul problema del riconoscimento di uno strato elettricamente polarizzabile mediante misure magnetotelluriche (in italian), in atti del ii convegno di geomagnetismo ed aeronomia, edited by a. meloni and b. zolesi (istituto nazionale di geofisica e vulcanologia, roma), 239-250. di maio, r., p. mauriello, d. patella, z. petrillo, s. piscitelli, a. siniscalchi and m. veneruso (1997): self-potential, geoelectric and magnetotelluric studies in italian active volcanic areas, ann. geofis., xl (2), 519-537. di maio, r., d. patella, z. petrillo, a. siniscalchi, g. cecere and p. de martino (2000): application of electric and electromagnetic methods to the study of the phlegrean fields caldera, ann. geofis., 43 (2), 375-390. fink, j.b., e.o. mcalister, b.k. sternberg, w.g. wieduwilt and s.h. ward (editors) (1990): induced polarization: applications and case histories, in investigations in geophysics, society of exploration geophysicists, tulsa, oklahoma, vol. 4, pp. 414. giammetti, s., d. patella, a. siniscalchi and a. tramacere (1996): the siena graben: combined interpretation of des and mt soundings, ann. geofis., xxxix (1), 189-200. madden, t.r. and t. cantwell (1967): induced polarization, a review, in mining geophysics, society of exploration geophysicists, tulsa, oklahoma, vol. 2, 373-400. mauriello, p., d. patella and a. siniscalchi (1996): the magnetotelluric response over two-dimensional media with resistivity frequency dispersion, geophys. prosp., 44, 789-818. mauriello, p., d. patella, z. petrillo and a. siniscalchi (2000): an integrated magnetotelluric study of the mt. etna volcanic structure, ann. geofis., 43 (2), 325-342. mauriello, p., d. patella, z. petrillo, a. siniscalchi, t. iuliano and c. del negro (2004): a geophysical study of the mt. etna volcanic area, in mt. etna: volcano laboratory, edited by a. bonaccorso, s. calvari, m. coltelli, c. del negro and s. falsaperla, american geophysical union, geophysical monograph series, 143, 273-291. nabighian, m.n., and c.l. elliot (1976): negative induced polarization effects from layered media, geophysics, 41, 1235-1255. patella, d. (1987): tutorial: interpretation of magnetotelluric measurements over an electrically dispersive onedimensional earth, geophys. prosp., 35, 1-11. patella, d. (1993): i principi metodologici della magnetotellurica su mezzi generalmente dispersivi (in italian), ann. geofis., xxxvi (5/6), 147-160. patella, d. (2003): on the role of the j-e constitutive relationship in applied geoelectromagnetism, ann. geophys., 46, 589-597. patella, d. and r. di maio (1989): on the analysis of cole-cole relaxations transients in the induced polarization prospecting method, in inverse modeling in exploration geophysics, edited by a. vogel, r. gorenflo, b. kummer and c.o. ofoegbu (f. vieweg & sohn, braunschweig/wiesbaden), 205-219. patella, d., a. tramacere, r. di maio and a. siniscalchi (1991): experimental evidence of resistivity frequency-dispersion in magnetotellurics in the newberry (oregon), snake river plain (idaho) and campi flegrei (italy) volcano-geothermal areas, j. volcanol. geoth. res., 48, 61-75. pelton, w.h., s.h. ward, p.g. hallof, w.r. sill and p.h. nelson (1978): mineral discrimination and removal of inductive coupling with multi-frequency ip, geophysics, 43, 588-603. pelton, w.h., w.r. sill and b.d. smith (1983): interpretation of complex resistivity and dielectric data. part i, geophys. trans., 29, 297-330. safonov, a.s., i.a. mushin, e.s. kiselev and a.s. goryunov (1996): a structural-formation model as the physical-geological basis for high-resolution electroprospecting, geofizika, 2, 12-23. seigel, h.o. (1959): mathematical formulation and type curves for induced polarization, geophysics, 24, 547565. stoyer, c.h. (1976): consequences of induced polarization in magnetotelluric interpretation, pure appl. geophys., 114, 435-449. stratton, j. (1941): electromagnetic theory (mcgrawhill, new york). sumner, j.s. (1967): the problem of negative ip anomalies, in proc. of the symposium on induced electrical polarization, engineering geoscience, department of mineral, technical university of california, berkeley, 32-50. sumner, j.s. (1976): principles of induced polarization for geophysical exploration (elsevier, amsterdam). svetov, b.s. and v.v. ageev, (1999): high resolution electromagnetic methods and low fequency dispersion of rock conductivity, ann. geofis., 42 (4), 699-713. wait, j.r. (editor) (1959): overvoltage research and geophysical applications (pergamon, oxford). wait, j.r. (1982): geo-electromagnetism (academic press, new york). vol51,1,2008_delnegro 16-02-2009 21:28 pagina 165 041_053 adg v–5 n01.pdf annals of geophysics, vol. 45, n. 1, february 2002 41 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov institute of solar-terrestrial physics sd, russian academy of science, irkutsk, russia abstract using a new technology for global gps detection of ionospheric disturbances, globdet, it has been established that a drastic increase in the time derivative of the magnetic field strength during magnetic storms is accompanied by an almost simultaneous decrease in mid-latitude total electron content on the entire dayside. the corresponding correlation coefficient is not below −0.8; the delay with respect to the time of a magnetic storm sudden commencement is about 3-10 min. this is most pronounced for magnetic storms with a wellmarked sudden storm commencement. the sudden storm commencements presented in the paper were observed during the initial storm phase. the analysis reported here was made for a set of from 90 to 300 gps stations for 10 days in 1998-2001 with a different level of geomagnetic activity (dst from −6 nt to −295 nt, and k p from 0 to 9). the «simultaneous» total electron content response for the events under consideration was 0.1-0.4 tecu, and the travel velocity of the disturbance from the dayside to the nightside was in the order of 10-20 km/s. results obtained are consistent with earlier ionospheric parameter measurements obtained using high temporal resolution methods. 1. introduction the ionospheric response to a sudden storm commencement (ssc) should be set off from the large number of phenomena observed during different phases of magnetospheric storm development. many morphological research efforts have been made to puzzle out the dynamic nature of the processes accompanying ssc. ssc response characteristics in mid-latitude vlfemissions were considered in mullayarov and muzlov (2000). observations of post-ssc ionospheric fluxes and particle precipitation events were presented in nielsen and honary (2000). the solar wind velocity and density effects on the cyclic behavior of the ssc occurrence frequency were investigated in makarov (1994). distinctive decreases in maximum ionization (n m ) and in ionization at fixed levels (n h ) after ssc were observed for a strong magnetic storm of july 11-19, 1959 (potapova and shapiro, mailing address: p r o f . e d w a r d l . a f r a i m o v i c h , institute of solar-terrestrial physics sd, russian academy of sciences, p.o. box 4026, irkutsk, 664033, russia; e-mail: afra@iszf.irk.ru key words global ionospheric response – sudden storm commencement – magnetic storms 42 edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov 1961). a valley in the diurnal variation of the f2-layer critical frequency (f0 f2), and a drastic increase in the height of the f2-layer maximum (h max f 2 ) occurred almost simultaneously with ssc on the entire day-side (for mid-latitude stations). the valleys during evening ssc are hidden by an abrupt diurnal change in the parameter f0 f2; therefore, a scatter of the delay time τ between the f 0 f 2 minimum and the ssc time was observed. the duration (∆t ) of the f0 f2 response to the ssc varied from 1 to 22 min. table i presents the following characteristics: magnetic storm date; reference to a relevant publication; τ delay between the ssc time and the ionospheric response; ∆t duration of the ionospheric response to the ssc; ∆ | a| amplitude increment relative to quiet values; v displacement velocity of disturbances, and n largest number of stations involved in the investigation. the post-ssc ionospheric response for the magnetic storm of june 15, 1965 was investigated in great depth by shashunkina (1968, 1972), based on data from a number of ionospheric stations located at different sites of the globe. using data on f 0 f 2 , the f 2 -layer maximum electron density (n max f 2 ), the maximum height hmaxf2, the f2-layer thickness y m , and electron density distribution profiles with height (n h ), it was found that the delay of the maximum of the ionospheric response to ssc varied from 0 to 90 min. the response duration according to f0 f2 data varied from 90 (low solar activity) to 180 min (high solar table i. characteristics of the ionospheric response to ssc. n date reference technique parameters τ, min ∆t, min ∆| a| v, m/s n 1 11-19.07-1959 potapova and sphiro i f0 f2, n(h) 0-138 1-22 0.2-2 mhz − 14 (1961) nmf2, nh 2 15.06.1965 shashunkina i f0 f2, n(h) 0-90 90-180 1-1.3 mhz 0.72-2.5 20 (1968, 1972) hmf2, nmf2, h′f2, ym 3 25-26.05.1990 buonsanto frlps, tec, nmf2, 108-300 102-360 10-32 tecu, − 4, (1995) isr h m f 2 180-300 150 km 1 4 25-26.09.1958 rüster (1965) i hmf2, f0 f2 150 120 100 km − 1 5 13-14.09.1959 rüster (1965) i hmf2 180 120 150 km − 1 6 3-4.10.1959 rüster (1965) i f0 f2 90 90 100 km − 1 7 8.07.1958 bowman i ∆h′f 3-3.5 180-540 20-100 km 0.37-0.72 5 (1965) hmf2 8 12-14.03.1989 huang and cheng i f0 f2, tec 0-0.6 30-60 5 mhz, − 1, (1991) frlps 65 tecu 3 9 15.02.1967; huang et al. fds ∆f 0-0.07 1-4 0.5-2 hz − 2 13.03.1970 (1973) 10 6.01.1998; − globdet tec 4-10 15-30 0.08-0.38 10-20 90-300 23.04.1998; 6.04.2000; 8.06.2000; 13-15.07.2000; 31.03.2001; 4-11.04.2001 i = ionosonde; frlps = faraday rotation of the linearly-polarized signal; isr = incoherent scatter radar; fds = frequency doppler shift measurements along the hf path; 1tecu = 1016m−2. 43 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement activity) depending on the station’s latitude. the delay of the ionospheric response to ssc increased from high to low latitudes. some publications (huang and cheng, 1991; buonsanto, 1995) contain estimates of variations of ionospheric parameters after ssc made on the basis of data from incoherent scatter stations (iss) and by determining the value of the total electron content (tec) from faraday effect measurements of the signal from geostationary satellites (goes2, goes7). the delay of the ionospheric response to ssc for the magnetic storm of may 25-26, 1990 was 108-300 min (according to tec measurements) and 180-300 min (according to data from iss) (buonsanto, 1995). the response duration ∆t was 102-360 min. for the magnetic storm of march 12-14, 1989, τ ranged from 0 to 36 min according to tec and f 0 f 2 data. the response duration ∆t was 30-60 min (huang and cheng, 1991). information on the ionospheric response to ssc for the magnetic storms of september 2526, 1958, september 13-14, 1959, october 3-4, 1959, and july 8, 1958 according to ionosonde data is reported by bowman (1965) and rüster (1965). the delay τ for these storms averaged 90-210 min. the ionospheric response to an instantaneous change in the magnetic field lasted 90-540 min. the ionospheric response to a ssc of the magnetic storms of february 15 and september 19, 1967; october 29, 1968; may 14, 1969; april 28, 1969, and of march 31, 1970, was investigated by measuring the frequency doppler shift along an hf path (huang et al., 1973). the delay τ between a maximum frequency doppler shift and the ssc time was 0-4 min; the response duration was 1-4 min. of all the abovementioned data these results are distinguished for the highest time resolution and hence for the highest reliability of determination of the response delay τ. investigations of the ionospheric response to ssc made by a number of authors to date, contributed significantly to the study of the phenomena described above. a demerit characteristic for most investigations is the low spatial and temporal resolution of measuring facilities (time steps of 5 or 15 min; the largest number of ionospheric stations involved in the study of the same event does not exceed 20). for that reason, in spite of the large body of experimental evidence available, it is impossible to obtain reasonably reliable estimates of the main parameters of ionospheric response to ssc. because of this, the results presented in table i are most likely to refer to different manifestations of geomagnetic disturbances in the mid-latitude ionosphere. thus, in some cases (lines 2 and 7 of table i) the ionospheric parameter variations were caused by large-scale traveling ionospheric disturbances (ls tid) with typical time periods of 1-2 h and wavelengths in the order of 1000 km (bowman, 1965; shashunkina, 1968, 1972). it is generally believed that ls tids are manifestations of acoustic-gravity waves (agw) whose generation regions lie in equatorial zones of the northern and southern hemispheres. these concepts received further support through recent experiments using gps data (ho et al., 1998; afraimovich et al., 2000b). the ls tid response delay of about 1-2 h is consistent with the idea that tids are generated in auroral regions and travel equatorward at a velocity of about 300-400 m/s. in other cases (lines 9 and 10 of table i) the response delay is too small to be consistent with the above mechanism associated with the generation and propagation of agw. a global spatial averaging of tec variation, obtained using data from the international gps network, seems to have been used for the first time by afraimovich et al. (2000a) in investigating the global ionospheric response to ssc. an «instantaneous» ionospheric response to a sudden commencement, ssc, of a strong magnetic storm of april 6, 2000 was detected; it appeared as a single negative tec disturbance with a duration of about 20 min, with about 200s delay with respect to the ssc at mid-latitudes, almost simultaneously on the entire dayside. the study reported by afraimovich et al. (2000a) used the new technology, globdet, developed at the istp sd ras for global detection of ionospheric disturbances of natural and artificial origins using data from the international network of two-frequency multi-channel receivers of the navigation gps system which substantially improves the sensitivity and space-time resolution of experiment (afraimovich, 2000). 44 edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov the objective of this paper is to obtain statistically more significant estimates of the main parameters of the response to ssc for magnetic storms with a different level of geomagnetic activity. the ssc presented in the paper were observed during the initial storm phase. 2. the method for determining the global ionospheric response to a sudden storm commencement we now outline the procedure of processing gps data which is directly used in this paper. the input data in our experiment are represented by series of «oblique» tec values along the beam to the gps satellite at selected receiving points, as well as the corresponding series of values of the elevation θ (t) and azimuth α (t) of the beam calculated from our developed convtec program to convert standard (for the gps system) rinex-files available through the internet (gurtner, 1993). the variations of the «oblique» tec i (t) are determined on the basis of phase measurements at each of the spatially separated two-frequency gps receivers using the known formula from hofmann-wellenhof et al. (1992) (2.1) where l 1 λ 1 and l 2 λ 2 are the increments of the radio signal phase path caused by the phase delay in the ionosphere (m); l1 and and l2 stand for the number of complete phase rotations, and λ 1 and λ 2 are the wavelengths (m) for the frequencies f1 = 1575.42 mhz and f2 = 1227.60 mhz, respectively; const is some unknown initial phase path (m); and nl is the error in determining the phase path (m). the sensitivity of phase measurements in the gps system permits irregularities and wave processes to be detected over a wide range of amplitudes (as large as 10–3 of the diurnal tec variation) and periods (from several days to a few minutes). standard data available via the internet have a time resolution of 30 s. ionospheric data provide their data with a time resolution of 5-15 min. hence the gps data have higher time resolution compared with ionosondes and incoherent scatter stations. to determine the characteristics associated with disturbances involves selecting several series of tec i(t) measurements with a duration of 1 h at least. to exclude the influence of the signal reception conditions, we used only observations with satellite elevations θ (t) larger than 30°. for the purpose of eliminating the variations of a regular ionosphere as well as trends introduced by the satellite motion, use is made of the procedure of removing the linear trend by preliminarily smoothing the initial series with the selected time window of a duration in the order of 30 min. the globdet software package used in this study performs a global coherent accumulation of tec variations over the time interval chosen for the analysis (afraimovich, 2000) (2.2) where di i is the filtered tec series; and n is the number of beam. the correction coefficient k i is required for converting the «oblique» tec to an equivalent «vertical» value (klobuchar, 1997) (2.3) where r z is earth’s radius; and hmax is the height of the ionospheric f2-layer maximum. 3. geometry of the experiment the investigation reported in this paper is based on data from the global network of gps receiving stations available via the internet. at present (as of february 2001), data from no fewer than 800 gps stations can be accessed through the internet. figure 1a illustrates the geometry of part of the global network used in nl+ + ]const i f f f f l l= − −( ) +[ 1 40 308 1 2 2 2 1 2 2 2 1 1 2 2. λ λ s t di ki i i n = × = ∑ 1 ( ) k r r h i z z i= +         θcos arcsin cos max ( ) 45 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement 09:10 17:10 01:10 09:10 lt 0 60 120 180 240 300 360 90 60 30 0 -30 -60 -90 la tit ud e, d eg . longitude, deg. 0 60 120 180 240 300 360 90 60 30 0 -30 -60 -90 la tit ud e, d eg . a b fig. 1a,b. the geometry of the part of the global gps network used in this study when analyzing the ionospheric response to the ssc of the strong magnetic storms of june 8, 2000 (263 stations) (a). dots correspond to the location of gps stations. the upper scale indicates the local time lt, corresponding to the time 09:10 ut. panel (b) shows the coordinates of subionospheric points for the height of the f 2 -layer maximum h max = 300 km for all satellites visible at the ssc time june 8, 2000, for each of the gps stations marked on panel a (a total of 1325 los). 46 edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov this case study of the magnetic storm of june 8, 2000 (263 stations). as is evident from fig. 1a, the set of stations we selected from the part of the global gps network available to use, covers rather densely north america and europe, and to a much lesser extent asia. gps stations are more sparsely distributed on the pacific and atlantic oceans. however, such coverage of the surface of the globe makes it possible, even today, to tackle the problem of global detection of disturbances with hither to unprecedented spatial accumulation. this ensures the number of statistically independent series which is, as a minimum, by two orders of magnitude larger than could be realized when taking records with other measuring facilities. thus, in the western hemisphere the corresponding number of stations can reach no less than 500 today, and the number of beams to the satellite can be no less than 2000-3000. the upper scale in fig. 1a shows the local time (lt), corresponding to 09:10 ut (ssc time for the magnetic storm of june 8, 2000). for a variety of reasons, slightly differing sets of gps stations were selected for different events to be analyzed; however, the geometry of experiment for all events was virtually identical. the station coordinates are not given here for reasons of space. this information may be found at the electronic address http://lox.ucsd.edu/cgibin/allcoords.cgi? figure 1b presents the locations subionospheric points for the height hmax = 300 km for the whole set of gps satellites during the ssc of june 8, 2000, for each of the gps stations marked off in fig. 1a (the number of beams is 1325). 4. ionospheric response to a sudden commencement of the strong magnetic storm of april 6, 2000 figure 2a-d plots the tec time dependence i(t) on the dayside for april 6, 2000 (a), and the di(t) variations obtained upon removing the linear trend from the i(t) series and smoothing with the 30-min time window (b) for stations algo (prn02, thick line) and ohig (prn01, thin line). the same applies for the nightside for stations irkt (prn21, thick line), kerg (prn22, thin line) (c) and (d). as is evident from fig. 2b, the decrease of di(t) relative to the ssc time, which is characteristic for the algo and ohig stations, occurs with a delay τ of about 5 min. for the irkt and kerg stations (fig. 2d), there is a delay of about 32-37 min between the valley in s(t) and the ssc time. the geographic coordinates of gps stations ohig and algo are shown in fig. 2b, and those for stations irkt and kerg are indicated in fig. 2d. for the time interval 16:00-18:00 ut, april 6, 2000, fig. 3a plots the variations in magnetic flux at geostationary orbit of the goes10 satellite (135°w), fig. 3b the variations of the h component of the magnetic field h(t), and fig. 3c the time derivative of the h-component (thin line) for station irkutsk (52.20°n; 104.30°e). results of a global summation of s(t) are presented for the dayside (472 trajectories) in fig. 3d. the dependence s(t) for the nightside (245 trajectories) is plotted in fig. 3e. for comparison, fig. 3c plots the inverted curve s(t) for the dayside (thick line). the ssc time (16:39 ut) in fig. 3a-e is shown by the vertical dashed line. an analysis of the tec data for our entire chosen set of gps points for the time interval 16:30-17:30 ut, including the ssc time, shows a negative disturbance of a duration of about 20 min. the delay σ between a maximum of the dh(t)/dt and a maximum of the inverted s(t) series was 6 min. the delay τ between the ssc time and a maximum of the s(t) curve was 7 min. figure 4a plots the time of a minimum t min of the filtered tec series versus latitude, obtained for each beam; the amplitude of the tec response to the ssc of the strong magnetic storm of april 6, 2000 on the dayside; and the distribution of t min and of the response amplitude a as a function of local time (lt) (for the time interval 16:24-17:36 ut). the ssc time in fig. 4a,c is shown by the thin horizontal line. the scatter of the time t min is due to the fact that there exist background tec fluctuations with periods similar to periods of oscillations caused by the ssc; therefore, to improve the signal/noise ratio we apply a global coherent accumulation of di(t) series. 47 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement fig. 2a-h. time dependencies of tec i(t) on the dayside on april 6, 2000 (a), and variations of the di(t) with the linear trend removed and a smoothing with the 30-min time window (b) for stations algo (prn02 thick line) and ohig (prn01 thin line). same on the nightside for the station irkt (prn21 thick line) and kerg (prn22 thin line) (c) and (d). time dependencies of tec i(t) on the dayside on june 8, 2000 (e), and variations of the di(t) (f ) for stations pfan (prn03 thick line). same on the nightside for the station mem2 (prn02 thick line) and riog (prn01 thin line) (g) and (h). all panels show gps station names, prn numbers of the gps satellites and geographical coordinates of stations. 0 1 0 2 0 3 0 4 0 i ( t ) , t e c u o h i g a l g o 1 .8 1 .2 0 .6 0 0 .6 1 .2 d i ( t ) , t e c u 0 1 2 3 4 i ( t) , t e c u 1 6 :3 0 1 7 :0 0 1 7 :3 0 t i m e , u t 0 .4 0 .2 0 0 .2 0 .4 d i ( t ) , t e c u i r k t k e r g 0 4 8 1 2 i ( t ) , t e c u 0 .1 6 0 .0 8 0 d i ( t ) , t e c u p f a n s s c ( 6 3° n ; 3 0 2 ° e ) ( 4 6° n ; 2 8 2 ° e ) ( 4 9 .3° n ; 7 0 ° e ) ( 5 2° n ; 1 0 4 .3° e ) 0 2 4 6 i ( t ) , t e c u m e m 2 r i o g 0 9 :0 0 0 9 :3 0 1 0 :0 0 t i m e , u t 0 .2 0 .1 0 0 .1 0 .2 d i ( t ) , t e c u ( 3 5 .4 6 ° n ; 2 7 0 ° e ) ( 5 3 .8° n ; 2 9 2 .2° e ) s s c (47.5° n; 9.8° e) april 6, 2000 june 8, 2000 a b c e d g h f 48 edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov an examination of figs. 3a-d and 4a-d, suggests the conclusion that on the dayside of the earth the largest value of the tec response amplitude was 0.4 tecu (1-2% of the background value of the tec i0; 1tecu = 10 16 m–2). the value of i0 was determined from standard ionex-files (mannucci et al., 1998), available via the internet, by averaging the tec values for the latitude, longitude and time ranges of our interest. the duration of the ionospheric response to the ssc was 20 min. the delay τ between the largest value of the tec response amplitude and the ssc time, in mid-latitudes on the dayside, was 6 min. in high latitudes the delay τ lasts for more than 15 min. on the nightside, a = 0.1 tecu, and τ = 45 min. 5. ionospheric response to other magnetic storms similar results were also obtained for some other magnetic storms: of january 6 and april 23, 1998; june 8 and july 13, 14 and 15, 2000; and of march 31 and april 4, 11, 2001. data on the magnetic field were received from the goes spacecraft (the satellite numbers are 9 and 10 for april 23, 1998 and june 8, 2001; march 31 and april 4, 11, 2001, respectively). figure 2 (right) plots the time dependencies of the tec i(t) on the dayside for june 8, 2000 (e) and the di(t) variations (f ) for gps station pfan (prn03). the same applies for the nightside for stations mem2 (prn02, thick curve) and riog (prn01, thin curve). it is apparent from fig. 2f that for station pfan lying on the dayside, the delay τ between the ssc onset time and a minimum di(t) is about 3 min. on the nightside, τ = 7-9 min for stations mem2 and riog. the geographic coordinates of gps station pfan and for stations mem2 and riog are shown in fig. 2f,h respectively. figure 4e-h shows the same parameters as in fig. 4a-d, but for the magnetic storm of june 8, 2000. the response amplitude a in fig. 4h is plotted as a function of local time (lt) for the time interval 08:42-09:42 ut. figure 5a-e plots the same dependencies as in fig. 2a-h, but for the time interval 08:00-10:00 ut, june 8, 2000. the result of a global coherent summation for the dayside (fig. 5d) is shown for 406 beams. such a result for s(t) on the nightside (632 beam) is presented in fig. 5e. figure 6a-e plots the same dependencies as in fig. 2a-h, but for the time interval 15:30-17:30 ut, april 11, 2001. the result of a global coherent summation for the dayside (fig. 6d) is shown for 403 beams. such a result for s(t) on the nightside (97 beam) is presented in fig. 6e. fig. 3a-e. the variations in magnetic flux (a) at geostationary orbit of the goes10 station (135°w), the variations of the h-component of the magnetic field h(t) (b), and of the time derivative dh(t)/dt (c) (thin line) for the time interval 16:00-18:00 ut, april 6, 2000, at station irkutsk (52.2°n, 104.3°e). the result of a global spatial averaging of s(t) (d) for 472 los on the dayside. a similar result for 245 los on the nightside (e). for comparison, panel (c) presents the inverted curve s(t) (heavy line) on the dayside. the time of ssc is shown on panels (a-e) by a vertical dashed line. a b c d e 49 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement fig. 4a-h. the latitudinal dependencies (obtained for each los) of the tmin (a) and of the amplitude a (b) of the ionospheric response to the magnetic storm ssc on april 6, 2000, on the dayside; the total distributions of tmin (c) and a (d) as a function of local time lt (for the time interval 16:24-17:36 ut). the latitudinal dependencies of the t min (e) and of the amplitude a (f ) of the ionospheric response to the magnetic storm ssc on june 8, 2000, on the dayside; the total distributions of t min (g) and (h) as a function of local time lt (for the time interval 08:42-09:42 ut). the ssc times is shown on panels (a, c, e, g) by a horizontal straight line. 8 0 4 0 0 4 0 8 0 1 6 :3 0 1 7 :0 0 1 7 :3 0 t m i n , u t 6 8 1 0 1 2 1 6 :3 0 1 7 :0 0 1 7 :3 0 t m i n , u t 8 0 4 0 0 4 0 8 0 l a ti t u d e , d e g . 1 .5 1 .0 0 .5 0 .0 a , t e c u 6 8 1 0 1 2 t i m e , l t 2 .0 1 .5 1 .0 0 .5 0 .0 a , t e c u s s c s s c 8 0 4 0 0 4 0 8 0 0 9 :0 0 0 9 :1 1 0 9 :2 3 0 9 :3 5 0 9 :4 7 t m i n , u t 9 1 2 1 5 0 9 :0 0 0 9 :1 1 0 9 :2 3 0 9 :3 5 0 9 :4 7 t m in , u t 8 0 4 0 0 4 0 8 0 l a t i t u d e , d e g . 0 .4 0 .2 0 .0 a ,t e c u 9 1 2 1 5 2 t im e , l t 0 .6 0 .4 0 .2 0 .0 a ,t e c u s s c s s c june 8, 2000april 6, 2000 1 8 2 1 1 4 1 6 1 4 1 6 1 8 2 1 a b c e d g h f 50 edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov an examination of figs. 4a-h and 5a-h suggests that the ionospheric response to the ssc lasted 20 min. the delay σ between a maximum of the inverted curve s(t) and a maximum of the curve dh/dt is 3.5 min. the delay τ between the ssc time and a maximum of the s(t) curve was 4.5 min. on the dayside of the earth the largest value of the response in amplitude and the delay τ with respect to the ssc time were 0.1 tecu and 2 min, respectively. on the nightside, these values were 0.06 tecu and 8 min, respectively. the travel velocity of the disturbance from midto high-latitudes and from the dayside to the nightside averaged 10-20 km/s. fig. 5a-e. the same as fig. 2a-h, but for the time interval 08:00-10:00 ut, june 8, 2000. the result of a global spatial averaging of s(t) for 406 los on the dayside (d). a similar result for 632 los on the nightside (e). fig. 6a-e. the same as fig. 2a-h, but for the time interval 15:30-17:30 ut, april 11, 2001. the result of a global spatial averaging of s(t) for 403 los on the dayside (d). a similar result for 97 los on the nightside (e). the main values characterizing the magnetic storms being analyzed are presented in table ii: dst at the ssc time (according to the data from the goes satellite); the ssc time; max dh/dt maximum value of the derivative h(t); ∆t time interval under investigation; m number of averagings (of beams) in the case of a global coherent accumulation;a∑modulus of a maximum response amplitude for the s(t) series (it is shown on a percentage basis how much our measured value of the tec amplitude differs from the mean background value of i 0 calculated from standard ionex-files); σ delay between maxima of the series s(t) (inverted values) and 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 n t 1 0 1 2 d h (t )/ d t, n t /s ec 0 .1 0 0 .0 5 0 .0 0 s (t ), t e c u s s c n = 4 0 6 0 8 : 0 0 0 8 : 3 0 0 9 : 0 0 0 9 : 3 0 1 0 : 0 0 t i m e , u t 0 .0 4 0 0 .0 4 0 .0 8 s (t ), t e c u n = 6 3 2 4 0 0 4 0 8 0 h (t ), n t s ( t ) i r k u t s k june 8, 2000 g o e s 1 0 ( 1 3 5 0 w ) a b c d e a b c d e n = 403 n = 97 irkutsk 51 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement dh/dt; τ delay between a minimum of the s(t) series and the ssc time; and τ is the correlation coefficient between s(t) and dh/dt. the distinguishing features of the storms of april 6; june 8, july 15, 2000 and april 4, 2001 are the relatively high maximum values of dh/ dt at the ssc time 0.2, 0.4 nt/s, respectively. for these storms, high values of the correlation coefficient of s(t) and dh/dt, – 1 and – 0.9, were obtained. it is evident from the table that the global negative tec disturbance is quite well correlated with the derivative of the magnetic field strength (the correlation coefficient not below – 0.8) but is delayed with respect to these variations by about 3-9.5 min. 6. discussion and conclusions using the new technology for global gps detection of ionospheric disturbances, globdet, it has been ascertained that a drastic increase in the time derivative of the magnetic field strength at the time of magnetic storms is accompanied by an almost simultaneous table ii. statistics of experiments. n date dst, nt time of max dh/dt, ∆t, ut m | a∑|, σ, τ, r ssc, ut nt/s tecu min min 1 6.01.1998 102.25 14:16 0.1 14 15 395 0.1 −6 −6.5 −0.8 2 23.04.1998 148.84 18:25 0.1 18 18:50 401 0.1 −3 −4 −0.8 0.4 i 0 = 14.5 3 6.04.2000 144.82 16:39 0.2 16:30 18 472 2.6% −6 −7 −1 0.1 i 0 = 35.3 4 8.06.2000 114 9:10 0.2 8:55 9:52 406 0.3% −3.5 −4.5 −0.9 0.2 i 0 = 35.3 5 13.07.2000 88.32 9:42 0.1 9:19 10:32 227 0.3% −3 −13.5 −0.9 0.1 i 0 = 37.3 6 14.07.2000 101.45 15:32 0.1 15 16 350 0.4% −4.5 −6 −0.8 0.1 i 0 = 35.3 7 15.07.2000 166.5 14:37 0.4 14 15:21 203 0.3% −9.5 −10 −0.9 0.2 i 0 = 25.9 8 31.03.2001 143 00:52 0.15 00:22 01:19 103 0.8% −4.5 −7.5 −0.9 0.1 i 0 = 39.6 9 04.04.2001 132 14:55 0.15 14:32 15:17 91 0.2% −6 −3.5 −0.9 0.3 i 0 = 35.3 10 11.04.2001 30.6 15:47 0.2 15:30 17:30 403 0.8% −9.0 −51 −0.9 52 edward l. afraimovich, eugene a. kosogorov, ludmila a. leonovich, oleg s. lesyuta and igor i. ushakov decrease of the mid-latitude tec throughout the dayside. the corresponding correlation coefficient is not below – 0.8; the delay with respect to the time of the sudden magnetic storm commencement is about 3-10 min. this is most pronounced for magnetic storms with a clearly-defined ssc. the amplitude of the «instantaneous» tec response for the events under consideration was 0.1-0.4 tecu, and the travel velocity of the disturbance from the dayside to the nightside was in the order of 10-20 km/s. these results are in reasonably good agreement with data obtained by recording the ionospheric response to a sudden commencement of strong magnetic storms, based on measurements of the frequency doppler shift along a hf path (huang et al., 1973). thus the disturbances which we have analyzed do not fit in the model of acousticgravity waves (agw), and the appropriate physical mechanisms should be pursued when modeling the electromagnetic set of phenomena accompanying a strong geomagnetic disturbance. effects caused by electric fields in the ionosphere influence the behavior of its parameters and manifest themselves differently with a different latitude, which is due primarily to the geomagnetic field geometry. under quiet conditions, the electric field has no marked effect on the behavior of ionospheric parameters as a function of time. however, during magnetic disturbances when the electric field is enhanced significantly (kozelova et al., 2001), large variations in the height of the f2-layer maximum are observed, which can be caused by the vertical component of the e→ × β→ drift of ionospheric plasma (rüster, 1965). according to bryunelli and namgaladze (1988), ionospheric effects of magnetospheric electric fields in mid-latitudes are caused by the divergence of the vertical ion flux produced by the horizontal electric field. the electric field-induced transport during a disturbance in the f2-region with the e → × β→ drift velocity is effective both on day and night. the electric field can reach 10-15 mv/m in midlatitudes (ogawa et al., 1975); the unperturbed values of the field are smaller by a factor of 2-3. the influence of the vertical component of the e→ × β→ drift on the height distribution of electron density was investigated by different methods (tanaka and irao, 1973). as the e→ × β→ drift velocity is virtually independent of the f 2 layer height, the profile of the electron density distribution is displaced, under the action of the electric field, as a single whole without changing its form substantially (bryunelli and namgaladze, 1988). the eastward-directed electric field causes the height of the f2-layer maximum to rise, while the westward-directed field is responsible for its lowering. the h m f 2 variations are quite well correlated with zonal electric field variations, with a small delay, in the order of 20 min, at night and with a still smaller delay during the daytime. n m f 2 variations at the time of an instantaneous change of the field are poorly pronounced and show up as a decrease in critical frequencies by about 0.1-0.3 mhz. as suggested in park (1974), such effects corresponded to electric fields with an amplitude of 5-10 mv/m. unambiguous interpretation of the ionospheric effects in terms of the action of electric fields in earlier studies was made difficult by the fact that similar effects can also be caused by agw generated by auroral electrojets during substorms. the deciding criterion for separating the effects of electric fields and agw is the delay between geomagnetic and ionospheric effects. in the former case, where the influence of electric fields is predominant, the delay is a few minutes (bryunelli and namgaladze, 1988; volkov and namgaladze, 2001). in the latter case, where ionospheric effects are triggered by agw, the delay averages 45-60 min (shashunkina, 1968, 1972; afraimovich et al., 2000b). the analysis made in this paper which describes the occurrence of ionospheric effects on abrupt changes of the electric field, and also the data on tec variations together with dh/dt obtained in this study, point to the penetration of electric fields of magnetospheric origin with typical nonstationarity times of 30-60 min, into the mid-latitude ionosphere during periods of drastic magnetic disturbances. acknowledgements the author is grateful to n.n. klimov and e.a. ponomarev for their encouraging interest in this study and active participation in discus53 gps detection of the instantaneous response of the global ionosphere to strong magnetic storms with sudden commencement sions. we are grateful to s.a. nechayev and v.v. kharchenko for providing data from magnetic observatory irkutsk. thanks are also due to v.g. mikhalkovsky for his assistance in preparing the english version of the t e x manuscript. we greatly appreciate the referee’s efforts to improve the submitted manuscript. this work was done with support from both the russian foundation for basic research 00-05-72026 and rfbr grant of leading scientific schools of the russian federation 00-15-98509. references afraimovich, e.l. 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(1972): research results on the sudden magnetic storm commencement effect, ionos. issled., 20, 154-165 (in russian). tanaka, t. and k. hirao (1973): effects of an electric field on the dynamical behavior of the ionospheres and application to the storm time disturbance of the f-layer, j. atmos. terr. phys., 35, 1443-1452. volkov, m.a. and a.a. namgaladze (2001): influence of the magnetospheric convection electric field on the formation of field-aligned currents of the substorm break-up phase, geomagn. aeron., 41, 33-38 (in russian). the effect of recent venus transit on earth’s atmosphere the effect of recent venus transit on earth’s atmosphere s. s. de1, b. k. de2, s. k. adhikari1, b. k. sarkar1, s. k. sarkar3, a. guha1, p. k. mandal1, s. k. mandal1, h. p. sardar1 and m. ray1 1centre of advanced study in radio physics and electronics, university of calcutta, kolkata 700 009, india. 2department of physics, tripura university, agartala 799 130, india. 3department of physics, university of calcutta, kolkata 700 009, india. abstract some experiments on june 8, 2004, the day of transit of venus across the sun, were undertaken at kolkata (latitude: 23034( n) to observe effect, if any, of transit of venus on fwf, elf and vlf amplitudes. the result shows good correlation between their temporal variations during the transit. the observation was unbelievable as the venus subtends only 1/32th of the cone subtended by sun on earth. this anomaly may be explained on the assumption that the height of venusian atmosphere with high content of co2, and nitrogen which absorbs electromagnetic and corpuscular radiations from sun, depleting the solar radiation reaching the earth to a considerable extent. as a result, relevant parameters of earth’s atmosphere are modulated and here we show how these changes are reflected in identical behaviour of fair weather field and elf and vlf spectra. key words venus transit, effects on atmospheric electric parameters 1. introduction the quasistatic electric field observed near earth’s surface, i.e., fair weather field (fwf), is maintained by global thunderstorm activities (rycroft and price 2000; bering et al., 1998). thunderstorm activity and lightning again put the earth-ionosphere waveguide into resonance producing characteristic spectra in elf and vlf ranges (nickolaenko, 1997; rycroft and cho, 1998). __________ mailing address: prof. s. s. de, centre of advanced study in radio physics and electronics, university of calcutta, 1, girish vidyaratna lane kolkata 700 009 india e-mail: de_syam_sundar@yahoo.co.in various parameters of the atmospheric electricity are directly related to the global thunderstorm activity and solar radiation. the discrete spectra of frequencies 8, 14, 20… hz due to schumann resonances are generated by electromagnetic emission from the lightning strokes and can be regarded as excitation of an ac global circuit (bering et al., 1998). some frequency changes of the peak values and also amplitude changes are found to be present in the observed data which are attributed by the uncertainties arising from spatial distribution of lightning sources exciting the schumann resonance modes (de et al., 2004). schumann resonance phenomenon as well as their measurement techniques and other aspects have been described by many workers (williams, 1992; tzanis and beamish, 1987; satori, 1996; huang et al., 1999). in three areas, namely, atmospheric potential gradient, elf and vlf, we are taking continuous records where some significant deviations are observed in the values of the involved parameters during the transit of venus on june 8, 2004. the transit started at 1044 hr local time and continued up to 1653 hr local time, which was visible from kolkata (latitude: 22034´n) throughout this period. the local sunrise and sunset times were 0452 hr and 1822 hr respectively. in this paper, the results of experimental observations during venus transit will be presented. 2. experimental arrangement we take observations from the roof top at a height of about 26 meters from ground. the vertical electric field is measured with an ac field-mill which has an aluminum rotor plate of 12 cm. in diameter. the output from the amplifier is recorded through computer sound card at a sample rate of 44,100 per second. the rms value of the recorded signal is used to find the required electric field from the calibration chart. we calibrated the field-mill in a vertical field setup between two large aluminum cover plates, electrically isolated at a given potential through a fixed distance between them. the outer shield of the field-mill is grounded properly to ensure possible field distortions. the sensitivity of the field-mill is (0.33±0.03) vm-1. for the observation of integrated field intensity (ifi) of vlf sferics at 9 khz, an 8 swg straight copper wire of 120 meter in length is used as the antenna. the antenna is sensitive to the vertical electric field component of the electromagnetic signal. it is installed horizontally at a height of about 30 meters above the ground. the signal processor is tuned to the desired frequency. the overall q-factor of the tuning circuit is around 300. the signal from the tuning stage is rectified and fed to a log amplifier. the time constant of the output from the signal processor is 10 seconds. the data is recorded using a 12 bit adc at a sample rate of 5. for observations on schumann resonances, two square loop antennas in series combination have been used. the length of each side of one of the loops is 1 meter with a total of 75 turns and other with 90 turns having a length of 1.3 meter for each side. the antennas are sensitive to the magnetic field component of the natural schumann resonance electromagnetic signals. a stereo-preamplifier-integrated circuit with la3161 chip, whose low frequency response starts below 5 hz, has been used to pre-amplify the signal from antenna. the chip is capable of detecting signals in the microvolt region. the pre-amplified signal is then amplified with a low pass amplifier with upper cut-off at 35 hz. the amplified signal is recorded using computer sound card at a sample rate of 44,100 per second. 3. analyses of the observed data (a) schumann resonances the temporal variations of the amplitudes of first three schumann resonance modes on 8th june, 2004, during the venus transit, together with their averaged values of 18 days are shown in figure 1. standard deviations from the average values have been plotted as error bars in the average plot. standard deviations have been calculated over 2nd and 3rd quartile of the frequency distribution curve. it is seen that the average amplitude of the first mode line in the average value graph is 0.025 a.u. (arbitrary unit) except almost a sharp 3 fold increase at 14 hr local time, whereas on the day of transit, the corresponding amplitude decreases gradually from 10 hr to 16 hr from 0.06 a.u. to 0.03 a.u. after 16 hr, the amplitude increases. the second mode amplitude in the average value graph remains steady at 0.01 a.u. except almost a two fold increase at 14 hr local time. on the day of transit, the corresponding amplitude shows the gradual decrease from 0.035 a.u. to almost zero at 13 hr local time which then gradually increases. the third mode amplitude in the average graph remains the same except a depression between 13hr to 14 hr local time, whereas on the day of transit, the amplitude decreases from 10 hr local time to 13 hr local time and shows a peak at 14 hr and then the amplitude decreases up to 15 hr local time and then again increases. (b) vlf atmospherics a comparative picture of the ifi at 9 khz atmospherics on 8th june, 2004, during the venus transit (blue line) and its 18 days’ average value over the same period (black line) is presented in figure 2. the dotted curves represent standard deviation. on the day of transit, there are some unique peaks that appear just before and after the beginning of the transit. the peaks are in decreasing order of magnitude. the ifi then slowly increases to a steady value showing considerable fluctuations before and after the end of the transit also. it is worthwhile to mention that in both the measurements of potential gradient and vlf ifi, just after the beginning of transit and before the end of transit, their values lie beyond the standard deviation limits of the average variation. in case of potential gradient, it decreases from higher value to a lower value whereas in case of vlf ifi, it increases from lower value to a higher value. in schumann resonance also, we obtained the similar trend. (c) potential gradient figure 3 represents the temporal variation of vertical potential gradient on 8th june, 2004 during the venus transit (red line) and its values averaged over other 18 days around that date (blue line) with standard deviations from the average value plotted as error bars. firstly, we notice that the maximum and minimum values of the potential gradient on the day of transit are around 300 vm-1 and 50 vm-1 respectively. but during the same period on other days, it is around 205 vm-1 and 152 vm-1, respectively. the high values owe to the higher level of pollution at the place. secondly, the curve during the transit shows significant deviation from the average values since the variation is well beyond the standard deviation of the average trend. there is sudden enhancement of potential gradient up to around 300 vm-1 after the start of the transit and then the gradual decrease extends up to around 50 vm-1. in the cases of potential gradient and vlf sferics and also in the case of first and second mode of schumann resonance, the “effect” of the venus transit reverses its nature at about 1400 hour local time (figure 1-3). this is probably due to the contribution from regional thunderstorm centre in se asia. the behaviour of the third mode of schumann resonance curves are not properly understood (figure 1). the atmospheric temperature recorded at an indian meteorological department observatory about 10 km from observation site did not show any outstanding change during transit. relative humidity was a little bit higher than normal throughout the day. 4. discussion the commonly accepted model explains fwf near the ground by the dynamic equilibrium between charging currents from lightning activity and the leakage current of the earth-ionosphere spherical capacitor (bering et al., 1998; fullekrug et al., 1999; rycroft et al., 2000).the idea is corroborated by the measurements of fullekrug et al. (1999) who showed a similarity in the monthly averaged pattern of fwf and elf amplitude. during the transit of venus, it obstructs 1/1000th part of incident solar radiation coming to the earth, since the angular diameter of venus compared to that of the sun as seen from the earth is of the ratio 1:32. therefore, apparently the transit of venus should have no significant effect on any terrestrial phenomenon. but, in spite of this fact, the changes observed in the atmospheric parameters at kolkata during the transit of venus are quite significant. this anomaly may be explained in terms of very large extent of the atmosphere of venus with about 96% co2 that produces atmospheric pressure of about 92 bars (http://pr.erau.edu/~holmesg). because of very high content of co2, and nitrogen (warby, 1943), its atmosphere absorbs high frequency radiations, protons as well as other ionizing particles from the sun thereby depleting to a considerable extent the solar radiation reaching the earth. hence, during the transit, there will be some changes in the depth of interaction between earth’s magnetosphere and solar radiations. it is expected that collision frequency would be affected little by such interaction. the main influence would be on the rate of ionization within the earth-ionosphere cavity. this in turn introduces perturbation in the atmospheric electrical conductivity (roble, 1991) which may give rise to the changes that have been observed. the modification of solar atmosphere under the influence of tidal forces from the venus during transit may also contribute the observed effects (harnischmacher and rawer, 1981). each plot has two distinct time zones, one up to 1330 to 1400 hr lt and other after that. a prominent feature of our observation is that the deviation of the average values of measured parameters changes signs at the boundary of two time zones. atmospheric potential gradient begins to increase from 2 hours before onset, reaches a maximum after 1 hour from onset and then gradually decreases and reaches a minimum at almost the same time as the end of transit. then it gradually increases and becomes equal to the average value about 4 hours after the end of transit. the beginning and end of transit mean the transit of solid core of venus. the times of 1st and 4th contact of atmosphere of venus are different than the times of transit, 1st contact being earlier and 4th contact later than the recorded time of transit. the cause of the observed phenomenon may probably due to the rise of lower boundary of the ionosphere. changes of convection current due to change in intensity of solar radiation may also contribute to the cause. of particular interest is the increase in potential gradient from sunrise which continues upto 1 hr after onset of transit. similar observation in case of solar eclipse is known (dhanorkar et al., 1989). the amplitudes of 3 modes of schumann resonance show gradual increase from average value from sunrise and this tendency decreases from 10 hr lt, i.e., one hour before transit. the gradual decrease continues up to 13 hr lt, after which the variations assume the characteristics of regional thunderstorm activity in asia (nickolaenko et al., 2002). amplitudes of 9 khz sferics descend from the average value about 1 hour before transit and then it slowly rises after 1 hour. it crosses the average graph gradually and again meets it after 1 hour from the end of transit (figure 2). at the end, it shows a fluctuation which may have its origin in local thunderstorm activity. the amplitude is significantly lower from about 1 hour before transit to about 14 hr lt. this portion of the ifi curve is structurally similar to the average curve. apparently, venus transit affects the attenuation parameter. for its explanation, calculation of amplitude under “eclipse” condition is necessary. atmospheric potential gradient on the day of transit of venus just before its onset increases which is probably due to the scattering of solar radiation and ionizing solar particles by the protruding part of venusian atmosphere (figure 3). after the onset of transit, absorption of solar radiation takes over causing gradual decrease of fwf, fft amplitude and ifi [figure 1 (red line), figure 2 (blue line) and figure 3 (red line)]. the peak around 1400 hr local time in figure 1 correlates with lightning activity of regional thunderstorm centre in asia (nickolaenko et al., 2002). it is interesting to note that all the modes of schumann resonance do not respond equally to the lightning activity, probably because of differences in nonlinear interaction with transverse resonance and differences in attenuation rates. 5. conclusion the transit of venus is an event encouraging detail study of its terrestrial effects. observations on schumann resonance spectra and earth’s atmospheric electric field were undertaken during the transit of venus. the results have been compared with our ongoing records. how so small a speck in the backdrop of solar disc can cause so large effect is an open question. we have tried to explain this anomaly in this presentation. acknowledgements the work has been funded by indian space research organization (isro) through s. k. mitra centre for research in space environment, institute of radio physics and electronics, university of calcutta, kolkata 700 009, india. references bering, e. a. iii, few a. a. and benbrook, j. r. (1998): the global electric circuit, physics today, 51, 24-30. de, s. s., saha, a. k. and de m. (2004): measurement of elf emission in the upper atmosphere over kolkata due to schumann resonances, ind. j. radio sp. phys., 33, 32-34. dhanorkar, s., deshpande, c. g. and kamra, a. k. (1989): atmospheric electricity measurements at pune during solar eclipse of 18th march 1988, j. atmos. terr. phys., 91, 1031-1034. fullekrug m., fraser smith a. c., bering e. a. and few a. a. (1999): on the hourly contribution of global cloud to ground lightning activity to the atmospheric electric field in the antarctic during december 1992, j. atmos. solar terr. phys., 61, 745-750. harnischmacher, e. and rawer, k. (1981): lunar and planetary influences upon the peak electron density of the ionosphere, j. atmos. terr. phys., 43, 643-648. huang, e., williams, e., boldi, r., heckman, s., lyons, w, taylor, m., nelson, t. and wong, c. (1999): criteria for sprites and elves based on schumann resonance observations, j. geophys. res., 104, 16943-16964. http://pr.erau.edu/~holmesg/ nickolaenko, a. p. (1997): modern aspects of schumann resonance studies, j. atmos. solar-terr. phys., 59, 805-816. nickolaenko, a. p. and hayakawa m. (2002): resonances in the earth ionosphere cavity, kluwer academic publisher, page 97. roble, r. g. (2000): on modeling component processes in the earth’s global electric circuit, j. atmos. solar-terr. phys., 53, 831-847, 1991.rycroft, m. j. and price, c., the global atmospheric electric circuit, solar activity and climatic change, j. atmos. solar-terr. phys., 62, 1563-1576. rycroft, m. j. and cho, m. (1998): modelling electric and magnetic fields due to thunderclouds and lightning from cloud top to the ionosphere, j. atmos. solar terr. phys., 60, 889-893. satori, g. (1996): monitoring schumann resonances-ii. daily and seasonal frequency variations, j. atmos. terr. phys., 58, 1483-1488. tzanis, a and beamish, d. (1987): time domain polarization analysis of schumann resonance waveforms, j. atmos. terr. phys., 49, 217-228. warby, r. e. (1943): phys. rev., 64, 207. williams, e. r. (1994): global circuit response to seasonal variations in global surface air temperature, monsoon weather review, 122, 1917-1929. williams, e. r. (1992): the schumann resonance: a global tropical thermometer, science, 256, 1184-1187. figure captions figure 1. variation of peak magnetic amplitude of the three modes of schumann resonance spectra on the day of venus transit, 8th june, 2004 (red line) and average variation of the same parameter around 8th june for 18 days (black line) figure 2. variation of atmospheric integrated field intensity (ifi) at 9 khz vlf on the day of venus transit, 8th june, 2004 (blue line) and average variation of the same parameter around 8th june for 18 days (black line) figure 3. variation of atmospheric vertical potential gradient on the day of venus transit, 8th june, 2004 (red line) and average variation of the same parameter around 8th june for 18 days (blue line) figure 1 figure 2 figure 3 page 6 649_656 vernon 12.pdf annals of geophysics, vol. 45, n. 5, october 2002 649 regional gps receiver networks for monitoring local mid-latitude total electron content angela vernon and ljiljana r. cander rutherford appleton laboratory, chilton, didcot, oxon, u.k. abstract two regional gps receiver networks from the ordnance survey u.k. (os) and the italian space agency (asi) have been used for monitoring mid-latitude total electron content (tec) during quiet and disturbed ionospheric conditions in the current solar cycle. a few quiet and disturbed days in march and april 2002 were examined. these showed how the temporal and spatial patterns of changes develop and how they are related to solar and geomagnetic activity for parameter descriptive of plasmaspheric-ionospheric ionisation. use is then made of computer contouring techniques to produce snapshots of daily maps of tec for these different regional areas. 1. introduction it is well known that the ionosphere, as a major region of the earth’s environment, shows great temporal and spatial variations. temporal variations include cyclical effects with diurnal, seasonal and solar cycle periods. spatial variations deal with geographical and also significant variations in ionospheric behaviour in different ionospheric layers. in studying these variations, particularly important are ionospheric disturbances with their damaging effects on radio communications systems that involve links between earth and space at frequencies up to at least 4 ghz (bishop et al., 1996). the determination of whether the ionospheric conditions will generate significant system degradation involves a continuous monitoring and extensive study of ionospheric variability at local, regional and global levels. in this paper the emphasise is on the regional ionospheric variability as seen from os and asi gps receiver networks in fig. 1a,b during the solar-terrestrial conditions given in table i. most of the studies so far indicate that in order to realistically specify the global undisturbed ionosphere, hourly ionospheric observation every 30° of longitude and 15° of latitude are needed. such a minimum world-wide grid network would provide at least 20% improvement in the specification of the ionosphere over the median condition (rush, 1972). however, during geomagnetic disturbances, the observations should be more frequent both in time and space (bradley and cander, 2002). such a network of observing gps sites is currently in existence (e.g., os and asi) giving the potential improvement to propagation forecasting. the scale size of the changes in the structure and ionisation content are quite variable and are investigated here on regional basis. mailing address: angela vernon, rutherford appleton laboratory, radio communications research unit, chilton, didcot, oxon ox11 0qx, u.k.; e-mail: angela.vernon@rl.ac.uk key words regional ionospheric modelling – ionospheric variability – ionospheric storm – total electron content (tec) 650 angela vernon and ljiljana r. cander the vertical tec estimations along all gps satellite links in view for elevation angles greater than 10° are derived using a technique developed by ciraolo (1993, 2000) and recently discussed by cander and ciraolo (2002). the residual rms values of the observations result generally in the order of approximately 2 tec units (1 tec unit = 1016 el/m2). 2. spatial and temporal regional tec structure the possible origins of plasmasphericionospheric variability can be the solar ionising flux, the interaction of the solar wind with the earth’s magnetic field and even meteorological influences (prölss, 1995). typical multi-point observational tec results for asi sites are given in figs. 2a-c. they show clearly geomagnetic activity effects at the regional plasmasphericionospheric conditions. evidently there is a part of scatter in figs. 2a-c that can be only attributed to the higher level of geomagnetic activity on both 5 and 6 march than on 8 march 2002, displaying pronounced upper atmospheric sensitivity to the rapidly changing solar-terrestrial activity as given in table i. in particular, ionospheric vertical tec variations are found to have relatively small magnitude (12 tecu during daytime and 8 tecu during night-time) for quiet conditions on 8 march 2002. thus, it is unlikely that short-term solar euv changes can produce the observed spatial tec variability. figures 2a-c show that there is a marked tec variation from hour to hour as well as within 1 h related to the receiver’s locations in a rather complicated way that cannot generally be linked directly to any single solar_terrestrial event. if the difference is of order of 30 tecu during daytime and 20 tecu during night-time between one location and another in the area of 15° longitude and 10° latitude on 5 and 6 march 2002, it is expected that the behaviour of the plasmasphere-ionospheres structure during major geomagnetic disturbances departs even more significantly from that described in figs. 2a-c. although daytime variability in tec is found to be considerably larger than night-time, fig. 3a,b shows important difference between two regional areas tec during nights hours on 4 march 2002. fig. 1a,b. maps of the os (a) and the asi (b) gps sites considered in this paper as two regional areas. a b 651 regional gps receiver networks for monitoring local mid-latitude total electron content date rc 10 cm ak bkg m x 2002 mar 03 178 183 012 b9.8 0 0 2002 mar 04 − 175 011 b7.3 0 0 2002 mar 05 − 172 024 b6.4 0 0 2002 mar 06 165 178 017 b9.9 0 0 2002 mar 07 − 180 008 b7.2 0 0 2002 mar 08 − 177 004 c1.0 0 0 2002 mar 29 − 181 007 b7.7 0 0 2002 mar 30 145 189 018 c1.8 2 0 2002 mar 31 179 204 024 c1.4 1 0 2002 apr 01 165 207 018 c1.0 0 0 2002 apr 02 220 206 012 c1.4 0 0 2002 apr 03 − 209 015 b8.1 0 0 2002 apr 04 248 216 006 c1.0 2 0 2002 apr 05 213 217 004 c1.3 0 0 2002 apr 06 249 206 004 c1.1 0 0 2002 apr 07 211 208 010 c1.5 0 0 table i. solar-terrestrial conditions in march and april 2002 (sidc (rwc-belgium) http://sidc.oma.be)). rc = sunspot index from catania observatory (italy); 10 cm = 10.7 cm radioflux (drao, canada); ak = ak index wingst (germany); bkg = background goes x-ray level (noaa, u.s.a.); m, x = number of x-ray flares in m and x class, see below (noaa, u.s.a.) fig. 2a. daily vertical tec at asi during disturbed 5 march 2002 day. 652 angela vernon and ljiljana r. cander fig. 2b. daily vertical tec at asi during disturbed 6 march 2002 day. fig. 2c. daily vertical tec at asi during quiet 8 march 2002 day. 653 regional gps receiver networks for monitoring local mid-latitude total electron content fig. 3a,b. night-time vertical tec from os (a) and asi (b) on 4 march 2002. a b 654 angela vernon and ljiljana r. cander the possibility of using the kriging contouring technique, which behaves nonlinearly with respect to the spatial co-ordinates (samardjiev et al., 1993; stanisl/awska et al., 1996; cander and vernon, 2000; stanisl/awska et al., 2002), for tec mapping over a limited geographical area in europe is further explored in this paper. grid values are generated at 10 min intervals to show the spatial extent of ionospheric tec changes over the os and asi areas. the kriging contouring of the gps vertical tec data from all stations shown in fig. 1a,b was performed for selected days in march and april 2002. results are presented only for 0005, 1205 ut over the asi area and 0805, 1405 ut over os area in figs. 4a,b and 5a,b, respectively. they reveal that the maps differ significantly by several tec units, which is expected from the structures in tec variations clearly seen at figs. 2a-c and 3a,b. this emphasises a unique opportunity to monitor the ionosphere regularly by numerous gps ground sites on the regional basis. accordingly, there is a considerable spatial/ temporal structure over limited asi and os areas that can be seen on the maps in figs. 4a,b and 5a,b, respectively. the 5 and 31 march minor storms affected the plasmasphere-ionosphere over these two areas in such a way that the tec variation appeared as a negative-value phase. more importantly the common pattern consists of significant tec variability at limited geographical areas. thus, it is obvious that such a fig. 4a,b. maps showing the ionospheric vertical tec over the asi for 6 and 8 march 2002 generated by kriging. contours are labelled in units of tec with fit of data from 1205 (a) and 0005 (b) ut. a b 655 regional gps receiver networks for monitoring local mid-latitude total electron content spatial/temporal structure in the ionospheric and plasmaspheric ionisation during minor geomagnetic storms creates the great difficulty in matching the rapid tec variations in forecasting models and data as storms develop. the physical mechanisms that lead to such a complex behaviour even at this limited area neither can be explained nor forecast easily with current knowledge (fuller-rowell, 1998). 3. discussion and conclusions advantage of the limited area tec observations with the regional gps networks is not only its wide coverage of space but its high spatial and temporal resolution, and wide temporal coverage. it helps in further understanding the relationships between the regional plasmaspheric-ionospheric structure and associated solar-terrestrial events. this should lead to a regional storm forecasting technique based on continuous and rapid measurements over limited areas as these at the asi and os. it is also important for determining the required geographical spacing of tec monitoring locations for different practical applications. the present study has been an area-centred one in which specific examples of regional tec variability were investigated. although ionospheric storms are global phenomena, the findings here emphasise the means for monitoring fig. 5a,b. maps showing the ionospheric vertical tec over the os for 31 march and 5 april 2002 generated by kriging. contours are labelled in units of tec with fit of data from 1405 (a) and 0805 (b) ut. a b 656 angela vernon and ljiljana r. cander and possibly forecasting storms and perturbations of the ionosphere over different spatial and temporal scales. the aim of any trans-ionospheric propagation-modelling project is to study the variability of the various physical parameters, such as the ionospheric critical frequency f0f2 and tec and relate them to solar-terrestrial events. therefore, the database of high time resolution tec gps receiver networks can be used for developing regional plasmaspheric-ionospheric model as a part of global ionospheric and space weather models. coupled with global real-time monitoring and warning systems, such a model will be able to identify the onset and location of the ionospheric storm conditions and forecast the immediate future. acknowledgements the radiocommunications agency of the dti has funded this work. the authors would like to express appreciation to the italian space agency and the ordnance survey u.k. for providing rinex data as well as to dr l. ciraolo for using his ‘tecmake’ software for tec evaluation. references bishop, g.j., a.j. mazzella, e.a. holland and s. rao (1996): an overview of ionospheric effects and mitigation in rf communication, navigation and surveillance, in proceedings of the ionospheric effects symposium, alexandria, u.s.a., 11-21. bradley, p.a. and lj.r. cander (2002): proposed terminology for the classification and parameters for the quantification of variability in ionosphere morphology, ann. geophysics, 45 (1), 97-103. cander, lj.r. and a. vernon (2000): tec mapping over europe by kriging contouring technique, paper presented at millennium conference on antennas and propagation, april, davos, switzerland, cd-rom p1172.pdf. cander, lj.r. and l. ciraolo (2002): first step towards specification of plasmaspheric-ionospheric conditions over europe on-line, acta geod. geophys. hung., 37 (2-3), 153-161. ciraolo, l. (1993): evaluation of gps l2-l1 biases and related daily tec profiles, in proceedings of the gps/ ionosphere workshop, neustrelitz, germany, 90-97. ciraolo, l. (2000): results and problems in gps tec evaluation, in proceedings of the radio communications research units, 1st gps tec workshop, chilton, u.k., 47-60. fuller-rowell, t.j., m.v. codrescu, r.j. moffett and s. quegan (1998): response of thermosphere and ionosphere to geomagnetic storms, j. geophys. res., 99, 3893-3914. prölss, g.w. (1995): ionospheric f-region storms, in handbook of atmospheric electrodynamics, edited by h. volland (crc press/boca raton), vol. 2, 195-248. rush, c. (1972): improvements in ionospheric forecasting capability, afcrl-72-0138, environmental research papers, n. 387, air force cambridge research laboratory, bedford, massachusetts, u.s.a. samardjiev, t., p.a. bradley, lj.r. cander and m.i. dick (1993): ionospheric mapping by computer contouring techniques, electron. lett., 29 (20), 1794-1975. stanisławska, i., g. juchnikowski and lj.r. cander (1996): kriging method of ionospheric parameter f 0 f 2 instantaneous mapping, ann. geofis., 39 (4), 845-852. stanisławska, i., g. juchnikowski, lj.r. cander, l. ciraolo, p.a. bradley and a. swiatek (2002): kriging method of tec instantaneous mapping, adv. space res., 29 (6), 1017-1020. (received may 6, 2002; accepted october 4, 2002) s u l l e m o d e r n e m i s u r e g r a v i m e t r i c h e e s e g u i t e i n a f r i c a giovanni boaga d u r a n t e gli a n n i c h e p r e c e d e t t e r o l a s e c o n d a g u e r r a m o n d i a l e v e n n e r o e f f e t t u a t e nel c o n t i n e n t e a f r i c a n o n u m e r o s i s s i m e m i s u r e g r a v i m e t r i c h e da p a r t e di o p e r a t o r i i n g l e s i , f r a n c e s i ed i t a l i a n i . 1 r i s u l t a t i finora o t t e n u t i , i n t e g r a l i da s u c c e s s i v e d e t e r m i n a z i o n i e c o l l e g a l i fra l o r o , p o r t e r a n n o n u o v a l u c e sui p r o b l e m i g e o l o g i c i e g e o f i s i c i c o n p e d a l e r i g u a r d o a q u e l l i c h e i n t e r e s s a n o l e d i s l o c a z i o n i s o t t e r r a n e e di q u e s t a v a s t a r e g i o n e , c h e p r e s e n t a : l u n g o t u t t o il suo p e r i m e t r o e s t e s e z o n e con ( p i o t e p i c c o l e , i n f e r i o r i a 2 0 0 m e t r i : f r a t t u r e in a l l i n e a m e n t i m e r i d i a n i c o s t i t u i t e dai laghi \ i a s s a , t a n g a n i c a , r o d o l f o , \ i n o l i a , a l b e r t o , c h e poi c o n t i n u a col c o r s o del n i l o . q u e s t o s i s t e m a di fi at t u r e , c o m p r e s o nel fuso m e r i d i a n o di c i r c a c i n q u e g r a d i di a m p i e z z a , l i m i t a l o d a l l e l o n g i t u d i n i 3 0 " e 3 5 " ad f.s| di g r e e n w i c l i . p r e s e n t a n o t e v o l i v a r i a z i o n i di d e n s i t à t e r r e s t r e e di m a g n e t i s m o : c c d r a n c h e di a r e e s i s m i c h e di q u a l c h e i m p o r t a n z a e t a l v o l t a da m a n i f e s t a z i o n i v u l c a n i c h e . si n o l a n o a n c o r a m a s s i c c i m o n t a g n o s i c h e r a g g i u n g o n o f a c i l m e n t e i 3 0 0 0 m e t r i e p e r f i n o i -1000 m e t r i ed o l t r e : c s i si s v i l u p p a n o lungo la p a r i e o c c i d e n t a l e e m e r i d i o n a l e del c o n t i n e n t e : ira questi n o t e v o l e è l ' a l t o p i a n o e t i o p i c o c h e si s v i l u p p a nel fuso m e r i d i a n o a d i a c e n t e a q u e l l o d i a n z i a c c e n n a l o d e l l e f r a t t u r e e c l i c c o m p r e n d e a n c h e n e l l a p a r i e m e r i d i o n a l e il k e n i a ed il k i l i m a n g i a r o c l i c sj e l e v a n o a q u o t e m o l t o e l e v a l e (52-12 m e 5 9 3 0 m ris p e t t i v a m e n t e ) . la p a r t e s e t t e n t r i o n a l e e q u e l l a n o r d o r i e n t a l e è car a t t e r i z z a t a dai d e p o s i t i e o l i c i , da f o r m a z i o n i p a l e o z o i c h e « a r e n a r i e d e s e r t i c h e » e più o m e n o f o s s i l i f e r e ( s a h a r a , e c c . ) . n e l l a p a r t e (•( lil i a l e ed in q u e l l a o c c i d e n t a l e -ì n o l a n o i s e d i m e n t i a r c a i c i e a l g o n k i a n i a l l e v o l l e più o m e n o m e t a m o r f o s a t i e r o c c e c r i s t a l l i n e a n t i c h e . nella p a r i e o c c i d e n t a l e a n o r d d e l l ' e q u a t o r e fino q u a s i a l l a l a t i t u d i n e lo" p r e d o m i n a n o le r o c c e e r u t t i v e di e t à r e l a t i v a m e n t e r e c e n t e e f o r m a z i o n i del g i u r a l i a s e del p a l l o g e n c p r e v a l e n t e m e n t e m a r i n e . g l i o c e a n i a t l a n t i c o ed i n d i a n o c h e b a g n a n o le coste o r i e n t a l e ed o c c i d e n t a l e del c o n t i n e n t e , r a g g i u n g o n o g r a n d i p r o f o n d i t à ( c i r c a 6 0 0 0 n i ) s l u . e m o d e r n e s i i s i r e g r a v i m e t r i c h e e s e t . l i t e in a f r i c a 3 2 5 a n c h e a n o n g r a n d e distanza d a l l e c o s t e . iv i n d u b b i o c h e in t a l e c o n t i n e n t e , c h e p r e s e n t a cosi n o t e v o l i e m a r c a t e c a r a t t e r i s t i c h e f i s i c h e , g r a n d e d e v e r i s u l t a r e il gioco d e l l e a n o m a l i e g r a v i m e t r i c h e ed int e r e s s a n t i le c o n c l u s i o n i che si p o s s o n o t r a r r e . n e l l ' a f r i c a o r i e n t a l e gli i n g l e s i ( b u l l a r d , 1933-193-1, h o r s l i e l d e l i s s e l l , 1 9 3 5 ) o s s e r v a r o n o c o i i i p l c s > i v a m c i i l c 9 4 s t a z i o n i g r a v i m e t r i c h e , così d i s t r i b u i t e : 2 3 n e l k e n i a , 16 n e l l ' u g a n d a , 8 nel congo b e l g a . 12 n e l t a n g a n i c a , 2 n e l s u d a n — ossia n e l l a f a s c i a e q u a t o r i a l e c o m p r e s a f r a le l a t i t u d i n i + 4a ( n o r d ) e — 10" ( s u d ) — ed una s t a z i o n e a c a p o t w o n , a l l a l a t i t u d i n e di 34° a sud. l e m i s u r e v e n n e r o e f f e t t u a t e c o n d u e p e n d o l i di invai' o s c i l l a n t i c o n t e m p o r a n e a m e n t e in a r i a r a r e f a t t a . l e d e t e r m i n a z i o n i di t e m p o v e n n e r o eseguite c o n ricezioni r a d i o t e l e g r a f i c h e . n e l l e p r e d e t t e s t a z i o n i i l b u l l a r d m o l t o o p p o r t u n a m e n t e inc l u s e tre stazioni ( d a r c s s a l a a m . m o s c h i n i . p a n g a n i ) già o s s e r v a l e nel 1 8 9 9 1 9 0 0 da k o h l s c h ù l t e r , o t t e n e n d o v a l o r i i n f e r i o r i di 5 , 6 , 7 m i l l i g a l r i s p e t t i v a m e n t e , da q u e l l i o t t e n u t i da k o h l s c h ù t t c r . c l i c p r e s e n t a n o e r r o r i m e d i d e l l ' o r d i n e di : 5 m i l l i g a l . m e n t r e quelli inglesi h a n n o i n c e r t e z z e i n f e r i o r i o e g u a l i a 1 m i l l i g a l . p a r t i c o l a r e i m p o r t a n z a p r e s e n t a la s t a z i o n e di c a p o t w o n , dove si h a n n o o s s e r v a z i o n i g r a v i m e t r i c h e , r i p e t u t e p e r o l t r e un s e c o l o . il v a l o r e d e t c r m i n a t o da b u l l a r d ( 9 7 9 , 6 4 5 g a l ) r i s u l t a s u p c r i o r e di app e n a 3 m i l l i g a l d i q u e l l o d e t e r m i n a t o nel 1897 da r e i l c r c l a n k e sis t e m a t i c a m e n t e i n f e r i o r e di 10 e p e r f i n o 2 0 m i l l i g a l dai v a l o r i det e r m i n a t i da a l t r i o s s e r v a t o r i negli a n n i .1910. 1 8 9 8 , 1 8 9 5 , 1890. 1 8 3 9 . 1 8 2 9 e 1 8 1 8 . ìnella m e m o r i a di b u l l a r d ( ' ) dove v e n g o n o d a t e a m p i e r e f e r e n ze sulle m i s u r e e s e g u i t e , v e n g o n o a l t r e s ì discussi i r i s u l t a t i o t t e n u t i e r i p o r t a t i i n t e r e s s a n t i grafici e c a r t e r i p r o d u e c n t i le c u r v e i s o a n o m a l e p e r la c o r r e z i o n e in a r i a l i b e r a o di l ' a v e , p e r q u e l l a di b o u g u e r e p e r la c o r r e z i o n e i s o s t a t i c a h a v f o r d i a n a estesa fino a l l a p r o f o n d i t à di i 13.7 k m . d a l l ' e s a m e d e l l e a n o m a l i e si p u ò c o n c l u d e r e c h e l ' a l t o p i a n o a f r i c a n o — n e l l a r e g i o n e b a t t u t a — è in e q u i l i b r i o i s o s t a t i c o , f a t t a e c c e z i o n e p e r la \ a l l e del h i l l , dove a p p a r e una f o r t e d e f i c i e n z a g r a v i m e t r i c a . l e q u o t e d e l l e stazioni v e n n e r o d e t c r m i n a t e col m e t o d o t e r m o b a r o m e t r i e o . n e l l e c o r r e z i o n i , p e r la d e n s i t à t e r r e s t r e s u p e r f i c i a l e , si t e n n e c o s t a n t e m e n t e il v a l o r e m e d i o 2 , 6 7 . d e l l e 9-1 s t a z i o n i b e n 61 s u p e r a n o i 1 0 0 0 in di q u o t a , di q u e s t e . -1 s u p e r a n o i 2 0 0 0 m . l a staz i o n e pili alta ( t h o m s o n ' s f a l l s ) h a la q u o t a 2 3 3 9 i n . a d o t t a n d o p e r il c a l c o l o d e l l a g r a v i l a n o r m a l e la f o r m u l a di g i o v a n n i boaga h e l m e r t ( 1 9 0 j ) le a n o m a l i e i s o s t a l i c h e r i s u l t a n o i n p a r t e p o s i t i v e ed in p a r t e n e g a t i v e . i n v a l o r i a s s o l u t i r i s u l t a n o c o s i d i s t r i b u i t e : l e a n o m a l i e i s o s t a l i c h e c o m p u t a n d o la g r a v i t à n o r m a l e con la f o r m u l a i n t e r n a z i o n a l e ( 1 9 3 0 ) -i o t t e n g o n o da q u e l l e r i c a v a t e con la f o r m u l a di h e l m e r t , a g g i u n g e n d o — 19 m g a l . g l i o p e r a t o r i f r a n c e s i ( l a g n i l a , e c c . ) ( ) n e g l i a n n i 1 9 3 3 , 1 9 3 5 e 1 9 3 6 e s e g u i r o n o n e l l e c o l o n i e f r a n c e s i a f r i c a n e n . 5 0 s t a z i o n i g r a v i m e t r i c h e allo scopo di o t t e n e r e l e a n o m a l i e del m a s s i c c i o di b o u z a r è o h , li. 3 0 stazioni in a l g e r i a , n . 7 n e l m a r o c c o , n . 13 n e l g r a n d e k a b y e i e . esse c o m p r e n d o n o d e t e r m i n a z i o n i e f f e t t u a t e c o l p e n d o l o ( r e t e f o n d a m e n t a l e ) e d e t e r m i n a z i o n i di d e t t a g l i o e s e g u i t e eoi g r a v i m e t r i h o h v c c k l e j a y . il c a p . r e i g n i e r h a eseguito con la m e n s o l a b i p e n d o l a r e m i o n i , a p p a r t e n c i i t c a l c o m i t a t o n a z i o n a l e f r a n c e s e e c o r r e d a t a da q u a t t r o p e n d o l i in b r o n z o , t i p o s t e r n e c k , il r i l e g a m e n l o di p a r i g i c o n l"()ss c r v a t o r i o d i a v e r r h o è s , n e l l e v i c i n a n z e d i b e r r c c h i d ( m a r o c c o ) o t t e n e n d o il v a l o r e g = 9 7 9 , 5 6 2 g a l . -velie c a m p a g n e d e l 1 9 3 4 e 1 9 3 5 l e 5 0 d e t e r m i n a z i o n i sono state d i s t r i b u i t e f r a le l a t i t u d i n i 3 1 ° 3 0 ' e 3 6 m 8 ' ( n o r d ) e f r a l e l o n g i t u d i n i 5 " 2 5 ' est e 8 1' ovest r i s p e t t o a g r e e n w i c h . n . 15 s t a z i o n i sono s t a t e e f f e t t u a l e a q u o t e s u p e r i o r i a 5 0 0 m e di q u e s t e 3 a q u o t e s u p e r i o r i ai 1 0 0 0 i n . l a s t a z i o n e p i ù a l t a ( a i n ' n ' s o u r ) è in a l g e r i a ( q u o t a 1 0 9 0 m ) . a i v a l o r i osservali v e n n e r o p o r t a t e solo le c o r r e z i o ni di f a y e e di b o u g u e r , q u e s t ' u l t i m a senza c o r r e z i o n e t o p o g r a f i c a , a d o t t a n d o p e r d e n s i t à m e d i a t e r r e s t r e s u p e r f i c i a l e v a l o r i c o m p r e s i f r a 2 , 2 e 2 , 7 . n o n v e n n e c a l c o l a t a la c o r r e z i o n e i s o s l a t i c a . l e a n o m a l i e di b o u g u e r , c a l c o l a t e c o n la f o r m u l a i n t e r n a z i o n a l e ( 1 1 3 0 ) p e r la gravità n o r m a l e , s o n o in p a r t e p o s i t i v e ( n . 2 4 ) ed i n p a r t e n e g a t i v e ( n . 2 6 ) . q u e l l e p o s i t i v e o s c i l l a n o f r a -r 2 e + 9 0 m i l l i g a l , q u e l l e negative f r a — ' 7 7 e — 8 m i l l i g a l . nel 1937 s e m p r e dai f r a n c e s i ( l a g n i l a e r o u x ) v e n n e r o f a t t e a l t r e 82 stazioni n e l s u d a l g e r i n o , nel s a h a r a e n e l s u d a n , in r e g i o n i c o m p l e t a m e n t e i n e s p l o r a t e dal p u n t o di vista g r a v i m e t r i c o , e n e l m a r o c c o . d i q u e s t e , 13 f u r o n o f a t t e n e l s a h a r a s e t t e n t r i o n a l e , 2 4 in q u e i da 0 a 15 m g a l )> 16 » 3 0 » » 3 1 » 5 0 « o l t r e 5 0 m g a l » l i . 5 4 s t a z i o n i » 2 9 » ). 8 » » 3 » s l i . i . e m o d e r n e m i s u r e cra vi m e t r i c h e e s e g u i t e in a f r i c a lo c e n t r a l e c n e l h o g g a r c 21 fra il m a s s i c c i o di h o g g a r ed il l a g o t c l i a d , 2 4 n e l m a r o c c o . n e l 1938 ( l a g n i l a ) v e n n e r o e f f e t t u a t e a l t r e 15 stazioni n e l l a reg i o n e degli a l t o p i a n i , n. 16 n e l l a zona di c o s l a n t i n a , n . 65 in i un i s i a , d o v e la c a m p a g n a fu c o n t i n u a t a fino al 1939. si h a cosi un c o m p l e s s o di 2 7 9 s t a z i o n i e s e g u i t e d a i f r a n c e s i . c i r c a i r i s u l t a t i c o n s e g u i t i con le u l t i m e c a m p a g n e ( 1 9 3 7 3 9 ) osserv i a m o solo c h e p e r la t u n i s i a le c u r v e i s o a n o m a l e grosso m o d o seg u o n o il c o n t o r n o d e l l i t o r a l e , l e s t a z i o n i di b i s e r t a e r a s e l a i n m e t t o n o in e v i d e n z a il c a r a t t e r e m a r i n o del l a g o di b i s e r t a . nella zona di s a l i c i si n o t a l ' e s i s t e n z a di u n a z o n a c o n a n o m a l i a n e g a t i v a : tale zona è p r e s u n t a p e t r o l i f e r a e qui c o n v e r r e b b e e s e g u i r e u n a r e t e g r a v i m e t r i c a di d e t t a g l i o p i ù fitta e c o r r e d a r e l e m i s u r e da a l t r e ind a g i n i . n e l l ' i s o l a di d j e r b a la s t a z i o n e e f f e t t u a t a a a g l i i r p r e s e n t a u n a a n o m a l i a n e g a t i v a l o c a l e . n e l l e p u b b l i c a z i o n i r i c h i a m a l e n e l l a b i b l i o g r a f i a a n n e s s a . s 0 n o r i p o r t a t i tutti i v a l o r i o s s e r v a t i , le q u o t e d e l l e s t a z i o n i , le a n o m a l i e . a n c h e p e r q u e s t e d e t e r m i n a z i o n i v a l g o n o le o s s e r v a z i o n i fatte p e r le p r e c e d e n t i e s e g u i t e n e g l i a n n i 1 9 3 5 , 1 9 3 6 e 1937 p e r q u a n t o si r i f e r i s c e a l l e c o r r e z i o n i a p p o r t a t e a l l e g r a v i t à o s s e r v a t e . i n a l g e r i a su iii d e t e r m i n a z i o n i si h a n n o s o l t a n t o li a n o m a l i e di b o u g u e r p o s i t i v e . n e l s u d a n e n e l s a h a r a su 4 7 d e t e r m i n a z i o n i s o l t a n t o 4 a n o m a l i e d i b o u g u e r r i s u l t a n o p o s i t i v e e di i m p o r t i + ' i, 49 . 1 1 . + 36 m i l l i g a l . l a m a s s i m a a n o m a l i a n e g a t i v a si b a ad a d a r a j e r e n e ( — 125 m i l l i g a l ) di (piota 211)0 i n . nel m a r o c c o si h a n n o 7 a n o m a l i e p o s i t i v e -u 2 4 d e t e r m i n a z i o n i . l a m a s s i m a a n o m a l i a n e g a t i v a p e r u n a s t a z i o n e di ([nota 1 5 0 0 m r a g g i u n g e il v a l o r e 97 m i l l i g a l ( e n j i l des a l t l a h s e n c ) . i n a l g e r i a su 37 d e t e r m i n a z i o n i si n o t a n o 13 a n o m a l i e p o s i t i v e ed in t u n i s i a su 65 d e t e r m i n a z i o n i sì h a n n o 33 a n o m a l i e di b o u g u e r posi ti ve. questa c a m p a g n a h a m e s s o in e v i d e n z a un e r r o r e s i s t e m a t i c o e quasi c o s t a n t e di u n a v e n t i n a di m i l l i g a l f r a l e m i s u r e f a t t e ad mgeri da d e f f o r g e s e b o u r g e o i s nel 1 8 9 0 e da b o u r g e o i s n e l 1 8 9 2 : a p h i l i p p e v i l l e , a n l e d . r i m i n o m i e m è d é a l i , a n c o r a osservate da b o u r geois negli a n n i 1 8 9 1 , 1 8 9 2 e 1 8 9 3 . i m p o r t a n z a p a r t i c o l a r e p o i a s s u m o n o l e m i s u r e g r a v i m e t r i c h e eseg u i t e in e t i o p i a d o p o il 1937 da o p e r a t o r i i t a l i a n i : b a l l a r i n ( ' ' ) , e p a c c l l a ( ' ) . il p r i m o di questi o p e r a t o r i p r e s e p a r t e con la m i s s i o n e g e o l o g i c a p r o m o s s a dall a . g . i . p . per r i l e v a m e n t i geofisici ad h a r r a r , •128 g i o v a n n i boaga g i g g i g a , d a g a b u r , a u b a r r e , aiscia. d e s i è , s a r d o , t e n d a h ò , b a l i e , d i r e d a m i , g a u a s i . i ilio dove v e n n e r o p u r e eseguite ( d a b a l l a r i n ) n u u r e di l a t i t u d i n e a l r o n o m i e a secondo il m e t o d o di h o r r e b o w l a l c o t t ed in t a l u n e ( a z i o n i a n c h e m i s u r e di l o n g i t u d i n i a s t r o n o m i c h e . l e osservazioni g r a v i m e t r i c h e c l i c v e n n e r o e f f e t t u a l e in tale o c c a s i o n e dal b a l l a r i n -i r i f e r i s c o n o a sei l o c a l i t à : h a r r a r , g i g g i g a , d a g a b u r , a u b a r r e , a i c i à . d i r e d a u a , a q u o t e v a r i a b i l i f r a 7 6 0 m c 1b47 m . l e f o r m a z i o n i g e o l o g i c h e d e l l e l o c a l i t à c i r c o s t a n t i le s t a z i o n i nell ord i n e c i t a t o , s o n o : g r a n i t o i n i e t t a t o in c i s t i c r i s t a l l i n i , s a b b i e a l l u v i o n a l i con sottosuolo p r o f o n d o g r a n i t i c o e scisto c r i s t a l l i n o , s a b b i e a l l u v i o n a l i c o n s o t t o s u o l o c a l c a r e o , scisti c r i s t a l l i n i , b a s a l t i s l r a t o i d i , s a b b i e a l l u v i o n a l i su a r e n a r i e r i c o p r e n t i c a l c a r i . g l i s t r u m e n t i a d o p e r a t i sono q u e l l i stessi a d o p e r a t i dal p r o f . dot e e dal p r o f . b a l l a r i n n e l 1 9 2 8 p e r le m i s u r e g r a v i m e t r i c h e eseguite s u l l ' a p p e n n i n o l o s e o e m i l i a n o , e c i o è : a p p a r a t o q t i a d r i p c n d o l a r e b a i n b e r g c o r r e d a l o di c a m p a n a p n e u m a t i c a . 11 t e m p o v e n n e d e t e r m i n a l o o con m i s u r e a s t r o n o m i c h e ( h a r r a r , g i g g i g a ) o c o n d e t e r m i n a z i o n i r a d i o t e l e g r a f i c h e ( p e r le a l t r e s t a z i o n i ) r i c o r r e n d o ai segnali o r a r i trasmessi da r u g b y . c r o n o m e t r i i m p i e g a l i : d c n t n . 1 9 9 6 2 , h a w c l k n . 5 0 e p e n d o l o r i e f l e r n . 1 0 9 6 ; i n s e r i t i n e l c i r c u i t o d e l r e l a i s d e l l e c o i n c i d e n z e . a l l e g r a v i t à o s s e r v a l e , o t t e n u t e con le o s c i l l a z i o n i m e d i e dei q u a t t r o p e n d o l i , s o n o s t a t e a p p o r t a t e le r i d u z i o n i di f a y e e di b o u g u e r , a d o t t a n d o p e r d e n s i t à s u p e r f i c i a l e il v a l o r e 2 . 6 7 . l e g r a v i t à n o r m a l i , c o m e è c o n s i g l i a t o d a l l a a . g . g . i . , sono state c a l c o l a t e c o n la f o r m u l a i n t e r n a z i o n a l e . l e q u o t e d e l l e singole stazioni sono s t a t e d e t c r m i n a t e p e r m e z z o di b a r o m e t r i ( f o n i t i ) a s s u m e n d o c o m e q u o t a ili p a r t e n z a il v a l o r e 1 2 0 0 m c o r r i s p o n d e n t e al p i a n o dei b i n a r i d e l l a s t a z i o n e f e r r o v i a r i a di d i r e d a u a . ed a p p l i c a n d o la f o r m u l a di l a p l a c e r i i h l n i a n n . l e p r e c i s i o n i c o n s e g u i t e n e l l e d e t e r m i n a z i o n i g r a v i m e t r i c h e r i s u l t a n o degli i m p o r t i c o m p r e s i n e l l ' i n t e r v a l lo 1 0 , 9 0 m i l l i g a l . 0 . 9 9 m i l l i g a l . l e a n o m a l i e di f a y e sono tutte p o s i t i v e , q u e l l e di b o u g u e r tutte n e g a t i v e . e s s e sono r i a s s u n t e n e l l o s p e c c h i o s e g u e n t e : stazione altezza gravità osservata anom. faye anom. bouguer i f a n a r it, 17 m 9 7 7 . 6 5 4 pai + 38 m g a l — 171 n m a l 1 6 3 4 , 7 1 1 + 2 9 156 d a g a h u r 1 0 8 9 . 8 2 3 + 4. — 120 a u b a r r e 1 6 0 3 . 7 5 5 + 50 — 131 a i c i à 760 9 7 3 . 0 4 2 + 47 4 5 d i r e d a u a 1 2 0 5 9 7 7 . 8 2 1 0 1 3 6 . s u l l e m o d e r n e m i s u r e g r a v i m e t r i c h e e s e g u i t e in a f r i c a 3 2 9 i l g r u p p o di d e t e r m i n a z i o n i g r a v i m e t r i c h e e s e g u i t e d a l p r o f . g . b . p a c e l l a , p e r i n i z i a t i v a e con m e z z i e s t r u m e n t i del n o s t r o i . g . m . c o m p r e n d e 1 4 s t a z i o n i d i s t r i b u i t e f r a l e l a t i t u d i n i 8 ° 5 4 ' e 1 1 " 3 0 " n o r d e di q u o t e c o m p r e s e f r a 8 0 0 m e 2 9 0 0 m . v e n n e i m p i e g a t o u n app a r a t o g r a v i f o t o g r a f i c o c o n y n g a m c o n tre p e n d o l i in i n v a r e c o l t e l l i di s t e l l i l e , c r o n o m e t r o m e r c e r , c r o n o g r a f o \ a r d i l i , a p p a r a t o r a d i o i . g . m . p e r la r e c e z i o n e dei s e g n a l i o r a r i p e r l e d e t e r m i n a z i o n i di t e m p o . v e n n e r o a l t r e s ì u s a t e p e l l i c o l e f e r r a n i a e lo s p o g l i o di q u e s t e v e n n e e f f e t t u a t o c o l m e t o d o di m e i n e s z . l a p r e c i s i o n e d e l l e d e t e r m i n a z i o n i g r a v i m e t r i c h e è di c i r c a 5 m i l l i g a l . l e q u o t e f u r o n o d e t e r m i n a t e col m e t o d o t e r m o b a r o m e t r i c o e f u r o n o a n c h e c a l c o l a t e l e r i d u z i o n i di f a v e e di b o u g u e r . o t t e n e n d o i s e g u e n t i r i s u l t a t i : stazione altezza gravità osservala unum. i'aya animi. bouguer m e l a h a r a 9 4 8 i i l 9 7 7 8 6 3 g a l — 1 6 n i g a l 1 2 7 n i g a l a d d i s a b e b a 2 4 2 6 , 4 6 5 + 3 8 — 2 4 6 p o n t e a u a s c 8 3 3 , 8 9 7 — 2 2 — 1 1 9 h a r r a r 1 8 5 4 . 6 3 3 + 2 1 — 1 9 6 g i g g i g a 1 6 3 3 , 7 1 4 + 33 — 1 5 8 d i r e d a u a 1 2 1 6 . 8 2 4 + 7 — 1 3 5 d e l i r a d e r h a 2 8 5 4 . 4 1 1 + 9 7 — 2 3 7 d e l i r a s i n a 2 7 1 6 •423 62 — 2 5 6 g i a i r à 1 4 5 6 . ( 5 7 — 1 5 — 1 8 5 c o n i l t o l e i à 1879 , 6 5 4 — 5 — 2 2 5 d e s s i è 2 7 0 3 . 4 7 2 6 6 — 2 5 0 b a l i e 1 6 7 6 , 7 6 0 + 3 4 — 1 6 2 c a n t i e r e , 4 4 9 1 3 , 9 5 3 — 1 0 — 1 1 7 u s c i a l l i 1 8 7 9 , 6 3 7 — 3 7 — 2 5 7 l e a n o m a l i e sono s t a t e d e t c r m i n a t e c o n la g r a v i t à n o r m a l e f o r n i t a d a l l a f o r m u l a i n t e r n a z i o n a l e ( 1 9 3 0 ) . t r e d e l l e p r e c e d e n t i s t a z i o n i e p r e c i s a m e n t e : h a r r a r , g i g g i g a , d i r e d a m i sono state o s s e r v a t e a n c h e dal p r o f . b a l l a r i n . p o i c h é n e i d u e p r o s p e t t i le q u o t e d e l l e s t a z i o n i sono d i f f e r e n t i , il c o n f r o n t o d e v e e s s e r e f a l l o c o n l e a n o m a l i e di f a v e . si o t t e n g o n o c o s ì l e d i f f e r e n z e : + 17 m i l l i g a l , -f 4 m i l l i g a l , — 7 m i l l i g a l , c h e r i e n t r a n o t i n t e e n t r o il t r i p l o d e g l i e r r o r i m e d i d e l l e d i f f e r e n z e s i e s t e , c o n s i d e r a t e c o m e d i f f e r e n z e di q u a n t i t à o s s e r v a t e , ed a f f e t t e da u n e r r o r e m e d i o n o n s u p e r i o r e a 6 m i l l i g a l . nei c a l c o l i p e r la c o r r e z i o n e di b o u g u e r il p r o f . p a c e l l a t e n n e p e r d e n s i t à m e d i a s u p e r f i c i a l e t e r r e s t r e il v a l o r e 2 , 8 . :j:ì() i . 1 0 v a n m hoai.a d o m e os-c'ivu il p r o f . macella, in nn t e r r e n o c o m e i a e r q e o r o ahi-.-ino "i i n i ] ) o r r e l ) h e una m a g g i o r cur a nella a p p l i c a z i o n e della riduz i o n e di b o u g u e r . c l i c in ogni m o d o d o v r e b b e , a p p e n a p o s s i b i l e , essere c o m p l e t a t a a l m e n o d a l l a r i d u z i o n e t o p o g r a f i c a fino a 4(1 k m dai punti staziono e c i ò o p r a i i ut i o p e r il v a l o r e c h e si de\c a t t r i b u i r e alla d e n s i t à s u p e r f i c i a l e nei singoli c o m p a r t i m e n t i . c o n f r o n tando le a n o m a l i e di b o u g u e r r i p o r t a t e noi due s p e c c h i , con le q u o t e d e l l e s t a z i o n i , si vedo s u b i t o c h e r i s u l t a n o c o n f e r m a t o le ben n o t e teor i e i s o s t a l i c h e . ci a u g u r i a m o c h e questi lavori u r a v i m e t r i o i . già iniziati su v a s t a -cal a da ii ' inglii i t e r r a , d a l l a f r a n c i a e d a l l ' i t a l i a , sui l e n i t o l i a f r i cani d e l l e loro g i u r i d i z i o n i p o l i t i c h e , n a n o al più p r e s t o r i p r e s i e p o t e n z i a t i , o n d e p o t e r in b r e v e a d d i v e n i r e a d e l l e c o n o s c e n z e c i r c a la s t r u t t u r a s o t t e r r a n e a di q u e s t o g r a n d e c o n t i n e n t e c h e p r e s e n t a forti legami geologici con l ' e u r o p a . roma — istillilo (li geodesia e tojìografia, delia facoltà ili ingegneria 1918. ri tssu\to si riferisce sulle misure gravimetriche eseguite in ijrica dal 1933 al 1939 da operatori inglesi, francesi, italiani, con apparati ]>cndolari e cori gravimetri e si riportano i valori ottenuti dagli operatori italiani nelle misure effettuate in etiopia. b i b l i o g r a f i \ l'i e . ('.. b l u \iii>. (irarily measuremeiiis in fusi lirica. « l ' h i l . t r a n s , o ! t h e r o v a i s o c i e t y ot l o n d o n ». s e r i e \. m a t l i e n i . and l ' i n s . s e . il. 7 5 7 . v o i . 235j). 1-15-531. i'-| j. laciiii.a. mesures ile iinlensilé ile hi iicsantctir eri urit/ne ila \oril. « 0 . l i . acuii. s c i . « t . 1 9 9 , 1 9 3 4 . p . 1 5 8 6 : t . 2111. 1 9 3 5 . p . 1 0 9 5 : i'. 2(11. 1 9 . ( 7 . p . 1 0 8 ; t . 2 0 6 . 1 9 3 7 . p . 1 5 5 3 : t . 2 0 8 . 1 9 3 9 . p . 7 3 1 : t . 2 0 8 . 1 9 3 9 . p . 1 2 0 7 : t . 2 0 8 . i 9 3 9 , p . 1627. g . r o l x . mesures ile l'intonsi/è de in /.esunteur un murar, u ( . . i ! . \ a d . s c i . „ t . 2117. 1 9 3 8 . p . 7 1 3 . i :> s . lìai.lmun. misure asirinmniiche e «eojìsieue. l s l r a t l o ila « m i s s i o n e g e o l o gica nella d a n c a l i a m e r i d i o n a l e e nel l ì a r r a r i n o ». p r o m o s s a d a l l a \ . g . 1 . i ' . . v o l u m e i l r o m a . i'm2. r e a l e \cad. d ' i t a l i a . o n l r o l u d i p e r i a f r i c a o r i e n l a l e i t a l i a n a . l ' i g . il l ' a d j h . deterniiiinziiini griiiimeiriclie in l'jìopia. t< h o m i . s e i . \ccad e m i a n a z i o n a l e dei l i n c e i ». :iniu> 1 9 1 8 . r o m a . coseismic effects of the 2016 amatrice seismic sequence: first geological results annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 1 coseismic effects of the 2016 amatrice seismic sequence: first geological results emergeo w.g.: pucci s. *, de martini p.m. *, civico r. *, nappi r. *, ricci t. *, villani f. *, brunori c.a. *, caciagli m. *, sapia v. *, cinti f. r. *, moro m. *, di naccio d. *, gori s. *, falcucci e. *, vallone r. *, mazzarini f. *, tarquini s. *, del carlo p. *, kastelic v. *, carafa m. *, de ritis r. *, gaudiosi g. *, nave r. *, alessio g. *, burrato p. *, smedile a. *, alfonsi l. *, vannoli p. *, pignone m. *, pinzi s. *, fracassi u. *, pizzimenti l. *, mariucci m.t. *, pagliuca n. *, sciarra a. *, carluccio r. *, nicolosi i. *, chiappini m. *, d’ajello caracciolo f. *, pezzo g. *, patera a. *, azzaro r. *, pantosti d.*, montone p. *, saroli m.° *, lo sardo l. ° *, lancia m. °, *istituto nazionale di geofisica e vulcanologia °university of cassino and southern lazio stefano.pucci@ingv.it abstract since the beginning of the ongoing amatrice seismic sequence on august 24, 2016, initiated by a mw 6.0 normal faulting earthquake, the emergeo working group (an ingv team devoted to earthquake aftermath geological survey) investigated coseismic effects on the natural environment. up to now, we surveyed about 750 km2 and collected more than 3200 geological observations including differently oriented tectonic fractures together with intermediateto smallsized landslides. the most impressive coseismic evidence was found along the known active mt. vettore fault system, where surface ruptures with clear vertical/horizontal offset were observed for more than 5 km, while unclear and discontinuous coseismic features were recorded along the laga mts. fault systems. i. introduction n august 24, 2016, at 1:36 utc an mw 6.0 earthquake hit a large portion of the central apennines fold and thrust belt, between the towns of norcia and amatrice, central italy (fig.1). the event nucleated at a depth of 8.2 km, with epicenter close to the village of accumoli (fig. 1) [ama_loc the mainshocks caused heavy damage in a wide area between the towns of norcia and amatrice and resulted in 298 fatalities, thousands of injured and over 3000 displaced people. focal mechanisms of the two main events show nw-se striking normal faulting (fig. 1). the seismic sequence was confined within the o mailto:stefano.pucci@ingv.it annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 2 upper 10 12 km of the crust and the volume affected by the aftershocks extends for about 50 and 15 km in the nw-se and ne-sw directions, respectively. this area of the central apennines is characterized by a quaternary extensional regime overprinting ne-verging thrust-sheets (i.e. vai and martini, 2001 and references therein), mostly made of meso-cenozoic carbonate rocks and miocene flysch deposits. the resulting dense array of nw-se and nnw-sse striking, mainly swdipping, up to 30 km-long active normal fault systems (galli et al., 2008 and references therein; fig. 1) accommodate the present-day 2-4 mm/yr regional ne-sw extension (galvani et al., 2013, and references therein). in this paper we present the dataset of coseismic effects on the natural environment collected by the emergeo working group together with a preliminary discussion. figure 1. location of the measurements and observations collected in the field. normal faults are compiled from geological maps [centamore et al., 1992; pierantoni et al., 2013]. hillshade from tinitaly 10m-dem [tarquini et al., 2012]. the 24 august 2016 mainshock and largest aftershock are reported with red stars. tdmt focal mechanisms available at http://cnt.rm.ingv.it/event/7073641 and http://cnt.rm.ingv.it/event/7076161. dgsd (deep-seated gravitational slope deformations) with evidence of coseismic fracturing are shown. http://cnt.rm.ingv.it/event/7073641 http://cnt.rm.ingv.it/event/7076161 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 3 ii. methods field surveying of the coseismic effects by emergeo was mainly performed by using smartphones equipped with a range of software that use the phone's gps, compass and orientation sensors. we used the rocklogger application (rockgecko.com) to measure and store the data of the coseismic effects (type of observation, strike, vertical dislocation, fracture opening, etc.). gps track loggers were used for monitoring and subsequent planning the survey coverage. low-altitude, easy deployable, aerial platforms (helikite and unmanned aerial vehicles uavs), together with a helicopter flight, have been used to support the post-earthquake documentation of surface ruptures. a webbased data sourcing was utilized to gather information on coseismic geological effects from local people. the collected data have been stored in a georeferenced-database (for details see emergeo working group, 2012). figure 2. map of the coseismic geological effects of the mt. vettore area. geological map from pierantoni et al. [2013]. hillshade from tinitaly 10m-dem [tarquini et al., 2012]. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 4 iii. results we surveyed an area of about 750 km2, recording more than 3200 measurements and observations. among them, we collected: 230 coseismic fracture (<1 cm displacements) and 2600 coseismic rupture (>1cm displacements) measurements; evidence for 160 mediumto small-sized coseismic landslides, 28 coseismic features related to the shaking (e.g. clast extrusions, soil remobilization, etc.), and 130 locations where we documented the lack of coseismic effects. in addition, a fracturing pattern is hypothesised in the trench sector and along the upper limit of 3 dgsd (deepseated gravitational slope deformations) between the villages of castelluccio and norcia as due to coseismic-induced acceleration (fig. 1). the inspection of the laga mts. fault system provided only few sparse observations of discontinuous (maximum 300 m-long) equivocal ruptures with small displacements (maximum 5 cm), mostly concentrated along its northern sector. a few ruptures exhibit a trend parallel to the mapped fault, whereas most of them do not have a systematic pattern. in contrast, in the central-southern portion of this fault, no clear coseismic fracturing or rupturing was observed, but several coseismic landslides (rotational and rock falls) occurred, possibly due to the shear strength of the outcropping pelitic formations. some of them are pre-existing landslides re-activated, partly known and mapped after the 2009 l’aquila earthquake (miccadei et al., 2013). conversely, to the north, along the mt. vettore fault system, a continuous alignment of newly formed coseismic ruptures was mapped for more than 5.2 km. the coseismic ruptures follow the cima del redentore and mt. vettoretto sw-dipping and n155°-striking normal faults (fig. 2). the coseismic ruptures occur along a cumulative fault scarp made by bedrock at the footwall (mostly corniola fm.) and by highly fractured bedrock as well as unconsolidated deposits (generally cryoclastic debris) at the hanging wall. a few additional coseismic ruptures parallel continuously a southern fault splay for more than 400 m along the sw-flank of the mt. vettore, and another discontinuous set occurs at the slope foot, resulting in an alignment 1.2 km long (fig. 2). in detail the 5.2 km-long rupture consists of 56m long segments, each up to 5-6 m-long, mostly displaying a right-stepping en-echelon arrangement (fig. 3, 4a). the deformation zone width is generally confined on a single ground crack, rarely exceeding 5 m where maximum three parallel ground cracks and ruptures were found. the orientation of the different elements composing the coseismic rupture is: 1) n125°-140° for the coseismic ruptures (with offset) affecting unconsolidated deposits; 2) n105°-140° for coseismic fractures (with no offset) affecting unconsolidated deposits; 3) n130°-175° for coseismic ruptures occurring on the bedrock fault planes (coseismic free faces) (fig. 3). notably, the trend of the ruptures south of mt. vettoretto is oblique with respect to the slope, while northward, it becomes perpendicular to the slope of the southwestern mt. vettore flank (fig. 3). the coseismic rupture pattern is not influenced by the bedrock bedding attitude, while locally it re-utilizes joints. the rupture trace preserves its trend, even when cutting through debris cone apexes. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 5 figure 3. geometrical pattern of the mt. vettore coseismic ruptures. the rose diagram shows different trends of the coseismic free face, rupture and fracture measurements with respect to the bedrock bedding and the average trend of the fault trace. on bedrock fault planes, the newly formed free face looks like a fresh, light-coloured stripe of rejuvenated bedrock surface, with no lichens and soil shade (fig. 4b). the coseismic ruptures always down-throw the sw side with vertical displacement being mostly below 16 cm and exceeding 35 cm in few locations of its northern portion (fig. 5a). moreover, the vertical displacement of the coseismic free faces is mostly between 16 and 20 cm. the width of open cracks is mostly below 16 cm with few outliers (up to 40 cm) along its central portion for both coseismic ruptures and free faces (fig. 5b). notably, the almost 2 km long coseismic ruptures south of mt. vettoretto tend to revert the topography. figure 4. photos of the mt. vettore coseismic ruptures. a) en-echelon coseismic ruptures (black arrows) affecting unconsolidated slope deposits and a trail, the red arrows mark the mt. vettore fault scarp; b) coseismic free face on bedrock fault plane (black arrows). iv. discussion and conclusion based on the characteristics and distribution of the coseismic effects related to the 24 august 2016 amatrice earthquake, we identified two areas: the laga mts. fault system, to the south and the mt. vettore fault system to the north. along the northern portion of the laga mts. fault system few sparse evidences of discontinuous coseismic ruptures with small displacements were surveyed, while in the central-southern portion of this fault, no significant fracturing at the surface was observed. in general, along the laga mts. fault system there is no clear deformation as expression of the seismogenic fault plane motion occurred at depth. a possible explanation for this negative evidence is due to the thick sequence (ca. 1-2 km) of flysch and marls, which may accommodate the coseismic annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 6 deformations without a purely brittle behavior. along the mt. vettore fault system sector we documented a localized and continuous concentration of coseismic ruptures with offset. main characteristics are: i) the considerable length of the continuous alignment (more than 5 km); ii) the consistency of the structural pattern (often characterized by right en-echelon arrangement with kinematically coherent oblique transfer zones); iii) the downthrown side is systematically the sw one; iv) the structural pattern, the kinematics and the offset distribution is not influenced by topography and morphology (e.g. erosional scarp and debris cone crosscut, figs. 3 and 5), v) the constant rupture characteristics over both bedrock and loose deposits. all these features lead to infer that the observed displacement could be defined as primary surface faulting (i.e. propagation up to the surface of the seismogenic motion occurred at depth). however, shaking/gravity effects may have locally contributed to part of the observed displacement. shaking/gravity effects may have locally contributed to part of the observed displacement. a number of authors (p. burrato, m. carafa, d. di naccio, u. fracassi, v. kastelic, r. vallone and p. vannoli) are doubtful in straightforwardly interpreting the surveyed brittle features as effects of primary surface faulting. rather, they maintain that the geologic and morphologic conditions have favored the instability of jointed and faulted rocks and unconsolidated sediments under dynamic conditions. therefore, the hypothesis of the observed fractures as secondary earthquake effects driven by blind faulting cannot be ruled out a priori. we all agree on the fact that future detailed comparison with other datasets like seismicity relocations, satellite data (insar and gps) and modeling inversions of different seismic parameters will help in better understanding and evaluating the gravitational component vs tectonic displacement. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 7 figure 5. variation in displacement along the strike of the mt. vettore coseismic ruptures. distribution of a) vertical displacement and b) opening values. the inset shows the frequency of the displacements for ruptures in coincidence of loose deposits (coseismic rupture) and along preexisting bedrock fault planes (coseismic free face). references ama_loc working group (2016). amatrice 2016 main events re-location (v1.0 – 20160902 12.00). istituto nazionale di geofisica e vulcanologia. doi 10.5281/zenodo.61371 centamore, e., adamoli, l., berti, d., bigi, g., bigi, s., casnedi, r., cantalamessa, g., fumanti, f., morelli, c., micarelli, a., ridolfi, m., and salvucci, r. (1992). carta geologica dei bacini della laga e del cellino e dei rilievi carbonatici circostanti. in: studi geologici camerti, vol. spec. università degli studi, dipartimento di scienze della terra. selca, firenze. emergeo working group: alessio, g., l. alfonsi, c.a. brunori, p. burrato, g. casula, f.r. cinti, r. civico, l. colini, l. cucci, p.m. de martini, e. falcucci, f. galadini, g. gaudiosi, s. gori, m.t. mariucci, p. montone, m. moro, r. nappi, a. nardi, r. nave, d. pantosti, a. patera, a. pesci, m. pignone, s. pinzi, s. pucci, p. vannoli, a. venuti and f. villani (2012). technologies and new approaches used by the ingv emergeo working group for real time data sourcing and processing during the emilia romagna (northern italy) 2012 earthquake sequence, annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6117. galli, p., f. galadini and d. pantosti (2008). twenty years of paleoseismology in italy. earth-science reviews, 88, 89–117. galvani, a., m. anzidei, r. devoti, a. esposito, g. pietrantonio, a.r. pisani, f. riguzzi and e. serpelloni (2013). the annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7195 8 interseismic velocity field of the central apennines from a dense gps network, annals of geophysics, 55, 5, doi: 10.4401/ag-5634 miccadei, e., t. piacentini, n. sciarra and r. di michele (2013). seismically induced landslides in abruzzo (central italy): morphostructural control. in landslide science and practice (pp. 315-320). springer berlin heidelberg. pierantoni, p., g. deiana and s. galdenzi (2013). stratigraphic and structural features of the sibillini mountains (umbria-marche apennines, italy). italian journal of geosciences, 132(3): 497-520. tarquini, s., s. vinci, m. favalli, f. doumaz, a. fornaciai and l. nannipieri (2012). release of a 10-m-resolution dem for the italian territory: comparison with global-coverage dems and anaglyph-mode exploration via the web. computers & geosciences, 38(1), 168-170. vai, g.b., and l.p. martini (ed.), (2001). anatomy of an orogen: the apennines and adjacent mediterranean basins, kluwer academic publishers, dordrecht, the netherlands, 633 p. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 1 building bridges through science: increased geoscience engagement w ith canada’s n orthern communities pascale groulx office of th e ch ief scien tist, n atu r al resou rces can ad a p ascale.gr ou lx@can ad a.ca don n a kirkwood ch ief scien tist, n atu ral resou rces can ad a d on n a.kirkw ood @can ad a.ca dan iel lebel geological su rvey of can ad a, n atu ral resou rces can ad a d an iel.lebel@can ad a.ca abstract a decade ago, data uptake by industry was held as the principal indicator of success of the geological survey of can ada’s geo-mapping for energy and m inerals (gem ) program, an initiative aimed at modernizing geological knowledge of the country’s n orth to spur economic growth. upon renewal in 2013, the geoscience program evolved its approach for engaging local communities, putting principles of geoethics into practice. this cultural shift has not only enriched the gsc as a whole; but has set an example for other science endeavours in the n orth. it has nurtured enhanced dialogue and relationships, fostered more sustainable economic growth, and helped position the gsc as a more welcome partner to n orthern communities. 1. backgroun d eoeth ics is d efin ed in th e “cap e tow n statem en t on geoeth ics” as d ealin g “w ith th e eth ical, social an d cu ltu ral im p lication s of geoscien ces kn ow led ge, ed u cation , research , p ractice an d com m u n ication , an d w ith th e social role an d resp on sibility of geoscien tists in con d u ctin g th eir activities” (di cap u a et al., 2017). in can ad a, th ere are good exam p les in w h ich geoeth ics p rin cip les are r eflected in th e geoscien ce research at th e geological su rvey of can ad a (gsc), p art of th e fed eral govern m en t’s d ep artm en t of n atu ral resou rces (n rcan ). th e gsc is can ad a’s old est scien tific agen cy. it w as fou n d ed in 1842 to h elp d evelop a viable can ad ian m in eral in d u stry. as th e cou n try m atu red , p rovin ces an d territories establish ed th eir ow n geological su rveys to ad van ce r esou rce d evelop m en t allow in g th e gsc to sh ift its focu s to ad d ress broad er issu es of n ation al relevan ce. th is sh ift is exp licitly d efin ed in th e in tergovern m en tal geoscien ce accord (iga) w h ere roles, resp on sibilities an d p rin cip les of coop eration am on gst th e su rveys w ere first e n tren ch ed in 1996 (du ke, 2010). un d er th e iga, th e gsc is w ell p osition ed to exercise its lea d ersh ip an d con ven e th e cou n try’s geoscien ces to collaborate on n ation ally im p ortan t th em es. th is ch an ge also brou gh t th e gsc in lin e w ith th e fed eral govern m en t’s exp ectation s th at its organ ization s exercise resu lts-focu sed lead ersh ip by p lacin g in creasin g em p h asis on socio econ om ic con texts at h om e an d abroad . tod ay, th e gsc m obilizes scien ce to in form p u blic p olicy d evelop m en t on issu es associated w ith h igh social im p act su ch as grou n d w ater, g an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 2 clim ate ch an ge, n atu ral d isasters, an d m in eral an d en ergy su p p ly (bobrow sky et al., 2017). th is sh ift, in creasin gly relevan t in all asp ects of th e gsc’s w ork, is also con sisten t w ith th e grow in g in tern ation al tren d to ap p ly geo eth ical id eas, p rin cip les, an d p ractices to scie n tific research abou t th e earth an d its m in eral resou rces an d lan d scap es, as w ell as t o m in in g an d su stain able d evelop m en t (gill an d bu llou gh , 2017; n ickless, 2017; n u rm i, 2017). 2. a geoscien ces program for the can ad ian n orth can ad a is a geologically resou rce-rich n ation th at covers 9,984,670 km 2 , ap p roxim ately 40% of w h ich in is th e n orth (i.e., can ad a’s th ree territories an d th e n orth ern p ortion s of six p rovin ces). th is vast exp an se h as h istorically rep resen ted a d au n tin g m ap p in g ch allen ge. in fact, before 2008, 60% of can ad a’s n orth w as n ot m ap p ed to m od ern geological stan d ard s. th at’s w h en th e govern m en t of can ad a in itia ted th e geo-m ap p in g for en ergy an d min eral (gem) p rogram as p art of its n orth ern strategy a strategy w h ich in clu d ed a stron g p osition on ad v an cin g can ad ian sovereign ty in th e n orth an d th e p rioritization of n orth ern in d igen ou s en gagem en t. lau n ch ed w ith an in itial in vestm en t of $100 m illion over five years (gem -1: 2008-2013), gem w as th en ren ew ed for seven m ore years th rou gh an ad d ition al fu n d in g of $100 m illion in 2013 (gem-2: 2013-2020). ru n by th e gsc u n d er th e p u rv iew of n rcan , th e p rogram w orks to p rom ote an d m od ern ize geological kn ow led ge in th e n orth to sp u r econ om ic be n efits. in keep in g w ith th e iga, th is is ach ieved in collaboration w ith p r ovin cial an d territorial cou n terp arts. th e p rogram also w orks w ith n atio n al an d region al in d igen ou s organ ization s, territorial govern m en ts, an d th e fed eral d ep artm en t of in d igen ou s an d n orth ern affairs to h elp n av igate th e d istin ct socio-cu ltu ral an d econ om ic con text of can ad a’s n orth . 3. a geoethical evolution 3.1 the local context socio-econ om ic con d ition s are ch allen gin g in can ad a’s th ree n orth ern territories, yu kon , n orth w est territories (n wt), an d n u n avu t. th e u n em p loym en t rate is h igh er an d th e ed u cation level low er th an th e n ation al n orm (th e con feren ce board of can a d a, 2011). op tion s for econ om ic p rosp erity in th e territories are lim ited an d h eavily relian t on th e n atu ral r esou rce sectors, w ith m in in g oversh ad ow in g con tribu tion s from all oth er sectors (th e con feren ce board of can ad a, 2010). th e territories are h om e to a sm all p op u lation of ap p roxim ately 119,000 p eop le sp read across few er th an 100 sm all com m u n it ies in a rem ote lan d scap e. geoeth ics th erefore p resen ts an excellen t fou n d ation for en gagin g com m u n ities in th e n orth w h ere large p ortion s of th e p op u l ation are in d igen ou s p eop les w h o h ave a d eep con n ection to th e lan d an d a rich cu ltu ral h ist ory in th e region (pep p olon i an d di cap u a, 2012). in can ad a, p rin cip les of m u tu al recogn ition , resp ect, an d sh ared resp on sibility gu id e fed eral en gagem en t related to gov ern m en t activities th at in volve in d igen ou s lan d s, resou rces an d in som e cases su b-su rface righ ts. th ese p rin cip les are in ad d ition to resp on sibilities set ou t u n d er section 35 of th e constitution a ct, 1982 th at recogn izes an d affirm s th e existin g aborigin al an d treaty righ ts of th e aborigin al p eop les of ca n ad a. to su ccessfu lly establish research p r ogram s in th e n orth , it th erefore is im p erative to u n d erstan d an d con sid er local cu ltu ral p ractices, com m u n ity h istory, an d can ad a’s evolvin g relation sh ip w ith in d igen ou s p eop les. 3.2 changing scientific culture through gem th e gem p rogram ’s p rim ary aim w as, an d r em ain s, m ap p in g can ad a’s n orth . th ou gh th is m an d ate h as n ot ch an ged , th ere h as been a p rogressive sh ift in h ow gem is im p lem en ted . as w ith m an y p u blic geoscien ce p rogram s, gem w as origin ally d evelop ed to focu s on freely p rovid in g p u blic geoscien ce (e.g., m ap s an d syn th esis rep orts) to stim u late in d u stry in vestm en t (bern kn op f et al., 2007; du ke, 2010). fou r years in to th e p rogram , th e govern m en t an n ou n ced its resp on sible resou rce develo p m en t in itiative w h ich w ou ld in p art “prom ote p ositive an d lon g -term relation sh ip s w ith ab oan n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 3 rigin al com m u n ities in ord er to im p rove reco n ciliation an d facilitate greater p articip ation of aborigin al p eop le in th e d irect an d in d irect ben efits of n ew resou rce p rojects”. th e in itiative d em on strates th e fed eral p olicy sh ift t ow ard s in creased in d igen ou s en gagem en t. wh en th e tim e cam e to ren ew th e p rogram , m an agem en t d rew from in sigh ts gain ed d u rin g th e first years of th e p rogram , d irection from th e evolvin g p olicy con text, an d sp ecifics tools th at gu id e fed eral action w ith in d igen ou s p e op les (govern m en t of can ad a, 2011). gem’s m an agem en t evolved th e origin al a p p roach to also in clu d e m ore active in volvem en t by n orth ern ers. n orth ern er s, in clu d in g in d igen ou s com m u n ities, w ere con sid ered as an in tegral p art of ach ievin g gem’s m an d ate to su p p ort a stron g n orth ern econ om y. th is led to a m ore d yn am ic en gagem en t ap p roach w h ich evolved in to a p rogram firm ly rooted in geoeth ics (pep p olon i an d di cap u a, 2016). tod ay, th e p rogram is en tren ch ed in a belief th at th e m ore n orth ern com m u n ities kn ow abou t geology an d th e n atu ral resou rces arou n d th em , th e better able th ey are to m ake d ecision s abou t th eir fu tu re. gem n ow co n n ects n orth ern com m u n ities w ith critical d ata th at in form s th eir resou rce d evelop m en t an d lan d -u se d ecision s, an d it in vites th em to p a rticip ate in th e geo-m ap p in g p rocess. th is ap p roach is bu ild in g im p roved relatio n sh ip s betw een n orth ern ers an d govern m en t agen cies. n orth ern com m u n ities, in d u stry an d govern m en ts are exp ected to all ben efit from th is ap p roach . 4. the evolvin g approach lookin g forw ard , th e gem p rogram n ow in corp orates geoeth ical n orth ern en gagem en t th rou gh ou t its activities su ch as seekin g n ort h ern ers’ p ersp ectives, u n d ertakin g field w ork, an d su p p ortin g n orth ern er’s d ecision m akin g . 4.1 impact on gem leadership alth ou gh th e p ath to ach iev in g resu lts w as focu sed on gettin g in d u stry to u se th e n ew in form ation , an ad visory grou p of n orth ern ers (agn ) w as form ed as a b od y th rou gh w h ich th e gsc’s sen ior lead ersh ip an d n orth ern re p resen tatives cou ld sh are valu able kn ow led ge an d in form ation . its recom m en d ation s w ere rep orted to th e assistan t dep u ty min ister of th e earth scien ce sector th at oversees th e ge ological su rvey. o rigin al m em bers w ere p rim a rily d raw n from lon g -stan d in g p artn ers fam iliar w ith geoscien ces, su ch as in d u stry, territorial govern m en ts, an d n orth ern train in g organ iz ation s. over tim e an d w ith gu id an ce from th e agn , it becam e clear th at focu sin g on servin g th e n eed s of on ly on e en d u ser th e exp lor ation in d u stry w as n ot com p atible w ith th e socio cu ltu ral an d econ om ic realities of th e n orth . in creased en gagem en t w ith n orth ern ers n ee d ed to be p art an d p arcel of th e ren ew ed p r ogram ’s d esign for su ccess. with th e p rogram ’s ren ew al an d th e evolvin g geoeth ical sh ift tow ard s greater en gagem en t of n orth ern ers, a revived agn becam e a crit ical m ech an ism to seek p ersp ectives on h ow to m axim ize th e p rogram ’s ben efits to n orth er n ers. th e n ew grou p brin gs togeth er a d iv ersity of n orth ern stakeh old ers, in clu d in g com m u n ity eld ers, you th , acad em ics, local an d territorial govern m en t rep resen tatives, in d igen ou s association m em bers, an d in d u stry rep resen tatives. meetin gs occu r in p erson an n u ally an d teleco n feren ces su p p ort on g oin g d iscu ssion s betw een m eetin gs. in p erson m eetin gs leverage creative facilitation tech n iqu es su ch as op en sp ace tech n ology th at en cou rage p articip an ts’ in te rests d rivin g th e d iscu ssion s (sw an son , 2006), resu ltin g in m em bers id en tifyin g an d lead in g w orkin g grou p s on top ics th ey are p assion ately in terested in ad v an cin g. mem bers’ in sigh ts focu s on issu es im p ortan t to n orth ern ers su ch as: bu ild in g cap acity of n orth ern com m u n ities; en su rin g com m u n ication s p rod u cts su ch as vid eos, en gagem en t le tters, an d fin al rep orts are su itable; facilitatin g th e u se of d ata an d kn ow led ge by n orth ern ers; im p rovin g en gagem en t p rotocols, an d ad d res sin g n orth ern ers’ con cern s regard in g th e field w ork. th ere are several exam p les of tim es w h en th e agn p rovid ed con crete in sigh t th at reflected its u n iqu e n orth ern p ersp ective. most n otably, th e agn ad vised on h ow to in volve n orth ern stu d en ts in gem p rogram activities an d h elp ed gsc staff by review in g com m u n ication s p lan s an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 4 an d p rod u cts to ad ap t th em to n orth ern co n sid eration s an d realities. 4.2 impact on science culture th rou gh th eir “boots on th e grou n d ” activities, gsc geologists h ave an op p ortu n ity to sh are th eir in terest in an d resp ect for th e lan d w ith local com m u n ities. th ey act as im p artial in form ation brokers, sh arin g d ata an d kn ow led ge freely an d equ ally w ith all in teres ted p arties. com m on p ractice is to sh are resu lts op en ly, en su rin g th at com m u n ities an d in d u stry can access th e in form ation at th e sam e tim e. to su p p ort en gagem en t op p ortu n ities, tools an d gu id elin es w ere d evelop ed to h elp gsc team s en gage in d igen ou s com m u n ities. th ese in clu d e a strategy, gu id elin es, an an n u al critical p ath , an d en gagem en t p lan s. th e key to m ain stream in g en gagem en t con sid eration s as p art of th e field w ork p lan n in g w as establish in g an agreem en t am on gst th e gsc field team s on a critical p ath an d en gagem en t p lan s. th e an n u al critical p ath h elp s team s p lan th eir en gagem en t by settin g ou t tim elin es for key p h ases of en gagem en t lead in g u p to a p r op osed field cam p aign . th e critical p ath takes in to accou n t th e tim in g requ ired for logistics p lan n in g an d p erm its ap p lication s w h ile accom m od atin g th e rh yth m of life in th e co m m u n ities. resp ectfu l an d m ean in gfu l en gag em en t in clu d es recogn izin g w h en com m u n ity m em bers are m ore likely to be aw ay livin g on th e lan d an d n ot available to an sw er calls, co rresp on d , or atten d m eetin gs (see fig . 1). th e en gagem en t plan s are at th e core of p la n n in g com m u n ity en gagem en t an d are based on research , an alysis, an d valid ation as reco m m en d ed in th e u p d ated fed eral gu id elin es (govern m en t of can ad a, 2011). th e research an d an alysis p h ase seeks to cross referen ce p rop osed scien tifically relevan t sam p lin g sites w ith com m u n ity in terests an d righ ts. research h elp s th e team s learn abou t th e com m u n ities an d can reveal th e lan d scap e of ov erlap p in g settled an d u n settled lan d claim s, selfgovern in g n ation s, trad ition al lan d u se, organ ization al stru ctu re, an d h istory. th e an alysis p h ase th en com p ares th e p rop osed field w ork w ith th e com m u n ity -related in form ation to id en tify w h o an d h ow to en g age in a w ay th at resp ects com m u n ity govern an ce. valid ation w ith n orth ern ers is critical becau se th e avail able resou rces for research can be ou td ated an d p erson al con n ection is very im p ortan t to in d igen ou s com m u n ities. th e team s rely on several ap p roach es to valid ate in form ation , in clu d in g w orkin g w ith terr itorial govern m en t colleagu es, region al in d ig en ou s organ ization s, an d d irectly callin g th e com m u n ity to con firm n am es an d p osition s of key com m u n ity lead ers. en gagem en t p lan s are iterative an d evolve th r ou gh ou t th e p roject as team s are exp ected to be resp on sive to co m m u n ity feed back an d ad ju st th e en gagem en t in ten sity accord in gly. plan s are created at th e ou tset of each n orth ern research activity. th ey track p rogress to en su re th at su itable en gagem en t activities are con d u cted d u rin g th ree d istin ct p eriod s: figure 1: a nnual critical path for engagement by field work teams. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 5 pre-field w ork pre-field w ork activities in clu d e corresp on d en ce exch an ges an d com m u n ity visits. th ese aim to in trod u ce th e p rogram , p resen t p r op osed field p lan s, gath er feed back on h ow best to resp ect cu ltu rally sen sitiv e sites an d d iscu ss h irin g op p ortu n ities. critical lesson s learn ed in clu d e u sin g p lain lan gu age to d escribe th e p rop osed scien ce an d an ticip atin g qu estion s from local com m u n ities, su ch as “wh at is th e govern m en t d oin g h ere?” “are th ey g oin g to im p act ou r lan d , w ild life, or w ay of life?” “will th ere be ben efits for u s?” team s m ake u se of tow n h all com m u n ity m eetin gs, social m ed ia p latform s, com m u n ity rad io p resen tation s, an d ou treach tables set u p in local grocery stores, for exam p le. gsc staff som etim es offer training in com m u nities, su ch as a field school hosted in collaboration w ith the canad a n u navu t geosciences office. the team held half-d ay w orkshop s that w ere open to the com m u nity and provid ed an overview of basic elem ents of geolog ical research su ch as geo-caching, su rficial and bed rock geology, gis, am ong other top ics (see fig. 2). field w ork th e team s w ork w ith com m u n ity organ ization s d u rin g p re-field w ork en gagem en t activities to valid ate a h irin g p rocess con sisten t w ith th eir govern an ce, often th rou gh th eir h u n ter an d trap p er organ ization s. gem staff h ires locals to w ork in an d m an age th e field cam p s, an d h ire local h elicop ter op erators, w ild life m on itors, field gu id es, an d tran slators. th rou gh th ese p articip ation op p ortu n ities, local resid en ts becom e m ore aw are of th e field w ork an d can th en sh are th eir exp erien ce w ith th e com m u n ity. research ers also take ad van tage of bein g in th e com m u n ity to con d u ct sch ool an d com m u n ity visits, h ost op en in form ation se ssion s, w orksh op s, an d p r esen tation s to p rovid e an u p d ate on th eir research activities. for in stan ce, th e n askap i n ation an d th e fon d min ier d u qu ébec in vited a gem p rogram scien tist to sp en d a d ay w ith stu d en ts, p resen tin g basic con cep ts of geology w ith d isp lays of ore sam p les, an d review in g th e elem en ts u sed in m an y con su m er p rod u cts. th ey also d iscu ssed careers related to n orth ern scien tific research , m in eral exp loration , an d th e m in in g in d u stry. betw een an d a fter-field w ork regu lar com m u nications and visits p r ovid e an op p ortu n ity to u pd ate the com m u nity on r evised plans and share p relim inary resu lts. at an ou treach event in colville lake, n wt, for exam p le, the science crew p resented d isp lay -qu ality specim ens of local rocks and fossils collected d u ring the field season and d onated them , along w ith several sets of topograp hic m ap s, to the local com m u nity. ad d itionally, all gem p rojects pu blish an annu al rep ort that is shared w ith relevant com m u nities. upon requ est by the com m u nity, gem scientists m ay even retu rn to an area w hen the p roject is com plete to present resu lts and help ensu re that gem know led ge can be u sed by the com m u nity. th rou gh early an d on goin g en gagem en t, gem’s p roject team s h ave con n ected w ith over 60 com m u n ities an d h ave visited over 30 of th em sin ce th e lau n ch of th e ren ew ed gem p rogram in 2013. gem field w ork related en gagem en t h as in creased op p ortu n ities for op en com m u n ication , en h an ced u n d e rstan d in g, an d tru st all kn ow n m ech an ism s for bu ild in g an d stren gth en in g relation sh ip s am on g stakeh old ers (alm an y et al., 2010) (see fig. 3). 4.3 impact on funding tools figure 2: gem field school hosted in taloyoak, n u. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 6 gran ts w ere origin ally on ly p rovid ed to ge oscien ce acad em ics. as th e p rogram evolved it recogn ized th at allocatin g gran t fu n d s d iffe ren tly cou ld be a w ay to p rovid e n orth ern ers w ith tan gible ben efits. a sp ecial stream of p r ogram fu n d in g w as th erefore d ed icated to su p p ort th e d evelop m en t of ap p roach es an d tools th at facilitate th e u se of geoscien ce d ata an d kn ow led ge by n orth ern ers. n orth ern organ iz ation s are w ell p osition ed to id en tify th e w ays in w h ich gem d ata is m ost relevan t to th eir d ecision -m akin g. fou r calls for p rop osals w ere issu ed , in vitin g n orth ern in st itu tion s to su bm it ap p lication s for p rojects th at su p p orted th eir ability to in corp orate gem d ata in to th eir d ecision -m akin g p rocesses. th e p rogram fu n d ed p rojects th at are led by n ort h ern in stitu tion s su ch as colleges an d in d igen ou s organ ization s. a total of 16 p rojects w ere su p p orted th rou gh d irect in vestm en ts of n early $1m . 5. results 5.1 culture change at a leadership le vel th e agn p rovid ed a n op p ortu n ity for d ialogu e w ith sen ior lead ersh ip an d served as a sou n d in g board on th e n orth ern p ersp ective. en gagin g gem’s m ost sen ior lead ersh ip d irectly w ith n orth ern ers h ad a m u ltip lier effect, as d ecision s tricklin g d ow n from m an agem en t a ffect th e en tire p rogram . th e agn ’s m ost sign ifican t im p act is beyon d an y sp ecific in sigh t it p rovid ed on p rod u cts p resen ted to it for r eview . th e agn in flu en ced th e cu ltu re of th e p rogram ’s lead ersh ip on h ow to see an d u n d erstan d th e p rogram from a n orth ern er’s p e rsp ective. th e evolu tion of th e p rogram is a te stam en t to th at evolu tion in a cu ltu re th at starts at th e top an d can becom e h ard w ired th rou g h ou t th e p rogram . figure 3: m ap of communities in n orthern canada engaged since gem program renewed in 2013. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 7 5.2 culture change in field w ork planning en gagem en t p lan s h ave led to sign ifican t cu ltu ral ch an ges w ith in th e scien ce at th e gsc. en gagem en t p lan d evelop m en t is a team effort an d allow s for op en d iscu ssion s am on gst scie n tific team m em bers, en gagem en t officers, m a n agers, an d execu tives. each team m em ber p r ovid es th eir in p u t w h ile m akin g th e lin k w ith n orth ern ers p ersp ectives. en gagem en t officers w ith a backgrou n d in in d igen ou s relation s, lead th e research an d an alysis to id en tify relevan t com m u n ities, an d en su re th at everyon e h as th e op p ortu n ity to con tribu te in sigh ts from th eir exp erien ce an d m ake con n ection s w ith th e com m u n ities in qu estion . scien tists p rovid e geoscien ce targets for field w ork relevan t to th eir h yp oth esis, an d are ch allen ged to exp lain th eir ap p roach in p lain lan gu age. th ey also id en tify n orth ern ers’ p ercep tion s of th e kin d of sam p lin g, cam p set u p , an d equ ip m en t u sed (in clu d in g h elicop ters). th ey su ggest h ow com m u n ities cou ld be in volved , for exam p le in clu d in g servin g as th e base for th e field cam p . project m an agers u se th e p lan to h elp m an age bu d get an d field p lan n in g im p lication s an d p rep are for region al en gagem en t. th e a ccou n table execu tive ap p roves th e p lan an d u ses it to u n d erstan d th e local com m u n ity co n text. en gagem en t p lan s en su re th at all team m em bers are on board , u n d erstan d th e r ation ale for en gagem en t activities, an d can ad ap t th e p roject’s in ten sity based on com m u n ity resp on se. retu rn in g to com m u n ities h as been p articu larly n oted by n orth ern ers as a sign of resp ect an d op en n ess. 5.3 culture change in funding ad d in g a fu n d in g stream , th at d irectly su p p orted n orth ern in stitu t ion s’ u se of gem d ata an d resu lts, p aid d ivid en d s. gem fu n d in g an d kn ow led ge su p p orted th e d evelop m en t of th e qau jisarn ik n u n am ik ed u cation program (qn ep) at th e n u n avu t arctic college’s en v iron m en tal tech n ology program . stu d en ts th ere create m ap s th at lin k geoscien ce, in u it kn ow led ge, an d oth er relevan t d ata togeth er in an en gagin g an d in form ativ e w ay. th is ed u cation al p rogram h as been offered sin ce 2014, an d exp an d ed in 2016 to in clu d e p rofession al gis u sers an d an in stru ctor train in g p rogram so th at n u n avu m m iu t can teach th e cou rse th rou gh ou t th e territory (see figu re 4). in th e n orth w est territories, th e ɂehdzo got’ın̨ę gots’ę̨́ nákedı (sahtú renewable resources board) compiled and integrated existing gsc published geocience data and dene/métis landscape traditional knowledge into the online sahtú atlas. program fu n d in g also su p p orted w orksh op s to h elp sch ools an d com m u n ities learn abou t th is u ser-frien d ly m ap p in g tool. fu rth er w est, th e teslin tlin git cou n cil, a selfgovern in g first n ation sp an n in g th e yu kon british colu m bia bord er, in tegrated geoscien ce d ata as p art of its d igital m ap of areas w ith p oten tial for m in in g an d en ergy d evelo p m en t. th ese n ew kn ow led ge ap p lication op p o rtu n ities are ju st a few exam p les of th e p r ogram ’s su p p ort for com m u n ities’ lan d -u se an d resou rce d evelop m en t d ecision m akin g. com m u n ities can u se th e tools to recon cile r esou rce-based op p ortu n ities in relation to in d igen ou s kn ow led ge an d valu es 6. con clusion s prior to gem, th ere w as a geoscien ce gap for m ost of th e n orth . in m an y areas, th e geology w as p oorly u n d erstood an d th ere w as in su fficien t evid en ce to su p p ort in vestm en t resou rce d ecision s. sin ce 2008, th e p rogram h as closed m ore th an 40% of th is kn ow led ge gap , an d co n tin u es th is w ork w ith sign ifican t in p u t from n orth ern ers d esp ite th e ch allen gin g logistics of th e n orth . figure 4: qn ep students from a rctic college working on generating gis maps. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7512 8 th ou gh th ere w ere en gagem en t op p ortu n ities betw een p u blic geoscien ce an d n orth ern ers d u rin g th e first p h ase, th ese w ere sign ifican tly bolstered an d con sid ered essen tial d u rin g gem-2. in large p art d u e to th e fact th at ap p roxim ately h alf of gsc staff are in volved in d eliverin g th e p rogram , in d igen ou s en gag em en t before, d u rin g, an d after field w ork is n ow m ore p art of th e organ ization ’s cu ltu re an d a key con sid eration in p rogram p lan n in g exercises. th rou gh th e p rogram , scien ce d ip lom acy by th e gsc h as p layed a role in d efin in g a r en ew ed relation sh ip w ith n orth ern p op u lation s. gsc lead ersh ip h ave su p p orted th is objective by takin g a d eliberate an d ad ap tive ap p roach of com bin in g levers like gran ts an d p a rtn ersh ip s, ad visory m ech an ism s, an d p rocu rem en t. th e gsc’s m an d ate to m ap th e n orth rem ain s th e sam e, bu t its m eth od olog y h as ch an ged . usin g a m ore geoeth ical ap p roach h as brou gh t a rich n ess to geoscien ce p rogram s, created be tter relation sh ip s w ith com m u n ities, fostered econ om ic grow th in th e n orth , an d h elp ed p osition th e gsc as a m ore w elcom e scien ce p ar tn er in n orth ern com m u n ities. ackn owledgemen ts a d eep h earted th an ks to lin d a rich ard for h er lead ersh ip role, su p p ort an d en cou ragem en t in tu rn in g th is vision in to reality. we are gratefu l for everyon e th at h as been p art of th e gem coord in ation office, th e gem scien ce p roject team s an d th e gsc execu tive com m ittee, th is tran sform ation cou ld n ot h ave h ap p en ed w it h ou t you r su p p ort. jou rn al review ers p rovid e d th ou gh tfu l com m en ts th at h elp ed im p rove th e fin al version of th is p ap er. referen ces alm an y g., h am ilton r., william son d., ev an s r., jon es g., mataw ai m., potu ku t., rh od es k., ru ss g., saw yn ok b. 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(ed s.), geoethics at the heart of all geoscience, annals of geophysics, vol 60, fast track 7. du ke j. (2010). govern m en t geoscien ce to su p p ort m in eral exp loration : p u blic p olicy r ation ale an d im p act. prosp ectors develop ers association can ad a (pdac), h ttp :/ / w w w .n gsccan ad a.com / files/ gover n m en t_geoscien ce_du ke_pdac.p d f (accessed 11 may 2018). gill j.c. an d bu llou gh f. (2017). geoscien ce en gagem en t in global develop m en t fram ew orks, 10.4401/ ag-7460. in : pep p olon i s., di cap u a g., bobrow sky p., cron in v.s. 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(2016). geoeth ics: eth ical, social, an d cu ltu ral valu es in geoscien ces research , p ractice, an d ed u cation , p p . 17-21, d oi: 10.1130/ 2016.2520(03). in : wessel, g.r., an d green berg, j.k., ed s., geoscien ce for th e pu blic g ood an d global develop m en t: tow ard a su stain able fu tu re: geological society of am erica sp ecial pap er 520, isbn : 978-0-8137-2520-8. sw an son j. (2006). a mod el for aborigin al facilitation : an op en , em p ow erin g way to get con sen su s an d action  : in trod u cin g op en sp ace tech n ology. th e jou rn al of aborigin al man agem en t. th e con feren ce board of can ad a (2010). map p in g th e econ om ic p oten tial of can ad a’s n orth . con feren ce board of can ad a. th e con feren ce board of can ad a (2011). bu ild in g labou r force ca p acity in can ad a’s n orth . con feren ce board of can ad a . adg vol5 n02 iuli 431_438.pdf annals of geophysics, vol. 45, n. 2, april 2002 431 advanced magnetic visualization of mt. vesuvius shallow plumbing system by probability tomography teresa iuliano (1), paolo mauriello (2) and domenico patella (1) (1) dipartimento di scienze fisiche, università «federico ii», napoli, italy (2) istituto per le tecnologie applicate ai beni culturali, cnr, roma, italy abstract this paper reports the results of the application of a new magnetic probability tomography imaging method to the aeromagnetic data set collected by agip on mt. vesuvius in 1978. a magnetic dipole occurrence probability function is defined, in order to contain all information about the whole class of equivalent sources compatible with the original survey data set. an advanced 3d visual system is also applied for the first time to visualise the spacial pattern of the magnetic dipole occurrence probability function. it is shown that in the case of vesuvius, the shallow plumbing system is characterised by a high probability of occurrence of a dipole at 2 km of depth b.s.l., nearly directed along the present main earth’s magnetic field. this leads to hypothesize that the top terminal portion of the volcanic apparatus is completely filled with magnetised volcanic material. 1. introduction probability tomography was first introduced to image self-potential anomaly sources (patella, 1997a,b) and then extended to analyse geoelectrical (mauriello et al., 1998; mauriello and patella, 1999a), natural electromagnetic induction (mauriello and patella, 1999b, 2000), gravity (mauriello and patella, 2001a,b) and magnetic data (mauriello and patella, 2001c). it is an interpretation tool, which accounts for the inherently uncertain nature of the geophysical inversion problem, due to the many different source configurations that can be modelled compatibly with a given data set. probability tomography has been already applied to the analysis of self-potential, geoelectrical, gravity and magnetic data sets collected in the volcanic area of mt. vesuvius (naples, italy) (di maio et al., 1998; patella and mauriello, 1999; iuliano et al., 2001). the purpose of this paper is to repropose the to-mographic imaging of the magnetic sources beneath mt. vesuvius, using a new formulation of the magnetic probability tomography theory (mauriello and patella, 2001c) coupled with a 3d object-oriented advanced visual system (avs). in the following sections, the new magnetic probability tomography imaging method and the most important aspects of a 3d avs visual development tool are described. then mailing address: prof. domenico patella, dipartimento di scienze fisiche, università «federico ii», complesso universitario monte s. angelo, via cintia, 80126 napoli, italy; e-mail: patella@na.infn.it key words probability tomography – magnetic anomaly – vesuvius volcano 432 teresa iuliano, paolo mauriello and domenico patella the results of the application to the vesuvius aeromagnetic data set collected by cassano and la torre (1987) are presented and discussed. 2. probability tomography theory 2.1. the 3d magnetic occurrence probability function we assume a coordinate system with the (x, y)plane at sea level and the z-axis positive upwards. let b(r) be a static magnetic induction field, evaluated at a grid of points r s, where s is a portion of the earth’s surface characterized by a topography function z (x, y). in terms of total magnetization vector m(r), b(r) can be expressed as (jackson, 1998; mauriello and patella, 2001c) where v is a volume containing all the magnetic sources at r v and n is the unit vector in the direction of r-r . assuming the magnetic field on the earth’s surface as due to a finite distribution of q elementary magnetic sources, eq. (2.1) can be discretized as follows: where n q is the unit vector in the direction r r q and the q-th generic element is a small volume v q centred at r q with magnetization m(r q ) and magnetic moment d q given by using the principles of the probability tomography (patella, 1997a,b), at first the total power u , associated with the component b u (r) of b(r) along a generic direction identified by the unit vector u, is defined as (2.4) where the explicit expressions of the three uv (r – r q ) functions ( = x, y, z) are given by (2.5a) (2.5b) (2.5c) with n qv ( = x, y, z) being the components of the unit vector nq. then, a three-component magnetic occurb r n n m r' m r' r r' ( ) = ( )[ ] ( )µ0 3 4 3 dv v b r n n d d r r q q q q q ( ) = ( ) = 3 1 3 q q d m rq= ( ) µ0 4 dvq q . u u s b ds= ( ) = ( ) 2 r s d b dsqv u uv sv x y zq q = ( ) ( ) ( )==1 r r rq , , uy( ) =r rq = 1 r rq 3 3n n n nqy qx qy qzi u j u k u j u+ +( )[ ] uz r rq( ) = n n n nqz qx qy qz r r i u j u k u k u q = + +( )[ ] 1 3 3 ux( ) =r rq n n n nqx qx qy qz= + +( )[ ]3 1 3 r r i u j u k u i u q (2.1) (2.2) (2.3) 433 advanced magnetic visualization of mt. vesuvius shallow plumbing system by probability tomography rence probability (mop) function uv (m)(r q ) is defined as (mauriello and patella, 2001c) ( = x, y, z), (2.6) where ( = x, y, z), (2.7) and g (z), called the topographic surface regularization factor, is (2.8) each uv (m) function satisfies the bounding condition 1 uv (m)(r q ) +1 and three uv (m) values ( = x, y, z) can be computed at each r q . each value is interpreted as the probability, with which the homologous component m ( = x, y, z) of a m-field can at r q be considered responsible for the measured component for the magnetic field. in geophysical exploration, either the zcomponent or the modulus of the earth’s magnetic field is usually measured and a scalar secondary field (the anomalous field) is evaluated in order to identify local sources of magnetic anomaly. in the case of the so-called total field survey, the scalar anomalous field is obtained by subtracting from the measured modulus of the earth’s magnetic field the uv m( )( ) =rq uv m u y y x x uvc b g z dxdy ( ) = ( ) ( ) ( )r r rq , modulus of the known primary magnetic field. since the secondary field is always a very small fraction of the primary field, it can be readily shown that in the case of a total field survey the scalar secondary field is the projection of the secondary vector field along the direction of the primary vector field (blakely, 1996). this direction can generally be assumed uniform within the areas normally considered in geophysical prospecting (parasnis, 1997). thus, from now on we will consider b(r) as the secondary vector field, and the scalar component of b(r) along any fixed direction as the object of analysis in the new tomographic approach. 3. 3d probability tomography procedure the 3d tomography procedure for imaging the sources of a magnetic field, measured on a generally non-flat topography, consists in a reiterated computation code, involving the scanner functions u and the b u (r) field data set. in practice, since we do not know the position of the real sources generating the anomalous magnetic field, we use a synthetic source of unitary strength to scan the x, y, z half-space below the survey area (tomospace), in order to search where the real sources can be located in probabilistic sense. the scanning operation is performed by computing the crosscorrelation integrals in eq. (2.6) for each point (x q ,y q ,z q ) of a regular grid within the tomospace. at each point, the value of each integral is interpreted as the probability of occurrence of the relative magnetic source component, whose positive or negative orientation depends on whether it is > 0 or < 0. by scanning the tomospace, using, e.g., a sequence of horizontal slices spaced from each other by a constant depth interval, we can finally obtain a 3d image of the equivalent magnetic source distribution underground in a probabilistic sense. in order to improve the filtering property of the scanning procedure, for each r q of the tomospace it is advisable to use varying sizes of the integration surface in eq. (2.6). the smallest size is the [– x, x ] × [– y, y ] domain fully uv mc( ) = u y y x x uv y y x x b g z dxdy g z dxdy= ( ) ( ) ( ) ( )r r rq / 2 2 1 2 ,, g z z x z y .( ) = +( ) +( ) 1 2 2 434 teresa iuliano, paolo mauriello and domenico patella containing the surface trace of the magnetic response of the scanning source element placed at r q . the greatest surface is, of course, the largest rectangle fitting to the whole survey area. the highest (r q ) is taken with its sign as the most appropriate source occurrence probability at r q . 4. advanced data visualization in the most general term, data visualization is the process of representing data as a graphic display on a computer screen. in practice, data visualization gives an insight into data allowing the interpreter to graphically view structure and changes in the data. generally speaking, there are two things the interpreter wants to do with data: analyse it to obtain information and present it to others to share that information. in either case, an effective visualization is an important tool. an advanced visual system (avs) extends the obvious advantages of standard visualization by sophisticated 3d representations of data to be developed. modern 3d avs softwares are considered object-oriented, visual development tools, enabling one to build reusable application components and complex visualization applications. avs softwares become very useful when they can support different object-oriented techniques, such as encapsulation of data and methods, class inheritance, templates and instances, object hierarchies and polymorphism. very shortly, a 3d avs tool can be considered a visual development environment that can be used to connect, define, assemble and manipulate objects and related applications through mousedriven operations. the objects and applications that are connected and assembled control how data is processed and how it is displayed. the interpreter can even add a user interface to create a complete application that can be delivered as a stand-alone application. we have applied a 3d avs software (advanced visual systems inc., 1998) to visualize the results of the application of the magnetic probability tomography to the volcanic area of mt. vesuvius. of course, as we cannot reproduce all the dynamical capabilities of the 3d avs, we show in the following section only a particular perspective of the tomographic images from the most effective point of view. 5. mt. vesuvius magnetic tomography and avs imaging vesuvius is considered one of the most risky active volcanoes in the world, because of its volcanic history and closeness to the city of naples, italy. the results achieved so far (iuliano et al., 2001, and references therein) indicate that the shallow part of the volcano is made of a unique central plumbing system, entirely filled of altered volcanics in the summit portion. this peculiarity makes the volcano extremely hazardous, since explosion becomes a highly probable event in case of renewal of extruding activity. the magnetic study can notably help to ascertain whether such a condition really occurs. to this purpose, we show the results of the application of the probability tomography to the aeromagnetic total field data set collected in 1978 by the italian oil company agip to study the volcano-geothermal structure of mt. vesuvius. a cesium optical pumping magnetometer was used at the constant altitude of 1460 m a.s.l. figure 1 shows the original total magnetic field map (after cassano and la torre, 1987). subtracting a constant regional field in the survey area with intensity 30 100 nt, inclination 56.30° and declination 1°, the residual effect is a large and intense positive anomaly with peak value of approximately 1800 nt, located in the central part of the volcanic system. the 3d avs pictures in fig. 2a-c show the pattern of the mop functions ux (m), uy (m) and uz (m), respectively, drawn by plotting the mop isosurface of the lower limit of the coloured palette corresponding to the decimal step where the highest absolute mop values occur. only absolute mop values equal or greater than 0.6 have been considered in fig. 2a-c. in particular, fig. 2c shows a well defined nucleus centred at about 2 km b.s.l., including the lowest computed values of uz (m), falling in the range from – 0.9 to – 1. accordingly, fig. 2b shows a smaller nucleus, centred again at about 2 km b.s.l., including the highest computed values of uy (m), falling in the 435 advanced magnetic visualization of mt. vesuvius shallow plumbing system by probability tomography 0 5 10 15 20 25 x (km) 0 5 10 15 20 25 30 35 y ( k m ) 29900 30500 31100 31700 total magnetic field (nt) castellamare s.anastasia mt somma vesuvius torre annunziata pomigliano naples trecase well fig. 1. the total field aeromagnetic map in the volcanic area of mt. vesuvius (after cassano and la torre, 1987). 436 teresa iuliano, paolo mauriello and domenico patella z (km) -1 -3 -5 -7 z (km) -1 -3 -5 -7 a b ux (m) 0.6 0.6 uy (m) 0.7 1 uz (m) 0.9c fig. 2a-c. 3d avs imaging of the magnetization occurrence probability function along the x-axis (a); y-axis (b) and z-axis (c). the top slice is the experimental map of the residual total magnetic field. z (km) -1 -3 -5 -7 range from 0.6 to 0.7. no signal above the threshold of 0.6 is present in the ux (m) tomography of fig. 2a. combining all these features, the relevant information is the high occurence probability of an inclined dipole centred at x = 12 km, y = 17 km and z = 2 km, with dominant negative vertical component and positive horizontal component directed along the y-axis. figure 3a,b shows the comparison between the experimental map of the residual total magnetic field (fig. 3a) and the synthetic map (fig. 3b) obtained assuming a magnetic dipole centred at x = 12 km, y = 17 km and z = 2 km, with magnetic moment of 110 nt, inclination 56.30° and declination – 1°. the misfit between the two maps is in modulus less than 2%. therefore, the synthetic dipole can represent a simple model of the observed residual magnetic anomaly field in the vesuvius volcanic area. concluding, this leads to hypothesize the existence of a uniformly magnetised material filling the top terminal portion of the volcano, exactly where gravity data indicate a low density (iuliano et al., 2001). 437 advanced magnetic visualization of mt. vesuvius shallow plumbing system by probability tomography references advanced visual systems inc. (1998): avs/express manual. waltham, ma, usa. blakely, r.j. (1996): potential theory in gravity and magnetic applications (cambridge university press), pp. 461. cassano, e. and p. la torre (1987): geophysics, in: somma vesuvius, edited by r. santacroce, quad. ric. sci., 114/8, 175-196, consiglio nazionale delle ricerche, rome. di maio, r., p. mauriello, d. patella, z. petrillo, s. piscitelli and a. siniscalchi (1998): electric and electromagnetic outline of the mount somma-vesuvius structural setting, j. volcanol. geotherm. res., 82, 219-238. fig. 3a,b. comparison between the observed map of the residual total magnetic field (a) and the synthetic map (b) obtained assuming a magnetic dipole centred at x = 12 km, y = 17 km and z = 2 km, with magnetic moment 110 nt, inclination 56.30° and declination – 1°. 0 5 10 15 20 25 x (km) 0 5 10 15 20 25 30 35 y ( k m ) 29900 30500 31100 31700 total magnetic field (nt) castellamare s.anastasia mt somma vesuvius torre annunziata pomigliano naples trecase well y ( k m ) 0 5 10 15 20 25 x (km) 0 5 10 15 20 25 30 35 castellamare s.anastasia mt somma vesuvius torre annunziata pomigliano naples trecase well a b iuliano, t., p. mauriello and d. patella (2001): looking inside mount vesuvius by potential fields integrated probability tomographies, j. volcanol. geotherm. res. (in press). jackson, j.d. (1998): classical electrodynamics (j. wiley and sons, new york), pp. 832. mauriello, p. and d. patella (1999a): resistivity anomaly imaging by probability tomography, geophys. prospect., 47, 411-429. mauriello, p. and d. patella (1999b): principles of probability tomography for natural-source electro magnetic induction fields, geophysics, 64, 1403-1417. mauriello, p. and d. patella (2000): a physical pattern recognition approach for 2d electromagnetic induction studies. ann. geofis., 43 (2), 343-359. mauriello, p. and d. patella (2001a): gravity probability 438 teresa iuliano, paolo mauriello and domenico patella tomography: a new tool for buried mass distribution imaging, geophys. prospect., 49, 1-12. mauriello, p. and d. patella (2001b): localization of maximum-depth gravity anomaly sources by a distribution of equivalent point masses, geophysics, 66, 1431-1437. mauriello, p. and d. patella (2001c): magnetic buried sources discrimination by probability tomography, geophys. prospect. (submitted). mauriello, p., d. monna and d. patella (1998): 3d geoelectric tomography and archaeological applications, geophys. prospect., 46, 543-570. parasnis, d.s. (1997): principles of applied geophysics (chapman & hall, london), pp. 429. patella, d. (1997a): introduction to ground surface selfpotential tomography, geophys. prospect., 45, 653-681. patella, d. (1997b): self-potential global tomography including topographic effects, geophys. prospect., 45, 843-863. patella, d. and p. mauriello (1999): the geophysical contribution to the safeguard of historical sites in active volcanic areas. the vesuvius case-history, j. appl. geophys., 41, 241-258. on the radiative forcing of volcanic plumes: modelling the impact of mount etna in the mediterranean on the radiative forcing of volcanic plumes: modelling the impact of mount etna in the mediterranean pasquale sellitto1, pierre briole2∗ 1 laboratoire de météorologie dynamique (lmd), cnrs-umr8539, institut pierre simon laplace, école normale supérieure, école polytechnique, université pierre et marie curie, paris, france psellitto@lmd.ens.fr 2 laboratoire de géologie, cnrs-umr8538, école normale supérieure, paris, france abstract the impact of small to moderate volcanic eruptions on the regional to global radiative forcing and climate is still largely unknown and thought to be presently underestimated. in this work, daily average shortwave radiative forcing efficiencies at the surface (rfesurfd ), at top of the atmosphere (rfe toa d ) and their ratio (f), for upper tropospheric volcanic plumes with different optical characterization, are derived using the radiative transfer model uvspec and the libradtran suite. the optical parameters of the simulated aerosol layer, i.e., the ångströem coefficient (α), the single scattering albedo (ssa) and the asymmetry factor (g), have been varied to mimic volcanic ash (bigger and more absorbing particles), sulphate aerosols (smaller and more reflective particles) and intermediate/mixed conditions. the characterization of the plume and its vertical distribution have been set-up to simulate mount etna, basing on previous studies. the radiative forcing and in particular the f ratio is strongly affected by the ssa and g, and to a smaller extent by α, especially for sulphates-dominated plumes. the impact of the altitude and thickness of the plume on the radiative forcing, for a fixed optical characterization of the aerosol layer, has been found negligible (less than 1% for rfesurfd , rfe toa d and f). the simultaneous presence of boundary layer/lower tropospheric marine or dust aerosols, like expected in the mediterranean area, modulates only slightly (up to 12 and 14% for rfesurfd and rfe toa d , and 3 to 4% of the f ratio) the radiative effects of the upper tropospheric volcanic layer. i. introduction v olcanoes can have a direct effect on the atmospheric radiation budget, because of the the absorption and scattering of solar and terrestrial radiation by the emitted primary and secondary aerosols. the most important radiative forcing from volcanic emissions is the shortwave (solar) interaction by long-lived secondary sulphate aerosols, formed by the conversion of sulphur dioxide emissions [oppenheimer et al., 2011]. primary ash particles can also significantly modulate the radiation transfer. volcanic emissions forcing, ∗this work has been partially supported by the eu 7th framework program under the grants 603557 (stratoclim) and 308665 (med-suv) 1 mailto:psellitto@lmd.ens.fr francescocaprara typewritten text annals of geophysics, 58, fast track 3, 2015 francescocaprara typewritten text in particular from sulphate aerosols, has been proposed as one of the possible causes of the global warming hiatus observed in the last 15 years [santer et al., 2014]. a more detailed estimation of the radiative effects of volcanic emissions is hampered by the incomplete knowledge of their physico-chemical processes, especially in the upper troposphere, and in particular the mechanisms of new particle formation leading to sulphate aerosol droplets [andreae, 2013]. mount etna is a hotspot sulphur dioxide source in the mediterranean basin, accounting for more than 0.7·106 t s / yr, 10 times stronger and with injection altitudes significantly higher than anthropogenic sulphur emissions in the same area [graf et al., 1997]. despite its potential to form long-lived sulphate aerosols, the radiative forcing of mount etna’s emission is largely overlooked in aerosols impact studies in the mediterranean. in this paper, the surface and top of the atmosphere (toa) radiative forcing of simulated upper tropospheric volcanic plumes, with different optical characterization, is derived with a forward radiative transfer model. the aerosol layer (optical properties and vertical distribution) and environmental conditions (atmospheric profiles, surface albedo and properties of simultaneous aerosol layers at lower altitudes) are set-up to mimic mount etna emissions and the mediterranean environment. methods are described in section ii, results are discussed in section iii and conclusions are given in section iv. ii. methods the potential radiative impact of mount etna emissions in the mediterranean basin are estimated by means of the shortwave surface and toa radiative forcing. these two quantities are calculated using the uvspec radiative transfer model and the libradtran suite [mayer and kylling, 2005]. the general set-up of our simulations is reported in table 1. table 1: general set-up of uvspec. (sdisort=[pseudo-]spherical discrete ordinate radiative transfer, lowtran=low resolution atmospheric radiance and transmittance model, afgl=air force geophysics laboratory) radiative transfer solver sdisort [dahlback and stamnes, 1991] solar spectrum [kurucz, 1994] gas absorption model lowtran [pierluissi and peng, 1985] atmospheric state afgl mid-latitude summer standard surface albedo 0.09, as in [meloni et al., 2005] spectral range 300.0-3000.0 nm spectral resolution 0.1 nm the volcanic aerosol layer is modelled by means of its (wavelength-independent) optical properties and vertical distribution. the optical characterization of the aerosols has been defined by means of the single scattering albedo (ssa), ångström exponent (α) and asymmetry parameter (g). these three parameters describe the absorption properties, the size distribution of the aerosols, and the angular distribution of the radiation field after the interaction of the layer, respectively. different simulations are performed with varying parameters values; the explored intervals are summarized in table 2. these simulations are discussed in section iii.1. to gather indications on the impact of mount etna aerosols and gaseous precursors emissions in the mediterranean, this work is focused on aged volcanic plumes, i.e., plumes where the gas-to-particle conversion of so2 emissions has enriched the plume of sub-micrometric sulphate droplets and/or the coarse ash component is significantly reduced by gravitational settling. at these conditions, typical values of the mentioned optical parameters vary from 1.0 [hervo et al., 2012, e.g.] to more 2 francescocaprara typewritten text sellitto and briole than 1.6 [watson and oppenheimer, 2001, specific determination for a non-ash-bearing plume of mount etna] (α), from 0.8-0.9 [derimian et al., 2012] to 1.0 (ssa) and from 0.7-0.8 [derimian et al., 2012] to 0.7 (g), for ash and sulphate aerosols, respectively. in general ash is characterized by bigger (smaller α), more absorbing (smaller ssa) particles. the variability of g is of less immediate interpretation because it depends on both the size and the shape of the aerosol particles. in any case, slightly smaller values are expected for sulphate aerosol layers, due to the very small mean sizes of these particles. the variability of the aerosol optical properties of these simulations are then tought to mimic plumes dominated by sulphate aerosols, ash and mixed layers of these two compositions. finally, a total of more than 1000 (11x13x7 optical properties combinations) aerosol scenarios are tested. table 2: variability of the aerosol properties. (par.=parameter, min=minimum, max=maximum, incr.=increment, n=number of different values) par. min max incr. n ssa 0.80 1.00 0.02 11 α 0.6 1.8 0.1 13 g 0.60 0.90 0.05 7 the vertical distribution of the plume has been modeled by setting its aerosol optical depth (aod) profile. in our simulations, the aod has a maximum value at 11 km and linearly decreasing to zero at higher (up to 13 km) and lower altitudes (down to 8 km), see the distribution in red in figure 1. the thickness of the plume is then 6 km. different altitudes of the plume, i.e., with a rigid shift of the distribution 2 km upwards and downwards (distributions in orange and brown in figure 1) are also tested to estimate the impact of the plume altitude on the radiative forcing. the impact of different thicknesses of the plume have also been investigated by compressing the vertical distributions in a 3 km vertical interval, with constant aod (distributions not shown in figure 1). the results of these simulations are discussed in section iii.2. finally, the impact of the the presence of different aerosol typologies in the lower troposphere (lt, vertical distribution in blue in figure 1) is studied and the results are discussed in section iii.3. figure 1: vertical aod profiles used in the simulations. (v=volcanic, lt=lower troposphere, m=maritime, r=rural) a further baseline simulation is carried out, with the same atmospheric state but without aerosols. starting from the high spectral resolution spectra, the instantaneous shortwave radiative forcing has been calculated as the difference between the net flux with and without aerosols, integrated in the whole spectral range. the radiative forcing per unit of aod, also called radiative forcing efficiency (rfe), is discussed in the following rather than the absolute radiative forcing. this is done to obtain more general results and because it is difficult to single out the optical depth of volcanic-only aerosols in real world mixed aerosol layers. the simulations are made at different solar zenith angle values (from 0 to 90◦, with 15◦ steps) and 3 francescocaprara typewritten text on the radiative forcing of mt. etna's plume the daily mean value of the radiative forcing efficiency at the surface and at the toa (rfesurfd and rfetoad ), with the different aerosol characterization, is finally calculated (equinox simulations). the ratio f between rfesurfd and rfetoad is important to determine the reflective/absorbing properties of the layer and is also discussed in the present work. iii. results and discussion iii.1 dependence on volcanic aerosol optical properties the radiative forcing of the simulated volcanic plume varies significantly with the three varying aerosol optical parameters. rfesurfd and rfetoad vary between about -12 and -118 w/m2/aod and about 0 and -66 w/m2/aod, respectively (results not shown here). the variability of the ratio f is shown in figure 2. f varies between about 5 (very absorbing layer; higher values of f for small ssa and big g; slightly facilitated for high α) and 0.8 (very reflective layer; smaller values of f for bigger ssa and small g; at these conditions f is weakly sensitive to α). the most sensitive parameter determining f in our simulations is ssa, while g can also be important. the red cross in figure 2 indicates the measured values of ssa, α and g for the test case of [sellitto et al., 2015]. plume dispersion simulations and observations for that case study suggest that the plume is composed of sulphate aerosol; an f of about 1.0 was found. the present simulations suggest that, in this area of the ssa-α-g space, f does not vary rapidly with varying parameters. we conclude that sulphate aerosols plumes tend to have stable radiative forcings (reflective) for moderate oscillations of the optical properties (e.g., disregarding the micro-physics and composition of the sulphates or in presence of an ash component). ashy plumes, on the contrary, can be very absorbing (comparable with dust or urban tropospheric aerosols in the mediterranean, see e.g. [di biagio et al., 2010]) and their radiative forcing has the potential to vary more abruptly with varying optical properties. 5.0 2.9 0.8 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.80 0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.020.80 0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.020.80 0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.02 å n gs tr ö m ex p o n en t single scattering albedo f figure 2: f ratio as a function of single scattering albedo ssa and ångström coefficient α, for asymmetry factor g varying between 0.6 (a) to 0.9 (g). the red star indicates the ssa, α and g values for the case study discussed in [sellitto et al., 2015]. iii.2 dependence on volcanic plume altitude and depth the impact of different altitudes (orange and brown distributions in figure 1) and thicknesses of the plume are investigated on selected optical properties, i.e., sulphates-typical values of 0.98, 1.6, and 0.7 for ssa, α and g, respectively. the aod has been kept constant in these simulations. the rfesurfd , rfe toa d and f varied for less than 1% with these different configurations, indicating that the altitude and the vertical extent of the volcanic plume has a negligible effect on its radiative forcing. these results are consistent with earlier observations of 4 francescocaprara typewritten text sellitto and briole the small impact of the aerosol vertical distribution on radiative forcing [meloni et al., 2005]. iii.3 impact of the presence of other aerosols types in the lower troposphere the impact of the presence of a lt aerosol layer, at lower altitudes than the volcanic plume, is also investigated. to this aim, simulations are performed with: 1) a volcanic-only case (distribution in red in figure 1), 2) a volcanic plus maritime lt layer (distributions in red + blue in figure 1), 3) a volcanic plus rural lt layer (distributions in red + blue in figure 1). the optical characterization of the different layers is derived from [shettle, 1985]. for the volcanic plume, moderate volcanic conditions are selected. maritime and rural optical properties are selected to mimic sea salt-preponderant aerosols (maritime) and the dust component (rural), which are two typical conditions for the lt aerosols in the mediterranean. background conditions are then chosen as: 1) no aerosol (for the volcanic-only case), 2) lt maritimeonly aerosols (for the volcanic plus maritime case), 2) lt rural-only aerosols (for the volcanic plus rural case). the rfesurfd , rfe toa d and f for the 3 cases are reported in table 3. the interference of simultaneous lt aerosol layers on the volcanic plume radiative forcing are found relatively small. while the individual surface and toa components can reach up to 6 w/m2/aod smaller values, with respect to the volcanic-only case, the ratio f is only slightly perturbed, indicating a very small additional absorption component. please note that more consistent variations of the radiative forcing, due to the simultaneous presence of lt aerosols, are found for other aerosols types, like dust, in the mediterranean [gómez-amo et al., 2010]. one marked difference with respect to our simulations is the smaller altitude of the dust plume (1.5-4.0 km). table 3: radiative forcing efficiencies at the surface and the toa, and their ratio f, for 3 cases: volcaniconly (v-only), volcanic + maritime (v+m), volcanic + rural (v+r). conf. rfetoad rfe surf d f v-only -47.80 -47.83 1.00 v+m -41.37 -42.85 1.04 v+r -41.04 -42.08 1.03 iv. conclusion simulations have been carried out to estimate daily average shortwave radiative forcing efficiencies (rfesurfd ) and (rfe toa d ), and their ratio f, for upper tropospheric volcanic plumes with different optical characterization. we have shown how the optical characterization of the layer affects the reflective/absorbing nature of the plume. the dominating factors are ssa and g, with α being less important, especially with modelled sulphate-dominated layers. the altitude and thickness of the plume has a negligible impact on the radiative forcing, for a fixed optical 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(2001). photometric observations of mt. etna’s different aerosol plumes. atmos environ, 35, 3561– 3572. 6 francescocaprara typewritten text sellitto and briole introduction methods results and discussion dependence on volcanic aerosol optical properties dependence on volcanic plume altitude and depth impact of the presence of other aerosols types in the lower troposphere conclusion variation of the earth tide-seismicity compliance parameter during the recent seismic activity in fthiotida, central greece annals of geophysics, 59, 1, 2016, s0102; doi:10.4401/ag-6795 s0102 variation of the earth tide-seismicity compliance parameter during the recent seismic activity in fthiotida, central greece dimitrios n. arabelos, michael e. contadakis*, georgios vergos, spyrous spatalas university of thessaloniki, department of geodesy and surveying, thessaloniki, greece abstract based on the results of our previous studies concerning the tidal triggering effect on the seismicity in greece, we consider the confidence level of earthquake occurrence tidal period accordance as an index of tectonic stress criticality, associated with earthquake occurrence. then, we investigate whether the recent increase in the seismic activity at fthiotida in greek mainland indicates faulting maturity and the possible production a stronger earthquake. in this paper we present the results of this investigation. 1. introduction recent analyses on the problem of tidal triggering on earthquakes indicate that this effect is manifested [heaton 1982, rydelek et al. 1992, tsuruoka et al. 1995, vidale et al. 1998, wilcock 2001, tanaka et al. 2002, tolstoy et al. 2002, tanaka et al. 2006, cadicheanu et al. 2007, stroup et al. 2007]. in these studies, not only tidal triggering at the global [tanaka et al. 2002] and local [tanaka et al. 2006, cadicheanu et al. 2007, contadakis et al. 2012] scales was found. in addition, in the last three studies, an increase in the reliability of the tidalearthquake occurrence correlation was shown to be a precursory phenomenon for strong earthquakes. in the frame to this international effort, applying the hist(ogram)cum(ullating) method [van ruymbecke et al. 2007], we have analyzed the series of the earthquakes occurred in the last 50 years in seismic active areas of greece. these areas are: (a) the mygdonian basin [contadakis et al. 2009], (b) the ionian islands [contadakis et al. 2012], (c) the hellenic arc [vergos et al. 2015] and (d) the santorini [contadakis et al. 2013]. the result of the analysis for all the areas indicate that the monthly variation of the earthquake frequency of is in accordance with the period of the tidal lunar monthly and semi-monthly (mm and mf ) variations. similarly, the same happens with the corresponding daily variations of the earthquake frequency with the diurnal luni-solar (k1) and semidiurnal lunar (m2) tidal variations. in addition the confidence level for the identification of such period accordance between earthquakes frequency and tidal period varies with seismic activity. the higher confidence level corresponds to time periods with stronger seismic activity. these results are in favor of a tidal triggering process on earthquakes when the stress in the focal area is near the critical level. based on these results, we consider the confidence level of earthquake occurrence tidal period accordance, p, as an index of tectonic stress criticality for earthquake occurrence. we call it “earth tide-seismicity compliance parameter”. then we check on posterior weather the variation of the earth tide-seismicity compliance parameter p, indicate the fault matureness in the case of the recent seismic activity in the prefecture of fthiotida, central greece. in this paper we present the results of this test. 2. seismicity in the broader area of fthiotida on august 7, 2013, a magnitude 5.2 earthquake occurred in the prefecture of fthiotida, central greece followed by series of 18 earthquake with m > 4 until april, 2013. the area is a well known tectonically active area in greece. on april 20 and 27, 1894, two earthquakes of 6.4 and 6.6 ml with more than 250 dead and hundreds of injured in human casualties and cased extensive damages. [papazachos and papazachou 2003, ganas et al. 2006]. figure 1 displays the area and the shocks with magnitudes greater than 3.5 ml which occurred during 2013. in this figure the main faults of the area, as published by ganas et al. [2006], are presented. ganas et al. [2014], analyzing the aftershock sequence, conclude that the spatial distribution of the aftershock sequence points towards the reactivation of a n70° ± 10° e striking normal fault at crustal depths between 8 and 13 km. a nnw–sse cross-section imarticle history received may 14, 2015; accepted january 12, 2016. subject classification: earth tides, seismicity, hi(stogram)cum(ulating) method, schuster test. aged the activation of a steep, south dipping normal fault. they interpret the kallidromon seismic sequence as release of extensional seismic strain on secondary steep faults inside the fokida–viotia crustal block. table 1 displays the earthquakes with magnitudes greater than m = 4 that occurred in the area during 2013. in addition a seismic electrical signal (ses) activity, initiated on january 8, 2013, was recorded at a station of van network, labeled lam, in central greece. additional ses activities were recorded at lam from arabelos et al. 2 date time distance from the nearest city { degree m degree depth (km) magnitude 1 2013/12/11 13:00:53 17.4 km nw of lamia 39.01 22.29 21 4.2 2 2013/11/22 15:12:03 16.8 km n of lamia 39.05 22.41 21 4.2 3 2013/11/12 18:09:28 31.2 km nne of atalanti 38.92 23.10 17 4.8 4 2013/09/20 02:05:18 32.0 km nne ofkorinthos 38.18 23.13 23 4.4 5 2013/09/17 07:39:44 24.0 km wnw of atalanti 38.70 22.73 13 4.0 6 2013/09/16 15:01:14 23.8 km wnw of atalanti 38.72 22.74 17 4.9 7 2013/09/16 14:42:39 24.0 km wnw of atalanti 38.70 22.73 20 4.5 8 2013/08/18 16:39:21 24.8 km wnw of atalanti 38.70 22.72 13 4.0 9 2013/08/18 10:42:54 24.8 km wnw of atalanti 38.70 22.72 14 4.0 10 2013/08/09 13:10:10 30.0 km se of lamia 38.69 22.65 17 4.7 11 2013/08/09 11:49:56 28.0 km w of atalanti 38.69 22.68 9 4.6 12 2013/08/09 11:49:23 25.7 km wnw of atalanti 38.70 22.71 19 4.8 13 2013/08/07 13:44:32 29.8 km w of atalanti 38.69 22.66 15 4.7 14 2013/08/07 09:56:35 27.4 km wnw of atalanti 38.70 22.69 14 4.0 15 2013/08/07 09:06:51 28.2 km w of atalanti 38.70 22.68 8 5.2 16 2013/08/07 09:02:45 29.1 km w of atalanti 38.70 22.67 14 4.3 17 2013/04/28 04:49:55 14.7 km e of aegion 38.26 22.25 55 4.2 18 2013/01/30 04:27:25 35.2 km ene of atalanti 38.78 23.37 22 4.0 figure 1. the seismic activity of 2013 in fthiotida (shematic fault of the area; ganas et al. [2006]). (1) edipsos, (2) sperchios, (3) kadilli, (4,5) kamena vourla, (6) kallidromon, (7,8) atalanti, (9,10) tithorea. table 1. earthquakes with ml > 4.0, in the study area of figure 1. 3 march 31 to april 11, 2013 [sarlis et al. 2015]. this indicate that the area of central greece enter to a tectonically critical stage. then sarlis et al. [2015] analyzed the earthquake sequence from that moment on up to august 12, 2013, in the natural time domain, using the procedure developed in varotsos et al. [2005]. they found that the probability prob(l1) of the l1 values of seismicity in the area of the earthquake of august 7, maximized at l1 = 0.070 at times between 13:10 ut and 19:49 ut of august 9, 2013, exhibiting magnitude threshold invariance in the broad magnitude range mthres= 2.6 to 3.6 (which suggests that the system approaches the critical point). in conclusion, different approaches indicate that the area of fthiotida is found in tectonic excitation. for this area we apply the hi(stogram)cum(ulating)method in order to see if tidal triggering effect is been detected and if this effect is better traced in the period of the increased microseismicity i.e. the years 2011 february of 2012. in our analysis we use the seismological data of the earthquake catalogue of noa (http://www.gein. noa.gr). the set of data consist of a series of 33281 shallow and 769 of intermediate depth earthquakes with ml ranging from 0.2 to 6.3, occurred within the time interval from january 1964 to december 2013, in an area bounded by 38° ≤{≤ 39° and 21° ≤m≤ 23°. however, since 1964 the network of noa subjected three main upgrades. (1) in 1995 the instrumentation and processing passed from analog-to-digital mode. (2) gradually from the end of 2007 to 2011 developed the hellenic unified seismological network (husn), which combined the noa network to three university networks (athens, patras, and thessaloniki), and (3) in early 2011 the magnitude determination software has been upgraded [chouliaras 2009, chouliaras et al. 2013, mignan and chouliaras 2014]. as a result the completeness magnitude mc of the noa catalogue varies. thus in order to realize if the development of the seismicity from 1964 up to 2014 is real and not artificial due to the variation of the magnitude completeness mc, we pick earthquakes with magnitudes m > mc of the most insensitive period, from the catalogue. according to mignan and chouliaras [2014] recommendation, for statistical studies mc = mbmc + 3v, where mbmc and v are the bayesian magnitude completeness and the respective standard deviation. from the respective maps (figures 3 and 4) of the paper of mignan and chouliaras [2014] we get mbm = 2.5 and v = 0.1 for the most insensitive period, which is 1964-1995, for fthiotida. thus the magnitude completeness is mc = 2.8. the resulting development of the seismicity is similar to the seismicity development shown by the original catalogue. we conclude that the variation of the seismicity, shown by the original catalogue, is real. figure 2 displaces the development of the seismicity in the area from 1964, using both catalogues, the original and the one with m > 2.8. it is seen that the number of earthquakes is greatly increased since 2009. finally, it earth tide-seismicity compliance parameter in fthiotida symbol period t (min) signal/noise amplitude (nms-2) origin k1 1436 525.1 487.840 lunar and solar declination wave o1 1549 379.7 352.816 lunar principal wave m2 745 1208.5 510.350 lunar principal wave s2 720 564.5 238.393 solar principal wave figure 2. the development of the seismicity in the selected area from 1964 until december of 2013 using both the original and the one with m > 2.8 table 2. the strongest components of earth tides in thessaloniki. should be noted that only for the years 1964 and 1965 the total number of earthquakes was less than 30, rendering the results of shuster’s test doubtful. 3. tidal effect tidal effects on a rigid earth can be computed theoretically from the ephemerides of the moon, the sun and the planets. the earth tides are the combined effect of the mentioned celestial bodies and the reaction of the solid earth (like an elastic body) to the tidal forces. the ocean tides follow the law of hydrodynamics, with strong disturbances affecting adjacent seas, so that the ocean loading effect has to be taken into account. earth tides are discussed extensively in melchior [1983], baker [1984], torge [2001]. the constituents of the earth tides for the area of thessaloniki were determined gravimetrically by arabelos [2002]. table 2 displays the strongest components of the earth tides for thessaloniki. although the amplitude of the lunar tidal component m1 is equal 27.091 nms-2 (see arabelos [2002], table 3), i.e. it is much weaker than the listed components, we consider in addition the possible effect of this constituent by means of the lunar synodic month (i.e. period from new moon to new moon which is 29d.530589) as well as by lunar anomalistic month (i.e. time period between two successive passages of the moon from perigee which is 27d.554551). table 3 displays the actual ocean corrected tidal parameters of o1 and m2 for sofia, instabul and thessaloniki, and the corresponding values from the model of wahr-dehant-zschau [dehant 1987, dehant and zchau 1989], expressing the dependency of the tidal parameters on the latitude. as it is shown from table 3 the amplitude factors of the principal o1 and m2 tidal constituents agree within their error of estimation with the model. for the latitude of 38° which is the mean latitude of the area under consideration, the extrapolated model amplitude factors for o1 and m2 are equal to 1.156 and 1.158 respectively. consequently, the amplitudes of o1 and m2 might be changed to about 408 and 591 nms-2 respectively, which are very close to the amplitudes observed in the tidal station of thessaloniki. however, this estimation does not take into account the actual elastic properties of the lithosphere in the ionian zone. 4. method of analysis as we have done in previous studies [contadakis et al. 2009, contadakis et al. 2012, vergos et al. 2015], in order to check the possible correlation between earth tides and earthquake occurrence we investigate the time of occurrence of each earthquake in relation to the sinusoidal variation of earth tides and investigate the possible correlation of the time distribution of the earthquake events with earth tides variation. since the periods of the earth tides component are very well known and quite accurately predictable in the local coordination system we assign a unique phase angle within the period of variation of a particular tidal component, for which the effect of earthquake triggering is under investigation, with the simple relation: (1) where zi = the phase angle of the time occurrence of the i earthquake in degrees, ti = the time of occurrence of the i earthquake in modified julian days (mjd), to = the epoch we have chosen in mjd, td = the period of the particular tidal component in julian days. we choose as epoch to, i.e. as reference date, the time of the upper culmination in thessaloniki of the new moon of january 7, 1989, which has mjd = 47533.8947453704. thus the calculated phase angle for all the periods under study has 0 phase angle at the maximum of the corresponding tidal component (of course m2 and s2 has an upper culmination maximum every two cycles). as far as the monthly anomalistic moon concern the corresponding epoch to is january 14, 1989, which has mjd = 47541.28492. intt t t t tdtd 360i i i tdtd0 0 #z = --q qv v" "% %f fi i arabelos et al. 4 sofia (latitude = 42.71°) istanbul (latitude = 41.07°) thessaloniki (latitude = 40.63°) amplitude factor phase degree amplitude factor phase degree amplitude nm factor phase degree o1 1.1493±0.0014 -.1590±.060 1.1564±0.0035 -.281±0.174 1.1536±0.003 -.201±0.151 model 1.1540 -0.2 1.1542 -0.2 1.1543 -0.2 m2 1.1541±0.0005 -.207±0.026 1.1587±0.0011 -.039±0.026 1.1639±0.001 -.195±0.001 model 1.1541 -0.2 1.1583 -0.2 1.1583 -0.2 table 3. ocean corrected parameters of o1 and m2 in 3 neighboring stations. 5 we separate the whole period in 12 bins of 30° and stack every event according to its phase angle in the proper bin. thus we construct a cumulating histogram of earthquake events for the tidal period under study. in order to check the compliance of the earthquake frequency distribution periods with the tidal periods we use the well known shuster’s test [shuster 1897; see also tanaka et al. 2002, tanaka et al. 2006, cadicheanu et al. 2007]. in shuster’s test, each earthquake is represented by a unit length vector in the direction of the assigned phase angle ãí. the vectorial sum d is defined as: where n is the number of earthquakes. when ai is distributed randomly, the probability to be the length of a vectorial sum equal or larger than d is given by the equation: thus, p < 5% represents the significance level at which the null hypothesis that the earthquakes occurred randomly with respect to the tidal phase is rejected. this means that the smaller the p is the greater the confidence level of the results of the cumulating histograms is. finally it should be noted that the total number of the shocks for each year is greater than 30 for all the years since 1966. this means that the normal distribution approach on which shuster test is based is valid for all the years since 1966. 5. results figures 3 to 8 display the cumulating histogram for the 6291 earthquakes which occurred in the time interval from january 1st, 2013, to december 31, 2013. these figures correspond to the tidal periods of: anomalistic monthly period (i.e. time period between two successive passages of the moon from perigee which is 27d.554551) (figure 3), synodic monthy period (i.e. period from new moon to new moon which is 29d.530589) (figure 4), diurnal luni-solar constituents k1 (figure 5), diurnal luni-solar constituent o1 (figure 6), semidiurnal solar constituents s2 (figure 7), and semi-diurnal lunar constituent (figure 8), for the last year of the 50-year analyzed data, i.e. 2013. it is obvious that there is a perfect compliance of tidal and earthquake occurance distribution for monthly anomalistic and synodic period, diurnal luni-solar k1 and semi-diurnal solar s2 periods and a smaller compliance for the diurnal lunisolar o1 and semidiurnal lunar period. this is shown in table 4. this table displays the corresponding confi,cos sind a aii n ii n2 1 1 2w2w= + = = r r2w 2w| | expp n d2 = -a 2 d earth tide-seismicity compliance parameter in fthiotida figure 3. hist(ogram)cum(mulating) for the anomalistic monthly period for 2013. figure 4. hist(ogram)cum(mulating) for the synodic monthly period for 2013. figure 5. hist(ogram)cum(mulating) for the luni-solar k1 period for 2013. (2) (3) dence levels for all six tidal components for 2013 together with the same quantities for a year of low seismic activity, i.e. 1994 and the mean confidence levels for the 50 years. year 1994 was a year with apparently smaller seismic activity than 2013. table 4 displays the confidence level of earthquake-earth tide correlation for all earthquakes of the broader area of fthiotida for the year 2013. in comparison the mean values of the confidence levels for the last fifty years as well as those for the year 1994, a relatively quiet seismically year. this table indicates that the confidence level of the compliance of earthquake frequency distribution over the tidal period is very sensitive to the seismicity of the area. this is also shown in figures 9 to 14. these figures display the variation of the confidence level parameter in the time period 1964 to 2013 together with the earthquakes occurrence for each year. finally it has to be understood that the confidence level parameter p indicate that the tectonic stress in the area has reached a critical point. this also has been found by sarlis et al. [2015]. the magnitude of the potential earthquake depends on the tectonic morphology in the stress regime and seismic history of the area. the high confidence level of the monthly tidal components, despite their small intensity, may indicate that they provide in general favourable conditions for the action of the much stronger tidal components k1 and m2. in this point we may refer to the fact that the monthly tidal barometric variations are quite sensitive to the seismic activity [arabelos et al. 2008]. perhaps this peculiar coincidence merits further investigation. arabelos et al. 6 figure 6. hist(ogram)cum(mulating) for the luni-solar o1 period for 2013. figure 7. cumulation histogram for the semi diurnal solar s2 period for 2013. figure 8. hist(ogram)cum(mulating) for the semi diurnal lunar m2 period for 2013. manom. msynod k1 o1 s2 m2 mean 0.172 0.182 0.387 0.420 0.306 0.435 1994 0.971 0.481 0.470 0.791 0.816 0.812 2013 0.000 0.000 0.000 0.101 0.000 0.159 table 4. the confidence level of the correlation between earthquake and earth tide for all earthquakes of the broader area of fthiotida for 2013 in comparison to the corresponding mean of the last fifty years as well as those of 1994. 7 6. conclusions in this paper we investigate the tidal triggering evidence on the earthquakes of the area of fthiotida in greece. the result of our analysis using the hicum method, indicate that the monthly variation of the frequencies of earthquake occurrence is in accordance with the period of the tidal lunar monthly (mm) variations. the same happens with the corresponding diurnal and earth tide-seismicity compliance parameter in fthiotida figure 9. the confidence level parameter p between seismicity and tidal anomalistic monthly period. arrows indicate the 5.2 ml earthquakes at aegion on 2010 and fthiotida on 2013. figure 10. the confidence level parameter p between seismicity and tidal synodic monthly period. arrows indicate the 5.2 ml earthquakes at aegion on 2010 and fthiotida on 2013. figure 11. the confidence level parameter p between seismicity and diurnal tidal luni-solar k1 period. arrows indicate the 5.2 ml earthquakes at aegion on 2010 and fthiotida on 2013. figure 12. the confidence level parameter p between seismicity and tidal diurnal luni-solar o1 period. arrows indicate the 5.2 ml earthquakes at aegion on 2010 and fthiotida on 2013. figure 13. the confidence level parameter p between seismicity and tidal semi diurnal solar s2 period. arrows indicate the 5.2 ml earthquakes at aegion on 2010 and fthiotida on 2013. figure 14. the confidence level parameter p between seismicity and tidal semi diurnal lunar m2 period. arrows indicate the 5.2 ml earthquakes at aegion on 2010 and fthiotida on 2013. semi-diurnal variations of the frequencies of earthquake occurrence with the diurnal (k1), (o1) and semi-diurnal solar (s2) and semidiurnal lunar (m2) tidal variations. the confidence level of the tidal-earthquake frequency period compliance is very sensitive to the seismicity of the area and we call it tidal-earthquake frequency compliance parameter. we suggest that this parameter may be used in earthquake risk assessment. references arabelos, d. 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(2001). geodesy, 3rd ed., walter de gruyter, berlin-new york. tsuruoka, h., m. ohtake and h. sato (1995). statistical test of the tidaltriggering of earthquakes: contribution of the ocean tide loadingeffect, geophys. j. int., 122, 183-194. van ruymbeke, m., p. zhu, n. cadicheanu and s. naslin (2007). veryweak signals (vws) detected by stacking method according todifferent astronomical pearabelos et al. 8 9 riodicities (hicum), nat. hazards earth syst. sci., 7, 651-656. varotsos, p.a., n.v. sarlis, h.k. tanaka and e.s. skordas (2005). similarity of fluctuations in correlated systems: the case of seismicity, physical review e 72, 041103. vergos, g., d.n. arabelos and m.e. contadakis (2015). evidence for tidal triggering on the earthquakes of the hellenic arc, greece, phys. chem. earth, 85-86, 210-215. vidale, j.e., d.c. agnew, m.j.s. johnston and d.h. oppenheimer (1998). absence of earthquake correlation with earth tides: an indication of high preseismic fault stress rate, j. geophys. res., 103, 24567-24572. wilcock, w.s.d. (2001). tidal triggering of microearthquakes on the juande fuca ridge, geophys. res. lett., 28, 3999-4002. * corresponding author: michael e. contadakis, university of thessaloniki, department of geodesy and surveying, thessaloniki, greece; email: kodadaki@eng.auth.gr. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. earth tide-seismicity compliance parameter in fthiotida << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice experimentation of new tools for scientific learning in the eduseis project: the e-learning experiment new tools for scientific learning in the eduseis project: the e-learning experiment a. bobbio1, l. cantore2, n. miranda3 and a. zollo2 1 istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano -, via coroglio 156, 80124 napoli, italy. tel: +39-081-2420319; fax: +39-081-2420334; e-mail: bobbio@na.infn.it 2dipartimento di scienze fisiche, università di napoli “federico ii”, italy. 3itis “e. majorana”, somma vesuviana, napoli, italy. abstract the educational seismological project (eduseis) is a scientific and educational project, the main aim of which is the development and implementation of new teaching methodologies in earth sciences, using seismology as a vehicle for scientific learning and awareness of earthquake risk. within this framework, we have recently been experimenting with new learning and information approaches that are mainly aimed at a high school audience. in particular, we have designed, implemented and tested a model of an e-learning environment in a high school located in the surroundings of the mt. vesuvius volcano. the proposed e-learning model is built on the eduseis concepts and educational materials (web-oriented), and is based on computer-supported collaborative learning. ten teachers from different disciplines and fifty students at the i.t.i.s. “majorana” technical high school (naples) have been taking part in a cooperative e-learning experiment in which the students have been working in small groups (communities). the learning process is assisted and supervised by the teachers. the evaluation of the results from this cooperative e-learning experiment has provided useful insights into the content and didactic value of the eduseis modules and activities. the use of network utilities and the “learning community” approach promoted the exchange of ideas and expertises between students and teachers and allowed a new approach to the seismology teaching through a multidisciplinary study. key words elearning – jigsaw method – eduseis web tools introduction the educational seismological project (eduseis) is a research and experimentation project that is aimed at involving research centres, universities, scientific museums and schools in the consolidation and maintenance of a prototype of a seismographic network that is formed of high technology and low cost components (bobbio and zollo, 2000). the project is designed to promote the earth sciences (especially seismology and geophysics) and an to increase awareness of seismic risk at the level of high schools and the public at large through the “città della scienza” science centre in naples. a network of digital seismographs has been installed in a number of high schools in central and southern italy (cantore et al., 2003). through the continuous updating of seismic data, and following its analysis and processing, relevant events are selected and made available on the eduseis website (http://eduseis.na.infn.it). the project has many educational, scientific and social goals. the main educational objectives are to train students and teachers in the analysis and interpretation of seismological data. for this purpose, modules and didactic activities have been designed and tested using informatics and web-oriented tools. the scientific objective is to introduce into high schools the use of advanced instruments and experimental methods that are usually restricted to research laboratories, with the main product being the creation of an earthquake waveform archive. thus a large amount of this data can be used by the students and teachers for educational purposes. for the social objectives, the project represents an effective instrument for informing and creating an awareness of the seismic risk, for experimentation into the efficacy of scientific communication, and for an increase in the direct involvement of schools and the general public. 1. the e-learning experiment in the i.t.i.s. majorana technical high school e-learning has been defined as the use of new multimedia technologies and the potential of the internet to improve the quality of teaching/ learning. according to this definition, rather than being a new tool for formal education that can be used in the classroom, e-learning should be considered as a modern method for teaching and learning that integrates and completes the work in classroom. on the “alpi” platform of the edulab cultural association, an e-learning model has been built that is based on the eduseis concepts and its related educational materials. this model is based on computer-supported collaborative learning (ligorio, 1995; trentin., 1998), which is an innovative method for teaching and learning that adopts modern information and communication technologies. the eduseis educational activities (e.g. earthquake location and magnitude estimate, seismogram analysis) are mainly based on web and internet communication tools and are suitable for this kind of experimentation. determination of their didactic value is also one of the main educational objectives of this project. a number of students at the i.t.i.s. “majorana” technical high school (naples) who are following the didactic strategy of the “learning community” and the “jigsaw” method (clarke, 1994) have been involved in this cooperative e-learning experiment. the basic idea is that the students work in small groups (communities) where each member evolves gradually from an initial stage as a “fellow learner”. this is followed by an intermediate level as a “teacher”, and then finally as a “scientist”. the learning process is assisted and supervised by the teachers, who have already improved their use of the internet and their knowledge of earth science. 1.1. the jigsaw method at the majorana school the jigsaw method uses the cooperative concept to promote individual learning, whereby this kind of structure creates interdependence and status equalization (clarke,, 1994). this jigsaw method is a very useful tool for helping students to integrate knowledge and understanding that has been drawn from various sources and experts. indeed, the basic idea behind the jigsaw method is very simple: a general theme and a number of related topics are identified, and the application of this strategy is carried through the following steps: 1. define the general theme that the class will work on; 2. split the class in small groups of students; 3. assign to each group a topic related to the general theme. each group member will become an expert in this topic, using didactic material and support from the teachers; 4. re-mix the students into new groups that include these experts of each topic; 5. reassemble the original groups where each member teaches the rest of the group what they have learnt about the other topics; 6. the groups each present their own analysis of the general theme to the entire class 1.2 trial of the e-learning platform with the eduseis tools ten teachers from different disciplines and fifty students at different educational levels have been involved in the trials around this eduseis-based e-learning platform. the fifty students were divided in five groups, so that students of different levels were present in each of these groups (miranda, 2004). the general theme chosen for this experiment was seismology. this choice was prompted by the collaboration between the school and the research staff of the eduseis project. indeed, during this collaboration, teachers of scientific disciplines trained themselves on and experimented with the tools provided by the eduseis website, which they evaluated as being suitable for this e-learning experiment. at the same time, to test the background knowledge of the students participating in the project, students were asked to initially complete a multiple-choice questionnaire on seismology and earth sciences in general. during the “kick-off” meeting, the teachers showed the “edulab” platform architecture to all of the student groups and illustrated the guidelines for using the tools built on this platform (edulab, 2001). one of each of the five didactic units concerning different topics on seismology that have been set-up on the eduseis website were attributed to each group; these five units relate to the following: 1. world-wide seismicity and earthquake genesis; 2. seismometry and seismic networks; 3. seismic waves and their propagation; 4. earthquake classification according to epicentral distance; 5. observation and interpretation of seismograms (complexity analysis). in the first step, each group became the experts on their own topic. this was achieved by their using both the didactic material available from the eduseis website, and advanced searching on the internet to improve their knowledge and to acquire further expertise on the subject (fig. 2). the exchange of data and information among the students within each group occurred through the internet, using the tools within the chat and forum services for document up-loading and down-loading, which were all made available through the platform. for each student the first approach is to consult and use the eduseis material and tools available on the website, improving the knowledge on the topic through an internet research, the results of which have shared with each member of its own group using the forum, the e-mail and the chat services. most of student activity for the e-learning experiment was carried out via computer both in the school and at home, only a 30% of all experimentation phases were face-to-face with and without the teachers presence. in this phase, all of the students were considered as being apprentices (or “fellow learners”), and they were involved in: · the learning of new concepts, and being opening to questioning their own knowledge; · the accessing of new information using various internet communication channels; · the discussing of their own ideas, doubts and problems, through the chat and forum services available through the platform. as a major result of this phase, a number of documents (e.g. powerpoint presentations, word documents, excel data and analysis sheets) were produced by each group, and the high quality of the content demonstrated their enthusiastic participation and the success that they had obtained through working in these collaborative communities. a typical starting point for the realization of each product in the e-learning experiment is the generation of conceptual maps which are diagrams showing the relationships between concepts in the form of propositions. concepts are included within boxes whereas the relations between them are explicated by means of connecting lines. these maps can often be used to communicate complex ideas. an example of a conceptual map produced by students is shown in fig.3 in the second phase, each group was split, and five new groups were formed in such a way that an expert on each seismology topic covered (see above) was present in each new group, so that each student could then be a teacher for the other members of their new group. in this phase, all of the students were considered as “teachers”, and this involved: · the sharing with each other of their own stock of knowledge; · the teaching of their acquired expertise to the other members of their new group; · the need to demonstrate the reasonableness of their own opinions to the other members of their new group. in the last phase, the original five groups were re-assembled, and each member progressed further on the basis of the experience they had gained in the previous phases. having acquired a deeper knowledge of these seismological topics, each group was ready to make full use of the eduseis activities implemented on the website, by using either the web tools supplied or analogue devices in the school laboratories. at the end of the experiment, each group presented its work to the whole community, i.e. to the classes and the teachers involved in the experiment. in this phase all of the students were considered as “scientists”, and they were faced with the need to demonstrate: · their acquiring of expertise within a new theme (i.e. seismology) that is not a part of the scholastic curricula; · their ability to promote the diffusion of seismological concepts and an increased seismic risk awareness to a widespread community. 2. the eduseis didactic unit: an example the didactic unit chosen for showing the architecture of a typical didactic unit is: “seismic waves and their propagation” four topics are correlated to this theme: · section 1: seismic wave propagation. · section 2: the interior of the earth. · section 3: the seismic wave. · section 4: travel-time curves. each section contains one or more activities that are designed around the use of web tools. in the following, the classical structure of an activity will be illustrated, with this example relating to the activity: “an analysis of the p and s waves on seismograms”. · objectives measurement of p-wave and s-wave first arrivals with the help of theoretical estimates from the jeffreys and bullen (1940) travel-time curves (these give the travel times of all of the main seismic phases from different depths of hypocentre to surface stations at different distances). · area of expertise: seismology the activity is subdivided into two main phases: first phase: inferring arrival times from jeffreys and bullen curves. ingredients: 1. an event selected from the eduseis archives; 2. the “distance applet” for the calculation of epicentral distance (fig. 4); 3. jeffreys and bullen tables; 4. a ruler. second phase: estimation of p-wave and s-wave arrival times inferred from seismograms. ingredients: 1. an event selected from the eduseis archives; 2. the “seisgram2k applet” (lomax, 2000) for the visualization and analysis of seismograms; 3. the origin time of the earthquake inferred from national and/or international seismic alerting agencies; 4. the theoretical values of the arrival times inferred from the jeffreys and bullen curves. in this activity, the use of web tools under the supervision of the teachers allows the students to compute the epicentral distance (the distance from the earthquake epicentre to the point of observation) of the chosen event, and to recognize the first arrival of the p and s waves using the jeffreys and bullen curves to provide reference values. the basic idea of each of these activities is to lead the student through a “learning by discovering” process. 3. results and conclusions one of the most interesting factors that has arisen from this e-learning experiment has been to promote the working of the students and teachers within a "community of learners”. in this model, the classroom is changed in a real community where everyone has different, but changing, roles, as apprentice, teacher and scientist, with the consequent exchanging of tasks and responsibilities. a selection of the products from this experiment have been published on the website of the school, and these have become a useful support for the didactic activities. the involvement of students in this high-social-impact activity represents an opportunity for the increasing of their own awareness of earthquake risk. this is thus seen as improving their understanding of seismic phenomena and the potential damage arising therefrom, along with their knowledge of the measures that can be implemented to mitigate these effects. fig. 5 shows a comparison between the input and output values of a questionnaire administered at the beginning and the end of this e-learning experiment for two different age-level classes. based on the number of correct answers, a quantitative value can be put on the knowledge that has been gained about the topic under consideration, and therefore the positive effects of this e-learning experiment. the evaluation of the results from this e-learning experiment has also provided useful insights into the contents and the didactic value of the eduseis didactic modules and activities. the success of this e-learning experiment is due to the great involvement and motivation of the students, who have provided a large number of high quality products that testify to their enthusiasm for working in cooperative communities. the use of the network utilities for the elaboration of the final products has provided each student with the opportunity to feel that they are themselves a protagonist of the experimentation, and it has promoted the exchange of ideas and expertise between students and teachers. this innovative method that is based on the strategy of the “learning community” has, moreover, allowed a new approach to seismology through a multidisciplinary study, promoting the ability to deal with such complex themes as the study of seismology. acknowledgements we are grateful to all of the teachers and students of the i.t.i.s. majorana technical school in naples who, with much enthusiasm, took on active roles in this project. we wish also to thank angela petrillo, mariella simini, andré herrero and luca d’auria for their contribution in the realization of eduseis web-site. we acknowledge the anonymous referee for his fruitful comments and suggestions. references bobbio, a. and zollo, a. (2000): the educational broadband seismic network at naples (southern italy), orfeus newsletter, 2, no 3. cantore, l., bobbio, a., di martino, f., petrillo, a., simini, m. and zollo, a. (2003): the eduseis project in italy: a tool for training and awareness on the seismic risk, seismological research letters, 74, no 5, 596-602. clarke, j. (1994): pieces of the puzzle: the jigsaw method, in sharan, s. educ., handbook of cooperative learning methods, greenwood press, westport, ct., 1994. edulab (2001): associazione culturale per la ricerca sulla didattica multimediale education laboratory, costituita il 27 luglio 2001. (http://www.edulab.it) jeffreys, h. and bullen, k.e. (1940): seismological tables, british association for the advancement of science, gray milne trust. l. cantore, a. zollo, a. bobbio, flora di martino, m. simini (2005): il progetto eduseis un triennio di esperienze didattiche in sismologia a cura di, edizioni cuen, settembre 2005. ligorio, m.b. (1995): “community of learners": strumenti e metodi per imparare collaborando, td tecnologie didattiche, vol. 1, n. 4 lomax, a. (2000): seisgram2k: seismogram visualization software for the internet. anthony lomax scientific software mouans-sartoux, france. miranda, n. (2004): eduseis con l’ambiente alpi, e-learning: formazione, modelli, proposte – seminario di studio università degli studi di macerata facoltà di scienze della formazione. april 2004. trentin g. (1998): insegnare e apprendere in rete, ed. zanichelli captions: fig. 1. the eduseis website (http://eduseis.na.infn.it) fig. 2. a scheme of the e-learning experiment at the i.t.i.s. majorana technical school, naples. fig. 3. an example of a typical product produced by the students in the 1st phase of the process. the products are included on the website and made available for the community. fig. 4 a web tool for a didactic activity fig. 5. a comparison between before and after values of a questionaire administered at the beginning and the end of the experiment for 1st and 2nd leveles (a) and 3rd and 4th levels (b). fig. 1. fig. 2. fig. 3. fig.4 (a) (b) fig. 5. page 1 annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 1 geoscience engagement in global development frameworks joel c. gill british geological survey, environmental science centre, nicker hill, keyworth, nottingham, ng12 5gg, uk geology for global development, uk joell@bgs.ac.uk florence bullough geological society of london, burlington house, piccadilly, london, w1j 0bg, uk florence.bullough@geolsoc.org.uk abstract during 2015, the international community agreed three socio-environmental global development frameworks, the: (i) sustainable development goals, (ii) sendai framework for disaster risk reduction, and (iii) paris agreement on climate change. each corresponds to important interactions between environmental processes and society. here we synthesize the role of geoscientists in the delivery of each framework, and explore the meaning of and justification for increased geoscience engagement (active participation). we first demonstrate that geoscience is fundamental to successfully achieving the objectives of each framework. we characterize four types of geoscience engagement (framework design, promotion, implementation, and monitoring and evaluation), with examples within the scope of the geoscience community. in the context of this characterization, we discuss: (i) our ethical responsibility to engage with these frameworks, noting the emphasis on societal cooperation within the cape town statement on geoethics; and (ii) the need for increased and higher quality engagement, including an improved understanding of the science-policy-practice interface. facilitating increased engagement is necessary if we are to maximize geoscience’s positive impact on global development. 1. introduction he agreement of three global development frameworks in 2015 reflects ‘a global consensus that business as usual is no option any longer, that changing the development trajectory is necessary’ (spangenberg, 2016, p.1). the un sustainable development goals (sdgs), sendai framework for disaster risk reduction (sfdrr) and cop21 paris climate change agreement (paris agreement) will be at the forefront of national and international policy discourse for the next 15 years. collectively they aim to shape the strategies that guide economic growth, human welfare, access to natural resources, and environmental management. each of the sdgs, sfdrr, and paris agreement relates to the interaction of human activities with the natural environment. advances in science and technology, including geoscience, are central to each framework (e.g., lubchenco et al., 2015; aitsi-selmi et al., 2016; boucher et al., 2016; gluckman, 2016; gill, 2017). for example, managing natural resources, characterizing natural hazards, or modelling future climate all require multi-scale (spatial and temporal) understanding of earth materials and/or processes. this requirement for geoscience input presents an opportunity for the geoscience community. it also places t annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 2 upon us a social responsibility to engage, which we define to mean ‘actively participating in framework design, promotion, implementation, monitoring and evaluation’. scientific business as usual, however, will not be sufficient, with changes to geoscience practice required for successful engagement (lubchenco et al., 2015). in this paper, we describe each global development framework and opportunities for geoscientists to help deliver their objectives (sections 2-4). we then discuss engagement by geoscientists, reflecting upon types of engagement, our ethical responsibility to engage, catalyzing increased engagement, and characterizing effective engagement (section 5). 2. sustainable development goals in september 2015, member states of the united nations formally adopted the sustainable development goals (sdgs), an ambitious set of 17 goals and 169 targets (un, 2015a). the sdgs aim to eradicate global poverty, end unsustainable consumption patterns, and facilitate sustained and inclusive economic growth, social development, and environmental protection over a 15-year period, 2015-2030 (un, 2015a). the sdgs have been described as ‘science intensive’ (gluckman, 2016), with their environmental focus meaning geoscience is essential to their success (lubchenco et al., 2015). gill (2017) produced a matrix, which illustrates the role of geoscience in the sdgs (fig. 1). the matrix was populated by analyzing the text of the specific sdg sub-goals and targets, identifying links between sdg requirements and geoscience. interconnections between many sdgs (nilsson et al., 2016) results in this approach giving a conservative estimate of the true impact of geoscience interventions. for example, goals on education (sdg 4) and gender equality (sdg 5) do not specifically refer to access to water/sanitation (sdg 6), but increased access to water/sanitation can support both. fig. 1 shows a role for geoscience within all 17 of the sdgs. contributions will be required from all sectors and sub-disciplines of geoscience, including those working in research, industry, the public sector and civil society. examples of geoscience activities helping to deliver the sdgs include research projects, industry engagement, and civil society activities. we provide specific examples in section 5. 3. sendai framework for disaster risk reduction (sfdrr) 2015-30 the sfdrr was adopted at the 3rd un world conference on drr in march 2015, supported by the un office for disaster risk reduction (unisdr). through its implementation, the sfdrr aims to reduce substantially disaster risk and losses in all forms (unisdr, 2015). the sfdrr includes four priorities for action (pfa), with a 2016 unisdr conference demonstrating the scope for science and technology in delivering each (aitsi-selmi et al., 2016). we introduce each pfa in table 1, with a description of geo-sciences’ role and examples of engagement. underpinning the four pfa of the sfdrr are 13 guiding principles, many of which require geoscience input. for example, one guiding principle requests that decisionmaking use a ‘multi-hazard approach’. unisdr (2017) defines multi-hazard as considering interrelationships between natural hazards, including hazardous events occurring simultaneously, in cascades, or cumulatively over time. geoscientists have experience in contributing to the understanding and communication of multi-hazard dynamics. for example, following the 2015 m7.8 earthquake in nepal, the british geological survey compiled inventories of triggered landslides (bgs, 2017a). inventories, and associated maps, demonstrate where landslides block rivers (potentially triggering floods), and can be used by organizations responding to disasters. other guiding principles can inform change within the geoscience community, helping to improve engagement in the sfdrr. for example, research collaborations should reflect on the principle ‘international cooperation to be effective, meaningful and strong’. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 3 figure 1: a matrix highlighting the role of geoscientists in helping to achieve the sdgs (gill, 2017). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 4 table 1: geoscience and the sendai framework for disaster risk reduction. priority for action (pfa)1 description and use of geoscience example output 1. understanding disaster risk research on earthquakes, volcanic eruptions, tsunamis, landslides, subsidence, and other hazards addresses this priority. pfa-1 also requests comprehensive surveys of multi-hazard disaster risk, regional assessments and maps, and enhanced access to and support for long-term multihazard research. an interrelated hazards approach to anticipating evolving risk (duncan et al., 2016). 2. strengthening disaster risk governance to manage disaster risk geoscience information informs laws, regulations and policy tools. for example, understanding ground conditions is a necessary input to building codes. pfa-2 also emphasizes the development of strategies to strengthen environmental resilience, environmental and resource management standards, and policies to prevent settlement in disasterrisk prone zones. earthquake science in drr policy and practice in nepal (oven et al., 2016). 3. investing in disaster risk reduction for resilience resilience is enhanced through investment in both structural and non-structural measures. for example, retrofitting critical infrastructure to the effects of earthquakes (structural), and ensuring coherence of drr and urban development strategies (non-structural). pfa-3 seeks to mainstream disaster risk assessment into land-use policy development and implementation. it also encourages cooperation between scientific networks and the private sector to develop new products/services to reduce risk. setting, measuring and monitoring targets for reducing disaster risk (mitchell et al., 2014), with comment on insurance and catastrophe modelling. 4. enhancing disaster preparedness for effective response and to ‘build back better’ in recovery, rehabilitation, and reconstruction pfa-4 requests development and maintenance of peoplecentered multi-hazard, multisectoral forecasting, early warning systems, and hazard-monitoring communications. geoscience information will need integrating with appropriate knowledge in communications, development, and psychology. pfa-4 also encourages preparedness, response and recovery exercises, and sharing of resources. using video games for volcanic hazard education and communication (mani et al., 2016). 1 see unisdr (2015) for full description, and lists of local/national and regional/global objectives. 4. paris climate change agreement geoscience has significantly contributed to our understanding of anthropogenic climate change. for example, evidence of climate change in the geological record forms an important, independent evidence base for anthropogenic climate change (gsl, 2010). the paris agreement, published at the end of the 21st conference of the parties (cop21) in december 2015, secured a legislative agreement with a long-term goal to limit climate change to well below 2˚c above pre-industrial averages (un, 2015b). at the time of writing 132 parties ratified this agreement. the paris agreement consists of an opening statement and 29 ‘articles’ which detail the component parts of the agreement. many articles refer to requirements for which geoscience expertise and capacity are essential, as described in table 2. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 5 table 2: geoscience and cop21: the role of geoscience in delivering the agreement. relevant articles (un, 2015b) contribution of geoscience to article example output article 2.1(a): ‘holding the increase in the global average temperature to well below 2°c above preindustrial levels and to pursue efforts to limit the temperature increase to 1.5°c above pre-industrial levels.’ exploring for and extracting fossil fuels with a lower carbon impact; researching and implementing carbon capture and storage (ccs); investigating geothermal energy sources; and working to develop geological disposal for radioactive waste from nuclear power stations. geological disposal of depleted, natural and low enriched uranium (rwm, 2016) article 2.1(b): ‘increasing the ability to adapt to the adverse impacts of climate change and foster resilience…in a manner that does not threaten food production.’ for secure food production, geoscience is essential to (i) the mapping and understanding of groundwater resources to maintain water security for agriculture, (ii) mineral extraction for fertilizer, and (iii) mapping of soil quality. soil type influences crop mineral composition in malawi (joy et al., 2015) article 4.1: ‘… reach global peaking of greenhouse gas emissions as soon as possible… undertake rapid reductions thereafter in accordance with best available science… achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases…’. globally we need to reduce greenhouse gas emissions to a point where there is a sustainable balance between gas emission and sequestration. this can be through both natural carbon sinks and ccs implementation. locating suitable reservoirs and characterizing these for co2 sequestration over large timescales will require geoscience expertise in stages of design, testing, and implementation. co2 sequestration and storage capacity at sleipner in the north sea (bgs, 2017b). article 7.1: ‘parties hereby establish the global goal on adaptation of enhancing adaptive capacity, strengthening resilience and reducing vulnerability to climate change, with a view to contributing to sustainable development and ensuring an adequate adaptation response…’ geoscientists support research into climatelinked hazards (e.g., flooding, landslides, drought). engineering, hydroand structural geology are essential for effective siting of infrastructure and homes. long-term monitoring data (e.g., slope movement) can be used to inform new development. geologists’ understanding of climate change in the deep past, and its impact on environments can inform mitigation and resilience strategies. resilience assessment for geotechnical infrastructure assets (shah et al., 2014) article 10: ‘parties share a longterm vision on the importance of fully realizing technology development and transfer in order to improve resilience to climate change and to reduce greenhouse gas emissions.’ the technical capacity required to realize the ambitions of the paris agreement will come, in part, from geoscientists. cooperation is needed over areas such as technology transfer and knowledge exchange. sharing of appropriate disciplinary knowledge across political and geographic borders will support the implementation of the paris agreement. collaborative geoscience research, such as that funded by the uk government’s global challenges research fund. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 6 5. discussion in sections 2-4, we describe geoscientists’ role in the sdgs, sfdrr and paris agreement, noting the significant scope for geoscientists to engage in all three. engagement can take many forms, as noted in table 3. here we outline four types of engagement, with examples of actual/potential activities associated with each. the examples in table 3 are illustrative, rather than exhaustive, and intended to promote discussion. in the remainder of this section, we consider this diversity of engagement in the context of (i) our ethical responsibility to engage, (ii) catalyzing increased engagement, and (iii) ensuring effective engagement for maximum development impact. 5.1 ethical responsibility to engage the geoscience community have a professional and social responsibility to reflect on the engagement required to help deliver these frameworks. there is a professional responsibility as the geoscience sector must be equipped and ready to respond to the demands placed on us by government and industry. there is a social responsibility, as our failure to engage, or engage well, can limit what is achieved or reduce sustainability. poor quality engagement (e.g., a weak understanding of the social context of a project, or limited dialogue with stakeholders) may detrimentally impact a project (gill, 2016). we discuss this in section 5.3. the cape town statement on geoethics (di capua et al., 2016) includes a set of geoethical values that help to frame our responsibility to engage in global development frameworks. for example, it encourages sharing knowledge and a spirit of cooperation, and promotes geoeducation and outreach to further sustainable development. the broad range of organizations supporting the cape town statement (e.g., american geophysical union, european federation of geologists, african association of women in geosciences) is indicative of the widespread international support for an outward looking geoscience community. 5.2 catalyzing increased engagement throughout this contribution, we have included examples of activities, projects, and publications that demonstrate existing engagement by the geoscience community in global development. there is scope, however, for this to expand (lubchenco et al., 2015; stewart and gill, 2017), as illustrated by one example. consider the engagement labelled ‘framework promotion’ in tab. 3. the 2017 european geosciences union (egu) general assembly included 1059 scientific sessions and side events (egu, 2017). in the session descriptions, only nine (0.85%) of these 1059 sessions referred to the sustainable development goals, five (0.47%) to the sendai framework, and five (0.47%) to the paris agreement or cop 21. the remaining 1040 (>98%) sessions did not refer to any of the global frameworks, despite many being on pertinent topics. the proactive promotion of development frameworks, including in settings such as the egu general assembly, would help improve awareness and foster greater engagement. it would also demonstrate the role of geoscience to other disciplines and the broader policy-making community. improved awareness could catalyze other types of engagement. for example, helping to shape new research questions, or improving research dissemination to policy makers. 5.3 effective engagement engagement must be effective, culturally appropriate, and sustainable. as previously noted, poor quality engagement can hinder development progress and does not serve society well. the cape town statement on geoethics (di capua et al., 2016) presents a helpful articulation of the necessary values if the geoscience sector is to make a full and positive contribution to the delivery of global development frameworks (e.g., honesty, integrity, competence, commitment to life-long-learning). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 7 table 3: types and examples of engagement. type of engagement example of engagement sfdrr sdgs paris agreement a. framework design: informing the process that determines what is included, defining key terms, determining indicators of success. submission (individual or institutional) to unisdr expert working group on indicators and terminology. early-career scientists engaging with the un major group for children and youth submission to sdg negotiations. research contributions to ipcc assessment reports (ipcc, 2013). b. framework promotion: ensuring that members of the geoscience community are aware of the framework, and potential geoscience inputs. panel discussion on ‘geohazards: from sendai to the sdgs’ at a gfgd conference. sdgs workshop at the european geosciences union general assembly. joint learned societies’ ‘climate communiqué’ (gsl, 2015). c. framework implementation: research, outreach, and industry activities to support the successful delivery of the framework. research: triggered landslides after the 2015 m7.8 earthquake in nepal (bgs, 2017a). research: ‘unlocking the potential of groundwater for the poor’ (upgro, 2017). research: carbon capture and storage (nerc, 2017). practice: developing tools to support earthquake education (parsquake, 2017). practice: construction of sustainable water points (e.g., boreholes) and sanitation facilities. research: groundwater resilience to climate change in africa (macdonald et al., 2011). d. framework monitoring and evaluation: assessing the efficacy of interventions to support implementation. evaluation of landslide education to assess its impact on perceptions of landslide triggering. data collection on access to geoscience training, monitoring progress on sdg 5 (gender equality). long-term monitoring of ocean acidification (ioccp, 2017). professional and learned societies, such as the geological society of london (see www.geolsoc.org.uk), also play an important role in ensuring effective engagement through their focus on professionalism. chartership and the emphasis on continued professional development, encourages the geoscience workforce to reflect on the skills and experiences required to serve society. effective engagement is also rooted in understanding the science-policy-practice interface. this includes, for example, determining the information needs of stakeholders (e.g., policy makers, community groups, development ngos), how they will use this information, and how best to present it to support policymakers. translating geoscience knowledge into tools to support policy and practice requires dialogue and partnerships between geoscientists and other stakeholders (lubchenco et al., 2015). engaging diverse stakeholders early in the research-process helps to ensure a shared perception of the problem, defines data needs, and ultimately results in the production of useful knowledge (weichselgartner and kasperson, 2010). increased dialogue, critical to our contributions being relevant, may also require the geoscience community to invest in additional and complementary skills (gill, 2016). the geoscience annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7460 8 community readily embraces advances in technology, informatics, and other physical sciences to advance their science. in contrast, whereas cultural and ethical understanding, crossdisciplinary communication, and social science research approaches can also support effective engagement and enhance our science, they are rarely included in a geoscientist’s education (stewart and gill, 2017). to engage with policymakers, for example, we should enhance our socio-political understanding (e.g., how government works), and recognize the complexity of policy-making and the role of science as one form of evidence in this process (boyd, 2016; gluckman, 2016). dissemination approaches may also need to change if geoscience engagement is to be most effective. geoscientists are well trained in the skills required to collect, analyze and publish data in scientific journals, and present information at (geo)scientific conferences. these are important opportunities to communicate with other scientists, but may not be the most appropriate medium for communicating with other stakeholders (marker, 2016). priority for action 1 of the sfdrr, for example, includes an objective ‘promote the collection, analysis, management and use of relevant data and practical information and ensure its dissemination, taking into account the needs of different categories of users, as appropriate’ (unisdr, 2015). to realize this objective, we should embrace forms of communication other than the scientific journal, and be proactive at presenting information across disciplinary silos. 6. conclusions in this article, we have highlighted the role of geoscientists in three development frameworks, designed to address global priorities of sustainable development (sdgs), disaster risk reduction (sfdrr), and climate change (paris agreement). these frameworks offer the geoscience community an exciting opportunity for innovative research and application of our science. the successful implementation of these frameworks through 2015-30 will require increased engagement from the geoscience community. this engagement can take many forms, and we include in this contribution examples that demonstrate this broad scope. common across all engagement is the need for it to be of the highest quality, embracing the values and skills required to work at the science-policypractice interface. a geoscience community that invests in the skills and understanding that are required for effective engagement is wellpositioned to help deliver a sustainable future. acknowledgements this paper is the extended version of a contribution made at the 35th international geological congress, 2016 (cape town). we thank martin smith, peter bobrowsky and giuseppe di capua for their comments and guidance, and nic bilham for supporting fb’s contribution. this article is published with the permission of the executive director, british geological survey (nerc). references aitsi-selmi, a., murray, v., wannous, c., dickinson, c., johnston, d., kawasaki, a., stevance, a.s. and yeung, t. 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(2010). barriers in the science-policy-practice interface: toward a knowledge-action-system in global environmental change research. global environmental change, 20(2), 266– 277. microsoft word 4_ferreira_appendix_ba_150721.docx 1 annals of geophysics, 64, 3, se324, 2021; doi:10.4401/ag-8559 appendix to using non-structural mitigation measures to maintain business continuity: a multi-stakeholder engagement strategy mónica amaral ferreira*,1, carlos sousa oliveira2, mário lopes3, francisco mota de sá4, gemma musacchio5, rajesh rupakhety6, danilo reitano7, isabel pais8 1 ceris, instituto superior técnico, ulisboa, portugal, monica.ferreira@tecnico.ulisboa.pt 2 ceris, instituto superior técnico, ulisboa, portugal, csoliv@civil.ist.utl.pt 3 ceris, instituto superior técnico, ulisboa, portugal, mariolopes@tecnico.ulisboa.pt 4 ceris, instituto superior técnico, ulisboa, portugal, fmotasa@gmail.com 5 ingv, istituto nazionale di geofisica e vulcanologia, rome, italy, gemma.musacchio@ingv.it 6 eerc, earthquake engineering research centre, selfoss, university of iceland, rajesh@hi.is 7 ingv, istituto nazionale di geofisica e vulcanologia, catania, italy, danilo.reitano@ingv.it 8 consultant, instituto superior técnico, ulisboa, portugal, pais.isa@gmail.com keywords: business continuity; earthquake; resilience; non-structural elements; critical infrastructures annex 1. stakeholder’s perspective on non-structural elements priorities: diy= do-ityourself; ne= no engineer is needed; er= engineer is needed; h = high; m= medium; l= low nº class category life safety property loss functional loss required intervention priority stakeholder’s priority 1 furniture, fixtures, & equipment computer and communication equipment h h l diy h h 2 furniture, fixtures, & equipment hazardous materials storage h m h diy h h 3 furniture, fixtures, & equipment computer and communication equipment m h m ne h m 4 furniture, fixtures, & equipment miscellaneous contents m h h diy h h 5 mechanical, elec-trical, & plumbing ductwork m h l ne h m 6 furniture, fixtures, & equipment bookcases, shelving h m m diy h h 7 furniture, fixtures, & equipment miscellaneous ff&e h m m diy h h 8 mechanical, elec-trical, & plumbing light fixtures h m m ne h m 9 mechanical, elec-trical, & plumbing light fixtures h m m ne h m 10 furniture, fixtures, & equipment miscellaneous contents h l l diy h m 11 architectural components ceilings l h h ne h h 12 furniture, fixtures, & equipment computer and communication equipment l h m diy h l 13 architectural components canopies, marquees, and signs h h m er m h 14 architectural components exterior & interior walls h h h er m h 15 architectural components exterior wall components h h l er m h 16 architectural components exterior wall components h h l er m h 17 architectural components exterior wall components h h m er m h 18 architectural components interior patitions h h h er m h 19 architectural components parapets and appendages h h l er m h 20 architectural components stairways h m h er m h 21 mechanical, elec-trical, & plumbing storage tanks and water heaters h h l er m h 22 mechanical, elec-trical, & plumbing storage tanks and water heaters h h l er m m 23 architectural components interior floors m h h er m m 24 architectural components interior patitions m h m er m h 25 architectural components interior patitions m h h er m m 26 mechanical, elec-trical, & plumbing electrical and communications equipment m m h er m m 27 mechanical, elec-trical, & plumbing elevators and escalators m h h er m h 28 architectural components canopies, marquees, and signs h m m er m h 3 29 architectural components exterior wall components h m m er m h 30 architectural components exterior wall components h m m er m h 31 architectural components chimneys and stacks m m m ne m h 32 furniture, fixtures, & equipment miscellaneous ff&e m m m ne m m 33 mechanical, elec-trical, & plumbing electrical and communications equipment l m h er m m 34 mechanical, elec-trical, & plumbing electrical and communications equipment l h m er m l 35 mechanical, elec-trical, & plumbing electrical and communications equipment l h h er m l 36 architectural components ceilings l l m ne m h 37 furniture, fixtures, & equipment miscellaneous contents l l m diy m m 38 furniture, fixtures, & equipment miscellaneous contents l l l diy m m 39 mechanical, elec-trical, & plumbing electrical and communications distribution m m m er l h 40 architectural components exterior wall components h m m er m l 41 mechanical, elec-trical, & plumbing storage tanks and water heaters m m m er l l 42 mechanical, elec-trical, & plumbing electrical and communications equipment l m m er l l 43 mechanical, elec-trical, & plumbing mechanical equipment l m m er l l 44 furniture, fixtures, & equipment mechanical equipment h l l ne m m 45 architectural components canopies, marquees, and signs h m l ne h h 46 architectural components canopies, marquees, and signs h h m er m h 47 furniture, fixtures, & equipment miscellaneous contents m l l diy m h muography applied to nuclear waste storage sites annals of geophysics, 60, 1, 2017, s0106; doi:10.4401/ag-7381 s0106 muography applied to nuclear waste storage sites raffaello d’alessandro1,2,*, fabio ambrosino3,4, lorenzo bonechi2, luigi cimmino1,2, david g. ireland5, ralf kaiser5, david f. mahon5, nicola mori1,2, pasquale noli4, giulio saracino3,4, craig shearer6, lorenzo viliani1,2, guangliang yang5 1 università di firenze, dipartimento di fisica e astronomia, sesto fiorentino, florence, italy 2 istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy 3 università di napoli federico ii, dipartimento di fisica, naples, italy 4 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 5 supa, school of physics and astronomy, university of glasgow, glasgow, scotland, uk 6 national nuclear laboratory, central laboratory, sellafield, seascale, cumbria, england, uk abstract legacy storage sites for nuclear waste can pose a serious environmental problem. in fact, since certain sites date from the middle of the last century when safety protocols had not been properly established and strict bookkeeping was not enforced, a situation has evolved where the content of storage silos is basically known only with a large uncertainty both on quantity and quality. at the same time maintenance work on old storage structures is becoming ever more urgent and yet this work requires exactly that information which is now lacking on the type of waste that was stored inside. because of the difficulty in accessing the storage silos and the near impossibility of making visual inspections inside, techniques have to be developed which can determine the presence or absence of heavy elements (i.e. uranium) within the structures. muography is a very promising technique which could allow the survey of previously inaccessible structures. we have begun an evaluation performing feasibility studies using simulations based on real case scenarios. this paper will outline the storage site scenarios and then present some of the results obtained from the monte carlo simulations. 1. introduction the monitoring of legacy storage sites for nuclear waste is growing the interest of the nuclear industry. the limited knowledge of the content of some storage facilities coming from an epoch when there were no strict rules for bookkeeping impairs the ability to perform the needed maintenance operations. investigation campaigns on these sites are strongly constrained by factors like the difficult access to high-radiation environments and the impossibility to make visual inspections. given the critical importance of this topic in the frame of environment conservation, new inspection methods which cope with these difficulties must be developed. muon radiography is a technique that is attracting much attention in many fields related to the investigation of the internal density structure of a given target body. its application to the detection and monitoring of high-z materials (such as uranium) inside various kind of containers ranging from a nuclear reactor [morris et al. 2014, miyadera et al. 2013] to cargos [blackwell and kudryavtsev 2013, riggi et al. 2013] has been studied recently. monte carlo simulations also show that muon radiography can be successful in detecting cm-sized uranium samples inside small storage canisters filled with concrete [clarkson et al. 2015]. this result, however, does not provide any firm evidence that the technique can scale up to storage facilities with typical dimensions of several meters. when enlarging the size of the inspected structure, two kinds of problems arise. the first one is connected to the geometric acceptance of the detection device and thus to the data acquisition time needed to collect a reasonable amount of muons. typically, the presence of high-z materials is investigated by measuring the scattering position inside the target and the scattering angle of an incoming muon: a large number of muons will be scattered at large angles when traversing dense regions inside the inspected structure. this technique requires at least two tracking devices, one upstream of the structure, to measure the direcarticle history received july 5, 2016; accepted october 5, 2016. subject classification: muons, radiography, nuclear waste. tion of the incoming muon, and one downstream, to measure the direction after traversing the structure. the distance between the trackers when they are placed at opposite sides of a large storing facility will reduce the geometric acceptance of the detection device, making the inspection of large volumes unfeasible (although the monitoring of a localized sample with known position would still be possible by means of a proper arrangement of the detectors). the second problem with large facilities is the increased background due to multiple scattering inside the structure itself. a shield of concrete which is several meters thick can add a non-negligible contribution to the scattering angle given by the passage through the high-z region, and increase the probability of a large-angle scattering in the concrete itself. these effects must be assessed by means of dedicated studies. in this paper we present the preliminary results of a monte carlo study of the performance of a muon radiography technique based on muon absorption when applied to large storage volumes. muon absorption is widely used in investigating large geological structures [tanaka et al. 2001, ambrosi et al. 2011, cârloganu et al. 2013] thanks to its large acceptance given by the usage of a single, downstream detector. in the next sections we describe the measurement scenario (storage silo, detector, measurement principle) and the data analysis techniques and then we briefly discuss the results. 2. measurement scenario in order to obtain results which are relevant to the real world, a real storage scenario has been considered. the storage facility is a cylindrical silo filled with concrete, with radius 3.5 m and height 4 m: this kind of facility is present e.g. in the storage site managed by sellafield ltd. (uk). the hypothesized uranium content of the real silos is an unknown number of cm-sized uranium debris in unknown locations. a monte carlo simulation based on geant4 [agostinelli et al. 2003] for the above described scenario has been set up. the simulated silo contains various uranium cubes with sizes of 2, 5 and 10 cm at different locations, and a cubic air bubble with a size of 10 cm in order to assess the effects of eventual low-density regions inside the concrete. the detector is made of two parallel planes spaced by 50 cm; each plane is (2×2)m2 wide and consists of two orthogonal layers of scintillating bars made of poly-vinyl-toluene. the assumed spatial resolution of the detector is 0.3 cm. these figures have been chosen in order to resemble a realistic, scaledup muon detector for geological surveys [ambrosi et al. 2011]. the simulation setup is shown in figure 1. a total number of 1.2×109 events inside the detector acceptance have been simulated, which corresponds to a data acquisition time of about 160 days. the energy and angular spectra of the muon flux used for random particle generation have been parameterized from ground measurements performed with a magnetic spectrometer [bonechi et al. 2005]. the simulated energy range is 0.7 130 gev, while the polar angle ranges from 0 (i.e. vertical muon) to 80 degrees. 3. data analysis the absorption technique seeks for missing muons due to enhanced absorption when traversing high-density regions inside the target. this requires an estimate of the expected number of detected events coming from any direction when no density fluctuation is present, in order to compute the amount of missing events from a given direction which will be proportional to the column density along that direction. for large targets like geological structures this method can only provide information about the column density of the target along a line of sight, since the detector is point-like when compared to the dimension of the target, thus resulting in a two-dimensional density map. however, in the storage silo scenario the dimension of the detector is not negligible, and this fact can be exploited to obtain a full three-dimensional estimate of the target density. as shown in figure 2, missing muon tracks due to a localized high-density region inside the target are detected along different directions, allowing for a stereoscopic reconstruction of the position of the absorption point. detected tracks can be back-extrapolated to the silo region, the intersections with yz planes at a given x can be computed and from those a yz histogram of intersection points can be built. this histogram (the “back projection map”) will show a deficit of events with respect to the d’alessandro et al. 2 figure 1. schematic representation of the simulated scenario. the direction of arrival of detected muons is from right to left. 3 homogeneous target case in the portion of the histogram (the “signal region”) where the yz plane intersects the high-density region inside the silo. together with the x position of the back-projection plane, the yz position of the deficit will give the full three-dimensional location of the high-density region. if the plane does not intersect the high density region then the signal region will be wider and the signal-to-noise will be less, analogously to what happens to optical images outside the focal plane of the optical system. many back-projection layers at different x coordinates can thus be considered, and the x position of the high-density region will be that of the layer giving the smallest signal region with the highest signal-to-noise ratio. this method is described in details in bonechi et al. [2015]. the above described method requires the knowledge of the expected event count for a silo containing no uranium debris. this information cannot be measured directly since the content of the silos is unknown and thus there is no known uranium-free silo. however, since uranium debris are much smaller than the silo and their positions are scattered randomly in the silo volume, it seems reasonable to assume that the position of a debris inside a silo will not match any of the positions of the debris inside another silo, so that the signal regions of the back projection maps at a given x for the two silos will never overlap. with this assumption, subtracting the back projection map of silo a from that of silo b will show signal regions for b as regions with a deficit of events (direct signal), and those for a as an excess of events (inverse signal). in order to avoid the necessity of acquiring data for two silos, the mirror symmetry about the z axis of the measurement scenario can be exploited. assuming that no uranium debris will lie exactly on the z axis, one can mirror the back projection map about the z axis and subtract it from the direct map. in this way, every direct signal region at coordinate (y, z) will have a mirrored inverse signal region at (−y, z). to investigate the presence of a signal from uranium debris and study its variation with acquisition time, simulated data have been analyzed by placing back projection layers so that they intersect the debris inside the silo. in a realistic scenario this would not be possible, since the positions of the debris would be unknown. a full stack of planes with a suitable regular spacing along x would be required to perform a tomographic scan of the silo, and suitable algorithms to find the plane containing the signal region with minimum extension and maximum signal-to-noise would have to be implemented in order to determine the position of the debris. this is far beyond the scope of this work, which only aims at assessing the eventual presence of a signal and its time dependence. so for this goal we relied on the knowledge of the positions given by the monte carlo truth. the simulated data have been processed with the mirror map technique in order to produce event count difference maps. each bin of the difference maps is computed as the difference of two independent poisson random variables (i.e. event counts), and thus follows a skellam distribution whose rms is given by the square root of the sum of the expectation values of the two poisson variables, which have been estimated with the event count themselves. signal-to-noise maps have thus been built by dividing each bin of the difference maps by its skellam rms. an example of these maps is shown in figure 3. the back projection layer is set at x = −100 cm, where muography applied to nuclear waste storage sites figure 2. measurement principle: missing tracks (dashed lines) due to absorption in a high-density region (black box) “hit” the detector with different angles, allowing for a stereoscopic view of the absorption point. figure 3. signal-to-noise map for an acquisition time of about 160 days computed at x = −100 cm. the direct (red) and inverse (blue) signal regions are clearly visible at z ~ 350 cm. a 5 cm uranium cube is present at coordinates (y, z) = (−15, 350) cm (see figures 1 and 2 for the details of the reference frame). the signal is clearly visible with a signal-to-noise value for the single bin around 6, thanks to the uranium sample being placed near to the detector and in the upper part of the silo. this condition increase the statistics since the muon flux increase with decreasing polar angle, and makes the effect of multiple scattering in the concrete less effective due to the reduced thickness of traversed material. indeed, the same 5 cm sample placed near the center of the silo does not produce a detectable signal. on the contrary, a 10 cm sample in the same position produce a signalto-noise of about 7. the detection capability is thus strongly influenced by the position of the uranium debris, other than their size. to put a lower limit on this last parameter, 2 cm samples have been placed in various positions, but none of them resulted to give a detectable signal with the available statistics. also, the 10 cm air bubble placed near the center of the silo resulted to be not visible as well. in order to define the lower signal-to-noise threshold for detection, the probability of having a statistical fluctuation in a single bin giving a signal-to-noise greater than a given value has been studied. when taking into account the look-elsewhere effect given by the ~2000 bins corresponding to the silo region in the signal-to-noise map, it turns out that the probability of having one or more bins with a signal-to-noise ratio greater than 4 due to statistical fluctuations is about 9%, decreasing to 0.08% when considering a threshold of 5. a signal-to-noise value of 5 have thus been assumed as a safe detection threshold. to assess the acquisition time needed to reliably detect a uranium sample, the analysis has been repeated by considering subsets of the whole simulation data set. a plot showing the dependence of the signal-to-noise ratio on the acquisition time for the 5 cm and the 10 cm visible samples is shown in figure 4. an acquisition time of 1-2 months result to be sufficient to detect both the 5 cm and the 10 cm samples. 4. discussion in this work the feasibility of searching legacy nuclear waste storage silos for uranium presence leveraging the absorption of atmospheric muons has been investigated. this is a novel application of a well established and developed technique in the field of the monitoring of geological structures, which present many peculiar aspects when applied to the nuclear storage scenario. this preliminary analysis based on monte carlo simulations shows that in a realistic case there is a good discovery potential for well-positioned uranium debris with a typical size greater than 5 cm. the required data acquisition time is of the order of a couple of months. the study of systematic effects given by e.g. non-uniform concrete density, the presence of other heavy debris like steel rods, surrounding buildings, different parameterizations of the muon flux etc. are beyond the scope of this preliminary, and will be addressed in a future work. references agostinelli, s., al. (2003). geant4 a simulation toolkit, nucl. instrum. meth. a, 506, 250-303. ambrosi, g., et al. (2011). the mu-ray project: volcano radiography with cosmic-ray muons, nucl. instrum. meth. a, 628, 120-123. blackwell, t.b., and v.a. kudryavtsev (2013). identification of nuclear materials in cargo containers using cosmic rays, nuclear science symposium and medical imaging conference (nss/mic), 1-8. bonechi, l., m. bongi, d. fedele, m. grandi, s. ricciarini and e. vannuccini (2005). development of the adamo detector: test with cosmic rays at different zenith angles, proceedings of the 29th international cosmic ray conference, 9, 283-286. bonechi, l., r. d’alessandro, n. mori and l. viliani (2015). a projective reconstruction method of underground or hidden structures using atmospheric muon absorption data, j. instrum., 10, p02003. cârloganu, c., et al. (2013). towards a muon radiography of the puy de dôme, geosci. instrum. meth., 2 (1), 55-60. clarkson, a., d.j. hamilton, m. hoek, d.g. ireland, j.r. johnstone, r. kaiser, t. keri, s. lumsden, d.f. mahon, d’alessandro et al. 4 figure 4. signal-to-noise ratios for the 10 cm (open squares) and the 5 cm (full squares) samples as functions of the data acquisition time. error bars represent the statistical errors. 5 b. mckinnon, m. murray, s. nutbeam-tuffs, c. shearer, g. yang and c. zimmerman (2015). characterising encapsulated nuclear waste using cosmicray muon tomography, j. instrum., 10, p03020. miyadera, h., k.n. borozdin, s.j. greene, e.c. milner, c.l. morris, z. luki , k. masuda and j.o. perry (2013). imaging fukushima daiichi reactors with muons, aip advances, 3, 052133. morris, c.l., et al. (2014). analysis of muon radiography of the toshiba nuclear critical assembly reactor, appl. phys. lett., 104, 024110. riggi, s., v. antonuccio-delogu, m. bandieramonte, u. becciani, a. costa, p. la rocca, p. massimino, c. petta, c. pistagna, f. riggi, e. sciacca and f. vitello (2013). muon tomography imaging algorithms for nuclear threat detection inside large volume containers with the muon portal detector, nucl. instrum. meth. a, 728, 59-68. tanaka, h., k. nagamine, n. kawamura, s.n. nakamura, k. ishida and k. shimomura (2001). development of the cosmic-ray muon detection system for probing internal-structure of a volcano, hyperf. int., 138 (1), 521-526. *corresponding author: raffaello d’alessandro, istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy; email: candi@fi.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. muography applied to nuclear waste storage sites << /ascii85encodepages false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 131_143 adg v5 n01 mizrahi.pdf annals of geophysics, vol. 45, n. 1, february 2002 131 statistical properties of the deviations of f0 f2 from monthly medians eti mizrahi (1), ayşe h. bilge (1) and yurdanur tulunay (2) (1) department of mathematics, istanbul technical university, maslak, istanbul, turkey (2) faculty of aeronautics and astronautics, istanbul technical university, maslak, istanbul, turkey abstract the deviations of hourly ƒ0f2 from monthly medians for 20 stations in europe during the period 1958-1998 are studied. spectral analysis is used to show that, both for original data (for each hour) and for the deviations from monthly medians, the deterministic components are the harmonics of 11 years (solar cycle), 1 year and its harmonics, 27 days and 12 h 50.49 m (2nd harmonic of lunar rotation period l 2 ) periodicities. using histograms for one year samples, it is shown that the deviations from monthly medians are nearly zero mean (mean < 0.5) and approximately gaussian (relative difference range between %10 to %20) and their standard deviations are larger for daylight hours (in the range 5-7). it is shown that the amplitude distribution of the positive and negative deviations is nearly symmetrical at night hours, but asymmetrical for day hours. the positive and negative deviations are then studied separately and it is observed that the positive deviations are nearly independent of r12 except for high latitudes, but negative deviations are modulated by r12. the 90% confidence interval for negative deviations for each station and each hour is computed as a linear model in terms of r 12 . after correction for local time, it is shown that for all hours the confidence intervals increase with latitude but decrease above 60°n. long-term trend analysis showed that there is an increase in the amplitude of positive deviations from monthly means irrespective of the solar conditions. using spectral analysis it is also shown that the seasonal dependency of negative deviations is more accentuated than the seasonal dependency of positive deviations especially at low latitudes. in certain stations, it is also observed that the 4th harmonic of 1 year corresponding to a periodicity of 3 months, which is missing in ƒ0f2 data, appears in the spectra of negative variations. 1. introduction the ultimate goal of the work related to the ionospheric critical frequency ƒ 0 f 2 is prediction and forecast. however determination of reliability bounds is also important for planning in hf communication, radar and navigation systems. the median values for each hour can be considered as a first approximation to the data. the prediction of the median values, based on standard trigonometric expansions and parabolic models in terms of the 12 month smoothed sunspot number r12, proves to be satisfactory, with the mean square errors in the range of 3-7% (baykal, 1998; bilge and tulunay, 1998). the estimation of the actual hourly values of ƒ0f2 is called «forecasting», and it deals with neural network (tulunay et al., 2000) and feedback methods (bilge and tulunay, 2000), in addition to more standard autocorrelation techniques (stanislawska et al., 1999). in this paper, we concentrate on the deviations of ƒ0f2 from monthly medians, denoted by f = ƒ 0 f 2 – mailing address: dr. eti mizrahi, istanbul technical university, department of mathematics, 80626 maslak, istanbul, turkey; e-mail: mizrahi1@itu.edu.tr key words ƒ0f2 – critical frequency – variability 139 statistical properties of the deviations of f 0 f 2 from monthly medians that in night hours the negative deviations are below the positive deviations, while in day time the roles are reversed. the local time dependency can also be seen as a shift of the crossing points as we move longitudinally. we have obtained similar graphs for each latitude group in table i, where the similarities of the shapes of the curves inside each group justify the subdivisions with respect to latitude. 3.4. seasonal dependency as we worked with 1-year samples in the statistical analysis, we overlooked the seasonal dependency of the deviations. we can however make certain qualitative remarks based on our time domain plots and our frequency spectra. the time domain plots of f given in fig. 7a,b show that at high latitudes, for example uppsala, both positive and negative deviations have seasonal dependency while at lower latitudes, for example at rome, only negative deviations have seasonal dependency. the amplitudes are higher during equinoxes. this is consistent with the spectral analysis results given in fig. 3b, where it can be seen that the periodicity of 6 months has higher power for negative deviations. 3.5. long-term trend analysis in previous sections, we considered ( f +/ ) as a function of r12. it is also of interest whether there is a trend irrespective of the solar conditions. for this purpose, we selected years with r 12 = 100, r 12 = 50 and r 12 = 20 in rising and falling phases, obtaining 6 data groups. for example for r12 = 100 in the falling phase we have the years (1960, 1970, 1982, 1991). in each data group we take the difference of the data for consecutive years that we denote as f +/ . for example, if there is an increase in f +/ from 1982 to 1991, at a given hour and station, the corresponding entry in f +/ will be positive. the relative «strength» of the positive values in f +/ will be an indication of an increasing trend. in order to quantify these trends, we separated the positive and negative parts of f +/ and computed their norms, denoted as n p and n m . if the ratio n p /n m is larger than 1 there is a trend to increase from one year to the next in the same group, while if n p /n m is less than 1 there is a trend to decrease. we found that for positive deviations from monthly medians the ratio n p /n m fig. 5. comparison of the histogram plots of the deviations from monthly medians for rome at 24 ut and 9 ut. fig. 6. positive (solid line) and negative (dotted line) deviations for slough, juliusruh, kaliningrad, moscow (in the 50°n-57°n latitude band). plots are shifted each by 2 units. 140 eti mizrahi, ayşe h. bilge and yurdanur tulunay is larger than one for 15 out of 17 times, while for negative deviations from monthly medians, it is larger than one only for 10 out of 17 times. thus we can conclude that for positive variations there is a trend to increase with time irrespective of the solar conditions. in the literature, linear trends in ƒ 0 f 2 are tied to the lowering of the f 2 layer caused by the increasing greenhouse effect (bremer, 1992) and the increase in magnetic storm activity (clilverd et al., 1998; danilov, 2001). 4. modelling positive and negative variations separately for each station year and hour, we computed the 90% confidence interval, i.e. the value ( f +/ )c such that ( f +/ ) is less than ( f +/ )c with a probability of 0.9, for the given station, hour and year. we have seen that these upper deciles for positive deviations are virtually independent of r12 and we decided to investigate the negative deviations. in general ( f ) c for a given hour (ut) is expected to depend on r12, the latitude and longitude of the station and the month of the year. as we worked with yearly samples of data, we lost the information related, for example, to the semi-annual and seasonal dependencies. on the other hand, the time domain plots in fig. 6 suggest that the longitude dependency reflects in a shift of the curves, hence using a local time correction we may assume that ( f )c is a function of the latitude, local time and r12. we shall look for a linear model with respect to r 12 . as an example, in fig. 8 we show ( f ) c for sverdlovsk for each hour as a function of r12 . the data are quite scattered, but we can still look for a linear trend using least squares approximation: for each station and each hour we obtained the slope and the intercept of an approximating line, by minimizing the least squares error between the data and the linear model. for selected representative stations and hours these errors range between 9.9% and 21.3%, which indicates the existence of some linear trend. as a result, we obtain a model where we express the 90% confidence interval in terms of r 12 as below. ( f )c (latitude, local time, r12) = = a (latitude, local time) r 12 + + b (latitude, local time). where the «coefficients» a and b depend on the latitude and local time. in order to analyse the dependency on the latitude, we evaluate ( f )c (4.1) fig. 7a,b. yearly variation of the positive and negative deviations for uppsala (high latitude) and rome (low latitude). a b 141 statistical properties of the deviations of f 0 f 2 from monthly medians as given by eq. (4.1) for a representative value of r 12 , say the median, corresponding to (r 12 ) median = 62.63. this gives a representative value of ( f ) that we denote as ( f )m given below. ( f ) m (latitude, local time) = = a (latitude, local time)(r12) median + + b (latitude, local time). note that ( f )m is a function of two variables only. for each station, we plot ( f )m with respect to local time and present the results in fig. 9a-d grouped with respect to their latitudes. it can be seen that the graphs for stations between 60°n-70°n are less coherent, the maxima are located at 12-14 lt, and the amplitudes are less than 14 mhz. for the stations in the 50°n-57°n band, the variations fig. 8. upper deciles of ffor sverdlovsk, with respect to r 12 for all hours. are extremely coherent, there is a distinct peak at 12 lt, and the amplitudes reach 18 mhz. down to 45°n-51°n band, all graphs have a peak again around noon, but their amplitudes seem to have a shift, their respective maxima ranging from 10 to 16 mhz. in the lowest latitude band, 40°n-43°n, the peaks are located around 10-11 lt, the maxima range from 7 to 14 mhz, but the lowest value of 7 mhz, for tortosa is questionable, because the data for tortosa has large gaps. the incoherency of the graphs of equal latitude/ different longitude stations shows that the local time correction at high latitudes may not compensate for spatial dependency. on the other hand, graphs for lower latitude station groups are coherent; hence we may say that local time correction is successful at least below 60°n latitude. we also note that the variation curves shift up with latitude up to 143 statistical properties of the deviations of f 0 f 2 from monthly medians deviations have larger amplitude at 12 ut and the histograms are wider at daytime. the negative deviations are modulated by r12 at all latitudes, but positive deviations have a r12 dependency only at high latitudes. also from the frequency spectrum we observed that the 27 days periodicity disappears at night time (24 ut). at high latitudes both positive and negative deviations have seasonal dependency while at low latitudes positive deviations are nearly independent of season. the longitude dependency reflects as a shift with respect to local time. we quantified the r12 dependency of the positive and negative deviations as a linear model for the %90 confidence interval for each station. we found that the positive deviations were rather independent of r12, and we computed representative values for the upper deciles of negative variations for each station and local time. on the other hand, we found an increase in the 90% confidence intervals of positive deviations after elimination of r12 effects, which were not apparent in the negative deviations. for stations below 60°n, the longitude dependency was eliminated by a local time shift and the upper deciles have a coherent behaviour for stations grouped according to latitudes. the location of the peaks moves from 11 lt to 14 lt from south to north, and the peak amplitudes start from 10 mhz, increase to 18 mhz and then decrease to 16 mhz above 60°n. this latitude dependency agrees with recent investigations where kouris et al. (2000) also observed an increase in muf with latitude followed by a decrease above 60°n. as we worked with 1 year samples of data for the study of the deviations from monthly medians, we overlooked any seasonal dependencies in the quantitative study of the deviations. seasonal variations appeared only in frequency spectra. semi-annual (rishbeth et al., 2000) variations as well as the seasonal variations of the hysteresis effects (buresova and lastovicka, 2000) are discussed in the literature. acknowledgements this work is partially supported by the cost271 eeeag-tubitak project. the authors are indebted to the comments by the referees; section 3.5 was added on referee’s comments. references baykal, s.a. (1998): a model study of ionospheric critical frequencies for telecommunication purposes, m. sc. thesis, middle east technical university, ankara. bilge, a.h. and y. tulunay (1998): semi empirical single station modelling of f0f2 variations: spectral analysis, poster presented at the 23rd general assembly of european geophysical society, nice, france. bilge, a.h. and y. tulunay (2000): a novel on-line method for single station prediction and forecasting of ionospheric critical frequency f 0 f 2 1-hour ahead, geophys. res. lett., 27, 1383-1386. bremer, j. (1992): ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effects, j. atmos. terr. phys., 54, 1505-1511. buresova, d. and j. lastovicka (2000): hysteresis of f 0 f2 a t european middle latitudes, ann. geophysicae, 18, 987-991. clilverd, m.a., t.d.g. clark, e. clarke and h. rishbeth (1998): increased magnetic storm activity from 1868 to 1995, j. atmos. sol.-terr. phys., 60, 1047-1056. danilov, a.d. (2001): f2-region response to geomagnetic disturbances, j. sol.-terr. phys., 63, 441-449. kouris, s.s., d.n. fotiadis and b. zolesi (1999): specifications of the f-region variations for quiet and disturbed conditions, phys. chem. earth (c), 24 (4), 321-327. kouris, s.s., d.n. fotiadis and r. hanbaba (2000): on the day-to-day variation of the muf over europe, phys. chem. earth (c), 25 (4), 319-325. rishbeth, h., i.c.f. müller-wodarg, l. zou, t.j. fuller-rowell, g.h. millward, r.j. moffett, d.w. idenden and a.d. aylward (2000): annual and semiannual variations in the ionospheric f 2 layer:ii. physical discussion, ann. geophysicae, 18, 945-956. stanisl/ awska, i., lj. r. cander, e. tulunay, y. tulunay, a.h. bilge, g. juchnikowski, m. kadioglu, c. özkaptan and z. zbyszynski (1999): instantaneous mapping of 1-hour ahead f0f2 forecasting values over europe, in cost 251 4th workshop, madeira, portugal, 22-26 march 1999, 235-241. tulunay, e., c. özkaptan and y. tulunay (2000): temporal and spatial forecasting of the f 0 f 2 values up to twenty four hours in advance, phys. chem. earth (c), 25, 281-285. 703_708 ferrari.pdf annals of geophysics, vol. 45, n. 5, october 2002 703 letters in the earth sciences: their historic value and present-day scientific relevance graziano ferrari sga storia geofisica ambiente, bologna, italy 1. introduction scientific letters, of great importance in the history of science and scientific instrumentation, have a particular importance for those sectors of earth sciences in which structured observation networks are distributed across the territory, such as seismology, meteorology, geomagnetism and astronomy. it may come as a surprise to know that in the historical scientific letter collections there can be information having a certain scientific novelty. the current scientific relevance for seismology of the content of some historical letters is one of the aspects that most of all motivate an investment into this kind of research, more by the seismologists than by the historians of science. the experiences conducted up until now within the tromos project (ingv-sga) framework have led us to identify the following types of information: description on the effects of seismic events; scientific comments to theories, to publications, etc.; graphs; reproductions of seismograms; news of loans of recordings; information and drawings of instruments, their location and orientation within the observatories. the first three information types do not require particular comments, while the others are very important for seismological research and it is worth deepening several aspects. among the hundreds of the world’s seismological observatories that have recorded earthquakes ever since the early 1890s (ferrari, 2000), unfortunately not all of them have kept their recordings archives intact. in many cases these were partly or totally destroyed for a great variety of causes: human (state of abandonment, deliberate destruction or wars) and natural (fires, earthquakes, floods, etc.). at times the recordings of the most important earthquakes have been lost, with serious damage done to research. the reproductions of their seismograms and/or related information that can be gleaned from several letters, appropriately re-evaluated according to the current methods of analysis, can still provide important elements and return part of the information lost. within the framework of a broad international collaboration in the projects iaspei subcommittee historical instruments and documents in seismology and the esc working group history of seismometry (ferrari, 2000), realised on the experience of the tromos project, the idea of starting up a specific research strand and collaboration into the scientific correspondence in the earth sciences was born. it is true that the experience hitherto performed mainly refers to the seismological field and at most to the meteorological field, but is should nevertheless be observed that the disciplinary distinction is rather recent and that in any case in italy the meteorological and seismic observation has often been conducted in long-established astronomical observatories. thus the materials, mailing address: dr. graziano ferrari, sga storia geofisica ambiente, via bellombra 24/2, 40136 bologna, italy; e-mail: ferrari@sga-storiageo.it 704 graziano ferrari the observations and history of distinct disciplines mingle together. the astronomical observatories of brera at milan, turin, piacenza, the collegio romano in rome, capodimonte in naples and palermo, to mention just a few, are some examples of how a historical italian astronomical observatories have constituted the natural home that hosted meteorological observation first, followed by seismological observation (ferrari, 2002). numerous and largely identified files of historical correspondence of interest for meteorology and astronomy lie idle, uncatalogued, and even risking dispersion. the problem of cataloguing the files and above all managing them it with as view to consultation for study purposes has often been a disincentive that has prevented the preservation bodies from ever getting off the ground. within the scope of the tromos project, we are digitally scanning the files of scientific correspondence of some of the most important italian seismological observatories. the catalogues and scans are conducted in collaboration with the conservatories of documentation, both public and private, and is made for their benefit. the letters of the files of timoteo bertelli (1826-1905), pietro tacchini (1838-1905) and part of that of giulio grablovitz (1846-1928) have already been submitted to electronic scanning, respectively held at the centro studi storici barnabiti of rome (transferred there from its original site of production and preservation, the «alla querce» boarding school of florence), the ufficio centrale di ecologia agraria in rome and at the home of the descendants of grablovitz (temporarily kept at sga). it is no coincidence that the first systematic digital scan was performed on the tacchini file: he was an astronomer, ‘meteorologist’ and ‘seismologist’, at least from the institutional point of view. the digital archive hitherto produced has brought together over 7000 letters, for a total of over 11 000 pictures, received by tacchini, bertelli and grablovitz from over 800 correspondents, all surveyed in the tromos database and documented by all the biographical information available. modern computer technologies allow us to store and distribute, via dvd-rom and the internet, the most diverse information forms and thus make it possible to realise what only a few years ago was unthinkable: a single letters archive concerning the scholars of earth sciences. the project, enlarged to embrace a broader community of scholars, aims to promote and foster the identification, cataloguing, and electronic scanning of the letters, thus making available the files to of interest for the study of the history of earth sciences and astronomy. parallel to the project we also aim to enhance the recovery in electronic format of everything hitherto published: catalogues of letters, registers, integral letter transcriptions; this phase should also be flanked by a digital scanning of the letters themselves. cataloguing, scans and online availability are performed respecting the rights established by the law and by the specific agreements stipulated with the public or private bodies that preserve the files subjected to the study. in order to discuss the methodological aspects of the study of the scientific correspondence and their scientific relevance, a seminar has been organised for 10th may 2002, promoted by the author in collaboration with the istituto nazionale di geofisica e vulcanologia (ingv) and the ufficio centrale di ecologia agraria (ucea), at the congress hall of the ucea, historic headquarters of meteorology and seismology in italy. at the seminar, apart from the scholars most committed to studies of this kind over the past few years, the descendants of the most illustrious italian scholars of the earth sciences have also been invited: mario baratta (1868-1935), pietro caloi (1907-1978), michele stefano de rossi (18341898), giulio grablovitz (1846-1928) and quintino sella (1827-1884), besides a prestigious representation of the public, private and ecclesiastic italian archives, of interest to this sector. 2. the seminar the seminar was held under the splendid renaissance ceilings of the ucea, furbished to host congresses and exhibitions. in order to represent the various situations existing in this particular study sector and the 705 letters in the earth sciences: their historic value and present-day scientific relevance multidisciplinary and interdisciplinary characteristics of the various approaches, the day of study developed along five strands: 1) scientific letters: study traditions and prospects; 2) fragments of the european context; 3) case studies in italy; 4) places of observation; 5) places of preservation. domenico vento (director of the ucea, rome) opened the session with a welcoming message that traced back the stages from the birth and the transformations of the office he is directing. vento briefly discussed the great historic and scientific value of the documentary and instrumental assets of the ucea, generally acknowledged for some time. he also recalled the long and well-established tradition, within the tromos project, between the ucea on the one hand and ingv and sga on the other, and he expressed his hope that the renewed cultural understanding between these bodies could achieve new and important objectives. 2.1. scientific letters: study traditions and prospects this section was dedicated to the relating of research experiences having a consolidated tradition, and two talks on astronomy and seismology were given. there also emerged the international character that this kind of research can have, given the universal nature of the research itself and the relationship between the scholars. pasquale tucci (institute of general applied physics, university of milan) with his talk on the correspondence between otto struve and giovanni virginio schiaparelli (1864-1904), two eminent astronomers who lived between the 19th and 20th centuries, presented by means of the study of an important scientific collection, a clear-cut example of the potential that this kind of documentary evidence has for the history of science, embedding it in the cultural and political context of the day. in reconstructing, through some 280 letters preserved in milan and pulkovo (russia), the major stages in the scientific and public lives of the two scholars, tucci also paused to reflect on the specific value of the testimonies provided by this kind of documentation. for example, its crucial role in the reconstruction of the history of international scientific institutions such as the international astronomical union, established and developed thanks to those scholars whose collaboration went far beyond the official availability granted by the various states. graziano ferrari (sga storia geofisica ambiente, bologna) with his talk on correspondence in seismology, historical value and present-day scientific relevance outlines the role and the importance of the letters between scholars of seismology, developing and exemplifying some of the aspects alluded to in the introductory part of this text. he also mentioned the cataloguing project, the electronic reproduction and the dissemination of the scientific correspondence in the earth sciences, started up within the tromos project, inviting those in attendance to join in as well. 2.2. fragments of the european context a couple of experiences of recovery and analysis of correspondence performed in the european field help us to contextualise the italian experience in this sector. in one case it is a matter of a very advanced experience, belonging to the british tradition, which documents the status and the study method of the new dictionary of national biography (dnb) and the materials available to the researchers. the second case deals with a research experience performed in spain, similar to the one performed in italy within the tromos project. anita mcconnell (research editor, new dnb oxford university press) with her talk on the ‘new dictionary of national biography’: a new look at british earth scientists presented the contents of the dnb, with particular regard for the british earth scientists, and the places, the materials and the instruments of their research. the new dictionary of national biography: (http://www.oup.co.uk/newdnb) contains some 60 000 names of british people active from ca. 60 b.c. to 2000 a.d., in scotland, ireland, north america and other colonies, and of foreign people who were active in britain. it includes more than 100 people who were active in the earth sciences. in preparing their articles, 706 graziano ferrari searches were made for their correspondence and papers. the survival and whereabouts depends largely on the date, affiliation, and family interests of the scientists concerned. mcconnell also made an overview of the main institutions holding documents of interest for the dnb, such as university and the scientific societies archives, the public records office in london and the john milne library. josep batlló (crht dept. de matematica aplicada i, barcelona) with his contribution on scientists and scientific associations in catalonia (spain) in the early 20th century: manuscript versus published research brought his study experience within the seismological and meteorological field, relating to published and unpublished documentation on scholars, observatories and scientific associations in catalonia. he also highlighted the present-day relevance of the various kinds of documentation, and in particular the letters, in regard to the research in the meteorological and seismological fields, with some examples for catalonia of the reconstruction of the macroseismic maps of historical earthquakes and climatological maps. 2.3. case studies in italy one of the main figures in the history of the earth sciences at the end of the 18th century is pietro tacchini, a leading personality in astronomy, who from 1879 had the scientific, political and organisational skills to set up and develop the national meteorological and geodynamic services within the scope of the central meteorology office (and from 1887 also geodynamics). three talks outlined the complex biographical profile of this scientist seen from different disciplinary perspectives. letizia buffoni and edoardo proverbio (astronomical observatory of brera, milan) with the contribution of angelo secchi and pietro tacchini to the birth of earth and solar meteorology: the role of correspondence, developed some aspects of the scientific and human relations existing between pietro tacchini and angelo secchi, two of the greatest 18th century italian astronomers, through the study of the letters sent by tacchini to secchi preserved at the archives of the pontifical gregorian university, rome. ileana chinnici (astronomical observatory, palermo) in his talk on pietro tacchini ‘astronomer’ in the scientific correspondence preserved at the ucea outlined tacchini’s scientific biography with special regard for the contribution of this great scientist in reorganising the network of astronomical observatories, documented by the letters of the tacchini epistolary archive preserved in the library of the ucea. franca mangianti (central office of agrarian ecology, rome) pietro tacchini ‘meteorologist’ in the epistolary documentation of the ucea (1879-1899) went over the experience of tacchini as a meteorologist and the first director of the ucea, documenting it with letters from the tacchini epistolary archive of the ucea (wholly digitally scanned by the tromos project) and that of the archive of the pontifical gregorian university. francesco obrizzo (ingv-ov, naples) and edvige schettino (department of physics, university of naples) presented a contribution on the machines of the bourbons in naples between collection and museum: verbal sources and others (1840-1850) in which they recalled how the royal cabinet, made up of an initial nucleus of 200 scientific instruments, where magnetic, meteorological and time measurements were also made, acted as a teaching laboratory for the heir to the throne francis ii of bourbon. the instrumental resources recovered in the ten-year period 1985-1995 are the subject of study, supported by the documentary and epistolary evidence. 2.4. places of observation the changing strategies for monitoring the observation networks, both seismological and meteorological, many of which are often centralised and heavily automated, have radically modified the functions of several ancient observatories, the surviving ones, transforming them in some cases into places for the preservation of the historical materials of their history of observation. in other cases these 707 letters in the earth sciences: their historic value and present-day scientific relevance scientific institutes have been able to adapt to the new requirements of modern research and represent the places where modern monitoring systems cohabit with the preservation and the valorisation of the material traces of the long historical tradition of observation. in the hope of providing a broad picture of the sites having a long scientific tradition in the seismological and meteorological field, the representatives of the various observatories that are still active were invited, amongst which the «vincenzo nigri» observatory of foggia, still kept working by the descendants of the founder, and the ximenian observatory of florence of the scolopi fathers. owing to other commitments, only the piacenza observatory was able to accept the invitation. graziano ferrari (sga storia geofisica ambiente, bologna) and matteo cerini (director of the observatory «alberoni», piacenza) presented a contribution on the seismological correspondence of the ‘alberoni’ observatory of piacenza, first of all recalling that the observatory of piacenza is one of the rare cases of a long scientific tradition, still active, that has its roots in the 1870s, and where the modern instruments are side by side with the ancient ones and the related historical documentation. from the observatory’s letters archive, which cerini is patiently recomposing, the more recent history of the observatory and its instruments is emerging: from the events that brought them to piacenza in the 1920s and their characteristics. all these elements are of great scientific value today of we consider the fact that the observatory has intact over 15 000 original seismograms of enormous scientific value. 2.5. places of preservation the archive and the library of the barnabite study centre, the central state archive, the library of the neapolitan society for local history and the archive of the pontifical gregorian university are very important examples of the contribution that can come from these ‘historical containers’ both because of the importance of the historical documentary resources contained therein, and the availability to collaborate demonstrated by the bodies which took part in the seminar and the expertise of their representatives. filippo lovison (barnabite historical studies centre, rome) in his the earth sciences in the epistolary archives of the barnabite scientists briefly but incisively recalled the contribution of several 19th and 20th century barnabite scientists to the progress in several sectors of the earth sciences such as: timoteo bertelli, francesco denza (1834-1898), camillo melzi d’eril (18511929), to mention some of the most important. in recalling the long collaboration of the barnabites with the research performed within the tromos project, lovison stressed how important it is for the historical sites preserving the ancient epistolaries not to be considered merely as ‘document containers’, but as centres for the study and the valorisation of the historical documentation they are preserving. it is important for the archive to benefit, as in the case of the tromos project, of scientific collaborations that can help to draw from the documentation the historical and currently relevant scientific contents. in confirming the centre’s availability and its willingness to collaborate with the scholars and with the project on the scientific correspondence as described by ferrari, lovison underlined the need to reflect on some aspects of the dissemination of the digital copies of the letters, especially via the internet. nella eramo (central state archive, rome) with her talk on the archive of the general directorate of agriculture in the central state archive, outlined a brief historical and archival profile of the archive of the general directorate of agriculture, with special regard to the repercussions on the history of the earth sciences. these materials often document the reasons, the conditioning, the motivations behind the choices made and the paths taken by scientists and public administrators in the development of the meteorological and seismological observational networks. eramo also recalled the important documents in the history of geology, citing some documents dating back to the early 1870s in which the urgent need for a geological map of the kingdom of italy was manifest. paola milone (neapolitan society for the local history, naples − snspn) with her presentation on the seismic archive of the nea708 graziano ferrari politan society for the local history, milone first of all recalled the present-day knowledge on the origins of this important archive, established upon the nucleus of the library and archive belonging to the french naturalist alexis perrey, acquired by the club alpine of naples (cai), and the reasons that brought this wealth of information to naples in 1878, later given by the cai to the snspn in 1893 or 1894. the society is currently cataloguing all the library, documentary and iconographic material available in the archive, in which there stands out the substantial epistolary collection of perrey himself, that goes from 1842 to 1877 with some gaps, having nonetheless an indisputable historical and scientific value for seismology. lydia salviucci insolera (director of the archive of the pontifical gregorian university, rome) with her talk on the scientific archives of the pontifical gregorian university: research methods, coordination and future potential outlined the principal contents of the archive with particular regard to the most important jesuit scholars, particularly in physics and astronomy. she confirmed the opening and the great attention and interest of the gregorian university for collaboration with the scientific world and the historians of science. she put forward some proposals, amongst which that of setting up a communications network among the scholars who may be a point of reference for the archive’s operators, and selecting together the archive’s materials relating to jesuit scholars of special interest for the earth sciences. the scholars who took part in the seminar left one another hoping for further collaboration within the scope of the project, subsequently named letters in earth sciences, for the recovery, reproduction and valorisation of the scientific correspondence in the earth sciences. the seminar has also been followed up on a european scale during the 28th esc general assembly in genoa, where the letters in earth sciences project was presented, arousing positive feedback from a number of seismologists operating in the historical sites of european seismology preserving an important epistolary documentation. contacts are being fostered to formalise the collaboration between the ingv and bodies and scholars interested in participating in the project. a web site is under construction, for the moment a demo version is available (at the address http:// 80.117.141.2/letters), which will allow us to disseminate the historical epistolary materials, the reproductions, the elaborations and more generally the products of the collaborations developed. the project is open to all the researchers and the institutions that may be interested, and they are invited to get in touch with the author. references ferrari, g. (2000): the new iaspei sub-committee historical instruments and related documents in seismology: goals, objectives and first results, in proceedings of the xxii iugg general assembly, birmingham, 19-30 july 1999, sismol. res. lett., 71 (5), 553-561. ferrari, g. (2002): census, filing and elaboration of scientific letters in the earth sciences, nuncius, 17 (1), 307-320. annex 2 to: trace elements mobility in soils from the hydrothermal area of nisyros (greece) 1 annex 2 (doi: 10.4401/ag-6760) trace elements mobility in soils from the hydrothermal area of nisyros (greece). daskalopoulou, k., et al. a.1. analytical techniques all soil samples were air dried, thoroughly mixed and split into subsamples for analysis. particular care was taken using a riffle-type sample splitter to ensure that representative subsamples were obtained. all analyses were made after sieving the soil samples through a 2-mm sieve and, except for ph determination, ground with an agate mortar. xrd analysis was carried out for all soil samples with the use of xd diffractometer bucker (siemens) d5005, fully equipped with the software diffractplus of bucker (siemens). ph values were measured using a specific combination electrode on soil suspensions made with deionized water with a soil/solution weight ratio of 1/2.5 (thomas, 1996). samples were treated for chemical determinations, using hno3 + hcl (closed microwave digestion) in order to determine the pseudo-total composition and with ultrapure water (leaching extraction) to determinate the soluble fraction. the first case, about 100 mg (precision ±0.1 mg) of the sample dried at 40 °c overnight was dissolved in a teflon vessel with 4 ml hno3 (merck suprapur), 2 ml hcl (panreac suprapur) and 9 ml milliq water using a microwave oven. after cooling down, the solution was filtered (0.45 µm) and diluted with milliq water to 50 ml. the reproducibility of the chemical analysis was verified by using repetition of several random samples. for most of the elements the reproducibility was in the order of 2-5 %, except for be, cr, ni, zn about 10%. the reliability of results was checked analysing four certified references materials (crm) and quality control was assessed by comparing our results with the certified values. the recovery values, that are a measure of the difference in percentage between measured and certified values for each element, for most of the elements were in the range between 80 and 120 %. several replicates of the crm and random samples confirmed a good reproducibility of analytical procedures (microwave digestion and chemical analysis) between about 5 to 15 % difference, depending on the element. a second oven-dried sample aliquot was leached with milliq water with a sample/water weight ratio of about 1/50 for 4 hours with constant stirring. the solution was subsequently centrifuged and filtered through 0.45 µm filter. method blanks and duplicate samples were analysed too but certified standard was available for such kind of treatment. method blank values were subtracted from the raw concentrations of all samples. all the solutions were analysed for major and trace element contents by inductively plasma spectrometry (icp-ms and icp-oes). trace elements (al, as, b, be, bi, ba, cd, co, cr, cs, cu, fe, la, li, mn, mo, ni, pb, rb, sb, se, sr, te, th, ti, tl, u, v, y and zn) were analyzed by icp-ms using an agilent 7500ce instrument. the instrument was equipped with a standard peristaltic pump, a micro mist concentric nebulizer, a peltier-cooled spray chamber, the plasma forward power, the shield torch system, and a collision/reaction cell system. an autosampler (asx 520, cetac technologies) was employed to introduce the samples into the plasma of the icp-ms. introduction was done with a micro mist concentric nebulizer and scott-type quartz glass spray chamber. helium was used as the collision gas for as, co, cr, cu, fe, mo, ni, ti and zn analyses while hydrogen was used as a reaction gas only for se analysis. no cell gas was used for the other elements. the instrumental setting were optimized to minimize both oxide levels and double charges (ceo+/ce+ ratio < 1.5% and ce++/ce+ ratio <3%). francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text 2 calibration solutions of all investigated elements were prepared daily by appropriate dilution of 100 mg l-1 and 1000 mg l-1 stock standard solutions (merck) with 0.14 mol l-1 high-purity nitric acid. the accuracy of the method was checked analysing certified reference materials of natural waters (nist 1643e, environment canada tm-24.3 and tm-61.2, spectrapure standards sw1 and sw2) at regular intervals during sample analysis. the experimental concentrations determined in this study agreed well with these certified values (within ±5%). matrix induced signal suppressions and instrumental drift were corrected by internal standardization. indium was used for elements up to mass 138 (ba138), and rhenium was used for heavier elements. major elements (al, ca, fe, k, mg, na, s and si) were measured with icp-oes (jobin yvon ultima2). references [thomas, g.w., 1996] thomas, g.w. (1996). soil ph and soil acidity. in: methods of soil analysis – part 3 chemical methods (sparks d.l. ed.). soil science society of america book series n. 5, 475-490. vol. 48, 01, 05ok.qxd 19 annals of geophysics, vol. 48, n. 1, february 2005 key words eger rift – french massif central – eifel – carbon dioxide – gas fractionation – isotope composition 1. introduction in areas with both low co2 abundance and contents in the gas phase as for example in complexes of crystalline basement or in areas with younger volcanic activity, one frequently encounters co2 with isotopic compositions of δ13c values lesser than –10 ‰. in many cases, such values are interpreted as biogenic/organic co2 or mixtures between magmatic and biogenic co2 or respectively mantle and crustal end members (e.g., griesshaber et al., 1992). it appears that co2 gases with low δ 13c values occurring in the margin areas of regions with ascending magmatic co2 as for example in mineral springs of the western eger rift, the eifel or the french massif central (batard et al., 1982) confirm this interpretation. however, due to the high solubility of co2 in water and the hco3 formation, fractionations of the co2-rich gases take place (e.g., batard et al., 1982; capasso et al., 1997; d’alesandro et al., 1997; chiodini et al., 1999; weinlich et al., 1999). commonly, the isotopic data are compared and calculated with a single equilibration. however, in the peripheral areas of regions with magmatic co2 marked by longer migration mailing address: dr. falk h. weinlich, referat gasund isotopengeochemie, bundesanstalt für geowissenschaften und rohstoffe (bgr), stilleweg 2, 30655 hannover, germany; e-mail: falkweinlich@gmx.de isotopically light carbon dioxide in nitrogen rich gases: the gas distribution pattern in the french massif central, the eifel and the western eger rift falk h. weinlich bundesanstalt für geowissenschaften und rohstoffe (bgr), hannover, germany abstract based on characteristics of the distribution pattern of the western eger rift spring gases, a distribution pattern is presented for the gases of the french massif central. the central parts of these areas with ascending magmatic co2 are characterised by high gas fluxes, high co2 contents of up to 99.99 vol% and isotopially heavy co2. in the peripheries, the decrease of δ 13c values of co2 and co2 contents in the gas phase is compensated by a rise in n2 contents. it can be demonstrated that gas fractionation in contrary to mixtures with isotopically light biogenic or crustal co2 controls the distribution pattern of gas composition and isotopic composition of co2 in these spring gases. dissolution of co2 results in formation of hco3 – causing isotope fractionation of co2 and an enrichment of n2 in the gas phase. with multiple equilibrations, values of about –17 ‰ or lower are obtained. the scale of gas alteration depends on the gas flux and the gas-water ratios respectively and can result in n2-rich gases. essential for the interpretation are gas flux measurements with mass balances derived for most of the springs. without such mass balances it is not possible to discriminate between mixture and fractionation. the processes of isotopic and chemical solubility fractionations evidently control the gas distribution pattern in other regions as well. 20 falk h. weinlich pathways, it can be assumed that gases migrate in different water systems, for example along various faults where dissolution of co2 and fractionation with concomitantly formed hco3 – can take place several times. this results in a comprehensive gas fractionation concerning both gas composition and isotopic composition of co2 and can yield n2-rich gases. 2. gas distribution pattern in european areas with magmatic co2 2.1. western eger rift (czech republic) in the western eger rift the main release of magmatic co2-rich gases is bound to gas release-centres in the cheb basin and karlovy vary north and mariánské lázně south of the main structures of eger rift (weinlich et al., 1998). these structures of the krušné hory (erzgebirge) main fault together with the central fault both dipping to south and the litoměřice deep fault dipping to north form a y-structure. this y-structure splits the gas flux and forms a shielded gas free zone within the eger rift. the gases of the mofettes and springs in these areas with the highest gas fluxes are characterised by high co2 contents of up to 99.99 vol% and δ 13c values of –3.9 up to –1.9 ‰ (fig. 1). in some mofettes and springs the gas flux reach values up to 28 m3/h in bublák and 35 m3/h in soos (cheb basin) or about 100 m3/h in the mariiny mofette in mariánské lázně. the magmatic nature of these gases is indicated, besides these δ 13c values, by high proportions of mantle-derived helium with r/ra values up to 6 (weinlich et al., 1999). with increasing distances from these gas release-centres the gas flux falls and related to it the co2 becomes isotopically lighter whereas the co2 contents decrease and are compensated by a rise of the n2 and he contents. to the east and south of konstantinovy lázně the co2 contents drop linked with gas fluxes of lower than a half l/h up to values of 87 and 67 vol%, respectively. to the north of the eger rift nitrogen contents of 98 vol% are attained apart from 0.7 vol% co2 as in the schönbrunn fluorite mine or in other spring gases in the erzgebirge (weinlich, 1989). linked with the decrease of the co2 contents in the gas phase is a decrease of the δ13c values of co2. south of the eger rift, the δ13c values fall to –7.4 ‰ in the konstantinovy lázně area and to –8.8 ‰ in bavaria, respectively. in the same way, the δ13c values of co2 decrease north of the eger rift down to – 6.0 ‰ in bad elster and in the n2-rich gases of schönbrunn down to a value of –17.4 ‰. the gases in the mofettes and mineral springs in the western eger rift migrate upwards along faults in the area with exposed metamorphic rocks of variscian in the north and of moldanubian consolidated basement in the south or with exposed variscan granite intrusions. the metamorphic rocks in this area have very low potential if any for a co2 release because all organic carbon is fixed thermodynamically as very stable graphite. in the course of the variscan, metamorphism mobilised and displaced organic carbon with δ 13c values of – 14 ‰ is present in the form of co2 gases among others in the fluid inclusions of granitic quartz in schönbrunn. with a decrepitation temperature of 800°c (weise et al., 2001), the release of co2 gases by mineral waters is today hardly possible. 2.2. eifel (lower rhine graben, germany) co2-rich gases linked with quaternary volcanic activities occur also in the eifel. griesshaber et al. (1992) report isotopic compositions of co2 ranging from –7.8 ‰ up to –3 ‰. the lighter isotopic values are explained with the aid of lower r/ra values due to mixing of magmatic and biogenic co2. may (2002) describes the occurrence of co2-rich gases linked with higher gas fluxes within the central west eifel and the decrease of co2 contents in its margin areas (fig. 2). so gases with up to 98.3 vol% co2 predominate for e.g., in wallenborn in the central part of the west eifel and with up to 99 vol% in laach lake in the east eifel. the δ 13c values of co2 in the west eifel range from –5.7 up to –2.0 ‰ (hubberten, 2004, pers. comm.) and in the central parts of the east eifel from –5.1 up to –3 ‰ (griesshaber et al., 1992). in contrast, away from these areas with high gas fluxes, gases with higher n2 contents occur 21 isotopically light carbon dioxide in nitrogen rich gases fig. 1. distribution pattern of gas flux (free gas), gas composition (air-free) and δ 13c co2 values of gases in mineral springs and mofettes in the western eger rift, czech republic (data taken from weinlich et al., 1998, 1999, 2003). legend: ml – mariánské lázně; fl – františkovy lázně; half-filled squares – uranium mines with gas blowouts. fig. 2. gas flux distribution (moving average for circles with 8.6 km diameter), isotope composition of co2 and schematic profile of co2 flux, spring density and gas composition across the west eifel, after may (2002). carbon isotope data from griesshaber et al. (1992) and hubberten (2004, pers. comm.). the brubble spring of wallenborn (wallen – seethe, born – spring) is a geyser-like spring with gas release of 11.3 m3 per eruption period (may, 2002) the spring with the highest gas flux and the highest δ 13cco2 value (–2.0 ‰) in the west eifel. 22 falk h. weinlich in the spring gases of aachen (27 vol% n2, 72 vol% co2) located to the northwest of eifel or in the southeast at bad bertrich (90 vol% n2, 6.2 vol% co2) and bad wildstein (98.5 vol% n2, 0.13 vol% co2) (may, 2002). congruent with the alteration of gas composition beyer (1995) reports δ13c values of co2 ranging from –10.2 up to – 6.3 ‰ for the thermal springs of aachen. in the n2-rich gas of bad bertrich, δ 13c value of co2 amounts to –13.9 ‰ (own analysis). the δ13c values of co2 in these marginal springs are significantly lower than in the central parts. the gases ascent along faults in an area with exposed devonian sediments and are occur mainly in the areas of the maar-type quaternary volcanism in the east and west eifel. fig. 3. distribution of gas flux (free gas), gas composition and δ 13c co2 values of the gases of the mineral springs of the french massif central. compiled after data from moureu and lepape (1912), moureu (1923), baubron et al. (1979), schoeller and schoeller (1979), batard et al. (1982) and matthews et al. (1987). based on the hydrologic map (risler et al., 1973) and tectonic map (autran et al., 1980). fig. 4. rise of n2 (air-free) in gases with decreasing free gas-water ratio due to increased gas fractionation, i.e. selective co2 solution in water. the unavoidable scattering in the data is due to varying air proportions in the gases by different partial pressures influencing the bubble point pressure of the gas-water systems (data from weinlich et al., 1998, 2003). 23 isotopically light carbon dioxide in nitrogen rich gases 2.3. french massif central in the french massif central co2 occurs with δ13c values ranging from –23 ‰ up to – 4 ‰. the co2-rich gases of vichy, royat or mont dore and cezallier (>99 vol% co2) are bound to the area of limagne depression or its direct vicinity. matthews et al. (1987) proved the mantle-derived nature of these gases with r/ra of up to 5.5. gas flux measurements (moureu and lepape, 1912; batard et al., 1982) carried out on these spring gases in the above region also show that the co2-rich gases are linked with high gas fluxes. according to the gas composition and isotope data by moureu (1923), schoeller and schoeller (1979), baubron et al. (1979), batard et al. (1982) and matthews et al. (1987) a similar distribution pattern also prevails there. in the central part of the massif central isotopically heavy co2 with δ 13c values of –7 up to –5 ‰ occurs linked exclusively with co2-rich gases whereas isotopically lighter co2 with δ 13c values ranging from –23 up to –12 ‰ is linked with n2-rich gases of evaux (89-93 vol%), sail-les-bains (97.2 vol%) maizières (88.2 vol%) or santenay (86-88 vol% n2) in the margin areas (fig. 3). the latter values are in the range of «typical» biogenic co2. apart from the limagne depression filled with oligocene – quaternary sediments where the spring gases migrate upward along marginal faults and the volcanic complex of mont dore these spring gases migrate upward along faults in areas with exposed variscian metamorphic rocks or granites. 3. co2 fractionation 3.1. the gases in the western eger rift in contrast to other gases, co2 is very vulnerable to fractionation processes. firstly, due to its good solubility in water compared to n2, hc’s and rare gases, the gas composition can be altered solely by solubility fractionations. this results in enrichment of the inert gases as observed by an aureole of n2-richer gases in the surroundings of all regions with co2-rich magmatic gases in europe. figure 4 demonstrates that with an ongoing solution of co2 resulting in a decrease of the gas/water ratios the gases in the western eger rift are enriched in n2 in the gas phase (weinlich et al., 1998). secondly, linked with the solution of co2 are decreasing ph values of these waters. this results in leaching of cations from the adjacent rocks and formation of hco3 – ions. between the newly formed hco3 – and co2 in the gas phase exists an isotope fractionation of about 10 ‰ (at 10°c) (wendt, 1968). with ongoing hco3 – formation, the remaining co2 in the gas phase becomes isotopically lighter. consequently, the decreasing gas flux correlates with decreasing δ 13c value of co2 in the gas phase as demonstrated in the distribution pattern for the eger rift gases. however, isotopically lighter co2 can be also a result of mixing of biogenic and magmatic co2. the key for discrimination between mixing or fractionation is the compilation of complete mass balances of co2 for each mineral spring with a 24 falk h. weinlich scribed by the following equation: m m m m c c wdiss gas hco gas 13 13 gas total total total hco3 3 = + d d f f _ ` i j free gas phase consisting of gas flux, isotopic and chemical gas composition, contents of hco3 – and dissolved co2 contents and water discharge. according to wendt (1968) the isotope balance for co2 in the free gas phase can be defig. 5. dependence of δ 13c co2 values in free gas phase from the ratio of hco3 – transport (mhco3) in water to total co2 flux in mineral springs and mofettes in the western eger rift with complete mass balances, according to the isotope balance formula (see text) (data of hco3 –, dissolved co2 and water discharge are taken from kolářová and myslil, 1979 and weinlich et al., 1999, 2003). lines starting from the scattering range of the dry gas vents (on the y axis) in the mofettes wrap the field of theoretical fractionation according to the fractionation factor ε hco3-gas at 10°c. between these lines, the δ 13c values for the free gas are exclusively a result of fractionation by the means of formed hco3 – during a single equilibration, without the necessity to assume an additional biogenic carbon. the δ 13c values below the lines can be explained by twice equilibration. this results in an increase of n2. multiple equilibrations with solely dissolved co2 and single equilibration with hco3 – result in occurrence of n2-richer gases, which fall in between the fractionation lines. table i. measured and calculated δ 13c values of co2 for springs in the vicinity of mofettes in north bohemia demonstrate that the differences between mofettes and springs are solely caused by hco3 --fractionation. for the δ 13c primary (total) – value the isotope signature of the neighbouring mofettes, the hco3 as well as dissolved co2 and the water discharge of the respective spring were used according to isotope balance formula (data from weinlich et al., 1998, 1999). locality, spring co2 gas flux water co2 flux δ 13c co2 discharge free gas dissolved hco3 − total measured calculated vol% l/h l/h mol/h mol/h mol/h mol/h ‰ ‰ cheb basin soos, mofettes 99.946 21100 0 941.46 0 0 941.46 −− 2.9 soos, cisař sý 99.941 7600 2520 339.08 143.18 60.7 542.96 − 3.6 − 3.64 františkovy lázně kostelní spring 99.017 2500 8600 110.51 418.08 84.31 612.90 − 3.6 − 3.34 mariánské lázně smrad’och mofettes 99.923 5200 0 231.96 0 0 231.96 −− 2.27 farská spring 99.610 149 1166 6.63 67.73 11.55 85.91 − 2.8 − 2.55 mariiny mofette 99.990 87000 0 3883.54 0 0 3883.54 −− 2.7 ferdinand spring ii 99.969 8240 1440 367.74 70.85 71.86 510.45 − 4.0 − 3.88 kř ížový spring iii 99.625 135.6 72 6.03 4.74 3.27 14.04 − 3.9 − 4.52 martinov 99.280 4 90 0.18 5.28 1.06 6.52 − 3.5 − 3.22 chotěnov 85.570 4.1 171.1 0.16 8.54 2.35 11.05 − 4.0 − 3.75 dolní kramolín 99.370 164 324 7.28 19.29 1.58 28.15 − 2.6 − 2.35 25 isotopically light carbon dioxide in nitrogen rich gases where m is the amount of co2 and ε the fractionation factors of –1.3 ‰ for ε diss-gas and 9.6 ‰ for ε hco3-gas at 10°c (wendt, 1968; mook et al., 1974; zhang et al., 1995). in case of the mofettes the water discharge is 0, i.e. the ratios mdiss /mtotal and mhco3/mtotal are 0. consequently, the measured δ13c value of the free gas phase is identical to the total, i.e. the primary isotopic composition of magmatic co2 in this area. therefore mhco3/mtotal ratios near 0 can be used to identify the primary isotopic composition. in the case of mineral springs with a continuous transport of leached cations, mainly ca++ and mg++, isotopic fractionation occurs with contemporaneously formed hco3 – whereas remaining co2 in the gas phase becomes isotopically lighter and co2 contents can decrease. figure 5 exhibits this dependency of the δ13c value of co2 in the gas phase from the mhco3/mtotal ratios in the springs. in cases of spring gases in the close vicinity of mofettes it can be demonstrated that the differences in the isotopic composition are solely caused by hco3 – fractionation (table i). the spring gases, which are further away from the main gas release-centres, can be transported within more than one fault system and thus in different waters. therefore, the equilibration between co2 in the gas phase and hco3 – can occur several times and the calculated δ 13c co2-values are consequently higher than expected in a single equilibration. changes in isotopic composition can also be explained by mixing with lighter biogenic co2. however, the common change in isotopic and chemical composition (fig. 6) points to fractionation processes caused by multiple equilibrations as increasing nitrogen contents are not linked with biogenic co2 admixtures. in the gases of the western eger rift a correlation between n2 and he contents can be observed (fig. 7) which indicates one source and 26 falk h. weinlich fig. 6. plot of n2 content versus δ 13c values displays the common variations in gas and isotope composition of gases from the eger rift area, e.g., from the cheb basin/south vogtland area (cb-sv), konstantinovy lázně area (kl) and bavaria (by). these variations are caused by fractionation (co2 solution and hco3 – formation). in the case of waters with low tds contents and without hco3 – formation, the fractionation can only take place with dissolved co2 and the isotopic heavy co2 remains therefore in the gas phase. fig. 7. correlation between nitrogen and helium contents in gases of the western eger rift (data from weinlich et al., 1998). a continuous enrichment by solubility fractionation (weinlich et al., 1998). the assumption of an additional n2-source in case of the n2-richer gases is therefore not necessary. compared to the gas release centres the nitrogen flux decreases in the springs in the marginal areas. an exception is the water inflow about 500 m below the surface with 100 l/h n2 in the schönbrunn fluorite mine, caused by pressure release in this mine. a special case of these fractionations are springs with very low ca-mg-hco3 contents and with isotopic composition of co2 being still nearly unchanged due to lack of extensive hco3 – fractionation and where the co2 contents are solely decreased by the solution of co2. this can be explained by fractionation processes. however, in case of an interpretation of isotopically lighter co2 in the gas distribution pattern of the western eger rift caused by mixing with lighter biogenic co2 there should be no reason to elucidate why the mixing should not occur in springs with low total dissolved solids (tds) contents. regarding longer migration pathways as mentioned above it is considered that the gas migration occurs within different hydrological systems and therefore these fractionations can take place repeatedly during the migration. this results in a drastic drop of the co2 contents and the δ13c values in the remaining gas phase. figure 5 demonstrates the isotopic composition of the n2-rich gases in the fluorite mine of schönbrunn (one of the most northern springs shown in fig. 1) with a δ 13c value of –17.4 ‰ which can be explained alone under the assumption that the co2-hco3 – system is equilibrated twice. certainly, an admixture of biogenic co2 cannot be excluded but in this mine about 3.6 m3 of co2 gas and 3664 m3 of dissolved co2 per year were released. facing the fact that the granite surface is located only about 650 m below both the thermal water and gas inflows (kuschka and hahn, 1996) this amount is hardly explainable with noticeable proportions of biogenic co2. it is problematic to derive the nitrogen from crustal sources in terms of the 100 l/h n2 in the free gas phase and about 450 l/h dissolved n2 (air-free over dissolved ar; procedure in weinlich et al., 1998) accompanied by 0.45 l/h he and 5.72 l/h dissolved he (in total 54 m3/yr he). it should be considered that due to the intrusion of variscian granites the metamorphic rocks were exposed to far higher temperatures, as is the case today. therefore, the n2 is probably also mantle derived, because the n2 gas release including metamorphic co2 sourced from these crustal rocks took place during the variscian intrusions. the nitrogen isotope composition with δ 15n 0.7 (weinlich et al., 1999) exhibits a tendency to more positive values of a plume-like mantle (marty and dauphas, 2003) which occurs in central europe (wilson and downes, 1992). recalculating the co2 and n2 in the gas phase together with the dissolved n2, and co2 including the hco3 – the whole fluid system contains about 90 vol% co2 (possible caco3 precipitations would additionally increase this co2 content) and about 10 vol% n2 and thus this system is comparable with the eger rift gases. 3.2. the gases of the french massif central the distribution pattern and the isotopic signature of – 23 up to –12 ‰ of the n2-rich spring gases of the french massif central could be explained in the same way. as in the western eger rift, the co2-rich gases with δ13c values of –7 up to – 5 ‰ are linked with high gas fluxes in the concerned mineral springs. figure 8 demonstrates this correlation between δ13c values and the gas composition. batard et al. (1982) calculated initial isotope composition for some gases in this area with mass balances according to a single equilibration co2-hco3 – . the authors concluded a biogenic or mixed origin for the co2 because the isotopic composition of the total carbon ranges between –16 and –11 ‰. however, due to multiple equilibrations, it cannot be excluded a priori but only the mhco3/mtotal ratios near 0 should be used to avoid misinterpretations (the calculated δ13 c value of co2 for schönbrunn assuming a single equilibration is also –11 ‰). for the gases of the french massif central, it can be assumed that the gas of royat with a δ 13c value of –6.4 ‰ is unfractionated owing to its mhco3/mtotal ratio of 0.007 and displays the primary composition. further, geothermometer calculations (pauwels et al., 1997) indicate that 27 isotopically light carbon dioxide in nitrogen rich gases 28 falk h. weinlich fig. 8. correlation of gas composition and isotopic composition of co2 in gases of the massif central (data from batard et al., 1982 and matthews et al., 1987). fig. 9. mass balance plot displays δ 13c values versus ratio hco 3 – transport (mhco3) to total co2 flux (m0) for gases of mineral springs in the french massif central (data from batard et al., 1982). these waters emerging in the mont dore area reach temperatures of about 100-130°c at depth and in saint-nectaire (δ 13c value co2 –7.0 ‰) temperatures of 160-175°c at depth respectively. according to mook et al. (1974), the fractionation between co2 and hco3 – at temperatures of around 120°c is zero. based on the isotope composition of royat it can be shown that the low values of –12 and –23 ‰ can be reached (fig. 9) under the assumption fig. 10. long-term observation of isotopic composition of the gas of the eisen spring in bad brambach, eger rift (weise et al., 2002) and typical annual changes in the co2 production in soils (andrews and schlesinger, 2001). growing seasons – light grew. 29 isotopically light carbon dioxide in nitrogen rich gases that the equilibration between co2 and hco3 – takes place only twice and in two cases three times. therefore, it is not absolutely necessary to assume biogenic contributions in the region as well as in the eger rift. just two isotope values, which represent gases of santenay and saint honoré, situated at the edge of the morvan horst and which are associated with naso4 waters are lighter than those lying in the field of gases equilibrated three times. however, an uncertainty of these mass balances lies within the possibility of influence of non-mineralised groundwaters and/or mineral waters of different type, which «dilute» the hco3 – -rich mineral water in the respective springs. in the cézallier area, négrel et al. (2000) demonstrate such mixtures of mineral waters with meteoric and different mineralised waters in line with the ree distribution and strontium isotope ratios. pauwels et al. (1997) state similar processes in the mont dore region on the basis of the main element distribution in spring waters. this effect of «dilution» of these mineral waters can produce lower mhco3/mtotal ratios, present during equilibration in deeper regions as higher mhco3/mtotal ratios. since the co2 of these n2-rich gases is completely fixed as hco3 – , it is no longer possible to form new hco3 – in these waters. the δ 13c values were altered by the formed hco3 – and remained unchanged in less mineralised waters. this results in a shift of these gases in plot of the ratio of hco3 – versus total co2 flow (fig. 9) and acts as if a thrice equilibration took place. according to schoeller and schoeller (1979) the tds contents and especially the hco3 – contents decrease with increasing distances from the area of vichy-cantal-devès. 30 falk h. weinlich as in the western eger rift, the n2-rich gases in the massif central are enriched in helium. the extreme enrichment of helium, whose contents are the highest in europe, points rather to a complete fractionation than to a simple mixing with biogenic components. according to the gas flux measurement of batard et al. (1982) 30 l/h of n2 are also released in the co2rich gases of royat. on the other hand, n2 release in the case of n2-rich gases are ca. 2.8 l/h in bourbon-lancy, 29.6 l/h in evaux-les-bains, 0.5 l/h in sail-les-bains and 1.7 l/h n2 in the lithium spring in santenay. thus only the gas composition is fractionated and it is not necessary to assume additional n2 sources. an additional argument contradicting the influence of mixing processes is that outside these areas with magmatic co2 there are no springs with biogenic/organic co2 in the gas phase. the production rates of biogenic co2 in soils (andrews and schlesinger, 2001) are too small to nourish a free gas phase. a long-term measurement of isotope composition of co2 in the gas phase of the wettin spring in bad brambach (weise et al., 2001) compared with the biogenic co2 production rates (andrews and schlesinger, 2001) demonstrate that there is no influence (fig. 10). it is also problematic to derive biogenic/organic co2 from sedimentary rocks, especially in areas of metamorphic rocks, since these waters and gases circulate within fault systems. there, either a far-reaching co2 exchange between the gases migrating along fault pathways and the surrounding country rocks is impeded or the ascending magmatic co2 saturates the groundwater with co2 gas, as it is the case in the cheb basin. the co2 concentration gradient in the close vicinity of the faults prevents the admixture of co2 from other sources like for example the biogenic/organic co2. 4. conclusions as demonstrated, it is possible to elucidate low δ 13c values with gas fractionation, i.e. by isotope fractionation with formed hco3 – and not necessarily and exclusively by mixing with biogenic or organic co2. however, without complete mass balances it is not possible to discriminate between either or give reasons to prefer one of the interpretations. in some cases, it will not be possible to educe the «last proof» for the interpretation. therefore, it should always be considered that even enhanced co2contents in the soil air encountered in the vicinity of fractured rocks can also represent completely fractionated magmatic co2. however, if we have to assume that the isotopic composition and contents of co2 in the gas phase can be alternated by fractionation processes, an influence on the c/3he ratios should also be assumed. marty et al. (1989) described abating c/3he linked with reduced co2 contents. acknowledgements the author would like to express his gratitude to fausto grassa, istituto nazionale di geofisica e vulcanologia sezione di palermo, for providing constructive comments to the review. the support of h.w. hubberten, a. wegner inst. f. polar marin res., potsdam in his permission to use his unpublished data is greatly appreciated. k. pütz, staatsbad bad bertrich is thanked for supplying gas samples. for constructive discussions i would like to thank jolanta kus and franz may, bgr hannover. references andrews, j.a. and w.h. schlesinger (2001): soil co2 dynamics, acidification, and chemical weathering in a temperate forest with experimental co2 enrichment, global biochem. cycles, 15, 149-162. autran, a., j-p. breton and j.c. chiron (coordinator) (1980): carta tectonquie de la france 1:1 000 000, mem. brgm 110, pp. 52. batard, f., j.c. baubron, b. bosch, a. marcé and j.j. risler (1982): isotopic identification of gases of a deep origin in french thermomineral waters, j. hydrol., 56, 1-21. baubron, j-p., b. bosch, p. desgranges, m. leleu, j.j. risler and c. sarcia (1979): recherches géocimiques sur les eaux thermals de la bordure oust de la limagne, bull. minéral, 102, 676-683. beyer, f. 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(1968): fractionation of carbon isotopes and its temperature dependence in the system co2-gas-co2 in solution and co3-co2 in solution, earth planet. sci. lett., 4, 64-68. wilson, m. and h. downes (1992): mafic alkaline magmatism associated with the european cenozoic rift system, tectonophysics, 208, 172-182. zhang, j., p.d. quay and d.o. wilbur (1995): carbon isotope fractionation during gas-water exchange and dissolution of co2, geochim. cosmochim. acta, 59, 107-114. vol. 48, 01, 05ok.qxd 117 annals of geophysics, vol. 48, n. 1, february 2005 key words radon – atmosphere – uranium – thorium – rock 1. introduction radon is released into the atmosphere mainly from soils and underlying rocks, from ground waters (especially thermal ground waters), but also from oceans, natural gases, caves and mines (gesell, 1983; khatir sam and holm, 1995; lozano et al., 2000; jha et al., 2001; papastefanou, 2001). spatial variation of outdoor radon activity and its dependence on the geological conditions, on the type of cover of earth surface (ocean, ice cap or snow cover) and on the variations in soil moisture (grasty, 1991) was investigated in many areas worldwide (gesell, 1983; gundersen, 1991). there is a large number of measurements of radon activity in the air above the ground surface, but due to the application of different methods, the results are not comparamailing address: dr. agnieszka anna ochmann, institute of geological sciences wroc l⁄ aw university, pl. m. borna 9, 50-204 wrocl⁄ aw, poland; e-mail: ochmannn@yahoo.com distribution of radon activity in the atmosphere above wzgórza niemczańsko-strzelińskie (south-west poland) and its dependence on uranium and thorium content in the underlying rock and indirect ground basement agnieszka anna ochmann institute of geological sciences wrocl⁄ aw university, wrocl⁄ aw, poland abstract radon activity in the atmosphere and its behavior in the environment have been investigated using lr-115 nuclear track detector. the complex geological structure of wzgórza niemczańsko-strzelińskie (south-west poland) enabled this problem to be studied in various geological conditions. the eu and eth content in rocks and soil was measured by gamma-spectrometer gr-320. uranium content of bedrock reached its maximum value of 15 ppm in the case of quartz-graphite schist. thorium reached its maximum value of 35 ppm in the case of granodiorite. radon activity was measured by means of long-term exposure of lr-115. the mean value of atmospheric radon activity was 21 bqm–3 in the air 2 m above the ground surface. the highest radon activities were measured in the area of granite and quartz-graphite schist outcrops and in the area of mylonitic rocks of the niemcza zone. radon activity in close to ground cup detectors varies from 25 to 300 bqm–3, these values depend on uranium and thorium content in indirect ground basement (soil and weathered rocks). not only uranium and thorium content but also rock disintegration due to tectonic events (shear zones) influenced atmospheric radon activity. seasonal variation is not strong, although higher values were measured in the autumn-winter period. 118 agnieszka anna ochmann ble. gesell (1983) compared the data from the area of united states, where the average radon activity in the air varies between 4-15 bqm–3. in canada outdoor radon activity, measured by one of the passive methods with an exposure period of 3 months, varied from 11 bqm–3 to 67 bqm–3 (the mean value for manitoba is 59 bqm–3 and saskatchewan − 61 bqm–3) (grasty, 1991). according to unscear (1983 fide wilkening 1990) the typical radon activity in the air above the ground surface averages 10 bqm–3. the aim of this investigation was to enrich the knowledge of radon behavior and to construct a map of spatial variation of radon activity on the eastern area of foresudetic block. 2. geological setting wzgórza niemczańsko-strzelińskie are located in the eastern part of the foresudetic bloc (sw part of poland). this area includes the periphery of the sowie góry block, niemcza zone, metamorphic of niemcza-kamieniec ząbkowicki, which is extended on the ne from the niemcza zone, and a crystalline massif of wzgórza strzelińskie (fig. 1). the north-east part of sowie góry block is composed mainly of paragneisses and migmatic gneisses. in the gneisses small bodies of amphibolite, gabbro and serpentinite are inserted (dziedzicowa, 1987). in the vicinity of the boundary of niemcza zone there are intrusions of quartz monzodiorite, which belongs to the plutonic rocks of niemcza zone (dziedzicowa, 1987). niemcza zone is extended along the east edge of góry sowie block. it is the zone of dislocation of the mass of rocks and is built of mylonites originated from deformations of gneisses of góry sowie (scheumann, 1937 fide dziedzicowa, 1987; mazur et al., 1995). the mylonitic rocks include the small bodies of amphibolites, quartz-graphite schist, the enclaves of gneisses and intrusions of plutonic rocks (mazur et al., 1995). in the south part of niemcza zone there are vast outcrops of serpentinite and gabbro (puziewicz and radkowska, 1990). the eastern edge of niemcza zone adjoins the series of mica schist or more general series of metamorphic rocks. this area is known as a metamorphic of niemcza-kamieniec ząbkowicki and its contact with the crystalline massif of wzgórza strzelińskie (strzelińskie hills) in the eastern part is covered with quaternary sediments (dziedzicowa, 1966). metamorphic of niemcza-kamieniec ząbkowicki is built mainly of mica schist with the inserts of quartz-feldspathic schist, quartz-graphite schist, amphibolites and marbles (dziedzicowa, 1966). between the metamorphic of niemcza-kamieniec ząbkowicki and wzgórza strzelińskie, near górka sobocka village, there are outcrops of granite intrusion and its southern metamorphic cover – gneisses of wzgórza lipowe (lipowe hills) (wojnar, 1977; bartz and puziewicz, 1999). according to oberc-dziedzic and szczepanski (1995) the granite from górka sobocka is the westernmost part of the crystalline massif of wzgórza strzelińskie. the series of crystalline rocks of the wzgórza strzelińskie massif can be divided into four groups (oberc-dziedzic, 1991): gneisses, the elder schist series, the younger schist series and plutonic rocks. the most common are gneisses. the rocks of the elder schist series (amphibolites, mica schist, limestone and marbles) appear in gneisses and are crimped with the rocks of younger series. the younger schist series, named jeglowa series, is composed of quartzite, quartz-sericite schist, mica-sillimanite-quartz schist. the youngest group of rocks are plutonic rocks: quartz diorite, tonalitie, granodiorite and granite. granite predominates in the northen part of massif (oberc-dziedzic, 1991). on the whole area of investigation there are the intrusions of tertiary basalts. complex geological structure of this area, a wide variety of types of rocks and the presence of tectonic zone between sowie góry block and crystalline massif of wzgórza strzelińskie (mazur et al., 1995), enabled the discussed problem to be studied in various geological conditions. 3. methods the fundamental equipment of this investigation was the lr-115 detector (production of kodak), which is one of many types of solid 119 distribution of rn activity in the atmosphere and its dependence on u and th content in the ground state nuclear track detectors. the application of solid state nuclear track detectors is based on the creation of structural defects on the sensitive surface of the detector due to alpha-particle hit against it (fleischer et al., 1965). the structural defects were enlarged during the process of etching, which enabled us to observe them even at slight magnification. a lr-115 detector was used for measuring the radon activity in the air (both in the open air and inside of a closed can). according to the information of its manufacturer, the lr-115 detector records the alpha-particles in a range of 1.24.8 mev, meaning that the alpha-particles emitted in a well-defined distance from the detector could be registered. this characteristic facilitates the elimination of recording alpha-particles emitted from the solid products of radon decay. in order to carry out the measurements of radon activity in the outdoor air, the detectors were fixed to the inner surface of a black plastic cup (of 8 cm diameter). the plastic cups provided shelter from the sun light and precipitations. we chose 27 monitoring points were situated on the outcrops of the different types of rocks. each monitoring point consisted of 4 cups fixed 2 m, 1 m, 0.5 m and 0.05 m above the ground surface (fig. 2a,b). the higher the level above the ground surface where the detector was fixed the more representative the measure was for the more exfig. 1. geological scheme of wzgórza niemczańsko-strzelińskie (scheme prepared on the basis of the geological maps made by badura, berezowska, cwojdzinski, dziemianczuk, gawronski, gazdzik, jerzmanski, oberc, trepka, walczak-augustyniak, wòjcik). 120 agnieszka anna ochmann tensive area. furthermore, detectors situated 0.05m above the ground registered thoron rn220 beside the radon rn222. the time of exposure was 6 months, twice a year: october-march (autumnwinter period) and april-september (spring-summer period). a long measurement period enables us to obtain the average values for the time of exposure and reduces the effect of diurnal fluctuations of pressure and temperature. the two sets of measurements were repeated the following year and the values obtained were similar to these which had been taken the year before. the radon activity in the outdoor air was calculated dividing the number of traces on 1 cm2 of the detector surface by the number of days of exposure and the result obtained multiplied by the calibration coefficient (in case of the atmospheric air it is 13.8), this model was elaborated by srivastawa et al. (1995). the lr-115 detector was also used during the measurement of the radon emanation from different types of rocks from the investigated area. to measure the radon emanation a modified method of the «can technique» was applied. the «can technique» was developed by alter and prize (1974 fide azam et al., 1995) and then applied with some modification, among others, by karamadoust (1988), azam et al. (1995) and solecki (1999). the basis of this method is that the total amount of radon in the investigated material consists of two parts, one – the atoms of radon stuck in the mineral crystals, the other – the radon atoms free to migrate in the interstitial spaces. the second part of radon atoms is the part which can get out of the rock material, enter the air above and could be registered by the detector. the ratio of these parts is defined by the emanation coefficient. the emanation coefficient was calculated introducing the total radium activity (content) ctra and effective radium activity cera k c ce era rat= . the total radium activity ct ra is the total amount of radon in the rock material calculated on the basis of gamma-spectrometric analysis of radium activity. in this case the analysis was made by radiometric laboratory of gig in katowice, using semi-conductor detector hpge, according to the polish norm pn-89/z-70073. the effective radium content cera is the fraction of radium which corresponds to the part of radon which has emanated from the sample. effective radium activity is expressed in bq/kg unit and can be calculated from the equation c kmt v e e ra = t where ρ is the track density on cm2 of the detector surface, v the volume of free space in the can, k a calibration constant (0.0245 track on cm2 in one day), m the mass of sample in kg and te the time of exposure in days (azam et al., 1995). crushed rock material (grain size of 3.51 cm) was closed in a hermetic can of the known fig. 2a,b. the measurements of radon activity in the outdoor air, a) a plastic cup with detector; b) diagram presenting the construction of the monitoring point. a b fig. 3. the instrument for measuring radon emanation from the rock material by «can technique» method; 1 – magnet, 2 – detector, 3 – metal plate. 121 distribution of rn activity in the atmosphere and its dependence on u and th content in the ground volume. on the inner surface of the cap, there was fixed a detector to register the alpha particles of radon in the air above the sample of rock (fig. 3). the important element is to start exposure after stabilization of the secular equilibrium between ra226 and rn222. the equilibrium (of 98%) between them stabilizes after 3 weeks in the closed space. because of this period of stabilization, 3 weeks after closing the can, the detector was covered by metal plate, which was held by a magnet from outside of the can. after this period the metal plate was removed by taking away the magnet and the detector starts to be exposed. this is a modification of the azam method introduced by solecki (1999). the detector mainly registers the alpha particles which come from the decay of rn222 and not from rn220. taking into account diffusion coefficients of radon in the air and in the crushed rock material, the half-life time of the thoron rn220 and the distance between the detector and the surface of the sample, it was estimated that alpha particles of thoron could be registered in 10%. the emanation coefficients of 14 different types of rocks were measured and estimated. moreover the emanation coefficient was measured for a grain size smaller than 1 cm and for a moisture content of 13% and 24% wt for each type of rock. these variations of conditions changed the ke values but the relative differences of ke between the different types of rocks remained the same. in this paper ke values are presented which were obtained for dry rock material of grain size of 3.5-1 cm. the field measurements of the uranium and thorium content in rocks and soil were performed using gamma-spectrometer gr-320. the gamma-spectrometer gr-320 is equipped with a detector, with a source cs of 0.5 mci (18.5 kbq). the gamma-spectrometer gr-320 measures contents of bi214 and tl208 and on this basis, it estimates the contents of u238 and th232. therefore the results are presented as an equivalent content of uranium (eu) and thorium (eth). a few zero values of measurements obtained on the outcrops of gabbro and serpentinites prove that the cosmic radiation does not influence the results of measurements. the 29 sets of measurements (in each place – 30 single measurements, each one lasting 300s) were collected above the outcrops of different type of rocks and above the soils which cover them. 4. results 4.1. the eu and eth content in soil and underlying rocks the mean eu contents in the rocks of investigated area vary from values close to zero (gabbro, serpentinite) up to 12 ppm, in the quartzgraphite schist in the south part of niemcza zone (fig. 4). the quartz-graphite schist which crops out in the north part of metamorphic of niemcza-kamieniec ząbkowicki, contains 7 ppm eu. the mean values for granitoides are 3-6 ppm eu, in mica schist of metamorphic of niemczakamieniec ząbkowicki and schist of wzgórza lipowe the measured contents are 2-4 ppm eu. a relatively high value was obtained for basalt in the north-east part of the metamorphic of niem122 agnieszka anna ochmann cza-kamieniec ząbkowicki – 3.2 ppm. the lowest values of uranium content were measured on amphibolites, quartzite, quartz-feldspathic schist and some basalts. the thorium content has a decisive impact the thoron activity in the environment. the relatively high eth contents (> 20 ppm) were measured on the outcrops of some granite and schist of wzgórza lipowe (fig. 4). the highest mean value – 32 ppm, was found on granodiorite near niemcza. the slightly lower thorium contents (20-15 ppm) were recorded in quartz-feldspathic schist of metamorphic of niemcza-kamieniec ząbkowicki, mica schist of wzgorza lipowe and mica-sillimanite schist in south part of wzgorza strzelinskie. the high value eth of the basalt in the north-east part of the metamorphic of niemcza-kamieniec ząbkowicki – 12.9 ppm is noteworthy. the uranium and thorium content in soil usually corresponds to the contents in the underlying rocks. in most places the eu and eth contents in soil (mean values: 2 ppm eu and 8 ppm eth) are slightly reduced in relation to the contents in underlying rocks (mean: 3 ppm eu and 12 ppm eth), but not in every place. on the areas composed of amphibolite, serpentinite, gabbro and quartzite the values measured on soil are higher than the values measured on the underlying rocks. it could be explained by the presence of loess material in the soil, which contains, after solecki (2000), 2-3 ppm eu and about 10 ppm eth. fig. 4. the mean eu and eth contents in the rocks of wzgórza niemczańsko-strzelińskie. 123 distribution of rn activity in the atmosphere and its dependence on u and th content in the ground 4.2. the emanation coefficient of the samples of selected rocks emanation coefficient (ke) is interpreted as a ratio of the amount of radon that emanated to the air above the sample to the whole radon which was generated in the sample. emanation coefficient is a feature of the material and quantifies the ability of this material to emanate radon-gas. the emanation coefficient of samples of 14 selected rocks vary in the range 0.003-0.13 (table i). according to unscear, the emanation coefficient varies in range of 0.01 – 0.8 (1988 fide robé and labed, 1995; markkanen and arvela, 1992). the emanation coefficients of investigated rocks fit in the lower range of the values published by unscear. the lower emanation coefficients were measured for basalts and the highest values of emanation coefficient were obtained for quartz-feldspathic schist, quartz-graphite schist and granite (górka sobocka). solecki (1999) measured for the metamorphic schist an emanation coefficient of 0.29. the higher emanation coefficient of metamorphic schist than that of the magmatic rocks could be explained by the textures of metamorphic schist, which were formed as a result of numerous transformations and deformations. therefore it could be presumed that the migration inside these rocks will be faster than in the solid magmatic rock. effective radium activity (content) (cera) quantifies the radon that come out to the atmosphere from the definite quantity of crushed rock. effective radium activity is the result of the total activity of ra228 and ra226 in rock and the emanation coefficient of this rock. the measurements inside the closed can partially include the thoron emanation, however in the atmosphere the presence of thoron is limited up to the height of 30-40 cm. therefore in the case of atmospheric radon the effective ra226 activity should by taken in consideration. the effective ra226 activity for the investigated samples is shown in the last column of table i. 4.3. atmospheric radon activity in the air 2 m above the ground surface the radon activity distribution above the area of wzgórza niemczańsko-strzelińskie is illustrated in fig. 5, the values are year-average. the highest radon activity, above 20 bq m3, is observed above the area of the outcrops of mylonite of niemcza zone, where the maximum values were measured above the outcrops of mylonitic gneisses, granodiorite and quartz-graphite schist. another area of high values is the region of górka sobocka, wzgórza lipowe and north-west part of wzgórza strzelińskie, with the maximum values above the outcrops of granites near górka sobocka and strzelin. these two geological situations of the higher radon activity in the atmosphere demonstrate the two general factors table i. emanation coefficient (ke) and effective ra226 activity of selected rocks from investigated area. type of rock ke cera226 [bq/kg] gabbro 0.01 0.03 serpentine 0.03 0.13 quartzite (wzgórza strzelińskie) 0.07 0.45 amphibolite (ne part 0.06 0.80 of sowie góry) basalt (e periphery of góry 0.007 0.26 sowie) basalt (metamorphic of niemcza0.003 0.17 -kamieniec ząbkowicki) basalt (s part of wzgórza 0.003 0.08 lipowe) quartz-feldspathic schist 0.13 3.39 (metamorphic of niemcza -kamieniec ząbkowicki) quartz-graphite schist 0.11 6.80 (niemcza zone) gneisses (wzgórza strzelińskie) 0.01 0.55 quartz monzodiorite 0.01 0.51 (ne part of sowie gory) granodiorite (niemcza zone) 0.06 4.43 granite (górka sobocka) 0.09 5.25 granite (strzelin) 0.03 1.90 124 agnieszka anna ochmann that influence the radon exhalation: uranium/ radium content in the rocks and their emanation coefficient. the influence of the high contents of uranium in rocks on the radon activity in the atmosphere is obvious. much more interesting seems to be the influence of the mylonitisation, fracturating and weathering of rocks in the tectonic zone, which can be observed above the niemcza zone. the increased migration of radon from fractured and weathered rocks was described among others by gates et al. (1990), ball et al. (1991), gundersen (1991), dubois et al. (1995) and ciężkowski and przylibski (1997). faults and cracks increase the surface of contact between rock and ground water, what promotes the possibility of radon migration too (ball et al., 1991; strzelecki and wolkowicz, 1993). the mean value of atmospheric radon activity, measured 2 m above the ground, on the area of wzgórza niemczańsko-strzelińskie was 21 bqm–3. this value corresponds to the radon activity measured above the central mountainous areas of the usa. the mean radon activity in the atmosphere on the area of usa varies in the range of 4-15 bqm–3, on the colorado plateau these values reach 18.5-27.8 bqm–3 (gesell, 1983). in canada outdoor radon activity measured 3 m above the ground surface reached the mean values for the provinces of manitoba 59 bqm–3 and saskatchewan 61 bqm–3 (grasty, 1991). fig. 5. atmospheric radon activity distribution above the area of wzgórza niemczańsko-strzelińskie. 125 distribution of rn activity in the atmosphere and its dependence on u and th content in the ground 4.4. radon activity in the air close to ground, 0.05 m above the ground surface the year-average values of radon activity close to the ground are shown in table ii. the values are not interpolated over the whole area because this parameter varies strongly, depending on the exhalation from the limited local ground. moreover, because of the way the cups with detectors are positioned (they were placed directly on the ground), the measurements were not influenced by dispersion in the atmosphere. the measurements carried out close to the ground include rn222 and rn220, so the radon activity values are influenced by both uranium and thorium content in soil and underlying rocks. the highest values of radon activity (>200 bqm–3) in air close to the ground were measured on granodiorite and granite, a little lower values (100-200 bqm–3) were measured on mylonitic gneisses of niemcza zone, on mica schist and quartz-feldspathic schist of metamorphic of niemcza-kamieniec ząbkowicki and on some basalts. the high values of radon activity are caused by the relatively high uranium and thorium content in soil (2-3 ppm eu and 7-18 ppm eth) and underlying rocks (3-6 ppm eu and 1126 ppm eth) in these places. the lowest activities (25-45 bqm–3) were measured on serpentinite and gneisses of góry sowie, where the uranium and thorium contents are: close to zero for eu and 23 ppm eth. the relatively low radon activity of 52 bqm–3 on quartz-graphite schist, where the contents of eu and eth in rock are 13 ppm and 6.6 ppm respectively, results from the unavoidable location of the detector outside of the old quarry, near the gneiss outcrops. the high values of radon activity in the air in relation to the low uranium and thorium contents in underlying rocks, were measured on amphibolites, gabbros and quartz. in these places the soil contains more radioactive elements (1.5-2 ppm eu and 6-9 ppm eth) than the rocks (0.1-0.5 ppm eu and 0.7-3.7 ppm th), which could result from the uranium and thorium concentration during the soil genesis process and from addition of loess material. 4.5. seasonal variation of radon activity in the air it is possible to observe slight seasonal variations of radon activity in the air at a height of 2 m (fig. 6). in spite of the fact that the modal value is located in the same range of activity valtable ii. radon activity in the air close to the ground above the outcrops of different rocks. number of type of rn activity monitoring underlying 0.05 m above point rock the ground (fig. 1) [bqm–3] 1 mylonitic gneisses 193 of niemcza zone 2 basalt 171 3 granodiorite 303 4 mica schist 90 of niemcza zone 5 amphibolite 118 6 quartz-graphite schist 52 7 gneisses of sowie góry 25 8 quartz monzodiorite 66 9 serpentinite 45 10 granodiorite 212 11 serpentinite 37 12 gabbro 85 13 amphibolite 165 14 mica schist 67 15 quartz-feldspathic schist 131 16 mica schist 137 17 mica schist 180 18 mica schist 155 19 basalt 87 20 basalt 101 21 granite 240 22 granite 200 23 gneisses 48 of wzgórza strzelińskie 24 gneisses 47 of wzgórza strzelińskie 25 quartzite and quartz schist 40 26 granite 145 27 erlanes 75 126 agnieszka anna ochmann ues for the autumn-winter and spring-summer period (10-20 bqm–3), in the autumn-winter period there are more measurements in the range of 20-30 and 30-40 bqm–3 than in the range of 0-10 bqm–3, the opposite situation is observed on the histogram for the spring-summer period. the slightly higher values in the autumn-winter period could be explained by increased exhalation in the conditions of temperature difference between the air in the ground and air above the ground surface. in the autumn-winter period the air temperature above the ground usually is lower than that in the ground, which causes convection of the air from ground to the atmosphere. these phenomena were described by wilkening (1990), hakl et al. (1995) and robé and labed (1995). the snow cover, described as a factor which decreases the radon activity in the atmosphere (juzdan et al., 1985; somogyj et al., 1986; feichter and crutzen, 1989; dörr and münnich, 1990; jacob and prather, 1990; ennemoser et al., 1995), is not the dominant factor because on this area the period of snow cover is very short. 5. conclusions the mean value of radon activity in the air 2 m above ground surface was 21 bqm–3. the highest values were measured in the area of granite and quartz-graphite schist outcrops (rocks of the high eu content) and in the area of mylonitic rocks of the niemcza zone. these observations confirm that the radon activity in the atmosphere depends on uranium/radium content in the rocks and their emanation coefficient and on the mylonitisation and fracturating grade of rocks in the tectonic zone. the slight seasonal variation of radon activity in the air is the result of weather conditions which control radon migration from soil-gas to atmosphere. radon activity close to the ground surface varies from 25 to 300 bqm–3 and accurately reflects the uranium and thorium content in the indirect ground basement (soil and weathered rocks). references azam, a., a.h. naqvi and d.s. srivastava (1995): radium concentration and radon exhalation measurements using lr-115 type ii plastic track detectors, nucl. geophys., 9 (6), 653-657. ball, t.k., d.g cameron, t.b. colman and p.d. roberts (1991): behaviour of radon in the geological environment: a review, quart. j. eng. geol., 24, 169-182. bartz, w. and j. puziewicz (1999): orientotion of quartz axes in the quartz-graphite schist of niemcza zone and the niemcz-kamieniec ząbkowicki metamorphic unit (lower silesia, poland) as an indicator of metamorphism and deformation conditions, arch. miner., lii (l), 113-129 (in polish). ciężkowski, w. and t.a. przylibski (1997): radon in waters from health resorts of the sudety mts. (sw poland), appl. radiat. isot., 48 (6), 855-856. dörr, h. and k.o. münnich (1990): rn-222 flux and soil air concentration profiles in west-germany. soil rn222 as tracer for gas transport in the unsaturated soil zone, tellus, 42b, 20-28. dubois, c., a. alvarez calleja, s. bassot and a. chambaudet (1995): modelling the 3-dimensional microfissure network in quartz in a thin section of granite, in gas geochemistry, edited by c. dubois, d. klein, a. chambaudet and m. rebetez (northwood: science reviews), 357-368. dziedzicowa, h. (1966): the schist series east of the niemcza zone in the light of new investigations, z geologii ziem zachodnich, wroc l⁄ aw, 101-118 (in polish). dziedzicowa, h. (1987): structural evolution and metamorphism of eastern border of the góry sowie, acta univ. wrat. no. 788, prace geologiczno-mineralogiczne, x, 221-247 (in polish). fig. 6. distribution of radon activity values, measured in the air 2 m above the ground surface in the two periods of the year, which demonstrates slight seasonal variation. 127 distribution of rn activity in the atmosphere and its dependence on u and th content in the ground ennemoser, o., s.m.g. giacomuzzi, p. brunner, p. schneider, v. stigl, f. purtscheller and w. ambach (1995): radon measurements in soil to predict indoor radon concentrations in new buildings in an area with unusually high radon levels, sci. total environ., 162, 209-213. feichter, j. and p.j. crutzen (1990): parameterization of vertical tracer transport due to deep cumulus convection in a global transport model and its evaluation with radon-222 measurements, tellus, 42b, 100-117. fleischer, r.l., p.b. price and r.m. walker (1965): solid-state track detectors: applications to nuclear science and geophysics, ann. rev. nuc. sci., 15, 1-28. gates, a.e., l.c.s. gundersen and l.d. malizzi (1990): comparison of radon in soil over faulted crystalline terranes: glaciated versus unglaciated, geophys. res. lett., 17 (6), 813-816. gesell, t.f. (1983): background atmospheric 222rn concentrations outdoors and indoors. a review, health phys., 45, 289-302. grasty, r.l. (1991): a cross canada radon survey, in proceedings of the fifth international symposium on the natural radiation environment, salzburg austria. gundersen, c.l.s. (1991): radon in sheared igneous and metamorphic rocks, u.s. geol. surv. bull. (on line: http://sedwww.cr.usgs.gov:8080/radon/shearl-6). hakl, j., i. hunyadi and a. varhegyi (1995): the study of subsurface radon transport dynamics based on monitoring in caves, in gas geochemistry, edited by c. dubois, d. klein, a. chambaudet and m. rebetez (northwood: science reviews), 391-398. jacob, d.j. and m.j. prather (1990): radon-222 as a test of convective transport in a general circulation model, tellus, 42b, 118-134 . jha, s., a.h. khan. and u.c. mishra (2001): a study ofthe technologically modified sources of 222rn and its environmental impact in an indian u mineralised belt, j. environ. radioact., 53 (2), pp. 183-197. karamadoust, n.a., s.a. durrani and j.h. fremlin (1988): an investigation of radon exhalation from fly ash produced in the combustion of coal, nucl. tracks, 15, pp. 667. khatir sam, a.k. and e. holm (1995): the natural radioactivity in phosphate deposits from sudan, sci. total environ., 162, 173-178. lozano, j.c., f. vera tome, v. gomez escobar and p. blanco rodriguez (2000): radiological characterization of uranium mine with no mining activity, appl. radiat. isot., 53, 337-343. markkanen, m. and h. arvela (1992): radon emanation from soils, radiat. prot. dosimetry, 45 (1/4), 269-272. mazur, s., j. puziewicz and d. józefiak (1995): the niemcza zone – a regional-scale shear zone between two areas of contrasting tectono-metamorphic evolution, in przewodnik lxvi zjazdu ptg «geologia i ochrona środowiska bloku przedsudeckiego», wroc l⁄ aw, 221-240 (in polish). oberc-dziedzic, t. (1991): geological setting of the strzelin granitoids, acta univ. wratisl., 1375, prace geol. miner., xxix, pp. 295-324 (in polish). oberc-dziedzic, t. and j. szczepański (1995): geology of the wzgórza strzelińskie cristalline massif, in przewodnik lxvi zjazdu ptg «geologia i ochrona środowiska bloku przedsudeckiego», wroc l⁄ aw, 111-126 (in polish). papastefanou, c. (2001): radiological impact from atmospheric releases of 238u and 226ra from phosphate rock processing plants, j. environ. radioact., 54, pp. 75-83. puziewicz, j. and m. radkowska (1990): morphology of the crystals of zircon from plutonic rocks of the niemcza zone (sudetes, sw poland), arch. miner., xlvi (1-2), 124-139 (in polish). robé, m.c. and v. labed (1995): explaining the variation in soil radon concentrations: a study of the influence of some intrinsic properties of a rock matrix on the radon emission factor, in gas geochemistry, edited by c. dubois, d. klein, a. chambaudet and m. rebetez (northwood: science reviews), 535-542. solecki, a.t. (1999): radon emanation of selected sudetic rocks, in 5th international conference on rare gas geochemistry, august 30-september 3, 1999, debrecen, hungary. solecki, a.t. (2000): radiometric anomalies of the central part of the sudetic foreland and their relation with geological environment, acta univ. wrat. no. 2210, prace geologiczno-mineralogiczne, lxix, 1-91 (in polish). somogyj, g., a.f. hafez, i. hunyadi and m. tóth-szilágyj (1986): measurement of exhalation and diffusion parameters of radon in solids by particle track detectors, int. j. radiat. appl. instrum., part d, nucl. tracks radiat. meas., 12 (1/6), 701-704 srivastava, d.s., p. singh, n.p.s. rana, a.h. naqvi, a. azam, t.v. ramachandran and m.c. subba ramu (1995): calibration factor for lr-115 type ii track detectors for environmental radon measurements, nucl. geophys., 9 (5), 487-495. strzelecki, r. and s. wolkowicz (1993): geological control on radon emanations, nukleonika, 38 (4), 109-120. wilkening, m. (1990): radon in the environment, studies in environmental science no. 40, pp. 138. wojnar, b. (1977): petrography of granite from górka sobocka in the foresudetic block, acta univ. wrat. no. 378, prace geologiczno-mineralogiczne, vi, 139-156 (in polish). preliminary analysis of radon time series before the ml=6 amatrice earthquake: possible implications for fluid migration annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 preliminary analysis of radon time series before the ml=6 amatrice earthquake: possible implications for fluid migration valentina cannelli, antonio piersanti, elena spagnuolo, gianfranco galli istituto nazionale di geofisica e vulcanologia valentina.cannelli@ingv.it abstract on august 24, 2016 a ml=6.0 earthquake occurred in central apennines, italy, between the towns of norcia and amatrice, causing severe destruction and casualties in a wide area around the epicenter. we present a preliminary analysis of continuous radon concentration data collected from the second half of 2012 to the day after the earthquake by a long term radon monitoring station, installed at cittareale (ri, italy), about 11 km south-west of the epicenter. we combine the field data analysis with the outcome of dedicated laboratory experiments, aimed to study real time radon emission dynamics from rock samples subject to normal and shear stress loads in absence of fluid transport and migration phenomena. our results suggest the possibility of a minor role played by phenomena related to fluid migration for the amatrice seismic event with respect to other recent apennine earthquakes. i introduction c urrent research on soil radon emanation in terms of analyses of long time series has revealed its potential informative power regarding the link between temporal variation of this noble gas concentration and seismogenic processes [stefansson(2011), piersanti et al.(2016)]. in fact, the radioactive nature of radon makes it a potentially efficient marker to study and monitor fluid flows. in recent years, new laboratory experiments gave unambiguous evidence of the link between the rock state of stress and variations in the radon emanation properties [tuccimei et al.(2010), mollo et al.(2011)]. however, the analysis is complicated by the susceptibility of radon emissions to meteorological parameters and site-specific features [jaishi et al.(2014), piersanti et al.(2015)]. we had the possibility of analyzing a three-years long time series of radon concentration acquired by a monitoring station installed at about 11 km south-west of the 24 august, 2016, ml=6.0 amatrice earthquake epicenter. we present the preliminary results obtained limiting the impact of meteorological effects on the measured radon time series and combining them with the results of a series of laboratory experiments to study radon concentration variations in connection with a process of gradual deformation in shear. ii methods ii.1 soil radon observations radon data were collected by a high sensitivity, high efficiency active radon monitoring station based on a lucas cell [lucas(1957)], installed at cittareale (cttr, 42◦37′3.0′′n 13◦9′33.5′′e) in may 2010, at about 960 m 1 mailto:valentina.cannelli@ingv.it annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 above sea level. in august 2012 a new radon concentration detector replaced the previous one. in this work we decided to employ only data from the latest (and currently working) detector. the station is located in a basement hosting the municipal archive of the city (occasionally accessible to technical staff only) and measures radon concentration with an adjustable acquisition time (currently is 115 minutes); its efficiency is 0.06 count min−1/bq m−3 and the minimum detectable concentration is as low as 6 bq m−3. cttr acquires simultaneously local temperature values by means of a specific sensor co-located with the radon one. all other daily values of meteorological parameters employed in our analysis (external temperature, pressure, precipitation) are approximated as short term (12-24h) weather forecast by an italian weather website (http://www.ilmeteo.it/). the complete time series of radon concentration data recorded at station cttr from august 1, 2012 to august 25, 2016, together with the time series of local temperature are shown in figure 1a and figure 1b, respectively. cttr radon concentration data show a marked seasonal signal, ascribable to a major correlation with temperature (see blue monthly moving-average of radon time series in figure 1a and daily average of local temperature in figure 1b), as laboratory tests [iskandar et al.(2004)] and long term radon monitoring studies [jaishi et al.(2014), piersanti et al.(2015)] indicate. the meteorological corrected radon concentration values that we will show and discuss in the following sections have been obtained applying the procedure described in [piersanti et al.(2016)]. jul12 oct12 jan13 apr13 jul13 oct13 jan14 apr14 jul14 oct14 jan15 apr15 jul15 oct15 jan16 apr16 jul16 oct16 50 100 150 200 250 300 350 400 450 a) r a d o n c o n c e n tr a ti o n ( b q /m 3 ) jul12 oct12 jan13 apr13 jul13 oct13 jan14 apr14 jul14 oct14 jan15 apr15 jul15 oct15 jan16 apr16 jul16 oct16 6 8 10 12 14 16 18 20 22 24 b) in te rn a l te m p e ra tu re ( ° c ) figure 1: a) radon concentration (bq m−3) at cttr for the period august 1, 2012 august 25, 2016 both as daily average (yellow dots) and as monthly moving-average (blue line). b) daily averaged time series of local temperature for the same period as in (a). ii.2 laboratory we conducted four laboratory experiments to study radon concentration variations in connection with a process of gradual deformation in shear. the use of controlled conditions of loading and ambient conditions (temperature and humidity), helps reducing the number of implicated variables affecting the natural radon emanation process and simplifies the interpretation of the results. we use the rotary shear apparatus shiva (slow to high velocity apparatus) installed in the high pressurehigh temperatures (hp-ht) laboratory of the ingv of rome simulating close to natural seismic deformation condition at depth of the upper-crust [di toro et al.(2010)]. the sample assembly is made of two cylinders 50 mm of external diameter sandwiched in frictional contact under a constant normal load of 5 mpa on tuffs and 15 mpa on tonalities, within a pressure-vessel. the vessel [violay et al.(2013)] is a device made of stainless steel equipped with sealing o-rings that ensure isolation of the sample pair and guarantee fluid confinement. rock type selection (tuff and tonalite) 2 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 was driven by the radon initial concentration, porosity and shear modulus. we used the same instrument employed in the field to continuously acquire radon/thoron concentration variations during the progress of the experiment, with an acquisition time of 1 sec. we use a simple pump to flux air in closed loop between the inlet valve of the vessel and the outlet flange of the radon detector to allow air circulation within the vessel and from the vessel to the detector (figure 2). variations of radon concentration are referred to an initial condition set at the achievement of secular equilibrium. secular equilibrium was ensured pre-stressing the sample pair in a uniaxial press for four days. experiments consisted in a step-wise increase in shear-stress (τ ≈ 0.2mpa, dt ≈ 5 min) under constant normal load until sample failure, resulting in a fast rotation of the rotary column at prescribed velocity and pronounced wearing and axial shortening of the contact surfaces. the shearstress was then re-established and increased again, for several cycles, until the initial bare rock is crushed to powder. deviation from the prescribed experimental conditions were possible due to the onset of mechanical oscillations in shear stress or a fast increase in the deformation rates, requiring a manual intervention. all experiments were well reproducible in both the mechanical behaviour and recorded radon emission variations. figure 2: the experimental apparatus (shiva, on the left modified from [di toro et al.(2010)]) essentially made of a rotary axes, a stationary axis and a sample chamber. the experimental assembly consists of the radon detect. (labelled with a), the connection (b) allowing for close loop circulation of fluids from the vessel (c) to the radon detect.; the air pump for air flux (d). iii results iii.1 soil radon observations our analysis is based on the phenomenological observation of the trend of radon concentration time series during the months of july and august for the four years from 2013 to 2016 and successively on their correlation with variations of meteorological parameters measured in the same periods, through an empirical correction procedure aimed at limiting the impact of their variations on the measured radon concentration levels (see for details [piersanti et al.(2016)]). in figure 3a daily averaged time series of radon concentration for the period july-august (20132014-2015-2016) are shown. radon concentration data acquired at station cttr show for these four time windows values variability comparable with the one of the complete time series but with higher absolute concentration values, as expected for summer months [iskandar et al.(2004), jaishi et al.(2014)]. namely, we registered sharp peaks ranging 3 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 from a few tens up to 400 bq m−3. the average values of radon concentration evaluated for the period july-august and for the month of august alone are reported in table 1, in the second and in the fourth column respectively. it is worth noting that 2016 average value is the lowest of all the four years and it is lower than the average values registered during the previous three years by more than 3 standard deviations (see last two rows of table 1). table 1: mean values of radon concentrations in (bq m−3). jul-aug jul-aug aug aug (uncorr.) (corr.) (uncorr.) (corr.) 2013 227 149 212 96 2014 228 129 222 90 2015 233 158 216 179 2016 214 71 196 63 avg. 229 145 217 122 std.dev. (± 3) (± 15) (± 5) (± 50) 25−jun 05−jul 15−jul 25−jul 04−aug 14−aug 24−aug 03−sep 50 100 150 200 250 300 350 400 450 a) r a d o n c o n c e n tr a ti o n ( b q /m 3 ) 2013 2014 2015 2016 25−jun 05−jul 15−jul 25−jul 04−aug 14−aug 24−aug 03−sep 0 200 400 600 800 1000 1200 b) r a d o n c o n c e n tr a ti o n c o rr e c te d ( b q /m 3 ) 2013 2014 2015 2016 figure 3: a) daily-average time series of radon concentration for the period july-august for 2013,2014,2015,2016. the black vertical line marks the occurrence of the ml=6 earthquake. b) the same as in (a) but with daily average time series corrected for meteorological parameters (ctmax , crmax = 5; cpmax = 1). in figure 3b we show the time series of figure 3a corrected for meteorological parameters, according to the empirical correction procedure developed by [piersanti et al.(2016)] in order to remove the effect of meteorological phenomena on measured radon concentrations. as described in cited work, the correction parameters are determined through a numerical optimization scheme whose results, in agreement with results obtained for the pollino range stations (south italy, calabrian arc), indicate temperature and precipitations as the most impacting variables on radon concentration data (ctmax , crmax = 5), while the role of the atmospheric pressure variations is not well constrained (cpmax = 1). the features of all the considered time series change noticeably after the correction. we focus our attention on the 2016 july-august corrected one (cyan solid line in figure 3b) that is significantly flattened with respect to all the others (see also the average values of corrected radon concentration in the third and fifth column of table 1). indeed, the 2016 july-august uncorrected time series show three low concentration peaks, the latest occurring two days before the earthquake (figure 3a). the peculiar behavior of 2016 july-august radon timeseries is confirmed also by the observation of relative variations of radon concentration during the period selected for our analysis. figure 4 shows that the largest relative variations of the whole radon time series occur just in the 90 days preceding the earthquake. 4 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 nov−12 jun−13 jan−14 jul−14 feb−15 aug−15 mar−16 sep−16 −0.5 −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 0.5 r n r e la ti v e v a ri a ti o n 14 aug 21 jun 23 may figure 4: relative variations of radon concentrations for the period january 1, 2013 august 25, 2016 iii.2 laboratory we conducted two experiments on tonalite and two on tuff to test the instrument sensitivity and the reproducibility of radon concentration variations in case of laboratory faults. in the following we will focus on tonalite since we consider it more representative of a seismogenic setting with respect to tuff. experiments conducted at normal load of 15 mpa (experiments n. 1063 and s1095) show that the experimental fault responds to a gradual increase in shear stress (blue solid curve in figure 5) by sliding at slip rates (black solid curve) varying from less than a few mm s−1 up to 4 cm s−1. these episodic slip events are concomitant to a large increase in the axial shortening which typically ranges from 0 to 2 mm (red solid curve). after the application of the normal load and with the progressive increase in shear stress the number of radon counts (converted in counts per hour in figure top panel) was gradually decreasing (figure 5 top panel) until the onset of the largest event where we observed the largest decrease in radon counts. it is worth noting that this large negative radon concentration variation occurred, in the case of both the experiments, before sample failure and still far from a condition of seismic sliding (slip rates > 0.1m s−1). moreover, the correlation between the state of loading if the sample pair and the radon concentration variation follow almost instantaneously. the negative radon concentration variation prior sample failure was also reported in previous static analysis ([tuccimei et al.(2010)]) on tuff where the radon concentration variation was measured as a cumulative value prior and after the application of a normal load under an uniaxial press. 0 5 1 h , 2 4 ', 5 0 .8 '' 0 0.02 0.04 )s / m( et ar pi l s s h e a r s tr e ss ( m p a ) time 9 a p ri le . 2 0 1 5 0 8 .5 6 :1 6 ( g m t ) axial shortening 2mm 09:00 10:00 11:00 08:00 10:00 12:00 14:00 700 650 600 550 compression compression + shear 12:00 13:00 14:00 # / h o u r figure 5: radon variation concentration (top panel) as a function of the shear stress increase (blue solid line, bottom panel) in case of experiment number s1095. the mechanical response of the system made of fault + experimental apparatus is also shown (bottom panel) in terms of slip rate (black solid curve) and axial displacement (shortening, red solid line). the number of radon counts (averaged in counts/hour in top panel) decreases with increasing shear stress at constant normal load (compressive load of 15 mpa) and has a maximum negative slope corresponding to the highest recorded slip pulse with slip-rate of 0.04 m/s. the axial shortening (generally associated to sample grinding and wearing) was of the order of 0.1 mm. iv discussion we have presented the results of an analysis of radon concentration data acquired at cttr station from 2013 to the day after the ml=6.0 5 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 amatrice earthquake supplemented with data obtained from dedicated, original laboratory experiments. the field observations confirm the strong impact of meteorological parameters variations on observed radon time series, especially temperature and precipitations. at the same time, both daily average time series and the ones corrected for meteorological parameters evidence for july-august 2016 a different behavior with respect to the same time window of the previous years, showing overall lower average values of radon emanations and an increase of relative variations among different detections. when a meteorological parameter correction is applied, the previous behaviors are confirmed and a flattened trend in the days preceding the earthquake can be also evidenced. data obtained from laboratory experiments, aimed to study real time radon concentration variations in connection with a process of gradual deformation in shear and in absence of fluid transport and migration phenomena, give results compatible with the cttr radon time series behavior during the 2016 july-august period. these combined observations could pave the way to the hypothesis of a minor role played by processes associated with fluid transport and migration for the amatrice seismic event with respect to other recent apennine earthquakes. nevertheless, the available data, being limited to a single station, do not allow us to rule out the possibility that different portion of the seismogenic volume could have been subject to different styles of fluid dynamics related phenomena. in this respect, a multi-station monitoring of seismogenic areas would represent an important evolution of the presented investigative approach. acknowledgment the experimental part of this work was supported by the erc cog nofear project 614705 (p.i. giulio di toro) references [di toro et al.(2010)] di toro, g., niemeijer, a.r., tripoli a.; et al. (2010), from field geology to earthquake simulation: a new state-of-the-art tool to investigate rockfriction during the seismic cycle (shiva), rendiconti lincei, 21(supp.1), s95-s114, pp. 1-20, doi:10.1007/s12210-010-0097-x. [iskandar et al.(2004)] iskandar, d., yamazawa, h., and t. iida (2004), quantification of the dependency of radon emanation power on soil temperature, appl radiat isot, 60, 971–973. [jaishi et al.(2014)] jaishi, h., singh, s., tiwari, r. p.; et al. (2014), analysis of soil radon data in earthquake precursory studies, annals of geophysics, 57(5) s0544, doi:10.4401/ag-6513. [lucas(1957)] lucas, henry f. (1957), improved low-level alpha-scintillation counter for radon, review of scientific instruments, 28(9), 680–683, doi:10.1016/j.cageo.2003.08.011. [mollo et al.(2011)] mollo, s., tuccimei, p., heap, m. j.; et al. (2011), increase in radon emission due to rock failure: an experimental study, geophysical research letters, 38(14;), doi:10.1029/2011gl047962. [piersanti et al.(2015)] piersanti, a., cannelli, v. and g. galli (2015), long term continuous radon monitoring in a seismically active area, annals of geophysics, 58(4), s0437, doi:10.4401/ag-6735. [piersanti et al.(2016)] piersanti, a., cannelli, v. and g. galli (2016), the pollino 2012 seismic sequence: clues from continuous radon monitoring, solid earth, 7, 13031316, doi:10.5194/se-7-1303-2016. 6 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7166 [stefansson(2011)] stefansson, r.(2011), advances in earthquake prediction, springer-verlag, berlin, heidelberg, 300 pp, doi:10.1007/978-3-540-47571-2. [tuccimei et al.(2010)] tuccimei, p., mollo, s., vinciguerra, s.; et al. (2010), radon and thoron emission from lithophysae-rich tuff under increasing deformation: an experimental study, geophysical research letters, 37(5), doi:10.1029/2009gl042134. [violay et al.(2013)] violay,m., nielsen, s., spagnuolo, e.; et al. (2013), pore fluid in experimental calcite-bearing faults: abrupt weakening and geochemical signature of co-seismic processes, epsl, 361, 74-84. 7 introduction methods soil radon observations laboratory results soil radon observations laboratory discussion microsoft word 7360-18377-1-sp_lopera et al_final.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 1 public policies, social perception and media content on fracking: an analysis in the spanish context emilia hermelinda lopera-pareja centro de investigaciones energéticas, medioambientales y tecnológicas (ciemat) emilia.lopera@ciemat.es ana garcía-laso domingo alfonso martín-sánchez polytechnic university of madrid ana.garcia.laso@upm.es domingoalfonso.martin@upm.es abstract energy supply and security is a topic that connects geoscience and ethics to economic, social and sustainable development. it also involves a wide range of interests and social actors, such as politicians, experts, industry, environmental ngo and citizen organizations and platforms. the fossil fuel extraction industry using hydraulic fracturing, and commonly known as fracking, is one of the most recent and controversial issues arising from the application of energy technologies in north america, south america and europe. this paper examines the public debate in spain on the topic of fracking from a multi-level and complementary perspective, based on data collected during 2012-2016 by exploring the development of public policies at national and regional levels, monitoring public perception through a survey, and analyzing the media content. from the theoretical frameworks of social amplification of risk and post-materialist values, a content analysis was applied to assess the relevance and the social, cultural and human values distributed by the press when dealing with fracking in spain. our results concerning public policies show that there are different positions on the matter between national and regional authorities, even if both authorities belong to the same political party. regarding social perception, almost 60 per cent of spaniards have admitted to having no opinion about fracking. finally, media discourse has been driven by pessimistic views, focused mainly on cost-benefit analysis and emphasizing how human health, natural resources and local communities might be negatively affected by fracking activities. 1. introduction urrently hydraulic fracturing techniques to extract fossil resources often do not meet with general social acceptability. the implementation of fracking has aroused public opposition in many local communities, becoming a controversial issue both in the united states and in european countries due to its potential consequences on the environment and human health (jaspal and nerlich, 2014; negro, 2012). in addition to economic and technical feasibility, the lack of public acceptance emerges as a complementary research field. in this context, this contribution examines the development of public policies, social perception and media discourse on fracking in spain with the aim to understand the dynamics and synergies that may be influencing the emergence and evolution of public opposition and the potential underlying ethical implications. for this purpose different sources and data were used: a) following and monitoring of public policies related to the topic at national c annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 2 and regional level from 2012 to 2016; b) data collected from the national survey on social perception of science and technology conducted in 2014 by the spanish foundation for science and technology; and c) data resulting from a content analysis applied to the press coverage on fracking published by the premium general information newspaper in spain, el país, during the timeframe 2013 and 2014. 2. results public policies spanish public policies on energy issues have experienced great changes since 2012 in updating and adapting the national regulatory framework in order to speed up and promote the implementation of fracking extractive technique. the sustained increase of oil prices from 2009 to 2014 and, as a consequence, the interest shown by fossil fuel companies to explore and exploit shale gas fields in several areas of the spanish territory were some of the key factors behind legal reforms developed by the national conservative government. at the same time, public policies on renewable energies also experienced significant changes following the entry into force of the decree law 1/2012 on the abolition of the subsidies to renewable energies (official state gazette, 2012). hydraulic fracturing was expressly included among the standard technologies for the exploitation of unconventional shale gas in spain under the law 17/2013 on the security of energy supply (official state gazette, 2013a), amending law 34/1998 on hydrocarbons (official state gazette, 1998). the environmental impact assessment for fracking projects was also included in law 21/2013 (official state gazette, 2013b), although preliminary projects to study viability through this technique were exempt. whereas licenses for exploration activities are granted by regional authorities when the whole area is located in a single region, this responsibility falls to the national authorities when the activity covers two or more regions. given this regulatory situation, three of the spanish northern regions – cantabria, la rioja and navarra – were adopting respective regional laws on the ban of fracking activities in its territories throughout 2013. but the national government immediately appealed these laws to the spanish constitutional court. at the european level, in january 2014 the european commission published a set of recommendations (2014/70/eu) on minimum principles for the exploration and production of hydrocarbons using high-volume hydraulic fracturing (official journal of the european union, 2014). but instead of setting a common framework, member states were simply ‘invited to’ plan developments, assess environmental impacts, ensure the best practice standards, check the quality of the local water, air and soil, control air emissions and inform the public. some months later, the three regional laws banning fracking were repealed by the constitutional court. the respective rulings were based on the existence of a conflict of political and administrative competences between regional and national authorities in the field of exploratory drilling and extraction of no conventional hydrocarbons. since hydrocarbon resources are deemed strategic, the regulation falls within the competence of national authorities. unlike in the united states, spanish landowners do not have property rights over the exploitation of fossil fuels from the subsoil. in fact, according to the aforementioned hydrocarbon law, gas and oil fields are considered strategic resources of state public domain. however, the strong opposition against fracking expressed by local and regional authorities plus the public rejection and protest movement promoted by local platforms on social networks and in the streets could have motivated the adoption of a new national regulation in 2015 that provided for economic benefits for individuals, local, and regional authorities. in fact, in order to further promote the use of fracking despite the controversial social debate at the local level, the new law 8/2015 called for a system of financial compensation and incentive for property owners, municipalities, and regions (official state gazette, 2015); in this latter case through taxes and fees paid by annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 3 the operating industry. but the promise of economic benefits did not prevent catalonia and the basque country from banning fracking in their territories over the course of 2015. at present, these laws await a ruling by the spanish constitutional court. disagreement over fracking was also a reason for the confrontation between regional and national political leaders, especially within the right-wing party, partido popular. generally, local and regional institutions geographically closer to the citizen daily life shared the same view against fracking including worries about possible negative effects derived from such energy facilities. the opposition to fracking shown by the conservative regional government of la rioja is a paradigmatic example of this situation. social perception and public interest the last edition of the spanish survey on social perception regarding science and technology (fecyt, 2015) featured two new questions about the consequences of scientific and technological development in general (q.14) and about specific technological applications (q.15) such as hydraulic fracturing and nuclear energy (muñoz-van den eynde, 2015). according to the results, almost 60 per cent of the respondents admitted they did not have an opinion on fracking whereas only 9 per cent reported the same answer regarding nuclear energy, a much more controversial issue in spain and abroad for a much longer time. from the remaining 40 per cent of respondents with an opinion on fracking, 24 per cent considered that it represented higher risks than benefits whereas 7 per cent believed the opposite. finally, for 10 per cent of the interviewers risks and benefits were balanced (fig. 1). figure 1: social perception on technological applications, fracking and nuclear energy in spain. data source: muñoz-van den eynde (2015). the fact that six out ten spaniards do not have an opinion on fracking should not be interpreted as a lack of concern or interest to know more about this matter. as in a previous study analyzing the fracking debate in the united states and united kingdom (mazur, 2016), we used the monthly frequency of web searches on “fracking” as reported by google trends as a measure of public interest (filtering on spain), along with media trends in the time period of 2013 and 2014. google trends and el país media attention is shown in fig. 2 using normalized data, that is, adjusted so that the month with the higher web searches and articles were indexed as 100, and numbers of searchers and articles for other months are a proportion of that number. this is an effective strategy to compare public and media interests on a similar issue. data represented in fig. 2 show that while media attention suffered great fluctuations during the period in question, the public interest remained more stable and increased on december 2014. accordingly, it could be interpreted that in some manner media attention boosted public interest during 2013 although public interest on fracking remained active and even surpassed media attention for most of the year 2014. by regions, web searches on fracking were much more frequent in northern regions of spain more affected by exploratory efforts and where local authorities tried to ban this activity: cantabria, basque country, la rioja and navarra. concern, public opposition and confrontations beannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 4 tween regional and national governments could have made the issue more salient for online users from these communities. figure 2: spain web searched on “fracking” and reporting in el país, 2013-2014. media content on a quarterly basis (fig. 3), our results reveal that the tone of media discourse evolved from a perspective mainly dominated by pessimism concerning the viability of fracking in 2013 – representing the technique as a threat rather than as an alternative source of energy and prosperity – to an increase of news stories written from a neutral view in 2014. optimistic approaches supporting the implementation of this type of extraction were clearly scant throughout the studied timeframe. figure 3: tone of media discourse (el país) on fracking in spain, 2013-2014. in order to have a comprehensive understanding of how pessimistic and optimistic views about fracking were constructed in the spanish press coverage we applied a content analysis (data collection, sampling and complete list of variables are available in: lopera et al., 2015) whose risk and benefit variables were inspired by the sociological theory of post-materialism (inglehart, 2008). according to this theoretical framework it is assumed that western societies have been gradually switching from the acceptance of material values (calculated as benefit variables) to the defense of post-materialist values (calculated as risk variables) (table 1). table 1: categories and subcategories of the variables coded in the content analysis applied to the press coverage on fracking. risk variables (worries) environment water soil air quality human health induced seismic activity benefit variables (promises) economy job opportunities competitiveness energy supply and security post-materialism emphasizes non-material goods focusing on a clean and healthy environment instead of materialism positions that argue that economic growth is strongly articulated with energy security and supply. fig. 4 shows a quantitative snapshot of fracking social representation through the daily press in spain in terms of risks and benefits. our findings revealed that media content primarily highlighted certain endangered postmaterialist values, mainly focused on negative impacts on human health (51 per cent of the news articles) and several environmental aspects at a local scale such as water resources (groundwater) (49 per cent). to a lesser degree soil (23 per cent) and air quality (17 per cent) were shown to be under risk as well. possible induced seismic activity was also a narrative element taken into account by the news media (16 per cent). the connection between seismic movements and fracking was not only based on describing the tremors that annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 5 have occurred abroad (lancashire, in uk, and oklahoma, in usa) (el país, 2013a; el país, 2013b), but also those in spain along the mediterranean coast, in valencia region (el país, 2014). the occurrence of several earthquakes, with magnitude (m between 3.8 to 4.3) (juanes et al., 2017), forced the closure of a megaproject that was supposed to store injected natural gas using the old facilities of an oil exploitation in amposta, known as the castor project. the new york times made a connection between potential impacts of fracking and the severe consequences of the british petroleum oil spill in the gulf of mexico on april 2010 (mazur, 2016), whereas in spain el país connected the tremors occurred due to the gas storage of the castor project in 2013 to the potential impacts of induced seismic activity resulting from fracking activities. on the other hand, materialist values were highlighted as key narrative elements in the articles offering a neutral and/or optimistic approach, especially the economic benefits (39 per cent) and energy security and supply (30 per cent). we stress that whereas these benefits were depicted at a national scale in a very diffuse way, the aforementioned risks were represented as a close threat for local and traditional lifestyles based on agriculture, livestock and tourism. this results fall in line with the data obtained by davis and fisk (2014) when analyzing public attitudes toward fracking use and policies in the usa. previous studies analyzing representations of fracking on youtube videos from a cost and benefits angle identified threats to environmental, human and economic values (jaspal et al., 2014). social and psychological impacts on small communities were highlighted as well. figure 4: risks and benefits of implementing fracking according to the media content. 3. conclusions this paper reveals that the prevailing media discourse on the use of fracking in spain has been pessimistic and neutral, almost always underlining the potential negative impacts on human health, environment and local natural resources rather than as an exclusive energy issue providing economic growth arising from energy security and supply. this was combined with the lack of a clear public opinion at a national level despite strong local opposition in the affected regions. moreover, the analysis and monitoring of the public policies on this issue indicate a deep confrontation between national and regional governments, especially controversial in those regions where exploration studies have been completed or are being planned in order to study the economic and technical viability of the fracking projects: cantabria, la rioja, navarra, catalonia and basque country, among others. while national policies are designed to promote the implementation of fracking attending to the industry claims, the main goal of regional policies was to avoid the use of fracking in their territories. in this regard, local and regional authorities showed to be more sensitive and receptive to the instances of the public opposition than the directives of the national government. acknowledgment this work was carried out as part of the project “concept and dimensions of scientific culture” (mineco-15-ffi2014-58269-p) funded by the spanish ministry of economy and competiannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 6 tiveness and by the department of economy and employment of the principality of asturias for the “grupo de estudios cts” (fc-15grupin14-128). references davis, c., and fisk, j.m. (2014). energy abundance or environmental worries? analizing public support for fracking in the united states, review of policy research, 31(1), 116. el país (2013a). el ‘fracking’ atrae réplicas de terremotos lejanos (7/12/2013). available from: https://elpais.com/sociedad/2013/07/ 12/actualidad/1373660760_579342.html (accessed 2 october 2017). el país (2013b). terremotos fruto de la actividad humana (3/27/2013). available from: https://elpais.com/sociedad/2013/03/ 27/actualidad/1364419029_235833.html (accessed 2 october 2017). el país (2014). los expertos confirman la relación entre los seísmos y el proyecto de gas castor (5/12/2014). available from: https://elpais.com/sociedad/2014/05/12/ actualidad/1399897547_384101.html (accessed 2 october 2017). fecyt (2015). cuestionario de la vii encuesta de percepción social de la ciencia y la tecnología en españa, percepción social de la ciencia y la tecnología 2014, madrid, fecyt, 365-396. available from: https://icono.fecyt.es/sites/default/files/filepu blicaciones/cuestionario_epscyt204.pdf (accessed 2 october 2017). inglehart, r. (2008). changing values among western publics from 1970 to 2006, west european politics, 31(1-2), 130-146. jaspal, r., and nerlich, b. (2014). fracking in the uk press: threat dynamics in an unfolding debate, public understanding of science, 23(3), 348-363. jaspal, r., turner, a., and nerlich, b. (2014). fracking on youtube: exploring risks, benefits and human values, environmental values, 23(5), 501-527. juanes et al. (2017). coupled flow and geomechanical modeling, and assessment of induced seismicity, at the castor underground gas storage project. assessment of induced seismicity at the castor project – final report, april 2017. available from: http://www.minetad.gob.es/eses/gabineteprensa/notasprensa/2017/doc uments/castor_final_report_final_signed.p df (accessed 2 october 2017). lopera-pareja, e. h., garcía-laso, a., and martín-sánchez, d. a. (2015). discourses and values underpin public debate on fracking in spain: a case study at the crossroad, geoethics for society: general aspects and case studies in geosciences european general union 2015 general assembly, vienna. mazur, a. (2016). how did the fracking controversy emerge in the period 2010-2012?, public understanding of science, 25(2), 207-222. muñoz-van den eynde, a. (2015). factores que contribuyen a construir la imagen pública de la ciencia. la relación entre percepción, interés y conocimiento, percepción social de la ciencia y la tecnología 2014, madrid, fecyt, 15-44. negro, s.e. (2012). fracking wars: federal, state and local conflicts over the regulation of natural gas activities, zoning and planning law report, 35(2), 1-16. official journal of the european union (2014). recommendations 2014/70/eu on minimum principles for the exploration and production of hydrocarbons (such as shale gas) using high-volume hydraulic fracturing. available from: http://eurlex.europa.eu/legal-content/en/ txt/pdf/?uri=celex:32014h0070&from=es (accessed 2 october 2017). official state gazette (1998). law 34/1998 on hydrocarbons. available from: https://www.boe.es/boe/dias/1998/10/08/pd fs/a33517-33549.pdf (accessed 2 october 2017). official state gazette (2012). decree law 1/2012 on the abolition of the subsidies to renewable energies. available from: https://www.boe.es/boe/dias/2012/01/28/pdfs /boe-a-2012-1310.pdf (accessed 2 october 2017). official state gazette (2013a). law 17/2013 on the security of energy supply. available from: https://www.boe.es/boe/dias/2013/10/30/pd fs/boe-a-2013-11332.pdf (accessed 2 october 2017). official state gazette (2013b). law 21/2013 on environmental assessment. available from: https://www.boe.es/boe/dias/2013/12/1 annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7360 7 1/pdfs/boe-a-2013-12913.pdf (accessed 2 october 2017). official state gazette (2015). law 8/2015 on tax and non-tax measures in respect of the exploration, research and exploitation of hydrocarbons. available from: https://www.boe.es/boe/dias/2015/05/2 2/pdfs/boe-a-2015-5633.pdf (accessed 2 october 2017). rapolla 733_751.pdf annals of geophysics, vol. 45, n. 6, december 2002 733 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic and seismically active areas antonio rapolla (1), federico cella (2), maurizio fedi (1) and giovanni florio (1) (1) dipartimento di scienze della terra, università degli studi di napoli «federico ii», napoli, italy (2) dipartimento di scienze della terra, università della calabria, arcavacata di rende (cs), italy abstract geopotential data may be interpreted by many different techniques, depending on the nature of the mathematical equations correlating specific unknown ground parameters to the measured data set. the investigation based on the study of the gravity and magnetic anomaly fields represents one of the most important geophysical approaches in the earth sciences. it has now evolved aimed both at improving of known methods and testing other new and reliable techniques. this paper outlines a general framework for several applications of recent techniques in the study of the potential methods for the earth sciences. most of them are here described and significant case histories are shown to illustrate their reliability on active seismic and volcanic areas. 1. introduction gravity and magnetism are classical branches of the earth physics. potential field interpretation represents one of the oldest and most suitable approaches followed by geophysical researchers. this is also due to the high cost-benefit ratio which characterizes the application of these methods and by the relatively simple logistic management of a gravity and magnetic survey. the amount of computation often required by numerical algorithms has long prevented the mailing address: prof. antonio rapolla, dipartimento di scienze della terra, università degli studi di napoli «federico ii», largo san marcellino 10, 80138 napoli, italy; e-mail: rapolla@unina.it key words potential fields – gravimetry – geomagnetism – signal enhancement techniques – boundary analysis – southern italy full use of many theoretical efficient methods of analysis and interpretation of the potential fields. this is the case of several methods involving computation of spatial derivatives. many of these interpretation techniques fall in the class of the enhancement methods (blakely, 1996). in the following, improvements to well established enhancement methods (downward continuation, vertical and horizontal derivatives, euler deconvolution and 3d boundary analysis) are described and illustrated by applications to italian seismic and volcanic active areas. 2. enhancement methods the advantages of using the vertical derivative of gravity were first recognized by evjen (1936). at the beginning of 80’s, rather different directions were suggested to develop techniques useful to geologic interpretation of gravity and magnetic maps. the horizontal derivative method 734 antonio rapolla, federico cella, maurizio fedi and giovanni florio (cordell and grauch, 1985) is the best example of such techniques. by this technique, it is possible to locate the horizontal position of the density or magnetization boundaries. afterwards, this method and others were implemented to give also an estimation of the depth to the sources. they assume particular source-models for which the depth estimates are valid, like the werner deconvolution (hartman et al., 1971), the analytic signal (nabighian, 1984; roest et al., 1992) and the enhanced analytic signal (hsu et al., 1996, 1998). information on the depth and other geometrical source parameters are provided by the method known as «euler deconvolution». it was first proposed by thompson (1982) and extended to the analysis of maps by reid et al. (1990). thurston and smith (1997), smith et al. (1998) and thurston et al. (1999) by their source parameter imaging method obtained essentially the same information as the euler deconvolution by computing the so-called ‘local wavenumber’. finally, another rapidly developing family of methods is that based on the continuous wavelet transform (cwt; see moreau, 1995; hornby et al., 1999, sailhac et al., 2000). using a particular class of wavelets, based on the poisson semigroup kernel, these authors explored the information contained in the continued and derived field. in such a way 2d magnetic problems may be solved for both the source depth and the homogeneity degree that is in turn related to the source shape. the determination of depth and other source parameters are here obtained from the knowledge of the field at several altitudes (or scales), similarly to what was already achieved by other authors (paul et al., 1966; mcgrath, 1991; fedi and rapolla, 1999). 2.1. stable downward continuation by integrated second vertical derivative (isvd) method: an application to the gravity field of the campanian plain (italy) 2.1.1. computational approaches the shape of a gravity and magnetic anomaly is intrinsically smoothed with respect to that of its source. a physical way to improve resolution of potential fields is to calculate the field to a level close to the sources by means of the downward continuation. nevertheless, the inherent instability of the process at high frequencies may limit its practical application. the downward continuation frequency response is defined as (2.1) where h is the difference of altitude between the original survey and the target level, u and v are spatial wavenumbers in the two orthogonal horizontal directions. the positive exponent in eq. (2.1) implies that an excessive amplification of high frequency noise, present in real data, cannot be avoided. thus, in some cases the computation of the downward continued fields could not lead to meaningful results. several approaches have been suggested to prevent this problem, e.g.: the attenuation of the highest frequencies of the potential field by means of the smoothing process (see dean, 1958; grant and west, 1965); the definition of a limit value of the continuation level (baranov, 1975); the attenuation of the amplitude spectrum only at higher wavenumbers according to a function determined by wiener filtering theory (pawlowski, 1995). an alternative approach to the fourier domain filtering is based on the approximation of the downward continued field by means of a taylor series expansion (evjen, 1936) (2.2) where f (x, y, z) is the potential field, z 0 is the measurement level and z the continuation level. therefore, the computation of m-order vertical derivatives of the potential field f (x, y) is needed. to calculate any order of vertical derivatives, fedi and florio (2001) suggested the integrated r u v e h u vdown ,( ) = + +2 2 f x y z f x y z f z z z z , , , ,( ) = ( ) + ( ) +0 0 0 f z z z m f z z z z m m z m + ( ) + ( ) ( ) 2 2 0 2 0 0 0 1 2 1 ! ! 735 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... second vertical derivative (isvd) method, by which the first vertical derivative of the potential field is computed in two different steps: a) vertically integrating the field by means of a frequency domain operator; b) computing the second vertical derivative of the integrated field by means of the sum of its second horizontal derivatives, according to the laplace equation. other vertical derivatives are computed by the laplace equation starting from the field (any order even derivatives) and its first vertical derivative (any order odd derivatives). rather stable horizontal derivatives may be computed in the space domain by finite-differences or by means of splines. such a scheme for the computation of a downward continued field (fedi and florio, 2002a) involves an intrinsically stable transformation in the fourier domain and the use of methods like finite differences for the computation of the second horizontal derivatives. the advantage is represented by the stability and the accuracy of the computation of the first vertical derivative, and consequently of all the higher order odd derivatives. the sum of stable vertical derivatives ensures the control of high wavenumber amplification in the downward continued field. 2.1.2. case history an example of isvd downward continuation concerning the application of the algorithm to the real case of the gravity field of the campanian plain (southern italy) is shown here (fedi and florio, 2002a). the gravity data set available for the area consists of the bouguer anomaly map (cassano and la torre, 1987; reduction density: 2.2 g/cm–3 ). the anomalies were digitized with a sampling step of 0.5 km (fig. 2a). fig. 1. regional sketch map of the campanian plain (southern italy). windowed areas indicate sectors specifically investigated: 1) phlegrean fields gulf of pozzuoli; 2) parete area. gray shading represents topography (darker colors = higher values). coordinates utm, zone 33n. 736 antonio rapolla, federico cella, maurizio fedi and giovanni florio a b 737 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... the parete volcanic and geothermal area is located within the campanian plain (italy). this is a plio-pleistocene tectonic depression filled with sediments of alluvial and volcanic origin and limited at nw, ne and se by mesozoic carbonate platforms and at sw by the tyrrhenian coast (fig. 1). the sinking of the carbonate platform reaches some thousands of meters (ippolito et al., 1973). its formation is thought to be related to the tectonic events accompanying the opening of the tyrrhenian sea and the anticlockwise rotation of the italian peninsula (scandone, 1979). these events produced a regional rise of the mantle and caused a phase of intense potassic volcanism along the italian perityrrhenian border, specifically in some areas, as the campanian plain, where the carbonate basement deepens of thousands of meters. here, volcanic complexes of quaternary age are located (somma-vesuvius and phlegrean fields). older evidence of volcanism was found in geothermal boreholes near parete (calc-alkaline andesitic and basaltic lavas 1.5 km thick; ortolani and aprile, 1978), and northward, in the villa literno area. a gravity low characterizes the campanian area. it is justified by the sediments filling the depression. it is split into three parts by the relative highs in the parete area and corresponding to the somma-vesuvius volcanic complex. the gravity high near parete was interpreted as due to a buried volcanic structure (baldi et al., 1976; aprile and ortolani, 1979) or to a buried carbonate horst with magmatic intrusions along its bordering faults, as inferred by studying the local magnetic signature (carrara et al., 1973). the reconstruction of the buried structure generating the gravity high in the parete area would be helped by a more detailed image of the gravity field. the isvd downward continuation to 0.7 km is shown in fig. 2b. this continuation level was c fig. 2a-c. a) bouguer gravity anomaly field in the parete area (near naples, italy); b) downward continuation to 0.7 km by isvd method; c) downward continuation to 0.7 km by using the fft linear filter. 738 antonio rapolla, federico cella, maurizio fedi and giovanni florio selected after a spectral depth estimation by the method of spector and grant (1970) modified by fedi et al. (1997). the maximum number of terms used in the taylor series was five. the increase in resolution is clearly visible with respect to the original gravity field (fig. 2a). for example, north of parete two highs are now clearly singled out instead of the single positive anomaly visible in the gravity field. the northeastern one assumes an elongated shape in the nw-se direction, while the south-western one shows now an elongation towards e and se. the gradients between the highs and the se low have increased considerably and are clearly w-e oriented in the southern part and n-s oriented in the northern one. starting from these results, interpretative steps can now be greatly improved in resolution. on the contrary, the downward continuation to the same level obtained by means of the frequency response in eq. (2.1) (fig. 2c) shows a large enhancement of noise in the data and, consequently, any later interpretation of this map should be necessarily preceded by a lowpass filtering step. 2.2. multiscale analysis by enhanced horizontal derivative method: an application to the southern apennines seismic active area (italy) 2.2.1. computational approaches potential fields are the result of the interference among anomalies of different extent presumably due to sources with different shape and extension located at different depths. therefore, methods allowing their analysis at different scales are needed. the multiscale derivative analysis technique (mda; fedi, 2002) yields valuable results without resorting to any sharp separation of the effects related to frequency ranges. mda is based on the enhanced horizontal derivative (ehd) properties (fedi and florio, 2001). ehd may be defined as (2.3) where (2.4) and f (1),…, f (m) are the m-order derivatives of the field f. finally, w0…wm is a set of weights. ehd involves numerical computation of high-order vertical derivatives. to this task some stable derivation scheme is needed as the integrated second vertical derivative method (isvd; fedi and florio, 2001). the horizontal derivative of the sum of eq. (2.4) tends to enhance the signal over the source boundaries and to suppress the other spurious maxima due to the single derivatives of different order. source boundaries are therefore defined by considering the location of maxima of the ehd function (eq. (2.3)). if a general case of sources of different depth/ extent is assumed so as to generate effects at various scales, the choice, of weight set, the starting term and the last term are decisive to obtain different images of the source boundaries. in case of correct choice, analysis of ehd for different values of the derivatives order m corresponds in practice to a multiscale boundary analysis (mda), that reinforces the potential field effects at a specific scale. mda technique acts similarly to a filtering process, however mda does not involve true separation of field components, but different mda signals related to different field ‘details’ are determined by a selective combination of (derivative-based) components, each one having a different inherent resolution. thus a multiscale boundary analysis consists in producing a number of ehd maps having selective enhancements, and since the concept of resolution is a relative one, we will refer in the following to large, intermediate and short scale maps. 2.2.2. case history mda should give the most interesting results where the superposition of effects due to local/ regional or shallow/deep sources, is likely to occur. therefore, a typical test site to evaluate the efficiency of the mda method can be rex y w f x y w f x y, , ,( ) = ( ) + ( ) +( )0 1 1 w f x y w f x ym m, ... ,+ ( ) + + ( )( ) ( )2 2 ehd x y x y ,( ) = +( ) ( )2 2 739 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... presented by a geological domain characterized by a complex setting made-up of different structural elements, located at various depths and with local and regional extension, overlapping each other. these conditions are typically observed in systems resulting in a thick pile of tectonic units like, for example, the southern sector of the apennine chain (fedi et al., 2002). in fact, such a chain can be described as a complex thrust and fold belt system, built from lower cretaceous to quaternary, as a consequence of the convergence between african and european plates (finetti and del ben, 1986; dewey et al., 1989; finetti et al., 1996). most of the structural and geological patterns outcropping along the southern apennine chain and surrounding areas, which cannot be easily described by means of the simple analysis of the potential field or of other traditional methods of signal enhancement, are instead clearly recognized by the mda method. however, the most significant result is represented by the existence of several structural lineaments poorly or not completely correlated to any outcropping feature, but well evidenced by mda and with an information content often so rich as to need a very careful geological interpretation. mda was carried out on both gravity and aeromagnetic data sets. gravity data windowed from the bouguer gravity anomaly map of italy published by the cnr (carrozzo et al., 1986; reduction density: 2.4 g/cm3) were sampled with a step grid of 1 km (fig. 3). aeromagnetic field was windowed from a data set prepared by agip (1981), merging flight lines data to an unique altitude of around 2600 m a.s.l. (8500 feet), with a 2 km sampling step (fig. 5). the example shown is relative to the area from the tyrrhenian to the adriatic sea, included between 40°30′n and 41°30′n. fig. 3. bouguer gravity anomaly field in the central-southern italy. grid step = 1 km. 740 antonio rapolla, federico cella, maurizio fedi and giovanni florio fig. 4a,b. a) intermediate scale mda map of bouguer anomaly gravity field in the area windowed as shown in fig. 3. b) short scale mda map of bouguer anomaly gravity field in the area windowed as shown in fig. 3. lineaments are represented by trends of maxima, which are graphically recognised by the colorbar in normalized units. a b 741 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... gravity data: intermediate scale map – the mda map showing the gravity source patterns at intermediate scale of the selected area (fig. 4a), was obtained by computing ehd starting from the gravity field as first term of eq. (2.2), and considering derivatives up to m = 7 (fedi et al., 2002). as far as the structural setting, the tectonic style and the shallow geological features are concerned, the regional pattern of mda reveals clearly the abrupt change in tectonic style passing from to western to the eastern side of the central and southern apennines. west of the front chain, the mda signal shows the prevalence of short, arc-shaped and variously oriented trends, together with some linear trends with anti-apennine direction. most of the mda trends within the chain seem to be related to its structural elements and generally coincide with normal faults systems and major overthrust fronts. this is consequence of the geodynamic evolution of the area, since the tectonic history of the structural units within the chain is much more complex than that at the external (eastern) side of the chain front. in fact, since tortonian an intense crustal shortening occurred, due to a complex, poli-phasic compressive regime and to the consequent mountain building forming the current apenninic belt. along the eastern side of the apennines, several long lineaments are clearly shown with predominantly nw-se direction. they may be fig. 5. aeromagnetic anomaly field of southern italy (agip survey, 1980). grid step = 2 km. 742 antonio rapolla, federico cella, maurizio fedi and giovanni florio related to the deformation of the external apennines domains and should evidence a series of regional thrust fronts with linear trends. more eastward, the mda map outlines the hidden deep normal fault systems bordering the western side of the undeformed foreland, represented by the gargano-apulia carbonate platform. gravity data: short scale map – the short scale mda map (fig. 4b) enhances the finest gravity source patterns. it was computed starting from the gravity field as first term and calculating ehd up to m = 9 (fedi et al., 2002). the great improvement in describing structural-geologic patterns with small extension and local significance is immediately evident, although some of them do not have a clear structural meaning to be inferred on the base of the outcropping features. one of the stronger ehd maxima in this area is an uncorrelated, long regional trend extended from abruzzo to the gulf of taranto, marking with high precision the position of the eastern boundary of the allochtonous chain front as inferred by well information and seismic data. another example is represented by a noticeable linear trend running from the gargano to the matera area. its meaning could be related to the presence of an inner margin of the apuliagargano platform sunk along a regional normal fault system and buried by the bradano units. the southern end is partially related to structural features at the surface, where the deep carbonate block outcrops near matera. northward, the trend is divided in two different branches. the first one continues in the same direction and is clearly visible at the short scale. the second one turns northward and probably indicates the internal margin of a buried carbonate sector connecting the outcropping murge and gargano carbonate blocks. the northern segment of this branch is clearly evidenced also at the intermediate scale. magnetic data: short scale map – the mda of the magnetic field of the southern apennines foreland area (fig. 6) was obtained starting from the magnetic scalar potential as first term of the summation and calculating ehd up to m = 8 (fedi et al., 2002). here, the main lineaments corresponding to ehd maxima are evidenced. they are mostly ne-sw oriented (anti-apenninic trend), but some of them show nw-se trends. the lineaments that roughly follow a nesw trend tend to disappear just w of the front of the appenines chain thrusts, and display an anastomosing and intersecting pattern. the absence of most of these lineaments in the gravity multiscale analysis can be explained by a small or zero density contrast along these lineaments and/or by a limited throw between the two faulted blocks. on the other hand, if faulting were ascribed to an old tectonic phase, only rocks actually lying at high depths would be affected, with a possible involvement of the crystalline basement. this may imply changes of magnetic properties, but without significant density contrasts across the fault. 2.3. euler deconvolution with no a priori definition of structural index: the case of the phlegrean fields volcanic district (italy) 2.3.1. computational approaches euler deconvolution is one the most popular methods based on the derivative of potential fields, due to its suitability to manage several classes of sources. an ‘automatic’ interpretation of magnetic profiles using the euler equation was proposed by thompson (1982) based on early studies by hood (1965) and slack et al. (1967). the computation was extended to the use of map data (reid et al., 1990) and to the gravity field and its vertical derivative (marson and klingélé, 1993). some difficulties in its use in practical cases are given by the a priori selection of a constant, called structural index, that depends on the shape of the source, and by the possible existence, in a small area, of sources characterised by different structural indexes. to solve these problems, it was proposed to repeat the deconvolution several times in a moving window, varying the structural index and analysing the degree of clustering of the solutions. it was shown that in the presence of a zero background, the deconvolution problem may be solved also for the structural index (roy et al., 2000). the theoretical base of the euler deconvolution method is that magnetic and gravity fields (and their horizontal or vertical derivatives) 743 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... of simple sources are homogeneous (e.g., stavrev, 1997; blakely, 1996) (2.5) where n is the degree of homogeneity. if f is homogeneous of degree n, it may be demonstrated that . (2.6) equation (2.6) is referred to as euler’s equation. if we consider a one-point source (that is a geometrical figure characterized by the coordinates of a unique singular point) located at (x0, y0, z0) and a magnetic (or gravity) field m measured on the plane z m(x, y) = f [(x – x0), ( y – y0), (z – z0)] eq. (2.6) can be rewritten as (thompson, 1982) f tx ty tz t f x y zn, , , ,( ) = ( ) fig. 6. short scale mda map of magnetic data field in the foreland area windowed as shown in fig. 5. lineaments are represented by trends of maxima, which are graphically recognized by the shown colorbar in normalized units. x f x y f y z f z nf+ + = 744 antonio rapolla, federico cella, maurizio fedi and giovanni florio (2.7) and rearranged as (2.8) in eqs. (2.7) and (2.8), n = – n represents the structural index. thompson (1982) pointed out that the presence of regional fields or dc offsets masking the absolute level of an anomaly, makes unstable the application to real data. thus thompson (1982) rearranged eq. (2.8) considering that the measured potential field is the sum of the anomaly field and a background: t = m + b. substituting in eq. (2.8) the expression m = t – b (2.9) the knowledge of the anomalous field and of its derivatives in eq. (2.9) leads to a linear system of equations. in the most common approach (thompson, 1982; reid et al., 1990), this system may be solved in a moving overlapping window for the unknown coordinates of the anomaly source (x 0 , y 0 , z 0 ) and b, assuming an a priori knowledge of the structural index n. the inversion is repeated using different values for n until the tightest clustering of solutions for a particular feature is finally selected. serious drawbacks must however be mentioned: – existence of sources with shapes intermediate between two simple models, thus having fractional structural indexes with a value variable with the distance between source and observations (e.g., slack et al., 1967; steenland, 1968). – implicit assumption that in each analysed window only the effect of a single source is present. – variation of n depending on the analysed single portion of the considered anomaly and, consequently, influence of the direction of the total magnetization vector (in the magnetic case) on the parameters estimation (ravat, 1996). – impossibility to simultaneously estimate the depth source and n, because they are linearly dependent. a new approach to euler deconvolution was recently suggested by fedi and florio (2002b). the technique consists in the application of eq. (2.8) to a n-order vertical derivative of the field, instead of the field itself (derivative euler deconvolution, ded). the application of eq. (2.8) to a n-order vertical derivative suppresses the constant background level, reduces long period trends and, increasing field spatial resolution, reduces the problems of interference between nearby anomalies. as a consequence, this allows eq. (2.8) to be solved simultaneously both for the position of the sources and the structural index, giving additional information about geologic structures. in this way, use of euler equation is generalized to any source model and there is no need to assume any a priori (and then subjective) value for the structural index. such an approach can be very useful in complex structural regions. obviously, the application of this method also implies the computation of k + 1 order horizontal and vertical stable derivatives. the problem can be solved by means of stable algorithms such as the integrated second vertical derivative method (isvd, see section 2.1; fedi and florio, 2001). 2.3.2. case history the derivative euler deconvolution provides a classification of the sources basing on their shape, so giving valuable information for the interpretation of complex geologic structures like a volcanic district. a volcanic environment is in fact characterized by very different structure typologies causing gravity effects (faults zones, lithology contacts, igneous bodies, dikes, etc.). phlegraean fields (fig. 1) is an area of active volcanism west of the town of naples (italy). in this area, the volcanic activity of the last 37 000 years was mainly explosive. most of the products x m x y m y z m z nm0 0 0+ + = x m x y m y z m z = + + . x t x y t y z t z nb0 0 0+ + + = x t x y t y z t z nt= + + + . x x m x y y m y z z m z nm( ) + ( ) + ( ) =0 0 0 745 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... are pyroclastic rocks, with composition ranging from trachybasalts to phonolitic alkali-trachytes (e.g., rosi and sbrana, 1987). great thickness of light pyroclastites was confirmed (san vito wells) in the central part of the district by geothermal exploratory well logs (cassano and la torre, 1987), together with the lack of thick lava complexes that, instead, are present along the peripheral sectors (agip, 1987). this supports the hypothesis of a caldera formed as a consequence of major eruptions which occurred in this area (37 000 years b.p. campanian ignimbrite eruption; 12 500 years b.p., neapolitan yellow tuff eruption). since about 5 ka, besides a recent minor eruption (monte nuovo eruption, 1538), the major tectonic event is represented by a series of uplifts inside the caldera. the last ones occurred in 1970-1972 and 1982-1983, centered in the area of the town of pozzuoli (i.e. orsi et al., 1996) with a net rise of about 3.5 m. these uplifts were interpreted as the consequence of the arrival of new magma into a shallow reservoir (bonafede, 1990; de natale et al., 1991) or as the combined effect of the latter with a pore pressure increase (casertano et al., 1976). gravity data were digitised from the agip (1987) bouguer anomaly map data (reduction density: 1.4 g/cm3; barberi et al., 1991). the most significant feature is represented by a circular gravity low centred on the gulf of pozzuoli and surrounded by a belt of positive anomalies. an intense positive anomaly is present in the sw area between procida and monte di procida, while the lower values are located in the ne area. the existence of a main structure reflecting the geometry of a collapsed caldera structure, was already inferred by florio et al. (1999) by analysing the horizontal derivative of the gravity and magnetic maps of phlegraean fields. other structures are present inside the caldera, and a good correspondence with epicenters of recent seismicity disclosed young ages for these features. other lineaments probably having regional importance were also found, with sw-ne direction. ded was applied (fedi and florio, 2002b) to the vertical derivative of gravity field (fig. 7a) to recover 3d coordinates of sources (fig. 7b) and structural indexes (fig. 7c). ded clearly delineates the sw sector of the inner boundary of the caldera. in the western sector it shows structural indexes typical of limited-throw faults (0.4 < n < 0.7), the top of which is computed to a depth of about 0.15 km. in the eastern sector, the structural index has higher values and the top of this structure seems to be at about – 0.2 km of depth. the same model (limited-throw faults) is confirmed for the most part of the outer boundary of the pericalderic magmatic intrusions, where computed depths range between – 0.1 and – 0.35 km. in correspondence with the belt of gravity highs, a number of structures having a higher n value (sheet-like sources, 0.75 n < 1.75) show up, consistently with the fact that in this area sources are expected to be sills. solutions associated with the highest structural index values (linear or point like sources, 1.75 n < 3.0) concentrate in a few points of the map that correspond to known volcanic vents. this is consistent with the probable presence of dense igneous bodies along the conduit. 2.4. boundary analysis of the 3d gravity anomaly field of southern apennines active seismic area 2.4.1. description of the method as broadly treated in the previous sections, the study of derivatives of the potential fields represents a signal enhancement technique experimented since a long time. it allows the information content inside the signal to be amplified without implying any arbitrary assumption. the simplest procedure is represented by the analysis of the horizontal derivative of the potential field, defined as follows: (2.10) where m is the potential field and h is its horizontal derivative. the horizontal derivative assumes the highest values just in correspondence with the lateral boundaries of the source anomaly. this additional h x y m x m y ,( ) = +( ) ( )2 2 746 antonio rapolla, federico cella, maurizio fedi and giovanni florio information, not immediately evident from the natural field, helps to provide a reconstruction of the geological setting of the structures investigated. however, this technique is fully reliable only in the case of vertical or sub-vertical discontinuities. in fact, in the presence of oblique surfaces, maxima of the horizontal derivative are located on the vertical correspondence of a point of the discontinuity located at an intermediate depth (grauch and cordell, 1987), without any information on the dipping direction. therefore, the efficacy of this simple method is good in the case of structural features typical of an extensional regime (normal faults, transform faults) or in the presence of dykes, volcanic conduits, plutonites, etc. on the contrary, it is scarcely exploitable when the structural setting is characterised by a compressive tectonic style (presence of inverse faults, transpressive faults, multiple thrust systems, etc.). a strategy to improve this method, consisting in several steps, was proposed (cella et al., 2000): a) application of a hanning tapering window to the potential field to prevent edge effects in the following step; b) upward continuation to different increasing levels of the potential field; c) computation of the horizontal derivative; d) automatic location and thresholding of maxima values of the gradient of the field continued at different levels, following the method suggested by blakely and simpson (1986). if the anomaly field, due to a density contrast along an oblique discontinuity, is upward continued at increasing levels, the resulting signal will be related to progressively deeper sectors of the discontinuity. therefore, the position of the maxima of the horizontal derivative of the continued field will be laterally shifted toward the dipping direction of the discontinuity, proportionally to the continuation level. synthetic tests, made on several kinds of simple sources, confirmed this prediction and showed that the shifting rate of maxima of the gradient is subjected to an asymptotic decrease with increasing continuation levels. thus, major shift is observed at lower continuation levels. fig. 7a. bouguer gravity anomaly field (step grid = 0.15 km) in the phlegrean fields (near naples, italy). 747 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... fig. 7b,c. b) ded solutions shown as function of their depth; c) ded solutions shown as function of the related structural index. the found depths, associated with n estimates between 0 < n < 3, are in the range + 0.12 > z 0 > – 1.28 km. gray shading represents topography (darker colours = higher values). coordinates utm, zone 33n. c b 748 antonio rapolla, federico cella, maurizio fedi and giovanni florio fig. 8. high pass filtered bouguer gravity anomaly field in the central-southern italy. fig. 9. trends of local maxima of the horizontal gradient of the field shown in fig. 8, upward continued to several increasing levels (from 1 to 30 km). 749 improved techniques in data analysis and interpretation of potential fields: examples of application in volcanic ... only in the presence of a regional trend or in complex cases (when several significant sources lay very close), the interference can be so intense as to hide the described effect. removing the long period component of the signal or, in the second case, decreasing the step grid of the data set can help to obtain good results. this allows the range of upward continuation levels to be concentrated to the lowest values, where the effect due to the dipping sense prevails. another strategy is the use of the vertical derivative instead of the horizontal one. the vertical gradient in fact allows a further increase in resolution of the local anomalies due to a higher signal decay with distance. another positive feature of 3d boundary analysis is that it allows a fast qualitative evaluation of the depth to sources basing on the progressive disappearance of the trends of maxima of the horizontal gradient increasing the continuation level. the higher the level at which the trend disappears, the deeper the source. 2.4.2. case history a preliminary application of the method on the bouguer gravity field of the central-southern apennine is presented (cella et al., 2000). to prevent the effect due to the presence of a regional field, data were filtered by using a high-pass numerical filter. the residual field (fig. 8) at several continuation levels was horizontally derived and maxima were plotted as points with different colours depending on the continuation level (fig. 9). as predicted by several tests, the results consist in many trends of maxima variously shaped and showing a different shifting with increasing continuation levels. this allows vertical discontinuities (no shifting) to be distinguished from oblique surfaces and, in this case, to recognise their dipping direction. consequently, many complex structural settings can be identified along the apennine belt (cella et al., 2000). 3. conclusions the spatial derivative-based techniques discussed above are included in a large number of methods that do not imply a data interpretation by means of classical approaches (forward or inverse method), but enhance the gravity and magnetic signals to extract a large amount of information inherent in such data sets, but not directly evidenced by the measured fields. this information generally concerns the horizontal position of the boundaries separating the anomaly sources. as the process of derivation is inherently unstable, to compute high order derivatives, it is necessary to use specially designed algorithms, such as the integrated second vertical derivative (isvd; fedi and florio, 2001). in this paper we illustrated several improvements of classical enhancement techniques. these improvements regarded: 1) a stable downward continuation scheme, based on a taylor series approach and on the computation of isvd vertical derivatives. 2) a multiscale boundary analysis (mda), based on the enhanced horizontal derivative technique. by mda it is possible to produce a number of boundary maps related to different scales without any true separation of field components. 3) a version of the euler deconvolution (derivative euler deconvolution) method in which there is no need to assume a model of source, but instead the type of source is one of the searched parameters, together with the 3d position of sources. 4) a 3d version of the horizontal derivative method, where the boundary analysis is performed on a set of upward continued fields. this can give additional information, specifically about the inclination of density/magnetization contrasts. acknowledgements thanks are due to the italian national volcanology group for a research grant to one of us (a.r.). references agip (1981): carta aeromagnetica d'italia (scala 1:500 000), att. min., direz. espl. idrocarburi, s. donato milanese. agip (1987): modello geotermico del sistema flegreo (sintesi), internal report (in italian). 750 antonio rapolla, federico cella, maurizio fedi and giovanni florio aprile, f. and f. ortolani (1979): sulla struttura profonda della piana campana, boll. soc. nat. napoli, 88. baldi, p., g.m. cameli, b. d’argenio, a. oliveri del castillo, t. pescatore, l. puxeddu, a. rossi and b. toro (1976): geothermal research in western campania (southern italy): a revised interpretation of the qualiano-parete structure, in proceedings of the congress on thermal waters geothermal energy and volcanism of the mediterranean area, athens. ba r a n o v, w. 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(1982): euldph: a new technique for making computer-assisted depth estimates from magnetic data, geophysics, 47, 31-37. thurston, j.b. and r.s. smith (1997): automatic conversion of magnetic data to depth, dip and susceptibility contrast using the spi method, geophysics, 62, 807-813. thurston, j.b., r.s. smith and j.c. guillo (1999): modelindependent depth estimation with the spi method, in 69th seg meeting, expanded abstracts. layout 6 annals of geophysics, 61, 1, se106, 2018; doi: 10.4401/ag-7424 reply to the comment on “assessing cn earthquake predictions in italy” by g. molchan, a. peresan, g.f. panza, l. romashkova, v. kossobokov matteo taroni1, warner marzocchi1, pamela roselli1 1 istituto nazionale di geofisica e vulcanologia, rome, italy article history received april 3, 2017; accepted january 16, 2018. subject classification: earthquake prediction; model validation; italy. se106 molchan et al. [2018] raised concerns on the reliability of the main taroni et al.’s [2016] conclusion that reads “considering the data available so far, the molchan test does not show that cn prediction performance is significantly better than predictions based on the stationary poisson model.” in particular, molchan et al. [2018] discuss two main issues: 1) the taroni et al.’s [2016] results are based on too few data to achieve robust conclusions, and 2) the parimutuel gambling score (pgs) produce unfair results in comparing predictive models. we thank molchan et al. [2018] to give us this opportunity to clarify further some aspects of our paper, but we anticipate that we do not see any compelling reason to modify our original conclusion. 1. conclusions drawn from too few data molchan et al. [2018] dissert on the influence of few data on the outcome of a statistical test, claiming that “a priori the standard statistical methods may not be effective in any of the cn sub-regions”. in particular, molchan et al. [2018] argue that the rejection of the null hypothesis at a specific significance level of the test is unstable when the test is made with a few data. we agree with the general (and trivial) fact that the fewer the data, the smaller the power of the test. at the same time, we do not agree that with this number of target earthquakes in each sub-region, unavoidably, we cannot reach any meaningful conclusion. for example, if the cn alarm rate would have been smaller, say 0.10, 4/5 hits in the northern region would have been an excellent and robust proof of a significant superiority of cn predictive capability with respect to the poisson reference model (the null hypothesis of the test); in fact for 4/5 hits the p-value is 4.6 10-4, and if we remove one hit, i.e., 3/5 hits, the p-value is still very low (8.6 10-3). so, it is not only matter of how many target earthquakes we have used for the test, but also of the alarm rate imposed by the model that does not depend on the number of target earthquakes. if the alarm rate is high (as for cn), of course we will need many more earthquakes to detect a possible difference with the reference poisson model. but, in this case, we can still draw some conclusions about the cn predictive capability, e.g., (at best) it requires the occurrence of many earthquakes to show some statistically significant gain with respect to the reference poisson model. in taroni et al. [2016] we have tested each sub-region independently to avoid the problem of sub-region overlapping. conversely, molchan et al. [2017] adopt a strategy to test the cn model aggregating the three sub-regions. this aggregation introduces some further assumptions, but it allows the authors to test the model using simultaneously all target earthquakes. worthy of note, the results of molchan et al. [2017] do not contradict the ones showed in taroni et al. [2016]; in fact molchan et al. [2017] find a p-value of about 0.08 for the pooled test. the same result (p-value of 0.08) can be obtained aggregating the three p-values shown in taroni et al. [2016] through a classical fisher method [fisher 1925], once the p-values have been corrected for continuity. this is certainly interesting because it confirms the reliability of the analysis and results shown in taroni et al. [2016]. in their table 3, molchan et al. [2017] show other apparently “more significant” results (even if they never achieve a p-value less than 0.01) only review articles in two cases that we deem as inappropriate: i) when the target earthquake that occurred both in central and southern region (9/9/1998) is considered only as a success for the central region, and not also as a missed target earthquake for the southern region despite the latter was not in alarm (note that in taroni et al., 2016 this case has been considered both as a success for central region and as a missed target earthquake for southern region); ii) when the norcia earthquake (10/30/2017) is included as a target earthquake. the latter is unacceptable because it contradicts the rules imposed by the authors, which aim at predicting only independent mainshocks. in fact, the norcia earthquake is clearly related to the target amatrice earthquake (8/24/2017) that occurred nearby about two months before [marzocchi et al. 2017]; hence, if the norcia earthquake is considered as a target earthquake, the poisson assumption used for the reference model is violated, and the results of the test become unacceptable. as a final consideration on this section, we underline that in taroni et al. [2016] the null hypothesis h0 consists of the equality of predictive performance of cn with a reference model that is the poisson model (cn aims to predict only mainshocks and do not consider aftershocks). taroni et al. [2016] concluded that with the available data we do not reject this h0. at the risk of being trivial we emphasize that this does not mean that cn and poisson prediction performance are really equal, but only that the available data do not show any significant discrepancy from the null hypothesis. note that we never use the terms “verified” or “proved” as claimed by molchan et al. [2018]. at this point, it is worth explaining why we decided to carry out this analysis with this limited amount of data. since 2011, peresan et al. [2011] claimed that cn provides “successful and stable results” in italy when compared to the poisson model, cn predictions are “so far the only formally validated tools for anticipating the occurrence of strong italian earthquakes” [peresan et al. 2012], and the same authors repeatedly advocate through italian mass-media that cn predictions should be used by civil protection for risk reduction purposes (they provide these predictions to the civil protection of friuli venezia giulia since many years). we think that any prediction model to be used for practical purposes must prove its reliability and superior skill with respect to the present state of knowledge through prospective tests, i.e., using independent data [jordan et al. 2011]; the taroni et al.’s [2016] results show that so far cn prediction capability does not appear superior to random guess, even aggregating the results of the sub-regions (see before). 2. on the parimutuel gambling score (pgs) before addressing this comment, we emphasize that pgs is not an alternative method to the approach discussed before. pgs is a scoring procedure that tends to give higher score to the best performing model according to a specific metric. in other words, pgs is not used for hypothesis testing, and it gives a different type of information with respect to the previous test. in taroni et al. [2016] we apply correctly pgs, comparing the cn model with a random forecast (rf). molchan et al. [2018] is right when they claim that, under some specific conditions, pgs rewards more any reasonable random process with respect to the cn prediction scheme. according to equation 12 in molchan et al. [2017], these conditions are met when the alarm rate is much higher that the target earthquake rate (using as time unit the length of the prediction time interval), as in the present case of cn model. however, it is worth noting that when this condition is met, it says a lot on the prediction capability of the model, because it implies that the false alarm rate is large. at the same time, we acknowledge the fact that if we decide that a false alarm is less important from a practical point of view than a missed target earthquake, pgs should be applied weighing differently these kinds of errors. we do not do that in taroni et al. [2016], where both successes and errors are weighed symmetrically (i.e. hit and the correct negative give the same gain, likewise missed target earthquakes and false alarm give the same loss). the choice to weigh any kind of error in the same way is neither right nor wrong. it is just a possible decision for scoring, and others are possible. in taroni et al. [2016] we avoid to give different weights to a failure in terms of missing target earthquakes or false alarms, because this requires nonscientific information (e.g. the cost of a false alarm versus the cost of a missed target earthquake) that has to be decided by the decision makers. molchan et al. [2018] also claim that using rf instead of random guessing (rg) is not the same thing. as mentioned at the beginning of this section, we did not apply pgs to add a further significance test to the cn model, but to score cn against rf from a uniform poisson model, which is a standard model used in such a kind of comparison [e.g. rhoades et al. 2011]. taroni et al. 2 3 response to comment by g. molchan, a. peresan, g.f. panza, l. romashkova, v. kossobokov 3. additional considerations finally, we take this opportunity to add a few further considerations. in taroni et al. [2016] we have used the classical neyman-pearson approach of statistical testing with a significance level of 0.05. although this value is still widely used in science, we argue that this choice is very generous for the cn model; in fact, the recent general tendency is to use much smaller significance levels in testing any null hypothesis [singh chawla 2017]. on the other hand, if we decide to interpret the results of taroni et al. [2016] avoiding the somehow subjective choice of a significance level, we may interpret the p-value as a graded measure of the strength of evidence against the null hypothesis or the reference model [amrhein et al. 2017]. this raises the problem of the scientific quality of the reference model that has been used to evaluate the cn model; in fact, a weak reference model may easily lead to get small p-values [e.g. marzocchi et al. 2003]. in essence, the reference model used for testing is a spatially homogeneous poisson process inside each sub-region (and in all aggregated sub-regions). we think that this is a very crude reference model, and it does not represent what seismologists already know. in fact, we know that there is a substantial spatial variability of independent mainshock occurrences inside these regions (see, e.g., werner et al. [2010], and references therein; and the spatial variability of the seismicity rate model used for the new seismic hazard model for italy; meletti et al. [2017]). we also know that, even after applying some declustering procedures, in time intervals less than one year the poisson assumption for the mainshocks is not always applicable. more in general, since forecasting models for italian seismicity already exist [e.g. marzocchi et al. 2014], they could be more proper reference models to be used in testing cn. moreover, in taroni et al. [2016] we follow the same testing rules used by the cn authors. however, we think that these rules should be modified in the future to make more meaningful tests. for instance, if the ultimate goal is to predict earthquakes inside the italian territory, target earthquakes that occur in italy outside the macro-zones used by cn must be considered as failures, because the definition of the spatial macro-zones is part of the cn model that we aim to test. the two most remarkable examples of earthquakes in the italian territory, but not considered for cn testing are the m 6 earthquake occurred just offshore palermo on sept 9, 2002, and the m5.9 occurred in molise region on october 31, 2002 (a new region covering this part of italy has been included only after this earthquake). of course, the inclusion of these failures would strengthen the taroni et al.’s [2016] conclusions. 4. conclusions we thank again molchan et al. [2018] for their comments and for pointing out some interesting features of the pgs test. we would like also to emphasize that this scientific discussion is possible because the cn authors did a good job in allowing independent researchers to evaluate their predictions; this is a positive and rare attitude in this field. in this reply we show that we did not make any mistake in taroni et al.’s [2016] paper, and we do not feel to have oversold our conclusions. for this reason, we conclude this reply re-stating again that “considering the data available so far, the molchan test does not show that cn prediction performance is significantly better than predictions based on the stationary poisson model.” references amrhein, v., f. korner-nievergelt, and t. roth (2017). the earth is flat (p > 0.05): significance thresholds and the crisis of unreplicable research. peerj, 5, e3544. fisher, r.a. (1925). statistical methods for research workers. oliver and boyd (edinburgh). jordan, t.h., y.-t. chen, p. gasparini, r. madariaga, i. main, w. marzocchi, g. papadopoulos, g. sobolev, k. yamaoka, and j. zschau (2011). operational earthquake forecasting: state of knowledge and guidelines for utilization. ann. geophys.,54, 315-391. marzocchi, w., l. sandri, and e. boschi (2003). on the validation of earthquake-forecasting models: the case of pattern recognition algorithms. bull. seismol. soc. am., 93, 1994-2004. marzocchi, w., a.m. lombardi, and e. casarotti (2014). the establishment of an operational earthquake forecasting system in italy. seismol. res. lett., 85(5), 961-969. marzocchi, w., m. taroni, and g. falcone (2017). earthquake forecasting during the complex amatrice-norcia seismic sequence. sci. adv., 3, e1701239. meletti, c., marzocchi, w. and mps16 working group (2017). the 2016 italian seismic hazard model, 16th world conference on earthquake engineering, santiago de chile, 9-13 january. molchan, g., a. peresan, g.f. panza, l. romashkova, and v. kossobokov (2018). comment on “assessing cn earthquake predictions in italy” by m. taroni, w. marzocchi, p. roselli. ann. geophys., this volume. molchan, g., l. romashkova, and a. peresan (2017). on some methods for assessing earthquake predictions. geophys. j. int., 210, 1474-1480. peresan, a., e. zuccolo, f. vaccari, a. gorshov, and g.f. panza (2011). neo-deterministic seismic hazard and pattern recognition techniques: time-dependent scenarios for north-eastern italy, pure appl. geophys. 168, 583-607. peresan, a., v.g. kossobokov, and g.f. panza (2012). operational earthquake forecast/prediction, rend. fis. acc. lincei. rhoades, d., d. schorlemmer, m. gerstenberger, a. christophersen, j. zechar and m. imoto (2011). efficient testing of earthquake forecasting models. acta geophys., 59(4), 728-747. singh chawla, d. (2017). big names in statistics want to shake up much-maligned p value. nature, 548, 16-17 taroni, m., w. marzocchi, and p. roselli (2016). assessing cn earthquake predictions in italy. ann. geophys., 59(6), s0648. werner, m. j., j. d. zechar, w. marzocchi, and s. wiemer (2010). retrospective evaluation of the five-year and ten-year csep-italy earthquake forecasts. ann. geophys., 53, 11-30. *corresponding author: matteo taroni, istituto nazionale di geofisica e vulcanologia, rome, italy; email: matteo.taroni@ingv.it © 2018 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. taroni et al. 4 annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 1 geoethics and professionalism: the responsible conduct of scientists david w. mogk* montana state university mogk@montana.edu abstract the primary focus of geoethics has traditionally been on the responsible conduct of science: issues such as plagiarism, fabrication or falsification of data, and topics related to the doing of science. equally as important to the success of the scientific enterprise are the personal attributes required of being a scientist and the responsible conduct of scientists in their personal interactions with colleagues and the public. in this contribution, numerous aspects of professionalism are addressed, which encompass behaviors and attitudes that impact the ability of scientists to do their work in the social environment of the workplace and with the public. principles of professionalism have their foundations in concepts related to power, trust, respect, and responsibility. these are topics that are typically not directly addressed in day-to-day workplace communications or in classrooms, but they are essential in providing a safe, inclusive and productive work environment. administrators, faculty, staff, students, managers and co-workers may encounter all manner of interpersonal conflicts that may affect the safety and productivity of their work environment. in particular, recent revelations about instances of sexual harassment and bullying in the geoscience workplace are of immediate concern and dictate that there must be changes in the geoscience work culture. know how to recognize the signs of potential trouble, intervene early to prevent a bad situation, know the rules, and have a plan in place about how to respond to and mitigate impacts of unprofessional behaviors. 1. introduction rofessionalism refers to the attitudes and behaviors that dictate the "climate" of the work environment through the many types of interpersonal relations between managers, colleagues, students and the public. the norms and standards established for these interpersonal relations have direct impact on the ability of scientists to do their work and for science to progress. in day to day practice, it is often the case that very little is said or acknowledged about the social environment in which we work, and lack of attention to interpersonal conflicts can lead to dysfunction in the workplace, exposure to civil liability, or even criminal prosecution. everyone has a right to a safe, inclusive, supportive, and productive work environment. professionalism addresses these fundamental rights. professionalism is built on concepts and principles such as power, trust, respect, responsibility, and justice that define the foundations for healthy professional relations. an awareness of these concepts provides a vehicle for self-reflection and institutional assessment: are you and your colleagues doing all you can to meet the highest standards of professional behaviors? start the conversation in your workplace! be proactive! the purpose of this contribution is to help geoscientists be cognizant of established standards and norms of professional behavior, recognize unprofessional behaviors as they emerge, prevent these issues from having destructive and irreversible consequences, and have the tools to act and mitigate to resolve these issues. continued education about professionalism in the classroom for students, and in professional development programs for p annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 2 working geoscientists, is needed for the longterm health of our profession. 2. principles of professionalism it’s all about power. in any social structure or work environment there is inevitably a hierarchical, asymmetric power structure that involves leaders and how they interact with subordinates that presents great potential for unethical behaviors. judicious use of power can support, enable and inspire. however, inappropriate use of power can lead to coercion, manipulation, intimidation, retribution, or denigration of individuals. it is worthwhile to reflect on the power structures in your work environment, where does power reside and how is it adjudicated? and in your personal interactions with peers and subordinates, are you aware of your power over others, and is this power used to enhance and enable or to erect barriers (whether deliberate or not) to the work of others? an important corollary of power is the concept of trust. the many dimensions of trust are realized in interpersonal relations between coworkers as part of the scientific enterprise, and with the larger civic community. “the scientific enterprise is built on a foundation of trust. society trusts that scientific research results are an honest and accurate reflection of a researcher's work. researchers equally trust that their colleagues have gathered data carefully, have used appropriate analytic and statistical techniques, have reported their results accurately, and have treated the work of other researchers with respect” (national academy of sciences, 2009). resnik (2011a) identified many key aspects of trust among individuals: relationships between or among people; between individuals and groups or the profession; to facilitate cooperative social interactions; as an expression of shared expectations of behavior; to enable risk taking; the expectation of appropriate use of skills and sound judgement; an expectation of competence, experience and good will; adherence to ethical and legal duties; and an obligation to do what is expected. trust in scientific research is closely aligned with responsible conduct of research (national academy of science, 2009). trust among scientists, and in the integrity of science as a discipline, is required to promote cooperative relationships and activities among researchers, such as collaborative work, publication, peer review, sharing data, replication of research results, teaching, and mentoring. among scientists, there must be trust regarding the fidelity of data including accuracy, repeatability and in reporting of inherent uncertainties related to the data. the bonds of trust between scientists and society are particularly important. society has made great investments in support of science, and must trust that scientific results are an honest and accurate reflection of a scientist’s work (national academy of science, 2009; 2017a). the public must be able to trust the scientific process to produce outcomes to inform public policy decisions (obama, 2009). scientists often serve on government advisory bodies and regulatory boards, give expert testimony to legislative committees or as major factor in criminal cases, products liability litigation, and malpractice lawsuits. trust in scientific leadership is necessary to help the public gain acceptance of new technologies, and in consideration of risks and benefits of new technologies. as with trust, there are many aspects of respect that bear on professional behavior: respect for individuals, the processes and products of science, equipment and their safe use, institutions, the profession, and the earth. the progress of science is inherently adversarial, based on hyperbolic skepticism (“... never accept anything as true that i did not know evidently to be so; that is, carefully to avoid precipitous judgment and prejudice, and to include nothing more in my judgments than what presented itself to my mind with such clarity and distinctness that i would have no occasion to put it in doubt …” (descartes, 1637)); testing and refutation of hypotheses (e.g., popper, 1959, 1963); and in the geosciences, inculcation of the scientific method (gilbert, 1886) notes that [the researcher] “... must be fertile in the invention of hypotheses and ingenious in the application of tests ... the great investigator is primarily and preeminently the man who is rich in hypotheses ... the man who can produce but one, cherishes and champions that one as his own, and is blind to its faults. with such men, the testing of alternative hypotheses annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 3 is accomplished only through controversy. crucial observations are warped by prejudice, and the triumph of the truth is delayed” and chamberlain’s (1890) method of multiple working hypotheses wherein “the dangers of parental affection for a favorite theory can be circumvented”. respect for the heritage of the accepted practices of science is essential, particularly through scrutiny of ideas, methods and outcomes, and by challenging research findings based on the strength of evidence. however, in doing so, the integrity and dignity of individuals who conducted the research must be respected and protected. appropriate professional behavior dictates that there is never cause to humiliate, denigrate, marginalize or abuse; ad hominem attacks on individuals and their personal attributes cannot be tolerated in the profession. responsibility is a key attribute of professionalism. scientists will have professional responsibilities at many levels, to: science, the profession, colleagues, students, employers and employees, clients and end users, the public and humanity. responsibility entails a personal duty or obligation to satisfactorily perform according to personal commitment or professional/social standards. a failure to act responsibly can lead to loss of personal or professional credibility, and possibly even legal actions. responsible conduct of research is closely connected to professional behaviors in many ways, and this has been directly addressed in numerous reports (e.g., national academy of science, 2009 and 2017) and in scientific integrity policies defined by federal agencies (e.g., u.s. department of the interior http://www.doi.gov/ scientificintegrity/index.cfm, accessed on 6 november 2017 and u.s. department of agriculture http://www.usda.gov/documents/ usdascientific-integrity-policy-handbook.pdf accessed on 6 november 2017). the singapore statement on research integrity (2010) establishes four basic principles that also articulate professional responsibilities: honesty in all aspects of research; accountability in the conduct of research; professional courtesy and fairness in working with others, and good stewardship of research on behalf of others. the cape town statement of the international association for promoting geoethics further articulates the responsibilities of geoscientists, “.... to improve both the quality of professional work and the credibility of geoscientists, to foster excellence in geosciences, to assure sustainable benefits for communities, as well as to protect local and global environments; all with the aim of creating and maintaining the conditions for the healthy and prosperous development of future generations” (di capua et al., 2017). one important professional behavior required of scientists is the need to self-police and report scientific misconduct, and there is a growing literature about how to address these issues (keithspeigel, 2010; koocher and keith-speigel, 2010). responsibilities to society have been articulated by aaas (1998): “... if the u.s. is to respond effectively to the challenges of the 21st century, we must find ways to reorganize our science and technology enterprise to address tomorrow’s needs and aspirations: maintaining global sustainability, improving human health, addressing economic disparities, understanding our place in the universe, promoting peace and security, and directing the products of technology toward the betterment of society, nationally and worldwide”. these values were affirmed in the aaas (2015) survey on social responsibility of scientists, engineers and health professionals. geoscientists have a particularly important responsibility in communicating the nature of science and results of scientific research to the general public, particularly with regard to uncertainties in the areas of natural hazards and resources (e.g., pollack, 2003; lelliott et al., 2009; lark, 2015; lachner and kirchengast, 2015; bilham, 2015; tinti et al., 2015). 3. the role of professional societies american geosciences institute (2016; boland and mogk, 2017) established guidelines for ethical professional conduct that articulates both personal and communal responsibilities of geoscientists. the american geophysical union (2017a) has updated its policies on scientific integrity and professional ethics that defines specific responsibilities of its members, and has developed related policies on conduct at meetings and publication ethics (american geophysannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 4 ical union publication ethics. https://ethics.agu.org/home/publicationethics/, accessed 6 november 2017). the geological society of america code of conduct (https://www.geosociety.org/gsa/members hip/code_of_conduct/gsa/membership/co de_of_conduct.aspx?hkey=1a11cac9-2183-4e8f8cbd-3d930591fb56, accessed 6 november 2017) articulates responsibilities of members to the sciences and profession, students and colleagues, employer and employees, clients and end users, the public and humankind, and the environment and natural resources, and also defines principles of conduct for its members. the international association for promoting geoethics (http://www.geoethics.org) has further articulated the fundamental values of geoethics in the cape town statement of geoethics (di capua et al., 2017). these are just a few examples of the many codes of conduct that have been developed by sister professional societies in the geosciences. the important point is that our discipline has made issues of geoethics and professionalism a high priority, and has set high standards for behaviors that are expected and accepted in the geoscience community. 4. (un)professional behaviors: when things go bad to be blunt, we have a problem. recent revelations in the news have exposed an epidemic of unprofessional behaviors in the form of sexual harassment and bullying across society. in academic settings, libarkin (2017) reports 569 cases of academic sexual harassment (list updated november 9, 2017). these issues are not restricted to the academy. recent reports also demonstrate a culture of male entitlement and sexual hostility in the united states forest service and national park services (joyce, 2016). a shocking study by clancy et al. (2014) reports on the results of a survey of trainees in field settings, and over 70% of the respondents reported that they had experienced or observed unwanted sexual remarks and over 20% reported they had personally experienced sexual assault (i.e. physical sexual harassment, unwanted sexual contact without consent). the issues of sexual harassment and bullying in the geosciences have been directly addressed by marín-spiotta et al. (2016) and st. john et al. (2017). the geological society of america (2017) has proactively responded as they have instituted the respectful, inclusive, scientific events (rise) to the top program to ensure a safe and welcoming environment at its meetings. similarly, the american geophysical union (2017b) has sponsored safe agu: ethics, response to harassment, and work-climate related events at its annual meeting. most significantly, the agu scientific integrity and professional ethics policy (2017a) now specifically defines sexual harassment and bullying as a form of scientific misconduct with tough sanctions: “this policy takes a much stronger stance against harassment by including it in the definition of research misconduct and expanding its application to agu members, staff, volunteers, and non-members participating in agusponsored programs and activities including agu honors and awards, and governance” (davidson et al., 2017; mcphadden et al., 2017). these issues are not restricted to the geosciences, and a much larger initiative is in progress to address sexual harassment across the stem disciplines (e.g., national academy of sciences, engineering and medicine (2017b), workshops on impacts of sexual harassment in academia; american geophysical union (2016), workshop on scientific societies speak out against sexual harassment (wendel, 2016)). sexual harassment and bullying have had devastating consequences for the individuals involved and for our profession. too many colleagues, particularly women and those from underrepresented groups, have had to endure harassment, bullying, discrimination, and other offensive conducts, that have compromised their ability to work to their full potential, and all-too-often have driven these colleagues out of the field. in today's work climate, there is no excuse to not be aware; there is no justification for engaging in these behaviors; there is no place for these behaviors in the geoscience profession. there are many other professional relations that have the potential for abuse of power and http://www.geoethics.org/ annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 5 breach of trust. in any of these relations, it is important to recognize the inherent imbalance of power, the vulnerability of subordinates, and potential consequences for all parties involved. consider the following relations: • mentoring; mentors are more than advisors, they play a prominent role in the mentee's development, supporting professional development, facilitating networking, creating opportunities. mentors are in a position of trust, and yield great power over the professional prospects of their mentees. • letters of recommendation; colleagues and students rely on letters of recommendation to pursue their career goals. whitaker [2016] provides some practical advice on how to write a good letter of recommendation: decide whether you can write a good letter for the applicant; it's ok to say no; request as much information about the application before you begin writing; do some research about the job or appointment; provide ample time to write a good letter; be specific; avoid clichés and platitudes; organize the letter chronologically or thematically; be honest, original and genuine; and, follow through. be aware of biases that you may reveal in your letters. dutt et al. (2016) report: “our results reveal that female applicants are only half as likely to receive excellent letters versus good letters compared to male applicants ... these results suggest that women are significantly less likely to receive excellent recommendation letters than their male counterparts at a critical juncture in their career”. • publication ethics also encompasses a variety of interpersonal interactions that can be included in professionalism: relations between editors, authors and reviewers and among co-authors raise issues of confidentiality, conflicts of interest, attribution and credit, and related issues of trust. the committee on publications ethics (cope: https://publicationethics.org/, accessed 6 november 2017) has detailed guidelines and resources related to these issues, the national academy of sciences (2009) addresses issues of authorship and allocation of credit, and professional societies have broadly disseminated their guidelines on publication (e.g., agu’s scientific ethics for authors and reviewers). resnik (2011b) provides a nice summary of these issues in his essay “a troubled tradition, it’s time to rebuild trust among authors, editors and peer reviewers”. • data rights and policies; data provide the factual basis for scientific work, and the integrity of research depends on all aspects of data management including the collection, use and sharing of data. all researchers have an interest in, and responsibility for, protecting the integrity of the research record. data management presents many interesting questions that relate to professional behaviors: who controls information? is there (or should there be) an expectation of making information universally available? who has the right to use information, and under what circumstances? how should sources be cited or provided attribution for data that may not be published or placed in a repository? many types of data are of a proprietary nature and may have economic value. how scientists handle this type of data has implications for confidentiality, privacy, and conflict of interest. it is particularly important for all members of a research group, principal investigators, research staff, post-docs and students, to have a clear understanding of who has access and rights to publish data, and under what circumstances, to avoid potential interpersonal conflicts. • workplace safety; a culture of safety must be established in all work environments, in the lab, while traveling and in the field. professional standards must be adhered to in areas such as safe handling and disposal of laboratory materials, safe operation of instruments and equipment, emergency response plans are in place, and safety education programs are in place (national academy of sciences, 2009). field safety is a particular concern and has been addressed by oliveri and bohacs (2005) and whitmeyer and mogk (2013). workplace safety standards will vary according to jurisdiction (e.g., national, state, institution) and it is incumbent on managers and workers to obtain any required licensures, certifications, annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 6 and training, and to adhere to all relevant regulations to ensure the safety of workers, the workplace, and the environment. • contractor-client relations; professionalism in relations between contractors and clients may be dictated to some extent by licensure and certification requirements, although these requirements may vary according to national or state jurisdictions. practitioners need to be aware of issues related to competence, confidentiality, negligence, accurate representation of abilities, knowledge and expertise, and reporting standards regarding uncertainty. geoscientists canada (2014) have provided a comprehensive summary of a competency profile for professional geoscientists, and specific ethical guidelines for the profession. • serving as an expert witness; geoscientists may be called upon to serve as expert witnesses in civil or criminal cases. the role of the expert witness is to provide state-of-theart information pertaining to the issue, not to be an advocate for one side or the other. american association for the advancement of science (aaas) provides specific advice for court appointed scientific experts (case: https://www.aaas.org/page/courtappointed-scientific-experts-case, accessed 6 november 2017). the principles of professionalism described in the first part of this contribution provide a good road map for navigating the complex and diverse interpersonal relations that contribute to the landscape of professional activities in the geosciences. 5. cultivating a civil work environment this is a good time for the geoscientists across the profession, in departments, institutions, professional societies and in companies, to ask the question: is your department/workplace welcoming and inclusive for all people? professional behaviors of individuals in aggregate contribute to the workplace “climate”. awareness of these issues is the first step. but action is also needed. the following is a list of topics that could be considered in department/company meetings, in formal training sessions, and in day-to-day interactions to help create a safe, inclusive and productive work environment. • civility: uncivil behavior in the workforce impacts workers’ physical and mental health, productivity, and retention. cortina et al. (2001) report on “... interpersonal mistreatment in the workplace by examining the incidence, targets, instigators, and impacts of incivility (e.g., disrespect, condescension, degradation) ... negative effects on job satisfaction, job withdrawal, and career salience. uncivil workplace experiences were also associated with greater psychological distress”. does your department/company have an explicit statement of policy on inclusiveness, diversity, or intolerance? some examples from geoscience departments at universities in the united states include: baylor university where members of the baylor geosciences department, as part of the international geoscience community, are expected to abide by the agu scientific integrity and professional ethics policy; boise state university has advertised a strong statement of inclusiveness on their departmental webpage; and, colorado state university has developed a departmental values statement to “... represent our departmental culture, guide us as we interact as professional colleagues, influence how we make decisions, and frames a vision for our future”. • diversity: diversity is an opportunity, not an obstacle. our profession is stronger if we embrace the diversity of people, interests, experiences and abilities. we can’t afford to waste human capital. is your department/program/workplace welcoming for all people? are you proactively recruiting to broaden participation in the geosciences? the related topic of cultural sensitivity and literacy is increasingly important for geoscientists working in a globally interconnected world (see: integrate module on develop cultural competency: https://serc.carleton. edu/integrate/programs/diversity/dev_cult ural_comp.html, accessed 6 november annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 7 2017), and has been identified as a key competency by the summit on the future of undergraduate geoscience education (employers’ workshop, may 2015, http://www.jsg.utexas.edu/events/futureof-geoscience-undergraduate-education/, accessed 6 november 2017). • implicit bias: implicit bias refers to the attitudes or stereotypes that affect our understanding, actions that are activated involuntarily without an individual’s awareness or intentional control (american women in science aws: https://awis.siteym.com/?policyissues, accessed 6 november 2017). implicit bias refers to attitudes and stereotypes that affect perception and judgment without our being aware of it. there is a large literature documenting evidence of gender bias in academic hiring (e.g., moss-racusin, 2012; handley et al., 2015). do you have mechanisms in place to detect and prevent implicit biases in your selection of candidates, hiring practices, and rewards and recognition policies? related aspects of implicit bias include: the halo effect which is a cognitive bias where the overall impressions of an individual affect how we perceive other attributes of their character. for example, someone who appears to be physically attractive might also be considered to be a good leader, smart, funny, or well-liked. anchoring bias is a term used to describe the human tendency to rely too heavily on one trait (and often the first piece of information) when making decisions. beware of first impressions! confirmational bias is realized as people make decisions that confirm beliefs that are already developed. thoughts and actions are commonly influenced by ingrained stereotypes. aspects of confirmation bias include biased (or selective) searches for information, biased interpretation, and biased memories. • microaggressions are the casual degradation of any marginalized group through insults or dismissals, or other denigrating words or actions. whether intentional or not, the impacts are real, cumulative, and can lead to diminished self-confidence and self-image and potentially can lead to mental health problems such as depression, anxiety and trauma. jokes, comments, exclusion from group efforts have a cumulative negative affect on individuals (see runyowa, 2015, microaggressions matter). tolerance of these behaviors can lead to a hostile and even toxic work environment. • addressing bias, empowering bystanders: there are well established strategies to address unconscious bias in professional life. a compilation of these strategies developed by numerous u.s. national science foundation advance programs can be found at the virginia tech advance diversity and equity portal (http://www.portal.advance .vt.edu/index.php/categories/diversity, accessed 6 november 2017). to promote safety and prevent hostile situations, policies and training can be adopted to empower bystanders to intervene. the university of new hampshire prevention innovations research center has developed resources to help bystanders “identify behaviors on a continuum of violence, develop empathy or those who have experienced violence, practice safe and appropriate intervention skills, and commit to intervene before, during and after an incident occurs” (https://cola.unh.edu/preventioninnovations-research-center accessed 6 november 2017). the national sexual violence resource center also has resources on engaging bystanders to prevent sexual violence: a guide for preventionists (https://www.nsvrc.org/publications/nsvr c-publications-guides/engagingbystanders-prevent-sexual-violence-guide accessed, 6 november 2017). this is a sampling of aspects of professional life that can have profound effects, either positive or negative, on individuals, the work environment and the profession. 6. what is needed: be prepared, be proactive changes in the geoscience culture are needed at every level: personal, institutional, and as a profession. geoscientists must do all they can to make sure their work environments adhere annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 8 to the highest professional standards, so that all workers feel safe, welcomed and included in the profession. as a profession, we can’t afford to lose human capital by driving people out due to unprofessional behaviors. our workplaces, departments or companies, cannot afford to be exposed to liabilities for civil or even criminal charges when behaviors transgress from rude and boorish to illegal. an ounce of prevention is the best policy. here are some suggestions about what can be done to promote professionalism in the geoscience workplace: • be explicit in addressing the issues of workplace climate with managers, coworkers, faculty, students and the public (mogk, 2015). the cultural changes that are needed must be addressed openly and systemically from day-to-day interpersonal interactions to codified institutional or corporate polices. • develop a strategic plan that creates a collective vision for a healthy, successful science organization with clear targets and key performance indicators that directly address issues of discrimination, bias, harassment, bullying and other forms of abuse. take a collective stand against harassment and publish antidiscrimination/harassment policies on webpages and in departmental brochures. build a department that is inclusive, welcoming and safe. • be proactive and have policies in place to deal with professional issues; look for the warning signs, be aware of microaggressions and implicit bias; • build relations with the human resources department, affirmative action officer, campus security, counseling and psychological services, and other services to help prevent, and know how to respond to, disruptive behavioral issues if they should arise. • have an incident plan in place, know the policies and chains of communications so you can act quickly and appropriately if necessary. protecting personal safety must be your highest priority, but protecting privacy and confidentiality is also important. • recognize that leadership comes from the top, and that senior colleagues (department heads, deans, managers) have a responsibility to set the professional expectations for the work group. set the standard every day, every way. • make it easier to report abuse and harassment, and empower bystanders to act without fear of retribution or retaliation. • make safety in the work environment a day-to-day priority. • in matters of harassment and bullying, no one can be neutral. silence is not an option. the impacts on the individuals targeted by these behaviors will be devastating and irreversible. and, there will always be collateral damage to other workers and to the department or company. put a stop to harassment and bullying before the situation causes irreparable damage. • performance review (and recommendations for promotion and rewards) should include aspects of professionalism. in the academy, annual reviews are typically limited to research productivity (published papers and grants), teaching (student credit hour production, course evaluations, number of undergraduate or graduate students supervised), and some minor component of service to department, institution, profession or community. i recommend that a fourth category be included in performance reviews: call it civility, collegiality, or citizenship, but recognition should be given to those who have embodied the highest standards of professional behavior. those who have engaged unprofessional behaviors should be subject to lower performance reviews, required training or remediation, suspension, or even termination depending on the severity of the transgression. • there are additional responsibilities of department chairs, section heads, and managers and supervisors: keep the lines of communication open to everyone in your department; be aware of the climate in your department, and be proactive by intervening in interpersonal situations before they get out of hand; know the rules, what procedures are in place, and who to report to so that you can respond quickly and approannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 9 priately when situations arise; realize that intent is irrelevant, and that actions have consequences whether intended or not. • finally, given the importance of geoethics and professionalism, it’s fair to ask where and how should professional ethics be taught? mogk et al. (2017) provide a detailed exploration of this question. the short answer is: everywhere. good professional behavior should be modeled by faculty and working geologists day-to-day in all our work environments. discussions of professionalism can be explicitly introduced into formal classes throughout the undergraduate and graduate geoscience curricula, and in research lab working groups. corporations can institute in -house training sessions. professional development workshops and short courses are routinely presented at meetings of professional societies, and students, peers, and colleagues should be encouraged to attend. professional behaviors are learned behaviors. all members of the geoscience profession have an interest in, and responsibility for, establishing and reinforcing the highest ethical standards of the profession. 7. closing thoughts most working scientists are knowledgeable about what is meant by responsible conduct of research. we are less conversant about responsible conduct of scientists. few geoscientists have the training, knowledge or interest to deal effectively with unprofessional, interpersonal behaviors as they arise. we have a lot to learn from our colleagues in the social sciences. the purpose of this contribution is to introduce some fundamental principles that inform professionalism, and how we can best work together when attending meetings, or when at work in the office, field or laboratory. as a discipline, we cannot afford to ignore the causes and consequences of interpersonal conflicts that may arise in the workplace, particularly with respect to discrimination, sexual harassment and bullying. hopefully, this brief introduction will provide the incentive for the geoscience community to be proactive in our personal lives and professional situations, and to make sure that all our colleagues are treated with the dignity and respect that they deserve. reflect personally, start the conversation among colleagues, take appropriate actions to make sure that high standards of professional behaviors are established and expected, so that we all will have a safe, welcoming and inclusive work environment. we are all better when we work together. acknowledgements work by the author on professionalism was originally supported by the nsf ethics education in science and engineering (eese) program, grant number nsf 133874 to develop the teaching geoethics across the geoscience curriculum website. subsequent participation by the author in the agi committee to revise the code of conduct for geoscientists, agu ethics task force, gsa committee on supporting success for women in the geosciences, and iapg sponsored events at igc meetings contributed greatly to the development of the ideas presented. reviews by giuseppe di capua and anonymous reviewers greatly improved the clarity of this contribution. more extensive resources on geoethics can be found at the associated website: https://serc.carleton.edu/74990. any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the national science foundation or professional societies referred to in this contribution references american association for the advancement of science, board of directors (1998). a framework for federal science policy, (cited at resources for research ethics education. http://research-ethics.net/topics/socialresponsibility/), accessed 6 november 2017. american association for the advancement of science (2015). investigating the perceived social responsibilities of scientists, engineers and health professionals. https://mcmprodaaas.s3.amazonaws.com/ s3fspublic/aaas%20social%20responsibil ity%20questionnaire%20report_a%20preli minary%20inquiry.pdf, accessed 6 november 2017. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7584 10 american geophysical union (2016). scientific societies speak out against sexual harassment. https://news.agu.org/pressrelease/scientific-societies-speak-outagainst-sexual-harassment, accessed 6 november 2017. american geophysical union (2017a). agu scientific integrity and professional ethics, https://ethics.agu.org/files/2013/03/scien tific-integrity-and-professional-ethics.pdf, accessed 6 november 2017. american geophysical union (2017b). safe agu: ethics, response to harassment, and work-climate related events, http://fallmeeting.agu.org/2017/safeagu/, accessed 6 november 2017. american geosciences institute (2016). agi guidelines for ethical professional conduct. https://www.americangeosciences.org/com munity/agi-guidelines-ethical-professionalconduct, accessed 6 november 2017. bilham r. 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(2013). safety and liability issues related to field trips and field courses. eos, 94, 40, doi: 10.1002/2013eo400002 * further correspondence with the author can be sent to: david mogk, dept. of earth sciences, montana state university, bozeman, mt 59717 adg vol5 n02 aver247_258.pdf annals of geophysics, vol. 45, n. 2, april 2002 247 depth model building by constrained magnetotelluric inversion paolo dell’aversana and sergio morandi enterprise oil italiana s.p.a., roma, italy 1. introduction an apparent paradox can arise when many different data have to be used to build a model of a complex geological system: the more data are available the less reliable and stable the model. apparently it seems that collecting more «objective» information has the effect of increasing the «subjective» character of the solution for a given problem. this situation is common for many categories of complex problems, where there is a predominance of nonlinear relationships and non-linear processes between data and model spaces and when the data set is itself something like an advanced interpretation. in exploration in a thrust belt environment, that situation is frequent, especially when the quality of the near vertical reflection seismic data is poor or very poor: in that case many efforts are addressed to apply alternative geophysical tools, such as wide angle reflection seismic, magnetotelluric, gravity and so on. also, during the last ten years, special efforts have been oriented to improve the efficiency of the seismic acquisition layouts in order to improve the quality of the data in difficult areas. in particular, the technological improvements of the recording systems (especially, but not only, in terms of number of available recording channels) yielded seismic data characterised by a fold much higher than in the past. for the same reasons, also the possibility to collect large offset seismic data has been increased significantly, without losing the benefits offered by multi-fold data sets. mailing address: dr. paolo dell’aversana, enterprise oil italiana s.p.a., via dei due macelli 66, 00187 roma, italy; e-mail: paolo.dellaversana@rome.entoil.com abstract in this paper we describe an approach aimed at integrating seismic and magnetotelluric data in a complex geological setting, characterised by thrust structures, in southern apennine, southern italy. seismic data were collected by the «global offset» technique that is designed to record high fold data in a wide range of offsets, without losing the benefit of near vertical reflection seismic. first arrivals picked from short to long offsets and the main reflections were inverted in order to produce a tomographic velocity-interface model. it was converted into a resistivity section applying an empirical relationship, obtained by well logs, between resistivity and velocity. that section was used as a reliable reference model for 2d inversion of magnetotelluric data collected along a parallel section very close to the seismic profile. the process was iterative and interactive and was aimed at producing consistent velocity and resistivity sections, honouring seismic and mt data set. the final mt model fits very well the observed apparent resistivity and phase, reproduces the main geological trends and is constrained by a well drilled close to the line. key words magnetotelluric seismic inversion integration 248 paolo dell’aversana and sergio morandi based on that point, during the last five years enterprise oil developed an acquisition technique, called «global offset», that is aimed to optimise simultaneously the benefits of both near-vertical reflection seismic, wide angle seismic and transmission-reflection tomography (dell’aversana et al., 1999, 2000). that goal can be obtained by using multi-channel recording systems typical of standard reflection seismic and conventional dynamite shots. the particularity is that the geophone layout is kept active throughout the seismic profile, so that each shot can be recorded, simultaneously in the whole range of available offsets, by densely spaced stations. the final data sets recorded by that kind of layout contain a huge amount of information (in terms of large offset turning rays and wide angle reflections) that is commonly lost when standard seismic, short offset layouts are used. at same time, the receiver coverage is maintained very high. a similar data set can be considered optimal for joint inversion of first breaks and reflections or, in other words, for transmission/reflection tomography. based on that idea, in the last few years, enterprise oil has been acquiring global offset data in s. apennines (s. italy), also using additional standalone stations in case of long seismic profiles and/or 3d surveys. on the other hand, strong improvements have recently been obtained for non seismic methods too, such as in the case of magnetotelluric, in terms of both recording equipment, acquisition scheme, processing tools and inversion algorithms. it is clear that the effects of these improving technologies reflect also on the exploration approach. especially in difficult areas where standard seismic methods fail to produce interpretable data volume, a possible solution can be to try to integrate the benefits derived by complementary methodologies and data sets, such as near vertical reflection seismic, wide-angle seismic, tomography, magnetotelluric and so on. in that viewpoint, in the period 1998-2000, eni agip and enterprise oil, together with the european union, founded a research project aimed to optimise the integration of global offset seismic data with magnetotelluric and gravity data in a difficult thrust belt area of s. italy (dell’aversana et al., 2001). in this paper we discuss a similar approach based on the integration of a tomographic seismic 2d model, obtained by global offset data, and a magnetotelluric section obtained by mt data collected along a profile parallel to the seismic line. the two parametric models (velocity and resistivity sections) have been linked together using empirical relationships between the two physical parameters, obtained by sonic and resistivity logs. the integration approach was based on an iterative and interactive procedure: before running any 2d mt inversion, a reliable reference model was obtained by inverting the whole data set recorded by a global offset acquisition layout. after obtaining a reliable tomographic velocity model, a resistivity section was derived from it using empirical relationships. also 1d mt inversions and borehole data were used to constrain the resistivity initial model. the successive step was to use that section as a reference model for 2d mt inversion based on a perturbative approach. using that procedure, the final resistivity model can be considered the results of an integrated process based on borehole, mt and global offset data. in that sense, we can say that the 2d mt inverse problem is solved constraining the solution in the direction of t h e g l o b a l o ff s e t t o m o g r a p h i c m o d e l . obviously that approach does not guarantee a unique solution for such a complex inverse problem, but at least it produces a consistent model honouring independent data set. it can be considered itself an interesting objective to realise in a difficult exploration area. in the present paper, we discuss mainly the magnetotelluric aspects of our integration approach. additional details on global offset data, transmission/reflection tomographic section, resolution synthetic tests and gravity data integration can be found in other specific papers (dell’aversana and morandi, 2000; dell’aversana, 2001). 2. seismic data figure 1 shows the final stack section produced applying a conventional processing 249 depth model building by constrained magnetotelluric inversion flow on a seismic line about 15 km long recorded, in s. apennines (s. italy), by using a standard roll along the layout. low reflectivity and poor quality of the stacked data are quite evident, although the theoretical fold was 60%. it is clear that an interpretation based only on that section could lead to a wrong model in time and in depth domains. the complexity of the geophysical/geological problem requires a more appropriate approach integrating the standard seismic data with other independent information. in our specific case, the acquisition layout consisted of a symmetrical spread (maximum offset = 3.6 km, move-up = 60 m, intertrace = 30 m, shot distance = 60 m). that layout was integrated by a series of stand-alone stations in order to define a «global offset» acquisition layout. vertical 4.5 hz geophones every 90 m and horizontal inline geophones every 450 m were kept active along the whole seismic line, in order to record the seismic energy produced by each single dynamite shot, in the whole range of available offsets (dell’aversana et al., 1999, 2000; improta et al., 1998, 2000a,b, 2001). fig. 1. conventional time-stack section obtained by a conventional processing flow in cdp domain. many factors can negatively affect the quality of seismic imaging in thrust belts, such as low signal to noise ratio, scattering phenomena, bad static corrections and so on. the final effect can be a seismic section very difficult to interpret, like that shown in the figure. 250 paolo dell’aversana and sergio morandi 3. magnetotelluric data assuming the presence of a conductive flysch above the resistive carbonate target (on the basis of a regional understanding of the geological setting), it was expected that magnetotelluric would have been useful to highlight the top of the platform. to obtain that goal an mt survey was performed in the same area. in particular, about 20 mt sites were acquired in a profile parallel to the seismic line shown in fig. 1. the first interpretation step of the mt data was 1d inversion for each site along the profile. assuming that te mode is less affected or distorted than tm by 3d lateral heterogeneity, 1d inversions were performed on transverse electric mode. in a second step, inversion tests of tm and 1d invariant mode were also performed. the one-dimensional models show the main features of the explored medium (fig. 2), with resistive thrusts alternating with more conductive layers. in particular, a thick (2-3 km) conductive body seems to be present above the resistive basement in almost all mt sites. the basement shows a lower resistivity (both apparent and modelled by 1d inversion) than expected; in fact the resistive layer at about 6.57.5 km depth could correspond to the limestone platform, which usually presents a much higher resistivity. a possible explanation is that the thick conductive layer overlaying the carbonates induces an effect of under-estimation in the modelled resistivity associated with the platform. also, the apparent resistivities are pushed down at middle-low frequencies. it is clear that the 1d inversion process alone cannot solve the problem of a correct definition of a so complex structure, although it discloses some of its important general features. 4. mt rotational analysis figure 2 shows resistivity and phase, for both te and tm modes on the left panel for each mt site. on the right panel, 1d models are shown: the blue curve represents the bostic model; the purple curve is the occam model; the green curve is the corresponding layered model (guess layered model, fixing the number of possible layers). lateral and vertical complexity is quite evident from the examples that are shown: mt curves vary significantly with location and frequency. the presence of resistive thrusts and conductive layers is clear, down to the resistive basement, although 1d modelling is not sufficient for a quantitative estimation of true resistivity and thickness: 2d and 3d effects can still affect the model parameter definition. in any case the presence of a thick conductive layer covering the resistive basement is evident, especially in the first two examples. it produces an under-estimation effect on the apparent resistivity of the basement itself. in order to improve the model building process, an accurate 2d magnetotelluric inversion flow was performed. in that approach, a 2d structure was assumed, although a 3d hypothesis would be more appropriate for the investigated geological setting. however, our hypothesis seems to be partially confirmed by rotational analysis. in fact we analysed several impedence parameters for varying rotational angles. figure 3 shows impedence skew and ellipticity, for site bg06br (at km 12 on the profile). the tipper, resistivity and phase curves are also shown. the skew can be considered a measure of three-dimensionality. it is an invariant for rotation of co-ordinates and should be zero for noise-free data, in 1d and 2d cases. on the other hand, impedence ellipticity varies with setup direction: it is zero only for 1d data and in 2d case when x or y axis is along the strike direction. in the example of fig. 3, the skew is about constant, for different rotational angles and frequency values, showing low values of about 0.1, due to some noise in the data. ellipticity varies with angles: the lower values appear for rotation of 15°, indicating the direction of the strike (or the orthogonal direction) in the area. tipper values vary between 0.1 and 0.4, remaining within an acceptable range for the main range of frequency. in any case, the 2d hypothesis is not perfectly valid for all mt sites along the profile and for the whole range of frequency. for example, in site aa02r, the skew values range between 0.1 and 0.5 and increase at low frequency, indicating 251 depth model building by constrained magnetotelluric inversion fig. 2. three examples of mt curves and corresponding te 1d models (aa04r, aa22r, ab26r, starting from top). see details in the text. 1 10 10010 252 paolo dell’aversana and sergio morandi fig. 3. rotational analysis for the mt station bg06br. several rotations were applied to the axis in order to evaluate possible 3d effects. for that purpose impedence skew and ellipticity were analysed for each rotation. a 253 depth model building by constrained magnetotelluric inversion fig. 4. polar diagram for mt site aa02r, at the beginning of the profile. 3d effects. they are also confirmed by looking at polar diagrams (fig. 4), where it is clear that, especially at low frequencies, z xx impedence is far from zero, as it should be in a perfect 2d case. it means that our 2d-inversion approach can solve only partially the problem of reconstructing a very complex three-dimensional structure, and that 3d inversion would be more appropriate to that exploration case. at the moment we are testing the possibility to apply 3d inversion algorithms to our data, but it will be the subject of a future study. 5. 2d magnetotelluric inversion returning to our profile we see that mt data distribution is quite irregular. in some cases, the mt stations projected on the line appear very close (less than 1 km) whereas, in other parts of the profile, the spacing is 6 km or more (aa16r and aa22r). that is a consequence of the variations in data quality (strongly dependent on local noise effects) that induced a necessary selection of the data to be inverted: in fact some bad data were rejected producing a gap of information. in that condition, it is highly probable that the mt 2d inverse problem is, in the best case, mixed determined (menke, 1989) and, in the worst case, underdetermined; as a consequence non-uniqueness problems have to be expected. this is a quite common problem in mt surveys, apart from those cases where a redundant acquisition layout is used. it is clear that using good constraints and a reliable starting model could be a good strategy to reduce the space of acceptable geophysical models. from that standpoint, we performed a series of 2d mt inversions using a robust reference model built on the basis of all available global offset seismic data. in particular, the large amount of seismic data collected within a wide range of offsets (using stand alone stations too), proved very useful to image shallow and deep structures by transmission/reflection tomography. all the first breaks and some reflection events were inverted to produce a parametric zxy impedence zxx impedence impedence strike tipper mag. strike ind. arrow o 1 0 1 * 254 paolo dell’aversana and sergio morandi depth section to be used as a guide for building a reliable reference model for 2d mt inversions. in other words we performed a transmission/reflection tomography using all first breaks available, near vertical and wide-angle reflections. after obtaining that tomographic model we transformed it into a resistivity model using an empirical relationship between velocity and resistivity. for that purpose, we used sonic and resistivity log information available from a well drilled along the profile. the analytic form of that relationship is v = a [ln (ln (rho))] + b where a and b are two constants, v is the velocity by sonic logs, rho is the resistivity by logs. the above resistivity section was completed by using the indications derived by 1d mt inversions. finally it was used as a reliable reference model for 2d magnetotelluric inversion. the used mt inversion method was a non-linear conjugate gradient algorithm (rodi and mackie, 1998). it is based on searching a «regularised solution» minimising an objective function, , defined by (m) = (d – f(m))t v -1 (d – f(m)) + mt lt l m for given , v and l. f is a forward modelling function, d is the data vector. is called «regularisation parameter» and is a positive number. v is a variance-covariance matrix of the error vector e. the second term of (m) is a «stabilising functional», where lm approximates the laplacian of log . a satisfactory fit has been obtained for almost all mt stations (figs. 6a and 6b) for both te and tm models. figure 5 shows the final mt model. colours represent the «true» resistivity distribution. high resistivity thrusts (in yellow) are very clear. some low resistivity (in blue) basins appear in the fig. 5. final magnetotelluric model. it honours mt data and, at the same time, it derives from a starting model based on global offset data. note the presence of a high resistivity thrust system, low resistivity shallow basins, low resistivity layers before reaching the high resistivity platform at about 6 km b.s.l. z (m) 1000 0 -1000 -2000 -3000 -4000 -5000 -6000 x (km) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 4096 2048 1024 512 256 128 64 32 16 8 4 2 rho (ω•m) 255 depth model building by constrained magnetotelluric inversion shallowest part of the section. also a quite continuous low resistivity layer is present below the thrusts; at a depth of about 6 km b.s.l., the high resistivity layer of the carbonate platform appears. on the basis of synthetic tests, the accuracy of the final resistivity model was estimated. the final model was used to simulate the real mt experiment using the true acquisition geometry: the synthetic curves were inverted using the same reference model based on seismic data. the final model was reproduced with an accuracy of about 5%. although this result does not represent a true estimate of the resolution, it confirms the stability of the 2d inversion when a reliable starting model is used. that is also true in the case of sparse data, as in our real experiment. finally, another local confirmation of the good model accuracy was obtained by comparing the final mt section with the available borehole information. fig. 6a. pseudo-sections for te mode. te and tm calculated pseudosections were compared with the observed pseudo-sections in terms of resistivity (upper panels) and phase (lower panels). the fit ranges from good to very good, indicating that the model of fig. 5 honours the mt data. app rho obs app rho calc phase obs phase calc 10-2 10-1 100 101 102 103 262144 131072 65536 32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 app. rho ( •m) 90 80 70 60 50 40 30 20 10 phase (deg.) 10-2 10-1 100 101 102 103 10-2 10-1 100 101 102 103 10-2 10-1 100 101 102 103 256 paolo dell’aversana and sergio morandi fig. 6b. pseudo-sections for tm mode. te and tm calculated pseudosections were compared with the observed pseudo-sections in terms of resistivity (upper panels) and phase (lower panels). the fit ranges from good to very good, indicating that the model of fig. 5 honours the mt data. 6. conclusions in complex geological settings, such as thrust belt environment, the standard seismic approach often produces low quality images in both time and in depth domains. in those cases, a mt survey can significantly improve the process of depth model building. on the other hand, many additional problems can be associated with mt data utilisation, such as non-uniqueness and low resolution of the final models. 1d inversions present all the limits associated with a strong simplification of the real 3d nature of the problem. on the other hand, especially when the distance between stations is large and irregular, the reliability of 2d inversion can be very low and the final solution of the mt inverse problem can be extremely unstable. if the inversion is based on a perturbative approach, the reference model plays a fundamental role in the process. 257 depth model building by constrained magnetotelluric inversion in this sense, the choice of a robust starting model is very important in order to obtain reliable final 2d models. in this paper, we discussed an approach aimed to produce consistent and reliable magnetotelluric models by 2d mt inversions based on reliable reference models. the basic idea to build realistic reference sections was to use the whole available seismic information contained in a range of offsets much wider than the standard range of the conventional layout. in fact, increasing the maximum offset is a way to produce supercritical reflections and turning rays exploring at considerable depth. the tomographic inversion of the first breaks and of the wide-angle reflections produced a reliable parametric model in depth that guided the process of building a reliable starting model for 2d mt inversion. this approach was applied to a real case of exploration in a thrust belt environment (s. italy). an integrated model honouring the consistency between global offset, mt, borehole and geological data sets was obtained. it reflects the real complexity of the explored geological system, although additional 3d analysis could be appropriate in our case. strong lateral and vertical geophysical variations appear, consistently with the presence of high resistive thrusts alternating with low resistive layers. the top of the resistive carbonate platform has been clearly highlighted. the resolution and the level of accuracy have just been estimated: more accurate and appropriate tests are required to establish the level of non-uniqueness of the final solution. the process of depth model building described in this paper is clearly iterative and based on a non-linear approach. it means that a full comprehension of the real accuracy and resolution is not easy. on the other hand, it was possible to test the stability of the solution of the mt inverse problem by synthetic tests. they confirmed that the model obtained using a reliable starting model is very stable and consistent with the borehole information available along the line. references dell’aversana, p. (2001): integration of seismic, magnetotelluric and gravity data in a thrust belt interpretation, first break, 19 (6), 335-341. dell’aversana, p. and s. morandi (2000): integrated inversion of seismic and magnetotelluric data sets in thrust belt, in 62nd eage conference, glasgow (extended abstract). dell’aversana, p., s. morandi, l. improta and a. zollo (1999): tomographic images from «global offset» seismic data inversion in southern apennines, in 61st eage conference, helsinki (extended abstracts). dell’aversana, p., e. ceragioli, s. morandi and a. zollo (2000): a simultaneous acquisition test of high density «global offset» seismic in complex geological settings, first break, 18, 87-96. dell’aversana, p., d. colombo, s. morandi and m. buia (2001): thrust belt exploration by «global offset» seismic and reflection/refraction tomography, in 63rd eage conference, amsterdam (extended abstracts). improta, l., p. dell’aversana, a. herrero, s. morandi and a. zollo (1998): tomografia sismica di strutture crostali superficiali in appennino meridionale mediante inversione di dati acquisiti in configurazione «global offset», in 17th gngts meeting, rome, italy (abstract). improta, l., m. frattini, a. zollo, j. virieux, a. herrero and p. dell’aversana (2000a): non-linear inversion of global offset reflected arrivals in highly heterogeneous shallow crustal structure, in european geophysical society meeting, nice, france (abstract). improta, l., a. zollo, j. virieux, a. herrero and p. dell’aversana (2000b): mapping interfaces in an overthrust region by non linear travel time inversion of reflection data, am. geophys. un., fall meeting (abstract). improta, l., a. herrero, a. zollo, p. dell’aversana and s. morandi (2001): 2d non-linear travel time inversion of first arrivals and reflection data: application to the southern apennines thrust-belt (italy), in seg, san antonio (abstract). menke, w. (1989): geophysical data analysis: discrete inverse theory, int. geophys. ser., vol. 45. rodi, w. and r.l. mackie (2001): non-linear conjugate gradients algorithm for 2d magnetotelluric inversion, geophysics, 66 (1), 174-187. characteristics of local earthquake seismograms of varying dislocation sources in a stratified upper crust and modeling for p and s velocity structure: comparison with observations in the koyna-warna region, india annals of geophysics, 58, 6, 2015, s0656; doi:10.4401/ag-6714 s0656 characteristics of local earthquake seismograms of varying dislocation sources in a stratified upper crust and modeling for p and s velocity structure: comparison with observations in the koyna-warna region, india v. g. krishna retd. csir – national geophysical research institute, hyderabad, india abstract vertical component record sections of local earthquake seismograms from a state-of-the-art koyna-warna digital seismograph network are assembled in the reduced time versus epicentral distance frame, similar to those obtained in seismic refraction profiling. the record sections obtained for an average source depth display the processed seismograms from nearly equal source depths with similar source mechanisms and recorded in a narrow azimuth range, illuminating the upper crustal p and s velocity structure in the region. further, the seismogram characteristics of the local earthquake sources are found to vary significantly for different source mechanisms and the amplitude variations exceed those due to velocity model stratification. in the present study a large number of reflectivity synthetic seismograms are obtained in near offset ranges for a stratified upper crustal model having sharp discontinuities with 7%-10% velocity contrasts. the synthetics are obtained for different source regimes (e.g., strike-slip, normal, reverse) and different sets of source parameters (strike, dip, and rake) within each regime. seismogram sections with dominantly strike-slip mechanism are found to be clearly favorable in revealing the velocity stratification for both p and s waves. in contrast the seismogram sections for earthquakes of other source mechanisms seem to display the upper crustal p phases poorly with low amplitudes even in presence of sharp discontinuities of high velocity contrasts. the observed seismogram sections illustrated here for the earthquake sources with strike-slip and normal mechanisms from the koyna-warna seismic region substantiate these findings. travel times and reflectivity synthetic seismograms are used for 1-d modeling of the observed virtual source local earthquake seismogram sections and inferring the upper crustal velocity structure in the koyna-warna region. significantly, the inferred upper crustal velocity model in the region reproduces the synthetic seismograms comparable to the observed sections for earthquake sources with differing mechanisms in the koyna and warna regions. 1. introduction mobile seismograph networks in seismogenic regions provide significantly large database of local earthquake seismograms. record sections displaying the seismograms in the reduced time versus epicentral distance frame, similar to those obtained in seismic refraction profiling, can be assembled for an average source depth using the processed seismograms of earthquakes of nearly equal source depths with similar source mechanisms and recorded in a narrow azimuth range. vertical component seismogram sections, assembled from a state-of-the-art koyna-warna digital seismograph network, illuminating the upper crustal structure demonstrate this approach. the koyna-warna seismic region located in the western ghats, in the south-western part of the deccan volcanic province (dvp), in western india, reveals prolific seismicity confined to an area of 20 × 30 km2 and mostly originating in the upper crustal depths up to ~11 km in the north and 6-8 km in the south [srinagesh and sarma 2005]. figure 1a shows the location map with the epicenters of some well-located earthquakes in the koyna-warna seismic region [rai et al. 1999]. the seismicity pattern in this region starts as a single seismic tract in the north trending ne-sw, further branching into two distinct zones with nw-se trend in the south (see inset in figure 1a). figure 1b shows the depth distribution of seismicity in these three zones [rai et al. 1999]. the koyna-warna seismic activity has been attributed to both regional tectonism [wadia 1968, krishna et al. 1973] and to reservoir-triggered seismicity [gupta and rastogi 1974, gupta et al. 1980, gupta 1992, talwani 1997, gupta 2002, gupta 2005, srinagesh and sarma 2005, shashidhar et al. 2011]. rai et al. [1999] suggest that seismicity is related to lithology that extends into the lower crust. gahalaut et al. [2004], srinagesh and sarma [2005], and sarma and srinagesh [2007] sugarticle history received december 24, 2014; accepted october 28, 2015. subject classification: seismology, earthquake source and dynamics, waves and wave analysis, computational geophysics, data processing. gest that sustained seismicity in koyna-warna seismic zone is related to the geometry of existing faults and their interaction through stress transfer. most researchers classify the seismicity as resulting from reservoir induced high pore pressure on existing stressed tectonic faults. the structure and nature of these faults, however, are poorly known because the entire koyna-warna region is covered at the surface by up to ~1.5 km of deccan traps basalt. the ongoing seismic activity in the koynawarna region is primarily considered as the most outstanding example of reservoir-triggered seismicity [e.g., gupta et al. 1969, gupta et al. 1972, gupta and rastogi 1976, gupta 1985, gupta 1992, talwani 1997, gupta et al. 2012] essentially due to its proximity to the two reservoirs in the region, the koyna (impoundment in 1962) and the warna (impoundment in 1985). the seismicity in the koyna region is considered to be possibly related to the presence of a large number of faults and lineaments, predominantly striking nnw-ne, inferred from landsat images as well [langston 1981]. the inferred faults and lineaments tend to define a broad en-echelon zone that parallels the western ghats in this area. it is well known that essentially two different types of faulting mechanisms, strike-slip and normal dominate in the koyna-warna seismic region [talwani 1997]. further, there seems to be also a rapid variation of focal mechanisms from strike-slip type in the koyna region earthquakes to normal type in the warna region earthquakes [talwani 1997, sharma 2000, srinagesh and sarma 2005]. figure 1c illustrates the rapid variation of the earthquake focal mechanisms from a predominantly strikeslip type in the north in the clusters c1, c2, c3, c8 in the koyna region to a predominantly normal type in the south in the clusters c4, c5, c6, c7 in the warna region (compiled by sharma [2000]). a recent study by inversion of broad-band waveform data of warna earthquakes also indicates predominantly normal type focal mechanism solutions, some having a component of strike-slip [shashidhar et al. 2011]. upper crustal p and s velocity structure can be inferred reliably from the observed seismogram sections assembled from local earthquake sources as well by the synthetic seismogram modeling techniques. it is important to note that the seismogram characteristics of the local earthquake sources vary drastically for different source mechanisms as will be shown here. the amplitude variations for varying source mechanisms may often exceed those expected from velocity model variations. therefore in seismogenic regions of sharply varying source mechanisms (within a short distance range), it is necessary to assemble the seismogram sections separately for earthquakes of similar source mechanism. an understanding of local earthquake seismogram characteristics is important for determining the velocity structure models of the upper crust as well as the character and amplitude of near-source ground motions. the rupture directivity size, that controls the amplitude of near-source ground motion, depends on the dip angle and the direction of slip (rake angle) of the dislocation source [aagaard et al. 2004]. further, the local earthquake seismogram characteristics may vary significantly for different source regimes (e.g., strike-slip, normal or reverse) and even within each regime for different sets of source parameters (strike, dip, and rake) rather than depending only on the seismic velocity and q structure of the krishna 2 figure 1. (a) location map of the koyna-warna seismic region showing some of the seismograph stations (filled triangles) of the digital network operated, and the well located epicenters (filled circles) of the local earthquakes. the inset shows the observed seismicity pattern in the region described in the text. (b) depth distribution of seismicity along three sections ab, cd, and ef observed in the region. (figures 1a and 1b, after rai et al. [1999]). (c) composite source mechanism solutions for different earthquake clusters ci, c2, c3, and c8 in the koyna, and c4, c5, c6, and c7 in the warna seismic region (after sharma [2000]). 3 host region. some combinations of the source parameters of the upper crustal earthquakes may result in seismograms at local distances that do not reveal any signatures (like reflections and conversions) related to the vertically inhomogeneous seismic structure with presence of sharp interfaces of large velocity contrasts. a systematic study by numerical simulations for different source mechanisms can provide the necessary insight to understand the characteristics of local earthquake seismograms and their modeling for the upper crustal velocity structure. in the recent years, only a few studies [oglesby et al. 2000, aagaard et al. 2001a, aagaard et al. 2001b, aagaard et al. 2004] have systematically explored how the source parameters affect the near-source ground motions. similar efforts for studying feasibility of velocity modeling of record sections of local earthquake seismograms for varying source regimes are, however, yet to be initiated. because it is essential that the seismograms should be revealing and prominent phases as reflections, conversions and any other energetic phases generated by the existing velocity stratification are well recognized. a source effect leading to apparently missing or extremely low amplitude phases in a seismogram section should not be interpreted in terms of the velocity model features alone that suggest absence of prominent discontinuities. the source effect can be properly predicted by a systematic study of the characteristics of the seismograms for different source regimes. in the present study we investigate this problem by a large number of numerical simulations. reflectivity synthetic seismograms in near offset ranges are obtained for a stratified upper crustal model having sharp discontinuities with 7%-10% p and s velocity contrasts. the synthetics are obtained for different source regimes (e.g., strike-slip, normal, reverse) as well as for different sets of source parameters (strike, dip, and rake) within each regime. the characteristics of local earthquake seismograms generated for different dislocation sources in a stratified upper crust are systematically examined. the scenarios considered vary from strike-slip to normal faulting regimes of the upper crustal seismogenic zones, besides a few dip-slip and reverse (thrust) faulting models. various numerical simulations for different source regimes considered are obtained here for the same upper crustal p and s velocity models similar to those given by krishna et al. [1999] for the 1993 latur earthquake area in the deccan volcanic province (dvp), in the western indian shield. the p and s velocity models have sharp discontinuities with velocity contrasts of 7%-10% at the top and bottom boundaries of two prominent low-velocity layers (lvls), i.e., with significant stratification in the upper crust. it is the purpose of the present study to examine how well the existing velocity stratification is revealed and can be modeled from the seismograms of local earthquakes for different source regimes considered. seismogram sections of local earthquakes with dominantly strike-slip mechanism are found to be clearly favorable in revealing the velocity stratification in the upper crust for both p and s waves. in contrast the seismogram sections for earthquakes of other source mechanisms, particularly with rake angles around ±90°, seem to display the upper crustal p and p-to-s converted phases poorly with low amplitudes even in presence of sharp discontinuities of high velocity contrasts. the significance of this study has been realized as several local earthquake seismogram sections are assembled in the koynawarna seismic region in the dvp and their modeling attempted for obtaining the upper crustal p and s velocity structure [krishna, under review]. a few observed seismogram sections are illustrated here for the strikeslip and normal faulting regimes from the koyna-warna seismic region. the observed seismogram sections illustrated here for the earthquake sources with strikeslip and normal mechanisms from the koyna-warna seismic region in the deccan volcanic province in the western indian shield substantiate these findings. travel times and synthetic seismogram computations by the reflectivity method, are used for 1-d modeling of the observed virtual source local earthquake seismogram sections and inferring the upper crustal velocity structure in the koyna-warna region. significantly, the inferred velocity model for the upper crust in the region reproduces the synthetic seismograms comparable to the observed sections for earthquake sources with differing mechanisms in the koyna and the warna regions. earlier studies [e.g., talwani 1997, sharma 2000, srinagesh and sarma 2005] in this region reveal left-lateral strike-slip faulting in the north and rapidly changing to normal faulting in the south escarpment zones of the koyna seismic region, and normal faulting in the warna seismic region as well. 2. local earthquake data set as more reliable data sets became available over time in the koyna-warna seismic region, location estimates steadily improved. the csir national geophysical research institute (ngri) deployed a temporary seismic network consisting of 20 stations during 19961998, and rai et al. [1999] used some of those stations to relocate more than 400 of the recorded events. their events had estimated timing errors less than a few milliseconds, and 93% of their events had epicentral resolutions of 0.2 km or less and hypocentral resolutions of 1.0 km or less, but the locations were based on only about characteristics of local earthquake seismograms six stations. srinagesh and sarma [2005] and sarma and srinagesh [2007] applied double-difference methodology in more precisely relocating more than 600 earthquakes from this dataset. shashidhar et al. [2011] used the velest algorithm to jointly locate hypocenters and to determine the velocity structure in the koynawarna area, and their data were recorded on 11 seismographs. in an attempt to better characterize the seismogenic crust in the koyna-warna region, dixit et al. [2014] recently acquired seismic data using a portable network consisting of 97 taurus® seismographs, which is claimed to be denser than the previously deployed networks in the area. in the present study, we utilize the vertical component seismograms of local earthquakes well recorded by a state-of-the-art koyna digital seismograph network deployed by csir-ngri during 1996-1998 [rai et al. 1999]. the events were recorded (at 100 samples/s) by various stations of the seismograph network equipped with 24 bit reftek/passcal digital recorders of short period three-component seismometers and gps timing system. figure 1a shows the location map of the koyna-warna seismic region with the epicenters of the local earthquakes and the seismograph stations used in the present study. the inset shows the observed seismicity pattern in this region. table 1 gives the epicentral data of the local earthquakes in the koyna (clusters c2, c3) and warna (clusters c4, c5) regions considered here for creating the two vertical component seismic record sections ck-fr-sg, and ck-fr for an average source depth of 8 km. statistical analysis of a dataset of about 400 local earthquakes of the region by rai et al. [1999] reveals well-constrained estimates of the epicentral locations and the hypocentral depths for various events in the depth range to ~11 km of the seismogenic upper crust. srinagesh and sarma [2005] relocated more than 600 earthquakes in the koyna-warna seismic zone, separately for three spatially distinct clusters, namely the north escarpment zone (nez), south escarpment zone (sez) and warna seismic zone (wsz), based on the inferences by rai et al. [1999] during the analysis of seismicity patterns in this region. the constrained focal mechanisms show distinct styles of faulting amongst the various fault segments with nez predominantly showing the strike-slip faulting with left lateral motion on the ne-sw fault. in sez and wsz the focal mechanisms are basically normal on the steeply dipping nnw-sse to n-s faults [srinagesh and sarma 2005]. the epicentral-hypocentral resolutions estimated by rai et al. [1999] are found to be good enough for various events considered for investigating the problem krishna 4 eq no. cluster epicenter focal depth magnitude epicentral distances lat.° long.° (km) (mcoda) (km) record section ck-fr-sg (figure 9a) 9 c3 17.32 73.74 8.9 1.6 18.1, 12.8, 13.7 23 c3 17.28 73.74 7.3 1.6 16.3, 14.4, 10.7 24 c3 17.28 73.73 7.4 2.4 25 c2 17.35 73.75 8.2 2.1 20.7, 13.9, 16.7 30 c3 17.30 73.73 8.0 1.9 16.5, 13.2, 11.5 36 c2 17.33 73.73 7.8 2.3 12.0, 14.1 record section ck-fr (figure 10a) 21 c4 17.23 73.72 8.5 1.9 17.0, 13.5 26 c5 17.20 73.76 7.4 2.6 22.1 39 c5 17.22 73.76 7.7 1.6 20.2 cluster focal depth (km) strike° dip° rake° c1 4–7 40 90 10 c2 7–11 30 82 6 c3 7–11 30 82 6 c8 1–4 50 85 9 c4 1–11 340 50 -90 c5 1–11 340 50 -90 c6 1–11 340 50 -90 c7 1–11 340 50 -90 table 1. koyna (clusters c2, c3) and warna (clusters c4, c5) earthquakes (eq) data for the seismogram sections ck-fr-sg, ck-fr, for an average source depth of 8 km. table 2. source parameters for various earthquake clusters in the koyna-warna seismic region (after sharma [2000]). 5 being addressed in the present study, and thus no attempts are made here to further relocate the hypocentral parameters. the results of a preliminary study of composite focal mechanism solutions obtained by sharma [2000] are given in table 2, at different depth ranges of the spatially different clusters shown in figure 1c. as shown in figure 1c, analysis of the composite fault-plane solutions in the koyna-warna seismic region reveals two distinct faulting mechanisms, predominantly left-lateral strike-slip faulting in the koyna region in the north rapidly changing to predominantly normal faulting in the warna region in the south. 3. characteristics of seismograms of local earthquake sources with different source mechanisms in a stratified upper crust in order to examine the influence of different source mechanisms on the observed local earthquake seismograms for a stratified upper crustal structure with presence of sharp velocity discontinuities (e.g., with p and s velocity contrasts up to 7%-10%), a simulation study is thus attempted as shown in the following. a good number of reflectivity synthetic seismogram sections are obtained for different source regimes (e.g., strikeslip, normal, reverse) as well as for different sets of source parameters (strike, dip, and rake) within each regime for the same model of p and s velocity structure given in figure 2. the characteristics of the local earthquake seismograms, as will be shown here, vary significantly for different source mechanisms. the amplitude variations for varying source mechanisms may often exceed those expected from velocity model variations. therefore in seismogenic regions of rapidly varying source mechanisms within a short distance range, specifically in the koyna-warna seismic region of the present study, it is indeed necessary to assemble the seismogram sections separately for earthquakes of similar source mechanism. 3.1. synthetic seismogram gathers for varying source parameters a simulation study a large number of reflectivity synthetic seismograms [kind 1985, müller 1985] are computed in near offset ranges out to about 40 km for an arbitrary 6 km deep earthquake source in the upper crust with the vertically inhomogeneous p and s velocity models shown in figure 2. a double-couple point source with the source-time function of brüstle and müller [1983] is used for various computations. source mechanisms similar to the strike-slip, normal, reverse, and dip-slip models are considered for different computations. in each case the set of source parameters (strike, dip, and rake) are systematically varied (at 10°-20° intervals) and the synthetics are obtained for several combinations. while a qp/qs ratio of 9/4 is used in all the computations other ratios, e.g., 1 and 1/2 are used for a set of strike-slip and normal mechanisms to examine the resulting seismograms. various synthetics obtained as described above are displayed by using identical scaling of the true amplitudes, so that the seismogram sections shown in the following may be directly compared. figures 3-7 show the vertical component reflectivity synthetic seismogram gathers (true-amplitude plots with reduction velocity 6 km/sec) for the common offset range of 28 km. the theoretical travel-times computed for the vp and vs models (given in figure 2) are also shown as indicated on various seismograms by their onsets (p and s: refractions; p1, p2, p3, p4, and s1, s2, s3, s4: primary reflections). the source parameters, for which the seismograms are generated, are indicated with each gather in figures 3-7. each of the gathers illustrates variation of the seismogram characteristics with variation of one of the source parameters, with the other two parameters remaining unchanged. figures 3a and 3b and figures 4a and 4b reveal the seismogram characteristics varying due to the rake characteristics of local earthquake seismograms figure 2. upper crustal p and s velocity models used to compute the synthetic seismogram gathers shown in figures 3-7 to illustrate variations of the local earthquake seismogram characteristics for different source mechanisms. note the significant stratification in the upper crust that is examined as to how well the p and s reflections and the p-to-s conversions can be recognized in various synthetics for different source mechanisms. krishna 6 figure 3. (a) vertical-component reflectivity seismograms gather (true amplitudes plot with reduction velocity 6 km/sec) for a common offset range of 28 km showing amplitude variations of different phases due to rake angle variations of various source mechanisms. all the seismograms are computed for the source depth of 6 km using the p and s velocity models shown in figure 2. onsets of various travel-time branches are given to the right of the seismograms. note the p phase amplitudes are however recognizable for rake angles in the range of at least −20° to +20°. (b) same as figure 3a for the normal faulting mechanism. note very low amplitude p phases for the intermediate rake angles near −90°. figure 4. (a) same as figure 3a for the right-lateral strike-slip (rake 180°) and the dip-slip (rake ±90°) mechanism. note very low amplitude p phases and relatively high amplitude s phases for the rake angles ±90°. (b) same as figure 3a for the reverse (thrust) mechanism. note very low amplitude p phases for the intermediate rake angle 90°. 7 angle variations for the strike-slip, normal, reverse, and dip-slip mechanisms. it is clear that the amplitudes of the p phases (refractions and reflections) as well as the p-to-s conversions are significantly low for the intermediate rake angles around ±90° and these phases are recognizable only for the extreme rake angles around ±180° and 0°. seismograms for the strike-slip mechanisms generally appear to be favorable for revealing the phases of p velocity stratification (see figure 3a for the left-lateral strike-slip case, and the seismogram for rake angle 180° in figure 4a for the right-lateral strike-slip case). the s phases do not reveal significant amplitude variations as the p phases, although for intermediate rake angles around ±90° they are relatively low for the normal and reverse mechanisms. variations of the seismogram characteristics due to the dip angle variations are illustrated in figures 5a and 5b for the strike-slip and the normal mechanisms. amplitudes of the p phases and the p-to-s conversions are relatively low for the normal mechanisms as compared to the strike-slip mechanisms. however the amplitudes of the s phases are stronger for the normal mechanisms. it is again clear that the seismograms for the strike-slip mechanisms appear to be more favorable to delineate both p and s velocity stratification. similar differences, especially concerning the p phase amplitudes, between the seismograms for the strike-slip and normal mechanisms are also evident from figures 6a and 6b. in these figures the seismograms are shown for varying strike but the same dip and rake angles. figure 7 shows the differences between the seismograms for the strike-slip and normal mechanisms for varying qp/qs ratio. the source parameters (strike, dip, and rake) of the two source regimes considered here correspond to those inferred for some upper crustal earthquakes in the koyna-warna seismic region of the dvp in the western indian shield [sharma 2000]. qp/qs ratios in the upper crustal layers equal to or even less than 1 are not uncommon and similar ratios have been found in the 1993 latur earthquake area of the dvp [krishna et al. 1999], and the eastern dharwar craton [krishna and ramesh 2000] in the south indian shield. similar smaller qp/qs ratios, if exist in the koyna-warna region, may further degenerate the amplitudes of the p phases in the seismograms of the events with normal mechanisms as can be seen from figure 7b. the major result of interest in this simulation study is that, seismogram sections of local earthquakes with dominantly strike-slip mechanism are clearly favorable in revealing the velocity stratification in the upper crust for both p and s waves. in contrast the seismogram sections for earthquakes of other source mechanisms, particularly with rake angles around ±90°, seem to display the upper crustal reflected p and p-to-s converted phases poorly with low amplitudes even in presence of sharp discontinuities of high velocity contrasts. thus it is clearly necessary to assemble the seismogram sections from local earthquakes of similar source mechanisms for recognizing prominent phases from potential velocity discontinuities and their modeling leading to reliable upper crustal p and s velocity structure. 4. modeling of the koyna-warna local earthquake seismogram sections the development of 1-d crustal velocity models, particularly in seismogenic regions as the koyna-warna region, is the basis for a range of applications in earthquake seismology. a well-developed 1-d crustal model characteristics of local earthquake seismograms figure 5. same as figure 3a showing amplitude variations due to dip angle variations for (a) strike-slip, and (b) normal mechanisms. note higher amplitude p phases for the strike-slip mechanism. has been widely used to determine the source parameters of earthquakes. the computational efficiency to calculate green’s functions is a primary advantage of the 1-d model, which can be used in studies of the rupture processes of earthquakes [e.g., hartzell and heaton 1983, dreger and helmberger 1993]. furthermore, the 1-d krishna 8 figure 6. (a) same as figure 3a showing amplitude variations as the strike varies for the strike-slip mechanism. (b) same as figure 6a for the normal mechanism. note relatively low amplitude p phases as compared to those for the strike-slip mechanism. figure 7. same as figure 3a showing amplitude variations due to different qp/qs ratios in seismograms for (a) strike-slip, and (b) normal mechanism. note higher amplitude p phases for the strike-slip mechanism. 9 model can be used as an initial model for developing more complex 2-d and 3-d velocity models. the traditional way to develop a 1-d velocity model utilizes the traveltimes of seismic phases. synthetic seismograms modeling is recognized as the more efficient technique to determine the velocity structure mitigating nonuniqueness of the models obtained from the arrival times of a few recognized phases. by the approach given by krishna et al. [1999] seismogram sections in near distance ranges are assembled for a few common source depths using well located earthquakes with hypocentral depths agreeing within 1 km. figure 8 illustrates one such composite section for a common source depth of 8 km assembled for earthquakes recorded by various stations in the region. this record section is considered as a composite section generated by a virtual source at 8 km depth, because this section includes the seismograms from various azimuths and also those for different source mechanisms. it is clear that the direct and refracted p and s phases are well aligned in this section. however, any later arriving phases, in the p and s windows, may not be evidently recognizable in this section. therefore it is considered necessary to assemble only seismograms for a similar source depth as well as similar source mechanism and recorded in a narrow azimuth range from various seismograph stations. the earthquake epicentral data used for this purpose are given in table 1. the record sections thus assembled are shown in figure 9a for events with dominantly strike-slip mechanism and in figure 10a for events with normal mechanism. these record sections are band-pass filtered (5-20 hz) and plotted with a reduction velocity of 6 km/sec and amplitudes trace-normalized. significant variations in the seismogram characteristics as described in the earlier sections are clearly observable in the record sections for the strike-slip (figure 9a) and normal events (figure 10a). it should be considered here that the crustal volume sampled by the ray paths involved for the seismograms shown in figures 9a and 10a is of limited dimensions only. thus it is quite unlikely that the observed systematic variations of the seismogram characteristics can be caused by lateral velocity perturbations. both p and s velocity tomographic inversion using local earthquake arrival times revealed a maximum lateral variation of about 4% [rai et al. 1999]. further p and s velocity-depth profiles modeled in the nearby 1993 latur earthquake area of the dvp [krishna et al. 1999] reveal significant stratification of the upper crust with presence of prominent discontinuities having velocity contrasts of about 7%. the vp and vs models similar to those given in figure 2, used for computation of travel time curves in figures 9a and 10a also indicate similar order of the velocity contrasts at various interfaces in the upper crust in the koyna region as well [krishna 2006]. the high amplitude secondary phases in figure 9a fit the p2, p3, p4, and s2, s3, s4 travel-time curves which are primary reflections at the discontinuities with similar order of velocity contrasts. these travel-time curves shown in figure 10a also fit the lower amplitude secondary phases correlated in the record section. it is only the different source mechanism that causes the apparently low amplitudes of the secondary phases. synthetic seismograms thus obtained clearly resolve this problem by generating low amplitude secondary phases even in presence of discontinuities with similar high velocity contrasts. therefore the local earthquake seismogram sections as shown in figure 10a should be modeled only by considering the appropriate source mechanism, otherwise the existing stratification in the upper crust may be misinterpreted as a vertically hocharacteristics of local earthquake seismograms figure 8. composite observed record section with amplitudes normalized, in the epicentral distance versus reduced time (reduction velocity 6 km/s) frame, assembled from processed seismograms of local earthquakes with average source depth of 8 km. mogeneous medium. this is particularly applicable for regions like the koyna-warna seismic zones where the source mechanisms sharply vary from strike-slip to normal within a short distance range. the methodology for modeling the record sections in the present study is as follows. preliminary determinations of layer parameters are obtained from travel times modeling for inferring earth’s structure. further, synthetic seismograms are computed using the inferred p and s velocity models and initial set of source parameters (strike, dip, and rake) appropriate for the source depth of the record section. several forward problem runs are made by varying the source parameters to improve the synthetics in explaining various features of the seismogram. in the following, 1-d model computational results of the travel times and synthetic seismograms are presented for modeling the record sections of local earthquake seismograms available out to a maximum range of about 22 km as shown in figures 9 and 10. the virtual source parameters inferred from modeling of these record sections are further used to compute synthetics. by this modeling approach, plausible models of the p and s velocity structure of the upper crust are inferred. krishna 10 (a) (b) figure 9. (a) observed record section assembled from seismograms of local earthquakes with similar source mechanisms (the dominant mechanism is strike-slip), aligned to an average source depth of 8 km, and recorded at a combination of the seismograph stations ck, fr, and sg in the koyna-warna region. a reduction velocity of 6 km/sec is used for plotting. the amplitudes are trace-normalized. a band-pass filter 5-20 hz is applied. computed travel-time curves are shown for direct and refracted p and s phases, and primary reflection phases pi and si, as well as for the p and s phases leaving the source and reflected at the free-surface followed by reflection at an upper crustal boundary (unlabeled dotted curves in the time-increasing-order for reflections pfpi and sfsi, [i = 1-4]). the seismograph stations and their azimuth range are indicated on the top of the record section. (b) synthetic seismogram section computed by the reflectivity method for the virtual source parameters and using the inferred models of the upper crustal p and s velocity structure in the region. the amplitudes are trace-normalized. the left panel shows the p velocity model. 11 4.1. travel times modeling the upper crustal p and s velocity models inferred from the invsp (inverse vertical seismic profiling) gather [krishna 2006] for the station wr, are used as the initial models for travel times modeling computations. the following wave codes are considered for the purpose of illustration and modeling. p, s: up-going direct waves from a source as well as down-going refracted waves bottoming in the upper crustal layers and terminating on the free surface; pi, si: down-going waves from a source, reflected on i-th interface and terminating on the free surface (travel time curves are shown by continuous lines); and also pfpi, sfsi, unlabeled and dotted: up-going waves from a source reflected on the free surface followed by a reflection on i-th interface and terminating on the free surface (travel time curves are shown by unlabeled dotted curves in the order for reflection on the i-th interface, i = 1-4 for both p and s phases) figure 11 illustrates the ray paths of various wave types described above. it is clear that these wave codes are by no means exhaustive, but they certainly represent a set of prominent p and s phases being considered for modeling the upper crustal velocity structure using the travel times. possibly a large number of other multiples as well as converted phases from potential interfaces in the stratified upper crust, may be generated in the full wave synthetic seismogram computations ilcharacteristics of local earthquake seismograms (a) (b) figure 10. (a) same as figure 9a observed record section assembled from seismograms of local earthquakes with similar source mechanisms (the dominant mechanism is normal), aligned to an average source depth of 8 km, and recorded at a combination of the seismograph stations ck, and fr in the koyna-warna region. note significantly lower amplitudes of various phases, particularly p reflections and p-tos conversions, as compared to those in figure 9a. (b) same as figure 9b synthetic seismogram section computed for a different set of the virtual source parameters and using the same inferred models of the upper crustal p and s velocity structure in the region. the amplitudes are trace-normalized. the left panel shows the same p velocity model as in figure 9b. lustrated. travel time computations for the p and s as well as the later arriving pi and si and the unlabeled phases (pfpi and sfsi), consistent with the ray geometry (figure 11) in the stratified upper crust as illustrated in figure 2, for a good number of plausible velocity models starting with the initial models, are obtained. the travel time curves computed for the final models of upper crustal p and s velocity structure inferred in the study region are illustrated in the two observed record sections in figures 9a and 10a. as can be seen from these figures, the travel times modeling substantiates qualitatively the viability of the upper crustal p and s velocity models and the stratified structure with presence of low-velocity layers (lvls) inferred from the invsp gather for the station wr in the koyna-warna region [krishna 2006]. the synthetic seismogram sections, also illustrated in figures 9b and 10b, further confirm these models as discussed in the following. 4.2. synthetic seismograms modeling the travel times modeling of the koyna-warna local earthquake record sections given in the earlier section is limited to only a few prominent p and s phases in the stratified upper crust for inferring the preliminary models of the p and s velocity structure. the velocity models thus inferred are further substantiated with the aid of full wave synthetic seismogram computations for a set of source parameters (strike, dip, and rake) which are also progressively revised to generate the synthetic sections comparable to the observed sections (figures 9a and 10a). the synthetics are generated for the 1-d isotropic earth models in the present study. computation of synthetic seismograms in a layered half-space is an efficient technique for modeling the earth’s structure as well as the dynamic process of earthquake sources from well recorded seismic data. many numerical techniques are well developed for generating synthetic seismograms for point sources in plane layered media in order to model observed datasets. the generalized ray and wavenumber integration techniques are however more commonly used in seismic modeling studies. fourier transform techniques require some form of wavenumber integration [e.g., fuchs and müller 1971, kennett and kerry 1979, wang and herrmann 1980, bouchon 1981, kind 1985, müller 1985]. wavenumber krishna 12 figure 11. schematic ray diagram illustrating the ray paths of prominent wave types considered for computing the p and s travel time curves given in figures 9a and 10a. a: illustrates the upper crustal model, b: illustrates the ray paths for p (or s), c: illustrates the ray paths for pi (or si), and d: illustrates the ray paths for pfpi (or sfsi). 13 integration method for the generation of synthetic seismograms gives a complete solution, but can be computationally intensive. the complete solution, rather than individual rays, is considered in this full wave theory approach. this method can handle a larger number of plane layers, but requires considerable computational effort, especially at high frequencies. the reflectivity method for computing synthetic seismograms for an earthquake source [kind 1985] is used for modeling the observed seismogram sections shown in figures 9a and 10a. a trial and error approach of forward computations, initially for refining the p and s velocity models and later for fixing the virtual source parameters (strike, dip, and rake), yielded acceptable fits (based on visual check) of the synthetic seismogram sections with the observed sections. starting with the initial set of source parameters given in table 2, each of the three parameters are varied fixing the other two, and in the process a good number of synthetics are computed for the same set of p and s velocity models obtained from travel times modeling. the virtual source parameters (strike, dip, and rake) are inferred for the record section (assembled from earthquakes with different sets of the source parameters) being modeled. the azimuth used in the computations is the average azimuth for each of the observed sections in figures 9 and 10. qp and qs in the upper crustal layers are initially used as those inferred in the nearby 1993 latur earthquake area [krishna et al. 1999]. the qp/qs ratio is however suitably improved for the upper crust in order to obtain the synthetics that fit the observed sections. the final synthetic sections are selected based on the overall fits with the prominent phases in the observed sections, and shown in figures 9b and 10b. it may be noted here that the same set of the p and s velocity models, but different sets of the virtual source parameters (strike, dip, and rake), are used for modeling the sections in figures 9a and 10a. it is significant to note that the same set of final p and s velocity models as shown in figure 12 are able to reproduce the prominent features in both the seismogram sections, but different sets of the source parameters have to be used in order to generate the synthetics reasonably matching the observed sections. the viability of the inferred velocity models for the study region is thus established. 5. results and discussion the p and s velocity models and the resulting travel time curves given in figure 12 are inferred from modeling the seismogram sections (figures 9a and 10a) recorded at stations ck, fr, and sg. the maximum characteristics of local earthquake seismograms figure 12. inferred set of final p and s velocity models and the resulting travel time curves (for ray paths illustrated in figure 11) reproducing the prominent features in both the seismogram sections (figures 9 and 10), but with different sets of the source parameters in order to generate the synthetics reasonably matching both the observed sections. offset in the record sections modeled here is within 22 km. thus the inferred models are applicable to a limited area surrounding the epicentral region (see figure 1a). these p and s velocity models are consistent with those obtained from modeling the 28-km constant offset invsp gather at the seismograph station wr in the study region [krishna 2006]. significantly, the alternating lvls with an average velocity reduction of about 0.30 km/s (4%-5%) for p and about 0.18 km/s (4%-5%) for s waves at depths of 6.1-8.1 km and 10.8-12.6 km appear to be compatible with the travel times and synthetic seismograms generated (figures 9 and 10 using different sets of virtual source parameters) in the present study. the ratio of p and s velocities vp/vs in the upper crust is generally found to be ~1.7 in the study region. the p and s velocities in the upper crust (below a 1 km thick deccan traps layer with vp 4.65 km/s and vs 2.80 km/s as used in the present study) in the koyna-warna region are found to be varying from 6.34-6.45 km/s and 3.70-3.75 km/s (1.0-6.1 km depth), 6.45-6.48 km/s and 3.78-3.83 km/s (below the lvls up to 12.6 km depth), and 6.15 km/s and 3.57 km/s in the lvls. knowledge of the deccan traps thickness in the koyna region and the nature of the underlying basement has been a subject of considerable interest. recent results of scientific drilling (reported in 2013) through the deccan traps near koyna has revealed about a 933 m thick pile of basaltic flows, which is found to underlain by granitic basement rocks. the koyna bore hole-1 (kbh-1) located very near the koyna dam, in close proximity to the 1967 koyna earthquake of magnitude 6.3, reached a depth of 951 m at the time of the report [roy et al. 2013]. the transition from basalt to granite is marked by the occurrence of quartz and pink feldspar, followed by typical coarse grained granite deeper down. the p velocity model considered by srinagesh and sarma [2005] for relocating the local earthquakes recorded by the koyna-warna digital network has a 1 km of the deccan traps layer with vp 4.70 km/s underlain by the layer with vp 6.04 km/s and increasing in steps to 6.56 km/s at 16 km depth. shashidhar et al. [2011] have estimated a 1.2 km thickness for the deccan traps layer with vp 4.40 km/s underlain by a layer with krishna 14 figure 13. vp iso-velocity plots given by dixit et al. [2014] for a limited region (as shown by a square near the seismograph station dh in figure 1a) along with the ray paths obtained in the present study for two source depths at ~4 km and ~6 km and the resulting travel time curves for the direct p waves in the available offset range −8 to 8 km. 15 vp 5.96 km/s in their model. however, the lower crustal layers in their model could not be resolved unambiguously. a recent study by kilaru et al. [2015], based on the digital data set acquired by a dense network of 97 seismograph stations in the region, inferred a 1km thick deccan traps layer with vp 4.81 km/s, followed by an upper crustal layer with vp 5.94 km/s down to 10 km, and a middle crustal layer with vp 6.47 km/s extending to 25 km depth. a comparison of these models with the velocity model inferred in the present study reveals that the deccan traps layer thickness and the vp as well as the vp/vs ratio agree quite well, and also the upper crustal vp at ~10 km agrees well within 1%-2% in all the models. it may be noted here that all the 1-d velocity models inferred by srinagesh and sarma [2005], shashidhar et al. [2011], and kilaru et al. [2015] have been determined by velest software [kissling 1988, kissling et al. 1994]. it is generally recommended that, the velest runs have to begin without low-velocity layers (setting lowveloclay=0 by default) since they have strong effects on the ray paths and, thus, they increase the non-linearity of the problem. it is not clear whether presence of low-velocity layers in the upper crust in the study region is tested by any of these models. the inferred velocity models in the present study as well as those given by krishna [2006] in this region significantly differ and clearly suggest presence of alternating lowvelocity layers in the upper crust based on interpretation of the coherent later arriving secondary p and s phases as plausible reflections from them. these model features are also consistently well revealed by at least more than 20 similar record sections compiled in the study region. it is thus believed that the upper crustal velocity models inferred in the present study are quite compatible with majority of the seismic record sections compiled by using the local earthquake recordings revealing the wave-field in the available range in the study region. an attempt has also been made here to ray-trace through the complex velocity model recently given by dixit et al. [2014] and generate the first arrival travel times in the near-offset range available in their model to ~8 km distance from two sources at ~4 km and ~6 km depths. figures 13a and 13b show their vp iso-velocity plots along with the ray paths from the two source depths, and the resulting travel time curves for the direct p waves in the offset range −8 to 8 km. the region of study by dixit et al. [2014] is indicated as a square area in figure 1a, near the seismograph station dh. figures 14a and 14b show two record sections compiled in the region in the present study using the recordings at dh-wr stations for a source depth of 4 km and at dh-kn stations for a source depth of 6 km. the p and s travel time curves for the ray paths as given in figure characteristics of local earthquake seismograms figure 14. two record sections compiled in the region in the present study, using the recordings at dh-wr stations for a source depth of 4 km and at dh-kn stations for a source depth of 6 km. the p and s travel time curves for the ray paths as given in figure 11 are also given in these two record sections. the direct p travel time curves computed for the model given by dixit et al. [2014] are also indicated below these record sections. 11 are also given in the two record sections. the direct p travel time curves computed for the model given by dixit et al. [2014] are also indicated below these record sections. these travel times are found to be late by 0.030 s as compared to those obtained in the present study. further, it is clear that the two record sections given in figure 14 are more revealing as regards the upper crustal velocity structure in the region brought out with the aid of coherent and energetic secondary (reflection) phases. this is the great advantage of compiling the record sections using the local earthquake recordings in order to visualize the entire wave-field of interest and model the p and s velocity structure of the upper crust in the region of study. the recent and denser dataset collected by dixit et al. [2014] may be more resourceful to generate a large number of record sections as shown in the present study. the presence or absence of the upper crustal low-velocity layers may also be investigated reliably by this approach. it is also well known that the presence of low-velocity layers is a challenge to first-arrival tomography, because the ray paths stay above these layers. this point is generalized to a notion that it is challenging for seismic tomography, especially travel time tomography, to resolve low-velocity anomalies because ray paths tend to stay away from them. considering these problems, it is suggested compiling and modeling of a large number of local earthquake seismic record sections for several virtual sources in seismogenic regions as done in the present study may lead to reliable velocity models for the upper crust. 6. conclusions the present study, by several numerical simulations for varying source parameters, investigates an important problem concerning local earthquake seismogram characteristics, and their constraints for modeling the upper crustal velocity structure. seismogram sections of local earthquakes with strike-slip mechanism are clearly more favorable in revealing the velocity stratification in the upper crust for both p and s waves. specifically in seismogenic regions like the koyna-warna, where more than one type of source mechanisms are dominant within a short distance range, special care has to be taken to assemble the record sections of seismograms from events of similar source mechanisms. seismogram sections of local earthquakes with normal, reverse, and dip-slip mechanisms particularly with rake angles around ±90° seem to display the upper crustal p and p-to-s converted phases poorly with low amplitudes, even in presence of sharp discontinuities of high velocity contrasts. such record sections have to be modeled by considering the appropriate source mechanism and possibly by testing whether the synthetics obtained for significant stratification and high velocity contrasts consistently display low p amplitudes well matching the observed sections. it is clear that quite often the stratification resolved in the upper crust has important implications for the structure and physical state of the seismogenic crust. especially the upper crustal low-velocity layers well resolved in seismogenic regions help understand the physical state of the earthquake source regions [krishna et al. 1999]. 7. data and resources a state-of-the-art digital seismograph network was deployed in the koyna region during 1996-1998 to provide a reliable database for studying the earth’s structure and the seismicity in the region [rai et al. 1999]. in the present study, we utilized the vertical component seismograms of local earthquakes well recorded by 3 stations selected from the koyna digital seismograph network. the events were recorded (at 100 samples/s) by various stations of the seismograph network equipped with 24 bit reftek/passcal digital recorders of short period three-component seismometers and gps timing system. local earthquake seismogram sections illustrated here are assembled by an approach discussed by krishna et al. [1999]. synthetic seismogram computations are made by using the reflectivity software developed by kind [1985] for an earthquake source. acknowledgements. i am grateful to prof. dr. harsh k. gupta, panikkar professor at the csir national geophysical research institute, hyderabad, for inviting me to process and model the valuable dataset of local earthquake seismograms to derive the upper crustal velocity structure in the koyna-warna seismic region, as well as for his helpful discussions concerning this research. i gratefully acknowledge the kind advice of prof. dr. r. kind, gfz, germany, for successful implementation of his reflectivity software in the windows 7-cygwin 64 system, extensively used for rapid computation of a large number of synthetic seismogram sections in the present study. prof. dr. friedemann wenzel of kit, universitaet karlsruhe, germany contributed by helpful discussions. prof. dr. s.s. rai while at ngri, made available the digital seismograms data of local earthquakes acquired by their research group in the koynawarna seismic region illustrated here. computations were made on a vax-3100 system, and on a windows 7-cygwin 64 system. references aagaard, b.t., j.f. hall and t.h. heaton (2001a). characterization of near-source ground motion with earthquake simulations, earthq. spectra, 17, 177-207. aagaard, b.t., t.h. heaton and j.f. hall (2001b). dynamic earthquake ruptures in the presence of litho static normal stresses: implications for friction models and heat production, b. seismol. soc. am., 91, 1765-1796. aagaard, b.t., j.f. hall and t.h. heaton (2004). effects of fault dip and slip rake angles on near-source ground krishna 16 17 motions: why rupture directivity was minimal in the 1999 chi-chi, taiwan earthquake, b. seismol. soc. am., 94, 155-170. bouchon, m. 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(2000). focal mechanism studies in koynawarna seismic zone, m.tech. dissertation, kurukshetra univ., india, 38 p. shashidhar, d., n. purnachandra rao and h. gupta (2011). waveform inversion of broad-band data of local earthquakes in the koyna-warna region, western india, geophys. j. int., 185, 292-304. srinagesh, d., and p.r. sarma (2005). high precision earthquake locations in koyna-warna seismic zone reveal depth variation in brittle-ductile transition zone, geophys. res. lett., 32, l08310; doi:10.1029/ 2004gl022073. roy, s., n.p. rao, v.v. akkiraju, d. goswami, m. sen, h. gupta, b.k. bansal and s. nayak (2013). granitic basement below deccan traps unearthed by drilling in the koyna seismic zone, western india, news and notes, j. geol. soc. india, 81, 289-290. talwani, p. (1997). seismotectonics of the koyna-warna area, india, pure appl. geophys., 150, 511-550. wadia, d.n. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49_1_2006def 253 annals of geophysics, vol. 49, n. 1, february 2006 key words landslide movements – damage evaluation – differential sar interferometry – remote sensing of land surface – maratea 1. rationale and methods natural disasters caused by meteorological phenomena (such as hail, storms, floods, landslides), are affecting countries of the european union with increasing frequency, and are producing heavier and heavier damage, partly hampering the economic development of these countries. timely assessment of damage caused by natural disasters and, above all, their continuous monitoring, are of fundamental importance in a modern well organised society to understand the effects of climatic changes on the social and economic fabric. in fact, a fast and synoptic data acquisition is useful to guarantee rapid adequate intervention, and, on the other hand, it is fundamental in planning proper long-period countermeasures. in addition, loss pricing is highly interesting for institutions for socio-economic analysis, and also for insurance companies. most of such institutions and companies produce their analysis or set their annual premium on the basis of estimates of damage in the last period. currently, these estimates are based on very rough damage assessment performed immediately after the disaster by central or local administrations of the various european counsatellite differential sar interferometry for the evaluation of effects of hydrogeological disasters: definition of a scale for damage evaluation vincenzo rizzo (1) and antonio iodice (2) (1) istituto di ricerca per la protezione idrogeologica (irpi), cnr, rende (cs), italy (2) dipartimento di ingegneria elettronica e delle telecomunicazioni, università degli studi di napoli «federico ii», napoli, italy abstract in this paper we present the results of monitoring soil movements over an about 10 km2 area around the border between the calabria and basilicata regions in italy. monitoring has been performed using the satellite differential sar interferometry measurements integrated with gps measurements. in particular, we used ers data acquired at time interval of several months (about two acquisitions per year), and after particularly strong pluvial events. terrain displacement spatial and temporal analysis has been performed by employing the original method described in berardino et al. (2002, 2003). obtained results allow us to characterise unstable areas, and can be used within projects aimed at territory classification and characterisation, and at performing damage evaluation. accordingly, this technique has been included in the framework of a proposed ue project (wedelop project) aimed at developing an integrated methodology to devise a damage scale for hydrogeological disasters. this scale is of great interest in many fields; in particular, definition of a damage scale would be highly desirable from the viewpoint of insurance companies. mailing address: dr. antonio iodice, dipartimento di ingegneria elettronica e delle telecomunicazioni, università degli studi di napoli «federico ii», via claudio 21, 80125 napoli, italy; e-mail: iodice@unina.it 254 vincenzo rizzo and antonio iodice tries. this assessment is largely approximated, and usually direct and indirect costs for damage fixing and for productive activity re-starting tend to significantly increase with respect to the initial estimate, due to necessarily unpredictable factors. in general, objective and uniform criteria for damage evaluation are still lacking, also in the case of low intensity, but more frequent, events. direct on site evaluation of damaged objects is sometimes impossible, and is often very costly. the consequence of such a situation is that large-scale reliable estimates are not available. by using modern well-established satellite remote sensing techniques, integrated with on site studies and measurements, an ad-hoc research can be conducted, aimed at defining damage scales. they will be functions of the event typology, its intensity, the urbanization degree, and so on. in order to do that, researches are needed on «sample events» used as a reference and to be investigated in detail. damaging events can be defined and coded first of all in terms of physical intensity using remote sensed data related to the considered event (e.g., height or duration of rain, wind strength, etc.). to these elements, some others can be added, relative to the intensity of effects on the physical environment. in fact, nowadays sufficiently experimented tools and methods are available for the analysis of soil changes and for the evaluation of the physical intensity of events. physical effects at the soil produced by strong meteorological events consist of changes that are partly temporary (such as soil saturation, flooding, sea strength, etc.) and partly permanent (such as debris cover, terrain displacements, variation of coastline, etc.). such effects can be detected and measured using remote sensing techniques (spectral and multispectral sensors, differential sar interferometry, etc.), and then coded in terms of their intensity. the coding of the intensity of physical effects measured on the territory (for instance in terms of percentage of affected areas or number of damaged sites per investigated area) will be then related to the coding of the event intensity. in order to obtain a better homogeneity of data, it is useful to consider a first distinction among different homogeneous physical and morphological environments (alpine or appenninic areas, coastal area, basins extension, kinds of rocks, etc.). accordingly, a number of possibly recent and well documented «sample events» that correspond to the above cited coding will be chosen. this set of events will be the basis for a detailed damage estimate, limited to areas that are representative of different urbanization degrees. obtained data will allow the availability of a large survey to establish a scale of relations among events, physical effects produced, and damage that allows an estimate relying on objective coded criteria. this general procedure was recently suggested within the 6th framework programme («weather damage evaluation and loss pricing», or wedelop proposal) for a large number of weather risks. in particular with regard to landslide activity, permanent effects should be monitored in terms of slope movements, and a specific methodology could be assessed using remote sensing data. fig. 1. block scheme for the damage scale definition procedure. 255 diffsar for the evaluation of effects of hydrogeological disasters: a scale for damage evaluation 2. application to landslide damage evaluation following the procedures proposed in wedelop we could also define a scale of damage for landslides scattered by rainy events whose elements are shown in fig. 1. the combination of these elements produces a large number of damage situations (and sampled events) to price: each combination of values of the geological settings, event magnitude, and urbanization degree indexes leads to one class of damage. however, to assess damage on large areas, some simplification can be made because only some factors are dominant. for example with regard to geological setting the «landsliding» and «morphodynamic» indexes are generally related: geodynamic areas showing a considerable amount of landslide are usually also characterized by a percentage increase in active landslide. more properly in certain cases (where available) the indexes of morphodynamic activity could be represented by the percentage of landslide remobilisation scattered by fig. 2. the investigated area. boxes a, b, c, and d show the locations of areas in the maps of fig. 5. 256 vincenzo rizzo and antonio iodice table i. interferometric pairs combined to generate the map of fig. 3. dates of sar acquisitions spatial baseline temporal baseline percentage of selected points b⊥ [m] [days] (coherence > 0.3) 13/08/1997 31/12/1997 30 140 33.11% 31/12/1997 15/04/1998 15 105 46.74% 13/08/1997 15/04/1998 45 245 30.34% 15/04/1998 02/09/1998 110 140 30.85% 02/09/1998 09/06/1999 2 280 33.17% 02/09/1998 14/07/1999 6 315 35.28% 09/06/1999 14/07/1999 4 35 52.46% 09/06/1999 05/01/2000 6 210 32.05% 14/07/1999 05/01/2000 10 175 29.53% destabilising events. finally, the degree of urbanisation is very important in relation to the potential damage, and should be immediately related to the events magnitude. geological setting (landslide and morphodynamic indexes for various lithological complexes) could be fixed on sampling areas; while differential sar interferometry could help in assessing and setting up landslide activity, so that the morphodinamic index (d, e, f) can be updated. 3. results of diffsar application on earth slope movements performed at maratea differential synthetic aperture radar (sar) interferometry (diffsar) is able, in principle, to measure very small movements of the ground and to cover in continuity large areas (godstein et al., 1994; franceschetti and lanari, 1999; ferretti et al., 2001), so that it can be considered an ideal tool to investigate landslides and other slope instability (acache et al., 1995; carnec et al., 1996). in our implementation of the diffsar technique, the problem of decorrelation noise is faced using a phase unwrapping approach that allows us to process sparse data, and the impact of atmospheric artefacts is reduced by performing a temporal analysis of the deformations observed in successive interferograms. as an example of the potentiality of this technique, we here show some results obtained by using it to improve our knowledge of the slope instability of a well investigated area (the maratea valley, see fig. 2) affected by continuous slow movements (berardino et al., 2002, 2003). in particular, using this technique, and employing the sar acquisitions reported in table i, we analysed the time evolution of ground movements from 1997 to 2000. this time interval had also been explored in the past using other techniques, such as distancemeter (edm) and gps measurements: gps and diffsar results were then compared. obtained results are summarised in figs. 3 to 5. in the map of fig. 3, by visual inspection it is possible to verify the agreement between data obtained by the sar technique (represented by the color scale defined in the bottom left of fig. 3) and those obtained by in situ gps measurements (blue arrows). a 3d view of the map is reported in fig. 4, and a table of sar versus gps results is shown in table ii. a full discussion of the comparison between sar and gps measurements can be found in (berardino et al., 2003). we here want to emphasize that the employed technique is also able to detect small scale events, possibly with antropic origin, as shown in fig. 5. results of fig. 3 have shown that it is possible to perform a temporal analysis of continuous slow landslide movements using a limited number of ers sar data sets and low precision topographic information. all the acquired data (edm, gps and diffsar) are consistent and allow a kinematic model of instability within the investigated time interval to be sketched. a map 257 diffsar for the evaluation of effects of hydrogeological disasters: a scale for damage evaluation fig. 3. diffsar displacement map relative to the period from july 1997 to january 2000, and gps data acquired in almost the same time interval. displacements must be understood as projected along the sar line of sight (about 23° from vertical and 104° from north direction, descending orbit). fig. 4. dem of the investigated area, showing diffsar displacements. the same color scale of fig. 3 is used, except for blue areas, showing slight positive vertical displacements: those are often low coherence area. 258 vincenzo rizzo and antonio iodice fig. 5. examples of geological validation for small phenomena, also of antropic nature, in the maratea area (parts of maps, in scale 1:25000, of the vertical soil movements in the interval 1996-1998, only for pixels with coherence coefficient greater than 0.3). top left (a): the maratea dump (a few pixels large) is perfectly visible and well delimited, and turns out to be moving towards the sensor (i.e., upwards). bottom left (b): last part of the noce river, where detected upward and downward movements are coherent with erosion and deposition phenomena, with dumps, quarries, and so on. top right (c): central part of the noce valley, where detected upward and downward movements are in reasonable agreement with low and progressive landslide movements. bottom right (d): movements to be verified by gps measurements, probably related to deep gravitative movements or tectonic alignments. 259 diffsar for the evaluation of effects of hydrogeological disasters: a scale for damage evaluation table ii. comparison of gps and diffsar measurements. displacements are both relative to the period from june 1997 to march 2000 and are projected along the sar line of sight. the obtained values show good agreement in the pixels at higher coherence. however, by considering all the points, the difference between gps and diffsar measurements has a mean value equal to 2.3 cm (diffsar underestimates the movements) and a standard deviation equal to 3.3 cm. these discrepancies can be explained by considering that in the low-coherence points the diffsar measurements are obtained by interpolating results obtained in surrounding high-coherence points, so that they can be considered as average displacement over a rather wide area, whereas gps measurements are strictly referred to each single ground point. gps gps diffsar coherence gps-sar benchmarck displacement displacement difference number (mm) (mm) (mm) 1 − 52 −54 0.3 2 2 −49 −63 0.6 14 3 −65 −63 0.5 −2 4 −52 −54 0.7 2 5 −69 −22 0.2 −47 6 −75 −24 0.6 −51 7 −18 −15 0.2 −3 8 −51 −8 0.3 −43 9 no −25 0.6 10 −28 −6 0.2 −22 11 −84 −56 0.2 −28 12 −13 −36 0.2 23 13 −105 −83 0.7 −22 14 −73 −52 0.3 −21 15 no −47 0.3 16 −32 −65 0.3 33 17 −17 −38 0.7 21 18 −112 −46 0.6 −66 19 no −14 0.4 20 −41 −15 0.4 −26 21 −102 −27 0.6 −80 22 no −58 0.3 23 −97 −51 0.3 −46 24 no −69 0.3 25 no −69 0.3 26 −104 −56 0.6 −48 27 −24 −75 0.2 51 28 −31 −18 0.3 −13 29 no 2 0.2 30 −47 −41 0.3 −6 31 −24 −18 0.2 −6 32 −120 −33 0.1 −87 33 −97 −13 0.2 −84 34 −83 −68 0.4 −15 35 −66 −61 0.5 −5 260 vincenzo rizzo and antonio iodice of slopes subject to different velocities and vertical displacements was delineated, modifying previous knowledge (berardino et al., 2003). within the valley a progressive and almost linear displacement over time was confirmed. displacement maps in figs. 3 and 5 detect areas subjected to movements, and hence the active landslides. this information can be used to set the morphodynamic index. 4. conclusions results of research on diffsar application to the monitoring of western basilicata (maratea and noce valley) show the possibility to use algorithms and operative conditions (integrated use of reference gps networks) such that slow and progressive soil deformations can be detected, even in small areas and in medium-low coherence areas. however, the obtained final maps have some limitations: they do not cover the entire monitored area (in the considered case-study for about 40% of the territory the coherence coefficient was below 0.3) and show some visual artefacts due to the sensor viewing geometry. obtained data, however, can be integrated by ground based observations and by other remote sensing images. these results encourage applications aimed at studying landslide movements and erosion and deposition processes close to rivers. therefore, the outlined procedures are suitable to develop a project aimed at monitoring permanent changes produced by hydrogeological events and at devising criteria for the definition of a relative damage scale. references achache, j., b. fruneau and c. delacourt (1995). applicability of sar interferometry for operational monitoring of landslides, in proceedings of the 2nd ers applications workshop, london, 165-168. berardino, p., m. costantini, g. franceschetti, a. iodice, l. pietranera and v. rizzo (2002): differential sar interferometry for the study of slope instability at maratea, italy, in proceedings of the international geoscience and remote sensing symposium, toronto, canada, 2693-2695. berardino, p., m. costantini, g. franceschetti, a. iodice, l. pietranera and v. rizzo (2003): use of differential sar interferometry in monitoring and modelling large slope instability at maratea (basilicata, italy), eng. geol., 68, 31-51. carnec, c., d. massonnet and c. king (1996): two examples of the use of sar interferometry on displacement-fields of small spatial extent, geophys. res. lett., 23 (24), 3579-3582. ferretti, a., c. prati and f. rocca (2001): permanent scatterers in sar interferometry, ieee trans. geosci. remote sensing, 39 (1), 8-20. franceschetti, g. and r. lanari (1999): synthetic aperture radar processing (crc press, boca raton, fl), 25-28. goldstein, r.m., h.a. zebker, c.l. werner, p.a. rosen and a. gabriel (1994): on the derivation of coseismic displacement field using differential radar interferometry: the landers earthquake, j. geophys. res., 99, 19618-19634. table ii (continued). gps gps diffsar coherence gps-sar benchmarck displacement displacement difference number (mm) (mm) (mm) 36 −56 −38 0.2 −18 37 −37 −15 0.2 −22 38 −24 −56 0.2 32 39 −17 −9 0.2 −8 40 −25 −25 0.4 0 41 −15 −21 0.2 6 42 −9 −26 0.3 17 vol49_2_2006 659 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words long-term multidisciplinary seafloor observatories – geophysical and environmental seabed monitoring 1. introduction the european experience on seafloor monitoring started in early 1990s with the ec mast (acronyms and abbreviations are listed before the references) programme. feasibility studies commissioned by the ec were addressed to identifying the scientific requirements (thiel et al., 1994) and to establishing the possible technological solutions for the development of seafloor observatories (abel, berta et al., 1995). in parallel, other studies and activities, such as desibel (ria fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor paolo favali (1) (2), laura beranzoli (1) , giuseppe d’anna (1) , francesco gasparoni (3) , jean marvaldi (4) , günther clauss (5) , hans w. gerber (6) , michel nicot (7) , michael p. marani (8) , fabiano gamberi (8) , claude millot (9) and ernst r. flueh (10) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) università degli studi di roma «la sapienza», roma, italy (3) tecnomare-eni spa, venezia, italy (4) ifremer, centre de brest, plouzané, france (5) technische universität berlin, germany (6) tfh berlin – university of applied sciences, berlin, germany (7) sercel-underwater acoustic division (former orca instrumentation), brest, france (8) istituto di scienze marine (ismar), cnr, sezione di geologia marina, bologna, italy (9) laboratoire d’océanographie et de biogéochimie (lob), la seyne-sur-mer, france (10) ifm-geomar, kiel, germany abstract seafloor long-term, multiparameter, single-frame observatories have been developed within the framework of european commission and italian projects since 1995. a fleet of five seafloor observatories, built-up starting from 1995 within the framework of an effective synergy among research institutes and industries, have carried out a series of long-term sea experiments. the observatories are able to operate from shallow waters to deep-sea, down to 4000 m w.d., and to simultaneously monitor a broad spectrum of geophysical and environmental processes, including seismicity, geomagnetic field variations, water temperature, pressure, salinity, chemistry, currents, and gas occurrence. moreover, they can transmit data in (near)-real-time that can be integrated with those of the on-land networks. the architecture of the seafloor observatories follows the criteria of modularity, interoperability and standardisation in terms of materials, components and communication protocols. this paper describes the technical features of the observatories, their experiments and data. mailing address: dr. paolo favali, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: paolofa@ingv.it 660 paolo favali et al. gaud et al., 1998), were carried out at ec level, aimed at defining needs and expectations for long-term investigations at abyssal depths. meanwhile, the most technologically advanced countries have launched a large number of projects and programmes addressed to long-term and multiparameter seafloor monitoring. favali and beranzoli (2006) review these international efforts. a widely accepted definition of seafloor observatories has progressively been affirmed at numerous international conferences and workshops (e.g., chave et al., 1990; montagner and lancelot, 1995; utada et al., 1997; romanowicz et al., 2001; beranzoli et al., 2002; kasahara and chave, 2003). this definition outlined by nrc (2000) is: « [...] unmanned system of instruments, sensors and command modules connected either acoustically or via seafloor junction box to a surface buoy or a cable to land. these observatories will have power and communication capabilities [...]». accordingly, a seafloor observatory is characterised by a data acquisition and control systable i. requirements for the instrumentation used in seafloor observatories. sensor typical sampling data acquisition installation constraints rates (bits) three-component 20÷100 hz 24 – positioning (error ≤100 m). broad-band seismometer – orientation to the north (known ≤1°). – good ground coupling. – fine levelling (if required). hydrophone 80 ÷ 100 hz 24 – positioning (error ≤100 m). gravity meter 0.01 ÷ 1 hz 24 – positioning. – temperature controlled. – fine levelling. scalar magnetometer 1 sample/min 16 – minimisation of possible electro-magnetic interferences. tri-axial fluxgate 1 sample/s 24 – minimisation of possible electro-magnetic interferences. precision tilt meter (x, y) 10 hz 24 – northwards orientation. tri-axial single-point 2 hz 16 – avoiding frame interference. current meter adcp 300 khz 1 profile/h – avoiding frame interference. transmissometer 1 sample/h – avoiding frame interference. ctd 1 sample/10 min (or 1 sample/h) ch4 sensor 1 hz 24 h2s sensor 1 sample/10 min 24 (averaged on 30 samples/s) ph sensor 1 sample/6 h (*) – ampling and self-calibration programmable – self-calibration every 24 samples (*). water sampler – 48 bottles, sampling depending on the mission targets. (*) orion-geostar-3 configuration. 661 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor tem, multiple sensors, long-term autonomy, communication systems, remote re-configuration of mission parameters, accurate positioning. another important constraint to be considered is a unique time reference for all measurements, giving us the chance to compare different processes for exploring possible reciprocal relationships. the sensors themselves are suitable for long-term operation, when properly installed to provide highly reliable data. the requirements for the instrumentation, used in seafloor observatories, are shown in table i. between 1995 and 2001 the ec funded the geostar and geostar-2 projects (beranzoli et al., 1998, 2000a,b; favali et al., 2002) which designed, developed and operated a prototype autonomous deep-sea observatory (hereafter geostar) hosting a wide range of sensors in a single frame and providing facilities for external experiments. geostar satisfied the definition of seafloor observatory mentioned above with multidisciplinary, long-term monitoring capabilities providing time-referenced data series, and the chance to transmit data in (near)-real-time through a surface buoy. moreover, the management of the observatory from the sea surface has represented an innovative approach exportable to other seafloor monitoring and survey applications. the geostar system has performed experiments both in shallow and deep waters, which confirmed the reliability and the feasibility of the deployment/recovery procedure even in a moderately perturbed sea state (jourdain, 1999; beranzoli et al., 2000a; favali et al., 2002). two paths were followed after the geostar experience: the development of other single-frame observatories devoted to specific applications and the enhancement of geostar as principal node of a network of seafloor observatories. these paths have led to the current availability of four more geostar-class observatories and the first european prototype of a deep seafloor observatory network. sn-1 and gmm systems were developed (favali et al., 2004a) among the single-frame geostar-class observatories. sn-1 is addressed to seismological, oceanographic and environmental measurements developed within a gndt-funded project (favali et al., 2003). gmm, built within the ec assem project (blandin et al., 2003) is devoted to seafloor gas monitoring (marinaro et al., 2004). within the framework of the ec oriongeostar-3 project (beranzoli et al., 2004), geostar was implemented to act as the main node of an underwater network of deep-sea observatories of geostar-class with the capability of (near)-real-time communication. in addition to this main node, two more observatories, with the function of satellite nodes (orion nodes 3 and 4), were built and equipped with seismological and oceanographic sensors. the concomitant running of the oriongeostar-3 and assem projects has given us the chance to integrate one of the orion nodes in the shallow water assem system during the assem pilot experiment in corinth gulf. this integration has been dedicated to demonstrating the compatibility of the two seafloor networks and the chance to operate a «coast-to-deep-sea» monitoring system in the near future. this paper gives a technical description of the five above-mentioned seafloor observatories, together with the presentation of the acquired data. a sixth single-frame system, called mabel, is being developed for polar sea applications within the framework of the italian pnra (calcara et al., 2001). a short description of mabel is also given. 2. the geostar system geostar is a single-frame autonomous seafloor observatory, based on three main sub-systems (beranzoli et al., 1998): a) the bottom station, that is the monitoring system; b) modus, the dedicated deployment/recovery vehicle; c) the communication systems. geostar is capable of long-term (more than one year) multidisciplinary monitoring at abyssal depths. at present, the maximum operative depth is 4000 m. 2.1. bottom station the bottom station (fig. 1) is a four-leg marine aluminium frame hosting the monitoring system including lithium batteries for power 662 paolo favali et al. supply; electronics mounted inside titanium vessels; hard disks for data storage; the underwater part of the communication systems; scientific and status sensors. the bottom station mission is driven and controlled by a central data acquisition and control unit (named dacs; gasparoni et al., 2002). geostar dacs (fig. 2) can perform the following tasks: management and acquisition from all scientific packages and status sensors; preparation and continuous update of hourly data messages to be transmitted on request including detection of events; actuation of received commands (e.g., data request, system reconfiguration, re-start); data back-up on internal memory. dacs manages a wide set of data streams at quite different sampling rates (from 100 hz to 1 sample/day) tagging each datum according to a unique reference time set by a central high-precision clock (stability within a fig. 1. geostar seafloor observatory: bottom station with modus vehicle on the top. 663 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor fig. 2. dacs equipped with central bottom station high-precision clock (left-bottom) provided by sercel (former orca instrumentation). table ii. dacs main technical characteristics of the geostar-class platforms (e.g., gasparoni et al., 2002). geostar sn-1 orion orion gmm mabel(1) node 3 node 4 configuration 4 cpu 3 cpu 3 cpu 3 cpu 1 cpu 2 cpu (mcu, sdu, (mcu, sdu, (mcu, sdu, (mcu, sdu, (mcu, sdu, hdu, dau) hdu) hdu) hdu) mass memory 3×8 gb 3×8 gb 3×8 gb 3×8 gb 512 mb 30 gb (2 hds sdu, (2 hds sdu, (2 hds sdu, (2 hds sdu, (flash) (hd sdu) 1 hd hdu) 1 hd hdu) 1 hd hdu) 1 hd hdu) 3×64 mb 1 gb 3×64 mb 3×64 mb 1 gb (flash mcu, (flash mcu) (flash mcu, (flash mcu, (flash mcu) sdu, hdu) sdu, hdu) sdu, hdu) 512 mb 2×64 mb 128 mb (flash dau) (flash sdu, hdu (flash sdu not used in rtl) power supply 24 vdc 12 vdc 12 vdc 12 vdc 12 vdc 12 vdc (battery) (battery or cable) (battery) (battery) (battery) (battery) power 70 ma (id) 200 ma (id) 120 ma (id) 120 ma (id) 80 ma (id) < 80 ma (id) consumption 300 ma (mm) 450 ma (mm) 350 ma (mm) 400 ma (mm) 150 ma (mm) < 200 ma (mm) 550 ma (peak) communication modus (2) modus (2) modus (2) h acoustics cable telemetry modus (2) interfaces v acoustics v acoustics h acoustics v acoustics h acoustics fibre-optic messengers telemetry (1) the first polar experiment started at the end of 2005; (2) during deployment. mcu – mission control unit; sdu – seismometer data acquisition unit; hdu – hydrophone data acquisition unit; dau – data acquisition unit; hd – hard disk; rtl – real time link [mode]; id – idle mode: all sensors switched off; cpus waiting command from the operator; mm – mission mode: all sensors switched on; cpus and communications active; v – vertical; h – horizontal. range 10–9 to 10–11, accordingly to supplier specifications and verified during the experiments controlling the clock drift). the sensors were selected also in order to keep power consumption lower than 350 ma at 24 v. table ii describes the geostar dacs’ main technical characteristics. devices were designed and implemented to install the seismometer and magnetometers with the aim of reducing the disturbances caused by the observatory frame and electronics. the former, installed in a benthosphere by the supplier, was included inside a heavy cylindrical housing. then the whole package was released by a special device after the bottom station touch down to guarantee a good coupling with the sea bottom, and was kept linked to the bottom station frame by a 664 paolo favali et al. slack rope. special care was taken in the choice of the electronic components of the ingv fluxgate magnetometer prototype. the resolution of this prototype is 1 nt and an absolute accuracy 5 nt. the magnetometers, used in the early version, were scalar (overhauser magnetometer) and bi-axial fluxgate (horizontal axes), then in all the subsequent experiments the ingv fluxgate prototype was fully tri-axial. they were installed at the end of two booms attached at opposite angles of the bottom station frame to keep them as far as possible from electronic noise sources. the booms, kept vertical during the descent, were opened by command from the surface through the umbilical cable, once the observatory was placed on the seafloor. the direction of the three components of the geomagnetic field was reconstructed using the scalar information (total field) deduced from the overhauser magnetometer and from calibrating the fluxgate magnetometer in the air close to the geomagnetic observatory of l’aquila (central italy). the results were also confirmed when compared with the horizontal component as deduced from a land magnetic station running during the first deep mission close to ustica island (sicily, italy) in 2000-2001 (see also de santis et al., 2006). 2.2. modus modus, a simplified rov, is the special vehicle for the deployment/recovery procedures (clauss and hoog, 2002; clauss et al., 2004; gerber and clauss, 2005). modus is remotely controlled from the ship through a dedicated electro-opto-mechanical cable. the telemetry system also provides the primary communication link with the observatory during the deployment phase. it is equipped with a latch/release device and thrusters mounted on a frame around the cone that assists the docking. the aim is to load, deploy and recover the bottom station in surface-assisted mode. the modus frame is also equipped with video cameras for visual seabed inspection, compass, sonar and altimeter. the main modus characteristics are listed in table iii, while the system is shown in fig. 3a-e including the latch/release scheme. table iv contains the main features of the handling system (winch, hydraulic unit and sheave) and cable (fig. 4). 2.3. communications systems two independent communication systems were originally developed for geostar, based on different principles (marvaldi et al., 2002). the first one consists of buoyant data capsules, named messengers, releasable upon surface command or automatically, when full of data or in case of emergency. two types of messengers are available: a) expendable (data storage capacity 64 kb); b) storage (data storage capacity larger than the expandable, 40 mb). they can transmit their position at sea surface and small quantities of data via argos satellites. the second communication system is based on a bi-directional vertical acoustic link table iii. modus main characteristics (clauss and hoog, 2002; clauss et al., 2004; gerber and clauss, 2005). purpose umbilical-driven frequent operations material aluminium (frame) stainless steel (docking device) titanium (pressure vessels) weight in air (kn) 10 weight in water (kn) 7 total length-l (m) 2878 total width-w (m) 2348 total height-h without cable 1700 termination (m) maximum payload (kn) 30 power (kw) 25 horizontal thrusters (n) 4×700 vertical thrusters (n) 2×700 altimeter range (m) 100 heading accuracy (degrees) 1 tilt accuracy (degrees) 1 360° sonar range (m) 300 video cameras (+ lights) 6 videos and recorders 4 depth rated (m) 4000 665 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor fig. 3a-e. modus, the geostar deployment/recovery vehicle: a) docking cone and b) pin; c) modus on the deck of r/v urania; d) modus on-board console; e) bottom station signature displayed on the sonar monitor. table iv. main characteristics of the winch (macartney) and cable (rochester). item dimensions (m) weight (kn) max payout load (kn) max pull (kn) notes speed (m/min) winch 3.80×2.35×2.40 181 70 (a) 80 (c) 102 (a) −20 ÷ + 45°c (l×w×h) 51 (b) 75 (b) remote control hpu (1) 1.77×1.15×1.71 20 325 bar (l×w×h) 75 kw (3×380 v-50 hz) sheave 1.05 (ø) 0.2 100 (d) instrumented (cable out, pull, speed) cable 0.0254 (ø) 22 (e) (in air) 89 (d) 3 optic fibres 4000 (length) 18 (e) (in water) 205 (f) 3×3000 vac-6a (1) hydraulic pump unit; (a) 1st layer; (b) 10th layer; (c) static, top layer; (d) working load; (e) kn/km; (f) breaking strength. a b c d e 666 paolo favali et al. fig. 4. the geostar handling system: power unit (right), cable spooled on the winch (left) and system console (insert) . fig. 5a-d. a) messengers installed on the bottom station (height 1.3 m); b) messengers storage and expendable-type in brest ifremer laboratory; c) surface buoy (weight: 35 kn; volume: 5 m3) geostar-2 version on board r/v urania; d) surface buoy orion-geostar-3 version just deployed. a b c d 667 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor with a ship of opportunity or moored buoy, called mats-12 (frequency: 12 khz; speed: up to 2400 bit/s). a surface relay buoy, equipped with a surface telemetry unit and radio/satellite transmitters, assures the (near)-real-time communication between a shore station and the observatory on the seafloor. pictures of the messengers and the buoy are shown in fig. 5a-d. 3. single-frame systems derived from geostar 3.1. sn-1 sn-1 is a reduced-size version of geo-star (fig. 6) and represents the recent effort of italian marine research and technology addressed to the fig. 6. sn-1 and modus (left) on the deck of the cable-vessel pertinacia before deployment; the rov connecting sn-1 observatory to the cable interface (top-right); sn-1 on the seafloor during the cable connecting operations (bottom-right). the cable route from catania harbour to 25-km east in the ionian sea is shown in top-left panel. 668 paolo favali et al. development of a seafloor network around italy (favali and beranzoli, 2006). sn-1 has the same features as geostar in regard to deployment/ recovery procedures based on modus, the data acquisition system (sn-1 dacs, see table ii) and the special device for seismometer installation developed in the geostar projects (see section 5 for details). compared with geostar, sn-1 hosts a reduced set of sensors, mainly seismological and oceanographic. like geostar, sn-1 has a vertical acoustic link from the seafloor to a surface unit managed by a ship of opportunity, while it is not supported by a surface-moored buoy. from october 2002 to may 2003 sn-1 successfully completed the first long-term experiment off-shore from catania (southern italy, eastern sicily) at 2105 m w.d. in autonomous mode (favali et al., 2003). after this experiment, sn-1 was fitted with a fibre-optic telemetry interface so as to be compatible with the electro-optical cable owned and deployed off-shore from catania by infn. in january 2005, the observatory was deployed at the same site as the first mission (about 25 km east from catania at 2060 m w.d.) by modus and connected to the submarine cable. the sea operations were carried out using the c/v pertinacia (elettra tlc spa) and the sn-1 connection was performed by the on-board deep-rated rov. sn-1 receives power from the shore, can communicate in real-time with the shore station located in the lns-infn laboratory inside catania harbour, and is integrated in the ingv landbased networks. sn-1 is the first real-time seafloor observatory in europe and one of the few in the world. it is also the first seafloor observatory operative in one of the «key-sites» planned in the ec project esonet (priede et al., 2003, 2004). these achievements were fulfilled thanks to a mou between ingv and infn, which is going to use the site for the nemo pilot experiment addressed to the underwater detection of neutrinos (favali et al., 2003; favali and beranzoli, 2006). 3.2. gmm designed and built within the framework of the assem project (blandin et al., 2003), gmm is another system developed on the basis of the geostar experience (marinaro et al., 2004). gmm is an autonomous station designed to monitor the gas seawater concentration close to the seabed. gmm is based on a light benthic circular tripod of aluminium alloy (fig. 7). it can also operate interfaced to external units (e.g., other seafloor nodes of an underwater network, on-shore stations) via a submarine cable. the system can be reconfigured either to be integrated in more complex observatories (like geostar) or operated as a payload of submarine vehicles for surveys. in particular, the gmm design allows for modification of the frame-top and the installation of the mechanical interface to be managed during deployment/recovery procedures by modus. gmm electronics performs similar tasks as the geostar dacs (see table ii). fig. 7. gmm module on the ship before the deployment in the corinth gulf. 669 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor 3.3. mabel mabel is another deep-sea multiparameter seafloor observatory under development specifically addressed to the acquisition of geophysical, geochemical, oceanographic and environmental time series in polar regions (calcara et al., 2001). mabel, sponsored by the italian pnra, is designed to operate autonomously for at least one year and will be the first seafloor observatory deployed in antarctica. the characteristics of its dacs are shown in table ii. its mechanical and electronic behaviour at low temperatures was already tested in 2002 at hsva basin (hamburg) in simulated polar conditions (air: −15°c, and icy waters: −2°c) (fig. 8; cenedese et al., 2004). the first antarctic mabel experiment started at the end of the 2005 having deployed the observatory in the weddell sea at over 1800 m w.d., with the logistic support of the r/v polarstern managed by awi, and it will last for at least one year. 4. orion-geostar-3 system within the framework of the ec oriongeostar-3 project, the geostar bottom station, the surface relay buoy and modus have been upgraded in order to be able to manage a network of geostar-class observatories, as a significant step towards deep-sea networking (beranzoli et al., 2004). two additional observatories have been developed (orion nodes 3 and 4) being able to communicate via acoustics with geostar bottom station. the communication system has been implemented in order to enable the geostar observatory to operate as the main node (gateway) of the orion network, exchanging data and status parameters with the satellite nodes and transferring data to the sea surface. a picture with the general scheme of orion-geostar-3 is shown in fig. 9. the bottom station has thus also been equipped with horizontal acoustics devoted to the communication among the nodes, based on mats modems. through the horizontal communication, geostar receives automatic messages from the satellite nodes, while the vertical communication to the surface buoy, enhanced with respect to the original version, is used to transmit data from both geostar and the nodes. connection between the buoy and a shore station is ensured by radio and satellite links. data, specifically pieces of seismic waveforms, can be retrieved on request. the horizontal modems use omni-directional transducers, whereas the vertical acoustic link is based on directional transducers. the buoy transmission system (drts) comprises an electronic unit (meu) managing the communications and interfacing the acoustic transmission system with two buoy-to-shore data links, vhf radio or iridium satellite. in case of vhf-link failure, a switch to the satellite transmission is automatically performed. fig. 8. mabel during the low temperature tests at hsva basin in hamburg. 670 paolo favali et al. to achieve the new required functionality, the dacs has been properly enhanced (beranzoli et al., 2004). the sampling rate of some sensors (e.g., gravity meter) has been increased and new sensor packages installed (e.g., electrode analyser, hydrophone). accordingly, additional acquisition channels have been made available. the following function was implemented: automatic event detection on the seismometer and hydrophone data, transmission of seismometer waveforms. the dacs interface to the communication system was properly enfig. 9. scheme of management and operation of the orion-geostar-3 deep-sea network of geostarclass seafloor observatories. table v. list of the geostar-class platforms and some specifications. platform overall dimensions (m) weight (kn) weight (kn) depth rated (m) (l×w×h) (in air) (in water) geostar 3.50×3.50×3.30 25.4 14.2 4000 sn-1 2.90×2.90×2.90 14.0 8.5 4000 orion node 3 2.90×2.90×2.90 14.0 8.5 4000 orion node 4 2.00×2.00×2.00 6.6 3.4 1000 gmm 1.50×1.50×1.50 1.5 0.7 1000 mabel 2.90×2.90×2.90 14.0 8.5 4000 671 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor hanced in order to make data and status parameter available for transmission to the communications system. the communications can be started by any of the orion-geostar-3 network nodes. the dacs hardware has been also upgraded in order to increase functions/capabilities and reliability with reduced power and volume requirements (see table ii): a) new cpu boards with increased power; b) new status boards with additional sensors, scientific data acquired at 24 bits; c) status sensors acquired at 16 bits (12 bits in the previous version); d) boards managing up to 32 gb on hard disk and 1 gbyte on flash card (see table ii). as already mentioned, the ec requested the orion-geostar-3 and assem networks to be compatible. accordingly, common communication protocols were defined and implemented in the nodes of both networks in regard to data communication. for this purpose, an orion node (node 4) was deployed and tested together with the assem nodes in the corinth gulf pilot experiment. the list of the geostar-class platforms with some specifications are summarised in table v. 5. experiments, data and prototyping activity the sea experiments performed are depicted in table vi including specific information and the sensors used in each experiment. figure 10 shows the map of the locations. all the experiments were carried out by means of mediumsize vessels with dgps and dp, like, for instance, the cnr r/v urania. only for the deployment of sn-1 and its connection to the electro-optical cable was a larger cable vessel used (c/v pertinacia). geostar performed its first sea demonstration mission in shallow waters in 1998 (jourdain, 1999; beranzoli et al., 2000a,b). the observatory was deployed on the seafloor of the adriatic sea (northern italy) about 50-km east of ravenna harbour. the selection of the mission site was based both on the knowledge of geological and geotechnical soil characteristics (flat and consolidated seabed, distance from turbulence sources, absence of pockmarks and gassy sediments) and safety factors (shallow water depth, vicinity to harbour logistics). the starting mission procedure foresaw that after the bottom station had touched down, all the sensor packages and devices were switched on through modus telemetry and their correct functioning was checked. after the positive outcome of this operation, the bottom station was released by modus and left on the sea bottom (beranzoli et al., 2000a,b). during the shallow water demo mission around 346 mb of data were acquired over roughly 440 operational hours, corresponding to 98% of the mission’s duration, see table vi for the list of the used sensors. an expandable messenger was automatically released and transmitted data via the argos satellite. a storage messenger was release acoustically just before the bottom station’s recovery. the experiment demonstrated the functionality of the whole system, including modus. temporary magnetic and seismological stations were also installed on land as a reference for geostar measurements. analysis of data acquired, even if during only 21 days, pointed out the reliability of the measurements and their scientific potentiality as a unique time-referenced multiparameter data. some interesting events, like regional earthquakes, water current and thermocline depth variations, and a magnetic storm were recorded (beranzoli et al., 2003). the first geostar long-term deep-sea mission was performed between september 2000 and april 2001 at about 2000 m w.d. in southern tyrrhenian sea (see table vi). the communication system was enhanced adding a surface moored buoy, equipped with the interface of the acoustic system and a radio/satellite link for (near)-real-time transmission between the bottom station and on-shore sites. data acquired, 4160 h corresponding to about 174 days (out of 205 because the batteries were exhausted), amount to more than 65 mb mostly from the gravity meter. an external self-recording hydrophone acquired 4 gb of data. also in this longterm experiment, the data quality was high, as demonstrated by de santis et al. (2006), iafolla et al. (2006), and etiope et al. (2006) pointed out ocean-lithosphere interactions at bbl level. during the 2002-2003 experiment off-shore from catania (southern italy, eastern sicily; 672 table vi. list of the seafloor experiments performed with geostar-class platforms and the sensors used (see fig. 10 for the map). experiments location depth year(s) days platform(s) sensors used (m) geostar northern adriatic 42 1998 21 geostar three-component broad-band (demo mission) sea (italy) seismometer; scalar magnetometer; fluxgate magnetometer (only x-y); adcp 300 khz; ctd; transmissometer; precision tilt meter (x, y). geostar-2 (1) southern tyrrhenian 1950 2000 205 geostar gravity meter; scalar magnetometer; (1st deep-sea sea (italy) 2001 tri-axial fluxgate magnetometer; mission) adcp 300 khz; ctd; transmissometer; tri-axial single-point current meter; precision tilt meter (x, y); water sampler (off-line); hydrophone (off-line). (2) sn-1 western ionian sea 2105 2002 213 sn-1 three-components broad-band (first mission) (off-eastern sicily, 2003 seismometer; hydrophone; gravity italy) meter; ctd; tri-axial single-point current meter. assem gulf of patras 40 2004 198 (3) gmm ch4 sensors (3); h2s sensor; ctd. (pilot experiment, (greece) 2005 in a pockmark) assem gulf of corinth 380 2004 214 orion three-component broad-band (pilot experiment) (greece) node 4 seismometer; hydrophone; (orion-geoch4 sensor. star-3 -assem clustering) orion-geo tyrrhenian sea 3320 2003 477(4) geostar (g) three-comp. broad-band star-3 (deep(marsili seamount, 2005 and orion seismometers (g, n3); sea networking) italy) node 3 (n3) hydrophones (g, n3); gravity meter (g); scalar magnetometer (g); tri-axial fluxgate magnetometer (g); adcp 300 khz (g); ctd (g); transmissometer (g); tri-axial singlepoint current meter (g); ph sensor (g); precision tilt meter (x, y) (g); water sampler (off-line) (g). nemo – sn-1 western ionian sea 2060 2005 ongoing sn-1 three-component broad-band (cabled january (off-eastern sicily, seismometer (5); hydrophone; 25, 2005) italy) gravity meter; scalar magnetometer; ctd; tri-axial single-point current meter. (1) this experiment included originally also a three-component broad-band seismometer and a chemical analyser prototype. these instruments were not used in the experiment, due to failures that occurred during the sea operations preceding deployment; (2) provided by ifm-geomar; (3) 91days from april 26 to july 26, 2004, and 107 days from september 29, 2004 to january 14, 2005; (4) 134 days from december 14, 2003 to april 26, 2004, and 337 days from june 14, 2004 to may 23, 2005; (5) installed in a titanium sphere. paolo favali et al. 673 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor table vi), sn-1 acquired in autonomous mode, around 10 gb of data, 7.65 gb of which belong to 100 hz sampling rate broad-band seismometer, guralp cmg-1t (favali et al., 2003). the double housings of seismometer, comprising a titanium benthosphere inside an external bell, and the relative simple procedure to release it allowed protection from sea-current effects and good coupling of the instrument to the seabed. these solutions, already used in the previous geostar experiments, were validated and allowed to collect high-quality seismological data (monna et al., 2005). the signals showed noise in the underwater environment (webb, 1998) with a level comparable with «quiet» terrestrial seismic stations, well within the high and low background noise reference models (peterson, 1993). it is worth noting that in our case, unlike the ocean experiments, long-period noise on the vertical component caused by infragravity waves is not a first-order effect. in fact, the energy of infragravity waves in the mediterranean sea is small as compared with the pacific and atlantic oceans. thanks to its good s/n ratio sn-1 demonstrated the relevant improvement of the seismic event detection recording hundreds of local events not recorded by the dense on-land networks (favali et al., 2004b). examples of the data collected are shown in fig. 11a-f. gmm was deployed in an active pockmark in the gulf of patras (corinth shelf, greece) in april 2004 as one of the nodes of the assem system (see table vi). the system was simply lowered down to the seafloor (40 m w.d.) with a mechanical cable and positioned in the right place by divers. gmm was linked to a submarine cable for real-time data transmission to an on-shore modem. the 12 v, 960 ah lithium battery pack made six-month autonomous operation possible. a remote link to the on-shore modem was active for the system checks and data retrieval. through this daily link, a malfunctioning in all of the methane sensors was detected at the end of july, so the system was recovered at the end of september, the ch4 and h2s sensors were replaced, and the mission re-started after one day. gmm was operating until mid-january 2005. data analysis is in progress. fig. 10. map of the seafloor experiments performed with geostar-class platforms, see table vi for details. 674 paolo favali et al. the first long-term mission of the oriongeostar-3 deep-sea network started in december 2003 (see table vi and fig. 12). the deployment site lies in the southern tyrrhenian sea at more than 3300 m w.d. at the nw base of the marsili complex volcanic seamount, one of the largest seamounts of the mediterranean basin (marani et al., 2004). the network configuration for this mission includes geostar as main node and one satellite (orion node 3) in horizontal acoustic communication with geostar deployed 1 km apart. a surface buoy enables the connection with geostar via vertical acoustics and the radio/satellite link with the on-shore station located at the ingv observatory of gibilmanna (northern coast of sicily). due to a malfunctioning in the acoustic communication link with the nodes (underwater fig. 11a-f. sn-1 measurements acquired in ionian sea at over 2000 m w.d. in stand-alone mode (first mission, 2002-2003) and in real-time acquisition mode (cable connected, since end of january 2005): a) regional earthquake of 27 december 2002, not reported by land-network bulletins, showing p-, s-, and t-phase arrivals; b) 2002-2003 mt. etna eruption, seismic activity of the volcano over one hour (27 october, 2002, 2:00-3:00 a.m.), including the major event of the sequence (ml = 4.8); c) temperature measured by ctd sensor over the mission period (the mean value is around 13.74°c); d) teleseism occurred in kuril islands (mw = 7.3) and recorded by the gravity meter on 17 november, 2002; e) water current velocity components over the mission period, the n-s component, running along the sicilian coast from/to the messina strait shows the most significant values (in average 5 cm/s); f) real-time acquired waveforms of the 28 march, 2005 sumatra earthquake (mw = 8.7). a c e b d f 675 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor part), they were recovered at the end of april 2004 and re-deployed at the same site at the middle of june until the final recovery in may 2005, always using the r/v urania. examples of the collected data are shown in fig. 13a-d. parallel to sea experiments with the geostar-class platforms, sensor prototypes also had to be developed, due to the lack of reliable instruments to collect long-time data series especially in the deep-sea environment. a fluxgate magnetometer (first version bi-axial, then tri-axial) built at ingv and subsequently manufactured industrially by tecnomare (a company of eni group) has been successfully used since geostar demo mission. its resolution is 0.1 nt, the absolute accuracy 5 nt, and the power consumption reduced to 2 w. the thermal drift of the three-component magnetometer (0.2-0.5 nt/°c for typical fluxgate magnetometers) is expected to be negligible because the sea temperature is quite constant at the working depths of more than 2000 m, within a fraction of 1°c. another prototype is a gravity meter derived from a prototype built for space applications, its marine version was developed in a joint venture between ingv and ifsi-inaf, and has been successfully used since geostar-2’s first deep-sea mission in the tyrrhenian sea. the main characteristics of the gravity meter are sensitivity 10−9 g hz−1/2; frequency range 10−5 to 10−1 hz; power consumption 300 mw; volume 10 cm3; weight 2 kg (iafolla and nozzoli, 2002). the last prototype developed and tested both in the laboratory and in the deepsea (in the orion-geostar-3 project) is an automatic electrode analyser. this analyser with self-calibrating capability is capable of performing longterm (six months) experiments. the instrument was developed and validated in a joint activity between ingv and tecnomare. at present, it is equipped with a ph electrode (amt), which is the only commercially electrode for the deepsea, but it can be equipped with other electrodes. the main characteristics of ph electrode fig. 12. geostar gateway seafloor observatory (right) and orion node 3 (left) on the deck of the r/v urania before the deployment at the base of marsili underwater volcano (orion-geostar-3 first mission). 676 paolo favali et al. are in ph units: range 2 to 11; accuracy 0.05; resolution 0.01. the electrode can operate at the maximum pressure of 600 bar, and at a t range from −2 to +38°c. all these prototypes are managed by the dacs. other sensors, like a nuclear spectrometer, are undergoing development. 6. conclusions geostar, derived platforms and the orion-geostar-3 deep-sea observatory network, have been tested during long-term missions (maximum duration over 330 days). the assets of these platforms lie in the reliability of the whole system, the chance to have (near)-realtime communications, and the data quality. the chance to perform quick comparisons of unique time-referenced data series of different sensors makes the development of multiparameter data analysis quite easy. the geostar-class platforms represent a fleet of five seafloor observatories among the twenty-eight available worldwide already validated at sea (favali and beranzoli, 2006). these platforms are perfectly compatible and can be easily re-configured according to the specific applications. all these features fit the requirements outlined within the framework of specific programmes, such as the esa-eu gmes joint programme. fig. 13a-d. orion network measurements acquired at the base of the marsili seamount: a) one month of magnetometer measurements (april 2004, red line) compared with the italian land reference observatory (l’aquila) in central italy; b) ph measurements by the electrode analyser compared with the chemical analysis (stronzium) performed on the samples collected by the water sampler; c) local event of the southern tyrrhenian sea (3 march 2004, ml = 4.6) recorded by the seismometer; d) teleseismic event recorded by the gravity meter (26 december 2003, ms = 6.8, iran). a b c d 677 a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor acknowledgements first of all, we are deeply indebted to the ec that supported our activities and funded many projects within the framework of the mast and the environment programmes. the authors also wish to thank everyone who worked in the european and italian projects (a) geostar (ec), (b) geostar-2 (ec), (c) assem (ec), (d) orion-geostar-3 (ec), (e) sn-1 (gndt), (f) mabel (pnra): ingv (co-ordinator: a, b, d, e and f; partner: c): laura beranzoli, thomas braun, lili cafarella, massimo calcara, paolo casale, giuseppe d’anna, roberto d’anna, angelo de santis, domenico di mauro, manuela ditta, giuseppe etiope, paolo favali, francesco frugoni, louis gaya-pique, matteo grimaldi, cristina la fratta, nadia lo bue, luigi innocenzi, luigi magno, giuditta marinaro, sabrina mercuri (till 2003), caterina montuori, stephen monna, paolo palangio, giuseppe passafiume, giovanni romeo, stefano speciale, tiziana sgroi, giuseppe smriglio, roberto tardini, roberta tozzi. ismar-cnr (partner: a, b, d and e): fabiano gamberi, michael p. marani. tecnomare spa (partner: a, b, d, e and f; sub-contractor: c): marco berta (up to 1997), ercole boatto, gian mario bozzo (till 2000), daniele calore (till 2004), renato campaci, stefano cenedese, roman chomicz, felice da prat, flavio furlan, francesco gasparoni, mascia lazzarini, andrea marigo (till 1998), luciano pedrocchi, carmelo pennino (up to 1997), walter prendin, fabio zanon, marco zordan (till 2002). tub (partner: a, b, d, e and f): günther clauss, haiko de vries, sven hoog (till 2004), jorg kruppa, peter longerich. tfh (partner: a, b, d, e and f): hans w. gerber, wilfried langner. ifremer (co-ordinator: c; partner: a, b and d): yannick aoustin, jérôme blandin, gérard guyader, yvon le guen, david le piver, gérard loaëc, jean marvaldi, roland person, christian podeur, jean-francois rolin. lob-cnrs (partner: a and b): jean-luc fuda, claude millot, gilles rougier. sercel-underwater acoustic division, former orca instrumentation (partner: a, b and d; sub-contractor: c): gerard ayela, dominique barbot, david fellmann (till 2002), jeanmichel coudeville, michel nicot, alain priou. ifsi-inaf (partner: e; sub-contractor: b and d): emiliano fiorenza (till 2004), valerio iafolla, vadim milyukov, sergio nozzoli, matteo ravenna. ipgp (partner: b and c): pierre briole, jeanpaul montagner. ifm-geomar (partner: d): joerg bialas, ernst r. flueh. ogs (partner: e and f): renzo mosetti, marino russi. hcmr (partner: c): vasilios lykousis. university of patras (partner: c): dimitris christodoulou, george ferentinos, george papatheodorou. capsum technologie gmbh (partner: c): michel masson. ngi (partner: c): per sparrevik, james m. strout. fugro engineers (partner: c): david cathie. university of roma-3 (partner: e): andrea billi, claudio faccenna. university of catania (partner: e): stefano gresta. university of messina (partner: e): giancarlo neri. university of palermo (partner: e): dario luzio. special thanks to: claudio viezzoli and marcantonio lagalante (marine logistics); capts. emanuele gentile and vincenzo lubrano, and the crew of r/v urania, vessel owned by cnr and managed by so.pro.mar.; capt. alfio di giacomo and the crew of m/p mazzarò, vessel owned by gestione pontoni spa; capt. vincenzo primo and the crew of c/v pertinacia, owned by elettra tlc spa (giuseppe maugeri, chief of mission). the authors are very grateful to the reviewers for their comments able to greatly improve the clarity and quality of the paper. this paper is dedicated to the memory of luc floury and giuseppe smriglio, who embarked on this adventure many years ago, believing in the potential of this «new» science. 678 paolo favali et al. list of acronyms and abbreviations used in the text abel – abyssal benthic laboratory. adcp – acoustic doppler current profiler. argos – advanced research and global observation satellite (www site: http://www.cls.fr/html/argos/ welcome_en.html). assem – array of sensors for long-term seabed monitoring of geo-hazards (www site: http://www.ifremer.fr/ assem). awi – alfred-wegener-institut für polarund meeresforschung (www site: http://www.awi-bremerhaven.de). bbl – benthic boundary layer. cnr – consiglio nazionale delle ricerche (http://www.cnr.it) cnrs – centre national de la recherche scientifique (www site: http://www.cnrs.fr). ctd – conductivity, temperature and depth. c/v – cable vessel. dacs – data acquisition and control system. desibel – deep-sea intervention on future benthic laboratory (www site: http://dbs.cordis.lu/cordis-cgi/ srchidadb?caller=projadvancedsrch&srch&qf_ep_ rcn_a =27267&action=d). dgps – differential global positioning system (www site: http://chartmaker.ncd.noaa.gov/staff/dgps.htm). dp – dynamic positioning. drts – data radio transmission system. ec – european commission (www site: http://europa.eu.int/ comm). eni – ente nazionale idrocarburi (www site: http:// www.eni.it). esa – european space agency (www site: http:// www.esa.int). esonet – european seafloor observatory network (www site: http://www.abdn.ac.uk/ecosystem/esonet). eu – european union (www site: http://europa.eu.int). geostar – geophysical and oceanographic station for abyssal research (www site: http://www.ingv.it/ geostar/geost.htm) geostar-2 – geostar 2nd phase: deep-sea mission (www site: http://www.ingv.it/geostar/geost2.htm). gmm – gas monitoring module (www site: http:// www.ifremer.fr/assem/corinth/photo_gallery/photo_ gallery.htm). gmes – global monitoring for environment and security (www site: http://www.gmes.info). gndt – gruppo nazionale per la difesa dai terremoti (www site: http://gndt.ingv.it). hcmr – hellenic centre for marine research (www site: http://www.hcmr.gr). hsva – hamburgische schiffbau-versuchsanstalt gmbh (www site: http://www.hsva.de). ifm-geomar – leibniz-institut für meereswissenschaften an der universität kiel (www site: http:// www.ifm-geomar.de). ifremer – institut français de recherche pour l’exploitation de la mer (www site: http://www.ifremer.fr). ifsi-inaf – istituto di fisica dello spazio interplanetarioistituto nazionale di astrofisica (www site: http:// www.inaf.it). infn – istituto nazionale di fisica nucleare (www site: http://www.infn.it). ingv – istituto nazionale di geofisica e vulcanologia (www site: http://www.ingv.it). ipgp – institut de physique du globe de paris (www site: http://www.ipgp.jussieu.fr). ismar – istituto di scienze marine-cnr, sezione di geologia marina di bologna (www site: http:// www.bo.ismar.cnr.it). lns – laboratori nazionali del sud (www site: http:// www.lns.infn.it). lob – laboratoire d’océanologie et de biogéochemie (www site: http://www.com.univ-mrs.fr/lob). mabel – multidisciplinary antarctic benthic laboratory (www site: http://www.ingv.it/geostar/mabel.html). mast – marine science and technology (www site: http://www.cor-dis.lu/mast). mats-12 – multimodulation acoustic transmission system-12 khz (www site: http://www.sercel.fr). meu – multipurpose electronic unit. modus – mobile docker for underwater sciences mou – memorandum of understanding. m/p – moto pontoon. nemo – neutrino mediterranean observatory (www site: http://nemoweb.lns.infn). ngi – norges geotekniske institutt (www site: http:// www.ngi.no). nrc – national research council (www site: http:// www.nationalacademies.org/nrc). ogs – istituto nazionale di oceanografia e geofisica sperimentale (http://www.ogs.trieste.it). orion-geostar-3 – ocean research by integrated observation networks (http://www.ingv.it/geo-star/orion.htm). pnra – programma nazionale di ricerche in antartide (http://www.pnra.it). rov – remote operated vehicle (http://my.fit.edu/ ~swood/rov_pg2.html). r/v – research vessel. sn-1 – submarine network-1 (www site: http://www.ingv.it/geostar/ sn.htm). tfh – techniche fachhochschule (www site: http:// www.tfh-berlin.de). tub – technische universität berlin (www site: http:// www.tu-berlin.de). vhf – very high frequency. references beranzoli, l., a. de santis, g. etiope, 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(1998): broad-band seismology and noise under the ocean, rev. geophys., 36 (1), 105-142. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 1 surface faulting during the august 24, 2016, central italy earthquake (mw 6.0): preliminary results livio f. (1), michetti a.m. (1), vittori e. (2), gregory l. (4), wedmore l. (9), piccardi l. (6), tondi e. (11), roberts g. (3) and central italy earthquake working group blumetti a.m. (2), bonadeo l. (8), brunamonte f. (1), comerci v. (2), di manna p. (2), ferrario m.f. (1), faure walker j. (9), frigerio c. (1), fumanti f. (2); guerrieri l. (2), iezzi f. (3), leoni g. (2), mccaffrey k. (5), mildon z. (9), phillips r. (4), rhodes e. (7), walters r.j. (5), wilkinson m. (10). (1) università dell’insubria – dipartimento di scienza ed alta tecnologia – via valleggio 11, 22100 como (it). (2) ispra via brancati, 48 00144 roma (it). (3) birkbeck university of london malet st, london (uk) (4) university of leeds leeds ls2 9jt (uk) (5) centre for observation and modelling of earthquakes, volcanoes and tectonics (comet) durham university (uk) durham dh1 3le (uk). (6) cnr istituto di geoscienze e georisorse igg via g. la pira 4 50121 – firenze (it). (7) sheffield university western bank, sheffield s10 2tn, (uk). (8) ingv via di vigna murata, 605, 00143 roma (it). (9) institute for risk and disaster reduction, university college london, gower st, london, wc1e 6bt (10) geospatial research ltd, department of earth sciences, durham university, durham, dh1 3le, u.k. (11) università di camerino piazza cavour 19f, camerino – mc (it) abstract we present some preliminary results on the mapping of coseismically-induced ground ruptures following the aug. 24, 2016, central italy earthquake (mw 6.0). the seismogenic source, as highlighted by insar annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 2 and seismological data, ruptured across two adjacent structures: the mt. vettore and laga faults. we collected field data on ground breaks along the whole deformed area and two different scenarios of on-fault coseismic displacement arise from these observations. to the north, along the mt. vettore fault, surface faulting can be mapped quite continuously along a well-defined fault strand while such features are almost absent to the south, along the laga fault, where flysch-like marly units are present. a major lithological control affects the surface expression of faulting, resulting in a complex deformation pattern. i. introduction n august 24, 2016, an mw 6.0 earthquake hit central italy between norcia and amatrice (fig. 1), some tens of kilometers northwest of l'aquila, epicentre of a slightly larger earthquake in 2009 (mw 6.3). the epicenter of the 2016 normal faulting event was located near the village of accumoli at the border of umbria, marche and latium regions. one hour later, an mw 5.3 extensional event followed, with epicenter close to norcia. this region hosts several quaternary normal faults arranged along the axis of the apennines and capable of producing surface faulting (e.g., blumetti et al., 1993; calamita et al., 1994; blumetti, 1995; cello et al., 1997; piccardi et al., 1997; galadini and galli, 2003; roberts and michetti, 2004; mildon et al., 2016). for this reason, these are included in the ithaca database, that is, the inventory of capable faults in italy, managed by ispra (fig. 1). according to preliminary seismological, geodetic and geologic data (i.e. gruppo di lavoro ingv sul terremoto di amatrice, 2016b; ingv working group "gps geodesy", 2016), the seismogenic structure ruptured across a relay zone between two major nnw trending normal faults: the mt. vettore and laga faults. total rupture length is ca. 25-30 km and rupture width ca. 10-12 km. ii. earthquake surface faulting initial modeling of coseismic insar data suggests a complicated pattern of rupture with slip across two faults separated by a right-stepping relay zone (fig. 1). fault geometry at depth in this critical linking section is presently poorly understood. we started surveying the day after the o figure 1: on-fault ground breaks, possibly linked to surface faulting, and insar-derived vertical coseismic surface deformation: contours every 20 mm (sentinel-1; marinkovic and larsen, 2016). location of main shocks and focal mechanisms (http://cnt.rm.ingv.it/tdmt) are also shown; capable faults (after ithaca database https://goo.gl/55fwdh) are mapped for comparison. http://cnt.rm.ingv.it/tdmt https://goo.gl/55fwdh annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 3 mainshock and our observations were collected quite continuously for almost 15 days, including data on after-slip on mapped ground ruptures at one location?. in the following, we describe our collected evidence of on-fault ground rupturing along the mt. vettore and laga faults. the mt. vettore fault the western slope of mt. vettore is the morphologic expression of a nnw-sse trending sw dipping primary extensional tectonic element (total length 30 km), cutting through carbonates of the umbria-sabina mesocenozoic succession (i.e., umstz after pierantoni et al., 2013) with a total stratigraphic offset of ca. 1000 m. a major neogene thrust (i.e., the sibillini thrust), bordering the east side of the apennines, displaces the units of the usmtz over the miocene siliciclastic turbiditic deposits outcropping within the foothills and to the se of mt. vettore. compressional structures were later dissected by quaternary normal faults, mostly nnw-sse trending (e.g., the mt. vettore fault), and this is generally interpreted as soling into the sibillini thrust (e.g., pizzi & galadini, 2009; pierantoni, 2013). the south end of the fault has been coseismically reactivated in the august, 2016 earthquake. here the slope is marked by at least two major holocene bedrock fault scarps. the lower scarp runs at the base of the vettore mountain front and bounds the castelluccio basin. the upper scarp (i.e., “cordone del vettore”; e.g., pierantoni et al., 2013) runs very close to the drainage divide. we found evidence of ground ruptures on both these fault strands; nevertheless, major evidence of reactivation are reported for the upper scarp and will be described in the following. a set of ground ruptures was mapped in the southern sector of the vettore fault, along the the e slope of mt. vettoretto (fig. 2a a-a’ sector). these ruptures show normal movement (slip vector ca. n240) with a minor leftlateral component, strike ca. n150, and commonly show en-echelon right-stepping. data on recorded displacement are reported in figure 2a. ground breaks run continuously, from the road sp34 (fig. 2b) at the south end, up to the end of mt. vettoretto slope, where the ruptures bend slightly downslope, figure 2: a) map showing of the mt. vettore fault area with mapped ground breaks (in red): structural data (displacement vectors and great circles for bedrock faults) and displacement profiles are also reported google earth base image (8/9/2010); b – f) examples of ground breaks: note the typical right-stepping enechelon pattern and the post-seismic movement documented in b). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 4 for a total distance of ca. 1.7 km (fig. 2a, between b and f). other roughly north-south ground ruptures were found south of the road sp34, quite well aligned with the major rupture but less continuous and not clearly associated to a bedrock fault plane. they mainly trend n120-150, with individual rupture lengths generally shorter than some tens of meters and opening of some centimeters. the observed end-to-end length of this rupture zone is 1.2 km. vertical offsets here may exceed 20 cm, but a gravity component may well add to the possible tectonic displacement. in the uphill sectors, ground ruptures were found along the “cordone del vettore” scarp (fig 2a, i.e., between point f and the northern tip), for a length of about 2.4 km. here, we record an average strike of n134 and the slip vector is ca. n230. the fractures generally affect colluvium and soil, close to the bedrock fault plane (e.g., fig. 2e) or, less frequently, at a distance of a few meters. here, in the first days, the observed throw ranged from 2 to 25 cm, while heave reached 10 cm. the end-to-end length of this surface rupture zone is thus, at least, 4.5 km. repeated field surveys have clearly pointed out a post-seismic evolution of the rupture, whose vertical offset is continuing to increase with time (as of 1st september). the phenomenon is clearly evident where the fault crosses the road sp34 (fig. 2b). in the castelluccio basin, a short ne-sw trending ground fracture (throw ≥ 10 cm; length ca. 100 m) runs just south of the village of castelluccio (fig. 2c), possibly along a secondary fault. discontinuous cracks were also recorded, over a length of ca. 400 m, at the foot of mt. vettore (fig. 2a), along a subdued fault trace investigated by galadini and galli (2003). the laga fault the laga fault, considered the source of the oct. 7, 1639, amatrice earthquake (diss working group, 2015 io = x mcs according to the cpti15 catalogue; rovida et al. 2016), is a normal structure running at the base of the mt. gorzano range and displacing flysch and marly units (i.e. infilling of the miocene laga basin; e.g., pierantoni et al., 2013) for a stratigraphic offset of ca. 1500 m (boncio et al., 2004). our survey along the n part of the fault, between accumoli and amatrice, has not detected unambiguous evidence of surface faulting to date. nevertheless, some effects may be in some way related to the coseismic slip on the fault, as suggested by insarderived surface deformation. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 5 for instance, in the lower part of the slope between san tommaso village and the valley floor, several ground breaks were observed (fig. 3). these fractures follow the slope orientation and consist of 20 to 25 m long surface breaks, 5 to 20 cm wide, n140 n160 trending and west-dipping. the observed ground breaks are located near the head of a large landslide, suggesting that they may be tension cracks connected to the semicrescentic crown of a seismically-triggered slide. the surface ruptures affect the colluvial deposits covering the area andwe did not observe, if any, evidence of ruptured bedrock . nevertheless, the affected area corresponds to the zone where, according to insar data, significant deformation occurred (i.e., max. 15 – 18 cm of los vertical component near this area, see figure 1). we suppose that the reason why little or no ground rupture occurred is that the local clayey and arenaceous lithology in the amatrice area is less prone to reveal the expected modest surface faulting phenomena when compared to the limestone and hard compacted slope debris of mt. vettore. figure 3: a) map of the laga fault area: ground breaks are mapped in red; two known landslides are indicated (landslide crown in yellow –iffi database; https://goo.gl/be7utn ); b) and c) location are indicated figure 4: most recent and significant seismic sequence that hit central apennines during the last decades (modified after: gruppo di lavoro ingv sul terremoto di amatrice, 2016a). https://goo.gl/be7utn annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 6 iii. final remarks the assessed primary surface faulting along the mt. vettore fault is relevant to earthquake geology and paleoseismology, and therefore to seismic hazard assessment based on geological investigation. this earthquake is consistent with the observation that the threshold for surface faulting during extensional events in the apennines is between mw 5.5 and 6.0 (e.g., serva et al., 2002). however, it would be difficult to recognize such small slip events in paleoseismological trenches. for example, the paleoseismic site excavated by galadini and galli (2003) across a fault on the hangingwall of the mt. vettore fault revealed surface faulting events probably associated, according to the authors, with larger earthquakes (possibly mw ≈ 6.5-7.0). given the series of events with roughly decadal inter-event times that have hit the region between gubbio and the high sangro valley since 1984 along the apennine fault system, the 2016 central italy earthquake in some ways partially? fills the “gap” between the 1997 colfiorito and the 2009 l’aquila earthquakes. a similar “gap” may exist for the area southeast of l’aquila, between the 1984 and the 2009 earthquakes (e.g., tondi and cello, 2003). seismic gaps (sensu lato) and redistribution of coulomb stress from recent earthquakes need to be studied in more detail because they may suggest an increased probability of occurrence of an m ≥ 6 event on the laga fault between amatrice and campotosto. the overall scenario emerging from this earthquake, from macroseismic observations and earthquake-induced environmental effects (guerrieri et al., this volume) is complex. given that the seismogenic source ruptured in a relay zone between two major fault segments, with contrasting topography, lithology and buoldings vulnerability we have two different outcomes along these two segments: a) to the north, along the mt. vettore fault, evident primary and distributed surface faulting, tipically as re-activation of preexisting free-face scarps in limestone. little damage was recorded, due to the sparsely inhabited area (i.e., castelluccio plain) and to retrofitting of the buildings, performed following the sept. 19, 1979 norcia event (mw 5.9); b) to the south, little or no ground rupture, due to an evident lithological control, but extensive damage to villages with near-field site effects. the ongoing studies will hopefully shed light on this matter of great relevance for reliable seismic hazard assessment. references blumetti, a. m., dramis, f. and michetti, a.m. (1993). fault generated mountain fronts in the central apennines (central italy): geomorphological features and seismotectonic implications. earth surface processes and landforms, 18, 203-223. blumetti, a. m. 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(1995). quaternary evolution of the castelluccio di norcia basin (umbro-marchean apennines, central italy), il quaternario 8, 149– 166. diss working group (2015). database of individual seismogenic sources (diss), version 3.2.0: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas. http://diss.rm.ingv.it/diss/, istituto nazionale di geofisica e vulcanologia; doi:10.6092/ingv.it-diss3.2.0 faure walker, j., roberts, g. p., cowie, p. a., papanikolaou, i., michetti, a. m., sammonds, p., ... & phillips, r. j. (2012). relationship be-tween topography, rates of extension and mantle dynamics in the actively-extending italian apennines. earth and planetary science letters, 325, 76-84. galadini, f. and galli, p. (2003). paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults. annals of geophysics. gruppo di lavoro ingv sul terremoto di amatrice, . (2016a). primo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24 agosto 2016 (italia centrale). zenodo. doi: 10.5281/zenodo.61121 gruppo di lavoro ingv sul terremoto di amatrice. (2016b). secondo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24 agosto 2016 (italia centrale). zenodo. doi: 10.5281/zenodo.154400 ingv working group "gps geodesy (gps data and data analysis center)", . (2016). preliminary co-seismic displacements for the august 24, 2016 ml 6, amatrice (central italy) earthquake from the analysis of continuous gps stations. zenodo. doi 10.5281/zenodo.61355 marinkovic p. and larsen y. (2016). mapping and analysis of the central italy earthquake (2016) with sentinel-1 a/b interferometry. zenodo, doi 10.5281/zenodo.61133 mildon, z. k., roberts, g. p., walker, j. p. f., wedmore, l. n. and mccaffrey, k. j. (2016). active normal faulting during the 1997 seismic sequence in colfiorito, umbria: did slip propagate to the surface?. journal of structural geology, 91, 102-113. piccardi, l., sani, f., bonini, m., boccaletti, m., moratti, g. and gualtierotti, a. (1997). deformazioni quaternarie nell’appennino centro-settentrionale: evidenze ed implicazioni. il quaternario, 10 (2), 273-280. pierantoni, p., deiana, g., and galdenzi, s. (2013). stratigraphic and structural features of the sibillini mountains (umbria-marche apennines, italy). italian journal of geosciences, 132(3), 497-520. pizzi, a. and galadini, f. (2009). pre-existing cross-structures and active fault segmentation in the northern-central apennines (italy). tectonophysics, 476(1), 304-319. roberts, g. p. and michetti, a.m. (2004). spatial and temporal variations in growth rates annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7197 8 along active normal fault systems: an example from the lazio–abruzzo apennines, central italy. journal of structural geology, 26(2), 339-376. rovida a., locati m., camassi r., lolli b., gasperini p. (eds) (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. istituto nazionale di geofisica e vulcanologia. doi:http://doi.org/10.6092/ingv.itcpti15 serva, l., blumetti, a. m., guerrieri, l., michetti, a. m. (2002). the apennine intermountain basins: the result of repeated strong earthquakes over a geological time interval. bollettino della società geologica italiana, 121(1), 939-946. tondi, e. and cello, g. (2003). spatiotemporal evolution of the central apennines fault system (italy). journal of geodynamics, 36(1), 113-128. adg vol5 n02 nuzzo 321_337.pdf annals of geophysics, vol. 45, n. 2, april 2002 321 application of 3d visualization techniques in the analysis of gpr data for archaeology luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta osservatorio di fisica e chimica della terra e dell’ambiente, dipartimento di scienza dei materiali, università di lecce, italy abstract in this work, some results of a gpr survey carried out in a 10 000 m2 large archaeological site, located in lecce (italy) near to a necropolis dating from the messapian to the roman imperial age, are reported. after a preliminary survey, performed on the entire area along parallel 1 m spaced profiles using a 200 mhz and a 500 mhz antenna in single-fold continuous mode, some smaller areas were selected, where the survey was repeated decreasing the profile spacing down to 0.50 m for the lower frequency antenna and to 0.25 m for the higher one. for two selected zones (d and b) the processed data were visualized in 3d space not only by the standard time slice technique, but also by two recently proposed approaches, namely by iso-amplitude surfaces of the complex trace amplitude and by 3d projection of energy and envelope stacks. the immediacy in revealing the spatial positioning of highly reflecting bodies, such as the anomaly interpreted as an old refilled cistern in zone d, makes 3d visualization techniques very attractive in archaeological applications of gpr. their sensitivity to the signal/noise ratio is, on the other hand, highlighted by the quite poor performance in zone b, where the only reliable result provided by all the techniques was the soil/bedrock reflection, whereas none of them could effectively enhance the visibility of weak dipping reflections noted on 2d sections and probably related to fractures or bedding planes in the calcarenitic basement. the performance of the various techniques in these two different situations allowed insights into their main advantages and drawbacks to be gained. 1. introduction ground-penetrating radar (gpr) is a fast and cost-effective electromagnetic (em) method which, in favourable conditions, i.e. mainly resistive non-magnetic environments, can provide valuable information on the shallow subsurface. since it is based on the propagation and reflection of em waves, it is sensitive to variations of the em parameters in the subsoil, especially the dielectric constant and the electric conductivity (davis and annan, 1989). despite its relatively low penetration depth (especially with high-frequency antennae and in moderately conductive environments), the gpr resolution capability (also depending on frequency and soil properties), by far greater than that obtained by other geophysical methods, makes this technique suitable for high-resolution shallow studies like archaeological applications and shallow stratigraphy mapping. the increasing need for detailed 3d imaging of the shallow subsurface makes 3d gpr one mailing address: dr. luigia nuzzo, dipartimento di scienza dei materiali, università di lecce, via per arnesano, 73100 lecce, italy; e-mail: luigina.nuzzo@unile.it key words ground-penetrating radar – archaeological investigations – visualization techniques 322 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta of the most important current topics. although the significant advantages of 3d georadar surveying are well documented, especially for mapping geological features (grasmueck, 1996; grandjean and gourry, 1996; sigurdsson and overgaard, 1998), the higher horizontal and vertical resolution required in archaeological applications makes 3d gpr more expensive for large-area surveys. because of the small size of common archaeological targets, for a proper 3d acquisition a submeter line separation is generally needed, but in most cases 1 m spacing is used. subsequently data are processed by means of 1d and 2d techniques, since the crossline spacing is still too large to obtain remarkable improvements from 3d processing techniques. furthermore, the latter are very time-consuming and rarely available on common gpr processing software. so far, although widely appreciated, 3d acquisition techniques, using submeter spacing, have been applied only in test-areas of limited extensions or, using a coarser spacing, also in larger areas, but to image only large-scale archaeological features (goodman et al., 1994; malagodi et al., 1996; pipan et al., 1999; basile et al., 2000). the use of 3d visualization techniques is o f primary importance in archaeological applications in order to display complex data in an easily understandable fashion, thus improving the quality and efficiency of the archaeological interpretation. the most widespread way to display 3d radar data is in «time slice» (or depth slice) maps (conyers and goodman, 1997). horizontal slices may not be the more suitable visualization technique in the case of great subsurface complexity since, for example, false amplitude anomalies can occur when the slicing planes cross dipping or undulating reflectors. however, time slices still remain the easiest and most rapid means to provide a plan synthetical view of the anomaly pattern, especially for large areas. for small zones a more complete understanding of the subsurface can be achieved by means of various 3d data presentations, including 3d cubes, chair views and slices parallel to the axes or along arbitrary directions. two interesting visualization approaches, involving the extraction and 3d visualization of the most promising signal attributes, have been proposed in recent papers (zanzi and valle, 1999; valle et al., 2000), where they have been successfully applied in imaging three dimensional bodies in mine detection and non-destructive testing applications. in the present work, a trial of application of the two above-mentioned 3d visualization techniques, along with classical time slice representation, in archaeological prospecting has been made. the results obtained from two different situations encountered in an archaeological case history («buon pastore» lecce, italy) are presented, which allowed a comparison of the performance of the different visualization techniques, outlining the main advantages and drawbacks of each one. 2. site description the study area (fig. 1) is located in lecce (italy), about 500 m nw of «porta napoli», near the present cemetery and the norman monastery of «ss. niccolò e cataldo» (12th century). the site is just outside the north-western border of the ancient city. it has remained almost unchanged from the messapian to the modern age, as testified by the local coincidence of the quite well-preserved aragonese walls with the roman and the messapian ones. the potential archaeological interest of the site arise from the documented recovery of both messapian (4th2nd century b.c.) and roman tombs in the neighbourhood. the typology and sizes of the tombs recovered in lecce are quite variable. the burials can be classified in three main types: – «ipogei» (hypogeum type: dimensions of the order of 3 m × 1.5 m × 1.5 m). – «tombe a fossa» (grave pit type: average dimensions of 1.8 m × 0.5 m × 0.5 m; some of which covered by stone slabs). – «urne cinerarie» and «urne a cassetta in pietra» or «ciste litiche» (cinerary urn and stone chest types: dimensions of the order or smaller than 0.5 m × 0.5 m × 0.5 m). the second type seems to be the most common. also the depth of burial is highly variable from a minimum of 1 m to a maximum of about 4 m from the present ground level (giardino, 1994). as well as for the classical age, 323 application of 3d visualization techniques in the analysis of gpr data for archaeology the site is of archaeological interest also for the middle ages, since in past times it could have been belonged to the «ss. niccolò e cataldo» monastery and, potentially, remnants of structures related to the monastic property could be buried there. at present the site belongs to the university of lecce, which intends to enlarge the existing building, named «buon pastore», by adding further edifices. the risk of destruction of potential archaeological structures during the building works motivated archaeological investigations. the relatively large dimension of the area (almost 10 000 m2), together with time and cost constraints, made necessary the recourse to geophysical investigations as a faster means to ascertain the presence of relevant archaeological features and to delineate the geological fig. 1. location map of the «buon pastore» study site in lecce, italy. buon pastore ss. niccolò e cataldo porta napoli messapian and roman tombs n 324 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta stratigraphy, with particular regard to the calcarenite bedrock. since the most important targets (tombs or wall remnants) were expected to be located at the soil-bedrock boundary, its depth was an important parameter for planning archaeological tests. 3. methodology 3.1. data acquisition and preliminary analysis the interpretation of a previous four-lines seismic refraction survey (s1, ..., s4 in fig. 2) led to a simple two-layer model for the entire area: an upper soil or weathered layer (v p 500600 m/s), extending from ground level to a relatively constant depth of about 1 m (locally increasing to less than 2 m), and a deeper layer, interpreted as the calcarenite bedrock (v p 2000 m/s). taking into account the two-fold objective (stratigraphical and archaeological) and the seismic indications, a gpr survey, using a gssi sir-system2 equipped with 200 mhz and 500 mhz antennae, was performed in autumn 1999 and spring 2000. because of the variety of sizes of the expected archaeological targets, a 1 m line spacing was used for the reconnaissance survey, carried out in continuous mode on all the accessible parts of the site. apart from rare exceptions, due to the presence of trees, shrubs and other obstacles, almost all profiles were surveyed with both antennae. subsequently, on the basis of the preliminary survey indications or proximity to zones being excavated in the meantime (test 1 to 7), some areas were selected (zones a, b and c), where the crossline spacing was lowered to 0.50 m and 0.25 m for the 200 fig. 2. location map of the geophysical surveys: seismic refraction (dashed lines s1,..., s4) and gpr (continuous lines); heavy lines refer to profiles whose sections are shown in the next figures. a, b and c denote zones of detailed 3d gpr surveys. the dots mark the centres of the cmp gathers carried out for velocity estimation. tests 1 to 7 denote archaeological excavations. 325 application of 3d visualization techniques in the analysis of gpr data for archaeology mhz and 500 mhz surveys, respectively. moreover some cmp gathers (dots in fig. 2) were carried out for velocity estimations. despite some small differences, they allowed us to estimate an average velocity of 0.09 m/ns for the entire area, also confirmed by the velocity analysis made on diffraction hyperbolas in the radar sections. this average value was used for depth conversion and migration. the radar sections shown in figs. 3a-d and 4a-c, exemplify the signal behaviour in different parts of the site, probably related to different rock and soil conditions and to variable moisture content. figure 3a-d shows the sections relative to two profiles acquired in the southern part of the site (l2 and l1 in fig. 2). on the 200 mhz section relative to profile l2 (fig. 3a) at time ranging from about 25 ns to about 35 ns (1.20 m ÷ 1.60 m in depth), a reflection event, slightly undulating and sometimes interrupted by diffraction hyperbolas, is easily identifiable. because of its high amplitude, denoting a strong electromagnetic contrast, this event was interpreted as due to the soil-bedrock interface. the same reflection, on the other hand, is barely recognisable on the 500 mhz section (fig. 3b), due to the proximity of the antenna frequency to the «clutter frequency» (annan and cosway, 1994), which causes an enhancement of the soil heterogeneity diffraction effect. apart from the bedrock reflection, a very strong anomaly (as that denoted by c in fig. 3c,d) was noted on the radar sections relative to l1 and to some profiles close to it. in fig. 4a,b which refers to a profile acquired in the northern part of the site (l4 in fig. 2), the bedrock reflection appears shallower (almost flat at about 20 ns) and more continuous. moreover, the overlying soil appears more homogeneous, while at times greater than 20 ns, although disturbed by noise, a slightly dipping event can be identified, probably related to fractures or bedding planes in the calcarenite bedrock. in such more favourable conditions the bedrock reflection is quite well distinguishable also on the 500 mhz section, while the more visible diffraction hyperbolas better mark its interruptions. as can be seen from the section (fig. 4c) relative to a profile from the middle part of the site (l3 in fig. 2), probably because of more gradual changes, the bedrock reflection is less evident even on the 200 mhz antenna (and sometimes is overlain by a shallower reflection). moreover, the signal behaviour in the deeper part of the section could be due to a lower degree of bedrock integrity (fractures) in this zone. from the interpretation of all the radar sections, the estimated bedrock depth varied between 1.2 and 2.0 m in the southern part and between 0.8 and 1.2 m in the northern part of the area, in agreement with the results of the seismic survey and of the archaeological excavations. moreover, the bedrock reflection was generally characterized by marked roughness (although less or more evident in different parts of the site) and in the overlying soil layer many anomalies of various types were noted. at a first sight, both the reflector roughness and the soil anomalies could be due to natural or anthropic causes, so recourse to visualization schemes which reveal geometrical (and in particular horizontal) relationships between the anomalies appeared unavoidable to resolve this ambiguity. as a first trial, two zones of different radar signal complexity were selected to test the methodology, namely zone d (fig. 2), where a very strong anomaly of limited size was noted (fig. 3c,d), and zone b (fig. 2) characterized by weaker but more complex gpr signal (fig. 4c). 3.2. data processing and visualization radar data were previously processed using standard 1d and 2d techniques, mainly consisting of: trace editing and horizontal normalization (0.1 m inline spacing), spectral analysis and band-pass filtering, horizontal highpass filtering (background removal), gain control and 2d kirchhoff migration using the average velocity value of 0.09 m/ns. 3d kirchhoff migration was tested on the 500 mhz data from zone a, but the minor improvements obtained in comparison to 2d migration, confirmed that even in this case the spacing (0.25 m) was too large for 3d migration to be effective. 2d migration appeared the best-suited method for the largerspaced 200 mhz data. the processed data were then visualized with three different techniques whose main characteristics are briefly outlined. 326 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta fig. 3a-d. examples of radar sections from profiles relative to the southern part of the site: profile l2, 200 mhz (a) and 500 mhz (b); profile l1, 200 mhz (c) and 500 mhz (d). i denotes the soil-bedrock reflection; h a hyperbolic diffraction probably due to a ditch in the bedrock; c a strong anomaly probably due to an old refilled cistern. a b c d 327 application of 3d visualization techniques in the analysis of gpr data for archaeology time slice maps are built averaging the amplitude (or the square amplitude) of the radar signal within consecutive time windows of width t. sometimes a particular complex-trace attribute, the instantaneous amplitude or envelope (modulus of the hilbert transform), is used instead. being a measure for reflectivity strength, it helps to evidence high amplitude anomalies. previous spatial averaging is also useful to reduce small-scale heterogeneity noise. finally data are interpolated and gridded on a regular mesh (conyers and goodman, 1997). selecting the various parameters involved (basile et al., 2000) and in particular the width of the slice, t, is crucial. typically t must be of the order of the dominant period, but different widths can be used to enhance particular features. in common practice, non-overlapping time windows are fig. 4a-c. examples of radar sections from profiles relative to the northern part of the site: profile l4, 200 mhz (a) and 500 mhz (b); profile l3, 200 mhz (c). i denotes the soil-bedrock reflection. a b c 328 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta chosen, although sliding windows could be used instead, with the advantage of greater resolution but higher computational costs. a two-fold approach for visualizing 3d radar data has been proposed by zanzi and valle (1999) for automatic mine detection. in the first case, after an appropriate processing of radar data, a 3d image of the sought diffracting or reflecting object could be easily obtained by: 1. extraction of a particular complex signal attribute (trace envelope). 2. thresholding. 3. 3d contouring by means of iso-amplitude surface. whereas this was effective in the case of a laboratory experiment, the low signal-noise ratio observed in a real case induced the authors to propose an alternative approach consisting of: 1. extraction of the most promising complex signal attributes (trace energy and envelope). 2. three stacks separately performed along each coordinate axis, providing separate 2d results: stacking of the energy along the depth or z axis, in order to obtain a plan view of the high-energy suspected zones; stacking of the trace envelope along x; stacking of the envelope along y. 3. thresholding. 4. 3d rendering of the presumed target by projection in 3d space of the automatically selected thresholded data. as pointed out by the authors, in both cases the threshold calibration is a very delicate task. in the following figures two examples of application of the three visualization procedures in an archaeological context are given. in both cases the 200 mhz gpr data were used because they are less noisy than the 500 mhz ones. for every figure the viewpoint is from the south corner. 4. results 4.1. zone d (crossline spacing: 1 m) in fig. 5a,b two time slice representations, using the absolute amplitude, are shown for zone d, obtained using the same grid cell sizes, but different time intervals: 8 ns (or approximately 0.36 m depth windows) and 16 ns (about 0.72 m). a clear rectangular anomaly, 6 m long and 4 m wide, is visible from the 8 ns (0.36 m depth) to the 32 ns (1.44 m) slices. moreover, although weak and thin, a ne-sw linear anomaly is visible on 24 ns and 32 ns slices ending at the base of the main anomaly. it corresponds to the hyperbola seen on radar sections (as, for example in fig. 3a) slightly deeper than the soil-bedrock reflection. the higher mean amplitude on slice 24 ns (1.08 m) is related to the soil-bedrock reflection, whose roughness is evident on this slice. the same main lineaments are also evident in fig. 5b, but the coarser slicing in t axis could lead to a less correct depth estimation, since for the non-overlapping window choice the depth error is at least half the depth window. as expected, amongst the other parameters, the time window selection appeared the most critical. apart from minor differences, using the square amplitude or the modulus of the hilbert transform led to analogous conclusions. in fig. 6a-e the same data set is displayed with iso-amplitude surfaces using two threshold values: 50% (a) and 40% (b) of the maximum complex trace amplitude. obviously, lowering the threshold value, increases the visibility of the main anomaly and smaller objects (the linear anomaly denoted by an arrow in fig. 6b), but also heterogeneity noise. to better evidence the main characteristics, three different views, obtained by simple rotations of the 3d volume (for the 40% threshold) are displayed: plan (c), front (d) and lateral view (e). in particular, the latter figures clearly show that the small-size anomalies are located at depth corresponding to the soilbedrock boundary (about 1.2 m), while the main body has a nearly flat top at about 0.5 m and a thickness of about 1 m. following the third approach, the trace energy and envelope were extracted from the processed data, stacked along the depth (energy) and along x and y (envelope) axes, normalized to the respective maximum and visualized as the three coordinate planes of a 3d volume (fig. 7a); after a careful selection of the threshold value (fig. 7b), data were projected in 3d space to produce the final display (fig. 7c). although a relatively strong continuous reflection is visible on the stacked and thresholded volumes (between 20 329 application of 3d visualization techniques in the analysis of gpr data for archaeology and 30 ns), the backprojection procedure images only the most energetic 3d anomalies. using a different attribute for the top plane, e.g., the complex trace amplitude, no major differences were noted in this case, provided a careful selection of the threshold value is made, which appeared in both cases the most delicate parameter. 4.2. zone b (crossline spacing: 0.5 m) the same visualization techniques were employed on the zone b processed data set. in the time slice technique the lower signal/ noise ratio than in zone d is clearly apparent from the more chaotic anomaly pattern on nearly every slice (fig. 8a). a general character fig. 5a,b. zone d. time slices obtained averaging the absolute amplitude of the data within different time windows: 8 ns (a) and 16 ns (b). the strong rectangular anomaly c is probably related to an old refilled cistern; arrows denote a weak and thin linear anomaly probably due to a ditch in the bedrock. a b 330 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta is the amplitude variation between the western and the eastern corners, from the slice corresponding to the soil-bedrock boundary (16 ns) to the deepest one (48 ns). while in the fig. 6a-e. zone d. 3d visualization by means of iso-amplitude surfaces of the complex trace amplitude. perspective views (from the south) using different thresholds: 50% (a) and 40% of the maximum (b), and different views using the same threshold (40%): plan (c), front (d) and lateral view (e); the arrow denotes a linear anomaly probably due to a ditch in the bedrock. a b c d e shallower one the high-amplitude anomalies mainly concentrate on the western corner, the opposite situation occurs moving to greater depths, and a central low-amplitude zone 331 application of 3d visualization techniques in the analysis of gpr data for archaeology a b c fig. 7a-c. zone d. 3d visualization by means of a procedure consisting of: complex attribute extraction, thresholding and 3d rendering. energy stack on top and envelope stack on lateral sides (a), thresholding (b) and 3d rendering (c). separates the two areas. moreover, some localized strong anomalies are visible on different slices at the same location, probably indicating near vertical objects, although we could not rule out the possibility that the vertical distribution of energy could also be due to reverberations occurring at the top of the targets, because of local antenna/soil characteristics. the west-east trend of the amplitude anomalies with depth and the central low-amplitude zone may be more apparent using the 16 ns time window (fig. 8b), but this implies less precision in the vertical direction. an even lower threshold value (42% or 37%, as in fig. 9a,b), than that used for zone d, is needed in this case for the iso-amplitude visualization, because of the absence of highly reflecting bodies with respect to the host environment. the already outlined west-east differences are easily visible through this technique both in the perspective (b) and in the plan views (c), whereas the lateral views (d and e) better evidence the shallower location (less than 1 m) of the soil-bedrock anomalies and, to a much lesser extent, some dipping features (lines). within the third technique, by stacking the envelope along x and y coordinates, the visibility of the weak dipping features was slightly increased with respect to the host diffraction energy (fig. 10a). nevertheless, the thresholding and 3d rendering procedure inherently enhanced the contribution of threedimensional bodies in comparison to that of 2d (surfaces) or 1d (linear) features, so that the final image (fig. 10c) is quite similar to that obtained by the previous technique (fig. 9b). in this case, besides the threshold value, the technique is also rather sensitive to the complex attribute (energy or envelope) used for 332 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta the top plane (in this case the envelope was used also for the top plane instead of the energy). with respect to the results obtained in zone d, a general consideration is that, in this lessfavourable case, every technique suffered from the lower signal/noise ratio. 4.3. data interpretation in zone b the anomaly concentration on the western corner at the soil-bedrock boundary, indicating a local higher roughness of the interface, could be compared with the results of the nearest archaeological excavation (test 6 in fig. 8a,b. zone b. time slices obtained averaging the absolute amplitude of the data within different time windows: 8 ns (a) and 16 ns (b). a b 333 application of 3d visualization techniques in the analysis of gpr data for archaeology fig. 9a-e. zone b. 3d visualization by means of iso-amplitude surfaces of the complex trace amplitude. perspective views (from the south) using different thresholds: 42% (a) and 37% of the maximum (b), and different views using the same threshold (37%): plan (c), front (d) and lateral view (e); the lines denote anomaly alignments probably related to dipping fractures. a b c d e fig. 2). in fact, in test 6 the bedrock was found at a depth varying from 0.9 and 1.1 m, below two different-coloured main soil units (fig. 11). moreover its surface was characterized by a complex network of artificial cuts, mainly oriented in the ns and ew directions. their average horizontal dimensions (about 1.6 m × 0.5 m) were comparable to grave pit sizes, but the vertical one (only 0.15 ÷ 0.25 m) was lower than the average grave pit thickness. for this reason, 334 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta a b c fig. 10a-c. zone b. 3d visualization by means of a procedure consisting of: complex attribute extraction, thresholding and 3d rendering. envelope stack on top and lateral sides (a), thresholding (b) and 3d rendering (c). archaeologists interpreted them as ancient (maybe messapian) quarry works. their limited sizes (close to the vertical and lateral resolution allowed by the selected antennae), along with their closeness to each other, prevented their identification as individual anomalies in the gpr survey. on the other hand, the presence of boulders and natural bedrock undulations of almost the same sizes, could have masked the anthropical gpr anomalies. so only qualitative estimations, c o n c e r n i n g t h e i r h i g h e r o r l o w e r c o n centration in particular places, could have been derived from the gpr investigation. nevertheless, despite the low signal-noise ratio in zone b, some anomalies were noted on gpr data below the soil-bedrock reflection that could be related to particular geological features. almost all the walls of the subsequent founding pit revealed the presence of numerous various-sized colluvial deposits (most of them quasi-vertical), testifying the karstic character of the area, and some dipping fractures. in correspondence to zone b they dip south-eastward (fig. 12a,b), in agreement with the weak dipping reflections observed in gpr radar sections. although the proposed visualization techniques evidenced local alignments of anomalies following the geometric development of the fractures (figs. 9a-e and 10a-c), their performance in this zone was quite poor and strongly affected by the low signal/ noise ratio. a good correspondence was noted between the colluvial deposit location and the low-amplitude zone. the strong squared anomaly found in zone d seemed unlikely to be an hypogean tomb, both in base of gpr results and oral tradition. its shape and size, clearly evidenced by all the visualization techniques adopted, and the 335 application of 3d visualization techniques in the analysis of gpr data for archaeology connected linear anomalies, visible on particular slices using proper time windows, suggested that it could have been an old refilled cistern with connected water canals probably excavated in the rock. on the other hand, no evident grave pit anomaly was found by the analysis of the detailed gpr survey data, neither indication of grave pit presence was derived by the reconnaissance survey, although in the last case the large crossline spacing (1 m) could not completely exclude this possibility. however, no grave pit was found in any of the archaeological tests. fig. 11. photo of the excavation results from test 6: artificial cuts in the bedrock interpreted as ancient (maybe messapian) quarry works. 5. discussion and conclusions the primary importance of 3d visualization comes from the fact that it can provide a powerful and intuitive means of communicating complex information to non-geophysicists. aside from the archaeological meaning of the features imaged, the present study allowed a comparison of the performance of different visualisation techniques of gpr data in relatively complex environments. – time slice maps, used mainly to enhance the horizontal relationships between amplitude anomalies at the almost flat soil-bedrock 336 luigia nuzzo, giovanni leucci, sergio negri, maria teresa carrozzo and tatiana quarta interface, evidenced some alignments along directions consistent with those of the artificial cuts observed in the nearby excavations and allowed distinguishing zones probably more heavily quarried from other less disturbed. obviously this method failed in imaging dipping events. they are a more objective representation of the results than the other methods explored, but do not furnish an instantaneous view of the entire volume with the same immediacy. – the 3d contouring and isoanomaly plotting suffered from a certain degree of subjectivity in selecting the threshold value, but furnished very impressive pictures of 3d (and to a lesser degree also 2d or 1d) reflecting bodies, provided the data had a relatively high s/n ratio. – the alternative visualisation approach proposed, which involves a procedure of stacking, thresholding and 3d rendering of selected data attributes, has a higher degree of subjectivity (selecting the trace attribute and the threshold value), but is the most selective and effective in imaging only true 3d bodies. in the archaeological case study proposed, none of the visualisation methods was able to resolve the singular bedrock cuts due to intrinsic limits: – low thickness and width in comparison to the vertical and spatial resolution allowed by the antennae and acquisition geometry used. – relatively high depth that prevented the use of higher frequency antennae. – proximity of the archaeological target sizes to the natural bedrock undulation wavelengths and to the dimensions of the host heterogeneity. – generally weak electromagnetic contrast. in reality, 3d visualisation techniques, which enhance particular characteristics present in the data, can give very impressive images for understanding the 3d spatial relationship and, hence are very helpful for archaeological or geological interpretation. nevertheless the fundamental rule remains the same: features must be detected before fig. 12a,b. photo of the founding pit wall close to zone b revealing the presence of colluvial deposits and fractures in the calcarenite bedrock dipping to the south (a) and east (b). a b 337 application of 3d visualization techniques in the analysis of gpr data for archaeology they can be enhanced, so that particular care is needed both during the acquisition and the processing stages. acknowledgements the authors are grateful to the technicians: s. corriero, m. luggeri and d. fortuzzi for their valuable collaboration during data acquisition. they also wish to thank the archaeologists from the department of cultural heritage of the university of lecce (and in particular prof. l. giardino and prof. p. arthur) for giving them the opportunity to test the proposed methodology by correlation with the excavation results, and the referees for their valuable comments. references annan, a.p. and s.w. cosway (1994): gpr frequency selection, in proceeding of the fifth international conference on ground penetrating radar (gpr ’94), june 12-16, kitchener, ontario, canada, 747-760. basile, v., m.t. carrozzo, s. negri, l. nuzzo, t. quarta and a.v. villani (2000): a groundpenetrating radar survey for archaeological investigations in an urban area (lecce, italy), j. appl. geophys., 44, 15-32. conyers, l.b. and d. goodman (1997): groundpenetrating radar – an introduction for archaeologists (altamira press, a division of sage publications, inc.), pp. 232. davis, j.l. and a.p. annan (1989): ground-penetrating radar for high-resolution mapping of soil and rock 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(1994): per una definizione delle trasformazioni urbanistiche di un centro antico attraverso lo studio delle necropoli: il caso di lupiae, in studi di antichità 7 (congedo editore), 137-203. goodman, d., y. nishimura and k. tobita (1994): gprsim forward modeling software and time slices in ground penetrating radar surveys, in proceedings of the fifth international conference on ground penetrating radar (gpr ’94), june 12-16, kitchener, ontario, canada, 31-43. grandjean, g. and j.c. gourry (1996): gpr data processing for 3d fracture mapping in a marble quarry (thassos, greece), j. appl. geophys., 36, 19-30. grasmueck, m. (1996): 3d ground-penetrating radar applied to fracture imaging in gneiss, geophysics, 61 (4), 1050-1064. malagodi, s., l. orlando, s. piro and f. rosso (1996): location of archaeological structures using gpr method: three-dimensional data acquisition and radar signal processing, archaeol. prospect., 3, 13-23. pipan, m., l. baradello, e. forte, a. prizzon and i. finetti (1999): 2d and 3d processing and interpretation of multi-fold ground penetrating radar data: a case history from an archaeological site, j. appl. geophys., 41, 271-292. sigurdsson, t. and t. overgaard (1998): application of gpr for 3d visualization of geological and structural variation in a limestone formation, j. appl. geophys., 40, 29-36. valle, s., l. zanzi and g. lenzi (2000): 2d and 3d focusing of ground penetrating radar data for ndt, in proceedings of the 8th international conference on ground penetrating radar, may 23-26, gold coast, australia, cd-rom. zanzi, l. and s. valle (1999): elaborazione di dati gpr 3d per la ricerca di mine antiuomo, in atti del 18° convegno nazionale del gruppo nazionale di geofisica della terra solida, novembre 1999, roma, cd-rom, file 04.12. soldati 6603_revfc:layout 6 annals of geophysics, 57, 6, 2014, s0652; doi:10.4401/ag-6603 s0652 tomography of core-mantle boundary and lowermost mantle coupled by geodynamics: joint models of shear and compressional velocity gaia soldati1,*, lapo boschi2,3, steve della mora4, alessandro m. forte5 1 istituto nazionale di geofisica e vulcanologia, rome, italy 2 sorbonne universités, institut des sciences de la terre paris (istep), paris, france 3 cnrs, institut des sciences de la terre paris (istep), paris, france 4 institute of geophysics, department of earth sciences, eth zürich, switzerland 5 geotop, université du québec à montréal, canada abstract we conduct joint tomographic inversions of p and s travel time observations to obtain models of dyp and dys in the entire mantle. we adopt a recently published method which takes into account the geodynamic coupling between mantle heterogeneity and core-mantle boundary (cmb) topography by viscous flow, where sensitivity of the seismic travel times to the cmb is accounted for implicitly in the inversion (i.e. the cmb topography is not explicitly inverted for). the seismic maps of the earth's mantle and cmb topography that we derive can explain the inverted seismic data while being physically consistent with each other. the approach involved scaling p-wave velocity (more sensitive to the cmb) to density anomalies, in the assumption that mantle heterogeneity has a purely thermal origin, so that velocity and density heterogeneity are proportional to one another. on the other hand, it has sometimes been suggested that s-wave velocity might be more directly sensitive to temperature, while p heterogeneity is more strongly influenced by chemical composition. in the present study, we use only s-, and not p-velocity, to estimate density heterogeneity through linear scaling, and hence the sensitivity of core-reflected p phases to mantle structure. regardless of whether density is more closely related to por s-velocity, we think it is worthwhile to explore both scaling approaches in our efforts to explain seismic data. the similarity of the results presented in this study to those obtained by scaling p-velocity to density suggests that compositional anomaly has a limited impact on viscous flow in the deep mantle. 1. introduction our understanding of the dynamics of earth's mantle is largely based on a precise imaging of its velocity structure. combining tomographic inversion with geodynamic modelling, and using seismicallyinferred density variations as a proxy for cmb topography, soldati et al. (2012) obtained mantle yp models which are physically sound (their geodynamic coupling to cmb topography is accounted for) and which fit the seismic data (isc data sets of p-wave arrivals) at least as well as models obtained from seismic data alone. their method requires that a tomography model of mantle seismic velocity be interpreted in terms of equivalent density anomalies via a constant or radially varying scaling factor. however, density and velocity heterogeneity are proportional to one another only if mantle heterogeneity is of purely thermal origin, and no compositional heterogeneity is present. this is certainly not strictly true, and it is still questioned to what extent it is a valid approximation of the real earth (e.g. karato, 2003; deschamps and trampert, 2003; trampert et al., 2004; della mora et al., 2011), at least for the uppermost and lowermost regions of the mantle. shear and compressional velocity are in principle sensitive to both composition and temperature, but we do not know a priori the relative importance of the two effects. while soldati et al. (2012) assume a linear relationship between temperature t and yp anomalies, we investigate here the other end-member model of density/velocity scaling, constraining t and ys. our goal is simply to try and explain seismic data with a broader range of seismic/geodynamic models, and we do not claim at this point that t and density are more closely related to ys than to yp (or vice-versa). we present here a new method to conduct joint article history received june 12, 2014; accepted november 28, 2014. subject classification: geological and geophysical evidences of deep processes, mantle and core dynamics, tomography and anisotropy, inverse methods. soldati et al. 2 ( ) ( ( ), ( ), ( )) ( , ) t v r v r s s s ds k c c pcp p p path pcp b b 2d d dh u h u = +# ( ) ( ) ,c b r r dr 1 lm cmb l c a lmd t dt d = # , t t t a a a a k v v 0 0 s p pcp mantle p mantle pcp mantle s cmb pcp pcp p s $ d d d d d = j l k kk f d n p o oo p n inversions of pand s-waves, which are no longer coupled via any assumption on their scaling; they are now indirectly coupled through the cmb, which is computed by integration of the dys heterogeneity structure weighted by the geodynamic sensitivity kernels (forte and peltier, 1991). this is different from previous joint p-s tomographic models (e.g., su and dziewonski, 1997; vasco and johnson, 1998; kennett et al., 1998; saltzer et al., 2001; kennett and gorbatov, 2004; houser et al., 2008), in that the inversion is now also constrained by the expected physical coupling between mantle and cmb, and no a-priori value for dys-to-dyp scaling is prescribed. furthermore, only ys, and not yp, is attached to density structure, while yp heterogeneities are completely free parameters. the assumption that dys (rather than dyp) be proportional to density anomaly is motivated by the observation of, e.g., boschi et al. (2007, 2008) that the distribution of deep plume roots correlates much better with lowermost-mantle dys than dyp. while density/velocity scaling in the mantle remains an open question, this could be an indication that dys is more sensitive than dyp to thermal variation, while dyp is more strongly affected by chemical heterogeneity. the mantle and cmb models we find following this new hybrid approach after scaling ys velocity to density are almost coincident with the ones obtained by soldati et al. (2012) scaling yp to density; this is a strong indication that the potential presence of compositional heterogeneity in the lowermost mantle, while it may have important local effects, does not heavily affect the viscous convective flow (simmons et al., 2009). 2. method we assume a linear relationship between travel time data and seismic velocities (boschi and dziewonski, 2000), and use the lsqr method (paige and saunders, 1982) to iteratively invert for dyp and dys the following system of equations m of equations (1) (2) (3) where r = r(s), i = i(s), z = z(s) is the ray-path equation, (ib, zb) are the coordinates at which the pcp raypath is reflected off the cmb, and kpcp the sensitivity of dt to cmb undulations (defined e.g. by dziewonski and gilbert (1976)). the cmb topography dc is the other unknown function to be determined; forte and peltier (1991) show that its spherical harmonic coefficients dclm coincide with the harmonic coefficients dtlm of density perturbation modulated by the viscositydependent cmb sensitivity kernels bl(r) (4) with c and a denoting the reference, mean radii of the cmb and earth’s surface, respectively, and dtcmb the density jump across the cmb according to prem (dziewonski and anderson, 1981). the sensitivity kernels bl are computed adopting the radial viscosity profile selected by mitrovica and forte (1997) on the basis of the fit to geoid and post-glacial rebound data, neglecting the effect of lateral viscosity variations (moucha et al., 2007). replacing dc in equation (3) with its harmonic expansion 4, and expressing dyp/yp and dys/ys as a linear combination of voxels (soldati et al., 2012), the system of equations above may be summarized in the compact formula (5) where the submatrices apmantle and a p cmb represent the sensitivity of the seismic phase p (s, p, pcp) to mantle and cmb structure, respectively. taking into account the mechanical relationship between deep mantle heterogeneity and cmb deflections allows the equations for dyp and dys to be coupled via the sensitivity kernels kpcp, and the resulting velocity models to be physically consistent with each other and with the cmb topography, obtained from equation (4) after scaling dys to dt. relative density anomalies are indeed assumed to be proportional to shear-velocity ones through the constant factor dlnt/dlnys = 0.27 (average of the profile proposed by karato (1993) on the basis of mineralogical experiments). this simplistic assumption has been proven to be a good approximation of the mineral properties of the mantle through a series of experiments using the depthdependent scaling factors employed by simmons et al. (2009) and shown in their figure 3. we employ a database of ~630,000 summary rays travel times of p waves and ~63,000 of pcp waves extracted from the international seismological centre (isc) bulletin, as corrected by antolik et al. (2001), plus ( ) ( ( ), ( ), ( )) t v r vp r s s s dsp ppath 2d d h u = # ( ) ( ( ), ( ), ( )) t v r v r s s s ds p path p p 2d d h u = # 3 the ~170,000 s waves arrivals computed by houser et al. (2008) via a cross correlation technique. this is a preliminary data set that we use in a first exploration of our new methodology; a more complete data set of core-related phases will be assembled in the future. with respect to our previous study (soldati et al., 2012), we neglect here the data set of pkp travel times because they are also sensitive to possible outer core structure. the resolving power of the data employed has been discussed in detail in soldati et al. (2012) (see their figures 4 and 5). the combination of the pand s-wave data sets provides a better coverage throughout most of the mantle and allows us to get a significant increase in information. the velocity models are parametrized in terms of 15 layers (200 km-thick) of 1656 equal area voxels each (plus 1656 pixels to describe the cmb topography), measuring 5°× 5° at the equator. multi-parameter inversions may be strongly influenced by factors like the weighting associated with different data or the misfit functions. following boschi and dziewonski (1999); soldati et al. (2012), we assign to each travel time a weight (exponential function) depending on its deviation from prem predictions. we select different values for the cutoff of our subsets of data, depending on their standard deviation, while we assume the same relative weight for each data set, despite the difference in the number of phase arrivals. the lsqr linearized inversion is regularized via radial and lateral roughness damping only (e.g. boschi and dziewonski, 1999). 3. results 3.1. mantle velocity and cmb topography models we first compute dyp and dys models of the earth’s mantle and the corresponding cmb topography map via classic tomographic (t) inversions of the entire tomography of core-mantle boundary... mantle/ cmb model s (%) p (%) pcp (%) dyp (t) / 25.4 10.2 dys (t) 60.3 / / dyp, dys (tg) 61.3 25.3 10.5 table 1. variance reductions of different databases (columns) achieved by models t and tg, as indicated. note that model t consists of the results of two entirely decoupled inversions (first and second row), as in della mora et al. (2011), while model tg (third row) is an individual, joint model including both ys and yp anomalies. figure 1. maps of relative yp and ys velocity variations (%) at (top to bottom) five different depths in the mantle, obtained from the entire p, pcp, s data set. the maps are derived by jointly inverting isc and houser et al. (2008) data with a purely tomographic approach t (columns 1, 3), and with a tomographic-geodynamic approach tg (columns 2, 4). the scale for the dyp maps is ±2% at 100 km depth and ±1% elsewhere, that for the dys maps is ±4% at 100 km depth and ±3% elsewhere; blue regions denote higher than average velocity, and red regions denote lower than average velocity. data set. in this case the inverse problem 5 is totally decoupled (the geodynamic part of the matrix being null) and therefore equivalent to separate inversions of the s, p, pcp data sets. the solution model dyp, shown in figure 1 (column 1), is consistent with previously published ones (e.g. boschi and dziewonski, 1999, 2000; tkalĉić and romanowicz, 2002; tkalĉić et al., 2002; soldati et al., 2003; young et al., 2013). the same holds true for the model of mantle dys anomalies (column 3), in close agreement with the results found by kennett et al. (1998); ritsema et al. (1999); houser et al. (2008); soldati et al. (2012); auer et al. (2014). compared to the dyp solutions, dys anomalies are considerably larger and the slow structures beneath southern africa and pacific ocean emerge much more clearly; this suggests a stronger sensitivity of ys to thermal anomalies. we also show in figure 1 the results of inverting the entire data set with the tomographic-geodynamic (tg) approach described in section 2, assuming the radial viscosity profile by mitrovica and forte (1997). the dyp and dys anomaly maps so obtained (column 2, 4, respectively) are almost coincident with the corresponding t models, derived without considering the mantle-cmb coupling. table 1 shows the variance reductions of inverted data associated with our models. as a general rule, inverting different data sets jointly results in lower variance reduction with respect to inversions of a single data set. the value of pcp variance reductions are only slightly lower than those found by soldati et al. (2012) (figure 10), while the variance reduction of p data is more significantly reduced. the same is true for s-data variance reduction with respect to the recent model of auer et al. (2014), who inverted similar data; but notice that auer et al.’s (2014) model included a much higher number of free parameters, since it allows for radial anisotropy and is parameterized in terms of an adaptive-resolution grid often much denser than ours. the tg model of dyp achieves similar or higher variance reduction to the different subsets of data than that associated with the purely tomographic t one, and similarly the tg model of dys fits the s data better than the corresponding t model. this is a nontrivial result, since the number of free parameters is reduced in the tg inversion (e.g. soldati et al., 2012). that a model achieves a higher variance reduction with a lower number of free parameters is an indication that the added regularization provided by geodynamic constraints helps the solution to converge to a better model. the dyp model of figure 1 is in agreement with those found by soldati et al. (2012) using the same approach to invert solely the isc p-wave data set, and scaling dyp (instead of dys) to dt heterogeneity. we show in figure 2a our model of cmb topography, obtained directly from the t inversion, and in 2b the model we obtained stepwise from the tg inversion and integration of dys as described by soldati et al. (2012). both models are dominated by harmonic degree 2, corresponding to systematically negative cmb topography under the circumpacific ring. this is generally consistent with the observations of, e.g., morelli and dziewonski (1987); boschi and dziewonski (1999); rodgers and wahr (1993); forte et al. (1995); soldati et al. (2003, 2013). with respect to the t model, the tg one is characterized by stronger cmb depression at very high and low latitudes, and an overall smaller amplitude (with a depthto-valley amplitude of 13.3 km, vs. a value of 14.6 km for the t model). 3.2. comparison of yp and ys models we show in figure 3a the correlation between shear and compressional velocity anomalies and that between shear and bulk sound velocity (yz) anomalies for t and tg mantle models. we observe a positive correlation between dyp and dys throughout the mantle, and a corresponding decorrelation between dyz and dys, with little or no difference between t and tg inversions. all the correlation curves tend to decrease in the midmantle and at d" depth, as also found by della mora et al. (2011) inverting direct p and s waves with a classic tomographic approach and with finer radial parameterization. the simultaneous drop of correlation dyp-dys and anticorrelation dyz-dys at the base of the mantle has been already observed by several studies (e.g. su and dziewonski, 1997; kennett et al., 1998; becker and boschi, 2002; antolik soldati et al. 4 figure 2. maps of cmb topography (km) obtained from the entire p, pcp, s data set. the maps are obtained by jointly inverting isc and houser et al. (2008) data with a purely tomographic approach t (a), and with a tomographic-geodynamic approach tg (b), integrating the mantle shear velocity anomaly modulated by the sensitivity kernels as in equation (4). the tg approach assumes a laterally constant scaling between seismic velocity (ys here) and density. 5 et al., 2003; simmons et al., 2010), and may indicate a non-thermal origin for the velocity heterogeneities (karato and karki, 2001; saltzer et al., 2001; hirose, 2006). another indicator of potential compositional heterogeneity is the ratio rs,p between shear and compressional velocity anomalies, which may be computed following various approaches. figure 3b shows in red the depth dependence of rs,p computed from equation (1) of della mora et al. (2011), while blue and green curves refer to the ratio of the rms of models dys and dyp, multiplied/not multiplied by the pearson’s correlation coefficient. this coefficient acts as a weight: wherever the correlation between dyp and dys is poor, it is meaningless to assume that they are proportional, and to compute their ratio. independently from the formula applied, and from the approach used to invert the data (t/tg), rs,p is positive throughout the mantle and increases (of different amounts depending on the formula used to compute it) below 2100 km depth, with a relative maximum around 2600 km depth. this trend is consistent with previous studies (saltzer et al., 2001; tkalĉić and romanowicz, 2002; della mora et al., 2011), and suggests at least a partial influence of chemical composition on the seismic velocity heterogeneities (karato, 2003). again, the importance of compositional effects does not appear to be altered by the inclusion of geodynamic constraints (tg) in the inverse problem. 4. discussion using a large data set of p, pcp and s travel time data and the fact that cmb deflections should be gravitationally related to velocity structure of the deep mantle, we apply the method by soldati et al. (2012) to joint inversion for both compressional and shear velocity anomalies. in this approach, cmb topography is not explicitly inverted for, but required to be coupled to mapped mantle heterogeneities according to the viscous flow theory of forte and peltier (1991). this is a entirely new approach to joint inversion, in that it does not require any assumption on the scaling between yp and ys anomalies, which are instead only coupled by the cmb. this study represents a significant extension to that of soldati et al. (2012), in that no scaling was assumed between yp and density anomalies, but rather between ys and density anomalies. interestingly, despite this important difference in approach, the results of soldati tomography of core-mantle boundary... figure 3. (a) radial correlation between ys and yp heterogeneities (red) and between bulk sound velocity and ys heterogeneities (blue), for t (solid) and tg (dashed) mantle models. (b) ratio between ys and yp heterogeneities as a function of depth (red); same ratio computed via the rms of both models as a function of depth (blue); same as the latter, but multiplied by the ys-yp correlation coefficients (panel a, red lines) (green). solid and dashed lines as in panel (a). et al. (2012) are confirmed: neither the cmb topography, nor the pattern of mapped low/high velocity are strongly perturbed with respect to theirs. given the lower sensitivity of ys to compositional heterogeneity, scaling density with ys is equivalent to assume a purely thermal origin of velocity anomalies. thus, the agreement between mantle models that we obtained by scaling density with ys and yp may interpreted as an indication of the limited influence on mantle flow of compositional heterogeneity. this has been proven even at the african superplume, where, despite the large chemical component of velocity anomalies, the viscous flows appear to be driven by thermal (buoyancy) forces (e.g. forte and mitrovica, 2001; simmons et al., 2009). the robustness of this result suggests that, while considerable compositional heterogeneity exists in the lowermost mantle, a flow model controlled by temperature/density heterogeneity alone is consistent with the mapped cmb topography and pattern of low/high seismic velocity in the mantle. acknowledgements. we thank hrvoje tkal i and an anonymous reviewer for helpful reviews of this manuscript. references antolik, m., g. ekström, and a. m. dziewonski (2001), global event location with full and sparse data sets using three-dimensional models of mantle p-wave velocity, pure appl. geophysics, 158 (1-2), 291-317. antolik, m., y. j. gu, g. ekström, and a. m. dziewonski (2003), j362d28: a new joint model of compressional and shear velocity in the earth’s mantle, geophysical j. int., 153 (2), 443-466. auer, l., l. boschi, t. w. becker, t. nissen-meyer, and d. giardini (2014), savani: a variable-resolution whole-mantle model of anisotropic shear-velocity variations based on multiple datasets, j. geophys. res., 119 (4), 30063034, doi:10.1002/2013jb010773. becker, t. w., and l. boschi (2002), a comparison of tomographic and geodynamic mantle models, geochemistry geophysics geosystems, 3; doi:10.12 9/2001gc000,168. boschi, l., and a. m. dziewonski (1999), high and lowresolution images of the earth’s mantle: implications of different approaches to tomographic modeling, j. geophys. res., 104 (b11), 25,567-25,594. boschi, l., and a. m. dziewonski (2000), whole earth tomography from delay times of p, pcp, and pkp phases: lateral heterogeneities in the outer core or radial anisotropy in the mantle?, j. geophys. res., 105 (b6), 13,675-13,696. boschi, l., t. w. becker, and b. steinberger (2007), mantle plumes: dynamic models and seismic images, geochemistry geophysics geosystems, 8, q10,006, doi:10.10292007gc001733. boschi, l., t. w. becker, and b. steinberger (2008), on the statistical significance of correlations between synthetic mantle plumes and tomographic models, phy. earth. planet. int., 167 (3-4), 230-238. della mora, s., l. boschi, p. j. tackley, t. nakagawa, and d. giardini (2011), low seismic resolution cannot explain s/p velocity decorrelation in the lower mantle, geophys. res. lett.; doi:10.1029/2011gl 047559. deschamps, f., and j. trampert (2003), mantle tomography and its relation to temperature and composition, phys. earth planet. inter., 140 (4), 277-291. dziewonski, a. m., and f. gilbert (1976), effect of small, aspherical perturbations on travel times and a re-examination of corrections for ellipticity, geophys. j. roy. astron. soc., 44 (1), 7-17. dziewonski, a. m., and d. l. anderson (1981), preliminary reference earth model, phy. earth. planet. int., 25 (4), 297-356. forte, a. m., and w. l. peltier (1991), viscous-flow models of global geophysical observables 1: forward problems, j. geophys. res., 96, 20,131-20,159. forte, a. m., j. x. mitrovica, and r. l. woodward (1995), seismic-geodynamic determination of the origin of excess ellipticity of the core-mantle boundary, geophys. res. lett., 22 (9), 1013-1016. forte, a. m., and j. x. mitrovica (2001), deep-mantle high-viscosity flow and thermochemical structure inferred from seismic and geodynamic data, nature, 410 (6832), 1049-1056. hirose, k. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice s i i p e r i o d i d e l l a f a s e m a s s i m a d i t e r r e m o t i l o n t a n i maurizio giorgi l a lunga e s p e r i e n z a f a l l a n e l l o s p o g l i o q u o t i d i a n o dei s i s m o g r a m mi per 1 i n i c r p r c l a z i o n e dei dati presso la s t a z i o n e s i s m i c a s p e r i m e n t a l e di (ionia dell i s t i t u t o .nazionale di g e o f i s i c a , m i h a p e r m e s so di f a r e r i l i e v i m o l l o i n t e r e s s a n t i n e i r i g u a r d i d e l l e p a r t i c o l a r i t à (iei vari tipi di o n d e s i s m i c h e c h e si p r e s e n t a n o n e l l e r e g i s t r a z i o n i . in ijtiesla c o m u n i c a z i o n e mi p r o p o n g o di r i f e r i r e c i r c a a l c u n e oss e r v a z i o n i r i g u a r d a n t i i p e r i o d i d e l l a f a s e m a s s i m a di t e r r e m o t i lont a n i . l n o t o c h e la cosi d e l l a « lase m a s s i m a » o p r i n c i p a l e di un sism o g r a m m a è c o s t i t u i t a di o n d e s u p e r f i c i a l i di d i v e r s a s p e c i e , tra le q u a l i le più i m p o r t a n t i e più o m e n o e v i d e n t i in tutti i t e r r e m o t i n o r m a l i ( a d i p o c e n t r o non t r o p p o p r o f o n d o ) o n o le o n d e « l » ( o « q ») e le o n d e <i m ». l e p r i m e sono o n d e t r a s v e r s a l i t a n g e n z i a l i c i o è non h a n no c o m p o n e n t e v e r t i c a l e e si p r o p a g a n o lungo la s u p e r f i c i e del globo con la v e l o c i t à d e l l e o n d e di d i s t o r s i o n e . l e « ivi ». i n v e c e , si prop a g a n o , s e m p r e in s u p e r f i c i e , c o n v e l o c i t à un poco i n f e r i o r e a q u e l l a d e l l e o n d e « l » . l e m a s s i m e a m p i e z z e sono g e n e r a l m e n t e a s s o c i a l e a q u e t o g r u p po. da q u e s t o f a t t o t r a e o r i g i n e il n o m e c l i c spesso si ( ]à ad esse di " o n d e m a s s i m a l i » : v e n g o n o a n c h e c h i a m a t e « p s e u d o o n d e di ravleigh » p e r c h è , c o m e è p r e c i s a t o più s o t t o , p u r a v e n d o a l c u n e c a r a t t e r i s t i c h e d e l l e o n d e di r a v l e i g h . d i f f e r i s c o n o da q u e s t e p e r m o l t i a s p e l l i . ft1 n o l o c h e la t e o r i a di r a v l e i g h ( 1 ) . s v i l u p p a l a s u c c e s s i v a m e n t e da l a m h ( " ) , l o v e (''), g a l i t z i n ( ' ) . e i e . , p r e v e d e un t i p o di o n d e s u p e r f i c i a l i c h e h a n n o d e t e r m i n a l e c a r a t t e r i s t i c h e c h e -i a v v i c i n a n o a q u e l l e p r e s e n t a l e d a l l a p a r t e più vistosa di un s i s m o g r a m m a . i n f a t t i d a l l ' e q u a z i o n e g e n e r a l e dei m o v i m e n t i di un c o r p o s o l i d o s o t t o p o s t o a s o l l e c i t a z i o n i e l a s t i c h e : q — — = (à f 11) prtid div s + u a s [ 1 ] dis c i fkhiodi dei.i.a fase m a s s i m a [il teillie.moti i.ontani 1 1 7 ( d o v e q ò la d e n i l à del m e z z o , s lo s p o s t a m e n t o d o v u t o alla p e r t u r b a z i o n e c l a s t i c a , à c li sono le così d e l l e « c o s t a n t i di l a m é ») d i s c e n d e , o l t r e ai s i s t e m i di o n d e l o n g i t u d i n a l i e t r a s v e r s a l i , un a l t r o simonia di o n d e l'ormalo d a l l a s o v r a p p o s i z i o n e dei tipi p r e c e d e n t i . i r a d u c c n d o i n l'orma a n a l i t i c a la [ i ] . c h i a m a l e con //, r . ir le c o m p o n e n t i di s -i trova elle la i i j a m m e t t e le s egu en ti s o l u z i o n i : (ii, v, w) = (a,!!,(.) e ' '"> [2] d o v e v. j j , c , cj, i, g, p. sono c o s t a m i . s e si s o s t i t u i s c o n o le [ 2 ] n e l l e e q u a z i o n i e a r t e s i a n e p r o v e n i e n t i d a l l a | 1 ] , n e l l a i p o t e s i c l i c per i m a t e r i a l i d e l l o s t r a t o s u p e r f i c i a l e terr e s t r e il « c o e f f i c i e n t e di poisson » a, c o m e è a t t e n d i b i l e , a b b i a il valore di 0 . 2 5 . (/. : u), si a r r i v a ad u n ' e q u a z i o n e di 3 " g r a d o c l i c anim e l l e d e r a d i c i r e a l i : 3 / . " 2 4 7 . s + 5 6 7 . 3 2 = 0 [3] dove : vf * + è * v'i ( i m, l s, sono r i s p e l l i v a i n e n l e la v e l o c i t à d e l l e o n d e in q u e s t i o n e e la v e l o c i t à d e l l e o n d e t r a s v e r s a l i ) . d e l l e ire r a d i c i reali d e l l a [ 3 ] solo q u e l l a m i n o r e dell u n i t à (•/ 0 . 8 4 5 3 ) soddisfa al p r o b l e m a p e r c h è d e v e v a l e r e la c o n d i z i o n e : ( 1 x ) ( / f g " ) = <[~ [5] e s i s t e q u i n d i un o l o s i s t e m a di o n d e la cui v e l o c i t à si o t t i e n e d a l l a (''): r u = y ì ~ j\ = 0 , 9 1 9 4 f , [6] r e l a z i o n e valida n e l l e p r e s u p p o s t e c o n d i z i o n i di o m o g e n e i t à ed isotropia (hdlo i r a t o s u p e r f i c i a l e t e r r e s t r e . l" n o l o ( h e esistono a n c h e a l t r i sistemi di o n d e s u p e r f i c i a l i e m o l l i r i c e r c a t o r i ( c a l o i ( " ) , g u t e n b e r g . sczavva. s o m i g l i a m i ed a l t r i ) h a n n o m e s s o in e v i d e n z a , sia dal p u n t o di vista t e o r i c o c l i c da q u e l l o d e l l ' e s p e r i e n z a , gli a s p e t t i f o n d a m e n t a l i c h e li c a r a t t e r i z z a n o . le o n d e di l ì a v l c i g h . s e c o n d o la t e o r i a , v i b r a n o nel p i a n o p r i n c i p a l e , m e n t r e le « m » o s c i l l a n o in p i a n i diversi da q u e s t o ( ' ' ) ; i n o l tre l ' a m p i e z z a della c o m p o n e n t e v e r t i c a l e di q u e s t e u l t i m e o n d e è spesso s e n s i b i l m e n t e i n f e r i o r e ( 7 ) a l l ' a m p i e z z a d e l l a c o m p o n e n t e ori/.118 " u u i u z i o c t o n i a z o n l a l e , m e n t r e la t e o r i a di lord r a v l e i g h . in un m e z z o in cui r, 0,2.~). c o n d u c e ad un r a p p o r t o it a la p r i m a e la seconda di c i r c a i .."> = 1 , 4 6 8 ) . l v e r o elle ciò p o t r e b b e s p i e g a r s i s u p p o n e n d o una d i v e r s i t à dei coefficienti di a s s o r b i m e n t o p e r le due c o m p o n e n t i , m a è alleile v e r o c h e t a l e r a p p o r t o non si è p o l l i l o a n c o r a v e r i f i c a r e con s i c u r e z z a , alm e n o n e l l e v i c i n a n z e dell e p i c e n t r o , s o p r a l u l t o per la difficoltà di ind i v i d u a r e d e l l e o n d e n e i s i s m o g r a m m i r e l a t i v i a l l e p i c c o l e d i s t a n z e c p i e e n l r a l i ( s ) , d a t o elle a r r i v a n o s o v r a p p o s t e ad a l t r i lipi di o n d e c h e a l l e slesse d i s i a n z e c o n s e r v a n o a n c o r a a m p i e z z e c o m p a r a b i l i , se non m a g g i o r i , di ([nelle d e l l e o n d e ili q u e s t i o n e . m e n t r e le o n d e s p a z i a l i -i s m o r z a n o p i u t t o s t o r a p i d a m e n t e c la loro a m p i e z z a si r i d u c e n o t e v o l m e n t e c o n la d i s t a n z a , l e o n d e m a s s i m a l i , a causa del l o r o c a r a t t e r e s u p e r f i c i a l e , p o s s o n o p r o p a g a r s i a l l e m a g g i o r i d i s t a n z e e sono l e u l t i m e a d i s s i p a r s i ; il r a p p o r t o i r a le a m p i e z z e d e l l e o n d e m a s s i m a l i e d e l l e o n d e s p a z i a l i c r e s c e q u i n d i con l ' a u m e n t a r e d e l i a d i s i a n z a . j \ c i t e r r e m o t i v i o l e n t i di n o r m a l e p r o f o n d i t à ( 1 0 ^ '(ì k m ) possono p r o d u r s i o n d e m a s s i m a l i di a m p i e z z a e c c e z i o n a l e c l i c h a n n o la p o s s i b i l i t à di p e r c o r r e r e a n c h e più di u n a volta il g i r o del g l o b o ( w n . \\ iti, e i e . ) c o n s e r v a n d o u n a e n e r g i a sufficiente a p o t e r s i r i l e v a r e med i a n t e s t r u m e n t i di d i s c r e t a s e n s i b i l i t à . l i n i z i o del g r u p p o d e l l e « m », c o m e è n o l o , non è s e m p r e fac i l m e n t e i n d i v i d u a b i l e . e s o è più o m e n o g r a d u a l e e r a r a m e n t e b r u sco e n e l l o : c i ò è in a c c o r d o con le c o n c l u s i o n i a cui a r r i v a il g i a p p o n e s e i l .nakano ( ') c h e ha s t u d i a t o le p a r t i c o l a r i t à d e l l a p r o p a g a z i o n e di q u e s t o t i p o di o n d e pur l i m i t a n d o s i t r a t t a r e il p r o b l e m a b i d i m e n s i o n a l e . con le c o n s u e t e i p o l e s i di o m o g e n e i t à ed i s o t r o p i a del m e z z o . l ' i n i z i o di q u e s t a f a s e è s p e s s i s s i m o i n c e r t o p e r c h è -i c o n f o n d e c o n le u l t i m e o n d e d e l l a fase « l » : i p e r i o d i d e l l e o n d e di quesl ult i m a fase, n e i t e r r e m o t i r e m o l i e l o n t a n i , sono s e m p r e n o t e v o l m e n t e s u p e r i o r i a q u e l l i c l i c »i r i s c o n t r a n o n e l l a f a s e m a s s i m a l e , m a , p e r e f f e t t o d e l l a d i s p e r s i o n e , v e r s o la fine il g r u p p o stesso v i e n e ad a v e r e p e r i o d i c o n f o n d i b i l i c o n q u e l l i c h e si o s s e r v a n o agli inizi d e l l a fase (i m » e q u i n d i n e r i s u l t a una c o n f i g u r a z i o n e di s o v r a p p o s i z i o n e c h e 11011 l a s c i a i n d i v i d u a r e se n o n con e s t r e m a difficoltà il p u n t o di a l l a c c i a m e n t o d e l l e d u e f a i . 11 s i s m o g r a m m a d e l l a c o m p o n e n t e v e r t i c a l e s u i periodi della fase m a s s i m a di terremoti lontani 1 1 9 è q u e l l o elle p e s s o dà l ' i n d i c a z i o n e p i ù a t t e n d i b i l e p e r s t a b i l i r e i in i z i o . l a f a s e p r i n c i p a l e fino a l l e o n d e di c o d a elle v a n n o a t t e n u a n d o s i più o m e n o r a p i d a m e n t e a s e c o n d a d e l l a i n t e n s i t à e d e l l a n a t u r a d e l la p e r t u r b a z i o n e i n i z i a l e , si p r e s e n t a p e s s o c o m e la s u c c e s s i o n e di diverbi treni di o n d e d i s t i n t i e , t a l v o l t a , m o l t o d i s t a n z i a l i c o n a m p i e z ze m a s s i m e r e l a t i v e v i a via d e c r e s c e n t i . l ' a m p i e z z a m a s s i m a è in g e n e r e ad u n a d i s t a n z a di t e m p o dall ' i n i z i o d e l l a f a s e elle varia c o n la d i s t a n z a e p i e e n t r a l c , m a senza u n a legge c l i c possa p r e v e d e r n e l ' a n d a m e n t o . n o n è i n f r e q u e n t e il caso in i ni essa -i r i s c o n t r a a n o t e v o l e d i s t a n z a d a l l ' i n i z i o ; se n e p u ò a v e r e u n ' i d e a dai d a l i d e l l a t a b e l l a 11. 2 r i p o r t a t a più s o t t o . l a p a r t i c o l a r i t à p i ù s a l i e n t e di q u e s t o l i p o di o n d e , sulla q u a l e i n t e n d o s o f f e r m a r e i a t t e n z i o n e , è q u e l l a r i f e r e n t e i ai p e r i o d i elle più f r e q u e n t e m e n t e r i c o r r o n o n e l l e r e g i s t r a z i o n i r e l a t i v e a t e r r e m o t i n o r m a l i . con d i s i a n z e c p i c c n t r a l i c o m p r e s e t r a 3 . 0 0 0 e 2 0 . 0 0 0 k m . i p e r i o d i c l i c si osservano p e r d e t t e o n d e e n t r o t a l i d i s t a n z e var i a n o ali i n c i r c a da un m a s s i m o di 28' .'ilf e a . , r i s c o n t r a b i l e ali i n i zio o in v i c i n a n z a d e l p r i m o m a s s i m o , ad un m i n i m o di 14s <—̂ 16' d e l l e o n d e di c o d a , p a s s a n d o p i u t t o s t o g r a d u a l m e n t e a t t r a v e r s o i valori i n t e r m e d i . p . c a l o i , in 11110 i n d i o s u l l ' a n a l i s i p e r i o d a l e d e l l e o n d e sismielle ( j " ) . ila t r o v a l o elle il s i s m o g r a m m a è c o s t i t u i t o dal c o n t i n u o susseguirsi di o n d e di r i n c a l z o aventi p e r i o d o c v e l o c i t à via v i a m i n o r i . i p e r i o d i p i ù f r e q u e n t i sono q u e l l i di 2 2 ~ 24 s n e i t e r r e m o t i con d i s t a n z e c p i c c n t r a l i da 5 . 0 0 0 k m in p o i . 1 p e r i o d i c o m p r e s i tra 2 4 e 3 0 sono 1111 p ò m e n o f r e q u e n t i anzi -i p u ò d i r e <die p e r d i s t a n z e m o l l o g r a n d i , d a i 1 1 . 0 0 0 k m ai 2 0 . 0 0 0 k m , i p e r i o d i i n i z i a l i , c l i c g e n e r a l m e n t e c r e s c o n o con la d i s t a n z a , reg r e d i s c o n o e si m a n t e n g o n o i n f e r i o r i ai 24 s ( v e d i t a b e l l a 11. 2 ) . 1 p e r i o d i di 16 ~ ifi sono g e n e r a l m e n t e p r e s e n t i e si p o s s o n o o s s e r v a r e m o l l o p r i m a d e l l a fine del s i s m o g r a m m a , sia p u r e con a m piezze i n f e r i o r i a q u e l l e d e l l e o n d e di m a g g i o r p e r i o d o c l i c li p r e c e d o n o : sono i p e r i o d i c l i c p e r m a n g o n o p i ù a lungo n e l l e r e g i s t r a z i o n i . il s i s m o g r a m m a si e s l i n g u e con o n d e a v e n t i p e r i o d o c o m p r e s o fra i 1 2 ^ 1 6 s e c o n d i ; in a l t r e p a r o l e , fra q u e s l i v a l o r i sono c o m p r e s i i p e r i o d i c o r r i s p o n d e n t i a l l e m i n i m e a m p i e z z e d e t e r m i n a b i l i . l ' a s p e t t o c o m p l e s s i v o c l i c p r e s e n t a q u e s t a fase in u n s i s m o g r a m m a è d o v u t o senza d u b b i o a l l ' e f f e t t o di d i s p e r s i o n e : la d i s p e r s i o n e è n o r m a l e . c i o è la v e l o c i t à c r e s c e col p e r i o d o . l e c a u s e c l i c p r o d u c o n o la 1 2 1 ) .\i u m / m cloni.i d i s p e r s i o n e pò.—ono essere m o l t e p l i c i ( n ) : la g r a v i t à , la c u r v a t u r a d e l l a t e r r a , il p r o g r e s s i v o a u m e n t o d e l l a v e l o c i t à con la p r o f o n d i t à , le d i c o n t i n u i t à n e l l a d e n s i t à e n e l l a r i g i d i t à , e i e . i. p e r i o d i più g r a n d i , m o l t o spesso a s s o c i a t i al m a s s i m o d e l l am p i e z z a . si t r o v a n o g e n e r a l m e n t e a l l e p i ù g r a n d i d i s t a n z e e c i ò è d o v u t o a n c h e a l l ' e l l e n o di e s t i n z i o n e . g i à f . k o e s e n e r ('"') aveva c e r c a t o di (une una s p i e g a z i o n e del l a t t o c h e i p e r i o d i d e l l e o n d e i i n i e l i e o s s e r v a t e sui s i s m o g r a m m i presenta no la t e n d e n z a ad a u m e n t a r e m a n m a n o elio c r e s c e la d i s i a n z a c p i e e n l r a l e . il coefficiente di a s s o r b i m e n t o k.(a) è f u n z i o n e ili x ed è più g r a n i l e p e r i p e r i o d i più b r e v i ( ' ) ; c l e o n d e di p e r i o d o più lungo b a n n o m a g g i o r e v e l o c i t à «li q u e l l e a v e n t i m i n o r e p e r i o d o , i citctto d e l l a d i s p e r s i o n e sarà l a u t o più n o t e v o l e p e r q u a n t o più g r a n d e è la d i s t a n z a c p i e e n l r a l e ed è p r o p r i o q u e l l o c l i c si verifica g e n e r a l m e n t e sui s i s m o g r a m m i : le o n d e di m a g g i o r p e r i o d o h a n n o m a g g i o r e r i l i e v o m a n m a n o clic a l i m e n t a la d i s i a n z a c p i e e n l r a l e . b i s o g n a p u ò t e n e r p r e s e n t e alleile un a i t i t i l'atto f o n d a m e n t a l e c h e r i s u l t a d a l l e o s s e r v a z i o n i . i n t a n t o , c o m e a b b i a m o già o s s e r v a l o o p r a , i p e r i o d i m a s s i m i r i s c o n t r a t i in t e r r e m o t i a v v e n u t i a d i s t a n z e grand i s s i m e , r e g r e d i s c o n o e -i m a n t e n g o n o al d i s o t t o dei 22" ~ 21 : e >e poi -i c o n s i d e r a n o le o n d e c h e h a n n o c o m p i l i l o t r a g i t t i n o t e v o l m e n t e lunghi s t i l l a s u p e r f i c i e l e r r e s t r e . c o m e le « \v », « ^ », e i e . , fino c i o è a • o p e r a r e un g i r o e o l t r e lungo il c e r c h i o m a s s i m o p a s s a n t e per l ' e p i c e n t r o e la s t a z i o n e , si o s s e r v a n o p e r i o d i a t t o r n o ai l<> . o r a m e n t r e la d i p e r i o n e e i a s s o r b i m e n t o possono giusti f i c a i e p i e n a m e n t e i a s p e t t o della f a e m a s s i m a in un s i s m o g r a m m a , non p i e g a n o da soli il l a t t o d e l l a p r e v a l e n z a del p e r i o d o s o p r a i n d i c a t o [ielle o n d e c h e h a n n o p e r c o r s o un c a m m i n o l u n g h i s s i m o . d o b b i a m o i n v o c a r e q u a l c h e a l t r o f a t t o r e c o m e causa d e t e r m i n a n t e di q u e s t a s p e c i e di t r a s p a r e n z a p r e f e r e n z i a l e p e r d e t t o p e r i o d o . i t a l u l l e le s v a r i a l e t e o r i e c h e sono s t a l e e l a b o r a l e nei r i g u a r d i d e l l e o n d e -upeittciali. da l o r d r a v l e i g h in p o i . nessuna p r e v e d e q u o t o f a t t o . il c o m p l e s s o dei r i s u l t a t i dell e s p e r i e n z a può s p i e g a r s i q u a l i t a t i v a m e n t e c o s i : la e n e r g i a d e l l e o n d e s u p e r f i c i a l i , c h e si g e n e r a n o c i o è n e l l a c r o s t a t e r r e s t r e , ha al! o r i g i n e un p a r t i c o l a r e s p e t t r o di d i s t r i b u z i o n e c h e d i p e n d e d a l l a n a t u r a «iella scossa i n i z i a l e e d a l l e c a r a t t e r i s t i c h e d e l l o s t r a t o più e s t e r n o ( n a t u r a , s p e s s o r e , c o s t a n t i fisiche, e i e . ) . s i i l'ili unii deli. v fase m a s s i m a di tehhe.m0t1 lontani 1 2 1 q u e l l o p e l i l o , inali m a n o itile l ' e n e r g i a si p r o p a g a , va m o d i f i c a n d o s i p e i p e r d i l e d o v i n e a r i f l e s s i o n i , r i f r a z i o n i , d i s p e r s i o n e , a s s o r b i m e n t o s e l e l l i v o ( p e r p u r l i c o l a r i s t r a l i f i c a z i o n i ) , a s s o r b i m e n t o p e r ( ' i l l u s i o n e , e i e . , a s e c o n d a d e l l a d i r e z i o n e ili p r o p a g a z i o n e , p e r c h è con la d i r e / i o n e di p r o p a g a z i o n e v a r i a n o le c o s t a n t i e l a s l i c b e e le p r o p r i e t à fisiche d e l l o s t r a t o s u p e r f i c i a l e c h e n a t u r a l m e n t e è b e n l o n t a n o d a l i e s s e r e o m o g e n e o e i s o t r o p o c o m e (piasi t u l l e l e t e o r i e p r e s u p p o n g o n o , s a l v o a l c u n e c l i c t u l l ' a l p i ù -i l i m i t a n o a c o n s i d e r a r e d u e o t r e s t r a t i f i c a z i o n i o r i z z o n t a l i d i f f e r e n t i . l e v a r i a b i l i più i m p o r t a n t i c h e p o s s o n o m o d i f i c a r e la s t r u t t u r a d e l l o s p e t t r o e n e r g e t i c o s o n o : l o s p e s s o r e e s t e r n o ( p e r e s . . l o p e o r c a u m e n t a s o t t o l e g r a n d i c a l c i l e di m o n t a g n e e s o t t o gli o c e a n i è i n i n o i e c h e s o l t o i c o n t i n e n t i ) e le e t e r o g e n e i t à i n c o n t r a t e o r i z z o n t a l m e n t e c h e v a r i a n o n e l l a m a n i e r a p i ù i m p r e v e d i b i l e ( c i ò s p i e g a la g r a n d e d i v e r s i t à c h e p r e s e n t a n o i s i s m o g r a m m i r e l a t i v i a t e r r e m o t i p r o v e n i e n t i d a d i v e r s e d i r e z i o n i a n c h e p e r u n a slessa p r o f o n d i t à i p o c e n l r a l e e p r e s s o c h é i d e n t i c a n a t u r a d e l l a scossa o r i g i n e , s p e c i a l m e n t e a l l e p i e cole. d i s i a n z e c p i c c n t r a l i ) . 1 p i c c o l i p e r i o d i s o n o q u e l l i c l i c m a g g i o r m e n t e r i s e n t o n o d e l l e e t e r o g e n e i t à i n t e r e s s a n t i z o n e di l i m i t a l a e s t e n s i o n e d i s s e m i n a t e l u n go il t r a g i t t o d e l l a p e r t u r b a z i o n e e q u i n d i v e n g o n o r a p i d a m e n t e e s t i n t i . n e l l a t a b e l l a c h e s e g u e ( ') si p u ò c o n s t a t a r e c o m e v a r i a i n m a n i e r a a c c e n l u a l i s s i m a il c o e f f i c i e n t e di a s s o r b i m e n t o g l o b a l e p a s s a n d o d a i pei i o d i b r e v i ai p i ù l u n g h i . si p u ò v e d e r e , p e r e s . . c o m e il c o e f ficiente di a s s o r b i m e n t o g l o b a l e p e r un p e r i o d o (li 4 è p i ù d i m i l l e v o l l e m a g g i o r e di q u e l l o c o r r i p o n d e n t e ad un p e r i o d o di 2 0 " . t a b e l l a n. 1 . i in see. 5 (> 8 10 12 15 20 k 155. ir.-" 50.1c-:1 20,6.10-" 5,58.10-2,0.10-1,02.1c-0,11.10-0,14.10-i n k m 1 l e m o d i f i c a z i o n i d e l l o s p e t t r o d e l l ' e n e r g i a s o n o q u i n d i tali c l i c , v e n e n d o ad e s s e r e m a g g i o r m e n t e a s s o r b i t e le v i b r a z i o n i di b r e v e l u n g h e z z a d ' o n d a , le o n d e di m a g g i o r p e r i o d o p o s s o n o p r o p a g a r s i a l l e m a g g i o r i d i l a n z c e r i s u l t a così c l i c l o s p e t t r o d e l l a f a s e m a s s i m a , c o m e si p r e s e n t a in un s i s m o g r a m m a di un l e r r e m o t o l o n t a n o ( a = 5 0 0 0 ~ 2 0 . 0 0 0 k m ) finisce p e r c o n t e n e r e p e r i o d i v a r i a b i l i d a i 3 0 s ai 1 6 " ; i n f a t t i . 1 2 2 m a u r i z i o giorgi più g r a n d e d i v i e n e la d i s i a n z a , p i ù s e m p l i c e è l ' a s p e t t o di q u e s t a p a r t e del s i s m o g r a m m a . in un t r a g i t t o c l i c c o m p r e n d a m a g g i o r i d i s t a n z e p e r c o r s e da q u e l l o l i p o di o n d e , le e t e r o g e n e i t à e la v a r i a b i l i t à degli s p e s s o r i d e l l a s t r a t i f i c a z i o n e p i ù e s t e r n a '(ino tali c l i c s o l t a n t o le o n d e con p e r i o d i a t t o r n o ai 16' r i e s c o n o a r e s i s t e r e a l l ' u s u r a e c o s t i t u i s c o n o l ' u l t i m o r e s i d u o d e l l a f i l t r a z i o n e . p e r c o n f e r m a r e q u a n t o sopì a è s t a t o i l l u s t r a t o e p e r d a r e u n ' i d e a d e l l a d i s t r i b u z i o n e d e i p e r i o d i d e l l e o n d e « m » al v a r i a r e d e l l a <lil a n z a e p i e e n t r a i e . ito c r e d u l o o p p o r t u n o o f f r i r e c o m e e s e m p i o p r o b a tivo, i d a t i d e l l e o s s e r v a z i o n i n l ' e r e n l i s i ad un q u a l s i a s i p e r i o d o di t e m p o . i r i s u l t a t i clic si sono o t t e n u t i -0110 in o t t i m o a c c o r d o con q u e l l i trovati da c a i n i . g u t e n b e r g , m a c e l w a n c . y i s s e r , w e g e n c r ed a l t r i . t a b e l l a n . 2 . d a t a £ s c o o r d i n a t e e p i c c n + r a l i h a u t o r i t à ' a m j m m h _ r r u t r a in z w w 1 1 t s ì! m z m d a t a £ s a h a u t o r i t à ' a m j m m h _ r r u t r a in z w w 1 1 t s ì! m z m d a t a £ s a h m s a u t o r i t à ' km h m 5 mc m m h m s 5cc. h rn s òtc ì! m z m 1 5 gc-naiol946 167° e 16° s 19 57ì u.s.c 6 5. 16 250 21 15 20 26 7 o 7 7 21 « 2 5 21 54 45 . , 2 12 „ .. 14 7°5 w 59.0 n 20 25.7 8 600 21 08 05 1 9 •12 3 45 5 02 10 18: 5 20 •• .. 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' 15 23 .. >u m0° e 51° 5 — 15.950 0 6 13 25,~ 06 17 58 2 2 06 51 0 0 il 16 3 maggio » " u 159°5 e 27.% 5 22 00.! b c i s 16 6 0 0 23 co 8 4 1 82 2 23 38 50 26.23 00 18 14 7 17 8 9 8 ° e 1° s 0 5 202 u s c ù s 9 700 06 11 25 71 •> 51 2 4 8 . 6 j 18 ,. ,. „ > u 143° e 2° s 09 456 b.c i . s . 13 300 10 51 56 2 3 6 5 . 3 66 2 11 0 2 38 19 1 ) 58 00 1 6 10 7 19 15 ., .. 9 6 ° w • 6 ° n 22 10.s 1 5 a 10 3 0 0 22 5 9 0 0 4 3 4 51 5 25 0 8 20 9 3 i i 20 2 g i u g n o 121° e 2 3 ° n 01 09.1 b.c 1 s 9 750 01 5 8 06 4 9 . 0 4 8 . 8 0 2 0 7 0 9 169 0 li 21 6 10° w 7 ^ 5 10 39 10 u r 5 . 5 . 5 950 ii 07 28 16 s 2 8 . 3 30 4 14 2s jj 22 7 " u 94°6 w 16?7 n 04 13 18 j . 5 . a 10.100 05 00 50 2 2 47 1 5 0 6 od 05 26 19 12 .. „ ' u i44°3 e 12?j n 16 0 8 27 b. c 1 5 12 250 17 09 06 6 0 6 6 1 . 0 16*50 2 0 ló 28 00 1 6 7 7 ' 24 15 „ „ 129° e 3° 5 19 29. i 12 300 20 >1 00 26.6 1 . 7 6 1 . 3 20 46 24 2 0 21 47 00 18. 1-1.4 ji 1 25 20 .. „ i r 6 6 ° e 2 9 ° n 00 34 55 5.000 01 01 0 0 16 • 2 6 1 01 10 30 : 9.5 "26 26 „ ,. ' u i7i°5 e 43°; s 12 34u.s.c s s. 18 300 14 05 4 0 22 91.0 9 0 . 6 ;4 12 55 2 0 14 21 30 17 6 9 27 28 „ „ " 07 12» " " 0 8 4 4 30 2291.7 03 55 10 1909 02 00 16. 10.7 i il s i i periodi 111:1.1. \ l'asi: m a s s i m a 111 terremoti lontani 1 2 . 3 f ' s l a t o p r e s o il p e r i o d o da! i g e n n a i o al 3 0 g i u g n o 1 9 4 6 , l e n e n d o presenti sol l a u t o i t e r r e m o t i l o n t a n i le r u i regi-t r a z i o n i f o s s e r o a b b a stanza c i l i a r e da p o l e n t e t r a r r e dati s i c u r i . q u e l l i s o p r a r i p o r t a t i -i r i f e r i s c o n o a l l e m i s u r e f a t t e sulle r e g i s t r a z i o n i d e l l a -ola c o m p o n e n t e v e l l i c a l e di un s i s m o g r a f o l i p o w i l i p g a l i l z i n a r e g i s t r a z i o n e l'olografica d e l l a s t a z i o n e s p e r i m e n t a l e di r o m a d e l l ' i s t i t u t o n a z i o n a l e di g c o f i s i c a . nella t a b e l l a 11. 2 v e n g o n o r i p o r t a t i i d a t i d e l l e osservazioni r i g u a r danti i t e m p i di a r r i v o e i p e r i o d i d e l l e o n d e « m » e « w n » ed i n o l t r e i t e m p i ed i p e r i o d i r e l a t i v i al m a s i m o d e l l ' a m p i e z z a . l e c o o r d i n a t e g e o g r a f i c h e e t e m p o r a l i d e l l e scosse sono s t a t e p r e s e dai b o l l e t t i n i s i s m i c i n a z i o n a l i e s t r a n i e r i e ad essi si fa r i f e r i m e n t o nella t a b e l l a . le d i s t a n z e c p i c c n t r a l i sono state c o m p u t a l e su una c a r t a d e l l e l i n e e e q u i d i s t a n t i ed i s o a z i m u t a l i a v e n t e c e n t r o in r o m a ( ' " ) e c i ò e sufficiente ai fini d e l p r e s e n t e studio. d a l l a t a b e l l a i n t a n t o a p p a r e s u b i t o c o n f e r m a t o il d i v e r s o valored e l i a v e l o c i t à di p r o p a g a z i o n e d e l l e o n d e « m » r e l a t i v a m e n t e ai tragitti s u b a t l a n t i c i e c o n t i n e n t a l i : la v e l o c i t à nei p r i m i è m a g g i o r e c l i c n e i s e c o n d i : q u e s t o f a l l o è l a t o già osservato da insigni s i s m o l o g i a ( g u t e n b e r g , c a l o i ( i n 1 e i e . ) . dai dati d e l l a t a b e l l a a p p a r e in m a n i e r a e v i d e n t e una d i v e r s i t à nei t e m p i di t r a g i t t o r e l a t i v i a t e r r e m o t i c o n e p i c e n t r o noli e m i s f e r o o c c i d e n t a l e r i s p e t t o al m e r i d i a n o di r o m a e n e l l ' e m i s f e r o o p p o s t o . nella t a b e l l a si è posto, a fianco del t e m p o o s s e r v a t o , q u e l l o c o r r i s p o n d e n t e al l a d i s t a n z a d a t a , l e t t o s u l l e t a b e l l e del m a c e l w a n e . si n o t a così c l i c le « m » sono in a n t i c i p o r i s p e t t o ai t e m p i d e l l e t a b e l l e p r e d e l l e q u a n d o si r i f e r i s c o n o a t e r r e m o t i avvenni i ad o c c i d e n t e della l a z i o n e . q u a n t o ali i n t e r v a l l o di i e m p o tra i i n i z i o d e l l a fase ed il m a s i m o d e l l ' a m p i e z z a non -i può t r a r r e a l c u n f a l l o n o t e v o l e e n o n v ' ò a l c u n i n d i z i o c l i c lasci i n t r a v e d e r e una q u a l s i a s i d i p e n d e n z a c o n la d i s t a n z a (•piceni l'ale. nel d i a g r a m m a c h e segue sono stati r i p o r t a l i i p e r i o d i in c o r r i s p o n d e n z a d e l l e d i s i a n z e e p i c c n t 1 ali e -0110 stati c o n t r a s s e g n a l i con i m b o l i d i v e r s i i p e r i o d i i n i z i a l i r e l a t i v i a l l e o n d e » m » di t e r r e m o t i aventi e p i c c n i r o ad f l e ad ovest di r o m a . a n a l o g a m e n t e è l a l o f a l l o per i p e r i o d i a s s o c i a l i al l a m a s s i m a a m p i e z z a . s i vede c h i a r o l ' e f f e t t o di d i s p e r s i o n e e a s s o r b i m e n t o : il p e r i o d o i n i z i a l e e dei m a s s i m i c r e s c e con la d i s t a n z a c p i c e n l r a l e . i dilli n e l l ' i n s i e m e n o n p e n n e l l e r e b b e r o di c a l c o l a r e una q u a l siasi legge di v a r i a z i o n e dei p e r i o d i i n i z i a l i con la d i s i a n z a , o n d e verificare le f o r m u l e d e t t a l e da a l t r i r i c e r c a t o r i ( s e w a z a . g a l o i . g u t e n b e r g , e i e . ) ; p e r ò la d i s t i n z i o n e m e d i a n t e diversa p u n t e g g i a t u r a , i n c i l e in evidenza un f a l l o m a i finora o s s e r v a t o n e l l a vasta l e t t e r a t u r a s c i e n t i fica d e l l e o n d e s u p e r f i c i a l i , e c l i c s a r e b b e d ' a l t r a p a r i e o p p o r t u n o sott o p o r r e a r i c e r c h e più r i g o r o s e -u s t a t i s t i c h e più n u m e r o s e : i p e r i o d i i n i z i a l i d e l l e « ìy1 » di t e r r e m o t i con e p i c e n t r o ad lisi di r o m a s o n o , g e n e r a l m e n t e , s e n s i b i l m e n t e m i n o r i , a p a r i t à di d i s t a n z a , dei c o r r i r p o n d c n l i p e r i o d i di t e r r e m o t i a v e n t i o r i g i n e n e l l ' e m i s f e r o o p p o s t o . la d i s t r i b u z i o n e dei p e r i o d i a s s o c i a t i a l l e m a s s i m e a m p i e z z e non ì t a l e da p e r m e i t e l e di f a r e a n a l o g a d i s t i n z i o n e ; e c i ò è s p i e g a b i l e in q u a n t o i p e r i o d i i n i z i a l i d i p e n d o n o d a l l ' e l l ' e l i o di d i s p e r s i o n e e a=sorbii<icnlo c o m b i n a l i , m e n t r e i p e r i o d i a s s o c i a l i a l l e m a s s i m e am piezze d i p e n d o n o d a l l a n a t u r a della scossa e del m e z z o in cui si ,'• g e n e r a l a . 1 p e r i o d i osservali p e r le « ;> c o n f e r m a n o ( p i a n t o s o p r a è s i a l o d e l l o . essi r e s t a n o c o m p r e s i f r a i 1-1 e 18\ 121 maurizio gloltci t tnsec. '26 9 11 1 3 1 5 1 7 1 9 2 1 2 5 a i n m i g l i a i a di k m . s i i neuroni h i j . i . a f a s i ; m a s s i m a ni t e i i i i k m o t i i . o n t a n i riassunto do/io niellilo considerazioni sulle /trincipilli caratteristiche delle onde massimali, con speciale riguardo ai periodi che si riscontrano sulle regi si razioni di terremoti lontani, si dà una spiegazione ipialitativa del fallo che i periodi delle onde che hanno percorso tragitti lunghissimi, come, per es., le « i! ^ ». si aggirano attorno a! valore di ](>•'. i engono quindi riportati i dati delle osservazioni riferentisi al pi imo semestre dell'anno 1916. le risultanze di delle osservazioni costituiscono una ullerìoic conferma di quelle a cui sono pervennii. in stilili analoghi, emiliani si smologhi. i iene successivamente messa in evidenza una sensibile differenza, a parità di disianza epicentrale. nei periodi iniziali della fase massima relativamente a terremoti con epicentro ad est di roma e nell emisfero opposto. j ì i r l i o g r a f i a ! 1 1 r a y i i : m ; h . i.nril lstrilli. j . w . l , oli naves propngntcd aloni: the illune surface ii/ un elusile solili. p r o r . malli. s u r . l o n d o n 1 7 : h i < 1835). l~l i .a ai il i i . . ()n lite propalinoli of tremors over lite solfaci' of un elusile solili. p l i i l . t r a n s . r o y . s o r . l o n d o n i \ i 2 0 3 : 1-12 (1904'). ( ; t l i . o m : . some probleins of geodynaniics. c a m b r i d g e l niversily p r e s s , l ' i g a i . i t / i n h . . i orlesungen iibcr seistnonielrie. i ) c a i . n i p . . sopra alcuni nuovi sistemi iìì mule sismiche a dirimere supi'ificiale oscillanti nel pianti principale. r e m i . r . are. d ' i t a l i a . ci. s e . i'i>. m a l . e nat. s. \ 11 \. ii puh. 1111 p w n o c u i i a g . . sluiiio sulla fuse massima ili un lerremoto lontano. r i e . s r i e n l . 12. 595-613 il<11 l i . i 7 i i 11 r 1.. don. empiricid investigatimi of surface icares generateli hy disumi ciirlhipiah'es. l'ulti. dominion ohserraiory. otlawa 1931. ( s ) glolu.i m.-\ al i.]-; p . e . , c.ontribulo allo studio delle onde « 1/ ... ann. ili g e . i l i i r a . 1. 19181. i ' 1 ) y a k a n o i i . . on raylcigh tiare.s. j a p . j o u r n . of a l r . and g e o p l i . 5. 233326 119251. i j " i c a i . o i p._ inalisi periodale delie onde sismiche e problemi, mi i ssa connessi. r i e . s e i e n i . a. \ n. 1 119391. ( l ' i s t o n k i . k y r . . dispersimi ni seismie ienres. m . n . r a . s . geophy>. s n p p l . voi. i . 11925). i ' i r o i : s i : m : h i ' . . ì'ergleicliende t nlersiieliungen iibcr die periodai der erdb " benwcllen inil hesonderer lieriicksichtigiing iler \ncliliiitfcnccllcn. g e r l a n d s lìeilriige z. g e o p l i y s i k 12. 2117-276 119121. ( j ; 1 i cai ni i ' . . onde sismiche superficiali e loro assorbimento da parie del. mezzo. 1 nivorso, a. x x i i i . n. 7 ( 1 9 1 2 ) . ( l r) l.o s i uno i.a registrazione e lo studio dei fenomeni sismici nell'istituto nazionale di geofisien del c.is.r. r i e . s c i e n t . a. xi n. 10 ( 1 9 1 0 ) . vol49_2_2006 715 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words adcp sensitivity – current lines inclination – observatory’s wake 1. introduction adcps have now proven their capabilities to sample phenomena ranging from surface waves, to small-scale internal waves, to mesoscale eddies and to general circulation. however, adcps are generally operated in current fields that can be assumed as homogeneous at a given level and at beam-spacing scale; hence, they generally work at best, which does not reveal their utmost performances. as shown hereafter, less favourable conditions, for instance when a relatively huge structure prevents from having a homogeneous mean current field around it, reveal unexpected aspects of their performances. the geostar group has developed a prototype observatory aimed at being deployed with a carrier as deep as ∼6000 m mainly to study geophysical phenomena (beranzoli et al., 2000, fig. 1). the observatory is a three-ton, semi-cubic (2.5×2.5×1.0 m3) structure on which is set, among other oceanographical and geophysical instruments (ctd, seismometer, gravimeter, etc.), a 300-khz adcp (workhorse sentinel from rd instruments) whose beams (20° angle) are unobstructed by the docking/undocking tetrahedral armature on top of the observatory. this relatively short-range (∼150 m) adcp was set instead of the long-range (∼500 m, 75 khz) initially planned because of funding cuts, even though the observatory is obviously too massive to allow the accurate study of near-bottom dyanalysis of adcp data above a bottom observatory jean-luc fuda (1), claude millot (1), sven hoog (2) and hans w. gerber (3) (1) laboratoire d’océanographie et de biogéochimie (lob), la seyne-sur-mer, france (2) institute of naval architecture and ocean engineering, technical university of berlin, germany (3) tfh berlin – university of applied sciences, berlin, germany abstract a 300-khz adcp was set on geostar, a six-m3 deep-sea observatory. it was operated with cells of 80 cm during a three-week test experiment at 42-m water depth in the northern adriatic sub-basin. although it provided valuable data on the horizontal current field over most of the water column, it also specified the wake disturbances induced by the observatory. these disturbances are characterised by vertical velocities that are significant up to ~ 20 m above seafloor (echo intensity data suggest that the wake can even reach the surface), and by inclinations of the bottom nepheloïd layer (as deduced from differences in echo intensities from beam to beam). our analysis is validated by consistent relationships between the horizontal current direction and speed on one side and the characteristics of both dynamic (vertical velocity) and non-dynamic (echo intensity) parameters on the other. it is in good agreement with the simulations from a numerical model, and hence specifies the sensitivity (especially with respect to echo intensity) and accuracy of an instrument usually operated within fields of current and scatterers not disturbed by the device supporting it. in addition, the error velocity parameter displays specific characteristics that easily specify the thickness of the layer disturbed by the observatory, thus providing a technique to validate the quality of data acquired in similar conditions. mailing adress: dr. claude millot, laboratoire d’océanographie et de biogéochimie (lob), bp 330, f-83507 la seyne-sur-mer, france; e-mail: cmillot@ifremer.fr 716 jean-luc fuda, claude millot, sven hoog and hans w. gerber namic phenomena. although wake disturbances induced on the near current field by such an observatory were anticipated, we did not expect the adcp to be able to specify them and only expected to get data noisier than usual. during a three-week test, from august 13 to september 1, 1998, at a depth of ∼ 42 m in the northern adriatic sub-basin, eastern basin of the mediterranean sea, the adcp was checked for oceanographic purposes (since we did not imagfig. 1. the geostar observatory during deployment. the conical upper part, the base of which is protected by a black rubber fender, is the mobile docker that is removed when the observatory is set on the bottom; it masks the upper part of the docking/undocking tetrahedral armature. then, the docker is used only for recovery, hence as a female, to clamp the observatory via a pin system set on top of the four inclined tubes. the two vertical arms terminated by yellow spheres that contain magnetometers will be lowered on the bottom, i.e. away from the observatory. during the experiment, the adcp is thus nearly on top of the observatory. also shown is the deployment location. 717 analysis of adcp data above a bottom observatory ine being able to specify the wake disturbance). hence, it was set to record earth co-ordinates velocities calculated from 200 pings evenly transmitted during 10 min every hour (were it operated to specify the wake disturbances around the observatory, it would have been set to record single-ping values). figure 2a specifies the beams numbers and the orientation of the adcp with respect to the observatory and to magnetic north; the adcp axis was roughly vertical (pitch and roll ∼ 0.2°). profiles were set to 50 cells 80 cm thick. due to the adcp location and to the blank zone, cell #1 was ∼ 4.4 m above seafloor (asf). due to the secondary lobe reflection on the sea surface, data are available up to a depth of ∼ 5 m (cell #42). figure 2b provides a sketch of the general experimental conditions. the overall circulation in the study area is usually thought to be southwards along the largescale isobaths (orlic et al., 1992). the circulation encountered during the experiment in the deep layer below the seasonal pycnocline, as illustrated by the progressive vector diagrams from cells #1-30 (fig. 3), was thus unexpected and rather complex. diagrams from cells #31-42 are a b fig. 2a,b. a) actual orientation of the adcp with respect to the observatory and magnetic north. b) beams configuration and tens of cell numbers as seen when looking towards north; the upper and lower blank zones are delimited by dashed lines; the arrows represent an homogeneous current flowing towards east while the dotted lines schematise the current lines close to the bottom, and thus the structure (in both shape and concentration of suspended particles) of the nepheloïd layer as disturbed by the observatory. fig. 3. progressive vector diagrams from cells #130 plotted with respect to north (upwards) and with a 100-km scale. 718 jean-luc fuda, claude millot, sven hoog and hans w. gerber markedly different from long-term ones too, and from the deeper ones since they were always in the surface mixed layer; hence, they are not plotted for clarity. most of the variability shown by fig. 3 comes from tidal and inertial oscillations that induced relatively intense currents towards nearly all directions. as indicated by vertical profiles from a shiphandled transmissometer operated at the beginning and end of the experiment and the time series from a transmissometer mounted on the observatory, a bottom nepheloïd layer several metres thick was present in permanence during the experiment. however, for sake of simplicity, we just consider hereafter that the concentration in suspended sediments decreases upwards. as schematised in fig. 2b, this layer is obviously uplifted by the station; just above the adcp, one can thus easily expect that the layer will generally be inclined, so that echo intensity must differ from beam to beam in a way related to the current direction and speed. these expected characteristics of dynamic (vertical velocity) and non-dynamic (echo intensity) parameters will be analysed thoroughly. in addition, we have analysed the error velocity parameter that represents the heterogeneity of both horizontal currents and the sums of beam-paired vertical velocities. we show that the error velocity distribution displays characteristics that allow specifying the depth range over which the current field is made significantly heterogeneous by the observatory or by a similar bottom irregularity. we thus provide criteria to check the current homogeneity, hence to validate bottom adcp measurements. to understand the data analysis in an easier way, the current field around the observatory simulated with a numerical model is described first (section 2). adcp characteristics and the speed data screening steps are presented in section 3. the dynamic and non-dynamic data as well as the error velocity parameter are analysed in sections 4 and 5, respectively. 2. the numerical current field around the observatory numerical simulations of the current field around the observatory were performed using the commercial cfd (computational fluid dyfig. 4a. 2d current in a plane through the centre of the observatory with a far-field speed of 300 mm/s along a diagonal. note that current is coming from the left. 719 analysis of adcp data above a bottom observatory namics) software fluent (2000). the effect of turbulence was considered by means of the rng (re-normalization-group) code for a κ-ε model derived from the instantaneous navierstokes equations. the analytical derivation results in a model with constants different from those in the standard κ-ε model, and additional terms and functions in the transport equations for κ and ε. the near-wall layer was approximated by a standard logarithmic function approach. the observatory was schematised by a block of 2.5×2.5×1.0 m3 and by the 4 booms of the docking/undocking frame on top, which gives a total height of ∼ 2.7 m asf. several sets of experiments were performed: two values of the mean speed (100 and 300 mm/s), two configurations for the observatory orientation (mean current parallel to a side or to a diagonal) and two domains (30 m/x – direction of the mean current –, 20 m/y and 20 m/z; 45 m/x, 20 m/y and 40 m/z) were investigated. parameters were adjusted (i.e. seafloor roughness down to 0) to obtain vertical velocity values as large as possible (i.e. more similar to the observed ones). although the simulated vertical velocities were still lower than the observed ones, they reached the surface, consistently with the measured echo intensity data. the results illustrated in fig. 4a,b represent the smaller domain (to provide more visible details of the current field around the observatory) and a mean current of 300 mm/s parallel to a diagonal. although the adcp is then located at ±∼ 1 m from the observatory centre in both x and y, simulations are considered in a vertical plane through the centre of the observatory. although the current vectors at some distance from the observatory do not seem in fig. 4a to be strongly modified (due to the large difference between the vertical and horizontal components), significant vertical velocities are in fact induced over the whole water depth (fig. 4b). maximum vertical velocity values that can be compared with those from the adcp, i.e. at ∼ 4-5 m asf (cell #1), reach ∼ 10 mm/s, when values of ∼ 1 mm/s are still encountered at ∼15 m asf. slopes of the simulated current lines (i.e. lines of iso-concentration in suspended sediments) are thus maximum (∼ 1/30) at ∼ 4-5 m asf. there, they correspond to a difference in depth along two opposite beams of ∼ 6 cm that is much lower than the cell size (80 cm), and thus a priori hard to be shown. this is a fortiori valid for the upper cells since w decreases more rapidly than the beam separation. 3. the adcp data processing to verify the basic assumption for adcp technique, the 3d current field must be homogefig. 4b. vertical velocity in a plane through the centre of the observatory with a far-field speed of 300 mm/s along a diagonal. note that current is coming from the right. 720 jean-luc fuda, claude millot, sven hoog and hans w. gerber neous horizontally, i.e. over ∼2 m for cell #1 and ∼15 m for cell #40. from the four beam velocities, each pair of opposite transducers (1-2 and 34) gives one horizontal and one vertical components. the two horizontal components are directly associated with u and v. with a homogeneous current field, the two vertical components (w12 and w34) are identical; hence, the 3d current profile is defined with some redundancy. practically, a mean vertical velocity w=(w12+w34)/2 is computed, as well as an error velocity ev= c ⋅ ⋅ (w34−w12) where c=1/(sqrt(2).tan(20°)) is a normalisation constant (rdi technical booklet, 1998). another parameter to deal with is the echo intensity (eib in db) for each beam (#b) and cell, which represents the amount of energy echoed at given distances. the workhorse sentinel model uses the broadband method based on the propagation delay computed from correlation between one ping and its echo (gordon, 1996). in fact, single-ping beam velocities are generally not accurate enough so that ensemble-averages are performed (we used 200 pings emitted during 10 min every hour). in the earth-coordinates mode, and for each single ping, there are three kinds of screening on velocity data: the correlation test, the fish rejection algorithm and the error velocity test. the correlation test compares the correlation level and results to thresholds. beam velocities either associated with a poor signal-tonoise ratio (that occurs in waters too much depleted in scatterers) or larger than a pre-defined ambiguity velocity are rejected. these problems were not often encountered. the fish rejection algorithm detects an abnormal beam velocity for a specific cell or an echo much greater from one beam than from the others, which would make the strong echo from the «fishy» beam heard by the others, and the same velocity computed for all beams. the large concentration and gradient of suspended particles in the bottom nepheloïd layer, and the sometimeslarge inclination of this layer, often led to such problems. in some cases, more than 80% of single-ping beam velocities were rejected. in a heterogeneous current field, w12 and w34 are markedly different in general, so that ev is large. this often occurred for the lower cells, resulting in uncertainties larger than usual on the horizontal components. however, it is «frustrating» to eliminate all horizontal current values associated with an ev larger than a threshold, and not convenient to have gaps in a time series (i.e. for a given cell; we used the option «3-and-4beam solution»). an analysis of the ev characteristics will show that it is possible to appreciate, in a statistical way, the thickness of the layer significantly disturbed by such an observatory. 4. the adcp data analysis 4.1. the dynamic parameters up to 15-20-m asf, w (fig. 5) reached relatively large upward values (up to 40-50 mm/s) when ev (fig. 5) displayed variations that were similar both in time and on the vertical. the similarity between large w (i.e. large w12+w34) and large ev (i.e. markedly different w12 and w34) is consistent with what was expected. the relationship between w and the horizontal current speed m=[u 2+ v 2]1/2 is also noteworthy. the regression lines for some specific cells and all directions (fig. 6) indicate that w∼ 0 when m = 0 everywhere, that w increases with m, and that this increase diminishes with the cell number (i.e. distance from the observatory). for cells numbers >15, w values are relatively small and no more depend on m. these features are consistent with the deformation of the schematised (fig. 2b) and simulated (fig. 4) current field. simulated w’s can also be compared with the measured ones (fig. 7). obviously, the measured w’s are more scattered than the simulated ones, due to the ranges in both speed and direction of the in situ currents, and to the inaccuracies in the w computations, especially those due to the heterogeneity of the current field. therefore, only mean measured w’s were plotted, hence showing that they are ∼ 3 times larger than the simulated ones. however, the mean curves have a roughly similar shape and they all reach ∼ 0 at ∼ 20 m asf. we are unable to specify which kind of curves is the most realistic one. furthermore, vertical velocities are in fact induced up to the surface, as shown hereafter (although not just above the observatory, i.e. non-measurable by the adcp and not in the domain considered 721 analysis of adcp data above a bottom observatory for the simulations). since inaccuracy in w computation does not necessarily mean large w, we are tempted to think that measured ws are, on average, more realistic than simulated ws. 4.2. the non-dynamic parameters as schematised in figs. 2b and 4a,b, the nepheloïd layer being uplifted and inclined should induce a marked heterogeneity in the echo intensity eib along every beam and for every cell. in the open sea, eib mainly provides information on the amount and distribution of zooplankton. in the present case, this information mainly concerns i) the zooplankton diurnal migration on the vertical, and ii) the concentration and thickness of the bottom nepheloïd layer above the observatory, which partially depends on the current speed and direction. plots fig. 5. vertical velocity w (up), absolute value of the error velocity ev (middle) and standard deviation of the echo intensity ei in arbitrary units (down) as a function of depth (i.e. cells #) and time (julian days). 722 jean-luc fuda, claude millot, sven hoog and hans w. gerber of the measured eib versus depth versus time (not shown) thus depend on several phenomena and are not clearly instructive. it can be assumed that «natural» eib variations (due to both the nepheloïd layer per se – i.e. undisturbed by the observatory – and the zooplankton vertical migrations) are horizontally homogeneous. in order to eliminate them, we consider eib variations along a specific beam relatively to variations along the other beams (eibs must first be normalised since transducers are not of equal quality). figure 8 illustrates, for the 4 beams (b), the difference dib between a given eib and the average over the 3 other eibs as a function of the current direction (north is upward) and depth (represented by the cell #1-42). dib thus represents relative echo intensity. mainly for cells close to the observatory (cells #1-15) and for upper cells, a specific dib appears to be larger or lower than the others depending on the current direction. for instance, di4 is relatively large when the current is directed towards south to west. for most cells, this corresponds to a nepheloïd layer denser along beam #4 than along the other beams above the observatory. as expected from figs. 2a,b and 4a,b, the nepheloïd layer is higher along beam #4 than along the other beams for currents towards south to west. similar features occur for the other beams. the analysis of relative echo intensity signals thus allows showing slopes of the bottom nepheloïd layer that were not thought, according to simulation, to be measurable. especially for di4 and di2, large values are encountered over the whole water depth (∼42 m), hence showing that the wake of the observatory sometimes reached the surface, thus probably inducing changes in particles concentration, i.e. in colour, at the surface, eventually downstream from the zone sampled by the adcp. the reverse (dib relatively low) occurs, for all beams, in the opposed and perpendicular directions (i.e. where one of the others dibs is relatively large). also to be noticed is that the negative parts of di2 and 6 7 fig. 6. relationships and linear regression lines between w and the current speed m (in mm/s) for cells #3, 7, 11 and 15 (i.e. at 6, 9.2, 12.4 and 15.6 m above the bottom). fig. 7. simulated ws (+) for mean currents of 100 (black) and 300 (red) mm/s parallel to a diagonal are those obtained in a prism centred on the observatory and having a 4-m side at ∼20 m asf. to get a sufficient number of measured ws to compare with, we considered altogether currents towards sectors of 45° roughly aligned with the diagonals (∼45°, 135°, 225° and 315°) and in the ranges of 80-120 mm/s (solid black) and 250-350 mm/s (solid red). since measured values are much more scattered than simulated ones, we just reported the corresponding averaged profiles. the blue dashed line represents the averaged ws whatever the mean current direction and module. 723 analysis of adcp data above a bottom observatory di4 are more negative than those of di1 and di3. these asymmetrical features are related to the fact that more beams are in the wake of the observatory (i.e. eibs are more similar and dibs are lower) when the current is towards east than when it is towards west. this is due to the fact that the adcp is located on the eastern side of the observatory and is consistent with the distribution of the data computed from either 4 beams (p4) or 3 beams only (p1) as indicated in fig. 9a-c for a fig. 8. differences dib between the normalised eib and the average over the 3 other eibs versus the current direction (north is upward) and cell number. the diagrams can be analysed as if the observatory were in between them. fig. 9a-c. a) distribution of samples for cells #1-5 as a function of the current direction and speed up to 150 mm/s (north is upward). b) distribution of the percentage of data computed from 3 beams only (p1) and corresponding scale. c) distribution of the percentage of data computed from 4 beams (p4) and corresponding scale. ab c 724 jean-luc fuda, claude millot, sven hoog and hans w. gerber given cell (#15). indeed, more data are computed from the 4 beams when the current is towards west, hence not strongly disturbed above the adcp, than when it is towards east, hence the adcp being in the wake of the observatory. it can thus be concluded that: i) the wake induced by the observatory can extend several tens of metres above it, and ii) the slightly out of centre location of the adcp (∼ 1 m) is consistently reflected on the wake signature (over several tens of metres). the concentration in particles within the nepheloïd layer per se increases downwards and varies with time. hence, it is impossible (without any measurement in distance to the observatory) to identify and separate the influences, on the dibs, of either the concentration in particles or the inclination of the nepheloïd layer. in order to deal with a somehow synthetic parameter representative of the eibs heterogeneity, we computed, for each cell and over time, the eib standard deviation [1/4⋅σb(eib−eim)2]1/2, whereas eim is the average of the eibs over b. it is clear that the eib standard deviation (fig. 5c) and w (fig. 5a) have very similar distributions, especially in the 15-20-m bottom layer where both can be large. this direct correspondence between a priori independent (one non-dynamic and one dynamic) parameters clearly validates fig. 2a and 4a,b and shows that the larger the inclination of the nepheloïd layer, the larger the vertical velocity. it must be emphasised that the eib standard deviation, and consequently the inclination of the surfaces of equal concentration in suspended particles, is computed from differences between eib’s in cells that are separated horizontally by a few metres only (e.g., ∼2 m for cell #1 and ∼15m fig. 10. spectra of u (red), v (blue), w (black), eim (orange) and di4 (green) at cell #5 averaged over 2 pieces to give 4° of freedom. the confidence interval is the 90% one. 725 analysis of adcp data above a bottom observatory for cell #40). whatever the current field homogeneity is, and thus whatever the quality of the current computations, this analysis demonstrates that the echo intensity is a very sensitive signal that provides significant information (not perceptible with a homogeneous current field). 4.3. spectral analysis figure 10 displays dynamic (u, v, w) and non-dynamic (eim, di4) parameters spectra at cell #5 (representative of the 15-20-m bottom layer). classically, the u and v spectra show a similar amount of energy at the inertial frequency (clockwise-polarised circular oscillation at ∼ 0.06 cph) while the semidiurnal (∼ 0.08 cph) and diurnal (∼ 0.04 cph) frequencies are shown mainly on the v spectrum (north-south rectilinear currents in the northern adriatic). since eim and w are purely integrated (over beams) parameters, they do not change markedly according to a circularly polarised current (the inertial frequency will not appear on the spectra) while they change according to a rectilinear current (at twice the corresponding tidal frequency). both eim and w are also sensitive to the zooplankton diurnal migration, which is not a purely diurnal signal so that energy is also introduced at the semidiurnal frequency (and higher harmonics as well). therefore, the fact that only tidal frequencies, i.e. not the inertial frequency, are shown on both eim and w is consistent with our analysis. in addition, all peaks (diurnal, inertial, semidiurnal) must appear on the spectrum of a parameter such as di4 (insensitive to zooplankton diurnal migration) since tidal currents and inertial currents induce an inclination of the nepheloïd layer at corresponding frequencies. as shown by fig. 10, a peak (at the inertial frequency) on the spectrum of a non-dynamic variable (di4) coming from a purely dynamic phenomenon (the inertial oscillations) definitely validates our analysis. 5. more on the error velocity parameter theoretically, a homogeneous current field leads to an error velocity ev=0. practically, ev≠ 0 indicates that the data are not accurate and/or that the current field is heterogeneous. let us consider an adcp suitably mounted (nearly centred) on a symmetrical (with respect to current direction) structure, and collecting data at best (in terms of e.g., pings averaging). the analysis below shows that the ev characteristics allow specifying which cells are markedly influenced by the structure, hence validating the data from a statistical point of view. with a heterogeneous current field, and for a given cell, ub and wb (b = 3, 4) define the actual current components from beams 3 and 4, and vb and wb (b = 1, 2) the actual current components from beams 1 and 2 (fig. 11a). f being the angle from the adcp axis, the radial velocities rb and the computed vertical velocities are (for more details see van haren et al., 1994) r1 = − v1sinf−w1cosf r2 = v 2sinf−w2cosf r3= − u3sinf−w3cosf r4 = u4sinf−w4cosf w34=−(r3+r4)/2cosf=−[(u4−u3)tanf+ +(w3+w4)]/2 w12=−(r2 + r1)/2cosf = − [ (v 2−v1)tanf+ +(w1+w2)]/2. in case the actual current field is homogeneous, u3=u4=u, w3=w4=w34, v2=v1=v, and w1= =w2=w12, with the consequence that the u-v-w field is specified with measurement inaccuracies that are quantified by ev/c = (w34 −w12). in case that the current field is heterogeneous, with du = u3−u4 and dv=v1−v2, it results in ev/c=(du−dv)tanf+(w3+w4)−(w1+w2). assuming a simplified cubic observatory oriented northwards (fig. 11b), and an adcp located on its eastern side, two relationships can be expected. for currents of similar speed towards directions a and al that are symmetrical with respect to the east-west axis, identities between the ub and vb of both situations lead to eva=−eval. for currents towards either east or west, w2 = w4, w1=w3, u3=v1 and u4=v2; hence ev=0 (also obvious from the previous re726 jean-luc fuda, claude millot, sven hoog and hans w. gerber a b fig. 11a,b. a) beams and current components configuration in case of an heterogeneous current field; b) horizontal components of currents towards directions a and al symmetrical with respect to east-west, and components correspondence. fig. 12a-c. a) distribution of samples for cells #1-20 as a function of the current direction and speed up to 200 mm/s (north is upward). b) distribution of the error velocity (scale on the left) for cells #1-20 versus current direction and speed. this diagram displays neither the number of samples (shown in a) in a specific area of the diagram, nor the variance in that specific area. the error values that are indicated are interpolated from the errors associated with each sample, so that errors for areas with few samples are less significant. c) the same for the vertical velocity (scale on the right). ab c 727 analysis of adcp data above a bottom observatory lationship). if the adcp were centred, ev would have been = 0 for currents towards either north and south, and eva= ev− a for currents towards opposed directions (a and −a). in a given data set, any indication of such relationships will account for a significant heterogeneity of the current field. conversely, if ev does not display any specific distribution (i.e. is only noisy), the current field can be considered homogeneous. considering these relationships over depth can specify, from a statistical point of view, the thickness of a significant wake disturbance. verifying these ev specific relationships with our data set can be hampered by the fact that both the speed and the direction are not homogeneously distributed. to get a distribution as homogeneous as possible, we considered all couples speed-direction from cells #1-20 for the whole experiment (fig. 12a) and their associated ev (fig. 12b) and w (fig. 12c) values. we have neglected the difference between north and the 3° actual orientation. figure 12b shows that, whatever the horizontal speed is, the ev sign changes when crossing the eastwest axis, which validates the corresponding theoretical relationship (ev= 0) for currents towards east or west. also, for all directions and speeds, the theoretical relationship eva= − eval is clearly validated. to be noticed is the (sole) other sign change near directions towards southeast and northeast, i.e. the north-south axis is clearly not a symmetry axis as would have been the case for a adcp located in the centre of the observatory. in such a case, the relationships (ev= 0 for north and south directions, and eva = ev−a or opposed directions) are clearly not validated. therefore, the ev distribution in fig. 12b clearly shows that i) the current field is heterogeneous and ii) the adcp is not centred. although the interpolation procedure is rough for very large speeds due to the low number of points, it can be considered that the larger/lower the speed, the larger/lower the ev absolute values. for what concerns w, most values are positive and, the larger the speed the larger w. also, w values are larger for currents towards west than for currents towards east. these results are consistent with what was intuitively thought and previously demonstrated with the simulation (fig. 4b) and the data analysis (fig. 6). it can thus be concluded that, in case heterogeneity of the current field is induced by some semi-cubic structure set on the bottom, the ev distribution actually displays those characteristics that are expected from the algorithms. analysing this parameter’s distribution can thus allow specifying the layer thickness modified by such a structure. obviously, these remarks also apply to a structure set at intermediate depths and to the current field above and below it. they can also be extended to structures that are parallelepipedic and currents that are symmetric with respect to the adcp location. 6. conclusions a 300-khz adcp set on a semi-cubic (2.5× ×2.5×1.0 m3) bottom observatory has allowed specifying the wake disturbances induced on the current field. with a far field general current of 20-40 cm/s, upward vertical velocities of 40-50 mm/s were computed in a 15-20-m bottom layer. although these experimental values are noisier than usual, mainly due to the heterogeneity of the current field above the observatory, they are consistent with (although larger than) simulated values, and hence considered significant. in such conditions, the wake of the observatory can probably extend ∼ 40 m above seafloor at least (even if not directly above the observatory). differences in echo intensities along the beams have demonstrated that surface of isoconcentration in suspended particles were inclined over the observatory, consistently with what was expected from the general current direction and speed. the relationships between the horizontal current direction and speed on one side and the characteristics of both dynamic (vertical velocity) and non-dynamic (mean echo intensity as well as relative echo intensities along the beams) parameters on the other side are also consistent with the oscillatory character of mesoscale (i.e. inertial and tidal) currents. indeed, spectral characteristics expected for all these parameters, in association with currents polarised at the inertial frequency as well as with rectilinear currents at diurnal and semidiurnal frequencies, are clearly retrieved. 728 jean-luc fuda, claude millot, sven hoog and hans w. gerber finally, we have shown that the error velocity parameter displays specific characteristics that specify the thickness of the layer disturbed by such a semi-cubic observatory from a statistical point of view. this parameter can thus be used to check the validity of adcp data collected in such a way, hence providing a criterion for qualifying this or that time series. more generally, our analysis demonstrates how sensitive (especially for what concerns echo intensity) and accurate is an instrument usually operated in more favourable conditions (such as on a mooring line or directly on the bottom with a profiled mooring frame), which does not allow its utmost performances to be appreciated. considering the importance of induced vertical velocities compared to that of undisturbed horizontal currents (up to o(10−1) within a depth range corresponding to a few times the observatory height), such a massive observatory is obviously not adequate to study finely currents close to the bottom. such an observatory can efficiently host long range adcps devoted to the study of the general circulation and of mesoscale processes within bottom layers several 100s-m thick. acknowledgements the geostar group has been supported by eu (grants mas3-ct95-0007 and mas3ct98-0183). references beranzoli, l., t. braun, m. calcara, d. calore, r. campaci, j.-m. coudeville, a. de santis, g. etiope, p. favali, f. frugoni, j.-l. fuda, f. gamberi, f. gasparoni, h.w. gerber, m. marani, j. marvaldi, c. millot, p. palongio, g. romeo and g. smriglio (2000): european seafloor observatory offers new possibilities for deep-sea study, eos, trans. am. geophys. un., 81 (5), 45-49. fluent (2000): cfd software, online manuals (fluent germany gmbh, darmstadt). gordon, r.l. (1996): acoustic doppler current profilers. principles of operation: a practical primer (rd instruments publication), 2nd edition for broadband adcps. orlic, m., m. gacic and p. la violette (1992): the currents and circulation of the adriatic sea, oceanol. acta, 15 (2), 109-124. rdi technical booklet (1998): adcp coordinate transformation; formulas and calculations (rd instruments publication). van haren, h., n. oakey and c. garrett (1994): measurements of internal wave band eddy fluxes above a sloping bottom, j. mar. res., 52, 909-946. vol. 48, 01, 05ok.qxd 129 annals of geophysics, vol. 48, n. 1, february 2005 key words antarctica – radon – uranium – thorium – spectrometry 1. introduction the area under investigation (fig. 1) is a part of the shetland microplate which originated at the subducting pacific margin of gondwana and contains cretaceous-early miocene island arc extrusives (mainly basalts and andesites) and intrusives (gabros, diorites, monzonites) (birkenmajer et al., 1991). oceanic basalts are normally of low uranium and thorium content. and atmospheric radon activity is extremly low over oceanic island areas. in such a situation any migration of air from the continents, where radon activities are ten times higher can be easily detected, making radon a useful tracer of atmospheric circulation (see lambert et al., 1970; turekian et al., 1977; reiter, 1978; polian et al., 1986; balkanski and jacob, 1990; jacob et al., 1997). however, locally, even within oceanic islands, magmatic differentiates of andesitic and granodioritic character and hydrothermal zones occur. they can be enriched in uranium and thorium making them a significant local source of radon emmanation. this possibility should be taken into account in the case of any meteorological interpretation. a pioneering paper on radon activity in the king george island area published by evangelista, pereira (2002) did not contain elements of radiometric mapping, in spite of the consolidated geophysical experience of both authors probably because of logistic problems. because this, mapping was performed during austral summer 2002/2003 by the present mailing address: dr. andrzej tomasz solecki, institute of geological sciences wrocl⁄ aw univeristy, pl. m. borna 9, 50-204 wrocl⁄ aw, poland; e-mail asol@ing.uni.wroc.pl radon and thoron daughter activities in the environment of the king george island (west antarctica) andrzej tomasz solecki institute of geological sciences wrocl⁄ aw univeristy, wrocl⁄ aw, poland department of antarctic biology polish academy of sciences, warsaw, poland abstract results of 253 gamma spectrometric analyses of radon daughters in soil and bedrocks of the king george island (west antarctica) are presented. measured values range from 0.1 to 58.4 bq/kg, and from 4.9 to 75.5 bq/kg for 214bi and 208tl respectively, but most measurements fall in the lower part of this range due to predominantly basaltic character of the geological basement. obtained gamma spectrometric data correspond well to very low soil gas radon content measured by means of kodak lr115 being below 454 bqm–3. low soil gas radon content and characteristic type of architecture is responsible for low indoor radon activity in arctowski station being as low as 1015 bqm–3. the highest 105 bqm–3 indoor rn activity has been measured in the greenhouse bungalow of the station. this increased value was probably connected with the presence of about 1000 kg of imported soil material in the greenhouse room of the bungalow. local zones of increased uranium and thorium content, discovered as a result of radiometric mapping, can potentaily influence atmospheric radon used for meteorological interpretation. 130 andrzej tomasz solecki author by means of a portable gammaspectrometer gr-320. 2. analytical method measurements of bedrock radon daughter activity were performed using an exploranium gr-320 gamma radiation spectrometer with the standard nai (tl) gpx-21a detector of 0.35 l volume. calibration of the detector was performed by the manufacturer using traceable test pads. impulses supplied by the detector units were classified using channels 70-204 of the 256 channels of the spectrometer covering the energy window 850-2810 kev. three bands (regions of interests roi) corresponding to energy windows of radionufig. 1. localisation of the study area, three northernmost sites are located in the keller peninsula close to ferraz station. table i. gamma ray spectrometer gr-320 energy window characteristics. band radionuclide peak channels energy window (kev) sensitvity ro/2 40k 1460 kev 109-122 1370-1570 0.661 cps/% ro/3 214bi 1760 kev 129-142 1660-1860 0.067 cps/ppm ro/4 208tl 2620 kev 179-204 2410-2810 0.025 cps/ppm 131 radon and thoron daughter activities in the environment of the king george island (west antarctica) clide peaks 40k, 214bi and 208tl were set up (table i). the problem of stabilization of energy windows of channels was solved by means of continous measurement of caesium 662 kev photons from internal source in the band ro/1 covering channels 51-60 (600-730 kev). gain parameter responsible for fitting channels to energy windows was continously updated using the least-squares fit of a gaussian caesium peak shape every time the 5000 level of cs counts was exceeded. this ensured that system gain was always correct and selected channels corresponded to the desired energy windows. measured activities of radon and thoron daughters were recalculated into uranium and thorium concentrations (in ppm) assuming existence of equilibrium in uranium and thorium decay series, a common practice in this kind of measurements marked by eu and eth notation instead of u and th. for each measurement the detector was deployed in the field using a geometry as close as possible to 2π. sampling time ranged from 15 to 30 min depending on local radionuclide content to reach statistically significant counts in all roi’s. ambient temperature during measurements was in the range –2 to + 6°c, well above the recommended limit of –10°c radon activity was measured by means of the kodak lr115 solid state nuclear track detectors. in the case of indoor measurements detectors were placed on the wall at a height of 2 m. soil-gas radon activity was measured in five sites at a depth of 20 cm and the detectors were fixed inside a plastic cap of 15 cm internal diameter. in both cases free air space around the detector was greater than 7.5 cm to avoid irradiation by plateout particles. real soil with organic matter horizon (3 cm thick) existed only in one case in other cases physically weathered fine grained material of bedrock prevailed. 3. results average measured bedrock radon and thoron daughter activity was 14.6 and 18.9 bq/kg for 214bi and 208tl, respectively, reaching its maximum values 58.4 and 75.5 bq/kg for granodioritc dropstone (table ii). in the case of basalts, 214bi activity was often below detection limit, while for 208tl the lowest recorded value was 4.9 bq/kg. trimodal distribution of the obtained data was especially well visible in the case of thoron activity (fig. 3). three groups of radon activities (214bi): 4, 16 and 36 bq/kg visible in the fig. 2 corresponded to basement composed of: basalts, andesites and granodioritic quarzt lode, respectively. the same rocks in the case of thoron activity (208tl) data yielded three maximas: 4, 20 and 52 bq/kg (fig. 3). quartz lodes of 2.8 ppm eu and 12.6 ppm eth seem to be a potentialy high source of radon, and especially thoron emanation. its outcrop tens of meters wide was covered with high table ii. mean values of rn daughter activities for various rocks types. 214 bi activity 208 tl activity eu ppm calculated eth ppm calculated number of bq/kg bq/kg from 214 bi activity from 208 tl activity measurements basalt 5.6 8.7 0.45 ± 0.07 2.12 ± 0.29 35 lahar (tillite) 13.7 18.5 1.10 ± 0.07 4.50 ± 0.15 24 cobble beach 18.2 20.0 1.46 ± 0.11 4.87 ± 0.27 16 andesite 21.0 25.4 1.69 ± 0.12 6.19 ± 0.43 11 tuffite-zeolite 21.9 26.5 1.76 ± 0.13 6.45 ± 0.32 20 granodiorite 58.4 75.5 4.70 18.4 1 ezcurra fault zone 19.4 23.3 1.56 ± 0.15 5.68 ± 0.46 8 keller peninsula 35.2 51.9 2.83 ± 0.12 12.65 ± 0.36 12 mineral vein 132 andrzej tomasz solecki amounts of crushed material which resulted due to physical weathering in polar climate. locally significant chemical weathering was superimposed due to abundance of pyrite. all these factors resulting in a strong increase in porosity and permeability can be responsible for significant radon flux towards brazilian ferraz station located few tens meters down slope. the highest result of five measurements of rn soil gas activity 454 bqm–3 was measured in the weathered volcanoclastic-zeolitic material of 21 and 26 bq/kg for 214bi and 208tl respectively. indoor radon activity values obtained were as low as 10-15 bqm–3 (five measurements) in social rooms of polish henryk arctowski station. the highest 105 bqm–3 indoor rn activity was measured in the greenhouse bungalow of the station. this increased value was probably connected with the presence of about 1000 kg of imported soil material in greenhouse room of the bungalow. 4. conclusions obtained results indicate that king george island area is of extremely low rn potential due to low uranium and thorium content and low emanation coefficient of the chemically unweathered bedrock material. measured values are slightly lower than those based on 22 gamma-spectrometric measurements published by evangelista and pereira (2002), but a much better fit to godoy et al. (1998) data and world scale data for areas of such a geology. however, the local increase in uranium and thorium concentration observed locally can result in significant radon flux in favourable meteorogical conditions. rapid pressure changes connected with cyclones can promote radon flux from such zones (see schery and gaeddert, 1982), which can be interpreted as influx of remote continental air. from this point of view, results obtained at brasilian ferraz station (evangelista and pereira, 2002) should be interpreted very carefully since it is located downslope of the quartz lode outcrop covered with a high dump of crushed and weathered rocky material of relatively high uranium and thorium content. acknowledgements the author would like to thank prof. s. rakusa-suszczewski and other colleagues from the department of antarctic biology polish academy of sciences for their kind support, help and companionship in the remote area of west antarctica. the author is also grateful to brazilian colleagues form ferraz station for their kind hospitality. research was partly funded by grant no. 2022/w/ing of the institute of geological sciences wroclaw university. references balkanski, y.j. and d.j. jacob (1990): transport of continental air to the subantarctic indian ocean, tellus, 42b (1), 62-75. birkenmajer, k., l. francalanci and a. peccerillo (1991): petrological and geochemical constraints on the genesis of mesozoic-cenozoic magmatism of king george island, south shetland islands, antarctica, antarctic sci., 3 (3), 293-308. fig. 2. histogram of radon activity distribution. fig. 3. histogram of thoron activity distribution. 133 radon and thoron daughter activities in the environment of the king george island (west antarctica) evangelista, h. and e.b. pereira (2002): radon flux at king george island, antarctic peninsula, j. environ. radioact., 61, 283-304. godoy, j.m., l.a. schuch, d.j.r. nordemann, v.r.g. reis, m. ramalho, j.c. recio, r.r.a. brito and m.a. olech (1998): 137cs, 226ra, 228 ra and 40k concentrations in 0-5 cm soil samples collected at several south shetland islands, j. environ. radioact., 41 (1), 33-45. jacob, d.j., m.j. prather, p.j. rasch, r. shia, y.j. balkanski, s.r. beagley, d.j. bergman, w.t. blackshear, m. brown, m. chiba, m.p. chipperfield, j. grandpre, j.e. dignn, j. feichter, c. genthon, w.l. grose, p.s. kasibhatla, i. koehler, m.a. kritz, k. law, e.j. penner, m. ramonet, c.e. reeves, d.a. rotman, d.z. stockwell, p.f.j. van velthoven, g. verver, o. wild, h. yang and p. zimmermann (1997): evaluation and intercomparison of global atmospheric transport models using 222 rn and other short-lived tracers, j. geophys. res., 102 (d5), 5953-5970. lambert, g., g. polian and d. taupin (1970): existence of periodicity in radon concentrations and in the largescale circulation at latitudes between 40 and 70 south, j. geophys. res., 75, 2341-2345. polian, g., g. lambert, b. ardouin and a. jegou (1986). long-range transport of continental radon in subantarctic and antarctic areas, tellus, 38b, 178-189. reiter, e.r. (1978): atmospheric transport processes – part 4: radiactive tracers (technical information center, us department of energy). schery, s.d. and d.h. gaeddert (1982): measurements of the effect of cyclic atmospheric pressure variation on the flux of 222 rn from the soil, geophys. res. lett., 9 (8), 835-838. turekian, k.k., y. nozaki and l.k. benninger (1977). geochemistry of atmospheric radon and radon products, ann. rev. earth planet. sci., 5, 227-255. a possible point of contact between cosmic ray physics and archaeology: muon absorption radiography at the tharros phoenician-roman site annals of geophysics, 60, 1, 2017, s0107; doi:10.4401/ag-7382 s0107 a possible point of contact between cosmic ray physics and archaeology: muon absorption radiography at the tharros phoenician-roman site lorenzo bonechi1,*, fabio ambrosino2,3, luigi cimmino2,3, raffaello d’alessandro1,4, nicola mori1,4, pasquale noli3, giulio saracino2,3, paolo strolin2,3, lorenzo viliani1,4 1 istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy 2 università di napoli federico ii, dipartimento di fisica, naples, italy 3 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 4 università di firenze, dipartimento di fisica e astronomia, sesto fiorentino, florence, italy abstract several on-going activities exploiting the potential of the muon-absorption radiography technique are focusing on the study of large volcanic structures located in different geographical regions. nonetheless, the possible application of this non-destructive surveying method to other fields is also under consideration by different groups. looking backward to the history of muon radiography, we can learn how the first important success of this technique was achieved in the archaeological field by the physicist luis w. alvarez in the 1960s. further examples of possible applications can be found concerning geological and mining applications. in this paper a possible application of muon absorption radiography in the context of the archaeological research is proposed. results of a simulation of a simplified case study are presented. 1. introduction many activities exploiting the muon absorption radiography (or muon transmission radiography) technique are currently under consideration or development in fields other than volcanology, as partially reported in other contributions to this workshop. this technique, which can be properly contextualized in the generic framework of monitoring or imaging of material structures, has the potential to be applied in many different areas. each area of application, and often each application itself, requires an independent feasibility study for this technique to be validated. this is due to several reasons, which can be summarized in the following main points: 1) the different materials and material thicknesses encountered by the incident muons, 2) the muon energy range relevant for the selected case study, 3) the relative detector-target geome-try, 4) the type of detector which can be used for the measurement and 5) the purpose of the measurement itself. the first practical cases where muon absorption radiography was successfully applied were in the fields of geology and archaeology. in the 1950s e.p. george measured the thickness of the ice burden over the guthegamunyang tunnel in australia [agostinelli et al. 2003]. a decade later l.w. alvarez studied the interior of the second pyramid of chephren in egypt, looking for a hypothetic hidden chamber [bonechi et al. 2015]. in the latter case, alvarez was able to demonstrate the absence of any large chamber hidden over the already discovered one. drawing on the idea of alvarez, in this work we propose a possible application of muon absorption radiography in the framework of archaeology, based on the use of a robust, low cost and low power consumption particle detector. in section 2 a description of the case study is presented; in section 3 the preliminary results of a simulation are shortly discussed; finally, in section 4 the first activities for the production of a prototype detector are presented. 2. detecting hidden tombs at tharros the tharros site (see figure 1) is a beautiful punicroman site located at the south end of the sinis peninsula near the town of oristano in the sardinia island (italy). in 2014 we had contacts with the managers of the site who illustrated some relevant details about the anarticle history received july 5, 2016; accepted october 5, 2016. subject classification: muon radiography, muon absorption, archaeology, tharros necropolis. cient town. in summer, we had the possibility to visit the site to evaluate a possible usefulness of exploiting muon radiography for a survey of unknown parts of the site. apart from the main ruins of tharros, related to the old town itself, a large area of the peninsula hosts an important necropolis, which is still hidden underground for the greatest part. only a few points have been brought to light by excavations and later covered again to preserve the status of the site. part of these excavations are still visible (see figure 2). surveys by means of a gpr device have been only partially conducted due to the difficulty of moving this kind of device over large areas covered by the mediterranean scrub, which is protected by the italian legislation. therefore, the presence of tombs in several parts of the site (e.g. figure 3) is currently only a guess. thanks to the muon radiography technique, the placement of a muon detector in a fixed measuring position, possibly ten or fifteen metres underground or under a cliff, would give the possibility to make a survey of some hundred square metre surface searching for empty cavities. the minimum volume of a hidden tomb that can be reasonably detected with this method varies from one to a few cubic metres, assuming the tomb is located a few metres underground. to assess the possibility of identifying such hypothetic hidden tombs by exploiting the information carried by the detected muons, a data analysis method [anastasio et al. 2013] has been proposed, based on the comparison of the measured flux with a simulated expected flux, calculated for the same data acquisition time. the required simulations must take into account all the relevant aspects of the problem, from the functional shape of the muon energy/angular spectra at ground level, to its modification while traversing the material along the detector’s field of view. this method is based on the back-projection of the detected and simulated muon tracks to parallel planes at increasing distances from the detector, or to spherical surfaces centred on the detector’s centre for increasing radius. two hit map distributions are therefore produced for each surface and then subtracted from each other. as a result of the different muon absorption in the real and in the simulated cases, the resulting “difference map” should present regions with an accumulation or deficit of hits beyond the statistical fluctuations in correspondence of a sufficiently large unknown empty cavity or a place containing a material much heavier than the surrounding earth or rock. 3. preliminary study with the geant4 high energy physics simulation package preliminary simulations of a simplified case study have been implemented based on the geant4 simulation package [agostinelli et al. 2003], a toolkit that is commonly used in high energy physics for simulating the passage of particles through matter. details of this study are published in bonechi et al. [2015]. a homogeneous terrain with a flat surface has been considered, with a generic “reasonable” chemical composition. a realistic muon detector, based on the technology used for the muray experiment [anastasio et al. 2013], has been positioned 7 m underground and a one cubic metre empty cavity has been placed 4 m undersoil on its field bonechi et al. 2 figure 1. view of the tharros remains. figure 2. tombs found in the tharros necropolis. figure 3. the region of the excavation shown in figure 2. a necropolis is thought to occupy large parts of this place. 3 of view, 8 m far from it. a sketch of the com-plete geometry implemented in geant4 is shown in figure 4. a realistic muon flux generator has been integrated in the geant4 code. this generator is based on experimental measurements of cosmic rays at ground level by the adamo experiment [bonechi et al. 2005] in the momentum range between 100 mev/c and 130 gev/c, which is relevant for this application. two different simulated data sets, with and without the presence of the empty cavity, have been generated and the results compared (see bonechi et al. [2015] for details). for each of these simulations a number of muon events, corresponding to a data acquisition time ranging from two weeks up to 130 days, entering the detector’s field of view and passing through a region surrounding the cavity has been generated. muon tracks are reconstructed and the final trajectories saved to file only if crossing all tracking planes. the sets of reconstructed tracks are back projected to vertical planes or spherical surfaces and a two dimensional hit density map is produced for both simulations in correspondence of each surface, as shortly described at the end of the previous section. from the comparison of the two simulations, a hit map difference is obtained for all the selected surfaces. due to geometrical reasons, a focused image of the cavity is expected as the back projection surface approaches the position of the cavity. in figures 5, 6 and 7 the difference maps on spherical surfaces of radii 1 m, 4 m and 7 m are shown in the azimuth-zenith plane. these figures show how the image of the cavity gets more detailed while the radius of the back projection sphere increase from 1 m to 7 m (see the rms values of the distributions). as shown in bonechi et al. [2015], the best focusing of the image of the cavity can be obtained when the back projection sphere (or plane) crosses a region in proximity of the real position of the cavity itself. in figure 8 the dependence of the width of the reconstructed image along the azimuth axis, as function of the radius of the back projection sphere (equivalent to the distance from the centre of the detector) is shown. the minimum of this function is found for a distance d = 6.1 ± 0.7 m, approximately compatible with the position of the face of the cavity closer to the detector. the angle subtended in this example by the cavity with respect to the detector’s centre is approximately 125 mrad. a detector with an angular resolution of 10÷50 mrad is therefore required for this measurement. 4. development of a prototype borehole tracking detector based on the experience derived from the muray experiment, a prototype borehole muon tracker is under development for a demonstration of the technique promuon radiography in archaeology figure 4. a sketch of the geometry implemented in the geant4 simulation. figure 5. projection on a 1 m radius spherical surface. figure 6. projection on a 4 m radius spherical surface. figure 7. projection on a 7m radius spherical sur-face. posed in bonechi et al. [2015]. this detector is a 40 cm × 40 cm × 40 cm cubic tracker made of two x and two y planes. each plane is made of 22 scintillator bars with isosceles triangular section (figure 9). the scintillator material is ups-923a by de-tecrad, which has a good light yield and a fast rising time. the bar section has a 40 mm base and 20 mm height and the total length of each bar is 40 cm. each bar is enveloped in an aluminized mylar film to guarantee the optical insulation of the scintillator from the external environment (figure 10). the triangular bars are assembled one with the base facing upwards and the adjacent one with the base facing downwards, in such a way to fill a 20 mm height parallelepiped volume (with oblique faces at two ends). signals from the two adjacent bars crossed by a muon can be used to determine the crossing point of the muon tracks with a better resolution with respect to the simple digital algorithm. for the single tracking plane, made of 22 bars, we expect a spatial resolution around 3÷4 mm, according to previous tests with similar bars [carlà 2009/2010]. considering two planes at 40 cm distance, it means therefore an angular resolution of 10 mrad or better. each bar is readout by two silicon photomultiplier (sipm), one on each side. the selected model for this device is asd-nuv4s-p by advansid, which is a 4×4 mm2 device optimized for near ultra violet light detection. a transparent epoxy layer protects the surface of each sipm, therefore the optical contact between sipm and scintillator bar can be achieved by means of a thin layer of optical grease directly applied to the surface of the sipm. these sensors are kept faced to the triangular faces of the bars thorough dedicated plastic supports produced at a 3d printer. a photo of such a plastic support is shown in figure 11. these parts are fixed head on to the bars thanks to standard double sided tape. some pieces of kapton tape is finally applied for safety to fix the support laterally. the readout electronics is currently under development. it is based on the easiroc chip by omega, a 32 channels chip dedicated to readout sipm detectors. bonechi et al. 4 figure 8. dependence of the rms value associ-ated to the image of the cavity as a function of the radius of the back projection sphere. a minimum can be identified for a radius around 6-7 m. figure 9. one scintillator bar used for the con-struction of the prototype muon tracker. figure 10. one scintillator bar enveloped in the aluminized mylar film. figure 11. plastic support use to fix the sipm sensors to the scintillator bars. at the centre a slot used to host the sensor is visible, with a small hole to drive the signal to the readout electronics. 5 the same chip is used for the muray and muraves projects. 5. conclusions the possibility to exploit the potential of muon absorption radiography for surveys in the archaeological field is under consideration. in particular, the application of this technique for the study of the necropolis of the tharros site in the sardinia island is under consideration. for this purpose a prototype borehole muon tracker based on scintillators and silicon photomultipliers in under development. the readout electronics will be similar the muray and muraves projects. the first test of the technique is foreseen for 2016 in a controlled underground environment at the infn unit of florence (italy). acknowledgements. we would like to thank prof. anna chiara fariselli, who has the ministerial concession for archaeological research at tharros, dr. federica boschi from university of bologna and dr. roberto carrus, president of cooperativa “penisola del sinis”, for their availability for discussions and for the invitation to visit the site of tharros. we would like also to thank mr. massimo falorsi, head of the workshop of the department of physics and astronomy of university of florence, and mr. marco manetti of infn unit of florence for the production of scintillators and mechanics. references agostinelli, s., et al. (2003). geant4 a simulation toolkit, nucl. instrum. meth. a, 506, 250-303. anastasio, a., et al. (2013). the muray experiment. an application of sipm technology to the understanding of volcanic phenomena, nucl. instrum. meth. a, 718, 134-137. bonechi, l., m. bongi, d. fedele, m. grandi, s. ricciarini and e. vannuccini (2005). development of the adamo detector: test with cosmic rays at different zenith angles, proceedings of the 29th international cosmic ray conference, 9, 283-286. bonechi, l., r. d’alessandro, n. mori and l. viliani (2015). a projective reconstruction method of underground or hidden structures using atmospheric muon absorption data, j. instrum., 10, p02003. carlà, m. (2009/2010). misura della risoluzione spaziale di un odoscopio realizzato con scintillatori a sezione triangolare, bachelor’s degree in physics, academic year 2009/2010, university of florence, italy (in italian). *corresponding author: lorenzo bonechi, istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy; email: lorenzo.bonechi@fi.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. muon radiography in archaeology << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice preliminary macroseismic survey of the 2016 amatrice seismic sequence annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7172 1 preliminary macroseismic survey of the 2016 amatrice seismic sequence mariano angelo zanini*, lorenzo hofer, flora faleschini, paolo zampieri, nicola fabris, carlo pellegrino university of padova, dept. of civil, environmental and architectural engineering, padova, italy *marianoangelo.zanini@dicea.unipd.it abstract after the recent destructive l’aquila 2009 and emilia-romagna 2012 earthquakes, a sudden mw 6.0 seismic event hit central italy on august 24, 2016. a low population density characterizes the area but, due to its nighttime occurrence, about 300 victims were registered. this work presents the first preliminary results of a macroseismic survey conducted by teams of the university of padova. macroseismic intensities were assessed according to the european macroseismic scale (ems98) for 180 sites. i. introduction n august 24, 2016, at 3:36 local time (1:36 utc), most of the inhabitants of central italy were woken up by a mw 6.0 earthquake that occurred at the boundaries of lazio, umbria, marche and abruzzo regions. the epicenter was located by the istituto nazionale di geofisica e vulcanologia (ingv) seismic network [iside 2016] at 42.70°n and 13.24°e, between the municipalities of accumoli and amatrice, in the lazio region. the central apennines sector is highly prone to seismic hazard: in recent years other devastating events occurred with epicenters located in a range of 30 km far from the actual one, like the april 6, 2009 (mw 6.29) l’aquila event and the 1997 umbria seismic sequence, characterized by a mw 5.97 mainshock. according to the parametric catalogue of italian earthquakes (cpti15) [rovida et al. 2016], historical information reveals that the seismic activity in the monti sibillini area is frequent and in the past centuries was characterized by highly destructive events: from the eighteen century a mean annual rate of about 0.25 events with magnitude mw higher than 4.0 was observed. the first evidences of damage induced by earthquake occurrences in the area of accumoli were related to the effects of the july 1627 monti della laga event (mw 5.3, io 7-8 mcs) [monachesi and castelli 1992]. it was followed by the severe october 7, 1639 (mw 6.21, io 8-9 mcs) [castelli 2013] amatrice earthquake, that seemed very similar to the actual seismic scenario. the accumoli area suffered extensive damages also after the most destructive january 14, 1703 (mw 6.92, io 10 mcs), and was re-struck by the may 12, 1730 valnerina event (mw 6.04, io 7 mcs) [guidoboni et al. 2007]. after about 150 years these territories were hit again by the november 7, 1883 monti della laga earthquake (mw 5.10, io 7 mcs) and in the twentieth century by other significant events with io mcs 7-8 in 1916, 1950 [tertulliani et al. 2006] and 1979. in the days following the august 24 event, teams of the university of padova organized a field survey of the damaged areas with the aim to develop an exhaustive macroseismic assessment of the o annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7172 2 earthquake scenario, according to the european macroseismic scale (ems98) [grünthal 1998]. the survey was continuously updated until september 6, 2016 to better define the damage effects induced by the mainshock event. ii. survey methodology the teams surveyed 180 sites, assessing the intensity level on the basis of the structural damage suffered by the residential building stock and classifying it according to the european macroseismic scale (ems98) [grünthal 1998]. industrial buildings were not taken into account due to their lack of representativeness for intensity assessment purposes in the region. short interviews on the perception of the effects experienced by the inhabitants integrated damage data. a vulnerability class was identified for each analyzed building, and failures were classified according to the ems98 damage grades. the most common residential building types in the area are: one-or-two-story old masonry buildings in stone and low quality mortar, with lack of connections (vulnerability class a and b); recently retrofitted masonry buildings (mostly renovated after the 1997 umbria sequence, to which a vulnerability class d was assigned); and two-story seismically designed reinforced concrete frame structures (vulnerability class c). historical buildings like churches, castles and towers were considered for the intensity assessment purposes only in sites where the building stock was undamaged or slightly affected. iii. intensity assessment differently from the 2012 emilia-romagna seismic sequence, characterized by relevant aftershocks comparable to the may 20 mainshock, only one event was characterized by a magnitude mw greater than 5.0, about one hour after the 1:36 utc mw 6.0 mainshock: for this reason, the intensity map can be reasonably viewed as the effects of the main event. intensity values were defined on the basis of the damage grades detected on the different vulnerability classes observed in the analyzed villages. in some centers, difficulties in the intensity assessment were experienced mainly due to differences between damage levels observed in old centers and on recent reinforced concrete buildings: hence, teams surveyed such sites more times, to reduce subjectivity judgment. figure 1 illustrates the intensity iems distribution over the struck territories and table 1 lists iems values for each site surveyed. major intensities were observed in amatrice and pescara del tronto, which were mainly justified by a diffused level 5 of damage to several masonry buildings (figure 2). damages compatible with a iems 8 were detected in villages in an area within a radius of about 12 km far from the instrumental epicenter: in these cases, most of the vulnerability class b residential masonry buildings suffered level 3 of damage. some particular situations of very slight damage were observed close to the instrumental epicenter zone in the villages of vezzano, colle d’arquata and spelonga, probably due to beneficial site effects. the iems 6 area instead extended asymmetrically northwards, about 30 km from the instrumental epicenter, whereas southwards effects seemed significantly attenuated. some difficulties were encountered in the intensity evaluation of damage in the southeastern towns of the abruzzo region, since many buildings were still presenting damage caused by the 2009 l’aquila sequence, and the relatively slight level of ground shaking induced in these areas by the mw 6.0 mainshock, if compared to the epicentral zones. another challenging issue was related to the assessment in the umbria towns, since most of them were seismically retrofitted after 1997 sequence: here the critical issue annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7172 3 figure 1: macroseismic intensities observed after the august 2016 seismic sequence (damage up to september 6, 2016). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7172 4 table 1: localities surveyed after the august 2016 seismic sequence. d(*) municipality locality iems d(*) municipality locality iems ap arquata del t. pescara del t. 10-11 ri amatrice nommisci 6-7 ri amatrice saletta 10 pg norcia savelli 6-7 ri amatrice amatrice 9-10 pg norcia valcadara 6-7 ri accumoli illica 9 ap arquata del t. trisungo 6-7 ri amatrice san lorenzo fl. 9 ri accumoli roccasalli 6 ap arquata del t. arquata del t. 9 ri amatrice pasciano 6 ri accumoli accumoli 8-9 ri amatrice san giorgio 6 ri accumoli fonte del campo 8-9 ri amatrice santa giusta 6 ri accumoli villanova 8-9 pg norcia agriano 6 ri amatrice casale 8-9 pg norcia campi 6 ri amatrice preta 8-9 ap acquasanta t. acquasanta t. 6 ap arquata del t. capodacqua 8-9 ap arquata del t. colle 6 ap arquata del t. tufo 8-9 ap arquata del t. faete 6 ri accumoli grisciano 8 ap arquata del t. spelonga 6 ri accumoli san giovanni 8 ap arquata del t. vezzano 6 ri accumoli tino 8 ap montegallo montegallo 6 ri amatrice casteltrione 8 fm montefortino montefortino 6 ri amatrice cornelle di sotto 8 mc tolentino tolentino 6 ri amatrice cossito 8 mc visso visso 6 ri amatrice mosicchio 8 ri accumoli terracino 5-6 ri amatrice petrana 8 ri amatrice cornillo nuovo 5-6 ri amatrice retrosi 8 ri amatrice forcelle 5-6 ri amatrice sommati 8 ri amatrice varoni 5-6 pg norcia castelluccio 8 pg cascia cascia 5-6 ap arquata del t. piedilama 8 pg cascia castel s. maria 5-6 ap arquata del t. pretare 8 pg cascia civita 5-6 ri accumoli macchia 7-8 pg norcia ospedaletto 5-6 ri amatrice bagnolo 7-8 pg norcia pescia 5-6 ri amatrice capricchia 7-8 pg norcia piè la rocca 5-6 ri amatrice collepagliuca 7-8 pg preci preci 5-6 ri amatrice colli 7-8 ap montegallo bisignano 5-6 ri amatrice cornillo vecchio 7-8 ap montegallo castro 5-6 ri amatrice moletano 7-8 ap montegallo uscerno 5-6 ri amatrice scai 7-8 fm amandola amandola 5-6 ri amatrice torrita 7-8 fm montefortino cerretana 5-6 pg norcia san pellegrino 7-8 mc castelsantangelo castelsantangelo 5-6 ri amatrice collegentilesco 7 mc sarnano sarnano 5-6 ri amatrice colleposta 7 mc ussita ussita 5-6 ri amatrice configno 7 aq campotosto poggio cancelli 5-6 ri amatrice patarico 7 aq montereale aringo 5-6 ri amatrice poggio vitellino 7 ri borbona borbona 5 ri amatrice roccapassa 7 ri cittareale cittareale 5 ri amatrice san benedetto 7 ri cittareale santa croce 5 ri amatrice voceto 7 ri posta bacugno 5 pg cascia avendita 7 ri posta fontarello 5 pg norcia fontevena 7 ri posta posta 5 pg norcia frascano 7 pg norcia aliena 5 pg norcia norcia 7 pg norcia fogliano 5 pg norcia nottoria 7 pg norcia maltignano 5 aq montereale santa lucia 7 pg norcia puro 5 ri accumoli collespada 6-7 pg norcia sant’andrea 5 ri amatrice collemoresco 6-7 pg preci collescille 5 ri amatrice domo 6-7 pg preci piedivalle 5 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7172 5 d(*) municipality locality iems d(*) municipality locality iems pg preci saccovescio 5 aq capitignano capitignano 5 ap ascoli piceno castel trosino 5 aq capitignano collenoveri 5 ap ascoli piceno mozzano 5 aq capitignano pago 5 ap acquasanta t. arli 5 aq capitignano paterno 5 ap acquasanta t. centrale 5 aq capitignano sivignano 5 ap acquasanta t. corneto 5 aq montereale casale bottone 5 ap acquasanta t. novele 5 aq montereale castiglione 5 ap acquasanta t. paggese 5 aq montereale cesaproba 5 ap acquasanta t. ponte d’arli 5 aq montereale cesariano 5 ap acquasanta t. quintodecimo 5 aq montereale lonaro 5 ap acquasanta t. san martino 5 aq montereale marana 5 ap acquasanta t. santa maria 5 aq montereale montereale 5 ap montegallo forca 5 aq montereale piedicolle 5 ap montemonaco montemonaco 5 aq montereale san vito 5 ap montemonaco pignotti 5 aq montereale santa vittoria 5 ap montemonaco rocca 5 aq montereale verrico 5 ap montemonaco san giorgio isola 5 aq montereale ville 5 ap roccafluvione roccafluvione 5 te cortino cortino 5 mc caldarola caldarola 5 te crognaleto alvi 5 mc camerino camerino 5 te crognaleto cesacastina 5 mc camerino san luca 5 te crognaleto crognaleto 5 mc camerino san marcello 5 te crognaleto nerito 5 mc castelsantangelo gualdo 5 te crognaleto poggio u. 5 mc gualdo gualdo 5 te crognaleto san giorgio 5 mc muccia muccia 5 te crognaleto tottea 5 mc pieve torina apennino 5 te valle castellana ceraso 5 mc pieve torina capriglia 5 te valle castellana cerquito 5 mc pieve torina pieve torina 5 te valle castellana morrice 5 mc pievebovigliana pievebovigliana 5 te valle castellana pascellata 5 mc ripe s. ginesio ripe s. ginesio 5 te valle castellana pietralta 5 mc san ginesio san ginesio 5 te valle castellana san vito 5 mc ussita calcara 5 te valle castellana valle castellana 5 mc ussita cuore di sorbo 5 ap ascoli piceno ascoli piceno 4-5 mc ussita frontignano 5 ap ascoli piceno casette 4-5 mc visso borgo s. antonio 5 ap venarotta venarotta 4-5 aq campotosto campotosto 5 mc belforte belforte 4-5 aq campotosto ortolano 5 mc muccia maddalena 4-5 (*) districts: rieti (ri), perugia (pg), ascoli piceno (ap), fermo (fm), macerata (mc), aquila (aq), teramo (te). was to define the most reliable ems98 vulnerability class, for properly derive the iems degree. a review of the local strengthening techniques suggested to adopt, in such cases, a vulnerability class d for masonry structures. unreinforced buildings, if present, were also considered in these localities to check the reliability of the assigned intensity value. iv. conclusions results of a macroseismic survey performed in the area struck by the august 24, 2016 amatrice sequence were presented. a total number of 180 sites were surveyed and for each of them a iems value was defined. according to the results shown in this work, it can be concluded that site effects annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7172 6 figure 2: diffused collapses in amatrice (left) and aerial view of pescara del tronto (right, retrieved on www.meteoweb.eu). significantly influenced the 2016 central italy sequence. additionally, a key aspect of this work related the evaluation of the effects induced by the analyzed earthquake to predamaged and seismically retrofitted residential buildings, for a proper derivation of the a iems value. finally, a preliminary analysis based on statistical inference of the ems intensities assessed was performed, evidencing how the macroseismic epicenter seems located 4 km southeastwards with respect to the instrumental one. references castelli, v. 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i e r i c a r i c a v a t a i n u n t e r r e n o , p r e f e r i b i l m e n t e r o c c i o s o ( f i g . 1). d e t t a c a v i t a c o m u n i c a con l ' a t m o s f e r a p e r m e z z o di u n t u b o t a n c h e r i p i e n o in p a r t e di l i q u i d o . u n p i c c o l o c a p i l l a r e m a n t i e n e c o s t a n t e il l i v e l l o del l i q u i d o e fa sì c h e non -i r i s e n t a n o l e v a r i a z i o n i l e n t e di v o l u m e , c a u s a t e d a v a r i a z i o n i di t e m p e r a t u r a o di p r e s s i o n e a t m o s f e r i c a . l n ' o n d a l o n g i t u d i n a l e i n c i d e n t e fa v a r i a r e il v o l u m e d e l l a f e r a e q u i n d i p r o d u c e u n o s p o s t a m e n t o del l i v e l l o del l i q u i d o n e l t u b o . s i a s = a s e n cn|/ — 1 e q u a z i o n e d e l l o n d a l o n g i t u d i n a l e r i f e r i t a al c e n t r o d e l l a s f e r a ( e è la v e l o c i t à eli p r o p a g a z i o n e d e l l e o n d e l o n g i t u d i n a l i nel t e r r e n o ) : il l i v e l l o del l i q u i d o nel t u b o 7 -i s p o s t e r à a l l o r a di h = —co a cos cot [ 2 ] ac dove v„ è il v o l u m e d e l l a f e r a ed i l a s e z i o n e del t u b o . q u e s t a form u l a è s l a t a s t a b i l i t a d a .benioil' p a r t e n d o d a l l a v a r i a z i o n e d i v o l u m e a v= — i a d v [ 3 ] d o v e o è la c o n d e n s a z i o n e d o v u t a a l i o n d a i n c i d e n t e d e f i n i t a d a ss a — , e f a c e n d o la s e g u e n t e i p o t e s i : c/) la v i b r a z i o n e l o n g i t u d i n a l e i n c i d e n t e h a u n a l u n g h e z z a di onda m o l t o p i ù g r a n d e del r a g g i o d e l l a f e r a : q u e s t a i p o t e s i è q u a s i t ' j bknioff 11.. b u l l , ol' t h e s e i i u o l o g i c a l s o e . o f . a n i . 1 9 3 5 , 25, p a g . 3 0 7 . 2 0 8 r e n a t o c i a l d e a s e m p r e v e r i f i c a l a nel caso d e l l e o n d e s i s m i c h e o r d i n a r i e , c l i c h a n n o in g e n e r a l e u n a l u n g h e z z a vi o n d a m o l l o g r a n d e . m a e v i d e n t e m e n l e b e n i off. n e l «no c a l c o l o , s u p p o s e i n o l i l e : b) l ' i m p e d e n z a m e c c a n i c a d e l l ' a p p a r a t o è t r a s c u r a b i l e ; e ) l ' o s c i l l a z i o n e d e l l e d i v e r s e p o r z i o n i d e l l a s u p e r f i c i e di separ a z i o n e ira il t e r r e n o ed il l i q u i d o è i d e n t i c a p u n l o p e r p i l l i l o a l l a o s c i l l a z i o n e i n c i d e n t e p e r a m p i e z z a e f a s e . v e d i a m o a n z i t u t t o c o m e i n l l u i s e a l ' i m p e d e n z a m e c c a n i c a s u l l o spos t a m e n t o d e l l i v è l l o del l i q u i d o nel tulio t , s e m p r e s u p p o n e n d o val i d e l e i p o t e s i a ) e e ) : a p p u n t o i n b a s e a tali i p o t e s i , il c a l c o l o è m o l t o s e m p l i f i c a t o , g i a c c h é la v a r i a z i o n e di p r e s s i o n e , d o v u t a a l l a v i b r a z i o n e i n c i d e n t e , si p u ò r i t e n e r e c o s t a n t e , i s t a n t e p e r i s t a n t e , in tutti i p u n t i d e l l a s f e r a . si h a i n f a t t i p e r q u e s t a v a r i a z i o n e d i p r e s sione a p = e a [ 4 ] dove £ è il m o d u l o di c o m p r e s s i b i l i t à del l i q u i d o ; o r a , se si s u p p o n e la l u n g h e z z a d ' o n d a d e l l a v i b r a z i o n e i n c i d e n t e m o l t o p i ù g r a n d e del r a g g i o d e l l a s f e r a , si p u ò r i t e n e r e a c q u i n d i a n c h e a p , c o s t a n t e p e r ogni punto d e l l a s f e r a , i s t a n t e p e r i s t a n t e . si c o n s i d e r i ora ( f i g . 1} la sfera d i r a g g i o k. il t u b o t d i sez i o n e ( , r i e m p i l o di l i q u i d o f i n o a l l ' a l t e z z a l e sia li l o s p o s t a m e n t o del l i v e l l o r i f e r i t o a l l a sua p o s i z i o n e d i e q u i l i b r i o . d a l l a b e n nota r e l a z i o n e di e l a s t i c i t à a p = — e — [ 5 ] v si p u ò t r o v a r e il v a l o r e d e l l a p r e s s i o n e e s e r c i t a t a n e l l i q u i d o p e r effetto d e l l a v a r i a z i o n e di v o l u m e : q u e s t a p r e s s i o n e a g i s c e sul l i q u i d o c h e si trova nel t u b o t , p r o d u c e n d o v i u n a v a r i a z i o n e d i l i v e l l o li: p e r c i ò nel a l d e l l a [ 5 ] si d e v e c o n s i d e r a r e non solo l a v a r i a z i o n e di v o l u m e d o v u t a a l l a c o n d e n s a z i o n e , m a a n c h e la v a r i a z i o n e li i dovuta al f a t t o che il l i q u i d o v i e n e ad o c c u p a r e un v o l u m e m a g g i o r e , in s e g u i t o a l l ' i n n a l z a m e n t o li del l i v e l l o n e l t u b o t ; n e l d e n o m i n a t o r e d e l l a [ 5 ] p o i , al posto di p , si p o t r à p o r r e il v o l u m e ' o d i tutto il l i q u i d o c o n t e n u t o s i a n e l l a s f e r a , sia n e l t u b o ; ciò si p u ò a m m e t t e r e , -e si s u p p o n e c h e i « sia m o l l o p i ù g r a n d e del v o l u m e spostato d u r a n t e mi o s c i l l a z i o n e . la p r e s s i o n e a l l ' i n t e r n o s a r à q u i n d i a v+ha a i ' = — e \ , . n. „ — \ l t e g l s t k v z i o n e d e i . i . e o n d e l o n g i t u d i n a l i 2 0 9 s e -i s u p p o n e i n o l t r e c l i c l ' a l t e z z a / del tubo -iu p i ù p i c c o l a d e l l a lunghe •zza d ' o n d a d e l l a v i b r a z i o n e nel l i q u i d o , si p u ò a m m e t t e r e c h e la m a s s a del l i q u i d o c o n t e n u t o n e l tubo t si -posti tutta i n s i e m e : si p u ò a l l o r a a p p l i c a r e a q u e s t a m a s s a m l ' e q u a z i o n e d i m o l o , e -i h a così, t r a s c u r a n d o gli a t t r i t i , d'h a a> m — =—e a) —e — li di» r„ v0 e cioè in b a s e a l l a ! i ) r i f e r i t a a l c e n t r o d e l l a s f e r a ed a l l a [ i j d-h aaa< o m -(e — li — e eos o)/ dii n t a l e e q u a z i o n e a n i m e l l e c o m e è n o l o l ' i n t e g r a l e p a r t i c o l a r e f amo ,, eos mi j_o j mc({}~—or) dove la p u l s a z i o n e di r i s o n a n z a 12 d e l l a p p a r a t o è d a t a da q = 1 ' t e . = r , i ' i o » " o q u e s t a f o r m u l a è i d e n t i c a a ( l u c i l a d e i r i s o n a t o r i di h e l m h o l t z : c i ò è e v i d e n t e , e s c n d o s i a l o f a l l o un r a g i o n a m e n t o a n a l o g o ( q u i e , è la v e l o c i t à di p r o p a g a z i o n e d e l l e v i b r a z i o n i nel l i q u i d o ) . nel caso poi c h e si c o n s i d e r i una f r e q u e n z a i n c i d e n t e l a u t o picc o l a da p o t e r t r a s c u r a r e o>2 r i s p e l l o a d la |') | d i v e n t a j 0 nm li = — eos (0/ airi iv >i i d e n t i l i c a con la |2| t r o v a t a d a b e n i o i l . s e l ' i p o t e s i t/) non è p i ù v a l i d a , la c o n d e n s a z i o n e non p u ò p i ù r i t e n e r s i c o s t a n t e in ogni p u n t o d e l l a f e r a : q u i n d i la p r e s i o n e v a r i a da p u n t o a p u n t o e si p r o p a g a n e l l ' i n t e r n o . i n o l t r e , se si avessero d e l l e v i b r a z i o n i la cui l u n g h e z z a d o n d a fosse tanto p i c c o l a da g e n e r a r e d e l l e o n d e s t a z i o n a r i e n e l l ' i n t e r n o d e l l a s f e r a , l ' a t t a c c o del tubo 7 p o t r e b b e c a p i t a r e in u n a p o s i z i o n e q u a l u n q u e r i s p e t t o a l l e zone vent r a l i ed a ( i n c i l e n o d a l i : -i u d r e b b e r o a v e r e q u i n d i in q u e s t o p u n t o d e l l e p r e s s i o n i c h e d i p e n d e r e b b e r o non solo d a l l a m p i e z z a e d a l l a f r e q u e n z a d e l l ' o n d a i n c i d e r n e , m a a n c h e e « o p r a t u t t o d a l l a d i r e z i o n e di p r o v e n i e n z a d e l l ' o n d a . p e r e v i t a r e q u e s t o i n c o n v e n i e n t e c h e p o t r e b b e p r e s e n t a r s i q u a l o r a 58 r e n a t o c i a l d e a -i v o l e s s e r o r e g i s t r a r e d e l l e v i o l a z i o n i di p i c c o l a l u n g h e z z a d o n d a , o c c o r r e f a r e u n a v a r i a n t e a l l ' a p p a r a t o s o p r a d e s c r i t t o . i a l e v a r i a n t e c o n s i s t e n e l p o n e il t u b o t i n t e r n a m e n t e a l l a s f e r a ( f i g . 2), in m o d o c h e l a s u a e s t r e m i t à a p e r t a i n f e r i o r e si trovi p r o p r i o al c e n t r o . o g n i p o r z i o n e d e l l a s f e r a si p u ò a l l o r a c o n s i d e r a r e c o m e u n a s o r g e n t e c h e e m e t t a , d u r a n t e la sua v i b r a z i o n e , d e l l e o n d e p r o p a g a n i c s i n c l l i n t e r n o d e l l a s f e r a : ora t u t t e le a z i o n i di q u e s t e o n d e a r r i v e r a n n o al c e n t r o con u n o stesso r i t a r d o r e l a t i v a m e n t e a l l e v i b r a z i o n i d e l l e r i s p e t t i v e s o r g e n t i , c i o è d e l l e d i v e r s e p o r z i o n i d e l l a s u p e r f ì c i e d e l l a s f e r a . si p u ò cosi f a r e 1 i p o t e s i c h e la p r e s s i o n e n e l c e n t r o a l l ' i s t a n t e i sia p r o p o r z i o n a l e a l l a v a r i a z i o n e di v o l u m e d e l l a s f e r a a l l ' i s t a n t e ! — ( e , è la v e l o c i t à ili p r o p a g a z i o n e di u n a v i b r a z i o n e e l a s t i c a nel l i q u i d o ) ed a l l a v a r i a z i o n e di v o l u m e d e l l i q u i d o c o n t e n u t o nel t u b o t l ' e l a t i v a ali i s t a n t e t: a n c h e o r a si s u p p o n e c h e l ' a l t e z z a / d e l l i v e l l o del l i q u i d o sia p i c c o l a r i s p e t t o a l l a l u n g h e z z a d ' o n d a d e l l a v i b r a z i o n e n e l l i q u i d o : si h a q u i n d i d a l l a s o l i l a r e l a z i o n e di e l a s t i c i t à — ( t 7 > r r ' b p e r c a l c o l a r e la v a r i a z i o n e di v o l u m e d e l l a s f e r a , d o v u t a ad u n ' o n da l o n g i t u d i n a l e i n c i d e n t e di e q u a z i o n e [ 1 ] , m p o t r e b b e p a r t i r e d a l l a registrazione delle onde longitudinali 2 1 1 | i j . p o n e n d o p e r ò a f u n z i o n e d e l l a p o s i z i o n e e del t e m p o . m a si p u ò p i ù s e m p l i c e m e n t e c o n s i d e r a r e il v o l u m e d e l s o l i d o i n f i n i t e s i m o g e n e i a t o da una corona c i r c o l a r e c o a s s i a l e a l i asse v. d u r a n t e la sua o s c i l l a z i o n e e p o i i n t e g r a r l o a t u t t a la s f e r a . c o n s i d e r i a m o a l l o r a u n a corona c i r c o l a r e e l e m e n t a r e c o m p r e s a tra d u e coni di a p e r t u r a (| e cp-j-ezrp ( l i g . 3 ) : t u t t a la c o r o n a -i s p o s t e r à di un t r a t t o s d a t o d a l l a [ 1 ] , s u p p o n e n d o c l i c l ' a l t e z z a del s e g m e n t o sia tanto p i c c o l a da r i t e n e r e s c o s t a n t e su tutta la c o r o n a ; in l a i m o d o si v i e n e a f o r m a r e u n sol i d o i n f i n i t e s i m o c o m p r e s o ira la sfera i n i z i a l e e la s u p e r f i c i e d e l l a corona s p o s t a l a : il suo v o l u m e è d a t o e v i d e n t e m e n t e d a l l a s u p e r f i c i e d e l l a corona p e r l ' a l t e z z a , clic è r a p p r e s e n t a t a d a l l a p r o i e z i o n e di s sulla n o r m a l e a l l a s f e r a e q u i n d i da 3 "\ \ d t 1 1 v •'vx i x . . . 1 \ i r / / dv — 2.t r~ seni| d'p a cosip senio 1 ed essendo .v = — r cosip, si h a di -.. 2.-t ra senip cos<| s e n ( t r c o s q s e n 2rc i — -4 t }. [ 7 ] la |t] va i n t e g r a t a p e r i|> v a r i a b i l e da 0 a -"tl i n t e g r a z i o n e è i m m e d i a t a e -i o p e r a la s o s t i t u z i o n e 2 re r y — — — cosi) = p eosij 1. con la q u a l e si o t t e n g o n o i d u e i n t e g r a l i . , 1 a/~ , 1 a/r , al — — / — senco/y cosv dy — — cosco? v s e n v a y ' 2:r * " / 2,-t di cui il p r i m o è n u l l o e l ' a l t r o v a l e a)'~ , / sen (3—-j3cos|3 \ a f — -j coso)/ (sen|"3 — |3eos|3) = -j4,-t r~a coscof. re \ ) [ 8 ] 60 r e n a t o c i a l d e a p e r v a l o r i di (3 m i n o r e d i 1. cioè p e r l u n g h e z z e d ' o n d a m o l l o p i ù g r a n d i di r. la [ 8 ] si p u ò s v i l u p p a r e i n s e r i e , o t t e n e n d o a v = — 4jt r a z (—1)" — lì'2"-1 cos cot (271+1)! e se -i t r a s c u r a n o i t e r m i n i di g r a d o s u p c r i o r e , si o t t i e n e i i v,ao) a 1 = cos co/ da cui -i p u ò f a c i l m e n t e o t t e n e r e la f o r m u l a 1 2 ] di b e n i o f f . per t r o v a r e la v a r i a z i o n e h dei l i v e l l o d e l l i q u i d o , si a p p l i c h i l ' e c p i a z i o n e di m o t o a l l a m a s s a d e l l i q u i d o c o n t e n u t o n e l tubo t. e d a l l a [ 6 ] d'h , a~ , 4 jt za s e n t ì —rcosfi / r\ m e — h = r~a cos co l d'1 v0 p 0 \ ) t a l e e q u a z i o n e a m m e t t e c o m e i n t e g r a l e p a r t i c o l a r e 4 ite r''aa i sentì—bcostì / r li = — • 1 — • cos col t — m i ' , q2—co2 pr d o v e c o m e al solito c )'• ± < i • in q u e s t a f o r m u l a c o m p a i o n o v a r i f a t t o r i , d e i q u a l i è b e n e f a r e u n a r a p i d a a n a l i s i . o l t r e a l l ' a m p i e z z a a d e l l a v i b r a z i o n e i n c i d e n t e , il p r i m o f a t t o r e c o n t i e n e a l c u n e g r a n d e z z e c a r a t l e r i s t i c l i e d e l l a s f e r a e del l i q u i d o a d o p e r a t o . il s e c o n d o f a t t o r e l i e n e conto c h e l u t t o il sistema ha u n a p r o p r i a f r e q u e n z a di r i s o n a n z a . il terzo f a t t o r e è u n a f u n z i o n e di (3 di cui si r i p o r t a n e l l a fi<r. f il d i a g r a m m a p e r v a l o r i di [j c o m p r e s i tra 0 e 8 c a . l u l t i m o f a t t o r e i n f i n e è la v a r i a z i o n e nel t e m p o e la d i f f e r e n z a d i f a s e t r a il m o t o di h e la v i b r a z i o n e i n c i d e n t e r i f e r i t a a l c e n t r o d e l l a s f e r a ; è n a t u r a l e c l i c q u e s t a f a s e p e r (3 p r o s s i m i a 0, cioè p e r l u n g h e z z e d ' o n d a m o l l o g r a n d i , t e n d a ad ann u l l a r s i , r i c a d e n d o cosi n e l l a f o r m u l a di b e n i o f f . registrazione: delle onde longitudinali 2 1 3 in q u a n t o a l l a reg i s i r a z i o n e ' d e l l e vib r a z i o n i i n c i d e n t i , essa p u ò e s s e r e f a l l a con un q u a l s i a s i m e t o d o o m e c c a n i c o o ottico o e l e t t r i c o c h e possa reg i s t r a r e g l i s p o s t a m e n ti d i un, g a l l e g g i a n t e p o s t o l i l l a s u p e r f i c i e d e l l i q u i d o nel tulio t. nel caso p e r ò elicla r e g i s t r a z i o n e a v v e n ga con u n m e t o d o clic l i v e l i non g l i spostam e n t i , m a le v a r i a z i o ni d e g l i s p o s t a m e n t i nel t e m p o ( p e r es. u n m e t o d o c i e l i r o m a g n e tieo), la r i s p o s t a d e l l ' a p p a r a l o si m o d i f i c a : co-i le e l o n g a z i o n i d di u n o s t r u m e n t o r e g i s t r a t o r e , -olio p r o p o r <|il ilici i t e. ri ne 1 sen|j — peosfl . .. dh / m i n a l i a di • sen ii, l s e n 8 — 3 c o s s d e l l a f u n z i o n e n e l l a fig. -1. romu istillilo fazionate ili geofìsica v i e n e r i p o r t a t o a n a l o g o g r a f i c o febbrai o 1918. riassunto nella presente nota t iene studiati) il coni portamento di un re. gistralore ili onde longitudinali, basato rullìi condensazione: l i. introduce quindi una modifica all' appurato, suggerito dal rcnioff nel caso particolare delle ondi• sismiche, per migliorarne la risposta. 145_153 adg v–5 n01 pietrel.pdf annals of geophysics, vol. 45, n. 1, february 2002 145 could we find any signal of the stratosphere-ionosphere coupling in antarctica? marco pietrella (1), edward s. kazimirovsky (2), giorgiana de franceschi (1), paolo grigioni (3) and carlo scotto (1) (1) istituto nazionale di geofisica e vulcanologia, dept. rm2-uf/faa, roma, italy (2) institute of solar-terrestrial physics, russian academy of sciences, irkutsk, russia (3) enea casaccia, s. maria di galeria (rm), italy abstract an investigation searching for a possible coupling between the lower ionosphere and the middle atmosphere in antarctica is here performed on the basis of stratospheric vertical temperature profiles and ionospheric absorption data observed at the antarctic italian base of terra nova bay (74.69°s, 164.12°e) during local summer time. the result obtained by applying a multi-regression analysis and a superimposed epoch analysis (sea) shows a statistically significant ionosphere-stratosphere interaction. in particular, by selecting stratospheric temperature maxima occurring at different heights as the referring epoch («zero day») for the sea approach, the ionospheric absorption is found to show a positive and/or negative trend (several days) around it. the tendency for an increasing/decreasing absorption is obtained for temperature maxima occurring below/above the stratospheric level of about 17-19 km, respectively. mailing address: dr. marco pietrella, istituto nazionale di geofisica e vulcanologia, dept. rm2-uf/faa, via di vigna murata 605, 00143 roma, italy; e-mail: pietrella@ingv.it 1. introduction geophysicists have extensively discussed the problem of a stratosphere-ionosphere interaction during the last decades. it has become apparent that the ionosphere cannot be studied properly as merely magneto-active plasma without considering the general properties of the atmosphere. the lower ionosphere is the region where the solar activity (controlling the higher ionospheric regions) and the meteorological influences (controlling the lower atmospheric regions) become comparable. under some circumstances, the meteorological parameters control the variability in the lower ionosphere. it is now accepted that some reasons for ionospheric variability could be sought in the dynamic interaction between lower and upper atmosphere. many discoveries and experimental results have confirmed the existence of detailed correlations between the parameters of the lower atmosphere and the ionosphere (see, e.g., kazimirovsky and kokourov, 1991 and references therein). as part of the physical mechanism for the «coupling from below» the influence of upward propagating internal atmospheric waves may be considered. planetary waves, tides and acoustic-gravity waves are essential sources of key words polar ionosphere – middle-upper atmosphere interaction 146 marco pietrella, edward s. kazimirovsky, giorgiana de franceschi, paolo grigioni and carlo scotto energy and momentum for the thermosphere and ionosphere (wickwar and carlsson, 1999). it seems likely that there could be some events in the mid-latitude ionosphere due to the effect of atmospheric oscillations such as: winter anomaly of the ionospheric absorption of radio waves, sporadic-e layer (e s ) occurrence and structure, thermospheric wind regime, travelling ionospheric disturbances (tid), day-to-day ionospheric variability, spread-f events, etc. i n particular, the winter anomaly is under «meteorological control» (taubenheim, 1983) and the major mechanisms are the increased ion production rate due to an increase in no concentration, and the decreased loss rate due to removal of water-cluster ions. both may be regarded as the effects of a large-scale transport of species from the auroral zone due to changes in the circumpolar wind pattern reaching from the stratosphere up to the ionospheric d-region. ionospheric absorption is the most suitable parameter to «feel» the meteorological influences on the lower ionosphere because it depends on the relative changes in the electron density and the collision frequency between electrons and neutral air particles. these changes could be subjected to variations in the lower thermospheric composition, which can originate from dynamical processes. these processes, in turn, appear to be (at least partly) related to stratospheric changes. the first attempts to find the signal of the stratosphere-ionosphere interaction were carried out on the basis of ionospheric absorption and stratospheric temperature data: the discovery of a relation between increases in temperature at the 10 mb level and the absorption of radiowaves in the ionospheric d-region, established for the first time the existence of a link between the stratosphere and the lower ionosphere (bossolasco and elena, 1963). the discovery of a correlation between the winter anomaly observed at middle latitudes and the sudden stratospheric warming which, as known at present, are connected with planetary wave activity (shapley and beynon, 1965; bossolasco and elena, 1967), became a significant new part of the upper atmosphere physics concerning the stratosphere-ionosphere coupling which may be generally defined as any association between the variability of a parameter in the stratosphere with one in the ionosphere. the majority of the results concern the northern hemisphere at middle latitudes: by using the superimposed epochs analysis (sea) a very clearly positive association between the winter anomaly and the stratospheric warming was established, and a tendency for subsidiary absorption maxima to recur at an interval of about 5-8 days before and after a stratospheric warming was found (shapley and beynon, 1965). some experimental investigations have been carried out on the stratosphere-ionosphere coupling in the southern middle latitudes hemisphere (40-50 s): the sea performed to investigate the effect of stratospheric warming on the ionospheric absorption as measured by f min (the lowest frequency of reflection wave recorded in the ionogram), still established a positive correlation between temperature and ionospheric absorption, but, unlike the northern hemisphere, a day-to day variation in f min maxima is of about 2-4 days. moreover, the fmin values several days before the temperature maximum are greater than those observed several days afterwards. so particle precipitation should have a marked effect on the stratosphere as well as on the ionosphere, but ionospheric absorption is still a useful measure for investigating the effects due to meteorological disturbances (essex and morton, 1974; fraser and thorpe, 1976). very few observational studies for stratosphere-ionosphere coupling have been made at high latitudes. there were some results for north-east siberia, i.e. at northern subauroral latitudes (kershengolts, 1986), where some correlations between stratospheric temperatures and f 2 -region parameters have been found by concluding that the sign of correlation depends on the epoch of solar activity. to extend our knowledge on the middleupper atmosphere interaction, here the preliminary results are reported concerning multi-regression and superimposed epoch analyses applied to ionospheric absorption and stratospheric temperature data observed in antarctica during austral summer time. the data could we find any signal of the stratosphere-ionosphere coupling in antarctica? 147 analysis is described in section 2 and the results are shown in section 3. concluding remarks are finally given in section 4. 2. data used and method of analysis since 1993, in the frame of pnra (italian program for antarctic researches), the ingv (istituto nazionale di geofisica e vulcanologia rome, italy) maintains an ionospheric observatory and two riometer stations at the italian antarctic base of terra nova bay (geographic coordinates: 74.69 s, 164.12 e; geomagnetic coordinates: 77.34 s, 279.41 e) for the vertical ionospheric sounding and for the cosmic noise measurements at the frequencies of 30 mhz and 38.2 mhz. at the same location, during austral summer time, measurements of tropospheric and stratospheric parameters, acquired by balloons launched each day around 12.00 ut and 00.00 ut (bacci and pellegrini, 1996; grigioni, 1996), are regularly provided by the enea (ente per le nuove tecnologie l’energia e l’ambiente, rome, italy). ionospheric absorption data, derived from the cosmic noise measurements at 30 mhz (a2 method, de franceschi et al., 1997), stratospheric temperature data, derived from the vertical temperature profiles obtained by balloons in the range of geopotential altitudes 13-26 km, the solar flux f 10.7 , and the time weighted magnetic index ap ( = 0.8) (see, e.g., perrone and de franceschi,1999 and reference therein) are considered as the data base during the period 1 december 1994 15 february 1995. as the stratospheric temperature values are available at 00.00 ut and 12.00 ut, the ionospheric absorption and the magnetic index used in the analyses are related to the same time of the day, while for the solar index the daily value is processed. the choice of ap ( = 0.8) to represent the magnetic activity influence on the ionosphere (at least in the f region) is due to the fact that several recent investigations pointed out a better correlation between the magnetic perturbation and the ionospheric variability if a time weighted magnetic index, such as ap ( = 0.8), is used instead of the well-known planetary ap. the multi-regression and superimposed epoch analyses, applied separately to the case of h = 00.00 ut and 12.00 ut and for each geopotential height l = 13,14...26 km, are briefly summarised as follows. 2.1. multiregression analysis the stratospheric temperature and the ionospheric absorption data series (t(h,l ) and a(h)) are considered with the daily solar flux f 10.7 and the index ap ( = 0.8, h) in the following multi-regression formula: a(h) = a(h,l) + b(h,l)t(h,l) + c(h,l)f10.7 + (2.1) +d(h,l)ap( = 0.8,h). the numerical coefficients a(h,l), b(h,l), c(h,l), and d(h,l), have been calculated by means of the least squares method to evaluate the meteorological, solar and magnetic influence on the ionospheric absorption. fisher’s test (confidence level = 95%) has been performed to check the statistical significance of the model. 2.2. superimposed epochs analysis (sea) the sea has been applied to search for a possible influence of the stratospheric temperature on the ionospheric absorption. the temperature data series, t(h,l) (whose values ranged between about – 32 °c and – 42 °c), and absorption data series, a(h) (whose values ranged between about 0.014 and 0.11 db) from 1 december 1994 to 15 february 1995 have been considered. firstly the temperature maxima tmax(h,l) were selected as the «zero day» epochs and 6 days before and after each «zero day» were taken into account: – for each t max (h,l), the corresponding values of the ionospheric absorption were considered to calculate the average value of the absorption corresponding to the «zero day». – then, the daily average absorption values were calculated at 1, 2...6 days before and after 148 marco pietrella, edward s. kazimirovsky, giorgiana de franceschi, paolo grigioni and carlo scotto the «zero day». in this way the average behaviour of the ionospheric absorption 6 days before and 6 days after tmax(h,l) was evaluated for each h and l. – the average levels of ionospheric absorption and the standard deviations before and after the «zero day» were also calculated to define the statistically significant results. table ii. results of multi-regression analysis in the range 13-26 km for data set at 00.00 ut: statistically significant results are in bold. l a(h,l) b(h,l) c(h,l) d(h,l) f f 13 km .09 .0013 .0010 .0014 1.61 2.76 14 km .06 .0039 .0018 .0014 2.46 2.76 15 km .29 .0105 .0023 .0013 8.80 2.76 16 km .26 .0086 .0016 .0012 8.80 2.76 17 km .25 .0079 .0012 .0016 6.33 2.76 18 km .23 .0073 .0012 .0017 5.29 2.76 19 km .27 .0081 .0009 .0022 6.28 2.76 20 km .18 .0054 .0008 .0018 3.50 2.76 21 km .15 .0051 .0010 .0019 3.33 2.76 22 km .12 .0048 .0012 .0019 3.11 2.76 23 km .06 .0030 .0011 .0018 1.89 2.76 24 km .07 .0033 .0011 .0019 2.10 2.76 25 km .08 .0034 .0011 .0013 1.75 2.76 26 km .09 .0044 .0014 .0015 1.82 2.76 table i. results of multi-regression analysis in the range 13-26 km for data set at 12.00 ut: statistically significant results are in bold. l a(h,l) b(h,l) c(h,l) d(h,l) f f 13 km .01 .0002 .0007 .0003 0.41 2.76 14 km .08 .0034 .0014 .0003 1.40 2.76 15 km .24 .0079 .0017 .0003 6.76 2.76 16 km .32 .0091 .0011 .0004 7.88 2.76 17 km .25 .0063 .0006 .0006 4.31 2.76 18 km .21 .0053 .0005 .0006 2.83 2.76 19 km .20 .0047 .0003 .0006 2.58 2.76 20 km .11 .0024 .0004 .0006 .089 2.76 21 km .07 .0015 .0005 .0006 0.55 2.76 22 km .05 .0008 .0004 .0004 0.27 2.76 23 km .02 .0008 .0007 .0005 0.39 2.76 24 km .07 .0005 .0013 .0002 0.90 2.76 25 km .09 .0011 .0013 .0001 0.89 2.76 26 km .09 .0012 .0013 .0001 0.81 2.76 could we find any signal of the stratosphere-ionosphere coupling in antarctica? 149 fig. 1. results from sea for data set at noon in the range 13-18 km: statistically significant results were found at 13, 14 and 15 km for which the difference between the mean values (mv) of the ionospheric absorption are greater than both standard deviations (sd ) calculated before and after the «zero day». fig. 2. results from sea for data set at midnight in the range 13-16 km: statistically significant results were found at 13, 14 and 16 km for which the difference between the mean values (mv) of the ionospheric absorption are greater than both standard deviations (sd ) calculated before and after the «zero day». 150 marco pietrella, edward s. kazimirovsky, giorgiana de franceschi, paolo grigioni and carlo scotto 3. results tables i and ii show the results of the multiregression analysis. for data set at h = 12.00 ut (table i), the contribution of the temperature is greater than the contribution due to the solar and magnetic activity in the range 15-18 km. for data set at h = 00.00 ut (table ii), the contribution of the temperature is greater than the contribution due to the solar and magnetic activity in the range 15-22 km. fisher’s test, reported in the last two columns, shows that these results are statistically significant, the parameter f being greater than fα selected for a confidence level = 95%. figures 1 to 4 show the results of sea application for h = 00.00 ut and 12.00 ut and for l = 13, 14...26 km. ionospheric absorption (db) and the days from t max (h,l) or «zero day» are placed on the y and x axis, respectively. the horizontal lines represent the average levels of the ionospheric absorption (the numerical values are also reported on the plot with the corresponding standard deviations). different responses of the ionospheric absorption to the stratospheric temperature can be noted: at the lower stratospheric altitudes, from 13 km up to 16-18 km, the ionospheric absorption on the average increases after the «zero day» (figs. 1 and 2), while at higher stratospheric altitudes, starting from 17-19 km up to 26 km, the ionospheric absorption on the average decreases after the tmax (h,l) day (figs. 3 and 4), independently of the time of the day (h = 00.00 ut and/or 12.00 ut). this different behaviour could depend on the different meteorological conditions in the stratosphere (e.g., the character of the meridional wind). many results are also statistically significant, the difference between the mean values of the ionospheric absorption being greater than both standard deviations calculated before and after the «zero day». fig. 3. results from sea for data set at noon in the range 19-26 km: statistically significant results have been found at 19, 22 and 23 for which the difference between the mean values (mv) of the ionospheric absorption are greater than both standard deviations (sd ) calculated before and after the «zero day». could we find any signal of the stratosphere-ionosphere coupling in antarctica? 151 fig. 4. results from sea for data set at midnight in the range 17-26 km: statistically significant results were found at 19, 23, 24, 25 and 26 km for which the difference between the mean values (mv) of the ionospheric absorption are greater than both standard deviations (sd ) calculated before and after the «zero day». 4. conclusions terra nova bay station is located close to or even inside the polar (southern) cusp region, where the geomagnetic field lines provide a direct connection between the ionosphere and the magnetosheath. it means that in this area the ionosphere should be predominantly controlled by the interaction with the magnetosphere. so it is difficult to distinguish between the ionospheric variability due to meteorological effects «coming from below», if they exist, from the 152 marco pietrella, edward s. kazimirovsky, giorgiana de franceschi, paolo grigioni and carlo scotto effects caused by the influx of the energetic particles that increase the electron density and then the ionospheric absorption in the lower ionosphere (e.g., pca phenomenon). nevertheless, during the period under consideration (rather low solar activity, f10.7 < 100 and quiet/ normal magnetic conditions, mean value of ap ( ) is about 10) strong absorption events were not recorded: in fact, on the average, the absorption values ranged between 0.014 db and 0.11 db. this could be why we pointed out the existence of a statistically significant signal of the stratosphere-ionosphere coupling (at least for some stratospheric levels) suggesting possible meteorological effects on the lower ionosphere. in particular, the analyses performed on the short period of data simultaneously available gave the following main results: – the ionospheric absorption at 30 mhz shows a correlation with the stratospheric temperature variations occurring between 1518 km (12.00 ut) and 15-22 km (00.00 ut). moreover, fisher’s test confirms that these results are also statistically significant for a confidence level equal to 95% (tables i and ii). – the temporal trend of the ionospheric absorption related to the maximum stratospheric temperature depends on the height where this maximum occurs, by pointing out a sort of increasing or decreasing absorption behaviour if t max (h,l) is in the lower or in the upper stratosphere, respectively. from the sea applied to the cases under investigation, the separation level is found to be located around 17-19 km. in fact, on the average, in the range 13-18 km (at 00.00 ut and 12.00 ut) the ionospheric absorption increases after the «zero day» (figs. 1 and 2), and, on the contrary, in the ranges 19-26 km (at 00.00 ut and 12.00 ut) the ionospheric absorption decreases after the «zero day» (figs. 3 and 4). moreover, for several levels between 13-26 km. at 12.00 ut and 00.00 ut the result of stratosphere-ionosphere coupling is statistically significant. although this investigation does not deal with stratospheric warming events, some general considerations can be made. the existence of a daily absorption variability, i.e. maxima and minima recurring 2-4 days around the «zero day» (figs. 1, 2, 3 and 4) and, on the average, a growing absorption after the «referring epoch» (figs. 1 and 2), is similar to results obtained by essex and morton (1974) who applied the sea to investigate the effect of stratospheric warming on the ionospheric absorption in the southern hemisphere. the comparison between previous studies and the preliminary results here obtained suggests that further investigations need to confirm, if any, the coupling «coming from below», between the upper and middle atmosphere at polar latitudes, also by making use of total ozone content data, and meteorological parameters, such as the meridional and zonal wind (lastovicka et al., 1990), and of longer time data series than those here analysed. acknowledgements this study was supported by the italian program of antarctic researches (pnra). the authors thank dr. l. perrone and mr. m. cerrone for the riometer stations measurements and the derived ionospheric absorption data, and the space environment center (noaa, u.s. dept. of commerce, boulder-co, u.s.a.) for supplying the solar flux f10.7 data. references ba c c i, g. and a. pe l l e g r i n i (1996): operational meteorology at terra nova bay, in italian geophysical observatories in antarctica, edited by a. meloni and a morelli (istituto nazionale di geofisica, roma), 151-162. bossolasco, m. and a. elena (1963): absorption de la couche d et temperature de la mesosphère, c.r. acad. sci., paris, 256, 4491-4493. bossolasco, m. and a. elena (1967): stratospheric warmings and ionospheric absorption in winter, geophys. meteorol.,16, 45-48. de franceschi, g., a. de santis and l. perrone (1997): analysis of riometer 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(1996): the italian antarctic meteorological observatory: technical activity and some climatological results, in: italian geophysical observatories in antarctica, edited by a. meloni and a. morelli (istituto nazionale di geofisica, roma), 135-149. kazimirovsky, e.s. and v.d. kokourov (1991): the tropospheric and stratospheric effects in the ionosphere. a review, j. geomagn. geoelectr. (japan), 43 (suppl.), 551-562. kershengolts, s.z. (1986): stratosphere-ionosphere coupling, ph.d. thesis, institute of cosmophysics and aeronomy, siberian department of u.s.s.r. academy of sciences, jakutsk. lastovicka, j., d. pancheva and x.j. zhang (1990): on some difference in the behaviour of the upper and lower part of the lower ionosphere in relation to meteorological parameters, ann. geophysicae, 8 (9), 599-608. perrone, l. and g. de franceschi (1999): a correlation study between time-weighted magnetic indices and the high latitude ionosphere, phys. chem. earth (c), 24 (4), 389-392. shapley, a.h. and w.j.g. beynon (1965): winter anomaly in the ionospheric absorption and stratospheric warming, nature, 206 (4990), 12421243. taubenheim, j. (1983): meteorological control of the d-region, space sci. rev., 34, 397-411. wickwar, v.b. and h.c. carlsson (1999): ionospheric and thermospheric couplings: vertical, latitudinal and longitudinal, j. atmos. sol. terr. phys., 61, 141-152. cetacean behavioral responses to noise exposure generated by seismic surveys: how to mitigate better? annals of geophysics, 59, 4, 2016, s0436; doi:10.4401/ag-7089 s0436 cetacean behavioral responses to noise exposure generated by seismic surveys: how to mitigate better? clara monaco1,2,*, jesús m. ibáñez3,4, francisco carrión3,4, l. mario tringali1 1 ketos cultural scientific association, catania, italy 2 università di catania, dipartimento di agricoltura, alimentazione e ambiente, catania, italy 3 instituto andaluz de geofísica, universidad de granada, granada, spain 4 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy abstract cetaceans use sound in many contexts, such as in social interactions, as well as to forage and to react in dangerous situations. little information exists to describe how they respond physically and behaviorally to intense and long-term noise levels. effects on cetaceans from seismic survey activities need to be understood in order to determine detailed acoustic exposure guidelines and to apply appropriated mitigation measures. this study examines direct behavioral responses of cetaceans in the southern mediterranean sea during seismic surveys with large airgun arrays (volume up to 5200 ci) used in the tomo-etna active seismic experiment of summer 2014. wide angle seismic and multi-channel seismic surveys had carried out with refraction and reflection seismic methods, producing about 25,800 air-gun shots. visual monitoring undertaken in the 26 daylights of seismic exploration adopted the protocol of the joint nature conservation committee. data recorded were analyzed to examine effects on cetaceans. sighting rates, distance and orientation from the airguns were compared for different volume categories of the airgun arrays. results show that cetaceans can be disturbed by seismic survey activities, especially during particularly events. here we propose many integrated actions to further mitigate this exposure and implications for management. 1. introduction seismic surveys used for monitoring reserves of fossil-fuel and also to study tectonics and dynamics of the seafloor crust, are anthropogenic noise sources that may interfere with natural functions of cetaceans, producing different behavioral reactions in relation to the context, the species and their sex or age [gordon et al. 2004; hildebrand 2005]. marine mammals have a great ability to perceive biologically important sounds in water, but this capacity is endanger in presence of acoustic disturbance human-generated that significantly contribute to noise in the marine environment. seismic surveys are characterized by intermittent sound pulses that are more intense than the continuous noise emitted by most industrial noises in the ocean, with peak frequency bands overlapping those used by cetacean in many contexts, such as in social interactions, as well as to forage and in dangerous situations [finneran et al. 2000; southall et al. 2007; heide-jørgensen et al. 2013]. noise generated by seismic surveys may overlap with the low frequency sounds of baleen whales, masking long distance communication between individuals and precluding the detection of other feeble sounds [evans and nice 1996]. moreover odontocetes generally show some form of avoidance during survey operations by vacating the area [stone and tasker 2006; castellote et al. 2012]. temporal avoidance of cetaceans could be also a consequence of avoidance reaction of fish during seismic surveys. these factors may induce in cetaceans different stress responses and behavioral alterations [gordon et al. 2004] but little information exists to describe how they respond physically and behaviorally to intense and long-term noise levels. strengthen mitigation measures and investigate their effectiveness is an urgent need to safeguard cetaceans and the entire marine environment. effects from seismic survey activities need to be better understood in order to regulate detailed acoustic exposure guidelines and to apply mitigation measures appropriated to species and areas of investigation. for this reason, direct observation is very important to detect cetaceans and take action when necessary. visual monitoring can ensure that the source is shut down whenever an animal is observed within or article history received january 12, 2016; accepted june 3, 2016. subject classification: seismic surveys, cetaceans, behavioral responses, monitoring, mitigation protocol. about to enter the safety zone, furthermore it allows to understand behavioral boundaries of a population in relation to the space, the environment and certain life functions that can result in response to a losing access to its optimal conditions [stone 2000; beckman 2012; stone 2015]. observing cetaceans during seismic surveys is essential to develop precautionary approaches in order to undertake these, following appropriate regulations as mitigation measures. dedicating right effort and tools to monitor populations it is possible to collect important information on animals, improving the capability to understand phenomena, predict potential effects, and minimize impacts in the future [stone 2015; castellote and llorens 2016]. comparing different behavioral responses during periods of airguns-firing and periods when airguns are silent allow also investigating short-term responses [gordon et al. 2004]. in summer 2014, the granada university (spain) and the national institute of geophysics and volcanology (ingv) section of catania (italy), in collaboration with the italian navy and other academic institutions, conducted the active seismic experiment tomo-etna which has foreseen seismic surveys on eastern sicily and aeolian islands (italy) in order to investigate tectonic structures extending from the southern tyrrhenian sea towards the ionian sea and their interaction with the etna volcano plumbing system. the research was included in the mediterranean supersite volcanoes (med-suv) and eurofleets-2 european projects [coltelli et al. 2016, in this volume; ibáñez et al. 2016a, 2016b, in this volume]. in this study, we examine data recorded during the tomo-etna research cruise in order to investigate further the effects of seismic airgun activity on cetaceans. the expedition has provided both shooting times and moments with airguns silent, so it was possible to compare the different phases of firing occurred. 2. the area transects carried out for the research have affected the ionian sea, closeness to the eastern coast of sicily, and the southern tyrrhenian sea, in the aeolian islands area (figure 1). these zones have a high level of primary and secondary productivity despite they are subjected to a strong anthropogenic impact deriving from human activities such as different categories of fishing, coastal engineering, marine traffic, waste and litter dumping, causing habitat alteration, resources over-exploitation and noise pollution [halpern et al. 2008; vindigni et al. 2016]. thermosalinograph parameters recorded in the period june 25 july 18, 2014, on board the “sarmiento de gamboa” oceanographic vessel (csic-utm, spain), indicate the following minimum and maximum values of water in ionian and tyrrhenian sea: sigma t 25-27.5 sinu; salinity 37.1-38.2 psu; temperature 19.2-26.8°c. from the morphological point of view, the area is highly heterogeneous. within the volcanic archipelago of the aeolian islands a number of submarine secondary eruptive centers rise from depths ranging between −400 and −1100 m. submerged shelves are also located along the western flanks of vulcano and lipari, the western and northern flanks of alicudi, the north-eastern and the south-western flanks of stromboli, on the whole submerged summit of panarea and in most of the coastal sector of filicudi and salina. narrower insular shelf borders the islands coastline (down to 100-120 m b.s.l.). an irregular seabed is characterized by alternating ridges and gullies that extend to −2000 m depth, and depressions bound by escarpments and shallow-water saddle with minimum depth at −50 m that connects different volcanic edifices. in the aeolian arc are also recognizable outcropping volcanic features and prodmonaco et al. 2 figure 1. map of the area under study with covered transects. 3 ucts of recent submarine eruptive activity. however, smaller-scale mass-wasting processes on the submarine flanks of all the islands contribute to transport large amounts of volcaniclastic sediments towards the base of the volcanic edifices [romagnoli et al. 2013]. after the semi-enclosed basin of the messina channel reaching up to −1000 m of depth, on the ionian side, from cape taormina to riposto, the coast is characterized by pebble beaches interposed with bays and inlets in which imposing calcareous cliffs run out. here, the continental shelf is almost absent and the bottom slopes steeply down to −100 m within a mile from the coast. the central region, up to ognina di catania, presents high depths closeness its jagged coastline composed by vulcanites and igneous rocks. because of a large continental shelf, the southern area of the gulf of catania is distinguished by the wide sandy strip of the playa littoral zone and shallow-water with maximum depth at −20 m until 3 miles from the coast. the platform has a narrow and elongated shape, it slopes quietly up to about −90 m, beyond the slope increases. in the far south of the area, close to brucoli, the continental shelf shrinks and limestone cliff of the carbonatic hyblean plateau prevails. offshore the gulf of catania the bottom extends from −1000 to −2000 m depth. the variety of environments described, in particular the presence of a reduced continental shelf and submarine canyons in the tyrrhenian sea; the rivers that flow into the ionian sea (alcantara, fiumefreddo, simeto, san leonardo); upwelling and currents that arise from hydrodynamic phenomena that occur in the messina channel, involving aeolian water and generating a residual flow in favor of the eastern coast of sicily; positively affect the biological richness of the area and help to maintain an equitable distribution of marine life during the year [amore et al. 1982; cognetti et al. 2008; carrà et al. 2014]. this, together with the other factors, favours also the presence of coastal and pelagic species of cetaceans, some of which are presents in the international union for conservation of nature (iucn) red list and are protected by many international agreements such as the agreement on the conservation of cetaceans in the black sea mediterranean sea and contiguous atlantic area (accobams; 1996), the habitats directive (council directive 92/43/eec of may 21, 1992), the convention on the conservation of migratory species of wild animals (bonn convention or cms; 1979), the convention on the conservation of european wildlife and natural habitats (bern convention; 1979), the convention for the protection of the mediterranean sea against pollution (barcelona convention; 1976), the convention on international trade in endangered species of wild fauna and flora (washington convention; 1975), the united nations environment programme (unep; 1972). fin whale (balaenoptera physalus; figure 2) is present along the north and eastern coasts of sicily, especially during the summer period, because of the passage through the strait of messina [tringali et al. 2008; sciacca et al. 2015]. bottlenose dolphin (tursiops truncatus; figures 3 and 4) occupies the whole coastal strip, partly in conjunction with the striped dolphin (stenella coeruleoalba) that makes also daily offshore migrations. in the proximity of the continental slope there cetaceans’ responses to seismic surveys figure 2. fin whale during survey in ionian sea. (photo by ketos association). figure 3. bottlenose dolphin in the gulf of catania with etna volcano shape. (photo by ketos association). figure 4. bottlenose dolphin mother and cub in ionian sea during summer. (photo by ketos association). are the risso’s dolphin (grampus griseus; figure 5), the cuvier’s beaked whale (ziphius cavirostris) and rarely also the pelagic common dolphin (delphinus delphis; figure 6) [tringali et al. 2008] but most present in aeolian region. bioacoustics data of the national institute for nuclear physics (infn-lns) demonstrate the presence of sperm whale (physeter macrocephalus; figure 7) all year round, especially offshore [internullo et al. 2008; caruso et al. 2015]. 3. methodology data were collected during the tomo-etna experiment where seismic explorations at etna volcano’s roots and surrounding areas were conducted from june 24 to july 20, 2014. the seismic survey vessel “sarmiento de gamboa” using airguns to release pulses of compressed air into the water column, was capable to determine substrata structure due to the creation of sound pressure waves capable to penetrate the seafloor. the escaping component of the generated compressed air contains energy converted to sound thereby generating a seismic signal capable to travel into the earth’s surface. the research required wide angle seismic (was) and multi-channel seismic (mcs) surveys using refraction and reflection seismic methods. airguns had a total volume up to 5200 cubic inches generated at about 2000 p.s.i. (1.382 × 107 pa) pressure. the array was composed by two batteries, each having 8 guns (sercel, ggun ii). during was surveys they were kept at a depth of 15 m below the sea level and the shots were carried every 90 seconds. for mcs surveys the depth was 7,5 m and the range shots 20 seconds [ibáñez et al. 2014; ibáñez et al. 2016a, 2016b, this volume]. during 26 survey days on board the vessel, we conducted visual marine mammals monitoring, traveling at a speed of less than 5 knots. observations were aimed to ensure the implementation of the joint nature conservation committee ( jncc) guidelines, in order to reduce the potential risk of injury or death to cetaceans in close proximity to blasting operations to negligible levels. before the start of the experiment, the scientific team (mainly students and geophysicists), has been subjected to an anonymous questionnaire in order to understand their degree of experience in cetacean monitoring. consequently, some lessons about the implementation of the jncc guidelines, data recording and marine mammal identification have been given. therefore one experienced marine biologist, with the support of the members that have received training, carried out visual monitoring by naked eye and using binoculars. all days, observers, located simultaneously on bridge deck and weather decks of the survey vessel, conducted observational scans covering a range of 360° during daylight and in good sea condition (≤ 3 of the douglas scale). an observation survey was completed recording data on time, survey vessel (position, route, speed, seismic activity), weather conditions (sky, visibility, sea, wind, precipitation), water (temperature, salinity), marine traffic and notes (presence of animals, large litter, fishing gears, buoys, etc.). these informamonaco et al. 4 figure 5. risso’s dolphin during survey offshore the gulf of catania. (photo by ketos association). figure 6. common dolphins during survey in ionian sea. (photo by ketos association). figure 7. sperm whale during survey offshore the gulf of catania. (photo by ketos association). 5 tion were recorded at the beginning and at the end of the observation activity, hourly, and each time a change occurred. furthermore vessel position was track automatically and continuously by the gps integrated system on-board. during cetacean sightings the ad libitum sampling method [altmann 1974; mann 1999] was applied adding information on time and position (coordinates, distance, radial angle and direction of swim from the vessel). therefore an additional survey was completed collecting data on species, group size, presence of juveniles, behavior (approaching, leaving, indifferent towards the vessel, surface behavioral patterns according with shane et al. [1986], weaver [1987] and monaco [2006]), other vessels, airguns status, associations with other animals or human activities. sightings have been recorded in order to understand any behavioral response to seismic surveys. for this reason, changes in volume of airguns arrays have been classified according to table 1 for identifying any ambiguous or avoidance reaction comparing airguns power shooting. distance and orientation from the airguns were also compared for different powers of the airguns. data on cetacean occurrence were also analyzed using the percentage of encounter (number of sighting case for each behavioral category *100/total number of sighting case during a specific seismic activity) as sighting rate. 4. results during the 26 survey days, 2454 nautical miles were covered, generating approximately 24,800 airgun blasts. on about 180 hours of observation we sighted about 400 individuals during 21 sighting of identified species (1, balaenoptera physalus; 1, physeter macrocephalus; 1, ziphius cavirostris; 3, tursiops truncatus; 15, stenella coeruleoalba). sightings often lasted few seconds or minutes and frequently animals were at far distance. furthermore the geo-scientific team was not very specialized to distinguish cetacean species. consequently, a lot of animals (51% of total sightings) were not classified by species (2 sighting of unidentified big cetaceans and 24 unidentified small cetaceans). sighting rates for fin whales, sperm whales and cuvier’s beaked whale did not decrease when airguns were off despite they were on, but, considering their normal aerial of distribution, a general avoidance of the area was remarked. instead, cetacean sightings at low distance from the vessel increase when airguns were turned off (figure 8). during seismic shooting, smaller odontocetes appeared to be feeding; often when they were closed to the vessel, within the 500-meter mitigation zone (mz), they seemed to swim faster moving away. the percentage of encounter calculated in good sea conditions (sea state ≤3 of the douglas scale) increase cetaceans’ responses to seismic surveys code before si gh ti n g after aa high power shooting high power shooting ar high power shooting medium power shooting ad high power shooting airguns turned off ss soft start soft start hold steady sr soft start lower soft start sd soft start airguns turned off ds airguns turned off soft start dd airguns turned off airguns turned off sa medium power shooting high power shooting table 1. classification of power changing events of noise source. figure 8. distance sighting in relation to airguns power. hp = high power; ss = soft start; mp = medium power; off = source off. with reducing volume (0.53 cases with airguns off; 0.24 with airguns on). furthermore, during the soft start (ss) or other phases with medium power shooting (mp) animals travelled towards the vessel or adopted an indifferent behavior (figures 9 and 10). big cetaceans probably vacate the area during exposure to seismic since contemporary marine mammal surveys, made in proximate areas, verified the presence of fin whales. in fact, phone and radio communications on cetacean occurrence come from the navy, fisherman, pleasure boaters and local research organizations (ketos association, mpas). smaller odontocetes may remain in the area but increasing their distance from the noise in relationship to their activities, e.g., small cetaceans were observed during feeding several times in the same zone. probably, they have not completely avoided the area in order to continue to respond to their life needs, anyway remaining at a certain distance from the noising source. it is important to underline that the jncc protocol doesn’t prevue power changing of airguns in presence of cetaceans as the sa event reported in table 1, that is a variation of the volume from medium power shooting to high power shooting. this recorded episode occurred during a line turn phase that, normally, was realized as a ss. in this case, because of a bad communication between the observers on the bridge deck and technicians in the control’s room, when four small odontocetes were approaching the vessel, airguns volume was modified from 1700 ci to 3000 ci and then to 4000 ci, so dolphins have disappeared immediately. we highlight that during seismic activities and in months after their end no stranding occurred [podestà et al. 2015]. ketos, the local research association that since 1997 studies cetaceans of the investigated area, had also continued its monitoring activities to control cetacean population dynamics and their potential long-term responses to noise exposure (results will be showed in other studies). 5. discussion since seismic surveys are used not only for academic researches but ever more mainly by oil companies to find and monitor reserves of fossil-fuel, is important to establish standardized plans to mitigate better impact from generated noise. existing protocols were not applied in all the oceans and not always the implemented strategies are adapted monaco et al. 6 figure 9. cetacean responses within the 500 m mz, according with classification on power changing in table 1. figure 10. cetacean responses beyond the 500 m mz, according with classification on power changing in table 1. 7 to survey areas. however, appropriately conducted long-term monitoring studies were strongly recommended in order to investigate cetacean populations and to measure, with standardized methods, effects deriving from seismic surveys and in general from noise pollution. results of this study, according with other works, show that cetaceans can be disturbed by seismic exploration [hermannsen et al. 2015]. seeing as behavioral responses to noise exposure can be also highly variable and context dependent, on-board mitigation measures represent the primary method to protect them. based on the tomo-etna experience and on the dynamics that usually emerge within seismic studies, here we suggest some potential broader measures for cetacean protection in order to mitigate better impacts of noise exposure during next seismic surveys. the protocol marine mammal mitigation guidelines should be adopted by all types of research that require use of airguns. whilst alternative seismic technologies must be developed. it is needed that each country recognizes the critical importance of precautionary on-board mitigation measures, regulating with laws and mandatory protocols access to their marine areas. the start of firing activities must be always preceded by a ss event. the period of increasing power shooting must provide all the intermediate possible volume of airguns during not less than 20 minutes. in case of cetacean sighted within the mz when air guns are firing, the source must be shut-down immediately. if animals leave the security area after more than 5 minutes, new firing activities have to be preceded by a full ss that require also a pre-firing observation. marine mammal observers (mmos) mmos must be specialized and motivated. they have to monitor during firing activities with shifts of 1-2 hours to avoid excessive fatigue of the observer and without stopping monitoring when guns are firing. monitoring has to be realized also during pre-firing phases. every mmo must have available essential personal equipment such as binocular and radio to communicate directly with the crew working on the firing. a minimum of four mmos should be simultaneously in observation; they must cover a range of 360°. passive acoustic monitoring visual monitoring should be integrated with passive acoustic monitoring (pam) by specialized operators. when pam is not possible, the number of specialized mmos must be increased in the morning and firing activities must be stopped at night or during adverse weather conditions (douglas sea state ≥4, heavy rain or fog). adapted measures seismic surveys must be approved by supporting scientific basis, as it has been the case in this experiment tomo-etna. at the same way, duration of the prefiring observation and extent of the mz should be increased according with species distribution and migration in proximity of the surveys areas, with particularly attention to critical habitats. only if national and international authorities recognize seismic airgun noise as a serious marine environmental pollutant, it will be possible to apply ad hoc actions and mitigation measures for cetacean conservation. divulgation of works on seismic surveys is necessary to encourage their intervention. acknowledgements. this paper has been partially funded by the following research projects: the european project med-suv funded by the european union’s seventh framework program for research, technological development and demonstration under grant agreement no. 308665; the spanish cocsabo project (cocdi-2011-08); the european project eurofleets2 (seventh framework programme, grant agreement no. 312762) through transnational access to the research vessels “sarmiento de gamboa” operated by csic (spain) and “aegaeo” by hcmr (greece); the geophysical instrument pool potsdam (gipp) from gfz (potsdam) with the project (seismic tomography of etna volcano and eolian islands, italy, using active and passive seismic data). we would like to thank the following supporting institutions: dipartimento regionale della protezione civile, regione siciliana; dipartimento azienda regionale foreste demaniali, ufficio provinciale di catania; ente parco dell’etna; unidad de tecnología marina-csic in barcelona (spain); stato maggiore marina (italian navy general staff ), cincnav (command in chief of the fleet) and marisicilia (navy command of sicily); coastal guard of messina and riposto; to obtain support and navigation permissions for the oceanographic cruises: spanish foreign office and italian foreign office. this paper has been partially supported by the spanish projects tec2015-68752-r (mineco/feder), knowaves and cgl2015-67130-c2-2. this manuscript has been largely improved by the insightful comments of dr. arcangeli and de domenico by the editor josé morales. references altmann, j. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice midorakawa 769_778.pdf annals of geophysics, vol. 45, n. 6, december 2002 769 importance of damage data from destructive earthquakes for seismic microzoning damage distribution during the 1923 kanto, japan, earthquake saburoh midorikawa department of built environment, tokyo institute of technology, yokohama, japan abstract to verify the results of seismic microzoning and to improve techniques, the damage data of past destructive earthquakes is an important key reference. the damage data of the 1923 kanto, japan, earthquake in the epicentral region are collected and compiled to produce the most reliable and detailed damage map. the damage map is compared with the results from the existing damage assessment and is used to discuss revision of the site amplification evaluation. 1. introduction seismic microzoning has been conducted as the basis for earthquake disaster mitigation planning. earthquake ground motions are directly related to the seismic force acting on structures and to geotechnical hazards such as liquefaction and slope failure. therefore, zoning for ground motion is an essential part of seismic microzoning. many approaches for ground motion zoning have been proposed such as: 1) damage survey of destructive earthquakes. mailing address: prof. saburoh midorikawa, department of built environment, tokyo institute of technology, 4259 nagatsuta, midori-ku, yokohama 226-8502, japan; e-mail: smidorik@enveng.titech.ac.jp key words seismic microzoning – damage data – the 1923 kanto earthquake – japan 2) questionnaire seismic intensity survey of lo cal earthquakes. 3) geological and geomorphological classifica tion. 4) microtremor analysis, 5) ground response analysis. in practice, appropriate methods for each area should be selected from these approaches since the available data vary from region to region (e.g., technical committee for earthquake geotechnical engineering, tc4, issmge, 1999). the results obtained by zoning techniques are preferably verified with the observation data. therefore, the damage survey of destructive earthquakes plays two important roles, such as: 1) conducting a zoning technique simply from the damage distribution, and 2) collection of material for verification of zoning results. destructive earthquakes occur with low frequency even in high seismicity areas. as a result, it is important to collect, compile, and interpret the information from old or historical earth770 saburoh midorikawa quakes. this paper shows the importance of the damage data from destructive earthquakes for seismic microzoning through a case study of the 1923 kanto earthquake in japan. 2. importance of the 1923 kanto earthquake for zoning of the tokyo metropolitan area to produce the zoning map of tokyo, imamura (1913) compiled the damage data of the 1855 tokyo earthquake (m 6.9) in which more than 7000 people were killed. from the damage map of the earthquake together with the old topographic map, he divided the city area into four zones such as low, medium, high and very high hazard zones, as shown in fig. 1. this map was the first microzoning map proposed in japan. ten years after his work, tokyo was attacked fig. 1. seismic microzoning map of tokyo (imamura, 1913). fig. 2. damage distribution in tokyo during the 1923 kanto earthquake (geological survey of japan, 1925). 771 importance of damage data from destructive earthquakes for seismic microzoning again by the 1923 kanto earthquake (m 7.9). the damage distribution of the kanto earthquake was surveyed by the geological survey of japan (1925a), as shown in fig. 2. as most of downtown of tokyo was burned out by the earthquake fire, they may have overlooked some of the areas damaged by ground shaking. considering the incompleteness of the damage map shown in fig. 2, the damage distribution of the kanto earthquake is consistent with imamura’s zoning map and could be predicted by the imamura’s map. this demonstrates the effectiveness of the damage data of the past destructive earthquakes. now thetokyo metropolitan area has grown to a huge densely populated area. the area consists of tokyo, kanagawa, saitama and chiba prefectures, with a population of 27 million. the location of the area is shown in fig. 3. if tokyo metropolitan area is attacked again by the kanto earthquake, the amount of the direct loss is estimated as more than two trillion dollars which is about 10 times of that of the 1995 kobe earthquake (risk management solutions, 1995). as shown by a square in fig. 3, the seismic microzoning map by imamura (1913) covers only the central part of tokyo prefecture and is no longer proper to evaluate the seismic hazard in tokyo metropolitan area. since the 1923 kanto earthquake was the last destructive event for tokyo metropolitan area, the damage data of the kanto earthquake will provide the basic information for the seismic microzoning of the tokyo metropolitan area. several reports on the damage to houses during the kanto earthquake have been published, as described by takemura and moroi (2001a). among them, the damage data compiled by matsuzawa (1925) have often been referred. matsuzawa’s data are based on the reports from local governments as of a few weeks after the earthquake. in the most disastrous regions, the damage survey had not been completed by that time because of the aftermath of the disaster. some of the reported data might be preliminary rough estimates, and furthermore there seem to have been some mistakes in the reported data. as a result, some parts of the data set do not seem reliable, as pointed out by matsuzawa himself (matsuzawa, 1925). about two months after the earthquake, the ministry of domestic affairs ordered the local governments to conduct a complete damage survey. based on the damage investigation, the statistical damage data were published by the ministry of domestic affairs (1926). the published statistical data, however, were not well defined. for some towns, the numbers of the damaged residential houses were described, but for the other towns, the numbers consisted of both residential and non-residential houses. unfortunately, many of the original documents of the survey data of the disastrous regions were lost mainly due to fires during the second world war. the geological survey of japan also conducted the damage investigation in tokyo, kanagawa, chiba, saitama, yamanashi, shizuoka, ibaragi, tochigi and gunma prefectures (inoue, 1925). locations of these prefectures and the source area of the kanto earthquake are shown in fig. 3. they published the reports on the damage in tokyo, chiba, saitama, ibaragi, tochigi and gunma prefectures (geological survey of japan, 1925a,b). the reports describe the damage for each sub-division of the towns and villages. from the descriptions, the detailed fig. 3. locations of epicenter and fault plane of the 1923 kanto earthquake. 772 saburoh midorikawa intensity maps of tokyo, chiba, and saitama prefectures are derived and the effects of surface as reported by moroi and takemura (2001) and takemura and moroi (2001b,c,d). unfortunately, the reports on the damage in kanagawa, yamanashi and shizuoka prefectures were not published probably due to financial problems at the time of the earthquake. among these three prefectures, kanagawa prefecture is located just on the epicentral area of the earthquake as shown in fig. 3 and suffered the most disastrous damage. as mentioned earlier, some of the statistical damage data for kanagawa prefecture have been published, but the reliability of these data has not been fully examined, so that the damage distribution of kanagawa prefecture is still unclear. 3. damage distribution of wooden houses in kanagawa prefecture in order to reveal the accurate damage distribution in kanagawa prefecture, three data set by nishisaka (1924), kanagawa prefecture (1927) and the agricultural association of kanagawa prefecture (1925), in addition to the statistical damage data by matsuzawa (1925) and the ministry of domestic affairs (1926) mentioned earlier, were collected from local institutes for compilation of historical material (shinbo and midorikawa, 2001). as a result of examination of the five groups of data, the data of the agricultural association of kanagawa prefecture (1925) seem most reliable. the data have not been referred by the previous studies on the damage of the kanto earthquake because that the agricultural association of kanagawa prefecture was a local organization whose reports were not noticed by engineering communities. the data compiled by the agricultural association of kanagawa prefecture (1925) are the numbers of the damaged residential houses for each village and town. since the major industry in kanagawa prefecture was agriculture at that time, the agricultural association of kanagawa prefecture was strongly related to the local governments. the data might be based on the original data of the damage survey as ordered by the ministry of domestic affairs. the distribution of the ratio of the collapsed houses based on the agricultural association of kanagawa prefecture (1925) is shown in fig. 4. since the data for the urban areas such as yokohama and yokosuka cities are not included in the data of the agricultural association of kanagawa prefecture (1925), the supplementary data (takahama et al., 2001; committee for publication of yokosuka city earthquake disaster report, 1932) are also used to draw the damage distribution. the damage distribution based on the data by matsuzawa (1925) which have often been referred to, is also shown in fig. 5. in comparison with the damage map by matsuzawa (1925), the damage tends to be smaller as shown in fig. 4. this is mainly because the data by matsuzawa include the damage to both residential and non-residential houses and the damage ratio was calculated dividing the number of damaged houses by the number of households which is equivalent to the number of residential houses. as shown in the geological map of kanagawa prefecture (see fig. 6), the damage seems to be larger on alluvial deposits, suggesting local site effects. in particular, in most towns and villages on alluvial deposits along the sagami river, the collapsed ratios are higher than 50%. the earthquake damage assessment has been conducted by kanagawa prefecture. figure 7 shows the japanese-scale seismic intensity map calculated for the kanto earthquake by kanagawa prefecture (1999). the map is calculated by the empirical attenuation relationship of ground motion intensity and the site amplification evaluation technique based on the geological and geomorphological classification. to verify the calculated intensity map with the observed damage map, the correlation between the intensity and the damage ratio is necessary. although there are uncertainties between the intensity scale and the damage ratio, a recent study suggests that the intensities 6 and 7 correspond to the collapsed ratios higher than 1% and 50 to 80%, respectively (nobata and midorikawa, 2002). if the collapsed ratios higher than 1%, 10%, and 50% are assumed to correspond to the intensities 6 –, 6 +, and 7, respectively, the calculated intensity map seems to be consistent with the observed damage map 773 importance of damage data from destructive earthquakes for seismic microzoning fig. 4. the damage distribution of wooden houses in kanagawa prefecture by this study. fig. 5. the damage distribution of wooden houses in kanagawa prefecture by matsuzawa (1925). 774 saburoh midorikawa fig. 6. geological map of kanagawa prefecture. fig. 7. calculated seismic intensity map for the kanto earthquake (kanagawa prefecture, 1999) 775 importance of damage data from destructive earthquakes for seismic microzoning fig. 8. detailed damage distribution and geomorphological conditions along the sagami river. 776 saburoh midorikawa in a broad sense, but in a more strict sense some discrepancies are found between both maps. for example, in the area along the sagami river, the observed damage is highest 5 to 15 km north from the coast and lower near the coast. the calculated intensity is highest from the coast to the north for a length of about 15 km. the calculated high intensity zone seems to shift to the south in comparison with the observed damage zone. as the source of the earthquake is beneath the area as shown in fig. 3, this discrepancy may be related to the site amplification evaluation. 4. local site effects on the damage distribution in order to discuss the local site effects, the detailed damage distribution was investigated in the area along the sagami river where heavy damage was observed. the area of investigation is shown by a square in fig. 4. more than 100 data sources such as the reports on local history or folklore, administrative documents, and the booklets of school anniversaries were collected. the data were interpreted and used to determine the ratio of collapsed houses at each section of towns or villages. figure 8 shows the detailed distribution of the collapsed ratios. in the figure, the geomorphological conditions are overlaid. the correlation between the damage ratio and the geomorphological condition can be found; the collapsed ratios tend to be more than 70% at back marsh, valley plain, natural levee and alluvial fan, and less than 50% at sand dune and terrace. figure 9 shows the collapsed ratios at different geomorphological conditions. although there is a large variation of the collapsed ratios in the same geomorphological classification, the average collapsed ratios are about 60% and 30% at lowland except sand dune and at sand dune and terrace, respectively. this suggests that the site amplification is smaller at sand dune and terrace. according to the previous studies (e.g., matsuoka and midorikawa, 1995), however, the site amplification has generally been considered to be larger at lowland including sand dune than at terrace. figure 10 shows the soil profile along the sagami river (kanagawa prefecture, 1985) together with the collapsed ratio. lateral variations of the soil profiles perpendicular to the section are not considered significant because of the wide valley shape, and the effects of the three dimensional structure will be small. at sand dune, there is a stiff gravel layer beneath a sand layer at the surface. the gravel layer contributes to the smaller amplification at sand dune. at back marsh, the collapsed ratio increases with the increase in the thickness of surface deposits. the collapsed ratio which should be strongly related to the site amplification can be accurately estimated from the geomorphological land classification together with the thickness of surface deposits. the results suggest that it is important for more accurate microzoning to consider the regional characteristics of soil conditions and that the damage data can be the basic data for verification and revision of the site amplification evaluation techniques. 5. concluding remarks many techniques for seismic zoning have been proposed. to verify the results and to fig. 9. collapsed ratios at different geomorphological conditions. 777 importance of damage data from destructive earthquakes for seismic microzoning improve the techniques, the damage data of past destructive earthquakes are still an important key reference. in this paper, as an example, the damage data of the 1923 kanto, japan earthquake in the epicentral region are collected and compiled to produce the most reliable and detailed damage map. the damage map is compared with the results from the existing damage assessment and is used to discuss revision of the site amplification evaluation. through the case study of the kanto earthquake, it was reconfirmed that digging, collection and compilation of the damage data of past destructive earthquakes should be continuously promoted for improvement of seismic microzoning. acknowledgements the author wishes to express his sincere thanks to hiroshi shinbo and kazuo fujimoto for preparation of the figures. references agricultural association of kanagawa prefecture (1925): report on reconstruction of houses damaged by the kanto earthquake, j. agric. ass. kanagawa prefecture, 196, 1-26 (in japanese). committee for publication of yokosuka city earthquake disaster report (1932): yokosuka city fig. 10. soil profile along the sagami river (kanagawa prefecture, 1985) and collapsed ratios. 778 saburoh midorikawa earthquake disaster report (in japanese). geological survey of japan (1925a): report on the damage investigation of the 1923 kanto earthquake, vol. 1 (in japanese). geological survey of japan (1925b): report on the damage investigation of the 1923 kanto earthquake, vol. 2 (in japanese). imamura, a. (1913): seismic intensity distributions in tokyo and osaka, rep. imp. earthquake invest. comm., 77, 17-42 (in japanese). inoue, k. (1925): preliminary note on the physiographical investigation of the great earthquake of s.e. japan, rep. imp. earthquake invest. comm., 100 (b), 95-96 (in japanese). kanagawa prefecture (1927): report of the earthquake disaster in kanagawa prefecture (in japanese). kanagawa prefecture (1985): earthquake disaster assessment of kanagawa prefecture (volume of geological and soil data) (in japanese). kanagawa prefecture (1999): report on earthquake disaster assessment of kanagawa prefecture (in japanese). matsouka, m. and s. midorikawa (1995): gis-based integrated seismic hazard mapping for a large metropolitan area, in proceedings of the fifth international conference on seismic zonation, 2, 1334-1341. matsuzawa, t. (1925): intensity distribution deduced from the damages on wooden buildings, rep. imp. earthquake invest. comm., 100 (a), 163-260 (in japanese). ministry of domestic affairs (1926): report on the great kanto earthquake disaster (in japanese). moroi, t. and m. takemura (2001): comparison of damage data for the 1923 kanto earthquake and strong ground motion in tokyo, j. struct. constr. eng, aij, 540, 65-72 (in japanese with english abstract). nishisaka, m. (1924): memorandum on earthquake disaster in kanagawa prefecture (in japanese). nobata, a. and s. midorikawa (2002): near-field ground motion intensity estimated from damage distribution of wooden houses during the 1948 fukui earthquake, j. struct. constr. eng., aij, 553, 27-32 (in japanese with english abstract). risk management solutions (1995): what if the 1923 earthquake strikes again? a five-prefecture tokyo region scenario. shinbo, h. and s. midorikawa (2001): damage distribution of wooden houses in kanagawa prefecture during the kanto earthquake and its relation with surface geology, j. soc. saf. sci., 3, 109-116 (in japanese with english abstract). takahama, t., s.midorikawa, h. shinbo and s. abe (2001): damage distribution of wooden houses in yokohama city during the 1923 kanto earthquake, in proceedings of the 26th jsce earthquake engineering symposium, 1, 105-108 (in japanese). takemura, m. and t. moroi (2001a): comprehensive list of the regional damage data sets for the 1923 kanto earthquake, j. seismol. soc. jpn., 53, 285-302 (in japanese with english abstract). takemura, m. and t. moroi (2001b): detailed seismic intensity distribution due to the 1923 kanto earthquake based on the data of the report by the geological survey of japan. part 1. chiba prefecture, j. jpn. ass. earthquake eng. 1, 1-26 (in japanese with english abstract). takemura, m. and t. moroi (2001c): damage and strong ground motion distribution from the 1923 kanto earthquake. part 3. saitama prefecture, abstracts, 2001 japan earth and planetary science joint meeting, sp-p009 (in japanese with english abstract). takemura, m. and t. moroi (2001d): damage and strong ground motion distribution from the 1923 kanto earthquake. part 4. ibaragi prefecture, programme and abstracts, the seismological society of japan, 2001, fall meeting, a51 (in japanese). technical committee for earthquake geotechnical engineering, tc4, issmge (1999): manual for zonation on seismic geotechnical hazards (revised version), the japanese geotechnical society. layout 6 1 annals of geophysics, 64, 4, se433, 2021; doi:10.4401/ag-8566 o p e n acc e s s spatial and time variations of seismicity before strong earthquakes in the southern part of the balkans emil oynakov, emil botev* department of seismology and seismic engineering national institute of geophysics, geodesy and geography bulgarian academy of sciences (niggg-bas) article history: received september 1, 2020; accepted may 10, 2021 abstract a retrospective analysis of the spatial and time variations of three main statistical parameters of the seismicity before recent 4 stronger earthquakes (2015 – 2020) in the southern balkans is presented. the modern extended software package zmap with various advanced seismological functions for earthquake catalog analysis is used for estimating the spatial-time variations in avalue (seismic activity), b-value (slope of the recurrence graph) and z-value (parameter of the relative seismic quiescence). the catalog data from constantly updated catalog of the university of athens for the period 1964-2020 and spatial window 32° 44° n and 10° – 30° e are used for the various statistical interpretations. the main result of the whole analysis is that the abnormally low b-values and high z-values, defining the zones of relatively seismic quiescence, may be an indicator of imminent release of more significant stress in areas adjacent to the zones of relatively high a-values. thus, the result of the proposed joint interpretation of the spatial-time variations of these three statistical parameters of seismicity could be considered as a kind of predictor of the stronger recent seismic events in the southern part of balkans. keywords: earthquake catalog; b-value; a-value; z-value; relatively seismic quiescence. 1. introduction the territory of the balkan peninsula is characterized by active geodynamics. this is the most geodynamical active region in central and eastern europe. a number of dangerous geodynamic processes of endogenous (earthquakes, modern crustal movements, mud volcanoes) and exogenous origin (natural and man-made) are observed in the area. today’s geodynamics of the balkan region is controlled by the active tectonic processes in the eastern mediterranean: the collision of the adriatic (apulian) microplate with the dinarides; the subduction of the oceanic ionian and levantine lithosphere under the aegean arc system; the collision between the eurasian and arabian plates and the subsequent displacement of the anatolian microplate to the west along the northern anatolian fault. the main goal of this study is to map and analyze the spatial and time variations of three main statistical parameters of seismicity before 4 stronger earthquakes (ml > 6.0) that occurred in the southern balkans in the annagrazia cross-out last decade (2015-2020). to quantify the variations of the three parameters a-value (seismic activity), b-value (slope of the recurrence graph) and z-value (seismic quiescence), catalog seismic events from 1964 to 2020 are used. there are many publications considered geographical distribution of b-value (sometimes and a-value) in the seismic zones of southern balkans [for instance: hatzidimitriou et al., 1985; papazachos, 1999; chouliaras, 2009; d’alessandro et al., 2011; popandopoulos and baskoutas, 2011; popandopoulos et al., 2016; vamvakaris et al., 2016]. some regularities in the time variations of b-values before one weak and one moderate earthquakes in two local zones in greece are presented by popandopoulos and baskoutas, [2011]. the authors suggest the obtained features of the time series of b to be used for the expected time prediction of the local main earthquakes. in our study an alternative approach to calculate the regional spatial-time variations of the statistical parameters of seismicity (b-, a-, z-values) by means of the extended software package zmap [westerhaus et al., 2002] is used. the software package is open source software used to assess various seismic hazards [westerhaus et al., 2002; wiemer and wyss, 1994, 2000]. zmap contains many advanced seismological functions for earthquake catalog analysis, including for estimating variations in b-, a-, z-values, time series analysis, temperature reduction, and more. as an example of the previous successful use of the capabilities of zmap for studying the spatial-time variations of the b-value before strong earthquakes, we can mention damanik’s study [damanik et al., 2010] for the region of the island of java (106–115.5°, –0, 7 to –11.5°), which uses data from the us geological survey (usgs) earthquake catalog and the engdahl catalog for very strong earthquakes (мmin = 4.7). we are not familiar with any examples of a combined study of the spatial-time variations of the above 3 statistical parameters of seismicity (b-, a-, z-values) as predictors of nowadays strong seismic events in the southern balkans. 2. preparation of catalog data the assessment of spatial-time variations in seismic activity in a given region is important for understanding the processes in the seismic-tectonic environment development, as well as for the seismic analysis of the danger of an imminent natural disaster. to quantify seismic variations in an area, we need information in the form of an earthquake catalog [burton, 1990]. in this study a constantly updated catalog of the university of athens for the period 1964-2020 is used (http://dggsl.geol.uoa.gr/en_index.html). the catalog of earthquakes covers data in a spatial window 32° 44° n and 10° – 30° e. the total number of included earthquakes is 295029, as the depths vary from 0 ≤ h ≤ 220 km, and the magnitude estimates ml are in the range of 2.0 < мl < 7.0. duplicate events were recognized and removed algorithmically, and later checked by visual assessment. the catalog of independent events is defined as final after the identification and removal of clustered events by the gardner and knopoff algorithm [gardner and knopoff, 1974]. any earthquake that occurred within the spatialtime window around and after each larger event is taken into account and is considered as a cluster event. the spatial-time window is larger in size for stronger previous events according to gardnerand knopoff dependency. this step is relevant merely for our catalog data comprising large events with long aftershock and foreshock,series. figure 1 shows the distribution of the epicenters of the earthquakes after declustering. to guarantee the completeness of data, analysis comprise only events with magnitudes equal to or larger than the threshold magnitude mc. so, the next step has been the determination of minimum value of magnitude mc for catalog completeness. the value of mc usually decreases with time, as the number of seismic stations and their sensitivity increase [wiemer and wyss, 2000]. the method of maximum curvature (mmax curvature) is used to verify the estimates of mc [wiemer, et al., 1999; wyss et al., 2004] (figure 2 a, b). the very final catalog includes 24750 events with mc ≥ 3.5 limited mainly to a depth of 40 km (shallow earthquakes) – maximum depth is 70 km (figure 3). emil oynakov et al. 2 3 seismic variations before some balkan events figure 1. map of epicenters of the independent seismic events obtained after declustering of the permanently updated earthquakes catalog data of the university of athens. http://dggsl.geol.uoa.gr/en_index.html figure 2. a) estimation of the minimum magnitude of catalog completeness by the method of maximum curvature. the arrows indicate the breakpoints of the cumulative distribution, marking the minimum magnitude mc for completeness of the catalog; b) spatial distribution of the magnitude threshold mc. http://dggsl.geol.uoa.gr/en_index.html emil oynakov et al. 4 3. analysis of the b-value analysis of the b-value in the studied region was performed before 4 stronger earthquakes with magnitude ml > 6.0. these are the earthquakes from: 16.04.2015 with coordinates 26.82° e, 35.23° n; m1 = 6.1, h = 37 km and t0 = 18:07; 12.06.2017 with coordinates 26.36° e, 38.84° n; m1 = 6.1, h = 28 km and t0 = 12:28; 20.07.2017 with coordinates 26.56° e, 35.64° n; ml = 6.2, h = 63 km and t0 = 22: 31 and 20.05.2020 with coordinates 25.69° e, 34.40° n; m1 = 6.0, h = 51 km and t0 = 12:51. a polygon locked in a spatial window with coordinates 24.20° 29.98° e, 34.40° 41.00° n presented in figure 4, and containing 2637 events with mc > 4 (figure 4, b). the value of b for the tested region is calculated using the maximum likelihood method [aki, 1965] (figure 5), then mapped before each of the main studied events using the usual technique with the grid with a cell length of 5 km and a fixed number of earthquakes (ni = 50) [wiemer and wyss, 2000], as shown in figs. 6a, b, c, d. in the graph of the value of b (figure 5) we can distinguish four periods: 1964-1987 average value of b ≈ 1.42; 1987-2003 average value of b ≈ 1.9; 2003-2009 average value of b≈1.4 and 2009-2020 average value of b ≈ 1.0. the last period, which includes the studied events, has the highest average value of the b-parameter, which implies increased heterogeneity and reduction of stresses [görgün et al., 2009]. the significant decrease in the value of b can be associated with the increasing effective level of stress before major earthquakes [schorlemmer et al., 2004; wu, 2008]. the spatial variations of b-value for the studied region are estimated for the period as follows: from 1964 to 01.03.2015 one month before the earthquake on 16.04.2015 (ml 6.1) (figure 6, a); from 1964 to 01.05.2017 one month before the earthquake on 12.06.2017 (ml 6.1); from 1964 to 01.07.2017 one month before the earthquake on 20.07.2017 (ml 6.2) and from 1964 to 01.04.2020 one month before the earthquake on 02.05.2020 (ml 6.0). in the figures (figure 6, a, b, c, d) in ascending order are marked from low to high values of the b-parameter. the spatial differences in the value of b illustrate variability in plan, as relatively low values can determine the places where an earthquake would most likely occur [schorlemmer et al., 2003; westerhaus et al., 2002]. the zones with relatively low values of b (0.9 1.2) are clearly delineated and the epicenter of the earthquake falls into them. according to wyss et al. [2004] and motaghi et al. [2010] low values of b indicate that stresses accumulate in these zones until the activation of the main event. we note that the zones with a low value of b in which the epicenters fall are very close to zones with relatively high values. figure 3. histogram of the depth distribution of the earthquake hypocenters. 5 seismic variations before some balkan events figure 4. a) epicentral map for the tested polygon; b) estimation of the minimum magnitude mc for completeness of the catalog for the selected polygon. figure 5. time variations of the b-value of earthquakes with ml > 4 within the studied polygon. the moments of the studied earthquakes from 2015 and 2017 are marked with red squares. 4. analysis of the a-value the spatial distribution of the a-value characterizing the seismic activity before the studied strong earthquakes shows that the epicenters of strong earthquakes fall in areas with relatively low values of seismic activity (a = 6-7) (figure 7 a, b, c, d). the estimates were performed for the same periods before the earthquakes as in the b-value studies. without exception, however, the epicenters of strong earthquakes are located in close proximity to areas with relatively high activity (a = 10-12). this result is consistent with the general idea of creating non-equilibrium stresses in the vicinity of the zones of high seismic activity and their subsequent redistribution with migration of stronger events in the direction of the low active seismic zones. emil oynakov et al. 6 figure 6. spatial distribution of the bvalue before the earthquake: a) on 16.04.2015 (ml 6.1); b) on 12.06.2017 (ml 6.1); c) on 20.07.2017 (ml 6.2); d) on 02.05.2020 (ml 6.0); epicenters of earthquakes. 5. analysis of the z-value the method of the z-test of seismic catalogs for determining the spatial-time areas of seismic quiescence before strong earthquakes is based on the research of wyss and habermann and [wiemer and wyss, 2002; habermann, 1987]. this method is focused on the determination of spatial-time blocks in the seismically active zone with a significant change in the intensity (rate) of the seismic activity in the selected energy range. the analysis is based on the statistical function of the z-test. to determine the seismic quiescence, the study area is covered with a fixed node grid. for each network node at a given moment, the function z(t) is calculated z(t) = , [1] where r1 and r2 are respectively the average values of the velocity of earthquakes flow (number of earthquakes in r₁ – r₂ σ₁2 +σ₂2 n₁ n₂� ——— 7 seismic variations before some balkan events figure 7. spatial distribution of the a-value before the earthquake: a) on 16.04.2015 (ml 6.1); b) on 12.06.2017 (ml 6.1); c) on 20.07.2017 (ml 6.2); d) on 02.05.2020 (ml 6.0); epicenters of earthquakes. a time window 30 days) for two-time intervals (one with a duration of not less than a year and tied to the test date, the other includes all other time intervals); σ1 and σ2 standard deviations of r1 and r2 respectively for the first and second time interval, and n1 and n2 number of earthquakes for the respective period. earthquakes with a hypocenter depth of up to 70 km are considered for each node of the network. high (positive) values of z indicate a decrease in the rate of the seismic activity, and respectively low z-values indicate increase [maeda k., wiemer s., 1999]. all nodes of the network with z > 3 correspond to 99% reliability of determination of seismic quiescence [wiemer, 2001; wiemer and wyss, 1994]. the calculated z-values in the nodes of the network are combined on the principle of spatial-time adjacency and determine the areas with seismic quiescence. the spatial distribution of the z-value before the earthquakes (figure 8 a, b, c) are calculated for the same polygon, comparing two time periods as follows: t1 = 30.03.2012-30.09.2013 / t2 = 30.09. 2013-30.03.2015 for the earthquake on 16.04.2015 (ml 6.1) (figure 6, a); t1 = 30.05.2014-30.11.2015 / t2 = 30.11.2015-30.05.2017 for the earthquakes on 12.06.2017 (ml 6.1), and on 20.07.2017(ml 6.2) (figure 6, b) and t1 = 28.02.2017-30.08.2018 / t2 = 30.08.2018-30.04.2020 for the earthquake on 02.05.2020 (ml 6.0), (figure 6, c). the high (positive) z-values of the maps can be interpreted as a decrease in the rate of seismic activity (seismic quiescence) compared to the first period, and the low (negative) z-values represent an increase in the seismic rate. earthquake density and distribution is a critical factor in interpreting z-value variations. large areas of constant color (value) could show the same density of earthquakes for different periods of time, i.e., may show a homogeneous degree of seismicity in this area. from the figures below (figure 8 a, b, c) it is observed that the epicenters of the stronger events fall in areas with relatively high values of the z-value (4-5), which means that the selected periods before the earthquakes are periods of relative seismic quiescence. the relatively high values of z = 4 – 5 > 3 show 99% reliability of the results. 6. conclusions the time variations of the b-value for the period 1964 2020 shows a minimum of the average value of b for the period 2013-2020 (b ≈1), which includes the four studied stronger earthquakes. large decreases in the value of b may be associated with increasing effective levels of stress before major earthquakes. the increase in the b-value after these earthquakes may indicate an increase in the heterogeneity of the earth’s crust and a decrease in shear stress, but after the study period the value of b does not increase significantly which may indicate imminent strong seismic events in the study area. the change in the spatial distribution of the b-value before the stronger earthquakes shows that the area with an abnormally low value of b covers the epicenters of the studied stronger earthquakes. these low values of b can be interpreted as potentially locked areas or areas with accumulated high stress before major earthquakes. the spatial distribution of the α-value characterizing the seismic activity before the studied stronger earthquakes shows that the epicenters of these earthquakes fall in areas that have relatively low values of seismic activity but are close to areas with relatively high activity. the results of the conducted research show that the epicenters of the selected stronger earthquakes are located in zones of relatively high z-value (z ≈ 4.2 > 3). this fact definitely indicates a statistically reliable marking of an area with a relatively seismic quiescence before the corresponding stronger earthquake. as a result of the whole analysis we can summarize that the abnormally low b-values and high z-values, defining the zones of relatively seismic quiescence, may be an indicator of imminent release of more significant stress in areas adjacent to the zones of relatively high a-values. thus, the joint interpretation of the spatial-time variations of these three statistical parameters of seismicity can be interpreted as predictor of strong seismic events and also could be used for the assessment of seismic hazard in the southern part of the balkan. acknowledgments. this study is supported by the bulgarian national science fund, contract dn 14-1/11.12.2017 “exploration of changes in some geophysical fields preceding the occurrence of earthquakes in the balkans” and particularly supported by the ministry of education and science (mes) of bulgaria (agreement no д01-322/18.12.2019). emil oynakov et al. 8 9 seismic variations before some balkan events references aki k. 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e-mail: tsapanos@geo.auth.gr key words bayesian estimation – local intensity – random variable p – probabilities of exceedance – japan many relationships have been proposed by different authors to model the variations of macroseismic intensity versus distance (algermissen et al., 1976; anderson, 1978; chandra, 1979; ambraseys, 1985; grandori et al., 1991; papazachos et al., 1993; musson and winter, 1997; slejko et al., 1998) among others. their leit-motiv is that macroseismic data could, in some way, be used like instrumental ones for determining earthquake source parameters and properties of wave propagation utsu (1988) derived a relation between the seismic intensity near the epicenter, the focal depth and the magnitude of earthquakes which is of the form (1.1) this is an empirical formula and holds for seismic intensity (which is in jma-japanese-scale) i 0 > 0, m i i h= + + +0 23 0 105 1 2 1 3 0 0 2 . . . .log 658 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson for m ranged between 2.0 and 8.0 and focal depth h from 3 up to 100 km. utsu noted that the magnitude used was the m (of jma) which is roughly equal to ms in the interval 5.0 to 8.0. the eq. (1.1) was determined using 1114 data (earthquakes). the decay of intensity with distance and its spatial distribution are performed when the observed intensities of an earthquake are a sufficient number of points. a number of attenuation laws were constructed for numerous regions of the world, and their parameters have been evaluated using simple radiation model (s) involving a point source and by applying a least squares technique to the radii d i of the isoseismals. in order to model the uncertainty in the data (meroni et al., 1991) the propagation itself fig. 1. sites-points where the observed intensity is reported. 659 a method for bayesian estimation of the probability of local intensity for some cities in japan of the intensity, from the epicenter to a site, has been considered as a process which is affected by randomness due to the qualitative character of the seismic intensity, as well as to the complex relationships between intensity decay and local seismotectonic features. in order to make this aspect clear the normalized intensity decay d i is considered as a random variable following the beta distribution given the epicentral intensity i0 and the distance r from the epicenter to the site of interest. the paper confines itself to the estimation of the probability distribution of the intensity i s at some cities of japan, only considering the observed intensity points (fig. 1) of a sufficient number of earthquakes (fig. 2) belonging to a zone which has the same attenuation. no complex attenuation laws are needed because in the study the estimates are based only on the data. table i shows the events used with the available observed intensity data points occurring in different cities which are within the zones of japan. the number of the data points for each intensity level is listed in the right part of the table. fig. 2. the examined earthquakes which caused the intensities studied in the present work. 660 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson table i. events used with the available observed intensity data points occurring in different cities within the zones of japan. the number of data points for each intensity level is listed in the right part of the table. date lat. n lon. e i 0 vi v iv iii ii i 1972/12/04 33.20 141.08 6 1 9 15 6 4 1982/03/21 42.07 142.60 6 1 8 10 3 7 1994/10/04 43.37 147.68 6 1 3 2 2 4 1 1994/12/28 40.43 143.75 6 1 3 6 15 10 4 1995/01/17 34.59 135.04 6 2 3 21 17 22 13 1952/03/04 41.80 144.13 5 4 8 13 4 2 1953/11/26 33.98 141.72 5 2 18 15 5 4 1961/02/27 31.60 131.85 5 3 7 9 15 3 1962/04/23 42.23 143.92 5 2 4 18 2 2 1964/06/16 38.35 139.15 5 5 14 15 12 11 1968/04/01 32.28 132.53 5 2 15 25 11 4 1968/05/16 40.73 143.58 5 7 21 5 2 1968/05/16 41.42 142.85 5 2 12 11 5 2 1970/07/26 32.07 132.03 5 3 5 9 13 2 1971/08/02 41.23 143.70 5 1 7 11 13 1 1973/06/17 42.97 145.95 5 2 6 6 9 2 1973/06/24 43.29 146.43 5 1 1 6 8 1 1974/05/09 34.57 138.80 5 1 7 10 12 9 1978/01/14 34.77 139.25 5 2 8 16 17 6 1978/06/12 38.15 142.17 5 5 20 14 9 5 1983/05/26 40.35 139.07 5 3 6 14 10 10 1993/07/12 42.78 139.18 5 4 6 7 6 6 1995/01/07 40.22 142.31 5 2 4 15 12 11 1952/03/10 41.70 143.72 4 2 8 9 9 1955/07/24 35.77 140.62 4 1 3 9 6 1955/07/27 33.73 134.32 4 1 8 13 7 1960/03/21 39.83 143.43 4 6 8 5 6 1961/08/19 36.02 136.77 4 10 16 13 8 1962/01/04 33.63 135.22 4 4 18 10 8 1962/04/30 38.73 141.13 4 5 5 7 6 1963/10/13 44.89 149.56 4 2 1 1964/07/12 38.52 139.32 4 1 3 8 10 1965/08/03 34.27 139.30 4 1 1 1966/06/26 36.55 138.35 4 1 1 2 5 1967/11/04 43.48 144.27 4 2 3 4 5 1968/01/29 43.52 146.72 4 1 4 1 2 1968/06/12 39.42 143.13 4 5 13 7 7 1968/09/21 36.82 138.27 4 1 1 1 3 1969/04/21 32.15 132.12 4 3 9 5 11 1969/08/12 43.44 147.82 4 3 2 1 1971/02/26 37.13 138.35 4 1 2 6 8 1972/01/14 34.80 139.32 4 1 1974/01/25 41.83 144.27 4 1 3 1 1 1981/01/19 38.60 142.97 4 3 9 5 8 661 a method for bayesian estimation of the probability of local intensity for some cities in japan is the epicentral intensity of an earthquake then the probability that this earthquake will cause a site intensity i s equal to i is (2.1) the parameter p depends on epicentral intensity date lat. n lon. e i 0 vi v iv iii ii i 1982/03/07 36.47 140.65 4 2 5 6 6 1982/07/23 36.18 141.95 4 6 13 11 6 1983/06/21 41.26 139.00 4 4 6 7 10 1983/08/08 35.52 139.03 4 4 12 14 13 1983/10/31 35.41 133.93 4 1 9 8 15 1984/08/07 32.38 132.16 4 6 12 8 11 1984/09/14 35.82 137.56 4 4 28 11 14 1985/05/13 32.99 132.58 4 1 7 7 3 1985/10/18 37.66 136.93 4 1 3 2 11 1989/07/09 34.99 139.11 4 1 6 6 5 1989/11/02 39.85 143.05 4 4 8 8 9 1990/02/20 34.76 139.23 4 7 7 14 11 1991/04/24 42.71 144.85 4 1 2 2 1 1993/07/12 43.10 139.23 4 1 1 2 6 1993/08/08 41.96 139.89 4 2 8 2 8 1994/08/14 38.66 142.39 4 1 3 10 15 1994/10/09 43.55 147.81 4 1 3 1 2 1953/07/14 42.07 139.93 3 1 1 2 1954/04/14 32.87 134.42 3 3 6 5 1955/06/23 35.30 133.38 3 4 9 7 1961/01/16 36.02 141.92 3 1 6 9 1966/11/12 33.07 130.27 3 4 5 4 1970/05/28 40.15 143.25 3 3 6 6 1976/07/05 38.77 140.68 3 1 1 1980/12/31 45.99 151.47 3 1 2 4 1984/09/15 35.79 137.47 3 12 18 21 1986/11/13 43.80 141.84 3 1 5 2 1994/08/16 37.83 142.60 3 4 7 15 1952/03/04 42.00 144.30 2 4 5 1959/10/27 45.91 151.15 2 1 6 1970/09/29 34.43 133.30 2 5 8 1963/11/13 34.28 139.22 1 2 1964/01/20 44.05 145.22 1 2 1975/06/10 43.18 147.36 1 4 table i (continued ). p i i i i p i i p ps i i i = =( ) = −( ) −/ , .0 0 0 1 0( ) 2. method applied the observed intensity at the site, i s , is considered as a random variable having the binomial distribution. in our case, the binomial distribution simulates values of probability (in the same way that an exponential or power function simulates the values of intensity in an attenuation law, when using a non-probabilistic approach). that is, if i0 662 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson i 0 and epicentral distance r. it is sensible to discretize the values of epicentral distance, r j and round all epicentral distances r to the closest r j . the values of epicentral distance increase with a step of 10 km. in order to apply a bayesian model the parameter p must be considered as a random variable, as well. a prior beta distribution is required for p and is expressed by (2.2) where α and β are the beta distribution parameters, γ is the gamma function and x is the integration variable. meroni et al. (1994) also proposed that the prior expected average of α and β for epicentral intensity i 0 = i 0 and epicentral distance r = r j are (2.3) β = 1–α (2.4) where and c is a constant such that the prior estimate of p, is almost equal to 0.99. the physical meaning of this, is that for the smallest value of j ( j = 1) which corresponds to the smallest epicentral distance (r 1 ), it is almost certain that the intensity will be equal to the epicentral intensity. equations (2.2), (2.3) and (2.4) combined, have as a result that the prior estimator of p is equal to (2.5) it was found that the posterior values of p are not sensible to changes in the prior values of α and β, so we adopted the proposed prior values. it follows that the posterior mean of p is (2.6) where α and β are the parameters of the beta distribution of p, i s is the n-th intensity point at an epicentral distance rj and nj is the number of observed intensities caused by earthquakes having an epicentral intensity of i0 and an epicentral distance r j . this is the computed p value, used in the calculation of the probability of exceedance of given intensity. using eq. (2.6) we calculated values of p for epicentral intensities 3-6 (jma) and epicentral distances 0-500 km. following meroni et al. (1994) if there were no data for a value of r j , we assigned to it the same p with r j–1. the prior and the posterior estimates of the parameter p, for the epicentral distance 0-500 km, are shown in fig. 3a and fig. 3b, respectively. for distances greater than 500 km there appears to be no dependence of p on distance (fig. 4), mainly because of lack of data. however a large value of intensity in the long distance exists in fig. 4. this can due to an error in the estimation of intensity for any reason (like the wrong determination of shock’s epicenter or/ and its corresponding magnitudes). an inspection to figs. 3a,b and 4 illustrates that the posterior estimates of p do not decrease monotonically with increasing distance from the source. a relevant bayesian theory was given by rhoades and evison (1993, eq. (2.10)). the main difference from the present study is that instead of a prior beta distribution for a single value p, the above mentioned authors assume a prior dirichlet (multivariate beta) distribution for a collection of ordered binomial parameters. an alternative approach would be to smooth the posterior estimates of p over radii, for a given value of i0. for this purpose we applied smoothing by 7-point moving average. an example prepared for tokyo where we estimated the probability of exceedance used for this purpose the alternative smoothing procedure (in table iii, as tokyo a). no significant difference from the original results observed. the procedure suggested by rhoades and evison (1993), would be very useful if we focused to the attenuation and not to hazard assessment. in our case the parameter p in not strictly decreasing with distance, but its mean value shows such trend. in order to base the results on reliable data only, it was decided to choose 500 km as the upper limit for epicentral distance. in fig. 5 we can see the smoothed posterior value of p for the chosen distance of 500 km. p̂ i i n j i s n n n j j 0 1 0 = + + + ( ) = ∑α α β r r ci j' /= ˆ ,p i 1 0 0 b p x x dx p ; ,α β α β α β α β( ) = +( ) ( ) ( ) −( )− −∫ γ γ γ 1 1 0 1 α = +( ) 1 1 r i ' 1 0 / i ˆ , ' p r i i 1 0 0 1 1 = + = + α α β ( ).1 0/ i 663 a method for bayesian estimation of the probability of local intensity for some cities in japan fig 3a,b. a) the prior estimates of the p parameter given the epicentral distances 0-500 km and i0 = vi, v, iv and iii, and (b) the posterior estimates of the parameter p given the epicentral distances 0-500 km and i 0 = vi, v, iv and iii. a b 664 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson fig. 4. the posterior distribution of the values of the parameter p for epicentral distances 0-1000 km and i 0 = vi, v, iv and iii. fig. 5. the smoothed posterior estimates of p for the chosen distance of 500 km. 665 a method for bayesian estimation of the probability of local intensity for some cities in japan the computations the a and b values very recently stimated by koravos (2000) for the zones in which japan and the surrounding area are divided. in table ii the values of a and b parameters used, are listed. in order to take into account the uncertainties of eq. (1.1), it was assumed that its values have a standard error of σ = 0.5. so, instead of obtaining a single value for m, we calculated a normal distribution with a mean value of m and standard deviation σ. applying this, the probability p(i 0 ≥ i 0 ) is the one obtained by the average value of eq. (2.8), weighted over all possible values of m using a weighing factor equal to n(m,σ 2 ), where n is the probability function of the normal distribution, m is the magnitude given by (eq. (1.1)) and σ is the standard error (2.9) where p(i0 ≥ i0| m) is the probability of exceedance of epicentral intensity i 0 given that the magnitude is m and n(m; m, σ) is the probability density function of the normal distribution at m. in comparison with the method proposed by meroni et al. (1994), we showed an alternative approach. these authors utilised the probability distribution of the epicentral intensity. contrary to the aforementioned method we express the probability distribution of i0 through the probability distribution of m i (eq. (2.8)) in combination with utsu’s magnitude-epicentral intensity relation (eq. (1.1)). in addition to this we assumed that there is an uncertainty in this relation, which is incorporated in our method with the use of eq. (2.9). the second factor of eq. (2.7), is calculated by eq. (2.1) with the proper value of p for the specific epicentral distance and epicentral intensity. for every subdivision of every source, the probability in question is the sum of the probabilities that correspond to epicentral intensities of i0 = is , i0 = is + 1, i0 = is + 2 and so on, up to the maximum value of i0, which is 6 for the japanese intensity scale (2.10) in order to assess the seismic hazard in a specific site, in terms of probability of exceedance of a given intensity in a given time span, the following steps must be taken. first, for every subdivision of every source and for every possible value of i0, the probability of exceedance of is in the given site is evaluated (2.7) where p (i0 = i0) is the probability of occurrence of an earthquake with an epicentral intensity of i 0 , in the given subdivision of the given zone within the given time span and is the probability of an earthquake with an epicentral intensity of i0 , to cause an intensity of is or greater at the site. the first factor of eq. (2.7) is calculated indirectly. the magnitude of an earthquake that is expected to have an epicentral intensity of i0 , is given by eq. (1.1). a mean value of h equal to 15 km was calculated from the data. then using the gutenberg-richter law parameters a and b (1944) for the specific source, the probability of exceedance of the magnitude m is calculated (2.8) where m 0 = m (i 0 ) see eq. (1.1) and then it is normalized for an area equal to the area of the subdivision. in the present study we adopted for zone a b ± σ0 j1 5.09 j2 3.01 j3 5.49 j4 4.19 j5 4.84 j6 3.86 j7 2.89 j8 5.52 j9 4.19 j10 3.31 table ii. the values of the a and b (± uncertainty) parameters used in the present study as computed by koravos (2000). 0.09 0.11 0.07 0.08 0.07 0.09 0.16 0.12 0.06 0.07 0.92 0.62 0.88 0.74 0.85 0.73 0.57 1.00 0.78 0.68 ˆ ; ,p i i p i i m n m m dm0 0 0 0≥( ) = ≥( ) ( ) −∞ +∞ ∫ σ p p i i pj s= =( ) =ˆ i i i i p i is i i s = = ={ } ⋅ ={ } = ∑ , ˆ .pr pr0 0 0 0 6 0 p p i i p i i i is= =( ) ≥ =( ) 0 0 0 0 p i i i is ≥ =( )0 0 pr =i i i i 0 0 0 0 ∈ +( ){ }, ∆ = pr m m m m e dmbm 0 0} ∈ +( ){ } ∝ −, ∆ 666 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson table iii. probabilities of exceedance for the site intensities 6.0, 5.0, 4.0 and 3.0 in 1, 2, 5, 10, 20, 25, 50, 100, 200 and 500 years for the 12 japanese cities examined. the epicentral distances vary from 0 to 500 km. an example prepared for tokyo (see tokyo a) used for this purpose the smoothing by 7-point moving average. i s 1 2 5 10 20 25 50 100 200 500 kushiro 6.0 0.0050 0.0146 0.0281 0.0478 0.0793 0.0921 0.1380 0.1848 0.2173 0.2291 5.0 0.0660 0.1077 0.1822 0.2463 0.3023 0.3177 0.3619 0.4027 0.4301 0.4397 4.0 0.1726 0.2358 0.3307 0.4085 0.4824 0.5046 0.5708 0.6293 0.6661 0.6783 3.0 0.4281 0.5170 0.6380 0.7305 0.8131 0.8363 0.8970 0.9379 0.9578 0.9634 hachinohe 6.0 0.0088 0.0147 0.0280 0.0466 0.0744 0.0849 0.1183 0.1462 0.1623 0.1665 5.0 0.0673 0.1042 0.1613 0.2089 0.2546 0.2680 0.3056 0.3357 0.3535 0.3586 4.0 0.1705 0.2339 0.3258 0.4017 0.4777 0.5009 0.5666 0.6178 0.6470 0.6556 3.0 0.4547 0.5558 0.6894 0.7852 0.8662 0.8874 0.9368 0.9639 0.9755 0.9784 sendai 6.0 0.0082 0.0142 0.0268 0.0448 0.0721 0.0825 0.1163 0.1448 0.1620 0.1678 5.0 0.0615 0.0974 0.1559 0.2041 0.2518 0.2663 0.3069 0.3390 0.3582 0.3644 4.0 0.1626 0.2266 0.3220 0.3974 0.4729 0.4965 0.5642 0.6166 0.6459 0.6548 3.0 0.4394 0.5421 0.6787 0.7737 0.8546 0.8766 0.9292 0.9581 0.9702 0.9734 hitachi 6.0 0.0083 0.0141 0.0268 0.0456 0.0756 0.0876 0.1284 0.1628 0.1800 0.1845 5.0 0.0617 0.1032 0.1752 0.2283 0.2746 0.2890 0.3327 0.3683 0.3873 0.3926 4.0 0.1723 0.2379 0.3387 0.4161 0.4905 0.5142 0.5849 0.6401 0.6686 0.6766 3.0 0.4541 0.5528 0.6877 0.7807 0.8591 0.8809 0.9345 0.9640 0.9754 0.9782 tokyo 6.0 0.0079 0.0132 0.0250 0.0426 0.0720 0.0842 0.1280 0.1694 0.1929 0.1994 5.0 0.0581 0.0994 0.1749 0.2321 0.2798 0.2941 0.3381 0.3768 0.3986 0.4046 4.0 0.1681 0.2332 0.3332 0.4106 0.4833 0.5064 0.5775 0.6364 0.6678 0.6764 3.0 0.4396 0.5357 0.6699 0.7642 0.8444 0.8672 0.9253 0.9593 0.9726 0.9758 tokyo a 6.0 0.0033 0.0114 0.0207 0.0346 0.0578 0.0675 0.1024 0.1355 0.1543 0.1595 5.0 0.0556 0.0951 0.1677 0.2235 0.2709 0.2853 0.3302 0.3699 0.3922 0.3984 4.0 0.1658 0.2316 0.3338 0.4132 0.4880 0.5118 0.5850 0.6453 0.6773 0.6861 3.0 0.4308 0.5301 0.6688 0.7663 0.8488 0.8720 0.9304 0.9636 0.9762 0.9792 osaka 6.0 0.0061 0.0096 0.0164 0.0269 0.0451 0.0531 0.0838 0.1176 0.1406 0.1482 5.0 0.0327 0.0579 0.1119 0.1611 0.2034 0.2150 0.2501 0.2848 0.3076 0.3150 4.0 0.1080 0.1590 0.2398 0.3092 0.3750 0.3952 0.4595 0.5200 0.5573 0.5690 3.0 0.3286 0.4158 0.5457 0.6485 0.7411 0.7683 0.8459 0.9029 0.9309 0.9387 kochi 6.0 0.0072 0.0111 0.0197 0.0327 0.0537 0.0623 0.0919 0.1174 0.1307 0.1347 5.0 0.0315 0.0556 0.1069 0.1528 0.1915 0.2021 0.2337 0.2609 0.2765 0.2820 4.0 0.0953 0.1429 0.2202 0.2866 0.3490 0.3680 0.4266 0.4770 0.5064 0.5178 3.0 0.2868 0.3705 0.4994 0.6054 0.7024 0.7311 0.8126 0.8700 0.8979 0.9081 fukuoka 6.0 0.0066 0.0096 0.0161 0.0257 0.0409 0.0468 0.0658 0.0790 0.0842 0.0862 5.0 0.0222 0.0387 0.0740 0.1061 0.1336 0.1414 0.1639 0.1810 0.1899 0.1945 4.0 0.0658 0.1009 0.1609 0.2135 0.2640 0.2795 0.3271 0.3656 0.3881 0.4003 3.0 0.2112 0.2813 0.3968 0.4972 0.5943 0.6242 0.7122 0.7762 0.8098 0.8270 667 a method for bayesian estimation of the probability of local intensity for some cities in japan is 1 2 5 10 20 25 50 100 200 500 sapporo 6.0 0.0082 0.0125 0.0232 0.0391 0.0654 0.0763 0.1165 0.1578 0.1844 0.1921 5.0 0.0539 0.0899 0.1574 0.2154 0.2647 0.2782 0.3191 0.3581 0.3834 0.3912 4.0 0.1513 0.2106 0.3005 0.3752 0.4454 0.4667 0.5324 0.5928 0.6304 0.6420 3.0 0.3988 0.4876 0.6117 0.7081 0.7937 0.8180 0.8844 0.9316 0.9547 0.9611 hiroshima 6.0 0.0064 0.0101 0.0173 0.0281 0.0455 0.0526 0.0766 0.0968 0.1073 0.1106 5.0 0.0262 0.0460 0.0887 0.1275 0.1609 0.1701 0.1973 0.2203 0.2333 0.2379 4.0 0.0783 0.1188 0.1855 0.2437 0.2994 0.3164 0.3691 0.4143 0.4407 0.4510 3.0 0.2406 0.3153 0.4332 0.5340 0.6299 0.6590 0.7442 0.8076 0.8402 0.8526 niigata 6.0 0.0078 0.0117 0.0214 0.0359 0.0601 0.0703 0.1086 0.1497 0.1793 0.1900 5.0 0.0468 0.0792 0.1423 0.2001 0.2565 0.2728 0.3183 0.3576 0.3837 0.3928 4.0 0.1440 0.2090 0.3073 0.3845 0.4601 0.4836 0.5528 0.6111 0.6463 0.6576 3.0 0.4137 0.5140 0.6487 0.7436 0.8256 0.8487 0.9075 0.9438 0.9601 0.9646 gifu 6.0 0.0061 0.0096 0.0164 0.0271 0.0457 0.0539 0.0862 0.1226 0.1486 0.1575 5.0 0.0356 0.0628 0.1202 0.1723 0.2179 0.2306 0.2693 0.3083 0.3349 0.3438 4.0 0.1203 0.1761 0.2646 0.3393 0.4105 0.4325 0.5018 0.5670 0.6078 0.6207 3.0 0.3607 0.4550 0.5916 0.6951 0.7859 0.8121 0.8839 0.9335 0.9563 0.9623 table iii (continued ). p p j n j= − −( ) − 1 1 1 π the contribution, in terms of probability of a whole source is the sum of the probabilities of all the subdivisions of the source. finally, the contributions of all the sources are combined using the relation (2.11) where p j is taken as a computation from eq. (2.10). the seismicity of the examined area is of the highest in the world and observed to occur in the front and the back-arc area where great interplate and intaplate earthquakes are generated, respectively. in both areas, high risk seismic sources were created because of their proximity to major centers of dense population and heavy industry. the probability of exceedance of a given intensity in a given time span was assessed for 12 sites in japan (fig. 6). these sites belong both to the front and the back-arc parts of japan and they are found within zones of the area defined by papazachos et al. (1994) and matsuda (1990), respectively. these sites are the following cities (from north to south): sapporo, kushiro, hachinohe, sendai, niigata, hitachi, tokyo, gifu, osaka, hiroshima, kochi and fukuoka. 3. results and discussion figure 6 shows the seismogenic sources of japan (matsuda, 1990; papazachos et al., 1994) and the 12 examined sites (cities). the method effectively estimates the probability of the local intensity i s through the bayes statistics. there is no need for an attenuation law, but the seismic hazard can be evaluated directly from the observed data points. the obtained results can be considered as a measure (under poissonian hypothesis) of the uncertainty in seismic hazard evaluation. looking at fig. 7, presenting the probability of exceedance of various intensities i s (vi, v, iv and iii) for the next 50, 100, 200 and 500 years, we can observed that kushiro is most likely to experience an intensity vi degrees of the japanese scale. hitachi and tokyo follow, 668 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson while fukuoka has the lowest probability of exceedance of an intensity vi in the next 50 years. the seismic hazard of japan is illustrated in fig. 8, where the probabilities of exceedance in the next 50 years (475-years mean return period) of an intensity vi for the twelve examined sites-cities are presented by black circles. this plotted curve taken into account the cities from north towards south and thus fig. 8 is an interesting picture of the fluctuations of the seismic hazard profile of japan. we can observe that the general trend of the hazard profile decreases southwards. another interesting observation is that almost all the sites-cities in the back arc area are characterized by low probability of exceedance. these cities are: sapporo, niigata, gifu, hiroshima and fukuoka. but there are also cities which belong to the front area of japan, where take place the underthrusting process, like hochinohe and sendai which are cited in the inland part of japan away from the centers of the very large earthquakes and thus their values on the hazard curve are low. the highest values appeared in the cities of kushiro, fig. 6. the seismic zones into which japan was divided (matsuda, 1990) and the 12 sites-cities for which the seismic hazard is estimated. 669 a method for bayesian estimation of the probability of local intensity for some cities in japan fig. 7. the probability of exceedance of various intensities i 0 = vi, v, iv and iii in the next 50, 100, 200 and 500 years for the 12 examined sites-cities of japan. fig. 8. seismic hazard curves for the next 50 years (475-years mean return period) and for the next 200 years (1000-years mean return period) presented by black circle and black triangles, respectively for the 12 examined sites-cities of japan. the name of the cities and their corresponding point number on the curves are: 1 sapporo; 2 kushiro; 3 hachinobe; 4 sendai; 5 niigata; 6 hitachi; 7 tokyo; 8 gifu; 9 osaka; 10 kochi; 11 hiroshima and 12 fukuoka. 0. 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1. 0 ku sh iro ha ch in oh e se nd ai h ita ch i to ky o o sa ka ko ch i fu ku ok a sa pp or o hi ro sh im a n iig at a g ifu 0. 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1. 0 ku sh iro ha ch in oh e se nd ai h ita ch i to ky o o sa ka ko ch i fu ku ok a sa pp or o hi ro sh im a n iig at a g ifu i=6 i=5 i=4 i=3 0. 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1. 0 ku sh iro ha ch in oh e se nd ai h ita ch i to ky o o sa ka ko ch i fu ku ok a sa pp or o hi ro sh im a n iig at a g ifu 0. 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1. 0 ku sh iro ha ch in oh e se nd ai h ita ch i to ky o o sa ka ko ch i fu ku ok a sa pp or o hi ro sh im a n iig at a g ifu i=6 i=5 i=4 i=3 probability of exceedance of intensity i in 500 years probability of exceedance of intensity i in 100 years probability of exceedance of intensity i in 50 years probability of exceedance of intensity i in 200 years p ro ba bi li ty p ro ba bi li ty p ro ba bi li ty p ro ba bi li ty site site site site �� �� �� � � � �� � � � ���� ��� ���� ��� ���� � � � � � � � �� �� �� ������������ � � � � � �� � � �� � � � � � � � � � � �� �� � �� � ��� ���� ���� ���� 670 theodoros m. tsapanos, odysseus ch. galanis, george ch. koravos and roger m.w. musson tokyo and hitachi. the problem becomes greater for tokyo because it is either a high dense populated city and moreover there are centers of high risk around (heavy industry, electricity power and chemical plants, etc). the upper part of fig. 8 demonstrates the seismic hazard profile of japan, where the probabilities of exceedance in the next 200 years (1000years mean return period) of an intensity vi for the twelve examined sites-cities are presented by black triangles. it is expected to follow almost the same pattern as the hazard profile for 50 years, admittedly with the exception of niigata. niigata, number 5 (fig. 8), shows relatively higher hazard, in comparison with the previous profile it belongs to those sites of low hazard, and this can be due to the different time period examined. another critical point is that tokyo still dominates in the range of the high probabilities of exceedance (the seismic hazard profile). the majority of earthquakes, particularly the large ones (m ≥ 8.0) occur along the japan trench and nankai and sagami troughs. earthquakes along the plate boundaries generally show lowangle thrust mechanisms and are interpreted to denote that the pacific and philippine sea plates are subducting beneath the eurasian plate (ando, 1975; scholz and kato, 1978). earthquakes are also occur in the back-arc area. these are referred to as intraplate seismicity (wesnouski et al., 1982). the largest intraplate events are generally about one magnitude unit less (7 < m < 8) than the great interplate earthquakes, although they are a major danger because of their proximity to centers of high population. an integration of geological and seismological data for probabilistic seismic hazard in japan was carried out by wesnousky et al. (1984). in particular, he estimated the probabilities of the ground shaking of jma intensity ≥ v during the next 20, 50, 100 and 200 years, induced by either interplate and intraplate earthquakes. for the 50 year period, he suggested nankai trough and portions of the japan trench adjacent to east coast on northeast honshu will experience major events. the 200 years map projects the probable occurrence of a major earthquake along sagami trough. the cities of tokyo and hitachi (point 6 and 7 in fig. 8) are very close to sagami trough and both can be considered sites of high seismic risk. especially for the back-arc area and for the next 200 years, wesnousky et al. (1984) estimated high probabilities in the north and central part of honshu. the seismogenic sources j-8 and j-9 (fig. 6) are parts of this high seismic risk area, where the cities of sapporo and niigata are located. in the present study, both of these cities show a relatively high probability of exceedance for the next time span of 200 years. the regions of high probability expand as longer time periods are sampled. the results obtained by wesnousky et al. (1984) in general are not directly comparable with ours, mainly because we search the ground shaking in sites (cities), whereas wesnousky and his colleagues covered the whole territory of japan. despite this, we can observe that few of the cities examined in this study which present high seismic hazard parameter are within the areas of high risk according to wesnousky et al. (1984). rikitake (1991) estimated the probability of exceedance for intensities greater than or equal to v and vi of the jma within the next 10 years in the broad area of tokyo. his results for intensity ≥ v are 0.40 while ours is 0.23, and for intensity ≥ vi he found 0.049 while our estimation is 0.043. the plots of fig. 7 in comparison with table iii, give useful information for the seismic risk assessment in japan for earthquake resistant design (erd). acknowledgements the authors want to express their sincere thanks to prof. t. utsu, for his kind help during the present study. many thanks are also due to the anonymous reviewers for their constructive criticism of the study. the contribution of r.m.w. musson to this paper is published with the permission of the executive director of bgs (nerc). references algermissen, s.t., d.m perkins, w. issherwood, d. gordon, g. reagor and c. howard (1976): seismic risk evaluation of the balkan region, proceedings sem. seismic zoning maps, unesco, skopje 1975, 2, 68-171. 671 a method for bayesian estimation of the probability of local intensity for some cities in japan ambraseys, n. (1985): intensity-attenuation and magnitudeintensity relationships for northern european earthquakes, earthquake eng. struct. dyn., 13, 733-778. anderson, j.g. (1978): on the attenuation of modified mercalli intensity with distance in the united states, bull. seismol. soc. am., 68, 1147-1179. ando, m. (1975): seismotectonics of the 1923 kanto earthquake, j. phys. earth, 22, 263-277, chandra, u. (1979): attenuation of intensity in the united states, bull. seismol. soc. am., 69, 2003-2024. grandori, g., a. drei, f. perrotii and a. tagliani (1991): macroseismic intensity versus, epicentral distance: the case of central italy, tectonophysics, 193, 165-171. gutenberg, b. and c.f. richter (1944): frequency of earthquakes in california, bull. seismol. soc. am., 34, 185-188. koravos, g.ch. (2000): the study of the seismicity of the seismogenic zones in the circum-pacific belt and the seismic hazard assessment of the large cities, msc. dissert., aristotle university of thessaloniki, geophysical laboratory, pp. 208 (in greek). matsuda, t. (1990): seismic zoning map of japanese islands, with maximum magnitudes derived from active fault data, bull. earthq. res. inst. univ. tokyo, 65, 289-314. meroni, f., r. rotondi, f. cella and g. zonno (1994): bayesian estimation of the probability function for the local intensity, in proceedings of 24th e.s.c. assembly, 19-24 september 1994, athens, vol. 3, 1408-1418. meroni, f., v. petrini, r. rotondi and g. zonno (1991): expected damage for alternative seismic hazard evaluations, in proceedings of 4th icsz-stanford, california, vol. 2, 801-818. musson, r.m.w. and p.w. winter (1997): seismic hazard maps for the u.k, natural hazards, 14, 141-154. papazachos, b.c., ch.a. papaioannou, v.n. margaris and n.p. theodoulidis (1993): regionalization of seismic hazard in greece based on seismic sources, natural hazards, 8, 1-18. papazachos, b.c., e.e. papadimitriou, g.f. karakaisis and t.m. tsapanos (1994): an application of the time and magnitude predictable model for the long-term prediction of strong shallow earthquakes in the japan area, bull. seismol. soc. am., 84, 426-437. rhoades, d.a. and f.f. evison (1993): long-range earthquake forecasting based on a single predictor with clustering, geophys. j. int., 113, 371-381. rikitake, t. (1991): assessment of earthquake hazard in the tokyo area, japan, tectonophysics, 199, 121-131. scholz, c.h. and t. kato (1978): the behavior of a convergent plate boundary: crustal deformation in the south kanto district, japan, j. geophys. res., 83, 783-797. slejko, d., l. peruzza and a. rebez (1998): seismic hazard maps of italy, ann. geofis., 41 (2), 183-214. utsu, t. (1988): relation between seismic intensity near the epicenter, focal depth and magnitude, bull. earthquake res. inst. univ. tokyo, 63, 23-31 (in japanese). wesnousky, s.g., c.h. scholz and k. shimazaki (1982): deformation of an island arc: rates of moment release and crustal shortening in intraplate japan determined from seismicity and quaternary fault data, j. geophys. res., 87, 6829-6832. wesnousky, s.g., c.h. scholz, k. shimazaki and t. matsuda (1984): integration of geological and seismological data for the analysis of seismic hazard: a case study of japan, bull. seismol. soc. am., 74, 687-708. (received may 20, 2002; accepted september 27, 2002) vol50,1,2007 21 annals of geophysics, vol. 50, n. 1, february 2007 key words ground deformation – magnetic anomalies – inverse modelling – genetic algorithms – mt. etna 1. introduction mt. etna is one of the most active and investigated volcanoes in the world. over recent decades, new modern techniques of volcano monitoring have also been implemented to improve the knowledge of eruptive processes. in particular, monitoring involves geodetic and magnetic techniques that have provided essential information on the eruption mechanism including magma storage and transport within the volcano edifice. continuous ground deformation monitoring at mt. etna is performed with permanent tilt and gps networks (bonaccorso et al., 2002). for the different lateral eruptions that occurred between 1981 and 2001, ground deformation data analysis and modelling show early indications of intrusions initiating months to years earlier. the modelling showed that the intrusion source, i.e. the tensile mechanisms inside the volcano edifice, always involved the same structure oriented approximately nnw-sse, which coincides with a main tectonic trend crossing the volcano (bonaccorso and davis, 2004). since the end of 1998, a permanent magnetic network equipped with overhauser magnetometers has been operating on mt. etna (del negro et al., 2002). during the more recent lateral eruptions of mt. etna, significant correlations were observed between volcanic activity and changes in the local magnetic field, up to ten nanoteslas (del negro and currenti, 2003). these observations are consistent with those calculated from integrated inversion of ground deformation and magnetic data at etna volcano using a genetic algorithm technique gilda currenti (1), ciro del negro (1), luigi fortuna (2) and gaetana ganci (1)(2) (1) istituto nazionale di geofisica e vulcanologia, sezione di catania, italy (2) dipartimento di ingegneria elettrica, elettronica e dei sistemi, università degli studi di catania, italy abstract geodetic and magnetic investigations have been playing an increasingly important role in studies on mt. etna eruptive processes. during ascent, magma interacts with surrounding rocks and fluids, and inevitably crustal deformation and disturbances in the local magnetic field are produced. these effects are generally interpreted separately from each other and consistency of interpretations obtained from different methods is qualitatively checked only a posteriori. in order to make the estimation of source parameters more robust we propose an integrated inversion from deformation and magnetic data that leads to the best possible understanding of the underlying geophysical process. the inversion problem was formulated following a global optimization approach based on the use of genetic algorithms. the proposed modeling inversion technique was applied on field data sets recorded during the onset of the 2002-2003 etna flank eruption. the deformation pattern and the magnetic anomalies were consistent with a piezomagnetic effect caused by a dyke intrusion propagating along the ne direction. mailing address: dr. gilda currenti, istituto nazionale di geofisica e vulcanologia, sezione di catania, piazza roma 2, 95125 catania, italy; e-mail: currenti@ct.ingv.it 22 gilda currenti, ciro del negro, luigi fortuna and gaetana ganci volcanomagnetic models, in which the magnetic changes are generated by stress redistribution due to magmatic intrusions at different depth and by the thermal demagnetization at a rather shallow depth. the magnetic data not only allowed the timing of the intrusive event to be described in greater detail but also, together with other volcanological and geophysical evidence, permitted some constraints to be set on the characteristics of propagation of shallow dikes (del negro et al., 2004). geodetic and magnetic investigations have been playing an increasingly important role in studies on mt. etna eruptive processes. the amount of available data collected represents a valuable database, but no major effort has been made for an effective integration of this knowledge. ground deformation and magnetic data are usually interpreted separately from each other and the consistency of the geophysical models coming from the different methods is checked only qualitatively. when the cause of their variations can be ascribed to the same volcanic source, a joint inversion of ground deformation and magnetic data would be advisable to identify the source parameters with a greater degree of accuracy. indeed, the integrated approach involving geophysical data of different kinds ensures a more accurate solution than when single data types are considered (nunnari et al., 2001) and leads to the best possible understanding of the physical process adding further constraints to the interpretation. we develop an inverse technique that is robust enough with respect to the complexity of the problem and able to model different geophysical data simultaneously. the aim of inverse modelling is to find out as much as possible about the source parameters using the observations. elaborated inverse methods combine forward models with appropriate optimization algorithms to automatically find out the best model parameters that minimizes the difference between the model values and the observations (tiampo et al., 2000; jousset et al., 2003). the forward problem involves deriving a mathematical relationship that relates observations and model. investigations on the geophysical models have revealed that they are highly non-linear and, as a consequence, the function to be minimized exhibits inherent discontinuities and multiple local minima. in such a case, techniques based on gradient methods or linearization of the optimization problem are hardly effective. that entails the need for global optimization methods. in this regard, we investigate the ability of an inversion procedure based on genetic algorithms (gas) for the estimation of the volcanic source parameters from available data (magnetic and ground deformation). among others, the gas are considered due to their capability of performing a much broader search over the parameter space with a greater likelihood of finding the global optimal solution even in presence of local minima in the objective function (goldberg, 1989). this modelling inversion technique was applied on field data sets recorded during the onset of the 2002 etna eruption. in order to provide better constraints on the intrusion mechanism, we investigate joint inversion of magnetic and deformation data during the period encompassing the opening of the eruptive fissures on the ne flank in 2002-2003. 1.1. the inversion technique inverse problems in volcanology involve the analysis of appropriate numerical strategies to identify the source parameters of a wide spectrum of geophysical signals recorded on active volcanic areas. as considered models are highly non-linear, the inverse numerical problem could be difficult to solve by applying local optimization techniques. in particular, nonlinear models engender in the function to be minimized inherent discontinuities and local optima, where local optimization methods are likely to get stuck. on the other hand, a trial and error procedure for solving the inversion problem may take a long time and the quality of the results might not be accurate enough. since the parameter space is typically very large, multimodal, and poorly understood, gas have the advantage of performing a much broader search over the space parameter using a random process with a greater likelihood of finding a near optimal solution (holland, 1975). roughly speaking, the adopted ga search algorithm randomly generates a set of n initial 23 integrated inversion of ground deformation and magnetic data at etna volcano using a genetic algorithm technique parameters (initial population), whose values are constrained within a prefixed range. using the chosen forward model, a population of potential solutions is computed for this set of parameters. the solutions are tested by an objective function that quantifies the degree of correlation between computed and observed data. the solutions are ranked, from the best to worst, according to the objective function and a new set of likely solutions is generated from the best solutions by the laws of evolutionary genetics. the evolutionary algorithm uses a collection of heuristic rules to modify the population of trial solutions in a way that the next generation tends to be on average better than its predecessor. heuristic selection and replacement operators are then used to sample the best solution in the current population for reproduction at each generation (goldberg, 1989). the population evolves from one generation to the next using probabilistic transition rules defined by genetic operators (crossover and mutation). the evolution is tuned by specifying crossover (pc) and mutation (pm) probabilities (michalewicz, 1992). this procedure is repeated for a large number of generations until the best solution is reached. a simplified schematic diagram of the procedure is shown in fig. 1. the ga search strategy is determined by two crucial issues: i) exploration of the search space to investigate new and unknown areas and ii) exploitation of the knowledge found at the previous step to improve the solution. genetic algorithms are able to find a balance between these two contradictory requirements (exploration and exploitation of the search space) by combining elements of deterministic and stochastic search. since the randomly generated population progressively evolves by means of a more deterministic sampling of the parameter space, the ga algorithm progressively leads the population to converge efficiently and rapidly toward the most promising solution that minimizes the objective function. 1.2. the objective function the goal of inversion methods consists in determining the best model that produces the solution closest to the observed measurements. the model can be represented by a set of parameters vector m={m1, ... , mk}∈ m, where m is the «space parameter» and k is its dimensionality. therefore, the best model corresponds to a parameters set, which on the basis of the forward problem formulation produces ground deformation and magnetic anomalies that well fit the observed data. for a given model m, the objective function quantifies the degree of misfit between observed data and the values computed by the forward problem. in this way, it helps to define the best parameter set m. fig. 1. schematic of ga inversion. 24 gilda currenti, ciro del negro, luigi fortuna and gaetana ganci geodetic and magnetic data are characterized by their own physical units and number of data points. as a result, the simultaneous inversion of both data sets without equalization may yield a solution that is dominated by one set. to perform the integrated inversion of ground deformation and magnetic data, we minimize the value of chi-square that accounts for the measurements error defined by the standard deviation σ, as (1.1) where mk are the measured data, ck the calculated deformation or magnetic variations and k the number of available measurements (bevington and robinson, 1992). 1.3. forward modelling the forward models used by the ga provide a mathematical formulation that relates magnetic anomalies, displacements and stress/strain fields associated to a particular volcanic source. on the whole, we have considered spherical pressure source (mogi model in volcanology) and rectangular sources. mogi’s model is quite appropriate for modelling the inflation/deflation of the magma reservoir that can be used to describe eruptive events associated with summit eruptions. on the contrary, rectangular sources seem more realistic for interpreting eruptive phenomena in volcanic areas such as mt. etna, where eruptions have their origin in dykes opening from a certain depth toward the surface. volcanic processes are complex geophysical systems and it is difficult to derive a forward model, unless important simplifications and approximations are taken into account (mctigue, 1987). surface displacements in a homogeneous elastic half-space have been described by mogi (1958) for a spherical source and by okada (1985, 1992) for a rectangular fault. also, over recent decades, the analytical formulation of the different mechanisms, which can be the cause of volcanomagnetic signals, has advanced considerably. a series of analytical solutions have been devised and widely used in the literature for mod( )m c k k k k 2 2 2 χ σ= −/ elling volcano-related variations (adler et al., 1999). the main geophysical mechanisms leading to magnetic anomalies in active volcanic areas have been investigated: i) piezomagnetic processes related to stress-induced changes in rock magnetization, ii) electrokinetic effects arising from fluid flow within the volcano edifice in the presence of an electric double layer at the solid-liquid interface, and iii) thermal demagnetization or remagnetization phenomena. while thermomagnetic effects are mainly concerned with temperature changes within the volcano edifice, piezomagnetism and electrofiltration process are both related to stress variations. if the volcano can be assumed to be elastic, the electrokinetic and piezomagnetic fields vary with time like the strain field and the ground deformation (zlotnicki and le mouel, 1988). hence, volcanomagnetic variations are expected to accompany crustal deformation (johnston, 1997). therefore, an integrated inversion of the ground deformation and magnetic data could give more insights on the state of volcano than when only one geophysical parameter is considered. as for the piezomagnetic field, sasai (1991) succeeded in devising an analytical expression for the mogi model, while utsugi et al. (2000) fig. 2. the okada source: a fault occurring in homogeneous and isotropic elastic half-space with a uniformly magnetized upper layer. depth is the distance between the origin o and the upper edge of the fault; strike is the orientation of the fault with respect to the north; dip is angle of the fault plane with respect the horizontal plane; length and width are the length and width of the fault respectively; slip is the vector of the displacement. 25 integrated inversion of ground deformation and magnetic data at etna volcano using a genetic algorithm technique calculated the solutions for strike-slip, dip-slip, and tensile-opening of a rectangular fault with an arbitrary dip angle (fig. 2). as for electrokinetic effects, the analytical form of magnetic fields by an inclined vertical source in inhomogeneous media was devised by murakami (1989). 2. the 2002 etna flank eruption in the following we interpret and model magnetic anomalies and ground deformation data that were observed simultaneously with the onset of the 2002 etna lateral eruption. during the night of 26-27 october 2002, two fissure systems opened on the s flank and ne rift of the volcano, feeding explosive activity and two distinct lava flows. the mainly explosive s flank eruption lasted 3 months (26 october 2002-28 january 2003), while the vents on the ne rift erupted only for 8 days and their activity was mainly effusive. the early stages of this lateral eruption were accompanied by local magnetic field changes and ground deformation recorded at the continuously operating stations set up on the volcanic edifice (aloisi et al., 2003; del negro et al., 2004). the most significant changes to occur here are associated with the opening and propagation of the vents along the ne rift. the dataset used for the inversion consists of the total magnetic field measurements at dgl and pdn stations, where positive variations of about 5 and 4 nt were recorded respectively between 26-27 october (fig. 3). as for ground deformation data, we inverted the static offsets recorded in the tilt components (at 6 stations) and in the gps horizontal and vertical changes (at 8 stations) during the same period (aloisi et al., 2003). a critical choice is the selection of the model used to interpret the geodetic and magnetic data. the magnetic variations could have resulted from stress redistribution due to dike emplacement at a rather shallow depth, which took place in a few hours. in such a case, the piezomagnetic effect is the principal mechanism accompanying the fractures opening, since it could justify both the amplitude and the time-scale of the magnetic changes. it is worth noting that the magnetic field changes might also be expected to accompany magmatic intrusions as a result of electrokinetic effects generated by fluid flow (fitterman, 1979). however, to explain the observed rapid and reversible changes in terms of fig. 3. plot of 10-min means of differential magnetic field at pdn and dgl stations with respect to the csr reference station between 20 october and 03 november 2002. 26 gilda currenti, ciro del negro, luigi fortuna and gaetana ganci electrokinetics would require rapid and implausibly intense fluid flow (murakami, 1989; adler et al., 1999). thus, even if the electrokinetic mechanism cannot be ruled out, we favor a more straightforward explanation in terms of the piezomagnetic effect. starting from the assumption of a piezomagnetic fault model (fig. 2), we apply okada (1985)’s elastic dislocation theory for computing the ground displacements and utsugi (2000)’s formulation for estimating the expected piezomagnetic field. we disregard the southern dyke since it made only a minimal contribution on the recorded pattern deformation (aloisi et al., 2003) and it was located rather far away to affect the magnetic field at dgl and pdn stations (fig. 5). hence, we invert ground deformation and magnetic data to infer the positions and the geometrical parameters of the source leading the geophysical changes. the fault model involves ten parameters: m={δ, α, d, w, l, us, ut, ud, xc, yc}, whose descriptions and value ranges are reported in table i. the ga was requested to determine the best solution for the fault parameters in the search space. the set of likely solutions is significantly constrained by the location of the observed surface ruptures. the values of the parameters of the magnetoelastic medium used in these calculations are shown in table ii. lame’s constant was set up to λ = µ that leads for poisson’s ratio of 0.25, a reasonable approximation to the values estimated in the upper crust on mt. etna. the success of the search depends on the ga control parameters, explicitly i) the size of the initial population n; ii) the crossover probability pc; and iii) the mutation probability pm. a large number of initial individuals ensures more possibilities to explore the overall parameter space but makes the inversion procedure more time-consuming. high values for the crossover and mutation probabilities guarantee a fine exploitation and exploration, respectively, but decrease the rate of convergence toward table i. search range for the fault model parameters. the coordinates are in utm-wgs84 projection. parameters for the piezomagnetic fault source modelling the deformation and magnetic data recorded during the onset of 2002 ne etna flank eruption. the standard deviation of each parameter is computed over 50 different runs of ga. fault model parameters ranges obtained parameters standard deviation δ, dip fault 0°-90° 28° 2.71° α, azimuth (° from north) 0°-180° 84° 2.91° d, depth 50-3000 m 0.04 m 216.5 m w, fault width 200-8000 m 4140 m 556.2 m l, fault length 500-6000 m 6541 m 730.8 m us, strike-slip 0.1-3 m 0.7 m 0.18 m ut, tensile opening 0.1-3 m 1.1 m 0.17 m ud, dip-slip 0.1-3 m 0.1 m 0.12 m xc, north coordinate 4173.90-4185.71 km 4182.7 km 135.31 m yc, east coordinate 499.89-504.27 km 500.400 km 159.02 m table ii. the magneto-elastic parameters estimated in the upper crust on mt. etna. magneto-elastic symbol value parameters average magnetization j 2 a/m curie depth h 15000 m magnetic inclination i 53° magnetic declination d 1° rigidity µ 1.6×105 bar poisson’s ratio ν 0.25 stress sensitivity β 0.0001 bar−1 27 integrated inversion of ground deformation and magnetic data at etna volcano using a genetic algorithm technique the solution that minimizes the objective function. initial tests suggested using a crossover probability pc of about 0.7, which seems to be a good compromise between exploration and exploitation as well as between efficiency and computation time. in particular, we used a population of between 50 and 150, and pm in the range of 0.3-0.7. the best results for ga setting parameters were found to be n = 150, pc = 0.7 and pm = 0.6 (fig. 4) for which the objective function reaches a minimum value. the reduced χ2 value is computed by (2.1)d 2 2 χ χ =t where d = n−p is the degree of freedom, n is the number of data points, p is the number of parameters. the best solution held a reduced of 3.18. we statistically compute the error for each parameter over 50 different inversions. in all of the 50 runs of the ga, the algorithm converges to the close neighborhood of the best solution starting from different random initial populations (table i). the ground deformation and the magnetic observations are reproduced well by the obtained fault model (fig. 5). the computed magnetic anomalies were about 4.7 and 4.3 nt at dgl and pdn respectively. the fault location matched well the position of the eruptive fissure observed at the surface. 2χt fig. 4. testings on ga setting parameters. the population converges toward the minimum of the objective function from its initial random distribution in the parameter space. 28 gilda currenti, ciro del negro, luigi fortuna and gaetana ganci 3. conclusions intensive monitoring of recent lateral eruptions on mt. etna has produced a large amount of ground deformation and magnetic data. although there have been numerous attempts to independently model these data, no investigations have been carried out to jointly interpret these observations. moreover, the consistency of the models coming from these geophysical methods has been verified only a posteriori. such a lack of multidisciplinarity and integration of observations and theoretical studies has certainly been prejudicial to the development of a deeper understanding of the volcano dynamics. in order to make the description of volcanic sources more robust we propose an integrated inversion from deformation and magnetic data. the inversion problem was formulated following a global optimization approach based on the use of genetic algorithms. the gas are considered due to their capability of performing a much broader search over the model parameters with a greater likelihood of finding the global optimal solution even in presence of local minima in the objective function. attempts at modelling changes in different geophysical parameters recorded in volcanic areas often involve a great deal of effort due to the complexity of the problem. the inversion problem in geophysical modelling suffers from the ambiguity and instability of its solutions. the ambiguity arises because different combinations of source parameters may lead to similar observations. moreover, geophysical inversion problems are notoriously unstable. if the cause of ground deformation and local magnetic changes can be ascribed to the same geophysical process, a joint inversion of different data sets would be desirable in order to yield more robust estimates fig. 5. schematic map of the 2002-2003 etna eruption showing lava flows erupted from south and ne-trending fissures. locations of gps, tilt and magnetic stations are also indicated. recorded and computed horizontal displacements and tilts vectors are shown as well. 29 integrated inversion of ground deformation and magnetic data at etna volcano using a genetic algorithm technique of source parameters and reduce the ambiguity of solution. we applied this procedure to model the data set recorded during the 2002-2003 etna flank eruption. the simultaneous inversion of geodetic measurements and magnetic data provides a coherent interpretation of the observations and allows delineating with more accuracy the magma intrusion process occurred at the onset in the ne flank. even if the fault model provides a quite reasonable representation of the dyke intrusion occurred on the northern flank, it is not able to justify all the details. the simplified assumption in the forward model of elastic half-space medium disregards effects caused by the topographic variations and the presence of a multilayered medium that could play a role in the modelling procedure. to overcome this intrinsic limitation and provide more realistic models which consider topographic effects as well as complicated distribution of medium properties, more elaborated forward models are required. in this regard, numerical solutions could provide a new insight into the expected variations that cannot be achieved through analytical models, and in particular can help in distinguishing features that can be very useful for hazard evaluation. acknowledgements this work was developed in the frame of the tecnolab, the laboratory for the technological advance in geophysics organized by dieesunict and ingv-ct. references adler, p., j.l. le mouël and j. zlotnicki (1999): electrokinetic and magnetic fields generated by flow through a fracture zone: a sensitivity study for la fournaise volcano, geophys. res. lett., 26 (6), 795-798. aloisi, m., a. bonaccorso, s. gambino, m. mattia and g. puglisi (2003): etna 2002 eruption imaged from continuous tilt and gps data, geophys. res. lett., 30 (23), 2214, doi: 10.1029/2003gl018896. bevington, p.r. and d.k. robinson (1992): data reduction and error analysis for the physical sciences (mcgraw-hill, new york), pp. 328. bonaccorso, a. and p.m. davis (2004): modeling of ground deformation associated with recent lateral eruptions: mechanism of magma ascent and intermediate storage at mt. etna, in etna volcano laboratory, edited by a. bonaccorso, s. calvari, m. coltelli, c. del negro and s. falsaperla, am. geophys. un., geophysical monogr. ser., 293-306. bonaccorso, a., m. aloisi and m. mattia (2002): dike emplacement forerunning the etna july 2001 eruption modelled through continuous tilt and gps data, geophys. res. lett., 29 (13), 1624, doi: 10.1029/2001gl014397. del negro, c. and g. currenti (2003): volcanomagnetic signals associated with the 2001 flank eruption of mt. etna (italy), geophys. res. lett., 30 (7), 1357, doi: 10.1029/2002gl015481. del negro, c., r. napoli and a. sicali (2002): automated system for magnetic monitoring of active volcanoes. bull. volcanol., 64, 94-99. del negro, c., g. currenti, r. napoli and a. vicari (2004): volcanomagnetic changes accompanying the onset of the 2002-2003 eruption of mt. etna (italy), earth planet. sci. lett., 229 (1-2), 1-14. fittermann, d.v. (1979): theory of electrokinetic-magnetic anomalies in a faulted half-space, j. geophys. res., 84, 6031-6040. goldberg, d.e. (1989): genetic algorithms in search, optimization, and machine learning (addison-wesley, ma), pp. 432. holland, j.h. (1975): adaptation in natural and artificial system (university of michigan press). johnston, m.j.s (1997): review of electric and magnetic field accompanying seismic and volcanic activity, surv. geophys., 18, 441-475. jousset, p., h. mori and h. okada (2003): elastic models for the magma intrusion associated with the 2000 eruption of usu volcano, hokkaido, japan, j. volcanol. geotherm. res., 2607, 1-26. mctigue, d.f. (1987): elastic stress and deformation near a finite spherical magma body: resolution of the point source paradox, j. geophys. res., 92, 12931-12940. michalewicz, z. (1992): genetic alogorithms+data structures=evolution programs (springer, new york, ny). mogi, k. (1958): relations between the eruptions of various volcanos and the deformations of the ground surfaces around them, bull. earthquake res. inst. tokyo, 36, 99-134. murakami, h. (1989): geomagnetic fields produced by electrokinetic sources, j. geomagn. geoelectr., 41, 221-247. nunnari, g., l. bertucco and f. ferrucci (2001): a neural approach to the integrated inversion of geophysical data types, ieee trans. geosci. remote sensing, 39 (4), 736-748. okada, y. (1985): surface deformation due to shear and tensile faults in a half-space, bull. seismol. soc. am., 75, 1135-1154. okada, y. (1992): internal deformation due to shear and tensile faults in a half-space, bull. seismol. soc. am., 82, 1018-1040. sasai, y. (1991): tectonomagnetic modeling on the basis of the linear piezomagnetic effect, bull. earthquake res. inst., univ. tokyo, 66, 585-722. tiampo, k.f., j.b. rundle, j. fernandez and j.o. langbein (2000): spherical and ellipsoidal volcanic sources at long valley caldera, california, using a genetic algorithm inversion technique, j. volcanol. geotherm. res., 102, 189-206. 30 gilda currenti, ciro del negro, luigi fortuna and gaetana ganci utsugi, m., y. nishida and y. sasai (2000): piezomagnetic potentials due to an inclinated rectangular fault in a semi-infinite medium, geophys. j. int., 140, 479492. zlotnicki, j. and j.l. le mouel (1988): volcanomagnetic effects observed on piton de la fournaise volcano (reunion island): 1985-1987, j. geophys. res., 93, 91579171. varenna workshop report. operational earthquake forecasting and decision making annals of geophysics, 58, 4, 2015, rw0434; doi:10.4401/ag-6756 rw0434 operational earthquake forecasting and decision making warner marzocchi1,*, thomas h. jordan2, gordon woo3, conveners 1 istituto nazionale di geofisica e vulcanologia, rome, italy 2 southern california earthquake center, university of southern california, los angeles, usa 3 risk management solutions, london, uk a workshop on operational earthquake forecasting and decision making was convened in varenna, italy, on june 8-11, 2014, under the sponsorship of the eu fp 7 reakt (strategies and tools for real-time earthquake risk reduction) project, the seismic hazard center at the istituto nazionale di geofisica e vulcanologia (ingv), and the southern california earthquake center (scec). the main goal was to survey the interdisciplinary issues of operational earthquake forecasting (oef), including the problems that oef raises for decision making and risk communication. the workshop was attended by 64 researchers from universities, research centers, and governmental institutions in 11 countries. participants and the workshop agenda are listed in the appendix. the workshop comprised six topical sessions structured around three main themes: the science of operational earthquake forecasting, decision making in a low-probability environment, and communicating hazard and risk. each topic was introduced by a moderator and surveyed by a few invited speakers, who were then empaneled for an open discussion. the presentations were followed by poster sessions. during a wrap-up session on the last day, the reporters for each topical session summarized the main points that they had gleaned from the talks and open discussions. this report attempts to distill this workshop record into a brief overview of the workshop themes and to describe the range of opinions expressed during the discussions. 1. background the concept of operational earthquake forecasting was developed by the international commission on earthquake forecasting for civil protection (icef; jordan et al. [2011]) in a report requested by the italian government after l’aquila earthquake of april 6, 2009 (available at http://www.annalsofgeophysics.eu/index. php/annals/article/view/5350). according to the icef definition, oef comprises procedures for gathering and disseminating authoritative information about the time dependence of seismic hazards to help communities prepare for potentially destructive earthquakes. the icef report has stimulated a broad discussion about the development of oef and its practical utility in managing seismic risk. the search for “diagnostic precursors” has so far been unsuccessful; no signals have yet been discovered that can reliably predict the location, time, and magnitude of an impending earthquake with high probability. but the icef concluded that earthquake clustering models can reliably forecast earthquakes at low probabilities over short time intervals. according to these models, the short-term probabilities of large earthquakes may increase by up to 2-3 orders of magnitude during seismic sequences, though they rarely exceed about few percent per week. decision making in such a “low-probability environment” is problematic and poorly studied. an even more formidable challenge is how to communicate the hazard information and risk implications to various stakeholders, including the public at large, in a comprehensible and effective way. 2. science of operational earthquake forecasting two sessions on oef science highlighted the formulation and testing of probabilistic forecasting models based on seismic clustering [e.g., ogata 1988, reasenberg and jones 1989, michael 2012], and a third surveyed how oef systems are developing in italy, new zealand, and the united states [marzocchi et al. 2014, gerstenberger et al. 2014, field et al. 2015]. a hot topic was the seismicity induced by deep fluid injection, which is rapidly changing the seismicity of the u.s. continental interior [e.g. ellsworth 2013] and has become a article history received march 3, 2015; accepted july 22, 2015. subject classification: operational earthquake forecasting, decision-making, seismic risk, risk communication. varenna workshop report political as well as scientific issue in several countries. speakers pointed out the difficulty of accommodating the rapid rise of induced seismicity into long-term probabilistic seismic hazard assessments (psha) that are the basis for most national seismic hazard maps, and this motivated discussions of how oef systems could be useful for projecting time-dependent earthquake probabilities in regions experiencing anthropogenic seismicity increases. short-term clustering models, such as the shortterm earthquake probability (step; gerstenberger et al. [2005]) and epidemic type aftershock sequence (etas; ogata [1988]) models, currently show the highest information gains relative to long-term forecasts, making them the prime candidates for oef applications. it was reported that various versions of these models are now being continuously evaluated against seismic data in several testing regions around the world by the collaboratory for the study of earthquake predictability (csep). csep is an international infrastructure for evaluating earthquake forecasting models through prospective forecasting experiments that are blind, transparent, and reproducible. csep testing experiments, which currently involve over 400 variations of statistical forecasting models, are running in california, new zealand, italy, japan, western pacific, and at global scale; other testing regions under consideration include iceland and part of china. csep has been enhancing its procedures to provide more robust statistical evaluation of the models and to assess a wider range of model types [e.g. jordan 2006, zechar et al. 2010]. model developers emphasized the role of csep testing in validating models, or model combinations, for the practical purposes of oef. csep experiments are also aimed at addressing important scientific questions, for example testing the hypotheses that clustering is self-similar from small to large earthquakes. the null hypothesis is that earthquakes do not encode any information about the magnitude of the future events; the alternative hypothesis is that the preparatory phase of a large shock is somehow different, implying more predictability. the limitations of csep were also discussed, particularly regarding the infrequency of large earthquakes, which limits the power of the prospective tests for that class of events. it was generally agreed that, in order to get better statistics at higher magnitudes, prospective testing in the existing natural laboratories should be augmented with experiments in new natural laboratories and the testing of worldwide forecasts, as well as through well-structured retrospective testing. it was reported that both activities are under active development at the csep testing centers. the clustering models involve generic statistics that do not explain the distinctive features observed in specific seismic sequences. some participants argued that such deficiencies rendered the models inadequate for operational purposes. others disagreed, noting that, though the models are uncertain and cannot explain everything, their forecasting performance has been validated by prospective testing in the csep natural laboratories, and they should therefore be useful within their stated limitations and epistemic uncertainties. all participants agreed that the skill of existing shortterm forecasting methods is very modest. it is rare for the weekly probability of large shocks to reach 1% (usually after another large shock). researchers are exploring how to increase forecasting skill by incorporating other types information into the forecasting models, including constraints on stress changes and rupture nucleation processes, as well as non-seismic precursory activity. potential improvements may also come from the explicit incorporation of fault-based rupture models, such as the short-term extensions of the third uniform california earthquake rupture forecast (ucerf3), which is being developed by the working group on california earthquake probabilities [field et al. 2015]. knowledge of the fault geometries and slip rates better defines the distribution of the large ruptures that could be triggered during seismic sequences, and it allows the concept of elastic rebound and stress relaxation to be incorporated into the clustering models. a major topic in the workshop discussions was the need for consistency among forecasting methods across a full range of forecasting time scales. new zealand researchers reported that, in the canterbury region, earthquake forecasting is now being done using hybrid models that incorporate the short-term step model, the longterm psha model, and a medium-term model called eepas (every earthquake a precursor according to scale), developed in new zealand [rhoades and gerstenberger 2009]. there was much interest in the lessons learned from recent canterbury sequence, which began with the 2010 darfield earthquake and has since provided a datarich environment for evaluating oef practice and its impact on society. the new zealanders provided interesting perspectives on the important question of how expert opinion should be used in combination with earthquake clustering models. in recent practice, experts have defined a set of plausible scenarios that span the possibilities for the evolution of the seismic sequence, attaching to each scenario a probability that is consistent with the probability given by the earthquake clustering models [gerstenberger et al. 2014]. representatives from other advisory groups described different approaches. in italy, for example, the marzocchi et al. 2 3 grand risk commission (grc) relies on expert opinion about the peculiarities of each seismic sequence rather than short-term forecasting models, which results in qualitative rather than quantitative assessments of the sequence’s possible evolution. in united states, the california earthquake prediction evaluation council and the national earthquake prediction evaluation council have not yet structured their procedures to release homogeneous information consistently during seismic sequences, although the u.s. geological survey has been routinely, automatically producing generic assessments of aftershock probabilities after every m≥3.5 in california for almost 20 years. presently, usgs is working to expand the approach nationwide and to more rigorously handle situations where a straight omori-gutenbergrichter probability assessment may not be the best representation of the hazard, and considering approaches for continuous production of probabilities. there was a consensus among the participants that expert opinion should be included in oef, but some disagreement on how to integrate it with quantitative models. most modelers in the audience favored the quantitative formulation of expert opinion; for example, through well-structured expert elicitations. some stressed the importance of casting all oef information in the form of probabilities, both aleatory and epistemic, to allow an adequate separation between science of hazard assessment and the more socially complex process required to formulate an appropriate spectrum of risk-mitigation options. 3. decision making in a low-probability environment oef provides probabilistic forecasting information, but the formulation of mitigation options requires that the earthquake hazard, as described in terms of faulting, shaking, and secondary effects such as ground failures, be translated into seismic risk, as measured in dollars, casualties, and social functionality. the engineering and policy approaches to this risk-analysis problem were the topic of the fourth session, and related decision-making issues were addressed by several speakers in the fifth and sixths sessions. it was noted that the term “low-probability” associated with the hazard is an incomplete description of oef problem, because the consequences for some stakeholders (e.g., critical facility operators) may be much higher than for ordinary citizens. the higher risk levels perceived by such stakeholders will more frequently warrant mitigation actions. usually, but not always, a costbenefit analysis will dictate actions that are relatively low in cost, commensurate with the low probabilities. earthquake engineers emphasized the quantification of risk through well-defined loss metrics normalized to quiescent reference conditions, such as the expected damage rates to structural models or expected fatality rates, and they illustrated plausible metrics with examples from structural engineering [iervolino et al. 2015]. there was a substantial agreement among the attendees to separate the formulation of scientific information about hazard from the risk assessments that inform decision making. according to the icef, the oef role is to deliver robust, authoritative hazard information, usually cast in terms of probability. this information has to be translated into risk, i.e. into different kinds of expected losses. risk assessment is the natural framework in which to formulate mitigation options and to choose the proper actions according to a rationale cost-benefit analysis. although widely accepted in principle, the precise nature of this hazard/risk separation, as well as the means to achieve it, was debated by the participants, mostly around the question of whether scientists should act as decision makers in evaluating and prioritizing the mitigation options. one clear example discussed at the meeting is the policy of releasing oef information only during aftershock sequences [e.g. wang and rogers 2014, jordan et al. 2014]. most participants agreed that oef probabilities during vigorous aftershock sequences should be made publically available. after large earthquakes, the probabilities are higher and, according to advocates of the “aftershocks-only” policy, more easily comprehended by the public. others argued that such a policy violates the hazard-risk separation principle, because it puts oef in an inappropriate decision-making role; namely, of judging at which probability levels the users of oef should be informed. moreover, it violates the transparency principle, also articulated by the icef, which states that authoritative scientific information about future earthquake activity should not be withheld from the public. those issues aside, it was widely agreed that the utility of forecasting is strongly correlated with the amplitude of the probabilities. higher probabilities can be more easily used by the decision makers to establish a set of mandatory mitigation actions based on a quantitative rational framework based on cost-benefit analysis. the practical use of forecasts with high-hazard probability rates of less than, say, 5% per week to mitigate the risk is much less clear, and the attending issues have not been yet been sufficiently explored. some attendees argued that current short-term forecasting methods are effectively useless for practical purposes, owing to the high false alarm rates implied by the low-probability rates the “crying-wolf ” problem [wang and rogers 2014]. others countered that even small probabilities can be helpful in nudging people towards beneficial self-protection actions that are varenna workshop report consistent with their own aversion to risk [thaler and sunstein 2009, jordan et al. 2014]. some might choose to minimize their time in seismically unsafe buildings, for example. once citizens are correctly informed about the earthquake threat and possible actions to mitigate their personal risks, they can act as rational decision makers for their own families and communities. it was noted that nudging citizens towards rational actions through the release of authoritative information is commonly employed to defend against many other low-probability hazards, such as terrorism threats and the spread of communicable diseases. the recent canterbury seismic sequence in new zealand has provided an excellent testbed for the use of oef in decision making on an urban scale. it was reported how quantitative oef information was delivered, interpreted, and applied to risk mitigation during this sequence, and how the oef system set up by new zealand seismologists continues to guide the reconstruction of christchurch, which was badly damaged by the darfield aftershocks of 2011 [wein and becker 2013, gerstenberger et al. 2014]. oef systems will have to service a wide variety of information requirements, most obviously those of the general public and responsible governmental agencies, but also those of other stakeholders, such as those who manage critical infrastructures and insurance/re-insurance companies. the various needs and levels of risk aversions among the many stakeholder groups imply that oef systems will have to provide scientific information in different formats across a range of forecasting intervals. one notable problem is maintaining the consistency of short-term oef forecasts with the longterm hazards quantified in psha. to avoid confusion during seismic crises, it was recommended that the probability thresholds for significant risk-reduction actions be described in protocols prepared in advance of a crisis through negotiations among scientists, risk analysts, political decision-makers, experts in communication, and interested stakeholders. these protocols should describe the way in which the authoritative scientific information is to be delivered and how high-priority risk mitigation actions are to be implemented. 4. communicating hazard and risk the discussions of how oef information should be communicated to decision makers, including the public at large, dominated the fifth and sixth topical sessions. the social scientists attending the workshop outlined some general findings from their research on risk communication. humans have the ability to perceive and balance a large number of risks regularly encountered in the natural environment, including high risks at low levels of probability. however, when new information about lowprobability, high-consequence events is received, people tend to dichotomize their response: they are either concerned and motivated to act, or they are unconcerned and don’t. studies of this threshold-type response has led to some general guidelines for risk messaging [e.g., wood et al. 2012, and references therein]: (i) communication about specific risks should be layered, with basic information broadcast in high-level messages and more detailed information, including hazard and risk probabilities, made easily available to those who seek it. (ii) high-level messages should contain authoritative information about the hazard, identify the sources of the information, and be explicit in recommending specific actions to reduce the risk. (iii) messaging is most effective when consistent information comes from multiple sources. (iv) the public should be educated about hazards and risks through a continual dialog with scientists and decision makers and within their own communities. considerable debate was centered on how to communicate increases in earthquake hazard and risk when the absolute probability of high hazard remains low. some geoscientists expressed particular concern over the widespread public illiteracy about probability, which may limit the ability of non-scientists, including decision makers, to comprehend the meaning of the aleatory variability and epistemic uncertainty intrinsic to earthquake forecasting. techniques for improving public preparedness through oef have not yet received sufficient attention, although relevant research was conducted in california during the parkfield prediction experiment and following the 1989 loma prieta earthquake [mileti and derouen 1995]. studies were reviewed that show the ability of even primitive societies to comprehend probability-based statements of risk, as long as the statements are properly formulated and clearly communicated [fontanari et al. 2014]. it was emphasized that communication can build trust in authoritative information over time, and that honesty about uncertainty can enhance credibility. experts in risk communication argued that, when faced with confusing and possibly hazardous situations, people respond positively to authoritative statements from official sources about the actions they should take, even when the future is highly uncertain; properly delivered, authoritative information about impending threats has rarely, if ever, caused panic among an informed populace [e.g. clarke 2002]. these considerations underline the need for transparency and continuity in broadcasting authoritative scimarzocchi et al. 4 5 entific information about the time-dependence of earthquake hazards. several participants noted that the timely communication of authoritative forecasts can benefit the public by filling information vacuums that set the stage for amateur earthquake predictions and misinformation. seismic crises can also be teachable moments, when people abandon their usual apathy about earthquake preparedness. timely oef communication can thus be very effective in teaching people how to reduce seismic risk both in the short and in the long-term. references clarke, l. 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(2012). fundamental questions of earthquake statistics, source behavior, and the estimation of earthquake probabilities from possible foreshocks, b. seismol. soc. am., 102, 2547-2562. mileti, d.s., and j. derouen (1995). societal response to revised earthquake probabilities in the san francisco bay area, int. j. mass emer. dis., 13, 119-145. ogata, y. (1988). statistical models for earthquake occurrences and residual analysis for point processes, j. am. stat. assoc., 83, 9-27. reasenberg, p.a., and l. m. jones (1989). earthquake hazard after a mainshock in california, science, 243, 1173-1176. rhoades, d.a., and m.c. gerstenberger (2009). mixture models for improved short-term earthquake forecasting, b. seismol. soc. am., 99, 636-646. thaler, r.h., and c.r. sunstein (2009) nudge: improving decisions about health, wealth and happiness, penguin press. wang, k., and g.c. rogers (2014). earthquake preparedness should not fluctuate on a daily or weekly basis, seismol. res. lett., 85, 569-571. wein, a., and j. becker (2013). communicating aftershock risk: roles for reassuring the public, risk frontiers, 13 (3), 3-4. wood, m.w., d.s. mileti, m. kano, m.m. kelley, r. regan and l.b. bourque (2012). communicating actionable risk for terrorism and other hazards, risk anal., 32, 601-615. zechar, j.d., d. schorlemmer, m. liukis, j. yu, f. euchner, p. maechling and t.h. jordan (2010). the collaboratory for the study of earthquake predictability perspective on computational earthquake science, concurrency comput pract. ex., 22, 1836-1847. corresponding author: warner marzocchi, istituto nazionale di geofisica e vulcanologia, rome, italy; email: warner.marzocchi@ingv.it. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. varenna workshop report appendix a: workshop agenda session 1: the problem of earthquake forecasting the problem of earthquake forecasting (t. jordan) earthquake forecasting when the background rate is non-stationary: the case of the u. s. midcontinent (b. ellsworth) forecasting induced seismicity: state of the art and future directions (s. wiemer) panel discussion (moderator: i. main; reporter: m. page) session 2: testing short-term earthquake forecasting models the collaboratory for the study of earthquake predictability (d. schorlemmer) the two-way street between next-day seismicity forecasts and operational earthquake forecasting ( j. zechar) csep developments in support of operational earthquake forecasting: a future perspective (d. rhoades) panel discussion (moderator: n. hirata; reporter: m. werner) session 3: operational earthquake forecasting operational earthquake forecasting in italy (w. marzocchi) operational earthquake forecasting in the united states (n. field) recent experiences in oef in new zealand: the good and the bad (m. gerstenberger) panel discussion (moderator: r. stein; reporter: m. blanpied) session 4: from short-term hazard to risk operational earthquake loss forecasting: framework and proof of concept (i. iervolino) evaluating risk and providing advise for low probability-high impact events in the presence of large uncertainties: the experience of the italian high risk commission, 2011-2013 (d. giardini) current practice in the united states, and plans for improved dynamic earthquake likelihood forecasting (m. blanpied) panel discussion (moderator: m. dolce; reporter: m. gerstenberger) session 5: decision-making process and risk communication in a low-probability forecasting environment (i) participatory decision making under uncertainty (g. woo) the structure and content of effective risk messages communicated to the public (d. mileti) managing seismic sequences in italy: civil protection current practice and questions for the future (m. dolce) panel discussion (moderator: w. marzocchi; reporter: t. sellnow) session 6: decision-making process and risk communication in a low-probability forecasting environment (ii) case histories of communicating uncertain earthquake hazard and loss estimates (d. wald) risk communication strategies when you understand that your audience is about as smart as you are (t. patt) strategies for communicating science-based messages in times of uncertainty (t. sellnow) panel discussion (moderator: d. mileti; reporter: g. woo) session 7: summary of the meeting reports on the topic sessions (m. page, m. werner, m. blanpied, m. gerstenberger, t. sellnow, g. woo) wrap-up discussion posters sessions: a short-term earthquake forecasting experiment in japan (n. hirata) operational short-term earthquake forecasting in japan (k. doi) corssa: community online resource for statistical seismicity analysis ( j. d. zechar) evaluating earthquake predictions by using the gambling score ( j. zhuang, j. d. zechar, c. jiang) effect of data quality on a hybrid coulomb/step model for earthquake forecasting (s. steacy) practice and enlightenment of earthquake prediction in china (xiadong zhang) an operational earthquake forecasting system in metropolitan area around beijing, china (yongxian zhang) short-term forecasting and preparation of earthquakes (d. albarello) preparing for an operational earthquake forecast experiment in iceland (f. panzera) improving earthquake and aftershock risk communication: lessons from the canterbury earthquakes, new zealand (s. potter) the effect of 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice microsoft word 7473-18531-1-rv_peppoloni-di capua.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7473 1 geoethics: ethical, social and cultural implications in geosciences silvia peppoloni giuseppe di capua istituto nazionale di geofisica e vulcanologia, rome, italy iapg – international association for promoting geoethics silvia.peppoloni@ingv.it, giuseppe.dicapua@ingv.it abstract humans are recognized as a “geological force”, capable of modifying natural environments, and in virtue of this prerogative they have an ethical responsibility towards the planet. indeed, studying and managing the earth system, exploiting its geo-resources, intervening in natural processes are actions that involve great responsibilities towards society and the environment, of which perhaps we, as geoscientists, are not sufficiently aware. only by increasing the awareness of this responsibility, can we work with wisdom and foresight, and respect the balances that exist in nature while guaranteeing a sustainable development for future generations. in order to define acceptable solutions to current global challenges, we need to take into proper consideration the ethical and social aspects involved in geoscience issues. geoethics was born to define a conceptual substratum of categories, useful as framework of reference for geoscientists, to help them develop a new way of thinking and interacting with the earth system. geoethics widens the cultural horizon of geoscience knowledge and contributes to orient scientists and society in the choices for responsible behavior towards the planet. the paper provides an overview of the emerging field of geoethics, focusing on theoretical and practical aspects, by showing the trajectory that has led to the current point of development of geoethics and suggesting some cues for thought for further advancements of ethical thinking in geosciences. 1. introduction ince geosciences have ethical and social implications (bobrowsky et al., 2017; stewart and gill, 2017; wyss and peppoloni, 2014), geoethics should become a key requirement in every geoscientists’ curricula (bobrowsky et al., 2017). to achieve this goal, geoethics needs above of all to obtain dignity and credibility within the scientific community. geoethics can represent a new way of thinking and interacting with the earth system, and a new way to approach the global problems facing the planet. but the field of geoethics is still in the early stages of development, and its status as a new discipline has to be fully characterized, assessed and clarified, as well as its relationship with “environmental ethics” (hourdequin, 2015). to start this process, in recent years a conceptual substratum has been developed following progressive steps, helpful to provide a clearer framework of categories and contents within geoethics: a formal definition, as well as four levels of content including analyses, topics, goals, values. this framework is necessary to give a rational structure to geoethics, and to assure its authoritativeness. the founding of the iapg – international association for promoting geoethics (iapg, 2017) in 2012 has stimulated the creation of a large geoscience community to share ideas, to build contents and to address geoethical issues. 2. meaning of the word “geoethics” the first step was to clarify the etymological origin of the word "geoethics" (peppoloni and di capua, 2014). “geoethics” is the union of the prefix “geo” and the word “ethics”. the prefix “geo” refers to “gaia”, which means “earth” in greek. although its ancient sumerian meaning is “home, dwelling place”. thus s annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7473 2 “geo” refers to the place where humans dwell. the word “ethics” has a double meaning: first, “ethics” contains a sense of belonging to the social dimension of life, second, it is related to the individual sphere of each person. in both these two existential conditions (social and individual) the etymological root of the word “ethics” calls upon human beings to face their responsibilities. this concept stresses that geoethics means (social and individual) responsibility towards the earth, an ethics for the planet. 3. responsibility and four levels of analysis in geoethics the second step was to define one of the key concepts at the base of geoethics: the responsibility (individual and social), that can be referred to four different levels of analysis (bobrowsky et al., 2017; tgeagc, 2017); responsibility: • we, as geoscientists, have towards ourselves in conducting our work to the best of our abilities (this implies the importance of each scientist’s individual conscience); • towards our colleagues, with whom we have to work with a cooperative, respectful and honest attitude, with the common goal to find solutions to geoscience problems under a multidisciplinary approach; • towards society that we, for our expertise, have the duty to serve to allow its development, as far as possible minimizing the environmental impact and respecting natural dynamics; • towards the earth system, as our irreplaceable home, that we have to preserve and to entrust to future generations. we, colleagues, society and the earth system are fundamental ambits for the geoethical analysis, to be considered in a hierarchical order. in fact, before our responsibilities towards society and the earth system, we refer to our individual conscience and identify our own reference values, both ethical and professional. at the base of geoethics there is definitely an honest relationship with ourselves and consequently with our colleagues, society, and environment. 4. a definition of geoethics when a new discipline begins to be developing, definitions are fundamental to clarify to what we refer. starting from the concept of ethics proposed by aristotle (384-322 bc), and putting together the concept of responsibility related to the four levels of analysis and etymological remarks, geoethics has been finally defined as the “research and reflection on the values which underpin appropriate behaviours and practices, wherever human activities interact with the earth system. geoethics deals with the ethical, social and cultural implications of geoscience knowledge, education, research, practice and communication, and with the social role and responsibility of geoscientists in conducting their activities” (bobrowsky et al., 2017; iapg, 2017; peppoloni and di capua, 2012). this definition implies three important matters: • understanding when an ethical issue arises. • building a framework of common values to be adopted by the geoscience community and society as a whole. • identifying an ethical criterion that can orient geoscientists, on which to base technical decisions. 5. ethical issue and ethical decision an ethical issue arises when we are faced with at least two possible options and wherein one option is likely preferable to the other. if one option is clearly better than the other, then the decision to be taken is quite simple. however it is more complicated when we face ethical dilemmas, in which both the solutions generate negative consequences. in this case, which is the best decision from an ethical point of view? the definition of geoethics states that deciding and acting ethically, in other words taking an ethical decision, means to adopt “appropriate behaviours and practices”. an ethically correct and well-considered choice, and so “appropriate”, has to be based on identified common values, shared by all those with whom one will share also the effects of that choice (positive or annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7473 3 negative effects). certainly we tend to base our decisions on practical and technical considerations. so we take into account the economic and temporal implications of the action to be taken (for example: a certain technical solution should be adopted because it costs less, or because it allows one to achieve the same result in a shorter time). in addition, our decisions may also take into account the greater social benefit that they can entail (for example, decisions that aim to protect the lives of citizens, as well as the economic activities in an area more effectively). and even in this latter case there could be important economic implications. finally, we can base our decisions on specific environmental considerations, where we will choose interventions that aim to protect nature, its dynamics and balances. all these considerations are legitimate: they are the result of our personal beliefs and of the social values to which we refer. a thorough scientific analysis of a problem to be solved, rationally articulated and developed, must lead to that point of equilibrium that is able to optimize the sum of the effects. it is from this balance that a choice can arise, the decision that is ethically sustainable for the human community and the environment; both affected by the same problem. and it is clear that the human community and the environment cannot exist in contrast with each other. if we don't reach that point of equilibrium, and if our decision doesn’t take into account the balance between the different variables involved, we may lose the overview of the problem, its vision in the long-term perspective, such that conflicts and problems might arise among those who would like to act in different ways. it is reasonable to think that professional, social and environmental considerations are connected with each other, so they should be carefully considered under a common perspective. decisions are ethical if they are shared, contemplated, scientifically balanced, technically constructed and considered in their multi-facets. 6. identifying and sharing values in order to adopt “appropriate behaviors and practices”, we must identify the framework of our reference values, on which to base our choice, which can orient us in the search for the "best" possible balance between the various options, the optimal choice or at least the one most acceptable in relation to a specific situation. if the decisive choice to solve a problem is based on a defined framework of reference values, on which to articulate the couple reflection-action, then one’s decisions will not be biased, limited in time nor socially questionable. in fact, the choice of the "best" solution to a problem between multiple existing solutions is possible when the reference values have been clearly defined and shared within the professional or, more in general, the societal community. only on the basis of these values can we answer the question: what do we want to achieve with our choice, with our decision? which is our goal? the ethically correct solution will not be the result of a simplistic choice between what is right and what is wrong. in fact, right and wrong cannot be discerned if reference values have not been clearly established. so, what are these values? sustainability, conservation of nature, future generations, human health and progress are some examples. but what takes priority? and is it possible to find a balance between conservation of nature and human development? taking a well-weighed ethical decision is impossible if one doesn’t answer these questions and fix shared values that can orient them in the problem solving. 7. ethical choices and ethical dilemmas reference values should take into due account the different cultural, economic and social contexts, existing in different parts of the world. otherwise we run the risk of a conflict between values. “globalization is a fact. it is a contradiction to recognize ourselves immersed in a globalized world and at the same time to claim the right to act for our own interest, without considering the interrelationships among ourselves and the world in which we live. geosciences teach us that such relationships operate on a global scale” (peppoloni and di capua, 2016). but if we do not take proper account of local conditions, we risk believing in the possibility of exporting local values to a global context, as if they were commodities. often, especially in more developed annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7473 4 countries, mining is considered as a threat to human health and nature. but surely mining is also a great opportunity for development especially in low-income countries. reducing the carbon footprint is a priority of more developed countries, but is it ethical to demand the same reduction to less developed countries, constraining their economic development, in order to reduce their contribution to the global co2 emissions? without doubt, a large dam has a significant impact on the environment. it is likely that the construction of the dam will lead to the destruction of numerous natural habitats, but at the same time it ensures protection from floods and manages water and energy supply for thousands of people. is its construction unethical? 8. ethical dilemmas and the approach of geoethics the examples above highlight that often geoscientists face especially ethical dilemmas when they have to take decisions or suggest choices between two or more options, each of them with a possible negative impact on the earth system and/or the population. in such cases, we can take a (geo)ethical decision only if we justify it adequately from a scientific and technical point of view, if we clearly indicate pros and cons of the choice we are proposing, including a cost/benefit analysis also in societal and environmental terms, and if we include in our scientific analysis probabilities and uncertainties. from these considerations, it is evident that the role of geoscientists has not only a technical-scientific value, but also a sociocultural one. in fact, geoethics implies not only the definition of standards and procedures (best practices), but also the constant search for universal values to be shared, in the light of geoscientists’ expertise and their specific ability to approach environmental problems. 9. ethical, social and cultural values of geoethics on the basis of these considerations, some reference values can be identified, grouped into three different categories (peppoloni and di capua, 2016): • ethical values: they concerns both the individual and social sphere of geoscientists. the singapore statement on research integrity (2010), the hippocratic-like oath published in the form of a “geoethical promise” (matteucci et al. 2014), and deontological codes of ethics/conduct of scientific and professional associations and societies (iapg, 2017) include, in different and partially complementary ways, several ethical values to be followed, such as honesty, integrity, awareness, accuracy, cooperation, inclusiveness, courtesy and fairness. • social values: geosciences are essential to help society in facing great challenges, such as climate change, the search for raw materials and new sources of energy and the best management of the current ones, the need for a sustainable approach to the environment, the defense against geo-hazards, and the development of a society of knowledge. sustainability, prevention and education are social values on which to base a new vision for future years. “sustainability” means a prudent and prolonged use of a natural resources and low consumption of energy from two perspectives: in the near term, it aims to develop strategies and technologies for reduced use of energies and minerals, and to encourage the percentage increase of renewable energies. in the long term, it means building a new model of economic development for our societies that aims to give new generations the possibility of discovering and exploiting other ways to produce energy and use natural resources. “prevention” implies developing an ethical way of thinking for the protection of populations from damages related to geo-risks. a preventive approach should replace an approach focused solely on emergency, in order to improve the community’s resilience; that is their ability to cope with and recover from a disaster. finally, “geo-education”, that is transferring geologic knowledge to the public, can give geosciences a fundamental role in contributing to build a knowledgeable society by raising awareness about how the earth system operates and evolves. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7473 5 • cultural values: geosciences have also a great cultural value, capable of influencing current and future ways of thinking about the earth. geoethics enhances cultural values such as geodiversity, geological landscape and geoheritage to strengthen the relationship between communities and the land they inhabit, and transforms those values into economic resources, such as geoparks and geotourism, that represent not only the synthesis of those values, but also an opportunity for a country’s development. 10. the (geo)ethical criterion: responsibility what is the criterion that allows us to take (geo)ethical decisions, to deal with (geo)ethical dilemmas, in the light of the reference values that we set, on the basis of which we can follow "appropriate behaviours and practices, wherever human activities interact with the earth system"? the ethical criterion must be sought in the concept of responsibility, since geoethics is grounded in an ethics of responsibility towards the earth. the term "responsibility" derives from the latin verb "respòndere", which means to respond. it expresses the commitment to answer to someone for our actions and their consequences. in other words “the duty or obligation to satisfactorily perform or complete a task that one must fulfill, and which has a consequent penalty for failure” (www.businessdictionary.com/definiti on/responsibility.html, accessed 30 october 2017). for the scientific community the "penalty for failure" must be intended not only in legal terms (for example: if geotechnical calculations to analyse a slope are wrong owing to negligence and a disaster occurs, the geoscientist will pay for the consequences). a penalty for failure is also the loss of credibility of science, the loss of credibility of our role of experts of natural dynamics, the failure of our scientific and cultural role to guide society in managing the earth system. it is the loss of the reason for being geoscientists. the ethics of responsibility is considered the “bedrock” of geoethics, the ethical criterion underlying geoethics. it implies the ethical commitment of geoscientists towards their individual conscience, towards their colleagues, society and the earth system. for all these reasons, geoethics should become an essential element in the geoscientist's training. the “geoethical promise” (matteucci et al., 2014), recently introduced as a fundamental part of the “cape town statement on geoethics” (di capua et al., 2017) can be the means through which early-career geoscientists become aware of this responsibility, and take care to adopt an ethical approach in conducting their future activities (bohle and ellis, 2017). 11. the prerequisite of the responsibility: intellectual freedom intellectual freedom is a fundamental prerequisite for practicing the ethics of responsibility. its absence prejudices the possibility of taking an ethical decision. when we are not free to choose between alternative solutions, we can take only one road and act only in one way, whether it means adopting a right or a wrong solution. harassment, discriminations, bullying, conflicts of interest, and pressures at work (st. john at al., 2016; gawthrop, 2014; neuberg, 2014), can result in a lack of freedom, since they threaten the serenity of the working environment and more generally limit freedom of choice. a respectful working environment is fundamental to maintain a high level of professionalism and to assure an ethical conduct while practicing geosciences. in particular, harassment, discriminations, political and business pressures offend the dignity of the person, and seriously undermine integrity, quality and credibility of the geoscience community. these kinds of behaviours prevent individuals from taking ethical decisions. geoethics cannot be practiced if professional ethics are constrained. 12. ethics of responsibility and ethics of the tool sometimes, we believe that to improve the working conditions of a scientific and professional community, by increasing its ethical level, it may be sufficient to adopt codes of ethics or conduct, which prohibit wrong practices and foster correct ones. certainly ethical codes are the result of a careful ethical reflection and are a very useful tool to try to prevent, monitor annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7473 6 and control inappropriate practices and poor policies within the geoscience community. but despite the existence of such codes, more and more often we learn about cases of "bad practices", "research misconduct" (allison and spencer, 2016; oig, 2016) or "conflict of interests" (andrews, 2014). it could depend on the tendency to confuse "the ethics of responsibility" with "the ethics embodied by the tool". the observance of practices deemed ethical (contained in codes of ethics) should not be confused with the essential ethics education that each geoscientist should have if he/she wants to reach a higher level of integrity, respectability and credibility within the professional community. too many geoscientists will continue to ignore ethical codes, if they don’t assimilate in their professional training those values that address the ethics of responsibility. geoethics, implies a conscious and rational way of acting, scientifically constructed, respectful towards ourselves, colleagues, society and earth system. an ethical decision can only come from a responsible choice, made freely. this ethical decision may be "certified" by a code of ethics. 13. conclusion geoethics recognizes that human beings are a geological force capable of acting on natural environments, and in virtue of this prerogative assigns to them an ethical responsibility towards the earth system. the awareness that humans can influence the processes of nature was earlier highlighted in the nineteenth century by the italian geologist antonio stoppani (1824-1891), who first recognized the potential of human beings to be a "geological force”, capable of influencing natural processes and changing environments (peppoloni and di capua, 2014; lucchesi, 2017). his intuition has gradually led to the modern concept of the “anthropocene” (crutzen, 2002), and to large and multi-faceted discussions, developed within the “environmental ethics” (bohle, 2016). today environmental challenges impose responsible answers. regardless of the value we want to give to the anthropocene, whether merely a political necessity or a real scientific item (zalasiewicz et al., 2015; finney and edwards, 2016; waters et al., 2016), its cultural importance remains (hamilton et al., 2015). that is to have understood that mankind is a modifier of the earth system, and for this reason it has an ethical responsibility towards itself and towards all that is other than itself. in virtue of this prerogative, geoethics can propose a responsible neo-anthropocentrism, where human beings assume responsibly the role of critical and rational conscience of the earth system, which is the synthesis of the biotic and abiotic elements of nature (bobrowsky et al., 2017). when we are in front of a problem, geoethics remind us to consider not only "what we have to do", but also "why we have to do it" and "what are the consequences of our doing". therein lies the value of geoethics. references allison m.l. and spencer j.e. 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(2015). when did the anthropocene begin? a midtwentieth century boundary level is stratigraphically optimal. quaternary international, 383, 196-203, doi: 10.1016/j.quaint.2014.11.045. vol51,1,2008_delnegro 303 annals of geophysics, vol. 51, n. 1, february 2008 key words rst – earthquake – satellite thermal infrared – hector mine – izmit – geostar 1. introduction the earth’s thermally emitted radiation in the thermal infrared (8-14mm) spectral range, when measured from sensors on board satellite platforms, is referred to as a tir signal wich us usually given in units of brightness temperature [k]. in the last few decades, a number of papers have been published suggesting the existence of a possible relation between «anomalous» space-time fluctuations (hereafter «thermal anomalies») of such a tir signal and earthquakes occurrence (e.g. gorny et al., 1988; qiang et al., 1991; qiang and dian, 1992; tronin, 1996; qiang et al., 1997; salman et al., 1992; tronin, 2000; singh et al., 2002; tronin et al., 2002; day and singh, 2003; ouzounov and freund, 2004; cervone et al., 2006; saraf and choudhury, 2005a,b). these papers have been greeted with some scepticism by the scientific community mainly for the poor methods used for the analysis of satellite tir images (see tramutoli et al., 2001a) and for the interpretation of results (e.g. geller, 1997). in fact, the main problems in the above-mentioned studies were the lack of a rigrobust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives carolina aliano (1), rosita corrado (1), carolina filizzola (2), nicola genzano (1), nicola pergola (1)(2) and valerio tramutoli (1)(2) (1) dipartimento di ingegneria e fisica dell’ambiente, università degli studi della basilicata, potenza, italy (2) istituto di metodologie per l’analisi ambientale (imaa, cnr), tito scalo (pz), italy abstract in the last few years, robust satellite data analysis techniques (rst) have been proposed which significantly improved present capabilities to investigate possible relations between tir signal fluctuations and earthquake occurrence. this paper, starting from a critical survey of results achieved by applying different rst-based algorithms to different satellite sensors to approximately ten earthquakes (two of them are discussed here for the first time) which occurred in three different continents, tries to offer a first assessment of main achievements, residual limits and perspectives of such studies. even if it is still not possible to relate (or to exclude) observed anomalous tir transients definitely to impending earthquakes, such studies demonstrate at least: a) the strong improvement of s/n ratio achievable moving from polar to geostationary satellites; b) the further s/n improvement achievable by using tir sensors which also offer split-window possibilities; c) the crucial role played by a space-time persistence test to select tir anomalies candidate to be associated to impending earthquakes; d) the possibility of identifying and correctly discarding tir anomalies related to clouds and to image navigation errors; e) the scarce importance of spatial resolution of observations which encourages the use of passive mw sensors which are less affected by atmospheric conditions. mailing address: dr. carolina aliano, dipartimento di ingegneria e fisica dell’ambiente, università degli studi della basilicata, via dell’ateneo lucano, 85100 potenza, italy; e-mail:carolaliano@gmail.com vol51,1,2008_delnegro 7-04-2009 21:54 pagina 303 304 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli orous definition of anomalous tir signal fluctuations, the absence of a convincing testing step based on a validation/confutation approach and the scarce attention paid to the possibility that other causes (e.g. meteorological) different from seismic activity could be responsible for the observed tir variations (tramutoli et al., 2005). within this context, a different approach has been proposed which, unlike preceding methods, permits a statistically based definition of tir anomaly even in the presence of highly variable contributions from atmospheric (e.g. transmittance), surface (e.g. emissivity and morphology) and observational (time/season, but also solar and satellite zenithal angles) conditions. it was initially proposed for avhrr-noaa (advanced very high resolution radiometer onboard noaa, national oceanographic and atmospheric administration) data, and therefore it was named «rat» (robust avhrr techniques, tramutoli, 1998), but its full exportability on different satellite systems suggested a more generic name «rst» (robust satellite technique, tramutoli, 2005), that will be used from now on throughout this paper. the rst possible applications to satellite tir surveys in seismically active regions has already been tested in the case of approximately ten earthquakes (see a selection in table i) by using a validation/confutation approach, devoted to verify the presence/absence of anomalous space-time tir transients in the presence/absence of seismic activity. this paper discusses the progress and limits of the use of the rst method on seismically active area monitoring by comparing results achieved on different earthquakes which happened in different geographic areas by using different satellite sensors. progresses in reducing the most important noisy contributions to table i. seismic events which have been studied by applying the rst approach: the used variable v(r,t), the sensor, the length (years) and homogeneity rules (months of the year, time of the day) which have been applied to build the historical data-set (used to compute reference images µv and σv) and the minimum s/n ratio of tir anomalies are indicated, together with the references. event technique homogeneity rules s/n (date and magnitude) and time extension of data sets (sensor, month, years, hour) polar satellites 23 november 1980, <⊗∆t(r)> – monthly average avhrr – november irpinia-basilicata-italy (tramutoli et al., 2001a) (1994-1998) –17:00 -19:00 0.6 ms=6.9 23 november 1980, <⊗lst(r)> – monthly average avhrr november irpinia-basilicata, italy (di bello et al., 2004) (1994-1998) –17:00 -19:00 1 ms=6.9 7 september 1999 ⊗lst(r,t) – daily analysis avhrr august and september athens, greece (filizzola et al., 2004) (1995-1998) 1:00-4:00 gmt 1.5 ms=5.9 geostationary satellites 17 august 1999, ⊗t(r, t) – daily analysis meteosat august kocaeli-izmit, turkey (this paper) (1995-2000) –24:00gmt 2 ms=7,8 26 september 1997, ⊗∆t(r, t) – daily analysis meteosat september umbria, italy (aliano, 2004) (1992-2000) – 24:00 gmt 2 ms=5.9 to 6.4 vol51,1,2008_delnegro 7-04-2009 21:54 pagina 304 305 robust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives table i (continued). event technique homogeneity rules s/n (date and magnitude) and time extension of data sets (sensor, month, years, hour) 3-7-12-14 october 1997, ⊗∆t(r, t) – daily analysis meteosat -october umbria, italy (abbate, 2005) (1992-2000) –24:00 gmt 2.5 ms=5.7 max 16 october 1999, ⊗∆t(r, t) – daily analysis goes october hector mine, california (this paper) (1996-1999)24:00 gmt 2.5 ms=7,4 7 september 1999 ⊗∆t(r, t) – daily analysis meteosat august &september athens, greece (filizzola et al., 2004) (1995-1998) 24:00 gmt 3 ms=5.9 21 may 2003 ⊗∆t(r, t) – daily analysis meteosat april and may zemmouri, algeria (aliano et al., 2006) (1992-1999) 24:00 gmt 3 ms=6.9 28 may 1995, ⊗∆t(r, t)– daily analysis meteosat may patras, greece (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 4.7 29 may 1995, ⊗∆t(r, t)– daily analysis meteosat may cyprus (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 5.3 3 june 1995 ⊗∆t(r, t) – daily analysis meteosat june crete, greece (corrado et al., 2005) 1992-1999) 24:00 gmt 3 mb 4.2 18 june 1995, ⊗∆t(r, t) – daily analysis meteosat june crete, greece (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 4.9 4 may 1996, ⊗∆t(r, t) – daily analysis meteosat may erzurum,turkey (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 4.2 13 june 1996, ⊗∆t(r, t) – daily analysis meteosat june ionian sea (corrado et al., 2005) (1992-1999) 24:00 gmt 3 (southern greece) mb 4.2 16 june 1996, ⊗∆t(r, t)– daily analysis meteosat june patras, greece, (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 4.3 17 june 1996, ⊗∆t(r, t) – daily analysis meteosat june crete, greece (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 4.0 29 june 1996, ⊗∆t(r, t) – daily analysis meteosat june isparta, turkey (corrado et al., 2005) (1992-1999) 24:00 gmt 3 mb 5.1 26 january 2001, ⊗∆t(r, t) – daily analysis meteosat january &february gujarat, india (genzano et al., 2007) (1999-2004) 24:00 gmt 3 ms=7.9 17 august 1999, ⊗t(r, t) – daily analysis meteosat august kocaeli-izmit, turkey (tramutoli et al., 2005) (1992-1998, 2000) 24:00 gmt 3.5 ms=7,8 vol51,1,2008_delnegro 7-04-2009 21:54 pagina 305 306 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli the measured signal will be discussed by considering new explanatory cases. 2. a robust satellite-based tir index a wider description of the rst approach and its implementation on different satellite sensors to monitor seismic areas can be found in different papers (tramutoli et al., 2001a; di bello et al., 2004; filizzola et al., 2004; corrado et al., 2005; tramutoli et al., 2005; aliano et al., 2007; genzano et al., 2007). the above mentioned papers used specific indexes, such as the retira (robust estimator of tir anomalies, after tramutoli et al., 2005) which belongs to a more general class of alice (absolutely llocal index of change of the environment) indexes which give a statistically well based definition of llocal signal anomalies (since tramutoli, 1998 the double l is used to make reference not only to a specific place rl but also to a specific time tl). using a general formula, alice indexes are computed on the image at hand as follows: (2.1) where r≡(x,y) represents location coordinates on a satellite image; t is the time of image acquisition with t∈τ , where t defines the homogeneous domain of satellite imagery collected during the years in the same time-slot of the day and period (e.g. same month) of the year; v(r,t) is the value of a variable v at location r≡(x,y) and at acquisition time t; µv(r) is the time average value of v(r,t) at location r≡(x,y) computed on cloud free records belonging to the selected data set (t∈τ); σv(r) is the standard deviation of v(r,t) at the location r≡(x,y) computed on cloud free records belonging to the selected data set (t∈τ). the choice of the variable v(r,t) depends on the specific application and on particular effects that we want to take into account. rst has been indeed successfully applied to the major natural and environmental hazards related to: volcano activity (pergola et al., 2001; tramutoli et al., 2001b; di bello et al., 2004; pergola et al., 2004a,b; bonfiglio et al., 2005; marchese et al. 2006; filizzola et al. 2007); flood risk (tramu( , ) ( ) ( , ) ( ) r r r r t v t v v v 7 σ µ = toli et al., 2001c; lacava et al., 2005; lacava et al 2006); forest fires (cuomo et al., 2001), etc. for its application to seismic area monitoring, different variables have been used (2.2) where t(r,t) is simply the tir radiance at the sensor; lst (land surface temperature) is a product of satellite data analysis, which is expected to give, starting from observations in two spectrally very close tir bands (split-windows), an estimate of the earth’s surface temperature corrected for the effects of atmospheric water vapour content (see di bello et al. 2004 and reference herein); t(t) and lst(t) are spatial averages of t(r,t) and lst(r,t) computed in place on the image at hand considering cloud-free pixels only, separately for land and sea: only sea pixels are used to compute ∆t(r,t) and ∆lst(r,t) if r is located on the sea; only land pixels are used on the other hand if r is located on land. the use of the excesses ∆t(r, t) and ∆lst(r,t), instead of the simple t(r,t) and lst(r,t), is expected to reduce the possible contributions (e.g. occasional warming) to the signal due to the year-to-year climatic changes and/or season time-drifts which usually affect near-surface temperature at a regional scale (tramutoli et al., 2005). by construction, the alice indexes turn out to be useful tools for a robust identification of tir anomalies and allow us to estimate them in terms of signal-to-noise (s/n) ratio. in fact, the local excess v(r,t) − µv(r), which represents the signal (s) to be investigated for its possible relation with seismic activity, is evaluated by comparison with the corresponding observational/natural noise (n) represented by σv(r). it is important to note that σv(r) includes all (natural and observational, known and unknown) sources of the overall (llocal) variability of s as historically observed at the same site in similar observational conditions (same platform, time of day, month, etc). this way, the relative im( , ) ( , ) ( , ) ( , ) ( ) ( ( , ) ) ( , ) ( , ) ( ) the corresponding index is also named retira lst lst lst v t t t t t t t t t t t t t r r r r r r r t7/ / / ∆ ∆ − − ∆ z [ \ ] ] ] ] ] ] vol51,1,2008_delnegro 7-04-2009 21:54 pagina 306 307 robust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives portance of the measured tir signal (or the intensity of anomalous tir transients) can naturally be evaluated in terms of s/n ratio by the alice indexes. as a result, alice indexes are intrinsically resistant to false alarms and, in addition, may assure a complete exportability to different geographical areas and a free choice of sensors to be used. 3. possible choices for the tir-based variable v(r, t): results for different satellite sensors as indicated in the previous section, different choices, described in (2.2), are possible for the variable v(r,t) in the general expression (2.1). the first application of the rst approach to seismically active areas was performed using polar satellite data (noaa-avhrr) and retira index on the irpinia-basilicata (23 november 1980, ms=6.9) earthquake (tramutoli et al., 2001a). in that case, the monthly average <⊗∆t(r)> of the ⊗∆t(r,t) index was considered to compare the mean signal observed during the month of the earthquake (november 1980) with the one observed during the same month in different years. these preliminary studies showed how the rst approach is able to strongly reduce site effects (e.g. related to horography, land cover, etc). in addition, validation results showed more anomalous pixels than in «unperturbed» years (confutation) and in a better correlation with spatial distribution of seismogenic areas (as in valensise et al., 1993). however the monthly average <⊗∆t(r)> of the ⊗∆t(r,t) index rarely reached values greater than 0.6. a significant improvement of s/n ratio (see also fig. 1) was achieved by using the monthly averages <⊗∆lst(r)> of the ⊗∆lst(r,t) index, instead of <⊗∆t(r)> (pictures on the right in fig. 2). in fact, as lst(r,t) gives an estimate of the land surface temperature corrected by the effects of atmospheric water vapour content, it is expected that σ∆lst(r,t)<σ∆t(r,t) with a consequent increase in s/n ratio. in the case of the irpiniabasilicata earthquake, computing lst values with becker and li (1990) algorithm, an increase in s/n ratio from 0.6 to 1 was achieved (di bello et al., 2004). fig. 1. s/n evolution after the application of the rst approach to different satellite sensors and different alice indexes. vol51,1,2008_delnegro 7-04-2009 21:54 pagina 307 308 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli figure 2 clearly shows this improvement: for november 1980 (validation) the spatial distribution of pixels with ⊗∆t(r,t)>0.6 and ⊗∆lst(r,t)>1 is rather similar, but, for the unperturbed november 1998 period (confutation), pixels with <⊗∆lst(r)> > 1 almost disappear, differently from pixels with <⊗∆t(r)> > 0.6. moreover, in both cases thermal anomalies seem to occur even far away (up to several hundred kilometres) from the epicentre zone. a subsequent study (filizzola et al., 2004) on avhrr data demonstrated that a significant increase in s/n ratio is achievable moving from monthly (<⊗∆lst(r)>) to daily (⊗∆lst(r,t)) indexes. in this case ⊗∆lst(r,t) values greater than 1.5 were observed to draw anomalous spacefig. 2. the rst approach applied to polar satellite data (noaa-avhrr). comparison between the results of the analysis on monthly average <⊗∆t(r)> of ⊗∆t(r, t) (left) and <⊗∆lst(r)> of ⊗∆lst(r, t) (right) indexes, computed for (top) november 1980 (year of irpinia’s earthquake) and (bottom) november 1998, one of seismically unperturbed (i.e. no earthquakes with m>4 in the study area) years (adapted from tramutoli et al., 2001a; di bello et al., 2004). the unperturbed signal behaviour, in terms of µ∆t(r) and σ∆t(r) or µ∆lst(r) and σ∆lst(r), was determined on the basis of five years of noaa-avhrr records collected in similar observational conditions (see table i). november 1980 <⊗ t(r)> > 0.6 november 1998 seismic epicentre main seismogenic faults november 1980 november 1998 <⊗ lst(r)> > 1 vol51,1,2008_delnegro 7-04-2009 21:54 pagina 308 309 robust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives 7 o c tob e r 1 999 o c tob e r 1 999 21 o c tob e r 1 999 ⊗ t (r, t) > 2.5 31 o c tob e r 1 999 1 o c tob e r 1 999 25 o c tob e r 1 999 ⊗ t (r, t) > 2 clouds seismic epicentre main tectonic lineaments 21 o c tob e r 1 999 22 o c tob e r 1 999 24 o c tob e r 1 999 25 o c tob e r 1 999 20 o c tob e r 1 999 26 o c tob e r 1 999 23 o c tob e r 1 999 main zone affected by tir anomalies fig. 3. the rst approach applied to geostationary satellite data (goes). results of the analysis of the daily ⊗∆t(r, t) index on the epicentral area before and after earthquake (ms=7.4) of hector mine (california) occurred on october 16th 1999. left side: results of the analysis performed on goes images collected from october 1st to 31st with pixels having ⊗∆t(r, t) >2.5 depicted in red. right side: results of the analysis performed from october 16th to 26th with pixels having ⊗∆t (r, t) > 2 depicted in red. note that, images corresponding to mostly overcast days or without any thermal anomaly on the scene are not shown in the figure (see text). vol51,1,2008_delnegro 7-04-2009 21:54 pagina 309 310 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli time tir patterns affecting the epicentre area a few days before the athens (ms=5.9) earthquake which occurred on september 7th, 1999. the use, for the first time, of daily ⊗∆lst(r,t) observations demonstrated the importance of a temporal and spatial persistence analysis of tir anomalies in order to discriminate meaningful anomalous transients from signal outliers with similarly high s/n values (see also tramutoli et al., 2005; genzano et al., 2007). in the same study, filizzola et al. (2004) compared results they achieved by using v(r,t)=∆lst(r,t) on avhrr data with the ones achieved by using on the contrary v(r,t)=∆t(r,t) on meteosat observations. due to the absence of split-windows tir bands on meteosat, it was impossible to use the same retira index for both, the polar avhrr and the geostationary meteosat, satellites. however, this circumstance allowed the authors to make a first analysis of the trade-off between the best choice of v(r,t) in (2.1), which is also related to satellite sensor spectral capabilities, and satellite attitude itself. in fact, compared with polar satellites, geostationary satellites guarantee for each ground location, even if at lower spatial resolution, constant view angles with the same ground resolution cell size and precise image-to-image co-location. time-slots which are no longer than 30 min guarantee higher homogeneity of imagery time series further reducing the contribution to the signal variability simply due to image-to-image changes in observational time. for this reason, achievable s/n is expected to improve, as a consequence of reduction of σv(r,t), not only moving from v(r,t)=∆t(r,t) to v(r,t)=∆lst(r,t), but also moving from polar to geostationary satellite attitudes. in the case of the athens september 7th 1999 earthquake, it was demonstrated that geostationary attitude, more than the availability of tir split-window sensors on board polar satellites, allows us to have a major reduction of observational noise with the possibility of reaching a higher s/n levels (see fig. 1). the same good performances on geostationary data (meteosat, in this case) were observed by corrado et al. (2005) in the case of mediumlow magnitude events which occurred in greece and turkey (see table i). also in that case, thermal anomalies were observed in known tectonic lineaments even if the most spectacular case of such a correspondence remains the case of gujarat’s earthquake (january 26th, 2001, ms=7.9), where the whole indiaeurasia boundary plate seems to be drawn by tir anomalies (genzano et al., 2007). the indication for possible large scale effects confirms, also in this case, that high spatial resolution is not a requirement for this kind of study. 3.1. two notable study cases: hector mine and izmit-kocaeli, 1999 earthquakes potentialities and limits of the rst approach have been assessed starting from a critical analysis of results achieved by applying: a) different alice indexes to b) different satellite sensors in c) different geographic areas affected by d) different kinds of earthquakes. in this paper, the rst approach has been applied to two new test-cases (see table i and fig. 1): 1) the seismic event of ms=7.4 which occurred on october 16th, 1999 in california, along the san andreas fault with the epicentre located near hector mine. in this case, rst performances were evaluated, for the first time, on goes (geostationary operational environment satellites operated by noaa) satellite tir data and on the american continent. 2) the kocaeli-izmit earthquake of ms=7.8 which struck turkey, along the north anatolian fault (naf), on august 17th 1999. in this case, rst performances were evaluated, for the first time, by applying the alice index based on the choice v(r,t) = t(r,t) to meteosat tir observations. it should be mentioned that important analogies exist between the san andreas fault system in california (e.g. usgs, 2000) and the naf zone (nafz) in turkey. it is similar is the apprehension for the possible impact of a large magnitude earthquake in such densely populated areas. it is also notable that both the above mentioned fault systems give a unique opportunity of studying to which extent seismic stress transfer alters the probability of subsequent earthquakes, and for this reason they are among the most studied tectonic areas around the world. vol51,1,2008_delnegro 7-04-2009 21:54 pagina 310 311 robust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives 6 august 1999 11 august 1999 12 august 1999 13 august 1999 15 august 1999 16 august 1999 17 august 1999 (izmit earthquake) 19 august 1999 20 august 1999 21 august 1999 23 august 1999 a) b) ⊗t (r, t) > 2 seismic epicentre naf clouds fig. 4a,b. a) the rst approach applied to geostationary satellite data (meteosat). results of the analysis of the daily ⊗t(r,t) index computation on the epicentral area before and after the august 17th 1999 izmit earthquake; b) tectonic map of the region including the north anatolian fault (modified from barka, 1992). vol51,1,2008_delnegro 7-04-2009 21:54 pagina 311 312 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli in the case of the hector mine event, the analysis was carried out by using goes-9 and goes-10 i-m imager data. four years of satellite observations, from 1996 to 1999, were used for computing σ∆t(r,t) and µ∆t(r,t) reference fields. figure 3 (left side) shows that few, but well localized, pixels with ⊗∆t(r,t)>2.5 appear a few weeks before and a few days after the seismic event. except on october 7th, all anomalous transients are located in a space around the seismic epicentre and arrange themselves in lineaments which seem to run parallel to tectonic structures, among which there is the san andreas fault, responsible for the earthquake. this circumstance appears evidently not casual after the inspection of fig. 3 (right-side) showing the distribution of tir anomalies, of lower (⊗∆t(r,t)>2) relative intensity, which occurred after the earthquake (from october 16th until october 26th). their particular shape and alignment with the san andreas fault system, together with their space-time persistence, allow us to attribute a particular significance also to the isolated, anomalous transients observed before the earthquake on october 1st and october 7th (see fig. 3, left side). in fact, october 7th anomalies actually appear isolated in space-time while some spatial persistence, in relation to the previous mentioned tir anomalies sequence, is quite evident in the case of october 1st anomalies. therefore, in spite of the scarcity of anomalous pixels, a certain space-time correlation between tir anomalies and the considered seismic event cannot be excluded. the spatial correlation between tir anomalies and tectonic structures was highlighted, even by using ⊗t(r,t), the simplest of the alice indexes, instead of a more robust ⊗∆t(r,t) retira index, as demonstrated in the test case of izmit earthquake, which has been studied using both indexes on the same meteosat-7 geostationary data (fig. 4). notwithstanding the same spurious effects on the caucasus region due to a seasonal warming over the area, which can be reduced by using the ⊗∆t(r,t) index, as demonstrated in tramutoli et al. (2005), some thermal anomalies with an intensity of ⊗t(r,t)>2 clearly show a trend which seems to gather along the naf a few days before the occurrence of the earthquake. it should also be noted that the increase in tir anomaly spatial distribution along the whole nafz until august 13th (i.e. 4 days before the earthquake) the decrease of tir anomalies until the day of the seismic event, the new increase in tir anomalies in the post seismic period, describe a spatial-temporal dynamics which is in good agreement with the model proposed in scholz et al. (1973) to explain different phenomena possibly associated to earthquake preparation phases. it should be highlighted that, even in comparison with the best results (s/n~1.5) previously achieved on polar satellite sensors using the most refined variable v(r,t)=∆lst(r,t), the application of the rst approach to geostationary data leads to better (s/n~2) results already with the simplest v(r,t) choice. this confirms the main role played by the stability of observational conditions associated with the geostationary attitude in improving rst performances with respect to sensor spectral capabilities and, also in addition, to the choice of the v(r,t) variable in (2.1). on the other hand, the comparison with results achieved by using a more robust index ⊗∆t(r,t) (see tramutoli et al. 2005, for details), straddling the day of izmit earthquake (see fig. 5), clearly shows the effectiveness of the retira index not only in reducing spurious effects related to occasional warming at a regional scale, but also in improving s/n ratio up to the highest levels (⊗∆t(r,t) > 3,5). at this stage and on the basis of the tests performed up to now on several earthquakes which occurred in 3 different continents, the combination of tir sensors on board geostationary satellites and the choice v(r,t)=∆t(r,t) demonstrated to guarantee the best rst performances. 4. present limits and perspectives seismic area monitoring by tir satellite surveys finds an intrinsic limitation when many of cloudy pixels dominate the scene. in fact, cloud cover always prevents earth’s emitted tir signal from reaching the satellite sensor so that surface or low atmospheric phenomena are not detectable if clouds are present. moreover, if the cloudiness is very wide and persistent, the scene can be so «obscured» that it is difficult to appreciate space-time continuity of observed tir anomalies which, as in the case of hectorvol51,1,2008_delnegro 7-04-2009 21:54 pagina 312 313 robust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives mine earthquake previously discussed, can be crucial in order to assess the actual significance of observed anomalous signal transients. the opacity of clouds to tir radiation introduces further problems when alice indexes, and particularly retira ones, are used. actually, the temporal persistence of clouds over the same area, determining data missing in the observational tir data-set, may influence multi-temporal analysis. in our case, the reduction in the population of satellite tir time-serie records usable to build historical reference fields may locally affect the computed values of alice indexes in (2.2). in addition, the geographic distribution of clouds over a thermally heterogeneous scene turns out to heavily affect the computation of the spatial averages t(t) and lst(t) used to compute ∆t(r,t) and ∆lst(r,t) values in the expression of retira indexes in (2.2). in fact, as emerged from our analysis on hundreds of satellite images, if clouds gather mainly over the warmest portions of a scene. t(t) spatial averages will result lower than the expected clear sky values. this eventuality may cause false ∆t(r,t) anomalies which spread over almost all clear portions of the scene. this is due to spurious excesses ∆t(r,t)=t(r,t)−t(t), as a result of the underestimation of t(t) because of the exclusion, due to the presence of clouds, of the warmest fraction of the scene which normally contribute to increase t(t) value. the particular spatial distribution of this kind of tir anomalies normally allows us to easily identify them and, in any case, to distinguish them from the spatially and temporally persistent ones possi13 august 1999 19 august 1999 20 august 1999 15 august 1999 16 august 1999 17 august 1999 (izmit earthquake) ⊗ t (r, t) > 2.5 ⊗ t (r, t) > 2 ⊗ t (r, t) > 3 ⊗ t (r, t) > 3.5 seismic epicentre naf clouds fig. 5. the rst approach applied to geostationary satellite data (meteosat). results of the analysis of the daily ⊗∆t(r,t) index computation on the epicentral area of the day of izmit’s earthquake. the unperturbed signal behaviour, in terms of µ∆t(r) and σ∆t(r), was determined on the basis of eight years of satellite records collected in similar observational conditions (adapted from tramutoli et al., 2005). vol51,1,2008_delnegro 7-04-2009 21:54 pagina 313 314 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli bly related to an impending earthquake, even in the case they have similar intensity (aliano, 2004; filizzola et al., 2004; abbate, 2005). in this context, it is worth noting that the overestimation of cloudy pixels could also emphasize similar drawbacks. a reliable cloud-mask is then mandatory to minimize this problem. to this aim, the rst approach has also been successfully applied for cloud detection (as shown in pietrapertosa et al., 2001; and in cuomo et al., 2004), in order to avoid an overestimation of cloudy pixel number across the scene. the sporadic presence of residual anomalies which are isolated enough in space and/or so ephemeral to survive the space/time averaging processes at the bases of the rst method, should be also denounced. in fact, the retira index, being based on time averaged quantities, is intrinsically not protected from such signal outliers, which turn out to be related to very local, in space and time, natural/observational conditions (examples in filizzola et al., 2004). however, it is expected that a time-averaging process carried out on more extended time-series, increasing the frequency of appearance of the phenomena responsible for such tir outliers, should reduce or vanish their occurrence. at the same time, the request for spatial-temporal persistence of tir anomalies turns out particularly important to identify the ones which are more significant in the context of seismically active area monitoring. the partial overcoming of the limits related to cloud presence is expected to be obtained by means of the use of microwave (mw) sensors which, by penetrating (not-raining) clouds, allow us to observe the earth’s surface in whatever weather condition. even if passive mw sensors operate with a spatial resolution (10-50 km nadir view) which is much lower than the one (1-5 km) achievable by tir sensors, it remains largely sufficient to monitor thermal anomalies fig. 6. occurrence of non-persistent anomalies related to cloud passage (on the left) and navigation errors (on the right): the rst approach applied to polar satellite data (noaa-avhrr). results of analysis of the daily ⊗∆lst (r, t) index computation on the epicentral area of september 7th 1999 athens’s earthquake. the black arrows indicate the cloud system which is probably responsible for the anomalous patterns, of similar shape, affecting crete island. in the right image, spurious effects due to wrong co-location are quite evident along southern coasts of peloponnesus and crete (adapted from filizzola et al., 2004) . vol51,1,2008_delnegro 7-04-2009 21:54 pagina 314 315 robust tir satellite techniques for monitoring earthquake active regions: limits, main achievements and perspectives which we always observed at a wider scale around the epicentre zone. passive mw sensors are presently on board polar satellite platforms only, but the presence of a mw sensor aboard a geostationary platform has already been proposed by jpl-nasa. such a satellite package (named geostar: geostationary synthetic thinned aperture radiometer), combining all the previously described advantages offered by mw sensors and geostationary platforms, represents at the moment the best choice for further, significant, s/n ratio improvements (see fig. 1). however, the use of mw sensors does not allow us to eliminate all spurious effects due to cloudiness. this is the case of the appearance of tir anomalies which may be explained by night time cloud passages. in fact, if clouds are present, some of the heat emitted from the earth’s surface is trapped by the clouds, which are composed of water vapour (a greenhousegas) and reemitted back towards the ground, leading to an increase of the surface temperature and producing a local warming effect. such thermal anomalies can be comparable in intensity to the ones observed in some correlation with seismic events. however, also in this case, the spatial signature and the temporal dynamics of such anomalies may allow us to identify their true origin. in fact, the shape of the thermal anomaly is similar to the shape of the cloud responsible for it (pictures on the left in fig. 6) and its time permanence is limited to the cloud life/passage. other spurious effects can be introduced by an uncorrected image navigation which may cause false, intense thermal anomalies where sea pixels turn out to be erroneously co-located over land portions. as a consequence, it may happen that in the computation of ⊗v(r,t), the v(r,t) value of a sea pixel is compared with a reference value µv(r) of a land pixel, which is lower than the expected value for a sea pixel during the night. this way, anomalous values of the alice indexes are sometimes recorded along coast lines. this is not rare for polar platforms, as shown in filizzola et al. (2004) and in fig. 6 (right side), but the use of geostationary satellite data is expected to strongly reduce such co-location/navigation errors. it should be highlighted that, even if very rarely, geostationary platforms are not completely exempted from this kind of errors. therefore, a position of anomalous values along the sea-land boundary should make us suspicious about a possible colocation error, so that in such cases an additional check is strongly recommended. waiting for a geostar-like system, tir sensors having split window capabilities onboard geostationary satellites already operating could be usefully employed. in fact the use of a retira index based on the choice v(r,t)= =∆lst(r,t) will allow us to strongly reduce, in the absence of clouds, the contribution to the observational noise related to the variability of water vapour content in the atmosphere. in the case of the avhrr sensor, this noise reduction, obtained moving from v(r,t)=∆t(r,t) to v(r,t)= =∆lst(r,t), produced an increase in s/n ratio from 0.6 to 1.0. in the case of geostationary tir sensors, s/n ratios between 2 and 3.5 were achieved by using v(r,t)=∆t(r,t) so that s/n ratios between 3 and 6 could be expected by using v(r,t)=∆lst(r,t). at present tir sensors with the split window capability are available onboard the following main geostationary satellites: – goes-8/11, but the same package is no foreseen on-board goes-12 and on the next generation of goes (m to p series) satellites at least for the next 10 years. – metsat satellites operated since 2005 by japan meteorological agency, will become suitable for our research purposes in the next few years as soon as a longer time-serie of observations will be available. – msg (meteosat second generation, launched by eumetsat in 2003) with its seviri (spinning enhanced visible and infrared imager) sensor, will soon start to guarantee time series of records (on the hemisphere containing europe and africa) long enough to implement rst analysis with a retira index like ⊗∆lst(r,t). 5. conclusions this paper has tried to offer a first assessment of the achievements, residual limits and perspectives, related to the use of an advanced satellite vol51,1,2008_delnegro 7-04-2009 21:54 pagina 315 316 c. aliano, r. corrado, c. filizzola, n. genzano, n. pergola and v. tramutoli data analysis technique (rst) to monitor earthquake active regions. the analysis has been performed on the basis of results achieved by applying different algorithms to different satellite data in the case of approximately ten earthquakes which occurred in three different continents. the new test-cases of hector-mine (california, october 16th 1999, ms=7.4) and kocaeli-izmit (turkey, august 17th 1999, ms=7.8) earthquakes have been discussed, demonstrating rst performances achievable on satellite sensors (goes), geographic areas (north america) and algorithms (v(r,t)=t(r,t)) which have never been tested before. even if such an analysis does not allow us to relate (or to exclude!) observed thermal anomalies definitely to impending earthquakes, which was not the purpose of this paper, it demonstrated at least: a) that a strong improvement of s/n ratio (around the doubling) is achievable simply moving from polar to geostationary satellites; b) that significant s/n improvements are achievable by using tir sensors with split-window capabilities; c) the crucial role played by a space-time persistence test in order to select tir anomalies possibly associated to impending earthquakes. problems still remain in interpreting thermal signals in a seismogenetic region: understanding whether or not the observed anomalous tir signals are in statistical significant relation with time and place of incoming earthquakes or they are on the contrary related to other natural phenomena. however, the analyses performed on hundreds of satellite images allowed authors to highlight the role that tir anomaly shape and space-time persistence analyses can play to decide whether or not they could be related to seismic activity. on the other hand, if the presence of clouds limits the applicability of such a space-time persistence test on tir anomalies, the use of satellite packages operating in the mw spectral region could help to overcome such limitations: the lower spatial resolution offered by passive mw sensors does not seem to be a vital requirement for such studies indeed. major improvements are moreover expected by the combination between mw sensors and satellite geostationary attitude, which happens in geostar. while we are waiting for the geostar satellite, geostationary satellite sensors having split window capabilities (goes-8/11 for the past, metsat and msg for next years) can already be used, which are expected to play the major role in the next few years. acknowledgements the authors would like to thank prof. paul menzel and dee wade from cimss-ssec data centre (university of wisconsin madison) for their kind support and for providing goes data used to study the hector mine earthquake. this work was supported by the italian space agency (contract no. i/r/173) within the framework of «seismass» (seismic area monitoring by advanced satellite system) project and by ec and esa through the network of excellence «gmoss» (contract no. sne3ct-2003-503699) in the framework of gmes program. references abbate, a. 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(2000): thermal ir satellite sensor data application for earthquake research in china, inter. j. remote sensing, 16, 3169-3177. tronin, a.a., m. hayakawa and o.a. molchanov (2002): thermal ir satellite data application for earthquake research in japan and china, j. geodyn., 33 (45), 519-534. usgs (2000): implications for earthquake risk reduction in the united states from the kocaeli, turkey, earthquake of august 17, 1999, usgs circular 1193 (available at the web site http://pubs.usgs.gov/ circ/2000/c1193/index.html/). valensise, g., d. pantosti, g. d’addezio, f.r. cinti, and l. cucci (1993): l’identificazione e la caratterizzazione di faglie sismogeniche nell’appennino centro-meridionale e nell’arco calabro: nuovi risultati e ipotesi interpretative, in proceedings of the gngts-cnr annual meeting, roma, 331-342. vol51,1,2008_delnegro 7-04-2009 21:54 pagina 318 layout 6 annals of geophysics, 61, 3, se338, 2018; doi: 10.4402/ag-7516 1 “dynamics of mikir hills plateau and its vicinity: inferenceson kopili and bomdila faults in northeastern india through seismotectonics, gravity and magnetic anomalies„ sangeeta sharma1, jogendra nath sarma1 and saurabh baruah1,* (1) geoscience and technology division, csir-north east institute of science and technology, assam, india (2) department of applied geology, dibrugarh university, assam, india 1. introduction the northeastern region (ner) of india is a seismically active region which lies in the most active zone v (bmtpc, 2003) (figure 1a) of seismic zones of india. the region jawed in between the himalayan collision arc to the north and northeast and the indo burmese subduction arc to the south and southeast has produced two great earthquakes (1897 and 1950) and the whole region experienced 20 large earthquakes (m > 7.0) during the last 200 years [angelier and baruah, 2009]. this region is tectonically divided into several mosaics by deep-rooted faults/thrust along which episodic block/thrust/strike-slip movements are reported [nandy, 1986, 2001]. the seismicity pattern of ner shows relatively higher concentration of events in the shillong and mikir plateaus, arunachal himalaya and indomyanmar subduction zone. amongst a number of active faults in ner, the kopili and the bomdila faults are very active, which is evidenced from the occurrence of 24 large magnitude (mw≥5.5) seismic events in recent past (figure 1b). concentration of events along both the kopili and bomdila faults are considered to be an intraplate seismotectonic domain. these intraplate seismic activities are fairly intense in this region due to its complex stress regime [nandy, 2001, angelier and baruah, 2009]. the kopili valley region has produced two large earthquakes article history receveid august 3, 2017; acceptedfebruary 2, 2018. subject classification: mikir hills; kopili fault; bomdila fault; seismotectonics; gravity contour; magnetic contour. abstract the mikir hills plateau is encompassed by two prominent faults bomdila fault to the east and the kopili fault to the west characterized by strike-slip kinematics. the kopili fault has a dip of 75° towards ne. simultaneously, the bomdila fault dips with 50-55° towards the nne. an integrated approach based on seismotectonics, gravity and magnetic data is utilized to understand the tectonic activity of the kopili and bomdila faults. the bottom of seismogenic zones is inferred to be 45±2 km and 50+2 km for the kopili fault and the bomdila fault region respectively. so far gravity anomaly is concerned; it varies between -110 to +60 mgals along the kopili bomdila fault regions from the belt of schuppen to the mct. the low gravity values over the bomdila fault area indicate presence of thick alluvial deposits while along kopili fault lesser sediment thickness is observed. simultaneously, basement being at shallower depth, lower magnetic values indicate presence of thick alluvial deposits in and around bomdila fault. the curvatures and closures of the gravity contours along the fault lines indicate structures involving basement and indicate influence of bomdila fault up to the basement. simultaneously, it is observed that kopili and bomdila faults are neotectonically active. all these are the prime input to the seismic hazard assessment of the region. (m>7.0) in 1869 and 1943 and kopili fault is thought to have triggered the 2009 bhutan earthquake [kayal et al., 2012] (figure 1b). kayal et al. [2006, 2010, 2012] have identified this region as a potential region of generating future large magnitude earthquakes. recently, it is argued that kopili fault cuts across the himalayas and caused displacement and curvilinear structure at the main boundary fault and main central thrust zones [kayal et al., 2010, 2012]. it is also observed that topographic features (e.g., higher hillocks, valleys, rivers and elevation) represent a departure from hydrostatic equilibrium, and earth’s reaction to surface load according to certain rheological laws provide a means of distributing the stress over depth within the earth [nandy, 2001]. from historical point of view, the first estimate of stress in the earth was based on gravity anomalies. gravity anomaly also infers that there exist mass anomalies and, therefore, deviatoric stresses, that are unrelated to the surface elevations [mc nutt, 1980]. gravity modeling generally uses density variation, whereas magnetic uses susceptibility contrast to map basement depth [ghosh et al., 2010]. seismological work in these areas are done by nandy [2001], dasgupta [1977], dasgupta et al., [1987], kayal [1987, 1996, 2001], kayal et al., [2006, 2010, 2012], baruah et al., [1997], de and kayal [2004], bhattacharya et al., [2002, 2008, 2010]. paleoseismic investigation in the kopili fault zone by kumar et al. [2016] sharma et al. 2 figure 1. a) geological location map of the study area (within latitudes 25°00΄-28°00΄ n and longitudes 91°00΄96°00΄ e shaded in grey) showing the major structures (modified after dasgupta, 1977 and nandy, 2001). the major rivers along the study area are: 1. kopili r., 2. dhansiri (north) r., 3. dhansiri (south) r. and 4. bargang r. inset: b) showing major earthquakes along with tectonic features (red lines) in northeast india region (after murthy, 1969; nandy, 2001 and baruah and hazarika, 2008). the great 1897 shillong earthquake (ms~8.7) in the region is shown by star, and the large earthquakes (mw>5.5) by circles. the digital seismic stations are shown by triangles. c) and d) are the frequency distribution of magnitudes and focal depths of 9376 earthquakes of northeast india during the period 1984-2015. indicates seismogenic liquefaction features near kopili and kalang rivers which correspond to the occurrence of causative seismic events. gravity and magnetic studies in different parts of northeast india was done by verma and mukhopadhyay [1977], tiwari et al. [2006], saha et al. [2008], ghosh et al. [2010], saha [2011] and sharma et al. [2012]. this study deals with seismotectonics, gravity and magnetic data to understand the dynamics of the kopili, bomdila faults and its vicinity. the variation of gravity and magnetic anomaly besides inferences on focal mechanism solutions, orientation of p & t axis and the primary faults planes are the main inputs which ascertains the dynamics associated. the major question addressed is that these two faults are seismically and neotectonically active and are capable of producing a >m7.0 earthquake. moreover these two faults intersect with one gigantic tectonic domain the main boundary thrust in arunachal himalaya. a critical assessment of these interactions and their persistent associated features are the major issues addressed in this study. the inferences from this study is that these two faults show recent tectonic activity to the east of kopili fault and west of bomdila fault, and is a region of stress build up with high strain of about (64-256)* 10-9 nanostrain/year [kreemer et. al., 2003]. hence, these two faults and its vicinity are seismically a potential site of a future large earthquake. 2. tectonic setting geologically, the kopili and bomdila faults comprise neogene-quaternary sediments, which were deposited directly over the archean basement. the kopili fault zone is approximately 300 km long and 50 km wide. it is a nw-se trending strike-slip fault [kayal et al., 2006, bhattacharya et al., 2008, 2010]. the tectonic disposition of this fault delineates the two precambrian massifs on either side the shillong plateau and the mikir hills. it is bounded by the main boundary thrust (mbt) to the north and by the ne-sw trending belt of schuppen to the south (figure 1a). the bomdila fault trends along wnw-ese direction and is about 400 km long strike slip fault. the northern part of the fault mostly lies in the gondwana, paleogene and neogene sediments. this fault is bounded to the east and south by the belt of schuppen, to the west by the mikir massif. in the north, the fault also cuts across the himalayan fold belt [nandy and dasgupta, 1991]. the kopili and the bomdila faults are the two main seismically active structures under study. the kopili fault was the seat of two large earthquakes (figure 1b). one of these events occurred on 1869 (m 7.7) in the southeastern end of the fault transgressing naga-disang thrust while the other event (1943; m 7.2) occurred farther north of 1869 event within a span of about 75 years [kayal, 2008] (figure 1b). intense seismic activity is observed down to a depth of about 50 km beneath the kopili fault, and the activity continues to the main central thrust (mct) in the bhutan himalaya. although mct is dormant [ni and barazangi, 1984], however intense activity is observed at the region where kopili fault meets the mbt and mct [nandy, 2001, kayal et al., 2010, 2012]. this is evidenced by the august 19, 2009 earthquake (mw 5.1) in the assam valley that occurred in the center of the kopili fault zone and the september 21, 2009 strong bhutan himalaya earthquake (mw 6.3) that occurred at the northern end of the kopili fault where it hits the mct [kayal et al., 2012]. the earthquakes of august 19, 2009 and the september 21, 2009, are shallow focus (depth ~ 10 km) showing right lateral strike-slip faulting [kayal et al., 2010]. this indicates that the kopili fault zone is under compressional stress from the indo-burma arc to the east and from the himalayan arc to the north and is characterized by transverse tectonics. the bomdila fault lies in a tectonically active region which crisscrosses the mct, mbt and naga-disang thrust along nw-se direction. most of seismic events that occur along this fault have shallow focus depth [kayal, 2008], except for a few events in the nw trending wedge-shaped block lying in between the kopili and the bomdila faults. the earthquake events in this tectonic block occur in a diffused pattern having post-collisional intracratonic characteristics [nandy and dasgupta, 1991]. characteristically, the upper brahmaputra valley between the bomdila fault and almost near nw-trending mishmi thrust in the northeast is almost devoid of any earthquakes, which is termed as the assam gap [khattri, 1983]. 3. data selection and methodology a database of 9376 events consisting of hypocentral parameters and 188 focal mechanism solutions (fms) are prepared for the periods 1984-2015 and 1963-2015, respectively for seismotectonic study. the hypocentral database is prepared from the seismological bulletin published annually by csir north east institute of science and technology (neist)-jorhat and csir national geophysical research institute (ngri)-hyderabad. phase data bulletins of indian meterological department (imd), shillong, manipur university, mizoram university and 3 dynamics of kopili and bomdila faults, ne india sharma et al. 4 gauhati university are also taken into consideration wherever needed. the barest minimum magnitude of the earthquake events are considered to be 1.5. the uncertainties involved in the estimates of origin time for these events are 0.03 to 0.1 sec. the uncertainties involved in the estimates of focal depths are ± 3 km. on the other hand the uncertainties involved in the estimates of longitude (erln) and latitude (erlt) of epicenters lies within the range of 0 to 2.0 km. the maximum number of earthquakes lies within the magnitude band of 2.5 5.5, which is around 80 % (figure 1c). as regards distribution of depth, most of the events occur within the depth of 50 km range (figure 1d). the fms database is prepared from global centriod moment tensor (gcmt) (http://www.globalcmt.org) and published literatures [chen and molnar, 1990, kayal et al., 2012 and nandy, 2001]. five events of 12th may 2012 (m 5.4) obtained from and recorded by more than 5 local seismic stations of csir-neist have been located using the hypocenter program of lienert et al. [1986] with an average rms 0.03 sec, epicenter and depth error <1km. fms for these events are determined using focmec (http://www.iris.edu/pub/programs/focmec/). in order to correlate, the fms and depth section plots within a span of 50 km on either side along and across the kopili and bomdila faults, three sections are considered: (a) kopili fault (section ab; 25.18° n, 93.27° e 27.58° n, 91.48° e), parallel to the trend of the fault. (b) kopili bomdila (section cd; 24.42° n, 90.60° e 27.38° n, 94.23° e), perpendicular to the trend of the faults and (c) bomdila fault (section ef; 25.73° n, 95.48° e 27.75° n, 91.25° e), parallel to the trend of the fault. a total number of 1159, 1213 and 1160 events of m>1.5 along kopili fault, across kopili-bomdila and figure 2. a): inset: seismotectonic map of the kopili-north dhansiri and south dhansiri-bargang valleys. ab is the section along the kopili fault, ef is the section along the bomdila fault and cd is the section across the kopili and bomdila faults. the bounding lines indicate a distance of 50 km on either side of the section lines. b) structural map of the study region showing distribution of 188 numbers of focal mechanism solutions (black and coloured circles) within latitudes 24° – 28° n and longitudes 90° – 96° e. the dashed brown lines indicate the sections lines. ab is the section along the kopili fault (fms shown by red circles), cd is the section across the kopili and bomdila faults (fms shown by blue circles) and ef is the section along the bomdila fault (fms shown by green circles). 5 dynamics of kopili and bomdila faults, ne india figure 3. focal mechanism solutions (fms) of the earthquake events using p-wave first motion and data compiled from various sources a)i. 40 events along the kopili fault (a-b), b)i. 39 events along the bomdila fault (e-f) and c)i. 42 events occurring across the kopili and the bomdila faults. a), b) and c) ii illustrates a) the depth distribution of events, b) projected depth view of focal mechanism solutions, c) nodal planes projected in the depth section and d) and e) orientation of p and t axes, along section ab, ef and cd respectively. sharma et al. 6 across bomdila fault are plotted respectively. out of these, a total number of 42 fms along the kopili, 43 fms across kopili-bomdila and 40 fms along bomdila fault are considered. cross-sections of these events are plotted [utilizing rake software; louvari and kiratzi, 1997] with the variation of hypocenters, depth view of focal mechanism solutions, plots of the primary fault planes, p-t axes with respect to the depth to know about the dynamics underneath. for a better understanding of structures and tectonics of a region, it is essential to have an insight into the deep crust down to the upper mantle. one of the tools for drawing such inferences is the analysis of the gravity and magnetic data especially in the area covered by thick sediments. a detailed gravity and magnetic data are compiled from grace satellite by usgs (https://grace.jpl.nasa.gov/data/) to observe gravity and magnetic anomaly associated with major tectonic elements in the region. this anomaly supplements the seismotectonics analysis of this region. grace measure changes of gravity and magnetic field in time and register mass movements caused by global and regional changes. also literature data and maps by different workers on geophysical gravity studies are compiled [e.g. verma and mukhopadhyay, 1976, 1977; nandy, 2001; tiwari et al., 2006; saha et al., 2008; ghosh et al., 2010; saha, 2011; vaish and pal, 2015] and a thorough study is done to find out evidence which support the findings. geomorphologically, various features including disposition of the faults and the main trends of some major rivers trunks (kopili-dhansiri (n) and dhansiri (s)-bargang) are parallel to the main fault trends (kopilibomdila). the anomalous linear courses and the abrupt right angle turn in many places of the rivers indicate relatively some influence of these structures on the regional drainage of the area. moreover, geomorphological evidences like, development of terraces, upliftment, tectonic depressions, sag ponds, swamps, fault scarps, drainage anomalies and abandoned channels along the faults suggest that these two faults are highly active. in addition the disposition of the gravity and the magnetic contours clearly depicts the trend of these two faults which can be traced from the belt of schuppen upto mct through these rivers. 4. results the significant indicator of ongoing tectonic activity in the kopili and the bomdila faults is derived from seismotectonic, gravity and magnetic studies supplemented by neotectonic evidence. the important parameters which incidentally discover the internal mechanism, its coupling, the anomalous tectonic behavior besides defining crustal structure are: 4.1 seismotectonic study 4.1.1 kopili fault: the epicentral map indicates that the kopili fault remains highly active towards northern part in comparison to the southern part (figure 2a). the seismic activity of the region to the north where the kopili fault intersects mbt and mct portray current tectonic activity. it is also observed that the seismicity in the eastern part of the fault is more frequent than in the western part (figure 2a). in order to study the source characteristics of the kopili fault region, a total number of 40 fault plane solutions along the section ab are used (figure 3a,i). however among these focal mechanism solutions (figure 3a,i), different types of mechanisms are observed (thrust = 5, thrust with strike-slip component = 4, strike-slip = 23, normal = 1, normal with strike-slip component = 6) in the demarcated area. a total of 10 (event number 1, 32, 21, 29, 10, 14, 2, 27, 42 and 38) focal mechanism solutions could be associated with the kopili fault. out of these, 7 (event numbers 1, 32, 21, 29, 14, 2 and 38) events are characterized by strike-slip faulting, 2 (event numbers 10 and 42) are characterized by thrust with strike-slip component events and 1 event (event number 27) indicate thrust component. event number 29 characterizes the kopili fault with strike-slip mechanism having the fault plane very clearly oriented along nw-se. similarly event numbers 2, 14, 38, 32 clearly ascertain the trend of the fault plane which is pertinent to the regional trend of the kopili fault. similar strike-slip type faulting mechanisms are also observed far north where kopili fault crosses the mbt. further north, in the same alignment of the kopili fault, crossing the mct, the region is characterized by preferably thrust mechanisms as depicted by event numbers 31 and 17. considering the primary nodal planes of the 10 fault plane solutions which coincide with the regional nw-se trend of the kopili fault, it is observed that the fault dips towards the ne direction with an average dip angle around 75°. for example, two solutions numbered 33 and 29 are well associated with the kopili fault. the solution 33 indicates the focal parameters as strike=321°, dip=73°, rake=173°, while solution 29 indicate strike=342°, dip=80°, rake=-175°. these parameters and a few solutions involve predominantly the strike-slip mechanisms of the kopili 7 dynamics of kopili and bomdila faults, ne india fault having dip angle ~ 75° to the ne. figure 3a,ii illustrates the distribution of events along the kopili fault (section ab in figure 2a). the depth section indicates very clearly the variation of focus between 40 km and 75 km from southeast to northwest which comprises mainly kopili fault, mbt and mct, successively. notably, the bottom of seismogenic zone is within 45±2 km in the kopili where large concentrations of earthquakes are observed. however, sharp increase in depth is observed starting from north of brahmaputra towards north up to mct (figure 3a,ii,a). the projected depth view of the fms (figure 3a,ii,b), the inferred primary fault plane (figure 3a,ii,c), orientation of p and t axes (figures 3a,ii,d,e) in kopili depict the characteristics of each of associated focus. 4.1.2 bomdila fault: similarly, the epicentral map during 1984-2015 illustrates less seismicity in and around bomdila fault in comparison to the kopili fault zone (figure 2a). however, the segment of the fault beyond the mct towards north does display a clustering of events. this shows that the bomdila fault remains highly active towards northern part in comparison to the southern counterpart. it is seen that the seismicity in the western part of the bomdila fault is much higher as compared to the eastern part. most of the seismic events that occur along this fault have shallow focus depth [kayal, 2008] except for a few events in the nw trending wedge shaped block lying in between the kopili and the bomdila faults. a total number of 39 fault plane solutions have been used for source characteristics of the bomdila fault (figure 3b,i). figure 3b,ii illustrates the distribution of events along the section ef which are used to study the bomdila fault from south to north. however among these focal mechanism solutions (figure 3b,i), different types of mechanisms characterize (thrust = 3, thrust with strike-slip component = 10, strike-slip = 23, normal = 1, normal with strike-slip component = 2) the area demarcated. a total of 12 (event numbers 32, 9, 11, 22, 10, 20, 28, 27, 3, 23, 29 and 17) of fms could be associated with the bomdila fault. out of these, 9 (event numbers 32, 9, 11, 20, 28, 3, 23, 29 and 17) events are characterized by strike-slip faulting; besides 2 (event numbers 10 and 27) events having thrust with strike-slip component and 1 event (event number 22) showing normal with strike-slip component. the strike-slip events clearly ascertain the trend of the fault plane which is pertinent to the regional trend of the bomdila fault. considering the primary nodal planes of the 12 fault plane solutions lying close to the bomdila fault, which coincide with its regional nw-se trend, it is observed that the fault dips towards the nne. for example, two solutions numbered 30 and 26 are well associated with the bomdila fault. the solution for number 30 indicates the focal parameters as strike=315°, dip=53°, rake=26° and solution for number 26 indicates strike=290°, dip=50°, rake=-160°. these parameters and a few solutions involve predominantly with the strike-slip characteristic of bomdila fault having dip angle ~ 50-55° to the nne. simultaneously, figure 3b, ii illustrates the distribution of events along the section ef (figure 2a) aligning the bomdila fault. the depth section indicates very clearly the variation of focus from 40 to 65 km from south to north, which comprises belt of schuppen, assam plains, arunachal himalayas and mbt. notably, the bottom of the seismogenic zone is within 50±2 km beyond belt of schuppen where large concentration of earthquakes is observed towards north (figure 3b,ii,a). however, sharp increase in depth starts from arunachal himalayas towards further north. the depth section plots of fault plane indicate the characteristics orientation of the plane co-located with the focus. the projected depth view of the fms (figure 3b,ii,b), the inferred primary fault plane (figure 3b,ii,c) and orientation of p and t axes (figures 3b,ii,d,e) show dominant nw-se trending paxes in the bomdila fault while the t-axes are mostly oriented along nnw direction. the orientation of paxes conforms and confirms the transverse tectonics of bomdila fault. 4.1.3 across kopili-bomdila faults: a total number of 42 fault plane solutions are used to study the source characteristics across both the kopili and bomdila faults (figure 2a), as illustrated in figure 3c,i. different types of mechanisms (figure 3c,i), are observed among these focal mechanism solutions (thrust with strike-slip component = 10, strike-slip = 21, normal = 2, normal with strikeslip component = 9). figure 3c,ii illustrates the depth section of events distributed along the section cd (figure 2a) across the kopili and the bomdila faults. the depth section (figure 3c,ii,a) indicates very clearly the variation of focus from 40 km to 60 km along section cd (from west to east) which comprises shillong plateau, assam valley, kopili and bomdila faults. seismicity is seen to be relatively lower at and to the east of the bomdila fault. the bottom of seismogenic zone is within 45±2 km in the kopili which increases towards the bomdila fault, consequently. in this illustration as well, a large concentration of earthquakes are observed in the kopili valley area. the projected depth view of the focal mechanisms (figure 3c,ii,b), the inferred primary fault plane (figure 3c,ii,c) and orientation of p and t axes (figure 3c,ii,d,e) show dominant nnw and nne trending pressure axes in both kopili and bomdila while the t-axes are mostly oriented along e-w and nw-se direction. 4.2 gravity study: 4.2.1 gravity anomaly in and around kopili fault: the gravity contours of the area range from -110 to 60 mgals and almost follow the regional structural trend of the region (figure 4a). the gravity contours along the western part of the kopili fault trend in the e-w direction. but in the central part of the fault i.e., towards the brahmaputra and in the southwestern part (near the fault end) we observe gravity anomaly which sharma et al. 8 figure 4. a) and b): gravity and magnetic anomaly map at 20 mgal contour intervals derived from grace satellite showing the kopili and the bomdila faults. the red lines indicate the geological structures and the blue lines indicate the major rivers under study. the yellow lines (aa΄, bb΄, cc΄, dd΄, ee΄, ff΄, gg΄, hh΄ and ii΄) are the gravity and the magnetic profile lines across the kopili and the bomdila faults respectively. may indicate some structures or anisotropic material involving the basement (figure 4a). the 20 mgals interval contour indicates a higher thickness of alluvial deposit across kopili and bomdila faults along the bramhaputra flood plain. the whole brahmaputra valley show a low value of -70 to -110 mgals while the value increases towards both the northern and southern side of the brahmaputra valley up to +40 mgals. to the southern side of the brahmaputra river (along the kopili fault), the contours trend from an e-w direction to a nw-se direction (figure 4a). while in the northern side of the brahmaputra river, the contours trend in an e-w direction. the contours to the north of the kopili fault are more closely spaced by. the steeper gradients suggest faulting up to the basement and maximum change in crustal thickness [verma and gupta, 1973]. the sparse distribution of average normal gravity estimates in the central part of the kopili fault is due to the thick deposit of alluvium. here the basement assumes a north dipping bowl shaped basin with thickest sediment in the area north of nagaon. the nw-se kopili fault is clearly discernible by the disposition of these gravity contours. the contour show deviation from the average normal gravity estimates where the fault passes through. the gravity profiles (aa΄, bb΄, cc΄, dd΄, ee΄, ff΄, gg΄, hh΄ and ii΄) in figure 4a across the kopili fault indicate low gravity value along the fault. but the value increases as we move from the north to the south of the fault indicating less sediment thickness (shallow basement) as we approach the belt of schuppen area. 4.2.2 gravity anomaly in and around bomdila fault: the gravity contour varies in between -110 mgal to +60 mgal over the bomdila fault (figure 4a). towards the northern and the southern end of the fault they show closely spaced contours with u-shaped pattern. the gravity contours around golaghat are low which range from -110 to -20 mgals. the curvatures and the closures by the sides of the fault indicate some structures involving basement or may be due to the influence of the bomdila fault, which extends down to the basement. around 26°20΄ n latitude and 94°00΄ e longitude, where the bomdila fault meets the belt of schuppen, the contours takes sharp southward turn with steeper gradient (figure 4a). this indicates that the northeast extension of the basement complex (foreland spur) plunges sharply near to the area where the bomdila fault meets the belt of schuppen as inferred by nandy [2001]. increase in gravity value, steep gradient and flexure of the contours near the northern end of the bomdila fault may be due to a deep seated fault located south of the himalayan frontal thrust [nandy, 2001]. this may also suggest the extension of the bomdila fault up to the basement. small local partial closures to the west of bokakhat may indicate local sag in the basement. this area is a part of the kaziranga national park. low gravity value over this area is due to the presence of thick alluvial deposits, which progressively become thinner towards nw and se of the bomdila fault. it is observed that the trend and the gradient of the gravity contours follow the trend of the bomdila fault. the gravity profiles (aa΄, bb΄ and cc΄), in figure 4a across the bomdila fault indicate higher gravity value along the bomdila fault than in the kopili fault indicating lesser sediment thickness in this area. but profile lines dd΄, ee΄, ff΄, gg΄, hh΄ and ii΄ (figure 4a) indicate lower gravity values in the bomdila fault than in kopili fault which indicates that the sediment thickness increases towards south along the bomdila fault. from naharbari towards golaghat and beyond, the value of gravity decreases indicating presence of thick sediment thickness around golaghat. 4.3 magnetic study: 4.3.1 magnetic anomaly in and around kopili fault: the values of the magnetic anomaly map along the kopili fault range from -80nt to +90 nt (figure 4b). from the mct along the kopili fault, the magnetic value change from a positive value of +50 nt, reaching a very high value of +80 nt near raha to a negative value of -80 nt at the southeastern end of the fault (figure 4b). the value rises again in the belt of scuppen indicating highly susceptible material underlying the places where we find high magnetic anomalies. from the profiles aa΄, bb΄, cc΄, dd΄, ee΄, ff΄, gg΄, hh΄ and ii΄ drawn across the kopili fault it is seen that from mct up to raha shown by profiles aa΄, bb΄, cc΄ and dd΄, the magnetic value is positive indicating shallower basement. on the other hand the profiles ee΄, ff΄, gg΄ and hh΄ show lower magnetic value indicating deeper basement. 4.3.2 magnetic anomaly in and around bomdila fault: the anomaly contour map of the bomdila fault area show magnetic value from -80 nt to +90 nt (figure 4b). we observe magnetic high to the east of the bomdila fault and to the west of the kopili fault along bramhaputra flood plain while magnetic low is observed near diyungmukh. mixed magnetic anomaly prevails within 9 dynamics of kopili and bomdila faults, ne india the region encompassed by both the faults right from naharbari to dimapur and bokakhat to raha. this trend clearly indicates the presence of the nw-se bomdila fault. the presence of the positive and negative anomalies on either side of the bomdila fault along with the trend of the magnetic contours indicates large crustal heterogeneities in the area. profile lines aa΄, bb΄, cc΄, dd΄, ee΄, ff΄, gg΄, hh΄ and ii΄ in figure 4b drawn across the bomdila fault show that the fault from mct to mbt (profiles aa΄, bb΄ and cc΄) has lower magnetic values indicating greater sediment thickness. from mbt towards the belt of schuppen, positive magnetic value is observed all along the bomdila fault indicating the presence of susceptive materials and basement at shallower depth. the average elevation along the profile cd, the seismicity depth section plot, gravity and magnetic anomalies are combined to develop a model of the structure underneath (figure 5). the computed gravity anomaly in shillong plateau is comparatively higher than mikir plateau. this leads us to infer that moho is located at shallower depth in shillong plateau and at deeper depth in mikir plateau respectively. this contrast is reflected prominently in magnetic anomaly throughout the profile where (ff΄ and gg΄) we observe lower magnetic anomaly. considering the relationship between the gravity, magnetic and seismicity, the higher gravity anomaly is due to presence of high density contrast. the occurrence of less seismicity confirms as well the presence of high density and rigid material. it is also assumed that the occurrence of seismicity is restricted to the upper to lower crustal domain far above moho. 4.4 neotectonic evidences: 4.4.1 kopili fault: the neotectonic activities in and around kopili fault are characterized by: i) anomalous linear courses of two major rivers (kopili and the dhansiri (n) rivers), ii) development of large number of low lands in later period [1971], and iii) development of kinks on existing structures like mct and belt of schuppen. such evidence as illustrated in figure 6a indicate that the kopili fault is neotectonically active and is responsible for the existing regional landform and drainage trends in this area. moreover, this fault can be traced from the belt of schuppen along the kopili river and the dhansiri (n) river up to the mct. 4.4.2 bomdila fault: bomdila fault is a major geological structure across the brahmaputra where parts of the courses of the riversthe brahmaputra, dhansiri (s), bargang and many others are aligned along this structure (figure 6b). the influence of this structure on the courses of these rivers has been studied in detail using topographic maps, satellite data and field evidences. the following main points: a) an unusually linear course of the lower part of the dhansiri (s) river from golaghat up to dhansirimukh, b) the abandonment of the westerly course of the earlier dhansiri (s) river (flowing through kaziranga) towards the present nw direction by avulsion, c) knick bends in the mbt-mct and naga thrust of belt of schuppen, (d) a linear 15 m high topographic scarp on the left bank of the dhansiri (s) at numaligarh and e) an anomalous se-nw trending course of the brahmaputra from dhansirimukh up to hartamuli, along with the parts of the rivers buroi and bargang on sharma et al. 10 figure 5. map showing the topography of the study area (topmost) along profile cd having the relationship among a) the average elevation profile, b) the depth view of the seismicity, c) gravity and d) magnetic profiles across the kopili and bomdila faults. the north in the same trend infer the influence of a fault-type structure. since all these linear segments of the rivers align over the nw-se trending bomdila fault, they suggest the influence of the later on the courses of these rivers. moreover, it is believed that the neotectonic activity along this fault might have created the linear high scarp and abandonment of earlier river courses. 5. discussion and conclusions the kopili and bomdila faults are two major morphotectonic features of northeast india influencing the neotectonic activities of the region. both these faults are seismically active which is evidenced from occurrence of large to moderate magnitude seismic events in recent past. interrelationship between these faults with seismotectonics, gravity, magnetic and topographic profiles is well observed in this region. the epicentral map shows that the kopili fault remains highly active towards the northern and the eastern parts while the bomdila fault is active towards the northern and western parts. both these faults are characterized by strikeslip kinematics with kopili fault dipping towards ne direction with an average dip angle of about 75° and bomdila fault dipping towards nne direction with an average dip angle of ~50-55°. intense seismic activity occurs to a depth of 45±2 km in the kopili fault zone while in the bomdila fault zone it is 50±2 km where large concentrations of earthquakes are observed. however in kopili fault, the activity continues with sharp increase in depth from north of brahmaputra towards north up to mct and in bomdila fault increase in activity is observed up to the north. although mct is dormant [ni and barazangi, 1984], intense activity is observed at the region where kopili fault meets the mbt and mct [nandy, 2001, kayal et al., 2010, 2012]. this is evidenced by the august 19, 2009 earthquake (mw 5.1) in the assam valley that occurred in the center of the kopili fault zone and the september 21, 2009 strong bhutan himalaya earthquake (mw 6.3) that occurred at the northern end of the kopili fault where it hits the mct [kayal et al., 2012]. the kopili fault planes 11 dynamics of kopili and bomdila faults, ne india figure 6. drainage maps showing the a) anomalous linear courses of kopili-dhansiri (n) rivers along the alignment of the kopili fault, abandoned channels and the bils/swamps and b) anomalous linear courses of the dhansiri (s)–bargang rivers along the alignment of the bomdila fault, abandoned channels and the bils/swamps. the kopili fault and bomdila fault are indicated on the drainage pattern as red dashed lines. show varying orientation dominantly along nnw and nne. the p-axes indicates a nne orientation while the t-axes are mostly oriented along nw-se direction. the bomdila fault shows dominant nw-se trending p-axes while t-axes are mostly oriented along nnw direction. it is observed that the entire kopili and bomdila fault zones are dominated by mostly strike-slip faulting and is believed to extend transversely below the himalayas to the north and the belt of schuppen to the south. the orientation of inferred bomdila fault plane, typically average nnw, suggests transverse tectonics which is in agreement with regional tectonics down to the depth of bottom of seismogenic zone. the fault planes of both the kopili and the bomdila are characteristics of each associated focus at that particular depth. remarkably at shallower depth near kopili fault, the p-axes orientations of few earthquakes are found to be north-south. these variations of axes at depths may be due to transverse tectonics with the availability of geological fractures imaged as lvzs which are possibly zones of denser rocks (archaen gneiss) under compressional stress in the region. the gravity and magnetic values over the entire kopili and bomdila fault region varies between -110 and +60 mgals and -80 nt to +90 nt respectively. the curvatures and closures of the gravity and the magnetic contours along the fault lines indicate some hidden structures involving basement and indicate the influence of the faults down to the basement. the sparse distribution of average normal gravity estimates in the central part of the kopili fault is due to the thick deposit of alluvium. here basement is assumed to be a north dipping basin with thickest sediments. the gravity profiles across fault indicate low value along the kopili fault. increase of gravity value to the south of this fault indicates less overburden of sediment thickness. on the other hand the low gravity value over the bomdila fault area indicates presence of thick alluvial deposits which progressively become thicker se of the fault. the gravity profiles from mct up to north of naharbari indicate higher gravity value along the bomdila fault than in the kopili fault indicating lesser sediment thickness in this area. from naharbari towards golaghat and beyond (along bomdila fault), the value of gravity decreases indicating presence of thick alluvial deposits around golaghat. the magnetic anomaly map shows high magnetic value near raha (along kopili fault) and bokakhat (along bomdila fault), indicating denser as well as highly susceptible shallow basement including presence of ultrabasic rocks. the broad magnetic closures at the southern end near diyungmukh (along kopili fault) indicate change in rock composition within the basement. the magnetic profiles show that from mct up to raha the basement is at shallower depth while from raha up to the southeastern part of the fault the basement is deeper. the bomdila fault indicates large crustal heterogeneities. profile lines show that the fault from mct to mbt has lower magnetic values indicating deep basement. from mbt towards the belt of schuppen, positive magnetic value is observed all along the bomdila fault indicating presence of highly magnetic susceptibility materials and basement at shallower depth. basement being at shallower depth, lower magnetic values indicate presence of thick alluvial deposits. the neotectonic activities in and around kopili and bomdila fault are suggested by anomalous linear courses of some major rivers, river abandonment by avulsion, development of large number of low lands in later period, topographic scarps and development of kinks on existing courses and network of river system which subsequently gave rise to some drainage anomalies. thus, the study clearly indicates that both the kopili and the bomdila faults are tectonically active and their trend is clearly discernible from the gravity and magnetic maps. the neotectonic activity is responsible for the existing regional landform and drainage trends in these areas. the indepth study of these faults through multidisciplinary approach helps us in understanding the transverse tectonic behavior of these structures. moreover, these studies might lead to an understanding of the probable occurrence of any seismic event in the vicinity of these structures in near future and also contribute to the seismic hazard assessment of the region. acknowledgements. we thank dr. d. ramaiah, director, csirnorth east institute of science and technology (neist), jorhat for his kind permission to publish the work. we also thank prof. harsh k. gupta, chairman-research council, csir-neist for his constant encouragement. ministry of earth sciences (moes), new delhi is highly acknowledged for sponsorship of this study vide sanction no. moes/p.o. 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gravity, and tectonics of northeast india and northern burma, bulletin of the seismological society of america, 66, 1683-1694. verma, r.k. and m. mukhopadhyay (1976). tectonic significance of anomalyelevation relationship in northeastern india, tectonophysics, 34, 117-133. verma, r.k. and m. mukhopadhyay (1977). an analysis of gravity field in northeastern india, tectonophysics, 42, 283-317. *corresponding author: saurabh baruah, geosciences and techonology division, csir-north east institute of science and techonology, assam, india email: saurabhb_23@yahoo.com © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. sharma et al. 14 d u e e s t e n s i o n i d e l c o n c e t t o d i t e m p e r a t u r a a lessano r ( > a merio i . — i e m p e r a t m a di una p a r t i c e l l a e l e m e n t a r e i s o l a l a . l a t e m p e r a t u r a di un c o r p o è d e f i n i t a m e d i a n t e u n t e r m o m e t r o p o s t o i n i n t i m o c o n t a t t o col c o r p o stesso. s e q u e s t o c o r p o è r i d o t t o a u n a p a r t i c e l l a e l e m e n t a r e : p r o t o n e , n e u t r o n e , e l e t t r o n e , e c c . . . il t e r m o m e t r o a d a t t o n o n c ' è p e r c h é d o v r e b b e e s s e r e i n f i n i t a m e n t e p i c c o l o . in tal c a s o la d e f i n i z i o n e d e l l a t e m p e r a t u r a n o n p u ò e s s e r e c h e l e g a t a ali e n e r g i a c i n e t i c a d e l l a p a r t i c e l l a c h e si c o n s i d e r a . i n i n s i e m e di q u e s t e p a r t i c e l l e c o s t i t u i r e b b e un gas m o n o a t o m i c o e se a u n g r a m m o a t o m o di q u e s t o a t t r i b u i a m o il c a l o r e m o l e c o l a r e a v o l u m e c o s t a n t e c v . f o r n e n d o g l i la q u a n t i t à di c a l o r e rlq, la sua t e m p e r a t u r a a u m e n t e r e b b e di (it s e c o n d o la r e l a z i o n e f ìq = cv(it l e n e r g i a c i n e t i c a t o t a l e p o s s e d u t a da q u e l g r a m m o a t o m o di gas a l l a t e m p e r a t u r a t è . e s s e n d o c o s t a n t e il c a l o r e s p e c i f i c o : q (\t [1] p o s t o t /" k, 1 e n e r g i a p o s s e d u t a dal g r a m m o a t o m o è d a t a dal c a l o r e a t o m i c o . l cner<jia di o g n u n a d e l l e p a r t i c e l l e si o t t i e n e d i v i d e n d o a m b i i m e m b r i d e l l a [ 1 ] p e r il n u m e r o \ di a v o g a d r o . e s s a è e, = — 7 [ 2 ] n e se p o n i a m o x la [ 2 ] d i v e n t a e.r = e t [ 3 ] er r a p p r e s e n t a l ' e n e r g i a c h e la p a r t i c e l l a p o s s i e d e a t" k e e q u e l l a c h e essa p o s s i e d e a u n g r a d o k . la fi è una c o s t a n t e u n i v e r s a l e p o i c h é non d i p e n d e d a l l a m a s s a d e l l a p a r t i c e l l a c l i c si c o n s i d e r a . ks-a è i 3 2 d e l l a c o s t a n t e di b o l l z n i a n n . 3 3 2 a l e s s a n d r o a m e r i o c o m e t e m p e r a t u r a d e l l a p a r t i c e l l a e l e m e n t a r e i s o l a t a si d e f i n i s c e q u i n d i il r a p p o r t o f r a l ' e n e r g i a c i n e t i c a c l i c essa p o s s i e d e e q u e l l a i h e e a p o s s e d e r e b b e a 1" k . c i ò : t = — [ 4 ] 8 a a l e a d i r e si c o n s i d e r a c o m e t e m p e r a t u r a di u n a p a r t i c e l l a i s o l a t a q u e l l a c h e a v r e b b e un gas m o n o a t o m i c o c h e fosse c o s t i t u i t o d a p a r t i c e l l e d o t a t e t u t t e d e l l a sua slessa e n e r g i a c i n e t i c a di t r a s l a z i o n e o d o t a t a in m e d i a d e l l ' e n e r g i a c i n e t i c a c h e essa p o s s i e d e . d a l l a d e f i n i z i o n e s t a b i l i t a segue c h e le t e m p e r a t u r e di p a r t i c e l l e e l e m e n t a r i i s o l a t e sono p r o p o r z i o n a l i alle r i s p e t t i v e e n e r g i e c i n e t i c h e . v e d i a m o q u a l c h e conse<iuenza; p e r c i ò c o m i n c i a m o con l ' e s p r i m e r e l ' e n e r g i a di u n a p a r t i c e l l a a i " k. p e r il p r i n c i p i o di e q u i p a r t i z i o n e d e l l ' e n e r g i a b a s t e r à d e t e r m i n a r l a p e r u n a m o l e c o l a di e l i o . p e r l ' e l i o , essendo 2 , 9 9 6 il c a l o r e m o l e c o l a r e a v o l u m e c o s t a n t e in p i c c o l e c a l o r i e , l ' e n e r g i a c i n e t i c a di una sua m o l e c o l a a 1° k è 2 9 9 6 — = 0 . 1 9 6 7 . 1 0 p . c . o 2 , 0 8 . 1 0 j o u l e , o 2 . 0 8 . 1 0 16 e r g . 6 , 0 3 . 1 0 p e r c i ò la [ 4 ] d i v e n t a f t=——. 1 0 l 6 = 4 . 8 0 et . 10'5 [ 5 1 2,08. si usa e s p r i m e r e l ' e n e r g i a di u n a p a r t i c e l l a i s o l a l a i n v o l t e l e t t r o n i . u n v o l t e l e t t r o n e v a l e p e r d e f i n i z i o n e 1 , 6 0 . i o 1 9 j o u l e ossia 1 , 6 0 . 1 0 e r g . l a t e m p e r a t u r a c o r r i s p o n d e n t e a u n v o l i e l e t t r o n e i n c o n s e guenza d e l l a [ 5 ] è 0 = 1 , 6 0 . i o " 1 . 4 , 8 0 . i o 1 5 = 7 6 8 0 ° k [ 6 ] o r a , p o i c h é la t e m p e r a t u r a di u n a p a r t i c e l l a i s o l a l a è p r o p o r z i o n a l e al i a sua e n e r g i a c i n e t i c a , il r a p p o r t o f r a d u e t e m p e r a t u r e s a r à e g u a l e a q u e l l o d e l l e c o r r i s p o n d e n t i e n e r g i e ; p e r c i ò se i n d i c h i a m o con 7' e t1 le due t e m p e r a t u r e , c o n e ed el l e d u e e n e r g i e s a r à t _ e d u e e s t e n s i o n i dei. c o n c e t t o di t e m p e r a t u r a 3 3 3 da cui t= t, s e i n d i c h i a m o le energie in volt-elei t r o n i , posto e = cj . l\ = cj\ dove c è la c a r i c a di un e l e t i r o n e , f e j , w n o i r i s p e t t i v i potenz i a l i , sarà 7 t r ^ i s e ora p e r t1 p r e n d i a m o la t e m p e r a t u r a della p a r t i c e l l a clic ha un v o l t e l e t t r o n e di e n e r g i a e che a b b i a m o i n d i c a t o con 0 , esprcssasa ili gradi k s a r à : t= 0 . 1 = 7 6 8 0 . v [ 8 ] l a c a r a t t e r i s t i c a « temperatura di una particella y> è e q u i v a l e n t e a q u e l l a della sua energia c i n e t i c a . a p p l i c h i a m o questo a un fascio di raggi c a t o d i c i p r o v o c a l i da una c a d u t a di p o t e n z i a l e di 2 0 0 0 v o l t ; s a r à : t = 7 6 8 0 . 2 0 0 0 = 1 , 5 3 6 . 10 r k p o i c h é le v e l o c i t à sono p r o p o r z i o n a l i d i r e t t a m e n t e alle radici q u a d r a t e d e l l e t e m p e r a t u r e assolute e la velocità degli e l e t t r o n i p e r p u r a a g i t a z i o n e t e r m i c a a 273%2 assoluti è 1 . 1 1 5 . 1 0 ' cui see, alla t e m p e r a t u r a c a l c o l a t a risulta di 2 , 6 3 . i o 3 cm sec. questo r i s u l t a t o c o i n c i d e con q u e l l o che si può o t t e n e r e direttam e n t e con la r e l a z i o n e , in questo caso sufficientemente a p p r o s s i m a t a , 1 m e — e •> dove è posto m 9 , 0 9 . i o 2 8 , c4 . 8 . 1 " . 1' = 2 0 0 0 / 3 0 0 . n e i raggi c o s m i c i si h a n n o e n e r g i e di 10'', 10!1 e oltre v o l t e l e t t r o n i . a questi raggi c o r r i s p o n d o n o q u i n d i per la i 8 ] t e m p e r a t u r e di 7 , 6 8 . i o 9 ak o 7 , 6 8 . 1 0 1 2 "k e o l t r e . n e l l o r o spettro il m a s s i m o di energia si p r e s e n t a poco al di sopra di 10" v . e . a questa energia secondo la f o r m u l a [ 8 ] c o r r i s p o n d e r e b b e la t e m p e r a t u r a di 7 , 6 8 . i o 1 2 °/v. ai.essa.ndho a m e r i o o r b e n e c o n s i d e r i a m o u n a r a d i a z i o n e y c o s m i c a a v e n t e q u e l l ' e n e r g i a . s e -i t i e n e c o n t o d e l l a r e l a z i o n e /iv = e = l , 6 . 1 0 , : f = l , 6 . 1 u 3 e r g si b a v = — io2 4 da c u i : = 1 . 2 4 . 1 0 3 c m . 6.6 o r a se '/. fosse la lunghezza d ' o n d a emessa col m a s s i m o di e n e r gia n e l l o s p e l i l o di un c o r p o n e r o , a q u e s t o c o r r i s p o n d e r e b b e la t e m p e r a t u r a di 1 0 = 2 . 3 7 . 1 0 » "a' 1 . 2 4 questa s a r e b b e d u n q u e la t e m p e r a t u r a se q u e s t o f o t o n e y t a c e s s e p a r t e della e m i s s i o n e di un c o r p o n e r o . s e si c o n s i d e r a la d i s p a r i t à d e l l e c o n d i z i o n i di e m i s s i o n e di q u e sti raggi r i s p e t t o a q u e l l a del c o r p o n e r o , c ' è da r e s t a r s o r p r e s i c l i c la ( l i p a r i l à dei r i s u l t a t i non sia m o l t o m a g g i o r e . s e si c o n s i d e r a infine la r e l a z i o n e k = mc2 e si s u p p o n e elle t u t t a la m a s s a tri si t r a s f o r m i in e n e r g i a , r i d u c e n d o s i al f o t o n e y c o n s i d e r a l o si vede c h e la m a s s a c o r r i s p o n d e n t e d o v r e b b e a v e r e il v a l o r e 1 , 7 8 . 1 0 g r a m m i c l i c è a l l ' i n c i r e a q u e l l a di un p r o i o n e ( 1 . 6 6 . 1 0 ' ) . s e si a m m e t t e la p o s s i b i l i t à c h e un i n s i e m e di m a s s e e l e m e n t a r i eguali al p r o t o n e si t r a s f o r m i in e n e r g i a , q u e s t a p o t r à e s s e r e dis t r i b u i t a in uno s p e l t r o il q u a l e d o v r e b b e p r e s e n t a r e il suo m a s s i m o n e l l ' i n t o r n o di io" v o l i e l e t t r o n i . 2 . — l ' a l i l a e s t e n s i o n e d e l c o n c e d o di t e m p e r a t u r a è p e r il v u o t o . cosa si deve i n t e n d e r e p e r l e r u p e r a t r a di uno spazio v u o t o di m a t e r i a ' ! il c o n c e t t o non è a l t r e t t a n t o s e m p l i c e c o m e q u e l l o c l i c si è p o t u t o s t a b i l i r e por la p a r t i c e l l a m a t e r i a l e . e v i d e n t e m e n t e non p o s s i a m o c l i c r i f e r i r c i alle r a d i a z i o n i c h e a t t r a v e r s a n o q u e s t o s p a z i o , m a non si potrà a s s u m e r e r o m e t e m p e r a t u r a di q u e s t o quel la c l i c s ' i n t e n d e per t e m p e r a t u r a della r a d i a z i o n e , t e m p e r a t u r a c h e è d a t a d a l l a dis t r i b u z i o n e d e l l ' e n e r g i a n e l l o s p e t t r o e c l i c si r i f e r i s c e i n v e c e a l l a t e m p e r a t u r a di e m i s s i o n e d e l l a s o r g e n t e . lo zero a s s o l u t o si a v r e b b e o v v i a m e n t e r nessuna r a d i a z i o n e al t r a v e r s a s s e q u e l l o spazio ed e v i d u e e s t e n s i o n i d e l c o n c e t t o iii t e m p e r a t u r a d c n t c m e n l e q u e s t o z e r o assoluto non j u i ò e s i s t e r e c o m e non esiste per i c o r p i , ili q u a n t o lo spazio dovrà s e m p r e c o n t e n e r e d e l l a m a t e r i a o e s s e r n e c i r c o n d a t o , e la m a t e r i a non può essere a l l o z e r o a s s o l u t o . c o m i n c i a m o a c o n s i d e r a r e un caso s e m p l i c e nel (piale non si possono a v e r e d u b b i . s u p p o n i a m o una c a v i t à ruota le cui p a r e l i a b b i a n o la t e m p e r a t u r a a s s o l u t a u n i f o r m e t c un c e r t o p o t e r e e m i s s i v o . questa c a v i t à c o s t i t u i s c e un c o r p o n e r o e p e r c i ò la r a d i a z i o n e c h e vi regna ha la d e n s i t à d a t a da ò = ( t t 1 [ 1 b i s ] dove (7 = 5 , 7 8 . 1 0 5 unità c . c . s . o 1 , 3 8 . 1 0 'p i c c o l e c a l o r i e per sec o n d o . c o m e è n o t o questa r a p p r e s e n t a la q u a n t i t à di e n e r g i a e m e s s a da i e n r di s u p e r f i c i e n e r a in un s e c o n d o . si p u ò a t t r i b u i r e a q u e s t o spazio v u o t o di m a t e r i a e c o n t e n e n t e la sola e n e r g i a i r r a d i a l a la t e m p e r a t u r a d a t a d a l l a | 1 b i s ] . [ 9 ] ci c i ò è in a c c o r d o col fatto c h e se in q u e s t o spazio n i e l l i a m o un c o r p o n e r o , o a n c h e non t a l e , p u r c h é d o t a l o di un q u a l c h e p o t e r e a s s o r b e n t e a. questo c o l p o tende ad a s s u m e r e la t e m p e r a t u r a t. i n fatti esso p e r ogni e n r e in ogni s e c o n d o , ricevo a n i ', e se l è la sua t e m p e r a t u r a e m e t t e adi'; la / va c r e s c e n d o finche al l i m i t e , e g u a g l i a n d o s i e m i s s i o n e ed a s s o r b i m e n t o , sia tt. l o slesso si h a se l ' a m b i e n t e c o n s i d e r a t o è p i e n o di a r i a ; in lai caso a n c h e se il c o r p o l'osse un r i f l e t t o r e p e r f e t t o , per c o n l a t t o con q u e s t a , finirebbe con a s s u m e r e la stessa t e m p e r a t u r a . d i v e r s o è il c a s o in cui lo s p a z i o vuoto sia a t t r a v e r s a l o da una r a d i a z i o n e u n i d i r e z i o n a l e , c o m e a v v i e n e in t a l e -vicinanza di' una s t e l l a , c h e si possa t r a s c u r a r e l ' i r r a g g i a m e n t o d e l l e a l t r e . c o n s i d e r i a m o ad e s e m p i o il caso p a r t i c o l a r e d e l l o s p a z i o i n t o r n o al s o i e . sia i la c o s t a n t e s o l a l e , r i f e r i t a p e r c o m o d i t à al m i n u t o , 1 , 9 2 s e c o n d o . il suo v a l o r e alla s u p e r f i c i e d e l l a i erra <• i 0 . 0 3 2 0 1 60 p'.c. e p o i c h é si p u ò a m m e t t e r e c h e vari in r a g i o n e i n v e r s a del q u a d r a l o della d i s t a n z a , p r e s a la d i s i a n z a f e r i a s o l e c o m e u n i t à , a l l a distanza il dal s o l e la c o s t a n t e r e l a t i v a è ( , ' [ 1 0 ] 3 3 6 a l e s s a n d r o a m e r i o il valore di questa costante dà la q u a n t i t à di energia che a t t r a versa in un secondo i area di im cm" n o r m a l e ai raggi. si p o t r e b b e c o n v e n i r e di a s s u m e r e c o m e definizione d e l l a temp e r a t u r a in un p u n t o d e l l o spazio la t e m p e r a t u r a c h e a s s u m e r e b b e un dalo corpo nero clic si trovi i n v e s t i t o dalla r a d i a z i o n e . p e r ò questa t e m p e r a t u r a d i p e n d e dalla f o r m a del c o r p o . s u p p o n i a m o c h e il c o r p o sia sferico, n e r o e a b b i a il raggio r. f.sso sarà investito in ogni secondo dalla radiazione zi r" a, e m e t t e r à 4 i r o t , ' e se è raggi un lo l ' e q u i l i b r i o 4 a t j l = a, da cui / > i y r u ] f 4 a questa p o t r e b b e quindi essere c o n s i d e r a t a c o m e la t e m p e r a t u r a dello spazio c o n s i d e r a l o , ma la cosa è puramente convenzionale, perc h é un corpo di a l t r a forma p o t r e b b e a s s u m e r e una t e m p e r a t u r a diff e r e n t e e questa d i p e n d e r e b b e a n c h e d a l l ' o r i e n t a m e n t o del c o r p o ris p e t t o ai raggi i n c i d e n t i . i n f a t t i se il c o r p o fosse una l a m i n e t t a sottilissima p i a n a , n o r m a l e ai raggi s o l a r i , n e r a dalla p a r t e r i v o l t a al s o l e e p e r f e t t a m e n t e spec u l a r e d a l l ' a l t r a , r i c e v e r e b b e per ogni c m 2 e ad ogni secondo l ' e n e r gia /, c i r r a d i e r e b b e a i v . l ' e q u i l i b r i o della t e m p e r a t u r a -i a v r e b b e p e r i — t„= ] / ^ l [ 1 2 ] r rr che c o i n c i d e con la [ 9 ] e dove r 8 = r l v / t [ 1 3 ] si può essere i n c e r t i n e l l a scelta f r a queste due t e m p e r a t u r e : la t r a p p r e s e n t a la m a s s i m a t c m j j c r a t u r a c h e può e s s e r e r a g g i u n t a in quel dato spazio da un c o r p o il q u a l e sia esposto l i b e r a m e n t e alla radiazione ( e quindi non sia c i r c o n d a l o da uno strato che gli p e r m e t t a di r i c e v e r e raggi di una data specie e di e m e t t e r n e a l t r i i d u e e s t e n s i o n i d e i . c o n c e t t o di t e m p e r a t u r a 3 3 7 q u a l i non lo possano a b b a n d o n a r e per q u a l c h e artifizio, c o m e a v v i e n e p e r i p i a n e t i in grazia d e l l ' a t m o s f e r a che li avvolge). l a t e m p e r a t u r a d e f i n i t a con la [ 1 3 ] c o r r i s p o n d e q u i n d i m e g l i o al c o n c e t t o di temp e r a t u r a di un c o r p o c h e è la m a s s i m a c h e v e r r e b b e raggiunta da un t e r m o m e t r o posto in c o n t a t t o del c o r p o . q u e l l a definita m e d i a n t e la [ 1 1 ] p o t r e b b e r a p p r e s e n t a r e la temp e r a t u r a m e d i a tra q u e l l e clic a s s u m e r e b b e un r i c e v i t o r e a l a m i n a p i a n a , con una f a c c i a nera c l ' a l t r a s p e c u l a r e , n e l l e v a r i e direzioni in cui venisse p o s t a . v e d i a m o ora q u a l c h e r i s u l t a t o sia a p p l i c a n d o la [ 1 1 ] c h e la [ 1 2 ] . alla distanza della t e r r a dal sole la [ 1 1 ] dà r _ y 0 . 0 3 2 0 = 2 7 6 „ k = 3 , 4 . 1 , 3 8 . 1 0 " e la [ 1 2 ] dà •i t= | n ' 0 3 2 0 = 3 9 0 tv = 117 c. 1 , 3 8 . 1 0 " p o i c h é la costante i, v a r i a ili ragione inversa del q u a d r a t o delle distanze) c le t e m p e r a t u r e v a r i a n o in r a g i o n e diretta della r a d i c e (pulita della costante, le t e m p e r a t u r e v a r i a n o in r a g i o n e inversa d e l l e radici q u a d r a t e d e l l e distanze e p e r c i ò «i h a n n o le due serie s e g u e n t i : a l l e distanze dal sole d i : o o c g ? £ s > 3 c 5 o qj 5 o 3 x ..j i uì z l, tv 4 4 3 3 2 5 2 7 6 2 2 4 121 89 63 50 4 4 1 7 0 52 3 — 4 9 — 1 5 2 — 1 8 4 — 2 1 0 — 2 2 3 — 2 2 9 t , "k 6 2 6 4 5 9 3 9 0 3 1 6 171 126 89 71 62 :,c 353 186 117 43 — 1 0 2 — 1 4 7 — 1 8 1 — 2 0 2 —21 1 le due serie d e d o t t e non sono c h e dei casi p a r t i c o l a r i , j i c r ò s o iio sufficienti p e r m o s t r a r e c h e la temperatura che può raggiungere un corpo in uno sjxtzio ruolo in <~ui la radiazione non sia uniformemente distribuita, può essere molto caria. essa p o t r e b b e raggiungere valori m o l l o superiori ai p r e c e d e n t i in casi p a r t i c o l a r i clic sfuggono ad una definizione p a r t i c o l a r e . s e ad e s e m p i o si ha un c o r p o c i r c o n d a t o da uno strato c h e sia b e n traspar e n t e p e r la p a r t e più r i f r a n g i b i l e dello s p e t t r o , o p a c o p e r le grandi 3 3 8 a l e s s a n d r o a m e r i o l u n g h e z z e di onda s u p e r i o r i a un c e r i o v a l o r e , p. c s . al l i m i l e rosso di v i s i b i l i t à , esso r i c e v e l ' e n e r g i a c l i c , p r o v e n e n d o da u n a s t e l l a , è r i c c a n e l l e p i c c o l e l u n g h e z z e <1 o n d a , si s c a l d a e i r r a d i a ; m a p o i c h é la sua t e m p e r a t u r a è m o l l o b a s s a , i r r a d i a in e n o r m e p r e v a l e n z a rad i a z i o n i a g r a n d e lunghezza d ' o n d a p e r le q u a l i lo s t r a t o a v v o l g e n t e è o p a c o , q u i n d i la sua t e m p e r a t u r a deve c r e s c e r e e p o t r e b b e c r e scere a n c h e m o l t o r i s p e l l o ai valori d e l l a t a b e l l a . infatti se ad e s e m p i o la p a r t e d e l l a r a d i a z i o n e i n c i d e n t e c h e . 1 v i e n e a s s o r b i t a d a l l o s t r a t o a v v o l g e n t e è del t o t a l e . i e n e r g i a , c h e , n n—1 a t t r a v e r s a t o lo s t r a t o p r o i e t t o r e giunge al c o r p o è • a , ; il c o r p o n in p a r t e l ' a s s o r b e e i r r a d i a , m a d e l l a sua r a d i a z i o n e solo una p a r t e m o l l o p i c c o l a può s f u g g i r e , ad es. d e l t o t a l e , si c o m p r e n d e q u i n d i p c h e la t e m p e r a t u r a possa s a l i r e n o t e v o l m e n t e al di s o p r a dei v a l o r i c a l c o l a t i , i n f a t t i a m m e s s a a n c o r a la f o r m a sferica con l'aggio r, il c o r p o r i c e v e r e b b e n — 1 e (n — 1 ) . ir r ' l e n e t r a t t e r r e b b e r i, n n p e r ogni s e c o n d o : se si a n i m e l l e c h e a b b i a la stessa t e m p e r a t u r a in ogni p u n t o , e m e t t e r e b b e a, . 1 .t r ~ a t l 1 e di questa u s c i r e b b e la f r a z i o n e . i> l ' e q u i l i b r i o si a v r e b b e p e r t ' 1/ a p — 1 > ' a i n cr e se n e p sono grandi r i p e t t o ali u n i t à , i; e <i( p o c o diversi ira di l o r o , questa t e m p e r a t u r a p u ò o p e r a r e di m o l l o q u e l l e d e f i n i t e per lo spazio vuoto m e d i a n t e le [ 1 1 ] e [ 1 2 ] . (ionie si è visto n e l l a d e d u z i o n e di due ca-i a b b a s t a n z a t i p i c i , q u e l l o della sfera e q u e l l o d e l l a l a m i e r a s o t t i l e , n e r a d a l l a p a r t e esposta ai raggi s o l a r i e p e r f e t t a m e n t e r i f l e t t e n t e d a l l ' a l t r a , la t e m p e r a t u r a d i p e n d e a n c h e d a l l a l'orma del c o r p o : essa d i p e n d e p u r e da a l t r e c a r a t t e r i s t i c h e , ad es. d a l l a r e l a z i o n e fra p o t e r e r i f l e t t e n t e e angolo d incidenza dei raggi. d u e e s t e n s i o n i dei. c o n c e t t o di t e m p e r a t u r a 3 3 9 q u i n d i si può p e r f e t t a m e n t e p i e g a r e il f a t t o r e c e n t e m e n t e oss e r v a t o , c o n e s p l o r a / i o n i in a l t i s s i m a a t m o s f e r a , clic a l l ' a l t e z z a di un c e n t i n a i o di k m si siano r e g i s t r a t e t e m p e r a t u r e di 1 0 0 e p i ù g r a d i c e n t i g r a d i . i n f a t t i , a q u e l l e altezze si può r i t e n e r e c l i c la p r e s s i o n e i a tali' ( q u a l c h e c e n t e s i m o di m m di m e r c u r i o o a n c h e m e n o ) c h e u n term o m e t r o non r e g i s t r i più la t e m p e r a t u r a d e l l ' a r i a , m a c h e le sue ind i c a z i o n i s i a n o c o m a n d a t e e s c l u s i v a m e n t e d a l l a r a d i a z i o n e e p e r t a n t o m e n t r e con 1111 r i c e v i t o r e s f e r i c o esposto alla r a d i a z i o n e solare si dov r e b b e r o o t t e n e r e t e m p e r a t u r e i n t o r n o a l l o zero o r d i n a r i o , con un r i c e v i t o r e p i a n o c h e sia n e r o d a l l a p a r t e esposta n o r m a l m e n t e ai raggi solari e s p e c u l a r e dall a l t r a p a r t e , e si possano o t t e n e r e t e m p e r a t u r e di 100° e p i ù . le osservazioni p r e c e d e n t i pei m e t t o n o a n c h e di s p i e g a r e 1 a n d a m e n t o del g r a d i e n t e di t e m p e r a t u r a . f i n c h é la m a s s a d ' a r i a i n t o r n o al p u n t o di cui si e s p l o r a la t e m p e r a t u r a è n o t e v o l e , il t e r m o m e t r o registra una t e m p e r a t u r a c o m plessa dovuta in p a r t e al c o n t a t t o con l ' a r i a , in p a r t e a l l a r a d i a z i o n e . p e r c i ò la t e m p e r a t u r a va d i m i n u e n d o al c r e s c e r e dell a l t e z z a , p e r c h é d i m i n u i s c e la t e m p e r a t u r a d e l l ' a r i a la q u a l e r i c e v e s e m p r e m e n o cal o r e d a l l a l'erra e ne i r r a d i a s e m p r e p i ù l i b e r a m e n t e verso il c i e l o . p e r ò l ' e f f e t t o d e l l a r a d i a z i o n e d i v e n t a s e m p r e più s e n s i b i l e r i s p e t t o a ([nello del c o n t a t t o con l ' a r i a , a m a n o a m a n o c h e la massa d a r i a in c o n t a t t o col t e r m o m e t r o d i m i n u i s c e . questa p o t r e b b e essere la r a g i o n e per cui n e l l a s t r a t o s f e r a ila una i n v e r s i o n e c l i c è stata r e g i s t r a t a p e r la p r o f o n d i t à li v a r i c h i l o m e t r i , talvolta estesa fra 12 2 0 k m . m a fino a 20 k m la d e n s i t à d e l l ' a r i a è a n c o r a n o t e v o l e : m a g g i o r e o u s u a l e a della n o r m a l e e q u i n d i essa p u ò essere a n c o r a il p r i n 1 3 c i p a l e f a t t o r e c h e d e t e r m i n a la t e m p e r a t u r a . i n v e c e a l l ' a l t e z z a di 100 km q u e s t a d e n s i t à è t r a s c u r a b i l e , dell ' o r d i n c del c e n t e s i m o (li m i l l i g r a m m o per dm'1 e p e r c i ò il suo effetto nel d e t e r m i n a r e la t e m p e r a t u r a del t e r m o m e t r o d i v e n t a t r a s c u r a b i l e e la t e m p e r a t u r a è p e r c i ò d e t e r m i n a t a d a l l a r a d i a z i o n e . l s s a p o t r à q u i n d i a p p a r i r e con valori diversi a seconda del t i p o di t e r m o m e t r o a d o p e r a t o . i 11 t e r m o m e t r o a b u l b o sferico a n n e r i t o p o t r e b b e i n d i c a l e q u a l c h e grado in più o in m e n o d e l l o z e r o ordinai i o : un t e r m o m e t r o a b u l b o p i a t t o , n e r o d a l l a faccia r i v o l t a al s o l e e s p e c u l a r e d a l l ' a l t r a i n d i c h e r e b b e di p i ù , u n a l a m i n a d ' a r g e n t o spe3 i n a f . k s s i m i k o a m e r i o c u l a i e a n n e r i t a o l o d a l l a p a r t e r i v o l t a a! s o l e , 1 0 0 " e p i ù . p e r ò n e l l e a t t u a l i c o n d i z i o n i d e l l a n o s t r a c o n o s c e n z a non si p o t r e b b e r o p r e v e d e r e t e m p e r a t u r e s u p e r i o r i a q u e l l a c a l c o l a t a c o n la [ 5 ] a n c h e sal e n d o a v a r i e c e n t i n a i a di ktn sul l i v e l l o del m a r e a m e n o c h e i l c o r p o r i c e v e n t e n o n f u n z i o n i nel m o d o i n d i c a t o a p a g . 8 . milano — istituto (li fisica 'lei politecnico — aprile 1 9 4 8 . riassunto si. parla (li (lue estensioni del concetto di temperatura. la prima riguarda l'estensione ci una particella elementare e si jonda sull'energia cinetica della stessa. '"ve risulla che lo sjiettro di energia (hi raggi cosmici presenta il massimo che corrisponde a una temperatili u dello stesso ordine di quella di un corpo nero nel cui spettro d massimo si abbia per un fotone di ente quella stessa energia. la seconda è sulla temperatura del vuoto ed è fondata sul! intensità della radiazione che lo atti uversa. uri cor/io situato nel vuoto assumerà una temperatura di pendente dalle sue condizioni di forma, di superficie, di posizione. a e risulta che a grandi altezze nella stratosfera un corpo potrebbe raggiungere lem perai lire superiori a 1 0 0 c. adg vol5 n02 loddo 289_292.pdf annals of geophysics, vol. 45, n. 2, april 2002 289 generation of synthetic wide-band electromagnetic time series mariano loddo, domenico schiavone and agata siniscalchi dipartimento di geologia e geofisica, università di bari, italy abstract the estimation of the earth transfer functions in mt prospecting method poses the greatest difficulty. as in the seismic prospecting method, this task requires the development of advanced processing techniques. in order to assess the performance of each technique, controlled synthetic data and different noise types, which simulate the observed signals, are required. this paper presents a procedure to generate a wide-band noise-free electromagnetic field to be used both for magnetotelluric and audio-magnetotelluric studies. furthermore, an effort was made to extend the simulation procedures to generally stratified and simple inhomogeneous earth structures. the discretetime magnetic field values are generated through the inverse fourier transform of a continuous amplitude spectrum and a sampling procedure. the electric field time series are obtained by the convolution of the magnetic field time series, calculated in the interested frequency band, with a non-causal impedance impulse response. polarized fields, which are important when inhomogeneous media are considered, are also generated. 1. introduction computer simulation of electromagnetic field components has frequently been used for testing estimation techniques of the magnetotelluric impedance tensor (goubau et al., 1978; mcmechan and barrodale, 1985; yee et al., 1988; larsen et al., 1996) and for assessing preprocessing methods of extraction of the stationary and coherent part of signals corrupted by noise (san filipo and hohmann, 1983; lamarque, 1999). the testing and assessing tasks are generally accomplished in the frequency domain through the power spectra of the measured signals. hence, the simulation is carried out in the frequency domain. some authors have pointed out the numerous problems arising in the frequency-domain approach and have proposed estimation methods in the time-domain (kunetz, 1972; ernst, 1981; mcmechan and barrodale, 1985; yee et al., 1988; spagnolini, 1994). moreover, for some purposes (e.g., noise identification and removal, filling missing data, search for outliers) the time-domain methods are also useful in a frequency-domain context (egbert, 1992; egbert et al., 1992). therefore, these considerations justify the interest in generating synthetic time series for simulation purposes. in the above-mentioned papers, the synthetic data are constructed by means of different techniques. larsen et al. (1996) used measured magnetic time series and an estimated 1d transfer mailing address: prof. mariano loddo, dipartimento di geologia e geofisica, università di bari, campus universitario, via e. orabona 4, 70125 bari, italy; e-mail: loddo@geo.uniba.it key words magnetotelluric – electromagnetic field simulation 290 mariano loddo, domenico schiavone and agata siniscalchi function, modified by a distortion function, to generate the electric time series in a test area. simple pseudo-random numbers were used to represent the noise-free random components of the time series (lamarque, 1999) or the real and imaginary parts of the incident magnetic field components (goubau et al., 1978). the latter authors calculated the electric field spectra using a bi-dimensional impedance tensor made by simple complex relationships. san filipo and hohmann (1983) and yee et al. (1988) assume a function which tends to zero exponentially with increasing values of frequency to represent the main spectral characteristics of the natural magnetic field, simulating this behaviour through recursive digital filters. these same authors generated the magnetic time series by convolving a pseudo-random number sequence with the filter coefficients. yee et al. (1988) computed the noise-free electric field components by convolving in time the magnetic time series with the impedance impulse responses modelled as rational forms. uncorrelated noise is simulated by adding random numbers either to the spectra (goubau et al., 1978) or to the time series (yee et al., 1988; larsen et al., 1996). some authors introduce the noise by adding sinusoids to the signal. these are characterized by frequencies and amplitudes which are typical of industrial and atmospheric noise (lamarque, 1999). in recent years, the english version of a systematic study on the mt synthetic data generation, carried out by russian researchers, was published (varentsov and sokolova, 1995). the study was conducted within the framework of the project to compare mt data processing techniques, using synthetic data sets (comdat project). the discrete values of the spatial components of the magnetic field spectrum, considered in the mt frequency band, were constructed by opportunely multiplying selected random functions with a simple realistic field structure (the form was used with < 0). the electric field spectra were calculated via arbitrary one-dimensional scalar impedances. finally, the time series were obtained by a discrete inverse fourier transform of the spectral data approximated by a piecewise constant function. noise components were added both in the frequency and in the time domain. the present paper describes an approach to generate wide-band electromagnetic time series. the procedure adopted is represented in a flow chart (fig. 1). the aim was to simulate real acquisition techniques to be used both in magnetotelluric and audio-magnetotelluric studies. in this first work only noisefree signals have been considered. the results were then applied to 1d and simple non 1d structures. 2. em field simulation the noise-free horizontal electromagnetic field components are related by means of the earth transfer functions in a two-input/twooutput linear system (cantwell, 1960). in the time-domain, the relation between the timevarying input magnetic field h(t) and the timevarying output electric field e(t) can be written in terms of convolution equations among the field components e x (t) = z xx (t) h x (t) + z xy (t) h y (t) (2.1) e y (t) = z yx (t) h x (t) + z yy (t) h y (t) where z xx , z xy , z yx , z yy are the elements of the impedance impulse response tensor. the input and output channels in the system are perturbed by added noise (swift, 1967; sims et al., 1971) giving the observed field components e x ob(t) = e x (t) + n ex (t) e y ob(t) = e y (t) + n ey (t) (2.2) h x ob(t) = h x (t) + n hx (t) h y ob(t) = hy(t) + nhy(t). for application purposes, the continuous-time convolutions of eq. (2.1) must be replaced by discrete forms. this task can be accomplished b y a p p l y i n g t h e w e l l k n ow n c l a s s i c a l sampling theorem (freeman, 1965) to lowpass (anti-alias) filtered continuous signals. in the following, the procedure adopted to 291 generation of synthetic wide-band electromagnetic time series fig. 1. flow chart outlining the procedure for generating the electromagnetic field time series. construction of a continuous function representing the magnetic field spectrum h(ω ) in the 10-4-104 hz frequency range (eq. (2.5) and fig. 2) construction of the analytical expression for the frequency domain earth impedance z (ω) for a stratified model (eq. (2.9)) inverse fourier transform of h(ω ) (eq. (2.7)) inverse fourier transform of z(ω ) (eq. (2.12)) input the subband to use in simulation and the resulting sampling rate sampled magnetic field impulse response (eq. (2.8) and fig. 3a) sampled impedance impulse response (eq. (2.13) and fig. 5) sampled electric field impulse response (fig. 6) input a sequence of random numbers noise-free magnetic field time series noise-free electric field time series dft dft h(∆ω ) fig. 4 e(∆ω ) fig. 7 292 mariano loddo, domenico schiavone and agata siniscalchi generate the noise-free electromagnetic field components and the impedance impulse response is illustrated. 2.1. h-field generation the natural em field is due to a variety of causes. at frequencies above 6 hz, roughly, it is primarily due to spherics, which are em transients generated by lightning strokes. below 6 hz, the field is of geomagnetic and ionospheric origin. the general features of the natural field in different frequency bands have been illustrated by many authors (campbell, 1967; filloux, 1973; serson, 1973; mcnae et al., 1984; labson et al., 1985; malergue et al., 1986). the horizontal components of the field present average amplitudes, generally increasing with longer periods and reaching a value of about 10 nt at 10 4-10 5 hz. moreover, two distinct minima in the 1 hz to 6 hz and 1 khz to 2 khz frequency ranges are observed. these minima represent the separation between ionospheric pulsations and spherics and between lowfrequency and high-frequency spherics, respectively. a smoothed, mean spectrum of the magnetic field is shown in fig. 2. to obtain a relationship which describes the field, the plotted behaviour can be modelled through a linear time invariant system. depending on the goal of the study (geomagnetic,magnetotelluric, audiomagnetotellurics prospecting), the wide-band field is used in a limited frequency range. this operating characteristic influences the procedure for the generation of the magnetic field time series. in fact, in accordance with some authors (san filipo and hohmann, 1983; yee et al., 1988), the approximation of the frequency characteristics of the field could be performed directly in the discrete domain by writing the transfer function of the system in terms of poles and zeros in the z domain. an alternative is to transform the transfer function from the continuous to the discrete domain through a bilinear transformation (oppenheim and schafer, 1975). in the present study, these methods cannot be adopted for the following reasons: 1) for each sub-band, the power of the h field at frequencies greater than the nyquist one is not negligible, thus resulting in the aliasing problem when the normal z transform is used. 2) with regards to the bilinear z, this method maps the complex continuous s plane into the complex sampled z plane without aliasing, but with a frequency distortion (warping) (oppenheim and schafer, 1975). this distortion, resulting from the non-linear character of the transformation, is compensated when the frequency-magnitude curve of the system can be approximated by a piecewise constant function in which only two or three characteristic frequencies need to be transformed without distortion (prewarping procedure) (golden and kaiser, 1964). this requirement is not fulfilled in the simulation of a physical field which is characterized by a complex morphology spectrum. considering the previous arguments against the use of a discrete-time relationship, a continuous function for the h field was constructed. the sampled impulse response was 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 frequency (hz) 10 -3 10 -2 10 -1 10 0 10 1 h o r iz o n t a l m a g n e t ic ( n t ) fig. 2. smoothed, mean amplitude of the horizontal electromagnetic field component variations simulated through a linear time invariant system. 293 generation of synthetic wide-band electromagnetic time series obtained by a windowed integral fourier transform, computed with the time interval dictated by the nyquist frequency of the studied band. in particular, by using the poles and zeros form in the continuous (laplace) domain, a system function can be expressed (seshu and balabanian, 1959) as (2.3) where, s = σ + iω is the laplace complex variable with σ the damping factor of the system and ω the frequency. the complex numbers s 0k are the m zeros while the s ph are the n poles of the function, respectively; k is a scale factor. the analytic properties of the function are determined by its poles and zeros. in order for the system to be stable, the poles must be in the left half of the complex s-plane and any pole on the imaginary iω axis must be simple. moreover, since the network function must be real for a real variable s, it follows that the poles and zeros must either be real or in complex conjugate pairs. the simple zeros and poles and the complex conjugate pairs in the numerator and denominator of (2.3) can be factored in the following form (gatti et al., 1966) (2.4) where the indexes i, j, l, k indicate the generic simple zero, simple pole, complex conjugate zero and complex conjugate pole, respectively. the scale factor k 1 and the correction factors ζ l and χ k are defined by the following equations: if s = iω is inserted into eq. (2.4), the magnetic field relationship in the frequency domain can be obtained. the frequency and characteristics of the poles and zeros are selected by considering that simple poles and zeros introduce a 20 db/decade slope change in the function curve on a cartesian decibel versus the log (ω) representation (bode’s plot), while the complex conjugate pairs double these changes. moreover, the ζ l and χ k quantities act as damping factors at each critical frequency. considering the magnetic field spectrum shown in fig. 2, the downward change of the slope (energy decrease) can be modelled by poles, while the upward changes (energy increase) by zeros. in particular, four poles and four zeros, with values as shown in table i, were sufficient to represent the changes in the spectrum. the simple poles and zeros were chosen to be real, while for the complex conjugate pairs, correction factors ζ l = 1 and χ k = 1 were adopted. these conditions are equivalent in considering real double poles and double zeros. for the sake of system stability, all the poles have negative values. finally, the scale factor k1 was purposely selected in order to obtain a low frequency asymptote of 10 nt for the field spectrum. with the above conditions eq. (2.4) becomes (2.5) h s k s s s s s s s s s s s s m p p pn ( ) −( ) −( ) ⋅ ⋅ ⋅ −( ) −( ) −( ) ⋅ ⋅ ⋅ −( ) = 01 02 0 1 2 k k s s s s ii ll pjj pkk 1 0 0 2 2 = [ ] [ ] [ ] [ ] ∏ ∏ ∏ ∏ ζ l l l s s = − ℜ( )0 0 . ( )s s s s s si i l l l l 0 2 0 2 0 1 2 1−         + +        ∏ ∏ ( )ζ ( )s s s s s spj j pk k pk k −         + +        ∏ ∏1 2 1 2 2( )χ χ k pk pk s s = − ℜ( ) h s( ) = ( )( )( )( ) ω ω ω ω ω ω ω2 1 2 1 2 2 2 2 3 2 4 2 4 2 1 2 1 1 2 1 p i p p i p i p p i p + + − + + − − + + k= 1 ( )( )( )( ) i z z i z i z z i z ω ω ω ω ω ω 1 2 2 2 2 3 2 4 2 4 1 2 1 1 2 1− − + + − − + + h ω( ) = k= 1 294 mariano loddo, domenico schiavone and agata siniscalchi the continuous-time impulse response of the em field can be obtained from the inverse fourier transform of eq. (2.5) (2.6) if ω l and ω h are the low-frequency and highfrequency cut-off of the band, and h r (ω) and h i (ω) the real and imaginary parts of the complex field h(ω), respectively, then eq. (2.6) becomes (2.7) if ∆t is the sampling period and t the maximum acquisition time, being h r (ω) and h i (ω) even and odd functions, respectively, the sampled impulse response can be written as (2.8) both for the numerical testing of the procedure, discussed at the end of section 2.3, and for the generation of polarized fields in section 3, the magnetic field in the frequency range 0.0005-5 hz has been computed. the latter range is adequate for investigating the earth models considered. the resulting sampling period and maximum period were 0.1 s and 2000 s, respectively. the sampled magnetic field impulse response, calculated from eq. (2.8), is shown in fig. 3a. the discrete fourier transform (dft) of the impulse response (fig. 3b) is in perfect agreement with the continuous magnetic field amplitude spectrum, within the frequency range of interest. in order to generate a magnetic field time series, the impulse response of fig. 3a was convolved with a sequence of pseudo-random numbers with a zero mean and unity-variance. the dft of the resulting time series is shown in fig. 4. the generation of the electric field could be performed in the frequency domain as the product of h(ω) and z(ω). however, if eq. (2.5) for h(ω) and, for example, eq. (2.9) for z(ω) are considered, their product is a very complex function; thus, its inverse fourier transform is performed with difficulty. hence, it was decided to inverse transform the two functions separately, thereby obtaining the electric field by a convolution procedure in the time domain. the determination of the discrete-time impedance impulse response which produces the same outputs as the continuous-time counterpart remains a problem which is thoroughly and elegantly discussed by egbert (1992). as the continuous-time impedance impulse response is table i. characteristics of the poles and zeros of the system function used in the generation of the magnetic field variations. all poles and zeros are real and shown as absolute values. critical frequencies frequency value (hz) multiplicity p 1 0.002 double z1 0.006 simple z 2 0.8 double p 2 7.0 double z3 10.0 simple p3 100.0 simple z 4 1500.0 double p 4 20000.0 double h t h e di t( ) = ( ) −∞ ∞ ∫ 1 2π ω ωω . = ( ) + ( )[ ] ( ) + ( )[ ] + − − ∫ 1 2π ω ω ω ω ω ω ω cos sin h ih t i t dr i h l + ( ) + ( )[ ] ( ) + ( )[ ]∫ 1 2π ω ω ω ω ω ω ω h ih t i t dr i l h cos sin . h t( ) = h n t∆( ) = = ( ) ( ) − ( ) ( )[ ]∫ 1 2 π ω ω ω ω ω π π cos sin h n t h n t dr i t t ∆ ∆ ∆ . 295 generation of synthetic wide-band electromagnetic time series unbounded, the non-causality of the corresponding discrete form generally follows. hence, the discrete-time impulse response can be estimated from the frequency-domain impedance by band limiting and sampling it, as well as, by taking into consideration the negative time values. this procedure, applied to a stratified model, is presented in the following paragraph. 2.2. impedance impulse response the analytical expression of the frequency domain mt impedance, relating magnetic and electric field components, can be written for a stratified earth as (kunetz, 1972) (2.9) a b fig. 3a,b. a) impulse response of the electromagnetic field in the frequency range 0.0005-5 hz, sampled every 100 ms; b) theoretical amplitude spectrum of the magnetic field in the frequency range considered (continuous line) and dft of the impulse response in a) (dashed line). fig. 4. amplitude spectrum of the magnetic time series generated by the convolution of the impulse response of fig. 3a with a sequence of pseudo-random numbers. z i i q em m am i( ) = + = = 1 1 2 4 4 1 2 e q e i m am i m = + = 4 1 2 4 14 1 2 296 mariano loddo, domenico schiavone and agata siniscalchi where the first term on the right hand side represents the impedance over a half space of constant resistivity 1, the second one results from the reflections of the q m images on the boundaries between the layers and , h1 being the thickness of the first layer (see kunetz, 1972, for the determination of the images from the reflection coefficients). the corresponding continuous time impulse response can be written as (2.10) if the low-frequency and high-frequency limits of the band are indicated, as previously, with l and h , respectively, then eq. (2.10) becomes (2.11) by integrating by parts, eq. (2.11) yields (2.12) by placing l = 2 /t and h = / t, with t and t the sampling period and the longest period, respectively, the solution of the integrals on the right side of (2.12) leads to the following algorithm for the discrete-time impedance impulse response for a stratified earth: (2.13) the above expression can be easily calculated numerically for a given sequence of resistivities and thicknesses. figure 5 shows an example of the impulse response for an h-type three-layer fig. 5. non-causal discrete-time impedance impulse response for a three-layer earth model (shown in the upper part of the figure). e q e e d i m am i m i t h l = + + = 1 2 4 1 24 1 2 4 1 e q e e d i m am i m i t l h .+ + = 1 2 4 1 24 1 2 4 1 z n t n t d t t ( ) = ( ) +13 2 22 4 cos( ) z t( ) = z t t d h l ( ) = ( ) +13 22 4 cos( ) q em m am h l cos+ =1 2 22 4 t am d2 2 . q e n tm m t t am+ =1 2 2 4 2 2 cos am d2 2 . z t z e di t( ) .= ( ) 1 2 a h= 1 110 297 generation of synthetic wide-band electromagnetic time series earth (the model is shown in the same figure). the impulse response shows a structure which is mainly localized around the origin, with oscillating decreasing values. moreover, a different behaviour between positive and negative time lags is observed. the procedure previously adopted for layered earth models could be extended and applied to more complex situations, the analytical solutions of which are available, e.g., vertical discontinuity (d’erceville and kunetz, 1962), vertical dyke (rankin, 1962), anisotropic stratified sequence (douglas et al., 1967). 2.3. e-field generation the electric field time series is calculated by convolving in time the magnetic field time series with the earth impulse response (eq. (2.13)). the procedure adopted for the convolution operation requires further discussion. if m values for h( t) and n values for z( t) are considered, with n m, the e( t) resulting from the convolution will contain m + n – 1 values. however, there are (n – 1) values both at the beginning and at the end of the convolved series which are not fully constrained by the data. a solution to this problem is to strip off these edge regions from the data, thus obtaining m – n + 1 e( t) values. moreover, in order to generate magnetic and electric field time series of the same length as well as a causal electric field, the (n – 1)/2 values at the beginning and at the end of h( t) must be discarded. thus, the value e(n) will be generated by h[(n + 1)/2]. figures 6 and 7 show the impulse response and the spectral amplitudes of the noise-free electric field for the 1d structure of fig. 5. for the generation of the electric field impulse response 20 000 samples of the magnetic field impulse response, together with 4001 samples of the impedance impulse response were used. for the purpose of numerically testing the procedure, the impedance transfer function z( ), obtained as e( )/h( ) for a single time series, has been computed. the computed discrete impedance transfer function (fig. 8) fits the theoretical curve well in all but low frequency values, due to the number of samples used. fig. 6. electric field impulse response resulting from the convolution in time of the magnetic field impulse response (fig. 3a) with the non-causal impedance impulse response of fig. 5. fig. 7. amplitude spectrum of the electric field generated by the magnetic field time series. 297 generation of synthetic wide-band electromagnetic time series earth (the model is shown in the same figure). the impulse response shows a structure which is mainly localized around the origin, with oscillating decreasing values. moreover, a different behaviour between positive and negative time lags is observed. the procedure previously adopted for layered earth models could be extended and applied to more complex situations, the analytical solutions of which are available, e.g., vertical discontinuity (d’erceville and kunetz, 1962), vertical dyke (rankin, 1962), anisotropic stratified sequence (douglas et al., 1967). 2.3. e-field generation the electric field time series is calculated by convolving in time the magnetic field time series with the earth impulse response (eq. (2.13)). the procedure adopted for the convolution operation requires further discussion. if m values for h( t) and n values for z( t) are considered, with n m, the e( t) resulting from the convolution will contain m + n – 1 values. however, there are (n – 1) values both at the beginning and at the end of the convolved series which are not fully constrained by the data. a solution to this problem is to strip off these edge regions from the data, thus obtaining m – n + 1 e( t) values. moreover, in order to generate magnetic and electric field time series of the same length as well as a causal electric field, the (n – 1)/2 values at the beginning and at the end of h( t) must be discarded. thus, the value e(n) will be generated by h[(n + 1)/2]. figures 6 and 7 show the impulse response and the spectral amplitudes of the noise-free electric field for the 1d structure of fig. 5. for the generation of the electric field impulse response 20 000 samples of the magnetic field impulse response, together with 4001 samples of the impedance impulse response were used. for the purpose of numerically testing the procedure, the impedance transfer function z( ), obtained as e( )/h( ) for a single time series, has been computed. the computed discrete impedance transfer function (fig. 8) fits the theoretical curve well in all but low frequency values, due to the number of samples used. fig. 6. electric field impulse response resulting from the convolution in time of the magnetic field impulse response (fig. 3a) with the non-causal impedance impulse response of fig. 5. fig. 7. amplitude spectrum of the electric field generated by the magnetic field time series. 298 mariano loddo, domenico schiavone and agata siniscalchi 3. field polarization the fields simulated through the previously described procedure were generated by considering a horizontally stratified isotropic half-space. in this case, it is well-known that the polarization of the waves is not important (porstendorfer, 1975). on the contrary, in an earth having a complicated electrical structure, the measured electric and magnetic fields depend strongly on the orientation of the primary electric and magnetic fields. the effect of an electrical inhomogeneity varies as the orientation of the primary field varies. thus, the total field polarization is a function of the current sources in the ionosphere. since, in general, the location of the sources changes with time, or several sources may contribute simultaneously to the field, the total electromagnetic field observed along arbitrary directions often changes with time at a given frequency. the representation of the impedance as a tensor instead of as a scalar quantity was developed to take into account the orientation of the primary field and the structural complexities. in considering the most general polarization of the magnetic field, that is the elliptical one, an electric field, itself elliptically polarized (porstendorfer, 1975), results. a 2d structure is now considered and measuring axes which coincide with the principal axes of the anisotropy tensor, are assumed. remembering that any elliptically polarized magnetic wave can be synthesized from two linearly polarized waves, the field may be divided into two parts, parallel and perpendicular to the strike of the structure, respectively. in this particular case, each of the electric field components relates to the perpendicular magnetic field component alone. thus, the magnetotelluric eqs. (2.1) become e x (t) = z xy (t) h y (t) e y (t) = z yx (t) h x (t) where z xy and z yx are the impedance impulse responses of two different 1d models. fig. 8. fit of the impedance transfer function generated by synthetic data (solid line) to the theoretical curve (dashed line) for the three-layer model in fig. 5. table ii. two 1d layered earth sections representing a weakly anisotropic stratified structure, used to generate the impedance impulse response along the anisotropy directions. z xy z yx resistivity ( m) thickness (km) resistivity ( m) thickness (km) 16 1 19.36 1 1 0.75 1.21 0.75 16 19.36 299 generation of synthetic wide-band electromagnetic time series fig. 9. horizontal components of the time series filtered at 5 s and total field polarization of short sections. the ellipses are centered at each of the sections and are in the same scale as the corresponding field. 300 mariano loddo, domenico schiavone and agata siniscalchi in summary, to preserve the real field behaviour it must be reminded that it presents (grillot, 1975): 1) an impulsive nature, occurring in short bursts. 2) a distinct, randomly variable, polarization in each burst. 3) more generally, an elliptic polarization. together with the previous characteristics of the fields, the field components must be correlated to each other. in order to satisfy the previous requirements, the magnetic time series have been modulated with a pseudo-random sequence, uniformly distributed in the interval (– 1,1). this modified time series was assumed to represent one component of the field. the other component was obtained by modulating the same initial field with a sequence of numbers, complementary to the pseudo-random ones. as earth model, a stratified anisotropic structure, characterized by the two 1d models shown in table ii has been selected. the electric field components were generated by the procedure discussed in the previous sections. in accordance with grillot (1975), the occurrence of the field characteristics was tested through a band-pass filtering of the time series. figure 9 shows a 1600 s specimen of the horizontal components of the field time series filtered with a band-pass centered at 5 s and a bandwidthcentre frequency ratio of 0.3. in the figure the polarization ellipses of the total horizontal field vectors for short sections of the data have also been plotted. the figure shows ellipses well defined over 2-3 cycles with polarizations changing from section to section. as expected, the magnetic and electric ellipses are perpendicular. considering independent sets of field components at a given frequency, which occurs when the sections have different polarizations, is the basis for any statistical estimation of the earth impedances. 4. conclusions a procedure to generate synthetic wide-band electromagnetic time series has been developed and described in detail. it attempts to take into account and to reproduce the main characteristics of the actual electromagnetic field. the main intent was to carry out a simple yet accurate mt experiment simulation, consisting in the interaction of a synthetically generated magnetic field with assigned resistivity models of the underground. these models were complex with regards to their analytical representation. the electric field time series were calculated in the timedomain through a non-causal impedance impulse response. the procedure was extended to simple inhomogeneous structures considering the field polarizations. further studies will include: 1) more complex earth structures. 2) the effects of different noise types. 3) the relative performances of different statistical approaches for a reliable impedance estimation. references campbell, w.h. (1967): geomagnetic pulsations, in physics of geomagnetic phenomena, edited by s. matsushita and w.h. campbell (academic press, new york), vol. 2, 821-909. cantwell, t. (1960): detection and analysis of low frequency magnetotelluric signals, ph.d. thesis, mit, boston. d’erceville, i. and g. kunetz (1962): the effect of a fault on the earth’s natural electromagnetic field, geophysics, 27, 651-665. douglas, p., d.p. o’brien and h.f. morrison (1967): electromagnetic fields in an n-layer anisotropic halfspace, geophysics, 32, 668-677. egbert, g.d. (1992): noncausality of the discrete-time magnetotelluric impulse response, geophysics, 57, 13541358. egbert, g.d., j.r. booker and a. schultz (1992): very long period magnetotellurics at tucson observatory: estimation of impedances, j. geophys. res., 97 (b11), 15,113-15,128. ernst, t. (1981): a comparison of two methods of the transfer function calculation using the least-square criterion in time and frequency domain, in publ. inst. geophys. pol. acad. sci., 143, 13-24. filloux, j.h. (1973): techniques and instrumentation for study of natural electromagnetic induction at sea, phys. earth planet. inter., 7, 323-338. freeman, h. (1965): discrete-time systems (john wiley & sons, new york) pp. 241. gatti, e., p.f. manfredi and a. rimini (1966): elementi di teoria delle reti lineari (casa editrice ambrosiana, milano), pp. 351. 301 generation of synthetic wide-band electromagnetic time series golden, r.m. and j.f. kaiser (1964): design of wideband sampled-data filters, bell. syst. tech. j., 43, 1533-1546. goubau, w.m., t.d. gamble and j. clarke (1978): magnetotelluric data analysis: removal of bias, geophysics, 43, 1157-1166. grillot, l.r. (1975): calculation of the magnetotelluric tensor impedance: analysis of band-limited mt signal pairs, geophysics, 40, 790-797. kunetz, g. (1972): processing and interpretation of magnetotelluric soundings, geophysics, 37, 1005-1021. labson, v.f., a. becker, h.f. morrison and u. conti (1985): geophysical exploration with audiofrequency natural magnetic fields, geophysics, 50, 656-664. lamarque, g. (1999): improvement of mt data processing using stationary and coherence tests, geophys. prospect., 47, 819-840. larsen, j.c., r.l. mackie, a. manzella, a. fiordalisi and s. rieven (1996): robust smooth magnetotelluric transfer function, geophys. j. int., 124, 801-819. macnae, j.c., y. lamontagne and g.f. west (1984): noise processing techniques for time-domain em systems, geophysics, 49, 934-948. malergue, g., j. aissa, c. herisson and j. rainaud (1986): recent developments in the magnetotelluric prospecting method, first break, 4, 23-28. mcmechan, g.a. and i. barrodale (1985): processing electromagnetic data in the time domain, geophys. j. r. astron. soc., 81, 277-293. oppenheim, a.v. and r.w. schafer (1975): digital signal processing (prentice-hall international, london), pp. 585. porstendorfer, g. (1975): principle of magneto-telluric prospecting (gebruder borntraeger, berlin-west, stuttgart), pp. 118. rankin, d. (1962): the magnetotelluric effect on a dike, geophysics, 27, 666-676. san filipo, w.a. and g.w. hohmann (1983): computer simulation of low-frequency electromagnetic data acquisition, geophysics, 48, 1219-1232. serson, p.h. (1973): instrumentation for induction studies on land, phys. earth planet. inter., 7, 312-322. seshu, s. and n. balabanian (1959): linear network analysis (john wiley and sons, new york). sims, w.e., f.x. bostick jr. and h.w. smith (1971): the estimation of magnetotelluric impedance tensor elements from measured data, geophysics, 36 (5), 938-942. swift, c.m. jr (1967): a magnetotelluric investigation of an electrical conductivity anomaly in the southwestern united states, ph.d. thesis, mit, boston. spagnolini, u. (1994): time-domain estimation of mt impedance tensor, geophysics, 59 (5), 712-721. varentsov, i.m. and e.yu. sokolova (1995): generation of synthetic magnetotelluric data, izvestiya, phys. solid earth, 30, 554-562. yee, e., p.r. kosteniuk and k.v. paulson (1988): the reconstruction of the magnetotelluric impedance tensor: an adaptive parametric time-domain approach, geophysics, 53, 1080-1087. preface to special issue “the amatrice seismic sequence: preliminary data and results” annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7373 1 “the amatrice seismic sequence: preliminary data and results” preface carlo doglioni, marco anzidei, silvia pondrelli, fabio florindo istituto nazionale di geofisica e vulcanologia, rome, italy i. introduction he m=6.0 earthquake that struck central italy at 01:36 utc (universal time coordinated) on august 24, 2016, marked the beginning of a long, still-ongoing seismic sequence, which culminated in the mw 6.5 event at 06:40 utc on october 30, 2016, while this volume was already in preparation, and reactivated again when this preface was almost complete. this dramatic seismic sequence, which on january 18, 2017, released four additional events of m between 5.0 and 5.5 in a few hours, caused 298 casualties, hundreds of injuries, and the practically total destruction of several villages across a wide area of the central apennines, covering the italian regions of lazio, umbria, marche and abruzzo. in particular, the historical village of amatrice was completely destroyed. this seismic sequence represents an important new case study for earth scientists only 5 years after the ml 5.9 destructive event in the emiliaromagna region in 2012 [scognamiglio et al., 2012] and 7 years after the mw 6.3 l’aquila earthquake of april 6, 2009 [chiarabba et al., 2009]. during these few years, the skill of the scientific community and its reaction to the emergencies of large destructive earthquakes have improved: the earthquake epicenter and source parameters were precisely located within a few minutes of the mainshock, while the acquisition of on-site multiparametric data started just a few hours after the onset of the seismic sequence. seismological, geodetic, geological and geochemical data were rapidly collected and analyzed to identify the features of the seismogenic source and the geological structures responsible for these earthquakes. the level of damage, the effects on the environment and the extension of ground deformations were rapidly assessed with the ultimate goal of better understanding earthquake characteristics in order to respond to civil protection requirements. preliminary data were shared worldwide through the internet via the www.ingv.it webpages, while scientific information to the media was rapid and exhaustive, also through dedicated pages (https://ingvterremoti.wordpress.com/). less than three months after the mainshock, the istituto nazionale di geofisica e vulcanologia (ingv) is now publishing a collection of the first scientific results in this special issue of fast track papers of annals of geophysics, vol. 59, n. 5, 2016, entirely dedicated to the amatrice seismic sequence: preliminary data and results, as in the case of the 2012 emilia earthquake [anzidei et al., 2012]. papers are freely available online at the web site of annals of geophysics (http://www.annalsof geophysics.eu/), for the rapid distribution of these scientific results. this special issue represents an important goal for the ingv and the whole scientific community which was involved in the study of this seismic sequence. ii. organization and volume contents the large amount of information is collected in 57 brief, but exhaustive, papers. readers will find preliminary results of scientific interpretat annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7373 2 tions of instrumental data and field observations. namely, the spatial and temporal evolution of the seismic sequence, responsible faults, surface geological effects, coseismic crustal deformations, changes in geochemistry of groundwater and the considerations of seismic hazards for central italy. some papers introduce the reader to the geological [falcucci et al., 2016-this issue; bonini et al., 2016-this issue; moro et al., 2016-this issue; pucci et al., 2016-this issue] and historical seismological features [castelli et al., 2016-this issue; valensise et al., 2016-this issue] and the regional stress field [mariucci and montone, 2016-this issue] of the earthquake area, while others refer to the collection and analysis of instrumental data during the seismic sequence. seismological data are shown and discussed by ciaccio et al. [2016-this issue], scognamiglio et al. [2016-this issue], marzorati et al. [2016-this issue], roselli et al. [2016-this issue], michele et al. [2016-this issue], lanzano et al. [2016-this issue], marchetti et al. [2016this issue], pondrelli et al. [2016-this issue], moretti et al. [2016-this issue], massa et al. [2016-this issue], while cheloni et al. [2016-this issue] and avallone et al. [2016-this issue] report on static and dynamic coseismic crustal deformation observations. the analysis of remote sensing data from insar observations is described by romaniello et al. [2016-this issue] and bignami et al. [2016-this issue], while geochemical data, including trends in radon concentrations at some continuous monitoring stations, are presented by ciotoli et al. [2016-this issue] and cannelli et al. [2016-this issue]. these studies all explore different aspects of the earthquake, providing an overview of the first interpretations and the available multiparametric data sets rapidly collected and analyzed during the seismic crisis. finally, this issue concludes with a few contributions about the dissemination of information through popular social media communication channels [pignone et al., 2016a-this issue; pignone et al., 2016b-this issue; musacchio and piangiamore, 2016a-this issue; musacchio et al., 2016a-this issue]. although these papers are not strictly scientific papers, they highlight the relevance of rapid and exhaustive dissemination of the scientific information to population. most of the contributions presented in this issue, independent of their principal subject, show the rapid reaction of the emergency groups right after the event. their organization was based and tuned on the experience gained during the previous seismic crises that struck the italian peninsula during the last two or three decades. the sismiko team rapidly installed a set of temporary seismographic stations [moretti et al., 2016-this issue], while the emergeo team [pucci et al., 2016-this issue] and others [aringoli et al., 2016-this issue; livio et al., 2016-this issue] provided data on the geological and geomorphological co-seismic effects observed at the topographic surface. measurements of the co-seismic crustal deformations are presented by the gps team that analyzed data from the available permanent gnss networks and installed a set of temporary geodetic stations on the existing benchmarks within a few hours of the mainshock. macroseismic data are presented by the quest group [azzaro et al., 2016-this issue], and other teams [zanini et al., 2016-this issue; galli et al., 2016-this issue; hofer et al., 2016-this issue] that surveyed and mapped the damage to hundreds of buildings, using the most recent macroseismic scales. rapid information on the extension of the area throughout which the earthquake was felt across italy was retrieved by online questionnaires [de rubeis et al., 2016-this issue]. side by side with these on-site activities, are described seismological data and products, such as the location and magnitude of seismic events [michele et al., 2016-this issue; marchetti et al., 2016-this issue], even for earthquakes of m<1.0, often hidden by the waveforms of larger seismic events. the huge number of earthquakes recorded by the seismic networks during the seismic sequence required advanced management of the large amount of digital waveform data, including preliminary analysis and careful storage [pintore et al., 2016-this issue]. the annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7373 3 figure 1. paper classification published in this volume, based on their main topics (ou, outreach; gc & hy, geochemistry and hydrology; sg & rs, space geodesy and remote sensing; ge, geology; ms & en, macroseismic and environment; se, seismology). seismic moment tensors were calculated using different methods by multiple independent groups [pondrelli et al., 2016-this issue; scognamiglio et al., 2016-this issue], providing key information on the size of the earthquakes, the mw moment magnitude and the focal mechanisms, to allow extended fault modelling. the latter is derived from a combination of different data sources including seismological [magnoni and casarotti, 2016-this issue; saccorotti et al., 2016-this issue], geodetic [cheloni et al., 2016-this issue; avallone et al., 2016-this issue] and insar [bignami et al., 2016-this issue]. the directivity of the earthquake propagation during the rupture is investigated by spagnuolo et al. [2016-this issue], while montuori et al. [2016-this issue], meletti et al. [2016this issue] and murru et al. [2016-this issue] focus on the seismic hazard assessment and the b-value of the seismic sequence. seismological data also focus on the features of the ground shaking and related site effects. in this regard, the strong motion data presented by cultrera et al. [2016-this issue], massa et al. [2016-this issue], ladina et al. [2016-this issue], pischiutta et al. [2016-this issue], together with the shake-maps by faenza et al. [2016-this issue], represent key information for the interpretation of structural damage experienced by buildings that responded with different modes to ground shaking depending on their features and locations [gaudiosi et al., 2016-this issue; masi et al., 2016-this issue; caserta et al., 2016-this issue]. on this topic, iervolino at al. [2016-this issue] present preliminary engineering data while de silva et al. [2016-this issue] write on the response of shaking rigid bodies, to seismic waves. ground deformations, as estimated by analyzing data from the available continuous monitoring gps stations, were elaborated by different groups, providing a consensus solution on the amount and trend of horizontal and vertical ground displacement and locating the responsible faults at depth [cheloni et al., 2016-this issue]. high rate geodetic data (hr-gps), collected in the range 1-10 hz, were also used for the seismological analysis of the co-seismic ground movements [avallone et al., 2016-this issue]. gps data also supported the remote sensannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7373 4 ing observations of insar data that provided very high resolution images of the features and extension of continuous surface deformations in relation to surface faulting, from which were estimated different models of the buried faults [bignami et al., 2016-this issue]. the latter were also compared with the evidence of surface faulting reported by the emergeo team, to separate primary and secondary superficial co-seismc effects, or even gravitational motions [albano et al., 2016-this issue; aringoli et al., 2016-this issue; falcucci et al., 2016-this issue; bonini et al., 2016this issue; valensise et al., 2016-this issue]. finally, some papers describe the dissemination of earthquake information. this subject has become increasingly important in the recent years due to the large impacts that earthquakes have had on the media [musacchio and piangiamore, 2016-this issue]. particularly important is the role of science feeding the information system, using also the most popular social media, like twitter or blogs (i.e. ingvterremoti blog) [pignone et al., 2016b-this issue], and showing unprecedented images of wave propagation [casarotti et al., 2016-this issue] and gis maps [pignone et al., 2016-this issue]. iii. future perspectives in 1980, when the irpinia area was struck by an m 6.8 earthquake that caused 3000 deaths, 9000 injured and 100,000 homeless [bernard and zollo, 1989], it took several days to locate the earthquake epicenter and several months were required to understand the very first features of the earthquake. nowadays, 36 years after this destructive and tragic event, thanks to the great development of ground geophysical networks and the advent of spatial techniques and with the experience gained from previous seismic crisis in italy (i.e. the umbria-marche, 1997; molise, 2003; l’aquila, 2009; emilia, 2011; and finally the 2016 central italy seismic sequence) and elsewhere, the scientific community was prepared to face this new challenge and its actions allowed the collection of an unprecedented amount of multiparametric data. these represent the basis for new scientific advancements for the comprehension of earthquakes that will help to delineate improved monitoring strategies. in this regard, etas modelling [lombardi, 2016this issue] and probabilistic seismic hazard analysis (psha) [peruzza et al., 2016-this issue], are two of the most attractive techniques for the prediction of an aftershock sequence during a seismic crisis, while spatial data from artificial satellites, namely insar and gps, will improve the continuous monitoring of the movements of the earth’s surface during a seismic cycle. results presented in this volume are already enhancing our knowledge of the seismicity and seismic hazard of the italian region. acknowledgements the preparation of a special issue in such a short time has been a great challenge that would not have become a reality without the valuable contributions of the many reviewers who enthusiastically supported us. their work has been of the utmost importance, and for this reason we wish to sincerely acknowledge the colleagues who worked towards this goal. most of studies and activities described in this volume benefit from the support of the department of civil protection. we dedicate this volume to our beloved colleagues bruno de simoni and marco mucciarelli, who both passed away in 2016. bruno was a research director of the ing and later of the ingv. his long-lasting work focused on the first realization of the modern national seismic network. he also directed the sismos project for the recovery of historical seismograms and seismographs. marco was research director at the ogs and his activity mainly focused on seismic hazard. his studies provided great improvements on the knowledge of the historical and current seismicity of italy and surrounding regions. we are thankful to bruno and marco for their valuable work, always carried out with enthusiasm and cheerfulness. annals of geophysics, 59, fast track 5, 2016; 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hazard implications, annals of geophysics, 59, fast track 5; doi:10.4401/ag-7215. zanini, m.a., l. hofer, f. faleschini, p. zampieri, n. fabris and c. pellegrino (2016). preliminary macroseismic survey of the 2016 amatrice seismic sequence, annals of geophysics, 59, fast track 5; doi:10.4401/ag-7172. ------------------------ © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. c 0 n v e g n 1 e congressi le celebrazioni p e r il 75" anniversario d e l l ' i s t i t u t o geografico m i l i t a r e italiano i n o c c a s i o n e d e l l a c e l e b r a z i o n e p e r il 75" a n n i v e r s a r i o dell l l i tuto g e o g r a f i c o m i l i t a r e si è ten u l o a f i r e n z e , nei giorni dal 27 al 31 o t t o b r e 1 9 4 7 , un convegno s c i e n t i f i c o i n t e r n a z i o n a l e , d u r a n t e il q u a l e v e n n e r o discussi i seguenti t e m i : 1) c o m p e n s a z i o n e d ' i n s i e m e d e l l e reti g e o d e t i c h e e u r o p e e ; 2 ) t r i a n g o l a z i o n e ae rol'o log r a m i n e t r i c a ; 3 ) r e c e n t i progressi nel calc o l o degli o b b i e t t i v i f o t o g r a f i c i ; 1) n o r m a l i z z a z i o n e nel c a m p o dell ' o t t i c a . s i e b b e r o p u r e p u b b l i c h e riun i o n i d e l l a c o m m i s s i o n e g e o d e t i ca i t a l i a n a e del c e n t r o studi per la g r a v i m e t r i a c l ' i o t a s i a . al convegno — c h e e b b e a p r e s i d e n t e il p r o f . g i n o c a s s i n i s . d i r e t t o r e del p o l i t e c n i c o di m i l a n o , p r e s i d e n t e d e l l a c o m m i s s i o n e g e o d e t i c a i t a l i a n a , e a s e g r e t a r i o il p r o f . a n t o n i o m a r u s s i , ingegnere g e o g r a f o d e l l ' i s t i t u t o g e o g r a fico m i l i t a r e e r a n o l a r g a m e n t e r a p p r e s e n t a t e i s t i t u z i o n i scientific h e n a z i o n a l i ed e s t e r e . si e b b e r o r a p p r e s e n t a n z e di 23 n a z i o n i es t e r e . p a r t e c i p a r o n o i n o l t r e al conv e g n o il p r o f . p i e r r e t a r d i , seg r e t a r i o d e l l ' a s s o c i a z i o n e i n t e r n a z i o n a l e di g e o d e s i a , i a m m i r a g l i o c h e s t e r l . n i c h o l s , per il b u r e a u 1 1 v d r o g r a p h i q u e i n t e r n a t i o n a l , ed il c o l . f l o y d w o o d w o r t h h o u g h . c a p o d e l l a d i v i s i o n e g e o d e t i c a d e h ' a r m v m a p s e r v i c e degli s . i . a . il c o n v e g n o fu i n a u g u r a t o il 27 o t t o b r e con un saluto del s i n d a c o di f i r e n z e e un d i s c o r s o i n t r o d u t tivo del p r o f . c a s s i n i s . il g e n . f . g e l i d i , d i r e t t o r e d e l l ' i s t i t u t o g e o g r a f i c o m i l i t a r e , i l l u s t r ò q u i n di il c o n t r i b u t o p o r t a t o d a l l ' i s t i tuto a l l a soluzione di alcuni imp o r t a t i l i p r o b l e m i g e o d e t i c i . le sedute a sezioni r i u n i t e del c o n v e g n o i n i z i a r o n o il 28 ottob r e . f u r o n o t r a t t a t e q u e s t i o n i di « t r i a n g o l a z i o n e a e r o f o t o g r a i n m e t r i c a » ( p r o l ' f . d o r è e s o l a i n i , sig. s a n t o n i e d o t t . b a r l o r c l l i ) . con i n t e r v e n t i del p r o f . zelicidel p o l i t e c n i c o di z u r i g o e del p r o f . cassinis, il (piale a u s p i c ò la form a z i o n e di u n ' o r g a n i z z a z i o n e int e r n a z i o n a l e d i s t u d i e di esper i e n z e nel c a m p o della t r i a n g o l a zione a e r e a . 11 2 9 o t t o b r e -i passò a l l a diseussione sul t e m a d e l l a c o m p e n sazione d ' i n s i e m e d e l l e reti geod e t i c h e e u r o p e e . d o p o una trattazione p a n o r a m i c a del t e m a , seg u i r o n o le r e l a z i o n i dei p r o l i . b o a g a , m a r u s s i e m a r c a n t o n i . s u l l o stesso a r g o m e n t o si e b b e r o convegni e congressi 1 2 7 c o m u n i c a z i o n i del p r o f . r u n e di s t o c c o l m a e dell a m m i r a g l i o slal u n i t e n s e n i c h o l s — il q u a l e oss e r v ò c h e con i m o d e r n i m e t o d i e l e t t r o n i c i i n t r o d o t t i n e l l a navigaz i o n e ( r a d a r , e c c . ) le e s i g e n z e di p r e c i s i o n e si sono t a l m e n t e a c c r e s c i u t e , c l i c una u n i f i c a z i o n e geod e t i c a m o n d i a l e si i m p o n e — dei proli', b o a g a e p a e e l l a c d e l f i n g . p a i o l i . 1 lavori sul p r i m o tema c o n t i n u a r o n o il g i o r n o 3 0 con c o m u n i c a z i o n i d e i proli'. m a n i — i e h r i l o v di s o f ì a , del c o l . limigli deli l i . s . a n n ) ' ma]) s e r v i c e , del p r o f . b o a g a , d e l l ' i n g . d e campo» g u e r c i a di m a d r i d , del p r o f . t a r di e del p r o f . c a s s i n i s , c h e riass u m e la d i s c u s s i o n e e d i c h i a r a c h i u s o il c o n v e g n o . l e r i u n i o n i p u b b l i c h e d e l l a c o m m i s s i o n e g e o d e t i c a i t a l i a n a e del c e n i l o s t u d i p e r la g r a v i m e t r i a e l t s o s l a s i a , f u r o n o presied u t e dal p r o f . c a s s i n i , si e b b e r o c o m u n i c a z i o n i dei proli". d o r è , c a s s i n i s , t a r d i . d i r e t t o r e del b u reau c e n t r a l d e l l ' a s s o c i a t i m i int e r n a t i o n a l de g e o d e s i e , m a i l e r d e l l a t c c h n i s c h e h o c h s c h u l e di v i e n n a , m a r u s s i . t e n a n i , s o l a i n i , p a e e l l a , m o r e l l i e b o a g a . i n f i n e il g e l i . g e l i d i r i n g r a z i a il p r o f . cassinis e tutti i c o n v e n u t i per il l o r o c o n t r i b u t o alla c e l e b r a z i o n e del 75" a n n i v e r s a r i o di vita d e l l ' i s t i t u t o g e o g r a f i c o m i l i t a r e . i ila r e l a z i o n e s u l l e discussioni dei due t e m i di o t t i c a , a v v e n u t e al c o n v e g n o ( p a r a l l e l a m e n t e a q u e l l e sui t e m i di g e o d e s i a e f o t o g r a m m e t r i a ) si trova nel b o l l e t t i n o d e l l ' a s s o c i a z i o n e o t t i c a i t a l i a n a . 1 , " i s t i t u t o n a z i o n a l e di g e o f ì s i ca fu r a p p r e s e n t a l o al c o n v e g n o dal suo d i r e t t o r e , p r o f . a n l o n i n o l o s u r d o . adg vol5 n02 di fi 417_429.pdf annals of geophysics, vol. 45, n. 2, april 2002 417 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets boris di fiore (1), paolo mauriello (2), dario monna (2) and domenico patella (3) (1) dipartimento di geofisica e vulcanologia, università «federico ii», napoli, italy (2) istituto per le tecnologie applicate ai beni culturali, consiglio nazionale delle ricerche, roma, italy (3) dipartimento di scienze fisiche, università «federico ii», napoli, italy abstract we present the results of the application of the tensor resistivity method to the assessment of the state of conservation of some architectonic features of the s. giovanni a carbonara monumental monastery (naples, italy) and to the recognition of buried remains in the archaeological site of the roman port of claudius at fiumicino (rome, italy). the 3d tomographic approach, based on the concept of resistivity anomaly source occurrence probability, has been used for the analysis of the invariant parameter related to the trace of the determinant of the apparent resistivity tensor. 1. introduction high-resolution acquisition techniques favour the application of 3d tomographic imaging in non-invasive microgeophysical testing for monument preservation and in nearsurface geophysical prospecting for archaeology. in particular, probability tomography is gaining increasing importance for its objectivity and reliability. it has the notable advantage of not requiring any a priori assumption about the structures to be imaged. the theoretical principles of 3d probability tomography were originally established for the self-potential method (patella, 1997a,b) and then extended to the geoelectrical method for both the scalar (mauriello et al., 1998; mauriello and patella, 1999) and tensor (mauriello and patella, 2001) apparent resistivity acquisition modes. the aim of this paper is to illustrate the results of the application of the tensorial resistivity probability tomography (trpt) method to the assessment of the conservation state of a few architectural features in the san giovanni a carbonara ancient monastery (naples, italy) and the recognition of a marble cargo sunk in the roman port of claudius at fiumicino (rome, italy). 2. methodological synopsis in performing a resistivity survey on a flat surface s, identified by a reference (x, y) plane, the adoption of two current bipoles (fig. 1) allows mailing address: prof. domenico patella, dipartimento di scienze fisiche, università «federico ii», complesso universitario monte s. angelo, via cintia, 80126 napoli, italy; e-mail: patella@na.infn.it key words applied geoelectrics – apparent resistivity tensor probability tomography – cultural heritage 418 boris di fiore, paolo mauriello, dario monna and domenico patella an apparent resistivity tensor (a) to be determined in terms of the x and y components of the electric field e i and the primary current density j i , with i = 1,2 indicating the active bipole (bibby, 1977). in order to obtain structural information independent of the electric field direction and the location of the exciting current bipole, tensor invariants p can be deduced from (a). we consider the invariant p related to the trace of (a) (bibby, 1986). the trpt method is based on an integral elaboration of the whole dataset of the trace of (a), and aims at imaging the most probable location of the sources of the observed apparent resistivity anomalies through the analysis of a resistivity anomaly occurrence probability (raop) function (mauriello and patella, 2001). the raop function is formally derived from the partition of the expression of the total geoelectrical signal power on the surface into a set of elementary contributions. the investigated volume is, in fact, divided into a set of q cells with sufficiently small volume, and each cell is assumed to contribute to the observed total signal. the 3d trpt method consists in a crosscorrelation between a scaled version of the collected data set p = p – p 0 and the scanner function is the tensor invariant for a reference homogeneous half-space and is the frechet derivative of p0 for a perturbation of the half-space resistivity in the qth cell (q = 1,2,.....,q). the raop function q is defined as =q qp p0 0 p q0 fig. 1. a schematic depiction of the high-resolution 2d acquisition procedure for the determination of the matrix elements of the apparent resistivity tensor. = c pq q qs 419 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets fig. 2a,b. a synthetic three-prism resistivity model: a) plan view; b) section view. a b 420 boris di fiore, paolo mauriello, dario monna and domenico patella where the normalisation factor c q is given by each q value is interpreted as the probability with which a resistivity increment ( q > 0) or decrement ( q < 0) in the qth cell, with respect to the selected reference resistivity, can be considered responsible for the whole observed apparent resistivity data set. an elementary cell with unitary strength is used to scan the whole target-space in order to search where resistivity variations with respect to the chosen reference resistivity are located in probabilistic sense. before showing the results of the application of the trpt method to real cases, we illustrate the whole procedure by examining a synthetic model. the model consists of three prismatic blocks with resistivities of 5, 20 and 105 m, respectively, buried in a uniform half-space with a resistivity of 100 m. we assume that the 105 m block represents the target and that the two other blocks represent shallow disturbing bodies. figure 2a,b shows a plan and a section view of the three-prism model in the upper and lower pictures, respectively. a 1 b 1 and a 2 b 2 is a pair of orthogonal current bipoles used to simulate an apparent resistivity tensor survey. a standard finite element program has been edited to draw the synthetic response due to this model. each current bipole is 19 m long, and a passive 0.5 m long dipole is assumed to be moved at steps of 0.5 m along crossed profiles spaced 0.5 m apart, within a survey area of 9 × 9 m2. a total of 361 apparent resistivity data were processed to obtain the trpt simulation. the 100 m background resistivity was taken as the uniform resistivity of the reference homogeneous half-space. the results of the trpt imaging are depicted in fig. 3. the top slice in fig. 3 shows the anomaly map of the fig. 3. tomographic images of the resistivity anomaly occurrence probability for the three-prism model of fig. 2a,b. =c pq q 2 21 s s . 421 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets invariant p, and the following slices show the relative trpt imaging results. the trpt slices display two weak shallow negative raop nuclei in correspondence with the two disturbing bodies, and a deeper, more intense positive raop nucleus in the range 1-3 m, closely in correspondence with the position of the target body. the probability image of the resistive prism appears much better delineated than the images of the two conductive bodies, due to its greater resistivity contrast with the background material. 3. examples of application we show now the results of the application of the 3d trpt method to the study of some architectural features in the monumental monastery of san giovanni a carbonara (sgc), located in the historical centre of naples, and to the recognition of a marble cargo sunk in the ancient roman port of claudius, located in an area outside the terminals of the international airport of fiumicino (rome). 3.1. san giovanni a carbonara at naples the construction of the sgc complex started in 1344, based on a plan that has been expanded and redrawn in the course of the centuries, before reaching the present structure (fig. 4). the monastery was definitively locked in 1799. then, the building was used to house military hospitals, boards and barracks. in recent times, it has also housed offices of the public attorney, which have been now completely removed. all these events have caused a complex and articulated architectural bedding, as well as a high deterioration of the primal, very valuable structures. in the frame of a total restoration project led by the neapolitan authority for architectonic and environmental property, geoelectrical (ge) analyses were included within the preliminary studies aimed at assessing the state of conservation of the monument. the sgc monastery has, in fact, been retained as an ideal laboratory to test accuracy and reliability of scaled down ge methods applied to architectonic heritage, using a new type of absolutely non-invasive capacitive electrodes. in particular, the aims of the ge tests in the sgc complex were to help to diagnose the conservation state of the stone slab facing of one of the pillars supporting the arcades of the balcony at the first floor of the 16th century pietra serena cloister, and to locate a blind valuable architectural feature along a plastered portion of wall at the first floor of the same cloister. 3.1.1. pietra serena cloister: the experiment on the pillar the ge measurements were carried out on two adjacent faces of a pillar of the pietra serena cloister, inside an area of 1 × 1 m2 on each face. a 128 hz alternating current was injected using two perpendicular energizing bipoles, and the potential differences were measured between any two adjacent points spaced 0.5 cm inside a 2d regular grid (fig. 1). the potential differences were measured in both the parallel and perpendicular directions with respect to the bipole axis, for both bipole orientations, in order to estimate the matrix elements of the apparent resistivity tensor. in each survey face, the average value of all the determinations of the tensor invariant has been assumed as the reference value for the computation of the raop function in the trpt imaging. the pillars have an original facing consisting of slabs of pietra serena, a typical tuscan sandstone, successively covered by grout beds, and an internal structure made of mortars and blocks of various materials. figures 5a and 6a show the contour maps of the selected tensor invariant, obtained on the two adjacent faces. the trpt images were drawn on a sequence of vertical slices up to a penetration of 20 cm from the freeface (figs. 5b and 6b). the first remarkable feature appearing in the tomographies of figs. 5b and 6b is the presence of well defined elongated positive raop spots, which show the highest values on the slice at 12.5 cm penetration. these alignments are interpreted as portions of vertical and horizontal contact zones between adjacent slabs of pietra serena. the occurrence of positive raop values 422 boris di fiore, paolo mauriello, dario monna and domenico patella fig. 4. structural plan of the san giovanni a carbonara monumental monastery, located in the historical centre of naples, italy. 423 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets fig. 5a,b. the tensor resistivity probability tomography applied to the first face of the selected pillar in the pietra serena cloister at san giovanni a carbonara (naples, italy). a) the surface map of the invariant parameter related to the trace of the determinant of the apparent resistivity tensor. b) a sequence of tomographic slices at increasing penetration. a b b 424 boris di fiore, paolo mauriello, dario monna and domenico patella fig. 6a,b. the tensor resistivity probability tomography applied to the second face of the selected pillar in the pietra serena cloister at san giovanni a carbonara (naples, italy). a) the surface map of the invariant parameter related to the trace of the determinant of the apparent resistivity tensor. b) a sequence of tomographic slices at increasing penetration. a b b 425 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets is explained by admitting a breaking up of the pasting mortars. the local increase in resistivity is, in fact, fully compatible with the presence of void fissures between adjacent slabs. the second remarkable feature is the presence of either elongated or rounded negative raop spots, which show the lowest values starting from 15 cm up to 20 cm, at least. these negative spots can be explained as due to condensation of humidity just behind the pietra serena slabs, or the presence of alteration zones of the same slabs, or a combination of both factors. 3.1.2. pietra serena cloister: the experiment on the wall the ge survey was carried out on an almost totally plastered portion of wall at the first floor of the pietra serena cloister inside an area of 2 × 2 m2. this survey had been required in order to locate one of the external edges of the caracciolo di vico chapel, inglobed in the wall of the pietra serena cloister during the renaissance. it consists of a block of piperno, a typical welded, perlitic pyroclastic stone, inserted into blocks of neapolitan yellow tuff, constituting the pietra serena cloister wall all around. we used the same acquisition procedure as in the previous example (fig. 1). figure 7a shows the contour map of the selected tensor invariant obtained over the wall portion. the trpt images have again been drawn on a sequence of vertical slices up to a penetration of 20 cm from the freeface (fig. 7b). the first remarkable feature appearing in the tomographies of fig. 7b is the presence of a well defined squared sequence of negative raop spots in the central lower sector of the slices, which reach the highest values on the slice at 20 cm of penetration. this squared sequence is interpreted as the signature of the boundary of a piperno block of about 60 cm of height and 90 cm of width. the contact zones of the piperno block with the surrounding yellow tuff wall are, in fact, preferential paths for humidity to rise by capillarity from the ground and cause a local decrease of resistivity. another negative raop spot appears in the central top part of the slices in a zone where the wall had been exposed and presented clear signs of humidity along an injection of mortar seaming two adjacent yellow tuff blocks. the second remarkable feature is the presence of a continuous positive raop anomaly at the right-hand margin of the slices, from top to bottom. the presence of positive raop values in this area can be explained by admitting a local compactness of the yellow tuff wall. 3.2. port of claudius at fiumicino (rome) during the construction of the international airport of fiumicino (rome), marble blocks and planking were dug out suggesting the presence of a cargo transported by one of several boats, which possibly sunk near the right mole of the roman port of claudius. a geophysical intervention was required to help identify the sunken ships, through the localisation of the cargo, now buried in the sediments piled up in the area due to a sea water regression of about 3 km (see fig. 8). among the various geophysical methods, the ge method was deemed the best suited to differentiate the container (sediments) from the content (transported lapidary material) buried at approximately 2 m of depth. again, the same technique depicted in fig. 1 was adopted with 0.5 m long receiving dipoles. figure 9a shows the survey map of the selected apparent resistivity tensor invariant. in this map, which was the largest area that could be prospected, the black circles indicate the points where some archaeological material was dug out by chance inside a trench of 3 × 3 m2 cross-section, which was excavated along the longer median axis of the surveyed rectangle and then filled with reworked sediments. in particular, the circle at the left-hand side indicates the point where some marble remnants were found, while the circle on the right-hand side indicates the point where planking was dug out. the most notable features appearing in the tomographies of fig. 9b are the presence of a sequence of negative nuclei along the main axis of the surveyed rectangle, and a small positive nucleus located about the lower left-hand corner of the area. both anomalies reach the highest absolute probability values around 3 m of depth. 426 boris di fiore, paolo mauriello, dario monna and domenico patella fig. 7a,b. the tensor resistivity probability tomography applied to a portion of wall in the pietra serena cloister at san giovanni a carbonara (naples, italy). a) the surface map of the invariant parameter related to the trace of the determinant of the apparent resistivity tensor. b) a sequence of tomographic slices at increasing penetration. a b3 b2 b1 427 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets fig. 8a,b. the archaeological site of the roman port of claudius at fiumicino (rome, italy). the rectangle indicates the area prospected by the geoelectrical survey. we interpret the sequence of negative nuclei as a trace of the trench, whose refilling sediments appear less resistive than around because of major water content due to increased permeability. the positive nucleus can instead be interpreted as a possible site where marble remnants can still be dug out. 4. conclusions the examples which we have dealt with in the previous section show how effective the imaging capability of the trpt method is in outlining buried features for architectonic and archaeological purposes. trpt is indeed a powerful combination of two specific improvements made in modern high-resolution geoelectrical prospecting. the first step was the realisation of a non-invasive survey technique aimed at highlighting all the diagnostic properties of the apparent resistivity tensor, using a pair of independent current sources and measuring the two horizontal components of each generated electric field. the second step was the theoretical formulation of the probability tomography method for tensorial geoelectrics and the introduction of the raop function as a suitable tool for imaging the most probable locations of the physical targets, which are responsible for the observed matrix elements of the apparent resistivity tensor. a remarkable aspect of this study is that the resistivity signatures have been disclosed only 428 boris di fiore, paolo mauriello, dario monna and domenico patella b a fig. 9a,b. the tensor resistivity probability tomography applied to a portion of land in the archaeological site of the roman port of claudius at fiumicino (rome, italy). a) the surface map of the invariant parameter related to the trace of the determinant of the apparent resistivity tensor. the two black dots over the map indicate the places where remnants were dug out. b) a sequence of tomographic slices at increasing penetration. 429 examples of application of tensorial resistivity probability tomography to architectonic and archaeological targets from a probabilistic viewpoint, which conforms to the inherently uncertain nature of the geophysical interpretation process. the trpt approach is the consequence of the intrinsic nonunivocity of the geophysical solution, as many models provide, within the measuring accuracy, equivalent responses which cannot be discriminated without external constraints. references bibby, h.m. (1977): the apparent resistivity tensor, geophysics, 42, 1258-1261. bibby, h.m. (1986): analysis of multiple-source bipolequadripole resistivity surveys using the apparent resistivity tensor, geophysics, 51, 972-983. mauriello, p. and d. patella (1999): resistivity anomaly imaging by probability tomography, geophys. prospect., 47, 411-429. mauriello, p. and d. patella (2001): introduction to multiplesource resistivity probability tomography, geophysics (submitted). mauriello, p., d. monna and d. patella (1998): 3d geoelectric tomography and archaeological applications, geophys. prospect., 46, 543-570. patella, d. (1997a): introduction to ground surface self-potential tomography, geophys. prospect., 45, 653-681. patella, d. (1997b): self-potential global tomography including topographic effects, geophys. prospect., 45, 843-863. annals of geophysics, 65, 2, 2022; doi:10.4401/ag-8835 o p e n acc e s s 1 special issue epos a research infrastructure in solid earth: open science and innovation preface this special issue of annals of geophysics is dedicated to present and discuss a research infrastructure whose goal is integrating and providing access to solid earth science data, research products and services as well as promoting physical access to research facilities. this research infrastructure is the european plate observing system (epos, www.epos-eu.org). the idea of the special issue was matured considering the centrality of research infrastructures in the research process, promoting open science and enabling new forms of research and collaborations by providing solutions to share and distribute data and scientific products. research infrastructures play a key role in establishing and consolidating interactions between research organizations and academia to govern and manage virtual access to scientific data and physical access to research facilities. moreover, by providing access to scientific data, products, and information they represent a collaborative framework to foster innovation directly contributing to scientific progress and education in specific scientific domains. the epos research infrastructure resulted to be an excellent candidate for the special issue because of its mission to establish and maintain sustainable and long-term access to research data, services, and facilities for the whole solid earth science domain. moreover, epos has integrated the diverse national research infrastructures under a common federated framework, thus ensuring a pan-european dimension and a global perspective for its challenge to long-term sustainability. managing research infrastructures is, indeed, a key challenge to tackle for many research organizations both at national and international level. sharing the experiences of a pan-european infrastructure like epos is certainly of interest for scientists, data providers, as well as for those committed to operate services for science and society. the special issue contains several papers aimed at presenting the general aspects of the epos research infrastructure, such as its architecture focusing on the adopted solutions for the sustainable operation of the epos services and the fair1 data management, together with other specific papers describing the data and service provision of the different research communities participating to the epos integration plan. overall, the special issue offers both a description of the epos research infrastructure and a presentation of the contents of its delivery framework. in other words, these articles allow readers to understand the epos architecture and the adopted solutions to operate its delivery framework; likewise, they describe the data and service provision to users and stakeholders, fostering the aware usage of the services to access, analyse, process, and use both individual and aggregated data sets. the special issue is divided in two volumes of annals of geophysics, namely volumes 2 and 3 of the journal, published in april and june 2022, respectively. the papers included in this special issue of annals of geophysics dedicated to the epos research infrastructure also represent an interesting contribution to show and discuss how different research communities belonging to solid earth science have joined resources, skills and experiences to foster the culture of data sharing coherently with fair data management, thus contributing to open science. the special issue allows readers to understand the governance of the data lifecycle coordinated by the thematic communities (tcs), in charge of ensuring quality control and the adoption of shared standards for data and metadata, which have been established in epos for this specific 1 fair acronym stands for findable, accessible, interoperable, re-usable. http://www.epos-eu.org 2 reason. the thematic communities integrate and make available both data that are already accessible through community services and data that would not be available without the new services established in epos. it is important to emphasize that in epos the scientific community is the key enabler of innovation in data management and interoperability, effectively supporting open science. this makes the special issue of relevance for a broad readership. in our opinion, there are several reasons for publishing a special issue dedicated to the epos research infrastructure, from discovering the contents of the epos data and service provision to understanding the new challenges to be tackled for the sustainable operation of a research infrastructure and management of data and scientific products. in this framework, the special issue can be a further contribution to foster multi-disciplinary research in solid earth science and cross-disciplinary investigations in environmental science. the special issue represents an effective presentation of the epos science case described through the multidisciplinary contents of its delivery framework, spanning from seismology, geodesy, geology, volcanology, and geomagnetism to the chemistry and physics governing the earth’s dynamics. as guest editors of this special issue, we have appreciated the motivated and enthusiastic approach shown by the epos community in sharing their experiences and promoting in the culture of data sharing and open science. list of papers in the first volume of the special issue cocco, m., c. freda, k. atakan, d. bailo, k. saleh contell, o. lange, j. michalek (2022). the epos research infrastructure: a federated approach to integrate solid earth science data and services, ann. geophys., this volume. saleh contell, k., k. karlzén, m. cocco, h. a. pedersen, k. atakan, d. bailo, o. lange, d. mercurio, g. maracchia, a. sangianantoni, d. piras, m. fredella and c. freda (2022). long-term sustainability of a distributed ri: the epos case, ann. geophys., this volume. bailo d., k. g. jeffery, k. atakan, l.trani, r. paciello,v. vinciarelli, j. michalek, a. spinuso (2022). data integration and fair data management in solid earth science, ann. geophys., this volume. atakan, k, m. cocco, b. orlecka-sikora, r. pijnenburg, j. michalek, c. rønnevik, d. olszewska, b. górka-kostrubiec, m. r. drury (2022). national epos initiatives and participation to the epos integration plan, ann. geophys., this volume. marti, m., f. haslinger, s. peppoloni, g. di capua, h. glaves, i. dallo (2022). addressing the challenges of making data, products, and services accessible: an epos perspective, ann. geophys., this volume. haslinger, f., r. basili,r. bossu, c. cauzzi, f. cotton, h. crowley, s. custódio, l. danciu, m. locati, a. michelini, i. molinari, l. ottemöller, s. parolai (2022). coordinated and interoperable seismological data and product services in europe: the epos thematic core service for seismology, ann. geophys., this volume. wessels, r., g. ter maat, e. del bello, l. cacciola, f. corbi, g. festa, f. funiciello, g. kaviris, o. lange, j. lauterjung, r. pijnenburg, g. puglisi, d. reitano,c. rønnevik, p. scarlato, l. spampinato (2022). transnational access to research facilities: an epos service to promote multi-domain solid earth sciences in europe, ann. geophys., this volume. babeyko, a., s. lorito, f. hernandez, j. lauterjung, f. løvholt, a. rudloff, m. sørensen, a. androsov, i. aniel-quiroga, a. armigliato, m. a. baptista, e. baglione, r. basili, j. behrens, b. brizuela, s. bruni, m. d. cambaz, j. cantavella-nadal, f. carrilho, i. chandler, d. chang-seng, m. charalampakis, l. cugliari, c. denamiel, g. güney dogan, g. festa, d. fuhrman, a.-a. gabriel, p. galea, s. j. gibbons, m. gonzalez, l. graziani, m.-a. gutscher, s. harig, h. hebert, c. ionescu, f. jalayer, n. kalligeris, u. kânoğlu, p. lanucara, j. macías sánchez, s. murphy, ö. necmioğlu, r. omira, g. a. papadopoulos, r. paris, f. romano, t. rossetto, j. selva, a. scala, r. tonini, k. trevlopoulos, i. triantafyllou, r. urgeles, r. vallone, i. vilibić, m. volpe, a. c. yalciner (2022). towards the new thematic core service tsunami within the epos research infrastructe, ann. geophys., this volume. g u e s t e d i to r s: massimo co cco, istituto nazionale di geofisica e vulcanologia, rome, italy paola m o n to n e, istituto nazionale di geofisica e vulcanologia, rome, italy laura_jgl testo inserito eliminato “2” laura_jgl testo inserito eliminato “2” laura_jgl testo inserito eliminato “6” laura_jgl testo inserito eliminato “13” the august 24th 2016 accumoli earthquake: surface faulting and deep-seated gravitational slope deformation (dsgsd) in the monte vettore area annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 1 the august 24th 2016 accumoli earthquake: surface faulting and deepseated gravitational slope deformation (dsgsd) in the monte vettore area domenico aringoli, piero farabollini, marco giacopetti, marco materazzi, silvia paggi, gilberto pambianchi, pietro paolo pierantoni, eugenio pistolesi, alan pitts, emanuele tondi marco.materazzi@unicam.it geology division, school of sciences and technology, university of camerino (italy) abstract on august 24th 2016 a mw=6.0 earthquake hit central italy, with the epicenter located at the boundaries between lazio, marche, abruzzi and umbria regions, near the village of accumoli (rieti, lazio). immediately after the mainshock, this geological survey has been focused on the earthquake environmental effects related to the tectonic reactivation of the previously mapped active fault (i.e. primary), as well as secondary effects mostly related to the seismic shaking (e.g. landslides and fracturing in soil and rock).this paper brings data on superficial effects and some preliminary considerations about the interaction and possible relationship between surface faulting and the occurrence of deep-seated gravitational slope deformation (dsgsd) along the southern and western slope of monte vettore. i. introduction on august 24th 2016 a mw=6.0 earthquake struck a vast area in central italy causing many fatalities (297) and lot of damage to houses and infrastructures. the epicentre of the mainshock was located near the village of accumoli (rieti) and thousands of aftershocks continuing to shake the area since until the end of september 2016, when we are writing this paper (for details about the seismic record of the sequence, please see the reports available on this web site: http://terremoti.ingv.it/it/). the focal mechanism of the mainshock and the distribution of the aftershocks suggest the activation of a nnw-sse oriented and wsw dipping normal fault, with a minor left-lateral component of motion. immediately after the main event and also in the following days, the geological survey was carried out around the epicentral area and along the previously mapped active faults (cello et al., 1997: galadini and galli, 2000; tondi and cello, 2003; boncio et al., 2004). the focus of the survey was to detect both the efhttp://terremoti.ingv.it/it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 2 fects related to the reactivation of the active faults (i.e. primary) and the seismic shaking (e.g. landslides and fracturing in soil and rock), generally classified as secondary effects. figure 1 – a) geological and structural sketch of the study area with location of the main epicenters (updated september 25th 2016 – source ingv): 1) quaternary continental deposits; 2) mainly arenaceous-pelitic bedrock; 3) mainly calcareous and marly calcareous bedrock; 4) monte vettore fault system; 5) main dip-slip and transverse faults; 6) main thrusts; 7) main dsgsd’s and large landslides; 8) trace of cross-section (see below). b) schematic cross section: 1) mainly calcareous formations; 2) marly calcareous formations; 3) calcareous and marly calcareous formations; 4) arenaceous-pelitic formations annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 3 this paper presents field data of the superficial effects observed in the area of monte vettore, where both an active fault and a deep-seated gravitational slope deformation (dsgsd) was previously mapped (cello et al., 1997; aringoli et al., 2010a; pierantoni et al., 2012). moreover, some preliminary considerations about the interaction and the possible relationship between the characterized primary and secondary effects are proposed. ii. geological and seismic framework the castelluccio basin is an intramontane depression, located in the central apennines, and filled by pleistocene to holocene fluviallacustrine deposits; bedrock units are instead represented by limestone and pelagic marls of jurassic to miocene age (figure 1). the main geomorphological modeling of the landscape started during the late pliocene, when arid or sub-tropical humid climatic conditions, favorable to planation processes, created a “paleo”landscape with gentle relief. the subsequent tectonic phase, active since the early-middle pleistocene until now, is characterized by dip and oblique-slip faults and strong uplift. these processes interrupted and dissected the previous landscape, forming the tectonic depression, same as the other nearby basins (colfiorito, norcia, cascia, leonessa, ecc.) within the central apennines (aringoli et al., 2012, 2014 and references within.). the high relief which was generated, together with the stratigraphic setting, produced a very intense morphodynamic activity. this condition led to the significant remodeling of the previous landscape, also activating large landslides and deepseated gravitational slope deformations (dsgsd’s) (aringoli et al., 1996; aringoli et al., 2010b); some of these, as in the ussita and casali areas, have shown evidences of reactivation during the present seismic sequence and are presently being monitored (figure 2). figure 2 – the dsgsd of ussita; data about the reactivation are currently being acquired concerning the seismicity, two main historical and instrumental destructive earthquakes, january 14th, 1703 (x mcs, mw=6.6) and september 19th, 1979 (ms = 5.9, focal depth of 6-8 km) have been documented in the norcia area, even though many others have occurred in time (rovida et al., 2016). fault data and earthquake focal mechanisms show a predominant ne-sw extension, but strike-slip and even reverse mechanisms have also been determined (cello et al., 1997). these historical earthquakes, as the recent one, have been associated to the activity of a coherent and interacting active fault system (the central apennines fault system in cello et al., 1997) running in a nnw-sse direction from colfiorito to l’aquila. this fault system (figure 3) has also annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 4 been interpreted as the superficial expression of several deep seated seismogenic faults (tondi and cello 2003). figure 3 the central apennines fault system and relative interpreted seismogenic sources (source: tondi and cello, 2003) iii. the monte vettore active fault the main active tectonic element in the monte vettore seismic area is a well preserved fault scarp exposed in bedrock, for a length of about 7 km along the western slope of monte vettore (figure 1). the fault scarp branches into two splays: the eastern one strikes n150° (named in the official topographyc map of igm as “cordone del vettore”), whereas the western fault branch maintains a roughly constant northsouth trend. along the eastern fault scarp, kinematic indicators (striae) from shear surfaces in bedrock record a main left-lateral strikeslip motion along the fault planes oriented nnw-sse and a normal behaviour, with a minor sinistral component of slip, along the fault planes oriented nw-se. on figure 4 is shown the pattern of coseismic surface faulting along the monte vettore fault based on the results of our survey. figure 4 – pattern of ground ruptures observed along the monte vettore fault. see text for details. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 5 in particular, a set of ground ruptures was surveyed on the southern slope of monte vettoretto in correspondence to the intersection between the monte vettore fault and the sp34 (province road). these en-echelon ruptures mainly strike between nnw-sse, with a slight left-lateral horizontal component. they generally affect colluvium and soil, whereas midway up the slope, ground ruptures are very close to the bedrock fault plane. this portion of the rupture is continuous for about 1.7 km. continuing uphill to the nw, even more evident ground ruptures are found in correspondence to the nw-se trending fault segment running just below the top of cima redentore (namely the “cordone del vettore” fault). here the ruptures have been surveyed in detail for about 2.4 km. the end-to-end length of this rupture zone is greater than 4.5 km. according to the preliminary report provided by several authors (http://www.isprambiente.gov.it/files/notizieispra/notizie-2016/sisma-italiacentrale/report_amatrice_en_2016_09_16.compres sed.pdf), ground ruptures are also present to the south of the sp34 forming a total length of surface ruptures of at least 5.3 km. along the sp34, where the fault crosses the road, repeated field surveys have clearly displayed a postseismic evolution of the rupture, whose vertical offset is continuing to increase with time. iv. the deep-seated gravitational slope deformation (dsgsd) of monte vettore at the top of monte vettore, around 2230m a.s.l., along the alignment punta di prato pulito–monte vettore, several trenches and double ridges, roughly e-w oriented, can be observed (figures 5 and 6a). all these features, showing lengths of more than 100m, width up to 4 meters and depth over than 1m and favoured by dip-slope attitude, were partially filled with debris and vegetated at the bottom. inside, crack and fractures around 20 cm deep and 5cm wide, and free-faces on bedrock, as a consequence of the compaction of the filling material during the earthquake, have been evidenced (figure 5). also the south-western slope is strongly deformed and shows a convex morphology with presence of small dips and rounded counterslopes, filled by debris and soil material; despite the lithology is the same, the strata attitude changes along the slope with respect the top of the relief. besides, compressive shear planes and small superficial landslides reactivated by the earthquake have been observed. figure 5 – fractures and cracks observed along the bottom of the double ridge of monte vettore http://www.isprambiente.gov.it/files/notizie-ispra/notizie-2016/sisma-italia-centrale/report_amatrice_en_2016_09_16.compressed.pdf http://www.isprambiente.gov.it/files/notizie-ispra/notizie-2016/sisma-italia-centrale/report_amatrice_en_2016_09_16.compressed.pdf http://www.isprambiente.gov.it/files/notizie-ispra/notizie-2016/sisma-italia-centrale/report_amatrice_en_2016_09_16.compressed.pdf http://www.isprambiente.gov.it/files/notizie-ispra/notizie-2016/sisma-italia-centrale/report_amatrice_en_2016_09_16.compressed.pdf http://www.isprambiente.gov.it/files/notizie-ispra/notizie-2016/sisma-italia-centrale/report_amatrice_en_2016_09_16.compressed.pdf annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 6 the above mentioned morphological elements enable us to confirm the presence of a deepseated gravitational slope deformation (dsgsd), already recognized by aringoli et al. (2010a, 2014 and references within), that would affect the highest portion of the slope with a depth of hundreds of meters and partially covered by debris material (figure 6a and b). the genesis of this phenomenon, that can be defined as a deep rock creep, can be associated to the high relief, generated by the quaternary tectonic uplift, and to the interaction between the fault system of monte vettore and the portion of the monti sibillini thrust. the triggering factors can be related to seismic events (as in this case) or the effects following extreme weather events. figure 6 – a) panoramic view of the southern slope of monte vettore, with indication of the main geomorphological elements (source google earth): b) schematic cross section evidencing the geomorphological model of the dsgsd of monte vettore (see text for explanation) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 7 v. preliminary conclusions this paper shows the result of preliminary field surveys carried out soon after the earthquake of august 24th 2016 in the area of monte vettore, where both an active fault and a previously mapped deep-seated gravitational slope deformation (dsgsd) are present. the survey carried out on the western and southern slope of the monte vettore, allowed us to document the geometry, the kinematics and the dimensional properties of the ground ruptures along the previously mapped active fault. on the base of our findings, we may interpret these ground ruptures as primary effects related to the mainshock. geophysical data (e.g. distribution of the aftershocs, insar and accelerometry data etc.) suggest that the fault causative of the mainshock which occurred on august 24th extends from south of amatrice basin to the north of castelluccio basin for a total length of 20-25km. however, geological data indicates for these two areas (castelluccio and amatrice basins) two distinct seismogenic faults (tondi and cello, 2003). on the base of both geological and geophysical data, we may suggest for this recent event a possible short term interaction between the two seismogenic faults or a linkage at depth, as interpreted by tondi et al. (1997). on the other hand, the survey carried out on the previously mapped deep-seated gravitational slope deformation (dsgsd) along the southern slope of monte vettore, allowed us to report evidences of reactivation of the phenomenon, mostly located in the uppermost portion of the slope itself. the reactivation is definitely associated with the effect of shaking related to the earthquake even though, given the structural setting and the typology of movement, a continuous deformation process, favored by particularly intense weather events cannot be excluded. references aringoli, d., gentili, b. and pambianchi, g. (1996). the role of recent tectonics in controlling the deep-seated gravitational deformation of mount frascare (central apennine). geogr. fis. dinam. quat., 19, 281-286. aringoli, d., gentili, b. and pambianchi, g. (2010a). deep-seated gravitational slope deformations in active tectonics areas of the umbria-marche apennine (central italy). geogr. fis. dinam. quat., 33 (2), pp. 127-140 aringoli, d., gentili, b., materazzi, m. and pambianchi, g. (2010b). mass movements in adriatic central italy: activation and evolutive control factors. in: werner, e d. and friedman h.p., eds., landslides: causes, types and effects, pp. 1-72 aringoli, d., cavitolo, p., farabollini, p., galindo-zaldivar, j., gentili, b., giano, s. i., lòpez-garrido, a. c., materazzi, m., nibbi, l., pedrera, a., pambianchi, g., ruano, p., ruiz-constàn, a., sanz de galdean,o c., savelli, d., tondi, e. and troiani, f. (2014). morphotectonic characterization of the quaternary intermontane basins of the umbriamarche apennines (italy). rend. lincei scienze fisiche e naturali boncio p.; g. lavecchia; milana g.; rozzi b. (2004). seismogenesis in central apennines, italy: an integrated analysis of minor earthquake sequences and structural data in the amatrice-campotosto area. annals of geophysics, 47/6, 1723-1742. cello, g., mazzoli, s., tondi, e. and turco, e. (1997). active tectonics in the central apennines and possible implications for seismic hazard analysis in peninsular italy. tectonophysics 272, 43–68. diss working group (2015). database of individual seismogenic sources (diss), verannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7199 8 sion 3.2.0: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas. http://diss.rm.ingv.it/diss/, istituto nazionale di geofisica e vulcanologia; doi:10.6092/ingv.it-diss3.2.0 galadini, f. and galli p. (2000). active tectonics in the central apennines (italy) – input data for seismic hazard assessment. natural hazards, 22/3, 225–268. pierantoni, p.p., deiana, g., galdenzi, s. (2013). stratigraphic and structural features of the sibillini mountains (umbria-marche apennines, italy). italian journal of geosciences 132 (3), 497-520. rovida, a., locati, m., camassi, r., lolli, b., gasperi (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. istituto nazionale di geofisica e vulcanologia. doi:http://doi.org/10.6092/ingv.it‐cpti1 5 tondi, e., cello g., mazzoli s. (1997). strutture sismogenetiche in appennino centrale: potenziale sismico, analisi frattale e processi di crescita. il quaternario, 10 (2), 409414. tondi, e. and cello, g. (2003). spatiotemporal evolution of the central apennines fault system (italy). journal of geodynamics, 36, 113-128. vol. 48, 01, 05ok.qxd 43 annals of geophysics, vol. 48, n. 1, february 2005 key words geochemical monitoring – seismicity – helium – thermal waters 1. introduction it is well-known that earthquakes can provoke modifications in the natural setting of a seismic area and may induce modifications in the fluid phases, such as a decrease/increase in the water level of wells, changes in the temperature and/or chemical composition of the groundwaters, and variations both in the gas discharge flow-rates and in the chemical and isotopic composition of the gases (e.g., see king, 1986; thomas, 1988; toutain and boubron, 1999). at times, precursory time and duration of the anomalies seem to increase as the magnitude and epicentral distance grow, but sometimes this is not the case. on the basis of previous observations carried out in several italian seismic areas, some authors (favara et al., 2001; italiano et al., 2001, 2004; caracausi long-term geochemical monitoring and extensive/compressive phenomena: case study of the umbria region (central apennines, italy) antonio caracausi (1), francesco italiano (1), giovanni martinelli (2), antonio paonita (1) and andrea rizzo (1) (1) istituto nazionale di geofisica e vulcanologia, sezione di palermo, italy (2) arpa – agenzia regionale prevenzione e ambiente dell’emilia-romagna, sezione provinciale di reggio emilia, italy abstract long-term geochemical monitoring performed in the seismic area of the umbria-marche region of italy (i.e. central apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical and isotopic variations of both the thermal springs as well as the gas vents. coincident with the last seismic crisis, which struck the region in 1997-1998, an enhanced co2 degassing on a regional scale caused a ph-drop in all the thermal waters as a consequence of co2 dissolution. furthermore, much higher 3he/4he isotope ratios pointed to a slight mantle-derived contribution. radon activity increased to well above the ± 2σ interval of the earlier seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the single deep-located shock (march 26, 1998, 51 km deep). the anomalous co2 discharge was closely related to the extensional movement of the normal faults responsible for the mw 5.7, 6.0 and 5.6 main shocks that characterized the earlier seismic phase. in contrast, a clear compressive sign is recognizable in the transient disappearance of the deep-originating components related to the mw 5.3, 51 km-deep event that occurred on march 26, 1998. anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic crisis had terminated. we argue that the observed geochemical anomalies were driven by rock permeability changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the detection of anomalies reveals that a seismogenic process is developing. indeed, comprehensive, long-term geochemical monitoring can provide new tools allowing us to better understand the development of seismogenesis. mailing address: dr. a. caracausi, istituto nazionale di geofisica e vulcanologia, sezione di palermo, via u. la malfa 153, 90146 palermo, italy; e-mail: caracau@pa.ingv.it 44 antonio caracausi, francesco italiano, giovanni martinelli, antonio paonita and andrea rizzo et al., 2002a,b) have suggested that the best way to approach the broad-ranging problem of fluid geochemistry and seismogenesis was to collect as many locally significant geochemical data as possible and to evaluate it within an interpretative geochemical model. thus, the long-term geochemical monitoring of any seismic area enables us to define the background and detect any anomalies, which, besides the knowledge of fluid circulation and interaction, allow us to define the behaviour of these anomalies as a seismogenic process develops. since the surface chemical/isotopic composition is a consequence of deep physical-chemical conditions and may change due to re-equilibrations at depth, the geodynamic context of the area may deeply influence the surface composition. in fact, shallow-originating fluids could mix with others coming from different depths of the crust and/or from the upper mantle. the mixing proportions may change with time due both to seasonal variations and/or to the development of seismogenesis (stress accumulation, deformation, strain release, etc.). the seismic event in itself, however, is not the main goal of this research, although its forecasting does in some ways represent a sort of final target. the monitoring activity focuses on the evaluation of the effects that the development of a seismogenic process has on the circulating fluids which may or may not generate a «seismic shock». starting from the beginning of the seismic crisis (september 26, 1997), thermal water samples (bagni di triponzo, parrano and stifone) and samples from gas vents (montecastello di vibio, umbertide, san faustino) were collected on a weekly basis during the period october 1997-july 1998 and at longer intervals thereafter (i.e. twice a week, then monthly and then once every season). the data collected during a rather quiet seismic period (1999-2002) allowed us to identify the background values for some geochemical parameters that could characterise the study area. all the values outside the ±1σ interval were considered as being real variations. however, the observed fluctuations were wider than the ± 2σ interval. some previously published results have pointed out that: a) the thermal waters are caso4– enriched due to their circulation in the deep evaporitic basement and have changed their mixing proportions with cold waters typically equilibrated in shallow carbonate systems (favara et al., 2001); during the seismic period deep-originating waters contaminated shallow aquifers exploited for human purposes (bazzoffia et al., 2002); b) the gases dissolved in the thermal waters, co2-dominated with the presence of ch4 and an excess of n2 in respect of the atmosphere, displayed various influxes of both deep-originating and atmospheric-derived components (italiano et al., 2004); the 3he/4he ratios of venting gases showed that although the region is located in a typical crustal environment, the contribution of a mantle-derived component could be detected (italiano et al., 2001). this paper contains new data mainly collected at a single site, namely the thermal spring of bagni di triponzo, where the emitted fluids have undergone the widest geochemical modifications of all the monitored sites (e.g., the temperature of the thermal water, 29.6 ± 0.2°c constant over time and with no seasonal effect, dropped by 2°c two weeks before and > 20°c two days before the occurrence of the march 26, 1998 shock). this paper focuses on the relationships between the geochemistry of the circulating fluids and the mechanical properties of the hosting fig. 1. location of the selected sampling sites for thermal waters (umbria region, central italy) and the main boundaries between extensional and compressional zones of the central apennines (after barba and basili 2000). squared symbols show the epicentral locations of the main shocks; in red the 26th march 1998 deep located event. 45 long-term geochemical monitoring: case study of the umbria region (central italy) rocks, highlighting the geochemical phenomena related to extension versus those related to the compressive behaviour of the faults involved. 2. seismic activity and geotectonic settings the umbria apennine is affected by frequent seismic activity associated with an extensional strain field. earthquakes affecting the area generally occur in the sedimentary cover at a shallow depth (deschamps et al., 1984; haessler et al., 1988). at least 22 ml ≤ 5 local earthquakes occurred between 1279 and 1984 (boschi et al., 1998). focal mechanisms of the main shocks occurred in 1979, 1984 and 1997. this has also been confirmed by further stress indicators (montone et al., 1997), highlighting the active extension processes in the crust characterized by ne-sw and e-w directions (frepoli and amato, 1997; amato et al., 1998). selvaggi and amato (1992) demonstrated that deep subcrustal seismicity also affects the central and northern apennines. subcrustal seismicity may indicate the ongoing activity of subductive processes. after a relatively long period of quiescence, a seismic swarm, marked by the ml 4.5 massa martana mainshock, occurred in the umbria region in may 1997 about 40 km south of the village of «colfiorito», which was the area most damaged during the subsequent months of september and october 1997. the strongest event, which occurred in 1998 (ml 5.7 on march 26), had a subcrustal hypocenter located at a depth of about 51 kms (morelli et al., 2000). a descending lithospheric slab, linked to an ongoing subduction process, may account for subcrustal events in the northern-central apennines (selvaggi and amato, 1992). the entire seismogenic process caused a crustal deformation with a maximum horizontal co-seismic displacement of 14 ± 1.8 cm and a maximum co-seismic subsidence of 24 ± 3 cm. these were detected by means of sar differential interferometry and gps data (stramondo et al., 1999), while a post-seismic long-term deformation process was detected by means of levelling data (basili and meghraoui, 2001). the inner part of the umbria-marche apennine was affected by a relatively extensional regime able to generate a set of grabens during plio-pleistocene times within the frame of a possible, limited rifting process (elter et al., 1975; liotta et al., 1998). the basement of the central apenninic rock is made up of phyllitic rocks from the carboniferous age, underlying conglomerates of the verrucano group attributable to the middle triassic age. the sedimentary cover continues with the burano anhydrites, contorta limestone and calcare massiccio (late triassic-early lias). it is followed by a pelagic, calcareous and marly multilayer (middle lias-oligocene) and ends in a flisch sequence (middle to upper pliocene) (bally et al., 1986; ponziani et al., 1995). travertine rocks occur in the vicinity of co2 gas emissions of the umbria apennine, evidencing the ascent of deep fluids towards the surface (barbier and fanelli, 1976). co2 emissions are widely distributed across the umbria region and can be related to the slightly anomalous heat flow pattern (cataldi et al., 1995), that may be responsible for the thermo-metamorphic processes at depth (chiodini et al., 1999), while intense local faulting (e.g., cello et al., 1997) can be considered responsible for co2 uprising. the existence of the magmatic structures of san venanzo (stoppa and sforna, 1995) in the vicinity of montecastello di vibio, and colle fabbri (stoppa, 1988) in the vicinity of massa martana, provides clues to the possible increase in local crustal permeability. the triponzo spring sources are located in a limestone formation (calcare massiccio) of the hettangian-sinemurian age characterized by high porosity. this favours locally important karst phenomena. spring sources are located at the bottom of a canyon generated by an important regional fault, which cut the limestone formations allowing the groundwaters to reach the underlying evaporitic rocks (giaquinto et al., 1991). 3. sampling and analytical methods water composition was determined by both field measurements (temperature, ph, electrical conductivity and rn activity) and 46 antonio caracausi, francesco italiano, giovanni martinelli, antonio paonita and andrea rizzo laboratory analyses. in the field the radon activity was measured by an eda-rd 200 radon detector. a 130-ml aliquot of water was degassed and the extracted radon was measured in a zns (silver activated) coated cell. the analytical errors have been estimated to be ± 10%. the temperature was always measured using the same mercury thermometer (scale 0-50°c; resolution 0.1°c), while ph and conductivity were measured by electronic instruments calibrated in situ using buffer solutions (error below ± 5%). the chemical composition was determined by high-pressure liquid chromatography (hplc, dionex 2001) using a dionex cs-12 and a dionex as4a-sc column for cation and anion determinations, respectively. the hco3 content was measured in the laboratory by standard titration procedures on samples sealed in the field following the same method for the dissolved gas analyses. for the analysis of the dissolved gases, samples were collected and sealed in the field in 240 ml pyrex bottles. keeping the bottles immersed, they were filled with the thermal water, and then a gas-tight septum was fitted on using special pliers. the chemical gas-analyses were carried out using the methodology based on the equilibrium partitioning of gases between the liquid and gas phase (sugisaki and taki, 1987), and following the procedure previously suggested by capasso and inguaggiato (1998). the chemical analyses and helium isotope measurements were made by gas-chromatography and mass spectrometry, respectively; the sample was split into two gas aliquots concomitantly extracted from the same sampling bottle (italiano et al., 2004). chemical analyses of he, o2, n2, ch4 and co2 were carried out using a perkin elmer 8500 gas-chromatograph equipped with a 4 m carbosieve 5a column and double detector (flame ionization detector with methanizer and hot wire detector). the detection limits were 5 ppmv for he, o2, and n2, and 1 ppmv for ch4 and co2. analytical errors were ± 5% for he, and ± 3% for the other species. a 0.3 cm 3 aliquot was admitted at a known pressure to the purification uhv line for helium isotope analysis. helium was purified in three stages in separate sections of an all-metal uhv preparation system. first, the reactive gases were adsorbed into a charcoal trap held at 77 k; then two saes getters worked simultaneously to adsorb residual nitrogen at 250°c and hydrogen at room temperature. the third step, after measuring the 4he/20ne ratio and checking for residual 40ar on an in-line quadrupole mass spectrometer (qms, vg quartz), was to completely separate helium from neon by means of a charcoal trap held at 40 k. the isotopic analyses of the purified helium fraction were performed with a modified static vacuum mass spectrometer (vg5400tft, vg isotopes) that allows for the simultaneous detection of 3he and 4he-ion beams, reducing the 3he / 4he measurement error to very low values. typical uncertainties in the range of low-3he (radiogenic) samples are below ± 5%. 4. results and discussions the chemical composition of the sampled thermal waters was established to be a mixture of a carbonate component, equilibrated with calcite and dolomite, and a selenitic component circulating in the evaporitic triassic basement (chiodini et al., 1982, 1999; quattrocchi et al., 2000). however, no information was available regarding temporal modifications before the occurrence of the last seismic crisis (favara et al., 2001). the langelier-ludwig diagram (langelier and ludwig, 1942) in fig. 2 highlights the composition changes of the major constituents during the seismic crisis. it should be noted that not only did the composition change, but the physical-chemical features and the flow rate of the thermal waters did as well (favara et al., 2001; see also fig. 7). figures 3 and 4a,b show both the background anomalies and the recorded ones for some anions and cations of the investigated springs (favara et al., 2001; caracausi et al., 2002a). using the wateqp software (appelo, 1988) we were able to calculate the saturation index (si) expressed as the logarithm of the ion activity ratio, pertinent to a mineralogical phase, and the equilibrium constant of its solubility product. the results plotted in figs. 5a,b and 6 show the water47 long-term geochemical monitoring: case study of the umbria region (central italy) fig. 2. langelier-ludwig classification diagram. the plotted analyses show the modifications that occurred over time at all the selected thermal springs. the dates show the modifications that happened at triponzo at the end of march 1998. the arrows display the chemical modifications caused by cold water mixing. concentration units are in meq/l. rock interactions occurring at the thermal waters of parrano stifone and triponzo, respectively. all the calcite-dolomite si show that the discharged thermal waters are normally from under-saturated to close-to-saturation with respect to both of the two mineralogical species, fig. 3. temporal variation of so4 and mg concentration (meq/l) at the thermal spring of parrano between june 1997 and june 2002. the occurrence of seismic events with ml > 4 is reported as a bar diagram. fig. 4a,b. temporal variation of ca and cl concentration (meq/l) at the thermal springs of triponzo «pipe» and «pond» between june 1997 and december 2001 and their relationship to the seismicity as shown by the associated bar diagram. even though the two springs are located several dozen meters apart at the same thermal area, they followed very different trends during the monitoring period. the «pipe» spring is the one referred to by favara et al. (2001), while the «pond» site is the one sampled by quattrocchi et al. (2000). a b fig. 6. saturation indices calculated for the thermal water samples of triponzo «pipe». the faulting behaviour that marked the seismic events of septemberoctober 1997 and march 1998, respectively, induced significant and different effects that can be recognized in the paths followed by the water-bodies when returning to initial conditions, i.e. chemical equilibrium. the dates refer to the month of march 1998. 48 antonio caracausi, francesco italiano, giovanni martinelli, antonio paonita and andrea rizzo fig. 5a,b. calcite versus dolomite and calcite-gypsum saturation indices (si) calculated for the thermal springs of parrano (a) and stifone (b). the lowest si are related to the event that occurred in march 1998. a b as this variability is related to seasonal variations. however, they varied simultaneously as time passed (figs. 5a,b and 6) to become decisively under-saturated over a short time-scale. the calcite-dolomite si pattern is similar for all the thermal waters, but parrano and stifone (fig. 5a,b) show variable si even for gypsum. these results are consistent with an initial equilibration within evaporitic (selenitic) reservoirs, then thermal waters mix with shallower cold waters equilibrated in carbonatic reservoirs during their uprising towards the surface. all the sampled thermal waters displayed the lowest si values during the earlier phase of the seismic crisis (september-october 1997), in agreement with the extension that marked the faulting activity. then the si increased as time passed. normal faulting (barba and basili, 2000) increased crustal permeability allowing a higher co2 flow rate to the surface (heinicke et al., 2000; italiano et al., 2001), thus inducing a higher aqueous co2 concentration in the thermal waters. as a consequence of this, the aggressiveness of the groundwaters increased, thereby causing more limestone to dissolve. however, considering the chemical reaction kinetics, the equilibrium with the hosting rocks was far from being achieved and the calculated si displayed a waterbody that became less and less under-saturated in both calcite and dolomite (fig. 6). the evidence that the gypsum saturation index and the emission temperature remained unchanged means that the crustal permeability increase did not enhance the thermal waters uprising from the evaporitic basement. the gypsum si at triponzo shows that the water is constantly gypsum-under-saturated with (si = − 0.4). furthermore, the calcite si is close to saturation and varies as time passes (fig. 6). the thermal water from triponzo showed the highest calcite and dolomite si in the sample collected four days before 26th march, ml = 5.7, deep located event (morelli et al., 2000). the water samples taken during the days that followed showed some very interesting behaviour: all the calcite and dolomite si dropped to typical under-saturated values, whereas the sample taken on 1st april was again in the range recorded at the beginning of the seismic crisis. since the observed modifications were consistent with other observations that denoted a transient disappearance of the thermal water component (favara et al., 2001), they reveal a compressive phenomenon that involved various crustal layers. the compression was already acting on the deep crustal layers many days before the seismic shock, obstructing the rise of co2. the water sample dated 22nd march shows a decrease in the dissolved co2 that modified the equilibrium conditions of the water body toward lower si values. the samples taken in the days that followed exhibit a trend toward a new chemical equilibrium, with a wider decrease in the dolomite si in respect of the calcite si. the sample collected on the 28th, two days after the shock, shows the lowest si for both the mineralogical species. the trend in gypsum-calcite si (fig. 6) highlights another aspect of the compressive mechanism: gypsum si did not change very much before the earthquake, however afterwards it displayed the lowest value (sample dated 28th march). this is related to the transient disappearance of the thermal water component, as pointed out by the dramatic water temperature drop, meaning that the compression obstructed the uprising pathways located at the level of the evaporitic basement (where waters become «thermal») at the end of the compression. an alternative explanation, based on the occurrence of mixing phenomena, has to be ruled out due to the very low temperature (8.4°c) reached by the thermal waters that cannot be a mixture of a thermal component, having a temperature of about 30°c, with water colder than the infiltrating waters (normally as cold as 810°c). the graph log pressure co2 versus temperature (fig. 7) shows that, two weeks before 26th march seismic shock, the thermal component was already mixing with cold waters or, alternatively, that the relative flow-rates were changing. the 20°c drop occurred two days before the shock and the time the temperature took to return to the same pre-seismic range (i.e. some months) is consistent with a longlasting process (compression and relaxation). figure 8 shows the radon activity trend versus ph. the different behaviour of rn at the times of the two seismic periods in 1997 and 1998 is clearly evident. in fact, during the former, rn increased, reaching its peak activity, while, coincident with the latter, it decreased to its lowest value. the activity of rn returned to background values about 15 weeks after the remarkable geochemical changes observed in concomitance with the last, deep seismic shock (march 1998). the ph variation pattern is similar, although antithetical. a 0.3 ph-unit decrease, that lasted for 6 months, was observed at the 49 long-term geochemical monitoring: case study of the umbria region (central italy) fig. 7. log pco2, temperature and seismicity versus time at the triponzo «pipe» spring. data refer to the sampling period that lasted from june 1997 to august 1999. fig. 9. radon activity (bq/l) and 3he/4he isotope ratio in the thermal water of triponzo «pipe». during the march 1998 seismicity the radon activity displayed a marked antithetical trend in respect of the 3he/4he ratio, showing that radon content and 4he are related to a common radiogenic origin. the increase in the r/ra values is a consequence of the dissolution of an atmospheric component in the thermal waters. the displayed helium isotope ratios, expressed as r/ra values (r = sample ratio; ra = atmospheric ratio = 1.39 × 10− 6), have not been corrected for atmospheric contamination. 50 antonio caracausi, francesco italiano, giovanni martinelli, antonio paonita and andrea rizzo fig. 8. radon activity (bq/l), flow-rate (litres per second) and ph versus time at triponzo «pipe» thermal spring. the flow rate changes because of seasonal effects but does not affect the radon activity, the ph or the temperature (see fig. 7). in coincidence with the deep event of 26th march 1998, both the flow rate and radon concentration dropped to the lowest values ever recorded (see text). beginning of the seismic crisis. the values of ph increased towards the pre-seismic/background values at the end of february 1998, and exhibited a further, sudden increase of the order of 0.8 units in concomitance with the lowest radon activity, the lowest water temperature and flow-rate, and the highest 3he/4he ratio (i.e. the variations related to the 26th march 1998 shock). it then went back to within the ± 2σ background interval, displaying variations, probably due to seasonal oscillations, consistent with those of rn, pco2, etc. rn and water flow rate followed very similar trends (fig. 8) that occurred at an almost constant water temperature (apart from the 20°cdrop preceding the 26th march 1998 seismic event). the absence of significant flow-rate-related temperature variations implies that local hydrological settings control the flow rate, thus its temporal variation does not represent variable mixing proportions between the deep selenitic thermal component and the shallow cold, carbonatic water, but variable hydrostatic loads. since the mixing ratios of the thermal water are almost the same on a temporal scale, the ph decrease recorded at the beginning of the seismic crisis cannot be related to compositional variations but to the massive co2 output in the area. this evidence leads to the consideration that at the beginning of the seismic crisis the excess radon release was triggered by the co2 outflow (i.e. dongarrà and martinelli, 1995 and references therein). rn reached its lowest value when only the carbonatic water was outflowing, highlighting how the background of this noble gas is kept at levels of about 40 bq/l by the thermal component. figure 9 shows the inverse correlation between the trends of the radon activity and the 3he/4he isotope ratio. this pattern, that gives support to the hypothesis of an enhanced crustal permeability that allowed co2 to be released and to act as carrier for the heavy, low-mobile radon, highlights the same radi15th, 1988), the epicentre of which was located about 80 km southwards, on the same main tectonic structure. the results show that the geochemistry of the fluids in a seismic area reflects the faulting behaviour and the properties of the hosting rocks, so it is closely related to the local tectonic, geologic and hydrologic settings, and that the findings collected in one selected area cannot be applied directly to another. taking into consideration all the results we obtained, we argue that the geochemical variations are linked to a more general seismogenic process, involving crustal deformation, rather than to single events. the results strongly outline that without long-term geochemical monitoring, it is impossible either to interpret or to create a model of the geochemical anomalies and the temporal variations occurring during the development of a seismogenic process. acknowledgements the authors wish to thank dr. h. woith, dr. j. heiniche and an anonymous referee for the helpful comments and suggestions that greatly improved the manuscript. mrs. penelope dyer is thanked for having revised the english. references amato, a., r. azzara, c. chiarabba, g.b. cimini, m. cocco, m. di bona, l. margheriti, s. mazza, f. mele, g. selvaggi, a. basili, e. boschi, f. courboulex, a. dechamps, s. gaffet, g. bittarelli, l. chiaraluce, d. piccinini and m. ripepe (1998): the 1997 umbria-marche, italy, earthquake sequence: a first look at the main shocks and aftershocks, geophys. res. lett., 25 (15), 2861-2864. appelo, c.a.j. (1988): wateqp program, institut voor aardwetenschappen, vrije universiteit, amsterdam bally, a., l. burbi, c. cooper and r. ghelardoni (1986): balanced sections and seismic reflection profiles across the central apennines, mem. soc. geol. ital., 35, 257-310. barba, s. and r. basili (2000): analysis of seismological and geological observations for moderate-size earthquakes: the colfiorito fault system (central apennines, italy), geophys. j. int., 141, 241-252 barbier, e. and m. fanelli (1976): main fractures of italy from erts satellite images and correlations with thermal springs, volcanoes and earthquakes, in volcanoes and tectonosphere, edited by h. aoki and s. iizuka (tokay university press, tokyo). basili, r. and m. meghraoui (2001): coseismic and postseismic displacement related with the 1997 earthquake se51 long-term geochemical monitoring: case study of the umbria region (central italy) ogenic origin for 4he and rn: they both originate from the deep crustal levels involved in the tensive and compressive phenomena of the seismogenetic process. 5. conclusions the long-term monitoring program, which started in july 1997, allowed us to interpret the geochemical information in such a way as to model the circulation pattern of the fluids released in the area. geochemical variations disclosed changes in crustal permeability that are to be referred to deformations associated with subductive processes that characterize the apennine chain. the most significant geochemical changes were apparently linked to 5 events characterized by ml ≤ 5 and limited to the period september 1997-april 1998; conversely the long-term geochemical trends show the persistence of long-lasting processes that have no relationship with the seismic shocks. the results collected reasonably denote extensive crustal movements at the beginning of the seismic crisis, in full agreement with the observations that the normal fault, known as colfiorito fault, was the first structure to move (basili and barba, 2000). the enhanced co2 release was one of the consequences that induced modifications in the chemical equilibrium of the water bodies. coincident with the 51 km-deep seismic shock that occurred on 26th march 1998, a dramatic permeability drop, capable of modifying the deep thermal water circuit, was responsible for the most extensive modifications observed during the entire seismic crisis. nevertheless the focal mechanism of the event was a combination of normal faulting and strike-slip (morelli et al., 2000), whereas the observed geochemical modifications reveal an opposite effect at shallower levels. it is worth noting that the variations occurred some days before and continued some days after the event, thereby supporting the hypothesis that the permeability drop was not a consequence of the earthquake itself. some strong geochemical variations also occurred in the period may-august 1998, when there was only one significant event (ml ≥ 4.8, august quence in umbria-marche 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(1988): geochemical precursors to seismic activity, pure appl. geophys., 126, 241-265. toutain, j.p. and j.c. baubron (1999): gas geochemistry and seismotectonics: a review, tectonophysics, 304, 1-24. 53 long-term geochemical monitoring: case study of the umbria region (central italy) vol49_1_2006def 45 annals of geophysics, vol. 49, n. 1, february 2006 key words radiometric calibration – mivis – atmospheric correction – modtran 1. introduction a quality index of radiometrically calibrated and atmospherically corrected data can be obtained by comparing spectral ground truths acquired at the same time of flight (clark et al., 2002). this ground truths area must be characterized by a good spectral and spatial homogeneity and a high reflectance to assure a high signal perceived at the sensor. such comparison between spectra measured by the sensor and ground truths has been applied to data collected during last mivis (multispectral infrared and visibile imaging spectrometer, aa500, daedalus inc.) campaigns performed by cnr. mivis is a whiskbroom scanner in which the light collimated by the scan head is distributed by means of dichroic filters into 4 spectrometers that simultaneously record the radiation coming from the earth (table i). the scanner geometry is characterized by a fov of 71.06°, an ifov 2 mrad and a scan rotation frequency varying from 6.25 hz to 25 hz; data are digitized at 12 bit (daedalus, 1993). mivis standard radiometric calibration sequence is carried out on a test bench (tb) made up of a black painted aluminium box with two calibration halogen lamps and a reference panel of barium sulphate (spectraflect, reflectance cerlaboratory activity for a new procedure of mivis calibration and relative validation with test data cristiana bassani (1), rosa maria cavalli (2), angelo palombo (1), stefano pignatti (1)(2) and fabio madonna (1) (1) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy (2) laboratorio aereo ricerche ambientali (lara), iia-cnr, tor vergata (rm), italy abstract remotely sensed data, recorded by means of the mivis hyperspectral scanner in the framework of the research activity of the cnr institutes iia-lara and imaa, have been calibrated to reflectance values and then quantitatively compared with ground data. a new procedure for radiometric calibration has been defined by utilizing the mivis test-bench and applying a wider radiance range with respect to the one provided by the manufacturing company. new calibration curves have been determined and applied in the pre-processing chain. for validation purpose ground spectra were measured during the campaign by means of a portable spectroradiometer. the atmospheric correction has been carried out by implementing an idl procedure to manage modtran4 input and output cards. mivis test data acquired over passo corese (roma) have shown how the new calibration coefficients significantly improve the radiometric accuracy. in particular, in the vis spectral region the percentage error, with respect to a ground truth spectrum, is about half of that occurring if the standard calibration coefficients are used. mailing address: dr. cristiana bassani, istituto di metodologie per l’analisi ambientale (imaa), cnr, c.da s. loja, 85050 tito scalo (pz), italy; e-mail: bassani@imaa.cnr.it 46 cristiana bassani, rosa maria cavalli, angelo palombo, stefano pignatti and fabio madonna tified in 1994) of known reflectance. the mivis sensor is mounted on the bench in the lab for the automatic acceptance test procedure (atp), which is run for a fast and periodic scanner calibration and a performance check (signal to noise ration – snr in fig. 1). such standard calibration associates the dn, measured during the test, to the panel radiance values tabulated for each channel, so producing the calibration curves, straight lines intersecting the axis at the origin, relating dn with tabulated radiance values. the straight line slope determines the radiance factor (rf) applied to the radiometric calibration to convert the dn to radiance while the standard deviation of the dn values has been considered an estimation of the snr. from 1994 to date, besides the common checks performed at the deadalus labs, a trial (lechi, 2000) of reliability of the rf values defined by atp has been carried out as regards possible variations of the tb parameters (lamps and panels). lechi’s work is not to be considered resolutive, but it certainly represents an outstanding experimental basis to define a more precise procedure. the present work describes a non standard procedure, implemented in the lab using the tb and a reference spectroradiometer to obtain new rf curves on the basis of a wider range of lighting conditions to better describe the natural irradiance values. the new calibration coefficients have been applied to the test image recorded on an cultivated area close to passo corese (rome, italy) on march 7, 2002 at an altitude of 1500 m a.s.l (pixel ground resolution 3 m/pixel) where spectral ground truth campaign has been performed for image validation purpose. furthermore, the paper includes an appendix to demonstrate analytically and experimentally how the optical defocusing occurring in the tb during atp does not invalidate the accuracy of the mivis calibration. thus, the presence of a collimator facing the scan head is not necessary for a correct calibration. table i. mivis spectral characteristics. spectrometer bands spectral range bandwidth (nm) (nm) 1 1-20 430-830 20 2 21-28 1150-1550 50 3 29-92 1983-2478 9 4 93-102 8180-12700 340÷440 fig. 1. mivis snr @ 25 hz, plus the radiance values from which it has been calculated. 47 laboratory activity for a new procedure of mivis calibration and relative validation with test data 2. mivis radiometric calibration in the atp the method used to calculate the panel radiance data for the tb is based on the following equation: (2.1) where nλ is the panel radiance at wavelength λ, hλ the panel irradiance at wavelength λ; ϕ the lamp to panel illumination angle; eλ the panel reflectance at wavelength λ, and π the factor for lambert surface. the physical quantities involved in eq. (2.1) are all monochromatic and the λ index corresponds to the central wavelength of each channel of the sensor. in this procedure the radiance factors for the first 92 channels are derived from this simple relation (2.2) where dnλ ref1 is the offset of dark current (dn of the first mivis internal blackbodies). during the analysis it was noted that the radiance received by the instrument during the atp is much lower than the one measured throughout instrument’s normal operation condn dn k n ref1= m m m m sin n h e2 $ $= r { m m mc m; e dition (summer clear sky) (fig. 2). therefore, to verify the behavior of the scanner operating under radiance conditions similar to those occurring during surveys, a new calibration procedure was tuned. the developed calibration procedure consists in building calibration curves with the use of different radiance values obtained by either varying the power of the lighting source (introducing additional lamps besides those normally used for atp) or employing different reflectance panels. in the experimental phase the combination of different irradiating powers and different panels produced 18 experimental conditions measured by both mivis and the portable spectrometer. the panels employed during non standard calibration are: the spectraflect (baso4) provided with the tb; the spectralon certified by labsphere inc. up to december 2002 (∆ρ ≤ ≤± 0.005 between 300-2200 nm and ∆ρ ≤ ≤± 0.020 between 2250-2500 nm); a 18% kodak grey panel and the tb black panel. they have been measured by means of the portable asd fieldspec pro fr spectroradiometer (asd). asd is a portable spectrometer covering the range between 350 nm and 2500 nm with a spectral resolution of 3 nm for the range 350-1000 fig. 2. radiances inherent to the radiometric calibration process: rad_wp: radiance of spectralon panel measured in open air at our latitude in summer time; rad_max (maximum radiance) and rad_min (lowest radiance) measured in tb in the new procedure; rad_atp: radiance in the tb during atp. 48 cristiana bassani, rosa maria cavalli, angelo palombo, stefano pignatti and fabio madonna nm, and of 10 nm for the 1000-2500 nm (milton et al., 1997). in accordance with the last calibration certificate the error associated with the radiance measures is: 1.4 × 10−9 w/cm2 nm sr @ @700 nm; 2.4 × 10−9 w/cm2 nm sr @ 1400 nm; and of 10.4 × 10−9 w/cm2 nm sr @ 2100 nm. the dnλ used to express the function relationship with the radiance are not those utilized by the tb software performing the atp, but they were obtained off-line by reading the dn directly from the raw files recorded on the mivis data tape. each raw file recorded for each panel differently illuminated has a length of about 400 scan lines at a scan rotation frequency of 25 hz. the dnλ used in the rf calculation are the mean computed on a 20 × 20 pixel square, selected in the middle of the scanner scan line, once the dark currents are subtracted. this window size was chosen to avoid image blurring due to the reduced distance between the panel and scan head, which does not permit panel images to be well focused, and to allow a significant statistical assessment. the square standard deviation takes into consideration both measurement variations typical of the scanner, which is a least fig. 3. best fit of the 18 radiance values for channel 21 (second polynomial fit) and channel 31 (linear fit) to derive the calibration curves. fig. 4. comparison of spectralon radiance measured in the tb. rad_atp curve is the mivis radiance calibrated with rf from atp; rad_new is the mivis radiance calibrated with the new procedure; rad_asd is the asd measured inside the tb. 49 laboratory activity for a new procedure of mivis calibration and relative validation with test data contribution, and the possible error due to the non homogeneities in the panel lighting. the 18 couples of values (radiance/dn) so defined for each channel, positioned on the graph of fig. 3, show that the linear trend, presumed as valid by the tb software during the atp, is not always perfectly verified. for example, some points of the channel 21 (fig. 3 on the left) wander off from the hypothesized linear trend at higher radiance values (close to those occurring under mivis normal working conditions). in the first 28 channels (first and second spectrometer, vis-nir regions) the best fitting curve is a second polynomial, while for the mir the trend is a perfect straight line (fig. 4 on the right). to determine the rf of the non linear channels, instead of a linear relationship, the following equation is used (2.3) where a, b and c are the fit coefficients (bassani et al., 2002). to ascertain the accuracy of the mivis radiance values obtained by applying the new calibration they were compared with the radiance values obtained by applying the rf from the atp, and the measured panel radiance by asd. figure 4 depicts how the panel radiance curves, obtained with the new calibration coefficients, better fits the reference asd curve to those calculated by using the atp. the difference between the atp and asd curves could be primarily due to variations of the tb lamp irradiance and the weathering conditions of the spectraflect panel (rollin, 2000). as a consequence, mivis radiometric correction was performed using the new calibration coefficients. 3. mivis atmospheric correction and validation to convert the instrument perceived radiance values to reflectance ρg(λ), an intrinsic property of the surface independent of the illumination and atmospheric conditions at the time of measurements, an atmospheric correction procedure was applied to the mivis data. in order to cordn rad rada b c2$ $= + + rect the atmospheric effects, the relationship between the upward radiance, l0(λ), measured by mivis and the surface reflectance, ρg(λ), has to be established by using the modtran4 radiative transfer model (slater, 1980; tanré, 1992; vermote et al., 1997). under the assumption of a cloudless sky, a plane-parallel atmosphere and a lambertian surface, the general model applied is given by (3.1) where l0(λ) is the radiance measured at the sensor; ld↑(λ) represents the upwelling atmosphere spectral radiance, scattered from the direct sunbeam into the sensor by the atmosphere (path radiance); ed ↓(λ) is the downwelling spectral irradiance at the surface due to the scattered solar flux in the atmosphere; τv↑(λ) is the atmospheric transmittance along the path from the ground surface to the sensor; τz↓(λ) is the atmospheric transmittance along the path from the sun to the ground surface; θz is the solar zenith angle, e0↓(λ) is the solar spectral irradiance at the top of the earth’s atmosphere. for applying the atmospheric correction to mivis data, an idl procedure has been developed to manage input and output cards, to run modtran4 (berk et al., 1999). the idl code is a stepwise procedure in which the different atmospheric parameters are computed separately. the first step takes into consideration the contribution of path radiance, ld↑(λ) along the scan line, while in the following step the quantities ld↑(λ), ed↓(λ), τv↑(λ), τz↓(λ), e0↓(λ) are computed. the procedure automatically resamples each modtran output quantity fi(λ) to mivis spectral channels by using the following formula: (3.2) where the index i is the wavelength step selected in the modtran simulations (1 nm in our case), j is the mivis channels index (i.e. 1-92), ( ) ( ) ( ) ( ) f r f r j i j i n i i n i j 1 1 $ =m m m m = = 6 6 @ @ / / ( ) ( ) ( ) ( ) ( ) ( ) ( ) cose e l l g v z z d d0 0 = + t m x m m i x m m r m m . .6 6 @ @ 50 cristiana bassani, rosa maria cavalli, angelo palombo, stefano pignatti and fabio madonna and [ri (λ)]j is the spectral response function for the j mivis channel in the modtran wavelength step. the determined fj(λ) are applied in eq. (3.1) to obtain the mivis reflectance value. the idl code was applied to correct the mivis survey on passo corese configuring modtran cards with a mid-latitude summer profile and with a rural aerosol model (visibility equal to 23 km). the validation process of the atmospheric correction is based on the comparison of the mivis reflectance with the spectral ground reflectance spectra acquired with the asd. to fulfil this task the selection of a wide areas of interest (aoi), as spectrally and spatially homogeneous as possible, is required, so to minimize the adjacency effects; moreover high reflectance values are required in order to have high snr at the sensor. as regards the image of passo corese, the cultivated fields were not used as target reference (mixture of soil and bean plants) because of the fig. 6. comparison of apt and new calibration spectrum of the aoi in concrete with respect to asd reflectance. asd measure is plotted with one sigma errors bars. fig. 5. comparison of apt and new calibration spectrum of the aoi in concrete with respect to asd reflectance. asd measure is graphed with one sigma errors bars. 51 laboratory activity for a new procedure of mivis calibration and relative validation with test data growing season and, therefore, the aoi were selected on a concrete platform covering a total area of about 200 m2. the following graphs in figs. 5, 6 and 7 show the mivis reflectance spectra obtained respectively by applying the radiometric calibration with the rf derived from the atp and the new calibration coefficients from the above described new procedure. on the graphs mivis spectra are compared with the asd measurements of the aoi. asd measurements have been averaged and the standard deviation has been considered like the spectral variability of the concrete spectrum (error bars on the graphs). the comparison of mivis and ground spectra outlined how the atp tends to overestimate the reflectance values in the vis region (fig. 5) where the reflectance value determined by means of the new procedures is to be considered equal within the error to the mean of the ground truths. in terms of percentage with the new calibration the reflectance errors are of about 15% with respect to the asd ground truth, while the atp carries an error of about 30%. the nir region is manifestly subject to a contribution of water vapour (fig. 6) as regards the ground truths as well. the high variability of the water vapour content in the ground spectra leads to a great uncertainty in determining the reference reflectance value so much that both mivis spectra fall within the asd measurements confidence interval. in the swir region (fig. 7), the two calibration procedures instead show comparable reflectance values, even though both highlight an underestimation of the image reflectance with respect to the reference ground truth spectra. 4. results and discussion in order to test the accuracy of the rf for the entire instrument dynamic range, a new procedure has been devised. such non-standard procedure is able to produce radiance values with a radiometric precision higher than that reachable by the atp procedure. the non-standard procedure, being developed not taking into account the atp tabulated values, decouples the scanner performance variation from that related to the tb. by comparing the new procedure with the atp in the data pre-processing chain to obtain reflectance values, it has been observed that the atp in the vis region overestimates the spectrum of the selected aoi. this result is congruent with the experimental results obtained from the calibration of the tb dn 18 images, showing the overestimation of the panels’ radiance in the atp. in the nir and swir spectral regions, the atp and the new calibration procedure attain two similar curves both occurring nevertheless in the secfig. 7. comparison of apt and new calibration spectrum of the aoi in concrete with respect to asd reflectance. asd measure is plotted with one sigma errors bars. 52 cristiana bassani, rosa maria cavalli, angelo palombo, stefano pignatti and fabio madonna ond spectrometer within the intrinsic aoi variability, while in the third mivis optical port they both appear below the asd reflectance curve. the result is satisfactory if compared to the purpose of this work: the new procedure brings about an indeterminacy in the reflectance measurement which is about the half of that occurring if the atp rf are used. once the experimental phase is over, this procedure can be applied at regular intervals throughout the year to verify, and possibly update, the atp routine calibration. the new radiometric calibration is going to be applied in the next mivis airborne campaign to be deployed in sweden in june 2003, where atmospheric correction will be performed using in situ atmospheric measures (i.e. radiosounding, solar direct measurements for optical thickness), thus permitting the uncertainty of modtran simulations to be reduced. acknowledgements the authors are grateful to maurizio poscolieri and roberto ponzianelli for valuable discussions and to angela mirabelli for helping to improve the paper. appendix. atp accuracy as function of the optical defocusing within the tb. to demonstrate how the distance between the tb panel and the mivis scan head is not affecting the calibration procedure, a twofold procedure has been followed: the former, analytic, according to the rules of geometric optics; the latter, experimental, to prove the independence of the radiance measures on the distance from a uniform and wide source. a telescope could be schematised with a thin lens (fig. a.1), in which, for distances smaller than the infinite, the image will be displayed at a distance i . the difference between i and the focal distance (f) is named defocalization (d). the distance i, for a point c located at z distance from the lens, is defined by the following equation (formula valid for thin lens) . (a.1) therefore, the defocalization is defined as . (a.2) from the above equations, it derives that the defocalization goes asymptotically to zero by increasing the distance. the defocusing phenomenon causes a lost of energy passing the field stop, coming from the single point of the viewed surface, that is due to the non coincidence of the image disc and the field stop located on the focal plane (fig. a.1-β). nevertheless, it is possible to demonstrate that: from a surface, sufficiently large and uniformly illuminated, the energy reaching the sensor after the field stop is not correlated to the distance. a generic element of the surface c at distance z from the lens, forming an angle θ with the optical axis, is focalised at position cl and it is spread on the focal plane f, as a circle (disc image) of diameter eh. the diameter of the above mentioned circle (2rc) is yielded by . (a.3) by means of a geometric consideration . (a.4) d i r ocgh t= l d eg i r oct= + l eg gheh = + d z f f = 2 i z f zf = 53 laboratory activity for a new procedure of mivis calibration and relative validation with test data where rt is the lens radius and ocl is the parallax (p) which is determined by the following relation: . (a.5) including eq. (a.4) in eq. (a.3), the new relationship is obtained . (a.6) from which it derives that (a.7) assuming e0 as the energy falling on the telescope from the surface viewed under the infinitesimal solid angle δω, it is spread on the focal plane within a circle of radius rc. the fraction of e0 passing . eh d i r eh z fr 2 2 t t= = eh d i r oc d i r oct t= + + -l l ( )tanp i= j fig. a.1. telescope scheme with one convergent lens. on the right (α) is the front view of the field stop (grey disc) and the image of c on the focal plane (light disc patterned with squares); (β) show the relative position and dimension of the discs for condition τ1; (γ) show the relative position and dimension of the discs for τ3; (δ) show the relative position and dimension of the discs for τ2. on β, γ, δ white area point out the two discs intersection. 54 cristiana bassani, rosa maria cavalli, angelo palombo, stefano pignatti and fabio madonna the field stop is described by the τ function. the τ function is defined according to three different formulas (fig. a.1). in the condition of partial overlap between field stop and disc image τ1 (fig. a.1-β) (a.8) where a represents the overlap area between disc image and field stop and is yielded by the relation (a.9) where p is the parallax and corresponds to the distance between the centres of field stop and disc image. in the condition of total overlap between field stop and disc image and in the case of rc ≥ rd (fig. a.1-γ) the τ function is expressed by . (a.10) in the situation of total overlap between field stop and disc image with rc ≤ rd (fig. a.1-δ) the τ function is expressed by . (a.11) the condition of rc = rd implies that and, therefore, for higher z values the disc image has dimensions lower than the field stop. the energy fraction ρ| , arriving to the sensor from the whole surface located at distance z, is obtained by integrating the τ function for the entire solid angle, lying within the sensor viewing angle. for energy fraction ρ| is obtained by (a.12) while for by (a.13) the integration boundaries θlmax and θ mmax are the limit angles beyond which the disc image on the focal plane: 1) does not totally overlap the field stop (θlmax); 2) is totally out of the field stop aperture (θ mmax). the geometric condition that identifies θlmax and θ mmax are respectively (a.14) utilizing eqs. ((a.1), (a.2)) and the first expression of eq. (a.4) and considering that ocl= p = itan(θ), the next relation is derived (a.15)arctan= . z r f r max t d+im b larctan= z r max til b l . eg oc eg oc r 0 d = = l l max max .sen send d2 23 0 1 max = +t r x i i r x i i i i il l m ! # # z fr rt d2 maxmax sensen dd 22 12 0 max = +t r x i ir x i i i iim l m ! ## z fr rt d# z fr rt d= 13 =x r r 2 c d 2 2 =x ( ) ( ) ( ) ( ) arccos arccosa r dr p r r r dr p r r p r r p r r p r r p r r 2 2 2 1 c c c d d d c d c d c d c d c d 2 2 2 2 2 2 2 2 = + + + + + + + + + + c cm m r a c 1 2=x r 55 laboratory activity for a new procedure of mivis calibration and relative validation with test data numerically resolving the integral of eq. (a.12) and eq. (a.13), the trend of ρ| energy fraction as function of the distance z is obtained. this ρ| trend, only for distances very close to f, shows a difference from the asymptotic value. the difference is related to the methods applied to resolve the integral; in fact it decreases with the increase of the steps number used in the integration increase. at the distance in the tb used for the mivis calibration ≅ 70 cm (distance panel – scan head ≅ 40 cm plus the optical path within the mivis from the scan head to primary mirror) the error is lower of 0.2% (fig. a.2). to obtain an experimental verification, an apparatus composed by a spectralon panel illuminated by two halogen lamps, and the asd has been arranged. the asd has been used with two foreoptics (focal distance respectively of 98 mm and 32 mm with radius rt of 25 mm and 9 mm) mounted on the fiber optic which operate as field stop with a diameter of 1.7 mm. radiance measurements have been performed varying the foreoptics and panel distance. in table a.i the rc values, calculated for both foreoptics at the various distances, are shown. the rc values are ten times the field stop (rd) dimension. the graph of radiance @ 750 nm (fig. a.2) depicts the nonfig. a.2. graph of the radiance trend with respect to the distance from the panel to asd foreoptics. the curves show the asd measured trend for fov 1° (f = 98 mm) and fov 3° (f = 32 mm), while energy fraction curve represents the numerical solution of eq. (a.12) and eq. (a.13). table a.i. rc as a function of the distance between field stop source for the two asd optics. z (cm) rc 98 (mm) rc 32 (mm) 17.5 14.00 1.65 45.0 5.44 0.64 93.0 2.63 0.31 222.0 1.10 0.13 437.0 0.56 0.07 56 cristiana bassani, rosa maria cavalli, angelo palombo, stefano pignatti and fabio madonna functional dependence on the radiance from the distance and the foreoptic field of view. also in the limit condition (i.e. distance of 10.5 cm optics of 98 mm) in which rc is equal to 14 mm (≅ 10 times rd) the radiance asd measure is basically the same as those measured in the other experimental configurations (rd >>rc). therefore, both analytically and experimentally demonstrations show how the optical defocusing, occurring in the tb during atp, does not invalidate the accuracy of the mivis calibration. distribuiti sulle bande dal visibile all’infrarosso termico dello scanner aerotrasportato mivis del cnr, rapporto interno cnr iia. milton, e.j., d.r. emery and c.h. kerr (1997): nerc equipment pool for field spectroscopy. the 21st century observations and interactions, in proceedings of the 23rd annual conference of the remote sensing society, nottingham, u.k., 159-164. rollin, e.m. (2000): monitoring the average spectral radiance of the daedalus ab532a calibration bench for daedalus 1268 atm scanner, in activities of the nerc equipment pool for field spectroscopy (epfs) in support of the nerc airborne remote sensing facility, british geological survey, university of southampton, 33-43. slater, p.n. (1980): remote sensing – optics and optical systems (addison-wesley publishing company, reading, massachusetts, u.s.a.), pp. 575. tanré, d., b.n. holben and y.j. kaufman (1992): atmospheric correction algorithm for noaa-avhrr products: theory and application, ieee trans. geosci. remote sensing, 30, 231-248. vermote, e.f., n. el saleous, c.o. justice, y.j. kaufman, j.l. privette, l.a. remer, j.c. roger and d. tanré (1997): atmospheric correction of visible to middle-infrared eos-modis data over land surfaces: background, operational algorithm and validation, j. geophys. res., 102 (d14), 17131-17141. references bassani, c., r.m. cavalli, a. palombo and s. pignatti (2002): la calibrazione dello scanner mivis (multispectral imaging visibile and infrared spectrometer) nelle campagne di misura realizzate dal cnr nel 2001-2002, in atti vi conferenza nazionale asita «geomatica per l’ambiente, il territorio e il patrimonio culturale», vol. i, p. 337. berk, a., g.p. anderson, l.s. bernstein, p.k. acharya, h. dothe, m.w. matthew, s.m. adler-golden, j.h. chetwynd jr., s.c. richtsmeier, b. pukall, c.l. allred, l.s. jeong and m.l. hoke (1999): modtran4 radiative transfer modeling for atmospheric correction, in optical spectroscopic techniques and instrumentation for atmospheric and space research iii, spie proc., 3756, 348-353. clark, r.n., g.a. swayze, k.e. livio, r.f. kokaly, t.vv. king, j.b. dal ton, j.s. vance, bw. rocwell, t. hoefen and r.r. mcdougal (2002): surface reflectance calibration of terrestrial imaging spectroscopy data: a tutorial using aviris, in proceedings of the aviris conference, pasadena, jpl, nasa. daedalus (1993): daedalus manual’s. maintenance manual aa5000 multispectral infrared and visibile imaging spectrometer (mivis), vol. 2. lechi, g., (2000): calibrazione radiometrica su 102 canali annex 1 to: passive degassing at nyiragongo (d.r. congo) and etna (italy) volcanoes 1 annex 1 (doi: 10.4401/ag-6757) passive degassing at nyiragongo (d.r. congo) and etna (italy) volcanoes. calabrese, s., et al. a.1. sampling and analytical methods volcanic emissions the technique for the assessment of the metal output from volcanoes was based on direct (inplume) collection of the plume on filter substrates. gas and aerosols in the volcanic plume have been sampled from the rims of the active craters. sampling was performed by pumping plume air (a mixture of gases and aerosol) for about two hours through an untreated whatman filter placed inside a teflon filter holder (urg-2000-30fg). this allowed solid and liquid particles to be trapped on the filter surface for later determination. acid gases (so2, hcl, and hf) were simultaneously collected trough three base-treated filters in series (“filter packs”), previously impregnated with 1 n naoh. portable battery-powered pumps were equipped with flow meters to assure a constant flow rate and to measure the volume of collected air (flow rate: 4 l min-1 for filter-packs and 20 l min-1 for aerosols). after sampling, filters were sealed in pre-washed plastic bags and stored in a refrigerator until analysis. acid species were extracted from the filters by leaching with double-distilled water and h2o2, and the solution was analysed for cland so42by ionic chromatography and for fby ion selective electrode. the concentrations of metals in the collected aerosols were measured by inductively-coupled plasma mass and optical emission spectrometry (icp-ms agilent 7500ce and icp-oes jobin yvon ultima2) after total digestion of the filters. the particles trapped on filters were totally dissolved in a mixture of strong ultrapure acids (hno3, hf, hclo4 – 3:1:1) in a microwave oven. the absolute concentrations were corrected for filter blanks. all determinations were performed by the external standard calibration method, using 115in as internal standards to correct instrumental drift; analytical errors ranged between 5 and 30% for the analysed elements. bulk deposition the rain gauges used were bulk collectors which remained open during the whole sampling period, thus receiving both wet and dry deposition. they were composed of a funnel and a polypropylene (pp) bottle. the funnel (büchner type, ø 240mm) can be separated in two pieces for easy and total cleaning. a sheet of filter paper (dp-400-240 basis weight: 80 ± 3 g m-²; thickness: 0.180-0.190 mm; retention of particles: 35-40 µm) was put inside the funnel, and a hdpe disc was inserted between the filter paper and the perforated büchner plate to avoid clogging of the holes. the filter-system (perforated plate, hdpe disc and filter paper) excluded a prolonged interaction between sampled rainwater and coarse particulate, and minimized evaporation. all the equipment was washed with a 2% nitric acid (merck suprapur) solution, and then rinsed several times with de-ionized (milliq) water, dried under hood, packed in double clean plastic bags, and zipped until exposure in the field. disposable plastic gloves were used whenever working with samples and sampling equipment. at the end of the sampling period, the sampling bottle was capped and transported in the laboratory for analysis. all water samples were analysed for 38 elements at ingv-pa. ion chromatography was used for the determination of both major anions (f-, cl-, so42-, no3-, and br-) and major cations (na+, k+, mg2+, and ca2+), using a dionex ion chromatograph ics1100 with suppressor and conductivity detector. anions were analysed with an as14a column and 1 mm sodium bicarbonate + 8 mm sodium carbonate solution as eluent (flow rate 1.1 ml/min), while francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text 2 cations were analysed with a cs12a column and a 20 mm methanesulphonic acid solution as eluent (flow rate 1.2 ml/min). with a 25 µl introduction system, the detection limits (µeq/l) were 0.1 for br-, 0.2 for f-, 0.3 for no3-, ca2+, 0.4 for cl-, so42and mg2+, 0.5 na+, and k+, with precision  3%. calibration curves were built using 6 calibration levels and accuracy was checked by analysing certified solutions (ontario-99 and lethbridge-03). silica concentrations were as measured by using a uv-visible spectrophotometer (detection limits 0.03 mg l-1). trace elements were analysed by icp-ms (agilent 7500ce). the quality assurance and quality control (qa/qc) procedures included: field and analytical blanks, cleaning procedures, field and inter-laboratory comparison. the analytical precision of trace element determinations was checked by analysis of two certified reference material (crm) specific for analysis of trace elements in fresh water (slrs-4 and 1643-e). the deviation between certified and measured values ranged from 0.5 and 27 %. vegetation leaves of senecio species were had picked using disposable gloves from at least five randomly chosen plants at each sampling site. the leaves were packed into paper containers, transported to the laboratory, oven-dried at 40°c for 48 h, and completely powdered by planetary ball mills. agate jars and disposable gloves were used to avoid metal contamination. an aliquot of the powder was dried at 40°c for 24 hours, and about 250 mg of each homogenised sample were mineralised for 60 min by closed vessel acid digestion in a microwave oven (cem mars xpress) using teflon vessels with 3 ml concentrated hno3 (ultrapure grade 65%), 2 ml h2o2 (ultrapure 39%) and 5 ml milliqwater. control samples were added to each batch of samples as follows: at least four certificate reference materials, three replicates of one random sample and one method blank (acid solution without any powder). digestates were further diluted with milliq-water up to a total volume of 50 ml and analysed by icp-ms (agilent 7500-ce) and icp-oes (jobin yvon ultima2) spectrometers. multi-element calibration standards were prepared freshly by dilution from certified stock solutions, and standard solutions of ultrapure quality, and milliq-water. matrix induced signal suppressions and instrumental drift were corrected by internal standardization (rh, in and re). method blank values were subtracted from the raw concentrations of all samples. vol49_2_2006 793 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words seismicity and volcanic activity in deep marine areas – fibre-optic cable failure – southern tyrrhenian sea 1. introduction this paper deals with some very interesting observations on a possible seafloor eruption in the southern tyrrhenian sea. we tried to demonstrate, also by laboratory experiments, that the occurrence of a strong off-shore earthquake (september 2002) triggered off a significant lava flow. this flow trapped the rome-palermo submarine communication cable, burning it and interrupting communications in the process. these events were part of a long sequence. in fact, 2002 was an anomalous year from a geodynamic point of view for the southern tyrrhenian sea and sicily (italy). in may of that year the off-shore earthquake was preceded by the strong high-energy «strombolian» activity of stromboli. it was then followed by an unusually explosive etna eruption with associated earthquakes starting in october, and by a remarkable degassing episode at panarea in november. finally, an unusual lava-flow eruption by stromboli associated with the sliding of a volcano flank occurred in december. the failure of the volcano’s submarine part triggered off a tsunami that caused substantial damage to the island’s coastline. it is hard to tell whether there are physical links between all the aspects of this sequence of events; however, this lies beyond the aim of our paper. 2. tectonic outline the tyrrhenian sea is a back-arc basin, with a maximum water depth of around 3600 m, a new active volcano in the tyrrhenian sea? paolo favali (1) (2), angelo de santis (1) (3), giuseppe d’anna (1), bruno di sabatino (3), mario sedita (4) and elio rubino (5) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) università degli studi di roma «la sapienza», roma, italy (3) università degli studi di chieti «g. d’annunzio», chieti scalo, italy (4) istituto nazionale di fisica nucleare, laboratori nazionali del sud, catania, italy (5) elettra tlc spa, telecom italia group, roma, italy abstract a strong earthquake occurred in 2002 off-shore from the northern coast of sicily in the southern tyrrhenian sea (italy), and was followed by a series of hundreds of aftershocks. communications through the fibre-optic cable between palermo and rome were interrupted a few hours after the occurrence of the main shock. after the required technical checks, the failure point was found a few kilometres away from the seismic sequence area. a few days later, a specialised cable ship reached the failure area. one side of the cable was completely burnt, while about three kilometres of cable was found locked. tests on slices of cable showed that the temperature at which the cable was heated went well above 700°c. we can speculate that the earthquakes triggered off the emission of a submarine lava flow that buried, trapped and burnt the fibre-optic cable. the revising of the bathymetric survey made before the cable’s deployment allowed for the identification of a low seamount in the vicinity of the rupture. this structure could represent the lava flow’s source volcano. mailing address: dr. paolo favali, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: paolofa@ingv.it 794 paolo favali, angelo de santis, giuseppe d’anna, bruno di sabatino, mario sedita and elio rubino which opened 10 ma ago. the opening of the basin progressed towards se in pleistocene with the formation of the magnaghi-vavilov (7-3.5 ma) and the marsili basins (1.7-1.2 ma) (kastens et al., 1988; mongelli et al., 2004). its opening has been related to the sinking of the adriaticionian lithosphere with a rollback mechanism (malinverno and ryan, 1986; faccenna et al., 1996). the tyrrhenian basin is also characterised by high heat flow values, up to over 200 mw/m2 (della vedova et al., 1991; mongelli et al., 2004). the tyrrhenian morphology is complex but similar to larger ocean basins in having an abyssal plain, seamounts and well-developed continental shelves (marani et al., 2004a-c). the seven aeolian islands and the seamounts form a volcanic necklace. they began to develop about one million years ago with the alicudi island being the youngest (see table i; beccaluva et al., table i. age (ma) of the seven aeolian islands and some seamounts, arranged from the youngest to the oldest (beccaluva et al., 1985; kilburm and mcguire, 2001). island age (ma) alicudi 0.06 vulcano 0.12 panarea 0.15 lipari 0.16 stromboli 0.23 palinuro 0.35 salina 0.43 enarete 0.73 eolo 0.81 filicudi 1.02 sisifo 1.30 fig. 1. sketch of the southern tyrrhenian sea (redrawn simplified from marani et al., 2004b,c), including coastlines and isobaths (every 500 m). all the volcanic islands (in grey in the figure) and submerged seamounts are indicated, except the vavilov seamount that lies at nw out of the map. proceeding from ne counter-clockwise: pl – palinuro; gb – glabro; al – alcione; la – lametini; st – stromboli; pa – panarea; vu – vulcano; li – lipari; sa – salina; fi – filicudi; ac – alicudi; eo – eolo; en – enarete; pr – prometeo; us – ustica; si – sisifo; gl – glauco (marani and gamberi, 2004; trua et al., 2004). the circles represent the main shock (mw 5.9) and its most energetic aftershocks according to table ii, where the errors in km can be found. the star indicates the cable failure point. 795 a new active volcano in the tyrrhenian sea? 1985; kilburn and mcguire, 2001). besides these islands and ustica, many volcanic seamounts have been recognised (marani and gamberi, 2004). the submerged aeolian arc starts from north with the palinuro volcanic complex (striking e-w for 75 km) and glabro seamount, the easternmost from palinuro. then the north-eastern part with three volcanoes, alcione and the twin cones of the lametini seamounts, lie in the calabrian slope. the submerged western arc is aligned in a nw direction and is located to the west of alicudi island, and consists of eolo, enarete, sisifo and glauco seamounts from se to nw. a lava field, called prometeo, has been recently discovered between these last sea-mounts and ustica (trua et al., 2003). the aeolian magmas belong to island arc basalt (iab)-type (trua et al., 2004). ustica is an inactive pleistocene island, which dates from 0.735 to 0.132 ma (etiope et al., 1999). its volcanics are classified, together with volcanic products of the prometeo lava field located slightly nw of area studied in this paper, as island ocean basalt (iob)-type (trua et al., 2003, 2004). the two central volcanoes, vavilov (elongated about 30 km in a n-s direction, with maximum width of 14 km) and marsili (elongated about 60 km nne-ssw, with a mean width of 16 km), are located in the middle of the two basins magnaghi-vavilov at nw and marsili at se, respectively (marani and gamberi, 2004). figure 1 shows a sketch of the southern tyrrhenian sea together with some information relevant for this work. 3. seismicity on september 6, 2002 (01:21 gmt) a mw 5.9 shallow earthquake (5-10 km) occurred offshore the northern coast of sicily in the southern tyrrhenian sea, wsw of alicudi (azzaro et al., 2004). the seismic event was followed by hundreds of aftershocks over the following three months. the main seismic event struck all of the northern coast of sicily, even causing some damage in the town of palermo. in an area 20 km of radius from the failure cable point, 654 aftershocks were localised by the ingv seismic network up to the end of december 2002 (115 shocks just in the first day). in particular, five earthquakes with magnitude between 3.5 and 4.3 occurred within less than one hour (see table ii; ingv, 2005), and the largest aftershock (ml 4.6) on september 27 (fig. 1). this seismic activity was very important if compared with the recent past. for instance, in the same area only 59 earthquakes were localised by the same network from january 1, 1994 to september 5, 2002 (maximum magnitude 2.8). this is also true if we consider that the seismic activity of the tyrrhenian basin is poorly monitored and localised, due to the presence of an on-shore network alone (favali et al., 2004). the epicentral area is located wsw of alicudi roughly in the middle between this island and ustica island. in an adjacent area (within 100 km) another seismic sequence recently occurred in 1998 with maximum magnitude 4.3 (agate et al., 2000). in the historical seismic cattable ii. list of the major shocks (m l ≥ 3.5) occurring on september 6, 2002 (ingv, 2005); the last column indicates the distance in km of each seismic event from the rupture in the fibre-optic cable. time (gmt) ml latitude (°n) longitude (°e) distance (errors in km) (errors in km) from the rupture (km) 01:21:28.6 5.6 (mw = 5.9) 38.381 (± 1.4) 13.701 (± 1.4) 8.6 01:33:15.9 4.2 38.380 (± 1.9) 13.670 (± 1.8) 6.4 01:39:53.6 3.9 38.339 (± 2.7) 13.734 (± 2.9) 13.6 01:45:30.3 4.3 38.435 (± 2.0) 13.731 (± 1.4) 10.7 02:08:58.2 3.6 38.443 (± 2.3) 13.773 (± 2.2) 14.4 02:15:53.6 3.5 38.490 (± 1.6) 13.807 (± 1.2) 19.0 796 paolo favali, angelo de santis, giuseppe d’anna, bruno di sabatino, mario sedita and elio rubino alogue other off-shore earthquakes that affected the northern coast of sicily are reported; among these the 1726 and 1940 earthquakes were the major events, both with significant damage also affecting palermo (boschi et al., 1997; azzaro et al., 2004). the 2002 earthquake focal mechanism is reversed like all the mechanisms of events located along the northern margin of sicily, having a contractional axis with an approximately n-s strike (vannucci et al., 2004). this is in accordance with the area’s geodynamic framework (malinverno and ryan, 1986; faccenna et al., 1996; agate et al., 2000). 4. communications cable failure communications between rome and palermo broke down at 04:55 gmt, 03:34 hours after the occurrence of the main shock. the total length of the optic cable connecting the two sites is 444.5 km, and 49 segments with four relays subdivide the cable. the fibre-optic cable is a pirelli light weight protected (lwp). this type of cable is generally used at any sea depth between 1500 m and 6000 m. it has an optic core with eight fibres placed inside a composite unit, comprising two layers of carbon steel wires and fig. 2. picture of the lwp-optic cable (left) and its actual section (right). fig. 3. photograph of the end of the burnt fibre-optic cable taken on board the cable ship just after the recovery. 2 table iii. main characteristics of lwp 8-fibre cable. aluminium tape thickness mm 0.30 overall diameter (nominal) mm 28.40 weight in air (approximately) kg/m 1.10 weight in water ( approximately) kg/m 0.45 breaking load kn 100.00 short-term allowable load (1 h) kn 85.00 torque nm/kn 0.40 reverse bend resistance (r = 1 m) cycles 30.00 hydrostatic pressure resistance mpa 70.00 minimum breaking strain of steel wires mpa 1900.00 aluminium tape d.c. resistance (10°c) w/km 1.30 capacitance mf/km 0.20 composite unit d.c. resistance (10°c) w/km 1.00 3 797 a new active volcano in the tyrrhenian sea? a high conductivity copper tube. the main characteristics of the cable are summarised in table iii and a picture is shown in fig. 2 together with the actual section. according to the tests carried out by palermo terminal station (ts) the cable presented all the fibres broken at 41.6 km from ts and at a depth of about 1450 m, based on the bathymetry performed before the cable deployment. the cable ruptured a few kilometres away from the seismic sequence area (fig. 1). a few days later (on september 14) a cable ship reached the area of the failure to repair the damage. the ship made many attempts to recover the two sides of the broken cable. on the palermo side, 5051 m of cable up to the fault were recovered. this end of cable appeared to be been broken by mechanical tension, probably due to the recovery operations. on the rome side, 3409 m of cable up to the fault were recovered, whereupon the end was found to be burnt (fig. 3). the optical tests performed from the ship confirmed that 2880 m of cable had gone missing. 5. laboratory test results one end of the cable was broken by mechanical tension with almost 3 km lost, while the other burnt end supports the hypothesis that something locked the cable at a temperature high enough to burn it. in order to confirm whether this hypothesis is feasible and to check the behaviour with temperature of the materials the cable was made of, especially the chemical elements with higher melting temperatures (see table iv; weast, 1976-1977), we decided to heat different original slices of cable at increasing temperature steps with an oven able to reach high temperature values. the oven reached a maximum temperature of 1100°c (model be43n bicasa spa). the initial steps were 130°, 230°, 250°, 350° and 750°c, at these stages the non metallic parts were seen to melt. to check the fusion time and temperature of the aluminium, we decided to perform another test at 810°c fig. 4. the results of the oven tests at 810°c with time of 5 min (left) and 15 min (right). table iv. density, fusion and ebullition temperatures for copper and aluminium, two of the elements the lwp cable is made of (weast, 1976-1977). chemical ρ (g/cm3) fusion ebullition element temperature temperature (°c) (°c) copper (cu) 8.92 1083.4 ± 0.2 2567.0 aluminium (al) 2.72 660.4 ± = 2467.0 798 paolo favali, angelo de santis, giuseppe d’anna, bruno di sabatino, mario sedita and elio rubino leaving the sample there for one, five and fifteen minutes. in the first two steps the time was not enough for the oven temperature to reach the desired level, whereas in the last test the aluminium finally melted. in this latter case, nacl was also added (fusion temperature 801°c) to have another independent control of the oven temperature. the results of the steps at 810°c with times of five and fifteen minutes are shown in fig. 4 (pictures on the left and right-hand sides, respectively). on the grounds of the test results, we concluded that the minimum temperature (at which the cable was heated) was well over 700°c for a length of time significant enough to melt and damage the cable definitively. 6. discussion the time difference (3:34 h) between the main seismic event and the line interruption makes it unlikely that trapping, pulling and breaking of the underwater cable could be due to fractures, faults, collapses directly linked to the seismic events. the distance between the cable and the epicentral area (≥ 8 km), the imprisonment of the cable – the failure to recover more than 2.8 km cable with an approximately n-s strike, parallel to the maximum inclination of the seabed – the traction and breakage of the cable, the disappearance of the internal resins, the fusion of the cylindrical aluminium ring (t≥660°c, see table iv), the temporal interval between seismic swarm and transmission interruption on the cable, are all data and clues coherent and plausible with a significant deepwater lava flow. moreover, the same data should underline a good enough fluidity, itself of basaltic-andesitic magmas that are characterised by a temperature of less than or equal to 1100°c at emission, and 700-750°c at time of tightening. the seismic activity’s time trend (septemberdecember 2002) and the seismogram records are typical of a so-called «seismic sequence», perfectly suited to the expected size of the seismogenic structure, with a main shock (the largest in magnitude) and hundreds of aftershocks in a few months (with magnitude over 4.0 in only a few cases). hence, given the area’s seismic «history», the main earthquake and its aftershocks are recognised as being the trigger of the lava effusion. finally, the shallow epicentral depth (5-10 km) and the time difference between the seismic event and the interruption allow us to evaluate the average speed of lava climbing at 3-8 km/h. these range values are completely coherent with the distance between the areas of the shock occurrence and the cable failure. last but not the least, the bathymetric survey that was performed to define the cable route before its deployment fig. 5. n-s bathymetric profile parallel to the cable route, based on the data collected to decide the cableway before its deployment. the figure shows, in the middle, a seamount about 10 km wide (n-s direction) and 0.6 km high. the arrow roughly indicates the cable rupture area. the vertical scale is exaggerated 10 times with respect to the horizontal scale. 799 a new active volcano in the tyrrhenian sea? shows the presence of a low seamount in the proximity of the rupture area (fig. 5). this seamount (about 10 km wide in n-s direction and 0.6 km high) roughly elongated n-s with steep flanks in the northern and southern edges striking e-w and the physiographic shape of a volcano structure, can be considered as the source of the lava flow. the cable rupture occurred on its flank. submarine eruptions in the italian seas are not that unusual, as historical events reported by mercalli demonstrate (mercalli, 1883). 7. conclusions a 5.9 earthquake occurred off-shore wsw alicudi, and 3 h later the communications along the main fibre-optic cable connecting palermo to rome were interrupted. the repair operations found the cable burnt, with almost 3 km of cable lost. in an attempt to clarify the cause-effect relationships between the earthquake occurrences and the cable interruption, and to model a feasible explanation, we decided to check the cable’s behaviour with the temperature in laboratory tests. by progressively heating up some cable samples up to 810°c we demonstrated that the cable had been heated to a minimum temperature well over 700°c. this temperature excludes the hydrothermal vents as a potential cause of the burning. therefore, we inferred that the earthquakes had triggered a deepwater lava flow, with this effusion causing the damage, trapping and burning the cable. the presence in the vicinity of the failure point of a volcano-shape seamount can corroborate the hypothesis of this structure as a source of effusion. finally this paper shows also the importance of considering geo-hazards (seismicity, volcanic activity and slope instabilities) in planning the deployment of network infrastructures on the seafloor, such as communications cables. this should allow us to avoid unnecessary risks for such infrastructures, also reducing the direct and indirect economic consequences, respectively, the repair and service costs, along with the even higher costs incurred by the breakdown in communication 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(editor) (1976-1977): handbook of chemistry and physics (section b) (crc press, cleveland), 57th edition, pp. 439. vol49_2_2006 853 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words seismic bulletin – magnitude – completeness analysis – errors of hypocentral locations – detection capacity 1. introduction during the first five years of the nineties, the istituto nazionale di geofisica (nowadays ingv) was involved in a european project named «eurobull». the aim of the eurobull group was to compile a high quality european bulletin to be used both as a reference, to evaluate bulletins produced by international networks, and for scientific purposes. italy, france and sweden collected data from about fifteen countries and, starting from their own bulletin procedures, processed the data independently from each other. i was directly involved in the software implementation and giuseppe smriglio, a very helpful person who was the manager of the data centre, let me modify the italian bulletin procedures adapting them to the european purposes. moreover, the eurobull group produced a series of control routines to test the quality of the new bulletin, so we had the opportunity to apply the same test to the italian bulletin. this work, that analyzes sixteen years of ingv bulletins, recalls the efforts to improve the network and the quality of the data for which giuseppe was one of the principal promoters. 2. database bulletins produced by ingv between 1988 and september 2003, are the database of the present work. figure 1 shows the area analyzed and the ingv national centralised seismic network (rsnc) configuration used in this paper. figure 2 illustrates the evolution of the rsnc during the analyzed period. colors of the stacked evaluation of sixteen years of ingv seismic bulletins rosalba di maro istituto nazionale di geofisica e vulcanologia, roma, italy abstract this work analyzes earthquake parameters published in the seismic bulletin of the istituto nazionale di geofisica e vulcanologia (ingv). the analysis and interpretation of the digital signals, done by specialist employees daily, produce most of the seismological information that comprises ingv’s earthquake bulletins. after a brief introduction on the criteria we use to obtain seismic parameters, this paper will review the processing procedures employed over a period of sixteen years from 1988 to september 2003.this study also addresses the issue of the comparison between magnitude calculated on signal duration (md) and on amplitude (ml) and the lack of a correct calibration between them. a completeness analysis of the whole bulletin performed using both the stepp and habermann techniques shows the importance of considering changes in the seismicity rate and in the geometry of the seismological network. to conclude this excursus, we calculated the errors of hypocentral locations and the detection capacity of individual seismic stations. this final step stresses the increasing improvement of the ingv seismic network over the past 16 years. mailing address: dr. rosalba di maro, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143, roma, italy; e-mail: rosalba.dimaro@ingv.it 854 rosalba di maro histograms are related to the years, the different amplitude of the columns is related to the different percentage working that year (since january 1988, the ingv shift workers draw up a daily report on the efficiency of each seismic station). stations are in descending order based on latitude, and the stations reported in the four hisfig. 1. ingv seismographic network configuration. the star symbols indicate the station locations, the red plus symbols the closed stations. the four colors of the stars are related to four sectors: north, center-north, center-south, south. fig. 2. evolution of the seismographic network during the analyzed period. colors of the stacked histograms are related to the years, the different amplitude of the columns is associated to the different percentage working that year. stations are in descending order based on latitude, and the stations reported in the four histograms are indicated, in the previous figure, by the four sectors respectively. 1 2 855 evaluation of sixteen years of ingv seismic bulletins tograms are indicated, in the previous figure, by the four sectors respectively. it is easy to identify from the color bars closed stations and new ones, most of which were installed in central italy. 3. local magnitude and duration magnitude before processing the database, a brief mention of the magnitude we used in our determinations is useful. for local events we apply the magnitude-duration relation (console et al., 1988) (3.1) where a = 2.0, b = 0.870 and c = 0.082. τi is the ith seismic signal duration and ∆i is the linear ( )logm a c bd i i10i )= + -τ ∆ distance of the i-th station compared to the epicenter determination. the (3.1) is derived from the observation that the absolute time (τi + c∆i) is constant, in other words independent from the station. most of the time, the automatic acquisition system calculates τi determining the end of the signal when its amplitude comes back to the noise level. when the signal duration exceeds 8-min (that is the maximum acquisition time) the operator evaluates the duration directly from the analog acquisition that has a continuous registration on the paper. the flowchart in fig. 3 represents the algorithm that attributes md value to a regional event. this quantity is calculated as arithmetic average among all the values. the maximum and (/or) the minimum are eventually removed if they are «very distant» from the average. the algorithm that attributes a local magnitude to an event (fig. 3), is the same as the one for md. in this case we eliminate values less than 1.5. the richter formula is used for stations whose distances, from the epicenter, are less than 600 km (3.2) the distance function f (∆) is the same published by richter in 1935 (richter, 1935). on the other hand, the deduced attenuation function for italy closely corresponds to richter’s one, as described by gasperini (2002). at the present time the rsnc is mostly composed of short period vertical seismometers (s-13 geotech). for this reason, in eq. (3.2), we use the maximum amplitude picked up automatically on the vertical seismic signal and corrected for the instrumental response, as described below. if asp is the short period maximum amplitude and hsp the transfer function of the whole instrumental chain, we can obtain the wood anderson amplitude (nm) by (3.3) where hwa is the wood anderson transfer function. for regional events (600-1600 km) we apply a modified nuttli formula (nuttli, 1973) a a h h i i sp i sp wa = ( ) ( ) .logm a a f 2l ns ew i 10i = + + ∆ fig. 3. flowchart of the algorithm that attributes the md value to a regional event. this quantity is calculated as the arithmetic average eventually removing the maximum and (/or) the minimum, if they are «very distant» from the average. 856 rosalba di maro (3.4) the original formula was modified by means of regression with italian data (1988-1993). 4. data processing 4.1. discrepancy between the duration and local magnitude in this paper we processed sixteen years of data: from january 1988 to september 2003 (inclusive). the ingv bulletin reports, for that period, 42154 locatable events («locatable» means that the event has been located using at least six phases). among them, 23595 events have both local and duration magnitudes and 9594 have at least five md values. figure 4 illustrates the annual trend of these quantities. ml and md are very different from each other most of the time. figure 5a,b illustrates this discrepancy: the first displays the frequency of the events for each class of magnitude md and ml. it is easy to notice that the ml histogram is shifted more to the left compared to the md ones. the dif.log+. ( )log2 34 ) ∆+.4 37-=( )m t a b i i i i 10 10lg c m ferent shape of the two histograms, provided by the same event data-set, shows that the italian seismicity in the last sixteen years is concentrated between 2.5 and 3.2 considering md, but under 2.5 if we consider ml. figure 5b illustrates that md and ml are not univocally determined. also the most probable value seems to be the medium one, considering md in lack of ml creates a non homogeneous data set. this ambiguity is due to the method of deriving md. the magnitude-duration relation (3.1) fig. 4. annual trend of italian seismicity. fig. 5a,b. a) discrepancy between the frequency of the events for class of magnitude md and ml. b) three-dimensional vision of the correspondences between md and ml. a b 857 evaluation of sixteen years of ingv seismic bulletins has been obtained analyzing 15 months of data that are probably not enough for a correct determination of parameters. another problem is related to the new formulation given by (3.1) of the absolute time. to validate the hypothesis (4.1) where τs is the absolute duration; ∆s is the total distance covered by waveforms that travel with velocity v; τi is the i-station signal duration; ti is the i-station arrival time; t0 is the origin time. we calculated τs and the related standard deviations for the whole data set. figure 6 illustrates this trend, the biggest magnitude values, that have the biggest number of recordings, are associated to high values of standard deviations. this means that the absolute duration is not independent from the station and (4.1) is not validated. 4.2. data set completeness to determine the completeness of our data set we apply the stepp method (stepp, 1972). this model assumes that: – earthquake distribution can be modeled by a poisson distribution whose standard deviation is ( ) const v t ts s i i 0= = + =τ τ ∆ (4.2) where λ is the average of the occurrences in the period t. – the seismic activity for the whole observed period is stationary i.e. λ= cost. let’s go back to our data set and consider first the local magnitude. observing the seismicity year by year, four steps of local magnitude and duration magnitude have been selected md ≤ 2.4 ml ≤ 1.8 2.5 ≤ md ≤ 2.8 1.9 ≤ ml ≤ 2.4 2.9 ≤ md ≤ 3.4 2.5 ≤ ml ≤ 3.0 md ≥ 3.5 ml ≥ 3.1. figures 7a and 8a show the cumulated number of events during the 16-year period, whereas figs. 7b and 8b show, in bi-logarithmic scale, s versus t (starting from 2003). according to stepp’s theory, when the mean rate of occurrences is constant, the related period of time is ‘complete’ and (4.2) has the linear trend shown in the figs. 7b and 8b, as reference, in green. the four steps of magnitude md have the same «critical» time tc under which the data set is considered incomplete. the four steps of magnitude ml have two «critical» times, one, in common with md corresponding to 1997, and the other for ml ≥ 3.1, corresponding to 1999. the explanation for this «critical» year could derive from fig. 2: numbers of stations were added, during 1997, in the middle-south part of italy. consequently, an apparent low seismicity has grown during the last period. the increment in the station density makes the data set for the period before 1997 «incomplete». moreover we can make another comment: stepp’s approach to the data-set completeness has the «poissonianity» of the event distribution underlying the hypothesis. of course clusters of events as well as aftershocks or foreshocks are not independent, so their spatial, temporary distributions are not «poissonian». in addition we should mention the «colfiorito» sequence that contributed with more than 5000 events during the 1997 and 1998. of course it is possible to make the database «poissonian» removing the clusters, but this is not the purpose of this paper. = t σ λ λ fig. 6. standard deviations of the absolute time (τs) versus md. the standard deviation values indicate that τs is not constant. 858 rosalba di maro habermann (1983) proposed another definition for the minimum magnitude of completeness. beginning from the gutenberg-richter (1944) relationship . (4.3) the determination of the minimum magnitude of log n a bm10 = completeness mc of a data set is obtained plotting (4.3) where n is the cumulative number of events, m the magnitude and a and b are constants. figure 9a,b shows the frequency-magnitude distribution of italian earthquakes during these sixteen years. the straight line represents the best fit of the observations in the area of «complete» recording indicated by mc. we can individuate this a b fig. 8a,b. a) cumulated number of events during the 16-year period for four steps of local magnitude. b) standard deviation, versus t (starting from 2003) in bi-logarithmic scale for four step of local magnitude. fig. 7a,b. a) cumulated number of events during the 16-year period for four steps of duration magnitude. b) standard deviation, versus t (starting from 2003) in bi-logarithmic scale for four steps of duration magnitude. a b 859 evaluation of sixteen years of ingv seismic bulletins values that for ml coincides with the minimum value 1.5 and for md is 2.3 ± 0.1. performing the regression (for both relations the correlation coefficient is approximately 1) we can write (4.3) as follows: . 4.3 hypocentral locations as mentioned before, in these sixteen years of bulletins, our qualified technicians processed more than 40 000 events, merging data from several seismic networks to obtain a high quality product. the hypocentral determinations are performed, using routines belonging to the hypo71 family. . .log n m1 03 5 8l10 = + . .log n m1 54 8 0d10 = + fig. 9a,b. a) minimum magnitude of completeness for md in accordance with habermann (1983). b) minimum magnitude of completeness for ml in accordance with habermann (1983). fig. 10. the first three events of the 2003 ingv bulletin. a b 860 rosalba di maro figure 10 shows an extract of the ingv bulletin, in particular the first three events of 2003. most of the events have the location fixed at a certain depth, sometimes the software itself fixes the depth, and sometimes because the depth value is not consistent the technicians fix them manually. by increasing the number of the stations, the number of events with fixed depth is decreasing as the fig. 11 demonstrates. figure 12a reports all the depths selected among events with the product of coordinate errors (lat error ∗ long error ∗ depth error) below 1000 km3. the interpolation is made on a grid of 30 km side. from the figure the very deep location of the events in the southern part of the tyrrhenian sea is clear. the depth of most of the other events is less than 40 km, as fig. 12b shows. regarding the errors on the hypocentral locations, the routine we use gives latitude, longitude and depth errors in kilometres instead of the ellipsoid error. to have an idea on the quality of the locations, let us take a look at the error location distribution. table i shows (in the second column) the percentage of the events with a certain error indicated in the first column. most of these events have a very small error, fig. 13a-d show the distribution of the smallmedium location error (below 1000 km3) separated for class of duration magnitude, it is remarkable that greatest errors are related with areas not included in the rsnc and, of course, fig. 11. percentage of the events whose hypocentral location has fixed (darker) depth. the percentages are calculated over a three year period, as indicated on the abscissa. fig. 12a,b. a) hypocentral depths map distribution. the events selected have an error on the coordinates (lat error ∗ long error ∗ depth error) below 1000 km3. the interpolation is made on a grid of 30 km per side. b) hypocentral depths distribution in class of 10 km each. the events selected have an error on the coordinates (lat error ∗ long error ∗ depth error) below 1000 km3. a b 861 evaluation of sixteen years of ingv seismic bulletins table i. evaluation of the error associated to hypocentral locations. the percentage only considers the events whose depth is not fixed. v (km3) lat error ∗ long error ∗ depth error percentage 0 ≤ v < 100 81.6% 100 ≤ v < 1000 15.6% v ≥ 1000 2.8% fig. 13a-d. a) distribution of the small-medium localization error (below 1000 km3 ) for events with md ≤ 2.4. the interpolation is made on a grid of 50 km per side. b) distribution of the small-medium localization error (below 1000 km3) for events with 2.5 ≤ md ≤ 2.8. the interpolation is made on a grid of 50 km per side. c) distribution of the small-medium localization error (below 1000 km3 ) for events with 2.9 ≤ md ≤ 3.4. the interpolation is made on a grid of 50 km per side. d) distribution of the small-medium localization error (below 1000 km3) for events with md≥3.5. the interpolation is made on a grid of 50 km per side. the bigger the event the better the hypocentral location will be. to complete the «panorama», fig. 13e summarizes everything about the 2.8% of the largest errors. the radius of the circumferences is increasing with md, the color is darkens with the error values and, in the end, the plus sign indicates the events which occurred in the last three years. we can notice that: a b c d 862 rosalba di maro – the hypocentral locations with the biggest error and magnitude are outside the network. – most are concentrated in the southern part of the tyrrhenian sea where the deepest events occur. – the increasing number of the stations in the last three years improved the quality of locations, only few of them, with low magnitude, are inside the network. 4.4. the detection capacity of the seismographic stations this last paragraph is dedicated to the performance of the rsnc stations. all the events recorded by each station have been considered, more precisely, all the arrival times that contributed to evaluate the hypocentral location have been considered (not all the phases recorded by the automatic system will contribute to the location process; it depends on the signal/ noise ratio and on the convergence of the location algorithm solution). the events are collected into four steps of duration magnitude, as usual md ≤ 2.4 2.5 ≤ md ≤ 2.8 2.9 ≤ md ≤ 3.4 md ≥ 3.5. the area around each station has been divided into twelve parts (30° each), each part has been divided into sectors whose dimensions are related to the distance from the center (i.e., the station). figure 14 explains the criteria. i selected, to be brief, three stations that, from north to the south are: trieste (tri), l’aquila (aqu) and sicignano (sgo). the selection was on the location basis and also because these stations are currently working. the complete analysis of all the stations will be the topic of a specific paper. the results of this partial investigation are reported in fig. 15a-l: 1) sgo has the best coverage for all the steps of magnitude. fig. 13e. largest errors (above 1000 km3) distribution. the radius of the circumferences is related to md value, the color changes with the error values and, in the end, the plus sign indicates the events which occurred in the last three years (see the legend as reference). fig. 14. criteria to determine the detection capacity of the seismographic stations. the area around each station has been divided in twelve parts (30° each), each part has been divided in to sectors whose dimensions are related to the distance from the station: for the first 200 km the sectors are 50 km by 50 km, from 200 km to 600 km the distances are 100 km each. the farthest sector has the distance, from the station, determined by the farthest event detected (marked in the figure by a red or white star). the white sectors, eventually not closed, indicate a lack of events. 863 evaluation of sixteen years of ingv seismic bulletins fig. 15a-h. a) percentage of events (md ≤ 2.4) detected by trieste station. b) percentage of events (md ≤ 2.4) detected by l’aquila station. c) percentage of events (md ≤ 2.4) detected by sicignano station. d) percentage of events (2.5 ≤ md ≤ 2.8) detected by trieste station. e) percentage of events (2.5 ≤ md ≤ 2.8) detected by l’aquila station. f) percentage of events (2.5 ≤ md ≤ 2.8) detected by sicignano station. g) percentage of events (2.9 ≤ md ≤ 3.4) detected by trieste station. h) percentage of events (2.9 ≤ md ≤ 3.4) detected by l’aquila station. the criteria are explained in fig. 14. green dots are the events recorded by the rsnc network in the range of duration magnitude indicated. a b c d e f g h 864 rosalba di maro 2) the farthest event detected is not in a sector with a high percentage of detection. 3) no privileged direction in the detections is visible. 4) a high percentage of detection does not correspond to those areas with a high seismicity. the «strategy» in the performing of hypocentral location is one possible explanation for the last point. in fact, in case of an abundant number of phases, the technicians exclude those arrival times belonging to far stations in order to obtain a robust solution for the location algorithm. this could be the reason for the second point also. 5. conclusions sixteen years ago, the possibility to acquire and process digital seismic signals gave a new impulse and speeded up the process of optimization of the rsnc. this overview emphasizes the effort to improve the software and the hardware related to the correct detection and characterization of the earthquakes. regarding the parameters of the hypocentral location the accuracy is not under discussion. the only weak link in the chain is, as indicated above, the magnitude and the correlation between the local and duration magnitude. nowadays the introduction of the new digital seismographic network, with new sensors, three-components and broadband, will allow a more correct definition of the local magnitude, so we could calibrate the duration magnitude having much more data to compare. references console, r., b. de simoni and a. di sanza (1988): riesame della relazione magnitudo-durata, in proceedings of vii yearly meeting of the gruppo nazionale di geofig. 15i-l. i) percentage of events (2.9 ≤ md ≤ 3.4) detected by sicignano station. j) percentage of events (md ≥ ≥ 3.5) detected by trieste station. k) percentage of events (md ≥ 3.5) detected by l’aquila station. l) percentage of events (md ≥ 3.5) detected by sicignano station. the criteria are explained in fig. 14. green dots are the events recorded by the rsnc network in the range of duration magnitude indicated. i j k l 865 evaluation of sixteen years of ingv seismic bulletins fisica della terra solida (gngts), november 30-december 2, rome, italy, 51-62. gasperini, p. (2002): local magnitude revaluation for recent italian earthquakes (1981-1996), j. seismol., 6, 503-524. gutenberg-richter, n. (1944): frequency of earthquakes in california, bull. seismol. soc. am., 35, 185-188. habermann, r.e. (1983): telesesmic detection in the aleutin island arc, j. geophys. res., 88, 5056-5064. nuttli, o.w. (1973): seismic wave attenuation and magnitude relations for eastern north america, j. geophys. res., 78 (5), 876-885. richter, c.f. (1935): an instrumental earthquake magnitude scale, bull. seismol. soc. am., 25, 1-31 stepp, j.c. (1972): analysis of completenessof the earthquake sample in the puget sound area and its effect on statistical estimates of earthquake hazard, in proceedings of the first microzonation conference, seattle, u.s.a., 897-909. s0662 annals of geophysics, 60, 6, s0662, 2017; doi: 10.4401/ag-7427 candidates for volcanoes under the ice of antarctica detected by their gravito-topographic signal jaroslav klokočník1,*, jan kostelecký2, 3, aleš bezděk1, ivan pešek4 1 astronomical institute, czech academy of sciences, ondřejov, czech republic 2 research institute of geodesy, topography and cartography, zdiby, czech republic, 3 department of geodesy and mining surveying, faculty of mining and geology, ostrava, czech republic 4 department of geomatics, faculty of civil engineering, czech technical university in prague, prague, czech republic article history received april 7, 2017; accepted august 31, 2017. subject classification: antarctica, gravity field models eigen 6c4 and ret 14, bedmap 2 bedrock topography, gravitational aspects, subglacial volcanoes. abstract we analysed the newest gravity field models (given in the form of spherical harmonic expansion to degree and order 2190) eigen 6c4 and ret 14, which consist basically from the gravity data from eigen 6c4 plus the bedrock topography from the bedmap 2 model, describing the topography of the ground under the ice of antarctica. from these models we computed the gravity anomalies, the marussi tensor of the second derivatives of the disturbing potential, the gravity invariants and their specific ratio, the strike angle and the virtual deformations. we applied these results for a selected part of antarctica; not too far from the lake vostok we discovered at least two objects that might be subglacial volcanoes. we present all predictors and arguments we have available to support this hypothesis, but we are well aware that the “last word” waits for other specialists. we provide geographic coordinates on the ice surface where to dig for the possible volcanoes. 1. introduction we use the best now available gravity data (the harmonic geopotential coefficients from the gravity field model ret 14 [hirt et al. 2016], focused on antarctica, transformed to the various gravity functions) and the bedrock topography data [bedmap 2, fretwell et al. 2013] to detect possible subglacial volcanoes and other features (like subglacial lakes and rivers). this paper is a continuation of klokočník et al. [2016]; it deals with possible subglacial volcanoes (glaciovolcanoes) discovered near the lake vostok (lv). volcanoes are common on this planet and there is about 30 volcanoes (not subglacial) already known in antarctica, see, e.g., antarcticglaciers.org, last updated in march 2014; lemasurier & thomson 1990 and lough et al. 2013 for discovery of two active subglacial volcanoes in marie byrd land from seismic observations. many more such volcanoes can still be hidden under the ice [e.g., walton 2013, p. 46]. we can only speculate about the number and age of the subglacial volcanoes in antarctica. knowledge of their location is important for further investigations on their role in glacial motion in antarctica as well as seismic activities and the tectonic setting. their possible role in glaciation and deglaciation of antarctica is also a highly interesting topic. so we have good reasons to seek for candidates of subglacial volcanoes. active subglacial volcanoes are known to be located in iceland, british columbia and yukon. what we could expect in antarctica due to the local conditions (location under the ice) are small slopes of the volcanoes normally associated with lava-dominated shield volcanoes of hawaiian type. as for detection of subglacial volcanoes in antarctica we do not consider seismic tools or magnetic data, but the density contrasts due to a volcano, inducing their specific “gravity signal”; we also use the bedrock topography. both data types are used simultaneously. we are very well aware of non-uniqueness of the gravity data (one and the same gravity signal observed on surface may mean more than one type of features hidden under the surface and generating the causative density anomaly). thus, we prepared a list of the gravity symptoms for a huge single mountain or volcano (section 4). at lv, a tectonically controlled subglacial lake, klokočník et al. 2 the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce sufficient amounts of thermal energy into the lake [e.g., studinger et al. 2003a]; a seismic activity was recorded nearby [studinger et al. 2003b, 2004], thus we speculate that volcanoes might be expected not far away. our study area (figure 1a) is around lv and in the west direction to gsm (gamburtsev subglacial mountains). the core of our method [klokočník et al. 2016, 2017b] is in the use of various gravitational (or gravity) aspects, namely the gravity anomalies or disturbances δg, the components of the marussi tensor γ of the second derivatives tij of the disturbing potential (especially the radial component tzz), the gravity invariants i1 and i1, their specific ratio i, the strike angle θ and the virtual deformations vd. each such gravity aspect contains different information and has a different sensitivity to the underlying causative density variations [see figure 2 in klokočník et al. 2016]; the traditional gravity anomalies are already not sufficient, more gravity aspects are used. the theory with many examples (figures) can be found in klokočník et al. [2014, 2016, 2017b]. the theory underlying our methodology is described mainly in pedersen and rasmussen [1990] and beiki and pedersen [2010]. many examples for local use of the gravity aspects can be found in the literature dealing with petroleum, metal, diamond, economic minerals, oil and gas deposits, fault location, groundwater explorations etc. [see, e.g., saad 2006, murphy and dickinson 2009 or mataragio and kieley 2009]. the marussi tensor provides more complex information than the ordinary gravity anomalies only; tzz informs about the setting (target location), the other components tij of γ refer to the orientation of the causative structure. the invariants can be looked upon as non-linear filters enhancing the sources having big volumes; they discriminate major density anomalies into separate units. the ratio i can indicate two dimensionality of the causative body. the condition i=0 is necessary but not a sufficient condition for two-dimensionality. the strike angle θ tells us how the gradiometer measurements (used to derive γ) rotate within the main directions of the underground structure; when i=0, the values of θ may indicate a dominant 2d structure [used to predict possible oilfields in klokočník and kostelecký 2015]. the virtual deformation vd is an analogy with the tidal deformation and characterizes the “tensions” (compression and dilatation) generated by the causative body; one can imagine the directions of such a deformation due to “erosion” brought about solely by the “gravity origin” klokočník et al. 2013, p. 90]. the vd provide a dynamical information although they are, as well as all the gravity aspects, computed from the static gravitational geopotential. the theory used (with our own contribution to it), the method and the notation (with various results for different parts of the world) can be found in kalvoda et al. [2013], klokočník and kostelecký [2015] or klokočník et al. [2010, 2014, 2016, 2017a,b]. the computation of all the aspects listed above is not trivial. it was done by means of software developed mainly by b. bucha [see bucha and janák 2013, where reader can find technical details] and by our own software. the input to the computations is always a set of harmonic geopotential coefficients of the given static gravity field model (here ret 14), the outputs are the individual gravity aspects, which are then plotted (individually with non-linear scales) with a 5x5 arcmin step in latitude and longitude. we do not use any filter during our computations. about plotting and coordinate systems used see klokočník et al. [2017b], sections 2.2 and 3.5. we have scanned the whole world by means of several recent gravity field models with a step of 5x5 arcmin in geodetic latitude and longitude, including eigen 6c4 [foerste et al. 2014]. this 5 arcmin step corresponds to the resolution of eigen 6c4 (and ret 14, too, see section 2.1) which is dictated by the spherical harmonic expansion of the gravity model available to degree and order 2190. we always use this full resolution. the ret 14 is intended only for antarctica, where it is valid (section 2.1); for many examples see kalvoda et al. 2013, klokočník et al. 2014, 2016, 2017 a, b. we work also with the bedrock topography; it is given in a network 1x1 km (in a specific x,y coordinate frame of bedmap 2), but it does not mean that everywhere the topography data have actually such a resolution (more about this topic is in sections 2.1 and 2.2). 2. notes on the data 2.1. the data files in general terms, the “gravity” is represented by a static model of gravity/gravitational field of the earth in terms of the harmonic geopotential coefficients (also known as stokes parameters) to a certain degree l and order m, clm, slm, derived from a combination of satellite and terrestrial data. the “topography” is represented by a model of the bedrock topography for antarctica achieved dominantly via the ice penetrating airborne radar (also called res, radio echo sounding) data. in particular, we use the best data now available candidate volcanoes under antarctic ice 3 for antarctica, i.e. ret 14 [hirt et al. 2016]. it is a degree 2190 gravity field model satgravret2014 (shortly ret 14), given as a set of harmonic geopotential coefficients, meaningful however only for the continent of antarctica (not for the whole world). roughly speaking it combines the global gravity field model eigen 6c4 and the bedmap 2 topography (see below). more precisely: the ret 14 combines the predecessors of eigen 6c4 with other and very important gravity data sets; it combines the itg-grace2010s and the unconstrained goce tim5 satellite gravity models to degree and order 180. these models describe the longand medium-wavelength components of the earth’s static gravity field from grace (gravity recovery and climate experiment e.g., https://www. nasa.gov/mission_pages/grace/index.html) and goce missions (gravity field and steady-state ocean circulation explorer, esa, e.g., http://www.esa.int/ our_activities/observing_the_earth/goce). the “non-gravity” data to the ret 14 came from the earth 2014 1 arcmin global topography model [hirt and rexer 2015] which incorporates the bedmap 2 bedrock topography as a subset and the other data sets like topography from the shuttle radar topography mission (srtm), greenland bedrock topography and srtm 30_plus bathymetry [for more details and references see hirt et al. 2016]. eigen 6c4 [european improved gravity model of the earth by new techniques, foerste et al. 2014] is a combined global static gravitational solution, to degree and order 2190, including also a complete set of gradiometry data from the goce mission (in addition to the grace satellite-to-satellite tracking data and data from observations of other satellites). bedmap 2 [fretwell et al. 2013] contains the bedrock elevation beneath the grounded ice sheet based on various types of ground, airborne and satellite data. the res data are the most contributing. the bed topography in bedmap 2 is given in a 1x1 km grid of heights of the bedrock above present day sea level (asl). the actual spatial resolution is, however, often lower than 1 km; the resolution and quality is still much worse for some areas without data or with sparse data and also the precision varies highly [see figures 3 and 12 and other material in fretwell et al. 2013, esp. pp. 379 and 388 and our figure 1b, this section]. some data incorporated into the bedmap 2 were taken from its predecessor known as bedmap [lythe et al. 2001, figure 2b]. ret14 increases the resolution of eigen 6c4 and decreases the resolution of bedmap 2; generally the spatial resolution of ret14 should be better than ~10 km over the whole antarctica (excluding the polar gap and two bigger areas without data in bedmap 2). the success of our approach lies in the simultaneous use of all the gravity aspects and the bed topography and in the high resolution data now coming on the scene (for us today it is the ret 14). but even after that we can speak only about candidates for subglacial volcanoes, nothing more. the final decision about the existence of the volcanoes is expected from analyses by other geoscientists, namely after drilling the ice on the appropriate places; we provide geographic coordinates on the ice cover where to do it (section 5.3). the area of the lake vostok (lv) has been studied. its bedrock topography, derived from bedmap 2, is shown in figure 1a. the lv is an elongated depression with two local minima. the hills in west and south directions (up and left in this figure) continue to the lakes 90 east, sovetskaya and others (“lake district”), via the ridge b on the surface to the western flanks of the gsm; to east and north, there is wilkes land (wl). the positions of our candidates for the subglacial volcanoes are encircled and numbered. 2.2 reliability of the data the data available were critically evaluated; we have a rich experience with various types of artefacts due to insufficient or missing data or their irregular, inhomogeneous distribution [see, e.g., klokočník et al. 2016, 2017a], so we should be able to avoid misinterpretations also here in antarctica. the gravity data. the noise in ret 14 can be only roughly estimated to be better than 10 mgal nearly everywhere in antarctica, based on information deduced from [pavlis et al. 2012, foerste et al. 2014, fretwell et al. 2013, hirt et al. 2016]. these values are available only for the gravity anomalies [in the fundamental source pavlis et al. [2012] they are called “commission error”]. but accounting for usually huge correlations among gravity anomalies inside the studied area and outside but not too far from it, we found about half values for the noise [klokočník et al. 2017a]. these are our values n (as noise): n=510 mgal (more at the end of section 4). the bedrock topography data. how we can rely upon figure 1a based on bedmap 2? can examples 1 and 2 be real features, do they exist in nature or are they only artefacts generated by bedmap 2 due to insufficient data? in figure 1b we show the bedmap 2 topography again (in black and white) and we add klokočník et al. 4 the tracks with res, transformed from figure 3 of fretwell et al. (2013) into the geographic coordinates, and from figure 2b of lythe et al. [2001]. while lv is well covered by the data [with density of ground tracks from res mostly in east-west oriented flights up to 7.5 km; see, e.g., studinger et al. 2004], the tracks around the lake are not so dense (~ tens kilometres). in figure 1b, the densely covered areas are shown in violet colour (the rectangles). they are there two large zones without data in bedmap 2, called (by the authors of bedmap 2) “poles of ignorance”, pi; one is in princess elizabeth land, see fretwell et al. 2013, pp. 379 and 388. with our example 2 we are close to it (figures 1 a-c). we use ret 14 and it is influenced strongly by bedmap 2. but they are some measurements exactly at or in vicinity of our examples 1 and 2, namely res data (see the tracks in figure 1b). one older surface gravity traverse goes (by a good chance) directly over the subglacial mountains in the case of example 2 in the older bedmap (shown in figure 1b as thick blue line); it is logical that such a data set was used also in bedmap 2. in the bedmap 2 black and white topography, figure 1b, we can see an evident trend to show details along the res flight lines, changing often positive and negative heights quickly, so the lines with measurements look like “beads on a string“ (figure 1b). we can see a trend to smooth the bedrock topography among the tracks in places without data (figure 1b, mostly its upper part, in west direction). but some topographic features are very pronounced so that they are not “averaged out” and that even sparser data can “catch” them. a helpful fact is that examples 1 and 2 are visible also by means of the gravity aspects from eigen 6c4 alone [klokočník et al. 2016, figure 17], but this test is strong only for example 1 (more in section 5). as we know, this gravity field model is completely independent of the both bedmaps; that provides an independent and evidential check. finally a note to noise of bedmap 2 heights. the estimated uncertainty in bed elevation grid is about 100 m at lv, but may reach 200 m in other parts we investigate [fretwell et al. 2013]. we answer our question about reliability of the data: example 1 and 2 might be real features, no artefacts. figure 1a. bedmap 2 topography [m] with two candidates for subglacial volcanoes, example 1 and 2, encircled; for more details see section 5. the west is up. the zones in blue colour are below asl. the hills to the west and south (up and left in this figure) continue from the lake vostok (lv) via the lake area (the largest lake here is 90 east degree, 90°e) to the gamburtsev subglacial mountains (gsm). to the east and north (down and right in this figure), there is a part of wilkes low land (wl). the symbol pi (pole of ignorance) labels the danger zone without data in bedmap 2 (more in text). candidate volcanoes under antarctic ice 5 3. gravity symptoms of a single mountain from the gravity aspects sections 3 and 4 serve as a preparation for presentation of our results (section 5). here we focus on the particular object (a single mountain or a volcano) and list its typical gravity signal expressed in all the aspects. in each individual case of our testing of known volcanoes over the world, a single mountain, impact craters or other objects, the gravity aspects (listed above in section 1) have a specific character and reach their local extrema at the objects and their vicinity. as a graphic representation to document our conclusions, we recommend to see figures 3d-g and 4b-g of this paper and many figures in our previous papers [klokočník et al. 2014, 2016, 2017b]; they cannot be (due to space reasons) repeated here. a single mountain or volcano outside a mountain belt causes δg, tzz, i2 and i to have their local maxima; in the case of tzz the maximum at the top of the object is surrounded by a ring with negative values of tzz. for i1 we observe a local minimum at its top. the quantities i1 and i2 have their extremes concentrated over the volcano’s caldera. the volcano and its close vicinity exhibits a “missing θ”. we compute θ for i < 0.3 and the localities with presence of flat causative bodies are indicated by one-side oriented values of θ; figure 1b. data gaps among the res ground tracks (blue lines) in bedmap 2 (and its predecessor bedmap) plotted over the bedrock topography, taken from bedmap 2 (in metres, asl) in geographic coordinates, near the lake vostok (lv). a gravity traverse from the older bedmap [lythe et al. 2001] is shown by thick blue line, see the upper right, just crossing our candidate for volcano example 2. our candidates for sub-glacial volcanoes are those two noticeable peaks aimed by red circles. reproduced from [fretwell et al. 2013] and adopted for our purpose. figure 1c. this close-up is a modification of figure 3 from [fretwell et al. 2013] by anonymous reviewer (courtesy of ) to show positions of our candidates for subglacial volcanoes with respect to one of the “danger” (red) zones without data in bedmap 2 [pi, fretwell et al. 2013, pp. 379 and 388]. scales of the xand y-axes are in kilometres in a local system of bedmap 2. klokočník et al. 6 for a 3d body like a mountain, the θ-values are missing. the virtual deformations vd show a strong dilatation (red colour) at the caldera which is surrounded by a belt of compression (blue colour). the gravity aspects and the bedrock topography must be accounted for together. if they all show what is expected for a volcano, then the tested mountain might be a volcano. but it is well known that the gravity information itself and alone does not provide unique results, so a special care is needed. the bedrock topography is needed, other data are welcome. however, this way one cannot distinguish between a single “normal” mountain and a volcano. thus, we modelled a volcano (section 4); for its given shape, size and density, we computed the theoretical δgm and tzz m for a reference point p above the top of the modelled object. then we compared δgm and tzz m with δgo and tzz o of the actual volcano (the case of mt sidley, the largest volcano in antarctica) and with the objects suspected to be a volcano, derived from the ret 14 (see below). the agreement of these values for δg and tzz support our hypothesis about a volcano (section 4). 4. model of a volcano this test should strengthen our predictions for the candidate volcanoes (objects) under the ice or under the sea level [for similar tests see section 6 of klokočník et al. 2016]. we developed our own simple model producing δg and tzz values for a single mountain. we considered various input data about the single mountain (model of volcano), shape, size, density, etc. and computed the model values δgm and tzz m. then we computed δg and tzz for the same object with ret 14, say δgo and tzz o. finally we compared both types of results. the reference point p for our computations of δgm and tzz m is put to the height hp vertically above the base of the object, always higher than all masses of the model (figure 2). we consider the object as an isolated single peak (first important simplification); it has a circular base of diameter d and homogeneous density ρ (further simplifications). the volcano is modelled by a homogeneous truncated cone with a crater on its top. the ice cover is approximated by a layer with a uniform thickness h big enough that a truncated part would not affect the results. in the part of the volcano embedded in the ice, the density ρ is reduced by the density of ice to avoid a duplication of the ice contribution. a volcano body is divided into a set of mass elements, which are treated as mass points. their gravity attraction is computed numerically and summed over the whole body. we performed many tests with varying the input values hp, d, ρ, and h. first we used an arbitrary known volcano in antarctica as a test to compute its δgm and tzz m and then we extrapolate to hypothetical volcano under the ice. we considered volcano mt sidley, the largest known non-active volcano in antarctica (4.2 km above sea level) with the upper part protruding through the ice about 2 km high, in the place known as the executive committee range (marie byrd land); the central part of the caldera of mt sidley has these geodetic coordinates: φ = 770 03‘41“ s, λ =2330 49‘ 52“ e. we take this mountain as an isolated single peak. we estimate d = 60 km, height hv = 3.8 km over the surrounding area and density ρ= 3 g/cm3. the altitude hp = 4.8 km of the point p was used, i.e. 1.0 km over the top of the volcano, to simulate a resolution comparable to that of ret 14. then our model yields δgm = 265 mgal and tzz m = 390 e. we ask whether our model can explain the actual values δgo and tzz o. these were computed with the ret 14 and they read: δgo = 260 mgal and tzz o = 380 e for the top of mt sidley. they compare very well with δgm = 265 mgal and tzz m = 390 e. then, we considered an object as an isolated single peak (example 1) completely under the ice of thickness h = 2 km. the height of the observation point hp = 2.5 km, the object’s height hv = 1.2 km, the object is surrounded by an approximately plain terrain. again d = 100 km and ρ =3 g/cm3 were used. our model yields: δgm = 85 mgal and tzz m = 60 e. this should compare to δgo = 68±10 mgal and tzz o = 85 e, computed by the ret 14 (see figures 4b and 4c). [for the estimate of the standard deviation of the gravity anomalies used figure 2. a simple model of volcano for estimation of δgm and tzz m. candidate volcanoes under antarctic ice 7 here see a note below.] a decrease of δgm and tzz m with a distance from a centre of the volcano below the point p is sharper for tzz m than for δgm; this is expected for the second derivative; the half values of δgm and tzz m in comparison with those for the top are found for d = 40 km and 20 km, respectively. we can vary hp, d or ρ. let us consider ρi ≠ ρ. then, under the ice (which has 1 g/cm3), we can use the result for ρ = 3 g/cm3 but with a multiple factor (ρi 1)/(ρ 1), so for ρi = 2.7g/cm 3 we get: (2.7 1) / (3 1) = 0.85, and δgm = 75 mgal and tzz m = 80 e. this time the comparison with δgo and tzz o from ret 14 comes out better. the previous test concerned example 1; similarly successful was our test for example 2. these examples are not a prove but can be used as one of the arguments that the suspicious objects examples 1 and 2 might be volcanoes. but they may be normal single mountains as well. we used the gravity symptoms (section 2) and the bed topography (section 3). we care about a statistical significance of our results, too. the noise of ret 14 is n = 5-10 mgal [section 2.2], following sect. “discrimination criterion” in klokočník et al. [2017a]. the signal (s) is taken from detailed listings belonging to the maps of the gravity disturbances over the objects 1 and 2. we seek for the s/n ratio; it should be > 3 to believe that we have statistically significant results. we find s/n = 7-14. 5. the results examples of possible subglacial volcanoes near the lake vostok at the lake vostok, a seismic activity was recorded [studinger et al. 2003b, 2004]; thus we speculate that volcanoes might be expected not far away. we prefer a single mountain to a mountain in a mountain belt, because it can be determined with higher probability from existing gravito-topographic data. we could choose many other places in antarctica and we certainly have a chance to find other candidates for subglacial volcanoes [klokočník et al. 2016]. 5. 1. test with known large volcano known volcano: mt sidley (figure 3c): φ = 770 03‘41“ s, λ = 233049‘52“ e. it is the highest non-active volcano known in antarctica. the simple modelling of this volcano (see above) shows that our model can approximate well the real volcano. it means that the model works fairly and can be used in an extrapolation for the volcano candidates (next section). the reader can follow the details below. all the symptoms (local maxima of δg, tzz, i2, i; min. i1, no θ, positive vd at the volcano above its caldera, negative vd in a belt around the object) are fulfilled, as expected for each volcano or a “strong” single mountain. here we show the bedrock topography from bedmap 2 in figures 3 a, b, the ice surface from google earth in figure 3 c; and the gravity aspects are in figures 3 d g; everywhere in these figures we make use of ret 14. figure 3 d shows δg with positive anomalies at the main volcanoes and a belt between them, similarly figure 3 e with tzz. very nice is figure 3 f with i2; the strong signal due to a density anomaly from the shallow depth is focused to the main volcanoes, to their calderas. the vd in figure 3 g show a pronounced dilatation at the volcanoes surrounded by the belts of compression. 5.2. candidates for volcanoes: example 1: φ = 79° 19‘s, λ = 1180 00‘e; example 2: φ = 750° 59‘s, λ = 930 57‘e. all the symptoms (local maxima of δg, tzz, i2, i; min. i1, no θ, positive vd at the volcano above its caldera, negative vd in a belt around the object) are fulfilled. here we show the topography from bedmap 2 in figure 1a and 4a and the gravity aspects in figures 4 b f. everywhere for the gravity aspects we make use of ret 14. figure 4 b shows δg with noticeable positive anomaly peaks for both example 1 and 2, similarly figure 4 c with tzz. very nice is figure 4 d with i2; both the examples are marked by big red spots. in figure 4 e, the strike angle θ show a oneside orientation inside the lake vostok, in a contrast to white places with missing θ indicating that there is really no 2d-causative body! the vd values in figure 4 f depict strong dilatations at the candidates for volcanoes surrounded by belts of compression, very nice and large especially for example 2. the topography (figure 5a) reveals an isolated non-sharp-edged peak for example 1, surrounded by a flatland and an oblong feature for example 2 with a mountain belt nearby (figure 5b). example 1. there is a fair agreement between the actual δgo and tzz o computed with ret 14 and the single mountain model for a volcano. the topography from bedmap 2 confirms the existence of an isolated peak 1200 m above the relatively plain surrounding terrain (figure 5a). candidate 1 cannot be an artefact due to insufficient data in bedmap 2 in this area, although the density of the relevant data here is not too high [see figure 3 in fretwell et al. 2013, our figure 1b]; the reason confirming existence of example 1 is that the same klokočník et al. 8 feature (a single massive mountain) computed with ret 14 (dependent of bedmap 2) is shown (with a lower resolution) also by eigen 6c4 (independent of bedmap 2), namely by tzz and vd [cf. figure 17b in klokočník et al. 2016]. note also that the bedmap 2 model used the data of the original bedmap model [lythe et al. 2001] and for this area there were some data already in this model [see figure 2b in lythe et figure 3b. bedmap 2 for the executive comm. range with mt sidley (the largest mountain in this picture) (m asl). figure 3c. surface topography from ©google earth. figure 3d. δg, all figures in this series are with ret 14. figure 3e. tzz. figure 3f. i2. figure 3g. vd. the series of figures 3 a-e. the gravity aspects with ret 14 and the topography for mt sidley and the executive committee range area. figure 3a. topography (from bedmap 2) in marie byrd land with the executive committee range (m); a zoom for mt. sidley is below in figure 3 b. mt sidley (visible on the ice): φ = 770° 03‘41“ s, λ = 233° 49‘ 52“ e. candidate volcanoes under antarctic ice 9 al. 2001 and section 2.2.1.2]. conclusion. all pieces of circumstantial evidence about example 1 lead to a conclusion that it is a candidate for a subglacial volcano (figure 5a). example 2. (i) all the aspects show values expected for a mountain zone. (ii) the comparison of the actual figure 4b. δg, everywhere in this series, the ret 14 was used. figure 4c. tzz. figure 4d. i2. figure 4e. the strike angle θ is expressed in degrees with respect to the local meridian and its demonstration in red means its direction to the west and in blue to the east of the meridian. figure 4f. vd. the series of figures 4 b-f. the gravity aspects and the bedrock topography from bedmap 2. figure 4a. bedrock topography according to bedmap 2 (asl) (m). no disturbing or misleading circles around examples 1 and 2 (see the other figures in this series with the circles and figures 1 a-b). klokočník et al. 10 δgo and tzz o computed with ret 14 with the single mountain model for a volcano works well. (iii) the topography from bedmap 2 shows three peaks together dominating the surrounding terrain, figure 5b. (iv) this area has data dense enough in the bedmap 2 [see figure 3 in fretwell et al. 2013]; it is the lv area, but nearby in the north-west direction, an extensive “pole of ignorance” (pi, a large area still without any data in bedmap 2) begins [fretwell et al. 2013, pp. 379 and 388], see figures 1a and c. one has to be very careful here to avoid a misinterpretation. the gravity data from eigen 6c4, which were helpful for example 1, do not help here too much, because there is no single mountain surrounded by a plain area but a mountain belt; the resolution of eigen 6c4 cannot distinguish the individual peaks and a smoothing takes place. therefore, the older surface gravimetric data [a gravity traverse used in bedmap, lythe et al. 2001], just crossing our candidate for subglacial volcano example 2, is very welcome (see figure 1b right upper corner, thick blue line). 5.3. geodetic coordinates and suggested names of the candidates for volcanoes we provide the geographic coordinates, i.e. geodetic latitude φ and longitude λ (in wgs 84) for the top of these objects under the ice (figures 5a, b), according to the maps from bedmap 2, together with the roughly estimated depths h of those tops beneath the ice layer given by the thickness of the local ice cover (derived from another source of bedmap 2). these data might be used for prospective future drilling. names for the hypothetical volcanoes: we suggest woman’s names dana and zuzana for the objects (1) and (2), respectively. example 1. geographic coordinates: φ = 79° 19‘ s, λ = 118° 00‘ e, h = 2.3 km name of candidate: dana. example 2. geographic coordinates: φ = 75° 59‘ s, λ = 93° 57‘ e, h = 2.5 km name of candidate: zuzana. 6. conclusion by using the bedrock topography bedmap 2 and a high resolution static gravity field model for antarctica ret 14, we computed the gravity anomalies/ disturbances, the marussi tensor of the second derivatives of the disturbing potential, the gravity invariants and their specific ratio, the strike angle and the virtual deformations. we found by extensive testing that these gravity aspects show specific values for a massive single mountain. this knowledge was applied for our study area in antarctica; not too far from the lake vostok we discovered at least two objects that might be subglacial volcanoes. we present all the arguments supporting this idea the expected values of the gravity aspects using ret 14, topographic signal based on bedmap 2 bedrock topography, and a good agreement between the gravity anomaly and the radial second derivative derived from ret 14 and from an independent simple volcano model. however, we are aware that the final decision about the respective volcanoes is on other specialists. we provide geodetic coordinates of the candidates for volcanoes (named dana and zuzana) and an estimate of the depth of their tops under the ice cover. acknowledgements. this work has been prepared in the frame of projects #13-36843s (grant agency of the czech republic, cr) and rvo #679 858 15 (czech academy of sciences), partly figure 5a. the bedrock topography from bedmap 2 (m) of example 1. one single peak with a relative height difference (with respect to surrounding landscape, mostly plain area) ~1000 m, the top has 1240 metres asl (sea level = black thick contour line with zero). a possible caldera below our resolution. interval of contour lines 20 m, the lines in step 100 m are shown as thick curves; those below sea level are in blue. figure 5b. the bedrock topography from bedmap 2 (m) of example 2. three peaks with the highest top 1400 metres asl. the detailed features at slope and below the mountains might be artefacts. interval of contour lines 20 m, the lines in step 100 m are as thick curves, below sea level in blue. candidate volcanoes under antarctic ice 11 supported by the project lo 1506 (puntis) from the ministry of education of the cr. we thank b. bucha for the computations of the gravity aspects with eigen 6c4 and ret 14 and h. d. pritchard for his help with the bedrock topography data. references beiki, m. and pedersen, l. b. 2010. eigenvector analysis of gravity gradient tensor to locate geologic bodies. geophysics, 75, 137-149; doi: 10.1190/1.3484098. bucha b. and janák j. 2013. a matlab-based graphical user interface program for computing functionals of the geopotential up to ultra-high degrees and orders, computers and geosciences, 56, 186-196. http://dx.doi.org/10.1016/j.cageo.2013.03.012. foerste ch., bruinsma s., abrykosov o., lemoine j-m. et al. 2014. the latest combined global gravity field model including goce data up to degree and order 2190 of gfz potsdam and grgs toulouse (eigen 6c4). 5th goce user workshop, paris 25-28, nov. fretwell p, pritchard h, vaughan d, bamber j, et al. 2013. bedmap2: improved ice bed, surface and thickness datasets for antarctica. the cryosphere, 7, 375-393; 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doi: 10.1007/s12517014-1491-y; issn 1866-7511, springer-verlag. klokočník j., kostelecký j. and bezděk a. 2016. on feasibility to detect volcanoes hidden under ice of antarctica via their “gravitational signal”, annals of geophys. 59, 5, s0539; doi: 10.4401/ag-7102. klokočník j., kostelecký j. and bezděk a. 2017a. a support for the existence of paleolakes and paleorivers buried under saharan sand by means of “gravitational signal” from eigen 6c4, arab. j. geosciences, on-line; doi: 10.1007/s12517-017-2962-8. klokočník j., kostelecký j. and bezděk a. 2017b. gravitational atlas of antarctica. springer on-line; isbn 978-3-319-56639-9. klokočník, j., j. kostelecký, i. pešek, p. novák, c. a. wagner and j. sebera 2010. candidates for multiple impact craters? popigai and chicxulub as seen by the global high resolution gravitational field model egm08. egu solid earth 1, 71-83; doi: 10.5194/se-1-71-2010. lemasurier, w.e. and thomson j.w. 1990. volcanoes of the antarctic plate and southern oceans, washington, d.c., agu, antarctic res. series, 487. lough a.c., wiens d.a., barcheck c.g., et al. 2013. seismic detection of an active subglacial magmatic complex in marie byrd land. antarctica, nature geosci. 6, 1031-1035; doi: 10.1038/ngeo1992. lythe m., vaughan d.g., the bedmap consortium 2001. bedmapa new ice thickness and subglacial topographic model of antarctica, j. geophys. res. 106, b6, 11,335-11,351. mataragio j. and kieley j. 2009. application of full tensor gradient invariants in detection of intrusion-hosted sulphide mineralization: implications for deposition mechanisms. mining geoscience, eage first break, 27, 95-98. murphy c. a. and dickinson j. l. 2009. exploring exploration play models with ftg gravity data. 11th saga biennal techn. meeting and exhib., swaziland, pp. 89-91, 16-18 sept. pavlis n. k., holmes s. a., kenyon s. c. and factor j. k. 2012. the development and evaluation of the earth gravitational model 2008 (egm2008). j. geophys. res. 17, b04406; doi: 10.1029/2011jb008916, 2012. pedersen b. d. and rasmussen t. m. 1990. the gradient tensor of potential field anomalies: some implications on data collection and data processing of maps. geophysics, 55, 1558-1566. saad a. h. 2006. understanding gravity gradients a tutorial, the meter reader, ed. b. van nieuwenhuise, august issue, the leading edge, 941-949. studinger, m., and 11 colleagues. 2003a. ice cover, landscape setting, and geological framework of lake vostok, east antarctica. earth and planetary science letters 205, 195-210; doi: http://dx.doi. org/10.1016/s0012-821x(02)01041-5. studinger, m., karner, g. d., bell, r. e., levin, v., raymond, c. a., tikku, a. a., 2003b. geophysiklokočník et al. 12 cal models for the tectonic framework of the lake vostok region, east antarctica. earth and planetary science letters 216, 663-677; doi: http://dx. doi.org/10.1016/s0012-821x(03)00548-x. studinger, m., bell, r. e., tikku, a. a. 2004. estimating the depth and shape of subglacial lake vostok’s water cavity from aerogravity data. geophys. res. letters 31, l12401; doi: 10.1029/2004gl019801. walton d. w. h. 2013. antarctica: global science from a frozen continent. cambridge university press, (editor) isbn 978-1107003927 *corresponding author: jaroslav klokočník astronomical institute, czech academy of sciences, ondřejov, czech republic; email: jklokocn@asu.cas.cz. 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved a geant4 framework for generic simulations of atmospheric muon detection experiments annals of geophysics, 60, 1, 2017, s0108; doi:10.4401/ag-7383 s0108 a geant4 framework for generic simulations of atmospheric muon detection experiments nicola mori1,4,*, fabio ambrosino2,3, lorenzo bonechi1, luigi cimmino2,3, raffaello d’alessandro1,4, pasquale noli3, giulio saracino2,3, paolo strolin2,3, lorenzo viliani1,4 1 istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy 2 università di napoli federico ii, dipartimento di fisica, naples, italy 3 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 4 università di firenze, dipartimento di fisica e astronomia, sesto fiorentino, florence, italy abstract a flexible and adaptive simulation framework based on geant4 is presented. the framework has been developed in order to speed up the deployment of full-featured monte carlo simulation codes for small/medium sized particle detection experiments with generic geometries. specific components related to atmospheric muon experiments have been developed and are presented as well. 1. introduction the detection of atmospheric muons for investigating the density structure of target bodies is a growing field with many real-world applications. modern fields of investigation relying on this technique ranges from geology [tanaka et al. 2005, ambrosi et al. 2011] to archaeology [menichelli et al. 2007]. novel possibilities opened up recently also for monitoring legacy nuclear waste storage sites [clarkson et al. 2014], nuclear power plants [morris et al. 2014] and nuclear contraband [gnanvo et al. 2008]. the different sizes of the target structure in the various scenarios, as well as the conditions of the environment where the measurement is performed (e.g. difficult access to the site), influence the design of the detectors, resulting in a wide variety of target-detector cases. monte carlo simulations are of the utmost importance for atmospheric muon detection experiments, in order to optimize the design of the detector, to estimate the level of the backgrounds and of the expected signals, and so on. currently, any simulation program for a specific experiment or measurement scenario must include the details of the geometry of the target and of the detector, the detector response, an accurate description of the muon flux etc. these details must be implemented for the specific target structure and detector involved, so the software development effort has to be repeated for each measurement scenario. however, a careful planning of the software architecture could allow to reuse those elements which are common to every atmospheric muon detection simulation, i.e. the generation of a realistic spectrum of muons. also, some elements can be implemented in a way that is sufficiently general that they can be reused in all scenarios where the required accuracy is not very high: for example, a generic detector may be useful in an early stage where a coarse estimation of the expected performance with a given target structure has to be assessed. in this paper a monte carlo simulation framework based on the geant4 [agostinelli et al. 2003] simulation toolkit is presented. the framework has been developed to enhance the speed of deployment of simulation software for atmospheric cosmic ray detection experiments by implementing all the common features (generation of muons, creation of the output files and so on) and providing a reference implementation of a detector to be used in early stages. a particular focus has been put on making the framework fully extensible to accommodate any experiment and/or scenario specific feature (e.g. the target structure, a more detailed description of the detector, a different muon generator), either as a side-by-side implementation or by actually replacing the reference ones. in the followarticle history received july 5, 2016; accepted october 5, 2016. subject classification: atmospheric muons, simulations, muon radiography. ing sections, the software architecture of the framework is described. 2. the monte carlo framework starting from the geant4 toolkit, a full monte carlo simulation stack has been developed. the implementation is layered with different levels, from the most generic up to the most specific, in order to maximize code re-usage and development efficiency. the foundation software layer all the software stack is implemented on top of the geant4 simulation toolkit. geant4 is a generic (i.e. experiment-independent) package providing tools for simulating the propagation of particles inside matter. it requires the end user to write some mandatory c++ classes describing the detector geometry, handling particle generation and managing the output, in order to produce a full simulation program. while giving the opportunity to finely tune the simulation details, this also increases the time required to deploy the simulation program. in order to mitigate this issue, an implementation of the mandatory geant4 classes, called ggs (generic geant4 simulation), has been developed. ggs implements all the experiment-independent bits of the mandatory geant4 classes in a generic and reusable way. in particular, it automatically detects the sensitive volumes (i.e. those volumes where the energy released by the traversing particles has to be recorded on the output files) in the geometry and adapts the format of the output files to the sensitive volumes layout in the current geometry. the implementation leverages the i/o engine of the root framework [brun and rademakers 1997] and the output is produced in the form of root files. while being experiment-independent, ggs has been designed to be fully customizable via plugin libraries. a plugin api allows the user to define the detector geometry and/or any particular particle generator, to add information to the output files and so on. all these eventual customizations can be implemented as c++ classes, and allow the introduction of experiment-specific features in the simulation while still taking advantage of the generic code paths implemented in ggs. the atmospheric muon software layer all the simulation details which are relevant for atmospheric muon detection have been implemented as ggs plugins. while the geometry of the target structure is a specific feature of the simulated scenario, and thus cannot be implemented in a generic way, a generic, reference muon detector can be defined. the chosen implementation is a tracking device made of n pairs of x-y tracking layers made of plastic scintillator, either segmented (in fibers or triangular bars; see ambrosi et al. [2011]) or monolithic (in this case the muon impact point can be determined only by means of the monte carlo truth). the user can set any parameter of the detector (number and size of planes, plane spacing, number of fibers etc.) at runtime. an arbitrary number of these detectors can be placed with any orientation in the geometry describing the target volume, allowing for a quick geometry development. the plugin infrastructure allows also to replace the reference detector with a more specific one if needed. two different muon generators based on a theoretical model [tanaka et al. 2001] and on ground measurements with a magnetic spectrometer [bonechi et al. 2005] have been implemented. these generators cover different ranges in kinetic energy and zenith angle and generate realistic spectra for vertical and nearly-horizontal muons up to some tev. besides particle generation inside the simulation, the generators are also used by the flux normalization tool, a program which converts the number of generated events to the corresponding data acquisition time. the software architecture allows for an easy definition and integration of additional models of the muon flux. the simulation runs can be very time consuming for large targets, since only a few muons hitting the target hit also the detector. those events which do not hit the target might not be relevant, while those not hitting the detector would be actually undetected. simulating and saving on disk these events increase the run time, the analysis time and the size of output files with no benefit whatsoever. thus to increase efficiency while retaining maximum accuracy, an optional twostep acceptance check has been implemented. the underlying assumption is that the space surrounding the target is filled with a light material in which the scattering of the primary muon is negligible, so that a straight line constitutes a good approximation of the real trajectory. this is generally true since often the target structure is surrounded by air. when the initial kinematics of a muon is generated the framework checks if the extrapolated trajectory hits the target volume: if not the event is discarded and another kinematics is generated, otherwise the muon is propagated until it exits from the target. at this point the framework checks if the extrapolated trajectory hits the detector: if not the event is discarded, otherwise the simulation is carried on. the user can enlarge or restrict the acceptance criteria in order to define enlarged or fiducial acceptances. this mechanism minimizes the time spent to simulate useless (i.e. undetected) events mori et al. 2 3 without artificially reducing the background coming from those events whose extrapolated track at generation lies out of the acceptance of the detector but are deviated inside the acceptance by multiple scattering inside the target. a sketch illustrating the mechanism is shown in figure 1. the scenario-specific simulations if no specific tuning is required (e.g. a particle generator implementing a different flux parameterization or a detailed detector) the simulation of a specific atmospheric muon detection experiment can be implemented simply by creating a c++ ggs geometry describing the target and then placing the reference muon detector(s) in the desired position(s) with the desired orientation(s). every kind of target can be simulated quickly with a minimum effort in terms of code writing. in fact the entity of the effort may vary according to the complexity of the target geometry: modeling the surface and the internals of a large geological structure like a volcano would require a considerable effort anyway. yet this is the only thing that the user has to take care of, reducing the amount of the needed scenario-specific software development to a bare minimum. the benefits of this approach are even more evident when simulating many different scenarios: besides the further enhanced reduction in development time, the user will also take advantage of having a common output format for all the different scenarios which will speed up also the production of data analysis codes. an example of a simulation scenario implemented with the framework is shown in figure 2. 3. discussion the architecture of a software framework for monte carlo simulations aimed at atmospheric cosmic ray detection has been presented. its design guidelines are geared towards code reuse and customizability, to speed up the simulation deployment leveraging standard implementations of common features while retaining the ability to finely tune the simulation by introducing experiment-specific bits. the framework is currently being used with success for simulating different muon detection scenarios (i.e. nuclear waste storage sites, archeological sites, geological structures), meeting all the design requirements. this framework is currently being used by the muraves collaboration for their mount vesuvius muography project, and the infn university of glasgow collaboration for their evaluation of muography applied to the sellfield nuclear stockage site. references agostinelli, s., et al. (2003). geant4 a simulation toolkit, nucl. instrum. meth. a, 506, 250-303. ambrosi, g., et al. (2011). the mu-ray project: volcano radiography with cosmic-ray muons, nucl. instrum. meth. a, 628, 120-123. bonechi, l., m. bongi, d. fedele, m. grandi, s. ricciarini and e. vannuccini (2005). development of the adamo detector: test with cosmic rays at different zenith angles, proceedings of the 29th international cosmic ray conference, 9, 283-286. brun, r., and f. rademakers (1997). root an object oriented data analysis framework, nucl. instrum. meth. a, 389, 81-86. clarkson, a., et al. (2014). geant4 simulation of a scintillating-fibre tracker for the cosmic-ray muon tomography of legacy nuclear waste containers, nucl. instrum. meth. a, 746, 64-73. gnanvo, k., p. ford, j. helsby, r. hoch, d. mitra and geant4 simulations of atmospheric muons figure 1. the acceptance checking algorithm: track 1 is propagated down to the detector, track 2 is killed at the exit of the target since its extrapolation does not hit the detector, track 3 is killed just after the generation of initial kinematics since its extrapolation does not hit the target. figure 2. a simulation scenario implemented with the framework. the target is a cylindrical storage silo for nuclear waste. the detector is constituted by two x-y pairs of layers with area 2 × 2 m2. the muons (red tracks) are generated from a horizontal surface placed above the silo. m. hohlmann (2008). performance expectations for a tomography system using cosmic ray muons and micro pattern gas detectors for the detection of nuclear contraband, ieee nucl. sci. symp. conf. record, 1278-1284. menichelli, m., s. ansoldi, m. bari, m. basset, r. battiston, s. blasko, f. coren, e. fiori, g. giannini, d. iugovaz, a. papi, s. reia and g. scian (2007). a scintillating fibres tracker detector for archaeological applications, nucl. instrum. meth. a, 572, 262-265. morris, c.l., et al. (2014). analysis of muon radiography of the toshiba nuclear critical assembly reactor, appl. phys. lett., 104, 024110. tanaka, h., k. nagamine, n. kawamura, s.n. nakamura, k. ishida and k. shimomura (2001). development of the cosmic-ray muon detection system for probing internal-structure of a volcano, hyperf. int., 138 (1), 521-526. tanaka, h.k.m., k. nagamine, s.n. nakamura and k. ishida (2005). radiographic measurements of the internal structure of mt. west iwate with near-horizontal cosmic-ray muons and future developments, nucl. instrum. meth. a, 555, 164-172. *corresponding author: nicola mori, istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy; email: mori@fi.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. mori et al. 4 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 63, 5, 2020; doi: 10.4401/ag-8594 special issue advances in multi-parametric, time-dependent assessment of seismic hazard and earthquakes forecast foreword until now proposed operational earthquakes forecast (oef) methods suffer of strong limitations in terms of their actual (too low absolute value of estimated probabilities) and general (forecast is substantially limited to earthquakes which are preceded by foreshocks) operational applicability [e.g. wang and rogers, 2014; panza et al., 2014]. they become of some (unfortunately still marginal) usefulness just when such a low-probabilities are multiplied for very high exposure and/or vulnerability factors in order to obtain significant amounts of the estimated seismic risk. mostly, due to these limitations, they have been scarcely used in the past while the interest of the scientific community for the study of additional, not just seismological, geophysical/geochemical parameters that could provide useful indications about the evolution of seismic hazard in the medium/short term, is every day increasing. if the deterministic earthquake prediction remains in our times “the holy grail of earth physics and seismology”, the real-time integration of multi-parametric observations is, day by day, demonstrating to have a huge potential for the development of operational systems for a time-dependent assessment of seismic hazard (t-dash) suitable for supporting decision makers with continuously updated forecasts and seismic hazard scenarios. a preliminary step in this direction is the identification of those parameters (seismological, geophysical, geochemical, etc.), whose multi scaled space-time dynamics and/or anomalous variability, could be associated with the complex process of preparation of major earthquakes. several of them (chemical, physical, biological, etc.) have been since long time proposed in literature [see for instance tronin, 2006 and reference herein; cicerone et al., 2009] together with physical models [e.g. scholz et al., 1973; tronin, 1996; freund, 2007; pulinets and ouzounov, 2011; huang, 2011b; tramutoli et al., 2013] suitable for explaining their possible correlation with earthquake preparation phases. however, until now, no one single measurable parameter, no one single observational methodology, has demonstrated to be sufficiently reliable and effective for the implementation of an operational earthquake prediction system [already in geller, 1997]. but, moving from the deterministic prediction to a probabilistic forecast goal, a multi-parametric approach could offer, already today, forecast probabilities orders of magnitude higher than the ones offered by traditional oef approaches strongly reducing the “alerted” space-time windows. to this aim, long-term correlation analyses are required in order to qualify each selected parameter in terms of its actual information content (in particular false positive rates), and application range (earthquake magnitude, affected space-time volume, etc.). for some of them preliminary studies already exist like for plasma frequency at the ionospheric f2 peak fof2 [e.g. liu et al., 2006], ionospheric ion density [e.g. li and parrot, 2013] recorded by demeter (detection of electromagnetic emissions transmitted from earthquake regions) satellite, ulf (ultralow frequency) geomagnetic signal [e.g. han et al., 2014; xu et al., 2013], earth’s emitted thermal radiation measured by geostationary satellites [e.g. elefteriou et al., 2015] – offering the indication of achievable false positive rates (i.e. anomalous transients occurring in absence of eqs of a prefixed minimum magnitude m within a pre-established, parameter-specific, space-time volume) lower than 50% (up to 7% for selected areas/parameter). so, as far as the number of combined parameters increases, it is easy to expect a progressive reduction of the combined probabilities of false alarms (even if not at the lowest limit expected for completely independent processes) as well as a strong reduction of the alerted space-time volumes (intersection of the ones associated to individual parameters). it is then already possible to generate real-time 3d (in the space time domain) representations of the timedependent seismic hazard with specific (combined) probabilities of eq occurrence for different areas and time-intervals starting from the probabilities (weights) of individual forecasts associated with each selected parameter. products for earthquake forecasting based on the analysis of seismic data, will be also incorporated in the same scheme provided that, like for all the other parameters, their correlation with earthquake occurrence, have been pre-qualified (in particular in terms of false positive rates) on a long-term training database. in this evolving scenario, we are honored to present this special issue which provides, through its collection of original papers, the state of the art of the emerging worldwide research potentially contributing to the development of t-dash systems. such contributes can be grouped in a) long-term analyses devoted to qualify specific parameters also in terms of reliability of associated forecasts; b) preliminarily studies devoted to emphasize the contribution to the reduction of the space-time forecast volume by a multi-parametric approach; c) preliminarily studies devoted just to support the use of specific parameters and/or new data analysis methodologies for specific areas/period of time.to the first group belongs the papers by genzano et al. [2020], an et al. [2020] and fidani et al. [2020]. the first one offers also for italy (after greece [elefteriou et al., 2016]) a full qualification of a parameter (anomalies in thermally emitted earth’s radiation estimated by the robust satellite technique/rst approach [tramutoli 1998; 2005; 2007]) already suitable for inclusion in a t-dash system. main characteristics of this parameter are: a) space forecast window ranging from 150km to rd (being rd the dobrovolsky radius, [dobrovolsky et al., 1979]) depending on the expected earthquake (m>4) magnitude; b) time forecast window from days to few weeks before event; c) false alarm rate around 40% (it was 7% for greece [elefteriou et al., 2015]) after a 11 years’ long-term correlation analysis. an et al. [2020] analyze more than 10 years of gef (geo-electromagnetic field) data from a single station in pingliang (china). main characteristics of the correlation test for this parameter are: a) space window ranging from 200 to 365 km, m>3,5, time window 60 days before and after the event; c) significant correlation found in the 200km/30 days space/time window (but not along all the directions). fidani et al. [2020] on the base of 11 years of electron bursts (30-100 kev) data obtained from noaa 15 satellite, evaluate the correlation with eqs (m>6) occurred in the indonesia and philippines regions. in two cases (over 63) anomalous ebs are reported in a very short (4-2 hours) time window before eq's occurrence. to the second group of papers belong the papers by arabelos et al. [2020], contadakis et al. [2020], ouzounov et al. [2020], who combine different parameters and/or measuring methods to reduce space-time forecast volumes (from regional/annual to very local/short-term scales) as well as those by huang et al. [2020] mostly devoted to better discriminate among signals possibly related to earthquake occurrence. in arabelos et al. [2020] two different parameters and three measuring methods (tidal seismicity compliance parameter p, and high-frequency limit, fo of the ionospheric turbulence, deduced directly from gps-tec observations and, indirectly, through a vlf transmission network) are jointly applied to the forecast of earthquake with m>5,5 occurred in greece in the years 2013-2015. the same ionospheric parameters are investigated by contadakis et al. [2020], in a limited period of time (from 24/07/2016 to 25/09/2016 from 25/10/2016 to 25/11/2016) around the major earthquakes that hit central italy (amatrice sequence) on august 24th (mw = 6.2, 42.71°n, 13.17°e), october 26th (mw = 6.1, 42.96°n, 13.07°e) and october 30th (mw = 6.6, 42.86°n, 13.09°e), emphasizing their potential in contributing to reduce (up to 15 days) the forecast time window. in their paper ouzounov et al. [2020] compare ground (air temperature t, relative humidity rhu and atmospheric chemical potential acp) and satellite based (outgoing longwave radiation olr measured from polar and geostationary platforms) observations around the time of 3 main earthquakes occurred in xinjiang province (china) on 02.12.2014 (m7.3), 08.12.2012 (m6.2) and 03.20.2008 (m7.2). forecast time windows, from few days up to 3-4 weeks, over a spatial range up to few hundreds kilometers, are reported moving from one parameter/measurement technique, to another, from one event to another one. to the third group of papers belong the ones of huang et al. [2020], novikov et al. [2020], scordilis et al. [2020], stănică et al. [2020], zhai et al. [2020], and zhu et al. [2020]. 2 3 in the paper of huang et al. [2020] the variations of the flux of high energetic particles from 300 to ~2000 km altitudes are analysed using data from wind, cluster ii and goes satellite systems, in order to investigate their occurrence in relation with m9.0 tohoku earthquake (march 11th 2011) in presence and absence of geomagnetic storms. in the paper of novikov et al. [2020] the possible triggering effects of strong solar flares are analyzed at the global scale starting from the case of the strongest flare over the past twelve years occurred on september 6, 2017. the time window for forecast is quite short (less than a week for earthquakes with m>2,5) compared with the global dimension of the alerted area. in their paper scordilis et al. [2020] consider the possible relation between ionospheric tec anomalies (and related turbulence frequency limit f0 and turbulent period limit p0) measured by 5 gps stations and strong earthquakes (m5.1 6.6) occurred in the eastern aegean in 2017. reported forecast time windows range from few days up to 2 weeks, over a spatial range from few hundreds up to 1000 kilometers and more. stănică et al. [2020] analyze a continuous time-series of geomagnetic data, collected during 2016 to emphasize possible relationships with two m5.7 and m5.6 earthquakes occurred in the seismic active vrancea zone on september 24 and december 28, 2016. reported forecast time window range from 3 to 7 days over a spatial range from 100 to 300 kilometers. zhai et al. [2020] compare six different data analysis methods to identify significant ir-olr anomalies from annual time-series of hirs4 radiances and hirs4 olr data/products. the best selected method (arima) is then used to identify several pre-earthquake anomalies occurring from 1 to 12 days before two strong events occurred in china (jiuzhaigou, ml = 7.0, august 8th, 2017) and in mexico (ml = 7.1, september 20th, 2017) in a spatial window of 1°x1° around the corresponding epicentres. zhu et al. [2020] analyse about 3 years of continuous records of stress and strain measured by a borehole strainmeter installed deeply into bedrock at the the guza station 153 km far from the epicentre of wenchuan earthquake (may 12th, 2008, ms = 8.0). by applying pca (principal component analysis) they identify a noticeable acceleration starting about 4 months before. all these papers, in different ways, contribute to the exploration and understanding of the complex earthquake related processes. most of the investigated parameters still requires long-term correlation analyses in order to qualify their false alarm rates in a prefixed space-time-magnitudes range, others, even without an actual predictive usefulness (e.g. too large alerted space windows compared with very low magnitude limits), still provide new stimulating insights on physical relations among phenomena apparently far each other. this is not unexpected considering that the construction of long-term observational dataset, having the required continuity in time and a sufficient density in the space domain, would require human and economic resources rarely devoted to such studies. we dedicate this volume to the memory of proff. oleg molchanov and alexander (sacha) rozhnoi brilliant examples for young geoscience researchers on the possibility of seriously applying to science, following their curiosity even when it takes them out of the safe waters of the research mainstream where it is sometimes easier to make a career and get financial support, not always achieving significant goals for humanity. acknowledgments. the guest editors are very grateful to the authors (belonging to several research institutions located in european, asian and american countries) for their valuable contributes and close cooperation to the preparation of this special issue. they want also warmly thanks all the international referees who offered their valuable time and expertise to guarantee and improve the quality of each single paper. a special thanks goes to the aog staff for their professional assistance and technical support during the entire publishing process leading to the realization of this special issue. references an z., y. zhan, y. fan, q. chen, j. liu (2020). investigation of the characteristics of geoelectric field earthquake precursors: a case study of the pingliang observation station, china. ann. geophys., this issue, doi:10.4401/ag-7982 arabelos d.n., m. e. contadakis, g. s. vergos , ch. skeberis, t.d. xenos, s. d. spatalas (2020). variation of some planetary seismic hazard indices on the occasion of lefkada, greece, earthquake of 17 november, 2015, ann. geohys, this issue, doi:10.4410/ag-7847 cicerone r.d., ebel j.e., britton j. 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(2014). earthquake preparedness should not fluctuate on a daily or weekly basis.seismological research letters (2014) 85 (3): 569–571.doi: 10.1785/0220130195. zhai d., x. zhang and p xiong (2020) detecting thermal anomalies of earthquake process within outgoing longwave radiation using time series forecasting models, ann. geophys. 63, 5, this issue, doi: doi.10.4401/ag-8057 zhu k., c. chi, z.yu, m. fan, k. li, h. sun (2020). the characteristics analysis of strain variation associated with wenchuan earthquake using principal component analysis, ann. geophys., this issue, doi:10.4401/ag-7946 guest editors valerio t ra m u to l i school of engineering, university of basilicata, potenza, italy filippos va l l i a n ato s department of geophysics and geothermy, national & kapodistrian university of athens, greece 117_124 adg v5 n01 feichte.pdf annals of geophysics, vol. 45, n. 1, february 2002 117 properties of the main trough of the f region derived from dynamic explorer 2 data erika feichter and reinhart leitinger institut für geophysik, astrophysik und meteorologie, karl-franzens-universität graz, austria abstract in winter and equinoctial season nights, the main trough of the f region is an important stable structure of the ionosphere at the border between mid and high latitudes. therefore it has to be taken into account in modelling and mapping approaches. werner and prölss (1995) derived a model for the position of the trough minimum which has found wide acceptance. the model is based on in-situ electron density data measured aboard the low orbiting dynamic explorer satellite de 2 (1981-1986). we present results for other properties of the main trough derived from the same data set. these results are sufficiently good for modelling purposes which need reliable information on the depth, the equatorward and poleward width and the steepness of the walls of the trough. because of the eccentric orbit of de 2 (orbit height between about 300-1000 km) it was necessary to «project» observed electron densities to the peak of the f 2 layer. this was done by means of the electron density model costprof. the database was restricted to those cases for which the height of de 2 was below 700 km. examples are shown for «typical» troughs observed under various conditions. 1. introduction our trough studies are based on data gained by the satellite dynamic explorer de 2 from 1981 to 1983. the altitude of its orbit was between about 300 and 1000 km (perigee: 309 km, apogee 1012 km, inclination: 89.99°, period: 98 min). the two satellites de 1 and de 2 became operational in 1981. the nearly polar and nearly coplanar orbits provided the unique capability of acquiring data at two altitudes along common magnetic flux tubes. data acquisition during passages of the spacecraft through specific geophysical regions like the dayside cusp and plasmapause were emphasised. the general objective point of the mission was the investigation of magnetosphere-ionosphereatmosphere coupling processes. from the de 2 data we first eliminated data from altitudes above 700 km to avoid «plasmaspheric» trough observations and investigated the northern hemisphere exclusively. additionally we compared published models of the variation of the latitudinal location of the trough minimum and decided on the use of the trough minimum model of werner and prölss (1995). other models have been published, e.g., that of annakuliev et al. (1997). s. werner created two empirical models basing on electron density data. for the time dependence he used magnetic local time (mlt) values and for the coordinates of the location of the minimum polar invariant latitude. he divided mailing address: dr. reinhart leitinger, institute for geophysics, astrophysics and meteorology, karlfranzens-universität graz, universitätsplatz 5, 8010 graz, austria; e-mail: reinhart.leitinger@uni-graz.at key words ionospheric f region main trough model dynamic explorer data trough shape parameters trough minimum model 118 erika feichter and reinhart leitinger the time into 10 min intervals and calculated the median of each data set. in model a he fitted a fourier series approximation (degree 2) to the medians. model b also includes a distinction between two possible trough locations between 3 and 9 mlt (a «new» trough at high polar cap latitudes and the «old» trough at a much more equatorward position). model b shows the time dependence of the minimum as an archimedian spiral (degree 3). werner and prölss (1995) also investigated the dependence of the trough minimum position of magnetic activity and seasons because the trough position moves towards lower latitudes during disturbed conditions and will also be influenced by the seasons. the models neglect as insignificant the influence of solar activity (putz and leitinger, 1987, 1988). in 1999 we investigated the usefulness of published models of the f-region trough. the f-region trough is a stable structure that occurs above all during winter and equinoctial months especially during the night. we can interpret the ionisation minimum of the trough as the border between «middle» and «high» latitudes. the high variability of the ionisation poleward of the trough minimum has consequences for modelling: it is not sufficient to use solar activity parameters as input but one has to add information on geomagnetic activity, too. this means, strictly speaking, that one has to leave the realm of pure monthly average models. the position of the trough is influenced by the ionospheric conditions at higher latitudes which depend on solar-terrestrial relations via the solar wind, the interplanetary magnetic field, and the reaction of the magnetosphere. cross polar cap plasma convection and precipitation of energetic particles play a major role in trough formation, movement and maintenance. since the latitudinal range of the trough is small it does not show in large scale empirical models. therefore we assume that a separate trough model is combined with a large scale model by multiplication: m (h, , ) = l (h, , ) t (h, , ) (m: resulting model, l: large scale model, t: trough model; h: height, : geographic latitude, : geographic longitude). 2. preparation of de 2 data and trough examples werner and prölss have used the de 2 electron density data directly to derive their trough minimum model. for any other parameter of the trough this is not possible because of the change in orbit height. to gain comparable conditions we kept to de 2 heights below 700 km and we have «projected» the electron densities to the peak of the f2 layer. this was done by means of the electron density model costprof (leitinger et al., 2001) n m (de 2) = n e (de 2) n m (costprof)/n e (costprof) n m (de 2): inferred peak electron density; n e (de 2): measured in-situ electron density; n m (costprof): peak electron density from costprof for the latitude and longitude of de 2; n e (costprof): electron density from costprof for the position of de 2. the «trough parameters» depth, half width (equatorward, poleward, total), steepness of the walls were gained in the following way: 1) the projected electron densities were low pass filtered. 2) the intersection points (e equatorward, p poleward) of the filter results with the unfiltered curve were used to define the borders of the trough. 3) the trough minimum (m) was found in the unfiltered projected electron densities. some trough examples are shown in fig. 1: peak electron density (gained by «projection») versus invariant latitude. de 2 orbit parameters for the trough minimum are found in the headers and foot lines of each graphic. the trough properties derived in the indicated way are found on top of each panel. 119 properties of the main trough of the f region derived from dynamic explorer 2 data fig. 1. examples for de 2 troughs, projected to the f2 peak. top: winter conditions; bottom left: summer conditions; bottom right: equinox conditions. peak electron density in units of 1012 m–3 versus invariant latitude (°n). header and foot line information: date (year and day number) / orbit number; ht: satellite height; h m f 2 : height of the f 2 peak; lng: geogr. longitude; lt: local time; mlt: magnetic local time. the «trough parameters» are listed above the panels (left: equatorward; right: poleward; «width»: half width, third value is total half width). 120 erika feichter and reinhart leitinger table i. trough parameter statistics based on all available de 2 data (years 1981-1983). l: lower quartile; m: median; u: upper quartile; mlt: magnetic local time; (e): equatorwards; (p) polewards. (1) all seasons day (08-18 mlt) night (18-08 mlt) l m u l m u rel. depth 37.8 50.5 63.3 41.3 56.2 67.7 percent half width (e) 2.6 4.1 5.8 2.9 4.6 6.3 degrees half width (p) 2.1 3.5 5.2 2.4 3.4 5.3 degrees total width 5.6 8.0 9.6 6.7 8.4 10.3 degrees rel. steepness (e) 0.28 0.41 0.78 0.19 0.39 0.91 1/degree rel. steepness (p) 0.28 0.52 0.91 0.37 0.60 1.08 1/degree total number of troughs read in: 1779 troughs with relative depth > 20%, height of satellite < 700.0 km and total width < 20.0: 506 day: 488, night: 1116 (2) winter day (08-18 mlt) night (18-08 mlt) l m u l m u rel. depth 42.4 56.1 66.6 45.7 59.4 69.4 percent half width (e) 2.8 4.1 5.9 3.6 5.3 6.8 degrees half width (p) 2.0 3.3 5.1 2.3 3.3 5.0 degrees total width 5.4 8.0 9.5 7.3 8.8 10.5 degrees rel. steepness (e) 0.29 0.40 0.73 0.17 0.32 0.74 1/degree rel. steepness (p) 0.31 0.61 0.93 0.41 0.65 1.22 1/degree total number of troughs read in: 1128 troughs with relative depth > 20%, height of satellite < 700.0 km and total width < 20.0:957 day: 328, night: 709 (3) equinoctial months day (08-18 mlt) night (18-08 mlt) l m u l m u rel. depth 33.1 44.6 54.8 36.6 52.7 65.3 percent half width (e) 2.2 3.7 5.6 2.4 3.7 5.2 degrees half width (p) 2.1 4.0 5.3 2.4 3.8 5.9 degrees total width 5.8 8.0 9.9 6.3 7.8 9.7 degrees rel. steepness (e) 0.25 0.43 0.82 0.25 0.52 1.05 1/degree rel. steepness (p) 0.23 0.40 0.76 0.29 0.51 0.91 1/degree total number of troughs read in: 547 troughs with relative depth > 20%, height of satellite < 700.0 km and total width < 20.0:476 day: 134, night: 351 (4) summer day (04-20 mlt) night (20-04 mlt) l m u l m u rel. depth 27.6 35.1 44.5 34.2 38.3 46.5 percent half width (e) 2.4 4.2 5.3 1.6 2.2 3.3 degrees half width (p) 2.7 3.7 5.7 1.9 3.1 5.1 degrees total width 6.6 8.2 9.8 4.2 6.3 7.7 degrees rel. steepness (e) 0.22 0.42 1.48 1.05 1.37 1.91 1/degree rel. steepness (p) 0.19 0.55 0.95 0.45 0.62 0.93 1/degree total number of troughs read in: 104 troughs with relative depth > 20%, height of satellite < 700.0 km and total width < 20.0:73 day: 46, night: 29 121 properties of the main trough of the f region derived from dynamic explorer 2 data fig. 2. histograms for the relative depth and the total half width of the trough under winter conditions. left hand side: daytime (08-18 mlt); right hand side: nighttime (18-08 mlt). 122 erika feichter and reinhart leitinger fig. 3. examples for the shape of the model trough: trough combined with peak electron density from the itu-r (ccir) coefficients (solid lines). dotted: without trough modulation. winter (january), nighttime (ut = 0 h), high solar activity (r 12 = 150), high level of geomagnetic activity (kp = 6). peak electron density in units of 10 12 m 3 versus geogr. latitude (°n), geographic longitudes: 0°e, 60°e, 120°e, 180°e, 240°e, 300°e (lt 0, 4, 8, 12, 16, 20 h). 123 properties of the main trough of the f region derived from dynamic explorer 2 data fig. 4. peak electron density from the itu-r (ccir) coefficients combined with the trough model. contours of constant electron density in units of 1010 m 3 over a geographic coordinate system. ut = 0 h, r 12 = 150, k p = 6. thick curve: position of the trough minimum. 3. trough model parameters we calculated the position of the trough minimum with the model a of werner and prölss (1995). for our additional calculations of the relative depth of the trough, the half widths equatorwards and polewards, the total width of the trough and the relative steepnesses (polewards and equatorwards) we decided to restrict modelling to northern hemisphere troughs. we used data from a satellite altitude < 700 km only, restricted the acceptable relative depth to > 20% and neglected troughs with a total width of more than 20 degrees in invariant latitude. we calculated histograms and the most important quantiles of the distributions of trough parameter data under various conditions. table i lists lower quartiles, medians and upper quartiles for the trough parameters used for modelling and fig. 2 shows graphics of a few selected histograms. the width of the trough was «measured» between the midpoints of the trough walls and split into an equatorward «half width» and a poleward one. the steepnesses also refer to the midpoints of the walls. the trough model makes use of the medians of the relative depth; the half widths; the relative steepnesses. after several tests with different formulations we settled on a composite. the trough now consists of two parts, the equatorward part and the poleward part. both parts use ellipse sectors for bottom and top joined together by straight lines in such a way that the first derivatives are 124 erika feichter and reinhart leitinger continuous. the equatorward and poleward parts meet at the trough minimum. the poleward part includes a 20% enhancement which fades out like a gaussian. the test trough formulations had disadvantages. the modified semi-gaussians presented in leitinger and feichter (1999) resulted in troughs which were too «rectangular» in the cases of larger steepness. despite their low number in the de 2 data we included «summer» trough properties in our model but the user should be aware that summer troughs are not as stable and reliable as winter and equinox troughs when the main trough is a normal feature of the ionosphere. nighttime summer troughs (if they occur) usually have special properties: they have considerably smaller width and the equatorward walls are much steeper than the poleward walls. for a global model it is necessary to have smooth transitions from «day» to «night» and from «night» to «day». we have ensured this by using exponentials in the following way: f (t) = { ( f1(t) exp[ (t t0)] + + f 2 } / {1+ exp[ (t t 0 )] } (t 0: mid-time of transition; : steepness parameter; f1: function for t > t0; f2: function for t < t 0 ). figure 3 shows examples for the latitude dependence of the trough model combined with f2 peak density derived from the itu-r (formerly ccir) coefficients for january 0 hours ut, high solar activity (r 12 = 150), high level of geomagnetic activity (kp = 6) and a selection of geographic longitudes. figure 4 shows contours of constant peak electron density in a geographic coordinate system for the conditions of fig. 3. the trough minimum is also shown. references annakuliev, s.k., v.v. afonin, m.g. deminov and a.t. karpachev (1997): empirical formulae for the main ionospheric trough position during magnetic storm, geomagn. aeron., 37, 183-187. leitinger, r. and e. feichter (1999): modelle für den tro g der f schicht: möglichkeiten und grenzen, kleinheubacher berichte, 42, 63-70. letinger, r., s. radicella, e. feichter and b. nava (2001): electron density models suitable for assessment studies which involve satellite to ground and satellite to satellite electron content, in proceedings beacon satellite symposium 2001, edited by p. doherty, on cr-rom. putz, e. and r. leitinger (1988): a proposed model for the tec main trough in winter and equinox nights, in investigation of the ionosphere by means of beacon satellite measurement, edited by cao chong, (international academic publishers, pergamon), 209216. werner, s. and g.w. prölss (1995): the position of the ionospheric trough – an empirical model based on de 2 observations, kleinheubacher berichte, 38, 143-154. 683_698 collettini 23_1.pdf annals of geophysics, vol. 45, n. 5, october 2002 683 hypothesis for the mechanics and seismic behaviour of low-angle normal faults: the example of the altotiberina fault northern apennines cristiano collettini dipartimento di scienze della terra, università degli studi di perugia, italy abstract widespread mapping of low-angle normal faults in areas of former continental extension continues to prompt debate as to whether such structures may be seismically active at very low dips (δ < 30°) in the upper continental crust. the northern apennines provide an example where an active low-angle normal fault (altotiberina fault, atf) can be studied. a set of commercial seismic reflection profiles plus deep boreholes have been used to constrain the geometry of the fault at depth. these data have been integrated with a microseismic survey showing that part of the microseismicity (m < 3.0) is consistent with the geometry of the atf as imaged by depth converted seismic reflection profiles. frictional fault mechanics under byerlee’s friction coefficient and vertical σ 1 (constrained from the inversion of the focal mechanisms) defines the peculiar condition for reactivation of the atf: small values of differential stress, σ 1 − σ 3 < 28 mpa, relatively high value of tensile strength of the fault surrounding rocks (t ~10 mpa) and tensile fluid overpressure p f > σ3 (i.e. λv > 0.93). the short-lived attainment of pf > σ3 along small fault portions, in an area characterised by large amounts of co2, account for the microseismic activity located along the atf, which occurs on rupture surfaces in the range of 10–1− 10–3 km2. 1. introduction the northern apennines consist of a ne verging thrust-fold belt formed as the result of the collision between the european continental margin (sardinia-corsica block) and the adriatic microplate (e.g., alvarez, 1972; reutter et al., mailing address: dr. cristiano collettini, dipartimento di scienze della terra, università degli studi di perugia, piazza dell’università 1, 06100 perugia, italy; e-mail: colle@unipg.it key words northern apennines – friction – normal faults – microseismicity – fluid pressure 1980). ne migration of compression is associated with hinterland extension. geological and geophysical data locate the compressional front, presently active close to the adriatic coast and the extensional active front near the axial zone of the northern apennines (fig. 1). the presence of adjacent sectors of compression and extension (lavecchia et al., 1994; mariucci et al., 1999) and the migration with time of the compressionextension pair from the tyrrhenian sea to the present position can find a possible explanation in the rollback and retreat of a subducting slab (reutter et al., 1980; royden and karner, 1984; doglioni, 1991; keller et al., 1994). the extending sector is characterised by a widespread high heat flow, more than 90 mw/m2 (mongelli and zito, 1991), long-wavelength 684 cristiano collettini positive bouguer gravity anomaly, ~ 30-40 mgals, (marson et al., 1998), a widespread flux of co 2 , likely to be related to mantle degassing (chiodini et al., 2000), a shallow lithosphere/ asthenosphere boundary, ~ 50 km deep (suhadolc and panza, 1989) and a shallow moho, 20-25 km deep (ponziani et al., 1995). the development of extensional deformation within the northern apennines is well known (elter et al., 1975), but the asymmetric character of extension has been only recently revealed. the crop 03 deep seismic reflection profile (pialli et al., 1998), which crosses the northern apennines from the tyrrhenian to the adriatic coast, shows that extension within the brittle upper crust is accommodated by a set of major ene-dipping lanfs, bounding syntectonic basins, associated with antithetic high-angle normal faults (fig. 1). continuous migration of extension from west to east is well constrained by the age of the syntectonic basins (fig. 1). extension started in the western part of tuscany during the tortonian, then migrated eastward and now is active in the axial zone of the northern apennines. in this paper: 1) the easternmost of these lanfs, the altotiberina fault (atf) which is located in the seismically active umbria region, has been studied by using seismic reflection fig. 1. schematic crustal-scale cross section along the crop 03 (barchi et al., 1998a). the profile shows that part of the extension is accommodated by upper-crust, east-dipping lanfs. the extensional processes in the tyrrhenian sea and tuscany have been active enough to favour exhumation accompanied with high heat flow, positive bouguer gravity anomaly, a shallow moho, deep release of co 2 . migration of extension from the tyrrhenian coast to umbria as calibrated by the time-space evolution of the syntectonic basins (data from seismic reflection profiles barchi et al., 1998b; pascucci et al., 1999). in the active region crustal thinning is continuing. crustal structure is from the crop 03 (barchi et al., 1998a), heat flow (mongelli and zito, 1991), gravity anomaly (marson et al., 1998), co2 release (chiodini et al., 2000). the position of the moho is calibrated by deep seismic refraction profiles (ponziani et al., 1995). 685 hypothesis for the mechanics and seismic behaviour of low-angle normal faults profiles and seismological data; 2) the andersonbyerlee frictional fault mechanics along with the debate related to the lanf argument is extensively revisited; 3) a mechanical model, invoking the local attainment of dramatic fluid overpressures is proposed to explain the microseismic activity of the atf. 2. active extensional faults in umbria extensional tectonics has been affecting the umbria region since the upper pliocene (ambrosetti et al., 1978). extension is accommodated by nnw-sse trending normal faults which bound syntectonic basins (fig. 2). the tiber basin, infilled by fluvio-lacustrine upper pliocene-quaternary syntectonic sediments, is the wider basin of the area and has a longitudinal continuity of more than 100 km. east of the tiber basin, the axial zone of the northern apennines is characterised by intramountain basins possessing sub-polygonal shape and relatively little longitudinal continuity (gubbio, gualdo tadino, colfiorito, norcia, castellucio). in the intra-mountain area the sw dipping (50°70°) basin-bounding faults severely affect the topography and show an extensional kinematics close to pure dip-slip (lavecchia et al., 1994; boncio et al., 1996; collettini, 2001). 2.1. seismic images of normal faults seismic reflection profiles provide a powerful tool to investigate the geometry of the faults up to the seismogenic depth and comparison with seismological data help much to discriminate the active systems. the subsurface setting of the area has been studied by interpreting and converting to depth a set of nine seismic reflection profiles (barchi et al., 1999): eight commercial profiles, provided by eni/agip division, and the crop 03 deep seismic reflection profile (see location in fig. 2). the stratigraphy of the study area consists schematically of four major lithological units, from top to bottom: miocene turbidites, jurassicoligocene carbonates, triassic evaporites, permian-triassic phyllitic basement. the seismic profiles show at least four main seismic markers, calibrated on boreholes data (bally et al., 1986; anelli et al., 1994), that can be traced throughout the region and correspond to: bisciaro fig. 2. schematic structural map of the umbria region. the map is derived from the compilation of available data and original geological surveys. instrumental seismicity 5.6 < m w < 6.0: 1 september 19, 1979, norcia earthquake m w = 5.8; 2 april 24, 1984, gubbio earthquake m w = 5.6; 3 september 26, 1997, 00.33, colfiorito earthquake m w = 5.7; 4 september 26, 1997, 09.40, colfiorito earthquake m w = 6.0; 5 october 14, 1997, sellano earthquake m w = 5.6. focal mechanism solutions 1, 3, 4 and 5 cmt; 2 westaway et al. (1989). historical seismicity (boschi et al., 1999) with epicentral intensity greater than ix on the mercalli-cancani-sieberg scale (mcs): a april 30, 1279, camerino earthquake; b december 1, 1328, norcia earthquake; c october 7, 1639, amatrice earthquake.; d january 14, 1703, norcia earthquake; e july 27, 1751, gualdo tadino earthquake. 686 cristiano collettini fm. (at the base of the miocene turbidites), marne a fucoidi fm. (a marly interval within the carbonatic sequence), top of triassic evaporites and top of permian-triassic phyllitic basement. the main result of the interpretation of the seismic profiles is that the major fault of the study area is an ene dipping lanf, named altotiberina fault, atf, which bounds the tiber basin (barchi et al., 1998a). a portion of the crop 03 (pialli et al., 1998) deep seismic reflection profiles (fig. 3), and a line drawing, derived from a commercial profile (fig. 4) are here reproduced along with their geological interpretations. in the crop 03 profile (fig. 3) the trace of the atf is represented by the eastward dipping alignments of the reflectors merging at surface into the western margin of the tiber basin. the alignment of the reflectors deepens towards ene and reaches a depth of ~ 5 s (twt), roughly corresponding to 13 km, below the apenninic belt. in the shallower part of the profile, an antithetic normal fault bounding the eastern part of the tiber basin can also be detected. along the atf fault trace, at depth ranging between 2.5 and 3.5 s (twt), a lens shaped, east-dipping package of reflectors discloses a mature fault zone, forming an extensional duplex. on the atf hangingwall the seismic markers effectively depict the previously formed compressional structures of the apenninic thrust and fold belt, dissected and down-thrown by the normal fault activity. these reflections are strongly interrupted against the eastward dipping reflections representing the atf and they form sharp angles. the geometry of the continental tiber basin (cf. also collettini et al., 2000) is highlighted in the western and shallower portion of seismic profile of fig. 4. the asymmetrical shape of the basin is consistent with movements along an eastward dipping normal fault: the tiber basin results from the activity of a high angle synthetic splay of the atf, whose break away zone is located 10 km westward (fig. 2). the structures in the atf hanging wall show the effects of the compressional tectonics: in particular, the geometry of the gubbio anticline, with the frontal thrust and the west-dipping normal fault displacing its back-limb, can be recognised. the tectonic framework of the area is well evidenced in the geological cross section constructed by integrating geological and geophysical data (fig. 5). the san donato borehole (anelli et al., 1994) drilled the atf close to its breakaway zone where it superposes the miocene turbidites of the marnoso arenacea fm. above the triasssic evaporites. the surface expression of the atf is represented by the complex normal fault system mapped at surface in the perugia mts. (minelli, 1992; brozzetti, 1995) and interpreted as a domino-like structure detached on the atf (boncio et al., 1998). the easternmost high-angle splay of the atf, which bounds the syntectonic tiber basin, has a maximum throw of ~ 1.5 km. the age of the sediments infilling the tiber basin constrain the initiation of the activity of the fig. 3. portion of the crop 03 deep seismic reflection profile (see location in fig. 2) and geological interpretation (after barchi et al., 1998a). a normal faults; b thrusts; c marne a fucoidi fm.; d top evaporites; e top of phyllitic basement. 687 hypothesis for the mechanics and seismic behaviour of low-angle normal faults fig. 4. line drawing and geological interpretation of a seismic profile (modified after barchi et al., 1999 see location in fig. 1). a normal faults; b thrusts; c marne a fucoidi fm.; d top evaporites; e top of phyllitic basement. fig. 5. interpretative geological cross-section through the study area, based on seismic profile of fig. 4, integrating borehole data (anelli et al., 1994), seismic refraction profile (ponziani et al., 1995), surface geology data (minelli, 1992; brozzetti 1995). 688 cristiano collettini fig. 6a-d. a) earthquake locations for the study area recorded by a detailed temporary microseismic survey mayjune 1987, focal mechanisms constrained by more than eight reliable p-wave first motion polarities. hypocentral errors < 2 km in the xy plane and < 4 km in the z direction (deschamps et al., 1989; boncio et al., 2000). b) summary of stress analysis for the main population of the microearthquake focal mechanisms (n = 19) homogeneously distributed in the study area (boncio et al., 1996, 2000). projected on the wulff net are: seismic planes, observed (arrows) and theoretical (dots) slip vectors: note some gently eastward dipping planes. orientation of the stress tensor, possessing vertical σ1, form inversion technique (after boncio et al., 1996, 2000). rose diagram constructed for the main population of the microearthquakes. c) plot of the microseismicity on a geological cross section obtained from a depth-converted seismic reflection profile: the events plotted possess errors in focal depths < 1 km and are located within a band with half-width of 5.0 km. d) 3d reconstruction of the atf constructed by using depth converted seismic reflection profiles. c a b d 689 hypothesis for the mechanics and seismic behaviour of low-angle normal faults easternmost splay of the atf to the upperpliocene hence suggesting a time-averaged slip rate of 1 mm/yr. the total displacement of the fault ranges from 5 to 8 km. with the exception of the shallower portion, the atf has a gently dipping attitude (20°); the sw-dipping normal fault which bounds the intramountain basins are antithetic to the atf. the atf cuts and displaces the compressional structures inherited by the compressional phase. seismic reflection profiles constrain the geometry of the atf over an area of 150 km2: the fault trends nnw-sse and has a constant dip (~ 20°) toward ene (fig. 6d) 2.2. seismotectonic setting during the last two decades, three seismic sequences have been recorded along the umbria fault system (ufs in fig. 2): norcia 1979, gubbio 1984 and colfiorito 1997-1998 (deschamps et al., 1984; haessler et al., 1988; westaway et al., 1989; amato et al., 1998). improved techniques in earthquake recording and location have led to high resolution definition of the colfiorito sequence but the earlier seismic sequences are less well defined. the three sequences are characterised by moderate earthquakes (5.6 < m < 6.0), epicentres located in the proximity of the intramountain basins (fig. 2) and extensional focal mechanism with one nodal plane parallel to the normal faults that border the extensional basins of the area, though this interpretation has been questioned for the norcia event (see cello et al., 1997). for each sequence, the aftershock distribution seems to highlight sw dipping nodal planes, with the mainshocks located at the base of the aftershock sequence at depth ranging from 5 to 15 km. the historical seismicity of the region (boschi et al., 1999), with the strongest events that reach intensity i = x, also follows the same nnw-sse alignment (fig. 2). these data identify the ufs as the active system of the area where the strongest earthquakes occur (e.g., barchi et al., 2000). the faults within the ufs dip in the range 50°-70° at surface and are antithetic to the atf in the seismic profiles (cf. also boncio et al., 2000). during may-june 1987 a seismic survey (fig. 6a) highlighted the area between perugia and the apenninic chain (deschamps et al., 1989). in a short time interval (2 months), nearly 400 earthquakes, with local magnitude ranging from 0.6 to 3.0, were recorded in the study area. the focal mechanisms (fig. 6a) show the extensional character of the microseismicity, also confirmed by stress inversion technique (boncio et al., 2000) that constrains a stress tensor with vertical σ1 and a ne trending sub-horizontal σ3 (fig. 6b). the slip orientation inferred by seismological data agrees with structural data collected in the perugia mts. (brozzetti, 1995) and with the dip of the fault obtained from seismic reflection profiles. the distribution of the microseismicity plotted on a geological cross section (fig. 6c) shows the hypocenters deepen from west to east, from about 4 km below the tiber basin to about 14 km below the axial zone of the apenninic belt. many events fit fairly well with the atf trace, whereas other earthquakes nucleate in the atf hanging wall; only a few scattered events are located in the atf footwall. considering the focal mechanisms available, it can be said that most of these earthquakes have focal mechanisms with one nodal plane oriented in the same direction as the atf, and some of these planes, though without being positively discriminated, are gently eastward dipping (fig. 6b). supporting evidence for the seismogenic role of the atf comes from: a) the hanging wall block has been stretching toward ne since upper pliocene; b) the sw dipping normal faults antithetic to the atf are active and seismogenic (5.0 < m < 6.0). these faults in the seismic profiles do not cut the atf. recent seismological data recorded in northern umbria by a temporary network show again that the area is characterised by abundant microseismicity (piccinini, personal comunication) consistent with the atf trace and its seismically active hanging wall. it is worth noting that the observed microseismicity is not controlled by temperature; models based on heat flow data for the study area (pauselli and federico, 2002) locate the brittle ductile transition between 15 and 20 km. the distribution of the microseismicity has to be 690 cristiano collettini controlled by some lithological and/or structural factors influencing frictional properties along the fault zone. 3. initiation, reactivation and frictional lockup of normal faults in a fluid saturated rock-mass with porefluid pressure, p f , effective principal stresses are 1 = ( 1 – pf ) > 2 = ( 2 – pf ) > 3 = ( 3 – pf ) (hubbert and rubey, 1959). within intact isotropic crust brittle faults should form in accordance with the coulomb criterion for shear failure (3.1) where and n are, respectively, the shear and normal stresses on the failure plane, and c (the cohesive strength) and µ i (the coefficient of internal friction) are rock material properties. experimentally determined values for internal friction generally lie in the range 0.5 < µ i < 1.0 (jaeger and cook, 1979), leading to the expectation that brittle faults should initiate at 32° > i > 22° to the maximum compressive stress 1 . for a simple ‘andersonian’ extensional regime with v = 1 (anderson, 1951), normal faults would thus be expected to form with initial dips in the range, 58° < < 68°. however, with increasing displacement individual faults may rotate within the stress field, especially if they occur within a parallel-dipping set which may ‘domino-rotate’ to lower dips as regional extension proceeds. continued reactivation (reshear) of existing cohesionless faults is then governed by amonton’s law (3.2) where µ s is the coefficient of sliding friction. from an extensive series of laboratory friction experiments byerlee (1978) determined that for most rocks µs typically lies in the range 0.6-0.85. the only prominent exceptions is for material rich in montmorillonite, where the coefficient may be lowered to the range, 0.2 < µ s < 0.4 (morrow et al., 1992); montmorillonite, however, is not stable at temperatures comparable with seismogenic depths. constraints for friction coefficient, µ s = 0.6, also come from recent interpretation of deep boreholes stress measurements (townend and zoback, 2000). for the 2d case where an existing fault containing the 2 axis lies at a reactivation angle, r , to 1 , eq. (3.2) may be rewritten in terms of the ratio of effective principal stresses as (3.3) defining the relative ease of shear reactivation for faults oriented at varying angles to 1 (sibson, 1985). the optimal orientation for frictional reactivation, when the stress ratio for reactivation is a positive minimum (fig. 7a) is given by r * = 0.5 tan–1(1/µ s ). as r decreases or increases fig. 7a,b. a) stress ratio, r, for the frictional reactivation of a cohesionless fault plotted against the reactivation angle, r , for µ s = 0.6 (after sibson, 1985). b) histogram of active normal fault dips (after collettini and sibson, 2001). µ µ= = ( )s n s n fp' µ µ 1 3 1 3 1 1 ' ' = ( ) ( ) = +p p f f s r s r ( )cot tan µ µ= + = + ( )c c pi n i n f' a b 691 hypothesis for the mechanics and seismic behaviour of low-angle normal faults away from this optimal position, the stress ratio required for reactivation increases. frictional lock-up (σ 1 ′/σ 3 ′ = ∞) occurs when θ r = 2θ r * = tan−1 (1/µ s ). in the field of severe misorientation, beyond frictional lock-up, reshear is only possible under the tensile overpressure condition (sibson, 1990) with σ 3 ′ = (σ 3 – p f ) < 0. for byerlee’s (1978) range for rock friction, optimal reactivation occurs when θ r = 25-30° (with faults close to their original ‘andersonian’ initiation attitudes) and frictional lock-up is expected at θ r = 50-59°. the anderson-byerlee frictional fault mechanics (vertical σ1; µs = 0.6) agrees with the dip range of active normal faults capable of generating big ruptures (fig. 7b): compilation of dip estimates prepared from focal mechanisms of shallow intracontinental normal-slip earthquakes (m > 5.5; slip vector raking 90 ± 30° in the fault plane) where the rupture plane is unambiguously discriminated extends from 65° > δ > 30° (collettini and sibson, 2001). due to the impossibility of sustaining p f > σ3 no big ruptures occur on normal faults dipping less than 30°. 4. the debate on low-angle normal faults the presence of low-angle normal faults, lanfs, (dip < 30°) has been extensively documented in areas of continental extension. lanfs were first recognised in the basin and range province (wernicke, 1981; lister and davis, 1989 for a comprehensive review) and then documented in other areas: in greece (lister et al., 1984; rigo et al., 1996; sorel, 2000); in the east african ridge system (morley, 1999) in the northern apennines (carmignani and klingfield, 1990; jolivet et al., 1998; pialli et al., 1998). though the lanfs topics is well covered in literature, it is still full of controversies. first, how do lanfs form (i.e. is their lowangle attitude an original feature or the result of rotation)? according to anderson-byerlee frictional fault mechanics, normal faults initiate at dips ~ 60° than domino rotate to frictional lockup angles, 40°-30° (sibson, 1985). dips lower than lockup angle would be achieved by domino rotation produced by successive normal fault sets (proffett, 1977), or isostatic adjustments producing footwall flexure and uplift (wernicke and axen, 1988). in marked contrasts some field observations constrain initiation and movements along lanfs at dips similar to their present attitude (scott and lister, 1992; john and foster, 1993). very low dips have been also explained as the result of dramatic departures from the andersonian state of stress induced by severe topography (abers et al., 1997) or high shear stress at the base of the brittle crust (westaway, 1999). second, can displacement be accommodated by lanfs and how (seismic, microseismic, aseismic)? there are no definitive examples of m > 5.5 normal slip earthquakes on fault dipping less than 30° (fig. 7b). on the contrary three possibly low-angle ruptures (10°< δ < 30°; 6.0 < <m w < 6.8), though without positive discrimination, have been adduced to a lanf active in the papua new guinea (png) region (abers, 1991; wernicke, 1995): the lack in the contemporary seismic record of moderate and large ruptures on lanfs has been suggested to be due to their long recurrence intervals (wernicke, 1995). in considering the png earthquakes as genuine ruptures on lanfs two points must be considered. firstly, it can be said that depth converted seismic profiles crossing the same area show the seismogenic low angle detachment dipping ~ 30° (taylor et al., 1999). secondly, it should be noted that in some peculiar conditions (i.e. areas of high extensional rates) shear stress at the base of the brittle layer can be high enough to perturb the stress field in the upper crust with the consequent rotation of the σ1 trajectories (westaway, 1999); in this case a lanf is not severely misoriented for reactivation and can nucleate moderate earthquakes. more recently, there have been suggestions of a microseismically active low-angle detachment dipping ca. 15° north beneath the rapidly extending gulf of corinth, greece (rietbrock et al., 1996; rigo et al., 1996), though this interpretation has since been questioned (hatzfeld et al., 2000). 5. mechanical consideration on atf in order to discuss the mechanical conditions to explain the microseismic activity of the atf (dip 20°) within a regional stress field charac692 cristiano collettini terised by a vertical σ1 (e.g., boncio et al., 1996; chiaraluce et al., 2001), frictional fault mechanics has been applied. since most of the earthquakes are due to local frictional instability on existing faults (scholz, 1998) and since the atf shows a normal slip kinematics, 2d reactivation theory has been applied. the analysis seeks to speculate on the conditions for reactivation of the atf, using mohr circles in a coulomb griffith failure diagram integrated with the re-shear condition (e.g., brace, 1960; hancock, 1985). under vertical trajectories of σ1 it is analysed with mohr circles the state of stress necessary for reactivation of the plane of weakness as a function of differential stress (σ 1 – σ 3 ), reactivation angle (θ r ) and fluid pressure (p f ). to do this, in the parametric equations of a circle (5.1) (5.2) it is written r = 1/2 (σ1 − σ3 ) and a = σ 3 ′ + 1/2 (σ1 − σ3 ). from eq. (3.3), the values of differential stress, (σ1–σ3), and effective least principal stress, σ3′, have been written (sibson, 2000) as (5.3) (5.4) by the eqs. (5.1)-(5.3) and (5.4), is possible to evaluate with mohr circles the conditions for reactivation of the atf as a function of the reactivation angle θ r , depth and fluid pressure. for the intact rock failure envelope in the compressional field µ i = 0.75 has been assumed, corresponding to the mid value for the internal friction coefficient (0.5 < µ i < 1.0) ( jaeger and cook, 1979). for the re-shear condition the friction coefficient µ s = 0.6 has been assumed, the lower end-member of the byerlee (1978) range. the average density of the crust ρ is assumed to be 2650 kg/m3 and g is the gravitational acceleration. the pore fluid factor λ v represents the ratio between fluid pressure p f and the lithostatic stress (σ v = ρgz) assumed to coincide with σ1. hydrostatic fluid pressure is represented by λ v = 0.4 while lithostatic by λ v = 1.0. figure 8a-c represents the computed mohr circles for a fault dipping 20°, corresponding to θ r = 70°, for different values of the pore fluid factor, at 10 km of depth and for a tensile strength of the rocks enclosing the fault t ~ 10 mpa. under these boundary conditions, the reactivation of the atf is met only for low values of differential stress (σ 1 − σ 3 < 28 mpa) and quasi lithostatic fluid pressures (λ v > 0.93), circle 1 not intercepting neither the griffith parabola nor the coulomb criteria. for lower values of the pore fluid factor (λ v < 0.93) the differential stress is higher (eq. (5.3)) and σ 3 ′ is negative for θ r = 70° (eq. (5.4)); under these conditions the atf cannot be reactivated. in fact, the situations represented by circles 2 and 3 cannot be reached. figure 8b represents the fault loading, assumed to be induced mainly by the increase in fluid pressure, for a differential stress equal to the diameter of circle 2. before reaching the condition for reactivation of the atf (circle 2, fig. 8b), extensional fractures or extensional-shear fractures occur as soon as the mohr circle intercepts the griffith parabola (e.g., point e on fig. 8b). this occurs for all values 0.86 < λ v < 0.93. for greater values of differential stress (circle 3), a new ‘andersonian’ fault develops as soon as the mohr circle intercepts the coulomb criterion (point c on fig. 8c), and this happens for all values λ v < 0.86. it has been tested the sensitivity of the calculations to the assumed friction coefficients, within the «byerlee law», and to different depths: under the conditions required for reactivation of the atf, the simulations are modestly sensitive to these parameters. x a r= − cos2α y r= ° sin 0 < < 90 2α α σ σ µ θ θ µ θ ρ λ1 3 1 1−( ) = +( ) +( ) −( ) s r r s r vgz tan cot cot σ µ θ µ θ ρ λ3 1 1 1' .= − + −( )s r s r vgh tan cot 693 hypothesis for the mechanics and seismic behaviour of low-angle normal faults the mechanical analysis says that the atf can be reactivated only in peculiar circumstances: low differential stress, t ~ 10 mpa and high fluid pressure in particular. in the next two paragraphs the specialised circumstances that in the study area may lead to localised fluid overpressures will be discussed. 5.1. local short lived attainment of tensile fluid overpressure a possible explanation for small ruptures as discussed previously, the key issue for reactivation of the atf is the attainment of the tensile overpressure condition p f > σ 3 (i.e. λ v > 0.93). as a consequence of slip, which produces grain comminution, porosity collapse and alteration of clay-rich material from the phyllitic basement, the atf represents an impermeable horizon trapping fluids coming from deeper levels favouring fluid overpressures. moreover, the phyllitic fault gouge itself can generate water from dehydration reactions (faulkner and rutter, 2001). it is unlikely, however, to sustain tensile fluid overpressure, p f > σ3, for wide fault portions. first, fluid loss through hydraulic extension fracturing limit the sustainability of the tensile overpressure condition required for reactivation. second, an increase in permeability under low effective stress has been documented by laboratory experiments (seront et al., 1998). third, the load-weakening behaviour for normal faulting induced by the preseismic decrease of the mean stress favours the increase in permeability (sibson, 1993). these considerations suggest that the overpressure condition is likely to develop only in small portions of the atf (fig. 9) and for a short time, being a possible explanation for the microseismic activity (1 < m < 3) recorded along the fault and involving rupture surfaces in the order of 10–1 – 10–3 km2 (sibson, 1989). the impossibility of sustaining the tensile overpressure condition over wide fault portions would prevent the nucleation of big ruptures (m > 5.5; rupture dimensions ca. 5 km) on normal faults dipping less than 30° (cf. fig. 7a,b). dealing with the influence of fluid pressure in rupture nucleation and propagation, miller et al. (1996) proposed a model coupling shear stress and high fluid pressure; the increase in fluid fig. 8a-c. a) differential stress (σ1 – σ3), required for reactivation of the atf, θ r = 70°, under a pore fluid factor λ v = 0.94, 0.90, 0.86, tensile strength of the rock t = 10 mpa, plotted in a composite coulomb-griffith diagram integrated with the re-shear condition of shear stress, τ, against normal stress σ n . only circle 1 represents the conditions for reactivation of the atf. b,c) fault loading mainly induced by the increase of fluid pressure starting from differential stress values represented by circles 2 and 3. b) the mohr circle intercepts the griffith parabola at point e, leading to the formation of extensional fractures before reaching the condition for reactivation of atf (circle 2). c) the mohr circle intercepts the coulomb criterion at point c, leading to the formation of a new ‘andersonian’ fault before reaching the condition for reactivation of atf (circle 3). c a b 694 cristiano collettini pressure within discrete cells creates zones of low effective stress which produce slip. the propagation of ruptures toward the other cells and thus the possibility to create earthquakes of different magnitudes, is a function of the state of stress of the surrounding areas. pressurised (p f > σ3 ) small portions of the atf, characterised by very low effective stress, can slip. when the slip takes place, it is unlikely to propagate in the surrounding regions because of: 1) strain hardening and velocity hardening of the phyllitic gouge (faulkner and rutter, 2001); 2) reduced rupture propagation rates on low angle normal faults (ofoegbu and ferrill, 1998). 5.2. why is there fluid overpressure in an extending crust? although fluid overpressures in extensional environments are not the rule, the study area represents a peculiar case. high flow rates of gas emission have been documented in the western sector of the tiber basin with isotopic signatures suggesting deep source (vasselli et al., 1997; minissale et al., 2000). this non-volcanic mantle derived co 2 flux is widespread in central italy (see also fig. 1) and extremely vigorous (1-3 ⋅ 1011mol ⋅ yr–1) with maximum fluxes in the order of 0.2 m3⋅m2⋅yr–1 (chiodini et al., 2000). the co 2 rich fluids on their ascent are likely to be entrapped when encounter stratigraphical (e.g., evaporites or phyllitic basement) or structural seal (e.g., atf). at the beginning, co2 tends to be dissolved in the pore water present at the base of the seal (fig. 10a). the increase in co 2 concentration leads to the formation of a free gas phase (chiodini et al., 1999) as soon as the total pressure of the reservoir (roughly corresponding to pco 2 ) exceeds pore fluid pressure at the base of the seal. this process produces gas bubbles (fig. 10b) that add to pore pressure and favour the attainment of the tensile overpressure conditions required for reactivation. the consequent slip on the fault creates permeability with post seismic fluid and gas discharge. this process accounts both for the presence in the western margin of the tiber basin of focussed vents and for the high pressure gas pockets documented in the deep drilling of the region (chiodini et al., 1999). it is worth noting that the san donato borehole, before being stopped for fluid overpressure, encountered pco2 of about 100 mpa at 4.7 km depth (chiodini and cioni, 1989) corresponding to a pore fluid factor at least λ v = 0.85. other examples of high fluid pressures in extensional environments have been documented by fluid inclusion studies in correspondence with fig. 9. cartoon showing the rupture dimensions for a m = 1, m = 3, m = 5.5 earthquake. the tensile fluid overpressure condition is sustained only along small fault portions capable of generating microseismicity. fig. 10a,b. schematic model for the attainment of tensile fluid overpressure. a) deep emissions of co2 rich fluids encounter a fluid rich reservoir below a structural seal (e.g., atf). at the beginning co2 tends to be dissolved within the fluid. b) increasing concentration of co 2 leads to gas bubble formation within the reservoir favouring the attainment of the tensile fluid overpressure p f > σ 3 . a b ���� ���� ��� ��� � �� 695 hypothesis for the mechanics and seismic behaviour of low-angle normal faults exhumed seismogenic normal faults in nevada and utah (parry and bruhn, 1990), and a pore fluid factor of 0.95 has been measured from well data in extending crusts beneath shale sequences in the gulf coast basin (bradshaw and zoback, 1988). 6. discussion in the study area, the strongest seismicity, represented by moderate extensional earthquakes (5.0 < m < 6.0), is related to a set of sw dipping normal faults which border the quaternary intramountain basins of the area. these faults dipping at the surface in the range of 50°-70° are antithetic to an ene-dipping lanf (atf), whose geometry has been reconstructed matching surface geology with boreholes data and seismic reflection profiles. the fault has an average dip of 20° and shows a good correlation with the microseismicity of the area. the application of reactivation theory (sibson, 1985, 1990) under vertical trajectories of σ1, suggests that the atf can be seismically reactivated only in specialised circumstances, characterised by tensile fluid overpressure, p f > σ 3 , (i.e. λ v < 0.93). this condition is difficult to maintain and it is likely to be sustained only for a small fault portion which can produce microearthquakes. friction coefficient too may influence the dip range of severely misoriented faults. if for some reason (not well documented by laboratory or borehole data, but see townend and zoback, 2001) the fault possesses very low friction coefficient at seismogenic depths (e.g., µ s ~ 0.35 at depth > 5 km), a 20° dipping normal fault is not in the field of severe misoriented fault hence in a condition to be reactivated without invoking the tensile fluid overpressure condition (p f > σ 3 ). in the peculiar situation of the northern apennines, large emissions of fluids, probably related to mantle degassing phenomena (chiodini et al., 2000), are likely to be the major cause for the weakening mechanism of the atf. when these fluids in their ascent meet the atf, they can be entrapped in small fault portions. the continuous co 2 flux forms gas bubbles that add to pore pressure and favours the attainment of localised overpressures which lead to the nucleation of small earthquakes. in any case, the microseismicity recorded along the atf cannot account for the displacement of the fault (2 km for the easternmost splay of the atf in the upper pliocene-quaternary, i.e. ~ 1 mm/yr) part of which is presumably accommodated aseismically. according to these considerations, the atf would represent a creeping lanf, with most of its trace within the phyllitic basement, producing part of its displacement aseismically and in some peculiar circumstances, nucleating microearthquakes as a consequence of local fluid overpressures. this seismic behaviour is in agreement with the mode of failure for a phyllosilicate-rich gouge found from laboratory experiments (faulkner and rutter, 2001). rubin et al., (1999) also documented that faults which creep largely aseismically, simultaneously produce abundant microearthquakes, and the total slip represented by the earthquakes amounts to only a small fraction of the long term observed slip rate. acknowledgements i thank massimiliano barchi and rick sibson for discussions, carlo cardellini and giovanni chiodini for help on the co2 topic. this research was supported by miur 01 ur perugia, massimiliano barchi grant. references abers, g.a. 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(received june 18, 2002; accepted october 21, 2002) vol50,1,2007 1 annals of geophysics, vol. 50, n. 1, february 2007 key words pressure stimulated current (psc) – damage parameter – fracture 1. introduction phenomena associated with fracture, particularly those concerning materials of inhomogeneous structure, such as geomaterials, in combination with occurring transient electric phenomena, have always attracted the interest of the scientific community (hayakawa and fujinawa, 1994; hayakawa, 1999). although there exist important similarities between the fracture of a pristine rock and an earthquake rupture, there are also important differences (turcotte et al., 2003). therefore, such phenomena are promising candidates of earthquake precursors (molchanov and hayakawa, 1995; tzanis and vallianatos, 2002). while deformed, geomaterial generates electrical signals that are assumed to be caused by crack generation and propagation in the earth’s crust (molchanov and hayakawa, 1998; vallia-natos and tzanis, 1998). in order to understand the mechanisms that produce these electric signals, a number of laboratory experiments of mechanical distress to fracture have been conducted on minerals and rocks (dry and saturated) (nitsan, 1997; ogawa and miura, 1985; enomoto and hashimoto, 1990; hadjicontis and mavromatou, 1994; o’keefe and thiel, 1995; takeuchi and nagahama, 2001). these experiments have been combined with numerous studies and recordings of acoustic emissions (tonolini et al., 1987) due to mechanical stress cause microcracking in rocks as well as the kaiser effect which takes place in rocks and materials subjected to cyclic loading/unloading (kaiser, 1953; beattie, 1983; lavrov, 2003). it is known that when mechanical stress is applied upon a rock sample it reacts by emitting electric current (hadjicontis and mavromatou, 1994; o’keefe and thiel, 1995; vallianatos et al., 2004). recent laboratory experiments conducted on penteli marble samples have confirmed that the application of a uniaxial stress on geomaterial samples is accompanied by the production of weak electric currents to which the term pressure stimulated currents (psc) has been attributed (stavrakas et al., 2003, 2004; anastasiadis et al., correlation of pressure stimulated currents in rocks with the damage parameter cimon anastasiadis (1), ilias stavrakas (1), dimos triantis (1) and filippos vallianatos (2) (1) department of electronic engineering, technological educational institution (tei) of athens, greece (2) department of natural resources & environment, technological educational institute of crete, chania, greece abstract pressure stimulated current (psc) experiments were conducted on marble samples to correlate psc with the damage parameter, d. the phenomena and procedures taking place in the vicinity of the fracture limit were observed and analytically described. psc recordings were conducted by application of uniaxial compressional stress, both at a constant stress rate and at a constant deformation rate. a linear relationship was shown to exist between the emitted psc and the damage parameter which quantifies the deviation from linear elasticity and the concentration of microcracks. mailing address: dr. cimon anastasiadis, department of electronic engineering, technological educational institution (tei) of athens, ag. spiridons street, gr12210 egaleo-athens, greece; e-mail: cimon@ee.teiath.gr 2 cimon anastasiadis, ilias stavrakas, dimos triantis and filippos vallianatos 2004; vallianatos et al., 2004). the above experimental procedure is described by the term «psc technique» and consists of recording the currents emitted by geomaterial samples when subjected to either an abrupt stress variation or an extended constant increase in stress rate up to failure. as the magnitude of stress increase reaches the plastic deformation range of the material, microcracks occur randomly starting at the heterogeneities and spreading within the bulk of the sample (jaeger and cook, 1979). in the beginning, the structural imperfections produced are uncorrelated but as the density of microcracks increases they interact and a correlated microcrack net structure starts to appear. the microcracks eventually coalesce leading to the irreversible fracture process. in the present paper psc emission is studied in two cases: first, psc emission when the applied stress s is a linearly increasing function of time (ds/dt = constant), and second psc emission at a constant strain, ε, rate (dε/dt = constant) up to sample fracture. the stress, s, on the material is given as a function of strain ε. for the linear elasticity range it can be stated that (1.1) where y0 is young’s modulus of the undamaged material which is constant in the elastic range. when the stress takes values that lead into the plastic deformation range, then microcracks occur. for a prescribed stress s, the strain ε is greater than the value given by eq. (1.1). accordingly (turcotte et al., 2003) (1.2) where yeff is the effective young’s modulus and it is no longer considered as constant. in the plastic range young’s modulus becomes progressively smaller while stress increases. a more descriptive approach to the results of this process is to introduce a damage parameter d so that (lemaitre and chaboche, 1990; krajcinovic, 1996) . (1.3) the damage parameter, d, quantifies the devia(1 )y y deff 0= − s yeff $ ε= s y0 $ ε= tion from linear elasticity and the concentration of microcracks. in general 0 ≤ d ≤ 1. when d = = 0, linear elasticity is obtained with eq. (1.1) valid, but when d = 1, failure occurs. the damage parameter, d, is a function of the applied stress, s, only. however in most cases of interest the development of damage in a material is a transient process, so that we have d(s). thus in the range where deviations from linear elasticity observed, eq. (1.2) can be written . (1.4) 2. material and experimental technique the samples used to perform the experiments were of marble collected from mt. penteli located in attica, greece (pentelicon or dionysos marble). its chemical composition is 98% calcite and 2% quartz and other minerals such as muscovite, sericite and chlorite. its porosity is low, of the order 0.371%. the geometric characteristics of the samples as well as the loading parameters are summarised below. all samples were cylindrical measuring 106.4 mm in height, with a diameter of 51 mm. those of the samples that were compressed at a constant stress rate suffered a pressure increasing at a rate of 20 kpa/s. those of the samples that were compressed at a constant strain rate, were contracted in one dimension and suffered shortening at a rate of 0.5 µm/s. the fracture limit in the former case was 101 mpa while it was only 88 mpa in the latter as they were measured on the basis of their stressstrain curves. stress-strain curves depict the stress required by a material sample to expand or contract by a specified percentage with respect to its initial dimensions. they depend on various factors such as stress history, humidity, water content and porosity. in order to prevent significant variations in the stress-strain curves of the sample under test, all samples were extracted together from the same rock mass and they were kept under the same conditions of temperature and humidity. figure 1 shows a typical stress-strain curve of one of the samples used. the values on the ( )s y d10 ε= − 3 correlation of pressure stimulated currents in rocks with the damage parameter stress axis correspond to stress, s, normalized by the maximum stress recorded before fracture (smax). each stress-strain, (s-ε), curve can be divided into three parts: in the first part a non-linear relationship between stress and strain is observed. this is a very short initial quasi-plastic range, that does not extend over 0.1 of smax, which can be attributed to the closing of pores and to probable pre-existing microcracks (griffith, 1920). the long linear part of the curve corresponds to the elastic range of the material. this range extends up to s = 0.7, where the plastic behaviour of the sample starts and linearity is no further maintained. this range continues up to fracture. hereafter, all stress values noted with s, will be normalized with respect to smax. the stress was applied by a loading machine (model mts-815) capable of applying a maximum force of ±1600 kn and a maximum deformation of ±50 mm. an integrated electronic micro-console (model mts-453.20), equipped with a load and displacement controller as well as a function generator unit, were used to provide a closed loop control of the servo-hydraulic system. a keithley electrometer (model 617) was used to measure psc and all data were stored in a computer hard disk through a gpib interface. a detailed description of the loading system and measuring system as well as sample mounting and data recording is given elsewhere (anastasiadis et al., 2004; stavrakas et al., 2004). 3. experimental results and discussion during the experiments either the stress rate or the deformation rate were kept constant. as a result, two series of experiments were realized, keeping in each of them either the stress or the strain rate constant. figure 2 represents the temporal behaviour of the pressure stimulated current when a marble sample is compressed uniaxially at a constant stress rate of 20 kpa/s up to the fracture while the inset diagram show the variation of the psc with respect to the normalized stress. the psc signal was recorded when the normalised stress exceeded a value of approximately 0.7. at this stress range irreversible structural changes occur due to plastic behaviour of the material. this observation was verified repeatedly (stavrakas et al., 2004) since at fig. 1. typical marble stress-strain curve on normalised stress axis. fig. 2. psc with respect to time, when stress at a constant rate is applied. the inset diagram shows the psc versus normalized stress. 4 cimon anastasiadis, ilias stavrakas, dimos triantis and filippos vallianatos approximately such stress values correspond to the beginning of the plastic range of the material where changes in structure are irreversible due to microcracks taking place (jaeger and cook, 1979; turcotte et al., 2003). in this range yeff gets gradually lower than y0. the normalised stress curve at values exceeding 0.7 exhibits a smooth ascending, and it seems to reach a maximum value slightly before fracture. the peak is not clear because the process develops quickly as the stress increase rate is high and the time elapse corresponding to the range 0.9 < s < 1.0 is relatively short. in order to show the peak in detail during the fracture process, uniaxial compression at constant deformation rate was applied. figure 3 depicts the temporal recording of psc when the marble sample is subjected to an increase in the uniaxial compression at a constant deformation rate of 0.5 µm/s. in the same diagram the temporal variation of the normalized stress s is represented too. when the material enters the plastic deformation range (s>0.7) the stress rate continuously decreases, and diminishes at s=1, when the sample fails. as can be seen in fig. 3 the psc peak at fracture is clearly distinguished. the emitted psc acquires a maximum value and consequently decreases; this corresponds to a decrease of the psc rate which becomes continuously smaller and slightly before fig. 4. variation of the damage parameter, d, with respect to the normalized uniaxial stress, s, applied to a typical marble sample. fig. 5. psc with respect to the damage parameter, d, when the sample is under constant stress rate conditions: open circles; psc with respect to the damage parameter, d, when the sample is under constant deformation rate: solid circles. 4 5 fig. 3. detailed representation of the temporal variations of both psc (curve a) and normalized stress applied on a marble sample (curve b) while the deformation rate is kept constant. 5 correlation of pressure stimulated currents in rocks with the damage parameter fracture (s ≈ 0.98) the psc gets to a maximum and an abrupt decrease follows. based on the experimental data of the stress-strain curve (fig. 1) the damage parameter, d, was determined by the use of eq. (1.4). the diagram of fig. 4 shows the variation of the damage parameter as a function of normalized stress s. it can be seen that the damage parameter, for values of normalized stress greater than 0.7 takes values different from zero and continuously increases. figure 5 depicts a correlation between the psc and the damage parameter, d, when the sample is under constant stress rate and constant deformation rate respectively. both diagrams exhibit a linear relationship between psc and d in the range of d < 0.8. 4. conclusions in this work we correlated the psc with structural damage introduced in a material by the external application of uniaxial stress. the damage is quantified by the parameter d that can be calculated with good approximation by the use of the stress-strain curve. correlating the emitted psc with the damage parameter, d, showed a linear relation. this finding holds for d < 0.8. since damage mainly occurs at stress levels close to plastic deformation and fracturing, experiments of constant deformation were performed for a better analysis of this process. these experiments validated the linearity between psc and d. from the experiments performed using a constant increase in the strain rate it was verified that psc reaches its maximum value just before failure at normalized stress of approximately s = 0.98. above this level the psc decreases rapidly. acknowledgements this work is supported by the project archimedes ii: «support of research teams of technological educational institute of athens», sub-project entitled «the electric behavior of geo-materials» in the framework of the operational programme for education and initial vocational training. it is co-funded 75% by the eu and 25% by the greek government. references anastasiadis, c., d. triantis, i. stavrakas and f. vallianatos (2004): pressure stimulated currents (psc) in marble samples after the application of various stress modes before fracture, ann. geophysics, 47 (1), 21-28. beattie, a.g. 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(1953): erkenntnisse und folgerungen aus der messung von gerauschen bei zugbeanspruchung von metallischen werkstoffen, archiv eisenhuttenwesen, 24, 43-45. krajcinovic, d. (1996): damage mechanics (elsevier, amsterdam), pp. 761. lavrov, a. (2003): the kaiser effect in rocks: principles and stress estimation techniques, int. j rock mech. min. sci., 40, 151-171. lemaitre, j. and j.-l. chaboche (1990): mechanics of solid materials (cambridge university press, cambridge), pp. 581. molchanov, o.a. and m. hayakawa (1995): generation of ulf electromagnetic emissions by microfracturing, geophys. res. lett., 22, 3091-3094. molchanov, o.a. and m. hayakawa (1998): on the generation mechanism of ulf seismogenic electromagnetic emissions, phys. earth planet. int., 105, 201-210. nitsan, u. (1997): electromagnetic emission accompanying fracture of quartz-bearing rocks, geophys. res. lett., 4, 333-337. o’keefe, s.g. and d.v. thiel (1995): a mechanism for the production of electromagnetic radiation during fracture of brittle materials, phys. earth planet. int., 89, 127135. ogawa, t.k. and t. miura (1985): electromagnetic radiation from rocks, j. geophys. res., 90, 6245-6249. stavrakas, i., c. anastasiadis, d. triantis and f. vallianatos (2003): piezo stimulated currents in marble samples: precursory and concurrent – with – failure signals, nat. hazards earth sys. sci., 3, 243-247. stavrakas, i., d. triantis, z. agioutantis, s. maurigiannakis, v. saltas and f. vallianatos (2004): pressure 6 cimon anastasiadis, ilias stavrakas, dimos triantis and filippos vallianatos stimulated currents in rocks and their correlations with mechanical properties, nat. hazards earth sys. sci., 4, 563-567. takeuchi, a. and h. nagahama (2001): voltage changes induced by stick-slip of granites, geophys. res. lett., 28, 3365-3367. tonolini, f., a. sala and g. villa (1987): general review of developments in ae methods, int. j. pressure vessels piping, 28, 179-201. turcotte, d.l., w.i. newman and r. shcherbakov (2003): micro and macroscopic models of rock fracture, geophys. j. int., 152, 718-728. tzanis, a. and f. vallianatos (2002): a physical model of electrical earthquake precursors due to crack propagation and the motion of charged edge dislocations, in seismoelectromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o.a. molchanov (terra scientific publishing co., tokyo), 117-130. vallianatos, f. and a. tzanis (1998): electric current generation associated with the deformation rate of a solid: preseismic and coseismic signals, phys. chem. earth, 23, 933-938. vallianatos, f., d. triantis, a. tzanis, c. anastasiadis and i. stavrakas (2004): electric earthquake precursors: from laboratory results to field observations, phys. chem. earth, 29, 339-351. origin of calderas: discriminating between collapses and explosions annals of geophysics, 59, 6, 2016, s0650; doi:10.4401/ag-7010 s0650 origin of calderas: discriminating between collapses and explosions izumi yokoyama the japan academy, ueno park, tokyo, japan abstract origins of calderas may differ according to their subsurface structure that may be characterized by high or low density deposits that may be observed as high or low gravity anomalies, respectively. in the introduction, the pioneering work of fouqué [1879] on santorini caldera is referred to in relation to definition of calderas. first, our discussion is focused on four calderas that were seen forming during the period from 1815 (the tambora eruption) to 1991 (the pinatubo eruption). coincidently, these four calderas are all low-gravity-anomaly type. their formation processes and subsurface structure are summarized by the existing data analyzed by various authors. these results are confirmed by results of drillings at some other calderas. then, caldera formation of both types is discussed: high-gravity-anomaly-type calderas are expected to originate from subsidence of high-density ejecta into the summit magma reservoir. on the calderas of this type, the genetic eruption believed to be accompanied by subsidences were not actually observed, and consequently three examples are mentioned only briefly. the low-gravity-anomaly-type calderas are discussed from standpoint of both the models of collapses and explosions. it is also emphasized that dynamic pressure of explosions is an important factor in the caldera formation, not only volume of the ejecta. to confirm the possibility that volcanic ejecta and edifices collapse into magma reservoirs, we discuss stress propagation from a depleted reservoir upward towards the earth surface. formation mechanisms of large calderas of this type are speculated; large calderas measuring about 20 km across may develop by successive merging of component calderas over a long period of times. a kamchatka caldera under enlargement during the holocene period is interpreted by successive merging of five component calderas. 1. introduction the term “collapses” means collapses or subsidences of volcanic ejecta and volcano edifices engulfed into the magma reservoirs resulting from series of violent eruptions. here, the positions of magma reservoirs are critical. if the magma reservoirs are located at shallow depths, explosions can easily cause roofs of magma reservoirs to collapse. on the other hand, “explosions” are outward volcanic actions producing depressions, and some parts of exploded materials fall back into vents or sometimes the depressions are eroded, both resulting in formation of flat floors. since the early 19th century, calderas were defined as a kind of volcanic depressions having diameters larger than 2 km accompanied by flat floors. this classical definition of calderas was rather primitive and vague, and has been improved by the development of earth sciences in the last two centuries. extensive literature exists, particularly, in the last 20 to 30 years that allows various approaches to classify calderas. a chief object in view of the present paper is reexaminations of the classification mainly from a viewpoint of geophysics. in the early period, geologists relied mainly on surface topographies of calderas that differed from other depressions in volcanic areas. generally, such calderas formed over long geological times and have altered due to various topographic deformations. here, reference is made to the pioneering work of fouqué [1879] on santorini caldera in greece is referred to. this caldera formed about 80 ka bp. around 1870 ad fouqué visited santorini erupting at the middle of the caldera, and he may have been immediately impressed with the precipitous caldera walls of about 300 m high above the sea that was different from usual crater rims. he was convinced this development was consistent with “collapse model” as indicated by the steep slopes of the escarpments. since then, the majority of geologists may have been influenced by this idea. however, the actual slope of the caldera wall on topographic maps is about 35 degrees in arc, on the average, which is different from subjective eyesight. after a caldera forms on the sea, or on the land, its walls would continue to retreat outward by surface foundering and erosion extending article history received march 18, 2016; accepted november 13, 2016. subject classification: origin of calderas, calderas of low-gravity-anomaly type, caldera deposits, magma reservoirs, collapse model, explosion model, composite calderas. over a lengthy period of time, repeatedly showing precipitous or gentle cliffs. bold sea cliffs are not specific to caldera walls. the caldera walls of santorini face to the inner sea, and hence their recession must have been very pronounced for about 80 × 103 years. from geophysical standpoint, in 1969, yokoyama and bonasia [1978] observed the gravity anomalies on santorini caldera as shown in figure 1 where the anomalies are corrected for topographies. their distribution suggests a possibility that the caldera is composed of 2 depressions, south and north. at the south, residual low anomalies amount to (about 125 to 115) 10 mgal due to a thick deposit of low-density material conforming to the caldera boundary. if we assume a vertical fault at the western side of thera island, in fault-model analyses, the fault line should pass the points of the largest gradient of the anomalies, namely along the 120 mgal contour and not the shoreline in figure 1. later budetta et al. [1984] carried out a more detailed survey of gravity on the same islands and they also did not detect faults at that shoreline. heiken and mccoy [1984, fig. 6] do not assume any faults at the basement in their geological cross sections of the caldera walls. at santorini caldera, fouqué [1879] insisted on the “collapse model”, in addition, mentioning the small amount of older rocks found in the pumice, but without showing any quantitative field data. this statement caused a problem for the younger generation of volcanologists. only 4 years after fouqué’s paper, tremendous caldera-forming eruptions occurred at krakatau. verbeek [1885] and later researchers made efforts to estimate volume of the ejecta from the 1883 krakatau eruption to prove validity of the collapse model. as shall be mentioned in section 2(b), volume estimations of the pyroclastic ejecta are very difficult especially at island volcanoes and their accuracy may remain to be plus or minus an order of magnitude. santorini caldera formed in early times, and was not observed in comparable detail as krakatau. with these considerations mentioned above, it is difficult to easily accept fouqué’s hypothesis of “collapse model”. in the 20th century, williams [1941] preferred classification of calderas into the following four types with their proper names as krakatau, kilauea, katmai and glen coe. the first three calderas shall be referred to later and the last is a tertiary basaltic “cauldron” in scotland probably revealing a local high gravity anomaly as those in adjacent areas (cf. mcquillin and tuson [1965]). from geophysical standpoint, yokoyama [1963] found low or high residual gravity anomalies on several calderas and, accordingly, classified the calderas into the two types that correspond to “krakatau type” and “glen coe type” according to the classification of williams [1941] as mentioned above. now, this geophysical classification that is a very simple and significant standard based on subsurface structure shall be used in the present discussion. calderas of low-gravity-anomaly (lga-type): the caldera deposits of low density are mainly “fallbacks” of siliceous pyroclastic ejecta and lithic breccias of the destructed volcano edifices, and develop low-gravityanomalies. williams and mcbirney [1979, p. 221] argued against this idea explaining that gravity anomaly may reveal either low or high depending on the nature of the ancestral cones and basement rocks. in fact, lithic breccias of disrupted volcano bodies also have low density due to dilatational effect: the potential theory indicates that the low gravity anomalies over a caldera are local and not due to regional structure of basements. the low gravity anomalies are characteristics of explosive caldera-forming eruptions of lga-type beside large amounts of their juvenile ejecta. calderas of high-gravity-anomaly (hga-type): the caldera deposits of high density are usually accumulation of basaltic lavas. at these calderas, eruptive activities are usually derived from shallow magma reservoirs, within about 5 km in depth. on calderas of hga-type, roofs of the magma reservoirs gravitationally collapse or subside stepwise, to fill up gaps caused by ejection or withdrawal of lavas or magmas. this will be further discussed in chapter 3. yokoyama 2 figure 1. santorini caldera, greece: the bouguer gravity anomalies are shown after yokoyama and bonasia [1978]. solid dots show gravity points. h and l denote residual anomalies, high and low, respectively. 3 the above classification of calderas is based on their subsurface structure, and necessarily related to their formation mechanism. hitherto, many researches have contributed to explain caldera formation. however, many of them based their discussions or experiments on the “collapse model”, and uniquely, leaving the “explosion model” out of consideration. even if they succeeded in modeling, they have to verify uniqueness of their solution. a rather recent book “caldera volcanism” edited by gottsmann and martí [2008] contains 14 papers covering various aspects of calderas and contributed by 40 active researchers. it is a pity that they scarcely refer to the four historical caldera formation, tambora, krakatau, novarupta and pinatubo, which afford main data to be discussed in the present paper. 2. calderas observed during their formation 2.1. calderas formed after the 19th century the four calderas that were observed during formation, tambora in 1815, krakatau in 1883, katmai in 1912 and pinatubo in 1991 shall be emphasized because discussions on old and dissected calderas tend to be vague. coincidently, these four calderas are all lgatype that is much more abundant in number than the other type. novarupta and pinatubo calderas are not fully proved to reveal low gravity anomalies at present but may be assumed as lga-type on the basis of funnel-shaped conduit and large amounts of juvenile silicious ejecta. by these four particular calderas alone, of course we cannot treat the great variety of different formation and structure of the calderas worldwide, and also the post-caldera activities such as resurgent doming and so on and hence we need to refer to the other calderas and volcanoes in the following discussion. main features of the four calderas are shown in table 1. in this table, all the items except the last one (ejecta volume) are observed values at the time of the caldera formation and probably reliable. “ejecta volume” is not always unique and some estimates widely range. the ejecta from some calderas are distributed in the surrounding sea and, at some calderas, the ejecta surveys were carried out long time after the events, and furthermore, evaluations of their errors are rather problematic (cf. yokoyama [2015a]). the ejecta volume is closely related to evaluation of eruption magnitude (or explosivity) as well as to dynamic pressure. “volcanic explosivity index” (vei) shall be discussed in section 5.3. in table 1, the largest magnitudes of earthquakes related to the events are mentioned as a possible measure of their activities. the observational conditions and the origins of the data of the four calderas are briefly summarized in the following. (a) the 1815 tambora eruption, sumbawa, indonesia this eruption was exceptionally violent but not scientifically observed. at that time, the monitoring network was not developed. we do not have any reliable research reports of this eruption. the eruption reached its paroxysm on april 10 and lasted until april 12, producing a caldera measuring 6 km in diameter. on april 10, a strong shock was felt in the town of surabaya 600 km away. if this was an earthquake, its magnitude (m) may have been nearly the m 7 class (cf. yokoyama [2001]) and it should have damaged structures considerably in the populated towns in bali island (at about 250 km distance), however, we have no particular reports of damages. probably the strong shock was caused by air shocks. eruptions finally ended on july 15, 1815. later in august 1819, heavy rumbling, earthquakes and fire-phenomena were observed. a small basaltic lava dome (doro afi toi) with a small lava flow may have formed from this activity. the post-activity of the 1815 eruption is important for discussions of the total magma activity. immediately after the caldera formation and until 1819, the magma conduit system functioned to support the faint activities within the new caldera and after that, the volcano has not shown any activity. this may suggest that the magma in the reservoir was almost depleted by the caldera-forming eruption, and totally emptied by the 1819 activity. in other words, the conduit system was origin of calderas caldera year diameter and depth (km) previous eruption earthquakes before the event max. earthq. magnitude m duration of climactic eruption ejecta volume (km3 dre) + missing edifices (km3 bulk) tambora 1815 6 0.6~0.7 ? 3 yr 7 (?) 3 days (12~60) + 46 krakatau 1883 5 0.25 1680 ad 99 days 5 class 23 hr (9~19) + 8 novarupta 1912 2 ca. 0.25 ? 7 days 7.0 14 hr 15 + 0.7 pinatubo 1991 2.5 0.2 ca.1490 ad 71 days 5.7 9 hr 4.5 + 2.5 table 1. main features of the four calderas observed during and after the formation. the origins of the data are given in the paragraph of each caldera. dre: dense rock equivalent. not blocked by the caldera-forming eruptions in 1815. the main conduit may have been blocked after the caldera formation, and a small basaltic lava flow probably leaked from depth after 4 years. in 1980s, several parties carried out geological studies on tambora. self et al. [1984] newly estimated the total volume of the pyroclastic ejecta at 50 km3 dre. recently kandlbauer and sparks [2014] used new ash thickness data and new methods to estimate a total volume of the ejecta and obtained about 41 ± 4 km3 dre, made of 23 ± 3 km3 dre ash fall and 18 ± 6 km3 dre pyroclastic flows (cf. figure 2). (b) the 1883 krakatau eruption, sunda, indonesia this eruption was once deemed to have formed the second caldera based on earlier reports. yokoyama [2014] disagrees with this hypothesis for two reasons: the marine depths shallowed toward the pre-caldera krakatau islands in the 1854 bathymetric chart (cf. yokoyama [2014], fig. 1) and the gravity values increased similarly toward the islands in the 1969 gravimetric map (ibid., fig. 2). these prove that the old krakatau islands were on a basement upheaval and did not indicate any traces of large depressions. it is important to know whether calderas can form repeatedly at the same vent or not. on may 20, perbuwatan, one of the parasites, began to erupt. on august 27, climactic eruptions began at the center of the islands and finally formed a submarine caldera. coincidently, a barograph and a tide gauge in batavia registered the eruptions. after that, in september, october, and february 1884, some activities were reported. comprehensive research reports of the eruptions were published by verbeek [1885] and symons [1888], and in addition, we may refer to “krakatau 1883: the volcanic eruption and its effects” compiled by simkin and fiske [1983]. in such extensive worldwide reports, it is not easy to find any facts explicitly related to “collapses”. the eruption resulted in formation of a submarine caldera having rather flat bottom about 200 m below sea level and after the event, in 1883 and 1884, some weak earthquake activities were reported. we have not received any report on volcanic activity at this submarine caldera. we suspect that the magma conduit system connecting to krakatau caldera has been blocked at some depth. in 1927, 44 years after the caldera formation, anak krakatau erupted as a parasite, intervening between former danan and perbuwatan, which were both parasites of the proto-krakatau that developed before the climactic eruptions. anak krakatau is a unique example of a parasite of the calderas observed during formation, and its activity is still continuing as of the 21st century. the new conduit to anak krakatau may have branched from the main conduit at a point deeper than the above-mentioned blockage. yokoyama [2014] interpreted formation of the parasitic conduit to have branched from the main conduit at a depth of about 4 km, based on a shear fracture model. the processes are schematically shown in figure 2. this interpretation does not assume “caldera collapses” into the magma reservoir situated at a depth of 10 km, but the magma conduit system used for the caldera-forming eruption was probably blockaded under the caldera deposits and the magma was supplied to the parasite through another path. according to bemmelen [1949], the magma of the 1883 eruption was hypersthene andesitic and that of anak krakatau was basaltic: the krakatau magma has differentiated in the reservoir after the caldera formation. this fact indicates that the magma reservoir has not been substantially affected by the 1883 caldera formation. only this eruption, out of the four cases in table 1 was not accompanied with any volcanic activity during the few years after the caldera was formed, but a new parasitic activity broke out 44 years after the caldera formation. as for volcanic ejecta, a pioneering survey of verbeek [1885] was carried out soon after the eruption but limited narrowly to an area around krakatau islands and to the flow deposits. following verbeek, various authors have estimated the total volume of magmatic ejecta at 9.0, 9.5, 12.2, 14.7 and 19.3 km3 dre [yokoyama 2015a]. such diverse estimations are due to the differences in interpretations and data acquisitions among the various authors. this suggests that volume estimates of the pyroclastic ejecta, falls and flows, are very difficult especially at island volcanoes influenced by ocean dispersion, and their accuracy may remain to be plus or minus of an order of magnitude. in figure 2, a probable volume of the magma reservoir is assumed to be 19 km3 [yokoyama 2015a]. yokoyama [2001] discussed the earthquake activity related to the 1883 eruption and concluded that the maximum magnitude of these earthquakes was in the m 5 class and that explosive disruption released much more energy than earthquakes. the caldera was reported to have formed by collapses in the most violent explosion at 10:02, august 27 (local time). but this is doubtful since the tsunami observed near krakatau islands all began with rises [yokoyama 1981]: namely the tsunami was generated by upward explosions, contrary to collapses. hence, the collapse model is not valid in this case. the second largest explosion occurred at 10:52 of the same day, and then we estimated that the climactic eruptions continued for about 50 minutes. probably this may not be sufficient time for volcano body to collapse into the depleted magma reservoir. yokoyama 4 5 (c) the 1912 novarupta eruption, alaska, usa hildreth [1983] discussed this eruption in detail and later, hildreth and fierstein [2012] summarized the results of various research of the 1912 eruption for centennial perspectives. the activity began with precursory earthquakes felt near novarupta on may 31. abe [1992] studied the katmai and novarupta earthquakes registered worldwide for this period: earthquakes of m 7 class occurred three times in june. according to hildreth [1983], the seismicity does not correlate with the eruptive events, and alternatively, much of the seismic activity seems to have been related to fitful subsidence of katmai and novarupta calderas. the explosions continued during june 6 ~ 9 and novarupta caldera is assumed to have formed around 13:00, june 6 (local time), simultaneously with tephra eruptions, but not with major earthquakes. in table 1, the eruption of novarupta is unique in having generated pyroclastic flows of a medium size in volume during climactic eruptions of a medium duration and resulted in formation of a caldera of a minimum diameter. these suggest the highest ejection rate of pyroclastic material during the caldera-forming eruption. hildreth [1983] analyzed a flaring funnel-shaped conduit of novarupta. once he questioned himself whether novarupta basin was a crater or a caldera. according to him, a blocky dacitic lava dome, 380 m in diameter and about 65 m in height was formed within the new caldera possibly several months after the last major pumice eruption. origin of calderas figure 2. proposed models of subsurface structure of the four calderas. tambora: probable volume of a magma reservoir is shown as 41 km3 (cf. kandlbauer and sparks [2014]) and its depth is not determined. krakatau (mainly by yokoyama [2015a]): synthesized from gravity anomalies and seismological data. dotted part consists of caldera deposits. vp/vs ratios are contoured and high s-wave attenuation is marked by stripes. volume of the magma reservoir is assumed as 19 km3 (cf. yokoyama [2015a]). novarupta (mainly by hidreth [1983]): m1 is a geological cross-section beneath the caldera, m2 is regional plexus of magmas in forms of sills, dikes and small chambers. pinatubo (mainly by mori et al. [1996]): magma bodies of 40 ~ 90 km3 are detected at a depth of 6 ~ 11 km. the black dots represent post-eruption earthquake foci from june 29 to august 16, 1991. the magma composition did not change drastically during the period. almost simultaneously with the novarupta eruption, a caldera collapsed at the summit of katmai, 10 km east of novarupta. hypothetical interpretations on the relationship between the two events were presented by the other authors in section 4.2. (d) the 1991 pinatubo eruption, luzon, philippines this eruption was monitored by a modern observational system. newhall and punongbayan [1996] compiled an interpreted research report of this eruption. before the climactic eruptions, a lava dome (andesitic) formed near the summit of pinatubo during june 7 to 12, 1991. this lava dome was destroyed during the climactic caldera-forming eruption of june 15, which lasted about 9 hours. this caldera measures 2.5 km across, almost the minimum requirement for the definition of calderas, and ejected pyroclastics of 4.5 km3 in volume is the smallest in table 1. thus, pinatubo may stand on the boundary between the definitions of craters and calderas. though we have no reports about gravity surveys on pinatubo caldera, this andesitic caldera is assumed to be lga-type because of its high explosivity and deep magmatic origin (cf. section 4.2). about one year after caldera formation, for about 4 months during july october 1992, a lava dome (hybrid andesite) extruded at the same position as the 1991 dome. pinatubo formed lava domes before and after the caldera formation and both the domes are compositionally similar. this means that the caldera formation had scarcely affected the system of magma reservoir and conduit beneath pinatubo mechanically. in short, the caldera formation was not accompanied with “collapse of the conduit”. generally, lava domes formed soon after caldera formations should not support “collapse model” considering that “collapses” should destroy the magma conduit systems and stop upward transportation of magmas. magmas, especially viscous ones, may be difficult to pass through fragmented conduits. according to scott et al. [1996], the climactic eruption began at 13:42, june 15 (local time), and earthquake series of the m 5 class started at 15:39. the strong earthquakes m 5.5 and m 5.7 occurred successively at 18:41 and 19:11. these earthquakes took place during the period of decreasing explosivity. if pre-caldera volcanoes collapse downward in a short time, we may observe earthquakes simultaneously. scott et al. [1996] suggest that this caldera collapsed at about 16:30 by climactic eruptions. however, about this time, major earthquakes were not reported. they model the processes in three stages as illustrated in three figures that are similar to the explosion model. it is probable that the caldera was formed by destructive explosions lasting about 9 hours, and not by collapses of volcanic material into the magma reservoirs. large earthquakes may result from adjustments of crustal stresses induced by magmatic activities in a wide area. 2.2. summary of the four calderas the similarity in volume between pyroclastic ejecta and disrupted volcanic edifices and the paucity of lithic fragments around the volcano are the most fundamental basis for the collapse model. it is also assumed that the latter volume compensates for the depletion of the magma reservoir caused by extrusions of magmatic material. williams [1941] agreed with this logic and stated that this was a powerful argument in favor of the collapse hypothesis. from the above data of the four calderas of lga-type (table 1), the similarity does not always hold true even if we assume adequate accuracy in the volume estimation. we must recognize that volume estimate of volcanic ejecta in the field is very difficult, and estimated errors ranging over an order of magnitude should be expected [yokoyama 2015a]. furthermore, “caldera deposits” have been left out of consideration in the volume estimations. even if the similarity holds true, this is not sufficient. we should prove the mechanism by which collapsed material of the volcano edifices reach magma reservoirs through magma conduit systems. the diameters of calderas are not strictly proportional to the volume of their ejecta as shown in table 1. this may be because explosivity or eruption magnitude depends on the explosion pressure, as well as the mass of the ejecta. we know many calderas larger than the four calderas in diameter, and we suggest a kind of partial overlapping of small calderas to form larger ones (cf. section 5.4). in table 1, formation of the four calderas was completed no later than the end of the eruptions within 3 days (tambora), 23 hr (krakatau), 14 hr (novarupta) and 9 hr (pinatubo). if “collapse model” assumes that collapses occur during violent explosions, it should be very difficult for crushed volcanic bodies to collapse downward against the upward force of the violent explosions. in fact, it cannot be absolutely certain that “collapses” occur during eruptions in all the cases. 3. formation of hga-type calderas the four calderas observed during their formation coincidentally belong to the lga-type. here we discuss formation of hga-type calderas independently of the above-mentioned calderas. unfortunately we have no historical data or direct observations on the formation of hga-type calderas because their development would proceed gradually or stepwise over long periods. yokoyama 6 7 therefore, we refer to ensuing events from the original caldera formation. at the calderas of this type, the deposits of high density are usually composed of accumulation of basaltic lavas. and their eruptive activities are usually derived from shallow magma reservoirs located within 5 km of depth. at calderas of this type, roofs of the magma reservoirs would collapse gravitationally to fill gaps caused by ejections or withdrawals of magma through lava flows. the following are two examples of similar collapses that are not caldera formation but occur as collapse stages of structure that we finally consider hga-type calderas. the last (miyake volcano) is from the genetic stage of caldera formation of the same type. dimensions of calderas of this type are not related to their explosivity, and not so large because their origins are relatively shallow. (a) the 1924 eruption of halemaumau crater in kilauea caldera, hawaii a major collapse of kilauea caldera and related volcanic and tectonic events occurred in 1790. a smaller collapse accompanied by low explosivity occurred at kilauea caldera in 1924 and the caldera floor was lowered of about 270 m. the subsidence occurred for 3 months while the multiple steam explosions occurred for 18 days within a month. decker [1987] pointed out that the volume rate of magma removal was limited and the process of caldera collapse went on over periods of a few months and interpreted the caldera-collapses as a result of steam explosions caused by contacts between molten lava filling the crater and the aquifer. the explosions blew off the caldera material including molten lava and the fallback rubble deposited in the crater at the deeper level than previous one. as a result, the explosions caused subsidence of the caldera floor. a shallow magma reservoir was an indispensable condition for the caldera collapse at kilauea caldera. (b) the 1968 eruption of fernandina caldera, galapagos islands, ecuador the caldera measures about 6 × 4 km2. in early june 1968, a small eruption occurred at the flank and was followed by a small explosion with ash-falls at the summit caldera rim. accompanying earthquake swarms continued for about 10 days. on june 21, the existing caldera bottom, with a surface area of about 8 km2, subsided about 300 m [filson et al. 1973]. there was no evidence of new lavas. small eruptions and earthquake swarms may have triggered the gravitational subsidence. hawaiian and galapagos calderas are named “basaltic lava-flow calderas” by lipman [1997]. these calderas are classified to hga-type by their subsurface structure. (c) the 2000 eruption of miyake volcano, japan this volcano island is composed of basaltic rocks. on june 27, a small submarine eruption occurred at a distance of 1.2 km west of miyake island. by the shear fracture model [yokoyama 2015b], this parasitic vent is assumed to have branched from the main vent at a depth of about 6 km beneath the center of the volcano, determined by taking the radial distance of the parasite vent from the center of the main vent at 5 km. the above branch point is not necessarily at the magma reservoir. on july 8, magma pressure beneath the volcano may have triggered volcanic earthquakes of m 2 ~ 3, resulting in the summit eruptions. simultaneously, the cylindrical summit part measuring about 900 m in diameter and 190 m in depth collapsed forming a summit depression. geshi and oikawa [2008] estimated volume of the tephra erupted as 1 × 107 m3 dre, corresponding to only 1.7 % of the total volume of the depression: this means that the summit collapse of miyake volcano in 2000 had minimal relation to the magma ejections, and may have been caused by internal removal of magmas from the summit part. before the eruption, there was a shallow pit of about 200 m in diameter at the summit, and the residual bouguer gravity anomaly around there showed a tendency to be high [yokoyama 2002]. from a long-ranged viewpoint, miyake volcano may repeat such gravitational subsidence and grow to form a caldera of hga-type. similar convulsions may have been repeated on the adjacent ooshima volcano island, 60 km away, that has a typical hga-type caldera at its summit, and its magma reservoirs are situated around two depths of 5 and 10 km. among the three eruptions discussed in this chapter, shallow summit reservoirs and basaltic magmas of high density and low viscosity are common. notably, collapses that occurred in response to removals of basaltic lavas or magma beneath summit craters or portions of the volcano, originate hga-type calderas. 4. structure of lga-type calderas calderas of this type are generally large in diameter, and accordingly, their structure will be on larger scale. in order to interpret formation mechanisms of lga-type calderas, the author will discuss both surface and subsurface structure of the calderas. and to confirm the interpretation, drilling results at some other calderas shall be referred to. in general, the geomorphologic characteristics of calderas common to both hgaand lga-types, are precipitous caldera walls and flat caldera floors. for caldera walls, a pertinent discussion is made in the introduction exemplifying santorini caldera. and the latter characteristic, caldera deposits with flat floors are much more abundant in lga-type origin of calderas calderas because of their larger dimension. hence, they shall be mentioned in the following section. 4.1. caldera deposits generally speaking, the residual gravity anomalies observed on and around calderas provide us useful knowledge on the caldera deposits and their boundaries. the gravity anomalies observed on lga-type calderas suggest funnel-shape caldera deposits of low density, and such structures are verified by drilling results at some calderas. at krakatau caldera, yokoyama [2014] determined the funnel-shape of caldera deposits by gravimetries, whereas the bathymetric charts show that the maximum gradient of the caldera wall at se of verlaten island is about 40 degrees in arc, not very steep and suggests a funnel-shaped depression. the northward precipice of rakata island is not located on the caldera rim and was formed by explosive breakdowns from the 1883 eruption. as for novarupta, from topographical observations, hildreth [1983] inferred a flaring funnel-shaped vent (cf. figure 2) probably produced by explosions. almost all calderas have flat floors overlying caldera deposits, large or small in area. the deposits are usually composed of “fallbacks” of juvenile ejecta and lithic breccias, both being of low density. flat caldera floors are usually one of the characteristics of calderas, distinguishing them from volcanic craters. at both novarupta and pinatubo calderas, post-eruptive domes developed at their bottoms as mentioned before. we believe such dome formation is a possible consequence of the calderic event. 4.2. subsurface structure of the four calderas with emphasis on magma reservoirs magma reservoirs are storage locations where magmas differentiate and are extruded out causing volcanic eruptions. their depths and configurations are closely related to caldera formations. geophysical techniques of locating magma reservoirs are expanding with time. the results of some recent researches are shown in figure 2 which indicates that the lga-type calderas are probably not underlain by shallow magma reservoirs. (a) tambora: no geophysical discussions on subsurface structure have been reported. after the 1815 eruption, the volcano has not discharged significant amount of magma, except from a small activity in 1819. if we assume that the magma stored in the reservoir was totally ejected by the 1815 eruption, volume of the reservoir is larger than that of krakatau in proportion to volume of the ejecta. we have no data related to the depth of the reservoir. only, volume of a magma reservoir assumed from the ejecta volume is shown in the figure. (b) krakatau: since 1960s, geophysical research has been carried out on and around krakatau islands. yokoyama [1981] proposed a model of subsurface structure of this caldera mainly from gravity anomalies. he assumed presence of caldera infilling deposits and tentatively estimated the depth of the reservoir at about 5 km. later, the bathymetries and gravimetries on land and sea clarified the subsurface structure beneath the caldera. ranging in the time-span from 1984 to 2006, temporary seismological observations were made to estimate seismic s-wave attenuation [harjono et al. 1989] and vp/vs variations [jaxybulatov et al. 2011], and both properties indicated the presence of fluid believed to be magma probably coinciding with the 1883 reservoir. yokoyama [2015a] synthesized these results as shown in figure 2, where the caldera deposits are saturated in sea water producing very high vp/vs ratios. this supports existence and origin of caldera deposits at krakatau. also, both authors similarly located magma reservoir beneath krakatau caldera at a depth of about 9 km. in 1927, anak krakatau erupted at the caldera rim as a parasite of krakatau volcano. the magma conduit for the parasite branched from the main vent at a depth of about 4 km [yokoyama 2014] and the resulting parasitic volcano has been intermittently active in the 21st century. (c) novarupta: hildreth [1983] studied ash-fall and ash-flow deposits from novarupta caldera and derived his geological interpretation of the shallow system or magma stop (m1: rhyolitic magma) shown in figure 2. his caldera model assumes caldera deposits of 2 km3 in volume, and also assumes the magmatic plumbing system (m2: andesitic and dacitic magma) beneath the katmai area. ward et al. [1991] suggest the system more likely to be a plexus of dikes, sills, and small chambers, rather than one large uniform magma chamber, from their seismometric results. these may be located at a depth ≦ 10 km beneath a regional area based on the observations made by matumoto [1971, fig. 6]. as for the relationship between the two calderas, katmai and novarupta, eichelberger and izbekov [2000] made an assumption that magma was transported to the shallow chamber beneath novarupta through planar dikes rising from depths. and hildreth and fierstein [2000] proposed that all magmas related to the katmai-novarupta eruption were stored under katmai. later, yokoyama [2015b] proposed a mechanical interpretation applying the shear fracture model: the depth (d) of the pressure source is obtained from 1.2 times r, where r denotes radial distance of the surface vent from katmai (10 km). this means that the conduit for novarupta branched from a depth of about 12 km beneath katmai volcano, and may have been accomyokoyama 8 9 panied with major earthquakes. evaluation of the structures through petrological data may help distinguish the above hypotheses. (d) pinatubo: mori et al. [1996] observed volcanic earthquakes following the climactic eruptions of june 15, 1991, and obtained the distribution of seismic pwave dissipation by means of velocity inversion. a cross section of the velocity perturbations from the average velocity is shown in figure 2 where a low velocity zone between 6 ~ 11 km in depth is inferred to be a magma reservoir. this is one of the most accurate localization of magma reservoirs beneath active volcanoes in the last century, and this suggests possible variety in the configuration of the chambers. in short, the above examples show that reservoirs of silicic magmas were located at depths between 5 and 10 km beneath the above-mentioned volcanoes. in the following discussion, this will be the presupposed characteristic of lga-type calderas. 4.3. results of drillings at some calderas of lga-type to study subsurface structure of calderas, various geophysical investigations are fundamentally necessary, and to confirm their results, drillings are very useful as the drill cores ultimately give a definitive geologic cross section in depths. no drillings have been carried out at the four calderas in question. at calderas of lga-type, deposits of low-density are mainly composed of silicic pyroclastic ejecta, and partly, portions of volcano edifices. results of drillings at some calderas may afford us specific information about the physical state and chemical composition of the deposit material and may help to analogize with the other calderas of the same type. (a) aso caldera, kyushu, japan: yokoyama [2014] previously reported the results of drillings near the center of the caldera (symbol ⊕ in figure 3a) that penetrated the 500 m thick layer of welded pyroclastic flow deposits [hoshizumi et al. 1997]. these results indicate that the profile of aso caldera is funnel-shaped and its caldera deposits are mainly composed of fallbacks, at least, at the drilling sites. (b) la primavera caldera, mexico: this caldera measures 13 km in diameter. since 1980, the comisión federal de electricidad of mexico has drilled 13 geothermal wells reaching depths of 668 ~ 2986 m around the center of the caldera. the drill cores provide definitive depth-section of the underlying deposits. the caldera deposits are mainly pyroclastics and partly mixed with lithified tuff layers. beneath the caldera deposits, there are basements of compact granite at a depth of about 3 km. yokoyama and mena [1991] determined the structure of the caldera combining the drilling results with gravity anomalies which are composed of two relative lows amounting to 14 and 9 mgal: the caldera basement is composed of partially overlapped depressions, about 1.5 and 1 km deep (cf. 5.4. composite calderas). (c) guayabo caldera, costa rica: this caldera lies between young stratocones and measures 11 km in diameter and forms a caldera lake. halliman and brown [1995] carried out micro-gravity surveys and examined stratigraphy to a depth of 2200 m at 11 boreholes within this caldera. the caldera is lga-type with a vshaped gravity anomaly. according to them, the gravity anomaly is related to the caldera fills of low density and not to brecciated caldera basement, and the caldera was originally formed in funnel-shape, and not pistontype subsidence, that later collapsed incrementally, and stratocones grew within the caldera. in conclusion, the results of sufficiently deep drillings carried out at several calderas of lga-type verify that caldera deposits are underlain with basement rocks and the basements appear funnel-shaped. caldera deposits are mostly composed of low density infilling deposits, made of pyroclastics and lithic breccias. furthermore, it has been established by these studies that summit magma reservoirs do not exist directly beneath the calderas of lga-type. 5. formation of lga-type calderas in the following, formation mechanism of lgatype calderas shall be interpreted on the basis of subsurface structure as discussed in the previous section. first, modeling of caldera formation characteristic of studying lga-type calderas is mentioned, and two hypotheses of caldera formation of the same type are separately discussed. last the discussion is extended to composite calderas. 5.1. modeling of caldera formation in nature, caldera-forming eruptions of lga-type are all extraordinarily explosive accompanying tremendous amounts of juvenile ejecta. at present, modeling of volcanic explosions is impracticable, and so the modeling of “collapses into magma reservoirs” cannot totally simulate the natural processes of formation of lgatype calderas. generally speaking, for physical modeling, the law of similarity between natural objectives and experimental media (usually fluid) should be provided. otherwise, the results remain only as qualitative deduction. modeling of caldera formation, whether experimental or numerical, should set the initial conditions. however, at present, the conditions are too much simplified in many cases. fundamental conditions for modeling of caldera formation are concerned in crustal structure beneath origin of calderas volcanoes (geological properties and mechanical strength), and furthermore magma reservoirs (depths and configurations): the latter has not been established except for very few calderas and so we should make allowances in interpreting the results of the modeling. experimental modeling: for an example, roche et al. [2000] carried out experimental study of caldera formation with considerations of scaling. they intended to clarify the depression of the earth surface caused by magmatic and hydrostatic pressure and adopted models of shallow reservoirs with flat tops. numerical modeling: for an example, gregg et al. [2012] studied caldera-forming eruptions by viscoelastic numerical models following the general elastic model formulation. they developed a model of magma reservoirs beneath caldera volcanoes as an ellipsoid at relatively shallow depths. their results show two cases of triggering eruptions of magma reservoirs according to the dimensions of the reservoirs: (1) dike fractures at the magma reservoir boundary, and (2) fault fracture within the crust. alternatively, the point of their argument covers eruption mechanism related to the explosion model of caldera formation. the initial conditions of both of the above modeling are not always similar to the actual volcanism. modeling study of caldera formation should be first applied to the four calderas observed during their formation (chapter 2), because such modeling may need fewer assumptions than the prehistoric ones. 5.2. hypothesis: “collapses into a magma reservoir form a caldera” in order to coordinate the long disputed problem on the formation of lga-type calderas, we consider the two main hypotheses in the following. the first hypothesis is concerned with “collapses”. this mechanism is consistent with that of hga-type caldera as discussed in chapter 3. here its application to lgatype calderas shall be discussed. the concept of “collapse model” has been determined with the assumption that the total volume (dre) of pyroclastic ejecta from a caldera-forming eruption is equal to the volume of disrupted volcano edifices, and that lithic breccias of volcano edifices are not found around the caldera. this has been long disputed by many researchers. yokoyama [2015a] reexamined this concept for the 1883 caldera formation of krakatau and did not agree because the volume estimate of volcanic ejecta requires an allowance of about an order of magnitude, and we cannot prove parity between the two. lipman [1997] discussed subsidence of ash-flow calderas presenting a caldera model that is characterized by rather thick and funnel-shaped layer of tuff fills. he interpreted many examples and corresponding models, but depths and configuration of the magma reservoirs remain arbitrary. the lga is derived from the caldera fills or fallbacks composed of juvenile ejecta and lithic breccias, all being low density (cf. section 4.1). as for magma reservoirs, the present author postulates that they are composed of sills and dykes, similar to that proposed by fisk and kinoshita [1969] in their description of the 1967~1968 activity of kilauea volcano, hawaii. a magma reservoir of similar constitution is assumed beneath novarupta caldera (cf. section 4.2). at such reservoirs, magma ejection is possible while magma withdrawal may be rather difficult. when a deep magma reservoir is emptied by eruptions partly or totally, the reservoir should be crushed immediately by lithostatic pressure and any mechanical effects should propagate outward from the reservoir as shown in the next. stress propagation from crushed magma reservoirs: possibility of collapses of volcano bodies into magma reservoirs depends on the depth of the reservoirs. when the reservoirs are located near the surface as in the case of calderas of hga-type (chapter 3), volcanic bodies destructed by explosions may collapse into the reservoirs. however, probable depths of magma reservoirs of silicic magmas are estimated at 5 ~ 10 km as shown in section 4.2 and so it is doubtful that destructed volcanic edifices can reach the magma reservoir in opposition to the outward explosive force during caldera-forming eruptions. apart from the collapse process itself, stress changes around a magma reservoir caused by magma discharge should be an obstructive factor to the collapse processes. a similar discussion by yokoyama [2015a] shall be briefly summarized in the following. for the sake of simplicity, we assume crushing force f to act concentrically at the center of a cavity that is the remnant of a reservoir, and to reach maximum instantly. timoshenko and goodier [1970] discussed “force at a point in an infinite solid”, and their results are qualitatively applicable to the present problem. beginning with the laplace’s equation, they derived the vertical stress in polar co-ordinates (the origin is the center of the cavity) as along the vertical axis, poisson’s ratio o is canceled and the stress distribution is given as (2) qualitatively, the stress induced by the crushing of a magma reservoir should decrease rapidly in inverse z r z z r z f 1 2 3 8 1 z / / 2 2 3 2 3 2 2 5 2 v o r o = + + + + q q q q v v v v e h f 4z, r 0v r== z 2 r yokoyama 10 (1) 11 proportion to z2. f in the above formulae is a concentrated force and acts simultaneously with discharge of magma. magma reservoirs of lga-type are most probably located at a depth of around 5 ~ 10 km, as mentioned in section 4.2. under such conditions, the reservoir should be crushed by lithostatic pressure as the magma is discharged by eruptions, provided that magma conduits have no influence on the above processes. then, the stress due to the crushing shall dissipate upward and instantly with almost no effect on the surface. in short, destructive stress caused by magma discharge from a reservoir at a sufficient depth does not normally reach the earth surface. now the collapse model should explain the mechanism of transfers of shattered materials from the earth surface to a deep magma reservoir. as a result, the hypothesis “collapses into a magma reservoir form a caldera” is not agreement for formation of lga-type calderas unless their magma reservoirs are located at shallow depths and the mechanism for crushed volcano bodies to reach the reservoir is logically interpreted. 5.3. hypothesis: “destructive explosions form a caldera” originally destructivity of volcanic eruptions is evaluated by dynamic energy of eruption that is expressed by mass of ejecta combined with ejection pressure. such dynamic energy accumulated in the earth crust is limited due to the elastic limit of the crust. explosion energy and formation of calderas: explosivity, or eruption magnitude, is defined by dynamic pressure of the explosions which is proportional to (mass of ejecta) × (velocity of ejecta)2. eruption scale was first discussed by tsuya [1955] who classified volcanic ejecta according to volume and type: the type distinguishes the ejecta into three categories: lava-flows, fragmentary ejecta only, and ejecta dominated by old detritus. the kind of ejecta is related to (velocity of ejecta) and thermal energy, but their quantitative estimations are actually difficult. later, newhall and self [1982] proposed “volcanic explosivity index” (vei) that actually classifies explosivity by the order of magnitude of tephra volume and by column height from the explosions that is a factor of dynamic pressure, though its quantitative estimation is possible only for present-day eruptions. caldera-forming eruptions should be positively characterized by extraordinarily high dynamic pressure, not only large volume of the ejecta. tsunamis and air waves caused by volcanic eruptions are typical phenomena of tremendously large dynamic pressure. caldera-forming eruptions should be evaluated by their power not only by ejecta volume. eruption magnitude has the upper limit that is usually assigned to the 1815 eruption of tambora. on the other hand, the highest limit of the earthquake magnitude related to volcanic eruptions is m 7, as instrumentally observed in the 1912 eruption of novarupta and the 1914 eruption of sakurajima. volcanological meaning of earthquakes of the m 7class has been not fully interpreted, but they may be due to readjustment of crustal stresses caused by large volcanic eruptions. empirically, they are not always indicative of caldera formation (cf. table 1). steinberg [1974] discussed the theory of strong throw-out blasts and the results in the 1956 eruption of bezymianny volcano, kamchatka, observed by gorshkov [1959], and concluded that the largest volcanic explosion with the total energy of 5×1018 joules, corresponding to an earthquake of m 9, can produce a crater of 10 km in diameter maximum, on the assumption that the center of the explosion is situated at the optimum depth. the above energy is derived from the total energy of the explosion and ejected material. in contrast, meteoric craters may have much larger diameters because of their higher impact energy. in table 1, the maximum diameter of the four calderas is 6 km at tambora. this may suggest validity of steinberg’s estimation. many calderas measuring more than 10 km across may have been produced by merging of adjacent and smaller component calderas. this shall be mentioned later, in section 5.4. gigantic eruptions observed during crater formation: main eruption data of the four calderas, tambora, krakatau, novarupta and pinatubo, are listed in table 1. they were all large eruptions forming calderas of diameters ranging from 2 to 6 km. beside these calderas, we know two examples of gigantic eruptions observed during crater formation in the 20th century: the bezymianny and mt. st helens eruptions. the 1956 event at bezymianny volcano ejected ashes and an agglomerate flow of 1.8 km3 bulk with the maximum column height of about 45 km, forming a crater measuring 1.5 × 2 km2 in dimension. gorshkov [1959] estimated its explosion pressure at 3000 atm. that is equal to lithostatic pressure of about 11 km in depth. and, the 1980 event at mt. st helens ejected 0.2 km3 dre of tephra, with the maximum column height was about 14 ~ 24 km, forming a crater measuring 2 km across (cf. lipman and mullineaux [1981]). these two eruptions were extraordinarily explosive with high velocity of ejecta and exploded the upper parts, but their volume was rather small, and consequently resulted in formation of relatively small craters. on the other hand, according to the classical definition, they may be just called calderas. if these two eruptions were accompanied with large amounts of juvenile ejecta, they may have formed proper calderas. in this respect, ejecta volume is one of origin of calderas the important factors for caldera formation. gigantic explosions accompanied by a large amount of pyroclastic ejecta mainly contribute to formation of calderas, larger than 2 km and maximum of 10 km in diameter. then, it may be assumable that large calderas, such as aso, aira, lake toba and others, are formed through the amalgamation of 2 ~ 3 smaller calderas as shall be discussed in the next section. as a result, the hypothesis “destructive explosions form a caldera” is in agreement with formation of lgatype calderas. 5.4. composite calderas worldwide, there are many calderas with large diameter (about 20 km). if we adopt the explosion model, the dimensions of the resulting depressions have an upper limit of about 10 km in diameter, as mentioned in the previous section. calderas usually expand outward by foundering and erosion of caldera walls over long periods of time. sometimes, a new caldera would be produced adjacent to an existing caldera and the two calderas eventually may be combined into a larger caldera. this is a “composite caldera”. therefore, our hypothesis is that calderas of large diameter (around 20 km) result from the merging of various smaller calderas. in this paper, we have called these various portions of the large caldera: “component calderas”. we may possibly distinguish “component calderas” by using gravity anomalies or topographic features. for an example, santorini caldera shown in figure 1 may be composed of the north and south component calderas judged by the topographies, and the south vent is active at present. in the following, three large calderas aso and aira in kyushu, japan and toba in sumatra shall be mentioned as typical examples. the first two calderas are located at about 150 km apart and have no direct influence with each other in volcanic activity. (a) aso caldera, kyushu: measures 15 × 20 km2 in dimension. komazawa [1995] carried out precise gravity surveys on and around this caldera, and analyzed the observed bouguer anomalies quantitatively, applying frequency analyses and filtering and achieved the residual anomalies of a shallow structure, comparable to a structure shallower than 5 km. such gravity anomalies disclose the subsurface structure covered by topographic features. thus, the anomalies in aso caldera are composed of five lows of residual gravity anomalies ranging 5 ~ 10 mgal, and the small and large dimension are marked by ⓛ in figure 3a. the three blocks of small gravity anomalies in the northern part are derived from three nested depressions, and collectively are represented by one component caldera (✻n). thus, we assume aso caldera is composed of three component calderas, (✻n), (✻m) and (✻s). at present, we cannot exactly determine the order of formation of the three component calderas. pyroclastic flows and falls from aso caldera have occurred four times since 300 ka bp, “aso-1” (ca. 270 ka bp) to “aso-4” (ca. 80 ka bp), and the last one was the largest, amounting to about 400 km3 bulk in volume of the fall deposits. symbol ⊕ at the southern rim of reyokoyama 12 figure 3. composite calderas in kyushu, japan, aso and aira: hatchured contours indicate the present caldera rims. ⊕ denotes drilling sites. thick solid lines outline the caldera formed by amalgamation of component calderas. (a) aso: ⓛ denotes a residual low gravity anomaly and triangles denote active cones. (✻n), (✻m) and (✻s) are component calderas and amalgamated to form aso caldera. (b) aira: thin outlines in the sea area of both (✻w) and (✻e) show flat bottoms, respectively. triangles in (✻e) are vents: “t” is active and another two are extinct. n and s on sakurajima volcano denote extinct north peak and active south peak, respectively. a b 13 gion (✻m) denotes a group of boreholes which detected the aso-4 deposits at roughly between 1000 ~1500 m depth from the caldera surface, as mentioned in section 4.3. this suggests that the caldera deposits in component caldera (✻m) are composed of considerable amount of the “fallbacks” from pyroclastics “aso-4”. such conditions may be found at other lga-type calderas. at aso caldera, if these fallbacks were found in the whole area of (✻m), their volume should amount to roughly 25 km3 in bulk. this may be not a negligible quantity and is suggestive of formation mechanism of aso caldera. if we can locate the original vents of the four units of pyroclastic flows and falls from aso caldera, we may better understand the sequences of formation of this caldera. the present activities are concentrated at the middle region (✻m). according to sudo et al. [2006], these activities originate from a magma reservoir situated at a depth of 5 ~ 6 km. it is not clear whether the present condition of aso caldera is in the process of developing activities of the component caldera (✻m), or it is in its post-caldera activities. (b) aira caldera, kyushu: measures 20 × 20 km2 in dimension. this caldera is located at the northernmost part of the kagoshima bay. gravity anomaly there reaches about 40 mgal, and relatively low at the center. the gravity was measured at the sea surface, and the resolution was not adequate to distinguish any component calderas. the two component ones, (✻w) and (✻e) are derived from topographic features in figure 3b. yokoyama and ohkawa [1986] analyzed the gravity anomalies observed in the caldera area and evaluated the subsurface structure of the caldera as follows: the caldera deposits of low density extend to a depth of 2.5 km and are underlain with basement rocks that possibly form a funnel-shape. in the component caldera (✻w), there is flat bottom of about 140 m (b.s.l.). in (✻e) region, there is also flat bottom of 200 m (b.s.l.). such flat floors are characteristic of calderas as mentioned in section 4.1. and at the eastern part, an active vent “t” and two extinct vents exist. it is assumable that region (✻e) is younger than (✻w) because “t” is the last remnant of the activities in this area. about 29 ka bp, during the period of the caldera formation, the eruptive activity generated a volcanic succession of 4 different units composed of pumice fall layers, pyroclastic flow deposits and co-ignimbrite ashfalls. the total volume of these ejecta amounts to 100 km3 dre. their original vents are not accurately located because the majority of the caldera is currently under the sea. the drilling site (⊕ symbol in figure 3b) is near the west shore of the island of sakurajima volcano, a post-caldera cone just outside the assumed caldera rim. the drilling reaches a depth of 810 m (b.s.l.). almost all drilled cores are composed of tuff, welded tuff and pyroclastics and it is not clear whether this site is inside the caldera or not. sakurajima volcano began its activity 22 ka bp as a parasite, probably just outside of aira caldera. north peak of the volcano is extinct, while south peak has been active and repeatedly erupted since 1955. (c) toba caldera, sumatra: one of the largest calderas on earth measuring roughly 20 × 80 km2. its long axis is parallel to the sumatra fault (s.f. in figure 4). the lake level is 906 m (a.s.l.) and the summit of samosir island is about 1916 m (a.s.l.) at the northern part. according to bemmelen [1949], in late pleistocene, siliceous magma rose beneath the present site of lake toba and paroxysmal explosions discharged roughly 2 × 103 km3 of pumiceous tuffs. later the new magma pushed up the floor producing resurgent domes and resulted in samosir island. nishimura et al. [1984] carried out gravity surveys at 332 points in this area for the period 1976~1980. yokoyama et al. [1984] supplemented the surveys at 68 points mainly at the central part in 1982. all the results of bouguer anomalies are simplified and shown in figure 4. in the figure, the residual gravity anomalies reach −30 ~ −85 mgal on and around lake toba. these anomalies are corrected for the topographies and display the structure below the sea level. a gravity-low at the se end of the lake suggests a component caldera of about 15 km across, and after the analogy of aso and aira calderas, another large and wide anomaly may be due to 2 component calderas probably bounded by the ccand ss-lines. gravity points are not enough in number, especially in the central part of the samosir island. in order to advance detailed interpretation, more dense gravity surveys are desirable. origin of calderas figure 4. composite caldera in lake toba, sumatra: distribution of bouguer gravity anomalies. ccand ss-lines denote probable boundaries of hypothetical component calderas. 5.5. calderas under enlargement if some calderas are recognized as composite calderas, there should be component calderas undergoing enlargement. here, we suggest ksudach caldera, kamchatka as a unique example of such enlargements. according to braitseva et al. [1996], the caldera boundaries at each period are shown in figure 5a, and ① and ② formed earliest during pleistocene time, and subsequently, ③ in 8.8 ~ 8.7 ka bp, ④ in 6 ~ 6.1 ka bp and ⑤ in 4 ~ 4.3 ka bp. after formation of ⑤, shtyubel’ cone has been active, leading to the 1907 eruption. it may be probable that the center of component caldera had migrated, as schematically shown in figure 5b where black dots denote the centers of component calderas. the southeastern part bounded by a broken line approximately shows the first caldera area ① and the solid line outlines the present caldera boundary. during these periods, the caldera area has been roughly doubled. volynets et al. [1999] applied geological, tephrochronological and geochemical methods to reconstruct the eruptive history of this caldera during the holocene period and presented sequence of five events, as shown in figure 5a. macias and sheridan [1995] studied the products of the 1907 eruption of shtyubel’ volcano and estimated the ejecta volume at 2.4 km3 bulk, roughly a half of that for the 1991 eruption of pinatubo (table 1). at ksudach caldera, eventually, several component calderas shall be successively merged into a larger caldera. 6. concluding remarks in the introduction of the present paper, transitions in the classification of calderas are traced, and “collapse model” of caldera formation proposed by fouqué [1879] who interpreted his observation on santorini caldera is criticized and its validity is questioned on the basis of mainly subsurface structure of the caldera. in the present paper, calderas are first classified into two types, lga-type and hga-type, according to subsurface structure. though the former is more abundant in number than the latter, the calderas of these two types have their own characteristics in explosivity, caldera deposits, and magma reservoirs. since the 19th century, four large volcanic eruptions resulting in formation of calderas were observed with the scientific understanding that prevailed at that time and are considered to provide reliable data. thus, these four calderas are critically compared and analyzed from geophysical viewpoints. their eruptions were all tremendously violent and their activities were realistically reported; however, it is difficult to settle the time of “collapses” during or after the eruptions. calderas of hga-type usually originate from summit reservoirs with basaltic magmas at shallow depths (≲5 km), while those of lga-type are usually underlain by deeper reservoirs of 5 ~ 10 km depth with silicic magmas. such differences cause activities to differ in the caldera formation between the two types. calderas of hga-type have not demonstrated their formation stages in our history. only their ensuing activities were observed on some calderas of this type: their magmas are usually basaltic and activities are not very violent. hence, calderas can form mainly by gravitational collapse or subsidence, compensating magma depleted from the reservoirs during eruptions. the role yokoyama 14 figure 5. ksudach caldera, kamchatka. (a) caldera areas of the five component calderas after braitseva et al. [1996]. (b) migration of the centers of component calderas. the arrow shows a synthesized migratory direction. 15 of “explosions” is rather small in such processes. this is a caldera of hga-type. on the other hand, historical cases of lga-type calderas studied in this work indicate that silicic magma reservoirs are usually located at 5 ~ 10 km depth and highly pressurized magmas play an important role in the caldera formation, as well as, the volume of the ejecta. physical and chemical properties of magmas depend on their depths and consequently may have much importance to the discussions of caldera formations. “magmatic explosions” accompanied by a large amount of pyroclastic ejecta mainly contribute to the formation of calderas, larger than 2 km and maximum of 10 km in diameter. “collapses” contribute to deposition of caldera deposits or fallbacks of disrupted volcano edifices, not to compensation for gaps produced in the magma reservoirs. this is a caldera of lga-type. large calderas of 10 ~ 20 km in diameter would be formed by successive merging of adjacent component calderas of lga-type after a lengthy period of time. these are composite calderas. in conclusion, some calderas are formed mainly by gravitational collapses, and others are mainly by explosions. both “collapses” and “explosions” are essential factors that contribute to caldera formation. variable circumstances emphasize one process or the other. acknowledgements. the present author owes his knowledge on various aspects of many calderas to innumerable pioneering researchers and wishes to express his cordial thanks to them. the author is obliged to mention only the references directly related to the present discussion. the author is grateful to prof. v.v. ponomareva who gave him introductory knowledge of ksudach caldera. the author expresses his heartfelt thanks to dr. r.y. koyanagi who kindly contributed to improve the quality of the paper. thanks also due to the two anonymous referees and dr. g. de astis for their critical comments on the early manuscript, which led to many improvements. references abe, k. 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(2015b). eruption pattern of parasitic volcanoes, annals of geophysics, 58 (3), s0327; doi:10.4401/ag-6557. corresponding author: izumi yokoyama, the japan academy, ueno park, tokyo, japan; email: iyokoyama@joy.ocn.ne.jp. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. origin of calderas << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 61, 1, se105, 2018; doi: 10.4401/ag-7374 comment on “assessing cn earthquake predictions in italy” by m. taroni, w. marzocchi, p. roselli george molchan1, antonella peresan2,3, giuliano francesco panza3,4,5, leontina romashkova1, vladimir kossobokov1,3 1 institute of earthquake prediction theory and mathematical geophysics, russian academy of sciences, moscow, russian federation 2 national institute of oceanography and experimental geophysics. crs-ogs, udine, italy 3 international seismic safety organization, arsita, italy 4 institute of geophysics, china earthquake administration, beijing, people’s republic of china 5 accademia nazionale dei lincei, roma. italy article history received february 5, 2017; accepted january 16, 2018. subject classification: statistical methods; earthquake interaction, forecasting and prediction; statistical seismology. se106 adequate assessment of prediction results is a fundamental step in earthquake prediction research and requires a correct application of appropriate statistical tools, respectful of their basic assumptions and data quantity/quality. for this reason we wish to draw the attention of the readers of annals of geophysics to some basic pitfalls of the paper by taroni et al. [2016], which may become a source of misleading interpretations and follow-up erroneous conclusions. in a nutshell, taroni et al. [2016] failed to assess correctly the results of the on-going cn prediction experiment in italy started in 1998 [peresan et al. 2005]. the main reasons for this are the following: (1) the cn forecast is implemented in three overlapping regions of italy, where the statistics of target events is small, and the analysis of the prediction statistical significance is forcedly applied to each region separately, where the statistic of target events is insufficient; (2) there are methodological errors in taroni et al. [2016], which are analyzed in detail in molchan et al. [2017]. taroni et al. [2016] declared intent to give “a careful assessment of cn prediction performances … using standard testing procedures.” this is an unlikely feasible goal when splitting the entire, yet small, sample of target earthquakes into smaller parts related to sub-regions of italy used in the cn algorithm application: the number of target events within each of the three sub-regions is 5, 3, and 1. a priori the standard statistical methods may not be effective in any of the cn sub-regions. molchan et al. [2017) show that such a small number of binomial trials implies, with necessity, low resolution of testing, and may lead to erroneous interpretation of the entire statistics in total. taroni et al. [2016] made their choice of splitting the total into parts to conclude that the model cn and the poisson model have comparable predictive performances. note another, although typical text-book, methodological error: “if a statistic falls in a reasonable part of the distribution, you must not make the mistake of concluding that the null hypothesis is “verified” or “proved”. that is the curse of statistics, that it can never prove things, only disprove them!” [press et al. 1992]. still, a single flip of a coin does not allow reliably assessing whether the coin is fair or not. as an example, let us illustrate the uncertainty of binomial testing with the best case statistics of 4 successful predictions out of 5 target events in the cn north sub-region [molchan et al. 2017]. the poisson model with the same rate of alarm, i.e., 35.7%, pr vides such or a better hit score in 5.8% cases. however, due to stability of the rate of alarm, an additional single one prediction success will change the hit score to review articles 5 out of 6, which corresponds to 2.4% cases of the same or better score by random guessing and rejection of the poisson model at significance level of 5%. on the other hand, an additional single one failure-to-predict will change the hit score to 4 out of 6 which corresponds to significance level of 12.5% that does not allow excluding the random guessing alternative to the cn predictions in north sub-region. the considerations made by molchan et al. [2017] do not contradict statements by peresan et al. [2011 and 2012] about the significance of cn results, since their conclusions are based on a significant number of target earthquakes, as provided by aggregate analysis of the three regions [following rules well specified in peresan et al. 2005]. the analysis and considerations by taroni et al. [2017] persistently overlook the basic definitions and rules of cn application, including declustering procedure and definition of target events [peresan et al. 2005], therefore their conclusions are fundamentally flawed. to strengthen the negative conclusion, taroni et al. [2016] apply, a modification of so-called pari-mutuel gambling score [zhuang 2010, zechar and zhuang 2014]. it is the zero-sum game approach whose results, in general, depend strongly on the “wagers and commission rate”. in the considered case, the applied methodology leads to practically complete loss of information on successful prediction. specifically [molchan et al., 2017), when, on average, the interevent time between target earthquakes is much larger than the time step of prediction updates (as in the case of cn algorithm application in italy), a negative verdict about significance of any prediction algorithm is predetermined a priori. the methodological error of taroni et al. [2016] is that the authors compare the alarm based method with the random forecast (rf), while the actual problem consists in the comparison of two alarm based predictions, namely cn versus random guessing (rg). let us recall the difference between rf and rg: rf admits the target event in each time bin with probability p; whereas the rg use this rule to generate alarm zone, suggesting that only within such zone the target event may happen. the comparison of two similar in nature, but different in content, prediction methods, namely pf vs rg, shows that in the framework of the pmgs approach and poisson seismicity model rf almost always wins against rg [molchan et al. 2017]. on the other side, the comparison of two alarm based methods, for example cn vs rg, in the framework of the game approach can be quite informative (see gs approach by zechar and zhuang [2010]). molchan and romashkova [2011] successfully applied gs in modified form to the analysis of the alarm-based forecasts produced by the m8 algorithm in non-homogeneous time-space bins. it is of common knowledge that a very limited amount of data is a serious obstacle for a reliable statistical analysis, in particular, when quantifying the performance of an earthquake forecast/prediction method at a regional level. an in-depth comprehensive discussion of methodologies for assessing earthquake prediction results can be found in molchan et al. [2017] and, earlier, in [molchan 1997, molchan and romashkova 2011]. we recommend the interested readers to consider carefully and critically the paper by molchan et al. [2017], which provides the basic elements for an objective independent assessment of taroni et al. [2016], who persistently bypass these elements. references molchan g., romashkova l., peresan a. (2017). on some methods for assessing earthquake predictions. geophys j. int., 210, 1474-1480, https://doi.org/10.1093/gji/ggx239. molchan, g. and romashkova, l., (2011). gambling score in earthquake prediction analysis. geophys. j. int., 184, 1445-1454. molchan, g. and romashkova, l., (2010). earthquake prediction analysis. based on empirical seismic rate: the m8 algorithm, geophys.j.int., 183(30,15251537. molchan, g.m. (1997). earthquake prediction as a decisionmaking problem, pure appl. geophys, 147(1), 1-15. peresan, a., kossobokov, v., romashkova, l., panza, g.f. (2005). intermediate-term middle-range earthquake predictions in italy: a review, earth sci. rev., 69, 97-132; doi:10.1016/j.earscirev.2004.07.005 peresan, a., e. zuccolo, f. vaccari, a. gorshov, and g.f. panza (2011). neo-deterministic seismic hazard and pattern recognition techniques: time-dependent scenarios for north-eastern italy, pure and appl. geophys. 168, 583-607. peresan, a., v.g. kossobokov, g.f. panza (2012). operational earthquake forecast/prediction, rend. fis. acc. lincei, 23, 131-138. doi 10.1007/s122100120171-7. press, w. h., s. a. teukolsky, w. t. vetterling, and b. p. flannery (1992). numerical recipes in c: the art of scientific computing 2nd ed.. new york: cambridge university press, 994 p. taroni, m., marzocchi, w., roselli, p. (2016). assessing molchan et al. 2 3 comment on “assessing cn earthquake predictions in italy” by m. taroni, w. marzocchi, p. roselli ‘alarm-based cn’ earthquake predictions in italy annals of geophysics, 59, 6, s0648; doi:10.4401/ag6889s0648. zechar, j.d., zhuang, j., (2014). a pari-mutuel gambling perspective to compare probabilistic seismicity forecasts, geophys. j. int. 199, 60-68. zhuang, j. (2010). gambling scores for earthquake forecasts and predictions, geophys. j. int., 181, 382– 390, doi:10.1111/j.1365-246x.2010.04496.x *corresponding author: antonella peresan, istituto nazionale di oceanografia e di geofisica sperimentale. crs, udine. italy email: aperesan@inogs.it © 2018 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. preliminary engineering analysis of the august 24th 2016, ml 6.0 central italy earthquake records annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 1 preliminary engineering analysis of the august 24th 2016, ml 6.0 central italy earthquake records iunio iervolino 1, georgios baltzopoulos 2, eugenio chioccarelli 2 1 dipartimento di strutture per l’ingegneria e l’architettura, università degli studi di napoli federico ii, italy. iunio.iervolino@unina.it 2 construction technologies institute itc-cnr, urt at dipartimento di strutture per l’ingegneria e l’architettura, università degli studi di napoli federico ii, italy. baltzopoulos@itc.cnr.it; chioccarelli@itc.cnr.it abstract an earthquake of estimated local magnitude (ml) 6.0 struck central italy on the 24th of august (01:36:32 utc) in the vicinity of accumoli (close to rieti, central italy) initiating a long-lasting seismic sequence that also featured events of larger magnitude within a few months. the earthquake caused widespread building damage and around three-hundred fatalities. ground motion was recorded by hundreds of seismic stations. this work uses accelerometric records for a preliminary discussion, from the earthquake engineering perspective, of strong motion caused by the earthquake. peak and integral ground motion intensity measures, are presented. the response spectra at some select stations are analysed with respect to the code-mandated design actions for various return periods at the recording sites. hazard disaggregation for different return periods is discussed referring to the site of the epicentre of the earthquake. finally, some preliminary considerations are made concerning the impact of rupture propagation on near-source ground motion; i.e., the records are scanned for traces of pulse-like forward-directivity effects. i. introduction the national accelerometric network of italy (ran), operated by the governmental dipartimento delle protezione civile (dpc), and the italian seismic network (rsn), operated by the istituto nazionale di geofisica e vulcanologia (ingv) have made available, rapidly after the event, the records of the earthquake, with epicentre located in the vicinity of accumoli, central italy, that struck on aug. 24 2016, at 1:36:32 am – utc. the moment magnitude (mw) declared by ingv is 6.0, while other international institutions claim mw 6.2. corrected records and processing details are available on the engineering strong-motion database website, while the uncorrected waveforms can be found on the ran and rsn websites (see section vi). mailto:iunio.iervolino@unina.it mailto:baltzopoulos@itc.cnr.it mailto:chioccarelli@itc.cnr.it annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 2 the present short article deals with some aspects of recorded strong ground motion of earthquake engineering interest. first, shaking intensity parameters for some of the ground motions recorded nearest to the fault are presented. then, response spectra for some of the stations closer to the source are compared to the code-mandated spectra. finally, nearsource ground motions are examined for impulsive characteristics to be possibly attributed to rupture directivity. ii. ground motion intensity measures and response spectra table 1 shows some peak and integral ground motion intensity measures (ims) for the records within 30km from the source (i.e., joyner-and-boore distance, jbr ). more specifically, data reported in the table are: the id of the station, the jbr distance, the soil class 1 according to the code (cs.ll.pp. 2008, ntc2008 hereafter), the peak ground acceleration of the east-west horizontal component  epga , and of the north-south component  npga , the angle with respect to the north (positive clockwise) to which corresponds the maximum recorded pga  pga , the corresponding pga (max pga ). the maximum pseudoacceleration response spectrum ordinate at 0.3s and 5% damping,  0.3sa , is given in the same table along with the angle to which it corresponds; i.e., max  0.3sa and , respectively. the same information is provided, with analogous notation, assuming an oscillation 1 asterisks indicate that soil classification is based on inferred, rather than measured, average shear velocity in the upper 30m. period of 1.0s. considered integral parameters are the arias intensity  ia and the significant duration  5 95d  for the horizontal components, estimated between 5% and 95% of ia . finally, if available, vertical pga values are also reported  vpga . 2 data reported in table 1 show that max pga and  0.3sa recorded by nrc and amt station were significantly higher than those recorded by all the other stations, while the highest max  1.0sa was recorded by the nor station. in particular, epga of the amt station is the highest horizontal pga recorded up to that point in italy (larger pgas may have occurred later in the sequence, yet this is not yet fully consolidated). among other stations, a few records have values somewhat departing from the general trend: fema, clf, foc and tre if pga and  0.3sa are considered, and rm33 and clf if  1.0sa is of concern (see also section iv). a similar situation applies to integral parameters. to have a more complete picture of the characteristics of the recorded ims, these have to be compared with ground motion prediction equations (gmpes). one of such comparison can be found in reluis-ingv workgroup (2016a), that also includes the complete response spectra (elastic and inelastic) for the 2 ims from amt station are derived from the revised records provided by ran website several weeks after the event. moreover, at the time of the submission of this paper, recordings from aqa and nor stations have been retracted pending possible revision by the dpc. aqa and nor data analysed here are those available prior to said revision. (0.3)sa annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 3 records considered here, as well as for others more distant from the source. another comprehensive comparison of observed ground motion peak parameters and predicted ims is provided in lanzano et al. (2016) with respect to two different gmpes. in that paper, it is underlined that the gmpes generally fit the observations for low spectral periods and short source-to-site distances and seem to underpredict the observed ims at distances larger than 80 km. iii. the records and the italian seismic code in figure 1 the pseudo-acceleration response spectra associated to the horizontal ground motions, recorded by some of the stations with the smallest jbr distance, are compared with the elastic design spectra provided by the italian building code, at the corresponding sites, for four different return periods (tr): 50, 475, 975 and 2475 years. before proceeding any further, it is worthwhile recalling that ntc2008 spectra are a direct approximation of the uniform hazard spectra computed via the probabilistic seismic hazard analysis (psha) discussed in stucchi et al. (2011). the east-west component of amt exceeds the 2475 years spectra in the 0s-0.4s range of periods, while at least one component of the same station exceeds the 475 years spectra for spectral periods up to 2.1s. nrc and fema (see figure 2a, for the position with respect to the source) exceed the 475 years spectra in the range of periods 0s-0.3s and 0.35s-0.5s respectively. this applies to at least one of the two horizontal components (for nrc record, the exceedance happens also for 0.67s-0.88s). the nrc record also exceeds the 2475 years spectrum for periods between 0.13s and 0.28s. however, at all the stations for longer oscillation periods, and as soon as the distance increases, spectral ordinates become comparable with code-spectra corresponding to return periods of a few tens of years. the maximum ratios of the peak of the pseudo-acceleration spectrum (5% damping) and the pga are equal to 5.1 and 2.7, for nrc and amt stations, respectively (these refer to the east-west and north-south directions without investigating other possible rotations). notwithstanding the not completely intelligible (so far at least) differences between the two horizontal components of amt, the shape and the amplitude of these spectra appear compatible with extensive damage in some villages, where the population of structures suffered significant damage or total collapse. it should be also discussed that exceedance of code spectra close to the source of a strong earthquake does not directly imply inadequacy of psha at the basis of the code spectra (iervolino, 2013). this is also because spectra from psha, are the results of an average of a series of scenarios considered possible (e.g., small and large source-to-site distances). such an average may be exceeded close to the source of an earthquake, even if the corresponding scenario is included in the psha. referring to the coordinates of the epicentre (42.70n; 13.24e), the hazard disaggregation (e.g., iervolino et al., 2011) was computed, by means of rexel v 3.5 (iervolino et al., 2010), for pga and for  1.0sa , for two return periods (475 and 2475 years). the entire set of disaggregation distributions is not shown here for the sake of brevity (although it can be found in annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 4 reluis-ingv working group, 2016a). nevertheless, the relevant scenarios for the considered return periods and ims are summarized here; they are the magnitude and distance intervals that have probability, of being causative for the exceedance of the corresponding im, larger than 0.5. all of these scenarios are characterized by the same distance range between 0km 20km. magnitude intervals for pga are 5.3 6.3 and 5.9 – 6.8 for 475 years and 2475 years, respectively. for  1.0sa , and the same return periods, magnitude intervals are 6.0–7.0 and 6.3-7.0, respecitively. it may be concluded that, according to the hazard analysis the code is based on, exceedance of highfrequency spectral accelerations, corresponding to 475 years and 2475 years tr, is most likely caused by a close moderate-magnitude earthquake that is loosely compatible to what was observed. figure 1. comparison between the observed ground motions and the elastic design spectra provided by ntc2008. iv. analysis of pulse-like directivity effects pulse-like near-source (ns) ground motions may be the result of rupture directivity. this can lead to a constructive wave interference effect, which is manifested in the form of a double-sided velocity pulse that delivers most of the seismic energy early in the record (somerville et al., 1997). clues of impulsive features in near-source ground motions have been probably found in italian seismic events of normal faulting style before (e.g., chioccarelli and iervolino, 2010). in this preliminary investigation for such pulse-like effects, the continuous wavelet transform algorithm suggested by baker (2007) was implemented for all recordings (horizontal components) within a closest-to-rupture distance of 30km from the fault and for all orientations. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 5 it should be noted that the adopted approach is purely phenomenological, extracting empirical evidence of impulsive characteristics directly from the recorded ns signals without attempting to assign a causal relation to specific effects falling under the banner of rupture directivity (i.e., rupture propagation, seismic source radiation pattern, motion polarization). models regarding the phenomenon through the prism of the physics of finite-fault rupture are also available (e.g., spudich and chiou, 2008) but not followed in this preliminary analysis. the surface projection of the fault rupture plane (tinti et al., 2016) is shown in figure 2a along with the equal-probability contours of the iervolino and cornell (2008) model for the probability of observing ns directivity pulses. it can be observed that, interestingly enough, some of the most prominent impulsive waveforms (nrc, nor, fema, rm33) have been recorded at sites where the empiricallycalibrated model assigns low probability of pulse occurrence due to directivity. this is indicative of the fact that more research is required into the phenomenon for the case of normal faulting, but it should also be mentioned that there were hardly any accelerometric stations present in the area where pulse-like effects were most probable to be observed according to the model. out of all the records investigated, six ground motions exhibited impulsive characteristics, as expressed by a pulse indicator (pi) score in excess of 0.85 (see baker, 2007). the amt record revealed two distinct pulses. one with pulse period p t of 0.41s being predominant in the fault-normal (fn) and another, longer duration pulse with p t = 0.98s in the fault-parallel (fp) direction (figure 2b). the latter might be attributed to the breakage of a nearby asperity on the fault plane (tinti et al., 2016). the fema record was also discovered to exhibit impulsive characteristics in both fn ( pt = 0.50s) and fp directions. the ground motions recorded at the mnf station and rm33 were found to contain pulses in the fn direction ( pt = 1.40s and 1.20s respectively) that also hinted at rupture directivity effects, despite the lower velocity amplitude due to the greater distance from the fault and consequent attenuation. this leaves the two ground motions recorded at norcia, nrc and nor, that were of particular interest. the nrc record was found to contain a 2.09s period pulse mostly towards orientations that lie between the fn and fp but somewhat more prevalent in the direction perpendicular to the strike. interestingly, it is known that the nrc site sits upon deep soil deposits characterized by an inversion of velocity profile at a depth of more than 30m (see bindi et al., 2011) and this cannot be disregarded when narrow-band characteristics are observed in the recordings. however, station nor was also found to be pulse-like, with a 1.63s pulse in the fn direction (see figure 2c). furthermore, records obtained at the base/ground-level of instrumented, seismically monitored buildings distant up to 500m from the two accelerometric stations, contained velocity pulses almost identical in orientation and period to the nor station. this consistency enhances the argument in favour of the presence of a directivity effect. one prominent characteristic of directivity-induced velocity pulses is that they tend to annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 6 dominate the spectral shape of the signal’s pseudo-velocity spectrum. this is also confirmed in the case examined here; see figures 2(d-e). furthermore, pulse period pt (as defined via the wavelet transform, see baker, 2007) is very well correlated with predominant ground motion period gt (vibration period where maximum pseudo-velocity occurs); also confirmed in this case. note that the extracted pulses typically correspond to some local maximum also on the pseudo-acceleration spectrum ( sa ), but do not necessarily account for the absolute maximum amplitude, which is usually determined by the higher-frequency content. finally, when the pulse periods extracted from the various ns sites examined are compared against the empirical regression model from baltzopoulos et al. (2016), they are found in reasonable agreement with what is expected for a mw 6.0 event ( pt geometric mean of 1.02s compared to predicted median of 1.29s). figure 2. equal probability of pulse occurrence contours, according to the model of iervolino and cornell (2008), plotted against the surface projection of the rupture plane (a), velocity time-history and extracted pulse of amt fn component (b) and nor fn (c), pseudo-velocity spectra (psv) of the extracted pulses plotted against psv of the original signal for the fn components of amt (d) and nor (e), pseudo-acceleration spectra of the extracted pulses plotted against sa of the original signal for the fn components of amt (f) and nor (g). v. conclusions this article provided a preliminary engineering-point-of-view analysis of the strong motion records obtained during the amatrice earthquake that struck central italy on 24th of august 2016. an overview of the ground motion intensity parameters typically associated with structural response was given, identifying stations with highest ims, with amt exhibiting the highest horizontal pga recorded annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 7 in italy so far. the comparison of some of the closest-to-rupture records’ spectra and the code spectra showed cases of exceedance of the latter by the former at tr 475 years and even tr 2475 years. it was then briefly discussed how this is to be expected, even though to claim otherwise can be a common pitfall. finally, it can be said that some indications of pulse-like ns motions were observed, which could be the result of rupture directivity. points of interest that merit further investigation include the potential role of site effects in the manifestation of some particular spectral shapes and the juxtaposition of inelastic response spectra with damages. vi. postscript this paper was prepared before the occurrence of other large earthquakes from same seismic sequence. indeed, since the first submission of this article, two further strong earthquakes have struck the area, the 26/10/2016 mw 5.9 ussita event and the 30/10/2016 mw 6.5 norcia event. as such, the 24/08 mw 6.0-6.2 amatrice earthquake is currently regarded as the initiating event of the long-lasting 2016 central italy sequence, with the norcia seismic event being typically nominated as the overall mainshock, so far. herein the first event was discussed, while the reader is referred to reluis-ingv workgroup (2016b) for a more comprehensive analysis of the sequence as a whole. vii. data and sharing resources records used herein were processed and provided to the authors by the itaca-esm working group of the istituto nazionale di geofisica e vulcanologia (ingv). they are also available at http://esm.mi.ingv.it/ (last accessed 21/11/2016). the unprocessed records can be accessed at http://ran.protezionecivile.it for the ran network and at the european integrated data archive (http://www.orfeus-eu.org/data/eida/) for the rsn. the parameters of the finite-fault geometry used are available at http://esm.mi.ingv.it and are attributed to tinti et al. (2016). accelerograms recorded at the monitored structures in norcia are available from the seismic observatory of structures of the national civil protection – www.mot1.it. viii. acknowledgments the study presented in this paper was developed within the activities of reluis (rete dei laboratori universitari di ingegneria sismica). ix. references baker j.w. (2007). quantitative classification of near-fault ground motions using wavelet analysis. bull. seismol. soc. am., 97(5):1486– 1501. baltzopoulos g., d. vamvatsikos and i. iervolino (2016). analytical modelling of nearsource pulse-like seismic demand for multilinear backbone oscillators. earth. engin. struct. dyn., 45(11):1797–1815. bindi d., l. luzi, s. parolai, d. di giacomo and g. monachesi (2011). site effects observed in alluvial basins: the case of norcia (central italy). bull. earthquake eng., 9:1941-1959. chioccarelli e., i. iervolino (2010). near-source seismic demand and pulse-like records: a discussion for l'aquila earthquake. earthq. engn. struct. dyn., 39(9):1039–1062. http://esm.mi.ingv.it/ http://ran.protezionecivile.it/ http://www.orfeus-eu.org/data/eida/ http://esm.mi.ingv.it/ http://www.mot1.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 8 cs.ll.pp. (2008). decreto ministeriale 14 gennaio 2008: norme tecniche per le costruzioni. gazzetta ufficiale della repubblica italiana, 29 (in italian). iervolino i., c.a. cornell (2008). probability of occurrence of velocity pulses in near-source ground motions. bull. seismol. soc. am., 98(5):2262-2277. iervolino, i. (2013). probabilities and fallacies: why hazard maps cannot be validated by individual earthquakes. earthq. spectra, 29(3):125– 1136. iervolino, i., c. galasso and e. cosenza (2010). rexel: computer aided record selection for code based seismic structural analysis. bull. earthq. eng., 8:339-362. iervolino, i., e. chioccarelli and v. convertito (2011). engineering design earthquakes from multimodal hazard disaggregation. soil dyn. earthq. eng., 31:1212-1231. lanzano g., l. luzi, f. pacor, r. puglia, m. d’amico, c. felicetta and e. russo (2016). preliminary analysis of the accelerometric recordings of the august 24th, 2016 mw 6.0 amatrice earthquake. annals of geophysics, 59, doi: 10.4401/ag-702. reluis-ingv workgroup (2016a). preliminary study of rieti earthquake ground motion records v5, available at http://www.reluis.it. reluis-ingv workgroup (2016b). preliminary study on strong motion data of the 2016 central italy seismic sequence v6, available at http://www.reluis.it. somerville p.g., n.f. smith, r.w. graves and n.a. abrahamson (1997). modification of empirical strong ground motion attenuation relations to include the amplitude and duration effects of rupture directivity. seismol. res. lett., 68:199–222. spudich, p., b.s.j. chiou (2008). directivity in nga earthquake ground motions: analysis using isochrone theory. earthq. spectra 24(1): 279-298. stucchi, m., c. meletti, v. montaldo, h. crowley, g.m. calvi and e. boschi (2011). seismic hazard assessment (2003–2009) for the italian building code. b. seismol. soc. am., 101:1885– 1911. tinti, e., l. sconamiglio, a. michelini and m. cocco (2016). slip heterogeneity and directivity of the ml 6.0, 2016, amatrice earthquake estimated with rapid finite-fault inversion. geophys. res. lett., 43(10): 10,745-10,752. http://www.reluis.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7182 9 table 1. ims of the recorded ground motions within 30 km from the source (distance in terms of jbr ). station id soil class jbr [km] epga [g] npga [g] pga [°] max pga [g] (0.3)sa [°] max  0.3sa [g] (1.0)sa [°] max  1.0sa [g] eia [cm/s] 5 95 ed  [s] nia [cm/s] 5 95 nd  [s] vpga [g] amt b* 1.4 0.867 0.376 90 0.867 96 1.833 13 0.400 188.52 3.74 72.23 3.16 0.399 nrc b 2.0 0.359 0.373 145 0.452 138 0.897 100 0.267 104.37 6.00 82.59 6.25 0.216 nor c* 2.3 0.202 0.180 48 0.203 141 0.474 87 0.405 55.95 10.27 34.33 17.02 mmo1 a* 9.7 0.118 0.139 154 0.149 129 0.442 161 0.127 7.12 7.13 19.40 9.05 rm33 b* 13.0 0.102 0.101 104 0.106 53 0.356 93 0.170 8.66 8.64 6.07 9.13 0.036 fema b* 13.9 0.247 0.189 84 0.248 103 0.444 92 0.116 30.49 5.02 12.42 7.68 spd b* 16.1 0.053 0.102 9 0.103 2 0.204 53 0.117 3.95 10.39 7.16 8.62 0.055 mnf a* 20.4 0.073 0.044 69 0.078 56 0.150 89 0.069 3.03 6.86 1.48 10.91 0.061 tero b* 22.1 0.057 0.085 176 0.085 79 0.122 131 0.036 4.70 11.22 7.50 9.75 0.036 lss a 22.2 0.022 0.018 112 0.024 53 0.080 37 0.016 0.74 16.72 0.67 18.90 0.015 pzi1 b* 22.7 0.045 0.046 58 0.052 23 0.161 88 0.062 2.38 11.63 2.87 14.89 0.026 ant a* 26.0 0.014 0.024 25 0.026 20 0.062 4 0.059 0.68 25.19 1.13 20.96 0.009 clf d 26.1 0.125 0.131 129 0.142 165 0.456 29 0.198 18.13 9.03 14.11 10.80 0.105 foc c* 26.3 0.261 0.329 20 0.351 104 0.522 156 0.029 36.24 5.60 38.48 4.22 0.128 fos b* 28.8 0.060 0.076 40 0.090 31 0.230 92 0.039 4.64 9.02 5.94 8.81 0.041 aqf b* 29.3 0.044 0.038 92 0.044 174 0.161 99 0.024 1.96 12.03 1.71 14.96 0.033 aqv b 29.3 0.061 0.046 84 0.061 127 0.103 112 0.065 3.00 14.23 2.82 15.89 0.023 aqa e 29.4 0.002 0.002 2 0.002 45 0.005 102 0.003 0.00 14.86 0.01 14.94 0.001 tre c* 29.6 0.064 0.111 33 0.118 39 0.276 27 0.097 5.10 14.62 10.62 14.52 0.046 trl a* 30.2 0.036 0.039 164 0.041 47 0.095 33 0.047 2.58 20.84 3.79 20.28 0.018 gsa b 30.7 0.036 0.037 110 0.038 66 0.071 53 0.020 1.37 17.12 1.75 15.33 0.019 spm a* 30.9 0.067 0.065 98 0.068 38 0.104 18 0.030 4.93 14.43 4.99 12.48 0.021 recovering of local magnetic k-indices from global magnetic kp-indices using neural networks: an application to antarctica annals of geophysics, 58, 4, 2015, g0440; doi:10.4401/ag-6719 g0440 recovering of local magnetic k-indices from global magnetic kp-indices using neural networks: an application to antarctica antoni segarra*, juan josé curto observatori de l’ebre (oe), csic universitat ramon llull, roquetes, spain abstract this paper describes a method to obtain local magnetic index, k, from the global index, kp. until now, however, for the cases of areas without magnetic observatories, to estimate the geomagnetic activity there, global indices were the only option. the methodology that we used to estimate local index was based on neural networks. this tool has a great potential for processing information from complex systems as in the case of the geomagnetic system. local k index calculated with this method resulted to be a better option than directly using the global index kp when we need an indicator of geomagnetic activity in a specific area. the best results of our method were for moderate and high geomagnetic activity, which are of major interest in space weather. 1. introduction the magnetic variations on the magnetograms are different at each observatory. they mainly depend on the latitude of the observatory and the local time. we need a quantitative measure of the amount of magnetic disturbance at any time and, consequently, we need to introduce magnetic indices. these indices are often used as inputs of different regional models to describe the geomagnetic or ionospheric behavior [e.g., zolesi and cander 1998, pietrella and perrone 2008, pietrella 2012]. there is a great variety of indices used to measuring the degree of magnetic disturbance for various purposes. in this paper, we will focus our attention in two indices: k-index (local) and kp-index (planetary), which are widely used in geophysical studies. the k-index is a local index. it was introduced by bartels in 1939 [bartels et al. 1939]. each observatory assigns a digit ranging from 0 to 9 to both h and d, two components of the magnetic field, for every three-hour interval starting at midnight, universal time (ut). the digit is based on the range of the element during the three-hour interval normalized on a standard quasi-logarithmic scale [mayaud 1980]. several scales are applied, each one being specified by the amplitude, in nt, that corresponds to k = 9. bartels chose the appropriate scale for each observatory, considering each one to have roughly the same frequency distribution of k-indices. the factors of proportionality between each scale were derived empirically for each observatory and reflect different geomagnetic effects depending on the observatory location. however, it was later shown that the “goal of proportional grids leading to similar significance of k values is achieved only in the subauroral regions, where the geomagnetic perturbations have similar local time and seasonal variations and where the distribution of k index is lognormal” [menvielle and berthelier 1991]. in order to derive this index, some caution is required when determining the three-hourly range on which the k-index is based, since what is wanted is not simply the range, but the range after the diurnal variation has been removed [mayaud 1967]. kindices have to measure the disturbance variations which are not related to the diurnal variation. as the diurnal variation differs from day to day, observers in the case of hand-scaled or algorithms in the case of automatically must exercise great care when estimating it [menville et al. 1995]. in some situations, it is very difficult to obtain the diurnal variation, for instance in the recovery phase after magnetic storms which modify the baseline of the diurnal variations. the other index that we take into consideration is the kp-index, which is derived from the preceding one. kp is a global index. it was introduced by bartels in 1949 [bartels 1949]. kp involves a digit ranging from 0 to 9 with a subdivision in thirds (0, 0+, 1–, 1, 1+,…) for each three-hour interval starting at midnight, ut. it is worked out from the k-index of a group of 13 observatories (11 in the northern hemisphere, mainly in north america and western europe, and only 2 in the southern hemisphere), this irregular distribution implies that kp-index is biased due to northern hemiarticle history received december 29, 2014; accepted june 30, 2015. subject classification: k-index, neural networks, local magnetic indices, antarctica. sphere. kp-indices are meant to measure and compare different disturbances at different times. bartels recognized that k indices from different observatories had to be “standardized” (ks, s from standardized) before they could be combined into a planetary index. bartels handled the latitudinal and local time/seasonal corrections. both corrections had the same condition: k values must have the same frequency distribution. to obtain the ks value, the latitudinal variation was corrected with the definition of k=9 scales for each observatory; and the local time/seasonal variation was corrected through conversion tables. in this way, each three-hour value from the group of 13 selected observatories was calculated on basis of latitude conversion table, k=9 scale, and after that, converted into ks using the local time/seasonal conversion tables [bartels 1949, 1951]. as a result of this process, each k measurement was transferred through statistical tables to a specific location, niemegk, and to a specific time, autumn, in order to be compared under the same conditions. the kp-index gives us information on global magnetic activity, but it is an abstract index which does not give specific information of the disturbance in a specific location at a specific time. a significant problem related to magnetic indices is the irregular distribution of the observatories. numerous in the northern hemisphere and very scarce in the southern hemisphere, these observatories, however, are the only source of local indices. the inverse process of obtaining local indices from global indices is our aim. it is not an easy task because there is not any relating algorithm. the process of constructing kp-index implies a transformation of local k through statistical tables. then, using selected observatories, an average of ks is calculated and this gives the kp. to obtain local indices from global indices at a specific place you need to have the statistical table for this place, and also be able to undo the calculation of the ks average. the main problem to undo this process remains on the statistical tables; these tables are constructed to have the same frequency distribution of k for every observatory, and are significantly different for each observatory. the observatories distribution over the world is very irregular. with the time, the number of observatories has increased, but, even now, there are still large areas without observatories. the aim of this work is making an assessment of the geomagnetic activity in remote areas which usually are poorly covered with observatories. having an assessment about geomagnetic activity in areas without observatories would be helpful to have a complete view of geomagnetic activity in any part of the earth. there are different ways to try to do this. when the k-value for an observatory is not available, the easiest method to obtain a good approximation is to use the k-values of the nearest observatory. however, we are interested in remote areas, with low number of observatories, and so, this method would not be suitable. a second method involves making an interpolation using other observatories, but, again, the reduced number of observatories and their irregular distribution in those remote areas also rule out the feasibility of this technique. finally, a third method involves attempting to undo the process whereby the kp has been calculated. in this way, we propose a method to obtain local assessment derived from a global index. we compute local index k from a global index, kp, through neural networks. the neural network models are known to have the property of learning from cases and with ability to handle complex nonlinear physical phenomena. this neural network capability in space weather, related to predictions, has been demonstrated in various studies [uwamahoro et al. 2012, watthanasangmechai et al. 2012]. in the field of geomagnetism, segarra and curto [2012] applied neural networks for sudden commencement automatic detection. lundstedt and wintoff [1994] used neural network to predict geomagnetic storms using solar wind data. wu and lundstedt [1996, 1997], lundstedt et al. [2002] and kugblenu et al. [1999] demonstrated the ability of neural network for dst forecast. takalo and timonen [1997] used neural networks for ae-index forecasting. duka and hyka [2008] checked multiple neural networks to predict the evolution of the three components of the global geomagnetic field. boberg et al. [2000] developed a method for the prediction kp-index based on neural networks, other authors used different methodology with the same aim, for example costello [1997] or wing et al. [2005]. a few models have been developed for the prediction of the local k-index [virjanen et al. 2008, kutiev et al. 2009]. as a validation of our method, we compute k values for a specific and remote place, livingston (liv) (62°39´44˝ s, 60°23´41˝ w geographic coordinates; 52°50´13˝ s, 8°57 42˝ e, geomagnetic coordinates). livingston observatory is an unattended observatory during the major part of the year, only there are staffs during summer season. there, when data are retrieved, local k indices are automatically calculated by the adaptive smoothing method recommended by intermagnet [novozynsky et al. 1991]. 2. methodology neural networks neural networks can be defined as a form of computation inspired in biological models. their name comes from their similarity with the human brain in two aspects: the fact that knowledge is acquired by learning; and that links, which are connecting the neusegarra and curto 2 3 rons, store the information using a weight function [hopfield and tank 1985]. neural networks consist of elements (neurons) and connections (weight functions) that are adjusted, or trained, so that a particular input leads to a specific target output. these elements are inspired by biological nervous systems. as in the natural world, network behaviour is possible due to the connections existing between individual elements. a network requires two or more layers. each layer consists of simple elements (neurons), with the ones in each layer connected to the neurons in the next layer through weighted links. the process of determining these weights is called “learning”. the first layer receives the external information; the last one produces the output or solution and the intermediate layers have no connection with the external “world”, so they are called hidden layers. a neural network can be trained to perform a particular function by adjusting the values of the weights. the weights are adjusted according to different rules for example backpropagation learning rule: a learning rule which weights and biases by error-derivative (delta) vectors back-propagated through the network. generally, in order to train neural networks, a particular input is introduced along with a target, which represents the desired output. the network begins to create different connections selecting those variables which may make the final output reach the desired target. when network training ends, the information has flowed through the network several times and the weights have been modified according to the accuracy of the output regarding the target. 2.1. back-propagation neural networks, in particular a feedforward backpropagation network, are very efficient in the recognition of patterns or hidden structures in a dataset [demuth and beale 1994]. feedforward neural network with error back-propagation is a method commonly used for mapping multivariable inputs to multivariable outputs [rumelhart et al. 1986].this kind of network extracts features from a data set and learns the associations between the inputs and the outputs. input information crosses the network (forward), being transformed by the intermediate layers, and the obtained output is compared with the target. the difference or error is returned back to the network, crossing the network backwards [rojas 1996]. repeating this process adjusts the weights of the connections in the network to minimize the difference between the current and desired output of the net. there is no conclusive way to determine the appropriate number of layers and neurons. these figures are determined on a trial-and-error basis. the amount of data and the distribution used to train the network is fundamental for an accurate determination of the network. for a successful manipulation, training data should include cases of every possible relevant option in the distribution [kröse and van der smagt 1996]. therefore, there is an aspect concerning the number of neurons in the hidden layer that should be emphasized: more neurons, which theoretically would entail a greater capacity of calculation, does not necessarily mean better results [demuth and beale 1994]. in order to find the appropriate network, two major selections should be considered [nakamura et al. 2007]: the parameters of the physical problem to be resolved, which will determine the input information for the neural network, and the parameters of the neural network architecture. furthermore, these choices have to be made in parallel. 2.2. input parameters in order to choose the input parameters, we need to consider the physics of the problem, which variables could affect the value of k-index. depending on the locations of the observatories, different effects from the flowing currents around the magnetosphere are recorded. thus the observatories located at night local time are more affected by the particles that came from the magnetotail, especially during substorm time [parkinson 1983]. thus, we take the geomagnetic latitude and geomagnetic longitude of each observatory as input variables. diurnal and seasonal variations also contribute to modulate regular and irregular variations, the ionospheric current systems are modulated by thermal effects of the sun and their incidence to the earth, so ionospheric current systems have a seasonal dependence [parkinson 1983]; consequently we also take the time and day of the year as input variables. and as our objective is transforming kp-index to local index, we must take kp-index as an input variable. the neural network input “vector” for each observatory was determined as: geomagnetic latitude and longitude, time, day of the year and kp-index. the neural network target is the corresponding k-index, but only for our training stations, as our test station, liv, is excluded from the training set. the network found the relationship between these variables and the target. we worked with the values of k obtained for the years 2003, 2004 and 2005 in the intermagnet network (international real-time magnetic observatory network). we have data from 68, 72 and 78 observatories respectively according to their availability. in addition to that, we have the data from the station in livingston (liv) for the same period [marsal et al. 2004, 2005], which is useful to test the results of our neural network. recovering of k-indices from kp-indices 2.3. data sets after defining the variables of physical interest we selected the observatories whose data we were going to use in order to train the neural network. we cannot directly include all the observatories because their irregular distribution around the earth, and as we commented above, kp-index is biased for northern america and western europe observatories. if we take all observatories the results would be biased due to the overweight of the overpopulated areas. as our test station is livingston, which is located in a very special region the antarctic region we had to select our data set even more carefully. to train the neural network we were faced with two possible options: either take observatories from all around the planet although well distributed -, or just choose observatories close to livingston. to sum up, we could either use a global data set network or a local data set one. we tried both options. 2.3.1. global network we need to attain a regular distribution of observatories, with special attention paid to the number of observatories in each hemisphere. figure 1 shows the global distribution of the observatories included in the study. we have 7 files containing the data of 6 observatories each, making 42 observatories in total. these files are intended to offer a regular distribution of observatories; two in the northern hemisphere, two in the southern hemisphere, and two near the equator. moreover, the training was carried out randomly, both in terms of which observatories were chosen for each group and also in which order those observatories were selected. 2.3.2. local network the other approach involves taking the local data set network with those observatories close to our test station, as shown in figure 2. in this case we take all the observatories in the southern hemisphere, except those in the auroral zone because of their different magnetic behaviour. livingston is an antarctic observatory but it is considered as a mid-latitude station from the point of view of geomagnetism. unlike the global data set network, all the available observatories were included in this second approach. there are just 11 available stations out of the 42 stations selected for the global data set network. livingston is not included in the training data set as we kept it apart for checking purposes. 2.4. neural network parameters the other elements to be chosen are the parameters of the neural network, such as the number of layers, the number of neurons in each layer, the transfer function included in the neurons, and the training algorithm. after running several tests to delimit our research, we concentrated on 2, 3 or 4 layers (counting the input, hidden and output layers), with a maximum of 40 neurons each. a higher number of neurons would involve more computing time but would not lead to a significant improvement in our results. due to the training method chosen, the neural network used the error back-propagation method, which means the weights keep adjusting as the information flows through the network. the transfer function was a log-sigmoid for the hidden layers and a linear function for the other layers. after running several segarra and curto 4 figure 1. global distribution of magnetic observatories used in the option one in the training process of the network. 5 tests, we chose these functions because they give us the best performance. the training method we used was a conjugate error gradient algorithm, based on the search of direction of the negative gradient. to accelerate the training process, we used the powell-beale condition [demuth and beale 1994] (1) where gk is the current gradient, gk t its transposed matrix and 1gk is the gradient computed in the previous cycle. the search of the negative gradient is made along the conjugate gradient direction; the direction search is periodically reset to the negative of the gradient. this technique restarts if there is very little orthogonality left between the current gradient and the previous gradient. if this condition is satisfied, the direction search is reset to the negative of the gradient. in order to choose between all the possible parameters of the neuronal network, we had to compare the results obtained in each case. we created and trained the neural network and then we compared the results obtained with the real data of the magnetic station of livingston. we ran a series of tests taking different numbers of layers and neurons per layer (the results are shown in table 1). we were interested in finding a network able to produce an output kn (n from network) with the high and low values of k. after a series of tests, we decided to work with three different architectures: a two layer network, with 40 and 1 neuron per layer respectively; a three-layer network, with 20, 10 and 1 neurons per layer respectively; and a four layer network, with 40, 10, 10 and 1 neurons per layer respectively. we decided work with these three neural networks because these neural networks were able to provide almost the full range of the k values. among the other options which were tested, the case of 4-layer network [40, 30, 20, 1] is worth mentioning: it provided values of kn over 9, and thus it was discarded. next, with our chosen parameters, we needed to check which kind of data set network, either global or local, may lead to better results. we ran series of tests on the selected neural networks with a global and local data set network of observatories (table 2). to decide which the best neural network for our aim is, we used the same criteria than above; we need a network able to produce low and high k values. we decided that the best neural network in our case is a three later network, with 20, 10 and 1 neuron per layer respectively. in all three network architectures, using a global data set network of observatories, the networks pro.g g g0 2k t k k1 2$recovering of k-indices from kp-indices figure 2. local distribution of magnetic observatories in the neighbourhood of our test station, livingston (liv), used in the option two in the training process of the network. number of layers neurons for layer kn range 2 layers [40 1] 0-7 [20 1] 1-5 3 layers [40 20 1] 0-7 [20 10 1] 0-8 [10 5 1] 0-5 4 layers [40 30 20 1] 0-13 [40 10 10 1] 0-7 [20 10 5 1] 0-6 [10 5 5 1] 1-7 data sets nn architecture kn range global [40 1] 1-4 [20 10 1] 1-6 [40 10 10 1] 1-4 local [40 1] 0-7 [20 10 1] 0-8 [40 10 10 1] 0-7 table 1. summary of the kn range for each network parameter set. table 2. summary of the kn range depending on data set and neural network architecture. duce k but they do not cover all the range of possible values. however, the local data set network of observatories does. so the option that leads to the best results is a local network of observatories. this is because when training the neural network with observatories from different regions, the network is not able to recognize the different latitudinal effects that affects the magnetic signatures of the different observatories, and tries to produce an average. the neural network can not correct the latitudinal effects on the basis of the input parameters. but when training the network with observatories from the same region, they are subjected to the same magnetospheric effects, and the neural network is able to recognize these patterns, because we used time and day of the year as input parameters and the neural network can correct the local time/seasonal variations. nevertheless, we must also verify the frequency distribution obtained by the neural network, the percentage of each class that the network obtains. we understand by class the subset of three-hourly periods having the same degree of magnetic activity determined by the value of k. we must also verify whether a neural network works better for high or low k values. figure 3 shows frequency distributions for a local data set network (grey bar), a global data set network (white bar), and the real frequency distribution of k at livingston (black bar). we compare kn obtained with a local data set network and kn obtained with a global data set network. clearly, the local data set network seems to be superior, especially for moderate k values. figure 3 illustrates another question: all these neural networks produce fewer kn = 0 than we expected. it seems that the neural network converts a significant number of kn = 0 into kn = 1. we detected this problem in a large variety of architectures of neural networks and with different data tests. on the basis of our empirical experience, we assume that the few number of kn = 0 is due to a systematic boundary problem of operational function of the selected network. the neural network gives us an integer number from 0 to 9, and if we do not apply this condition we can see that there is an important number of cases in which kn is around 0.5-0.7. so, taking kn<0.7 as the k=0 value, we can adjust our results with more accuracy. all the figures and results which we show below were obtained using these criteria. at the highest boundary, k = 9, we have another problem but from a different source. the neural network learns from cases but with three years of data only 0.6% of our training data set lies between k= 7 to k = 9. in this situation is remarkable that we have adequate predictions for these values. 3. results with the selected neural network, we represent the distribution of the differences between kn and k that the neural network gives for each class of real kindex at the magnetic station of livingston, figure 4. in this figure, for low values of k, the differences between kn and k are more than 90% centred around 0 and ±1. for k values greater than 5, the differences are biased to –1 value, with a ratio of more than 80% between 0 and ±1. in the strict sense, a good “hit” occurs when the difference between the k-computed by the network, kn, segarra and curto 6 figure 3. comparison between the frequency distribution of kn obtained through a local network (grey bar) and another kn obtained with a global network (white bar), and also k real frequency distribution in liv (black bar). figure 4. histograms with the percentage of differences between kn and k for each class of k-index. 7 and the k-real is zero. in order to determine if the results can be considered to be correct, we need to take into account that an error of ±1 may be accepted. such an error is “de facto” acceptable, mainly for low k values, since some uncertainty is involved in the determination of the k index mainly due to the problem in extracting the diurnal variation. a two-unit difference can already be considered problematic, and in the case of a three-unit difference, the error is unacceptable. in order to see the advantages of using the k of the neural network instead of directly using kp, we use the same kind of graphs, but now focusing on the differences between kp and k-real at livingston (figure 5). for all k values the differences are centered around 0, but have a distribution similarly to distribution showed in figure 4. to show the benefits to use kn we have to compare the frequency distributions of kp and kn (figure 6). we applied the same criteria used by bartels et al. [1939] to establish the k = 9 logarithmic scale for each observatory. we see that for low k values, the kp distribution matches better than kn to k values. for these ranges, when we have quiet solar conditions, this is especially significant because the diurnal variation is dominant in magnetograms, and this is usually the range with a greater error in the process of determining k. but for moderate and high magnetic activity (high k) when the most important situations in space weather happen -, the kn distribution is much better than the kp distribution when we compare them to the local k. on the basis of deviations between amplitudes in the frequency distribution, we can compare the differences between both distributions. if we take kp as the k-local, the difference rate is 3% for k=1, 11% for k=2, and the rate increases considerably for higher values of k (29% and 64% for k=4 and 5). however, if we take kn as the k-local, the difference rate is higher for low k (9.5% for k=1), but from k=2 upwards the difference lessens and keeps constant around 5% for k=2 to k=5. to show the advantages of our method, in the figure 7, we compare the frequency distributions of k values obtained with our method with those obtained with two different interpolation methods. we obtained k values for liv interpolating the k values of observatories close to liv. we checked two interpolating cases, recovering of k-indices from kp-indices figure 5. histograms with the percentage of differences between kp and k for each class of k-index. figure 6. comparison between the frequency distributions of kreal at livingston (black bar), kn calculated with the network (white bar) and kp (grey bar). figure 7. comparison between the frequency distribution of the longitudinal interpolation results (dark grey bar), the latitudinal interpolation results (light grey bar), kn calculated with the neural network (white bar) and the k-real at liv (black bar). one derived from a latitudinal interpolation and the other one from a longitudinal interpolation. we obtained k values of liv interpolating the k values of aia and trw observatories (light grey bar in figure 7), located around similar longitude. the other interpolation method implies the observatories of trw and czt (dark grey bar in figure 7), this two observatories are located around the same latitude. in the same figure, we plot the frequency distribution obtained by our method, kn (white bar in figure 7), and also the frequency distribution of the real k (black bar in figure 7). for all k ranges the distribution of kn match better the real values than the interpolation methods, specially for k = 1, k = 3 and k = 4. this comparison reinforces the goodness of the results obtained by the neural network. until now, this computation was applied to the case of livingston, and it is obvious that under conditions of disturbance, kn is a better choice than kp. at this point, it is worthy checking if the same neural network may be applied to other observatories in the same zone. thus, we had to check if our neural network works correctly at other observatories in the southern hemisphere. we could not take another observatory for testing directly, because we had already used all the available observatories for training. so, first we had to remove the data of the observatory we were interested in from the training set, and then we trained the network again and checked the new results. in figure 8, we represent the frequency distributions at 4 observatories, eyrewell (eyr), gnangara (gna), martin de vivies-amsterdam island (ams) and trelew (trw). the network responds well enough for all these observatories, showing the same features than livingston observatory. we can conclude that for the zone of middle latitudes in the south hemisphere, taking the kn calculated with the neural network is better than taking kp directly, especially in periods of magnetic activity. as this zone is very under populated with observatories, this method could be very useful. 4. conclusions k index is one of the most popular geomagnetic activity indicators. the aim of this research was to estimate a more accurate local k-index which would be especially useful for remote zones with not available observatories. our method adjusts the frequency distribution of local k better than when replacing the unknown local data with global indices kp, especially under conditions of moderate and high magnetic activity, which are, by far, the most interesting conditions for the modellers of space weather. we applied our method to the case of a remote observatory, livingstone, as a test. we demonstrated that under disturbance conditions this method is a better choice than kp. the same neural network is also able to produce appropriate values for a wide region under the same magnetic effects. in the future, the network would be improved by being trained with more cases of very high k-indices (k=8 or k=9) taking advantage of years with high geomagnetic activity. another interesting input variable to be considered might be the ordinal number of the year in the solar cycle or the wolf number, which segarra and curto 8 figure 8. comparison between the frequency distributions of k-real (black bar), kn (white bar) and kp (grey bar) for 4 observatories which are inside the influence zone of the network. 9 would allow us to relate the value of the indices to the phase of the solar activity cycle. these indices could be suitable to be introduced in regional models of space weather when local conditions are relevant because kn are statistically more accurate than the direct use of kp. acknowledgements. this research has been supported by spanish government projects ctm2008-03033-e, ctm2009-13843c02-01 and ctm2010-21312-c03-01 of mineco. and also supported by project 2014 sgr 175 of generalitat de catalunya, and the project 2014-url-trac-039 of ramon llull university obra social “la caixa”. the results presented in this paper rely 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(1996). neural networks a systematic introduction, § 7: the backpropagation algorithm, recovering of k-indices from kp-indices springer-verlag, berlin. rumelhart, d.e., g.e. hinton and r.j. williams (1986). learning representation by back-propagating errors, nature, 323, 533-536. segarra, a., and j.j. curto (2012). automatic detection of sudden commencements using neural networks, earth planets space, 65, 791-797. takalo, j., and j. timonen (1997). neural network prediction of ae data, geophys. res. lett., 24 (19), 2403-2406. uwamahoro, j., l.a. mckinnell and j.b. habarulema (2012). estimating the geoeffectiveness of halo cmes from associated solar and interplanetary parameters using neural networks, annales geophysicae, 30, 963-972. virjanen, a., a. pulkkinen and r. pirjola (2008). predictions of the geomagnetic k index based on its previous value, geophysica, 44 (1-2), 3-13. watthanasangmechai, k., p. supnithi, s. lerkvaranyu, t. tsugawa, t. nagatsuma and t. maruyama (2012). tec prediction with neural network for equatorial latitude station in thailand, earth planets space, 64, 473-483. wing, s., j.r. johnson, j. jen, c.-i. meng, d.g. sibeck, k. bechtold, j. freeman, k. costello, m. balikhin and k. takahashi (2005). kp forecast models, j. geophys. res., 110 (a04203); doi:10.1029/2004ja010500. wu, j.g., and h. lundstedt (1996). prediction of geomagnetic storms from solar wind data using elman recurrent networks, geophys. res. lett., 23, 319-322. wu, j.g., and h. lundstedt (1997). neural network modeling of solar wind-magnetosphere interaction, j. geophys res., 102 (a7), 457-466. zolesi, b., and l.r. cander (1998). advances in regional ionospheric mapping over europe, annali di geofisica, 41 (5/6), 827-842. corresponding author: antoni segarra, observatori de l’ebre (oe), csic universitat ramon llull, roquetes, spain; email: asegarra@obsebre.es. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. segarra and curto 10 << 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol. 48, 01, 05ok.qxd 135 annals of geophysics, vol. 48, n. 1, february 2005 key words crude oil – noble gas tracing – gangxi oil field – huanghua depression 1. introduction noble gases in natural gas have been widely applied in identifying natural gas genesis (jenden et al., 1989; xu yongchang et al., 1996a), migration (torgersen and kennedy, 1999; battani et al., 2000), gas source tracing (xu yongchang et al., 1998), interaction of the crust with the mantle (dai jinxing et al., 1995; tao mingxing et al., 1996), tectonics (oxburgh and o’nions, 1987; gulec et al., 2002), and geothermal flow (polyak et al., 1985). since crude oil is closely related to natural gas in formation, migration, and pool-forming of hydrocarbons, it could be deduced that detailed information on petroleum geology also exists in crude oil noble gases. however, it is a pity so far that little research has been done on the geochemistry of crude oil noble gases (pinti and marty, 1995). the purpose of this paper is to report he and ar isotopic values of the oil-dissolved gases and to discuss their application in tracing natural gas source, tectonics, geothermal flow and oil mailing address: dr. zhang xiaobao, state key laboratory of gas geochemistry, lanzhou institute of geology, chinese academy of sciences, 324 donggang west road lanzhou, 730000 gansu, people’s republic of china; email: zhangs@sun.ihep.ac.cn crude oil he and ar isotopic characteristics and their geochemical significance: an example from the gangxi oil field in the huanghua depression zhang xiaobao (1), ma liyuan (1), chen jianping (2), sun mingliang (1), tu jianqi (2), li xiufen (1) and song chenpeng (1) (1) state key laboratory of gas geochemistry, lanzhou institute of geology, chinese academy of sciences, gansu, people’s republic of china (2) the research institute of exploration and development, china national petroleum corporation, beijing, people’s republic of china abstract sampled from the gangxi oil field in the huanghua depression, bohaiwan basin, the crude oil he and ar isotopic compositions and their geochemical significance were investigated. the result shows that i) the 3he/ 4he values of the six oil samples free from air contamination indicate a mantle helium contribution ranging between 13.9% and 32.8% and averaging 24.1%, which means that the gangxi oil field once had a tectonic background of the mantle-derived helium input and higher geothermal flow with an average of 75.0 mwm–2; ii) deduced from the 40ar/ 36ar aging effect the gangxi oil should be derived from the tertiary; iii) water injection is the main reason for the air contamination for the gangxi oil noble gases and the relations between 3he/ 4he and 4he/ 20ne as well as 40ar/ 36ar and 4he/ 20ne are of an applicative potential in estimating the water injection effect and the connectivity of producing formations. the search for the crude oil he and ar isotopes provides a new approach to inferring natural gas and oil sources, tectonic backgrounds, geothermal flow and water injection effect. 136 zhang xiaobao, ma liyuan, chen jianping, sun mingliang, tu jianqi, li xiufen and song chenpeng source as well as their potential in identifying the water injection effect in the gangxi oil field of the huanghua depression. 2. geological background the indo-china movement during the mesocenozoic is thought to be a very important one which greatly changed the pre-mesozoic tectonic framework of china by subduction of the pacific plate under the asia-euro plate (tao mingxing et al., 1996; xu yongchang et al., 1998). after the movement china could be tectonically classified into the eastern rift tectonically active area, the middle craton tectonically stable area, the western tectonically substable area, and the qinghai-tibet plate collision compression area (tao mingxing et al., 1996; xu yongchang et al., 1998), of these areas the eastern rift tectonically active area could be further separated into the near-shore rift system and the cathaysion rift system by the tancheng-lujiang deep fault belt (tao mingxing et al., 1996; xu yongchang et al., 1998) (fig. 1). the near-shore rift system is located to the east of the tancheng-lujiang deep fault belt and the cathaysion rift system to the west. many petroleum-bearing basins distribute in these two systems. many mantle helium anomalies, which are considered to be associated with the tancheng-lujiang deep fault belt and mantle defig. 1. tectonic location of the huanghua depression in the huaxia rift system, china (ishibashi et al., 2002): 1 – subei-south huanghai basin; 2 – donghai basin; 3 – taixinan basin; 4 – zhujiangkou basin; 5 – songliao basin; 6 – bohaiwan basin; 7 – jianghan basin; 8 – sanshui basin; 9 – ordos basin; 10 – sichuan basin; 11 – baise basin. fig. 2. tectonic position of the gangxi oil field in the huanghua depression (song mingliang, 1991): 1 – oil field; 2 – studied area; 3 – fault; 4 – present shoreline; 5 – boundary of depression; 6 – present 5 m shoreline. 137 crude oil he and ar isotopic characteristics and their geochemical significance fig. 3. cross section of the gangxi oil field (song mingliang, 1991): nm-upper tertiary minghuazhen formation; ng-upper tertiary guantao formation; ed – lower tertiary dongying formation; mz – mesozoic; c-p – carbo-permian; o – ordovician; 1 – mudstone; 2 – sandstone; 3 – sandy conglomerate; 4 – conglomerate; 5 – limestone; 6 – oil; 7 – fault. 3. experiment glass was used to make a sample tube shown in fig. 4. the tube has two dependable airproof valves with the same shape on both sides and a volume of 100 ml3, with a 5 mm inner diameter and a 7 mm outer diameter of the piston tube. the upstream end of the tube is linked with a latex tube connecting with a mouth of a wellhead. the two valves of the tube were open first, then the switch of the mouth of the wellhead was opened and oil was allowed to flow through the tube. to remove air contamination, the tube was washed by oil for 5 min, then the switch of the wellhead and the valve on the downstream end of the sampling tube were closed simultaneously, and finally the valve on the upstream end of the tube was closed. after sampling, the two valves of the tube were fixed using an adhesive plaster. ten oil samples were collected from the gangxi oil field. to correlate the oil with natural gas in noble gas geochemistry, seven natural gas samples were collected from the lower tertiary dongying formation in the tangjiahe oil field, the baishuitou oil field, and the first member of the shahejie formation and the guantao formation and the minghuazheng formation in the madong oil field, respectively. of the samples the oils and natural gases of the xi 3-7-1 well and the xi 34-5-1 well were collected from the same productive formations. in the laboratory a glass tube filled with oils linked with a vacuumed empty glass container, was then connected with the sample inlet of a vg5400 mass spectrograph. then the downstream end of the vacuumed glass container was opened and the oil in the glass container was degassed. after the oil-dissolved gas was exposed to a zircon-aluminum getter, a titanium sponge gassing, have been found in natural gases of these basins (tao mingxing et al., 1996; xu yongchang et al., 1998). of which the bohaiwan basin is located in the huaxia rift system while the huanghua depression is a tectonic division of the basin (fig. 1). the oil in the huanghua depression is mainly derived and reservoired in the tertiary. the shahejie formation and the dongyin formation developed in the lower tertiary and the guantao formation and the minghuazheng formation in the upper tertiary from the lower to upper. the oil source rock is mainly distributed in the shahejie formation. the gangxi oil field is situated in the huanghua depression (fig. 2). the oil field occurs in a drape anticline trap formed by continuous arching of the pre-tertiary base rocks and being covered by the tertiary (fig. 3). oils of the oil field are preserved and sealed in the upper tertiary guantao formation and minghuazheng formation. the evidence of biomarkers in oils shows that the oil is chiefly derived from the lower tertiary shahejie formation source rocks. fig. 4. sample glass bottle. a – outlet tube; b – chamber; c – piston hole; d – piston tube; v – valve. 138 zhang xiaobao, ma liyuan, chen jianping, sun mingliang, tu jianqi, li xiufen and song chenpeng t ab le i. c ru de o il a nd n at ur al g as n ob le g as i so to pe s in t he h ua ng hu a oi l fi el d. o il f ie ld w el l s am pl e s tr at um d ep th w at er s am pl e 4 h e/ 20 n e r = 3 h e/ 4 h e r /r a 40 a r/ 36 a r m an tl e g eo th er m al no . ty pe (m ) in je ct io n (* ) co nt ai ne r he li um fl ow % m w m –2 g an gx i 35 -5 o il n g, n m 10 72 .2 -1 43 5. 0 in je ct io n g la ss b ot tl e 2. 3 (1 .6 3 ± 0. 04 )× 10 –6 1. 16 29 9 15 .0 2 72 .1 15 9 o il n g, n m 10 48 .5 -1 15 2. 4 u nin je ct io n g la ss b ot tl e 4 (1 .8 3 ± 0. 05 )× 10 –6 1. 30 30 5 16 .8 0 72 .9 152 o il n m 11 84 .1 -1 20 8. 6 u nin je ct io n g la ss b ot tl e 10 (1 .4 5 ± 0. 05 )× 10 –6 1. 04 29 7 13 .1 7 71 .2 35 -5 -1 o il n g, n m 11 90 .0 -1 44 5. 5 in je ct io n g la ss b ot tl e 21 (1 .2 8 ± 0. 06 )× 10 –6 0. 91 11 .4 8 70 .2 14 -8 o il n m 91 3. 6 -9 15 .6 u nin je ct io n g la ss b ot tl e 23 8 (3 .6 2 ± 0. 09 )× 10 –6 2. 58 35 0 32 .8 5 77 .5 15 -9 -1 o il n g 13 40 .0 -1 34 4. 0 in je ct io n g la ss b ot tl e 28 5 (2 .8 6 ± 0. 07 )× 10 –6 2. 04 31 7 25 .9 3 75 .9 17 6 o il n m 10 04 .5 -1 11 0. 0 u nin je ct io n g la ss b ot tl e 52 7 (3 .5 3 ± 0. 09 )× 10 –6 2. 52 49 1 32 .0 3 77 .4 14 -8 -4 o il n g 13 51 .5 -1 40 4. 1 in je ct io n g la ss b ot tl e 13 0 (2 .7 8 ± 0. 09 )× 10 –6 1. 98 55 5 25 .2 0 75 .7 34 -5 -1 o il n g 13 69 .4 -1 46 9. 9 in je ct io n g la ss b ot tl e 11 0 (1 .6 1 ± 0. 05 )× 10 –6 1. 15 38 7 14 .5 6 71 .9 371 o il n g, n m 10 45 .0 -1 20 7. 0 u nin je ct io n g la ss b ot tl e 14 1 (1 .5 4 ± 0. 04 )× 10 –6 1. 10 38 2 13 .9 2 71 .6 34 -5 -1 n at ur al n g 13 69 .4 -1 46 9. 9 in je ct io n s te el b ot tl e 99 .5 (1 .5 2 ± 0. 04 )× 10 –6 1. 08 36 2 13 .7 4 71 .5 ga s 371 n at ur al n g 10 45 .0 -1 20 7. 0 u nin je ct io n s te el b ot tl e 27 1 (1 .6 6 ± 0. 04 )× 10 –6 1. 18 31 1 15 .0 1 72 .1 ga s t an gj ia he 55 6 n at ur al e d 3 25 40 .6 -2 62 4. 1 u nin je ct io n s te el b ot tl e 25 .5 (4 .4 2 ± 0. 16 )× 10 –7 0. 32 31 3 62 .8 ga s 56 2 n at ur al e d 1 21 80 .0 -2 56 0. 0 u nin je ct io n s te el b ot tl e 16 7. 7 (2 .9 1 ± 0. 09 )× 10 –7 0. 21 36 0 59 .9 ga s 77 -1 k n at ur al e d 3 26 16 .8 -2 62 8. 9 u nin je ct io n s te el b ot tl e 97 .3 (3 .1 4 ± 0. 09 )× 10 –7 0. 22 32 9 60 .4 ga s m ad on g 14 -1 8 n at ur al b an i i + 34 99 .3 -3 76 3. 6 s te el b ot tl e 92 .5 (1 .3 0 ± 0. 04 )× 10 –6 0. 93 33 6 70 .4 ga s + b an i ii b ai sh ui to u 21 -2 n at ur al e s 1 , 23 01 .0 -2 65 6. 2 u nin je ct io n s te el b ot tl e 10 4 (5 .4 6 ± 0. 14 )× 10 –7 0. 39 33 0 64 .3 ga s b an i ii l an zh ou a ir 0. 31 8 (1 .4 0 ± 0. 03 )× 10 –6 1 29 5. 5 (* ) a ft er t he e xp lo it at io n re po rt o f th e d ag an g oi l fi el d. pump removed the active gases such as h2, o2, h2o and hydrocarbon gases. ar was then trapped in a charcoal trap held at a liquid n2 temperature. however, pure he was admitted to the analytical system of the mass spectrometer. resolving power of about 600 at 1% of the peak height was attained for complete separation of the 3he+ beam from hd+ and h3+ beams in the system. then ar was desorbed from the charcoal finger by warming and was also allowed into the analytical system. the standard gas was measured before analyzing samples; measurement results of samples were normalized to the standard gas, which was made by purifying air and periodically examined by air during measurement. all the analytical results are reported on the basis of the atmospheric values internationally used as standard, whose 3he/ 4he ratio is 1.40 × 10–6, and 40ar/ 36ar ratio is 295.5. all the results of helium were given in the 3he/ 4he ratio and r/ra ratio where r is the 3he/ 4he ratio of the sample and ra is that of air. the precise description of the measurement has been published (song mingliang, 1991; song mingliang, 1995; xu yongchang et al., 1998). the results of the measurement are listed in table i. it should be emphasized that based on our practice the samples collected using the method described above should be prepared and analyzed as quickly as possible, because the samples could be air contaminated if they are collected after more than 15 days. 4. discussion 4.1. 3he/ 4he versus 4he/ 20ne the two stable isotopes of helium, 3he and 4he, have different origins. 3he is essentially primordial and retained in the earth’s interior at the time of its formation whereas 4he is mainly produced in the crust by the decay of uand th-series isotopes. the different layers of the earth have different helium isotopic values. the atmospheric helium (a) has a 3he/ 4he ratio of 1.384 ± 0.0004×10–6 (clarke et al., 1976), the crust helium (c) has a 3he/ 4he ratio 2 × 10–8, and the mantle helium (including type m and type p) has a 3he/4he ratio greater than 1.1 ×10–5 (kaneoka and takaota, 1985) . therefore, 3he/ 4he could be used to trace helium sources. generally, 4he/ 20ne could be utilized to value the degree of air contamination in natural gases (wakita et al., 1990). through the search for 3he/ 4he and 4he/ 20ne, the source of helium could be discussed after the magnitude of air contamination has been estimated. figure 5 shows the relation of 3he/ 4he and 4he/ 20ne of the gangxi oils. in order to make a correlation, fig. 5 also shows the he and ar isotopic data of natural gases from the gangxi oil field and other oil fields in the huanghua depression. table i demonstrates that the oils and natural gases from the same productive formations as the xi 3-7-1 well and the xi 34-5-1 well fig. 5. plot of 3he/ 4he versus 4he/ 20ne of the crude oils from the gangxi oil field. 139 crude oil he and ar isotopic characteristics and their geochemical significance 140 zhang xiaobao, ma liyuan, chen jianping, sun mingliang, tu jianqi, li xiufen and song chenpeng have a similar distribution of he and ar isotopes, suggesting that the correlation of the crude oil he and ar isotopes with the natural gas he and ar is feasible. table i and fig. 5 display that the noble gases in natural gases and crude oils could be divided into the crust-mantle composite type, the crust type and the air-mixed type based on 3he/ 4he and 4he/ 20ne ratios. 4.1.1. crust-mantle composite type the 4he/ 20ne ratio of this type of noble gases ranges from 99.5 to 527.0, averaging 225.2 (8). most of the samples have 4he/ 20ne values three orders of magnitude greater than that of air (4he/ 20ne = 0.318) indicating that air contamination is negligible to the 3he/ 4he ratios of the samples. the 3he/ 4he ratios of these samples vary from 1.52×10–6 to 3.62 × 10–6 with an average of 2.39×10–6 (8). the r/ra ratios (where r/ra represents a 3he/ 4he ratio of a sample to that of air) of these samples ranges from 1.1 to 2.6 with a mean value of 1.7 (8). these show a mantle helium contribution. based on the calculation formula (xu yongchang et al., 1998) of the mixed percent of the crust-mantle end members, the percentage of the mantle-derived helium of these samples ranges from 13.7 to 32.8%, averaging 21.7%. the noble gases of the six oil samples from the gangxi oil fields are attributed to this type (table i and fig. 5). the basic conditions (dai jinxing et al., 1995; tao mingxing et al., 1996; xu yongchang et al., 1998) for the input of mantle helium into petroleum-bearing basins include that i) the mantle bursts upward and the crust thins, ii) deep faults develop, resulting in mantle degassing and serving as a passage for mantle helium migration. the existence of the mantle-derived helium in the gangxi oils of the huanghua depression indicates that the oil field is of the above-mentioned tectonic conditions. since the mesozoic the mantle underlying the petroleum-bearing basins in the cathaysion rift belt and the near-shore rift belt in eastern china (fig. 1) has uplifted and the crust has notably thinned, halfgraben-like basins have formed in the shallow crust and expanding faults have developed in the basins. in the mantle cut by the tancheng-lujiang deep fault belt located between the cathaysion rift belt and the near-shore rift belt, the pressure decreases due to deep cutting of the fault belt and largescale mantle degassing occurs. in addition the tancheng-lujiang deep fault belt and its accessary base faults provide the migration of the mantle helium with passages. therefore, a series of mantle helium anomalies (r/ra > 1), even some mantle helium commercial accumulations (xu yongchang et al., 1996a), are found in the basins or depressions on both sides of or in the fault belt. the huanghua depression is located in the cathaysion rift belt, west of the fault belt, therefore, it is also of the tectonic background for the input of the mantle-derived helium (dai jinxing et al., 1995; tao mingxing et al., 1996; xu yongchang et al., 1998). similar to the natural gas 3he/ 4he ratio, the crude oil 3he/ 4he ratio could also be used to estimate the value of geothermal flow. geothermal flow directly indicates the heat in the earth, and it mainly consists of radiogenic heat in the crust and the heat from the upper mantle. the crude oil 3he/ 4he ratio is also associated with the crust radio elements and mantle matter. therefore, the crude oil 3he/4he ratio can be used to indicate geothermal flow as well. table i lists values of geothermal flow in the gangxi oil field, and then calculates using the relation formula, q = 6.993ln(3he/ 4he) + 165.16 (polyak et al., 1985), between geothermal flow (q) and crude oil 3he/ 4he ratios. table i demonstrates that the values of geothermal flow range from 71.6 mwm–2 to 77.5 mwm–2, averaging 75.0 mwm–2 (6), which are coincident with the higher geothermal flow in the petroleum-bearing basins in eastern china (polyak et al., 1985). geothermal flow data are of important reference value to thermal evolution of source rocks, petroleum formation and petroleum resource assessment. 4.1.2. crust type this type of noble gases in oils has 4he/ 20ne values ranging from 25.5 to 167.7 and averaging 97.4 (5), most of them are three orders of 141 crude oil he and ar isotopic characteristics and their geochemical significance magnitude higher than that of air (4he/ 20ne = = 0.318), suggesting a negligible mixture of air. the 3he/ 4he ratios of the oils range from 2.91×10–7 to 1.30×10–6, averaging 5.79×10–7 (5), while the r/ra ratios from 0.2 to 0.9, averaging 0.4 (5). these are characteristic of a dominant crust radiogenic origin with a minor contribution of the mantle. the natural gases from the tangjiahe, madong and baishuitou oil field are attributed to this type, showing that these oil fields have no favorable conditions of the input of the mantle helium. additionally based on the calculated results of the relation formula between geothermal flow (q) and 3he/ 4he ratios, the geothermal flow values of these fields are lower than those of the gangxi oil field, ranging from 59.9 mwm–2 to 70.4 mwm–2, averaging 63.6 mwm–2 (5). in fig. 5 the 3he/ 4he and 4he/ 20ne ratios of the crustmantle composite type and the crust type constitute a positively linear correlation, reflecting the increase in the mantle helium input. 4.1.3. air-mixed type different from the above crust-mantle composite type and the crust type, the 4he/ 20ne ratios of this type of noble gases ranges from 2.3 to 21.0, averaging 9.3 (4), which are only one to two orders of magnitude larger than that of air (air 4he/ 20ne = 0.318). this in combination with the 40ar/ 39ar distribution near to the air 40ar/ 39ar ratio, suggests that this type of the noble gases in oils is characteristic of the mixture of the crust noble gas with the air noble gas. the oil for the xi 35-51 well has a 3he/ 4he value of 1.28×10–6, reflecting a mixture of the crust-derived noble gas and the air noble gas. the oils from the gang 159 well, xixin35-5 well and xi 1-5-2 well have 3he/ 4he values of 1.83×10–6, 1.63×10–6 and 1.45×10–6, respectively, which are higher than that of air, indicating a mixture of the crust-derived, mantle-derived and airderived noble gases. based on the relation formula between the 3he/ 4he and 4he/ 20ne ratios (ishibashi et al., 2002) the proportion of noble gases without air contamination could be estimated. by estimation, the percent of the mantle – derived noble gases varies from 11.48% to 16.80%, averaging 14.12% (4). in comparison with noble gases of the crust type and the crust-mantle composite type, an obvious feature is that noble gases of this type have a mixture with air. the noble gas datum of the oil from the gang 556 well is plotted between the crude oil noble gases of the air-mixed type and the crustderived type (fig. 4), representative of a mixture of the air mixed and the crust-derived noble gases, namely the proportion of the air-derived noble gases is lower than that of the air-mixed noble gas. 4.2. 40ar/36ar versus 4he/20ne 36ar is considered the original component of the earth and 40ar is formed by the decay of 40k, whose rate of production has an exponentially functional relation with time, therefore the natural gas 40ar/ 36ar ratio has an evident correlation with source rock age, i.e., the aging effect (liu wenhui and xu inching, 1993). by using the natural gas 40ar/ 36ar aging effect, source rock age can be estimated. the total ar concentration in natural gas ranges from n×10–3 to n×10–5, however, that in air is 0.93%, near 1%. therefore if 1% of air ar is mixed in natural gas, its content will be several times or several ten times greater than that in natural gas, so that the natural gas 40ar/ 36ar ratio will obviously be reduced (xu yongchang et al., 1998). thus the 40ar/ 36ar ratio is a sensitive indicator of air contamination in natural gas. as mentioned above, the 4he/ 20ne ratio is also an effective indicator of air contamination of crude oil and natural gas, therefore, through the search for 40ar/ 36ar and 4he/ 20ne ratios, gas source age can be estimated after air contamination is excluded. figure 6 displays the relation between crude oil 40ar/ 36ar and 4he/ 20ne ratios. for convenient correlation, the noble gas 40ar/ 39ar data of the other oil fields in the huanghua depression are also compiled in fig. 6. table i and fig. 6 show that the natural gas and crude oil noble gases can be classified into the aging type and the air mixed type based on 40ar/ 36ar and 4he/ 20ne ratios. 142 zhang xiaobao, ma liyuan, chen jianping, sun mingliang, tu jianqi, li xiufen and song chenpeng 4.2.1. aging type the 4he/ 20ne ratio of this type of noble gases ranges from 92.5 to 527.0, averaging 188.6 (12), which are two or three orders of magnitude higher than the air 4he/ 20ne ratio (air 4he/ 20ne = 0.318). coupled with 40ar/ 36ar distribution, the influence of air contamination on 40ar/ 36ar of this type can be negligible. the 40ar/ 36ar ratio of this type varies from 310.7 to 555.0 with an average of 375.9 (12). based on the mantle ar/he ratio (0.83) (xu yongchang et al., 1998) and the estimation that the highest mantle he proportion of the gangxi oil accounts for 32%, the calculated mantle ar concentration is 2.5 × 10–5; therefore, its influence on the sample ar contribution can be negligible. this is supported by the fact that the 40ar/ 36ar ratio of the most gangxi crude oils and natural gases is similar to those in the tanjiahe, madong, and baishuitou oil fields where the crust noble gas is dominant in crude oils. therefore, the crude oil 40ar/ 39ar ratio from the gangxi field is mainly of a crustal origin and can reflect an aging effect. by using 40ar/ 36ar the source rock age of the gangxi oil field can be estimated. according to the conclusions reached by xu yongchang et al. (1998) who summarized the 40ar/ 36ar distributions of the chinese natural gases from the source rocks with different geological ages, the 40ar/36ar ratio of the chinese tertiary natural gases varies from 302 to 600. the six crude oils from the gangxi oil field have 40ar/ 36ar ratios from 317.0 to 555.0, averaging 413.7 (6), therefore, they should be derived from the tertiary source rocks, which is consistent with the conclusion reached according to the oil source correlation of biomarkers. 4.2.2. air-mixed type different from the above type, the 4he/ 20ne ratio of this type of crude oil noble gases ranges from 2.3 to 21.0, averaging 9.3 (4), which is only one to two orders of magnitude larger than that of air (4he/ 20ne = 0.318). in combination with their 40ar/ 36ar ratios, it could be inferred that this type of the crude oil noble gases is heavily contaminated by air. the 40ar/ 36ar ratios of the oils from the gang 195 well, the xixin 35-5 well and the xi 1-5-2 well are 305, 299 and 297 respectively, near to that of air, characteristic air contamination. the noble gas datum of the natural gases from the gang 556 well in fig. 6 falls between the air-mixed type and the aging type, representative of a mixture of the two types of the crude oil noble gases. 4.3. relation of crude oil noble gases to water injection through the research on the noble gases of over 500 chinese natural gas samples, xu fig. 6. plot of 40ar/ 36ar versus 4he/ 20ne of the crude oil from the gangxi oil field. 143 crude oil he and ar isotopic characteristics and their geochemical significance yongchang (xu yongchang et al., 1998) concluded that the natural gas noble gases from wells without water injection were mainly derived from the mantle and the crust, therefore, the air derived noble gas in these natural gases could be negligible. recently, we analyzed the noble gas isotopes of natural gases from 18 wells without water injection in the zhongbu gas field and the suligemiao gas field in the ordos basin and found that these samples had 4he/ 20ne values larger than 1000, suggesting a negligible contribution of the air derived noble gases to these natural gases and further supporting the above view. in this contribution noble gases in the oils and gases from the madong, baishuitou and tangjiahe oil fields where water is not injected into wells are not air contaminated (table i). however, the gangxi oil field has a long developing history and 2/3 of its oil reserves have been exploited. to improve oil recovery, the measure of water injection has been taken in some wells. during water injection in the gangxi oil field air noble gases are inevitably carried into wells by water, which contaminates the noble gases in the oils and gases. besides, from the analytical results in table i it can be seen that the situation of air contamination in the crude oil noble gases from the gangxi oil field is complicated. of the wells with water injection, the noble gases in oil and gases from some wells are attributed to an air-mixed type, e.g., xixin35-5 well and xi35-5-1 well, however those from the other wells are attributed to a crust type or a crust-mantle composite type, e.g., xi159-1 well, xi14-8-4 well and xi34-5-1 well (table i, figs. 3 and 4). of the wells without water injection, the noble gases from oils and gases of the xi14-8well, gangxi176well and xi3-7-1 are not air contaminated, but those from gang159 well and xi1-5-2 well are air contaminated (table i, figs. 3 and 4). therefore it could be deduced that whether crude oil noble gases had been air contaminated depends not only on water injection but also on the connectivity of productive formations and the effect of water injection. this inspires us that the noble gases from oils and gases might be of a great applicable potential in identifying the possibility of water injection, the connectivity of productive formations, and the effect of water injection. 5. conclusions taking the gangxi oil field as an example, the characteristics of the crude oil noble gases were investigated and their tracing implication were discussed. the research shows that: i) the 3he/ 4he ratio of the six crude oils from the gangxi oil field is characteristic of a mantle contribution. the percent of the mantlederived gases averages 24.1%, suggesting that the gangxi oil field has a tectonic background for the contribution of the mantle helium and its geothermal flow values average 75.0 mwm–2. ii) the six crude oils from the gangxi oil field have an average 40ar/ 36ar ratio of 413.7, which is larger than that of air (297.5), basically characteristic of an aging effect. based on this inference, the gangxi oil should be derived from the tertiary source rocks. iii) the existence of the air-mixed type of the crude oil noble gases in the gangxi oil field could be attributed to water injection. the crude oil 4he/ 20ne, 3he/ 4he and 40ar/ 36ar distributions are of great potential in investigating the water injection effect and the linkage of productive formations. the research also shows that the tracing of oils by noble gases is similar to that of natural gases. by using crude oil 3he/ 4he, 40ar/ 36ar and 4he/ 20ne ratios one cannot only trace oil gas sources, but also gain information on the water injection effect during the development of oil fields. researching crude oil noble gas provides a new tool for identifying gas source, tectonic backgrounds, geothermal flow, gas/oil sources and water injection effect. acknowledgements this contribution was financed by the chinese national natural science foundation (grant no. 40172053), important direction project of knowledge innovation in resource and environment field, chinese academy of sciences (grant no. kzcx3-sw-128), and the foundation of the state key laboratory of gas geochemistry. we also thank the research institute of petroleum exploration and development, dagang petroleum group, for assistance in sampling in oil fields. 144 zhang xiaobao, ma liyuan, chen jianping, sun mingliang, tu jianqi, li xiufen and song chenpeng references battani, a., p. sarda and a. prinzhofer (2000): basin scale natural gas source, migration and trapping traced by noble gases and major elements: the pakistan indus basin, earth planet. sci. lett., 181, 229-249. clarke, w.b, w.j. jenkins and j. top (1976): determination of the tritium by mass spectrometric measurement of 3he, int. j. appl. radiat. isot., 27, 515-522. gulec, n., d.r. hilton and h. mutlu (2002): helium isotope variations in turkey: relationship to tectonics, volcanism and recent seismic activities, chem. geol., 187, 129-142. ishibashi, j., m. sato, y. sano, h. wakita, t. gamo and w.c. shanks (2002): helium and carbon gas geochemistry of pore fluids from the sediment-rich hydrothermal system in escanaba trough, appl. geochem., 17, 1457-1466. jenden, p.d. and i.r. kaplan (1989): origin of natural gas in the sacramento basin, california, am. ass. petrol. geol. bull., 73, 431-453. jinxing, d., s. yan, d. chunsheng, c. anfu, s. mingliang and l. yongshen (1995): inorganic gases and their pool-forming conditions in eastern china (beijing: science press), 62-200 (in chinese). kaneoka, i. and n. takaota (1985): noble gas state in the earth’s interior-some constraints on the present state, chemical. geol., 52, 75-95. mingliang, s. (1991): mass spectrum of helium isotope in natural gases, bull. anal. test, 19, 50-55 (in chinese). mingliang, s. (1995): mass spectrum of helium isotope in water, acta mass spectrum, 16, 34-38 (in chinese). mingxing, t., x. yongchang, s. ping and l. wenhui (1996): tectonic, geochemical characteristics and pool-forming conditions in accumulation belts of mantle gas pools in eastern china, sci. ch., 26 (d), 531536 (in chinese). oxburgh, e.r. and r.k. o’nions (1987): helium loss, tectonics and the terrestrial heat budget, sci., 237, 1583-1588. pinti, d. and b. marty (1995): noble gases in crude oils from the paris basin, france: implications for the origin of fluids and constraints on oil-water-gas interactions, geoch. cosmoch. acta, 59, 3389-3404. polyak, b.g., e.m. prasolov, v. cermak and a.b. verkhovskiy (1985): isotopic compositions of noble gases in geothermal fluids of the krusne hary mts., czechoslovakiy, and the nature of the local geothermal anomaly, geochim. cosmochim. acta, 49, 695-699. poreda, r.j., p.d. jenden, i.r. kaplan and h.craig (1986): mantle helium in sacramento basin natural gas wells, geochim. cosmochim. acta, 50, 2847-2853. torgersen, t. and b.m. kennedy (1999): air-xe enrichments in elk hills oil field gases: role of water in migration and storage, earth planet. sci. lett., 167 (3/4), 239-253, 1999. wakita, h., y. sano, a. urabe and y. nakamura (1990): origin of methane-rich natural gas in japan: formation of gas fields due to large-scale submarine volcanism, appl. geochem., 5, 263-278. wenhui, l. and x. yongchang (1993): significance of he and ar isotopic compositions in natural gases, chin. sci. bull., 38, 818-821. yongchang, x., s. ping, t. mingxing and l. wenhui (1996a): mantle helium geochemistry in natural gases in eastern chinese petroleum province, i. a new type of helium resource: industrial accumulations in sedimentary layers in the crust, sci. ch., 26 (d), 1-8 (in chinese). yongchang, x., s. ping, t. mingxing and l. wenhui (1996b): mantle helium geochemistry in natural gases in eastern chinese petroleum province, ii. helium, argon and hydrocarbons, sci. ch., 26 (d), 187-192 (in chinese). yongchang, x., s. ping, l. wenhui, t. mingxing, s. mingliang and d. jiangou (1998): noble gas geochemistry in natural gases (science press, beijing), 97-227 (in chinese). 439_454.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 439 key words ground motion indicators – attenuation relationships – umbria-marche area 1. introduction attenuation relations are mathematical functions relating a strong motion parameter to parameters characterizing: the earthquake, the propagation medium, local site geology and structure (campbell, 1985). in conventional ground motion models the earthquake source is represented by the magnitude; the effects of wave propagation from the earthquake source to the site region are specified by a distance; and the effects of the soil conditions by a site category (so-merville, 2000). accelerometric recordings are used to derive scaling equations that predict earthquake-induced ground motion in a certain analysis of strong ground motions to evaluate regional attenuation relationships gaetano zonno and valentina montaldo istituto nazionale di geofisica e vulcanologia, sezione «pericolosità e rischio sismico», milano, italy abstract italian attenuation relationships at regional scale have been refined using a data set of 322 horizontal components of strong ground motions recorded mainly during the 1997-1998 umbria-marche, central italy, earthquake sequence. the data set includes records generated by events with local magnitude (m l ) ranging between 4.5 and 5.9, recorded at rock or soil sites and epicentral distance smaller than 100 km. through a multiple step regression analysis, we calculated empirical equations for the peak ground acceleration and velocity, the arias intensity and for the horizontal components of the 5% damped velocity pseudo response spectra, corresponding to 14 frequencies ranging from 0.25 to 25 hz. we compared our results with well known predictive equations, widely used on the national territory for probabilistic seismic hazard analysis. the results obtained in this study show smaller values for all the analyzed ground motion indicators compared to other predictive equations. mailing address: dr. gaetano zonno, istituto nazionale di geofisica e vulcanologia, sezione «pericolosità e rischio sismico», via bassini 15, 20133 milano, italy; e-mail: zonno@mi.ingv.it area starting from a simplified empirical model. usually attenuation relationships are revised after an earthquake as new data are made available to the scientific community. the most recent scaling law for italy was published in 1996 by sabetta and pugliese, just before the earthquake sequence that hit central italy in 1997-1998. the purpose of this study is to generate new attenuation relationships using the strong motion data recorded during that sequence. the main criticism appointed to attenuation function is that usually regional data sets are statistically not significant, while national or global data sets often include records from very different seismotectonic settings that are not mixable (decanini et al., 2001). nevertheless, the amount of records available for the umbriamarche area in central italy is now sufficiently large to perform statistical analysis at regional scale. as fig. 1 shows, the data used in this study are nearly twice the number of records used by sabetta and pugliese (1996) for the whole national territory. attenuation relationships are used to estimate the pertinent characteristics of real ground motion, 440 gaetano zonno and valentina montaldo such as amplitude, frequency content and duration. several ground motion parameters have been proposed to quantify these characteristics, among which we used peak ground acceleration and velocity to describe amplitude, response spectra to account for frequency content, while arias intensity (arias, 1970) reflects all the characteristics (kramer, 1996). attenuation relations are used in the probabilistic seismic hazard analysis (psha) to determine the seismic hazard at a given site, taking into account a number of factors like seismic sources model, attenuation law and the uncertainties in the model parameters. usually the results of psha are hazard maps showing the ground motion level that will not be exceeded in a given time lag, with a given probability. other ground motion measures of engineering significance derived from psha are the uniform hazard spectra (uhs) and the ground motion time history (naeim and lew, 1995; bommer et al., 2000). in this study, we will focus on the derivation of the ground motion attenuation relationships and will compare our results with other predictive relations already used in italy. 2. ground-motion data the strong ground motions available for the umbria-marche area, central italy, include records provided by several italian institutions, namely servizio sismico nazionale (ssn), ente nazionale per l’energia elettrica (enel, now sogin) and ente per le nuove tecnologie, l’energia e l’ambiente (enea), which are all available in digital format (ssn and enel, 1998; imperial college et al., 2000). for this study, we used data from events occurred at different times: norcia-valnerina earthquake in 1979, gubbio earthquake in 1984 and the 19971998 umbria-marche earthquakes. all these earthquakes were generated by normal or oblique faults parallel to the apennine at focal depth ranging between 2.0 and 8.7 km (table i). the parameters listed in table i are derived from deschamps et al. (1984) for norcia-valnerina earthquake, haessler et al. (1988) for gubbio earthquake and cattaneo et al. (2000) for the 1997-1998 umbria-marche earthquakes. among all the data available for this area, we selected only the records produced by events table i. list of the umbria-marche, central italy, earthquakes used in this study. earthquake name date time latitude longitude m l depth n. records norcia-valnerina 19.09.79 21:35 42n43.80 12e57.60 5.9 6.0 7 gubbio 29.04.84 05:02 43n15.00 12e31.20 5.2 7.0 7 umbria-marche 03.09.97 22:07 43n00.92 12e52.77 4.5 8.7 1 umbria-marche 26.09.97 00:33 43n01.24 12e50.83 5.6 3.8 15 umbria-marche 26.09.97 09:40 43n01.41 12e50.83 5.9 6.5 21 umbria-marche 03.10.97 08:55 43n02.05 12e50.52 5.4 5.7 11 umbria-marche 04.10.97 16:13 42n55.96 12e56.14 4.8 2.1 5 umbria-marche 06.10.97 23:24 43n00.91 12e50.58 5.5 7.0 20 umbria-marche 12.10.97 11:08 42n54.64 12e56.99 5.3 2.2 10 umbria-marche 14.10.97 15:23 42n54.91 12e55.77 5.6 4.9 28 umbria-marche 16.10.97 12:00 43n02.06 12e53.42 4.6 2.0 7 umbria-marche 09.11.97 19:07 42n51.26 12e59.99 4.9 2.0 8 umbria-marche 21.03.98 16:45 42n57.09 12e54.86 4.6 4.1 6 umbria-marche 03.04.98 07:26 43n11.05 12e45.54 5.0 2.6 9 umbria-marche 05.04.98 15:52 43n11.41 12e46.35 4.7 5.4 6 analysis of strong ground motions to evaluate regional attenuation relationships 441 with local magnitude m l bigger than 4.5, recorded at less than 100 km of distance from the epicenter (fig. 2 and table i). the records of march 26th 1998, m l 5.6 earthquake were not considered in the analysis because this event occurred at 47 km depth; moreover, we did not use the records from the station of nocera umbra because it shows a strong amplification effect due to the presence of a sub-vertical fault and to highly fractured rocks (marra et al., 2000). the data set used for this study was therefore reduced to 161 records (see table i and fig.1) whose horizontal components have been uniformly corrected using the software bap (basic strong motion accelerogram processing software; converse and brady, 1992). all the records, sampled with a 0.005 s interval (200 samples/s), were baseline and instrument corrected and then band-pass filtered applying a high-cut filter between 23 and 28 hz and a low-cut filter represented by a bi-directional butterworth filter with corner frequency of 0.4 hz and rolloff parameter equal to 2. since the largest earthquake in the data set turned out to be 26th september, 1997 m l 5.9 earthquake (see table i), in this study we are allowed to use local magnitude because this scale is known to saturate at about magnitude 6.5 (lay and wallace, 1995). sabetta and pugliese (1996) used m l for small magnitudes, and m s when both m s and m l were larger than 5.5. according to the relation derived by rebez and stucchi (1996) for italy, the difference between m s and m l , in the range of magnitude considered in this study, is sufficiently small to allow the comparison of the two analysed attenuation relationships. also, bolt (1989) suggests that for shallow earthquakes, as in our case study, m l scale should be used for magnitudes from 3 to 7. in this study, all the stations are classified as either rock or soil (as in spudich et al., 1999), according to the superficial geology. the criterion used to define rock sites is the same as adopted in ambraseys et al. (1996) and in imperial college et al. (2000), and follows the scheme proposed by boore et al. (1993) based on the measured or estimated average shear-wave travel-time velocity to a depth of 30 m. according to this classification rock sites are characterized by shear-wave velocity larger than 750 m/s. the sites classified as stiff or soft soils (having respectively velocity between 360 and 750 m/s and smaller than 360 m/s) are put 0 10 20 30 40 50 4.5 5.0 5.5 6 6.5 magnitude n . r e c o rd s zm02 sp96 fig. 1. the histogram shows the number of data used in this study (zm02) and in sp96 for each magnitude level. data for sp96 are obtained from sabetta and pugliese (1987). 442 gaetano zonno and valentina montaldo together in a single class generically defined as «soil» (see table ii). sabetta and pugliese (1996) used a different classification based on geological and geotechnical information including the depth of the sediments, so for these authors rock sites are located on outcrops, stiff shallow soils have average shear-wave velocity greater than 800 m/s and are less than 20 m thick, while deep alluvium soils have thickness of more than 20 m and shear-wave velocity ranging between 400 and 800 m/s (see table ii). some problems may arise when comparing our results with sp96’s because we did not consider the thickness of the soil. as shown in table ii, according only to shear-wave velocity our «soil» class corresponds to sp96’s «deep soil» class (see also fig. 6a,b), while the «shallow soil» class used by sp96 should correspond to «rock» for zm02. this is proved not to be true by fig. 6a,b, showing that sp96’s «shallow soil» pseudo-velocity response spectrum has a peak at high frequencies. therefore we compared our «rock» curves with sp96 «rock» curves, when sites are defined as rock according to the classification by ambraseys et al. (1996) and in imperial college et al. (2000), while we suggest comparing sp96 «deep soil» curve with our «soil» curve for all the other cases. for each horizontal acceleration recording we calculated peak ground acceleration (in g), peak ground velocity (in cm/s), arias intensity (cm2/s3) and the pseudo-spectra of velocity (in cm/s) using a 5% damping and then regression analysis was performed to evaluate the attenuation function for each of these indicators. the next paragraph discusses the methodology adopted for the regression analysis. 10 0 10 1 10 2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 umbria-marche earthquakes epicentral distance (km) m l rock soil fig. 2. distribution of the records used in this study with respect to local magnitude m l , epicentral distance r (km) and site geology. analysis of strong ground motions to evaluate regional attenuation relationships 443 3. predictive equation many predictive relationships derived from regression analysis of strong motion data are available in literature for horizontal peak acceleration, velocity and response spectral values. several papers (for example, campbell, 1985; joyner and boore, 1988) offer a review of the most commonly used equations and regression techniques: the general form chosen by most investigators to model attenuation is given by the following expression: (3.1) where y is the strong motion parameter to be predicted, a is a coefficient that depends on frequency, f1(m) is a function of magnitude, f2(r) is a function of distance, f 3 (s) accounts for soil conditions and is the standard deviation of y. as said in section 1, the most recent attenuation relationships for italy were calculated by sabetta and pugliese (1996), from a data set of 95 three components accelerometric recordings, generated by 17 italian earthquakes of magnitude ranging from 4.6 to 6.8 and distances up to 100 km. this relation is generally used for the whole national territory, and its final form is the following equation: (3.2) where a is the offset term, b is the magnitude term, and c is the distance term, m is the magnitude (m l or m s , as described previously), r is the distance (fault or epicentral) in kilometers and h is the pseudo-depth determined by regression that accounts for all the factors that tend to limit or reduce the motion near the source and also includes any factors that tend to enhance the motion near the source, like directivity (joyner and boore, 1981). the terms e 1 s 1 and e 2 s 2 are related to the site classification (e 1 s 1 is for shallow soils, while e 2 s 2 is for deep soils), and is the standard deviation of the logarithm of y. the values of the coefficients in eq. (3.2) were estimated through nonlinear multiple regression analysis, although it is worth noting that coefficient c is set equal to –1 and does not vary because it represents the energy decay due to geometric spreading. another attenuation relationship often used in italy is the one proposed by ambraseys et al. (1996) that was calculated over a data set representative of european strong motions composed by 422 triaxial records generated by 157 earthquakes having magnitude m s between 4.0 and 7.9 and focal depth less than 30 km. the equation adopted by these authors has the following form: (3.3) where c1 is the offset term, c2 is the magnitude term, c3 is the distance term, m is the magnitude m s , and r is the distance calculated as in eq. (3.2). the coefficient c a s a accounts for stiff soils, while log log 10 1 2 3 10 y c c m c r( ) = + + ( ) + c s c s pa a s s+ + +=y 10 )(log a bm c r h e s e s 10 2 2 1 2 1 1 2 2 /= + + + + + ±log )( f y a f m f r f s( ) = + ( ) + ( ) + ( ) ±1 2 3 table ii. correlation between different soil classifications used for attenuation relationships. zm02 rock soil v s30 > 750 m/s v s30 750 m/s amb96 rock stiff soil soft soil v s30 > 750 m/s 360 v s30 750 m/s v s30 < 360 m/s sp96 rock shallow soil (h 20 mm) deep soil (h > 20 m) outcrop v s > 800 m/s 400 v s 800 m/s 444 gaetano zonno and valentina montaldo c s s s is for soft soils, is the standard deviation of the logarithm of y, and p is a coefficient that can take only the values 0 or 1. note that in the following paragraphs we will indicate the two cited papers and the relative attenuation relationships as sp96 and amb96. the attenuation relationship proposed in this paper is given by (3.4) where m is local magnitude (m l ), r is epicentral distance in kilometers, h is the pseudo-depth determined by regression, the coefficient e accounts for soil condition, is a dummy variable that can take only the value 0 (for rock sites) or 1 (for soil sites) as in sea99 (spudich et al., 1999), and is the standard deviation of the logarithm of y. the coefficients were calculated using a multiple step regression analysis, similar to the two-step procedure described by joyner and boore (1981), in which a and b are derived by linear regression for a given value of h, which is then determined, along with c, minimizing the sum of the squared residuals. after the twostage regression, the site coefficient was determined minimizing the residuals (see ambraseys et al.,1996). 4. predicted horizontal response spectra we solved eq. (3.4) for y corresponding to the average of the horizontal pseudo-spectra of velocity (psv) calculated with 5% damping (in cm/s), and the regression analysis described in the previous paragraph was performed on the data frequency-by-frequency for a total of 14 frequencies in the range between 0.25 and 25 hz. log 10 y a bm( ) = + + c r h e2 2 1 2 + +( ) + ± log10 / frequency (hz) a b c h e 0.25 3.002 0.773 1 2.1 0.157 0.329 0.33 3.012 0.809 1 3.5 0.171 0.331 0.50 3.169 0.890 1 5.5 0.243 0.348 0.67 2.900 0.852 1 4.6 0.336 0.350 1.00 2.280 0.745 1 4.9 0.292 0.319 1.33 2.067 0.715 1 4.6 0.230 0.308 2.00 1.608 0.635 1 3.1 0.101 0.295 2.50 1.373 0.595 1 2.0 0.065 0.297 3.33 0.878 0.505 1 2.1 0 0.288 5.00 0.391 0.411 1 3.3 0 0.270 6.67 0.038 0.310 1 3.2 0 0.269 10.00 0.194 0.225 1 3.6 0 0.269 15.00 0.036 0.225 1 3.1 0 0.274 25.00 0.737 0.292 1 2.8 0 0.274 pga (g) 1.632 0.304 1 2.7 0 0.275 pgv (cm/s) 1.275 0.458 1 1.9 0.051 0.289 i a (cm2/s3) 0.713 0.664 1.046 0 0.075 0.335 table iii. regression coefficients for eq. (3.2) for y equal to psv, pga, pgv or arias intensity. coefficients are to be used for 4.5 m l 5.9 and epicentral distance r 100 km. analysis of strong ground motions to evaluate regional attenuation relationships 445 the coefficients evaluated with our regression analysis are shown in table iii, along with the corresponding frequencies. figure 3a-c compares the spectral values (psv) of the data (fig. 3a) and the results of different attenuation functions: sp96 (fig. 3b) and this study (indicated by zm02, fig. 3c). in each plot, the horizontal axes are frequency (in hz) and epicentral distance (in km), while colors indicate different values of the logarithm of psv. these figures show that our attenuation function reproduces the same upper values of sp96 but it decays faster as distance increases. this result is due to the small amplitudes shown by our data at large distances (see fig. 2): in fact we have records from small, shallow events which have very small amplitude at large distance, while sp96 data are generated by strong events (magnitude larger than 5.5 and up to 6.8) and therefore they have larger amplitudes at the same distances. generally, the results of regression analysis are of good quality, as proved by relatively small standard deviations. figure 4a,b shows two examples of good quality of fit between recorded and simulated spectra. the first (in fig. 4a) was recorded during 26th september 1997, m l = 5.6 earthquake at cascia, which is a rock site (imperial college et al., 2000) located at about 35 km from the epicenter on calcareous marl aged paleogene (ssn and enel, 1998). the second example (fig. 4b) is a record of october 14th 1997, m l = 5.6 earthquake at the site of spoleto monteluco. this station is located at about 30 km from the epicenter on massif jurassic limestone (ssn and enel, 1998), therefore is classified as rock (imperial college et al., 2000). in each plot, the black solid curve is the average horizontal component of the spectrum, our results are in red, while sp96 is in blue. the figure shows that sp96 curves fit the higher values of the spectra calculated from the data, while zm02 seems to be smoothing the data. figure 5a shows the spectra calculated at different epicentral distances (5, 10, 30, 50, 100 and 200 km) for a fixed value of local magnitude (m l = 6) and fixed site geology (rock). all the curves have the same shape, characterized by a plateau between 0.50 and 5.0 hz, and amplitudes decrease with increasing distance. figure 5b fig. 3a-c. comparison between psv calculated from the data set (a) and two different attenuation functions: sp96 (b) and this study (zm02 (c)). data (c m /s ) (km) (hz) a a (hz) (c m /s ) (km) bb (hz) (c m /s ) (km) cc 446 gaetano zonno and valentina montaldo 10 0 10 1 10 0 10 1 frequency (hz) p s v ( cm /s ) record 1316 m l = 5.6 r = 27.31 rock psv ns psv ew average zm02 sp96 10 0 10 1 10 0 10 1 frequency (hz) p s v ( cm /s ) record 1188 m l = 5.6 r = 35.83 rock psv ns psv ew average zm02 sp96 fig. 4a,b. comparison between psv derived from processed records (black curves) and psv calculated using zm02 (red curve) and sp96 (blue curve). figure 4a shows the case of ra01188 recorded at the site of cascia during 26th september 1997, m l = 5.6 earthquake; in fig. 4b the case of ra01316, recorded of spoleto monteluco during 14th october 1997, m l = 5.6 earthquake. a b analysis of strong ground motions to evaluate regional attenuation relationships 447 10 1 10 0 10 1 10 2 10 1 10 0 10 1 r = 5 km r = 10 km r = 30 km r = 50 km r = 100 km r = 200 km frequency (hz) p s v (c m /s ) predicted spectra (5% damping) m l = 6 rock fig. 5a,b. figure 5a shows the predicted (5% damping) psv curves versus the frequency for different values of the epicentral distance (i.e. r = 5, 10, 30, 50, 100 and 200 km) calculated with the attenuation relations proposed by this study (see table iii) for a rock site and local magnitude m l = 6. figure 5b shows a comparison of the predicted (5% damping) psv versus the frequency for a fixed epicentral distance (r = 30 km) and at different levels of local magnitude (i.e. m l ranging between 4 and 7.5, each step is equal to 0.5). the red lines are relative to the results of this study (zm02) while the blue dashed curves are sp96 (raw curves). 10 1 10 0 10 1 10 1 10 0 10 1 10 2 4.5 5.5 6.5 7.5 frequency (hz) p s v ( cm /s ) predicted spectra (5% damping) r = 30 km rock sp96 zm02 a b 448 gaetano zonno and valentina montaldo 0.3, and ranging between 23 and 35 hz for the low-cut filter, with typical values of 25 hz. the different soil classification adopted in this study and in sp96 has been discussed previously (section 2). figure 6a represents the predicted response spectra calculated with zm02 attenuation relationship for the soil categories explained in table ii, while fig. 6b presents the results for sp96. if we compare fig. 6a and b, it is immediately noted that the soil class used in this study (dashed line in fig. 6a) is more similar to the deep soil class used by sp96 (dashed line in fig. 6b). the maximum value reached by the two curves is approximately the same (about 12 cm/s), but zm02’s peak occurs at 0.7 hz, while sp96 reaches the maximum at about 1.2 hz. some useful indicators of the strong motion’s spectral energy are the response spectrum intensity (si, housner, 1959) and the acceleration spectrum intensity (asi, von thun et al., 1988), that represent the response spectrum ordinates in period ranges of engineering significance and are defined as follows: (4.1) (4.2) where is the damping and t is the period. these parameters were calculated from the psv obtained with sp96 and zm02 attenuation functions for a rock site and different magnitudedistance levels (see tables iv and v; the numbers in italic refer to sp96). as expected from figs. 4a,b and 5b, zm02 outcomes are smaller than sp96 results because our predicted spectra have always smaller values up to m l = 6, but if we extend the calculation up to m l = 7, we obtain larger values of asi for all distances and larger values of si only for very small distances, and slightly smaller values elsewhere. therefore, if used in psha, our attenuation relationship will give smaller values of the ground motion indicators compared to sp96 (thus decreasing the level of hazard) in those source zones characterized by low or moderate size earthquakes. shows the comparison between the pseudospectra of velocity calculated at a fixed epicentral distance (r = 30 km) for eight values of local magnitude (from m l = 4 to m l = 7.5 with a 0.5 step) at rock sites. the results deduced by this study are in red, while the spectra by sp96 are represented by blue dashed lines. it is interesting that the two attenuation relationships give very different results both at small and large frequencies. in fact up to m l = 5.5 sp96 is systematically higher than our results at the small frequencies; while for higher magnitudes the opposite situation occurs. for large frequencies (above 5 hz), sp96 is always larger than our curves. figure 5a,b (for rock sites) shows the predictive values of the two attenuation relationships computed outside their theoretical limits of magnitude and distance, although it is common practice in psha to perform the analysis for magnitudes ranging between 4 and 7 and for distances up to 200 km. to explain these discrepancies on the results one might claim different facts: first of all, and most important, the different data set used. it is clear from fig. 1 that more than 50% of our records are from events with local magnitude m l smaller or equal to 5.5, while most of sabetta and pugliese (1996) data (nearly two thirds) are larger than that. as a consequence, small or intermediate magnitudes have more weight in our attenuation functions compared to sp96, thus providing smaller values of the ground motion. as a general statement, the attenuation function proposed in this study should not be used for m l greater than 6 since such magnitudes are not present in our data set, while sp96 can be extended more confidently toward large events because some records of magnitude 6.8 are included in their analysis. we can also attribute some differences to the fact that we used the average of the two horizontal components instead of the larger of the two (as sp96) and thus we have introduced some sort of smoothing in the data, eventually lowering high values. finally we recall the slightly different bandpass filtering frequencies used in our study (see section 2) compared to sp96. these authors used cutoff frequencies for the high-cut filter ranging between 0.2 and 0.7 hz, with typical values of si psv t dt( ) = ( ), . . 0 1 2 5 asi s t dta= =( )0 05 0 1 0 5 . , . . analysis of strong ground motions to evaluate regional attenuation relationships 449 10 0 10 1 10 0 10 1 frequency (hz) p s v ( cm /s ) zm02 predicted spectra (5% damping) m l = 6 r = 30 km rock soil fig. 6a,b. predicted velocity response spectra for different soil classes. figure 6a represents the results of this study for rock (thick solid line) and soil (dashed line); fig. 6b shows the results of sp96 for rock (thick solid line), shallow soil (thin solid line) and deep soil (dashed line). 10 0 10 1 10 0 10 1 frequency (hz) p s v ( cm /s ) sp96 predicted spectra (5% damping) m l = 6 r = 30 km rock shallow soil deep soil a b 450 gaetano zonno and valentina montaldo table v. values of acceleration spectrum intensity (asi, in cm/s) for rock sites, using the psv attenuation relationships zm02 and sp96 (numbers in italics correspond to sp96). asi (cm/s) epicentral distance (km) m l 5 10 30 50 100 200 4 8.48 5.09 1.83 1.10 0.55 0.28 22.47 13.71 4.95 2.99 1.50 0.75 5 46.15 27.93 10.06 6.08 3.05 1.52 86.96 52.91 19.06 11.51 5.77 2.89 6 260.98 159.09 57.45 34.71 17.41 8.71 344.57 209.02 75.18 45.41 22.77 11.39 7 1530.80 983.15 339.50 205.18 102.91 51.49 1395.90 844.30 303.22 183.12 91.81 45.94 table iv. values of response spectrum intensity (si, in cm) for rock sites, using the psv attenuation relationships zm02 and sp96 (numbers in italics correspond to sp96). si (cm) epicentral distance (km) m l 5 10 30 50 100 200 4 5.66 3.21 1.12 0.67 0.34 0.17 8.41 5.52 2.08 1.26 0.63 0.32 5 13.89 7.84 2.73 1.65 0.82 0.41 19.43 12.68 4.76 2.89 1.45 0.73 6 38.91 22.00 7.68 4.63 2.32 1.16 46.99 30.41 11.36 6.89 3.46 1.73 7 127.64 72.84 25.55 15.40 7.71 3.86 121.01 77.52 28.79 17.44 8.76 4.38 5. predicted peak ground acceleration, velocity and arias intensity several indicators may be used to characterize the ground motions for purposes of seismic design: these include peak acceleration, peak velocity, response spectral values and fourier spectral values. response spectral values were discussed in the previous paragraph, while peak ground acceleration, peak ground velocity and arias intensity will be analyzed in this section. peak ground acceleration (pga) is defined as the largest absolute value of horizontal acceleration derived from each accelerogram. the pga attenuation relationship proposed in this study is calculated by regression analysis using the approach described in section 3. the dependent variable y in eq. (3.4) is the larger horizontal component of each record, and the coefficients a, b, c, h and e determined by regression are listed in table iii. the attenuation relationship obtained in this study always predicts lower pga compared to amb96, while compared to sp96 it gives higher pga values only for small magnitudes and small distances. figure 7 shows the three different attenuation functions and the data with magnitude m l = 5.6, plotted with different symbols according to the soil category (squares are for rock, circles for stiff soils, and triangles for soft soils). figure 8 shows the same data, but the curves represented are the result of this study without standard deviation (solid red line), and analysis of strong ground motions to evaluate regional attenuation relationships 451 introducing the standard deviation (summed or subtracted). when the standard deviation is considered in the analysis the majority of data are inside the curve plus or minus one standard deviation. only a few data are slightly higher than the curve plus standard deviation and a few more data points are smaller than the curve minus standard deviation. the latter were recorded by digital instruments installed by servizio sismico nazionale and show very small pga because are located at large distance from the fault, near l’aquila. another parameter used to quantify the amplitude of the ground motion is peak ground velocity (pgv), which gives information particularly at intermediate frequencies. we derived pgv by integration of the accelerograms, and as for pga, we used the larger value of the two horizontal components of each record to define the coefficients in the attenuation function (eq. (3.4)), and the results are listed in table iii. finally, an attenuation function was derived for the arias intensity (arias, 1970), whose coefficients are listed in table iii (bottom line). the pseudo-depth h calculated from regression analysis results equal to zero. in fact the values of arias intensity (in cm2/s3) are quite large and depend only on distance because the effect of h, which is always very small, is almost irrelevant. 10 1 10 2 10 2 10 1 m l = 5.6 epicentral distance (km) p g a ( g ) zm02 sp96 amb96 fig. 7. different pga attenuation functions are plotted with respect to epicentral distance for local magnitude m l = 5.6. the red line is the curve calculated in this study (zm02), in blue is sp96 attenuation relationship and in green is amb96. solid lines are for rock sites, while dashed lines are the attenuation relationships for soil. the black symbols represent all the data with m l = 5.6: squares are for rock sites, triangles for soils (see table ii). 452 gaetano zonno and valentina montaldo 6. conclusions this study aimed at refining italian attenuation relationships at regional scale using a data set of 322 horizontal strong ground motions recorded in umbria-marche, central italy. the analysis was restricted to this area because the amount of records is now large enough to be statistically significant, thus overcoming one of the most common criticisms of attenuation relationships. the data set includes records generated by events with local magnitude (m l ) ranging between 4.5 and 5.9, and epicentral distance smaller than 100 km, which represent the limits beyond which the attenuation functions should be used. through a multiple steps regression analysis, we have calculated empirical equations for the peak ground acceleration and velocity, the arias intensity and for the horizontal components of the 5% damped velocity pseudo response spectra, corresponding to 14 frequencies ranging from 0.25 to 25 hz. the attenuation relationships were tested comparing the results to other attenuation relations generally assumed to be valid for the whole national territory. the comparison with sabetta and pugliese (1996) results, in terms of psv, shows that for small frequencies our curves are systematically smaller than sp96 up to m l = 6, while for higher magnitudes this situation is inverted. for high frequencies our curves are always smaller than sp96 for all magnitudes. 10 1 10 2 10 2 10 1 m l = 5.6 epicentral distance (km) p g a ( g ) zm02 + std std fig. 8. peak ground acceleration attenuation function is plotted with respect to epicentral distance for local magnitude m l = 5.6 (solid line), while the dashed and the dotted lines are respectively pga attenuation relationship plus or minus one standard deviation. the black symbols represent the observed pga: squares are for rock sites, triangles for soils (see table ii). analysis of strong ground motions to evaluate regional attenuation relationships 453 the use of indicators like si and asi helps understanding the response spectrum ordinates in period ranges of engineering significance and prove that sp96 outcomes are higher than zm02 results for m l 6. in terms of pga, the attenuation relationship obtained in this study gives higher values compared to sp96 only for small magnitudes and small distances. these results are not surprising because we mostly used records from events with local magnitude m l smaller or equal to 5.5, whereas only one third of sabetta and pugliese (1996) data belong to this range of magnitude. also in this study the highest magnitude is equal to 5.9, while sp96 uses data up to 6.8. as a consequence, in our study small or intermediate magnitudes weigh more than large magnitudes, and this means that our attenuation functions will provide smaller values of each ground motion indicator. acknowledgements this study was supported by the istituto nazionale di geofisica e vulcanologia in the framework of hazard and seismic risk topic studies. in particular we are grateful to dr. marco cattaneo at ingv for providing useful data, and to an anonymous reviewer for fruitful comments. references ambraseys, n.n., k.a. simpson and j.j. bommer (1996): prediction of horizontal response spectra in europe, earthquake eng. struct. dyn., 25, 371-400. arias, a. (1970): a measure of earthquake intensity, in seismic design for nuclear power plants, edited by r. hansen (mit press, cambridge), 438-483.. bender, b. and d.m. perkins (1987): seisrisk iii: a computer program for seismic hazard estimation, u.s. geol. surv. bull., 1772, pp. 48. bolt, b.a. (1989): the nature of earthquake ground motion, in the seismic design handbook, edited by f. naeim (van nostrand reinhold, new york),1-31. bommer, j.j., s.g. scott and s.k. sarma (2000): hazardconsistent earthquake scenarios, soil dyn. earthquake eng., 19, 219-231. boore, d.m., w.b. joyner and t. fumal (1993): estimation of response spectra and peak acceleration from western north american earthquakes: an interim report, u. s. geol. surv. open-file report 93-509. campbell, k.w. (1985): strong motion attenuation relations: a ten-year perspective, earthquake spectra, 1, 759-804. cattaneo, m., p. augliera, g. de luca, a. gorini, a. govoni, s. marcucci, a. michelini, g. monachesi, d. spallarossa, l. trojani and xgums (2000): the 1997 umbria-marche (italy) earthquake sequence: analysis of the data recorded by the local and temporary network, j. seismol., 4, 401-414. converse, a.m. and a.g. brady (1992): bap: basic strong-motion accelerogram processing software, version 1.0, u. s. geol. surv. open-file rep. 92-296a. decanini, l., f. mollaioli, g.f. panza, f. romanelli and f. vaccari (2001): probabilistic vs deterministic evaluation of seismic hazard and damage earthquake scenarios: a general problem, particularly relevant for seismic isolation, in proceedings of the 7th international seminar on seismic isolation, passive energy dissipation and active control of vibrations of structures, assisi, italy, october 2-5, 2001, vol. 1, 117-141. deschamps, a., g. iannaccone and r. scarpa (1984): the umbrian earthquake (italy) of 19 september 1979, ann. geophys., 2, 29-36. haessler, h., r. gaulon, l. rivera, r. console, m. frogneux, g. gasparini, l. martel, g. patau, m. siciliano and a. cisternal (1988): the perugia (italy) earthquake of 29 april 1984: a microearthquake survey, bull. seismol. soc. am., 78, 1948-1964. housner, g.w (1959): behavior of structures during earthquakes, asce j. eng. mech. div., 85, n. em14, 109-129. imperial college of science, technology and medicine, sogin-ssn, enea and ipsn (2000): european strong-motion database, european council, environment and climate research programme, cd-rom. joyner, w.b and d.m. boore (1981): peak horizontal acceleration and velocity from strong-motion records including records from the 1979 imperial valley, california, earthquake. bull. seismol. soc. am., 71, 2011-2038. joyner, w.b and d.m. boore (1988): measurement, characterization and prediction of strong ground motion, in earthquake engineering and soil dynamics ii – recent advance in ground-motion evaluation, geotechnical special publication 20, asce, new york, 463-481. kramer, s.l. 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(2000): new developments in seismic hazard estimation, in proceedings of the «sixth international conference on seismic zonation (6icsz)», palm springs, ca, 12-15 november 2000, cd-rom. spudich, p., w.b. joyner, a.g. lindh, d.m. boore, b.m. margaris and j.b. fletcher (1999): sea99: a revised ground motion prediction relation for use in extensional tectonic regimes, bull. seismol. soc. am., 89, 1156-1170. ssn and enel (1998): elaborazione delle principali registrazioni accelerometriche della sequenza sismica umbro-marchigiana del settembre-ottobre 1997, 3 vol. + cd-rom, roma, 21 maggio 1998 (in italian). von thun, j.l., l.h. rochim, g.a. scott and j.a. wilson (1988): earthquake ground motion for design and analysis of dams, in earthquake engineering and soil dynamics ii – recent advance in ground-motion evaluation, geotechnical special publicaton 20, asce, new york, 463-481. (received september 5, 2001; accepted february 21, 2002) di mauro 8.0_revfc:layout 6 annals of geophysics, 57, 6, 2014, g0656; doi:10.4401/ag-6605 g0656 geomagnetic polar observatories: the role of concordia station at dome c, antarctica domenico di mauro1,*, lili cafarella1, stefania lepidi1, manuela pietrolungo1, laura alfonsi1, aude chambodut2 1 istituto nazionale di geofisica e vulcanologia, rome, italy 2 école et observatoire des sciences de la terre, strasbourg cedex, france abstract a geomagnetic observatory is a permanent facility where magnetic declination and inclination are recorded in conjunction with the temporal evolution of the magnetic field components. polar regions are scarcely covered by observational points then the contributions from observatories located there are particularly relevant. the geomagnetic observatory at concordia station, dome c antarctica is located in the inner part of the continent, its position is favorable for two key reasons, i) data are unaltered by the "coastal effect” and ii) crustal effect is negligible due to the thickness, almost 3 km, of ice coverage. nevertheless, these latter conditions imply an unconsidered aspect which characterizes the entire station and every structure laying on the ice surface: the dome on which concordia station resides is sliding horizontally and moving vertically with a velocity of few millimeter to centimeters per year as indicated by independent geodetic observations. this slow and continuous movement has a puzzling effect on the trend of horizontal components of the magnetic field, sampled in a time window of a decade since the establishing of the observatory in 2005. during the international polar year (2007-2009) the observatory was upgraded with new equipment fulfilling the requirements of the intermagnet consortium, and becoming an observatory member in 2011. in this paper are illustrated the strategy adopted to track any possible displacement of the observatory reference points (i.e. the azimuth mark, the pillar position) and all the ordinary and extraordinary actions required for collecting high quality data. 1. introduction the ensemble of geomagnetic observatories spread around our planet is aimed to mark out the temporal evolution and the spatial configuration of the earth’s magnetic field at different time-space scales. observatory data enclose the superposition of natural signals generated by different physical processes from the earth’s core to mantle, lithosphere, ocean, ionosphere, magnetosphere, and the solar-terrestrial interaction. ideally, an even and dense coverage of observational points on the globe allows the computation of models of the geomagnetic field very close to the observed field, as the international geomagnetic reference field (igrf), adopted by the referenced community (http://www.ngdc.noaa.gov/iaga/vmod/igrf.html). this latter model, compiled and released every 5 years by the referenced community, is nothing more than a synthetic (mathematical) description of the field derived from the measured values collected continuously and for long time at each observatory. the possibility to expand the number of ground observatories, especially in scarcely covered areas like the polar regions, or modernize the existent obsolete instrumentation in pre-existent observatories should be one of the principal objective of the scientific community to accomplish the full understanding of the ongoing geodynamical processes both inside and outside the planet, from the earth's core where the main field is generated to the field interactions with external sources. since 2005, after 3 years of testing, a geomagnetic observatory is in operation on the iced plateau, at dome c, antarctica (geographic coordinate: 75° 6’ 4”s; 123° 19’ 51” e, elevation 3.232 m). the observatory is currently maintained by two scientific institutions, ingv for italy and eost for france. concordia station shares its facility between the two countries which agreed to establish and sustain a scientific base at dome c since the beginning of 2000’s; this statement became official in 2005 with the first article history received june 17, 2014; accepted december 22, 2014. subject classification: geomagnetic observatory, gnss, azimuth mark determination, dome c, concordia, antarctica. winter-over. the position of the geomagnetic observatory at dome c is particularly favorable since it is far from the electrical induction due to the conductive sea water coastal circulation and far from the effects of the crustal field contamination, a source of severe bias for those observatories located on magnetized basements. another important aspect of the dome c observatory is the location inside the polar cap, very close to the south invariant pole, and the geomagnetic pole, during the whole day. the invariant and geomagnetic poles are those points (small areas) around which the aurora ovals are generated and phenomena like magnetic storms, substorms and pulsations are enhanced and display strong variations within short distances [akasofu 2002]. hence, a denser networks of stations possibly with good instrumentation at both hemispheres would be of great support to a wide range of geophysical studies. moreover the typical dipolar configuration almost parallel to the earth’s spin axis (tilted of about 11°) implies a strengthening of the magnetic field at polar regions, and then close to the poles the field intensity is two times the one at equatorial or low latitude areas. data from the concordia observatory are also valuable for global and regional modeling based on ground and satellite measurements and for representing an absolute constraint and reference for the measurement obtained from satellite dedicated to the earth’s magnetic field observation as for swarm esa mission launched on november 22nd, 2013, (http:// www.esa.int). although the location of the concordia observatory at dome c is particularly favorable for the reasons described above, at the same time, the entire ice mass of the antarctic plateau, including the eastern sector, and every permanent structure put on its surface, undergoes relevant local and global displacements, as revealed by integrated analysis through geodetic, glaciological, geophysical, and insar (interferometric synthetic-aperture radar) space-based surveys [capra and dietrich 2008, rignot et al. 2008]. the sliding effect of the antarctic plateau could in principle invalidate the observations made on the assumption of a steady geographical position. one of the requirements for a geomagnetic observatory is the conservation through time of a stable position, and consequently the possible effect of the local displacement at dome c needs to be adequately measured and considered when monitoring the long-trend changes on the horizontal components of the geomagnetic field. the unexpected rate of change in the horizontal component and the magnetic declination at concordia in comparison with the known "secular variation" (figure 1), more relevant since 2009 suggested that a possible cause to the anomalous long-term trend was the slow but continuous displacements of the structures hosting the observatory. the sliding was effectively demonstrated by independent geodetic observations made at dome c by a gps network established in 1995. the 37 observational points surrounding the concordia station along concentric rings at increasing distances from 3 to 25 km pointed out that at the dome summit has an horizontal velocity of about 11 mm/yr ±0.6 mm/yr along a direction of 302° clockwise from true north (vn=5.8 mm/yr±0.4 mm/yr, ve=–9.2 mm/yr±0.4 mm/yr) and a vertical velocity vup=–152.6 mm/yr±1.3 mm/yr with respect to itrf2000. at 25 km from the summit of dome c a movement with velocity of 211 mm/yr was reported [urbini et al. 2008, vittuari et al. 2004]. 2. the geomagnetic observatory at concordia base, dome c, antarctica geomagnetic observatories are ground based non-magnetic structures (from a simple hut to a standard brick building) aimed to collect time-series measurements of the earth’s magnetic field. generally they are operated by academic or government institutions that voluntarily agree to disseminate collected data to scientific communities through world data centers or specific consortiums like intermagnet (www.in termagnet.org). according to the guidelines issued by the iaga (international association of geomagnetism and aeronomy) organization, a geomagnetic observatory stands in a place with very low magnetized basement and surrounding environment and it produces very high quality data under some specific requirements [jankowski and sucksdorff 1996]. at present, of approximately 130 observatories included in intermagnet only 8 are positioned beyond the arctic and 5 beyond the antarctic circle (table 1). among these polar observatories only two are placed on the inland layered iced surface, at vostok and concordia stations. a monitoring point of the earth’ magnetic field can be promoted as “observatory” if the vector components and the total intensity of the magnetic field can be collected for long time with the highest accuracy and quality at a minimum sampling rate of 1-minute. the vector elements are continuously recorded above an arbitrary level which has to be adjusted by means of a baseline, derived from the manual measurements of two essential angles, the magnetic declination d and inclination i, and from the measurements of the total field intensity f. those angles are measured by a fluxgate theodolite, a demagnetized optical device with a magnetometer mounted on the telescope (figure 2), positioned over a pillar. declination and inclination are estimated with respect to the true north by pointing di mauro et al. 2 3 the telescope towards a local azimuth mark. both the azimuth mark and the pillar were geographically referenced. the theodolite must stand over a pillar rooted into the ground to guarantee a stability over time. another important requirement is the stability of the room temperature where the instrumentations operate for avoiding variations due to the expansion/contraction of the mechanical parts of the instrumentation. among the three fundamental requirements for a geomagnetic observatory, i.e. 1) continuity in data collection of f and d and i angles, 2) stability of the position of the pillar and 3) stability of the room temperature, the durable and stable position of the pillar is one of the hardest to accomplish when no rock basement is available for rooting a pillar. this is exactly the case of the concordia station observatory. similar conditions should be addressed in vostok station but indications of strategy to minimize their effects are not known or, at least, not found by the authors in associated literature. the magnetic observatory at concordia station consists essentially of two amagnetic carbon fiber shelters, about 26 m apart, directly laying on the iced surface. one shelter hosts the automatic instrumentation, that is a vector magnetometer for the 3 components of the field recordings at 1 hz, and a scalar magnetometer for the total intensity of the field, with sampling rate every 10 seconds. the other shelter hosts a pillar, made of special polypropylene highly resistant against very low temperature with a marble plate on top [schott et al. 2005]. a special crystal glass with very low refraction index, 1.5 cm thick, used as window, allows to optically point from the pillar toward the azimuth mark, stuck on the external wall of the shelter hosting the automatic instrumentation the concordia geomagnetic observatory at the inland antarctica figure 1. comparison between the horizontal component (top) and declination (bottom) of the geomagnetic field from absolute measurements and from igrf [2010] at the concordia observatory. values from 2010 to 2012 are linearly interpolated since observed values are under processing. values for 2014 are provisional. note the progressive divergence from the initial years and the inverse trend between 2013 and 2014 for both graphs. this latter trend needs to be confirmed in the future years. however, it is also important to note that in polar regions the comparison of measured geomagnetic values with those from the igrf model is known to be somehow weak [de santis a. et al. 2002]. (figure 2). manual observations of the magnetic declination and inclination, called absolute measurements, are performed once a week/ten days by the staff all year round. in fact, permanent personnel is present at dome c also during the long polar night, the so-called winter-overs. besides the continuous availability of high quality data, the activity of the winter-overs for all year round and along the years within a rotation of staff, allowed the geomagnetic observatory of dome c to become a member of the intermagnet organization with the iaga code of dmc in may 11, 2011. according to the initial solar observation made in summer 2003, the azimuth between the pillar centre and the azimuth mark was at the angle 253° 56’ 52” from true north. over the years the snow accumulation forced to face the progressive coverage of the two shelters and consequently of the optical path from the pillar to the azimuth mark. nowadays a shallow tunnel, just below the surface, is protecting this optical path from obstruction of further snow accumulation (figure 2). the three-component field variation magnetometer and the scalar magnetometer installed at the dome c geomagnetic observatory are described by schott et al. [2005] and chambodut et al. [2009]. we only recall that the vector magnetometer is a suspended triaxial fluxgate, oriented along the local geomagnetic meridian. by combining the absolute measurements taken as baselines with the variation dataset one can obtain the absolute values for each component of the earth's magnetic field. di mauro et al. 4 observatory name (northern hem.) iaga code geog. latit. (°) north geog. long. (°) east elev. (m) corr. geom. latit. (igrf 2010) corr. geom. long. east (igrf 2010) year of estab./country abisko abk 68.36 18.82 380 60.114 114.40 1921/sweden barrow brw 71.32 166.76 16 69.72 246.92 1949/usa cambridge bay cbb 69.12 140.02 40 76.44 303.99 1972/canada qeqertarsuaq (godhavn) gdh 69.25 306.47 0 78.35 33.76 1991/greenland hornsund hrn 77.00 15.37 15 73.91 125.26 2002/norway-poland resolute bay res 74.69 265.10 30 82.81 304.69 1952/canada sodankyla sod 67.32 26.63 178 64.00 119.79 1991/finland qaanaaq (thule) thl 77.47 290.77 57 87.46 13.60 1991/greenland observatory name (southern hem.) iaga code geog. latit. (°) south geog. long. (°) east elev. (m) corr. geom. latit. s (igrf 2010) corr. geom. long. east (igrf 2010) year of estab./country vostok (on layered ice) vos 78.47° 106.82 3488 72.91 177.43 1958/russia scott base sba 77.85° 166.76 16 79.00 289.35 1957/new zealand dumont d'urville drv 66.67° 140.02 40 74.31 230.90 1956/france dome c (on layered ice) dmc 75.10° 123.39 3233 87.80 39.17 2005/italy-france mawson maw 67.60° 62.88 12 73.19 110.94 1955/australia table 1. list of intermagnet geomagnetic observatories located northernmost the arctic circle (66° 33' 44" n) (top frame) and southernmost the antarctic circle (66° 33' 44" s) (bottom frame). corrected geomagnetic coordinates are calculated for the igrf2010 by the online service http://omniweb.gsfc.nasa.gov/vitmo/cgm_vitmo.html. for a visualization of the position of the listed observatories on the world map visit the intermagnet website at the url address: www.intermagnet.org. some of these observatories, from opposite polar areas, can be considered as magnetically conjugated, that is they are linked by the same magnetic lines running from an hemisphere to the other. this aspect becomes clear when the geomagnetic coordinate reference system is taken into account. under this condition many interesting phenomena are reported and studied (e.g. in lepidi et al. 2011). 5 the concordia geomagnetic observatory at the inland antarctica figure 2. a compilation of pictures taken along the last decade at concordia station. in particular: 1) two amagnetic shelters hosting the automatic instrumentation and the pillar respectively as they appeared in 2005; 2) the shelter with automatic instrumentation and azimuth mark fixed on the external wall, visible from the opposite shelter; 3) a trench in the ice facilitated some activities at the shelters and surrounding areas; 4) an enlarged view of the azimuth mark; 5) full snow coverage of one of the two shelters and wooden plates laying over the shallow tunnel (not visible). the picture was taken in 2010; 6) the dark shallow tunnel (back), the illuminated azimuth mark and the geomagnetic theodolite placed over the pillar (close-up); 7) a trapdoor mounted on the roof of one of the two embedded shelters, used as entrance; 8) new external asset for gnss survey, in preparation for future recordings. an aluminum pole as stand for the receiver antenna screwed on top. 3. snow accumulation, movement of the entire dome and gnss surveys since the establishing of the geomagnetic observatory at concordia station, the two shelters described above, have been affected by a slow and steady coverage of snow, derived by wind transportation mechanism since the rate of precipitation at this site can be considered less effective [scarchilli et al. 2011]. it is important to remind that every object, no matter its dimensions, from a whole station to a single electronic box, left on the iced surface becomes rapidly an obstacle for the continuous ice/snow stream and so shortly covered. for allowing a safe access to the shelters which are, at present, completely sunk in the ice, the ultimate adopted solution was to open a new entrance, a trapdoor, mounted on their roofs. the previous approach of removing large quantities of snow/ice to free the standard doors, an operation which had to be made each year, wasting time, human power and fuel for the use of snowmobiles was disregarded. a long lasting solution to avoid a rapid coverage of snow, especially applicable for large buildings, is to super-elevate the structures using an appropriate support framework (pileworks, legs or pillars) like the new buildings of concordia station or other modern stations (figure 3). such solution cannot be functional for a geomagnetic observatory building because absolute measurements have to be done over a pillar firmly rooted on a fixed basement. it cannot stand suspended nor (partially) raised without any suitable protection against mechanical and/or thermal stress (wind strength and seasonal/diurnal temperature variations). the unique solution for a geomagnetic observatory based on ice, at least for the case of concordia station, is to check and record on a regular basis both the pillar and azimuth mark geographical positions. the accuracy of the azimuth determination has a crucial and direct effect on the accuracy of declination, inclination and other derived geomagnetic quantities. in geomagnetism, different methods can be used to determine azimuths with accuracy generally from 5 to 20" of arc, as the astronomical observations of sun, moon, or other celestial bodies (with accuracy of 5" of arc or better), the gyro-theodolite (with accuracy of 15"-20" of arc) and recently the gnss (global navigation satellite system) observations (with accuracy of 5" of arc or better under specific conditions). in the case of concordia geomagnetic observatory, after the initial determination in 2003, the progressive sinking of the two shelters did not allow any direct observation of celestial bodies. for physics-mechanical principles, the gyro-theodolite cannot provide stable measurements at such high latitude since its latitude working range is between 75° s and 75° n, and then not suitable for the concordia station observatory. gnss observations have been then used since 2005, and the survey was repeated in 2007, 2009 and 2013. today the gnss includes the american gps, the russian glonass and the newborn european galileo satellite constellations. in the near future the chinese and indian positioning systems will be progressively introduced. gnss observations provide absolute and relative determination of the position of the observational points. the former is used for navigation where lower level of precision is required, while the latter is mainly used for precise positioning and geodetic studies [šugar et al. 2012]. moreover, positioning methods can be static or kinematic depending on the stationary or roving mode selection by the gnss receivers, according to the purpose of the observations. the static relative survey approach was used at the geomagnetic observatory at concordia station, along a time windows spanning 9 years. two gps simultaneous observations were performed by two identical navcom® rt-3020m dual frequency receivers which allowed to post-process baseline vectors and azimuth determination between the pillar and the azimuth mark. at concordia station different conditions play an unfavorable role for good quality satellite signals. in the polar regions the satellite signals can be weak and unstable, this results from the orbital satellite paths that are low over the horizon of sight (elevation mask), implying a degradation or even loss of data transmission. in addition, ionospheric instabilities as scintillations [romano et al. 2013 and references therein] might limit satellite tracking at the receivers. the antarctic zone is outside the coverage of gnss correction services which may help in supplying additional corrective missing data segments. the surveyed points are located indoor of both shelter and their roofs as well as the lateral walls behave as reflectors for the satellites signals producing multipath propagations to the receivers. this aspects is particularly crucial since it introduces a further rate of error in position estimation. an appropriate network of receivers and triangulation among stations were used for tying all the points of gnss baselines together, this to minimize the errors in the azimuth determination. postprocessing software was then used to achieve stable solution and a typical spreadsheet of results is shown in figure 4. from an optical-geometrical point of view, the short distance between the pillar and the azimuth mark is an additional source of inaccuracy, because it enlarges the angular gap among the points of interest. under all those limitations the results of gps surveys are listed in table 2. in 2005 the geographic coordinate of both the pillar and the azimuth mark allowed the determination of the azimuth mark as 253° di mauro et al. 6 7 the concordia geomagnetic observatory at the inland antarctica figure 3. four examples of modern stations erected on iced layered surface, over special legs or pillars. from top to bottom: concordia station (italy france), new amundsen-scott south pole station (usa), halley vi (great britain), neumayer-station iii (germany). di mauro et al. 8 date of survey 28nov. 2005 point lat. (wgs84) dms long. (wgs84) dms horizontal standard deviation (mm) network: pillar p1 -75° 06' 8.88" 123° 19' 59.01" +-10 1 geodetic receiver trimble 5700 mark m1 -75° 06' 9.11" 123° 19' 55.88" +-10 note: azimuth (from pillar to mark): 253° 54' 24.4" rms (31 mm), sd (1.9 mm) difference to initial azimuth: -2' 27.6"=-147.6" the shelters are on the iced surface horizontal distance on ellipsoid: 26.001 m surface distance on geoid: 26.014 m free air distance: 26.015 m date of survey 19 dec. 2007 point lat. (wgs84) dms long. (wgs84) dms horizontal standard deviation (mm) network: pillar p2 -75° 06' 8.88" 123° 19' 59.05" +-20 none mark m2 -75° 06' 9.12" 123° 19' 55.93" +-20 note: azimuth (from pillar to mark): 253° 56' 01" rms (36 mm), sd (2.1 mm) difference to initial azimuth: -0' 51" the shelters are on the iced surface horizontal distance on ellipsoid: 25.921 m surface distance on geoid: 25.934 m free air distance: 25.936 m date of survey 28nov. 2005 point lat. (wgs84) dms long. (wgs84) dms horizontal standard deviation (mm) network: pillar p3 -75° 06' 8.90" 123° 19' 59.11" +-15 1 gnss receiver for scintillations novatel vgs4004 + 1 geodetic receiver trimble 5700 mark m3 -75° 06' 9.11" 123° 19' 56.02" +-15 note: azimuth (from pillar to mark): 254° 02' 04.5" rms (14.1 mm), sd (4.3 mm) difference to initial azimuth: +5' 12.5" the shelters are partially embedded in the ice horizontal distance on ellipsoid: 25.766 m surface distance on geoid: 25.779 m free air distance: 25.781 m date of survey 28nov. 2005 point lat. (wgs84) dms long. (wgs84) dms horizontal standard deviation (mm) network: pillar p4 -75° 06' 8.87" 123° 19' 59.00" +-34 3 gnss receivers for scintillations novatel vgs4004+ 1 geodetic receiver trimble 5700 mark m4 -75° 06' 9.10" 123° 19' 55.85" +-34 note: azimuth (from pillar to mark) 254° 12' 34" rms (13 mm), sd (12.9 mm) difference to initial azimuth: +15' 42" the shelters are entirely embedded in the ice horizontal distance on ellipsoid: 26.064 m surface distance on geoid: 26.077 m free air distance: 26.080 m table 2. summary results for positioning surveys performed in 2005, 2007, 2009 and 2013 (from top to bottom) aimed to determine precise angular direction between the pillar and the azimuth mark at concordia geomagnetic observatory. 9 54' 24.4" (rms=31 mm), the measure was close to the value obtained in 2003 via astronomical observations with a difference of -147,6" of arc towards south (negative difference, see table 2). at the time of that observation only the dataset from one additional geodetic gps receiver was available. this permanent point, equipped with a trimble 5700, is still working and positioned on the roof of the southernmost building at concordia station. for a target 30 m distant, a deviation of±10 mm corresponds to an angular width of±60"=1' of arc. assuming the moderate threshold of±20" of arc achievable with a gyro-teodolite as an acceptable accuracy for the azimuth determination, during the 2005 measure this limit was overcome by a factor of 9 (rms=31 mm associated to the azimuth determination). in 2007 the unavailability of the permanent gps receiver determines a poorer accuracy with a deviation of±36 mm. a lower level of error was achieved in 2009 when, besides the permanent receiver, an additional dataset was available in the framework of a project aimed to ionospheric scintillation observations via one gps recordings (novatel vgs4004). in 2009 the shelters were partially embedded into the ice. despite the availability of 3 identical gps receivers (novatel vgs4004) operating at 3 different points of the base and the availability of permanent receiver dataset generating a robust network of points for crossed triangulations, in 2013 the complete embedding of the shelters into the ice caused a standard deviation of±34 mm in the position of each single point (pillar p4, azimuth mark m4 in table 2) but a better accuracy in the azimuth determination. for this last determination a final difference of +15' 43" of arc towards north (positive difference) from the initial value was found. in 2009 and 2013 two direct measurements of the distance in free air between the pillar centre and the azimuth mark were executed with tape meter and laser meter, respectively. in the first case the total distance was m 25.76±0.01 (november 2009), very close to the horizontal distance on ellipsoid, and in the second case was m 25.729±0.001 (november 2013). in the limit of the correctness of a free air measurement performed manually outdoor in 2009 and assuming the superior intrinsic precision of the procedure with laser meter, the distance from the pillar to the azimuth mark decreased of about 3 cm in 4 years. the two shelters consequently are moving and getting closer. despite the inaccuracy of each measure shown in table 2, a directional evolution for the different azimuths is observed (figure 5). the limit of the computed errors cannot allow any quantitative indication on the strength of the global displacement. in fact, note that the free air distance between the pillar to the azimuth mark, from the 2009 to 2013 gnss observations is increased by 30 cm which is not realistic and does not validate the laser measurement. nevertheless, this analysis shows that both shelters are almost coherently moving along the years. from the first azimuth determination (line 1 in figure 5) to the third determination (line 3) a displacement toward se and a slight rotation clockwise are shown along the time interval 2005-2009. an opposite direction is visible in the transition from the third to the fourth and last determination (2009-2013) along the direction nnw (about 340° from north), comparable to the result from geodetic studies (direction 302°). it is also interesting to remark that the last azimuth mark position m4 (2013) is getting closer to the first position m1 the concordia geomagnetic observatory at the inland antarctica figure 4. graphic and computer results of a gnss survey carried out in nov. 28, 2005. p1 and m1 indicate geographic positions of the pillar and azimuth mark, respectively. (2005), in a sort of almost closed loop as shown in corresponding path of figure 5 (bottom graph, left side). this last condition may explain the reduced divergence between the measured values of h and d and those from igrf (figure 1), which is evident since 2013 and future observations are needed to follow up this trend. this behavior is due to the fact that relative rotation between pillar and azimuth mark has a stronger effect on the horizontal component and declination, both very sensitive to angular deviations along the years from a stable configuration. the progressive increasing of error associated to the azimuth determination via gnss observations over the years is certainly due to the progressive sinking of the observatory. the current situation cannot allow to run further indoor measurements, being currently the shelters fully incorporated into the ice. generally the condition to operate under a layered and compact ice coverage makes any aspiration to have good quality gnss dataset fruitless, at least at very high latitudes. at concordia, the recently defindi mauro et al. 10 figure 5. graphic rendering (from google earth tm) of concordia station and the evolution of the geographic positions for the pillar (p1 to p4) and azimuth mark (m1 to m4), in successive gnss surveys from 2005 to 2013, together with a schematic sketch of the current configuration of the whole observatory (middle figure). both shelters are completely embedded into the ice at present. each azimuth line is reported from 1 to 4 (bottom graph), according to the year of survey. a translation along the years in two opposite directions and slight rotations are reported also in the text. in the left side, the complete path followed by the azimuth mark position along the years resembles a nearly close loop. 11 itive solution adopted for retrieving gnss data with low rate of degradation consisted in installing some suitable points of observation joint to the roof of the two shelters which are at the same level of the free iced surface (figure 2, sub-picture 8). gnss measurements will be repeated at least once per year, and the additional contributions from frequent verification of the relative distance and rotation between the pillar position and azimuth mark will be tested in the coming years for getting a corrective term to the geomagnetic dataset. considering the prudent approach to assume that the entire effect of the measured movement of the system (azimuth mark and pillar position), whose incidence is within the order of few minutes (max 15') of degree over the years, we can only partially explain the observed features in the declination values, since the deviation seen with respect the igrf model is within a range of 2-3 degrees. investigations and longer series of data are needed to better understand such behavior over the future years. 4. discussion differently from any scientific installations rooted on solid ground thus subjected only to plate tectonic movements as first order of magnitude, generally negligible in geomagnetism, any point of observation laying on layered iced surface may suffer from additional source of displacement mainly due to ice movement. from place to place, local movements strongly influenced by topographic asset and other glaciological dynamics, are detectable. this additional contribution can be relevant and cannot be disregarded especially if issue of position of an object is a fundamental parameter to derive other quantities. this is exactly the case of the geomagnetic observatory established at concordia station, dome c on the antarctic plateau. to avoid biases in the recordings of geomagnetic quantities an occasional monitoring of the pillar and the azimuth mark positions was adopted at concordia observatory via gnss observations to track their spatial migrations along the years. no point of a floating/shifting system can be used as reference for local positioning unless monitored by gnss system or by other absolute positioning methods. any referenced point will be completely defined, giving the absolute geographic coordinates so derived and the date of the surveyed position, since they may change over time. at the concordia geomagnetic observatory the growing rate of error to the azimuth determination via gnss technique is due to the progressive sinking of the observatory, bringing to disregard any further indoor data collection since the shelters are sunk in the ice. the gnss signal degradation is strongly reduced when the displacement is monitored from the free iced surface above the shelters. some permanent points joint to the shelters have been recently selected and arranged in a way that gnss measurements can be routinely repeated. a permanent gnss station and a number of additional (temporary) receivers are necessary to generate a network of control points for a robust approach to the computation of a reliable determination of positions. the further contribution from direct measurements of the relative distance and rotation between the pillar position and azimuth mark will be also inspected in the coming years together with a reliable gnss determination of mutual positions aimed at establishing corrective terms which may effectively counterbalance the real migration of the observed displacements and their effects on geomagnetic dataset. acknowledgements. luca vittuari and vincenzo romano are acknowledged as data provider of gnss recordings. andrea piancatelli is thanked for the gnss survey in 2007. eost (école et observatoire des sciences de la terre, france), ingv (istituto nazionale di geofisica e vulcanologia, italy), ipev (institut polaire paul émile victor) and pnra (programma nazionale per le ricerche in antartide) are also acknowledged for their scientific and logistics support and contributions to the maintaining of the geomagnetic observatory at dome c. pascal bordais, jean-francois vanacker, lucia angioletto, jonathan zaccaria, erick bondoux, antonio litterio and paride legovino are thanked for their geomagnetic observational activities during the winter seasons from 2005 to 2014. references akasofu, s.-i. (2002). exploring the secrets of the aurora, kluwer academic publishers, netherlands. capra, a., and r. dietrich (eds.) (2008). geodetic and geophysical observations in antarctica, springerverlag berlin. chambodut, a., d. di mauro, j. j. schott, p. bordais, l. agnoletto and p. di felice (2009). three years continuous record of the earths’ magnetic field at concordia station (domec, antarctica). annals of geophysics, 52, 15-24. de santis, a., j. m. torta, and l. r. gaya-piqué, (2002). the first antarctic geomagnetic reference model (arm), geophys. res. lett., 29(8), doi:10.1029/2002gl014675. jankowski, j. and c. sucksdorff (1996). guide for magnetic measurements and observatory practice, international association of geomagnetism and aeronomy, boulder (usa), pp. 235. lepidi, s., l. cafarella, m. pietrolungo and d. di mauro (2011). daily variation characteristics at polar geomagnetic observatories, adv. space res., 48, 521-528. rignot, e., j. l. bamber, m. r. van den broeke, c. davis, y. li, w. j. van de berg and e. van meijgaard (2008). recent antarctic ice mass loss from radar inthe concordia geomagnetic observatory at the inland antarctica terferometry and regional climate modelling, nature geosci, 1, 106-110, doi:10.1038/ngeo102. romano v., g. macelloni, l. spogli, m. brogioni, g. marinaro and c. n. mitchell (2013). measuring gnss ionospheric total electron content at concordia, and application to l-band radiometers. annals of geophysics, 56 (2), r0219, doi: 10.4401/ag-6242. scarchilli, c., m. frezzotti and p.m. ruti (2011). snow precipitation at four ice core sites in east antarctica: provenance, seasonality and blocking factors, clim. dyn., 37, 2107-2125, doi:10.1007/s00382-010-0946-4. schott , j.j., d. di mauro, a., peres, l., cafarella, l. magno, a. zirizzotti and a. meloni (2005). towards the opening of a magnetic observatory at domec (antarctica), proceedings xith workshop on geomagnetic observatories, instruments, data acquisition and processing, (kakioka, november 17-24, 2004) šugar, d., m. brkić and d. špoljarić (2012). comparison of the reference mark azimuth determination methods, annals of geophysics, 55 (6), 1071-1083; doi: 10.4401/ag-5405. urbini s., frezzotti m., gandolfi s., vincent c., scarchilli c., vittuari l. and fily m. (2008) historical behaviour of dome c and talos dome (east antarctica) as investigated by snow accumulation and ice velocity measurements. global and planetary change. vol 60/3-4 pp 576-588 doi: 10.1016/j.gloplacha.2007.08.002. vittuari, l., vincent, c., frezzotti, m., mancini, f., gandolfi, s., bitelli, g. and capra, a. (2004). space geodesy as a tool for measuring ice surface velocity at the dome c site and between terra nova bay and dome c (east antarctica), ann. glaciol. 39, 402-408. *corresponding author: domenico di mauro, istituto nazionale di geofisica e vulcanologia, rome, italy; email: domenico.dimauro@ingv.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. di mauro et al. 12 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol50,6,2007 699 annals of geophysics, vol. 50, n. 6, december 2007 key words ionospheric scintillation – gps scintillation monitors – scintillation indices 1. introduction the amount of scintillation activity present in radio waves propagating through the upper atmosphere is estimated by means of scintillation indices which measure the stochastic fluctuating components of these electromagnetic radiations. the scintillation activity is assessed on both intensity and phase of radio signals received on the ground, after propagating through drifting irregularities of the electron density spatial distribution in the ionosphere. the scintillation index, used overall for estimating the fluctuations induced by drifting plasma density irregularities on the signals intensity, is the index s4 (briggs and parkin, 1963), i.e. the intensity standard deviation normalised to the average received intensity and is given by (1.1) where i is the received intensity and the 〈.〉 operator denotes an ensemble average. the phase scintillation index σφ is usually defined as the standard deviation of the received phase. of course, these standard deviations are aimed to estimate the signal fluctuations induced by radio wave scintillations. thus, the signal components subject to fluctuations due to ionoss i i i 4 2 2 2 2 = − on the relationship between the geometrical control of scintillation indices and the data detrending problems observed at high latitudes biagio forte centre for atmospheric research, university of nova gorica, slovenia abstract the experimental estimate of radio waves scintillation, caused by plasma density irregularities in the ionosphere, is usually attempted by means of scintillation indices which are essentially standard deviations of stochasticly fluctuating parts of the received radio wave intensity and phase. at high latitudes, provided that the propagation problem may be modelled by means of the weak scattering theory, the typical scintillation indices s4 and σφ depend on a geometrical factor which introduces some amplifications on their values. scintillation indices s4 and σφ measured at auroral latitudes are estimated by means of different boundary detrending conditions and the geometrical effect on those detrending conditions is investigated. in the case of the polar low earth orbiting (leo) satellite links considered here, high phase with low intensity scintillation events do not seem to be related to geometrical effects only, but rather to misleading data detrending. mailing address: dr. biagio forte, centre for atmospheric research, university of nova gorica, vipavska 13, po box 301, si-5001 nova gorica, slovenia; e-mail: biagio.forte@p-ng.si 700 biagio forte pheric scintillation (i.e., caused by drifting electron density irregularity structures in the ionosphere) need to be identified and separated from other signal components. this is usually accomplished by means of a detrending process applied to the intensity and phase of the received radio signal, filtering out those signal components not pertaining to the scintillation mechanism. the detrending process usually takes place in the frequency domain and is characterised by a low frequency edge below which signal fluctuations and trends are assumed to be not related to ionospheric scintillation. the choice of such a cut-off frequency depends on the geometry of the propagation problem considered. the key point is to choose the cut-off frequency low enough to catch the bulk of intensity scintillation, leaving s4 unmodified for all the experimental conditions, determined by the signal wavelength and by the ionospheric relative drift. the cut-off frequency choice clearly depends on the experimental features implying that a general value does not exist. for instance, in the case of the past wideband experiment based on a polar low earth orbiting satellite transmitting several radio beacon wavelengths, such a value was fixed to be 0.1 hz (fremouw et al., 1978). this has been the cut-off frequency value used overall, in both high and low latitude experiments dealing with generic beacon satellites, although not always leo. in the last decade, the rising interest in assessing the impact of ionospheric scintillation on satellite based navigation systems has led to the development of gps scintillation monitors allowing for the retrieval of the scintillation information from all the satellites on track (van dierendonck et al., 1993; beach and kintner, 1999). the detrending process used in these monitors relies on a fixed low frequency cut-off edge equal to 0.1 hz, as in the case of beacon satellites based experiments. it has been a common and accepted picture that gps scintillation monitors detect high phase scintillation in the presence of low intensity scintillation at high latitudes, implying that gps equipment is subject to disturbances in the phase rather than on the intensity of the signals, irrespective of how this may impact the functioning of gps receivers (doherty et al., 2000; pi et al., 2001; aquino et al., 2007). a possible explanation of «phase without amplitude» scintillation events routinely observed by gps scintillation monitors at high latitudes is based on erroneous data detrending, due to a fixed cut-off frequency (0.1 hz) not appropriate to the propagation conditions which may occur at high latitudes for gps signals (forte and radicella, 2002). moreover, there seems to be no clear evidence for a geometrical control of the scintillation indices as measured on gps links (forte and radicella, 2004), similar to what is observed with polar leo beacon satellites which leads to enhancements of the phase scintillation index whenever the ray path lies within an l-shell (fremouw, 1980). here, some scintillation events measured by means of polar leo beacon satellites are analysed to explain how the classical phase scintillation index σφ may introduce some troubles in the estimate of the phase scintillation activity on a given satellite link. this takes into account possible geometrical effects on σφ and an alternate index for the assessment of phase fluctuations induced by drifting electron density irregularities at ionospheric heights (forte, 2005). 2. experimental data the experimental data used here are based on radio signals coherently transmitted from tsykada beacon satellites at two frequencies: 150 mhz and 400 mhz. these radio signals are received at nearly aligned ground stations which are used overall for tomographic ionospheric reconstruction at european auroral latitudes (fig. 1). the intensity and phase of the received radio signals are sampled at a 50 hz rate making it possible to estimate the scintillation activity experienced while traversing drifting electron density irregularities in the ionosphere. no elevation mask angle is applied to the data, so that border effects at the beginning and end of the time series shown hereafter are associated mainly with low elevation angle phenomena. a detrending process is applied to the raw data to select only those stochastic fluctuations induced by drifting plasma density irregularities on both intensity and phase of the signals received on the ground. the detrending process is 701 geometrical and detrending effects on scintillation indices essentially a high-pass filter designed in the frequency domain and characterised by a fixed cutoff low frequency edge. all the fluctuations corresponding to frequencies below this cut-off are ruled out from the calculation of the scintillation indices, as they are considered not pertaining to ionospheric scintillation. the scintillation indices s4 and σφ are calculated for three different values of the cut-off low frequency edge: 0.1 hz, 0.3 hz, and 0.5 hz. the values assumed for the low frequency limit in the detrending process are consistent with the typical values for the fresnel fluctuation frequencies relative to the geometry of satellites links considered. the fresnel frequency is given by (2.1) where vrel is the relative drift at ionospheric altitudes composed of the satellite motion and the electron density irregularities drift, while df is the fresnel radius. considering that at high latitudes plasma density irregularity structures may have drift speeds ranging from about 100 m/s to about 1 km/s (aarons, 1982), the fresnel frequencies for the tsykada satellites links are expected to be around a few hz. this is also confirmed by the experimental data. figure 2a-d shows some scintillation events observed at kiruna (lat: 67.84°n, long: 20.41°e, f d v rel f f = fig. 1. the ground stations used for recording tsykada signals: tromso (tro), kiruna (kir), lulea (lul), and kokkola (kok). fig. 2a-d. intensity power spectral densities normalised to the mean received intensity, as measured for different satellite links. this shows that a fresnel frequency of a few hz may normally occur at auroral latitudes. the measurements are taken during the night of 30 october 2003 from a) kokkola, b) kiruna, c) and d) lulea. a b c d 702 biagio forte l: 5.6) during the night of 30 october 2003. the power spectral densities corresponding to 20 s time intervals show fresnel frequencies of a few hz as indicated, owing to real values for the relative ionospheric drift, for the distance of the receiver to the phase screen approximating the electron density structures, and for the satellite zenith angle. the values chosen for the cut-off frequency in the data detrending process are aimed to show how s4 and σφ estimates modify as the low frequency detrending limit is shifted towards higher fluctuation frequencies, owing to fresnel frequencies of a few hz for the links considered here. the key point is understanding whether large σφ values may always be explained in terms of geometrically induced enhancements or in terms of misleading data detrending conditions as well (forte, 2005). 3. discussion figure 3 shows scintillation indices measured for the 400 mhz radio signal during a satellite pass over the ground station located in lulea (lat: 65.58°n, long: 22.17°e, l: 4.7) on the night of 30 october 2003 starting at 19:30:29 ut. the top left plot shows s4 values for the three different low frequency detrending limits: the solid line refers to a cut-off frequency of 0.1 hz, the dashed line refers to a cut-off frequency of 0.3 hz, while the dotted line refers to a cut-off frequency of 0.5 hz. the top right plot shows σφ values in radians for the same cut-off frequency values. the bottom left plot shows a new parameter proposed for the estimate of phase scintillation, which is given by (forte, 2005) (3.1) where the 〈.〉 operator denotes an ensemble average and φ is the received and detrended phase. sφ values are given in rad/s. the bottom right plot shows the geometrical factor g (in arbitrary units) responsible for amplifications of σφ values. for axially symmetric irregularity structures g achieves a maximum only at the magnetic zenith, while for sheetlike structures g maximises whenever the propagation vector lies within the plane of the sheet which is also l-shell aligned (rino, 1979). the g factor s t 2 2 2φ =φ c m fig. 3. scintillation indices recorded during the night of 30 october 2003 from the ground station located in lulea. the solid lines refer to a detrending cut-off frequency of 0.1 hz, the dashed lines to 0.3 hz, and the dotted lines to 0.5 hz. also shown the geometrical factor g for this satellite link. 703 geometrical and detrending effects on scintillation indices shows a localised peak for a pierce point latitude between 60° and 70° which is associated with a local peak in both σφ and sφ occurring slightly after 19:35:00 ut. no similar localised peaks appear in s4 which takes low values in the neighbourhood of the localised peak in g. this seems to be a typical situation where a radio wave with a ray path aligned with an l-shell is weakly scattering on an electron density irregularity structure. for all the events shown here the g factor has been calculated assuming sheetlike electron density irregularities structures, which appear to be typical at auroral latitudes. the axial ratios which define g are those relative to sheetlike irregularities, i.e. 8:8:1 (rino and owen, 1980). the values of σφ indeed experience an enhancement due to a pure geometrical factor which is perfectly described by means of the weak scattering theory (rino, 1979). after this peak, an overall increase in the scintillation activity is recorded along the ray path which is not aligned with a magnetic field line anymore. the detrending effect is clearly visible on the scintillation indices: an evident rescaling of σφ occurs when increasing the value of the low cut-off frequency detrending limit, while such a rescaling is essentially negligible on both s4 and sφ. in particular, the rescaling observed on σφ shows how the detrending effect and the geometrical effect are separate and consistent with each other. the σφ enhancement caused by purely geometrical reasons is still evident on its rescaled values as well as on the sφ index. moreover, rescaled values of σφ are still tracking the higher scintillation event starting after 19:38:00 ut, in accordance with s4 and sφ behaviour. a slightly different situation is depicted in fig. 4, which shows scintillation indices measured for the 400 mhz signal during a satellite pass over the ground station located in kiruna on the night of 30 october 2003, beginning at 23:11:39 ut. high scintillation activity is present on this link as shown by all the three indices. there is a peak in g which appears to be soft, owing to a limited ray path alignement with lshells. the geometrical effect on σφ is probably taking place, but very difficult to notice, due to the mixing with an overall high scintillation activity present throughout the entire satellite pass. fig. 4. scintillation indices recorded during the night of 30 october 2003 from the ground station located in kiruna. the solid lines refer to a detrending cut-off frequency of 0.1 hz, the dashed lines to 0.3 hz, and the dotted lines to 0.5 hz. also shown the geometrical factor g for this satellite link. 704 biagio forte on the contrary, the detrending effect is clearly visible: again, an evident rescaling of σφ values occurs when the low frequency boundary of the detrending process is shifted towards higher fluctuation frequencies. such a rescaling does not occur for both s4 and sφ which show almost the same values in the three different detrending conditions. nevertheless, the σφ values corresponding to cut-off frequencies of 0.3 hz (dashed line) and 0.5 hz (dotted line) are still tracking the high scintillation event present throughout the link as their behaviour is consistent with both s4 and sφ trends. although s4 and sφ values remain unmodified after changing the detrending cut-off frequency, the question is still whether the correct cut-off frequency for σφ is 0.3 hz or 0.5 hz. this clearly depends on the fresnel frequency values corresponding to each link geometry. the fresnel frequency may vary even during a single satellite pass owing to the changing satellite position, to the drift of the irregularity structures along the ray path, and to the distance of the receiver from the phase screen associated with those structures.the difficulties related to the choice of a unique detrending cutoff frequency, suitable for a proper estimate of phase scintillation, make the classical σφ a problematic parameter (forte, 2005; beach, 2006). although this has been already observed in the case of gps satellites links at high latitudes and explained in terms of weak scattering results (forte, 2005; forte et al., 2004; beach, 2006), the data presented here show that the detrending problem may be a critical issue also in the case of different satellite links, unless a careful analysis of the propagation geometry and conditions is performed in order to establish the most appropriate cut-off frequency value. for instance, an example of such a careful analysis was provided in the wideband experiment (fremouw et al., 1978). figure 5 provides the last case study to be considered. scintillation indices observed on the 400 mhz link are plotted. these indices refer to the satellite pass started at 18:16:31 ut and received from the ground station located in kiruna. low scintillation activity is observed on this fig. 5. scintillation indices recorded during the night of 30 october 2003 from the ground station located in kiruna (for a time interval different than in fig. 4). the solid lines refer to a detrending cut-off frequency of 0.1 hz, the dashed lines to 0.3 hz, and the dotted lines to 0.5 hz. also shown the geometrical factor g for this satellite link. 705 geometrical and detrending effects on scintillation indices satellite pass. neglecting the initial and final epochs which correspond to low satellite elevation, s4 is around 0.3 first and mostly around 0.1 later. sφ is also mostly around 2÷3 rad/s, which correspond to low scintillation values, as may be noticed by comparing the sφ values appearing in fig. 5 and those in figs. 3 and 4 relative to high scintillation events. the classical phase scintillation index σφ records values around and in excess of 1 rad for a typical cut-off frequency of 0.1 hz, while for cut-off frequencies of 0.3 hz and 0.5 hz there is a significant drop to lower values, more appropriate to low scintillation conditions. both s4 and sφ maintain almost the same values for all the three different low frequency boundary conditions in the detrending process, as the difference between the corresponding values is negligible. the three case studies show how s4 and sφ are not affected by misleading data detrending, as they maintain their values unmodified when shifting the detrending cut-off frequency towards even higher fluctuation frequencies. this confirms the intrinsic weakness of σφ for the estimate of the phase scintillation activity, particularly at high latitudes, and the evident capability of the new sφ parameter to properly assess the impact of ionospheric scintillation on the phase of a radio wave. as may be noticed in fig. 5, a less pronounced peak is present in g which corresponds to a peak in σφ, still visible for the 0.3 hz and 0.5 hz lines. to some extent, such a peak is also reflected in sφ. this proves again that the geometrical effect and the detrending effect are separate and consistent with each other. the geometrical effect only cannot satisfactorily explain the occurrence of high sφ values with low s4 values, not only in the case of gps satellites links but also in the case of tsykada satellites links. even in the case of polar leo satellites links, it seems that a misleading data detrending may still be a critical issue in the estimate of the classical phase scintillation index sφ. 4. conclusions the analysis of the scintillation activity recorded on tsykada satellites links through the estimate of the classical indices s4 and sφ, coupled with the new parameter sφ, clarifies the relationship between the geometrical effect which may control σφ in presence of weak scattering and the detrending effect which may introduce misleading large σφ values, particularly at high latitudes. the case studies considered indicate that the geometrical effect and the detrending effect are separate and consistent with each other. enhancements in σφ owing to localised peaks in the geometrical factor g remain evident even after the rescaling produced by the increase in the low frequency boundary of the data detrending process. only the geometrical effect cannot explain the overall occurrence of high σφ values with low s4 values, as this effect is very localised and limited only to those satellite positions making the ray path aligned with an lshell. in case of satellite passes mostly dominated by high scintillation activity, the geometrical effect seems not to be very evident, while the detrending effect on σφ appears to be indeed very evident. the geometrical effect also seems to be present on the new parameter sφ which does not depend on the low frequency boundary conditions, while s4 does not show any geometrical or detrending effects. the sφ parameter appears to be a more robust indicator for the estimate of radio waves ionospheric phase scintillation, not only in the case of gps links but also for overall polar leo satellites links, such as the tsykada ones. it should be noticed that the sφ index is not related to the existing theory on radio wave scintillations, but more tailored to the operational aspects of satellite based navigation systems. such an index, indeed, is aimed to estimate the signal dynamics caused by ionospheric scintillations which may even lead to signal loss of lock. in this respect, sφ seems to provide some useful insights which may result of particular interest in designing future satellite navigation based devices. the events shown here refer to three different satellite passes at auroral latitudes. a larger statistics is nevertheless needed and will be investigated in forthcoming studies to confirm these initial insights. 706 biagio forte acknowledgements the work related to this paper was funded by the slovenian research agency. the author would like to thank the sodankyla geophysical observatory (university of oulu, finland) for kindly sharing some of the tsykada satellites data. references aarons, j. (1982): global morphology of ionospheric scintillations, proc. ieee, 70 (4), 360-378. aquino, m., m. andreotti, a. dodson and h. strangeways (2007): on the use of ionospheric scintillation indices as input to receiver tracking models, adv. space res., 40, 426-435. beach, t.l. (2006): perils of the gps phase scintillation index, radio sci., 41 (5), rs5s31, doi: 10.1029/ 2005rs003356 beach, t.l. and p.m. kintner (1999): simultaneous global positioning system observations of equatorial scintillations and total electron content fluctuations, j. geophys. res., 104 (a10), 22535-22565. briggs, b.h. and i.a. parkin (1963): on the variation of radio star and satellite scintillations with zenith angle, j. atmos. terr. phys., 25, 339-365. doherty, h.p., s.h. delay, c.e. valladares and j.a. klobuchar (2000): ionospheric scintillation effects in the equatorial and auroral regions, paper presented at institute of navigation gps 2000, september 2000, salt lake city, utah. forte, b. 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(received june 22, 2007; accepted march 3, 2008) e s p e r i e n z e di c o n t r o l l o s u l l a m i s u r a d e l l a t e m p e r a t u r a n e l l a s u b s t r a t o s f e r a giorgio fea n e l l a c o n f e r e n z a i n a u g u r a l e elei corso 1946-17 di m e t e o r o l o g i a p r o s o l a s c u o l a di i n g e g n e r i a a e r o n a u t i c a di r o m a il p r o f . l r e d i a -i s o f f e r m ò s u l l e r i c e r c h e del l u g e o n il ( p i a l e , in u n a s e r i e di r e c e n t i n o i e , ila m e s s o in d u b b i o la v a l i t l i l à dei chili di t e m p e r a t u r a r e l a t i v i a l l a s o l i s i r a l o s f e r a r i c a v a l i con le r a d i o s o n d e e in p a r t i c o l a r e nega l ' e s i s t e n z a d e l l a t r o p o p a u s a . il l u g e o n s o s t i e n e c h e la r a d i a z i o n e d a p a r i e d e l l ' a p p a r a l o r a d i o t r a s m i t t e n t e v e r s o l ' e l e m e n t o t e r m o m e t r i c o , n o n c h é la c o n d u z i o n e t e r m i c a a t l r a v c i s o i s u p p o r l i , s i a n o t a l i da g i u s t i f i c a r e il d u b b i o . l ' a u t o r e a n z i , n e l l a s u a p u b b l i c a z i o n e p i ù rec e n t e a noi p e r v e n u t a ( 1 ). così si e s p r i m e : « l ' o r d i n e di g r a n d e z z a d e g l i e r r o r i di t e m p e r a t u r a r a g g i u n g e l a p i d a m e n l e ( s o p r a i 91100-riimmm!; n i ) u n a d i e c i n a d i g r a d i n e l l a s e d i c e n t e s t r a t o s f e r a » . jn un c o l l o q u i o a v u l o con il p r o f . k r c d i a « l e o , p o c h i g i o r n i p r i m a d e l l a s u a c o n f e r e n z a , idi c o m u n i c a i p e r s o n i m i c a p i c o m e int e n d e v o e l i m i n a r e l ' i n c o n v e n i e n t e i n d i c a l o d a l l u g e o n e l a m i a int e n z i o n e di e f f e t t u a r e ( p i a n t o p r i m a u n a s e r i e eli e s p e r i e n z e . t u t t a v i a , p u r n o n o f f r e n d o il s i s t e m a d a m e i d e a t o a l c u n a p a r t i c o l a r e d i f f i c o l t à t e c n i c a , le e s p e r i e n z e , p e r r a g i o n i e s t r a n e e a l l a ric e r c a s t e s s a , si d o v e t t e r o r i m a n d a r e a m o l l o t e m p o d o p o e c i o è a l p e r i o d o tra l ' o s i a t e del 1947 e l ' i n v e r n o del 1948. 11 m e t o d o c o n s i s t e s e m p l i c e m e n t e nel d i s p o r r e la b a c c h e t t i m i di m a t e r i a l e c e r a m i c o ( a n a l o g o a l l a l i n c i l e ) . com'uucnlc la r e s i s t e n z a v a r i a b i l e con la t e m p e r a i l i r a di u n a r a d i o s o n d a a m e r i c a n a , al d i f u o r i d e l l a p a r t e r e l a t i v a m e n t e m a s s i c c i a , c o s t i t u e n t e il r a d i o t r a s m e t t i t o r e , ad u n a c e r t a d i s i a n z a d a q u e s t a e nel p r o t e g g e r e la stessa r e s i s t e n z a d a l l e r a d i a z i o n i di g r a n d e l u n g h e z z a d ' o n d i : c i o è d a l l e r a d i a z i o n i e v e n t u a l m e n t e p r o v e n i e n t i d a l r a d i o t r a s m e t t i t o r e , a c a u s a d e l l eccesso di t e m p e r a t u r a di q u e s l o ( l i s p c t l o a l l a p a r i e t e r m o m e t r i c a ) eccesso e l l e si v i e n e a c r e a r e nel c a s o d e l l a d i m i n u z i o n e di t e m p e r a t u r a l u n g o la v e l l i c a l e , p e r la g r a n d e d i f f e r e n z a d ' i n e r z i a t e r m i c a e s i s i e n t e a p p u n t o ira t e r m o e l e m e n t o e r a d i o t r a s m e t t i t o r e . 2 4 0 giorgio fea la p r o t e z i o n e ( f i g . 1) è c o s t i t u i t a d a t i c ( e t a l v o l t a ~olo d u e ) c i l i n d r i c o a s s i a l i di s o t t i l i s s i m a f o g l i a di a l l u m i n i o l u c i d o , a v e n t i r a g g i di 1,5 3 4 , 5 c e n t i m e t r i ; essondo lo p e s o r e d e l l a f o g l i a d ' a l l u m i n i o q u a l c h e c e n t e s i m o di m i l l i m e t r o . sono d o v u t i r i n f o r z a r e i c i l i n d r i a g l i e s t r e m i con un l e g g e r o a n e l l o di filo di o t t o n e di 5 d e c i m i di m i l l i m e t r o : l ' i n s i e m e d e i tre c i l i n d r i è fissato l a t e r a l m e n t e a l l a scatola d e l l a r a d i o s o n d a , m e d i a n t e 1 lìli ili o t t o n e da fi d e c i m i e l u n g h i c i r c a 25 c e n t i m e t r i ( f i ; ; . 2). l a p i c c o l i s s i m a m a s s a p e r u n i t à di s u p e r f i c i e d e l l a f o g l i a di a l l u m i n i o u s a t a e l ' o t t i m a c o n d u t t i v i t à d e l m e t a l l o , g a r a n t i s c o n o un inerzia t e r m i c a e s t r e m a m e n t e p i c c o l a p e r lo s c h e r m o r e a l i z z a t o , cosi c o m e il forte p o t e r e r i f l e t t e n t e ne a s s i c u r a un o t t i m o f u n z i o n a m e n t o p e r lo scopo p r e f i s s a l o . p e r p r o v v e d e r e a l l a e l i m i n a z i o n e d e l l effetto d e l l e r a d i a z i o n i , entro i l i m i l i di ogni p o s s i b i l i t à , p e r s u g g e r i m e n t o del d o t t . c i a l d c a e con ia sua g e n t i l e c o l l a b o r a z i o n e , si sono resi s p e c u l a r i gli stessi c l e m e n t i t e r m o s e n s i b i l i col s i s t e m a d e l l ' n a l l u m i n a t u r a « a d o t t a l a c o m e è n o l o n e l l a t e c n i c a o t t i c a m o d e r n a ( * ) . ( * i l o s c r i v e n t i ? a v e v a p e n s a l o , in u n p r i m o t e m p o , ili p r e p a r a r e ex noin p e r e v a p o r a z i o n e n e l v u o t o o p p o r t u n e r e s i s t e n z e s p e c u l a r i , m a p e r i l m o m e n t o n o n è s t a l o a n c o r a p o s s i b i l e r e a l i z z a r l e i n i n o l i o s o d d i s f a c e n t e e si è u t i l i z z i n o c o l i l e r i p i e g o il s i s t e m a i n d i c a t o . e s p e r i e n z a ih ( o n t i i o i . i o dei i.a t e v i p e k a t u h a n e u . a s l b s t h a t o s l eiia 2 1 1 p e r f a r q u e s t o si è u t i l i z z a t o u n i m p i a n t o p e r le e v a p o r a z i o n i nel vuoto d e i m e t a l l i e in p a r t i c o l a r e d e l l ' a l l u m i n i o , i m p i a n t o c o s t r u i t o d a l l o s c r i v e n t e e dal p r o f . < iocconi nel 1941-12 p r e s s o l ' i s t i t u t o di f i sica d e l l ' i diversità d i r o m a , a d a t t a n d o n e a l l o scopo i pezzi c o m p l e m e n t a r i ; in p a r t i c o l a r e si è s t u d i a t o un s i s t e m a c l i c p e r m e t t e s s e d i a l l u m i n a r e c o n t e m p o r a n e a m e n t e p i ù e l e m e n t i , m a n t e n e n d o l i in cont i n u a r o t a z i o n e , d u r a n t e l ' e v a p o r a z i o n e d e l l ' a l l u m i n i o nel v u o t o , p e r a s s i c u r a r e l ' u n i f o r m i t à d e l r i v e s t i m e n t o r i f l e t t e n t e . n a t u r a l m e n t e p r i m a di e i f e l t u a r c il d e p o s i t o di a l l u m i n i o , -i sono p r o t o n i g l i e l e m e n t i con u n a p e l l i c o l a i s o l a n t e di n i t r o c e l l u l o s a d e l l o p e s s o r e di q u a l c h e m i c t o n . gli e l e m e n t i così t r a t t a t i , sottoposti a l l ' a z i o n e d i r e t t a d e l l e rad i a z i o n i di g r a n d e l u n g h e z z a d ' o n d a p r o v e n i e n t i da u n a p o t e n t e s t i l l a e l e t t r i c a , h a n n o m o s t r a l o di s u b i r n e i i n f l u e n z a in m o d o e s t r e m a m e n t e p i ù p i c c o l o di e l e m e n t i non t r a t t a l i , e in p a r t i c o l a r e di a l t r i c h e . a t i t o l o di c o n f r o n t o , e r a n o stati « a n n e r i l i » m e d i a n t e d e p o s i t o , nel xilolo, di n e r o di b i s m u t o o di a n t i m o n i o . a n c h e q u e s t o d e p o s i t o si è e f f e t t u a l o m e d i a n t e un a p p a r a l o c o s t r u i t o d a l l o s c r i v e n t e e dal p r o f , ( a n i o n i i n s i e m e al p r e c e d e n t e p e r i d e p o s i t i s p e c u l a r i . jn a l c u n e d e l l e p r o v e , p e r e s i l i a r e i e v e n t u a l e d i s t u r b o s e g n a l a l o d a l lugeon ( c i o è il « i r a i n i g c ») i p i e l i e l e m e n t i a n n e r i l i sono s i a l i c o l l o c a l i n e l l a p o s t a z i o n e n o r m a l e d e l l ' e l e m e n t o t e r m o s e n s i b i l e , c i o è a l i i n t e r n o d e l l a r a d i o s o n d a , m e n t r e f u o r i -i t r o v a v a n o q u e l l i a l l u m i n a l i . m e n t r e d u n q u e il t e r m o e l e m e n t o è s c h e r m a t o d a l l a r a d i a z i o n e ( l i d i a s c a t o l a del m e t e o r o g r a f o ( * ) con i c i l i n d r i di a l l u m i n i o e con i a l l u m i n a t u r a , la e l i m i n a z i o n e di e v e n t u a l i d i s t u r b i d a p a r t e d e l l a c o n d u z i o n e t e r m i c a 4 è a s s i e m a l a co' l a r e , c o m e g i à a c c e n n a l o , m o l l o l u n g h i e sottili s i a i s u p p o r t i d e l l o s c h e r m o -ia i c o n d u t t o r i p o r t a n t i d a l l ' e l e m e n t o s e n s i b i l e a l l ' a p p a r a l o r a d i o : m a l g r a d o la s o t t i g l i e z z a di q u e s t i u l t i m i (()..'{ m i n i non -i a l t e r a n o in m o d o s e n s i b i l e la r e s i s t e n z a e l e t t r i c a d e l l ' i n t e r o c i r c u i t o t e r m o m e t r i c o e l e v a r i a z i o n i di q u e s t i , 1*1 si n o l i in p a r t i c o l a r e c l i c , p o i c h é n e l l a r a d i o s o n d a a m e r i c a n a 1 i n v o l u c r o . s t e r n o è d i c a n o n e , p e r la s c a r s a c o n d u t t i v i t à t e r m i c a d i q u e s t a s o s t a n z a , l a s u p e r l i c i e e s p o s t a al v e n t o r e l a t i v o r e s t a a t e m p e r a t u r a a s s a i p i ù v i c i n a a q u e l l a d e l l ' a r i a c h e n o n l a p a r l o i n t e r n a e q u i n d i l a d i f f e r e n z a 'li t e m p e r a t u r a c h e p u ò d a r l u o g o a s c a r n i l i p e r r a d i a z i o n e r i s u l t a n o t e v o l m e n t e r i d o t t a : di p i ù i r a ( d e m e n t i l e r i n o m e l r i c i e s c a t o l a , n e l l a d e l l a r a d i o s o n d a -i t r o v a u n c i l i n d r o di s o n i l e l a m i e r a d i l e r r o s t a g n a t o c o m p l e t a m e n t e c i r c o n d a t o d a l l a c o r r e n t e d e l v e l i l o r e l a t i v o ; q u e i o c o s t i t u i s c e u n ' u l t e r i o r e d i f e s a d a l l e r a d i a z i o n i . 242 c10hc10 ffa d a t o il v a l o r e d e l l a resistenza t e r m o m e t r i c a . clic è di 15.000 q c i r c a a t e m p e r a t u r a a m b i e n t e (15" c ) e il -no f o r t i s s i m o c o e f f i c i e n t e ili t e m p e r a t u r a ( c i r c a — 0 . 0 1 p e r g r a d o ) . l e e s p e r i e n z e sono state e f f e t t u a t e di n o t t e p e r e s c l u d e r e l ' e f f e t t o d e l l a r a d i a z i o n e s o l a r e e in cs-e sono t a t e usate, c o m e a c c e n n a t o , le r a d i o s o n d e di d o t a z i o n e d e l l e f o r z e yriuate a m e r i c a n e ; in q u e s t e -onde, ai m o r s e l l i d e l l a resistenza i g r o m e t r i c a si sono c o l l e g a t i i cond u t t o r i p o r t a n t i a l l a r e s i s t e n z a t e r m o m e l r i c a p r o t e t t a e s t e r n a , d o p o a v e r o p p o r t u n a m e n t e a d a t t a t a la c o n t a t t i e r a c o m a n d a t a d a l l e c a p s u l e b a r o m e t r i c h e , in (pianto essa n o r m a l m e n t e e s c l u d e a p p u n t o la p a r t e i g r o m e t r i c a al disotto ili una c e r t a p r e s s i o n e , cioè p r e s s ' a poco al ili sopra di 8000 i n . la resistenza t e r m o m e t r i c a n o r m a l e sj l a s c i a t a al suo posto .-alvo in q u a l c h e prova n e l l a ( p i a l e es~a è stata sostituita d a una res i l e n z a « a n n e r i l a » per lo c u p o già i n d i c a t o . nelle u l t i m e p r o v e -i e e l i m i n a t o il più i n t e r n o dei ire c i l i n d r i p r o t e t t i v i p e r c h é come schert a b e l l a 1 t a b e l l a 2 p ili li 1 "c l ili s 4011 — 34.1 350 40.11 7.75 300 50.0 250 — 58.5 8.31ì 200 — 51).'j 177 — 47.3 7.08 t a b e l l a 3 p mb t "c 1 ms -hill 35.0 350 — 41.8 8.32 300 10.2 250 — 56.6 9,211 200 — 52.0 150 — 53.0 6,92 125 — 55.8 p mb ' a " < ' n ° c ' a < n 1 111 s 400 — 36,7 — 36.2 — 0,5 350 — 43,0 12.3 0.7 3.07 3110 — 50.6 49,3 1.3 251) — 56.8 — 55.2 1.6 2.56 200 48,8 17.11 1.8 150 — 55.2 — 53.4 — 1.8 2.33 136 54.8 — 53,0 1-8 abella 4 p mb f a ° c ts°c « a ' . n km/s 401) — 36-1 — 35-2 — 0,9 350 43,6 — 42.8 — 0.8 8.20 300 51,8 — 51.0 — 0.8 250 — 53,0 — 53.1 + 0,4 6.13 2011 51.4 — 51.7 + 0.3 151) — 53.1 — 53.3 + 0.2 4,75 108 — 54,7 — 55.0 + 0,3 e s p e r i e n z e di c o n t r o l l o d e l l a t e m p e r a t u r a n e l l a s 1 ii s t r a t o s f e r a 2 1 3 ìlio è s u p e r f l u o , m e n i l e p u ò r e n d e r e m e n o a t t i v a la c i r c o l a z i o n e d ' a r i a i n l o r n o al l e r m o e l c m e n t o . s i n o a l l a q u o t a 15 k m i r i s u l t a l i sono stati a s s o l u t a m e n t e negativi nel senso c l i c non si è c o n s t a t a t a d i f f e r e n z a s e n s i b i l e ( v a l e a d i r e s u p e r i o r e a g l i s c a r t i di i -=2 "c n o r m a l m e n t e t o l l e r a t i ) tra i d u e e l e m e n t i e s t e r n o ed i n t e r n o , fosse q u e s t o a n n e r i t o o n o . e s o p r a l u t l o non -i è n o t a l o a l c u n o s c a r t o n e l l e d i f f e r e n z e di t e m p e r a t u r a tra i d u e c l e m e n t i d a l l a q u o t a 9000-:1.0000 m e t r i a 15000. s c a r t o c l i c a v r e b ' r 1 ' •• ! ^ ' 'i ' • i ( f . ; il \ . : . i .. ' f i g . 4 b e i n d i c a t o a p p u n t o un s e n s i b i l e « i r a i n a g e » a q u e l l a q u o t a p e r l a r a d i o o n d a u s a l a . c i ò è p r o v a l o in p a r t i c o l a r e d a i o n d a g g i e l l e t l u a t i a p p u n t o con e l e m e n t i a l l u m i n a l i e a n n e r i t i il 1 1 f e b b r a i o 1948 a l l e o r e 2 1 , 3 0 e il 24 f e b b r a i o 1918 a l l e o r e 18,-10; l e t a b e l l e 2 e 1 ne f o r n i c o n o i d a t i i l l u s t r a t i d a l l a lig. 4 in b e (/. a t i t o l o ili c o n f r o n t o la f i g u r a 1 in a e e r i p r o d u c e 1 a n d a m e n t o dei s o n d a g g i n o r m a l i e f f e t t u a t i a l l e ore lò d e l i i e a l l e 01 del 15 f e b b r a i o , m e n t r e i d a t i r e l a t i v i -imo r i p o r t a t i n e l l e t a b e l l e i e 3. in q u e s l i u l t i m i d u e s o n d a g g i a p p a r e un a n d a m e n t o di t e m p e r a t u r a q u a s i i d e n t i c o a q u e l l o r i s c o n t r a t o nel o n d a g g i o e f f e t t u a l o n e l i i n t e r v a l l o di t e m p i ; i r a i d u e a l l o -copi» s p e r i m e n t a l e : in p a r t i c o l a r e a p p a r e e v i d e n t e la i n v e r s i o n e a l l a b a e d e l l a s t r a t o s f e r a ( m i p e r doni l ' e s p r e s s i o n e il p r o f . l u g e o n ) i n v e r s i o n e p r o b a b i l m e n t e d o v u t a ad u n a s u h i d e i i z a d e l l a s u b s t i a l o s f e r a a c c o m p a g n a n t e la f o r t e i n v a s i o n e di a r i a f r e d d a m a n i f e s t a n t e i n e l l a t r o p o s f e r a in q u e l p e r i o d o . k* e v i d e n t e c h e . se a n z i c h é di u n a v e r a i n v e r s i o n e si t r a t t a s s e d i una p s e u d o i n v e r s i o n e d o v u t a al « t r a i n a g e » , essa s a r e b b e a p p a r s a con c a r a t t e r i s t i c h e assai d i v e r s e nel s o n d a g g i o d e l l e 21,3(1. effettuate, con v e l o c i t à r i d o t t a a m e n o di un t e r z o d e l l a n o r m a l e . d a l t r a p a r t e essa non a p p a r e nel s o n d a g g i o del 24 f e b b r a i o e f f e t t u a t o con le stesse m o d a l i t à . i s o n d a g g i ili p r o v a p i ù r e c e n t i sono stati e f f e t t u a t i con d u e p a l loni di cui il s u p e r i o r e di 2 kg di f o r z a a s c e n s i o n a l e e 1 i n f e r i o r e di 2 , 5 k g . v i n c o l a t i c o m e è i n d i c a l o d a l l a f i g . 3, o n d e e v i t a r e l e oscill a z i o n i f a s t i d i o s e c h e si h a n n o col i s t e m a a i . s i e d a t a u n a m a g g i o r forza a s c e n s i o n a l e al p a l l o n e i n f e r i o r e a f f i n c h é i pezzi di q u e s t o , u n a v o l t a a v v e n u t o lo s c o p p i o , s i a n o sostenuti d a q u e l l o s u p e r i o r e e non d a n n e g g i n o l a s o n d a . si p u ò c o n c l u d e r e c h e , p u r non n e g a n d o c h e a l c u n i t i p i ili s o n d a p o s s a n o p r e s e n t a r e u n e f f e t t o di « t r a i n a g e » verso i 10000 i n , i d a t i di t e m p e r a t u r a o t t e n u t i d a l l e r a d i o s o n d e d i t i p o a m e r i c a n o a t t u a l i , in c o n d i z i o n i n o r m a l i di l a n c i o non sono d i s t u r b a l i , a l m e n o s i n o a l l e q u o t e i n d i c a l e , dairli e f f c l l i t e r m i c i d o v u t i al c o r p o d e l l a r a d i o s o n d a slessa: p e r t a n t o le e v e n t u a l i i n d i c a z i o n i r e l a t i v e a l l a e s i s t e n z a d e l l a t r o p o p a u s a e i v a l o r i di t e m p e r a t u r a d e l l a s u b s l r a t o s f c r a con es-c r i c a v a l i sono a t t e n d i b i l i ; è d a n o t a r e c h e t a l i r a d i o s o n d e , c o n s e n t o n o di r a g g i u n g e r e i 20-25 k m di q u o t a e c l i c a t t r a v e r s a n o l ' a l t a troposfera e la s u b s t r a t o s f e r a con v e l o c i t à d e l l ' o r d i n e di 6 m e t r i s e c o n d o . l a p r e p a r a z i o n e tecnica d e l l e e s p e r i e n z e , e f f e t t u a t e con la c o l l a b o r a z i o n e del t c n . p a l l e t t a m e s s o la s e z i o n e r a d i o s o n d a g g i d e l l ' a e r o p o r t o di ( t a m p i n o per conto iella s e z i o n e s t u d i del s e r v i z i o m e t e o • o l o g i e o d e l l a e r o n a u t i c a , è « l a t a c o m p i u t a presso l ' i s t i t u t o n a z i o n a l e di g e o f i s i c a . roma — istituto nazionale di geofìsica — febbraio l'ilo. riassunto si è e f f e t t u a t a una serie di sondaggi sino a 15000 metri con radiosonde americane munite di due termoelementi in condizioni divrrsr e con diverso potere assorbente ; ir radiosonde sono siate fatti satire a velocità molto diverse; la concordanza dei dati rilevati sembra e s p e r i e n z e 1)1 lo.ntkoi.i.o d e l l a t e m p e r a t u r a n e l l a s u b s t r a t o * p e r a 2 4 5 mostrare chi' per (piente sonde, fino alla quota esplorata, l ' e f f e t t o di trainage" segnalato dal lugeon, sia trascuratile e che oltre alla esistenza di una netta discontinuità nel gradiente di temperatura <111 10000-12000 metri si possa confermare un gradiente (piasi nullo tra questa (piota e la (/nota 15000. b i b l i o g r a f i a i ' ) l i i;eon jean e ai.eermann p a u l , h'invraisembhtble stratosphère d'opre* no* inesures du trainage tliemiique de nonibrenx me/eorograplies. acles de la sor. h e l v e l i q u e d e s s e . n a t . f r i b o u r s r , 133 ( 1 9 4 5 ) . vol49_1_2006def 65 annals of geophysics, vol. 49, n. 1, february 2006 key words hyperspectral – airborne scanner – mivis – radiometric calibration – data quality 1. introduction the uncertainty introduced by the retrieval methods of remotely sensed measurements is related both to sensor calibration and its stability, as well as to atmospheric effects. as a matter of fact, the problem of data calibration has become a key problem for many remote sensing applications. the main task of the calibration process is to experimentally determine the relationship between the measured signals, expressed as instrumental counts, and the corresponding physical values of radiance of the investigated targets, and it has to guarantee the reliability and reproducibility of the measured object radiances not only in the laboratory but also throughout the mission. the methods used for sensor calibration can be grouped into three domains (dinguirard and slater, 1999): pre-launch, in-flight and indirect or vicarious approaches. as many authors stated (teillet et al., 1997; thome, 2002), to derive high quality data from remote sensed images, sensors must be time-stable instruments with well-understood characteristics. therefore, it is always a good norm to include calibration approaches that are independent from pre-flight in research programs. traditionally, the radiometric calibration of satellite systems has been accomplished by on-board calibration a new methodology for in-flight radiometric calibration of the mivis imaging sensor marco gianinetto and giovanmaria lechi laboratorio di telerilevamento, dipartimento di ingegneria, idraulica, ambientale, infrastrutture viarie, rilevamento (diiar), politecnico di milano, italy abstract sensor radiometric calibration is of great importance in computing physical values of radiance of the investigated targets, but often airborne scanners are not equipped with any in-flight radiometric calibration facility. consequently, the radiometric calibration or airborne systems usually relies only on pre-flight and vicarious calibration or on indirect approaches. this paper introduces an experimental approach that makes use of on-board calibration techniques to perform the radiometric calibration of the cnr’s mivis (multispectral infrared and visible imaging spectrometer) airborne scanner. this approach relies on the use of an experimental optical test bench originally designed at politecnico di milano university (italy), called mivis flying test bench (mftb), to perform the first on-the-fly (otf) calibration of the mivis reflective spectral bands. the main task of this study is to estimate how large are the effects introduced by aircraft motion (e.g., e.m. noise or vibrations) and by environment conditions (e.g., environment temperature) on the radiance values measured by the mivis sensor during the fly. this paper describes the first attempt to perform an on-the-fly (otf) calibration of the mivis reflective spectral bands (ranging from 430 nm to 2.500 nm). analysis of results seems to point out limitations of traditional radiometric calibration methodology based only on pre-flight approaches, with important implications for data quality assessment. mailing address: dr. marco gianinetto, laboratorio di telerilevamento, dipartimento di ingegneria, idraulica, ambientale, infrastrutture viarie, rilevamento (diiar), politecnico di milano, piazza l. da vinci 32, 20133 milano, italy; e-mail: gianinetto@polimi.it 66 marco gianinetto and giovanmaria lechi sources (such as lamps, diffusers, etc.) supported by vicarious calibration activities using natural targets, while nowadays several sensors are using the moon (barnes et al., 1999; kieffer and wildey, 1996) or stellar targets (bowen, 2002) as calibration references. unfortunately, most of the airborne sensors are not equipped with any inflight radiometric calibration device. for this reason, the calibration procedure of airborne systems usually relies only on pre-flight and indirect approaches that cannot assess the real performance of the sensor during the fly. this paper introduces an experimental approach that makes use of on-board calibration techniques to perform the radiometric calibration of cnr’s mivis (multispectral infrared and visible imaging spectrometer) airborne scanner. this approach relies on the use of an experimental optical test bench originally designed at the remote sensing laboratory of the politecnico di milano university (italy), called mivis flying test bench (mftb), to perform the first on-thefly (otf) calibration of the mivis reflective spectral bands. the otf of the mivis sensor was performed with a 1-h in-flight atp (acceptance test procedure), using the mftb as reference source mounted onto the mivis scan head. by rationing the digital numbers (dns) output from the mivis with the laboratory measured radiance of the optical test bench, the absolute otf radiometric calibration of the mivis hyperspectral scanner was achieved. the main task of this study was to estimate how large were the effects introduced by aircraft motion (e.g., noise, vibrations) and by operational conditions (e.g., environment temperature) on radiance values measured by the mivis hyperspectral scanner, and also to verify the temporal stability of the mivis sensor during the fly. all this to improve the radiometric calibration of mivis data. a brief overview of the mftb calibrator and first results of the mivis otf calibration tests are presented. 2. the mivis sensor the mivis is an airborne hyperspectral scanner for remote sensing applications belonging to italian cnr-iia, roma. the mivis optical system consists of a scanner and four spectrometers. line array detectors are used, requiring scanning to be performed mechanically in whisk broom mode (bianchi et al., 1995). one spectrometer line detector array exists for each of the visible (vis), near infrared (nir), shortwave infrared (swir) and thermal infrared (tir) portions of the spectrum, for a total of 102 spectral bands ranging from 430 nm to 12 . 680 nm. 3. radiometric calibration of the mivis sensor and the mftb the radiometric calibration of the mivis hyperspectral scanner is usually done prior to the flight with the optical test bench daedalus ab532 which was supplied from the manufacturer (daedalus enterprises, ann arbor, mi, u.s.a.). for vis, nir and swir calibration (ranging from 430 nm to 2.500 nm) the daedalus ab532 optical bench uses a white spectralon (polytetrafloroethylene) panel illuminated under a fix geometrical configuration, while for tir calibration (ranging from 8.200 nm to 12 .700 nm) it uses a blackbody flat plate. the dn to at-sensor-radiance conversion is performed using a self calibration procedure called acceptance test procedure (atp). in particular, for the 92 reflective bands (spectral bands from no. 1 to no. 92), the atp measures, for every mivis scanning frequency (from 8.3 hz to 25 hz), the radiance of the daedalus ab532 test bench illuminated at constant power (fig. 1). the limit in using the daedalus ab532 test bench for deriving the mivis radiometric calibration parameters (gains and offsets) is that the daedalus ab532 test bench is not a transportable device (dimensions: 890 mm × 640 mm × 1200 mm, weight: 160 kg), thus it is not possible to test the scanner’s performance during the flight. to estimate how large are the effects introduced by the aircraft motion and environment conditions (e.g., temperature effects) on the radiance values measured by the mivis sensor, a very compact and light calibration device was designed to be used for mivis otf radiometric calibration. 67 a new methodology for in-flight radiometric calibration of the mivis imaging sensor the mftb was designed at politecnico di milano university (italy) and built with the technical support of compagnia generale ripreseaeree (cgr), parma, italy (now a blom asa company). the project of the mftb concerned the definition of physical, optical and radiometric characteristics of the new test bench system, followed by the building of a prototype. the mftb is composed of two different sub systems: the test bench and the power supply. the test bench was shaped to fit the mivis scan head during the flight and was designed to fit in the aircraft casa 212c used to carry the mivis (140 mm × 340 mm × 210 mm). it was built using a 3mm aluminium sheet for the external chassis and a 10mm white teflon coat for the internal lambert diffuser. as light sources, four 10 w halogen lamps (with colour temperature 2900 k) were used, disposed with a symmetrical geometry. to supply the mftb both using the 220 v ac and the aircraft 28 v dc, a current controlled power supply with constant output (3.32 a at 12.5 v dc) was built. 4. field experiments and preliminary results after a laboratory check, the radiance of the mftb was measured using a laboratory analytical spectral device (asd) fieldspec fr spectroradiometer. the asd was chosen because of its spectral range similar to that of the mivis reflected bands (ranging from 350 nm to 2.500 nm) and its higher sampling frequency (1.4 nm at 350-1.000 nm and 2 nm at 1.000-2.500 nm). moreover, for the vis and the nir bands, the noise equivalent radiance (ner) of the asd is of one order of magnitude smaller than the ner of the mivis, and is of two order of magnitude fig. 1. example of acceptance test procedure (atp) self calibration procedure output file for the first 20 mivis spectral bands. 68 marco gianinetto and giovanmaria lechi smaller for the swir. therefore, the uncertainties in using the asd to validate the mftb calibrator is at least one order of magnitude smaller than the mivis precision. the construction phase of the mftb was followed by a period of validation tests at cgr, where the intercalibration between the daedalus ab532 and mftb optical test benches was achieved on the basis of a traditional laboratory atps. to realize the otf calibration for the mivis sensor, the mftb was mounted onto the mivis scan head and the sensor was flown to perform a 1-h in-flight atp under controlled conditions (fig. 2). to control the mftb calibrator stability during the tests, the otf atp was carried out controlling the current flowing from the mftb power supply to the mftb internal lamps (3.32 a at 12.5 v dc). concerning the use of the mftb during the flight, unpredictable differences in at-sensor radiance were found comparing the results of the laboratory calibration tests with those performed on-the-fly (fig. 3). while differences in at-sensor radiance measured in the vis (ranging from 430 nm to 830 nm) are small and limited to 1-2%, in the nir (ranging from 1.150 nm to 1.550 nm) they grow up to more than 6.5% for mivis spectral band no. 27 (1.450 nm-1.500 nm wavelength) and no. 28 (1.500 nm-1.550 nm wavelength). our otf experiments are in accordance with the differences found in the mivis survey of grado (italy) in winter 2000 (giardino et al., 2001), where, after performing on the mivis data atmospheric correction with the second simulation of satellite signal in the solar spectrum (6s) radiative transfer code (vermote et al., 1997), researchers found that the mivis nir data and the ground reference measurements taken with the asd during the survey diverged of about 10%. analysis of results points out the limitations of traditional calibration techniques for airborne sensors. using a traditional methodology (laboratory test bench or laboratory integrating sphere) fig. 2. relationship between dn and at-sensor-radiance computed with the otf procedure (otf) and with the traditional laboratory atp (laboratory). comparison of results for the mivis spectral bands no. 26 and no. 27 using the mftb calibrator. 69 a new methodology for in-flight radiometric calibration of the mivis imaging sensor for computing the instrumental gain and offset parameters, it is not possible to test the sensor in real conditions. for this reason, a precise data conversion from instrumental counts into radiance values can be only derived in post-processing using on otf procedure as that here described. as shown in our tests, sometimes these differences can be not negligible and also seems to be wavelength dependant, not only a constant bias. 5. conclusions for many remote sensing applications the use of calibrated data has become one of the most important points. besides, the use of a large number of narrow spectral bands forces hyperspectral sensors to have snr that decreases with increasing wavelength, and this can restrict the real use of hyperspectral data to only visible and near-infrared data, if not adequately calibrated (colombo et al., 2002). a new optical test bench for the mivis airborne hyperspectral scanner (mftb) was originally developed at politecnico di milano university to improve the use of remote sensed mivis data. the new approach to airborne otf calibration proposed in this paper and the use of the experimental mftb has shown that mivis data may be affected by considerable error if the radiometric conversion procedure is not based on real in-flight conditions, as giardino et al. (2001) showed with an independent study for grado (italy) mivis survey. fig. 3. comparison between ground tests and otf tests. differences in at-sensor-radiance stands out between 1% and 2% in the vis (ranging from 430 nm to 830 nm wavelength) and more than 6.5% in the nir (ranging from 1.150 nm to 1.550 nm wavelength). (*) = spectral band not working during the tests. 70 marco gianinetto and giovanmaria lechi acknowledgements this work has been carried out under a research framework founded by italian national research council (cnr), contract title «calibrazione radiometrica su 102 canali distribuiti fra le bande dal visibile all’infrarosso termico dello scanner iperspettrale aerotrasportato mivis del cnr» and by the italian space agency (asi) within the project «certificazione di qualità delle mappe tematiche e dei modelli digitali delle elevazioni (dem) ottenute con tecniche di telerilevamento e standardizzazione di legende, classificatori e caratteristiche spettrali radiometriche e geometriche» contract number i/r/132/01. the mftb development would not have been possible without the technical support of compagnia generale ripreseaeree, parma (italy). in particular, the authors wish to thank l. ferretti, a. cavazzini, a. mantovani and g. pizzaferri. the remote sensing institute of the cnr-irea, milano (italy) contributed additional technical support regarding measurements with the asd spectroradiometer. special thanks go to c. giardino (cnr-irea) and m. lualdi (dis – politecnico di milano university, italy) for their help in this research. references barnes, r.a., e.e. eplee, f.s. patt and c.r. mcclain (1999): changes in the radiometric sensitivity of the seawifs determined from lunar and solar-based measurements, appl. opt., 38, 4649-4664. bianchi, r., r.m. cavalli, l. fiumi, c.m. marino and s. pignatti (1995): cnr lara project, italy: airborne laboratory for environmental research, in proceedings of the fifth jpl airborne earth science workshop, 2327 january 1995, pasadena, u.s.a., 1, 15-18. bowen, h.s. (2002): absolute radiometric calibration of the ikonos sensor using radiometrically characterized stellar sources, in proceedings of the pecora 15/land satellite information iv/isprs commission i/fieos 2002 conference, 10-14 november 2002, denver, colorado. colombo, a., m. gianinetto and g. lechi (2002): dall’iperspettrale al superspettrale, in atti della vi conferenza nazionale asita, 5-8 november 2002, perugia, italy. dinguirard, m. and p.n. slater (1999): calibration of space-multispectral imaging sensors: a review, remote sensing environ., 68 (3), 194-205. giardino, c., p.a. brivio and m. sgavetti (2001): effetti della geometria della ripresa e della radianza dell’atmosfera sull’interpretazione di dati mivis, aitinforma, rivista italiana di telerilevamento, 20/21, 99-105. kieffer, h.h. and r.l. wildey (1996): establishing the moon as a spectral radiance standard, j. atmos. oceanic technol., 13 (2), 360-375. teillet, p.m., d.n.h. horler and n.t. o’neill (1997): calibration, validation, and quality assurance in remote sensing: a new paradigm, can. j. remote sensing, 23 (4), 401-414. thome, k.j. (2002): ground looking radiometric calibration approaches for remote sensing imagers in the solar reflective, in proceedings of the pecora 15/land satellite information iv/isprs commission i/fieos 2002 conference, 10-14 november 2002, denver, colorado. vermote, e.f., d. tanré, j.l. deuzé, m. herman and j.j. morcrette (1997): second simulation of satellite signal in the solar spectrum, 6s: an overview, ieee trans. geosci. remote sensing, 35 (3), 675-686. yokoyama 709_722.pdf annals of geophysics, vol. 45, n. 6, december 2002 709 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan izumi yokoyama (1) and antonio nazzaro (2) (1) higashi 1-17-7-1304, kunitachi, tokyo, japan (2) viale italia 419, avellino, italy abstract campi flegrei is a unique volcanic region located near naples, italy. anomalous crustal movements at pozzuoli in campi flegrei have been documented since the roman period. the movements were gradual and have continued to the present, occasionally accompanying swarms of local earthquakes and volcanic eruptions. generally the movements proceed with low seismicity. after the 1538 eruption of monte nuovo, pozzuoli had subsided monotonously, but it changed to uplift abruptly in 1969. the uplift accelerated in 1983 and 1984 reaching more than 2 m, and thereafter began to subside. many discussions of this event have been published. in japan, we have examples of deformations similar to those at campi flegrei, mainly in volcanic areas, and rarely in non-volcanic areas. the former includes iwojima, miyakejima and aira caldera while the latter is represented by cape omaezaki. iwojima is a volcano island, and its secular uplifts since the 18th century are recognized as an unusual event. miyakejima volcano and aira caldera exhibited anomalous movements with low seismicity after their eruptions. cape omaezaki is not situated in volcanic zone but near a subduction zone, and gradually and continuously subsides as a precursor to a large earthquake. in such cases as campi flegrei and the japanese localities, we would question whether the deformations are accompanied by normal seismicity or low seismicity. to examine quantitatively the relationship between seismicity and related deformation, seismic efficiency is generally useful. the crustal deformations in all the regions cited above are characterized by exceptionally low seismic efficiencies. in the present paper, the deformations at pozzuoli and iwojima are mainly described and a comparative discussion among these and other localities in japan is supplemented. it is concluded that such anomalous phenomena in volcanic areas are attributable to peculiar rheological aspects of the material composing the local upper crust, and the deformation in a non-volcanic area is of tectonic origin. 1. introduction we know many examples of ground deformation accompanying major earthquakes: they usually appear instantaneously with the mailing address: dr. izumi yokoyama, higashi 1-177-1304 kunitachi, tokyo 186-0002 japan; e-mail: iyokoya@aol.com key words crustal movements – low seismic efficiency – campi flegrei – iwojima – aira caldera – miyakejima – cape omaezaki earthquakes, and often accompany the formation of faults and fractures. on the other hand, there are a few examples of secular ground deformations with very low seismicity. in italy, these are called «bradisismi» (slow movements), often observed along shores. serapeo, the ruins of a roman market place at pozzuoli being located at the shore of campi flegrei, has been used as a benchmark for sea level changes for 20 centuries. the anomalous uplifts since 1969 centering on pozzuoli were remarkable and are called the pozzuoli event. in japan, secular ground movements are usually related to volcanic activities such as iwojima, aira caldera 710 izumi yokoyama and antonio nazzaro and miyakejima volcano. in addition, there is one of slow movement in non-volcanic area: cape omaezaki in the tokai district has subsided secularly since the first survey in 1962, and the subsidence is one of the precursors of a large subduction earthquake. in the following, the activities of campi flegrei and iwojima are briefly described mainly on basis of the published data, and then some similar examples of anomalous deformations such as aira caldera, miyakejima volcano and cape omaezaki, are mentioned. to examine quantitatively the relationship between seismicity and related deformation, seismic efficiencies of some earthquakes are estimated. in the closing section of this paper, their characteristics are comparatively discussed mainly from a standpoint of their origins. in the present paper, gradual ground deformations are limited to vertical ones. 2. campi flegrei campi flegrei is a caldera measuring 15 km in diameter and consists of multiple phreatic craters, monogenetic cones and fumaroles. a topographic sketch map of campi flegrei is shown in fig. 1 where the southern half of the caldera is bounded by three shallow banks in pozzuoli bay. the town of pozzuoli is located near the center of the flegrean caldera and has been populated since the roman period. 2.1. geologic setting of campi flegrei campi flegrei caldera was formed approximately 35 000 years b.p. by the eruption of 80 km3 of the campanian tuff (rosi et al., 1983). judged from the bouguer gravity anomalies there, the caldera is funnel-shaped and its center coincides with the town of pozzuoli. the caldera fill is the tuff and the fall-back which have lower density than the basement rocks (carbonatic and/or thermometamorphic rocks). according to fedi et al. (1991), the low density material (∆ρ = – 200 kg/ m3) fills the caldera to a depth of approximately 2 km at the center. 2.2. ground deformations and seismicity at pozzuoli anomalous upheaval since 1969 centering on pozzuoli in a densely populated area attracted interest of earth scientists all over the world. this is called the pozzuoli event (yokoyama, 1970). many research papers were published on this event: the special issues of bulletin volcanologique, vol. 47 (1984, eds. f. barberi, d. hill, f. innocenti, g. luongo and m. treuil) and journal of volcanology and geothermal research, vol. 48 (1991, eds. g. luongo and r. scandone) are devoted to the events in campi flegrei, including 20 and 15 papers, respectively. in the present paper, we owe various pieces of information and interpretation of the pozzuoli event to these papers. the height above sea level of serapeo at pozzuoli has been documented from time to time since the roman period. actually it may be rather difficult to establish the trend of deformation at serapeo, especially during the ancient periods. parascandola (1947) proposed the secular vertical deformation at serapeo as shown in fig. 2a where we can infer a relationship between the fig. 1. topographic sketch map of campi flegrei (after dvorak and gasparini, 1991). 711 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan deformation and large eruptions of vesuvius. actually the areas are connected through an underwater tectonic structure (nazzaro, 1985). serapeo subsided monotonously for approximately 9 centuries since the roman period, with the exception of short periods of uplift related to the 79 eruption of vesuvius and one other eruption. in the 10th century, the pattern changed to uplift and sharply rose in case of the 1538 eruption of monte nuovo. after 1538, it again subsided till 1969. according to dvorak and gasparini (1991), it is probable that no uplift larger than a few meters occurred during the period between 1538 and the 1820’s when the sea level measurements near pozzuoli were made for the first time. serapeo subsided at an average rate of 1.4 cm/yr between 1820’s and 1969. in 1969, it began to rise suddenly probably due to magma or steam intrusion beneath campi flegrei caldera. after 1969, the precise levels have been repeated around pozzuoli. oliveri del castillo and quagliariello (1969) found that pozzuoli had uplifted approximately 94 cm since 1953, by mareographic observations on february 5, 1970. the upheaval probably began in 1969. on this assumption, it may have continued to sub-side for 16 years from 1953 to 1969. if we assume that pozzuoli subsided 22 cm (= 1.4 cm/yr × 16 yr) from 1953 to 1969, we may conclude that it had uplifted 116 cm (= 94 cm + 22 cm) over a period of nearly 4 months; the rate is approximately 3.5 m/yr. since the end of february 1970, a seismometric network has been installed in campi flegrei. berrino and corrado (1991) and berrino (1998) discussed the sea-level changes and the temporal trends of seismicity at pozzuoli during the period 1970 ~ 1995 as shown in fig. 3 where the deformation curve has roughly similar trend to that of the seismicity diagram. this means qualitatively that the volume changes were accomodated by seismic failure. in the figure, there are two peaks on both trends, in 1973 and 1984. during 1970 ~ 1973, both deformation and seismicity were rather low: a few microearthquakes of magnitude m 0 ~ 2 were registered every day, and pozzuoli uplifted approximately 70 cm in total. this was a measurable ground deformation with low seismicity. during 1983 ~ 1984, the activities increased remarkably. according to dvorak and gasparini (1991), on may 15, 1983, an earthquake of m 3.4 took place, and was followed by shallow earthquake swarms for 15 months. the largest earthquake of m 4.2 occurred on october 4, 1983. considering the high upheaval rate at pozzuoli, amounting to 1.5 m/ 2 yr, we may say that seismic efficiency for the period was low. later in the paper, «seismic efficiency» will be discussed to compare energies of earthquakes with energy of ground deformation. seismicity and deformation – as for the relationship between volcanic earthquakes and associated deformations, we know the following example of the 1977 eruption of usu volcano, japan. yokoyama et al. (1981) found that the rate of seismic energy release is proportional to the rate of deformation (upheaval rate) as shown in fig. 4. this relationship is approximately valid fig. 2a,b. deformation trends of serapeo proposed by (a) parascandola (1947) and (b) casertano et al. (1976). eruptions: arrows indicate major local eruptions probably capable to effect pozzuoli (ve: vesuvius; so: solfatara; is: ischia; mn: monte nuovo). earthquakes: bars indicate major earthquakes in campania, and bars with letter p do earthquakes in the pozzuoli area. a b 712 izumi yokoyama and antonio nazzaro fig. 3. seismicity (monthly numbers of shocks) and uplifts (cm) at pozzuoli during the period 1970 ~ 1995 (after berrino, 1998). fig. 4. seismicity and deformation in the 1977 eruption of usu volcano, hokkaido (after yokoyama et al., 1981). daily discharge rate of seismic energy (erg/day) averaged for every 5 days, and daily upheaval rate (cm/day) of the summit dome averaged around observation periods. eruption magnitudes are shown in arbitrary scale. 713 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan for the pozzuoli event as seen in fig. 3 where, strictly speaking, the number of shocks must be converted into energy releases. it is remarkable that the pozzuoli event covering 20 years exhibits the same characteristics as the 1977 usu eruption accompanied with severe explosions covering 2 years. in fig. 3, the subsidence after 1985 was accompanied with very few earthquakes. this means that the recovery was not by elastic rebound, but rather by rheologic flow. in other words, serapeo tends to subside without any local shocks to disturb the pozzuoli area. local earthquakes originating at the pozzuoli area should be accompanied by local deformation provided that the deformation is caused by seismic failure. reports of felt shocks at pozzuoli in the historical documents suggest that these shocks may have been accompanied by some movement at pozzuoli. dvorak and gasparini (1991, table 1) examined reliable data of local felt earthquakes around pozzuoli after the 15th century. they mentioned 8 felt shocks in the 16th century after the 1538 eruption of monte nuovo, and no felt shocks in the 17th and 18th centuries. these felt shocks may have disturbed the ground level at serapeo. dvorak and gasparini (1991) concluded that no remarkable movements have occurred at pozzuoli during approximately 300 years after 1538. during that period, serapeo may have subsided smoothly and rheologically. seismic efficiency – to examine quantitatively the relationship between seismicity and corresponding deformation, seismic efficiency shall be considered. seismic efficiency η is defined as ratios of the elastic energy radiated seismically and the energy required for volumetric changes (mcgarr, 1976) where the volume changes are conditionally accomodated by seismic failure only. the latter is expressed as where σ is overburden stress at the hypocentral region, and volumetric change. then we get (2.1) where es denotes seismic energy. mcgarr (1976) determined seismic efficiencies ranged from 0.25 to 3% for his sampling. for instance, 0.3% to 3% for the denver earthquakes from 1962 through 1967, and roughly 1% for the matsushiro earthquake swarms between march and september 1966. seismic efficiency of volcanic earthquakes – the 1977 eruption of usu volcano shall be used as an example in the following: its seismicity was high and its deformation was remarkable, both being quantitatively monitored (yokoyama and seino, 2000). the summit part of the volcano had uplifted approximately 160 m and tilted 11 degrees of arc by upward pressure of the intruded magma during the period 1977-1979. volcanic earthquakes were closely related to ground deformations as already shown in fig. 4. the total seismic energy released for the 2 years is estimated at 8.5 × 1013 j, and the largest earthquakes were m 4. the volume of the upheaval during the 2 years is estimated at 1.2 × 108 m3. the overburden stress is calculated as 5 × 107 pa assuming the mean depth of hypocenters to be 2 km. then we obtain ηusu = 1.4% or roughly 1%. the magma of the recent activity of usu volcano is dacitic: under the same temperature, dacitic magma is more viscous than basaltic and andesitic magma. the latter magmas scarcely cause remarkable deformations in comparison with the former. hence, the seismic efficiencies of earthquakes accompanied with activities of basaltic and andesitic volcanoes must be comparable to ηusu or larger. seismic efficiency of the pozzuoli earthquakes – yokoyama (1988) calculated the seismic efficiency of the 1982-1984 activity of campi flegrei. the results for the three periods of 1983 are reproduced in table i (data after osservatorio vesuviano, 1984) where ηpoz ranges from 10 –3 to 10–2%. these are extremely small comparing with tectonic earthquakes and dacitic volcanic earthquakes. during the periods, several earthquakes of m 3 ~ 4 occurred, but the seismic efficiency remained low. this is because the caldera deposits are porous and contain much water, and behave anelastically under magmatic actions. σ ⋅ ∆v η σ= ⋅∑ es v/ ∆ ∆v , 714 izumi yokoyama and antonio nazzaro 2.3. interpretations of the pozzuoli event already in the previous part, we saw that serapeo tends to subside and sometimes instantly changes to uplift effected by local volcanic and seismic activities. it is clear that the anomalous deformations at pozzuoli derive mainly from physical properties of the medium and partly from particular external actions. as mentioned above, fedi et al. (1991) discussed the subsurface structure of the pozzuoli area from consideration of gravity data, and showed that the low density materials (∆ρ = – 200 kg/m3) are deposited to a depth of approximately 2 km at the center of campi flegrei caldera, of which the southern half is pozzuoli bay. oliveri del castillo and quagliariello (1969) proposed an idea, prior to their finding the rise of serapeo in february 1970, that serapeo would have a tendency to subside monotonously due to self-loading compaction of the caldera fills, and that it would turn to uplift instantly when major local earthquakes occurred such as in 63, 1488, 1536-1538, and so on, and also when the major eruptions broke out such as vesuvio (79 and 1631), solfatara (1198), ischia (1301) and monte nuovo (1538). the above compaction model can account for the variation of subsiding velocity from the borders to the center. furthermore, casertano et al. (1976) discussed hydrodynamics and geodynamics in campi flegrei and emphasized effects of the sea movement. they modified parascandola’s diagram as shown in fig. 2b. in this diagram, the height of serapeo increased instantly with a high rate when triggered by major local shocks or nearby volcanic activities, and recovered rheologically. as discussed above, during the pozzuoli event, seismic efficiency was extremely low: this means that the medium is anelastic and deforms by self-loading compaction. when magmatic forces act at the caldera or local earthquakes disturb it, the medium changes to reverse almost instantaneously with a large deformation, and when the external factors stop, the medium resumes subsiding rheologically. around the caldera of campi flegrei, there are post-caldera volcanoes such as monte gauro, solfatara and monte nuovo. it may be probable that the magma remains beneath the pozzuoli area although it is probably not so large. 3. iwojima iwojima is located at 1200 km south of tokyo, in the north-west pacific ocean and belongs to «volcano islands» (fig. 5). «iwo» means sulfur and «jima» (or shima) island. «sulfur island» means an island of many fumaroles. it is the largest of the volcano islands, measuring approximately 8 km in the ne direction and occupying approximately 20 km2 in area. the english exploration ship «resolution» visited there in 1779. the oldest sketch and description of iwojima can be found in the report locality period seismic volume elastic seismic energy change work efficiency (× 1010 j) (× 107 m3) (× 1014 j) (%) 1983 jan.-june 0.95 1.4 7.3 10–3 pozzuoli* june-sept. 2.7 0.4 2.1 10–2 sept.-dec. 15 (incl. m 4.2) 2.6 1.4 10–2 iwojima 1976-1984 2.3 0.7 3.4 10–2 * data after osservatorio vesuviano (1984). table i. seismic efficiencies of the activities of pozzuoli and iwojima. 715 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan of king (1785): «the shoreline where captain cook’s surviving crews landed in 1779 is now approximately 40 m above sea level». its secular upheavals were first studied in 1925 and monitored intensively after 1952. a special issue of journal of geography, vol. 94 (1985, eds. j. ossaka, h. takahashi and s. kaizuka) including 18 papers (in japanese with english abstract) was devoted to earth science studies of iwojima, especially anomalous deformations. recently modern techniques, such as gps and sar, are being applied to this anomalous deformations. our discussion mainly depends on the data published in these papers. 3.1. geologic setting of iwojima iwojima is the summit part of a large volcano standing from the sea bottom of 1 ~ 2 km deep and measuring approximately 40 km in basal diameter. the surrounding submarine bathymetry shows a caldera-like topography measuring approximately 10 km in diameter and the caldera rims are just submerged beneath the sea. the northern half of the island is the central cone. a geologic sketch map is shown in fig. 6, after morimoto et al. (1968). the island consists of three parts: 1) motoyama (mo, 115 m a.s.l.) in the ne part is not a constructional cone but a structural uplift, and is composed of alternating of tuff breccia and lava beds, both of which consist of trachyte andesite. this volcano was fig. 5. location map of iwojima, miyakejima and cape omaezaki. fig. 6. geologic sketch map of iwojima (after morimoto et al., 1968). 716 izumi yokoyama and antonio nazzaro built mainly under the sea, and later was thrust up above the sea level. 2) suribachiyama (su, 167 m) at the sw end, is a scoria cone with a crater, under which lie lava flows and pyroclastic deposits. all these rocks are also trachyte andesite. 3) chidorigahara (ch), an unconsolidated sandy isthmus connecting the two parts, was built by pyroclastic deposition combined with uplift. a residual low gravity anomaly of approximately 7 mgal centers on the ch area. this suggests a thick accumulation of porous volcanic clastics below. 3.2. ground deformation and seismicity on iwojima ground deformation – the first topographic survey of this island was carried out in 1911 and a topographic map of 1/50 000 in scale was published. homma (1925) examined the geology of this island, and noticed its remarkable uplift. topographic surveys were repeated in 1944, 1948, 1952, 1968 and 1979, and the maps have been revised. in the 1960s, the receding of the shoreline drew the attention of earth scientists as evidence of the unusual uplift of the island. secular uplift of the island since the 18th century is shown in fig. 7a,b after kaizuka et al. (1985) where (a) lines show the change in height above sea level of the lowest point at the isthmus at the northern foot of su and the 1779 value is deduced from the sketch and the description by king (1785). b) line shows the accumulated uplift of the same point obtained by instrumental surveys since 1911. from these two curves, we may say that the average uplift rate is approximately 20 cm/yr through the last 2 centuries. the distribution of the uplift during 16 years from 1952 to 1968 obtained by tsuji et al. (1969) using geodetic means is shown in fig. 8a. in the figure, the uplift increases towards the ne, and the ch area locally uplifted 7 m. kumagai and takahashi (1985) observed the deformation during the period 1976 ~ 1984 amounting to approximately 2 m uplifts at the ch and the mo areas. in short, iwojima as a whole island, has continued to rise. however, on the island, the center of uplift and that of subsidence may have migrated between the two areas. this means that movement of the whole island is controlled by a deeper magmatic force that is rather stable, on which is superimposed a shallower source whose position varies with time. the ground surveys on iwojima are based on mareographic observations which are not always easy to make. because of this, remote sensing techniques were found useful. ohkura (1998) analyzed interferometric sar images of iwojima obtained in 1993 and 1995, and found a concentric zone of subsidence at the mo area that agrees with the result of precise levels during the same period shown in fig. 8b. fig. 7a,b. secular movements of iwojima (after kaizuka et al., 1985). a) secular change in height (above sea level) of the isthmus at the north of su; b) accumulated uplift and annual rates after 1911. a b 717 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan volcanic activity – there are many solfatara, steam fumaroles, hot springs and hot grounds whose temperature is approximately 100 °c. since 1890, more than 15 steam explosions have occurred at several fumaroles on the island although they were not very explosive. around iwojima, there occurred several submarine eruptions in the 20th century: a submarine eruption occurred at a distance of 5 km nne of the island in 1904, and again in 1914, and eruptions also occurred near the same sea area in 1937 and 1968. in 1973, a submarine eruption began quite near nishinoshima (fig. 5) located at approximately 260 km n of iwojima, and a new island was formed. on and around iwojima, small eruptions have occasionally broken out. on september 21, 2001, a small submarine eruption occurred at the nw part of the island, 1 km off iwojima: sea water in a circle 10 ~ 15 m in diameter gushed up to a height of about 5 m and it continued for 1 h. on the island, about 20 microearthquakes were observed on that day. seismicity – iwojima has shown rather low seismicity in spite of the remarkable deformation and intensive fumarolic activity. monitoring of earthquakes with 5 seismometers on this island were begun in 1976 (kumagai, 1985). monthly means of daily numbers of volcanic earthquakes observed on the island are shown in fig. 9 where the arrows with numbers represent steam explosions counted since 1890. felt shocks (m ≥ 2) were observed once in march 1980 and 13 in november 1982. the latter accompanied the 15th explosion. the steam explosions were accompanied by several earthquakes every day, and small swarms (up to 40 events per day) every few months. their hypocenters were dispersed beneath the entire island, at depths between 0 and 4 km. their magnitudes ranged from 0 to 2. felt shocks had not been reported until november 1982 when an earthquake swarm with felt shocks continued for 6 days. seismic efficiency – during the period march 1976 to february 1984, both deformation and seismicity data are available (kumagai, 1985). the uplift amounted to 2.0 m at the ch area (fig. 6) during the period: the volume change is roughly estimated at 7 × 106 m3. during the same period, there were approximately 10 880 unfelt earthquakes (m ≤ 1). the total seismic energy fig. 8a,b. crustal movements at iwojima. a) uplift (m) during the period 1952 ~ 1968 obtained by geodetic methods (after tsuji et al., 1969); b) deformation (cm) during the period 1993 ~ 1995 obtained by precise levels. this is comparable with the result obtained by interferometric sar images (after ohkura, 1998). a b 718 izumi yokoyama and antonio nazzaro release during the period is roughly estimated at 2 × 1010 j. the hypocenters were distributed at depths from the surface to 4 km: the overburden stress is calculated as 5 × 107 pa assuming the mean depth of the hypocenters to be 2 km. the seismic efficiency during the period is estimated at η iwo = 10–2 % (table i). this is extremely small and of the same order of magnitude as that of campi flegrei. 3.3. origin of anomalous upheavals on iwojima iwojima originally formed as the central cone of a submarine caldera, and is composed of pyroclastic material. during the past 2 centuries, the island as a whole has continued to rise due to intrusion of magmatic material, causing intermittent steam explosions and fumarolic activity. as mentioned above, two magmatic forces, deeper and shallower, are assumed to control the deformation of the whole island and local surface changes, respectively. considering the frequent submarine eruptions in the surrounding sea and the 1973 eruption of nishinoshima (fig. 5) in addition to sporadic volcanic and seismic activities and remarkable ground movements on the island, we may say that the volcanic energy source around iwojima is enough reserved to maintain the present state for some time into the future. 4. secular ground deformations with low seismicity; volcanic and tectonic examples in japan aira caldera, kyushu – aira caldera, kyushu is approximately 20 km in diameter and a postcaldera volcano, sakurajima stands at the southern rim of the caldera (fig. 10a). the subsurface structure of aira caldera is discussed by yokoyama and ohkawa (1986). in 1914, sakurajima violently erupted effusing andesitic lava of 1.4 km3 in dense rock equivalent. during the eruption, a large earthquake of m 7.1 occurfig. 9. monthly mean of daily frequency of volcanic earthquakes at iwojima, observed for the period 1976 ~ 1985 (after kumagai, 1985). arrows indicate steam explosions counted since 1890. 719 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan red near the volcano (omori, 1914-1922). the precise levels around the caldera have been repeated since 1891. the secular height changes of a benchmark (bm 2474) at the western rim is shown in fig. 10b (yokoyama, 1986). it subsided approximately 75 cm almost simultaneously with the 1914 eruption, and additive 5 cm viscoelastically in the following 7 months. thereafter it gradually recovered for roughly 70 years without much earthquake activity. yokoyama (1996) interpreted the recovery from the standpoint of rheology of the crust beneath sakurajima. if we adopt a model of a kelvin solid, retardation time of a recovery process is given by µ /k where µ denotes viscosity and k bulk modulus. in fig. 10b, after the 1914 eruption, the retardation time is roughly estimated at 22 years. when we adopt k conventionally as 30 gpa, the viscosity of the medium µ is calculated to be 2.1 × 1019 pa.s. this may be plausible, granting the uncertainty of the k value. when we compare fig. 10b with fig. 2b for serapeo, we notice that both the deformations derive from rheological property of the media; the crust beneath aira caldera and the caldera fills in pozzuoli. on the other hand, the controlling forces are different; upward restoring force of the crust after instantaneous subsidence in the former, and downward compaction after instantaneous uplifts in the latter. miyakejima, seven izu islands – miyakejima is a basaltic volcano island at approximately 170 km south of tokyo (figs. 5 and 11a). in the 1983 eruption of the volcano, a benchmark used for precise levels (bm 10250) near the explosion site rose instantaneously approximately 17 cm as shown in fig. 11b. after the eruption, it recovered without any noticeable seismic activity. the retardation time is determined as approximately 9 years. the viscosity of the upper crust of miyakejima volcano is approximately 8.6 × 1018 pa.s which is a little smaller than that of aira caldera. if the difference of the viscosity is meaningful, it may be controlled by differences in the depth of deformation volume. figure 11b indicates that the 1983 deformation at that benchmark has not totally recovered as of 1997, and it may be permanently deformed. in june 2000, the volcano reawoke at the summit crater, and in september, the crater bottom collapsed approximately 300 m. during that event, deformations of the benchmarks along the coast were not noticeable. cape omaezaki, tokai – cape omaezaki is located along the pacific coast of the tokai district (fig. 5) and lies within a potentially hazardous area related to a tokai earthquake that is believed to be imminent and should be of m 8.4 class. this region is non-volcanic and lies above the subduction of the philippine sea plate beneath the eurasian plate. cape omaezaki at the edge of the eurasian plate is dragged downward and gradually subsides until another large earthquake fig. 10a,b. aira caldera. a) the subsidence (mm) caused by the 1914 eruption of sakurajima volcano, the hatched parts indicate the new lava flows (after omori, 1914-1922). b) secular deformation of bm 2474 referred to bm 2469. the asterisks show the interpolation (data after sakurajima volcano observatory). a b 720 izumi yokoyama and antonio nazzaro occurs. in this region, the last earthquake of m 8.4 took place in 1854 and since then, no large earthquakes have occurred. precise levels have been carried out since 1962 to monitor the activity of this area. the cape has subsided secularly approximately 20 cm during the period 1962 to 2000 and the subsidence is believed to be a precursor of a large subduction earthquake. through all the period, no microearthquakes have been observed in this region. the deformations extend to a distance of approximately 100 km from the cape. the secular changes in height of bm 2595 (near cape omaezaki) as referred to bm 140-1 (kakegawa) are shown in fig. 12 (geographical survey institute, 2001) where the surveys repeated four times every year after 1980, revealed the annual changes. the origin of the annual changes is not clear at present. the distance between the two benchmarks is approximately 20 km. cape omaezaki has continued to subside at a rate of approximately 0.5 cm/yr. the subsidence is gradual, but could be a significant precursor of the tokai earthquake. 5. concluding remarks: characteristics of various secular deformations with low seismicity in the above, we reviewed the examples of secular gradual deformations in both volcanic and tectonic areas; they have individual charfig. 11a,b. miyakejima. a) distribution of the benchmarks for the precise levels along the shore. a broken line indicates the line of the 1983 fissure eruption; b) vertical deformation (cm) of a benchmark near the explosion site (bm 10250) referred to the tide gauge station (ts). the arrow indicates the eruption and the hollow circles interpolated values (after yokoyama, 1996). a b fig. 12. secular deformation of cape omaezaki since 1962 (after geographical survey institute, 2001). 721 anomalous crustal movements with low seismic efficiency − campi flegrei, italy and some examples in japan acteristics controlled by the nature of the local crust, and by the local sources of stress. tectonic settings – the majority of secular gradual deformations are observed in volcanic areas, particularly in caldera regions: pozzuoli is located at the center of campi flegrei caldera, and iwojima is also the center of a submarine caldera. aira caldera contains an active postcaldera volcano, sakurajima. these calderas are characterized by low gravity anomalies which originate from thick deposition of coarse pyroclastic material. such materials behave anelastically against external forces in both the shortor long-term. miyakejima is a volcano island and may have a particular basement structure which reacts anelastically to magmatic intrusions. the phenomena rarely occur in tectonic areas: an example is near a subduction zone. cape omaezaki is located along the pacific ocean and over the subducting pacific plate. geodynamic forces – the secular subsidence of pozzuoli may be interpreted by self-loading compaction of the caldera deposits and sometimes has been reversed in the short-term by local earthquakes of magmatic origin or major volcanic eruptions. iwojima, as a whole, has risen continuously due to a deep magmatic force, and sometimes shallow magmatic steam pressure effects local deformations. secular deformations observed on aira caldera and miyakejima are recovery stages of the strains caused by volcanic eruptions; the former is a subsidence and the latter is an uplift. omaezaki receives a regional tectonic force which is long-term and should result in an instantaneous breakdown when the crustal strain reaches the elastic limits. time constants – the events mentioned in the present paper have the time constants covering a wide range. pozzuoli had subsided at a rate of 1.4 cm/yr after the 1538 eruption of monte nuovo and suddenly began to rise at a rate of 3.5 m/yr in 1970. if we adopt the model of self-loading compaction, it should finally approach equilibrium asymptotically in several thousands of years. iwojima, as a whole, has risen continuously at a rate of 11 ~ 33 cm/yr since 1800 with low seismicity. considering the past volcanic activities on the island and the submarine eruptions in the surrounding sea, we may say that the future activity and deformation on the island would depend on the volume of the remaining magma beneath. the time constant of the iwojima upheaval may be at least several hundreds of years. the upheaval around aira caldera and the subsidence of the 1983 eruption site of miyakejima both represent recovery of deformation caused by volcanic eruptions. their time constants are roughly 22 and 9 years, respectively. the secular subsidence observed at cape omaezaki is an example of tectonic ground deformation or a precursory of a large subduction earthquake. the cape subsides gradually with very low seismicity due to subduction of the pacific plate. its subsidence rate since 1962 is 0.5 cm/yr, the smallest rate of the many events mentioned in this paper. such subsidence may have continued since the last tokai earthquake in 1853, and should rebound instantly by release of the strain energy as a large earthquake. the recurrence time of the tokai earthquake may be around 150 ~ 200 years. in conclusion, the characteristics of anomalous crustal movements with low seismic efficiency derive from two factors: one is the property of the medium, with the exception of omaezaki where the crust is not involved, and the other is geodynamic forces acting on respective site. the crust is a complicated medium, particularly in volcanic area, and responds in various ways to various geodynamic forces, short-term and long-term. acknowledgements the authors are grateful to prof. a. rapolla for his critical discussion on the pozzuoli event. the useful information on the recent state of iwojima were afforded to us by dr. m. ukawa, to whom our thanks are extended. we extend our gratitude to dr. r. azzaro for his critical review of the manuscript. we appreciate the assistance of dr. thomas l. wright who provided helpful 722 izumi yokoyama and antonio nazzaro suggestions for improvements of the manuscript. we further thank the colleagues of the osservatorio vesuviano for their scientific friendship over many years. references berrino, g. (1998): detection of vertical ground movements by sea-level changes in the neapolitan volcanoes, tectonophysics, 294, 323-332. berrino, g. and g. corrado (1991): tidal signal in the recent dynamics of campi flegrei caldera (italy), j. volcanol. geotherm. res., 48, 93-101. casertano, l., a. oliveri del castillo and m.t. quagliariello (1976): hydrodynamics and geodynamics in the phlegraean fields area of italy, nature, 264, 161-164. dvorak, j.j. and p. gasparini (1991): history of earthquakes and vertical ground movement in campi flegrei caldera, southern italy: comparison of precursory events to the a.d. 1538 eruption of monte nuovo and activity since 1968, j. volcanol. geotherm. res., 48, 77-92. fedi, m., c. nunziata and a. rapolla (1991): the campania-campi flegrei area: a contribution to discern the best structural model from gravity interpretation, j. volcanol. geotherm. res., 48, 51-59. geographycal survey institute (2001): crustal movements in the tokai district, rept. coord. com. earthquake predic., 66, 277-289. homma, f. 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(1996): rheological behavior of volcanic deformations, proc. jpn. acad., 72, ser. b, 124-128. yokoyama, i. and s. ohkawa (1986): the subsurface structure of the aira caldera and its vicinity in southern kyushu, japan, j. volcanol. geotherm. res., 30, 253-282. yo k o ya m a, i. and m. se i n o (2000): geophysical comparison of the three eruption in the 20th century of usu volcano, japan, earth planets space, 52, 73-89. yokoyama, i., h. yamashita, h. watanabe and hm. okada (1981): geophysical characteristics of dacite volcanism. the 1977-1979 eruption of usu volcano, j. volcanol. geotherm. res., 9, 335-358. adg vol5 n02 losito 207_217.pdf annals of geophysics, vol. 45, n. 2, april 2002 207 physical parameters activating electrical signal distortions in polluted soils gabriella losito and rossana angelini dipartimento di ingegneria civile, università di firenze, italy abstract laboratory investigations and field measurements show that the electrical behaviour of polluted soils is strongly non-linear at low frequencies. this phenomenon can be related to the class and amount of pollutants. to measure this non-linearity, we used only monochromatic voltage waveform as input signal and analysed the current signals at first by means of the classical spectral analysis. in particular, the total harmonic distortion % (thd%) and the harmonic distortion %(hd%) measure the non-linearity level and identify the frequency interval where the nonlinear electrical behaviour is activated. this frequency interval can be related to the pollutant molecular size. open interpretative problems were the following: 1) phase localization of the signal deformation; 2) «local» amplitude of the applied signal activating the distortion, and 3) numerical fit of the distortion. we employed the wavelet analysis to study the phenomenon. the wavelet technique breaks up a signal into shifted and scaled versions of the original wavelet, which is a waveform of limited duration. these features of the wavelets allow us to obtain current components that can be interpreted on the bases of a real physical meaning. by using the wavelet analysis, we obtained the phase localization of the «oscillations» of the details and consequently the phase and amplitude of the applied signal. the sum of nine details provides a good numerical fit of the distorted signal. starting from the wavelet analysis, we determined the physical conditions activating each distortion, testing some parameters on experimental data. the parameters that resulted most significant are the phase of the distortion activation and the product vin t (vs) (where t is the time interval corresponding to the said and vin is the integral tension applied to the sample on t). the latter parameter is in a very good agreement with field data of advanced monochromatic spectral induced polarization (amsip) and restricts the physical interpretative hypotheses of distorted signals that are measured in the field. typical experimental results will be shown as examples. mailing address: dr. gabriella losito, dipartimento di ingegneria civile, università di firenze, via s. marta 3, 50149 firenze, italy; e-mail: losito@dicea.unifi.it key words induced polarization – wavelet decomposition – non-linear electrical behaviour – electrical physical parameters 1. introduction this paper concerns the analysis of the nonlinear electrical behaviour of soils, especially polluted soils, at very low frequencies (f 0.1 hz). in fact, at these frequencies, the electrical response of a water saturated porous rock is sensitive to the properties of the mineral-water interface and to the polluted particles floating inside the fluid. this fluid/solid interaction produces a non-linear electrical behaviour and consequently distortions of the output signals: if pure harmonic signals are used as input function, the frequency content of the output signals is not monochromatic (olhoeft, 1979). currently rocks and soils are characterised in the field by their dc resistivity ( m), but in the laboratory also by the complex electrical resistivity. if the phenomena are linear, the frequency dispersion of can correctly de208 gabriella losito and rossana angelini scribe the electrical polarizations; the standard induced polarization (ip) can be employed (both frequency domain fd and time domain td) and the classical cole-cole (1941) model can be used in the interpretation of electronic and ionic polarizations (patella, 1993). in the case of non-linear electrical behaviour, the electrical characterization of the medium must be completed by other parameters, both should be obtainable in the field and in the laboratory (finzi-contini, 1971). recent applications of advanced monochromatic spectral induced polarization (amsip), a technique based on the use of sinusoidal waveforms as input signals (losito et al., 1998), provides a quantitative evaluation of soil pollution by means of the relative total harmonic distortion % (thd%). this parameter is computed from the fourier spectral analysis applied to the output signals. here we must note that this parameter represents only a global measure of the non-linearity produced by the presence of pollutants at the mineral-water contact. it cannot discriminate the physical conditions activating the waveform distortions specific for each pollutant. to solve this problem, we present in this paper an application of the wavelet analysis to analyse the non-linear components of the output signals produced by the electrochemical interaction between some pollutants and silica sands. the wavelet analysis was selected, because this technique is based on functions that are time limited and amplitude modulated (grossman and morlet, 1984; holschneider, 1995) and defines the physical conditions (like time, phase and applied voltage) activating local perturbations. the research criteria and application results are described in section 4.1. 2. experimental studies on polluted samples the electrical behaviour of artificial and natural samples has been studied in laboratory as a function of frequency (0.002 hz 100 khz) and voltage amplitude (0.5-5 v), using the 4-electrode array technique (losito, 1989). hydrated artificial samples are: 1) a pure watersaturated silica sand (si-h2o); 2) si-h2o polluted by, respectively, motor-oil, trichloroethylene, dermo-nitrogen (si-h 2 o + dn), stomp (si-h 2 o + stomp) and nitrophoska gold (the last three are fertilisers); 3) a pure watersaturated calcite sand; 4) pure water-saturated calcite-silica sand polluted by, respectively, motor-oil and trichloro-ethylene. natural samples have been studied after water saturation and are: 1) soil fertilised by dermo-nitrogen (soil + dn); 2) natural soil (nat soil); 3) soils of dye-works; 4) soils and percolates of a waste deposit area (dondoli et al., 1994; losito et al., 1995, 1998). the stomp is a fertiliser that uses, as active principle, pendimethalin, an aniline derivative, while nitrophoska gold is an inorganic fertiliser. we focused our attention on dermonitrogen, a fertiliser used in nurseries (organic nitrogen with organic n: 11%, organic biological c: 40%; cr: 3%). we must outline its high content of cr that is a strong pollutant. an example of pollution effect on electrical signals is shown in fig. 1. this figure shows the comparison between the non-linear component of the current density (output signal) of pure silica sand and silica sand polluted by dermo-nitrogen. measurements were performed at 0.002 hz. we can observe that dermo-nitrogen produces higher distortion in the electrical signals than pure silica sand. laboratory investigations were carried out on the bases of the following experimental criteria: phase (rad) j (a /m 2 ) -3 -2 -1 0 1 2 3 5v 1.5v 3v silica + dermo-nitrogen + water silica + water π π////2222 3333 π////2222 2222 π fig. 1. non-linear component of current density signals j (a/m2), as function of phase (rad): pure silica sand (solid line) and silica sand polluted by dermo-nitrogen (dashed line). the working frequency is 0.002 hz. 209 physical parameters activating electrical signal distortions in polluted soils 1) only pure sinusoidal waveforms of the applied electrical potential were used; 2) the electrical parameters were calculated in the frequency range 0.002 hz-100 khz; 3) the electrical signals of samples were analysed by means of the fourier spectral analysis, to obtain a measure of the distortion phenomena; 4) the differences between the normalised input and output signals have been calculated to locate the critical phases and voltages that activate the signal distortions; 5) the sample electrical behaviour was interpreted on the bases of chemical analyses. for all samples the following electrical parameters were calculated: complex electrical resistivity ( m); loss tangent tan ; total harmonic distortion thd%( ) and harmonic distortion hd%( ) of the output electrical signals. joining the classical electrical parameters to thd%, it is possible: 1) to identify the polarization phenomena; 2) to know the class of polarization on the bases of the frequency range; 3) to evaluate the polarization distortion size using the thd% parameter. to evaluate the soil electrical behaviour in the field we used a combined procedure. the classical electrical methods vertical electrical sounding (ves) and induced polarization (ip) were combined with the new technique of advanced monochromatic spectral induced polarization (amsip). figure 2 shows field results related to an application of this combined technique to a soil polluted by dermo-nitrogen. the strong correlation of ( m), percent frequency effect (pfe) and thd% highest variations is evident if the energizing halfdistance (ab/2) is around 4 m. this behaviour can be attributed to a dermo-nitrogen level that is a high polarizable material. 3. the wavelet analysis application of fast fourier transform leaves various questions open. we tried to solve these questions using the wavelet analysis in order to: 1) study phase localization of deformation signals; 2) identify physical conditions that activate each distortion; 3) fit distorted signals, and 4) find a relation between a single component and a physical phenomenon. the non-linear component of rock electrical behaviour has been represented by the relative residuum between the output signal and its fundamental harmonic. we analysed relative residua of density current signals because the current and voltage output signals have a similar waveform, and, as is well known, the current flow inside the sample is the physical phenomenon induced by the voltage application (loeb, 1976). the percent relative residua of current density j (a/m2) give the phase and the frequency localization of distortion peaks (see fig. 3a,b concerning hydrated silica sand polluted by dermo-nitrogen) but the physical simulation of distortion remains an open problem. recently, fig. 2. electrical responses of a polluted soil with dermo-nitrogen: resistivity [ m], percent frequency effect (pfe) and total harmonic distortion % (thd%) of advanced monochromatic spectral induced polarization (amsip). note the strong trend correlation if the energizing half-distance ab/2 is around 4 m. 210 gabriella losito and rossana angelini to quantify the electrical signal distortions produced by pollutants/mineral interactions, and to define the physical conditions activating the non-linearities, we used the wavelet analysis. for the background theory relative to the use of wavelet analysis see daubechies (1992), coifman et al. (1992), meyer (1990), wicherhauser (1994). the signal wavelet analysis has been carried out on the following steps: 1) sample selection and evaluation of the distortion of j signals; 2) selection of a wavelet function; 3) wavelet decomposition of relative residua; 4) determination of physical conditions activating distortions. here we recall the mathematical definition of the continuous wavelet transform (cwt) of a temporal signal x(t) (3.1) where t (s) is the time, is the complex conjugate function of the mother wavelet (timelimited, made of phase and frequency modulated oscillations), b (s) is the time translation factor; a is a dimensionless scale factor (a < 1, compressed, a > 1, expanded). using the (3.1) we can calculate the c(a,b) coefficients, that are a correlation index between the wavelet mother translated by b and scaled by a, and the signal part under examination: high values of c(a,b) correspond to a high correlation. it is important to point out that the adjective «continuous» is referred not to t, but to a and b. in fact, the discrete wavelet transform (dwt) is the cwt with discrete values for a and b (3.2) where a = 2 j and b = na = n2 j. we selected a multiple-level wavelet decomposition technique (mallat, 1989) of j relative residua, with nine levels, according the following steps. first the signal is decomposed in approximation a1 (its low frequency component) and in detail d1 (its high frequency component). the process is iterative (fig. 4a) and produces linked approximations and details (3.3) (3.4) a b fig. 3a,b. percent non-linear component of j signals, concerning a silica sand polluted by dermo-nitrogen, as frequency (0.002 hz 200 hz) and phase (rad) function. the applied voltage is 3 v. a) 3d representation and b) the corresponding contour level representation. d t c j n tj n j n( ) = ( ) ( ), ,, a t d tj j j j ( ) = ( ) > . cwt x t c a b( )[ ] = ( ) =, a x t t b a dt) = ( ) 1 c j n x t t dtjn( ) = ( ) ( ) =, x t t n dtj j= ( ) ( )/2 22 t( ) 211 physical parameters activating electrical signal distortions in polluted soils several wavelet functions were tested. in our selection, we followed the following basic criteria: the details must help in the physical description of the phenomenon and a particular detail can simulate a non-linear polarization. finally we chose the mother wavelet sym8 (for its mathematical description see daubechies, 1992); its form is shown in fig. 4b. some basic observations about fourier and wavelet analysis are interesting. the fourier analysis is based on sinusoidal waves (so signal infinitely repeated in time and regular waveform) and so it decomposes a signal into sinusoidal waves. the wavelet analysis instead is based on mother wavelet ψ(t) (so time-limited and free fig. 4a,b. a) scheme of iterative procedure for multiplelevel wavelet decomposition. b) waveform of the selected wavelet: sym8. a b waveform) and it decomposes a signal into translated and scaled ψ(t) functions: so wavelet analysis is more suitable for decomposition of signals with low frequency component and large local changes. in fact, using lower scale factors (a < 1) we obtain compressed wavelets with a high time resolution. they can be useful in the simulation of a short signal part and its high frequency behaviour using low-level details. instead, using higher scale factors (a > 1) we use expanded wavelets with a minor time resolution. they can be compared with a long signal part to identify its low frequency components (i.e. its trend characterised by high level details). as regards the numerical simulation of local deformations, in fourier analysis they are periodic elements. in fact it hypothesises that the local deformations repeat themselves during the whole signal and this is obviously a not realistic situation in non-linear behaviour. the wavelet analysis instead is able to point out the single distortions and give their exact time localization, without their repetition throughout the whole signal, as in the fourier technique. so we can conclude that wavelet analysis components can be correlated with the physical parameters that produce the non-linear polarization of the samples. fitting examples calculated using the two methods are shown in fig. 5a-c. we can observe the higher fitting quality that is obtained by the wavelet method (w9, dash-dotted line). this fitting has been computed summing all nine details while fourier fitting (f, dashed line) is computed with the contribution of harmonics with percent amplitude not below 0.7% of the principal one. 4. application of wavelet analysis to electrical signals of polluted samples we performed a wavelet decomposition with nine levels of all residual signals at 0.002 hz using the sym8 function and we found that the details from 5th to 8th level are the most interesting and the largest components, that can describe polarization. in fact, more low-level details (d1, d2, d3, d4) represent principally noise, while d9 and a9 represent the low frequency components (trend). 212 gabriella losito and rossana angelini then, we made a final selection of the samples to study their behaviour in detail. first, we chose samples with very high non-linear behaviour (silica polluted by dermo-nitrogen and stomp). then, we chose fertilised soil to make a comparison between natural and artificial samples with the same pollutant. finally, we chose silica sand and natural soil similar to the fertilised one as calibration samples. we made a wavelet decomposition of all relative residua > 5% for the five selected samples. the computed details allow us to localize the perturbation phase and to state a relationship between detail waveforms and applied signal waveforms. in fact, it is possible to relate the presence of the most important peaks to the sign of energizing signal and its derivative. a common result for all the studied samples is that, for all the details (except d8) there are high distortions at the change of applied signal sign. this is a proof that the two semi-waves (positive and negative) introduce different deformations, as confirmed also by the asymmetry of the relative residuum. in fig. 6 we can see the decomposition results relative to silica sand polluted by dermo-nitrogen, with applied tension respectively of 1.5 v and 3 v. d8 is more influenced by the increment of applied tension, but in general, for all the levels, the peak presence in phase depends on applied tension values (see for example d5). 4.1. physical parameters activating the nonlinearity to find physical parameters activating the non-linearity, we focused our attention on positive peaks near π for the three first details and on the maximum positive peak for d8, trying to define the physical conditions activating this particular deformation. we selected the following parameters: 1) phase ϕ of distortion start (rad); 2) energizing time ∆ t (s); 3) integral tension (v); 4) the product v in ∆ t (vs). phase (rad) silica + water + dermo-nitrogen 0.002 hz v ap=5v -30 -20 -10 0 10 20 30 π////2222 π 3333 π////2222 2222 π os w9 f π////2222 π 3333 π ////2222 2222 π silica + water 0.002 hz v ap=1.5 v phase (rad) -15 -10 -5 0 5 10 15 os w9 f n o n li n e a r c o m p o n e n t o f j (% ) silica + water + dermo-nitrogen 0.002 hz v ap=1.5 v phase (rad) -60 -40 -20 0 20 40 60 80 π////2222 π 3333 π////2222 2222 π n o n li n e a r c o m p o n e n t o f j (% ) n o n li n e a r c o m p o n e n t o f j (% ) os w9 f fig. 5a-c. comparison between relative residua signals (os, solid) as phase (rad) function and its fitting obtained by using fourier analysis (f, dashed) and wavelet analysis (w9, dash-dotted): a) pure silica sand with v ap = 1.5 v; b) silica sand polluted by dermonitrogen with v ap = 1.5 v; c) the same sample with vap = 5 v. all the examples are obtained at a working frequency of 0.002 hz. the wavelet fitting is obtained summing all the 9 calculated details. the fourier fitting is obtained summing the harmonic components of relative amplitude a ≥ 0.7%. a b c apd=v vin ϕ 00 ϕ ∫ 213 physical parameters activating electrical signal distortions in polluted soils fig. 6. wavelet analysis of the j signals % non-linear component. details d5-d8 concerning a silica sand polluted by dermo-nitrogen. applied voltage signals: left column 1.5 v, right column 3 v. working frequencies as described in the legend. 214 gabriella losito and rossana angelini fig. 7. frequency trend of phase ϕ and the parameter v in ∆ t (see text) of the details d7 and d8 of the five selected samples. applied tension 1.5 v (upper part of the figure) and 3 v (lower part of the figure). si + dn: silica sand with dermo-nitrogen; soil + fert: soil fertilised by dermo-nitrogen; si + stomp: silica sand with stomp; si + water: pure silica sand; nat soil: natural soil. observe that d8 level allows to discriminate the presence of pollutant. 215 physical parameters activating electrical signal distortions in polluted soils fig. 8. comparison between frequency trend of ϕ and v in ∆ t obtained by applying a cosine signal (white point) and a sine signal (black point) to silica sand polluted by trichloro-ethylene. the applied tension is 3 v. note the no effect applied signal phase. 216 gabriella losito and rossana angelini we must note that in the calculation of v in (i.e. the total tension applied, at the working frequency, to the sample until the phase ) we considered the absolute value of the applied signal because the provided tension during the negative semi-wave is a further energy contribution to the sample and not an energy subtraction from the energy provided during positive semi-wave. then we selected the most interesting parameters: and v in t. the frequency trends of and v in t are shown in fig. 7. we can see that for d7 and d8 at 1.5 v the vin t parameter is able to discriminate the samples with dermo-nitrogen. it is interesting that for the two details we have an opposite situation. in fact, for d7 (more high frequency component) the distortions due to dermo-nitrogen presence are characterised by lower values of vin t product compared to the distortions in the other samples. instead, for d8 (more low frequency component) the maximum positive peak in the samples polluted by dermo-nitrogen is activated by major values of v in t parameter compared to the peak in the other samples. the low frequency component is probably influenced by polarization phenomena due to the largest molecules (for example organic ones) with a major electrical inertia. this means that, with the same applied signal frequency, the peak is present at higher values of : consequently both t and vin parameters will assume higher values and so their product. instead, the highest frequency component is probably influenced by polarization phenomena due to smaller molecules, with a minor electrical inertia, mobilised by minor values of vin t product. only at low tension can d7 discriminate dermo-nitrogen while d8 well discriminates it at all tensions. so we can conclude that 1.5 v is the most suitable value of applied signal in pointing out the non-linear polarization phenomena, while with high values there is a saturation effect, experimentally evaluated (marshall and madden, 1959; madden and cantwell, 1967; dondoli et al., 1994; losito et al., 1995). to verify and v in t we conducted the following experiment: we applied both sine (black point) and cosine (white point) waveform signals to an artificial polluted sample (silica sand with trichloro-ethylene). also in this case these parameters were independent of applied signal phase and consequently independent of local applied tension value, as we can see in fig. 8. this result indicates that the sample behaviour depends on applied tension history. we think that the laboratory results can be extended to the interpretation of field data that show high non-linearities (see fig. 2 where the value of 60% for thd% depends on a soil level polluted by dermo-nitrogen). in fact, even if the field measurements are obtained at energizing frequencies (i.e. in the range 0.01-0.1 hz), the output signals are acquired after a certain number of energizing cycles. in this way the total energizing time can be compared with the laboratory one obtained with minor working frequencies. 5. conclusions the principal aim of this work was to study the non-linearity of the electrical outputs produced by polarization phenomena in polluted rocks. the application of the advanced monochromatic spectral induced polarization (amsip) to laboratory samples represents a useful starting point. this technique allows to verify not only the presence of polarization phenomena, but it also yields a physical interpretation of the problem, on the basis of the polarization frequency range. in addition, the amsip technique gives a first «global» measure of the non-linearity entity, thanks to the total harmonic distortion % parameter, obtained starting from the fourier analysis. by comparison with the fourier analysis, the wavelet analysis allows a «local» study of the electrical signal deformations. this is due to the particular characteristics of the functions used in this type of analysis. in fact they are timelimited and made of phase and frequency modulated oscillations, and this makes them suitable to study signals with large local changes (i.e. high frequency component) put upon low frequency components. an important goal is that wavelet analysis, instead of fourier one, not only gives a good numerical fitting but also frequency components 217 physical parameters activating electrical signal distortions in polluted soils correlated with the real physical phenomenon. for fourier analysis, the local deformations are periodic elements while wavelet analysis permits their exact time localization. the analysis of relative residua and their details gives a precise localization in phase of the single distortions and how they develop throughout the output signal. the distortion localization in phase (rad) seems to give a way to determine the physical conditions for deformation activation. in fact, knowing the applied signal frequency, it is possible to evaluate t (s), the energizing time necessary to activate a particular distortion. in addition, it is also possible to evaluate v in (v), the total tension provided to the sample until . however, the most interesting parameter to define the physical conditions of activation is the product v in t (vs). the reason for this is that the sample electrical behaviour depends not only on the given tension, but also on the energizing time (i.e. the time during which this tension is given to the sample). we can see that the product v in t for d7 and d8 discriminates the samples with dermo-nitrogen. references coifman, r.r., y. meyer and m.v. wickerhauser (1992): wavelet analysis and signal processing, in wavelets and their applications, edited by m.b. ruskai et al. (jones and bartlett). cole, k.s. and r.h. cole (1941): dispersion and absorption in dieletrics – i: alternating current characteristic, j. chem. phys., 9, 341-351. daubechies, i. (1992): ten lectures on wavelets (siam), pp. 225. dondoli, a., g. losito and a. trova (1994): possibilità del metodo elettrico nell’individuazione di inquinanti in acquiferi e terreni, ing. sanit. amb., 3, 22-29. finzi-contini, g. (1971): theoretical study on membrane polarization and complex mobility in moist rocks, geol. surv. can. pap., 71-1, b, 53-65. grossman, a. and j. morlet (1984): decomposition of hardy functions into square integrable wavelets of constant shape, siam j. math. anal., 15, 723-736. holschneider, m. (1995): wavelets: an analysis tool (clarendon press, oxford), pp. 324. loeb, j. (1976): experimental and theoretical aspects of induced polarization – vol. 2: macroscopic and microscopic theories (gebrüder borntraeger, berlin stuttgart), pp. 335. losito, g. (1989): a new rock physics laboratory apparatus with programmed thermo-baric cycles and controlled pore fluid internal pressure (theory, hardware-software, electrical measurements), cnr-enea, pfe sp en. geot., lb-21, 1-150. losito, g. and a. trova (1999): wavelet technique applied to localization and interpretation of electrical signal distortions produced by pollutants in soils, in 5th eegses environmental and engineering geophysics, budapest (hungary) september 6-9, ext. abstract de10. losito, g., m. muschietti and a. trova (1995): electrical effects of pollutants in earth soils and sedimentary rocks, in 1st meeting of environmental and engineering geophysics, 57-60. losito, g., a. trova and i. d’urso (1998): frequency and dc electrical behaviour of polluted sediments in industrial areas: field and laboratory investigations, in eegs barcelona (spagna), 14-17 september 1998, extended abstract, 35-38. madden, t.r. and t. cantwell (1967): induced polarization. a review, min. geophys. (s.e.g.), ii, 373-400. mallat, s. (1989): a theory for multiresolution signal decomposition: the wavelet representation, ieee trans. pattern analysis and machine intelligence, vol. 11 (7), 674-693. marshall , d.j. and t.r. madden (1959): induced polarization. a study of its causes, geophysics, 24, 790-816. meyer, y. (1990): wavelets and operators, translated by d.h. salinger (cambridge univ. press, 1993), pp. 223. olhoeft, g.r. (1979): nonlinear electrical properties, in nonlinear behaviour of molecules, atoms and ions in electric, magnetic and electromagnetic fields, edited by l. neel (elsevier), 395-409. pat e l l a , d. (1993): i principi metodolo gici della magnetotellurica su mezzi generalmente dispersivi, ann. geofis., 36 (suppl. to n. 5-6), 147-160. wickerhauser, m.v. (1994): adapted wavelet analysis from theory to software algorithms (a.k. peters), pp. 241. seismic hazard in central italy and the 2016 amatrice earthquake annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 1 seismic hazard in central italy and the 2016 amatrice earthquake carlo meletti, francesco visini, vera d’amico, andrea rovida istituto nazionale di geofisica e vulcanologia, italy carlo.meletti@ingv.it abstract the amatrice earthquake of 24 august 2016 (mw 6.0) struck an area that in the national reference seismic hazard model (mps04) is characterized by expected horizontal peak ground acceleration (pga) with 10% probability of exceedance in 50 years higher than 0.25 g. after the occurrence of moderate-to-large magnitude earthquakes with a strong impact on the population, such as the l’aquila 2009 and emilia 2012 ones (mw 6.1 and 5.9, respectively), possible underestimations of the seismic hazard by mps04 were investigated, in order to analyze and evaluate the possible need for its update. one of the most common misunderstanding is to compare recorded pga with the pga value with 10% probability of exceedance in 50 years only. moreover, by definition, probabilistic models cannot be validated (or rejected) on the basis of a single event. however, comparisons of forecasted shakings with observed data are useful for understating the consistency of the model. it is then worth highlighting the importance of these comparisons. in fact, mps04 is the basis for the current italian building code to provide the effective design procedures and, thus, any modification to the seismic hazard values would also affect the building code. in this paper, comparisons between recorded ground motion during the amatrice earthquake and seismic hazard estimates are performed, in order to evaluate the consistency between predicted and observed accelerations. i. introduction ince 2006 italy has a reference seismic hazard model in accordance with the prime minister ordinance 3519/2006, which ratified the mps04 (mappa di pericolosità sismica 2004) seismic hazard model [mps working group, 2004; stucchi et al., 2011; http://zonesismiche.mi.ingv.it/] as the basic elaboration to be considered to update the seismic classification of municipalities. later in 2009, mps04 has been used also for the determination of the design spectra in the italian building code [norme tecniche per le costruzioni, 2008 (ntc08)]. the mps04 model was produced in the aftermath of the mw 5.7 san giuliano (southern italy) 2002 earthquake that struck an area not considered as seismic before. the main requirements of mps04 were the adoption of upto-date and worldwide accepted methodological approaches, updated input data, and transparent and reproducible procedures with clear description of the operating choices and related uncertainties. several input elements were specifically updated, such as the cpti04 earthquake catalog [cpti working group, 2004; http://emidius.mi.ingv.it/cpti04/], the zs9 seismic source zone model [meletti et al., 2008], and the diss database of potential seismogenic sources, originally defined in [valensise and pantosti, 2001]. two groundmotion predictive models, two available models based on italian and european data were selected [sabetta and pugliese, 1996; ambraseys et al. 1996], together with two newlydeveloped ones [malagnini et al., 2000 and 2002; morasca et al., 2002]. a logic-tree approach was then adopted in order to manage the epistemic uncertainty associated with the input elements, and the median hazard value s annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 2 (assumed as reference estimate) was computed together with the percentiles to show model uncertainties. the resulting mps04 model provides, on a 5km-spaced grid, pga and spectral accelerations computed for 10 periods (from 0.1 to 2 seconds), for 9 probabilities of exceedance in 50 years (from 2% to 81%, corresponding to return periods from 2475 to 30 years), for rocky soil conditions and flat topography. all the data of mps04 are accessible through a webgis application [http://esse1-gis.mi.ingv.it/; martinelli and meletti, 2008]. the 24 august 2016 amatrice (central italy) mw 6.0 earthquake occurred in an area with the highest seismic hazard in italy, where the pga values expected with a probability of exceedance of 10% in 50 years are higher than 0.25 g (fig. 1). figure 1. seismic hazard map (mps04) for central italy (top inset: map for whole italy) in terms of horizontal pga on rock with 10% probability of exceedance in 50 years. epicenters of the 24 august 2016 main shock (white star) and events with mw ≥4 (white dots) of the sequence in the first 40 days are also shown. green triangles show the accelerometric stations considered in this study. seismic source zones of the zs9 model are indicated with blue lines. the area is a portion of the apenninic belt, a young chain formed in the tortonian, when the tyrrhenian sea started its opening and the compressional thrust fronts migrated towards the east and the north east. at present, the extensional tectonic features reach the axial belt, whereas the compressional front is located along the adriatic coastline. the amatrice event struck a region that, in the zs9 seismic source model, belongs to a large area source [zs923], characterized by prevalent normal faulting focal mechanisms and a maximum magnitude of 7.2, and where a number of strong earthquakes occurred in the past (fig. 2), including the 2009 mw 6.1 l’aquila event. figure 2. seismic source zones of the zs9 model (blue lines) and earthquakes from the cpti04 catalogue (moment magnitudes mainly derived from historical data). white star is the epicenter of the 24 august 2016 mw 6.0 event. the seismic histories of villages located in the area confirm the occurrence of frequent and strong earthquakes in the last 400 years [castelli et al., 2016]. the seismogenic potential of zs923, in terms of activity rates and range of magnitude (mainly derived from historical annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 3 seismicity), shows that a mw 6.0 or larger has a rate of occurrence of about one event every 50 years (fig. 3). figure 3 also shows that using the last release of the italian parametric catalogue cpti15 [rovida et al., 2016] does not substantially modify the magnitude-frequency distribution of zs923, especially for rates of occurrence of mw≥6.0 events. figure 3. annual seismicity rates for the two approaches adopted in mps04 (i.e., gr=gutenberg-richter rates; ar=activity rates) for source zone zs923 and for the three share seismicity models: itas308 are the rates associated with the area source, seifa are the kernel-smoothed stochastic rates, fsbg are the rates obtained combining fault sources (for mw≥6.4) and background seismicity (mw<6.4). cpti15 are the rates obtained for source zone zs923 using the most recent release of the catalogue cpti15 [rovida et al., 2016]. in addition, figure 3 displays the seismicity rates for the sources that include the amatrice area according to the three alternative earthquake source models considered in the share european seismic hazard map [woessner et al., 2015]. they consist of: i) an area source model, ii) a combination of faults and background seismicity (fsbg), and iii) a kernel-smoothed, zonation-free stochastic earthquake rate model that considers seismicity and cumulated fault moment (seifa). in the first model, the area source itas308 corresponds to zs923 and is characterized by seismicity rates close to those of mps04, except for the different evaluation of the maximum magnitude. on the contrary, rates for the fsbg and seifa models are lower than those of mps04. this paper is focused on the 24 august mw 6.0 earthquake and does not take into account the following large events of 26 and 30 october 2016 (mw 5.9 and 6.5, respectively). ii. seismic hazard and ground-motion recordings of the amatrice earthquake recorded accelerations of the 24 august 2016 mw 6.0 mainshock (data are available at http://esm.mi.ingv.it) were compared with the hazard values provided by the mps04 model. as known, the comparison of the observed ground motion from a single event with the shaking values expected with a certain probability of exceedance in a given time period is not a correct validation of a probabilistic seismic hazard (psh) model. however, a comparison of recorded response spectra and peak ground-motion parameters with psh estimates allows evaluating the relative “position” of a given earthquake with respect to the range of psh values in order to understand their consistency, as shown by studies on other recent earthquakes [e.g. meletti et al., 2012]. as shown in figure 4, the maximum horizontal pga recorded by the amatrice accelerometric station (amt), the nearest to the epicenter (about 9 km; fig. 1 and table 1), was 0.87 g. such value is compared with the hazard curve, which represents the annual frequency of exceedance (afoe) of different levels of pga (afoe is expressed in figure 4 also as its inverse, that is the return period). the hazard curve in figure 4 is referred to the node of the mps04 computational grid closest to the amt station. to make the comparison meaningful, we corrected the mps04 hazard curve computed for soil class a (i.e. rock or very stiff soil with vs30 > 800m/s) by applying the coefficient for soil class b (the class of the amt station, [itaca working group, 2016]) prescribed by the italian building code (ntc08). the resulting hazard curve returns expected pga values ranging from 0.09 g to 0.54 g. figannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 4 ure 4 also shows the hazard curve evaluated by share for the same site, modified for soil class b as for the mps04 curve. pgas expected by the share curve are lower than mps04 ones for afoe ≥0.02 yr-1 (i.e. return periods ≤50 years), and higher than mps04 ones for lower afoe (i.e. longer return periods). at the longest return period considered by the mps04 model (2475 years, afoe = 0.0004 yr-1), the share curve gives a pga of ~0.7 g. to investigate the impact of the ground-motion predictive equations (gmpe) on seismic hazard estimates, we re-evaluated the mps04 curve retaining the same earthquake rate model but using the ita10 equation [bindi et al., 2011] instead of the originally adopted gmpes. the resulting hazard curve returns the highest estimates of pga, that is ~0.9 g at return period of 2475 years. figure 4. comparison of the maximum horizontal pga of the amatrice mw 6.0 mainshock recorded at the amt station with the hazard curves of mps04 (for soil class a and b), share (for soil class b) and mps04 modified by using ita10 gmpe (for soil class b) [bindi et al., 2011] at the node of the computational grids closest to amt site. because we have no data about the return period of the pga recorded at amt, it is shown as a vertical line. potentially, the intersection between this vertical line and the hazard curve returns the approximated return period of that horizontal ground shaking for that site. it is clear from figure 4 that the mps04 curve, even modified for soil class b, does not reach the pga recorded at amt. however, adopting a more recent gmpe shifts the hazard curves towards higher values, making the ground shaking recorded at amt consistent with expected pga for return period of 2475 years. moreover, we are considering the pga observed at a site very close to the epicenter. as shown in table 1, a strong decrease of the recorded pga is observed at the other acceleroannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 5 metric stations located at epicentral distances lower than 25 km. since the level of seismic hazard is almost the same at amt and at the other considered stations, one can see that the minimum and maximum observed pga (~0.1 and ~0.4 g) intersect the hazard curve of mps04 at return periods between ~30 and 975 years, respectively. in figure 5, the horizontal acceleration response spectra (5% damping) recorded at the amt station for the 24 august mainshock are compared with the uniform hazard spectra (uhs) computed by the mps04 model, corrected for soil class b, for probabilities of exceedance of 10% and 2% in 50 years, corresponding to return periods of 475 and 2475 years, respectively. as above, we then computed the uhs by replacing the gmpes adopted in mps04 with ita10 equation [bindi et al., 2011]. it is worth noting that the mps04 model returns estimates of the maximum component of the horizontal ground shaking, as defined in the adopted gmpes, whereas the [bindi et al., 2011] model estimates the geometrical mean of the horizontal components of ground shaking. figure 5 shows that, for the 475 years return period, the mps04 uhs is generally lower than the maximum of the e-w and n-s components of the recorded spectrum. for the 2475 years return period, the mps04 uhs is always higher than the n-s recorded spectrum, while it is higher than the ew spectrum at periods longer than ~0.5 s and lower at shorter periods. figure 5. comparison between the horizontal acceleration response spectra (5% damping, n-s and e-w components) of the 24 august mw 6.0 mainshock recorded at amt station and the uhs of mps04 for soil class b and of mps04 modified using the ita10 gmpe [bindi et al., 2011] (for soil class b), at the node of the computational grid closest to amt, for 2% and 10% probability of exceedance in 50 years. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 6 figure 6. comparison between the maximum horizontal acceleration response spectra (5% damping) of the amatrice mw 6.0 mainshock recorded at the stations located between 16 and 25 km from the epicenter and the uhs of mps04 for soil class b, at a node located at about 20 km to the southeast of amt, for 2%, 5% and 10% probability of exceedance in 50 years. the station codes refer to figure 1 and table 1. instead, the uhs obtained by mps04 using the [bindi et al., 2011] gmpe for 2475 years return period is always higher than the maximum recorded response spectrum. figure 6 compares the uhs evaluated by mps04, corrected for soil class b, with the maximum component of the recorded horizontal ground shaking for stations located at epicentral distances less than 25 km (fig. 1 and table 1). uhs in figure 6 are computed for a site located at about 20 km to the southeast of amt. the response spectrum at nrc (norcia) station, the second nearest station to the epicenter of the 24 august earthquake, shows a sharp peak at 0.2 s that is higher than the uhs for 2475 years and a second peak at ~0.7-0.8 s which exceeds the uhs for 975 years. at the other stations, instead, the maximum horizontal components are always lower than the mps04 uhs for 475 years return period. table 1: data of the accelerometric stations considered in this study (http://esm.mi.ingv.it). station code ec8 soil class epic. distance (km) pga (g) amt b* 8.5 0.8673 nrc b 16.0 0.3733 pcb b* 17.7 0.3077 csc b 19.6 0.1068 msc b* 21.3 0.1055 msct b* 21.3 0.1101 rm33 b* 21.9 0.1023 spd b* 23.2 0.1018 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 7 iii. conclusions a comparison between available recordings for the 24 august 2016 amatrice earthquake and expected accelerations provided by the reference seismic hazard model (mps04) for italy was performed. as shown in figures 4 and 5, at the accelerometric station closest to the epicenter (amt), the observed accelerations exceed the expected values given by mps04. recorded spectra for the other stations located at epicentral distances lower than 25 km are instead consistent with expected accelerations (figure 6). in order to understand possible reasons for the “failure” of the mps04 model at amt station, we investigated the impact of the adoption of different gmpes on psh assessment. to this purpose, we computed the hazard using the same earthquake rate model of mps04 but a more recent gmpe (i.e. ita10 [bindi et al., 2011]) than those originally adopted. results show a strong increase of expected values for both pga (fig. 4) and uhs (fig. 5), thus making the psh estimates consistent with the observations. as known, recent gmpes produce higher hazard estimates, due to the larger values of uncertainty (standard deviation) with respect to older gmpes (e.g. [bommer and abrahamson, 2006]). moreover, it is worth noting that the ita10 equation was derived from an italian strongmotion dataset that includes also recordings in the near field, that were lacking in previous gmpes used for the mps04 model (e.g. [sabetta and pugliese, 1996]). anyway, the very high accelerations recorded at amt (the largest pga ever recorded in italy) could be due to pulse-like motions in the near field as discussed in [reluis-ingv workgroup, 2016]; a similar effect was also observed for the l’aquila 2009 mw 6.1 earthquake [chioccarelli and iervolino, 2010]. as mentioned above, the aim of this study was not to validate the mps04 model, but rather to evaluate the “relative position” of the 24 august 2016 earthquake with respect to the range of psh values proposed in that model. any psh model, however, has to be considered with respect to the available data and knowledge at the time of its release. mps04 adopted the best input data available in 2004, and the same applies to the share project ten years later. as an example we showed how the use of a recent gmpe changes the hazard. for this reason, the italian reference seismic hazard model is currently under re-evaluation (https://ingvcps.wordpress.com/), taking into account state-of-the-art methodological approaches, input data, and computational codes. references ambraseys, n.n., simpson, k.a. and bommer, j.j. (1996). prediction of horizontal response spectra in europe. earthq. eng. struct. d., 25:371-400. bindi, d., pacor, f., luzi, l., puglia, r., massa, m., ameri, g. and paolucci r. (2011). ground motion prediction equations derived from the italian strong motion database, bull earthquake eng, 9(6): 1899–1920, doi:10.1007/s10518-011-9313-z . bommer, j.j. and abrahamson, n.a. (2006). why do modern probabilistic seismichazard analyses often lead to increased hazard estimates? bull. seismol. soc. am., 96:1967–1977; doi:10.1785/0120060043 castelli, v., camassi, r., caracciolo, c., locati, m., meletti, c. and rovida, a. (2016). new insights in the seismic history of monti della laga area. ann. geophys., this volume. chioccarelli, e. and iervolino, i. (2010). nearsource seismic demand and pulse-like records: a discussion for l'aquila earthquake. earthquake engineering and structural dynamics. 39(9):1039–1062. cpti working group (2004). catalogo parametrico dei terremoti italiani, versione 2004 (cpti04), ingv, bologna; http://emidius.mi.ingv.it/cpti04/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7248 8 itaca working group (2016). italian accelerometric archive, version 2.1, doi: 10.13127/itaca/2.1; available at http://itaca.mi.ingv.it/ malagnini, l., akinci, a., herrmann, r.b., pino, n.a. and scognamiglio, l. (2002). characteristics of the ground motion in northeastern italy. bull. seismol. soc. am., 92:2186-2204. malagnini, l., herrmann, r.b. and di bona, m. (2000). ground motion scaling in the apennines (italy). bull. seismol. soc. am., 90:1062-1081. martinelli, f. and meletti, m. (2008). a webgis application for rendering seismic hazard data in italy. seismol. res. lett., 79:68-78. meletti, c., galadini, f., valensise, g., stucchi, m., basili, r., barba, s., vannucci, g. and boschi, e. (2008). a seismic source zone model for the seismic hazard assessment of the italian territory. tectonophys., 450:85108; doi:10.1016/j.tecto.2008.01.003. meletti, c., d'amico, v., ameri, g., rovida, a. and stucchi, m. (2012). seismic hazard in the po plain and the 2012 emilia earthquakes. ann. geophys., 55(4):623-629; doi: 10.4401/ag-6158. morasca, p., malagnini, l., akinci, a. and spallarossa, d. (2002). ground-motion scaling in the western alps. seismol. res. lett., 73:251. mps working group (2004). redazione della mappa di pericolosità sismica prevista dall’ordinanza pcm del 20 marzo 2003 n. 3274, all. 1. ingv, milano-roma, italy, available at http://zonesismiche.mi.ingv.it/, 163 pp. (in italian). norme tecniche per le costruzioni, ntc (2008). norme tecniche per le costruzioni, ministerial decree 14/01/2008, official gazette n. 29, 4 february 2008 (in italian). prime minister ordinance 3519/2006 ministerial decree 28/04/2006, official gazette n. 108, 11 may 2006 (in italian). reluis-ingv workgroup (2016). preliminary study on strong motion data of the 2016 central italy seismic sequence v6, available at http://www.reluis.it/. rovida a., locati m., camassi r., lolli b., gasperini p. (eds) (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. ingv; doi:10.6092/ ingv.it-cpti15 sabetta, f. and pugliese, a. (1996). estimation of response spectra and simulation of nonstationary earthquake ground motions. bull. seismol. soc. am., 86:337-352. stucchi, m., meletti, c., montaldo, v., crowley, h., calvi, g.m. and boschi, e. (2011). seismic hazard assessment (20032009) for the italian building code. bull. seismol. soc. am., 101:1885-1911; doi:10.1785/ 0120100130. valensise, g. and pantosti, d. (2001). database of potential sources for earthquakes larger than m5.5 in italy, ann. geofis., supplement to vol. 44 (4), 180 pp., with cdrom. woessner, j., et al. (2015). the 2013 european seismic hazard model: key components and results. bull. earthq. eng., 13:3553– 3596; doi:10.1007/s10518-015-9795-1. annals of geophysics, 60, 2, 2017, a0327; doi: 10.4401/ag-6969 long-term monthly statistics of the mid-latitude ionospheric e-layer peak electron density in the northern geographic hemisphere during geomagnetically quiet and steadily low solar activity conditions anatoli v. pavlov 1,*, nadezhda m. pavlova1 1 pushkov institute of terrestrial magnetism, ionosphere and radio-wave propagation of the russian academy of sciences (izmiran), moscow, troitsk, russia article history received january 21, 2016; accepted february 20, 2017. subject classification: mid-latitude ionosphere, e-region, modeling and forecasting. abstract long-term hourly values of the ionospheric e-layer peak electron density, nme, measured from 1957 to 2014 by 4 mid-latitude ionosondes (wallops island, boulder, de l’ebre, and rome) in the northern geographic hemisphere were processed to select periods of geomagnetically quiet and low solar activity conditions using the 3-hour index, ap, of geomagnetic activity and the daily solar 10.7 cm radio flux index, f10.7, of solar activity. the selected ionospheric e-layer peak electron densities are used to calculate several descriptive statistics of nme close to noon for each month in a year, including the mathematical expectation of nme, the standard deviation of nme from the mathematically expected nme, and the coefficient of variations of nme. the month-to-month variability of these descriptors allowed us to identify months when they reach their extremes (maxima, minima). 1. introduction production and loss processes of ions and electrons of the mid-latitude ionospheric e-region are well studied [see, e.g., schunk and nagy, 2009; pavlov, 2012, pavlov and pavlova, 2013, 2015, and references therein]. analyzing production and loss rates of ions and electrons of the ionospheric e-region, it is possible to establish causes of day-to-day variations of the mid-latitude e-region peak electron density, nme, during each month of a year under given local time, latitude, and longitude. the major sources of this variability of nme are changes of x-ray and euv solar irradiance with solar activity, variations of number densities and temperature of neutral species with solar and geomagnetic activity, and changes of the solar zenith angle [e.g., moore et al., 2006, pavlov and pavlova, 2013, 2015]. day-to-day variability of ionospheric e-region electron densities remains a topic of interest [see, e.g., kouris and fotiadis, 2002; moore et al., 2006; nicolls et al., 2012, and references therein]. the hourly values of the critical frequency, foe (which is proportional to the square root of nme [piggot and raver, 1978]), of the ionospheric e-layer measured by 30 mid-latitude ionosondes during the period of 1964-1995 were used by kouris and fotiadis [2002] to evaluate day-to-day variability of foe. they found that positive and negative relative deviations of foe from the monthly median values of foe are within the range of 10 % for more than 90 % of the time. noontime day-to-day ionosonde and incoherent scatter radar measurements of the e layer parameters were used by moore et al. [2006] to calculate the standard relative deviations of nme from noontime nme mean values in the range of 5-7% at middle latitudes for 9–27 march 1999 and 4 october – 4 november 2002. slightly larger values of nme variability were estimated by nicolls et al. [2012] using the inversion technique from satellite-based radio occultation total electron content measurements. however, little attention has been given in these and other published morphological studies to nme data sorting due to apparent variability of solar and geomagnetic activity. thus, this published nme statistics in fact describes a mix of day-to-day variations of geomagnetically quiet nme at a steady solar activity and variations of nme in response to changing geomagnetic and solar activity conditions. the daily solar 10.7 cm radio flux index, f10.7, (or a daily sunspot number) and the 3-hour geomagnetic index, ap (or kp), are the most widely used indices for exploring causes and consequences of solar and geomagnetic activity [see, e.g., akasofu and chapman, 1972; schunk and nagy, 2009]. therefore, the f10.7 and a0327 pavlov and pavlova 2 ap indices are used in our work to describe dependencies of nme on solar and geomagnetic activities, respectively. no+ and o2 + ions are the main ions at the e-region altitudes of the ionosphere, and the characteristic time to approach the photochemical equilibrium by dissociative recombination reactions of these ions with electrons is less than one minute during daytime conditions close to the e-layer peak altitude [banks and kockarts, 1973]. the value of this characteristic time is much less than 3 hours. as a result, variations of nme caused by changes in geomagnetic activity during a period that is less than 3 hours are not described in terms of changes of ap and can be considered as random variations of nme if the ap index is used to study a dependence of nme on geomagnetic activity. the sun is not static throughout each day, and changing x-ray and euv irradiance with time periods less than 24 hours are not captured by the f10.7 index of solar activity or by any other daily index of solar activity [acebal and sojka, 2011]. hence, if the solar ionizing fluxes are changing significantly during a few hours due to flares and also an overall background change, the daytime nme will respond, but this response of nme cannot be described in terms of variations of f10.7. it should be also noted that the integral solar flux below 200 nm increases with a rise in the average index 0.5(f10.7+f10.7) only on average, and there are significant deviations from the linear correlation between this integral solar flux and this average index where f10.7 is the 81-day average of daily f10.7 solar activity indices centered on the day under study [solomon, 2006]. manson [1976] has also pointed out that the correlation of the integrated solar flux between 5.2 nm and 12 nm with f10.7 is poor. on the other hand, there is significant influence of variations in x-ray irradiance on nme [pavlov and pavlova, 2013, 2015, sojka et al., 2014]. from the above reasoning, differences between nme calculated by the one-dimensional time-dependent theoretical mid-latitude model of the e-region ion composition and nme measured by the boulder and moscow ionosondes can be explained by uncertainties up to a factor of 2 in predictions of x-ray radiation on the basis of changes in the f10.7 and f10.7 indices [pavlov and pavlova, 2013, 2015]. as a result, the use of the f10.7 index as an indicator of solar activity in statistical studies of nme can be one of sources of deviations of nme from the expected nme for the chosen level of solar activity, and these deviations caused by the use of the f10.7 index can be considered as random variations of nme. the quiet time ionospheric e-layer number densities measured by an ionosonde during a month depend on the solar zenith angle which value is changed during this month. however, the existence of a day in a month at low solar activity does not mean that all days in this month correspond to the low solar activity conditions under consideration. in addition to that, geomagnetically quiet time periods are randomly distributed during each month of a year. thus, day-today variability of nme at given local time during each month of a year caused by changes of the solar zenith angle is modified in a random way due to variations of solar and geomagnetic activity. the foregoing shows that the daytime value of nme measured by an ionosonde during geomagnetically quiet conditions at low solar activity under given local time during a month in a year can be considered as a random variable. the objective of this work is to apply the mathematical statistics, as described, for example, by johnson and leone [1977], to study this variability of nme using nme measured by the mid-latitude ionosondes at wallops island, boulder, de l’ebre, and rome in the northern geographic hemisphere from 1957 to 2014 during geomagnetically quiet conditions at low solar activity. for achievement of this purpose, we calculate the mathematically expected, <nme>, and most probable, nmemp ,values of nme, the standard deviations of nme from <nme> and nmemp , and the coefficients of variations of nme relative to <nme> and nmemp for each month of a year using data collected by 4 mid-latitude ionosondes in the northern geographic hemisphere. as a result, month-to-month variations of the above listed statistical parameters of nme over each ionosonde are studied. 2. data and method of data analysis the ionosonde data for this investigation were obtained from the noaa national geophysical data center (ngdc) in boulder, colorado, using its online space physics interactive data resource (spidr). first, we selected the hourly values of foe observed in 1957-2014 by 4 ionosondes (wallops island, boulder, de l’ebre, and rome) in the northern geographic hemisphere. table 1 provides the geographic latitude and longitude (ϕ, λ), and average geomagnetic latitude and longitude (φ, λ) of each ionosonde. the ionosonde stations presented in table 1 are listed in order of increasing geographic latitude. it follows from the calculations [details are described by pavlov and pavlova, 2014] that the geomagnetic coordinates of the ionosondes under consideration averaged over the time long-term monthly statistics of the e-layer peak electron density 3 period from 1957 to 2012 and given by pavlov and pavlova [2014] are practically the same as for the time period of 1957-2014, and these average values of φ and λ are presented in table 1. we believe that, under quiet geomagnetic conditions, the mid-latitude ionosphere is located between 30° and 55° geomagnetic latitudes in the northern geographic hemisphere. the stations used are all located within the 30° to 55° geomagnetic latitude interval, representing the mid-latitude ionosphere. nme values were obtained from foe using their well known relationship [e.g., piggott and rawer, 1978] nme=1.24·104 foe2, (1) where the units of nme and foe are cm-3 and mhz, respectively. the hourly foe records acquired from the ngdc archives were analyzed to select values corresponding to time points during the geomagnetically quiet periods defined below with ut closest to the solar noon in the solar local time, slt (see table 1). the relationship between ut and slt is defined by ut=slt-λ/15, where λ is the east geographic longitude of the ionospheric observatory in degrees, while slt and ut come in units of hours. it should be noted that the values of foe measured by the juliusruh ionosonde at 10:58 ut are provided by the ngdc archives for the time period from 26 march 2007 to 31 december 2014, and these measurements are used in our statistical study in place of missing juliusruh ionosonde measurements of foe at 11:00 ut for this time period (see table 1). the e-region ion and electron densities depend on geomagnetic activity due to variations of the neutral temperature and densities with geomagnetic activity. these changes in the neutral temperature and densities can be described by variations of 7 indices: the daily ap index, the 3-hour ap index for current time, the 3-hour ap indices for 3, 6, and 9 hours before current time, the average of eight 3-hour ap indices from 12 to 33 hours prior to current time, and the average of eight 3-hour ap indices from 36 to 57 hours prior to current time [hedin, 1987; picone et al., 2002]. the relationship between indices ap and kp is well established, and the value of ap=18 corresponds to kp=3 [akasofu and chapman, 1972]. the geomagnetically quiet conditions for the candidate noon foe values were identified by ensuring that each of the seven above-mentioned indices of geomagnetic activity was equal to 18 or was less than 18. to select periods of steadily low solar activity, we relied on analysis of three indices derived from observations of f10.7. the solar euv flux, primarily responsible for the daytime ionization, is approximately represented by f10.7 and f10.7 [richards et al., 1994]. the electron density also depends on the solar-controlled neutral temperature and densities whose dependences on f10.7p (f10.7 for a day preceding a day under consideration) and f10.7 are well established [hedin, 1987; picone et al., 2002]. all three indices, f10.7, f10.7p, and f10.7, were used to control selection of the foe data by retaining only those days for which these indices were within the 65 to 85 interval (in 10-22 w·m-2·hz-1). we consider sets of foe(ut,m) and nme(ut,m) for each month, m, in a year at the given ut for each location. the results of measurements of foe are presented in the database with the step, δfoe, of 0.05 mhz, i.e. the considered foe are given on the table 1. the ionosonde names and locations, and time ranges of foe measurements at the universal time, ut, closest to the solar noon in the solar local time, slt. ionosonde ϕ (°) λ (°) φ (°) λ (°) years ut slt wallops island 37.8 284.5 45.3 358.6 1967-2014 17:00 11:58 boulder 40.0 254.7 46.9 325.2 1958-1960, 1962-2002, 19:00 11:59 2004-2014 de l’ebre 40.8 0.3 40.8 76.7 1957-1980, 1982-1987, 12:00 12:01 1991-1995, 1998-2004, 2007-2014 rome 41.8 12.5 40.0 88.4 1976-2004, 2007-2014 11:00 11:50 pavlov and pavlova 4 uniform grid of foek=kδfoe, where k=1, 2, …k, and k is the maximal value of k. the measured critical frequencies foek and eq. (1) allow to determine the corresponding measured e-layer peak electron densities nmek at the corresponding non-uniform grid of nmek. the probability, pk(ut,m), to measure a geomagnetically quiet nmek(ut,m) is counted individually for each ionosonde, ut, and m as pk(ut,m)=fk(ut,m)/f(ut,m), (2) where fk(ut,m) is a number of nmek(ut,m), f(ut,m) = fk(ut,m) is the total number of selected nmek(ut,m) values. it follows from eq. (2) that it should be noted that the value of f(ut,m) turned out to be at least 101 or greater than 101 for each ionosonde. we believe that this value of f(ut,m) is enough large to carry out our statistical study. we define the mathematical expectation of nme to be <nme(ut,m)>= pk(ut,m)nmek (3) the standard deviation of nme from <nme> is calculated as σav(ut,m)={ pk(ut,m)[nmek <nme(ut,m)>] 2}0.5. (4) the coefficient of variations of nme relative to <nme> (the relative standard deviation of nme from <nme>) expressed as a percentage takes a form cvav(ut,m)=100σav(ut,m)/<nme(ut,m)>. (5) all values of nmek are not equally probable, and pk reaches its maximum at the most probable value, nmempp , of nme. the standard deviation, σmp, of nme from nmemp and the coeff icient, cvmpp , of variations of nme relative to nmemp (the relative standard deviation of nme from nmemp) are calculated as σmp ,(ut,m)=[ pk(ut,m)(nmek-nmemp) 2]0.5. (6) cvmp(ut,m)=100σmp(ut,m)/nmemp . (7) 3. results and discussion the use of the statistical approach to study month-to-month variations in the statistical parameters of nme is motivated by a variability of nme during geomagnetically quiet conditions for approximately the same solar activity at the same ut and location during each month. if these conditions are carried out then day-to-day variability of nme determines a dependence of pk(ut,m) on nmek. examples of this dependence are shown in figure 1 when the boulder ionosonde data at 19:00 ut (11:59 slt) are used in the statistical study. circles and pluses in figure 1 correspond to january and february (left top panel), march and april (left middle panel), may and june (left bottom panel), july and august (right top panel), september and october (right middle panel), and november and december (right bottom panel), respectively. it follows from the calculations that each dependence of pk(ut,m) on nmek (see figure 1) is a sequence of peaks in pk(ut,m), and a location of the largest peak in the nmek–axes determines the most probable, nmemp , value of nme for each month under consideration. figure 1 shows that, with the exception of the march dependence of pk(ut,m) on nmek, each of the dependences of pk(ut,m) on nmek has a peak in pk(ut,m) whose amplitude is not much less than that of the largest peak in pk(ut,m) for this dependence. it should be noted that the mathematical expectation of nme defined by eq. (3) takes into account the relative contribution of each value of nmek in accordance with the value of pk(ut,m) for this nmek, and all significant peaks in each dependence of pk(ut,m) on nmek under consideration are taking into account in <nme>. figures 2-4 show month-to-month variations in the calculated values of <nme> (crosses in figure 2), nmemp (squares in figure 2), σav (crosses in figure 3), σmp (squares in figure 3), cvmp (crosses in figure 4), and cvmp (squares in figure 4) over the wallops island (left top panels), boulder (left bottom panels), de l’ebre (right top panels), and rome (right bottom panels) ionosondes. it follows from our calculations that each statistical parameter of nme is changed from ionosonde to ionosonde for the same month due to differences in geographic latitudes and longitudes of the ionosondes and in the values of slt when these measurements were carried out by the ionosondes (see table 1). percent differences between nmemp and <nme> can be calculated for each month in a year as 200|nmemp <nme>|/(nmemp + <nme>). this difference depends on m, and reaches its maximum value, z(nmemp,<nme>), that varies from an ionosonde to an ionosonde. we found that z(nmemp,<nme>)=8.1, 6.1, 4.4, and 4.5 % for the wallops island, boulder, de l’ebre, and rome ionosondes, respectively. the calculated month-to-month variations of pk(ut,m)=1. k=1 k ∑ k=1 k ∑ k=1 k ∑ k=1 k ∑ k=1 k ∑ long-term monthly statistics of the e-layer peak electron density 5 <nme> and nmemp shown in figure 2 are both trying to estimate month-to-month variations of nme. what is better? it follows from the calculations of σav and σmp presented in figure 3 that σav(ut,m)< σmp(ut,m) (8) the standard deviations of nme from <nme> and nmemp measure spreads of distributions of nme about <nme> and nmemp , respectively. we believe that, the smaller this spread, the more efficient the estimation. as a result, the calculated month-to-month variations of <nme> presented in figure 2 can be considered as the most efficient estimation of month-to-month variations of nme for the ionosonde measurements under consideration. the calculations show (see figure 4) that, as a rule, cvav(ut,m)<cvmp(ut,m) with the exception of m=2, 6, 7, 10, and 11 for the rome, delebre, rome, wallops island, and delebre ionosonde, respectively, and a difference between cvav(ut,m) and cvmp(ut,m) is negligible for each of these exceptions. thus, the calculated month-to-month variations of <nme> can be considered as the most efficient estimation of month-to-month variations of nme if we compare not only σav(ut,m) with σmp(ut,m) but also cvav(ut,m) with cvmp(ut,m). based on this conclusion, the calculated month-to-month variations of nmemp , σmp, and cvmp are not discussed further. it follows from figure 2 that the annual maximum of <nme> is formed in may (wallops island and rome), june (boulder), and july (de l’ebre). the mathematical expectation of nme is lowest in its annual value in december. a ratio of the largest to lowest value of the mathematically expected nme characterizes the maximal month-to-month variability of this statistical parameter of nme in a year. this ratio is found to be 1.40, 1.41, 1.43, and 1.43 for the wallops island, boulder, de l’ebre, and rome ionosondes. we also point out that local minima of <nme> are formed in may, june, and july over the boulder, rome, and wallops island ionosondes, respectively. by analogy with the definition of the winter anomaly of the f2-layer peak electron density (see, e.g., pavlov and pavlova [2005, 2009], pavlov et al. [2010], and references therein), the winter anomaly of nme can be defined as follows. if the e-layer peak electron density is sometimes greater in winter than that in summer over the same earth’s surface point during geomagnetically quiet daytime conditions at the same universal time despite the reduced solar insolation in winter in comparison with that in summer then this ionospheric phenomenon can be designated as the winter anomaly of nme. it follows from figure 2 that the winter anomaly of <nme> is not observed over the wallops island, boulder, de l’ebre, and rome ionosondes. it is seen from figure 3 that the annual maximum of the standard deviation of nme from <nme> occurs in july over all ionosondes under consideration. the lowest annual value of σav is found to be in january (de l’ebre), february (wallops island), october (boulder), and december (rome). figure 4 shows that the coefficient of variations of nme maximizes in its annual value in july. this coefficient minimizes in its annual value in january (de l’ebre), february (wallops island), september (boulder), and october (rome). it follows from the comparison of cvav of all 4 ionosondes that cvav reaches its lowest and largest values in its month-to-month variations over the de l’ebre and wallops island ionosondes, respectively. if all 4 ionosonfigure 1. dependencies of pk(ut,m) on nmek over the boulder ionosonde. circles and pluses correspond to january and february (left top panel), march and april (left middle panel), may and june (left bottom panel), july and august (right top panel), september and october (right middle panel), and november and december (right bottom panel). pavlov and pavlova 6 des are considered then the value of cvav is located in the range of 5.1-11.9 %. we point out that the calculated coefficients of variations of nme relative to <nme> presented in figure 4 (6.7-10.6 % in march and 6.0-7.7 % in october) are comparable with that given by moore et al. [2006]. the primary source of metals existing in the mesosphere and low thermosphere as metallic layers [e.g., na, fe, ca, mg, and k] is ablation of meteoroids in the atmosphere [see, e.g., kopp, 1997; ceplecha et al., 1998; plane et al., 2015, and references therein]. after ablation, metal vapor densities are changed by diffusion and chemical reactions of metals with components of the atmosphere, forming altitude distributions of metal atoms at altitudes of the ionosphere. in addition to the ionization of metal atoms in their hyperthermal collisions with n2, o2, and o, chemical reactions of these metals with o2 + and no+ ionospheric ions produce metal ions at eand d-region altitudes of the ionosphere [see, e.g., pavlov, 2014, and references therein]. a sharp change in the direction of the hydrodynamic velocity of the atmosphere (wind shift) causes changes in number densities of metallic ions, forming a thin long-lived sporadic eslayer electron number density at middle latitudes, and the presence of slowly recombining metallic ions is responsible for these es-layers having the properties of their irregular appearance in time and space [see, e.g., whitehead, 1989; haldoupis, 2012, and references therein]. as a result of the ablation of meteoroids, metallic ions exist not only in the es-layer, but also at all e-region altitudes of the ionosphere above and below this es-layer, and number densities of metallic ions under consideration are changed in time and in space. furthermore, a par of metallic ions is transported along magnetic field lines by diffusion and plasma drift, and metallic ions are observed even at f-region altitudes of the ionosphere [e.g., fesen and hays, 1982; mcneil et al., 1996; carter and forbes, 1999; collins et al., 2002; and references therein]. thus, it can be assumed that a part of σav and a part of cvav are produced by variations of metallic ion number densities. the meteoroid influx into the atmosphere consists of showers when the earth passes through meteoroid streams and sporadic meteoroids that do not belong to any specific meteoroid stream. the showers are divided into the major and minor meteor showers, and basic parameters of major meteor showers (period of activity, maximum date and sofigure 2. month-to-month variations of the mathematically expected nme (crosses) and the most probable nme (squares) over the wallops island (left top panel), boulder (left bottom panel), de l’ebre (right top panel), and rome (right bottom panel) ionosondes. figure 3. month-to-month variations of the standard deviations of nme from <nme> (crosses) and from nmemp (squares) over the wallops island (left top panel), boulder (left bottom panel), de l’ebre (right top panel), and rome (right bottom panel) ionosondes. long-term monthly statistics of the e-layer peak electron density 7 lar longitude, duration defined as the width of the rate profile at one-quarter of the maximum, radiant, approximate local time of the radiant transit, geocentric velocity and orbital elements) are presented in table xxii given by ceplecha et al. [1998]. some showers have a not regular annual activity, sometimes low, sometimes high, and changes in structures of meteoroid streams result in occasional intense outbursts or enhancements in their activity [ceplecha et al., 1998]. as a result, month-to-month changes of basic parameters of major meteor showers may have an impact on a part of month-to-month variations of σav and cvav shown in figs. 3 and 4, respectively. in particular, the absence of major meteor showers in february, march, june, and august [see table xxii given by ceplecha et al., 1998] should manifest itself in the calculated values of σav and cvav. it follows from the calculations that the value of σav and cvav is changed from one ionosonde to other ionosonde at given month of a year (see figs. 3 and 4). we believe that these variations of σav or cvavare caused by changes of <nme> from one ionosonde to other ionosonde and by differences in meteoroid populations for locations of the meteoroid influx into the atmosphere at the locations of the ionosondes (i.e., due to irregular distributions of metallic ion clouds in latitude and longitude). 4. conclusions the long-term statistical analysis of the month-to-month variations of mid-latitude noon nme measured by the wallops island, boulder, de l’ebre, and rome ionosondes during quiet times in 19572014 revealed notable contributions to the commonly accepted morphology of these variations. the probability of the occurrence of a geomagnetically quiet nme measured by each from 4 mid-latitude ionosondes was calculated at ut close to noon for each month in a year. we found that there are at list two peaks in each dependence of this probability on nme, and, as a rule, the amplitude of the second-large peak is not much less than amplitude of the greatest peak that determines the most probable nme. we provide evidence that the calculated month-to-month variations of the most probable nme are the less efficient estimation of month-to-month variations of nme in comparison with the expectation, <nme>, of nme. the annual maximum of <nme> is formed in may (wallops island and rome), june (boulder), and july (de l’ebre), and the value of <nme> is lowest in its annual value in december. a ratio of the largest to lowest value of <nme> that characterizes the maximal month-to-month variability of <nme> in a year is found to be in the range of 1.40-1.43. we report evidence that there is no the <nme> winter anomaly. the standard deviation of nme from the mathematically expected nme and the coefficient of variations of nme relative to <nme> calculated in this paper allow to quantitatively describe day-to-day variability of mid-latitude noon nme during each month in a year at low solar activity. we found that the standard deviation of nme from <nme> maximizes in its annual value in july, while the lowest annual value of this statistical parameter of nme is found to be in january (de l’ebre), february (wallops island), october (boulder), and december (rome). the annual maximum of the coefficient of variations of nme relative to <nme> occurs in july over the ionosondes under consideration. this statistical parameter of nme is minimal in january, february, september, and october over the de l’ebre, wallops island, boulder, and rome ionosondes, respectively. the calculated value of this coefficient of variations of nme is in the range of 5.1-11.9 %. figure 4. month-to-month variations of the nme variation coefficients relative to <nme> (crosses) and nmemp (squares) over the wallops island (left top panel), boulder (left bottom panel), de l’ebre (right top panel), and rome (right bottom panel) ionosondes. pavlov and pavlova 8 acknowledgments. hourly critical frequencies foe from the ionospheric sounder stations were provided by the national geophysical data center at boulder, colorado. the authors would like to thank reviewers and the editor for their comments on the paper, which have assisted in improving the final version. 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(1989). recent work on mid-latitude and equatorial sporadic-e, j. atmos. terr. phys., 51, 401–424. ___________ *corresponding author: anatoli pavlov, pushkov institute of terrestrial magnetism, ionosphere and radio wave propagation (izmiran), russia academy of science,142190, troitsk, moscow, russia; e-mail: pavlov@izmiran.ru 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved 111_116 adg v5 n01 coisson.pdf annals of geophysics, vol. 45, n. 1, february 2002 111 comparisons of experimental topside electron concentration profiles with iri and nequick models pierdavide coïsson, sandro maria radicella and bruno nava abdus salam ictp, trieste, italy abstract a critical part of the vertical ionospheric electron concentration profile is the region above its maximum (topside ionosphere) and many attempts have been made to model this region because of the limited experimental data available. recently, many topside electron concentration profiles obtained with the intercosmos-19 satellite became accessible through the internet. the period analyzed corresponds to march 1979 december 1980, a time interval of high solar activity. the present work describes the comparison of these profiles with the iri and nequick model profiles obtained by driving the models with the values of the maximum electron concentration and its height given by the satellite. 1. introduction modeling of the vertical electron concentration profile in the region above the maximum of the ionosphere has not yet reached a satisfactory global representation. one of the reasons is the limited number of data available for that region, since the main sources of such data are satellite topside sounding. in the past decades, ionosondes installed on satellites yielded measurements of topside electron concentration profiles, but only recently have those data sets been made available to the scientific community through the web. these large databases are useful to compare and validate ionospheric electron concentration models. in this work we focus attention on two models: – iri: the international reference ionosphere is a well known empirical model of the ionosphere and widely used (bilitza, 1990, 1994). for a given location, time and date, iri describes the electron concentration, electron temperature, ion temperature, and ion composition in the altitude range from about 50 km to about 2000 km as well as the total electron content (tec) to the same altitude. iri provides monthly medians in the non-auroral ionosphere for magnetically quiet conditions. iri electron density profiles sources are the coefficients ( f 0 f 2 and m(3000)) produced by the radiocommunications sector of the international telecommunication union (itu-r), incoherent scatter radars (jicamarca, arecibo, millstone hill, malvern, st. santin), the isis and alouette mailing address: dr. pierdavide coïsson, abdus salam ictp, strada costiera 11, 34100 trieste, italy; e-mail: coissonp@ictp.trieste.it key words topside ionosphere ionospheric models electron density profiles 112 pierdavide coïsson, sandro maria radicella and bruno nava topside sounders, and in situ instruments on several satellites and rockets. it can also use experimental values of f2 peak electron concentration (i.e. f 0 f 2 ) and height as inputs. iri is updated periodically and has evolved over a number of years. – nequick is a quick-run model for ionospheric applications. it is based on the dgr «profiler» concept (di giovanni and radicella, 1990). it is an epstein layers description of the electron concentration derived from models developed under european commission cost 238 and cost 251 actions. the input parameters of the model can be various: itu-r (former ccir) coefficients for f0 f2 and m(3000)f2 and r12 and/or monthly mean f 10.7; measured values of f 0 e, f 0 f 1 , f 0 f 2 and m(3000)f 2 ; f 2 peak concentration and peak height; regional maps of f0 f2 and m(3000)f2 based on grid values constructed from data obtained at given locations. the output is the vertical or slant profile of electron density and the corresponding total electron content to any given height up to 20 000 km (hochegger et al., 2000; radicella and leitinger, 2001). 2. comparison technique the database used in this work is the izmiran database of topside profiles accessible through the web (http://antares.izmiran.rssi.ru/ projects/ik19/), which at the time of presentation of this work contained more than 5000 topside electron concentration profiles from the intercosmos-19 satellite, in the period of high solar activity from march 1979 to december 1980 (fig. 1). in that period r12, twelve-month smoothed relative sunspot number, was between 136.5 and 164.5; the maximum occurred in december 1979. the height range of the profiles goes from the electron concentration maximum in the f2 region up to 500-1000 km, this range is determined by the elliptic orbit of the satellite with a perigee of 502 km and an apogee of 995 km. all profiles considered reached the f 2 peak, being able to determine in all cases h m f2 and nm f2, used to drive the models. the iri and nequick electron concentration profiles were calculated driving the models with: peak parameters h m f2 and nm f2 derived from the topside profiles, r12, geographic coordinates and time at which the experimental profile was taken. fig. 1. geographic location of the analyzed profiles march 1979 december 1980. 113 comparisons of experimental topside electron concentration profiles with iri and nequick models fig. 3. scatter plot of computed for iri model profiles as function of modified dip latitude. -75 -50 -25 25 50 75 modip 25 50 75 100 125 150 175 200 epsilon % nequick model -75 -50 -25 25 50 75 modip 25 50 75 100 125 150 175 200 epsilon % iri model fig. 2. scatter plot of computed for iri model profiles as function of modified dip latitude. 114 pierdavide coïsson, sandro maria radicella and bruno nava to have a single number to measure the deviation of the model from the observed profile, we made use of the concept of epsilon of electron concentration as an objective technique for profile comparisons. (zhang et al., 1998) is a percentage value defined as the integral from the height hmin to the height hmax of the absolute value of the difference between the modeled and measured electron concentration, over the integral of the measured electron concentration between the same heights, multiplied by 100. table i. statistics of in bands of modified dip latitude, considering: profiles covering a height range of 100-300 km above h m f2, profiles covering a height range of 400-600 km above hm f2, all profiles. modip 100-300 km 400-600 km all ranges band nequick iri nequick iri nequick iri no. samples 26 39 97 mean 25.0 25.2 22.1 33.4 22.1 28.7 90° 60° st. dev. 11.3 17.4 12.4 34.2 12.3 28.0 max 52.1 69.7 50.7 117.8 52.1 117.8 rms 27.3 30.4 25.3 47.5 25.2 40.0 no. samples 253 405 946 mean 19.5 27.5 18.5 19.1 18.7 21.1 60° 30° st. dev. 13.1 29.8 11.5 23.5 12.0 24.9 max 67.3 176.8 53.7 174.0 67.3 176.8 rms 23.5 40.5 21.8 30.3 22.2 32.6 no. samples 384 473 1210 mean 17.9 20.5 15.2 18.4 16.0 19.4 30° 0° st. dev. 12.8 14.7 11.7 12.4 12.4 13.1 max 66.0 120.8 64.0 108.1 92.7 120.8 rms 22.0 25.2 19.1 22.2 20.2 23.4 no. samples 411 445 1220 mean 19.9 18.0 20.0 17.9 19.8 18.3 0° 30° st. dev. 15.7 10.6 18.4 8.8 17.5 10.0 max 95.3 58.7 115.0 49.1 120.9 60.6 rms 25.5 20.9 27.1 20.0 26.4 20.9 no. samples 320 474 1263 mean 31.1 17.8 31.1 23.2 31.4 22.6 30° 60° st. dev. 14.3 15.2 15.7 21.3 16.0 20.4 max 85.2 121.9 88.6 138.5 124.2 138.5 rms 34.2 23.4 34.8 31.5 35.2 30.4 no. samples 73 81 256 mean 47.6 51.7 38.4 54.2 40.8 52.5 60° 90° st. dev. 14.4 37.9 14.5 29.3 14.9 33.7 max 108.1 197.0 72.5 145.2 108.1 197.0 rms 49.7 63.9 41.1 61.5 43.4 62.3 = ( ) ( ) ( ) n h n h dh n h dh h h h h model measured measured min max min max 100. 115 comparisons of experimental topside electron concentration profiles with iri and nequick models other statistical calculations were computed with these data, like those shown in table i where the results in bands of modified dip-latitude are presented. the table contains statistics of for profiles covering a fixed range of lengths (profiles covering at least 100-300 km and 400-600 km above the f 2 peak) and for all the profiles independently of the range of integration covered by the topside sounding. for each modified diplatitude band the number of samples, the mean value, the standard deviation, the maximum value and the root mean squared of are shown. it appears that the different height ranges of the experimental profiles do not much affect the statistics of , which is a quantity dominated by the behavior of the models with respect to experimental data near the f 2 peak. it can be noticed that while for the nequick model there is a general tendency to a better agreement on longer profiles, the iri shows many cases in which increased height ranges produce greater . for the northern hemisphere, at high modiplatitudes, the nequick model is closer to the experimental data than the iri model, but at medium and low modiplatitudes the iri model has a better behavior. for the southern hemisphere, the situation is slightly different and the nequick model seems to remain closer to 3. results the izmiran database of intercosmos-19 profiles over a period of two years gives a global coverage of the whole earth. this means that it is not possible to have instantaneous global maps of electron concentration profiles, but only sets of sparse points, or quasi-longitudinal sections along the satellite path. nequick and iri topside profiles were computed for the conditions of each measured profile. all the were calculated and the results were statistically evaluated. in particular, it is interesting to examine the different behavior of the models as a function of position. examining the plots of the computed for each model as a function of the modified dip-latitude (figs. 2 and 3), in the case of iri it is clear that starting from more or less 30° the model shows a tendency to diverge from the experimental profiles and this trend increases with increasing modiplatitude. many cases show an greater than 100, up to nearly 200. in the case of the nequick model, the situation is very different, it shows a homogeneous behavior at all latitudes, even if the spreading is larger in the northern hemisphere. only few cases have greater than 100. fig. 4. cross section of tec from f2 peak to satellite height. -80 -60 -40 -20 20 40 60 80 modiplat 20 40 60 80 tec [ 10 16 m-2 ] iri nequick expe. 116 pierdavide coïsson, sandro maria radicella and bruno nava the experimental profiles: the mean value, the standard deviation, the maximum value and the rms are always lower than those for the iri model. another kind of analysis that can be done with satellite data is to compare the behavior of the models in predicting the electron content in the available height range of the profile. as an example, fig. 4 presents a quasi-longitudinal (from 204 to 254 e) section of tec from peak height to satellite height during a satellite orbit on 28 november 1979 at around noon local time. at very low modified dip latitude the agreement of both models with experimental data is good, but at high northern latitudes the two models behave in a very different way: the nequick slightly overestimates the tec by a more or less constant value at all modiplatitudes, but the iri model diverges as the modified dip latitude increases, showing much higher values of tec than the experimental data at high modiplatitude. this behavior cannot be confirmed for the southern hemisphere because of the limited extension of the experimental profiles, near satellite perigee. 4. conclusions in the present work, the izmiran database of topside electron concentration profile was used to compare the prediction of the iri and nequick models. more than 5000 profiles were used for comparison with each model, in many different geophysical situations, in a period of high solar activity. the concept of was used to give a single number to evaluate the divergence between the experimental and modeled profiles. the were computed for each profile. the two models have shown a similar global behavior, but different specific problems. cases were found in which there are large discrepancies, particularly in the case of the iri model at high latitudes. this means that topside modeling is still critical and both models have to be improved in that part of the profile. for the case of the nequick model, an improvement is being developed, modifying the empirically determined parameter governing the shape of the topside profile as contained in the present formulation of the model (radicella and zhang, 1995). references bilitza, d. (1990): international reference ionosphere 1990, nssdc 90-22, greenbelt, maryland. bilitza, d. (1994): topside models: status and future improvements, adv. space res., 14 (12), 17-26. di giovanni, g. and s.m. radicella (1990): an analytical model of the electron density profile in the ionosphere, adv. space res., 10, 27. hochegger, g., b. nava, s.m. radicella and r. leitinger (2000): a family of ionospheric models for different uses, phys. chem. earth, 25 (4), 307-310. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27 (1), 35-40. radicella, s.m. and m.-l. zhang (1995): the improved dgr analytical model of electron density height profile and total electron content in the ionosphere, ann. geofis., 38 (1), 35-41. zhang, m.-l., s.m. radicella and l. kersley (1998): measured topside electron profile and its modeling, in proceedings of the iri task force activity 1997, ic/ir/ 98/9 trieste, june 1998. preface annals of geophysics [special issue_60_01_2017] muographers 2014: muon and geo-radiation physics for earth studies. edited by hiroyuki k.m. tanaka, cristina cârloganu, roberto scarpa. [special issue_60_1_2017] muographers 2014: muon and geo-radiation physics for earth studies preface hiroyuki k.m. tanaka, cristina cârloganu, roberto scarpa s0101 articles alliance to penetrate mysteries of the earth gianpaolo bellini, paolo strolin, hiroyuki k. m. tanaka s0102 the muraves muon telescope: technology and expected performances giulio saracino, fabio ambrosino, lorenzo bonechi, alan bross, luigi cimmino, roberto ciaranfi, raffaello d’alessandro, flora giudicepietro, giovanni macedonio, marcello martini, vincenzo masone, nicola mori, pasquale noli, massimo orazi, giuseppe passeggio, anna pla-dalmau, lorenzo roscilli, paolo strolin s0103 the muraves telescope front-end electronics and data acquisition luigi cimmino, fabio ambrosino, lorenzo bonechi, roberto ciaranfi, raffaello d’alessandro, vincenzo masone, nicola mori, pasquale noli, giulio saracino, paolo strolin s0104 muography of the puy de dôme pasquale noli, fabio ambrosino, lorenzo bonechi, alan bross, luigi cimmino, raffaello d’alessandro, vincenzo masone, nicola mori, giuseppe passeggio, anna pla-dalmau, giulio saracino, enrico scarlini, paolo strolin s0105 muography applied to nuclear waste storage sites raffaello d’alessandro, fabio ambrosino, lorenzo bonechi, luigi cimmino, david g. ireland, ralf kaiser, david f. mahon, nicola mori, pasquale noli, giulio saracino, craig shearer, lorenzo viliani, guangliang yang s0106 a possible point of contact between cosmic ray physics and archaeology: muon absorption radiography at the tharros phoenician-roman site lorenzo bonechi, fabio ambrosino, luigi cimmino, raffaello d’alessandro, nicola mori, pasquale noli, giulio saracino, paolo strolin, lorenzo viliani s0107 a geant4 framework for generic simulations of atmospheric muon detection experiments nicola mori, fabio ambrosino, lorenzo bonechi, luigi cimmino, raffaello d’alessandro, pasquale noli, giulio saracino, paolo strolin, lorenzo viliani s0108 nuclear emulsion techniques for muography cristiano bozza, lucia consiglio, nicola d’ambrosio, giovanni de lellis, chiara de sio, seigo miyamoto, ryuichi nishiyama, chiara sirignano, simona maria stellacci, paolo strolin, hiroyuki k.m. tanaka, valeri tioukov s0109 muography of 1949 fault in la palma, canary islands, spain seigo miyamoto, josé barrancos, cristiano bozza, lucia consiglio, chiara de sio, pedro hernández, ryuichi nishiyama, germán padilla, eleazar padrón, chiara sirignano, simona maria stellacci, hiroyuki k.m. tanaka, valeri tioukov s0110 muography with nuclear emulsions stromboli and other projects valeri tioukov, giovanni de lellis, paolo strolin, lucia consiglio, andrey sheshukov, massimo orazi, rosario peluso, cristiano bozza, chiara de sio, simona maria stellacci, chiara sirignano, nicola d’ambrosio, seigo miyamoto, ryuichi nishiyama, hiroyuki k.m. tanaka s0111 development of nuclear emulsion for muography kunihiro morishima, akira nishio, masaki moto, toshiyuki nakano, mitsuhiro nakamura s0112 annals of geophysics kamland: geo-neutrino measurement in japan itaru shimizu s0113 borexino: geo-neutrino measurement at gran sasso, italy matteo agostini, konrad altenmüller, simon appel, gianpaolo bellini, jay benziger, daniel bick, giuseppe bonfini, david bravo, barbara caccianiga, frank calaprice, alessio caminata, paolo cavalcante, alexander chepurnov, davide d’angelo, stefano davini, alexander derbin, lea di noto, ilya drachnev, alexander etenko, kirill fomenko, andrey formozov, davide franco, federico gabriele, cristian galbiati, chiara ghiano, marco giammarchi, marianne göger-neff, augusto goretti, maxim gromov, caren hagner, ed hungerford, aldo ianni, andrea ianni, karol jedrzejczak, dominik jeschke, vladislav kobychev, denis korablev, gyorgy korga, didier kryn, matthias laubenstein, bjoern lehnert, evgeny litvinovich, francesco lombardi, paolo lombardi, livia ludhova, georgy lukyanchenko, igor machulin, szymon manecki, werner maneschg, simone marcocci, emanuela meroni, mikko meyer, lino miramonti, marcin misiaszek, michele montuschi, pablo mosteiro, valentina muratova, birgit neumair, lothar oberauer, michel obolensky, fausto ortica, marco pallavicini, laszlo papp, laura perasso, andrea pocar, gioacchino ranucci, alessandro razeto, alessandra re, aldo romani, romain roncin, nicola rossi, stefan schönert, dmitrii semenov, hardy simgen, mikhail skorokhvatov, oleg smirnov, albert sotnikov, serguei sukhotin, yury suvorov, roberto tartaglia, gemma testera, jan thurn, maria toropova, evgenii unzhakov, alina vishneva, robert bruce vogelaar, franz von feilitzsch, hanguo wang, stefan weinz, juergen winter, marcin wojcik, michael wurm, zachary yokley, oleg zaimidoroga, sandra zavatarelli, kai zuber, grzegorz zuzel s0114 annals of geophysics preface the objective of this italian/japanese bilateral workshop is to advance, with research and technology, the field of new earth observation techniques performed in conjunction with experimental nuclear and particle physics. such topics have been specially developed in italy and japan, both of which have similar geological backgrounds (including earthquakes and volcanoes). opportunities at this workshop for discussion amongst researchers will help to promote and strengthen long-term relationships between the two countries along with increasing the presence, scope and influence of this research at the international level. the workshop is co-hosted by the national institute for nuclear physics [infn], the national institute for geophysics and volcanology [ingv] and the university of tokyo, with the support of the executive program following the italian/japanese bilateral cooperation agreement on science and technology and of the italian embassy in tokyo. guest editors hiroyuki k.m. tanaka cristina cârloganu roberto scarpa earthquake research institute, centre national università di salerno, the university of tokyo, de la recherche scientifique, dipartimento di fisica, tokyo, japan paris, france fisciano 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice a multisensor approach for the 2016 amatrice earthquake damage assessment annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7185 1 a multisensor approach for the 2016 amatrice earthquake damage assessment vito romaniello*1, alessandro piscini1, christian bignami1, roberta anniballe2, and salvatore stramondo1 1 istituto nazionale di geofisica e vulcanologia 2 diet, sapienza university of rome *vito.romaniello@ingv.it abstract this work proposes methodologies aimed at evaluating the damage occurred in the amatrice town by using optical and synthetic aperture radar (sar) change features obtained from satellite images. the objective is to achieve a damage map employing the satellite change features in a classifier algorithm, namely the features stepwise thresholding (fst) method. the main novelties of the proposed analysis concern the estimation of derived features at object scale and the exploitation of the unsupervised fst algorithm. a segmentation of the study area into several buildings blocks has been done by considering a set of polygons, over the amatrice town, extracted from the open source open street map (osm) geo-database. the available satellite dataset is composed of several optical and sar images, collected before and after the seismic event. regarding the optical data, we selected the normalised difference index (ndi), and two quantities coming from the information theory, namely the kullback-libler divergence (kld) and the mutual information (mi). in addition, for the sar data we picked out the intensity correlation difference (icd) and the kld parameter. the exploitation of these features in the fst algorithm permits to obtain a plausible damage map that is able to indicate the most affected areas. i. introduction n earthquake damage map, available right after a seismic event (from few hours up to few days, depending on the satellite data availability), can guide the rescue teams interventions towards the most affected areas. satellite data can be very useful for this purpose, thanks to the wide coverage and the high spatial resolution, especially to map damage in large regions, where villages cannot easily accessed. satellite damage assessment is based on detection techniques that are capable to identify changes of an object by observing it at different times (ingram (1981); jenson (1983)). both optical and radar sensors can be exploited for change detection purposes and several examples are reported in the literature a annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7185 2 (matsuoka and yamazaki (2004); hoffman et al. (2004); yonezawa and takeuchi (2001); stramondo et al. (2006); chini et al. (2013)). by exploiting both optical and radar sensors, and using change features achieved from the statistical analysis, a more accurate and reliable damage mapping can be obtained. bignami et al. (2004) have analysed the possible advantages of combining radar and optical satellite data. regarding the sar data, the intensity correlation difference (icd) demonstrated to be a very good damage proxy (stramondo et al. (2006)). kld and mi (brunner et al. (2010)) are also suitable features that can contribute to damage estimation. as far as the optical data is concerned, the most significative results are related to the normalised difference index (ndi), kullbacklibler divergence (kld) (kullback and leiber (1951)), and mutual information (mi) indexes (xie et al. (2003)). all these features show a good sensitivity to the collapse ratio. in this work, we used the ndi, kld and mi indexes for the optical data, and the icd and the kld parameters for the sar data, following the outcomes carried out during eu funded project aphorism (advanced procedures for volcanic and seismic monitoring, www.aphorism-project.eu). the main novelties of the proposed analysis concern the estimation of derived features at object scale and the use of an unsupervised algorithm for damage estimation purposes. in order to exploit the object-oriented approach, a segmentation of the study area into several regions was performed. in particular, testing an operational approach, we have segmented and then generated the damage map of amatrice by considering a set of polygons extracted from the open source open street map (osm) geo-database. finally, an unsupervised algorithm to estimate the damage level by satellite features, namely in the paper features stepwise thresholding (fst), has been used (roaniello et al. (2016)). ii. data and methods the available dataset is made up of two sentinel-2 optical images (1 preand 1 postseismic), three cosmo-sky sar images (2 preand 1 post-seismic) and a buildings footprint layer extracted by the open street map service (see table 1 for eo data characteristics). both optical and sar data have a spatial resolution of 10 m. datatype satellite acquisition optical sentinel-2 2016/08/14 optical sentinel-2 2016/09/04 sar cosmo-sky 2016/07/19 sar cosmo-sky 2016/08/20 sar cosmo-sky 2016/08/28 table 1: eo data list the amatrice footprint layer extracted from osm, which is at single building scale, was modified to obtain polygons surrounding more than one building. in this way, there are more pixels associated to each polygon leading a better estimation of change features over the polygon itself. the resulting layer consist of 112 polygons, inside an area of about 1 km2. previous studies, carried out during the european aphorism project, allowed us to identify the features having a good sensitivity to damage at object scale (romaniello et al. (2016)). following the project outcomes, in the present work we used the ndi, and two quantities coming from the information theory, the kld and the mi. in addition, for the sar data, we picked out the icd parameter. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7185 3 the ndi parameter is defined as: 𝑁𝐷𝐼𝑖 = 𝑃𝑂𝑆𝑇𝑖−𝑃𝑅𝐸𝑖 𝑃𝑂𝑆𝑇𝑖+𝑃𝑅𝐸𝑖 (1) where prei and posti indicate the mean values of intensity, respectively for preand postseismic images, associated to i-th polygon. the ndi parameter was computed, considering preand post-seismic sentinel-2 images, for each polygon obtained from osm service. fig. 1 shows the resulting ndi map. figure 1: ndi map over amatrice town obtained from preand post-seismic sentinel-2 image actual ndi values range from -0.080 and 0.311, with the highest values centred on the historical centre of amatrice town, where most severe damage were registered (azzaro et al. 2016). the kld parameter is defined as: 𝐾𝐿𝐷𝑖 = (𝑃𝑅𝐸𝑖−𝑃𝑂𝑆𝑇𝑖) 2+𝑉𝑎𝑟(𝑃𝑅𝐸𝑖)+𝑉𝑎𝑟(𝑃𝑂𝑆𝑇𝑖) 2 × ( 1 𝑉𝑎𝑟(𝑃𝑅𝐸𝑖) + 1 𝑉𝑎𝑟(𝑃𝑂𝑆𝑇𝑖) ) − 2 (2) where prei and posti are the same parameters in the eq. 1, and var(prei) and var(posti) are their variances within the i-th polygon. nominally, the kld parameter has the same behaviour of ndi: kld increasing values correspond to increasing damage level. the kld values, calculated from the two sentinel-2 optical images, range from 0.0 and 8.5. the mi index measures the correlation between preand post-seismic images (see equation 3). 𝑀𝐼𝑖 = −𝑙𝑛(1 − 𝑟𝑖 2) ∗ 0.5 (3) where ri2 is the correlation between the preand post-seismic pixels within each polygon. mi is related to the spatial arrangement of the pixels, and it is inversely proportional to the damage grade. for the present case study, its values range from 0.0 to 3.0. as far as the sar data is concerned, the kld feature is derived in the same way as the optical data, while the icd can be computed on the base of the pearson correlation coefficient (ρi) estimated on the pre-seismic sar image pair icpre and on the co-seismic sar image pair iccos. these two intermediate output are then used to obtain the icd: 𝐼𝐶𝐷𝑖 = 𝐼𝐶𝑝𝑟𝑒 − 𝐼𝐶𝑐𝑜𝑠 (4) each feature provides a qualitative assessment of the damage distribution within the town. in order to achieve a damage map useful in the rescue operations, the unsupervised algorithm fst (romaniello et al. (2016)) and based on these satellite features, has been employed; the scheme of the algorithm is shown in the fig. 2. the algorithm is able to use all change features, both from optical and sar data, analyzing the distribution of values of each parameter. the entire ranges of values of each parameter, obtained excluding the outliers, are divided into three intervals corresponding to three damage levels. this classification is repeated n-times (in the present analysis, n is annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7185 4 equal to 21) varying the width of the three intervals at each iteration. figure 2: fst method block scheme so, we obtain several damage level estimations for each polygon (specifically, 21 iterations x 5 features). after this operation, in order to assign the final damage level (low, medium and high) to a polygon, the modal value of all previous estimations is considered for that polygon. the output is a map of the damage level occurred in amatrice. it is worth noticing that the damage classification (low, medium and high) is based on a relative scale. romaniello et al. (2016) have shown the correlation between this kind of damage classification and the collapse ratio, defined as the ratio between the number of collapsed buildings and the total number of buildings within a polygon. therefore, high damage can indicate many collapses within a city block. iii. results a damage-retrieved map can be obtained by using both optical and sar satellite images as input for the fst method. the data fusion approach showed the best damage assessment. the resulting damage map is reported in the fig. 3. from the classification, 86 polygons are associated to the lowest damage grade, 12 to the medium and 14 to the highest. figure 3: damage classification by using all change features (both optical and sar) over amatrice town the most affected area is the historical centre of the town and in particular the west side as visible in fig. 4. figure 4: damage classification in the historical centre. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7185 5 the data fusion approach has overcome false alarms problems occurred in the single sensor approach for several buildings. in particular, the analysis performed using only optical features has led to false positive cases recognized by the comparison with a very high resolution vhr image acquired after the seismic event (fig. 5). figure 5: amatrice postearthquake damage. in the classification maps reported in fig. 3 and fig. 4 the most affected area is the west side of the historical center with several collapsed buildings. this confirms the soundness of the fst method. nevertheless, a quantitative assessment will be performed in the next future to draw much more detailed conclusion on the accuracy of the derived products. iv. discussion the proposed method is a quite robust approach being based on unsupervised classification and employing several change features, both from optical and sar data. furthermore, this method, being based on city blocks areas, is reliable with respect to changes not related to earthquake, because we can guess that modifications caused by temporal changes are small (within the blocks) if compared with the changes due to such catastrophic event. in other words, these sources of errors are averaged within the blocks. moreover, it is worth noticing that the proposed damage classification is on a relative scale. indeed, the method identifies always three classes of damage associated with the low, medium and high grades of changes for the specific regions we are analyzing. on the one hand, this characteristic can be considered as a limitation of the algorithm, but from an operational point of view, we think that during rescue activities it is important to know where to intervene promptly, independently from the absolute value of damage. finally, a comparison with the macroseismic survey will be done, as soon as a complete survey will be available. acknowledgements the research leading to these results has received funding from the european union’s seventh framework programme (fp7/20072013) under grant agreement number 606738, aphorism project. references azzaro, r., and the quest working group (2016). "the 24 august 2016 amatrice earthquake: macroseismic survey in the damage area and ems intensity assessment", ann. geophys., vol. 59, no. 5. bignami, c., chini, m., pierdicca, n. and stramondo, s. (2004). comparing and combining the capability of detecting earthquake damage in urban areas using sar and optical data. proceedings of ieee igarss, 1, pp. 55–58. brunner, d., lemoine, g. and bruzzone, l. (2010). "earthquake damage assessment of buildings using vhr optical and sar imageannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7185 6 ry", ieee trans. geosci. remote sens., vol. 48, no. 5, pp. 2403-2420. chini, m., piscini, a., cinti, f.r., amici, s., nappi, r., de martini, p.m. (2013). the 2011 tohoku (japan) tsunami inundation and liquefaction investigated through optical, thermal, and sar data. ieee trans. geosci. remote sens., 10, 347–351. hoffman, j., roth, a. and voigt, s. (2004). interferometric displacement and damage assessment for the 2003 bam earthquake. in proceedings of the 2004 envisat and ers symposium, salzburg, austria, 6–10 september 2004, esa sp-572. ingram k., knapp e., and robinson j. w. (1981). change detection technique development for improved urbanized area delineation, technical memorandum cscitm-81/6087, computer sciences corporation, silver springs, maryland, u.s.a. jenson j. r. (editor) (1983). urban/suburban land use analysis. manual of remote sensing, vol. 2, second edition (falls church, virginia: american society of photogrammetry), pp: 1571-1666. kullback, s.; leibler, r. a. (1951). on information and sufficiency. ann. math. statist. 22, no. 1, 79--86. doi:10.1214/aoms/1177729694. http://projecteuclid.org/euclid.aoms/1177729 694. matsuoka, m. and yamazaki, f. (2004). use of satellite sar intensity imagery for detecting building areas damaged due to earthquakes. earthquake spectra, 20, pp. 975–994. romaniello, v., piscini, a., bignami, c., anniballe, r. and stramondo, s. (2016). earthquake damage mapping by using remotely sensed data: the haiti case study, journal of applied remote sensing, (submitted). stramondo, s. bignami, c., chini, m., pierdicca, n., tertulliani, a. (2006). satellite radar and optical remote sensing for earthquake damage detection: results from different case studies. international journal of remote sensing, 27, 4433 4447. h. xie, l. e. pierce, and f. t. ulaby (2003). “mutual information based registration of sar images,” in proc. ieee igarss, toulouse, france, jul. 2003, pp. 4028–4031. yonezawa, c. and takeuchi, s. (2001). decorrelation of sar data by urban damage caused by the 1995 hoyogoken-nanbu earthquake. international journal of remote sensing, 22, pp. 1585– 1600. vol. 48, 01, 05ok.qxd 55 annals of geophysics, vol. 48, n. 1, february 2005 key words fluids – earthquakes – methane – gas emissions – flux anomalies 1. introduction fluid pressure and its variation is an important mechanism for the transport of stress in the earth’s crust (hickman et al., 1995). small perturbations of pore pressure are able to modify the effective stress and can induce seismic events (zoback and harjes, 1997; shapiro et al., 2003). an aftershock sequence triggered by a fluid pressure pulse propagation coming from a deeper reservoir is discussed by miller et al. (2004). crampin (2003) proposed an increase of hydrostatic fluid pressure as being one part of the required stress before brittle faulting. that means that local stress variation will be transmitted by fluids through the open fracture system (gudmundsson, 2000). muir-wood and king (1993) described these effects of stress propagation to the fluid transport system by seismic energy release. preand co-seismic effects of changes in crustal permeability attributable to seismotectonic activity were reported by toutain and baubron (1999 and references therein), and roeloffs et al. (2003 and references therein). similar effects were expected in the test site area of val d’agri (southern apennines). the influence of crustal deformation processes, recognisable by a relatively high seismicity, on the local fluid dynamics was therefore investigated by means of hydrogeochemical and results of gas flux records in the seismically active area of val d’agri (southern italy) gerardo colangelo (1), jens heinicke (2), ulrich koch (2), vincenzo lapenna (1), giovanni martinelli (3) and luciano telesca (1) (1) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy (2) saechsische akademie der wissenschaften zu leipzig, arbeitsstelle umweltforschung mit radiometrisch-geochemischen methoden, institut für angewandte physik, freiberg, germany (3) arpa agenzia regionale prevenzione e ambiente dell’emilia romagna, sezione provinciale di reggio emilia, italy abstract the val d’agri area is well-known for oil exploration. an old 500 m deep exploration well in the northern part of this area has been used for long-term hydrogeochemical investigations. the well is characterized by a discharge of about 500 l/min of thermal water (27.8°c) and a simultaneous methane gas emission of about 200 l /min. gas analyses gave evidence that the methane come from a multiple deep reservoir. continuous records of gas emission showed some anomalous variations occurred during the past three years. the gas flux anomalies were in a distinctive coincidence with self-potential anomalies of one station close to the hydrogeochemical station. the present paper describes the interpretation of these anomalies in relation to the geodynamic activity in the area. mailing address: dr. luciano telesca, istituto di metodologie per l’analisi ambientale (imaa), cnr area della ricerca di potenza, c.da s. loja, 85050 tito scalo (pz), italy; e-mail: ltelesca@imaa.cnr.it 56 gerardo colangelo, jens heinicke, ulrich koch, vincenzo lapenna, giovanni martinelli and luciano telesca geoelectrical methods. for that purpose, the fluid emission of an unused exploration well was recorded. anomalous fluid behaviour in seismic as well as volcanic active areas was also reported by ito et al. (1999), heinicke and koch (2000) and matsumoto et al. (2002, 2003). 2. geological and seismological settings the investigated area is located in southern apennine chain, a neogene and quaternary foldand-thrust belt attributed to the subduction of ionian lithosphere beneath the adriatic plate (doglioni et al., 1996). this part of the apennines comprises a stack of east verging tectonic units representing an accretionary wedge composed of mesozoic-cenozoic sediments. those were originally deposited in different paleogeographic domains, both basinal (lagonegro) and shallow water platforms which were thrust onto the apulian foreland to the east (bonini and sani, 2000). in the present paper the val d’agri area, a nw-se trending intermontane basin located in the basilicata apennine, has been focused. this fault-bounded basin is about 30 km long and 12 km wide and was formed during quaternary in the interland of the fold and thrust belt after the major miocene-pliocene episodes of shortening (giano et al., 2000). the seismicity of the investigated area is related to the irpinia normal faults, the potenza strikeslip fault and the val d’agri normal faults. the val d’agri fault system consists of roughly n120° trending left-lateral strike-slip faults (monaco et al., 1998) and associated features including mainly: i) n020-n030 trending right-lateral/transtensional faults; ii) n090-n110 trending, left-lateral transtensional faults; and n120-n150 trending, left-lateral transpressional faults (see fig. 1a,b). about the potenza strike-slip fault the focal mechanism calculated for the main shock (may 5, 1990, ml = 5.2) shows a strike slip solution with a n70e t-axis and a n163e p-axis (azzara et al., 1993). for the irpinia area, a strike angle of 320° and a dip angle of 70° ne has been calculated for the main normal faults (vaccari et al., 1990). historically one of the most important events, the normal-faulting earthquake of december 16, 1857 (i = xi° mcs; me = 6.9), occurred in the nw part of the val d’agri (mallet, 1862; boschi et al., 1997). this earthquake was fig. 1a,b. a) map of the active faults, b) stress field geometry in val d’agri region (from cello et al., 2003) and the locations of the hydrogeochemical station of tramutola and the sp-station of marsico nuovo and villa d’agri. a b 57 results of gas flux records in the seismically active area of val d’agri (southern italy) characterized by an epicentral area which extended in a roughly nw-se direction from the northern vallo di diano to the val d’agri area, for a total length of about 60 km and a width of more than 10 km (cello et al., 2003). the stress field responsible for the genesis and evolution of the fault systems (σ1, sub-horizontal) is wsw-ene trending (cello et al., 2003). upgraded stress field data in the studied area have recently been reported by cucci et al. (2004) with a ne-sw direction of extension. on november 23, 1980, a strong earthquake (ms = 6.9) struck the area north of val d’agri, the campano-lucano apennines. the 1980 irpinia earthquake was characterised by three distinct subevents occurring within 40 s along different faults. seismic activity after the 1980 event consisted of medium intensity events (m < 5.5) located close to the border between the campania and basilicata regions (alessio et al., 1995). besides the strong historical seismicity, the local seismicity with low-middle strong events (ml = 1, ..., 5) is still active. we selected several seismic events from the ingv internet bulletin (www.ingv.it) for our interpretation (see fig. 2 and table i). important for our selection was an empirical frame: a distance to the hydrogeochemical station of less than 60 km (in coordinates: 40.0-40.8n and 15.3-16.1e), the foci not deeper than 25 km and an occurrence in a time window of three weeks before and after the anomaly. all seismic events used for our interpretation are marked by a symbol, the capital letters a-t, and documented in fig. 2. fig. 2. spatial distribution of epicentres of the seismic events a-t (black dots, see table i) during anomalous gas flux periods and their relation to the tramutola, marsico nuovo and villa d’agri station (white circle). table i. all seismic events occurring in val d’agri region within the co-ordinates of 40.0-40.8n and 15.3-16.1e and the foci not deeper than 25 km with possible temporal relations to the recorded flux anomalies; focal depth in km (ingv bulletin). figure 2 show the epicentre with the symbols. date lat. long. m depth symbol 03.04.1996 40.683n 15.550e 4.9 8.0 a 09.09.1998 40.020n 15.570e 5.5 11.0 b 04.11.2001 40.393n 16.052e 2.0 10.0 c 04.11.2001 40.458n 16.162e 2.0 4.4 d 04.11.2001 40.492n 15.580e 2.0 13.1 e 21.11.2001 40.547n 15.814e 2.8 19.7 f 21.11.2001 40.560n 15.824e 2.6 11.3 g 21.11.2001 40.523n 15.776e 2.0 22.6 h 08.02.2002 40.239n 15.927e 2.6 10.0 i 14.02.2002 40.644n 15.557e 2.6 10.0 j 19.02.2002 40.699n 15.411e 2.4 9.4 k 26.02.2002 40.218n 15.822e 2.6 10.0 l 03.03.2002 40.529n 15.544e 2.1 10.0 m 24.01.2003 40.762n 15.384e 2.6 9.9 n 03.02.2003 40.753n 15.643e 3.6 8.4 o 03.02.2003 40.751n 15.669e 3.5 8.0 no 04.02.2003 40.783n 15.661e 2.8 9.7 no 04.02.2003 40.643n 15.659e 2.5 9.8 p 07.02.2003 40.459n 15.600e 2.3 5.0 q 12.02.2003 40.690n 15.608e 2.3 11.0 r 16.02.2003 40.367n 15.576e 2.2 15.9 s 16.02.2003 40.274n 15.735e 2.1 16.2 t 58 gerardo colangelo, jens heinicke, ulrich koch, vincenzo lapenna, giovanni martinelli and luciano telesca 3. analysis of hydrogeochemical and geoelectrical signals geological and seismological features, combined with a very low level of anthropogenic noise, allow us to consider the selected area a test site to study the possible correlations between tectonic activity and anomalous patterns in hydrogeochemical and geoelectrical signals. in 2000 the institute of methodologies for environmental analysis (national council of research, potenza, italy) installed a monitoring network to detect geophysical, hydrogeochemical and seismometric parameters in the basilicata region (balasco et al., 2001). two remote stations of this network are recording self-potential signals: marsico nuovo (40.416n, 15.722e) and villa d’agri (40.372n, 15.803e) and a third station is recording hydrogeochemical parameters: tramutola (40.322n; 15.759e) (figs. 1a,b and 2). 3.1. self-potential stations these geoelectrical recordings started at marsico nuovo station in november 2000 and in villa d’agri station in september 2001. the self-potential recording consists of measurements at the earth’s surface, recording the electrotelluric field developed in the subsurface by several mechanisms such as electrokinetic coupling (self-potential) and electrochemical effects (corwin and hoover, 1979). the electrical field is due to the relative displacement between the charged mineral grains and the pore water. this drags the excess of electrical charge, located in close vicinity to the pore water/mineral interface, into the socalled electrical double layer (revil and leory, 2001). this phenomenon produces a net current density, which serves as a source term in the maxwell equations. these electrokinetic effects are known as streaming potential. in addition, this electric field is strong enough to be recorded at the ground surface using non-polarizable electrodes (revil, 2002). other theories also attribute self-potential signal generation to charge motions in crystalline solid materials at depth (see also vallianatos et al., 1999 and references therein). the self-potential acquisition system used in the val d’agri stations consists of an array with 1.5 m long copper electrodes (6 in marsico nuovo and 2 in villa d’agri), put into the ground at 1 m depth. the electrodes spacing is between 6080 m. they are linked up with a high-resolution acquisition unit (16 bit) connected to a pc with a serial interface. the sampling rate is one second. 3.2. hydrogeochemical station after the strong 1857 val d’agri earthquake, an unusual natural emission of oily fluids took place close to the tramutola village. just a few tens of meters away a 500 m deep well for oil exploration was drilled in 1956. the well, which crosses clay, silicic clays, and silicic limestones belonging to the lagonegro units, encountered an artesian aquifer emitting thermal water and gas. the well is currently utilised for scientific purposes. italiano et al. (2001) investigated geochemical parameters and isotopes by manual sampling techniques. the gas emission is methanedominated (> 80%) and showed significant shortterm variations in the composition due to local seismogenic processes. the water, spontaneously discharged from the well at a rate of 500 l/min, is of a na-hco3-cl type. the water temperature is 27.8 °c. the isotopic composition disclosed a meteoric water origin, since δ 18 o and δ d values are on the local meteoric water line. the chemistry of the thermal water is related to the process of carbonate dissolution and subsequent cation exchange in deeper reservoirs. the r/ra ratio of the helium isotopes is > 1 and indicates a mantle contribution, which is also supported by the δ 13 c data of the co2 components of − 3.5‰. the hydrogeochemical station of tramutola, whose acquisition started in may 2001, is able to record several parameters. a gas counter was installed to record a part of the natural gas flux from the well. due to different installation set ups with different sampling tubes and gas counters, the recorded gas flux ranged between 4 and 50 l/min. one part of the emitted gas flux is collected in a long plastic tube of 30 cm diameter directly above the borehole. a smaller tube transports the gas to the counter. the gas flux is recorded by counting the electric pulses per 10 59 results of gas flux records in the seismically active area of val d’agri (southern italy) min by a drum gas counter. the recording error is less than 1%. other parameters recorded by the tramutola station are the atmospheric pressure, water and environment temperatures and, since may 2003, two self-potential dipoles along n-s and e-w directions. the records of gas flux show no response to tidal effects. the typical correlation to atmospheric pressure variation, observed on mineral springs (koch et al., 2003) or on volcanic gas emission sites (shimoike and notsu, 2000) was not observed due to the artesian discharge from the deep reservoir. figure 3 shows the first period of continuous gas flux records before the records were disturbed partly due to technical problems. a possible relation to local seismic events will be discussed in the following paragraphs. 3.3. previously recorded data during the past three years some anomalies in the gas flux have been recorded. the macroscopically observed flow rate as well as the chemical and isotopic compositions of the tramutola gas emission changed significantly in connection with the seismic event occurring on april 3, 1996, north of tramutola (ml = 4.9; epicentral distance 45 km, symbol a in fig. 2) (italiano et al., 2001). the gas emission changed from a methane dominated phase to a nitrogen dominated one and back to normal after the seismic crisis (table ii) mixing processes occurred which were probably induced by the seismogenic process. pore pressure increase in the upper nitrogen dominated reservoir, or gas pressure reduction in the deeper methane dominated reservoir could have generated the recorded anomaly. a second strong event (m = 5.5) which occurred in september 1998 induced no variation in the chemical composition of the water. the epicentre (epicentral distance 38 km, b) was probably outside the seismogenic influenced fluid system, which seems to be trending more n-nw. a parallel 130°n-trending seismogenic zone (cucci et al., 2004) provide further arguments for an open fluid system in that direction. fig. 3. temporal variation of the gas flux recorded at tramutola station during the first measurement period from november 2001 to march 2002 and their local seismicity. table ii. gas composition in normal conditions and during the seismic crisis. gas in vol% ch4 he (ppm) n2 co2 normal gas 83 230 13 1.4 composition during the 38 69 59 3.1 seismic crisis 60 gerardo colangelo, jens heinicke, ulrich koch, vincenzo lapenna, giovanni martinelli and luciano telesca 3.4. anomalous data recorded in november 2001 two periods of hydrogeological anomalies occurred in november 2001. at the beginning of our records on november 1, 2001, after a gap in recording, immediately we recorded a period of anomalous gas variations with higher gas flux of around 6-7 l/min up to november 8 (fig. 4). the following period shows the normal flux of around 3-4 l/min. barometric pressure variations were not responsible for this increased gas flux. if these anomalous phenomena were generated by the same causes that induced the anomalies recorded in the flux and in the gas composition in 1996 we infer that the observed anomalies can be related to local geodynamic processes testified by one of the seismic events of november 4 (table i and figs. 2 and 4). other events in that area and period occurred on october 13 and on november fig. 4. temporal variation of the gas flux and atmospheric pressure recorded at tramutola station in the period from november 1 to 13, 2001. the capital letters c, d and e are local seismic events in that period. their epicentres are presented in table i and fig. 2. fig. 5a,b. a) temporal variation of the gas flux, rain and atmospheric pressure recorded at the tramutola station and the self potential signal recorded at marsico nuovo in the period from november 11 to december 6, 2001. local seismic events in that period: f, g and h. b) enlarged part of the temporal variation of gas flux and self potential signal in the anomalous period from november 20 to 22, 2001. a b 21. the events of november 21, have occurred north of tramutola in the val d’agri area (table i), could be responsible for a further anomaly on the same day. the distance from the main event to tramutola station is about 18 km (fig. 2). an anomalous fluid emission had been recorded some hours before these events. figure 5a shows the second gas flux anomaly at tramutola station: an anomalous pattern in the signal is visible between november 20 and 25. this anomaly is characterized by a sharp decrease and increase occurring several hours before the earthquakes f, g and h (fig. 5b). the gas flux increased from 4 to 6 l/min at 10 p.m. on november 20. concomitantly, an anomalous decrease in the self-potential signal to a level of 25 mv in the e-w signal was recorded at marsico nuovo sp-station on november 20 followed by a positive anomaly co-seismically on november 21. no rain influenced that region during these days. 3.5. anomalous data recorded in february 2002 a third gas flux anomaly was recorded on february 27, 2002 as a spike like gas eruption with a duration of one hour (fig. 6). the gas flux increased from around 4.5 l/min to 6.9 l/min. in the same period on february 26 a m = 2.6 earthquake (l) occurred close to the tramutola station. furthermore, the geoelectrical signal recorded at marsico nuovo station, showed an anomaly characterised by a sharp negative variation on february 26 – ca. 7 h before the seismic event. no rainfall was observed in the same period. other seismic events close to the stations during that period are listed in table i. 3.6. anomalous data recorded in february 2003 a further example of significant gas flux anomaly occurred on february 8, 2003 (fig. 7). this short term peak lasting 3 h was not accompanied by a self-potential anomaly. the gas flux increased from 16 to 24 l/min. a seismic event of magnitude 2.3 occurred 9 h before the recorded anomaly. further seismic events are listed in table i. fig. 6. temporal variation of the gas flux and atmospheric pressure recorded at the tramutola station and the self potential signal recorded at marsico nuovo in the period from february 21 to march 7, 2002. l and m are local seismic events in that period. fig. 7. temporal variation of the gas flux and atmospheric pressure recorded at the tramutola station in the period from february 4 to february 16, 2003. p, q, r, s and t are local seismic events in that period. 61 results of gas flux records in the seismically active area of val d’agri (southern italy) 62 gerardo colangelo, jens heinicke, ulrich koch, vincenzo lapenna, giovanni martinelli and luciano telesca 4. discussions the four gas flux anomalies presented here show different patterns in their duration but also some common features. in particular a significant increase in gas flux characterises each of the anomalous periods, which could be related to a compression process. anomalous water temperature variations were not observed. the anomalies in the investigated period were represented by an increase in gas flux with long term variations in november 2001 and short term peaks in the following two years. italiano et al. (2001) showed that fluctuations in the fluid composition of tramutola well were recorded and accompanied by a macroscopically observed variation in the flow rate. in 1995, the nitrogen content increased from approximately 15% to 50% in comparison to methane content during the anomalous period. if the methane content was constant, the nitrogen content increased six times so that the whole gas flux increased from 100% to 170%. the relatively stable he3/4 ratio supports this idea. the recently recorded gas flux anomalies also show an increase of about 150%. these additional amounts of gas could be related to a variation in the local stress field. the influences on fault permeability during crustal deformation processes could induce the temporal variations in the gas emission. long-term gas flux anomalies having been recorded before the seismic events on november 2001 could be interpreted with a long-term pressure increase on the fluid conduits. obviously, not each of the stress accumulation stages before local events influences the fluid transport parameter. only a few anomalies were recorded in that seismically active region. the two short term anomalies occurring 10-13 h after the seismic events in 2001 and 2002 are so called post-seismic anomalies. geodynamic processes, like hydro-fracturing or a fast conduits compression, could possibly characterise this type of short term anomaly generation. matsumoto et al. (2003) suggest ground motion due to the earthquake for interpretation. in all cases, probably the upper nitrogen filled horizons are the resources for that increase in the flux rate. the nnw-sse oriented fault structure at tramutola as main fault gauge is influenced by the regional stress field of σ1 in wnw-ese (cello et al., 2003). stress accumulation could be responsible here as an influencing factor to induce fluid flux anomalies. figure 2 shows a distribution of all mentioned events a-t which could be related to the recorded anomalies. however, an unproven/weak correlation is recognisable: for all anomalies seismic events exist along the nw-se trending val d’agri fault line, in particular: a, e, (f, g, h), l and q. all epicentres, except b, c and d, were located in a n-nw trending fault zone which is characterised by the main compression zone, a suitable conduit for anomalous high pore pressure. the distances between the epicentres and tramutola are less than 50 km. furthermore self-potential anomalies were simultaneously recorded in two cases: november 2001 and february 2002. these anomalies were recorded only in marsico nuovo. probably, similar to the gas flux anomalies of tramutola, short term anomalies in the fluid transport process could here also be responsible for these sp variations. the idea of regional/local stress/pressure propagation will be supported due to the fact that both stations are located in the same part of the fault structure. temporal differences in the anomaly occurrence could express the dynamics of this process. the second self potential station at villa d’agri on the east side of the fault zone showed no anomalies during that period. 5. conclusions significant anomalies in hydrogeochemical and geoelectrical signals recorded in val d’agri by remote stations have been related to the occurrence of seismic events, whose epicentres are located close to the stations. since the influence of meteorological factors on the abnormal fluctuations of geoelectrical and geochemical data has been excluded, these fluctuations can be attributed to crustal deformation processes, though not all local seismic events were preceded or followed by this kind of fluid anomalies. 63 results of gas flux records in the seismically active area of val d’agri (southern italy) references alessio, g., f. esposito, a. gorini and s. proofed (1995): detailed study of the potation seismic zone in the southern apennines, tectonophysics, 250, 113-134. azzara, r., a. basili, l. beranzoli, c. chiarabba, r. di giovambattista and g. selvaggi (1993): the seismic sequenze of potenza (may 1990), ann. geofis., xxxvi (1), 237-243. balasco, m., d. chianese, g. di bello, m.r. gallipoli and v. lapenna (2001): a new multiparametric geophysical station to detect self-potential and seismometric signals at tito site (southern italy), ann. geofis., 44 (2), 193-203. bonini, m. and f. sani (2000): pliocene-quaternary transpressional evolution of the anzi-calvello and northern s. arcangelo basins (basilicata, southern apennines, italy) as a consequence of deep-seated fault reactivation, mar. pet. geol., 17, 909-927. boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (editors) (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990 (ing, roma sga, bologna), vol. 2, pp. 644. cello, g., e. tondi, l. micarelli and l. mattioni (2003): active tectonics and earthquake sources in the epicentral area of the 1857 basilicata earthquake (southern italy), j. geodyn., 36, 37-50. corwin, r.f. and d.b. hoover (1979): the self-potential method in geothermal exploration, geophysics, 44, 226-245. crampin, s. and s. peacock (2003): seismic evidence for fluid-driven deformation, j. geodyn., 36, 67-77. cucci, l., s. pondrelli, a. frepoli, m.t. mariucci and m. moro (2004): local pattern of stress field and seismogenic sources in the pergola-melandro basin and the agri valley (southern italy), geophys. j. int., 156, 575-583. doglioni, c., p. harabaglia, g. martinelli, f. mongelli and g. zito (1996): a geodynamic model of the southern apennines accretionary prism, terra nova, 8, 540-547. giano, i., v. lapenna, s. piscitelli and m. schiattarella (2000): electrical imaging and self-potential surveys to study the geological setting of the quaternary slope deposits in the agri high valley (southern italy), ann. geofis., 43 (2), 409-419. gudmundsson, a. 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[2005, 2006] applied the mqp model for target tracking in othr. baker and lambert [1989] and wang et al. [2009] estimated ranges for high-frequency single station location with this model. gasse et al. [1999] estimated mean bearing derivations with the mqp model. hou et al. [2015] employed the dynamic mqp model to compute sky-wave radar detection performance. li (x.d.) et al. [2012] developed a signal model by utilizing the mqp ionospheric model to derive an optimum detector for mimo-othr. liu (r.y.) et al. [2008] applied the mqp model to shortwave ray tracing in the ionosphere. wang et al. [2010] presented an f1 layer inversion method based on mqp model. the data obtained from the incoherent scatter radar indicate the presence of a valley between the e and f layers; the depth and width of the valley are significantly less in daytime than at night [mahajan et al. 1990], but ionosonde could not sound that region. no incoherent scatter radar has been developed in china; therefore, the mqp model used at present does not include an efvalley. this lack reduces the accuracy of systems which use the ionosphere model. the depth of the valley from iri2012 is a relative value, and the corresponding width is an absolute value; the former is closer to the actual situation than the latter is. the current work combines the predicted values of the ef-valley from iri2012 with the other qp layer parameters derived from vertical sounding to generate two improved mqp models with ef-valley, namely, the mqp model which uses the depth of valley (mqp-vd) and the mqp model which uses the depth and width of valley (mqp-vdw). oblique sounding ionograms demonstrate the group delay dependence over a defined frequency range; this group article history received april 29, 2015; accepted january 12, 2016. subject classification: wave propagation, ionosphere model, ef-valley, ray-tracing, oblique sounding ionogram. delay function depends upon the ionospheric structure in a significantly more complex manner than it does for vertical sounding [krasheninnikov et al. 1996]. thus, we generate one-hop oblique sounding ionograms between wuhan and beijing with the two improved mqp models and a conventional mqp model. we also compare the three oblique sounding ionograms with the experimental ionogram from the ionosonde. the results illustrate that in contrast to the conventional mqp model, the improved mqp models effectively increase the accuracy of ray tracing and reduce the errors in shortwave single location and othr registration. 2. conventional mqp model the qp model describes the profile of electron concentration in the ionosphere with a quasi-parabolic segment. a relationship exists between plasma frequency and electron concentration ( fn 2 = 80.8ne ); therefore, the qp model for a single layer is often determined with equations (1a) and (1b) [dyson and bennett 1988]. where r is the radial distance from the earth’s center, rb is the value of r at the layer base, ym is the layer semithickness, rm= rb + ym is the value of r where the plasma frequency reaches maximum, a = f0 2 ( f0 is the critical frequency of the layer), b is given by equation (2a) and rt is given by equation (2b). the mqp model is developed in this manner (see figure 1). the e and f1 layers are joined by the j1 joining layer, and the joining points are points a and b. the f1 and f2 layers are joined by the j2 joining layer, and the joining points are points c and d. the equations which describe the mqp ionosphere model can be written as equations (3a)-(3e); the parameters in these equations are similar to those in equation (1). the joining layers are described by the counter-qp function, as mentioned above; therefore, rmj1 and rmj2 represent the minimum radial distance of the j1 and j2 layers, respectively. the parameters of the joining layers and the values of the joining points are determined as follows: firstly, we assume that the j1 and e layers join at the peak of the latter. then aj1= ae, rmj1= rme. the j2 and f1 layers converge at the peak of the latter; subsequently, aj2= af1, rmj2= rmf1. rtf2 is the rt value of the f2 layer. secondly, the parameter of the j1 layer bj1 and the height of joining point b are computed by matching the plasma frequency and its gradient according to equations (4a) and (4b). then, the answers are given by equations (5a) and (5b). otherwiwise f n a b r r rb r rt1 0 f n m rb rt 2 2x 1 1= -s 2xg b a y rb rt rb y r rb m rb rt rb m m rb 2 = = t yz] []] \ ]] z]]]]]]]]]]]]]]]][]]][] ]][]]]]]]]]]]]]]]]] \ * e * * j1 * * * f1 * * * * j2 * * * * * f2 * * * * * * * * * * * * * * * f n a b r r f n aj bj r r f n a b r r f n aj bj r r f n a b r r lalayayer rb r ra joiningng lal yayer ra r rb lal yayer rb r rc joiningng lal yayer rc r rd lalayayer rd r rt 1 1 1 1 1 f ne e e e f njnj1 aj1 bj1 jm 1 f nf1 f1 f1 f1 f njnj2 aj2 bj2 jm 2 f nf2 f2 f2 f2 ra rqa rb rqb rc rqc rd rqd f2 m jm m jm m rt rq b 2 2 2 2 2 1 1 1 1 1 # # # # # = = = = = s rq s s rq s s rq rq rq x x x x x v v v v v z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]] \ ]]] \ ]] f n f n r f n r f n rb a a b rf re rf b rf re bj re re rb rf b rf rb 1 1 1 1 f nf f njnj f2 nf f2 njnj rb f e f rf re rf f rf re bj re re rb rf f rf rb r r r r r r r r 1 2 1 2 1 2 1 2 1 1 1 1 1 1 1 1 1 1 b b b b 2 f2 2 f2 = = = + = = = = = r r r r w w w w z] [] \ ]] z]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]] \ z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]]]]] \ sun et al. 2 figure 1. mqp ionosphere model. (1a) (2a) (2b) (1b) (3a) (4a) (4b) (5a) (5b) (3b) (3c) (3d) (3e) 3 finally, the parameter bj2 of the j2 layer and the height rd of joining point d are computed as described above. the values are represented by equations (6a) and (6b) 3. improved mqp model the vertical sounding ionosonde cannot obtain information with regard to the ef-valley. in this study, the parameters of this factor are obtained from iri2012. the real-time data from vertical sounding ionosonde are combined with the predicted data from iri2012 to develop the improved mqp models with ef-valley. the joining parameters of the ef-valley with the e and f layers are computed as follows. a. mqp-vd model the relative depth “m” value of the ef-valley can be obtained based on iri2012. the value of av is computed by av = m · ae. the heights ra and rb of joining points a and b are given by equations (7a) and (7b). the ef-valley of mqp-vd is denoted in figure 2 by a solid line. the functions which describe the mqp-vd model are represented by equations (8a)–(8c). b. mqp-vdw model both the relative depth “m” and absolute width “w” of the ef-valley obtained from iri2012 are used in the mqp-vdw model. the parameters of the joining layer are computed according to the values of av and w, and the joining segment should pass through point (ae, rq), (where rq= re+w) and converge with the straight line at the bottom of the valley, i.e., aj = av. the joining layer intersects with the f2 layer at point m (see figure 2). the gradients of this layer are continuous at point m. we obtain equation (9) according to the aforementioned rules, where rm, bj and rmj are unknown parameters. the valley of mqp-vdw is highlighted by the dotted and dashed line in figure 2. if the computed rmj value is smaller than ra, which reduces the valley depth, then only the first two formulas in equations (9a)-(9c) are used to compute rm and bj. this procedure maintains a satisfactory valley depth (i.e., rmj= ra). the values of rm and bj are given by equations (10a) and (10b). 4. regional reconstruction technology for the ionosphere parameters the transmitter is located in wuhan, and the receiver is located in beijing. therefore, we could obtain vertical sounding data of the two places only. we employ regional reconstruction technology for the ionosphere parameters to determine the parameters of various layers in the midpoint along the great circle path. the method is described below [liu (w.) et al. 2008]. firstly, ionosphere distance dij is computed by using equation (11), in which loni and lati represent the longitude and latitude of the ith place, respectively; the unit is degree; and i =1, 2; j =1, 2. the parameter sf refers to the scalar factor. the value of sf is 2 in the middle latitude area. ra a a re rb a a b rf rb r ra ra r rb f n a b r r rb r rt b f n a b r r f v a 1 1 1 1 ra e v re v ra ra rb f n f2 f2 e f ne e e e f v v rt rb f f rf f f rb f rb m m 2 2 2 2 2 2 1 1 1 # # # = = = = = q q s s q q q v v x x v v v z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]] \ ]]] \ ]] z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]] \ ]]] \ ]] rd a a b rf rf rf b rf rf bj rf rf rd rf b rf rd 1 1 1 1 f f f rf rf rf f rf rf bj rf rf rf f rf rd rd rd 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 = + = r r r r w w w w z] [] \ ]]] z]]]]]]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]] \ ]]]]]] \ f n f n r f n r f n f n a bj r r bj b r b r j a r j a r j b rj rf a a b r rm bj rj b rf bj r j b r 1 2 f njnj f nf2 f2 njnj f2 nf2 f njnj v bj jm bj f f f r j f r j v r j f rj rf f v f f rm bj rj f rf bj r j f f r r r r r r r r jm 2 2 2 2 2 2 2 2 2 2 2 2 m m m m 2 f2 2 f2 = = = + = + + = + + = = = = s q x v z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]]]]]]]]]] \ ]]]] \ z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]] \ ]]] \ ]] mqp ionosphere model with ef-valley figure 2. mqp-vd ionosphere model. (6a) (7a) (7b) (8a) (9a) (10a) (8b) (9b) (10b) (8c) (9c) (6b) secondly, weight coefficients w1 and w2 are computed with equations (12a) and (12b), in which n denotes the lagrange multipliers, and di0 is the ionosphere distance between the ith place and the midpoint. lastly, the parameters in the midpoint are estimated with the computed weight coefficients and the known parameters of the transmitting and receiving regions. for the parameters that could be obtained from iri2012, such as the peak heights and the critical frequencies of different layers, we utilise equation (13) as follows: the parameters of iri2012 are used as the background to retain the regional character of the ionosphere and to improve the accuracy of the reconstructed parameters, where z0 is the estimated parameter, and −zj j = 0,1,2 is the counterpart to parameter zj from iri2012. for the parameters which cannot be found in iri2012, we apply equation (14). 5. experiments three oblique sounding ionograms between wuhan and beijing are selected. the times at which the data were collected are listed as follows: january 3, 2014, at 16:22 (ut), january 4, 2014, at 03:07 (ut) and may 25, 2014, at 02:07 (ut), which correspond to the nighttime scenario without the f1 layer, the daytime scenario without the f1 layer and the daytime scenario with the f1 layer, respectively. the one-hop oblique sounding ionograms generated with the three mqp models are compared with the experimental oblique sounding ionogram, as depicted in figures 3, 4, and 5. the group path of the one-hop e layer is unaffected by the ef valley; therefore, the e layer group paths are not lon lonj latjlatd sfijij i nj i tj 2v 2v= + -q q2v 2v wj wj ,d d i 1 1 2ijij wj i j wj j 0 1 2 1 2 n+ = = = = = z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]] \ ]]] \ ]] | | wz z z j0 0 0 1 2= = r r |wj zj zj zj zj zj zj wj r r wjz zjwj zj j 0 1 2 = = | sun et al. 4 f(mhz) 2 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 pexp pmqp-v (km) -10 -14 -13 -21 -23 -26 -28 -24 -24 -24 -33 -110 pexp pmqp-vd (km) -79 -75 -65 -64 -56 -46 -35 -16 -1 16 25 -34 pexp pmqp (km) -93 -89 -78 -75 -66 -55 -40 -19 0 21 35 -19 figure 3. one-hop oblique sounding ionograms generated with the three mqp models and the experimental ionogram. time: january 3, 2014, at 16:22 (ut). table 1. difference values in the one-hop group paths of the experimental oblique sounding and of the simulation results. time: january 3, 2014, at 16:22 (ut). (11) (12a) (14) (12b) (13) 5 displayed in the figures. the corresponding difference values in the one-hop group paths of the experimental oblique sounding and of the simulation results of the three mqp models are listed in tables 1, 2 and 3, respectively. pexppmqp-v represents the one-hop group path of the experimental oblique sounding minus onehop group path of the mqp-v model, pexppmqp-vd represents the one-hop group path of the experimental oblique sounding minus the one-hop group path of mqp-vd model, pexppmqp represents the one-hop group path of the experimental oblique sounding minus the one-hop group path of the mqp model. figure 3 and table 1 illustrate that the oblique sounding ionogram generated by using the mqp-v model is similar to the experimental sounding ionogram and that the group path error is small, except for high frequencies close to muf. meanwhile, the ionograms generated with mqp-vw and the conventional mqp are less similar to the measured ionogram, and the group paths of the former change rapidly, but those of the latter shift gradually. the group path error observed at low frequencies is greater than 50 km; nonetheless, the ionogram generated with the mqp-vw model is slightly superior to that which was generated by using the conventional mqp model. abrupt changes do not occur in the ionograms generated with the three mqp models, unlike in the measured ionogram near muf, because the vertical data utilised to obtain the parameters of various layers are not highly accurate. figure 4 and table 2 illustrate that the oblique sounding ionogram generated by employing the mqpv model is highly similar to that of the measured ionogram. the group path error is small, and the changes at muf are also similar. meanwhile, the ionograms generated with mqp-vw and the conventional mqp are less similar to the experimental ionogram; the group paths change evidently with frequency, and the variations in mufs are greater than those in its experimental counterpart. the muf of mqp-v is 0.3 mhz less than that of the experimental counterpart, whereas the mufs of mqp-vw and mqp are 1.3 mhz less than that of the experimental ionogram. the errors observed at low frequencies of mqp-vw and mqp are large, and the maximum errors at 10 mhz are 40 and 44 km for mqpvw and mqp, respectively. nonetheless, the oblique sounding ionogram generated with the mqp-vw mqp ionosphere model with ef-valley f(mhz) f2 at a low angle f2 at a high angle 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5 18.5 pexp pmqp-v (km) -19 -26 -22 -27 -21 -18 -19 -13 -14 -13 -40 pexp pmqp-vd (km) -40 -40 -29 -26 -11 -1 6 20 25 ----46 pexp pmqp (km) -44 -43 -31 -27 -11 1 10 25 33 ----48 figure 4. one-hop oblique sounding ionograms generated with the three mqp models and the experimental ionogram. time: january 4, 2014, at 03:07 (ut). table 2. difference values in the one-hop group paths of the experimental oblique sounding and of the simulation results. time: january 4, 2014, at 03:07 (ut). model is slightly superior to the oblique ionogram generated with the conventional mqp model. figure 5 and table 3 indicate that the oblique sounding ionograms generated with the three mqp models differ slightly from one another. this variation is attributed to the low predicted depth and width values of the ef-valley from iri2012 in the daytime with f1, thereby leading to a shallow ef valley and a slight effect on radio propagation. 6. conclusion for the ionosphere in the daytime with the f1 layer, the oblique sounding ionograms generated by the three mqp models are less similar to the measured ionogram. by contrast, for the ionosphere without the f1 layer, the ionogram generated by using the mqp-v model is superior to those generated by using mqpvw and the conventional mqp model. the mqp-vd model maintains not only the valley depth parameter, but also the width of the valley bottom, whereas the mqp-vdw cannot guarantee the width of the valley bottom. this finding suggests that the depth and width of the valley bottom play an important role in electromagnetic propagation. compared with the conventional mqp model, the mqp-v model is more suitable for describing the ionosphere without the f1 layer to increase the accuracy of ray tracing and for reducing the error of parameter inversion based on the ionosphere model. acknowledgements. this study was supported by the national natural science foundation of china (grant no. 61172017). references baker, d.c., and s. lambert (1989). range estimation for ssl hfdf systems by means of a multiquasiparabolic ionospheric model, iee proceedings h (microwaves, antennas and propagation), 136 (2), 120-125. bourgeois, d., c. morisseau and m. flecheux (2005). quasi-parabolic ionosphere modeling to track with over-the-horizon radar. ieee/sp workshop on statistical signal processing, 962-965. bourgeois, d., c. morisseau and m. flecheux (2006). over-the-horizon radar target tracking using multiquasi-parabolic ionospheric modeling, iee proceedings-radar, sonar and navigation, 153 (5), 409-416. dyson, p.l., and j.a. bennett (1988). a model of the sun et al. 6 figure 5. one-hop oblique sounding ionograms generated with the three mqp models and the experimental ionogram. time: may 25, 2014, at 02:07 (ut). table 3. difference values in the one-hop group paths of the experimental oblique sounding and of the simulation results. time: may 25, 2014, at 02:07 (ut). f(mhz) f1 f2 at a low angle f2 at a high angle 10 10.8 10 10.8 11.6 12.4 12.8 12 12.4 12.8 pexp pmqp-v (km) -83 -28 16 -13 -25 -54 -118 123 118 158 pexp pmqp-vd (km) -83 -27 16 -13 -25 -53 -118 123 118 160 pexp pmqp (km) -81 122 17 -12 -25 -52 -117 123 118 160 7 vertical distribution of the electron concentration in the iono-sphere and its application to oblique propagation studies, journal of atmospheric and terrestrial physics, 50 (3), 251-262. gasse, v., d. lemur and l. bertel (1999). a 3d ray tracing procedure to study ionospheric tilts, physics and chemistry of the earth, part c: solar, terrestrial & planetary science, 24 (4), 379-383. hou, c.y., g. ke and y. fu (2015). the sky-wave radar detection performance computing based on the dynamic ionospheric model, neurocomputing, 151, 1305-1315. krasheninnikov, i.v., j.c. jodogne and l.f. alberca (1996). compatible analysis of vertical and oblique ionospheric sounding data, annali di geofisica, 39 (4), 763-768. li, w.m., d.l. su, z.w. yang and y. liu (2012). ionosphere hybrid modeling method for short-wave ray tracing, journal of beijing university of aeronautics and astronautics, 38 (4), 473-477. li, x.d., q. he, b.x. han and z.s. he (2012). detection performance of mimo-oth radar: advantages of multipath ionospheric propagation//computer and information technology (cit), 2012 ieee 12th international conference on ieee, 758-762. liu, r.y., g.h. liu, j. wu, b.c. zhang, j.y. huang, h.q. hu and z.h. xu (2008). ionospheric fof2 reconstruction and its application to the short-term forecasting in china region, chinese journal of geophysics, 51 (2), 300-306. liu, w., p.n. jiao, s.k. wang and j.j. wang (2008). short wave ray tracing in the ionosphere and its application, chinese journal of radio science, 23 (1), 41-48. mahajan, k.k., r. kohli, v.k. pandey and n.k. sethi (1990). information about the e-region valley from incoherent scatter measurements, advances in apace research, 10 (8), 17-20. wang, j., w. fu, x.z. feng and d.m. lu (2009). technique on short distance high frequency single station location, electronic warfare technology, 24 (1), 25-28. wang, s.k., w. liu, s.l. li, y.b. guo, j.m. fan and p.n. jiao (2010). inversion method of f1 layer on vertical sounding ionogram, chinese journal of radio science, 25 (1), 172-177. * corresponding author: xiao-juan zhang, key laboratory of electromagnetic radiation and sensing technology, chinese academy of sciences, beijing, china; email: xjzhang@mail.ie.ac.cn. © 2016 by the istituto nazionale di geofisica e 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice muography of 1949 fault in la palma, canary islands, spain annals of geophysics, 60, 1, 2017, s0110; doi:10.4401/ag-7385 s0110 muography of 1949 fault in la palma, canary islands, spain seigo miyamoto1,*, josé barrancos2,3, cristiano bozza4, lucia consiglio5, chiara de sio4, pedro hernández2,3,6, ryuichi nishiyama1, germán padilla2,3, eleazar padrón2,3, chiara sirignano7,8, simona maria stellacci4, hiroyuki k.m. tanaka1, valeri tioukov5 1 earthquake research institute, the university of tokyo, tokyo, japan 2 instituto tecnológico y de energías renovables (iter), santa cruz de tenerife, spain 3 instituto volcanológico de canarias (involcan), puerto de la cruz, tenerife, spain 4 università di salerno, dipartimento di fisica “e. r. caianiello”, salerno, italy 5 università di napoli federico ii, dipartimento di fisica, naples, italy 6 agencia insular de energía de tenerife (aiet), granadilla de abona, tenerife, spain 7 università di padova, dipartimento di fisica e astronomia “galileo galilei”, padova, italy 8 istituto nazionale di fisica nucleare (infn), sezione di padova, padova, italy abstract muography (muon radiography) is a new geophysical technique that allows investigation of inner structures of an edifice with a very detailed spatial resolution. it has been recently used for several volcanoes and different geoscientific targets. in 2011 tanaka et al. succeeded to find hidden ancient seismic faults. in 1949 there was a volcanic activity of the cumbre vieja, la palma, canary islands, spain and a 1km-long fault suddenly appeared during the active period. the fault might be the sign of a large scale land collapse. in order to get additional information, investigations by several geophysical exploration techniques are needed. we consider that muography can be applied to study the shallow part of the fault and it can clarify some important parameters: the bulk density, the width of the low density zone, and the depth. previous investigations allowed detecting two ancient seismic faults that have 20m-wide mechanically fractured zone consisting of highly damaged rocks. the 1949 fault might be the result of large scale land slide and the slide length might be just a few meter. therefore the expected width of the fault is only a few meter. in order to detect such narrow fault, the muon detector should have as high spatial resolution as possible. in addition, it is difficult to get continuous power supply near the fault. nuclear emulsions are a kind of photographic films that have high sensitivity for high energy charged particles. they also have high spatial resolution for high energy muon paths and do not need any power supply to be operated. we placed an emulsion detector having 0.19 m2 effective area near the 1949 fault. the exposure started on january 2014 and lasted 106 days. all the emulsion films were developed and they are under analysis. we also estimated the expected performance of this test exposure. assuming a very simple model, we evaluated the detectable region as a function of the low density zone width and of the depth from the ground surface as well. 1. introduction la palma is a volcanic island located in the northwest sector of the canary islands (figure 1). it spans 45 km in north-south direction and 28 km in east-west directions, respectively. the area is 706 km2 and the maximum altitude is 2423 m. la palma consists of two volcanic systems. the northern part of the island was created by the activity of the older volcano cumbre nueva (more than 3.0 ma; staudigel et al. [1986]) and the southern part has been built by the activity of cumbre vieja until article history received july 5, 2016; accepted october 5, 2016. subject classification: muon radiography, muography, volcano, fault, land slide. figure 1. the shape of the la palma island and the position of 1949 fault. the muon detector installation site is marked by the star. 123 ka ago. the length of the main north-south rift zone is about 20 km [ancochea et al. 1994, day et al. 1999]. the collapse of the mountain flank is ordinary for volcanoes and sometimes it can affect very seriously the nearby population. recent geological investigation shows that the 1949 volcanic eruption near the summit of the cumbre vieja might have been accompanied by the development of a west facing fault system along the crest of the volcano. it is considered to be the first surface rupture along a developing zone of flank instability [day et al. 1999]. this fault is not the surface of a dyke and might be a sign of large scale land collapse. there are many ancient deposits of debris flow and debris avalanches distributed on the seafloor around the canary islands and some of them are larger than the area of the island itself [krastel et al. 2001]. the vertical ground layer structure of la palma suggests that the weak layer is made of debris avalanche deposit and/or collapse distributed on the seafloor by the activity of older volcano cumbre nueva, and the eruption product of cumbre vieja accumulated on it. the magma bulge beneath the north-south rift zone may cause the rupture of the cumbre vieja flank with the product of collapse running into the sea at the speed of 100 m/s. this might generate a huge tsunami and give rise to a very serious damage to the islands and the north-west coast of africa and also to the east coast of america [ward and day 2001]. it is necessary to investigate the 1949 fault by various geophysical techniques, not only the standard ones, for proper risk estimation. cosmic-ray muon radiography (muography) is a new method that can be used to make a map of the inner density structures of large objects such as volcanoes. many investigations were performed on volcanoes by this method recently [tanaka et al. 2007a, 2007b, 2007c, 2009a, 2009b, 2010, 2014; lesparre et al. 2012, jourde et al. 2013, cârloganu et al. 2013, carbone at al. 2013]. the principle is similar to the one of ordinary x-ray photography. the source that corresponds to x-rays is provided by cosmic-rays from the sky, muon miyamoto et al. 2 figure 2. an image of the sensitive layer of nuclear emulsion by using an optical microscope. the size of the view is 120 micron*105 micron. each silver grain diameter is about 1 micron and the field depth of the objective is about 3 micron. aligned grains on the straight line are the track produced by high energy charged minimum ionization particle. figure 3. (a) picture of the rigid metallic frame. ecc made by ten 1mm-thick lead plates and eleven emulsion films are stacked inside the frame. the arrow shows the direction of the muon emerging from the fault. (b) the rigid frame was covered by polystyrene thick foils to prevent heating and rapid changes of the surrounding temperature. two gps antennas were attached to the frame to measure the position and the direction of the detector. (c) the schematic of the muon detector inside. four metallic frames were stacked in a vertical direction and five ecc were included in each frame. (d) the cross view of each ecc is represented as a schematic diagram. the root mean square of deflection angles (dii) of charged particle in the material is proportional to the inverse of momentum and the square root of the radiation length of the material. considering the angular resolution of emulsion films and the radiation length of the lead plates, most charged particles with momentum below 2 gev/c should be rejected. (a) (b) 3 detectors act as x-ray films, and the imaging target are volcanoes. the remarkable difference between x-rays and cosmic-ray muons is the attenuation length in the material. the typical attenuation length of x-rays is less than 100 g/cm2 “density length” (1 m water equivalent), while the attenuation length of cosmic-ray muons ranges from 10 m to more than 1 km water equivalent depending on the momentum. in the previous observation of the low density rock crushing zone in the ancient seismic fault in itoigawa, japan, the low density zone width was evaluated to be about 20 m [tanaka et al. 2011]. in the case of la palma, the expected width of the 1949 fault is just a few meters. a muon detector with high spatial resolution is needed. furthermore, it’s difficult to get stable and continuous electric power supply around the 1949 fault. the features of nuclear emulsions (figure 2) [nakamura et al. 2006], compared with the ones of any other muon detectors are the following: 1) intrinsic high spatial resolution; 2) no electric power supply needed. 2. detector installation and exposure of cosmic-rays in january 2014 a rigid metallic frame containing emulsion cloud chamber (ecc) and having 0.19 m2 effective area (figure 3a) was placed at the bottom of the ravine (figure 4). ecc has a modular structure made of a sandwich of passive material plates such as lead interleaved with emulsion film layers (figure 3c). we used ecc to remove low momentum particles by the detection of their large deflection angles in lead (figure 3d) [nishiyama et al. 2014]. the detector position and the orientation were measured by two gps receivers as shown in figure 3b. after 106 days’ exposure, the detector was removed and all films were developed. now the films are under data taking by automated readout systems [armenise et al. 2005; arrabito et al. 2006]. such systems acquire tomographic images in the emulsion sensitive layers and recognizes straight sequences of silver grains leaved by muons. the output data are the positions and the directions in xand y-axis, respectively. after the reconstruction of tracks in ecc by offline processing [tioukov et al. 2006], muon candidates are selected by average number of grains per length and average deflection angle. 3. estimation of the expected performance we estimated the detectable region in this exposure. the maximum detectable depth from the ground surface is determined by the topography around the fault and the detector. the side view of the topography along the fault is shown in figure 5. for example, if the elevation angle is 200 mrad, the maximum depth from the ground surface is about 20 m, while if elevation angle is 100 mrad, the path length is too long to get enough muon statistics to evaluate the density. the maximum detectable depth also depends on the width of the low density zone in the fault; as the width of the low density zone increases, the solid angle for low density path increases, consequently the available statistics of penetrating muon increases. in figure 6 the expected detectable region is shown as a function of the width of the low density zone and of the depth from the ground surface. the parameters we assumed are the following: a) effective area is 0.19 m2; b) exposure time is 106 days; c) muon flux model by honda et al. [2004] and the muon energy attenuation by groom et al. [2001] are used; d) the muon detection efficiency is 79% and it has no angular dependence; e) the bulk density of the rock around the fault is uniform (2.2 g/cm3) while a density of 2.0, 1.9, 1.8, 1.7 g/cm3 in the rock crushing low density zone is supposed. figure 6 shows our detection limit in this observation if no exmuography of 1949 fault in la palma, canary islands, spain figure 4. topological map around the 1949 fault. the color scale represents the altitude and the pitch of the contours is 20m. the acceptance of the muon detector is indicated by the white solid lines and it is about ±30 degree. the vertex is the position of the muon detector. the grid pitch is 200 m. figure 5. topographic side view along the fault. note that the scale is different between horizontal and vertical axis. blue lines emerging from the detector point indicate the muon paths; they have 0.1, 0.2, 0.3 and 0.4 tangent of the elevation angle respectively. pected contrast region is found. if so, the thickness of the low density zone of crushed rock should be small, less than 2 meter in the case of 1.7 g/cm3. 4. conclusion there is the possibility that the 1949 fault in la palma is the sign of a large scale land collapse. high spatial resolution is necessary to gather information about this fault. we applied the muon radiography method to investigate this fault with a nuclear emulsion based detector. all the films are under analysis. we estimated the detectable region for some important parameters assuming a very simple model. we might see/exclude inner bulk density anomalies in the detectable region at the end of emulsion measurements and data analysis. acknowledgements. this research was financed through a grant from the canary islands government research agency, aciisi (proid20100160), and supported by the cabildos insulares de tenerife and la palma. we acknowledge the gran sasso laboratory and infn for their support in the emulsion development activity. we also thank conny spelbrink for their logistical support during the fieldwork in cumbre vieja. references ancoechea, e., f. hernan, a. cendrero, j.m. cantragrel, j.m. fuster, e. ibarrola and j.coello (1994). constructive and destructive episodes in the building of a young oceanic island, la palma, canary islands, and genesis of the caldera de taburiente, j. volcanol. geoth. res., 60, 243-262. armenise, n., et al. (2005). high-speed particle tracking in nuclear emulsion by last-generation automatic microscopes, nucl. instrum. meth. a, 551, 261-270. arrabito, l., et al. (2006). hardware performance of a scanning system for high speed analysis of nuclear emulsions, nucl. instrum. meth. a, 568, 578-587. carbone, d., d. gibert, j. marteau, m. diament, l. zuccarello and e. galichet (2013). an experiment of muon radiography at mt etna (italy), geophys. j. int., 196 (2), 633-643; doi:10.1093/gji/ggt403. cârloganu, c., et al. (2013). towards a muon radiography of the puy de dôme, geosci. instrum. meth., 2 (1), 55-60. day, s.j., j.c. carracedo, h. guillou and p. gravestock (1999). recent structural evolution of the cumbre vieja volcano, canary islands: volcanic rift zones reconfiguration as a precursor to volcano flank instability?, j. volcanol. geoth. res., 94, 135-167. krastel, s., h.u. schmincke, c.l. jacobs, r. rihm, t.p. le bas and b. alibes (2001), submarine landslides around the canary islands, j. geophys. res., 106 (b3), 3977-3997. groom, d.e., n.v. mokhov and s.i. striganov (2001). muon stopping muon stopping power and range tables 10 mev-100 tev, atom. data nucl. data, 78, 183-356. honda, m., t. kajita, k. kasahara and s. midorikawa (2004). new calculation of the atmospheric neutrino flux in a three-dimensional scheme, phys. rev. d, 70, 043008; doi:10.1103/physrevd.70.043008. jourde, k., d. gibert, j. marteau, j. de bremond d'ars, s. gardien, c. girerd, j.-c. ianigro and d. carbone (2013). experimental detection of upward going cosmic particles and consequences for correction of density radiography of volcanoes, geophys. res. lett., 40, 6334-6339; doi:10.1002/2013gl058357. miyamoto et al. 4 figure 6. relation between the width of low density crushing zone and the maximum detectable depth from the ground surface. if both parameters are above the solid line, we can detect the low density zone of the fault with more than 90% statistical confidence level. 5 lesparre, n., d. gibert, j. marteau, j.c. komorowski, f. nicollin and o. coutant (2012). density muon radiography of la soufrière of guadeloupe volcano: comparison with geological, electrical resistivity and gravity data, geophys. j. int., 190, 1008-1019. nakamura, t., et al. (2006). the opera film: new nuclear emulsion for large-scale, high-precision experiments, nucl. instum. meth. a, 556, 80-86. nishiyama, r., s. miyamoto and n. naganawa (2014). experimental study of source of background noise in muon radiography using emulsion film detectors, geosci. instrum. meth., 3, 29-39. staudigel, h., g. feraud and g. giannerini (1986). the history of intrusive activity on the island of la palma (canary islands), j. volcanol. geoth. res., 27, 299-322. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida, h. ohshima, t. maekawa, h. watanabe and k. niwa (2007a). imaging the conduit size of the dome with cosmic ray muons: the structure beneath showa shinzan lava dome, japan, geophys. res. lett., 34, l22311; doi:10.1029/2007gl031389. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida, m. takeo, j. oikawa, t. ohminato, y. aoki, e. koyama, h. tsuji and k. niwa (2007b). high resolution imaging in the inhomogeneous crust with cosmic ray muon radiography: the density structure below the volcanic crater floor of mt. asama, japan, earth planet. sc. lett., 263, 104-113. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida and k. niwa (2007c). development of an emulsion imaging system for cosmic-ray muon radiography to explore the internal structure of a volcano, mt. asama, nucl. instrum. meth. a, 575, 489-497. tanaka, h.k.m., t. uchida, m. tanaka and h. shinohara (2009a). cosmic-ray muon imaging of magma in a conduit: degassing process of satsuma-iwojima volcano, japan, geophys. res. lett., 36, l01304, 1-5. tanaka, h.k.m., t. uchida, m. tanaka, m. takeo, j. oikawa, t. ohminato, y. aoki, e. koyama and h. tsuji (2009b). detecting a mass change inside a volcano by cosmic-ray muon radiography (muography): first results from measurements at asama volcano, japan, geophys. res. lett., 36, l17302, 1-4. tanaka, h.k.m., h. taira, t. uchida, m. tanaka, m. takeo, t. ohminato, y. aoki, r. nishitama, d. shoji and h. tsuiji (2010). three-dimensional computational axial tomography scan of a volcano with cosmic ray muon radiography, j. geophys. res., 115, b12332; doi:10.1029/2010jb007677. tanaka, h.k.m., h. miyajima, t. kusagaya, a. taketa, t. uchida and m. tanaka (2011). cosmic muon imaging of hidden seismic fault zones: rainwater permeation into the mechanical fractured zones in itoigawa-shizuoka tectonic line, japan, earth planet. sc. lett., 306, 156-162. tanaka, h.k.m., t. kusagaya and h. shinohara (2014). radiographic visualization of magma dynamics in an erupting volcano, nat. commun., 5, 3381, 1. tioukov, v., i. kreslo, y. petukhov and g. sirri (2006). the fedra-framework for emulsion data reconstruction and analysis in the opera experiment, nucl. instrum. meth. a, 559, 103-105. ward, s.n., and s. day (2001). cumbre vieja volcanopotential collapse and tsunami at la palma, canary islands, geophys. res. lett., 28, 3397-3400; doi:10.10 29/2001gl013110. *corresponding author: seigo miyamoto, earthquake research institute, the university of tokyo, tokyo, japan; email: miyamoto@eri.u-tokyo.ac.jp. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. muography of 1949 fault in la palma, canary islands, spain << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true /maxsubsetpct 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice adg vol5 n02 fer 219_232.pdf annals of geophysics, vol. 45, n. 2, april 2002 219 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies fausto ferraccioli, emanuele bozzo and egidio armadillo dipartimento per lo studio del territorio e delle sue risorse (dip.te.ris.), università di genova, italy abstract high resolution aeromagnetic surveys (hram) are a novel tool experimented in several countries for volcano and earthquake hazard re-assessment, ground water exploration and mitigation, hazardous waste site characterization and accurate location of buried ferrous objects (drums, uxo, pipelines). the improvements achieved by hram stem from lower terrain clearance coupled with accurately positioned, real-time differential navigation on closely spaced flight grids. in field cultural noise filtering, advanced data processing, imaging and improved interpretation techniques enhance data information content. development of hram approaches might also contribute to mitigate environmental hazards present throughout the italian territory. hence an hram field test was performed in july 2000 in friuli, north-eastern italy to assess the capabilities and limitations of hram over a buried pipeline and a domestic waste site. a cesium magnetometer in towed bird configuration was used on two separate grids. profile line spacing was 50-100 m and bird nominal ground clearance was set to 50 m. microlevelled total field magnetic anomaly data forms the basis for subsequent advanced processing products including 3d analytic signal, maximum horizontal gradient of pseudo-gravity and 3d euler deconvolution. the magnetic signatures we detected and enhanced over the environmental test site area in friuli are also compared with similar but more extensive hram signatures recently observed in other countries. 1. introduction environmental characterization and hazard studies may benefit from improvements in the resolution power of relatively low cost aeromagnetic surveys. the increased capabilities of hram (high resolution aeromagnetic) surveys compared to reconnaissance campaigns stem from lower terrain clearance, tighter line spacing, more accurate real-time magnetic and positioning data acquisition, and enhanced data processing and display techniques. the main advantages of airborne geophysical surveys over ground based studies relate to improved access to remote, hazardous and contaminated areas coupled with extensive, yet rapid sampling (raymond and blakely, 1995). hram surveys are based upon preprogrammed flight lines which assist pilot navigation during airborne or helicopter-borne surveys. hram terrain clearance is typically of 50-200 m, mailing address: dr. fausto ferraccioli, dipartimento per lo studio del territorio e delle sue risorse (dip.te.ris.), università di genova, viale benedetto xv, 5, 16132 genova, italy; e-mail: magne@dipteris.unige.it key words hram applications – environmental hazards – italian test site 220 fausto ferraccioli, emanuele bozzo and egidio armadillo profile line spacing is of 50-200 m, with tie lines spaced less than 500 m. differential global positioning system (dgps) techniques have been the key to widening applications of highresolution airborne geophysics. real-time differential gps pseudorange navigation ensures that the horizontal positional accuracy is 5 m or better and that the vertical positioning is accurate at least to 10 m (grauch and millegan, 1998). laser and radar altimeter systems with much higher accuracy (about 10 cm) can also be added as part of integrated airborne platforms. improved positioning implies that magnetic errors related to location can now be reduced to about 1 nt over many terrains allowing for more confident recognition of subtle, low-amplitude anomalies. optically pumped magnetometers have a resolution of 0.001 nt and cycle at 10 hz so ground sampling rate can be reduced to 2-5 m. this leads to precise delineation of much shorter wavelength magnetic anomalies related to shallow-source, man-made or natural, geologic features. however, high-resolution aeromagnetic applications demand detailed in-field quality control in conjunction with review of flight videos, followed by cultural noise editing and filtering (peirce et al., 1998). microlevelling of hram data may also be applied to reduce residual flight-line related corrugation, which might otherwise hinder successful application of digital enhancement, transformation and filtering techniques (ferraccioli et al., 1998). prime current applications of hram include volcano and earthquake hazard assessment, ground water exploration and mitigation problems, waste site characterization and location of buried man-made ferrous objects, as briefly described hereafter. high-resolution aerogeophysics may provide for example unprecedented views of the 3d internal structure of active volcanoes with hazardous collapseprone alteration zones, such as verified over mt. rainier in the u.s. (finn et al., 2001). hram investigations also improve and extend fault delineation and structural characterization in seismically active regions, in particular in areas concealed by alluvial deposits, vegetation and urban development and therefore mostly inaccessible to other geophysical methods (blakely et al., 1995; liberty et al., 1999). furthermore, the extent and character of ground water hydrogeological systems, whether profitable or hazardous, may be imaged and interpreted using integrated aeromagnetic and airborne electromagnetic approaches (grauch et al., 2000; wightman et al., 2000; siemon et al., 2000a). waste site characterization from airborne geophysical surveying is a valuable tool for rapid screening of hazardous disposal areas, minimizing personnel risk and directing subsequent ground follow-up (lerssi et al., 1997; doll et al., 2000; siemon et al., 2000b). detection of hazardous unexploded ordnance (uxo) has also recently been achieved by very high-resolution low level helicopter-borne surveys (lahti et al., 2001). many of these diverse natural and man-made environmental problems also pose well recognized hazards for the italian territory. nevertheless the utility of high resolution airborne geophysics to assess such hazards appears to have been severely underestimated over the national territory which is largely devoid of such studies compared to other countries world-wide. existing aeromagnetic data acquired for the national italian oil company in the late 70’s (agip, 1981) and a newly compiled ground magnetic anomaly map (chiappini et al., 2000) are much too regional in character to contribute towards assessment of environmental hazards. an attempt to perform higher resolution aeromagnetic surveying in the area of roccamonfina volcano was made with an italian helicopterborne platform in 1994 and was mainly targeted to regional structural mapping rather than to volcanic hazard (chiappini et al., 1998). in 1999 some active italian volcanic regions were covered with flight specifications broadly similar to hram type surveys by using the geological survey of austria airborne geophysical platform (supper et al., 2000). adaptation of airborne geophysics to detailed environmental research over italy is not straight forward but requires testing, experimentation and experience as verified for example in the u.s. (doll et al., 2000). hence, in july 2000 we performed the first hram field test in italy over friuli to test the capabilities and limitations of the currently available aeromagnetic system for environmental characterization, in parti221 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies cular for location of buried ferro-metallic bodies. the magnetic signatures we identified and enhanced over a major buried pipeline and over a small domestic waste site are described in comparison with similar but more extensive hram patterns detected by others outside the italian territory. 2. aeromagnetic system and survey layout in july 2000 an hram field test was performed near trieste, northeast italy, in friuli (inset in fig. 1a). the elifriulia company provided an ecureil 350 helicopter, fuel and logistic support for the test. the aeromagnetic system we adopted was used in the last decade to perform reconnaissance, 4-km line spacing field work over the antarctic targeted to crustal exploration (bozzo et al., 1999; ferraccioli and bozzo, 1999). the system has also been employed on a single 500-m line spacing, low-altitude (125 m above sea level) campaign for an antarctic drill site survey (bozzo et al., 1997). a scintrex cesium magnetometer (cs2) measuring the total magnetic field to an accuracy of 0.001 nt was installed in towed bird configuration. the magnetometer was set to cycle at 10 hz to achieve an approximately 2-3 m ground sampling rate. the bird was towed on a 25 m long cable also serving for data transmission to the acquisition system placed inside the helicopter. the position of the helicopter was recorded by means of a magnavox gps rover. for later differential gps correction a magnavox gps base station was installed at a small airfield in the premariacco area where the scintrex magnetic base station was also deployed. inflight data control was accomplished by the operator through an electro-luminiscent screen. the navigator assisted the pilot over the flight grid using a picodas navigation system interface. no video is included in this aeromagnetic system. therefore an extra operator spotted and noted the main cultural features which were positioned in-flight by placing digital event marks using the acquisition system. no laser altimeter is included either, limiting altitude measurement accuracy. the hram test flight included two grids. area 1 flight grid was located to the southwest of premariacco (see igm sheet premariacco) and spans 1.5 km2 (figs. 1 to 4). area 2 flight grid was placed just south of premariacco; its extent is equal to 3 km2 (figs. 6 to 8). line spacing was selected to be 100 m over a buried pipeline known to occur within area 1. a more detailed 50 m section was flown over a small exposed and active domestic waste site in area 2 (figs. 7 and 8). the whole survey area is a flat, low-lying alluvial plain with mean elevation of 100 m above sea level. safety regulations impede flying lower than 50 m terrain clearance under standard conditions. since the bird is placed about 20-25 m beneath the helicopter during survey operations 175 m was the nominal altitude selected for barometric mode surveying. the aeromagnetic system had never been tested before on such a tight line spacing and low altitude survey. the fairly low level of urban development and the weakly magnetic alluvial plain were favourable local conditions for improved ferro-metallic target delineation in both areas. conversely the small size of the selected targets was a challenging test for assessing the capabilities and limitations of the reconnaissance magnetic exploration instrumentation to much higher resolution environmental research. 3. data processing the baseline of magnetic data processing was adapted from standard procedures extensively used for regional magnetic anomaly exploration programmes (bozzo et al., 1999). these included repositioning of flight lines by means of differential gps corrections, removal of external field magnetic time variations using base station magnetic data, removal of the igrf reference field using the provisional igrf2000 model, statistical levelling and microlevelling (ferraccioli et al., 1998). no additional filtering of anomalies linked to cultural features was attempted so as not to affect the rest of the data. more careful levelling and microlevelling compared to standard processing was however required. root mean square residual errors are estimated to be less than 0.5 nt outside areas of cultural noise. 222 fausto ferraccioli, emanuele bozzo and egidio armadillo p ip e l in e friuli italy c. zucco c. pitassi le li a b 223 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies 4. advanced hram mapping to locate buried ferro-metallic bodies over the test site areas 4.1. aeromagnetic anomaly maps over the test pipeline the total field aeromagnetic anomaly map over survey area 1 was gridded at a 25 m cell size, equal to 1/4 of line spacing (fig. 1a). it was compiled to verify if the buried pipeline exhibited a magnetic signature in the area and, if so, if it could be used to relocate the pipeline itself. this is a segment of the transalpine oil pipeline, a major 465 km long feature built in 1967. the oil pipeline connects trieste industrial harbour in northeastern italy with ingolstadt in southeastern germany. the presently functioning pipeline has an outer diameter of 1016 mm, a thickness of 8.74 mm and has an annual oil capacity of up to 54 million tons. it is estimated to be buried at about 1.5-2 m below the ground. a 1.2 v negative dc current is applied to the pipeline for cathodic protection. the buried pipeline clearly shows up in the hram image as a linear nw trending high-low magnetic pair with mean peak to peak amplitudes of 14 nt. the aeromagnetic anomaly amplitudes and shapes we detected over the test site are comparable to those mapped with high resolution magnetic data over buried natural-gas pipelines with cathodic protection over the albuquerque c fig. 1a-c. a) microlevelled total field hram map over a buried pipeline in friuli (see inset) located in test site area 1. the pipeline anomaly signature is evident despite high-frequency noise corresponding to a low voltage electric line between colli pitassi and colle zucco. b) upward continued version at 100 m above original observation level. c) downward continued version at 25 m below original observation level. 224 fausto ferraccioli, emanuele bozzo and egidio armadillo basin, new mexico (grauch and millegan, 1998; grauch, 2001). the typical anomaly we observed over the pipeline is asymmetric with a negative peak of 2 nt on the south western flank and a positive peak of +12 nt on the north eastern flank (see also fig. 3). the shape of the observed anomaly deviates from the one expected for a linear nw trending 2d body with induced magnetization only. this suggests that remanent magnetization or the effect of the dc cathodic protection current is responsible for the transalpine pipeline magnetic anomaly signature. indeed grauch and millegan (1998) showed that the polarity of the typical high-low magnetic pair reversed where there was a change in current direction along the albuquerque basin pipelines. however bozzo et al. (1994) used vertical gradient ground magnetic measurements over liguria in northwestern italy to argue that remanent magnetization was likely to determine the leading magnetic anomaly signature of oil pipelines, but that current effects were less significant. smaller amplitude, short wavelength, 1.5 to 5 nt mostly positive anomalies, forming a nw trending magnetic chain parallel to the pipeline correlate with a low voltage dc powerline. this feature does not appear to severely hinder recognition of the pipeline itself. an upward continued map (fig. 1b) was produced to verify the fall-off rate of the pipeline anomaly at a distance of 100 m above observation level, i.e. equal to a 150 m sensor-ground distance. this map shows that the broadened positive component of the anomaly decays in amplitude to about 6 nt while the negative peak is now close to 0. the signature of the low-voltage line is also considerably broader and tends therefore to merge with the one of the pipeline at grid edges making recognition of the pipeline much less confident. indeed in-flight experimentation on a single profile flown at a 200 m bird ground clearance suggests no clear pipeline signature. conversely a map downward continued to 25 m enhances the pipeline magnetic signature, which is marked by an almost 20 nt peak-to-peak linear anomaly (fig. 1c). despite hanning convolution filtering, noise is clearly discerned especially along grid edges. in flight experimentation at this lower level was not possible owing to safety regulations. the exact plan location of the pipeline is not obvious from the total field aeromagnetic anomaly maps because of the asymmetric dipolar character of the associated anomaly. we therefore compiled a 3d analytic signal map (fig. 2a) which is computed from the square root of the squared sum of the horizontal derivative and vertical derivative of the total field aeromagnetic anomaly data (roest et al., 1992). the analytic signal might be appealing for pipeline location because it is considered to exhibit maxima over magnetization contrasts, independent of the ambient magnetic field and of source magnetization directions (roest et al., 1992). some authors pointed out however that the magnetization direction may influence analytic signal results (e.g., linping and zhining, 1998). notwithstanding this, the pipeline is better located by using the maxima of the 3d analytic signal than by total field data. analytic signal peaks were picked by applying the blakely and simpson (1986) automated detection approach, conventionally used for pseudo gravity anomalies. the width of the analytic signal is however fairly broad (about 200 m) and some individual peaks are de-focussed and apparently located along the flight lines. this may relate to the vertical derivative needed to calculate the analytic signal which appears to enhance residual flight line-related noise. we then tested the maximum horizontal gradient of the pseudo-gravity method (cordell and grauch, 1985) for aeromagnetic pipeline location because this approach does not require vertical derivatives. on the other hand pseudogravity involves reduction to the pole which may be critical when the assumption of magnetic anomalies related to induced magnetic effects does not hold and when magnetization direction parameters cannot be well estimated (macleod et al., 1993). regardless of this probable limitation, we found an improved plan location of the pipeline using this method (fig. 2b). figure 3 compares the typical total field, 3d analytic signal and pseudo-gravity signature over the pipeline and electric power line. aiming at refining the horizontal location of the pipeline and estimating its depth we then applied 3d euler deconvolution to the total field data (reid et al., 1990). this method might be 225 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies p ip e l in e p ip e l in e a p ip e l in e fig. 2a,b. a) three-dimensional analytic signal map in the pipeline area. local peaks over the buried pipeline are displayed as black triangles. b) maximum horizontal gradient of pseudo gravity with local peaks indicated with black crosses. b 226 fausto ferraccioli, emanuele bozzo and egidio armadillo particularly suitable for pipelines because it was thought to be insensitive to magnetic inclination, declination and remanence. more recently, the method has been claimed to exhibit a magnetization direction dependence (ravat, 1996). an appropriate structural index for the source body must be introduced and then euler’s homogeneity relationship is solved by leastsquares inversion. the structural index essentially relates to the rate of change with distance of the field (reid et al., 1990). we found that a structural index equal to 2.0 could adequately represent the pipe-like source, broadly consistent with results from a ground magnetic test site targeted to buried ferro-metallic body detection in columbia (yaghoobian, 1993). the plan location of the pipeline is visually well defined by spatially clustered solutions (fig. 4), with minimal de-focussing, if solutions are plotted according to a spatial window defined by previous analytic signal and pseudo-gravity mapping. mean depth estimate is also fairly accurate, within 10% of the real mean depth level which is known in the test site area to be about 91 m, i.e. 1.5-2 m below ground. this euler deconvolution result is encouraging since 2391300 2391400 2391500 2391600 2391700 2391800 2391900 2392000 1 2 3 4 5 6 7 p se u d o -m g a l/ m (1 0 ) -4 pseudo-gravity total field 3d analytic signal horizontal gradient 0 2 4 6 8 10 12 nt 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 3 d a n a lytic sig n a ln t /m fig. 3. profile view comparing hram total field data with calculated 3d analytic signal and pseudo gravity signatures. note the main peaks over the pipeline and over the electric line to the west. fig. 4. three dimensional euler deconvolution map over the buried pipeline. plan location of the pipeline is well defined and most depth solutions plot in 80-90 m elevation interval, i.e. fairly consistent with real depth. 227 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies 2397000 2397500 2398000 2398500 -60 -50 -40 -30 -20 to ta lm a g n e ti c fi e ld (n t ) -10 0 10 fig. 5. profile view displaying microlevelled total field hram signature over the domestic waste located in test site area 2. previous ground magnetic modelling efforts in liguria over a buried pipeline were claimed not to lead to reliable depth determination because of its remanent magnetization (bozzo et al., 1994). 4.2. aeromagnetic anomaly maps over the test waste site a typical total field aeromagnetic anomaly profile over the largest waste site of area 2 is displayed in fig. 5. a smooth dipolar aeromagnetic anomaly was consistently detected over the exposed domestic waste site with a mean peak to peak amplitude of 70 nt. the typical anomaly is asymmetric with a negative peak of −60 nt on the western flank and of + 10 nt on the eastern one. this may indicate the existence of ferrometallic objects or at least higher apparent susceptibility waste site infill, enriched in ferrometallic components with respect to background values. outside the waste site area anomaly values decay to an average close to 0 nt over the alluvial plain where such infill is lacking. relative highs (5-10 nt) superimposed upon the regionally flat anomaly field were recognized in field as being other cultural anomalies (e.g., roads, pylons, small villages). we then compiled the 3d analytic signal, the maximum horizontal gradient of pseudo-gravity and 3d euler deconvolution maps. the nne strike of the 3d analytic signal anomaly over the waste site is evident in fig. 6. within the waste site, area 1, which is the saturated part of the waste site body, features a relative high with respect to relative lows over the western part of areas ii and iv. an individual peak is identified just north of area 2 along the north-western margin of the waste site. moreover, a small positive anomaly parallels the main waste site body to the east speculatively indicating a satellite disposal site. it appears that the edges of the «regional» 3d analytic signal high over the waste site slightly underestimate the true dimensions of the waste site (fig. 6). on the contrary, the local peaks of the maximum horizontal gradient of pseudogravity appear to overestimate the true dimensions of the waste site, particularly on the north-western flank (fig. 7). it is unknown if this reflects some errors in apparent inclination due to non-induced magnetization components within the waste site body (mac leod et al., 1993). it is 228 fausto ferraccioli, emanuele bozzo and egidio armadillo i ii iii iv 100 0 100 200 metre 5 1 0 1 4 0 0 5 1 0 1 5 0 0 5 1 0 1 6 0 0 5 1 0 1 7 0 0 5 1 0 1 8 0 0 5 1 0 1 9 0 0 5 1 0 1 4 0 0 5 1 0 1 5 0 0 5 1 0 1 6 0 0 5 1 0 1 7 0 0 5 1 0 1 8 0 0 5 1 0 1 9 0 0 2396500 2396600 2396700 2396800 2396900 2397000 2397100 2397200 2397300 2397400 2397500 2397600 2396500 2396600 2396700 2396800 2396900 2397000 2397100 2397200 2397300 2397400 2397500 2397600 4 6 °3 '2 0 " 4 6 °3 '3 0 " 4 6 °3 '2 0 " 4 6 °3 '3 0 " 13°24'20" 13°24'30" 13°24'40" 13°24'50" 13°25' 13°24'20" 13°24'30" 13°24'40" 13°24'50" 13°25' i ii iii iv 100 0 100 200 metre 5 1 0 1 4 0 0 5 1 0 1 5 0 0 5 1 0 1 6 0 0 5 1 0 1 7 0 0 5 1 0 1 8 0 0 5 1 0 1 9 0 0 5 1 0 1 4 0 0 5 1 0 1 5 0 0 5 1 0 1 6 0 0 5 1 0 1 7 0 0 5 1 0 1 8 0 0 5 1 0 1 9 0 0 2396500 2396600 2396700 2396800 2396900 2397000 2397100 2397200 2397300 2397400 2397500 2397600 2396500 2396600 2396700 2396800 2396900 2397000 2397100 2397200 2397300 2397400 2397500 2397600 4 6 °3 '2 0 " 4 6 °3 '3 0 " 4 6 °3 '2 0 " 4 6 °3 '3 0 " 13°24'20" 13°24'30" 13°24'40" 13°24'50" 13°25' 13°24'20" 13°24'30" 13°24'40" 13°24'50" 13°25' fig. 6. three-dimensional analytic signal map in the waste site area. note contrasting signatures over different parts of the waste site. fig. 7. maximum horizontal gradient of pseudo gravity over the waste site area with local peaks indicated with black crosses. note relative peak over the saturated part of the waste site (i). 229 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies noteworthy however that a relative pseudo-gravity maximum corresponds to area 1, consistent with the analytic signal results. the 3d euler deconvolution map for structural index 2.0 (fig. 8), is appropriate for 2d finite dimension bodies as might be encountered within a waste site (e.g., drums, pipes, etc.) (yaghoobian, 1993). this map shows preferential clustering of solutions in areas i and iii with respect to areas ii and iv. the advanced processing products (figs. 6 to 8) might collectively be interpreted to indicate that areas ii and iv are «cleaner» areas, i.e. with lower apparent susceptibility infill, possibly reflecting volumetrically less significant ferro-metallic bodies. our aeromagnetic anomaly maps delineate the waste site quite well. our local finding is consistent with much more extensive aeromagnetic imaging of waste sites in tennessee, in the u.s., revealing that the boundaries of waste sites may be delineated fairly accurately using hram data (doll et al., 2000). in our aeromagnetic test site no higher frequency anomalies were detected and related to individual ferro-metallic waste site bodies despite ground sampling rate of about 2 m. only areas of enhanced apparent magnetization may be delineated. it is clear that considerably lower terrain clearance and tighter line spacing would be necessary to delineate individual ferrous bodies (e.g., drums, pipes, scrap material) within the waste site, if indeed present. doll et al. (2000) attempted to delineate such thresholds for magnetic sensitivity over waste disposal areas in relation to system noise, acquisition parameters, geologic noise and transient noise effects. their field test indicated that a single 55-gallon barrel could be detected under ideal circumstances at 5 m bird altitude (i.e. 10 times less than our test), but that generally it was not possible to distinguish fewer than 10 barrels from background heterogeneity. this low-level aeromagnetic figure is very promising and would be comparable with results from a ground magnetic test site performed in central italy (marchetti et al., 1998). siemon et al. (2000b) presented higher resolution waste site imaging compared to our field test over a specially prepared test area (150 i ii iii iv 100 0 100 200 metre 5 1 0 1 4 0 0 5 1 0 1 5 0 0 5 1 0 1 6 0 0 5 1 0 1 7 0 0 5 1 0 1 8 0 0 5 1 0 1 9 0 0 5 1 0 1 4 0 0 5 1 0 1 5 0 0 5 1 0 1 6 0 0 5 1 0 1 7 0 0 5 1 0 1 8 0 0 5 1 0 1 9 0 0 2396500 2396600 2396700 2396800 2396900 2397000 2397100 2397200 2397300 2397400 2397500 2397600 2396500 2396600 2396700 2396800 2396900 2397000 2397100 2397200 2397300 2397400 2397500 2397600 4 6 °3 '2 0 " 4 6 °3 '3 0 " 4 6 °3 '2 0 " 4 6 °3 '3 0 " 13°24'20" 13°24'30" 13°24'40" 13°24'50" 13°25' 13°24'20" 13°24'30" 13°24'40" 13°24'50" 13°25' fig. 8. three dimensional euler deconvolution map over the waste site area. note clustering of euler solutions in area i and iii of the waste site. a single depth solution range of 80-90 m is plotted. 230 fausto ferraccioli, emanuele bozzo and egidio armadillo m × 300 m) containing typical waste site materials including steel drums, a tank, ordinance, scrap metal, and a pipeline. using the test site indications, they later performed a detection survey covering an area of 86 km2 surveyed with a nominal flight-line spacing of 50 m and achieved a high detection level over small waste occurrences. lerssi et al. (1997) described more extensive high-resolution airborne geophysical studies including hram over three potentially contaminated landfills over finland. in the helsinki region, southern finland, results from a repeat survey were used to assess changes in the environment prior to and after installation of a large municipal landfill. magnetic anomalies with magnitudes up to 1000 nt were detected in the repeat survey over non-magnetic granite bedrock clearly indicating the presence of metallic waste. 5. conclusions high-resolution aeromagnetic surveys have recently addressed environmental characterization and hazard studies. hram experience is lacking in italy to date, despite the variety of environmental problems which the italian territory and population faces. amongst the broad range of such problems we used newly acquired hram data in friuli to test airborne location of buried ferro-metallic bodies. previously such an application was one most universal and well established targets of environmental ground magnetic field studies. our hram test demonstrates that it is possible to identify well-resolved magnetic anomaly signatures over a buried pipeline and over a small domestic waste site. because of the vector nature of the magnetic field and of remanence, it appears necessary to apply transformations and advanced digital analysis to total field aeromagnetic data to simplify aeromagnetic location of buried ferrometallic objects. we found that the maximum horizontal gradient of pseudo gravity was efficient in determining the plan location of the pipeline. three dimensional analytic signal mapping was effected to a higher degree by flight-line related noise, but still leads to an enhancement in pipeline location. the most useful characteristic of three-dimensional euler deconvolution is its relative insensitivity to the magnetization direction, which can be exploited for improved plan location and for approximate depth determination of the pipeline. over the second area of the test site, we found that the true dimensions of the waste site appear to be slightly underestimated by 3d analytic signal and overestimated by local peaks of the maximum horizontal gradient of pseudo-gravity. both techniques delineate an area of enhanced magnetic signature over the saturated part of the waste site, which may reasonably contain larger volumes of magnetized ferro-metallic (?) materials. clustering of euler deconvolution solutions also highlights possible differences in volumes of magnetized materials between individual waste site areas. hram is envisaged as a good remote approach for rapid waste site screening in terms of location of unmapped or poorly documented waste site areas. once individual anomaly areas have been delineated within a waste site these may be more efficiently selected for very lowaltitude surveying or ground-follow up, if safe, aiming at recognition of potentially hazardous materials in ferro-metallic containers or scrap materials. acknowledgements the authors wish to thank the pilot, engineer and mechanics of elifriulia for providing assistance, the helicopter, fuel and logistic support during the high-resolution aeromagnetic test site over friulia. dr. marco gambetta is acknowledged for his help in test site survey planning and execution. giorgio caneva of the dipartimento per lo studio del territorio e delle sue risorse performed technical installation work of the aeromagnetic rack on board and deployed base stations on the ground. the ufficio tecnico del comune di premariacco kindly sent us a 1:2000 scale map of the surveyed waste site. dr. giorgio macorini of the società italiana per l’oleodotto transalpino s.p.a. provided necessary background information on the oil pipeline. this 231 a high-resolution aeromagnetic field test in friuli: towards developing remote location of buried ferro-metallic bodies research was funded with the finanziamenti a progetti di singoli e/o giovani ricercatori (d.r. 226 del 25.10.2000). discussions with experts in airborne environmental geophysicists such as r. supper of the geological society of austria, d. eberle of bgr, hannover and c. finn of the u.s. geological survey, denver, colorado were highly constructive. in particular, c. finn provided access to many references during f.f’s visit to the usgs. the suggestions of two anonymous reviewers and of m. chiappini improved the clarity and focus of the manuscript. references agip spa (1981): italia, carta magnetica – anomalie del campo magnetico residuo, scala 1:500 000, s. donato milanese (italy). blakely, r.j. and r.w. simpson (1986): approximating edges of source bodies from magnetic or gravity anomalies, geophysics, 51, 1494-1498. blakely, r.j., r.e. wells, t.s. yelin, i.p. madin and m.h. beeson (1995): tectonic setting of the portlandvancouver area, oregon and washington: constraints from low-altitude aeromagnetic data, geol. soc. am. bull., 107 (9), 1051-1062. bozzo, e., a. caneva, g. caneva and s. 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workshop on geoelectromagnetism, programme and abstracts, maratea (italy), p. 83. supper, r., k. motschka and w. seiberl (2000): airborne measurements in italian volcanic regions (vulcano, vesuvio, campi flegrei), in viii workshop on geoelectromagnetism, programme and abstracts, maratea (italy), p. 49. wightman, e., t.b. bakaya, a. kumar and i.h. ngwisanyi (2000): application of airborne and ground geophysical surveys in elucidating the geology and water resources potential of the lower okavango delta, northwestern botswana, url: http://www.terraplus.com/papers/ wight1.htm. yaghoobian, a., g.a. boustead and t.m. dobush (1993): object delineation using euler’s homogeneity equation, sageep, 613-632. 629_647 jodogne 35_1.pdf annals of geophysics, vol. 45, n. 5, october 2002 629 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database jean-claude jodogne and stanimir m. stankov royal meteorological institute of belgium, brussels, belgium abstract presented is a review of the ionospheric storm research carried out at the royal meteorological institute (rmi) of belgium. the studies are based on the opportunities offered by the long-term measurements and the accumulated comprehensive database managed by the rmi geophysics centre at dourbes (4.6°e, 50.1°n). reported are case studies of major storms, and also results in developing forecasting and density reconstruction methods. 1. introduction he ionospheric storm, defined as the set of large perturbations in the geomagnetic field and the ionosphere-plasmasphere system characteristics induced by solar flares and coronal mass ejections, is still a challenging subject of investigation. observations of the ionosphereplasmasphere system response to storms have been well documented for several decades. nevertheless, many questions remain unanswered and many early studies are now being questioned and improved by new discoveries. recently, particular importance has been attribuited to studies of «space weather» (all geospace mailing address: dr. stanimir m. stankov, royal meteorological institute, av. circulaire 3, b-1180 brussels, belgium; e-mail: sstankov@oma.be key words ionospheric storms – synthetic geomagnetic index – electron profile reconstruction phenomenae caused by the radiation and atomic particles emitted by the sun and other stars). traditionally, the geomagnetic storm effects have been investigated by means of vertical radio-sounding measurement techniques (ionosondes) providing long series of data for the ionised atmospheric layers. later, with advances in satellite technology, in situ observations of the plasma composition and behaviour have helped the research. most recently, measurements of the total electron content (tec), both groundand space-based, contributed significantly to the knowledge of the storms’ global manifestation. in addition to the above methods, numerical modelling approaches allow the combination of various observations, better coupling with theoretical investigations, and spreading the information on essential processes and parameters over areas which are not directly observed. the purpose of this paper is to present the ionospheric storm research carried out at the royal meteorological institute (rmi) of belgium. the studies are based on the opportunities offered by the long-term measurements and the accumulated comprehensive database managed by the rmi geophysics centre at dourbes 630 jean-claude jodogne and stanimir m. stankov (4.6°e, 50.1°n). reported are case studies of two major storms in view of some important topics of interest, and preliminary results in developing new forecasting and electron density reconstruction methods. 2. rmi dourbes geophysics centre: history and observations the rmi geophysics centre at dourbes is a complex observational site (fig. 1) consisting of several observatories – ionosphere sounding, atmospheric, geomagnetic, cosmic rays, tec, etc. – all connected with optical-fibre communication lines. a fast link allows communication with the rmi in brussels, the royal observatory of belgium (rob) and the belgian institute for space aeronomy (isa). some information is shared and available via the world wide web (www). first measurements at dourbes started more than half a century ago and a vast amount of data have been accumulated during the years. 2.1. vertical ionospheric sounding the ionospheric soundings are carried out by a digital sounder (dgs-256) developed by the university of massachusetts lowell and featuring the operational characteristics given in table i. the dgs-256 ionosonde automatically calculates the electron density profile assuming a model valey and outputs the coefficients of a chebyshev polynomials expansion. it is very important because it provides an opportunity for comparison of the ionosonde tec (computed using ionosonde measurements) with the gps tec (gps-based observations of tec). 2.2. gps-based total electron content some of the major advantages of using gps to derive tec are: the use of high frequencies (so that ionospheric absorption and geomagnetic field effects on the radio signal are small), electron content evaluated up to a very high altitude (20 000 km, providing the plasmasphere contribution to the content), operational global coverage (e.g., using the international gps geodetic network), cost efficient observations (continuous signals and commercially available receivers). at the rmi-dourbes, a gps receiver is collocated with the digital ionospheric sounder. using the gps signals on two coherent carrier frequencies (l1/l2 = 1575.42/1227.6 mhz), the tec computation procedure (warnant, 1997, 1998) is based on the «geometry-free» combinations of gps code (p i p,gf ) and phase (f i p,gf ) measurements (2.1) where p i p is the code measurement made by receiver p on i – th satellite, f i p is the carrier phase measurement made by receiver p on the i – th p p p f f f f fp gfi p li p li p gfi p li l l p li, , , , , ,/= − = − ( )1 2 1 1 2 2 , fig. 1. the measurement and communication network of the royal meteorological institute of belgium. safir – thunderstorm alert system using radio interferometry. a 2p connection consists of two cables with two optic fibres in each cable. 631 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database satellite, and f l1 , f l2 – the frequencies on the l1, l2 carriers respectively. rewritten as functions of tec, the above equations read (2.2) (2.3) where n i p is the phase ambiguity, tec i p is the slant electron content (along the i – th satellite raypath) in tecu, di, d p are the i – th satellite and p− th receiver differential group delays. the ambiguity is eliminated by the following combination of «geometry-free» code and phase measurements: (2.4) where λ l1 is the l 1 carrier wavelength. the formula requires the estimation of the receiver and satellite group delays, which are obtained from (2.2) after modelling tec by means of a simple polynomial depending on latitude and local time. the conversion to vertical tec is performed by assuming that the ionosphere is a layer of infinitesimal thickness located at a «mean ionospheric» height of 350 km and using a simple cosine function of the zenith angle at the «ionospheric point» (the raypath’s point at the mean ionospheric height). finally, the tec value is calculated from (2.3). to obtain a tec value, representative of the ionosphere above a given observing station, the following is applied: first, selected are all tec values within a latitude difference of 1.5° from the latitude of the observing station, and second, computed is the mean of these tec values over 15 min periods. measurements have been conducted in dourbes on a regular basis since 1994 and a large tec database has been created for the best part of the current solar-activity cycle. p d d p gf i p i p i , .= − × + −( )−1 05 10 17 tec p f d d np gfi l p gfi p i l p gfi, , ,− = −( ) −λ λ1 1 f np gfi pi p gfi, ,.= − × +−5 52 10 17 tec operational parameter range db049 ionogram settings frequency range 0.5 30 [mhz] 1-16 frequency scale linear or logarithmic linear frequency steps 5, 10, 25, 50, 100, 200 [khz] 100 [khz] range resolution 2.5, 5.0, 10.0 [km] 5.0 [km] number of range pixels 128, 256 128 range start selectable 60 [km] pulse repetition range 50, 100, 200 [s– 1] 100 [s– 1] pulse width 66, 133 [µs] 66 [µs] transmitter rf power ≤ 10 [kw] 5 [kw] duration of ionogram selectable 4 [min] phase code interpulse pseudo-random biphase coding digitization 12 bit linear amplitude resolution 0.25 [db] phase resolution 1.4° doppler resolution 4 [hz] or 12 [hz] 4 [hz] wave polarization o/x data display video and paper data storage disk, cartridge table i. the dourbes (db049) digital ionosonde – operational parameters and settings (after reinisch, 1996). 632 jean-claude jodogne and stanimir m. stankov 2.3. geomagnetic measurements from the beginning, the goal of the designers was the best measurements of the geomagnetic induction. the observatory building was erected without any ferromagnetic material even for the external paint. double-core fluxgate magnetometers are used for measurements (primdahl, 1979; jankowski and sucksdorff, 1996). these magnetometers are robust, have reliable electronics, and are capable of recording not only the field variations but its full strength as well. absolute declination, inclination and induction values are manually determined with theodolite, diflux and overhauser protons magnetometer respectively. an optical pumping cesium magnetometer provides data each second. the precision level is 0.1 nt for the induction and 0.001° for the angles. data are sent each hour to the meteosat satellite in the intermagnet format. the large data bank on magnetic support started in 1957. a team uses also cryogenic magnetometers to make paleoand archeo-magnetic studies. the horizontal component is deduced from measurements of the total field (b) and inclination (i ), and their trigonometric relation h = b.cosi. local magnetic observations can be very helpful in many aspects. one particular advantage, concerning the present study, is the opportunity for near real-time detection of geomagnetic storm onset. such detection can be performed by identifying the ssc using observations of the geomagnetic field’s horizontal component variations. the apparent shape of ssc(h) is not a simple increase but rather manifests some variety. morphological ssc studies (matsushita, 1967) reveal that the shape of ssc(h) depends on local time and latitude, and can be sorted in the following major types: i) a small negative impulse preceding the main positive impulse; ii) a main positive impulse alone; iii) an increase (lasting up to about six minutes) followed by a decrease to a level lower than the initial pre-ssc level. however, the ssc detection is not always easy, particularly when performed on a real-time basis. a similar abrupt disturbance without succeeding large changes, called «sudden impulse», can be mistaken for a ssc. 2.4. cosmic rays the interplanetary space is continuously traversed by very energetic nuclear particles called cosmic rays. their origin is not definitely determined − some certainly originate from the sun and other stars, but others are associated with various energetic processes in the galaxy. cosmic rays consist mostly of protons (~ 94%) and helium nuclei (~ 5.5%), with few nuclei of heavier elements (galactic component of the cosmic rays). the particles have a broad energy range, from less than 1 gev (109ev) to more than 1011 gev (1020ev). in interplanetary space, the cosmic ray flux is in the order of 0.6 particles cm– 2s– 1, the energy flux is 5 gev cm– 2s– 1, and the space density – approximately 0.8 × 10– 10 particles cm– 3 (akasofu and chapman, 1972). the high energy of the galactic cosmic rays means that they also have a very high penetrating power. near the earth, their motion is influenced by the geomagnetic field. when penetrating deep into the atmosphere, the cosmic rays collide with the atmospheric constituents and the results are different according to the energy input. protons with energies of about 0.1 gev isotropically scatter the products of the collisions, such as protons, α-particles and neutrons. the paths of these products appear as radial lines from the collision point, forming a «star». if the energy of the protons is of higher order (several gev), then π mesons are also produced, which are scattered forward and their paths appear as a «shower». when the energy of the incident protons is more than 103 gev, the products are scattered into a narrow cone called a «jet». the protons produced collide with new atmospheric particles generating similar reactions. thus, the newly-generated rays/particles permeate the atmosphere and can even reach ground level. eleven-year cycle variation of the cosmic rays is detected, affecting the lower-energy cosmic rays much more than the higher-energy ones. the interplanetary shocks and their drivers (the fast coronal mass ejections) play an important role in the modulation of the cosmic rays. since the earliest observations of the cosmic rays (forbush, 1938) it has been recognized that transient decreases in the cosmic ray intensity (forbush decreases) occur at the times of sudden 633 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database commencement geomagnetic storms and that the cosmic ray intensity varies regularly and inversely to the sunspot cycle, with the ray flux most pronouncedly decreased at high solar activity. therefore, the forbush decreases (produced by large-scale solar-wind disturbances) can be successfully used for investigating ionospheric storms and related phenomena (barnden, 1973; barouch and burlaga, 1975; zhang and burlaga, 1988; cane et al., 1993, 1994; shadrina et al., 2002; gonzalez et al., 2002). at the rmi-dourbes, a 9nm-64 neutrons monitor provides measurements of the secondary neutron component of the cosmic rays on the ground. after pressure correction this component faithfully follows the primary cosmic rays intensity. a large data bank of the hourly intensities has been existing since 1967. 3. case studies of storms in a broad sense, the «ionospheric storm» is defined as the set of disturbances that appear in the ionosphere immediately following a geomagnetic storm commencement (hargreaves, 1992). the mechanisms accounting for the storm effects are composition changes, neutral winds, electric fields, etc. the individual case studies of geomagnetic storms and effects are considered as powerful instruments deepening our understanding of the inter-related processes of energy and momentum transfer, and of the kinetic, chemical, and electrodynamic coupling in the chain of solar-terrestrial relations. the main reason for this is the fact that during ionospheric storms the ionospheric processes are intensified and a phenomenon normally occurring over a long period is observed during a relatively short time period. two major topics were investigated: the neutral winds effect on the ionosphere and the electrodynamic mechanism. the global manifestation of the neutral wind effect starts at high latitudes where the perturbations in the interplanetary medium and the magnetosphere yield increased energetic particle precipitation, convection electric fields and associated joule heating. the joule heating raises the temperature of the thermosphere at auroral latitudes resulting in enhanced equatorward winds and travelling ionospheric disturbances in all local time sectors. on the night-side the poleto-equator wind field is enhanced. on the dayside, the equatorwards wind counteracts (and often overpowers) the solar-driven poleward winds. as a result of the air drag, the day-time f-region experiences upward motion and an ion density increase. this contributes to the «positive phase» in the critical frequency and total electron content values, whose increase is usually observed on the first day of the storm. large-scale structures of the earth’s plasma environment are determined by electrodynamic processes of magnetospheric and solar wind origin. for example, the electrodynamic drift can redistribute plasma to regions of either high or low chemical loss. moreover, the e × b effect of the electric fields mapped down the geomagnetic field lines during storms is instantaneous. unanswered questions are: the exact interplay of processes during the positive phase, the degree to which magnetospheric convection effect can penetrate to midlatitudes, coupling of atmospheric regions and processes, etc. (szuszczewicz et al., 1998; namgaladze et al., 2000; foerster and jakowski, 2000). geomagnetic storms are predominantly investigated using magnetic, ionosonde, and gpsbased measurements, all available at the dourbes geophysics centre. the geomagnetic activity characterisation is based on evaluation of the geomagnetic disturbances with two major types of indices in use – the k and dst indices (menvielle and berthelier, 1991). the ionosonde measurements are important not only with the critical frequency ( f 0 f 2 ) measurements but also with the peak density (n m f 2 ), peak height (h m f 2 ), and propagation factor m3000f2: a) the f0f2 values respond quickly to the storm, and together with tec measurements, they can provide a crucial information on the slab thick-ness. b) the n m f2 values will be used for calculating the slab thickness τ, where τ [km] = 806.405 tec [1016m– 2]/f 0 f 2 2 [mhz]. this parameter can provide information on the plasma uplifting processes, plasmasphere fluxes, etc. c) the h m f2 will give more information on the profile and the f 2 layer response to the storm, particularly in connection to the uplifting process. 634 jean-claude jodogne and stanimir m. stankov the tec measurements are used together with the ionosonde measurements. no storm studies seem possible nowadays without the tec value, especially after the introduction of the gps system providing opportunities for a global coverage. 3.1. the storm on 10th january 1997 as a first example, we present the study of the storm on 10th january 1997, which has been well documented on a global scale because of the pre-observed and announced large coronal mass ejections on 6th january 1997 (jakowski et al., 1999; mikhailov and foerster, 1999). this storm is characterised by a pronounced positive response from both tec and f 0 f 2 parameters – for example, the noon tec values are more than 100% higher than the monthly median (fig. 2c). the basic storm features are summarised in table ii. several aspects of the ionospheric response can be investigated from dourbes and it will be demonstrated on this particular storm. the positive storm phase is generally accepted to be induced by the strong southward thermospheric winds. the most significant consequence is the plasma uplifting effect – the strong meridional winds push the f2-layer plasma upward, leading to reduced f2-layer peak density and increased h m f 2 and tec. this effect can be easily detected from measurements through the slab thickness shape parameter τ; increasing slab thickness (e.g., rising tec accompanied by constant or decreasing f0f2) indicates plasma uplifting. a clear demonstration of this process is detected in the 09:00-10:00 ut period when tec is rising, f0f2 is almost unchanged, and hmf2 also increases (fig. 2c,d). the wind-induced uplifting takes place along the geomagnetic field lines, and is therefore most efficient near the 45° geomagnetic inclination. situated at 50.1°n, the dourbes geophysics centre provides an excellent database for investigating the above-mentioned effect. other storm effects, the plasma fluxes from the plasmasphere, are due to the compression of the geomagnetic field during the onset phase of the storm (foerster and jakowski, 2000) and/or inter-hemispheric flows (menietti et al., 1988). such plasmaspheric fluxes can be easily detected: e.g., a high slab thickness value during nighttime is certainly an indication of an enhanced plasma influx from above, and this is exactly the situation observed near 02:00ut (fig. 2d). the next effect of interest is associated with the generated convection electric field. the sharp fig. 2a-d. ionosphere-plasmasphere response to the 10th january 1997 geomagnetic storm, as observed at the site of the dourbes ionosonde station db049 (4.6°e, 50.1°n). a b c d 635 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database decrease of the slab thickness after 02:00 ut shows the reaction to this field. there is no unique explanation for the decrease; a possible explanation is the occurrence of zonal disturbance electric fields. a westward electric field moves the plasma from higher to lower l-shells with smaller volumes. thus, the plasma pressure in the flux tubes is increased, causing an enhanced plasma flux from the plasmasphere down to f2 region altitudes. an eastward electric field results in the opposite effect of plasma decompression so that the plasmaspheric flux decreases (mikhailov and foerster, 1999). the ionospheric storms can be much better investigated on a global scale. for the purpose, the rmi and rob jointly upgrade the existing gps tec measurements to provide the tec values at practically every point in the european region. the propagation velocity of the perturbation is one of the important storm characteristics that can be investigated in this way. some authors (hernandez and roble, 1978; proelss et al., 1991; proelss, 1997; jakowski et al., 1999; foerster and jakowski, 2000) estimate this velocity in the order of 400-900 ms– 1. theoretically, due to the reduced ion drag on the night side, the propagation should be much faster than on the day side. it is possible to estimate the time delayed ionospheric reaction using simultaneous gps tec and ionosonde data. for example, if we observe the exact time of the day-time tec peak (fig. 2c) at various geomagnetic latitudes near 0°e, we find that the peak is observed at 65°n in 11:30 ut, at 50°n (dourbes) in 12:30 ut, at 35°n in 13:00 ut. thus, it obviously takes about 90 min for the plasma uplifting to move from 65°n to 35°n, which corresponds to a wind velocity of approximately 600 ms– 1. it is of crucial importance to have higher resolution data (more frequent measurements) during storm times. for example, if we work with hourly data, the above peak would be observed in 11:00, 12:00, 13:00 ut, hence the time delay would be 120 min resulting in a different wind velocity estimate. at the dourbes geophysics centre we are able to provide 15 min sampling rate for gps tec and even better rate for the digital ionosonde. thus, the tec peak is observed at 12:30 ut which ensures better estimation of the propagation velocity. 3.2. the storm on 15th july 2000 another example is the geomagnetic storm observed in the period 15th-17th july 2000. the ssc is recorded at 14:35 ut and follows a disturbed period starting approximately 24 h earlier. it is a very intense storm (fig. 3a,b) showing a pronounced negative response in both the tec table ii. summary of the 10th january 1997 storm characteristics. feature value comments storm onset (ssc) 01:00 ut 01:00 ut, 10/1/1997 dst (min) – 78 nt 09:00 ut, 10/1/1997 total storm period 10 h 08:00 ut (10/1/1997) – 17:00 ut (10/1/1997) (dst ≤ – 50 nt) initial plus main phase 8 h 01:00 ut (10/1/1997) – 09:00 ut (10/1/1997) (ssc → dst (min)) main phase 1 h 08:00 ut (10/1/1997) – 09:00 ut (10/1/1997) ( – 50 nt → dst (min)) recovery phase 9 h 09:00 ut (10/1/1997) – 18:00 ut (10/1/1997) (dst (min) → – 50 nt) 636 jean-claude jodogne and stanimir m. stankov and f 0 f 2 values. around 03:00 ut on 16th july 2000, the relative differences increase to more than – 80% for tec and – 50% for the critical frequency (fig. 3d). the most important storm characteristics are given in table iii. at high latitudes, the negative phase is attributed to the increased molecular concentrations of the thermosphere, enhancing the recombination processes and reducing the electron density. the standard explanation for a negative phase at middle and low latitudes (szuszczewicz et al., 1998) is the enhanced equatorward wind system advecting the modified thermospheric composition to lower latitudes. this equatorward penetration is deeper at night-time, and theoretically, the corotation, coriolis forces, and zonal winds transport the modified thermospheric composition also to the day-time hemisphere and maintain the negative storm phase. elements which complicate this simple picture involve: penetrating electric fields of magnetospheric origin, dynamo-driven fields, vertical and horizontal interactions between the wind system, thermospheric composition, ionospheric plasma, storm intensity, storm onset time (and associated local times in different longitude sectors), variations in pre-storm conditions, evolution of the controlling interplanetary-magnetospheric inputs, etc. this storm is interesting in that it reveals another phenomenon, the so-called «ionogram g condition». under g conditions, a measurement is influenced or impossible because the ionization density of the layer is too small to deduce the measurement value accurately. such conditions occur during ionospheric disturbances associated with geomagnetic storms. then, the electron density in the f 2 layer becomes equal to or lower than the electron density in the f 1 layer. in the result, f0f2 is expressed by the value of f0f1 with the descriptive letter g. this is exactly the case on 16th july 2002 (fig. 4a): f 0 f 2 is equal to f 0 e during the day, except for a few hours (15:00-17:00 lt) in the afternoon. however, it is important to separate the g condition from two other confusing cases. the first case is when the f 2 trace disappears due to either interference or attenuation. the other case is when the trace cannot be recorded because of its extremely high virtual height exceeding the height range limit (descriptive letter w ). this is obviously what happens from 21:00 ut on 15th july 2000 (h’f2 = 520 km) to 03:00 ut on 16th july 2000 (h’f2 = 513 km) when h’f2 is above these values and above the height range limit (fig. 4b). in all of the above cases, the automatic ionogram scaling may fail to properly identify the f0f2 and hmf2 values, may not produce electron density profiles, or deliver profiles which can be quite different from the real ones. the fig. 3a-d. ionosphere-plasmasphere response to the 15th-17th july 2000 geomagnetic storm as observed at the site of the dourbes ionosonde station db049 (4.6°e, 50.1°n). a b c d 637 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database associated itec (ionosonde-based tec) calculations, derived from such erratic profiles, can be very misleading. concurrent gps-based tec measurements can be extremely useful. on the one hand, tec measurements provide opportunities for identification and correction of ionosonde measurements under such g/w conditions (fig. 3c,d). on the other hand, the tec values can be used to reconstruct the electron profiles using the reconstruction method described in the following sections. 4. synthetic index of local response to the geomagnetic activity the response of a given ionosphere characteristic f (like f 0 f 2 , m 3000 f 2 , or tec) is better analysed through the relative deviation, f rel = = (fmeas – fmed)/fmed, of the actual observed characteristic from its median value, fmed. the response strongly depends, in a highly non-linear fashion, on geomagnetic indices such as the planetary indices a p and k p (menvielle and berthelier, 1991). for developing geomagneticallycorrelated short-term forecasting methods, a new synthetic index is required (stankov et al., 2001; muhtarov et al., 2002; and the references therein). this new synthetic index is actually a function showing the frel dependance on the geomagnetic index (say k p ) used for the forecast. however, the new synthetic index (function) fig. 4a,b. ionosonde measurements during the 15th 17th july 2000 geomagnetic storm recorded at the dourbes ionosonde station db049 (4.6°e, 50.1°n). table iii. summary of the 15th july 17th july 2000 storm characteristics. feature value comments storm onset (ssc) 14:35 ut 14:35 ut, 15/7/2000 dst (min) – 300 nt 21:00 ut, 15/7/2000 total storm period 49 h 17:00 ut (15/7/2000) – 18:00 ut (17/7/2000) (dst ≤ – 50 nt) initial plus main phase 6.5 h 14:35 ut (15/7/2000) – 18:00 ut (15/7/2000) ( ssc → dst (min)) main phase 4 h 17:00 ut (15/7/2000) – 21:00 ut (15/7/2000) (– 50 nt → dst (min)) recovery phase 45 h 21:00 ut (15/7/2000) – 18:00 ut (17/7/2000) (dst (min) → – 50 nt) a b 638 jean-claude jodogne and stanimir m. stankov demonstrates strong seasonal and spatial variability and should be determined for each measurement site separately. at dourbes, there are two types of measurements that can be used for the purpose – ionosonde ( f 0 f 2 ) and gps tec measurements. 4.1. the f 0 f 2 response to geomagnetic activity the f0f2 response to geomagnetic activity was first deduced from f0f2 measurements carried out at dourbes over the period from july 1957 to june 1989. it is calculated in the following manner. all values of the f0f2 relative deviations in a given month of the year are sorted according to the three-hour values of the planetary index kp, which index is recorded in step values of 0.00, 0.33, 0.67, 1.00, 1.33,..., 8.67 and 9.00. then, for each level of k p and each month, the basic statistics are calculated – average, standard deviation, data number, etc. taking into account the expected seasonal variations in the ionospheric response to geomagnetic storms (fullerrowell et al., 2000), calculations were made and the results presented in fig. 5. the top four panels show the relative deviations of f 0 f 2 for the months of january, april, july, and september; the contour plot at the bottom shows the detailed annual behaviour of the averages. the first important observation was the distribution of the zero value of the f 0 f 2 relative deviation ( f0f2rel = 0), i.e. when f0f2meas = f0f2med. this is the value representing the «average» magnetically quiet conditions. it is supported by the fact that the majority of the observations cluster at and around the median. it becomes obvious from the bottom contour plot that frel = 0 is observed for k p = 2.67 except for the winter months when f rel varies between the higher values of 3.00 and 4.00. considering the definition of a quiet magnetic activity (0 < k p < 4), it follows that even within this interval a trend exists in the observations of frel: a generally positive response for k p < 2.67 and negative response for kp > 2.67. the second important fact is that the response of f0f2 under intense geomagnetic activity (k p > 4) is generally negative, i.e. during storms the measurements are below their monthly medians. this negative response is particularly strong near the equinoxes (april and september) when the decrease reaches 25-30% during severe geomagnetic storms (k p > 7). on the other hand, the maximum positive values of f rel are also observed during april and september for small values of k p . 4.2. the gps tec response to geomagnetic activity it is interesting to compare the critical frequency response with the response of the gps tec. the gps tec time-series data available at dourbes (july 1994 october 2001) are not so long as the f0f2 series, and data scattering is rather large, but some important differences have already been detected. the tec relative deviations (tec rel) have been sorted in the same way as the f0f2 deviations and the results are provided in fig. 6. again, the top four panels show the tec relative deviations for the months of january, april, july, september, while the contour plot details the entire annual behaviour. for the gps-based tec measurements, our calculations show that the tec response is generally stronger and much more complex than the f0f2-based observations. first, the «quiet» behaviour, tec rel = 0, is detected at lower k p values of around 2.0, which is a sign for a stronger response to the geomagnetic activity. second, it is obvious that, for quiet magnetic conditions (k p < 4), the dependence of the relative tec on k p is similar to that of f 0 f 2 . third, for higher values of the k p index, the relative tec stops decreasing and in many cases increases significantly particularly during winter months. at this stage of investigation, the nature of this positive response is attributed mainly to the prolonged positive phase in the tec storm-time behaviour. it is also clear that in november and december the increase of tecrel during storms can be as much as 20-25%. there are indications that such an increase can be expected also during january and even february. alternatively, pronounced decreases are observed in both april and september for values of k p > 4, although not as strong as in the f 0 f 2 case. 639 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database fig. 5. the f 0 f 2 relative deviations ( f 0 f 2rel ) response to geomagnetic activity for the site of ionosonde station db049 (4.6°e, 50.1°n) based on data from the 1957-1989 period. the average values of f 0 f 2rel (dashes) are approximated by 3−rd degree polynomials (solid lines). the vertical bars represent the standard deviations (doubled), and circles represent the data number (scaled on the right-hand axis). 640 jean-claude jodogne and stanimir m. stankov fig. 6. the gps tec relative deviations (tec rel ) response to geomagnetic activity for the site of ionosonde station db049 (4.6°e, 50.1°n) based on data from the 1994-2001 period. the average values of tec rel (dashes) are approximated by 3−rd degree polynomials (solid lines). the vertical bars represent the standard deviations (doubled), and circles represent the data number (scaled on the right-hand axis). 641 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database however, more gps tec measurement data and further analysis are required to better evaluate the tec response to stronger geomagnetic activity, including the local time variation of the storm effects (proelss, 1993). statistical studies show (davies, 1990) that, especially in middle latitudes, a geomagnetic storm follows different patterns depending on the local time of the sudden storm commencement (ssc). on the one hand, a geomagnetic storm in which the ssc occurs during daytime generally produces a tec increase. on the other hand, the tec during a storm in which the ssc occurs at night-time either goes directly to the negative stage, or increases for a short period during the afternoon of the following day. nevertheless, the case gps tec degree month 0 1 2 3 january 4.8198 × 10– 2 – 2.2835 × 10– 2 – 0.3204 × 10– 2 0.1971 × 10– 2 february – 1.9767 × 10– 2 4.0903 × 10– 2 – 1.4147 × 10– 2 0.1828 × 10– 2 march 6.5163 × 10– 2 – 7.4277 × 10– 2 2.5464 × 10– 2 – 0.2299 × 10– 2 april 3.8189 × 10– 2 0.9663 × 10– 2 – 1.4048 × 10– 2 0.1399 × 10– 2 may 1.7888 × 10– 2 – 1.0004 × 10– 2 – 0.3160 × 10– 2 0.0603 × 10– 2 june – 3.4079 × 10– 2 8.8947 × 10– 2 – 3.7321 × 10– 2 0.3786 × 10– 2 july 7.1668 × 10– 2 – 7.1640 × 10– 2 2.6913 × 10– 2 – 0.2537 × 10– 2 august 1.3113 × 10– 2 – 1.9644 × 10– 2 1.1797 × 10– 2 – 0.2037 × 10– 2 september 4.8579 × 10– 2 – 4.0954 × 10– 2 1.3645 × 10– 2 – 0.1542 × 10– 2 october 7.6576 × 10– 2 – 1.5230 × 10– 1 6.0279 × 10– 2 – 0.5746 × 10– 2 november – 4.0179 × 10– 2 – 1.4980 × 10– 2 2.1028 × 10– 2 – 0.1998 × 10– 2 december – 0.8146 × 10– 2 2.1755 × 10– 2 – 0.2223 × 10– 2 0.0500 × 10– 2 f 0 f 2 degree month 0 1 2 3 january 2.4165 × 10– 2 – 2.5847 × 10– 2 1.2986 × 10– 2 – 1.8028 × 10– 3 february – 1.0463 × 10– 3 2.4421 × 10– 2 – 6.4418 × 10– 3 – 1.4525 × 10– 4 march 2.4913 × 10– 2 1.0363 × 10– 2 – 8.2630 × 10– 3 3.6672 × 10– 4 april 4.7779 × 10– 2 – 9.7067 × 10– 3 – 2.2601 × 10– 3 – 2.1101 × 10– 4 may 4.5731 × 10– 2 – 1.6177 × 10– 2 – 1.4128 × 10– 3 – 1.8441 × 10– 4 june 4.3341 × 10– 2 – 3.1992 × 10– 2 7.2534 × 10– 3 – 1.0285 × 10– 3 july 3.6014 × 10– 2 – 1.8669 × 10– 2 2.7797 × 10– 3 – 5.9729 × 10– 4 august 4.2705 × 10– 2 – 2.9214 × 10– 2 7.4882 × 10– 3 – 1.1622 × 10– 3 september 6.0539 × 10– 2 – 4.4463 × 10– 2 1.1422 × 10– 2 – 1.5466 × 10– 3 october 3.6506 × 10– 2 – 2.6692 × 10– 2 7.4757 × 10– 3 – 1.2045 × 10– 3 november – 1.1878 × 10– 2 3.6587 × 10– 2 – 1.0752 × 10– 2 4.7759 × 10– 4 december – 5.0906 × 10– 3 2.2692 × 10– 2 – 6.7083 × 10– 3 3.3266 × 10– 4 table iv. coefficients for the 3– rd degree polynomial approximation of the average monthly response to geomagnetic activity; based on f0f2 (top table) and gps tec (bottom table) measurements at the site of the dourbes ionosonde station db049 (4.6°e,50.1°n). 642 jean-claude jodogne and stanimir m. stankov studies presented here reveal that there are storms which do not follow the common pattern proving the necessity of further investigations of such a complex issue. 4.3. synthetic index – approximation formulae for the local ionospheric response considering the above analysis, it is possible to deduce simple analytical expressions of the f0f2 and tec average response to the activity on a local basis. such expressions can be used as a new synthetic index replacing the traditional a p and kp indices. the purpose of creating such index is the necessity to «linearize» the relationship between the relative deviations (frel) of the ionospheric characteristic and the standard geomagnetic index (e.g., kp), which is absolutely necessary for a reliable forecast based on the same characteristic. it is known that the mutual correlation between two random variables is highest when they are linearly dependent (muhtarov et al., 2002). new analytical expressions, using thirddegree polynomials, have been deduced for the synthetic index of local ionospheric response to geomagnetic activity. the polynomial coefficients are given in table iv for the site of the dourbes ionosonde station. 5. reconstruction of electron density profiles many ionosphere-plasmasphere processes, like the ionospheric storms and propagation delays in gnss, can be estimated and corrected much more easily if the (vertical) electron density profile is available at a given location on a realtime basis. as the theoretical ionospheric models are not sufficiently good for the purpose, actual on-line measurements immediately available should be considered. 5.1. electron profiles deduced from ionosonde measurements the dgs-256 ionosonde automatically calculates the electron density profile assuming a model valey and outputs the coefficients of a chebyshev polynomials expansion (huang and reinisch, 1996, 2001). the deduced electron density profile is very important as it provides opportunity to compare the tec computed using ionosonde measurements with the gps tec available at the same site. the ionosonde total electron content (itec) is computed in the following manner (warnant and jodogne, 1998). the bottomside vertical electron density profile is obtained after standardized inversion of the h’( f ) traces on vertical incidence ionograms with an accuracy of about 10 km as long as the profiles are monotonic. as the topside profile cannot be measured by the ground-based ionosonde, it is modelled with the β-chapman function from the peak height up to the gps satellite height of 20 200 km (5.1) where z = (h – h max )/h t and h t is the top-side scale height. the topside scale height is calculated as the mean of the scale heights computed at three points of the bottom-side profile located at 88%, 90%, and 92% of the nmax. the itec is obtained after numerically integrating the electron profile. for the low solar activity period (1995-1996), the estimated mean and standard deviations of the (tec-itec) distribution are 0.46 and 1.72 [1016m– 2], respectively. 5.2. electron profiles deduced from ionosonde and gps tec measurements a new technique has been recently developed (stankov and muhtarov, 2001, stankov et al., 2002a) for retrieval of electron density profiles from concurrent gps tec and ionosonde data. the ionosonde measurements are used primarily for obtaining the bottom-side profile. ionosonde data, together with simultaneously-measured tec, are required for the topside electron profile determination. the unique determination of the top-side profile also depends on the availability of the o+-h+ transition level (upper transition level, utl) value. the following «reconstruction» formula is proposed for calculation of the top-side (h > h m f 2 ) electron density profile: n h n z z t ( ) = − − −( )( )maxexp exp1 643 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database (5.2) where h o+ and h h+ are the top-side o+ and h+ scale heights, n o+ (h m f 2 ) and n h+ (h m f 2 ) are the o+ and h+ densities at the f2-layer peak height (h m f2 ), and sech(h) = 1/cosh(h), cosh(h) = = 0.5(exp(h) + exp(– h)). several assumptions have been made: first, the height of the o+ density maximum is equal to the height of the h+ density maximum; second, the neutral particles and ions have the same scale height; third, the hydrogen and oxygen ion temperatures are equal. there are four unknown variables in the proposed formula: the ion densities n o+(h m f 2 ) and nh+(hmf2), and the ion scale heights ho+ and hh+. however, the number of unknown variables can be decreased by considering that h h+ ≈ 16.h o+ following the definition of scale height. therefore, the next system is constructed to determine the remaining three unknowns (5.3) (5.4) (5.5) the first equation represents the principle of plasma quasi-neutrality and the fact that at the peak height, the only ion densities that really count are the oxygen and hydrogen ion densities. the second equation denotes the fact that the hydrogen and oxygen ion densities are equal at the o+-h+ transition level (h tr ); the level is obtained from a model based on satellite and rocket measurements (stankov, 2002). the third equation is obtained after integrating the proposed n e (h) «reconstruction» formula (5.2) from h m f 2 to infinity. the above system (5.3)(5.5) is solved numerically. 5.3. exemplary reconstruction results the reconstruction is demonstrated on actual gps tec and digital ionosonde data obtained at the rmi geophysics centre and using its measurements and communication network (fig. 1). the deduced electron density distribution is plotted in fig. 7 (top panel). the required input values of gps tec, f0f2, f0e, m3000f2, utl and h m f 2 are given in the middle and bottom panels of fig. 7. the tec and f 0 f 2 show stronglycorrelated diurnal behaviour; both quantities increase sharply in the early morning, reach their absolute maximum just before noon, and then start gradually decreasing. relatively high values are maintained throughout the afternoon, followed by a 10% increase in the early evening. after that, both tec and f0f2 fall rapidly to their corresponding absolute minima observed at 02:00-03:00 lt. in the bottom panel, the f 2 peak-density and o+-h+ ion transition level are also provided. the level, starting from 680 km at midnight, increases up to 1000 km at noon and then decreases in a symmetrical fashion during the second half of the day. the h m f2 has highest values at midnight (around 400 km) and lowest values during day (varying between 300 and 330 km). notice the detailed density distribution above the h m f 2 . it is easy to detect the changes in the calculated topside scale height and resulting density distribution as they develop in time. 5.4. a note on the operational reconstruction recently, an operational procedure has been developed for real-time reconstruction of the vertical electron profile at a single ionosonde location (stankov et al., 2002b). in general, it is a stand-by procedure; its execution is triggered by either time control system n h n h f h h f h e m m( ) = ( ) − ++ + o o sech2 2 2 2 ( ) . n h f h h f h m m+ ( ) − h h sech+ + 2 2 2 2 ( ) . tec o o o ht m mh n h f h n h f= ( ) + ( )+ + + +2 322 2. . . . . n h f n h f n fm m mo h+ +( ) + ( ) =2 2 2 n h f h h f h m tr m o o sech+ + ( ) − =2 2 2 2 ( ) . n h f h h f h m tr m h o sech+ + = ( ) − 2 2 2 32 ( ) . 644 jean-claude jodogne and stanimir m. stankov fig. 7. diurnal behaviour reconstruction: reconstructed vertical electron density (log scale, m– 3) (top), actual gps tec and ionosonde measurements (middle), and model-based utl and h m f 2 (bottom). 645 ionosphere-plasmasphere response to geomagnetic storms studied with the rmi-dourbes comprehensive database or new measurements. thus, it relies heavily on regular discrete influx of (ionosonde and tec) measurement data. both types of observations should be synchronized, so representative data are obtained for given location and time. highest flexibility, in terms of time resolution, is offered by the digital ionosonde – a new block of measurement data is available within a delay of about 5 min. a longer delay is expected for receiving the gps tec value because the tec derivation procedure (warnant, 1997, 1998) requires time and a sufficient number of measurements to evaluate the hardware (receiver plus satellite) biases and detect possible irregularities (tid and scintillation effects). in practice, a tec value can be obtained every 15 min, which is sufficient for most applications. several distinct stages are observed in the operational reconstruction procedure: transmission of measurement data and retrieval of input parameters, construction of the bottomand topside electron profile, backup and display of results. the measurement data are transmitted using the file transfer protocol; the utl values are provided by the empirical formulae (kutiev et al., 1994) incorporated into the reconstruction software. if the tec value is not available on time, it is possible to use the ionosonde-based tec value. the final stage of the operational procedure is to store and display the results. as said before, the next round of calculations can be triggered by time control or the arrival of new measurements. 6. conclusions presented was a review of some important ionospheric response studies carried out at the dourbes geophysics centre of the royal meteorological institute. the studies were carried out with the help of the local comprehensive database. three major topics of importance to the ionospheric response investigations were covered – case studies of storms, development of local geomagnetic activity indices for use in the shortterm forecasting, and reconstruction of the vertical electron density distribution. according to each of the above subjects, the following conclusions can be made: ionospheric storm studies: – the storm studies are much more reliable and efficient if combined ionosonde and gps tec measurements are used. – higher resolution data (e.g., every 15 min) are required for a better understanding of the dynamic ionospheric storm behaviour on a global scale. – the local measurements of the horizontal geomagnetic field component can be used successfully as a reference for storm onset and subsequent main and recovery phases. short-term forecasting of f 0 f 2 and tec: – the development of local synthetic geomagnetic activity indices is of crucial importance to the f0f2 and tec short-term forecasting. such indices are local storm-response characteristics of high precision. – differences are observed between the f0f2 and gps tec behaviour response to intense magnetic activity (k p > 4), mainly with the detected positive response of tec during winter in contrast with the all-negative f0f2 relative deviations. electron profile reconstruction: – a recently-developed electron density reconstruction technique can be successfully utilized in investigating the local storm-time ionosphere development. – a new operational procedure is being developed, which will provide the reconstructed electron density on a near real-time basis. – an advantage of this method is its applicability on a global scale through the evergrowing gps tec and ionosonde measurements network. in summary, the overall evaluation of the dourbes measurement database proved a reliable research reference that will help to accomplish the above-stated goals. acknowledgements this research was financially supported by the belgian federal office for scientific, technical and cultural affairs and by the royal me646 jean-claude jodogne and stanimir m. stankov teorological institute of belgium. the authors thank dr. 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(received april 10, 2002; accepted october 15, 2002) 563_573.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 563 ground magnetometric surveys and integrated geophysical methods for solid buried waste detection: a case study marco marchetti, lili cafarella, domenico di mauro and achille zirizzotti istituto nazionale di geofisica e vulcanologia, roma, italy abstract the detection of illegal buried waste by means of geophysical techniques has recently become a major effort in shallow geophysical investigations. in particular, detection and location of underground metallic storage tanks can be accomplished using different instruments and techniques. in this paper we describe the results of an investigation carried out in a tuff quarry in riano flaminio (north rome, italy). a preliminary magnetometric survey revealed the existence of anomalous zones in the analysed region. excavation in some of the selected areas confirmed that the anomalies were generated by underground magnetic material: over 160 steel drums were found. after their removal, a new magnetometric survey was performed. on the basis of the new map, a multifrequency induction survey, a geoelectrical profile and gpr measurements were taken to extend the characterization of the subsoil. 1. introduction the ever-increasing interest in environmental problems has recently spurred the scientific community to improve geophysical methods aimed at the detection of illegal waste disposal sites. in particular, detection and location of buried steel storage tanks can be done through several geophysical methods (pierce and de reamer, 1993; foley, 1994; vogelsang, 1995; gibson et al., 1996; marchetti et al., 1998). generally in these cases the magnetic surveys are those most frequently used allowing a quick detection of underground metallic refuse. typically, such method consists of two different kind of measurements: the total intensity of the local magnetic field and its vertical gradient. moreover, detection of metallic buried waste using magnetometric surveys is advantageous for the low cost of its non invasive execution, and the rapidity of data collection and progressing. the exploration and localization of hazardous, toxic buried waste with only magnetic signature, depends on the depth of the target and on the material the hidden body is made of, since a nonmagnetic body is completely transparent to detection by this kind of investigation. this limitation can be removed if other techniques such as active electromagnetic, gpr (ground penetrating radar) and geoelectrical methods, based on different physical properties of the inspected matter, are used. this paper describes different investigation techniques used in an old tuff quarry at riano flaminio (north of rome) where the presence of mailing address: dr. marco marchetti, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: marchettim@ingv.it key words environmental pollution geophysical surveys buried waste magnetic anomalies 564 marco marchetti, lili cafarella, domenico di mauro and achille zirizzotti buried toxic waste in the subsoil was suspected by the «government waste recycling committee». thanks to the first magnetic investigation, many steel drums were found in some points of the quarry. after a preliminary reclaiming, a multifrequency induction survey, a geoelectrical profile and gpr measurements were carried out in order to characterize the subsoil in the study area. 2. magnetic measurements istituto nazionale di geofisica e vulcanologia in rome has been involved for some years with the italian government waste recycling committee (ufficio di presidenza della commissione parlamentare d’inchiesta sul ciclo dei rifiuti e sulle attività illecite ad esso connesse) in performing geophysical investigations in order to check the presence in the subsoil of buried toxic waste; one of the latest investigations was undertaken in an old tuff quarry at riano flaminio (north of rome, central italy, fig. 1). as is known, magnetometers can be successfully used to detect ferromagnetic targets because they can measure the superposition of the earth’s magnetic field and the induced magnetic field in the targets, that depends on their volume magnetic susceptibility. in this way, local variation in the geomagnetic field can be used to identify and to locate the presence of buried ferromagnetic bodies. shallow exploration to locate and quantify buried hazardous and toxic waste filled drums with magnetic signature can then be advantageously performed by this kind of survey (for an example of a test site see marchetti et al., 1998) especially for the low cost and rapidity of collection and data elaboration. of course the success of magnetometric measurements essentially depends on the depth of the target, since the resolution of potential field data decreases rapidly with distance from the sources and depends on the material a hidden body is fig. 1. location of study area. riano flaminio is a small town near rome (central italy). extracted from the italian geomagnetic total field map at year 2000.0 (coticchia et al., 2001). 565 ground magnetometric surveys and integrated geophysical methods for solid buried waste detection: a case study 1000-1500 nt. they reveal the presence of some ferromagnetic bodies in the subsoil. the diggings up to 4 m in depth corresponding to the anomalies brought to light over 160 rusty steel drums containing toxic waste, dangerous materials and different kind of refuse (photo 1). the drums were mainly placed in horizontal position, and they have already been recovered at a starting depth of 1.0-1.5 m from ground level. after a first reclamation of this area, the original morphology was restored. some months later, a new magnetic survey in the eastern sector was performed using a optical pumped caesium magnetometer, geometrics mod. g-858. the new survey was carried out along profiles 3 m apart with a sampling step of 1 m on a more restriced area of 5600 m2 (fig. 2). the magnetometer was used in the gradiometer configuration (two sensors mounted on a vertical staff at a distance of 0.8 m apart) in order to measure the vertical gradient of the magnetic field and the intensity of the field. a new magnetic anomalies map (total field intensity) was drawn and the result is shown in fig. 4a. from the comparison of the new map with the previous one, a strong decrement of anomalies due to the absence of the ferromade of. other techniques based on different physical properties of the inspected matter are also useful to detect different kind of waste (gilkeson et al., 1992; cochran and dalton, 1995; dahlin and jeppsson, 1995). in order to ascertain the existence and location of underground buried storage tanks in an old quarry (of about 10 000 m2 ), a magnetic survey was planned. this area is located at riano flaminio, north of rome (fig. 1). magnetic data were collected using a geometrics g-856 proton precession magnetometer. 36 profiles spaced 3 m, with a sampling step of 1 m along each line were executed. on the whole, about 1500 measurements were made (fig. 2). the diurnal variation of the geomagnetic field and the electromagnetic noise produced by the near railway line were monitored with a second identical magnetometer set close to the area. it was considered a reference station to perform the magnetic data reduction. figure 3 shows the map of the magnetic anomalies of the geomagnetic total intensity field. five anomalous areas clearly emerge: dipolar magnetic anomalies with the main axis n-s oriented, reaching an intensity range of about fig. 2. general scheme of the different geophysical surveys carried out on the whole analysed area. 566 marco marchetti, lili cafarella, domenico di mauro and achille zirizzotti photo 1. some steel drums dug out after the first magnetometric survey performed at riano flaminio (north of rome, italy). fig. 3. first map of the magnetic anomalies of the total intensity field on the whole area. 567 ground magnetometric surveys and integrated geophysical methods for solid buried waste detection: a case study fig. 4a,b. new magnetic anomaly map of the total intensity field (a) and the magnetic vertical gradient map (b) after the removal of 160 steel drums on the restricted sector (about 5600 m2). a b 568 marco marchetti, lili cafarella, domenico di mauro and achille zirizzotti magnetic material removed from the subsoil is clear. on the other hand, some anomalies enhanced, as in the case of the anomaly identified by the letter a in fig. 4a, where no excavation was made. anomaly b is reduced to about 380 nt with respect to the corresponding anomaly in fig. 3. anomaly c is about 600 nt while anomaly d reaches about 1000 nt. the gradient data are especially useful for detecting objects buried at shallow depth (breiner, 1973). figure 4b also shows the vertical gradient of a sector of the area: this map discloses some details of the anomalies of the geomagnetic field. as a matter of fact, anomalies a and c are more evident than in the total intensity map whereas b and d are very weak. as we have pointed out before, the deeper a given 3d source is, the lower the gradient measurements. the differences between the two maps can be explained if we assume that the bodies responsible for the b and d anomalies are deeper than the a and c ones. in particular d seems to be divided in two parts: this is probably due to the presence in the subsoil of two close masses of ferromagnetic waste. starting from the vertical gradient and from the anomalies of the total field intensity, we can obtain an approximation of the depth of the sources associated with their anomalies, z = 3t/(dt/dz), where t (nt) is the geomagnetic intensity and dt/dz the value of the vertical gradient. in our case we estimated a depth, for the ferromagnetic masses, of about 1 and 2 m for anomalies a and c respectively, 4 m for b and d. these results confirm our considerations about the depth of the magnetic bodies deduced from a comparison of the maps showed in fig. 4a,b. 3. electromagnetic induction measurements the electromagnetic induction method can detect lateral and vertical variations of electrical conductivity. the method is useful for detection of landfills and buried waste because the electrical conductivity of these materials is greater than the surrounding geologic layers. the instruments used in this kind of survey include a transmitter and a receiver loop. the transmitter generates an alternating magnetic field (primary field) which induces a flow of currents within the earth below. this induced current, in turn, generates a secondary magnetic field detected by the receiver coil. the secondary field is resolved into quadrature and in-phase components against the primary field strength. the ratio between the amplitude of quadrature-component and the primary field depends on the electrical conductivity of the geologic materials. the ratio between the in-phase component and the primary field depends on the resistivity of the material and is more sensitive to metallic materials (gilkeson et al., 1992). the electromagnetic survey was performed using gssi gem 300 equipment. this instrument works in the frequency domain and can be configured to measure up to 16 user-defined frequencies between 330 hz and 20000 hz. in this way, using different frequencies, it is possible to map the subsurface at different depths. the field detected by the receiver loop is then split into in-phase and quadrature components which are expressed as a percentage intensity of the signal against the primary field strength. this technique also offers the possibility to draw a map whose anomalies are almost monopolar, centred directly above the target giving an easier interpretation of the position than dipolar magnetic anomalies (in which the target is commonly located at the slope of an anomaly, won et al., 1996). in our case, the survey was carried out continuously along profiles e-w oriented and at a distance 2 m, using 2010-3210-5010-795012570-19950 hz frequencies. the results show a good correlation with the results given by magnetic method. in fig. 5a, the in-phase anomaly maps at 19950 hz and 5010 hz (expressed as a percentage in intensity of the signal), show two monopolar anomalies corresponding to the a and c magnetic anomalies. in south direction two separated, electromagnetic monopolar anomalies, close to the single d magnetic anomaly, are shown. this separation, as we referred before, is also visible in the map of the vertical gradient shown in fig. 4b. similar information emerged from the in-phase maps at different frequencies (not reported here). on the contrary, in these maps the anomaly b is hardly visible, probably due to the presence of a poor amount of buried 569 ground magnetometric surveys and integrated geophysical methods for solid buried waste detection: a case study fig. 5a,b. the in-phase (a) and quadrature (b) component anomaly maps at 19950 and 5010 hz (expressed as a percentage intensity of the signal). a b 570 marco marchetti, lili cafarella, domenico di mauro and achille zirizzotti waste. the in-phase map (fig. 5a) roughly confirms the consideration described in the previous paragraph, showing anomalies corresponding to the area already identified by the magnetic surveys. the quadrature component map (fig. 5b) does not show additional anomalies besides those linked to the metallic target found through the in-phase map. 4. geoelectrical measurements electrical imaging techniques have been broadly used in recent years, especially in environmental field where shallow soundings with high resolutions are required. in many geological situations, 2d imaging surveys can give useful results that are complementary to the information obtained by other geophysical methods. the dipole-dipole array is very sensitive to horizontal changes in resistivity, so it can be addressed to map vertical structures such as dykes and cavities, and buried objects. on the other hand, wenner array can give useful information on vertical discontinuities, so the integration of the two methods can be very important to understand the subsoil structure (mcneill, 1994; jordan and costantini, 1995; loke, 1999). for a better characterization of the magnetic anomaly a (where the excavation was not made) a profile 23 m long oriented in the s-n direction was performed using a resistivity meter irissyscal kid. the profile was carried out by means of wenner and dipole-dipole electrode arrays using 24 electrodes with 1 m spacing. results are shown in fig. 6 where two resistivity regions (150-200 m) and a low resistivity area of about 20 m at a depth of about 1.5-4.0 m are recognizable. this effect can be explained if we assume that the low resistivity sector is related to the presence of many metallic bodies which determine a considerable lowering of the ground resistivity. the depth of this sector corresponds to the depth of the recovered metallic drums in other points of the quarry. these considerations suggest that probably there are many buried drums in the proximity of the magnetic anomaly a (disclosed by all geophysical techniques). 5. ground penetrating radar measurements among the geophysical techniques used to study refuse disposal site, ground penetrating radar (gpr) is not as widely used as the techfig. 6. geoelectrical profile (along magnetic anomaly a, fig. 4a) showing a low resistivity area between two high resistivity sectors. 571 ground magnetometric surveys and integrated geophysical methods for solid buried waste detection: a case study fig. 7. ground penetrating radar profile (along magnetic anomaly d, fig. 4a). data were acquired in continuous mode using a trace acquisition rate of 40 scan/s. right side axis indicates the signal penetrating depth. niques described in the previous paragraphs. however, as is well known, the contribution of different kind of geophysical information can provide the best results. the advantage of gpr is that it provides better high lateral and vertical resolution of the investigated area in a very short time (daniels, 1996; orlando and marchesi, 2001). moreover, it does not depend on the particular nature of the searched target. in this case, the gpr measurements were done by a sir10b gssi equipped with a low frequency, portable lightweight antenna with very high penetration, the 300 mhz subecho radarteam antenna. such antenna is configured so that it can be suspended over the ground, handy especially in hard terrain. data were acquired in continuous mode using a trace acquisition rate of 40 scan/s, along the profile shown in fig. 2. in order to eliminate noise, band pass filters were applied during the acquisition of the profiles. acquired data have been migrated in order to eliminate the diffraction hyperbolas and a hylbert transform was applied to disclose signal amplitude differences. the gpr profile in fig. 2 was 26 m long and was carried out parallel to the geoelectrical profile described in the previous paragraph, across the large geomagnetic anomaly in the s-e part of the area. results are shown in fig. 7. the two visible anomalies are about 12 m long and located at about 4 m from the beginning of the profile over the area identified previously by the magnetic and electromagnetic surveys (d in magnetic map, fig. 4a). the magnetic anomaly d (and also the corresponding anomaly from the gradiometric 572 marco marchetti, lili cafarella, domenico di mauro and achille zirizzotti and electromagnetic surveys) in this case was neither limited nor sharp. this point suggests that probably in the located zone there are two large heaps of heterogeneous buried rubbish mixed with ferromagnetic waste. if we assume a value of 8 or 9 for the relative dielectric constant of the ground, the depth of that waste is about 3-4 m as it was concluded by the previous considerations. 6. conclusions this paper reports the results of two ground magnetometric surveys and other geophysical methods performed in a tuff quarry in riano flaminio (north rome). suspected buried waste disposal of ferromagnetic targets was investigated on behalf of the government waste recycling committe. detection and location of buried waste and steel drums were done through four different geophysical techniques. the results of a preliminary magnetometric survey were validated at all sites: over 160 steel drums corresponding to the magnetic anomalies were found. after a preliminary reclaim, a new magnetic investigation followed by an electromagnetic survey was performed. geoelectrical and gpr profiles were carried out on two sectors of the studied area in order to obtain a better characterization of the anomalies previously detected. a good correlation between magnetic and electromagnetic anomalies emerges from the results presented. moreover the electromagnetic anomalies are centred above the target giving an easier interpretation of its position. these described surveys remarkably reduce the cost and time of investigations and the precise indication from the anomaly maps allows excavations to be undertaken only in limited sectors of the whole analysed area without introducing substantial modifications of the territory. the combined use of these techniques with electrical tomography and gpr measurements determines both size and location of the detected underground refusals not only of metallic nature. acknowledgements the authors wish to thank prof. e. boschi, president of ingv, for his continuous encouragement towards the development of the research in this field. the authors are also very grateful to drs. g. morelli and s. del ghianda of the geostudi astier ltd, livorno, for their valuable support in the realization of this work. references breiner, s. 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(received april 14, 2002; accepted july 9, 2002) vol49_2_2006 751 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words ionian sea – chirp sonar – echo character – coral mound – mud volcano 1. introduction and geological setting in march 2002 a cruise onboard the r/v urania, collected about 1100 km of chirp 2 sonar profiles in the ionian sea, from the inner parts of the calabrian arc accretionary prism to the apulian foreland, crossing the taranto trench (fig. 1). part of the chirp data were collected along a trackline which follows crop seismic line m5, oriented sw-ne (doglioni et al., 1999; merlini et al., 2000). a chirp sonar grid has been acquired off-shore puglia, where living deep water corals (n.d.r. lophelia pertusa) have been recently dredged (tursi and mastrototaro, pers. comm.). according to several authors (gasparini et al., 1982; cristofolini et al., 1985; malinverno and ryan, 1986; royden et al., 1987; patacca and scandone, 1989; boccaletti et al., 1990; doglioni et al., 1994, 1996; selvaggi and chiarabba, 1995) the apennines belt is the result of the eastward retreat of the adriatic-ionian litosphere, during the last 30 ma. the southern apennines and calabria, located in the most arcuate part of the belt, migrated eastward at the fastest rates (about 3-4 cm/yr, patacca and scandone, 1989; patacca et al., 1990; doglioni, 1991). evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey nicoletta fusi, alessandra savini and cesare corselli dipartimento di scienze geologiche e geotecnologiche, università di milano «bicocca», milano, italy abstract a chirp sonar survey in the ionian sea investigated the calabrian margin, the calabrian accretionary wedge, the taranto trench and the apulian foreland. shallow tectonic structures have been related to deeper ones, recognised on crop seismic profiles. the identified echo characters have been compared with those described in the modern literature and have been related to different kinds of sediments, on the basis of core samples. based on echo character and morphology we have recognised: 1) a widespread presence of mounds, up to 50 m high, occurring on the apulian plateau as isolated mounds in the deepest zones (1600-800 m) and in groups in the shallower ones (800-600 m); they have been interpreted as coral mounds, according to a recent discovery of living deep water coral colonies in this zone. 2) some mud diapirs, isolated or in groups of two or three elements, widespread in the whole study area. similar to what has been observed on the mediterranean ridge, their presence suggests the activity of deep tectonic structures (thrusts and faults) and a reduced thickness (or absence) of messinian evaporites in this part of the ionian sea. mailing address: dr. nicoletta fusi, dipartimento di scienze geologiche e geotecnologiche, università di milano «bicocca», piazza della scienza, 4, 20126 milano, italy: e-mail: nicoletta.fusi@unimib.it 752 nicoletta fusi, alessandra savini and cesare corselli the apennine belt consists of thrust sheets of the sedimentary cover (mostardini and merlini, 1986; sella et al., 1988; patacca and scandone, 2004), later cross-cut by deep normal faults, along which fluids migrate upwards (doglioni et al., 1996). the southern apennines belt is marked by compressive thin-skinned tectonics at the front and to the east and extensional thick-skinned tectonics in the main chain and to the west of it (merlini et al., 2000). seismic line m5, underlying our chirp survey, runs from off-shore eastern calabria to off-shore southern puglia and is considered one of most representative lines of the italian project crop (doglioni et al., 1999). on the calabrian margin this section crosses extensional faults, which inland are well known to form grabens (val d’agri, vallo di diano; merlini et al., 2000) and are responsible for the high seismicity of the southern apennines (amato and selvaggi, 1993); on the eastern side the section crosses the apulian swell which appears rather as a 100 km wide buckling anticline, deforming the entire lithosphere (merlini et al., 2000). objectives of our study are: 1) to identify the main shallow active tectonic structures; 2) to integrate these structures into the regional framework, defined by deeper seismics (crop seismic line m5); 3) to eventually relate regional fluid flow with the presence of living coral colonies and with active tectonics. 2. methods the chirp survey was carried out using the ship’s datasonic dsp-661 chirp 2 profiler. trigger rates varied from 2 to 4 s, depending on water depth, ranging from about 600 m to about 2500 m; the spectrum of frequencies of sound signal is between 3 to 7 khz. the chirp survey was carried at a rather constant speed of about 10 knots, except for the lines on the apulian fig. 1. map of northern ionian sea, showing the chirp lines and the main core locations, obtained during the urania cruise, with the main structural tectonics elements of the central mediterranean region shown in the right down corner. 753 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey plateau, where our main interest concentrated, which where carried at a lower speed of 4 knots. data were displayed on an epc 9800 and recorded on magneto-optical disks in seg-y format, with the exception of the first part of line calabrian ridge, which was not recorded due to technical problems. synchronization of this instrument with the navigation system (navpro) allows precise location of structures on chirp profiles and precise localization for core sampling sites (gravity cores, fig. 1). sediment samples were collected with a 10 cm diameter gravity corer at 6 stations (fig. 1); data from multi-corer, used mainly for biological interest, were also taken into account. the digital chirp data were initially processed with suitable software to provide a useful image with proper geographic locations of features identified in the imagery. the processing steps included the replying data in several different vertical distance range windows, applying image corrections and digitising sub-bottom. the imagery was then visualised in its proper geographic location and exported from the software package as tiff images with associated georeference information. the chirp profiles yield a detailed image of the surficial structure up to 100 ms twt (corresponding to 80 m of sediments, using an interval velocity of 1600 m/s) depending on seafloor dip and characteristics. the interpretation of seismic facies identified on chirp profiles was compared with those identified by other authors (damuth and hayes, 1977; damuth, 1980; lee et al., 2002) and ground truth by means of core logs. 3. results and discussion the chirp survey (fig. 1) crosses the following structural domains, from sw to ne: the calabrian margin, the accretionary wedge, the taranto trench and the apulian foreland. the seismic section identified by means of chirp profiling can be referred to plio-pleistocene units on the calabrian margin, the accretionary wedge and the taranto trench, whereas on the apulian plateau neogene – pleistocene units outcrop, forming the apulian swell; for stratigraphic and structural interpretation of crop seismic line m5 we refer to doglioni et al. (1999) and merlini et al. (2000). seismic facies were related, if possible, to echo types identified by lee et al. (2002) and indicated following their nomenclature (roman number + arabic one). 3.1. calabrian margin in this area the chirp 2 system has generally a good penetration, ranging from 50 to 75 ms twt. the seismic sequence is characterised by strong reflectors alternating with transparent intervals, which mantle the topography, pinching out on the top of topographic highs. prevalent echo type of this area can be attributed to type i-2. this part of the seismic section is characterised by widespread dip-slip faults, which offset the seafloor for several hundred meters, the total throw of the calabrian margin being of about 1500 m. these faults cross-cut both the plio-quaternary sedimentary successions and the buried pre-messinian back-thrusts (see fig. 3 of merlini et al., 2000); their activity seems thus to be plio-quaternary. this active tectonics results in an articulated topography, with several horsts (off-shore punta stilo and rocella ionica) mantled by sediments (fig. 2). the steepest flanks of horst structures (about 1.5° gradient) are affected by mass movements: landslide scarps, about 20 m high, can be identified, due to sharply truncated reflectors. landslide deposits, with chaotic reflectors inside, (type iv-1) extend for about 8.5 km, both on the lower part of the slope and in the basin. landslides seem to involve only the superficial and probably unconsolidated part of the sedimentary succession. several deeply incised canyons (fig. 2) are probably related to the outlet of nearby rivers. off-shore punta stilo south-eastward dipping reflectors (fig. 3) are truncated by an erosional surface at about 150 m depth. a subhorizontal erosional surface, mantled by reflectors and probably related to the last glacial lowstand, can also be identified in this area at about 80-100 m b.s.l. at least two non reflective mounds, 100150 m high and 2500 m across, can be identified in this area, off-shore rocella ionica; multi-corer, put down twice on the top of one of 754 nicoletta fusi, alessandra savini and cesare corselli fig. 2. chirp profile across the calabrian ridge (calabrian ridge line – see fig. 1 for location), with the insets of the main structural features identified (a, b, c). a – example of a canyon; b – onlapping reflectors on the top of a horst; c – landslide scarp on the steepest flanks of horst structures and relative deposits. fig. 3. chirp profile across the calabrian ridge and the western section of calabrian margin (ionica 1 line – see fig. 1 for location). the zoom (a) shows the subhorizontal erosional surface of the shelf break, mantled by reflectors and probably related to last glacial lowstand at about 80-100 m b.s.l. 755 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey these mounds, recovered no sediments at all, thus confirming the total lack of soft sediments on the top of the mounds. on the basis of seismic response and morphology they are interpreted as mud volcanoes. 3.2. accretionary wedge the accretionary wedge is characterised by a rough topography, ranging between 1400 m and 2000 m depth. the shallow sediments are gently deformed by symmetrical and asymmetrical waves, referred to echo type iii-3 and interpreted as the result of creeping on the slope of the accretionary wedge; some patches of debrites (echo type iii-2) are also present. steep flanks are characterised by slumps (fig. 4). a distinctive character of this area is a flat plateau, delineated by 1600 m and 1800 m isobaths and gently dipping south-eastward (0.2°), here called punta stilo plateau (fig. 4). it is characterised by an abrupt change in seismic character: the sea bottom is highly reflective and reflects nearly all the acoustic energy; only occasionally very few reflectors can be identified below sea bottom (echo type i-1). micro undulations of the sea bottom of this plateau (echo type iii-2) are visible only on arco calabro line (se-nw). punta stilo plateau is interpreted as the outcropping of more resistant lithologies. we suggest that these lithologies are coarse grained turbidites, on the basis of «rocella» core, which recovered several meters of mud intercalated with sandy layers; this interpretation is supported by other seismic surveys in this area, which outline the pinching fig. 4. chirp profile across the calabrian margin (calabrian arc line – see fig. 1 for location), with the zoom of the main geophysical features that could be related to mud volcanism (a, b, c, d). a – two mounds with asymmetric flanks mantled by sediments, characterised by slumpings with the location of the gc-19 core (see fig. 1 for location). b – isolated symmetric mound with chaotic sediment, with the ring shaped depression filled by re-deposited sediments surrounding the volcano, that is a typical character of mud cones. c – the u-shaped valley (thorgatten), with sub-horizontal reflectors on the flat bottom, bordered by two acoustically transparent mounds, interpreted as mud volcanoes. d – particular in rough topography area of a transparent symmetric mound at the top of a horst (interpreted as mud volcano). 756 nicoletta fusi, alessandra savini and cesare corselli out of reflectors below the sea bottom, suggesting lobate and basin-fill geometry of deposits (merlini, pers. comm.). the origin of sediments involved in turbidites is probably the steep continental margin off-shore calabria. 3.2.1. geophysical evidence of mud volcanism several transparent mounds, 100-150 m high and from 2000 m to 3500 m across, have been identified. they occur in small groups of two or three elements and in many cases they present asymmetric flanks; they can be mantled by sediments, sometimes characterised by slumpings (fig. 4). dimensions of the mounds are consistent with the mud volcanoes and mud cones identified on the mediterranean ridge (camerlenghi et al., 1992, 1995; limonov et al., 1994; fusi and kenyon, 1996 among others). two of these mounds were investigated by a detailed survey, that included sampling. core gc 19, taken on top of one mound (fig. 1), recovered only soft mud, alternated with a tephra layer and some sandy layers; it can thus be inferred that the mud breccia, if present, is buried below several meters of layered sediments, as suggested by reflectors on top of the mound. a peculiar morphologic and seismic feature of this area is an u-shaped valley, here called thorgatten, about 130 m deep (fig. 4); sub-horizontal reflectors can be seen on its flat bottom, about 3.5 km wide. it is bordered by two acoustically transparent mounds, interpreted as mud volcanoes, presenting steep internal flanks, whereas the outer flanks are characterised by dipping reflectors sometimes chaotic, due to slumping. the thorgatten lies on the outcrop of the minor southwestward verging thrusts, identified on crop line m5. a similar relationships between mud volcanoes and compressive deep tectonics has been delineated on the mediterranean ridge (camerlenghi et al., 1995). although we did not observe any chimney cross cutting the sedimentary sequence below the mud volcanoes, we did observe (fig. 4) a typical character of mud volcanoes worldwide, that is the ring shaped depression filled by re-deposited sediments surrounding the volcano (see fig. 15 of camerlenghi et al., 1995). the presence of mud volcanoes on the calabrian ridge was already known: some few small isolated mud cones were identified by means of a discontinuous side scan sonar survey (gloria; fusi and kenyon, 1996); they have a round shape and seem to recur as isolated spots, whereas on the mediterranean ridge mud volcanoes are found in groups, of different shapes and dimensions, on the northern edge of the ridge (camerlenghi et al., 1992, 1995; limonov et al., 1994; fusi and kenyon 1996); mud volcanoes on the calabrian ridge are generally smaller than those on the mediterranean ridge (see fig. 3 of fusi and kenyon, 1996). on the basis of gloria data (fusi and kenyon, 1996) it can be hypothesized that most mud volcanoes on the calabrian ridge are circular or sub-circular, but it cannot be excluded that some of them, especially those located along thrust planes, such as thorgatten, are mud ridges. 3.3. taranto trench the taranto trench appears as an almost symmetrical depression (fig. 1), about 2500 m deep (fig. 5) and about 9.5 km wide; both ne and sw slopes of the trench have the same gradient (about 18°). its axis, which follows the outcropping of the accretionary wedge front, has an apenninic direction (nnw-sse) in this part of the ionian sea. along its axis, the topography of the taranto trench is characterised by two main steps of about 200 m each at 2800 m and 2700 m depth (fig. 6). the deepest part of the line is characterized by sharp surface echo, with no sub-bottom reflectors, referred to echo type i-1. the flat area starting after the second steps, from 2600 m to 2400 m depth shows two main characteristics: a) reflectors, generally mantling the topography but in some cases pinching out on small mounds; b) small basins, about 1 km wide and 50 m deep, filled up by about 20 m of sediments. the upper part of the trench is characterised by a hummocky topography with some chaotic reflectors, referable to echo type iv-3, interpreted as interlayered debrites and turbidites. as for the punta stilo plateau, we suggest that the taranto trench is filled by coarse grained turbidites; reflectors identified on crop 757 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey fig. 5. chirp profile across the taranto trench and the eastern section of apulian plateau (apulian 1 line – see fig. 1 for location), that rises through westward dipping deep seated faults. the zoom shows the sharp surface echo, with no sub-bottom reflectors, that characterised the taranto trench. fig. 6. chirp profile across the axis of taranto trench (apulian 6 line – see fig. 1 for location), with the zoom of the main features identified (a, b): a – a hummocky topography with some chaotic reflectors and small basins filled up by sediments; b – the deepest part of the trench with steplike topography. 758 nicoletta fusi, alessandra savini and cesare corselli seismic line m5 in the taranto trench, which onlap the apulian platform, suggest different turbiditic episodes. turbidity currents, probably originating from the inner parts of the taranto gulf, are channelled along the taranto trench axis, eroding in part former deposits and filling small basins. 3.4. apulian plateau the apulian plateau rises from more than 2400 m to about 200 m depth through several westward dipping deep seated faults, with a total throw of about 1500 m, also affecting the sea-bottom. merlini et al. (2000) tentatively interpret these south-western scarps on the apulian plateau as an example of foreland inversion and thus they hypothesise two thrust planes (fig. 5 of merlini et al., 2000). on chirp we have no evidence of such faults. few probable mud volcanoes were identified on the slope toward the taranto trench and are probably related to these deep seated structures. in this area the seismic signal has no penetration, due to the steepness of relief. the diffuse presence of irregular overlapping hyperbolae (echo type iii-1) prevents identifications of the regional westward dipping monocline visible on crop line m5. at about 500 m depth, three antiforms can be recognised on chirp profiles (figs. 7 and 8); their axis is appeninic (nnw-sse) and parallel to the main dip slip faults of the apulian plateau. the first one from sw is the more evident, both from bathymetry and from seismics. the chirp survey suggests an asymmetric antiform, with its ne flank mantled by reflectors, 50 ms thick, onlapfig. 7. chirp profile across the apulian plateau (apulian 2 line – see fig. 1 for location), with the insets (a, b, c, d) of the main tectonic and geophysical features that mark the transparent mounds recognised in this area and the location of the main cores collected. a – group of transparent mounds on the top of the fault steps; b – isolated transparent mound on a fault step; c – the asymmetric antiform, with the ne flank mantled by reflectors, onlapping the crest of the structure, and the south-western flank characterised by sub-horizontal reflectors, truncated by a sharp erosional surface; d – area with widespread transparent mounds and ponded sediments. 759 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey fig. 8. chirp profile across the apulian plateau (apulian 9 line – see fig. 1 for location), with the zooms (a, b) of the main geophysical features that characterise the mound fields recognised in this area. see on zoom b the highest transparent mound identified on apulian plateau. fig. 9. log of cores gc 11, gc 12, gc 13, and gc 16 (see figs. 1 and 7 for location). 760 nicoletta fusi, alessandra savini and cesare corselli fig. 10. chirp profile across the apulian plateau (apulian 4 line – see fig. 1 for location), with the zooms of the main geophysical features that characterise the mound fields recognised in this area (a, b) and the diapiric intrusions (c). ping the crest of the structure and probably disrupted by a normal apenninic fault, which could be interpreted as a conduit for fluid expulsion (see 3.4.1). the south-western flank of this structure presents sub-horizontal reflectors, truncated by a sharp erosional surface at about 750 m depth; the outcrop or subcrop of old consolidated sedimentary successions can be reasonably supposed. on the top of this anticline small reverse faults were identified, suggesting a recent activity of this structure. this structure (figs. 7 and 8) can be recognised also on crop line m5, where it is underlined by two reverse faults which isolate a wedge, extruded upward. these reverse faults seem to affect both the mesozoic apulian carbonate platform and the neogenepleistocene sediments (fig. 6 of merlini et al., 2000). a gravity core taken at the top of this anticline structure (gc 13, 666 m depth) recovered 261 cm of mud, stiffer toward the bottom of the core, with pteropod fragments and foram tests in the upper 13 cm (fig. 9). this structure is followed to the ne by two other antiforms, less evident from their morphology, but still identifiable by seismics (fig. 8). on the basis of chirp data we interpret both the three anticlines and the reverse faults as minor structures, ductile and tensile respectively, connected to the apulian lithospheric anticline. on the apulian plateau an erosional surface at about 900 m depth is cut by several diapiric structures, sometimes crosscutting each other, and covered by a transparent sedimentary interval, which is thinner on structural highs and thicker in small basin (fig. 10). some strong reflectors, thicker north-westward, mantle some of the diapiric structures; they are reduced southeastward to one single reflector, which is cut by the diapiric structures. a semi transparent sedimentary interval mantles all the diapiric structures, onlapping south-eastward on the swelling of a nearby horst structure. in analogy with the other transparent mounds, these structures are interpreted as sub-cropping mud diapirs, deforming all the sedimentary sequence. their intrusion is thus interpreted as an actual deformation structure, since it offsets the sea bottom; as761 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey suming this hypothesis, the sediment deformation rate is higher than the sedimentation rate. 4. geologic and structural interpretation of the study area a synthetic structural-geological map and a cross section of the study area were produced (figs. 11 and 12) on the basis of: a) the echo characters and shallow structures identified on chirp survey; b) core logs collected as ground truth data; c) deeper seismics (crop line m5) and its structural interpretation. the transition between the extensional tectonics of the calabrian margin and the compressive tectonics of the accretionary wedge is represented by the punta stilo plateau (figs. 11 and 12), dominated by the sedimentation of coarse grained turbidites and not affected yet by active tectonics. this plateau can be interpreted as a sort of huge talus, which collect coarse sediments, deriving from the erosion and mass movements from the steep calabrian margin and from the main land. the accretionary wedge, marked to the front by an eastward verging thrust, is topographically and structurally deeper than the foreland and fig. 11. synthetic structural-geological map of the northern ionian sea produced on the basis of the echo characters and shallow structures identified on chirp survey and the core logs. 762 nicoletta fusi, alessandra savini and cesare corselli is mainly subsiding (doglioni et al., 1999; merlini et al., 2000). on chirp profiles there is no sign of the compressive tectonics identified on crop line m5 and in particular of the accretionary wedge front, but the widespread presence of creep deposits and debrites suggests a recent activity of this area. the apulian foreland, separated from the accretionary wedge by the narrow taranto trench, filled up by coarse turbidites, is folded by a 100 km wide crustal-lithospheric anticline, currently uplifting (merlini et al., 2000). superficial anticlinal folds and reverse faults identified on chirp profiles (figs. 7, 11 and 12) are interpreted as minor tensional and ductile compressive structures, connected with the hinge zone of the apulian lithospheric anticline. in the study area mud volcanoes do not seem to be related to any particular geodynamic context, since they occur both on the accretionary wedge and on the margin of the apulian foreland (see 4.4.2). this difference from the mediterranean rigde, where deep origin mud volcanoes occur only at the ne edge of the ridge due to reduced thickness of messinian evaporites and to the presence of thrusts (camerlenghi et al., 1995) suggests that: a) the source rock is not as deep as on the mediterranean ridge or there are possibly different source rocks at different depths; in fact, according to well logs (merlini, pers. comm.), different age clays are widespread in all this area; b) mud diapris are related to deep seated tectonic structures, such as thrusts and/or dip slip faults. c) messinian evaporites are absent or highly reduced in thickness, as suggested both by well logs on the mainland (casero, 2004) and seismic sections, where a messinian unconformity can be traced on the calabrian margin and on the accretionary wedge (doglioni et al., 1999; merlini et al., 2000). in the ionian sea the only deep well drilled (odp site 964 on the pisano plateau) recovered 112 m of plio-pleistocene nannofossil ooze (shipboard scientific party, 1996), adding no further information to the presence/absence of evaporites. 5. geophysical evidence of coral mounds the apulian plateau is characterised by the widespread presence of transparent mounds, about 20 m high and 200-300 m large each; they occur as isolated mounds on the top of the fault steps in the deepest zones (1600-800 m – fig. 7) and become widespread from 800 m depth up, both in groups and as single mounds separated by reflectors. in some areas the mounds are underlined by concave downward reflectors, repeating the morphology of the mounds themselves. a particularly high (about 50 m) transparent mound was identified at a depth of about 750 m (fig. 8). three gravity cores have been collected on these mounds (figs. 1 and 7). core gc 11, taken on a fault step with a transparent mound, at fig. 12. cross section of the northern ionian sea with shallow tectonics structures identified by chirp survey, related to deeper ones from doglioni et al. (1999) and merlini et al. (2000). 763 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey 1170 m depth recovered only 93 cm of brown mud. core gc 12 (800 m depth), taken on an isolated transparent mound, and core gc 16 (750 m depth), taken in the area of widespread transparent mounds, recovered respectively: 100 cm of grey mud with forams and pteropods at the top and coral fragments in the upper 40 cm; 341 cm of mud, with forams and pteropods at the top (fig. 9). these mounds are here interpreted as coral mounds, on the basis of their acoustic and morphological characters. in fact, due to the high reflectivity at their top, reef structures mask the acoustic response further down, appearing thus transparent. furthermore, these transparent mounds rise from the sea bed with a relatively high angle, that is about 50°, as described also for norwegian reefs (hovland and thomsen, 1997). seismic data are supported by the recent discovery in this area of dredged living corals (tursi and mastrototaro, pers. comm.) and of coral fragments in core gc 12. hovland and thomsen (1997) proposed that the presence of cold deep water coral could be associated with microseepage from the seabed of light hydrocarbons which provide nutrients for bioconstructor organisms. coral colonies become widespread from the apenninic dip slip fault disrupting the anticline (fig. 7); this part of the apulian plateau is intensively fracturated by conjugate dip slip faults affecting the neogenepleistocene sedimentary sequence (merlini et al., 2000) and providing a local micro-seepage through the seabed of light hydrocarbons, useful for the life of cold water corals (hovland and thomsen, 1997). the areal extension and shape of coral colonies is currently unknown. on the basis of our survey, the area characterised by the presence of coral mounds should cover about 250 km2. in any case, this coral site on the apulian plateau seems to be one of the more extended and richer deep water coral site of the mediterranean sea and is worth additional detailed investigations. fig. 13. different scenarios of the geophysical evidence of transparent mounds identified on the apulian plateau: 1a – isolated mounds, located on the slopes; 1b – small groups near the main dip slip fault steps; 2a – widespread mounds, cross cutting each others, below which no reflector can be detected; 2b – widespread mounds, cross cutting each others, underlined by upward convex reflectors; 2c – small mounds separated by ponded sediments. 764 nicoletta fusi, alessandra savini and cesare corselli according to our survey coral mounds are present in two main different scenarios (fig. 13): 1) on the slope of the apulian swell rising from the taranto trench, in water depth as deep as 1600 m, coral mounds occur as: a) isolated mounds, located on the slopes; in this case no deep structure can be reliably connected with biohermes and probably these bio-constructions can be seen as a sort of pioneer organisms; b) small groups of several bioconstructions; their presence is strictly related to the dip slip fault steps, which offset the slope of the apulian swell in this area. we infer that the presence of coral reefs at this water depth is connected to fluids rising along faults and providing a food source for suspensionfeeders, such as corals (hovland and thomsen, 1997). 2) in the shallower part of the apulian plateau, from about 700 to 400 m water depth, coral mounds are widespread, covering an area of at least 250 km2; furthermore, coral reefs are located immediately above a nearly surfacing up-dipping and high-reflective layer, that can be interpreted as partly gas charged and an upward micro-seepage site (hovland and thomsen, 1997). they can occur as: a) groups of several tens of high reflective mounds, cross cutting each other, below which no reflector can be detected. due both to concentration of reefs and to their particular height (up to 50 m) we interpret this scenario as characterised by fast growing bio-constructions; b) groups of several tens of high reflective mounds, cross cutting each other, underlined by upward convex reflectors, interpreted as dead coral reefs. this context suggests superimposed cycles of biohermes; it can thus be inferred that the presence of corals in this area is long lasting; c) small coral mounds, few meters high, separated by ponded sediments; one of this coral reefs grows on one small fault, offsetting the sea bottom, with a throw of about 3 m. we interpret this situation as isolated, relatively slow growing bioconstructions, separated by ponded sediments. 6. conclusions the peculiarity of the study area is the passage from an extensional tectonic regime (calabrian margin) through a compressive one (calabrian ridge) to a stable zone (apulian foreland). the main results of our chirp survey in this area are the following: 1) the transition between the extensional tectonics of the calabrian margin and the compressive tectonics of the accretionary wedge is represented by a wide flat plateau (punta stilo plateau), gently dipping south-eastward (0.2°) and dominated by the sedimentation of coarse grained turbidites; no evidence of active tectonics can be identified on this plateau. 2) superficial anticlinal folds and reverse faults, identified on the apulian plateau, are interpreted as minor tensional and ductile compressive structures, connected with the hinge zone of the apulian lithospheric anticline. 3) some mud volcanoes have been identified in the whole study area. in analogy with what has been observed on the mediterranean ridge, their presence suggests the activity of deep tectonic structures (both thrusts and faults) and/or a reduced thickness (or absence) of messinian evaporites in this part of the ionian sea. 4) on the apulian plateau, the widespread presence of mounds, up to 50 m high, occurring as isolated mounds in the deepest zones (1600800 m) and in groups in the shallower ones (800-600 m), has been interpreted as deep water coral bioconstructions, according to a recent discovery of living deep corals in this zone. the intense fracturation of the apulian plateau and the possible related cold seepage of hydrocarbons could sustain the deep coral communities through the presence of chemiosyntetic bacteria as primary producers. acknowledgements authors are grateful to saverio merlini for constructive discussions. angelo camerlenghi kindly revised the first draft of the manuscript. references amato, a. and g. selvaggi (1993): aftershock location and p-wave velocity structure in the epicentral region of the 1980 irpinia earthquake, ann. geofis., xxxvi (1), 3-15. boccaletti, m., n. ciaranfi, d. cosentino, g. deiana, 765 evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resultion chirp sonar survey r. gelati, f. lentini, f. massari, g. moratti, t. pescatore, f. ricci lucchi and l. tortorici (1990): palinspastic restoration and paleogeographic reconstruction of the peri-tyrrhenian area during the neogene, paleocl., paleoec., paleogeog., 77, 41-50. camerlenghi, a., m.b. cita, w. hieke and t.s. ricchiuto (1992): geological evidence of mud diapirism on the mediterranean ridge accretionary complex, earth planet. sci. lett., 109, 493-506. camerlenghi, a., m.b. cita, b. della vedova, n. fusi, l. mirabile and g. pellis (1995): geophysical evidence of mud diapirism on the mediterranean ridge accretionary complex, mar. geophys. res., 17, 115-141. casero, p. (2004): structural setting of petroleum exploration plays in italy, in geology of italy, edited by u. crescenti, s. d’offizi, s. merlino and l. sacchi, spec. vol. ital. geol. soc. for the igc 32, 189-199. cristofolini, r., f. ghisetti, r. scarpa and l. vezzani (1985): character of the stress field in the calabrian arc and the southern apennines (italy) as deduced by geological, seismological and volcanological information, tectonophysics, 117, 39-58. damuth, j.e. (1980): use fo high-frequency (3.5-12 khz) echograms in the study of near-bottom sedimentation processes in the deep-sea: a review, mar. geol., 38, 51-75. damuth, j.e. and d.e. hayes (1977): echo character of the east brazilian continental margin and its relationship to sedimentary processes, mar. geol., 24, 73-95. doglioni, c. (1991): a proposal of kinematic modelling for w-dipping subductions: possibile applications to the tyrrhenian-apennjnes system, terra nova, 3, 423-434. doglioni, c., f. mongelli and p. pieri (1994): the puglia uplift (se italy): an anomaly in the foreland of the apenninic subduction due to buckling of a thick continental lithosphere, tectonics, 13 (5), 1309-1321. doglioni, c., p. harabaglia, g. martinelli, f. mongelli and g. zito (1996): a geodynamic model of the southern apennines, terra nova, 8, 540-547. doglioni, c., s. merlini and g. cantarella (1999): foredeep geometries at the front of the apennines in the ionian sea (central mediterranean), earth planet. sci. lett., 168: 243-254. fusi, n. and n.h. kenyon (1996): distribution of mud diapirism and other geological structures from longrange sidescan sonar (gloria) data, in the eastern mediterranean sea, mar. geol., 132, 21-38. gasparini, c., r. scarpa and l. vezzani (1982): seismotecotnics of the calabrian arc, tectonophysics, 84, 276-286. hovland, m. and e. thomsen (1997): cold-water corals – are they hydrocarbon seep related?, mar. geol., 137, 159-164. lee, s.h., s.k. chough, g.g. back and y.b. kim (2002): chirp (2-7 khz) echo characters of the south korea plateau, east sea: styles of mass movement and sediment gravity flow, mar. geol., 184, 227-247. limonov, a.f., j.m. woodside and m.k. ivanov (editors) (1994): mud volcanism in the mediterranean and black seas and shallow structure of the eratosthenes seamount, in initial results of the geological and geophysical investigation during the third unesco-esf «training through research» cruise of r/v, june-july 1993, gelendzhik, unesco rep. mar. sci., 64, pp. 173. malinverno, a. and w.b.f. ryan (1986): extension in the tyrrhenina sea and shortening in the apennines as a result of arc migration driven by sinking of the lithosphere, tectonics, 5, 227-245. merlini, s., g. cantarella and c. doglioni (2000): on the seismic profile crop m5 in the ionian sea, boll. soc. geol. ital., 119, 227-236. mostardini, f. and s. merlini (1986): appennino centromeridionale. sezioni geologiche e proposta di modello strutturale (agip), p. 59. patacca, e. and p. scandone (1989): post-tortonian mountain building in the apennines. the role of the passive sinking of a relic lithosphere slab, in the lithosphere in italy, edited by a. boriani, m. bonafede, g.b. piccardo and g.b. vai, acc. naz. lincei, atti dei convegni lincei, 80, 157-176. patacca, e. and p. scandone (2004): the plio-pleistocene thrust belt – foredeep system in the southern apennines and sicily (italy), in geology of italy, edited by u. crescenti, s. d’offizi, s. merlino and l. sacchi, spec. vol. ital. geol. soc. for the igc 32, 93-129. patacca, e., r. sartori and p. scandone (1990): tyrrhenian basin and apenninic arcs, kinematic relations since late tortonian times, mem. soc. geol. ital., 45, 425-451. royden, l., e. patacca and p. scandone (1987): segmentation and configuration of subducted lithosphere in italy: an important control on thrust-belt and foredeepbasin evolution, geology, 15, 714-717. sella, m., c. turci and a. riva (1988): sintesi geopetrolifera della fossa bradanica, mem. soc. geol. ital., 41, 87-108. selvaggi, g. and c. chiarabba (1995): seismicity and pwave velocity image of the southern tyrrhenian subduction zone, geophys. j. int., 121, 818-826. shipboard scientific party (1996): site 964, proceedings of the ocean drilling program, init. rep. (college station tx, ocean drilling program), 160, 85-123. 587_597 chelidze 18.pdf annals of geophysics, vol. 45, n. 5, october 2002 587 laboratory study of electromagnetic initiation of slip tamaz chelidze, nodar varamashvili, marina devidze, zurab tchelidze, victor chikhladze and temur matcharashvili institute of geophysics, georgian academy of sciences, tbilisi, georgia abstract recently russian seismologists reported the triggering effect of mhd soundings on microseismic activity in the central asia test area. the paper focuses on an experimental test of the possibility of triggering the mechanical instability of a system that is close to critical state by a series of electromagnetic pulses. the mechanical system consisted of two pieces of rock; the upper piece can slip on the fixed supporting sample if the latter one is tilted up to the critical angle. in this state, the triggering of mechanical instability by some weak impact such as electrical pulse became more probable. the slope of support in the experiment is an analogue of tectonic stress in natural conditions. the preliminary experiments, carried out in a dry environment, at the humidity of atmosphere 30-50%, show that a strong em-pulse induces sliding of a sample of rock (granite, basalt, labradorite) placed on the supporting sample which is inclined at the slope close to, but less than, the critical angle with a probability 0.07. 1. introduction in the experiments, initially aimed at finding resisitivity precursors of strong earthquakes in the upper layers of earth crust by mhdsounding, an unexpected effect of microseismicity activation was discovered (tarasov et al., 1999). the field of mhd-dipole is quite small at distances in the order of 100 km, where the effect was observed: for resistivity of rocks of the order of 100-1000 ω ⋅ m (which is the resisitivity of rocks at the depth 5-10 km in the test area, according to tarasov et al. (1999), at the distance of the order of 100 km from the dipole source (this is the distance to the area where the activation of microseismicity was noted) the intensity of dipole field for the experimental source is of the order of 10−5-10−6 v/m. the second peculiarity of the activation effect was that it occurred with 2-4 day delay after em impact. the laboratory data on acoustic emission during compression of artificial samples (sobolev et al., 2000) confirm the possibility of initiation of fractures by strong electromagnetic (em) field. thus it is necessary to find at least a phenomenological physical model explaining these two important details: long-range action and delayed response. 2. phenomenological approach the system, which is close enough to the critical state, can manifest anomalous sensitivity to small external impacts. according to recent investigations, the earth’s crust in seismically active regions can be in a critical state or in a state of self-organized criticality (bak et al., 1988; scholtz, 1990). this can explain the known phenomenon of seismic activation on filling large reservoirs; this additional loading gives an mailing address: dr. tamaz chelidze, institute of geophysics, georgian academy of sciences, 1 alexidze str., tbilisi 380093, georgia; e-mail: chelidze@ig.acnet.ge key words electromagnetic triggering − mechanical instability 588 tamaz chelidze, nodar varamashvili, marina devidze, zurab tchelidze, victor chikhladze and temur matcharashvili insignificant contribution to the existing tectonic strains. other examples are seismicity activation by pumping of water in the boreholes (sibson, 1994) and remote aftershocks of landers earthquake. according to king et al. (1994), aftershocks recorded very far from the epicenter of the earthquake were generated by only one-half bar increment in stress provided by the mainshock. one of the possible mechanisms can be the direct dielectric breakdown of rocks driven by tectonic stress to the critical state. that means that the em pulse should be strong enough, which can be expected only in the near-source zone. in the far zone, the pulse should be amplified in order to cause dielectric breakdown. dielectric breakdown as a rule is accompanied by emission of elastic waves. this class of models, which can be called «underground thunderstorms», was advanced by a. vorobyev et al. (1975). the em field amplification can be realized in following ways: 1) amplification of em field by wedge-type inclusions – local electric field near inclusion with conductivity g2, much larger than conductivity of the host medium g 1 , can exceed the intensity of the mean macroscopic field. the largest effect is expected for the needleor lensshaped inclusions. it is well known that elongated conductive impurities in transformer oil enhance dielectric breakdown (kharitonov, 1983). as the cracks in rocks are usually saturated with conducting pore fluid and have the form of lens or needle, some amplification of the applied field can be expected: theoretically at the tip of the needle, the local field intensity is infinitely high. in order to assess the effect more precisely, some experiments were performed on a physical (two dimensional) model. the aluminum «double wedge» inclusion was glued on the bottom of a flat glass vessel, which was filled with tap water. specific resistivities of metal and fluid were 2.810−8 ω⋅m and 100 ω⋅m respectively, i.e. the resistivity ratio was 1010. the voltage 20 v was applied to electrodes, separated by 38 cm; the uniform potential gradient is thus 0.526 v/cm. figure 1 shows electric field intensity e along profiles, parallel to the long axis, which are far from the inclusion (1-1), close to it (2-2) and crossing it (3-3). we see that at the tip of the inclusion, the local field can be orders of magnitude larger than the mean macroscopic field; that can give amplification of intensity from µv to mv which still is not enough to cause dielectric breakdown: the intensity should be of the order of 104-105 v/cm. 2) amplification of em field in the random lattice model – besides above discussed «wedge» model, there is another class of models that also explain local amplification of applied voltage, namely, percolation (random lattice) models. it has been shown (de archangelis et al., 1986; benguigi, 1988) that distribution of voltages on the random lattice of resistors and insulators is multifractal and at the percolation threshold p c , the maximal voltage drop vmax occurs on singly connected (so called red) bonds, which carry the total current passing through the network. there are two models of electrical breakdown of the random lattice, dielectric and fuse. in the first model, the system is a conductor-loaded insulator. an insulating element becomes conducting (breaks) at a voltage higher than v th . then, the whole system becomes conducting at the breakdown voltage v b , which strongly depends on the concentration of conducting bonds p. in the second (fuse) model, the system is an insulator-loaded conductor. here the conducting element is fused, i.e. it becomes an insulator if the current flowing through it is larger than the threshold current j t h . again, the whole system fuses (becomes an insulator) on carrying the current j b which strongly depends on the concentration of conductors p. near percolation threshold p c when the infinite cluster of conducting bonds spans the whole sys-tem, the breakdown voltage v b has a typical power law form (archangelis et al, 1986; benguigi, 1988) vb = ( p – pc) ν (2.1) and goes to zero as p → pc . here ν is the exponent of correlation length ξ. for two-dimensional 589 laboratory study of electromagnetic initiation of slip systems ν = 4/3. according to (2.1) at p → pc the breakdown (applied) voltage becomes very low. the geological formations can be considered as mosaics of insulating (minerals) and conducting (brine-saturated pores and cracks) components, which in some areas are close to the percolation threshold. this in principle can explain the local breakdown phenomenon, as at p→ pc the intensity of the field, necessary for initiation of breakdown tends to zero. thus, theoretically the local breakdown voltage can be achieved at very small applied field if the system is close to the percolation threshold. the em-activation mechanism can also be related to pore fluid action. 3) seismohydraulic mechanism of activation of seismicity – we have noted that in order to initiate fracturing by em pulses, the system should be close to the critical state; in our case close to the state of mechanical instability. the critical shear stress on the fault τ c is τc = c + µ (σn – pf) (2.2) where c is resistance to fault displacement due to its partial cementation, µ is friction coefficient, σn is the stress component normal to the fault plane, pf is fluid pore pressure. it is evident that, in order to provoke mechanical instability, empulse should affect at least one of parameters of the above formula and so change τc . it seems that µ ,σn and pf can be affected by em pulse. for example, due to the piezoelectric effect µ and σn can be favorably changed. the weakening of faults under em impact can also be connected with interaction of pore fluid and mineral backbone, namely: a) application of em pulse can drive pore fluid into the «dry» cracks. this decreases surface fracture energy, which means enhancement of crack growth either by griffith’s model of fracture or just by decrement of friction coefficient µ. b) electrical impact can provoke electrokinetic flow in the porous rock (electroosmosis) and thus increase pore pressure pf. c) strong em impact can generate so called electrohydrodynamic effect (ehd) which was discovered in the 1950’s (nesvetailov and serebryakov, 1966). ehd effect means that water saturated porous solid can be destroyed by strong fig. 1. electric field intensity (e) due to elongated (lense-like) conductive inclusion along profiles 1-1, 2-2 and 3-3 (see text). 590 tamaz chelidze, nodar varamashvili, marina devidze, zurab tchelidze, victor chikhladze and temur matcharashvili enough em pulses; this has even found industrial applications. the mechanism of ehd effect is not exactly known. the most popular explanation is cavitation or generation of small gas bubbles by applied em pulse; their collapse generates transient stress fields in the fluid, which destroys a solid. the first two mechanisms in principle can explain the considerable time lag between impact and (remote) response as the pore fluid migration or diffusion under electric field is a relatively slow process. rigorous quantitative testing of the above hypotheses is very difficult as all these models contain some parameters that cannot be measured exactly: say, closeness to the percolation threshold of geological formation at the depth 5-10 km, water content, stress state, etc. thus the most important thing now is to prove the potential of em pulses to trigger mechanical instability in laboratory experiments and to create reasonable physical models of the phenomenon. the present paper is devoted to experiments on em initiation of slip in relatively dry environment, at the humidity of atmosphere 40-50%. 3. experimental set up the experimental set up was designed in such a manner that the mechanical system could easily be driven to the critical state where the triggering fig. 2. the mechanical part of the experimental set up: 1 = supporting sample; 2 = sliding sample; 3 = slope regulating unit; 4 = acoustic sensors; 9 = sponge for shock . 591 laboratory study of electromagnetic initiation of slip of mechanical instability by some weak impact such as electrical pulse became more probable. the system consists of two pieces of rock; the upper piece can slip on the fixed supporting sample if the latter one is tilted up to the critical angle. the slope of support in the experiment is an analogue of tectonic stress in natural conditions. the set up can be used for experiments in various conditions, «dry» and «wet». in the latter case the sample holder should be immersed in the bath with water. the general view of the set up is shown in fig. 2. 3.1. mechanical part the device brings the support-slider system to the critical state, that is, to the critical slope of the support. the regulation and fixing of the tilt is realized by means of the vernier system with an accuracy of 15'. the second important detail is the acoustic emission sensor, which responds to elementary slip events of the upper sample on the support. it is the relatively low frequency sensor (25-100 khz). the sensor was glued to the supporting (fixed) sample. 3.2. electrical part the electrical part consists of a em pulse generator and acoustic signal amplifier. for experiments, it was necessary to assemble a system generating high intensity ac and dc fields. for this, a special high voltage linear amplifier was built. it has two outputs, for ac and dc voltages. the signal from the standard generator with amplitude 0.5-5 v is applied to the input of the amplifier and goes out from the output with amplitude up to 1300 v. the high voltage pulses are applied to co-planar electrodes, placed under the supporting rock sample. the electrodes are of various forms (fig. 3) and can be connected in various combinations. another amplifier was designed to record acoustic signals from the sensors, which respond to the slip events. this is a two-channel low-noise amplifier; the amplification rate of channels can be regulated separately. the amplifier’s output voltage is sufficient for recording acoustic signals by a sound card of the pc. in the scheme, the output clipper is installed, which protects pc fig. 3. the configuration of electrodes, located under the supporting sample. �� �� � � ������ �� �� � � �� �� 592 tamaz chelidze, nodar varamashvili, marina devidze, zurab tchelidze, victor chikhladze and temur matcharashvili input from voltage overload and indicates overload of low-noise amplifier as well. the pc used was a pentium ii. the scanning of the process was performed on the frequency 96 khz, i.e. the sampling rate was 1/96 000 s. 3.3. samples the supporting and the slipping blocks were prepared from glass, basalt, labradorite and granite. in all experiments, the support and the sliding block were of the same material. their finish after sewing was either crude (basalt) or fine (other samples). the height of surface protuberances in the former case was in the range 0.1-0.2 mm and in the latter case – 0.050.1 mm. 4. experimental results 4.1. the critical angle measurements the natural critical slopes of slip for various couples of rocks were determined. for each sample, 20 measurements of critical angle were made. the results are shown in the fig. 4. for all samples, except basalt, the critical angle was in the range 8-13°; for basalt it was 25°. the data are scattered around these mean values with a standard deviation of the order of the 0.5°. the experiment on glass discs shows that the area and thickness of samples do not change the critical angle notably. 4.2. assessment of friction coefficient from elementary physics it is known that the friction coefficient µ for the body, placed on the inclined plane, can be calculated from the condition that at the critical angle α c, when the body slips down the plane, the friction force f equals zero α where m is the mass of the upper (slipping) sample, g is gravity force acceleration, or µ = tgαc + 0(α). using data on the critical angle, obtained on the tested materials (fig. 4) the average values of friction coefficient for finished samples are between 0.14 and 0.23 and for the basalt sample µ = 0.47. 4.3. laboratory modelling of em-quakes (experimental procedure and case studies) the main objective of experiments was to find out whether the em-pulse could indeed displace the rock sample placed on the supporting sample at the slope of support less than but close to the critical slip angle. surprisingly, all tested samples except glass were eventually displaced (slipped) by strong em-pulses. the amplitude of dc-pulses was 1200 v; duration from 5 to 10 s, and interval between pulses was also from 5 to 10 s. in the following experiments, the negative pole of the generator was applied to electrodes a and b, and the positive one to electrodes c and d (fig. 3). fig. 4. the values of critical angles of slip for the following pairs of materials: stars = basalt; diamonds = glass (diameter 2 cm, thickness 10 mm); quadrangles = glass (diameter 7.5 cm, thickness 10 mm); triangles = glass (diameter 9.5 cm, thickness 15 mm); crosses = glass (diameter 14 cm, thickness 12 mm); circles = labradorite. f mg c c = −( ) =µ α αcos sin 0 593 laboratory study of electromagnetic initiation of slip after finding the critical angle, the slope of support was decreased by 0.1-2°. in this state, the upper sample was not moving for many hours (days), which means that other sources of instability such as building vibration by trucks, elevator, wind, etc. are not strong enough to initiate the slip. as the critical angle for the rough surface varyies (it is impossible to exactly reproduce the arrangement of asperities between support and slipping block), the sample was kept for at the angle α < αc for 10 min and only after this subjected to em-impact. that allows correct assessment of the statistics of em-activation, as the probability of slip in time intervals without em-impact can be compared with that in time intervals covering the whole em-activation period (including gaps between pulses). actually, the probability of slip without em-impact at α < αc was zero: no slip was observed during 10 min rest periods. application of em-pulse initiates slip either during pulses (i.e. in the active phase), or after them (i.e. in the passive phase). on the experimental recordings (figs. 5 to 7a,b), the horizontal axis is the time and y-axis is the amplitude of ae (upper trace) or applied em field pulse duration (lower trace). when the em field is switched on, the trace is bold, during gaps it is thin. the vertical bars on ae channel correspond to switching on and off of the em field; they are caused by the sound of the switch. on both channels, the y-axis shows the amplitude in decibels (db). a zero value of db scale corresponds to maximum absolute amplitude and negative fig. 5. em-slip on the basalt sample. slip happens in the passive phase. in figs. 5, 6 and 7a,b on both channels the y-axis shows the amplitude in decibels (db). a zero value of db scale corresponds to maximum absolute amplitude and negative infinity (– inf) corresponds to the complete absence of a signal. on the time axis of figs. 5, 6 and 7a,b the first two numbers correspond to hours, next two positions show minutes and the last numbers are for seconds and milliseconds from the beginning of the experiment (for example, 00.02.40.800 means 00 h, 02 min, 40 s and 800 ms). 594 tamaz chelidze, nodar varamashvili, marina devidze, zurab tchelidze, victor chikhladze and temur matcharashvili infinity (-inf) corresponds to the complete absence of a signal. as the figures are screen print outs, time marks are not very clear; corresponding comments are given in captions. small perturbations of ae channel during gaps are caused by action of pc and generator fans. the case histories of slip events are as follows (here only some typical cases are shown): 1) figure 5 presents the history of the test on the samples of basalt. in this test, the ae sensor was applied to the lower side of the organic glass frame under the supporting sample. here and in the following tests, the upper trace shows the acoustic sensor’s output and the lower one records the switching on and off of the em-pulse (the thick part of the lower trace corresponds to the active phase, i.e. current on). in this case, slip was initiated in the passive phase. 2) figure 6 shows in more detail the initial stage of slip on the same basalt sample. 3) figure 7a,b presents the results of labradorite sample testing. the em-induced slip has been observed soon after the application of pulse during the active phase. 4.4. assessment of electrical field intensity (efi) on the slip surface in the following experiments, only basalt samples were studied. the voltage applied to the samples was 1200 v; nevertheless it is important to know the electrical field, acting on the slip surface. the calculation of electrical field on the slip surface can be carried out using standard equations of electrostatics. for the pair of co-planar electrode systems used in our experiment, the efi on the height 2.5 cm above the electrodes, put under voltage 1250 v, varies from approximately 10 v/cm to 40 v/cm if the space is filled by material with dielectric constant 5 (characteristic for basalt in room conditions). the calculated values are for homogeneous media. real samples are very heterogeneous; that means that the efi should also be measured experimentally. efi on the edges of interface between supporting and sliding samples was measured by a digital multimeter with accuracy ± 0.01 v. the voltage was measured relative to fig. 6. em-slip on the same basalt sample test; the time scale of the initial stage of slip (from 00.02.40.800 to 00.02.4.000) is stretched. 595 laboratory study of electromagnetic initiation of slip fig. 7a,b. em-slip of the labradorite sample; slip happens in the active phase. a) the whole recording; b) the initial part of recording stretched. a b 596 tamaz chelidze, nodar varamashvili, marina devidze, zurab tchelidze, victor chikhladze and temur matcharashvili the negative pole. the distribution of efi proved to be rather mosaic: the values of v along the perimeter of slip surface vary from 0.01 to 57 v and the average value is around 8 v for the separation of probe sites of the order of 1 mm. these values are in quantitative accordance with above theoretical calculations. we can conclude that electrical impact of the order of several volts per millimeter can induce slip in the system close to the critical state. 4.5. assessment of mechanical equivalent of electrical impact for assessment of mechanical equivalent of electrical impact, both direct and theoretical methods were used. in the first case, the mechanical force, initiating slip at the same angle α <αc set in experiments with em-impact, was measured by spring (accuracy ± 0.01 n) and torsion (accuracy ± 0.005 n) dynamometers. both methods give comparable results. the force, equivalent to slipinitiating em-impact is of the order of 0.2 n. another way to obtain the mechanical equivalent is to calculate it from the general equation of balance of forces for a sample placed on the inclined plane as far as µ is known (µ = 0.47), slip-initiating force can be calculated for any angle. for example, if αc equals 25°, at α = 24°50′ the initiating force is 0.42 n. this value is of the same order as in direct experiments (0.2 n = 2 104 dyne). 4.6. statistics of em-slips it is well known that surface phenomena are rather complicated and influenced by many factors: surface roughness, configuration of asperities, humidity, temperature, chemistry of contacting materials, etc. so it is quite natural that, in the case of contact of rough surfaces, only a small part of the empulses initiate motion, namely the pulses that are applied at the most favourable arrangement of asperities on the slip surface. a general idea on the efficiency of impact can be given by the results of one series of experiment on the basalt sample in which 94 pulses in 30 tests (each test contained several pulses) were applied to the system and 7 of them were accompanied by slip. the total probability of em-initiation p is thus 7/94 = 0.07; here the probability of slip during the pulse is 0.04 and that of slip during gap between pulses is 0.03. the probabilities of initiation for various stages of impact for this series are: during first pulse – no case; after first pulse – 0.01; during second pulse – 0.02; after second pulse – 0.01; during third pulse – 0.02, etc. of course, these probabilities vary from one series to another one but the total probability does not change considerably. it should be noted that the observed process is extremely complicated and its mechanism is not clear. nevertheless, the experiments show that em-initiation of slip in mechanical system that is close to the critical state is quite real. 4.7. probable physical mechanism (dry conditions) in order to understand the physics of emslip, it is necessary to consider fundamentals of surface phenomena. intermolecular and intersurface forces, responsible for adhesion and friction, can be loosely divided into three categories: i) purely electrostatic, arising from the coulomb interaction between charges; ii) polarization forces arising from the dipole moments, induced by internal (bound charges, dipoles) or external electric field; iii) quantummechanical forces, responsible for covalent bonding and steric interactions. all these forces can act simultaneously, resulting in some total adhesion (friction) force. for friction we have ff = µ fn where µ is friction coefficient and fn is the normal component of force acting on the body (gravity, compression). from the above classification, it can be deduced that in principle the external electrical field f mg= −( )µ α αcos sin . 597 laboratory study of electromagnetic initiation of slip can affect the intersurface adhesion (friction) forces, changing either µ or fn and thus initiating slip of the body, placed on the inclined plane. of course, em-impact can also affect fn of the body containing piezoelectric materials. as far as the em-activation is clearly observed on samples that are practically free of piezoelectric minerals (basalt), we exclude the piezoelectric effect as a principal mechanism of em-slip. having in mind that quite often initiation of slip was observed after switching off the power, it seems reasonable to accept the hypothesis that the applied em-pulse affects the electrostatic and/or polarization component of the intersurface adhesive forces (forces due to polarization are acting even in passive periods). 5. conclusions a phenomenological analysis of the possible physical mechanisms of mhd-induced microseismicity has been done. an experimental set up was built for laboratory modeling of the impact of electromagnetic (em) pulses on the stability of the mechanical system, which is close to the critical state. the slope of the supporting plane is considered as an analogue of the tectonic stresses for natural faults. the preliminary experiments, carried out in dry environment, show that a strong em-pulse (1200 v) may induce sliding of a sample of rock (granite, basalt, labradorite) placed on the supporting sample, which is inclined at the slope, close to, but less than the critical angle. of course, the em field in our experiments was larger than in the field conditions; nevertheless the experimental proof of possibility of em-initiation of mechanical instability seems to be important. the mechanical equivalent of electrical force, initiating slip, was evaluated both theoretically and experimentally: it was of order of 0.20 n for the slopes of support used in experiment. the statistics of em-slips were studied: the probability of em-activation by series of pulses was found to be in the order of 0.07. references de archangelis, l, s. redner and a. coniglio (1986): multiscaling approach in random resistor and random superconducting networks, phys. rev., 34, 4656-4673. bak, p., c. tang and k. wiesenfeld (1988): self-organized criticality, phys. rev., a38, 364-374 benguigi, l. (1988): simulation of dielectric failure by means of resistor-diode random lattices, phys. rev, b 38, 7211-7214. vorobyev, a., m. samokhvalov and r. ibragimov (1975): search of effects due to existence of high local electric fields in the lithosphere, in seismology of uzbekistan, tashkent, 213-221 (in russian). kharitonov, e. (1983): dielectric materials with heterogeneous structure, moscow, radio and communication (in russian), pp. 127. king, g., r. stein and j. lin (1994): static stress changes and the triggering of earthquakes, bull. seismol. soc. am., 84, 935-953. nesvetailov, g. and e. serebriakov (1966): theory and practice of electrohydraulic effect, minsk (in russian). scholz, c. (1990): the mechanics of earthquakes and faulting (cambridge univ. press, cambridge), pp. 439. sibson, r. (1994): crustal stress, faulting and fluid flow, in geofluids: origin, migration and evolution of fluids in sedimentary basins, edited by j. parnell, the geological soc. london, 69-84. sobolev, g., a. ponomarev, a. avagimov and v. zeigarnik (2000): initiating acoustic emission with electric actions, report on esc conference. tarasov, n., h. tarasova, a. avagimov and v. zeigarnik (1999): the effect of high-power electromagnetic pulses on the seismicity of central asia and kazakhstan, volcanology and seismology (moscow), 4-5, 152-160 (in russian). (received february 18, 2002; accepted october 8, 2002) vol49_2_2006 595 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words decommissioned submarine cables – ocean bottom observatory – real-time telemetry – ecological monitoring – junction box – obs 1. introduction during the last several decades, intense geophysical and geochemical investigations on the seafloor have been carried out in regions such as subduction zones, mid-ocean ridges, and abyssal areas away from plate boundaries, and results from these studies have greatly improved the details of our geophysical understanding of the earth. however, very few investigations of temporal changes in these geophysical/geochemical phenomena have been carried an experimental multidisciplinary observatory (venus) at the ryukyu trench using the guam-okinawa geophysical submarine cable junzo kasahara (1), ryoichi iwase (2), tadashi nakatsuka (3), yoshiharu nagaya (4), yuichi shirasaki (5), katsuyoshi kawaguchi (2) and ju’ichi kojima (6) (1) earthquake research institute, university of tokyo, japan (2) japan marine science and technology center (jamstec), yokosuka, japan (3) national institute of advanced industrial science and technology (aist) , tsukuba, japan (4) hydrographic and oceanographic department, japan coast guard, tokyo, japan (5) institute of industry and technology, university of tokyo, tokyo, japan (6) kddi laboratory, kddi co. ltd., kasumigaseki, saitama, japan abstract a multidisciplinary ocean bottom observatory (mdobo) was installed on venus (versatile eco-monitoring network by undersea-cable system) at a depth of 2170 m on the slope of the ryukyu trench. in this context, «eco-»refers to both economic (e.g., earthquake hazard mitigation) and ecological motivation. the first step in this instillation was to insert a telemetry/power system into the submarine coaxial cable; this system could then service the mdobo, which consists of seven major bottom sensor packages. during august-september 1999, using a deep-towed unit and both manned and unmanned submersibles coupled with precise ship navigation, the mdobo system and its attendant cables were deployed over a range of distances from 80 m to 1 km from the telemetry system, with several meters allowance for navigational uncertainty in positioning. the unmanned submersible then extended the multi-conductor extension cables from the instrument units toward the telemetry system and connected them to undersea mateable connectors on a junction box installed on the submarine cable. the mdobo collected one and half months of continuous records. several kinds of useful data were collected after installation, including an aftershock (ms = 6.1) of the 1999 chi-chi earthquake (ms = 7.7) in taiwan. mailing address: dr. junzo kasahara, earthquake research institute, university of tokyo, 1-1-1 yayoi, bunkyo, tokyo 113-0032, japan; e-mail: junz_kshr@ybb.ne.jp now at: japan continental shelf survey, ntc bldg. 3f, 1-11-2 kyobashi, choo-ku, tokyo 104-0031, japan. 596 junzo kasahara et al. out. it has been widely recognized that such temporal investigations are of great importance, since geophysical and geochemical phenomena have episodic natures and some phenomena such as earthquakes, landslides, or submarine volcanic eruptions require real-time observation. several subduction zones surround the japanese archipelago. subduction of the pacific and philippine sea plates beneath the japanese archipelago generates destructive earthquakes along the plate boundaries beneath the forearc slopes. to minimize casualties and damage to buildings caused by large earthquakes, it is important to study both the nature of these earthquakes and the seismic structures along subduction zones. real-time seismic observation on the deep-seafloor has been proposed from the viewpoint of understanding these earthquake hazards. in addition to hazards, real-time monitoring of earthquakes is very important to understand ongoing movements along subduction zones. environmental measurements on the deep-seafloor are important from the viewpoint of monitoring of ecological changes due to the disposal of chemical materials in the sea and/or accidents involving nuclear submarines. thus, long-term and realtime geophysical/geochemical observations are essential for understanding earthquake hazards and environmental changes in the deep-sea. one of the best methods currently available for collecting and transmitting real-time observations is the use of submarine cables, which have long histories of technological development and proven field use in telecommunication. although fiber-optic submarine cables use very advanced and reliable technology, using new fiber-optic submarine cables is extremely costly. another kind of submarine cable is the coaxial cable, which can provide electrical power and real-time telemetry similar to fiberoptic cables (submarine cable association, 2003). due to the rapid growth of fiber-optic technology and large demands for global telecommunication, a number of fiber-optic submarine cables with giga-to-tera bit capacity have been deployed. although many submarine cable obss have been deployed around the japanese coast during the past two decades (e.g., mikada et al., 2003), each system required a huge investment. construction of similar systems far from the shore is both more difficult and more expensive. for example, a submarine cable obs along the izu-bonin arc would require much longer cables and using a fiber-optic system is neither practical nor economical. with the installation of fiber-optic systems, the older tpc-1 (trans pacific cable-1) and tpc-2 (trans pacific cable-2) coaxial submarine cables were removed from commercial service in 1990 and 1994 after 26 and 18 years of near-continuous use, respectively. tpc-1 (kdd, 1964) was the first japan-us submarine cable, constructed in 1964, and tpc-2 (kdd, 1976) was the second, constructed in 1976. by reusing such resources, real-time geophysical observatories on the deep-seafloor can be installed and implemented with high reliability and at reasonable cost (nagumo and walker, 1989; kasahara et al., 1995; kasahara, 2002). sections of the tpc-1, between ninomiya, japan and guam island, and the tpc-2 between okinawa island, japan and guam island were donated to the eri (earthquake research institute) and the incorporated research institutions for seismology (iris) in 1990, and to the eri in 1996, respectively. both cables cross geophysically important regions (fig. 1). tpc-1 is routed from guam, along the mariana-boninizu trenches to sagami bay (kdd, 1964). tpc-2 is routed from the mariana trough across the mid-philippine sea plate to the ryukyu trench (kdd, 1976). these areas contain very seismically active subduction zones, many active volcanoes, and a rifting backarc basin. we developed two cabled ocean bottom observatory systems using decommissioned submarine cables, the first is called geo-toc (geophysical and oceanographical trans ocean cable using tpc-1 cable; kasahara et al., 1995) and the second venus; we describe the latter in this paper. in 1997, an obs (izu-obs; kasahara et al., 1995) using the geo-toc was deployed at 2800 m depth on the forearc slope of the izubonin trench (fig. 1). it consisted of three-component accelerometers with 24-bit a/d resolution, a hydrophone, and quartz temperature597 experimental multidisciplinary observatory (venus) pressure sensors. it transmitted uninterrupted seismic and pressure-temperature data to tokyo for more than 5 and a half years, an exceptional record for any seismometer, let alone one on the seafloor far from japan. data transmission was abruptly terminated on october 1, 2002 just after a large typhoon passed above the shore station in ninomiya, japan. the cause of termination was found to be cable faults located 9 km from the shore station, probably related to turbidity currents caused by flooding of the nearby hayakawa river. in fall, 2003, we decided to use only off-shore segments of tpc-1 for a new installation of sea-earth at 400 km distance from tokyo, because of heavy cable crossings by commercial telecommunication lines ∼ 9 km from the coastal region of japan. these crossings make it impossible to repair the cable faults. the tpc-2 submarine cable was used to develop multidisciplinary ocean bottom observatory (venus project). in this paper, we describe the outline of the venus system and show examples of the data obtained by this system. fig. 1. cable routes of the geo-toc and gogc submarine cables, and locations of the izu-obs and the venus-mdobo. 598 junzo kasahara et al. 2. venus-gogc observation system in contrast to the geo-toc obs, the venus project (1995-1999) was intended to develop new technologies for using decommissioned submarine cables for environmental measurements at the ocean bottom. in the fall of 1999 a multidisciplinary observatory using the gogc cable (kasahara et al., 2000) was installed at a depth of 2200 m on the forearc slope of the ryukyu trench, ∼50 km from okinawa island (fig. 1). the objectives of the venus project (kasahara et al., 2000, 2001) were to develop a multidisciplinary station to study deep-sea environmental changes due to the subduction of the philippine sea plate at the ryukyu trench. eight japanese institutions cooperated on this project. the cable length of the gogc is 2400 km. the system uses a so-called sfsystem with 1.5 inch-diameter coaxial cables (kdd, 1976; submarine cable association, 2003). the former tpc-2 system had 845 voice channels. although +1.080v dc from okinawa and −1.080v dc from guam were supplied to the cables at a constant current during commercial use, the electric power supply for the venus project was modified to use a single 3.000v dc source from okinawa. the observatory system comprises seven ocean bottom sensor units (kasahara et al., 2000, 2001), an ocean bottom telemetry system, and several coaxial cables for connection to the submarine cable. the land system comprises a shore station and a data center. the total power dissipation caused by the bottom units (see fig. 2) was approximately 53.5 w (kasahara et al., 2000; kojima et al., 2000). the telemetry system in the data telemetry unit used 20 w, and the sensor packages used the rest. to minimize corrosion during the long observation period, all pressure cases and the major parts of frames for the botfig. 2. instruments and telemetry system configuration for the venus-mdobo. 599 experimental multidisciplinary observatory (venus) tom units were made of titanium and plastic. plastic insulators protected titanium and stainless steel interfaces. the sensor units comprise ocean bottom broadband seismometers (obbs), a tsunami pressure sensor, a hydrophone array, a multi-sensor unit, geodetic instruments, geoelectric-geomagnetic instruments, and a mobile unit (fig. 2) (kasahara, 2000; kasahara et al., 2000, 2001). the obbs uses gimbal-mounted «guralp cmg-1t» tri-axial broadband seismometers with response bands between 300 and 0.05 s. obbs outputs are digitized at 24-bit resolution and 100 hz sampling. the tsunami gauge uses a quartz pressure sensor and the resolution for sea-level change is 0.5 mm (katsumata et al., 2000). the multi-sensor unit comprises shortperiod seismometers, a hydrophone, a digital camera, a ctd sensor, a current meter, a light transmission meter, and sub-bottom temperature probes (iwase et al., 2000). the hydrophone array is composed of five hydrophones with 700 m spacing (wider than optimum due to budgetary limitations) (watanabe, 2000). during the experiment, sixteen-bit data were transmitted to shore. the geodetic changes were acoustically determined by precise baseline measurements between two transponders (nagaya, 2000). three units were placed in a triangular formation and the distance between two units was approximately 1 km. the estimated accuracy of geodetic measurements is a few cm/yr, which is estimated to be less than the expected precursory crustal deformation near the trench if a m 7 or greater earthquake occurs just beneath the site (kasahara et al., 1998a,b). the geoelectric-geomagnetic unit comprises a proton magnetometer, flux-gate magnetometers, and orthogonal geo-potentiometers, 20 m long each (nakatsuka et al., 2000). the mobile unit consists of an acoustic communication unit and a remote instrument (iidaka et al., 2000). a separate report describes the details of each sensor (kasahara, 2000; kasahara et al., 2000, 2001). the bottom telemetry system comprises a data-coupling unit, a data-telemetry unit and a junction box (fig. 2) (kasahara et al., 2000; kojima et al., 2000). the first dc-dc converter in the data-coupling unit creates 100 v dc power using 880 v dc extracted from 136 ma constant cable current, and then the second dc-dc converter in the data-telemetry units does 24 v dc using 100 v dc for the sensor packages. the power separation filter in the data-coupling unit separate the high-voltage dc component from the highfrequency carriers, and later re-mix the high-frequency carrier with the dc component. the datatelemetry unit multiplexes the data and sends them to shore using a 240-khz-carrier bandwidth. the transmission rate for the multiplexed data is 96 kbps. each instrument, however, uses a unique transmission rate, e.g., 19.2 kbps for the obbs. hydrophone data uses another 240-khz bandwidth. if an instrument does not operate correctly, users can shut it down remotely from the land base. the junction box has nine so-called rov (remotely operated vehicle) undersea mateable connectors (fig. 3). the rov connector has 8 conductor pins: two pins for 24 v dc, three for data from sensors to telemetry, and three for command signals to sensor. the extension cables between sensor packages and the junction box have 8 conductors with od 17 mm filled inside by jelly-like insulating material and jacketed by plastic. the rov connectors allow different units to be switched in or out of the sensor array on the ocean floor using either submersibles or rov. the route of the gogc cables was identified on the ocean floor (under a few centimetres sediment cover) using a deep-tow camera on the r/v yokosuka in february 1998. usually, submarine cables shallower than 1500-200 m depths are buried to minimize any damage from fishery activities. in the tpc-2 venus cable, the cables near the test site were not buried in the sediment because of the depth. in march 1998, the manned submersible shinkai 6500 (dive # 411/yk9802-leg3) cut the gogc at 25°44ln, 12°02.5le at a water depth of 2200 m using a newly developed cable cutter. three major operational legs were carried out by the m/s kuroshio-maru, the r/v kaiyo, and the r/v karei/rov-kaiko for deployment of the telemetry system, deployment of instruments, and extension-connection of cables, respectively. instruments were confined to an area within an approximately 1-km radius around the junction box. the data-coupling unit was spliced into the main cable on the deck of the m/s kuroshio-maru in august 1999. the m/s kuroshio-maru installed the bottom telemetry 600 junzo kasahara et al. system with the tsunami sensor and the hydrophone array on the ocean floor. the location of the telemetry system is 25°44.53ln and 128°03.52le (wgs84) at a water depth of 2.157 m. the mother ships used gps system for navigation, and the submersibles (shinkai 6500 and rov kaiko) and the deep-tow unit used by lbl systems based the gps navigation. the deep-tow equipment on the r/v kaiyo was used to install five instrument units on the ocean floor in september and october 1999. the rov-kaiko coupled each male termination of the rov connectors on the extension cables of nine instruments to the corresponding female connectors on the junction box installed on the main cable october 1999 (fig. 3). the obbs was placed ∼80 m north of the bottom telemetry system at a water depth of 2154-m (fig. 4). the obbs was not buried in sediment, as there was no shovel installed on the rov-kaiko. instead of burial, 80 kg weights were placed on the obbs frame to anchor it to the ocean floor; however, this method does not seem to be sufficient to reduce low frequency noise caused by infra-gravity waves (kasahara and sato, 2000; kasahara et al., 2001). the venus specific equipment was installed in the kdd okinawa shore station. some shore equipment was obtained from the previous station used by the tpc-2 system. the shore-receiving unit demodulated signals and sent them to yokosuka, japan, using two 64-kbps lines, and supplied 3100 v to the cable. data from ocean bottom instruments were archived in the data storage unit at jamstec (japan marine science and technology center). 3. data obtained by venus system some of the results obtained by the venus experiment are presented in this section. the fig. 5. photo taken by a digital camera incorporated in the multi-sensor system. a fish is passing near the sub-bottom temperature probe. fig. 4. venus broadband seismometer (photo taken from rov kaiko). fig. 3. junction box and rov kaiko manipulator during installation operations on the ocean floor. 601 experimental multidisciplinary observatory (venus) tilting caused by infra-gravity waves (approximately, 100-200 s) (webb, 1998). the data obtained by the multi-sensor system are shown in fig. 8a-c. the water temperature data (fig. 8a) exhibit nearly identical variations with the electrical conductivity data in time, showing both 15-day and diurnal cycles. the salinity is almost constant except for spikes, which seems to be caused by heat convection generated by the electronics of data transmission unit attached under ctd sensor. the temperature measured by sub-bottom temperature probes (fig. 8c) shows similar variation with the water temperature. on the other hand, the eastwest component of bottom current (fig. 8b) shows semi-diurnal change, although northsouth component mostly shows diurnal change except for the spring tides. vertical transfer of water with temperature gradient of the water column mainly causes the water temperature change. diurnal change in both water temperature and north-south component of water current fig. 6. ms = 6.1 earthquake, november 1, 1999, which occurred off taiwan. ns (34.4 × 10−5 m/s full scale), ew (the same scale as ns) and z (14.1× 10−5 m/s). horizontal axis: 20 min record. multi-sensor system had a digital camera and transmitted photo images to land every hour. figure 5 shows a fish passing near the sub-bottom temperature probe. a number of earthquakes, including two large events in southern california (ms = 7.3) on october 16 and the taiwan earthquake (ms = 6.1) of november 1, 1999, were observed (fig. 6) (kasahara and sato, 2000). the taiwan event was one of the aftershocks of the september 21, 1999 chi-chi (taiwan) earthquake (ms = 7.7). the noise levels in data obtained from the broadband seismometers seem to be extremely high (e.g., ≅ 5 µm/s at 300 s for horizontal components) (fig. 7). the change in amplitudes with time is several tens of times higher on the horizontal components than on the vertical components. the reason for large noise amplitudes on the horizontal components is partly because the obbs was not buried in ocean-bottom sediments (kasahara and sato, 2000; kasahara et al., 2001). another reason is related to 602 junzo kasahara et al. f ig . 7. e xa m pl e of o ce an b ot to m n oi se o bs er ve d by o b b s . 30 -m in r ec or ds . t op t hr ee r ec or ds : ve rt ic al s ca le i n ± 1. 3 × 10 − 2 cm /s . b ot to m t hr ee re co rd s: ve rt ic al s ca le i n ± 2. 36 × 10 − 3 cm /s f or n s a nd e w c om po ne nt s, an d ± 1. 89 × 10 − 3 cm /s f or z co m po ne nt . f ig . 8a . r ec or ds o bt ai ne d by t he m ul ti -s en so r sy st em ( fr om t op t o bo tt om ): sa li ni ty ,e le ct ri ca l co nd uc ti vi ty ,w at er t em pe ra tu re ,w at er p re ss ur e, li gh t tr an sm is si on t hr ou gh w at er . 7 8a 603 experimental multidisciplinary observatory (venus) fig. 8b,c. records obtained by the multi-sensor system. b) sea-water bottom current estimated by measurements collected using the ns and ew components of an electromagnetic current meter. note: there are some offsets due to lack of absolute level calibration. c) sub-bottom temperature measurements from three probes at 0 cm, 10 cm and 20 cm below the ocean floor compared to ctd temperature. b c 604 junzo kasahara et al. fig. 9. records obtained by the geoelectric-geomagnetic unit. 42-days observation at the okinawa deep-sea site (top) and data from kakioka magnetic observatory (middle) are well-correlated, with slight differences due to the difference in latitude. variations related to a magnetic storm took place on october 22, 1999 (jst), and subsequent electro-magnetic phenomena are shown on the bottom graph. electric field data are represented as the voltage between two electrodes separated by about 20 m. the disturbances in the electric field data became much larger after november 06, which might indicate some unknown process related to the progress to the stop of whole observation system. 605 experimental multidisciplinary observatory (venus) is probably due to the regional topography of southeastward or south-south-eastward slope, although more regional observation is necessary to confirm the result. the geoelectric-geomagnetic unit was operated continuously for 42 days (fig. 9), until system failure occurred as an unfortunate result of water leakage in that sensor’s telemetry unit (kojima, 2003). the continuous geomagnetic observation data exhibit field variations similar in pattern, but different in absolute values, from those observed at the kakioka magnetic observatory in central japan, which is quite reasonable as the venus deep-sea site is about 1.600 km sw of kakioka. during this period of observation, a remarkable magnetic storm took place on october 22, 1999. the fine-scale 3-day variations of electric and magnetic fields subsequent to this event are illustrated in fig. 9. our observations clearly detected electro-magnetic variations related to this storm and subsequent phenomena, which could be the subject of further investigations. electric field data have a somewhat noisy character, probably due to the effects of bottom currents. we consider that electric field data express both the induction of magnetic field variation such as magnetic storm and the electro-motive force from the bottom current in the geomagnetic field, though there is a somewhat noisy character in it. the electric field disturbances became larger after november 06. this increase in noise levels may be related to some unknown process, or alternatively may be related to the progressive breakdown of the observation system due to water leakage. 4. conclusion and future plans a multidisciplinary ocean bottom observatory was specifically developed with the aim of utilizing decommissioned submarine cables: the venus multidisciplinary station using the gogc cable at the ryukyu trench. the geotoc-izu obs used digital data acquisition, but it borrowed many methods, techniques and technologies from traditional submarine cable technology. in contrast, the venus system is more sophisticated than geo-toc, and was deployed by manned and unmanned submersibles, deeptow equipment, and cable ships. many new technologies have been developed for both obs systems. the venus system comprises six major bottom observatory stations, and one land based observation-recording-transmitting station. some of the results are presented here. the obbs records from the venus observatory highlight the necessity of installing or burying seismometers in sediments to reduce noise (e.g., webb, 1998). the venus system proved the usefulness of decommissioned submarine cables for multidisciplinary measurements on the deep-seafloor. however, it also reveals the difficulties in deployment, long-term observation recording and monitoring, and methods of instrument installation and maintenance. the submarine cable between okinawa and ninomiya (okinawa cable) will be used to monitor future large earthquakes along the nankai trough. the areana (advanced real-time earth monitoring network in the area) plan is another potential use of these cables to monitor deep-sea environmental change in areas surrounding the japanese archipelago (kasahara et al., 2003; shirakaki et al., 2003). the neptune project in the western u.s.a. and canada will install a submarine cable network off seattle and vancouver island to observe temporal changes in a wide array of multidisciplinary phenomena. real-time, long-term measurement will remain a major objective for future geophysical/geochemical ocean bottom observatories, and the (re)utilization of submarine cables will continue to be a crucial technology for both installation and monitoring of these systems. acknowledgements the ministry of education, science, culture and sports, japan supports the geo-toc project. the science and technology agency, japan supports the venus project. eights institutions (jamstec, jma, eri, gsj, kdd lab., crl, etl, and hydrographic office) joined in this research. the author thanks jamstec for its great assistance in operating research vessels and submersible vehicles. the author also thanks eri for permission to use the geo-toc and gogc cables for seismological research. 606 junzo kasahara et al. references iidaka, h., t. doi, t. saito, h. nakkano, s. sato and y. fujinawa (2000): development of maneuverable and flexible deep ocean observation by acoustic transmission, in final report on the venus project, edited by j. kasahara (eri), 47-56 (in japanese with english abstract). iwase, r., k. kawaguchi and k. mitsuzawa (2000): development of the multi-sensor observation equipment, in final report on the venus project, edited by j. kasahara (eri), 47-56 (in japanese). kasahara, j. (editor) (2000): final report on the venus project (eri), pp. 206 (in japanese). kasahara, j. 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(1998): broadband seismology and noise under the ocean, rev. geophys. space phys., 36, 105-142. layout 6 annals of geophysics, 60, supplement to 6, se668, 2017; doi: 10.4401/ag-7520 attenuation characteristics, source parameters and site effects from inversion of s waves of the march 31, 2006 silakhor aftershocks in western iran somayeh ahmadzadeh1, stefano parolai2,3, gholam javan-doloei*,1, adrien oth4 1 international institute of earthquake engineering and seismology, tehran, iran 2 helmholtz centre potsdam, gfz german research centre for geosciences, potsdam, germany 3 now at: istituto nazionale di oceanografia e di geofisica, sperimentale ogs, sgonico (ts), italy 4 european center for geodynamics and seismology, luxemburg article history received august 15, 2017; accepted october 30, 2017. subject classification: generalized inversion technique; source parameter; site response; bootstrap analysis; zagros. se668 abstract a nonparametric generalized inversion technique (git) has been used to derive attenuation characteristics, source parameters and site amplification effects from s wave spectra of the 2006 silakhor earthquake aftershocks recorded at 10 temporary stations in the epicentral area of the main shock along the main recent fault in zagros region. we apply a total number of 971 spectral amplitudes from 213 aftershocks with ml 1.5-4.4 in the frequency range 0.5-35hz. the obtained nonparametric attenuation curves decay monotonically with distance for the entire frequency range and the estimated s wave quality factor shows low q values with fairly strong frequency dependence. we fit the omegasquare source model to the inverted source spectra to derive earthquake source parameters. the estimated stress drops range from 0.04 to 5.3 mpa with an average of 0.9 mpa, within the range typically observed for crustal earthquakes in other regions of the world. the evaluated apparent stresses vary from 0.009 to 0.9 mpa and its relation with stress drops is generally consistent with the theoretically expected relation assuming brune model. the local magnitude ml shows a linear correlation with the moment magnitude mw that underestimates the mw scale by about 0.1-0.5 magnitude units for mw 1.5-3.0. we found a gradual increase in corner frequency values against seismic moments that may be considered as an evidence for a deviation from self-similarity scaling though the magnitude range of this study is fairly small to deduce a firm conclusion. the comparison of estimated git site amplification functions with the evaluated horizontal to vertical (h/v) spectral ratios shows different levels of amplification in site responses due to amplification of the vertical component; however, the overall shape of site spectra provided by both methods are relatively similar. 1. introduction the iranian plateau among the alpine-himalayan seismic belt is limited between the convergent movements of the arabian plate in the southwest and the eurasian plate in the northeast. this convergence makes iran one of the most seismically active areas of the world. iran has been frequently shaken by great earthquakes accompanied with heavy casualties and economic losses. the zagros fold thrust belt, known as one of the youngest and most active continental collision zones on earth, is the result of the continued convergence between arabia and eurasia plates [snyder and barazangi 1986]. therefore, the zagros region can be considered as one of the major active seismotectonic provinces with frequent and large earthquakes. the main recent fault (mrf) is an important element of tectonic units defined within the zagros composed of a series of right-lateral strikeslip faults. earthquakes occurring on or near the mrf have larger magnitudes than those in the other parts of the zagros region. also, the mechanism of earthquakes and seismic potential of the mrf is quite different from those earthquakes occurring within the rest of the zagros range. the most devastating earthquake of the region, the 1909 january 23 silakhor earthquake (mb 7.4), occurred on the main recent fault and produced over 40 km surface rupture [tchalenko and braud 1974]. based on the report of ambraseys and melville [1982], this historical event was strongly felt in a large area with more than 100 damaged villages and more than 6000 fatalities. the latest large event along the main recent fault (mrf), the silakhor earthquake of 31 march 2006 with ml 6.1 (iiees), occurred in the silakhor plain in the south of broujerd city in the lorestan province in west iran. a hypocentral depth ranging between 6 and 14 km was reported by several agencies [peyret et al. 2008, ghods et al. 2012]. according to field investigations no significant surface rupture was observed and a maximum intensity of viii was reported by moghadam et al. [2006]. this event severely affected about 300 villages in the silakhor plain with 68 fatalities and more than 1400 injuries. the 2006 silakhor earthquake was preceded by two fairly large foreshocks with magnitudes of 4.6 and 5.2, respectively, and followed by a large number of aftershocks. the occurrence of two relatively large foreshocks effectively alarmed many people to leave their homes leading to a lower number of casualties than expected. earthquake ground shaking at a specified location depends on three main region-specific parameters including seismic-source, wave propagation parameters and seismic-site effects. the study of these region-specific parameters is required to predict accurate ground-motion amplitudes in order to enable reliable seismic hazard assessment. the source, path and site effects can be separated from each other in the fourier amplitude spectra (fas) of recorded ground motion by implementing the common generalized inversion technique (git). this method was firstly proposed by andrews [1986] recasting the spectral ratios method into a generalized inverse problem, then decomposing the body-wave spectra into source, site, and path contributions, simultaneously. several authors have used and developed this approach to earthquake ground-motion data worldwide [castro et al. 1990, boatwright et al. 1991, hartzell 1992, parolai et al. 2000, salazar et al. 2007 among others]. for instance, castro et al. [1990] introduced a two-step nonparametric inversion method to isolate source, site and path effects along the guerrero, mexico, subduction zone. in recent years, a few studies have been conducted to investigate seismic-source and wave propagation parameters in various regions of the iranian plateau. among these, however, only a limited number are related to the zagros region. zafarani and hassani [2013] applied the git to 148 s-wave amplitude spectra of 35 events recorded at 40 stations of the iranian strong-motion network, which have been installed by the building and house research center (bhrc). source parameters and site response of swaves in two parts of the zagros region were examined for the events in the magnitude range from 4.2 to 6.2. rahimi et al. [2008] estimated the coda q, qc, for the zagros zone by analyzing the coda waves of 51 local earthquakes recorded on three stations of the iranian national seismic network (insn) with magnitudes ranged from 3.1 to 4.9 during march and april 2006.most of the analyzed events were foreshocks and aftershocks of the 2006 silakhor earthquake. using the sh-wave acceleration spectral data, hamzehloo et al. [2010] estimated the source parameters and frequency dependent shear wave quality factor of the 2006 silakhor earthquake and only one of its foreshocks and one aftershock that were recorded at 8 stations. in our study, we applied the nonparametric git to a large number of 2006 silakhor earthquake aftershocks, with a dataset composed of 971 records from 213 events, to evaluate source, site and path effects, simultaneously in the epicentral area of silakhor earthquake. the obtained nonparametric attenuation functions are parameterized to obtain an estimate of the s-wave frequency-dependent quality factor and the results are compared with other studies. the evaluated source spectra are interpreted using the omegasquare brune [1970] model to retrieve seismic moments and stress drop values. finally, we compare and discuss the site response functions estimated from git with those from horizontal-to-vertical (h/v) spectral ratios. 2. database on 31 march 2006, the ml 6.1 (iiees) silakhor earthquake occurred along the zagros main recent fault (mrf) in the lorestan province, west iran. the earthquake was followed by a large number of aftershocks. most of them were recorded by a local temporary seismic network including 10 short-period stations that was deployed around the epicenter of the main shock, 5 days after the mainshock for about 2 months from 5 april to 10 june 2006. the network consisted of five three component short-period guralp cmg-6td seismometers and five guralp cmg5td accelerometers with a sampling rate of 100 hz. more than 1300 aftershocks were recorded during network installation. the local magnitudes of these events were determined using seisan software [havskov and ottemöller 1999]. we relocated events using the velocity model for the region [sepahvand et al. 2012] and selected a subset of 213 events with wellconstrained locations (location error <3 km and gap <180°) with ml>1.5 (figure 1). each of the selected events is recorded by at least three stations and each of the used stations recorded at least three events. the final database is composed of 971 records from 213 earthquakes with a magnitude in range of ml 1.5-4.4 and hypocentral distances ranging from 3 km to 41 ahmadzadeh et al. 2 3 attenuation, source and site effects from inversion of the silakhor aftershocks km. figure 1 shows the location of 2006 silakhor mainshock and 213 selected aftershocks as well as temporary local stations around the zagros main recent fault with the source-station travel paths. the distribution of data in terms of magnitude versus hypocentral distance and focal depth is presented in figure 2. the dataset spans the range of ml = 1.5-4.4 with focal depths ranging between 0.5-14 km. the majority of our events have magnitudes in the range 1.5 to 3 and the smallest hypocentral distance in the dataset is 3 km. since near-field terms normally become most important at distances shorter than a few source dimensions, this effect is not expected to considerably affect the analysis. figure 1. epicenters of 2006 silakhor earthquake (star) and aftershocks (circles). inverse triangles indicate the location of 10 stations installed around the epicenter of the main shock. the source-station ray paths are shown as gray lines. figure 2. distribution of earthquake magnitude (ml) versus hypocentral distance (left) and depth (right). for the spectral analysis, the raw recordings were corrected for instrument response and a time window of 5 seconds length was selected on the horizontal components starting 1 second before the s-wave arrival. each selected window is cosine-tapered (5%) and the acceleration fourier amplitude spectra (fas) have been calculated. to compute the signal-to-noise ratio (snr), a pre-event noise windows with the same ahmadzadeh et al. 4 figure 3. three instrument-corrected ew component velocity seismograms at stations absa, vana and kabo (left) and calculated acceleration fas of the seismograms (right). the smoothed fas using the konno-ohmachi window with b=20 are indicated with gray lines for both signal and noise spectra. 5 attenuation, source and site effects from inversion of the silakhor aftershocks length as the s-wave windows were considered (for some events the available pre-p noise were less than 5 seconds) and only records with a snr larger than 3 at each frequency in the selected frequency range was taken into account for the present research. the selected frequency range of this study is limited to 0.5-35 hz, due to the signal-to-noise ratio (snr) constraint. the obtained amplitude spectra were smoothed using the konno and ohmachi [1998] windowing function with b=20 and sampled at 40 frequency points equidistant on log scale between 0.5 and 35 hz. the root-mean-square average of the two horizontal components was considered in the analysis. the above data processing procedures are shown in figure 3 for three examples. 3. generalized inversion technique the generalized inversion technique (git) has been widely used to retrieve the source spectra, attenuation characteristics and site effects in many regions of the world [e.g. castro et al. 1990, 1996; dutta et al. 2003; oth et al. 2011]. here we provide a brief description of the git method and refer the reader to the above-mentioned papers for more in-depth details of this technique. the observed fourier amplitude spectrum (fas) of the ground motion can be expressed as multiplication of the following three terms in frequency domain: uij(f, mi, rij) = si(f, mi). a(f,rij).gj(f) (1) where uij(f, mi, rij) is the observed fas at site j from source i with magnitude mi; rij is the hypocentral distance; si (f, mi) represents the source spectrum of the ith event; a (f, rij) denotes the attenuation along the travel path, and gj (f) is the site amplification function of the jth station. by taking the logarithm, the above equation can be linearized as equation 2: log10uij(f, mi, rij) =log10 si(f, mi) +log10 a(f,rij) + log10gj(f) (2) equation (2) provides a system of linear equations of the form ax=b, with data vector b including the observed spectral amplitudes, model vector x and system matrix a. this system of equations can be solved in a least squares sense for each frequency using appropriate inversion algorithms [e.g. svd and lsqr] [andrews 1986, menke 1989]. according to andrews [1986] one undetermined degree of freedom exists in equation (2) that can be resolved by assuming a predefined source spectral model [boatwright et al. 1991, salazar et al. 2007] or setting the site response of one or more rock sites in the dataset equal to one [andrews 1986, castro et al. 1990, parolai et al. 2000]. following castro et al. [1990] the inversion can be implemented using either a parametric or nonparametric approach. in parametric scheme [e.g. salazar et al. 2007] a predefined function is considered for attenuation along the travel path while in the nonparametric approach [e.g. oth et al. 2009] the attenuation is constrained to be a smooth function of distance including all attenuation effects like anelastic attenuation or geometrical spreading that takes the value of 1 at the reference distance r0, which is set to 9 km in present work. we solve the linear system of equations in one step implementing the lsqr algorithm [paige and saunders 1982] to determine the source spectra, site responses, and attenuation characteristics, simultaneously. to eliminate the undetermined degree of freedom [andrews 1986], one or several appropriate sites can be considered as a reference condition by setting either the amplification of one station or the average of several stations to be approximately one, irrespective of frequency. in order to select an appropriate reference condition, the h/v spectral ratio and geological setting of all stations as well as the results of trial runs using various reference conditions have been considered. the h/v spectral ratio calculated from the amplitude spectra at two gosh and vana stations are almost flat and close to unity (figure 4). on the other hand, these two stations are located on the rock outcrop based on field observations. for these reasons, in addition to the results of trial runs leading to reasonable spectral shapes of the source and site terms in the inversion, we conclude that selecting stations gosh and vana as reference stations and setting the average of site responses at these two stations to be equal to one is the most appropriate choice for this study. it is noteworthy that all site amplification and source spectra derived in the inversion are relative to the selected reference sites. the bootstrap method [efron 1979)] is applied in order to evaluate the stability of inversion results following parolai et al. [2000, 2004]. in this method, a new dataset is obtained by means of randomly selecting each row from the original system matrix with the same size. a certain row can either be selected several times or never. repeated inversions of the bootstrap data set are performed at each frequency and then the means and standard deviations of these bootstrap samples assessed. we carried out 100 bootstrap inversions and calculated the mean and standard deviation of these 100 iterations for each model parameter. 4. inversion results 4.1 attenuation characteristics applying a one-step git inversion to the selected spectral amplitudes, the nonparametric attenuation functions are determined. the hypocentral distance ranging from 3km to 41km is divided into 13 bins of 3 km width. we acknowledge that with this limited distance range available in the dataset, only limited conclusions can be drawn on the overall attenuation characteristics and in particular its frequency dependence. nonetheless, as shown below, the attenuation curves are robust and the data coverage within the limited distance range is excellent. the obtained attenuation functions for two selected frequencies together with the observed amplitudes corrected for source and site terms are shown in figure 5. the curves decay monotonically with distance and the observed data points are fairly described by the calculated attenuation curves. the attenuation functions for all 40 frequency points selected for the analysis between 0.5 hz and 35 hz are displayed in figure 6. a 1/r decay function is also indicated for comparison. as we set the reference distance to 9 km, the attenuation function takes the value log a(f,r)=0 at 9 km. in general, the curves show a simple shape consistently decreasing with distance almost for the entire frequency range with a more rapid decay for the high frequency attenuation curves than the low frequency ones, as expected. the low frequency curves decay similarly to a 1/r function up to about 20km whereas at larger distances the curves attenuate more quickly than 1/r for the entire frequency band. ahmadzadeh et al. 6 figure 4. the h/v spectral ratio calculated from the amplitude spectra at station gosh (a) and vana (b). the spectral ratios are almost flat and near to one. figure 5. nonparametric attenuation functions for two selected frequencies. the gray shaded area denotes one standard deviation calculated by bootstrap analysis around the mean. the observed spectral amplitudes corrected for source and site terms are indicated as gray crosses. 7 the obtained nonparametric attenuation term contains all attenuation characteristics including apparent quality factor and geometrical spreading. assuming a simple geometrical spreading model, the apparent quality factor can be estimated through fitting the following equation to the attenuation function: (3) where r is the hypocentral distance in km, c is the reference distance for the fit, q(f ) denotes the apparent swave quality factor, vs is the shear-wave velocity and b is the geometrical spreading exponent. due to the existing trade off in estimation of b and q(f ) together, we set the b=1 and restrict our calculation to the definition of q(f ). the nonparametric attenuation function, a (f,r) is corrected for the geometrical spreading (g(r) = c/r) and then the frequency dependent quality factor is evaluated from the slope of a least squares regression to log a(f,r) logg(r) versus distance. we fit a power function of the form q(f) = q0f n to the obtained q(f ) values through a least square fit to determine q0 and n. as shown in figure 7, the frequency dependent quality factor is approximated with the relation: q(f)=35.99 f 0.77 (4) the defined apparent quality factor with n=0.77 demonstrates fairly strong frequency dependence in our studied area located in zagros region. this evaluated apparent q factor with low q0 and value of n near to 1 indicates high attenuation characteristics in this region and is similar to the results have been found for other tectonically active regions of iran and the world [rovelli 1982, hellweg et al. 1995, yoshimoto et al. 1993, castro et al. 1996, bindi et al. 2006, motazedian 2006 amongst others]. it is important to point out that due to the tradeoff between q and the geometrical spreading function, we discuss q in the framework of the simplified geometrical spreading model adopted here. in addition since the nonparametric attenuation function (a(f,r)) includes all effects leading to attenuation of seismic waves along the travel path, the apparent quality factor obtained by fitting equation (3) to the attenuation curves include both intrinsic and scattering attenuation. however obtained apparent quality factor is frequency dependent, we cannot definitely conclude that the intrinsic q is also frequency dependent. 4.2 source spectra we fit the ω2 source model [brune 1970, 1971] to the inverted nonparametric source spectra in order to retrieve earthquake source parameters including corner frequency, fc and seismic moment, m0. the acceleration source spectra in terms of the ω2 model can be expressed by: (5) and (6) where rθφ is the average shear wave radiation pattern, v=1/√2 accounts for the division of s-wave energy into two horizontal components, f=2 is free surattenuation, source and site effects from inversion of the silakhor aftershocks ! !!! ! ! !!! !"# !!"!! ! !!! ! !! ! figure 6. nonparametric attenuation functions versus hypocentral distance for all frequencies. for comparison, a 1/r curve is depicted as a dashed line. note that loga(f,r)=0 at the reference distance r0=9 km. figure 7. solid black line indicates the obtained apparent q(f ) model from the analysis of the attenuation functions shown in figure 6. the individual q values derived at each given frequency are denoted by dots and error bars. s( f )=(2! f )2 rθϕvf 4!ρυs 3r0 m( f ) m( f )= m0 1+( f / fc ) 2 face amplification, ρ = 2.8 g/cm3 and vs=3.4 km/s are density and shear wave velocity estimates in the vicinity of source region, respectively, r0 denotes the reference distance, and m(f) represents the moment rate spectrum. figure 8 shows four examples of inverted source spectra (solid black lines, with standard deviation as gray shading) along with an ω2 source model fitted to the spectra (dashed lines) using non-linear least squares. estimated corner frequencies and seismic moments are defined by this fitting procedure. taking into account the standard deviations obtained from 100 bootstrap analyses, the evaluated source spectra are generally very stable and the spectra are appropriately fitted by ω2 model. under the assumption of the ω2 model the acceleration spectra increase as f 2 at low frequencies with a plateau at high frequencies. generally, the observed acceleration fourier spectra show a decay at high frequencies above a certain frequency which is characterized by an exponential of the form exp(-πκf ) [anderson and hough 1984]. the high-frequency spectral decay (κ) is often considered as a site effect originating from near surface attenuation [hanks 1982, anderson and hough 1984] or a source related parameter [papageorgiou and aki 1983]. since we did not consider this high-frequency κ decay term in the reference site constraint (stations gosh and vana), it can be reasonably expected to shift into the estimated source spectra as indicated by oth et al. [2011]. in order to evaluate the influence of the exponential κ term on the source spectra we fitted equation (7) to the high-frequency part of the spectra to estimate the κ values oth et al. [2011] as follows (7) where f e is the lower frequency limit. taking into account the magnitude range of the events, we set f e = 12 hz (we checked results from applying both f e = 12 and f e = 15 hz without remarkable differences) to be adequately above the corner frequency range of the events [parolai and bindi 2004] to obtain reliable κ estimates, however essentially f e would have to be properly selected for each individual spectrum. ahmadzadeh et al. 8 figure 8. examples of acceleration source spectra derived from git inversion (solid black lines) with related standard deviation (shaded area) of four events. the best fitted ω2 model is represented by dashed lines. the white squares trend indicates the high frequency κ correction at frequencies larger than 12 hz. 9 the distribution of κ values is depicted in figure 9. the obtained values are normally distributed and very small, with the mean value of 0.011 s. these small κ values indicate the attenuation corrected spectra are subjected to relatively slight high frequency diminution effect. to investigate how these κ values affect the source spectra, the spectra have been corrected for κ decay for frequencies larger than 12 hz (indicated with white squares at high frequencies in figure 8). as can be seen from the figure 8, the κ effect slightly affects the high frequency part of source spectra in our dataset. we determine the stress drop values based on the estimated m0 and fc values using the following relations [brune 1970, 1971]: (8) (9) moreover, we compute the apparent stress (σa) for each event as follows: (10) where μ is the rigidity modulus (μ=vs 2.ρ) and es is the radiated s-wave energy that can be calculated from the inverted source spectra using the relation [e.g. izutani and kanamori 2001]: (11) figure 10a represents the evaluated corner frequency versus seismic moment with the corresponding standard deviations indicated as horizontal and vertical error bars, respectively. dashed lines represent constant stress drop values from 0.01 to 10 mpa. the inferred corner frequencies range vary from 1.7 to 17 hz whereas the seismic moments vary within 4 orders of magnitude, from 10^11 nm to 10^15nm. we found a less rapid decrease of corner frequency with increasing seismic moment than expected from selfsimilar scaling considerations, as can be seen in figure 10a. this behavior may be considered as an evidence for a break in self similarity scaling as reported in some studies [malagnini et al. 2008, mayeda et al. 2007, mayeda 2009, 2010] though the magnitude range of our data (ml 1.5-4.4) with the vast majority of earthquakes (about 90 percent) with ml 1.5-3 is fairly limited to allow us to deduce a firm conclusion about non self-similar scaling. the evaluated stress drops range from about 0.04 to 5.2 mpa, with an average of 0.9 mpa (figure 10b). these values are close to the stress drops reported by hamzehloo et al. [2010] for three events of the silakhor earthquake that are not included in our database. they obtained a stress drop value of 4.3 mpa for the mainshock (mw 6.1) and 3.2 mpa and 2.8 mpa for the selected foreshock and largest aftershock with mw = 5.1 and 4.9, respectively. the obtained apparent stresses range from about 0.016 to 2.5 mpa, with an average of 0.2 mpa. a comparison of the evaluated apparent stress and stress drop values is represented in figure 11. the solid line indicates the theoretical ratio of ∆σ⁄σa =4.3 [singh and ordaz 1994]. considering the errors in figure 11, the relation between estimated apparent stress and stress drop values is approximately consistent with the theoretically expected relation. the comparison of estimated mw versus local magnitude ml is depicted in figure 12. the moment magnitude mw and the local magnitude ml are linearly correlated. the local magnitude ml generally underestimates mw by about 0.1 0.5 magnitude units for mw 1.5-3.0 and larger events have approximately the same mw and ml. the relation between the estimated moment magnitudes mw and local magnitude ml can be expressed using least squares regression as follows mw = (0.68 ± 0.03) ml + (0.91 ± 0.07). (12) there are only a few studies that have investigated magnitude conversion relations for iran [shahvar et al. 2013, zare et al. 2014, shoja-taheri et al. 2007]. considering the magnitude range of events, to our knowledge this study is the first one looking at the relation between mw and ml within the magnitude attenuation, source and site effects from inversion of the silakhor aftershocks r = 2.34vs 2! fc !σ = 7m0 16r3 σa =µ es m0 es = 4! 5ρvs f  m( f ) 0 ! " 2  df figure 9. distribution of the estimated κ values evaluated from the high-frequency part of the inverted source spectra for f >12 hz. the obtained κ values have approximately normal distribution. range ml 1.5-4.4 in iran and is in good agreement with previous studies in iran that investigated different magnitude ranges [e.g. shahvar et al. 2013 and zare et al. 2014 relationships that are shown in figure 12]. 4.3 site response the git site amplification terms have been derived for both horizontal (h) and vertical component (z) data for all stations. we performed the git inversion for the vertical (z) component applying the same s-wave windows and reference condition as the horizontal (h) component to investigate the z component amplification effect. we provide a comparison between the git site response functions and the horizontal to vertical (h/v) spectral ratios calculated directly from the observed spectra. the h/v method was first applied by lermo and chavez-garcia (1993) to the swaves of earthquake recordings to evaluate site effects. this technique assumes that the vertical component of the ground motion is not considerably affected by the local site conditions. figure 13 depicts the evaluated git site amplification function ratio of the horizontal and vertical components (git h/z) (solid line) and the h/v spectral ratios (dashed line) as well as the individual git amplification curves for horizontal (git h) and vertical (git z) components for all stations. as previously mentioned we considered stations gosh and vana as reference constraint for which the h/v spectral ratios are reasonably flat and near to one over the entire frequency range of analysis (as can be seen from the figure 13). the amplification functions are generally well constrained with small standard deviations obtained from the bootstrap analysis indicated in the git h curves as gray shaded areas with the maximum amplification of about 20 with respect to the reference sites. overall the observed git h/z and h/v curves are similar with small discrepancies. these small variations most probably originate from the differences in the reference site's amplification function, as we set the average response functions of stations gosh and vana to 1 for git method though it is not exactly one and slightly larger than unity for h/v case. therefore h/v ratios underestimate the git h/z results in most cases. ahmadzadeh et al. 10 figure 11. stress drop versus apparent stress with error bars. the theoretical relation is indicated by solid line. figure 10. (a) corner frequency versus seismic moment plot with constant stress drop values indicated by dashed lines. (b) stress drop versus moment magnitude. stress drop values range from 0.04 to 5.2 mpa. figure 12. local magnitude ml versus moment magnitude mw estimated from source spectral fitting. the solid line shows the best least squares fit. the dashed line indicates a one-to-one scaling. 11 the largest amplification based on git h/z is observed at station chag with a factor of 7 with peak near 1 hz. the amplification peaks almost range from 1 to about 8 hz for the majority of stations. the git h/z and h/v site functions represent significant differences with git h curves in the level of amplification. these differences are due to amplification of vertical component as can be seen for instance at stations lenj and chag at frequencies 10 and 25 hz respectively. based on geological maps the five stations (i.e. vana, dina, gosh, deht and kabo) are deployed on bedrock, for which the trends of the git h and h/v ratio are most consistent. the small differences between these stations in the high frequency attenuation, source and site effects from inversion of the silakhor aftershocks figure 13. site amplification functions resulting from the git in comparison with h/v spectral ratios (dashed lines) directly calculated from the observed ground-motion spectra for all stations. the git h/z (solid lines) denotes the git site response of h component divided by git site response of z. the individual git site response functions for horizontal (solid lines with circles) and vertical (solid lines with squares) components are also indicated. the gray shaded areas mark the mean ± one standard deviation from 100 bootstrap analyses for the git h site responses. range are most probably due to the complex geological setting (e.g. sedimentary rocks; dolomite limestone and igneous rocks; granite and grano-diorite) around stations. the remaining stations (i.e. absa, chal, lenj, zagh and chag) are mostly placed on a thin layer of quaternary alluvium. the observed decrease in the ratio of h/v and git h/z caused by amplification of vertical component in the high frequency range at stations chag, lenj and zagh is most probably due to a sharp contrast interface in this area and can be related to s-p conversions at the bottom of the soft layer in the case of high-impedance contrasts as parolai and richwalski [2004] discussed. 5. discussion and conclusions we applied the generalized inversion technique (git) to the data recorded by 10 temporary stations installed around the epicenter of 2006 silakhor earthquake for 2 months. we investigated the source parameters, attenuation characteristics and site response functions using the non-parametric one step git to the observed spectral amplitudes of 213 events within the magnitude range ml 1.5-4.4. the attenuation curves decay monotonically with distance for the entire frequency range with a more rapid decay for the high frequency attenuation curves than the low frequency ones. the attenuation is almost consistent with 1/r decay function for the low frequency curves up to about 20 km however beyond this distance the curves attenuate more quickly than 1/r for all frequency range. a comparison of the obtained apparent quality factor in this study with other studies for iran is described in figure 14 (see also table 1). hamzehloo et al. [2010] applied the sh-wave acceleration spectral data of the 2006 silakhor earthquake and only one of its foreshock and aftershock to estimate the shear wave quality factor, qs(f) = 121f 0.55 in the six selected frequency bands from 1 to 24 hz. hassani et al. (2011) estimated the s-wave quality factor as qs(f)=151f 0.75 using git with a dataset consisting of 40 earthquakes recorded in east-central iran for the frequency range 0.4-15 hz. farrokhi and hamzehloo (2016) investigated the attenuation of p and s waves in alborz and north central part of iran. they applied the extended coda normalization method (cnm) for 380 local earthquakes in the seven frequency bands between 0.4 to 24 hz to define qs(f)=83 f 0.99 and qs(f)= 68f 0.96 in alborz region and north central iran, respectively. another quality factor relation in north of iran is reported by motazedian [2006] as qs(f)=87f 1.46 based on the vertical component of 22 earthquakes. the estimated s wave quality factor in our study shows fairly strong frequency dependence and high attenuation in seismically active region of zagros which is compatible with results reported for other parts of iran. however, considering slight differences between these relations, it should be noted that in these studies different methods have been applied to obtain quality factors implementing various geometrical spreading functions in different frequency ranges sampling different crustal volumes. furthermore, the study areas of these models are different (except hamzehloo et al., 2010) and each relation is linked to various seismotectonic provinces of iran with distinct seismotectonic characteristics [e.g. mirzaei et al. 1998]. the inverted source spectra were fitted by the omega square source model satisfactorily to retrieve earthquake source parameters. the inferred corner frequencies are ranging between about 1 and 17 hz spanning within 4 orders of magnitude. we found a less rapid decrease of corner frequency with increasing seismic moment than expected from self-similar scaling considerations. this result can be considered as an evidence for a break in self-similar scaling, however bearing in mind the small magnitude range of this study (ml 1.5-4.4) including the vast majority of earthquakes with ml 1.5-3 (about 90 percent), more work is needed to derive a definitive conclusion about non self-similarity in this sequence. the estimated stress drop values range between 0.04 and 5.2 mpa with an average of 0.9 mpa. the obtained rather low stress drops are near the lower limit of the typical worldwide observations [~0.1-100, e.g., kanamori 1994]. the values are however in the same range as observed for crustal earthquakes in japan [oth 2013] and are in agreement with the expected smaller stress drops for interplate earthquakes (such ahmadzadeh et al. 12 figure 14. the comparison of the qs(f ) estimated in this study (solid black line) with other studies for iran. 13 as silakour events within tectonically active zagros area) compared to higher stress drop values observed for intraplate events [e.g., allmann and shearer 2009]. our estimated stress drop values were compared with the evaluated apparent stresses. the relation between these two parameters is generally consistent with the theoretically expected relation assuming brune model. comparison of evaluated mw versus local magnitude ml indicates that the local magnitude ml generally underestimates the moment magnitude mw in the order of 0.1-0.5 magnitude units. this result is in good agreement with previous studies in iran [e.g. shahvar et al. 2013 and zare et al. 2014]. considering the magnitude range of events, our study is the first one that investigated the relation between mw and ml within the magnitude range ml 1.5-4.4 in iran. to investigate the influence of kappa parameter on the source spectra, this parameter was estimated from the high-frequency part of the spectra. the evaluated κ values with small mean value of 0.011 sec indicate the attenuation corrected spectra are subjected to small high frequency diminution effect. the results of site amplification functions obtained from git, represent the amplification peaks, almost range from 1 to about 8 hz for the majority of stations with the maximum amplification of about 20 with respect to the reference sites. the different site responses for stations depend on the geological situation beneath the seismic stations, individually. some stations are not strongly affected by amplification effects and are supposed to be located on rock. the provided git z site responses for the vertical components present considerable amplifications leading to reduction in the h/v amplitude in comparison with the git h amplification functions. however, the overall shape of site spectra estimated by both methods is almost similar. finally, the results of our study including the region-specific source, attenuation functions and site responses provide important elements for further strong motion simulations and seismic hazard assessment. acknowledgements. the authors acknowledge international institute of earthquake engineering and seismology (iiees) for providing earthquake waveform database used in this study. this research was 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dependent attenuation of p and s waves in the kanto area, japan, based on the coda normalization method, geophys. j. int., 114, 165-174. zafarani h, and b. hassani (2013). site response and source spectra of s-waves in the zagros region, iran, j. seismol., 17, 645-666. zare, m., h. amini, p. yazdi, k. sesetyan, m.b. demircioglu, d. kalafat, m. erdik, d. giardini, m. asif khan and n. tsereteli (2014). recent developments of the middle east catalog, j. seismol., 18, 749-772. *corresponding author: gholam javan-doloei, international institute of earthquake engineering and seismology, tehran, iran; email: javandoloei@iiees.ac.ir. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. attenuation, source and site effects from inversion of the silakhor aftershocks 1 annals of geophysics, 65, 3, dm322, 2022; doi:10.4401/ag-8845 o p e n acc e s s communication strategy and plans for research infrastructures: the epos case enrico balli*,1, barbara angioni2, simona cerrato4, silvia filosa2, claudio goffi1, luca postpischl3, rita silva1, dorina stanculescu5, massimo cocco2 (1) epos eric, rome, italy (2) istituto nazionale di geofisica e vulcanologia, rome, italy (3) istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy (4) ecsa, berlin, germany (5) insilicotrials technologies, trieste, italy article history: received may 13, 2022; accepted july 7, 2022 abstract strategic research communication has found an increasing recognition in recent years. research infrastructures (ris) are called upon to effectively communicate the scientific research they foster in order to ensure that they attract users and their findings may influence both policy-makers and society at large. not to mention that many funding bodies are making communication a requirement when it comes to allocating research funds. the current paper reflects on the experience of developing a communication strategy for the european research infrastructure epos (european plate observing system) and highlights some challenges and best practices to set up and maintain the critical links between people, ideas and information that are vital for the success of every communication plan. the complex nature of the epos ri revealed a series of challenges and opportunities that need to be fully embraced if epos research findings are to have maximum possible impact and demonstrate their worth for all stakeholders involved. we started with an in-depth analysis of the epos mission, vision and value proposition, then moved on to identifying weaknesses and strengths to build on and eventually envisage pathways for improving internal and external communication and further engaging the different epos communities and stakeholders. keywords: communication strategy; research infrastructures; epos case 1. introduction the european plate observing system (epos) is a pan-european research infrastructure governed by a consortium (epos-eric, eu regulation no 1291, 2013) committed to enabling excellent science through the integration, access, use and re-use of solid earth science data, research products and services, as well as by promoting physical access to research facilities. its governance model relies on a federated approach to engage scientific communities, committed to ensure the data and service provision through nine thematic core services (tcs) and enrico balli et al. 2 one candidate tcs into its novel e-infrastructure, the integrated core service central hub (ics-c) governed by the eric [cocco et al., 2022, in this same volume). the scientific communities involved in epos decided since the conception phase of the research infrastructure (hereinafter ri) to design and implement a single ri for the whole solid earth science sector in full awareness of the challenges to tackle and the opportunities to effectively enable cross-disciplinary research in earth sciences. figure 1. the epos architecture relies on key elements designed to ensure the epos data and service provision: namely, national research infrastructures (nris), thematic core services (tcs) and integrated core services (ics) composed of the central hub (ics-c) and distributed core services (ics-d). the epos ri is coordinated by epos eric and designed to ensure a sustainable data and service provision. taking into account the complex epos’ architecture (figure 1), internal and external communication with all stakeholders is necessary and strategic. professional communication is all the most necessary given that epos’ vision and mission both embed a great impact on science, economy and society at large. solid earth science offers answers on how to maintain earth a safe, prosperous, and habitable planet, hence potentially addressing many of the 17 united nations sustainable development goals (un-sdgs). through the integration of data, models, and facilities, epos will allow the earth science community to make a step change in developing new concepts and tools for key answers to scientific and socio-economic questions concerning geo-hazards and geo-resources as well as earth sciences’ applications to the environment and to human welfare. epos is a distributed research infrastructure involving 25 countries from the european and mediterranean region, 14 of which have formally joined epos eric and 21 are involved in the data and service provision [cocco et al., 2022]. nearly 140 research organizations are engaged in epos, 65 of which are formally involved in the thematic communities responsible for the data, metadata and service provision [atakan et al., 2022]. these numbers give an idea of the pan-european dimension of the distributed epos ri, as well as of the community building behind this science-oriented initiative. this also emphasizes the importance of an effective structured communication relying on shared planning and the adoption of effective tools and appropriate languages. epos is becoming one of the major players in europe in providing data and services for earth sciences, and good communication can have enormous potential for empowering researchers, students and citizens in the much invoked and more than ever necessary dialogue between science, technology and society. epos is proposing a federated approach to the integration of scientific data fostering fair (findable, accessible, interoperable and reusable) data management in solid earth science [cocco et al., 2022; bailo et al., 2022]. the long-term sustainable operation of the epos distributed ri requires an effective plan to engage human resources and skills to be included through time in the governance and organizational structures [saleh-contell et al., 2022]. this emphasizes the need of structuring both the internal communication to have the right people at the right time and place as well as the external communication to engage scientists and disseminate the content of the epos delivery framework. communication strategy and plans: the epos case 3 epos is based on an open science approach, seeking collaboration, transparency and fairness; its main goal is to facilitate the integrated use of data, data products, and facilities from the solid earth science community in europe. this approach must be reflected in the communication strategy as well. that is why between september 2019 and march 2020 a communication plan has been devised, which is now being implemented. the epos communication plan indicates the path to make research results fully accessible to earth scientists, national research infrastructures, ict (information & communication technology) experts, decision-makers, and the public to develop new concepts and tools for accurate, durable, and sustainable answers to societal questions concerning geo-hazards and those geodynamic phenomena (including geo-resources) relevant to the environment and human welfare. this paper describes the case study of the development and adoption of the epos communication plan based on a true alliance between science communicators, researchers and technicians with the hope it will inspire other scientists, communication professionals, scholars and practitioners to take the same path. 2. principles and methods an effective and useful communication plan provides the guidelines to envisage, structure and manage external and internal communication activities for the various identified audiences. it includes strategic objectives, messages, target audiences, types of media, assigned roles, and resources. it must adapt to the rhythm marked by the changes in the organization and the needs of the communities and stakeholders. the communication officers must be able to interpret and adopt the plan, to anticipate the needs and take on the challenges that will gradually come up, to develop new tools and messages, while remaining faithful to the original structure, objectives, audiences and stakeholders identified as strategic. with these clearly defined principles in mind, a co-design and participatory approach was adopted to write and implement the epos communication plan. in developing the communication plan, representatives of the entire community of epos together with experts in science communication were involved according to their interest, expertise and role. the spirit that drove the working team was to make sure that the epos communication strategy benefitted from both the best international practices and the real needs of the research community. the work was divided into work packages. periodic, collective meetings allowed to update everyone on the work done, check progress and deal with any critical issues. all the documentation, reports, discussions were progressively made available to the entire team. given that epos addresses a very diverse community – with various objectives, methods, perspectives, languages – the communication had to be aimed at a multi-stakeholder audience [kalmár and stenfert, 2020]. this is in line with the recent trends in science communication, where communication itself is intended as a collaborative effort embedded in a complex environment, and as such may cause a power shift in society, achieve mutual learning and make sense of today’s challenges. epos does not intend communication as a mere unidirectional transfer of information to various audiences. rather, communication must become an important tool for strengthening the consortium through awareness and trust, for building its management capacity and increasing the sense of belonging and dedication of the community to the shared epos’ vision and mission. achieving these goals requires a well-structured internal communication, as it will be discussed later on in this article. furthermore, external communication should also enhance users’ engagement and the exploitation strategy of the research infrastructure, producing a significant structural change and increasing awareness of the role of public research and its protagonists. figure 2 illustrates the different communication frameworks identified to implement and adopt the epos eric strategic communication plan. aware of this frame, the team who worked on the communication plan was composed of representatives of all these various components. communicators of science with different specializations (theory and practice of science communication, journalism, social media, training, web development, leadership and management in scientific environments, etc.), researchers and technicians dealing with solid earth science data, research products and services, it experts in charge of the development of the e-infrastructure and the integrated core service central hub, and representatives of the epos managerial bodies. the epos communication plan, currently under implementation through the new communication office at the epos eric headquarters in rome, is an emblematic example of the quadruple helix model [carayannis and campbell, 2009], in which media and culture based public relations are also added to the traditional triplet made of academia/universities, industry/business, state/governments. enrico balli et al. 4 the principles on which the plan is based fit perfectly into what jensen and gerber [2020] call “evidence-based science communication” as it puts the experience of science communicators – who work at the border where practitioners and scholars meet – together with the expertise of the professionals in charge of the infrastructure. evidence-based science communication is still a relatively unexplored land and further reflection and discussion are certainly needed. we, therefore, invite the more experienced scholars to exercise constructive criticism, finding areas for improvement. this would be consistent with the spirit of open science and mutual learning in which the initiative was born and developed, and will benefit the entire community of science communicators and researchers. 3. background before getting to the heart of the communication strategies, it was useful to reflect on and discuss the epos’ vision, mission and values in order to strengthen these key messages, making them coherent with the communication framework. in line with the core objectives of the communication plan and with the best practices in the field [albrecht, 2006; kotter, 1996], an accessible and to the point version of the epos vision has been implemented in order to strengthen epos’ key messages, rendering them unambiguous and appealing to the internal communities, the public and other external target audiences. 3.1 epos vision “to ensure sustainable and universal use and re-use of multidisciplinary solid earth science data and products fostering state-of-the-art research and innovation.” the mission statement was shared with the epos’ community since the design and preparatory phase of the research infrastructure. the different areas of expertise of the team engaged and committed to implement the epos’ communication plan allowed the rephrasing of the mission statement, which had to be concise, outcome-oriented and inclusive to be shared by the entire epos’ community. it has been therefore collectively decided to adopt the following mission statement: 3.2 epos mission “to establish and underpin a sustainable and long-term access to solid earth science data and services, integrating diverse european research infrastructures under a common federated framework.” together, mission and vision guide strategy development, help communicate the organization’s purpose, and inform the goals and objectives set to determine whether the strategy is on track. strategy begins with goals, which naturally follow from an entity’s mission and vision. but for very practical reasons, goals cannot stand in isolation. they are backed up by a feeling about the external environment and the organization’s inner capabilities. the strategic choices available to each organization derive from the process of looking outside for opportunities and threats and inside for strengths and weaknesses. a swot analysis was, therefore, carried out to complete the picture framed with the epos’ vision and mission definition. what makes swot especially compelling is that it can help epos take precautions in case threats emerge, advance quickly and capitalize on opportunities, further sustain and develop internal strengths and work on improving its weaknesses. a thorough analysis has identified clear vision and effective planning, innovative approach to data science, sharing multidisciplinary data research results, federated governance, esfri landmark and eric establishment as epos’ main internal strengths. as for the internal aspects to be improved the following elements have been pointed out: complex architecture, maintenance of tcs-ics interaction, user strategy, automated ingestion of tcs metadata into ics, tools and skills for communication. looking outside for opportunities to capitalize on, epos can take advantage of: european ever-increasing needs for transnational cooperation to deal with societal challenges [martì et al., 2022, this volume], e-science innovation to foster open data science trends, reaching beyond the world of academia and extending cooperation to industry. when it comes to external factors outside its control, epos needs to carefully monitor: long-term governmental communication strategy and plans: the epos case 5 commitment, financial viability and long-term sustainability, dependence on harmonized national contributions, compliance with a global legal framework for data sharing, commercial competitors for service operation and data preservation. after having laid out epos’ mission and vision and analyzed its strengths, weaknesses, opportunities and threats, it is possible to have a clearer understanding of what its goals are. epos’s communication goals for internal and external audiences can be summarized as follow: – foster the exploitation of the contents of the epos delivery framework – increase epos’ impact and visibility by communicating scientific perspectives, innovation and added values for science and society – promote international cooperation by positioning epos in a global dimension – engage users, stakeholders, national authorities, and – build-up interest and trust from stakeholders and society at large. analyzing the journey of epos from its conception to the present days (figure 2) has been extremely important for two reasons: first because its long and rather extraordinary history and progress in its lifecycle can be a strong message to be delivered through both external and internal communication. secondly, because the transition from the current stage of the ri to the fully operational phase in 2023 requires the exploitation of a structured communication through key messages, effective tools and continuous interactions with selected target audiences. the epos journey started in 2002, successfully passed the preparatory and implementation phases, and is bound to the operational phase in 2023 (figure 2). figure 2. epos timeline and progress in the lifecycle of the research infrastructure during nearly two decades. the epos preparatory phase (pp) ran from 2010 to 2014 and was funded under the european commission fp7 work program. during this phase, the conditions for the integration of existing and future ri were created and the epos architecture was built. the implementation phase (2014-2019) was marked by the establishment of the eric, and the subsequent implementation of a technical, legal, and financial framework for all its components. the epos communication strategy at this stage focused on the engagement of a broader range of stakeholders, including other ri, european commission bodies and feedback user groups, while strategies to engage the private sector and society have been discussed and envisioned. the latter will be further explored and expanded during the currently on-going epos sustainability phase (epos sp) european project. the transition from the implementation phase to the operational phase has been recognized as a delicate stage that cannot be done instantaneously. a three-year period has been dedicated to this transition (2020-2023), corresponding to the current pilot operational phase (epos pop) that aims to consolidate a sustainable operation of the epos ri. indeed, to tackle the challenge of keeping the communities engaged and committed to supporting the epos integration plan, while strengthening the financial viability of the ri, the pilot operational phase (pop) has been designed according to the strategic plan 2020-2022 to coordinate this peculiar stage of epos’ lifecycle. enrico balli et al. 6 figure 3. epos stakeholder categories (highlighted with colors) and target audiences identified since the epos preparatory phase and engaged at different times according to the communication strategy. figure 4. diagram showing the internal and external communication perimeters and the involved key elements of the epos architecture and organizational structure. epos eric, with the eco and the ics-c, the tcs and internal structural projects (likely coordinated by epos eric) characterize the internal communication. relationships between tcs and epos eric are illustrated in the figure. external communication involves nri, national and international research organizations and other research projects. communication strategy and plans: the epos case 7 the full establishment of the executive coordination office of epos eric, and in particular of the epos eric communication unit during this stage has been foreseen in the communication plan as a necessary step to have the adequate skills and human resources available to address the envisioned tasks. having established the basic principles and values, the epos eric communication plan has been structured to address the following issues: – internal communication as a tool to improve management, strengthen cohesion in the community, share experiences and knowledge. – external communication for different stakeholders and audiences – relation between data sharing and communication – training. the communication actions have been organized in different phases involving different audiences (figure 3) and clearly separated between internal communication, aimed at the members within the vast community of epos delivery framework (internal users), and external communication, aimed at target audiences (figures 3 and 4). understanding and defining these two spaces unambiguously is essential for a distributed research infrastructure with a complex architecture as epos, since the boundaries between internal and external communication characterize the interactions and the engagement strategies necessary to ensure exploitation, participation and the sharing of information to foster trust and awareness. 4. internal communication internal communication is first of all the exchange of information between the members of an organization or between the functional components of the organization. yet it does not remain confined to the field of mere information. through efficient internal communication, a strong organization culture can be built, where everyone feels valued and empowered. greater self-determination of personnel, creation and strengthening of a sense of belonging to a common home, the overcoming of conflicts, the reduction of the distance between the various components and between the components and the management, the mutual understanding, the sharing of contents, objectives, language, a greater coordination of initiatives are among the most important objectives of internal communication [interact, 2018]. internal communication is essential to enable effective governance of an enterprise because it ensures awareness of on-going processes and alignment between involved actors in charge of coordinating activities and workflows. while the need of internal communication is acknowledged and its objectives are clear, it can be difficult to achieve it in practice, and in fact it is lacking in numerous scientific institutions, and it is a challenge to be tackled for a complex and articulated reality such as epos. epos relies on a federated approach to integrate solid earth science data according to the architecture shown in figure 1. this federated approach relies on the construction of the thematic core services (tcs), where data and metadata are generated, quality-controlled and standardized, as well as of the integrated core services (ics) where data and metadata can be integrated and used. the epos delivery framework is composed of the data integration system represented by the tcs and the ics (tcs-ics system) as well as epos eric (see figure 1 and 4). its federated approach and pan-european dimension is what makes epos unique since it involves different european countries, diverse national and international initiatives, and numerous data and service providers belonging to many different research organizations. internal communication assumes an essential role to ensure effective governance, awareness and trust among data, service providers and epos eric as well as to share resources and skills to maintain the envisioned roadmap for the ri operation. in epos, internal communication is the communication that takes place within the delivery framework, while external communication refers to stakeholders and everything that takes place outside this framework, including, for example, communication to users who access data and services made available by epos. therefore, internal communication concerns the communication within epos eric according to its organizational structure, among the key components belonging to the eric framework (namely, the eco and the ics) as well as between epos eric and the tcs and among and within tcs as illustrated in figure 4. epos eric is responsible for the coordination and management of the research infrastructure, as well as for its long-term sustainable operation. epos coordination includes governance-related communication with the general enrico balli et al. 8 assembly and the external advisory boards. communication between epos eric and the tcs as well as between the partners involved in the structural and development projects and the research organizations engaged by tcs needs to be strengthened and structured and this is the main goal of the communication plan. the communication internal to tcs is also essential in order to keep the communities engaged, harmonize and promote the participation of involved research organizations as well as to reinforce the awareness of the epos added value for the communities. the assessment of the impact of internal communication is also useful to implement external communication, because scientists, researchers, technicians, academics and in general all those who fall into the “internal” community are also part of the earth science community and society [cerrato et al., 2018]. research, science, technology are complex systems that cannot in any way be considered detached entities, especially due to the body of values of society itself in which science and technology develop [m.c. evan, 2018]. the boundary between external and internal communication needs to be well identified, monitored and managed with care because a given target audience can be engaged through both the communication frameworks. this implies harmonizing and synchronizing the different communication initiatives undertaken in each framework. 5. external communication society is not a compact block, but a heterogeneous entity formed of different audiences with specific needs. in recent years communication in the public sector has changed profoundly [bucchi and trench, 2014]. the original overall communication function was to inform and to influence, and was performed in a top-down way, mostly through mass communication, where citizens were not given any chance to take an active part. now “public sector communication has become a much more diverse and multipurpose activity – an activity that is based on and mobilized by a wider set of principles” in order “to reinvent and craft more efficient, accountable, and responsive organizations” [friedriksson and pallas, 2018]. being a scientific organization supported by public funds, epos is morally and legally bound to be open and transparent and must be able to listen and to respond to the voices of a variety of stakeholders, other organizations, citizens, experts, and national authorities. the context of communication activities for epos is indeed unique for the following characteristics: (i) epos’ audience is large, diverse and evolving, (ii) epos’ service proposition is large, diverse, complex and evolving, (iii) epos resources depend on the participation of the communities and on governments’ commitment. creating efficient and effective external communication is a great challenge. yet, communication can be a crucial element in the long-term sustainability of epos. it is also important to underline the often underestimated link between internal and external communication. if internal communication works well, internal staff will be well informed and empowered, and can become ambassadors towards the external, through social media, direct contacts with stakeholders, etc. they will thus have a significant impact on external communication and enhance epos’ trust and reputation. epos external communication refers to all audiences who are not directly committed to operate the delivery framework. its scope is broad, and it encompasses a huge variety of audiences, needs, methods and communication tools, skills, practices, as well as specific strategies for each case. the target audiences identified in epos for the external communication are: – researchers, master and phd students, users – policy makers and national authorities, including environmental and civil protection agencies – private sector and industry – media – society. researchers, master and phd students, users can benefit from epos’ unique data collection and services on solid earth science, from career development opportunities within a european network and in a collaborative environment. they are a well-known audience and are reached through dedicated communication initiatives and tools, such as websites, newsletters and social media. national authorities have been a key target audience since the beginning of the epos preparatory phase. specific actions have been undertaken to keep them involved not only through participation in the epos eric general assembly. in the framework of the epos sustainability phase project (epos sp) a national authorities consultation communication strategy and plans: the epos case 9 board (nacb) has been established. the representatives of 14 countries are formally involved in the epos eric general assembly, the decision body and the main financial contributor to the epos ri. the representatives of further 9 countries are engaged in the nacb. this board is expected to be maintained as a key element to sustain communication with this target audience and to address the long-term sustainability of the epos ri. further to national authorities, other funding and civil protection agencies are also key target audiences in epos. interactions with them are going on to prepare for their engagement in the epos delivery framework. yet epos has lot to offer to them: sharing data and services to promote and facilitate the scientific support to evidence-based policy making, advice on how to support research and manage ris, practices for open science and data management, a reliable source of information and data, a strong connection with solid earth science community, practices for delivering information on hazard and risk to different stakeholders, access to products associated with hazard and risk, policies and rules. a specific section on the website will be set up, along with periodic policy briefs, personal meetings, short videos of supportive policy makers, existing channels with environmental and civil protection agencies and tailored training events. these activities belonging to external communication require a strong and effective communication with involved tcs, corroborating the relevance of harmonizing and synchronizing internal and external communication. the epos relationships with industry are also important and need to be preserved and further strengthened. in general, stakeholders from the private sector can be users, suppliers or customers of the ri. formal relationships exist as those related to induced seismicity and anthropogenic hazards, a relevant issue related to the exploitation of georesources. in this case, industry is a supplier of data concerning the monitoring of geo-resource sites and a user of services concerning anthropogenic hazards. the epos eric communication unit and the epos community are working to strengthen awareness of potential benefits deriving from an active partnership between epos ri and stakeholders from the private sector. several pilot initiatives have been undertaken, also in the framework of european projects (such as epos sp), in order to strengthen relationships and implement good practices and case studies to be disseminated and replicated to foster new collaborative frameworks. further activities are envisioned in the communication plan, such as participation in fairs and conferences, the collaboration with field intermediaries (industry liaison officers, knowledge and innovation communities, industrial associations) as well as an industry dedicated area on the website. the creation of dedicated communication channels that will help epos develop more efficient and tighter relationships with the industry and private sector will be considered. by involving the private sector more actively, epos will make an important step forward in achieving long-term sustainability, while also maximizing the impact of its activities. media are the mediator towards all the other audiences. for the media, epos represents a reliable source of information and data, a source of engaging stories, a strong connection with the solid earth science community which can be translated into newsworthy information and answers to their queries. media can be reached by a section on the website, social media, the press office constant activity, and special events. communication with the media will be implemented in the epos operational phase coherently with the strategic approaches to engage society. society is a variety of publics, all with their attitudes, expectations, knowledge, fears, and visions of the future. understanding science in modern society and its contributions to decision-making requires literacy, awareness and preparedness, which can be strengthened through the communication of scientific information with appropriate lay language and tools. epos can be a reliable source of information and data and also offer engaging stories and a direct connection with the solid earth science community. epos can support education, putting at disposal a vibrant scientific community and insights into ongoing research and data science. to open a dialogue with society epos can use all possible channels, such as a section on the website, social media, special events (festivals, open days, etc.), education materials, workshops for students and teachers, citizen science projects, discussion games and other participatory activities. all this is envisioned in the epos communication plan towards the external public, it will be implemented gradually, involving different sectors of the public and providing the appropriate tools to the personnel involved in the communication initiatives. external communication is important to describe the contents of the epos delivery framework as well as to engage the public, allowing it to influence and be influenced by the use of the epos ri. the communication and dissemination of the contents of the epos delivery framework to a widely heterogeneous audience of citizens is an overwhelming challenge, that will be tackled in the near future in full awareness that it requires effective and dedicated planning and tools. external communication with private sector, decision-makers and society at large requires addressing ethical issues through ethical guidelines to preserve credibility, trust and impartiality for public good. communication with society at large is, therefore, scheduled for a later phase. yet it is good to start planning and training the people who will take care of it immediately at the inception of the communication actions. furthermore, enrico balli et al. 10 some of the actions that are intended for internal communication will also have an external impact, since epos is not a sealed system and needs a lot of support in order to arrive at the operational phase as structured as possible. besides having different attitudes, expectations and knowledges, stakeholders vary in their interest in epos, being either passive users of its services, or active users, engaged providing data and services to the ri. in addition, their influence in epos can also be classified as passive or active according to their level of engagement and influence on the decision-making process within the ri. national governments that become members of epos eric, for instance, are actively engaged in the epos framework, thus having both an active interest and influence in the research infrastructure. on the contrary, a large portion of society will have a passive influence and interest in epos. this category, which includes citizens who are informed by epos, requires a targeted and specific communication approach. 6. data science and communication tim berners-lee, the inventor of the world wide web, said in his famous ted talk the next web, in [2009]: now, i want you to put your data on the web. turns out that there is still huge unlocked potential. there is still a huge frustration that people have because we haven’t got data on the web as data. (…) i want you to think about a world where everybody has put data on the web and so virtually everything you can imagine is on the web and then calling that linked data. this is the environment in which epos develops: the web has made billions of linked documents available, now it’s the time for data sharing. epos is here precisely for this and the communication plan facilitates the exploitation of this potential. in communication, it is necessary to find elements of uniqueness or novelties that distinguish the object to be communicated from the rest and trigger the interest of the media and the publics. these elements should be translated in unique and clear messages, which represent the key assets of the organization, and must be reiterated at every occasion. epos offers many ideas for developing effective and meaningful messages for many different audiences. the very fact that it has such a long history and many years ahead of innovative development is an element that must be enhanced in internal and external communication. in addition to its long and complex history, the uniqueness of epos lies in the fact that a research structure has been created for the first time to offer open data and products for all solid earth science, through a european and federated approach. epos offers harmonized petabytes of data on all solid earth science for the same time span. this has never been done before and it represents an excellent opportunity for young scientists and for collaborations with the private sector. this level of complexity and interoperability has never been achieved before at european scale in any scientific field. the realization of the fully operational phase represents both a technical and a cultural challenge that must be fully seized by all communication activities. here we want to highlight some aspects that could be common to other similar initiatives: 7. integrating national research structures epos integrates european solid earth science research into a single, sustainable, permanent and distributed infrastructure, optimizing access to data and offering a wide variety of powerful modeling tools. this new environment is expected to foster innovative research, taking full advantage of big data analytics. 7.1 innovative approach to data science epos has been developed as a novel e-infrastructure for promoting fair data management and is currently working to create data interoperability: integrated metadata and interoperability services. communication strategy and plans: the epos case 11 7.2 sharing multidisciplinary data and research results the strengths of epos’ approach include cost reductions in the use of research infrastructure, trust-building and development of a common understanding among researchers, as well as standardization of data collection, coordination of research methods and open access to research data. 7.3 harmonized data quality management across tcs the long-term sustainability of epos, as that of many ris, involves the generation and exploitation of data, products – including software – and services. epos produces and is dependent on rapidly increasing amounts of data. the data it produces need to be made open and easily available to researchers, across a wide range of fields. to render this goal feasible, the data need to be managed, stored and preserved in a cost-efficient way, with appropriate quality and safety assurances, fostering access across borders. thus e-science solutions for enabling access, storage, preservation and curation of large amounts of data are made available with epos. 7.4 community building and knowledge transfer community building and training, dissemination of results and knowledge transfer to end-users are increasingly taken up with epos, highlighting the importance of both data and infrastructure-sharing. data infrastructure enables researchers and other stakeholders from research, education, society and business to use, re-use and exploit data for the benefit of science and society. 8. engaging the community with a training program consistent with the federated approach of epos, the communication unit has been structured as a hub, with a central office in rome and a communication team made up of representatives from all communities corresponding to the tcs and ics. the communication team is an essential link with the communities: it must be an active part of communication and should not be used in an instrumental and passive way. for this reason, it is important for all members of the communication team to acquire the necessary skills, to be aware of the communication plan that will be adopted and to participate in its evolution. a specific training programme will be proposed to the representatives of the communities who will be part of the communication team in each tcs and ics, supporting and collaborating with the central communication office. this training programme is communication-focused and goes beyond the training offered to researchers within the epos sp project on how to use the platform and better understand data usage. training in research communication is necessary to equip researchersinvolved in communication programmes, public engagement activities or acting as the relation with the mediawith techniques and competences that allow them to successfully run the activities, take into account the public views while respecting scientific knowledge, and ensuring a constructive outcome for the benefit of science and society [newman, 2020; cerrato, 2020]. the courses will be highly professional and can be considered as a reward, or at least as tangible public recognition for their commitment to internal and external communication. in addition, skills in research communication and commitment to outreach and public engagement activities are now an element assessed for career progression, for the evaluation of grant proposals, for the selection in research job positions. these courses will allow participants to acquire skills today deemed necessary in any profession in the field of research, and will therefore represent a valuable element for their career. for this reason, we recommend facilitating and encouraging the presence of young researchers, rather than senior ones, in the communication team of each tcs and ics. the courses will present settings and materials, so to best assist participants in using existing formats or designing new ones, tailored specifically for the needs of the epos communication plan. they will include a theoretical background and practical workshops leading to the development of concrete programmes. enrico balli et al. 12 9. conclusions the whole communication plan must be understood as a living document, “to become sustainable over time, european research infrastructures must increase their visibility, both individually and collectively. for this, their communication must become more effective and consistent” [costa abecasis and pintar, 2020]. working on the epos communication plan meant identifying the guidelines to envisage, structure and manage future external and internal communication activities for the various identified audiences. it included strategic objectives, messages, reference audiences, types of media, assigned roles, and resources. however, different research topics deriving from the different thematic areas tackled by epos will require different communication approaches in order to be effective. our plan revealed that in order for epos to be able to disseminate scientific information to society and the wide public it needs first to become stronger internally. one way of reinforcing the internal communication could be setting up a communication community of practice that provides ongoing support to those involved in communicating research. a community of practice has the advantage of supporting researchers both with ideas and resources, and what’s most important, it will also nurture a strong sense of belonging to a community that shares a common concern for communication. moreover, developing strong partnerships with research final users, other research organizations, knowledge intermediaries as well as other stakeholders will prove crucial for effective communication. one of the challenges for the communication and dissemination of the epos delivery framework is to cultivate and promote bidirectional communication between the tcs and the communities. tcs represent an innovation in the operation of the epos distributed ri that can yield an added value to data sharing and to the provision of data and service for solid earth science. at this stage of the epos lifecycle, it is crucial to consider that the engagement and the communication with national and international research organizations need to be harmonized, and strengthened where necessary, in a suitable governance model preserving the centrality of tcs. another thorny aspect to consider when devising a strategic communication plan is risk communication and the ethical issues it triggers. ethical issues emerge whenever a decision or action has the potential to affect another person [sellnow et al., 2008] and we strongly recommend writing a code of ethical communication and making it public to all members of the organization. on the risk side, risk communication planning needs to acknowledge that decision-making is difficult, because there are often trade-offs to be made among competing objectives and perspectives. risk communication has been defined as any two-way communication between stakeholders about the existence, nature, form, severity, or acceptability of risks [muralikrishna, valli manickam, 2017]. it is vitally important to know and understand what the basic rules of risk communication are and to ensure that communication among various stakeholders is part of the risk communication strategy. an accurate stakeholder analysis will provide useful information and tools for consideration in decision-making and communication processes. epos will manage risk communication through an approach designed to respect the roles and commitments of involved research organizations at national and regional level as well as the roles of international organizations and communities engaged in tcs. this requires to share with the involved communities the approach and the contents of risk communication as well as to implement suitable communication protocols with other stakeholders on issues related to geo-hazards and risks. as outlined by the european commission, research infrastructures already actively engage their thematic communities. however, their long-term sustainability also demands the engagement with society and a variety of stakeholders whose financial and strategic support might derive from their understanding of the significance and the benefits of the services delivered by the research infrastructures. the epos communication plan aims to provide the research infrastructure with tools and guidelines to design communication strategies that will allow epos to effectively engage all its stakeholders. this is a roadmap traced to favor context-appropriate communication, in view of increasing epos’ visibility and impact on its target audiences, and at the same time creating a fertile land for international collaboration or other growth opportunities. the epos communication plan reflects the evolving nature of a complex, distributed infrastructure. therefore, the current plan is to be considered an important starting point of epos’ communication journey, but not its final destination. continuous monitoring, evaluation and adjustment to emerging future scenarios are fundamental for the success of all epos communication activities. communication strategy and plans: the epos case 13 references aa.v., eu regulation no 1291/2013. establishing horizon 2020 the framework programme for research and innovation (20142020). article 2 (6) 11 december 2013. 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(2020), theory and practices in science communication training, routledge, new york 2020, isbn 978-1-138-47815-2 saleh contell k., k. karlzén, m. cocco, h.a. pedersen, k. atakan, d. bailo, o. lange, d. mercurio, g. maracchia, d. piras, a. sangianantoni, m. fredella, c. freda (2022). long-term sustainability of a distributed ri: the epos case, ann. geophys., 65, 2, dm209, https://doi.org/10.4401/ag-8786 sellnow t.l., r.r., ulmer, m.w. seeger, r. littlefield (2008). effective risk communication: a message-centered approach, springer science and business media, https://doi.org/10.1007/978-0-387-79727-4. *co r r e s po n d i n g a u t h o r: enrico b a l l i, epos eric, rome, italy, e-mail: enrico.balli@epos-eric.eu https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj https://ec.europa.eu/info/research-and-innovation/strategy/strategy-2020-2024/our-digital-future/european-research-infrastructures_en 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mailto:enrico.balli@epos-eric.eu ishikawa 833_841.pdf annals of geophysics, vol. 45, n. 6, december 2002 833 landscape planning for a safe city mikiko ishikawa environmental information, keio university, endoh fujisawa city, kanagawa, japan abstract to create a safe city free from natural disasters has been one of the important criteria in city planning. since large cities have suffered from large fires caused by earthquakes, the planning of open spaces to prevent the spread of fires is part of the basic structure of city planning in japan. even in the feudal city of edo, the former name of tokyo, there had been open spaces to prevent fire disasters along canals and rivers. this paper discusses the historical evolution of open space planning, that we call landscape planning, through the experiences in tokyo, and clarifies the characteristics and problems for achieving a safe city. 1. park system introduced as the infra structure for a safe city (1923-1950’s) in japan, the first city planning law was enacted in 1919. however, ironically enough, the first comprehensive city plan was carried out as the reconstruction plan after the kanto big earthquake which occurred in 1923. about 1 484 000 people were affected, whereas the total population in tokyo at that time was 2 309 000. the most important goal of this project was how to build a fireproof city. considering economic and cultural standards, it was impossible to construct stone or brick buildings. the government chose the method of separating dense urban areas by open spaces such as parks and parkways. people knew the fact that trees and open spaces prevented the spread of fires, since 1 570 000 people escaped into parks and saved their lives. this method was called the «park system». the park system is a simplified terminology of the «parks, parkways and boulevards system», rapidly developed in the united states after the big fire of chicago in 1871. figure 1 shows the original reconstruction plan of the kanto big earthquake, and large parks originated from the feudal stocks were deliberately connected with major roads. although the plan was reduced for financial reasons, 3 parks, 52 children’s playgrounds and 52 streets (width 22-73 m, total length 117 065 m) were created. the fundamental structure of modern tokyo was thus created. 2. the park system as the reconstruction project (1950-1960’s) the second wave of landscape planning for a safe city was the reconstruction project after world war ii. in 1946, the special city planning law for the reconstruction was enacted. the characteristic of this law was to establish the basic land use plan. the city planning areas were divided into three zones: urban zone, green belt zone, and urbanization pending zone. within the urban zone, building a fireproof city was the main theme. therefore the park system and the land mailing address: prof. mikiko ishikawa, environmental information, keio university, 5322 endoh fujisawa city, kanagawa prefecture 252-8520, japan; e-mail: mikiko@sfc.keio.ac.jp key words fire disasters − urban planning − safe city − park system − fireproof city 834 mikiko ishikawa readjustment project were adopted as the basic method of reconstruction (fig. 2). the greenbelt zone was designated along the fringe area of 23 wards to prevent urban sprawl (fig. 3). however, rapid urbanization took place and the reconstruction projects were decreased. the land readjustment area was reduced from 20 130 ha to 1652 ha. the greenbelt zone was reduced from 18 010 ha to 9 870 ha in 1955, and after 29 modifications, finally abolished in 1969. figure 4 shows the present location of the dense wooden housing areas. it extends over a radius of 15-20 km from tokyo station. this area is the middle area between the land readjustment zone and the greenbelt zone, where no comprehensive policies were implemented. fig. 1. reconstruction plan after kanto big earthquake (1923) (ishikawa, 2001). 835 landscape planning for a safe city fig. 2. reconstruction plan after the world war ii (1946) − park system (ishikawa, 2001). fig. 3. reconstruction plan after the world war ii (1948) − green belt (ishikawa, 2001). 836 mikiko ishikawa 3. hanshin-awaji big earthquake and its reconstruction projects (1995-2002) the third wave was the effect of the hanshinawaji big earthquake in 1995. 110 117 houses were completely destroyed and 6398 people died. small parks and open spaces in dense urban areas played important roles in preventing the spread of fires and became the immediate refuges for affected people (pictures 1 to 3). table i shows how people used small parks immediately after the earthquake. it has been said that small parks were not important when the big earthquake and fires occurred. however, their actual use had proved the importance of community-oriented small parks. picture 1. burned down area of former dense wooden houses (hanshin-awaji big earthquake 1995) (by m. ishikawa). fig. 4. dense wooden housing areas and intensive district for safe living environment zone project in tokyo (tokyo metropolitan government, 1996). 837 landscape planning for a safe city table i. the use of parks in kobe city after the hanshin-awaji big earthquake (january 24-25, 1995). the purpose of park usage overlaps. parks surviving parks used purpose of usage parks after refuse supporting supporting shelter total earthquake activities center small park ~ 1 000 m2 151 47 23 13 1 0 37 1 000 ~ 2 500 m2 95 49 31 26 0 0 57 2 500 m2 ~ 54 40 36 36 2 0 74 neighbourhood park 31 24 17 33 6 2 58 district park 11 9 6 13 3 0 22 comprehensive park 5 3 0 4 2 0 6 others 20 4 4 0 1 0 5 total 367 176 117 125 15 2 259 during the reconstruction process after the hanshin-awaji big earthquake, diverse reconstruction projects were implemented, involving citizen participation. the basic structure was chosen to enforce the park system, and in addition to this, many different community parks were created. figure 5 is the reconstruction plan in the matsumoto district in kobe. in this district, 80% of wooden houses were burned down. citizens had a strong opinion that if there had been water sources in the neighborhood, the damage would have been decreased. in may 1995, right after the big earthquake, the matsumoto area community development council was established and the picture 2. small parks prevented the extension of fires in dense wooden housing areas (hanshin-awaji big earthquake 1995) (by m. ishikawa). picture 3. small parks became immediate refuges and supporting centers for people (hanshin-awaji big earthquake 1995) (by m. ishikawa). first plan of a community development proposal was submitted to the city in december 1995. in 1996, the detailed district plan was determined, and the actual reconstruction process started. the characteristic of this plan is that it introduced a small stream in the middle of the district. the crucial issue for the implementation of this plan was the source of water for this stream. this problem was solved by utilizing the high treated sewage water from new towns behind the rokko mountains. this is a new collaboration between the existing old town and the new town. in november 2000, the construction of the stream started and the project was completed in sep838 mikiko ishikawa tember 2001. it took 6 years, but finally the new community park for safe city was implemented. 4. safe living environment zone and revitalization of park system in tokyo soon after the hanshin-awaji big earthquake, the tokyo metropolitan government and 23 wards carried out a comprehensive study and designated the intensive planning area where immediate actions for safe city were required (fig. 4). the basic strategy was to create the «safe living environment zone» surrounded by main roads or rivers. the safe living environment zone is divided into sub-units, and in the middle, small parks or open spaces are planned as centers of the community. water tanks and food were provided for emergencies. figure 6 shows the plan of the safe living environment zone in higashi-ikebukuro, toshima ward, where dense wooden houses were concentrated. it is hard to create a large park in this area. the characteristic of this plan is to open the cemetery as a parklike open space. also, by transforming 2 story old wooden houses into 5-6 story condominiums, it will become possible to create new small parks as a core of the community and a safe city. another important project for a safe city in tokyo is to re-enforce the metropolitan park system. figure 7 is the structure of green and water corridors in metropolitan tokyo, and fig. 8 is focused on the central 23 wards area. the core of tokyo is the imperial palace area where tokyo has had a huge stock of open spaces since the edo era. these open spaces are used as the site of public institutions, parks and the imperial palace. it is important to create a new public access to contribute to a safe city as refuges for many workers in the central business district. the second target of the new park system is the waterfront area along tokyo bay. because of drastic changes in the function of tokyo port, the land-use of waterfront areas shifted from industrial uses to commercial and residential uses. therefore, the basic policy of the new park system is to create natural environments along the seashore, and to establish a large park which fig. 5. the reconstruction plan of matsumoto area in kobe (matsumoto area community development council and kobe city, 2001). 839 landscape planning for a safe city fig. 6. the plan of the intensive district for safe living environment zone (tokyo metropolitan government, 1996). fig. 7. the park system in tokyo (tokyo metropolitan government, 2001). will serve for refuges and the supporting center if central tokyo is damaged. the third target of the new park system for a safe city is to create open space corridors along riversides. during the rapid economic growth in the 1960-1970’s, the rivers in tokyo were completely neglected. figure 9 shows the reclaimed rivers in tokyo. picture 4 shows the worst case of riverside landscape in tokyo. series of projects for revitalization along the riverside have been implemented. picture 5 is the super bank project along the arakawa river. this project has three pur840 mikiko ishikawa fig. 8. the park system in 23 wards in tokyo (ishikawa, 2001). fig. 9. rivers in tokyo (ishikawa et al., 2001). 841 landscape planning for a safe city poses. the first is to strengthen the function of the riverbank, the second is to redevelop the dense wooden industrial area into a residential area, and the third is to create river-side community parks. 5. conclusions this paper has discussed the historical evolution of landscape planning for a safe city. in japan, green open spaces and water have been regarded as the essential factors for a safe city. we have faced the era of the revitalization of inner cities. to introduce a new park system is the key issue to achieve a safe city. references ishikawa, m. (1996): the park system and its role as prevention disasters planning, 31st japan city planning conference, 133-138. ishikawa, m. (2001): city and green space, (iwanami co. ltd). ishikawa, m. et al. (2001): revitalization of inner city tokyo and kanda river. matsumoto area community development council and kobe city (2001): the reconstruction plan in matsumoto area. sato, a. (1977): the historical evolution of parks and open spaces planning in japan (city planning associates inc). tokyo metropolitan government (1996): the master plan for safe city. tokyo metropolitan government (2001): the vision for tokyo in 2020. picture 4. river surrounded by buildings (shibuya river, tokyo) (by m. ishikawa). picture 5. super bank project along the arakawa river in tokyo (by m. ishikawa). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 1 recent geoethical issues in moroccan and peruvian paleontology juan carlos gutiérrez-marco in stitu to d e geocien cias (csic, ucm), mad rid , sp ain jcgrap to@u cm .es artur a. sá dep artam en to d e geologia, un iversid ad e d e trás -os-mon tes e alto dou ro, vila real, por tu gal cen tro d e geociên cias, un iversid ad ed d e coim bra –pólo ii, coim bra, portu gal asa@u tad .p t diego c. garcía-bellido sch ool of biological scien ces, un iversity of ad elaid e, au stralia diego.garcia-bellid o@ad elaid e.ed u .au césar a. ch acaltan a in gemmet, av. can ad á 1470, lim a, peru cch acaltan a@in gem m et.gob.p e abstract the cases of joan corbacho from spain, and klaus hönninger and carlos a . v ildoso from peru, considered by some as prestigious paleontologists in their countries, are discussed here. the first one is a fossil collector and trader that, without a minimal scientific knowledge, published ca. 20 papers with proposals for a dozen new trilobite taxa coming from different paleozoic formations in the m oroccan a nti-a tlas. descriptions of new taxa seem formally valid but are rather inadequate, often based on poorly preserved material of dubious geological provenance, and mostly published as papers without peer review in a local jou rnal, managed by a private museum connected to the seminary of barcelona. besides this, part of the published and figured trilobite specimens were later offered for sale in the internet, sometimes accompanied with a ‘certificate of authenticity’ signed by the m useum director. m r. corbacho is acting also in spain as a judicial expert in paleontology while he is not more than an amateur fossil collector. in peru, the two cited pseudo-paleontologists lead their official-looking businesses, the ‘m eyerhönninger palaeontological m useum’ and the ‘peruvian institute of paleovertebrate studies’, respectively, under names of institutional appearance. the ‘scientific research’ activities of m r. hönninger have been basically deactivated by authorities of the m inistry of culture, and following a public complaint for fraud in the first international s ymposium of palaeontology of peru held in 2014 in lima. however, the dealings of the m r. v ildoso, who has long claimed to have a title on paleontology from the university of la plata (a rgentina), which he has never been able to show, has experienced a considerable increase, with paleontological heritage contracts with mining companies and the organization of the dakar rally. the prominence of the position he has attained is such that he was offered the presidency of the organizing committee of the ix latin a merican congress of palaeontology held in 2016 in lima, which has ended in an organization disaster and a money scandal. 1. in trod uction s occu rred w ith th e gu p ta affair of th e h im alayan ‘m isp laced ’ or ‘recycled ’ fossils (talen t, 1989; ru ffel et al., 2012; sh ah , 2013), or m ore recen tly w ith th e mid d le east fossils of th e im am case (agu irre, 2004; gran ier et al., 2009), an in crease of p aleon tolo gical d ata from relatively rem ote or u n exp lored p laces of th e w orld h as attracted th e in cu rsion of op p ortu n ists in to th e research field . h ere w e p resen t th e case of variou s u n eth ical p ractices a an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 2 d etected in p aleon tological stu d ies on th e o rd ovician of morocco, as w ell as oth ers affectin g m ore gen eral asp ects of peru vian p aleon tology. th e african case in volves ten y ears of p alaeon tological m iscon d u ct by a sp an ish citizen , an d is p resen ted h ere for th e first tim e. the tw o other cases of u nethical p roced u re by peru vian citizen s h ave been p artially d iscu ssed in a p rev iou s p ap er in sp anish (gu tiérrez-marco et al., 2016), w hich is m ostly tran slated and u p d ated here w ith new in form ation. the qu estionable activities of these th ree ind ivid u als w ere a lread y know n in circles of sp ecialists, bu t the p u rp ose of this note is to m ake these p ractices kn ow n ou tsid e their local areas, as w ell to the m ed ia an d nation al ad m inistration s, so th at n o one can claim in the fu tu re the ign orance of these p ro blem s. 2. the ‘corbacho affair’ this case affecting the n orth -african p aleontology began nine years ago, w hen joan corbacho, an am ateu r fossil collector settled in sabad ell (p rovin ce of barcelon a, n w sp ain) started to p u blish p ap ers on ord ovician trilobites of m orocco w ith ou t a m in im al scientific know led ge. at the beginning mr corbacho coau thored w ith em briologist dr. j.a. vela (university of barcelon a), an am ateu r trilobite collector w h o is co nnected to the geological mu seu m of the sem inary of barcelon a, later going alone or in colla boration w ith other p eop le, m ost of them non sp ecialists as w ell. th e geological m u seu m of barcelon a d eserv es atten tion becau se it h as a great resp on sibility in th e origin an d d evelop m en t of th is affair. th is is a n on -govern m en tal in stitu tion th at w as e stablish ed in 1874 as th e n atu ral h istory cab in et of th e cath olic sem in ary (vilella, 1999; ta ylor et al., 2012), an d w h ose collection m an ag em en t is in th e h an d s of a large grou p of fossil am ateu rs an d volu n teers. from 1987, th e m u seu m p u blish ed regu larly th e jou rn al batalleria, as w ell as th e n on -p eriod ic series m usei geologici seminarii barcinonensis (series palaeontologica), w h ich started in 2006. th e p roblem is th at both jou rn als p u blish ed p ap ers w ith in ad equ ate or totally lackin g p eer review , esp ecially w h en relatin g to ‘d iscoveries’ an d con tribu tion s su bm itted by th e mu seu m ’s m ain collabor ators. th is lack of scien tific con trol is in th e origin th at mr. corbach o, p u blish in g p refere n tially in th ese tw o jou rn als (see list of refe ren ces), h as flood ed th e literatu re on n orth a frican trilobites w ith p u tative n ew taxa, in th eir m ajority very p oorly d escribed an d lackin g p recise geological con text. th e im p act of th is scien tific “n oise”, m aterialized in alm ost tw e n ty p ap ers in ten years (2007–2017), varies accord in g to th e op in ion of d ifferen t sp ecialists, bu t its rep ercu ssion s can n ot be fu lly en visaged as yet. tw o view s are bein g im p osed am on g th e p rofession al scien tists: th ose w h o p refer to ign ore th e n ew taxa p rop osed by mr. corbach o an d co-au th ors, an d th ose w h o p rovision ally cou ld accep t som e of th ese p roblem atic n am es, in th e h op e of a fu tu re in -d ep th review of th eir d iagn osis, syn on ym ies an d p recise stratigrap h ic circu m stan ces th at w ill su rely be a d ifficu lt an d tim e-con su m in g job. as an exam p le of the first view , martin et al. (2016) rep orted a general list of th e low er o rd ovician trilobites id entified from the fez ou ata sh ale, w ith no reference of the new taxa d escribed by mr. corbacho from the sam e fo rm ation (vela and corbach o, 2007; corbacho, 2008, 2014b; corbach o an d vela, 2010, 2013; corbach o an d lóp ez-sorian o, 2012), w ith the single excep tion of a nacheirurus adserai (vela an d corbach o, 2007). in sen ior syn on ym y w ith th e latter th ere are p robably u p to six oth er moroccan sp ecies d escribed for th e sam e area by vela (2007), vela an d corbach o (2007) an d corbach o (2008), p artially review ed by vela an d corbach o (2011). most of th e figu red sp ecim en s from th ese ‘n ew sp ecies’ certain ly rep r esen t p reserv ation al or m ech an ical varian ts eith er tap h on om ic or cru d ely retou ch ed in of a sin gle form of th e gen u s a nacheirurus, w h ich is a ju n ior syn on ym of lehua (martin et al., 2016). as an exam p le of th e secon d op tion gu tiérrez-marco et al. (2017) h ave u sed th e n am e m egistaspis (ekeraspis) hammondi (corbach o an d vela, 2010), bu t sta tin g th at ‘th e origin al d escrip tion of th e sp ecies n eed s rev ision , becau se th e d iagn ostic ch aracters are barely recogn izable on th e h olotyp e, an d th e p u rp orted d ifferen ces w ith th e su bsp ecies m . (e.) hammondi forteyi cou ld be d u e to p rep ar ation by fossil d ealers’. on th e oth er h an d , th e sam e late trem ad ocian trilobite w as cited as m . an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 3 (e.) cf. filacovi by martin et al. (2016), in th e h op e of a m ore com p lete stu d y w ith better p reserved m aterial. a recu rren t p roblem in th e p ap ers by mr. corbach o is th at m ost of th e origin al typ e m aterial of h is n ew sp ecies w ere in ad equ ately p rep ared , or recon stru cted by lofigure 1: composition of past screen captures showing m r. corbacho’ m oroccan trilobites on sale in internet. on the left-half, a topotype specimen of uralichas hispanicus tardus figured in corbacho (2011, pl. 3, fig. 1), and offered with a ‘certificate of authenticity’ signed by the geological m useum of the seminary of barcelona. despite the specimen being described as 100% real, it shows clear cosmetic reconstructions in cephalon, thorax and pygidium. to the right is a clu ster of paratype specimens of m egistaspis (ekeraspis) hammondi, figured by corbacho and v ela (2010, pl. 5c) and also offered by ebay (bottom center) for a total of 32,317.64 euros. bottom left is an image of the holotype specimen of parvilichas marochii, figured by corbacho and v ela (2013, pl. 1, fig. 7) and found cited as ‘sold’. big upper arrows indicate plates and placement of specimens in the corresponding publication s, lower ones refer to the same in the web list of sales. a ccording to their records, the name ‘elfosil’ of m r. corbacho is used on ebay since february 28, 2004. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 4 cal d ealers, becau se a great m ajority of h is figu red sp ecim en s w ere certain ly acqu ired from moroccan w orkers an d fossil sh op s. evid en ces of cru d e cosm etic im p rov em en ts are often visible in m an y of th e tr ilobites illu strated in h is p ap ers, as w ell as p robable recon stru ction s of sp ecim en s p u t togeth er from d ifferen t in d ivid u als, a rath er com m on p ractice in som e local fossil w orksh op s. th is p roced u re is stran ge for a p erson w h o acts as a ‘legal exp ert in p aleon tology an d gem ology’ an d is ‘r esp on sible for th e fossil au th en tication an d tr ilobites’ in th e geological mu seu m of th e sem in ary of barcelon a. lackin g an y tertiary ed u cation in p aleon tology, mr. corbach o h as o btain ed th ree titles from th e catalon ian sch ool of crim in ology r elated to th e ‘gem ology an d metales p reciosos’ (=gem ology an d preciou s metals), ‘peritajes y tasación ju d icial’ (=official ap p raisin g) an d a d ip lom a in ‘falsificacion es y tasacion es paleon tológicas’ (=paleon tological fa lsification an d ap p raisals). th ese w ere all received from a p rivate cen ter, collaboratin g w ith th e also p r ivate eu rop ean un iversity migu el d e cervan tes of vallad olid , w h ich offers access ‘w ith ou t acad em ic requ isites’. mr. corbach o is p resid en t of h is ow n ‘associaciò d e perits i taxad ors d e c atalu n ya’ (five m em bers). abou t fossil fakes an d th eir recogn ition an d legal exp ertise in p aleon tology h e h as w ritten several p ap ers w ith very elem en tal an d obviou s exa m p les (corbach o, 2009, 2015; corbach o an d martín ez, 2014, 2015; corbach o an d sen d in o, 2015; corbach o et al., 2007, 2011, 2015) an d som e m ad e-u p statistics (corbach o et al., 2015, fig. 1). desp ite th is p r esu m able exp erien ce an d exp ertise an d , as m e n tion ed above, mr. corbach o establish ed n ew sp ecies of trilobites based on m aterial coarsely recon stru cted or h igh ly retou ch ed by moroccan d ealers. th is is th e case, for in stan ce, of a saphellus cuervoae (corbach o an d lóp ezsorian o, 2012), a trilobite w ith excep tion ally lon g an d tran sverse gen al sp in es, obviou sly gen erated as an ‘im p rovem en t’ w ith ep oxy p aste, p robably based on p oorly p reserved sp ecim en s of a saphellus stubbsi (fortey, 2009), a trilobite ch aracterized by lon g m ou stach e-like gen al sp in es. th e fact th at th e typ e m aterial of a . cuervoae com p rises retou ch ed in d ivid u als w ith 6 an d 7 th oracic segm en ts, in stead of 8 as occu rs in all oth er asap h id s, also p oin ts to p aleon tological falsification . bu t th ere’s m ore: regard in g th e sam e sp ecies an d m at erial –all of com m ercial origin , corbach o an d calzad a (2014) sp ecu late w ith th e existen ce of a p ossible sexu al d im orp h ism d efin ed by th e size an d th eir occu rren ce in p airs, w h ere th e y in terp ret th at th ese cou p les cou ld h ave fossilized w h en m atin g, after a su d d en d eath by ‘gases of vo lcan ic origin ’. th is is th e scien tific level of p ap ers p u blish ed in batalleria, th at som e often listed in th e ackn ow led gem en ts p eop le h avin g exp licitly sh ow n th eir d isagreem en t w ith th e con ten t of certain m an u scrip ts, an d th at latter see th eir n am e ackn ow led ged in th e p u blication (com . p ers. dr. j. esteve, dr. s. zam ora an d dr. i. rában o). with regard to th e p aleon tological d escrip tion s of n ew trilobite sp ecies, all of th em are very sim p listic an d often bold , fu ll of m easu rem en ts taken from recon stru cted m aterial bu t ou tsid e com m on taxon om ic p ractice. h e h as often ch osen th e w orst sp ecim en (least com p lete or w orst p reserved ) as h olotyp e, leavin g th e p ar atyp es for p rivate collection s. occasion ally tw o h olotyp es h ave been selected for th e sam e sp ecies (as for symphysops stevaninae; lóp ezsorian o an d corbach o, 2012), an d som etim es th e h olotyp e lacks th e d iagn ostic ch aracters (like th e p aratyp e w ith eyes an d –abrad ed – p ygid iu m , as op p osed to th e h olotyp e cran id iu m of degamella sendinoae; corbach o, 2011). on ce th ere is th e ad d ed valu e of h avin g been illu strated in a p u blication , mr. corbach o h as o ffered som e p aratyp es for sale on ebay or th rou gh h is com p an y’s w eb site (fig. 1), an d even th e h olotyp e ap p eared as “sold ”. after th ese p ractices w ere d en ou n ced by th e au th ors in tw o in tern ation al m eetin gs (october 2015 an d au gu st 2016), mr. corbach o took off th e w ebp age section on ‘trilobites for sale’, from w h ich w e h ave com p iled th e im ages in fig. 1. mr. corbach o, h ow ever, keep s p u blish in g p ap ers m ain ly regard in g ord ovician trilobites, n ot on ly d ealin g w ith th ose of th e low er o rd ovician fezou ata sh ale (corbach o an d lóp ez sorian o, 2016; corbach o et al., 2017), th e fo rm ation w h ich p reserves th e fezou ata biota (lefebvre et al., 2016), bu t also th e m ore u n kn ow n up p er ord ovician tafilalt biota (vela an d corbach o, 2009; corbach o, 2011; lóp ez sorian o an d corbach o, 2012; corbach o an d an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 5 kier, 2011; corbach o an d lóp ez-sorian o, 2013c; corbach o et al., 2014). th rou gh h is ‘fosilart’ com p an y h e h as an n ou n ced tw o m ore p ap e rs an d a book on ord ovician moroccan trilobites, ed ited by th e geological mu seu m of th e sem in ary of barcelon a. besid es th is geological p er iod , mr. corbach o h as exp an d ed occasion ally sen d in g m aterial to p rofession al p aleon tologists to th e stu d y of th e cam brian trilobites of morocco (geyer an d corbach o, 2015) as w ell as devon ian trilobites from th e an ti-atlas (corbach o an d lóp ez-sorian o, 2013a, 2013b; co rbach o, 2014a, 2014c; bign on et al., 2014). fin ally, mr. corbach o h as sen t som e n on -trilobite fossils to a czech sp ecialist p rod u cin g tw o join t n otes (valen t et al., 2013; valen t an d corbach o, 2015), an d recen tly h as w ritten gen eral p ap ers (corbach o et al., 2016; corbach o a n d h am m on d , 2017). most of th e coau th ors of mr. co rbach o’s p ap ers are also am ateu r p aleon tologists (j.a. vela, f.j. lóp ez -sorian o, c. kier), fossil collectors (k. h am m on d , d. martín ez bau tista, j.a. cu evas, e. moren o), n on -trilobite sp ecialists (a. ait ad d i, s. calzad a, c. sen d in o) or even fossil d ealers (m. tah iri). 3. the ‘hön n in ger affair’ in 2009, th e peru vian m ed ia rep orted th at klau s h ön n in ger mitran i, com p u ter en gin eer w orkin g as an am ateu r p aleon tologist, h ad e stablish ed in lared o (tru jillo) th e ‘first peru vian p aleon tological m u seu m ’ accord in g to som e n ew sp ap ers, or ‘th e first latin am erican p aleon tological m u seu m ’ accord in g to oth er sou rces. th is is th e m useo paleontológico m eyerhönninger, a p rivate com p an y registered as an association n cp (= ‘o f u n classified activity’) on au gu st 17, 2009 by mr. h ön n in ger, a ch iclayan born of germ an an d peru vian d escen t. th is e n th u siast of p aleon tology h ad assem bled a co llection of gen u in e fossils an d d in osau r rep licas d u rin g h is stay in germ an y, an d in ten d ed to d issem in ate th is asp ect of scien ce in h is cou n try, raisin g p u blic aw aren ess abou t th e im p ortan ce of kn ow in g an d p r otectin g th e rich peru vian p aleon tological h eritage. h is exh ib ition of fossils tou red variou s sites (tru jillo, ch iclayo, p lu s th e d istricts of su rco an d villa el salv ad or, in lim a). at th e sam e tim e, som e jou rn alists began to regard h im as a tru e p r ofession al p aleon tologist of great n ation al an d in tern ation al p restige. th is v iew w as fu elled by mr. h ön n in ger h im self th rou gh wikip ed ia an d th e social n etw orks, w h ere h e w as even treated as a ‘p u blic figu re’ in facebook. th e m ed ia h istory of mr. h ön n in ger in clu d es alleged d iscoveries of im p ortan t peru vian fo ssils, n ever backed by scien tific p u blication s bu t aired to th e m ed ia w ith obviou s ign oran ce of th e p reviou s p aleon tological literatu re, su ch as ‘th e first skeleton of m egalodon fou n d in th e w orld ’ (2006), a com p lete smilodon sku ll (2009), a n eogen e brain case m old of kentriodon (2010), seven n ew albian sp ecies from th e am azon ia (2011), miocen e am ber w ith in sects, arach n id s, m icrofossils an d rod en t h airs (2011), etc. som e of th em even broad casted by th e bbc in ap ril 2011. h e even claim ed th e au th orsh ip of w h at h e called ‘h ön n in ger h yp oth esis’, w h ich su ggests th at th e m ass extin ction of th e k/ pg bou n d ary w ou ld h ave begu n , d u e to clim atic cau ses, five m illion years before th e fall of th e yu catan asteroid , lead in g to th e d w arfin g of certain foram in ifera th at h e h as n ever sp ecified . th e backgrou n d of mr. h ön n in ger’ p aleon t ological kn ow led ge w as ap p aren tly based on tw o fake d egrees in th e sp eciality: th e first on e obtain ed at th e ‘pr aeh istorica in stitu t’ of h an au (germ an y), w h ich is a p riv ate fossil p rep aration laboratory, p artly w ith com m ercial p u rp oses, d irected by mr. in go meyer. th e secon d p u tative title com es from th e us ‘wyom in g din osau r cen ter’, an oth er p riv ate com p an y th at m ain tain s a p aym en t p rogram (dinosaur a cademy) to teach h ow to d ig d in osau rs to h igh sch ool stu d en ts. in its d eleted p age in wikip ed ia, mr. h ön n in ger w as p roclaim ed an ‘h on o rary m em ber’ of both in stitu tion s. in h is m ost p op u lar years in th e m ed ia, mr. h ön n in ger starred in an alleged ‘en cou n ter in th e th ird p h ase’ in th e ocu caje d esert, w ith lon g-h ead ed extraterrestrials th at rem in d ed h im of th e sku lls d eform ed by th e paracas civ ilization . it w as at th at stage w h en h e w as in te rview ed by th e sp an ish tv p rogram ‘fou rth millen n iu m ’, d ep loyed to p eru to th at effect (fig. 2). in itially in vited by th e peru vian cu ltu ral a u th orities to in terven e in th e regu lation of th e n ation ’s p aleon tological resou rces (sep tem ber 2012), mr. h ön n in ger soon becam e en viou s of an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 6 th e m em bers of th e cu rren t min istry of cu ltu re, for m ain tain in g stron g p erson al op p osition to th e traced itin erary of th e ‘rally dakar’ to cross th e d eserts of ica an d ocu caje in 2013 an d 2015. th is race cau sed in 2012 sev ere d am age in th e exten sive d ep osits w ith fossil cetacean s an d oth er m arin e vertebrates, n ot on ly by th e veh icles, bu t also th eir p articip an t’s cam p s an d sp ectators. th is op p osition led h im to r ep ort to th e m ed ia th at h e received d eath th reats again st h im (u sin g blood -stain ed colom bian bills), an d th at a grou p of 50 righ t livelih ood aw ard w in n ers (d escribed by som e as th e “a ltern ative n obel prize”) w rote to th e peru vian govern m en t requ estin g p erson al p r otection for mr. h ön n in ger (may 2013). follow in g h is d isagreem en t w ith th e au th or ities of th e min istry of cu ltu re, w h o sin ce 2013 began to m on itor th e legality of h is p aleon t ological p rosp ects, mr. h ön n in ger w as also qu estion ed at an in vited con feren ce, given in sep tem ber 2013, d u rin g th e first in tern ation al sym p osiu m on peru vian paleon tology h eld in lim a, w h ere h is m u ltip le op p ortu n istic activ ities an d p rofession al in tr u sion s cam e to ligh t (gu tiérrez-marco, 2014). it w as th en w h en h e began to d eclin e in h is w ork as a p seu d op aleon tologist, sh ortly after mr. h ön n in ger sign ed agreem en ts w ith th e p riv ate u n iversity ricard o palm a (lim a), an d also w ith th e m u n icip ality of lim a, to bu ild th e so-called ‘din op ark’ in th e h u áscar zon al park of villa el salvad or (a d istrict sou th of lim a). th is p roject w as p u blicized as th e ‘first peru v ian m u seu m of paleon tology’, h avin g a bu d get of abou t us$ 1.3 million . bu t after an in form ative exh ibition w as h eld for several years at th e site of th e fu tu re m u seu m , w ith an op en -air d isp lay of rep licas of skeleton s an d d in osau r m od els, th e p roject w as ‘frozen ’ an d w as fin ally d ism issed at th e begin n in g of 2015, bein g r ep laced by th e con stru ction of sp ort in stallation s in th e sam e area. h ow ever, dinopark h as m an aged to becom e a fu tu re ‘paleon tological th em e park an d mu seu m of h u m an ity’ w h ich , also u n d er th e d irection of mr. h ön n in ger, is exp ected to be located in th e city of tacn a, h op efu lly w ith exclu sively in form ative an d d iscon n ected objectives of research or p aleo n tological rescu e. tod ay, mr. klau s h ön n in ger n o lon ger p u blicly boasts of h is p ast statu s as a p aleon tologist bu t, after h is fru strated p assage th rou gh p olitics (in 2014 op ted for th e p resid en cy of th e region al govern m en t of lam bayequ e), n ow ad ays h e sp lits h is tim e betw een th e d irection of th e ‘meyer-h ön n in ger mu seu m ’ (virtu al in th e p resen t m om en t?) an d th e p osition s of ‘prin cip al in vestigator’ an d ‘director of in tern ation al affairs’ of th e instituto de investigaciones y estudios socio-territoriales (in stitu te of research an d socio-territorial stu d ies–iiest). th e latter is a n ew p riv ate com p an y (n cp -association , lim a 2015), w h ose n am e d en otes in stitu tion al a p p earan ce, an d w h ere h ön n in ger’s cu rricu lu m is alm ost u n related to paleon tology, alth ou gh it m ain tain s several d u biou s referen ces regard in g its p ast an d p resen t n on -p aleon tological activ ities. figure 2: a bove: screen capture of m r. klaus hönninger, participating in the television program ‘cuarto m ilenio’ (‘fourth m illenium’, channel ‘cuatro’, spain, 2012). the subscript reads ‘professor and director of the m useo m eyer-hönninger of peru’. below: m r. klaus hönninger with a dinosaur replica in his exhibition in lima (peru), from facebook (open access). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 7 4. the ‘vild oso affair’ th is case overlap s in tim e w ith th e p reviou s on e bu t even m ore con volu ted , given th at its p rotagon ist, th e p u blicist carlos an ton io vild oso morales, claim ed at th e begin n in g to h ave grad u ated as a p aleon tologist in 1990 from th e un iversity of la plata (argen tin a). a statem en t d en ied by h is acad em ic record , w h ich p roves th at h e on ly atten d ed seven su bjects of th e d egree in biology, of w h ich h e p assed th ree, lo sin g h is statu s as a regu lar stu d en t in 1991. after bein g in volved in variou s con troversies for breach of con tracts in th e mu seu m of la plata (argen tin a) an d in th e n ation al mu seu m of p eru , in 1995 h e fou n d ed th e in stitu to peru an o d e estu d ios en paleovertebrad os– ipep (= peru vian in stitu te of stu d ies in paleovertebrates). th is is a p rivate com m ercial com p an y registered as an association of un classified activity (n cp), th rou gh w h ich mr. vild oso gets co n tracts of p rojects w ith m in in g com p an ies for rescu e an d p rotection of p aleon tological elem en ts, as w ell as w ith th e fren ch com p an y o rfigure 3: a bove: screen captures and composition of images taken from the website of the ix latin a merican congress of paleontology, showing the president of the organizing committee, m r. carlos a . v ildoso, with his curriculum. below: paramilitary esthetics and propaganda among the participants of the company ‘instituto peruano de estudios en paleovertebrados’. bottom left: the president, m r. carlos a . v ildoso. images from facebook (open access). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 8 gan izin g th e rally dakar 2014, in ord er to p r oceed to th e d em arcation of ‘in situ ’ vert ebrate fossil skeleton s th at cou ld be affected by th e layou t of th is fam ou s race (fig. 3). oth er sou rces of fu n d in g for th e in stitu te in clu d e th e organ ization of itin eran t p aleon tological exh ibition s, th e sale of ed u cation al m aterials an d th e p u blication of a m agazin e (vild oso m orales, 2012). th e jou rn al contribuciones paleontológicas del instituto peruano de estu dios en paleovertebrados, th e ‘official p u blication ’ of th e ipep, is n ot traceable by bibliograp h ic m ean s. in th e cover im age of its first n u m ber (vol. 1 for th e year 2013, accord in g to th e w ebp age of th e ipep) it in d icates th e issn 225-2797 (sic). bu t in th e issn global d atabase th e jou rn al ap p ears as in itiated in 2010 u n d er th e isbn n u m ber 22252797, lackin g a d ep osit in th e n ation al library of peru as sh ou ld be exp ected . an oth er an om aly is th at th is jou rn al is n ot in clu d ed in th e world cat d atabase, w h ich raises d ou bts over its existen ce. bein g an active p erson in th e m ed ia of h is cou n try an d in social n etw orks, mr. vild oso regu larly w orks as a research p aleon tologist, d esp ite th e absen ce of recogn ized p aleon tolog ical train in g, w h ich can be qu alified as a kin d of u n qu alified p ractice. th e resu lts of h is alleged in vestigation s, in som e cases totally in ven ted p aleon tological d iscoveries, w ere ‘p u blish ed ’ by th e jou rn al of th e ipep an d in som e con fe ren ce abstracts th at h ave assu m ed , in good faith , th e in tegrity of its au th ors. th ere is n o record of in tern ation al research p u blication s w ith mr. vild oso as au th or or co-au th or, n or d oes h e ap p ear w ith an y in th e n ation al directory of scien ce an d tech n ology research ers of peru . from th e jou rn alistic in form ation it is d ed u ced th at mr. vild oso p ossess a valu able collection of peru vian fossils, estim ated by h im self at abou t 50,000 sp ecim en s, th at h e ap p aren tly keep s in h is p riv ate resid en ce, accord in g to a p erson al in terview p u blish ed by th e peru vian jou rn al la rep ú blica. on ly in th is early article (jan u ary 14, 2013), h e qu alified h im self as a ‘lover of p aleon tology –bu t n ot a p aleon tologist’. h ow ever, in march 2016, an d actin g as presid en t of th e ix latin am erican con gress of paleon tology, h is cu rricu lu m vitae ch an ged t o ‘a research er w ith alm ost th ree d ecad es of e xp erien ce in vertebrate p aleon tology’, teach in g for several years in th e p rogram of arch eology of th e un iversity san lu is gon zaga of ica (fig. 3). th e p osition of mr. carlos vild oso as ch airm an of th e organ izin g com m ittee of th is con gress d eserves m ore atten tion . th is m eetin g w as h eld in th e m ain bu ild in g of th e min istry of cu ltu re in lim a betw een sep tem ber 20–24, 2016, w ith th e ipep in ch arge of treasu ry th rou gh a p erson al ban k accou n t of its d irector. th e n u m ber of accou n tin g irregu larities w ere su ch , th at mr. josé ap olín , rep resen tative of th e min istry of cu ltu re an d actin g as ch airm an of th e eth ics an d regu lation s of th e peru vian con gress d efin ed th e h an d lin g by mr. vild oso a ‘p lan n ed em bezzlem en t’, w h ich w ou ld h ave affected th at m in istry an d scien tists from 14 cou n tries. th e econ om ic balan ce p resen ted by th e p resid en t at th e en d of th e con gress w as very u n satisfactory also in w ord s of dr. jean n oël martín ez, w h o acted as ch airm an of th e scien tific com m ittee of a con gress th at h ad a m in u scu le local p articip ation (6 p osters from peru ). 5. fin al remarks exam p les of recen t m eth od ological an d eth ical m iscon d u ct in p aleon tological scien ce affectin g morocco an d peru are p resen ted h ere. most of th e form er rem ain u n k n ow n ou tsid e th e lim ited circles of sp ecialists bu t cou ld in trod u ce con fu sion in th e fu tu re. th ere is an im p ortan t risk th at th ese p oor d escrip tion s an d con tam in ated moroccan d ata m ay be u tilized for r egion al geology or in global p aleon tological d atabases, affectin g th e con clu sion s reach ed in p aleogeograp h ical, stratigrap h ical or oth er ge ological an alyses. th e latter p roblem is th e social p rojection so far reach ed by th e false peru vian p aleon tologists d eterioratin g th e im age of tru e p rofession al p aleon tolog ists of th is an d ean cou n try. th e activities of th e sp an ish fossil collector an d trad er mr. joan corbach o led to th e p u blication of several d ozen n ew trilobite taxa com in g from morocco, in absen ce of tru e scien tific ta xon om ical stu d y an d p recise geological d ata abou t th e d ifferen t typ e localities. th e typ e m aterial of th e n ew sp ecies selected to be h ou sed in in stitu tion al rep ositories is barely rep r ean n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 9 sen tative, w h ile m u ch better sp ecim en s rem ain in p rivate h an d s. th e best -p reserved p aratyp es h ave been in p art offered for sale in in tern et (even th e figu red sp ecim en s) w ith a ‘certificate of au th en ticity’ sign ed by th e d irector of th e geological mu seu m of th e sem in ary of barcelon a, th e sam e in stitu tion th at ed ited th e jou rn als w h ere th e p ap ers d escribin g th e n ew moroccan sp ecies w ere p u blish ed w ith ou t an y scien tific con trol or p eer -review p rocess by sp ecialists. mr. corbach o con sid ers h im self as a tru e ‘exp ert in p aleon tology’ in th e m u seu m an d d em on strates h is con d ition by p u blicizin g h im as a m em ber of th e sp an ish society of paleon tology (w h ich is op en to all typ es of p u blic). desp ite h is p rivate titles in p aleon t ological falsification s an d ap p raisals, m an y of th e n ew trilobite sp ecies p rop osed by mr. co rbach o are based on sp ecim en s retou ch ed by moroccan d ealers. th is fact, ad d ed to th e lack of p recise stratigrap h ic in form ation an d a good taxon om ic stu d y, in trod u ces m ore p roblem s th an ben efits to n orth african p aleon tology. th e false h istory as p aleon tologist of mr. klau s h ön n in ger w as d eleted from wikip ed ia in early 2014, after bein g classified as ‘p rom otion al’ by several ed itors, an d becau se it exclu sively con tain ed referen ces to n on -scien tific jou rn alistic articles. ap art from th at, h e is th e au th or of a sm all book en titled buscando al gigante (‘lookin g for th e gian t’, san marcos ed ., lim a 2014), referrin g to th e cam p aign s to fin d th e ‘megalod ón ’ (= th e gian t sh ark otodus (m egaselachus) megalodon ) in th e ica d esert. th e con ten ts th at mr. h ön n in ger n ow d issem in ates in social n etw orks are m ostly im p regn ated w ith p h ilosop h ical, en viron m en tal an d arch e ological con n otation s, or sh ow in g its on goin g p erson al op p osition , for exam p le, to "trash tv" an d th e retu rn of th e rally dakar to peru , bu t w ith on ly few p aleon tological referen ces. h ow ever, by th e begin n in g of n ovem b er 2016 mr. h ön n in ger erron eou sly ap p eared in th e list of th e ‘five peru vian scien tists ad m ired by th e w orld ’, an in itiative from cienciactiva (= ‘active scien ce’), w h ich d ep en d s from th e consejo n acional de ciencia, tecnología e innovación tecnológica (con cytet, an equ ivalen t to th e usa n ation al scien ce fou n d ation n sf). up on th is error bein g realized , th e p eop le resp o n sible rep laced mr. h ön n in ger’s n am e by th at of a peru vian scien tist of u n qu estion able in tern ation al rep u tation . h ow ever, mr. carlos vild oso w as recen tly r ep orted to th e min ister of cu ltu re of peru by th e society of vertebrate paleon tology, requ estin g an in vestigation in to th e organ ization an d fin an cin g of th e ix latin am erican con gress of paleon tology. th is in tern ation al society a ccu sed h im of seriou s irregu larities exp erien ced by m em bers of th e p aleon tological scien tific com m u n ity w h o p articip ated in th e m eetin g, su ch as keyn ote sp eakers n ot issu ed w ith airlin e tickets, sch ed u led field trip s n ot celebrated an d w ith u n retu rn ed fees to p articip an ts th at h ad p aid in ad van ce, besid es a gen eral d iso rgan ization an d lack of scien tific coord in ation an d atten tion to p articip an ts. a sim ilar stat em en t h as been p u t ou t by the geological society of peru as w ell as by dr. rod olfo salas gism on d i, d irector of th e dep artm en t of vert ebrate paleon tology of th e n atu ral h istory m u seu m (n ation al un iversity of san marcos, lim a). th e latter h as p u blish ed on th e in tern et n u m erou s accred ited d ocu m en ts on th ese accu sation s an d oth ers on th e lack of eth ics of mr. vild oso. th is w as rep lied by mr. vild oso askin g for a p u blic retraction of th ese statem en ts in febru ary 10, 2017, w h ich to th is d ate h as n ot taken p lace. parad oxically, research ers from th e n atu ral h istory mu seu m of lim a are alm ost th e on ly on es th at p rod u ce relev an t p aleon tological r esu lts m easu red in term s of th e im p act factors establish ed by in tern ation al d atabases (wok, scop u s...), follow ed by few oth er u n iversities an d by p aleon tologists of in gemmet (th e p eru vian geological su rvey). with th e e xcep tion of th ose from th e un iversity of piu ra an d tw o oth er lim ean p rivate u n iversities, m ost of th e ren ow n ed peru vian p aleon tologists w ere n ot in vited in d u e tim e to join th e can d id acy of lim a as th e site of th e ix latin am erican co n gress of paleon tology, fin ally p resid ed by c.a. vild oso. accord in g to w ritten stat em en ts by dr. salas-gism on d i, mr. vild oso is su sp ected to be in volved in th e n u m erou s an on ym ou s facebook p ages fu ll of racist, sexist an d h om op h obic assertion s, in ten d ed to in su lt an d d isqu alify th e m em bers of th e dep artm en t of vertebrate paleon tology of th e n atu ral h istory mu seu m . an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 10 ackn owledgmen ts we are gratefu l to th e variou s am erican an d eu rop ean colleagu es w h o h ave p r ovid ed u s w ith a very valu able d ocu m en tary safegu ard , w ith w h ich to su p p ort all th e statem en ts m ad e. th an k you to dr. abd elou ah ed lagn aou i (un iversity of el jad id a, morocco) an d an an on ym ou s referee for th eir con tribu tion s im p rovin g th e m an u scrip t. referen ces agu irre j. 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(2016). platypeltoid es ham m ond i (trilobita, n ileid ae): a new species from the up per trem ad oc of the dra valley, morocco. batalleria, 23, 11-19. corbacho j. and martínez d. (2014). perito ju d icial en p aleontología: una nu eva visión. com u nicats inst. cat. miner. gem m ol. paleontol., 3839, 71-72. corbacho j. and martínez bau tista d. (2015). sobre peritajes y tasaciones en p aleontología. com u nicats inst. cat. miner. gem m ol. paleontol., 5, 40-41. corbacho j. and send ino c. (2015). fossil fakes and their recognition. dep osits mag., 30, 3540. corbacho j. and vela j.a. (2010). giant trilobites from low er ord ovician of morocco. batalleria, 15, 3-32. corbacho j. and vela j.a. (2011). revisión d e las especies d e lehu a d e la región d e zagora (marru ecos). batalleria, 16, 46-49. corbacho j. and vela j.a. (2013). parvilichas m arochii: n ew genu s and species of lich id ae from the zagora region (morocco); early ord ovician (floian). scrip ta mu s. geol. sem inar. barcinon ensis [ser. p alaeontol.], 14, 3-13. corbacho j., corbacho i., morrison s. and valent m. (2015). últim as técnicas d etectad as en la falsificación d e fósiles. batalleria, 22, 22-27. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7475 11 corbacho j., h am m ond k., moreno e. and morrison s. (2016). paleontología y coleccionism o. batalleria, 23, 43-46. corbacho j., lópez-soriano f.j., lem ke u. and h am m ond k. (2017). platyp eltoid es carm enae: a new n ileid ae (trilobita) from the low er ord ovician (trem ad ocian) of gu elm im area; western anti-atlas, morocco. batalleria, 25, 20-29. corbacho j., morrison s. and ait ad d i a. (2014). dionid e carlottae: una nu eva especie d e dionid id ae (trilobita) d el ord ovícico su p erior d e marru ecos. batalleria, 21, 13-21. corbacho j., send ino c. and tahiri m. (2011). palaeontological fakesbatalleria, 16, 37-45. corbacho j., vela j.a. and cu evas j.a. (2007). falsificación d e trilobites d el norte d e áfrica. batalleria, 13, 29-36. fortey r.a. (2009). a new giant asap hid trilobite from the low er ord ovician of morocco. mem . assoc. au stralas. palaeontol., 37, 9-16. geyer g. and corbacho j. (2015). the bu rlingiid ae (trilobita): revised generic com p osition, str atigrap hy, and the first species from the early mid d le cam brian of west gond w ana. gff, 137, 212-225. granier b., feist m., h ennessey e., bu cu r i.i. and senow bari-daryan b. (2009). the im am case. ad d itional investigation of a m icrop aleontological frau d . carnets géol., 2009/ 04, 1-14. gu tiérrez-marco j.c. 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(2015). pau xillites thad d ei a new low er ord ovician hyolith from morocco. acta mu s. n at. pragae, ser. b-h ist. n at., 71(1-2), 51-54. valent m., corbacho j. and martínez d. (2013). h yolith localities of zagora region (morocco), up per fezou ata form ation (low er or d ovician). batalleria, 19, 20-23. vela j.a. (2007). three new sp ecies of lehu a from low er ord ovician of dra valley (morocco), scripta mu s. geol. sem inar. barcinonensis [ser. p alaeontol.], 4, 24-37. vela j.a. and corbacho j. (2007). a new esp ecies sicof lehu a from low er ord ovician of dra valley of moroccobatalleria, 13, 75-80. vela j.a. and corbacho j. (2009). n ew trilobites from up p er ord ovician of el kaid er ram i, morocco. batalleria, 14, 99-106. vild oso morales c.a. (2012). paleontology in peru : ju st beginning. palaeontologia electronica, 15, 2-3e, 7 pp . vilella j. (1999). 125 años d el mu seo geológico d el sem inario d e barcelona (1874-1999). com u nicats inst. cat. miner. gem m ol. paleontol. [5], 8-9, 27-28. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 1 communicating seismic risk: the geoethical challenges of a people-centred, participatory approach iain s. stewart joh an n a ickert su stain able earth in stitu te, un iversity of plym ou th , u n ited kin gd om iain .stew art@p lym ou th .ac.u k joh an n a.ickert@p lym ou th .ac.u k robin lacassin in stitu t d e ph ysiqu e d u globe d e paris, sorbon n e paris cité un iversity, cn rs, fran ce lacassin @ip gp .fr abstract the sendai framework for disaster risk reduction (sfdrr) encourages scientists to participate in bottom -up risk communication approaches that directly engage hazard-prone populations. effective communication of seismic risks not only has economic impacts in terms of hazard mitigation but also provides social value in potentially empowering the marginalized populations that disproportionately live in high-risk areas. this emphasis on community-focused disaster preparedness, however, presents a novel set of communication challenges for geoscientists. few scientists have training in or experience of translating their science for lay publics, and conveying complex risk information is especially difficult in circumstances where scientific issues are socially contested and politically charged. recognising that disaster threats can create troublesome information battlegrounds, this paper explores the ethical and practical aspects of seismic risk communication, motivated by an early-career earth scientists’ workshop in istanbul that voiced the concerns of young geoscientists confronted firsthand by at-risk publics. those concerns form the basis of a wider review of the risk communication issues that are likely to be encountered if community -centred participatory drr approaches are to be adopted by earthquake science researchers. 1. in trod uction here has to be a broader and a more people-centred preventive approach to disaster risk. disaster risk redu ction practices need to be multi-hazard and multi-sectoral, inclusive and accessible in order to be efficient and effective. (...) there is a need for the public and private sectors and civil society organizations, as well as academia and scientific and research institutions, to work more closely together and to create opportunities for collaboration.’ (sen d ai fram ew ork , 2015, p .7). th e sen d ai fram ew ork for disaster risk red u ction (sfdrr) offers a n ew global in str u m en t for con fron tin g n atu ral h azard s, settin g ou t an am bitiou s h olistic strateg y th at em braces th e n eed for a ‘… full and meaningful participation of relevant stakeholders at appropriate levels’ an d th e ‘… empowerment and inclusive, accessible and non-discriminatory participation of the most marginalised publics’ (un isdr, 2015). as su ch , th e sen d ai fram ew ork d efin es a n ew social con tract betw een th e h azard scien tist an d th e w id er p u blic (ism ail-zad eh et al., 2017). it is a ‘t an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7593 2 con tract th at en cou rages th e scien tific com m u n ity to en d eavou r, alon gsid e th eir existin g tech n ical exp ertise, to ‘… support action by local communities and authorities; and support the interface between policy and science for decision -making’ (un isdr, 2015). th is vision of citizen -orien ted research is m im icked in oth er areas of geosocietal con cern s, su ch as en ergy , clim ate ch an ge an d in frastru ctu re d evelop m en t (e.g. pid geon et al., 2014; kam lage an d n an z, 2017), an d is on e in creasin gly en d orsed by m ajor in tern ation al sp on sors of scien tific research . th e eu rop ean com m ission , for exam p le, ch an ged th e title of th eir ‘scien ce an d society’ p r ogram m e to ‘scien ce in society’ an d u n d er h orizon 2020 d evelop ed gu id elin es arou n d ‘resp on sible research an d in n ovation ’ (rri) w ith th e d eliberate goal of stim u latin g reflexivity an d in volvin g a ran ge of social actors scien tists, citizen s, p olitician s an d bu sin esses m ore closely in scien tific en d eav ou rs th at w ere cod esign ed an d co-p rod u ced by society (o w en et al., 2012). in h eren t in th is sh ift from th e con ven tion al ‘top -d ow n ’, ‘exp ert-led ’ ap p roach to th e em ergen t ‘bottom -u p ’, ‘com m u n ity-led ’ ap p roach is th e ch allen ge of ‘th e last m ile’ a term borrow ed from th e telecom m u n ication s sector, in w h ich th e fin al con n ection betw een th e con su m er an d th e tech n ology d eterm in es h ow effective it is for th e vast m a rket of u sers. th e ch allen ge of reach in g th e last m ile (in th is case, reach in g th ose p eop le d irectly at risk) h as b ecom e a critical n otion th at in creasin gly in form s ou r th in kin g abou t a far w id er ran ge of n atu ral risk ch allen ges (e.g. sh ah , 2006), alth ou gh in keep in g w ith th e sh ift to a p eop le-cen tred focu s of d isaster risk red u ction d iscou rse it h as becom e re-fram ed as ‘th e first m ile’ (e.g. kelm an an d glan z, 2014; bau d oin et al., 2016). in ad d ition to in form in g civic officials an d d issem in atin g to p olicy m akers, com m u n icatin g th at first m ile to reach th e p eop le w h o d irectly face extrem e h azard th reats ou gh t to be a fairly u n con ten tiou s com p on en t in h azard p rep are d n ess an d m itigation efforts. yet, a p articip atory ap p roach m arks a m eth od ological m ove aw ay from th e p revailin g m od e of kn ow led ge tran sfer tow ard s m ore in clu siv e tran sd iscip lin ary strategies th at in corp orate p eer -role m od els, ad op t social n etw ork -based strategies an d d irectly en gage w ith com m u nities in m otivatin g p rep ared n ess action s (sch n eid ew in d et al., 2016; sch losser an d pfirm an , 2012; drake et al., 2015; ben d ito an d barrios, 2016; ism ail-zad eh et al., 2017). th is n ew , tran sd iscip lin ary scien ce h as been con troversial (sch n eid ew in d an d brod ow ski, 2014) an d th ere are cu rren tly n o gu id elin es for w h at con stitu tes su ccessfu l p a rticip ation an d w h at m easu res p rom ote bu ild in g tru st betw een civil society an d its organ iz ation s an d scien ce. desp ite th e in tern ation al p u sh for p articip atory ap p roach es, th ere is a lack of social sp aces an d in teractive form ats th at en able exch an ge an d join t learn in g b etw een tech n ical sp ecialists an d lay p u blics. a review of p eop le-cen tred ap p roach es for d isa ster risk m an agem en t d escribed : ‘… a complex landscape characterized by insufficient resources at the local level, and lack of willingness among the public at risk to share responsibility for disaster risk management with authorities. if official authorities are to implement the new people-centred approach, they must better understand residents’ perspectives and responsibility expectations, become more competent comm unicators, and be willing to engage in long-term dialogue with communities’ (scolobig et al., 2015, p .202). th e ch allen ge of h ow to com m u n ica te effectively to at-risk com m u n ities, th erefore, lies at th e h eart of th e p eop le-cen tred ap p roach to d isaster risk red u ction . desp ite th is, few ge oscien tists h ave been train ed in con veyin g th eir tech n ical kn ow -h ow beyon d th e acad em ic an d p rofession al w orld , an d , for th ose th at h ave, th at train in g u su ally p rioritises p eer -to-p eer com m u n ication skills an d h ow to m an age relation s w ith jou rn alists an d better access th e broad er p rin t an d broad cast m ed ia (liverm an 2008). in con trast, ord in ary p eop le ‘on th e grou n d ’ from local com m u n ity grou p s to civic au th orities ten d to be less fam iliar an d m ore rem ote (i.e. h ard er -to-reach ) au d ien ces for m ost scien tists (liverm an 2008, stew art an d n ield 2013). in ad d ition to bein g ‘h ard er to reach ’, p u blic au d ien ces often m eet ‘scien ce’ at tim es of crisis. in em ergen cy situ ation s, scien tific u n d erstan d in gs bu ilt u p grad u ally over m an y d ecad es are exp ected to be d elivered by ‘exp erts’ in n eat an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 3 m ed ia sou n d -bites an d u n am bigu ou s p u blic statem en ts (stew art an d n ield , 2013). distillin g com p lex tech n ical kn ow led ge in to d igest ible p op u lar risk m essages th at can be read ily co n su m ed by lay au d ien ces is a p ersisten t ch allen ge for th ose w orkin g at th e scien ce/ p u blic in terface. in th e im m ed iacy of an em ergin g d isaster, p eop le ten d to beh ave in w ays an d m ake d ecision s th at are n ot an ticip ated by scien tific exp erts an d by em ergen cy m an agers. ou tsid e of crisis situ ation s, com m u n icatin g u n certain ty to at-risk p u blics is associated w ith several ch allen ges, su ch as id en tifyin g th e facts relevan t to recip ien ts’ d ecision s w h ile d eterm in in g th e relevan t u n certain ties, estim atin g th eir im p act, form u latin g p ossible m essages, an d evalu atin g th eir su ccess (fisch h off an d davis, 2014). in ad d ition , bu ild in g com m u n ity aw aren ess of p oten tial risks can also be d ifficu lt if p eop le h old seriou s m iscon cep tion s abou t basic scien ce con cep ts, if th e scien tific issu es are socially con tested , an d if th e h azard th reat is p olitically ch arged (stew art an d lew is 2017). th e resu lt is th at th e scien ce/ p u blic kn ow led ge in terface can qu ickly becom e m ore like th e fron tlin e of an in form ation battlefield . in su ch com bative circu m stan ces, th ose con veyin g th e risk of n atu ral h azard s to com m u n ities th rea ten ed by th em can fin d th em selves n avigatin g a carefu l com m u n ication cou rse b etw een th e tech n ical n u an ces an d u n certain ties of extrem e n atu ral even ts on th e on e sid e an d th e n orm ative n u an ces an d u n certain ties of m ed ia p ra ctice an d h u m an beh aviou r on th e oth er. th e d ilem m as faced by geoscien tists in co m m u n icatin g risk d u rin g seism ic crises h ave been m ost exten sively d ocu m en ted an d acu tely d issected for th e 2009 l’aqu ila (italy) eart h qu ake an d su bsequ en t legal trial (alexan d er , 2010 an d 2014; marzocch i, 2012; jord an , 2013; di cap u a an d pep p olon i, 2014; dolce an d di bu cci, 2014; mu cciarelli, 2014; yeo, 2014; cocco et al., 2015; devasto et al., 2016). alth ou gh in itially fram ed as a trial of scien tists w h o ‘failed ’ to p red ict an earth qu ake, it is m ore w id ely in terp reted as a failu re in risk com m u n ication , as oreskes (2016, p .254) n otes: ‘the case centered not on the matter of whether or not earthquakes can be predicted, but on political questions about the social obligations of scientists speaking in official advisory capacities, and epistemic questions about the appropriate manner in which risk assessments should be performed. the questions at stake were what information scientists should have offered the public, and how that information should have been communicated. they were not so much matters of scientific facts, but matters of how those facts were rendered and communicated.’ su ch ‘fau lty’ com m u n ication s arose becau se seism ic crises are n ot sim p ly geop h ysical p h en om en a bu t th ey are also socio -p olitical in cid en ts. th eir th reat can m otivate vested bu sin ess in terests to op p ose th e scien ce (gesch w in d , 1997 an d 2011) an d th eir in cid en ce can be catalysts of cu ltu ral ch an ge or triggers for p olitical u p h eaval (clan cey , 2006). as is evid en t in oth er areas of scien ce, n otably clim ate ch an ge d iscou rse, con flicts of in terest can resu lt in ‘m an u factu red u n certain ty’ an d th e d eliberate obfu scation or m isrep resen tation of in fo rm ation for w h ich th ere is broad scien tific co n sen su s (mich aels, 2005; mich aels an d mon forton , 2006; oreskes an d con w ay, 2007). in su ch con tested social sp aces, th e w isd om an d resp on sibility of geoscien tific exp erts in offerin g gu id an ce or ad vocacy h as been qu estion ed : ‘w hether scientists providing expert input into policy issues should be guided by extra-scientific values (and if so, whose values, and which values) in either the conduct or communication of their science, and even whether they should go beyond their scientific competence strictly speaking and provide advice about policy options, is open to debate’ (yeo 2014). th is societal en tan glem en t h as p oten tially tran sform ative im p lication s for th e geoscien tists th at stu d y th em , as ackn ow led ged by oreskes (2016, p .261): ‘earthquake safety has never been simply a matter of geophysics, but most earthquake scientists … have traditionally understood their job to be to study how, when, and why eart hquakes happen, and only to a lesser extent (if at all) how to communicate that knowledge to engineers and officials responsible for mitigation, or to the general public … but in the contemporary world, the inter-relationship between knowledge and safety is not easily disentangled. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7593 4 seismology is no longer simply a matter of geophysics, if it ever was. it involves consideration of ethics, values, and monetary and social costs. l’aquila shows that scientists can no longer ignore the social factors that affect and even control how damaging a particular earthquake may be. earthquake prediction is a social science.’ in this p ap er, w e carry forw ard oreskes’s prov ocation that earthqu ake science is a social science by consid ering the issu e of how to com m u nicate seism ic risk in a pu blic sphere in w hich the science is socially contested and p olitically charged . ou r consid eration arises not from italy bu t from istanbu l, another trou blesom e earthqu ake hot spot, and view ed not from the perspective of senior hazard specialists bu t instead from that of ea rly-career geoscientists. motivated by their em ergent concerns, w e exam ine key them es of risk com m u nication that m ight be im p ortant if neighbou rhood -based particip atory drr is to be ad opted by the earthqu ake science com m u nity. 2. case study: a seismic confrontation in istanbul istan bu l, a m ega-city of 14.5 m illion resid en ts, faces a m ajor earth qu ake th reat (h ori et al., 2017). over th e last cen tu ry, a w estw ard m igratin g sequ en ce of large eart h qu akes h as left on e p rom in en t segm en t of th e n orth an atolian fau lt u n ru p tu red (arm ijo et al., 1999; stein et al., 1997; le pich on et al., 2003). th at segm en t lies im m ed iately sou th of th e city, b en eath th e w aters of th e marm ara sea an d seism ologists exp ect th is seism ic gap to be filled by a m >7 earth qu ake in th e com in g d ecad es (pa rson s et al., 2000; parson s, 2004; boh n h off et al., 2013). th e leth ality of th e th reat is ev id en t from a d estru ctive earth qu ake th at stru ck to th e east of istan bu l in au gu st 1999, killin g 17,000 p eop le an d m akin g ap p roxim ately h alf a m illion p eop le h om eless (özerd em , 1999). risk scen a rios for a fu tu re marm ara sea earth qu ake an ticip ate sign ifican t fatalities an d w id esp read d am age to resid en tial h ou sin g an d u rban in fr astru ctu re. for exam p le, a fu tu re m 7.25 earth qu ake on th is offsh ore segm en t is exp ected to h eavily d am age or d estroy 2-4% of th e n ear 1,000,000 bu ild in gs in istan bu l, w ith 9-15% of th e bu ild in gs receivin g m ed iu m d am age an d 20-34% of th e bu ild in gs ligh tly d am aged (erd ik et al., 2011; erd ik, 2013). sh aken by th e 1999 earth qu akes, istan bu l’s civ ic au th orities a ttem p ted to ad d ress th e acu te seism ic vu ln er ability of th e city th rou gh a series of legislative figure 1: there is a strong cultural influence on people’s perspectives on the perceived earthquake threat. based on interviews with residents in earthquake-prone parts of usa , japan and turkey, most participants indicated a high awareness of seismic adjustment but there was much variation in belief about the levels of human agency and control. turkish participants tended strongly toward fatalism, regarded earthquakes as divine providence but displayed a height ened focus on vulnerabilities caused by institutional and collective failings, implying that the consequences of earthquakes were largely determined by the actions of people and society. redrawn from joffe et al. (2013, figure 4). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 5 m easu res facilitatin g u rban ren ew al. th e co n troversial im p lem en tation of th e 2005 ren ew al law n o. 5366 in th e city’s h istoric d istricts a u th orised th e cen tral govern m en t h ou sin g d evelop m en t agen cy (toki) to u n d ertake regen eration p rojects in seism ically vu ln erable gecekondu (squ atter) n eigh bou rh ood s, p rojects th at m et sign ifican t local resistan ce (karam an , 2008; un sal, 2015). th at resistan ce reflects legislative ch an ges th at h ave been im p osed by civic au th orities. in 2012, th e u rban regen eration law n o. 6306 exten d ed regen eration beyon d th e h istoric d istricts, targetin g n eigh bou rh o od s th at are gen erally n ot th ose m ost vu ln erable to earth qu ake d estru ction bu t in stead rep resen t areas w h ere red evelop m en t is h igh ly econ om ically p rofitable (gibson an d gök şin , 2016). desp ite a recogn ition th at ‘seismic risk in the buildings in istanbul is mostly dominated by building vulnerability, not hazard’ (yaku t et al., 2012, p .1533), th ere is w id esp read d istru st of ista n bu l’s retrofittin g an d recon stru ction m easu res even am on g resid en ts of som e of th e city’s m ost at-risk qu arters (green , 2008; islam , 2010; karam an , 2013; ku yu cu , 2014). th e roots of th is d istru st go d eep in to th e tu r kish ‘earth qu ake p sych e’. a com p arison of p o p u lation s livin g in earth qu ake -p ron e areas in jap an , usa an d tu rkey revealed th at esp ecially stron g an d varied em otion s p erm ea te tu rkish earth qu ake p ercep tion s an d attitu d es (joffe et al., 2013) (fig. 1). direct exp erien ces w ith th e 1999 earth qu akes h ave p rovoked h eigh ten ed feelin gs of w orry, fear an d an xiety, bu t in ad d ition th ere w ere stron g exp ression s of corru p tion an d in com p eten ce of p olitician s, civil ser v an ts, p lan n in g regu lators an d th e con stru ction in d u stry (fig. 2). for m an y, it w as th is en d em ic corru p tion , greed an d selfish n ess th at w as seen to p rod u ce u rban vu ln erability, an d in tu rn created a h eigh ten ed sen se of d isso n an ce (fatalism ) an d w eaken ed sen se of con trol an d selfefficacy. th u s, d esp ite a su bstan tial aw aren ess of th e earth qu ake risk, joffe et al. (2013) rep ort th at tu rkish resp on d en ts w ere far less likely th an th eir us or jap an ese equ ivalen ts to ad op t seism ic ad ju stm en t m easu res, a ten d en cy also ap p aren t in oth er stu d ies (ru stem li an d karan ci, 1999; özerd em , 1999; eraybar et al., figure 2: the emotional and moral responses of people to earthquakes vary in strength and character between three eart hquake-prone countries – usa , japan and turkey. fear and anxiety-related emotions dominate in all three countries, but turkish participants show a greater prevalence of griefand traumarelated emotions and display considerably more emotions relating to moral issues such as corruption (e.g., anger, distrust, blame). from joffe et al. (2013, figure 3). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7593 6 2010; tekeli‐yeşil et al., 2010a, 2010b, an d 2011; oral et al., 2015; taylan , 2015). in d eed , joffe et al. (2013) rep ort th at th e tu rkish resp on d en ts w h o felt th em selves m ost v u ln erable to eart h qu akes d isp layed th e low est ad o p tion of an tiseism ic ad ju stm en t m easu res. in su ch a socially p olarised an d p olitically sen sitive con text, d evelop in g n eigh bou rh ood -based p articip atory strategies for seism ic risk com m u n ication w ou ld ap p ear d ifficu lt. to exp lore th is d ifficu lty, a com m u n ication train in g w orksh op for early-career geoscien tists h eld in istan bu l d irectly con fron ted tw elve ph d an d p ostd octoral r esearch ers w ith th e overtly p oliticised n atu re of th e istan bu l eart h qu ake th reat (ickert an d stew art, 2016). as p art of th e w orksh op th e you n g geoscien tists visited at-risk n eigh bou rh ood s w ith in th e city to h ear from resid en ts an d com m u n ity lead ers abou t h ow th e city’s seism ic risk w as m an ifest ‘on th e grou n d ’, w itn essin g firsth an d th e effects of th e u n p op u lar u rban ren ew al tran sform ation p r ogram m e. follow in g th is field p rovocation , ickert an d stew art (2016) rep ort h ow th e w or ksh op p articip an ts w ere in vited to d iscu ss th eir p erceived role as com m u n icators. across th e grou p of you n g research ers th ere w as broad agreem en t on th e relevan ce an d im p ortan ce of th eir exp ert kn ow led ge reach in g at-risk com m u n ities, bu t con sid erable d iscu ssion abou t th e ap p rop riate w ay an d level of en ga gin g w ith th e p u blic. researcher 1: if you know that something will happen [...] that many people could die [...] you will have to communicate that. y ou have to communicate that in order to prepare people. alth ou gh th is view w as w id ely sh ared , m u ch d ebate cen tred on th e p articip an ts’ in d ivid u al u n d erstan d in gs of th e role an d resp on sibility of geocom m u n icators, an d w h at im p lication s th is h ad for th eir p rofession al life. for som e, th e d isin cen tives to com m u n icatin g m ore w id ely w ere p ractical as w ell as m oral: researcher 8: (...) it is our responsibility. but the problem is: w e are not paid for that. w e have to maintain a career as well. a nd this is only one of the little aspects that are very relevant. w e have to do it for the sake of it. w e do a lot of things for science which are for free. a nd we also have a hard time to maintain a pace...and to do publications, to find the next position and so on. so it is a very difficult balance. researcher 7: there is no real reward. researcher 8: w ell, it depends on how you interpret reward. som e p articip an ts fou n d it cru cial n ot to blu r th e bou n d ary betw een scien tists an d n on scien tists an d to retain th eir role as “objectiv e exp erts”, feelin g u n easy w ith th e p rosp ect of op eratin g beyon d th e geoscien ce realm . researcher 11: i think you should do your best to improve your analyses and get proper results and publish and explain these results to proper people. for example, the government or the administration. a nd these people should know what to do with this. y ou can give them suggestions what you think is the best idea to use the results and how to protect the people, but the decision belongs to them. researcher 4: in my humble opinion, science has something to do with knowledge. policies, hazard mitigation, those are things related to judgement, to decision -making. those are two completely different things. for som e p articip an ts, a d irect en gagem en t w ith resid en ts, p articu la rly in p oliticized con texts, w as con sid ered as n egatively affectin g th eir role as scien tists, p oten tially riskin g a loss of rep u tation , tru st an d scien tific cred ibility d u e to actu al or p erceived ad vocacy p o sition s. oth ers, w h ilst ackn ow led gin g th ese w orries, stressed in stead th e ‘m oral an d p rofession al d u ty’ to d irectly p rovid e th eir exp ertise to com m u n ities, esp ecially in situ ation s w h ere in h abitan ts face an acu te risk an d op en ly requ est closer collaboration s w ith scien tists. for th em , th ere w as a ‘risk of losin g p u blic tru st’ w h en n ot reactin g to sh ortcom in gs of com m u n ication , bu t at th e sam e tim e an an xiety abou t d riftin g in to th e role of ‘ad vocate’ or even ‘a ctivist’, as th is exch an ge d em on strates: researcher 8: a hypothetical case, let’s imagine the scientific community has a very clear view that the m armara earthquake is going to happen in five years time, and it is going to be magn itude 8. then what is your responsibility, when people are not reached by standard geoscience comm uan n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 7 nication? this is how i face this problem. then you really have to push the boundaries and tell the people that they should move away from the boundary [… ] but i am already in the activist part. researcher 2: y ou’re looking at the human aspect, not at the scientific aspect. a s a human being, when you see that something bad will happen very soon, then of course you will push people and try to fix the problem [...]. a s a scientist you just have to do the research, get the information and share it. researcher 8: but i absolutely don’t feel like this – this is my scientific part and this is my human part [...] i don’t understand why geoscience should be communicated in a very specific, narrow way, for example centred on geohazards. then people might know something about the physics, but they don’t really do anything in their daily lives. a nd this is the challenge. researcher 7: y ou could make sure that you inform the public better, so that they can find a way around this corrupt system so that people are informed to really make decisions. researcher 10: but this is really complicated. th is d isagreem en t abou t roles an d resp on sibilities of geoscien tists in th e risk com m u n ication p rocess su ggest th at th e m u ltitu d e of factors th at in flu en ce h ow risk com m u n ication is p e rceived , in terp reted an d tran slated by in h abitan ts of at-risk com m u n ities is equ ally reco gn ised by scien tists. desp ite th is, th ere w as co n sen su s am on g th e grou p on th e n ecessity to m ore effectively con n ect w ith at -risk com m u n ities in ord er to red u ce th eir seism ic vu ln erability. in ad d ressin g th is, h ow ever, th ere w as u n certain ty abou t w h eth er th e you n g geoscie n tists h ad th e ap p rop riate skillset to su ccessfu lly en gage w ith lay au d ien ces. som e felt con fid en t in th is asp ect, givin g p erson al exam p les of scien ce–p u blic in teraction s, su ch as en cou n ters w ith local resid en ts in th e cou rse of th eir field w ork, in cid en ts in w h ich th ey “h ad to get in form ation from local p eop le”, an d w ere asked to “exp lain ” w h at “th ey are d oin g”. in th at co n text, som e su p p ort for th e valu e of a m ore p a rticip atory ap p roach em erged : researcher 6: w hy do you think that only the geoscientists give the information? m aybe there are things that you don’t know, and that only an ordinary person knows. for example when you go to the field, [...] to a little village, if you are working on a recent event of that region, you go to the manager of the village, and you talk to him, for example “have you ever had any floods in this area?” it is a communication situation and you learn from a person that is not a geoscientist. more gen erally, h ow ever, th e research ers felt in su fficien tly skilled in com m u n ication to reach be-yon d th e acad em ic an d p rofession al geoscien ce com m u n ity. in teraction w ith lay au d ien ces w as ju d ged a ‘rath er u n kn ow n territory’. given th is p erceived skills d eficit, d ebate em erged abou t w h eth er it w as m ore effective to ‘p in p oin t th e com m u n ication talen ts’ w ith in th e geoscien ce com m u n ity or in stead to en gage in in terd iscip lin ary research collaboration s. liaisin g w ith social scien tists w ere p r op osed , as w as w orkin g w ith in term ed iaries or tran slators m ed ia rep resen tatives, n gos or even artists to m ore effectively sh are kn ow led ge w ith p e op le on th e grou n d . th is brief exch an ge cap tu res th e essen ce of th at d isagreem en t: researcher 10: our responsibility is to produce science and use other scientists who can talk to people, like anthropologists, sociologists or people who have studied philosophy, psychology, this kind of stuff... m y point is that we need a bridge to communicate with the people. w e cannot communicate directly. w e need a translator. researcher 3: or translate it ourselves. researcher 2: it won´t be that easy for us. th e im p lication s arisin g from th ese w orksh op d iscu ssion s are exam in ed in d etail by ickert an d stew art (2016), bu t h ere w e h igh ligh t th e basic d ilem m a: com m u n icatin g d irectly w ith at-risk com m u n ities is recogn ised as bein g im p ortan t bu t th ere w as a gen eral an xiety abou t h ow easily or effectively geoscien tists can ad op t su ch a p articip atory ap p roach . in th is regard , th e early -career geoscien tists raise critical qu estion s abou t h ow best to in tegrate oth er d iscip lin ary p ersp ectives, p articu larly th ose from th e social scien ces, in to th eir geo -risk exan n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7593 8 p ertise. stim u lated by th ese m eth od ological d ialogu es, th e follow in g section s su m m arise som e key fin d in gs th at h ave em erged from broad er social scien ce d iscou rses on risk co m m u n ication . 3. faulty commun ication s toward s a social scien ce of con veyin g seismic risk 3.1 the risk perception paradox ‘the majority of people at risk from eart hquakes do little or nothing to reduce their vulnerability’ (solberg et al., 2010, p .1663). th e con ven tion al w ay of com m u n icatin g risk is th rou gh ed u cation cam p aign s th at raise p u blic aw aren ess of h azard th reats. th is ap p roach rests on th e assu m p tion th at in d ivid u als or com m u n ities w ith h igh h azard aw aren ess are m ore likely to resp on d to w arn in gs an d u n d e rtake p rep ared n ess m easu res th an in d ivid u als or com m u n ities w ith a low er/ d eficien t h azard aw aren ess. in crease an in d ivid u al’s p ercep tion of a th reat, th e assu m p tion goes, an d you im p rove th eir p rep ared n ess. dem eritt an d n or bert (2014) d escribe th is ap p roach to risk co m m u n ication eith er u n d er th e term “risk m e ssage m od el”, referrin g to th e belief th at sou n d risk com m u n ication is abou t faith fu lly tran sm ittin g risk in form ation w ith ou t d isto rtion , or u n d er th e term of a “risk in stru m en t m od el”, a com m u n ication ap p roach w ith th e goal to elicit certain cogn itive or beh aviou ral resp on ses in th e target au d ien ces th at are ad d ressed (d em eritt an d n orbert, 2014). h ow ever, several d ecad es of social scien ce research in d icates th at th ere can be little or n o correlation betw een th e p rovision of scien tific in form ation abou t ge op h ysical h azard s an d risks an d th e ad op tion of p rep ared n ess m easu res by in d ivid u als or com m u n ities (palm an d h od gson , 1992; palm , 1998; sp ittal et al., 2008; solberg et al., 2010; wood et al., 2011). wh ile th e h azard scien tist is steep ed in p robabilistic or d eterm in istic th in kin g abou t th e ch an ces or im p acts of an extrem e even t, th e statistical lik elih ood of a d isaster is barely taken in to accou n t w h en ord in ary p e op le m ake ju d gm en ts abou t p erceived risk le vels, an d th e p erceived m agn itu d e of a d isa ster seem s equ ally to be of little im p ortan ce . in stead , th ere is a grow in g ap p r eciation of th e role of socio-cu ltu ral, cogn itive an d em otion al variables in risk p ercep tion an d beh aviou r. solberg et al. (2010) p rovid e a com p reh en sive review of h ow p eop le th in k, feel an d act abou t seism ic risk ad ju stm en t, argu in g th at in d ivid u al or collective aw are-n ess is sh ap ed by a m yr iad of social factors, p sych ological biases an d cu ltu ral n orm s, in clu d in g exp erien ce, op tim ism , d em ograp h ic ch aracteristics (gen d er, age, statu s), p ercep tion s of fate an d fatalism , in d ivid u al an d com m u n ity feelin gs of con trol, self-efficacy an d em p ow erm en t, as w ell as th e d egree of tru st in exp erts an d a u th orities. all of th ese risk m ed iators are sen sitive to cu ltu ral an d p olitical con texts, an d all n eed to be co n sid ered if risk com m u n ication is to be tru ly e ffective. th e ‘risk p ercep tion p arad ox’ (wach in ge r et al., 2013) con ten d s th at if risk p ercep tion is on ly loosely related to risk a d ju stm en t, th en sim p ly d issem in atin g in form ation on seism ic h azard an d societal vu ln erability to exp osed p op u lation s m ay n ot m otivate m ean in gfu l risk red u ction beh aviou r. th is reap p raisal rests on a broad er rejection by com m u n ication p ractitio n ers of th e over -relian ce on factu al in form ation in con veyin g scien tific issu es to th e p u blic (bu rn s et al., 2003; n isbet, 2009). after all, th e ‘facts’ arou n d com p lex scien tific issu es a re often con tested even by th e exp erts, an d th e sam e tech n ical p roblem can be p resen ted in very d ifferen t w ays to elicit m arked ly con trastin g r esp on ses. in th e febrile atm osp h ere of n atu ral em ergen cies an d crisis situ ation s it can be e xp ected th at ‘facts will be repeatedly misapplied and twisted in direct proportion to their relevance to the political debate and decision-making’ (n isbet an d moon ey, 2009, p .56), w ith th e resu lt th at even ‘… compelling scientific information often runs aground almost as soon as it is launched into the choppy waters of public discourse’ (weber an d word , 2001, p .488). th e social p sych ology of h ow p eop le receive an d p rocess in form ation abou t risk d ecision s is com p lex an d con tested , an d th e im p lication s th is h as for scien ce com m u n ication in gen eral are d iscu ssed elsew h ere (e.g. jam ieson et al., 2017; n ation al acad em ies of scien ces, en g ian n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 9 n eerin g, an d med icin e, 2017). h ow ever, th e h ead lin e m essages th at h ave em erged from several d ecad es of h u m an an d beh aviou ral r esearch are n eatly su m m arised by corm ick (2014) (fig. 3): 1. ‘when information is complex, people make decisions based on their values and beliefs’; 2. ‘people seek affirmation of their attitudes or beliefs, no matter how strange those views are’. th is ten d en cy m ean s th at in d ivid u als w ill reject in form ation or evid en ce th at are cou n ter to th ose attitu d es an d beliefs (kah an et al., 2010); th e fact th at n ew in form ation con sisten t w ith on e’s beliefs is m ore easily seen as reliable an d in form ative th an in form ation th at d iscred its on e’s in itial beliefs exp lain s w h y beliefs ch an ge very slo w ly an d are qu ite en d u rin g in th e face of co n trary evid en ce (n isbett an d ross, 1980). 3. ‘people most trust those whose values mirror their own’. th ey ten d to look to oth ers arou n d th em for social clu es on h ow to act, w h ich can eith er accen tu ate or d ecrease social accep tan ce of th e risk of a giv en issu e (kah an et al., 2010). as a con sequ en ce, in d ivid u als gen erally m ake m ore risky or extrem e d ecision s as p art of a grou p th an as an in d ivid u al; 4. ‘attitudes that were not formed by logic or facts, are not influenced by logical or factual arguments’; 5. ‘public concerns about contentious science are almost never about the science and scientific information therefore does little to influence these concerns.’ th e n otion th at logical an d factu al argu m en ts m ay be su bord in ate to valu e ju d gem en ts an d in stin ctive th in kin g in d eterm in in g h ow p e op le m ake sen se of tech n ically com p lex issu es h as im p ortan t im p lication s for h ow th e geoscien ce com m u n ity con veys its scien ce to th e p u blic. after all, con ven tion ally, geoscien tists ten d to bu ild com m u n ication strategies arou n d co n veyin g clear, sim p le exp lan ation s of th e tech n ical d etail. th ey d o so becau se th at is w h at geoscien tists h ave been train ed to d o, becau se it is th at tech n ical kn ow -h ow th at d efin es th eir ow n u n d erstan d in g of th e p roblem , an d b ecau se oth er cru cial stakeh old ers regu lators, en gin eers, p lan n ers an d law yers d em an d it. figure 3: social science research highlights a few headline messages of how people make decisions about complex and contested environmental concerns (after cormick, 2014). an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7593 10 figure 4: in the context of community conflicts, sandman (1993) argues that ‘risk’ is a product of ‘hazard x outrage’. r educing risk can be achieved by lowering outrage through adjusting the levels of the primary components of community anxiety. bu t m arsh allin g th e scien tific facts, illu stratin g th em w ith sim p le grap h ics, an d exp lain in g th em u n clu ttered by jargon , seem s u n likely to ad d ress p u blic con cern s: ‘it is not enough to assure that scientifically sound information – including evidence of what sci-entists themselves believe – is widely disseminated: cultural cognition strongly motivates individuals – of all worldviews – to recognize such information as sound in a selective pattern that reinforces their cu ltural predispositions. to overcome this effect, communicators must attend to the cultural meaning as well as the scientific content of information ’ (kah an et al., 2010, p .23). 3.2 risk = hazard x outrage un d erstan d in g th e social an d cu ltu ral con stru ction of risk is recogn ised to be at th e h eart of com m u n ity-cen tred p articip atory ap p roach es to d isaster risk red u ction . it is a n otion rooted stron gly in th e p sych om etric m od el of risk p e rcep tion (fisch h off et al., 1978; slovic, 1987 an d 1989; slovic et al., 1991; fisch h off, 1995), w h ich d econ volves th e com p osite p u blic view of ‘risky’ scien tific issu es. th e ‘p sych om etric’ risk p arad igm h as been w id ely ap p lied by san d m an (1987, 1989, an d 1993), w h o con ten d s th at m ost local en viron m en tal con troversies com p rise tw o com p etin g fram es. th e first is a tech n ical fram in g of th e p roblem , in volvin g arg u m en ts abou t th e scien tific an alysis of th e h a zard s th at are p erceived to th reaten a com m u n ity. th e secon d relates to th e social con text w ith in w h ich th ose h azard s exist an d th e p r ocesses by w h ich a com m u n ity’s an xieties fu els collective an ger. accord in g to san d m an (1993), th at com m u n ity an ger is rooted in a ran ge of con cern s – in clu d in g tru st, con trol, volu n tar in ess, d read , an d fam iliarity – w h ich h e collectively term s ‘ou trage factors’ (fig. 4). sand m an (1993) contend s that w hen the experts and the pu blic d isagree abou t the technical aspects (su ch as the m agnitu d e of a p articu lar threat or its p robability of occu rrence), the exp erts are an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 11 m ore likely to be correct. and yet, althou gh scientists read ily p oint ou t how the pu blic often m isperceives the hazard , they rarely acknow led ge that they them selves p ay little attention to that com p onent of the p erceived risk that is socially constru cted . ‘the experts, when they talk about risk, focus on hazard and ignore outrage. they therefore tend to overestimate the risk when the hazard is high and the outrage is low, and underestimate the risk when the hazard is low and the outrage is high because all they are doing is looking at the hazard. the public, in precise parallel, focuses on outrage and ignores hazard. the public, therefore, overestimates the risk when the outrage is high and the hazard is low, and underestimates the risk when the outrage is low and the hazard is high’ (sand m an, 1993, p.8). the pu blic’s concerns the social risk is frequ ently d ism issed by scientific experts as being irrational, u nfou nd ed or m anipu lated , even thou gh it is evid ent from com m u nity p rotests that the resu lting anxiety, fear and anger is argu ably m ore tangible and m easu reable than the u nd erlying hazard . in the context of com m u nity conflicts, sand m an su ggests that the technical view of risk as a prod u ct of ‘hazard x vu lnerability’ is m ore u sefu lly reform u lated as being a p rod u ct of ‘hazard x ou trage’. this, in tu rn, sets the tem p late for risk com m u nication strategies. ‘two things are true in the typical risk controversy: people overestimate the hazard and people are outraged. to decide how to respond, we must know which is mostly cause and which is mostly effect. if people are outraged because they overestimate the hazard, the solution is to explain the hazard better. but if they overestimate the hazard because they are outraged, the solution is to figure out why they are outraged and change it.’ 3.3 risk d ialogues and honest brokers ‘… public participation measures are probably the most effective means to create awareness of potential disasters, to enhance trust in public authorities, and to encourage citizens to take more personal responsibility for protection and disaster preparedness. it will be a major challenge for risk management and also an important research topic for future research to understand people’s responses to natural hazards as well as a combination of natural and technological hazards and to design the most appropriate measures for effective risk communication, stakeholder involvement, and emergency preparedness’ (wach in ger et al., 2013). an altern ative ap p roach to con ven tion al risk m essage m od el of com m u n ication is th e “risk d ialogu e m od el”, w h ich is based on th e belief th at th e d iverse exp ertise of m ix of civil society actors m u st be system atically an ch ored if a relevan t solu tion to red u ce v u ln erability is to be fou n d (dem eritt an d n orbert, 2014). en gagin g w ith th e w id er civil society in a m ore p articip atory w ay abou t earth qu ake th reats m ean s fra m in g seism ic risk com m u n ication n ot sim p ly as th e con ven tion al on e-w ay tran sfer of in fo rm ation from th e tech n ical exp ert to th e ‘en d u ser’, bu t also as a tw o -w ay exch an ge w ith stakeh old er grou p s abou t w h at con cern s th em . in sh ort, p arap h rasin g latou r (1994), it requ ires a m in d set sh ift from con veyin g ‘m atters of fact’ to d evelop in g d ialogu es arou n d ‘m atters of con cern ’, recogn izin g th at sh ared rath er th an u n id irection al flow s of in form ation are m ore likely to p rom ote kn ow led g e an d attitu d e ch an ge (stew art an d lew is, 2017). scien tific an d tech n ical in form ation are n ecessary for th is p rocess, bu t are n ot th e sole basis on w h ich d ecision s or action s are m ad e. wh en accessible scien tific in form ation abou t a con tested scie n tific issu e is p resen ted in w ell-organ ized social sp aces in w h ich ord in ary p eop le can form an d exp ress th eir op in ion s, th e p u blic can actively p articip ate in scien tific d ecision m akin g: ‘it appears that people understand some things quite well, although their path to knowledge may be quite different from that of the technical experts… given an atmosphere of trust in which both experts and lay persons recognize that each group may have something to contribute to the discussion, exchange of information and deepening of perspectives may well be possible’ (slovic, 1985, p .170). wh eth er d irected at th e p u blic or at p olicy m akers, m ore effective com m u n ication em er ges from p articip atory en gagem en t an d d ialogu e an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7593 12 w ith in d ivid u als an d com m u n ities (wach in ger et al., 2012). particip ation p rocesses allow th e p u blic to gain kn ow led ge an d p erson al agen cy w ith resp ect to risks an d p rotective m easu res, an d au th or ities to gain kn ow led ge from th e “lay e xp erts”/ th e p u blic an d to collect id eas for m easu res th at are effective for th e given p op u lation . in d ivid u als an d com m u n ities can be risk am p lifiers or in h ibitors. th u s, as w ell as gau gin g th e in trin sic vu ln erability of th ose livin g in h azard -p ron e settin gs, collective con versation s p rovid e th e exp ert w ith a valu able in form ation arch ive. th ese d ialogu es can reveal a social m em ory of p ast even ts an d h igh ligh t h ow p reven tative m easu res m ay be bu ilt in to trad itio n al p ractices an d vern acu lar arch itectu re. fin a lly, a sh ared resp on sibility for solu tion s in su ch a d ialogu e situ ation can also p rom ote th e social im p lem en tation of th em . yet, d esp ite its ap p aren t ben efit s, a m ore p articip atory ap p roach raises eth ical d ilem m as for a geoscien tist, n ot least of w h ich is th e p ote n tial d rift aw ay from bein g a n eu tral m ed iator of in d ep en d en t kn ow led ge to a p articip an t th at is en gaged w ith , or p erh ap s even em bed d ed in , com m u n ity con cern s. th e scien tist as an ‘ad v ocate’ or even ‘activist’ for a sp ecific cou rse of action is on e th at m an y exp erts an d n on exp erts alike are u n com fortable w ith . in d istin gu ish in g d ifferen t ‘kin d s’ of scien tist, pielke (2007) h igh ligh ts th e im p ortan t role of th e ‘h on est broker’ th e sp ecialist w h o can in t egrate stakeh old er con cern s w ith available scien tific kn ow led ge to op en u p an d in form a ran ge of op tion s. draw in g on th e fau lty co m m u n ication s an d th eir ep istem ological roots d u rin g th e tu h oku an d fu kish im a catastrop h e, lacassin an d lavelle (2016) h igh ligh t th e r esp on sibility of com m u n icatin g u n certain ties th rou gh su ch ‘h on est brokers’. accord in g to th e au th ors, th ey n eed n ot on ly to focu s on th e d ecision -relevan t elem en ts of th at com m u n icated u n certain ties, bu t also u n cover th e u n certain ties th at scien tists fail or avoid to m en tion becau se of d om in an t com m u n ication p ar ad igm s, argu in g th at: ‘… to promote real democratic and open debate and choices, [scientists] have the responsibility to communicate an d properly explain all uncertainties and unknowns to the technical and political sphere as well as to the rest of the society’ (lacassin an d lavelle, 2016, p .57). as h on est brokers, effective geoscien ce co m m u n ication becom es n ot sim p ly a case of secu rin g p u blic accep tan ce, bu t, rath er, of secu rin g p u blic tru st. it is argu ably m ore im p ortan t to bu ild tru st th an to bu ild tech n ical u n d erstan d in g becau se tru st is u sed by p eop le as a su rr ogate tool a cogn itive sh ortcu t for red u cin g th e com p lexity of scien t ific in form ation . an d it is th at tru st th at w ill be cru cial later if w arn in gs an d oth er typ es of vital in form ation are to be taken seriou sly d u rin g an em ergen cy (wach in ger et al., 2013). th ose scien tific br okers w h o gen u in ely en gage w ith affected com m u n ities are likely to h ave a p articu larly p rivileged p lace in th e d eliberative p rocess b ecau se, in ad d ition to th eir grasp of tech n ical com p lexity, th ey w ill be afford ed a h igh d egree of p u blic tru st. yet th at elevated d egree of in flu en ce, an d th e an ticip ated co-p rod u ction of kn ow led ge th at accom p an ies a com m u n ity cen tred ap p roach , m ay m ake m an y geoscie n tists an xiou s th at th eir m u ch valu ed in d ep en d en ce w ill be com p rom ised . 4. con clusion s th e eth ical resp on sibility of scien tists is th e com m u n ication of balan ced factu al in form ation , yet th e relative p rom in en ce given to th ose facts is cru cial. to con vey a scien tific m essage in a w ay th at gain s w id e accep tan ce requ ires a sim p lified m essage strip p ed of th e u su al tech n ical caveats. th e eth ical bu rd en th at th is p laces on th e scien ce com m u n icator as an h on est broker of kn ow led ge is obviou s. th e clim ate scien ce com m u n icator step h en sch n e id er d u bbed it th e ‘d ou ble eth ical bin d ’, n otin g th at ‘each of us has to decide what the right balance is between being effective and being honest. i hope that means being both’ (sch n eid er, 2002, p .498). th e ch allen ge for th ose w orkin g in d isaster risk com m u n ication is h ow to con vey w h at th ey kn ow h on estly an d effectively to th ose p eop le w h o can ben efit from th at kn ow led ge. in term s of th e form er, it requ ires h azard scien tists to better u n d erstan d th e social p sych ology of h ow an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ ag-7593 13 p eop le receive an d p rocess in form ation , an d in d oin g so learn h ow best to fram e th e in tricacies, u n certain ties an d lim itation s of th eir in tr icate tech n ical scien ce in w ays th at are m ore easily grasp ed by lay au d ien ces. in u n iversities an d research in stitu tes it w ill be cru cial to train th e n ext gen eration of geoscien tists in th e scien ce an d art of com m u n ication , bein g m ore jou rn alistic an d m ed ia -savvy in th e w ay w e com m u n icate, an d m ore im agin ative in exp lo rin g n ew com m u n ication ch an n els, su ch as w eb based p latform s, social m ed ia. th e n ew reality is th at in tim es of th e crisis factu al scien tific in form ation s can be qu ickly (m in u tes to h ou rs) tran sm itted th rou gh social m ed ia (e.g. tw itter, facebook), th ereby h elp in g p eop le to ap p reciate th e geop h ysical u n d erstan d in g of an on g oin g n atu ral even t. in th e sam e w ay, h o w ever, m isin form ation or m iscon ceived facts can equ ally p rom in en tly be in stan tly relayed , m ean in g th at p u blic tru st in h on est scien tific brokers m u st be m ain tain ed . for th is reason , alon gsid e th e obviou s ap p eal of n ew , rem ote m obile tech n ologies an d d igital com m u n ication s, face-to-face en cou n ters w ill be critical for effective com m u n ity-cen tred d isaster d ialogu es. in th ose en cou nters, geoscien tists sh ou ld exp ect to m eet th e p u blic in th eir ow n com m u n ity sp aces in sch ools, p laces of w orsh ip , civic h alls an d social clu bs. in ad d ition , d eliberative form ats su ch a ‘reallabore’ (ma rqu ard t an d west, 2016), scien ce sh op s (sch lierfs an d meyer, 2013) or variou s form ats of in form al, d ialogu e-orien ted citizen p articip ation (kam lage et al., 2018) are im p ortan t foru m s w h ere m ore d irect tw o -w ay com m u n ication ap p roach es betw een d ifferen t stakeh old ers can take p lace. bu t as w ell as bein g m ore effective, geoscien tists n eed to better ap p reciate th at th eir seism ic risk com m u n ication s are u n d ertaken in co n tested socio-p olitical con texts an d brin g eth ical d ilem m as. th e exp ert tech n ical kn ow led ge of th e earth qu ake geoscien tist m ay be afford ed a h eigh ten ed d egree of tru st by th ose in d ivid u als an d com m u n ities m ost at risk, bu t th at brin gs resp on sibility. iron ically, th e m ore effectively a p oten tial seism ic th reat is com m u n ica ted by th e geoscien tist, th e m ore th at scien tific m essage w ill be n orm alized in to th e com p lex, ch aotic an d con tested d iscou rses of d aily life. particip atory -based en gagem en t strategies an ticip ate tech n ical exp erts takin g accou n t of th ese local socio-cu ltu ral, em otion al an d even p olitical d im en sion s of risk in w orkin g d ir ectly w ith vu ln erable com m u n ities. in su ch circu m stan ces, scien tific risk m essages can becom e h ijacked by or assim ilated in to social, econ om ic an d p olitical con troversies. th e ch allen ge for th ose w orkin g at th e con tested fron tlin e of seism ic risk com m u n ication s w ill be balan cin g th e eth ical bin d s th at con tin u e to con strain u s in ou r role as h on est brokers ackn owledgemen ts all th ree au th ors gratefu lly ackn ow led ge th e fin an cial su p p ort of th e eu marie cu rie in itial train in g n etw orks p rogram m e ‘alert’ (an atolian p lateau clim ate an d tecton ic h azard s) an d is w as ad d ition ally su p p orted by th e n erc-esrc-ah rc global ch allen ges research fu n d p roject 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(2014). fau lt lin es at th e in terface of scien ce an d p olicy: in terp retative resp on ses to th e trial of scien tists in l’aqu ila. earth scien ce review s, 139, 406-419. annex 2 to: passive degassing at nyiragongo (d.r. congo) and etna (italy) volcanoes annex 2 (doi: 10.4401/ag-6758) passive degassing at nyiragongo (d.r. congo) and etna (italy) volcanoes. calabrese s. et al. plume francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text rain vegetation 4 i n m o v o c o m p l e s s o p f , r i l r i l e v a m e n t o s i s t e m a t i c o d e l l e c a r a t t e r i s t i c h e t o n o s f e r t c h k antonio bolle n e l l ' i n t e n t o di d o t a r e l ' i s t i l l i l o n a z i o n a l e di g c o f i s i c a di u n a c o m p l e t a a p p a r e c c h i a t u r a p e r il r i l e v a m i i l i o e la r e g i s t r a z i o n e dei f e n o m e n i i o n o s f e r i c i , la d i l e z i o n e del p r e d e t t o i s t i t u t o m i h a affid a t o l ' i n c a r i c o del p r o g e t t o e d e l l a r e a l i z z a z i o n e d e l l a s t e s s a . n e l l a p r e s e n t e n o t a i n t e n d o r i f e r i r e a p p u n t o sul l a v o r o da m e c o m p i u t o a t a l e f i n e , e d i s c u t e r e l ' a p p a r e c c h i a t u r a r e a l i z z a t a , l a ( p i a l e , f o n d a l a sui p i ù m o d e r n i s u g g e r i m e n t i d e l l a t e c n i c a , e s p l i c a u n a p o t e n z a di i m p u l s o di c i r c a 2(1 k\\ , r e a l i z z a un p e r f e t t o a l l i n e a m e n t o d e l t r a s m e t t i t o r e col r i c e v i t o r e -u ogni p u n t o di u n a g a m m a a s s a i a m p i a ( 2 , 5 4 2 0 m e ) , e p e r m e t t e u n a r e g i s t r a z i o n e c o n t i n u a c s i c u r a d e g l i e c h i a n c h e in u n a z o n a p a r t i c o l a r m e n t e s o g g e t t a a r a d i o d i s t u r b i ( p i a l e i i n t e r n o di un g r a n d e a g g l o m e r a t o u r b a n o . i ra i si~tcmi e s c o g i t a t i p e r l ' e s p l o r a z i o n e d e l l a i o n o s f e r a . ( p i c i io più u s a t o è il m e t o d o d e g l i i m p u l s i di b r e i t e t u v e ( j ) . i n t r a s m e t t i t o r e m o d u l a t o a d i m p u l s i l a n c i a d e i t r e n i d ' o n d a di b r e v e d u r a t a ( i i i ' • r a . ) m a d i n o t e v o l e p o t e n z a . q u e s t i t r e n i d ' o n d a rif l e s s i d a l l a i o n o s f e r a v e n g o n o r a c c o l t i d a l r i c e v i t o r e r i g o r o s a m e n t e sintonizzato s u l l a stessa l u n g h e z z a d ' o n d a e , r i v e l a t i , l a s c i a n o una t r a c c i a s u l l o s c h e r m o d e l l ' o s c i l l o g r a f o u s a t o p e r la o s s e r v a z i o n e o p e r la r e g i t r a z i o n e . d a t a la n e c e s s i t à di r i p e t e r e q u e s t e e s p l o r a z i o n i , sia p u r e a d i n t e r v a l l i , in ogni ora del g i o r n o e d e l l a n o t t e , p e r lo s t u d i o sistem a t i c o si r i c o r r e a l l a r e g i s t r a z i o n e a u t o m a t i c a . p o i c h é i n t e r e s s a con o s c e r e il c o m p o r t a m e n t o d e g l i s t r a l i i o n i z z a t i p e r t u t t e le f r e q u e n z e c o m p r e s e e n t r o u n a c e r t a g a m m a , d a ! a ut m e c i r c a , è n e c e s s a r i o c l i c ogni r e g i s t r a z i o n e v e n g a e f f e t t u a t a a f r e q u e n z a v a r i a b i l e . e ' a l l o r a u n p r o b l e m a d i s o l u z i o n e n o t e v o l m e n t e d i f l i c i l e q u e l l o (li t e n e r e cos t a n t e m e n t e s i n t o n i z z a t i con la p r e c i s i o n e n e c e s s a r i a i d u e a p p a r a t i r i c e v i t o r e e t r a s m e t t i t o r e . l a s o l u z i o n e c o n c e t t u a l m e n t e p i ù s e m p l i c e è di d o t a r e i s i s t e m i di a c c o r d o dei d u e a p p a r a t i di o p p o r t u n i v i n c o l i m e c c a n i c i clic c o n s e n t a n o il c o m a n d o s i m u l t a n e o d e i c o n d e n s a t o r i o d e l l e b o b i n e di s i n t o n i a d e l t r a s m e t t i t o r e e del r i c e v i t o r e i n u n nuovo complesso sistematico caratteristiche ionoseeriche 165 i m o d o da o l l e n e r e i n ogni p l i n t o d e l l a g a m m a d ' o n d a la e s a t t a sint o n i a . e ' f a c i l e a c c o r g e s s i , c h e , ( p i a n d o i c i r c u i t i a c c o r d a l i sono d i c a r a t t e r i s t i c h e m e c c a n i c h e ed e l e t t r i c h e n o t e v o l m e n t e d i v e r s e , ( p i a l i possono e s s e r e q u e l l i di u n t r a s m e t t i t o r e d i n o t e v o l e p o t e n z a e d i u n r i c e v i t o r e di g r a n d e s e n s i b i l i t à si i n c o n t r a n o difficoltà m o l t o g r a n d i p e r o t t e n e r e il r i s u l t a t o i n d i c a t o , a n c h e con l a m a g g i o r e p r e c i s i o n e u s a t a n e l l o s t u d i o e n e l l a r e a l i z z a z i o n e d e i v i n c o l i m e c c a n i c i . i n f a t t i ili tal caso si p u ò o t t e n e r e l a s i n t o n i a e s a t t a solo in a l c u n i p u n t i d e l l a g a m m a , ed o c c o r r e c o n t e n t a r s i n e g l i a l t r i d i s o l u z i o n i d i c o m p r o m e s s o . s e i n o l t r e si c o n s i d e r a il n o t e v o l e n u m e r o di s t a z i o n i t r a s m i t t e n ti d i l u n g h e z z a d ' o n d a c o m p r e s a n e l l a g a m m a da e s p l o r a r e , si v e d e clic l ' i n t e r f e r e n z a d i q u e s t e u l t i m e r i d u c e a n c o r a il n u m e r o dei p u n t i e s p l o r a b i l i . l e r e g i s t r a z i o n i c h e -i o t t e n g o n o con q u e sto s i s t e m a a p p a i o n o in g e n e r e p o c o s o d d i s f a c e n t i . l a t e c n i c a m o d e r n a si 50p/s f ì r . 1 o r i e n t a t a r e c e n t e m e n t e ("') a l l a r e a l i z z a z i o n e di a p p a r a t i c l i c , s e b b e n e m o l t o p i ù c o m p l e s s i dal p u n t o d i v i s t a e l e t t r i c o , c o n s e n t o n o di o t t e n e r e la s i n t o n i a del r i c e v i t o r e e del t r a s m e t t i t o r e m e d i a n t e l a v a r i a z i o n e d e l l a f r e q u e n z a in u n u n i c o o s c i l l a t o r e clic s e r v e quindi p e r q u e s t i d u e a p p a r a t i , con la c o n s c g u e n t e e l i m i n a z i o n e del s i s t e m a d i a c c o p p i a m e n t o m e c c a n i c o , b a s t a n d o solo g l i o r g a n i n e c e s s a r i a l c o m a n d o di u n u n i c o c o n d e n s a tore v a r i a b i l e . i n o s c i l l a t o r e i, m o d u l a t o ad i m p u l s i , g e n e r a dei g r u p p i di o s c i l l a z i o n i d i f r e q u e n z a fissa /„, d e l l a d u r a l a d i c i r c a 10 1 s. i n sec o n d o o s c i l l a t o r e b a f r e q u e n z a v a r i a b i l e / ira i v a l o r i /, e; fè f a t t o b a t t e r e col p r i m o p e r m e z z o di u n a v a l v o l a m e s c o l a t r i c e ; a l l ' u s c i t a d i q u e s t a si h a n n o i b a t t i m e n t i d e l l e f r e q u e n z e /—/,, c o m p r e s e t r a fl—/,, ed {'.,—/„, c l i c v e n g o n o u t i l i z z a t i , o l t r e a q u e l l i d i f r e q u e n z e f + f„. e n a t u r a l m e n t e o l t r e a l l e f r e q u e n z e / ed /,, d e g l i o s c i l l a t o r i , a l l e l o r o a r m o n i c h e ed ai b a t t i m e n t i d i q u e s t e u l t i m e , o s c i l l a z i o n i t u t t e c h e o r i s u l t a n o di p i c c o l a a m p i e z z a o p p u r e v e n g o n o o p p o r t u n a m e n t e e l i i-i a n t o n i o uoi.i.e m i n a l e . l a f r e q u e n z a u t i l e / /,, v i e n e a m p l i f i c a l a ( l a u n a m p l i f i c a t o r e d i t e n s i o n e a p e r i o d i c o a l a r g a b a n d a c l i c p i l o t a u n o s t a d i o t r a s m e t t i t o r e a n c h ' e s s o a p e r i o d i c o e d a l a r g a b a n d a . s u l l a v a l v o l a c o n v e r t i t r i c e d e l r i c e v i t o r e , c h e è d e l t i p o s u p e r e t e r o d i n a , g i u n g e la o s c i l l a z i o n e d i f r e q u e n z a i /.,, d i r i t o r n o d a g l i s t r a t i i o n i z z a t i , r i c e v u t a d a l l a a n t e n n a . q u c » l a o s c i l l a z i o n e si f a i n t e r f e r i r e c o n q u e l l a g e n e r a t a d a l l ' o s c i l l a t o r e 11. p e r b a t t i m e n t o si o t t i e n e a n c o r a u n a o s c i l l a z i o n e d i f r e q u e n z a /„ u g u a l e a q u e l l a l i a g e n e r a l a d a l l ' o s c i l l a t o r e i, l a q u a l e , a m p l i f i c a t a e r i v e l a t a , v i e n e c o n d o t t a a l l ' o s c i l l o g r a f o . in d e f i n i t i v a i g r u p p i d i o s c i l l a z i o n i e l e t t r i c h e p r o d o t t i d a l l ' o s c i l l a t o r e .•/, d i f r e q u e n z a /., v e n g o n o p e r b a t t i m e n t i c o n o s c i l l a z i o n i p r o d o t t e d a l l ' o s c i l l a t o r e 11 d i f r e q u e n z a / c o m p r e s a t r a /, e d f-2, t r a s f o r m a l e in o s c i l l a z i o n i d i f r e q u e n z a / —/„, c o m p r e s a t r a /, —/,, e d f2—/•>. i r r a d i a t e d a 11 a n t e n n a t r a s m i t t e n t e , e , r i c e v u t e v e n g o n o t r a s f o r m a t e a n c o r a , p e r b a t t i m e n t o c o n l e s t e s s e o s c i l l a z i o n i d e l l ' o s c i l l a t o r e l i , i n o s c i l l a z i o n i d i f r e q u e n z a /,, u g u a l e a q u e l l a d e l l ' o s c i l l a t o r e i. da (pianto ora d e t t o r i u l t a c l i c è p o s s i b i l e o t t e n e r e l a s i n t o n i a r i gorosa sia del t r a s m e t t i t o r e c h e del r i c e v i t o r e p e r ogni v a l o r e d e l l a freq u e n z a c o m p r e s a i n tutta u n a g a m m a col -olo c o m a n d o di 1111 p i c c o l o o s c i l l a t o r e , c c h e , s c e g l i e n d o o p p o r t u n a m e n t e l e f r e q u e n z e d e i d u e o s c i l l a t o r i i e 11, e p o s s i b i l e , con la r o t a z i o n e di 1111 solo c o n d e n s a t o r e v a r i a b i l e e senza a l c u n c a m b i o di b o b i n e , c o p r i r e u n a g a m m a d ' o n d a m o l t o e s t e s a , q u a l e non s a r e b b e p o s s i b i l e con m e l o d i a l t r e t t a n t o semp l i c i . oscillatori e mescolatore. f a n n o p a r i e dell a p p a r e c c h i a t u r a i o n o f e r i c a i s e g u e n t i o r g a n i : l ' o s c i l l a t o r e p i l o t a a f r e q u e n z a fi-sa c o m a n d a t o d a g l i i m p u l s i r i t m i c i p r o d o t t i dall a p p o s i t o g e n e r a t o r e , l ' o s c i l l a t o r e p i l o t a a f r e q u e n z a v a r i a b i l e , l ' a m p l i f i c a t o r e a p e r i o d i c o di t e n s i o n e , q u e l l o a p e r i o d i c o di p o t e n z a ( t r a s m e t t i t o r e ) , il g e n e r a t o r e dì i m p u l s i r i t m i c i di c o m a n d o , il r i c e v i t o r e . 1 a p p a r a t o di r e g i s t r a z i o n e e v a r i g r u p p i di a l i m e n t a t o r i . l ' o s c i l l a t o r e p i l o t a a f r e q u e n z a fissa ( f i g . 2), u s a 1111 t r i o d o 6 j 5 con r e a z i o n e sul catodo ed è del t i p o h a r l l e y . la scelta del c i r c u i t o oscill a t o r e è c a d u t a su q u e s t o t i p o p e r i suoi r e q u i s i t i di s t a b i l i t à di freq u e n z a , f a c i l i t à di o s c i l l a z i o n e e b u o n a f o r m a d ' o n d a . questo o s c i l l a t o r e , c h e d e v e g e n e r a r e g r u p p i di o s c i l l a z i o n i d i d u r a t a m o l t o b r e v e , 1" 1 s, con c a d e n z a d e t c r m i n a t a , fi c o m a n d a l o dal g e n e r a t o r e d i i m p u l s i r i t m i c i . la g r i g l i a d e l l a 615 c l i c è n o r m a l m e n t e p o l a r i z z a t a u n n u o v o c o m p l e s s o s i s t e m a t i c o d e l l e c a k a t t e h i s t 1 c 1 i e i o n o s e e i u c h e k > 7 ali i n t e r d i z i o n e . a l l ' i s t a n t e d e l l a t r a s m i s s i o n e v i e n e p o r t a l a , d a l l ' i m p u l s o p o s i t i v o p r o d o t t o d a l g e n e r a t o r e , a p o t e n z i a l e z e r o , c o n s e n t e n d o così di s b l o c c a r e l ' o s c i l l a t o r e p e r t u t t a la d u r a l a d e l l ' i m p u l s o . p e r la c o n v e r s i o n e d i f r e q u e n z a l a f o r z a e l e t t r o m o t r i c e a l t e r n a t a di q u e s t o o s c i l l a t o r e , p r e l e v a t a d a l l a b o b i n a o s c i l l a l r i c e , v i e n e m a n d a t a a l l a g r i g l i a di soppressioik' di u n a 6 a c 7 c l i c f u n z i o n a d a m e s c o l a t r i c e . l / o s c i l i a l o r e p i l o t a a f r e q u e n z a v a r i a b i l e , è , c o m e l ' a l t r o a f r e q u e n z a fissa j2t1 al ricevitore f i g . 2 e p e r le stesse r a g i o n i , d e l t i p o l l a r t l e v , e c o m e q u e l l o u s a u n a 6j."> con r e a z i o n e sul c a t o d o . p o i c h é esso s e r v e a c o m a n d a r e sia l a f r e q u e n z a d e l t r a s m e t t i t o r e c h e q u e l l a d e l l ' e t e r o d i n a d e l r i c e v i t o r e è p r i v o di m o d u l a z i o n e , o s c i l l a con c o n t i n u i t à e c o p r e u n a c e r t a g a m m a di f r e q u e n z e : la c a p a c i t à del mio c i r c u i t o o s c i l l a n t e è p e r c i ò c o s t i t u i t a d a u n c o n d e n s a t o r e v a r i a b i l e clic, c o m a n d a t o a t t r a v e r s o u n g r u p p o di i n g r a n a g g i r i d u t t o r i d a 1111 m o t o r e e l e t t r i c o , c o m p i e u n a s e m i r o t a z i o n e i n c o n s e n t e n d o in q u e s t o t e m p o i e s p l o r a z i o n e di t u t t a la g a m m a d o n d a p e r cui è • •ostruito i a p p a r e c c h i o . q u e s t o o s c i l l a t o r e non è d i r e t t a m e n t e a c c o p p i a l o a l l a v a l v o l a m e s c o l a t r i c e m a a t t r a v e r s o u n o s t a d i o s e p a r a t o r e clic u s a u n t u b o 6 ag 7 con c i r c u i t o a c c o r d a t o s u l l a p l a c c a . q u e s t o s t a d i o o l t r e a filtrare l e \ v i ti n io uoi.i.e ine\ i t a b i l i a r m o n i c h e d e l l ' o s c i l l a t o r e , c o m p i e l ' i m p o r t a n t e f u n z i o n e eli p r o l e g g e r e l ' o s c i l l a t o r e p i l o t a d a l l e v a r i a z i o n i di f r e q u e n z a c l i c p o t r e b b e r o d e r i v a r e d a l l a r e a z i o n e d e i c i r c u i t i s u c c e s s i v i . l ' a c c o p p i a m e n t o con la m e s c o l a t r i c e è f a t t o s u l l a p r i m a g r i g l i a m e d i a n t e un c i r c u i t o s e c o n d a r i o a b a s s a i m p e d e n z a , d e b o l m e n t e a c c o p p i a t o al c i r c u i t o a c c o r d a l o d e l l a p l a c c a d e l l a s e p a r a t r i c e . c o m e g i à è s t a t o d e t t o , la v a l v o l a m e s c o l a t r i c e e u n a 6 a c 7 . s u l l a cui g r i g l i a v i e n e m a n d a t a la t e n s i o n e p r e l e v a t a d a l l o s c i l l a t o r e p i l o t a a f r e q u e n z a v a r i a b i l e , m e n t r e , q u e l l a p r e l e v a l a d a l l ' o s c i l l a t o r e p i l o t a a f r e q u e n z a fissa ò m a n d a t a s u l l a g r i g l i a cii s o p p r e s s i o n e . l e t e n s i o n i d e g l i a l t r i e l e t t r o d i sono s c e l t e , c e l a l i \ a m e n t e a l l e t e n s i o n i a l t e r n a t e p r e l e v a t e d a i d u e o s c i l l a t o r i , in m o d o t a l e c l i c la v a l v o l a non l'unzioni liei t r a t t o r e t t i l i n e o d e l l a c a r a t t e r i s t i c a , c o n s e n t e n d o la funzionecii m e s c o l a i r i c e . ad e v i t a r e c l i c , q u a n d o m a n c a n o le o s c i l l a z i o n i d e l p r i m o o s c i l l a t o r e p i l o t a , v e n g a n o a m p l i f i c a l e q u e l l e d e l s e c o n d o , il c l i c s a r e b b e d a n n o s o al b u o n f u n z i o n a m e n t o d e l r i c e v i t o r e , q u e s t a v a l v o l a v i e n e a n c h ' e s s a c o m a n d a l a a d i m p u l s i d a l l o stesso g e n e r a t o r e . il sopp r e s s o r e t e n u t o n o r m a l m e n t e a d 1111 p o t e n z i a l e n e g a t i v o s u f l ì c i e n l c m c n l c a l l o d a i n t e r d i r e il f u n z i o n a m e n t o d e l l a v a l v o l a , n e l l ' i s t a n t e d e l l a t r a s m i s s i o n e v i e n e p o r t a t o a d 1111 p o t e n z i a l e m o l t o v i c i n o a z e r o d a l l ' i m p u l s o p o s i t i v o p r o d o t t o dal g e n e r a t o r e . il c a r i c o a n o d i c o d i q u e s t a v a l v o l a è c o s t i t u i t o d a u n a r e s i s t e n z a e d a u n a i m p e d e n z a in s e r i e u l c i r c u i t o d i a l i m e n t a z i o n e , e d a u n a c a p a c i t à c h e s e r v e a l l ' a c c o p p i a m e n t o con i a m p l i f i c a t o r e d i t e n s i o n e . l ' u s o d e l l a i m p e d e n z a in s e r i e con la l ' e s i s t e n z a è s t a t o p r e v i s t o a l l o s c o p o di c g n a l i z z a r c il g u a d a g n o d e l l a v a l v o l a p e r l u t t o il c a m p o di f r e q u e n z a c h e i n t e r e s s a . implifìcatore ili tensione e trasmettitore. l a m p l i f i c a t o r e di t e n s i o n e a l a r g a b a n d a ( f i g . 3 ) con a c c o p p i a m e n to a r e s i s t e n z a e c a p a c i l à è a c i s t a d i ; idi c u i i p r i m i d u e u s a n o v a l v o l e d e l t i p o 6 a c 7 , il t e r z o u n a 6 a g 7 , il q u a r t o u n a 8 0 7 . il q u i n t o d u e 8 0 7 con i n v e r s i o n e d i f a s e , il s c s i o d u e 71315 in c o n t r o f a s e . il c a r i c o a n o d i c o di l u t t i g l i s t a d i è c o s t i t u i t o , c o m e p e r l a m e s c o l a i r i c e , d a res i s t e n z e ed i m p e d e n z e i n s e r i e s u l l ' a l i m e n t a z i o n e e d a c a p a c i t à p e r i a c c o p p i a m e n t o , a l l o scopo d i r e n d e r e c o s t a n t e il g u a d a g n o p e r t u t t o il c a m p o di f r e q u e n z e c h e d e v e e s s e r e a m p l i f i c a l o . q u e s t a s c e l l a d e l l e v a l v o l e è g i u s t i f i c a l a d a l l a n e c e s s i t à d i u s a r e , v i a v i a c l i c la t e n s i o n e v i e n e a m p l i f i c a l a , d e l l e v a l v o l e c l i c u n i s c a n o o_ (5 — 5 . — ~ i 1 'l % — z « "" o < ° ~ ' — o " s? o o ' o p ì r 'o ^ t 3 o — '/. s o o ^ o _ cl , 3 o o — 2 o •/ o o ;7 o j o o /. '/) •] n ~ 130 0 40 0_ _ + 15 00 ,+ 5 0 0 0 f i s . 3 av v b al ' g en er at or e di i m pu ls i ito antonio bolle c a t o d i a r i s c a l d a m e n t o i n d i r e t t o c l i c j>o-sono f o r n i r e c o r r e n t i di e m i s sione di p a r e c c h i e d i e c i n e di a m p è r e . due c o n d e n s a t o r i da 500 cm c o s t i t u i s c o n o l ' a c c o p p i a m e n t o d i a n t e n n a . il g u a d a g n o d e l l ' a m p l i f i c a t o r e di t e n s i o n e è di 4 6 di» p e r le f r e q u e n z e p i ù b a s s e e d i 4 0 p e r le più a l l e , a l l ' u s c i t a d e l l e d u e iì()7, e d i c i r c a 70 db a l l ' u s c i t a d e l l e 7 1 5 1 ' . generatore 01 impulsi a l l ' o s c i l l a t o r e b a l l a m p l i f i c a i . di p o t e n z a b a a a f i g . 4 l a p o t e n z a d i cresta d e l l o s t a d i o di u s c i t a , d e t e r m i n a t a in b a s e a l l a m i s u r a d e l l a p o t e n z a m e d i a e d e l l a d u r a t a d e g l i i m p u l s i è di c i r c a 20 k\\ p e r l e f r e q u e n z e p i ù b a s s e c d i c i r c a la m e t à p e r l e p i ù a l l e . generatore eli impilisi. c o m e g i à è s t a l o d e t t o , il g e n e r a t o r e di i m p u l s i r i t m i c i ( f i g . 4 ) a b a s s a f r e q u e n z a , è d e s t i n a t o a f o r n i r e s e g n a l i r e t t a n g o l a r i d e l l a d u r a t a di 10 ' s atti a p r o v v e d e r e al c o m a n d o d e l l a v a l v o l a o s c i l l a t r i e e , d e l 1 u l t i m o s t a d i o d e l l ' a m p l i f i c a t o r e d i t e n s i o n e e d e l l ' a m p l i f i c a t o r e d i p o t e n z a . deve a v e r e p e r t a n t o t r e l i s c i l e i n d i p e n d e n t i c l i c f o r n i s c o n o i m p u l s i s i n c r o n i i n a d i d i f f e r e n t e a m p i e z z a . p o i c h é l a g e n e r a z i o n e d e g l i i m p u l s i è s i n c r o n i z z a t a con la f r e q u e n z a d e l l a r e t e ( 5 0 p s)_ s u l l a g r i g l i a d e l l a p r i m a v a l v o ' a , u n a 6 \c7 r e t t a n g o l a t r i c e p e r s a t u r a z i o n e , v i e n e i n v i a t a la t e n s i o n e a l t e r n a t a d i a l i m e n t a z i o n e . l ' o n da r e t t a n g o l a r e c h e -i r i c a v a d a l l a p l a c c a a t t r a v e r s o u n o p p o r t u n o acc o p p i a m e n t o a r e s i s t e n z a e c a p a c i t à ( c i r c u i t o d e r i v a t o r e ) v i e n e i n v i a t a s u l l a g r i g l i a di u n a s e c o n d a 6 a c 7 clic h a lo scopo di amjili.fi.care i p i c c h i di t e n s i o n e c o r r i s j j o n d e n l i ai t r a t t i di s a l i t a d e l l ' o n d a r e t t a n g o l a r e . questa v a l v o l a è p o l a r i z z a l a s u f f i c i e n t e m e n t e da b l o c c a r e il seg n a l e c o r r i s p o n d e n t e al t r a t t o di d i s c e s a , s i c c h é p e r ogni p e r i o d o d e l l a u n n u o v o c o m p l e s s o s i s t e m a t i c o d e l l e c a r a t t e r i s t i c h e i o n ' o s l e r i c h e 1 7 1 t e n s i o n e a l t e r n a t a (li a l i m e n t a z i o n e si h a m i solo i m p u l s o . i na opp o r t u n a l i n e a d i r i t a r d o c o s t i t u i t a d a un c o m p l e s s o di i n d u t t a n z e i n s e r i e e c a p a c i t à in p a r a l l e l o , posta s u l l a p l a c c a di epiesta v a l v o l a , s e r v e ad a l l u n g a r e la d u r a l a d e l l ' i m p u l s o fino al v a l o r e v o l u t o . p e r il com a n d o d e l l ' o s c i l l a t o r e p i l o t a a f r e q u e n z a fìssa fu e del m e s c o l a t o r e è p r e v i s t a u n a v a l v o l a 6 l 6 a c c o p p i a t a a r e s i s t e n z a c a p a c i t à a l l o s t a d i o p r e c e d e n t e . d a l l a p l a c c a d i q u e s t a v a l v o l a clic d à u n i m p u l s o p o s i t i v o , a t t r a v e r s o d u e v a l v o l e del t i p o 6 v 6 clic h a n n o l o scopo d i i n v e r t i r n e il s e g n o , si va a l l e g r i g l i e di d u e v a l v o l e del iiito 81)7 c l i c , a l i m e n t a t e con 5 00 v e 1500 \ r i s p e t t i v a m e n t e a t t r a v e r s o d u e r e s i s t e n z e , forn i s c o n o l ' i m p u l s o p o s i t i v o di c o m a n d o a l l o s t a d i o di u s c i t a d e l l ' a m p l i f i c a t o r e d i t e n s i o n e e a q u e l l o di p o t e n z a . ricevitore.. il r i c e v i t o r e è del t i p o s u p e r e t e r o d i n a ( f i g . 5} con d o p p i a conv e r s i o n e di f r e q u e n z a . s u l l a p r i m a c o n v e r t i t r i c e , u n a 6 s a 7 . il cui c i r c u i t o di i n g r e s s o è a p e r i o d i c o , a r r i v a n o l e o s c i l l a z i o n i r a c c o l t e d a l l ' a n t e n n a , i r r a d i a t e d a l t r a s m e t t i t o r e di f r e q u e n z e c o m p r e s e t r a i v a l o r i /1—/„ ed /j /,, ( p r e c i s a m e n t e 2 . 5 — 2 0 m e ) e l e o s c i l l a z i o n i g e n e r a t e d a l secondo o s c i l l a t o r e p i l o t a . il c o l l e g a m e n t o con q u e s t o si e f f e t t u a s u l l a terza g r i g l i a d e l l a 6 s a 7 a t t r a v e r s o u n s e c o n d a r i o a b a s s a i m p e d e n z a d e b o l m e n t e a c c o p p i a t o con il c i r c u i t o a c c o r d a t o s u l l a p l a c c a d e l l a s e p a r a i r i e c . p o i c h é l a f r e q u e n z a di q u e s t ' u l t i m o o s c i l l a t o r e v a r i a t r a i v a l o r i /, ed f-, ( 3 0 — 5 0 m e ) p e r b a t t i m e n t o con il s e g n a l e c h e a r r i v a d a l l ' a n t e n n a , s u l l a p l a c c a d e l l a c o n v e r t i t r i c e si o t t i e n e il s e g n a l e a f r e q u e n z a /„ ( 2 7 . 5 m e ) u g n a l e a q u e l l a del p r i m o o s c i l l a t o r e p i l o t a . l a p r i m a m e d i a f r e q u e n z a del r i c e v i t o r e è a p p u n t o a c c o r d a t a su q u e s t o v a l o r e . l ' a m p l i f i c a t o r e d e l l a p r i m a m e d i a f r e q u e n z a è a d u e stadi ed usa r i s p e t t i v a m e n t e u n a v a l v o l a 6 a c 7 ed u n a v a l v o l a 6 3 g 7 . si h a q u i n di l a s e c o n d a c o n v e r s i o n e di f r e q u e n z a c h e a b b a s s a il v a l o r e d e l l a freq u e n z a i n t e r m e d i a da 2 7 . 5 me a 2 . 5 m e . a n c h e la s e c o n d a c o n v e r t i t r i c e è u n a v a l v o l a del t i p o 6 s a 7 c h e f u n z i o n a c o m e a u t o o s e i l l a l r i e e . s e g u o no a l t r i d u e s t a d i di m e d i a f r e q u e n z a con v a l v o l a 6 s g 7 c l i c p r e c e d o n o la r i v e l a t r i c e c h e è u n a (>t16. la v a l v o l a d i u s c i t a è u n a (>s\7 con i d u e t r i o d i in p a r a l l e l o ed il s e g n a l e clic si p r e l e v a dal c a t o d o v i e n e m a n d a l o a l l ' o s c i l l o g r a f o . l a scelta del v a l o r e di 2 . 5 m e p e r l a sec o n d a i n e d i a f r e q u e n z a è stata f a t t a t e n e n d o conto d e l l a d u r a t a d e l l ' i m p u l s o o n d e o t t e n e r e u n s e g n a l e a b b a s t a n z a f e d e l e . l a s e n s i b i l i t à b . n m o n o c o m p l e s s o s i s t e m a t i c o d e l l e c a r a t t e r i s t i c h e i o n o s f k k i c i i e i t . s p o n d e n z a del s e g n a l e d i r e l l o e d e g l i c e l l i , d e l l e z o n e o s c u r e d o v u t e a l l a d e v i a z i o n e del faseello di r a g g i c a t o d i c i p e r effetto d e i s e g n a i 1 p r o v e n i e n t i dal r i c e v i t o r e a l l e a l t r e d u e p l a c c h e d e v i a i l iei. davani a l l o .schermo è p o s t a u n a c a m e r a f o t o g r a f i c a e n t r o la q u a l e il filn scome con c o n t i n u i l a d a v a n t i a l l ' o b b i e t t i v o coi. m o t o u n i f o r m e , coniali d a t o a t t r a v e r s o un g r u p p o di i n g r a n a g g i da un m o t o r e e l e t t r i c o . 1.; l u n g h e z z a del f i l m i m p r e s s i o n a l o d u r a n t e ogni r e g i s t r a z i o n e è di ( ( i n . k p r e v i s t o un c o m a n d o a z i o n a l o dal m o t o r e d e l l ' o s c i l l a t o r e ; f r e q u e n z a v a r i a b i l e p e r d a r e f o t o g r a f i c a m e n t e sul film u n a s c a l a d e l l i f r e q u e n z e a l l o scopo di a g e v o l a r e la l e t t u r a d e l l e r e g i s t r a z i o n i . il c o m p l e s s o di a l i m e n t a z i o n e d e l l ' a p p a r e c c h i a t u r a eonsi>te i n e : a l i m e n t a t o r i d i s t i n t i ; d u e di e s s i , r i s p e t t i v a m e n t e d a 5000 e d a 1501 \ servono p e r lo s t a d i o t r a s m e t t i t o r e e lo s t a d i o d i l i s c i l a d e l l ' a n i p l i f i c a l o r e di t e n s i o n e . gli a l t r i q u a t t r o servono i n v e c e p e r l ' a l i m e l i l a / i o n e d e l g e n e r a t o r e di i m p u l s i r i t m i c i , d e l r i c e v i t o r e , d e l l ' a m p l i l i c a l o r e di t e n s i o n e e p e r le p o l a r i z z a z i o n i . il f u n z i o n a m e n t o d e l l ' a p p a rocchio è c o m p l e t a m e n t e a u t o m a t i c o : un o r o l o g i o p r o v v e d e i n f a t t i at t r a v e r s o un s i s t e m a di r e l è a l l a -ila p e r i o d i c a i n s e r z i o n e , m e n t r e k stesso m o t o r e c h e c o m a n d a 1 o s c i l l a t o r e a f r e q u e n z a v a r i a b i l e p r o v v e d i a l l a d i s i n s e r z i o n e a l l a fine d i ogni r e g i s t r a z i o n e . i . " a n t e n n a t r a s m i t t e n t e a t t u a l m e n t e u s a t a è u n d i p o l o . p o i c h é q u e sto t i p o di a n t e n n a lui un r e n d i m e n t o m o l l o basso p e r t u t t e l e i r e q i i e n z e a l l e q u a l i non r i s u l t a a c c o r d a t a , ne è p r e v i s t a la sostituzioni con una a n t e n n a a p e r i o d i c a r o m b o i d a l e v e r t i c a l e . conclusioni. dalle e s p e r i e n z e e s e g u i t o con q u e s t o n u o v o c o m p l e s s o è r i s u l t a t i c o m e esso sia d o l a l o d i q u a l i t à c h e lo r e n d o n o p a r t i c o l a r m e n t e a d a t t i al r i l e v a m e n t o s i s t e m a t i c a d e l l e c a r a t t e r i s t i c h e i o n o s f e r i c h c , m e d i a n i » l i p l o r a z i o n e c o n t i n u a e s e n z a i n t e r v a l l i d i un g a m m a d i f r e q u e n z i assai a m p i a , e con la p o s s i b i l i t à di o t t e n e r e u n a c h i a r a r e g i s t r a z i o n i a n c h e i n p r e s e n z a d i l'orli d i s t u r b i a t m o s f e r i c i ed i n d u s t r i a l i . questi q u a l i t à , u n i t e a l l a r i l e v a n t e p o t e n z a del s e g n a l e e m e s s o , c o n s e n t o n i l ' a t t u a z i o n e di u n s e r v i z i o s i s t e m a t i c o di r e g i s t r a z i o n e , s p i n g e n d o l ei ficeaza e la p r e c i s i o n e ad un g r a d o assai elevate, e p e r m e t t o n o quinti u n ' a n a l i s i c o m p l e t a e p r o f o n d a dei f e n o m e n i i o n o s f e r i c i . roma istillilo nazionale (li geofisica gennaio 1918. .174 a n t o n i o eoi.i.e rlassli n io si destri ve il nuovo uppuruto ionosferico dell istituto \ azionale di ( j e o f i s i c a . ou• siii apparato costruito secondo moderni criteri consente l'esplorazione continua e sicura di una gamma di frequenze particolarmente ampia (ila 2.5 a 20 me) senza alcun cambio delle bobine dei circuiti oscillanti, <on una potenza d'impulso notevole. i comandi della sintonia del trasmettitore e del ricevitore sono ri i/lizzati con un sistema particolarmente semplice senza l ausilio di -pedali vincoli meccanici. infatti poiché le frequenze trasmesse venu'/no ottenute per battimento tra due oscillatori di cui uno a frequenza lì ssa 127..) me) e l'altro a frequenza variabile ira 30 e 50 me basta <i rotazione del solo piccolo condensatore variabile dell accordo di questo oscillatore per coprire la gamma (i onda predetta. il ricevi• 'ore del tipo su pere/erodimi a doppio cambiamento di frequenza liti il circuito della griglia della prima valvola aperiodico e la sintonia c ittenuta utilizzando per la con versione di frequenza le oscillazioni prodotte dallo ste<so oscillatore a frctpicnza variabile del trasmettitore. opportuni amplificatori aperiodici e a larga banda accoppiati a resistenza e capacità, consentono li amplificare le oscillazioni e l e t t r i , he ottenute per battimento dai due oscillatori. j ali ale ili nuovo t i p o , -pedali per oscillatori ad impulsi, permettono di ottenere dall'amplificatore una potenza ili uscita compresa per ogni punto della gamma tra 20 e io l,w. b i b u o c r m i \ i b i n a r mici t i ve. nature, 1 1 6 . 3 5 7 ( 1 9 2 5 ì . gii.i.il.ani) unii narton. b u i . s i a m i . jour. r e . . vii. 1083 ( 1 9 3 1 ) . ranzi. r i c . s r i e n t . . i i . 2 5 8 2 7 1 ( 1 9 3 8 ) . ranzi. r i c . s r i e n t . . 11. 1 3 3 1 3 8 i 194(1). • ) si i.ziti, electronics, july ' 1 9 1 6 ) . layout 6 1 annals of geophysics, 64, 6, se656, 2021; doi: 10.441/ag-8610 o p e n acc e s s uncertainty analysis of strong earthquake hazard estimation based on the generalized pareto distribution model: a case study of the northeastern tibetan plateau mengyi ren institute of geophysics, china earthquake administration, beijing, china article history: received november 30, 2020; accepted july 12, 2021 abstract a statistical method to analyze the uncertainties of strong earthquake hazard estimation is proposed from generalized pareto distribution (gpd) model using the northeastern tibetan plateau earthquake catalogue (1885–2017) data. for magnitude threshold of 5.5, the magnitude return levels in 20, 50, 100, 200, and 500 years are 7.19, 7.70, 7.99, 8.22, and 8.45, respectively. the corresponding 95% confidence intervals are [6.77, 7.60], [7.27, 8.12], [7.53, 8.44], [7.71, 8.72], and [7.84, 9.06], respectively. the upper magnitude limit obtained from this gpd model is 9.07 and its 80% confidence interval is [8.16, 9.98]. the sensitivity analysis by the morris method indicates that the input magnitude threshold has a relatively large influence on the estimation results. thus, threshold selection is important for the gpd model construction. the sensitivity characteristic ranking of input factors become increasingly stable with the increasing of return period, which implies that gpd model is more suitable for estimating strong earthquakes magnitude return levels and upper magnitude limit. the gpd modeling approach and qualitative uncertainty analysis methods for strong earthquake hazard estimations proposed in this paper can be applied to seismic hazard analysis elsewhere. keywords: strong earthquake hazard estimation; generalized pareto distribution; uncertainty analysis; northeastern tibetan plateau. 1. introduction seismic activity models are often used to describe spatial, temporal and intensity distributions of earthquakes. this model forms the basis for seismic zoning and hazard analysis [jiang and dai, 1993]. the improvement of seismic activity models is thus an ongoing focus for seismologists. generalized pareto distribution (gpd) model from extreme value theory is developed in recent years, which uses historical large magnitude earthquake record. it assumes that magnitude distribution of large earthquakes conforms to the gpd and the earthquake occurrence follows poisson distribution over time. pisarenko and sornette [2003] used gpd model to analyze shallow earthquake distribution in 18 seismic regions from the harvard earthquake catalogue. qian et al. [2013] chosed gpd model to analyze the magnitude distribution of strong earthquakes in boundary zones of active blocks in china. pisarenko et al. [2014] proposed a method to estimate the unknown parameters involved in the two limit theorems corresponding to the generalized extreme value distribution and gpd. they applied this method for the global harvard and fennoscandia catalogues. tian et al. [2017] estimated the seismic hazard of potential earthquake tsunami source areas in the manila subduction zone according to the gpd model. ren [2018] used a gpd model to obtain the upper magnitude limit of earthquake in the longmenshan region. dutfoy [2019] combined the gpd and poisson distribution to analyze the tail characteristics of the earthquake magnitude distribution in southern france and calculated the annual maximum earthquake magnitude there. gpd seismicity model can take full advantage of large earthquake data in a catalogue. there is no need to designate a lower earthquake magnitude limit a priori when processing an earthquake catalogue. it is suitable for areas with long historical records of big earthquakes. however, it must be emphasized that the uncertainties of strong earthquake hazard estimation are inevitable when constructing a gpd model. two input factors of the model should be considered: the earthquake catalog start time and magnitude threshold. the selection of these two model input factors is somewhat subjective. dutfoy [2019] used a bootstrap statistical method to obtain the gpd model parameter distribution. he discussed a particular case where the probability density distribution of the gpd model shape parameter is bimodal, which may introduce uncertainties in the seismic hazard estimation. nevertheless, the analysis of specific situation in which the seismicity model input parameters change independently is not enough but rather a systematic analysis of two situations where the input parameters change simultaneously and the input parameters interact with each other. thus, the global sensitivity analysis method should be used when discussing the uncertainties. morris method, one of the global sensitivity analysis method, as a process by which the uncertainty of strong earthquake hazard estimations can be analyzed. this method is more computationally efficient and it provides generally qualitative sensitivity measures, which is capable of ranking the input factors in order of importance. in this paper, gpd is adopted to construct a strong earthquake activity model and strong earthquake hazard estimation (magnitude return level and upper magnitude limit and their corresponding confidence intervals) are obtained by using historical seismic data of large earthquakes from 1885 to 2017. the morris method is used to qualitatively analyze the influence of input factor (earthquake catalog start time and magnitude threshold) uncertainties and the interaction of factors on the earthquake hazard estimation uncertainty. this approach is also applicable in seismic zoning and hazard analysis. 2. method to model the gpd consider the model set 𝑋₁, 𝑋₂, ··· , 𝑋� to be a sequence of independent random variables having a common distribution function. if there exist sequences of constants {𝑎� > 0} and {𝑏�} such that 𝑃�{(𝑀� − 𝑏�) / 𝑎� ≤ 𝑥} → 𝐺{𝑥}, 𝑛 → ∞ (1) where g is a nondegenerate distribution function, then g is called the extreme value distribution. fisher and tippett [1928] indicated the three major theorems of the extreme value distribution, pointing out that g must belong to one of the three extreme value distributions, namely, the gumbel distribution, fréchet distribution or weibull distribution. the generalized extreme value theory unifies these three distribution types into a single family of models, having distribution functions of the form: (2) which is represented by three parameters, namely, the position parameter 𝜇, scale parameter 𝜎 and shape parameter 𝜉 [coles, 2001; shi, 2006]. 𝐺(𝑥) = 𝑒𝑥𝑝 {− �1 + 𝜉 � ��⁻¹/� }, 1 + 𝜉 (𝑥−𝜇) / 𝜎 > 0�⁻� � mengyi ren 2 then, for a large enough threshold 𝑢, the distribution function of (𝑋 − 𝑢), conditional on 𝑋 >𝑢, is approximately obeys the gpd: (3) defined on {𝑦:𝑦 > 0 and (1 + )>0}, where 𝜎� = 𝜎 + 𝜉 (𝑢 − 𝜇). two methods are available to select a reasonable threshold that provides a reasonable approximation for the model. one is an exploratory technique carried out prior to the model estimation, which means that the magnitude excess and magnitude threshold should satisfy a linear relationship in a plot of the magnitude mean excess distribution. the other method is a stability assessment of the parameter estimates based on the model fitting across a range of different thresholds. the shape parameter and modified scale parameter estimates should remain fairly constant with increasing threshold. suppose that a gpd with parameters 𝜎� and 𝜉 is a suitable model for exceedances of a threshold 𝑢 by a variable 𝑋. that is, for 𝑥 > 𝑢 , (4) it follows that, (5) where 𝜁� = 𝑃�{𝑋 > 𝑢}. hence, the level 𝑥� that is exceeded on average once every 𝑚 observations is the solution of (6) rearranged, (7) 𝑥� is the 𝑚-observation return level. the 𝑁-year return level is the level expected to be exceeded once every 𝑁 years. if there are 𝑛� observations per year, this corresponds to the 𝑚-observation return level, where 𝑚 = 𝑁 × 𝑛�. hence, the 𝑁-year return level is defined by (8) the parameters of the gpd can be estimated by maximum likelihood. only negative values of the form parameter 𝜉 are acceptable, which give the following estimation of upper limit value: 𝑥� = 𝑢 − 𝜎�� / ξ� (9) standard errors or confidence intervals for 𝑥� can be derived by the delta method [coles, 2001], that is, the approximate (1 −𝛼) confidence interval for the return level or upper limit would be: (10) where 𝑉𝑎𝑟(𝑥� ) ≈ 𝛻𝑥�𝑉𝛻𝑥, 𝑉 is the variance-covariance matrix of (ζ�� , 𝜎��, ξ�), 𝛻𝑥� is the value of � � evaluated at (ζ�� , 𝜎��, ξ�) and 𝑍₁₋ is the 1− quantile of the standard normal distribution. 𝑃�{𝑋 > 𝑥|𝑋 > 𝑢} = �1 + 𝜉 � ��⁻¹/��⁻� �� 𝑃�{𝑋 > 𝑥} = ζ� �1 + 𝜉 � ��⁻¹/��⁻� �� ζ� �1 + 𝜉 � ��⁻¹/� = �⁻� �� ¹ � 𝑥� = 𝑢 + [(𝑚ζ�)� − 1]�� � 𝑥� = 𝑢 + [(𝑁𝑛�ζ�)� − 1]�� � 𝐻(𝑦) = 1 − (1 + )⁻¹/��� �� , ,� ₂ 𝑥� ± 𝑍₁₋ �𝑉𝑎𝑟(𝑥� )� ₂ 3 global sensitivity analysis based on gpd mengyi ren 4 3. qualitative method of uncertainty analysis the morris method [morris, 1991] is composed of individually randomized ‘one-factor-at-a-time’ experiments and evaluates the impact of changing one factor at a time. each input factor may assume a discrete number of values, called levels, which are chosen within the factor variation range. assume that the model output factor is 𝑦 = 𝑓(𝑥₁, 𝑥₂, ··· ,𝑥�) and 𝑥 = (𝑥₁, 𝑥₂, ··· ,𝑥�) is an input factor with 𝑘 components. combining the probability distribution of each input factor, the morris method maps the value range of each input factor into the interval [0, 1] and discretizes it into 𝑝 levels. the morris design begins by randomly selecting a “base” value 𝑥*. each component 𝑥� of 𝑥* is sampled from {0, ¹/�₋₁, ²/�₋₁, ··· , 1} where 𝛥 = ¹/�₋₁ is the predetermined amount of change. the first sampling point, 𝑥⁽¹⁾, is obtained by increasing one or more components of 𝑥* by 𝛥. the second sampling point is generated from 𝑥* with the distinction that it differs from 𝑥⁽¹⁾ in its 𝑖�� component, which has been either increased or decreased by 𝛥. the design then produces a succession of (𝑘+1) sampling points 𝑥⁽¹⁾, 𝑥⁽²⁾, ··· , 𝑥⁽�⁺¹⁾ with the key property that two consecutive points differ in only one component. the succession of sampling points defines what is called a trajectory in input space. once a trajectory has been constructed and the model evaluated at its points, an elementary effect for each factor 𝑖, 𝑖 = 1, 2, ··· , 𝑘, can be computed. the elementary effect of the 𝑖�� input factor is defined as: (11) the mean and standard deviation 𝜎 of the 𝑟 elementary effects are defined as: (12) (13) two sensitivity measures are proposed by morris for each input factor: (1) 𝜇, which estimates the overall effect of the factor on the output and (2) 𝜎, which estimates the ensemble of the secondand higher-order effects in which the factor is involved [saltelli et al., 2004]. the absolute mean value of the elementary effect, 𝜇*, is used instead of 𝜇 because it is considered a more robust sensitivity metric, especially for the case of nonmonotonic functions [campolongo et al., 2007]. the euclidean distance between (𝜇*, 𝜎) and the origin can also be used for parameter ranking [pappas et al., 2013]. the key input factors of the gpd model can be determined based on a scatter plot of 𝜇* and 𝜎 or their euclidean distance for the output variables. the method relies on the elementary effect that uses incremental ratios and is apparently a local measure. however, the final measures, 𝜇* and 𝜎, are calculated by averaging several elementary effects, which are obtained by the different trajectories. the first sampling point of the trajectory has randomness and the method can thus be regarded as global. a large 𝜇* value indicates that the factor has a strong effect on the output. a large 𝜎 value implies that either the factor is interacting with other factors or has nonlinear effects on the output. in contrast, a small 𝜎 value indicates very similar values of the elementary effects, implying that the effect of this factor is almost independent of the values obtained from the other factors. if 𝜇* and 𝜎 are both large, the associated factor has a strong influence on the output involving nonlinearity or interactions; if the case is the opposite, the input factor has a negligible influence on the output [ren et al., 2018]. 4. case study: northeastern tibetan plateau 4.1 seismic and geological background the northeastern tibetan plateau is located at the intersection of the qinghai-tibetan plateau block with the ordos block and alxa block. the present tectonics of northeastern tibet resulted from sw–ne compression induced 𝑑�(𝑥) = ��⁽�¹, ... , ��₋¹,��⁺�, ��₊¹, ... , ��⁾⁻�⁽�⁾� 𝛥 𝜇 =� 𝑑�/𝑟� �₌₁ 𝜎 =�� (𝑑� − 𝜇)²/𝑟� �₌₁ by the collision between india and eurasia, which initiated ~50 ma [taylor and yinn, 2009; yin and harrison, 2000]. recent global positioning system (gps) measurements [gan et al., 2007] show that approximately 10 mm/yr of ne– sw or nne–ssw horizontal shortening is accommodated in northeastern tibet. the intense tectonic activity and frequent seismic activities in this region have thus been a hotspot of geoscience research for many years [zhao et al., 2004; chen et al., 2005; li et al., 2006; chang et al., 2008; tian and zhang, 2013; bao et al., 2013; li et al., 2017; zhu et al., 2018; sun et al., 2019]. five strong earthquakes with a magnitude greater than or equal to 8 are found in the historical record: gulang (ms 8.0) in 1927; haiyuan (ms 8.5) in 1920; wudu (ms 8.0) in 1879; pingluo (ms 8.0) in 1739; and tianshui (ms 8.0) in 1654 (figure 1). huang et al. [1994] conducted a detailed analysis on the historical seismic data of most parts of mainland china and showed that the seismic data with ms ≥ 5 from the northeastern tibetan plateau are essentially complete since 1885. the data used in this study are mainly from the chinese historical earthquake catalogue database and the national seismic network earthquake catalogue database of the china earthquake data center (http://data.earthquake.cn). the historical earthquake catalogue of the northeastern tibetan plateau region (99–107°e, 32–40°n) between 1100 and 2017 contains a total of 834 recorded earthquakes with a magnitude greater than 4. according to the collected seismic data, the northeastern tibetan plateau region is dominated by shallow earthquakes, including 5 earthquakes with ms ≥ 8, 15 with 8 > ms ≥ 7, 50 with 7 > ms ≥ 6, and 173 with 6 > ms ≥ 5 (figure 1). 5 global sensitivity analysis based on gpd figure 1. map of main active faults and earthquake distribution (ms ≥ 4) in northeastern tibetan plateau (according to li et al. [2006] revised). aftershocks in the earthquake catalogue should be efficiently removed before using the model for seismic hazard estimation. seismic hazard analysis from studies worldwide have widely adopted the space-time window method to remove aftershocks to account for the calculation simplicity and engineering operability. in this method, the spatial window is determined according to: lg 𝑅 = 𝑎𝑀 + 𝑏 (14) where 𝑎 and 𝑏 are fixed parameter values, which for china are equal to 0.5 and −1.78, respectively [liu et al., 1996]. the temporal window is shown in table 1. after removing the aftershocks using the spatiotemporal window method, 560 earthquakes with a magnitude greater than 4 were collected from the northeastern tibetan plateau region. m-t map of the earthquakes with ms ≥ 4 from 1885 to 2017 is shown in figure 2 according to the earthquake catalogue data after aftershocks deleting. since 1960, with the update of seismic monitoring instruments, more and more small magnitude earthquakes have been fully detected. in this paper, earthquakes used in gpd model are greater than magnitude threshold, so the lack of small magnitude earthquakes in historical catalogue will not have influence on the seismic hazard estimation results. table 1. aftershock time windows of different magnitudes [console et al., 1979; chen et al., 1998]. 4.2 strong earthquake hazard estimation when constructing a gpd model of strong earthquakes, the magnitude threshold 𝑢 is first selected according to the method introduced in section 2, and then the magnitude greater than the threshold 𝑢 in the selected historical main shock/ms lasting days/d main shock/ms lasting days/d 4 42 6.5 790 4.5 83 7 915 5 155 7.5 960 5.5 290 8 985 6 510 8.5 985 mengyi ren 6 figure 2. m-t map of the earthquakes from 1885 to 2017 in northeastern tibetan plateau (ms ≥ 4). seismic records in a certain period is taken as random variables. it is assumed that the magnitude satisfies the condition of independent and identically distribution and the magnitude excess conforms to the gpd. assuming that there are 𝑛 historical seismic record data in the selected 𝑡 years, the 𝑛� observation per year can be obtained by 𝑛/𝑡, put it into equation (8) to calculate the magnitude return level in the 𝑁 years. the upper magnitude limit is calculated according to equation (9). using the delta method, the confidence intervals of the magnitude return levels and the upper magnitude limit can be calculated according to equation (10). this study analyzes the earthquake catalogue data between 1885 and 2017 to estimate the magnitude return level and upper magnitude limit of the northeastern tibetan plateau region using the gpd model. figure 3 shows the mean excess distribution plot with an approximate 95% confidence interval for the earthquake data. the mean magnitude excess is approximately linearly distributed when 𝑢 is in the range of (5.5, 6.0). to make the most reasonable selection of 𝑢, it is necessary to also consider the actual situation of the sample data. too low of a threshold is likely to violate the asymptotic basis of the model, leading to bias; too high of a threshold will generate few excesses with which the model can be estimated, leading to high variance. in conjunction with figure 4, the estimated values of the shape parameter and modified scale parameter in the range of (5.5, 6.0) remain relatively unchanged. the threshold 𝑢 =5.5 is therefore selected. the maximum likelihood estimation is used to estimate the gpd model parameters. the estimated values of the modified scale parameter and shape parameter are 1.23 and −0.35, respectively. diagnostic plots for the fitted gpd model are shown in figure 5. the probability and quantile plots indicate the fit between the model and data. the goodness-of-fit in the probability and quantile plots is highly convincing. the curve in the return level plot is convex and asymptotically approaches a finite value because the estimated value of the shape parameter is negative, which confirms that the upper magnitude limit of the northeastern tibetan plateau region can be obtained. the corresponding density estimate is generally consistent with the histogram of the data. all four diagnostic plots therefore support the fitted gpd model established in this study. the estimated magnitude return levels for each return period and their corresponding confidence intervals with a 95% confidence coefficient can be obtained using the magnitude threshold 𝑢 and estimated values of 𝜎� and 𝜉 in equation (8) (table 2). following equation (9), the upper magnitude limit of the northeastern tibetan plateau region is 9.07. 7 global sensitivity analysis based on gpd figure 3. plot of magnitude mean excess distribution. mengyi ren 8 figure 5. diagnostic plots for threshold excess model fitted to magnitude data (a) probability plot; (b) quantile plot; (c) return level plot; (d) probability density plot. figure 4. parameter estimates against threshold for earthquake magnitude data. 9 global sensitivity analysis based on gpd the standard deviation of the upper magnitude limit is 0.71 and the confidence interval with an 80% confidence coefficient of the upper magnitude limit is [8.16, 9.98]. table 2. estimation of magnitude return level for northeastern tibetan plateau. 4.3 uncertainty analysis of earthquake hazard estimation the input factors of gpd model are earthquake catalog start time (st) and magnitude threshold (u). the outputs are magnitude return level from different periods, upper magnitude limit, and their corresponding confidence intervals. the range of start times for earthquake catalog in the northeastern tibetan plateau is based on a study of the seismic data integrity [huang et al., 1994]. the range of magnitude threshold values is considered in the two methods discussed above to select the threshold (section 2). supposing that the range of st values is between 1880 and 1890 and the range of u values is between 5.5 and 6.0, the two input factors both follow a uniform distribution pattern, as shown in table 3. table 3. probability distribution of input factors. the parameters of the morris experiment are set to 6 levels and 10 trajectories. by using the monte carlo sampling method, 30 sets of gpd model parameters are obtained and the sampling results are listed in table 4. table 4. gpd model parameters sampling combination. based on the sampling results used to construct the corresponding gpd models of the northeastern tibetan plateau, 30 sets of the earthquake hazard estimation results (tables 5-10) are obtained. in order to show the distribution of the results more intuitively, the results are presented in the form of boxplots (figure 6). the left subplots of figure 6 show the distribution of the earthquake hazard estimation with different earthquake catalog start time st for fixed trajectory samples 1 (5.7,1888) (6.0,1888) (6.0,1883) 2 (5.7,1889) (5.7,1884) (6.0,1884) 3 (5.9,1883) (5.9,1888) (5.6,1888) 4 (5.6,1888) (5.6,1883) (5.9,1883) 5 (5.6,1888) (5.6,1883) (5.9,1883) 6 (5.9,1884) (5.9,1889) (5.6,1889) 7 (5.8,1889) (5.8,1884) (5.5,1884) 8 (5.7,1889) (5.7,1884) (6.0,1884) 9 (5.6,1884) (5.9,1884) (5.9,1889) 10 (6.0,1886) (5.7,1886) (5.7,1881) input factors probability distribution earthquake catalog start time (st) st ~ u(1880, 1890) magnitude threshold (u) u ~ u(5.5, 6.0) return period /a 20 50 100 200 500 magnitude 7.19 7.70 7.99 8.22 8.45 confidence interval [6.77, 7.60] [7.27, 8.12] [7.53, 8.44] [7.71, 8.72] [7.84, 9.06] magnitude threshold 𝑢, while the right subplots of figure 6 show the distribution of the earthquake hazard estimation with different magnitude threshold 𝑢 for fixed earthquake catalog start time st. the seismic hazard estimation in right subplots is more discrete than in left subplots. it shows that, compared to st, the different selection of 𝑢 has greater influence on the dispersion degree of the estimation results. mengyi ren 10 11 global sensitivity analysis based on gpd figure 6. boxplots of the seismic hazard estimation based on the two input factors (figures 6a-f are based on the results in the table 5-10. x-axis in the left subplots show the magnitude threshold 𝑢, and x-axis in the right subplots show the earthquake catalog start time st. each input factor is divided into 6 levels, so x-axis corresponds to 6 values. there are three groups of estimation values in each subplot, from small to large, corresponding to the low end of confidence interval, magnitude return level or upper magnitude limit, upper end of confidence interval). table 5. magnitude return level in 20 years and its corresponding confidence interval with 95% confidence coefficient. table 6. magnitude return level in 50 years and its corresponding confidence interval with 95% confidence coefficient. table 7. magnitude return level in 100 years and its corresponding confidence interval with 95% confidence coefficient. trajectory magnitude return level and its corresponding confidence interval 1 7.10 [6.69,7.52] 7.14 [6.68,7.61] 7.13 [6.66,7.59] 2 7.10 [6.68,7.52] 7.09 [6.68,7.49] 7.13 [6.66,7.59] 3 7.05 [6.64,7.46] 7.08 [6.66,7.50] 7.14 [6.73,7.56] 4 7.14 [6.73,7.56] 7.12 [6.72,7.53] 7.05 [6.64,7.46] 5 7.14 [6.73,7.56] 7.12 [6.72,7.53] 7.05 [6.64,7.46] 6 7.05 [6.64,7.46] 7.08 [6.65,7.51] 7.14 [6.72,7.56] 7 7.12 [6.68,7.55] 7.09 [6.68,7.51] 7.18 [6.77,7.60] 8 7.10 [6.68,7.52] 7.09 [6.68,7.49] 7.13 [6.66,7.59] 9 7.12 [6.72,7.53] 7.05 [6.64,7.46] 7.08 [6.65,7.51] 10 7.14 [6.67,7.60] 7.10 [6.69,7.51] 7.09 [6.68,7.49] trajectory magnitude return level and its corresponding confidence interval 1 7.63 [7.15,8.10] 7.68 [7.19,8.17] 7.67 [7.18,8.15] 2 7.63 [7.14,8.11] 7.61 [7.13,8.08] 7.67 [7.18,8.15] 3 7.58 [7.07,8.09] 7.61 [7.10,8.11] 7.66 [7.20,8.11] 4 7.66 [7.20,8.11] 7.64 [7.19,8.09] 7.58 [7.07,8.09] 5 7.66 [7.20,8.11] 7.64 [7.19,8.09] 7.58 [7.07,8.09] 6 7.58 [7.07,8.09] 7.62 [7.11,8.12] 7.66 [7.20,8.12] 7 7.64 [7.16,8.13] 7.62 [7.14,8.09] 7.70 [7.27,8.12] 8 7.63 [7.14,8.11] 7.61 [7.13,8.08] 7.67 [7.18,8.15] 9 7.64 [7.19,8.09] 7.58 [7.07,8.09] 7.62 [7.11,8.12] 10 7.67 [7.19,8.16] 7.62 [7.14,8.10] 7.61 [7.13,8.08] trajectory magnitude return level and its corresponding confidence interval 1 7.95 [7.38,8.53] 7.99 [7.47,8.51] 7.98 [7.46,8.50] 2 7.96 [7.38,8.54] 7.94 [7.36,8.52] 7.98 [7.46,8.50] 3 7.94 [7.29,8.58] 7.95 [7.33,8.57] 7.97 [7.45,8.48] 4 7.97 [7.45,8.48] 7.95 [7.43,8.47] 7.94 [7.29,8.58] 5 7.97 [7.45,8.48] 7.95 [7.43,8.47] 7.94 [7.29,8.58] 6 7.94 [7.29,8.58] 7.96 [7.34,8.58] 7.97 [7.45,8.50] 7 7.97 [7.41,8.53] 7.94 [7.38,8.51] 7.99 [7.53,8.44] 8 7.96 [7.38,8.54] 7.94 [7.36,8.52] 7.98 [7.46,8.50] 9 7.95 [7.43,8.47] 7.94 [7.29,8.58] 7.96 [7.34,8.58] 10 7.99 [7.47,8.50] 7.95 [7.38,8.52] 7.94 [7.36,8.52] mengyi ren 12 13 global sensitivity analysis based on gpd table 8. magnitude return level in 200 years and its corresponding confidence interval with 95% confidence coefficient. table 9. magnitude return level in 500 years and its corresponding confidence interval with 95% confidence coefficient. table 10. upper magnitude limit and its corresponding confidence interval with 80% confidence coefficient. trajectory magnitude return level and its corresponding confidence interval 1 8.23 [7.52,8.95] 8.24 [7.65,8.83] 8.23 [7.64,8.82] 2 8.24 [7.51,8.97] 8.22 [7.50,8.95] 8.23 [7.64,8.82] 3 8.25 [7.39,9.11] 8.25 [7.45,9.05] 8.22 [7.61,8.84] 4 8.22 [7.61,8.84] 8.21 [7.59,8.83] 8.25 [7.39,9.11] 5 8.22 [7.61,8.84] 8.21 [7.59,8.83] 8.25 [7.39,9.11] 6 8.25 [7.39,9.11] 8.25 [7.45,9.06] 8.23 [7.60,8.86] 7 8.24 [7.56,8.92] 8.22 [7.52,8.93] 8.22 [7.71,8.72] 8 8.24 [7.51,8.97] 8.22 [7.50,8.95] 8.23 [7.64,8.82] 9 8.21 [7.59,8.83] 8.25 [7.39,9.11] 8.25 [7.45,9.06] 10 8.24 [7.64,8.83] 8.23 [7.52,8.94] 8.22 [7.50,8.95] trajectory magnitude return level and its corresponding confidence interval 1 8.54 [7.59,9.49] 8.49 [7.74,9.25] 8.49 [7.74,9.24] 2 8.55 [7.58,9.52] 8.54 [7.57,9.51] 8.49 [7.74,9.24] 3 8.62 [7.39,9.85] 8.59 [7.47,9.71] 8.49 [7.71,9.27] 4 8.49 [7.71,9.27] 8.49 [7.70,9.28] 8.62 [7.39,9.85] 5 8.49 [7.71,9.27] 8.49 [7.70,9.28] 8.62 [7.39,9.85] 6 8.62 [7.39,9.85] 8.59 [7.47,9.71] 8.50 [7.70,9.30] 7 8.53 [7.63,9.43] 8.53 [7.59,9.48] 8.45 [7.74,9.24] 8 8.55 [7.58,9.52] 8.54 [7.57,9.51] 8.49 [7.74,9.24] 9 8.49 [7.70,9.28] 8.62 [7.39,9.85] 8.59 [7.47,9.71] 10 8.49 [7.74,9.24] 8.54 [7.59,9.48] 8.54 [7.57,9.51] trajectory magnitude return level and its corresponding confidence interval 1 9.85 [7.33,12.38] 9.23 [7.76,10.70] 9.23 [7.76,10.70] 2 9.87 [7.26,12.47] 9.98 [7.18,12.78] 9.23 [7.76,10.70] 3 10.89 [4.95,16.84] 10.35 [6.15,14.55] 9.42 [7.87,10.97] 4 9.42 [7.87,10.97] 9.47 [7.83,11.11] 10.89 [4.95,16.84] 5 9.42 [7.87,10.97] 9.47 [7.83,11.11] 10.89 [4.95,16.84] 6 10.89 [4.95,16.84] 10.24 [6.30,14.17] 9.43 [7.83,11.04] 7 9.62 [7.46,11.78] 9.83 [7.24,12.42] 9.07 [8.16,9.98] 8 9.87 [7.26,12.47] 9.98 [7.18,12.78] 9.23 [7.76,10.70] 9 9.47 [7.83,11.11] 10.89 [4.95,16.84] 10.24 [6.30,14.17] 10 9.23 [7.76,10.70] 9.85 [7.33,12.38] 9.98 [7.18,12.78] by using the morris method, the sensitivity analysis results are shown in figures 7-9, where the sensitivity measures (𝜇*, 𝜎) of the two factors plotted for each output of interest. recall that 𝜇* reflects the overall influence of an input factor on the output, and 𝜎 indicates whether the factor has nonlinear effects or interacts with other factors. the euclidean distance 𝜀 between (𝜇*, 𝜎) and the origin is also used for parameter ranking. a comparison of the two input factors (figure 7) shows that 𝑢 has a relatively high mean value 𝜇*, which indicates a relatively strong influence on the estimation of the magnitude return levels and their corresponding confidence intervals. moreover, with the increasing of return period, 𝑢 has not only a high 𝜇* value but also a relatively high 𝜎, which indicates that the interaction with the other input factor also has a significant effect on the outputs. mengyi ren 14 figure 7. absolute means (𝜇*) and standard deviations (𝜎) of the elementary effects of the two input factors for magnitude return levels, upper magnitude limit and their corresponding confidence intervals (𝑢1 and st1 represent the input factors for magnitude return levels or upper magnitude limit. 𝑢2 and st2 represent the input factors for low end of confidence interval. 𝑢3 and st3 represent the input factors for upper end of confidence interval). 15 global sensitivity analysis based on gpd figure 8 shows that 𝑢 has a more important impact than st on the uncertainty of the magnitude return level and its corresponding confidence intervals on the whole. the influence of st on the estimated results is relatively stable with increasing return period, whereas the influence of 𝑢 on the estimated results significantly increases, especially at the both ends of the confidence intervals. the effects of uncertainty of the two input factors on the uncertainty of the upper magnitude limit and its corresponding confidence interval are also shown in figures 7 and 9, where 𝑢 is well separated from the other value because of its high mean 𝜇*. when considering 𝜇* and 𝜎 together, one can conclude that 𝑢 is not only important for the upper magnitude limit estimation and its corresponding confidence interval but also has a significant effect that involves either interaction or curvature. hence, the rationality of the 𝑢 selection should be carefully considered when estimating strong earthquake hazards using the gpd model. moreover, the sensitivity characteristic ranking of the input factors become more stable with increasing return period, which also indicates that the gpd model is more suitable for estimating the strong earthquakes magnitude return level and upper magnitude limit using the tail feature of the magnitude sequence. figure 9. euclidean distance 𝜀 of the two input factors for upper magnitude limit and its corresponding confidence interval (blue box represents the input factor magnitude threshold 𝑢. orange solid circle represents the input factor earthquake catalog start time st). figure 8. euclidean distance 𝜀 of the two input factors for magnitude return levels and their corresponding confidence intervals (blue box represents the input factor magnitude threshold 𝑢. orange solid circle represents the input factor earthquake catalog start time st). 5. discussion and conclusions in this paper, a qualitative uncertainty analysis process for strong earthquake hazard estimation is constructed. the selection of input factors for strong earthquake gpd model involves uncertainties, thus the seismic hazard estimation obtained by using this model is inevitably uncertain. global sensitivity analysis method of strong earthquake hazard estimation based on gpd has not been previously reported. in this paper, the morris method is introduced to handle the uncertainty of the gpd model. the influence of two input factors, earthquake catalog start time (st) and magnitude threshold (𝑢), and their interaction on the hazard estimation uncertainty is analyzed. for northeastern tibetan plateau seismic catalogue data, a gpd model is established to assess the magnitude return levels with different return periods and their corresponding confidence intervals. the model also predicts the upper magnitude limit and its corresponding confidence intervals for northeastern tibetan plateau. with magnitude threshold of 5.5, the magnitude excess distribution follows a gpd, and the modified scale and shape parameters are 1.23 and −0.35, respectively. the magnitude return levels in 20, 50, 100, 200, and 500 years are 7.19, 7.70, 7.99, 8.22, and 8.45, respectively. the corresponding 95% confidence intervals are [6.77, 7.60], [7.27, 8.12], [7.53, 8.44], [7.71, 8.72], and [7.84, 9.06], respectively. the upper magnitude limit from the gpd model is 9.07 and its 80% confidence interval is [8.16, 9.98]. the gpd modeling approach developed in this paper can make full use of historical earthquake catalogues and construct seismic activity models. by using the morris method, the influence of input factors and their interaction with the gpd model on the uncertainty of strong earthquake hazard estimations is qualitatively analyzed using the sensitivity index absolute mean (𝜇*) and standard deviation (𝜎). the analysis result indicates that the input magnitude threshold (𝑢) has a relatively large influence on the seismic hazard estimation. thus, meaningful magnitude threshold selection is the most important step for the successful gpd modeling of seismic activities in a region. this method can be applied to the sensitivity analysis of other seismicity models. the results also can improve the understanding of seismicity models and increase the modeling efficiency. acknowledgments. we would like to thank dr. jiang changsheng and dr. bian yinju (institute of geophysics, china earthquake administration) for their assistance during this work, the reviewers for 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arnórsson and gunnlaugsson, 1985), based on the variation of gas ratios with temperature in producing aquifers. also methods to estimate the reservoir excess steam have been proposed (giggenbach, 1980; d’amore and celati, 1983), which are useful when the reservoir temperature is available. the reservoir excess steam is very important in the assessment of vapor-dominated geothermal fields to calculate the in situ liquid saturation. methods to estimate both the reservoir temperature and the reservoir excess steam were developed by considering gas equilibria and mineral buffers in the reservoir (d’amore and truesdell, 1985, 1995; d’amore, 1998). d’amore (1998) developed a method based on equilibria for the fischer-tropsch reacmailing address: dr. rosa maría barragán reyes, instituto de investigaciones eléctricas, gerencia de geotermia, reforma st. 113, col. palmira, 62490 cuernavaca, morelos, méxico; e-mail: rmb@iie.org.mx gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico rosa maría barragán (1), victor m. arellano gómez (1), enrique portugal (1), fernando sandoval (2) and nuria segovia (3) (1) instituto de investigaciones eléctricas, gerencia de geotermia, cuernavaca, morelos., méxico (2) comisión federal de electricidad, residencia los azufres, campamento aguafría, los azufres, michoacán, méxico (3) instituto de geofisica, universidad nacional autónoma de mexico (unam), méxico d.f., méxico abstract gas data of the los azufres geothermal field were analyzed using a method based on equilibrium of the fischer-tropsch (ft) reaction: ch4 + 2h2o = 4h2 + co2 and on the combined pyrite-hematite-magnetite (hsh2) reactions: 5/4 h2 + 3/2 fes2 + 3/4 fe2o3 + 7/4 h2o = 3 h2s + fe3o4 in order to estimate reservoir temperature and excess steam. the solution of equilibrium equations produces a grid (ft-hsh2). this method is suitable for reservoirs with relatively high h2s but low h2 and nh3 concentrations in the fluid as is the case of the los azufres well discharges. reservoir temperature and reservoir excess steam values were estimated for initial and present conditions in representative wells of the field to study the evolution of fluids, because of exploitation and waste fluids reinjection. this method was very useful in estimating reservoir temperatures in vapor wells, while in two-phase wells it was found that as the well produces a smaller fraction of water, the reservoir temperature estimation agrees qualitatively with results from cationic or silica geothermometers. for liquid-dominated wells the reservoir temperature estimations agree with temperatures obtained from the well simulator welflo. this indicates that ft-hsh2 results provide the temperature of the fluid entering the well where the last equilibrium occurs. results show a decrease in reservoir temperatures in the southern zone of the field where intensive reinjection takes place. with exploitation, it was also noted that the deep liquid phase in the reservoir is changing to two-phase increasing the reservoir steam fraction and the non-condensable gases in well discharges. 146 rosa maría barragán, victor m. arellano gómez, enrique portugal, fernando sandoval and nuria segovia tion and the combined expressions for pyritemagnetite and pyrite-hematite mineral buffers. in this method more local oxidant conditions are assumed, implying the occurrence of high concentrations of h2s and relatively low concentrations of both h2 and nh3 in the fluid. the method was suitable for gas data from the los humeros geothermal system in order to understand the dynamics of the reservoir at initial and present conditions (arellano et al., 1998; barragán et al., 1999, 2000a,b). siega et al. (1999) described this method and an additional method based on the pyrite-pyrrhotite reaction. they concluded that the method based on ft-hsh2 equilibria best fits mature and magmatic systems data. the objective of this work is to investigate the effects of exploitation in los azufres reservoir with time through the changes in both the reservoir temperature and the excess reservoir steam of wells. 2. the los azufres geothermal field the los azufres geothermal field is an intensely fractured, two-phase, volcanic hydrothermal system located in the northern part of the mexican volcanic axis, in the state of michoacán at an average elevation of 2800 m a.s.l. (fig. 1). at present it is the second in the country generating 188 mwe (torres, 2003, pers. comm.). the field fig. 1. location of the los azufres geothermal field and wells. az-was removed of the name of wells. 147 gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico table i. average chemical composition of vapor phases of the los azufres geothermal wells. ts – separation temperature (°c); ysep – steam fraction at separation conditions (dimensionless); xg – ratio of millimoles of non condensable gas per mol of water vapor and concentration of gas species in ‰ molar (dry basis). average values of ft and hsh2 are also given. well year ts ysep xg co2 h2s h2 ch4 n2 nh3 ft hsh2 az-5 1985 180.7 0.709 13.020 978.200 14.600 3.140 0.724 2.684 0.642 –15.02 –5.94 az-5 1986 174.3 0.720 8.583 976.267 15.457 3.007 0.662 2.953 1.664 –15.75 –6.15 az-5 1987 179.7 0.706 11.007 976.833 14.557 3.191 0.965 3.062 1.428 –15.42 –6.08 az-5 1998 176.1 0.850 9.898 972.915 17.625 1.560 0.542 6.662 0.700 –16.27 –5.38 az-5 1999 178.9 0.860 15.360 972.023 16.000 2.680 0.738 7.661 0.880 –14.68 –5.46 az-5 2000 174.3 0.871 10.183 965.901 18.836 2.382 2.086 7.885 2.887 –16.04 –5.49 az-5 2001 175.0 0.892 13.916 902.032 12.813 11.298 1.246 66.408 5.987 –12.55 –6.58 az-6 1983 192.0 1 31.700 989.000 5.900 0.900 0.490 3.700 0.400 –14.87 –5.50 az-6 1985 182.3 1 35.333 982.000 6.567 1.100 0.807 6.100 3.200 –14.56 –5.39 az-6 1986 179.7 1 34.367 979.333 6.033 1.100 0.860 8.533 3.833 –14.63 –5.52 az-6 1987 169.3 1 31.525 979.750 6.875 1.450 0.943 9.550 1.200 –14.34 –5.57 az-6 1988 170.7 0.98 42.193 978.650 5.000 1.700 0.617 13.254 0.639 –13.41 –5.86 az-6 1998 178.1 0.940 43.280 983.950 3.200 1.200 0.221 10.763 0.617 –13.60 –6.27 az-9 1984 148.0 0.456 2.100 920.100 62.300 7.412 0.385 1.083 8.685 –17.22 –6.24 az-9 1987 182.5 0.407 2.335 944.250 42.970 3.673 0.494 6.232 2.383 –18.55 –6.35 az-9 1999 175.9 0.410 1.640 913.340 64.900 5.810 1.612 14.279 0.025 –18.88 –6.32 az-9 2000 175.3 0.424 1.350 859.076 62.455 10.600 1.340 59.896 5.714 –18.06 –6.82 az-9 2001 173.5 0.467 1.565 774.574 57.755 25.747 0.536 99.396 41.744 –15.74 –7.22 az-13 1985 172.5 0.5785 7.630 971.600 21.000 3.748 0.560 1.781 1.311 –15.89 –6.12 az-13 1986 172.3 0.584 5.733 961.933 27.303 5.854 0.730 2.044 2.150 –15.71 –6.23 az-13 1987 171.0 0.586 7.270 967.800 20.125 6.691 0.897 2.062 2.454 –15.15 –6.52 az-13 1998 182.3 0.940 6.268 953.860 31.375 2.640 1.457 9.167 1.488 –16.42 –5.19 az-13 1999 175.8 0.945 9.080 959.440 28.500 2.470 1.289 5.805 2.490 –15.82 –4.99 az-13 2000 184.0 0.999 9.164 950.878 29.613 2.960 1.566 7.053 7.855 –15.49 –4.99 az-13 2001 188.5 0.992 9.969 894.289 29.350 9.487 1.546 58.146 6.661 –13.35 –5.58 az-17 1984 174.0 1 17.850 987.500 6.450 2.450 0.195 1.400 1.600 –13.73 –6.37 az-17 1985 159.0 1 33.675 980.000 9.175 2.275 0.363 5.625 2.200 –13.03 –5.39 az-17 1986 200.0 1 15.967 975.667 11.300 1.967 0.303 5.500 5.200 –14.50 –5.60 az-17 1987 185.8 1 15.900 973.500 12.875 3.400 0.290 7.625 1.725 –13.54 –5.73 az-17 1988 169.0 1 12.367 968.333 13.500 4.033 0.473 12.167 1.433 –13.90 –5.95 az-17 1998 173.5 1 9.700 883.860 12.500 7.030 0.197 96.323 0.124 –13.01 –6.54 az-17 2000 173.5 1 12.550 875.190 12.800 7.130 0.309 104.177 0.444 –12.74 –6.32 az-17 2001 179.0 0.999 13.552 859.988 12.440 10.515 0.540 109.734 6.283 –12.18 –6.51 az-33 1984 206.0 0.842 27.570 986.600 7.640 1.956 0.610 2.424 0.815 –14.16 –5.82 az-33 1998 180.3 0.560 13.560 961.030 8.900 2.080 0.629 25.384 1.989 –16.02 –6.51 az-33 1999 177.6 1 9.010 942.830 19.500 6.820 0.062 30.193 0.633 –12.66 –6.00 az-33 2000 181.7 0.530 16.430 934.820 9.000 3.540 0.435 52.008 0.227 –14.71 –6.68 az-33 2001 178.4 0.608 16.519 937.275 10.240 6.210 0.964 39.919 5.290 –13.83 –6.71 148 rosa maría barragán, victor m. arellano gómez, enrique portugal, fernando sandoval and nuria segovia is divided into two zones: maritaro a liquid-dominated zone in the north and tejamaniles a steamdominated zone in the south. iglesias et al. (1985) found that in its natural state the los azufres geothermal field consists of a deep aquifer where the ascending fluid starts boiling at about 1200 m a.s.l. the twophase liquid dominant region extends upwards from 1200 m a.s.l. to about 1700 m a.s.l. where steam becomes the dominant phase. the twophase steam dominated region extends up to about 2400 m a.s.l. where a region of dry or superheated steam is located. according to cathelineau et al. (1985), the most important hydrothermal minerals occurring at the los azufres geothermal system are: chlorite, pyrite, hematite, epidote, calcite, albite, adularia, zeolite and quartz which were formed by alteration of primary minerals: olivine, pyroxene/ amphiboles, biotite, feldspar and rock-matrix. the los azufres reservoir geochemical model was proposed by nieva et al. (1987). this model was based on the spatial distribution of chemical and isotopic species at reservoir conditions. the reservoir excess steam y was estimated for the wells by a method based on equilibrium of the fischer-tropsch reaction (giggenbach, 1980; nieva et al., 1987). the y values were obtained at the reservoir temperature estimated by the cationic (ccg; nieva and nieva, 1987) and silica (fournier and potter ii, 1982) geothermometers in two-phase wells, whereas measured temperatures were taken in vapor wells. the y values were used to correct the chemical and isotopic composition of the total discharge to obtain the reservoir «reference» values. as the concentration of volatile species (co2) was larger in shallower strata and concentrations of non-volatile species (chlorides and oxygen-18 in the fluid) increased with depth, the model establishes the occurrence of a reservoir steam up-flow with partial condensation process, to explain the distributions of species as observed. according to the geochemical model, this behavior seems to be dominant in the southern zone while for the north, the presence of two different liquid phases (with slightly different isotopic composition) was proposed. the separated water in the los azufres twophase wells is of sodium-chloride type with neutral ph at separating conditions. the deep reservoir fluid contains up to 1600 ppm of chloride and the molar fraction of co2 has been calculated between 0.3 to 8.3 (nieva et al., 1987). the ph of fluids at the reservoir is neutral with values between 5.5 and 7.4 (barragán et al., 1988). the ccg geothermometer (nieva and nieva, 1987) provided reservoir temperatures of more than 300°c for the north zone, whereas slightly lower values between 270 and 290°c were estimated for the south. the well discharges in the los azufres geothermal field contain a relatively high amount of non-condensable gases compared to other fields (table i), i.e. the vapor well az-6 contains more than 40‰ molar of non condensable gas in steam, though liquid dominated wells such as az-4 contain only about 2‰. typically, the main constituent in dry gas is co2 (average 94 vol%), then h2s (average 2.5 vol%) while h2, ch4, n2 and nh3 concentrations are minor (average 3.5 vol% all of them). in wells affected by reinjection, n2 concentration has increased since a mixture of water/air is injected into the reservoir. 3. gas equilibria the chemical composition of geothermal gases depends mainly on temperature, pressure, vapor/liquid gas distribution processes and those secondary processes caused by different kinetic responses to changes in temperature and redox potential during the rise of the fluid to the surface (giggenbach, 1980). thus, large variations in gas compositions occur in different geothermal fields, but also in the same field among different wells. giggenbach (1980); d’amore and truesdell (1985, 1995) and d’amore (1998) developed methods based on gas equilibria considering mineral buffers in order to estimate physical characteristics of reservoirs. the mineral buffers controlling the hydrogen-hydrogen sulfide geothermal reaction at temperatures above 300°c were pyrite, pyrrhotite and magnetite while for temperatures around 240°c iron-aluminium-silicate minerals replace magnetite. calcite and anhydrite can also be used as buffers for geothermal gases since they are commonly found in geothermal systems, but anhyfig. 2. rh values for total discharge fluids of los azufres wells. 149 gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico drite is likely to reveal more oxidizing conditions occurring at the periphery of geothermal systems (giggenbach, 1980). however the fluid-mineral reactions may be different for different geothermal reservoirs depending not only on temperature but also on the rock type (d’amore and truesdell, 1985). giggenbach (1987) and taran et al. (2002) suggest evaluating the ratio of fugacities rh = = log ( f h2 /f h2o), of h2 and h2o in order to investigate predominating redox conditions within the reservoir. for most hydrothermal fluids the redox state is controlled by feofeo1.5 and hematite-fayalite-quartz assemblages, rh values are temperature independent at –2.8. for vapor dominated geothermal systems values of rh close to –2.8 are found. values above –2.8 are considered to be typical of reducing conditions and are found in high-temperature (about 800°c) volcanic gases (taran et al., 2002) whereas values below –2.8 are considered oxidizing conditions and are characteristic of geothermal systems. figure 2 (modified from giggenbach, 1987 and taran et al., 2002) shows total discharge rh values for some los azufres wells versus the reservoir temperature. as expected, the rh values indicate oxidizing conditions at reservoir (rh values less than –2.8) for all the wells. d’amore (1998) developed a method using the fischer-tropsch and another reaction for the h2s-h2 couple which was obtained from the combined pyrite-hematite, pyritemagnetite equilibrium named hsh2. this method was fully described by d’amore (1998), siega et al. (1999), barragán et al. (1999, 2000a, 2001). it is based on reactions (3.1) and (3.2) ( )ft ch h o h co2 44 2 2 2+ = + (3.1) ( ) / / / / . hsh h fes fe o h o h s fe o 5 4 3 2 3 4 7 4 2 3 2 2 2 3 2 2 3 4 + + + + = + (3.2) the thermodynamic equilibrium constant k for each reaction is given in eqs. (3.3) and (3.4), in terms of the partial pressures log log log log log p k p p p 2 4ft h co ch h o 2 2 4 2 + + = (3.3) / / log log log log p p p k 3 5 4 7 4 hsh h s h h o 2 2 2 2 = + (3.4) and writing the constants in terms of the water partial pressure, according to (d’amore, 1992) /log log log logp n n a pi i ih o h o2 2= +^ h (3.5) where (ni/nh2o) is the molar ratio of i component regarding the total water. the coefficient a for every species i is defined as a function of temperature and the steam fraction y ( ) ( ) < a y y b y a b y yb y if if 1 0 1 1 0 i i i i i $= + = + -^ h bi is the distribution coefficient for every gas and it is a function of temperature (giggenbach, 1980; d’amore, 1992). for temperatures between 100 and 340°c (t in °c) . . . . . . log log log b t b t b t 4 7593 0 01092 6 0783 0 01383 4 0547 0 00981 co ch h s 2 4 2 = = = 150 rosa maría barragán, victor m. arellano gómez, enrique portugal, fernando sandoval and nuria segovia . . .log b t6 2283 0 01403h2 = by substituting in eqs. (3.3) and (3.4) each p expression, as given by eq. (3.5) log log log log log log log log k a a a p n n n n n n 4 2 4 ft h co ch h o h h o co h o ch h o 2 2 4 2 2 2 2 2 4 2 + + + = + + ` ` ` j j j (3.6) / / . log log log log log k a a n n n n 3 5 4 3 5 4 hsh h s h h s h o h h o 2 2 2 2 2 2 2 + = -` `j j (3.7) the left side of eqs. (3.6) and (3.7) are defined as the ft and hsh2 parameters log log log log log a a a p ft k 4 2 ft h co ch h o 2 2 4 2 + + + = (3.8) / .log logk a ahsh 3 5 42 hsh h s h2 2 2= + (3.9) according to d’amore (1992), the expressions for the equilibrium constants (logkft and logkhsh2) are given by . ( / ) . ( )log logk t t4 33 8048 4 635ft = + (3.10) . ( / ) . ( )log logk t t7 609 6087 0 412hsh2 = (3.11) . ( / )log p t5 51 2048h o2 = (3.12) where t is given in k. the graphic solution of eqs. (3.8) and (3.9) provides a grid in the coordinates (hsh2, ft). the parameters ft and hsh2 are obtained from the gas composition according to eqs. (3.13) and (3.14) log log logft h h o co h o co h o 4 2 2 2 2 4 2 + + = _ _ _ i i i (3.13) / log5 4-loghsh h s h o h h o32 2 2 2 2= _ _i i (3.14) where concentrations of gas species are taken in the total fluid. the following trends and interpretations were given by d’amore and truesdell (1995): – increase t, decrease y: contribution of fluid from a hotter and deeper source with high liquid saturation. – increase t, increase y: apparent increase in t and y due to lateral source of steam, with practically zero liquid saturation and with a strong local accumulation of gas. – decrease t, decrease y: local source of pure and low temperature water with no gas content as in the case of reinjection fluids or fast meteoric water. – decrease t, increase y: caused by either recharge from peripheral fluids rich in gas; or sulfides precipitation caused by local over-production with blockage of main fractures. 4. results 4.1. tejamaniles steam-dominated south zone the southern zone has been intensively exploited since 1984. the wells az-6, az-17 and az-33 were selected as representative. table ii gives the height (m a.s.l.) of producing zones of the wells. in fig. 1 the location of wells in the field is given. figure 3 shows the ft-hsh2 grid diagram for the steam well az-6. in the diagram, the dark mark corresponds to the reference point found by nieva et al. (1987), calculated through equilibrium of the ft reaction for the measured reservoir temperature. the points represent average values for the indicated years, all data available were included. the figure shows that the grid temperature of about 290°c obtained for 1983 probably indicates that of the source of steam, since the reference temperature based on measured resertable ii. height of producing zones of studied wells. well elevation (m a.s.l.) az-5 1740 ± 330 az-6 2015 ± 100 az-9 945 ± 350 az-13 1620 ± 100 az-17 2200 ± 30 az-33 2190 ± 40 151 gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico voir temperatures is only 260°c. for the 1983 point, the vapor fraction (y) was very small, about 3% compared to that obtained for the reference 25%. an overall tendency with 19831998 data shows an important decrease in temperature, of about 30°c and an increase in y from 3 to 12%. according to d’amore and truesdell (1995), this trend is related to a decrease in the ratio h2s/h2o which could be caused either by recharge from peripheral fluids rich in gas or by sulfides precipitation. in well az-6 there is evidence of interference of reinjection fluids which are rich in gas. however the increase in y occurred only for the period 1983-1988, since points for 1988-1998 show a constant value of 13%. thus, for the period of 1988-1998 the drop in temperature was due to exploitation that lowered the reservoir pressure. figure 4 shows the ft-hsh2 grid for well az-17. the figure shows that the grid temperature of starting conditions was about 290°c while the reservoir steam y was about 10%. as in well az-6, the reference point indicates a lower temperature obtained from temperature logs. the overall tendency of the points in the grid shows a drop in the temperature but an increase in reservoir steam, which is the same trend as observed in well az-6. these results are a consequence of exploitation that causes pressure drop and boiling. deep liquid boiling produces two-phase fluids rich in non condensable gases that increases y values at reservoir. data for 2001 in the grid, where y is about 50%, could reflect drilling operations in the field that caused anomalous results regarding gas equilibrium. the anomalous point with a high y value (about 40%) in fig. 4 corresponds to the sample taken one week before the september 1985 earthquake (ms = 8.1). according to the grid, the reservoir steam/liquid ratio temporarily changed as a result of the tectonic event (barragán et al., 2001). well az-33 produces two phase fluids with an average steam fraction of 0.71 at wellhead fig. 3. ft-hsh2 grid diagram for well az-6. the dark dot shows the conditions taken as reference before exploitation (nieva et al., 1987). circles show the average values for the indicated years. see text for discussion. 152 rosa maría barragán, victor m. arellano gómez, enrique portugal, fernando sandoval and nuria segovia fig. 4. ft-hsh2 grid diagram for well az-17. the dark dot shows the conditions taken as reference before exploitation (nieva et al., 1987). circles show the average values for the indicated years. see text for discussion. fig. 5. ft-hsh2 grid diagram for well az-33. the dark dot shows the conditions taken as reference before exploitation (nieva et al., 1987). circles show the average values for the indicated years. see text for discussion. fig. 6. ft-hsh2 grid diagram for well az-5. the dark dot shows the conditions taken as reference before exploitation (nieva et al., 1987). circles show the average values for the indicated years. see text for discussion. 153 gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico and it is affected by reinjection. the grid for well az-33 is given in fig. 5. the reference point taken in 1987 indicated an initial reservoir temperature of 270°c and an excess steam y of 20% while results in the grid for the year 1984 show a temperature of 283°c and a y of 6.5%. the scattering shown by data in the grid is related to the influx of cooler fluids in an intermittent way, depending on reinjection. the data for 1984-1988 show a decrease in both the temperature and y. according to d’amore and truesdell (1995), this pattern indicates recharge of pure and low temperature water with no gas content as is the case with reinjection. for 1997-1998 data the ccg reservoir temperature was 256°c while the grid provided 265°c. the relatively low value obtained by ccg is due to the admixture of lower temperature waters to the reservoir from reinjection. ccg provides the temperature «far» from the well, although the reinjected fluid has been heated during flow to the reservoir. the trend of variation found for 1998-2001 data shows the same pattern observed at wells az-6 and az-17, a decrease in temperature and an increase in y. the decrease in temperature is caused by the mixing of reservoir-cooler reinjection fluids whereas the y increase is due to the deep boiling process induced by the drop in pressure that also releases non condensable gases. the same pattern observed for this well is shown at well az-46 which is also affected by reinjection. 4.2. maritaro liquid-dominated northern zone wells az-5, az-13 and az-9 have been selected as representative of the north zone. figure 6 shows the ft-hsh2 grid of well az-5. this well produced two-phase fluids in the past with an average steam fraction of 0.73 at separating conditions but at the present time the steam fraction has increased towards 0.89 in average. in fig. 6, the points corresponding to 1985-1987 indicate reservoir temperatures of 275-280°c while the reference temperature (obtained by ccg) was 300°c. overall tendency for the points indicates an increase in temperature and a decrease in y. 154 rosa maría barragán, victor m. arellano gómez, enrique portugal, fernando sandoval and nuria segovia this pattern is interpreted as a result of the contribution of a hotter fluid with high liquid saturation from a deep source. however, production data indicate that an important boiling process takes place in this zone of the reservoir. the estimations of reservoir temperature obtained by silica and ccg geothermometers and the temperature obtained by the well simulation using the simulator welflo (goyal et al., 1982) with time were compared according to truesdell et al. (1995), (fig. 7). the silica geothermometer provides the temperature «close» to the well while the simulator provides the temperature of the fluid entering the well, thus both temperatures agree in most cases. in contrast, ccg provides the temperature «far» from the well because of its relatively slow reequilibration rate. as is seen, for 1985-1987 the grid temperatures are close to the silica estimations, indicating that temperature at which the last gas equilibria occur. the comparison of the gridccg temperatures for los azufres wells suggest that as the wells produce higher steam fractions both temperatures approach each other (barragán et al., 2002) as in the case of the 1998-2000 data. as noticed, a very low grid temperature for the year 2001 (230°c) was found. the average values of co2 at 2000 and 2001 changed from 966 to 902‰ molar (dry basis) whereas the n2 changed from 7.9 to 66.4‰ molar (dry basis). this change is related to drilling operations in this zone during 2001, that allowed the influx of air to the reservoir causing gas deviations regarding equilibrium. reservoir steam values y range between 0.5 and 5%, indicating a high liquid saturation in the reservoir fluid. the ft-hsh2 grid for well az-13 is given in fig. 8. in the past this well produced two-phase fluids but at present it emanates only steam. the figure shows that the grid reservoir temperature for initial conditions is 270°c which is rather low compared to the reference temperature of 300°c, but, well-bottom temperature obtained by the fig. 7. estimation of reservoir temperatures in well az-5 with time. tccg: temperature estimated by ccg geothermometer; tsio2: temperature estimated by silica geothermometer and tsim: temperature estimated by the well simulator welflo. 155 gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico fig. 8. ft-hsh2 grid diagram for well az-13. the dark dot shows the conditions taken as reference before exploitation (nieva et al., 1987). circles show the average values for the indicated years. see text for discussion. fig. 9. ft-hsh2 grid diagram for well az-9. the dark dot shows the conditions taken as reference before exploitation (nieva et al., 1987). circles show the average values for the indicated years. see text for discussion. 156 rosa maría barragán, victor m. arellano gómez, enrique portugal, fernando sandoval and nuria segovia well simulator yields 275°c. as the well changed towards vapor phase the grid method indicates reservoir temperatures close to (in 1998) or higher than (in 1999, 2000) ccg estimations given in the past. the temperatures higher than ccg estimations found for 1999 and 2000 (grid temperature of 310°c) could be due to near-well boiling processes which increase the gas content in the fluid. the comparison of temperature estimations according to the method proposed by truesdell et al. (1995) confirmed that near-well boiling occurred within the well until 1997 (barragán et al., 2002). data of 2001 were affected by drilling thus show a decrease in temperature (295°c) and a higher value (6%) of y. figure 9 shows the ft-hsh2 grid for well az-9. this well produces two phase fluids from the deep compressed liquid zone of the reservoir. the steam fraction at separating conditions was 0.32 in 1998. the grid temperature found for initial conditions was 270°c while the reference value based on ccg was 320°c. the grid temperature in this well compares well with the temperature obtained by the well simulator (280°c) providing the temperature of the fluid entering the well. the detailed analysis of chemical and isotopic data (arellano et al., 2003) suggested that in spite of exploitation, this well shows stable conditions with time. this is also seen in fig. 9 since the points for 1987 and 1999 are located very close to each other in the grid. the point for the year 2000 indicates a reservoir temperature of 260°c which agrees with the well simulator temperature at present time. the year 2001 sample has been altered due to drilling in the field as in well az-5. 5. conclusions gas equilibria prove to be suitable in los azufres geothermal field in defining a conceptual model for the natural state conditions at reservoir. changes due to exploitation were observed in both zones of the field in the gas equilibria by means of ft-hsh2 grid. representative wells were studied using the ft-hsh2 method. for the south zone where reinjection is important, the ft-hsh2 grids for wells az-6, az-17 and az-33 show a decrease in temperature and an increase in y. this is due to the influx of fluids rich in gas as in the case of reinjecting fluids being water/air mixtures. the concentration of non condensable gases has increased in the southern zone of the field because of both processes: a deep boiling producing two-phase fluids rich in gas and water/air reinjection. in the northern zone the wells az-5, az-13 and az-9 were studied. in wells az-5 and az13 an increase in temperature and a decrease in y values are seen. this result is due to the higher vapor fraction produced by both wells at present time compared to lower values produced before. in the past, the wells were liquid-dominated and ccg and silica geothermometers indicated higher reservoir temperatures than those indicated by ft-hsh2 grid. as the grid provides the temperature for the last equilibrium, which occurs when the fluid enters the well, grid temperatures may not correspond to the fluid temperature in the «undisturbed» reservoir. thus, the grid underestimates temperatures of both wells at initial conditions. this fact is clearly seen in well az-9 where, in spite of exploitation, stable conditions have been inferred. as this well is liquid-dominated, the grid temperatures approach those obtained by the welflo simulator. these are lower than those estimated by liquid phase geothermometers (ccg or silica), providing the temperature of the fluid entering the well. when the vapor fraction is higher, the fthsh2 grid provides better estimations of reservoir temperature at los azufres wells, comparable to ccg results. although ft-hsh2 method is very useful in studying data from dry steam wells where no liquid phase is available, additional evidence based on production data should be investigated in order to better understand the trends in the ft-hsh2 grid. acknowledgements the authors thank the comisión federal de electricidad, residencia los azufres, eng. m. a. torres r. for providing data and allowing publication of this work. the results were part of the project «isotopic study of production and injection well fluids from the los azufres geot157 gas geochemistry for the los azufres (michoacán) geothermal reservoir, méxico hermal field» developed by cfe-iie in 2002. the authors also acknowledge dr. j. heinicke and dr. j. walker and two anonymous reviewers 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algorithm 1. introduction interpretation of geoelectrical induction data in seismoactive and volcanic areas often requires a directed search for characteristic structural details and quantitative parameters of the deep structure that could be directly compared to results of other, generally non-electrical investigations. typical questions may be, e.g., correlation stochastic interpretation of magnetotelluric data, comparison of methods václav červ (1), michel menvielle (2)(3) and josef pek (1) (1) geophysical institute, academy of sciences of the czech republic, prague 4, czech republic (2) centre d’études des environnements terrestre et planétaire, saint maur des fosses cedex, france (3) département des sciences de la terre, université paris sud xi, orsay cedex, france abstract global optimization and stochastic approaches to the interpretation of measured data have recently gained particular attraction as tools for directed search for and/or verification of characteristic structural details and quantitative parameters of the deep structure, which is a task often arising when interpreting geoelectrical induction data in seismoactive and volcanic areas. we present a comparison of three common global optimization and stochastic approaches to the solution of a magnetotelluric inverse problem for thick layer structures, specifically the controlled random search algorithm, the stochastic sampling by the monte carlo method with markov chains and its newly suggested approximate, but largely accelerated, version, the neighbourhood algorithm. we test the algorithms on a notoriously difficult synthetic 5-layer structure with two conductors situated at different depths, as well as on the experimental coprod1 data set standardly used to benchmark 1d magnetotelluric inversion codes. the controlled random search algorithm is a fast and reliable global minimization procedure if a relatively small number of parameters is involved and a search for a single target minimum is the main objective of the inversion. by repeated runs with different starting test model pools, a sufficiently exhaustive mapping of the parameter space can be accomplished. the markov chain monte carlo gives the most complete information for the parameter estimation and their uncertainty assessment by providing samples from the posterior probability distribution of the model parameters conditioned on the experimental data. though computationally intensive, this method shows good performance provided the model parameters are sufficiently decorrelated. for layered models with mixed resistivities and layer thicknesses, where strong correlations occur and even different model classes may conform to the target function, the method often converges poorly and even very long chains do not guarantee fair distributions of the model parameters according to their probability densities. the neighbourhood resampling procedure attempts to accelerate the monte carlo simulation by approximating the computationally expensive true target function by a simpler, piecewise constant interpolant on a voronoi mesh constructed over a set of pre-generated models. the method performs relatively fast but seems to suggest systematically larger uncertainties for the model parameters. the results of the stochastic simulations are compared with the standard linearized solutions both for thick layer models and for smooth occam solutions. mailing address: dr. václav červ, geophysical institute, academy of sciences of the czech republic, boční ii/1401, 14131 prague 4, czech republic; e-mail: vcv@ig.cas.cz 8 václav červ, michel menvielle and josef pek of clusters of seismic sources with boundaries of blocks with a significant conductivity contrast, detection, spatial mapping and conductivity estimates of magmatic plumes, delineation of groundwater layers and of deep aquifers, etc. due to the inherent ambiguity of the inverse problem solution with noisy and incomplete data, those structural features may be missed single model linearized inversions, or smeared over large intervals of depths if smoothness of the inverse model is emphasized by applying a roughness penalty for regularization. stochastic approaches to the interpretation of measured data have recently gained attention in geophysical applications (e.g., mosegaard and tarantola, 1995). they are particularly attractive as tools for estimating the model parameters from samples from their probability distributions, for quantifying uncertainties in the estimated model parameters, and for structural hypotheses testing. the advantage of the stochastic simulations is that they aim at performing the search throughout the parameter space and pick up the models according to their probabilities measured by the misfit and priorities for specific model features. the main difficulty of the stochastic inversions is that they are carried out via often extremely computer intensive simulations. mapping the parameter space has long been an ambition of global minimization methods in geophysical inversions (e.g., senn and stoffa, 1995). though these methods primarily aim at searching for a global minimum of a function, they always operate on a large population of models and rely on random factors within their optimization strategy to reduce the risk of being trapped at local minima of the target function. by this mechanism, the optimization procedure generates a topography map of the target function along a series of randomly perturbed paths in the parameter space, with usually dense coverage in the vicinity of the minima. the model population generated by a global optimization procedure does not generally follow the true probability density function of the models. in this paper we aim at demonstrating how ensembles of models produced by selected global optimization procedures compare with results of single model linearized inversions as well as with results of a probabilistic sampling. as representatives of the global minimization procedures, we have selected the controlled random search (crs) method (price, 1977) and the neighborhood algorithm (na) by sambridge (1999a), both being relatively fast global minimizers with only low demands on a model-conditioned finetuning of the optimization parameters. a fully stochastic sampling is based on a simple implementation of the markov chain monte carlo (mcmc) procedure adopted from (grandis et al., 1999) and modified for thick layer 1d mt inversion with both variable resistivities and layer thicknesses. the results are also compared with the standard occam smooth inversion (constable et al., 1987) and layered linearized inversion by weaver and agarwal (1993). as to the model structures, we restrict ourselves to 1d layered mt models here that are better suited if structural details like sharp electrical boundaries, thin sheets or sandwich structures with a tectonic significance are targets of the search. moreover, the easiness of 1d mt direct solutions allows us to carry out all the computer intensive tests more completely than would be possible under the serious limitations imposed by using 2d or even 3d direct modelling codes. specifically, we test the selected optimization procedures on a simple coprod1 benchmark model and then analyze a synthetic model with two conductive layers separated by a non-conductor in detail. the latter example presents a generally non-elementary problem, especially as regards the detection and resolution of the deeper conductor, and is typical of tectonic settings in many active areas. 2. global optimization and stochastic sampling methods 2.1. controlled random search controlled random search (crs) is a simple yet often very efficient optimization algorithm first suggested by price (1977). it starts with a sufficiently large pool of randomly generated models, say pi, i=1, ..., np with the target values ϕ( pi), where each vector pi consists of ns parameters that describe the physical model. the crs algorithm then proceeds by employ9 stochastic interpretation of magnetotelluric data, comparison of methods ing a set of heuristic rules to generate a new test model from the current pool, say pt = r(p1, ..., pnp, rand) where ‘rand’ symbolizes a random factor that diversifies the model population and reduces the risk of the algorithm being trapped in the local minima of the target function. if the new test model pt is better, in terms of the target misfit, than the currently worst model in the pool it is used to replace the latter. if, moreover, ϕ( pt) is less than the currently best model misfit in the pool then many heuristics suggest carrying out an additional detailed local search in the vicinity of this successful point. by repeatedly applying the above steps to the model population the model pool develops and moves towards regions with better target values until a termination criterion is met, defined via a target threshold, by a minimum of a parameter change between successive iterations, or by setting a limit on the number of iterations. particular versions of the crs algorithm largely differ by the heuristics used for generating the new test model. the original crs algorithm by price (1977) uses a simplex of ns + 1 models randomly chosen from the current pool. the new test model is generated by mirroring the (ns+1)−st model from the simplex through the centroid of the first ns models, . this crs version will be further referred to as crs1. a number of other heuristics are discussed and compared, e.g., in tvrdík et al. (2002). one of the approaches showing good performance in 1d mt inversions is a heuristics proposed by montaz et al. (1997), referred to as crs6 in what follows. it generates the new test model by component-by-component fitting parabolas through the currently best model and two further models selected randomly from the current pool. the parameters of the new trial model pt are then given by the minima of the individual parabolas. for a given number of model parameters to be estimated the performance of the crs algorithms in 1d mt inversions depends primarily on the size of the pool from which new test models are generated. the results indicate that = n p p p 2 t s i n i n 1 1 s s − + = / crs6 is largely superior to crs1 as regards the convergence speed. however, the crs1 simplex algorithm seems to better explore the parameter space and shows lower tendency to end up in secondary local minima. in general, crs with repeated runs presents a relatively fast and reliable global optimization algorithm, which provides us rapidly with information about the complete spectrum of the minima of the target function under study. 2.2. neighborhood algorithm recently, a new global optimization procedure was suggested by sambridge (1999a) for a seismic inversion, based on an iterative evolution of the initial model pool by randomly sampling the target function in the immediate vicinity of the pool models. the vicinity of a model is defined in a most natural way, as a subregion in the parameter space that comprises all the nearest points with regard to this particular model. formally, the nearest neighborhood of a pool model pi, i∈{1, ..., np} consists of all models p, which meet the condition , j∈{1, ..., np}, j ≠ i . in this way, the whole parameter space is divided into a system of convex voronoi cells, each of them representing the nearest neighborhood to a particular pool model. the optimization by the neighborhood algorithm (na) proceeds by carrying out np /nr random steps within nr selected voronoi cells with the best target values, generating thus np new models in the regions of the minimum misfit in each algorithm step. the random walks within the voronoi cells are carried out by using the standard gibbs sampler (gelman et al., 1995; also see next section) restricted to the selected cell. sambridge (1999a) demonstrated both the optimization and numerical efficiency of the na algorithm which adapts fast according to the shape of the underlying target functions and relatively rapidly provides a map of domains around the target minima. a particular choice of the factor nr controls the preference of the algorithm for a local search or for global exploration. numerical tests for a 1d inverse mt problem indicate that the na shows quite a similar performance as the crs algorithms above. p p p pi j− − 2 2 # 10 václav červ, michel menvielle and josef pek 2.3. monte carlo with markov chains stochastic sampling methods are used to generate model samples distributed proportionally to the likelihood of the models. the sample probability distribution can then be used to numerically approximate bayesian integrals that provide single parameter estimates or information about their credibility. the monte carlo methods with markov chains (mcmc) are algorithms to generate sample ensembles for the monte carlo integration. in the bayesian approach, the inverse problem solution is represented by a posterior probability distribution of the model given the data and prior information on the model (e.g., grandis et al., 1999) where prob(p) is the prior probability distribution, and the likelihood function prob(d/p) in the form given above assumes that the data items follow a normal distribution law n(dlobs, δdlobs), l = 1, ..., nd. if no specific prior information on the model parameters is available, we can assume, for a layered earth’s model, that logarithms of both the resistivity and thickness assume constant values within sufficiently broad limits for each layer. we use this non-informative prior in all the mcmc simulations presented in section 4. without going further into detail here (see, e.g., gelman et al., 1995; grandis et al., 1999, for reference), the idea behind the mcmc is to create a random walk, or markov process, that has prob(d/p) as its stationary distribution and then to run the process long enough so that the resulting sample closely approximates a sample from prob(d/p). the gibbs sampler is one of the particular methods used to construct such a markov process. the markov chain relies on updating the model parameters in the process of successively scanning the parameter domain under study. in each scan, we update one single model param( ) ( / ) ( ) ( / ) ( ) prob prob prob prob exp d d d d p d p p d p 2 1 obs mod obs l l l l n 2 1 d \ δ − − = ( / )prob p d = < f) 3/ eter, say pk, by drawing a new value from the one-dimensional conditional probability distribution prob(pk /d, p1, ..., pk−1, pk+1, ..., pns), i.e. with all parameters except the k-th one fixed at their current values. one mcmc step is completed after all ns model coordinates have been updated. in this way, an ergodic markov chain is designed with an invariant probability law identical with prop(p/d). the posterior marginal probability distributions for the parameters as well as various bayesian integrals are then estimated from the successive simulated values of this markov chain. 2.4. resampling with the neighborhood algorithm (nar) mcmc is a computer intensive method since a series of direct problem solutions is required for each parameter scan within the gibbs sampler. if an ensemble of direct solutions already exists from computations carried out previously, an effective resampling procedure can be applied to this model ensemble as suggested by sambridge (1999b). the models generated, e.g., by previous global optimization runs are used to approximate the misfit function by the neighborhood algorithm, i.e. by a piecewise constant function in a voronoi mesh with cells centered at the models available. then, the mcmc sampling is carried out for that surrogate target function in exactly the same way as the full mcmc simulation above, but without any additional direct problem solution required. clearly, the success of this approach highly depends on how good a support the underlying model ensemble gives to the true misfit function, which is a delicate question in multidimensional parameter spaces, and might be also a problem-dependent issue. 3. test data sets we selected two specific data sets to analyze the performance of the stochastic optimization procedures in situations often encountered in various interpretations of electromagnetic induction data. the first data set (dsi) is synthet11 stochastic interpretation of magnetotelluric data, comparison of methods ic and is derived from a model with a shallow conductor over a deeper conductive target. this general setting occurs in a variety of practical situations, especially if a layer of conductive sediments screens deeper layers containing fluids, but also in large-scale settings like those of an electric asthenosphere screened by a conductive lower crust. it is generally known that the deep conductor is difficult to resolve unless its conductance is considerably higher than that of the shallower screening layer. the particular data generating model used here was adopted from (grandis et al., 1999). model for dsi is shown in fig. 1a-d, marked by a gray-white contrast in all sub-panels, and its parameters are given in the figure caption. conductances of the layers, s = h/ρ, are 2.4, 16, 20 and 25, in siemens, from the top to the bottom, and indicate that there is only little chance to resolve the deep thin conductor situated at the depth of 2 km. it is clear from fig. 1a-d that for 5% of gaussian noise added to the synthetic mt data no common linearized inversion gives any indication that a deeper conductor exists beneath the shallow conductive layer. they all rather extend the intermediate third layer to greater depths and smear the deep thin conductor over a depth range of more than 2 km into the high-resistivity basement. the second data set (dsii) is the experimental coprod1 set introduced and analyzed in detail by jones and hutton (1979a,b), later used for demonstrating the performance of the occam 1d inverse approach by constable et al. (1987), and commonly employed as a benchmark data set for performance tests of 1d inverse techniques in magnetotellurics. the coprod1 data represent a set of 15 pairs of log apparent resistivity and mt phase data items within the period range of 20 to 2000 s and fig. 1a-d. results of four different linearized inversion routines applied to the synthetic data set dsi. the data generating model is marked by a gray-white contrast in all sub-panels, and its layer parameters are: layer 1 – h1 = 0.6 km, ρ1=250 ωm; layer 2 – h2 = 0.4 km, ρ2=25 ωm; layer 3 – h3 = 2 km, ρ3=10 ωm; layer 4 – h4 = 0.25 km, ρ4=10 ωm; basement 5 – ρ5=1000 ωm. mt data were generated from this model for 41log-regularly spaced periods within the range from 10−3 to 10 s, and the synthetic impedances were further contamined with gaussian noise with the standard deviation of 5% of the maximum impedance element for each period. inverse results in the sub-panels correspond to: a) occam inversion with roughness penalty (constable et al., 1987); b) occam inversion with total variation penalty (portniaguine and zhdanov, 1999); c) occam inversion with gradient support penalty (portniaguine and zhdanov, 1999); d) inversion for a minimum number of layers according to weaver and agarwal (1993). in the boxes below the panels the respective regularization weights (reg), misfit (rms2) values, and other routine-specific parameters are given. ba c d 12 václav červ, michel menvielle and josef pek correspond to a relatively simple structure with one pronounced conductive layer at a lower crustal/upper mantle depth. nonetheless, the data set is by far not elementary, especially owing to the data error structure that does not seem to follow any simple noise model. 4. search in the parameter space: model simulations for the test data sets 4.1. synthetic data set in the subsequent numerical tests we concentrate on the comparison of the performance of the selected global and stochastic optimization procedures for parameter estimation and uncertainty quantification in simple but practically relevant magnetotelluric settings. availability of relatively simple and reliable linearized inverse procedures for 1d mt problems makes it possible to assess the extra information we can infer from sampling the whole parameter space as compared to searching directly for one specific inverse solution. a first series of numerical experiments aimed at comparing the selected methods applied to the synthetic data set dsi. the underlying model for dsi is a 5 layer structure with a deep thin conductor that is clearly hard to detect in the surface mt data. we experimented successively with models consisting of 3 to 6 layers, including the basement, to test the outputs of the individual optimization procedures. we intentionally used very broad a priori limits for all layer parameters, specifically 10−3≤hi≤10 km, 1 ≤ ρi ≤ 104 ωm, i=1, ..., number of layers to test the convergence as well as the degree of risk of the individual methods to end up in false minima. with 3-layer test models the best attainable rms2 misfit was 1.88 and none of the methods could fit the data satisfactorily. both the crs6 (pool of 100 models, 5000 iterations, 200 runs, accepted all models with rms 2≤ 4) and mcmc (50 000 steps, burn-in 10000, thinning 100) could recover the resistivity of the first layer and that of the basement with high accuracy, but they were not able to satisfactorily simulate the fine resistivity structure in the intermediate part of the model section by one single layer. the second layer does, however, provide the true integral conductance of the three original conductive layers with good accuracy, within less than 5% of the true value. both methods compensate for the insufficient resistivity contrast between the first and second layer by systematically decreasing the depth to the conductor by about 100 m as compared to the true model. figure 2 indicates the mean values of the individual parameters are displayed, and ranges comprising 50 and 90% of all the parameter values provided by the simulation techniques. though not so comprehensive as complete parameter histograms, these plots qualitatively provide a good idea about both the correctness of the recovered parameters, their uncertainty and skewness of the underlying histograms. after having increased the number of layers to four, the fit to the data improved substantially. parameters of the crs6 and mcmc procedures were the same as those specified for the case of 3 layers above; from the crs outputs only models with rms2≤ 1 were accepted now for constructing the histograms. in the 4-layer case, both the crs6 and mcmc recover the top layer resistivity almost exactly, while the thickness of the first layer is systematically greater by about 50 m than the true value and is scattered over about 40 m for the mcmc estimate, if the interval between the 25 and 75% quartiles of the parameter samples is used to measure the uncertainty. as the crs6 was always terminated after a fixed number of 5000 iterations, repeated values of h1 during the final stage of the iteration process lead to overoptimistic estimates of the parameter uncertainties, specifically 20 m for h1, as compared to the probabilistic mcmc sampling. contrary, the parameters of the second layer, i.e. the shallow conductor, are recovered only with large uncertainty, especially as regards its thickness. nevertheless, both methods produce relatively sharp estimates of the conductance of this layer, within 1.5 s, which are by about 4 s lower than the true value of s2=16 s. the deficit of the conductance in the second layer is compensated by increased conductance of the layer beneath, which tries to account for the summary effect of the third layer in the true model as well as for that of the deep thin conductor situated on the top of the basement. 13 stochastic interpretation of magnetotelluric data, comparison of methods as the mcmc sampling is an extremely time-consuming procedure, even for a simple 4layer mt model, we also tested the relatively fast neighborhood resampling algorithm (nar) by sambridge (1999b) on the present data set dsi. we applied this procedure to a complete set of models generated by the crs6 algorithm in 20 successive runs with different initial pools, 100 models each, and with 5000 iteration steps in each run, which makes altogether about 100 000 to 150 000 individual models distributed all over the parameter space, but with most of them concentrated in close neighborhoods of the main minima of the target function. we performed 20 000 iterations of the nar to produce a resampled set of models for further stochastic inference. we can see from fig. 2 that the nar resampling gives results similar to those of the mcmc, but with clearly more diffuse statistical ranges for those model parameters that have been recovered sharply by the mcmc sampling above. comparison of true target misfits with those obtained by the na interpolation in the voronoi mesh during the nar runs shows that the na interpolation may extend the domain of the target minimum widely. as a consequence, models with large misfits may be accepted more frequently into the resampled chain than it would be appropriate to their true likelihood. for the 4-layer mt model tested here, the difference between the mcmc and nar outputs can be illustrated by stacked model samples shown in fig. 3 (top row of panels). here, the gray scale is used to visualize the relative number of models from the sample ensemble which pass through the individual cells in the log ρ−logz plane. the plots show limits for the resistivity indicated by the sample models as a function of depth. the fuzziness of the nar results could be partly reduced by using a more fig. 2. mean values (vertical bars) and ranges comprising 50 and 90% of all parameter values (black and gray zones, respectively) provided by various global optimization and stochastic sampling runs applied to the dsi data set with 5% of gaussian noise. the vertical bars in the top row indicate the true values of the parameters of the synthetic model. for specific routine parameters see the text. for the 4 and 5-layer models, the row labeled lin shows parameter estimates (long bars) and variances (short bars) obtained for classical linearized inverse solutions. the classical parameter variances for the 5-layer model from a truncated svd analysis (only the seven largest from altogether nine singular values were used to compute the parameter variance-covariance matrix) are shown by full lines, and those obtained with addition of the nul space of the solution are indicated by dotted lines. black rectangles show the restrictions to the parameter acceptability range for the non-linear model (models with rms less than one here). 14 václav červ, michel menvielle and josef pek accurate, but also greatly more time-consuming, interpolation scheme employing a standard inverse distance power procedure (with the power of 4 here). unfortunately, other experiments indicate that this improvement is not systematic and the performance of the nar with various interpolants depends highly on the dimension of the problem and the particular distribution of the interpolated models throughout the model space. for comparison with a classical single-model inverse solution, we also present in fig. 2 results of a posteriori linearized uncertainty analysis based on the singular value decomposition (svd) of the sensitivity matrix for the 4layered model obtained earlier by the inversion algorithm by weaver and agarwal (1993) (see fig. 1a-d for the model structure). the structure of the singular vectors corresponding to the four largest singular values of the sensitivity matrix for this model weighted by data variances indicates that the most significant parameters in the model are h1, s3 = h3/ρ3, ρ1, and s2=h2/ρ2, while the two smallest singular values suggest the resistivity of the basement, ρ4, and the resistance of the second layer, t2 = h2 ρ2, to be the least significant parameters. this ranking is also clearly manifested in fig. 2 by the individual parameters’ error bars, computed as square roots of the diagonal elements of the parameter covariance matrix (menke, 1989). these error bounds approximately demarcate the range of models that are equivalent, as to their fit to the experimental data, with the original model analyzed. though the classical variances are qualitatively in accord with the parameter uncertainties suggested by the stochastic simulations, their magnitudes are known to be underestimated (menke, 1989). fig. 3. gray-shade plots of stacked models produced by crs, mcmc and nar runs applied to dsi data set with 5% of gaussian noise. top row of panels show the results for a 4-layer model approximation, bottom panels are for inversion with 5-layer models. the degree of gray color indicates the relative number of models that pass through 0.1× 0.1 sized cells in the log ρ−logz plane. 15 stochastic interpretation of magnetotelluric data, comparison of methods obtaining more realistic error bounds within the classical analysis would require more sophisticated approaches to the estimates of the parameter limits to be applied, as those suggested, e.g., by pous et al. (1985) for linear or by meju and hutton (1992) for non-linear problems, or simply performing a random search for acceptable models in a certain vicinity of the original inverse solution. for test models with 5 or more layers, the inverse problem becomes ill-posed and most of the inverse methods suffer in some way from parameter indefiniteness or inter-dependence. often, the physical sense of the parameters gets lost in multi-layered models since characteristic conductive/resistive features of the real structure cannot be attributed to the model parameters in an unambiguous way. in particular, the histograms of the model parameters are difficult to interpret in terms of marginal distributions since they can, in fact, mix together true parameters of more than one physical domain. the bottom panels in figs. 2 and 3 show results of the global and stochastic sampling for 5-layer test models. except for the resistivities of the first and last layer, which are well constrained by the data, all the other parameter histograms are diffuse and, on a more detailed scale, even multimodal. the concentrated histograms resulting from the nar algorithm with the inverse distance power (power 4) interpolant rather suggest that the method stayed trapped in a vicinity of one specific minimum dictated by the particular distribution of the crs set of models. the stacked model plots in fig. 3 provide a clearer visual representation of the model samples with respect to the resistivity-depth section. they show very similar conductivity patterns for the shallow part of the structure down to the bottom of the first conductor for both the 4 and 5-layer models. the third layer is recovered sharply but already shows a typical highresistivity ‘shadow’ in the 5-layer case, which is an artifact due to a low sensitivity of the mt field to resistive domains beneath the first, shallow conductor. as regards the manifestation of the deep thin conductor at the depth of 3 km, less than 10% of the models in both the crs and mcmc patterns indicate a layer of increased conductivity between 2 and 3.5 km. the standard nar algorithm gives a largely diffuse image of the resistivity structure which indicates that the underlying set of crs models (from 20 runs of crs6, with the pool of 100 models and 5000 iteration steps) does not cover the parameter space properly. this situation did not change for better even when the support models were generated by the na minimization algorithm instead of crs. due to a massive inter-dependence of the parameters in multilayer cases, standard mcmc gibbs sampler mixes poorly and requires very long chains, of the order of hundreds of thousands of samples, to walk throughout the parameter space with sufficient density. nevertheless the resistivity pattern in fig. 3 remains quite stable after a few tens of thousands of the mcmc steps, though its detailed probabilistic interpretation may be questionable for chains so short. the stacked model resistivity image shows stable structural features also for increasing number of layers in the test models, though especially the high-resistivity artifacts become more pronounced in runs with many layers. the classical svd analysis of the 5-layer linearized inverse solution allows similar conclusions to be made as regards the parameter uncertainty. contrary to the 4-layer model studied earlier, the svd spectrum for the 5-layer model indicates that at least two singular vectors form the nul space of the inverse solution, specifically those corresponding to the resistances of the second and fourth layers, t2 = h2 ρ2 and t4 = h4 ρ4. then the linearized variances for the parameters involved in these combinations are not bounded within the range of the physical acceptability of the models. figure 2 shows parameters of one particular classical 5-layer inverse solution along with their linearized variances for the case of a truncated svd spectrum, with only seven largest singular values from nine left for the computation of the parameter covariance matrix, as well as for the case of the full svd spectrum. in the latter case, the projection of the nul space solutions onto the space of the physical parameters gives very large, or almost infinite, error bounds for the poorly resolvable parameters. the predicted linearized parameter variances have to be further verified, 16 václav červ, michel menvielle and josef pek and properly reduced, by an extra procedure so that they do not exceed the region of acceptable models for the underlying non-linear problem. one additional difficulty of the uncertainty analysis based on a linearized inverse singlemodel solution is that the svd spectra may differ not negligibly for different inverse models due to the non-linearity of the underlying direct problem, especially if the experimental data are sufficiently noisy, suggesting thus sufficiently different nul spaces for different inverse models. in the stochastic sampling procedures, this equivalency across various classes of models may substantially deteriorate the convergence properties, but for long enough chains all the model classes should be visited and properly sampled in the course of the stochastic walk. 4.2. experimental coprod1 data set we carried out a similar series of numerical simulations also for the experimental data set dsii (coprod1) with the standard data errors adopted from constable et al. (1987). this data set is specific especially due to its error structure. we show in fig. 4 a projection of all the rms2 misfit values obtained from twenty crs6 runs (3 layer test models, pool size 100, 3000 iterations) as a function of the resistivity of the first layer. the figure clearly shows a bimodal character of the misfit, with a deep minimum at ρ1 close to 170 ωm, but with rms2> 1 and with another broad, but very flat zone, ρ1> 300 ωm, with misfits rms2< 1. the former target minimum attracts most of the solutions of both the crs and mcmc runs, and especially for mcmc with 3-layer models does not allow the chain to leave its ‘gravity’ in reasonable time even in case that the chain has been started from the minimum of the target function, provided by the crs. in this case, the crs map of the target minima provides valuable information for constraining the parameters for the mcmc simulation. figure 5 summarizes the results of crs, mcmc and nar simulations with 4-layer models for the dsii data set in the form of stacked log ρ−logz gray-shade plots. similarly as in the synthetic case, very broad a priori parameter ranges were chosen to simulate minimum prior knowledge of the parameter limits. specifically, we have set the layer thicknesses to be between 3 and 300 km and the resistivities between 1 and 105 ωm, except for the first layer with ρ1 > 180 ωm to avoid the ‘target trap’ described above. the resistivity patterns from the crs6 (200 runs, pool size 100, 3000 iterations) and mcmc (50000 steps, burn-in 10000, thinning 100) runs show very similar structures, though the crs pattern is sharper due to excessive accumulation of the solutions close to the target minima. the models seem to prefer very high resistivity values of the top layer, but constraining this parameter does not change the deeper resistivity pattern noticeably. similarly as in the synthetic case above, the nar resampling algorithm smears the resistivity patterns over a broader range of values. in the case studied here, no dramatic improvement was achieved if the inverse distance power interpolant was used within the fig. 4. projection of all rms2 misfits versus the resistivity of the top layer obtained in the course of 20 crs6 runs applied to the coprod1 (dsii) data set with 3-layer test models. along the axes, the corresponding histograms of the rms2 and ρ1 values are shown. 17 stochastic interpretation of magnetotelluric data, comparison of methods nar algorithm instead of the voronoi tessellation. figure 5 also presents the fit of the 4-layer model data from the above simulation runs to the experimental apparent resistivities and phases. 5. discussion and conclusions we applied three common methods of global and stochastic optimization, specifically the crs, mcmc and na/nar, to simple 1d magfig. 5. left column: gray-shade plots of stacked models obtained from crs, mcmc and nar runs applied to the coprod1 (dsii) data set with 4-layer test models. the dashed line indicates a preferred 4-layer solution by jones and hutton (1979b) who minimized the misfit function without considering the confidence weights. the full line shows a 4-layer solution from the inverse procedure by weaver and agarwal (1993) applied to the data supplemented with error bars according to constable et al. (1987). central and right columns: data fit for the apparent resistivities and phases, respectively, for models generated by the crs, mcmc and nar runs. the experimental coprod1 resistivities and phases are shown by white circles with error bars according to constable et al. (1987), the black dots are solutions for the model by jones and hutton (1979b). 18 václav červ, michel menvielle and josef pek netotelluric models to test their performance in problems that target at searching for and at verifying distinct structural features in geoelectrical sections, such as boundaries with a large conductivity contrast or conductors screened by shallow conductive structures. similar structural elements are often typical of tectonically active areas, and their detection, as well as estimation of their geometrical and electrical parameters, along with a proper specification of the uncertainty of these estimates, are indispensable prerequisites for carrying out any analysis of possible correlations between electrical structural factors and non-electrical indicators of the tectonic activity. crs and na are relatively simple, in terms of the tuning complexity, but effective global optimization algorithms that are suitable both for providing a quick inspection of the target function topography and for searching for the inverse problem solution. performing repeated runs of these algorithms with different randomly generated initial model populations gives not only a good idea of the character and multimodality of the underlying target function, but also draws an image of the parameter bounds (uncertainties), though not in quite fair proportions with regard to the likelihood of the models. the parameter distributions provided by the crs or na are affected by the tuning conditions of the minimization procedure and may be, e.g., excessively peaked if the procedure is terminated after a fixed (and large enough) number of iteration steps, or diffuse and shifted if the optimization is stopped after a certain portion of models in the pool, or all of them, reach a predefined misfit threshold. stochastic sampling techniques aim at providing unbiased model samples distributed according to the likelihood of the models. for layered mt models with very broad a priori limits for the model parameters, the mcmc gibbs sampler procedure shows a good convergence provided the underlying test models are not largely underdetermined. in the opposite case, the markov chain may mix poorly unless closer and more realistic parameter bounds are specified, especially for the layer thicknesses. with broad parameter ranges, mcmc shows frequent irregular transitions between fine modes of the target function and does not provide stable parameter marginal distributions unless a very large number of mcmc steps is carried out. it can, however, provide relatively quickly a stable approximate image of the resistivity versus depth distribution via stacking the models from the chain. resampling model ensembles produced by relatively fast global optimization procedures, such as crs or na, is another way to estimating the model parameters and their true uncertainty. the nar resampling based on approximating the misfit function by constants in an ensamblerelated voronoi mesh gives results similar to those of mcmc if sufficiently narrow bounds for the mt model parameters can be specified a priori. for broad parameter limits and highly underdetermined models, nar tends to produce more diffuse posterior distributions for the parameters as compared to mcmc. one of the likely reasons for this behavior may be that models produced by a fast converging crs procedure do not provide sufficient support for the na interpolation throughout the parameter space. as a consequence, the high probability region may expand via the na interpolation far beyond its true limits. in some cases, using a more accurate interpolation technique (e.g., inverse distance power interpolant here) could improve the results, but it cannot be generalized. as compared to standard single-model linearized inversions, the global and stochastic inverse methods provide additional information, especially as regards the possible ranges of model parameters and uncertainty of the parameter estimates conditioned on the observed data, though for the price of higher, and sometimes enormous computation times. they are of particular interest in situations like, e.g., directed search for specific structural features, inversion with various kind of prior information, or in jointly solving a problem of parameter estimation and model selection (malinverno, 2002). acknowledgements financial assistance of the grant agency of the czech republic under grants no. 205/04/0740 and no. 205/04/0746, and of the ministry of education, youth and sports of the czech republic, 19 stochastic interpretation of magnetotelluric data, comparison of methods grant no. me 677 is gratefully acknowledged. we thank v. spichak and an anonymous reviewer for helpful comments. references constable, s.c., r.l. parker and c.g. constable (1987): occam’s inversion: a practical algorithm for generating smooth models from em sounding data, geophysics, 52, 289-300. gelman, a., j.b. carlin, h.s. stern and d.b. rubin 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(2002): parsimonious bayesian markov chain monte carlo inversion in a nonlinear geophysical problem, geophys. j. int., 151, 675-688. meju, m.a. and v.r.s. hutton (1992): iterative mostsquares inversion: application to magnetotelluric data, geophys. j. int., 108, 758-766. menke, w. (1989): geophysical data analysis: discrete inverse theory (academic press, london), 2nd edition, pp. 289. montaz, a., a. torn and s. viitanen (1997): a numerical comparison of some modified controlled random search algorithms, tucs technical report no. 98. mosegaard, k. and a. tarantola (1995): monte carlo sampling of solutions to inverse problems, j. geophys. res., 100, 12431-12447. portniaguine, o. and m.s. zhdanov (1999): focusing geophysical inversion images, geophysics, 64, 874-887. pous, j., x. lana and a.m. correig (1985): generation of earth stratified models compatible with both ellipticity and phase velocity observations of rayleigh waves, pure appl. geophys., 123, 870-881. price, w.l. (1977): a controlled random search procedure for global optimization, computer j., 20, 367-370. sambridge, m. (1999a): geophysical inversion with a neighbourhood algorithm, i. searching a parameter space, geophys. j. int., 138, 479-494. sambridge, m. (1999b): geophysical inversion with neighbourhood algorithm, ii. appraising the ensemble, geophys. j. int., 138, 727-746. senn, m.k. and p.l. stoffa (1995): global optimization methods in geophysical inversion, advances in exploration geophysics (elsevier, amsterdam), 4, pp. 294. tvrdík, j., l. mišík and i. křivý (2002): competing heuristics in evolutionary algorithms, in intelligent technologies: from theory to applications, edited by p. sincák, v. kvasnicka, j. vascák and j. pospíchal (ios press, amsterdam), 159-165. weaver, j.t. and a.k. agarwal (1993): automatic 1d inversion of magnetotelluric data by the method of modelling, geophys. j. int., 112, 115-123. tema_finalonline:layout 6 annals of geophysics, 57, 5, 2014, g0548; doi:10.4401/ag-6611 g0548 archaeological, archaeomagnetic and thermoluminescence investigation of a baked clay kiln excavated at chieri, northern italy: contribution to the rescue of our cultural heritage evdokia tema1,2,*, fulvio fantino3,4, enzo ferrara5, silvia allegretti3, alessandro lo giudice3, alessandro re3, federico barello6, silvia vella5, luigi cirillo3, monica gulmini7 1 università degli studi di torino, dipartimento di scienze della terra, torino, italy 2 alp-alpine laboratory of palaeomagnetism, peveragno, italy 3 università degli studi di torino, dipartimento di fisica, and infn torino, torino, italy 4 tecnart srl., università degli studi di torino, torino, italy 5 inrim istituto nazionale di ricerca metrologica, torino, italy 6 soprintendenza per i beni archeologici del piemonte e museo antichità egizie, torino, italy 7 università degli studi di torino, dipartimento di chimica, torino, italy abstract combining different dating techniques is fundamental to constrain the ages of archaeological findings, mainly when direct evidences for their chronological context are not available. this paper presents the results of a combined archaeological, archaeomagnetic and thermoluminescence study of a kiln discovered during a rescue excavation at chieri, northern italy. the archaeological site is quite complex mainly due to the interposition of different stratigraphic levels that span from roman times to present day. the studied kiln belongs to the post-medieval stratigraphic level but the lack of datable diagnostic objects strongly limits the possibility of its accurate dating only by means of archaeological evidences. archaeomagnetic study was performed on 26 baked clay samples, isolating stable characteristic remanent magnetizations. the statistic comparison of the averaged direction with reference secular variation curves suggests two possible dating intervals. independent dating from thermoluminescence study of 2 samples is consistent with the second time interval proposed by archaeomagnetism, suggesting that the kiln was abandoned at the beginning of the 17th century. the successful combination of the two independent laboratory techniques provides accurate dating of archaeological baked clays and can be used as routine for future archaeological investigations of rescue excavations. 1. introduction during last decades, the number and the variability of scientific laboratory techniques applied for the investigation of archaeological finds have been significantly increased and nowadays the natural sciences offer valuable and well-established tools for better understanding the traces of our past. this process is particularly important in the cases where the findings do not include ceramics or other date diagnostic objects. obtaining as much information as possible, including precise dating of an archaeological site, is even more significant nowadays, since the rapid expansion of new constructions inevitably involves the destruction of archaeological structures in urban areas, often even unearthed, causing the loss of important knowledge about the past. in the case of such excavations, usually called rescue excavations, scientific techniques such as archaeomagnetic and thermoluminescence dating can offer information about the last firing of baked clay material and contribute to the chronological reconstruction of the studied archaeological sites [liritzis and thomas 1980, becker et al. 1994, schnepp et al. 2003, tema et al. 2013]. such information is very important for the evaluation of our cultural heritage and better understanding of our past. archaeomagnetic and thermoluminescence (tl) dating present the advantage that both can be applied to the same materials (e.g. kilns, bricks, pottery) and date the same event (last heating of the material). archaeomagnetism is based on the fact that baked clay archaeological artefacts contain magnetic particles that, when heated and cooled in the presence of the earth’s magnetic field, acquire a thermal remanent magnetizaarticle history received june 23, 2014; accepted september 24, 2014. subject classification: dating, archaeomagnetism, thermoluminescence, baked clay, cultural heritage. tion (trm) with direction parallel to the local field. archaeomagnetic dating can be obtained after comparison of the remanent magnetization measured on the undisturbed archaeological artefacts with reference secular variation (sv) curves. similarly, tl dating is based on the light emission that occurs from the sample when the charges trapped in structural defects within the crystal lattice of several common minerals, such as quartz and feldspar, are thermally annealed. tl enables the evaluation of the time elapsed since mineral grains that carry the entrapped charges were last exposed to daylight or heated (at least 400°c), as in the case of the heating of a kiln during its last use. this paper presents the results of a combined archaeological, archaeomagnetic and thermoluminescence dating of a baked clay kiln discovered during a rescue excavation at chieri, northern italy. results obtained from the different techniques are compared and used for dating the last use of the kiln. this combined approach highlights the potential of archaeomagnetic and thermoluminescence techniques on dating baked clay archaeological artefacts and the importance of multidisciplinary studies for the rescue of our cultural heritage. 2. site context and archaeological time constrains the studied kiln was discovered during a rescue excavation for the construction of a modern residential complex at chieri, via dei molini 4 (45°0' n, 07°49' e), torino province, northern italy. the planimetry of the excavation (figure 1a) is quite complicated because of the interposition of several structures with ages varying from roman times to present day. the most important evidences of the roman period can be found at the southeastern sector of the excavation, comprising several walls oriented from north-east to south-west, that were probably part of ancient residential buildings situated in the western limit of the roman town called karreum potentia. other residential levels from few centuries ago were also recognised in the site [barello et al. 2013]. the remains of the studied kiln were found in the north-western part of the excavation. the kiln belongs to the post-medieval stratigraphic level, just few centimetres beneath the present-day surface. only the lower part of the combustion chamber is preserved, excavated in a clay layer that covers roman age deposits [barello et al. 2013]. the combustion chamber is roughly squared (3.7 × 3.8 m; with a maximum height of 0.9 m) and the walls were made by clay, which was baked and consolidated due to the repeated exposition to heat during the use of the kiln. two long feeding channels are recognised, at a distance of about 1.2 m from each other (figure 1b,c). the channels were filled with ash, carbonaceous deposits and wooden trash, probably residues of the kiln’s last combustion. the use of the tema et al. 2 figure 1. (a) general planimetry of the archaeological site excavated in via dei molini, chieri; (b-d) photos of the studied kiln; (e) in situ orientation of the collected samples for archaeomagnetic dating. 3 kiln is not yet well known. archaeological evidence suggests that it was probably used for the production of bricks. in fact, imprints of bricks are visible on the bottom of the chamber, indicating that the clay blocks to be baked were probably stacked directly on the floor by head and/or edge, arranged on parallel rows oriented from north to south (at least six), plus a single row oriented from west to east along the northern wall. little information about the age of the kiln is available. its stratigraphic position suggests a medieval chronology as several post-medieval to modern structures cut the kiln. these structures could likely belong to modern buildings with frontage in the today’s via dei molini [barello et al. 2013]. the studied kiln also presents structural features similar to two other kilns excavated in the wider area: one found in via tana (chieri) next to the renaissance walls and possibly used for their construction [pantò and vaschetti 2010] and one excavated at cerrione (near biella, about 40 km north of chieri) which can be dated around 1600–1700 ad, as suggested by the dimensions of the bricks employed for its construction [spagnolo garzoli and barberis 2012]. nevertheless, the sporadic ceramic findings and the lack of diagnostic objects strongly limit the possibility of accurately dating the kiln by means of archaeological evidence. 3. archaeomagnetic dating 3.1. archaeomagnetic sampling for archaeomagnetic dating, cylindrical samples were collected with a portable rock driller. samples were taken from the west and southern walls of the kiln (figure 1d); we also tried to collect material from the kiln’s floor but the clay was not consistent enough and all but one core were broken during drilling. a total of 26 baked clay cores were collected and all of them were independently oriented in situ with a magnetic compass and an inclinometer (figure 1e). in addition, some nonoriented samples were collected in order to perform rock magnetic analysis and tl dating. few days after the sampling, the site was destroyed to let the pending project of the new building construction to proceed at the place of the archaeological excavation. 3.2. magnetic mineralogy the magnetic properties of representative samples have been investigated by isothermal remanent magnetization (irm) acquisition and back field curves, thermal demagnetization of three irm components [lowrie 1990] and hysteresis loops. the irm acquisition and back field curves were investigated at the alp palaeomagnetic laboratory (peveragno, italy) using an asc pulse magnetizer to impart the irm, applying stepwise increasing magnetic fields up to 1.6 t, and a jr6 spinner magnetometer (agico) to measure the remanent magnetization. a three axis irm was also induced along the three orthogonal axes of the sample, applying first the maximum field (1.6 t) along the z-axis, then an intermediate field (0.5 t) along the y-axis and finally a minimum field (0.1 t) along the x-axis [lowrie 1990]. the composite three axes irm was subsequently thermally stepwise demagnetized with a tsd-2 schonstedt furnace. hysteresis loops were obtained at inrim (torino, italy) using a lake shore 7400 vibrating sample magnetometer (vsm). 20 mg to 50 mg clay samples were prepared, extracting material from the larger cylindrical samples. the clayey samples underwent magnetization cycles up to a maximum applied field of 1 t. the hysteresis cycles (figure 2) show two major ensembles of magnetic contributions: 1) magnetite/ti-magnetite type cycles (e.g. samples ch-3, ch-17, ch-20), with single domain (sd) and pseudo single domain (psd) ferrimagnetic particles, together with a minor paramagnetic contribution; 2) faint sd and psd ferrimagarchaeometric dating of a baked clay kiln figure 2. hysteresis cycles for representative samples. two main groups are individuated according to their magnetic behavior: presence of sd and psd plus minor superparamagnetic grains (left side) and dominant paramagnetic grains plus faint sd and psd contributions (right side). netic particles (e.g. samples ch-15, ch-16 and ch-19) together with a dominant paramagnetic contribution. in all cases, magnetic saturation is reached at about 0.5 t that is the field value at which magnetite-like minerals are expected to saturate. the irm acquisition curves for different samples show similar results to those obtained from hysteresis analyses, indicating that saturation is reached at fields around 0.4 to 0.6 t while only very few samples do not get completely saturated after 1.6 t peak field (e.g. sample ch-18, figure 3a). the application of a back field (figure 3b) points to the dominance of a magnetic mineral of low remanent coercive force (around 50 mt). these results are also confirmed by the stepwise thermal demagnetization of composite irm [lowrie 1990] curves showing that most of the magnetization is carried by the magnetically soft fraction (<0.1 t) while the medium and high-coercivity components are generally very small (figure 4). these results suggest the presence of magnetite and/or ti-magnetite as the main magnetic carrier. 3.3. archaeomagnetic direction the natural remanent magnetization (nrm) of 26 cylindrical samples was measured using a jr-6 spinner magnetometer at the alp palaeomagnetic laboratory (peveragno, italy). subsequently, all samples were stepwise thermally demagnetized up to 580°c using a tsd-2 schonstedt furnace. during demagnetization, 5 of the 26 samples were broken; therefore measurements were successfully completed on 21 samples. demagnetization diagrams show that the magnetic remanence is stable and the characteristic remanent magnetization (chrm) is clearly isolated for the majority of the samples (figure 5a). only 4 samples with disturbed and not lineal zijderveld diagrams were rejected. in some samples, a secondary component of viscous origin was removed during thermal demagnetization. the direction of the chrm for each sample has been obtained from principal component analysis [zijderveld 1967, kirschvink 1980]. all directions at sample level are reported in table 1, together with the maximum angular deviation values (mad) that for all the studied samples are lower than 5°. the mean direction for the kiln has been calculated according to fisher statistics [fisher 1953] based on 17 samples and it is: d = 18.2°, i = 66.8° with a95 = 2.6° and k = 184 (figure 5b). 3.4. archaeomagnetic dating results the mean kiln’s direction has been used for archaeomagnetic dating after comparison with the reference sv curves calculated from the scha.dif.3k regional geomagnetic field model [pavón-carrasco et al. 2009]. the reference curves have been directly calculated at the site geographical coordinates avoiding in this way any relocation error. possible ages at 95% confidence level have been calculated using the matlab archaeo_dating tool [pavón-carrasco et al. 2011]. taking into consideration the stratigraphy of the site which reasonably excludes that the kiln could be older than roman times, the possible dating intervals have been calculated only for the last 2000 years. the final tema et al. 4 figure 4. stepwise thermal demagnetization of composite three axes irm. symbols: dot = soft(0.1 t); triangle = medium(0.5 t); square = hard(1.6 t) coercivity component. figure 3. (a) isothermal remanent magnetization (irm) and (b) back field acquisition curves for representative samples. (a) (b) 5 archaeometric dating of a baked clay kiln sample temperature range (°c) d (°) i (°) mad ch-3 80-580 17.4 64.0 3.4 ch-4 200-580 6.2 66.3 2.6 ch-5 320-540 19.3 69.2 0.7 ch-6 320-540 354.2 66.7 2.1 ch-7 260-500 12.6 67.2 1.5 ch-10 320-500 32.1 68.4 2.5 ch-12 260-580 5.8 67.5 2.5 ch-14 320-500 15.1 73.5 1.4 ch-15 260-460 32.1 67.8 1.1 ch-18 380-500 22.3 67.2 2.9 ch-19 320-500 35.7 63.1 4.6 ch-21 200-580 20.2 63.7 4.7 ch-22 200-580 21.1 63.1 2.1 ch-23 200-580 28.4 70.3 3.2 ch-24 260-580 358.0 60.1 2.8 ch-25 140-580 16.9 68.2 1.8 ch-26 80-500 32.3 61.7 4.4 mean value: n = 17 dm = 18.2° im = 66.8° k = 184 a95 = 2.6° table 1 (right). archaeomagnetic analysis results. columns: sample; temperature interval used for the calculation of the direction of the chrm at sample level; declination (°); inclination (°); mad: maximum angular deviation; mean value: n = number of independently oriented samples; dm = mean declination; im = mean inclination; k = precision parameter; a95 = 95% semi-angle of confidence. figure 5 (top and above). (a) stepwise thermal demagnetization results illustrated as zijderveld diagrams for representative samples. symbols: full dots = declination; open dots = apparent inclination; (b) equal area projection of the chrm directions at sample level together with the kiln’s mean direction as calculated from fisher statistics (pink dot). (a) (b) dating of the kiln is obtained after the combination of the separate density functions for declination and inclination (figure 6). two possible dating intervals are proposed: a first one from 808 to 994 ad, and a second one from 1535 to 1606 ad. combining the available archaeological evidence [barello et al. 2013], the first time interval can be excluded suggesting that the kiln was abandoned around 1535–1606 ad at 95% of confidence. 4. thermoluminescence dating thermoluminescence (tl) dating has been performed at the physics department, university of torino (torino, italy). two baked clay cores (named tl1 and tl2), were drilled at the same place along with the collected archaeomagnetic samples and were used for tl analysis. the tl study was executed applying the fine grains method [aitken 1985]. due to the position of the analysed samples, coming from the internal wall of the kiln (figure 1d), the calculated thermoluminescence age is related to the kiln’s last use. 4.1. archaeological dose evaluation the two terms needed for the determination of the archaeological dose or palaeodose (p) are: the equivalent dose (ed) and the supralinearity (i), a phenomenon that occurs at the initial history of the sample after its last firing for low irradiation doses [aitken 1985]. in order to evaluate the archaeological dose, about 2 g of clay have been extracted from the two cores that present different color and high porosity and friability. the chemical procedure described by vieillevigne et al. [2007] was applied for sample preparation. the ed was determined using the additive dose method for multiple aliquots [fleming 1979] and a calibrated radioactive sr-90 beta source was used to supply the artificial dose. after irradiation, pre-heating was performed in an oven at 150°c for 120 sec to reduce the contribution to the tl signal coming from unstable traps. all thermoluminescence measurements were carried out by means of a tl2000-ipses reader in nitrogen atmosphere using a 10°c/s heating rate. the measured tl glow curves for sample tl 2 are shown in figure 7. by means of the plateau test, the range 260°c–300°c was identified for integration and comparison of tl signals between natural and irradiated samples (for both cores). in figure 8 the results and the linear fit for the ed calculation of tl2 sample are shown. in order to measure the supralinearity, a second glow curve analysis at artificial irradiation doses was carried out after removing natural tl by means of a four-hour heating at 450°c [aitken 1985]. tema et al. 6 figure 6. archaeomagnetic dating results. up: declination (left) and inclination (right) reference secular variation curves calculated from the scha.dif.3k model (red curve with error band) and the kiln’s measured direction (blue line with green error band); middle: calculated probability density functions for declination (left) and inclination (right); down: combined probability density function for declination and inclination. 7 archaeometric dating of a baked clay kiln finally, the anomalous fading was also estimated. sample aliquots were irradiated with 6 gy dose and stored in the dark at room temperature for 18 days together with other not irradiated aliquots. then, the not irradiated aliquots were exposed to the same dose and their tl signal was compared with the tl of the previously irradiated ones. no anomalous fading was detected for the samples under investigation. all the results of ed, supralinearity i, and archaeological dose p for tl1 and tl2 samples are shown in table 2. 4.2. annual dose evaluation the annual dose can be calculated as the sum of contributions from alpha, beta and gamma particles generated by radioactive decays. about 1 g of untreated clay from each core was employed to perform alpha decay counting by means of calph-ipses apparatus. the measured radioactivity was utilized to calculate the annual dose due to alpha particles and also to evaluate the contribution to the activity from uranium and thorium decay chains. the alpha activity has to be corrected with the k-factor [aitken 1985], that is the figure 8. linear fit for equivalent dose (ed) calculation for the tl 2 sample. figure 7. tl glow curves for sample tl 2. beta irradiations were performed in the range 2 gy – 6 gy. for each irradiation two samples were prepared and measured. sa m p le e q u iv al en t d o se (g y) su p ra li n ea ri ty (g y) p al eo d o se (g y) a lf a co u n ts (c o u n ts / k s) p o ta ss iu m (w t. % ) w (s at u ra ti o n co n te n t % ) k -v al u e a lp h a d o se ra te (m g y/ ye ar ) b et a d o se ra te (m g y/ ye ar ) g am m a d o se ra te (m g y/ ye ar ) a n n u al d o se ra te (m g y/ ye ar ) a ge (y ea rs a go ) t l 1 1. 13 ± 0 .0 6 a b se n t 1. 13 ± 0 .0 6 8. 3 ± 0 .3 1. 15 ± 0 .0 2 34 ± 1 0. 13 ± 0 .0 1 1. 20 ± 0 .1 3 1. 13 ± 0 .1 2 1. 21 ± 0 .1 5 3. 54 ± 0 .2 5 32 0 ± 2 5 t l 2 1. 25 ± 0 .1 3 a b se n t 1. 25 ± 0 .1 3 8. 4 ± 0 .2 1. 12 ± 0 .0 2 32 ± 1 0. 11 ± 0 .0 2 1. 02 ± 0 .2 1 1. 14 ± 0 .1 2 1. 21 ± 0 .1 5 3. 37 ± 0 .2 8 37 0 ± 4 0 t ab le 2 .t he rm ol u m in es ce nc e an al ys is r es u lt s. n o an om al ou s fa di ng w as r ec og ni ze d on t l 1 a nd t l 2 s am pl es a nd th e pl at ea u te st w as s u cc es sf u l i n th e ra ng e 26 0– 30 0° c fo r bo th s am pl es . t he a lp ha d os e ra te is c or re ct ed b y kfa ct or . g am m a do se r at e w as e st im at ed b y m ea ns o f l if d os im et er s. efficiency of the alpha particles compared to beta particles in producing a tl signal, and it depends on the material. it was calculated comparing the ed values obtained respectively by means of alpha (241am in vacuum) and beta particle artificial irradiations. the measured alpha particle activity was also utilized to calculate the annual dose due to beta particles from uranium and thorium decay chains. the beta contribution of potassium to the annual dose was calculated after measuring its content in the baked clay samples by means of icp-oes (inductively coupled plasma-optical emission spectroscopy) at chemistry department, university of torino by considering the isotopic abundance of the 40k isotope. the measurement of the environmental dose in situ (gamma dose from the soil and cosmic radiation) has been performed with two series of lif dosimeters, positioned and buried in proximity of the kiln’s wall for a period of 90 days, before sampling. a value of 1210±150 µgy/year was estimated. finally, the annual dose contributions for each sample were corrected for the water content; the saturation content (w) has been evaluated by means of a sartorius ma35m thermobalance; the fraction of saturation was arbitrarily considered f = (80 ± 20)% [aitken 1985]. radon escape is supposed to be absent. all the results on annual dose have been calculated using doserate conversion factors [guérin 2011] and are summarized in table 2. 4.3. thermoluminesce dating results considering all the measurements described above, the age of the last heating of the kiln has been calculated. sample tl1 is dated 1690 ± 25 ad and sample tl2 is dated 1640 ± 40 ad (table 2). due to the macroscopic differences between the two cores and the impossibility to verify the absence of radon escape and anomalous fading for long periods, the expanded uncertainty was adopted to establish the range of possible ages, considering a 95% confidence level (2-sigma) for each measurement. therefore, the proposed thermoluminescence dating interval for the last firing of the kiln is 1560–1740 ad. this is in good agreement with the archaeological evidence and coincides with the second dating interval obtained by the archaeomagnetic analysis. 5. discussion and conclusions in this study, a multi-disciplinary approach has been applied for dating a rescue excavation kiln discovered at chieri, combining archaeological, archaeomagnetic and thermoluminescence analysis. from archaeomagnetic dating, two possible dating intervals (808–994 ad and 1535–1606 ad) are proposed; when archaeological evidences are taken into account, the first of the two ages can be excluded, thus suggesting that the kiln was for last time used in 1535–1606 ad. furthermore, the tl results suggest that the last firing of the kiln occurred around 1560–1740 ad. both archaeomagnetic and tl results are consistent (figure 9) and in accordance with the archaeological information available for the site, suggesting that most probably the kiln was abandoned at the beginning of the 17th century (even though tl results do not exclude its use up to 1740 ad). this study confirms that both archaeomagnetic and thermoluminescence techniques can be successfully applied on baked clay archaeological structures [becker et al. 1994, schnepp et al. 2003, tema et al. 2013]. however, at the same time it shows that different dating techniques, even when applied to the same material, can give somewhat different results. this suggests that a cross-checking of dating methods is necessary for precise dating of an archaeological site. considering that the largest part of rescue excavations get destroyed in a short time after their discovery, this study also aims to encourage the collaboration between archaeologists and physics scientists in order to obtain the maximum number of information from a rescue archaeological site before it gets destroyed. even in the cases that financial support is not immediately available, the systematic collection of samples by specialists is still very important. a properly performed sampling campaign offers the possibility of preserving important material and makes possible performing further investigations in the future. in this way, precious information about our cultural heritage, often preserved underground for thousands of years, will not get permanently lost even when the archaeological structures themselves could not be preserved. acknowledgements. the archaeologist valentina cabiale (ft studio) is sincerely acknowledged for her help during sampling, for the planimetry of the site, and for fruitful discussion on the archaeological findings. tema et al. 8 figure 9. archaeomagnetic and thermoluminescence dating results of the kiln excavated at chieri. 9 references aitken, m.j. 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(1967). ac demagnetization of rocks: analysis of results, in: d. collinson, k. creer and s. runcorn (eds.), methods in paleomagnetism, elsevier, new york, 254-256. *corresponding author: evdokia tema, università di torino, dipartimento di scienze della terra, torino, italy; email: evdokia.tema@unito.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. archaeometric dating of a baked clay kiln << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice adg vol5 n02 mak 303_311.pdf annals of geophysics, vol. 45, n. 2, april 2002 303 mt-data analysis from a survey in the mygdonia basin (n. greece) john p. makris (1), alexandros s. savvaidis (2) and filippos vallianatos (1) (1) department of electronics, technological educational institute of crete, chalepa, chania, crete island, greece (2) laboratory of geophysics, department of geology, aristotle university of thessaloniki, greece abstract a magnetotelluric (mt) profile comprising eight measuring sites was conducted in the mygdonia basin, a european seismological test site. the mt-stations were deployed in a direction almost perpendicular to the major axis of the sedimentary basin to cover the area between the lagada and volvi lakes. the earth beneath the measuring points was considered to consist of a 2d-regional basement coupled with local, near surface, 3d-inhomogeneities. impedance tensor decomposition analysis in conjunction with the magnetotelluric study of mohr circles was implemented in the mt-data. from the electric field polarization diagrams and the interpretation of the mohr circles results, it is inferred that the local structure at all the sites polarizes the electric field in comparable directions ranging from n60°e to n80°e, except for the cases of the two stations located at the northern and southern borders of the basin, where the boundary conditions produce a deviation. furthermore, the deeper structure seems to be two-dimensional. the strike-direction of the 2d-basement was found to be ~ n70°e. although the regional geological strike in the area is nw-se, the regional geoelectric strike is more compatible with the orientation of the faults in the area. 1. introduction in october 1995, eight magnetotelluric (mt) soundings were carried out in the mygdonia basin, a european seismological test site (euroseistest, 1995). they were arranged on a 6 km long profile with a spacing of about 1 km (fig. 1). the frequency range was 128-0.004 hz and nine channels were employed for each sounding. five of them served as base station, namely e x , e y , h x , h y , h z , and four as reference station, named e x r, e y r, h x r, h y r. the data were collected in eight frequency bands using different sampling rates. the last three low frequency bands overlap. the mt-measurements were conducted with the spam mkiii instrument (dawes, 1990), developed at the university of edinburgh. an up-to-date summary on the instrument used can be found in ritter et al. (1998). the area of study is located in central macedonia (fig. 1) and occupies the inner of the hellenic mountain belt, including the serbomacedonian massif (sm) to the east and the circum rhodope belt (crb) to the west (kauffman et al., 1976). the rocks of these zones consist of the pre-alpine and alpine basement of the area that has a nw-se strike similar to mailing address: dr. filippos vallianatos, technological educational institute of crete, department of natural resources engineering, laboratory of geophysics and natural hazards, 3 romanou st., chalepa, gr-73133 chania, crete island, greece; e-mail: fvallian@chania.teiher.gr key words magnetotelluric – tensor decomposition – mohr circles – mygdonia basin – greece 304 john p. makris, alexandros s. savvaidis and filippos vallianatos the general «dinaric direction» of the hellenic mountain belt. upon these rocks were deposited post-alpine sediments with alluvial deposits after a geotectonic event during neogene and quaternary on which a series of graben with e-w to nw-se trends were formed by normal faulting (premygdonia basin). at the end of early pleistocene, a new tectonic action faulted the premygdonia basin and a new graben of an e-w direction was framed, forming the mygdonia lake. the present lakes volvi and lagada are the remnants of the mygdonia lake. in previous works (savvaidis, 1998; savvaidis et al., 2000), the electromagnetic response of the sub-surface hemi-space is considered as unified and the impedance tensor is not decomposed. from the mt-polar diagrams of the impedance tensor elements a strike-direction pointing n65°e was determined; the inspection of the dimensionality indices calculated (skew, ellipticity) indicated that they are compatible with a 2d-structure, although the skew values for frequencies above 1 hz indicated a departure from this high order symmetry and the apparent resistivity curves for higher frequencies are not similar in all stations. the latter result is an indication for the presence of local near-surface inhomogeneities (vallianatos, 1995; makris et al., 1997). the main objective of this work is to present a multi-station extended magnetotelluric study of this area by means of the decomposition of the measured impedance tensor and the mt-representation of mohr circles. 2. impedance tensor decomposition small-scale, near-surface 3d-inhomogeneities act as semi-static scatterers of the regional electric field then the conventional mt-procedure fails to determine the correct principal impedances and the principal induction axes. in such a case, a decomposition of the measured impedance tensor fig. 1. rough geotectonic map of mygdonia basin (the area between the lakes lagada and volvi). the numbers shown identify the mt-sites of the survey. the bold line represents a theoretical mt-section onto which the actual locations of the measuring points are projected. the diamond sign (tst) indicates the position of the borehole and the dashed line denotes the seismic profile (jongmans et al., 1998). the right part of the figure is a simplified geotectonic map of greece and the continuous line box encloses the area of this study. 305 mt-data analysis from a survey in the mygdonia basin (n. greece) by introducing the complex «telluric vectors» in the measuring coordinate frame (bahr, 1988, 1991) and (2.4) where (x → ns, y → ew), two angular deviations of the telluric vectors e→ x and e→ y are determined in the coordinate frame of the regional geoelectric structure tan (β1) = – c2 /c4 and tan (β2 ) = c3 /c1 (2.5) hereafter termed «skew angles». in cases where the local inhomogeneities produce strong local channeling of the regionally induced electric currents, it is expected |β 2 – β 1 | ≈ 90°. furthermore, the decomposition procedure suggested by groom and bailey (groom, 1988; groom and bailey, 1988, 1989, 1991; groom et al., 1993) proceeds to decomposition also of the distortion tensor (2.6) where t ↔ is the twist distortion tensor, s ↔ is the shear distortion tensor, a ↔ is the anisotropy distortion tensor, n i are normalizing factors and g a real scalar characterizing the measuring site. then the measured impedance tensor is described may conduce to separate, if possible, local distortion parameters and regional 2d-parameters (bahr, 1988, 1991; groom, 1988; groom and bailey, 1989; groom and bahr, 1992; groom et al., 1993; chave and smith, 1994). consider a 2d-regional geoelectric structure, where (2.1) the impedance tensor that refers to its principal coordinate system. the scattering of the electric field by local inhomogeneities can be described by an impedance tensor in the following form: (2.2) where the distortion tensor c ↔ is real and frequency independent (at low frequencies) [superimposition model, 3d(local)/2d(regional)]. thus, in the principal axes system (and not in an arbitrary measuring coordinate system) the elements in each column of z ↔ m should have the same phase. a set of parameters is defined (bahr, 1988, 1991). the rotationally invariant scalar parameter, η, termed as the regional skew, which is a measure of lack of symmetry (if η > 0.3 then the regional structure is 3d), the rotationally invariant parameter, µ, which is a measure of the phase difference (thus an indicator of a regional 1dstructure), the conventional skew index, s, which in case of presence of distorting bodies or structures, depends only on the distortion tensor real elements (2.3) the ratio ξ = η/s represents the relative contribution of the local distortion to s, i.e. if ξ << 1 then s is totally determined by this distortion. t z b2 0 0d = − ( )α s z z z z c c c c xx yy xy yx 3 2 1 4 ≡ + − = − + r e z x z yx xx yx= +ˆ ˆ t tt t c gtsa= = t a gn t t n e e n s st e s = − + − =( ) ( ) ( )1 1 1 1 1 0 0 1 g t e s = +( ) +( ) +( ) ⋅ 1 1 12 2 2 t z c c c c bm 1 2 3 4 0 0 = − = α( )( ) c b c c b c 2 1 4 3 = − − α α ( ) r e z x z yy xy yy= +ˆ ˆ s te s e t s e t s te ⋅ +( ) −( ) −( ) −( ) +( ) +( ) −( ) +( ) 1 1 1 1 1 1 ( ) . 306 john p. makris, alexandros s. savvaidis and filippos vallianatos by the equation (2.7) with (2.8) where r̂ is the operator for the clockwise rotation of the measuring coordinate system, in order to coincide with the principal axes of the 2d-structure. the groom et al. (1993) methodology successfully determines the dimensionality of the dominant structure and recovers the regional impedance responses (in case, of course, where the regional structure can be characterized approximately as 1d or 2d). in addition, the principal directions of the regional induction are determined but with the ambiguity concerning the strike-direction. in cases of strong local channeling, for the shear-distortion parameter holds: |e| 1. the decomposition analyses, introduced by bahr and groom and bailey, were implemented = +( ) ( ) t z g s s b ' 2 0 1 1 0d ( ) = = t tt t t tt t z rgtsaz r rtsz rm t tˆ ˆ ˆ ˆ' 2 2d d fig. 2. dimensional and distortion parameters versus period determined by impedance tensor decomposition analyses (bahr, groom and bailey) of the mt-data from all the sites of the mt-survey. 307 mt-data analysis from a survey in the mygdonia basin (n. greece) in the mt-data collected at the eight measuring sites of the mt-survey (fig. 1). from the interpretation of the results (fig. 2) the following conclusions are drawn: i) the model of the geoelectric structure that better fits the low frequency mt-data is 2d-(regional)/3d(local). a constrained study of the high frequency data (e.g., 0.1 hz) concludes that cagniard (1953) and swift (1967) models of a 1d and 2d-conductivity distribution are valid (skew parameter, s, very small, see fig. 2). by combining them with models obtained by gravity-data (savvaidis, 1998; savvaidis et al., 2000) and seismic soundings (jongmans et al., 1998) and information gained from a borehole at the position tst (fig. 1), the upper geoelectrical layers and geological formations were successfully resolved (savvaidis, 1998; savvaidis et al., 2000). ii) the regional principal axes found to deviate from the measuring coordinate axes (x ns, y ew) ~ 70° clockwise. iii) moderate twist distortion (| t | < 1) of the regionally induced electric field is present at all the sites of the mt-profile. iv) strong shear distortion (local channeling) of the regionally induced electric field is a remarkable characteristic of the area (common at all the sites) due to the facts that the relevant shear-parameter, e, is close to unity and the skew angle addition, 2 – 1, found to be ~ 90° for periods greater than 9 s and for all the measuring sites (fig. 2). the latter results indicate that the measured electric field is linearly polarized. we draw the attention of the reader to the fact that this was independently verified by the observed polarization diagrams (fig. 3), which exhibit directional angles (local strikes) of the local channeling diversified from site to site (this is more evident for the cases of the two sites located at the northern and southern borders of the basin, due to geological conditions at its boundaries). this implies that the area under study is inhomogeneous. 3. mohr circles analysis mohr circles are commonly met in the analysis of mechanical stress but they can be used similarly to depict magnetotelluric impedance tensor information and to determine site 15 site 16 site 12 site 14 fig. 3. observed polarization diagrams of the electric field measured at the sites 15, 16, 12 and 14 of mygdonia mt-profile. at each diagram, the gradient of the trend-line corresponds to the local polarization angle (deviating clockwise from ns-direction) of the electric field at the respective site. this calculation was done for all the measuring sites. 308 john p. makris, alexandros s. savvaidis and filippos vallianatos the characteristic strike-directions in local and/or regional scale (lilley et al., 1989; lilley, 1993a,b; makris, 1997; makris et al., 1997, 1999). figure 4 depicts, for various frequencies, the mohr circles z xx ( ) versus z xy ( ) – solid line circle groups – constructed separately for the real and imaginary parts, for site 12 of mygdonia basin (similar diagrams were constructed for the rest of the sites of the mt-survey). especially for the low frequency range (e.g., 0.1 hz), these circles pass very close to or approximately through the origin for all the measuring sites and independently of the consideration of either the real or the imaginary parts of the impedance tensor elements. this result implies that irrespectively of the polarization of the incident magnetic field, the electric field is linearly polarized in the direction of the y ( l )-axis. the directional angles (local strikes) obtained by this analysis (fig. 5) are in agreement with the gradients of the trend-lines of the relevant observed electric field polarization diagrams for each site (fig. 3). a conjugate form of the aforementioned mohr circles was introduced by makris (makris, 1997; makris et al., 1997, 1999). the relevant mohr circles for site 12 are also depicted in fig. 4 (dashed-line circle groups). they exhibit the same characteristics, i.e. again they pass very close to or approximately through the origin for all the measuring sites and independently of the consideration of either the real or the imaginary parts. this result refers to the h-polarization mode of an ideal 2d-regional geoelectric structure (e.g., a vertical boundary), if the measuring site lies on the conductive medium but close to the fig. 4a,b. mohr-circles for various periods, by taking (a) the real and (b) the imaginary parts respectively of the z xx and z xy impedance tensor elements [left swarm of circles at diagrams (a) and (b)] and of the z xx and z yx impedance tensor elements [right swarm of circles at diagrams (a) and (b)], constructed using the mt-data from site 12 of mygdonia area. [cf., a line connects the center with the respective first point (which corresponds to a rotation angle equal to 0°) of each circle. also, the last two points, which correspond to the rotation angles 179° and 180° respectively, are intentionally omitted in order to depict the counterclockwise way of circle construction]. real imaginaryz'xx z'yx z'xy z'xx z'yx z'xy area 12 f=71hz f = 35hz f =11hz f=4hz f=2hz f =0.9hz f = 0 .3hz f = 0 .1hz f = 0 .04hz f =0.02hz f = 0 .01hz f = 0 .005hz a b 309 mt-data analysis from a survey in the mygdonia basin (n. greece) resistivity contrast (fischer et al., 1992). the directional angle (regional strike) was found to be approximately n70°e, at all the measuring sites for periods above 0.1 s, but for periods higher than 3 s it deviates clockwise tending to coincide with the dominant geological strike, i.e. ~ n120°e (fig. 5). 4. discussion in this work, the impedance tensors, measured at all the sites of the mt-survey, were decomposed using bahr’s and groom’s and bailey’s procedures, which consider the model of a regional 2d-structure where 3d, fig. 5. directional angles (clockwise from ns-direction) of local strike, l and regional strike, r versus period. they were determined from the mohr-circles, which have been constructed using the mt-data (by considering the real parts of the impedance tensor elements) for each one of the sites of the mygdonia basin survey. 310 john p. makris, alexandros s. savvaidis and filippos vallianatos or 2d-inhomogeneities are embedded (superimposition model). the study of the dimensionality parameter µ indicated that the regional structure could not be characterized as 1d (µ > 0.1 throughout the investigated frequency range and for all the measuring sites). the regional skew, (a measure of the departure of the mt-data from the superimposition model) has values: 0.05 < < 0.3 for almost all periods, thus indicating that such a model is appropriate to fit the mtdata from all the sites. furthermore, the decomposition analysis indicated the existence of an inhomogeneous structure that produces strong channeling for periods equal to or greater than 9 s. the latter was confirmed from the observed electric field polarization diagrams that exhibit linear polarization. the scalar parameter obtains for these periods very small values and this result suggests that the conventional skew, s, is totally determined by the distorting structure. the principal axis system of the underlying regional 2d-structure found to deviate from the measuring coordinate system (x ns, y ew) by approximately 70° clockwise. the mt-study of mygdonia basin based on the magnetotelluric representation of mohr circles provided an independent verification of the aforementioned results and resolved the ambiguity concerning the strike-direction of the 2d-structure. the results were obtained mainly from the real parts because the experimental error was much smaller. from the conventional mohr circles resulted the local strike direction at each site ranging from n60°e to n80°e, except for the cases of the two sites located at the northern and southern borders of the basin, where the boundary conditions produce a deviation. from the conjugate mohr circles, the regional strike direction for all the sites was found to be n70°e ± 10°. it is underscored that although the regional geological strike points nw-se, the regional geoelectric strike is more compatible with the orientation of the faults in the area. at all the measuring sites and for periods higher than 3 s, the regional geoelectric strike deviates clockwise tending to coincide with the dominant geological strike. references bahr, k. 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(1988): the effects of inhomogeneities on magnetotellurics, ph.d. thesis, university of toronto, canada. groom, r.w. and r.c. bailey (1989): decomposition of magnetotelluric impedance tensors in the presence of local three-dimensional galvanic distortion, j. geophys. res., 94, 1913-1925. groom, r.w. and k. bahr (1992): corrections for near surface effects: decomposition of magnetotelluric impedance tensor and scaling corrections for regional resistivities: a tutorial, surv. geophys., 13, 341-379. groom, r.w., d.r. kurtz, g.a. jones and e.d. boemer (1993): a quantitative methodology to extract regional magnetotelluric impedances and determine the dimension of the conductivity structure, geophys. j. int., 115, 1095-1118. jongmans, d., k. pitalakis, d. demanet, d. raptakis,c. horrent, g. tsokas, k. lontzetidis and j. riepl (1998): euroseistest: determination of the geological structure of the volvi graben and validation of the basin response to one earthquake and one shot, bull. seismol. soc., 2 (1), 3-14. kauffman, g., f. kockel and h. mollat (1976): notes on the stratigraphic and paleogeographic position of the svou1a-formation in the innermost zone of the hellenides (northern greece), bull. soc. geol. fr., 18, 225-230. lilley, t. 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(1998): geophysical study of the northwestern part of chalkidiki peninsula, ph.d. thesis, dept. of geology, aristotle university of thessaloniki, greece. savvaidis, a.s., l.b. pedersen, g.n. tsokas and g.j. dawes (2000): structure of the mygdonia basin (n.greece) inferred from mt and gravity data, tectonophysics, 317, 171-186. swift, m.c. jr. (1967): a magnetotelluric investigation of an electrical conductivity anomaly in the southwestern united states, ph.d. thesis, mass. inst. of tech., ma., u.s.a. vallianatos, f. (1995): galvanic distortion of the mtimpedance tensor: application to ioannina vanstation (greece), acta geophys. pol., 42 (2), p. 115. 404 not found a t t i v i t à ' d e l l ' e t n a n e g l i a n n i 1 9 1 3 1 9 4 6 g . p o n t e r i a " l i m i a m o i f e n o m e n i più i m p o r t a n t i t h e si s o n o m a n i f e s t a t i s u l l ' e t n a d u r a n t e il q u a d r i e n n i o 1 9 4 3 4 6 . d o p o l ' e r u z i o n e d e l 5 l u g l i o 1 9 4 2 il c r a t e r e c e n t r a l e r e s t ò p e r f e t t a m e n t e c a l m o . l a lava c l i c e r a s t a t a e r u t t a t a a f o n t a n a e d il m a t e r i a l e n o n c o e v o p r o i e t t a t o d u r a n t e l ' a t t i v i t à p l i n i a n a a v e v a n o c o p e r t o e l i v e l l a t o l a t e r r a z z a c r a t e r i c a c e n t r a l e ove si n o t a v a n o d e l l e s p a c . c a t u r e d i r e t t e v e r s o i l c o n o i n l e r c r a t e r i e o d e m o l i t o . l a b o c c a s u b t e r m i n a l e d i n e , i n v e c e , e r a r i e n t r a t a i n a t t i v i t à e s p l o s i v a m o d e r a t a , c h e d i v e n n e f o r t e d u r a n t e t u t t o il m e s e di d i c e m b r e e spesso f u \ i s t a la n e v e , l i n o a l l a \ a l l e d e l b o v e , o s c u r a t a d a l l a c e n e r e c a d u t a . n e l 1 9 4 3 , m a l g r a d o l a g u e r r a ci t e n e s s e i n g r a n d e a l l a r m e , n o n f u r o n o t r a l a s c i a t e le o s s e r v a z i o n i q u o t i d i a n e c l i c f u r o n o f a t t e da c a t a n i a f i n o a t u t t o il m e s e di a p r i l e e da t a o r m i n a f i n o ai p r i m i di a g o s t o . d o p o l ' o c c u p a z i o n e a n g l o a m e r i c a n a d e l l a s i c i l i a f u r o n o r i p r e s e l e o s s e r v a z i o n i da c a t a n i a , m e n t r e il c u s t o d e d e l l ' o s s e r v a t o r i o e t n e o , \ i n c c i i z o b a r b a g a l l o , r i m a s t o a l l a s t a z i o n e c a n t o n i e r a d u r a n t e t u t t o il p e r i o d o d e l l ' e m e r g e n z a , e b b e o c c a s i o n e di v i s i t a r e v a r i e v o l t e il c r a t e r e c e n t r a l e e l a b o c c a s u b t e r m i n a l e di n e c l i c si m a n t e n n e r o a m b e d u e c a l m e . n e l g e n n a i o 1 9 4 4 n u l l a di n o t e v o l e fu visto s u l l ' e t n a . l a m a t t i n a d e l l ' l l f e b b r a i o a p p a r v e s u l l a n e v e d e l c o n o t e r m i n a l e u n f o r t e i n t o r b i d a m e n t o p e r l a c e n e r e i v i c a d u t a d u r a n t e la n o t t e e c h e vi e r a s t a t a l a n c i a t a da e s p l o s i o n i a v v e n u t e n e l l a b o c c a d i n e ; d i f a t t i i l 2 7 f e b b r a i o il c u s t o d e d e l l ' o s s e r v a t o r i o , e l l e la v i s i t ò , la t r o v ò i n f o r t e a t t i v i t à e s p l o s i v a c o n l a n c i o di s c o r i e r o v e n t i p r o v e n i e n t i da d u e c r a t e r i . t a l e a t t i v i t à si p r o t r a s s e fino a l l ' a p r i l e . i n m a g g i o r i t o r n ò la c a l m a . l a t e r r a z z a c r a t e r i c a c e n t r a l e , a l l o r a , p r e s e n t a v a u n t r a t t o d e l s u o l o a b b a s s a t o di c i r c a 6 m sul i a t o s w e si v e d e v a n o d e l l e s p a c c a t u r e c l i c d e l i m i t a v a n o l a p a r t e s p r o f o n d a t a c h e si e s t e n d e v a f i n o al p o s t o da d o v e il 5 l u g l i o 1 9 4 2 e r a v e n u t a f u o r i l a l a v a a f o n t a n a di c u i a b b i a m o f a t t o c e n n o . f i n o a l l a m e t à d e l s e t t e m b r e 1 9 4 4 il c r a t e r e c e n t r a l e e la b o c c a 3 4 2 g . i ' o n t e i i i b l c r r n i n a l c -i m a n t e n n e r o c a l m i ; il 15 a v v e n n e r o delle e s p l o s i o n i , p r o h a b i l m e n l e d a l l a b o c c a u d d e t l a e g i u n s e fino a c a t a n i a della cen e r e f i n i s s i m a . i n s e g u i l o l u l l o r e s t ò c a l m o fino al giugno 1 9 4 5 . nei p r i m i di q u e s t o m e s e r i p i g l i ò i n t e n s a l ' a t t i v i t à e s p l o s i v a alla hocca . u b l e r i n i n a l c ; le c o l i e v e n i v a n o l a n c i a t e da due c r a t e r i g e m e l l i fino a l l ' a l t e z z a d e l l ' o r l o del s o p r a s t a n t e c r a t e r e v e n t r a l e . l a t t i v i t à esplosiva c o n t i n u ò c o n i n t e r v a l l i vari di q u i e t e fino al m e s e di o t t o b r e , c o m e p o t è r i s c o n t r a r e il c u s t o d e d e l l ' o s s e r v a t o r i o d u r a n t e v a r i e v i s i t e l a t t i ' al c r a t e r e c e n t r a l e o v e , il g i o r n o 2 8 o t t o b r e , osservò c h e si e r a a p e r t a una b u c a di s p r o f o n d a m e n t o nel s e t t o r e v k d e l l a t e r r a z z a c r a t e r i c a il cui d i a m e t r o a l l o r a e r a di c i r c a iti m . da q u e s t a b u c a e s a l a v a n o p o c h i v a p o r i s o l f o r o s i . .nel n o v e m b r e e nel d i c e m b r e 1 9 1 5 e p e r t u t t o il g e n n a i o del 1946 i e t n a r e s t ò c a l m o . la m a t t i n a del l f e b b r a i o la lieve c h e c o p r i v a il v e r s a n t e s e del v u l c a n o a p p a r v e l i e v e m e n t e i n t o r b i d a t a e il g i o r n o e g u e n t e la p a r t e o s c u r a t a si fece più m a r c a t a e si estese lino a l l a \ a l l e del l e o n e . nei mesi seguenti si poi»; n o t a r e da c a t a n i a una l i e v e a t t i v i t à alla b o c c a s u b t e r m i n a l e , c h e fu c o n f e r m a t a d a l l e o s s e r v a z i o n i l'atte dal cus t o d e in v a r i e v i s i t e al c r a t e r e . il 2 3 luglio 1916 l'assis|enle d e l l ' i s t i t u t o di v u l c a n o l o g i a , d o t t . cueuzza. si r e c ò s u l l ' e t n a a c c o m p a g n a l o dal c u s t o d e b a r b a g a l l o e potè o s s e r v a r e dall o r l o sud del c r a t e r e c e n t r a l e c h e la b u c a di s p r o f o n d a m e n t o aveva f o r m a c i r c o l a r e del d i a m e t r o di c i r c a 15 m : le sue p a r e t i s c e n d e v a n o a p i c c o ed un l i e v i s s i m o f u m o e s a l a v a d a l l a sua p a r t e p r o f o n d a . l a b o c c a s u b l e r m i n a l e era a t t i v i s s i m a : d e l l e s c o r i e roventi v e n i v a n o l a n c i a t e fino a l l ' a l t e z z a del c r a t e r e c e n t r a l e fra a b b o n d a n t i s s i m o f u m o s o l f o r o s o . s a l v o q u a l c h e g i o r n o d ' i n t e n s i s s i m a a t t i v i t à e s p l o s i v a , c o m e il 9 s e t t e m b r e , c h e f u r o n o visti b a g l i o r i da c a t a n i a d u r a n t e la n o t t e , la b o c c a s u b l e r m i n a l e si m a n t e n n e in m o d e r a t a a t t i v i t à e s p l o s i v a e co-i c o n t i n u ò p e r lutto il m e s e di o t t o b r e . nel n o v e m b r e 1 9 1 6 la c i m a d e l l ' e t n a restò q u a i s e m p r e c o p e r t a . ii g i o r n o 6 si p r e s e n t ò , tallio al c r a t e r e c e n t r a l e c h e alla b o c c a subl e r m i n a l e di .ne. un bel p e n n a c c h i o di f u m o b i a n c o , c h e fu vi-to a n c h e il 2 7 . nei p o c h i giorni del d i c e m b r e c h e l ' e t n a restò s c o p e r t o il f u m o a p p a r v e s o l t a n t o sulla b o c c a di ne con volute i n t e r m i t t e n t i , s e g n o c a i ' a t l e r i t i c o di a t t i v i t à e s p l o s i v a . da (pianto a b b i a m o riassunto dei f e n o m e n i osservati s u l l ' e t n a nel a t t i v i t à d e l l ' e t n a n e g l i a n n i 1 9 4 3 1 9 4 6 3 1 q u a d r i e n n i o 1 9 4 3 1 9 4 6 il v u l c a n o -i è p r e s e n t a t o p e r lo p i ù in at t ività e s p l o s i v a a l l a b o c c a s u b l e r m i n a l e di n e c h e trovasi a un l i v e l l o n o t e v o l m e n t e più basso d e l l a terrazza c r a t e r i c a c e n t r a l e . d a l l a b u c a di s p r o f o n d a m e n t o c e n t r a l e r a r a m e n t e è a p p a r s o del f u m o e solo il i ~> n o v e m b r e e l ' i ! giugno 1 9 4 6 . d u r a n t e u n a e c c e z i o n a l e c a l m a a t m o s f e r i c a , f u r o n o vis|i f|uc a l t i f u m a i u o l i sulla c i m a del v u l c a n o , u n o e s i l e p r o v e n i e n t e d a l l a b u c a del c r a t e r e c e n t r a l e e l ' a l t r o m o l t o più gl'omo e più denso d e l l a b o c c a s u b l e r m i n a l e . n e l l e g i o r n a t e v e n t o s e , c h e s u l l ' e t n a sono le più f r e q u e n t i , i due f u m a i u o l i a p p a r i v a n o confusi in u n i c o p e n n a c c h i o . catania — istillilo i ulcanologico etneo dcu'lnivcrsilà — 1917. i l t e r r e m o t o d e l t u r k e s t a n det. 2 n o v e m b r e 1 9 4 6 l\ calci f . peronaci 11 2. n o v e m b r e 1916 fu r e n i < l r a i a r o m a u n a v i o l e n t a scossa «li t e r r e m o t o con i n i z i o a l l e o r e 181, 36"' 3.v c a . t . m . ( i r . i n a p r i m a a p p r o s i m a l i v a v a l u t a z i o n e r i p e r m i s e di l o c a l i z z a r e l ' e p i c e n t r o nel t u r k e s t a n . z o n a ad a l l a s i s m i c i t à , t r i s t e m e n t e n o t a p e r p r e c e d e n t i c a l a s t r o f i s i s m i c h e . n o t i z i e s u c c e s s i v e c o n f e r m a r o n o la z o n a di p r o v e n i e n z a , p r e c i s a n d o elio la scossa e r a s t a t a p a r t i c o l a r m e n t e d i s t r u t t i v a n e i d i s l r e l l i d e l l l s b c k i s l a n e d e l k i r g h i s i i a n . l e s i n g o l a r i c a r a t l e r i s t i i ile d e l l e r e g i s t r a z i o n i o t t e n u t e a r o m a , in cui a lisi i n i z i a l i di a n o r m a l e a m p i e z z a , f a c e v a n o r i s c o n t r o o n d e t r a s v e r s a l i di l i e v e e n t i t à , u n i t e a l l a p r e s e n z a di c h i a r i e s e m p i di o n d a t i p o c. ci h a n n o i n d o t t o a s o t t o p o r r e il t e r r e m o t o a c c e n n a t o ad u n o s t u d i o p a r t i c o l a r e . l e r i c h i e s t e di s i s m o g r a m m i , i n o l t r a t e in t u t t o il m o n d o , h a n n o a v u t o s o l o p a r z i a l e a c c o g l i m e n t o : p a r t i c o l a r m e n t e l a m e n t a t o , ai fini d e l l a r i c e r c a , il m a n c a t o i n v i o di s i s m o g r a m m i d a p a r t e d e l l e s t a z i o n i d e l l a r e t e r u s s a ( l ' a t t a t u ì i x i . v stazioni a (qradi) p p p h o r o i n i z i o [ e m p i c p r o p a g . p per h = 6 2 k r n o r a origine p'c a per b e l g r a d o 3 7 ° , 0 3 1 35" .', 1 ' ' 2 8 m 3 5 ' 1 7 m 0 5 ® s 3 6 ° , 6 2 u p i a l a 3 6 , 7 9 8 1 0 , 7 5 6 3 5 4 6 , 3 7 0 5 . 2 4 1 . 1 7 0 8 . 4 3 7 . 1 8 c o p e n a g h e n 3 9 , 5 0 5 1 6 . " 6 0 3 6 0 9 . 7 7 2 7 , 1 4 ? , 6 7 3 1 . 8 4 0 , 0 8 1 praga 3 0 . 6 3 2 14.8 7 6 3 6 0 5 . 2 7 2 8 . 3 7 . 2 7 2 7 . 3 3 9 , 5 3 | t r i e i t e •il , 2 8 2 1 3 . 3 6 2 36 1 8 , 7 7 4 1 , 6 3 7 , 1 7 4 0 , 7 4 1 , 1 7 1 r o m a 1 3 , 1 5 0 11,6 5 7 3 6 3 4 , 6 7 5 8 , 5 3 6 . 1 7 5 6 . 7 4 3 , 2 3 i p r o t o •13 . 6 6 6 1 2 . 5 6 0 3 6 3 7 . 5 8 0 0 , 2 3 7 , 3 7 5 9 , 6 4 3 . 6 0 : c a t a n i a •13 , ' 1 3 0 1 3 . 6 6 2 3 6 3 5 , 2 7 5 8 , 8 3 6 . 4 7 5 7 . 3 4 3 . 2 6 i z u r r g o 4 4 , 0 9 7 1 4 , 3 6 8 3 6 41 .3 8 0 3 . 5 3 7 . 6 8 0 3 , 4 4 4 . 0 7 : b a i i l c a 1 1 , 6 7 6 1 0 . 4 5 0 3 6 4 5 . 3 8 0 8 . 1 3 7 . 2 6 0 7 , 3 4 4 . 5 7 n c u c h à f e l 1 5 . 2 6 i 1 2 . 5 5 7 3 6 1 9 . 5 8 12.6 3 6 . 9 8 1 1 . 6 4 5 . 1 3 u c c i e 1 5 . 5 9 9 1 2 , 5 6 3 6 5 5 , 8 1 5 , 0 4 0 , 0 8 1 7 , 1 4 5 , 8 7 m e d i a 6 2 . 2 k m m e d i a 2 8 m 3 7 * 9 e c c e z i o n e p e r cpiclli di s v e r d l o v s k ) . c o m u n q u e , a v u t i a d i s p o s i z i o n e i d i a g r a m m i di u n a q u a r a n t i n a d i s t a z i o n i sis m i c h e , n e l l a m a g g i o r a n z a e u r o p e e , ci s i a mo p r e o c c u p a t i , p e r p r i m a c o s a , di p r o c e d e r e a d u n ' a c c u r a t a d e t e r m i n a z i o n e d e l l e c o o r d i n a l e i p o e e n l r a l i . data la m a n c a n z a di d a t i d i s t a z i o n i i l t e r r e m o t o deit u r k e s t a n dei. 2 n o v e m b r e 1 9 4 6 2 1 7 p r o s s i m e a l i e p i c e n t r o , lo s c o p o p o t e v a e s s e r e r a g g i u n t o solo r i c o r r e n d o a m e t o d i c l i c si v a l e s s e r o d e i t e m p i d e l l e o n d e /'„ o di q u e l l i d e l l e o n d e l ' n e s „ . a b b i a m o r i t e n u t o di non f a r uso del noto m e t o d o di g e i g e r , a n c h e p e r c h é t a l e m e t o d o p r e s c i n d e d a l v a l o r e d e l l a prof o n d i t a , c h e . l i t i caso in e s a m e , a p p a r i v a un p o ' s u p e r i o r e a l i a n o r m a l e . a b b i a m o p r e f e r i t o , nel c o m p u t o d e f i n i t i v o , r i s o l v e r e il p r o b l e m a i n t r o d u c e n d o c o n t e m p o r a n e a m e n t e le q u a t t r o g r a n d e z z e i n c o g n i t e in g i u o c o c o o r d i n a l e e p i e e n t r a i i . p r o f o n d i t à e t e m p o o r i g i n e c o n f o r m e m e n t e a q u a n t o f e c e r o a l t r i r i c e r c a t o r i , s e g u e n d o m e t o d i d i v e r s i . 1 il p r i m o v a l o r e a p p r o s s i m a t o d e l l e c o o r d i n a t e e p i e e n t r a i i fu d e t c r m i n a t o a p p l i c a n d o il m e t o d o d e l l a p i o i c z i o n e s t e r o g r a f i e a con i t e m p i d e l l e /'„ e d e l l e sn r e l a t i v i a l l e s t a z i o n i di r o m a , l p p s a l a . z u r i g o c c a r t u j a . r i t e n e n d o la p r o f o n d i t à i p o c e n t r a l e , c a l c o l a t a i n b a s e a l l e d i f f e r e n z e /)/"' 1' o t t e n u t e n e l l e p r e d e l l e s t a z i o n i , p a r i a 6 0 k m e a . l ' e p i c e n t r o p r o v v i s o r i o è r i s u l t a t o d i c o o r d i n a t e g e o g r a f i c h e <l0 = 4 3 " 1 0 ' nord , x0 = 72" 4 8 ' est data i i n c e r t e z z a , a l l a ( p i a l e si è g i à a c c e n n a t o , d e l l i n i z i o d e l l e o n d e ,s. q u a l e si o s s e r v a p e r q u a s i t u t t e le s t a z i o n i , a b b i a m o p r o c e d u t o a d un u l t e r i o r e a p p r o s s i m a z i o n e con n i t e m p i d e l l e l'„ o s s e r v a l i n e l l e sed u c i l i i 12 s t a z i o n i : b e l g r a d o , l pps a l a . ( c o p e n h a g e n , p r a g a . t r i e s t e , r o m a . p r a t o . c a t a n i a , z u r i g o , l ì a i i c a . n e u c h à t c l e l c e l e . d e t e r m i n a l i la p r o f o n d i t à d i 62 k m in b a s e a l l e d i f f e r e n z e />/' /> e a l l e d i s i a n z e a p p r o s . i m a t i v e g i à eonol'ig. 1 s c i n t e si è p r o c e d u t o a l l a d e t e r m i n a z i o n e d e l l ' o r a o r i g i n e p r o v v i s o r i a ( 18h 28'" 37",9) e a l l a r i d e t e r m i n a z i o n e d e l l e d i s i a n z e e p i e e n t r a i i a' ( v e d i t a b e l l a i ) . p e r l a d e t e r m i n a z i o n e d e l l e d i s t a n z e e p i e e n t r a i i -1 è f a t t o uso d e l l a b e n n o t a f o r m u l a eosa = sincp0 sincp -feosc('0 coscp cos(/.—/.„) | 1 ] s e i n d i c h i a m o con (x l ' a z i m u t d e l l ' e p i c e n t r o {e) r i s p e t t o a c i a s c u n a s t a z i o n e ( s ) d a l t r i a n g o l o s f e r i c o e!\s ( v e d i figura) c o n e g u e ; p . c a l o 1 " i \ p e k o n a c i cos cp s i n (à—à.) s 1 1 1 u = , sin a sinrn c o s c i . — sincpn cosip cos (à — /„) cos u = s i n a [2] p e r c u i . so 5fp0) òa„ r a p p r e s e n t a n o le c o l l e z i o n i da a p p o r t a r e a l l e c o o r d i n a t e p r o v v i s o r i e d e l l ' e p i c e n t r o , p e r a d a t t a r s i a l l e n u o v e dis t a n z e e p i e e n t r a l i c a l c o l a t e , p o i c h é d e t t e c o r r e z i o n i possono r i t e n e r s i s u f f i c i e n t e m e n t e p i c c o l e , a v r e m o con l ' a u s i l i o d e l l a [ l | e d e l l e [ 2 | 5 a — cos a . 6cp0 — cos cp0 s i n a . b).a [ 3 ] l ' a p p l i c a z i o n e d e l l a [ 3 ] a l l e d o d i c i s t a z i o n i a s s e g n a t e c o n d u c e a 12 e q u a z i o n i di c o n d i z i o n e , r i s o l t e le q u a l i con il m e t o d o d e i m i n i m i q u a d r a t i si o t t e n g o n o i s e g u e n t i v a l o r i p i ì i p r o b a b i l i d e l l e i n c o g n i t e , con r e l a t i v i e r r o r i m e d i : òrp0 = — 1 ° , 2 0 8 0 0 , 2 7 , b l n = + 0 ° , 9 1 0 3 0 , 2 1 le n u o v e c o o r d i n a t e e p i e e n t r a l i cp0 = 4 p 5 7 ' , 5 0°,27 , a0 = 71" 5 3 ' , 4 0 , 2 1 ci c o n s e n t o n o una n u o v a d e t e r m i n a z i o n e d e l l e d i s t a n z e e p i e e n t r a l i e q u i n d i , s e m p r e s u l l a b a s e d e l l a p r o f o n d i t à i p o c e n t r a l e d i (i2 k m e v a l e n d o s i d e l l e t a v o l e d e i t e m p i di t r a g i t t o c a l c o l a t e d a j e f f r e y s ( 1 9 1 0 ) . un n u o v o v a l o r e p e r il t e m p o o r i g i n e del t e r r e m o t o . i c a l c o l i sono s i a l i l i m i t a t i a 23 s t a z i o n i . o s s e r v i a m o a q u e s t o p u n t o c h e . >e i n d i c h i a m o con <„, à„, i| „ e /;,,. i valori, a p p r o s s i m a t i d e l l e i n c o g n i t e , con òl„, òa„, 8(jp„ e òh„ l e p i c c o l c c o r r e z i o n i da c a l c o l a r e con il m e t o d o d e i m i n i m i q u a d r a t i p e r o t t e n e r e i v a l o r i p i ù p r o b a b i l i d e l l e i n c o g n i t e , e se t„ r a p p r e s e n t a il t e m p o di p r o p a g a z i o n e d e l l e o n d e jpn> l i m i t a n d o l o s v i l u p p o i n s e r i e a i t e r m i n i di p r i m o o r d i n e , in ò?.„, 6cp„, 5/i„, s u p p o s t i sufficient e m e n t e p i c c o l i , si h a ' = / ( * . , t . a ) + [ 4 ] s k 3fpo 3 / jo n e l l a [ 4 ] f (kol cp0j h„) è il t e m p o di p r o p a g a z i o n e di p„ c l i c corr i s p o n d e a l l a d i s t a n z a c p i c e n t r a l e d e l l a s t a z i o n e c o n s i d e r a t a , q u a l e r i s u l t a ili b a s e ai v a l o r i a p p r o s s i m a t i d i rp„; m e n t r e 5//s/'.. -i ott i e n e d a l l e d r o m o c r o n e , r i t e n e n d o c o s t a n t e a n e l v a l o r e \0 e asseg n a n d o ad li ( a p a r t i r e d a /i„) u n i n c r e m e n t o p a r i a l l ' u n i t à d e l l a d i o i l t e r r e m o t o deit u r k e s t a n dei. 2 n o v e m b r e 1 9 4 6 217 i n o e r o n a p r e s c e l t a ( n e l n o s t r o caso 0.(11 del r a g g i o t e r r e s t r e , s u p p o s t a 'a terra p r i v a d e l l a c r o s t a r i t e n u t a di spessore m e d i o p a r i a 33 k m e a . ) . he d a r à il c o r r i s p o n d e n t e i n c r e m e n t o p o s i t i v o o n e g a t i v o del t e m p o di t r a g i t t o . s i p u ò s c r i v e r e , i n o l t r e , c o m e si usa l'are nel m e t o d o di g e i g e r . 3 / = d f _ dj_ £\„ dlb sa,, sà0 ' s'i o sa,, 3cp0 ' in ( ui a è la d i s t a n z a e p i c e n t r a l e c o r r i s p o n d e n t e a l l a s t a z i o n e eon3f s i d e r a t a di c o o r d i n a t e /.„. ((>„. la d e r i v a t a p a r z i a l e si c a l c o l a p e r sa,, mezzo d e l l e d r o i n o c r o n e , s u p p o n e n d o c o s t a n t e h ( i n h„) e d a n d o a a„ ] i n c r e m e n t o di 1° p e r a v e r e q u e l l o c o r r i s p o n d e n t e del t e m p o di sa 3 \ t r a g i t t o . p e r q u a n t o c o n c e r n e le d e r i v a l e — " —" , esse sono g i à 3<fo s t a t e c a l c o l a l e e v a l g o n o sa,, . sa,, = — cos(p0 sino. , = — c o s a . [ 6 j ^•o s , f o i n d i c a n d o a l l o r a con (p„) i t e m p i d i p„ r e g i s t r a l i n e l l e s t a z i o n i di c o o r d i n a t e }.„, (p„, p r e s c i n d e n d o d a e r r o r i di o s s e r v a z i o n e c di r e g i t r a z i o n c . d e t t i t e m p i r i s u l t e r a n n o d a l l a s o m m a d e l l ' o r a i n i z i a l e del t e r r e m o t o e del t e m p o di t r a g i t t o c o r r i s p o n d e n t e , p e r c u i . c o n s i d e r a n d o le j l ] . [ 5 ] , [ ( ' ] v a r r à 1 e q u a z i o n e s f _ s f òtn — cos i( 0 s u i a . ò/.0 cos a • . ò(| 0 + sa„ sa,, p. r j . bh 0 = (p„) t0 / ( à 0 cr0.7?0). [ 7 ] ' lie è l ' e q u a z i o n e di c o n d i z i o n e r e l a t i v a a l l a s t a z i o n e c o n s i d e r a t a , di d i s t a n z a e p i c e n t r a l e a„ ; il s e c o n d o m e m b r o è c o s t i t u i t o d a (pianti tà n o t e . la [ 7 ] n a t u r a l m e n t e va r i p e t u t a p e r tutto le stazioni e il s i s t e m a « h e così -i o t t i e n e , r i s o l t o con il m e t o d o dei m i n i m i q u a d r a l i . i ila p r i m a a p p l i c a z i o n e d e l l a | 7 ] fu f a t t a a 23 s t a z i o n i : i d a t i r e l a t i v i sono i n p a r t e c o n t e n u t i n e l l a t a b e l l a 11. l ' a p p l i c a z i o n e del m e t o d o dei m i n i m i q u a d r a t i a l l e 23 e q u a z i o n i di c o n d i z i o n e c h e ne. 2 5 0 p . c a l o ! " f . peronaci t a b e l l a 1 1 [ a ( g r a d i ) s e n a c o s a t e m p o d i ^ a n oec) a f / t n 1 ! s t a z i o n i a ( g r a d i ) s e n a v. c o s a r e g i s f r a z . t r a g i t t o o r i g i n e ^ a n oec) / a h 1 5cc.1 m w l • b e l g r a d o 36° 51',6 0 , 9 2 4 5 6 4 2 9 2 ° 23 ' 9 + 0 , 3 8 1 0 4 3 5 m 4 l : ; 7 7 m 0 3 * 7 2 6 m 3 8 3 0 8 s , 3 5 — 6 , 3 0 3 5 " "42*2 t r i e s t e 41 11.7 0 , 9 0 1 5 3 2 9 5 3 8 , 3 + 0 , 4 3 2 6 9 3 6 , 1 8 . 7 7' 3 9 , 8 3 8 , 5 6 ,20 — 6,40 36 1 8 , 3 r o m a 43 1 4 , 6 0 , 9 3 4 6 5 2 9 0 4 7 , 6 + 0 , 3 5 5 0 7 36 3 4 , 6 7 5 6 , 6 3 8 , 0 8 ,05 6 , 4 5 36 3 5 , 1 ; c a t a n i a 4 3 0 6 , 5 0 , 9 7 1 3 2 2 8 3 4 5 , 3 + 0 , 2 3 7 7 7 3 6 3 5 , 2 7 5 5 , 5 3 9 , 7 8 , 0 5 6 , 4 5 36 3 4 , 0 z u r i g o 4 4 0 6 , 5 0 , 8 6 9 5 2 2 9 9 3 5 , 6 + 0 , 4 9 3 8 9 3 6 41 , 3 6 0 3 , 6 3 7 , 7 8 ,00 6 , 5 0 36 4 2 , 1 p r a t o 4 3 3 2 . 1 0 , 9 1 3 4 0 2 9 4 0 0 , 3 + 0 , 4 0 7 0 6 36 3 7 , 5 7 5 8 , 9 3 8 , 6 6 , 0 5 6 , 4 5 36 3 7 , 4 ;! u c c i e 45 4 5 . 3 0 , 8 1 5 3 3 3 0 5 2 2 , 8 + 0 , 5 7 8 9 9 36 5 5 , 0 6 16, 7 3 8 , 3 7 , 9 0 6 , 5 5 36 5 5 , 2 ;' u p s a l a 37 1 3 , 5 0 , 6 7 3 7 8 3 1 7 3 8 , 4 + 0 , 7 3 6 9 2 3 5 4 6 , 3 7 0 6 , 9 3 9 , 4 8 , 3 0 6 , 3 0 35 4 5 , 4 ; . p r a g a 39 4 1 . 8 0 , 8 4 7 1 1 3 0 2 0 6 , 1 + 0 , 5 3 1 4 2 3 6 0 5 , 2 7 2 7 , 4 3 7 , 8 8 , 3 0 6 , 3 5 3 6 0 5 , 9 ì p a v i a 4 4 2 3 , 7 0 , 8 9 5 3 8 2 9 6 2 6 , 5 + 0 . 4 4 5 2 9 3 6 4 4 , 0 6 0 5 , 9 3 8 , 1 8 , 0 0 6 , 5 0 36 4 4 , 4 f i r e n z e a r e . 4 3 2 8 , 1 0 , 9 1 5 0 0 5 2 9 3 4 7 , 7 + 0 , 4 0 3 4 7 36 3 7 , 2 7 5 8 , 4 3 6 , 8 8 , 0 5 6 . 4 5 3 6 3 6 , 9 p i a c e n z a 4 4 0 6 , 5 0 , 8 9 6 0 8 2 9 6 0 5 , 6 + 0 , 4 3 9 6 4 36 4 2 , 0 0 0 3 , 6 3 8 , 4 8 , 0 0 6 , 5 0 36 4 2 , 1 c o i r à 4 3 3 8 , 0 0 , 6 7 7 9 6 298 3 6 , 2 + 0 , 4 7 8 7 4 3 6 3 6 , 6 7 5 9 , 7 3 6 , 9 8 , 0 5 6 , 4 5 36 3 8 , 2 h e l s i n k i 33 3 7 , 7 0 , 6 5 6 1 6 3 1 8 5 9 , 5 + 0 , 7 5 4 6 1 3 5 1 5 , 2 6 3 6 , 3 3 6 , 6 6 , 6 0 6 , 1 5 35 1 4 . 8 d e b i l l 4 4 5 4 , 6 0 , 7 9 9 1 9 3 0 6 5 6 , 8 + 0 , 6 0 1 0 7 37 5 0 , 0 6 1 0 , 0 4 0 . 0 7 , 9 0 6 5 0 37 4 6 , 5 a t e n e 36 3 8 , 6 0 , 9 6 4 0 6 2 8 0 1 4 , 2 + 0 , 1 7 7 8 0 35 4 1 , 2 7 0 2 , 0 3 9 , 3 8 , 4 0 6 , 3 0 35 4 0 , 5 ! z a g a b r i a 39 3 9 , 0 0 , 9 0 4 5 6 2 9 5 14,2 + 0 , 4 2 6 3 6 3 6 0 5 , 5 7 2 7 , 0 3 8 , 5 6 , 2 5 6 , 3 5 36 0 5 , 5 : k e w 4 8 2 0 , 8 0 , 7 9 3 8 5 3 0 7 2 7 , 2 + 0 , 6 0 8 1 3 37 1 4 , 2 3 3 7 , 0 3 7 , 2 7 , 7 0 6 , 6 0 37 1 3 , 5 , ! s . c o l l e g e 92 5 0 , 0 0 , 3 8 1 7 4 3 3 7 3 3 , 4 + 0 , 9 2 4 2 5 41 4 6 , 0 13 0 6 , 7 39 3 4 , 55 7 , 4 5 41 4 5 , 2 ! s t r a s b u r g o 4 4 16,0 0 , 8 5 2 7 1 301 2 9 , 5 + 0 , 5 2 2 3 7 36 4 4 , 0 8 0 4 , 6 3 9 , 2 8 . 0 0 6 , 5 0 3 6 4 3 , 3 j r k o u b k 2 4 0 5 , 2 + 0 , 6 0 3 8 1 3 5 3 2 9 , 7 + 0,5 9 4 8 9 33 , 5 1 , 0 5 1 0 , 3 4 0 , 7 9 , 70 5 , 6 0 33 4 6 , 8 k z a r a 29 18,a 0 , 9 9 7 3 9 2 7 4 0 6 , 5 0 , 0 7 2 2 2 3 4 3 7 , 0 5 5 8 , 5 3 8 , 4 8 , 9 0 5 , 9 0 3 4 3 7 , 1 [ h e l w a n 3 4 3 7 , 3 0 , 9 9 2 2 5 2 7 7 0 8 , 2 0 , 1 2 4 1 2 4 35 2 1 , 5 6 4 4 , 9 3 6 , 6 8 , 6 0 6 , 2 0 35 2 3 , 4 1 m e d i a 2 8 " 3 6 s 5 c o n s e g u o n o , f u f a l l a s e c o n d o u n o s c h e m a r i p o r t a t o i n un p r e c e d e n t e l a v o r o d i uno d i n o i ( t ) . f v a l o r i p i ù p r o b a b i l i d e l l e i n c o g n i t e f u r o n o 8cp0 = — 0 ° , 0 8 6 : b 0 . 1 1 : 5>0 = — 0 ° , 1 8 5 0 , 0 8 5 f 0 = + 1 b ,0 3 3 , 7 8 ; bh0 = + 0 , 0 6 0 8 6 0 , 6 1 d e l l ' u n i t à p r e s c e l t a . tenendo c o n t o d e i v a l o r i di p a r t e n z a , si h a p e r t a n t o cp0 = 4 1 ° 5 2 ' , 3 , à0 = 71° 4 2 ' , 3 , t0 = 2 8 m 3 9 s , 5 , h0 =68,6 km. u n ' u l t e r i o r e a p p r o s s i m a z i o n e con i d a t i q u i r i p o r t a t i , h a cond o n o a c o r r e z i o n i a n a l o g h e ( t a b e l l a i i i ) . a b b i a m o r i t e n u t o o p p o r t u n o e l i m i n a r e dal c a l c o l o i d a t i r e l a t i v i a l l e sei s t a z i o n i con v a l o r i d e l l a d i f f e r e n z a o-c m a g g i o r i d e l l ' u n i t à ( c a t a n i a , c o i r à , de 13ilt, a t e n e , k e w e h e l w a n ) . r i a p p l i c a t o il m e il terremoto deit u r k e s t a n dei. 2 novembre 1 9 4 6 217 t a b e l l a i i i [stazioni a ( g r a d i ) szn oc ci cos a t e m p o d i a f / a i n n.'c j ( wc ) t n 1 m i w o i i [stazioni a ( g r a d i ) szn oc ci cos a r c g n t r a z t r a g i t t o o r i g i n e a f / a i n n.'c j ( wc ) t n 1 m i w o i i | belgrado 36° 45 55".8 0 , 9 2 4 2 0 9 292' 27' 2",2 0,381887 35r "4is,7 7r 2 s ? 28m39?0 8s,35 6.30 0.5 ; t r i e s t e 41 6 30 .6 0.901292 295 40 17 ,7 0.433212 36 1 8 . 7 7 3 8 , 6 3 9 , 9 6 .2 6.40 4 0.4 | r o m a 43 6 41 .2 0,934783 290 46 2 4 . 8 0,355219 36 3 4 , 6 7 5 5 . 4 3 9 , 2 8 .05 6.45 0.3 ] ; catania 42 59 40 ,2 0,971334 263 45 7 . 0 0,237719 36 3 5 , 2 7 5 4 , 2 41 ,0 6 .05 6 , 4 5 + 1.5 : 1 z u r i g o 44 1 27 ,2 0,669295 299 37 2 3 , 5 0,494294 36 41 .3 6 2 . 6 3 8 , 7 8 .0 6 . 5 0 0.3 i p r a t o 43 26 41 ,9 0,913261 294 2 24,2 0,407375 36 3 7 , 5 7 5 7 , 9 3 9 . 6 8 ,05 6,50 + 0,1 1 u c c i e 45 41 3 4 , 1 0,815096 305 2 4 ii ,3 0,579326 36 55 .0 3 1 5 . 9 39.1 7 .9 6 . 5 5 0.4 ' | u p i a l a 37 11 47 ,7 0,672652 317 43 4 0 , 5 0.739959 35 46 .3 7 6,3 4 0 , 0 8 .3 6.30 + 0.5 ] p r a g a 39 37 3 0 , 0 0,646635 302 9 8 . 6 0.532174 36 0 5 , 2 7 2 6 . 5 3 8 . 7 6 . 3 6 . 3 5 0.6 j ! p a v i a 44 16 34,75 0,695252 296 27 3 3 . 1 0,445560 36 4 4 , 0 6 4 , 6 3 9 , 2 6 .0 6 , 5 0 0.3 ! f i r e n z e a r 43 22 3 7 , 9 0,914924 293 48 1 8 , 2 0,403626 36 37 ,2 7 5 7 . 3 3 9 , 9 8 ,05 6 . 4 5 + 0.4 ! p i a c e n z a 44 1 2 7 . 8 0,697908 296 6 5 6 , 1 0,440163 36 42 ,0 8 2 . 6 3 9 , 4 6 ,0 6 , 5 0 0.1 1 c o i r à 43 33 1 4 , 2 0,877740 298 37 4 4 , 6 0,479137 36 36 ,6 7 5 6 , 8 3 7 , 8 6 .05 6 , 4 5 1,7 ' h c b i n k i 33 36 1 3 . 9 c.654567 319 6 4 2 , 0 0.755987 35 15 ,2 6 3 5 , 6 3 9 . 4 6 .6 6 . 1 5 0.1 de b i l t 4 4 51 1 1 , 3 0,798865 306 58 41 , 3 0,601510 37 50 ,0 3 9,2 40. a 7 ,9 6 , 5 0 + 1,3 i a t e n e 36 31 3 5 , 6 0,984005 280 15 41 , 3 0,176141 35 41 ,2 7 0 . 7 4 0 , 5 3 , 4 6.30 + 1.0 , z a g a b r i a 39 33 4 7 , 0 0.904240 295 16 4 4 , 0 0.427025 36 05 ,5 7 26.0 3 9 , 5 6 .25 6.35 0,0 ' k e w 48 17 30,8 0,793691 307 28 5.6 0.606321 37 1 4 , 2 8 36.3 3 7 , 9 7 ,7 6.60 1.6 ' s college 92 51 4 0 , 4 0,383869 337 25 35.3 0,923388 41 4 6 , 0 13 6 . 5 39.' 5 4 , 5 5 7.45 0.0 : s t r a s b u r g o 4 4 11 4 7 , 5 0,852439 301 31 1 9 , 4 0,522827 36 4 4 , 0 « 3 , 9 4 0 , 1 8 . 0 6,50 + 0.6 k s a r a 29 10 il ,8 0.997403 255 52 1,1 " 0,072072 34 3 7 , 0 5 5 7 , 0 4 0 , 0 8 , 9 6.00 + 0.5 : i j r k o u h k 24 15 00 + 0,802775 53 23 4 5 , 3 0,596282 33 51 ,0 5 1 1 . 6 3 9 , 4 9 ,6 5.75 0,1 ; h c l w a n 34 28 26 ,1 0,992195 262 50 12.6 0 , 1 2 4 6 9 5 35 21 ,5 6 4 3 . 3 3 8 , 2 6 . 5 6,20 1.3 media 28m39;4; tempo origine 2óm39;'15l i l o d o d e i m i n i m i q u a d r a t i a l l e a l t r e it e q u a z i o n i di c o n d i z i o n e , s i a m o p e r v e n u t i ai s e g u e n t i r i s u l t a t i f i n a l i òffo = + 0 ° , 0 5 2 8 0 , 0 8 7 ; òà0 — 0 ° , 0 0 7 5 0 , 0 4 9 st„ = + 0 " , 6 6 8 5 2 . 2 6 6 ; = + 4 , 7 8 2 2 , 7 km. si e b b e i n o l t r e , a c o n f e r m a d e i r e s a t t e z z a dei c a l c o l i . ! it] ~ |//. l ] = 2 . 9 3 7 9 4 3 . i n d e f i n i t i v a , ceco l e c o o r d i n a t e i p o c e n t r a l i e il t e m p o o r i g i n e d e l t e r r e m o t o o g g e t t o di q u e s t o s t u d i o : rr„ = 41° 5 2 ' , 3 5 + 5 ' , 2 n ; x0 = 71° 4 2 ' , 2 9 2 ' , 9 e ; /*„ ^ 7 3 23 km f 0 = 18h 28'" 4 0 ' , 2 : i : 2»,3 q u e s t i v a l o r i ci s e r v i r a n n o p e r l ' u l t e r i o r e s v i l u p p o d e l l a r i c e r c a , di c u i d a r e m o n o t i z i a in u n a p r o s s i m a n o t a . hoimi — istillilo \azionale ili geofisicu — gennaio 1 9 4 8 . r . c a l o i f . p e d o n a c i riassunto contiene la prima parte di una ricerca sul jorte terremoto del turkestan ilei 2 novembre 1916. con i tempi delle onde longitudinali p„, registrate in 23 stazioni sismiche, si determinano, con un nuovo metodo, le coordinate ipocenlrali ed d tempo origine. b i b l i o g r a f i a ! j ! cai.oi i ' . : caratteristiche sismiche fondamentali dell'europa centrale quali risultano dallo studiti di 17 terremoti centro-europei. boll, della società sismologica italiana, voi. xl (1913). psha after a strong earthquake: hints for the recovery annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 1 psha after a strong earthquake: hints for the recovery l. peruzza1, r. gee1,2, b. pace3, g. roberts4, o. scotti5, f. visini6 , l. benedetti7, m. pagani2 (1) istituto nazionale di oceanografia e di geofisica sperimentale ogs, trieste, italy (2) hazard team, gem foundation, pavia, italy (3) disputer, università “g. d’annunzio” chietipescara, chieti scalo, italy (4) department of earth and planetary sciences, birkbeck, university of london, uk (5) institut de radioprotection et de surete nucleaire irsn, fontenay-aux-roses, france (6) istituto nazionale di geofisica e vulcanologia, sezione di pisa, italy (7) aix-marseille université, cerege cnrs-ird umr 34, aix en provence, france lperuzza@inogs.it abstract we perform aftershock probabilistic seismic hazard analysis (apsha) of the ongoing aftershock sequence following the amatrice august 24th, 2016 central italy earthquake. apsha is a time-dependent psha calculation where earthquake occurrence rates decrease after the occurrence of a mainshock following an omori-type decay. in this paper we propose a fault source model based on preliminary evidence of the complex fault geometry associated with the mainshock. we then explore the possibility that the aftershock seismicity is distributed either uniformly or non-uniformly across the fault source. the hazard results are then computed for short-intermediate exposure periods (1-3 months, 1 year). they are compared to the background hazard and intended to be useful for post-earthquake safety evaluation. i. introduction he event of august 24th, 2016 in central italy caused devastating damage and 298 deaths, in spite of the well-known high seismic hazard of the region. the earthquake provoked not only a prompt mobilization of rescue teams but also an unprecedented mobilization of national and international scientific teams. preliminary scientific data have been progressively provided, complemented and updated [e.g. gruppo di lavoro ingv, 2016a, b; marinkovic and larsen, 2016; reluis-ingv workgroup, 2016; gl irea-cnr and ingv, 2016;ran.protezionecivile.it/it/dettaglio_evid. php?evid=340867;www.gsi.go.jp/cais/topic16 0826-index-e.html; www.eqclearinghouse.org /2016-08-24-italy/]. since august 30th, some participants of the informal fault2sha working group* opened a forum [http://earthquake2016.prophpbb.com] to circulate ideas, publish material, draft papers and news within the scientific community. this attempt to debate scientific issues in real-time helped to focus investigations in the field, and motivated and boosted this work. * the wg has been formally approved during the xxxv esc general assembly: to join the wg fill the form at https://sites.google.com/site/linkingfaultpsha/ fault2sha-esc-wg t annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 2 figure 1: faults and earthquakes overlain on the e-w component of insar data [marinkovic and larsen, 2016], blue means eastwards; white pins for m>4 before sep, 28th, 2016 (cnt.rm.ingv.it), dotted for m>5; a=mt. vettore fault, b=laga fault, c=sibillini thrust. in this paper we gathered pieces of information for a first formulation of aftershock probabilistic seismic hazard assessment (apsha, as defined by yeo and cornell [2009]) in the damaged area. the topography of the area, 3d geometry of the faults and timedependent estimates of seismicity rates enter in our computations. the results should supersede the standard psha practice for the intermediate-term (months to year). thus, these estimates may be suitable for the impending activities of microzonation, retrofitting and rebuilding. we hope this work will form a basis for gathering more detailed information and to support activities aimed at post-earthquake recovery. ii. methods active normal faulting in the central apennines has been recognized for many decades; several authors [e.g. barchi et al., 1999; galadini and galli, 2003; boncio et al., 2004a, b; roberts and michetti, 2004; benedetti et al., 2013] have described potential seismic sources in the region, which has been recently and historically hit by deadly earthquake sequences. although fault segmentation, expected magnitude, recurrence time and associated historical earthquakes differ somewhat among authors, there is a clear agreement concerning the lack of surface faulting between the southern termination of mt. vettore fault and the northernmost limit of mt. gorzano fault (hereinafter laga fault) where the main shock of the 2016 amatrice earthquake is located and most of the insar deformation is concentrated (fig. 1). figure 2: surface effects after the 24 aug earthquake: a) location map of ruptures by emergeo wg [2016], piccardi et al. [2016], pace et al. [2016]; b) rupture detail at mt. vettore. a) b) after august 24th, researchers that went into the field documented coseismic ground ruptures with maximum throw up to 25 cm, detectable along segments 3-9 km long (fig. 2). most of these surface deformations can be ascribed to coseismic exhumation (rejuvenation) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 3 of existing sw dipping normal faults outcropping along the western flank of mt. vettore; minor features were reported also along the laga fault. the expected magnitude predicted by these displacements and surface rupture length, using existing empirical magnitude vs size relations, is compatible with that of the mainshock, but quite different values (mw 6.06.2, downdip length 4-8 km or more) can be expected due to the uncertainty in the scaling relationship and field measurements. within the fault2sha scientific forum, we faced the parameterization of the causative seismogenic source, based on very preliminary and uncertain data, with the aim to discern between the activation of one or more seismogenic sources. as the aftershock sequence evolved, the potential link at depth of mt. vettore and laga faults became more evident: this interpretation is actually the most widely accepted one after the quake (see e.g. lavecchia et al. [2016]). figure 3: geometry of the fault source proposed for the 2016 earthquake sequence: a) 3d model implemented in oq; b) surface projection of point sources (black dots) on earthquake locations, fault traces and rupture model; c) fault surface projection on the topography. a) b) c) adopting surface traces based on our mapping activities that acknowledge earlier work on existing published maps, focal mechanisms released from various seismological agencies, and the hypothesis of one unique source, we modeled a sw dipping plane representing a single fault at depth that splits into separate branches towards the surface as the mt. vettore and laga faults: it represents the preliminary fault geometry for the purpose of our calculations. this complex fault geometry is one way to explain the two distinct patches of slip distribution obtained by insar data inversion located on 50 degree dipping planes [gl irea-cnr and ingv, 2016]. it is coherent with outcropping observations of two steeper dipping planes (60-70 degrees) separated by about 5 km, in the area between the localities of arquata del tronto and accumuli, where negligible surface expressions have been observed in the field. the hypothesized fault geometry is compatible with the preliminary aftershock locations, but we do expect refinement and a better resolution when high-quality hypocentral locations will be released. conversely, this geometry does not account for the alternate stripes of east-westwards movement detected by insar (see fig. 1) for which two distinct conjugate faults may be needed. as these patterns suggesting east-dipping planes are controversial, we decided not to model such a plane in this study. figure 3a is the 3d sketch of the fault source, created as input for the openquakeengine software [pagani et al., 2014; oq, www.globalquakemodel.org/openquake/]. due to its complex geometry, the surface is represented by a set of point sources (fig. 3b) on which the global seismicity rate of the fault system is partitioned. the red rectangle roughly corresponds to the rupture area as modeled by gdl ingv [2016a]. considering the preliminary hypocentral distribution of aftershocks, we limit the fault source to a depth of 12 km, annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 4 although the surface itself may extend down to depths of 18 km [e.g. boncio et al., 2004b]. the basic principle for an apsha is to treat the earthquake sequence with time-dependent seismicity rates and superimpose it on the traditional poisson estimate. the traditional psha we use as reference is the one stated by law [http://zonesismiche.mi.ingv.it/] since 2004, namely mps04. for the earthquake sequence, we assume an omori-utsu decay of earthquakes with time after the main event. this simple model can be assumed as a realistic one provided that: • the seismicity belongs to a unique source, with no triggering of nearby cascade events; • the coefficients are properly calibrated. we therefore downloaded the ingv bulletin [http://cnt.rm.ingv.it/, data accessed on sep 17, 2016], for deriving the coefficients of the theoretical model that forecasts the number of events (n) versus time (t) as: n(t) = k/(c+t)p (1) where k, c, and p are empirical constants related to a particular aftershock sequence. the fitting was performed by zmap code, at www.seismo.ethz.ch/prod/software/zm ap/. a preliminary gutenberg-richter (g-r) analysis of this dataset suggests that b-value is close to 1, with completeness magnitude at about m2. we fit different sub-samples, by varying the minimum magnitude threshold and learning period: the best candidate by checking the total number events within the longest period available at the time of the analysis, is obtained with mmin≥ 1.8 and 20 days of learning period, represented in fig. 4a. we are conscious that during the first hours or days small events have probably gone undetected, merged in the coda of bigger events; similarly the completeness and location quality has increased with the deployment of temporary stations. nonetheless, the coefficients representing model 1 in fig. 4a, are suitable to represent the global seismic activity detected during the first weeks, as shown in fig. 4b, whilst a too rapid decay (black line) is predicted by 10-day learning period only. finally, we integrate eq.(1) to derive the cumulative number of events predicted by the theoretical curve in 30 days, 3 months, and 1 year, starting on october 1st. in table 1, the number of events and corresponding a-values of a g-r distribution are given, assuming b=1. figure 4: omori-utsu decay for the 2016 sequence: a) fit on the first 20 days of ingv bulletin data with m ≥ 1.8; b) observed versus predicted n(t) from aug 23 until sep 16 (24th day), model 1 of fig. 4a by light green curve. a) b) note that despite the a-value increases with time, the seismicity decay is preserved if the prediction time in psha is set equal to the observation time of the omori-utsu modelling. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 5 with this fundamental limitation (i.e. no extrapolation to other observation times), there is no need to generalize the seismicity rates into non-poissonian earthquake probabilities. thus in oq, g-r distributions truncated at mmin=3.5 and mmax=5.5 (based on magnitude of the largest aftershock) and a-values given in tab. 1 are partitioned on the point sources representing the fault plane. table 1: apsha, maximum pga/sa at 10% probability of exceedance in different observation times since oct, 1st, 2016; values obtained with uniform and non-uniform partitioning of aftershock seismicity rates on the fault. fault model pga /sa(0.3s) (g) time oct, 1 num m≥1.8 a-value (a-year) uniform nonuniform 30 days 745 4.672 (5.751) 0.13/0.22 0.13/0.23 90 days 1597 5.003 (5.605) 0.18/0.34 0.19/0.50 1 year 3236 5.310 (5.310) 0.24/0.47 0.25/0.50 similarly to yeo and cornell [2009], we set uniform and non-uniform partitioning of the seismicity rates: in the second case the a-value decreases as a function of distance away from the patches with highest coseismic slip, as it has been observed that aftershocks are often clustered at the ends of faults [e.g. das and henry, 2003]. these hypotheses are speculative and uncertainties can be handled as branches of a logic tree. finally, by applying the new oq features specifically developed for volcanic areas [gee et al., 2016], we introduce topography in the computation by defining sites in terms of their 3d location, via a dem (1km horizontal resolution, fig. 3c) that results in some minor changes in rupture-to-site distances (rrup). the ground motion prediction equation (gmpe) of chiou and youngs [2014] (cy14) is used, because it is defined in terms of rrup; it has been derived using earthquakes from active shallow crustal regions, and is applicable down to mw 3.5. some examples showing the impact of different ingredients in the apsha (fault geometry, distance metric, topography, seismicity rate distribution onfault) are given as electronic supplement. iii. results we briefly describe here the results in terms of peak ground acceleration (pga is defined as spectral acceleration sa at 0s) and sa(0.3s) for rock sites (vs30= 800m/s) obtained with the fault aftershock models only (fig. 5), thus comparing the proxy hazard curve for the 3 observation times with the ones given by mps04 (fig. 6). figure 5: apsha maps showing: a) pga for rock sites (cy14) at 10% in 1 year from october 1, 2016, using uniform and non-uniform rates with topography; b) the same but sa (0.3s) in the next 3 months. a) b) note the effects of the inclusion of a proper fault geometry, distance metrics and of topography (fig. 5 and esm) in the hazard calculations: for the town of amatrice, the aftershock hazard is slightly lower when the non-uniform annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 6 distribution of a-values is considered, compared to the assumption of uniform seismicity, due to the proximity of the city to the epicenter of the mainshock. we see the opposite trend, approximately same amount, for cities located at the tip ends of the fault, e.g. the town of visso. note also that the values obtained at 10% p.e. in 1-year since october 1, 2016 (approximately the time in which the theoretical decay is flattening, at a rate of about 2 events per day with m ≥ 1.8) are higher than the values expected in mps04 in 50 years. hypotheses of a more rapid decay, e.g. stated by the black curve in fig. 4b, lead to lower values of about 20-40%. fig. 6 shows that the hazard curves computed in this study (apsha) and those considered in the italian law (mps04) for the town of amatrice, for both pga and sa at 0.3s, cannot be reconciled, at least if we do not consider epistemic uncertainties of the whole logic tree. prospective work should aim at merging apsha with mps04, or with updated release of the italian reference hazard map; future efforts should include also the new information about site-effects that are being currently acquired by several institutions [www.centro microzonazionesismica.it/it/attivita/41-ilcentroms-per-il-terremoto-italia-centrale-2016]. figure 6: hazard curves in amatrice. iv. discussion we set up a model for apsha, in central italy, after the devastating event of august, 24. the theoretical approach is not novel [yeo and cornell, 2009; iervolino et al., 2014], but this is the first time it has been applied in a real case in italy, modeling aftershocks with a fault plane, within an ongoing seismic sequence. in the aim of supporting the impending recovery and rebuilding actions, we set up investigation times of 1 month/1 year since october 1st, 2016, as this is a reasonable time frame for the sequence extinction, if no triggering of nearby faults happen. the 3d fault geometry is essentially derived from data available before the sequence, some hypothesized details have to be refined, or reshaped with a community consensus, after that more accurate data will be available. however, the introduction of a realistic fault surface and proper computation of distances from ruptures via topography leads to a hazard map that captures complexities that are as detailed as those obtained via full ground wave propagation modeling. through a comparison of observations with event-based scenarios we are planning further analyses, in order to check to what extent this hazard map representation is realistic, and whether it can also be adopted in microzonation studies. the simple assumptions here adopted (omori-utsu decay of the earthquake number with time, non-uniform partitioning of seismicity rates outside the rupture area) cannot at present predict more complex fault interactions which, however, are strongly speculative: this will be an interesting aim of future work. the time-dependent seismic hazard in pga and sa suggests that the region may experience acceleration values in the next year since october 1st that are comparable or higher to the ones stated by the italian law in 50 years. these results cannot be extrapolated to different periods than the ones in the omori-utsu forecast. this is the first time that forecasts have been made for aftershocks based on information annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 7 gathered rapidly in the wake of the mainshock. previous knowledge of fault geometries was crucial to define the fault plane that hosted the earthquake rupture. we believe the inclusion of such information improves the forecast relative to those that do not include such information. it is particularly interesting because this unusual earthquake ruptured two separate faults as mapped at the surface. our approach allows us to deal with this complex geometry by using realistic fault geometries that are well known in the structural geology literature where en echelon faults at surface merge downwards into a single fault at depth [walsh et al., 1999]. a similar style of faulting may apply to other earthquakes such as the 2009 l'aquila earthquake [wilkinson et al., 2015], or to less known faults not as recently activated. it is worth remembering that central italy is often subjected to earthquake clusters, and that several faults are indicted of high timedependent earthquake occurrence probabilities [peruzza et al., 2011]. they motivated post l’aquila earthquake temporary seismometric monitoring [romano et al., 2013] in the southern middle aterno valley, still silent today. in addition, the modelling of static stress variation (see pace et al. [2014] and references therein) suggested an increase of the probability of occurrences of earthquakes in the northern laga fault that ruptured in 2016, as well as southwards. the coupling of earthquakes on different fault segments is the main limitation of this study that we hope to overcome by this new coupling of knowledge from psha and structural geology. we hope this work may be an important new avenue for re-insurance companies to appreciate losses in real-time after a major earthquake as well as for research to improve disaster response. v. epilogue on oct 26th when the revision of this paper was near the end, a mw 5.5 (data of qrmt from http://autorcmt.bo.ingv.it/quicks.html) occurred at 17:10 in the visso area; it was compatible with the aftershock g-r and mmax adopted by this study, and with the nonuniform distribution of earthquakes as given in fig. 5. but 2 hours later, another “main” event (mw 6.1) broke the northernmost patch of mt. vettore fault, followed on oct 30th at 6:40 utc by the actual biggest earthquake of the amatrice mt. vettore sequence (mw 6.6). thus, one basic assumption of this study, that no triggered and cascading events occur, does not hold anymore, as pinpointed by the reviewer, and the results provided since oct 1st are no longer valid. in light of the recent events, considered that the failure of a test hypothesis is itself a result, we recomputed our results for 30 days starting on sep, 20th. this exposure period is prior to the mw 6.1 earthquake of oct 26th and posterior to the learning period of the o-u calibration. the number of m>1.8 events increases (see table 2) as the limits for the integration of omori-utsu decay curve shifts left; maximum pga/sa values rise as well but the pattern remains the same, with maxima nearby amatrice assuming a uniform rate distribution, and northeast of visso in the non-uniform case. these are the results we plan to analyze under rigorous testing against real observations, in future work. for the purpose of this special issue, we decided to keep the manuscript as it was at its first submission, with the exception of this last chapter, for the following reasons: 1) the source we designed in september, largely based on field work existing before the sequence, is still coherent with the most recent events and observations; 2) the many novelties introduced in space and time characterization of the source depict a new hazard picture, much closer to deterministic modeling than ever; 3) the time-dependency introduced for a “simplistic” decay of seismicity with no triggering of similar-sized ruptures demonstrates that the survived buildings and temporary installations after a major earthquake are statistically exposed to similar/higher shaking than those expected for long return periods. we beannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 8 lieve this approach could be useful if applied in nearly real-time procedures, and valid in most of the sequences, as no triggered events occur. this analysis should also be a new way for identifying what a triggered event is, a debated issue in the whole scientific community. table 2: apsha, maximum pga/sa at 10% probability of exceedance since sep, 20st, 2016, see table 1. fault model pga /sa(0.3s) (g) time sept, 20 num m≥1.8 a-value (a-year) uniform nonuniform 30 days 932 4.769 (5.849) 0.14/0.25 0.14/0.26 acknowledgements for their work in the background, a special mention is due to g. weatherill, j. faurewalker, s. baize, h. jomard. thanks to the personell from various institutions that in labs and in the field guarantee new data acquisitions. we simpathize with the victims families and population, hoping a prompt and enduring recovery. references barchi m., f. galadini, g. lavecchia, p. messina, a. m. michetti, l. peruzza, a. pizzi, e. tondi, e. vittori (1999). sintesi delle conoscenze sulle faglie attive in italia centrale: parametrizzazione ai fini della caratterizzazione della pericolosità sismica. cnr-gndt, ftp://ftp.ing v.it/pro/gndt/pubblicazioni/barchi_et_alii/b archi.htm benedetti l., i. manighetti, y. gaudemer, r. finkel, j. malavieille, k. pou, m. arnold, g. aumaître, d. bourlès, and k. keddadouche (2013). earthquake synchrony and clustering on fucino faults (central italy) as revealed from in situ 36cl exposure dating. j. geophys. res. se, 118:4948–4974, doi:10.1002/jgrb.50299. ! boncio p., g. lavecchia, and b. pace (2004a). defining a model of 3d seismogenic sources for seismic hazard assessment applications: the case of central apennines (italy). j. seism., 8:407–425. boncio, p., g. lavecchia, g. milana, and b. rozzi (2004b). seismogenesis in central apennines, italy: an integrated analysis of minor earthquake sequences and structural data in the amatrice-campotosto area. ann. geophys., 47:1723–1742. chiou b. s.j., and youngs, r. r. (2014). update of the chiou and youngs nga model for the average horizontal component of peak ground motion and response spectra, earthquake spectra 30, 1117–1153. das s., and c. henry (2003). spatial relationship between main earthquake slip and its aftershock distribution, rev. geophys., 41(3), 1013, doi:10.1029/2002rg000119. emergeo working group (2016). the 24 august 2016 amatrice earthquake: coseismic effects. doi: 10.5281/zenodo.61568 galadini f., p. galli (2003). paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts.faults. ann. geophys., .46:815-36. gee r., l. peruzza, b. pace, r. azzaro, s. d’amico, and m. pagani (2016). when probabilistic seismic hazard climbs volcanoes: how 3d topography and scaling relationships influence hazard estimates. an example from mt. etna (italy) http://meetingorganizer. copernicus.org/esc2016/esc2016-545-1.pdf gruppo di lavoro ingv (2016a). primo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24/8 2016 (italia centrale). doi: 10.5281/zenodo.61121 gruppo di lavoro sul terremoto di amatrice (2016b). secondo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24/8/2016 (italia centrale). doi: 10.5281/zenodo.154400 gruppo di lavoro irea-cnr and ingv (2016). sequenza sismica di amatrice: risultati iniziali delle analisi interferometriche satellitari. doi: 10.5281/zenodo.60935 iervolino i., giorgio m., polidoro b. (2014). sequence-based probabilistic seismic hazard annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7257 9 analysis. bulletin seism soc america, 104(2): 1006-1012 lavecchia g., brozzetti f., boncio p., de nardis r., cirillo d. and ferrarini f. (2016). controllo strutturale sulla sequenza sismica di accumoli 2016 – analisi preliminare. 80° conv. soc. geol. it., www.youtube.com/watch?v =7imos36ydcu&app=desktop marinkovic p. and y. larsen (2016). mapping and analysis of the central italy earthquake (2016) with sentinel-1 a/b interferometry. zenodo, doi: 10.5281/zenodo.61133 pace b., bocchini g.m. and boncio p. (2014). do static stress changes of a moderate-magnitude earthquake significantly modify the regional seismic hazard? hints from the l’aquila 2009 normal-faulting earthquake (mw 6.3, central italy). terra nova, 26-6: 430-439. pace b., visini f. and valentini a. (2016). 24 august 2016, amatrice earthquake: field observations. doi: 10.13140/rg.2.2.35994.85445 pagani m., d. monelli, g. weatherill, l. danciu, h. crowley, v. silva, p. henshaw, l. butler, m. nastasi, l. panzeri, m. simionato, and d. vigano (2014). openquake engine: an open hazard (and risk) software for the global earthquake model. seism res lett 85:692–702. peruzza l., b. pace and f. visini (2011). fault-based earthquake rupture forecast in central italy: remarks after the l’aquila mw 6.3 event. bul seism soc am, 101(1):404-412. piccardi l., blumetti a.m., comerci v., di manna p., fumanti f., guerrieri l., leoni g., pompili r., vittori e, ferrario f., frigerio c., livio f., michetti a.m., bonadeo l., brunamonte f., sani f., tondi e., wedmore l., roberts g., faure-walkers j., iezzi f., mildon z., gregory l., phillips r., walters r., mccaffrey k., wilkinson m., cowie p., rhodes e. (2016). the august 24, 2016, amatrice earthquake (mw 6.0): field evidence of on fault effects. http://www.isprambiente.gov.it/files/ notizie-ispra/notizie-2016/sismaitaliacentrale /report_amatrice_en_2016_09_16.compress ed.pdf reluis-ingv workgroup (2016). preliminary study of rieti earthquake ground motion records v1.0, available at http://www.reluis.it roberts g. p., and a. m. michetti (2004). spatial and temporal variations in growth rates along active normal fault systems: an example from the lazio–abruzzo apennines, central italy. j. struct. geol., 26(2):339–376. romano m. a., r. de nardis, m. garbin, l. peruzza, e. priolo, g. lavecchia, and m. romanelli (2013). temporary seismic monitoring of the sulmona area (abruzzo, italy): a quality study of microearthquake locations. nat. hazards earth syst. sci., 13:2727-44, www.nathazards-earth-syst-sci.net/13/2727/2013 walsh j.j., j. watterson, w.r. bailey, c. childs (1999). fault relays, bends and branchlines. journal of structural geology 21:10191026. wilkinson m., g.p. robert, k. mccaffrey, p.a. cowie, j.p. faure walke, i. papanikolaou, r.j. phillips, a.m. michetti, e. vittori, l. gregory, l. wedmore, z.k. watson (2015). slip distributions on active normal faults measured from lidar and field mapping of geomorphic offsets: an example from l'aquila, italy, and implications for modelling seismic moment release. geomorphol., 237:130-41. yeo g.l. and c.a. cornell (2009). a probabilistic framework for quantification of aftershock ground-motion hazard in california: methodology and parametric study. earth. eng. struct. dyn., 38:45–60. 105_110 adg v5 n01.pdf annals of geophysics, vol. 45, n. 1, february 2002 105 extreme values in ionospheric radio propagation peter a. bradley pandora, farnham common, slough, berks, u.k. abstract proposals are made for earth-space and space-space paths to apply the term ‘propagation degradation probability’, analogous to ‘basic circuit reliability’ adopted within the radiocommunication sector of the international telecommunication union (itu-r) to quantify the performance of hf terrestrial ionospheric radio circuits. the principal elements characterising propagation impairments are described and formulae and a table given by the itu-r are reproduced relating many of these effects to the path total electron content. expressions are provided enabling estimates of the probabilities of occurrence of user-specified extreme values to be estimated in terms of their median quantities and day-to-day variabilities. the need is stressed for additional data collection and analyses both to refine the median values and to derive the required reference variability factors. 1. introduction characterisation of extreme values for planning is important in many disciplines. for example, in insurance such figures are needed for risk assessment. in agriculture, they provide vital information about the yields of particular fields and the likely influence of application of specific fertilisers. in civil engineering, windloading factors are required when designing outdoor structures, and in oceanographic applications knowledge of largest waves likely to be encountered must be taken into account when designing offshore structures. radio system design likewise needs information concerning extreme propagation effects that may arise. in all cases extreme values must be derived from past measurement data, either on the assumption that the future will be like the past, or with incorporation of a judicious trend factor. sometimes the parameter being evaluated may be determined indirectly, using measurements of a different parameter, but the point to note is that always some form of measurement data is involved. measurement data sets, however large, have to be of finite size. possibly for an infinitely large measurement data set the extreme value would be infinite (or zero), but that is not considered to be a useful result. more typically though, the largest (or smallest) values in successive data sets will not be the same, and so this raises the question how extreme values are to be determined. traditionally expressions such as ‘worst case’ have been used, but this terminology is not considered here to be useful, and indeed is positively misleading. worst case values would have to be successively revised using more and more data samples. extreme values from within samples are known to be much less stable to characterise than say the median values. key words ionosphere – space radiowa ve propagation – circuit performance assessment mailing address: prof. peter a. bradley, pandora, scotlands drive, farnham common, slough, berks sl2 3es, u.k. 106 peter a. bradley it is more precise then to express extreme values with regard to some selected probability of occurrence. for example, in deriving risks associated with artificial satellite failure, there may be a need to characterise the ‘ten-year event’. for offshore structures the requirement may be to specify the 100 year wave. of course the ten-year event or the 100 year wave may come tomorrow, but unless there is an a-priori reason to anticipate that situation, the statistical figure derived from past measurements remains the one to adopt. from the foregoing it is seen that not only must measurement data be processed to yield extreme values, but also there must be a specification of what is meant by the word «extreme». that specification, which transposes to a particular probability of occurrence, has to be a subjective factor. it is derived with regard to the importance attached to the estimate, particularly where cost has to be balanced against risk. one problem associated with extreme estimates is that often users of such information are unable to quantify the occurrence probabilities for which they want these to apply. indeed, it is only meaningful to do so, if those people analysing the measurement data can specify these in a probabilistic fashion. where that is not possible, which was often the case in the past, judicious ‘safety factors’ are introduced to allow for unforeseen factors. it is suggested here in the ionospheric propagation case that extreme values be determined for several different nominal probabilities of occurrence, such as 10%, 5%, 1% and 0.1%. four separate propagation situations with potential ionospheric influences need to be addressed: i) reception of wanted signals from a ground-based transmitter, received at a grounded location after ionospheric reflection; ii) reception at a grounded location of co-channel and adjacent channel interfering signals from diverse sources; iii) reception at the ground or at a spacecraft of signals which traverse the ionosphere on an earth-space link, and iv) reception at a spacecraft of signals originating from another spacecraft. except in the case of local interference not further addressed here, scenarios (i) and (ii) rely on the ionosphere for propagation support, whereas with (iii) and (iv) the presence of the ionosphere leads to signal corruption – change of ray path, delay, polarisation, attenuation etc. the treatment in these cases then needs to be different. 2. hf reliability and compatibility itu-r recommendation p.842 (itu-r, 2001a) defines the terms reliability and compatibility as the probabilities of satisfactory signal reception at hf in the presence of the natural and interference backgrounds, and when co-channel and adjacent channel signals are present respectively. for a particular radio circuit and in the presence of natural noise alone the reliability is known as basic circuit reliability (bcr). when also interference is allowed for, the corresponding quantity is referred to as circuit reliability (cr). formulae to compute these terms are given. bcr and cr are estimated in terms of predicted prevailing mean amplitudes and dayto-day variabilities of a wanted signal, and a background consisting of natural noise, manmade noise and interference, as appropriate. the required signal/background level must be specified. compatibility is similarly determined from the mean amplitudes and variabilities of a wanted signal and a co-channel or adjacent channel unwanted signal. in this case, the protection ratio between wanted and unwanted signals must be chosen. table i reproduces the reference decile dayto-day deviations of signal strength from the monthly median values used in recommendation p. 842. these values are combined on an rms basis with the corresponding withinthe-hour decile deviations of the signals and with the day-to-day and within-the-hour variations of the background, so that thereby the upper and lower decile deviations, d u sn and d l sn respectively of the signal/ background ratio are given. for natural noise alone and with monthly median signal/noise ratio s/n, then, following bradley and bedford (1976), for a required signal/noise ratio s/n r and with all quantities expressed in decibels, 107 extreme values in ionospheric radio propagation the bcr as a percentage is given as: bcr = 130 − 80/(1+(s/n − s/n r )/d l sn) or 100, whichever is smaller for s/n ≥ s/n r = 80/(1+(s/n r − s/n)/d u sn) − 30 or 0, whichever is greater for s/n < s/n r it is of note that for a particular transmitterreceiver path, separate reliability and compatibility values are derived because of changes in signal propagation and background levels for the different hours, months and solar epochs. so in choosing system parameters, e.g., transmitter power and types of antennas to achieve a desired service performance, there must also be subjective specification of what that desired service performance is. this is referred to as service probability. the service probability might for example be derived from just nighttime predictions over only a few months of a particular year, or it might embrace all hours over a full sunspot cycle, depending on the operational requirement. the above terms and the methods of their evaluation seem well established, and there would appear no need to seek to change these. 3. pdp for earth-space paths the determination of the signal components of bcr for a point-to-point hf radio circuit is a two-stage process. firstly the probability of ionospheric signal support has to be evaluated using reference maximum usable frequency (muf) day-to-day variability statistics, and then given that propagation is possible, the probability has to be assessed that a required signal/noise ratio is met. in the case of earth-space or space-space paths the situation is somewhat different in that it is supposed to a first order signals are received over line of sight trajectories with the various ionospheric effects representing a corruption of the free-space signals. the need is then to determine the probabilities that user specified tolerable propagation effect levels are met, rather than signal/noise values being achieved as at hf. (2.1) table i. lower decile (ld) and upper decile (ud) deviations from the predicted monthly median value of available receiver power of the wanted signal and interfering signals, arising from day-to-day variability. geomagnetic latitude (1) < 60° > 60° transmitting frequency/ ld ud ld ud predicted basic muf ≤ 0.8 8 6 11 9 1.0 12 8 16 11 1.2 13 12 17 12 1.4 10 13 13 13 1.6 8 12 11 12 1.8 8 9 11 9 2.0 8 9 11 9 3.0 7 8 9 8 4.0 6 7 8 7 > 5.0 5 7 7 7 (1) if any point on that part of the great circle which passes through the transmitter and the receiver and which lies between control points located 1000 km from each end of the path, reaches a geomagnetic latitude of 60° or more, the values of > 60° have to be used. 108 peter a. bradley nonetheless, formulae to determine position within the associated probability distributions are the same in the two cases. so, instead of, but by analogy with bcr, here the term propagation degradation probability (pdp) is judged more appropriate for space paths. the principal ionospheric propagation effects on earth-space paths may be listed as: faraday rotation, propagation delay, refraction, absorption, dispersion and scintillation. for an effect e we may write, by analogy with eq. (2.1): pdpε = 130 − 80/(1+(εm− εr)/dlε) or 100, whichever is smaller for ε m ≥ ε r = 80/(1+(ε r − ε)/d u ε) − 30 or 0, whichever is greater for ε m < ε r where ε m is the median value, d l ε and d u ε are reference lower and upper decile deviations from the median for effect ε and ε r is the required value that must not be exceeded. in particular, with the reference decile deviations assumed given, eq. (3.1) requires knowledge of both the median and required values. recommendation p. 531 (itu-r, 2001b) gives formulae and figures whereby median effect values may be estimated. many such values are a function of the prevailing tec. alternative methods of determining tec by electron-density height-profile integration are offered, using either the international reference ionosphere (iri) produced by a cospar-ursi task group (bilitza, 1990) or the model nequick (leitinger et al., 1999; hochegger et al., 2000), developed within cost251 (leitinger and feichter, 2000). the iri model applies only for heights up to 2000 km whereas nequick, still under revision within cost271 (radicella and leitinger, 2001) and with its allied models costprof and neuog-plas (leitinger, 1998a), now includes a plasmasphere topside (leitinger, 1998b, 1999). use of the iri or nequick models enables median tec to be given separately for ray paths in different geographical regions, and to show the dependence on time-of-day, season and epoch of the solar cycle, for the near full range of conditions for which they apply. there are two independent approaches to tec determination, the one involving profile integration as mentioned above, and the other from direct measurement of spacecraft signals. the latter method enables full allowances for electron content at the greater heights, but available data lack universal coverage. various methods of combining the two data sets are being investigated within cost271 (leitinger et al., 2001). use of incoherent-scatter radar standardisation of gps tec-derived values is addressed in lilensten and cander (2001). there are obvious advantages in seeking to characterise tec from a spacecraft using ionospheric tomography (dabas and kersley, 2001) or by means of occultation techniques (garcia-fernandes et al., 2001; jakowski et al., 2001; mitchell et al., 2001). it seems then likely that improved median tec specifications will emerge in due course, in particular providing more realistic figures with greater spatial resolution than hitherto. some stated «estimated maximum» values of the principal ionospheric propagation effects likely to be encountered at 1ghz for a one-way ray path traversal of the ionosphere for an elevation angle of about 30° are quoted in itu-r recommendation p.531 (itu-r, 2001b) and these are reproduced in table ii. for those parameters which exhibit a well-known frequency dependence and are directly related to tec, values can readily be converted to corresponding figures for other elevation angles and frequencies. it is suggested here though that instead one should follow the eq. (3.1) statistical approach and endeavour to model the decile deviations of the various effects. as a first step this involves trying to specify the tec decile deviations. moreover, it seems probable that these decile tec would better come from limited direct measurements, rather than from profile integration, because in the profile case, although the variabilities of the separate ‘anchor values’ such as layer peak densities and heights are generally well known, the correlations in the changes among them are not established. (3.1) 109 extreme values in ionospheric radio propagation 4. pdp for space-space paths ionospheric effects may be estimated from ray-tracing results through an appropriate ionospheric model such as neuog-plas to derive the corresponding tec values on satellite-tosatellite paths (hochegger and leitinger, 2000). at this time it is not clear how median tec may be characterised, but some amalgamation of separate results to give representative figures will no doubt be necessary. despite variability between cases, it ought to be possible to provide results whereas none are currently available. likewise, the study of tec variability on spacespace paths remains to be carried out. 5. combining pdp values for different propagation effects signals received over an hf point-to-point ionospheric path experience time spread and frequency dispersion as well as attenuation due to passage through the ionosphere. hence, particularly in the case of digital transmissions, circuit performance criteria have to take account not only of the probability of achieving a user-specified required signal/ background ratio, but also specified time spreads and frequency dispersions that must not be exceeded. bradley and muhtarov (2000), first proposed via itu-r (1997), have developed formulae to combine the bcr values for each of these effects on the assumption that they are independent of oneanother. for effects ε 1 , ε 2 and ε 3 we have that the combined bcr is bcr = bcrε1 . bcrε2 . bcrε3. (5.1) adaptation of this formulation may also be applied to evaluations for earth-space and spacespace paths when considering independent effects such as absorption and scintillation, leading to pdp = pdpε1 . pdpε2 . pdpε3 (5.2) but in the case of effects directly dependent on tec, these have to be regarded as completely correlated, in which case the pdp is taken as the smallest of those separately evaluated for the different effects. 6. combining pdp values for different times and paths pdp may be determined as wanted. pdp estimates may be used either to predict the performance of an established circuit, or to permit the design specification to meet stated performance objectives. in either case, pdp values are likely to need combining for different times and different satellite locations. this means that criteria to be achieved may well involve multiple effect tolerances for stated fractions of days, hours, months and years, as well as for a certain percentage of satellite positions. an example of a performance specification might be that the group delay must not exceed 0.2 µs for more than 5% of the nights table ii. itu-r recommendation p.531 ‘estimated maximum values’. effect magnitude frequency dependence faraday rotation 108° 1/f 2 propagation delay 0.25µs 1/f 2 refraction < 0.17 mrad 1/f 2 variation in direction of arrival 0.2 min of arc 1/f 2 absorption (polar cap absorption) 0.04 db ∼1/f 2 absorption (mid-latitude) < 0.01 db 1/f 2 dispersion 0-4 ns/mhz 1/f 3 scintillation limited amplitude information only quoted 110 peter a. bradley between march 2003 and september 2005 for a polarorbiting spacecraft over 90% of the world ocean area. the formulae given above permit these types of figures to be derived. on the other hand, it can be appreciated that for some applications, having regard also to uncertainties in ionospheric predictions, a single all-embracing effect estimate achieved say for 95% of occasions and covering all times and satellite locations might be favoured. this is a matter of user choice. 7. conclusions extreme value estimates in ionospheric radio propagation should be determined from variability statistics. the concept of basic circuit reliability (bcr) developed to characterise the probability that the signal/noise ratio for an hf circuit attains a required threshold is extended to earth-space and space-space paths giving the propagation degradation probabilities (pdp) that acceptable levels are achieved of the different propagation effects related to total electron content (tec) such as refraction, dispersion and faraday rotation, as well as arising from ionospheric absorption and scintillation. formulae are quoted giving pdp when multiple tolerance criteria must be met, both for uncorrelated and fully correlated effects. the particular need is identified to permit these evaluations for the derivation of reference decile deviations of tec from the monthly median values, and it is concluded that such information will best come from analyses of direct measurement data, rather than from electrondensity height profile integration. references bilitza, d. (1990): the international reference ionosphere 1990, national space science data center report 90-22, greenbelt, maryland. bradley, p.a. and c. bedford (1976): prediction of hf circuit availability, electron. lett., 12, 32-33. bradley, p.a. and p. muhtarov (2000): basic circuit reliability for digital hf ionospheric communications, fisica de la tierra, 12, 353-361. dabas, r.s. and l. kersley (2001): radio tomographic imaging as an aid to modelling of ionospheric electron density, in proceedings of the cost271 sopron workshop (cd-rom obtainable from the project secretariat at: angela.vernon@rl.ac.uk). garcia-fernandes, m., m. hernandes-pajares, j.m. juan, j. sanz and r. orus (2001): ionospheric data combination as a way to improve electron density estimates, in proceedings of the cost271 sopron workshop (cd-rom obtainable from the project secretariat at: angela.vernon@rl.ac.uk). hochegger, g. and r. leitinger (2000): inversions of satellite to satellite electron content: simulation studies with neuogplas, phys. chem. earth, 25 (4), 353-357. hochegger, g., b. nava, s. radicella and r. leitinger (2000): a family of ionospheric models for different uses, phys. chem. earth, 25 (4), 307-310. itu-r (1997): basic circuit reliability for digital modulation systems, doc. 3l/33, rapporteur group 3l-rg1, international telecommunication union, geneva. itu-r (2001a): computation of reliability and compatibility of hf radio systems, itu-r recommendation p.842-2, international telecommunication union, geneva. itu-r (2001b): ionospheric propagation data and prediction methods required for the design of satellite services and systems, itu-r recommendation p.531-6, international telecommunication union, geneva. jakowski, n., a. wehrenpfennig, s. heise, ch. reigber and h. luhr (2001): gps radio occultation measurements of the ionosphere on champ initial results, in proceedings of the cost271 sopron workshop (cd-rom obtainable from the project secretariat at: angela.vernon@rl.ac.uk). leitinger, r. (1998a): neuog-plas an easy to use global electron density model for the ionosphere and plasmasphere, in proceedings of the 2nd workshop of cost251, 140-150. leitinger, r. (1998b): a magnetic field aligned approach to model the topside f2 layer, adv. space res., 22, 789-792. leitinger, r. (1999): magnetic field aligned modeling for the upper f region and for the plasmasphere, phys. chem. earth, 24, 299-303. leitinger, r. and e. feichter (2000): costtec the electron content monthly median map adopted by cost251, phys. chem. earth, 25 (4), 311-314. leitinger, r., s. radicella, b. nava, g. hochegger and j. hafner (1999): nequick-costprof-neuog-plas, a family of 3d electron density models, in proceedings of the 4th workshop of cost251, 75-89. leitinger, r., s. radicella and b. nava (2001): electron density models for assessment studies new developments, in proceedings of the cost271 sopron workshop (cdrom obtainable from the project secretariat at: angela.vernon@rl.ac.uk). lilensten, j. and lj. cander (2001): calibration of the tec derived from gps or from models using the eiscat radar, in proceedings of the cost271 sopron workshop (cdrom obtainable from the project secretariat at: angela.vernon@rl.ac.uk). mitchell, c.n., c. chong and p.s. spencer (2001): observing electron concentration using radio-occultation and groundbased gps observations, paper presented at the cost271 workshop, sopron, hungary, 25-29 september 2001. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27 (1), 35-40. the muraves muon telescope: technology and expected performances annals of geophysics, 60, 1, 2017, s0103; doi:10.4401/ag-7378 s0103 the muraves muon telescope: technology and expected performances giulio saracino1,2,*, fabio ambrosino1,2, lorenzo bonechi3, alan bross4, luigi cimmino1, roberto ciaranfi3, raffaello d’alessandro3,5, flora giudicepietro6, giovanni macedonio6, marcello martini6, vincenzo masone1, nicola mori3,5, pasquale noli1, massimo orazi6, giuseppe passeggio1, anna pla-dalmau4, lorenzo roscilli1, paolo strolin1,2 1 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 2 università di napoli federico ii, dipartimento di fisica, naples, italy 3 istituto nazionale di fisica nucleare (infn), sezione di firenze, sesto fiorentino, florence, italy 4 fermilab, batavia, illinois, usa 5 università di firenze, dipartimento di fisica e astronomia, sesto fiorentino, florence, italy 6 istituto nazionale di geofisica e vulcanologia (ingv), sezione di napoli osservatorio vesuviano, naples, italy abstract the muraves project aims to study the inner structure of the upper part of the mt. vesuvius volcano by muon radiography (muography) technique. very high energy muons, produced by cosmic rays in the atmosphere, can penetrate large thickness of rocks. by measuring the attenuation of the muons flux trough the volcano cone is possible to obtain a 2d image of the density structure. internal discontinuities, with a spatial resolution of about 10 m, can be, in principle, resolved. an absolute average density measurement can be provided too. the project, funded by the italian ministry of university, research and education (miur), is led by ingv and infn. in this article the mechanical structure of the detectors and background suppression techniques are reported. 1. introduction the muraves project represents the first attempt for a muography of the upper part of mt. vesuvius. this challenging project implies a strong background rejection and a higher sensitivity with respect to previous muographies. a muon telescope consisting of an array of four identical detectors will be used, for a total sensitive area of 4 m2. the detectors are designed on the basis of the experience acquired in the mu-ray r&d program [ambrosi et al. 2011; anastasio et al. 2013a, 2013b; ambrosino et al. 2014, 2015] using the same technology of plastic scintillator bars of triangular cross-section, equipped by wave-length shifting (wls) fibres that are coupled to silicon photomultipliers (sipms). major technical upgrades have been introduced in the mechanical structure, in the photo-sensors, in the front-end electronics and in daq board. the last three technical upgrades are discussed in a dedicated contribution to these proceedings [cimmino et al. 2017]. here we will focus on the mechanical structure and present some considerations on the crucial issue of background suppression. 2. mechanical structure the basic “module” of the telescope consists of 32 plastic scintillator bars with their 32 wls fibres conveyed to an optical connector. the optical connector is located on a printed circuit board (pcb) that hosts the 32 sipms. a front-end electronic board (called slave) provides the voltage bias to the sipms and gives a local trigger (lt) digital signal when at least one of the 32 sipms produces a signal above the discriminator threshold. the lt signals of all modules are processed by a data acquisition board (called master), which gives a global trigger (gt) signal according to its programmable trigger logic. when a gt is sent back to the slave boards, all the relevant data are acquired and stored by the master. two modules are pre-assembled side-by-side in a “layer”, having about 1 m2 active area. each layer is enclosed, equipped by the sipms and the slave boards, article history received july 5, 2016; accepted october 5, 2016. subject classification: muon radiography, muography, volcanoes, geophysical imaging, particle detectors. in an aluminium shell that protects the modules and their electronics. the layers weigh about 60 kg and can be easily handled by two persons. circular military type connectors make cabling very easy. the layers give the coordinate perpendicular to the bars and lying in its geometrical plane. two adjacent layers with orthogonal bars form a “x-y plane”. these planes can be placed vertically thank to special foots that can be fixed on a frame or directly on the floor. alternatively the planes can be mounted horizontally on a support frame, as shown in figure 1. the modularity of the system allows building up muon telescopes with flexible geometry. the x-y planes are self-sustaining and can be placed easily to different distances from each other. since the master can control up to 32 slaves, up to 8 x-y planes can be connected to form a tracker with more layers and/or with a larger surface. 3. background sources the recorded events may correspond to muons that crossed the volcano coming from the observed direction, or may be due to background. a strong background rejection becomes essential for low muon fluxes, as in the case of rock thickness of order of 1 km. the most likely sources of background are fake tracks or real tracks produced by electrons, positrons and background muons. fake tracks fake tracks are reconstructed from hits of spurious origin, either temporally correlated or not. temporally uncorrelated hits may originate from dark counts or from one or more uncorrelated particles, hitting different layers of the detector (figure 2). the corresponding background can be reduced by lowering the dark count rates, by reducing the time coincidence window among the hits, or by increasing the number of x-y planes or the spatial resolution, in order to have tighter constraints in the alignment of the track. a way to measure this background is to put out of time the signal coming from the different layers. a fake track may result also from several particles temporally correlated, as those produced in the same cosmic shower (figure 3). in addition to tighter constraints, a time resolution enabling to measure the time of flight of the candidate muon helps its rejection. saracino et al. 2 figure 1. three x-y planes placed horizontally in the cosmic test stand. figure 2. example of a fake track generated by two not temporally correlated particles. figure 3. example of a fake track generated by two temporally correlated particles coming from the same cosmic shower. 3 real tracks produced by electrons and positrons at the ground level, part of the cosmic radiation consists of electrons, positrons and gammas. some of them may produce a track if they cross the detector in the backward-forward (bf) or forward-backward (fb) direction. a technique to reduce this background is to introduce a slab of material (typically iron) between tracking planes, so that these particles are absorbed or produce an electromagnetic shower that can be identified by the presence of high multiplicity hits. due to the poor alignment of the hits, a good spatial resolution can help in reducing significantly this background. real tracks produced by background muons this background may result from muons coming from the “wrong” direction, i.e. from the backward direction (bf muons) or from the “right” direction, ie from the volcano direction (fb muons). bf muons can be rejected by a time of flight measurement. fb muons are not distinguishable from signal muons. the origin of background fb muons is not completely understood. one possible source could be the volcano edifice itself. a possible source is multiple scattering (ms) in the volcano, in particular affecting muons that exit the volcano with a low energy (see figure 4). to quantify this background is hard, as in principle a simulation over the entire volcano edifice is required. a test measurement has been performed at the showa-shinzan lava dome [nishiyama et al. 2014], using two different muon telescopes consisting of nuclear emulsions films with interposed lead plates. due to the micrometric spatial resolution of the nuclear emulsion, it is possible to discriminate muons with different energies measuring the muon scattering in the lead sheets. it has been shown that by rejecting muons with energy below 2 gev the background level was reduced and the signal was compatible with the muon flux expected to survive crossing a few hundred meters of rock in the volcano. with a 200 mev energy cut, the background level was not compatible with any reasonable value of rock density. for electronic detectors, the approach followed by tanaka et al. [2014] to reduce the background level consisted in introducing lead and iron layers into a detector consisting of several planes of plastic scintillators. a drastic background reduction was obtained. 4. background rejection in muraves the good spatial resolution (≈3 mm), the large amount of scintillator bars per layer (64), the usage of at least three x-y layers to reconstruct the track, the short coincidence time window (40 ns) and the time resolution (1 ns) adequate for a time of flight measurement are expected to reduce significantly most of the backgrounds discussed in the previous sections. the insertion of lead layers is expected to reject a notable part of the background from low energy muons, the main source of fb background they introduce an energy threshold through two effects. the first one is that muons below a minimal energy cannot cross the lead. table 1 shows the minimum energy as a function of lead thickness, in the range 5-50 cm [olive et al. 2014]. the second effect is the multiple scattering suffered by the muons that cross the shield, that is strongly correlated to the muon energy and can be used for its estimate. however, the use of a large amount of lead can be applied only in detector locations where the lead can be transported and installed. for the measurement campaign at the mt. vesuvius this will not be a real problem. in fact, the possible sites selected for the muon telescope can be reached by a truck and geologthe muraves muon telescope: technology and expected performances lead thickness (cm) e min (mev) 15 300 20 350 25 400 30 450 35 550 40 600 45 650 50 750 table 1. the minimum energy required to a muon to cross a certain thickness of lead. figure 4. pictorial view of a muon entering the volcano with a certain direction and exiting with a different one because of the multiple scattering inside the rock. ical surveys showed the stability of the ground. in general, one of the main challenges for the next generation of muon telescopes is the introduction of an energy threshold in a more practical way than by using a large amount of lead. the energy threshold on the muons decreases, of course, the number of signal muons. the expected loss in the signal can be evaluated from the energy spectrum of the signal muon crossing mt. vesuvius, as observed from the telescope. figure 5 shows the thickness of mt. vesuvius, as seen from one of the possible detector sites. as an example, figure 6 shows the energy spectrum after 1012 m of rock at an elevation angle of 6°. figures 7, 8 and 9 show the fraction of muons surviving after propagation through volcano that arrive at detector with energies above the energy threshold, in three particular cases corresponding to rock thicknesses of 400 m, 800 m and 1310 m. figure 10 shows this fraction as a function of elevation and azimuth angles, for two different energy cuts of 1 gev and 2 gev. muons have been generated assuming the energy distribution described in tanaka et al. [2003]. 5. rejection of low energy muons in muraves we have studied a basic detector configuration consisting of three x-y planes, with a lead layer between the second and the third x-y plane further downstream with respect to the muon direction (figure 11). the first two x-y planes define the muon track, before multiple scattering in the lead. the displacement of in the third x-y plane with respect to the track projection is correlated to multiple scattering. in a preliminary study we have used as a multiple scattering estimator the |2 of the alignment of the three hits. figure 12 gives the selection efficiency as saracino et al. 4 figure 5. rock thickness of mt. vesuvius as seen from the muraves observation point. figure 6. example of an energy spectrum of muons exiting from the vesuvio after about 1012 m of rock thickness and an elevation angle of 6°. muons have been generated according to the momenta distribution described in tanaka et al. [2003]. figure 7. efficiency of the muon selection as function of the energy cut in the case of 400 m of rock thickness. figure 8. efficiency of the muon selection as function of the energy cut in the case of 800 m of rock thickness. figure 9. efficiency of the muon selection as function of the energy cut in the case of 1300 m of rock thickness. 5 a function of the muon energy, for lead thicknesses of 25 and 50 cm. in both cases the |2 cut has been set so that the efficiency for muon with energy of 2 gev is 95%. we are considering the possibility of adding a fourth x-y plane downstream of the third (figure 13) in order to reconstruct the muon track before and after the lead, so as to measure the distance of closest approach of the two tracks and the scattering angle. figure 14 gives the muon efficiency versus energy for 25 and 50 cm lead thickness. the cuts on the two above variables are such that the efficiency is, for a muon of 2 gev, the 95% for the 25 cm thickness and 90% for the 50 cm. the muraves muon telescope: technology and expected performances figure 10. map of the efficiency of selection of signal muons for a energy cut of 1 gev (left) and 2 gev (right). figure 11. pictorial view of the placement of the x-y planes (blue rectangles) and the lead shield (gray rectangle) in the case of three x-y planes per detector. signal muons from the volcano are coming from the left (black arrow). figure 12. selection efficiency for muons in the case of two different lead thickness. the cut is based on the chi-square of the track fit and is such that at 2gev the efficiency is of 95%. figure 13. pictorial view of the placement of the x-y planes (blue rectangles) and the lead shield (gray rectangle) in the case of four xy planes per detector. signal muons from the volcano are coming from the left (black arrow). figure 14. selection efficiency for muons in the case of two different lead thickness. the cut is based on the chi-square of the track fit and is such that at 2gev the efficiency is the 95%. 6. conclusions the muraves project aims to the first muographic observation of the upper part of the mt. vesuvius. an array of four muon detectors of 1 m2 sensitive area is under construction, upgrading the technology developed in the frame of the mu-ray r&d program. the use of three or four x-y tracking planes, the high segmentation and spatial resolution, the measurement of the muon time of flight and the insertion of a lead layer are expected to lead to the required background rejection. however, the complexity of the background is such that the final word will come from experience on the field. references ambrosi, g., et al. (2011). the mu-ray project: volcano radiography with cosmic-ray muons, nucl. instrum. meth. a, 628, 120-123. ambrosino, f., et al. (2014). the mu-ray project: detector technology and first data from mt. vesuvius, j. instrum., 9, c02029. ambrosino, f., l. bonechi, l. cimmino, r. d’alessandro, d.g. ireland, r. kaiser, d.f. mahon, n. mori, p. noli, g. saracino, c. shearer, l. viliani and g. yang (2015). assessing the feasibility of interrogating nuclear waste storage silos using cosmic-ray muons, j. instrum., 10 (6), t06005. anastasio, a., et al. (2013a). the mu-ray detector for muon radiography of volcanoes, nucl. instrum. meth. a. 732, 423-426. anastasio, a., et al. (2013b). the mu-ray experiment. an application of sipm technology to the understanding of volcanic phenomena, nucl. instrum. meth. a, 718, 134-137. cimmino, l., f. ambrosino, l. bonechi, r. ciaranfi, r. d’alessandro, v. masone, n. mori, p. noli, g. saracino, p. strolin (2017). the muraves telescope front-end electronics and data acquisition, annals of geophysics, 60 (1), s0104; doi:10.4401/ag-7379. nishiyama, r., s. miyamoto and n. naganawa (2014). experimental study of source of background noise in muon radiography using emulsion film detectors, geosci. instrum. meth., 3, 29-39. olive, k.a., et al. (2014). particle data group, chin. phys. c, 38, 090001. tanaka, h., k. nagamine, n. kawamura, s.n. nakamura, k. ishida and k. shimomura (2003). development of a two-fold segmented detection system for near horizontally cosmic-ray muons to probe the internal structure of a volcano, nucl. instrum. meth. a, 507, 657-669. tanaka, h.k.m., t. kusagaya and h. shinohara (2014). radiographic visualization of magma dynamics in an erupting volcano, nat. commun., 5, 3381. *corresponding author: giulio saracino, istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy; email: saracino@na.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. saracino et al. 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol. 48, 01, 05ok.qxd 65 annals of geophysics, vol. 48, n. 1, february 2005 key words radon – geodynamical precursor – mt. etna 1. introduction the release of radon from natural minerals has been known since the 1920s and it depends on the activity concentrations of 226ra in the earth’s crust, soil, rock and water. radon monitoring has recently been used as a possible tool for earthquake prediction. the origin and mechanisms of radon anomalies and their relationship to earthquakes is poorly understood, although several experiments, mathematical modelling and in situ hydraulic experiments have been performed (sing et al., 1991; virk et al., 1994; virk, 1995; al-hilal et al., 1998). the observed radon anomalies correlated with geophysical events may be considered as having two possible origins. either it is produced at and comes from a deep source or it is only locally displaced by other pore fluids whose motion is triggered by geodynamic events. in order to explain radon anomalies and geophysical events several models have been proposed. sing (sing et al., 1991) and planinič (planinič et al., 2001) suggested that the radon anomalies were related to mechanical crack growth in the rocks or to changes in flow rate of groundwater. this may allow either an opening of new cracks, a widening or closing of old cracks or a redistribution of opened and closed cracks. the diffusion coefficient of radon in the rocks will then be significantly changed, determining a change in the amount of radon which will escape from the rocks. an alternative mechanism is the stress corrosion theory, first proposed by anderson and grew (sing et al., 1991). it attributes the radon mailing address: dr. daniela morelli, dipartimento di fisica e astronomia, università degli studi di catania, via s. sofia 64 (cittadella universitaria), 95123 catania, italy; email: daniela.morelli@ct.infn.it gas radon emission related to geodynamic activity on mt. etna giuseppina immè (1), santo la delfa (2)(3), salvatore lo nigro (1), daniela morelli (1) and giuseppe patanè (2) (1) dipartimento di fisica e astronomia, università degli studi di catania, italy (2) dipartimento di scienze geologiche, università degli studi di catania, italy (3) irma srl, istituto di ricerca medica e ambientale, acireale (ct), italy abstract we report preliminary observations on possible correlations between anomalies of subsoil radon concentration and geodynamical events on mt. etna. in recent years several studies have been carried out on radon as a precursor of geophysical events, most of them performed either on tectonic or volcanic areas. the peculiarity of our investigation lies on the choice of the etnean region, in which tectonic and volcanic features are both present. in order to characterize mt. etna features by investigating radon gas in soil, two stations were located along the ne-sw direction on mt. etna. each of the two stations is fitted with a radon detector, a 3d seismic station and a meteorological station. differences in the radon concentration trend in the data from north and south flanks could be linked to different faulting mechanisms and then to different mechanisms of radon uprising. the increase in soil radon concentration could be related to both seismic and volcanic events. 66 giuseppina immè, santo la delfa, salvatore lo nigro, daniela morelli and giuseppe patanè anomalies to slow crack growth controlled by stress corrosion. the mechanism of stress corrosion suggests that the occurrence of radon anomalies may depend on strain rate and local conditions. moreover, according to the compression mechanism proposed by king (sing et al., 1991), the anomalous radon concentration may be due to an increase in crustal compression, impending an earthquake, that squeezes out the soil-gas into the atmosphere at an increased rate. in volcanic areas (monnin et al., 1991) a mechanism invoked to account for variation of radon concentration in soil is linked to an increased heat flow or dry steam discharge that would push up the available underground radon. another hypothesis is that increased radon activity in the soil is due to the collapse of pores volume and the following up flow of deeper radon-rich ground gas. the present study investigated an area in which tectonic and volcanic features are both present. continuous measurements of radon, environmental parameters and seismic signals were recorded, studying the possible correlation between radon anomalies and geophysical events. 2. sites of investigation the etnean area is characterized by tectonic structures. the sites where we located two stations were chosen considering the most active faults. in the etnean area several fault systems became very active before and sometimes during eruptive events. the most extended among the cropping up structural discontinuities was chosen, which lies along the ne-sw direction through the volcano. the first site (biancavilla) is in the sw flank, the second (vena-piedimonte etneo) in the ne flank. several researchers (benina et al., 1984; patanè et al., 1995) have observed that the fault segment in the south west mt. etna sector, becomes significantly seismically active when an eruptive event is coming; instead the segment in the ne sector and in the valle del bove valley shows a seismicity at less energy, however it is the site of sometimes very dangerous eruptions for the neighbouring villages (for example the 1928 eruption that destroyed the mascali village). figure 1 shows a view of the fault system, where it can be noted that the set of faults, which characterizes the south-western flank near biancavilla (where the first station is located) represents the extension, along the nesw direction, of north-eastern fault-system of the naca and piedimonte area (in which the second station is located). 2.1. biancavilla station the station is located at about 1111 m a.s.l., between biancavilla and ragalna, in a region in which the biancavilla fault and the ragalna fault are predominant (romano, 1982). the biancavilla faults, close to the site, are small ground fractures with prevailing nesw strike (patanè and imposa, 1996). they have been reported as faulting effects of the earthquake on 27th march 1983 (azzaro, 1999). extensional movements with small vertical displacements were observed in the locality of calcerana; the occurrence of a secondary set of cracks trending in the nw-se direction seems also to suggest left-lateral components (azzaro, 1999). the ragalna fault is in the ne-sw direction between santa maria di licodia and ragalna east villages. the first evidence occurred in connection with the santa maria di licodia earthquake in may 1898 (azzaro, 1999). discontinuous cracks along ne-sw direction develop in a narrow extent up to 300 m in length at the southern side of ragalna east village. this fault is less active than the biancavilla one. the frequent seismicity along the trend suggests that the ragalna fault is partially hidden and has a longer development, at least 2.5 km more than its morphological evidence would indicate (barbano et al., 1980; azzaro, 1999). 2.2. vena station the station is located in vena (a village near piedimonte) at about 740 m a.s.l. (romano, 1982). 67 gas radon emission related to geodynamic activity on mt. etna the structural belt system is that of the naca and piedimonte fault-system. on the basis of its morphostructural aspects, this highly tectonic area can be divided into two structurally linked systems where the ne-sw trend is always the tectonically dominant one. in fact, the whole north-eastern flank of the etnean complex is characterized by disturbances in that orientation, in particular along a strip developing almost continuously by the naca and piedimonte fault-system. the naca fault system is a couple of subparallel step faults facing the southeast with an overall extent of about 200 m. they give rise to escarpments, locally called ripe della naca, dislocated by minor faults running n-s. the eruptive events of 1928 were associated with this system, originating from effusive cracks along one of the main fault lines. the piedimonte fault system is the repetition at ne of the naca fault system of the dominant ne-sw trend. it is an step fault between piedimonte and fiumefreddo, variously dislocated by faults on nw-se average trend; the most developed of these partly coincides with the upper course of the fogliarino river. the structural trend of the defined naca and piedimonte fault-system is dislocated by minor faults generally running wnw-ese. the main faults are in steps facing the east. the trend can be considered one of the most important elements for interpreting correctly the structural pattern of the volcano. in fact the swarm of eruptive cracks characterizing the northern slope of the valle del bove and the southern area of piano provenzano can be correlated with that trend. it is particularly interesting to observe that the disturbances connected with the trend appear dislocated on the north-eastern flank by transverse faults along the nw-se trend. locally these faults seem to show large strike-slip movement. the division of the structural trend into the above described two systems is probably connected with a disturbance having a nw-se trend, completely covered by the scorciavacca lavas and therefore with no in situ evidence to support it (lo giudice et al., 1982). 3. experimental devices usually radon measurements, especially in inaccessible zones, are performed by means of solid-state nuclear track detectors, but these can give only integrated results and they need a frequent substitution to read the tracks. we preferred an active device, with sampling of soil gas into the detecting instrument. the choice of an active detection allowed radon monitoring for short time periods and both short and longterm analysis. in particular we performed continuous measurements of soil radon concentrations using a portable system that uses an ionization chamber to detect alpha-particles from radon decay. the drawing system of soil gas consists of a drilling rod with an exchangeable drilling tip, with air-lock closed by a rivet and a capillary probe. the drilling rod is driven into the ground and a capillary probe is inserted into the drilling rod, the highest part of the capillary probe is connected to a filter in order to eliminate the moisture, then to a progeny filter that allows the 222rn pass only. the system is connected to the ionization chamber by means of a pump. counting time was 10 min, flow rate 0.05 l/min, and the probe was driven into the soil at 1 m depth to reduce meteorological influence. fig. 1. map of the sites location: sw biancavilla; ne vena (piedimonte etneo) and fault-system in the etna region. 68 giuseppina immè, santo la delfa, salvatore lo nigro, daniela morelli and giuseppe patanè the seismic station is composed of a three directional 1 hz seismometer connected with a portable digital acquisition data system and a gps time signal receiver. the gps gives the time indication relative to the universal time. 4. experimental results measurements started on july 2001 and they are still in progress. the period of investigation has been characterized by geodynamical instability, in fact, two eruptive events occurred: the july-august 2001 and the october 2002january 2003 eruptions. since we started measurements just when the first eruption began it has not been possible to determine the background value of radon concentration, namely radon concentration recorded in a quiescent period, to take as a reference level. however, we can compare the radon concentration values during the eruptive phenomena, and in the interval between them. figures 2 and 3 report the daily averages of radon concentration obtained at the southwestern and north-eastern stations respectively. the trend of the barometric pressure, the most influential meteorological parameter, is also reported. the first evidence from our results is the different behaviour of radon concentration shown at the two flanks, corresponding to vena and biancavilla sites. first of all, while in the vena station the values are always higher than 9000 bqm-3, in the biancavilla one they do not exceed the value of 8000 bqm-3. the different behaviour is not due to different kinds of soil, in fact it consists, in both sites, of lava and volcanic ash in alternated layers, moreover the uranium content is the same in both sites, as observed by means of a previous gamma spectrometry investigation of the soil. moreover the comparison with the barometric pressure trend, the same happens for the other meteorological parameters, indicates that the radon trend cannot be related to the meteorological trend: even if the atmospheric conditions are the same in both sites the radon concentration trend is opposite. the differences could be originated by the different way in which the dislocations in the two fault segments under investigation occur. in fact in the south-western sector, where the fault system has a tectonic origin, the earthquake fault plane solutions are related to horizontal strike-slip mechanisms, instead in the north-eastern flank, where both tectonical and volcanic behaviours are present. the earthquake fault plane solutions could also be associated to dip-slip mechanisms or to widening of the eruptive fractures (montalto et al., 1996; patanè et al., 1996; la delfa et al., 1999; vinciguerra et al., 1999). that could determine different mechanisms of radon uprising, as this is made easier in the north-eastern sector where the permeability is higher due to the enlarged fracturations, where in the recent past several eruptions occurred (timpe della naca 1928). our aim was to investigate the correlations between earthquakes and radon concentration in the etnean area. several studies conducted in tectonic areas disclosed a relation to earthquakes of magnitude larger than 3 (virk et al., 1994; igarashi et al., 1995; al-hilal et al., 1998). the etnean area is characterized by a large number of earthquakes, up to thousands per day before an eruptive period (benina et al., 1984; patanè et al., 1995), but with low magnitude (< 3) and rarely they exceed magnitude 4. moreover mt. etna has a very complex structure, due to the occurrence of both tectonical and volcanic phenomena, so it is difficult to correlate radon anomalies and seismic events or volcanic phenomena. if we look at the ne flank radon concentration trend (fig. 3) we can distinguish an anomaly in the radon behaviour: concentration values start to increase on 27th october 2002, reach a maximum on 1st november 2002 and a minimum on 3rd november 2002. during this period several earthquakes of magnitude higher then 3 occurred, some of them reached values up to md 4.5 (29th october 2002 time 09:02:00 epicentral area of santa venerina). in order to find possible relations in a period between 1st september 2002 and 30th november 2002 that includes the eruption beginning (27th october 2002), we plotted (fig. 4) the trend of radon concentration together with the daily rate of earthquakes and the strain release. the latter was calculated for each event by means of the following relations (patanè et al., 1995): fig. 2. daily means of radon concentration (open square) and of atmospheric pressure (full circle) in the biancavilla site; july 2001-september 2003. fig. 3. daily means of radon concentration (open square) and of atmospheric pressure (full circle) in the vena site; july 2001-september 2003. 69 gas radon emission related to geodynamic activity on mt. etna 70 giuseppina immè, santo la delfa, salvatore lo nigro, daniela morelli and giuseppe patanè only some local events. in fact, in this study we focused on 3rd november event with md 3.5, because the epicentral zone was close (less then 1 km) to the vena station (ne station) and because it occurred just after the radon anomaly (fig. 3), and is associated with evident soil fractures. according to the observations, it is more appropriate to extend the investigation to the volcanic aspects, in particular to the eruption which started on 27th october 2002, that involved all the geostructures chosen for our investigation. if we then look at the radon trend recorded in the ne flank, under the hypothesis of volcanic activity, we can explain that the plateau region of higher radon activity (december 2001-may 2002) could indicate a relation with the magma uprising. the magma, moving inside the subsoil, could have induced a larger convection of radon gas owing to the increasing subsoil temperature, that could facilitate the gas escaping from the rocks. previous studies in volcanic areas (el chichon volcano and krafla volcano; monnin and seidel, 1991) have shown that radon measurements in the ground could be used as a precursor signal of eruptive phases. fig. 4. radon concentration (black line), daily earthquakes rate (black column bar) and strain release (grey histogram) measured in the period between 1st september 2002 and 30th november 2002 (vena station). , . . log log m r2800 0 35 0 17 maxwaeq ven = + + n i { ∆ ` j: d (4.1) where mwaeq is the earthquake magnitude woodanderson equivalent, µmax is the maximum peak amplitude (in mm) at vena station, r(θ,ϕ) is the radiation pattern for sh-waves, ∆ven is the distance between the station and the hypocentre; and . . .log e m m2 9 1 9 0 024ri waeq waeq 2 = + (4.2) where eri is the energy determined from the formula of richter (richter, 1958), then the strain release is e ri . ∆ven was determined by means o correspondence between the s-p time interval (ts-p) read on the seismograms and the hypocentral distance recorded in vena during the 19911993 eruption; approximately at ts-p ≈ 1s ∆ven of 4 km corresponds (patanè et al., 1995). from fig. 4 we can observe that, as well as the radon rises the earthquake daily rate and strain release rise correspondingly at the eruption beginning. by a first analysis we cannot confirm that the radon gas is a seismic event precursor except for 71 gas radon emission related to geodynamic activity on mt. etna 5. conclusions we have monitored radon gas concentration in two sites on mt. etna to find correlations to volcanic and tectonic geophysical events. the results of our investigation allow us to characterize the fault systems in the two chosen sites. from our data a different behaviour is evident between the north-eastern and the south-western flanks. in particular our data exhibit: i) higher values in vena then in biancavilla station; ii) more fluctuating daily values in biancavilla than in vena; iii) opposite seasonal trends in the two sites. these results seem to exclude meteorological influence and suggest a link to different faulting mechanisms. even if the two fault systems lie in the same direction, at present the structural discontinuities near biancavilla are typical of tectonic areas, while those near vena have both tectonic and volcanic character, the latter being historically involved by eruptive events. according to the literature, we confirm that radon anomalies are evident when seismic events occur at magnitudes higher than 3. in fact, the most important result seems to be that of the 3rd november 2002 earthquake (md 3.5) that affected a zone near the station and that followed a radon anomaly. however, the occurrence of volcanic events during the observation period also suggests a possible correlation between radon concentration trend and eruptive activity of mt. etna. in the graph of fig. 4, an evident radon uprising can be observed during the seismic phenomena before and after the eruptive event, the same trend is also observed after the main seismic phase. since the investigated period was characterized by high geodynamic activity, the increase in radon flow could be linked to both seismic and eruptive phenomena. references al-hilal, m., m.r. sbeinati and r. darawcheh (1998): radon variation and microearthquakes in western syria, appl. rad. isot., 49 (1-2), 117-123. azzaro, r. (1999): earthquake surface faulting at mount etna volcano (sicily) and implications for active tectonics, j. geodyn., 28, 193-213. barbano, s., m. cosentino, g. lombardo and g. patanè (1980): isoseismal maps of calabra and sicily earthquakes (southern-italy), cnr-progetto finalizzato geodinamica, publication no. 341, pp. 116. benina, a., s. imposa, s. gresta and g. patanè (1984): studio macrosismico e strutturale di due terremoti tettonici avvenuti sul versante meridionale dell’etna, in atti iii convegno annuale del gngts, 931-946. igarashi, g., s. saeki, n. takahata, k. sumikawa, s. tasaka, y. sasaki, m. takahashi and y. sano (1995): ground-water radon anomaly before the kobe earthquake in japan, science, 269, 60-61. la delfa, s., g. patanè and c. centamore (1999): the geodynamics of mt. etna volcano during and after the 1984 eruption, ann. geofis., 42 (3), 545-563. lo giudice, e., g. patanè, r. rasà and r. romano (1982): the structural framework of mount etna, mem. soc. geol. it., 23, 125-158. monnin, m.m. and j. seidel (1991): radon and volcanic surveillance, in radon measurements by etched track detectors, application in radiation protection, earth science and the environment, edited by a. saeed durrani and i. radomir, 274-285. montalto, a., s. vinciguerra, s. menza and g. patanè (1996): recent seismicity of mount etna: implications for flank instability, in volcano instability on the earth and other planets, edited by w.c. mcguire, a.p. jones and j. neuberg, geol. soc. spec. publ. 110, 169-177. patanè, g. and s. imposa (editors) (1995): atlas of isoseismal maps of etnean earthquakes from 1971 to 1991 (università degli studi di catania), pp. 89. patanè, g., a. montalto, s. imposa and s. menza (1994): the role of regional tectonics, magma pressure and gravitational spreading in earthquakes of the eastern sector of mt. etna volcano (italy), j. volcanol. geotherm. res., 61, 253-266 patanè, g., g. coco, m. corrao, s. imposa and a. montalto (1995): source parameters of seismic events at mount etna volcano, italy, during the outburst of the 1991-1993 eruption, phys. earth planet. inter., 89, 149-162. patanè, g., a. montalto, s. vinciguerra and j.c. tanguy (1996): a model of the onset of the 1991-1993 eruption of etna (italy), phys. earth planet. inter., 97, 231-245. planinič, j., v. radolič and ž. lazanin (2001): temporal variation of radon in soil related to earthquakes, appl. rad. isot., 55, 267-272. richter, c.f. (1958): elementary seismology (freeman and co., san francisco, ca), pp. 768. romano, r. (editor) (1982): mount etna volcano: a review of recent earth sciences studies, mem. soc. geol. it., 23, pp. 205. singh, m., r.c. ramola, b. singh, s. singh and h.s. virk (1991): radon anomalies: correlation with seismic activities in northern india, in radon monitoring in radioprotection, environmental and/or earth science, edited by a. saeed durrani and i. radomir, 354-375. vinciguerra, s., s. garozzo, a. montalto and g. patanè (1999): eruptive and seismic activity at etna volcano between 1977 and 1991, volcanoes in the quaternary, geol. soc., 161, 89-107. virk, h.s. (1995): radon monitoring of microseismicity in the kangra and chamba valley of himachal pradesh, india, nucl. geophys., 9 (2), 141-146. virk, h.s. and b. singh (1994): radon recording of uttarkashi earthquakes, geophys. res. lett., 21 (8), 737-740. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 1 site effect studies following the 2016 mw 6.0 amatrice earthquake (italy): the emersito task force activities giovanna cultrera1*, ezio d’alema2, sara amoroso3, barbara angioni1, paola bordoni1, luciana cantore3, fabrizio cara1, arrigo caserta4, rocco cogliano5, maria d’amico 2, giuseppe di giulio 3, deborah di naccio3, daniela famiani1, chiara felicetta2, antonio fodarella5, sara lovati2, lucia luzi2, claudia mascandola2, marco massa2, alessia mercuri1, giuliano milana1, francesca pacor2, marta pischiutta1, stefania pucillo5, rodolfo puglia2, gaetano riccio5, gabriele tarabusi1, maurizio vassallo3 1istituto nazionale di geofisica e vulcanologia, sezione roma1, roma, italy 2istituto nazionale di geofisica e vulcanologia, sezione di milano, milano, italy 3istituto nazionale di geofisica e vulcanologia, sezione roma1, l'aquila, italy 4istituto nazionale di geofisica e vulcanologia, sezione cnt, roma, italy 5istituto nazionale di geofisica e vulcanologia, sezione roma1, grottaminarda (av), italy * giovanna.cultrera@ingv.it abstract on august 24, 2016, at 01:36 utc a mw 6.0 earthquake struck an extensive area of the central apennines (italy) between the towns of norcia and amatrice. due to the mainshock magnitude and the widespread damaging level of buildings in the epicentral area, the emersito task force has been mobilized by the istituto nazionale di geofisica e vulcanologia (ingv). the aim of emersito is to carry out and coordinate the monitoring of local site effects, caused by geological and geomorphological settings. during the first days of the seismic emergency, emersito installed a temporary seismic network for site effect studies at 4 municipalities close to the epicentral area (amandola, civitella del tronto, montereale and capitignano), using 22 stations equipped with both velocimetric and accelerometric sensors. the selection of the sites where stations have been installed was mainly driven by the proximity to the epicentral area (without interfere with the rescue operations) and by peculiar geologic and geomorphologic settings (topographic irregularities, fault zones, alluvial plains). preliminary analyses performed on ambient noise and aftershocks signals show that directional amplification effects may have occurred at stations installed on the top of topographic irregularities. we also observed the lengthening and amplification of the seismograms and a variability of the peaked frequency across the sedimentary basin between montereale and capitignano, probably related to a different thickness of the deposits. further analyses are necessary to assess the correlation with surface geology. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 2 i. introduction he earthquake ground-motion depends on the source process, on the propagation of the incoming wave-field through the crustal medium, and on the transmission of the seismic waves from bedrock to the earth surface (free-field motion). the last contribution is known as site effect, and substantially modifies the amplitude, the frequency content and duration of the seismic waveforms. three main types of site effects can be identified: impedance contrast between bedrock and softer soil, the lateral heterogeneities in the elastic and/or geometrical properties of the soil and the topographic settings that can produce 2d or 3d effects. site effects due to soft soil layers (e.g. alluvial or sedimentary cover) overlapping hard rock produce seismic wave amplification whose extent depends on the impedance contrast between different layers. these effects are usually modeled by 1d wave propagation through the soil column. when lateral heterogeneities of the subsoil mechanical properties are not negligible (e.g. sediment filled valleys), 2d or 3d geometries are more appropriate to describe the complexity of the ground motion (kawase, 1996; bindi et al., 2009; chaljub et al., 2010; di giulio et al., 2016). in the last decade particular attention has been devoted to the ground motion directional amplification in fault damage zones, related to fracture networks and the resulting stiffness anisotropy of the rock mass (pischiutta et al., 2012). concerning the topographic effects, a correlation between pattern of damage and topographic elevation has been recently investigated (paolucci, 2002; massa et al., 2014), although there are fewer evidences compared to the previous two types of site effects. usually, these studies use seismic noise and background seismicity because stronger events are available for a target area during a seismic sequence only. emersito is a task force of the istituto nazionale di geofisica e vulcanologia (ingv) that involves several headquarters (milano, roma, l’aquila, grottaminarda; italy). its aim is to coordinate and carry out seismic monitoring to investigate possible site effects caused by seismic events of moderate–to-large magnitude (m≥5) in the italian territory. in case of relevant seismic sequences, a rapid installation of temporary seismic stations is fundamental to seismic monitoring, to the research in the field of seismic hazard and, in some cases, to support post-emergency activities (e.g. microzonation studies). emersito operated for the first time during the emilia 2012 sequence (bordoni et al, 2012), and it was officially established in 2015 (ddg, 2015); mobilization procedures have been test during the ingv emergency simulation performed on 2015 (pondrelli et al, 2016). on august 24, 2016 (01:36 utc) a mw6.0 earthquake struck an extensive area in the central apennines (italy) between the municipalities of norcia and amatrice. the epicenter was located by the ingv close to the accumoli village. a large aftershock (mw 5.3, august 24 02:33 utc) and over 9000 aftershocks have been located up to september 16, 2016, in an t annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 3 extended area of about 50 km in the appennine direction (from the town of ussita to the north, to the lake of campotosto in the south), and about 15 km width (ingv working group, 2016). in the first days of the seismic emergency (august 24th-25th, 2016), we have chosen several urban areas located at the margin of the epicentral area and characterized by geological and structural features able to produce local site effects (figure 1). several issues were considered for the selection of the areas: availability of geological maps and first level microzonation studies, location of recording stations of temporary and/or permanent seismic networks (italian national seismic network operated by ingv and italian strong motion network operated by the italian department of civil protection), rescue activities in the epicentral area. the available information was centralized in a web geographic information system (arcgis on-line) to facilitate the data sharing to all emersito members (figure 1). during august 26th-31st, 22 temporary seismic stations were installed in four municipalities: amandola (fm), civitella del tronto (te), montereale (aq), and capitignano (aq). additional 5 recording stations were set in acquasanta terme (ap) by the ingv national earthquake centre in cooperation with the emersito task force. figure 1. thematic levels of emersito arcgis on-line with the 2016 amatrice seismic sequence (from august 24th to september 28th) together with the selected sites and the temporary network. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 4 ii. instrumental setting the instrumental setting was composed of 22 digitizers (reftek 130 and quanterra), equipped with both lennartz-5s velocimeter and episensor accelerometer (emersito report, 2016). all the stations recorded in continuous mode: five of them transferred data to a remote server in real-time, while the remaining stations were in a stand-alone configuration. data will be distributed through the european integrated data archive (eida, http://eida.rm.ingv.it/), by using the international xo code assigned on august, 29th to the temporary emersito network by the federation of digital seismograph networks (www.fdsn.org/networks/detail/xo_2016). the xo network was named “seismic network for site effect studies in amatrice area (central italy)” and the station codes were registered at the international seismological centre (isc) on september 6th. table 1 reports network and station code, municipality, site coordinate, digitizer, sensor, type of seismic data acquisition (real-time vs local), and installation period. iii. description of the selected sites and preliminary results amandola the village of amandola is located 30 km northeast from the epicentral area (figure 1). it extends along a relief, with sw-ne trend, constituted by clayey sandstones. several buildings of the historical center, and also the hospital, suffered some damage after the mainshock. figure 2 shows the location of the 5 stations (table 1), which were installed along the ridge and at the base of the relief, with the aim of investigating possible topographic effects. in particular, the hospital area was monitored by am03 station. figure 2. stations installed at amandola. all stations installed at amandola sent data in real time to the remote ingv acquisition center and were soon available (i.e. in average 2 hours later each event origin time) and downloadable from the real time ingv strong motion database (ismd; http://ismd.mi.ingv.it). due to the broadening of the sequence towards north (gruppo di lavoro ingv, 2016), am05 was integrated to the italian seismic network with the aim to improve the location operated by the ingv national earthquake centre. a preliminary analysis of the horizontal to vertical spectral ratio (hvsr) was performed considering at all stations 1 hour of ambient noise (running window length of 120 seconds, 4th order butterworth filter in the frequency band 0.1-15 hz, tapering 5%, konno-ohmachi logarithmic smoothing with b=20). possible transient signals were deleted by visual inspection. in general, all stations installed at the top of the ridge show hvsr with amplification ranging from 3 to 4 in the frequency range 3-4 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 5 hz. the maximum amplification was observed in the n-nw direction (figure 2); am05, installed at the base of the ridge, can be considered the reference site in the range of interest for topographic effect, showing amplification at frequency > 8 hz due to a very thin layer of landfill. further investigations are needed to ascribe the recordings behavior to a topographical effect. figure 3. hvsr on noise recorded at am02, am03 and am05: (top) quadratic mean of the two horizontal components ± 1 standard deviation; (bottom) rotated spectral ratio at different azimuths. montereale and capitignano the villages of montereale and capitignano are located 20 km south of the mainshock and in correspondence with the southernmost epicentres of the sequence (figure 1). both municipalities suffered slight damage after the mainshock and the larger events. figure 4 shows the 12 stand-alone stations installed to study topographic and basin effects, as well as to investigate possible fault effects close to cp04, cp05 and cp06 (table 1). figure 4. stations installed at montereale and capitignano, superimposed to the geological map of italy at 1:100,000 scale. displayed seismograms refer to the recorded ns component of the ml 3.6 earthquake (01/09/2016 03:53, hypocentral distance r=16km); rose diagrams indicate the horizontal polarization averaged on 19 earthquakes with ml≥ 3 recorded in the period 30/081/09/2016. mn04, mn08 and mn06 are located on the top of a topographic irregularity, characterized by a maximum height of 140 m from the nearby level ground and maximum slope of 18°. they are on miocene flysch units (figure 4) and, as a first approximation, they can be considered stiff rock sites. signal polarization analysis performed in the time domain (jurkevics, 1988) indicates a predominant n-ne/s-sw direction of the horizontal motion at these sites (figure 4). the directional amplification effect is confirmed by the hvsr performed on noise and on earthquakes (mn04 in figure 5). the stations on quaternary alluvial deposits are affected by the lengthening and amplification of the seismograms (mn02 and mn03 in figure 4). at those stations, signal polarization analysis shows an isotropic distribution of poannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 6 larization azimuths, i.e. the site tends to produce a similar amplitude level on the two horizontal components. moreover, the hvsr on noise recorded at mn02 and cp07 show an amplification peak at 1.5 hz and 6 hz, respectively, that it is probably related to a different thickness of the basin deposits (figure 5). further investigations are needed to verify the persistence of the behavior and the correlation with the topography. figure 5. rotated hvsr on noise (left) and on earthquakes (right) recorded at mn04, mn02 and cpt07 of figure 4. the hvsr on earthquakes is averaged on 19 events with ml=3.0÷3.9 recorded in the period 30/081/09/2016; cp07 data were not available in this period. civitella del tronto civitella del tronto, a village located at about 30-40 km east of the epicentral area (figure 1), did not suffer significant structural damages for the august 24th mainshock. the municipality is located on a topographic relief, with a sw-ne trend, consisting of travertine overlaying sandstones. five stand-alone stations were installed along the top and at the base of the relief to detect possible topographic effects (figure 6). data were not available at the time of this study because the stations were dismounted at the end of september 2016. figure 6. location of the stations installed at civitella del tronto. iv. conclusive remarks the emersito efforts during the 2016 amatrice seismic emergency consisted in the deployment of 22 stations equipped with both velocimeter and accelerometer within few days after the mainshock of august 24th 2016, with the aim of studying site effects for selected areas. usually, these studies use background seismicity while during a seismic sequence stronger events are available and they can be recorded with a rapid installation of a temporary seismic network. due to the morphology of the settlements stroke by the earthquake, we focused in the investigation of the site effects of topography, small intra-mountain basin and fault zone to seismic motion. preliminary results suggest the presence of such effects, requiring further annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 7 analyses to assess the correlation with surface geology. the collected dataset will be available in eida node and could support other seismological studies (e.g. source and directivity effects, role of fluids, earthquake location). the remarks in this paper refer to the first phase of the central italy seismic sequence, up to the end of september 2016; at the time of publication, several large magnitude earthquakes that contributed to modify the damaging level of the studied villages have hit the area. acknowledgements we are grateful to the local authorities and the inhabitants of the investigated municipalities that supported us during the network management. the study benefited from funding provided by the italian presidenza del consiglio dei ministri, dipartimento della protezione civile (dpc); scientific papers funded by dpc do not represent its official opinion and policies. references [bindi et al., 2009] bindi, d and parolai, s and cara, f and di giulio, g and ferretti, g and luzi, l and monachesi, g and pacor, f and rovelli, a (2009). site amplifications observed in the gubbio basin, central italy: hints for lateral propagation effects. bulletin of the seismological society of america, 99(2a), 741-760. [bordoni et al, 2012] bordoni, p., azzara, r.m., cara, f., cogliano, r., cultrera, g., di giulio, g., fodarella, a., milana, g., pucillo, g.,riccio g., rovelli, a., augliera, p.,luzi, l., lovati, s.,massa, m., pacor, f., puglia, r., ameri, g. (2012). preliminary results from emersito, a rapid response network for site-effect studies. annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6153. [chaljub et al, 2010] chaljub, e., moczo, p., tsuno, s., bard, p. y., kristek, j., käser, m., stupazzini, m., kristekova, m. (2010). quantitative comparison of four numerical predictions of 3d ground motion in the grenoble valley, france. bulletin of the seismological society of america, 100(4), 1427-1455. [ddg, 2015] decreto direttore generale ingv n. 337 e allegati, 2015; http://istituto.ingv.it/lingv/amministrazione-trasparente1/contenuti-at-vari/decreti/anno-2015decreti-presidente-e-direttore-generale). [di giulio et al, 2016] di giulio, g., de nardis, r., boncio, p., milana, g., rosatelli, g., stoppa, f., lavecchia, g. (2016). seismic response of a deep continental basin including velocity inversion: the sulmona intramontane basin (central apennines, italy). geophysical journal international, 204(1), 418-439. [emersito report, 2016] emersito working group (2016). rapporto preliminare sulle attività svolte dal gruppo operativo emersito a seguito del terremoto di amatrice mw 6.0 (24 agosto 2016, italia centrale). doi: 10.5281/zenodo.151655 [ingv working group, 2016] ingv working group on the amatrice earthquake (2016). second summary report on the m6.0 amatrice earthquake of august 24, 2016 (central italy), doi: 10.5281/zenodo.16624 jurkevics, a. (1988), polarization analysis of three component array data, bull. seismol. soc. am., 78, 1725–1743. [kawase, 1996] kawase, h. (1996). the cause of the damage belt in kobe: the basin-edge effect, constructive interference of the direct s-wave with the basin-induced diffracted/rayleigh waves. seismol res lett, 67:25–34. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 8 [massa, 2014] marco massa, simone barani and sara lovati, overview of topographic effects based on experimental observations: meaning, causes and possible interpretations, geophysical journal international, doi: 10.1093/gji/ggt341. [paolucci, 2002] paolucci, r. (2002). amplification of earthquake ground motion by steep topographic irregularities. earthq. eng. struct. dyn., 31:1831–1853. [pischiutta et al., 2012] pischiutta, m., salvini, f., fletcher, j., rovelli, a., & ben-zion, y. (2012). horizontal polarization of ground motion in the hayward fault zone at fremont, california: dominant fault-high-angle polarization and fault-induced cracks. geophysical journal international, 188(3), 1255-1272. [pondrelli et al., (2016)] pondrelli, s., amato, a., massa, m., montone, p., crescimbene, m., la longa, f., emergenza sismica working group (2016). "pianificazione e gestione di un’emergenza sismica: esercitazione ingv del 26 novembre 2015 effettuata nell’ambito della linea di attività t5 “sorveglianza sismica e operatività post terremoto”. quaderni di geofisica, 317, novembre 2016. issn 1590-2595 [in italian]. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7189 9 table 1. emersito seismic stations installed at the beginning of the 2016 amatrice seismic emergency (network code xo) station code municipality lat. [°] lon. [°] elev. [m] digitizer vel. sensor acc. sensor acquisition installation period am01 amandola 42.980556 13.358708 549 reftek130 le3d-5s episensor real-time 29/08-10/10/2016 am02 amandola 42.979597 13.353573 516 reftek130 le3d-5s episensor real-time 30/08-10/10/2016 am03 amandola 42.981622 13.362768 511 reftek130 le3d-5s episensor real-time 29/08-10/10/2016 am04 amandola 42.983345 13.365007 455 reftek130 le3d-5s episensor real-time 30/08-10/10/2016 am05 amandola 42.977404 13.352786 464 reftek130 le3d-5s episensor real-time from 30/08/2016 cv01 civitella del tronto 42.772736 13.666229 642 q330 le3d-5s episensor stand-alone 31/08-26/09/2016 cv02 civitella del tronto 42.773104 13.669496 600 q330 le3d-5s episensor stand-alone 30/08-26/09/2016 cv03 civitella del tronto 42.772644 13.672969 540 q330 le3d-5s episensor stand-alone 30/08-26/09/2016 cv04 civitella del tronto 42.772273 13.666690 585 q330 le3d-5s episensor stand-alone 31/08-26/09/2016 cv05 civitella del tronto 42.771551 13.663721 605 q330 le3d-5s episensor stand-alone 31/08-26/09/2016 cp01 capitignano 42.52458 13.28758 885 reftek130 le3d-5s episensor stand-alone 29/08-17/09/2016 cp02 capitignano 42.52389 13.29596 898 reftek130 le3d-5s episensor stand-alone 29/08-19/09/2016 cp04 capitignano 42.52529 13.30163 957 reftek130 le3d-5s --stand-alone 30/08-16/09/2016 cp05 capitignano 42.52052 13.30267 925 reftek130 le3d-5s episensor stand-alone 29/08-16/09/2016 cp06 capitignano 42.53498 13.28591 900 reftek130 le3d-5s episensor stand-alone 29/08-17/09/2016 cp07 capitignano 42.50748 13.27508 842 reftek130 le3d-5s episensor stand-alone 30/08-16/09/2016 mn02 montereale 42.51393 13.26704 827 reftek130 le3d-5s episensor stand-alone 26/08-16/09/2016 mn03 montereale 42.51916 13.25532 823 reftek130 le3d-5s episensor stand-alone 26/08-17/09/2016 mn04 montereale 42.52906 13.23519 977 reftek130 le3d-5s episensor stand-alone 27/08-16/09/2016 mn06 montereale 42.52400 13.24480 923 reftek130 le3d-5s episensor stand-alone 26/08-16/09/2016 mn08 montereale 42.52624 13.24125 916 reftek130 le3d-5s episensor stand-alone 27/08-16/09/2016 mn09 montereale 42.52987 13.24546 827 reftek130 le3d-5s episensor stand-alone 27/08-16/09/2016 historical and recent sea level rise and land subsidence in marina di ravenna, northern italy annals of geophysics, 59, 5, 2016, p0546; doi:10.4401/ag-7022 p0546 historical and recent sea level rise and land subsidence in marina di ravenna, northern italy ines cerenzia1,2, davide putero3,2, flavio bonsignore1, gaia galassi4, marco olivieri5,*, giorgio spada4,5 1 arpae emilia-romagna, bologna, italy 2 università di bologna, dipartimento di fisica e astronomia (difa), bologna, italy 3 istituto di scienze dell’atmosfera e del clima (isac-cnr), bologna, italy 4 università di urbino, dipartimento di scienze pure e applicate (dispea), urbino, italy 5 istituto nazionale di geofisica e vulcanologia (ingv), sezione di bologna, italy abstract the regions facing the northern adriatic sea are particularly vulnerable to sea-level rise. several trade ports are located there, and the area is important from social and economical viewpoints. since tourism and cultural heritage are a significant source of income, an increase in sea-level could hinder the development of these regions. one of the longest sealevel time series in the northern adriatic, which goes back to the late 1880s, has been recorded at marina di ravenna, in emilia-romagna region. the record is anomalous, showing a rate of increase that largely exceeds that observed in nearby stations. during the last few decades, geodetic campaigns based on geometric high precision leveling, sar interferometry, and gps have monitored the ravenna area. in this work, tide gauge observations are merged with yet unpublished geodetic data, aiming at a coherent interpretation of vertical land movements. we confirm that land subsidence is the major cause of relative sea-level change at marina di ravenna, at least during the period allowing for a quantitative analysis (1990-2011). the rate of absolute sea-level change (2.2±1.3 mm yr−1 during the same time period), given by the difference between the rate of relative sea-level change and the rate of subsidence, is consistent with the rate of absolute sea-level change observed by altimetry in the northern adriatic sea. 1. introduction sea-level change is one of the key indicators of global warming. according to the latest report of the intergovernmental panel on climate change (ipcc), global sea level rose at a rate in the range between 1.3 and 1.7 mm yr−1 between 1901 and 1990 [church et al. 2013], with an acceleration of ~0.01 mm yr−2 [spada et al. 2015]. however, satellite data [wunsch and stammer 1995] and hydrographic observations [roemmich and owens 2000], in agreement with climate models [spada et al. 2013], show that sea level has not been rising uniformly across the oceans. in some regions, rates are even several times larger than the global mean sea-level rise, while in others sea level is falling [cazenave and llovel 2010]. globally, sea-level rise is mainly driven by the thermal expansion of water masses and by the melting of glaciers and ice sheets [lombard et al. 2005, plag 2006, cazenave and remy 2011], but both factors also imply significant local and regional variations [spada and galassi 2016]. these are also controlled by changes of atmospheric pressure and, in the case of semi-enclosed basins like the mediterranean sea, by the forcing exerted from nearby ocean masses driving variations of relative sea level [umgiesser et al. 2011]. other causes are significantly contributing, such as the deposition and compaction of sediments, tectonic movements, the soil reaction to fluid or gas withdrawal, and the response of the earth to changes in surface loading [milne et al. 2009]. understanding the mechanisms driving sea-level changes is important to promote mitigation measures, when these are possible [mcgranahan et al. 2007, nicholls and cazenave 2010]. it has long been recognised that the northern adriatic sea is particularly vulnerable to sea-level rise. this stems from the combined effect of low elevation coastlines, subsidence and intense urbanization [armaroli et al. 2012, perini et al. 2015]. the area is of great importance from the social and economical standpoints, mainly for several ports located here (e.g., venice, trieste, ravenna, dubrovnik, split; see figure 1), as well as for article history received march 30, 2016; accepted september 14, 2016. subject classification: sea-level change, tide gauges, land subsidence. tourism and cultural heritage [e.g. cori 1999, preti and de nigris 2009]. an important cause of relative sealevel change in some parts of this area facing the northern adriatic sea is land subsidence [bitelli et al. 2010], of both natural and anthropogenic origins [brunetti et al. 1998, carminati et al. 2003, stright jr et al. 2008, baldi et al. 2009]. the effects of subsidence are particularly important in the area of ravenna in emilia-romagna region, which belongs to the delta portion of the po river plain (figure 1). due to its sedimentary origin, the po valley is subject to natural subsidence, which is partly compensated by the transportation of sediments and by the advance of the coastline [teatini et al. 2005]. over the last century, the observed subsidence showed a significant increase in the area of ravenna, with rates as large as tens of millimetres per year between 1972 and 1977 [bertoni et al. 1973, carbognin et al. 1978, carbognin et al. 1984, teatini et al. 2005]. a first cause of subsidence is the ground water withdrawal from subsurface aquifers, promoted by the economic growth after the end of world war ii and by the development of tourism along the coast [preti and de nigris 2009, bitelli et al. 2010]. another source of subsidence is the gas extraction from deep on-shore and off-shore reservoirs, with the first producing a diffuse sinking along the coasts and the latter having much more localised effects [gambolati et al. 1991, 1999]. in addition to these causes, sea-side processes as the steep drop in the sand transportation by rivers due to the overbuilding of the river banks, or the reclamation of wetlands and lagoons areas on the north, are playing a significant role [teatini et al. 2005, preti and de nigris 2009]. for its vulnerability and for the high subsidence rates, the area of ravenna has been intensively studied. on one hand, efforts from integrated modelling approaches [gambolati et al. 1991, 1999] have allowed a quantitative evaluation of the main causes impacting the stability of the whole area. on the other hand, various geodetic surveys have provided new insights on the recent rates of subsidence. over time, the surveys have been planned by the italian geographic military institute (igm), the regional environmental service (arpae, formerly arpa), the consorzio di bonifica di ravenna, the geological service of the ravenna municipality, and the italian oil company eni-e&p. furthermore, in the northern adriatic sea, the porto corsini tide gauge sited in marina di ravenna (formerly known as porto corsini) has collected a long record (1897-2014) of relative sea level, in a location that during last century has been particularly affected by land subsidence [teatini et al. 2005]. no other comparatively long sea-level time series is available from instruments deployed along the coasts of emilia-romagna region. it has long been known (see e.g., caputo [1971]) that long-term relative sea-level variations at the porto corsini tide gauge are strongly affected by the vertical land movements and possibly by pier instability. consequently, the correlation with sea-level signals from other tide gauges deployed along the adriatic and northern mediterranean coast (venice, trieste and genoa) is poor, as also pointed out by raicich [2011] and tsimplis et al. [2012]. indeed, the mean rate of relative sea-level change shown cerenzia et al. 2 figure 1. map of the northern adriatic sea and surrounding regions, showing the toponyms of interest for this work. italy and emiliaromagna region are coloured in light grey and yellow, respectively. a blue line marks the path of the po river. 3 by the porto corsini tide gauge record exceeds by 3-6 times the typical mediterranean rates during the period 1897-2011 [tsimplis et al. 2012]. since they can help to assess the long-term subsidence, the relative sea-level data from the tide gauge of porto corsini are of remarkable importance. in the framework outlined above, the present study aims at investigating how the sea-level rise observed at the porto corsini tide gauge is reflecting the observed local land subsidence. during the last few decades, this is accomplished by means of yet unpublished geodetic data, which allow for estimating the rate of change of absolute sea level. we believe that this study may improve the reliability of the porto corsini tide gauge observations, which are now included in the quality-controlled section of the permanent service for mean sea level database (psmsl, see holgate et al. [2012]) only for a very limited time span (1969-1972). the paper is organised as follows. in section 2, we describe the data and the methods employed in our study, in section 3 we first estimate the long-term relative sea level and subsidence at the porto corsini tide gauge, and then we focus on the last few decades. the results are discussed in section 4 and the conclusions are drawn in section 5. 2. data and methods 2.1. sea level measurements the porto corsini tide gauge station (44.49° n, 12.28° e) is placed on the italian adriatic coastline in the municipality of marina di ravenna, east of the town of ravenna in emilia-romagna region (see figure 1). here, instrumental sea-level observations are available since 1897, when the station became part of the newly born italian tide gauge network. the annual mean sea-level data of porto corsini used in this work come from several sources: (i) the permanent service for mean sea level (psmsl) archive (data retrieved on january 1, 2016, from http://www. psmsl.org/), for the time period 1897-1922 [woodworth and player 2003, holgate et al. 2012], (ii) annali idrologici di bologna (hydrology annals of bologna) for the period 1934-1979 [annali 1934-1979], and (iii) dstn/apat/ispra (institute for environmental protection and research) for the period 1980-2014 (data are available from http://www.mareografico.it). a plot of the annual mean sea-level record for porto corsini is shown in figure 2. in order to obtain a coherent time series, the different datasets have been homogenised relying upon the information on benchmarks and data overlaps (see also mosetti [1969]). a relocation of the tide gauge was, for example, necessary due to works for the re-construction of the dock next to the marina di ravenna lighthouse. the sea-level measurements at the porto corsini tide gauge are historically referred to the igm benchmark located in the marina di ravenna lighthouse, sited 150 m away from the tide gauge. the lighthouse was connected in 1885 to the first igm italian leveling network [salvioni 1957], gathering at the time the main italian harbours, with the establishment of the porto corsini tide gauge. however, this initial network resulted inadequate and after the end of world war ii it was completely re-established. the first leveling of the lighthouse benchmark with respect to the national altimetric datum, defined by the tide gauge in genoa in 1942, was accomplished in 1970. subsequently, other subsidence in marina di ravenna figure 2. annual values of mean sea-level at the porto corsini tide gauge in marina di ravenna during 1897-2013. the blue line represents the long-term trend obtained by applying the least squares regression method over the entire time series, while the red one shows the trend during 1970-2013. level surveys were performed by igm in 1977 and 1990, which provided a reference for the sea-level data collected by the porto corsini tide gauge since 1990. in 2012, the lighthouse benchmark was connected to the igm network and since september 2013 the height observed in that occasion became the reference value used by the tide gauge. 2.2. land subsidence measurements in this work, different measurements of land altitude and of its rate of change are considered and compared, during different time periods. as mentioned in the introduction, land subsidence in the ravenna area has been monitored by several agencies. here, we consider only locations geographically or historically related to the porto corsini tide gauge site. a summary of the monitoring campaigns is presented in table 1. concerning the igm measurements, points prior to world war ii resulted inconsistent with respect to the rest of the period. salvioni [1957] homogenized the 1897-1957 dataset by using a compensation approach and the same igm datum. other surveys were performed by the consorzio di bonifica di ravenna in 1949 and 1972, with the establishment of a local leveling network in ravenna municipality. the elaboration in contour maps of these igm and consorzio di bonifica di ravenna surveys for the periods 1897-1957 and 1949-1972 [salvioni 1957, carbognin et al. 1984, arca and beretta 1985] was later georeferenced and gridded in gis maps by teatini et al. [2005], to which we refer to qualitatively characterise the land subsidence at marina di ravenna prior to 1970. first idroser spa and afterwords the arpae agency, on behalf of emilia-romagna region, monitored the subsidence by using the high precision geometric leveling method, assuming as a stable reference the igm benchmark at sasso marconi in the apennine mountains [regione emilia-romagna arpa 2006]. many sites along the ravenna coast were monitored, including the marina di ravenna lighthouse and the tide gauge station on the dock (table 1). the results from the 1993 survey campaign are not considered here, since doubts on their validity have been raised [regione emilia-romagna arpa 2001]. since 2005, arpae continued the land subsidence monitoring by the support of sar interferometry (psinsartm technique), to provide rates of subsidence [regione emiliaromagna arpa 2007, 2012]. two different calibration methods were used, providing rates of vertical movement for periods 19922000, 2003-2006, and 2006-2011 (see table 1). for the first two periods sar interferometry was calibrated by 2005 leveling surveys [regione emilia-romagna arpa 2007], while, for the last period, a network of 16 gpsgnss permanent stations located in the emilia-romagna plain [bitelli et al. 2014, 2015] was used [regione emilia-romagna arpa 2012]. since none of these gps stations is located in or near marina di ravenna, we did not include any gps data in the present analysis. we consider the scatter points available in the vicinity of the tide gauge site, i.e., on the dock structure and on top of the lighthouse or in its neighbourhood (an example of these points is shown in figure 3). when several scatter points were available on the same structure (this occurred during period 2003-2006), averaged values have been considered. table 2 reports raw data for the height measurement and for the rate of vertical movement, cerenzia et al. 4 agency date type of survey notes igm and consorzio di bonifica di ravenna 1897, 1949, 1972 geometric leveling gis maps in teatini et al. [2005] igm 1970, 1977, 1990 geometric leveling lighthouse benchmark idroser 1984, 1987 geometric leveling lighthouse benchmark and tide gauge benchmark on the dock arpae 1999, 2005 geometric leveling lighthouse benchmark and tide gauge benchmark on the dock arpae 1992-2000 insar calibrated on 2005 geometric leveling two scatter points: one on the dock and one over the lighthouse arpae 2003-2006 insar calibrated on 2005 geometric leveling four scatter points: two on the dock and two over the lighthouse arpae 2006-2011 insar calibrated on gps permament network two scatter points: one on the dock and one over the closest building to the lighthouse table 1. main surveys carried out in the marina di ravenna area by various agencies, with data used in the present work. 5 collected on the dock structure and on to the lighthouse or in its neighbourhood from 1970 to 2011. 3. results 3.1. long-term relative sea level and land subsidence in marina di ravenna the historical time series of relative sea level at the porto corsini tide gauge is shown in figure 2 (the data sources are summarized in section 2.1). here, annual mean relative sea-level data are presented, over a period exceeding one century (1897-2014). by considering the whole time series, the total variation of relative sealevel is ~1.0 m. to assess the rate of relative sea-level change, we applied the least squares regression, which provided a value of 8.5±0.2 mm yr−1. this trend, displayed as a blue line in figure 2, is comparable to the one that can be obtained by applying the theil-sen estimator [theil 1950, sen 1968], i.e. 8.7 mm yr−1 (the 95% confidence interval for the rate ranges between 8.1 and 9.5 mm yr−1). to obtain this statistically significant (p < 0.001) trend, we adopted the theil-sen implemensubsidence in marina di ravenna date/interval lighthouse dock level (m a.s.l.) rate (mm yr−1) level (m a.s.l.) rate (mm yr−1) 1970 0.9325 1977 0.7896 1984 0.6866 1.0688 1987 0.6315 1.0187 1990 0.6273 1999 0.5205 0.9298 2005 0.4639 0.8848 1992-2000 −6.7±1.0 −5.9±1.0 2003-2006 −7.2±2.6 −5.7±2.6 2006-2011 −5.3±2.0 −3.1±2.0 figure 3. aerial view of marina di ravenna showing the porto corsini tide gauge station and the surroundings. the arpae benchmark is sited in the same place of the tide gauge. the altimetric reference is the benchmark of the lighthouse. red dots mark the scatter points from the interferometric satellite considered in this work, for the period 2006-2011 (units are mm yr−1). table 2. direct and undirect height measurements at the lighthouse and at the dock of marina di ravenna from the subsidence surveys presented in table 1. a distinction is made between data sampled as height a.s.l. (e.g. levelling data, column “level”) and as average rates during specific time intervals (e.g. insar, column “rate”). tation in the openair software [carslaw and ropkins 2012], after having filled the gaps in the time series by applying a linear interpolation. such rate of relative sealevel rise largely exceeds that obtained for other stations along the northern adriatic and mediterranean coasts over comparable time periods. according to raicich [2011] and tsimplis et al. [2012], the rates of relative sea level observed in venice, trieste and genoa are 2.6, 1.3 and 1.2 mm yr−1, respectively. by applying a quadratic (three-parameter) regression, we find that the value of the sea-level acceleration at marina di ravenna is 0.08±0.02 mm yr−2, which provides evidence for a significant increase in the long-term rate of relative sealevel change. although existing estimates of global long-term sea-level acceleration are scattered (see review in spada et al. [2015], and references therein), the value for marina di ravenna largely exceeds all of these. the historical subsidence in the area of marina di ravenna is qualitatively inferred from the gis slicing of the leveling surveys along the shoreline of ravenna municipality from the work of teatini et al. [2005], by associating the subsidence between the intervals 18971949 and 1949-1972 with the benchmark elevation measured by igm in 1970 at the marina di ravenna lighthouse (see figure 14 in teatini et al. [2005]). the time history of subsidence is plotted in figure 4, where the observations are referred to the lighthouse of marina di ravenna. coherently with the general trend in the ravenna area (see the case study of porta adriana in the town of ravenna [carbognin et al. 1978]), the subsidence in the area of marina di ravenna experienced a marked increase during 1950-1970, with rates close to 24 mm yr−1. these exceed the “natural trend” during 1897-1950 by about 6 mm yr−1. in the ensuing years, the rate decreased, but still exceeding the natural trend. the last values obtained from the interferometric satellite analysis during 2006-2011 are close to 5.5 mm yr−1. overall, these data are consistent with the increase in the groundwater withdrawal between the 50s and late 70s and its subsequent decline. while the mainland appears to be substantially stable, many sites along the coast are still subsiding at rates exceeding the natural trend [teatini et al. 2005]. marina di ravenna is coherent with this general behaviour, although the most recent interferometric analysis (2006-2011) has evidenced a recovering of the slower natural trend of subsidence. new data from the interferometric analysis scheduled for year 2016 will be useful to confirm or deny this trend. 3.2. sea level at marina di ravenna since 1970 during the time period 1970-2013, monthly averaged values of relative sea level are available for marina di ravenna, characterised by a high degree of completeness. we have applied the least squares regression to this relatively short time series, obtaining a trend of 7.7±0.3 mm yr−1, displayed as the red line in figure 2. again, this trend resulted comparable to the significant (p < 0.001) trend obtained by the theil-sen method, i.e. 7.5 mm yr−1 (the 95% confidence interval for the rate ranges between 7.0 and 8.1 mm yr−1). to analyse the sea-level variability coherently, seasonal cycles should cerenzia et al. 6 figure 4. altitude measurements in marina di ravenna at various epochs. given the various elaborations applied to data before 1970 (see section 2.2), the association with the 1970 igm leveling is approximate (dashed line). data after 1970 refer to different sources and survey methods, marked by various symbols (see legend). further details are given in tables 1 and 2. 7 be removed, since these derive from regional coastal effects. in many coastal areas, a large fraction of the seasonal cycle is associated with the steric component of sea-level change, induced by density variations in response to temperature and salinity fluctuations [tsimplis et al. 2012]. however, due to the influence of regional hydrology, the non-steric component of the seasonal cycle of sea level was found to be important in some cases. the seasonal cycle and the amplitude of coastal sea-level height thus represent the local response to a seasonal pattern of large-scale sea-level variability and hydrology (see bergant et al. [2005], and references therein). in marina di ravenna, we estimated the “climatological” annual variation considering monthly values, although the details of this analysis are not shown here. the minimum sea-level height was found in february (1.51±0.26 m), while two maxima were found in november (1.64±0.25 m) and june (1.55±0.22 m). this is consistent with with the results of bergant et al. [2005], showing a minimum during the first months of the year, and two maxima in november and april. however, by a mann-whitney “u test” [mann and whitney 1947], we have found that the difference between the amplitude of the two peaks is not statistically significant (p = 0.764). to detect possible cyclic components in the tide gauge time series of porto corsini since 1970, we have employed the empirical mode decomposition (emd) method of huang et al. [1998], in the improved version of torres et al. [2011] (the complete ensemble emd with adaptive noise). the eemd has been adopted by breaker and ruzmaikin [2011] to analyse the san francisco tide gauge record and, more recently, by spada et al. [2014] to study the nuuk/godthab (sw greenland) time series, by galassi and spada [2014] to characterise the cyclic sea-level variations across the adriatic sea and by vecchio et al. [2014] to study the transient effect of a tsunami in the mediterranean sea. the eemd allows to split non-linear and non-stationary time series into a limited sequence of empirically orthogonal “intrinsic mode functions” (imfs) that describe cyclic modes not necessarily characterised by a constant amplitude nor phase. differently from traditional spectral methods, the eemd is not requiring assumptions on the functional expression of the regression model. figure 5 shows the seven imfs that compose the non monotonic portion of the porto corsini tide gauge record. imf2 and imf3 capture the semi-annual and annual pesubsidence in marina di ravenna figure 5. results of the emd analysis of the marina di ravenna relative sea-level time series since 1970, with the imfs in the top frames and the residual (res) in the bottom, respectively. units are cm throughout. riodicities, respectively. imf4 corresponds to an oscillation at about 1.7 years whereas the imf5 reveals a period of about 3.6 years; this latter has been detected, adopting a similar approach, on the whole adriatic sea scale (see galassi and spada [2014]). imf6 and imf7 show longer periods, with 8.4 and 14.2 years, respectively. res represents the non-cyclic residual, revealing the long-term natural trend of the time series [huang et al. 1998] that remains once all the cyclic components have been sifted. the linear trend of res since 1970 is 6.8 mm yr−1, somewhat smaller than the one obtained above by the theil-sen method. 3.3. land subsidence at marina di ravenna since 1970 to estimate the rate of subsidence in marina di ravenna since 1970, we have considered the full dataset from the igm and arpae leveling surveys and the results of the interferometric analysis (see tables 1 and 2). values for years 1992, 2000, 2003, 2006 and 2011 have been derived from the three interferometric analyses, by computing the height variation corresponding to the interferometric velocity multiplied by the time difference from the previous or the following leveling datum. this has allowed to compute regression curves and to estimate the rates of subsidence of the ground settlement at the lighthouse and at the tide gauge scatter points located over the dock, separately. the data shown in figure 6, clearly indicate that the rate of land subsidence at the lighthouse benchmark (vl ) has been significantly larger than the rate at the tide gauge point over the dock (vd ). indeed, by two linear regressions we obtain vl = −13.0±0.7 mm yr −1 (1970 2011) (1) and vd = −8.3±0.4 mm yr −1 (1984 2011), (2) for the two points, respectively. the different rates of subsidence may be explained as a local effect due to the weight of the structures on which the lighthouse and the dock are located, being the embankment lighter compared to the base of the building. the same conclusion can be reached only considering the level surveys data, available for the period 1984-2005, which provide the values vl = −10.3±0.7 mm yr −1, and vd = −8.3±0.7 mm yr−1, for the lighthouse and for the dock, respectively. this further motivates the choice of considering separately the rates observed on the dock structure and those observed close to the lighthouse. an acceleration in the rate of land settlement is found at both points, with comparable amplitudes. from a quadratic regression we obtain al = 0.19±0.03 mm yr −2 for period 1970-2011 and ad = 0.18±0.05 mm yr −2 during 1984-2011. 4. discussion a simultaneous evaluation of the sea-level rise and land subsidence in marina di ravenna is possible during the time interval 1990-2011. in fact, the elevation of the dock structure was measured for the last time in 2011 and the reference altitude value used by the tide gauge was not recoverable before 1990, when the igm level of 1990 was adopted. during 1990-2011, the dock cerenzia et al. 8 figure 6. elevation measurements at the porto corsini lighthouse (red) and dock (green) benchmarks at various epochs. the various data sources and sampling methods are denoted by different symbols, as indicated in the legend (see tables 1 and 2 for details). 9 structure subsided by 167±2 mm, where this value is derived from the linear regression of the dock time series that indicates a rate of subsidence of 7.6±0.1 mm yr−1. during the same time span, in marina di ravenna, relative sea-level rose by 216±29 mm, a value obtained by a linear regression of the annual averaged time series between 1990 and 2013 that provides a rate of change of 9.8±1.3 mm yr−1. on longer time periods, the analysis becomes more problematic, since the land movement is measured only at the marina di ravenna lighthouse since 1970 and in the previous years only one estimate across the whole marina di ravenna area was available. it is likely that the vertical movement at this site had a dominant role on the observed relative sea level, although the available data do not allow to obtain any quantitative estimate. our results firmly indicate that, during the period 1990-2011, local land subsidence was the major responsible for relative sea level rise observed at the site of porto corsini. anthropogenic factors, as water extraction and soil compaction, are known to be the major causes of land subsidence, contributing to the relative sea-level variations observed, as explained in teatini et al. [2005]. other possible factors contributing to sealevel change in the adriatic sea are worth to be discussed, although their role appears to be less important. the modelled contribution of glacial isostatic adjustment (gia) to the rate of relative sea-level change at the porto corsini tide gauge location only amounts to 0.1 mm yr−1, according to the ice-6g model (http://www.atmosp.physics.utoronto.ca/~peltier/ data.php) [peltier et al. 2015]. this implies that for the time period 1990-2011, only ~2 mm of the observed relative sea-level change can be attributed to the effects of gia. the contribution of co-seismic and postseismic vertical deformation to relative sea-level change at tide gauges has been assessed at global level by melini et al. [2004] and melini and piersanti [2006]. using data from the seismic moment tensor catalogue, they have clearly shown that earthquakes have cumulatively the tendency to produce a positive sealevel trend of ~ 0.2 mm yr−1 at the tide gauges locations. the low level of seismicity along the coasts of the northern adriatic sea may suggest that this global value may represent an upper bound for the site of marina di ravenna. in addition, in their work on the effects of seismicity on subsidence in foreland basins with an application to the case study of venice, carminati et al. [2007] have suggested that accelerations in the rate of subsidence due to co-seismic displacements associated to earthquakes that occurred along active faults around and within the po plain are negligible. according to the “sea-level equation” (see, e.g., spada [2016]), the rate of absolute sea-level change at the porto corsini tide gauge can be estimated by subtracting the rate of subsidence from the rate of relative sea-level change. during the time period 1990-2011, we obtain a trend of 2.2±1.3 mm yr−1. therefore, despite the large influence of land vertical movement at marina di ravenna, the absolute sea-level increase can be recognised combining the rate of subsidence with the tidal record. since the porto corsini tide gauge is not facing the open sea (see figure 3), a direct comparison with the rate of absolute sea-level change observed by altimetry is not feasible. however, the satellite altimetry trend during 1993-2012 in the northern adriatic sea has been close to 3 mm yr−1 [bonaduce et al. 2016], in fair agreement with the value obtained from in situ observations at the porto corsini tide gauge. 5. conclusions the analysis performed in our work has permitted, for the first time, the simultaneous evaluation of the sea-level rise and of land subsidence in marina di ravenna, along the north-adriatic coast of italy. although long records exist for both the relative sealevel variations at the porto corsini tide gauge (1897-2014) and for the historical land movements in the marina di ravenna area (1897-2011), a quantitative analysis has been possible only for the period 1990-2011, due to missing information about tide gauge setup before 1990 and since the last useful data of height measurement dates 2011. we have enlightened the importance of studying land movements at the porto corsini tide gauge by relying upon measurements collected on the dock structure above which the tide gauge is located. indeed, the analysis of the available geodetic data indicates that the reference point at which the tide gauge is usually connected (i.e., the lighthouse) has been subject to a rate of subsidence significantly exceeding that of the dock structure. in the time interval 1990-2011, the land subsided at a rate of 7.6±0.1 mm yr−1 while relative sea-level rose at a rate of 9.8±1.3 mm yr−1. therefore, our study confirms that land subsidence has been the major cause of relative sea-level change at marina di ravenna, at least during the period permitting a quantitative comparison (1990-2011) between relative sea level and vertical land motion signals. the residual (rate of relative sea-level change minus rate of land subsidence) of 2.2±1.3 mm yr−1 is found to be coherent with the altimetric observations in the northern adriatic for the rate of change of absolute sea-level during the same time period. subsidence in marina di ravenna acknowledgements. some of the porto corsini 1993-2014 data were obtained by the dstn/apat/ispra website. the seacoast unit of regione emilia-romagna is also acknowledged for useful discussion. this work has been partly funded by a research grant of the department of pure and applied sciences (dispea) of the urbino university “carlo bo” (cup h32i160000000005 and h32i15000160001). fabio raicich and two anonymous reviewers have greatly contributed to improve the manuscript. references annali (1934-1979). annali idrologici, tech. rep., ufficio idrografico del po, parma; 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ionospheric parameter modelling and anomaly discovery by combining the wavelet transform with autoregressive models annals of geophysics, 58, 5, 2015, a0550; doi:10.4401/ag-6729 a0550 ionospheric parameter modelling and anomaly discovery by combining the wavelet transform with autoregressive models oksana v. mandrikova1,2, nadegda v. fetisova (glushkova)1,2, riad taha al-kasasbeh3,*, dmitry m. klionskiy4, vladimir v. geppener4, maksim y. ilyash5 1 institute of cosmophysical research and radio wave propagation (far-eastern branch of the russian academy of sciences), paratunka, kamchatka region, russian far east, russian federation 2 kamchatka state technical university, petropavlovsk-kamchatsky, russian federation 3 al-balqa applied university, amman, jordan 4 saint petersburg electrotechnical university “leti”, saint petersburg, russian federation 5 saint petersburg state university, saint petersburg, russian federation abstract the paper is devoted to new mathematical tools for ionospheric parameter analysis and anomaly discovery during ionospheric perturbations. the complex structure of processes under study, their a-priori uncertainty and therefore the complex structure of registered data require a set of techniques and technologies to perform mathematical modelling, data analysis, and to make final interpretations. we suggest a technique of ionospheric parameter modelling and analysis based on combining the wavelet transform with autoregressive integrated moving average models (arima models). this technique makes it possible to study ionospheric parameter changes in the time domain, make predictions about variations, and discover anomalies caused by high solar activity and lithospheric processes prior to and during strong earthquakes. the technique was tested on critical frequency fof2 and total electron content (tec) datasets from kamchatka (a region in the russian far east) and magadan (a town in the russian far east). the mathematical models introduced in the paper facilitated ionospheric dynamic mode analysis and proved to be efficient for making predictions with time advance equal to 5 hours. ionospheric anomalies were found using model error estimates, those anomalies arising during increased solar activity and strong earthquakes in kamchatka. 1. introduction one of the most important tasks of ionospheric data analysis is connected with ionospheric state control along with anomaly discovery with subsequent interpretation of anomalies occurring during ionospheric perturbations [afraimovich et al. 2000, afraimovich et al. 2001, nakamura et al. 2007, pervak et al. 2008, hamoudi et al. 2009, liming et al. 2011, mandrikova et al. 2012a, mandrikova et al. 2012b]. ionospheric anomalies may be caused by increased solar activity; in seismically active regions they can also be encountered during the periods of high seismic activity [mandrikova et al. 2012a, 2012b]. the earth’s ionosphere is the area of the atmosphere extending from 80 km to 1000 km and affecting radio wave propagation [maruyama et al. 2011, nakamura et al. 2009, watthanasangmechai et al. 2012]. the dynamic mode of the ionosphere is determined by the distribution of its parameters over altitude, atmospheric density, chemical composition, and article history received january 21, 2015; accepted august 10, 2015. subject classification: ionospheric parameters, perturbation, earthquake, anomaly, prediction, wavelet transform, autoregressive integrated moving average model, critical frequency data, total electron content data. graphical abstract. spectral characteristics of solar radiation. ionospheric studies are mostly based on analyzing environmental parameters, which can dramatically change with altitude and depend on a solar activity cycle, geomagnetic conditions, geographical coordinates and contain characteristic diurnal and seasonal changes [afraimovich et al. 2000, afraimovich et al. 2001, nakamura et al. 2009, watthanasangmechai et al. 2012, danilov 2013, roux 2012]. among the main parameters characterizing the ionospheric state are variations of critical frequency of f2-layer of the ionosphere (fof2) and total electron content. ionospheric critical frequency data (fof2) are registered by vertical radio sounding using an impulse radar (ionosonde) and these data are represented as a time series. variations of total electron content are obtained using readings of double-frequency ground gps receivers [afraimovich et al. 2000]. variations of fof2 and total electron content characterizing electronic concentration of the ionosphere largely depend on diurnal and seasonal variations of solar radiation, solar activity cycles, and geomagnetic activity [afraimovich et al. 2000, nakamura et al. 2007, hamoudi et al. 2009, mandrikova et al. 2012a, mandrikova et al. 2012b]. abrupt fluctuations of the ionospheric electron density arising due to increased solar and/or geomagnetic activity lead to abnormal behavior of ionospheric critical frequency variations and total electron content variations [afraimovich et al. 2000, nakamura et al. 2007, hamoudi et al. 2009, mandrikova et al. 2012a, mandrikova et al. 2012b]. ionospheric state control and anomaly extraction are important tasks of ionospheric parameter analysis [afraimovich et al. 2000, afraimovich et al. 2001, liu et al. 2008, nakamura et al. 2009, roux 2012, watthanasangmechai et al. 2012, danilov 2013, ezquer et al. 2014, zhao et al. 2014]. ionospheric parameters affect many spheres of our life and can have a negative effect on satellite systems and radio communication propagation. an ionospheric anomaly is a very complex phenomenon in time and space [afraimovich et al. 2000, afraimovich et al. 2001, danilov 2013] and they mostly arise in periods of increased solar and geomagnetic activities [afraimovich et al. 2000, afraimovich et al. 2001, danilov 2013, roux 2012]. in seismically active regions they can also be observed in periods of increased seismic activity [afraimovich et al. 2000, afraimovich et al. 2001, nakamura et al. 2007, nakamura et al. 2009, watthanasangmechai et al. 2012, klimenko et al. 2012]. the registered ionospheric parameters contain anomalies manifesting themselves as a significant deviation (rise or decrease) from the background level. the main reason for anomalies in lower ionospheric layers (regions e and d) is ionization rate variation due to corpuscular invasion. variations in region f are caused by indirect factors neutral composition and dynamic processes [prolls 1995]. morphology and physics of ionospheric phenomena were described in many previous works and review papers [prolls 1995, rees 1995, buonsanto 1999, mendillo 2006, danilov 2013]. the ionospheric structure, being variable and heterogeneous, is normally studied by variation analysis of environmental parameters registered by special equipment. lack of knowledge on the structure of registered parameters and an insufficient number of formal models for parameter representation make the task in question rather complicated to deal with. despite the fact that contemporary technologies of near-earth space monitoring are developing rather steadily and rapidly, the capabilities to control and predict the ionospheric state are still rather limited [nakamura et al. 2007, mandrikova et al. 2012a, mandrikova et al. 2012b]. the complex structure of ionospheric parameter variations results in the conventional techniques being inefficient for any kind of mathematical modelling or computer simulation. the conventional techniques mentioned above are mostly based on smoothing procedures and do not allow us to explore subtle features of data [nakamura et al. 2007, pervak et al. 2008, hamoudi et al. 2009, liming et al. 2011, mandrikova et al. 2012a, mandrikova et al. 2012b], the latter usually containing important information about the processes under study. low sensitivity of the registered parameters is the reason why their processing and analysis have become even more complicated and time-consuming. one of the popular approaches to data simulation and analysis is the application of arima models [kay and marple 1981, huang et al. 2013]. they make it possible to reveal steady characteristics of data and some possible ways of data prediction. multiple practical research has confirmed the capability of this mathematical apparatus to deal with various applied problems in the area of geophysics [box and jenkins 1970, basseville and nikiforov 1993, huang et al. 2013]. nonetheless, arima methods have certain restrictions on working with particular kinds of data [nakamura et al. 2009, huang et al. 2013]. wavelet-based approaches have been broadly applied to multiple areas of signal processing and related fields in the last twenty years [al-kasasbeh 2004, mandrikova and bogdanov 2007, zhang et al. 2008, li et al. 2009, vargas-bracamontes et al. 2009, beltrán and ponce de león 2010, hafez et al. 2010, mandrikova et al. 2010, hafez and ghamry 2011, liming et al. 2011, mandrikova et al. 2011, wang et al. 2011, gwal et al. 2012, mandrikova et al. 2012b, mandrikova et al. 2012c, bakhshi et al. 2013, castillo et al. 2013, han and chang 2013, mandrikova et al. 2013a]. these methods encompass some mandrikova et al. 2 3 elements of the approximation theory [donoho and johnstone 1998, mallat 1999] and filtering [mitra 1998] and in many cases they help us to solve the tasks in question [vargas-bracamontes et al. 2009, mandrikova et al. 2012a, mandrikova et al. 2012b, mandrikova et al. 2012c, mandrikova et al. 2013a]. owing to a wide range of existing wavelet bases with compact support, wavelets provide special opportunities for detailed studies of complex data structure. fast computational algorithms for wavelet decomposition [mallat 1999, percival and walden 2000, daubechies 2001] undoubtedly pose a substantial advantage for real-time signal processing, which is indispensable for quick data analysis. the suggested technique of ionospheric parameter simulation and analysis is based on combining the wavelet transform with arima models. the underlying idea is concerned with employing multiresolution analysis [mallat 1999, percival and walden 2000, daubechies 2001], which acts to decompose the original dataset into a finite number of multiscale components with different time resolutions. application of the wavelet transform to fof2 data in order to extract trend and fluctuations was also suggested in the work by roux [2012] and showed its efficiency for finding connections between ionospheric parameters and solar and geomagnetic activities. the components obtained by decomposition have a more simple structure compared to the original data. this paper is focused on arima methods used for approximation. this approach has been applied by the authors since 2003 and was successfully adapted for anomaly extraction in subsoil radon. identification of component models includes noise suppression and determination of stable data characteristics. in this paper we suggest joining multiscale component models together so that they will make a general parametric construction characterizing data variability in time and allowing us to make predictions. using residual error estimates for final prediction we have developed a numerical algorithm for anomaly extraction. multicomponent time series models of ionospheric critical frequency fof2 (data were registered by the university of cosmophysical research and radio wave propagation, paratunka village, kamchatka region, russian far east) and tec (data were obtained from bi-frequency ground-based gps receivers) [afraimovich et al. 2000, afraimovich et al. 2001, erdoğan and arslan 2009] in kamchatka and magadan were constructed. they confirmed the efficiency of the suggested technique and made it possible for scientists to analyze regular diurnal and seasonal parameter changes. using the estimate of deviation from the background level ionospheric anomalies were extracted with a duration ranging from several dozens of minutes to several hours. these anomalies arise during ionospheric perturbations. our detailed analysis has shown that the anomalies described above occur during the increased solar activity and strong earthquakes in kamchatka (several seismic events of energy class k > 12.5 have been analyzed to make this conclusion). 2. suggested technique 2.1. model construction consider a closed space vj=closl2(r) 2 j{ 2jt−k :k∈z) with scale j = 0 formed by a scaling function {∈l2r. using multiresolution decompositions to level m we can represent the original data the following way [mandrikova et al. 2011, 2012c, 2013a]: f0(t) = -m ∑ j=-1 g2 jt + e2 jt + f 2-mt, (1) where: f 2-mt∈v-m, g2 jt∈wj; wj is a space of scale j formed by wavelet basis wj,k(t) = 2 j/2w(2jt − k); f 2-mt = ∑ k c-m,k{-m,k(t) is an approximating component; c-m,k=〈f ,{-m,k〉 are decomposition coefficients characterizing trend; g2 jt = ∑ k dj,kwj,k(t) are detailed components; dj,k =〈f ,wj,k〉:( j,k)∈i j are decomposition coefficients representing multiscale details; e2 jt=∑ k ej,kwj,k are noise components; ej,k= 〈f ,wj,k〉:( j,k)∉i j are decomposition coefficients. expression (1) can be used for representing a time series as a sum of multiscale components f 2-mt, g2jt, and e2jt, where j = −1, — −m —— . in order to suppress noise components e2jt and extract components f 2-mt and g2jt characterizing stable characteristics of data the following sequence of steps is needed: (1) reconstruction of the components f 2-mt, g2jt, and e2jt, j = −1, — −m —— to the original scale j = 0 and calculation of the reconstructed components f0 n(t)=∑ k cn0,k{0,k(t), g0 n(t)=∑ k dn0,kw0,k(t), e0 n(t)=∑ k en0,kw0,k(t), where n is a component number. (2) application of a conventional approach [basseville and nikiforov 1993] for determining an arima model in order to approximate each reconstruction component f0 n(t)=∑ k cn0,k{0,k(t), g0 n(t)=∑ k dn0,kw0,k(t), e0 n(t)= ∑ k en0,kw0,k(t). (3) performance of diagnostic tests for component models. if diagnostic tests for a component model confirm its adequacy, the component model reflects stable characteristics of data (according to arima model theory). (4) combination of the extracted component models to create a general parametric construction (all other components from expression (1) will be considered as ionospheric parameter modelling noise components). the outcome of this combination is a parametric multicomponent model representing data changes in the time domain: snj,k(t) b n j,k t, where: snj,k(t)=∑ pnj l=1 cnj,l~ n j,k−l t−∑ hnj n=1 inj,na n j,k−n(t) is an estimated value of the n-th component; cnj,l are autoregressive parameters of the n-th component; ~nj,kt =∇ onbnj,k(t), b 1 j,k= c 1 j,k, b n j,k= d n j,k, n= 2,m — , on− is a difference order of the n-th component; pnj is an autoregressive model order of the n-th component; hnj, i n j,k are model orders and parameters of moving average of the n-th component model; anj,k are residual errors of the n-th component model; m is the number of modelled components representing stable characteristics of data; nnj is the length of the n-th component; b1j,k ={j,k is a scaling function; b nj,k=wj,k, n= 2,m — is a wavelet basis of the n-th component; j is scale. remark 1. expression (2) is valid for any scale j. thus, identification of the suggested model can be carried out without reconstructing the components to the original scale j = 0 (see step 1 above). by changing a decomposition level (see expression (1)) we can obtain various models of type (2) for representing a time series. minimization of the residual error leads to the best multicomponent model of a time series. the resulting estimate can be further improved by applying various wavelets. 2.2. data prediction and anomaly discovery prediction of s nj,k+q, q≥1 (see expression (2)) determines the predicted value s nj,k at the moment t = k with time advance q. value s nj,k+q can be found via model (2): the residual errors of the n-th component at scale j are determined as a difference between predicted and actual values of data at time point t = k + q: model (2) represents regular changes in approximated data. if an anomaly has occurred, the data structure will be modified, but the absolute residual errors will rise. anomaly extraction can be performed using a residual error estimate of component models when performing predictions. anomaly discovery in a component with number n at scale j will be based on the following condition: where taj is a threshold given in advance (this threshold defines the presence of anomalies on scale j in data), uj is the window length on scale j. 3. experimental research the experimental research was conducted using critical frequency fof2 data collected in kamchatka and magadan. the critical frequency fof2 data contain missing values, which might impede further simulation and detailed analysis. to reduce errors it was decided to choose time periods with the smallest number of misses. due to seasonal features of ionospheric processes, the data were first split into seasonal sets and then simulated separately. below there is a detailed description of the critical frequency data simulation for winter and summer seasons. the model construction was based on hourly values of fof2 critical frequency data for the period from 1968 to 2013. these data were obtained by vertical radio sounding of the ionosphere. simulation of quiet variations of ionospheric parameters was performed using time intervals of a relatively calm geomagnetic field (intervals where the resulting k-index ∑k did not exceed 24). these intervals did not contain any strong seismic events (intervals where there were no earthquakes with the energy class ks≥12, which happened no farther than 300 km from the station of the ionospheric radio sounding). taking into account the seasonal variability of an ionospheric process we performed separate data simulation for different seasons. tables 1 and 2 contain time intervals for identification and model estimation. the solar activity was estimated using the value of radiation corresponding to a wavelength of f10.7. if f10.7 <100 the activity was taken as low, otherwise the activity was considered to be high. by changing the decomposition level (see (1)) and applying various wavelet functions (daubechies wavelets were taken) different models of type (2) were obtained for representing time changes in fof2. model identification and diagnostics for different time series components (steps 2,3 in 2.1) have shown that the original time series fof2 and their approximating components of the 1st and 2nd decomposition levels have a complex ( )f0 t , km f0 1 = n= | , n1 j = n | ___ ___ ( ) ( ) .s t t j t, , , , ,j k q jc l l p j~ k q l ji n a j k q n n h j 11 h jj jc j~ ji= n n n n n + == + + n n | | ( ) ( ) ( ) .a j t t s ts, , , , actual, predicteda j k q j k q j k q= n n n + + + ,d uj a j t ta 1 ,uj uj a j k q k uj ta 1 2 uj uj j 2= n + = " %| mandrikova et al. 4 (3) (2) 5 structure and cannot be approximated by an arima model. the reason is non-zero correlation of model residuals. residual error minimization is the way of finding the best time series model fof2, which includes 3 (three) components for winter and summer seasons. these components have the following form in the wavelet domain: where: f 2-3t =∑ k c-3k{-3k(t) characterizes trend; g2jt =∑ k dj,kwj,k(t), j =−2,−3 are components representing multiscale details; e2-1t is a noise component. both of the reconstructed components f0 1(t) =∑ k c10,k {0,k(t) and g n 0(t) =∑k d n 0,kw0,k(t), n = 2,3 have a more simple structure than the original time series f (t). in figure 2 we can see the distribution estimates of the registered time series fof2 and the components f0 1(t), gn0(t), n = 2,3. the model parameter estimation is performed for each extracted component. each reconstructed component f0 1(t) =∑ k c10,k{0,k(t) (j =−3 before reconstruction, time periods are 8 and longer) and gn0(t) =∑k d n 0,kw0,k(t), n = 2,3 (j =−2−3 before reconstruction, time periods range from 2 to 4 hours and from 4 to 8 hours, respectively) was used for identifying arima models of the 2nd order. in table 3 there are estimated model parameters ( ) ,f0 t f t g t e t2 2 2 j jf0 3 2 3 1= + +=-| ionospheric parameter modelling high solar activity winter season summer season 20.12.1968–09.01.1969 31.07.1969–20.08.1969 03.01.1970–22.01.1970 30.07.1970–16.08.1970 11.02.1970–02.03.1970 07.06.1979–24.06.1979 03.01.1982–20.01.1982 20.07.1989–10.08.1989 01.01.1991–23.01.1991 10.07.1990–28.07.1990 30.11.2000–19.12.2000 27.06.1998–15.07.1998 19.12.2011–08.01.2012 20.06.2001–07.07.2001 29.12.2012–24.01.2013 18.06.2002–06.07.2002 28.05.2002–18.06.2002 low solar activity 09.01.1984–29.01.1984 24.06.1983–14.07.1983 11.02.1985–27.02.1985 26.05.1983–12.06.1983 22.01.1987–10.02.1987 20.06.1984–10.07.1984 29.12.2005–14.01.2006 28.06.2004–17.07.2004 15.01.2006–04.02.2006 13.08.2007–01.09.2007 09.02.2008–27.02.2008 high solar activity winter season summer season 15.12.1970–29.12.1970 23.06.1969–05.07.1969 07.02.1981–21.02.1981 04.06.1971–21.06.1971 07.02.2002–25.02.2002 12.06.1989–29.06.1989 05.12.2011–18.12.2011 13.08.2000–26.08.2000 30.01.2012–11.02.2012 03.08.2002–17.08.2002 04.02.2013–18.02.2013 06.07.2002–18.07.2002 low solar activity 13.12.1983–29.12.1983 25.06.1987–06.07.1987 06.01.1993–25.01.1993 09.08.2010–22.08.2010 05.01.2007–18.01.2007 table 1. time intervals of fof2 data used for model identification. table 2. time intervals of fof2 data used for model diagnostics. figure 1. approximated components in the wavelet domain (highlighted in black). figure 2. distribution estimates using histograms: (a) histogram of the original time series fof2; (b,c,d) histograms of the components f0 1(t), g30(t), g 2 0(t). (4) mandrikova et al. 6 analyzed period f0 1(t) g30(t) g 2 0(t) parameter parameter parameter 1st 2nd 1st 2nd 1st 2nd 27.06.2005–10.07.2005 (kamchatka) 1.013 -0.291 0.828 -0.339 0.443 -0.572 16.01.2006–04.02.2006 (kamchatka) 1.012 -0.278 0.829 -0.344 0.371 -0.622 09.02.2011–27.02.2011 (kamchatka) 1.009 -0.266 0.805 -0.355 0.437 -0.473 16.01.2006–04.02.2006 (magadan) 1.009 -0.267 0.821 -0.348 0.366 -0.624 general parameters (summer and winter seasons) 1.01 -0.27 0.81 -0.35 0.35 -0.65 table 3. component model parameters. figure 3. simulation and prediction results for fof2 data. black: original data fof2; grey: prediction. (a) prediction for f0 1(t) with time advance q = 1; (b) prediction of the sum f0 1(t) and g30(t) with time advance q = 1; (c) prediction of the sum f0 1(t), g30(t), and g 2 0(t) with time advance q = 1; (d,e,f,g) prediction of the sum f0 1(t), g30(t), and g 2 0(t) with time advance q=2,5 —, respectively; (h) prediction error with time advance q = 1; (i) absolute prediction error with time advance q = 1; ( j) absolute prediction error with time advance q = 5. the arrow indicates a seismic event in kamchatka (energy class k = 14.1). 7 of these components for data with different time periods. due to close parameters of the corresponding components a general time series model was determined for fof2 for winter and summer seasons: where s10,k(t) = (1.01 · ~ 1 0,k−1− 0.27~ 1 0,k−2) + a 1 0,k(t); s 2 0,k(t) = (0.35· · ~20,k−1 − 0.65~ 2 0,k−2) + a 2 0,k(t); s 3 0,k(t) = (0.81 · ~ 3 0,k−1 − − 0.35~30,k−2) + a 3 0,k(t); bl~n0,k(t) = ~ n 0,k−l(t); an0,k(t) are residual errors of the n-th component model; ~n0,k−l t =∇b n 0,k(t); b10,k = c 1 0,k; bn0,k = d n 0,k , n = 2,3; an0,k are residual errors of the n-th component model; n is time series length; b10,k ={ 0,k is a scaling function; b n0,k = w 0,k , n = 2,3 is a wavelet basis of the n-th component model. figure 3 shows the simulation and prediction results for fof2 data from the paratunka observatory. these results were found by model (5) (the period is 14.02.2011–23.02.2011). figure 4 also illustrates the simulation error estimates for fof2 data obtained by arima methods and the suggested technique. these estimates were calculated using fof2 data from paratunka for the following periods: 13.07.1969–21.08.1969, 26.07.1970– 01.09.1970, 26.07.1972–22.07.1972, 31.05.1979–03.07.1979, 26.05.1983–11.06.1983, 22.05.1984–18.06.1984, 21.07– 16.08.1987, 21.07.1989–16.08.1989, 30.07–21.08.1992, 30.07.1999–21.08.1999, 13.06.2000–28.06.2000, 18.06.2002– 06.07.2002, 27.06–10.07.2005. analysis of figures 3 and 4 confirms the efficiency of the suggested technique and shows that the multicomponent model is suitable for making a prediction for ionospheric parameters with time advance equal to 5 hours. at some time points there are rises in prediction errors (figure 3h,i,j). in order to analyze the prediction errors, our simulation results were compared with geomagnetic data (h-component of the geomagnetic field was used), which were needed for estimating the geomagnetic perturbation intensity for the analyzed time periods (the geomagnetic perturbation intensity was estimated using the technique expounded in [mandrikova et al. 2013b]), eb=∑a|wwfb,a|, wwfb,a:=|a| -1/2 ∞ ∫ −∞ f (t)w (t−b—a )dt, f∈l 2(r),a,b∈r,a≠0. the results provided in figure 5 illustrate the fact that prediction error increase associated with an ionospheric anomaly is observed prior to a strong earthquake in kamchatka (20.02.2011, energy class e=14.1, the anomaly is shown in figure 5 by the dashed line). testing the suggested technique on tec data has confirmed possible connection between ionospheric anomalies and seismic events in kamchatka [mandrikova et al. 2013c]. such effects in the ionosphere with a duration from several minutes to several hours arising in periods of increased seismic activity in different seismically active regions are also specified in a number of earlier works [afraimovich et al. 2000, afraimovich et al. 2001, nakamura et al. 2007, nakamura et al. 2009, ( ) ( ) ,f0 t t b ts , ,, , kk k n f0 1 3 00 1 = n n n = = | | ___ ___ ionospheric parameter modelling figure 4. simulation error for fof2 data. solid line: using the wavelet transform; dashed line: without using the wavelet transform. solid line is ewin mcm = k ∑ k=1 3 ∑ l=1 (alj,k) 2, k is the number of observations. dashed line is ewin arima= k ∑ k=1 e2k, ek is arima model errors, k is the number of observations. (5) klimenko et al. 2012, watthanasangmechai et al. 2012]. the issues connected with the nature and origin of such effects are still to be studied more thoroughly. during the increased geomagnetic activity we can also observe a prediction error rise. ionospheric anomalies extracted in periods of strong and moderate magnetic storms were large-scale ones and their duration was equal to one day or even longer, which is caused by significant changes of electronic concentation level relative to the background. they reached their maximum intensity during minor geomagnetic perturbations. it was also possible to observe small-scale anomalies connected with local fluctuations of the electron density of the ionosphere. the results described in the paper agree with those published before [mendillo 2006, roux 2012, danilov 2013]. such a complex behavior of the ionosphere is illustrated in figures 5, 7 and 8. ionospheric anomalies discovered in periods of solar flares and ejection of coronal material mostly had small scales and arose against a background of weakly disturbed and quiet geomagnetic field. the morphology and physics of such a behavior are expounded in review papers [prolls 1995, rees 1995, buonsanto 1999, danilov and laštovička 2001, danilov 2013]. the statistical analysis mandrikova et al. 8 figure 5. detailed analysis of fof2 data for the period 09.02.2011–27.02.2011: prediction was performed using model (5) with time advance q = 1; the arrow indicates a seismic event. figure 6. prediction error estimates for fof2 data for different years. eyear= k ∑ k=1 3 ∑ l=1 (alj,k) 2 values are indicated on the axes oy, k = 384 is the number of observations per year. 9 of fof2 data for different years has shown a substantial connection between anomaly intensity and anomaly occurrence frequency in the ionosphere, on the one hand, and solar and geomagnetic activities, on the other hand. this conclusion agrees with works [afraimovich et al. 2000, afraimovich et al. 2001, roux 2012]. during the periods of increased solar activity the general error level goes up; when anomalies occur the deviation from the background level rises dramatically. we can confirm this by the results shown in figure 6. the joint analysis of the simulation results (data from kamchatka and magadan) reveals some general trend. the h-component of the geomagnetic field (paratunka observatory) is shown in figure 7 with the aim of analyzing the geomagnetic activity. when the geomagnetic activity is increased, the prediction error rises for data from kamchatka and magadan. at some time points the prediction errors for data from kamchatka exceed significantly those for data from magadan (this period is shown in figure 7 by the dashed line), which is probably connected with seismic events in kamchatka. due to wavelet transform properties [mandrikova et al. 2010] and remark 1 given earlier in this paper, we performed arima model identification to approximate the components f 2-3t = ∑ k c-3,k{-3,k(t) and g2 -3t= ∑ k d-3,kw-3,k(t). the approximation was made without data reconstruction to the original scale j = 0. figure 8 illustrates the simulation of these components. the comparison with kp-index [helmholtz-centre potsdam: inionospheric parameter modelling figure 7. simulation results for fof2 data from kamchatka and magadan for the period 16.01.2006–04.02.2006: prediction was made using model (5) with q = 1; the arrow indicates a seismic event in kamchatka (energy class k = 12.7). figure 8. simulation results for fof2 data from kamchatka for the period 01.01.2006–26.02.2006. (a) prediction error for the component f 2-3t in the sliding time window equal to 24 hours; (b) prediction error for the component g2-3t in the sliding window equal to 24 hours. dices of global geomagnetic activity; bartels 1957a, bartels 1957b, joselyn 1970, mayaud 1980, golovkov et al. 1989, rangarajan 1989, menvielle and berthelier 1991, sucksdorff et al. 1991, menvielle et al. 1995, siebert and meyer 1996, jach et al. 2006, nowozynski 2007, xu et al. 2008] values shows the connection between the prediction error and the geomagnetic activity level, which accords with the results discussed above. 4. conclusions the suggested technique of ionospheric parameter simulation and analysis based on combining the wavelet transform with arima models was used to approximate trend in critical frequency data from kamchatka and magadan. the experiments have confirmed the efficiency of the suggested technique and its advantages for studying regular changes in ionospheric parameters and making predictions with time advance equal to 5 hours. the residual error estimates of the models were employed for discovering ionospheric anomalies during ionospheric perturbations with a duration from several dozens of minutes to several hours. the changes in ionospheric parameters have different scales and arise prior to and during strong earthquakes in kamchatka. the variation analysis of the parameters during the periods with different solar activity and comparison of simulation results with geomagnetic data have shown the connection between anomaly intensity and anomaly occurrence frequency, on the one hand, and solar and geomagnetic activities, on the other hand. during the periods of increased geomagnetic activity there is a significant rise of deviation from the background level due to anomaly occurrence. acknowledgements. the data for research were kindly provided by kamchatka branch of geophysical survey of the russian academy of sciences (gs ras). the paper and the research are supported by the grant of the russian science foundation no. 14-1100194 (mov) and contract no. 02.g25.31.0058 dated 12.02.2013 (ministry of education and science of the russian federation). references afraimovich, e.l., e.a. kosogorov, k.s. palamarchouk, n.p. perevalova and a.v. plotnikov (2000). the use of gps arrays in detecting the ionospheric response during rocket launchings, earth, planets, and space, 52 (11), 1061-1066. afraimovich, e.l., n.p. perevalova, a.v. plotnikov and a.m. uralov (2001). the shock-acoustic waves generated by the earthquakes, annales geophysicae, 19 (4), 395-409. al-kasasbeh, r.t. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49_1_2006def 71 annals of geophysics, vol. 49, n. 1, february 2006 key words uv-vis instruments – remote-sensing – actinic measurements – satellite data validation 1. introduction the current state of the earth’s climate and its future development under different scenarios has been a subject of increasing interest in the international scientific community over the last decade. many studies, based on experimental data obtained using a large variety of instruments, indicate the existence of strong links between anthropogenic activity and a number of chemical processes taking place in the atmosphere (ipcc, 2001). one of the major questions nowadays is to discover the critical margins of the anthropogenic impact on atmospheric parameters, beyond which climate-controlling factors become irreversible thus affecting the biosphere in an uncontrolled manner. in general, two approaches are adopted to improve knowledge in this field: a) the use of models to simulate possible scenarios and assess the impact of a large range of factors on climate; b) the collection of experimental data and the establishment of long-time data series necessary for the evaluation of ongoing climate changes. both approaches have shortcomings. in case a), the lack of data needed for the initialization of the models can create difficulties in reproducing the observed processes, thus preventing reliable predictions. case b) requires specific procedures for maintaining the technical characteristics of already deployed instruments. if new instruments are introduced into the monitoring neta multi-input uv-vis airborne gascod/a4r spectroradiometer for the validation of satellite remote sensing measurements ivan kostadinov (1)(3), giorgio giovanelli (1), daniele bortoli (1)(2), andrea petritoli (1), fabrizio ravegnani (1), giandomenico pace (4) and elisa palazzi (1) (1) istituto di scienze dell’atmosfera e del clima (isac), cnr, bologna, italy (2) geophysics centre of évora university of évora (cge-ue), évora, portugal (3) solar-terrestrial influences laboratory (stil), bulgarian academy of science, stara zagora, bulgaria (4) università degli studi di roma «la sapienza», roma, italy abstract the present paper describes a uv-vis spectroradiometer named gascod/a4r developed at isac-cnr for remote sensing measurements aboard stratospheric m55-geophysica aircraft, flying up to 21 km. obtained experimental data are used for retrieving of no2, o3 and of other minor gases atmospheric content, applying the doas (differential optical absorption spectroscopy) method. uv actinic flux and j(no2) are also derived. all these parameters are used for satellite data validation tasks. the specific results obtained during dedicated aircraft missions in different geographical areas have already been utilized for envisat validation. mailing address: dr. ivan kostadinov, istituto di scienze dell’atmosfera e del clima (isac), cnr, area della ricerca cnr, via gobetti 101, 40129 bologna, italy; e-mail: i.kostadinov@isac.cnr.it 72 ivan kostadinov et al. works it should be guaranteed that the measurement accuracy already established in the longtime data series will not be reduced, in order to not affect the trends of the studied parameters. the above-mentioned aspects are of particular importance regarding satellite remote sensing instruments, contributing to b). instruments installed aboard environmental satellites, such as odin, ers-1, ers-2, ilas, ace, etc. supply a large variety of data, e.g., gas concentration, temperature, water vapour, cloud top height. these data are derived from measurements carried out under very different environmental conditions and observational geometries. to overcome practical problems related to b) specific intercomparisons and validation campaigns are carried out. satellites, with payloads dedicated to explore the earth’s atmosphere are usually launched in polar orbit to allow coverage of all geographical regions. although the repetition rate of these satellites is of the order of a few days, groundbased measurements alone are not sufficient to fully validate satellite data. on the other hand the satellite overpass does not always coincide with the most appropriate conditions for groundbased observations. changes in the atmosphere, due to local, regional or large-scale dynamic effects can take place during the period between two overpasses for a given location, so different air masses would be observed from the space and from the ground, leading finally to difficulties in the assessment of satellite data quality. ground-based instruments, distributed all over the world, as uniformly as possible, would provide an opportunity to control the satellite data capability to reproduce the actual atmospheric parameters correctly. however, there are large unpopulated geographical areas, where ground-based measurements for validation tasks cannot be performed and only balloon and aircraft measurements can provide the possibility of enlarging the validation areas. in this regard the validation campaigns of satellite remote sensors, with the participation of airborne instruments, would appear to be essential for continuing satellite payload characterization. as a matter of fact the characterization of the scientific payload begins with the design and assembly of individual instruments and includes calibration and test procedures, carried out during the pre-flight phase. the aim of these procedures is to simulate the space conditions under which the instruments will operate in orbit. sophisticated laboratory equipment and models are included in these pre-flight operations in order to check the behaviour of spaceborne sensors under circumstances expected in space, e.g., sharp variations in temperature (sunlight-darklight parts of the orbit), different illumination levels within instrumental field of view (fov), etc. notwithstanding the efforts of scientists to reproduce space conditions in the laboratory during preflight tests, there are still factors which cannot be forecast and/or simulated correctly. for this reason it is necessary to perform specific tests after the launch to supply a complete characterization of the behaviour of the satellite instruments. the aim of the validation procedures also includes optimising methods for satellite data processing, improving the subroutines for better treatment of the observational scenarios. some of the reasons for discrepancies between ground-based and satellite data can arise from the fact that exactly the same air masses are not observed and hence the output results will be different. in order to evaluate these differences additional information, useful for better interpretation of derived atmospheric parameters, is indispensable. of course, part of the information can be obtained from models, but in order to adapt such models to any given scenario it is preferable to initialize them with experimental data obtained as near in the time as possible to the moment of interest. for this reason the development of instruments, which perform simultaneous measurements within spaceborne instrumental fovs, is considered as an advanced approach towards this end. 2. methods of measurements the first airborne version of gascod family instruments during the ape-1 mission in rovaniemi, operated only in doas mode. before the tropical ape-theseo campaign (http://ape.ifac.cnr.it) the instrument was upgraded by adding two 2r sr fov receivers for collecting of solar direct and diffuse radiation arriving at the flight level. after this modifica73 a multi-input uv-vis airborne gascod/a4r spectroradiometer for the validation of satellite remote sensing measurements tion the instrument became known as gascod/a4r providing additional measurements necessary for a better understanding of the atmospheric photochemistry. 2.1. doas mode gascod/a4r was initially developed as a ground-based instrument (bonasoni et al.,1993; evangelisti et al., 1995; giovanelli et al., 1998), implementing the doas measurement technique. it relies on the retrieval of the column content of minor atmospheric gases such as o3, no2, bro, oclo, no3, so2, etc. from their characteristic narrow-band absorption spectra, by applying the beer-lambert law in its differential form. brewer et al. (1973) first attempted to apply this method. since then, many authors contributed to improve the technique (e.g., noxon, 1975; noxon et al., 1979; platt and perner, 1983; johnston and mckenzie, 1984; solomon et al., 1987; roscoe et al., 1994; roscoe et al., 1996). 2.2. spectroradiometric mode many photochemical reactions take place in the atmosphere, contributing to the variations of the local concentration of minor gases. therefore, by knowing the incoming uv radiation flux at flight level provides an opportunity to better distinguish the role of photochemistry in overall atmospheric processes. to this end, gascod/a4r can also operate as a spectroradiometer and supply spectrally resolved upwelling and down-welling radiance (kostadinov et al., 1999). the channels with 2r sr fov used for this purpose are calibrated in absolute terms allowing to convert measured spectral radiation through these channels into photon flux. 3. instrumental design space restrictions aboard m55-geophysica aircraft and requirements for spectrometric instruments have been taken into account and gascod/a4r has been designed as a two-unit device. it includes an optical unit (ou) and an electronic unit (eu). the ou is built-up as a multi-input device probing the atmosphere remotely in different directions. such configuration provides possibility to perform additional measurements necessary for better understanding of the atmospheric photochemistry, underlying spectral reflectivity etc., widely used for improvement of the satellite data validation. 3.1. optical unit ou consists of five optical inputs named i0, i1...i4. three of them have narrow fields of view (nfov) of 1.1e-5 sr and point towards the nadir (i0), zenith (i2) and, horizontally, to the left, perpendicular to the flight direction (i4). these three inputs are dedicated to doas measurements. the combination of (i2) and (i4) measurements is used to derive the so-called no2 and o3 acila (average gas concentration inside the layer near the aircraft) values (petritoli et al., 2002). the third nfov (i0) channel is used to evaluate the underlying spectral reflectivity, or combining with (i2) measurements to derive the tropospheric and stratospheric part of the total vertical column of the gases under interest. such distinction appears a task of remote-sensing satellite measurements too, aimed to evaluate tropospheric pollution from space (giovanelli et al., 2006). the other two channels having 2r sr fov are dedicated to collect diffused up-welling radiance, (i1) and diffuse + direct down-welling radiance (i3). measured spectra through these channels are subsequently used to obtain spectrally resolved actinic flux need for calculation of photodissociation rate coefficients. the ou consists custom-built monocromator based on a jobin-yvon holographic spherical grating with n = 1200 grooves/mm with blaze maximised at 320 nm and a focal length of 300 mm and entrance slit 100 nm × 8.0 mm. the linear dispersion and resolution are about 2.4 nm/mm and 0.7 nm @ 350 nm, respectively, satisfying requirements for resolving power of doas type instruments. these two parameters depend upon n and any factor changing n will affect the linear dispersion and resolution. temperature variations appear to be the most impor74 ivan kostadinov et al. tant factor in this regard. it was found that if dt ± 0.2°c the changes of instrumental spectroscopic characteristics introduce negligible errors on the measured physical parameters. the maintenance of the required temperature regime is guaranteed by means of special internal insulating layer of the ou and thermo-electric cooling system. the full spectral range (295 nm ÷ 1100 nm), which can be examined by gascod/a4r is sampled within single sub-intervals of 60 nm. depending upon the task of any given mission, sets of sub-intervals can be previously selected, although spectral intervals up to 460 nm are always examined to retrieve o3, no2 and bro concentration, allowing uv actinic flux measurements and hence the calculation of j(no2) too. 3.1.1. ccd sensor the sensor appears 2d (1100 × 330 pixels) site ccd based on back illuminated technology. a peltier cooling system ensures a constant operating temperature of −30°c ± 0.1°c. the measured signal is binned into a matrix of 1092 columns and 11 rows. even though the rows are reduced from 300 to 11, the curvature of the spectral lines still remains. applying a «shift» procedure further improvement of the instrumental spectral resolution is achieved. the exposure time needed to achieve sufficient s/n ranges from 0.1-1.0s, under high sun elevation, to 15 s at a solar zenith angle of about 94°. the most appropriate exposure time which should be applied for single measurements during flight is calculated by means of an autoranging procedure included in the measurement duty cycle. however in certain situations, due to fast changes of environment conditions (e.g., broken clouds), ccd saturation can take place. in order to decrease the ccd memory effects to negligible levels wiping procedure is applied at the beginning of each measurement. 3.1.2. spectral and absolute calibration a small integrating sphere with diameter of 60 mm, equipped with hg and qj tungsten lamps, is incorporated into ou for in-flight control of ccd performance and overall instrumental spectral characteristics and spectral calibrations. for each spectral sub-interval there are previously selected spectral lines used for correct positioning of the diffraction grating within ± 1pixel (wavelength equivalent ≈ 0.05nm). shift and stretch procedures are applied during post-flight data processing to introduce the correct wavelength scale in the measured spectra. the channels with 2r sr fov used for calculation of j-values are calibrated in an absolute manner. this includes specific laboratory measurements to find the relationship between instrumental output signal and measured radiation. a method, based on use of an integrating sphere has been developed for this purpose (kostadinov et al., 1999). the advantage of this method arises from the possibility to irradiate simultaneously the whole fov of the 2r sr receivers as it take place during field measurements during the flights. 3.1.3. electronic unit this is a pressurised unit installed inside the aircraft and consists of a dc/dc converter providing the power, a computer based on a pc104 motherboard, sensors for internal and external pressure and temperature controls. a driving program guards the execution of a duty measurement cycle. this includes home positioning of the diffraction grating, scanning mirror, band-pass filters wheel, ccd wiping and dark current measurements, etc. 4. data processing the acquired data from the narrow fov horizontal channel (i4) are used to derive average concentration of no2 and o3 along the instrumental optical axis at distances of about 50-70 km from the aircraft. this approach is actually based on doas methodology combined with a 2d single-scattering radiation transfer model (rtm) (solomon et al., 1987), adapted to the specificity of the aircraft measurements (petritoli et al., 2000). the model uses ray-tracing in a 75 a multi-input uv-vis airborne gascod/a4r spectroradiometer for the validation of satellite remote sensing measurements spherical atmosphere, with optical paths integrated over individual 1-km shells, within which the atmosphere is divided for the model calculations. the rtm has been adapted in order to evaluate the probability density function (for scattering towards the instrument) and to calculate the effective absorber optical path under off-axis measurement geometry (slusser et al., 1996). such an approach reveals the possibility of converting the derived slant columns detected at the aircraft altitude into corresponding acila values and to use them for satellite data validation. while doas requires an appropriate reference spectrum, the calculation of actinic flux and subsequently deriving rate coefficients (jvalues) of photochemical reactions strongly depends upon the accurate correction of all types of sensor noise and dark current. for this purpose two particular measurements – one with 0.0 s and 2.0 s integration times, together with previously mapped ccd pixel properties, corresponding to different integration times, allow one to extract the pure output signal generated by the incoming radiation. the signal is converted into instrumental units versus time, (i.u./s), allowing signals detected under different environmental and navigation conditions to be meaningfully compared. next measured spectra are converted from (i.u./s) units into (mw/m2.nm) units by applying the results from laboratory, pre-flight and post-flight calibrations. all detected spectra through the upwardsfacing and downwards-facing 2r sr fov receivers, undergo a cubic-spline interpolation procedure, allowing to obtain equidistant time grids for all actinic measurements during a given flight. derived actinic flux is converted into photon fluxes used for calculation of the j-values for a number of photochemical reactions. for example, using the no2 absorption cross section (harder et al., 1997) and recommended data for primary no2 quantum yield (gardner et al., 1987), we can calculate j(no2). 5. satellite measurements requisites and aircraft measurements over the last few decades, spatial and time resolution of satellite remote sensing measurements have been substantially improved. however, the resolution depends upon the type of observation – nadir or limb and ranges from a few kilometres up to hundreds of kilometres. assuming m55-geophysica aircraft cruise speed of 10 km/min it means that a long time is needed to examine in situ airborne instruments air masses observed from space. of course in situ measurements have the advantage of providing results with high time resolution, but they are not sufficient for validation procedures, so airborne remote sensing measurements appear essential for the validation tasks. each of these two measurement approaches has its advantages and shortcomings, and only their simultaneous deployment helps to achieve scientifically valuable information for satellite data validation (kostadinov et al., 1999). gascod/ /a4r forms part of the remote-sensing chemical payload of the m55-geophysica together with two ftir instruments safire/a (o3, hno3, n2o and clo) and mipas-a (o3, hno3, n2o, ch4, h2o and t profiles), (http://ape.ifac.cnr.it/). by means of vertical scanning to the right, perpendicular to the flight direction, both ftir instruments probe atmospheric air masses observed from space and profiles of the aforementioned gases are retrieved. these profiles correspond to areas located at ∼150 ÷ 300 km away fig. 1. observational directions of mipas/a, gascod/a4r and safire /a instruments aboard m55geophysica aircraft. measured parameters are assigned to areas of ∼150 km÷300 km away the aircraft (mipas/a and safire/a) and ~50 km÷70 km (gascod/a4r). 76 ivan kostadinov et al. from the aircraft, where the strongest contributions of detected emissions arise. if we image a virtual scale defining the distances d, from the m55-geophysica aircraft to the zone with maximum contribution to the retrieved parameters, fig. 1, it can be concluded that the zone lying a few tens of kilometers away from the aircraft will remain unexplored without acila values supplied by ascod/a4r instrument. it took part in several validations operating successfully more than 95% of the flight time. however, the still limited quantity of available mipas-e and sciamaschy data at the time of the preparation of this work prevent wider use of obtained by gascod/a4r no2 and o3 quasi in situ measurements and hence it is hard to evaluate the ability of these satellite instruments to reproduce the atmospheric content of gases under interest. here we would like to emphasize that before assessment of satellite data a preliminary internal quality control of the data obtained by means of different in situ and remote sensing instruments aboard m55-geophysica be carried out (heland et al., 2002). we report below some examples of the results obtained by means of gascod/a4r instrument during campaigns dedicated for validation of mipas-e and sciamaschy data. the flight route of flight of 12 march 2003 during envisat high latitudes campaign (http://ape.ifac.cnr.it) is shown in fig. 2, while in fig. 3 mipas-e o3 profile sampled within 69.3n-71.9n and gascod/a4r o3 acila values are plotted. during this flight m55 aircraft flew up to 19.6 km, so mipas-e data only below this altitude can be compared directly to the aircraft data. in this case satellite measurements reproduce the general shape of vertifig. 2. m55-geophysica aircraft flight route (12/03/2003). mipas-e scan 21, orbit 5386 (08:49:37utc) is compared to gascod/a4r measurements. closest correlation corresponds to ea back leg around 69n÷71n (see fig. 3). color bar indicates the tangent height of mipas-e line of sight. 77 a multi-input uv-vis airborne gascod/a4r spectroradiometer for the validation of satellite remote sensing measurements cal distribution of o3 mixing ratio obtained by gascod/a4r. an overall reasonable coincidence between satellite and aircraft data sets is evident in this example. in general, the discrepancies of the results are due mainly to differences in time and spatial probing from both platforms. better consistency is evident for the data (filled diamonds in fig. 3) corresponding to the descent of m55 aircraft, when gascod/ /a4r probe almost the same area examined approximately two hours before by mipas-e. a similar comparison regarding results obtained during ape-envisat mid-latitude campaign are shown in fig. 4. here only the lowfig. 3. ozone mixing ratio derived in 12 march 2003 by gascod/a4r (diamonds) and mipas-e (black triangles) during ape-envisat high-latitude validation campaign. mipas-e profile corresponds to scan 21taken within 69.3 n÷71.9n latitude interval. filled diamonds correspond to ea back leg. fig. 4. ozone mixing ratio measured by mipas-e (v.4.61) at 09:19:25 utc, (45.7° n, 12.9°e), error = ± 3v (black diamonds) and gascod/a4r (open diamonds) around (45.2°n, 10.2e) zone. data are obtained during ape-envisat mid-latitude campaign held at forlì airport, flight 22 july 2003. 78 ivan kostadinov et al. fig. 6. able narid background signal @355 nm and gascod/a4r signal in 333 nm ± 30 nm spectral band obtained through nadir faced nfov channel during 28/02/2003 flight from kiruna within ape-envisat highlatitude validation campaign. due to lower time resolution of gascod/a4r some fine spatial structures of the underlying reflectivity are omitted. however for each gascod/a4r point detailed spectral properties of the scattering or reflecting objects under the aircraft is available. the axes of the nested figure represent up-welling radiance: abscise – wanelength, å; ordinate – normalised instrumental signal, a.u./s. fig. 5. j(no2) values (open circles) measured during flight of 28 february 2003 as a function of time, solar zenith angle, latitude and pressure. 79 a multi-input uv-vis airborne gascod/a4r spectroradiometer for the validation of satellite remote sensing measurements er part of the o3 profile measured by mipas-e, corresponding to gascod/a4r measurements around (45.7°n, 12.9°e) is presented. the better spatial-time coincidence of the aircraft and satellite measurements in this case leads to a closer consistency of the retrieved ozone mixing ratio by means of both instruments. despite this, the complete understanding and quantification of satellite measurements need additional 3d ctm modelling, (giovanelli et al., 2003) in order to account also for the transport and chemical transformation of the species under interest. this approach is applied by wetzel et al. (2004) using kasima 3d ctm model output for assessment of mipae-e no2 profiles. unfortunately at the present time it is not always possible to calculate the no2 profile from mipas-e measurements down to the m55 aircraft flight level, so direct mipas-e and gascod/a4r no2 comparison is limited. for instance mipas-e measurements do not match air masses probed by gascod/ /a4r in the (45.4°n, 11.6°e) region during the flight in 14 october 2002. however, as pointed out in wetzel et al. (2004), the comparison of kasima 3d ctm no2 profile with gascod/ /a4r no2 acila values is qualitatively reasonable. this and another comparisons allow us to state that gascod/a4r no2 and o3 quasi in situ products (acila) can be widely deployed for further validation analysis. it was mentioned above that gascod/a4r provides other kinds of products appearing important for atmospheric studies. figure 5 demonstrates the variation of j(no2) detected through the 2r sr fov receivers during the flight of 28/02/2003 in the ape-envisat campaign, at kiruna, sweden. the three sharp spikes are due to the partial saturation of the measured spectra. strong increasing of j(no2) during the return leg of the flight is caused by increasing available solar radiation as the sza decreases. these measurements, together with no measurements performed by means of the sioux instrument (ziereis et al., 2000) aboard the same aircraft, have been used to calculate the no2\no ratio (kostdinov et al., 2003). this ratio is one of the controlled parameters during adaptation of the models to the environmental conditions under which dedicated validation measurements have been carried out. another product supplied by gascod/ar instrument appears to underlie spectral reflectivity. this product was derivable also before the installation of the nadir nfov channel, but with very low spatial resolution, because it was possible only through the nadir 2r sr channel. adding the nfov nadir faced channel significantly improves the spatial resolution of this kind of measurements. we examined the ability of gascod/a4r to reproduce spatial patterns of the landscape and/or cloud coverage comparing integrated measured spectra within 333 nm ± 30 nm interval to the able (airborne lidar experiment) (fiocco et al., 1999; pace et al., 2003) background noise measurements @355 nm. the background noise measured by able, towards the nadir, is actually an uncalibrated radiometric measurement of the solar radiation reflected and scattered from the surface and by the atmosphere. figure 6 plots the data series obtained by means of both instruments during the flight of 28/02/2004, kiruna. it is evident that overall spatial structures detected by able are also reproduced by gascod/a4r. the advantage of such combined measurements arises from the opportunity to obtain underlying reflectivity with high spatial and spectral resolution. 6. conclusions an airborne version of the gascod family instruments has been deployed during several campaigns dedicated to the study of tropical upper troposphere/lower stratosphere regions, satellite data validation and antarctic ozone depletion studies. it operates successfully under a wide variety of environmental conditions. the acila values fill the gap in the spatial scale defined by the classical remote sensing and in situ instruments. simultaneous measurements of atmospheric constituents and actinic fluxes provided by gascod/a4r reveal the possibility of evaluating the impact of photochemical reactions on tropospheric and stratospheric chemistry. correlative analysis of gascod/a4r and able enlarges lidar and spectroradiometric measurements for a better understanding of cloud processes, underlying reflectivity, radiative transfer and other aspects of geophysical science. 80 ivan kostadinov et al. acknowledgements this work was supported by asi, esa, pnra and the eu ape-infra project. the author daniele bortoli was financially supported by the subprograma ciência e tecnologia do 3° quadro comunitário de apoio. thanks are due to joerg heland for providing mipas data and to g. fiocco and m. cacciani for providing able data. references bonasoni, p., f. evangelisti, g. giovanelli and f. ravegnani (1993): remote sensing doas system for measurements of atmospheric trace gases, phys. med., 9, 304-307. brewer, a.w., c.t. mcelroy and j. b. keer (1973): nitrogen dioxide concentration in the atmosphere, nature, 246, 129-133. evangelisti, f., a.baroncelli, p. 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95-105. roscoe, h.k., j.a.c. squires, d.j. oldham, a. sarkissian, j.-p. pommereau and f. goutail (1994): improvements to the accuracy of zenith-sky measurements of total ozone by visible spectrometers, j. quant. spectrosc. radiant. transfer., 52 (5), 639-648. roscoe, h.k., d.j. fish and r.l. jones (1996): interpolation errors in uv-visible spectroscopy for stratospheric sensing: implications for sensitivity, spectral resolution, and spectral range, appl. opt., 35, 427-432. 81 a multi-input uv-vis airborne gascod/a4r spectroradiometer for the validation of satellite remote sensing measurements solomon, s., l. schmeltekopf and r.w. sanders (1987): on the interpretation of zenith sky absorption measurements, j. geophys. res., 92, 8311-8319. slusser, j., k. hammond, a. kylling, k. stammes, l. perliski, a. dahlback, d. anderson and r. demajistre (1996): comparison of air mass computations, j. geophys. res., 101, 9315-9321. wetzel, g., m. pirre,f. goutail, a. bazureau, j.-p. pommereau, a. bracher, m. sinnhuber, m. weber, k. bramstedt, b. funke, d.y. wang, a. martinez, i. kostadinov, t. blumenstock, f. hendrick, m. van roozendael and m. de mazière (2004): mipas no2 (http://envisat.esa.int/workshops/acve2/), in second workshop on the atmospheric chemistry validation of envisat (acve-2), frascati, italy. ziereis, h., h. schlager, h. fischer, c. feigl, p. hoor, r. marquardt and v. wagner (2000): aircraft measurements of tracer correlations in the arctic subvortex region during the polar stratospheric aerosol experiment (polstar), j. geophys. res., 105, 24305-24313. supplement-information-7604-pdf-sitoannali 1 supplement of italian radon monitoring network (iron): a permanent network for near real-time monitoring of soil radon emission in italy valentina cannelli, et al. corresponding author: valentina.cannelli@ingv.it figures fig. s1 radon (cpm) soil time-series from badi station, as of november 2017. 2 fig. s2 radon (cpm) soil time-series from badi2 station, as of november 2017. 3 fig. s3 radon (cpm) soil time-series from cae station, as of november 2017. 4 fig. s4 radon (cpm) soil time-series from cdca station, as of november 2017. 5 fig. s5 radon (cpm) soil time-series from cmpl station, as of november 2017. 6 fig. s6 radon (cpm) soil time-series from cttr station, as of november 2017. 7 fig. s7 radon (cpm) soil time-series from frme station, as of november 2017. 8 fig. s8 radon (cpm) soil time-series from gall station, as of november 2017. 9 fig. s9 radon (cpm) soil time-series from mmn station, as of november 2017. 10 fig. s10 radon (cpm) soil time-series from mmng station, as of november 2017. 11 fig. s11 radon (cpm) soil time-series from murb station, as of november 2017. 12 fig. s12 radon (cpm) soil time-series from nrca station, as of november 2017. 13 fig. s13 radon (cpm) soil time-series from pofi station, as of november 2017. 14 fig. s14 radon (cpm) soil time-series from ptrl station, as of november 2017. 15 fig. s15 radon (cpm) soil time-series from rom9 station, as of november 2017. 16 fig. s16 radon (cpm) soil time-series from rom92 station, as of november 2017. 17 fig. s17 radon (cpm) soil time-series from spi station, as of november 2017. 18 fig. s18 radon (cpm) soil time-series from ssfr station, as of november 2017. 1 annals of geophysics, 66, 2, se204, 2023;doi:10.4401/ag-8997 o p e n a c c e s s special issue seismic anisotropy and shear-wave splitting: achievements and perspectives foreword this special issue of annals of geophysics “seismic anisotropy and shear-wave splitting: achievements and perspectives” originates from a session (s10) of the 37th general assembly of the european seismological commission esc 2021 conference which was planned to take place on 21 september 2021, in corfu greece, but due to the covid19 pandemic was virtual. the main theme of the session and of this special issue was the crucial role of seismic anisotropy in investigating the earth’s interior from the upper crust to the inner core. shear-wave splitting, one of the most effective ways to study seismic anisotropy, can identify the properties and the geodynamics of the upper mantle, and identify the presence of fluid-saturated microcracks, oriented according to the stress regime, in the upper crust. azimuthal anisotropy and radial anisotropy can be assessed from earthquake or ambient noise recordings to detect the seismic layered features and to rebuild the 3d seismic structure. the session was organised in three parts, each with an invited presentation: the first part was focused on anisotropy in the crust, starting with an interesting introduction by prof. martha savage, titled “seismic anisotropy on volcanoes and geothermal areas”. the second part included seismic anisotropy studies using receiver functions techniques and seismic tomography, in the framework of which the invited talk was given by prof. jeffrey park with the title “the expression of seismic anisotropy in ps an sp converted waves”. the third part was about shear-wave splitting analysis at mantle scale, with prof. stephen gao giving an invited talk with the title “on the interpretation of teleseismic shear wave splitting measurements. lesson learned in north america, africa and australia”. the session was an occasion, even if only in virtual mode, to have a fruitful exchange of information and ideas between seismologists working on seismic anisotropy to understand the earth’s interior, structure and processes. the session hosted 3 invited presentations, 14 oral presentations and 8 posters. the main purpose of the papers collected in this special issue, “seismic anisotropy and shear-wave splitting: achievements and perspectives” is to stimulate collaboration between the seismologists who assess seismic anisotropy with various techniques in very different environments and at different scales, to improve the database of shear-wave splitting available to the scientific community and to provide useful and reliable information to young scientists who start working on seismic anisotropy. the 12 articles included in this special issue can be divided into three groups. the first group, 3 contributions, is methodological and provides important and useful criteria about the selection of data and constraints of different techniques for studying seismic anisotropy. the second group, 5 contributions, includes studies related to the presence, the causes and the temporal evolution of seismic anisotropy in the crust. the third group, 4 contributions, deals with the presence of seismic anisotropy in the lithosphere or in the mantle and its significance. below, there is a brief review of the contents and authors of each work. 2 seismic anisotropy techniques the paper “crustal anisotropy from the birefringence of p-to-s converted waves: bias associated with p-wave anisotropy” by park et al. [2023] focuses on the anisotropic parameters and wave-propagation effects that help distinguish ps amplitude variations from thin anisotropic layers and ps splitting that accumulates throughout the crust. it is proposed that the standard minimum-transverse method for estimating shear-wave splitting can be applied to the radial and transverse receiver functions (rf) and that simple crustal models that include only shear anisotropy are estimated well by rf birefringence, whereas the ps amplitude variation induced by compressional anisotropy leads to misleading estimates of birefringence. synthetic seismograms for a total of 36 anisotropic crustal models are computed to examine the effects on ps birefringence of s or p anisotropy and the depth range of anisotropy. it is revealed that large variations in ps birefringence estimates with back-azimuth occur theoretically in the presence of p-wave anisotropy, which normally accompanies s-wave anisotropy. the paper “testing observables for teleseismic shear-wave splitting inversions: ambiguities of intensities, parameters, and waveforms” by rümpker et al. [2023] assesses the capabilities of different observables for the inversion of core-refracted shear waves (xks phases) to uniquely resolve the anisotropic structure of the upper mantle. a “brute-force” inversion approach is employed by sampling the complete model space and by directly comparing full waveforms and waveform proxies. canonical models of anisotropic structure in the upper mantle are considered and the study focuses on accessing the suitability of apparent splitting parameters, splitting intensity, and full waveforms for the inversions to infer the model parameters. the results show that it is not possible to fully resolve the anisotropic parameters of a model, but inversions of both waveforms and apparent splitting parameters lead to similar models that exhibit systematic variations of anisotropic parameters. it is concluded that xks-splitting inversions and related tomographic schemes are not sufficient to fully resolve the heterogeneous anisotropic structures of the upper mantle and that combinations with alternative methods are required. the paper “on the effects of wrongly aligned seismogram components for shear wave splitting analysis" [2022] by fröhlich et al. is investigating the problem of mixing wrongly aligned recording components. this may result in misleading and wrong data representations, including the particle motions in both the zne and the ray (lqt) coordinate systems and waveforms in the lqt coordinate system. the authors consider that the main pitfall is that start and end times of the single traces in general differ due to data storage details. this effect is regarded to distort under certain conditions splitting signals or to simulate non-existing ones. to overcome this problem, the users have to make sure that the traces are temporally aligned relative to each other in a correct way, either based on a reference time or by cutting the traces to their shared time window. seismic anisotropy in the crust the paper “shear-wave splitting perspectives from the intense aftershock sequence of damasi – tyrnavos” by kaviris [2023] performs an upper crust seismic anisotropy study in thessaly (greece), at the damasi – tyrnavos area that hosted a mw = 6.3 earthquake on 3 march 2021, followed by a rich seismic sequence. a complex upper crust anisotropic regime was identified. fast shear-wave polarization directions at stations to the north and to the east area strike wnw-ese to nw-se, consistent with the anisotropic poro-elasticity (ape) model and the strike of the local active fault planes. however, stations at the central part exhibited different anisotropy directions. in addition, the highest normalized time-delay values were obtained at the northern stations, possibly related to the migration of seismicity to the north during the initial stage of the sequence. the paper “shear-wave splitting patterns in perachora (eastern gulf of corinth, greece)” by spingos et al. [2023] is an upper crust seismic anisotropy study at the perachora peninsula, central greece, that hosted a seismic swarm in 2020, with the strongest event having a magnitude of 3.7. a complex state of anisotropy was identified, with ne-sw polarization directions prevailing before the swarm, while a dominant wnw-ese orientation was present during the swarm. furthermore, variations of normalized time-delays were observed during the seismic crisis. taking into account these results and the fluid diffusion during the swarm, shear-wave splitting in the upper crust of the eastern gulf of corinth was considered mainly driven by pressure gradients. the paper ”seismic anisotropy in the upper crust around the north segment of xiaojiang faults in the se margin of tibetan plateau” by wu et al. [2023] aims to better understand the relation among crustal deformation, fault 3 characteristics and regional stress. to achieve this goal, the authors employ near-field seismic waveform data from a dense temporary seismic array around the north segment of the xiaojiang fault zone (xjfz) to study the crustal anisotropy and to analyse the detailed spatial characteristics in the upper crust, as well as the crustal deformation implications. a total of 875 effective records were obtained at 50 stations. based on the spatial distribution of the polarization of the fast shear-wave (pfs), the study area is divided into two zones. the pfs in the north (n) zone is scattered, with the dominant direction being consistent with the regional maximum principal compressive stress. in the south (s) zone, the dominant pfs directions at most stations are nearly n-s, consistent with the strike of the xiaojiang fault. the paper “s wave splitting in central apennines (italy): anisotropic parameters in the crust during seismic sequences” by pastori et al. [2023] performs a review of the main anisotropic findings obtained in the last two decades along the central apennines. the authors improved the database with results for the montereale area, covering spatio-temporal gaps between two of the main seismic sequences in central italy (l’aquila 2009 and amatrice-norcia-visso 2016. the collected results reveal a general nw-se fast shear wave direction, consistent with the orientation of the extensional active quaternary and inherited compressive fault systems, focal mechanisms and local stress field. the results can be interpreted either with the extensive-dilatancy anisotropy (eda) model, where the anisotropic pattern is related to the local stress variation, or with the structural-induced anisotropy (sia) model, where the anisotropic pattern is related to the major structural features and most of the variability is visible only in space. the discrimination between stress and structural anisotropy is quite complex in a region where the directions of the extensional regime and the strike of major faults coincide. the paper “seismic anisotropy in the upper crust beneath the sanjiang lateral collision zone in the southeastern margin of the tibetan plateau revealed by s wave splitting from a temporary array” by li et al. [2023] employs local seismic data to obtain the essential properties of upper crustal seismic anisotropy. the dominant polarization of the fast shear-wave is found to strike nnw. the study area is divided into three subzones according to the dominant polarization direction, which gradually varies from nnw, ns to nne from the west subzone to the east one. the largest time delay is found at the east side of the western boundary of the sichuan-yunnan rhombus block. in addition, the authors identified an area of strong anisotropy stretching along the western segment of the lijiang-xiaojinhe fault. it is concluded that the crustal structure may be strongly controlled by the fault and block boundary strike. seismic anisotropy in the lithosphere and mantle the paper “azimuthal anisotropy of receiver functions in the central south china block and its tectonic implications” by shi et al. [2023] employs seismic data to obtain the crustal structure and the anisotropic parameters using receiver functions. the authors further analyse the lithospheric deformation and composition in the central south china block, where strong lateral heterogeneity in crustal structures was revealed. another result of the study is that the crustal thickness reduces from northwest to southeast, with significant differences across the boundary of the sub-blocks. in addition, the average crustal vp/vs ratio gradually increases from west to east, leading to high values in the coastal region, which suggests that the subduction of the pacific plate has possibly caused the underplating of magma or the upwelling of upper mantle material. the crustal azimuthal anisotropy of the orogen is considered to be related to the extension and deformation of the lithosphere. the paper “rayleigh phase velocity and azimuthal anisotropy from ambient noise data in the sanjiang lateral collision zone in the se margin of the tibetan plateau” by tian et al. [2023] employs the direct surface wave tomography method to obtain the 3d s-wave velocity for periods 2 s to 40 s and azimuthal anisotropy in the study area. the authors perform a forward calculation of the phase velocity and azimuthal anisotropy. the fast velocity directions are found to mainly align with n-s, while at periods 10 35 s they rotate clockwise from north to south. the phase velocity and azimuthal anisotropy obtained by forward calculation are mainly discussed, and the phase velocity results, obtained by the one-step method, are compared with those obtained by the conventional method. the intensity of anisotropy in the low-velocity zone is considered stronger than that in the high-velocity zone. the paper “peeking inside the mantle structure beneath the italian region through sks shear wave splitting anisotropy: a review” by pondrelli et al. [2023] reviews the results of more than 11.000 core phase measurements 4 by international projects and published papers. the work focuses on retrieving the anisotropic structure beneath italy, promoting advances in the knowledge of geological and geodynamic settings. the aim is to better understand the complex and active geodynamic evolution of the active and remnant subduction systems characterising this region and the associated apennines, alps and dinaric belts, together with the adriatic and tyrrhenian basins. measurements of core refracted phases are used as a measure of mantle deformation and interpreted into geodynamic models. images of anisotropy identify well-developed mantle flows around the sinking european and adriatic slabs, recognised by tomographic studies. slab retreat and related mantle flow are interpreted as the main driving mechanism of the central mediterranean geodynamics. the paper “upper mantle anisotropy beneath northern algeria from shear-wave splitting” by radi et al. [2023] conducts a lithosphere and mantle deformation study. seismic anisotropy is investigated using the sks shear-waves splitting method using recordings of broadband seismic stations installed in central and north-western algeria. the results show that seismic anisotropy can be described by two main orientations. the ene-wsw follows the general trend of the saharan atlas, particularly in the central and western parts, and the ese-wnw one follows the hodna mountains in south-eastern algeria. the obtained results are compared to velocity directions estimated from gps measurements, implying that anisotropy fast axes are nearly perpendicular to the convergence direction between the african and eurasia plates. acknowledgment. during the session s10 of the 37th general assembly of the european seismological commission esc 2021 conference, we remembered our recently deceased colleague vladislav babuška, who greatly contributed to the understanding of seismic anisotropy inside the earth. the guest editors and the authors are grateful to the editors of annals of geophysics for the opportunity to publish the special issue “seismic anisotropy and shear-wave splitting: achievements and perspectives” and to the reviewers, whose contribution has been fundamental to improve the quality of the articles. references fröhlich y, m. grund, j.r.r. ritter (2023). on the effects of wrongly aligned seismogram components for shear wave splitting analysis, ann. geophys., 66, this volume, doi:10.4401/ag-8781. kaviris g. (2023). shear-wave splitting perspectives from the intense aftershock sequence of damasi – tyrnavos, ann. geophys., 66, this volume, doi:10.4401/ag-8848. li, x., y. gao (2023). seismic anisotropy in the upper crust beneath the sanjiang lateral collision zone in the southeastern margin of the tibetan plateau revealed by s wave splitting from a temporary array, ann. geophys., 66, this volume, doi:10.4401/ag-8867. park j., x. chen, v. levin (2023). crustal anisotropy from the birefringence of p-to-s converted waves: bias associated with p-wave anisotropy, ann. geophys., 66, this volume, doi:10.4401/ag-8882. pastori m., p. baccheschi, d. piccinini, l. margheriti (2023). s wave splitting in central apennines (italy): anisotropic parameters in the crust during seismic sequences, ann. geophys., 66, this volume, doi:10.4401/ag-8844. pondrelli s., s. salimbeni, p. baccheschi, j. m. confal, l. margheriti (2023). peeking inside the mantle structure beneath the italian region through sks shear wave splitting anisotropy: a review, ann. geophys., 66, this volume, doi:1044.01/ag-8872. radi z., a. y. chaouche, s. guettouche, g. bokelmann (2023). upper mantle anisotropy beneath northern algeria from shear-wave splitting, ann. geophys., 66, this volume, doi:104401/ag-8839. rümpker g, a. kaviani a, f. link, m. c. reiss, a. komeazi (2023). testing observables for teleseismic shear-wave splitting inversions: ambiguities of intensities, parameters, and waveforms, ann. geophys., 66, this volume, doi:10.4401/ag-8870. shi y., y. gao, z. zhang, g. li (2023). azimuthal anisotropy of receiver functions in the central south china block and its tectonic implications, ann. geophys., 66, this volume, doi:10.4401/ag-8825. spingos i, v. kapetanidis, g. michas, g. kaviris, f. vallianatos (2023). shear-wave splitting patterns in perachora (eastern gulf of corinth, greece), ann. geophys., 66, this volume, doi:10.4401/ag-8829. 5 tian j., y. gao, y. li (2023). rayleigh phase velocity and azimuthal anisotropy from ambient noise data in the sanjiang lateral collision zone in the se margin of the tibetan plateau, ann. geophys., doi:10.4401/ag-8874. wu p, y. gao, l. xu (2023). seismic anisotropy in the upper crust around the north segment of xiaojiang faults in the se margin of tibetan plateau, ann. geophys., 66, this volume, doi:10.4401/ag-8852. g u e s t e d i to r s: george kav i r i s section of geophysics – geothermics, department of geology and geoenvironment, national and kapodistrian university of athens, athens, greece yuan g ao key laboratory of earthquake prediction, institute of earthquake forecasting, china earthquake administration, beijing, china lucia m a rg h e r i t i istituto nazionale di geofisica e vulcanologia, osservatorio nazionale terremoti, roma, italy annals of geophysics annals of geophysics, 60, 4, 2017, s0447; doi: 10.4401/ag-7480 preface paolo clemente, salomon hailemikael, giuliano milana, luciana orlando 1. introduction the preservation of cultural heritage is a crucial issue in areas prone to seismic and other natural hazards. the major challenge in pursuing such an important objective is related to the extreme fragility of ancient monuments and buildings exposed for centuries to the ravages of time. a suitable way to successfully proceed in the preservation effort of monumental structures should be based on then integrated use of different non-invasive diagnostic techniques, aimed at understanding the geological and geotechnical features of the areas where monuments are founded and the structural characteristic of the construction itself. in this perspective a team composed by researchers of enea, ingv and sapienza university of rome has recently carried out geophysical and structural investigations on the amphiteatrum flavium in rome, better known as colosseum, which is the symbol of monumental heritage in italy and well-known all over the world. the experimental campaign was only a preliminary analysis of the very famous archaeological site that allowed outlining the state of knowledge about the characteristics of the site and the structure and should be considered as a starting point for an in-depth investigation of the monument vulnerability. in our opinion, the benefits of such an integrated approach can steer the political and social choices related with the preservation of the cultural heritage at national or european level. the results obtained were very interesting and induced the authors to propose their publication in a special issue of an international journal. the scope of this choice was twofold: ensuring both a wide audience with these activities and a serious review process of the studies. therefore, we selected the most interesting results of the experimental campaign and invited a number of researchers involved in the field of “monitoring and seismic characterization of archaeological sites and structures” to contribute to this special issue. the relevance of this volume has been emphasized by the occurrence of the recent seismic sequence in central italy, whose mainshocks of august 24th and october 26th and 30th, 2016, caused 298 victims, hundreds of injured and the damage to most of the cultural heritage in the area. symbol of the open wound left by the seismic sequence on the monumental patrimony is the san benedetto cathedral in norcia. the morning of october 30th only the façade of the church, which is dedicated to the patron saint of europe and represents the emblem of the medieval architecture in central italy dating back to xiii century, was still standing. it is worth noting that the mainshocks were clearly felt in rome, at distance of about 100 km. 2. volume contents the issue includes twelve papers regarding both geophysical and structural topics, pointing out the importance of a multidisciplinary approach when studying archaeological sites and structures. in the first paper, bozzano et al. [2017-this issue] present the evaluation of the local seismic response (lsr) along the ancient roman road laurentina, which has been exhumed in several areas of southwest rome over the last decade during the construction of new structures. it is an example of lsr analysis applied to ancient and archaeological sites resting on alluvial valleys with some methodological inferences for the design of infrastructure and urban planning. the next four papers are dedicated to the amphiteatrum flavium. in the first one, cardarelli et al., [2017-this issue] present the results of non-invasive geophysical investigations carried out on the subsoil below the archaeological site. the new experimental results contribute to link the reference geologic model to the specific conditions of the anthropic layer and the seismic bedrock at the monumental site, as well as the foundations and the buried parts of the monument below the former arena, providing key input data for the assessment of the response of the colosseum under dynamic loading. in the second one, orlando et al., [2017-this issue] report the results of the non-invasive investigation of the foundations, carried out by georadar surveys along the so called “passage of commodus”, dug in the foundation. they demonstrate that the foundations of the colosseum are characterized by a heterogeneous multi-layer structure, with the presence of a cavity network and of buried anomalies, likely due to the extension of load-bearing structural elements underground. hailemikael et al. [2017-this issue] with the aim of understanding the seismic response of the area and investigating the soil-structure interactions, performed seismic experiments based on ambient vibration recordings. measurements were performed at the original ground level, on the foundations and at different floors of the monument. data were analysed in terms of standard fourier analysis (fas) and horizontal-to-vertical spectral ratio technique (h/v). finally, bongiovanni et al. [2017-this issue] studied the effects of ambient and traffic-induced vibrations on the external northern wall of the amphitheatrum flavium, with the two objectives of analysing the amplitudes of such vibrations and extracting the dynamic characteristics of the structure as part of preservation effort. data processing consisted in the time domain and frequency domain analyses, in which fourier transform, power spectral density and cross spectral density were used to extract resonance frequencies, modal shapes and damping. the successive papers deal with new methods and proposals aimed at the preservation of structures of historical importance, which are tested with other very important archaeological sites and monuments. sboras et al. [2017-this issue] propose a gis-based data-base, developed to collect and combine all necessary data about the monuments and their regional geological and seismotectonic conditions, and to assess seismic hazard for each and every monument using the most modern techniques. in the paper the structure and development of the data-base is presented, and a methodological procedure for estimating seismic hazard in greece for the structural assessment of historical structures is presented. azzara et al. [2017-this issue] report on a vibration measurements campaign performed on the medieval maddalena bridge, over the serchio river in borgo a mozzano (italy), one of the most fascinating in italy. a monitoring system has been installed on the bridge and the natural frequencies and mode shapes of the structure and the corresponding damping ratios have been obtained by analysing the recorded data using different techniques of operational modal analysis. the paper by masciotta et al. [2017-this issue] deals with the dynamic characterisation of a historical masonry chimney aimed at identifying the structural damage and assessing its seismic performance. the structure was severely damaged by a lightning accident and in-depth repair works were executed to re-instate its sound configuration. the case study is fully detailed, including the aspects of survey, inspection, diagnosis, and evolution of the dynamic properties of the system throughout the structural intervention. hinzen [2017-this issue] faces the problem of rigid blocks under seismic input. he uses simply structured discrete element models of four walls with different block geometries, perfect rectangular, an inka-type structure and two polygonal designs, to test their dynamic behaviour. he also uses measured strong motion signals as boundary conditions for the 3d wall models with varying height to width ratios. schweppe et al. [2017-this issue] propose to use ancient manmade structures, delicate in terms of stability and particularly those that have survived earthquake ground motions intact, for a rough estimation of the ground motion that has not been reached or exceeded at the site. they suggest using them as local seismoscopes for determining maximum upper ground motion bounds. they applied this concept to the ruin of the roman temple of kedesh, located in close proximity to a branch of the dead sea transform fault. in the paper by minos-minopoulos et al. [2017-this issue] an indicator-based method for assessing the vulnerability of archaeological sites to earthquakes is presented and tested, which is based on the definition of two indeces, the spatial susceptibility index and the temporal vulnerability index, whose sum give the archaeological site vulnerability index. the method is applied to 16 archaeological sites across greece, allowing an assessment of their vulnerability. this then allows the establishment of a regional and national priority list for considering future risk mitigation strategies. riccio et al. [2017-this issue] carried out a vibration study experiment on a masonry building, using six-channel stations equipped with three-component velocity-transducers and accelerometers and running in continuous modality. the analysis of weak motions from several local earthquakes, together with the 3d numerical modelling of the structure, allowed identifying the first three vibration modes of the target building. 3. conclusions the variety of the issues presented in the papers testifies the complexity of the analysis of the behaviour of archaeological sites and structures under dynamic loading and the evaluation of their seismic vulnerability, we believe that the monitoring represents an effective tool to improve significantly our knowledge towards the definition of seismic risk mitigation plans. references bozzano, f., buccellato, a., coletti f., martino, s., marra, f., rivellino, s., varone, c. (2017). “analysis of the seismic site effects along the ancient via laurentina in rome”, annals of geophysics, 60, 4; doi: 10.4401/ag-7140. cardarelli, e., cercato, m., orlando, l. (2017). “geometry and seismic characterization of the subsoil below the amphitheatrum flavium in rome”, annals of geophysics, 60, 4; doi: 10.4401/ag-7124. orlando, l., de donno, g., di giambattista, l., palladini, l. (2017). “investigating the foundation of the amphitheatrum flavium through the passage of commodus”, annals of geophysics, 60, 4; doi: 10.4401/ag-7183. hailemikael, s., milana, g., cara, f., vassallo, m., pischiutta, m., amoroso, s., bordoni, p., cantore, l., di giulio, g., di naccio, d., famiani, d., mercuri, a. (2017). “sub-surface characterization of the amphitheatrum flavium area in rome through single-station ambient vibration measurements”, annals of geophysics, 60, 4; doi: 10.4401/ag-7359. bongiovanni, g., buffarini, g., clemente, p., rinaldis, d., saitta, f. (2017). “dynamic characteristics of the amphitheatrum flavium northern wall from traffic-induced vibrations”, annals of geophysics, 60, 4; doi: 10.4401/ag-7178. sboras, s., dourakopoulos, j.a., mouzakiotis, e., dafnis, p., palantzas, t., karastathis, v., voulgaris, n., tselentis, g.a. (2017). “seismic hazard assessment for the protection of cultural heritage in greece: methodological approaches for national and local scale assessment (pilot areas of aighio, kalamata and heraklion)”, annals of geophysics, 60, 4; doi: 10.4401/ag-7154. azzara, r.m., de falco, a., girardi, m., pellegrini, d. (2017). “ambient vibration recording on the maddalena bridge in borgo a mozzano, italy: data analysis”, annals of geophysics, 60, 4; doi: 10.4401/ag-7159. masciotta, m.g., ramos, l.f., lourenço, p.b., vasta, m. (2017). “damage identification and seismic vulnerability assessment of a historic masonry chimney”, annals of geophysics, 60, 4; doi: 10.4401/ag-7126. hinzen, k.g. (2017). “rectangular blocks vs polygonal walls in archaeoseismology”, annals of geophysics, 60, 4; doi: 10.4401/ag-7127. schweppe, g., hinzen, k.g., marco, s., reamer, s.k., fischer, m. (2017). “the ruin of the roman temple of kedesh, israel: example of a precariously balanced archaeological structure used as a seismoscope”, annals of geophysics, 60, 4; doi: 10.4401/ag-7152. minos-minopoulos, d., dominey-howes, d., pavlopoulos, k. (2017). “vulnerability assessment of archaeological sites to earthquake hazard: an indicator based method integrating spatial and temporal aspects”, annals of geophysics, 60, 4; doi: 10.4401/ag-7157. riccio, g., cogliano, r., di giulio, g., fodarella, a., pucillo, s., rovelli, a. (2017). “a study of building vibrations induced by weak motions: effects of earthquake excitation, cultural noise and wind speed”, annals of geophysics, 60, 4; doi: 10.4401/ag-7149. paolo clemente salomon hailemikael giuliano milana luciana orlando enea casaccia research centre, rome, italy enea casaccia research centre, rome, italy istituto nazionale di geofisica e vulcanologia, rome, italy “sapienza” university of rome dicea, rome, italy miscellanea 655 annals of geophysics, vol. 51, n. 4 august 2008 key words etna – stromboli – vulcano – eruption, and earthquake 1. introduction earthquake-eruption and eruption-eruption triggering mechanisms are the subject of intense debate and represent the new frontier for several geophysicists and volcanologists (kanamori and givens, 1982; kimura, 1996; brodsky et al., 1998, 2000; gudmundsson and brenner, 2003; marzocchi et al., 2004; gudmundsson, 2006; feuillet et al., 2006; madonia et al., 2008). in recent years, a few case histories of eruptions that triggered earthquakes (jacques et al., 1996; azzaro et al., 2001; walter and amelung, 2006) or other eruptions (miklius and cervelli, 2003; gudmundsson and andrew, 2007), and of eruptions that hindered earthquakes (walter and amelung, 2004) have been reported. in contrast, several case histories of large earthquakes that triggered eruptions at near or remote volcanoes are now well documented (carr, 1977; marzocchi et al., 1993, 2002, 2004; barrientos, 1994; linde and sacks, 1998; nostro et al., 1998; manga and brodsky, 2006; cigolini et al., 2007; harris and ripepe, 2007; lemarchand and grasso, 2007; manga, 2007; mellors et al., 2007; walter and amelung, 2007; walter et al., 2007). eruptions triggering other eruptions at the southern italian volcanoes (i.e., etna and aeolian volcanoes, fig. 1) have been speculated since at least the 1st century bc (diodorus concurrent eruptions at etna, stromboli, and vulcano: casualty or causality? andrea billi and renato funiciello dipartimento di scienze geologiche, università «roma tre», roma, italy abstract anecdotes of concurrent eruptions at etna, stromboli, and vulcano (southern italy) have persisted for more than 2000 years and volcanologists in recent and past times have hypothesized a causal link among these volcanoes. here this hypothesis is tested. to introduce the problem and provide examples of the type of expected volcanic phenomena, narratives of the most notable examples of concurrent eruptions are provided. then the frequency of eruptions at each individual volcano is analysed for about the last 300 years and the expected probability of concurrent eruptions is calculated to compare it to the observed probability. results show that the occurrence of concurrent eruptions is often more frequent than a random probability, particularly for the stromboli-vulcano pair. these results are integrated with a statistical analysis of the earthquake catalogue to find evidence of linked seismicity in the etnean and aeolian areas. results suggest a moderate incidence of non-random concurrent eruptions, but available data are temporally limited and do not allow an unequivocal identification of plausible triggers; our results, however, are the first attempt to quantify a more-than-2000-years-old curious observation and constitute a starting point for more sophisticated analyses of new data in the future. we look forward to our prediction of a moderate incidence of concurrent eruptions being confirmed or refuted with the passage of time and occurrence of new events. mailing address: andrea billi, dipartimento di scienze geologiche, università «roma tre», largo s. l. murialdo 1, 00146 roma, italy; tel: +39 0657338016; fax: +39 0657338201; e-mail: billi@uniroma3.it miscellanea 9-03-2009 14:42 pagina 655 656 a. billi and r. funiciello siculus in bibliotheca historica). julius solinus (in collectanea rerum memorabilium, 4th century ad) wrote that there should have been subterranean conduits that linked and fed these volcanoes. de dolomieu (1783) also hypothesized direct feeding relationships between etna and the aeolian volcanoes, whereas mercalli (1879, 1888) supposed an indirect physical (i.e., dynamic) link between these volcanoes. recently, cigolini et al. (2007) provided evidence of linked volcanic phenomena at etna and aeolian volcanoes during 2002-2003. although southern italy is seismically active and several m≥7 earthquakes were recorded in historical times (boschi et al., 1997; neri et al., 2006; basili et al., 2008), eruptions at etna, stromboli, and vulcano triggered by large nearby earthquakes are not known. in particular, the six largest historical earthquakes (with epicentral mcs intensity≥ix) of eastern sicily and southern calabria (fig. 1) were not shortly followed by significant eruptions from the nearby volcanoes (table i), which, therefore, provide valuable case histories to study eruption triggers alternative to large nearby earthquakes (sharp et al., 1981). this paper searches for evidence for a link between eruptions at etna, stromboli, and vulcano. our hypothesis is that concurrent activity at these volcanoes may occur at a frequency higher than expected based on the background rate. this hypothesis is explored in three different ways. the eruption catalogue is statistically analysed. the statistical method has the advantage of being quantitative, but the disadvantage of providing little insight into the plethora of behaviour that can fall under the heading of eruption. to fill out this story and provide examples of the type of phenomena expected, narratives of the most notable examples of concurrent activity are provided. finally, the catalogue of instrumentally-recorded earthquakes is analysed to find evidence of linked seismicity in the etnean and aeolian areas. this paper extends previous studies on analogous subjects (e.g., mulargia et al. 1985; nercessian et al., 1991; cardaci et al., 1993). moreover, the problem of possible links between concurrent eruptions at nearby volcanoes has been previously studied in other regions (e.g., klein, 1982; bebbington and lai, 1996). 2. geological setting etna and aeolian volcanoes (fig. 1) are located along the active convergent margin between the african and eurasian plates in the central mediterranean region (malinverno and ryan, 1986). a narrow remnant of the ionian table i. largest historical earthquakes of sicily and southern calabria (southern italy, fig. 1) and subsequent eruptions at etna, stromboli, and vulcano. data are from simkin and siebert (1994) and from boschi et al. (1997). earthquake date epicentral mcs intensity epicentral area subsequent eruptions 4 feb. 1169 xi estern sicily and a doubtful eruption western calabria from etna on feb. 1169 10 dec. 1542 ix-x southeastern sicily none 11 jan. 1693 xi southeastern sicily none 5 mar. 1823 x northeastern sicily none 28 dec. 1908 xi messina straits none 15 oct. 1911 x southeastern flank of mt etna none miscellanea 9-03-2009 14:42 pagina 656 657 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? ��������������������������� ��� ������ ������ ������ ������������������������ ���������� ��� ������ ��� ��������������� ��� ������ ��������� ������ ������������������ � � �� � �� � � � �� � �� � � � �� � �� � � �� �� �� �� �� �� �� � �� �� �� �� �� �� �� � �� �� �� �� �� �� �� � ������� �� ��� ��� � � �� � �!� �!� �! ������������ "������"������"������ ������������������ �� � �� � �� � �� �� � # �� � � # �� � � # �� � $������� $������� $������� ��������%������� ��������%������� ��������%������� ������������������������������ ��������������������� ��������������� ��������������������� ������ ������ ���������&���������&���������& �������$�����������$�����������$���� ���������' ���������' ���������' � ��� ��� �� ( �!( �!( �! ��������������� ��������������� # ���# ���# ��� � ��������������������� � � �� �� �� �� �� �� �� � �� �� � � � �� �� �� �� �� �� �� � �� �� � � � �� �� �� �� �� �� �� � �� �� � ��&����������&����������&�������� ������������ )���)���)��� "������"������"������ ��������������������������������������������������������� � �� �� �� � � �� �� �� � � �� �� �� � *������*������*������ +�$���+�$���+�$��� , �� � , �� � , �� � ��������������������� ��������������� -�. -�. fig. 1a,b. (a) tectonic map of southern italy. main fold-thrust belts occurring along the african-european plate boundary (i.e., apennines, calabrian arc, and maghrebides) are drawn in red. (b) hillshade of north-eastern sicily, including the studied volcanoes (i.e., etna, stromboli, and vulcano). note that stromboli and vulcano are included in the aeolian archipelago. earthquake focal mechanisms (m ≥ 3.5) and arrows indicating active stresses are from neri et al. (2005). miscellanea 9-03-2009 14:42 pagina 657 658 a. billi and r. funiciello slab (african affinity) is subducting toward the northwest beneath the calabrian arc, whereas large slab windows occur beneath sicily and the southern apennines, where subduction is mostly inactive (gvirtzman and nur, 2001; faccenna et al., 2004, 2005). the apennine and maghrebian thrust-fold belt have grown since late paleogene time along the africa-eurasia suture zone, with eastward and southward vergence, respectively. to the north of sicily, the oceanic tyrrhenian backarc basin started to form since about late neogene time (malinverno and ryan, 1986; dewey et al., 1989; patacca et al., 1992). seismic and geodetic evidence from sicily and surrounding regions shows that, in the central mediterranean region, africa and eurasia are converging at a considerable rate (circa 3 mm/yr) and that both the etnean and aeolian areas are undergoing n-s-oriented tectonic compression (fig. 1b; barberi et al., 2000; neri et al., 2003, 2005; d’agostino and selvaggi, 2004; goes et al., 2004; montone et al., 2004; pondrelli et al., 2004). in the study area, main active tectonic elements are the roughly e-wtrending thrust belt in the southern tyrrhenian area and the nnw-striking tindari fault between the aeolian islands and mount etna (fig. 1). the compressional belt in the southern tyrrhenian area accommodates most of the africa-eurasia ongoing convergence in the central mediterranean area and is characterized by compressional and transpressional earthquakes with a maximum recorded magnitude of 5.9 (goes et al., 2004; billi et al., 2007). this belt involves the western islands of the aeolian archipelago, i.e., alicudi and filicudi, which are the summit of submarine volcanoes inactive since about mid-pleistocene time. the tindari fault is the eastern boundary of the southtyrrhenian compressional belt and is characterized by right-lateral strike-slip earthquakes with a maximum recorded magnitude of 5.5. this fault involves the central islands of the aeolian archipelago (salina, lipari, and vulcano) and extends toward the south near mount etna (ghisetti, 1979; tortorici et al., 1995; ventura et al., 1999; billi et al., 2006). the aeolian volcanoes (alicudi, filicudi, lipari, panarea, salina, stromboli, and vulcano, fig. 1) form a high-k calc-alkaline island arc, whose activity started about 1.3 myr ago and is still active on at least three of the seven major islands, namely lipari (last eruption in 729), vulcano (last eruptions in 1888-1892), and stromboli (simkin and siebert, 1994; behncke, 2001). hydrothermal activity offshore panarea has occurred in historical times and in recent years (e.g., gabbianelli et al., 1993; esposito et al., 2006). stromboli is characterized by a persistent volcanic activity consisting in continuous small explosions occurring at approximately regular intervals of a few minutes (i.e., strombolian activity). however, individual events, such as lava effusions and paroxysmal eruptions, occurred at stromboli during recent and historical times (rittmann, 1931; simkin and siebert, 1994). the volcanic activity of vulcano has been characterized by powerful and impressive explosions (i.e., vulcanian eruptions) alternated with less frequent effusive events (frazzetta et al., 1984; simkin and siebert, 1994). volcanism in the etnean area started about 0.5 myr ago with tholeiitic magmas (branca et al., 2008). the modern volcano formed by a succession of volcanic edifices consisting in alternating pyroclastic and effusive rocks (gillot et al., 1994; behncke, 2001; tanguy et al., 2007; branca et al., 2008). the volcanic activity of etna is, in fact, mostly effusive; however, explosive activity has often occurred at the summit craters (e.g., simkin and siebert, 1994). lateral effusive and mildly explosive eruptions have also been frequent at mount etna (rittmann, 1964; lanzafame et al., 2003; alparone et al., 2005). the source of etna magmatism is possibly connected with a mantle plume that montelli et al. (2004) imaged above 1000 km using seismic tomography. alternatively or complementarily, the voluminous melting under mount etna may result from suction of asthenospheric material induced by the backward migration of the descending ionian slab (gvirtzman and nur, 1999). 3. examples of concurrent eruptions eruption catalogues are notoriously subjective. eruptive activity is gradational and activity during a given month may mean a variety of miscellanea 9-03-2009 14:42 pagina 658 659 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? different phenomena. to fill out this story, before the statistical analysis of the eruption catalogue, brief narratives of four of the most notable examples of concurrent activity are provided. two of these episodes fall outside our statistical study period (see the following section) and three involve observations from another aeolian volcano (i.e., panarea), but these stories are anyhow included to provide examples of the type of phenomena expected. the acronym vei is henceforth used instead of volcanic explosivity index (newhall and self, 1982) and the related values are from simkin and siebert (1994). 3.1. concurrent activity during 2002-2003 the 2002 eruption of etna started the night between 26 and 27 october 2002 from the summit craters, concurrently with the formation of ground-surface fractures in this area. from the summit craters, fractures propagated toward the south and toward the northeast. newly generated vents and cones developed along these fractures. for about two months since the onset of the eruption, voluminous emissions of ash occurred. volcanic ashes fell in the nearby regions of sicily and calabria. after about 95 days of almost continuous eruption, on 29 january 2003, etna ceased most of its effusive and explosive activity. the 2002-2003 eruption of etna was highly explosive, possibly one of the most explosive of the last 350 years. early in the morning of 3 november 2002, local fishermen observed spots of anomalous seawater boiling and high mortality of fishes offshore panarea, and perceived smell of sulphurs. in particular, five major sites of gas emissions were found around the islets of lisca bianca, bottaro, and lisca nera. at these sites, ascent of gas bubbles with diameter up to 1 m on the sea surface signalled the presence of submarine gas-emissions. with the exception of one site close to lisca bianca, these sites were previously unknown as gas-emitting spots. gas emissions originated from nwand ne-striking fractures in volcanic rocks lying at depths between 8 and 30 m from the sea surface. gas emissions offshore panarea gradually ceased during the first months of 2003. on 28 december 2002, after 17 years of mild although continuous volcanic activity, an intense eruption began at stromboli. the eruption was preceded by seismic swarms and by an increase in volcanic tremors since 3 november 2002, concurrently with the onset of gas emissions offshore panarea. since 28 december 2002, lava flowed from the stromboli summit crater into the sea along the steep northwest flank. renewed lava flow occurred since 30 december 2002. the eruption mostly ceased by 5 april 2003, when a powerful explosion occurred at the summit crater of stromboli. the above information is mostly synthesized after acocella et al. (2003); dellino and kyriakopoulos (2003); patanè et al. (2003); caliro et al. (2004); calvari et al. (2005); ripepe et al. (2005); walter et al. (2005); esposito et al. (2006), and cigolini et al. (2007). 3.2. concurrent activity during 1886-1890 from 10 january to 31 march 1886, strombolian activity occurred at vulcano. on 22 january 1886, volcanic explosions occurred at stromboli. on 18 may 1886, an eruption began at the summit craters of mount etna. this eruption was highly explosive (vei = 3). during january, march, and november 1887, volcanic explosions occurred at stromboli. on 3 august 1888, an eruption began at vulcano. the eruptive activity of vulcano ceased in march 1990. the 1888-1890 eruption of vulcano was highly explosive (vei = 3). on 23 october 1888, volcanic explosions and lava fountains occurred at stromboli. on 29 november 1888, offshore vulcano, local fishermen observed boiling seawater and associated fish mortality, and perceived smell of sulphurs. the above information are mostly synthesized after mercalli (1888); silvestri (1890, 1893); baratta (1901) and simkin and siebert (1994). miscellanea 9-03-2009 14:42 pagina 659 660 a. billi and r. funiciello 3.3. concurrent activity during 1865 on 30 january 1865, an explosive (vei≥2) eruption occurred at etna. at the onset of the eruption, ne-striking fractures propagated across the summit crater of mount etna. several vents and cones formed along these fractures. the effusive and explosive activity at mount etna lasted for about 90 days. voluminous emissions of ash, which fell for about two months in the nearby regions of sicily and calabria, accompanied the eruption from the beginning. during the 1865 eruption of etna, emissions of sulphurous gases occurred offshore panarea. from 26 january to 2 february 1865, volcanic explosions occurred at stromboli. the above information is mostly synthesized after fouqué (1865); silvestri (1867); mercalli (1879, 1888); baratta (1901), and simkin and siebert (1994). most volcanic phenomena occurred at etna and aeolian volcanoes during 1865 are incredibly very similar to those occurred at the same volcanoes during 2002-2003. 3.4. concurrent activity during 126 bc paulus orosius, a latin historian lived in the 5th century. in his historiarum adversus paganos wrote: «under the consulate of m. aemilius laepidus and l. orestes, mount etna was violently shaken by a powerful tremble and poured out waves of fire globes. the day after, the lipari islands (i.e., also known as aeolian islands) and nearby the sea reached such a high boiling point that rocks were burnt and broken up. ships, axes, and wax were carbonized and melted, dead fishes were burnt on the sea surface, and several men, except those who were able to escape, were stifled and their inner organs burnt by breathing». strabo, a greek geographer, and julius obsequens, a latin historian, had reported the same event in documents dated back to the 1st century bc (geographia) and to the 4th century (de prodigiis), respectively. the events reported by the above-cited authors refer to the explosive eruption of mount etna started in june 126 bc and to the contemporaneous emissions of gas occurred in the offshore area between vulcano and panarea. 4. statistical analysis of the eruption catalogue this section analyses the historical frequency of simultaneous eruptions at two or three volcanoes among etna, stromboli, and vulcano. the analysed catalogue is the one compiled by simkin and siebert (1994). we preferred to use the printed version of this catalogue (simkin and siebert, 1994) rather than the online version available at www.volcano.si.edu, because this latter is a non-static document updated in subsequent years (siebert and simkin, 2002), where, however, reasons for amendments are not explained, at least for what concerns the studied volcanoes. the problem of possible errors contained in the historical catalogue is addressed below in the discussion section. as eruptions are often ongoing sequences of various and heterogeneous phenomena (see the preceding section), the problem studied here is simply whether or not eruptions at etna, stromboli, and vulcano overlapped more often than expected by chance. this means that we do not consider eruption onset, whose exact dates are often lacking of poorly constrained (simkin and siebert, 1994). instead, we analyse eruption durations, that is whether an eruption is in progress at some point in a specific time interval. to do so, simultaneous months and years including eruptions at two or three of the studied volcanoes are considered (tables iii and iv). the analysis does not attempt to distinguish between eruptive styles or locations within a given volcanic system. this leaves the eruptive triggers poorly constrained, but makes the statistics possible for the historical data. a null hypothesis can be constructed by using the observed record (tables iii and iv) to calculate the expected probability of concurrent eruptions if the volcanoes do not interact. for instance, if the observed historical probability of etna erupting in a given month or year (pe) is the number of months or years including etna’s eruptions divided by the total number of months or years included in the studied period, and the observed historical probability of stromboli (ps) is computed in the same manner, then the expected historical probability (or exmiscellanea 9-03-2009 14:42 pagina 660 661 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? pected probability) of concurrent eruptions at etna and stromboli pe-s is simply the product pe ps. in this section, the null hypothesis that eruptions at each of the volcanoes are statistically independent is evaluated by calculating the expected probability of concurrent eruptions (e.g., pe ps) at two or three volcanoes among etna, stromboli, and vulcano and by comparing it to the observed probability of concurrent eruptions. before evaluating the probabilities, the catalogue is limited by type of data and by time to provide uniform completeness over the study interval. the catalogue is expected to be complete for the largest, most easily observable eruptions, but progressively more incomplete at the smaller sizes. volcanic eruptions generally follow a power law distribution in sizes with many more small eruptions than large ones as quantified by vei. significant depressions at small vei relative to the trend at large vei suggest that there are missing eruptions in the catalogue. figure 2 suggests that the volcanic catalogue used for this region (simkin and siebert, 1994) is only complete for about vei ≥ 2. for this reason, only eruptions with vei ≥ 2 are used for the analysis. to find the temporal intervals significant for the statistical analysis, the temporal distribution of data (i.e., eruptions) for the studied volcanoes is analysed. the catalogue is significant, by number of data, since about the 18th century (fig. 3). about 1700 or younger times are therefore considered as the lower temporal boundary for the following statistical analysis. the limited catalogue is used to estimate the probability of eruption per month and per year of each individual volcano as above explained. results are plotted in probability versus time diagrams (fig. 4) to check whether the statistical analysis is sensitive to the time window. in each diagram, observed and expected probabilities of concurrent eruptions at two volcanoes between etna, stromboli, and vulcano are compared. etna-vulcano and stromboli-vulcano pairs of volcanoes generally erupt simultaneously more frequently than would be expected based on the individual rates (figs. 4b and 4c). this relationship is particularly evident for the stromboli-vulcano pair (fig. 4c). in contrast, the simultaneous eruptions of etna and stromboli are often less frequent than would be expected based on the individual rates (fig. 4a). the expected and observed probabilities of occurrence of simultaneous eruptions at the three studied volcanoes are also considered. within the 1700-1890 period, the expected probability of a triple eruption is 0.015% by using months as a time period, and 0.400% using years. within the same period (i.e., 1700-1890), the observed probability of a triple eruption is � ���� ��� �� � � � � *���������/$����0��������/�-*��. ! � � � � �� � %� � �� $ �� � � � �1 �* � � 1� 2�33�� 4 1��� 2� � 4 1�� 2�33�� 4 1�� 2�33�� 4 1�� 2�3 � 4 fig. 2. relationship between the number of eruptions and the «magnitude» of the eruption expressed by the volcanic explosivity index (vei) for etna, stromboli, and vulcano. data (tables iii and iv) are from simkin and siebert (1994). miscellanea 9-03-2009 14:42 pagina 661 662 a. billi and r. funiciello 0.044% by using months as time period, and 1.047% by using years (tables iii and iv). it follows that, in both cases, the observed probability is almost three times the expected probability. it is eventually analysed whether concurrent eruptions often occur in a particular order. during the 1700-1993 period, the order of eruption is often vulcano, etna, and then stromboli (tables ii and iii). if only two of the three erupt, the order is usually preserved. 5. statistical analysis of the earthquake catalogue to infer possible chronological correlations between the seismic energies released in the etnean and aeolian areas, the related monthly totals of seismic moment (tm0) are computed and compared. the problem studied in this section is simply whether or not the greatest tm0-values for the aeolian and etnean areas overlapped more often than expected by chance. to do so, the magnitude of earthquakes (i.e., only those less than 30 km deep) occurred in the aeolian and etnean areas during the january 1983-december 2007 period (i.e., 300 months) is extracted from the ingv catalogue of earthquakes available on-line at www.ingv.it (bollettino sismico italiano, istituto nazionale di geofisica e vulcanologia, rome, italy). date and geographic location of earthquakes (i.e., aeolian and etnean areas) are included in the catalogue. the � ��� ��� ���� ���� ���� ���� ���� �� ������� �� � � ��� � �� � ���� -�. � �� �� ���� ���� ���� ���� �� ������� �� � � ��� � �� � ��������� -�. � �� �� ��� ��� ���� ���� ���� ���� �� ������� �� � � ��� � �� � *������ -�. fig. 3a-c. relationships between the cumulative number of data (i.e., months including eruptions with vei ≥ 2) and time for (a) etna, (b) stromboli, and (c) vulcano. data (table iii) are from simkin and siebert (1994). table ii. time order of erupting volcanoes in the occasion of duple and triple concurrent eruptions between 1700 and 1993. data are from simkin and siebert (1994). type of concurrent erupting no. of times in which the volcano erupted before eruptions volcanoes the others in theoccasion of concurrent eruptions etna stromboli vulcano duple etna-stromboli 23 3 / duple etna-vulcano 0 / 4 duple stromboli-vulcano / 0 2 triple etna-stromboli-vulcano 0 0 1 miscellanea 9-03-2009 14:42 pagina 662 663 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? fig. 4a-c. expected and observed probabilities of occurrence of months (diagrams to the left) and years (diagrams to the right) including concurrent eruptions at (a) etna and stromboli, (b) etna and vulcano, and (c) stromboli and vulcano, versus time. data (tables iii and iv) are from simkin and siebert (1994). a rather high correlation of activity is observed in (b) and (c). ��� ���� �� ������� � �� �� �! " " � � ��� �� �# � �$ ����� !"������ % �% ��� ���� � % �! " " � � ��� �� �# � �$ ����� !"�������� �� ������� �% �� ������� �� ������� � �� �� �� �� �! " " � � ��� �� �# � �$ ����� !"������ ��� ���� �� ������� �&% �&� �&% &� &% �&� �! " " � � ��� �� �# � ����2���������-�. �$ ����� !"������ ��� ���� �� ������� � �� �� �! " " � � ��� �� �# � ����2*������-�. �$ ����� !"������ ��� ���� � � �! " " � � ��� �� �# � ���������2*������-�. �$ ����� !"������ � � �� ������� miscellanea 9-03-2009 14:42 pagina 663 664 a. billi and r. funiciello magnitude of earthquakes is converted into the seismic moment by applying the equation m = 2/3 log m0 – 10.7, where m is the magnitude and m0 is the seismic moment (hanks and kanamori, 1979). tm0-values are then computed (table v). by following the same probabilistic approach used above for concurrent eruptions, the expected and observed probabilities that the greatest tm0 of the aeolian and etnean areas are contemporary are computed and compared (fig. 5). figure 5 shows that, except for the two months with the greatest tm0-values (i.e., for no. of months ≥ 2 in fig. 5), the observed probability is always significantly greater than the expected probability. from the eruption catalogues of simkin and siebert (1994) and siebert and simkin (2002), it is inferred that, both for the aeolian and etnean areas, the greatest tm0-values are, in some cases, contemporary with or near to significant eruptions (e.g., october 1986, july 2001, october, 2002, and december, 2006 at etna; november, 1994 and july. 2001 at stromboli), whereas in some other cases there is no correlation with eruptions (e.g., december, 1997 and january, 1998 for etna; october, 1986, july-august, 1995, and april, 2002 for stromboli). 6. discussion the statistical analysis of the eruption catalogue shows that, during 1700-1993, concurrent eruptions at the studied volcanoes occurred, at least in some cases, at a frequency higher than expected based solely on random probability (fig. 4). this result, in particular, applies to the stromboli-vulcano and etna-vulcano pairs. it follows that concurrent eruptions at these volcanoes can be plausibly interpreted as non-accidental. the considered temporal interval is, however, limited and the statistical analyses may therefore be poorly representative of the real rate of concurrent eruptions. moreover, statistical results may reflect reporting biases and the analysed catalogues may contain errors. these issues make our results preliminary and our hypotheses very speculative at least in some instances. on the other hand, statistical analyses do not consider eruptions with vei < 2 and a series of volcanic phenomena such as degassing, unrest, or hydrothermalism, which are poorly reported in the catalogues but can be the manifestation of linked activities at etna and aeolian volcanoes (e.g., cigolini et al., 2007). it follows that the incidence of concurrent volcanic phenomena at the studied volcanoes may be greater than that obtained by the statistical analyses presented in this paper. the recent literature on eruption triggering suggests that the cause of concurrent eruptions may be a reciprocal stimulus or an external common one. among the possible external causes, earthquakes are considered the most common and viable ones (e.g., linde and sacks, 1998; manga and brodsky, 2006). earthquakes may trigger volcanic eruptions by dynamic stress (i.e., the shaking of rocks at the passage of seismic waves; e.g., walter and amelung, 2007). in addition to dynamic stress, fault slip induces a static stress change (i.e., the fault slip induces a permanent deformation in the nearby crust), which can be at the origin of volcanic eruptions (e.g., bautista et al., 1996; nostro et al., 1998). in the first case (i.e., dynamic stress), the earthquake and the volcano can even be very far (e.g., manga and brodsky, 2006). in contrast, the effect of the fault-related static stress change decays rapidly with the distance and, therefore, the fault has to be near the volcano to efficiently excite it. in addition to earthquakes and fault slip, the stress changes induced by the annual sea-level variations (mcnutt and beavan, 1987), by the earth and ocean daily tides (glasby and kasahara, 2001), the glacial loading (tryggvasson, 1973), by the seasonal atmospheric variations (ohtake and nakahara, 1999), and by the regional tectonics (mulargia et al., 1991; mcnutt, 1999; madonia et al., 2008) have been hypothesized as potential external triggers of volcanic eruptions. alternatively or complementarily, eruptions can reciprocally trigger other events. either the static stress generated by the movement of magma (miklius and cervelli, 2003; gudmundsson and andrew, 2007) or the shaking generated by explosive eruptions and associatmiscellanea 9-03-2009 14:42 pagina 664 665 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? ed earthquakes (kanamori and givens, 1982) can trigger adjacent volcanoes. eventually, concurrent eruptions from adjacent volcanoes can be triggered by a simultaneous increase of magma supply from the mantle (miklius and cervelli, 2003). the presented statistical and anecdotal evidence of concurrent activities at etna, stromboli, and vulcano does not allow an unequivocal identification of a plausible triggering mechanism. it cannot even be discounted the idea that the triggering mechanism may have changed from time to time or may have been a combination of the above-quoted triggering mechanisms. some simple inferences can, however, be drawn from the presented evidence. the sequential order of eruptions (table ii) suggests that eruption-eruption reciprocal triggering may have been important. as vulcano was often the most explosive of the volcanoes (simkin and siebert, 1994; tables iii and iv), it likely generated the strongest seismic waves and hence usually began the sequence (table ii). the vicinity of vulcano to both etna and stromboli (fig. 1) may explain why its relationships with these volcanoes are seemingly causal (figs. 4b and 4c), whereas concurrent activities at etna and stromboli, which are as far as about 130 km, seem accidental or less connected (fig. 4a). the presence of an active fault zone between vulcano and etna (i.e., the tindari fault, fig. 1) may have enhanced the directivity of seismic waves between these volcanoes, thus promoting the dynamic stress transfer. of � # � ( � # � ( $ �� � � � �� �� �� � �5 . ��'��%������� !"������ �!" "� ��� �$ ����� �!" "� ��� fig. 5. expected and observed probabilities of concurrence of months with the greatest monthly totals of seismic moment (tm0) for the aeolian and etnean areas (table v). for a number of months greater than 45, the divergence between the expected and observed probabilities increases. miscellanea 9-03-2009 14:42 pagina 665 666 a. billi and r. funiciello course, whether or not a given dynamic input successfully triggers an eruption depends on the state of the volcano, particularly its magma chamber. accordingly, it is not expected to observe perfect correlations or a deterministic sequence of eruptions. a reciprocal triggering similar to that proposed for iceland volcanoes (i.e., by direct mechanical interactions among volcanoes about 30 km distant; gudmundsson and andrew, 2007) seems less likely for the studied volcanoes, whose reciprocal distance is as small as 50 km (fig. 1). however, as shown by the reported anecdotal instances, the concurrent volcanic phenomena can be very heterogeneous and can involve other volcanoes in addition to etna, stromboli, and vulcano. for instance, increased degassing from panarea, which is only about 22 km from stromboli and 30 km from vulcano, may have been the consequence of direct mechanical interactions with the adjacent active volcanoes (e.g., gudmundsson and andrew, 2007). concerning external triggers, it is historically demonstrated that the studied volcanoes did not respond immediately or shortly to dynamic stresses generated by large nearby earthquakes (table i). moreover, there is no evidence – anecdotal or otherwise – that either local or remote large earthquakes triggered eruptions in the region (see the neic catalogue available on-line at neic.usgs.gov/neis/epic/ and the ingv catalogue available on-line at www.ingv.it); however, also the hypothesis of a stimulus by external earthquakes cannot be fully discounted. for instance, the eruptive cycle of 2002-2003 involving eruptions at etna and stromboli and degassing phenomena at panarea started about one month after the 6 september 2002, m 5.6, palermo (fig. 1) earthquake. this sequence of seismic and volcanic phenomena has been interpreted as causal by cigolini et al. (2007). alternatively, the concurrent eruptions at the southern italian volcanoes may have been the effect of the regional tectonic compression (fig. 1b; goes et al., 2004; neri et al., 2005). such a stress may have been periodically relieved by a large earthquake on one segment of the regional fault network (boschi et al., 1997; neri et al., 2006) or, more often, by swarms of eruptions and moderate-to-small magnitude earthquakes where the triggering threshold is low (i.e., in the active volcanic districts) (e.g., brodsky et al., 2000). the statistical analysis of earthquakes instrumentally-recorded between 1983 and 2007 (fig. 5) supports this hypothesis although, of course, coupled seismicity in the etnean and aeolian areas may alternatively be explained as the consequence of coupled volcanism triggered by some other cause such as a simultaneous increase of magma supply to etna and the aeolian volcanoes. a link between the regional tectonics and the volcanic activity of etna was previously proposed by mulargia et al. (1991). moreover, feuillet et al. (2006) demonstrated that the largest nearby earthquakes occurred after years-long-periods of volcanic activity of etna, this evidence possibly indicating a period of tectonic load that first triggered the volcanic activity and, eventually, a large earthquake. 7. conclusions analyses of eruption and earthquake catalogues show that causal links between concurrent eruptions at two or three volcanoes among etna, stromboli, and vulcano are plausible, particularly in the stromboli-vulcano and etnavulcano cases. these links are also supported by recent and historical anecdotal instances. limits of statistical results, however, compel further research to better constrain the temporal pattern of such phenomena and to identify their causes. we look forward to our predictions of a moderate incidence of concurrent eruptions being confirmed or refuted with the passage of time and with more sophisticated analyses of new events (e.g., jaquet and carniel, 2006). acknowledgements f. barberi, m. bebbington, e. brodsky, a. gudmundsson, c. jaupart, o. jaquet, d. peacock, and s. tavani are thanked for useful comments and suggestions. l. da riva helped with latin translation from the historiarum adversus paganos by paulus orosius. miscellanea 9-03-2009 14:42 pagina 666 667 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. eruption (with vei ≥ 2) occurrences by month between 0 and 1993 ad at etna, stromboli, and vulcano. vei = volcanic explosivity index; 0 = no occurrence; 1 = occurrence. data are from simkin and siebert (1994). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 252 2 1 3 0 0 1329 6 1 2 0 0 1329 7 1 3 0 0 1329 8 1 3 0 0 1381 8 1 2 0 0 1408 11 1 3 0 0 1444 2 0 0 1 1536 3 1 3 0 0 1536 4 1 3 0 0 1537 3 1 2 0 0 1537 4 1 2 0 0 1537 5 1 2 0 0 1537 6 1 2 0 0 1537 7 1 2 0 0 1541 7 1 2 0 0 1566 11 1 2 0 0 1603 7 1 2 0 0 1603 8 1 2 0 0 1603 9 1 2 0 0 1603 10 1 2 0 0 1603 11 1 2 0 0 1603 12 1 2 0 0 1604 1 1 2 0 0 1604 2 1 2 0 0 1604 3 1 2 0 0 1604 4 1 2 0 0 1604 5 1 2 0 0 1604 6 1 2 0 0 1604 7 1 2 0 0 1604 8 1 2 0 0 1604 9 1 2 0 0 1604 10 1 2 0 0 1604 11 1 2 0 0 1604 12 1 2 0 0 1605 1 1 2 0 0 1605 2 1 2 0 0 1605 3 1 2 0 0 1605 4 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 667 668 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1605 5 1 2 0 0 1605 6 1 2 0 0 1605 7 1 2 0 0 1605 8 1 2 0 0 1605 9 1 2 0 0 1605 10 1 2 0 0 1605 11 1 2 0 0 1605 12 1 2 0 0 1606 1 1 2 0 0 1606 2 1 2 0 0 1606 3 1 2 0 0 1606 4 1 2 0 0 1606 5 1 2 0 0 1606 6 1 2 0 0 1606 7 1 2 0 0 1606 8 1 2 0 0 1606 9 1 2 0 0 1606 10 1 2 0 0 1606 11 1 2 0 0 1606 12 1 2 0 0 1607 1 1 2 0 0 1607 2 1 2 0 0 1607 3 1 2 0 0 1607 4 1 2 0 0 1607 5 1 2 0 0 1607 6 1 2 0 0 1607 7 1 2 0 0 1607 8 1 2 0 0 1607 9 1 2 0 0 1607 10 1 2 0 0 1607 11 1 2 0 0 1607 12 1 2 0 0 1608 1 1 2 0 0 1608 2 1 2 0 0 1608 3 1 2 0 0 1608 4 1 2 0 0 1608 5 1 2 0 0 1608 6 1 2 0 0 1608 7 1 2 0 0 1608 8 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 668 669 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1608 9 1 2 0 0 1608 10 1 2 0 0 1608 11 1 2 0 0 1608 12 1 2 0 0 1609 1 1 2 0 0 1609 2 1 2 0 0 1609 3 1 2 0 0 1609 4 1 2 0 0 1609 5 1 2 0 0 1609 6 1 2 0 0 1609 7 1 2 0 0 1609 8 1 2 0 0 1609 9 1 2 0 0 1609 10 1 2 0 0 1609 11 1 2 0 0 1609 12 1 2 0 0 1610 1 1 2 0 0 1610 2 1 2 0 0 1610 3 1 2 0 0 1610 4 1 2 0 0 1610 5 1 2 0 0 1610 6 1 2 0 0 1610 7 1 2 0 0 1610 8 1 2 0 0 1614 7 1 2 0 0 1614 8 1 2 0 0 1614 9 1 2 0 0 1614 10 1 2 0 0 1614 11 1 2 0 0 1614 12 1 2 0 0 1615 1 1 2 0 0 1615 2 1 2 0 0 1615 3 1 2 0 0 1615 4 1 2 0 0 1615 5 1 2 0 0 1615 6 1 2 0 0 1615 7 1 2 0 0 1615 8 1 2 0 0 1615 9 1 2 0 0 1615 10 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 669 670 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1615 11 1 2 0 0 1615 12 1 2 0 0 1616 1 1 2 0 0 1616 2 1 2 0 0 1616 3 1 2 0 0 1616 4 1 2 0 0 1616 5 1 2 0 0 1616 6 1 2 0 0 1616 7 1 2 0 0 1616 8 1 2 0 0 1616 9 1 2 0 0 1616 10 1 2 0 0 1616 11 1 2 0 0 1616 12 1 2 0 0 1617 1 1 2 0 0 1617 2 1 2 0 0 1617 3 1 2 0 0 1617 4 1 2 0 0 1617 5 1 2 0 0 1617 6 1 2 0 0 1617 7 1 2 0 0 1617 8 1 2 0 0 1617 9 1 2 0 0 1617 10 1 2 0 0 1617 11 1 2 0 0 1617 12 1 2 0 0 1618 1 1 2 0 0 1618 2 1 2 0 0 1618 3 1 2 0 0 1618 4 1 2 0 0 1618 5 1 2 0 0 1618 6 1 2 0 0 1618 7 1 2 0 0 1618 8 1 2 0 0 1618 9 1 2 0 0 1618 10 1 2 0 0 1618 11 1 2 0 0 1618 12 1 2 0 0 1619 1 1 2 0 0 1619 2 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 670 671 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1619 3 1 2 0 0 1619 4 1 2 0 0 1619 5 1 2 0 0 1619 6 1 2 0 0 1619 7 1 2 0 0 1619 8 1 2 0 0 1619 9 1 2 0 0 1619 10 1 2 0 0 1619 11 1 2 0 0 1619 12 1 2 0 0 1620 1 1 2 0 0 1620 2 1 2 0 0 1620 3 1 2 0 0 1620 4 1 2 0 0 1620 5 1 2 0 0 1620 6 1 2 0 0 1620 7 1 2 0 0 1620 8 1 2 0 0 1620 9 1 2 0 0 1620 10 1 2 0 0 1620 11 1 2 0 0 1620 12 1 2 0 0 1621 1 1 2 0 0 1621 2 1 2 0 0 1621 3 1 2 0 0 1621 4 1 2 0 0 1621 5 1 2 0 0 1621 6 1 2 0 0 1621 7 1 2 0 0 1621 8 1 2 0 0 1621 9 1 2 0 0 1621 10 1 2 0 0 1621 11 1 2 0 0 1621 12 1 2 0 0 1622 1 1 2 0 0 1622 2 1 2 0 0 1622 3 1 2 0 0 1622 4 1 2 0 0 1622 5 1 2 0 0 1622 6 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 671 672 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1622 7 1 2 0 0 1622 8 1 2 0 0 1622 9 1 2 0 0 1622 10 1 2 0 0 1622 11 1 2 0 0 1622 12 1 2 0 0 1623 1 1 2 0 0 1623 2 1 2 0 0 1623 3 1 2 0 0 1623 4 1 2 0 0 1623 5 1 2 0 0 1623 6 1 2 0 0 1623 7 1 2 0 0 1623 8 1 2 0 0 1623 9 1 2 0 0 1623 10 1 2 0 0 1623 11 1 2 0 0 1623 12 1 2 0 0 1624 1 1 2 0 0 1626 3 0 0 1 1626 4 0 0 1 1646 11 1 2 0 0 1646 12 1 2 0 0 1647 1 1 2 0 0 1647 2 1 2 0 0 1647 3 1 2 0 0 1647 4 1 2 0 0 1647 5 1 2 0 0 1647 6 1 2 0 0 1647 7 1 2 0 0 1647 8 1 2 0 0 1647 9 1 2 0 0 1647 10 1 2 0 0 1647 11 1 2 0 0 1669 3 1 3 0 0 1669 4 1 3 0 0 1669 5 1 3 0 0 1669 6 1 3 0 0 1669 7 1 3 0 0 1682 9 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 672 673 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1693 12 1 3 0 0 1694 1 1 3 0 0 1694 2 1 3 0 0 1694 3 1 3 0 0 1694 4 1 3 0 0 1694 5 1 3 0 0 1694 6 1 3 0 0 1694 7 1 3 0 0 1694 8 1 3 0 0 1694 9 1 3 0 0 1694 10 1 3 0 0 1694 11 1 3 0 0 1723 11 1 2 0 0 1723 12 1 2 0 0 1724 1 1 2 0 0 1724 2 1 2 0 0 1724 3 1 2 0 0 1724 4 1 2 0 0 1724 5 1 2 0 0 1731 12 0 0 1 1732 1 0 0 1 1732 2 0 0 1 1732 3 0 0 1 1732 4 0 0 1 1732 5 0 0 1 1732 6 0 0 1 1732 7 0 0 1 1732 8 0 0 1 1732 9 0 0 1 1732 10 0 0 1 1732 11 0 0 1 1732 12 1 2 0 1 1733 1 1 2 0 1 1733 2 0 0 1 1733 3 0 0 1 1733 4 0 0 1 1733 5 0 0 1 1733 6 0 0 1 1733 7 0 0 1 1733 8 0 0 1 miscellanea 9-03-2009 14:42 pagina 673 674 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1733 9 0 0 1 1733 10 0 0 1 1733 11 0 0 1 1733 12 0 0 1 1734 1 0 0 1 1734 2 0 0 1 1734 3 0 0 1 1734 4 0 0 1 1734 5 0 0 1 1734 6 0 0 1 1734 7 0 0 1 1734 8 0 0 1 1734 9 0 0 1 1734 10 0 0 1 1734 11 0 0 1 1734 12 0 0 1 1735 1 0 0 1 1735 2 0 0 1 1735 3 0 0 1 1735 4 0 0 1 1735 5 0 0 1 1735 6 0 0 1 1735 7 0 0 1 1735 8 0 0 1 1735 9 0 0 1 1735 10 1 2 0 1 1735 11 1 2 0 1 1735 12 1 2 0 1 1736 1 1 2 0 1 1736 2 1 2 0 1 1736 3 1 2 0 1 1736 4 1 2 0 1 1736 5 1 2 0 1 1736 6 1 2 0 1 1736 7 1 2 0 1 1736 8 1 2 0 1 1736 9 1 2 0 1 1736 10 0 0 1 1736 11 0 0 1 1736 12 0 0 1 miscellanea 9-03-2009 14:42 pagina 674 675 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1737 1 0 0 1 1737 2 0 0 1 1737 3 0 0 1 1737 4 0 0 1 1737 5 0 0 1 1737 6 0 0 1 1737 7 0 0 1 1737 8 0 0 1 1737 9 0 0 1 1737 10 0 0 1 1737 11 0 0 1 1737 12 0 0 1 1738 1 0 0 1 1738 2 0 0 1 1738 3 0 0 1 1738 4 0 0 1 1738 5 0 0 1 1738 6 0 0 1 1738 7 0 0 1 1738 8 0 0 1 1738 9 0 0 1 1738 10 0 0 1 1738 11 0 0 1 1738 12 0 0 1 1739 1 0 0 1 1744 7 1 2 0 0 1744 8 1 2 0 0 1744 9 1 2 0 0 1744 10 1 2 0 0 1744 11 1 2 0 0 1744 12 1 2 0 0 1745 1 1 2 0 0 1745 2 1 2 0 0 1745 3 1 2 0 0 1745 4 1 2 0 0 1745 5 1 2 0 0 1745 6 1 2 0 0 1745 7 1 2 0 0 1745 8 1 2 0 0 1745 9 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 675 676 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1747 9 1 2 0 0 1747 10 1 2 0 0 1747 11 1 2 0 0 1747 12 1 2 0 0 1748 1 1 2 0 0 1748 2 1 2 0 0 1748 3 1 2 0 0 1748 4 1 2 0 0 1748 5 1 2 0 0 1748 6 1 2 0 0 1748 7 1 2 0 0 1748 8 1 2 0 0 1748 9 1 2 0 0 1748 10 1 2 0 0 1748 11 1 2 0 0 1748 12 1 2 0 0 1749 1 1 2 0 0 1749 2 1 2 0 0 1749 3 1 2 0 0 1752 12 1 2 0 0 1753 1 1 2 0 0 1753 2 1 2 0 0 1753 3 1 2 0 0 1753 4 1 2 0 0 1753 5 1 2 0 0 1753 6 1 2 0 0 1753 7 1 2 0 0 1753 8 1 2 0 0 1753 9 1 2 0 0 1753 10 1 2 0 0 1753 11 1 2 0 0 1753 12 1 2 0 0 1754 1 1 2 0 0 1754 2 1 2 0 0 1754 3 1 2 0 0 1754 4 1 2 0 0 1754 5 1 2 0 0 1754 6 1 2 0 0 1754 7 1 2 0 0 1754 8 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 676 677 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1754 9 1 2 0 0 1754 10 1 2 0 0 1754 11 1 2 0 0 1754 12 1 2 0 0 1755 1 1 2 0 0 1755 2 1 2 0 0 1755 3 1 2 0 0 1755 4 1 2 0 0 1755 5 1 2 0 0 1755 6 1 2 0 0 1755 7 1 2 0 0 1755 8 1 2 0 0 1755 9 1 2 0 0 1755 10 1 2 0 0 1755 11 1 2 0 0 1755 12 1 2 0 0 1756 1 1 2 0 0 1756 2 1 2 0 0 1756 3 1 2 0 0 1756 4 1 2 0 0 1756 5 1 2 0 0 1756 6 1 2 0 0 1756 7 1 2 0 0 1756 8 1 2 0 0 1756 9 1 2 0 0 1756 10 1 2 0 0 1756 11 1 2 0 0 1756 12 1 2 0 0 1757 1 1 2 0 0 1757 2 1 2 0 0 1757 3 1 2 0 0 1757 4 1 2 0 0 1757 5 1 2 0 0 1757 6 1 2 0 0 1757 7 1 2 0 0 1757 8 1 2 0 0 1757 9 1 2 0 0 1757 10 1 2 0 0 1757 11 1 2 0 0 1757 12 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 677 678 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1758 1 1 2 0 0 1758 11 1 2 0 0 1758 12 1 2 0 0 1759 1 1 2 0 0 1759 2 1 2 0 0 1763 2 1 2 0 0 1763 3 1 2 0 0 1763 6 1 3 0 0 1763 7 1 3 0 0 1763 8 1 3 0 0 1763 9 1 3 0 0 1766 4 1 2 0 0 1766 5 1 2 0 0 1766 6 1 2 0 0 1766 7 1 2 0 0 1766 8 1 2 0 0 1766 9 1 2 0 0 1766 10 1 2 0 0 1766 11 1 2 0 0 1771 2 0 0 1 1771 3 0 0 1 1771 4 0 0 1 1771 5 0 0 1 1778 3 0 1 2 0 1780 4 1 2 0 0 1780 5 1 2 0 0 1780 6 1 2 0 0 1781 3 1 2 0 0 1781 4 1 2 0 0 1781 5 1 2 0 0 1787 6 1 4 0 0 1787 7 1 4 0 0 1787 8 1 4 0 0 1791 2 1 2 0 0 1791 3 1 2 0 0 1791 4 1 2 0 0 1791 5 1 2 0 0 1791 6 1 2 0 0 1791 7 1 2 0 0 1791 8 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 678 679 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1791 9 1 2 0 0 1792 3 1 2 0 0 1792 4 1 2 0 0 1792 5 1 3 0 0 1792 6 1 3 0 0 1792 7 1 3 0 0 1792 8 1 3 0 0 1792 9 1 3 0 0 1792 10 1 3 0 0 1792 11 1 3 0 0 1792 12 1 3 0 0 1793 1 1 3 0 0 1793 2 1 3 0 0 1793 3 1 3 0 0 1793 4 1 3 0 0 1793 5 1 3 0 0 1797 12 1 2 0 0 1798 1 1 2 0 0 1798 2 1 2 0 0 1798 3 1 2 0 0 1798 4 1 2 0 0 1798 5 1 2 0 0 1798 6 1 2 0 0 1798 7 1 2 0 0 1798 8 1 2 0 0 1798 9 1 2 0 0 1798 10 1 2 0 0 1798 11 1 2 0 0 1798 12 1 2 0 0 1799 1 1 2 0 0 1799 2 1 2 0 0 1799 3 1 2 0 0 1799 4 1 2 0 0 1799 5 1 2 0 0 1799 6 1 2 0 0 1799 7 1 2 0 0 1799 8 1 2 0 0 1799 9 1 2 0 0 1799 10 1 2 0 0 1799 11 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 679 680 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1799 12 1 2 0 0 1800 1 1 2 0 0 1800 2 1 2 0 0 1800 3 1 2 0 0 1800 4 1 2 0 0 1800 5 1 2 0 0 1800 6 1 2 0 0 1800 7 1 2 0 0 1800 8 1 2 0 0 1800 9 1 2 0 0 1800 10 1 2 0 0 1800 11 1 2 0 0 1800 12 1 2 0 0 1801 1 1 2 0 0 1802 11 1 2 0 0 1803 12 1 2 0 0 1804 1 1 2 0 0 1804 2 1 2 0 0 1804 3 1 2 0 0 1804 4 1 2 0 0 1804 5 1 2 0 0 1804 6 1 2 0 0 1804 7 1 2 0 0 1804 8 1 2 0 0 1804 9 1 2 0 0 1804 10 1 2 0 0 1804 11 1 2 0 0 1804 12 1 2 0 0 1805 1 1 2 0 0 1805 2 1 2 0 0 1805 3 1 2 0 0 1805 4 1 2 0 0 1805 5 1 2 0 0 1805 6 1 2 0 0 1805 7 1 2 0 0 1805 8 1 2 0 0 1805 9 1 2 0 0 1805 10 1 2 0 0 1805 11 1 2 0 0 1805 12 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 680 681 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1806 1 1 2 0 0 1806 2 1 2 0 0 1806 3 1 2 0 0 1806 4 1 2 0 0 1806 5 1 2 0 0 1806 6 1 2 0 0 1806 7 1 2 0 0 1806 8 1 2 0 0 1806 9 1 2 0 0 1806 10 1 2 0 0 1806 11 1 2 0 0 1806 12 1 2 0 0 1807 1 1 2 0 0 1807 2 1 2 0 0 1807 3 1 2 0 0 1807 4 1 2 0 0 1807 5 1 2 0 0 1807 6 1 2 0 0 1807 7 1 2 0 0 1807 8 1 2 0 0 1807 9 1 2 0 0 1807 10 1 2 0 0 1807 11 1 2 0 0 1807 12 1 2 0 0 1808 1 1 2 0 0 1808 2 1 2 0 0 1808 3 1 2 0 0 1808 4 1 2 0 0 1808 5 1 2 0 0 1808 6 1 2 0 0 1808 7 1 2 0 0 1808 8 1 2 0 0 1808 9 1 2 0 0 1808 10 1 2 0 0 1808 11 1 2 0 0 1808 12 1 2 0 0 1809 1 1 2 0 0 1809 2 1 2 0 0 1809 3 1 2 0 0 1809 4 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 681 682 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1809 5 1 2 0 0 1809 6 1 2 0 0 1809 7 1 2 0 0 1809 8 1 2 0 0 1809 9 1 2 0 0 1809 10 1 2 0 0 1809 11 1 2 0 0 1811 10 1 2 0 0 1811 11 1 2 0 0 1811 12 1 2 0 0 1812 1 1 2 0 0 1812 2 1 2 0 0 1812 3 1 2 0 0 1812 4 1 2 0 0 1812 5 1 2 0 0 1819 5 1 3 0 0 1819 6 1 3 0 0 1819 7 1 3 0 0 1819 8 1 3 0 0 1822 10 0 1 3 0 1828 8 1 2 0 0 1828 9 1 2 0 0 1828 10 1 2 0 0 1828 11 1 2 0 0 1828 12 1 2 0 0 1829 1 1 2 0 0 1829 2 1 2 0 0 1829 3 1 2 0 0 1829 4 1 2 0 0 1829 5 1 2 0 0 1829 6 1 2 0 0 1829 7 1 2 0 0 1829 8 1 2 0 0 1829 9 1 2 0 0 1829 10 1 2 0 0 1829 11 1 2 0 0 1829 12 1 2 0 0 1830 1 1 2 0 0 1830 2 1 2 0 0 1830 3 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 682 683 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1830 4 1 2 0 0 1830 5 1 2 0 0 1830 6 1 2 0 0 1830 7 1 2 0 0 1830 8 1 2 0 0 1830 9 1 2 0 0 1830 10 1 2 0 0 1830 11 1 2 0 0 1830 12 1 2 0 0 1831 1 1 2 0 0 1831 2 1 2 0 0 1831 3 1 2 0 0 1831 4 1 2 0 0 1831 5 1 2 0 0 1831 6 1 2 0 0 1831 7 1 2 0 0 1831 8 1 2 0 0 1831 9 1 2 0 0 1831 10 1 2 0 0 1831 11 1 2 0 0 1831 12 1 2 0 0 1832 1 1 2 0 0 1832 2 1 2 0 0 1832 3 1 2 0 0 1832 4 1 2 0 0 1832 5 1 2 0 0 1832 6 1 2 0 0 1832 7 1 2 0 0 1832 8 1 2 0 0 1832 9 1 2 0 0 1832 10 1 2 0 0 1832 11 1 2 0 0 1833 3 1 2 0 0 1838 7 1 2 0 0 1838 8 1 2 0 0 1838 9 1 2 0 0 1838 10 1 2 0 0 1838 11 1 2 0 0 1838 12 1 2 0 0 1839 1 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 683 684 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1839 2 1 2 0 0 1842 11 1 2 0 0 1842 12 1 2 0 0 1843 11 1 2 0 0 1852 8 1 2 0 0 1852 9 1 2 0 0 1852 10 1 2 0 0 1852 11 1 2 0 0 1852 12 1 2 0 0 1853 1 1 2 0 0 1853 2 1 2 0 0 1853 3 1 2 0 0 1853 4 1 2 0 0 1853 5 1 2 0 0 1855 10 0 1 2 0 1856 7 0 1 2 0 1863 5 1 2 0 0 1863 6 1 2 0 0 1863 7 1 2 0 0 1864 8 1 2 0 0 1864 9 1 2 0 0 1865 1 1 2 1 2 0 1865 2 1 2 1 2 0 1865 3 1 2 0 0 1865 4 1 2 0 0 1865 5 1 2 0 0 1865 6 1 2 0 0 1868 11 1 3 0 0 1868 12 1 3 0 0 1873 9 0 0 1 1873 10 0 0 1 1873 11 0 0 1 1873 12 0 0 1 1874 1 0 0 1 1874 2 0 0 1 1874 3 0 0 1 1874 4 0 0 1 1874 5 1 2 0 1 1874 6 1 2 1 2 1 1874 7 1 2 0 1 miscellanea 9-03-2009 14:42 pagina 684 685 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1874 8 1 2 0 1 1874 9 0 0 1 1874 10 0 0 1 1874 11 0 0 1 1874 12 1 2 0 1 1875 1 1 2 0 1 1875 2 1 2 0 1 1875 3 1 2 0 1 1875 4 1 2 0 1 1875 5 1 2 0 1 1875 6 1 2 0 1 1875 7 1 2 0 1 1875 8 1 2 0 1 1875 9 1 2 0 1 1875 10 1 2 0 1 1875 11 1 2 0 1 1875 12 1 2 0 1 1876 1 1 2 0 1 1876 2 1 2 0 1 1876 3 1 2 0 1 1876 4 1 2 0 1 1876 5 1 2 0 1 1876 6 1 2 0 1 1876 7 1 2 0 1 1876 8 1 2 0 1 1876 9 1 2 0 1 1876 10 1 2 0 1 1876 11 1 2 0 1 1876 12 1 2 0 1 1877 1 1 2 0 1 1877 2 1 2 0 1 1877 3 1 2 0 1 1877 4 1 2 0 1 1877 5 1 2 0 1 1877 6 1 2 0 1 1877 7 1 2 0 1 1877 8 1 2 0 1 1877 9 1 2 0 1 1877 10 1 2 0 1 1877 11 1 2 0 1 miscellanea 9-03-2009 14:42 pagina 685 686 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1877 12 1 2 0 1 1878 1 1 2 0 1 1878 2 1 2 0 1 1878 3 1 2 0 1 1878 4 1 2 0 1 1878 5 1 2 0 1 1878 6 1 2 0 1 1878 7 1 2 0 1 1878 8 1 2 0 1 1878 9 1 2 0 1 1878 10 1 2 0 1 1878 11 1 2 0 1 1878 12 1 2 0 1 1879 1 1 2 0 1 1879 2 1 2 1 2 0 1879 3 1 2 0 0 1879 4 1 2 0 0 1879 5 1 3 0 0 1879 6 1 3 1 2 0 1879 7 1 2 0 0 1879 8 1 2 0 0 1879 9 1 2 0 0 1879 10 1 2 0 0 1879 11 1 2 0 0 1879 12 1 2 0 0 1880 1 1 2 0 0 1880 2 1 2 0 0 1880 3 1 2 0 0 1880 4 1 2 0 0 1880 5 1 2 0 0 1880 6 1 2 0 0 1880 7 1 2 0 0 1880 8 1 2 0 0 1880 9 1 2 0 0 1880 10 1 2 0 0 1880 11 1 2 0 0 1880 12 1 2 0 0 1881 1 1 2 0 0 1881 2 1 2 0 0 1881 3 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 686 687 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1881 4 1 2 0 0 1881 5 1 2 0 0 1881 6 1 2 0 0 1881 7 1 2 0 0 1881 8 1 2 0 0 1881 9 1 2 0 0 1881 10 1 2 1 2 0 1881 11 1 2 0 0 1881 12 1 2 0 0 1882 1 1 2 0 0 1882 2 1 2 0 0 1882 3 1 2 0 0 1882 4 1 2 0 0 1882 5 1 2 0 0 1882 6 1 2 0 0 1882 7 1 2 0 0 1882 8 1 2 0 0 1882 9 1 2 0 0 1882 10 1 2 0 0 1882 11 1 2 1 3 0 1882 12 1 2 0 0 1883 1 1 2 0 0 1883 2 1 2 0 0 1883 3 1 2 0 0 1885 3 0 1 3 0 1886 1 0 0 1 1886 5 1 3 0 0 1886 6 1 3 0 0 1888 8 0 0 1 1888 9 0 0 1 1888 10 0 1 2 1 1888 11 0 1 2 1 1888 12 0 1 2 1 1889 1 0 1 2 1 1889 2 0 1 2 1 1889 3 0 1 2 1 1889 4 0 1 2 1 1889 5 0 1 2 1 1889 6 0 1 2 1 1889 7 0 0 1 miscellanea 9-03-2009 14:42 pagina 687 688 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1889 8 0 0 1 1889 9 0 0 1 1889 10 0 0 1 1889 11 0 0 1 1889 12 0 0 1 1890 1 0 0 1 1890 2 0 0 1 1890 3 0 0 1 1891 6 0 1 3 0 1891 7 0 1 3 0 1891 8 0 1 3 0 1892 7 1 2 0 0 1892 8 1 2 0 0 1892 9 1 2 0 0 1892 10 1 2 0 0 1892 11 1 2 1 2 0 1892 12 1 2 0 0 1893 1 0 1 2 0 1893 4 1 2 0 0 1893 5 1 2 0 0 1893 6 1 2 0 0 1893 7 1 2 0 0 1893 8 1 2 0 0 1893 9 1 2 0 0 1893 10 1 2 0 0 1893 11 1 2 1 2 0 1893 12 1 2 0 0 1894 1 1 2 0 0 1894 2 1 2 0 0 1894 3 1 2 0 0 1894 4 1 2 0 0 1894 5 1 2 0 0 1894 6 1 2 0 0 1894 7 1 2 0 0 1894 8 1 2 0 0 1894 9 1 2 0 0 1894 10 1 2 0 0 1894 11 1 2 0 0 1894 12 1 2 0 0 1895 1 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 688 689 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1895 2 1 2 0 0 1895 3 1 2 1 2 0 1895 4 1 2 0 0 1895 5 1 2 0 0 1895 6 1 2 0 0 1895 7 1 2 0 0 1895 8 1 2 0 0 1895 9 1 2 0 0 1895 10 1 2 0 0 1895 11 1 2 0 0 1895 12 1 2 0 0 1896 1 1 2 0 0 1896 2 1 2 0 0 1896 3 1 2 0 0 1896 4 1 2 0 0 1896 5 1 2 0 0 1896 6 1 2 0 0 1896 7 1 2 1 2 0 1896 8 1 2 0 0 1896 9 1 2 0 0 1896 10 1 2 0 0 1896 11 1 2 0 0 1896 12 1 2 0 0 1897 1 1 2 0 0 1897 2 1 2 0 0 1897 3 1 2 0 0 1897 4 1 2 0 0 1897 5 1 2 0 0 1897 6 1 2 0 0 1897 7 1 2 1 2 0 1897 8 1 2 0 0 1897 9 1 2 0 0 1897 10 1 2 0 0 1897 11 1 2 0 0 1897 12 1 2 0 0 1898 1 1 2 0 0 1898 2 1 2 0 0 1898 3 1 2 0 0 1898 4 1 2 0 0 1898 5 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 689 690 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1898 6 1 2 0 0 1898 8 0 1 2 0 1899 7 1 3 0 0 1899 8 1 3 0 0 1900 8 0 1 2 0 1900 10 0 1 2 0 1903 1 0 1 2 0 1903 2 0 1 2 0 1903 3 0 1 2 0 1903 4 0 1 2 0 1903 5 0 1 2 0 1903 6 0 1 2 0 1903 11 0 1 2 0 1905 4 0 1 2 0 1906 4 0 1 2 0 1906 7 0 1 2 0 1907 1 0 1 3 0 1907 2 0 1 3 0 1907 3 0 1 3 0 1907 4 0 1 3 0 1907 5 0 1 3 0 1908 4 1 2 0 0 1910 3 1 2 0 0 1910 4 1 2 0 0 1910 12 1 2 0 0 1911 1 1 2 0 0 1911 2 1 2 0 0 1912 7 0 1 2 0 1912 8 1 3 1 2 0 1913 11 1 2 0 0 1913 12 1 2 0 0 1914 1 1 2 0 0 1914 2 1 2 0 0 1914 3 1 2 0 0 1914 4 1 2 0 0 1914 5 1 2 0 0 1914 6 1 2 0 0 1914 7 1 2 0 0 1914 8 1 2 0 0 1914 9 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 690 691 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1914 10 1 2 0 0 1914 11 1 2 0 0 1914 12 1 2 0 0 1915 1 1 2 0 0 1915 2 1 2 0 0 1915 3 1 2 0 0 1915 4 1 2 0 0 1915 5 1 2 0 0 1915 6 1 2 1 2 0 1915 7 1 2 1 2 0 1915 8 1 2 1 2 0 1915 9 1 2 1 2 0 1915 10 1 2 1 2 0 1915 11 1 2 1 2 0 1915 12 1 2 0 0 1916 1 1 2 0 0 1916 2 1 2 0 0 1916 3 1 2 0 0 1916 4 1 2 0 0 1916 5 1 2 0 0 1916 6 1 2 1 2 0 1916 7 1 2 1 2 0 1916 8 1 2 0 0 1916 9 1 2 0 0 1916 10 1 2 0 0 1916 11 1 2 0 0 1916 12 1 2 0 0 1917 1 1 2 0 0 1917 2 1 2 0 0 1917 3 1 2 0 0 1917 6 1 2 0 0 1917 7 1 2 0 0 1919 3 1 2 0 0 1919 4 1 2 0 0 1919 5 1 2 1 3 0 1919 6 1 2 0 0 1919 7 1 2 0 0 1919 8 1 2 0 0 1919 9 1 2 0 0 1919 10 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 691 692 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1919 11 1 2 0 0 1919 12 1 2 0 0 1920 1 1 2 0 0 1920 2 1 2 0 0 1920 3 1 2 0 0 1920 4 1 2 0 0 1920 5 1 2 0 0 1920 6 1 2 0 0 1920 7 1 2 0 0 1920 8 1 2 0 0 1920 9 1 2 0 0 1920 10 1 2 0 0 1920 11 1 2 0 0 1920 12 1 2 0 0 1921 1 1 2 0 0 1921 2 1 2 0 0 1921 3 1 2 0 0 1921 4 1 2 0 0 1921 5 1 2 0 0 1921 6 1 2 1 2 0 1921 7 1 2 0 0 1921 8 1 2 0 0 1921 9 1 2 0 0 1921 10 1 2 0 0 1921 11 1 2 0 0 1921 12 1 2 0 0 1922 1 1 2 0 0 1922 2 1 2 0 0 1922 3 1 2 0 0 1922 4 1 2 0 0 1922 5 1 2 0 0 1922 6 1 2 0 0 1922 7 1 2 0 0 1922 8 1 2 0 0 1922 9 1 2 0 0 1922 10 1 2 0 0 1922 11 1 2 0 0 1922 12 1 2 0 0 1923 1 1 2 0 0 1923 2 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 692 693 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1923 3 1 2 0 0 1923 4 1 2 0 0 1923 5 1 2 0 0 1923 6 1 2 0 0 1923 7 1 2 0 0 1930 2 0 1 2 0 1930 9 0 1 3 0 1930 10 0 1 3 0 1930 11 0 1 3 0 1930 12 0 1 3 0 1931 7 1 2 0 0 1931 8 1 2 0 0 1931 9 1 2 0 0 1931 10 1 2 0 0 1931 11 1 2 0 0 1931 12 1 2 0 0 1932 1 1 2 0 0 1932 2 1 2 0 0 1932 3 1 2 0 0 1932 4 1 2 0 0 1932 5 1 2 0 0 1932 6 1 2 0 0 1932 7 1 2 0 0 1932 8 1 2 0 0 1932 9 1 2 0 0 1932 10 1 2 0 0 1932 11 1 2 0 0 1932 12 1 2 0 0 1933 1 1 2 0 0 1933 2 1 2 0 0 1933 3 1 2 0 0 1933 4 1 2 0 0 1933 5 1 2 0 0 1933 6 1 2 0 0 1933 7 1 2 0 0 1933 8 1 2 0 0 1933 9 1 2 0 0 1934 2 0 1 2 0 1934 8 0 1 2 0 1935 7 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 693 694 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1935 8 1 2 0 0 1935 9 1 2 0 0 1935 10 1 2 0 0 1935 11 1 2 0 0 1935 12 1 2 0 0 1936 1 1 2 1 2 0 1936 2 1 2 0 0 1936 3 1 2 0 0 1936 4 1 2 0 0 1936 5 1 2 0 0 1936 6 1 2 0 0 1936 7 1 2 0 0 1936 8 1 2 0 0 1936 9 1 2 0 0 1936 10 1 2 0 0 1936 11 1 2 0 0 1936 12 1 2 0 0 1937 1 1 2 1 2 0 1937 2 1 2 0 0 1937 3 1 2 0 0 1937 4 1 2 0 0 1937 5 1 2 0 0 1937 6 1 2 0 0 1937 7 1 2 0 0 1937 8 1 2 0 0 1937 9 1 2 0 0 1937 10 1 2 0 0 1937 11 1 2 1 2 0 1937 12 1 2 0 0 1938 1 1 2 0 0 1938 2 1 2 0 0 1938 3 1 2 0 0 1938 4 1 2 0 0 1938 5 1 2 0 0 1938 6 1 2 0 0 1938 7 1 2 0 0 1938 8 1 2 0 0 1938 9 1 2 0 0 1938 10 1 2 0 0 1938 11 1 2 1 2 0 miscellanea 9-03-2009 14:42 pagina 694 695 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1938 12 1 2 1 2 0 1939 1 1 2 1 2 0 1939 2 1 2 1 2 0 1939 3 1 2 1 2 0 1939 4 1 2 1 2 0 1939 5 1 2 1 2 0 1939 6 1 2 1 2 0 1939 7 1 2 0 0 1939 8 1 2 0 0 1939 9 1 2 0 0 1939 10 1 2 0 0 1939 11 1 2 0 0 1939 12 1 2 0 0 1940 3 1 3 0 0 1940 4 1 3 0 0 1940 5 1 3 0 0 1940 6 1 3 0 0 1940 7 1 3 0 0 1940 8 1 3 0 0 1940 9 1 3 0 0 1940 10 1 3 0 0 1940 11 1 3 0 0 1940 12 1 3 0 0 1941 1 1 3 0 0 1941 2 1 3 0 0 1941 3 1 3 0 0 1941 4 1 3 0 0 1941 5 1 3 0 0 1941 6 1 3 0 0 1941 7 1 3 0 0 1941 8 1 3 1 2 0 1941 9 1 3 0 0 1941 10 1 3 0 0 1941 11 1 3 0 0 1941 12 1 3 0 0 1942 1 1 3 0 0 1942 2 1 3 0 0 1942 3 1 3 0 0 1942 4 1 3 0 0 1942 5 1 3 0 0 miscellanea 9-03-2009 14:42 pagina 695 696 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1942 6 1 3 0 0 1942 7 1 2 0 0 1943 12 0 1 2 0 1944 1 0 1 2 0 1944 2 0 1 2 0 1944 8 0 1 2 0 1944 9 0 1 2 0 1944 10 0 1 2 0 1947 1 1 2 0 0 1947 2 1 2 0 0 1949 6 0 1 2 0 1949 12 1 2 0 0 1950 1 1 2 0 0 1950 10 0 1 2 0 1950 11 1 2 0 0 1950 12 1 2 0 0 1951 1 1 2 0 0 1951 2 1 2 0 0 1951 3 1 2 0 0 1951 4 1 2 1 2 0 1951 5 1 2 0 0 1951 6 1 2 0 0 1951 7 1 2 0 0 1951 8 1 2 0 0 1951 9 1 2 0 0 1951 10 1 2 0 0 1951 11 1 2 0 0 1951 12 1 2 0 0 1952 6 0 1 2 0 1953 7 1 2 0 0 1954 2 0 1 2 0 1954 3 0 1 2 0 1954 12 0 1 2 0 1955 1 0 1 2 0 1955 2 0 1 2 0 1955 3 0 1 2 0 1955 4 1 2 1 2 0 1955 5 1 2 1 2 0 1955 6 1 2 0 0 1955 7 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 696 697 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1955 8 1 2 0 0 1955 9 1 2 0 0 1955 10 1 2 0 0 1955 11 1 2 0 0 1955 12 1 2 0 0 1956 1 1 2 0 0 1956 2 1 2 0 0 1956 3 1 2 0 0 1956 4 1 2 0 0 1957 2 1 2 0 0 1957 3 1 2 0 0 1957 4 1 2 0 0 1957 5 1 2 0 0 1957 8 1 2 0 0 1957 9 1 2 0 0 1957 10 1 2 0 0 1957 11 1 2 0 0 1957 12 1 2 0 0 1958 1 1 2 0 0 1958 2 1 2 0 0 1958 3 1 2 0 0 1958 4 1 2 0 0 1958 5 1 2 0 0 1959 5 0 1 2 0 1959 10 1 3 0 0 1959 11 1 3 0 0 1959 12 1 3 0 0 1960 1 1 3 0 0 1960 2 1 3 0 0 1960 3 1 3 0 0 1960 4 1 3 0 0 1960 5 1 3 0 0 1960 6 1 3 0 0 1960 7 1 3 0 0 1960 8 1 3 0 0 1960 9 1 3 0 0 1960 10 1 3 0 0 1960 11 1 3 0 0 1960 12 1 3 0 0 1961 1 1 3 0 0 miscellanea 9-03-2009 14:42 pagina 697 698 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1961 2 1 3 0 0 1961 3 1 3 0 0 1961 4 1 3 0 0 1961 5 1 3 0 0 1961 6 1 3 0 0 1961 7 1 3 0 0 1961 8 1 3 0 0 1961 9 1 3 0 0 1961 10 1 3 0 0 1961 11 1 3 0 0 1961 12 1 3 0 0 1962 1 1 3 0 0 1962 2 1 3 0 0 1962 3 1 3 0 0 1962 4 1 3 0 0 1962 5 1 3 0 0 1962 6 1 3 0 0 1962 7 1 3 0 0 1962 8 1 3 0 0 1962 9 1 3 0 0 1962 10 1 3 0 0 1962 11 1 3 0 0 1962 12 1 3 0 0 1963 1 1 3 0 0 1963 2 1 3 0 0 1963 3 1 3 0 0 1963 4 1 3 0 0 1963 5 1 3 0 0 1963 6 1 3 0 0 1963 7 1 3 0 0 1963 8 1 3 0 0 1963 9 1 3 0 0 1963 10 1 3 0 0 1963 11 1 3 0 0 1963 12 1 3 0 0 1964 1 1 3 0 0 1964 2 1 3 0 0 1964 3 1 3 0 0 1964 4 1 3 0 0 1964 5 1 3 0 0 miscellanea 9-03-2009 14:42 pagina 698 699 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1964 6 1 3 0 0 1964 7 1 3 0 0 1964 8 1 3 0 0 1964 9 1 3 0 0 1964 10 1 3 0 0 1964 11 1 3 0 0 1964 12 1 3 0 0 1966 1 1 2 0 0 1966 2 1 2 0 0 1966 3 1 2 0 0 1966 4 1 2 1 2 0 1966 5 1 2 0 0 1966 6 1 2 0 0 1966 7 1 2 0 0 1966 8 1 2 0 0 1966 9 1 2 0 0 1966 10 1 2 0 0 1966 11 1 2 0 0 1966 12 1 2 0 0 1967 1 1 2 0 0 1967 2 1 2 0 0 1967 3 1 2 0 0 1967 4 1 2 0 0 1967 5 1 2 0 0 1967 6 1 2 0 0 1967 7 1 2 0 0 1967 8 1 2 0 0 1967 9 1 2 0 0 1967 10 1 2 0 0 1967 11 1 2 0 0 1967 12 1 2 0 0 1968 1 1 2 0 0 1968 2 1 2 0 0 1968 3 1 2 0 0 1968 4 1 2 0 0 1968 5 1 2 0 0 1968 6 1 2 0 0 1968 7 1 2 0 0 1968 8 1 2 0 0 1968 9 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 699 700 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1968 10 1 2 0 0 1968 11 1 2 0 0 1968 12 1 2 0 0 1969 1 1 2 0 0 1969 2 1 2 0 0 1969 3 1 2 0 0 1969 4 1 2 0 0 1969 5 1 2 0 0 1969 6 1 2 0 0 1969 7 1 2 0 0 1969 8 1 2 0 0 1969 9 1 2 0 0 1969 10 1 2 0 0 1969 11 1 2 0 0 1969 12 1 2 0 0 1970 1 1 2 0 0 1970 2 1 2 0 0 1970 3 1 2 0 0 1970 4 1 2 0 0 1970 5 1 2 0 0 1970 6 1 2 0 0 1970 7 1 2 0 0 1970 8 1 2 0 0 1970 9 1 2 0 0 1970 10 1 2 0 0 1970 11 1 2 0 0 1970 12 1 2 0 0 1971 1 1 2 0 0 1971 3 0 1 2 0 1971 4 1 2 1 2 0 1971 5 1 2 1 2 0 1971 6 1 2 0 0 1971 9 1 2 0 0 1971 10 1 2 0 0 1971 11 1 2 0 0 1971 12 1 2 0 0 1972 1 1 2 0 0 1972 2 1 2 0 0 1972 3 1 2 0 0 1972 4 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 700 701 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1972 5 1 2 0 0 1972 6 1 2 0 0 1972 7 1 2 0 0 1972 8 1 2 0 0 1972 9 1 2 0 0 1972 10 1 2 0 0 1972 11 1 2 0 0 1972 12 1 2 0 0 1973 1 1 2 0 0 1973 2 1 2 0 0 1973 3 1 2 0 0 1973 4 1 2 0 0 1973 5 1 2 0 0 1973 6 1 2 0 0 1973 7 1 2 0 0 1973 8 1 2 0 0 1973 9 1 2 0 0 1973 10 1 2 0 0 1973 11 1 2 0 0 1973 12 1 2 0 0 1974 1 1 2 0 0 1974 2 1 2 0 0 1974 3 1 2 0 0 1974 4 1 2 0 0 1974 5 1 2 0 0 1974 6 1 2 0 0 1974 7 1 2 0 0 1974 8 1 2 0 0 1974 9 1 2 0 0 1974 10 1 2 0 0 1974 11 1 2 0 0 1974 12 1 2 0 0 1975 1 1 2 0 0 1975 2 1 2 0 0 1975 3 1 2 0 0 1975 4 1 2 0 0 1975 5 1 2 0 0 1975 6 1 2 0 0 1975 7 1 2 0 0 1975 8 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 701 702 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1975 9 1 2 0 0 1975 10 1 2 0 0 1975 11 1 2 0 0 1975 12 1 2 0 0 1976 1 1 2 0 0 1976 2 1 2 0 0 1976 3 1 2 0 0 1976 4 1 2 0 0 1976 5 1 2 0 0 1976 6 1 2 0 0 1976 7 1 2 0 0 1976 8 1 2 0 0 1976 9 1 2 0 0 1976 10 1 2 0 0 1976 11 1 2 0 0 1976 12 1 2 0 0 1977 1 1 2 0 0 1977 2 1 2 0 0 1977 3 1 2 0 0 1977 4 1 2 0 0 1977 5 1 2 0 0 1977 6 1 2 0 0 1977 7 1 2 0 0 1977 8 1 2 0 0 1977 9 1 2 0 0 1977 10 1 2 0 0 1977 11 1 2 0 0 1977 12 1 2 0 0 1978 1 1 2 0 0 1978 2 1 2 0 0 1978 3 1 2 0 0 1978 4 1 2 0 0 1978 5 1 2 0 0 1978 6 1 2 0 0 1978 7 1 2 0 0 1978 8 1 2 0 0 1978 9 1 2 0 0 1978 10 1 2 0 0 1978 11 1 2 0 0 1978 12 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 702 703 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1979 1 1 2 0 0 1979 2 1 2 0 0 1979 3 1 2 0 0 1979 7 1 2 0 0 1979 8 1 2 0 0 1979 9 1 2 0 0 1979 10 1 2 0 0 1979 11 1 2 0 0 1979 12 1 2 0 0 1980 1 1 2 0 0 1980 2 1 2 0 0 1980 3 1 2 0 0 1980 4 1 2 0 0 1980 5 1 2 0 0 1980 6 1 2 0 0 1980 7 1 3 0 0 1980 8 1 3 0 0 1980 9 1 3 0 0 1980 10 1 2 0 0 1980 11 1 2 0 0 1980 12 1 2 0 0 1981 1 1 2 0 0 1981 2 1 2 0 0 1981 3 1 2 0 0 1981 4 1 2 0 0 1981 5 1 2 0 0 1981 6 1 2 0 0 1981 7 1 2 0 0 1981 8 1 2 0 0 1981 9 1 2 0 0 1981 10 1 2 0 0 1981 11 1 2 0 0 1981 12 1 2 0 0 1982 1 1 2 0 0 1982 2 1 2 0 0 1982 3 1 2 0 0 1982 4 1 2 0 0 1982 5 1 2 0 0 1982 6 1 2 0 0 1982 7 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 703 704 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1982 8 1 2 0 0 1982 9 1 2 0 0 1982 10 1 2 0 0 1982 11 1 2 0 0 1982 12 1 2 0 0 1983 1 1 2 0 0 1983 2 1 2 0 0 1983 3 1 2 0 0 1983 4 1 2 0 0 1983 5 1 2 0 0 1983 6 1 2 0 0 1983 7 1 2 0 0 1983 8 1 2 0 0 1983 9 1 2 0 0 1983 10 1 2 0 0 1983 11 1 2 0 0 1983 12 1 2 0 0 1984 1 1 2 0 0 1984 2 1 2 0 0 1984 3 1 2 0 0 1984 4 1 2 0 0 1984 5 1 2 0 0 1984 6 1 2 0 0 1984 7 1 2 0 0 1984 8 1 2 0 0 1984 9 1 2 0 0 1984 10 1 2 0 0 1984 11 1 2 0 0 1984 12 1 2 0 0 1985 1 1 2 0 0 1985 2 1 2 0 0 1985 3 1 2 0 0 1985 4 1 2 0 0 1985 5 1 2 0 0 1985 6 1 2 0 0 1985 7 1 2 0 0 1985 8 1 2 0 0 1985 9 1 2 0 0 1985 10 1 2 0 0 1985 11 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 704 705 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1985 12 1 2 1 2 0 1986 1 1 2 1 2 0 1986 2 1 2 1 2 0 1986 3 1 2 1 2 0 1986 4 1 2 1 2 0 1986 5 1 2 0 0 1986 6 1 2 0 0 1986 7 1 2 0 0 1986 8 1 2 0 0 1986 9 1 2 0 0 1986 10 1 2 0 0 1986 11 1 2 0 0 1986 12 1 2 0 0 1987 1 1 2 0 0 1987 2 1 2 0 0 1987 3 1 2 0 0 1987 4 1 2 0 0 1987 5 1 2 0 0 1987 6 1 2 0 0 1987 7 1 2 0 0 1987 8 1 2 0 0 1987 9 1 2 0 0 1987 10 1 2 0 0 1987 11 1 2 0 0 1987 12 1 2 0 0 1988 1 1 2 0 0 1988 2 1 2 0 0 1988 3 1 2 0 0 1988 4 1 2 0 0 1988 5 1 2 0 0 1988 6 1 2 0 0 1988 7 1 2 0 0 1988 8 1 2 0 0 1988 9 1 2 0 0 1988 10 1 2 0 0 1988 11 1 2 0 0 1988 12 1 2 0 0 1989 1 1 2 0 0 1989 2 1 2 0 0 1989 3 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 705 706 a. billi and r. funiciello table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1989 4 1 2 0 0 1989 5 1 2 0 0 1989 6 1 2 0 0 1989 7 1 2 0 0 1989 8 1 2 0 0 1989 9 1 2 0 0 1989 10 1 2 0 0 1989 11 1 2 0 0 1989 12 1 2 0 0 1990 1 1 2 0 0 1990 2 1 2 0 0 1990 3 1 2 0 0 1990 4 1 2 0 0 1990 5 1 2 0 0 1990 6 1 2 0 0 1990 7 1 2 0 0 1990 8 1 2 0 0 1990 9 1 2 0 0 1990 10 1 2 0 0 1990 11 1 2 0 0 1990 12 1 2 0 0 1991 1 1 2 0 0 1991 2 1 2 0 0 1991 3 1 2 0 0 1991 4 1 2 0 0 1991 5 1 2 0 0 1991 6 1 2 0 0 1991 7 1 2 0 0 1991 8 1 2 0 0 1991 9 1 2 0 0 1991 10 1 2 0 0 1991 11 1 2 0 0 1991 12 1 2 0 0 1992 1 1 2 0 0 1992 2 1 2 0 0 1992 3 1 2 0 0 1992 4 1 2 0 0 1992 5 1 2 0 0 1992 6 1 2 0 0 1992 7 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 706 707 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iii. (continued). year month etna vei stromboli vei vulcano eruptions eruptions eruptions 1992 8 1 2 0 0 1992 9 1 2 0 0 1992 10 1 2 0 0 1992 11 1 2 0 0 1992 12 1 2 0 0 1993 1 1 2 0 0 1993 2 1 2 0 0 1993 3 1 2 0 0 table iv. eruption (with vei ≥ 2) occurrences by year between 0 and 1993 ad at etna, stromboli, and vulcano. vei = volcanic explosivity index; 0 = no occurrence; 1 = occurrence. data are from simkin and siebert (1994). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 252 1 3 0 0 526 0 0 1 3 925 0 0 1 3 1157 1 2 0 0 1160 1 2 0 0 1164 1 2 0 0 1194 1 2 0 0 1222 1 2 0 0 1250 1 2 0 0 1329 1 3 0 0 1333 1 2 0 0 1350 1 2 0 0 1381 1 2 0 0 1408 1 3 0 0 1444 1 2 0 1 3 1536 1 3 0 0 1537 1 2 0 0 1541 1 2 0 0 1550 0 0 1 3 1554 1 2 0 0 1558 0 1 2 0 1566 1 2 0 0 1595 1 3 0 0 miscellanea 9-03-2009 14:42 pagina 707 708 a. billi and r. funiciello table iv. (continued). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 1603 1 2 0 0 1607 1 2 0 0 1610 1 2 0 0 1614 1 2 0 0 1615 1 2 0 0 1616 1 2 0 0 1617 1 2 0 0 1618 1 2 0 0 1619 1 2 0 0 1620 1 2 0 0 1621 1 2 0 0 1622 1 2 0 0 1623 1 2 0 0 1624 1 2 0 0 1626 0 0 1 3 1646 1 2 0 0 1647 1 2 0 0 1669 1 3 0 0 1682 1 2 0 0 1693 1 3 0 0 1694 1 3 0 0 1723 1 2 0 0 1724 1 2 0 0 1727 0 0 1 3 1731 0 0 1 3 1732 1 2 0 1 3 1733 1 2 0 1 3 1734 0 0 1 3 1735 1 2 0 1 3 1736 1 2 0 1 3 1737 0 0 1 3 1738 0 0 1 3 1739 0 0 1 3 1744 1 2 0 0 1745 1 2 0 0 1747 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 708 709 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iv. (continued). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 1748 1 2 0 0 1749 1 2 0 0 1752 1 2 0 0 1753 1 2 0 0 1754 1 2 0 0 1755 1 2 0 0 1756 1 2 0 0 1757 1 2 0 0 1758 1 2 0 0 1759 1 2 0 0 1763 1 3 0 0 1766 1 2 0 0 1768 0 1 2 0 1770 0 1 2 0 1771 0 0 1 3 1778 0 1 2 0 1780 1 2 0 1 2 1781 1 2 0 0 1786 0 0 1 3 1787 1 4 0 0 1791 1 2 0 0 1792 1 3 0 0 1793 1 3 0 0 1797 1 2 0 0 1798 1 2 0 0 1799 1 2 0 0 1800 1 2 0 0 1801 1 2 0 0 1802 1 2 0 0 1803 1 2 0 0 1804 1 2 0 0 1805 1 2 0 0 1806 1 2 0 0 1807 1 2 0 0 1808 1 2 0 0 1809 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 709 710 a. billi and r. funiciello table iv. (continued). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 1811 1 2 0 0 1812 1 2 0 0 1819 1 3 0 0 1822 0 1 3 1 2 1823 0 0 1 2 1828 1 2 0 0 1829 1 2 0 0 1830 1 2 0 0 1831 1 2 0 0 1832 1 2 0 0 1833 1 2 1 2 0 1838 1 2 0 0 1839 1 2 0 0 1842 1 2 0 0 1843 1 2 0 0 1850 0 1 2 0 1852 1 2 0 0 1853 1 2 0 0 1855 0 1 2 0 1856 0 1 2 0 1863 1 2 0 0 1864 1 2 0 0 1865 1 2 1 2 0 1868 1 3 0 0 1873 0 0 1 3 1874 1 2 1 2 1 3 1875 0 0 1 3 1876 0 0 1 3 1877 0 0 1 3 1878 1 2 0 1 3 1879 1 3 1 2 1 3 1880 1 2 0 0 1881 1 2 1 2 0 1882 1 2 1 3 0 1883 1 2 0 0 1885 0 1 2 0 miscellanea 9-03-2009 14:42 pagina 710 711 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iv. (continued). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 1886 1 3 0 1 3 1888 0 1 2 1 3 1889 0 1 2 1 3 1890 0 0 1 3 1891 0 1 3 0 1892 1 2 1 2 0 1893 1 2 1 2 0 1894 1 2 0 0 1895 1 2 1 2 0 1896 1 2 1 2 0 1897 1 2 1 2 0 1898 1 2 1 2 0 1899 1 3 0 0 1900 0 1 2 0 1903 0 1 2 0 1905 0 1 2 0 1906 0 1 2 0 1907 0 1 3 0 1908 1 2 0 0 1910 1 2 0 0 1911 1 2 0 0 1912 1 3 1 2 0 1913 1 2 0 0 1914 1 2 0 0 1915 1 2 1 2 0 1916 1 2 1 2 0 1917 1 2 0 0 1919 1 2 1 3 0 1920 1 2 0 0 1921 1 2 1 2 0 1922 1 2 0 0 1923 1 2 0 0 1930 0 1 3 0 1931 1 2 0 0 1932 1 2 0 0 1933 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 711 712 a. billi and r. funiciello table iv. (continued). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 1934 0 1 2 0 1935 1 2 0 0 1936 1 2 1 2 0 1937 1 2 1 2 0 1938 1 2 1 2 0 1939 1 2 1 2 0 1940 1 3 0 0 1941 1 3 1 2 0 1942 1 3 0 0 1943 0 1 2 0 1944 0 1 2 0 1947 1 2 0 0 1949 1 2 1 2 0 1950 1 2 1 2 0 1951 1 2 1 2 0 1952 0 1 2 0 1953 1 2 0 0 1954 0 1 2 0 1955 1 2 1 2 0 1956 1 2 0 0 1957 1 2 0 0 1958 1 2 0 0 1959 1 3 1 2 0 1960 1 3 0 0 1961 1 3 0 0 1962 1 3 0 0 1963 1 3 0 0 1964 1 3 0 0 1966 1 2 1 2 0 1967 1 2 0 0 1968 1 2 0 0 1969 1 2 0 0 1970 1 2 0 0 1971 1 2 1 2 0 1972 1 2 0 0 1973 1 2 0 0 miscellanea 9-03-2009 14:42 pagina 712 713 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table iv. (continued). year etna vei stromboli vei vulcano vei eruptions eruptions eruptions 1974 1 2 0 0 1975 1 2 0 0 1976 1 2 0 0 1977 1 2 0 0 1978 1 2 0 0 1979 1 2 0 0 1980 1 3 0 0 1981 1 2 0 0 1982 1 2 0 0 1983 1 2 0 0 1984 1 2 0 0 1985 1 2 1 2 0 1986 1 2 1 2 0 1987 1 2 0 0 1988 1 2 0 0 1989 1 2 0 0 1990 1 2 0 0 1991 1 2 0 0 1992 1 2 0 0 1993 1 2 0 0 table v. monthly totals of seismic moment (tm0) for the etnean and aeolian areas, from january 1983 to december 2007 (i.e. 300 months). seismic moments are computed from earthquake magnitudes provided in the ingv catalogue of earthquakes available online at www.ingv.it (bollettino sismico italiano, istituto nazionale di geofisica e vulcanologia). focal depth for the considered earthquakes is less than 30 km. 0 is where no records are available. year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1983 jan. 0 0 1983 feb. 0 0 1983 mar. 8.61157e+21 0 1983 apr. 0 0 1983 may 0 0 1983 jun. 3.1335e+21 0 1983 jul. 0 0 1983 aug. 0 0 miscellanea 9-03-2009 14:42 pagina 713 714 a. billi and r. funiciello table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1983 sep. 0 2.21309e+21 1983 oct. 3.68198e+21 2.21309e+21 1983 nov. 0 0 1983 dec. 0 0 1984 jan. 0 7.53147e+20 1984 feb. 0 0 1984 mar. 0 0 1984 apr. 0 0 1984 may 0 2.78612e+20 1984 jun. 2.72893e+21 3.9355e+20 1984 jul. 1.50272e+21 0 1984 aug. 5.81437e+21 7.85236e+20 1984 sep. 1.10917e+21 1.56675e+21 1984 oct. 5.52259e+21 0 1984 nov. 3.67231e+21 5.55904e+20 1984 dec. 0 0 1985 jan. 0 0 1985 feb. 1.1714e+21 0 1985 mar. 4.33707e+21 0 1985 apr. 1.343e+21 0 1985 may 0 0 1985 jun. 3.9355e+20 0 1985 jul. 0 0 1985 aug. 5.55904e+20 0 1985 sep. 3.12608e+21 3.9355e+20 1985 oct. 2.21835e+21 0 1985 nov. 2.08488e+21 0 1985 dec. 1.68442e+22 9.24872e+20 1986 jan. 6.15005e+21 0 1986 feb. 1.97242e+20 0 1986 mar. 1.13198e+21 3.9355e+20 1986 apr. 2.78612e+20 0 1986 may 7.10694e+21 2.21309e+21 1986 jun. 3.77467e+20 1.97242e+20 1986 jul. 3.26358e+21 0 miscellanea 9-03-2009 14:42 pagina 714 715 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1986 aug. 4.75854e+20 0 1986 sep. 1.35443e+21 1.97242e+20 1986 oct. 1.96846e+22 7.94256e+21 1986 nov. 2.23793e+21 0 1986 dec. 2.48313e+19 0 1987 jan. 2.38492e+20 0 1987 feb. 4.05612e+21 4.9545e+19 1987 mar. 0 0 1987 apr. 0 0 1987 may 6.76753e+20 9.88553e+19 1987 jun. 9.88553e+19 0 1987 jul. 4.18381e+20 0 1987 aug. 1.12675e+21 2.96191e+20 1987 sep. 8.84091e+20 5.55904e+20 1987 oct. 6.99842e+19 9.88553e+19 1987 nov. 3.28157e+20 0 1987 dec. 4.9545e+19 2.67695e+20 1988 jan. 3.13687e+20 0 1988 feb. 3.50752e+19 0 1988 mar. 0 5.18173e+21 1988 apr. 1.40989e+21 2.36029e+21 1988 may 0 3.23671e+21 1988 jun. 1.27126e+21 3.84779e+21 1988 jul. 2.43216e+20 0 1988 aug. 2.70863e+21 4.7803e+21 1988 sep. 1.75792e+19 4.9545e+19 1988 oct. 6.99842e+19 1.44361e+20 1988 nov. 6.27679e+20 6.99842e+19 1988 dec. 7.96041e+20 9.88553e+19 1989 jan. 7.56663e+20 3.50752e+19 1989 feb. 3.44341e+21 5.58018e+20 1989 mar. 3.34351e+20 1.6884e+20 1989 apr. 2.18737e+20 1.05059e+20 1989 may 6.41844e+20 2.18385e+20 1989 jun. 2.37224e+21 1.39637e+20 miscellanea 9-03-2009 14:42 pagina 715 716 a. billi and r. funiciello table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1989 jul. 3.64362e+21 1.41525e+21 1989 aug. 1.28044e+22 2.44696e+20 1989 sep. 5.81645e+21 6.72162e+20 1989 oct. 7.8372e+21 2.83997e+20 1989 nov. 1.56675e+21 6.71243e+19 1989 dec. 6.99842e+19 6.99842e+19 1990 jan. 3.2323e+20 0 1990 feb. 2.3495e+20 1.19391e+21 1990 mar. 1.07113e+22 2.52683e+21 1990 apr. 5.3811e+20 3.34819e+20 1990 may 1.63593e+21 4.9545e+19 1990 jun. 2.78612e+20 0 1990 jul. 6.50577e+20 0 1990 aug. 1.43476e+21 2.48313e+19 1990 sep. 3.21509e+21 1.63863e+21 1990 oct. 1.69074e+20 4.18381e+20 1990 nov. 7.01504e+19 1.05059e+20 1990 dec. 4.36437e+20 0 1991 jan. 2.78516e+21 1.69192e+20 1991 feb. 3.80765e+20 0 1991 mar. 1.39637e+20 0 1991 apr. 1.58196e+21 7.03374e+20 1991 may 2.62578e+21 0 1991 jun. 5.05719e+20 0 1991 jul. 4.21968e+20 4.9545e+19 1991 aug. 3.28049e+20 5.99065e+19 1991 sep. 3.6788e+21 2.92271e+21 1991 oct. 8.92828e+20 0 1991 nov. 2.92217e+20 6.99842e+19 1991 dec. 7.58078e+20 0 1992 jan. 1.2995e+20 6.99842e+19 1992 feb. 6.82522e+20 6.99842e+19 1992 mar. 6.99842e+19 1.25758e+21 1992 apr. 1.75792e+19 9.48155e+19 1992 may 3.45643e+20 1.40218e+20 miscellanea 9-03-2009 14:42 pagina 716 717 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1992 jun. 1.19813e+20 1.41266e+20 1992 jul. 1.97242e+20 4.28625e+20 1992 aug. 9.88553e+19 1.39637e+20 1992 sep. 1.57216e+20 1.89513e+20 1992 oct. 0 0 1992 nov. 0 4.63534e+20 1992 dec. 0 3.9355e+20 1993 jan. 0 2.58995e+21 1993 feb. 6.99842e+19 4.9545e+19 1993 mar. 4.9545e+19 2.28308e+21 1993 apr. 1.19529e+20 1.97242e+20 1993 may 1.48635e+20 3.78461e+20 1993 jun. 4.98609e+20 0 1993 jul. 9.50481e+20 1.25316e+21 1993 aug. 3.50456e+21 5.90792e+20 1993 sep. 4.96627e+19 1.20803e+21 1993 oct. 4.9545e+19 5.92524e+20 1993 nov. 0 0 1993 dec. 2.79274e+20 0 1994 jan. 0 0 1994 feb. 1.8968e+20 8.46202e+19 1994 mar. 4.9545e+19 5.83666e+20 1994 apr. 9.88553e+19 2.48313e+19 1994 may 1.42953e+21 5.05719e+20 1994 jun. 8.47035e+19 6.99842e+19 1994 jul. 5.99065e+19 4.70778e+21 1994 aug. 6.99842e+19 0 1994 sep. 2.53066e+21 1.44361e+20 1994 oct. 3.89057e+20 2.94814e+21 1994 nov. 2.80103e+20 5.59765e+21 1994 dec. 7.81624e+20 5.92299e+20 1995 jan. 1.75792e+19 9.88553e+19 1995 feb. 6.44712e+21 2.38492e+20 1995 mar. 0 0 1995 apr. 0 3.58021e+20 miscellanea 9-03-2009 14:42 pagina 717 718 a. billi and r. funiciello table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1995 may 3.19881e+20 0 1995 jun. 9.48155e+19 0 1995 jul. 1.39637e+20 5.6121e+22 1995 aug. 8.22263e+20 1.25151e+22 1995 sep. 2.48313e+19 5.33685e+20 1995 oct. 2.38727e+20 9.51834e+20 1995 nov. 9.88553e+19 4.9545e+19 1995 dec. 7.87947e+19 0 1996 jan. 9.909e+19 4.35734e+20 1996 feb. 2.64367e+20 1.97242e+20 1996 mar. 1.23687e+20 1.39637e+20 1996 apr. 1.34014e+20 0 1996 may 0 5.7471e+20 1996 jun. 9.48155e+19 1.38779e+21 1996 jul. 6.99842e+19 8.24451e+20 1996 aug. 1.2659e+21 1.97242e+20 1996 sep. 6.27505e+20 6.25888e+20 1996 oct. 0 0 1996 nov. 5.23153e+20 0 1996 dec. 2.12613e+21 0 1997 jan. 3.36345e+21 3.9355e+20 1997 feb. 4.74944e+21 8.45102e+20 1997 mar. 4.92758e+20 1.25972e+21 1997 apr. 1.25142e+21 0 1997 may 7.16238e+20 1.77135e+21 1997 jun. 7.73346e+20 9.88553e+19 1997 jul. 7.36677e+20 1.05059e+20 1997 aug. 4.40469e+21 0 1997 sep. 2.60291e+21 2.42808e+21 1997 oct. 3.8698e+21 1.75792e+19 1997 nov. 9.91181e+20 2.3411e+21 1997 dec. 2.50586e+22 3.48596e+20 1998 jan. 2.10765e+22 0 1998 feb. 3.9807e+20 8.70253e+20 1998 mar. 4.77556e+21 0 miscellanea 9-03-2009 14:42 pagina 718 719 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 1998 apr. 2.34619e+20 9.12398e+19 1998 may 2.2054e+21 0 1998 jun. 6.07607e+20 6.79591e+20 1998 jul. 2.38492e+20 4.43095e+20 1998 aug. 1.0944e+21 1.39968e+20 1998 sep. 6.75551e+20 5.7471e+20 1998 oct. 4.30705e+20 0 1998 nov. 1.44361e+20 0 1998 dec. 2.34753e+21 0 1999 jan. 6.46722e+20 4.18249e+20 1999 feb. 2.18658e+21 6.53665e+20 1999 mar. 2.01256e+21 2.59166e+20 1999 apr. 5.63324e+20 1.97242e+20 1999 may 4.01157e+20 1.6884e+20 1999 jun. 9.30161e+20 0 1999 jul. 1.73232e+20 0 1999 aug. 3.52873e+21 3.9355e+20 1999 sep. 6.42838e+20 1.74712e+20 1999 oct. 1.55728e+21 1.23687e+20 1999 nov. 1.484e+20 0 1999 dec. 4.18381e+20 2.01055e+20 2000 jan. 1.3795e+21 0 2000 feb. 1.93858e+21 0 2000 mar. 1.71442e+20 0 2000 apr. 6.5476e+20 2.54997e+21 2000 may 2.46787e+20 0 2000 jun. 0 1.39637e+20 2000 jul. 5.34921e+20 4.28625e+20 2000 aug. 2.92692e+20 0 2000 sep. 9.89385e+20 1.39637e+20 2000 oct. 6.02056e+20 6.99842e+19 2000 nov. 1.68763e+22 0 2000 dec. 1.29617e+21 0 2001 jan. 2.26636e+21 0 2001 feb. 5.35177e+20 1.97242e+20 miscellanea 9-03-2009 14:42 pagina 719 720 a. billi and r. funiciello table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 2001 mar. 7.57161e+20 0 2001 apr. 5.69959e+21 0 2001 may 4.65916e+21 1.75792e+19 2001 jun. 3.43712e+20 4.9545e+19 2001 jul. 5.18235e+22 1.14118e+22 2001 aug. 1.68645e+21 8.69404e+20 2001 sep. 2.48313e+19 8.5522e+20 2001 oct. 5.96325e+21 5.69665e+20 2001 nov. 0 7.85236e+20 2001 dec. 6.99842e+19 0 2002 jan. 3.28157e+20 0 2002 feb. 1.74712e+20 1.39637e+20 2002 mar. 3.28819e+20 1.3422e+21 2002 apr. 2.09621e+20 9.86941e+21 2002 may 2.14345e+20 2.78612e+20 2002 jun. 1.89513e+20 8.34516e+20 2002 jul. 7.30429e+20 0 2002 aug. 1.53444e+21 1.39637e+20 2002 sep. 2.46787e+20 6.95541e+20 2002 oct. 1.07678e+23 1.19529e+20 2002 nov. 5.43328e+21 0 2002 dec. 2.25681e+21 3.50752e+19 2003 jan. 1.6884e+20 0 2003 feb. 8.95013e+21 2.78612e+20 2003 mar. 2.09621e+20 2.48313e+19 2003 apr. 0 0 2003 may 1.39968e+20 0 2003 jun. 1.97242e+20 5.75371e+20 2003 jul. 1.95985e+21 0 2003 aug. 2.78612e+20 0 2003 sep. 0 0 2003 oct. 2.18385e+20 0 2003 nov. 0 1.6884e+20 2003 dec. 1.6884e+20 0 2004 jan. 2.32317e+20 1.6884e+20 miscellanea 9-03-2009 14:42 pagina 720 721 concurrent eruptions at etna, stromboli, and vulcano: casualty or casuality? table v. (continued). year month tm0 etna (dyn cm) tm0 aeolian(dyn cm) 2004 feb. 7.65504e+20 0 2004 mar. 8.92783e+20 2.50113e+21 2004 apr. 0 3.17562e+21 2004 may 7.57776e+20 2.39508e+20 2004 jun. 2.20707e+21 6.99842e+19 2004 jul. 1.39578e+21 1.14425e+21 2004 aug. 2.38824e+20 0 2004 sep. 7.43764e+19 5.99065e+19 2004 oct. 3.50752e+19 5.07524e+20 2004 nov. 9.88553e+19 8.46202e+19 2004 dec. 6.99842e+19 0 2005 jan. 1.97242e+20 7.85236e+20 2005 feb. 1.39637e+20 0 2005 mar. 0 0 2005 apr. 4.97243e+20 6.23735e+18 2005 may 9.48155e+19 3.10687e+19 2005 jun. 4.38857e+19 1.79728e+19 2005 jul. 3.44522e+21 1.75792e+19 2005 aug. 1.42534e+21 4.24106e+19 2005 sep. 3.50752e+19 1.24451e+19 2005 oct. 7.30334e+21 1.45584e+21 2005 nov. 2.63897e+19 3.61799e+20 2005 dec. 7.227e+20 2.78612e+20 2006 jan. 9.03744e+21 2.38166e+19 2006 feb. 2.84242e+20 5.26544e+19 2006 mar. 1.74146e+21 7.01504e+19 2006 apr. 4.97334e+19 6.19877e+20 2006 may 6.12295e+21 4.58971e+21 2006 jun. 6.12458e+21 4.17313e+20 2006 jul. 2.22845e+20 1.10445e+19 2006 aug. 2.56611e+20 1.62649e+21 2006 sep. 8.24293e+19 3.0517e+20 2006 oct. 1.55791e+20 5.70032e+20 2006 nov. 7.19683e+20 3.23731e+19 2006 dec. 1.84017e+22 3.42244e+21 miscellanea 9-03-2009 14:42 pagina 721 722 a. billi and r. funiciello references acocella, v., b. behncke, m. neri and s. d’amico (2003): link between major flank slip and 2002–2003 eruption at mt. etna (italy), geophy. res. lett., 30, 2286, doi: 10.1029/2003gl018642. alparone, s., b. behncke, s. giammanco, m. neri and e. privitera (2005): paroxysmal summit activity at mt. etna (italy) monitored through continuous soil radon measurements, geophys. res. lett., 32, l16307, doi: 10.1029/2005gl023352. azzaro, r., m.s. barbano, r. rigano and s. vinciguerra (2001): time seismicity patterns affecting local and regional fault systems in the etna region: preliminary results for the period 1874-1913, j. geol. soc., lond., 158, 561-572. baratta, m. 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(received february 11, 2008; accepted june 12, 2008) miscellanea 9-03-2009 14:42 pagina 725 ag6430_cai annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6340 1 analysis of new species retrieved from mipas shaomin cai *, anu dudhia university of oxford cai@atm.ox.ac.uk abstract the michelson interferometer for passive atmospheric sounding (mipas) instrument which operated on the envisat satellite from 2002-2012 is a fourier transform spectrometer for the measurement of high-resolution gaseous emission spectra at the earth's limb. it operates in the nearto mid-infrared, where many of the main atmospheric trace gases have important emission features. the initial operational products were profiles of temperature, h2o, o3, ch4, n2o, hno3, and no2, and this list was recently extended to include n2o5, clono2, cfc-11 and cfc-12. here we present preliminary results of retrievals of the third set of species under consideration for inclusion in the operational processor: hcn, cf4, hcfc-22, cof2 and ccl4. i. introduction he michelson interferometer for passive atmospheric sounding (mipas), which operated on the envisat satellite from 2002-2012, is a fourier transform spectrometer for the measurement of emission spectra at the earth's limb. it operates in the nearto midinfrared where many of the main atmospheric trace gases have detectable emission features. the initial operational products were profiles of temperature, h2o, o3, hno3, ch4, n2o and no2, and this list was recently extended to include n2o5, clono2, cfc-11 and cfc-12. in this paper the preliminary results of a third set of species, hcn, cfc-14 (cf4), hcfc-22, cof2, ccl4, sf6, ocs, hocl and c2h6, retrieved using the oxford l2 processor, morse (multispectral optimal retrievals using sequential estimation), are presented. hcn, ocs and c2h6 are tracers of biomass burning (rinsland et al., 1998; li et al., 2000; notholt et al., 2003; logan et al., 1981). the relatively long lifetimes for hcn and c2h6 make them indicators of how transport redistributes pollutants on a global scale (rinsland et al., 2005). ccl4, cfc-14, hcfc-22 and sf6 are mainly from anthropogenic emissions. for example cfc-14 are produced primarily as a byproduct during electrolytic aluminum production and sf6 is mainly from insulating electrical equipment. sf6 and cf4 are extremely longlived species and often used as tracers for the ‘age of air’. hocl is considered to be a reservoir for active chlorine, clox, and odd hydrogen, hox, in the stratosphere (von clarmann et al., 2012). it is produced primarily by the reaction of clo and ho2 (peroxy radical) and is destroyed mainly by photo-dissociation, which returns oh and cl radicals (hickson et al., 2007). here we also examine fluorine species collectively measured by mipas. stratospheric fluorine has important effects on the atmosphere, especially fluorine in long-lived greenhouse t annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6340 2 gases. over the past few decades, the major sources of fluorine in the stratosphere are the man-made chlorofluorocarbons cfcs and hcfcs. photolysis of these compounds forms cof2. the cof2 molecule has been shown to be the second most abundant stratospheric fluorine reservoir (kaye et al., 1991). ii. zonal mean distributions the oxford l2 processor morse has been used to retrieve these new species with the microwindows shown in fig.1. here we analyze these new retrieved species to establish their zonal mean for the period of 2010 march. for each gas, we eliminated those data points that are cloud-affected and where a priori has a significant contribution, in other words, the retrieved random error greater than 50% of the volume mixing ratio (vmr). a 5σ global spike test is then applied to each species to eliminate unrealistic data. the percentage of profiles removed by the spike test is shown in table 1. the largest percentage showed in the table is 21.5% from cfc-14. reasons for these spike test failures have yet to be investigated. as the total number of profiles for each species for one month is over 32,000, there are still remaining a large number of profiles, over 25,000 profiles, to have a reliable sample basis for the further study. the remaining profiles are used construct monthly zonal mean together with a standard deviation defined as the standard deviation of profiles contribute to each grid point, which is a combination of random error and atmosphere variability. zonal plots of species ccl4, ocs, hcfc-22, sf6, c2h6 and cof2 in fig. 2 show the expected structure for tropospheric source gases. the table 1, percentage of removed profiles by 5σ spike test. species ratio (%) ccl4 1.9 cof2 12.5 hcfc-22 0.5 hcn 5.2 cfc-14 21.5 sf6 12.3 hocl 0.03 c2h6 18.7 cfc-11 1.6 cfc-12 2.6 ocs 2.0 fig. 1, microwindows for the new species. the upper panel shows the microwindows selected for these new species retrievals. the lower panel show the spectral signatures of these species. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6340 3 plot of cof2 also well represents the maximum of vmr around the middle of the stratosphere over equator. this occurs because cof2 is an intermediate product in the degradation of cfcs in the stratosphere, which results in an increase of vmr. also because cof2 is destroyed by photolysis and reaction fig. 2, zonal plots and standard deviations of the new species from mipas. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6340 4 vmr of fluorine retreived from mipas for 2010 march 0 500 1000 1500 2000 2500 vmr of fluorine (pptv) 1000.0 100.0 10.0 1.0 0.1 p re s s u re ( h p a ) f cfc-11 cfc-12 cfc-14 cfc-22 sf6 cof2 -50 0 50 latitude 1000 100 10 1 0.1 p re s s u re ( h p a ) zonal mean of expected hf from mipas (pptv) 200.00 200.00 20 0.0 0 400.00 400.00 400.00 4 0 0 .0 0 600.00 600.00 600.00 60 0.0 0 800.00 800.00 800.00 80 0. 00 1000.00 1000.00 1000.00 10 00 .00 1600.00 1600.00 1600.001600.00 2400.00 2400.00 2400.0024 00 .00 fig. 4, expected zonal plot of hf from mipas fig. 5, zonal plot of hf from ace-fts (jones et al., 2012) with o(1d) in the upper stratosphere, which results in a decrease of vmr. there are some unexpected features in the zonal plots for species cfc-14 and hcn. for example, a maximum vmr of hcn occurs near the stratopause over the equator, which seems unrealistic and requires comparison with other data sets. similar for cfc-14 there is a maximum in the upper stratosphere over equator, which is unrealistic as well. due to its complexity, hocl species need further study before the data could be used for quantitative research. iii. total fluorine the fluorine species measured by mipas are cof2, cfc-11, cfc-12, cfc-14, cfc-22 and sf6 from which the total fluorine can be computer as [f] = 2[cof2] + [cfc-11] + 2[cfc-12] + 4[cfc-14] + 2[cfc-22] + 6[sf6]. mahieu et. al. (2008) show most of the stratospheric fluorine species eventually form hydrogen fluoride (hf), which is not measured by mipas. other fluorine containing species are comparable small compared with those measured by mipas and hf. figure 3 shows the vertical profiles of all the fluorine species and total fluorine (f) measured by mipas. in the lower stratosphere and upper troposphere, the atmospheric fluorine is mainly contributed by cfc-12. cof2 contributes nearly half of the fluorine vmr at 10hpa pressure level, while cfc-14 dominates in the upper stratosphere. the total [f] decreases with altitude as they are eventually formed into hf, which is not measured by mipas. the expected hf zonal plot for 2010 march is constructed by the following, [hf] = [fmax]-[f], where [f] is the total fluorine zonal mean measured by mipas and [fmax] is the maximum value of [f]. this plot (fig. 4) is further compared with the zonal plot of hf (fig. 5) produced by jones et al., 2012. fig 5 is a threemonth combined zonal average from ace-fts data during september-october-november. fig4 and fig 5 is for brief comparison of mipas data with ace-fts data of hf zonal means. fig. 3, the vertical profiles of all the fluorine containing species and the total fluorine retrieved by mipas. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6340 5 iv. conclusions and future work plausible distributions of hcfc-22, cof2, ccl4, sf6, ocs, hocl and c2h6 retrieved by mipas, which cfc-14 and hcn have some anomalies requiring further investigation. however, further comparison with ace-fts, other mipas processor and model results are required for further justification. references e. mahieu et al., validation of ace-fts v2.2 measurements of hcl, hf, ccl3f and ccl2f2 using space balloon and ground based instrument observations, atmos. chem. phys. discuss., 8, 3431–3495, 2008 hickson, k. and l. keyser et al., temperature dependence of the ho2 + clo reaction.2. reaction kinetics using the discharge-flow resonance-fluorescence technique, j. phys.chem. a, 111, 8126–8138, 2007. jones, a. and k. a. walker et al., technical note: a trace gas climatology derived from the atmospheric chemistry experiment fourier transform spectrometer (ace-fts) data set, atmos. chem. phys., 12, 5207–5220, 2012 kaye, j. a. and a. r. douglass et al., twodimensional model calculation of fluorinecontaining reservoir species in the stratosphere, j. geophys. res. ,96, 12,865–12,881, 1991 li, q. and d. j. jacob et al., atmospheric hydrogen cyanide (hcn): biomass burning source, ocean sink?, geophys. res. lett., 27, 357– 360, 2000 logan, j. a. and m. j. prather et al., tropospheric chemistry: a global perspective, j. geophys. res., 86, 7210–7254, 1981 notholt, j. and z. kuang et al., enhanced upper topical tropospheric cos: impact on the stratospheric aerosol layer, science, 300, 307– 310, 2003 rinsland, c. p. and m. r. gunson et al., atmos/atlas 3 infra red profile measurements of trace gases in the november 1994 tropical and subtropical upper troposphere, j. quant. spectrosc. radiat. transfer, 60, 891–901., 1998 rinsland, c. p. and g. dufour et al., atmospheric chemistry experiment (ace) measurements of elevated southern hemisphere upper tropospheric co, c2h6, hcn, and c2h2 mixing ratios from biomass burning emissions and long-range transport, geophys. res. lett., 32, l20803, 2005 von clarmann, t. and b. funke et al., the mipas hocl climatology, atmos. chem. phys., 12, 1965–1977, 2012. muography with nuclear emulsions stromboli and other projects annals of geophysics, 60, 1, 2017, s0111; doi:10.4401/ag-7386 s0111 muography with nuclear emulsions stromboli and other projects valeri tioukov1,2,*, giovanni de lellis1,2, paolo strolin1,2, lucia consiglio1,2, andrey sheshukov1,2, massimo orazi3, rosario peluso4,5, cristiano bozza4,5, chiara de sio4,5, simona maria stellacci4,5, chiara sirignano6,7, nicola d’ambrosio8, seigo miyamoto9, ryuichi nishiyama9, hiroyuki k.m. tanaka9 1 università di napoli federico ii, dipartimento di fisica, naples, italy 2 istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy 3 istituto nazionale di geofisica e vulcanologia (ingv), sezione di napoli osservatorio vesuviano, naples, italy 4 università di salerno, dipartimento di fisica “e. r. caianiello”, salerno, italy 5 istituto nazionale di fisica nucleare (infn), gruppo collegato di salerno, salerno, italy 6 università di padova, dipartimento di fisica e astronomia “galileo galilei”, padova, italy 7 istituto nazionale di fisica nucleare (infn), sezione di padova, padova, italy 8 istituto nazionale di fisica nucleare (infn), laboratori nazionali del gran sasso, assergi (l’aquila), italy 9 earthquake research institute, the university of tokyo, tokyo, japan abstract the muon radiography is a novel imaging technique to probe the volcanoes interior, using the capability of high energy cosmic ray muons to penetrate large thicknesses of rock. in this way it is possible to derive a 2d density map along the muon trajectory of volcanic edifices and deduce information on the variations in the rock density distribution, like those expected from dense lava conduits, or low density magma supply paths. this method is applicable also to study geological objects as glaciers, faults, oil underground reservoirs, engineering constructions, where a density contrast is present. nuclear emulsions are well suited to be employed in this context for their excellent angular resolution; they are compact and robust detectors, able to work in harsh environments without need of power supply. on the other side, a long exposure time is required for a reasonable detector surface (~10 m2) in order to collect a sufficient statistics of muons, and a quasi-real time analysis of the emulsion data is rather difficult due to the scanning time needed by the optical microscopes. such drawback is on the way to be overcome thanks to a recent r&d program on ultra-fast scanning systems. muon radiography technique, even if limited to the summit part of the volcano edifice, represents an important tool of investigation, at higher spatial resolution, complementary to the conventional geophysics techniques. the first successful result in this field was obtained by a japanese group that observed in 2007 the conduit structure of mt. asama. since 2010, other interesting volcanoes have been probed with the same method: stromboli in 2011, mt. teide in 2012 and la palma in 2014. here we discuss the muon imaging technique reporting the nuclear emulsion detector design exposed at stromboli and results of the data analysis. 1. the muon radiography technique the internal structure of active volcanoes, is one of the most important geophysical questions. currently, the shallow earth crust is explored in great detail with echo-sounding, electromagnetic or gravimetric techniques, but it cannot be well resolved because of its strong structural heterogeneity and the potential difficulty to be accessed. actually such indirect methods suffer from inherent ambiguity, require spatially dense measurements in active areas and may not provide sufficient spatial resolution in the uppermost part of the conduit system. the muon radiography performed with cosmic-ray muons represents a complementary method able to provide a direct snapshot of the density profile in a volcano. in someway it is similar to the standard medical radiography: it allows to view inside volcanoes from a certain distance, thus reducing the risk for the operator, using instead of x-rays, atmospheric muon particles from cosmic rays. cosmic muons are the products of pion and kaon decays originated from the hadronic interaction of the primary cosmic rays (mainly protons and alpha particles) with the atmosarticle history received september 18, 2016; accepted october 5, 2016. subject classification: muon radiography, nuclear emulsion, stromboli, volcano, tracking detector. pheric nuclei. muons are the charged particles with the highest penetration capability in the matter. their mass (about 200 times the electron mass) and their long lifetime allow them to propagate through the layers of the atmosphere toward the earth’s ground from any direction. cosmic rays reach very high energies, which are partially inherited by the muons. the muon average energy at the sea level is around 4 gev near the zenith [gaisser 1990], but there are very large tails, up to several tev. the muon flux intensity and energy distribution depend on the zenith angle. the nearly horizontal muons, are more interesting for the radiography purpose because their higher energy, allows them penetrate larger rock thicknesses. a muon detector is usually positioned on a slope of the volcano as schematically shown in figure 1, pointing toward a prominent rock region of interest in such a way it’s possible to derive results for the rock portion located above the detector. typically the measured fluxes are relatively low and strongly dependent on the amount of the thickness crossed. for instance, a muon telescope with a surface of the order of 1 m2 with 1 steradian aperture angle can record about 103 muons per day crossing a rock thickness equivalent to 2 km of water. the energy spectrum of the atmospheric muons is well known; thus assuming also the precise knowledge of the mountain shape and a well understood muon detector it is possible to infer a map of the rock density distribution, by using the informations on the attenuation and the absorption of the muon flux. since denser materials absorb more muons (like dense materials such as bones absorb more xrays), this provides a basis for producing shadow images of the volcano interior. an important advantage of the technique is the spatial resolution that can be achieved: whereas current indirect methods can provide information with a spatial resolution of some hundreds of meters, muon radiography may provide mapping of internal structures to a resolution of some tens of meters. the resolution in solving the density contrast depends on the thickness of the rock traversed by the muons: the thicker it is, the fainter the muon flux is and the longer it takes to collect enough muons for a picture. the time needed can thus range from some weeks to several months. another interesting feature of this technique, is the possibility to perform a tomographic measurement by placing two or more cosmic ray telescopes around the object of interest. 2. nuclear emulsions for muon recording mainly there are two kind of detectors that can be used for muon radiography purpose: real time detectors (like plastic scintillators or wire chambers) and nuclear emulsion films used in our observations. emulsion detectors, have an incomparable position and intrinsic angular resolution (less than 1 µ and a 1 mrad, respectively), high data storage capabilities, mechanical robustness and compactness. they are not real time detectors and differently from the electronic ones do not require power supply and electronic front-end readout systems. moreover, their quite simple implementation in harsh environments like the ones encountered in active zones of a volcano and their easy portability, makes them ideal for geological applications. the very recent r&d program on new generation faster scanning systems currently in use for the opera experiment on neutrino oscillation search [acquafredda et al. 2009; agafonova et al. 2010, 2013, 2014] have recently encouraged the employment of nuclear emulsions as tracking detector in this field allowing to exploit widely their potential. the nuclear emulsion films used in our detector are made of two layers 45 µm thick of agbr crystals suspended in an organic gelatine poured on a plastic base of 200 µm. when a charged particle crosses the sensitive layer, some micro-crystals on the particle trajectory record its path as sequence of dark aligned grains (0.3 -1 µm diameter) as shown in figure 2 which can be analyzed after a chemical development of the films, by an optical microscope. the grain density in the emulsion is about 35/100 µm optimized for a mintioukov et al. 2 figure 1. the principle of muography. figure 2. cross sectional view of a nuclear emulsion layer. 3 imum ionizing particle. as final response, the detector will provide a precise measurement of the incident angle of the muons. a detailed description of the nuclear emulsion detector is given in nakamura et al. [2006]. the first experiments of muon radiography of volcanoes with nuclear emulsion technique have been successfully conducted on mt. asama [tanaka et al. 2007a] and showa-shinzan [tanaka et al. 2007b] in japan. after that, new emulsion projects involving italian laboratories have been carried on: stromboli volcano in 2011, teide mountain radiography (2012) and la palma exposure (2014) in tenerife. 3. the stromboli survey stromboli is a composite strato-volcano that steeply reaching ~900 m above sea level, allows to image a large portion of the volcano edifice by muon radiography. the stromboli activity is marked out by a shallow seismicity occurring at about 200 m depth below the cone [chouet et al. 2007] associated with eruptions as well as the continuous volcanic tremor and intermittent explosions of gas jets and volcanic materials that are concentrated at depths shallower than 200 m beneath the summit crater [auger et al. 2006]. one of the most interesting regions is the so-called sciara del fuoco since, according to some theoretical models of the volcanic activity, the main explosion mechanism would be due to some cracks opening from the main conduit of this region [macedonio 2009]. an advanced geophysical system is installed on the volcano slopes, for monitoring several parameters (seismic, infrasounds, thermal infrared, ground deformation, chemical composition of fumaroles), but the precise internal structure of the region below craters is not completely understood yet. the muon radiography could be an independent tool for investigating the inner structure of the cone and revealing the location and extent of the conduits that feed the continuous explosions. the region where the structures are expected is indicated by a black line in figure 3. possible cracks or channels transverse dimensions is expected as 10 meters or less. this resolution could be reached by muon radiography in a favorable conditions. 3.1. the muon telescope an emulsion telescope with an area of 0.96 m2 was installed in the end of 2011 at 640 m of altitude pointing toward the sciara region. the apparatus consists of 8 modules (figure 4) each one equipped with 10 emulsion cells as shown in figure 5. each of them is made of 2 doublets of emulsion films individually vacuum-packed in light shield envelopes, coated on both sides of a central metal plate of 26 cm × 80 cm size and 5 mm thick (figure 6). the emulsion doublets are coated to the central plate by 2 layers of elastic rubber while additional inox steel plates integrate the module structure, in such way to guarantee flatness and at the same time shield the soft component of cosmic rays. the total amount of emulsion films per module is 40. the total weight of one module is 26 kg while the overall weight of 8 modules including also the support frame is about 250 kg. in order to avoid possible rotation effects of any emulsion film a rigid support frame was realized. the detector position and orientation was obtained by recording gps data of two outer limits of the structure. the detector exposure went on for 5 months (from the end of october to the end of march); during this period, a continuous temperature muography with nuclear emulsions stromboli and other projects figure 3. a south-east view of stromboli. the sciara del fuoco region and the active craters are indicated. figure 4. picture of the detector structure. figure 5. front view of a single detector module. monitoring was performed in order to control that the emulsion working temperature would be kept inside an acceptable range (15°25°). after the exposure, the detector was disassembled and the emulsions chemically developed. during the data taking, the emulsion films were held very tightly, in order to guarantee a pattern matching between consecutive films. 3.2. data analysis and preliminary results emulsion readout is performed by fast automatic microscopes (figure 7) [arrabito et al. 2006] with a nominal scanning speed of 24 cm2/h recently upgraded to 40 cm2/h by keeping the current hardware configuration [alexandrov 2013]. a sequence of 16 tomographic images is taken in each emulsion layer, with a step of 2.5 µm in order to reconstruct the threedimensional tracks. all charged particle tracks, just after their production are recorded in each film up to the development phase, with no time information. the event time reconstruction is done offline by geometrical coincidences, that means at least two emulsion films are needed to get position pattern matching. in this way only tracks collected during the observation period match between adjacent films while the fake track coincidences due to combinatorial background are discarded. after the offline track reconstruction procedure widely detailed in arrabito et al. [2007], it is possible to draw a bidimensional histogram with the reconstructed track density (figure 8), tx and ty here are the directional tangents in respect to the telescope main axis. in this coordinate system the mountain contour visible by detector using the muons should coincide perfectly with the usual photo made from the same position. the color scale is the number of track found in each bin. the positive ty corresponds to tracks (muons) coming from the mountain direction, negative ty muons comes from the back side (mainly from a free sky). the shadow of mountain is clearly visible for positive ty. the free sky muon flux was computed on the basis of the hkkm04 model [honda et al. 2004] with a cutoff for energies below 1 gev while in order to simulate the muon rates in the crater region we used the conventional cosmic ray flux attenuation values through the matter for each angle, and the digital elevation map (dem) with 10 m resolution, kindly provided by the italian civil defense. the dem was also used to estimate the rock thickness shown in figure 9d. on this angular plot the color scale for each bin presents the mean muon path (in meters) passed inside the rock before reaching the telescope. the parameters used for the simulation are: a uniform rock density of 2.2 g/cm3, an exposure time of 154 days and an effective detector surface of about 0.6 m2. the mountain profile is well visible and its shape is well in agreement with the simulation. it is also possible to observe, in the plot of figure 9c, showing the difference between monte carlo and data, a 10% excess of muon tioukov et al. 4 figure 6. side view of a single module. figure 7. prototype of the european scanning system for emulsion data acquisition. figure 8. bidimensional histogram of the counted muons in the angular space distribution obtained on a partial statistics. 5 counting rates in the rock region (red circle labelled a) which seems to reveal the presence of a crater. on the other side, the muon flux in the free sky shows some discrepancies respect to the expected one; this effect has to be better investigated performing deeper studies on the detection efficiency as a function of track momentum, choosing appropriate selection criteria in order to keep low momentum background tracks under control. also a more precise monte carlo simulation taking into account the electromagnetic components of air showers and the cosmic muon scattering in the matter, should be provided. it is also necessary to consider the fact that the cosmic ray spectrum below 10 gev is affected by not negligible uncertainties due to several factors like geomagnetic latitude effects or solar modulation activity [adriani et al. 2013]. at this stadium of the analysis it’s not yet possible to infer finest effects; anyway the preliminary comparison between monte carlo and data seems to be promising. 4. conclusions and perspectives we described the muon radiography technique for imaging volcano structures, reporting the first preliminary results on the radiographic survey of stromboli volcano with a nuclear emulsion telescope, obtained on the 60% of the collected statistics. a good match between the mountain shape and monte carlo simulation has been observed, that means a good understanding both of detector position and the topographic map usage. simulated and observed data seem to be in a good agreement, but a more refined analysis is mandatory in order to better estimate the detection efficiency and background level for the final data interpretation. actually the main background source as described in nishiyama et al. [2014] is represented by low momentum tracks and one of the best tool to filter them is the usage of the emulsion cloud chamber, opera-like that is made of interleaved layers of high-z plates and emulsion plates. muon radiography of volcanoes with nuclear emulsions seems to be very encouraging and motivates further research to improve the capability of the detector, the scanning power and the ability in interpretating data also from a geological point of view aiming a closer cooperation with geologists. italian laboratories are involved in other projects as the mt. teide exposure (2012) under analysis and the recent exposure in la palma (2014) to investigate the shape of the fault i.e. the depth, the width, and the porosity of the crushing zone. nuclear emulsions are really suitable for muon radiography applications for several aspects already mentioned. the main drawback in using such detectors is the huge amount of scanning power required, but such difficulty is on the way to be solved. the nominal scanning speed of automatic microscopes is about 24 muography with nuclear emulsions stromboli and other projects figure 9. (a) expected muon rates in 154 days on a detector surface of 0.6 m2; (b) observed muon rates; (c) difference normalized to the unit between monte carlo and data; (d) rock thickness. (a) (c) (d) (b) cm2/h/layer so the minimum time required for the full scanning of a stromboli-like detector (1 m2 × 8 emulsion layers) was about 170 days using the scanning power equivalent to 2 microscopes working at 30% of the time. thanks to the recent software and hw upgrades the scanning speed increased up to 80 cm2/h/layer. this value could be further raised up to more than 200 cm2/h/layer. references acquafredda, r., et al. (2009). the opera experiment in the cern to gran sasso neutrino beam, j. instrum., 4, p04018. adriani, o., et al. (2013). time dependence of the proton flux measured by pamela during the july 2006-december 2009 solar minimum, astrophys. j., 765, 91. agafonova, n., et al. (2010). observation of a first ox candidate in the opera experiment in the cngs beam, phys. lett. b, 691, 138-145. agafonova, n., et al. (2013). new results on on→ox appearance with the opera experiment in the cngs beam, j. high energy phys., 1311036. agafonova, n., et al. (2014). evidence for on→ox appearance in the cngs neutrino beam with the opera experiment, phys. rev. d., 89, 051102. alexandrov, a. (2013). a novel approach for fast scanning of nuclear emulsions with continuous motion of the microscope stage, conference proceedings, nucl. instrum. meth a, 718, 184-185. arrabito, l., et al. (2006). hardware performance of a scanning system for high speed analysis of nuclear emulsions, nucl. instrum. meth. a, 568, 578-587. arrabito, l., et al. (2007). track reconstruction in the emulsion-lead target of the opera experiment using the ess microscope, 2, p05004. auger, e., l. d’auria, m. martini, b. chouet and p. dawson (2006). realtime monitoring and massive inversion of source parameters of very long period seismic signals: an application to stromboli volcano, geophys. res. lett., 33, l04301; doi:10.1029/2005 gl024703. chouet, b., p. dawson, t. ohminato, m. martini, g. saccorotti, f. giudicepietro, g. de luca, g. milana and r. scarpa (2007). source mechanisms of explosions at stromboli volcano determined from moment-tensor inversions of very-longperiod data, j. geophys. res., 108 (b1), 2019; doi:10.1029/2002jb 001919. gaisser, t. (1990). cosmic rays and particle physics, cambridge university press, new york. honda, m., t. kajita, k. kasahara and s. midorikawa (2004). new calculation of the atmospheric neutrino flux in a three-dimensional scheme, phys. rev. d, 70, 043008. macedonio, g. (2009). motivations for muon radiography of active volcanoes, earth planets space, 61, 1-5. nakamura, t., et al. (2006). the opera film: new nuclear emulsion for large-scale, high-precision experiments, nucl. instrum. meth. a, 556, 80-86. nishiyama, r., s. miyamoto and n. naganawa (2014). experimental study of source of background noise in muon radiography using emulsion film detectors, geosci. instrum. meth., 3, 29-39. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida and k. niwa (2007a). development of an emulsion imaging system for cosmic-ray muon radiography to explore the internal structure of a volcano, mt. asama, nucl. instrum. meth. a, 575, 489-497. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida, h. ohshima, t. maekawa, h. watanabe and k. niwa (2007b). imaging the conduit size of the dome with cosmic ray muons: the structure beneath showa shinzan lava dome, japan, geophys. res. lett., 34, l22311. *corresponding author: valeri tioukov, istituto nazionale di fisica nucleare (infn), sezione di napoli, naples, italy; email: valeri@na.infn.it. © 2017 by the istituto nazionale di 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49_2_2006 729 annals of geophysics, vol. 49, n. 2/3, april/june 2006 challenging goals posed by the scientific community involved in the various disciplines related to the deep-sea environment (such as geophysics, geochemistry, biology, oceanography). availability of suitable infrastructures, like research vessels, deep-sea rovs, manned submersibles and seafloor observatories represents another major limitation. in this scenario, projects built around clear and challenging scientific objectives, as well as sound technological background and innovation perspective, stand for a potential source of spinoffs and exploitation opportunities of the utmost importance. to make this potential become a reality is neither easy nor frequent. in the last decade, in the field of marine research, the eusponsored project geostar (geophysical and oceanographic station for abyssal research, the exploration of eastern mediterranean deep hypersaline anoxic basins with modus: a significant example of technology spin-off from the geostar program elisa malinverno (1), francesco gasparoni (2), hans w. gerber (3) and cesare corselli (1) (1) conisma lru, dipartimento di scienze geologiche e geotecnologiche, università di milano «bicocca», milano, italy (2) tecnomare-eni spa, venezia, italy (3) tfh berlin – university of applied sciences, berlin, germany abstract a significant example of technological spin-off from the geostar project is the special-purpose instrumented module, based on the deep-sea rov modus, developed in the framework of the eu-sponsored project biodeep. the goal to be achieved has been defined as the exploration of the deep hypersaline anoxic basins of the eastern mediterranean sea through real-time video images, measurements and accurate video-guided sampling at water depths well exceeding 3000 m. due to their peculiar characteristics, these basins are one of the most extreme environments on earth and represent a site of utmost interest for their geochemical and microbial resources. the paper presents the strategies and the main results achieved during the two cruises carried out within the biodeep project. mailing address: dr. elisa malinverno, conisma lru, dipartimento di scienze geologiche e geotecnologiche, università di milano «bicocca», piazza della scienza 4, 20126 milano, italy: e-mail: elisa.malinverno@unimib.it key words deep-sea – anoxic basins – rov – marine technology – exploration 1. introduction exploration and long-term observation of the deep-sea environment is one of the last frontiers of marine science and technology. this technology, including marine engineering and underwater acoustics, plays a primary role in the development of equipment which can make possible, and economically feasible, the fulfilment of the 730 elisa malinverno, francesco gasparoni, hans w. gerber and cesare corselli 1996-2001) represented one of the very few cases where successful results were followed by real exploitation. besides making the first european seafloor observatory available, geostar developed the special deep-sea rov, modus (mobile docker for underwater sciences), whose exploitation for the purposes of the eusponsored project biodeep (biotechnologies for the deep, 2001-2004) is the object of the present paper. together with a brief description of modus, this paper will report about its adaptation and successful use for the exploration of a highly peculiar environment in the deep mediterranean sea, the deep hypersaline anoxic basins (dhabs). 2. modus: general description modus is a deep-sea remotely operated vehicle (rov) originally designed for the purpose of geostar seafloor observatory deployment and recovery (fig. 1, gerber et al., 2002). suspended from an electro-mechanical umbilical cable, it is equipped with electrical thrusters ensuring mobility on the horizontal plane, while the dedicated winch onboard the support vessel regulates its ascent/descent. cable and winch are the infrastructure property of ingv that allow modus operation from vessels of opportunity. modus configuration (clauss et al., 2002) fills the gap between full 6d-space operation and simple hook deployment systems. this means there are no free-swimming capabilities typical for rovs (especially those equipped with tether management system). however this does not represent a disadvantage, since mo-dus is not required to carry out close inspection or manipulation tasks like typical rovs. on the contrary, modus can handle heavy loads (up to 30 kn; for comparison, typical payload of a commercial rovs is less than 1.5 kn), overcoming one of the basic limitations of the existing rovs. this peculiar characteristic of modus is essential for the geostar concept; its modular design concept opens a wide range of interesting opportunities for its utilisation in different contexts. some of these opportunities have already been explored: among these, the possibility to carry out visual and instrumental surveys in fig. 1. modus and geostar-stations onboard r/v urania. 731 the exploration of eastern mediterranean deep hypersaline anoxic basins with modus deep waters, and to serve as a carrier of special instrumented packages, ensuring the scientist a virtual presence and operational capabilities in the area of interest. in the short, but already significant, history of geostar spin-offs, the exploration of the deep hypersaline anoxic basins in the eastern mediterranean sea is the first of these opportunities. 3. the deep hypersaline anoxic basins of the eastern mediterranean and biodeep project the anoxic basins of the eastern mediterranean represent a peculiar deep-sea environment having extreme physical and chemical conditions. they are in fact characterised by the presence of hypersaline brines, separated from normal deep-sea water by a sharp physical and chemical interface. they have a variable ph and ionic composition, no oxygen and at some places high sulfide concentration, high temperature, and methane seepage (corselli et al., 1998). several dhabs, having diverse morphologies and dimensions, are present in the eastern mediterranean in different tectonic settings and at variable depths (3300-3700 m) along the mediterranean ridge (jongsma et al., 1983; scientific staff of cruise bannock 1984-1912, 1985; medriff consortium, 1995); their origin is due to the interaction among tectonic processes, fluid migration and dissolution of messinian evaporitic rocks present in the subsurface at shallow depth (westbrook and reston, 2002). the peculiar physical and chemical characteristics of the dhabs and their location in the deep-sea make such basins an especially interesting site for different fields of research and a new frontier for exploration. in particular, the eu-funded project biodeep was targeted to the investigation of microbial life in such extreme conditions. the challenging goal was to characterise, in four selected dhabs, the physiology and ecology of the extremophiles microbial communities, their cellular components or products and to identify how their features can translate into new biotechnological applications. although driven by biotechnological goals, the biodeep approach is multidisciplinary, involving geological, geochemical and hydrological tasks besides microand macro-biological studies. a fundamental requirement to fulfil the scientific purposes of the project is the execution of accurate sampling at the seawater-brine interface and visual surveys at the surface of the dhabs and at their margins. in particular the latter is intended to obtain a direct, although remotely driven, description of this peculiar environment. in fact the seawater-brine interface has only been detected by geophysical methods (jongsma et al., 1983; medriff consortium, 1985) and directly investigated through ctd measurement and sampling (de lange et al., 1990): no investigation proved that this transition is visually detectable. therefore, besides the sampling task, the main questions addressed are related to how the brine interface appears at a visual inspection, which features can be visually detected on it and in particular which features and structures are present at the beach, i.e. the line where the brine surface impinges the bottom. 4. modus adaptations for biodeep to perform the challenging tasks described above, conventional off-the-shelf equipment is not available. rovs and manned submersibles have never approached the dhabs close enough to obtain samples at the interface or to make accurate visual surveys, because of the peculiar characteristics of the brines, which can cause damage to the systems. until recently, sampling in the dhabs has been carried out using tools like ctd/rosettes deployed from the ship. this approach has three basic drawbacks: a) it is intrinsically inaccurate: the sensors and the sampling devices hang at the end of a very long cable (in the order of 3500 m), so that an accurate regulation of their position for sampling at the brine interface is unlikely. b) the scientific payload is limited to the essential, and the electro-mechanical cable serving the ctd/rosette has normally no provision for additional telemetry channels (especially high capacity channels imposed by tv cameras). 732 elisa malinverno, francesco gasparoni, hans w. gerber and cesare corselli c) the interaction of the scientists with the phenomena under observation is minimal; there is no possibility to see where the instrumented package is and in which conditions the measuring and sampling operations are performed. the task the biodeep team had to fulfill has therefore been to find a new approach, meeting the challenging scientific requirements and at the same time compatible with the constraints imposed by the project (cost-effectiveness, reduced risks and short development time). the solution developed by technische universität berlin, tfh berlin and tecnomare (the technological partners of biodeep project) was based on the adaptation of modus to serve as the carrier of a specially developed module (scipack – scientific package), the instrumented unit intended to enter the dhabs. in this concept, illustrated in fig. 2, modus becomes a powerful and stable platform, capable of being actively positioned and «flown» a few meters over the dhabs surface, moreover providing plenty of telemetry capabilities for the transmission of video images, data and control signals. to manage scipack, the original idea was to equip modus with an underwater deep-sea winch (like those of the rovs tether management systems). in this way scipack could remain sheltered inside modus during the launch/transfer/recovery phases and subsequently lowered into the brines like a «tethered satellite» when modus had reached the desired position over the dhab. this idea was then substituted by the simple low-cost solution where scipack is suspended under modus using a fixed length of cable. although this solution has some impact on the operability of the system (in particular the launch and retrieval procedures are more complicated), it maintains the basic functionalities of the innovative concept. modus has been adapted for biodeep purposes as indicated in fig. 2. the docking cone (visible in the foreground of fig. 2) – not necessary for this operation as no seafloor observatory is involved – has been disassembled and replaced by a frame (sciskid) housing mechanical and electronic equipment for the scipack operation, an easy procedure because of the modularity of the modus design concept. the fully assembled modus and the scientific module scipack (equipped with water samplers, ctd, echosounder, a tv camera with light) are shown in fig. 3. the operational sampling procedure is schematically shown in fig. 4: scipack is deployed from the vessel in a first step; it is followed by modus which is constantly communicating with it, allowing control of the procedures to be executed during surveying and sampling. as mentioned above, the deployment and control of the vertical position is performed with the deep-sea winch of the r/v. horizontal position is controlled by the modus pilot. by adopting umbilical cables of different lengths, it is possible to keep scipack more or less close to modus, according to the task to be undertaken: for exploration well inside the body of the brines, cables up to 200 m can be used, while for sampling at the interface shorter cables (10-20 m) are preferred, so that modus can more accurately manage scipack operations. during these operations it is possible, through the downward-looking tv cameras installed on modus, to get visual control of the position of scipack; this was an important innovation in the critical phase of sampling at the seawater-brine interface, fig. 2. modus with sciskid (background) and docking cone (foreground). 733 the exploration of eastern mediterranean deep hypersaline anoxic basins with modus providing for the first time the possibility to have a «virtual presence» in this unique environment. for the execution of visual surveys, the system configuration is modified: scipack and its umbilical cable are removed; all tv cameras are placed onboard modus/sciskid, that can now be flown over the surface of the dhabs. fig. 3. modus with sciskid (left) and scipack (right) onboard r/v urania. fig. 4. operational concept of biodeep mission for sampling and surveying: a – deployment of scipack from the ship; b – operation of the modus-scipack system at the interface of the dhab. 734 elisa malinverno, francesco gasparoni, hans w. gerber and cesare corselli two missions were carried within the biodeep project using this new technology. during these missions, four dhabs were explored (urania, bannock, discovery, l’atalante); in all of them sampling tasks were carried out, while three were visually investigated both at the brine surface and at their margins. 5. the missions the two cruises were performed with the italian r/v urania (august 17-september 4, 2001 and november 7-27, 2003) within the framework of the biodeep project. the positioning during the cruise was done using dedicated navigation software (navpro version dos 5.5 of the communication technology), interfaced with a dgps system. the reference cartographic system used during the cruise was the ed 50 ellipsoid, with utm (universal transverse of mercatore) projection. a detailed bathymetric survey was performed with two atlas deso-25 echosounders – 12 and 33 khz – at the margins of the basins, based on previous bathymetric maps of the area (medriff consortium, 1995), to identify the best sites to lower the modus system for the visual survey, i.e. areas with gentle slopes and absence of rough topography. configuration of the modus system for the sampling tasks (sampling at the seawaterbrine interface and sampling inside the body of the brines) followed two basic modes, one characterised by a short cable (10 m) and one by a long cable (200 m) connecting modus with scipack (fig. 5). configuration 1 is characterized by a 200 m secondary umbilical with double sided y connection, connecting the scipack during operations in deep zones of the brines. after major difficulties with the telemetry system and a short circuit in a cable, it was decided to leave the deep sampling task out and to shorten the umbilical to 10 m. this yields a detailed view of the sampling activities with cameras. due to the high number of revolutions of the scipack during descent and ascent, the umbilical situation was changed again to a twin cable configuration: this further prevented the payload from uncontrolled vertical turns (configuration 2). the final dive configuration was found after placing the daq-box from the scifig. 5. summary of the operations during cruise i and ii of biodeep in the four dhabs (d – discovery; a – l’atalante; u – urania; b – bannock): type of operation and configuration (colour code and number code in the legenda), operation depth and duration (length of the blocks and white squares). 735 the exploration of eastern mediterranean deep hypersaline anoxic basins with modus pack to the sciskid frame at modus (configuration 2). the latter allowed us to work with a single secondary umbilical, which significantly improved the quality of data transmission. configuration for the visual surveys (configuration 3, fig. 5) did not include scipack. in this case a simple white-painted iron ball with a white flag was hung at the end of a 7/10 m rope, to create a clear reference and dimension scale during the approach of modus to the interface and seabed. the strategy has been to lower the system in an area characterized by regular topography, as near to the beach as possible, thanks to the accurate bathymetric control, and then to move the ship toward the selected target, dragging modus along and keeping it straight by using its thrusters. two different approaches were followed, depending on the morphology of the selected area and on the wind and sea direction, as the ship had always to be directed with the bow against the wind, in order to maintain an accurate positioning at the low speed (1-1.5 knots) needed for the survey. the first approach is to move from the brine pool toward the normal bottom, i.e. upward. this kind of operation is more dangerous, as the bottom can rise quite rapidly, possibly causing modus to touch the bottom: the winch operator must be ready at any time to recover the cable. nevertheless this method allows a better depth control using the sonar and the altimeters. in fact the sonar can «see» the slope while approaching it, while the altimeters, ad hoc developed, can detect the bottom under the brines when these are shallow enough (around 10 m) and therefore disclose in advance when the beach is reached. the second approach is to move from the normal bottom toward the beach, i.e. downward, and then to proceed inside the basin. this operation allows safety conditions for modus, but the control on the bottom is less clear: the sonar as well as the altimeters can just see the normal bottom, which is also seen with the tv camera. therefore the difficulty in this case is that the moment at which the brines are reached remains unknown. in total seventeen dives were carried out for sampling and surveying during cruise i and three during cruise ii; fig. 5 illustrates the diving fig. 6a,b. images of the interface as seen by modus: a) view from modus to the suspended scipack (10 m mechanical cable) right before entering the urania basin; b) sequence of the survey at the interface of l’atalante (10 m cable): iron ball and flag approaching the interface (i-iii), entering the brines (iv), laying below the interface (v-vii) and coming out (viii). a b 736 elisa malinverno, francesco gasparoni, hans w. gerber and cesare corselli depth, the duration of each dive and the dive configuration at the four different anoxic basins. the operational performance of the system was successful, with up to three dives per day. only four dives were interrupted for technical reasons during cruise i (all related to failures of single components and not to design faults). the capabilities were confirmed during the recent cruise ii where, thanks to the adoption of more sophisticated tools (as a high resolution zoom camera), more accurate sampling within the dhabs and visual observations along the beaches were carried out. several samples, dedicated to geochemical and microbiological tasks, were obtained from the four selected dhabs along with data from the sensors mounted on scipack and with an accurate visual control (fig. 6a). sampling strategies and details are described, among others, in borin et al. (2002). three dhabs (urania, l’atalante, discovery) were investigated through visual survey during which their beaches were detected and explored. the brine interface, observed both during the sampling operations and during the dedicated surveys, appears in all basins as a sharp surface, acting as a «black hole». in fact objects hung under modus disappear when crossing the brine surface (fig. 6b). this mechanism can be due to the high light adsorption within the brines or at the boundary itself, due to the high density and optical contrast between the two media. 6. conclusions the modus system has shown good suitability for deep-sea operations not only for the deployment of stations, as foreseen in the original concept, but also as a surveying and supporting carrier for other scientific packages. the adaptation of this technology, developed during the geostar project, to the new aims proposed by the biodeep project, allowed the execution of sampling and surveying tasks which were up to now not feasible with other conventional equipment, even more sophisticated and expensive, like deep-sea rovs and manned submersibles. the biodeep project started april 2001, and the cruise where modus was used for the first time in the dhabs started mid august of the same year. this means that in four months a new concept for the exploration of the dhabs was developed, fully tested in the laboratory and finally made available fully operative for the first application. this would not have been possible without the availability of a carrier like modus. the application of modus technology to the study of the anoxic basins of the eastern mediterranean allowed for the first time: – to observe the seawater/brine interface, which is optically detectable as a light-absorbing surface, as demonstrated by the disappearance of objects when entering the brines; – to observe the beaches of the selected anoxic basins; – to accurately sample the brine interface with real-time visual control acknowledgements authors wish to dedicate this paper to the memory of giuseppe smriglio, coordinator of geostar project, who prematurely died in september 2001. biodeep project is carried out under the vfp of the european community (contract evk3-ct-2000-00042). captain and crew of r/v urania and the scientific team on board during the two cruises are warmly acknowledged. geostar project was carried out under eu contracts mas3-ct95-0007 (geostar-1) and mas3-ct98-0183 (geostar-2). references borin, s., d. daffonchio, t. brusa, f. gasparoni, d. calore, c. corselli and biodeep scientific team (2002): bacterial communities in the seawater-brine chemocline in deep anoxic hypersaline basins in the eastern mediterranean sea, in proceedings of the 8th symposium on aquatic microbial ecology, same8, october 25-31, 2002, taormina (me), italy. clauss, g.f., s. hoog, m. vannahme, h.w. gerber, f. gasparoni and d. calore (2002): modus: space shuttle for deepwater intervention, in off-shore technology 2002 conference proceedings, paper 14051 (cd-rom). corselli, c., b. della vedova, a. camerlenghi, g.j. de 737 the exploration of eastern mediterranean deep hypersaline anoxic basins with modus lange and g.k. westbrook (1998): emission of warm fluids and high temperature in the urania basin: observations from 1993 to 1998, in proceedings of the eu-workshop on extreme marine environments, november 19-22, 1998, kiel. de lange, g.j., j.j. middelburg, c.h. van der weijden, g. catalano, g.w.i. luther, d.j. hydes, j.r.w. woittiez and g.p. klinkhammer (1990): composition of anoxic hypersaline brines in the tyro and bannock basins, eastern mediterranean, mar. chem., 31, 63-88. gerber, h.w., g. clauss and s. hoog (2002): modus – remotely operated carrier for abyssal research – experiences in the mediterranean sea, in proceedings of the isope 2002 (international symposium for ocean and polar engineering), may 2002, kitakyushu, japan. jongsma, d., a.r. fortuin, w. huson, s.r. troelstra, g.t. klaver, j.m. peters, d. van harten, g.j. de lange and l. ten haven (1983): discovery of an anoxic basin within the strabo trench, eastern mediterranean, nature, 305, 795-797. medriff consortium (1995): three brine lakes discovered in the seafloor of the eastern mediterranean, eos, trans. am. geophys. un., 76 (33), 313-318. scientific staff of cruise bannock 1984-12 (1985): gypsum precipitation from cold brines in an anoxic basin in the eastern mediterranean, nature, 314, 152-154. westbrook, g.k. and t.j. reston (2002): the accretionary complex of the mediterranean ridge: tectonics, fluid flow and the formation of brine lakes, mar. geol., 186, 1-8. 673_682 bonforte 25.pdf annals of geophysics, vol. 45, n. 5, october 2002 673 gps surveys in the foreland-foredeep tectonic system of southeastern sicily: first results alessandro bonforte (1), marco anzidei (2), giuseppe puglisi (3), mario mattia (3), orazio campisi (3), giuseppe casula (2), alessandro galvani (2), arianna pesci (2), biagio puglisi (3), stefano gresta (1) and paolo baldi (4) (1) dipartimento di scienze geologiche, sezione di geologia e geofisica, università di catania, italy (2) istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy (3) istituto nazionale di geofisica e vulcanologia, sezione di catania, italy (4) dipartimento di fisica, settore geofisica, università di bologna, italy abstract the hyblean plateau (southeastern sicily) is characterised by three main tectonic structural trends: the first, nnw-sse striking, runs on the easternmost part of the plateau and is linked to the hyblean-maltese fault system; the second runs along the western part of the plateau with a nne-ssw direction and is characterised by a sinistral strike slip motion, like the scicli fault; the third ene-wsw striking, characterises the northernmost part of the area, including the scordia-lentini graben. we analysed gps data collected in a dense network located in the northern area of the hyblean plateau during 1998 and 2000, between the towns of catania and syracuse. data from noto, matera and cagliari igs stations, were included in the processing to connect this network to the international terrestrial reference frame (itrf). the comparison between 1998 and 2000 data sets shows an average northward motion of the gps stations located south of the gela-catania foredeep. site velocities decrease from south to north and show a weak internal deformation of the northernmost part of the hyblean plateau. 1. introduction the main geological structures of eastern sicily (fig. 1) are represented by: i) the crystalline units of the calabrian arc, which belong to the innermost unit of the chain; ii) the maghrebian mailing address: dr. marco anzidei, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, via di vigna murata 605, 00143 roma, italy; e-mail: anzidei@ingv.it key words ground deformations − gps − regional tectonics − hyblean plateau arc, which is a fold/thrust belt built up during neogene as an effect of the consumption of different paleogeographic elements of the african and apulian margins; iii) the catania-gela foredeep, running roughly ne to sw; iv) the hyblean plateau which represents the relatively undeformed foreland of the collision zone (ben avraham et al., 1990). the hyblean plateau belongs to the northernmost part of the pelagian block which extends southward to the malta islands (ben avraham et al., 1990). the plateau is bounded eastward by the nnw-sse hyblean-maltese escarpment, one of the largest fault systems of the mediterranean basin which separates the inland thick continental crust from the ionian mesozoic oceanic crust underthrusting the calabrian arc 674 alessandro bonforte et al. (ben avraham et al., 1995). on the western side, the hyblean plateau is bounded by the nnessw striking scicli fault, which separates it from the foredeep, filled by the gela nappe (i.e. the buried outermost thrust of the chain) through the vittoria plain (grasso et al., 2000). these two main structures isolate a block of the continental african foreland which resists thrusting/ subduction beneath the maghrebian chain due to its high buoyancy (ben avraham and grasso, 1991; ben-avraham et al., 1995). along the ionian margin a dense fault pattern form onshore a horst and graben sequence, oblique with respect to the hybleo-maltese escarpment, while the northern edge of the hyblean plateau facing the northern foredeep areas is down faulted by ne-sw trending normal faults, forming structures as the scordia-lentini graben (adam et al., 2000). this complex structural setting plays an important role in the recent seismic activity of the area, characterised by major earthquakes along the western, northern and eastern boundaries of the hyblean plateau. in particular, seismicity is characterised by lower magnitudes (m < 5) on the western side of the hyblean plateau with respect to its northern (m ≈ 6) and eastern (m ≈ 7) areas, where destructive earthquakes occurred in 1169 and 1693. it is worth noting that the latter was proceeded by a strong foreshock of m ≈ 6 which likely occurred along the same seismogenetic structure that produced an earthquake of m ≈ 6.5 in 1542, identified as the southern fault of the scordia-lentini graben (azzaro and barbano, 2000). more recently, on december 13, 1990 the area was struck by a m l = 5.4 seismic event occurring offshore from the town of augusta, which produced widespread damage between syracuse and catania. although this is the only earthquake for which instrumental data are available, the seismogenic structure cannot clearly be identified due to the ambiguous fault plane solutions (amato et al., 1995; giardini et al., 1995). azzaro and barbano (2000) suggested an e-w striking right lateral fault, located east of the scordia-lentini graben, as the most probable source for this earthquake. after this last earthquake, the area did not display important seismicity, except two weak seismic swarms between november 1999 and january 2000 (scarfì et al., 2001). in the frame of this complex tectonic setting, a preliminary ground deformation study was carried out along the istituto geografico militare italiano (igmi) levelling route, measured several times since 1970 (mulargia et al., 1991). in 1991, just after the 1990 earthquake, a geodetic monitoring program started with the aim of estimating the present day deformation of this area. one of the goals was to evaluate the relative kinematics of the tectonic structures of the hyblean area, such as the scordia-lentini graben. the latter was invoked as a causative structure in the historical earthquakes affecting this area (azzaro and barbano, 2000). in this paper, we analyse the active ground deformation of the hyblean plateau by means of a gps network, since it can provide data on the local kinematics of the area and on the role played by the local tectonic structures. fig. 1. sketch of the structural setting of eastern sicily. 1 = crystalline units of the calabrian arc; 2 = units of the maghrebian arc; 3 = hyblean plateau. the box indicates the spatial distribution of the gps network described in this paper. 675 gps surveys in the foreland-foredeep tectonic system of southeastern sicily: first results 2. geodetic networks and surveys just after the december 13, 1990 earthquake, a first gps surveys was performed in 1991 in the epicentral area, across the scordia-lentini graben, with the aim to detect the near field deformation related with this tectonic structure (achilli et al., 1995). the surveyed network consisted of eight stations with an average grid of a few kilometres, monumented by stainless steel nails located on the massive outcropping units. the gps network partially included the already existing local trilateration network (surveyed by the electromagnetic distance measurement technique) installed by the international institute of volcanology (iiv) of catania in the same period for similar aims (achilli et al., 1991) and the igmi levelling route. unfortunately, the first gps campaign partially failed due to electromagnetic noise at gps frequencies and receiver malfunctions, that prevented a detailed analysis of the data. after 1991 the gps network was extended and currently comprises 50 stations, including the older edm and gps benchmarks (fig. 2). the new geodetic monuments consist in iron and concrete pillars on top of which a screw allows the gps antenna to be directly installed (fig. 3). this installation allows instrumentation set-up errors to be avoided. in 1998 and 2000, two gps surveys were carried out and 26 stations of the northern part of network were occupied (table i). the network is located between the town of syracuse and catania and extends from the ionian coastline to the central hyblean plateau (fig. 2). it was designed with an average grid of fig. 2. the hyblean gps network. the gps stations, the main tectonic lineaments of the plateau, the location of the 1990 earthquake and its focal mechanism (star and balloon) (from amato et al., 1995) are reported in the map. 676 alessandro bonforte et al. about 10 km to constrain both the local seismic and aseismic crustal deformations related to the present tectonic activity. during the april 1998 survey, 16 dual frequency trimble gps receivers were used (models 4000 ssi and sse), equipped with choke ring or compact l1/l2 with ground plane antennas. a further dual frequency novatel propak ii receiver, equipped with a model 503 choke ring l1/l2 antenna, was installed at the northernmost station of the network (iiv station). due to the short baselines, which do not require long observation sessions to obtain an affordable data set for geophysical applications, survey sessions were planned spanning from 6 to 8 h for the rover receivers. the three igs stations of noto (noto), matera (mate) and cagliari (cagl), together fig. 3. typical station of the hyblean gps network (ip21 pillar). survey start end sessions obs local igs local discarded epoch time stations stations permanent baselines stations 1998 april 20th april 24th 5 6-8 h 26 3 1 5% 2000 may 8th may 12th 5 6-8 h 26 3 4 3% table i. general features of the 1998 and 2000 surveys. 677 gps surveys in the foreland-foredeep tectonic system of southeastern sicily: first results with catania (iiv), observed a continuous 30 s sampling rate throughout the campaign. during each survey session, about 15 stations were measured and each station was occupied at least twice to provide a robust daily data set (table ii). during may 2000, the gps survey was repeated (table iii). in order to avoid systematic errors we used the same trimble instruments as the previous surveys. five survey sessions were planned during the week-long campaign, by measuring up to 20 stations each day with a 6 h observation window at 30 s sampling rate. three stations were equipped and planned to acquire data continuously, together with catania (iiv) and the three igs stations used in the previous surveys. although this campaign was planned to occupy each station at least twice, thanks to the high number of available instruments, three measurements were carried out for almost all benchmarks of the network (table iii), achieving a higher redundancy with respect to the previous survey. 3. gps data processing and analysis of the results gps data collected during 1998 and 2000 surveys were processed using the trimble geotable ii. schedule of daily sessions of the 1998 survey. 1998 gps survey daily sessions april 20 iped irne iazz ispr ip13 isli ip03 im01 ip04 im02 ip07 ip06 iscp icdr ipnc iiv april 21 iped irne icrc ispr ip13 igat ip18 ip21 ip19 im02 ip12 ip06 imtr ipnc iscp april 22 iped iazz icrc ispr igat isli ip03 ip19 ip10 ip12 ip07 ip09 iscp ip06 imtr iiv april 23 irne icrc iazz isli igat ip13 im01 icdr ip04 ip21 ip18 ip09 imtr ip10 iiv april 24 ip03 ip19 ip21 im02 ip07 iiv table iii. schedule of daily sessions of the 2000 survey. the higher redundancy in station occupations, with respect to the 1998 survey, is due to the higher number of available gps receivers. 2000 gps survey daily sessions may 8 iiv ip18 ip15 ip07 ip12 iscp icdr imtr ipnc im01 iazz icrc irne isli igat ip09 ip10 im02 ip13 ip14 may 9 iiv ip18 ip15 ip07 ip06 ip13 icdr imtr ipnc ip12 iazz icrc irne isli igat ip04 ip19 ip16 ip21 ip14 may 10 iiv i18 ip15 ip07 ip09 iscp icdr imtr ip03 im02 iazz icrc irne isli igat ip04 ip19 im01 ip21 ip16 may 11 iiv ip18 ip15 ip07 ip14 iscp ip06 ip03 im02 ip10 ip12 ip13 ip04 ip19 im01 ip21 ip16 may 12 iiv ip18 ip15 ip07 im02 ip03 ip06 ip09 ip10 678 alessandro bonforte et al. matics office software, version 1.5 (trimble, 2001). precise ephemerides produced by the igs were used during processing to achieve a higher accuracy during the baselines computation (beutler et al., 1990). data from noto, mate and cagl igs stations, belonging to the itrf network, were added to the processing, in order to connect the hyblean network to the international frame. gps data were processed using both l1 and l2 frequencies. the iono free linear combination l3 was adopted to avoid the bias produced by the ionosphere delay on the gps signal propagation. a threshold of 10 km was introduced into the processing for the l3 application, taking into account that for short baselines the noise introduced by the l3 combination may be of the same order of the ionospheric bias. a summary of the baselines processing relevant to each survey is reported in table i. this table shows a decrease in the percentage of discarded station delta lat (m) dlat err (m) delta lon (m) dlon err (m) delta h (m) dh err (m) iazz 0.041 0.001 0.040 0.001 0.011 0.004 icdr 0.039 0.002 0.044 0.002 – 0.007 0.004 icrc 0.040 0.001 0.031 0.001 0.015 0.004 igat 0.040 0.001 0.040 0.001 0.012 0.004 iiv 0.034 0.001 0.032 0.001 – 0.092 0.004 im01 0.034 0.002 0.040 0.002 0.021 0.004 im02 0.038 0.001 0.049 0.001 0.014 0.004 imtr 0.033 0.001 0.027 0.001 – 0.032 0.004 ip03 0.042 0.001 0.031 0.001 – 0.001 0.004 ip04 0.022 0.002 0.052 0.002 – 0.034 0.004 ip06 0.046 0.002 0.044 0.001 0.009 0.004 ip07 0.039 0.002 0.043 0.001 0.010 0.004 ip09 0.039 0.002 0.040 0.002 – 0.002 0.004 ip10 0.039 0.002 0.046 0.002 0.006 0.004 ip12 0.041 0.002 0.043 0.002 0.013 0.004 ip13 0.035 0.001 0.041 0.001 0.011 0.004 ip16 0.051 0.003 0.043 0.003 0.020 0.007 ip18 0.048 0.002 0.040 0.002 0.018 0.004 ip19 0.054 0.001 0.048 0.001 0.007 0.004 ip21 0.047 0.001 0.044 0.001 – 0.014 0.004 ipnc 0.043 0.003 0.043 0.003 – 0.009 0.004 irne 0.037 0.002 0.043 0.002 – 0.025 0.006 iscp 0.040 0.002 0.042 0.002 0.014 0.004 isli 0.044 0.002 0.040 0.002 0.013 0.004 cagl 0.026 fixed 0.044 fixed 0.001 fixed mate 0.038 fixed 0.047 fixed – 0.002 fixed noto 0.036 fixed 0.043 fixed − 0.003 fixed table v. coordinate variations between 1998 (epoch 1998) and 2000 (epoch 2000). the average formal errors are about 3 mm for the horizontal component and about 4-5 mm for the vertical one. similarity transformation 1998 2000 parameters translation along x – 0.042 0.275 (m) ± 0.006 ± 0.001 translation along y – 0.033 0.164 (m) ± 0.006 ± 0.001 translation along h 0.006 – 0.136 (m) ± 0.008 ± 0.009 azimuth rotation – 0.0001 – 0.0082 (arc second) ± 0.0005 ± 0.0004 deflection in lat. – 0.0182 − 0.0178 (arc second) ± 0.042 ± 0.0027 deflection in long. 0.0167 0.126 (arc second) ± 0.024 ± 0.0014 network scale – 0.069 – 0.01 (mm/km) ± 0.003 ± 0.000 table iv. transformation parameters for 1998 and 2000 network adjustments. 679 gps surveys in the foreland-foredeep tectonic system of southeastern sicily: first results solutions for the second campaign; this is mainly due to the presence of a higher number of stations for which a 24 h set of data was suitable. this allowed us to recover some bad solutions, by reducing, changing and moving the processed time window, in order to use the best available data set. after processing the baselines, the networks were adjusted without introducing constraints, to verify the internal consistency of the data set and to estimate the positioning errors at each station. following this first step in the adjustment computation, once stable network solutions were obtained, the coordinates of the three igs stations were fixed. reference station coordinates were derived from the itrf2000 solution, using the velocity model produced for each station by the igs. the 1998 and 2000 campaigns were adjusted by fixing the coordinates on the noto, mate and cagl igs stations calculated at april 1998 table vi. transformation parameters for 1998 network adjustment, fixed at epoch 2000. similarity 1998 (fixed at transformation apoch 2000) parameters translation along x 0.001 (m) ± 0.006 translation along y 0.002 (m) ± 0.006 translation along h 0.001 (m) ± 0.008 azimuth rotation – 0.0009 (arc second) ± 0.0005 deflection in lat. – 0.0123 (arc second) ± 0.026 deflection in long. 0.0134 (arc second) ± 0.016 network scale – 0.063 (mm/km) ± 0.003 station delta lat (m) dlat err (m) delta lon (m) dlon err (m) delta h (m) dh err (m) iazz 0.006 0.001 – 0.003 0.001 0.017 0.004 icdr 0.004 0.002 0.000 0.002 – 0.003 0.004 icrc 0.005 0.001 – 0.012 0.001 0.020 0.004 igat 0.005 0.001 – 0.004 0.001 0.016 0.004 iiv – 0.001 0.001 – 0.011 0.001 – 0.086 0.004 im01 – 0.001 0.002 – 0.003 0.002 0.026 0.004 im02 0.003 0.001 0.006 0.001 0.020 0.003 imtr – 0.002 0.001 – 0.016 0.001 – 0.027 0.004 ip03 0.007 0.001 – 0.012 0.001 0.004 0.004 ip04 – 0.012 0.002 0.009 0.002 – 0.030 0.004 ip06 0.011 0.001 0.001 0.001 0.014 0.004 ip07 0.004 0.001 0.000 0.001 0.016 0.004 ip09 0.004 0.002 – 0.004 0.002 0.003 0.004 ip10 0.004 0.002 0.003 0.002 0.010 0.004 ip12 0.006 0.002 – 0.001 0.002 0.018 0.004 ip13 0.000 0.001 – 0.002 0.001 0.016 0.004 ip16 0.015 0.003 0.000 0.003 0.026 0.007 ip18 0.012 0.002 – 0.003 0.002 0.023 0.004 ip19 0.019 0.001 0.005 0.001 0.012 0.004 ip21 0.011 0.001 0.001 0.001 – 0.009 0.004 ipnc 0.008 0.003 0.000 0.003 – 0.004 0.004 irne 0.002 0.002 0.000 0.002 – 0.020 0.006 iscp 0.005 0.002 – 0.002 0.002 0.018 0.004 isli 0.009 0.002 – 0.003 0.002 0.018 0.004 cagl 0.000 fixed 0.000 fixed 0.000 fixed mate 0.000 fixed 0.000 fixed 0.000 fixed noto 0.000 fixed 0.000 fixed 0.000 fixed table vii. coordinate variations between 1998 and 2000, with both networks fixed at epoch 2000. the mean formal error is about 3 mm for the horizontal component and 4-5 mm for the vertical one. 680 alessandro bonforte et al. fig. 4. horizontal (vectors) and vertical (contours) velocities at the gps stations (mm/yr) between 1998 and 2000 computed by fixing the igs stations co-ordinates at epoch 2000.4. the contour interval is 5 mm/yr. large arrows indicate the maximum and minimum shortening strain axes. and may 2000 epochs, obtaining average formal errors of about 2 mm for the n, e components and of about 3 mm for the up component, at 95% confidence level. table iv lists the transformation parameters for the 1998 and 2000 constrained adjustment. the coordinate comparison between the two campaigns shows horizontal displacements up to 54 ± 3 mm, with a northeast trend at all stations (table v). a similar displacement is shown by the igs noto, mate and cagl stations, whose motion is imposed by the velocity model calculated by the igs. the vertical component, although less accurate than the horizontal components, displays weak motion. the apparent subsidence measured at iiv station is likely due to a wrong modelling of the gps antenna offset used in the 1998 survey during the data processing, that probably gives a false height value for that epoch. to estimate the internal deformation of the area at local scale, a further adjustment was made for the 1998 network. the three igs reference stations were fixed to the same co-ordinates used to adjust the 2000 network, obtaining a common reference frame for 1998 and 2000 surveys (epoch may 2000). table vi lists the transformation parameters for the 1998 adjustment constrained to epoch may 2000. the quality of the results of this adjustment are not different to the previous ones. the coordinate comparison between the two epochs referred to the same local frame (table vii) shows displacements within 20 mm for the n component, 16 mm for the e component 681 gps surveys in the foreland-foredeep tectonic system of southeastern sicily: first results and 30 mm for the up component; in this analysis iiv station was excluded due to problems with the antenna modeling during 1998 survey. 4. discussion and conclusions to analyse the internal deformation of the hyblean plateau, we compared the gps data set collected in two different campaigns performed in 1998 and 2000, computed in the same reference frame (fig. 4). it is remarkable that the velocity vectors (fig. 4), suggests a non uniform deformation for the whole network. a quite uniform northward motion trend is evident on the southern part of the network, whereas the northernmost stations show a more scattered orientation of velocity vectors. the northward motion of the southern part of the network implies an extension zone between this area and the noto station, which was considered fixed. inside the network, the velocities decrease from the southern to the northern stations, suggesting a crustal shortening while approaching the thrust of the gela nappe (fig. 5a). the horizontal strain tensor of the whole network was computed using the ellipsoidal distances, to obtain an overview of its areal deformation. the results of the computation (table viii) show a negative value of the dilatation of the network with a maximum shorb fig. 5a,b. a) northward component of velocities versus latitude; b) vertical component of velocities versus latitude. a value error strain components east strain component – 0.33 ppm 0.045 ppm shear strain component – 0.12 ppm 0.0804 ppm north strain component – 0.29 ppm 0.0485 ppm principal axes min shortening – 0.25 ppm 0.0485 ppm max shortening – 0.38 ppm 0.045 ppm min shortening azimuth – 35.9484° 20.2543° max shortening azimuth 54.05164° 20.2543° areal dilatation – 0.62 ppm 0.0935 ppm table viii. overall strain field computed for the geodetic network. the areal dilatation is the sum of the north and east components. negative values represent shortening. the network displays a general contraction with a maximum shortening axis trending ne-sw. 682 alessandro bonforte et al. tening axis trending ne-sw (arrows in fig. 4). deformation values are very weak (less than 1 ppm), and the overall compressional regime largely depends on the displacements affecting the northernmost stations, located on the foredeep. a detailed analysis of the strain distribution in the study area is useful to investigate the behaviour of the different sectors of the plateau as, for instance, the scordia-lentini graben area. moreover, although the data refer only to a two year time span, a vertical deformation affects the northern side of the network, whereas a weak uniform uplift occurs in its southern part (fig. 4 and 5b). the three stations ip04, irne and imtr, located on the northern part of the network are affected by subsidence. the ip04 station, which shows a vertical velocity of – 15 ± 2 mm/yr and a horizontal se displacement (fig. 4), lies on the north-western end of the network, along the gela-catania foredeep (see fig. 1). since this pillar lies on the catania alluvial plain, it is not yet clear if this motion is due to the regional kinematics, like the foredeep subsidence, or to soil compaction. imtr and irne stations, located on the massive outcropping units of the scordia-lentini graben, are subsiding at rates of – 13 ± 2 and – 10 ± 3 mm/yr, respectively. although their behaviour can be due to a general subsidence of this tectonic structure, the other stations of the gps network located on the same tectonic structure do not show comparable subsidence. furthermore, irne station does not show horizontal motion, whereas imtr displays a clear westward displacement that is quite different from the surrounding stations. in any case, the detected vertical deformation is near the height resolution of gps measurements, thus longer time span between surveys is required to obtain more definitive results. acknowledgements we thank massimo bacchetti, giuseppe capone, sergio del mese, giuseppe di grazia, cristiano guidi and andrea ursino for their helpful participation in field surveys. this work has been partially funded by italian space agency. references achilli, v., m. anzidei, p. baldi, r. velardita and f. broccio (1991): la rete geodetica gps della sicilia sud orientale, in contributi allo studio del terremoto della sicilia orientale del 13 dicembre 1990, istituto nazionale di geofisica, publication n. 537, 151-162. achilli, v., m. anzidei, p. baldi, f. broccio and r. velardita (1995): the southeastern sicily gps network, ann. geofis., 38 (2), 301-308. adam, j., c.d. reuther, m. grasso and l. torelli (2000): active fault kinematics and crustal stresses along the ionian margin of southeastern sicily, tectonophysics, 326, 217-239. amato, a., r. azzara, a. basili, c. chiarabba, m. cocco, m. di bona and g. selvaggi (1995): main shock and aftershocks of the december 13, 1990, eastern sicily earthquake, ann. geofis., 38 (2), 255-266. azzaro, r. and m.s. barbano (2000): analysis of the seismicity of southeastern sicily: a proposed tectonic interpretation, ann. geofis., 43 (1), 171-188. ben avraham, z. and m. grasso (1991): crustal structure variations and transcurrent faulting at the eastern and western margins of the eastern mediterranean, tectonophysics, 196, 269-277. ben avraham, z., m. boccaletti, g. cello, m. grasso, f. lentini, l. torelli and l. tortotici (1990): principali domini strutturali originatisi dalla collisione neogenico-quaternaria nel mediterraneo centrale, mem. soc. geol. it., 45, 453-462. ben avraham, z., v. lyakhovskyy and m. grasso (1995): simulation of collision zone segmentation in the central mediterranean, tectonophysics, 243, 57-68. beutler, g., w. gurtner, m. rotacher, u. wild and e. frei (1990): relative static positioning with the global positioning system: basic technical considerations, in global positioning system: an overview, edited by y. bock and n. leppard (springer verlag, berlin), 1-23. giardini, d., b. palombo and n.a. pino (1995): longperiod modelling of mednet waveforms for the december 13, 1990 eastern sicily earthquake, ann. geofis., 38 (2), 267-282. grasso, m., b. philips, c.d. reuther, p. garofalo, r. stamilla, g. anfuso, g. donzella and g. cultrone (2000): pliocene-pleistocene tectonics on the western margin of the hyblean plateau and the vittoria plain (se sicily), mem. soc. geol. it., 55, 35-44. mulargia, f., v. achilli, f. broccio and p. baldi (1991): is a destructive earthquake imminent in southeastern sicily? tectonophysics, 188, 399-402. scarfì, l., h. langer, g. di grazia, a. ursino and s. gresta (2001): analysis of two microearthquake swarms in southeastern sicily: evidence for active faults? ann. geofis., 44 (4), 671-686. trimble navigation limited (2001): trimble geomatic office vers. 1.5 (u.s.a.). (received june 11, 2002; accepted october 3, 2002) a t t i v i t à " s t r a o r d i n a r i a d e l l o s t r o m b o l i g . ponte i l d o t t . \ i t l o r i o f a m u l a r o r i m a s t o l i d i i s o l a d i s t r o m b o l i d u i ani e il p e r i o d o b e l l i c o h a a v u t o c u r a di r e g i s t r a r e g l i i n t e r e s s a n t i f e n o m e n i p r e s e n t a t i d a l v u l c a n o . dal d i a r i o , c l i c il d e t t o d o t t o r e h a i r a m e s s o a l l ' i s t i l l i l o v u l c a n o l o g i c o l i n e o r i a s s u m i a m o q u a n t o s e g u e : l o s t r o m b o l i , d o p o la c a t a s t r o f i c a e s p l o s i o n e d e l 1930, c l i c p r o d u c e la m o r t e di p a r e c c h i e p e r s o n e , c o n t i n u ò la s u a a t t i v i l a e s p l o s i v a a b i t u a l e e f u r o n o f r e q u e n t i le c o l a t e eli l a v a c h e spesso r a g g i u n s e r o il m a r e . il 22 a g o s t o 1941 a i l c o r e 2 0 il v u l c a n o e n t r ò i m p r o v v i d a m e n t e in g l a n d e e t e r r i f i c a n t e a t t i v i t à p l i n i a n a : m i r i a d i di s c o r i e r o v e n t i f u r o n o p r o i e t t a t e a f o n t a n a a p i ù di u n c h i l o m e t r o d ' a l t e z z a sul c r a t e r e f r a v i v i s s i m i b a g l i o r i , m e n t r e lu u d i t o un p r o l u n g a t o b o a t o , c o m e lo scopp i o c o n t i n u o di m o l t e m i n e , s e g u i l o d a u n v i o l e n t o s p o s t a m e n t o d a r i a c h e p r o d u s s e m o l t i d a n n i n e l l ' a b i t a t o . le s c o r i e r o v e n t i c l i c c a d d e r o u l l e e r b e s e c c h e d e l l a p a r t e a l t a del v u l c a n o s o p r a i 4 0 0 m . p r o v o c a r o n o v a s t i i n c e n d i a l i m e n t a t i d a l l i m p e t u o s o v e n t o di n w . gli i s o . l a n i d o p o u n g i o r n o di a c c a n i l i s f o r z i p o t e r o n o a s t e n t o d o m a r e g l i i n c e n d i . l e e s p l o s i o n i a n d a r o n o i n d e b o l e n d o d o p o il p r i m o s c o p p i o , m a c o n t i n u a r o n o i n i n t e r r o t t a m e n t e m e n t r e la l a v a t r a b o c c a v a d a l c r a t e r e , d e t t o d e l t o r r i o n e , c d i v i d e n d o s i i n d u e l a r g h e c o l a l e l u n g o l ' e r t o p e n d i o d e l l a s c i a r a del f u o c o r a g g i u n s e il m a r e con g r a n d e f r a c a s s o f r a v o l u t e i m p o n e n t i di v a p o r e a c q u e o . d o p o 15 g i o r n i di e r u z i o n e r i t o r n ò sul v u l c a n o l a o r d i n a r i a a t t i v i t à s l r o n i b o l i a n a c h e i p r o t r a s s e p e r p i ù di d u e a n n i s e n z a , p r e s e n t a r e s t r a o r d i n a r i f e n o m e n i . il 3 d i c e m b r e 1943 a l l e o r e 13 e 30 u n f r a g o r o s i s s i m o b o a t o a n n u n c i ò u n a v i o l e n t a r i p r e s a e s p l o s i v a con l a n c i o di s c o r i e r o v e n t i m i s t e a c e n e r e e a v a p o r e a c q u e o c h e i n s i e m e f o r m a v a n o d e l l e g r a n d i m a s s e v o r t i c o s a m e n t e s o l l e v a n t e s i u l c r a t e r e fino a g r a n d e a l t e z z a . m o l t i d a n n i f u r o n o a r r e c a l i a l l ' a b i t a l o ed a l l e c o l t i v a z i o n i e la p o p o l a z i o n e e b b e g r a n d e p a n i c o . f r a t t a n t o a b b o n d a n t i e c o n t i n u i cllliisa t t i v i t à s t r a o r d i n a r i a d e l l o s t r o m b o l i 2 0 1 -i l a v i c i r a g g i u n s e r o il m a r e . l e scorie r o v e n t i , a l c u n e d e l l a g r o s s e z z a • he s u p e r a v a la n o c e , c a d d e r o -u tutta i isola e le p e r s o n e p o t e r o n o a l v a i s i r i p a r a n d o s i n e l l e c a e . l ' a t t i v i t à e f f u s i v a fu c o n c o m i t a n t e con q u e l l a e s p l o s i v a ed e n t r a m b e non e b b e r o t r e g u a . 11 25 g e n n a i o avv e n n e un f o r m i d a b i l e b o a t o a v v e r t i l o a n c h e d a l l a costa -.iel.la. f o r t u n a t a m e n t e i e s p l o s i o n e non fece v i t t i m e , né a r r e c ò d a n n i , p e r c h é l u t t o i e n o r m e m a t e r i a l e p r o i e t t a t o fu s p i n t o da un i m p e t u o s o vento di \-\\v e c a d d e s u l l a l o c a l i t à d i s a b i t a t a di p u n t a d ' i o n i o e f o r g i a \ cec i l i a ove tutti gli a r b u s t i e le e r b e f u r o n o d i s t r u t t i e c o p e r t i . d o p o q u e s t a e s p l o s i o n e , c h e p u ò d i r i u n a d e l l e p i ù t e r r i b i l i e p r o l u n g a l e d e l p e r i o d o s t o r i c o , il v u l c a n o n o n si a c q u i e t ò : a b b o n d a n t i l o l a t e (li c e n e r e si s u s s e g u i r o n o f r e q u e n t e m e n t e a l l a r m a n d o m o l t o la p o p o l a z i o n e . -nella n o t t e dal 23 al 21 f e b b r a i o 191-1 a l l e ore 2 e 30 fu u d i t o d a g l i a b i t a n t i d e l l isola un g r a n d e f r a g o r e s i m i l e a q u e l l o p r o d o t t o da u n a i m m a n e f r a n a ed a p p e n a fece g i o r n o t u l i o il v u l c a n o a p p a r v e c o p e r t o di u n o s t r a l e r e l l o di l a p i l l i . a l t r e a b b o n d a n t i p r e c i p i t a z i o n i cìi c e n e r e , a c c o m p a g n a l e da forti e s p l o s i o n i si e b b e r o d a l 3 a l l ' i ! a p r i l e . a l l e o r e 10 de) 15 g i u g n o a p p a r v e sul c r a t e r e del t o r r i o n e un e n o r m e p i n o oscuro c h e . con v o r t i c o s e v o l u t e , s ' i n n a l z ò a s m i s u r a t a a l t e z z a e poi -i d i l e g u ò . il f e n o m e n o fu a c c o m p a g n a t o d a un p r o l u n g a l o boato. più g r a n d i o s a e p i ù t e n i b i l e fu l ' e s p l o s i o n e del 2(1 agosto d e l l o -lesso a n n o , a v v e n u t a a l l e o r e 7 e 3 0 ; f u r o n o l a n c i a t i d a ! " r a t e r e d e l l ' o r n c n e a b b o n d a n t i c o l i e roventi m i t e a c e n e r e lino a g r a n d i a l tezze fra v i v i s s i m i b a g l i o r i e p r o l u n g a l i b o a t i . in b r e v e t e m p o si -oll e v ò un g r a n d i o s o p i n o , m e n i l e un g i g a n t e s c o t o r r e n t e di l a v a -i riv e r s ò l u n g o la s c i a l a del f u o c o fino al m a r e ove -i a l z a r o n o e n o r m i c a v a l l o n i c h e p e r poco non o m m e r e r e una b a r c h e t t a n e l l a q u a l e -i t r o v a v a n o d e g l i a n i m o s i s t u d e n t i . knorrne m a t e r i a l e r o v e n t e . m i i o a blocchi a b b a s t a n z a grossi e a c e n e r e , c a d d e l i l l a p a r t e a l t a del vulcano e u l d e c l i v i o di f o r g i a v e c c h i a lino a l l a s p i a g g i a p r o t e n d e n d o s i nel m a r e p e r c i r c a pio m ove fu p r e s t o d e m o l i l o d a l l a f u r i a d e l l e o n d e . k la seconda v o l t a c h e a l l o s t r o m b o l i -i r i p e t e la v a l a n g a di c e n e r e r o v e n t e dopo q u e l l a del 1930, c h e fu c a l a m i t o s a p e r la m o r t e di v a r i e p e r s o n e . la v i o l e n t a e p h i o n e p r o d u s s e un forte s p o s t a m e n t o (i a r i a c h e m i s e in g r a n d e a g i t a z i o n e il m a r e i cui (lutti i n v a s e r o la p i a g g i a p e n e t r a n d o v i fino a 2 0 0 m cosi da r o v i n a r e a l c u n i m u r i di c i n t a ed una casa nei p r e s i di p i l l i l a l e n a , ("essalo il m a r c i n o l o r i m a s e r o s u l l a s p i a g g i a dei pesci m o r i i e m o l t i ne f u r o n o visti g a l 2 0 2 u , p o n t e l e g g i a r c nel m a r e nei pre—i d e l l a s c i a t a del f u o c o . i flutti d e l m a n r a g g i u n s e r o la costa di g i n o s t r a — p i c c o l o c e n t r o a b i t a t o a s\\ del l ' i s o l a —• c i r c a 10 m i n u t i do|)o la e s p l o s i o n e . a l l ' i s o l a di p a n a r i a m a r o s i g i u n s e r o d o p o 25 m i n u t i , a l i p a r i d o p o poco p i ù ili u n ' o r ; c a l l a costa d e l l a s i c i l i a d o p o d u e o r e . l e e s p l o i o n i con a b b o n d a n t i g e t t i di c e n e r e e le c o l a t e l a v i c l u c l i c r a g g i u n s e r o il m a r e p e r la s c i a l a del fuoco c o n t i n u a r o n o l i n i a l l ' a u t u n n o 1911: in s e g u i l o e b b e r o f i n e g l i e f f l u s s i l a v i c i e r i t o r n ò 1; s o l i t a m o d e r a l a a t t i v i l a e s p l o s i v a . la p o p o l a z i o n e d e l l o s t r o m b o l i d u r a n t e q u e s t o l u n g o p e r i o d o il a t t i v i t à del v u l c a n o -offrì m o l t o a n c h e p e r i d a n n i a l l e c a m p a g n e < p e r la f a t i c a c h e d o v e t t e s o s t e n e r e a r i p u l i r e f r e q u e n t e m e n t e i l e t i d e l l e c a s e o n d e e v i t a r e c h e 1 a c q u a p i o v a n a g i u n g e s s e t o r b i d a e c a r i e ; di sali nocivi n e l l e c i s t e r n e , u n i c a l'onte p e r l ' a l i m e n t a z i o n e iti ri e in q u e l l ' i s o l a senza s o r g e n t i . r i n g r a z i a m o ili q u e s t e i n t e r e s s a n t i n o t i z i e il d o t t . \ i l t o r i o l a m u l a r o . il q u a l e ila p i l i a n n i e g i i e con a t t e n z i o n e d i s i m l i o s o i fctic» m e n i d e l l o s t r o m b o l i v e r a m e n t e i n q u i e t a n t i per la p o p o l a z i o n e del i isola c h e . p e r la sua t e n a c i a nel c o n t i n u a r e a v i v e r e su di un vul c a n o m o l l o p e r i c o l o s o , m e r i t e r e b b e l ' a t t e n z i o n e del g o v e r n o . (.ninnili •— istituto 1 ulruiioloaico etneo dell l nirersità — l'mt. layout 1 annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 1 validation of the advanced infra-red water vapour estimator (airwave) total column water vapour using satellite and radiosonde products enzo papandrea1,2,*, stefano casadio1, erminia de grandis1, elisa castelli2, bianca maria dinelli2, bojan bojkov3,* 1serco s.p.a., frascati, (rm), italy 2istituto di scienze dell’atmosfera e del clima, isac-cnr, bologna, italy 3eumetsat, darmstadt, germany enzo.papandrea@serco.com 1. introduction w ater vapour plays a crucial role both for the weather and the climate. it is indeed the most abundant greenhouse gas in the atmosphere (kiehl and trenberth, 1997). it is a strong absorber of infrared radiation and can amplify the effect of other greenhouse gases through positive feedback. water vapour can condense to form clouds that greatly influence the heating rate and circulation of the atmosphere. in addition, it can influence the atmospheric composition through photochemical reactions. water vapour is highly variable in space and time. a good knowledge of its distribution is therefore essential for predicting the weather, monitoring the climate, and understanding the physics and chemistry of the atmosphere (wang et al., 2016). observations of atmospheric water vapour abundance have been made using different instruments and platforms in order to study, e.g., its variability and trends. among these we can list radio soundings (e.g., balloon-borne) and satellite missions, operating in different spectral regions (microwave (mw), near-infrared (nir), ultraviolet-visible (uv-vis)). water vapour distribution can also be derived using the global positioning system (gps) technology. in addition to its observation, the scientific community has made an effort in intercomparing and validating abstract the along track scanning radiometer (atsr) instrument series, on board the european space agency (esa)’s european remote sensing (ers) satellites, ers-1/2 and the environmental satellite (envisat), covered the time period from 1991 to 2012. using the infra-red channels at 11 and 12 microns (available for both day and night and for all the three instruments) and combining nadir and forward geometries, total column water vapour (tcwv) fields have been obtained over the sea in cloud free scenarios. the retrieval scheme, named advanced infrared water vapor estimator (airwave), exploits the instrument physical characteristics (dual view and two channels), in combination with advanced radiative transfer models and a sea surface spectral emissivity database. the algorithm makes use of a set of tabulated parameters, fixed along the whole globe. here, the airwave dataset is compared with the satellite data of the special sensor microwave/imager (ssm/i) and with available stations from the analyzed radiosoundings archive (arsa). both comparisons demonstrated the good quality of the airwave dataset. however, they also highlighted the presence of a dry bias at high latitudes. globally, we found an average correlativeairwave bias of 0.72 kg/m2 (rmse=5.79 kg/m2) vs ssm/i and 0.80 kg/m2 (rmse=7.77 kg/m2) vs arsa. annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 2 the water vapour datasets obtained using different sensors. the along-track scanning radiometer (atsr, delderfield et al., 1986) instrument series had as main objective the accurate retrieval of sea surface temperature for climate studies. however, casadio et al. (2016) demonstrated that it is possible to retrieve accurate total column water vapour (tcwv) fields, in both daytime and night-time conditions, combining the atsr brightness temperature collected from nadir and forward views in the channels at 10.8 and 12 µm for cloud-free sea scenes. the new airwave (advanced infra-red water vapour estimator) algorithm has been applied to the whole atsr mission to develop a new tcwv database. here we describe the validation of the airwave tcwv dataset against both the satellite-based (i.e. the special sensor microwave/imager, ssm/i) and in situ radiosonde measurements (the analyzed radiosoundings archive, arsa). 3. datasets description the advanced infrared water vapor estimator (airwave) the airwave algorithm was used to retrieve tcwv for the three atsr missions in the 19912012 time range. roughly, 21000 orbits for atsr1, 35500 orbits for atsr-2 and 48000 orbits for aatsr have been processed on the grid processing on demand (g-pod) esa infrastructure. information on the airwave/g-pod processor can be found here: http://wiki.services.eoportal.org/tiki-index.php?page=g-pod+services. the retrieval code is computationally very efficient as it makes use of tabulated parameters obtained with a dedicated radiative transfer model (rtm). a detailed description of the algorithm can be found in casadio et al. (2016). the native spatial resolution of the airwave products is 1x1 km2, however they have been also regridded at lower resolution (0.25°x0.25°, approximately 28x28 km2). according to casadio et al. (2016), the airwave random error component at native resolution is estimated to be about 12% for atsr-2 and aatsr, while it is about 30% for atsr-1. the airwave algorithm requires that the observed scene is a cloud free water surface. indeed, clouds affect the atmospheric radiation both because of their relatively high reflectance and because they partly shield the atmosphere below them, causing i.e. inaccurate gas abundance retrieval (see e.g., raspollini et al., 2006). the current limit of airwave to water scenes can be in principle overcome with the availability of an extremely accurate auxiliary information on land emissivity (casadio et al., 2016). the atsr level 1b products contain auxiliary information on the observed surface (land/sea) and on cloud coverage (cloud mask) for both nadir and forward views. therefore only a subset of available level 1b measurements was processed, selecting only the pixels above water and filtering out the pixels flagged as cloudy in either the nadir or in the forward view. the atsr cloud mask is the result of a series of nine independent tests using different threshold values applied to individual short wavelength infrared (swir), mid infrared (mir) or thermal infrared (tir) channels or to a combination of channels (závody et al., 2000; esa, 2007). monthly fields of the airwave tcwv products at 2°x2° spatial resolution, spanning from 2003 to 2008, are freely available on the global energy and water exchanges (gewex) water vapor assessment (g-vap) website (http://gewexvap.org/). the special sensor microwave/imager (ssm/i) the ssm/i series of instruments are near-polar orbiting satellites, with the defense meteorological satellite program (dmsp)-f08 being the first one launched in 1987. the imager operates at the following four frequencies: 19.35, 22.235, 37, and 85.5 ghz. exploiting the brightness temperature measured over the oceans in these channels, it is possible to retrieve important geophysical paannals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 3 rameters over the ocean, such as near-surface wind speed, columnar water vapour, and columnar cloud liquid water (wentz, 1997). the author estimated, for ssm/i water vapour products, a systematic error of 0.6 kg/m2 and a root mean square retrieval accuracy of 1.2 kg/m2. the ssm/i tcwv dataset was produced by remote sensing systems (wentz et al., 2012). among the available products, daily mean at spatial grid resolution 0.25°x0.25° have been used for this analysis. the v7 water vapour products of the dmsp-f13 satellite, used in this analysis, have been compared with gps in hilburn et al. (2010), founding a mean ssm/igps bias of -0.1 kg/m2 and a standard deviation of 1.83 kg/m2. the analyzed radiosoundings archive (arsa) the arsa database was produced by the laboratoire de météorologie dynamique (lmd) to fulfil the needs of different applications, e.g., for the validation of forward and inverse models or satellite measurements (http://ara.abct.lmd.polytechnique.fr/index.php?page=arsa). the arsa database was started in january 1979, and has been extended onwards, on a monthly basis. the data inserted in the archive have passed different quality control tests in order to be free of gross errors: format problems, redundant radiosoundings and levels, unrealistic jumps, physically implausible values, temporal and vertical inconsistencies in temperature and dew point temperatures. the dataset contains water vapour abundances reported at specific pressure levels (nominally 43 levels between sea level pressure and 0.0025 hpa). in arsa the coverage of the full vertical atmospheric range is assured: in case of partial missing information, indeed, other reliable data sources or an extrapolation procedure up to the lower pressure level of the atmosphere are used. an estimate of radiosounding mixing ratio uncertainty of 2/3% was obtained by vömel et al. (2016). they also found a bias of 3% in the lower troposphere, in about 15% of all instruments. however, they concluded that the impact on studies using a large number of soundings such as satellite comparisons is considered small. 4. methods for the comparison of airwave-tcwv with ssm/i data we used the f13 satellite products, because they cover a long time period (19952009) that overlaps with the measurements of all three atsr instruments. furthermore, the local time of the ascending node of the f13 satellite (about 18:00) is more stable in comparison to other available ssm/i satellites. in fact, it varies less than one hour from the start to the end of the mission (more details can be found at http://www.remss.com/support/crossingtimes). this stability over time makes the comparison free from seasonal bias and minimise potential year to year differences. the spatial grid used for the comparison coincides with the ssm/i resolution (0.25°x0.25°), thus the airwave tcwvs have been aggregated to this grid. in the aggregation process, the amount of available airwave products in any grid cell (“pixel coverage” or cov) is also stored. therefore only ssm/i products in coincidence with valid airwave measurements, i.e. cov>2% were used for the comparison, in order to reduce potential biases due to ssm/i tcwv retrieved in presence of clouds. in fact most of airwave missing data are due to cloud coverage of the observed scene. for the comparison with arsa we found, globally, about 1400 stations having data in the time period coincident with the atsr missions. out of them, only sites surrounded, even partially, by water surfaces have been selected since airwave data are available only above water surfaces. for each sounding station, the proximity to water surfaces has been estimated using the high-quality (0.083°x0.083°) land-sea mask annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 4 “lsmask-world8-ar.dist5.5.nc” from group for high-resolution sea surface temperature (https://www.ghrsst.org/) by computing the percentage of sea surface values in a radius of 100 km centred on the station’s location. the final number of arsa stations selected for this analysis is 615. for every selected station we averaged the airwave products (at their native resolution) collocated with the radiosonde measurements. the collocation criteria are: 1) separate matching for day and night; 2) airwave measurements must be located within the radius of 100 km around the position of the station. the arsa tcwv is obtained by integrating the humidity profile over the full pressure range. 5. results and discussion bi-dimensional histograms of the comparison between the tcwv retrieved with airwave and ssm/i (upper panel) and arsa (lower panel) are shown in figure 1 using a bin size of 2.5 kg/m2. the selected arsa measurements mainly come from mid-latitude regions located in the northern hemisphere (see figure 3) and, as a consequence, are more representative of tcwv ranging from 3 to 30 kg/m2 while ssm/i measurements, being more homogeneously distributed all over the globe, extend the range up to 50 kg/m2. globally, a good correlation is found for tcwv values larger than 7.5 kg/m2 both against satellite and balloon-borne measurements, while below this value airwave performances deteriorate. this is because the channels used by airwave are less sensitive to dry atmospheric conditions, causing, as can be seen in figure 1, non-physical negative tcwv values. figure 2 reports airwave tcwv zonal means as a function of latitude in bins of 2 degrees (blue dots) and their standard deviations (blue vertical lines). the correlative measurements ssm/i (left) or arsa (right), binned as airwave data are showed in black. the two panels highlight the good quality of airwave tcwv in the equatorial and mid-latitude bands (up to 45/50 latitude degrees). indeed, as already mentioned, airwave retrievals are less reliable for low tcwv, typical condition at high latitudes. moreover, as highlighted in casadio et al. (2016), airwave uses a single set of parameters over the whole globe, computed as the weighted average of mid-latitude and tropical atmospheric conditions and therefore worse figure 1: airwave tcwv vs ssm/i tcwv (up) or arsa tcwv (down). the bin size is 2.5 kg/m2. the colour scale indicates the number of elements of the histogram. annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 5 performances were expected for the other latitude bands. figure 3 shows the geographical distribution of the mean tcwv differences (top panel=absolute, bottom panel=relative) using, as correlative dataset, ssm/i and arsa (overplotted circles). the size of the symbol used for the latter is related to the number of matches with each radiosonde. once again, small bias values are found in the regions between 50s/50n latitude degrees. a small longitudinal structure is observed. this is probably caused by the cloud coverage distribution that has a different impact on different areas. indeed, considering the difference in the acquisition time of the two instruments and the fact that ssm/i measures in all weather conditions, the same scene, cloud-free on atsrs observations, may be cloudy when acquired by ssm/i. to be noticed that airwave tcwv appears to be often lower than correlative measurements in proximity of the coastal areas. in the southern hemisphere, close to the equator, a positive bias is found over the central and east pacific and over the atlantic oceans. figure 4 shows monthly mean trends of the tcwvs difference (and their standard deviation), between the correlative measurements (ssm/i in azure or arsa in blue) and airwave. the monthly average negative bias, present at figure 2: zonal means of tcwv as a function of latitude for airwave (blue) and correlative measurements (black): ssm/i (left) or arsa (right). the data have been averaged in 2 degrees latitude bins. airwave tcwvs standard deviations are also reported. figure 3: average absolute (top) and relative (bottom) tcwv difference (ssm/i-airwave) at 0.25°x0.25° spatial resolution. average arsa-airwave tcwv differences are overplotted with circles; the size of the symbols is proportional to the number of matches (see legend). annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 6 the beginning of the atsr-1 mission, is progressively reduced and, since 1995, the difference between airwave and the collocated measurements is quite stable over time. the reason of the behaviour in the early part of the atsr-1 mission is a downlink problem causing very little 1.6 µm data available prior to the failure of the 3.7 µm channel in may 1992. this, in turn, impacts the daytime cloud screening, which relies on the 1.6 µm channel (corlett, 2016). very consistent findings are reported both with satellite and radiosonde matches, with a higher seasonality observed for the selected radiosondes, that are located in coastal or inner water regions characterized by a pronounced variability of atmospheric and surface conditions. table 1 sumfigure 4: ssm/i-airwave tcwv (azure) and arsa-airwave tcwv (blue) monthly mean trends. the difference between the correlative measurements and airwave tcwv ± stdv is also reported. table 1: airwave tcwvs compared with ssm/i and arsa stations. absolute and relative differences along with the standard deviations and root mean square errors are reported for the global (all latitudes), equatorial (25s-25n), mid-latitude (25s60s and 25n-60n) and polar (>60n or >60s) scenarios. average values for the atsr-1, atsr-2 and aatsr are also provided. instrument scenario ssm/i-airwave arsa-airwave n·105 bias stdv rmse n·105 bias stdv rmse kg/m2 or (%) kg/m2 or (%) all global 3110 0.72 (2.7) 5.75 (21) 5.79 (22) 3.01 0.80 (3.2) 7.73 (31) 7.77 (31) all equatorial 1560 -0.17 (-0.5) 5.57 (15) 5.57 (15) 0.87 -2.40 (-6.0) 7.74 (19) 8.10 (20) all midlat 1380 1.12 (6.1) 5.89 (32) 5.99 (33) 1.80 1.69 (8.4) 7.44 (37) 7.63 (38) all polar 170 5.55 (62) 5.14 (57) 7.57 (84) 0.35 4.12 (39) 6.47 (61) 7.67 (72) atsr-1 global 190 1.15 (4.3) 6.17 (23) 6.28 (24) 0.48 0.23 (1.0) 7.62 (33%) 7.63 (33%) atsr-2 global 1390 0.80 (3.0) 5.87 (22) 5.92 (22) 1.00 1.13 (4.7) 7.65 (32) 7.73 (32) aatsr global 1520 0.58 (2.1) 5.77 (21) 5.80 (21) 1.53 0.75 (2.9) 7.81 (31) 7.84 (31) annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 7 marises the results of the validation exercise. it reports absolute and relative differences (bias = correlative airwave tcwv), standard deviations and root mean square errors for the global, equatorial, mid-latitude and polar scenarios. the number of collocations for ssm/i is about three orders of magnitude larger than for arsa. ssm/i comparisons show that in the equatorial band the agreement is very good, with average bias of -0.17 kg/m2 while a larger difference (-2.40 kg/m2) is found vs arsa. the more pronounced negative bias of the comparison with the radiosondes can be explained by the fact that they are located mostly around the southeast asia where a wet bias is present also in the comparison with ssm/i. at mid-latitudes, similar results are found both against ssm/i (1.12 kg/m2) and arsa (1.69 kg/m2). larger biases are present in the polar areas (5.55 kg/m2 vs ssm/i and 4.12 kg/m2 vs arsa). the analysis performed separately for the three atsr instruments does not show instrument related features, a part for the early stage of atsr1 measurements. atsr-1 data versus radiosondes, globally, show a lower bias with respect to the other two instruments. this is not observed with ssm/i, as f13 measurements start in 1995 and, thus, do not cover the initial part of atsr-1 mission (see figure 4). 6. conclusions in this work we show the results of an extensive validation of the airwave tcwv dataset. we obtain similar conclusions using both satellite and balloon-borne products. we find an overall bias of about 3% partially due to tcwv retrieved in polar and coastal regions. when analysing the whole dataset we find no evident drift in the biases and correlated standard deviations thus highlighting the stability of the airwave tcwv and its very good accuracy. a potential improvement of the dataset should aim at solving the problem of negative values and at the reduction of the bias (and spread), especially in polar areas. acknowledgements this work has been performed under the esaesrin contract no. 4000108531/13/i-nb and contract no: 4000111304/14/i-am. we thank the ara/abc(t)/lmd group for producing and making available the analyzed radiosounding archive (arsa) database. ssm/i and ssm/is data are produced by remote sensing systems. data are available at www.remss.com/missions/ssmi . references casadio, s., castelli, e., papandrea, e., dinelli, b., pisacane, g., burini, a., and bojkov, b. (2016). total column water vapour from along track scanning radiometer series using thermal infrared dual view ocean cloud free measurements: the advanced infra-red water vapour estimator (airwave) algorithm, remote sensing of environment, 172, 1-14. corlett, g.k. (2016). (a)atsr validation activities/validation issues report ref: ulaatsr-vir issue: 5d. delderfield, j., llewellyn-jones, d.t., bernard, r., de javel, y., williamson, e.j., mason, i., pick, d.r. and barton, i.j. (1986). the along track scanning radiometer (atsr) for ers-1. proc. spie 589, 114-120. esa, h. a. p. (2007). issue 2.2, european space agency, available at: http://envisat. esa. int/handbooks/aatsr/(last access: 19 july 2017), 4056, 5. hilburn k.a., wentz f.j., mears c.a., meissner t., smith d. k., description of remote sensing systems version-7 geophysical retrievals (2010). remote sensing systems, available at: http://www.ssmi.com/papers/hilburn/hilburn_v7_poster_ams_sat met_2010_annapolis.pdf. kiehl, j. t. and trenberth, k. e. (1997). earth’s annual global mean energy budget, bulletin of the american meteorological society, 78, 197-208. raspollini, p., belotti, c., burgess, a., carli, b., carlotti, m., ceccherini, s., dinelli, b. m., dudhia, a., flaud, j.-m., funke, b., höpfner, m., lópez-puertas, m., payne, v., piccolo, c., remedios, j. j., ridolfi, m., and spang, r. (2006). mipas level 2 operational analysis, atmos. chem. phys., 6, 5605-5630, https://doi.org/10.5194/acp-6-5605-2006. vömel holger, tatjana naebert, ruud dirksen, and michael sommer (2016). an update on the uncertainties of water vapor measurements using cryogenic frost point hygrometers, atmos. meas. tech., 9, 3755-3768. wang, h., gonzalez abad, g., liu, x., and chance, k. (2016). validation and update of omi total column water vapor product, atmospheric chemistry and physics, 16, 11 37911 393. wentz, f. j. (1997). a well-calibrated ocean algorithm for ssm/i. journal of geophysical research, 102(c4), 8703-8718. wentz, f. j., hilburn, k.a., and smith, d. k.: remote sensing systems dmsp ssm/i daily environmental suite on 0.25 deg grid, version 7 (2012). remote sensing systems, santa rosa, ca. available online at www.remss.com/missions/ssmi/(last access: 19 july 2017). závody, a. m., mutlow, c. t., and llewellynjones, d. t. (2000). cloud clearing over the ocean in the processing of data from the along-track scanning radiometer (atsr), journal of atmospheric and oceanic technology, 17, 595-615. * a thank you or further information. enzo papandrea serco s.p.a., frascati, (rm), italy; isac-cnr, bologna, italy. paps101@gmail.com annals of geophysics, 61, fast track 8, 2018; doi: 10.4401/ag-7524 8 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 results of the preparatory study “premier analysis of campaign data” elisa castelli1,*, samuele del bianco2, bianca m. dinelli1, daniel gerber3, hermann oelhaf4, wolfgang woiwode4, bärbel vogel5, björn-martin sinnhuber4, roland ruhnke4, ugo cortesi2 1istituto di scienze dell’atmosfera e del clima del consiglio nazionale delle ricerche (isac-cnr), bologna, italy, 2istituto di fisica applicata “n. carrara” del consiglio nazionale delle ricerche (ifac-cnr), firenze, italy, 3rutherford appleton laboratory (stfc-ral), chilton, uk, 4institute for meteorology and climate research (imk-asf), karlsruhe institute of technology (kit), karlsruhe, germany, 5institute for energy and climate research stratosphere (iek-7), forschungszentrum jülich gmbh (fzj), jülich, germany e.castelli@isac.cnr.it i. introduction he relevance of the utls (upper troposphere and lower stratosphere) region and the impact of limb emission measurements at millimetre and sub-millimetre wavelengths for investigation of chemical, dynamical and radiative processes occurring at these altitudes constitute a major focus of the atmospheric ret * corresponding author: elisa castelli, istituto di scienze dell’atmosfera e del clima (isac-cnr), bologna, italy, email: e.castelli@isac.cnr.it search activities supported by the european space agency (esa) in the last two decades. since the end of the 1990’s, several studies were conducted on this topic, like those related to the design of the soprano (submillimetrewave observations of processes in the atmosphere noteworthy for ozone) and master (millimetre-wave acquisitions for stratosphere/troposphere exchange) instruments [bühler, 1999], the latter originally conceived for the esa explorer mission acechem (atmospheric composition explorer for chem1 mailto:e.castelli@isac.cnr.it annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 istry and climate interaction). further evolution of the scientific objectives and of the measurement concept originally proposed for acechem lead to the selection of the premier (process exploration through measurements of infrared and millimetre-wave emitted radiation) mission as one of the three candidate core missions of esa earth explorer 7. within the scientific context of premier, first priority is assigned to the currently open issues on the sensitivity of surface climate to utls variability and to global circulation, and on the troposphere/stratosphere exchange processes. premier will address these topics using atmospheric composition data obtained in 3d by an infrared limb-imaging spectrometer and by a mm-wave limb sounder [esa, 2012] with unprecedented spatial and temporal resolution. as part of the premier preparatory studies, the pacd project (premier analysis of campaign data) aimed at the analysis of the measurements acquired by the marschals (millimetre wave airborne receivers for spectroscopic characterization in atmospheric limb sounding) instrument, an airborne precursor of the millimetre-wave spectrometer of the premier payload, during the field campaigns premier-ex (kiruna, sweden, march 2010) and essence (kiruna, sweden, december 2011) with the m-55 geophysica high altitude research aircraft. the scope of the project was to demonstrate the capabilities of marschals to measure the vertical distribution of atmospheric targets (namely temperature, h2o, o3, hno3, n2o, and co) by the inverse processing of its limb sequences. moreover, the study included the evaluation of the potential synergy between limb measurements in the infrared and in the millimetre-wave region and the comparison of marschals retrieval products with the state of the atmosphere simulated using two atmospheric chemical transport models: clams (chemical lagrangian model of the stratosphere) (e. g., [grooß et al., 2005], [konopka et al., 2010], and references therein) and emac (echam/messy atmospheric chemistry) [jöckel et al., 2006]. in this paper, we provide an overview of the activities covered by the pacd project and present the consolidated results for the analysis of marschals measurements, for the comparison with models and for the synergy with infrared measurements, the latter obtained exploiting the measurements of mipasstr, an infrared interferometer on board the same aircraft. full details on the results from investigation of the individual aspects addressed by the pacd project are reported in dedicated papers, already published [castelli et al., 2013] or currently in preparation. ii. the marschals instrument marschals is a millimetre-wave heterodyne spectrometer that operates on board the m-55 geophysica as a demonstrator of the millimetre-wave limb sounder for the proposed earth explorer 7 mission premier. it acquires atmospheric emission spectra over three spectral bands around 300 ghz (band b), 325 ghz (band c) and 345 ghz respectively (band d). limb scans consecutively iterate over single spectral bands. a description of the spectrometer can be found in oldfield et al., [2001] and in dinelli et al., [2009], while castelli et al., [2013] report on the instrument upgrades performed before the premier-ex and the essence campaigns. the marc code (millimetre-wave atmospheric retrieval code, [carli et al., 2007]) was designed for the analysis of marschals measurements. the code can retrieve vertically distributed quantities such as temperature, volume mixing ratios (vmrs) and external continuum (a retrievable profile used to ac2 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 count for continuum effects from cloud opaque in the millimetre-wave frequency range, [dinelli et al., 2009], [castelli et al., 2013]), as well as scalar quantities like frequency shift, pointing bias and radiative gain and offset. all spectra acquired during a limb scan are analyzed simultaneously. the following list denotes all the different species measured by marschals, grouped together in corresponding frequency bands. band b: o3 and n2o; band c: temperature, h2o, o3 and hno3; band d: hno3, o3 and co. band b contains some information on hno3, but it is not sufficient to produce significant results. the external continuum, which is retrieved for every scan, provides information on the impact of clouds on marschals measurements. marschals has been deployed on board the m-55 geophysica in several measurement campaigns: scout-o3 (2005) in the tropics [dinelli et al., 2009]; premier-ex test campaign (2009) at mid-latitudes; premier-ex science campaign (2010) and essence campaign (2011), both at polar latitudes. in the following sections we describe the results obtained by marschals during the premierex and essence science campaigns. iii. measurement campaigns results iii.i premier-ex campaign: flight on 10 march 2010 from kiruna (sweden) the premier-ex campaign took place on the 10 march 2010 from kiruna (sweden) and aimed at the quantification of horizontal mixing processes in the utls. the flight track followed a triangular pattern, aiming toward the west and then turning back to kiruna. during the flight marschals sampled air masses of different origin (polar vortex as well as midlatitude and mixed air masses). from marschals spectra we retrieved vmr profiles of temperature, h2o, o3, hno3, n2o, co. in particular, the hno3 retrieved profiles along the flight show that the marschals instrument was able to resolve regions of vortex and filament of vortex air. the results obtained during this campaign have been thoroughly described in castelli et al., [2013]. during the campaign the mipas-str instrument was deployed aboard the m-55 geophysica [woiwode et al., 2012], as a demonstrator of the infrared limb sounder instrument proposed for the earth explorer 7. mipas-str is a high resolution fourier-transform limb sounder with an unapodized spectral resolution of 0.036 cm-1. the spectral channel 1 utilized for the retrievals considered here covers the spectral range from 725 to 990 cm-1. depending on the measurement scenario and flight altitude, typically one limb scan is obtained each 25 to 45 km along flight track. the mipas-str measurements are inverted using the tikhonov-phillips regularization method, yielding vertical profiles of the target parameters with typical vertical resolutions between 1.0 to 2.5 km between flight altitude and the lowest tangent point (depending on the retrieval target). during the flight, mipas-str detected dense clouds in the first leg below 8 km and in the final leg up to an altitude of 10 km. h2o, o3 and hno3 profiles retrieved from marschals were then compared with mipas-str results, using only the collocated results. since the synergy between ir and mmwave measurements is one of the main characteristic of the premier mission, it was explored using the vmr profiles retrieved from the two limb sounders, using two data fusion techniques: the level 1 plus level 2 technique (l1+l2) and the measurement space solution (mss) method [ceccherini et al., 2009]. 3 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 table 1: dofs, gi increase in % and total retrieval error reduction in % with respect to mipas-str obtained from the (l1+l2) data fusion method for h2o, o3, and hno3 in each marschals band. quantifier band h2o o3 hno3 dofs b 2 7 negligible c 8 5 2 d 2 4 3 gi increase [%] b 3 13 negligible c 11 7 8 d 3 5 12 total error reduction (10-20 km range) [%] b negligible 60 negligible c 50 60 30 d negligible 60 30 figure 1: comparison of number of o3 dofs from individual retrievals for marschals band b (red), mipas-str (green) with the number of dofs from synergistic retrieval performed using the l1+l2 (blue) and the mss (purple) methods. iii.i.i investigation of potential synergy between mm-wave and infrared limb-sounding during the premier-ex campaign: l1+l2 and mss methods the potentiality of data fusion between mmwave and infrared limb-sounding of the utls was investigated combining collocated data acquired by marschals and by mipas-str and comparing the quality of the products from individual and synergistic retrieval of selected targets, i.e. h2o, o3, and hno3. the performance of the different methods was estimated using a set of quality quantifiers that included the degrees of freedom (dof) of the retrievals, the gain of information (gi) (both described in rodgers, [2000]) and the total retrieval error comprising the random and systematic components. both data fusion methods, l1+l2 and mss, were tested and a brief description of the results is given below. the l1+l2 method consists in repeating the analysis of marschals level 1 (l1) data using the optimal estimation (oe) method 4 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 [rodgers, 2000] with the mipas-str level 2 (l2) products (with the associated covariances) used as a priori information. the mss method is a post-retrieval processing and it has been thoroughly described in ceccherini et al., [2009, 2010]. this method is based on the concept that a retrieved profile is the sum of a component belonging to the measurement space, the mss, and a component belonging to the null space. the observations only provide information on the mss and leave completely undetermined the component belonging to the null space. if we compute the mss in the space that is the sum of the two independent atmospheric measurements spaces, we obtain the fused data. this new mss includes all the information contained in the observations without any bias due to the a priori information used in the single retrievals. the l1+l2 data fusion was performed on h2o, o3, and hno3 vmr vertical profiles retrieved from the best matching sequences of marschals and mipas-str. table 1 summarizes the results obtained for each retrieval target separated for the three marschals bands (b, c and d). since mipas-str l2 data are used as a priori, we report the improvements due to data fusion to those data. a significant improvement is found for h2o in band c for all the three considered quantifiers, while for the other two bands we have a slightly lower improvement for the dof and gi. for o3 we found an improvement (with a reduction of the total error of up to 60%) in all spectral bands with the best results obtained for the gi and dof in band b. in the case of hno3 retrievals, as expected, we did not obtain any improvement from band b since there is negligible information on the hno3 in this band. however an improvement is obtained in both bands c and d with a total error reduction in the altitude range from 10 to 20 km of up to 30%. the evaluation of the performances of the mss method has been performed for the o3 vmr only. here, we briefly report a direct comparison of the results obtained in band b, highlight the basic differences between the two approaches and describe the potentialities of synergy between mm-wave and infrared limbsounding measurements. in figure 1 the number of dofs from individual retrievals is compared with the number of dofs from synergistic retrieval performed using the l1+l2 and the mss methods. we recall that the number of dofs obtained using the l1+l2 method represents the independent information that can be retrieved from marschals measurements in addition to the a priori knowledge provided by mipas-str l2 data, while the mss number of dofs are related to the fused data. the improvement obtained with the mss approach is evident. this is due to the fact that the mss approach is capable to evaluate the contribution of the measurement space component with no impact of the used external constraints, such as the tikhonov regularization used for the mipas-str retrievals [woiwode et al., 2012], or the a priori information used in marschals retrievals. the conclusions of this exercise are that the application of the l1+l2 method for the mmwave and infrared measurements qualitatively demonstrates the improvements due to the synergy between the different techniques, but quantitative comparisons are difficult since the characteristics of the different retrieval methods are not fully compatible. in contrast, the mss method allows for a more clear comparison of the individual results and the results of the data synergy, since the measured components from the different techniques are treated in a similar way and the same external constraint is applied to both. the investigation of potential synergy between mm-wave and in5 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 frared limb-sounding measurements shows that the fused products (obtained with both mss or l1+l2 techniques) provide an improvement with respect to the single sensor retrieval. in particular, from figure 1 the improvement given by the use mm-wave with respect to infrared measurements in case of presence of high altitude clouds is evident (last part of the flight, from marschals scan 31 onward). iii.ii essence campaign: flight on 16 december 2011 from kiruna (sweden) the essence campaign consisted of two flights. the first flight was performed on the 11 december 2011 at the boundary of the arctic polar vortex while the second flight was performed on the 16 december 2011 inside the vortex. during the second flight the m-55 followed a rectangular flight pattern. marschals spectra acquired during the second essence flight were used to retrieve vmr profiles of temperature, h2o, o3, hno3, n2o, and co inside the arctic vortex. these profiles are compared with results of the chemistry transport models clams and emac. iii.ii.i comparison between marschals and model results from clams and emac in order to assess the capability of the marschals instrument to resolve utls processes the retrieval results are compared to results from established models that allow to characterize the atmospheric conditions observed by measurements. while the primary goal of this cross-validation is to prove that atmospheric structures are represented adequately by the marschals results, these results in turn provide information to which degree these structures and the associated processes are captured by the models. the emac model is a numerical chemistry and climate simulation system used to investigate the past and future evolution of the atmosphere. the model includes sub-models for the description of tropospheric and middle atmosphere processes and their interaction with oceans, land and human influences. the clams model, originally developed for the stratosphere, was extended to the troposphere and can be used to investigate atmospheric processes in high resolution studies. both models are well suited for the comparisons with the marschals data and complements each other. the complementarity is due to different model resolutions and initialization, to differences in treatment of tropospheric processes and to differences in model architecture/transport scheme, which allows to identify shortcomings in the data sets. the comparison between marschals measurements and model results has been performed through the use of a common grid hereafter indicated as the vertical and horizontal grid of the comparison or vhgc. the models results were extracted on their own horizontal (2.8°x2.8° for emac and about 100 km for clams) and vertical resolution and were interpolated considering the horizontal distribution of the marschals tangent point geolocations. the vhgc ranges from 0 km to 26 km, in steps of 0.5 km, with spatial (longitude, latitude, altitude) and temporal coordinates associated to each marschals limb scan. for each altitude level of the vhgc in the range between the lowest and highest marschals tangent points, we used the interpolated time, latitude and longitude of the tangent points immediately above and below the selected altitude, while for each altitude level of the vhgc 6 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 figure 2: from top to bottom: emac (left) and clams (center) results after convolution with marschals ak and marschals retrieved vmrs (right) for o3, hno3, h2o and n2o. above (or below) the highest (or the lowest) marschals tangent point, we used the time, latitude and longitude values of the highest (or of the lowest) marschals tangent point. the models provide the atmospheric status on a very fine vertical grid. retrieved data are obtained on a coarser grid, optimized for each target. in order to compare the model data with the retrieved vertical profiles we need to take into account the smoothing introduced by the retrieval procedure due to the information content of the marschals measurements. using the method described in ceccherini, [2012] we can take into account the retrieval effect on the model profiles using the expression reported below: xmodel=akh xmodel,h+(i-ak)xa+ak x̂ -akh x̂h (1) where xmodel is the vertical profile obtained by the models and reported on the vertical grid of the retrieval, xmodel,h is the vertical profile provided by the models on the high resolution altitude grid, x̂ is the marschals retrieved profile on the vertical grid of the retrieval, x̂h is the retrieved profile interpolated on the high resolution altitude grid, xa is the a priori profile represented on the vertical grid of the retrieval, ak is the square averaging kernel matrix on the vertical grid of the retrieval,is akh the rectangular high resolution averaging kernel matrix. figure 2 shows the comparisons of the convoluted emac (left) and clams (center) model results with marschals (right) retrievals for o3, hno3, h2o and n2o. the figure shows that there are some differences between the two models and that in general, the agreement between models results and marschals measurements is quite good. the observed differences between the two models can be due to 7 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 both the different chemical initialization (the clams model was initialized using mls data on the 1st november 2011 while emac was initialized with model data on the 1st january 2011) and to differences in the polar stratospheric clouds (psc) nat particles sedimentation scheme. for ozone the models are in good agreement below 17 km, while above 17 km emac results are systematically larger. within this uncertainty marschals ozone is in good agreement with the models. the hno3 comparison shows that below 15 km the models are in good agreement and the agreement with marschals retrievals is reasonably good. above 15 km the differences among marschals and the models are larger; however these differences are present also between the model profiles with clams results systematically larger then emac ones. nevertheless both marschals and the models consistently show a hno3 reduction at 20 km. models h2o is in quite good agreement above 12 km, while below 12 km clams results are systematically larger. at lower altitudes, marschals is in better agreement with emac results. the model results for n2o are in good agreement, while marschals results are systematically lower than the models below the flight altitude. co values from both models agree quite well in the stratosphere above 10 km, however co from marschals retrievals assumes much lower values (not shown here) substantially because of the interference of co with ch3cl [castelli et al., 2013]. iv. summary and conclusions in the frame of the premier preparatory activities, the activities conducted for the esa project “premier analysis of campaign data” aimed at demonstrating the capabilities of marschals to measure the vertical distribution of atmospheric targets (also through the comparison with chemical transport model results) and to evaluate of the potential synergy between limb measurements in the infrared and in the millimetre-wave region. the inverse processing of marschals limb spectra, during the premier-ex 2010 and the essence 2011 campaigns, clearly demonstrated the capability of the instrument, when operating on the three spectral bands (i.e. band b, c and d, centered respectively at 300, 325 and 345 ghz) to measure vertical profiles of temperature and vmr of water vapor, ozone, hno3, n2o and co. this capability was successfully tested both in clear-sky and cloudysky conditions and proved the advantages of using mm-wave limb measurements to sound the utls altitude range in presence of clouds that are opaque to the infrared sounding. significant results were obtained in the investigation of the potentiality of combining the information from collocated and independent data sets measured by mm-wave and infrared limb sounders. by using marschals and mipas-str data on the flight of 10 march 2010, we compared the performances of individual and synergistic retrieval based on different approaches to data fusion. both the l1+l2 and the mss methods showed that the fused data have higher quality than the single one. the improvement was quantitatively assessed through selected quantifiers (total retrieval error, dofs and gi). in addition, we proved that the mss method enables an easier evaluation of the quality improvements of the data fusion products. finally, the comparison of marschals retrieved profiles with results from clams and emac models showed that 8 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 for o3 a generally good agreement is found between retrieved profiles and model outputs. the agreement is good also for h2o and hno3, with discrepancies between model values and marschals products at some altitudes. for n2o and co marschals retrieved profiles are in general low compared to model simulations. acknowledgements we acknowledge ecmwf for having granted direct access to their archives of atmospheric data and j. langen for useful comments. references [bühler, 1999] bühler, s. (1999). microwave limb sounding of the stratosphere and upper troposphere, shaker-verlag, aachen, isbn 38265-4745-4. [carli et al., 2007] carli, b., bazzini, g., castelli, e., cecchi-pestellini, c., del bianco, s., dinelli, b.m., gai, m., magnani, l., ridolfi, m., and santurri, l. (2007). marc: a code for the retrieval of atmospheric parameters from millimetre-wave limb measurements, j. quant. spectrosc. ra., 105:476– 491. [castelli et al., 2013] castelli, e., dinelli, b.m., del bianco, s., gerber, d., moyna, b.p., siddans, r., kerridge, b.j., and cortesi, u. (2013). measurement of the arctic utls composition in presence of clouds using millimetre-wave heterodyne spectroscopy, atmos. meas. tech., 6:2683-2701. [ceccherini et al., 2009] ceccherini, s., raspollini, p., and carli, b. (2009). optimal use of the information provided by indirect measurements of atmospheric vertical profiles, opt. express, 17: 4944–4958. 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(2005). simulation of denitrification and ozone loss for the arctic winter 2002/2003, atmos. chem. phys., 5:1437-1448. [esa, 2012] esa (2012). report for mission selection: esa sp-1324/3 (3 volume series), european space agency, noordwijk, the netherlands. [konopka et al., 2010] konopka, p., grooß, j.u., günther, g., ploeger, f., pommrich, r., müller, r., and livesey, n. (2010). annual cycle of ozone at and above the tropical tropopause: observations versus simulations with the chemical model of the stratosphere (clams), atmos. chem. phys., 10:121-132. [jöckel et al., 2006] jöckel, p., tost, h., pozzer, a., brühl, c., buchholz, j., ganzeveld, l., hoor, p., kerkweg, a., lawrence, m.g., sander, r., steil, b., stiller, g., tanarhte, m., taraborrelli, d., van aardenne, j., and lelieveld, j. (2006). the atmospheric chemistry general circulation model echam5/messy1: consistent simulation of ozone from the surface to the mesosphere, atmos. chem. phys., 9 annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6333 6:5067-5104. [oldfield et al., 2001] oldfield, m., moyna, b., allouis, e., brunt, r., cortesi, u., ellison, b., eskell, j., forward, t., jones, t., lamarre, d., langen, j., de maagt, p. j., matheson, d., morgan, i., reburn, j., and siddans, r. (2001). marschals: development of an airborne millimetre wave limb sounder, spie, 4540:221– 228. [rodgers, 2000] rodgers, c. d. (2000). inverse methods for atmospheric sounding: theory and practice, vol. 2 of series on atmospheric, oceanic and planetary physics, world scientific. [woiwode et al., 2012] woiwode, w., oelhaf, h., gulde, t., piesch, c., maucher, g., ebersoldt, a., keim, c., höpfner, m., khaykin, s., ravegnani, f., ulanovsky, a. e., volk, c. m., hösen, e., dörnbrack, a., ungermann, j., kalicinsky, c., and orphal, j. (2012). mipas-str measurements in the arctic utls in winter/spring 2010: instrument characterization, retrieval and validation, atmos. meas.tech., 5:1205-1228. 10 vol49_2_2006 581 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words landers – multidisciplinary long term observatories – global change – seismology – environment 1. introduction in this paper we recall how, during the last twenty years, deep-sea investigation moved from scarce observations in a unknown environment to continuous measurements of a wide set of parameters in carefully selected critical areas. in support of the analysis of this evolution, we will give a brief description of representative equipment or systems which were successively deployed during the period. 2. bottom landers up to the 1980s deep-sea investigations relied on autonomous bottom landers while some submersibles were used for visual observations only (e.g., cyana). landers, i.e. free falling shuttles launched from the sea surface and acoustically released after some time spent on the seafloor, could vary in size and weight from 100 kg to more than 2 t for the heaviest scientific landers. main problems encountered with landers are: – the deployment period is limited to 1 to 3 months (because of power supply and data storage capacities). – a lander can only support a small number of sensors. – the free fall deployment of the instrument does not allow a precise location of the instrument to be choosen. – there is no control of the instrument and in the early days it often failed at the beginning of the mission and no data were collected. with the availability of deep-sea manned submersibles equipped with arms (e.g., nautile, alvin, shinkai), low power electronic devices and improved batteries, a new generation of instruments appeared in the mid 80’s. one example of this type of instrument was the three component ocean bottom microprocessor based seismometer developed by ifremer from 1984 to 1986 in cooperation with the university from bottom landers to observatory networks roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin ifremer, centre de brest, plouzané, france abstract for a long time, deep-sea investigation relied on autonomous bottom landers. landers can vary in size from 200 kg weight to more than 2 t for the heaviest scientific landers and are used during exploration cruises on medium periods, from one week to some months. today, new requirements appear. scientists want to understand in detail the phenomena outlined during exploration cruises, to elaborate a model for future forecasting. for this, it is necessary to deploy instrumentation at a precise location often for a long period. a new mode of ocean science investigation using longterm seafloor observatories to obtain four dimensional data sets has appeared. although this concept has been proposed for many years, the high level of investment required limits the number of projects implemented. only multidisciplinary programs, supported by a strong social requirement were funded. some observatories have been deployed. mailing address: dr. jean marvaldi, ifremer, centre de brest bp 70, 29280 plouzané, france; e-mail: jean.marvaldi@ifremer.fr 582 roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin of brest and insu (institut national des sciences de l’univers) (pascal et al., 1986). a seismometer measures the motions of the ground in which it is embedded. a well designed instrument will detect them with a high accuracy and with low noise over a broad range of amplitudes and frequencies. the ground instrument coupling problem is crucial (byrne et al., 1982; guennou, 1988). this obs (ocean bottom seismometer) (fig. 1) was one of the first to separate geophones from the main pressure vessel and to place them at a one meter distance from the main frame on the seafloor. this design reduces the mechanical noise. the small geophone vessel can be optimised to have optimum horizontal and vertical coupling. the seismometer was also levelled to less than 2° once laid on the seafloor. all broad band obss in use today are built on this concept. after the obs had reached the seafloor, a rigid arm swang down with the sensor package when an explosive bolt was shot; then the pin holding the geophone package to the arm was pulled out by another explosive bolt. after dropping the package, the arm was automatically retracted firmly against the main structure by the pull of a stainless steel spring. buoyancy was provided by syntactic foam cylindrical floats, allowing manipulation by the nautile (glass spheres are forbidden for security reasons). the autonomy of the instrument rated for 6000 m operation, was more than 6 months and the functioning of the system was controlled acoustically throughout the deployment. this prototype gave very good results during test deployments and scientific cruises on deep-sea sites off-shore portugal and crete. at the same time many other european laboratories developed their own instruments in relation to their scientific interests, such as bathysnap from soc (southampton oceanographic center) and landers from nioz (netherlands instituut voor onderzoek der zee) or geomar. bathysnap is a time lapse camera system operated in various forms by soc (lampitt and burnham, 1983). it is a simple but effective example of an autonomous observatory system and made several important pioneering discoveries in the deep-sea environment. 3. landers and submersibles the ability to position a lander on a very well defined location with the nautile led ifremer to develop new instruments: ot 6000, nadia2, samo. 3.1. ot 6000: lander and submersible add their skills the ot6000 (observatoire thermique 6000 m) thermal observatory (fig. 2) was a self-contained instrument designed to measure temperature in the water layer in contact with the sea bottom and down to the depth of 60 cm in the sediment. it deploys five thermistor temperafig. 1. the pascal&person three components ocean bottom seismometer. the syntactic foam provides buoyancy. a lever arm deploys the cylindrical seismometer array from the main structure. 583 from bottom landers to observatory networks ture sensors, one in the sea bottom water layer approximately 2 m above the seafloor. the four sediment sensors are at the tip of thin steel rods vertically driven into the sediment upon deployment by the weight of a 50 kg cylindrical steel plate attached to their upper ends. the steel plate which is about 8 cm thick lies on the surface of the sediment after deployment. the rods are of different lengths, 30, 40 and 60 cm and provide temperature measurements at the corresponding penetration depths. the ot6000 thermal observatory was deployed at a selected site on the seafloor by the submersible nautile. the instrument was used during the kaikonankai cruise (japanese-french cooperation, august 1989) to estimate the total fluid flow out of the clam colonies in an area of approximately 500 by 625 m located on the first anticline landward of the frontal thrust at the toe of the eastern nankai accretionary complex. 3.2. nadia: the third dimension nadia (montagner et al., 1994) was a system which made it possible to re-enter an odp hole and install different kinds of geophysical instrumentation in the borehole. this system was for example used during the ofm/sismobs experiment. nadia (fig. 3) was an autonomous shuttle which was moved and controlled by the nautile. it included the borehole device, a hydraulic winch, electric batteries and a hydraulic generator. an underwater mateable electric connector was used to connect the nautile to nadia. in a first phase, the shuttle is moved down to the ocean bottom. then the nautile dives and joins the shuttle. an auxiliary dead weight is released so that the weight of nadia in water is about 20 kg. then the submersible moves the shuttle to the borehole and positions it. in a second phase the submersible is connected to nadia using the mateable connector and the borehole device is deployed in fig. 2. the ot6000 thermal observatory is deployed and recovered as a lander (syntactic foam, acoustic release). the temperature needles are positioned by arms of the nautile. fig. 3. nadia being put in sea water. 584 roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin the borehole and the control is performed from the nautile. glass beads fill the portion of the hole containing the instruments to enhance the coupling with the surrounding rock medium while enabling the package to be extracted from the hole at the end of the experiment. during the ofm/sismobs cruise a bottom package containing a set of cmg3 three component broadband seismometers was deployed during 10 days inside borehole 396b at a depth of 294 m below seafloor, at 4450 m water depth. 3.3. samo: vision and near real time transmission samo (station abyssale de mesures océanographique) is a more complex instrument deployed by the nautile or a rov using a procedure similar to nadia. it was designed to continuously monitor a hydrothermal vent. colour pictures (fig. 4) are transmitted to the surface by an acoustic link upon acoustic command. physical parameters (four temperatures and current) are continuously sampled and recorded. these data can also be transferred to the surface by acoustic command. tests to transmit these pictures in real time from the seafloor of the rift in the pacific ocean to the oceanographic centre in brest – france – took place in 1991, using inmarsat satellite. 3.4. hydrogeo: long term 3d a further important step was a long-term observation in borehole. back in the mid-80’s, ifremer started to monitor sediment temperature during one year deployments. the evolution of this equipment found an interesting challenge in 1994-1995: one and half years of monitoring of an ocean drilling program hole (no. 984d) after it was drilled in the barbados accretionary prism. the hole was equipped with a sub seafloor monitoring instrumentation to determine the fluid pressure in the decollement and the thermal structure of the sediments after the drilling disturbances had dissipated. the hole 984d was closed by a «cork» (cirfig. 4. samo is following up the hydrothermal biota with a video camera during some weeks. 585 from bottom landers to observatory networks culation obviation retrofit kit) system (davis et al., 1992). a logger with an underwater pluggable connector and a sampling device were installed in the «cork». a string of sensors was hanging under the «cork», measuring both temperature and pressure down to 500 m below the seafloor (fig. 5). the technological choice was to have numerical sensors every 20 m (foucher et al., 1997). a sigma-delta communication links them to the logger in the «cork». the harsh sea water environment (5 to 40°c) was taken in account through a corrosion study and led to the choice of hastelloy containers and glass epoxy composite bolts. a testing methodology based on afnor standard nf x 10 800 was established and applied. the reliability was demonstrated and the retrieval by submersible nautile was successfully performed during the french-american cruise odpnaut. the sensors were calibrated before and after deployment; the drifts were found lower than 0.1°c and 7⋅104 pa. 3.5. new generation of european landers during this period similar developments were conducted by laboratories having access to submersibles (alvin, shinkhai, mir, ...). various scientific teams improved lander technology and developed more complex instruments. as an example we can cite dobo, bobo and university of göteborg and ifm-geomar landers. 3.5.1. dobo dobo lander (bagley et al., 2004) is equipped with a time-lapse 35 mm reflex lens still camera (m8s, ocean instrumentation, u.k.) to monitor the response of deep-sea scavenging animals to bait (artificial food-falls). the camera is controlled by a custom built on-board control microprocessor. the camera can be biased to take photographs at times of interest if required by external control. the camera can take up to 1600 colour photographs on 35 mm ektachrome colour reversal film (kodak, u.k.) at a programmable interval greater than 30 s. the camera is positioned at a 2 m height and at a fixed 82° angle, photographing a 2.3 × 1.6 m area of seafloor. to attract scavenging fauna in the field of view of the camera, bait is placed in the centre of the field of view. 3.5.2. bobo the modular lander bobo (van weering et al., 1998) is designed for long (up to one year) in situ measurements in the lowermost 3 m of the benthic boundary layer, directly above the seabed, in water depths down to 5000 m. the bobo frame consists of three 2 m high fig. 5. configuration of the sensor string under the cork in the hole 948d. 586 roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin legs. at the base of the legs, the bobo has a width of 4 m. the upper part of the bobo lander consists of an hexagonal frame with a diameter of about 2 m. the lander frame has been specially designed to remain on the seabed for periods of more than one year and the materials were selected in consequence. exceptional care has been taken to avoid corrosion or electrolysis by isolating constitutive parts and connections. additionally all instrumentation is mounted on delrin blocks and instrument housings are either made of titanium or various kinds of plastics. benthos glass spheres are attached to the upper part of the frame for buoyancy. the instrumentation is attached in the hexagonal frame and to the legs of the lander. electrical power for the instruments is supplied by a battery pack that is housed in a glass sphere. like any lander, bobo is deployed by free fall from a surface vessel. its descent speed is 57 m/min. recovery is done by activating an acoustic release. near-seabed current velocity and direction measurements are made by a customised 1200 khz high resolution broadband acoustic doppler current profiler (adcp) made by rd instruments. salinity and temperature of the water are measured by a sea-bird sbe-16 conductivity/temperature recorder mounted at 2.5 m height in the frame. as an alternative the lander can be supplied with a sea tech transmissometer. for measurement of the amount, the temporal variability and the composition of near-seabed particle fluxes, a technicap ps 4/3 sediment trap is fitted in the hexagonal frame. the bobo lander can be equipped with other types of equipment as well. 3.5.3. göteborg lander-multisensor the research group led by prof. per hall at göteborg university (sweden) has developed and operated autonomous landers (karageorgis et al., 2003) since the early 1990s. collaborative work between the group and research institutes in france, denmark and the u.s.a. has resulted in the development and use of 5 different lander systems. today two landers, one big and one small, are operated routinely in several european research projects. both landers are built of non corrosive materials (titanium and various plastics) as a modular system in which experimental modules can be exchanged as desired. the largest lander carries four experimental modules and has been successfully deployed about 80 times in water depths ranging from 20 to 5200 m. the landers basically consist of two parts, an inner and an outer frame. the outer frame serves mainly as a carrier platform for the syntactic foam buoyancy package, the ballast and the acoustic system for the ballast release. the inner frame is a versatile system that carries the experimental module(s). these modules can easily be exchanged as desired. the module that has been in operation on the landers so far includes: chambers, or planar optode microelectrodes. the large göteborg lander normally records data from up to 30 sensors including: turbidity and oxygen in the chambers and outside, salinity, depth and temperature sensors, current sensors (such as single point and profiling acoustic current meters) and a video camera. 3.5.4. geomar lander – the help of a launcher at present ifm-geomar operates a suite of 8 landers of modular design as universal instrument carriers for investigations of the deepsea benthic boundary layer. 2 of these 8 landers have a squared design and carry a large benthic chamber covering 1 m2 sediment surface area to channel and measure fluid fluxes emanating from the seafloor (vent sampler system – vesp). the second line, the «geomar lander system» (gml) is based on a tripod-shaped universal platform which can carry a wide range of scientific payloads to monitor, measure and perform experiments at the deepseafloor (tengberg et al., 1995). both types of landers can be either deployed in the conventional free-fall mode or targeted deployed on hybrid fibre optical or coaxial cables with a special launching device. the launcher enables accurate positioning on meter scale, soft deployment and rapid disconnection from the lander by an electric release. the bi-directional video and data telemetry provides online video transmission, power supply (< 1kw) and sur587 from bottom landers to observatory networks face control of various relay functions. these landers provide a supporting platform system for: – gas hydrate stability experiments; – quantification of gas flow from acoustic bubble size imaging; – integrated benthic boundary layer current measurements; – quantification of particle flux; – monitoring of mega-benthic activity; – fluid and gas flow measurements at the sediment-water interface; – biogeochemical fluxes at the sedimentwater interface (oxidants, nutrients). depending on the scientific mission and the material of the lander frame (stainless steel or titanium), the gml-system may carry a maximum payload of up to 450 kg. 4. observatories 4.1. concepts in the 1990s, in parallel to these efforts of landers, appeared the notion of benthic station and long term observatory. scientists wanted to obtain four dimensional (space and time) data sets and long term observatories seemed to be the answer. on the impulse of the ec, discussions between european scientists tried to specify the technical needs and to conceive and evaluate the different solutions. a useful set of concepts and definitions was provided by tecnomare spa. basic elements characterising a seafloor observatory are: – multiple payload; – autonomy; – capability to communicate; – possibility to be remotely reconfigured; – positioning accuracy; – data acquisition procedures compatible with those of on-shore observatories. the necessity for multidisciplinary and crossdisciplinary investigations was emphasised. three classes of observatories are currently identified: relocatable observatory – a system which is expected to be installed at a site for a limited period of time and capable of then being redeployed elsewhere. although cable connectivity to shore may be attractive for some applications, most probably this class of observatories will be supported by mooring with satellite or radio communications to shore installations. a communication/power riser may be deployed from the seafloor to the surface or alternatively a vertical acoustic link is only used and the bottom equipment is self powered. the relocatable observatory may support an array of devices on the seafloor which are acoustically, electrically or fibre-optically linked, as well as auvs and their docking stations. long-term observatory – an observatory which is expected to be installed at a site for decades or more. for some applications a mooring based installation may be convenient. but more often, this class of observatory will utilize an undersea cable from shore to provide power and communications. a long-term observatory will include a large number of nodes, each node supporting a range of devices. global/basin-scale observatory network – an observatory designed to provide basin or global coverage through a network of observatories. individual observatory nodes might be mooring or cable based. at the present time many relocatable observatories have been developed and some longterm observatories are operational. studies for global scale observatories were conducted, but there is as yet no network in operation. 4.2. geostar european effort was focused on geostar (beranzoli et al., 2000). more recently assem and orion projects allowed the concept of local network to be explored. these observatories in their present state belong to the class of relocatable observatories, but they may easily be transformed into long-term observatories. landers will play a vital role in these developments. targeted deployed landers with a wide range of instruments and sensors for physical, chemical, biogeochemical and biological parameters will be used in a single autonomous mode in relatively inaccessible areas (e.g., cold seep, hydrothermal vent and aphotic coral settings). they will also be used to test and qualify new 588 roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin sensors. right now bi-directional communication with the lander is possible by using an acoustic link through a modem. in fact no technical frontier exists between landers and observatories but the main differences are relevant to data management and permanent long term deployment. 4.3. assem the assem project (blandin et al., 2003) consists in developing optimised means to measure and monitor a set of geotechnical, geodesic and chemical parameters distributed on a seabed area in order to better understand the slope instability phenomena and to assess and possibly anticipate the associated risks. the means are studied and realised to deploy a selection of adapted sensors on a seabed area (some km2) and transmit their data to shore for exploitation. a modular design as well as standard connecting and installation interfaces allow the system to be easily configured to the site of interest, to add new sensors, and to replace components for maintenance. an array is composed of several nodes. each node (fig. 6) includes an electronic unit providing a set of enhanced sensors (pore pressure, methane, geodesy, tiltmeter, ctd, turbidity, currents, …) with the means to communicate with the external world through an underwater acoustic or cabled network and to locally store the produced data. alarms can also be generated by processing these data. the architecture is organised around an internal can/can open bus hosting sensors, communication and memory devices on a common transmission backbone. the software resources enabling a monitoring node to act as a network node (routing algorithms throughout the network, network configuration management, data transmission protocol and other network layers) are implemented in every electronic unit. alarms can be generated for example if a critical parameter, or a group of parameters, comes above a programmed threshold for a given time. this distributed architecture allows a monitoring node to be easily configured and a new function added without modifying the existing functions. the same modularity concept is applied to fig. 6. assem node. 589 from bottom landers to observatory networks the mechanical design. deployment and maintenance of the node imply a submersible or a rov. protection devices against trawling are used and the acoustic transmitter is installed on a special flexible arm. two complementary pilot experiments have been performed. the first took place at a site presenting a risk of slope instability in norway. the second took place in the gulf of corinth. compatibility between assem and orion communication systems (fig. 7) were demonstrated during this test. 4.4. outside europe – u.s.a. and japan in u.s.a., the hugo (hawaii undersea geo-observatory), an automated submarine volcano observatory, was installed on the summit of the undersea loihi seamount in october 1997 and connected to the shore via a 47 km long fibre optic cable. a failure appeared on 29th april 1998. no repair was possible. the instrumentation included a seismometer, a hydrophone and a pressure sensor. in september 1998, a permanent deep ocean scientific research facility – the hawaii-2 observatory or h2o – was installed on a retired at&t submarine telephone cable that runs between oahu, hawaii and the california coast. the facility consists in a seafloor junction box and scientific sensors located at 5000 m water depth near 28°n latitude, 142°w longitude, that is to say about half way between hawaii and california. the junction box draws 400 w of power from the cable to power both itself and user scientific instruments, and provides twoway communication through 8 digital ports with wet-mateable connectors. instruments may be connected to the junction box using a rov. initial instrumentation at the h2o site includes a broadband three-component seismometer, a short period geophone, a standard hydrophone and a pressure sensor. the h2o system is connected to the internet via the cable terminus on oahu and the university of hawaii. this offers marine scientists a new opportunity to deploy and operate instrumentation in the middle of the ocean. this long-term observatory in operation is the first seafloor node of the global seismographic network. in japan, eight cabled observatory systems are in operation (mikada, 2003). hatsushima island, in sagami bay, is the site of the first cabled observatory installed by the jamstec (japan’s marine sciences and technology center) in 1993, at a depth of 1174 m. this station consists of a ctd sensor, electromagnetic current meter, two video cameras, two ground thermometers, a seismometer and hydrophones. it is connected to the shore via a 8 km long fiber optic cable. data are directly transmitted to the jamstec centre in yokosuka. the area is a tectonically active region and a chemosynthetic community mainly composed of calyptogena (giant white clams) thrives on methane and sulfides found in water seeping from underground. during the swarm of earthquakes in march 1997 scientists were able to videotape the mudflow believed to have occurred following a submarine landslide on the western slope of the station. changes in ground temperature accompanied this event. it was the first long term mulfig. 7. orion observatory. 590 roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin tidisciplinary observatory in operation in the world. two other jamstec real-time cabled observatories are now also operating on the seafloor: – off muroto (120 km cable length); – off kushiro-tokachi (240 km cable). a first observatory was installed off muroto in 1995. it was completed in 1997 by another station to form a local observation network. this one includes two real-time stations immersed at 1290 m and 3570 m depths, connected to the shore by a 120 km fibre optic cable and fitted with two seismometers and two quartz pressure sensors (tsunami detection), and some «mobile» observatories. these mobile observatories are not cabled. they consist in a central unit which includes several sensors (among which a seismometer and a pressure gauge) and 4 satellite units. a satellite unit consists in a three-component digital obs with a storage capacity of 3 months. a synopsis of data collected in the central unit is transmitted to the shore every month via a messenger float sent to the surface to transmit data via argos. five other real time observatories (see fig. 7 in favali and beranzoli, 2006) are deployed by jma (japan meteorological agency), eri (earthquake research institute) and nied (national research institute for earth science and disaster prevention). moreover, two borehole autonomous observatories (fig. 8) are implemented on odp sites 1150 and 1151 (39°n, 143°e). 5. networked observatories (french mediterranean sea) another specific observatory family is that of neutrino detectors. one example is antares (french mediterranean sea). all these observatories are only long-term observatories often multidisciplinary ones. but, despite the fact that sometimes several units are deployed in the same area, they do not yet constitute a network (different operators for example). all the global/basin/plate-scale observatory networks are still at the state of proposals, with sometimes demonstration experiment. 5.1. neptune the goal of the us-canada neptune project (delaney, 2003) is to establish a coherent system of submarine high speed communicationcontrol links using fibre-optic cables to connect remote interactive experimental sites with landbased research laboratories (see fig. 6 in favali and beranzoli, 2006). the system will provide real-time flux of data to shore, interactive control over robotic vehicles on site and power to the instruments and the vehicles. the whole juan de fuca plate will be investigated during 20 or 30 years under a multidisciplinary approach: – subduction processes; – plate interiors; – spreading centers; – sediment transport; – upwelling and productivity; – biological diversity; – climate change. real-time two way communications at high rate (1-10 gbps) are required to support large numbers of seafloor instruments and to anticipate changes in oceanographic technologies. power between 50 and 100 kw can be derived from cables to instruments and robots.fig. 8. borehole observatory. 591 from bottom landers to observatory networks 5.2. mars the monterey accelerated research system (mars) cabled observatory will serve as the test bed for a state-of-the-art regional ocean observatory and represents the next step toward harnessing the promise of new power and communication technologies to provide a remote, continuous, long-term, high-power, large-bandwidth infrastructure for multidisciplinary, in situ exploration, observation, and experimentation in the deep-sea and an engineering test bed for neptune nodes. mars (fig. 9) was installed in 2005 in monterey bay off-shore the monterey bay aquarium research institute (mbari). it includes one science node on 62 km of submarine cable with expansion capability for more nodes in the future (see fig. 6 in favali and beranzoli, 2006). the science node provides 4 science ports and each port has a 100 mbit per second bi-directional telemetry channel. the node has the ability to deliver a total of 10 kw of power to the 4 ports. fig. 9. mars observatory. 592 roland person, yannick aoustin, jerome blandin, jean marvaldi and jean-françois rolin 5.3. venus in japan the venus project (see fig. 8 in favali and beranzoli, 2006) re-commissioned the second trans-pacific ocean cable between okinawa and guam. the goal of venus is to construct a multidisciplinary earth observation system. technology for coaxial electric and fiber optic cables that permits them to be connected and disconnected underwater will be developed. a technique for connecting transfer units at the seafloor using submersible or rov will also be developed. the geo-toc plans to install sensors in a relay unit. 5.4. esonet in europe esonet (priede et al., 2002) proposes a network of seafloor observatories around the european ocean margin from the arctic ocean to the black sea for strategic long term monitoring as part of a gmes (global monitoring for environment and security) with capability in geophysics, geotechnics, chemistry, biochemistry, oceanography, biology and fisheries. long-term data collection and alarm capability in the event of hazards (e.g., earthquakes) will be considered. esonet will be developed from networks in key areas where there is industrial seafloor infrastructure, scientific/conservation significance (e.g., coral mounds) or sites suitable for technology trials (e.g., deep water close to land). 6. conclusions in less than thirty years observation of the ocean has moved from pseudo random sampling to well controlled measurements. observatory networks will offer a new challenge by offering a complete set of continuous measurement and sampling at the same place. communication support and deployment techniques are ready. observatory networks will probably be deployed on a large scale (fig. 10) as soon as an extended set of reliable long term sensor packages is available. fig. 10. global scale observation network of pacific ocean. 593 from bottom landers to observatory networks references bagley, p.m., i.g. priede, a.j. jamieson, d.m. bailey, e.g. battle, c. henriques and k.m. kemp (2004): lander techniques for deep ocean biological research, underwater technol., 26 (1), 3-12. beranzoli, l., t. braun, m. calcara, d. calore, r. campaci, j.-m. coudeville, a. de santis, g. etiope, p. favali, f. frugoni, j.-l. fuda, f. gamberi, f. gasparoni, h.w. gerber, m. marani, j. marvaldi, c. millot, p. palangio, g. romeo and g. smriglio (2000): geostar the first european long term seafloor observatory, eos, trans. am. geophys. un., 81 (5), 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such survey is then enriched with information based on regulations and practical rules used during construction: the result of this step is the definition of homogeneus classes of buildings representing typical structures. the second step is based on the assessment of the seismic capacity of these structures using both refined models and parametric analysis: the outcomes can be extended to the defined classes to obtain vulnerability maps. a first application of the proposed strategy was made in catania and the results of that study on an urban area, characterized by rc buildings constructed in the ’60s and ’70s without seismic provisions, are briefly presented. 1. introduction the reduction of the seismic risk of existing structures has an high social and economical impact in italy; in fact the 43% of the italian territory is classified as exposed to seismic risk and, within this portion, only the 40% of structures have been designed according to seismic guidelines (de marco et al., 2000). the majority of the italian seismic areas were only officially recognized in the national regulation after 1981 (fig. 1). most of its buildings dates back to the late ’60s and to the sudden urbanization of that period. in 1961, 10.7 million apartments were counted, increasing to 22.5 millions in 1981; this number increased of only 4.5 million between 1981 and 1998 (censis, 1999). the seriousness of this potential risk is increased considering that, unlike the majority of european countries, italian urban planning politicies tend to preserve the landscape, avoiding the continuous reconstruction of buildings. moreover, in italy the quality of the construction process has been poor in comparison with other european countries, mainly for structures built from 1945 to 1980. this situation is due to illegal construction carried out with unsatisfactory criteria and poor quality materials. in addition, the entire design and construction process has been characterized by a poor quality control; rules and procedures relating to checks during construction have generally been neglected in the past. in this context, the development of a rational procedure for the seismic assessment of existing buildings is a priority toward the definition of actions required to reduce seismic risk, and the selection of the methodology for the analysis represents a crucial aspect. the idea of using for the buildings an approach that is different from mailing address: dr. edoardo cosenza, dipartimento di analisi e progettazione 5strutturale, università degli studi di napoli «federico», via claudio 21, 80125 napoli, italy; e-mail: cosenza@unina.it key words seismic assessment − vulnerability − buildings − concrete − steel 818 edoardo cosenza, gaetano manfredi and gerardo m. verderame those adopted for vulnerability studies of urban areas could be acceptable, even though not mandatory, as both goals and level of approximation may not be the same. the ec8 (eurocode 8, 1995) suggests the following assessment methods for vulnerability analyses: – level i: less than one hour of analysis, focusing only on simple information such as the building age and the structural configuration. – level ii: more detailed and longer than level i (about half day of analysis per building) including also some geometrical measurements. – level iii: much more accurate than the first two and including mechanical measurements. methods of fast assessment are generally preferred for the risk analysis in large areas (even with little information), whereas detailed methods are used for the evaluation of the seismic capacity of single buildings. the fast assessment is based on the observation of those characteristics of potential seismic damage; since the method is very conservative, it does not allow a detailed screening of all the most vulnerable buildings. depending on the structural configuration and the seismicity of the area, a vulnerability grade is associated to each building. such preliminary value is then changed based on other vulnerability properties characterizing the building and considered highly significant (i.e. irregularities in plan or elevation, unsatisfactory torsional behavior, soft story, etc.) (braga et al., 1982). the need for more detailed investigations is then deduced based on the total grade (representing an index of potential damage) and on the plan extension which gives a rough idea of the occupancy level. such methods allow for risk assessment on a large scale and are characterized by a high level of uncertainties. the fig. 1. areas exposed to seismic risk in the italian classification. a b 819 a new strategy for the seismic assessment of existing rc buildings philosophy for its use suggested by late codes (bia, 1996; atc, 1996) is to consider all of them as pre-screening methods toward the definition of the most appropriate procedure of analysis: those buildings showing deficiencies in both material and structural sides are then studied with more detailed methods. a more accurate approach for buildings was recently proposed by calvi (1999). it was derived from concepts and outcomes of detailed analyses on existing buildings and is based on the evaluation of allowable interstory drifts for each structural configuration. the method is analytical and needs few input data. despite these advantages, the uncertanites on the results are acceptable for large scale evaluations, while they are not adequate for single building assessment. this paper presents a new strategy for the seismic assessment of existing rc buildings. the proposed method is based on a two-step approach. a field survey of geometrical and mechanical buildings characteristics is first developed and enriched with information based on regulations and practise rules used during construction: the result of this step is the definition of homogeneus classes of buildings representing typical structures. the second step is based on the assessment of the seismic capacity of these structures using both refined models and parametric analysis: the outcomes of this analysis can be extended to the defined classes of buildings to obtain vulnerability maps. 2. a flow chart of the strategy the proposed new strategy of assessment is devoted to the vulnerability analysis of urban areas; it is based on some steps aimed at collecting basic information for the assessment of buildings and on the use of methodologies that are typical of the detailed analysis (fig. 2). the step related to the data acquisition is crucial since its unknowns could play a significant role in the final evaluation. a typical form allowing for recording the current status of a single building (structural system, configuration, materials) should be defined; a database on the mechanical properties of materials used in construction and a comprehensive recognition of rules and regulations in force when the building was constructed should also be carried out in order to have a set of information to use not only to implement but also to check acquired data. the survey form can be summarized as follows: – building selection depending on location and construction date. – properties concerning shape and size such as plan dimensions or number of storeys. − properties related to the structural configuration and type of elements such as columns, beams, stairs, slabs, etc. – characteristics of non structural elements such as infills. – level of observed damage. − condition of the building compared to the surrounding environment and buildings. the information, summarized in the proposed form, can be integrated by data obtained by checking libraries and technical offices, or interviewing contractors and engineers: all these data define design criteria and general rules for the details. the analysis of morphological and structural parameters will define homogeneous classes of buildings having the same morphology, structural configuration, number of storeys, materials and quality of construction. such classes are further specified considering ranges of construction ages characterized by the same code requirements, design criteria and pratical rules. within each category a representative virtual building is constructed based on the data obtained: in fact, its mechanical and geometrical properties are those of all buildings of that category, while its structural details are defined using the general rules previously classified for age and quality of construction. the specific study of existing buildings (among those already examined) which are similar to the representative construction confirms the dimensional properties of the structural elements as well as the reinforcement and the details (i.e. beam-column joints, etc.) characterizing the virtual structure. finally, each structural configuration, representative of a certain class, is studied by two different approaches to assess the seismic capacity of that class: the first is based on a parametric 820 edoardo cosenza, gaetano manfredi and gerardo m. verderame fig. 2. flow chart of the assessment strategy. 821 a new strategy for the seismic assessment of existing rc buildings analysis with simplified methods (i.e. limit analysis), the second uses the usual methods of detailed analysis (push-over or non-linear dynamic analysis). the parametric analysis starts from the representative building varying dimensional and mechanical properties within assigned ranges. the outcomes can be statistically treated in order to obtain the vulnerability characteristics of each class of structures needed to define scenarios of seismic risk. 3. the materials database a first step of the proposed strategy for the seismic assessment of the existing rc structures is the development of a statistical database on the mechanical properties of concrete and steel used. for concrete, the source of the analysis was the official laboratory of the department of construction sciences of the «federico ii» university of naples; test data on concrete cubes performed during the decade 1960-1970 were considered (verderame et al., 2001a). the data collection and the statistical processing were carried out on a population of strength having the same cement content equal to 300 kg/m3. the cement title was ignored since it was almost constant; the most frequent titles in the period 1960-1966 are 600 kg/cm2 and 670 kg/cm2, whereas between 1967 and 1970 the titles 730 kg/cm2 and 325 and 425 kg/cm2 (still existing) become common. the outcomes of the statistical analysis on the strength for 1960 and for the entire decade 1960-1970 are summarized below. figure 3a,b shows the diagram of frequencies for 1960; the average concrete strength of 1960 is equal to 27.07 mpa. table i details shape and scatter indexes as well as other parameters characterizing the samples of 1960. in order to extend the validity of the results of 1960, a statistical analysis of the data concerning the decade 19611970 was conducted. fig. 3a,b. concrete strength distribution (a) for the year 1960 and (b) for the decade 1960-1970. a b table i. strength distribution of concrete in the year 1960. analyzed data 4850 average strength [mpa] 27.07 standard deviation [mpa] 9.41 c.o.v. 34.76% 822 edoardo cosenza, gaetano manfredi and gerardo m. verderame the results confirm the main indexes of position and dispersion obtained only in 1960. concerning the average concrete strength, the trend appears to be constant as the average within the decade is 29.33 mpa compared to 27.07 mpa recorded in 1960. figure 3a,b shows the frequencies of the strength, while table ii reports the main indexes of shape and dispersion. a similar procedure was followed to define the types of steel produced and used during the decade 1960-1970 and to classify its mechanical properties: in this case, the results of tensile tests conducted during the reference period were considered (verderame et al., 2001b). the analyzed sample can be considered as representative of the entire area of campania and of the southern part of italy. the data were classified according to the code valid during the ’60 that distinguished three types of steel aq. 42, aq. 50 and aq. 60 based on yielding stress, ultimate stress and ultimate elongation. those three types of steel referred to special steel with ultimate stresses ranging between 42-50, 50-60 and 60-70 kg/mm2, respectively. the aq. 42 steel was the steel most adopted during the reference period; fig. 4a,b shows the frequencies of its yielding stress and ultimate elongation; table iii summarizes the main shape and dispersion indexes for the mechanical properties examined. fig. 4a,b. steel properties distribution for the year 1960: a) yielding stress; b) ultimate elongation. a b table iii. mechanical properties of steel aq. 42 in the year 1960. analyzed data: 729 f y f u f u /f y a10φ [mpa] [mpa] average value 325.4 467.1 1.440 28.8% max. value 397.4 499.9 1.812 39.2% min. value 265.0 420.1 1.096 20.0% standard dev. 23.1 21.7 0.086 0.031 c.o.v. 0.071 0.047 0.060 0.107 table ii. strength distribution of concrete in the decade 1960-1970. analyzed data 4955 average strength [mpa] 29.33 standard deviation [mpa] 9.08 c.o.v. 30.96% 823 a new strategy for the seismic assessment of existing rc buildings the high ductility of this steel can be emphasized, showing an average strain hardening equal to 1.440 and an average ultimate elongation on 10 diameters equal to 28.81%. in order to extend the results on 1960, a statistical characterization of steel aq. 42 was carried out for the period 1960-1970. 232 files were examined, recording geometrical (i.e. bar diameter) and mechanical (yielding and ultimate stress, ultimate elongation) parameters. such analysis confirmed the values obtained with reference to 1960; the investigated parameters were basically constant throughout the decade. the average yielding and ultimate stresses along the decade are 328.6 mpa and 470.0 mpa, respectively, just slightly higher than those recorded in 1960 and equal to 325.4 mpa and 467.1 mpa. therefore, the strain hardening ratio remained constant throughout the decade. table iv shows the main shape and dispersion indexes of mechanical properties of steel aq. 42 used during the period 1961-1970. 4. a numerical model for the seismic assessment of rc frames rc frames designed without seismic provisions have in many cases a structural behavior characterized by low available ductility and lack of strength hierarchy inducing undesirable global failure mechanisms. the lack of horizontal and vertical regularity and the high torsional deformation are also problems resulting in an unsatisfactory global behavior. details such as low confinement levels or insufficient anchorage of rebars can also represent potential critical zones, characterized by brittle mechanisms and low available ductility (cosenza and manfredi, 1997). in the case of the columns, the low confinement level (i.e. spacing of stirrups or presence of ties) can result in the flexural crisis at the bottom of columns located at low levels with instability of the compressive reinforcement, slippage of tensile rebars and crushing of the unconfined concrete. shear failure can occur in beams due to both insufficient steel reinforcement and increase in loads during the earthquake; for this specific problem, the non-linear interaction between shear and bending strongly influences the structural behavior. shear failure of short columns (i.e. stairs frames) can also be the cause of a brittle structural response. the behavior of joints represents another critical issue due to the lack of appropriate detailing. in the case of smooth rebars, the absence of hooks at the end can result in the pull out of the rebars; exterior joints are the most critical, even though the same type of failure could also involve interior joints with lap splices. shear failure of the panel can be observed if it is not confined and it is subjected to high shear stresses. in general, the frame deformation largely depends on the fixed end-rotation of the joint that is amplified in gravity load designed (gld) structures by high stresses in the reinforcement and poor anchorage. in some cases, another important contribution to the strength of the frame can come from the infill walls. the interaction between infill walls and bare frame should be considered since brittle failures can be originated by the concentration of high stresses transferred from the infills to both joints and columns. these remarks underline how many issues should be taken into account in the assessment of the seismic behavior of existing rc underdesigned frames. in general, all the brittle mechanisms can potentially play an important role; the models need to be able to predict them in order to provide reliable estimates of the seismic capacity. table iv. mechanical properties of steel aq. 42 in the decade 1960-1970. analyzed data: 232 f y f u f u /f y a 10φ [mpa] [mpa] average value 328.6 470.0 1.436 29.1% max. value 408.0 499.8 1.696 35.6% min. value 260.0 422.0 1.158 20.0% standard dev. 26.1 18.8 0.088 0.032 c.o.v. 0.079 0.040 0.061 0.111 824 edoardo cosenza, gaetano manfredi and gerardo m. verderame the push over analyses, used in the seismic assessment of rc existing structures, are carried out by a numerical tool developed at the university of naples. this model is able to predict the main mechanisms influencing non linear behaviour of reinforced concrete frames in terms of behavior mechanisms and material properties (cosenza et al., 2002a). the beam-column element is characterised by spread plasticity and distributed cracking: it belongs to the fibre model family. the mechanical properties of the transverse section are evaluated by considering the constitutive laws of materials. the hypothesis of perfect bond between concrete and steel is removed and the stress-slip bond constitutive law is explicitly introduced. such aspect allows for a more reliable assessment of the tension stiffening effect, for both smooth and deformed rebars, and avoids the approximations due to the assumption of the plastic hinge length. for the column, it is possible to consider the variation of axial forces due to lateral loads, and the related effects in terms of sectional strength and deformation capacity. the beam-column joint plays a significant influence on the structural response, in terms of strength (i.e. shear failure of the panel or pull out of the rebars) and deformation, due to concrete cracking and slippage of rebars. the proposed model for beam-column joints allows the fixed end rotation to be computed taking into account the bond between concrete and steel and the behaviour of the hooks. the influence of shear forces on the behaviour of the beams was modelled by priestley et al. (1994); such model is based on a reduction of the shear strength depending on the local ductility, expressed in terms of curvature varying with a linear trend. the introduced model represents an improvement of priestley’s because it directly determines the sectional ductility at any step of the analysis and then evaluates the shear strength of those sections located in the plastic regions. therefore, along with predicting ductile (i.e. flexural) and brittle (i.e. shear) fig. 5. rc frame modelling. 825 a new strategy for the seismic assessment of existing rc buildings failures, this method also determines failures characterised by low ductility due to the bendingshear interaction. the model for infill walls is based on the shear model by (fardis and panagiokatos, 1997). it takes into account the strength reduction due to the cracking of the panels and the post-strength degradation. it is based on four different steps: initial shear behaviour of the uncracked panel, behaviour of the cracked panel as equivalent strut, its instability after the maximum strength and final stage after complete failure characterised by constant residual strength (fig. 5). the proposed model was validated using different comparison with experimental tests. in fig. 6 a blind prediction of the pavia test (pampanin et al., 2001) is drawn, where is possible to appreciate its effectiveness (cosenza et al., 2002b). 5. the behaviour of a critical detail: the hook anchorage the anchorage of reinforcing steel in rc joint is a critical detail in the behaviour of underdesigned structures, especially when smooth rebars are used. in the following the research process for the implementation of the corresponding element in the model is described as example of the development of the numerical tool. as a first step, an exstended experimental program was performed to characterize the behaviour of straight rebars and hooked ends. the experimental program is divided into two phases, depending on the type of tests: beamtest for evaluation of bond properties of straight bars and service performances of hooked anchorages; pull-out tests to analyse the response of hooked anchorages both under service and ultimate load (fig. 7a,b). the first phase is characterised by 10 tests, 6 straight bars and 4 hooked anchorages, the main investigated parameter is the bar diameter, 12 and 16 mm, derived from the above mentioned review of design practice (fabbrocino et al., 2002a). the second phase is characterised by a more comprehensive set of investigated parameters: bar diameter, cast direction, concrete cover thickness, orientation of hooks and the type of fig. 6. blind prediction of the pavia test results. 826 edoardo cosenza, gaetano manfredi and gerardo m. verderame loading (monotonic or cyclic); at the present stage more than 20 tests have been carried out (fabbrocino et al., 2002b). in the following, an overview of the experimental results is reported to discuss the main aspects of load-slip behaviour of anchored smooth bars. reinforcements used are smooth rebars that are still available for secondary purposes in rc structures and have mechanical properties similar to steel classified as aq. 42 according to italian design code of ’60s; yielding stress is about 320 mpa, ultimate strain is equal to 430 mpa and ultimate uniform strain is about 20%. concrete was prepared according to typical mix rules of the ‘60s and tests on cubes 150 mm wide are used to define mean concrete strength. in the following, an overview of experimental results is presented. figure 8a summarizes the results of beam tests on straight bars. the shape of the bond stress-slip relationship points out that mechanical interaction is characterised by different phases, like the initial adhesion, and the final residual strength related basically to friction mechanisms. the bond stress for 16 mm rebars is about 2.35 mpa reached at a slip of about 0.20 mm; for 12 mm the values are 2.10 mpa and 0.07 mm respectively. some experimental results of modified pullout tests are reported in fig. 8b. the figure describe the mechanical behaviour of circular hooks and shows all the measured parameters. the tests are carried out up to bar failure, that is reaching strains higher than 20%, both bar strain-hook slip and stress-slip relationship for hooked anchorage show that the slip of the hook at yielding is constant and slip increases when strain-hardening is activated. furthermore, initial stiffness of the hook stress-slip relationship is very high, and even a stress range with no slip exists. a significant slip is obtained at bar yielding; it ranges between 0.80 and 2.49 mm and cannot be easily neglected when the deformation capacity of the anchored bar is concerned. this value increases when strain hardening is triggered and becomes about 8 mm at failure. after these, a numerical model was developed starting from the experimental results. the structural behaviour of straight bars anchored with hooks can be described using two different components: the straight part of bar, that is governed by the steel-concrete bond, and the hooked part, that presents a mechanical interaction with the concrete. this approach is briefly described in fig. 9, where the bond slip relationship is introduced for the straight part, while a specific force-slip relation, obtained from the experimental results, is introduced for the hook. fig. 7a,b. test setup: a) beam-test and b) pull-out. a b 827 a new strategy for the seismic assessment of existing rc buildings 6. buildings characteristics: in situ investigations in catania a first application of the proposed strategy was made in catania, where a large part of the city has been studied, defining different urban sectors, with homogeneous buildings characteristic in terms of structural typology and age of construction. particular attention has been devoted to reinforced concrete buildings constructed in the ’60s and ’70s. in each urban sector, a field survey of the buildings was performed, filling appropriate forms for each structure, that contains geometrical and mechanical data; this step is coupled with the acquisition of original design drawings and reports. the data are enriched with information based on regulations and practise rules used during the construction age and interview with practitioners and contractors: the results of this step is the definition of homogeneus classes of buildings representing typical structures. the elaboration of the data, related to an urban area examined in catania, showed that the majority of buildings have a rectangular plan and in particular the most common dimensions in the plan are about 40 × 10 square meters with three or four floors: a reference building has been chosen as representative of this class. in this particular application, the representative structure is similar to an existing building that was then assumed as typical building for the seismic assessment (mollaioli and verderame, 2000); in other words, the representative building was an existing structure. the building, described in the following, is representative of a structural typology very common in catania in the ’70s before the seismic classification and, therefore, it was designed only for gravity load. it is a four floor rc framed building with a rectangular plan with a transverse axis of symmetry; three floors are above the ground, the fourth is under ground. the difig. 8a,b. test results: a) bond stress-slip in the straight rebars; b) bond stress-slip in the hooked rebars. a b fig. 9. element for the hooked rebars. 828 edoardo cosenza, gaetano manfredi and gerardo m. verderame mensions of its plan are 41 × 10 m, while its height is equal to about 12 m (fig. 10). three floors have similar plan, while the fourth has a smaller area; its function is to cover the stairs volume and is neglected in the analysis. the columns distribution in the plan is quite regular; their dimensions range from a minimum of 30 × 50 cm to a maximum of 20 × 105 cm; the exterior beams have dimensions of 30 × 50 cm with a slab 20 cm thick. the geometric dimensions of the elements, the steel percentages, the structural masses and loads have been obtained from the original drawings. the constitutive relationship for the steel was introduced considering the average mechanical parameters obtained in the statistical analysis described in section 3; as regards the strength, it has been assumed that concrete and steel strength were equal to the design values of f ′ c = 25 mpa and f y = 380 mpa, respectively. 7. assessment of the seismic capacity of the catania building the seismic assessment of the reference building was developed following the atc40 (atc, 1996) and the seaoc (1998) procedures. the seismic capacity was evaluated using a pushover analysis. the push-over analysis was conducted using the numerical model mentioned above, which gives a reliable estimate of the contribution of the different mechanisms to the global behaviour, starting from the constitutive relationships of the materials. the push-over analysis was carried out on the analyzed structure fig. 10. geometry of the catania building: plane view. 829 a new strategy for the seismic assessment of existing rc buildings applying a pre-defined distribution of horizontal forces; in particular, the distribution was selected for the structure to respect the first mode of vibration. the analysis was conducted considering a 2d approach that is justified by the regularity in plan of the structures that minimizes torsional effects. the structural collapse of the reinforced concrete frame was achieved in a base column due to concrete crushing. figure 11 depicts the push over curve. using the method proposed by fajfar and gaspersic (1996), the equivalent sdof is calculated starting from the capacity curve obtained from the push over analysis and assuming a displacement shape indipendent on the drift level: in this case a displacement shape similar to the first mode of vibration is assumed. the dynamic properties of equivalent sdof in terms of base shear veq and mass meq can be defined starting from the corresponding properties of the mdof and the capacity curve can be obtained scaling the base shear-drift relationship obtained for the multistory structure. the definition of the equivalent sdof requires a linearization procedure of the push over curve that must be fitted with a bilinear curve, defined by the initial and the post-yielding stiffness. the linearization procedure also, defines the yielding strength and the ultimate ductility of the equivalent sdof. in this case, the procedure suggested by fajfar assumes that the initial stiffness of the equivalent system is equal to 50% of the stiffness of the mdof system, while the postyielding stiffness is assumed equal to 5% of the initial stiffness. table v summarizes the elastic period, the shear strength, the mass and the equivalent ductility obtained in the linearization procedure. the results were then compared with the inelastic spectra obtained using the synthetic fig. 11. push-over analysis of the catania building. table v. dynamic properties of the equivalent sdof for the catania building. t eq [s] v eq [kn] m eq [kns2/m] µ 0.86 590 709 4.42 830 edoardo cosenza, gaetano manfredi and gerardo m. verderame records generated by priolo (1999) for the destructive earthquake with a return period of 475 years expected in catania (fig. 12). the capacity of the structure is much lower than the expected actions. in order to obtain a demand comparable with the capacity it is necessary to reduce in average the expected records by about three times. 8. conclusions the assessment strategy presented in this paper is a procedure in progress that is continuously developed and refined considering experiences from field applications. the key issue of the procedure is the integration of information based on filed survey, regulations, practic rules used during the construction age, and interviews with practitioners and contractors. the method seems promising particularly for urban areas characterized by rc underdesigned buildings, in which all the brittle mechanisms can potentially play an important role and consequently refined numerical tools are required for a reliable assessment of the seismic capacity. the application to the city of catania evidences the potential of the procedure to achieve a quantitative assessment of the seismic capacity of rc underdesigned structures and shows their low capacity in comparison to seismic demand. references atc (1996): seismic evaluation and retrofit of concrete buildings, applied technology council (rep. n. atc 40), redwood city, california. bia (1996): the assessment and improvement of the structural performance of earthquake risk buildings, new zealand national society for earthquake engineering, june 1996, pp. 122. braga, f., m. dolce and d. liberatore (1982): a statistical study on damaged buildings and an ensuing review of the msk-76 scale, in 7th european conference on earthquake engineering, athens, 1982. calvi, g.m. (1999): a displacement-based approach for vulnerability evaluation of classes of buildings, j. earthquake eng., 3 (3), 411-438. fig. 12. spectral analysis of the catania building. elastic spectra and inelastic strenght spectra for fixed ductilities; thick line corresponds to µ = 4.42. 831 a new strategy for the seismic assessment of existing rc buildings censis (1999): xxxii rapporto sulla situazione sociale del paese (f. angeli ed., milano), pp. 653. cosenza, e. and g. manfredi (1997): some remarks on the evaluation and strengthening of underdesigned rc frame buildings, technical report, nceer-97-0003, state university of new york at buffalo, 157-175. cosenza, e., g. manfredi and g. verderame (1999): seismic assessment of rc structures: case studies in catania, in the catania project: earthquake damage scenarios for high risk area in the mediterreaneam, edited by e. faccioli and v. pessina, cnr, gruppo nazionale per la difesa dei terremoti, roma, pp. 255. cosenza, e., g. manfredi and g.m. verderame (2002a): a nonlinear model for underdesigned rc frames, in twelfth european conference on earthquake engineering, london, september 2002. cosenza, e., g. manfredi and g.m. verderame (2002b): seismic assessment of gravity load designed rc frames: critical issues in structural modeling, j. earthquake eng., 6, special issue 1, 101-122. de marco, r., m.g. martini, g. di pasquale, a. fralleone and a.g. pizza (2000): la classificazione sismica e la normativa sismica italiana dal 1909 al 1984, servizio sismico nazionale. eurocode 8 (1995): design provisions for earthquake resistance of structures, strengthening and repair of buildings, env 1998 1-4, cen, brussels. fabbrocino, g., g. verderame and g. manfredi (2002a): experimental behaviour of straight and hooked smooth bars in existing rc buildings, in twelfth european conference on earthquake engineering, london, september 2002. fabbrocino, g., g. verderame, g. manfredi and e. cosenza (2002b): experimental behaviour of smooth bars anchorages in existing rc buildings, in osaka congress, 13-19 october 2002, osaka, japan. fajfar, p. and p. gaspersic (1996): the n2 method for the seismic of rc buildings, earthquake eng. struct. dyn., 25, 31-46. fardis, m.n. and t.b. panagiotakos (1997): seismic design and response of bare and masonry-infilled reinforced concrete buildings part ii: infilled structures, j. earthquake eng., 1 (3), 475-503. mollaioli, f. and g.m. verderame (2000): caratteristiche degli edifici campione situati nell’area di catania, in comportamento sismico di edifici in cemento armato progettati per carichi verticali, edited by e. cosenza, cnr, gruppo nazionale per la difesa dei terremoti, roma, dicembre 2000, pp. 201. pampanin, s., g.m. calvi and g. magenes (2001): experimental test on a three storey reinforced concrete frame designed for gravity only, internal report. priestley, m.j.n., r. verma and y. xiao (1994): seismic shear strength of reinforced concrete columns, asce, j. struct. eng. , 120 (8), 2310-2329. priolo, e. (1999): 2d spectral element simulation of destructive ground shaking in catania (italy), j. seismol., 3 (3), 289-309. seaoc (1998): part 2, preliminary guidelines for performance based seismic engineering a forcedisplacement approach. verderame, g.m., g. manfredi and g. frunzio (2001a): le proprietà meccaniche dei calcestruzzi impiegati nelle strutture in cemento armato realizzate negli anni ’60, in x convegno nazionale «l’ingegneria sismica in italia», potenza e matera, 9-13 settembre 2001. verderame, g.m., a. stella and e. cosenza (2001b): le proprietà meccaniche degli acciai impiegati nelle strutture in cemento armato realizzate negli anni ’60, in x convegno nazionale «l’ingegneria sismica in italia», potenza e matera, 9-13 settembre 2001. vol50,6,2007 741 annals of geophysics, vol. 50, n. 6, december 2007 fied the geological environment of the mediterranean. given the low tidal excursion and the presence of geological indicators and archaeological remains, the mediterranean coasts are particularly suitable for the reconstruction of past sea-levels. a considerable number of studies on the relative sea-level (rsl) variations in this region have been published, based on regional analyses aiming at the interpretation of tectonic, eustatic, and glacio-hydro-isostatic contributions to sealevel change. for reviews of regional holocene observations, the reader is referred to flemming (1972) and pirazzoli (1991, 1996), while quantitative interpretations based upon models of glacio-isostatic adjustment have been recently proposed by lambeck and purcell (2005) and pirazzoli (2005). due to the relevance held by sealevel variations in the context of the present-day glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets paolo stocchi (1) (2) and giorgio spada (1) (1) istituto di fisica, università degli studi di urbino «carlo bo», urbino, italy (2) delft university of technology, delft, the netherlands abstract solving the sea-level equation for a spherically symmetric earth we study the relative sea-level curves in the mediterranean sea in terms of clark’s zones and we explore their sensitivity to the time-history of late-pleistocene ice aggregates. since the mediterranean is an intermediate field region with respect to the former ice sheets, glacioand hydro-isostasy both contribute to sea-level variations throughout the holocene. in the bulk of the basin, subsidence of the sea floor results in a monotonous sea-level rise, whereas along continental margins water loading produces the effect of «continental levering», which locally originates marked highstands followed by a sea-level fall. to describe such peculiar pattern of relative sea-level in this and other mid-latitude closed basins we introduce a new clark’s zone (namely, clark’s zone vii). using a suite of publicly available ice sheet chronologies, we identify for the first time a distinct sensitivity of predictions to the antarctic ice sheet. in particular, we show that the history of mid to late holocene sea-level variations along the coasts of se tunisia may mainly reflect the melting of antarctica, by a consequence of a mutual cancellation of the effects from the northern hemisphere ice-sheets at this specific site. ice models incorporating a delayed melting of antarctica may account for the observations across the mediterranean, but fail to reproduce the se tunisia highstand. mailing address: dr. giorgio spada, istituto di fisica, università degli studi di urbino «carlo bo», via s. chiara 27, 61029 urbino (pu), italy; e-mail: giorgio.spada@gmail.com key words sea-level variations – glacial isostasy – mediterranean sea 1. introduction the mediterranean sea-level variations result from complex geodynamical, geological, and metereological processes that span a wide range of time-scales. it is nowadays accepted that the melting of the holocene remote ice sheets and the subsequent global glacioand hydro-isostatic readjustment profoundly modi742 paolo stocchi and giorgio spada climate change, new geological investigations have been recently complemented by geomorphological and archaeological evidence on a regional scale (lambeck et al., 2004a). the latter have allowed us to better constrain the age of the paleo-sea-level indicators available in the tyrrhenian and to address the influence of tectonic motions upon the observed uplift. while sea-level signals caused by tectonic forces and other local mechanisms may exhibit a complex spatial and temporal variability (see e.g., carminati and di donato, 1999; for a regional case study), those associated with glacial isostasy are characterized by a smooth, longwavelength pattern that discloses various regions (or zones) sharing the same rsl signatures and named after clark (clark et al., 1978; clark and lingle, 1979). the mechanisms that determine the shape of the clark’s zones can be identified solving the sea-level equation that describes simultaneously the effects of ice and melt water loads, the gravitational interactions between the solid earth and the oceans, and the time-dependent earth’s viscoelastic response (farrell and clark, 1976). due to the limited computing resources, early determinations of the clark’s zones failed to resolve important details of their global pattern, including their contours and their possible fine structure in the mediterranean basin. the availability of highresolution solutions of the sea-level equation (e.g., lambeck, 1993, 1995) now allows us to scrutinize details of the shapes of the clark’s zones; in particular, the recent high-resolution approach of mitrovica and milne (2002) has shed new light on the mechanisms that determine their shape on a global scale. using the ice sheet chronology ice3g (tushingham and peltier, 1991) and assuming a moderate viscosity increase across the mantle, mitrovica and milne have shown that the bulk of the mediterranean and of other mid-latitude basins is presently subject to a sea-level rise of glacio-isostatic origin. taking advantage of the recognized sensitivity of sea-level predictions in the mediterranean to model parameters (e.g., lambeck and purcell, 2005), here we investigate the details of the mediterranean clark’s zones for various available ice-sheets chronologies, focusing on the role of remote ice sheets. the chronology of the antarctic ice sheet since the last glacial maximum (lgm) is still affected by large uncertainties. during the last two decades evidence supported by glaciological and glacio-isostatic adjustment modeling and ice core analysis have provided sometimes divergent estimates of its contribution to global sea-level (see e.g., kaufmann, 2002 and references therein). in the following, we will consider several plausible models for the holocene time-history of antarctica and we will evaluate the consequences on the rsl observations in the mediterranean. this is motivated by the preliminary results of stocchi et al. (2005b), who first noticed the sensitivity of rsl predictions for se tunisia and gulf of sirte to the time-history of remote ice sheets and particularly of antarctica. after a presentation of the methods, we will discuss the shape of clark’s zones in the mediterranean for a suite of ice models and we will qualitatively compare predictions to observations obtained from a public domain global database. to gain insight into the results presented, we will separately consider the effects of glacioand hydro-isostasy. in the results section, we will consider more recent and reliable rsl observations to address the role of remote ice sheets on firmer grounds. 2. methods 2.1. theory to define the contours of the clark’s zones for the mediterranean, we solve the sea-level equation (hereafter sle), the linear integral equation that describes the space and time variation of the geoid due to the mass exchange between the hydro-and the cryo-sphere and to the delayed response of a visco-elastic earth. paraphrasing farrell and clark (1976), the sle reads (2.1) where o is the ocean function (o=1 over the oceans and o= 0 elsewhere), z = os is sea( z)ρ+ − ( ) o g g g g z i i s z i s s eus s s ) ) ) ρ= − + + ω )7 a 743 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets the sle is solved according the «pseudospectral» approach of mitrovica and peltier (1991), by publicly available numerical tools (spada et al., 2004; spada and stocchi, 2006, 2007) based on the spatial discretization scheme introduced by tegmark (2002). here we neglect the effects of rotation on gia-induced sea-level variations and we assume a constant ocean function. however, for most regions the rigorous and accurate solution of the australian national university, which incorporates time-dependent shorelines and accounts for floating ice is recommended (lambeck et al., 2003). in the study region, the inclusion of a time-dependent ocean function may me particularly appropriate to model the sea-level rise in northern adriatic, as done by lambeck et al. (2004a). in our computations, the viscosity profile of the mantle is that implied in the «standard» deglaciation chronology ice3g of tushingham and peltier (1991, 1992), with a shallow upper mantle with viscosity ηsm = 1021 pa·s, a transition zone viscosity ηtz = ηsm , and a lower mantle viscosity ηlm = 2 ×1021 pa·s. following peltier (2004), we will refer to this viscosity profile to as vm1. the shear modulus and density profiles are the same as in cianetti et al. (2002). 2.2. ice sheet models in the following we will consider three distinct global ice sheets chronologies: ice1 (peltier and andrews, 1976), ice3g (tushingham and peltier, 1991), and ice5g (peltier, 2004). frames (a), (b), and (c) of fig. 1a-f show ice thickness for these models at time t = 18 kyrs bp. using ice sheets with increasing complexity and spatial resolutions will allow us to assess more robustly the sensitivity of the mediterranean data to the antarctic ice sheet and to review some previous results in the literature. minor constituents of the global ice sheets distribution, such as the alpine aggregate (lambeck and purcell, 2005; stocchi et al., 2005a), will not be considered here. frames (d), (e), and (f) of fig. 1a-f show equivalent sea-level (esl) as a function of time for the ice distributions considered in this study. here esl is defined as level change restricted to oceanic regions, s = = s(θ, λ, t) is sea-level change at colatitude θ, longitude λ and time t, i = i (θ, λ, t) is ice thickness variation, ρi and ρω are the density of ice and water, respectively, * denotes a convolution over time and over the whole surface of the earth, gs=gs /γ where gs is the (rheologydependent) sea-level green function, γ is mean surface gravity, and the over bar represents the average over the surface of the oceans. in eq. (2.1) seus represents the uniform sea-level change for a rigid, non-gravitating earth (gs≡ ≡ 0), with (2.2) where mice and ao represent the ice mass variation and the area of the oceans, respectively. in the analyses of section 3.3 it will be instructive to consider separately iceand oceanload induced sea-level variations. thus, following e.g., mitrovica and milne (2002), we write (2.3) where sice, seus, and soce account for glacio-isostasy, eustasy, and hydro-isostasy, respectively, with (2.4) and (2.5) where z represents the solution of eq. (2.1). due to the integral nature of the sle, however, soce depends from sice through z, so that the decoupling given by eq. (2.3) is artificial (mitrovica and milne, 2002; lambeck and purcell, 2005) and is used here for illustrative purposes. once the sle has been solved, the rsl variations at coordinates (θ, λ) are obtained by (2.6) where tbp and tp are time before present and present time, respectively. ( , , ) ( , , ) ( , , )rsl t s t s tbp bp pθ λ θ λ θ λ= − ( )g gs z zoce i s s) )ρ= − ( )g gs i iice i s s) )ρ= − s s s sice oceeus= ++ s a meus ice oρ = − ω 744 paolo stocchi and giorgio spada (2.7) where v is melt water volume. model ice1 describes the retreat of the holocene northern hemisphere ice sheets according to geological and geomorphological evidence, and stores ∼78 m of esl at the lgm (see fig. 1a, solid). it is characterized by a relatively low spatial resolution, being specified on a grid of 5°×5° spherical quadrilaterals; the thickness of each element varies at steps of 2 kyrs. details on how this chronology is incorporated within the pseudo-spectral method is given e.g., by spada and stocchi (2007). esl a vi oρ ρ = ω differently from ice1, model ice3g (fig. 1b) has been built to improve the fit to the global rsl data in the near field assuming the vm1 rheological profile and a 120 km thick elastic lithosphere (tushingham and peltier, 1991, 1992). while ice1 assumes stationary southern hemisphere ice masses, ice3g accounts for the melting of antarctica and south america, and consequently stores a larger water volume at the lgm, with an esl of ∼113.5 m (see fig. 1c). ice3g has an improved resolution compared to ice1, being composed by a set of disc-shaped elements with half-amplitude of ∼1°, with thickness varying at steps of 1 kyr. antarctica stores ∼27 m of esl at the lgm and its volume is constant until 9 kyrs fig. 1a-f. global ice thickness distribution according to ice1 (a), ice3g (b) and ice5g (vm1) (c) at 18 kyrs bp. this epoch corresponds to the lgm for ice1 and ice3g, while in ice5g the lgm is reached 21 kyrs bp. the contribution to esl of major constituents of the ice models are shown in the right frames as a function of time, where fen, and nam stand for fennoscandia and laurentide, respectively. with a3 and a5 we label the antarctic components of ice3g and ice5g, respectively. a b c d e f 745 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets bp; its deglaciation occurs between 9 and 5 kyrs bp, when the northern hemisphere ice sheets had already lost most of their mass. to fit the constraints of the glaciological studies of huybrechts (2002) and denton and hughes (2002), in the recent ice5g model of peltier (2004) the total esl of antarctica has been reduced (by ∼ 50%) with respect to ice3g (ice5g can be downloaded from http://www.sbl.statkart.no/projects/pgs/). such reduction is accompanied by an increased esl for the north american ice sheet to fit constraints from vlbi and gravity observations (argus et al., 1999). the spatial resolution of ice5g, whose esl is shown in fig. 1f as a function of time, is improved relative to ice35, being defined on a gaussian grid with a spacing of ∼ 0.7°. differently from ice3g, ice5g describes the whole history of ice thickness during the last glacial phase (i.e., since 122.0 kyrs ago), and assumes that the lgm occurred 21 kyrs bp. here ice5g has been implemented assuming the vm1 viscosity profile described above. with the purpose of evaluating the sensitivity of mediterranean clark zones to the timehistory of remote holocene ice aggregates, in the computations of section 4.1 we will also consider alternative chronologies for the antarctic ice sheet, which differ from those implicit in ice3g and ice5g. 3. clark zones here we will first qualitatively characterize the rsl variations in the mediterranean sea studying the shape of the clark zones and then we will compare results based on ice1, ice3g, and ice5g with observations available from the database of tushingham and peltier (1993). the pattern of sea-level change in the mediterranean will also be discussed by considering separately the ocean and the ice-load components soce and sice (see eq. (2.3)), and evaluating the role of individual regional components of the two ice sheet distributions. an in-depth analysis of the sensitivity of the mediterranean rsl variations to the ice sheets chronology will be presented in the results section with the aid of further sea-level indicators. 3.1. pattern of clark zones for the mediterranean sea as shown in fig. 2a-e, the solution of the sle discloses complex patterns of rsl change across the mediterranean that strongly depend upon the assumptions about the chronology of the far-field ice sheets. model ice3g (frame a) implies a late-holocene highstand that marks the end of deglaciation (5 kyrs bp) along most of the mediterranean coasts, with the exception of southern italy, greece and part of the coasts of algeria, lybia and of the southern levant, while submergence is predicted in the bulk of the basin. the regions of emergence and submergence are separated by narrow transition zones in which the sea-level nearly follows the eustatic curve. the pattern of sea-level change shown in fig. 2a might be mistakenly interpreted as a match of clark’s zones vi and ii, which characterize the far-field continental shorelines as a consequence of «continental levering» and the collapsing forebulge regions, respectively (walcott, 1972; clark et al., 1978). however, as shown by mitrovica and milne (2002), lambeck and purcell (2005) and lambeck et al. (2004b) (and independently confirmed by our computations), submergence in the central mediterranean sea mainly stems from hydro-isostasy, the direct glacio-isostatic effects of the northern hemisphere aggregates, characteristic of zone ii, being confined to the coasts of france and northern italy. in general, clark zone vi shows up as bands of offshore sea-level rise and onshore sea-level fall, with the size of the submerging areas that show significant spatial variability with the tendency to increase for concave coast lines (this is clearly visible in the map of fig. 1a-f of mitrovica and milne 2002). when the shorelines close on themselves to define a relatively small basin, as in the mediterranean, the area of submergence tends to cover the central portion of the basin possibly leaving narrow regions of highstand on shore (this is also observed for the black sea and in other mid-latitude basins in the global map of mitrovica and milne). to characterize the peculiar rsl pattern predicted for mid-latitude basins, where zone vi is manifest as a central submergence region contoured by a narrow highstand zone, we propose the name of 746 paolo stocchi and giorgio spada clark’s zone vii. the dominance of the influence of hydro-isostasy in the mediterranean region when all the ice aggregates of ice3g are considered is apparent from inspection of fig. 1a-f of mitrovica and milne (2002), in which the pattern of ṡoce is broadly similar that of the total signal ṡ, where the dot indicates the time-derivative computed at present time. such pattern has remained qualitatively stable in the last 5 kyrs, after the end of deglaciation of major ice sheets (mitrovica and milne, 2002). when model ice1 is considered (fig. 2b) the portion of zone vii characterized by highstands is narrowed significantly, being only present along the coasts of spain, northern morocco, tunisia and lybia. furthermore, the two transition regions of fig. 2a are not observed. differently from in ice3g, in ice1 it is assumed that the antarctic ice sheet is stationary during the holocene, but the two models also differ for details of the melting chronologies of the north american and fennoscandian ice sheets (see fig. 1a-f). to understand the origin of the distinct patterns in fig. 2a,b, fig. 2c we consider the ice3g-a3 chronology, in which ice3g is deprived of its antarctic component. the pattern obtained is strikingly similar to that of ice1 (b), indicating that the existence of lateholocene highstand in the mediterranean mainly result from the melting of the antarctic ice sheet implemented in model ice3g. when antarctica is built into model ice1 (fig. 2d, model ice1+ +a3), the configuration of clark zones closely matches that of ice3g (a). significant difference between the results obtained for ice3g and ice1+a3, visible along the northern adriatic and tyrrhenian coasts, can be attributed to differences in the time-histories of the northern hemisphere components of ice3g and ice1, with the relatively contiguous fennoscandian ice sheet that is likely to play a major role. in terms of clark zones, for model ice1+a3 zone ii counteracts the highstand of zone vii and merges with its core approximately north of the 42°n parallel. in fig. 2a-e. shape of mediterranean clark zones for models ice3g (a), ice1 (b), ice3g-a3 (c), ice1+a3 (d), and a3 (e). rsl variations expected within each zone, qualitatively shown in the legend, include a monotonous submergence, a late-holocene emergence with a highstand of a few meters, and possibly two narrow transition zones. in all computations, we have adopted rheological profile vm1. a c e b d 747 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets fig. 2e we only account for the effects of a3, the antarctic component of ice3g. in this case, zone vii disappears leaving an ubiquitous highstand that, as we have verified, extends north of the 45°s parallel. up to now, we have discussed the shape of clark zones in the mediterranean using quite dated models for the global history of deglaciation. for instance, ice1 is known to overestimate the extent and thickness of ice in the baltic region (e.g., nakada and lambeck, 1988), while ice3g does not match some well constrained rsl curves, such those relative to southern france (see our discussion below). in addition, neither ice1 or ice3g satisfy the far-field sea-levels for the entire period since the last glacial maximum (nakada and lambeck, 1988). in spite of their limitations, these two dated ice models have allowed us to appreciate how the shapes of the clark zones evolve with increasing spatial resolution of the surface load. however, the question of the validity of our results with respect to more recent models remains open. to address this point, we implemented ice5g (peltier, 2004) within our code, and repeated all the above computations. the results, not shown here, indicate that clark zones of ice5g match those of ice1+a3 very closely. a more detailed comparison of the outcomes of ice5g with those of ice3g is considered, in terms of rsl curves, in the following section. 3.2. synthetic rsl curves for the mediterranean figure 3a shows the location of mediterranean sites pertaining to the publicly available fig. 3a,b. a) location of the rsl sites considered in the analysis of section 3.2, namely roussillon (1), marseilles (2), civitavecchia (3), termoli (4), catania (5), messenia (6), sw turkey (7), cyprus (8), beirut (9), jaffa (10), djerba (11), and algiers (12). with the exception of djerba (filled triangle), all the sites shown here belong to the rsl database of tushingham and peltier (1993). b) full set of rsl observations available for the sites in (a) during the last 10 kyrs, where filled symbols denote observations from north africa and the levant sea. a b 748 paolo stocchi and giorgio spada database (see ftp://ftp.ncdc.noaa.gov/pub/data/ paleo/paleocean/relative_sea_level/) of tushinham and peltier (1993) (henceafter tp), while rsl indicators are displayed in fig. 3b. recent and more reliable observations, for which information on the exact meaning of geomorphological indicators used to derive sea-level curves can be more easily obtained from the literature, will be considered in section 4.2 below. focussing on the tp observations at this stage is useful since they are available on a mediterranean scale, and thus are useful for a global (although qualitative) comparison with predictions across the whole region. the tp sites shown in fig. 3a are quite evenly distributed across the mediterranean, but since only one indicator is available for north africa (i.e., algiers, site 12), here we also consider the site of djerba (tunisia), based on the work of jedoui et al. (1998). according to fig. 3b, the only indications of a sea-level highstand in the late-holocene come from beirut (9), djerba (11) and algiers (12). while rsl indications from beirut and algiers are possibly influenced by local tectonic deformations (meghraoui et al., 2004), those from tunisia mainly reflect glacio-isostatic rsl variations (morhange and pirazzoli, 2005). we observe that sites indicating an highstand are only situated along the southern and eastern continental mediterranean coasts: this is qualitatively consistent with the pattern of clark’s zones obtained for models ice1, ice1+a3, and ice5g while it appears to be at variance with predictions based upon ice3g (see fig. 2a-e). to assess the role of far field ice sheets, in fig. 4a-i we compare individual rsl indicators from the tp database with model calculations based on ice3g (solid lines), ice1 (dashed), ice1+a3 (dotted), a3 (dash-dotted), and ice5g (grey solid). consistently with the qualitative study of fig. 2e, rsl variations driven by the melting of a3 show a clear highstand at 5 kyrs bp in all of the sites considered, with a peak amplitude varying between ∼2 m in roussillon and ∼1 m in messenia. for models ice3g and ice1+a3, the curves are characterized by a knee at 5 kyrs bp caused by their antarctic components that with the exception of the aegean sites of messenia (d) and sw turkey (e) corresponds to a sea-level highstand. when ice1 is considered instead (dashed), rsl predictions clearly show a monotonous sea-level rise that broadly agrees with the trend of tp indicators from france, italy and sw turkey. in roussillon (a), marseilles (b) and civitavecchia (c), the offset between ice1+a3 and ice3g results reflects differences in the time-history of the northern hemisphere ice sheets. at lower latitudes, with increasing distance from the former margins of fennoscandia, the gap between solid and dotted curves is reduced. in the case of djerba (h), ice1 implies a highstand of a few centimeters at 5.0 kyrs bp, and the predictions by a3, ice1+a3, and ice3g almost overlap, to indicate that rsl observations at this site are virtually sensitive to the sole antarctic component of the ice sheets distributions. although evidence in favor of a late-holocene highstand at djerba is weak due to the large (and statistically dubious) data uncertainties, the sensitivity of north africa rsl observations to the chronology of antarctica merits further investigations in section 4.2.3. predictions based on ice5g (grey solid curves) are generally intermediate between those of ice1 and ice3g, as a direct consequence of the significant reduction of the total esl of antarctica relative to ice3g. even a cursory inspection of fig. 4a-i reveals that ice3g provides globally a poor fit to the tp rsl indicators across the mediterranean. conversely, with the sole exception of djerba (h) and of sites belonging to tectonically unstable regions such as beirut (mahamoud et al., 2005; morhange and pirazzoli, 2005) and algiers (12) (meghraoui et al., 2004), ice1 and ice5g broadly match the rsl trends, but this should be tested by a more rigorous misfit study. the results suggest that the antarctic component is the main responsible of the failure of ice3g, but it is also possible that the rheological profile vm1 is not fully suitable to describe the rsl variations in this region. we will return to these issues in sections 4.2 and in our future investigations. 3.3. ocean and ice-induced rsl variations since the mediterranean is moderately distant from the former late-pleistocene ice sheets, 749 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets fig. 4a-i. observed and synthetic rsl for some of the sites of fig. 3a. predictions are obtained by vm1 and models ice3g, ice1+a3, ice1, a3, and ice5g. rsl data and error bars are taken from the tp database with the exception of djerba (jedoui et al., 1998). since the tp rsl indicators are not calibrated, this comparison has mainly a qualitative character. in djerba the melting of antarctica provides virtually the whole signal during the last 6 kyrs (compare solid with dash-dotted curve in frame h). the ice-induced rsl variations in this region are not expected to dominate the ocean components, as it is the case in the near-field clark’s zones i and ii (lambeck and purcell, 2005). to address this point, using the rheological model vm1, in fig. 5 we separately consider the rsl variations of glacioand hydro-isostatic origin at roussillon, jaffa and djerba, representative of the northern, eastern, and southern coasts of the mediterranean basin, respectively (see fig. 3a,b). dashed and dotted lines show oceanand ice-induced rsl components, obtained by eq. (2.6) with s = s oce and s = s ice, respectively. since seus= 0 after the end of deglaciation, in the time window considered rsl variations are only given by oceanand ice-load effects. a b c d e f ihg 750 paolo stocchi and giorgio spada for both ice1 and ice3g, the trend of the ice-induced component of rsl (dotted) is constant across the mediterranean, being mostly determined by long-wavelength deformations driven by distant sources. however, they are clearly sensitive to the assumed time-history, with a sea-level fall for ice3g (top frames) and a sea-level rise for ice1 (bottom). as discussed below, this diametrically opposite trend is to be attributed to the sea-level changes driven by the melting of antarctica in ice3g. differently from the ice term, the ocean term in fig. 5 (dashed) shows a significant spatial variability, with a clear sea-level fall in roussillon and djerba, and a moderate sea-level rise in jaffa. such variability is caused by the sensitivity of this component of rsl to local effects related to the irregular shape of the shorelines (e.g., mitrovica and milne, 2002; lambeck and purcell, 2005). the trend of the hydro-isostatic term is the same for both ice sheets, but its amplitude tends to be larger for ice3g, due to the effect of the extra ocean load driven by antarctica. in fig. 6 we have repeated the computations above for the site fig. 5. iceand ocean-load induced components of rsl for the sites of roussillon, jaffa, and djerba (the sites location is shown in fig. 3a), using ice3g (top) and ice1 (bottom). since in ice1 the end of deglaciation occurs 8 kyrs bp (see fig. 1a-f), its eustatic component vanishes in this time window (not shown in figure). fig. 6. iceand ocean-load induced components of rsl for the site of djerba (see map of fig. 3a) obtained using model ice5g (vm1). 751 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets of djerba using the recently published deglaciation chronology ice5g (peltier, 2004). the reduced esl of antarctica in ice5g reduces the highstand amplitude relative to ice3g (compare with fig. 5c); the emergence following the peak is now controlled by the ocean component of rsl, to indicate a dominating role of continental levering at this site and for this specific surface ice load. in fig. 7 we consider individual contributions from major late-pleistocene ice sheets (north america, fennoscandia, and antarctica), neglecting other minor constituents. the results illustrate how the distance from the former ice sheets affects rsl curves. for roussillon, the sea-level rise produced by the relatively nearby fennoscandian component of ice3g is counteracted by antarctica and north america to produce a marked late-holocene highstand. this is only barely visible in jaffa, since during the last 5 kyrs the melting of the northern hemisphere aggregates almost exactly compensates the effects from the southern hemisphere. finally, for djerba, in the last 6 fig. 8. contributions to rsl due to individual components of ice5g (vm1) at the site of djerba (see fig. 3a). fig. 7. contributions to rsl due to individual components of the ice3g (top) and ice1 (bottom) global ice sheets distributions at the sites of roussillon, jaffa, and djerba (see fig. 3a). 752 paolo stocchi and giorgio spada kyrs, the north american and fennoscandian ice sheets have produced equal but opposite trends for both ice1 and ice3g that make the rsl observations from the coasts of tunisia particularly sensitive to the deglaciation of antarctica, as anticipated in section 3.2. in fig. 8 the computations of fig. 7c are repeated using ice5g. in djerba we still observe a substantial cancellation of the effects of north america and fennoscandia that shows the robustness of the ice3g results and motivates further data acquisition from tunisia, which is underway. 4. results assuming ice3g and vm1 as a priori models for the late-pleistocene ice sheets and for the earth’s rheological profile, respectively, in the previous sections we have confirmed that antarctica significantly affects the holocene sea-level variations in the mediterranean. however, from the results obtained (see in particular fig. 4a-i) it is apparent that ice3g qualitatively provides a poor fit to the field observations in this area, which are better reproduced by ice1, which assumes a stationary antarctic ice sheet throughout the holocene, and by ice5g, which implies a sizeable reduction of the esl of antarctica relative to ice3g. to evaluate to what extent recent mediterranean data might be useful to constrain the history of far-field ice sheets, here we will first review some existing deglaciation histories of antarctica and then we will implement them in model ice3g. our predictions will be compared with field observations from various locations digitalized from the compilation of pirazzoli (1991, 1996) and from more recent sources. with respect to the tb database, employed so far in our study to describe qualitatively the rsl trends across the mediterranean, the sea-level indicators used in section 4.2 below provide a better spatial coverage and are more reliable in a number of aspects. 4.1. three ice models for antarctica in the following, we will modify the original ice model ice3g by including the three distinct melting chronologies for antarctica that in fig. 9 are denoted by s, g, and d, respectively. all of them contribute 14 m of total esl (i.e., one half of the ice3g value). considering the distance of the mediterranean from antarctica, this ice sheet is modeled by a disc load of rectangular cross-section having an half-amplitude of 20 and a thickness of ∼355 m at the lgm. to fit the ∼113 m lowstand predicted by ice3g in the far-field sites at the lgm (see fig. 1d), we have increased the volume of the northern hemisphere ice3g components at this epoch keeping their isochrons unaltered. the global chronologies so obtained are named ice3g(s), ice3g(g), and ice3g(d), respectively. evidence in support of the chosen esl for antarctica comes from a number of sources. in his review on the volume of antarctica at the lgm, bentley (1999) proposed an esl in the range of 6.1-13.1 m that matches the 12 m estimate of huybrechts (1992) based on glaciological modeling. through a 3d thermo-mechanical modeling, recently huybrechts (2002) has derived an esl in the range of 14-18 m, consistent with the value of 14 m obtained from geologic constraints by denton and hughes (2002), whereas from an ice-dynamical fig. 9. time-histories of esl for the antarctic component of ice3g according to the s, g, and d models described in the text. they are constituted by a simple disc-shaped ice element that stores an esl of 14 m before the beginning of melting (12 kyrs bp). predictions for modified ice3g chronologies including these antarctic components are shown in fig. 11 and 12. 753 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets approach bintanja et al. (2002) estimated that antarctica has contributed ∼5 m of esl since the lgm. these estimates are consistently smaller (by at least a factor of 2) than former values based on the classical reconstruction of denton and hughes (1981), and of the ice3g figure (∼27 m). as we will show in the following, a reduced esl for antarctica improves the fit with the rsl observations in the mediterranean, mainly because the amplitude of the lateholocene highstand is significantly reduced. the simplest of the three time-histories considered for antarctica (solid curve labeled by s in fig. 9) is characterized by a constant rate of melting between 12 and 5 kyrs bp, when geological evidence indicates that deglaciation was complete (goodwin, 1996). however, it has been suggested that some additional melt water was still added to the ocean during the last 6 kyrs. since the main source of this lateholocene additional global sea-level rise of ∼3 m is assumed to be the antarctic ice sheet (nakada and lambeck, 1988), we have also implemented a delayed (d) melting phase (dotted). this second chronology approximately follows s until 7 kyrs bp but subsequently the rate of deglaciation decreases until the esl vanishes 1 kyr bp. the d chronology provides a late-holocene water release sufficient to increase sea-level by about 3 m since 6 kyrs bp (lambeck and bard, 2000), while its contribution since 3 kyrs bp is less than 1 m (fleming et al., 1998). support for a delayed model of deglaciation also comes from the work of stone et al. (2003), who analyzed surface exposure ages of glacial deposits in the west antarctic ice sheet. various pieces of evidence indicate a midto late-holocene sea-level highstand in the south pacific, indian ocean and in parts of the northern atlantic and pacific oceans. the subsequent sea-level fall is generally attributed to «ocean siphoning» (mitrovica and milne, 2002). however, based on glaciological and geological evidence, goodwin (1998) suggested that a late-holocene increase of the antarctic ice volume may be partly responsible for this sea-level fall. according to this hypothesis, the expansion of mountain glaciers, ice sheet margins and the thickening of the ice sheet interior could account for ∼1.0 m of the sea-level fall on mid-oceanic islands. hence, differently from d, the g chronology, shown by a dashed line in fig. 9, is characterized by a lateholocene re-advance causing a general eustatic sea-level fall of ∼1.0 m since 5 kyrs bp, with ∼0.7 m of sea-level fall between 4 and 2 kyrs bp (goodwin, 1998). 4.2. rsl data and predictions 4.2.1. french coasts from the collection of sea-level field observations of pirazzoli (1991), we have borrowed data from nine sites along the golfe du lion and the coasts of corsica, ranging from roussillon (1) to south corsica (9); sites locations are shown by filled circles in fig. 10. in the original compilation of pirazzoli (1991), only two sites show some indication of a lateholocene highstand, with an emergence of ∼2.0 m near cap romarin between 5 and 4 kyrs bp (aloisi et al., 1978), and a highstand in the range of 2 to 4 m ∼4 kyrs bp deduced from undated beach deposits in the nice area and in the surroundings (dubar, 1987). from a reexamination of the available record, lambeck and bard (2000) concluded that along the french coast holocene sea-levels have never exceeded the present level, the highstand being marked in the erosional notch at cap romarin of pleistocenic age (laborel et al., 1998). some further evidence against a late-holocene highstand comes from archaeological excavations of the ancient harbor of marseilles, which have provided a new set of high-precision data for the past 4 kyrs (morhange et al., 2001), and from the preservation of half-submerged paleolithic paintings on a wall of the cosquer cave near marseilles (vouve et al., 1996), showing that during the holocene the sea-level never never exceeded its present-day level. in our previous calculations of fig. 2a-e, ice models ice3g and ice1+a3 predicted a lateholocene emergence along the mediterranean coast of france. as shown in fig. 11, different results are obtained when the three modified antarctic chronologies of fig. 9 are implemented 754 paolo stocchi and giorgio spada in ice3g. the solution for ice3g(s) (solid lines) show a sea-level highstand of ∼1 m at 5 kyrs bp both in roussillon and in the rhone delta region. due to the reduced esl of ice3g(s) and to the 2 kyrs anticipation of the melting inception of its antarctic component (see fig. 9), the maximum transgression is diminished by about a factor of 2 with respect to ice3g (compare with fig. 4a-i). the amplitude of the highstand diminishes eastward until it vanishes at port cros (site 6); in corsica (sites 7-9) the highstand is canceled by the submergence that characterizes clark zone vii in the bulk of the mediterranean. model ice3g(s) generally provides a poor fit to the data. the lateholocene re-advance of the antarctic ice sheet, implied in the ice3g(g) chronology (dashed lines), enhances the development of highstands. those with largest amplitude (∼1.5 m) are observed at roussillon (1) and in the rhone delta (2). the predictions for this hypothetical chronology of antarctica systematically stand above those based upon ice3g(s) (solid) and are in clear disagreement with observations from southern france. ice3g(d) (dotted) implies a smooth sea-level rise through the late holocene, similar to that predicted by ice1 and ice3g-a3 (see fig. 2c,d) and a subsequent barely visible sea-level fall ending between 2 and 1 kyrs bp. the general agreement of ice3g(d) predictions with sea-level observations from this region is apparent. fig. 10. sites from southern france and corsica (sites 1-9), the tyrrhenian coast of italy (sites 10-16), northern adriatic (17), and tunisia (18) for which field observations of past sea-level variations are available from the compilation of pirazzoli (1991) (filled circles), and lambeck et al. (2004a) (open). predictions for these rsl sites are shown in figs. 11 and 12, respectively. 755 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets f ig . 11 . r s l o bs er va ti on s fr om s ou th er n f ra nc e an d c or si ca a cc or di ng t o th e co m pi la ti on o f p ir az zo li ( 19 91 ), to w hi ch t he r ea de r is r ef er re d fo r th e or ig in al s ou rc es . d at a un ce rt ai nt ie s ar e ta ke n fr om s ou rc es w it h th e ex ce pt io n of r ou ss il lo n (1 ) an d th e r ho ne d el ta ( 2) , fo r w hi ch a q ua li ta ti ve er ro r ba nd i s gi ve n. s ol id , da sh ed , an d do tt ed c ur ve s pe rt ai n to m od el s ic e 3g (s ), ic e 3g (g ), an d ic e 3g (d ), re sp ec ti ve ly . f ig . 12 . r s l o bs er va ti on s an d un ce rt ai nt ie s fr om t he t yr rh en ia n co as ts o f it al y (s it es 1 016 ) an d fr om t he n or th er n a dr ia ti c (1 7) a cc or di ng t o l am be ck e t a l. (2 00 4a ). p re di ct io ns a re b as ed o n th e th re e va ri an ts o f m od el i c e 3g a lr ea dy c on si de re d in f ig . 9. 11 1 2 756 paolo stocchi and giorgio spada 4.2.2. tyrrhenian and northern adriatic coasts of italy the pioneering investigations of alfieri and caputo (schmiedt, 1972) have shown the importance of the archaeological remains as sealevel indicators of past sea-levels along the tyrrhenian coasts of italy. for this region, they estimated a sea-level rise of ∼1.7 mm yr−1 between 600 bc and 100 ad; in particular, dating roman fish tanks and submerged harbors, they showed that between 100 years bc and 100 years ad the sea-level was ∼1.0 m lower than present. subsequently, the radiocarbon-based rsl curve of antonioli and frezzotti (1989) evidenced, for the southern coasts of lazio, a sea-level similar to the present one between 7 and 5.4 kyrs bp, followed by a slight oscillation below the present level (pirazzoli, 1991), while the cumulative tyrrhenian sea-level curve of alessio et al. (1994) confirmed that during the holocene the sea-level was never above the present level. from the recently published highquality data of lambeck et al. (2004a) we selected those relative to the tectonically stable tyrrhenian sites marked by open circles in fig. 10. rsl observations in fig. 12 clearly indicate a monotonous submergence since 8 kyrs bp with no evidence of highstands. with the exception of the northern adriatic (site 17), ice3g(d) (dotted) systematically overestimates rsl observations, but correctly reproduce their trends. in qualitative agreement with the results obtained for southern france (see fig. 11), chronologies ice3g(s) and ice3g(g) do not improve the fit with the observations. it should be remarked, however, that since the ice models scrutinized here do not account for the effects of melting of the alpine ice-sheet, a bias may be present in our computations (see stocchi et al., 2005a). in addition, in the case of northern adriatic, assuming a constant ocean function may provide a poor approximation due to the shallow water environment. more refined modelizations can account for time-evolving shorelines (e.g., lambeck et al., 2004a). 4.2.3. gulf of gabes, tunisia paskoff and sanlaville (1983) proposed a tentative sea-level curve for south tunisia in the last 8000 years (see also pirazzoli 1991). the fluctuating rsl curve of paskoff and sanlaville, relative to djerba (site 18) and reproduced in fig. 13 by filled squares, shows a maximum emergence of reproduce ∼2.0 m between 6 and 5 kyrs bp and a second minor highst and ∼3 kyrs bp. the subsequent study of jedoui et al. (1998) evidenced two fossilized bioclastic beaches at different elevations (see the diamonds in fig. 13a). the older palaeobeach deposit (6.44.3 years bp) is found at an elevation of about 40 to 100 cm above the present sea-level, while the younger, dated 1850 years bp, at present sea-level. uncertainties on the interpretation of past sealevel indicators from this area have been discussed by pirazzoli (1987) and lambeck et al. (2004a). recently morhange and pirazzoli (2005) published a tentative sea-level curve for southern tunisia based on new indicators collected between the gulf of gabes and the libyan border. the curve, reproduced by open circles in fig. 13. rsl observations for djerba according to various sources, and predictions using ice models ice3g(s) (solid line), ice3g(g) (dashed), ice3g(d) (dotted), and ice5g (solid grey). a dash-dotted curve shows results obtained assuming that the antarctic ice sheet model d of fig. 9 is the only active load since the lgm. 757 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets fig. 13, shows a transgression peak of 1.86±0.11 m between 6 and 5 14c kyrs bp. in fig. 13 we compare the rsl observations from djerba with predictions based on the three modified ice3g chronologies described in section 4.1. while ice3g(s) (solid lines) and ice3g(g) (dashed) support the late-holocene submergence suggested by the data of morhange and pirazzoli (2005) and paskoff and sanlaville (1983) in the last ∼5 kyrs, ice3g(d) shows a monotonous emergence in contrast with observations. however, since reliable confidence bands for dotted curves are lacking, a rigorous evaluation of misfit is not possible. provided that uncertainties reported by jedoui et al. (1998) are reliable, fig. 13 shows that none of the three time-histories considered is indeed in contrast with the observations. while ice5g (grey curve) is broadly consistent with jedoui et al. (1998), it does not account for the neat highstand suggested by paskoff and sanlaville (1983) and morhange and pirazzoli (2005), nor with its timing. in every instance, from the results obtained for this region it is clear that improved rsl observations for the coasts of tunisia could significantly contribute to constrain the time-history of antarctica in the last 6 kyrs. the cancellation of the effects from the northern hemisphere ice sheets that occur here once again is manifest observing that ice3g(d) (dotted) basically reproduces the effect from the sole antarctic component of this ice sheet (d, dash-dotted). 4.2.4. the levant sea, israel evidence for late-holocene sea-level higher than the present in israel has been reported by sneh and klein (1984) and by raban and galili (1985) who derived two similar sea-level curves for the site of dor, showing sea-level fluctuations of over 2.0 m of amplitude (pirazzoli, 1991) as portrayed in fig. 14b. in a subsequent study, nir and eldar (1987) proposed a fig. 14a. sites along the coasts of israel, ranging between haifa (site 21) and yavne yam (24). 758 paolo stocchi and giorgio spada curve characterized by small oscillations for the last 2.5 kyrs. from the investigation of submerged archaeological remains along the continental shelf of israel, between haifa and atlit, galili et al. (1988) obtained the rsl curve shown by a dashed line in fig. 14b, indicating a monotonous and smooth sea-level rise until ∼1 kyrs bp (pirazzoli, 1991). as shown in frame (c), archaeological evidence from the sites of tel nami (20), dor (21), michmoret (23) and yavne yam (24) indicate that 6 kyrs bp sea-level was ∼4 m lower than today and that it reached the present-day level between 3 and 2 kyrs bp (sivan et al., 2001). the observational limits derived from the coastal water wells in caesarea maritima (site 22) have extended the record of the late-holocene sea-level change to 1300 ad (sivan et al., 2004) and suggest that during the byzantine period sea-level was higher by ∼30 cm than today. the rsl curve obtained using ice3g(g) (dashed curve in fig. 14c) shows a highstand between 5 and 4 kyrs bp, followed by emergence and lastly by a negative oscillation. when the ice3g(s) and the ice3g(d) ice chronologies are considered (solid and dotted lines, respectively), the predicted late-holocene sea-level curves define a narrow band broadly consistent with the archaeological evidence until ∼2 kyrs bp. these latter predictions do not match the more recent sea-level data and thus appear to be inconsistent with the higher than present sea-level between 2 and 1 kyr bp observed at caesarea maritima according to sivan et al. (2004). 5. conclusions the main results can be summarized as follows: i) the pattern of sea-level change in the mediterranean can be described by means of two types of rsl curves. the first denotes a rising sea-level through the late-holocene, while the second exhibits a late-holocene highstand that implies a sea-level fall in the last ∼5 kyrs. the extent of the so-called clark’s regions characterized by these distinct sea-level patterns is strongly dependent upon the assumptions about the time-history of the late-pleistocene ice sheets surrounding the mediterranean. when the ice3g chronology is employed, a well developed highstand region is expected along the mediterranean coasts, while the submergence region covers the bulk of the basin. such peculiar pattern for closed basins has been named here as «clark’s zone vii». the assumption of a stationary antarctic ice sheet and the enhanced effect of the melting of fennoscandia disrupt zone vii when ice1 is employed, leaving highstand zones along the indented coast lines of the alboran sea and se tunisia. this clearly shows the significant role played by the melting of antarctica upon the holocene sea-level variations in the mediterranean. fig. 14b,c. in (b) we have reproduced tentative rsl curves from the existing literature, while in (c) we compare the data of sivan et al. (2001, 2004) with results based on the same models previously considered in fig. 13. b c 759 glacio and hydro-isostasy in the mediterranean sea: clark’s zones and role of remote ice sheets ii) using models ice3g and ice5g, we have shown that along the coasts of se tunisia (i.e., at djerba) the effects due to the melting of fennoscandia are almost exactly counterbalanced by that of north america. such fortuitous cancellation makes the highstand of ∼2 m predicted in this region only sensitive to the effects of the remote antarctica ice sheet. a highstand is indeed suggested by both available observations and tentative rsl curves at djerba, but their resolving power is not sufficient to constrain its amplitude unequivocally. incorporating within ice3g a suite of plausible models for the melting of antarctica during the last 6 kyrs, all characterized by a sensibly reduced esl at the lgm, it has been possible to fully enlighten the sensibility of rsl observations from southern tunisia to the details of the time-history. a late-holocene highstand of ∼ 1 m in this region is predicted when antarctica is assumed to melt at a constant rate between 12 and 5 kyrs bp, and its amplitude is enhanced (nearly doubled) if a late ice re-advance is assumed. on the contrary, a delayed melting of antarctica until 1 kyr bp is responsible for a regular sea-level rise since 6 kyrs bp. only an improved spatial coverage and sampling frequency of the rsl data, made however difficult by the large tidal excursions in the gulf of gabes (sammari et al., 2006), could help to put tight bounds on the details of the melting chronology of antarctica. model ice5g(vm1) is unable to account for the amplitude and timing of the highstand suggested by the recent observations of morhange and pirazzoli (2005). this motivates further data acquisition in se tunisia and a refinement of existing ice sheets models. iii) an assessment of the effects of mantle viscosity is beyond our purposes here. however, we have verified that an increase in ηlm from 2×1021 to 1022, a value more appropriate according to a number of authors (see e.g., nakada and lambeck, 1989) has a significant influence on the predicted rsl curves for the mediterranean. in particular, this implies the disappearance of the late-holocene highstand from the coasts of southern france, that would improve the agreement with rsl observations from this region. in general, an increase in lower mantle viscosity limits the variability of the predicted rsl curves corresponding to different assumptions about the chronology of antarctica. assuming a high-viscosity lower mantle, the confirmation of the existence of a late-holocene highstand in djerba would indicate for antarctica an esl consistent with the value implicit in ice3g, whereas an essentially stationary sealevel would be suggestive of a sensibly reduced esl. acknowledgements we thank fabrizio antonioli for a constructive review and maurizio bonafede for suggestions and encouragement. florence colleoni, kurt lambeck, jeffrey donnelly, and liviu giosan are acknowledged for discussions and encouragement. anna radi is acknowledged for help in the compilation of bibliography. ps has benefited from an exchange program at the department of geological sciences of brown university at providence (ri), that we acknowledge for having provided support and a stimulating environment useful for the preparation of this work. the code for solving the sle (selen, see http://flocolleoni.free.fr/ selen.html/) is freely available (spada and stocchi, 2007) and can be requested to gs (e-mail: giorgio.spada@gmail.com). the figures were drawn using the gmt public domain software (wessel and smith, 1998). research funded by miur (ministero dell’università, dell’istruzione e della ricerca) by the prin2006 grant «il ruolo del riaggiustamento isostatico postglaciale nelle variazioni del livello marino globale e mediterraneo: nuovi vincoli geofisici, geologici, ed archeologici». references alessio, m., l. allegri, f. antonioli, g. belluomini, s. improta, l. manfra and m. preite martinez (1994): la curva di risalita del mare tirreno negli ultimi 43 ka ricavata da datazioni su speleotemi sommersi e dati archeologici, mem. descr. carta geol. d’italia, lii, 261-276. aloisi, j.c., a. monaco, n. planchais, j. thommeret and y. thommeret (1978): the holocene transgression in the golfe du lion, southwestern france: paleogeographic and paleobotanical evolution, geogr. phys. quat., 32 (2), 145-162. antonioli, f. and m. frezzotti (1989): i sedimenti tardo760 paolo stocchi and giorgio spada pleistocenici e olocenici compresi nella fascia costiera tra sabaudia e sperlonga, mem. soc. geol. ital., 42, 321-334. argus, d.f., w.r. peltier and m.m. watkins (1999): glacial isostatic adjustment observed using very long baseline interferometry and satellite laser ranging geodesy, j. geophys. res., 104 (b12), 29077-29094. bentley, m.j. 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(received august 1, 2007; accepted january 19, 2008) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 1 the role of the ingvterremoti blog in information management during the earthquake sequence in central italy maurizio pignone1, concetta nostro2, alessandro amato2, carlo meletti3 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, grottaminarda (av), italy 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di pisa, pisa, italy concetta.nostro@ingv.it abstract in this paper, we describes the role of the ingvterremoti blog in information management during the first part of the earthquake sequence in central italy (august 24-september 30). in the last four years, we have been working on the ingvterremoti blog in order to provide quick updates on the ongoing seismic activity in italy and in-depth scientific information. these include articles on specific historical earthquakes, seismic hazard, geological interpretations, source models from different types of data, effects at the surface, and so on. we have delivered information in quasi-real-time also about all the recent magnitude m≥4.0 earthquakes in italy, as well as on the strongest events that occurred in the mediterranean and in the world. during the 2016, central italy earthquake sequence, the ingvterremoti blog has continuously released information with three types of posts: i) updates on the ongoing seismic activity; ii) reports on the activities carried out by the ingv teams in the field and by other working groups; iii) in-depth scientific articles describing some specific analysis and results. all the blog posts have been shared automatically and in real time on the other social media of the ingvterremoti platform, also to counter the bad information, and to fight rumors. these include facebook, twitter and the ingvterremoti app on ios and android. as well, both the main ingv home page (www.ingv.it) and the ingv earthquake portal (http://terremoti.ingv.it) have published the contents of the blog on dedicated pages that were fed automatically. the work done day by day on the ingvterremoti blog has been coordinated with the ingv press office that has written several press releases based on the contents of the blog. since august 24th, 53 articles were published on the blog, and they have had more than 1.9 million views and 1 million visitors. the peak in the number of views, which was more than 800,000 in a single day, was registered on august 24, 2016, following the m 6.0 earthquake. http://www.ingv.it/ http://terremoti.ingv.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 2 i. introdution hen an earthquake occurs, people look for easy and fast information on the web and social media. the ingv websites deliver information in quasireal-time about all the recent earthquakes in italy, the strongest events in the mediterranean and in the world through the ingv home page (http://www.ingv.it) and the earthquake list web page (http://cnt.rm.ingv.it) of the ingv centro nazionale terremoti (cnt, national earthquake centre). in the aftermath, people look for more indepth news, more specific and detailed information on the historical earthquake catalog, the seismic hazard and the evolution of the seismic sequence. during the 2009 l’aquila emergency and even more in the 2012 emilia emergency, as well as classical information on websites, many people looked for information and updates on social networks. for this reason, from 2009 the ingv started to test different social media, such as youtube, twitter, facebook, and developed an application for ios and android, to release earthquake information promptly. for all of these media, we observed relevant increases in the number of views and downloads corresponding to the important seismic events, when the attention was high [amato et al. 2012, nostro et al. 2012]. social media and websites have proven to be very important for information sharing during crises [earle et al. 2011; bruns et al. 2012; wendling et al., 2013]. for this reason, in the days after the may 20, 2012, emilia main shock, we decided to open also a new blog (on wordpress), called ingvterremoti, to provide quick updates and in-depth scientific information. providing timely information is particularly important when seismic sequences last for several weeks and are characterized by several earthquakes with magnitude m≥4. at the same time, we have worked to provide fast, but scientifically sound, information, constantly updated and distributed throughout the territory, also to counter the bad information, and to fight rumors which always arise during such crises. all the information published on the blog are shared on the ingvterremoti social media [pignone et al. 2012]. the emergence of new communication channels (facebook, twitter, blogs, among others) represents an opportunity to spread information to different segments of the population in times of emergency. these technologies have the potential to prevent communication breakdown through reliance on just one platform and thereby to reinforce the diffusion of authoritative information. ii. the seismic data management in real time a blog is a kind of website where the content is called "posts" and by default, they are displayed from newest to oldest -on the homepage. all the blog posts arrive to people have subscribed to and are automatically shared on social media and other websites. that means every time a new post is published, many readers are w annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 3 immediately alerted. this is exactly what you need especially during a seismic sequence. the ingvterremoti blog was created using the well-known wordpress platform (http://ingvterremoti.wordpress.com), a free cms hosting platform that makes it easy for anyone to publish online, also because it is not necessary to download software or manage a web server. wordpress.com has hundreds of themes with many functionalities and widgets and provides a few services including stats and the sharing function, which automates sharing to a few social networks. the blog was opened on may 21, 2012, initially for internal testing, and then open to the public on may 29, 2012, during the 2012 emilia seismic sequence [pignone et al. 2012]. we initially decided not to allow comments on the blog, but then we opened the ingvterremoti facebook page where the blog posts are shared with comments. the official language is italian, as the blog is mostly devoted to people living in the italian region. the blog ingvterremoti is a place where people could find updates and explanations on the ongoing seismic activity in the dynamic posts and information in the static pages, the former being the predominant part. the dynamic section contains mostly posts organized in different categories. the static section includes pages about the ingv (chi siamo/who we are), earthquakes in italy (i terremoti in italia), seismic hazard (la pericolosità sismica), seismic monitoring (il monitoraggio sismico), and faq & glossary (faq & glossario). when a magnitude m≥4.0 earthquake occurs, immediately we publish a post with the location of the earthquake, the magnitude and the list of the cities close to the epicenter. then, when other earthquakes occur in the same area, we create a new category with the name of the zone where the seismic sequence is taking place. in these four years, we have created, in addition to the category associated with the emilia earthquake, nine categories associated to as many seismic sequences occurred in italy. some of them are linked to earthquakes with magnitude ≥ 5.0 (the 2012/10/25 earthquake (m5.2) in the province of cosenza, “pollino sequence”; the 2013/06/21 event (m5.1) in the province of massa carrara, “lunigiana sequence”; the 2013/12/29 earthquake (m5.0) in the province of caserta, “monti del matese sequence”) and the others to sequences in città di castello e gubbio (pg, umbria), in adriatic sea, in garfagnana (toscana) and in the province of frosinone (lazio). in the last four years, we have worked to develop the blog with a series of static information pages and several thematic rubrics as, for instance, “italy earthquakes: earthquakes of the month” (italia sismica: i terremoti del mese) and the earthquakes in history (i terremoti nella storia). since its publication on line, in 52 months on the blog were published 464 articles that have had more than 14.5 million views and 3 million visitors. the peak in the number of views, which was more than 800,000 in a annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 4 single day, was registered on june 3, 2012, following the m 5.1 earthquake in the po plain region and on august 24, 2016, following the m 6.0 earthquake in central italy. it is interesting to note that during “quiet times”, ie without relevant earthquakes, the blog has a monthly average of about 50,000 views. iii. the 2016 central italy earthquake the ingvterremoti blog has continuously released information on the 2016 central italy seismic sequence with three types of posts: updates on the ongoing seismic activity; reports on the activities carried out by the ingv teams in the field and other working groups; in-depth scientific articles describing some specific analysis and results. figure 1: map showing the spatial and temporal evolution of the seismic sequence published september 3, 2016 on the ingvterremoti blog. red: last hour; orange: last 24 hours; yellow: last 72 hours; blue previous earthquakes (since august 24th). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 5 the updates have followed regularly the evolution of the sequence, starting at 04:30, soon after the 24 august m6.0 main shock occurred at 03:36, with a release every 12 hours approximately in the first two weeks and every 24 hours in the following two weeks. every article has been framed with a map showing in colors the temporal evolution of the seismic activity in the previous days and, of course, with symbol size proportional to magnitude. the chosen symbology has been conceived to provide an immediate impact of the ongoing activity, and is analogous to the symbology adopted in the cnt web pages (http://cnt.rm.ingv.it) and in the ingvterremoti app on ios and android. besides the map, each article contains a report on the recent activity, with some numbers and statistics on the number of events divided by magnitude classes. a table 1: the number of posts, weekly views (visite) and visitors (visitatori) of the ingvterremoti blog, during 5 weeks, from august 22, 2016 to september 25, 2016. figure 2: the number of weekly views (visite) and visitors (visitatori) of the ingvterremoti blog, during 13 weeks, from july 11, 2016 to october 3, 2016. the week 22-28 august (in red) shows the peak in the number of views (1,358,808) and visitors (680,889). http://cnt.rm.ingv.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 6 table of the strongest aftershocks was always inserted. the frequency of updates has decreased for one post every 12 hours in the first 12 days, to one every 24 hours later. the second type of articles includes the report of the ingv working groups (on sar, gps, strong motion data, etc.) including the ingv emergency groups (sismiko, emergeo, quest, emersito) with thematic maps, description of the activities, pictures, etc. the third type of posts includes scientific descriptions of some particular aspects of the ongoing research, such as source models from different type of data, effects at the surface, geological interpretations, shake maps, videos with wave propagation and epicentral locations, etc. in particular, some “hot topic” has needed a specific in-depth article, when the public has shown to be confused on a particular issue. it happened, for instance, when a controversy on the magnitude of the august 24 event was raised by some bloggers, with many requests and criticism. the publication of an article explaining why there can be several different magnitude estimates by different institutions, using different methods and data, was particularly useful as demonstrated by the use of the contents by mainstream media. all the blog posts have been shared automatically and in real time on the other social media of the ingvterremoti platform: facebook, twitter and the app on ios and android, through which it is possible to receive a notification with the news posted in the blog. as well, both the main ingv (www.ingv.it) and the ingv earthquake portal (http://terremoti.ingv.it) have published the contents of the blog on dedicated pages which were fed automatically. the work done day by day on the ingvterremoti blog has been coordinated with the ingv press office that has written several press releases based on the contents of the published posts. as a consequence, maps and figures from our posts were largely used by tv news, newspapers and websites. some of the articles included links to the “story maps”, a tool used in order to collect in a simple and interactive way all the information related to a specific phenomenon. in the case of the central italy earthquake, these included the historical and recent seismicity, the seismic hazard map and so on. since august 24th, on the blog were published 53 articles that have had more than 1.9 million views and 1 million visitors. the peak in the number of views, which was more than 800,000 in a single day, was registered on august 24, 2016, following the m 6.0 earthquake in central italy. iv. discussion an conclusion it is well known that the attention of people towards a specific issue tends to decrease very rapidly after the event. in the immediate aftermath of the strong earthquake, a broad discussion was raised and maintained on the media, also thanks to a diffuse participation of scientists in television and radio shows, and to several interviews published on the newspapers, on the web and so on. the attention was focused first on the dramatic aspects of the earthquake, then on the scientific explanation of the seismic activity, http://www.ingv.it/ http://terremoti.ingv.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 7 and soon after on the need of rapid and effective reconstruction and on the search for responsibilities of the disaster. however, as soon as the news with dramatic images disappear (collapsed buildings, desperate people, lucky recoveries, etc.), the interest on the earthquake has vanished quickly. for this reason, we have tried to keep the attention high keeping on publishing updates and scientific analyses, although with lower frequency. this tendency is well evident if we compare the number of earthquakes with the views at the ingvterremoti blog. the visits reached more than 800,000 on august 24th, figure 3: comparison between the daily number of earthquakes and views (light blue, visitors are in blue) of the ingvterremoti blog, from august 22, 2016 to september 20, 2016. in august 24 there is the peak in the number of views (829,503) and visitors (547,923). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7261 8 then decreased in the following days, as expected, but remained at high level (around 50,000-100,000 for several days, to decrease dramatically after september 5, only ten days after the main shock. this is not unexpected, but shows very clearly how the people’s interest for the seismic problem is strongly conditioned by the ongoing activity. a similar trend can be seen in other websites dealing with the earthquake, and in traditional media (a. cerase, personal communication). for this reason, it is extremely important to keep working in scientific communication on earthquakes, both during a sequence like this and during “quiet times”. in the future, we will try to analyze the impact of our communication differently and more systematically so that we can offer a service appropriate to the different situations. references amato, a., l. arcoraci, e. casarotti, r. di stefano and the ingvterremoti team (2012). the ingvterremoti channel on youtube. annals of geophysics, 55 (3), 403-408; doi: 10.4401/ag5546. bruns, a., j. burgess, k. crawford and f. shaw (2012). #qldfloods and @qpsmedia: crisis communication on twitter in the 2011 south-east queensland floods, brisbane, arc centre of excellence for creative industries and innovation; http://cci.edu.au/floodsreport.pdf crescimbene, m., f. la longa and t. lanza (2012). the science of rumors, annals of geophysics, 55 (3), 421-425; doi: 10.4401/ag-5538. earle, p.s., d.c. bowden and m. guy (2011). twitter earthquake detection: earthquake monitoring in a social world, annals of geophysics, 54 (6), 708-715; doi: 10.4401/ ag-5364. moretti, m., c. nostro, a. govoni, m. pignone, f. la longa, m. crescimbene and g. selvaggi (2011). l'intervento del centro operativo emergenza sismica in occasione del terremoto del 2009 a l'aquila, quaderni di geofisica, 92, ingv, 29 pp. nostro, c., a. amato, g. cultrera, l. margheriti, g. selvaggi, l. arcoraci, e. casarotti, r. di stefano, s. cerrato and the may 11 team (2012). turning the rumor of the may 11, 2011, earthquake prediction in rome, italy, into an information day on earthquake hazard, annals of geophysics, 55 (3), 413-420; doi:10.4401/ag5559. pignone, m., c. nostro, amato, a., e. casarotti, c. piromallo, (2012). the ingvterremoti blog: a new communication tool to improve earthquake information during the po plain seismic sequence. annals of geophysics, 55 (4), 837842; doi: 10.4401/ag-6179. wendling, c., j. radisch and s. jacobzone (2013), “the use of social media in risk and crisis communication”, oecd working papers on public governance, no. 24, oecd publishing, paris. http://dx.doi.org/10.1787/5k3v01fskp9s-en annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 1 estimation of the ground shaking from the response of rigid bodies filomena de silva1, stefania sica2, francesco silvestri3, stefano aversa4 1 department of civil, architectural and environmental engineering, university of napoli federico ii, napoli, italy; filomena.desilva@unina.it; 2 department of engineering, university of sannio, benevento, italy 3 department of civil, architectural and environmental engineering, university of napoli federico ii, napoli, italy 4 department of engineering, university of napoli parthenope, napoli, italy abstract the paper illustrates and compares simplified approaches to interpret the mechanisms of damage observed on rigid bodies in the cemetery of amatrice, after the first strong-motion event (august 24, 2016, mw=6.0) of the seismic sequence occurred in central italy. the final goal of the work is to link the observed movements of the fallen objects to specific characteristics of the ground motion recorded on site. i. introduction fter a strong earthquake, information on ground motion characteristics can be indirectly derived from the observation of the damage suffered by objects with quite simple geometry and known degrees of freedom, for which the collapse mechanism is straightforward and simple to back-figure. this methodology, borrowed from the archeoseismology, takes into account not only the inventory and amount of the occurred damage, but also the failure mechanisms and the pattern of displacement of the fallen objects. the back-analysis of these latter aspects may provide useful information on the amplitude of ground motion parameters (if no seismic stations are available nearby) or on other aspects of the seismic shaking (directivity, impulsive motion, etc.). previous works on such an issue may be found in yegian et al. (1994), athanasopoulos (1995) and lanzo et al. (2010). conversely to inhabited areas, most of the injured cemeteries are not closed soon after a strong earthquake or if seriously damaged they may be inspected in a second stage, before any reconstruction or restoration. the above mentioned literature showed that their inspection soon after the main shock, or even later, may be very useful to have additional insight into the ground motion occurred on site. during the geer reconnaissance operations (stewart et al., 2016) after the mw 6.0 first strong motion event (august 24, 2016, local time 3.32 a.m.) of the seismic sequence in central italy, many interesting cases of fallen gravestones, funerary monuments and statues were observed in the cemetery of amatrice a annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 2 (figures 1a). this is located about 400 m southeast and at approximately the same elevation of the main gate of the historical center (figure 1a), which suffered a local imcs=10.5 (galli et al., 2016). all the objects apparently fell down during the first event, and most of them along the ns direction (figures 1b-e). among the rigid objects showing regular geometry and clear damage pattern, an obelisk was selected in order to develop a simple analysis to relate the observed failure mechanism to the ground motion occurred at the site. (a) (b) (c) (d) (e) figure 1. location of the cemetery and of the seismic station at amatrice (a), damages to statues, monuments and graves in the amatrice cemetery (b-e). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 3 ii. the case study of the marble obelisk the selected case study concerns the failure of the marble obelisk shown in figure 2. the obelisk has a shape of a truncated pyramid and it is approximately 1.10m high. the width of the square cross section tapers with the height from 0.27m at the base up to 0.13m at the top. the base section was attached through a thin layer of mortar to a stone basement, about 0.80m high. the detected geometry and the height of the gravity center are reported in table 1. during the reconnaissance, the obelisk was found lying on the ground, apparently overturned along the ns direction (figure 2a), while the weak mortar connection failed (figure 2b). table 1. geometrical parameters of the obelisk. lower width upper width height gravity center height slenderness ratio b b h hg  (m) (m) (m) (m) (/) 0.27 0.13 1.10 0.49 1.78 (a) (b) figure 2. damaged obelisk in the amatrice cemetery (a), particular of the base section (b). iii. failure conditions neglecting the vertical component of the seismic acceleration, the forces acting on the obelisk during the earthquake (figure 3) are its weight, w, and a horizontal inertia force h=ma, corresponding to the seismic action on the mass, m. due to the high stiffness of the marble, both the basement and the obelisk behave as rigid bodies: thus, the horizontal acceleration of the obelisk can be considered equal to the ground acceleration, a(t), at the site of the cemetery, from the beginning of the shaking until when it detached from the basement n annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 4 and failed. since the likely failure mechanisms are brittle, it is possible to estimate a lower bound for the maximum ground acceleration from the horizontal force, hf, that produces the failure of the obelisk. being difficult to quantify the degree of the constraint exerted by the mortar layer, two limit conditions were considered: 1. the horizontal force is determined by the limit equilibrium in terms of sliding and toppling of the obelisk, assuming a purely frictional marble-mortar interface (figure 3a); 2. the horizontal force corresponds to the achievement of the strength, in terms of compression and shear stress, in the reacting zone of the base (figure 3b-c). according to the first approach, the acceleration amplitudes (expressed in g) required to produce the horizontal translation and the rotation around the y axis (figure 3a) can be respectively computed as follows: sa   (1) m 1 a 2   (2) where:  is the friction coefficient of the marblemortar interface,  is the slenderness ratio of the obelisk, given by: gh b   (3) (a) (b) (c) w h hg d b b b h w h hg b b b h h u w h hg d' b b b h 0.80u w b h h w w w h hg d b b b h w h hg b b b h h u w h hg d' b b b h 0.80u w b h h w w w h hg d b b b h w h hg b b b h h u w h hg d' b b b h 0.80u w b h h w w figure 3. approach n°1 (a) and n° 2, corresponding to the onset of the yielding (b) and to the collapse (c) of the obelisk. the first translational mechanism does not necessarily lead to a global failure: in fact, the obelisk can accumulate sliding displacements along the base recovering a final global equilibrium, according to the newmark (1965) rigid block model. in the case at hand, the width of the basement is similar to that of the base of the obelisk. for this reason, the accumulation of horizontal displacements can induce a sort of "second order" effect, leading to the toppling of the obelisk when the cumulated displacement overcomes half of its base width. in this way, the acceleration producing the sliding mechanism can be annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 5 considered as a lower bound of the maximum acceleration of the basement which is required to produce global instability. following maiorano et al. (2015), the onset of rotation with the second type of failure mechanism does not necessarily induce a toppling failure, since the block can recover a stable position after some bouncing on the base. also in this case, the acceleration amplitude that triggers the first rotation can be considered as a lower bound of that required for producing failure. the second approach is based on the hypothesis that the limit compression stress, σu, is achieved at the edge of the resisting zone of the base section, this latter assumed with a width equal to d. this condition corresponds to the onset of the first yielding in the mortar section. from figure 4a, the equilibrium of the obelisk is determined by: vertical translation: uw 0.5 db  (4) rotation around the gravity center of the base section: m g u b d ma h 0.5 db 2 3         (5) horizontal translation: sma cdb w  (6) in this latter equation, it is assumed that the shear strength at the interface obeys to the mohr-coulomb criterion. from the equilibrium under the vertical action w (eq. 4), the value of d, corresponding to the achievement of the compression limit stress at the edge of the base section, is calculated. from eqs. (5)-(6), the acceleration amplitudes corresponding to the ultimate moment, am, and to the shear strength, as, can be respectively computed. (a) (b) d  u w mamhg b/2-d/3  c 0.5udb d' 0.80u w mamhg (b-d')/2  0.80ud'b masmas figure 4. equilibrium of the base section for the second approach, corresponding to the onset of the yielding (a) and to the collapse (b). with the propagation of yielding, the full plastic failure occurs with non-linear stress distribution at the stone-mortar interface (see figure 4b). just like commonly assumed in concrete structures, it can be hypothesized that plastic failure corresponds to the achievement of a mean 0.80σu in the whole resisting zone of the base section, the width of which reduces to d’. in this condition, the equilibrium equations become: ' uw 0.80 d b  (7) ' ' m g u b d ma h 0.80 d b 2           (8) ' sma cd b w  (9) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 6 iv. results the two approaches illustrated in the previous section were applied to the case study of the obelisk, in order to compute two respective couples of limit accelerations, as and am. the geometrical dimensions, including the slenderness ratio are those reported in table 1, while the physical and mechanical parameters adopted in the calculations are summarized in table 2. table 2. parameters assumed in the calculations. unit weight weight compression limit stress shear strength app.  w σu c  (kn/m3) (kn) (mpa) (mpa) (/) 1 27.00 1.30 / / 0.40 2 1.00 0.34 0.40 in details: · w was evaluated from the geometry and the marble unit weight, ; · the compression limit stress, σu, was assumed on the basis of the lowest class (m1) of the european classification of the mortar (en 998:2), included in ec6 (en 1996-1-1; 2006); · the frictional strength coefficient, , is the typical value adopted for the mortar adopted in the masonry (en 1996-1-1; 2006); · the cohesion, c, was back-calculated from σu and  through the mohr-coulomb criterion, adopted to model the shear strength of the marble-mortar interface. since the only vertical load considered is the weight of the obelisk, the limit value σu is achieved in the section for a very low length of the resisting zone (d=0.01m; d’=0.005m). as a consequence, the solutions of the equilibrium in correspondence of the first yielding and of the full plastic failure are coincident. moreover, they are close to the results of the first approach, as synthesized in table 3. table 3. computed limit accelerations. approach 1 approach 2 am as am as (g) (g) (g) (g) 0.28 0.40 0.27 0.40 the obtained results highlight that: · the relatively high slenderness of the obelisk makes it overturn under a seismic action lower than the value corresponding to the onset of sliding, as well as the acceleration associated to the ultimate moment at the base section results lower than that related to the ultimate shear force; · according to both approaches, the ground acceleration in the ns direction, i.e. the falling direction of the obelisk, should have been higher than 0.27g. iv. comparison with the recorded ground motion figure 5a-b show the ns and ew components of the acceleration time history recorded at the amatrice seismic station (amt), located 75 m downhill and about 1 km away of the cemetery (see figure 1a). the data downloaded from the esm database (esm working group, 2015) are plotted in the most critical time window of the shaking, i.e. from t=6s to t=12s. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 7 (a) -1 -0.5 0 0.5 1 6 7 8 9 10 11 12 |a n s ||a e w | (g ) t (s) -1 -0.5 0 0.5 1 6 7 8 9 10 11 12a n s (g ) t (s) max(|ns|-|ew|) ns exceedance ew exceedance -1 -0.5 0 0.5 1 6 7 8 9 10 11 12a e w (g ) t (s) as am am as as am am as (b) -1 -0.5 0 0.5 1 6 7 8 9 10 11 12 |a n s ||a e w | (g ) t (s) -1 -0.5 0 0.5 1 6 7 8 9 10 11 12a n s (g ) t (s) max(|ns|-|ew|) ns exceedance ew exceedance -1 -0.5 0 0.5 1 6 7 8 9 10 11 12a e w (g ) t (s) as am am as as am am as (c) -1 -0.5 0 0.5 1 6 7 8 9 10 11 12 |a n s ||a e w | (g ) t (s) -1 -0.5 0 0.5 1 6 7 8 9 10 11 12a n s (g ) t (s) max(|ns|-|ew|) ns exceedance ew exceedance -1 -0.5 0 0.5 1 6 7 8 9 10 11 12a e w (g ) t (s) as am am as as am am as figure 5. ns (a) and ew (b) acceleration time histories recorded in amatrice station (amt) and difference (c) between the absolute values of the two horizontal components. the two limit accelerations of the obelisk, am and as, are exceeded by several peak values of the ew component (figure 5b), with the first of them occurring around t=9s. at almost the same time, the ns component trespasses the limit overturning acceleration, i.e. am = 0.27g, having approached this threshold already two times before (figure 5a). since the obelisk fell down along the ns direction, its collapse likely occurred when the ns acceleration component was higher than am, and before the ew acceleration component exceeded the same threshold value. the overturning mechanism might have therefore occurred in the time interval between 8.5 and 9s, i.e. when the difference between the absolute values of the ns and ew components (shown in figure 5c) is mostly positive. table 4 summarizes the acceleration values corresponding to the instants of the maximum difference between the two horizontal amplitudes (red dots in figure 5) and when the ns (blue dot) and ew (black dot) components achieve the am threshold for the first time. the ns component overcomes significantly the contemporary ew value at t=8.66s and t=8.82s. in addition, at the same instants, the ns acceleration amplitude are 0.22g and 0.24g, respectively, i.e. very close to am (0.27g). the overturning critical acceleration is attained almost simultaneously in both directions, with the ns component reaching the threshold 0.01s before the ew one. table 4. acceleration values corresponding to the instants of the maximum difference between the two horizontal components and to the attainment of the overturning threshold in both directions. a ia t ns ew | ns |-|ew| ns ew (s) (g) (g) (g) (m/s) (m/s) max (|ns|-|ew|) 8.66 -0.22 -0.02 0.20 0.08 0.06 max (|ns|-|ew|) 8.82 -0.24 0.00 0.24 0.14 0.10 ns exceedance 8.92 0.27 0.21 0.05 0.19 0.11 ew exceedance 8.93 0.33 0.27 0.04 0.20 0.12 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 8 figure 6 shows the evolution of the arias intensity with time, while table 4 reports the accumulated values at the four critical instants discussed above. although the final value of ia along ew results more than twice that relevant to the ns direction, the detail in figure 6 reveals that, before t=9s, more energy is accumulated along ns than in the ew direction. the position of the dots in the same plot and the relevant values of ia in table 4 confirm that the temporary predominance of the ns component on that along ew direction is coherent with the direction of the failure. 0 1 2 6 7 8 9 10 11 12 i a (m /s ) t (s) ew ns 0.00 0.25 0.50 8.6 8.8 9 9.2 max(|ns|-|ew|) ns exceedance ew exceedance figure 6. arias intensity of the ns and ew components, recorded in amatrice. a predominance of the ns component with respect to the ew has also been shown by the displacement inferred from the high-rate gps data by the ingv gps analysis group (2016), with the ns peak-to-peak value (≈15 cm) resulting twice that corresponding to the ew direction. moreover, the above interpretation of the acceleration recording could not account for site effects due to the different locations of the amt station, placed on the laga flysch formation representing the seismic bedrock, and the cemetery, lying on alluvial deposits constituting the hill of amatrice (regione lazio, 2016), close to the steep slope bordering the northern ridge. thus, stratigraphic and topographic amplifications of the recorded ground motion likely occurred at the cemetery during the earthquake. considering that the morphology of the amatrice hill is elongated along the ew direction, the topographic amplification might have affected more significantly the ns component with respect to the ew shaking. in such hypothesis, the shaking energy associated with the ns component in the cemetery area might have been even more significant than that relevant to the ew direction. it is expected that seismic response analyses, which will be soon carried out for the seismic microzonation of the area, will corroborate such assumption. iv. conclusion and future developments the paper illustrated two simple analytical approaches to interpret the observed failure of a funerary obelisk in the cemetery of amatrice, due to the first strong motion event of the central italy seismic sequence. a lower bound for the peak ground acceleration occurred at the site was estimated by backanalyzing different possible failure mechanisms of the object (sliding and toppling) with reference to global limit equilibrium and yield as well as plastic local failure. the estimated peak acceleration in the ns direction may be representative of the severity of the ground motion occurred in this urban area, located far enough from the available seismic station (amt) and in quite different site conditions. an assessment of such estiannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7296 9 mation is expected from seismic response analyses of the amatrice ridge. references athanasopoulos g.a. (1995). discussion of “1988 armenia earthquake ii: damage statistics versus geologic and soil profiles”, by m.k. yegian, v.g. ghahraman and g. gazetas, journal of geotechnical engineering, asce, 121 (4), 395-398. en 998:2 (2003). specification for mortar for masonry. part 2 masonry mortar. en 1996-1-1 (2006). eurocode 6: design of masonry structures part 1-1: general rules for reinforced and unreinforced masonry structures. esm working group (2015). engineering strong-motion database, version 1.0, doi: 10.13127/esm. galli p., peronace e. tertulliani a. (2016). rapporto sugli effetti macrosismici del terremoto del 24 agosto 2016 di amatrice in scala mcs. regione lazio (2016). seismic microzonation of amatrice municipality. attuazione art. 11 legge n.77/2009. draft to be approved by the civil protection dept. sensu opcm 4007/2012 (in italian). zimmaro p. and stewart j.p., (2016). engineering reconnaissance following the 2016 m6.0 central earthquake, ver 2. doi:10.18118/g61s3z,http://www.geerasso ciation.org/. ingv gps analysis group (2016). preliminary hrgps results for the amatrice earthquake. zenodo, http://doi.org/10.5281/ze nodo.61699. lanzo g., di capua g., kayen r.e., kieffer d.s., button e., biscontin g., scasserra g., tommasi p., pagliaroli a., silvestri f., d’onofrio a., violante c., simonelli a.l., puglia r., mylonakis g., athanasopoulos g., vlahakis v., stewart j.p. (2009). seismological and geotechnical aspects of the mw=6.3 l’aquila earthquake in central italy on 6 april 2009. international journal of geoengineering case histories, http://casehistories.geoengineer.org,1(4):206 -339. maiorano r.m.s., adinolfi m., aversa s. (2015). rocking of slender blocks under seismic excitation. rivista italiana di geotecnica, 2/2015. newmark n.m. (1965) effects of earthquakes on dams and embankments. géotechnique 15(2):139–160,doi 0.1680/geot.1965.15.2.139. yegian m.k., ghahraman v., gazetas g. (1994). 1988 armenia earthquake ii: damage statistics versus geologic and soil profiles, journal of geotechnical engineering, asce, 120 (1), 21-45. http://www.geerassociation.org/ http://www.geerassociation.org/ http://doi.org/10.5281/ze adg vol5 n02 mar 351_360.pdf annals of geophysics, vol. 45, n. 2, april 2002 351 an electromagnetic system for planetary geophysical exploration roberto marcialis (1), sergio pagnan (1), giuseppe bucci (2) and cinthya ottonello (2) (1) istituto per l’automazione navale, consiglio nazionale delle ricerche, genova, italy (2) dipartimento di ingegneria biofisica ed elettronica, università di genova, italy abstract the paper presents a study, carried out in the context of teseo project (sistema di prospezione tdem per l’esplorazione geofisica dei pianeti), for the design of a tdem (time domain electromagnetic method) system suitable for planetary subsurface exploration. the tdem technique has been selected since it is based on the parameter of electrical conductivity, which is generally capable of discriminating geological layered structures in a fully non-invasive manner, down to depths ranging several hundred meters. furthermore, tdem does not need any material sample and intrinsically satisfies the planetary protection and cross contamination requirements. finally, since the tdem is a time-domain technique, it is in principle possible to retrieve information from different depths with a single measurement. this last peculiarity makes the tdem technique much more expeditious and flexible than analogous techniques operating in the frequency domain. the paper describes the purposes of teseo project, the requirements due to space application and the preliminary system design. 1. introduction the surface of the bodies of the solar system has been investigated by several missions and others are planned. currently, mars global surveyor is complementing the global map of mars, producing high-resolution images of a small part of the planet by orbital missions. other missions provided detailed images of the jovian bodies, venus, and mercury. these investigations, initially based upon optical and remote sensors, are becoming much less «remote» with the landing of robots on the surface of solar system’s bodies (mars’ pathfinder type) and include a wider range of parameters and measurements. in the next exploration programs, semi-autonomous roving vehicles capable of conducting science experiments on the surface of the planets are expected to be applied. the italian space agency (asi) has also been engaged for several years in projects concerning in-situ planetary exploration. in the last call of the scientific research program (july 2000), asi recommended the submission of proposals focused on: – remote sensing of mars surface, to be employed for orbital missions. – geophysical sounding of mars soil and subsurface, for in-situ missions. – radar sounding of jovis satellite, europa. in particular, drilling systems for subsurface analysis and sample collection as well as instrumentation suitable for installation aboard rovers were recommended for mars exploration. in response to this call, the teseo project (sistema di prospezione tdem per l’esplomailing address: dr. roberto marcialis, istituto per l’automazione navale, consiglio nazionale delle ricerche, via de marini 6, 16149 genova, italy; e-mail: roberto@ ian.ge.cnr.it key words tdem – planetary exploration – subsurface sounding – signal processing 352 roberto marcialis, sergio pagnan, giuseppe bucci and cinthya ottonello razione geofisica dei pianeti) has been conceived and submitted to italian space agency’s review for the realization of an electromagnetic (em) system for geological prospecting of subsurface structure of the planetary bodies. the project brings together the scientific and technological knowledge of experts in the fields of geophysical prospecting, planetary exploration research and em signal processing. according to the expertise of the authors, the paper presents the main guidelines of teseo project, mainly focussing on the engineering and telecommunication aspects of the research. section 2 describes the purposes of the project and problems to be faced when application of consolidated technologies is transferred from ground to space application. the approach followed in order to design a system adapted to work in extreme environmental conditions and with reduced data collection and processing resources is presented in section 3. this section deals with a case study: the design of a tdem (time-domain electromagnetic measurements) system to be installed on board deedri, the italian drill developed for collecting samples of mars subsurface (mars sample return lander). in the section, the solutions adopted for signal acquisition and processing are particularly stressed. section 4 draws some conclusions and future prospective, addressing the impact that the research may have on systems used for earth’s sounding. 2. objectives of the teseo project the abundance of data from planets’ surfaces collected in past missions has provided a remarkable background for geological analysis. however, several of the questions which arose should be addressed to the investigation of the subsurface. teseo represents a possible solution to the request of performing non intrusive in-situ geophysical measures, which are of immediate interest for mars. several reasons induce tdem to be chosen for the development of a system for em measurements, among the many geophysical sounding techniques (nabighian, 1984, 1993). tdem is based on the parameter of electrical conductivity, which is generally able to discriminate geological layered structures. since it is a time-domain technique (recording of a relaxation transient following a proper excitation), it is in principle possible to retrieve information from different depths with a single measurement, speeding up the execution of highdensity surveys over large soil areas. this last peculiarity makes the tdem technique much more expeditious and flexible than analogous techniques operating in the frequency domain. it is a low-cost and light-weight technology. moreover, the resistivity of a rock can undergo notable variations due to water presence. therefore, it may result very useful in detecting water/ice signatures. unlike other techniques, tdem does not need material samples and intrinsically satisfies the planetary protection and cross contamination requirements. teseo system will be designed in order to achieve the following main objectives: – measuring electrical conductivity profiles of planet subsoil layers. – identifying ground water/ice signatures. – performing passive measurements of em environment. the above-mentioned objectives are perhaps attainable from a tdem system operating on the earth, but would be much more complex if these techniques had to be accomplished from space. there are several constraints that cannot be disregarded: – mass and power budget limitations. – possibility of transmitting to the ground only a reduced amount of collected data. – possibility of scanning a limited area of the planet’s surface by tdem. – em interferences with other science experiments. – hostile environmental working conditions. the aforementioned limitations may compromise inspection depth, measure quality and data interpretation. improvements are obtainable by adopting ad hoc mechanical and electronic solutions. in particular, as concerns the acquisition system and measure allocation, the design will be focused on the following aspects: choice of hardware components operating in very 353 an electromagnetic system for planetary geophysical exploration critical conditions, design of efficient data preprocessing procedures, design of data compression procedures. it is also necessary, during the early phases of teseo development, to understand the type, quality and interpretation possibility of the data that will be obtained by the instrument. the geophysical partners involved in the project will simulate the geological setting of the investigation targets and will model the possible data obtained in the selected geological settings, in order to suggest a possible output of the em sounding. accordingly, a set of experimental tests will be planned on the earth in representative geological areas for planetary application. such measures will be used for instrument validation as well as performance evaluation versus stringent restrictions imposed by space application. in particular, the main tasks of the project consist of: 1) design and implementation of the measuring equipment. 2) design and implementation of the hardware and software systems for data acquisition, processing and ground reconstruction. 3) experimental surveys for earth-based tests and instrument validation. 4) processing and interpretation of the experimental data sets. 5) system specifications for space application and evaluation of missions of interest for teseo installation. 3. a case study: the tdem system designed for deedri payload in order to give an idea of the problems to be faced when operating in space, in this section the case study of a tdem system, fig. 1. allocation of the tdem instrument on board of deedri. 354 roberto marcialis, sergio pagnan, giuseppe bucci and cinthya ottonello named master (mars subsurface sounding by time-domain electromagnetic measurements), is presented (tacconi et al., 2000). master has been designed for allocation aboard deedri, the italian drill developed for collecting samples of mars subsurface. deedri represents one of the ten additional payloads that are scheduled to fly aboard the mission mars sample return (fig. 1). master is based on the tdem technique. the basic configuration of a tdem instrument consists of the so-called central loop induction coil, which is used to carry out time-domain em soundings. the receiver, a sensitive magnetometer, may be placed concentric with the source loop or may be a separated loop. ground application systems generally use transmitting coils from 1 m up to some tens of meters wide, driven by currents from one ampere up to tens of amperes, depending on the required inspection depth. however, the installation aboard deedri imposes some specific requirements, such as: – minimizing the total mass of the system (total weight less than 1000 g). – using components working in a hostile environment (e.g., temperature range [– 130, 85] °c). – using very low power consumption components (available average power of the order of some watts). – reducing data memory allocation and computation time. – limiting ground transmission resources (transmission channel capacity less than 50 kbyte per martian day). according to these specifications, the system presented in section 3.1 was designed. in addition, in order to overcome drawbacks due to mechanical and electrical design limitations, which may compromise the obtainable data, acquisition and processing procedures for critical signal-to-noise ratio (snr) measure treatment were implemented. sections 3.2, 3.3 and 3.4 present the followed approach and some simulation results. 3.1. master instrument configuration the instrument developed for the deedri uses a set of two equal concentric coils (transrxt x proximity electronics remote electronics transmitted wave characteristics: • current square wave • ton = 1024 µs programmable • amplitude = 10 a • mean power = 10 w martian soil received wave characteristics: • exponential shape • tsampling = 1 µs programmable • bandwidth = [0-500 khz] • acquisition time = 16 384 µs fig. 2. scheme of the tdem system designed for deedri. 355 an electromagnetic system for planetary geophysical exploration mitter and receiver) with a diameter of the order of 150 mm. a logic scheme of master is shown in fig. 2. it is mounted on the bottom face of the drill box and does not interfere with the deedri drilling and sampling activities. the coils directly face the soil (i.e. no conductive material shall be either interposed between the coils and the soil or allocated within the coils themselves). the total weight is 110 g for both coils. coils are made up with copper conductors, with a section of 0.4 mm2. the coils should be contained in teflon (or other material) coil holders. the coil holders have the double task of preserving the coils and of supporting them slightly spaced from the lander fixing surface. the control electronics is divided into two blocks: proximity electronics near the coils, and remote electronics which is part of the main electronics unit science boards. the source signal or gate is a current square wave, the received signal is expected to have an exponential shape in the hypothesis of uniform laterally distributed electrical conductivity of the subsoil. data acquisition frequency is set to 1 mhz with an acquisition time set to 16 384 µs, but we foresee the operating option to scale it down to 500 khz, thus extending the acquisition time to 32 768 µs. tele-commands from ground control are just a few scalar parameters such as a gate timing and sampling acquisition interval. mass and power restrictions are presented in tables i and ii. they imply that the instrument is not presently configured as stand alone but shares some resources with the other subsystems of deedri, i.e. data pre-processing unit, experiment management unit, power distribution unit (for operation and survival), and thermal control of electronics itself. table ii. power budget of the tdem system designed for deedri payload. subsystem status operation mode power consumption transmitting coil during pulse generation 10/20 w keep-alive heaters (*) off 0.5 w proximity electronics on stand-by 0.8 w proximity electronics on stand-by and pulse generation 1.05 w main electronics (pc excluded) on conversion and ram management 2.50 w peak of consumption on measurement mode 3.55 w (*) the survival heaters switch off when proximity electronics reach the storage temperature range. table i. mass budget of the tdem system designed for deedri payload. subsystem type weight transmitting coil cu coil diameter 150 mm, section 0.4 mm2 55 g receiving coil cu coil diameter 150 mm, section 0.4 mm2 55 g coils holder teflon 130 g fixing bolts etc. stainless steel 30 g proximity electronics box al 70 g proximity electronics boards 90 g main electronics and harness 180 g total weight of the system 610 g 356 roberto marcialis, sergio pagnan, giuseppe bucci and cinthya ottonello 3.2. signal acquisition and processing strategy in order to overcome limitations on data memory allocation and computation time, a measurement cycle was defined, that is the set of measurements and pre-processing procedures aimed at obtaining as much information as possible by reducing data exchange and memorization. the operating procedure is based on a comparison of passive (no transmitted power) and active measured data. the sampled signal is processed following two different criteria: the signals received after the transmission of the gate, called active measures, are averaged, fast fourier transformed (fft) and filtered according to a procedure based on the comparison with the signals received without a previous transmission, called passive measures. the scheme in fig. 3 represents the data processing block diagram. the modified frequency domain kurtosis (mfdk) is applied to the vector of passive measures (ottonello and pagnan, 1994; ottonello et al., 1994). the mfdk is a fourth order statistics operator that assigns a level of stationarity to each signal spectrum component and enhances unsteady components over stationary sinusoids embedded in noise. it is applied in order to remove local noise n(t), which may be composed of slow time varying switching power supply sources (n s (t)), thermal noise (n w (t)), or em interferences (n imp (t)). it is defined as the ratio between the fourth order moment of the fft of the i-acquisition of the noise n (t) (n i ( f )), with respect to the square of the second order moment of the fft of the same signal, as follows: (3.1) this operator requires one fft, two vectors multiplications and four vector sums for computation, but is very powerful in enhancing drifting components over stationary sinusoids. theoretical results are interesting for detection of randomly occurring signals, when methods based on power spectrum may fail to accomplish this task. this function is used to set, in an adaptive way, the digital filter h(f). after this step, the useful signal is compressed maintaining only mfdk f q n f q n f i i q i i q ( ) = ( ) ( )         = = ∑ ∑ 1 1 4 1 1 2 2 r buffer buffer mean kurtosis fft h(f ) tx data codify adaptive filter tx to the ground remote electronics active measure passive measure fig. 3. data processing block diagram. . 357 an electromagnetic system for planetary geophysical exploration the information on the most powerful non stationary spectral lines. in detail, the operations timeline is as follows. 3.2.1. definition of a single measure (sm) a single measure implying a vector of length n (default n = 16 384) is defined primarily as the set of measurements each one taken in sequence at small random delays (e.g., from 0 to 20 s) with the drill mechanically stable and motors switched off. all the data processing can be made in parallel or serialized, in order to minimize memory allocation and can run with lowest priority in background. the sequence is: 1) read a number q (default q = 16) of vectors in passive mode (without transmitting gate). 2) upgrade the mfdk state vector using the passive measures and the old state vectors taken from memory. 3) read a number l of vectors in active mode and perform the mean of l active measures. the delay between measures can be random (e.g., 1 to 10 s) and depends at least from the capacitor charging time in the transmission coil. 4) perform the fft of the mean. 5) set to zero stationary frequency lines according to mfdk operator based processing (i.e. h( f ) filtering). 6) sort in order of power magnitude the remaining spectral lines. 7) take the first m components (default m = 1024) obtaining a reduced compressed vector. 8) send or stack it, adding routine information like date, time, drill position, external temperature. 3.2.2. martian day timeline (mdt) 1) take care of commands from earth, such as sample interval and programmable gate length. 2) if sample interval (default = 1 µs) or gate length has changed reset mfdk state vector to zero. 3) if the drill changed position execute a position measure procedure, by sending to ground a new vector of passive measures and boundary information. 4) distribute regularly or randomly over a martian day a number k of single measure (default k = 8). if this process became unrealizable because of frequent drill activation, the k single measures can be taken consecutively. 3.3. memory allocation and computation time by assigning the following default values to: – data vector length: n = 16 384; – number of repeated measurements in one sm: q = 16 (passive mode) and l = 16 (active mode); – reduced data vector length: m = 1024 complex; – number of sms per martian day: k = 4; – floating point of 5 bytes (40 bits); – sampling time tsampling = 1 µs, we obtain for one entire mdt: total acquisition time 2 s; total computation flops = 67.2 megaflops; total bytes to telemetry = 48 kbytes. in order to reduce the number of data to be sent to telemetry data subsystem, we may use, for example, k = 2 and q = 64, obtaining 24 kbytes per martian day. this working configuration, although representing a minimal scenario for master, covers the experiment’s scientific validity. 3.4. simulation results a set of simulations were performed to validate the proposed data acquisition and compression strategy and to verify the performances of reconstruction algorithm versus compression parameters (i.e. m value). at first, we assumed that the martian subsoil electrical conductivity is uniform laterally distributed. hence, the subsoil conductivity is free to change layer by layer in depth, but it remains constant in the same layer (see fig. 4) (hohmann, 1975; mcneill, 1984; keller, 1994). the relation between the subsoil electrical conductivity, the magnetic permeability and 358 roberto marcialis, sergio pagnan, giuseppe bucci and cinthya ottonello the frequency of the received wave is the «skin depth» (nabighian, 1994), defined as follows: skin depth ( µ )1/2 this well known relation allows geophysical interpretation of data, by correlating the information contained in the frequency spectrum of the returned signal with electrical conductivity. in the simulation, the received signal r (t) is modelled as the sum of two signals, as follows: r (t) = s (t) + n (t) where s (t) = i a i exp (b i t) represents the original signal and n (t) is the noise, which can be modelled as the sum of three contributes: n (t) = n w (t) + n s (t) + nimp (t) where n w (t) represents thermal noise (additive white gaussian noise, i.e. awgn), n s (t) is the noise derived by the lander on-board systems (sum of persistent sinusoids) and nimp (t) takes into account the impulsive components due to occasional em interferences. in order to test the performances of data processing and to evaluate its limitations, in the simulation tests different working scenarios were considered, changing the values of the operation timeline (i.e. n, l, q, m) and the characteristics of s (t) (e.g., amplitude, frequency, number of persistent sinusoids being within the noise bandwidth) and n (t) (e.g., impulsiveness). for each test, we compared the power spectrum of original signal s (t) with respect to the power spectrum of the raw acquired signal r (t) and of the reconstructed signal, after mfdk filtering and compression. in fig. 5, on the left the received raw signal is shown, while on the right, we can see the same signal after the processing. for comparison, the original signal s (t) is reported in the small window. the proposed algorithm proved efficient since: – it is able to remove the periodic signals from the received signal power spectrum. – it can attenuate the contribution of impulsive and occasional signals to noise. – it can be executed in real time. – it allows the maximum timeline flexibility. the described data processing presents, however, some limitations: if parameters go over a range of values, the results could differ from the expected ones. in detail, the minimum number of passive acquisitions is q = 16, the maximum time interval of an impulsive signal is 500 µs; the minimum number of spectral lines to be transmitted to ground without complete loss of information is m = 128. 4. conclusions the paper presented the purposes of a new project, teseo, which, collecting the results of the feasibility study of master system, intends to answer the growing interest in em sounding systems to be installed aboard carriers (landers, rovers, drills) to be delivered on solar system body surfaces. the presentation mainly concerned the engineering and telecommunication problems layer 0 µ0 , ε0 layer 1 µ1 , ε1 layer 2 µ2 , ε2 layer n µn , εn layer n+1 µn+1 , εn+1 z = -d0 z = -d1 z = -d2 z = -dn-1 z = -dn z x fig. 4. subsurface modelling. 359 an electromagnetic system for planetary geophysical exploration related to data acquisition and processing, not the geophysical interpretation aspects, which are far from the expertise of the authors. the tdem technique can be successfully applied for performing non intrusive in-situ geophysical measures on planetary bodies, although several technological problems still need to be addressed. for example for the master system, the following problems need to be assessed: – increasing transmitting coil current by using supercapacitor recharging systems. – implementing a receiving coil based on squid (superconducting quantum interference device) technology. – optimizing data compression and transmission algorithms. tdem instruments are strongly recommended as reference ground truth for the remote sensing missions where data is acquired by sounding radars from satellites. tdem data will serve as a comparison reference for other scientists willing to explore the subsurface properties, employing different scientific investigation techniques, thus inspiring a variety of new insights into geologic history and up-to-date structure of planetary bodies. applications will mainly concern the 2005-2007 mars sample return missions but may be extended to space missions on other bodies of the solar system: moon, mercury, venus, titan. solutions adopted for space application may also have a significant impact on ground surveying, as concerns for example the following aspects: – lightweight and portable systems may improve applications in irregular and inaccessible terrains and/or may speed the execution of measures over large areas; – reduced equipment dimensions are also required for applications concerning natural hazards prevention (e.g., volcanic eruptions) and cultural heritage inspection and monitoring. – solutions for reducing measure allocation resources (e.g., by using suitable compression algorithms) and implementing more efficient reconstruction and processing algorithms for noisy or corrupted data may be transferred from space to ground applications. acknowledgements the authors wish to thank the other partners involved in the teseo proposal preparation: prof. domenico patella, department of physical sciences, university «federico ii» of naples, 0 . 9 1 0 . 8 0 . 7 0 . 6 0 . 5 0 . 4 0 . 3 0 . 2 0 . 1 0 0 2000 4000 6000 8000 10000 12000 14000 16000 0 . 9 1 0 . 8 0 . 7 0 . 6 0 . 5 0 . 4 0 . 3 0 . 2 0 . 1 0 0 2000 4000 6000 8000 10000 12000 14000 16000 0 . 9 1 0 . 8 0 . 7 0 . 6 0 . 5 0 . 4 0 . 3 0 . 2 0 . 1 0 0 2000 4000 6000 8000 10000 12000 14000 16000 fig. 5. simulation results. on the left side: the received signal r (t). for the test the noise was modelled as the sum of two sinusoids with a maximum amplitude equal to 0.6, an awgn with a maximum amplitude equal to 0.2 and a spike with a maximum amplitude equal to 0.3 and a duration of 500 µs. on the right side: the processed signal obtained after filtering and compressing procedures; in the small window: the original signal s (t). 360 roberto marcialis, sergio pagnan, giuseppe bucci and cinthya ottonello prof. tiziano cantalupi, osservatorio geofisico «r. bendandi», faenza; dr. goro komatsu, international research school of planetary sciences, università d’annunzio, pescara. particular thanks to prof. giorgio tacconi for his valuable suggestions to the research. references hohmann, g.w. (1975): three dimensional induced polarization and electromagnetic modelling, geophysics, 40, 309-324. keller, g.v. (1994): the geoelectrical method in geophysical exploration (elsevier). mcneill, j.d. (1984): approximate calculations of the transient electromagnetic response from buried conductors in a conductive half-space, geophysics, 49 (7), 918-924. nabighian, m.n. (editor) (1984): special issue on time domain electromagnetic methods of exploration, geophysics, 49 (7), 849-1029. nabighian, m.n. (editor) (1993): electromagnetic methods in applied geophysics, soc. explor. geophys., 1-2. ottonello, c. and s. pagnan (1994): modified frequency domain kurtosis for signal processing, electron. lett., 30 (14), 1117-1118. ottonello c., s. pagnan and g. tacconi (1994): filtering of randomly occurring signals by kurtosis in frequency domain, in 12th international conference on pattern recognition, jerusalem, israel, october 9-13, 1994, vol. 3, 131-133. tacconi, g., s. pagnan, l. minna, r. marcialis, g. bucci and c. ottonello (2000): mars exploration by tdem based measurement system, in proceedings of piers, july 5-14, 2000, cambridge, ma, u.s.a., p. 1067. s u l l ' o r i g i n e d e l c a m p o m a g n e t i c o t e r r e s t r e enrico m e d i l o s c o p o d e l l a p r e s e n t e n o t a è di e s p o r r e n e l l e l i n e e g e n e r a l i , u n ' i p o t e s i c h e p o t r e b b e f o r n i r e l a s p i e g a z i o n e d e l l a e s i s t e n z a d e l c a m p o m a g n e t i c o d e l l a t e r r a . q u e s t a p r i m a e s p o s i z i o n e si l i m i t a e s c l u s i v a m e n t e a i p u n t i f o n d a m e n t a l i , s e n z a e n t r a r e n e l l a p a r t e c r i t i c a e n e gli a s p e t t i p a r t i c o l a r i d e l l a t e o r i a , c h e s a r a n n o d e t t a g l i a t a m e n t e e s p o sti in u l t e r i o r i n o t e . l ' i p o t e s i èi l a s e g u e n t e : u n a c o r o n a di e l e t t r o n i n e g a t i v i , a v e n t e g i a c i t u r a p r o s s i m a a q u e l l a d e l l ' e q u a t o r e , c i r c o n d a l a t e r r a ; il suo r a g g i o m e d i o sia r . g l i e l e t t r o n i si m u o v o n o l u n g o la c i r c o n f e r e n z a di r a g g i o r . c o n v e l o c i t à v n e l e n s o stesso d i r o t a z i o n e d e l l a t e r r a . e s s i c o s t i t u i s c o n o u n a c o r r e n te e l e t t r i c a , c l i c p r o d u c e u n c a m p o m a g n e t i c o h d i r e t t o , n e l l ' i n t e r n o d e l l a c o r o n a , n e l s e n s o c l i c v a dal p o l o n o r d g e o g r a f i c o a l p o l o s u d . a t t r i b u e n d o al p i a n e t a u n a s u s c e t t i v i t à m a g n e t i c a m e d i a "/., l ' a z i o n e di q u e s t o c a m p o p r i m a r i o i i dà l u o g o ad u n a i n t e n s i t à d i m a g n e t i z z a z i o n e m e d i a i : n e r i s u l t a un m o m e n t o m a g n e t i c o c o m p l e s s i v o s u l l ' o r i g i n e d e l c a m i ' o m a g n e t i c o t e r r e s t r e 319 m . il c a m p o da noi osservato sulla s u p e r f i c i e d e l l a t e r r a s a r e b b e dov u t o . a p p u n t o , alla p r e s e n z a di q u e s t o m o m e n t o m a g n e t i c o t o t a l e . l e p a r t i c e l l e e l e t t r i c h e si m a n t e n g o n o in e q u i l i b r i o lungo l ' o r b i t a d e l l a c o r o n a , per l ' a z i o n e di tre f o r z e : 1) la forza di l o r e n t z f , . , c h e t e n d e a p i e g a r e la t r a i e t t o r i a degli e l e t t r o n i v e r o la t e r r a . e s s a è p r o d o t t a d a l v e t t o r e i n d u z i o n e m a g n e t i c a b . ( d o v u t o al m o m e n t o m a g l i , t e r r e s t r e m ) c h e a g i c o sugli e l e t t r o n i in m o t o . il v e t t o r e b è o r t o g o n a l e al v e t t o r e v e l o c i t à v . 2 ) l n a forza e l e t t r o s t a t i c a f e , d e r i v a n t e dal f a t t o c l i c la t e r r a ha sulla sua s u p e r f i c i e una c a r i c a n e g a t i v a . questa l'orza f c , t e n d e ad a l l o n t a n a r e gli e l e t t r o n i d a l l a t e r r a . 3 ) u n a forza c e n t r i f u g a f , . q u e s t ' u l t i m a , d a t o il suo v a l o r e m o l to p i c c o l o , in r a p p o r t o a q u e l l o d e l l e due f o r z e p r e c e d e n t i , è trascur a l a liei c a l c o l i di p r i m a a p p r o s s i m a z i o n e , c h e s e g u o n o . d a l l a i m p o s t a z i o n e d e l l e i p o t e s i r i s u l t a c o m e la c o r o n a e l e t t r o n i c a sia causa della e s i s t e n z a d e l c a m p o m a g n e t i c o d e l l a t e r r a e q u e s t o , a sua v o l t a , c o n t r i b u i s c a al l a s t a b i l i t à d e l l a c o r o n a stessa. la s a t u r a z i o n e magnetica) dei m a t e r i a l i , c l i c c o s t i t u i s c o n o il p i a n e t a . p o t r e b b e essere la causa l i m i t a n t e il v a l o r e d e l l e g r a n d e z z e in g i o c o , d a n d o ad esse d e l l e c o n d i z i o n i s t a z i o n a r i e . e " p o s s i b i l e c h e a l t r i p i a n e t i a b b i a n o p u r e un c a m p o e l e t t r i c o e un m o m e n t o m a g n e t i c o di a n a l o g a o r i g i n e . così p e r il sole si p o t r e b b e f o r m u l a r e una s j n i i l e i p o t e s i , a m m e t t e n d o p e r ò d e i p r o c e s s i p a r l i c o e n k i c o m e d i l a r i di i n d u z i o n e e l e t t r o m a g n e t i c a , c h e , n e l l ' a t t u a l e e s p o s i z i o n e , n o n è il c a s o di s v o l g e r e . p r i m a di e s p o r r e la p a r t e a n a l i t i c a d e l l a t r a t t a z i o n e , è o p p o r t u n o p e r r a g i o n e di c o m o d i t à , d a r e u n a t a b e l l a c o n t e n e n t e il s i g n i f i c a t o d e i s i m b o l i usati e i v a l o r i d e l l e g r a n d e z z e fi-ielle, f o r n i t i d a l l a e s p e r i e n za. ( i l sistema u s a t o è il s i s t e m a a s s o l u t o c . g . s . ) . l> l'aggio d e l l a t e r r a = 6 , 3 7 . i o 8 e v a l o r e m e d i o d e l c a m p o e l e t t r i c o t e r r e s t r e = 3 , 3 3 . 1 0 " " q c a r i c a t o t a l e della t e r r a = 1 , 5 . i o ' 5 b v a l o r e del c . i n . l . a l l ' e q u a t o r e = 3.io"1 "/. s u s c e t t i v i t à m a g n e t i c a m e d i a = 10 / i n t e n s i t à i n e d i a d i m a g n e t i z z a z i o n e = 8 . ] 0 " 2 m m o m e n t o m a g n e t i c o d e l l a t e r r a = 8 , 1 . 1 0 2 j v v e l o c i t à d e l l e p a r t i c e l l e e c a r i c a d e l l ' e l e t t r o n e = 4 , 7 8 . i o " 1 0 m m a s s a d e l l ' e l e t t r o n e = 9.io"2 8 n n u m e r o di p a r t i c e l l e p e r c e n t , c u b o d e l l a c o r o n a s sezione d e l l a c o r o n a r r a g g i o d e l l a c o r o n a c v e l o c i t à d e l l a luce = 3 . i o 1 0 i i c a m p o p r i m a r i o i n d o t t o d a l l a c o r r e n t e i o n i c a sul p i a n e t a li v e t t o r e i n d u z i o n e m a g n e t i c a , dovuto ad m . e a g e n t e s u l l e fi) r l a f o r z a di l o r e n t z fi. è : c a r i c h e in m o t o v v e l o c i t à a n g o l a r e d e l l e p a r t i c e l l e . li e v = f, c «love : li = m •• r •• r ed m = — — s i q " i . n e d e r i v a c l i c — j r ^ i — 3 r"c c l a forza e l e t t r o s t a t i c a r e p u l s i v a è fe = —(? = e0« fe = e ^ r" s u l l ' o r i g i n e d e l c a m i ' o m a g n e t i c o t e r r e s t r e 319 l ' e g u a g l i a n z a dei v a l o r i assoluti d e l l e d u e forze dà la c o n d i z i o n e di e q u i l i b r i o d i n a m i c o : 4 qs e v e qe 3 " ti" c ~ h da cui si o t t i e n e £ = ± „ _ l j l [ i ] 1 3 r e e ' i n l e i e s s a n t e o s s e r v a r e c l i c , n e l l a e s p r e s s i o n e [ f ] . il c a m p o e l e t t r i c o del p i a n e t a e l ' i n t e n s i t à di m a g n e t i z z a z i o n e m e d i a sono l e g a t i al r a g g i o d e l p i a n e t a stes>o e al l a v e l o c i t à a n g o l a r e di r o t a z i o n e dello p a r t i c e l l e da u n a f o r m u l a m o l t o s e m p l i c e e g e n e r a l e . p e r o t t e n e r e l ' i n t e n s i t à di m a g n e t i z z a z i o n e f , è n e c e s s a r i o c l i c la c o r o n a p r o d u c a un c a m p o p r i m a r i o la cui m e d i a , n e l l o s p a z i o occup a l o d a l l a t e r r a , sia tale c l i c i = ' / f l d a t e le g r a n d e z z e di q e di r si p u ò s c r i v e r e con b u o n a a p p r o s s i m a z i o n e 2tt i jl — , = n e s v cr e q u i n d i : i = 2jlusev ' c r d a q u e s t a si o t t i e n e p e r co l ' e s p r e s s i o n e : v l e =0,4: r 2xnsex d i e c o n f r o n t a t a con q u e l l a r i c a v a t a d a l l a [ 1 ] d à : 3 ec _ l e i 4 j t q 2 j t n . s e / da c u i si o t t i e n e il p r o d o t t o d e l l a c o n c e n t r a z i o n e p e r la s e z i o n e d e l l a c o r o n a , ns= 2i~q = 1 , 7 1 . 1 0 " [ 3 ] 3 e e x s u p p o n e n d o la c o n c e n t r a z i o n e e g u a l e a 1 0 0 0 p a r t i c e l l e p e r c e n t , c u b o la s e z i o n e d i v e n t a e g u a l e a q u e l l a di un r e t t a n g o l o di a r e a 1 . 7 1 . 1 0 " t u r c i o è 1 7 . 0 0 0 k m 2 . p e r t r o v a r e il v a l o r e di r è n e c e s s a r i o s t a b i l i r e il v a l o r e di v, 3 4 8 e n r i c o m e d i in ogni m o d o r è c o m p r e s o f r a un v a l o r e m a s s i m o c o r r i s p o n d e n t e a r ~ c , e m i n i m o ( n o n p o s s i b i l e ) e g u a l e a c>. s e si dà a v il v a l o r e di un d e c i m o d e l l a v e l o c i t à d e l l a luce r i s u l t a , r = 6 , 3 8 . 1 0 ° l a c a r i c a totale d e l l a c o r o n a c l i c r i s u l t a è qc = 2nnesr [ 4 ] q u e s t a c a r i c a tende p e r r a g i o n i e l e t t r o s t a t i c h e a d i f f o n d e r s i n e l l o s p a z i o , p e r la r e p u l s i o n e f r a le c a r i c h e stesse c h e l a c o s t i t u i s c o n o . l ' a z i o n e di q u e s t a f o r z a s a r e b b e c o m p e n s a t a dal f a t t o c l i c u n a cal i c a p o s i t i v a sufficiente è d i s t r i b u i t a con s i m m e t r i a c i l i n d r i c a i n t o r n o al l a t e r r a , in un r a g g i o m i n o r e di r. t a l e c a r i c a p o s i t i v a n o n i n f l u i s c e sul c a m p o m a g n e t i c o , p e r c h é è t r a s c i n a t a con la v e l o c i t à a n g o l a r e stessa d e l l a t e r r a . c o n c l u d e n d o : se n e l l a f o r m u l a [ 4 ] si m e t t e il p r o d o t t o ns d a t o d a l l a [ 3 ] si h a 4 za o c)c = — 7 t — — r = 5 , 1 2 . 1 0 s r [ 5 ] 3 e / questa f o r m u l a ci dà la c a r i c a e l e t t r i c a t o t a l e causa del f e n o m e no l a s c i a n d o a r b i t r a r i o ( e n t r o c e r t i l i m i t i ) il r a g g i o r, c l i c l ' e s p e r i e n za ( p e r e s e m p i o la r i f l e s s i o n e di o n d e e l e t t r o m a g n e t i c h e ) p o t r e b b e fiss a r e . d a l l a t e o r i a e o 4 s u c c i n t a m e n t e e s p o s t a si possono t r a r r e d e d u z i o n i a l l e ad i l l u m i n a r e a l c u n i p a r t i c o l a r i p r o b l e m i del c . m . t . , d e i q u a l i p e r ò ora si dà solo un b r e v i s s i m o c e n n o . a) il c a m p o si p r e s e n t a a l l ' o s s e r v a t o r e t e r r e s t r e c o m e i n m a s s i m a p a r t e dovuto a c a u s e i n t e r n e a l l a t e r r a : i n f a t t i n o i m i s u r i a m o i l v e t t o r e i n d u z i o n e b c l i c d e r i v a d a l l a i n t e n s i t à di m a g n e t i z z a z i o n e int e r n a del p i a n e t a . il c a m p o p r i m a r i o h ci a p p a r e c o m e o p p o s t o a b, al di f u o r i d e l l a s u p e r f i c i e d e l l a t e r r a e ne r a p p r e s e n t a u n a p i c c o l a p e r c e n t u a l e : c o m e a p p u n t o r i s u l t a d a l l a e s p e r i e n z a . b) i l f a t t o , c h e s e m b r a d e d u r s i d a l l o studio d e l l e v a r i a z i o n i sec o l a r i . c h e c i o è il c a m p o a b b i a i n c e r t e e p o c h e m u t a l o s e n s o , si pot r e b b e p e n s a r e c o m e p r o d o t t o o da un c a m b i a m e n t o d e l segno d e l l o c a r i c h e r u o t a n t i o da una i n v e r s i o n e del senso di r o t a z i o n e degli e l e t troni n e g a t i v i . s u l l ' o r i g i n e d e l c a m i ' o m a g n e t i c o t e r r e s t r e 3 1 9 c ) le p r o f o n d e a n o m a l i e d e l l a d i s t r i b u z i o n e d e l c a m p o sulla sup e r f i c i e d e l l a t e r r a si r i c o l l e g a n o al l a a s i m m e t r i a d e l l a d i s t r i b u z i o n e dei m a t e r i a l i i n t e r n i d e l l a t e r r a e s p e c i a l m e n t e di q u e l l i c o m p r e s i in u n o s t r a t o s u p e r l ì e i a l o di a l c u n e d i e c i n e di c h i l o m e t r i , c h e «ono q u e l l i d o t a t i della m a g g i o r e p e r m e a b i l i t à m a g n e t i c a . e/) l e t e m p e s t e m a g n e t i c h e d i p e n d e n t i d a l l a a t t i v i t à s o l a r e , si p o t r e b b e r o s p i e g a r e c o m e d e r i v a n t i da u n a v a r i a z i o n e del m o m e n t o m a g n e t i c o s o l a r e , c h e i n d u c e n e l l ' o r b i t a d e l l a c o r o n a u n a forza e l e t t r o m o t r i c e , p e r la v a r i a z i o n e ili llu>-o c h e n e d e r i v a a t t r a v e r s o la sua -superficie. s i a m o forse ili p r e s e n z a eli gigantesco b e t a t r o n e c o s m i c o ? u n a s i m i l e c o n s i d e r a z i o n e a p p l i c a l a a l l e p a r t i c e l l e r u o t a n t i i n t o r n o al s o l e , p o t r e b b e d a r e la s p i e g a z i o n e della genesi di p a r t i c e l l e e l e t t r i z z a t e di g r a n d e e n e r g i a . e ) l e v a r i a z i o n i d i u r n e , l ' e f f e t t o l u n a r e e a l t r i a s p e t t i p a r t i c o lari -i p o t r e b b e r o i n q u a d r a r e in u n a u n i c a v i s i o n e . i n ogni m o d o l ' a n a l i s i p a r t i c o l a r e di c i a s c u n o di e>si sarà oggetto <li p r o s s i m e e s p o s i z i o n i . palermo — istillilo di fisica dell'università — giugno 1918. riassunto e' formulala una ipolesi che potrebbe dare ragione della esistenza del campo magnetico terrestre. unu corona di cariche elettriche ruotanti intorno alla terra genera un campo primario, da cui derivti. la intensità di magnetizzazione esistente sul pianeta. l equilibrio delle cariche della corona <>. donilo all'azione della forza di tjorentz e di ima forza di natura elettrostatica. vol49,6,2006 1209 annals of geophysics, vol. 49, n. 6, december 2006 key words venus transit – effects on atmospheric electric parameters 1. introduction the quasistatic electric field observed near earth’s surface, i.e. fair weather field (fwf), is maintained by global thunderstorm activities (bering et al., 1998; rycroft and price 2000). thunderstorm activity and lightning again put the earth-ionosphere waveguide into resonance producing characteristic spectra in elf and vlf ranges (nickolaenko, 1997; rycroft and cho, 1998). various parameters of the atmospheric electricity are directly related to the global thunderstorm activity and solar radiation. the discrete spectra of frequencies 8, 14, 20 ... hz due to schumann resonances are generated by electromagnetic emission from the lightning strokes and can be regarded as excitation of an ac global circuit (bering et al., 1998). some frequency changes of the peak values and also amplitude changes are found to be present in the observed data which are attributed by the uncertainties arising from spatial distribution of lightning sources exciting the schumann resonance modes (de et al., 2004). schumann resonance phenomenon as well as their measurement techniques and other aspects have been described by many workers (tzanis and beamish, 1987; williams, 1992; satori, 1996; huang et al., 1999). in three areas, namely, atmospheric potential gradient, elf and vlf, we are taking continuous records where some significant deviations are observed in the values of the involved parameters during the transit of venus on june 8, 2004. the transit started at 10:44 h local time and continued up to 16:53 h local time, which was visible from kolkata (latitude 22°34ln) throughout this period. the local sunrise and the effect of recent venus transit on earth’s atmosphere syam s. de (1), b.k. de (2), s.k. adhikari (1), b.k. sarkar (1), s.k. sarkar (3), a. guha (1), p.k. mandal (1), s.k. mandal (1), h.p. sardar (1) and m. ray (1) (1) centre of advanced study in radio physics and electronics, university of calcutta, kolkata, india (2) department of physics, tripura university, agartala, india (3) department of physics, university of calcutta, kolkata, india abstract some experiments on june 8, 2004, the day of transit of venus across the sun, were undertaken at kolkata (latitude: 22°34ln) to observe the effect, if any, of transit of venus on fwf, elf and vlf amplitudes. the result shows a good correlation between their temporal variations during the transit. the observation was unbelievable as the venus subtends only 1/32th of the cone subtended by sun on earth. this anomaly may be explained on the assumption that the height of venusian atmosphere with high content of co2, and nitrogen which absorbs electromagnetic and corpuscular radiations from sun, depleting the solar radiation reaching the earth to a considerable extent. as a result, relevant parameters of earth’s atmosphere are modulated and here we show how these changes are reflected in identical behaviour of fair weather field and elf and vlf spectra. mailing address: prof. syam s. de, centre of advanced study in radio physics and electronics, university of calcutta, 1, girish vidyaratna lane, kolkata 700 009, india; e-mail: de_syam_sundar@yahoo.co.in 1210 syam s. de et al. sunset times were 04:52 h and 18:22 h respectively. this paper presents the results of experimental observations during venus transit. 2. experimental arrangement we take observations from the roof top at a height of about 26 m from ground. the vertical electric field is measured with an ac field-mill which has an aluminum rotor plate of 12 cm in diameter. the output from the amplifier is recorded through computer sound card at a sample rate of 44100 per second. the rms value of the recorded signal is used to find the required electric field from the calibration chart. we calibrated the field-mill in a vertical field setup between two large aluminum cover plates, electrically isolated at a given potential through a fixed distance between them. the outer shield of the field-mill is grounded properly to ensure possible field distortions. the sensitivity of the fieldmill is (0.33 ± 0.03) vm−1. for the observation of integrated field intensity (ifi) of vlf sferics at 9 khz, an 8 swg straight copper wire of 120 m in length is used as the antenna. the antenna is sensitive to the vertical electric field component of the electromagnetic signal. it is installed horizontally at a height of about 30 m above the ground. the signal processor is tuned to the desired frequency. the overall q-factor of the tuning circuit is around 300. the signal from the tuning stage is rectified and fed to a log amplifier. the time constant of the output from the signal processor is 10 s. the data are recorded using a 12 bit adc at a sample rate of 5. for observations on schumann resonances, two square loop antennas in series combination have been used. the length of each side of one of the loops is 1 m with a total of 75 turns and other with 90 turns having a length of 1.3 m for each side. the antennas are sensitive to the magnetic field component of the natural schumann resonance electromagnetic signals. a stereopreamplifier-integrated circuit with la3161 chip, whose low frequency response starts below 5 hz, has been used to pre-amplify the signal from antenna. the chip is capable of detecting signals in the microvolt region. the pre-amplified signal is then amplified with a low pass amplifier with upper cut-off at 35 hz. the amplified signal is recorded using computer sound card at a sample rate of 44100 per second. 3. analyses of the observed data 3.1. schumann resonances the temporal variations of the amplitudes of first three schumann resonance modes on 8th june 2004, during the venus transit, together with their averaged values of 18 days are shown in fig. 1. standard deviations from the average values have been plotted as error bars in the average plot. standard deviations have been calculated over 2nd and 3rd quartile of the frequency distribution curve. it is seen that the average amplitude of the first mode line in the average value graph is 0.025 au (arbitrary unit) except almost a sharp 3 fold increase at 14 h local time, whereas on the day of transit, the corresponding amplitude decreases gradually from 10 h to 16 h from 0.06 au to 0.03 au. after 16 h, the amplitude increases. the second mode amplitude in the average value graph remains steady at 0.01 au except almost a two fold increase at 14 h local time. on the day of transit, the corresponding amplitude shows the gradual decrease from 0.035 au to almost zero at 13 h local time which then gradually increases. the third mode amplitude in the average graph remains the same except a depression between 13 h to 14 h local time, whereas on the day of transit, the amplitude decreases from 10 h local time to 13 h local time and shows a peak at 14 h and then the amplitude decreases up to 15 h local time and then again increases. 3.2. vlf atmospherics a comparative picture of the ifi at 9 khz atmospherics on 8th june 2004, during the venus transit (continuous line) and its 18 days’ average value over the same period (dotted line) is presented in fig. 2. the dotted grey lines represent standard deviation. on the day of transit, there are some unique peaks that appear just before fig. 2. variation of atmospheric integrated field intensity (ifi) at 9 khz vlf on the day of venus transit, 8th june 2004 (continuous line) and average variation of the same parameter around 8th june for 18 days (dotted line) and the dotted grey lines represent standard deviation. 1211 the effect of recent venus transit on earth’s atmosphere fig. 1. variation of peak magnetic amplitude of the three modes of schumann resonance spectra on the day of venus transit, 8th june 2004 (continuous line) and average variation of the same parameter around 8th june for 18 days (dotted line). 1212 and after the beginning of the transit. the peaks are in decreasing order of magnitude. the ifi then slowly increases to a steady value showing considerable fluctuations before and after the end of the transit also. it is worth mentioning that in both the measurements of potential gradient and vlf ifi, just after the beginning of transit and before the end of transit, their values lie beyond the standard deviation limits of the average variation. in case of potential gradient, it decreases from higher value to a lower value whereas in case of vlf ifi, it increases from lower value to a higher value. in schumann resonance also, we obtained a similar trend. 3.3. potential gradient figure 3 represents the temporal variation of vertical potential gradient on 8th june, 2004 during the venus transit (continuous line) and its values averaged over other 18 days around that date (dotted line) with standard deviations from the average value plotted as error bars. firstly, we notice that the maximum and minimum values of the potential gradient on the day of transit are around 300 vm−1 and 50 vm−1 respectively. but during the same period on other days, it is around 205 vm−1 and 152 vm−1, respectively. the high values owe to the higher level of pollution at the place. secondly, the curve during the transit shows significant deviation from the average values since the variation is well beyond the standard deviation of the average trend. there is sudden enhancement of potential gradient up to around 300 vm−1 after the start of the transit and then the gradual decrease extends up to around 50 vm−1. in the cases of potential gradient and vlf sferics and also in the case of first and second mode of schumann resonance, the «effect» of the venus transit reverses its nature at about 1400 h local time (figs. 1-3). this is probably due to the contribution from regional thunderstorm centre in se asia. the behaviour of the third mode of schumann resonance curves are not properly understood (fig. 1). the atmospheric temperature recorded at an indian meteorological department observatory about 10 km from observation site did not show any outstanding change during transit. relative humidity was a little bit higher than normal throughout the day. fig. 3. variation of atmospheric vertical potential gradient on the day of venus transit, 8th june 2004 (continuous line) and average variation of the same parameter around 8th june for 18 days (dotted line). syam s. de et al. 1213 the effect of recent venus transit on earth’s atmosphere 4. discussion the commonly accepted model explains fwf near the ground by the dynamic equilibrium between charging currents from lightning activity and the leakage current of the earthionosphere spherical capacitor (bering et al., 1998; fullekrug et al., 1999; rycroft and price, 2000). the idea is corroborated by the measurements of fullekrug et al. (1999) who showed a similarity in the monthly averaged pattern of fwf and elf amplitude. during the transit of venus, it obstructs 1/1000th part of incident solar radiation coming to the earth, since the angular diameter of venus compared to that of the sun as seen from the earth is of the ratio 1:32. therefore, apparently the transit of venus should have no significant effect on any terrestrial phenomenon. but, in spite of this fact, the changes observed in the atmospheric parameters at kolkata during the transit of venus are quite significant. this anomaly may be explained in terms of very large extent of the atmosphere of venus with about 96% co2 that produces atmospheric pressure of about 92 bars (http://pr.erau.edu/ ∼holmesg). because of very high content of co2, and nitrogen (warby, 1943), its atmosphere absorbs high frequency radiations, protons as well as other ionizing particles from the sun thereby depleting to a considerable extent the solar radiation reaching the earth. hence, during the transit, there will be some changes in the depth of interaction between earth’s magnetosphere and solar radiations. it is expected that collision frequency would be affected little by such interaction. the main influence would be on the rate of ionization within the earth-ionosphere cavity. this in turn introduces perturbation in the atmospheric electrical conductivity (roble, 1991) which may give rise to the changes that have been observed. the modification of solar atmosphere under the influence of tidal forces from the venus during transit may also contribute the observed effects (harnischmacher and rawer, 1981). each plot has two distinct time zones, one up to 13:30 to 14:00 h lt and other after that. a prominent feature of our observation is that the deviation of the average values of measured parameters changes signs at the boundary of two time zones. atmospheric potential gradient begins to increase from 2 h before onset, reaches a maximum after 1 h from onset and then gradually decreases and reaches a minimum at almost the same time as the end of transit. then it gradually increases and becomes equal to the average value about 4 h after the end of transit. the beginning and end of transit mean the transit of solid core of venus. the times of 1st and 4th contact of atmosphere of venus are different than the times of transit, 1st contact being earlier and 4th contact later than the recorded time of transit. the cause of the observed phenomenon may probably due to the rise of lower boundary of the ionosphere. changes of convection current due to change in intensity of solar radiation may also contribute to the cause. of particular interest is the increase in potential gradient from sunrise which continues upto 1 h after onset of transit. a similar observation in case of solar eclipse is known (dhanorkar et al., 1989). the amplitudes of 3 modes of schumann resonance show gradual increase from average value from sunrise and this tendency decreases from 10 h lt, i.e. one hour before transit. the gradual decrease continues up to 13 h lt, after which the variations assume the characteristics of regional thunderstorm activity in asia (nickolaenko and hayakawa, 2002). amplitudes of 9 khz sferics descend from the average value about 1 h before transit and then it slowly rises after 1 h. it crosses the average graph gradually and again meets it after 1 hour from the end of transit (fig. 2). at the end, it shows a fluctuation which may have its origin in local thunderstorm activity. the amplitude is significantly lower from about 1 h before transit to about 14 h lt. this portion of the ifi curve is structurally similar to the average curve. apparently, venus transit affects the attenuation parameter. for its explanation, calculation of amplitude under «eclipse» condition is necessary. atmospheric potential gradient on the day of transit of venus just before its onset increases which is probably due to the scattering of solar radiation and ionizing solar particles by the protruding part of venusian atmosphere (fig. 3). after the onset of transit, absorption of solar ra1214 syam s. de et al. diation takes over causing gradual decrease of fwf, fft amplitude and ifi [fig. 1 (continuous line), fig. 2 (continuous line) and fig. 3 (continuous line)]. the peak around 1400 h local time in fig. 1 correlates with lightning activity of regional thunderstorm centre in asia (nickolaenko and hayakawa, 2002). it is interesting to note that all the modes of schumann resonance do not respond equally to the lightning activity, probably because of differences in non linear interaction with transverse resonance and differences in attenuation rates. 5. conclusions the transit of venus is an event encouraging detailed study of its terrestrial effects. observations on schumann resonance spectra and earth’s atmospheric electric field were undertaken during the transit of venus. the results have been compared with our ongoing records. how so small a speck in the backdrop of solar disc can cause so large effect is an open question. we have tried to explain this anomaly in this presentation. acknowledgements the work has been funded by indian space research organization (isro) through s. k. mitra centre for research in space environment, institute of radio physics and electronics, university of calcutta, kolkata 700 009, india. references bering, e.a. iii, a.a. few and j.r. benbrook (1998): the global electric circuit, physics today, 51, 24-30. de, s.s., a.k. saha and m. de (2004): measurement of elf emission in the upper atmosphere over kolkata due to schumann resonances, ind. j. radio sp. phys., 33, 32-34. dhanorkar, s., c.g. deshpande and a.k. kamra (1989): atmospheric electricity measurements at pune during solar eclipse of 18th march 1988, j. atmos. terr. phys., 91, 1031-1034. fullekrug, m., a.c. fraser smith, e.a. bering and a.a. few (1999): on the hourly contribution of global cloud to ground lightning activity to the atmospheric electric field in the antarctic during december 1992, j. atmos. solar terr. phys., 61, 745-750. harnischmacher, e. and k. rawer (1981): lunar and planetary influences upon the peak electron density of the ionosphere, j. atmos. terr. phys., 43, 643-648. huang, e., e. williams, r. boldi, s. heckman, w. lyons, m. taylor, t. nelson and c. wong (1999): criteria for sprites and elves based on schumann resonance observations, j. geophys. res., 104, 16943-16964. nickolaenko, a.p. (1997): modern aspects of schumann resonance studies, j. atmos. solar-terr. phys., 59, 805816. nickolaenko, a.p. and m. hayakawa (2002): resonances in the earth ionosphere cavity (kluwer academic publisher), p. 97. roble, r.g. (1991): on modeling component processes in the earth’s global electric circuit, j. atmos. solar-terr. phys., 53, 831-847. rycroft, m.j. and m. cho (1998): modelling electric and magnetic fields due to thunderclouds and lightning from cloud top to the ionosphere, j. atmos. solar terr. phys., 60, 889-893. rycroft, m.j. and c. price (2000): the global atmospheric electric circuit, solar activity and climatic change, j. atmos. solar-terr. phys., 62, 1563-1576. satori, g. (1996): monitoring schumann resonances, ii. daily and seasonal frequency variations, j. atmos. terr. phys., 58, 1483-1488. tzanis, a. and d. beamish (1987): time domain polarization analysis of schumann resonance waveforms, j. atmos. terr. phys., 49, 217-228. warby, r.e. (1943): absorption spectrum of n2 in the extreme ultraviolet, phys. rev., 64 (7-8), 207-224. williams, e.r. (1992): the schumann resonance: a global tropical thermometer, science, 256, 1184-1187. (received november 29, 2005; accepted july 28, 2006) 455_472.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 455 probabilistic interpretation of «bath’s law» anna maria lombardi istituto nazionale di geofisica e vulcanologia, roma, italy abstract assuming that, in a catalog, all the earthquakes with magnitude larger than or equal to a cutoff magnitude m c follow the gutenberg-richter law, the compatibility of this hypothesis with « bath’s law » is examined. considering the mainshock m 0 and the largest aftershock m 1 of a sequence respectively as the first and the second largest order statistic of a sample of independent and identically distributed exponential random variables, the distribution of m 0 , m 1 and of their difference d1 is evaluated. in particular, it is analyzed as the distribution of d1 changes when only the sequences with the magnitude of the mainshock above a second threshold m c * m c are considered. it results that the distributions of m 0 , m 1 and d1 depend on the difference mc * m c and on the number of events in the sequence. moreover, the expected value of d 1 increases with increasing of m c * m c for every value of n. then it is shown that « bath’s law » could be ascribed to selection of data caused by the two thresholds m c and m c * and that it has a qualitative agreement with the model proposed. 1. introduction in 1958, richter, in a note to his book, wrote: «dr. bath has lately noted that in many instances the magnitude of the largest aftershock is about 1.2 less than that of the main shock» (richter, 1958, pag. 69). some years later, bath (1965) confirmed this issue, asserting that in a catalog for shallow shocks and for magnitudes based on the surface wave scale the sample mean of the difference d 1 between the magnitude of the mainshock m 0 and mailing address: dr. anna maria lombardi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: lombardi@ingv.it the respective largest aftershock m 1 followed the rule (1.1) independently of the sequences examined. he also provided some examples for which the formula (1.1) did not completely fit the data and tried to make changes to it. in spite of this, from then onwards, eq. (1.1) has become, under the name of « bath’s law », one of the most important and mentioned statistical laws concerning the distribution of the earthquakes in a seismic sequence. several papers have been published concerning the law and many related aspects (i.e. the distribution of the mainshock and the one of the larger aftershock; the distribution of the events in a sequence; the relation of m 0 , m 1 and d 1 with b or with the number of events in the sequence, ...) and in particular, diverse attempts have been made to provide a statistical explanation of the eq. (1.1). the most commonly accepted form for the magnitude distribution in a catalog is expressed key words magnitude distribution cluster size b-value order statistics d1 1 2= . 456 anna maria lombardi by the well-known formula (gutenberg-richter law) (1.2) where n (m ) is the number of events with magnitude larger than of equal to m (gutenbergrichter, 1954). this equation is equivalent to the statement that the magnitudes of events in a catalog are independent and identically distributed random variables: their distribution is exponential with parameter = b ln(10) (ranalli, 1969). considering that the estimated value of the parameter b in eq. (1.2) is close to unity, the value of is about 2.3. if m c is the cutoff magnitude (the lowest magnitude above which the data set is complete), the density function of the above-mentioned magnitudes is (ranalli, 1969) (1.3) utsu (1961, 1969) noted that if the magnitude of the mainshock was included in the sample of random variables with density function (1.3), the gutenberg-richter law and « bath’s law » were in strong contradiction. in fact, it is well-known by the theory of order statistics that if m 1 , ... m n are independent and identically distributed random variables with density function (1.3), then the difference between the largest order statistic m ( n ) and the second largest order statistic m ( n-1) is an exponential random variable with the same parameter as in (1.3), irrespective of the sample size n; furthermore, under the same assumptions, this difference is positively correlated with m ( n ) (feller, 1966; utsu, 1969; vere-jones, 1969). therefore utsu pointed out that, if the mainshock was included in the random sample, the observed sample mean of d1 (equal to 1.2) was considerably larger than the expected value 1/ (�0.5). moreover, the positive correlation between m ( n ) and m ( n ) m ( n-1 ), predicted by the probability theory, was in disagreement with the independence, implicit in « bath’s law», or with the marked negative correlation, observed by himself on the japanese aftershock sequences (utsu, 1961, 1969), log10 n m a bm( )[ ] = f m e m mm mc( ) = ( ) c . between d1 and m0. he inferred from these results that the mainshocks had a different distribution from the aftershocks: it was not acceptable to consider the mainshock as the largest order statistic of an exponential random sample. the statistical interpretation of utsu was confuted by vere-jones (1969): he ascribed the discrepancies observed by utsu to the bias in selecting data and not to intrinsic properties of the aftershocks. in fact, only the sequences with the magnitude of the mainshock above a second threshold m c * (larger than m c ) were included in the data set. this selection caused, in his opinion, a different distribution of order statistics, an expected value larger than 1/ for d1 and a negative correlation between m 0 and d 1 . therefore, he concluded that « bath’s law » and the results of utsu were compatible with the hypothesis that the mainshock is the largest member of a sample of independent exponential random variables. he corroborated this thesis in a second work (vere-jones, 1975). it does not seem that the following papers on distribution of d1 (papazachos, 1974; purcaru, 1974; tsapanos, 1990) have accepted the interpretation given by vere-jones, preferring to it the one given by utsu. moreover, in other papers, « bath’s law » is mentioned as a proof that the mainshock does not come from the same population as the aftershocks (evison, 1999; evison and rhoades, 2001; lavenda and cipollone, 2000). on the contrary, in my opinion, verejones provides a satisfying explanation of the problem. the purpose of this paper is to go on with the interpretation of vere-jones making a thorough mathematical analysis of « bath’s law » and of related matters, based only upon basic elements of the probability theory, and to verify if the hypothesis that all the events come from the same population and « bath’s law » are compatible. 2. data and statistical analysis considering the magnitudes of the mainshock and of its aftershocks as a sample of independent and exponential distributed random variables, it is possible to evaluate the density functions of m0, m1 and d1 conditioned by the event 457 probabilistic interpretation of «bath’s law» {m0 mc *} and the relative expected values. in appendix a, the mathematical elaboration is explained in detail: it is shown as the conditional ditributions of m 0 , m 1 and d 1 depend on b, m c * m c and on the number n of aftershocks in the sequence (see eqs. (a.5), (a.7), (a.8)). to verify the reliability of the hypothesis of full compatibility between the gutenberg-richter law and « bath’s law », the catalog compiled by the southern california earthquake data center, including events with magnitude equal to or larger than 2.0 occurred in the time period 19902001, was analyzed; the total number of events is 62 394. as fig. 1 shows, the gutenberg-richter law fits very well the magnitudes of events and the catalog can be considered complete. to decluster the above-mentioned catalog the reasenberg algorithm was used (reasenberg, 1985). the clusters with aftershocks identified number 1763 and the clustered events are 33 360. to estimate the b-value the maximum likelihood method was used (utsu, 1966). the value obtained was b̂ = 0.8851. the dependence of probability distributions of m 0 , m 1 and d 1 on n implies that, to make a statistical analysis, the data must be divided into groups according to the size of clusters. table i shows the results of the analysis of catalog: the data have been divided into groups and only the results relative to all clusters and to groups with at least 30 sequences have been reported. the clusters with m c *= 4.0 and m c = 2.0 have a very variable size and it is impossible to make a statistical analysis with data relative to a single value of n; then only the results relative to all clusters have been reported. comparing the sample mean of m 0 and d1 (m� 0, d�1 ), relative to clusters with number of events n (foreshocks have been left out) and with thresholds m c and m c * , with the corresponding fig. 1. gutenberg-richter law for clustered events (33 360) of southern california catalog, with magnitude 2.0 which occurred in the interval time 1990-2001. 458 anna maria lombardi efficient is inside the range [0.6-0.9]. also by the analysis of the catalog it results that d 1 and m 0 are positively correlated random variables and that the estimated values of the correlation coefficient agree with the theoretical ones. as table i shows, the overall estimated correlation coefficient between m 0 and d 1 for m c * = 4.0 and m c = 2.0 is very near to 0. as i show in following section, this results is not in contradiction with the positive values of the theoretical correlation coefficient plotted in fig. 2. 3. discussion unfortunaly, it is very difficult to verify, with a rigorous statistical test, if the theoretical averages expected by the model (ie[m0], ie[d1]), a substantial agreement can be noted: for the same value of n the sample mean for d 1 increases with increasing m c * m c and the overall mean value 1.2 can be justified only for a difference between the two thresholds equal to 2. the table also lists the estimated values of the correlation coefficient between m0 and d1 (est corr [m 0, d1] ) and the corresponding values predicted by the model . for the sake of brevity not all calculations to evaluate it by the model are reported. however in fig. 2 its theoretical values are plotted versus n for four values of mc * mc , n 100 and for b = 1. as fig. 2 shows, m 0 and d 1 are always positively correlated and the correlation co corr n d m1 0, /( )[( m0/{ mc }] ) table i. results of the statistical analysis of the southern california catalog divided according to number of events in every cluster and to the difference of the two thresholds. n is the number of events in the cluster (foreshocks have been left out); n cl is the number of clusters with number of events n (only values with n cl 30 are shown); is the sample mean of d 1 ; is the theoretical expected value of d 1 ; is the sample mean of m 0 ; is the theoretical expected value of m 0 ; estcorr[m 0 , d 1 ] is the estimated sample correlation coefficient; corr[m 0, d1] is the theoretical correlation coefficient. in columns relative to and , after the symbol ±, there is the standard deviation of the relative variable. d1 m 0 d1 n n cl �d1 ie [d1 ] �m 0 ie [m 0 ] estcorr [m 0, d1 ] corr [m 0, d1 ] all 1763 0.4487±0.4607 0.4907 2.8836±0.6914 0.7036 2 1077 0.4162±0.4329 0.4907 2.6713±0.5079 2.6860 0.8327 0.8944 mc * = 2.0 3 285 0.4368±0.4590 0.4907 2.8368±0.5460 2.8496 0.8063 0.8571 m c = 2.0 4 99 0.5515±0.5346 0.4907 3.1253±0.6428 2.9722 0.8259 0.8381 (1763 clusters) 5 58 0.5224±0.5991 0.4907 3.2879±0.6931 3.0704 0.7628 0.8266 6 36 0.5306±0.4720 0.4907 3.2083±0.6429 3.1521 0.8626 0.8189 7 32 0.4812±0.5727 0.4907 3.1000±0.5913 3.2222 0.8325 0.8133 all 653 0.8190±0.5212 3.5881±0.6141 0.4082 mc * = 3.0 2 275 0.9262±0.4867 1.1039 3.3629±0.3950 3.4578 0.6509 0.8003 m c = 2.0 3 100 0.8260±0.5266 0.9335 3.4320±0.4360 3.4751 0.6963 0.7782 (653 clusters) 4 54 0.8556±0.5375 0.8318 3.5852±0.4788 3.4926 0.7438 0.7740 5 37 0.7405±0.6453 0.7628 3.6676±0.5696 3.5102 0.7020 0.7742 mc * = 4.0 m c = 2.0 all 142 1.1718±0.6492 4.5204±0.5936 0.0766 (142 clusters) d 1[ ]ie m0[ ]ie m 0 459 probabilistic interpretation of «bath’s law» distribution evaluated in appendix fit the data used by utsu and other authors for their statistical analyses, most of all because the sample size n of every sequence is not known. however some results, presented in the works on « bath’s law », seem to agree with the model. for example, an explanation of purcaru’s results (1974), obtained in his analysis of japanese and greek data, can be provided. in his paper, he observed that in both data sets, m 0 followed the exponential distribution with the same parameter as the distribution of general earthquakes. vere-jones (1969) showed that this result was incompatible with the probability theory (the maximum of a sample of n independent and identically distributed random variables does not follow the same distribution as the elements of the sample) and that the distribution of m 0 was only asymptotically (i.e. when m c * + ) an exponential one. figure a.1a-e shows that for every value of n, m 0 is not an exponential random variable and that the mainshocks have a different distribution from that of any random variable in the sample: in fact it is not a generic variable of the sample, but it is the largest one. however, the distribution of m 0 converges rather fast to an exponential when m c * m c + and then the model is completely in agreement with their conclusions, considering that in the catalogs analyzed by the two authors m c * mc it is larger than or equal to 2; to be precise: in vere-jones paper, for the japanese catalog, it is m 0 6 and m 1 4; in purcaru’s paper, for the japanese catalog, it is m 0 6 and m 1 3.2; in purcaru’s paper for the greek catalog it is m 0 5.6 and m 1 3.5. it has been observed (solov’ev and solov’eva, 1962; vere-jones, 1969; lavenda and cipollone, 2000) that there is a linear relation between the sample mean of the magnitude of the mainshock and the natural logarithm of the number of events (or only of the aftershocks) in the sequences of a catalog. moreover, vere-jones fig. 2. correlation coefficient between m 0 and d1, relative to the conditional distributions of section 2, versus the sample size n for m c * = m c , m c + 1, m c + 2, m c + 3. 460 anna maria lombardi (1969) demonstrated that there is a linear relation between the expected value of m ( n ) and ln(n ) when m c * is equal to m c . as fig. 3 shows, eq. (a.6) is consistent with this result for m c * equal to m c , but when m c * is larger than m c , the relation is only asymptotically (i.e. when n + ) linear. with regard to the distribution of m 1, purcaru (1974) noted that it is closer to a normal distribution that an exponential one. considering the high difference between the two cut-off values of m 0 and m 1 in two data sets examined by him, this issue is predicted by the gutenbergrichter law (see fig. a.2a-d). moreover, there is an impressive similarity between the smoothed histograms for the distribution of m1, shown in figs. 14, 15 and 16 of the purcaru’s work, and the theoretical densities plotted in fig. a.2a-d of the present paper, for m c * m c = 2 and m c * m c = 3. moreover, comparing fig. a.1a-e to fig. a.2a-d, it is evident that the distribution of m 0 is not equal to that of m 1. therefore, the observed differences between the two above-mentioned distributions do not necessarily have to be ascribed to «different conditions in which the mainshocks and the largest aftershocks occur» (purcaru, 1974): m 0 and m 1 are not variables selected at random, but they are the first and the second largest observations of a random sample respectively and, in agreement with the theory of order statistics, they cannot have the same distribution. then, even considering the gutenberg-richter law true for all the events, the probability theory predicts that the mainshock and the largest aftershock come from different populations. as regards the distribution of d1, purcaru (1974) showed in his paper that it was not an exponential one: in fact the histograms of the differences between the mainshocks and the relative largest aftershocks for the catalogs tested by him did not agree with the exponential density. moreover, the evaluated values of the coefficient of variation (0.52 for japan and 0.5 for greece) were discordant with the value (equal to 1) of the coefficient of variation for an exponential variable. furthermore, in his conclusions, he pointed out that the observations did not confirm, fig. 3. conditional expected value of m 0 by the event {m 0 m c *} versus the natural logarithm of the sample size n for m c * = m c , m c + 1, m c + 2, m c + 3. 461 probabilistic interpretation of «bath’s law» 0.59 and the coefficient of variation increased from 0.54 to 0.9. as fig. a.3a-e, figs. 4 and 5 show, all these results completely agree with the model. in fact, d 1 is an exponential variable only when m c * m c is equal to 0 or when m c * m c is positive and n + . in fig. 4, the coefficient of variation is plotted versus n for b equal to 1 and m c * m c equal to 2.0 (value of m c * m c for the japanese catalog analyzed by utsu, 1961 and vere-jones, 1969), 2.1 and 2.8 (values of m c * m c for the japanese catalog and the greek catalog, respectively, analyzed by purcaru, 1974): these curves could be consistent with the values, close to 0.5, of the analyses of these authors. generally, « bath’s law » and that, when the same cut-off was chosen for m0 and m1, the distribution of d1 seemed to be an exponential one with a sample mean close to 0.5 (see purcaru, 1974: tables 7 and 8) and an estimated coefficient of variation close to 0.8. in the end, he observed that the sample mean of d1 increased with the increase in the difference between the cut-off value of m 0 and m 1 by a linear dependence. similar results had been obtained by vere-jones (1969) in his analysis of the catalog of japanese aftershock sequences compiled by utsu in 1961. in fact, he had shown that when the difference between the two thresholds decreased from 2 to 0, the sample mean of d1 decreased from 1.39 to fig. 4. coefficient of variation of d1 relative to the conditional density function of eq. (a.8) versus the sample size n for mc * mc = 2.0 (value of the difference between the two thresholds of the japanese catalog analyzed by utsu (1961, 1969)) and by vere-jones (1969)), for m c * m c = 2.1 (value of the difference between the two thresholds of the greek catalog analyzed by purcaru (1974)) and for m c * m c = 2.8 (value of the difference between the two thresholds of the japanese catalog analyzed by purcaru, 1974). 462 anna maria lombardi the plot in fig. 5 shows, for b equal to 1 and four values of n, the relation between the conditional average and the difference of the two thresholds m c * and m c : it is consistent with the linear relation observed by purcaru (1974: fig. 11), considering that in both catalogs analyzed by him m c * m c is larger than 2.0. in a more recent paper, drakatos and latoussakis (2001), in their description of spatial and temporal characteristics of sequences in greece, dealt with the distribution of d1. they concluded that data were fitted by a normal distribution with an average of 0.9. considering that the two thresholds chosen were m c = 3.2 and m c * = 5.0, this result is consistent with the model: the similarity between fig. 5 of the drakatoslatoussakis paper and fig. a.3a-d of the present work is evident. the only point that seems to be in disagreement with the results and conclusions of utsu and purcaru, is the correlation coefficient between m 0 and d1. in fact fig. 2 shows that it is always positive for every value of n and m c * m c . also the analysis of the southern california catalog, in the previous section, shows a positive correlation between the two variables, but the overall estimated correlation coefficient for m c = 2.0 and m c * = 4.0 is 0.08. to weigh the compatibility of this value with the model, a simulation was done of 1000 groups of 142 clusters of independent exponential variables with the same size as clusters identified in the california catalog and the same b-value. the values obtained for the correlation coefficient are inside the range [ 0.010, 0.522]; moreover 15 of them are less than 0.08 and 190 are less than 0.2. then, considering all clusters, apart from fig. 5. conditional expected value of d 1 by the event {m 0 m c *} versus the difference between the two thresholds m c * m c for n = 2, 10, 100, 1000. d m1 0/{ }[ ]mcie n 463 probabilistic interpretation of «bath’s law» their size, the value of the correlation coefficient between m 0 and d1 can be rather low. it is not easy to justify this result with the model proposed: it becomes necessary to consider a distribution function for the cluster size n and to study the distribution of m 0 and d1 for a generic cluster (aside from n ). however it is not impossible for m 0 and d1 to have a correlation coefficient very near to 0 or even negative, as for the data analyzed by utsu and purcaru. moreover, as the same authors noted, for some clusters of their catalogs, the values of d1 were not known and then they were excluded in the statistical analysis: this selection could have caused a bias in results. in my opinion, there are too many factors that influence the results: the declustering algorithm used, the precision in estimate of magnitudes and then of b-value, the number of events not recorded (it is known that soon after a mainshock with high magnitude, the aftershocks are not recorded). the question needs further enquiries, but, in my opinion, only by analysis of data, without hypotheses about the magnitude distribution, it is evident that the value 1.2 of the « bath law » is justified only if the catalog is selected with thresholds that have a difference equal to or larger than 2.0. moreover, it does not seem to me that the previous studies exhaustively justify the incompatibility between the gutenberg-richter law and the « bath law ». 4. conclusions considering the gutenberg-richter law true for all the earthquakes of a catalog and utilizing only the results of the probability theory about the order statistics, the conditioned distributions of m 0 , m 1 and d 1 by the event {m0 mc *} are evaluated. as shown in the previous sections, most of the results obtained in the past on «bath’s law» are consistent with the probabilistic analysis expounded in the second section. in particular, assuming that the mainshock is the largest event of a sample of independent and identically distributed exponential random variables, it results that: 1) m 0 converges in distribution to an exponential variable when m c * + : for low values of n the convergence is very fast and this could justify the frequent conclusion that the mainshock magnitude is an exponential variable. 2) m 0 and m 1 are random variables with different distributions, not because the mainshocks have a different nature from aftershocks, but because the order statistics are not independent and identically distributed random variables as the non-ordered random variables of the sample. 3) the choice of the two thresholds m c and m c * is crucial for the distributions of m 0 , m 1 and d1. moreover, these distributions depend on n and, of course, on b. in particular, when m c * is larger than m c , d1 is not an exponential variable, its expected value is higher than 1/ and it could be consistent with « bath’s law ». 4) there is a substantial agreement between the theoretical model presented and the results obtained in the past on the distribution of m 0 , m 1 and d 1 and those obtained by the analysis of the southern california catalog. in particular, the hypothesis of vere-jones, i.e. that « bath’s law » can be ascribed to the selection of data caused by the choice of the threshold of the mainshocks m c *, seems to be confirmed. 5) even if the model predicts a positive correlation between m 0 and d1 for every value of m c * m c and n, it has been shown by simulations that for all clusters, apart from n, the value of the correlation coefficient can become near to 0 or negative. this result could justify the negative or law correlation observed by utsu and purcaru, considering also that for catalogs analyzed by them, the values of d 1 were not all known and that a selection of data was made. the purpose of the present study was to show that « bath’s law » is justified only if catalogs are suitably selected and that it does not seem to be in disagreement with the very simple hypothesis that all the events follow the gutenbergrichter law. of course, further analyses must be made on this matter. first of all, the model has to be tested on a suitable data set: in fact the dependence on n of the distribution of d1 in the model requires that this be tested in a catalog with many aftershock sequences and that the size of every sequence is known. as observed in 464 anna maria lombardi appendix a. mathematical background. let m1, ..., mn be n independent and identically distributed random variables with density function (1.3) and let m (1) ... m ( n ) be the corresponding order statistics; then (feller, 1966; casella-berger, 1990) m (1), ..., m (n ) are not independent and the density function of m (i), 1 i n, is (a.1) the joint density function of m (i ) and m ( j ), 1 i < j n, is (a.2) where . let’s consider the random variables d 1 = m ( n ) m (n-1), d 2 = m (n-1) m (n-2), ..., d n-1 = m (2) m (1), d n = m (1); then d1, ..., dn are independent exponential random variables with parameter , 2 , ..., n , respectively. therefore the distribution of d1 is independent of the sample size n and its expected value is (a.3) f m n i n e e e m n m m m m i m m n i i c c c ( ) ( ) = ( ) ( ) ( ) ( )( ) ( ) ( ) ! ! !1 1 1 1 ; f m n m m n i j( ) ( ) ( ) =, , c e e e e e ei j n m m n m m m i m m n m j i n m n j c c c c c c , ( ) ( ) ( ) ( ) ( ) ( )( ) ( ) ( )1 1 1 c n i j i n j i j n , = ( ) ( ) ( ) 2 1 1 ! ! ! ! d1. of the magnitude scale used. in conclusion, the same probabilistic elaboration could be made utilizing different distributions from the exponential one for the magnitudes of a catalog (i.e. the kagan distribution; kagan, 1997). acknowledgements the author is grateful to dr. r. console (ingv) and to reviewers of this paper for their useful comments. d1 1[ ] = .ie section 2, when m c * m c 0, the size of sequences is highly variable and then it is impossible to make a statistical analysis for a value of n. moreover, the influences of declustering algorithms and of rules used to identify the sequences could be studied. in fact, the definitions themselves of «mainshock-aftershock sequences» chosen by some authors for their analyses (see, i.e. utsu, 1970; evison, 1981; evison and rhoades, 1993) could cause arbitrary selections of data and then they could further modify the distribution of d 1 . it could also be interesting to study the influence 465 probabilistic interpretation of «bath’s law» furthermore d 1 and m (n ) are positively correlated and the correlation coefficient is (a.4) (vere-jones, 1969). from now onwards we shall denote the largest order statistic m (n ) and the second largest order statistic m ( n 1) with m 0 and m 1 , respectively. let now m c * be a constant larger than or equal to m c . let’s compute the density function of m 0, m 1 and d1 conditioned by event {m 0 mc *}. a.1. the density function of m 0 conditioned by {m0 mc *} let’s denote the density function of m 0 conditioned by the event {m 0 mc *} with then (a.5) = the relative conditional expected value is (a.6) the conditional distribution of m 0 depends on as well as on the sample size n. furthermore it depends on difference between the two thresholds m c * and m c , but this dependence vanishes when n + (see fig. a.1a-d). in fig. a.1e the conditional expected value of m 0 (eq. (a.9)) is plotted versus the sample size n for four values of m c * m c and for b = 1: it is evident that the value of increases with the increasing of n and of m c * m c . = ( ) ( )( ) v ar v ar m n d1 f c n m m0 0/ . { }{ }m f m f m f x dxc n n c n m n m n m m m0 0/{ }{ } + ( ) ( ) ( ) = ( ) ( ) = m 0 if <m m c n e e e m m m m m m n m m n c c c c c ( ) ( ) ( ) ( ) ( ) 1 1 1 1 if . { = + ( ) ( ) ( ) ( ) = m e e k c m m n m m k k n c c c c1 1 1 1 1 1 . m m m m 0 0 0 0 / / { }[ ] = ( ) ={ }{ } + m mf m dm c m n m cc ie n m m0 0/{ }[ ]mcie n 466 anna maria lombardi fig. a.1a-e. a) conditional density function of m 0 by the event {m 0 m c *} (see eq. (a.5)) for m c *=m c , m c + 1, m c + 2, m c + 3, and for n = 2. b) the same as (a) but for n = 10. c) the same as (a) but for n = 100. d) the same as (a) but for n = 1000. e) conditional expected value of m0 by the event {m0 mc *} (see eq. (a.6)) versus the sample size n for four values of difference between m c * and m c (m c * m c = 0,1,2,3). a b c d e 467 probabilistic interpretation of «bath’s law» a.2. the density function of m 1 conditioned by {m 0 mc *}. let’s denote the density function of m1 conditioned by the event {m 0 mc *} with then = fig. a.2a-d. a) conditional density function of m1 by the event {m0 mc *} (see eq. (a.7)) for m c *= m c , m c + 1, m c + 2, m c + 3, and for n = 2. b) the same as (a) but for n = 10. c) the same as (a) but for n = 100. d) the same as (a) but for n = 1000. a b c d 0 if m m c f m n dn f n dn m m m m m n m m n m c c n n c n c ( ) ( ) ( ) ( ) ( ) < + + 1 , , if f m n dn f n dn m mm m n m m n m c n n n c ( ) ( ) ( ) ( ) ( ) > + + 1 , , if { f m c n m m1 0/{ }{ } ( ) = m f c n m m1 0/{ }{ }m . 468 anna maria lombardi = (a.7) as eq. (a.7) and figs. a.2a-d show, also the distribution of m 1 depends on b, n and mc * mc but the dependence on mc * mc is less marked than that of m 0. a.3. the density function of d 1 conditioned by {m 0 m c *}. let’s denote the density function of d1 conditioned by the event {m 0 mc *} with then = = { if >m m m m m m m1 1 0 1 1 0 1> +( ) + +( )m x m m m xx m mc c c c c c, , if m m m m m m m 1 1 0 1 1 0 1 > +( ) + +( )m x m x m m x x m m c c c c c c , , ip ip ipip 0 if m m c n n e e e e m m m m m m m m m n m m n c c c c c c c c ( ) ( ) < ( ) ( ) ( ) ( ) 1 1 1 1 2 if n n e e e m m m m m m n m m n c c c c c ( ) ( ) ( ) > ( ) ( ) ( ) 1 1 1 1 2 2 if { f c n d m1 0/{ }{ }m . d m d m m 1 0 1 0 0 { }( ) = ( ) ( ) =x m x m m c c c / , ip ip ip = >( ) + ( ) ( ) = m m m m m m m m m 0 1 0 1 0 1 0 1 0 x m m x m m m c c c c c , , , ,ip ip ip = > +( ) + ( ) +( ) ( ) m m m m m m m 1 1 0 1 1 0 1 0 m x m m x m m m x m c c c c c , , ,max c ip ip ip 469 probabilistic interpretation of «bath’s law» = computing the integrals and deriving the conditional distribution function, it results = if x m c * m c if x > m c * c (a.8) the relative conditional expected value is (a.9) the conditional distribution of d1 depends on , n and mc * m c . as fig. a.3a-e show, if m c * is equal to m c , there is no conditioning and d1 is an exponential variable. furthemore, for n + , d1 converges in distribution to an exponential random variable, for all values of m c * mc, but the lower is the value of m c * m c the faster is the convergence. consequently, it results (a.10) (see fig. a.3e). = + ( ) ( )( ) ( ) ( ) = 1 1 1 1 1 1ne e m m e k m m m m n c c m m k k n c c c c c c . d m1 0 1 /{ }[ ] +mc nie n { ds dt f s t ds dt f s t x m mm m m mms x c cmmss xm n n n ncccc if >( ) ( ) ( ) ( )( ) + ( )+++ 1 1, ,, , ds dt f s t ds dt f s t x m m m m m mm s x c cm x m s s x m n n n ncc c c if ( ) ( ) ( ) ( )( ) + ( ) +++ 1 1, , , , f x m n cd m1 0/{ }{ } ( ) = { e x d m d m 1 0 0 1 0 / / { }[ ] = ( ) ={ }{ } + m x f x dxc m n c ie n e e e ne e ex m m x n m m m m x n c c c c c c( ) ( ) ( )( ) ( )1 1 1 1 e m m n c c( )( )1 1 e m m n c c( )( )1 1 470 anna maria lombardi fig. a.3a-e. a) conditional density function of d 1 by the event {m 0 m c *} (see eq. (a.8)) for m c *= m c , m c + 1, m c + 2, m c + 3, and for n = 2. b) the same as (a) but for n = 10. c) the same as (a) but for n = 100. d) the same as (a) but for n = 1000. e) conditional expected value of d1 by the event {m0 m c *} (see eq. (a.9)) versus the sample size n for four values of difference between m c * and m c (m c * m c = 0,1,2,3)e a b c d 471 probabilistic interpretation of «bath’s law» given that the estimated values of b are very close to 1, the asymptotic value of the conditional average of d 1 is about 0.4343. table a.i shows the dependence of on the b-value for some values of n and of m c * m c and for b in the range [0.5, 1.5]. the kind of this dependence is highly variable and it does not have a monotone trend (for some values of m c * m c and n the conditional average of d1 decreases with the increasing of b, while, in other cases, it has a minimum for a value of b inside the range examined). however a variation smaller than 0.15 of the conditional expected value of d 1 corresponds to a variation of 0.1 for b. table a.i. values of the expected value of conditioned by the event {m 0 m c *} for some values of b, n, and mc * mc. d1 mc * = m c mc * = m c + 1 mc * = m c + 2 mc * = m c + 3 b all n n=2 n=10 n=100 n=1000 n=2 n=10 n=100 n=1000 n=2 n=10 n=100 n=1000 0.5 0.8686 1.3509 0.8855 0.8686 0.8686 2.1510 1.1602 0.8686 0.8686 3.0622 1.7605 0.8977 0.8686 0.6 0.7238 1.2476 0.7583 0.7238 0.7238 2.0887 1.1405 0.7248 0.7238 3.0297 1.8535 0.8499 0.7238 0.7 0.6204 1.1796 0.6782 0.6204 0.6204 2.0532 1.1576 0.6294 0.6204 3.0144 1.9557 0.9022 0.6206 0.8 0.5429 1.1328 0.6279 0.5429 0.5429 2.0323 1.1926 0.5783 0.5429 3.0071 2.0532 1.0078 0.5513 0.9 0.4825 1.0996 0.5974 0.4826 0.4825 2.0198 1.2356 0.5666 0.4825 3.0035 2.1409 1.1355 0.5373 1.0 0.4343 1.0755 0.5801 0.4343 0.4343 2.0122 1.2811 0.5846 0.4343 3.0017 2.2176 1.2677 0.5850 1.1 0.3948 1.0577 0.5720 0.3949 0.3948 2.0076 1.3260 0.6222 0.3954 3.0009 2.2839 1.3945 0.6751 1.2 0.3619 1.0444 0.5703 0.3624 0.3619 2.0047 1.3688 0.6718 0.3675 3.0004 2.3411 1.5113 0.7879 1.3 0.3341 1.0343 0.5730 0.3358 0.3341 2.0029 1.4086 0.7279 0.3564 3.0002 2.3905 1.6166 0.9091 1.4 0.3102 1.0266 0.5788 0.3147 0.3102 2.0018 1.4451 0.7865 0.3647 3.0001 2.4334 1.7104 1.0299 1.5 0.2895 1.0207 0.5868 0.2992 0.2895 2.0011 1.4784 0.8451 0.3900 3.0001 2.4709 1.7938 1.1450 d m1 0/{ }[ ]mcie n references bath, m. (1965): lateral inhomogeneities of the upper mantle, tectonophysics, 2, 483-514. casella, g. and r.l. berger (1990): statistical inference (wadsworth), pp. 650. drakatos, g. and j. latoussakis (2001): a catalog of aftershock sequences in greece (1971-1997): their spatial and temporal characteristics, j. seismol., 5, 137-145. evison, f. (1981): multiple earthquakes events at moderateto-large magnitudes in japan, j. phys. earth, 29, 327-339. evison, f. (1999): on the existence of earthquake precursors, ann. geofis., 42 (5), 763-770. evison, f. and d. rhoades (1993): the precursory earthquake swarm in new zealand: hypothesis tests, n. z. j. geol. geophys., 36, 51-60. evison, f. and d. rhoades (2001): model of long-term seismogenesis, ann. geofis., 44, (1), 81-93. feller, w. (1956): an introduction to probability theory and its applications (wiley and sons, new york), vol. 2, pp. 669. gutenberg, b. and c.f. richter (1954): seismicity of the earth and associated phenomena, princeton, pp. 273. kagan, y.y. (1997): seismic moment-frequency relationship for shallow earthquakes; regional comparisons, j. geophys. res., 102, 2835-2852. lavenda, b.h. and e. cipollone (2000): extreme value statistics and thermodynamics of earthquakes: aftershock sequences, ann. geofis., 43 (5), 967-982. papazachos, b.c.(1974): on certain aftershock and foreshock parameters in the area of greece, ann. geofis., 27, 497-515. purcaru, g. (1974): on the statistical interpretation of the bath’s law and some relations in aftershock statistics, geol. inst. techn. ed. study geophys. prospect., bucharest, 10, 35-84. ranalli, g. (1969): a statistical study of aftershock 472 anna maria lombardi sequences, ann. geofis., 22, 359-397. richter, c.f. (1958): elementary seismology (freeman, san francisco, calif.), pp. 768. reasenberg, p. (1985): second-order moment of central california seismicity, 1962-1982, j. geophys. res., 90, 5479-5495. solov’ev, s.l. and o.n. solov’eva (1962): exponential increase of the total number of an earthquake’s aftershocks and the decrease of their mean value with increasing depth, izv. akad. nauk. sssr, ser. geofiz. 12, 1685-1694, english translation, 1053-1060. tsapanos, m. t. (1990): spatial distribution of the difference between the magnitudes of the main shock and the largest aftershock in the circum-pacific belt, bull. seismol. soc. am., 80, 1180-1189. utsu, t. (1961): a statistical study on the occurrence of aftershocks, geophys. mag., 30, 521-605. utsu, t. (1966): a statistical significance test of the difference in b-value between two earthquake groups, j. phys. earth, 14, 37-40. utsu, t. (1969): aftershock and earthquakes statistics (i), j. fac. sci., hokkaido univ., 3, 129-195. utsu, t. (1970): aftershock and earthquakes statistics (ii), j. fac. sci., hokkaido univ., 3, 196-266. vere-jones, d. (1969): a note on the statistical interpretation of bath ’s law, bull. seismol. soc. am., 59, 1535-1541. vere-jones, d. (1975): stochastic models for earthquake sequences, geophys. j. r. astron. soc., 42, 811-826. (received december 13, 2001; accepted march 11, 2002) vol. 48, 01, 05ok.qxd 159 annals of geophysics, vol. 48, n. 1, february 2005 key words carbon isotope composition – total inorganic carbon dissolved in water (tdic) – stable isotopes – continuous flow irms measurements – on-line extraction technique 1. introduction the stable carbon isotope ratios of dissolved inorganic carbon (δ 13ctdic) have been successfully applied in studies aimed at the evaluation of sources, sinks and fluxes of carbon taking place in natural waters (atekwana and krisnamurthy, 1998). in active volcanic areas, on the basis of the carbon isotope balance among dissolved carbon species in solution (co2aq, hco3– and co32–), the pristine carbon isotope composition of co2 interacting with thermal waters has been estimated (favara et al., 1999; inguaggiato et al., 2000). temporal variations observed in the isotopic composition of tdic in some geothermal waters of ischia island were recognized as significant for volcanic surveillance (caliro et al., 1999). the determination of δ 13ctdic is based on isotope measurements of carbon dioxide extracted from the samples. the most common reactions to produce co2 from water samples are: i) reaction of 100% h3po4 with solid carbonate mailing address: dr. giorgio capasso, istituto nazionale di geofisica e vulcanologia, sezione di palermo, via ugo la malfa 153, 90146 palermo, italy; e-mail: g.capasso@pa.ingv.it on-line technique for preparing and measuring stable carbon isotope of total dissolved inorganic carbon in water samples (δδ 13ctdic) giorgio capasso, rocco favara, fausto grassa, salvatore inguaggiato and manfredi longo istituto nazionale di geofisica e vulcanologia, sezione di palermo, italy abstract a fast and completely automated procedure is proposed for the preparation and determination of δ 13c of total inorganic carbon dissolved in water (δ 13ctdic). this method is based on the acidification of water samples transforming the whole dissolved inorganic carbon species into co2. water samples are directly injected by syringe into 5.9 ml vials with screw caps which have a pierciable rubber septum. an analytical precision «carbonate prep system» was used both to flush pure helium into the vials and to automatically dispense a fixed amount of h3po4. full-equilibrium conditions between produced co2 and water are reached at a temperature of 70°c (± 0.1°c) in less than 24 h. carbon isotope ratios (13c/ 12c) were measured on an ap 2003 continuous flow mass spectrometer, connected on-line with the injection system. the precision and reproducibility of the proposed method was tested both on aqueous standard solutions prepared using na2 co3 with δ 13c = −10.78 per mil versus pdb (1σ = 0.08, n = 11), and at five different concentrations (2, 3, 4, 5 and 20 mmol/l) and on more than thirty natural samples. mean δ 13ctdic on standard solution samples is –10.89 < per mil versus pdb (1σ = 0.18, n = 50), thus revealing both a good analytical precision and reproducibility. a comparison between average δ 13ctdic values on a quadruplicate set of natural samples and those obtained following the chemical and physical stripping method highlights a good agreement between the two analytical methods. 160 giorgio capasso, rocco favara, fausto grassa, salvatore inguaggiato and manfredi longo precipitated from the water sample and ii) direct injection of phosphoric acid onto the liquid sample. both these methods include the use of an expensive vacuum line and/or dedicated sampling and analytical devices. the first procedure implies the use of a large water sample. furthermore, it requires several steps for sample preparation including precipitation of carbonates by using ba(oh)2 or sr(oh)2, filtering and drying under a controlled (carbon-free) atmosphere, homogenization of the precipitate and finally its reaction with acids to form a co2 gas phase into a vacuum line and gas extraction. all these laborious preparation procedures comprise several potential sources of error and they lead to slow analysis time. a more rapid procedure consists of the addition of phosphoric acid onto the liquid sample. the chemical equilibrium among dissolved carbon species in the solution is shifted towards co2. the carbon dioxide produced by means of this process can be extracted into a vacuum line (atekwana and krisnamurthy, 1998; favara et al., 2002) or dynamically extracted under a re-circulating flow of high-purity inert gas (mc nicol et al., 1994). in the following sections, we report a rapid and simple method for collection and preparation of water samples for δ 13ctdic analyses. the extraction technique discussed in this paper is based on the liberation of co2 yielded by acidification of water samples. it requires a low amount of sample (0.5-2 ml) and an equilibrium time of about 18 h, when full-equilibrium conditions between produced co2 and water are established. the use of an autosampler both for acid dosing and for sample injection into the mass spectrometer reduces many potential manual errors as well as preparation and analysis time. 2. sampling and preparation procedures all water samples were collected in the field and dispensed into glass bottles (ca. 50 cc) and quickly sealed using an aluminum crimp cap and gas-tight rubber/teflon plugs, taking care that no air bubbles are present in the samples. in the laboratory, about 10 cc of pure ar, or another carbonfree host gas, are injected with a syringe into the upturned bottles (fig. 1). the volume of water simultaneously withdrawn is collected in a syringe, fitted with a stopcock. the water sample is then injected through the rubber septum of a 5.9 ml screw-capped glass vial. the amount of water injected into the vials ranges from 0.2 up to 2 ml, depending on the dissolved inorganic carbon (dic) content of the sample. the lower instrument detection limit (see details in the next paragraph) allows the analysis of water samples with dic contents greater than 4 mmoles per liter. fig. 1. extraction of sample from sampling bottles in the laboratory. injection of about 10 cc of pure ar in the upturned bottles and simultaneous collection of water in a syringe with a stopcock. fig. 2a,b. a) schematic of the analytical precision «carbonate prep system»; b) schematic diagram of the analytical unit (analytical precision ap2003). a b 161 on-line technique for preparing and measuring stable carbon isotope of total dissolved inorganic carbon in water samples (δ 13ctdic) however, reducing the headspace of the vials, it is possible to lower the detection limit further. this was done by inserting a glass cylinder of about 1 cc into the vials thus improving the minimum detectable dic content to 2.0 mmol per liter. in order to reduce any air contamination, the vials were previously flushed with ultrapure helium (99.9996 vol%) using an analytical precision «carbonate prep system» (fig. 2a). this device consists of a modified gilson 222xl autosampler tray holding up to 132 samples kept in a thermostatic rack and equipped with a needle having three concentric capillary tubes. the same flushing needle provides the acidification of the water samples. a fixed amount (100-200 µ l) of 100% h3po4 is automatically dispensed into the vials by means of a valveless pump. addition of acid decrease the ph of the sample shifting the equilibrium of dissolved carbon species towards gaseous co2. this reaction takes place at 70° 162 giorgio capasso, rocco favara, fausto grassa, salvatore inguaggiato and manfredi longo ± 0.1°c. at the end of the process, the ph of water samples ranges between 0.5 and 1 ph unit. 3. sample analysis after 18 h, the sample vessels containing the co2 produced by acidification and the water sample are removed from the bath and manually transferred into the analytical unit where the vials are cooled for about 2 h (fig. 2b). the analytical unit consists of an injection apparatus (gilson 222xl autosampler) and a purification system (nafion® trap and gc column) directly connected to the mass spectrometer. carbon isotope ratios (13c/12c) were measured on an ap 2003 continuous flow mass spectrometer using he (5.6) as carrier and co2 (4.8) as reference gas. by using this configuration, instrument sensitivity is calibrated on a carbon dioxide content in the range 3-10 vol%. in order to improve the precision of the measurements, the analysis is repeated four times for each sample, resulting in a series of co2 peaks reaching the mass spectrometer. within the same analysis, two pulses of reference gas bracket the sample peaks. the isotopic 13c/12c ratios (r) are expressed in the conventional δ -notation as follows: r rc 1 10sample reference 13 3 )-=d _ i8 b . the value of δ 13c versus pdb international standard is calculated using an internal calibrated standard measured within the same batch. the entire process for each sample takes less than 10 min. the precision of the measurement is better than 0.1 permil. 4. results and discussion the precision and reproducibility of the proposed method was tested both on aqueous standard solutions and on some natural samples. the aqueous standard solutions were prepared with na2co3 powder at five dic concentrations (2, 3, 4, 5 and 20 mmol/l). the δ 13c value of the powder na2co3 was determined by reacting about 1.5 mg of solid carbonate with an excess of 100% h3po4 using the same apparatus previously described. analytical results on eleven replicates revealed a δ 13c = –10.78 ‰ versus pdb (1σ = 0.08). 4.1. equilibration time and correction factor tests on the evaluation of the time needed for a complete reaction and the achievement of equilibrium conditions between dissolved and gaseous carbon dioxide were carried out on the aqueous standard solution with a dic content of 4 mmol/l. at least triplicate aqueous standard solution samples were analyzed after different equilibration times from 1 to 24 h. with increasing equilibration time, the δ 13c composition of co2 yielded by acidification tends progressively towards an equilibrium value of –10.38 ‰ versus pdb (fig. 3). therefore the carbon isotope exchange between gaseous co2 and carbon dioxide dissolved in the aqueous standard solution can be considered complete after about 18 h. due to a carbon isotope fractionation process occurring between coexisting water and co2, the obtained δ 13cco2 values are enriched in heavy fig. 3. evaluation of equilibration times on the na2co3 aqueous standard solution with a dic content of 4 mmol/l. the error bar indicates the standard deviation of at least triplicate measurements. after about 18 h the carbon isotope exchange between gaseous co2 and carbon dioxide dissolved in the aqueous standard solution can be considered completed reaching an equilibrium value of –10.38 ‰ versus pdb. isotopes with respect to the carbonate used to make the standard solution. as a consequence, a correction factor to δ 13ctdic measurements has to be applied. as the experiment is carried out in a closed system, the number of stable isotope carbon species remains constant. at the ph values of the solution (0.5 ÷ 1), the only carbon species present in our system are gaseous and dissolved carbon dioxide. under these conditions, the carbon isotope balance can be expressed by the following equation: c c c c co c c co gas aq 13 12 13 12 2 13 12 2 tdic gas aq # # = + + | | (4.1) where 13c/12ctdic, 13c/12cgas and 13c/12caq stand for the carbon isotope ratios of total dissolved carbon species, gaseous co2 and dissolved co2 respectively while, χ co2gas and χ co2aq represent the molar fraction of gaseous and dissolved co2. in terms of delta notation, the eq. (4.1) begins c co co c co co c 1 tdic aq gas co aq gas co 13 2 2 13 2 2 13 aq gas2 2 # = + + d | | d | | d (4.2) where δ 13cco2gas is the value of measured gas, and δ 13cco2aq is calculated considering the car163 on-line technique for preparing and measuring stable carbon isotope of total dissolved inorganic carbon in water samples (δ 13ctdic) bon isotope enrichment factor between dissolved and gaseous co2 (ε 13caq-gas). in vials of known volume, the molar fraction of gaseous co2 can be computed as a function of the dic content in the injected sample volume and the headspace volume (table i). the molar fraction of dissolved co2 has been calculated taking into account the solubility of co2 in water (866 cc/l at 25°c calculated from capasso and inguaggiato 1998). for δ 13cco2aq the value of –1.04 δ per mil was subtracted from the measured δ13cco2gas according to the enrichment factor between dissolved and gaseous co2 (ε 13caq-gas) in closed systems at 25°c found by szaran (1998). hence, the eq. (4.2) begins . c co co c co co c 1 1 04 tdic aq gas co aq gas co 13 2 2 13 2 2 13 gas gas2 2 # = + + d | | d | | d_ i . (4.3) in a closed system, the extent of isotope fractionation between gaseous and dissolved co2 is a function of the headspace-water volume ratio, thus resulting in a difference between the measured δ 13cco2gas and the values. therefore, the δ 13ctdic values can be easily obtained from measured δ13cco2gas introducing a correction factor (cf) reported in table i. the cf values can be computed from eq. (4.3) fixing both the δ13cco22 gas value and the headspace-water volume ratio. table i. computed correction factor (cf) as a function of the dic content and the volume of water injected for vials with internal volume of 5.9 ml. dic contents of water samples ([co2aq] + [hco3−] + [co3− −]) are expressed in mmol/l. sample volume is expressed in milliliter. the molar fraction of dissolved co2 (χco2aq) has been calculated by henry’s law. for dic contents between 2 and 3.5 mmol/l, cf was computed considering the reduced headspace volume due to the insertion of the glass cylinder (see text). dic sample vol headspace vol χ co2aq χ co2gas cf 2.0-3.5 2* 2.9 0.60 0.40 0.62 4-8 2 3.9 0.44 0.56 0.46 8.5-17.5 1 4.9 0.18 0.82 0.18 20-60 0.5 5.4 0.08 0.92 0.08 60-100 0.2 5.7 0.03 0.97 0.03 2* samples analyzed with the glass cylinder inserted into the vial. 164 giorgio capasso, rocco favara, fausto grassa, salvatore inguaggiato and manfredi longo 4.2. tests on standard solutions to determine the accuracy and precision of the proposed method, ten measurements were carried out for each of the prepared aqueous standard solutions having five different dic contents (2, 3, 4, 5 and 20 mmol/l). mean δ13cco2 measurements and corrected δ13ctdic values are reported in table ii together with their standard deviation (σ). the average value of δ13ctdic is only 0.11 δ per mil lighter with respect to the carbon isotope composition of the na 2 co3 powder (δ 13cna 2 co3 = –10.78‰ versus pdb). as can be seen in fig. 4, all the δ13ctdic values are very close to the δ13cna 2co3 value, the maximum difference being 0.23 δ per mil for the 3 mmol/l solution. furthermore, on the basis of the number of analyses (n = 50), the obtained results also highlight excellent reproducibility of the measurements. the standard deviation values decrease with increasing of dic content in the prepared standard solutions. using this method, the carbon isotope reproducibility is better than ± 0.14 for dic content higher than 3 mmol/l. 4.3. tests on natural water samples to test the proposed method on natural waters more than thirty water samples were sampled and analyzed in replicate. the main physical-chemical features of these waters are reported in table iii together with the analytical results. the water samples were collected from springs or domestic wells belonging to different geologic environments, from active volcanic systems (vulcano island, etna, stromboli, popocatepetl-mexico) to carbonate sedimentary aquifers (western sicily) to cover a wide range of carbon isotope composition. for comparison on the same samples δ13ctdic values were also determined using the methodology proposed by favara et al., (2002) based on chemical and physical stripping of co2 from water samples. the δ13ctdic values reported in table iii are graphically shown in fig. 5 highlighting comparable results giving very small differences in the whole range of isotope values. the least-squares regression line for the entire data set is: y=1.011x + 0.04, r2 = 0.998. a set of samples analyzed 30 days after sampling highlights the fact that the storage of water samples both in the glass sampling bottles and in the vials do not compromise the carbon isotopic ratio. however, some precautions have to be takfig. 4. comparison between isotopic composition of solid na2co3 and na2 co3 aqueous standard solution at different dic concentration (2, 3, 4, 5 and 20 mmol/l). the error bar indicates the standard deviation of at least eight measurements for each of the aqueous solutions. mean δ 13ctdic values are corrected according to eq. (4.2) and table i. all the resulting δ 13ctdic values are very close to the carbon isotope composition of the na2co3 powder (dark area) the maximum difference being 0.23 δ per mil. table ii. mean δ 13cco2 measurements for the aqueous standard solutions at five different dic contents (2, 3, 4, 5 and 20 mmol/l). each value represents the average of ten analyses. δ 13ctdic values have been computed according to the correction factors reported in table i. all the isotope values are expressed in δ ‰ versus pdb international standard. dic δ 13cco2meas δ 13ctdiccorr standard deviation (1σ ) 2 –10.34 –10.96 0.29 3 –10.39 –11.01 0.20 4 –10.47 –10.93 0.14 5 –10.40 –10.86 0.05 20 –10.63 –10.71 0.05 average δ 13ctdiccorr –10.89 standard deviation (1σ ) 0.18 165 on-line technique for preparing and measuring stable carbon isotope of total dissolved inorganic carbon in water samples (δ 13ctdic) t ab le ii i. t es ts o n na tu ra l w at er s am pl es t og et he r w it h th ei r m ai n ph ys ic och em ic al p ar am et er s. p h is e xp re ss ed i n p h un it , d ic i n m m ol /l , t d s (t ot al d is so lv ed s ol id ) in m g/ l. s am pl e vo lu m e is r ep or te d in m il li li te r an d th e co rr es po nd in g c or re ct io n f ac to r (c f ) is t ak en f ro m t ab le i . a ll t he is ot op e va lu es a re e xp re ss ed i n δ ‰ v er su s p d b i nt er na ti on al s ta nd ar d. s ta nd ar d de vi at io n – st d. d ev . (1 σ ) is c al cu la te d, at l ea st , on f ou r re pl ic at es . s am pl e l oc at io n d at e p h d ic t d s s am pl e vo l c f a v. δ 13 c c o 2 m ea s. s td . de v. δ 13 c t d ic co rr ( 1 ) δ 13 c t d ic ( 2 ) r . c am pa na e tn a 30 .1 0. 20 02 7. 45 5. 5 64 6 2 0. 46 –3 .9 6 0. 03 –4 .4 2 –4 .8 3 g . p av on e e tn a 13 .0 2. 19 98 6. 53 10 .8 52 8 1 0. 18 2. 25 0. 10 2. 07 2. 06 p on te fe rr o e tn a 17 .0 1. 19 98 7. 49 14 .2 16 36 0. 5 0. 08 1. 04 0. 28 0. 96 1. 28 v al co rr en te e tn a 13 .0 2. 19 98 6. 92 21 .3 16 81 0. 5 0. 08 3. 92 0. 18 3. 84 4. 40 p 31 e tn a 30 .1 0. 20 02 5. 99 21 .7 80 4 0. 5 0. 08 0. 45 0. 11 0. 37 0. 21 a t p op oc at ep et l 18 .0 5. 20 02 6. 80 4. 6 17 28 2 0. 46 –7 .6 0 0. 25 –8 .0 6 –7 .9 8 a x p op oc at ep et l 18 .0 5. 20 02 5. 90 28 .7 62 0 0. 5 0. 08 –2 .3 1 0. 10 –2 .3 9 –2 .6 5 z ur ro s tr om bo li 31 .0 1. 20 03 6. 93 5. 9 30 99 7 2 0. 46 –0 .0 8 0. 28 –0 .5 4 –0 .2 5 z ur ro s tr om bo li 26 .0 1. 20 03 6. 93 6. 3 32 69 6 2 0. 46 0. 21 0. 13 –0 .2 5 –0 .5 8 o ss id ia na s tr om bo li 02 .0 2. 20 03 6. 93 6. 6 40 40 4 2 0. 46 –0 .3 6 0. 27 –0 .8 2 –0 .7 9 s ir en et ta s tr om bo li 31 .0 1. 20 03 7. 10 7. 5 18 88 8 1 0. 18 1. 10 0. 22 0. 92 0. 91 c us ol it o s tr om bo li 31 .0 1. 20 03 6. 77 10 .8 26 12 1 1 0. 18 1. 54 0. 25 1. 36 1. 37 c us ol it o s tr om bo li 27 .0 1. 20 03 6. 61 11 .0 24 60 5 1 0. 18 2. 07 0. 14 1. 89 1. 75 c us ol it o s tr om bo li 2. 02 .2 00 3 6. 75 11 .6 25 91 3 1 0. 18 1. 84 0. 12 1. 66 1. 74 f ul co s tr om bo li 26 .0 1. 20 03 6. 39 19 .3 24 48 4 0. 5 0. 08 1. 83 0. 18 1. 75 1. 96 f ul co s tr om bo li 31 .0 1. 20 03 6. 35 19 .5 12 32 3 0. 5 0. 08 0. 90 0. 19 0. 82 0. 99 f ul co s tr om bo li 02 .0 2. 20 03 6. 45 19 .7 12 93 6 0. 5 0. 08 0. 96 0. 12 0. 88 1. 22 c . s ic il ia v ul ca no i sl an d 13 .0 1. 20 03 7. 45 7. 2 64 67 2 0. 46 1. 12 0. 08 0. 66 0. 82 e as v ul ca no i sl an d 13 .0 1. 20 03 8. 42 19 .8 22 93 0. 5 0. 08 4. 47 0. 09 4. 39 4. 78 b am ba ra v ul ca no i sl an d 13 .0 1. 20 03 5. 73 27 .5 10 38 0. 5 0. 08 –2 .6 1 0. 26 –2 .6 9 –3 .0 0 d is ca ri ca v ul ca no i sl an d 13 .0 1. 20 03 6. 63 63 .2 47 88 0. 2 0. 03 0. 90 0. 09 0. 87 0. 51 c . b ai da w es te rn s ic il y 29 .0 1. 20 03 8. 11 2. 7 36 7 2* 0. 62 –1 4. 96 0. 16 –1 5. 58 –1 5. 45 c . b ai da ( a ) w es te rn s ic il y 29 .0 1. 20 03 8. 11 2. 7 36 7 2* 0. 62 –1 4. 53 0. 13 –1 5. 15 –1 5. 45 s co pe ll o w es te rn s ic il y 29 .0 1. 20 03 7. 87 2. 7 31 8 2 * 0. 62 –1 2. 84 0. 36 –1 3. 46 –1 3. 56 s co pe ll o (a ) w es te rn s ic il y 29 .0 1. 20 03 7. 87 2. 7 31 8 2* 0. 62 –1 2. 87 0. 26 –1 3. 49 –1 3. 56 f ra gi ne si w es te rn s ic il y 29 .0 1. 20 03 8. 16 3. 5 76 4 2* 0. 62 –9 .6 3 0. 34 –1 0. 25 –1 0. 29 f ra gi ne si ( a ) w es te rn s ic il y 29 .0 1. 20 03 8. 16 3. 5 76 4 2* 0. 62 –9 .5 4 0. 23 –1 0. 16 –1 0. 29 p io pp o w es te rn s ic il y 29 .0 1. 20 03 7. 15 4. 7 61 6 2 0. 46 –1 3. 95 0. 20 –1 4. 41 –1 4. 62 p io pp o (a ) w es te rn s ic il y 29 .0 1. 20 03 7. 15 4. 7 61 6 2 0. 46 –1 4. 06 0. 07 –1 4. 52 –1 4. 62 s . n uo va w es te rn s ic il y 29 .0 1. 20 03 6. 70 5. 9 16 95 2 0. 46 –2 .1 7 0. 23 –2 .6 3 –2 .6 8 s . n uo va ( a ) w es te rn s ic il y 29 .0 1. 20 03 6. 70 5. 9 16 95 2 0. 46 –2 .3 5 0. 09 –2 .8 1 –2 .6 8 m er la w es te rn s ic il y 29 .0 1. 20 03 7. 06 7. 4 97 2 2 0. 46 –1 4. 00 0. 21 –1 4. 46 –1 4. 17 m er la ( a ) w es te rn s ic il y 29 .0 1. 20 03 7. 06 7. 4 97 2 2 0. 46 –1 4. 01 0. 28 –1 4. 47 –1 4. 17 2* s am pl es a na ly ze d w it h th e gl as s cy li nd er i ns er te d in to t he v ia l; ( a ) s am pl es a na ly ze d af te r 30 d ay s; ( 1 ) t hi s m et ho d; ( 2 ) a na ly ze d fo ll ow in g th e st ri ppi ng m et ho d (f av ar a et a l. , 20 02 ). 166 giorgio capasso, rocco favara, fausto grassa, salvatore inguaggiato and manfredi longo fig. 5. comparison between the δ 13ctdic values obtained using the proposed method and those carried out following the chemical and physical stripping technique (favara et al., 2002) for some natural water samples the least-squares regression line for all the data set (y=1.011x + 0.04, r 2= 0.998) highlights comparable results over the whole range of isotope values. en for waters saturated with respect to carbonate minerals and/or organic matter rich waters that could modify the pristine tdic isotope composition. in the first case this is due to the precipitation of a solid carbonate phase, whereas the organic activity leads to enrichment of 12c in biologically synthesised organic compounds. 5. conclusions an automated technique for extracting and analyzing inorganic carbon isotope ratio (δ13ctdic) from 0.5 to 2 ml water samples has been developed. this method, based on the release of co2 yielded by acidification of water samples, is a modification of the procedure previously proposed by favara et al. (2002). the use of an autosampler both for acid dosing and for sample injection into the mass spectrometer not only greatly reduces a lot potential manual errors as well as reducing preparation and analysis time but also eliminates the use of expensive devices (vacuum preparation line, special grease, liquid nitrogen, etc.). analytical results gave good sample reproducibility and precision over a wide range of isotope composition and highlighted that this method is comparable to the other analytical procedures. acknowledgements we are indebted to jason newton for helpful suggestions and reviews. we also thank two anonymous reviewers for their constructive comments. references atekwana, e.a. and r.v. krishnamurthy (1998): seasonal variations of dissolved inorganic carbon and δ 13c of surface waters: application of a modified gas evolution technique, j. hydrol., 205, 265-278. caliro, s., c. panichi and d. stanzione (1999): variation in the total dissolved carbon isotope composition of thermal waters of the island of ischia (italy) and its implication for volcanic surveillance, j. volcanol. geotherm. res., 90, 219-240. capasso, g. and s. inguaggiato (1999): a simple method for the determination of dissolved gases in natural waters, an application to thermal waters from vulcano island, appl. geochem., 13, 631-642. favara, r., f. grassa and s. inguaggiato (1999): chemical and isotopic features of dissolved gases from thermal springs of sicily, in proceedings of the fifth international symposium on the geochemistry of the earth’s surface (ges-5), 16-10 august 1999, reykjavik, iceland (h. armannssonn publishing), pp. 574. favara, r., f. grassa, s. inguaggiato, g. pecoraino and g. capasso (2002): a simple method to determine the δ 13c of total dissolved inorganic carbon, geofis. int., 41, 313-320. inguaggiato, s., g. pecoraino and f. d’amore (2000): chemical and isotopical characterisation of fluid manifestations of ischia island (italy), j. volcanol. geotherm. res., 99 (1/4), 151-178. mc nicol, a.p., g.a. jones, d.l. hutton, a.r. gagnon and r.m. key (1994): the rapid preparation of seawater ε co2 for radiocarbon analysis at the national ocean sciences ams facilities, radiocarbon, 36 (2), 237-246. szaran, j. (1998): carbon isotope fractionation between dissolved and gaseous carbon dioxide, chem. geol., 150, 331-337. layout 6 annals of geophysics, 60, 6, s0660, 2017; doi: 10.4401/ag-7437 reliability of first-hand accounts on the study of historical tsunamis in northeastern venezuela (southeastern caribbean sea) franck a. audemard m.1,* and alejandra f. leal guzmán1 1 earth sciences dpt., venezuelan foundation for seismological research -funvisis-, caracas, venezuela article history received april 29, 2017; accepted july 29, 2017. subject classification: tsunami hazard; inundation; run-up; strike-slip faults; sliding; caribbean. s0660 abstract the occurrence of tsunami waves on the eastern caribbean venezuelan coasts during 5 venezuelan (local) earthquakes (01-ix-1530, 15-vii1853, 29-x-1900, 17-i-1929 and 9-vii-1997), have been confirmed through the search and evaluation of written accounts by primary sources (eye witnesses) of tsunami inundation during these events. among the outcomes of this new assessment are: 1) the run-up heights of several of those tsunamis have been substantially reduced. in fact, maximum run-up heights for the 5 tsunamis are: 5-7 m at cumaná for the 1530 event, 5 m at barlovento and 3 m at the neverí mouth for the 1900 tsunami, 3 m at western cumaná for the 1853 and 1929 events and about 1 m for the 1997 earthquake. 2) these new estimates on wave heights for local earthquakes restrict the search for tsunamites by trenching and coring to mainly the first 500-m-wide strip from the coastline in low-lying flatlands. the source of these tsunami waves may be complex. some are the result of coastal-submarine sliding (1929 ad, 1997 ad), tectonic slip on active strike-slip (or normal oblique slip) faults (1530 ad, 1900 ad) or combination of tectonic slip and sliding (1853 ad). appropriate numerical modeling of tsunami wave generation, migration and inundation are urgently in need to understand these tsunami mechanics. 1. introduction the 1983 national inventory of geologic hazards [singer et al. 1983] and the 1999 catalog of felt/destructive venezuelan earthquakes (1530-1998); [grases et al. 1999] reliably report the occurrence of tsunami waves on the caribbean venezuelan coasts, or phenomena that might be interpreted as substantial sea level modifications in the region, during local, regional and extra-regional earthquakes, as well as related to other natural phenomena. in this research, we narrow down the focus of this investigation in 4 ways: 1) the tsunami source is an earthquake (tsunamigenic earthquake); 2) the earthquake has happened on a local (venezuelan) tectonic source; 3) eyewitness (first-hand) accounts provide characteristics (actual occurrence, location, time, inundation height, run-up and so on) of the tsunami; and 4) the tsunamis must have affected the eastern coasts of venezuela. the last constraint responds to the area objective of project fonacit 2013000361, which funds the current research. this geographical constraint answers to the potential favorable conditions of preservation of tsunamites along the low-lying coasts of the ensenada de barcelona (barcelona embayment; from cabo codera to araya) envisaged from the early stages of this investigation, as already proven by few pioneering tsunamite studies [leal and scremin 2011, audemard 2012, 2014, oropeza et al. 2013 2015, leal et al. 2014], where most tsunami affectation in venezuela in last centuries is reported in both abovementioned catalogs (inventories) [audemard and leal 2012, 2013, 2014, 2015]. in such research project, search of tsunamite by short (over a 1 m long) coring and shallow-pit hand digging is the geologic approach implemented to corroborate the occurrence of past tsunamis in association with its local tsunamigenic source. last but not least, the use of firsthand accounts seeks to optimizing reliability on occurrence and characteristics of individual tsunamis. this research then is mainly substantiated by a documentary assessment, which also includes last century pertinent photographs to contextualize the event in time and space, all supported with fieldwork observations on topography, coastal geomorphology, river morphology and hydrology, geological and natural coastal environments, and anthropogenic (cultural) aspects. all this is carried out as to recreate the prevailing conditions at the time of occurrence of each of the 5 events, to which we would apply the itis 2012 [lekkas et al. 2013] and imamura-iida magnitude scale [iida, 1963]. 2. offshore active faults of venezuela northeastern venezuela at present is the most seismically active area nationwide, as documented by the instrumental seismicity catalog of the venezuelan foundation for seismological research -funvisisfor the period 1910-present (www.funvisis.gob.ve). in historical times, the seismic activity was not much different, as indicated by historical reports on damage in this region by earthquakes and tsunamis from the dawn of the spanish conquest [centeno-graü 1940, 1969, gómez 1990, grases 1990, grases et al. 1999]. such colonization started at the beginning of the 16th century with nueva toledo in 1515, known today as cumaná -the oldest spanish settlement on continental america-. in fact, this city has been repeatedly destroyed to some extent during historical earthquakes [audemard 2007], such as in 1530, 1629, 1684, 1766, 1797 and 1853, as well as during two events in the 20th century (1929 and 1997). all these events have been associated by different authors to the el pilar fault (epf) that is considered to be the second major seismic source in eastern venezuela after the southern end of the nw-dipping slab of the lesser antilles subduction zone that lies under trinidad and partly under the paria gulf and peninsula (figure 1). the epf, along with the right-lateral strike-slip boconó and san sebastián faults, appears to accommodate a large fraction of the dextral transcurrent motion (figure 1); [molnar and sykes 1969, minster and jordan 1978, pérez and aggarwal 1981, stephan 1982, aggarwal 1983, schubert 1984, soulas 1986, beltrán and giraldo 1989, speed et al. 1991, singer and audemard 1997, pérez et al. 2001, weber et al. 2001, audemard and audemard 2002, audemard et al. 2005, audemard 2006, jouanne et al. 2011, reinoza et al. 2015, among many others] within the over a 100km-wide transcurrent-compressive plate-boundary zone between the caribbean and south america plates. it is worth mentioning that the epf active fault trace to which this paper refers, corresponds to the fault mapped by beltrán et al. [1996]. this mapping is more easily accessible from audemard et al. [2000; figure 2]. audemard [1999, 2007, 2011], altez and audemard [2008] and audemard et al. [2007] have associated most of the offshore historical destructive earthquakes in eastern venezuela (1530, 1629, 1684, 1797, 1853, 1929 and 1997) with epf. moreover, each of these events has been associated with a particular segment of the fault. for instance, the mw 6+ earthquakes of 1629 [altez and audemard 2008] and 1797 and 1929 [audemard 1999, 2007, audemard et al. 2007] have all broken inside the cariaco gulf, just east of cumaná. except for the 1929 event, they seem not to have generated tsunami waves in the cariaco gulf, which has significant implications on the source mechanism for the 1929 tsunami waves. likewise, the 1812 and 1900 large offshore earthquakes in north-central venezuela have been associated to segments of the san sebastián fault (ssf), [audemard 2002, colón et al. 2015]. 3. tsunamigenic earthquakes from a recent chronology of tsunami reports in venezuela compiled by oropeza and audemard [2016] from published data, which essentially gathers only tsunami data from 6 catalogs [centeno-graü 1940, singer et al. 1983, grases 1990, grases et al. 1999, o’loughlin and lander 2003, altez and rodríguez 2009] and a published paper [schubert 1987], just five local earthquakes out of 24 events with clear reports of sea surface perturbations, have affected the eastern venezuela coasts, which are, in sequence: 1-ix-1530, 15vii-1853, 29-x-1900, 17-i-1929 and 9-vii-1997 (figure 3). although the 1766 event is the largest earthquake in eastern venezuela, it has dubious evidence of tsunamis; probably related to the fact that it is an intermediate depth event [audemard 2007, mocquet audemard and leal guzmán 2 figure 1. simplified geodynamic framework of northern south america (from audemard et al. 2000]. abbreviations: bf boconó fault; epf el pilar fault; lbf los bajos-el soldado fault system; oaf oca-ancón fault system; ssf san sebastián fault. figure 2. quaternary fault map of northeastern venezuela [after audemard et al. 2000]. ve-13 identifies the el pilar fault (epf) and the suffixes (a through d) label the different individual fault portions or segments. epicenters of the 1929 and 1997 earthquakes are shown as solid circles. faults in red have had fault surface ruptures, or portions of it, mapped. 3 reliability of testimonies on venezuelan historical tsunamis 2007]. all but the 1900 event, whose several proposals of epicenter locations all lies in north-central venezuela, have been generated by the epf. instead, the 1812 tsunami, also generated by the san sebastían fault (ssf) offshore north-central venezuela, is not herein treated, because its hypocenter lies too far west and seems not have affected eastern venezuela. the 1900 ad event is the second largest earthquake (ms 7.6) [pacheco and sykes 1992] in magnitude in the venezuelan seismic catalog, after the 1766 event (mw 7.8), whose felt area is twice as large as the surface of the entire country, but the 1900 ad event is the largest earthquake of all times at crustal depth. all five tsunamigenic earthquakes are considered to be crustal events [e.g., audemard 2007, colón et al. 2015]. the 1900 ad tsunami waves were reported along most of the ensenada de barcelona coast (barcelona embayment, w of cumaná) and los roques archipelago (figure 3 and inset), being this quake attributed to the ssf segment running offshore cabo codera by colón et al. [2015], based on fresh seafloor scarps detected by high resolution shallow reflection seismics in 2007. after audemard [1999, 2007], the 1530 and 1853 earthquakes were produced by the cariaco trough segment of the epf west of cumaná, within a restricted over-1000-m-deep marine pull-apart basin on the ssf-epf right-lateral releasing step-over (figures 2 and 3), whereas the 1929 and 1997 events occurred on the epf segment east of cumaná, inside the <100-mdeep cariaco gulf (figure 2). several authors have interpreted all those four tsunamis as the result of major submarine sliding inside the steep-walled trough. firsthand accounts by locals about the abnormal waves during the cariaco 1997 event, as well as the identification of coastal sliding at the manzanares river mouth at cumaná [gonzález et al. 2004], support this hypothesis at least for the two latest events, because of the small size of the tsunami-affected area (figure 3). in addition, recent seafloor monitoring [cariaco project; e.g., thunell et al. 1999, lorenzoni et al. 2012] has recorded turbidite currents in the cariaco trough and the manzanares canyon -the latter connecting the manzanares river mouth to the eastern deep basin of the cariaco trough-, during the cariaco mw 6.9 earthquake, and also with a smaller, mw. 5.2, august 2008 event. however, the 1900 tsunami, and the 1530 and 1853 tsunamis by extension, appears to result from right-lateral tectonic slip along the cariaco trough walls [audemard and leal 2012, 2013]. however, aguilar et al. [2016], in order to explain an improved water exchange between the cariaco gulf and open sea since around 1850 ad, which has made the gulf waters more salty in more recent times, propose that the submarine salazar sill slid during the 1853 earthquake, thus deepening and enfigure 3. map of eastern venezuela extracted from google earth showing the different localities with confident eye-witness reports of tsunami inundations, or sea surface oscillations, during local historical and instrumental earthquakes. epicenters of historical and 1929 earthquakes are indicated by stars, whose epicentral coordinates are provided in table 1. relative location of figure 11 is also depicted. larging the gulf entrance north of cumaná, and increasing open sea water inflow. in such a case, the tsunami waves during this event could have resulted from an enhanced combination of tectonic slip and submarine sliding. 4. description of historical tsunamis from first-hand accounts as mentioned earlier, this study intends to explore how first-hand or eyewitness accounts can be used to better constraining tsunami inundation and tsunami height and run-up elevation for the five earthquakes that are known from earthquake (and/or tsunami) catalogs to have inundated a locality or several of them along the barcelona embayment coasts. to do this, we have started this assessment by looking into the most recent historical event (the january 17th, 1929 earthquake) that should logically have the largest number of and more detailed accounts, which happens to be the case. 4.1 the january 17th, 1929 tsunami for this earthquake, the occurrence of tsunami waves at the city of cumaná is repeatedly reported. for instance, centeno graü [1940] indicates:“el mar se retiró de la playa como 200 mts y vino después a la costa una ola como de 6 metros de altura que barrió parte de las casas de la playa.” (the sea retreated some 200 m and came back to shore as a 6 m high wave that washed down houses on the beach; free translation by authors -ftba-). paige [1930] reports “a tidal wave followed the earthquake causing much damage.” the front page of the caracas newspaper la esfera on january 19th 1929 says “…embarcaciones de caleta que se encontraba en el muelle de puerto sucre fueron alejados al mar; éste, embravecido, lanzó sus aguas hacia la población, en sentido inverso a una tromba de lo que envolvió completamente al “commewynne” y otras embarcaciones, de las cuales algunas naufragaron. el mencionado vapor holandés se vio juguete de olas y sólo, gracias a la entereza de su capitán que ordenó hábiles maniobras, a posibles contingencias”. the ftba is as follows: “anchored boats at puerto sucre -cumaná seaportwere pulled offshore; the sea returned with strong surfs to the village, wrapping completely the steam vessel commewynne and other boats, of which some sunk. the dutch steam vessel (figure 4) seemed like a toy and only skillful manoeuveurs of its capitain saved it”. this is confirmed by another caracas newspaper on the same day january 19 (el nuevo diario), which expresses “…en puerto sucre, donde los daños fueron menores, hubo un violento mar de eleva. el vapor holandés por esta causa, tuvo que retirarse varias veces hacia alta mar, volviendo luego a la costa. ….los botes amarrados al costado del muelle se anegaron por completo.”the translation goes: “in puerto sucre (figures 5 and 6), where damage was minor, there was a violent tsunami wave. the dutch steamboat, due to the tsunami waves, had to sail to open sea several times, later returning to shore. …boats tied to the pier sides were completely drowned”. three days later, the same newspaper publishes the following: “dos detalles impresionantes del terremoto fueron el desbordamiento del río manzanares, que en su curso atraviesa la ciudad, cuyas aguas arrastraron cuanto encontraron a su paso, causando enormes daños. fue el otro la invasión del mar que, elevándose en una marejada de nueve pies sobre su nivel ordinario, se lanzó sobre “el salado”, inundándolo totalmente. este caserío sufrió considerablemente. la mayoría audemard and leal guzmán 4 figure 4. typical dutch vessel, named el holandés, that came to pto sucre seaport, cumaná, in the first half of the 20th century (anonymous photographer). figure 5. ground view of the puerto sucre seaport, cumaná, in 1925, 4 years before the 1929 earthquake. the wooden pier and shelter structures appear rather slim. in the far background, a typical steam boat of the time is anchored, as well as a sailing boat of the sloop type. on the foreground, “piragua” type boats, which are much different from the freshwater canoe or pirogue boats carved by indians into a single trunk. 5 reliability of testimonies on venezuelan historical tsunamis de sus viviendas, todas gente pobre y trabajadora, quedaron casi destruidas.” (two impressive facts of the earthquake were the overflowing of the manzanares river that crosses the city, whose waters destroyed everything, causing much damage. and the other was the sea invasion, which with an abnormal height of 9 feet struck el salado -a small fishermen settlement on the southern bank of the manzanares mouth (figure 7); now being part of cumaná-, flooding it completely. this settlement suffered considerably. most of the dwellings, belonging to poor and labor people, were almost destroyed -ftba-). the same newspaper on march 03, several weeks after the earthquake and tsunami, says: “el mar por un momento se retiró para luego tornar formando una extensa marejada de gran elevación, que inundó la población situada en el puerto…. el río hinchó violentamente sus ondas que se llevaron por delante cuanto a su paso encontraron”. translated into english: “the sea for a short spell retreated to return later as a tsunami wave of big height…… the river swelled violently destroying everything at its passage”. the last first-hand account we could find appeared on the nationwide newspaper el universal on march 22, 1929. it indicates “…en el preciso momento de la catástrofe algunos pescadores cuyas barcas estaban ancladas en puerto sucre vieron amenazante hacia el norte franco una negra y formidable nube que se levanta sobre las aguas del mar, y que creyendo fuera una borrasca o chubasco marino dieron la voz de alarma es decir, de amarrar o de asegurar sus embarcaciones, de la cual no tuvieron tiempo, pues al instante estalló el cataclismo y una ola colosal los arrojo con sus barcos sobre la playa” (…right at the moment of the catastrophe, some fishermen, whose boats were anchored at puerto sucre (figures 4 through 6 and 8), saw an amazing and black threatening cloud rising over the sea due north, and believing it was a storm, gave the alarm of fastening and securing the boats, but in such a short notice and having not enough time, a colossal wave rafted the boats ashore). these several accounts provide very useful information for the understanding of what precisely happened during the january 17th, 1929 earthquake and tsunami that struck cumaná, and particularly its western coast, extending between the manzanares river mouth to the north and the city port named puerto sucre to the south, and affecting the fishermen village of the el salado sitting on that coast stretch. it is worth noting that this western coast stretch of cumaná, although in a partly-protected large embayment opened to sea towards the west, is more exposed to open sea; reason why the seaport sits there. being this city well figure 6. aerial photograph of the seaport of cumaná, known as puerto sucre, after the 1929 earthquake, during the phase of reconstruction and upgrading in the 1930. in this view, the narrow lightcolored coastal sand barrier is easily recognizable along seashore. in the same way, the salt flats, of lower elevation than the barrier, are identifiable because of being flooded, particularly in the upper left corner of the picture. from the size, the boats appear to be “piraguas”, except for the large sloop tied to the pier. instead, some “peñeros” (small wooden rowing fisherman boats) lie on the beach. figure 7. nw-looking bird-eye view of the manzanares river mouth, in 1957, embanked on both sides. river width close to the mouth is in the order of some 50 m. on the right (northern bank) used to sit the francis rodel fish cannery, in the el dique neighborhood. on the southern bank has developed the el salado neighborhood; largely affected by most tsunamis that has struck cumaná through the years. the steel bridge, which used to exist during the first author’s youth, was named aristides rojas. coconut tree plantations are visible along the river margins. photograph by gerardo gonzález. known to the first author, this allows describing the preevent scenario rather profusely. the city of cumaná has grown on a salt-flat (figure 9) that had to be progressively artificially filled to be constructed and urbanized through the years. cumaná suffered from inundation after every single large rain or pour (figure 10). this situation was later overcome with the construction of the city drainage system in the 70’s, accompanied by the design and construction of a spillway for the manzanares river. the salt-flat originally formed behind a coastal sand barrier that hardly reaches 1.7-2.0 m amsl in elevation, and the salt flat floor used to be close to a 1 m below msl (figures 9 and 10a). salt exploitation by locals was still a common practice in 1960’s. it also implies that tsunami wave height and inundation height (flow depth) or run-up height at cumaná is about the same. it is of major importance to indicate that tidal range along the venezuelan coast is less than 35 cm. therefore, time of tsunami with respect to tide height plays almost no role on the inundation height. in terms of typical navigation vessels, it is relevant to mention that fishermen in northeastern venezuela in the first half of the 20th century most frequently used “piragua” boats that are 6 to 10 m long and have a draught between 1 and 2 m depending on the load aboard, which are still in use by present-day fishermen (figures 5 and 8). from all these accounts, the tsunami wave height retained by most scientists is the one from centeno graü [1940] of 6 m. it is the most straightforward and no interpretation is needed. but we have to bear in mind that centeno graü’s work is a catalog in itself; in other words, the result of a compilation and not a first-hand account. however, this value can be in some way tested, and even contested. as a matter of fact, another account indicates only 9 feet high waves (some 3 m; el nuevo diario of january 22nd, 1929). another estimate of the wave height can be derived from the following facts: a) the anchored boats at puerto sucre (probably of the piragua type; figures 5, 6 and 8) were rafted ashore by the waves; being these boats no more than 3 m deep and exhibiting draughts between 1 and 2 m, and taking into account that the coastal sand barrier is about 1.5 m high, to abandon the boats at the shore, as mentioned in the account, the tsunami waves were hardly over 3 m in height. if run-up height had been larger, the boats would have been abandoned much farther inland. b) audemard and leal guzmán 6 figure 8. modern equivalents to the boats that could have been tied to the puerto sucre pier in 1929: “piragua” in the far background and “peñero” in the foreground. 7 the fact that the “commewynne” steam-boat at puerto sucre could sail out to sea repeatedly, it implies that the captain had time to respond to arriving tsunami waves, which were not large and powerful enough to drift the ship ashore. c) several boats tied to the pier drowned. if these boats also are of the piragua type (figures 5, 6 and 8), a 2-3 m high wave would be largely enough to sink boat that could not freely float. and d) the sea along the west coast of cumaná, whose seabottom is very flat, retreated some 200 m before returning and overflowing the sandy beach. if this is compared to the first author’s observations collected during the 1997 event [gonzález et al. 2004], a sea retreat along the same stretch of beach of a 100 m, allowed estimating a wave height in the order of a 1 m. by analogy, at constant beach slope gradient, the retreat of some 200 m during the 1929 tsunami would be equivalent to a wave of a couple of meters at most. from these observations several other conclusions can be drawn: 1) several waves seem to have happened. 2) most eyewitnesses at cumaná first report a sea retreat, like for the 1997 earthquake. 3) the tsunami wave are reported on the west coast of cumaná, on the open seaside, completely opposed to the earthquake epicenter proposed by mocquet et al. [1996] and audemard [1999, 2007],(figures 2 and 3), which lies inside the cariaco gulf. therefore, it should be considered that this tsunami is necessarily related to a submarine slump or slide occurring in the cariaco trough, and probably inside the submarine manreliability of testimonies on venezuelan historical tsunamis figure 9. bird-eye view to the west of cumaná in 1950, showing t intersection between los mangles (future perimetral) and fernández de zerpa avenues. in the foreground, notice salt deposition. puerto sucre seaport is visible above the trees. in the far background, the shape outline of the relief of the eastern interior range stands out. figure 10. cumaná, a city prone to past flooding. a) view to south of the future perimetral avenue, at el dique in 1960, with aristides rojas bridge over the manzanares river in background. small ponds attest to recent rains. note typical vegetation: coconut trees on the riversides and mangrove trees in the foreground (picture by gerardo gonzález). b) cancamure avenue flooded after heavy rain. c) all flooded around the el indio roundabout in the 1960’s (picture by vitorio de benedetto). d) las palomas sector, upstream of the aristides rojas bridge, inundated by manzanares river overflow in 1960’s (picture by vitorio de benedetto). these flooding problems were minimized with the construction of a diversion channel, pouring to the east into the cariaco gulf in the 1970’s. a) c) b) d) zanares canyon, in a similar way to the tsunami wave reported during the 1997 earthquake, which shows evidence of subaerial sliding at the manzanares river mouth [gonzález et al. 2004], whose turbidite currents into the cariaco trough were reported by thunell et al. [1999] and lorenzoni et al. [2012]. and 4) two firsthand accounts (el nuevo diario on 1929/i/22 and 1929/iii/03) report the rise of the manzanares river waters along its final stretch, overbanking and flooding along the river course. this seems to point out to generation of a riverbore at the river mouth. applying the itis-2012 tsunami intensity scale proposed by lekkas et al. [2013], the western coast of cumaná, stretching from puerto sucre to the manzanares river mouth, can be classified as intensity vii (damaging) to viii (heavily damaging), based on boats drifted ashore and damage grade 4 to dwellings of high vulnerability (classes a and b), respectively. using the same level of damage, according to imamura-iida magnitude scale [iida, 1963], this tsunami should be graded 1, to which would correspond a wave height range of 2 to 5 m, which appears to be consistent to one of the eye-witness accounts, as well as to our evaluation. itis-2012 also indicates that tsunami wave for intensity viii is expected to be higher than 2 m, but smaller than 5 m (corresponding to intensity ix). 4.2 the october 29th, 1900 tsunami for the october 29th, 1900 earthquake-tsunami, the first-hand accounts tell us a similar story. the newspaper la linterna mágica, on november the 2nd, publishes: “en el puerto tuy el mar se separó como ocho cuadras, y luego se vino encima de la playa una gran mole de agua como de diez metros de altura que anegó los almacenes.” (at puerto tuy (figure 11), sea retreated like 8 “cuadras” -about 1 km; a “block” ranges between 100 a 150 m in length-, and then returned onto the beach as a huge wall of water of about 10 m high, which flooded the warehouses -ftba-). another account in the same newspaper indicates that: “una parte del pueblo de paparo se hundió en el agua, quedando las casas sólo con la mitad afuera.” (a portion of the village of paparo (located next to seashore; figure 11) was drowned, being only the upper half of the houses sticking out -ftba-). on la religión of november 6th, the following note was found: “en san josé, población situada a menos de dos millas de río chico, se desbordó el río que atraviesa el pueblo en ambas márgenes, bañando las calles y luego se volvió a su cauce natural, siendo más grave este fenómeno cuanto que una de las márgenes del río san josé tiene cuatro metros de altura. el mismo fenómeno se verificó en todo el curso del río, brotando a tierra un sinnúmero de peces que sirvieron de alimento a la población en aquellos instantes de pavor; y de diaudemard and leal guzmán 8 figure 11. google earth map of the barlovento coast between the tuy and guapo river mouths. all accounts from this region as to the 1900 tsunami come from this stretch of low-lying flat coastal plain. 9 vertimento durante todo el día a los chiquillos (...) media hora después del cataclismo crecieron las aguas de los ríos sin que antes ni después de aquel aciago momento hubiese llovido en sus lechos.” (in san josé, village located less than 2 miles from río chico (figure 11), the river that crosses town flooded over on both sides, watering the streets before returning to its natural riverbed. this is even worse if considering that one of the river bank is 4 m higher than the river course. this was seen all along the river course, throwing on land countless fish that fed the population during those dreadful days; and were used as toys by kids the day long ….. half an hour after the cataclysm, the river waters were raised without any rain before or after that terrible moment -ftba-). to understand the relevance of these accounts, the coastal area of barlovento, along the barcelona embayment shore, needs to be described (figures 3 and 11). the barlovento depression exhibits a flat-lying topography, bounded by a sandy beach that is usually less than 1.7 m high. this alluvial plain extends far inland with almost no relief for tens of kilometers, being the villages of paparo (ca. 0.7 km inland), san josé (ca. 7.5 km inland) and río chico (ca. 5 km inland) not far from the coastline, while puerto tuy sits right on the beach, at the mouth of the guapo river (figure 11). the october 29th, 1900 earthquake strikes this region before sunrise, still at full darkness (4:40-4:45 am local time) and the tsunami, as mentioned by one of the accounts, strikes the region around half an hour later, still at dark. we still actually wonder how an eyewitness could reliably see anything at nighttime, and furthermore, how he could survive a 10 m high wave in an area with neither any natural elevation nor vertical escape except for tall trees. if the other accounts are analyzed in detail, it would be more appropriate to believe that riverbore actually took place, in which case, the water elevation could reach up to 3-4 m, eventually to 5 m above riverbeds at most at up to 7 km inland from coastline, considering that houses were half drowned and they were on river banks (inundation height of 1.5 m above alluvial plain at most; equivalent to mid-height of bahareque houses), which are 3 to 4 m above the riverbed in normal conditions. at los roques archipelago (figure 3 inset), espíndola [1900] writes a note, entitled el terremoto en los roques, in the newspaper la religión, which appears published a week after the event (on november 06th, 1900). a pertinent extract goes: “una fuerte conmoción seísmica se ha sentido en esta isla y los islotes circunvecinos a las 4 y 40 minutos de la madrugada de hoy (...) se oyeron estampidos sordos, como de poderosa artillería, la atmósfera se oscureció por largo tiempo y el mar se retiró mugiendo a muchos metros de la playa. los tripulantes de las embarcaciones que han llegado a este puerto en lo que va ocurrido del día, y que navegaban en los canales de estas islas, me dicen haber oído las mismas detonaciones, al tiempo que sintieron sacudidas violentas y una agitación extraña en las aguas”. our free translation (ftba) goes: a strong shaking was felt on this island [gran roque] and other smaller keys at 4:40 am today (...) booms were heard, as those of powerful artillery, sky got dark for quite a while and sea retreated mooing at many meters from the beach. ship crews who have disembarked during the day, and were sailing between the archipelago islands, tell me they have heard the same booms, at the same time that they felt strong shaking and a strange motion of sea waters. combining, the descriptions of towns sitting on barlovento seashore and los roques archipelago, we could say that the 1900 earthquake epicenter would apparently seem closer to los roques than paparo and san josé, based only on earthquake noise hearing. since there is no noise reporting at the southern site, we do not know if it corresponds to an actual lack of noise or a lack of description by locals. on the other hand, this tsunami waves exhibit a striking feature. both at los roques archipelago and barcelona embayment at barlovento, significant initial sea retreat has been reported, about 1 km in barlovento and “many meters” at gran roque. considering the tsunami could be eventually triggered by tectonic slip along the ssf, each of the 2 abovementioned sites sits on opposite sides or blocks across the fault, where initial water oscillations would be expected to behave in opposed manner. would this imply a submarine slide origin, or even any other source, for this earthquake-related tsunami, as proposed by other previous authors?? from the damage to buildings at barlovento, where dwelling and warehouse drowning seems a common feature nearshore (puerto tuy) and several kilometers inland (río chico and san josé de río chico), inundation appears not to be catastrophic but rather slow instead, more like a rising flood, suggesting that riverbore took place at the lowermost stretch of several rivers. then, assigning an itis-2012 intensity level is not straightforward. inundation during this tsunami is reported up to 7.5 km inland but only close to river. little flow damage seems to have been recorded by dwellings. from damage descriptions, it appears to range between iv (largely observed) and v (strong). however, damage might have been minimized by the height of terrace risers (about 4 m above riverbed), on which houses were built, and inundation progression substantially slowed down by dense tall tropical vegetation few kilometers inland. if tsunami height (or reliability of testimonies on venezuelan historical tsunamis tsunami flow depth enhanced by riverbore effect of 4.55.0 m) is taken into consideration, the intensity of this tsunami may reach as high as ix (destructive). meanwhile, at los roques archipelago, intensity v (strong) may be assigned on the basis of most boats, while sailing between islands of the archipelago, felt strong water shaking. centeno graü [1900], a young civil engineer who would compile the first seismic catalog of venezuela many years later [centeno graü, 1940], writes a note in the newspaper la linterna mágica, on november 15th, 1900, entitled interesantísimo estudio. the following extract refers to the tsunami effects around barcelona: “en el mismo puerto "el rincón" que he mencionado anteriormente, uno de los caños del río neverí se angostó como más de 2 metros, los árboles de sus riberas están arrasados como por un temporal. este caño está en terreno de aluvión muy reciente. el nivel de las aguas del neverí se levantó a juzgar por uno de los caños: el de puente colorado. este caño no corría desde hacía tiempo porque su embocadura, con los detritus y sustancias vegetales, quedó más alta que las aguas del neverí. al día siguiente del movimiento seísmico y en mis exploraciones noté que dicho caño estaba corriendo en abundancia. a los pocos días volví, y ya no corría. fui a su embocadura, y me cercioré que esta es más alta, con un metro, que el neverí”. ftba is as follows: in the same el rincón seaport that i mentioned before, a channel of the neverí river (figure 3) got narrower in more than 2m, and the trees on its riverbanks are torn down like by a strong wind. this channel incises very recent alluvial sediments. water level of the neverí river seems to have been raised, based on one of its channel, the one under the colorado bridge. this channel had been dry for a long time because sediments and dead trees at its mouth raised its river bed above the neverí waters. the day after the earthquake, during my field explorations, i observed running water in that channel. few days later, it went dry again. i went to its mouth, and confirmed it was a meter higher than the neverí river level. the detailed descriptions by centeno graü [1900] clearly attest to his observational skills, not only as an engineer but also as a naturalist. by the way, the best descriptions of damage by this earthquake were undoubtedly his. he even estimated the possible epicentral area of this earthquake [see colón et al. 2015 for more details). based on his observations on water levels and relative river bed heights narrated above, we have estimated that water level has been raised at el rincón seaport by about 2 m, eventually 3 m, since water was running on the higher and commonly abandoned channel of the neverí river. in addition, riverbore seems to have happened here as well, enhancing the coastal flooding at the neverí river delta. in addition, the fact that trees (of unknown species and robustness) are torn or broken down along the river channel, it allows estimating the run-up height at least at 3 m. this effect (generation of garbage) in the environment allows to classify it as intensity vii [damaging; lekkas et al. 2013] at least. the herein estimated tsunami wave height may upgrade it up to intensity viii (heavily damaging). collected damage descriptors are useless to assign an imamura-iida magnitude (iida, 1963], but can be classified as grade 1 from estimated tsunami wave height. summing up: 1) the tsunami associated to the 1900 earthquake was recorded at three sites (figures 3 and 11): a) gran roque and others keys of los roques archipelago, b) barlovento coast (between paparo and puerto tuy) and as inland as río chico (5 km from seashore) and san josé de río chico (7.5 km from coastline), both on margins of the río chico river, and c) neverí river mouth, at el rincón seaport (barcelona). 2) riverbore is common to most localities on the barlovento coast (figure 11): paparo next to tuy river mouth, puerto tuy at el guapo river mouth, and río chico and san josé de rio chico along the río chico river. same situation happens at el rincón seaport, near el dorado bridge. 3) sea retreat was noticed at gran roque and at barlovento; up to 1 km at the latter one. no observation has been found for barcelona yet. 4) maximum run-up height is: a) 5 m above river level (1.5 m above alluvial plain) along the río chico river, b) at least 3 m above the water level of the neverí river at el rincón seaport (barcelona); and c) less than coastal sand barrier height at gran roque leeward side (less than 1.5 m). from the number of tsunami reports and their spatial distribution (size of their felt area), this is the largest tsunami of all tsunamis produced by a local earthquake in eastern venezuela. in fact, based on the extent of the area with tsunami reports, which is in the order of 250-300 km long (distance between los roques archipelago and barcelona; figure 3), the magnitude of this tsunami would be less than 3, and most likely magnitude 2, by applying the imamura-iida tsunami magnitude scale [iida, 1963]. 4.3 the july 15th, 1853 tsunami five different eyewitness accounts tell us about this earthquake and its associated tsunami. all accounts report on cumaná, but two of them refer to a strong aftershock that took place on august 3rd, almost 3 weeks later. these two accounts also provide information about two other localities: barcelona, which is located audemard and leal guzmán 10 11 some 30-40 km wsw of cumaná and farther into the barcelona embayment, and the coche island (figure 3). the first account that we could find, says: “el mar se retiró algunas varas al principio del terremoto, y como si hubiese recogido sus fuerzas, volvió después con un ímpetu extraordinario penetrando más de cien varas en todo el rededor de la población. algunos chorros de aguas termales brotaron de repente y las aguas del manzanares, que divide la población, se elevaron algunos pies en ese instante” (anonymous eye witness, 1853; la catástrofe de cumaná. folletos, academia nacional de la historia, caracas).ftba goes more or less like this: “sea got away some rods [vara = 0.835 m or about 33 inches] at the beginning of the earthquake, and as if it had put together all its strength, came back with an extraordinary rush, invading over a hundred rods [>85 m] all around town. some hot water springs suddenly appeared and the waters of the manzanares river, which run across town, were then raised few feet”. “como ahora siglos, el mar inundó una gran parte del salado. se desbordó el rio manzanares que atraviesa la ciudad” (diario de avisos y semanario de las provincias, caracas: 27 de julio de 1853). this second account can be translated as follows: “today as few centuries ago [probably referring to the predecessor 1530 tsunami], sea inundated a large portion of el salado [coastal neighborhood of cumaná]. the manzanares river that crosses the city, flooded over its banks” (ftba). the two following accounts refer to the large aftershock felt in cumaná, in august 3rd, 1853. “se nos informa al poner en prensa este número que el 3 del corriente ha sufrido cumaná otra fuerte conmoción de tierra que ha derribado muchas de las casas que quedaban en pie después del terremoto del 15. el neverí se ha desbordado, inundando gran parte de la ciudad de barcelona” (diario de avisos y semanario de las provincias, caracas: 10 de agosto de 1853). translation (ftba) goes: “we have been informed when printing this issue (of diario de avisos y semanario de las provincias in caracas on august 10th, 1853) that cumaná on august 3rd has suffered another strong ground commotion (shaking), which has pulled down many of the still standing houses spared by the earthquake on the 15th. the neverí [river] has flooded over, inundating most of the city of barcelona”. from this account, it is still unclear to the authors when the neverí flooding occurred: on july 15th during the main shock or on august 3rd with the large aftershock? grases et al. [1999] relate the account on the neverí flooding to the aftershock. “el último temblor de esta ciudad [cumaná] tuvo lugar en la noche del 3 de agosto. asegúrase que surgieron manantiales de agua subterráneos, y que el mar volvió a invadir parte de la costa por el lado del dique, dejando una profundidad de 14 brazas” (diario de avisos y semanario de las provincias, caracas: 13 de agosto de 1853). the translation (ftba) goes: the last earthquake in cumaná happened on the night of august 3rd. people are persuaded that underground waters were spouted, and sea invaded again the coast by el dique [neighborhood of cumaná located on the northern bank of the manzanares river near its mouth; figures 7. 9 and 10a]-, leaving a depth of 14 fathoms [≅ 25 m; a fathom is equivalent to 6 feet or 1.8 m approximately]”. the fifth account seems to confirm most of what the eyewitness of the first account reported: “el mar, salvando sus límites conocidos inundo en una extensión de 200 varas, las sabanas del salado y caigüire, sumergiendo un alijo cargado de maíz, que acababa de llegar a la playa. una embarcación sintió intensamente el sacudimiento en frente de la isla de coche” [beauperthuy 1853]; otra relación del terremoto de cumaná en: diario de avisos y semanario de las provincias, caracas: 10 de septiembre de 1853). this report goes: “sea, leaving its known limits, inundated an extension of 200 rods [about 170 m; vara ≅ 33 inches or 85 cm] in el salado and caigüire flatlands, drowning a corn supply or stockpile, which had just arrived to the beach. a boat felt strong shaking off coche island (figure 3). from these five accounts, several facts can be deduced: 1) a large aftershock on august 3rd accompanied the july 15th main shock, and both were felt in cumaná. 2) the 1853 tsunami is not the first one suffered by cumaná. when describing the 1853 event, the writer mentions it is not the first time that el salado is drowned by tsunami waves. 3) the sea retreated at cumaná before inundating the city on july 15th, 1853. 4) inundation was in the order of a hundred to two hundred “rods” (varas) on flat-lying areas such as caigüire or el salado (in fact, salt-flats; figure 12); in other words, actual inundation was between 85 to 170 m inland. 5) inundation takes place preferentially along rivers, both the manzanares in cumaná and neverí in barcelona. 6) caigüire is also inundated; not only el salado. caigüire lies at the former east end of cumaná (in 1960’s), on the southern coast of the cariaco gulf entrance (figure 12), and not on open sea, such as el salado or el dique. 7) it is still unclear to us whether the neverí inundation happened during the main shock on july 15th or a large aftershock on august 3rd, because of the way the news is reported. we however suspect it is associated to the main event. 8) the sea surface shaking is spotted off the coast of coche island, to the north (figure 3). 9) maximum run-up height is not easily discernable. for instance, the first account expressly says: “manzanares river waters were reliability of testimonies on venezuelan historical tsunamis raised few feet”, near the river mouth (el salado), while the second one indicates that went over its banks. nowadays, the manzanares river near its mouth hardly incises down by a meter or so (figure 13). this would imply that tsunami height, better to say riverbore height, would have needed to be over a meter (or two?). but the fifth account brings additional information about this inundation in low lying flatlands or saltflats. beauperthuy [1853] says that a corn stockpile on the beach was drowned. in doing so, tsunami waves would have required to be at least 2 to 3 meter high to go over both the coastal sand barrier and the corn supply sitting on it, since the current sandy beach around el salado and caigüire is already about 1.5 m high. this maximum run-up height of about 3 m seems to be in good agreement with some 200 m of tsunami inundation. hafeez [2008] measured, after the december 2004 indian ocean tsunami, inundation distances and run-up heights at different indian localities, obtaining a rather linear relationship between both parameters (his figure 3). particular interest for us shows two of his localities with similar topographic conditions and relative locations to our current study case. the rajakkamangalam and manavalakurichi lie at the mouth of the panniyar and valliyar river mouths, respectively [hafeez, 2008]. this author expressly indicates “these rivers also contributed to such a huge amount of inundation”. riverbore appears to have taken place in such cases. the inundation distance and run-up height at rajakkamangalam were 350 m and 3-4 m, respectively; and they were at manavalakurichi 250 m and 3-4 m, audemard and leal guzmán 12 figure 13. view of the manzanares river near its mouth. the simón berrizbeitia’s monalisa yatch stands for scale. riverbanks are less than 1 m high above river level. higher grounds correspond to river protection embankments, which are also visible in figure 7, coconut trees in both figures are comparable. figure 12. bird-eye to the nw of the caigüire sector, taken in 1925. note the mangrove trees by the seashore in the far background. today, the perimetral avenue runs there. also notice the low-lying flat topography of the salt-flat, on which are built the wooden huts of locals. that is the view of a typical fisherman village of the time. 13 respectively [hafeez, 2008]. in other words, a rough relation of 1 to 70-100 can be established between run-up height and inundation, for localities sitting on lowlying coastlands near rivers. consequently, a 200 m inundation distance appears to go well with a 3 m run-up height for the 1853 tsunami in cumaná. finally, the meaning of “leaving a depth of 14 fathoms”, equivalent to some 25 m, remains unclear to the authors. by applying itis 2012 scale (lekkas et al. 2013], the tsunami at coche island can be categorized as intensity iv (largely observed) at least, since it was felt onboard small vessels or boats. meanwhile, the facts that the manzanares river rose few feet and inundated close to 200 m inland along cumaná western shore, with no other damage descriptors, allow to establishing a minimum intensity of vii [damaging; lekkas et al. 2013]. its magnitude by applying imamura-iida scale, is at least 0 at cumaná. 4.4 the september 1st, 1530 tsunami three different testimonies describing the 1530 tsunami are here evaluated. the first account of the three is from fernández de oviedo y valdéz. it says: “el año de mill é quinientos é treynta, en el mes de septiembre, en un día sereno é tranquilo, en un instante, á las diez horas del día, en la provincia de cumaná se levantó el mar en altura de quatro estados é juntamente dio la tierra un horrible bramido, é inundóse la tierra, sobrepujando la mar sobre ella…” (fernández de oviedo y valdés, gonzalo [(1535)1851] historia general y natural de las indias. islas y tierra firme del mar océano. primera parte. imprenta de la real academia de la historia: madrid). the story goes: “in 1530, in september, in a quiet and peaceful day, at 10 in the morning, in the province of cumaná, the sea raised in 4 “estados” [over 13 m; 1 “estado o estadal” = 4 “varas” or stick = 4 x 0.835 m ≅ 3.34 m] and the earth bellowed horribly at the same time, and land was drowned, running waters over it…”. the second account on the 1530 earthquake and tsunami comes from a letter of deputy mayor andrés villacorta to the canonry of nueva cádiz that says in spanish: “oy jueves, primero de setiembre de mill e quinientos e treynta años, a ora de las nueve se levantó el mar de tal manera ques cosa milagrosa a los que la vieron y entró dentro de esta tierra donde estava sytuada esta fortaleza, e con ello dio un bramido la mar e tenbló la tierra tres vezes en media ora, e abrióse por muchos lugares, e con el tenblor cayó esta fortaleza hasta los cimientos” [carta de andrés villacorta, teniente de alcaide, al cabildo de nueva cádiz, a razón del terremoto de 1530, citado en otte, 1977: 243]. the ftba goes as follows: “today, thursday september 1st 1530, at around 9 in the morning, the sea raised in such a way that was a miracle who could tell about it, and went inland where the fortress is situated, and the sea bellowed once, and land shook three times in half an hour, and cracked wide open in many places, and the fortress fell down to its foundations”. the third testimony is much more elaborated and abundant in details in many different relevant aspects: “...y al otro día siguiente vino la nueva de cómo la fortaleza de cumaná era cayda del gran temblor e terremoto y se abrió la tierra en muchas partes y salió la mar de sus términos (...) y el mismo dia viniernon ciertos yndios... los quales dixeron que se avia anegado un pueblo questava a dos leguas de alli e que ellos avian salido a nado donde murieron quatro yndios e muchos muchachos e perdieron toda su ropa (...) temblo la tierra veynte y tres veces a cada vez que temblaba la tierra bramaba debajo della de una manera espantosa e vido viniendo por el dicho golfo... quel agua de la mar salio de sus limites y los dichos indios decian que bramó la mar como leon (...) e vido los arboles arrancados y encima de otros arboles a donde salio la mar de sus términos e vido algunos peces colgados netre las ramas [producto] del gran terremoto e vido otros arboles de mangle que son muy rezios e muy gordos quebrados del dicho terremoto e otros arrancados de rayz e otras cosas de mucha admiración” [información hecha en la isla de cubagua, 1530]. the free translation by authors (ftba) goes: “... and the day after, the news arrived that the fortress of cumaná was pulled down by the great tremor and earthquake, and land opened in many places and sea left its boundaries (...) the same day some indians came... who said that a village at 2 leagues away (about 11 km away; league = 6666.7 “varas” or sticks = 5572.7 m) was drowned and they were able to escape swimming while 4 indians drowned and many young people had lost all their clothing (...) land shook 23 times; each time earth trembled, it bellowed underneath in a horrible way, seen as coming by the gulf [cariaco gulf located due east of cumaná]... that seawater went out of its limits and those indians mentioned that sea roared like a lion (...) trees were seen unrooted and piled on top of others where sea inundated, and some fish were seen hanging in tree branches because of earthquake, and mangrove trees, which are very strong and robust, were broken by the earthquake and others fully unrooted, and others things of much admiration”. for grases et al. [1999], the latter part of the account by the indians takes place into the cariaco gulf, located due east of cumaná (figure 3). as expected, the information about this 1530 earthquake and associated tsunami is more complex to unravel, but many details, however, are provided. as to the earthquake, we can express the following: 1) day and time of occurrence is very well estabreliability of testimonies on venezuelan historical tsunamis lished: in midmorning (9-10 am) of thursday september 1st, 1530. 2) both earthquake and tsunami takes place. time lag between earthquake and tsunami seems short. 3) earthquake epicenter is close to cumaná. many accounts refer to loud noises, like roaring, booming or bellowing, accompanying shaking. 4) many earthquakes (aftershocks) were felt at cumaná after the main shock, up to 23; most of them also accompanied by loud noise from the ground. 5) roaring in some cases would seem to come from the cariaco gulf, due east of cumaná, which would be in contradiction with a focus on the cariaco trough, located west of cumaná, as proposed by audemard [2007]; unless some echo effect or reverberation is called upon. 6) the fort built by jácome castejón was torn down to its foundation by the earthquake and not by the tsunami. as to the tsunami, we can list the following facts: a) inundation is reported both at cumaná and at a village 11 km (2 leagues) away (figure 3). being this village around the cariaco gulf, the locality, which should suit favorable conditions for indian settling and land cultivating at the time of the earthquake and tsunami, as well as exhibiting a topographic situation prone to drowning during high waves inundation, is the delta plain of either guaracayal or marigüitar, that lie 18 to 23 km due east from cumaná (caigüire), respectively b) wave height of tsunami waves seems to be significantly high since someone reports “lucky ones those who could tell about the sea rise”. at the nearby village, indians had to swim to survive and four of them drowned. this also implies that run-up has to be few meters high. this seems to confirm the description provided by centeno graü [1969], which indicates that sea got some meters above its normal level, leaving the beach dry and when returning, it inundated most of the city existing at that time. c) however, waves as high as 13 m (4 estados) seem an exaggeration to us, although some reports indicate that waves went over treetops at manzanares river mouth [see audemard 2007, for more details]. d) using plants as a height reference, we must indicate that two main plant species are common to this seashores and river banks at cumaná: coconut and mangrove trees (figures 7, 10a, 12, 13 and 14), particularly rhizophora sp. in the referred case, waves as high as 5 to 7 m, as interpreted by audemard [2007], would be large enough to go over mangrove trees and leave fish hanging from its branches, as reported in the third account. e) a better bracketing of wave height can be provided from the fact that robust mangroves (probably rhizophora trees) were torn down and even some specimens have their roots pulled out completely, as mentioned in the third account. based on recent studies of damage to mangrove populations, in banda aceh, indonesia, and pakarang cape, thailand, as a result of the devastating indian ocean 2004 tsunami, yanagisawa et al. [2010] found that more than 50% of mangrove trees with 20-25 cm [very robust] stem diameter at breast height (dbh) could survive a tsunami of less than 6-7 m, although mangrove trees were destroyed completely under 7-9 m or higher tsunami waves. we feel that coastal morphology, plant populations, shelf configuration and profile, between these two indian ocean study sites and the cumaná region are pretty much similar. therefore, their conclusions can be extrapolated to our case. in a previous study, yanagisawa et al. [2009] categorize damage to mangrove trees into five patterns: (1) broken at stem or prop roots, (2) uprooted and inclined, (3) uprooted and fallen down, (4) uprooted and swept up by tsunami flow, and (5) lost bearing capacity because of ground erosion. however, the same authors indicate that rhizophora trees were destroyed only slightly in the patterns resembling patterns (2)-(4) because the prop roots of rhizophora trees are sufficiently thick to resist the tsunami flow. they add that when pattern (5) does not show, pattern (1) is mostly applicable as the predominant damage pattern of rhizophora trees. then, from the description of damage to mangrove populations during the 1530 tsunami waves given in the third account, it would seem that pattern (1) and (3) were the two most common, implying that low-lying coastal plain erosion did not take place then. although we do not know the actual stem diameter of the mangrove trees affected during the 1530 event in venezuela, it would appear that no total destruction of tree population occurred. it would seem to point to flow depths during the cumaná event below 7 m. f) audemard [2007] indicated that tsunami waves at the manzanares river mouth (north part of the old city) flooded inland as far as the ranges (caigüire hills?). as applied for the 1853 tsunami herein, the inundation distance may give as clues as to the run-up heights. in old maps of the city of cumaná, the river mouth was some 500 m away from the caigüire hills foothills, although they are just over 2 km away nowadays. from a map by beltrán and rodríguez [1995], a coastal advance, likely strongly influenced by anthropogenic processes since the early 50’s, of some 130 m has happened in less than 50 years, between 1937 and 1981. so, despite the low precision of old maps, before the instrumental -theodoliteera, the delta plain of the manzanares river since the foundation of the city of cumaná in 1515 ad, has surely substantially changed, particularly advanced through the years. in any case, an inundation distance of at least 500 m audemard and leal guzmán 14 15 would allow estimating a lower bound for the run-up height in the order of 5 m. consequently, the run-up height in cumaná, characterized by a flat morphology typical of a young prograding delta, would be ranging between 5 (from inundation distance) and 7 m (damage to rhizophora trees). this value range is consistent with an earlier estimate proposed by audemard [2007]. the occurrence of indians drowning (at village in the cariaco gulf) and mangrove trees tearing and uprooting (at caigüire, cumaná) during this tsunami both allows to estimating a minimum itis-2012 intensity of viii (heavily damaging; lekkas et al. 2013]. this also determines the tsunami magnitude to be grade 2 in the imamura-iida tsunami magnitude scale [iida 1963]. if the tsunami wave height happens to be between 5 and 7 m, as proposed after this research, the assigned itis 2012 intensity can be as large as x (very destructive). 5. concluding remarks the first striking commonality from the historical evaluation of eyewitness accounts is that inundation for all these earthquakes are reported at river mouths or settlements on their mouths (figures 3 and 11): tuy, río chico, guapo, neverí, manzanares. from comparison with modern analogs, such as the valdivia 1960, camaná 2001, sumatra 2004, and tohoku-ochi 2011 tsunamis, this is not surprising because inundation and wave invasion happens sooner, faster and more efficiently along rivers, through the river mouth and along the incised river channel. as a matter of fact, seawater invasion during tsunami wave progression shows a prominent acute salient at river mouths. the explanation is obvious: there is no natural barrier blocking the landward wave progression. in addition, the water invasion is eased by the lower level of the coastland area resulting from river incision. this also drives that maximum inundation tends to happen and occur along rivers. however, a major difference between the venezuelan historical tsunami events and those abovementioned needs to be underlined. it refers to the size of river discharge in venezuela. as a country lying in the tropics, river flow is hardly negligible and appears to be significantly higher than for the other cases. for instance, the manzanares river, which is a small river for our standings, has an average annual discharge of 560-770 x 106 m3, which is equivalent to some 15 x 106 m3/month, or 5,63 m3s-1 (ranging between 3.5 and 7.0 m3s-1) at river mouth [medina et al. 2013]. this then surely leads to the formation of riverbore when both waters (tsunami wave and river flow) encounter at river mouths, rising river level and significantly flooding over banks. then, this may have fundamental implications on the dynamics of the inundation process, as well as on characteristics of the tsunami waves, particularly as to wave height. in other words, flooding heights along rivers when riverbore occurs, are not necessarily the same as maximum run-up heights, but should tend to provide a rather precise lower bound. the other common fact is that the city of cumaná has been affected by tsunami waves associated to all 4 events occurring on the epf but not by the 1900 ad event, the largest of all 5 events assigned to ssf, whose epicenter lies close to the far west end of the barcelona embayment (figure 3). nevertheless, it is a fact that there are more frequent and more widespread eyewitness accounts on the tsunami waves during the 1900 ad event (figure 3). a larger area of affectation by tsunami waves definitely support its larger earthquake magnitude: gran roque island of the los roques archipelago and river mouths of tuy, río chico, guapo and neverí. all these rivers drain into the barcelona embayment; neverí on the se while all others on the sw (figure 3). another outcoming commonality is that the value of the run-up height (in all our cases equivalent to the water depth of the inundation, because all affected localities sit on low-lying coastlands) from such eye-witness or first-hand testimonies not necessarily are rid of subjectivity. however, combining a good knowledge of coastal physiography, shelf morphology, natural environments and cultural aspects (construction types, housing, customs, life style, vessels, among many others) of affected populations at the time of occurrence of tsunamis with a thorough and careful reading of the testimonies, may lead to find ways of bracketing both inundation distances and run-up heights for any given historical tsunami. for instance, an eye-witness during the 1900 earthquake reports tsunami waves of 10 m; or 13 m during the 1530 tsunamigenic earthquake. simireliability of testimonies on venezuelan historical tsunamis figure 14. beach at caigüire in 1960’s, where mangrove trees grow almost on the sand barrier, which is visible between trees. also notice the height of sand barrier above sea level. larly, 6 m high tsunami waves are reported in association with the 1929 earthquake. it is very likely that the actual height of tsunami waves in all 3 cases may be half of the reported height at most, based on sound argumentation constructed from the descriptions provided by the eyewitnesses themselves (table 1). as well, tsunami waves during the 1530 are reported to go over treetops. this may be very misleading if interpreted as it sounds. actually, maximum run-up heights for the 5 tsunamis herein evaluated are: 5-7 m at cumaná for the 1530 event, 5 m at barlovento and 3 m at the neverí mouth for the 1900 tsunami, 3 m at western cumaná for the 1853 and 1929 events and about 1 m for the 1997 earthquake (table 1). a difficult aspect to manage, audemard and leal guzmán 16 parameters earthquake/tsunami 01-ix-1530 15-vii-1853 29-x-1900 17-i-1929 09-vii-1997 epicenter coordinates 10.261°n, 64.313°w (estimate; rodríguez et al., 2006) 10.265°n, 64.359°w (estimate; rodríguez et al., 2006) 10.660°n, 66.080°w (estimate; rodríguez et al., 2006) 10.394°n, 64.000°w (estimate; rodríguez et al., 2006) 10.545°n, 63.515°w (estimate; rodríguez et al., 2006) mm i o (with most damage settlement) x (cumaná) ix (cumaná) ix (guarenasguatire & macuto) viii-ix (caigüire, east cumaná; audemard, 2007) viii (cariacocasanay) magnitude (ms) 7.1-7.3 (estimate) (audemard, 2007) 7.1-7.3 (estimate) (audemard, 2007) 7.6 (pacheco and sykes, 1992) 6.3 (audemard, 2007) ≤ 6.6 (mocquet et al., 1996) 6.8 (baumbach et al., 2004) causative fault el pilar -epf(segment a) (audemard, 1999, 2007) el pilar–epf(segment a) (audemard, 1999, 2007) san sebastián -ssf (east segment) (audemard, 2002) el pilar -epf(segment b-1) (audemard et al., 2007) el pilar -epf(segment b-2) (audemard et al., 2007) tsunami height from eyewitness accounts cumaná: ≅ 13 m (4 estados; fernández de oviedo y valdés, 1851) manzanares river rose few feet (la catástrofe de cumaná, 1853) puerto tuy (barlovento): ≈ 10 m (linterna mágica, 02-xi-1900) 6 m (centeno graü, 1940) west cumaná: 9 feet (3 m) (el nuevo diario, 22-i-1929) cumaná & cariaco: ≤ 1m (gonzález et al., 2004) recalculated tsunami height (this research) cumaná: 5-7 m (this ratifi es audemard, 2007’s estimate) cumaná (caigüire): 3 m barlovento: 5 m los roques: < 1.5 m el rincón: ≥ 3 m west cumaná: 3 m ≤ 1m (unchanged) (gonzález et al., 2004) itis-2012 tsunami max. intensity cumaná & cariaco gulf: ≥ viii (up to x on tsunami height) coche island: ≥ iv west cumaná: ≥ vii barlovento: iv-v (up to ix on tsunami fl ow depth) los roques: v el rincón: vii (viii on tsunami height) west cumaná: vii-viii cumaná & cariaco gulf: iv imamura-iida tsunami magnitude 2 (cumaná & cariaco gulf ) 0 (west cumaná) 1 (barlovento) 0 (los roques) ? (el rincón; 1 from tsunami height) 2 (from tsunami extent) 1 (west cumaná) -1 (cumaná & cariaco gulf ) table1. summary of major parameters of the five tsunamigenic local earthquakes assessed during this investigation. it compares how tsunami heights from eyewitness accounts have been substantially reduced for some earthquakes, when the event is contextualized in time and space. the itis-2012 tsunami intensity scale and imamura-iida tsunami magnitude scale have been applied to the data provided by eyewitness accounts as well. the earthquake epicenters are shown on figure 2 or 3. 17 though, is the eventual enhancement of the tsunami wave height by contributing underwater sliding, also triggered by the local earthquakes on steep submarine (even subaerial) walls, such as inside the cariaco trough, on the continental slope or along steep rocky coasts. in all the tsunamis analyzed in this paper, eyewitnesses report sea retreat first. that is the case for the 1530, 1853, 1929 and even 1997 tsunamis in western cumaná (el salado, el dique or puerto sucre seaport). testimonies for the latter event were collected by the first author in the first few days following the july 9th, 1997 earthquake. reports of sea oscillations inside the cariaco gulf during the 1997 event were also collected. this seems sound in case of the cariaco 1997 earthquake because the earthquake epicenter was located farther east; more precisely, on the land bridge between the cariaco and paria gulfs [baumbach et al. 2004], the latter gulf lying even farther to the east (figure 2). the sea surface oscillations detected on western cumaná during this 1997 earthquake, on the contrary, are attributed to subaerial and submarine slides taking place at the river mouth banks and along the submarine manzanares canyon, respectively [audemard and leal 2014,2015], as attested by deep turbidite currents in the cariaco trough detected by thunell et al. [1999] and lorenzoni et al. [2012]. this same argumentation is to be applied to the 1929 earthquake, whose epicenter also lies east of cumaná, into the cariaco gulf, but the tsunami waves occurred on the opposite side of cumaná, at the manzanares river mouth (el salado) predominantly. instead, a more complex scenario is to be conceived if the 1530 and 1853 earthquakes do actually take place in the deep cariaco trough, as proposed by audemard [1999, 2007], unless the simpler submarine slide hypothesis is accepted for all these tsunamis affecting cumaná. aguilar et al. [2016], in order to explain an increase of more open seawater circulation into the cariaco gulf, proposes a major change in the salazar sill depth (lying just north of northern cumaná, in the cariaco gulf entrance) at around 1853 ad, in association to a submarine slide. this could explain the occurrence of inundation of the caigüire saltflats during this earthquake, which seems not to be reported during other earthquakes and associated tsunamis. but the 1530 tsunami affectation is even more difficult to explain with an earthquake locus in the cariaco trough lying due west of cumaná, taking into account that an indian village at 2 leagues into the cariaco gulf was clearly inundated, with 4 casualties by drowning. now that run-up height for this 1530 tsunami is better bracketed, numerical modeling of different tsunami scenarios -combining earthquake epicenter west or east of cumaná, tsunami waves generated by (fully strike-slip or normal oblique) tectonic slip or submarine slide or combination of both-, should provide answers or constraints to this uncertainties. a few first attempts [audemard et al. 2014], using the piston modeling developed by the norwegian geotechnical institute -ngi, have been performed, trying to reproduce a 3 m high inundation at the western coasts of cumaná. actually, the barcelona embayment shelf configuration with a pure strike-slip motion on epf, attempting to recreate the 1853 tsunami, induces tsunami waves of only 30 cm in height [audemard et al. 2014], which appears insufficient to inundate western cumaná with 3 m high waves, unless other local considerations are incorporated (funneling effect of the manzanares canyon, riverbore formation, actual oblique -normal-dextralslip in a pull-apart setting, among others). very much remains to be done and tested in this field yet. speculating about the 1853 slide scenario proposed by aguilar et al. [2016], which surely inundates caigüire from the testimonies collected herein, one of their own figures (figure 15) is much worth discussing. the morphology of the coast and shelf bathymetry at the cariaco gulf entrance and just north of cumaná, and particularly north of caigüire, points out to a potential large slide that indented the coast at caigüire and associated shelf; and slid down to deeper waters, as suggested by the belly shape the isobaths exhibit. in terms of intensity, the itis 2012 scale [lekkas et al. 2013] has been applied to all 5 tsunamis (table 1). even for the recent 1997 event, the quantity and quality of the six types of descriptors integrated in the itis scale (quantities, impact on humans, impact on mobile objects, infrastructure, environment and structures) are definitely insufficient for a good well constrained assessment in all cases. however, in some cases, some diagnostic descriptors (e.g., damage to trees, drowned individuals, among others) help to establish the minimum tsunami intensity. in this research, the assessment of the tsunami wave height (or flow depth) by contextualizing the eyewitness account(s) as a whole, has led to more reliable values. in addition, the tsunami descriptions have been also converted into tsunami intensity values by applying itis 2012 scale [lekkas et al. 2013], as well as tsunami magnitude grades by applying imamura-iida [iida 1963] scale. this has actually led to more robust and better bracketed tsunami wave heights. however, we need to highlight that although descriptors in the six types of elements assessed in the itis 2012 scale appear to have a good level of detail for modern tsunamis, its application to historical tsunamis reliability of testimonies on venezuelan historical tsunamis in the modern world characterized by a rather short written history, seems more difficult to implement. last but not least, these new estimates on tsunami run-up or wave heights for earthquakes produced by local active tectonics have major implications on search for geologic past tsunami records along the venezuelan coasts. for the particular case of the eastern venezuelan coasts, the search for tsunamites by trenching and coring should be essentially restricted to the first 500-mwide strip from the coastline in low-lying flatlands, if natural (e.g., vegetation, coastal sand barrier height) and anthropogenic (e.g., building and housing, coastal embankments) obstructions to inundation are not significant, due to the modest size of tsunami waves produced by local earthquakes of magnitude up to mw 7.6, mostly on offshore strike-slip (or normal oblique) faults. acknowledgements. this is a contribution to the terpro commission of inqua, led by dr. alessandro maria michetti from the university of insubria (como, italy). funds to the research are provided by fonacit through grants to project fonacit 2013000361 and past projects fonacit-ecos nord pi-2003000090 and pi-2009000818. we wish to thank the funding support the first author received from istituto nazionale di geofisica e vulcanologia -ingvand istituto superiore per la protezione e la ricerca ambientale -isprato partly present these results at the vi inqua international workshop on paleoseismology, active tectonics and archaeoseismology (vi pata) held in pescina (aq) between 19 and 24 april 2015, as well as the invitation from the vi pata organizing committee. really, many thanks for sponsoring me!! it was a very enlightening and an unforgettable meeting in many ways: friendly talks, lively discussions, nice sightseeing, excellent mediterranean food, eye-filling window geology, lovely re-acquaintances with friends and colleagues, varied wine tasting, teaching fieldtrips (visit to the fucino 1915 and l’aquila 2009 surface ruptures and damage to the urban area of l’aquila itself; and some paleoseismic trenches too!), illuminating rome archaeoseismic visit, among many others….. it is also a contribution to working group 2 of iocaribe, the regional subsidiary body of the unesco intergovernmental oceanographic commission, which at present is focused on designing, building and implementing the tsunamis and other coastal hazards warning system for the caribbean sea and adjacent regions(caribe-ews). we thank raquel vásquez for helping us with some macroseismic data. last but not least, we are thankful to two anonymous reviewers and the sector editor dr. francesca bianco, for providing pertinent comments. references aggarwal, y., 1983. neotectonics of the southern caribbean: recent data, new ideas. acta científica venezolana 34(1):17 (abstract). aguilar, i., beck, c., audemard, f. a., develle, a-l., boussafir, m., campos c. and crouzet, c., 2016. last millennium sedimentation in the gulf of cariaco (ne venezuela): evidence for morphological changes of gulf entrance and possible relations with large earthquakes. comptes rendus geoscience 348: 70-79. altez, r. and audemard, f. a., 2008. el sismo de 1629 en cumaná: aportes para una nueva historia sísmica del oriente venezolano. boletín técnico del imme, ucv, 46(2): 53-71. altez, r. and rodríguez, j. a. (project coordinators), 2009. catálogo sismológico venezolano del siglo xx, fundación venezolana de investigaciones sismológicas, caracas, 2 vols., 833 pp. audemard, f. a., 1999. nueva percepción de la sismicidad histórica del segmento en tierra de la falla de el pilar, venezuela nororiental, a partir de primeros resultados paleosísmicos. vi congreso venezolano de sismología e ingeniería sísmica, mérida, venezuela (in cd-rom). audemard, f. a., 2002. ruptura de los grandes sismos históricos venezolanos de los siglos xix y xx, revelados por la sismicidad instrumental contemporánea. xi congreso venezolano de geofísica, caracas, venezuela, nov. 17-20, 2002 (8pp; exaudemard and leal guzmán 18 figure 15. geomorphic setting of the cariaco gulf and of its connection with the cariaco basin or trough. cab cerro abajo basin; ch: caigüire hills; gd: guaracayal deep; mrd: manzanares river delta; ss: salazar sill [after aguilar et al. 2016]. the circle points out the coastal and submarine features that do resemble a large submarine slide, with crown at seashore. 19 tended abstract in cd-rom). audemard, f. a., 2006. surface rupture of the cariaco july 09, 1997 earthquake on the el pilar fault, northeastern venezuela. tectonophysics 424(1-2):19-39. audemard, f. a., 2007. revised seismic history of el pilar fault, northeastern venezuela, after the cariaco 1997 earthquake and from recent preliminary paleoseismic results. journal of seismology 11(3): 311-326. audemard, f.a., 2011. multiple trench investigation across the newly ruptured segment of the el pilar fault in northeastern venezuela, after the cariaco 1997 earthquake. in audemard, f. a., michetti a. m. and mccalpin, j. 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[ 2 ] i valori [ 1 ] e [ 2 ] sono stati presi come valori di partenza per l'applicazione del metodo delle approssimazioni successive (3). se si indica con / il tempo di tragitto delle onde p, c o n ? 0 , cp0' le ('") i l p r o f . g u t e n b e r g m i h a g e n t i l m e n t e c o m u n i c a t o c h e l ' a n a l o g a s t i m a eseg u i t a a p a s a d e n a h a d a t o l o s t e s s o v a l o r e . 2 1 2 paolo k m 11.10 v a l l e coordinate geocentriche dell'epicentro, con h0 il tempo origine, con /; le differenze tra i tempi di arrivo calcolati mediante i valori di partenza e i tempi osservati, si ha per ogni stazione 7 . + n + r = o [ 3 ] 3x; 3 cp'i dove si è posto (<• 1, 2 e = x 0 — x 11=^0'—1qp' t 4 ] t =h0-h e nell'ipotesi che e, e r siano sufficientemente piccoli in modo da poter trascurare i loro prodotti e i loro quadrati, ti indica il numero delle stazioni. le derivate df/dx-, e 3//3cp'i si possono calcolare tenendo presente che si piiò scrivere axi a ai 3xi ' a cp'i 3a; 3cp'; la 3//3a; è stata ottenuta dai tempi di tragitto calcolati da jeffreys e le altre due derivate dalle espressioni 3 a c o s c p ' ( a ; s i n x — b , c o s à ) 3x; ~ sin a; [6] 3a sin cp' («i cos x-f hi sin x)—e, cos qp' 3 c p ' i sin ai nelle quali ait b;, ci sono i coseni direttori della tm" stazione. dato d i e le [ 6 ] non sono indipendenti, ma legate dalla relazione 1 " m ^ y i [ " ] v coscp' 3x; ) \ 3cpi y l a verifica dei calcoli può essere fatta agevolmente. la tabella i contiene i valori dei coefficienti e dei termini noti del sistema di equazioni [ 3 ] , ebe sono stati usati per il calcolo. i valori approssimati più probabili delle incognite e i rispettivi errori medi sono £ = — 0",02±0",04 t) = — 0",01+0°,04 [ 8 ] t = o'.o +0".4 il t e r r e m o t o j o n i c o del 2 2 a p r i l e 1 9 4 8 t a b e l l a i 2 4 3 li il n . s t a z i o n e sh a c p ' i *i s e c . g r a d i 1 sec. g r a d i 1 sec. 1 s o f i a — 4 , 9 — 1 2 , 7 — 0,7 2 r o m a + 9,4 — 7,3 — 0 , 7 3 b u c a r e s t — 6 , 1 — 1 1 , 7 — 1 , 0 4 t r i e s t e + 5 , 9 — 1 1 . 6 + u 5 b u d a p e s t + 1,2 — 1 3 . 7 — 0,7 6 g r a z + 4 , 0 — 1 2 , 8 + 0,6 7 l ' r a g a + 3,3 — 1 2 , 7 + 0,8 8 h e l w a n — 8,0 + 8 , 7 + 0,4 9 z u r i g o + 6,9 — 1 0 , 2 + 0,9 ]() b a s i l e a + 7,0 — 9,8 + 1 , 1 1 1 s t r a s b u r g o + 6,4 — 1 0 , 4 + 0 , 7 1 2 j e n a + 4,2 — 1 2 , 1 — 0 , 1 1 3 g o t t i n g a + 4,5 — 1 1 . 7 — 0,2 1 4 t o r t o s a + 9,8 — 3,3 — 0,9 1 5 p a r i g i + 7,2 — 8,9 — 1 , 0 1 6 u c c i e + 6,1 — 1 0 , 0 — 1 , 1 1 7 l u n d + 2 , 3 — 1 2 , 1 + 0 , 1 1 8 c o p e n h a g e n + 2,5 — 1 2 , 0 + 1-2 1 9 t o l e d o + 8,9 — 2.4 — 0 , 1 2 0 l i s b o n a + 7,6 — 1 , 7 + 1 , 0 2 1 b e r g e n + 2,4 — 9,2 — 0,9 t a b e l l a i i n . s t a z i o n e p li m s s — p sec. 1 a t e n e 1 0 4 3 2 4 , 0 33,2 2 s o f i a 5 5 , 6 5 7 , 7 3 r o m a 4 4 2 7 , 8 8 0 , 2 4 b u c a r e s t 3 1 , 6 8 5 , 0 5 t r i e s t e 5 0 , 0 1 0 3 , 8 6 b u d a p e s t 5 6 , 4 1 0 5 , 6 7 g r a z 5 9 , 1 1 0 8 , 9 8 p r a g a 45 4 0 , 0 1 3 9 , 0 9 z u r i g o 4 1 , 0 1 4 0 , 0 1 0 b a s i l e a 4 9 , 2 1 4 7 , 0 1 1 s t r a s b u r g o 5 7 , 4 1 5 2 , 6 1 2 t o r t o s a 4 6 2 4 , 4 1 7 5 , 6 1 3 p a r i g i 3 7 , 0 1 8 4 , 0 2 4 4 paolo e m i l i o valle questi valori delle correzioni da apportare ai dati di partenza sono sufficientemente piccoli e si rende quindi superflua un'ulteriore approssimazione. pertanto le coordinate definitive dell'epicentro e il tempo origine risultano u = 20",48+0",04 e cp0 = 38",49+0",04 n (geografica! [ 9 ] h0 — 10" 42'" 40s,8+0s,4 ( t.m.g.l si è già accennato al fatto che le registrazioni indicano che il centro di perturbazione è superficiale, tuttavia è sembrata opportuna una verifica del tempo origine con altro metodo indipendente dai precedenti. a tale scopo è stato usato il metodo di v a d a t i (41, il quale si serve, come è noto, dei tempi di arrivo delle onde p e delle differenze s-p. se le stazioni non sono troppo lontane dal centro di perturbazione, risulta approssimativamente s—p 1 p-h0 k — costante [10] la eostante è legata al coefficiente di poisson dalla relazione a — 0,5 — 0,5 a~ [11] 1 + 2 k la tabella ii contiene i tempi di arrivo delle onde p e le differenze s-p espresse in sec, relative alle stazioni usate. i dati di atene sono stati presi dal bollettino di strasburgo. la stazione di helwan è stata esclusa, perché le onde trasversali sembrano arrivare con un sensibile anticipo. questo fenomeno è stato già segnalato in un precedente lavoro (5). i valori approssimati più probabili di h0 e k con i rispettivi errori medi sono k = 1,272 + 0,012 [12] iio = 10" 42'» 42s,3 + l s , 5 (t.m.g.l ii tempo origine si accorda abbastanza bene con quello calcolato precedentemente, anche considerando che il metodo di wadati non consente un'elevata precisione. in base alla [11] il coefficiente di poisson risulta pari a 0,27. questo valore relativamente elevato deriva dal fatto che, sebbene non i l t e r r e m o t o j o n i c o del 2 2 a p r i l e 1 9 1 8 2 4 5 siano stati inclusi nel calcolo i dati delle stazioni eccessivamente distanti dal centro di perturbazione, anche quelle prese in considerazione sono un po' troppo lontane e quindi l'onda si è propagata fino ad una profondità piuttosto elevata. roma — istituto nazionale ili geofisica — gennaio 1951 riassunto allo scopo di contribuire alla localizzazione dei centri sismici che interessano l'italia e per essere in jtossesso dei dati fondamentali per una successiva ricerca, sono state calcolate le coordinate epicentrali e il tempo origine del terremoto jonico del 22 aprile 1948. b i b l i o g r a f i a ( ' ) gutenberg b . a n d f . c . richter: earlhquake magnitude, intensily, energy and acccleration b u l l . s e i s m . s o c . of a m . x x x i i . 1 6 3 1 9 1 ( 1 9 4 2 ) . — di filippo d . e l. marcelli : magnitudo ed energia dei terremoti in italia a n n a l i di g e o f i s i e a i i , 3 3 7 3 4 8 ( 1 9 5 0 ) . <-) jeffreys i l : the time oj p. s and sks, and the velocities of p and s g e o p l i . s u p p l . i v , 4 9 8 5 3 3 ( 1 9 3 9 ) . ( 3 ) geiger l . : herdbestimmung bei erdbeben aus den ankunftszeiten • n a c l i l i c l i t e n d e r k o n i g l i r h e n g e s e l l s e h a f t d e r w i s s e n s c l i a f t e n zu g o t l i n g e n , m a t l i . p l i y s . k l a s s e , 3 3 1 3 1 9 ( 1 9 1 0 ) . — valle f . e . : stilla determinazione delle coordinate epicentrali di un sisma lontano b o l l . s o c . s i s m . i t a l i a n a x l , 73-81 ( 1 9 4 2 ) . < ') wadati k . : tìn the travet time of earlhquake uaves. pari ii • g e o p l i . m a g . 7, 1 1 1 1 1 1 1 ( 1 9 3 3 ) . (•"') valle p . e . : contributo allo studio delle caratteristiche sismiche del mediterraneo centro-orientale a n n a l i di g e o f i s i e a : i, 2 6 6 2 7 8 ( 1 9 4 8 ) . vol49_1_2006def 83 annals of geophysics, vol. 49, n. 1, february 2006 key words hyperspectral imaging (hsi) – mineral mapping – aviris – eo-1 hyperion 1. introduction hyperspectral remote sensing, measuring hundreds of spectral bands from aircraft and satellite platforms, provides unique spatial/spectral datasets for analysis of surface mineralogy (goetz et al., 1985; kruse et al., 2003). these data allow mapping of key iron mineralogy such as hematite, goethite, and jarosite as well as alteration minerals such as kaolinite, dickite, alunite, and sericite (clark et al., 1990). their use for geologic applications is well established (goetz et al., 1985; kruse et al., 1999, 2003; rowan and mars, 2003). the los menucos, rio negro, argentina, site (fig. 1) is a fossil analog of hot springs similar to modern systems in other locations around the world. this area has the largest significant concentration of advanced argillic, altered permian ignimbrite and rhyolite assemblages in argentina (franco et al., 2000). alteration is related to the intrusion of triassic-age (?) rhyolite dome complexes below thick permian-age felsic volcanic rocks. associated with dome fields are large areas of phreatic breccias and hematiterich altered oxidized zones. alteration is characdistrict-level mineral survey using airborne hyperspectral data, los menucos, argentina fred a. kruse (1), sandra l. perry (2) and alejandro caballero (3) (1) horizon geoimaging, llc, frisco, colorado, u.s.a. (2) perry remote sensing, llc, englewood, colorado, u.s.a. (3) rio tinto mining & exploration ltd., santiago, chile abstract the los menucos district, rio negro, argentina, provides an excellent case history of a complex epithermal gold system mapped and explored using a combination of field mapping and multispectral/hyperspectral remote sensing. the district offers a host of argillic and advanced argillic alteration minerals at the surface, many of which are difficult to identify visually. a strategy utilizing regional targeting with landsat tm to optimize field mapping followed by district-level survey with hyperspectral imaging (hsi) data demonstrates the value added by high-spectral resolution aircraft data. standardized analysis methods consisting of spatial and spectral data reduction to a few key endmember spectra provides a consistent way to map spectrally active minerals. minerals identified in the los menucos district using the jpl airborne visible/infrared imaging spectrometer (aviris) include hematite, goethite, kaolinite, dickite, alunite, pyrophyllite, muscovite/sericite, montmorillonite, calcite, and zeolites. hyperspectral maps show good correspondence with the results of field reconnaissance verification and spectral measurements acquired using an asd field spectrometer. further analysis of hyperion (satellite-based) hyperspectral data indicates that similar mapping results can be achieved from satellite altitudes. these examples illustrate the high potential of hyperspectral remote sensing for geologic mapping and mineral exploration. mailing address: dr. fred a. kruse, horizon geoimaging, llc, p.o. box 4279, frisco, colorado 80443, u.s.a.; e-mail: kruse@hgimaging.com 84 fred a. kruse, sandra l. perry and alejandro caballero terized by vuggy silica, quartz stockwork, kaolin, and alunite. the los menucos gold district, with potential for low-sulfidation style gold mineralization, was discovered in 1998 by arminex, s.a. using regional exploration methods employing landsat thematic mapper (tm) satellite imagery and field investigation (gemuts and perry, 2000; perry and gemuts, 2000). over 100 sites were predicted as alteration anomalies resulting from digital enhancement of the landsat tm imagery analyzed by perry remote sensing, llc (prs). these results were used to drive field exploration, and in less than one year, a field crew of six geologists systematically visited and sampled all of these anomalies. eighty percent of the areas visited exhibited epithermalstyle alteration, and five percent were mineralized (perry and gemuts, 2000). the exploration effort led arminex to assemble 80 000 ha near the village of los menucos and established the area as the first gold district in rio negro province. early in 2000, rio tinto mining & exploration (rtz) took an option on the arminex property and agreed to continue drilling and testing at key prospect areas. based on this work, the los menucos region was submitted and selected as a nasa e0-1 collection site during 2000 to evaluate earth observation sensors, including hyperspectral (airborne visible/infrared imaging spectrometer (aviris) and satellite eo-1 hyperion) as well as multispectral data sets (landsat 7 enhanced thematic mapper and aster imagery). tm reconnaissance results are published in perry and gemuts (2000). fig. 1. los menucos, argentina site location in rio negro province, argentina. black box marks approximate extent of aviris survey conducted during february 2001. 85 district-level mineral survey using airborne hyperspectral data, los menucos, argentina initial aviris results are summarized in kruse et al. (2002a). an overview of the results of the combined tm reconnaissance and aviris analysis are presented in kruse et al. (2002b). preliminary hyperion, etm and aster results are presented in kruse et al. (2002c). additional hyperion results are summarized in kruse et al. (2002a,d). additional details and results for the aviris and hyperion data analysis are discussed here. the principal airborne hyperspectral remote sensing instrument used for this research was the airborne visible/infrared imaging spectrometer (aviris), a 224-channel hyperspectral sensor with approximately 10 nm spectral resolution covering the 0.4-2.5 nm spectral range (porter and enmark, 1987; green et al., 2003). aviris was flown by nasa/jet propulsion laboratory (jpl) over the los menucos district on a twin otter aircraft at low altitude on 14-15 february 2001. an aviris dataset was collected consisting of 6 overlapping flightlines (98 «scene» equivalents), covering an area approximately 11 km × 30 km at 3.5 m spatial resolution. these data are used to illustrate hyperspectral analysis methods and specific analysis results for the los menucos district. 2. methods a «standardized» hyperspectral data analysis methodology (fig. 2) has been tested for a variety of data (boardman et al., 1995; kruse et al., 1999, 2003). this approach is implemented and documented within the «environment for visualizing images» (envi) software system originally developed by aig scientists (now a research systems (rsi) commercialoff-the-shelf (cots) product) (rsi, 2001). this is not the only way to analyze these data, but we have found that it provides a consistent way to extract spectral information from hyperspectral data without a priori knowledge or requiring ground observations. additional details are available in kruse et al. (2003). the analysis approach consists of the following steps: – correction for atmospheric effects using an atmospheric model «acorn» (aig, 2001) (fig. 3). – spectral compression, noise suppression, and dimensionality reduction using the minimum noise fraction (mnf) transformation (green et al., 1988; boardman, 1993). – determination of endmembers using geometric methods (pixel purity index – «ppi») (boardman and kruse, 1994; boardman et al., 1995). – extraction of endmember spectra using ndimensional scatter plotting (boardman et al., 1995). – identification of endmember spectra using visual inspection, automated identification, and spectral library comparisons (kruse and lefkoff, 1993; kruse et al., 1993). fig. 2. aig «hourglass» hyperspectral data processing scheme. large hsi datasets are reduced to a few key spectra (at the neck of the hourglass) that explain the data using spatial and spectral data reduction techniques (boardman et al., 1995). pixel-based spectral mapping methods are then applied to the full hsi dataset. 86 fred a. kruse, sandra l. perry and alejandro caballero – production of mineral maps using a variety of mapping methods. «mixture-tuned-matchedfiltering» (mtmf) used for this work is basically a partial linear spectral unmixing procedure (boardman, 1998). – geometric correction of analysis results using sensor models and aircraft or satellite navigation information (boardman, 1999). 3. results 3.1. general each of the six aviris flightlines was processed using the above procedures and analyzed separately in reconnaissance mode (optimized for the complete dataset, not individual fig. 3. comparison of selected aviris radiance spectra (top) and corresponding acorn reflectance spectra (bottom). spectrum a is a kaolinite-bearing rock, spectrum b a clay-bearing rock, and spectrum c is green vegetation. 87 district-level mineral survey using airborne hyperspectral data, los menucos, argentina sites), then combined as a spatial mosaic to produce mineral maps of the entire coverage. two spectral ranges were analyzed; 1) 0.4-1.3 nm (iron oxides), and 2) 2.0-2.5 nm (clays, carbonates, etc). the results were map-corrected and combined into an image mosaic covering an approximately 10 km × 30 km area covering several key mineral prospects. while the complete dataset was analyzed, only los menucos aviris flightline #2, covering several of the fig. 4a,b. a) aviris endmember spectra and mapping results for los menucos aviris flightline #2. b) an example of an image-map produced using mtmf for the los menucos aviris flightline #2 swir data. mtmf mineral mapping was performed using the endmembers selected and identified using the n-dimensional visualization and spectral identification procedures described above and shown in a). mtmf scatterplotting was used to select the spectrally predominant mineral for each pixel at abundances greater than 10% and to color-code the selected pixels in colors matching the endmember spectra. a b 88 fred a. kruse, sandra l. perry and alejandro caballero rtz prospects is shown here as an example of typical aviris processing results using the above methods. minerals identified as above for the los menucos aviris flightline #2 swir data include several common alteration minerals such as kaolinite, muscovite, alunite, silica, pyrophylite, calcite, montmorillonite, chlorite-epidote, and zeolites (fig. 4a,b). additionally, because of the high snr of the aviris data, we were able to discriminate different within-species spectral differences. thus several varieties of kaolinite and muscovite were also identified (fig. 4a,b). field reconnaissance was conducted during april 2001 to validate areas delineated by the aviris analysis. aviris mineral maps were used along with landsat tm mapping as base maps for field verification. several prospects and other mineralogically interesting areas were visited, the rocks and alteration were examined, and samples were collected. these samples were analyzed utilizing an analytical spectral devices (asd) «fieldspec full range» field spectrometer (see http://www.asdi.com) provided by jet propulsion laboratory. the asd spectrometer covers the 0.35-2.5 nm range with approximately 3 nm (vnir) and 10 nm (swir) spectral resolution and 1 nm spectral sampling. a «wand» attachment containing a halogen light source was used to illuminate the samples. this results in a high-quality spectrum with 2151 spectral bands, allowing identification of specific minerals. over 160 spectral measurements were made of various rocks and soils from the los menucos area. spectral libraries were used to validate and refine aviris results, and to apply to eo-1 hyperion and landsat/aster multispectral evaluation. 3.2. aviris pyrophyllite discovery the aviris data pointed out several minerals and mineral assemblages that were not readily apparent utilizing conventional field mapping methods. in particular, several previously unknown pyrophyllite occurrences cross-cutting predominant structural trends were detected and mapped using the swir data (kruse et al., 2002a,b). these have generated some interest for further exploration. northwest-trending structures apparently controlled the distribution of both well-crystalline and poorly crystalline kaolinite (fig. 5a,b). both dickite and pyrophyllite, geochemical indicators of higher temperatures appear to terminate and/or crosscut the kaolinite occurrences (fig. 5b,c). on-the-ground field checking of the pyrophyllite areas showed nothing obvious at these locations. the ground surface was flat, no outcrop was exposed, and it consisted principally of soil with small rock fragments. map positioning was double checked using gps and asd field spectral measurements showed that indeed the principal soil mineral was pyrophyllite (fig. 5d). the spectral signatures, image patterns, and surface exposure indicate that these areas are highly weathered surface exposures of altered rock. further investigation is required to determine their economic potential. 3.3. los menucos, argentina hyperion example hyperion data for the los menucos, argentina, site were first acquired on 25 february 2001, close to the 14-15 february aviris acquisition date. hyperion is a satellite hyperspectral sensor covering the 0.4 to 2.5 nm spectral range with 242 spectral bands at approximately 10 nm spectral resolution and 30 m spatial resolution from a 705 km orbit (pearlman et al., 1999). unfortunately, the hyperion data were predominantly cloudy and because of low solar zenith angle, signal-to-noise ratios (snr) calculated for this scene were in the 20:1 range, marginal for successful mineral identification and mapping (kruse et al., 2003). this example, does, however, serve to demonstrate the potential of satellite hyperspectral data for mineral mapping. the «cuya» and «kaolinite hills» sites shown in fig. 6a-d were mostly clear on the 25 february hyperion acquisition date. these data were processed to geologic products using the approaches described above for extraction of mineralogic and geologic information. several characteristic mineral spectra (silica, kaolinite, muscovite) were extracted from the los menucos hyperion data (fig. 6a-d). mineral maps were produced and compared to those derived from the aviris data above (fig. 6a-d). com89 district-level mineral survey using airborne hyperspectral data, los menucos, argentina parison of the two datasets shows that hyperion identifies similar minerals and produces grossly similar mineral mapping results as aviris, however, it does not produce the level of detail available from the aviris data. some minerals are missed, and others are confused (dickite/kaolinite). the kaolinite doublet shown for aviris data (fig. 6c) is not fully resolved fig. 5a-d. geocorrected pyophyllite discovery aviris results. a) false color infrared composite (cir) image of bands 31, 19, 9 (0.66, 0.55, 0.45 nm) as rgb. b) swir mineral mapping results. note pyrophyllite occurrence in center of image crosscutting northwest-trending kaolinite distributions. c) aviris endmember spectra: red is well crystalline kaolinite, green is poorly crystalline kaolinite, blue is dickite, and orchid is pyrophyllite. d) asd field spectra for mineral occurrences mapped using aviris. colors are the same as for the aviris endmembers. a b c d 90 fred a. kruse, sandra l. perry and alejandro caballero fig. 6a-d. a) image-map of selected endmembers from geocorrected aviris swir data. dark green is cuya area with hydrothermal silica ± clay signature. red is kaolinite hills area with kaolinite signature. bright green area is poorly crystalline kaolinite (spectrum not shown). magenta shows areas with muscovite signatures. b) hyperion grayscale image with mtmf mineral map overlay for the same approximate area: colors are the same as for aviris. white areas are clouds and dark areas are cloud shadows. c,d) hyperion endmember spectra. by hyperion (fig. 6d). this is largely the effect of reduced hyperion signal-to-noise-performance compared to the aviris (∼ 20:1 and less for these hyperion data, compared to > 500:1 for aviris) (kruse et al., 2003). the hyperion data, however, illustrate the potential for worldwide hyperspectral mapping using satellite hsi systems. suggestions are, that based on improved detector performance and more sophisticated design, that future sensor systems will a b c d 91 district-level mineral survey using airborne hyperspectral data, los menucos, argentina likely approach current aviris mapping performance (r. green, 2003, pers. comm.). 4. discussion and conclusions the los menucos district aviris case history presented here illustrates the use of hyperspectral data to perform a district-level mineral survey. initially discovered using landsat tm multispectral imagery, the district was further mapped and explored using hyperspectral imaging systems (aviris and hyperion). high-quality, high spatial resolution (3.5 m) aviris data were the key to rapid assessment of the detailed mineralogy of this area, pointing out minerals and mineral assemblages that were not readily apparent utilizing conventional field mapping methods. nearly 100 aviris scenes covering an approximately 10 × 30 km area were analyzed using standardized hsi analysis methods consisting of spatial and spectral reduction of the hyperspectral data to just a few key endmember spectra. partial unmixing using mixture-tunedmatched-filtering (mtmf) provided a consistent way to process the multiple aviris flightlines allowing identification and mapping of vnirand swir-active minerals. common alteration minerals such as hematite, goethite, kaolinite, dickite, alunite, pyrophyllite, muscovite/sericite, montmorillonite, calcite, and zeolites were mapped. distinguishing between similar minerals such as kaolinite and dickite and mapping of muscovite variability related to cation substitution was possible because of the high snr of the aviris sensor. aviris mapping results closely agree with field reconnaissance and ground spectral measurements. analysis of a hyperion dataset generally validates inorbit mineral mapping and sets the standard for future satellite hyperspectral system performance. these examples illustrate the high potential of hyperspectral remote sensing for geologic mapping and mineral exploration. acknowledgements this research was partially funded by nasa under grant ncc5-495. additional financial support was provided by analytical imaging and geophysics, llc and perry remote sensing, llc internal research and development funds. the argentina test site was suggested by sandra perry, perry remote sensing llc, englewood, colorado, u.s.a. argentina field work and spectral measurements were partially supported by rio tinto mining & exploration ltd., santiago, chile, and conducted cooperatively between aig/prs/rtz. aviris data were provided by jpl. spectral measurements in argentina were made with jpl’s asd fieldspec pro field spectrometer. hyperion data were provided by nasa, goddard space flight center as part of the eo-1 science validation team effort. acorn is a trademark of imspec associates, llc. envi is a registered trademark of research systems inc., boulder, colorado. pixel purity index (ppi), and mixture-tuned matched filter (mtmf) are all trademarks of research systems inc. fieldspec is a trademark of analytical spectral devices, boulder, colorado. references aig (2001): acorn user’s guide, stand alone version, analytical imaging and geophysics, llc, p. 64. boardman, j.w. (1993): automated spectral unmixing of aviris data using convex geometry concepts, in summaries, fourth jpl airborne geoscience workshop, jet propulsion laboratory publ. 93-26, 1, 11-14. boardman, j.w. (1998): leveraging the high dimensionality of aviris data for improved sub-pixel target unmixing and rejection of false positives: mixture tuned matched filtering, in summaries of the seventh annual jpl airborne geoscience workshop, pasadena, ca, jet propulsion laboratory publ. 97-21, 1, p. 55. boardman, j.w. (1999): precision geocoding of aviris low-altitude data: lessons learned in 1998, in proceedings of the 8th jpl airborne earth science workshop, jet propulsion laboratory publ. 99-17, 63-68. boardman, j.w. and f.a. kruse (1994): automated spectral analysis: a geological example using aviris data, northern grapevine mountains, nevada, in proceedings of the tenth thematic conference, geologic remote sensing, 912 may 1994, san antonio, texas, i-407-i-418. boardman, j.w., f.a. kruse and r.o. green (1995): mapping target signatures via partial unmixing of aviris data, in summaries, fifth jpl airborne earth science workshop, jet propulsion laboratory publ. 95-1, 1, 23-26. clark, r.n., t.v.v. king, m. klejwa and g.a. swayze (1990): high spectral resolution spectroscopy of minerals, j. geophys. res., 95 (b8), 12653-12680. 92 fred a. kruse, sandra l. perry and alejandro caballero franco, s., n. puente, c. varela and i. gemuts (2000): mineralizacion aurifera en el distrito los menucos, provincia de rio negro, argentina, in proceedings of the argentina mining 2000, august 22-25, 2000, mendoza, argentina, 112-126. gemuts, i. and s. perry (2000): los menucos au district, rio negro province, argentina, abstract and presentation, northwest mining association, december 7-10, 2000, spokane, washington. goetz, a.f.h., g. vane, j.e. solomon and b.n. rock (1985): imaging spectrometry for earth remote sensing, science, 228, 1147-1153. green, a.a., m. berman, b. switzer and m.d. craig (1988): a transformation for ordering multispectral data in terms of image quality with implications for noise removal, ieee trans. geosci. remote sensing, 26 (1), 65-74. green, r.o., t.g. chrien and b. pavri (2003): on-orbit determination of the radiometric and spectral calibration of hyperion using ground, atmospheric and aviris underflight measurements, ieee trans. geosci. remote sensing, 41 (6), 1194-1203. kruse, f.a. and a.b. lefkoff (1993): knowledge-based geologic mapping with imaging spectrometers, remote sensing rev., 8, 3-28. kruse, f.a., a.b. lefkoff and j.b. dietz (1993): expert system-based mineral mapping in northern death valley, california/nevada using the airborne visible/infrared imaging spectrometer (aviris), remote sensing environ, 44, 309-336. kruse, f.a., j.w. boardman and j.f. huntington (1999), fifteen years of hyperspectral data: northern grapevine mountains, nevada, in proceedings of the 8th jpl airborne earth science workshop, jet propulsion laboratory publ. 99-17, 247-258. kruse, f.a., j.w. boardman and j.f. huntington (2002a): comparison of eo-1 hyperion and airborne hyperspectral remote sensing data for geologic applications, in proceedings of the ieee aerospace conference, 9-16 march 2002, big sky, montana (cd-rom), ieee cat. no. 02th8593c, paper #6.0102, p. 12. kruse, f.a., s.l. perry and a. caballero (2002b): integrated multispectral and hyperspectral mineral mapping, los menucos, rio negro, argentina, part i. landsat tm reconnaissance and aviris prospect mapping, in proceedings of the 11th jpl airborne geoscience workshop, 4-8 march 2002, pasadena, ca, jet propulsion laboratory publ. 03-4 (cd-rom). kruse, f.a., s.l. perry and a. caballero (2002c): integrated multispectral and hyperspectral mineral mapping, los menucos, rio negro, argentina, part ii. eo1 hyperion/aviris comparisons and landsat tm/ aster extensions, in proceedings of the 11th jpl airborne geoscience workshop, 4-8 march 2002, pasadena, ca, jet propulsion laboratory publ. 03-4 (cdrom). kruse, f.a., j.w. boardman, j.e. huntington, p. mason and m.a. quigley (2002d): evaluation and validation of eo-1 hyperion for geologic mapping, in proceedings of the igarss 2002, 24-28 june 2002, toronto, canada, paper 02_06_17:00 (cd rom); ieee operations center, piscataway, nj, i, 593-595. kruse, f.a., j.w. boardman and j.f. huntington (2003): evaluation and validation of eo-1 hyperion for mineral mapping, ieee trans. geosci. remote sensing, 41 (6), 1388-1400. pearlman, j., s. carman, p. lee, l. liao and c. segal (1999): hyperion imaging spectrometer on the new millennium program earth orbiter-1 system, in proceedings of the international symposium on spectral sensing research (isssr), systems and sensors for the new millennium, international society for photogrammetry and remote sensing (isprs), (cd-rom). perry, s. and i. gemuts (2000): new high sulfidation gold district: los menucos, rio negro province, argentina a landsat discovery, presented at fourteenth international conference, applied geologic remote sensing, november 6-8, 2000, las vegas, nevada. porter, w.m. and h.e. enmark (1987): system overview of the airborne visible/infrared imaging spectrometer (aviris), spie proc., 834, 22-31. rowan, l.c. and j.c. mars (2003): lithologic mapping in the mountain pass, california area using advanced spaceborne thermal emission and reflection spectrometer (aster) data, remote sensing environ., 84, 350-366. rsi (2001): envi user’s guide (research systems inc.), p. 948. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 1 recent seismicity before the 24 august 2016 mw 6.0 central italy earthquake as recorded by the resiico seismic network simone marzorati *, marco cattaneo, massimo frapiccini, giancarlo monachesi, chiara ladina istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, ancona, italy *simone.marzorati@ingv.it abstract we present the seismicity of the last four years before the mw 6.0 earthquake of august 24, 2016, 01:36 utc in central italy, with the aim of understanding the preparatory phase of the event. our preliminary results show that no significant seismic sequence occurred in the months before the mainshock of august 24, 2016 and that there is little similarity between seismicity clusters in the last four years and the aftershocks. we pay attention to differences between the preparatory phase of the amatrice earthquake and two other seismic sequences: the 2009 l’aquila earthquake that was preceded by a seismic sequence, and the 2013-2015 gubbio seismic swarm that, to date, did end without including any strong event.. i. introduction ontinuous seismic monitoring near active faults allows to follow and reconstruct the space-time evolution of seismicity, before, during and after a destructive earthquake. it can also be observed that seismicity often tends to cluster, both before and after strong earthquakes, also at great distance both in time and in space [kagan and jackson 1991; mulargia and geller, 2003; marzocchi and lombardi, 2008, 2009; kagan, 2014; parsons et al. 2015; gasperini et al., 2016]. therefore, seismic sequences that follow omori’s law [omori, 1894] and seismic swarms (a subcategory of the sequences characterized by a chaotic trend in magnitude over time) are often observed [de gori et al., 2015]. in literature there are many examples in which foreshocks are cited as precursors of strong earthquakes [ie.: aki, 1981; allen, 1982; guagenti et al., 1991; jones, 1985; vere-jones et al., 1998; bakun et al., 2005] . in italy, the theme vigorously reappeared after the april 6, 2009 l'aquila earthquake [grandori and guagenti, 2009; sugan et al., 2014; papadoupolous et al., 2010], characterized by an increase of seismic activity before the mainshock [chiaraluce et al, 2011; valoroso et al., 2013]. a detailed investigation of strong earthquakes and sequences in the area around l'aquila has shown that there is no link between the foreshocks seismic sequences and mainshocks [amato and ciaccio, 2012] and that in most cases earthquakes are not preceded by seismic sequences at all. more than twenty seismic sequences occurred in the last two centuries did not result in a strong seismic event [amato and ciaccio, 2012; galadini, 2013]. out of a total of 127 seismic sequences recorded in italy between 2008 and 2010, only one was followed by a destructive event [arcoraci et al., 2011]. finally, considering the catalog of italian earthquakes since 1950, only c annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 2 0.8% of large sequences was followed by a strong earthquake [marzocchi and zhuang, 2011]. gasperini et al. [2016] concluded that about half of all recorded mainshocks are not preceded by any kind of earthquake precursor in a reasonable temporal range. this work shows the seismicity of four years before the earthquake of august 24, 2016 (mw 6.0), recorded from the seismic data collected in the eastern central italy from the resiico network [monachesi et al., 2013], with the aim of understanding whether any link exists with the seismic sequence. to get a basis for comparison, the long 2013-1015 gubbio seismic sequence is also analyzed. up to now this recent sequence has not yet included any strong earthquakes, and it evidences a different style of seismic activity with respect to the patterns attested in the l’aquila (foreshocks-mainshocks-aftershocks) and amatrice (mainshocks-aftershocks) earthquakes. ii. seismicity from 2012 to 2016 in the epicentral area of the august 24, 2016 earthquake in the last four years, a high resolution seismic monitoring system has been developed in central italy [amato and mele, 2008; chiaraluce et al., 2014; monachesi et al., 2013]. seismic stations belonging to the ingv national seismic network, near fault observatory taboo and to the networks of the marches and umbria regions centralized in real time at the ancona ingv headquarters – were brought together to form the resiico network, including a total of 101 stations, 49 of which are equipped with velocimeters, 24 with accelerometers and 28 with both. in some cases, the density and geometry of the monitoring network allow to depict in great detail the geometrical characteristics of seismic patterns, even for limited extension [valoroso et al., 2013; marzorati et al., 2014]. the data collected by the resiico dense network allowed to develop accurate propagation models [1d – de luca et al., 2009, and 3d – carannante et al., 2013] with whose help a better constrain of the location of recorded earthquakes has become possible. in this work, we will analyze the seismic events of the last four years, starting from august 1, 2012, as derived by the routine location analysis procedure of the ingv-ancona working group, that integrates the results of an automatic p and s picking procedure [spallarossa et al., 2014; scafidi et al., 2016] with the human expert analysis for the most critical cases and m ≥ 2.5 events. both the automatic and human revision procedures use the hypoellipse location code [lahr, 1999], the 1-d model derived by de luca et al. [2009] and relevant station corrections, updated for the new stations. the same procedures are used also for the analysis of the 2016 sequence. a comparison of automatic and human-revised locations pointed out that, for the automatically welllocated events, more than 90% of the automatic locations presented a distance from the humanrevised one lesser than the estimated formal error [cattaneo et al., 2016]. from 2012 onwards the network geometry in this area was quite stable and the detection and location procedures were well established: the obtained catalog can thus be considered homogeneous [cattaneo et al., 2016; marzorati and cattaneo, 2016]. fig. 1 shows the recorded seismicity in the investigated area, subdivided in four 1-year panels, going from august 1 to july 31 of the next year. the whole dataset is composed by 17758 events, whose magnitude ranges from -0.8 to 3.9. the area interested by the august 24, 2016 mainshock and by the following sequence (“source area” in the following) is depicted in thin annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 3 lines. seismicity appears rather uniformly distributed, in space and in time, both within the source area and in the surrounding zones. small clusters of seismicity, often not related to any event with magnitude larger than 3.0, can be recognized in the source area. some events with ml ≥ 3.0 are present in all panels, with a relatively higher rate of occurrence in the 2013-2014 period. the b-value, computed on the whole analyzed area for the different time-periods, does not show significant deviations with respect to the mean term of 1.0, above a threshold of completeness magnitude ranging between 0.8 and 1.1. the seismicity rate does not show significant variations in the last four years, as depicted by the cumulative number of events (fig. 1e). also the moment release rate (fig. 1f), after two step in 2013, linked to two events of magnitude 3.8 and 3.9 respectively, seems to depict a steady state for the last two years and a half. in order to investigate if the previous seismicity within the source area affected the same structures activated with and after the mainshock, we applied a crosscorrelation technique to evaluate the similarity between the waveforms of the events before the sequence and the aftershocks. in particular 7 clusters of events were selected, concentrated in areas between 2 and 10 km of extension and in a time frame smaller than one year (fig. 2). cluster seq01 occurred in the 2012-2013 time frame, clusters seq02, seq03 and seq04 in 2013-2014 and are the closest to the mainshock epicenter, seq05 occurred in 2014-2015, seq06 and seq07 in 2015-2016. thus seq02, seq03 and seq04 are rather close in time, but clearly distinct in space; seq05 and seq06 are partially overlapping in space, but separated in time by more than one year; seq01 and seq07 are more closely linked in space, but are 3 years apart in time. it seems that the space-time evolution of these clusters does not show any particular relationship. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 4 figure 1. seismicity around the amatrice sequence between august 2012 and july 2016. red polygon: source area. black star: epicenter of the august 24, 2016 01:36 utc mw 6.0 earthquake. green triangles: seismic stations. blue circles: seismic events before the sequence. white circles: seismic events of the sequence; a): seismicity between august 2012 and july 2013; b): seismicity between august 2013 and july 2014; c): seismicity between august 2014 and july 2015; d): seismicity between august 2015 and july 2016; e): cumulative number of earthquakes in the last four years (yellow stars: events ml ≥ 3.0); f): cumulative seismic moment released in the last four years. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 5 the events of each cluster were cross-correlated with the aftershocks of the 2016 sequence, selecting just the events located in an area slightly larger than that depicted by the cluster, in order to take into account possible location errors. it is well known that a high similarity of waveforms among different events allows to hypothesize a common mechanism and closely spaced sources [waldhauser and ellsworth, 2000]. fig. 2 shows a synthetic view of the results: for each cluster, the correlation matrix is presented. each matrix is subdivided by thin gray lines in four quadrants: the lower left one represents the correlation of the events of the clusters, the upper right the correlation of the aftershocks and the other two the mixed correlations. figure 2. clusters of seismicity in the source zone of amatrice sequence. the cross-correlation coefficient matrix are show in the small panels. the seismicity clusters are colored following an annual time scale. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 6 looking at the lower left quadrants, it is quite evident that, as a general trend, each cluster presents a rather high level of similarity, sometimes subdivided in more sub-clusters (as in the case of seq01, where a clear sub-family subdivides the sequence). the upper right quadrants show significant levels of correlation, but in general lower values with respect to the previous case: this can be due to the larger extension of the selected events, but also to the larger magnitude span of these data with respect to the clusters. on the contrary, the mixed correlations (clusters vs aftershocks) show, as a mean term, lower values. this is particularly true for seq02, seq03 and seq04, the 3 clusters surrounding the mainshock epicenter: here the correlation values are marginal, with few exceptions, leading to hypothesize that the clusters activated structures well distinct from the structures responsible of the 2016 sequence. also seq07 shows very low values, while seq01, and in particular seq05 and seq06 present higher degrees of similarity. preliminary results show that the nucleation of the sequence affected structures different from those responsible for the previous seismicity and re-activated some shallow secondary structures of the northernmost sector of the sequence. in order to add further information, we can compare the depth distribution of the previous seismicity with those of the sequence. for this comparison we use the preliminary locations (mainly provided by the automatic procedure run in ancona), that are rather homogeneous with the previous catalog, also for what regards the 1d propagation model used for locations, and thus focal depths are more directly comparable. fig. 3a shows the epicentral locations of the bestlocated events of the first month of the sequence (9353 events), and the trace of 4 cross-sections, approximately perpendicular to the main trend of the sequence. the color scale and the symbol size represent the local magnitude. fig. 3b shows the cross-sections of the sequence: thin green lines are depicted just to highlight the main trends of the seismicity, mainly the most energetic one, without any presumption of structural interpretation but just to simplify the comparison with the previous seismicity. indeed the same lines are reported in fig. 3c, that show the seismicity from 2012 to 2016. it is quite evident that, even if some events of the previous seismicity fall in the areas activated by the sequence, the general trend of the depth distribution is rather different, mainly for the southernmost section. this observation is still more evident if we limit the analysis to the last 4 months before august 24, 2016 (fig. 3d): none of the geometries recognizable in the sequence are evident in these data; the seismicity is always present (1179 events) but of low energetic level (maximum magnitude 2.8) and without any clear cluster, in particular in the area of nucleation of the mainshock between the urban centres of accumoli and amatrice. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 7 figure 3. sketch of seismicity in depth: a) map of amatrice sequence; b) sections of amatrice sequence; c) sections of four years seismicity from 2012 to 2016; d) sections of seismicity of the last 4 months before the amatrice sequence. thin green lines: main trends of the seismicity. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7191 8 iii the 2013-2015 gubbio swarm while the sequence of l'aquila in 2009 was preceded by foreshocks [chiaraluce et al., 2011], the preliminary results of the previous section show that the seismicity recorded for months prior to the 2016 amatrice sequence had the same characteristics as in the previous four years. with the aim to add examples to stress the idea that seismicity can represent foreshocks of a greater earthquake only in some cases and not always, we show another type of sequence, similar to a swarm, which hit the area of gubbio (umbria) over a period of at least a year and a half. although most of the events were small (only two reached the maximum magnitude 3.9), they were perceived by the population over a lengthy period of time, and put local authorities on the alert. fig.4a depicts 69574 events in a magnitude range from -2.2 to 3.9 recorded from august 2009 to august 2016. this unique data set was collected thanks to the presence, in addition to the ingv national seismic network, of the near fault observatory taboo [chiaraluce et al., 2014]. the high-resolution network started recording 3 years before the beginning of the swarm, so the background seismic rate of the area is visible (fig.4b). in the early months of 2013, the rate increased and a few earthquakes of magnitude 3 started appearing. the rate remained very high over the whole of 2013-2014, starting to fall down only in the first half of 2015. at the end of 2015, the rate of seismicity returned to levels similar to those observed before the start of the sequence, no event of magnitude greater than 3 being recorded anymore. therefore, this prolonged seismicity cluster does not represent aftershocks or foreshocks of a destructive earthquake near in space and reasonable time. iv. conclusions on august 24, 2016 at 01:36 utc a mw 6.0 earthquake struck central italy, starting a sequence that is still ongoing while we are preparing this report. the seismicity recorded in this area during the past four years was analyzed in order to preliminarily investigate the preparatory phase of the earthquake. our preliminary results indicate a steady rate of seismicity up to the destructive event. within the four previous months, seismic sequences did not take place in the vicinity of the source and no magnitude ≥ 3 event was recorded. through cross-correlation techniques, we verified the low similarity between the clusters of seismicity between 2012 and 2016 and the aftershocks following the event on august 24. in addition, the different distribution in depth of the previous seismicity and the aftershocks shows, at a first glance, that the nucleation of the mainshock falls on a structure different from those concerned in the previous seismicity and the following sequence re-activate only some shallow structures in the northermost part. a more precise and reliable feature of these preliminary results will be obtained from future analysis on revised locations of the seismic sequence with all seismic data available and a 3d velocity model to study the presence of repeating earthquakes. the comparison between the amatrice sequence, the 2013-2015 gubbio swarm, that apparently ended in 2016 without producing a strong earthquake, and april 6, 2009 l’aquila earthquake introduces a further example of different behaviors of the seismicity in the preparatory phases of earthquakes in central italy. annals of geophysics, 59, fast track 5, 2016; 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(2001). a software package to analyse seismicity: zmap, seismol. res. lett., 72, 3, 373-382. vol50,1,2007 93 annals of geophysics, vol. 50, n. 1, february 2007 key words piezomagnetic effect – borehole magnetic measurement – the mogi model – long valley caldera 1. introduction boreholes are now widely used in geophysical observations, in particular for seismic and crustal strain measurements, since they greatly reduce noise resulting from artificial mechanical vibrations and increase tectonic signals as the sensor is closer to the source. improvement of s/n ratio by the use of a borehole can be similarly expected in the case of tectonomagnetic observations. to avoid cultural noises due to moving magnetic bodies (cars, etc.), high-powered electric devices and so on, we must isolate an undisturbed area of at least a few hundreds meters square. under the ground, however, high-frequency em noise is almost completely shielded and a moving car is not detected at a 200 m depth (yamamoto, 1990). furthermore, tectonomagnetic signals increase with depth. drag-out effect of piezomagnetic signals due to a borehole: the mogi source as an example yoichi sasai (1), malcom j.s. johnston (2), yoshikazu tanaka (3), robert mueller (2), takeshi hashimoto (4), mitsuru utsugi (3), shinya sakanaka (5), makoto uyeshima (6), jacques zlotnicki (7) and paul yvetot (7) (1) disaster prevention division, bureau of general affairs, tokyo metropolitan government, tokyo, japan (2) us geological survey, menlo park, ca, u.s.a. (3) graduate school of science, kyoto university, japan (4) graduate school of science, hokkaido university, japan (5) faculty of engineering and resource science, akita university, japan (6) earthquake research institute, the university of tokyo, japan (7) observatoire de physique du globe de clermont-ferrand, france abstract we show that using borehole measurements in tectonomagnetic experiments allows enhancement of the observed signals. new magnetic dipoles, which vary with stress changes from mechanical sources, are produced on the walls of the borehole. we evaluate such an effect quantitatively. first we formulate a general expression for the borehole effect due to any arbitrary source models. this is valid everywhere above the ground surface as well as within the cylindrical hole. a first-order approximate solution is given by a line of horizontal dipoles and vertical quadrupoles along the central axis of the borehole, which is valid above the ground surface and a slightly away (several tens of cm) from the top of the borehole. selecting the mogi model as an example, we numerically evaluated the borehole effect. it turned out that the vertical quadrupoles produce two orders of magnitude more intense magnetic field than the horizontal dipoles. the borehole effect is very local, i.e. detectable only within a few m from its outlet, since it is of the same order or more than the case without a borehole. however, magnetic lines of force cannot reach the ground surface from a deeper portion (>10 m) of a borehole. mailing address: dr. yoichi sasai, disaster prevention division, bureau of general affairs, tokyo metropolitan government, nishi-shinjuku 2-8-1, shinjuku-ku, tokyo, 163-8001, japan; e-mail: yosasai@zag.att.ne.jp 94 yoichi sasai et al. however, we need special considerations in the case of magnetic field observations. we make measurements within the source material of the magnetized crust. when we excavate a hole, additional magnetic poles are observed along its wall. when a magnetometer is placed under the ground, the effect of the shallower magnetic source is different from the on-the-ground observation in which the source always lies beneath the sensor. sasai (1994) evaluated both the effects in the case of a vertical rectangular strike-slip fault, and demonstrated that piezomagnetic signals are significantly enhanced at depth beneath the ground surface. the underground solution for the piezomagnetic field is defined as a limit of the field value when the radius of the excavated spherical cavity approaches zero (sasai, 1991b). we assume that the drilled hole is filled up with the same magnetic substance. in actual situations, a borehole would rarely be buried again, nor could the spherical cavity for a magnetometer ever be shrunk to zero volume. usually, a casing-pipe is inserted to protect the borehole, in which a non-magnetic tube (e.g., vinyl chloride) is preferable, at least for shallow boreholes. this is because long-term alteration of magnetic susceptibility in high µ metals could disturb tectonomagnetic changes. when we consider such a tectonomagnetic observation system, the existence of a borehole itself could produce an additional magnetic signal. as will be described later, we observed an interesting magnetic change associated with a volcano-tectonic event in long valley caldera, california, u.s.a. we put two proton magnetometers only 40 m apart, one of which is located close to a buried borehole. the simple difference between the two magnetometers showed a variation more or less similar to the strain change at the observation site. a possibility was that amplified tectonomagnetic signal resulted from the presence of the borehole. we will examine quantitatively how much local magnetic field will be produced on the ground simply by the existence of a borehole. 2. theory we assume that the earth is a homogeneous and isotropic elastic half-space with a uniformly magnetized top layer stressed by some pressure source. suppose that there is a vertical cylindrical borehole with radius a and length l as shown in fig. 1. for the sake of simplicity, the borehole is located outside the pressure source so that no mechanical singularity due to the source exists along the surface of the borehole. as a result of crustal stress, magnetic poles resulting from piezomagnetism of rocks appear on the wall of the borehole. given the displacement field due to the source, we can evaluate the influence of a borehole. the piezomagnetic potential wk ( r 0) of a stressed magnetoelastic body with its surface s and the displacement field uk(r) can be expressed as (sasai, 1983, 1991b) (2.1) where (2.2) in the above, k indicates the k-th cartesian component, n an outward normal to the surface s, r0 the observation point, r an arbitrary point on s and . ck is given by r r0ρ = − .div x u x u m u 2 3( )k l k k l kl2 2 2 2 δ∆ = + −c m ( ) ( ) ( ) ( ) d w c n u u n s r r m n r 3 2 2 1 1 ( ) k k k k s k 0 $ $ $ 2 2 2 2 λ µ λ µ ρ ρ ∆= − + + + + c m = f ( " 2 , ## fig. 1. schematic representation of a borehole, a pressure source and the magnetized crust. 95 drag-out effect of piezomagnetic signals due to a borehole: the mogi source as an example (2.3) in which β is the stress sensitivity, jk k-th component of the magnetization, λ and µ lame’s constants, and in particular µ the rigidity. let us consider the local effect of the borehole above the ground surface near around its top. the piezomagnetic potential due to the boreholewk(bh) consists of three parts (2.4) wk (a) is the potential produced by magnetic poles along the cylindrical wall, wk(b) by those over the circular bottom, and wk(c) by those from the vanishing portion of the ground surface, respectively. we will evaluate these terms separately. first we introduce the cartesian coordinates with x-y plane on the earth’s surface and z axis positive downward, of which origin is at the center of the borehole. we also define the cylindrical coordinates system which shares the origin and z-axis with the cartesian one as shown in fig. 2. 2.1. the contribution from the cylindrical wall of the borehole: wk(a) let the observation point outside the magnetoelastic body be q(x0, y0, z0)=q(r0, θ0, z0) and a moving point on the cylindrical surface of w w w w( ) ( ) ( ) ( )k bh k k k a b c= + − c j 2 1 3 2 k kβ µ λ µ λ µ = + + the borehole p(x, y, z) = p(r, θ, z). we find (2.5) (2.6) the integral over the cylindrical surface is given by (2.7) where (2.8) (2.9) . (2.10) since the radius of the borehole a is sufficiently small, the displacement at an arbitrary depth z is almost identical along the wall surface. it can be replaced with the value at the central axis r=0, which could be attained if the borehole were absent. fa(k), fb(k) and fc(k) are functions of z only, which are given by (2.11) (2.12) (2.13)( )f z u( )c k k r 0= =6 @ ( ) ( ) ( ) div f z y u x u u 3 2 3 2 3 3 2 2 ( ) b k k k y ky r 0 2 2 2 2 λ µ µ λ µ λ µ λ µ λ µ δ = − + − + + + + + + = < f ( ) ( ) ( ) div f z x u x u u 3 2 3 2 3 3 2 2 ( ) a k k k x kx r 0 2 2 2 2 λ µ µ λ µ λ µ λ µ λ µ δ = − + − + + + + + + = < f f u( )c k k r a= =6 @ ( ) ( ) div f u x u y u 3 2 3 2 3 3 2 2 ( )k k k k r a b y y 2 2 2 2 λ µ µ λ µ λ µ λ µ λ µ δ = − + − + + + + + + = < f ( ) ( ) div f x u x u u 3 2 3 2 3 3 2 2 ( ) a k k k kx r a x 2 2 2 2 λ µ µ λ µ λ µ λ µ λ µ δ = − + − + + + + + + = < f df θ d df fθ θ d z( )w ar = ( ) cos sin cos c a r 1 ( ) ( ) ( ) ( ) k k a a k l b k c k 0 0 2 0 0 2 3 0 0 0 2 ρ θ ρ θ ρ θ θ + − + − π π π ++ ; f ## # # ( , , 0) .cos sinen r θ θ= − = − − ( ) ( )cospq r a z z ar20 2 2 0 2 0 0ρ θ θ= = + + − − − fig. 2. the cartesian and cylindrical coordinate system used in section 2. 96 yoichi sasai et al. these terms are excluded from the integrals with respect to θ, and eq. (2.7) is reduced to (2.14) where (2.15) (2.16) (2.17) in which (2.18) and (2.19) i(m, n; l)’s on the righthand side of eqs. ((2.15) to (2.17)) are called the integrals of lipschitzhankel type, whose characteristics are investigated in detail by eason et al. (1955) . (2.20) they frequently appear in the potential problems with axial symmetry. how to reduce the integrals with respect to φ to the lipschitz-hankel type should be referred to sasai (1991a). the lipschitz-hankel integrals are expressed with the complete elliptic integrals (eason et al., 1955), which can be expanded into the tailor series with respect to k2=4ar0 / ρa2, where . in the present case, k2 is sufficiently small and even only the first term of the tailor expansions works as a good approximation. φ2 to φ4 are approximated by (2.21) moreover, ρa can be replaced with , , . ar ar 2 1 2 3 a a a 2 3 0 3 3 4 0 5 π ρ π ρ π ρφ φ φ= = = ( ) ( )a r z za 0 2 0 2ρ = + + − ( , ; ) ( ) ( ) di m n l j at j t e t trm ct l n 0 0= 3 −# ,cosr a ar r c z z22 0 0 2 0φ= − + = − r c2 2ρ = +φ (1, 1; 1) cos d c i4 3 0 ρ φ φ π φ = = φ π # (0, 0; 1) cos d i c3 30 ρ φ φ π φ = = φ π # (1, 1; 0) cos d i2 0 ρ φ φ πφ = = φ π # 2a d z= +( )w fr θ φsin+ ( ) cos c f f a r 1 ( ) ( ) ( ) ( ) k k a a k b k l c k 0 0 2 0 2 0 3 0 4 θ φ φ φ+ − 6 @ # because the sensor height z0 (∼ a few m) is, usually, much larger than the borehole radius a (∼10 cm). finally, eq. (2.14) can be expressed by (2.22) the first and second terms of the integrand in eq. (2.22) are horizontal dipoles in the x and y direction, while the third one a vertical quadrupole, respectively. the vertical quadrupole is defined as the differential of a vertical dipole in the vertical direction. 2.2. the contribution from the circular bottom of the borehole: wk(b) the moving point p(x, y, z) = p(r, θ, l) lies on the bottom circle and the outward normal is upward, and we find (2.23) . (2.24) the integral over the bottom surface is given by (2.25) where (2.26) . (2.27) since the radius of the bottom circle is small, the displacement at the bottom can be regarded almost identical to the one at the bottom center (r = 0 , z=l). fd(k) and fe(k) are excluded from the [ ]f u( )e k k z l= = ( ) ( ) ( ) div f z u x u u 3 2 3 2 2 3 2 3 ( ) d k kz k k z z l 2 2 2 2 λ µ µ λ µ λ µ δ λ µ λ µ = − + + + + − + + + = < f ( ) d d c w r f f l z r 1 1( ) ( ) ( ) k k b d k l e k l a 0 3 0 0 2 0 ρ ρ θ= + −π ; e## (0, 0, 1)en z= − = − == ( ) 2 ( )cospq r r z z rrl z l 0 2 2 0 2 0 0ρ θ θ+ + − − − = aπ d z=( )w r ( ) . c f x f y f z z r 1 2 ( ) ( ) ( ) ( ) k k a a k b k l c k 0 0 3 0 0 3 0 0 0 5 0 2 0 2 ρ ρ ρ + + + − − 2 < f # ( )z zr0 0 2 0 2ρ = + − 97 drag-out effect of piezomagnetic signals due to a borehole: the mogi source as an example integral with respect to θ and r; eq. (2.25) is reduced to (2.28) where (2.29) (2.30) in which (2.31) and (2.32) the lipschitz-hankel integral is now defined as (2.33) further integration with respect to r in eq. (2.28) is not available analytically, and we are to find an approximate solution. using the tailor series expansion for i(m, n; l)’s, we obtain . (2.34) the integrals in eq. (2.28) can be easily obtained as (2.35) . (2.36) if the observation point is far from the borehole, i.e. r0 >> a, eqs. ((2.35) and (2.36)) approach zero. even near around the borehole, i.e. r0 ∼ 0, (2.35) and (2.36) become negligibly small, because usually c >> a. thus we can neglect the contribution from the bottom circle of the borehole. ( ) d r a r c a r r c r c 1 3 a 0 0 2 2 0 0 2 2 0 2φ = + + + − + < f# ( ) d logr r r c r a r a ca 1 0 0 0 2 2 0 0 2 2 φ = + + + + + +# ( ) ( )r r c r r c /1 0 2 2 0 2 23 3 2 π π φ φ= + + = + +" , ( , ; ) ( ) ( ) .di m n l j rt j r t e t tm n ct l 0 0 = 3 −# , .cosr r r rr c l z20 2 2 0 0φ= + − = − r cl 2 2ρ = + ( , ; ) d c i 0 0 1 l 3 0 3 ρ φ π φ = = π # ( , ; ) d i 0 0 0 l 1 0 ρ φ πφ = = π # ( ) ( ) ( )d d c w f r r f r r r 1 2 0 2 0 ( ) ( ) ( ) k k b d k e k a a 0 1 0 3 0 $ $ φ φ = = + =# # 2.3. the contribution from the circular top of the borehole: wk(c) this term comes from the counterbalance that we have to subtract the contribution from the vanishing portion of the ground surface, which is the circular outlet of the borehole with radius a. now the moving point p(x, y, z)=p(r, θ, 0) is on the ground z = 0 and the outward normal is upward, and hence (2.37) . (2.38) we follow the same procedure as in the case of section 2.2, and easily find a solution by putting l = 0. thus wk(c) is given by (2.39) where (2.40) (2.41) φ1 and φ3 are defined as (2.42) (2.43) in which (2.44) and (2.45) with the aid of approximation formulas (2.34), we can evaluate the integrals on the right hand side of eq. (2.39), which are given by eqs. , ( .)cosr r r rr c z z2 0<02 2 0 0 0φ= + − = − r cs 2 2ρ = + ( , ; ) d c i 0 0 1 s 3 3 0 ρ φ π φ = = π # ( , ; ) d i 0 0 0 s 1 0 ρ φ πφ = = π # [ ]f u( )k k zh 0= = ( ) ( ) ( ) div f z u x u u 3 2 3 2 3 3 2 2 ( )k k k z kz z g 0 2 2 2 2 λ µ µ λ µ λ µ λ µ λ µ δ = − + − + + + + + + = < f ( ) ( ) ( )d d c w f r r f r r r 1 2 0 2 0 ( ) ( ) ( ) k k c g k h k a a 0 1 0 3 0 $ $ φ φ = = + =# # (0, 0, 1)en z= − = − == ( ) ( )cospq r r z z rr2s z 0 2 2 0 2 0 0 0 ρ θ θ+ + − − − = 98 yoichi sasai et al. ((2.35) and (2.36)) with c = ⎢z0⎢. we find again that wk(c) is negligibly small because ⎢z0⎢>> a. wk (c) may play a significant role when we observe the magnetic field very close to the surface outlet, i.e. r ∼ 0 and z0 ∼ a. in ordinary tectonomagnetic observations, however, we cannot achieve such measurements just above the ground surface because of high field gradient. hence wk(c) can be neglected as compared with wk(a). in conclusion, the piezomagnetic field produced by the existence of a borehole can be represented by the first term of the contribution wk(a), i.e. eq. (2.22). however, we may not disregard wk (b) and wk(c) when we consider the effect of the casing pipe which is usually made of high µ metals. they result in additional magnetic fields near the end of the pipe: the magnetic lines of force are absorbed into the metallic wall and they spread out from the end near the ground into the air. to evaluate this effect, the approximate solutions, i.e. eqs. ((2.35) and (36)), are no longer useful, and we must numerically conduct integrations with respect to r according to eq. (2.28) and eq. (2.39). 3. a case study: the mogi model suppose that there is a spherical cavity of radius b at a depth of d in an elastic half-space, within which a hydrostatic pressure ∆p occurs. such a simple model works effectively to interpret crustal deformation around volcanoes (mogi, 1958). the piezomagnetic field produced by the mogi model was investigated in detail by sasai (1991a). the displacement field of the mogi model is given by (3.1) (3.2) (3.3) where ( ) ( ) ( ) u c r z d z d r r z z d 2 3 1 6 z 1 3 2 3 2 5 2 µ λ µ λ µ λ µ = − + + − − + + − + ( 1 ( ) u c r r r z z dy y y 2 3 6 y 1 3 2 3 2 5µ λ µ λ µ = + + + − + ( 2 ( ) u c r x r x r xz z d 2 3 6 x 1 3 2 3 2 5λ µ λ µ µ= + + + − + ( 2 (3.4) and . (3.5) note that the origin of the cartesian coordinates for eqs. ((3.1) to (3.3)) is at the ground surface right above the center of the pressure source. we should rewrite eq. (2.22), of which coordinates origin is the center of the outlet of a borehole, to the one based on the new coordinates. let the center of the top of the borehole be (xb, yb, 0) as measured from the new origin. then we have (3.6) where (3.7) substituting the displacement field given by eqs. ((3.1) to (3.3)) into eqs. ((2.11) to (2.13)), we obtain (3.8) (3.9) (3.10) (3.11)( )f f x y( ) ( )b x ba y b*= $+=f ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) c r x z d r x z r dx r x z z d 2 3 2 3 4 3 3 2 5 4 3 5 3 3 2 9 23 12 3 3 2 3 4 30 ( ) a z b b b b 1 5 2 5 2 2 2 5 2 7 2 $ $ µ λ µ λ µ λ µ λ µ λ µ λ µ λ µ λ µ λ λµ µ λ µ λ µ + + − + + + + + + + + + − + + + $c c c m m m ( ) ( ) c f x y r r r z z d 2 3 2 3 3 4 1 3 1 10 ( ) a y b b 1 5 2 5 2 7 µ λ µ λ µ λ µ λ µ = + + + + + + − + ( 1 $−= −f ( ) ( ) ( ) ( ) ( ) ( ) ( ) c r r x r r x z z d r r x r r z d 2 3 2 3 4 1 3 3 2 3 4 3 1 3 3 2 6 3 4 1 3 2 1 3 5 8 ( ) a x b b b 1 3 1 5 2 2 3 2 5 2 2 5 2 7 2 3 2 5 2 2 µ λ µ λ µ λ µ λ µ λ µ λ µ λ µ λ µ λ µ µ + + − + + + + − + + + + − − + − + + $ c c c c m m m m .=r= ( ) ( ) ( )r z z x x y yb b b b b2 0 2 0 2 0 2ρ + − − + − d z( ) c w a f x x f y y f r z z r 1 2 ( ) ( ) ( ) ( ) k k bh a k b b b k b b l c k b b 0 2 3 0 3 0 0 5 2 0 2 π ρ ρ ρ = − + − + + − −^ h < f # c b p 2 1 3 ∆= − == ( ) ( )r x y z d r x y z d1 2 2 2 2 2 2 2+ + − + + + 99 drag-out effect of piezomagnetic signals due to a borehole: the mogi source as an example (3.12) (3.13) (3.14) (3.15) (3.16) in the above, r1 and r2 should be read as . (3.17) the magnetic field components are given by differentiation of the potential (3.6) with respect to r0(x0, y0, z0) (3.18) (3.19) (3.20) the 1d integrations in the above can be achieved accurately by the double exponential formula (cf., sasai, 1991a). ( ) ( ) ( ) ( ) ( ) ( ) . d a c z z f x x z z f y y z z f z z z z 1 3 3 9 15 ( ) ( ) ( ) ( ) k k a k b b l b k b b c k b b 2 5 0 0 0 5 0 0 5 0 7 0 3 π ρ ρ ρ ρ ∆ = − − + + − − + − + − − < f ( 1 # ( ( ) ( ) ( ) ( ) ) ( ) d a c y z f x x y y f f y y z z y y y y 1 3 1 3 3 15 ( ) ( ) ( ) ( ) k k a k b b b l b k b b b c k b b b b 2 5 0 0 0 3 5 0 2 5 0 7 0 0 2 π ρ ρ ρ ρ ρ ∆ = − − + + − + − + − + − − − < f ( (2 2 # ( ) ( ) ( ) ( ) ( ) ( ) d a c x z f x x f x x y y f x x x x z z 1 1 3 3 3 15 ( ) ( ) ( ) ( ) k k a k b b b l b k b b b c k b b b b 2 3 5 0 2 0 5 0 0 5 0 7 0 0 2 π ρ ρ ρ ρ ρ ∆ = − + − + + − − + − + − − − < f ( ( 2 1 # =r= ( ) ( )r x y z d x y z db b b b1 2 2 2 2 2 2 2+ + − + + + ++=f ( ) ( ) ( ) . c r z d z d r r z z d 2 3 1 6 ( ) c z 1 3 2 3 2 5 2 µ λ µ λ µ λ µ− + − − + − + = +f ( ) c r r r z z dy y y2 3 6 ( ) c b b by 1 3 2 3 2 5 µ λ µ λ µ + + − + = +f 6 ( ) c r x r x r x z z d2 3( )x b b b c 1 2 3 2 53 µ λ µ λ µ + + − + ( )f f x y( ) ( )b a b b z z *= ( )f f x y( ) ( )b a b b y x *= 4. the behaviour of the magnetic field above a borehole let us investigate the behaviour of the magnetic field above a borehole by assuming an appropriate model. table i summarizes the mechanical parameters of the mogi model, the size and the position of a borehole, together with some magnetic parameters. as for the geomagtable i. parameters of the mogi model and magnetic environments. parameter name abbr. maximum uplift ∆h 10 cm source depth d 3 km rigidity µ 3.5×1010 n/m2 average magnetization j 5 a/m stress sensitivity β 2×10−3 mpa−1 geomagnetic dip i0 60° geomagnetic declination d0 n16°e fig. 3. total intensity changes on the ground (2.5 m above the surface) due to the mogi model. unit in nt. this area shows 10 km×10 km square of which center is right above the pressure source. a solid circle indicates the horizontal position of a borehole (2 km north and 1 km east from the center). 100 yoichi sasai et al. netic environment, we assume the one in long valley caldera. figure 3 shows the total intensity change produced by the assumed mogi model, in which we used sasai’s (1991a) point source solution. the magnetic field is measured at 2.5 m above the ground surface. suppose that there is a borehole for example at a point indicated by a solid circle in fig. 3, i.e. 0.5 km north and 1 km east from the uplift center. figure 4a-d shows the four components of the magnetic field change at a height of 2.5 m over 10 m by 10 m square with its center just above the borehole. note that the effect of a borehole is extremely local, i.e. within several meters from its outlet. as we can easily imagine from the two horizontal components and particularly from the vertical component, the magnetic lines of force spread out from the ground surface and are absorbed into the borehole. such magnetic field corresponds to the third term on the right hand side of eq. (2.22), which is a line of vertical quadrupoles along the borehole axis. the magnetic lines of force of vertical quadrupoles effig. 4a-d. four components of the piezomagnetic change due to a borehole: a) h, b) d, c) z and d) f component. each square shows 10 m×10 m area of which center is the outlet of a borehole. unit in nt. a c b d 101 drag-out effect of piezomagnetic signals due to a borehole: the mogi source as an example fectively come out into the air, while those of a line of horizontal dipoles encircle within the borehole. it turned out that the contribution to the external magnetic field from the vertical quadrupoles was two orders of magnitude larger than that of the horizontal dipoles. however, even a line of vertical quadrupoles is successive arrangement of vertical dipoles fig. 5a,b. depth dependence of the contribution from the cylindrical surface of a borehole: a) 2.5 m just above the outlet of a borehole; b) 2 m north and 2.5 m above the outlet. a b 102 yoichi sasai et al. with opposite sign, where the magnetic field cancel with each other. the effect of deep-seated magnetic sources is difficult to reach the ground surface. figure 5a shows cumulative changes in each component of the magnetic field at a point 2.5 m just above the outlet of the borehole as it elongates downward. the depth dependence of the borehole effect is not so monotonous as compared with fig. 5a at another point, which is represented by fig. 5b. this figure shows cumulative magnetic changes at a point 2 m north and 2.5 m above the borehole. the total intensity shows its maximum around 1 m, and then it reaches the final value around 10 m length. generally, the magnetic field above the ground is determined by the contribution from the shallower part of a borehole from top to a 10 m length. in other words, a borehole cannot drag out the magnetic lines of force at depth. this is rather disappointing. the influence of a borehole appears very locally around its outlet. we investigated the distribution of total intensity changes by changing the sensor height as shown in fig. 6a at a height of 2 m and in fig. 6b 1.5 m, respectively. we may expect significant amount of enhanced magnetic signals only near around the outlet of a borehole. however, in such near field, the 1st order approximation solution eq. (2.22) may not be valid, because it gives a divergent magnetic field at the top of a borehole. 5. discussion since 1998 we have conducted continuous measurements of geomagnetic total force intensity and area survey of self potential (sp) in long valley caldera, california, in order to detect possible changes in the em fields associated with intrusive events at depth. actually, mueller and johnston (1998) observed remarkable magnetic changes associated with reactivation of the resurgent domes in the early 1990’s. at one observation site plv, we installed two proton magnetometers, one close to a buried vertical pipe (borehole) and the other 40 m away in order to discriminate a sub-nanotesla change if any. an episodic event, i.e. swarm earthquakes and crustal inflation, was triggered in the caldera by the october 16, 1999, hector mine earthquake of m 7.1 in southern california. figure 7 shows the simple differences in the total intensity between the two magnetometers at plv and the volumetric strain changes at a different observation site. total intensity difference showed gradual increase of up to 0.3 nt from the latter half of october to early november, which is fig. 6a,b. changes in the total intensity above a borehole: a) 2.0 m above and b) 1.5 m above the ground surface. unit in nt. a b 103 drag-out effect of piezomagnetic signals due to a borehole: the mogi source as an example more or less similar to the accelerated volumetric strain changes after the hector mine earthquake. however, it should be noted that no such a step-like change occurred in the geomagnetic field as observed in the strain data. johnston et al. (2000) obtained a source model for volumetric strain changes in long valley caldera which were triggered by the hector mine earthquake. this model involved aseismic normal faulting plus inflation, about 7 km wnw from plv site, at a depth of 7 km. the volumetric inflation source was required to explain the strain data. according to utsugi et al. (2000), a nearly vertical dip-slip fault produces negligible piezomagnetic field, while a mogi pressure source effectively generates an observable one (sasai, 1991a). the observed magnetic changes could thus be produced mainly by the volume source. hence the results shown in fig. 7 strongly motivated the present study. magnetic field change of any tectonic model is proportional to its moment or the intensity of the source. it greatly depends also on the depth of the source. the triggered normal faulting occurred on a slightly inclined fault of 1 km by 1 km wide with 1.4 cm dislocation at a depth of 7 km. its seismic moment is estimated as 4.9×1019 dyne-cm. on the otherhand, the moment of the mogi source given in fig. 3 is estimated as c=6.3×1023 dyne-cm. this source for the strain transient triggered by hector mine earthquake has a moment that is too small and is too deeply located to generate any observable magnetic change even with the aid of an enhancement effect of a borehole. hashimoto et al. (2003) summarized the magnetic observations in long valley caldera during the period from 1999 to 2001. they investigated the cause of annual variations prominent at several stations, which were ascribed to fig. 7. total intensity difference between two magnetometers (unit in nt) at plv in the upper curve and volumetric strain (unit in µ-strain) in the lower one at pop site during the period from september 28 to november 3, 1999. total intensity and strain were measured at every 10 min interval. 104 yoichi sasai et al. changes in the ground temperature (utada et al., 2000). they successfully compensated for the annual variations on the basis of ground temperature data. then they removed some apparent variations in the total intensity differences caused by the vectorial differences of the local main geomagnetic field as well as by the local induction effect, i.e. what is called the 3-components correction or the prediction-error filter technique. they concluded that there was no significant change larger than 0.5 nt at plv. as we have seen in fig. 7, the observed variation at plv, if any, is at most 0.3 nt or so, which does not conflict with hashimoto et al.’s (2003) conclusion. at plv site in long valley caldera, a vertical steel casing was buried in a nearby borehole. this produced an anomalously high field gradient, say 1000 nt/m or more. we thus installed the sensor within a few m of the borehole. yamamoto (1990) conducted an experimental observation of the geomagnetic field at the bottom of a borehole using flux-gate magnetometers. he used a vinyl chloride tube to avoid some diffculties in magnetic measurements caused by high-µ metals. however, he reported some steps in the magnetic data associated with small felt earthquakes most probably caused by movement of the sensor. although the most up-to-date overhauser type proton magnetometer works under a relatively high field gradients, only a few mm displacement of a sensor would result in an apparent variation. it would then be difficult to discriminate signals of piezomagnetic origin. an optimum arrangement for geomagnetic observations using a borehole would be a) a borehole of several hundreds meters length plus; b) a spherical cavity of radius 1 to 2 m in order to avoid the high field gradient at the bottom of the borehole. the magnetometer sensor is placed at the center of the cavity, which is fixed with silica or any non-magnetic substance. we don’t use the casing pipe of high µ metal for the protection of the borehole. the magnetic field at any position in such a system, even inside the borehole, can be represented rigorously by eqs. ((2.14), (2.28) and (2.39)) with the aid of lipschitz-hankel integrals. the same procedure is applicable to evaluate the effect of the spherical cavity at the bottom of the borehole. our next subject is to investigate the behaviour of the tectonomagnetic field under such an observation system. acknowledgements we are greatly indebted to drs. ciro del negro and antonio meloni who carefully read the manuscript and gave us useful comments to improve it. references eason, g., b. noble and i.n. sneddon (1955): on certain integrals of lipschitz-hankel type involving products of bessel functions, phil. trans. r. soc. london ser. a, 247, 529-551. hashimoto, t., y. tanaka, m.j.s. johnston, m. utsugi, y. sasai and s. sakanaka (2003): on the annual variations in geomagnetic differences observed in long valley caldera, california, ann. disas. prev. res. inst., kyoto univ., 46b, 765-777 (in japanese with english abstract). johnston, m.j.s., d.p. hill and a.m. pitt (2000): strain transient recorded in the long valley caldera during triggered seismicity from the october 16, 1999, m 7.1 hector mine, california, earthquake, eos, trans. am. geophys. un., 81, wp1384. mogi, k. (1958): relations between the eruptions of various volcanoes and the deformations of the ground surfaces around them, bull. earthqake res. inst., univ. tokyo, 36, 99-134. mueller, r. and m.j.s. johnston (1998): review of magnetic field monitoring near active faults and volcanic calderas in california: 1974-1995, phys. earth planet. inter., 105, 131-144. sasai, y. (1983): a surface integral representation of the tectonomagnetic field based on the linear piezomagnetic effect, bull. earthq. res. inst., univ. tokyo, 58, 763-785. sasai, y. (1991a): piezomagnetic field associated with the mogi model revisited: analytic solution for finite spherical source, j. geomag. geoelectr., 43, 21-64. sasai, y. (1991b): tectonomagnetic modeling on the basis of the linear piezomagnetic effect, bull. earthquake res. inst., univ. tokyo, 66, 585-722. sasai, y. (1994): enhancement of piezomagnetic signals within a borehole, in electromagnetic phenomena related to earthquake prediction, edited by m. hayakawa and y. fujinawa (terra scientific publishing co., tokyo), 51-54. utada, h., m. neki and t. kagiyama (2000): a study of annual variations in the geomagnetic total intensity with special attention to detecting volcanomagnetic signals, earth planets space, 52, 91-103. utsugi, m., y. nishida and y. sasai (2000): piezomagnetic potentials due to an inclined rectangular fault in a semi-infinite medium, geophys. j. int., 140, 479-492. yamamoto, t. (1990): geomagnetic three components observations using a borehole, in proceedings of conductivity anomaly symposium, 187-194 (in japanese). s u l l ' o r i g i n e d e l l e o n d e s u p e r f i c i a l i a s s o c i a t e a l l e o n d e s , s s . s s s , . . . p i e t r o catoi i n a l c u n e n o i e p r e c e d e n t i ( x ) l i o d a t o n o t i z i a d e l l ' e s i s t e n z a di a l c u ni s i s t e m i d i o n d e s i s m i c h e a c a r a t t e r e s u p e r f i c i a l e , o s c i l l a n t i n e l p i a n o p r i n c i p a l e , a s s o c i a t i a l l e o n d e s , ss, sss. n e l l e u l t i m e d u e — p a r t i c o l a r m e n t e n e l l ' u l t i m a — d e l l e n o t e c i t a l e , h o c e r c a t o d i s p i e g a r n e l a n a t u r a c il m e c c a n i s m o eli p r o d u z i o n e , r i t e n e n d o l i d o v u t i a l l ' u r t o d e l l e o n d e t r a s v e r s a l i c o n t r o l e tra ti ficazioni t e r r e s t r i . o g n u n a d e l l e o n d e i n j>aroia e r a r i t e n u t a o r i g i n a n t e s i ad u n a dis i a n z a d e t e r m i n a t a , i n s t r e t t a r e l a z i o n e c o n l e o n d e s , s s , . . . : l e c v e n i v a n o a t t r i b u i t e a l l ' u r t o d e l l e s , l e sl a l l ' u r t o d e l l e s s , e c c . s i t e n t a v a s p i e g a r e l ' a u m e n t o d e l p e r i o d o c o n la d i s t a n z a , i n v o l a n d o l ' a z i o n e d e l l a d i s p e r s i o n e e si c o n c l u d e v a r i t e n e n d o l e r i s p e t t i v e d r o m o e r o n c c o s t i t u i r e r a m i di i p e r b o l e t e n d e n t i a s i n t o t i c a m e n t e a l l e d r o i n o c r o n e d e l l e s , s s . r i s p e t t i v a m e n t e . a l c u n i f a t t i r i m a s t i senza p l a u s i b i l e s p i e g a z i o n e , m i h a n n o i n d o t t o a r i e s a m i n a r e il p r o b l e m a . o n e s t o r i e s a m e m i h a c o n d o t t o a f o r m u l a r e la s e g u e n t e n u o v a i p o t e s i , c h e r i t e n g o m o l l o p i ù a t t e n d i b i l e d e l l a p r e c e d e n t e . u n ' a t t e n t a o s s e r v a z i o n e m i h a c o n s e n t i t o di c h i a r i r e c h e le onde c , s l , s m , . . . non sono legate ad nn particolare ii]>o d'onda ( s , s s . s s s , rispettivamente) ma ad un determinato valore dell' angolo di incidenza delle onde s , s s , . . . ; nel senso che. p. es., l'onda c può essere. determinata sia dalle s , che dalle s s o dalle s s s . purché queste incidano sempre sotto quel detcrminato angolo. o g n i t i p o d ' o n d a h a q u i n d i d i v e r s e d i s t a n z e o r i g i n i , a s e c o n d a c l i c è d e s t a t o da o n d e s. ss, sss, ... a v v i e n e c o s ì c h e l ' o n d a c p u ò o r i g i n a r e a 7 0 0 0 8 0 0 0 k m ( p e r a z i o n e d e l l e -s), a 1 4 0 0 0 1 5 0 0 0 k m ( p e r a z i o n e d e l l e s s ) , e c c . n a t u r a l m e n t e , a n c h e l e ps, pss,... in d e t e r m i n a t e c o n d i z i o n i , p o s s o n o d a r o r i g i n e a d a n a l o g h i s i s t e m i d ' o n d e . p e r q u a n t o si r i f e r i s c e al m e c c a n i s m o di p r o d u z i o n e , v a o s s e r v a t o c h e l e o n d e s ( . s v ) i n c i d e n d o c o n t r o la s u p e r f i c i e e s t e r n a d e l l a s u l l ' o r i g i n e d e l l e o n d e s u p e r f i c i a l i a s s o c i a t e a l l e o n d e s . s s . s s s . t e r r a s o l l o l ' a n g o l o l i m i t e , p r e s e n t a n o il f e n o m e n o d e l l a riflessione totale ( 2 ) . i n q u e s t o c a s o , la t e o r i a p r e v e d e la c r e a z i o n e di un p a r t i c o l a r e t i p o di o n d a s u p e r f i c i a l e , a c a r a t t e r e l o n g i t u d i n a l e , c o n o s c i u t a sotto il n o m e di onda evanescente. c i ò p r e s e n t a q u a l c h e a n a l o g i a con il c o m p o r t a m e n t o d e l l a l u c e n e l caso a p p u n t o d e l l a r i f l e s s i o n e t o t a l e . e s i s t o n o , in q u e s t o c a s o , nel s e c o n d o m e z z o d e l l e v i b r a z i o n i i c u i p i a n i d ' o n d a sono i p i a n i a ; = c o i . (x. d i r e z i o n e di p r o p a g a z i o n e sulla s u p e r f ì c i e di s e p a r a z i o n e ) , e la cui a m p i e z z a è p r o p o r z i o n a l e a c~'"~ (ni. g r a n d e z z a d i p e n d e n t e d a l l a n a t u r a dei due m e z z i , e d a l l e e a r a t e r i s t i c h e d e l l a l u c e , è d e l l ' o r d i n e 2 -t di , e s s e n d o x la lunghezza d ' o n d a : z è p o i t i v o p e r il 2 ' m e z z o ) . à d a o s s e r v a r e c h e , n e l c a s o d e l l a l u c e , la riflessione non può essere totale, che se il secondo mezzo non è troppo sottile: il fenomeno della ri flessione totale fa quindi intervenire uno spessore finito del uppie/a & • 4150 km ksàrà a = iouofm 2 xh 1945 • l 2 111 1935 a-=-49*9 io"" ;'•', • i : <5 y''j ì ';• ., ' ' ' i i ! •! '' ' j / / f i g . 1 e s e m p i di o n d e t i p o 1 ( c , ' i . con p e r i o d o di 3 5 s c a . . o r i g i n a n t i a d i s t a n z a a a s s o c i a t e a l l a s : 2 a a s s o c i a t e alla s s : 3 aa s s o c i a t e alla s s s . i r i d u z i o n e c a . 3 / 5 ) . 3 5 2 p i e t r o c a l o i mezzo meno rifrangente. ( \ . p . es. a q u e s t o r i g u a r d o . a . b r u h a l , « c o u r s de p h y s i q u e g é n é r a l e o p t i q u e ». ma—on, p a r i s 1 9 4 7 , j). 3 4 9 ) . c o m p o r t a m e n t o non diverso d o v r e b b e r o p r e s e n t a r e le o n d e s , s s , . . . nel senso elle a n c h e p e r esse la r i f l e s s i o n e t o t a l e r i c h i e d e r e s i stenza di un s e c o n d o m e z z o finito, q u a l e p o t r e b b e essere la c r o s t a t e r r e s t r e , n e i c o n f r o n t i (jcc!f a-h/ 'jjj 4 = woo h~m ,1 .n | a ^ = + del « m a n t e l l o )>. da ; fi i \ a _ cui l e p r e d e t t e o n d e l'i i i ' 4 \ t"'•} lì, (v, a "j p r o v e n g o n o . / v l | u s | \ ( \ f \ i y p l a s e c o n d a c h e l ' a n " 1 \ i ) i v ì 5 s ì ' g o l o l i m i t e v i e n e r a g -\ i \ ! • ; g i u n t o m c o r r i s p o n (lenza d e l l ' u n a o del2 l ' a l t r a s u p e r f i c i e di di0\de .ì-k-ixu || s c o n t i n u i t à d e l l a erozi i505h km , i e • , /\f = m s t a , 1 o n d a s u p e r n c i a l e i \ c h e n e c o n s e g u e s a r à a p e r i o d o p i ù o m e n o g r a n d e ( m a s s i m o p e r la s u p e r f i c i e di discont i n u i t à p i ù i n t e r n a ) . j h i i i | iw " ^ ' v ^ ' u c o n s e g u e c h e , a p a r i j l| j : b di a l t r e c o n d i z i o n i , l ' i n c i d e n z a l i m i t e c o n t u . i 5 i tro u n a d e t e r m i n a t a u 2 s u p e r f i c i e d i d i s c o n t i n u i t à , d a r à o r i g i n e a d e s e m p i di o n d e t i p o 2 1 c * ' . p e r i o d o di 48* r a . . ° l l d e > " p « ' f i c i a l i d i i l o r i g i n a n t i a disianza \ a s s o c i a t e alla s : 2 asson o s t e s s o p e r i o d o ( / c u c i a t e alla s s . ( r i d u z i o n e e a . 2 / 5 i . liine/iie sia la d i s t a n z a e p i c e n t r a l e d e i successivi p u n t i in c u i si v e r i f i c a la r i f l e s s i o n e t o t a l e ( v e d i figg. 1 2 ) . e ' s i n t o m a t i c o , a q u e s t o r i g u a r d o , c h e l ' i n c i d e n z a sotto l ' a n g o l o l i m i t e si v e r i f i c a p e r le o n d e s c h e i n c i d o n o sotto lo s t r a t o del granilo ad una d i s t a n z a di 4 0 0 0 k m ea, m e n t r e lo stesso a n g o l o l i m i t e p e r i n c i d e n z a d e l l e o n d e s sotto lo s t r a t o del b a s a l t o v i e n e r a g g i u n t o ad una distanza di 7 0 0 0 k m e a . a q u e s t e d i s t a n z e ( e d ai l o r o r i s p e t tivi m u l t i p l i ) si v e r i f i c a n o le r e g i s t r a z i o n i di o n d e s u p e r f i c i a l i , rigom i.1. olili,im'. dl-j.i.e omili s ltehi-'ici ai.i associati-: al.i h ondi. s. ss. r o s a m e n t c o s c i l l a n t i nel p i a n o p r i n c i p a l e , con p e r i o d i di 3s c a . ( o n d e 6 ' , ) e 4 8 " c a . ( o n d e c „ ) r i s p e t t i v a m e n t e . p a g o , p e r o r a . di q u e s t o l i e v e c e n n o i n f o r m a t i v o , m i r i s e r v o di r i t o r n a r e sull a r g o m e n t o con u n a più s i s t e m a t i c a d o c u m e n t a z i o n e del i n g o i a r e f e n o m e n o . koinii — istillilo x azionale di grofisica — maggio 1918. hi 15 ialografia ( x ) cai.oi p . . due tipi ili onde cariilleristici ini le fusi s eil i. ili un telesismii: le onde se ed sm. « l^a r i c . s c i e n t i f i c a » . \ ( 1 9 3 4 ) . — .1 propostin delle mule si. ed sm... « b o l l . c o n i . g e o i l . e g e o l . d e l c . n . r . » , l v ( 1 9 3 4 ) . — due nuovi tipi ili onde sismiche alla luce di una teoria del somigliami. « r e n d . ace. l i n c e i » , x x i i i ' 1 9 3 0 1 . — a l l o c a onda a lungo periodo oscillarne nel piano principale. « b o l l . c o n i . g e o d . e g e o f . d e l c . n . r . », \ i h 9 3 6 i . — sur une onde longtie oscillimi ilans !<• pian principili, u l ' u b i . du b i l i c a l i c e n t r . s é i s m . i n t . . t r a v . s r i r n t . » x v ( 1 9 3 7 ) . • — sopra niellili nuoci sistemi di onde sismiche a carattere superficiale oscillanti nel piano principale. « a c e . d ' i t a l i a . r e n d . » . i i (19411). l'-'l i i . jeffreys, tlie rejlexion unii rcjraetion oj elastie ii nves. « r o \ a l \-t r. s o c . . g e o p l i y s . s u p p l . », i , 3 2 8 3 2 9 ( 1 9 2 6 ) . h . b o u a s s k . sóismes et sismographes, 8 5 8 6 ( 1 9 2 7 1 . gutenberg i ) . . tlieorie der erdbehenwelhn. « h a n d b u r h dei' geopli\>ik ». i v . 5 4 5 6 ( 1 9 2 9 ) ) . — energy ratio oj reflecied and rejracted seismic ii in i c s . « b u l l . s i ' i s . s o c . \ n i . ». \ \ \ l \ ( 1 9 4 4 ) . s t u d i o d e l l a r a d i o a t t i v i t a ' d e l l e l a v e d e l l ' e t n a c o l m e t o d o d e l l e e m u l s i o n i n u c l e a r i l . barbera m . curatolo m . m . indovina addario m . santangelo 1. — i n q u e s t i u l t i m i t e m p i sono state eseguite diverse r i c e r c h e sulla r a d i o a t t i v i t à dei m a t e r i a l i c o s t i t u e n t i la crosta t e r r e s t r e . questo s t u d i o lia i m p o r t a n z a in q u a n t o p u ò d a r e i n f o r m a z i o n i sui processi fisici e fisico-chimici, c h e i n t e r e s s a n o i p r o b l e m i connessi con la f o r m a z i o n e e con l ' e v o l u z i o n e della crosta t e r r e s t r e ( d i f f e r e n z i a z i o n e dei m a g m i , c a l o r e radioattivo...). t r a i m a t e r i a l i solidi le lave vulcan i c h e p r e s e n t a n o i n t e r e s s e p a r t i c o l a r e ; d u e s o n o i p r o b l e m i p r i n c i p a l i c h e si i n c o n t r a n o n e l l o s t u d i o della r a d i o a t t i v i t à d e l l e l a v e : a) determ i n a z i o n e del r a p p o r t o delle c o n c e n t r a z i o n i d i u r a n i o e di t o r i o : b) i n v e s t i g a z i o n e sull'esistenza o m e n o d e l l o stato di e q u i l i b r i o radioattivo dei c o m p o n e n t i l e d u e f a m i g l i e . r e c e n t e m e n t e s o n o state e f f e t t u a t e sulle l a v e del vesuvio delle m i s u r e ('), le q u a l i c o n f e r m a n o q u a n t o era già s t a t o a f f e r m a t o fin dal 1909 d a j o l y (2), cioè c h e d e t t e l a v e h a n n o u n c o n t e n u t o r a d i o a t t i v o d i p e n d e n t e d a l l a d a t a d ' e r u z i o n e . e p o i c h é è s t a t o a f f e r m a t o c h e in lave di a l t r i v u l c a n i n o n è stata r i s c o n t r a t a fin'ora q u e s t a c a r a t t e r i stica, a b b i a m o ritenuto o p p o r t u n o s o t t o p o r r e ad e s a m e q u e l l e d e l l ' e t n a , p e r v e d e r e se la l o r o a t t i v i t à d i p e n d e s s e o n o dalla d a t a d ' e r u z i o n e . p e r e s e g u i r e q u e s t e m i s u r e a b b i a m o u s a t o la tecnica d e l l e e m u l s i o n i n u c l e a r i , t e c n i c a già i m p i e g a t a da m o l t i a u t o r i p e r r i c e r c h e sulla rad i o a t t i v i t à d e l l e rocce (1), (3), (4), (5), (6), (7), (8), (»), ( ] 0 ). 2. — l a s t r e i l f o r d c^, spesse 50 [i, sono s t a t e esposte p e r 51 g i o r n i a l l ' i r r a g g i a m e n t o di sezioni l e v i g a t e di lave, a v e n t i u n o spessore > loou, cioè u n o spessore t a l e d a p o t e r s i r i t e n e r e i n f i n i t o rispetto al r a n g e m a s s i m o d e l l e p a r t i c e l l e a ; t a l i lave a p p a r t e n g o n o a u n ' e r u z i o n e preistorica (lava i), a u n a d e l 1381 (lava ii), a u n a del 1669 (lava i i i ) e i n f i n e a u n a d e l 1928 (lava iv) ( n ) . p o i c h é l ' a t t i v i t à d e l l e lave in gen e r e r i s u l t a m o l t o d e b o l e , si è s t i m a t o necessario p r o l u n g a r e il t e m p o 604 l . b a r b e r a , m . c u r a t o l o , m . m . i n d o v i n a a d d a r i o , m . s a n t a n f . e l o d i e s p o s i z i o n e ; p e r q u e s t o a b b i a m o p e n s a t o fosse o p p o r t u n o p r e n d e r e a l c u n e p r e c a u z i o n i p e r r e n d e r e m i n i m o il f a d i n g , p e r p è r d e r e il min o r n u m e r o possibile d i p a r t i c e l l e a e i n f i n e p e r d i m i n u i r e il f o n d o . n o n a b b i a m o u s a t o b a l s a m o d e l c a n a d a p e r a t t a c c a r e l e sezioni ai v e t r i n i , d a t o c b e q u e s t o p r o d o t t o p u ò s v i l u p p a r e p e r o s s i d o d ' i d r o g e n o , f i g . 1 mappa dell'attività d e l l a lava i. c o m p o s t o q u e s t ' u l t i m o c b e favorisce il f a d i n g ( 1 2 ) : a b b i a m o a t t a c c a t o p e r c i ò le sezioni con u n a colla di cellulosa v e r g i n e a l l ' a c e t o n e . s e m p r e p e r r e n d e r e m i n i m o il f a d i n g a b b i a m o t e n u t o , d u r a n t e t u t t o il p e r i o d o dell'esposizione, le l a s t r e e n t r o u n essiccatore, conten e n t e c l o r u r o a n i d r o di calcio, e in u n a m b i e n t e m a n t e n u t o alla t e m s t u d i o d e l l a r a d i o a t t i v i t à d e l l e l a v e d e l l ' e t n a 605 ji e r a tura c o s t a n t e d i 0 "c ( ' ) . a l l o s c o p o d i r i d u r r e gli e f f e t t i di f o n d o è s t a t a r i c o p e r t a d i p a r a f f i n a la s u p e r f ì c i e d e l l a l a s t r a n o n o c c u p a t a d a l l a s e z i o n e . p e r m a n t e n e r e l e s e z i o n i a c o n t a t t o c o l l ' e m u l s i o n e f o t o g r a f i c a e r e n d e r e m i n i m o la s t r a t o d ' a r i a t r a s e z i o n e e d e m u l s i o n e , a b b i a m o u s a t o d e i t o r c h i e t t i ( 1 2 ). l o s t r a t o d i g o m m a p i u m a è s t a t o p o s t o p e r r e n d e r e u n i f o r m e la p r e s s i o n e su t u t t a la s u p e r f i c i e d e l l a l a s t r a , i n m o d o d a e v i t a r e a l t e r a z i o n i . f i g . 2 mappa dell'attività della lava i i i . n e l l a c o s t r u z i o n e d e i t o r c h i e t t i a b b i a m o u s a t o b a c h e l i t e i n v e c e d i u n ' a l t r a s o s t a n z a , p e r c h é la b a c h e l i t e h a u n b a s s o c o n t e n u t o d i imp u r i t à r a d i o a t t i v e . l e l a s t r e s o n o s t a t e s v i l u p p a t e , s u b i t o d o p o l ' e s p o s i z i o n e , n e l seg u e n t e m o d o : 1) s v i l u p p o : 1 g a m i d o l , 8 g na2so.v 340 c m 3 h20: t e m p o 25 m i n ; t e m p e r a t u r a 16 °c; n e s s u n a a g i t a z i o n e ; 2) l a v a g g i o i n a c q u a p e r 5 m i n . ; 3) b a g n o d i f r e n a g g i o : a c i d o a c e t i c o al 2 % ; t e m p o 15 m i n . : t e m p e r a t u r a 16 ° c : f 604 l. b a r b e r a , m . c u r a t o l o , m . m . i n d o v i n a a d d a r i o , m . s a n t a n f . e l o 4) fissaggio: i p o s o l f i t o al 4 0 % ; t e m p o c i r c a 45 m i n . ; t e m p e r a t u r a 20 °c; n e s s u n a a g i t a z i o n e ; 5) l a v a g g i o i n a c q u a c o r r e n t e p e r 15 m i n . l ' e s p l o r a z i o n e è s t a t a e s e g u i t a c o n u n i n g r a n d i m e n t o d i 460 d i v i d e n d o l a l a s t r a i n c a s e l l e di 330 x 330li2; d i ogni l a s t r a è s t a t a e s e g u i t a u n a m a p p a (fig. 1 e fig. 2), r i p o r t a n d o l e p a r t i c e l l e t r o v a t e i n u n a c a s e l l a d i 330 x 330 u2 i n u n q u a d r a t i n o di 5 x 5 m m 2 . d u r a n t e l ' e s p l o r a z i o n e s o n o s t a t e s e g n a t e solo l e t r a c c e di p a r t i c e l l e o , a v e n t i u n a p r o i e z i o n e o r i z z o n t a l e c o m p r e s a t r a 5 |i e 50 3. — i r i s u l t a t i o t t e n u t i s o n o s t a t i r i p o r t a t i n e l l a t a b e l l a i , d a l l a q u a l e r i s u l t a c b e l ' a t t i v i t à d e l l e s e z i o n i c o n s i d e r a t e è p i u t t o s t o b a s s a ; essa è d e l l ' o r d i n e d i g r a n d e z z a d i q u e l l a d e l l e l a v e m e n o a t t i v e ( p i ù a n t i c h e ) d e l v e s u v i o . d a l c o n f r o n t o d i r e t t o d e i r i s u l t a t i p e r i q u a t t r o c a m p i o n i si p u ò d i r e c h e l ' a t t i v i t à d e l l e l a v e d e l l ' e t n a , e n t r o gli e r r o r i f i g . 3 lava i. 604 l . b a r b e r a , m . c u r a t o l o , m . m . i n d o v i n a a d d a r i o , m . s a n t a n f . e l o s p e r i m e n t a l i , n o n v a r i a col v a r i a r e d e l l a d a t a d i e r u z i o n e . i n v e r o l'attività d e l l a lava l ì p r e s e n t a u n o s c a r t o , l e g g e r m e n t e f u o r i degli err o r i , r i s p e t t o ai v a l o r i d e l l e a l t r e , q u e s t o f a t t o p e r ò n o n ci s e m b r a possa assumersi c o m e i n d i z i o d i u n a v a r i a z i o n e d e l l ' a t t i v i t à con il t e m p o , a n c h e p e r il f a t t o c b e l e l a v e i e i v , c o r r i s p o n d e n t i a d a t e e s t r e m e d ' e r u z i o n e , d a n n o v a l o r i p r a t i c a m e n t e u g u a l i . è s t a t a f a t t a l ' a n a l i s i statistica d e l l a d i s t r i b u z i o n e d e l l e t r a c c e n e l l e q u a t t r o m a p p e , d a t o c h e ad u n p r i m o e s a m e r i s u l t a n o e v i d e n t i d i s u n i f o r m i t à , c o m e si p u ò v e d e r e o s s e r v a n d o la fig. 1; a b b i a m o calc o l a t o col m e t o d o di p o i s s o n la f u n z i o n e di d i s t r i b u z i o n e d e l n u m e r o d e l l e caselle r i s p e t t o al n u m e r o d e l l e t r a c c e in esse c o n t e n u t e e al v a l o r e m e d i o di tracce p e r casella. n e l l e figg. 3, 4, 5, 6 le c u r v e sono c a l c o l a t e con i d a t i della t a b e l l a i, m e n t r e i p u n t i r a p p r e s e n t a n o i c o r r i s p o n d e n t i v a l o r i s p e r i m e n t a l i . d a l l ' e s a m e d e l l e c u r v e r i s u l t a c h e p e r le l a v e i i e i i i la d i s t r i f i g . 6 lava i v . 604 l . b a r b e r a , m . c u r a t o l o , m . m . i n d o v i n a a d d a r i o , m. s a n t a n f . e l o b u z i o n e s p e r i m e n t a l e si a d a t t a grosso m o d o a q u e l l a t e o r i c a d i poiss o n ; i n f a t t i la m a p p a della lava i i i (fig. 2) si p r e s e n t a con pocliissime d i s u n i f o r m i t à . i n v e c e la d i s t r i b u z i o n e s p e r i m e n t a l e p e r l e lave i e i v si a l l o n t a n a in m o d o p i ù m a r c a t o da q u e l l a di p o i s s o n : o s s e r v a n d o la m a p p a della figura 1, si v e d e c h e si h a n n o z o n e di a d d e n s a m e n t o di p a r t i c e l l e a e z o n e di r a r e f a z i o n e . 4. — ci è g r a d i t o r i n g r a z i a r e il p r o f . a n g e l o b e l l a n c a , d i r e t t o r e d e l l ' i s t i t u t o d i m i n e r a l o g i a d e l l ' u n i v e r s i t à di p a l e r m o , p e r a v e r e messo a n o s t r a disposizione p a r t e d e l l ' a t t r e z z a t u r a necessaria alla r i c e r c a ; r i n g r a z i a m o p u r e la l a u r e a n d a m a r i a g i a r d i n a p e r la p r e p a r a z i o n e d e l l e sezioni sottili. palermo — istituto di fisica dell'università — luglio 1952. riassunto si espongono i primi risultati di uno studio sulla radioattività delle lave dell'etna; si conclude che l'attività globale media di tali lave non varia con la data d'eruzione, essa ha un valore di <v> 5-6.10 4 particelle a per cni1 e per sec. summary the authors give some preliminary results on the radioactivity of the « lave etnee ». they conclude that the total media activity of illese « lave » do not chanae ivith the date of the eruption and it is oo 5-6.10 4 a/crn" sec. b i b l i o g r a f i a ( ] ) f . b a c h e l e t , m . f e r r e t t i s f o r z i n i e f . i p p o l i t o , nuovo cimento. 8, 851 (1951). < ' ) j . j o l y , phil. mag. 1 8 , 5 7 7 ( 1 9 0 9 ) . ( 3 ) i . c u r i e , journ. pliys. et rad. 7 , 3 1 3 ( 1 9 4 6 ) . l ' i e . e . p i c c i o t t o , bull. soc. heine gèo. paléont. el hydrol. 5 8 , 7 5 ( 1 9 4 9 ) . ( 3 ) e . e . p i c c i o t t o , bull. soc. belge gèo. paléont. et hydrol. 5 9 , 1 7 0 ( 1 9 5 0 ) . s t u d i o d e l l a r a d i o a t t i v i t à d e l l e l a v e d e l l ' e t n a 6 1 1 ( ° ) j. h. j. p o o l e e j. w . b r e m m e r , nature. 163, 130 (1919). ( 7 ) j. h . j. p o o l e e j. w . b r f . m m e r , nature. 1 6 1 , 881 (1948). ( 8 ) i . h . f o r d , nature. 1 6 7 , 2 7 3 ( 1 9 3 1 ) . ( " ) s . m i l o n e , t a m b u r i n o e a . s t e l l a , nuovo cimento. 9 , 2 5 3 ( 1 9 5 2 ) . ( 1 0 ) h . v . b l t t l a r e f . g. h o u t e r m a n s , geochim. et cosmochim. ada, 2, 13 (1951). ( 1 1 ) r i n g r a z i a m o il p r o f . g. p o n t e per averci g e n t i l m e n t e f o r n i t o i suddetti a m p i o n i di lava. ( 1 2 ) h . y a c o d a , radioactive measurementes with nuclear emulsions. ( n e w o r k , 1949). vol. 48, 01, 05ok.qxd 73 annals of geophysics, vol. 48, n. 1, february 2005 key words radon – electrical conductivity – ions – thermal springs – earthquakes 1. introduction significant variations in geophysical and geochemical parameters in soil gas and natural springs have been detected prior to or during earthquakes. the observed geophysical phenomena include variations in water level in deep wells, hydrostatic pressure, water flow rate from wells, electrical conductivity and water temperature. the most frequently studied geochemical phenomena have been concentrations of dissolved gas and ions in groundwater and variations in concentrations of crustal and mantle volatiles in ground gases. (wakita, 1978; king, 1980; nakamura and wakita, 1985; wakita et al., 1985, 1986; thomas, 1988; king et al., 1994; koch and heinicke, 1994; dongarra et al., 1995; heinicke et al., 1995; igarashi et al., 1995; shimizu et al., 1996; monnin and seidel, 1997; bella et al., 1998; di bello et al., 1998; woith et al., 1998; singh et al., 1999; zhang, 1999; biagi et al., 2000; heinicke et al., 2000; favara et al., 2001; koch et al., 2003). stress/strain fields have been reported to generate geochemical and geophysical anomalies (king, 1978, 1986, 1989; wakita et al., 1985, 1986; igarashi et al., 1995; zhang, 1999; biagi et al., 2000). even small strains are able to do so because they can be greatly amplified at pre-existing faults and fractures, where pre-existing stresses may be already near the critical levels and where pore fluids are abundant. the threshold strain changes required to cause the observed earthquake-related anomalies were estimated to be ∼ 10 – 8, which is comparable with tidal strain mailing address: dr. andreja popit, jožef stefan institute, jamova 39, 1000 ljubljana, slovenia; e-mail: andreja.popit@ijs.si geochemical and geophysical monitoring of thermal waters in slovenia in relation to seismic activity andreja popit (1), janja vaupotič (1) and tadej dolenec (2) (1) jožef stefan institute, ljubljana, slovenia (2) faculty of natural sciences and engineering, university of ljubljana, ljubljana, slovenia abstract pre-seismic related strains in the earth’s crust are the main cause of the observed geophysical and geochemical anomalies in ground waters preceding an earthquake. posočje region, situated along the soča river, is one of the most seismically active areas of slovenia. our measuring stations close to the posočje region were installed in the thermal springs at bled in 1998 and at zatolmin in 1999. since the beginning of our survey, radon concentration, electrical conductivity and water temperature have been measured continuously once every hour. in may 2002, the number of geochemical parameters monitored was extended to ionic concentration, ph and eh, which are analysed once a month. before seeking a correlation between geochemical and geophysical anomalies with seismic events, the influence of meteorological (atmospheric precipitation, barometric pressure) and hydrological (water table of the tolminka river) factors on observed anomalies were studied. results at zatolmin showed that some radon variation during the period from june to october 2002 may be related to seismic activity and not only to meteorological effects. 74 andreja popit, janja vaupotič and tadej dolenec amplitudes (king, 1986). wakita et al. (1986) and king (1989) indicated that chemical precursor signals are detectable especially in faults and at intersections of faults. much evidence demonstrates the major role of crustal fluids of deep origin in the earthquake cycle (varhegyi et al., 1986; toutain and baubron 1999). geochemical and geophysical anomalies are due to the slow growth of small cracks in the crust caused by stress corrosion by groundwater. associated micro-fracturing of aquifers may cause mixing of fluids from two or more chemically distinct aquifers (thomas, 1988; igarashi et al., 1995; singh et al., 1999; heinicke et al., 2000). the exposure of fresh rock surfaces, resulting from mineral fracture, causes leaching of minerals with groundwater and allows the escape of trapped gases from the rock matrix (thomas, 1988). thus, geochemical anomalies are strongly dependent on local conditions such as rock type, stress/strain field, degree of water saturation of rock pores, etc. several thermal springs and wells have been reported to discharge mixtures of deep crustal fluids and shallow meteoric components, and precursory changes in fluid composition were shown to be the result of increasing contributions of the deep component to the total flow (thomas, 1988). advection of carrier gases, such as co2 and ch4, may be the main migration process for transport of trace gases (rn, he) toward the earth’s surface. in contrast to gas advection, transport mechanisms of endogenous gases by gas diffusion or by water advection (unless under rare conditions of high-velocity resurgent water) are usually far too slow to cause the observed geochemical anomalies. bubble movement through micro-fractured rocks (fracture aperture of 0.01 to 10 mm at depths of thousands of metres) appears to be an effective mode of rapid (gas velocities of the order of 10 to 1000 m per day) and long-distance gas migration. the evolution from bubble regimes to continuous phase flow and vice versa is due to gas pressure and fracture width changes, which provide the most suitable mechanism for explaining surface geochemical anomalies linked to seismo-tectonic processes (varhegyi et al., 1986; dongarra et al., 1995; etiope and martinelli, 2002). a significant correlation between rn and co2 in spring gas was found in vogtland, germany (heinicke et al., 1995; koch et al., 2003) and in the southern apennines, italy (di bello et al., 1998). therefore, it should be useful in earthquake prediction research to continuously monitor springs that are abundant in co2 (koch and heinicke, 1994). geothermal waters are in contact with crustal rocks at various depths. this is why spring gases might be more representative of the local environment than soil gases. spring gases are much more enriched with deep gases and only slightly contaminated by atmospheric gases and have been proved to be better earthquake precursors (toutain and baubron, 1999). the first geochemical monitoring in slovenia connected with seismic activity was carried out on thermal waters in the ljubljana basin from 1981-1982 by measuring radon concentrations (zmazek et al., 2000). in 1999, radon measurements were extended to other thermal waters in nw and e slovenia (zmazek et al., 2002a) and to soil gas along a presumed fault in the krško basin (zmazek et al., 2002b, 2003). in addition to continuous (once per hour) measurements of radon concentration, electrical conductivity and water temperature were monitored, and gas was analysed monthly for he isotopes. geochemical and geophysical monitoring in thermal water at zatolmin and bled (both in nw slovenia) was extended from may 2002. besides continuously (once per hour) measured radon concentration, electrical conductivity and water temperature, ionic concentration, ph and eh in the springs were also analysed once per month. the aim of our investigation was to study changes in geochemical and geophysical parameters in thermal springs in relation to seismic activity. 2. geotectonic and seismogenic setting of slovenia the geotectonic position of the slovenian territory is relatively well understood within the framework of the plate tectonic model. the recent structural pattern is a cumulative result of tethyan evolution, where recent dynamics is determined by the closure of the tethys and the collision of several lithospheric units. slovenia 75 geochemical and geophysical monitoring of thermal waters in slovenia in relation to seismic activity lies at the junction of three tectonic plates (european, adriatic and tiszian), which were amalgamated during the tertiary period. the seismicity is not concentrated along the primary plate boundaries, but is rather spread in a broad zone along their deformed rims (poljak et al., 2000). there is general agreement between the tectonic pattern and its geological dynamics with the type of seismicity in a particular area. thus, seismogenic areas (eastern alps, southern alps, friuli region, external dinarides and transdanubian range) generally coincide with the main geotectonic units (poljak et al., 2000). our monitoring stations are installed at two thermal springs (fig. 1). the first is located at bled and wells on the bled fault on the eastern side of lake bled (46.37n, 14.11e) which belongs to southern alps. the thermal water springs from a deep triassic carbonate reservoir and rises through oligocene clay and shallower quaternary lake and glacial sediment. the thermal water at zatolmin springs from cretaceous limestone at the fault in the cave under the devil’s bridge (46.20n, 13.74e) on the southern edge of the southern alps, close to the border with the external dinarides. both waters belong to calcium-magnesium-hydrocarbonatesulphate water type. the southern alps are characterised by regional thrusts from the north to the south and a number of regional faults in a nw-se direction. these faults originate from the paleogene external dinaric tectonic evolution and they are superimposed by the neogene southern alps thrusts. they were reactivated in the post-neogene period. the seismicity of this zone is moderate: the strongest shock occurred in 1895 and damaged the city of ljubljana (intensity viii-ix msk) (poljak et al., 2000). 3. analytical methods radon concentration is measured continuously with a barasol mc 450 probe (algade, france). radon enters the detection chamber through a filter, which prevents radon decay products from entering. the detection unit is a solid-state silicon detector. the measurement is carried out by gross alpha counting. the counts were integrated over an hour. in addition to fig. 1. geotectonic situation of slovenia and locations of the thermal springs at zatolmin and bled. 76 andreja popit, janja vaupotič and tadej dolenec measurement of radon, water temperature is measured over the same time periods. the sensitivity of the radon detector is 50 bqm–3, and temperature is measured to an accuracy of ± 0.01°c. conductivity and water temperature are measured once per hour with ec 350 electrical conductivity sensor (greenspan, australia), employing an electromagnetic field for measuring conductivity. an increase in charged ion mobility or concentration causes a decrease in resistivity and a corresponding increase in the output of the ec sensor. water temperature is monitored by a separate sensor, which provides both a temperature output and a signal to normalise and temperature compensate the ec output. the sensitivities for the ec and temperature sensor are 0.01µscm–1 and 0.01°c, respectively. the temperature measured with the ec sensor was used for the interpretation. eh and ph are measured by portable redox combination electrodes (mettler-toledo) in situ once per month. water samples for cation analyses are collected once per month. dissolved cation concentrations are analysed in the actlabs laboratory in canada by inductively coupled plasma mass spectrometry (icp/ms). meteorological (average daily air temperature and barometric pressure, daily atmospheric precipitation) and hydrological data (daily average water table of the tolminka river) were provided by the environmental agency of the republic of slovenia (arso), office of meteorology and office of seismology. seismological data were obtained from arso, office of seismology. 4. results and discussions geochemical and geophysical monitoring in relation to seismic events was performed in two slovenian thermal waters. our research was focused primarily on precursory changes observed in radon concentration, water temperature, electrical conductivity, concentrations of dissolved ions, ph and eh. non-seismic interferences by meteorological (average daily barometric pressure and daily atmospheric precipitation) and hydrological parameters (daily average water table of the tolminka river) were studied. changes in observed geochemical and geophysical parameters exceeding two standard deviations (2σ) from the mean value were considered anomalies (heinicke et al., 1995; di bello et al., 1998; planinić et al., 2001; virk and walia, 2001), according to the normal distribution of measurements, where 95% of the observations are contained in the interval ± 2σ from the mean value. biagi et al. (2000) labelled each ion or gas concentration signal with amplitude greater than 3σ as an irregularity. 4.1. measuring station at zatolmin the thermal spring at zatolmin is located in a natural cave along the tolminka river, which overflows it constantly, but more intensively during heavy rains. this spring is characterised by co2 bubbling activity. the average water temperature and its standard deviation (± σ) between may 2002 and april 2003 was 21.5 ± 0.8°c. probes for radon and electrical conductivity measurements are installed in the spring-water at the bottom of the cave. the average electrical conductivity and radon concentration (both ± σ) in this thermal spring between may 2002 and april 2003 were 754 ± 47 µscm–1 and 33.4 ± 19 kbqm–3, respectively. variations in radon concentration, electrical conductivity and water temperature showed significant temporal fluctuations, resulting mainly from atmospheric precipitation. these variations were distinguished from those which could be earthquake related. electrical conductivity is a function of both ion concentration and water temperature. the correlation coefficients between electrical conductivity and calcium, potassium and silicon concentration were 0.88, 0.98 and 0.79, respectively. the linear trends of electrical conductivity and water temperature curves (fig. 2) were interrupted during the periods of heavy rain, when the water level of the torrential tolminka river increased and heavy mixing of both waters occurred. these events are characterised by an instant decrease in water temperature, electrical conductivity and dissolved ion concentrafig. 2. geochemical data measured in the thermal spring at zatolmin between may 2002 and april 2003. the first diagram (starting from the top) shows radon concentration (kbqm–3) and earthquakes (ml). e is the distance between the measuring station and the epicentre, while is d the strain radius. earthquakes with e / d ≤ 1 are drawn with bold lines and those with e / d >1 with dashed lines. the second diagram shows electrical conductivity (µ s cm–1) and water temperature (°c), and the third diagram shows rainfall (mm) and the level of the tolminka river (cm). table i. list of earthquakes near the thermal spring at zatolmin and bled in the period from may 2002 to april 2003. e (km) is the distance between the measuring station and the epicentre, while d (km) is the radius of the precursory manifestation zone. earthquakes with (e/d ≤ 1) are written bold. date time latitude longitude ml e d e/d (utc) 22.05.2002 17:30 46.34 14.09 0.9 4 sw 2 2.0 07.06.2002 12:38 46.32 14.11 1.2 6 s 3 2.0 02.06.2002 13:37 45.64 14.24 3.8 73 se 43 1.7 19.06.2002 19:07 46.14 13.59 1.8 14 sw 6 2.3 26.06.2002 11:13 46.24 13.70 1.6 5 nw 5 1.0 15.07.2002 09:04 46.29 13.69 3.2 10 nw 24 0.4 22.07.2002 18:19 46.31 13.59 2.0 18 nw 7 2.6 02.08.2002 02:09 46.22 13.76 1.4 4 ne 4 1.0 20.09.2002 12:19 46.12 13.66 2.6 10 sw 13 0.8 30.09.2002 02:48 46.33 13.61 3.5 18 nw 32 0.6 08.10.2002 22:24 46.34 13.63 2.0 17 nw 7 2.4 15.11.2002 09:15 46.12 13.55 2.1 17 sw 8 2.1 11.01.2003 14:32 46.24 13.77 1.6 5 ne 5 1.0 02.02.2003 01:43 46.26 13.48 2.3 21 nw 10 2.1 09.02.2003 23:22 46.25 13.70 1.5 6 nw 4 1.5 24.02.2003 19:38 46.22 13.79 0.6 4 ne 2 2.0 77 geochemical and geophysical monitoring of thermal waters in slovenia in relation to seismic activity 78 andreja popit, janja vaupotič and tadej dolenec tion due to dilution of spring water with river water. the correlation coefficient between water temperature and level of the tolminka river was – 0.75. the correlation coefficient between magnesium concentration in the tolminka river and in the spring water was 0.60 and for silicon, 0.62. the ca, k, na, mg and si concentrations increased by up to 50% of their initial values during the period from the middle of august to the end of september 2002 (fig. 3), and then decreased to approximately the original values by november 2002. electrical conductivity showed a similar trend (fig. 3). in the period from the middle of august to the end of september 2002 there was only a small amount of rainfall, while in the period from september to november 2002, precipitation was more intense, which is evident also by the increase in tolminka level (fig. 2). the ph and eh values measured monthly from march to may 2003 were relatively stable, with average values of 6.9 and 250 mv, respectively. small variations in ph could be explained by dilution of spring water with rain water, or by dissolved co2, which lowers the ph, and dissolved ch4, which increases ph by stripping co2. during the monitoring period between may 2002 and april 2003, six earthquakes of magnitude ml between 1.4 and 3.5 occurred in the vicinity of the thermal spring (fig. 2, table i). the distances from the spring to their epicentres (e) were shorter than the strain radii (d) that define the area in which the effects of the earthquake are, in principle, detectable according to the empirical formula: d = 10 exp (0.43 ml), introduced by dobrovolsky (1979). in the period between june and october 2002 when five of the earthquakes with e/d ratio equal to or below 1 occurred, radon concentration significantly increased, while in the period between january and april 2003, with three earthquakes of ml between 1.5 and 2.1 and e/d ratios greater than 1, radon level was much lower. this fact favours the theory of the stress-strain field defined by dobrovolsky. two decreases of radon concentration from 27 to 15 kbqm–3, which occurred between may 17 and june 6, 2002, could be explained by dilution of thermal water with river water as a consequence of rainfall (fig. 2). at the same time water temperature also decreased. radon concentration correlated positively with water temperature until june 6, 2002. between june 7 and june 10, 2002 the positive correlation between radon concentration and water temperature changed (fig. 4). radon concentration increased significantly during the heavy precipitation, when the level of the tolminka river increased and water temperature decreased. the first radon anomaly of 2σ above the average value occurred twelve days before the earthquake of ml = 1.6 (fig. 4). the distance from the measuring station to the epicentre corresponded to the strain radius (e / d = 1). dilution of radon concentration due to rainfall was insignificant, so that the radon anomaly might reflect changes in the regional stress field preceding the earthquake. radon concentration began to decrease soon after it reached the value 2σ above the average and continued to do so until the earthquake, which might be attributed to the stress relaxation prior to the earthquake and fig. 3. ion concentrations (mgl–1), electrical conductivity (µ cm–1) and water temperature (°c) in the thermal spring at zatolmin between may 2002 and april 2003. 79 geochemical and geophysical monitoring of thermal waters in slovenia in relation to seismic activity some subsequent sealing of micro-cracks leading to decreased radon emanation (king, 1989; igarashi et al., 1995). a second increase in radon level at the beginning of july 2002 started at the same time as the increase in water level and decrease in water temperature (figs. 2 and 4). a radon concentration of 1.8σ over the average value was observed nine days before an earthquake of ml = 3.2. the distance from the measuring station to the epicentre was shorter than the strain radius (e / d = 0.4). soon after the peak in radon concentration was reached, it decreased until the seismic event occurred. this decrease might be explained by stress relaxation associated with the earthquakes and not by dilution of thermal water with rainwater, because there was no rainfall during that period. subsequently, radon concentration increased, in spite of the small amount of precipitation and dilution of spring water with rainwater (figs. 2 and 4). it reached 1σ above the average value at the time of an earthquake of ml = 1.4. the distance from the measuring station to the epicentre corresponded to the strain radius (e / d = 1). a radon anomaly of 1.96σ was reached six days after this earthquake, coinciding with the start of heavy precipitation. soon, the torrential tolminka river caused violent mixing of thermal water with cold river water. this is evident from the decreases in water temperature and radon concentration. radon reached its minimum value one month after the last radon anomaly of 2σ. radon concentration again started to increase after september 1, 2002, when there was no significant rainfall, and reached 1σ above the average value at the time of an earthquake of ml = = 2.6 (figs. 2 and 5). the distance from the measuring station to the epicentre was shorter than the strain radius (e / d = 0.8). the radon concentration reached its maximum of 1.6 σ five days before an earthquake of ml = 3.2. the distance from the measuring station to the epicentre was shorter than the strain radius (e / d = 0.6). it then decreased until the seismic event took place. that might be explained by stress relaxation associated with the earthquakes but not by dilution fig. 4. rn concentration (kbqm–3), earthquakes (ml), rainfall (mm) and the level of the tolminka river (cm) in the thermal spring at zatolmin between may and august 2002. fig. 5. rn concentration (kbqm–3), earthquakes (ml), rainfall (mm) and the level of the tolminka river (cm) in the thermal spring at zatolmin between august and october 2002. 80 andreja popit, janja vaupotič and tadej dolenec of thermal water with rainwater, because there was no rainfall during that period. another two radon peaks of 1.8σ and 1.7σ occurred approximately one month before an earthquake of ml = 2.1 (fig. 2). the measuring station was outside the area, defined by the strain radius (e / d = 2.1). the influence of this earthquake on the radon anomaly was considered less significant. the last radon increase of 1.9σ occurred one and a half months before an earthquake of magnitude ml = 1.6 (figs. 2 and 6). the distance from the measuring station to the epicentre corresponded to the strain radius (e / d = 1). no significant variation of radon concentration was found from the middle of january to april 2003 (fig. 2), although four consecutive earthquakes occurred during this period. this could be explained by the fact that the measuring station was outside the area, defined by the strain radius (e / d = 2.1, 1.5, 2.0, 2.3, respectively). our explanation of radon anomalies and their relation to earthquakes, as well as our interpretation of physical processes causing radon variations, will be strengthened after a longer monitoring period. 4.2. measuring station at bled thermal water at bled wells out in a basin at the grand hotel toplice. it displays co2 bubbling activity. the average water temperature between may 2002 and april 2003 was 21.9 ± ± 0.2°c. the water is used for bathing and drinking. instruments are installed at the bottom of the collecting basin, from where the water is released into the swimming-pool. the average electrical conductivity of spring water between may 2002 and april 2003 was 869 ± 21 µscm–1. no correlation between meteorological parameters and electrical conductivity or water temperature was observed (fig. 7). the only two earthquakes during the monitoring period occurred on may 22, 2002 with a ml of 0.9 and on june 7, 2002 with a ml of 1.2 (table i). in both cases, the measuring station was outside the area, defined by the strain radius (e / d = 2.0). no anomalous values of electrical conductivity or water temperature were detected at that time. the electrical conductivity suddenly decreased from 895 to 860 µscm–1 in september 2002 and then returned to the previous value in april 2003, which cannot be attributed to seasonal variations, because of the rapidity of the changes; on the other hand water temperature changes were smaller and of shorter duration. a longer monitoring period is required to explain this decrease in electrical conductivity. the correlation coefficient between electrical conductivity and water temperature was 0.63. the ca, k, na, mg and si concentrations decreased in the period from december 2002 to march 2003, when no significant precipitation was recorded (figs. 7 and 8). thus, inflow of ca, mg, k, na and si to thermal water resevoir from meteoric water was much reduced during this time. eh and ph values measured monthly from february to may 2003 were relatively stable, and averaged 220 mv and 6.7, respectively. fig. 6. rn concentration (kbqm–3), earthquakes (ml), rainfall (mm) and the level of the tolminka river (cm) in the thermal spring at zatolmin between december 2002 and february 2003. 81 geochemical and geophysical monitoring of thermal waters in slovenia in relation to seismic activity the average radon concentration between may 2002 and april 2003 was 9 ± 2 kbqm–3. the radon curve showed a trend of slow decrease with small fluctuations. data accumulated in a quiescent period is useful as a baseline for recognizing possible precursory changes in the future (nakamura and wakita, 1985). 5. conclusions much of the variation in radon concentration, electrical conductivity, water temperature, ion concentration, ph and eh in the thermal spring at zatolmin could be assigned to nontectonic causes, including rainfall and the level of the tolminka river. in the period between june and november 2002, radon concentration was always above 1σ and a few times it even reached 2σ over the mean value. but in the period between january and april 2003 radon level was constantly up to 1σ below the mean value. if precipitation were the main cause for infig. 7. geochemical data measured in the thermal spring at bled between may 2002 and april 2003. the first diagram (starting from the top) shows radon concentration (kbqm–3) and earthquakes (ml). e is the distance between the measuring station and the epicentre, while is d the strain radius. the second diagram shows electrical conductivity (µ scm–1) and water temperature (°c), and the third diagram shows rainfall (mm) and barometric pressure (hpa). fig. 8. ion concentrations (mgl–1), electrical conductivity (µ scm–1) and water temperature (°c) in the thermal spring at bled between may 2002 and april 2003. 82 andreja popit, janja vaupotič and tadej dolenec creased radon level between june and november 2002, then we would expect a larger increase in radon concentration also during the rainfall in december 2002 and from march to april 2003. thus, radon behaviour cannot be simply related to meteorological factors. in the period with radon levels exceeding 1σ over the mean value, six earthquakes with e/d ratio equal or below 1 occurred while in the period with radon concentration up to 1σ below the mean value, three earthquakes with e/d ratio above 1 occurred. this fact is in accordance with the theory of the stress-strain field defined by dobrovolsky. our interpretation of physical processes causing radon variations will be improved after a longer monitoring period. changes in electrical conductivity and water temperature in the thermal spring at bled showed no correlation with meteorological parameters. fluctuations of radon concentration were insignificant due to low seismic activity over the period. data accumulated in a quiescent period will be useful as a baseline for recognizing possible precursory changes in the future. continuous monitoring (frequency of sampling and analyses once per hour) of radon concentration, electrical conductivity and water temperature in spring waters, together with grab sampling and analyses (once per month or immediately after the seismic event) of dissolved ion concentrations constitute an important basis for studying the relationship between the measured parameters and seismic activity. further monitoring of geophysical and geochemical parameters at zatolmin and bled thermal springs is required to be able to better distinguish 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10.4401/ag-6341 2 the outline of this paper is as follows. in section 2, a brief introduction of the mipas instrument aboard envisat, which was operational from 2002 until 2012, is given. then, the processing scheme from level 1b to level 2 is briefly described in section 3. section 4 recalls the history of the algorithm development, and section 5 presents the software validation concept. section 6 concludes this paper. ii. the mipas mission mipas, being one of three atmospheric chemistry instruments aboard esa's largest earth observation satellite envisat, is a limb-sounding ftir (fourier transform infrared) spectrometer observing atmospheric data in the mid-infrared range from 4.15 µm to 14.6 µm with high spectral resolution. the measured data set is very rich in information as each spectrum comprises of the order of 50000 spectral samples. mipas was designed to detect limb emission spectra in the earth’s lower, middle and upper atmosphere (cf. figure 1), covering tangent heights from 5 to 150 km. during the operational phase, mipas gathered spectrally high resolved data in five spectral bands, which can be used for absorption spectroscopy. after in-flight anomalies of the mipas moving mirror, starting in march 2004, the instrument operation had to be suspended. after an analysis period, however, mipas could be recovered starting from a 35% duty cycle in january 2005 to a full mission with an optimized spectral resolution (see also [raspollini et al., 2013]). the mipas in-flight operational performance stabilized gradually, until the observed anomaly pattern that led to the mission interruption had practically disappeared. as a result of the modified spectral resolution, also the ground processing algorithms needed to be adapted accordingly. mipas was operated in two viewing ranges, a rearward viewing range (in anti-flight direction of the satellite) and sideways. further, instrument calibrations were used for regular radiometric calibrations (using offset calibrations and measurements from an internal blackbody) and line-of-sight calibrations to correct for pointing errors. for further readings about the mission and the mipas instrument refer to [raspollini et al., 2013]. iii. processing the mipas data processing takes place sequentially as presented in figure 2. the level 2 processing forms the final part of a processing chain, taking as input the level 1b (l1b) product, which contains calibrated spectra. it outputs atmospheric profiles of pressure, temperature and dedicated trace gases as a function of altitude. the number of different trace gases in the retrieval scheme has increased from initially six species for the first ml2pp version (o3, h2o, hno3, ch4, n2o, no2) to ten species for version 6.00 (plus: f11, f12, n2o5, clono2) and to 15 species for ml2pp version 7.00 (plus: f22, f14, cof2, ccl4, hcn). the recently added species spectral range: 685 2410 cm -1 (5 bands) 1020-1170 ab 1215-1500 b 1570-1750 c 1820-2410 d 685-970 a cm-1 spectral range: 685 2410 cm -1 (5 bands) spectral range: 685 2410 cm -1 (5 bands) 1020-1170 ab 1020-1170 ab 1215-1500 b 1215-1500 b 1570-1750 c 1570-1750 c 1820-2410 d 1820-2410 d 685-970 a cm-1685-970 a cm-1 fourier transform ir (ftir) limb sounding spectrometer day and night measurements figure 1: mipas measurement scheme mipas onboard processing envisat instrument source packets level 1a processing raw interferograms level 1a level 0 level 1b processing calibrated spectra level 1b atmospheric profiles level 2 level 2 processing space segment ground segment l e v e l 2 p ro c e s s in g l e v e l 1 b p ro c e s s in g processing data product hardware data flow legend scope of ml2pp figure 2: mipas data processing annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6341 3 show weaker spectral signatures either due to low atmospheric volume mixing ratio or low emission/absorption per molecule or both. the capability to retrieve such weak species demonstrates the quality of the level 2 processing algorithm scheme and requires high accuracy of the implementation. the general processing approach is schematically shown in figure 3. besides the l1b product, several auxiliary files (pointing information, spectral data, cross-section data, initial guess data, and program settings) are required for the level 2 processing. optionally, also european centre for medium-range weather forecasts (ecmwf) files can be provided to the ml2pp. the level 2 algorithm works in a scan by scan fashion, where one scan consists of a set of calibrated spectra at different tangent altitudes (sweeps). for each scan at first the pressure and temperature profiles (p,t) are retrieved from co2 observations assuming a fixed atmospheric co2 profile. with the retrieved pressure and temperature profiles then kept fixed, the different trace gas retrievals are performed one after the other. the initial guess is the starting point for each of the retrievals, and plays a crucial role for its convergence. depending on availability, the initial guess is an optimized composition of climatological profiles, ecmwf profiles (for p,t retrieval only), or retrieved profiles from the same or the previous scan. the retrieval itself is based on a levenbergmarquardt algorithm, an iterative scheme that combines the advantages of the newton-gauss algorithm and the method of gradient descent for finding the minimum of a specific function. in the case of the mipas l2 processing, the function to be minimized is the difference between the measured spectrum and a simulated spectrum that is produced by the current set of atmospheric profiles in the iteration. if the defined convergence criteria are fulfilled, the retrieval is successfully terminated. otherwise, a new set of atmospheric profiles is determined based on the difference of the spectra of the previous iteration. for detailed information on the algorithm refer to [carli et al., 2013]. iv. level 2 algorithm development in section 1, three level 2 processors have been introduced. the dedicated purposes and their role during the mission are summarized in this section. orm the scientific processor to be used as scientific reference was determined in a shoot-out of several approaches to the level 2 processing scheme. eventually, the ifac institute, florence, italy, has won this competition. it is interesting to note that all other independently developed processors are still being maintained, advanced, and tested on mipas data. this allows for inter-comparisons of all level 2 algorithms, which is extremely useful for validating the orm itself on algorithm level (see also [laeng et al., 2013]). perform p,t retrieval compute initial guess for p,t retrieval receive/store p,t and altitude profiles preprocessing read level 1b product & auxiliary files compute initial guess for vmr retrieval species #n perform vmr retrieval species #n receive/store vmr profile species #n last species? last scan? write level 2 product stop start select next species select next scan yes yes no no level 1b product auxiliary files ecmwf files level 2 product processing data product file i/o data flow legend figure 3: level 2 processing algorithm annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6341 4 ml2pp the ml2pp is the industrial reference implementation of the scientific core implementation orm. the main purposes of developing this instance are to • critically review the algorithm, • implement the processor in a high level language following standard coding rules, • implement the framework around the scientific core with read/write routines for the input and output files as they are used in the operational ground processing unit, • develop a software validation plan and carry out the validation against the orm with dedicated test procedures, • prepare documentation on the software, the interfaces, the validation report, etc. the history of the ml2pp started with the development phase as early as the mid 1990's which resulted in the first validated delivery in 1998. when the first operational mipas data were downloaded and fed into the ml2pp, the processor converged without problems. this proved the quality of the algorithm. the ml2pp eventually turned into an operational processor when the version v6.00 became the designated processor for offline reprocessing activities for the full mission in 2012, providing l2 data sets to the user community. between these milestones lie the activities related to continuous improvement of the mipas processing in general and the l2-processing in particular. a major upgrade of the processing-scheme became necessary after a malfunction of the mipas instrument in 2004, which was ultimately solved by reducing the spectral resolution in 2005. the ml2pp history is still ongoing as version 7.01 is the designated software tool for the reprocessing of all valid mipas l1b data into l2 products in 2014. by then, a rich collection of science and data quality improvements will have been implemented that have been elaborated by the mipas quality working group (qwg) members and supporters while analyzing and processing mipas data during 10 years of service. ipf the ipf was operational in the ground segment, thus providing the operational level 2 data sets during the mission from 2002 to 2012. the ipf instance of the algorithm is validated against the ml2pp. having three algorithms validated against each other has certain advantages, but this ultimately became a problem after adjusting the algorithm for the optimized mipas resolution, due to the instrument malfunction in 2004. the sequential adjustment from the orm via ml2pp to the ipf actually delayed the operational ground processing for years. in order to speed up the validation phase, it was discussed to drop either the ml2pp or the ipf from the validation chain. due to the unexpected end of mission, it never came to that decision, and the ipf remained operational in the ground segment until end of mission. for the reprocessing campaign of the complete mipas data set the ml2pp was chosen as operational processor. v. validation concept the software validation concept of mipas level 2 processing is a two-step procedure. first, the ml2pp is validated against the scientific instance orm. secondly, the ipf is validated against ml2pp (cf. figure 4). having three independently coded instances of the same algorithm lowers the probability of coding errors, which is a true benefit for such a complex algorithm that is prone to cumulative errors. although the same level of quality can also be reached with two instances instead of three, the choice of having a third implementation was mainly motivated by political and not scientific interest. a clear disadvantage associated with this two-step validation is the danger of having long adjustments of all software versions when algorithmic changes are made. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6341 5 the validation procedure itself is a process containing several steps, and follows different comparison criteria. in order to compare software outputs it is necessary to compare intermediate results, especially for algorithms containing so many calculations and iterations as for the ml2pp. thus, the algorithms have been designed to write intermediate test data sets used for validation. as an example, the pressure, temperature and volume mixing ratios are being written to files after every iteration cycle for comparison of intermediate results. the driving question for the comparison of two files is the allowed margin for the numbers being compared. ultimately, a set of margins basically defines by how much a physical process may deviate from a clear mathematical formulation. the margins initially proposed have been determined by running the orm algorithm on different hardware platforms and comparing the results in terms of statistical evaluations. these statistics campaigns should lead to margins of the order of the hardware precision. in practice, such a precision is impossible to reach in different implementations, e.g. different programming languages (fortran vs. c++), different numerical precision during calculations (floating point vs. double point variables), and different implementations of sub-routines (usage of external libraries vs. self-coded routines). moreover, the numerical precision is also affected by low-level calculations such as taking the logarithm of a number, which reduces the accuracy. also, the sequence of adding, subtracting, dividing, and multiplying numbers can be important when very big and very small numbers are involved. the margins certainly had to be increased taking into account these fundamental differences. an analytical approach was not feasible because of the algorithm’s complexity. especially the large numbers of iterations during the processing of several scans in sequence let errors accumulate, and thus further degrade the numerical accuracy between both algorithms. the choice for margins was then solved by comparing the intermediate ml2pp results with orm results for a robust test case where both algorithms converged after the same number of iterations to a very similar result. margins have been defined for all parameters written to the intermediate files and to the level 2 product file. ultimately, these margins are much higher than the initially proposed hardware precision, but are still small enough to state that the processing results of both different algorithms can be labeled as being ‘identical’. the margins for all values of the test outputs as well as for the level 2 output have been reported in a corresponding ‘test definition and procedure document’. in order to provide an example of the selected margins, some results are shown in the conclusion section. all margins and thus the precision requirements were fulfilled by orm vs. ml2pp and ml2pp vs. ipf. thus, the operational software is considered validated. vi. results and conclusion this paper discussed the three esa processors orm, ml2pp and ipf for mipas l1b to l2 processing with focus on the ml2pp by presenting its • general processing scheme, orm abc ml2pp ipf orm sdc development instance baseline reference consolidated subset of functions prototype instance operational instance level 2 product test data (teps) level 2 product test data (teps) margins fulfilled? yes validated no not validated level 2 product test data (teps) level 2 product test data (teps) margins fulfilled? yes validated no not validated ml2pp prototype instance 1. validation 2. validation figure 4: validation concept annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6341 6 • role during the mission, • software validation concept. the operational software validation is done by comparing orm results with ml2pp results. for this comparison some margins were defined for deciding whether the outputs can be considered ‘identical’. these comparisons are made for all values in the level 2 product and for intermediate results. as an example, the temperature and water vapour retrieval results of the scientific processor (orm) and of ml2pp are plotted on the left hand side in figure 5. both plot lines appear congruent, which is the expected result. on the right hand side, the numerical differences are plotted on a logarithmic abscissa. the allowed margins (vertical red lines) are usually percentage values of the maximum retrieval value. for water vapour retrieval, the allowed error is two percent of the maximum value (here: maximum is around 7.5 ppm). the error is well below that margin by more than one magnitude, which means that a very good match of the outputs is achieved with relative errors below 0.1 percent (here: error is lower than 0.0075 ppm). for temperature retrieval, two percent of the maximum would mean a margin of a few kelvin, which is higher than the mipas accuracy requirement of 0.1 k. therefore, the plots show a stricter absolute margin of 0.1 k. again, the error is well below that margin with accuracy better than 0.01 k. as a conclusion, software implementation validation is an important component of the entire mipas level 2 product validation chain. the ml2pp has been performing this task for the mipas mission since mid 1990’s. eventually, the ml2pp was chosen to provide the operational mipas l2 products since 2012. references [carli et al., 2013] b. carli, m. carlotti, s. ceccherini, m. höpfner, p. raspollini, m. ridolfi, l. santurri, and l. sgheri (2013). mipas level 2 algorithm theoretical baseline document, issue 6.0 http://www2.fci.unibo.it/~ridolfi/mipas/docs/at bd/atbd_issue_6.0.pdf (december 19, 2013). [raspollini et al., 2013] raspollini, p., carli, b., carlotti, m., ceccherini, s., dehn, a., dinelli, b. m., dudhia, a., flaud, j.-m., lópez-puertas, m., niro, f., remedios, j. j., ridolfi, m., sembhi, h., sgheri, l., and von clarmann, t.: ten years of mipas measurements with esa level 2 processor v6 part 1: retrieval algorithm and diagnostics of the products, atmos. meas. tech., 6, 2419-2439, doi:10.5194/amt-6-2419-2013, 2013. [laeng et al., 2013] laeng, a, hubert, d, verhoelst, t, von clarmann, t, dinelli, b.m., dudhia, a., raspollini, p., stiller, g., grabowski, u., keppens, a., kiefer, m., sofieva, v., froideveaux, l., walker, k.a., lambert, j.-c., zehner, c., the ozone climate change initiative: comparison of four level 2 processors for the michelson interferometer for passive atmospheric sounding (mipas), submitted to remote sensing of environment, 2013. 180 200 220 240 260 280 0 20 40 60 80 a lt it u d e [ k m ] temperature [k] temperature orm abc 3.0 ml2pp v7.00 0.0001 0.001 0.01 0.1 1 0 20 40 60 80 abs. error [k] a lt it u d e [ k m ] temperature error − (snr=96.5 db) error margin 0 2 4 6 8 0 20 40 60 80 [ppm] a lt it u d e [ k m ] h2o retrieval orm abc 3.0 ml2pp v7.00 0.0001 0.001 0.01 0.1 1 10 0 20 40 60 80 rel. error wrt max. ppm in % h2o error − (snr=69.6 db) a lt it u d e [ k m ] error margin figure 5: comparison of processor outputs 1 annals of geophysics, 66, 2023; doi:10.4401/ag-8881 o p e n acc e s s improving seismic hazard assessment in the mediterranean region rodolfo console1,2 and paola vannoli*,1 (1) istituto nazionale di geofisica e vulcanologia (ingv), rome, italy (2) center of integrated geomorphology for the mediterranean area (cgiam), potenza, italy article history: received july 27, 2022; accepted november 16, 2022 abstract this paper is intended as a short presentation of the main limitations affecting seismic hazard assessment, revisiting possible methods available in the literature to be applied for this purpose. the convergence of the african plate with the eurasian plate is the cause of the high seismic activity characterizing the mediterranean region, with particular intensity in its eastern part. it is clear that the associated seismic risk requires appropriate measures for its mitigation. seismic risk, the amount of resources that the community is expected to pay to earthquakes in the long term, is the product of three factors, such as seismic hazard, vulnerability and value of the exposed goods. as earthquakes cannot be prevented, seismic risk can be mitigated by improving our knowledge of seismic hazard, which is largely based on statistical analysis of historical earthquake catalogs. nevertheless, historical records are affected by problems of reliability, completeness and shortness, as they commonly span time lengths of the same order of magnitude or even shorter than the inter-event time of the strongest earthquakes produced by specific seismic sources. in this respect, alternative methods can be proposed for integrating and improving our knowledge of seismogenic processes, and estimating both time-independent and time-dependent occurrence rates of strong earthquakes. we propose the application of physics-based earthquake simulators, requiring the knowledge of a robust geological-geophysical seismogenic model. keywords: seismic hazard assessment; statistical analysis; historical earthquake catalogs; earthquake simulators; mediterranean region 1. introduction we know that great earthquakes are likely but rare events in the mediterranean area, and even earthquakes of relatively low magnitude can cause great damage if the buildings are of low quality or if there are in particularly unfavourable conditions. this may happen especially for communities that have not been affected by strong earthquakes since long time ago [valensise et al., 2017]. even earthquakes in the magnitude range 5-6 can be catastrophic, as demonstrated by the 2012 emilia sequence in italy (mw 6.1), which caused casualties and severe damage to monumental and industrial buildings. therefore, the high rate of seismicity over wide areas of the mediterranean region produces a high level of earthquake risk, which requires appropriate mitigation measures. rodolfo console and paola vannoli 2 as we describe in section 3, these measures should aim at optimizing the balance between the costs of protection and the cost of losses due to this particular kind of environmental risk. the approach to the treatment of this thread implies a reliable assessment of earthquake hazard. this contribution to the special issue of annals of geophysics devoted to environmental hazards in the mediterranean basin is intended as a short review of the state of the art and possible progress of seismic hazard assessment in this region. earthquake hazard assessment is performed by statistical methods, in terms of probability of exceedance of given earthquake intensities during a given time interval in a specific site (e.g. rundle et al., 2021 and references therein). this process is largely based on the available information concerning past earthquakes, with important limitations concerning the availability of data reported in the historical records. in the mediterranean basin we have some of the best historical seismic catalogs in the world [e.g. guidoboni et al., 1994; guidoboni and comastri, 2005] but there are necessary limitations to our knowledge when it comes to going far back in time. in fact, as the average recurrence time between successive large earthquakes of a seismogenic source is generally longer than the period covered by the historical catalogs, there are seismogenic faults for which no large earthquakes are reported, and for most of them only the latest strong event is known [e.g. diss working group, 2021 and reference therein]. these limitations of historical catalogs preclude the application of statistical analysis for seismic hazard assessment, even if it can be partly overcome by geological and paleoseismological investigations on specific faults (see section 5.1). in this contribution we conclude that: a) it is necessary to support the studies on seismic hazard with geophysical and geological data about earthquake sources, such as the geometrical sizes of active faults and their long-term slip rates, and b) the application of recently developed physics-based earthquake simulators could be a useful tool for the improvement of our capacity of understanding and forecasting the seismic processes. 2. seismotectonics of the mediterranean area the mediterranean basin is composed of a complex system of structures generated by the interaction between the eurasian plate and the african plate (figure 1). this region is the result of a complex geodynamic history: there is still no single comprehensive theory to describe the history of this system, and numerous models have been proposed. according to the plate tectonics theory, the two plates have approached with a rotational motion during the last 300 million years, during which the intermediate zones between the two plates were deformed, slipping and rotating against each other, locally either overlapping (thus initiating mountain building) or leaving room for the opening of new internal basins. the orogenic system that has affected the mediterranean region forms a very broad and diffuse boundary between the african and european plates that includes intervening microplates (most notably adria and anatolia), remnants of closed ocean basins (ophiolites and subduction melanges) as well as still open (eastern mediterranean) or newly opened (western mediterranean) oceanic domains [see figure 1; e.g. schmid et al., 2020; hasterok et al., 2022]. the process of orogenic formation of the whole alpine-himalayan chain was initially driven by the subduction of the tethyan ocean beneath eurasia and has continued even as the ocean has closed in many regions. the continentcontinent collisions with eurasia were heterogeneous in time, beginning with the collision of apulia (adria) with europe ca. 65 ma [e.g. hasterok et al., 2022]. the subduction of ionian oceanic crust underneath the southern margin of the european plate is still active in the calabrian, hellenic and cyprus arcs [figure 1; see diss working group, 2021 and references therein; https://diss. ingv.it/diss330/dissmap.html]. deep seismicity and tomography data depict a well-developed calabrian slab down to 500 km depth (figure 2). the calabrian subduction zone is characterized by an active back-arc basin in the tyrrhenian sea, related to the retreat of the calabrian slab toward southeast. the tyrrhenian sea and a wide tyrrhenian coastal sector are characterized by thinned crust, very high heat flow [e.g. della vedova et al., 2001], and a large number of thermal springs and co2 emissions [vannoli et al., 2021] due to astenospheric uplift. in the hellenic arc, the distribution of the seismicity depicts a well-developed subducting slab beneath the aegean sea down to a depth of about 180 km (figure 2). the cycladic region and the aegean sea as a whole are known to be regions of north-south back-arc extension and thinning continental crust. nineteen submarine volcanic edifices occur within this small rift zone, the largest of these being kolumbo which last erupted explosively in 1650 ad, causing significant damage and fatalities on the nearby island of santorini [e.g. nomikou et al., 2012]. https://diss.ingv.it/diss330/dissmap.html https://diss.ingv.it/diss330/dissmap.html seismic hazard in the mediterranean region 3 figure 1. tectonic plate and geologic province model of the mediterranean area with the three subduction zones of the calabrian, hellenic and cyprus arcs (hasterok et al., 2022). the main orogens of the mediterranean region are part of the larger alpine-himalayan system and are labeled in yellow. white arrows show the plate motion vectors based on the direction of plate movement (from usgs; https://www.usgs.gov/media/files/plate-boundarieskmz-file). figure 2. tectonic plate and geologic province model [hasterok et al., 2022] and earthquakes with a magnitude greater than 4, from 2004 to 2022, in the mediterranean area. circle size is relative to the magnitude of the earthquake (data from european-mediterranean seismological centre – emsc, https://www.emsc-csem.org). white arrows show the direction of plate movement (from usgs; https://www.usgs.gov/media/files/plate-boundaries-kmz-file). https://www.usgs.gov/media/files/plate-boundaries-kmz-file https://www.usgs.gov/media/files/plate-boundaries-kmz-file https://www.emsc-csem.org https://www.usgs.gov/media/files/plate-boundaries-kmz-file rodolfo console and paola vannoli 4 the hellenic subduction zone is thought to be capable of generating very large earthquakes and tsunamis. this is mainly testified by the historical, archeological, and geological data available on the 365 ad earthquake and on the effects of its ensuing tsunami [guidoboni et al., 1994]. in the island of crete, pirazzoli and co-authors [1982] estimated a coseismic uplift of 9 meters through the study of the raised and tilted shorelines. such large coseismic uplift are always associated with very high magnitudes [m > 8; e.g. davies et al., 2018]. east of the mediterranean, the cyprus subduction zone is the less known in the region. the seismicity distribution seems to depict a wadati-benioff zone beneath the anatolian microplate down to a depth of about 140 km in the western part of the arc, while in its eastern portion no earthquakes are recorded beyond 60 km depth. in the eastern part of the arc, the subduction zone seems to be evolving towards continental collision, as the crust is not properly oceanic but more probably thinned continental. however, there is no well-defined volcanic arc [diss working group, 2021 and references therein]. in the central mediterranean region, the adria microplate played a significant role during the orogenic phases. interactions between its boundaries and the surrounding continental plates controlled the evolution of the adria-verging orogenic systems. these plate interactions caused the building of both the apennine and the dinaride chains (figure 1). the present-day structural setting of the apennines is similar to the other mediterranean post-collisional thrust-belts, and consists of basement and cover thrust-sheets developed in an ensialic context [e.g. cosentino et al., 2010]. the geometry of the chain, the diachronism of the eastward migrating foredeep basins, and the different ages of the forethrusts are consistent with a regional foreland propagation model for the apennines [e.g. bally et al., 1988]. the apennines were therefore shaped by contraction at the front of the accretionary wedge overlying the subducting adria microplate followed by extension at the wedge rear in response to eastward slab rollback. the contraction in the frontal parts of the chains is usually accompanied by extension in the hinterland, with both compressive and extensional fronts migrating towards the foreland with time. the mediterranean region can be therefore subdivided into dominantly extensional and dominantly contractional domains. the former can be further split into rifting and mantle upwelling (i.e. along the peri-tyrrhenian belt), and post-orogenic extension (i.e. along the axis of the apennines, where the region has undergone continuous regional uplift over the past 1-2 my). the purely contractional domain extends along the frontal portions of the chains (i.e. the adriatic nearshore and offshore areas). the mediterranean basin is also characterized by the presence of the transform boundary between the eurasian plate and the anatolian microplate, the north anatolian fault. this is an active, ca. 1200-km-long, right-lateral strike-slip system which bounds to the north the anatolian microplate (figure 1). this continental transform fault accommodates ca. 20 mm/yr of right-lateral motion, with peak values (up to 300 nanostrain/yr) in the marmara sea. in the mediterranean region, the highest deformation rates are measured, excluding active volcanic areas, in the gulf of corinth, with rates up to ca. 400 nanostrain/yr, and along the north anatolian fault [serpelloni et al., 2022]. 3. seismic risk seismic risk can be defined quantitatively (from an economic point of view) as the expected cost that the community pays in the long term due to the damage caused by earthquakes, given synthetically by the product: r = s × v × q where r = risk = cost per time unit (€/year) s = hazard = frequency of events with intensity exceeding a given level (year–1) v = vulnerability = fraction of value lost by the goods subject to an event of such intensity q = total value of the goods exposed to risk (€) as factor s (seismic hazard) of this product cannot be reduced by any kind of human action, assuming a fixed q (total value), the only way to reduce r (seismic risk) is acting on factor v (vulnerability). the vulnerability can vary according to the construction criteria adopted for the defense against earthquakes. in principle, the total cost faced seismic hazard in the mediterranean region 5 by the community living in an earthquake prone region can be divided into two parts, as shown in a qualitative way by figure 3: the cost of the protection measures that is incurred before the events (for example, adopting stronger criteria in the design of buildings), and the cost that is incurred for the repair of the damage produced by high intensity events. the trends in these costs as a function of the severity of the prevention measures are generally in the opposite direction, respectively decreasing for that of damage, and increasing for that of prevention measures. the curve of the total costs that the community will pay for the earthquake in the long term, typically has a minimum point, which defines the optimal economic criterion for mitigating the risk due to earthquakes. a strategy that adopted a level of protection below the minimum point of the total cost curve (such as ignoring the problem) would not only be criminal towards the safety of people, but also uneconomic. on the contrary, the adoption of a higher level of protection, at the expense of a cost greater than the minimum, would ensure greater safety for the population. how much it would be appropriate to raise the level of protection thus becomes a problem to be solved at a decision-making-political level, in which the necessary balance between the resources that must be distributed among all the possible causes of risk must also be taken into account. 4. the validity of seismic hazard estimates as we saw in the previous section, the knowledge of the destructive characteristics of earthquakes, and their probability of occurrence in certain time intervals, is a fundamental component of the knowledge that leads to the assessment of seismic risk and its effective mitigation. italy, like all countries in the world with high seismicity and economically developed, has a specific legislation for constructions in seismic areas, which includes a map of seismic hazard of the national territory [figure 4; stucchi et al., 2004]. an update of the italian seismic hazard map has been recently proposed, even if has not yet formally adopted by the italian government [meletti et al., 2021]. the new model is based on the catalog of strong earthquakes occurring on the national territory [cpti15; rovida et al., 2022], and in limited way on the updated information of seismogenic sources recognized as capable of generating earthquakes of magnitude equal to or greater than 5.5 in italy [diss working group, 2021]. the questions we can ask ourselves are: 1) is it possible to evaluate the validity of the seismic hazard models adopted for the prevention measures, based on the observations? 2) is the model adopted by current legislation the optimal one to minimize damage to people and property caused by earthquakes? figure 3. qualitative plot of the costs incurred by the community due to earthquakes as a function of the level of protection adopted: ce cost of damages; ca cost of prevention measures; ctot total cost. rodolfo console and paola vannoli 6 it is obvious that a negative answer to the first question precludes the possibility of answering the second. some considerations that may help to answer the first question may derive from the analysis of figure 5, which shows the annual frequency of earthquakes occurred in the central apennines as a function of their threshold magnitude for three different time intervals, within which the cpti catalog can be considered complete. in figure 5 the red symbols refer to the catalog starting from 1950, the year from which it is estimated that all earthquakes with a magnitude of at least 4 have been detected. the green and blue symbols represent the data with threshold magnitudes equal to 5 and 6, respectively complete from the year 1871 and 1530. figure 5 shows that the trend of the accruals, on a logarithmic scale, follows an approximately straight decreasing trend, according to which the number of earthquakes of a given magnitude is roughly one-tenth of those of a magnitude less than one unit (the rule well known as gutenberg and richter law). assuming this law as valid, it would be possible to estimate for any seismic zone the expected frequency of earthquakes of any magnitude as large as desired, simply by extrapolating the trend of the diagram in figure 5, even if such earthquakes have not yet been actually observed. this possibility would allow to estimate the seismic hazard of an area, simply based on modern and reliable instrumental data. in reality, there is no certainty as to what are the maximum magnitude values up to which the gutenberg-richter law can be assumed to be valid. from figure 5 it is also very clear that the statistical uncertainty on the obtained values of the occurrence rates, represented by the height of the vertical bars associated with each symbol, grows with the increase of the magnitudes considered. in fact, as established by poisson’s theory of errors, this uncertainty increases as the number of events considered in the analysis decreases. figure 5 also shows, with black symbols, the rates obtained for the same seismic zone in central italy, by a simulation algorithm based on deterministic physical laws [console et al., 2018]. this type of algorithm is based on discretized geometrical models of earthquake sources and heuristic rules of nucleation and stopping of earthquake figure 4. seismic hazard map of the italian national territory expressed in terms of maximum ground acceleration with 10% probability of exceedance in 50 years [after stucchi et al., 2004]. seismic hazard in the mediterranean region 7 ruptures. it can generate, in a few hours of calculation on a computer with good performance, a synthetic catalog lasting 100,000 years, containing over 300,000 earthquakes with a magnitude greater than 4. the comparison between the trend obtained from the simulator and the one from the real catalog, it shows a close analogy in respect of the gutenberg-richter law, but also some discrepancies, as in the magnitude interval between 5 and 6, in which the simulator shows a seismic activity lower than that observed. the most important discrepancy concerns, however, the trend of the curves at the largest magnitudes (higher than 6.5) which are the most relevant for the purposes of seismic hazard estimates. as has already been said, for such large magnitudes the real catalog is based on a very limited number of observed events (even of only one, as in the case of the avezzano earthquake of 1915 of magnitude 7.1, shown in figure 5) and presents very significant uncertainties. for magnitudes greater than 6.5, the black curve, which represents the computer-simulated catalog rates, shows a tendency to decrease faster and faster than the observed data. this depends on the fact that the fault model provided with the input data to the simulation algorithm places physical limits on the energy (and therefore on the magnitude) released by the fracture phenomenon that generates earthquakes: a clear reason that explains the existence of an upper limit of magnitude beyond which the gutenberg-richter law cannot be applied. 5. long-term time dependent hazard models 5.1 renewal models seismic codes adopted by developed countries are generally based on a time-independent occurrence probability of earthquakes, modelled by the poisson hypothesis. however, it is commonly observed that the earthquake figure 5. cumulative distribution of the annual rate of earthquakes as a function of magnitude, obtained from a synthetic catalog of 100,000 years for the central apennines (black dots), compared with those obtained from the parametric catalog of italian earthquakes (cpti15) in the same area for three different time intervals (red, green and blue symbols), within which the catalog itself can be considered complete. the vertical bars indicate the statistical uncertainty of the occurrence rate values obtained from historical and / or instrumental observations [after console et al., 2018]. rodolfo console and paola vannoli 8 occurrence rate in limited areas is strongly variable in time, both in the short(earthquake clustering) as in the long-term (pseudo-periodicity). long-term pseudo-periodicity of strong earthquakes is a simple consequence of the reid [1910] elastic rebound theory. a more recent expression of this theory can be recognized in the characteristic earthquake model [schwartz and coppersmith, 1984; wesnousky, 1994]. according to this model, belonging to the category of renewal models, strong earthquakes have a general inclination to repeat themselves along the same fault segment or plate boundary. the occurrence of a characteristic earthquake ruptures the entire segment and relieves tectonic stress within the segment. therefore, the regularity of the inter-event time is just a simple physical consequence. according to this hypothesis, the earthquake hazard is small immediately following the previous large earthquake and increases with time since the latest event on a certain fault or plate boundary. hence, the earthquake occurrence can be regarded as a quasi-periodic process [mccann et al., 1979; shimazaki and nakata, 1980; nishenko and buland, 1987]. console et al. (2002) carried out a statistical comparison of the characteristic earthquake model with respect to the plain memory-less poisson model, using data extracted by the database of earthquake recurrence data from paleoseismology [pantosti, 2000], limiting the analysis to four trenches sites in the mediterranean region (figure 6) for which at least two inter-event times were available from paleoseismological data. for their test they considered the parameter cv (also named aperiodicity), defined as the ratio between the standard deviation σ and the mean inter-event time tr of the observed inter-event times. this parameter is equal to 1 for a uniform random distribution. cv < 1 denotes a periodical occurrence of events and cv > 1 characterizes clustering of events. to take into account the uncertainties of the observed inter-event times, they used a monte carlo procedure, obtaining the following values: cv = 0.368 ± 0.148 for the irpinia fault, cv = 0.187 ± 0.017 for the fucino fault, cv = 0.806 ± 0.102 for the el asnam fault, cv = 0.656 ± 0.214 for the skinos fault. even if these values, especially that of the fucino fault, appear substantially smaller than 1, they are biased by the small number of inter-event times used in the test. so, by proceeding with the test in a rigorous and quantitative way, console et al. [2002] concluded that the null hypothesis of the poisson inter-event distribution can be rejected with a confidence level of 99.5% for the fucino fault, but it can be rejected only with a confidence level between 90% and 95% for the irpinia fault, while it can not be rejected for the other two cases. figure 6. location of the four faults for which a seismic history was built on the basis of paleoseismological data extracted from the database of earthquake recurrence data from paleoseismology [after console et al., 2002]. seismic hazard in the mediterranean region 9 5.2 effect of stress-interaction among faults taking a further step towards the physical modeling of the seismogenic process, statistical models were built that take into account the elastic interaction between faults. these models require the calculation of the actual stress change (coulomb stress change) produced by a seismic dislocation of a given fault on the probable fracture plane of another fault. using suitable algorithms, this stress variation (positive in the sense that it brings a fault closer to the fracture condition, negative in the sense of moving it away from this condition) is respectively translated into an advance or a delay of the next seismic event with respect to what is expected in conditions unperturbed. console et al. [2007, 2010] applied a model of this type to the database of the individual seismogenic sources (version 3.0.2; 2006; https://diss.ingv.it/data) for the area of the central-southern italy. taking into account the possible interaction of the faults with each other, in addition to the time elapsed since the last earthquake occurred on each single fault, it is possible to calculate the expected rate of an earthquake of the same type at the present time (hazard rate), and from the latter, the conditional probability of having such an earthquake in a given future time period. table 1 shows some results obtained from the analysis conducted through this modeling on the two seismogenic sources of the central-southern apennines shown in figures 7a and 7b. taking into consideration the sulmona fault (figure 7a), it can be seen that the avezzano earthquake of 1915 induced a negative coulomb stress on it, which tends to lighten the value accumulated by deformations of tectonic origin. on the contrary, the earthquakes that occurred on the transcurrent faults located east of sulmona provide a positive contribution, i.e. they tend to accelerate the tectonic loading process, and therefore to bring the moment of the fracture closer. the result, shown in table 1 (first column of values), compares the probability of occurrence of the next characteristic earthquake for the next 50 years, estimated on the basis of different hypotheses. for the model called “poissonian”, based only on the time of recurrence, and completely independent of time, the maximum probability of a new event in the next fifty years is 5.3%. taking into account that the time passed since the last earthquake that completely broke the sulmona fault, that of 3 december 1315, is 692 years, based on a renewal model, with memory, this probability increases to 8.4%. but if we also consider the fact that a possible future event may be favored by a variation in effort determined by what happened in the surrounding areas after 1315, this probability rises further to 11.8%. it must be noted that the most recent version of the diss database (https://diss.ingv.it/) re-examines and excludes the attribution of the 3 december 1315 earthquake to the sulmona fault. if the latest large event had occurred earlier, the current probability of occurrence obtained by the renewal model would be significantly higher. sulmona basin melandro‑pergola date of latest event 1315.12.3 1857.12.16 slip rate (mm) 0.24 ± 0.06 0.11 ± 0.04 recurrence time (years) 2300 ± 1370 4300 ± 3700 max. poisson probability for the next 50 years 5.3% 8.8% elapsed time (years) 692 150 max. renewal probability for the next 50 years 8.4% 1.8% max. δcff (mpa) 0.15 0.19 max. δt (years) 132 394 max. modified probability for the next 50 years 11.8% 18.6% table 1. results of the elastic interaction analysis for two seismogenic sources of the central-southern apennines [after console et al., 2007, 2010]. https://diss.ingv.it/data https://diss.ingv.it/ rodolfo console and paola vannoli 10 b) figure 7. maps of the stress variation caused by seismic events on the sources of the diss catalog version 3.0.2 (2006) following (a) the sulmona earthquake of 3 december 1315, and (b) the val d’agri earthquake of 16 december 1857. the changes are calculated by assuming on each point of the map an expected mechanism of any induced earthquake equal to that of the past events respectively considered in (a) and (b) [after console et al., 2008, 2010]. a) seismic hazard in the mediterranean region 11 going a little further south and taking into consideration the melandro-pergola fault, one of the two that gave rise to the val d’agri earthquake of 16 december 1857, we can see in figure 7b that the irpinia earthquake of november 1980, for which three sources have been identified that ruptured at a distance of only 20 seconds from each other, had a positive influence, that is, it tends to accelerate the next event on this fault. so, it is as if the seismogenic process had accelerated or, as they usually say, as if the earthquake clock had been put forward again. therefore, in this second case, as shown by the second column of the values of table 1, the maximum poissonian probability (uniform over time) for the next 50 years is equal to 8.8%. in fact, the minimum recurrence time (average value minus the uncertainty limit) assumed for the melandro-pergola fault (600 years) is less than that of the sulmona fault (930 years). however, for the melandro-pergola fault (val d’agri) the last characteristic earthquake occurred, so to speak, only 150 years ago. this means that the conditional probability provided by the renewal model drops to just 1.8% for the next 50 years. finally, including in the calculations the potential effect of the 1980 irpinia earthquake which, as mentioned, is positive, the conditional probability rises again to 18.6%. 5.3 simulation by a physics-based earthquake simulator as shown in figure 5 of section 4, here in the following we quote a recent case study to show the way by which the application of an earthquake simulator algorithm can provide robust information on the seismogenic process, in comparison with that inferred from a catalog of real observations of limited duration. console et al. [2022] applied a physics-based earthquake simulation algorithm to the seismicity of the corinth gulf fault system, obtaining a catalog spanning 100,000 years and containing more than 360.000 m ≥ 4.2 events with a maximum likelihood b-value equal to 1.07. the spatio-temporal pattern of large earthquakes exhibits features that are comparable with the observations. more than 40% of the m ≥ 6.0 earthquakes in the simulated catalog are caused by the simultaneous ruptures of significant parts of more than one segment, which appears to be a relevant fraction. in extreme cases, strong earthquakes rupturing five segments simultaneously are noted six times in the 100 kyr simulation period. typically, these events ruptured adjacent segments for a total length of 80-85 km, releasing a seismic moment of 1.3-2.3*e19 nm, equivalent to a 6.7-6.8 magnitude earthquake, which seems realistic for the study area. this supports the hypothesis that multi-segmented ruptures are necessary ingredients for the simulator to produce events of magnitude consistent with the largest historical magnitudes (m ≥ 6.4). figure 8 shows the annual cumulative frequency-magnitude distribution (fmd) of the earthquakes in the 100 kyr simulated catalog compared with the instrumental earthquake catalog compiled by the geophysics department of the aristotle university of thessaloniki (gd-auth, https://doi.org/10.7914/sn/ht) for the period january 1965 – september 2020. the rate of earthquakes with m ≥ 4.2 in the simulated catalog is 3.67/yr, while the corresponding rate of observed earthquakes with m ≥ 4.2 in the observed instrumental catalog is 2.98/yr. the duration of 55 years of this instrumental catalog, shorter than the typical inter-event time for m ≥ 6.0 earthquakes on single fault-segments, appears quite short for representing the long-term behavior of this fault system. thus, the comparisons between the long-term features must be retained as quite questionable. figure 8 further shows that the fmd of the simulated catalog does not follow a straight line as expected according to the gutenberg-richter law, but exhibits a change in its slope from smaller than 1.0 to larger than 1.0 around magnitude 4.9. this case could be justified by the fact that the source model adopted in our simulation does not include the numerous small sources, capable of producing only small and moderate magnitude events, which coexist in the brittle part of the crust along with the major sources. console et al. [2022] compared the changes in seismicity rates observed in their simulated catalog with those ascribed to the phenomenon of accelerated moment release (amr), frequently observed for years to tens of years before, and over tens to hundreds of kilometers from the epicenter of the anticipated main shock. the simulated catalog of 100,000 yrs was analyzed by a stacking technique, counting the total number of events with m ≥ 4.2 that preceded or followed any other event of m ≥ 6.0 in a time window of ± 100 yrs, in bins of 1 yr, and within a radius of 20 km. the result of this analysis is shown in figure 9, where an acceleration of seismic rate starting about 40 yrs before the occurrence time of any m ≥ 6.0 earthquake is clearly depicted. figure 9 also shows that quiescence follows the aftershock decay and takes 40-60% of the seismic cycle. activity is then observed to build up, to a more or less steady background rate, during the latter part of the cycle. a simple explanation for this behaviour is that the regional quiescence that follows the aftershocks results from the region being within the stress https://doi.org/10.7914/sn/ht rodolfo console and paola vannoli 12 figure 8. cumulative frequency – magnitude distribution of the earthquakes (m ≥ 4.2) in the 100,000 yrs simulated catalog (red diamonds) compared with observations, i.e. the events (1965-2020) whose hypocenters are within 10 km from the fault planes (blue triangles). error bars are provided for the observation. the straight red and blue dashed lines represent their linear regression, respectively. the grey dashed line shows the slope of b=1 for reference [after console et al., 2022]. figure 9. stacked number of m ≥ 4.2 events in the 100 yrs period in bins of 1 yr before and after an m ≥ 6.0 mainshock and within a radius of 20 km, in the 100,000 yrs simulated catalog of the cgfs [after console et al., 2022]. seismic hazard in the mediterranean region 13 shadow of the mainshock. the slip is highly variable within an earthquake rupture, and the fault can be considered to be composed of elements that are heterogeneous at all scales coupled by longand short-range elastic forces. 6. conclusions in this short, and necessarily incomplete review on seismic hazard, we have seen that the mediterranean area is subject in many parts to seismic activity of considerable intensity, which requires adequate prevention measures to ensure reasonable mitigation. in the absence of any possibility of formulating deterministic forecasts of earthquakes, statistical methods are used, which provide hazard estimates in terms of the probability of exceeding certain values of the effects of earthquakes. after all, even weather forecasts, despite being largely based on deterministic physical formulations, never express the certainty of future phenomena, but announce them within margins of error. we have also seen that the estimate of the seismic hazard is strongly linked to the information available about past earthquakes, but with serious limitations due to the rarity of large-magnitude earthquakes, which are the ones that determine the main component of the risk. from data sets with such a small number, no significant inference can be obtained by means of statistical analyses. not only that: given that the return times of large earthquakes generated by a seismic source (of the order of thousands of years) are typically longer than the period covered by the historical catalogs available, it can easily be assumed that the strongest earthquakes of a particular source seismic may not yet have been observed and reported in the catalogs themselves. at the same time, this can overestimate the risk of areas where a major earthquake actually occurred relatively recently. another typical limitation of the presently used seismic codes can be found in the full adoption of a memory-less poisson hypothesis of the seismogenic process. it can be shown that, at least in some cases, a renewal timedependent model can provide a better likelihood for earthquake forecasting. this alternative approach could be used in a decision-making process of optimization of the resources available for long-term earthquake risk mitigation measures. from the above considerations, it follows the need not to limit studies for the assessment of seismic hazard to the analysis of historical and / or instrumental seismic catalogs, but to integrate them with information related to the physical and geological characteristics of the sources of earthquakes (for example, the geometric dimensions of the active faults and the average velocity of the relative sliding motion between two plates). our conclusion is in agreement with what had been already stated by burrato et al. [2012], in their study of the emilia, 2012 earthquakes. moreover, in a more recent paper valensise et al. [2020] concluded that “given the typical length of the seismic cycle in italy (centuries to millennia), the mutual interaction between historical earthquake data and geological and instrumental evidence may be the only way to constrain a number of seismological issues that would be very hard to address using instrumental data alone”. in this respect, the application of earthquake simulators based on the physical modeling of seismogenic fault systems can constitute an additional tool contributing to a better understanding and predictability of seismogenic processes. acknowledgements. we acknowledge the contribution of all the authors of the papers quoted in this article, whose results were included as basic ingredients in several parts of our review. we are grateful to the ae, prof. eleftheria papadimitriou, and two anonymous reviewers for their encouraging comments and suggestions. references bally, a. w., l. burbi, c. cooper and r. ghelardoni (1988). balanced sections and seismic reflection profiles across the central apennines, mem. soc. geol. it. 35, 257-310. burrato, p., p. vannoli, u. fracassi, r. basili and g. valensise (2012). is blind faulting truly invisible? 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(1994). the gutenberg-richter or characteristic earthquake distribution, which is it, bull. seismol. soc. am., 84, 6, doi:10.1785/bssa0840061940. *co r r e s po n d i n g a u t h o r: paola va n n o l i, istituto nazionale di geofisica e vulcanologia, rome, italy e‑mail: paola.vannoli@ingv.it https://doi.org/10.13127/sh/mps04/ag https://www.sciencedirect.com/journal/international-journal-of-disaster-risk-reduction https://www.sciencedirect.com/journal/international-journal-of-disaster-risk-reduction http://dx.doi.org/10.1785/bssa0840061940 mailto:paola.vannoli@ingv.it vol51,2_3,2008 397 annals of geophysics, vol. 51, n. 2/3, april/june 2008 postseismic deformation and body forces shaping the apennines and adjacent sedimentary basins in umbria-marche riccardo e.m. riva (1) and abdelkrim aoudia (2) (1) dept. of earth observation and space systems, delft university of technology, delft, the netherlands (2) earth system physics section, the abdus salam international centre for theoretical physics, trieste, italy abstract the geodynamic complexity of the apennines and adjacent sedimentary basins in umbria-marche (north-central italy) makes the dynamics of the present day deformation and its relationships with the seismicity less well understood. in this paper, we argue that, further to buoyancy forces, postseismic deformation of earthquakes taking place on the apennines contributes to the regional deformation. we investigate the interaction between the normal faulting system responsible of the 1997 umbria-marche earthquake sequence (colfiorito fault) and the low angle normal faulting system bordering the sedimentary basins, namely the altotiberina fault. we set-up a 2d finite element model of the lithosphere-asthenosphere accounting for lateral heterogeneities and investigate how this heterogeneous structure is capable of localizing strain under the umbria-marche sedimentary basins, providing a way for the colfiorito fault to influence the evolution of the altotiberina fault. we show how the two different length and time scale processes, namely postseismic deformation and buoyancy, are complementary in shaping the apennines and adjacent sedimentary basins. the high resolution deformation patterns modeled in this study can hardly be reproduced by a model accounting only for external forces such as a rotating or subducting or retreating adria. key words viscoelastic relaxation – aseismic slip – body forces – normal faults 1. introduction the geodynamic complexity of the apennines and adjacent sedimentary basins in umbria-marche (north-central italy) makes the dynamics of the present day deformation and its relationships with the seismicity less well understood. the recent 1997-1998 umbria-marche earthquake sequence clearly highlighted complex deformation processes both in its coseismic (e.g. amato et al., 1998; zollo et al., 1999; salvi et al., 2000; chimera et al., 2003) and postseismic (riva et al., 2000; aoudia et al., 2003; riva et al., 2007) phases. the umbria-marche crustal normal faulting seismic sequence, which consisted of three moderate earthquakes (26/09/97 at 00:33, mw=5.7; 26/09/1997 at 09:40, mw=6.1 and 14/10/1997 at 15:23, mw=5.6), ruptured three distinct fault segments (e.g., galadini et al., 1999; meghraoui et al., 1999; barba and basili, 2000). in the same region an earthquake occurred in the upper mantle, on march 28, 1998, mw=5.2 (e.g., olivieri and ekstrom, 1999; chimera et al., 20003) highlighting the importance of deep processes in the lithosphere. the lithosphere velocity models beneath central italy clearly highlighted the important lateral heterogeneity of the physical properties of the crust-uppermost mantle system along with pronounced variations of the mailing address: dr. riccardo e.m. riva, dept. of earth observation and spaces system, delft university of technology, kluyverweg 1, 2629 hs delft, the netherlands; e-mail: r.e.m.riva@tudelft.nl vol51,2_3,2008 4-03-2009 10:28 pagina 397 398 r.e.m. riva and a. aoudia geometry of both the crust-mantle and lithosphere-asthenosphere boundaries (e.g. agostinetti et al., 2002; chimera et al., 2003; mele and sandvol, 2003). these models exhibit clear evidence of lithospheric roots without any continuous slab. this would imply that the slab has been eroded and no engine is actually left to drive relative subduction processes. therefore our favorite model of the dynamics of the ongoing continental deformation is the one reported by aoudia et al. (2007) that invokes body forces instead of external forces to satisfy the recent kinematics. aoudia et al. (2007) have shown that the buoyancy forces resulting from the heterogeneous density distribution in the lithosphere govern the present day deformation within central italy and can explain regional coexisting contraction and extension at shallow depth and unusual distribution of sub-crustal earthquakes. their modeled uppermost mantle flow supports the lithospheric delamination beneath the peninsula and provides a unifying background for petrologic and geochemical studies of recent magmatism and volcanism in tuscany. further to the buoyancy forces already reported in aoudia et al. (2007), this paper argues that postseismic deformation of earthquakes taking place on the apennines contributes to the regional deformation. we investigate the interaction between the normal faulting system responsible for the 1997 umbria-marche earthquake sequence (colfiorito fault, cf) and the low angle normal faulting system bordering the sedimentary basins, namely the altotiberina fault (af) (e.g. boncio and lavecchia, 2000; chiaraluce et al., 2007). by means of a half-space model we investigate the effect on the cf of prolonged slip on the af. after that, we set-up a 2d finite element model of the lithosphere-asthenosphere accounting for lateral heterogeneities and investigate how this heterogeneous structure is capable of localizing strain under the umbria-marche sedimentary basins, providing a way for the cf to influence the evolution of the af. 2. secular postseismic relaxation when approaching the study of postseismic deformation in the area of the 1997 colfiorito earthquake, different mechanisms have to be taken into account (e.g. viscoelastic relaxation, afterslip, poroelastic relaxation…). here, we are not interested in the details of present day relaxation (see, e.g., riva et al., 2007): instead, we want to estimate the cumulative impact of viscoelastic relaxation due to repeated seismic events in the area. according to historical records (boschi et al., 2000), the largest events that occurred in the direct neighborhoods of the colfiorito fault (cf) are the 1279 camerino in the north and the 1328 norcia in the south, with magnitudes 6.6 and 6.2 respectively. we model the cumulative effect of those two earthquakes by means of a 50-km-long normal fault rupturing at a depth of 8 km and dipping 45 degrees to the sw, which approximately corresponds to an extension of the main fault of the 1997 sequence by 15 km to the north and 25 km to the south. the moment magnitude is equivalent to the cumulative effect of a 6.6 and 6.2 event (m0 = 6x1018 nm). the full relaxation of this fault is modeled by means of a normal modes approach (sabadini and vermeersen, 1997), with a stratified earth model comprising a 15-km elastic upper crust, a moho at 30 km, a 20-km-thick mantle wedge and a homogeneous mantle. since we discuss the fully relaxed model (i.e. at infinite time), the viscosity profile which controls the temporal variations of the relaxation process is here irrelevant, the key earth parameter being the thickness of the upper elastic layer. results for vertical relaxation are shown in fig. 1, where the thin solid lines describe the surface projection of the boundaries of the three main patches that represent the altotiberina fault (af) (boncio and lavecchia, 2000), starting at the surface in the sw and dipping towards the ne, and smaller open rectangles represent the surface projections of the three main faults of the 1997 sequence (from the north: the 9:40 event on 26 sept., the 0:33 event on 26 sept, and the sellano event on 14 oct.). we see how the whole area between the cf and the surface expression of the af is dominated by subsidence: it is noteworthy; that in the same area we find deep sedimentary basins and most of the spread microseismicity observed in the umbria-marche apennines (deschamps et al., vol51,2_3,2008 4-03-2009 10:28 pagina 398 399 postseismic deformation and body forces shaping the apennines and adjacent sedimentary basins in umbria-marche 1989). the maximum deformation amounts to about 22 mm, providing a secular rate of 0.03 mm/a if we assume a recurrence time of 700 a, as suggested by pantosti et al. (1999) after interpreting the 1997 sequence as a reoccurrence of the 1279 earthquake. since the upper crust has brittle properties, modeled vertical relaxation at the surface is representative of the displacement of the whole seismogenic layer. therefore, we can project the vertical motion onto the af to obtain average slip rates between 0.02 mm/a and 0.12 mm/a, which are much smaller than the value of 1 mm/a suggested by chiaraluce et al. (2007) for the northern segment of the af. the average slip rates we have computed are meant to represent the long term effect of the cf relaxation on the evolution of the af. therefore, we are not interested in a detailed description of the relaxation process, which would have to take into account additional factors such as time-variable relaxation and nearly-free shear stress conditions on the af. depending on the viscosity profile, deformation rates in the first few years after each earthquake can easily reach values of several millimeters per year, as we discuss in a later section. moreover, allowing the af to slip under specific conditions would surely have an impact on the stress evolution within the upper crust that we have here modeled as a continuous elastic layer. however, the use of a more sophisticated model would not have a strong impact on the purpose of this section, i.e. showing that relaxation of the cf is capable to load the af, but it cannot represent the primary loading mechanism. 3. slip on the altotiberina fault since it appears that the average rates of postseismic relaxation due to earthquakes in the colfiorito area account for only a small fraction of the af slip, here we explore the possibility of an opposite interaction, namely an active role of the af in the evolution of the umbriamarche fault zone. for this purpose, we model coand post-sesimic deformation due to a 50km-long ne-dipping normal fault representing the af. following boncio and lavecchia (2000), we distinguish three fault patches along the dip, with the shallowest (0-4 km depth) and deepest (7-14 km depth) patches dipping 30o and the middle patch dipping 13o. we model the coseismic response by means of a half-space model (feigl and dupre, 1998) and viscoelastic relaxation as in the previous section. figure 2 shows the total (elastic plus full relaxation) vertical deformation due to 70 cm of slip on the af, consistent with a rate of 1 mm/a (chiaraluce et al., 2007) during a 700-year seismic cycle (pantosti et al., 1999). the whole area under the af is dominated by subsidence, with a maximum due to the elastic component where the af intersects the free surface and a second maximum due to viscoelastic relaxation below the cf. the large subsidence below the cf reaches a value of about 35 cm, comparable with the average slip accommodated on the largest faults during the recent 1997 sequence (zollo et al., 1999), suggesting that the af is capable of actively loading the cf throughout a seismic cycle. fig. 1. vertical postseismic relaxation due to slip on a 50-km-long normal fault, dipping 45 degrees to the sw and representing a prolongation of the largest shock of the 1997 sequence (northernmost open box). the three small open boxes represent the surface projections of largest shocks of the 1997 sequence, while the large boxes with bounded by thin lines represent the three segments of the altotiberina fault. vol51,2_3,2008 4-03-2009 10:28 pagina 399 400 r.e.m. riva and a. aoudiar.e.m. riva and a. aoudia 4. postseismic relaxation with a lateral heterogeneous lithosphere the first part of our study focused on the average features of postseismic relaxation after earthquakes on the cf. the purpose of the present section is to discuss an improved postseismic relaxation model consistent with lateral crustal variations present in the central apennines. by means of a 2d finite elements approach (govers and wortel, 1995), we model a sw-ne section through the central apennines extending for 250 km at each side of the cf, where the domain size has been chosen to minimize the effect of boundary conditions on the area of interest. lateral variations are introduced according to the results published by chimera et al. (2003). a large level of heterogeneity characterizes the topography of the moho, which is located at a depth of 30 km under the cf area and at 38 km under the adriatic sea, whereas on the left side it presents a minimum of 28 km west of the af and finally deepens to 33 km towards the tuscan domain. according to chimera et al. (2003), a 20-km-thick layer of hot lithospheric material, called mantle wedge, underlies the moho beneath the tuscan magmatic complex and one large part of the central apennines. this layer dies out before reaching the adriatic coast, where a colder thermal profile is expected is also observed by heat flow studies (della vedova et al., 2000). in our model domain, the two end members are represented by the tuscan magmatic domain on the left, with a thin lithospheric lid leading to high temperature and low viscosities, and the adriatic domain at the right side, with a thick lithospheric lid and a consequent cold isotherm and high viscosities. in the middle of the domain, under the cf and af, the presence of the hot mantle wedge induces low viscosities also in the lower crust, even if the area is located above thick lithospheric roots. we impose a laterally varying temperature profile consistent with the lithospheric structure and with the observed heat flow, and let the viscosity profile be controlled by empirical flow laws for rocks and by laboratory derived rheological parameters (e.g., ranalli, 1995) . since we have a large choice of possible rheologies and stress conditions, we have introduced an additional constraint by looking for those combinations of rheology and stress that provide viscosities anywhere not lower than 1017 pa s at the bottom of the lower crust and 1016 pa s in the mantle wedge. a viscosity of 1017 pa s at the bottom of the lower crust is consistent with laboratory rheological parameters for soft lower crustal materials (e.g. wet diorite in hansen and carter, 1982) under realistic conditions in regions of active tectonics, with a surface heat flow of 60-70 mw/m2 and a background stress of 10-100 mpa. in the mantle wedge, the presence of low viscosities is inferred from the very low seismic velocities (vs less than 4.2 km/s), suggesting that the wedge might represent a partially molten mantle (chimera et al., 2003; aoudia et al., 2007). with our approach, we aim to represent a plausible viscosity structure, where the choice of different parameters would influence the actual values, but not the relative horizontal viscosity variations, which represent our main point of interest. figure 3, shows the resulting postseismic strain rates one year after the colfiorito main shock, where the horizontal axis is centered at fig. 2. vertical deformation (elastic plus full relaxation) due to 70 cm of slip on the altotiberina fault (af). the large black boxes represent the three segments of the altotiberina fault, dipping towards the ne with angles of 30o, 13o and 30o (shallow, middle and deep segment respectively). the three small boxes represent the surface projections of the largest shocks of the 1997 sequence. vol51,2_3,2008 4-03-2009 10:28 pagina 400 401 postseismic deformation and body forces shaping the apennines and adjacent sedimentary basins in umbria-marche the cf and the fault is represented by a small white segment. the two top layers represent the crust (upper and lower), the third layer the mantle wedge, followed by the rest of the lithosphere and by the asthenosphere. the moho is indicated by a black line bounded in white. we can clearly see how the highest strain rates are localized in three distinct regions: in the first few kilometers around the fault, and in the bottom part of both the lower crust and the mantle wedge. the high strain rates in the upper crust are mainly due to the very large differential stresses induced by the earthquake, while localization of strain rates in the other layers is mainly controlled by the viscosity profile. the combined effect of viscosity jumps at the moho and at the base of the mantle wedge due to changes in rheology, and lateral viscosity variations driven by the thermal structure, leads to the localization of postseismic relaxation in a confined region below the umbria-marche apennines. vertical postseismic relaxation is shown in fig. 4 for the first 10 years after the earthquake: the choice of representing this particular timespan is due to the fact that large portions of the lower crust and mantle wedge have viscosities lower than 1019 pa s, meaning that they almost completely relax in the first few years. the upper panel shows viscoelastic relaxation for the lateral heterogeneous model, while the bottom panel shows the same result for a lateral homogeneous model with the same structure as the region below the af. for the case of the heterogeneous model, the deformation pattern is dominated by a large subsidence in a rather narrow region between 10 km and 75 km west of the cf, bounded by a narrow uplifting region around the cf and by a broader uplift further to the west. the large subsidence occurs where large sedimentary basins such as the foligno basin are mapped. these sedimentary basins are bordered by low-angle normal faults. the effect of lateral heterogeneities is mainly visible in the eastern side: when approaching the colder adriatic domain, we observe a drastic reduction in subsidence with respect to the homogeneous model. a further difference between the two models is visible towards the tuscan domain, where the combined effect of the thinner crust and very warm geotherm leads to an amplification of the uplifting bulge. according to those fig. 3. 2d section through the central apennines, showing postseismic strain rates at 1 year after the earthquake. the colfiorito fault (cf) is represented by the small white segment in the center of the domain, and the moho by the thin black line bounded in white. fig. 4. postseismic relaxation for the 2d approximation of the cf for both the lateral heterogeneous (top panel) and the homogeneous (bottom panel) model. the four lines depict vertical deformation at four different times after the earthquake: t = 1 a (solid), 2 a (long dashed), 5 a (short dashed) and 10 a (dotted). vol51,2_3,2008 4-03-2009 10:28 pagina 401 402 r.e.m. riva and a. aoudiar.e.m. riva and a. aoudia results, we think that a lateral heterogeneous lithospheric structure has the effect of localizing postseismic relaxation due to earthquakes in the colfiorito area in a rather narrow region below the af and the related sedimentary basins. we have verified that our conclusions are not dependent on the adopted 2d approximation, because there is no substantial qualitative difference between relaxation of a 50-km-long fault, representative of multiple ruptures in the colfiorito area between camerino and norcia, and that of an infinitely long fault. the key parameter controlling the wavelength of the relaxation signal, in fact, is represented by the thickness of the upper elastic layer, which is here about 15 km and therefore much shorter than the extended cf. by comparing relaxation in a vertically layered medium for finite faults of different length, we have further verified that the quantitative effect of the 2d approximation results in the amplification of the magnitudes of relaxation by about a factor of two, which is again not relevant for the purpose of this study. 5. discussion and conclusions here elaborate further on the possibility of an interaction between the low-angle ne-dipping altotiberina fault (af) and the sw normal faulting structures responsible for the 1997 colfiorito sequence. with loose constraints on the magnitude and location of historical earthquakes, as well as on their recurrence time, we have modeled a possible scenario of postseismic relaxation which appears to be consistent that the formation of major sedimentary basins at the western side of the umbria-marche faulting zone. the location of the postseismic relaxation signal favours the loading of the af, even if the slip rates that we have obtained are rather small: however, uncertainties are very large because those values are directly dependent on the average earthquake magnitude and recurrence time. on the other hand, after modeling the effect of slip on the af, we have shown that the af alone could be responsible for providing most of the load necessary to rupture the cf. when we compare average postseismic relaxation rates due to the cf with slip rates on the af, the latter appears to provide the dominating signal. however, for a realistic viscosity structure where the lower crust and the mantle wedge below the umbria-marche apennines are characterized by low viscosities (in the range 1016-1018 pa s), about half of the total postseismic relaxation is likely to take place in the first 10 years after the earthquake. this means that, at least locally, the postseismic loading on the af could easily exceed the average slip rates. in addition, the impact of stress relaxation on the evolution of the af can be increased by the presence of large lateral heterogeneities in the lithospheric structure, which have the effect of concentrating postseismic relaxation below the af. in conclusion, a possible scenario for the interaction between the af and the cf allows aseismic slip on the af to effectively load the cf, while postseismic relaxation after each rupture event on the cf plays the role of a feedback process, reloading sections of the af on short time-scales and potentially affecting the evolution of the af itself. from a regional perspective, the strain pattern retrieved from the 2d finite element modeling of the postseismic deformation mimics the modeled stress pattern that accounts only for buoyancy forces as reported by aoudia et al. (2007) along the same section and using the same geometry of the crust upper mantle. in fact the realistic pattern of localized vertical deformation reported in fig. 4 compares fairly well with its equivalent part reported in aoudia et al. (2007, fig. 3). this is evidence that the two different length and time scale processes, namely, postseismic deformation and buoyancy, are complementary in shaping the apennines and adjacent sedimentary basins. the high resolution deformation patterns modeled in this study as well as the one reported in aoudia et al. (2003) satisfying the ongoing observed and complex geodynamics across north central italy with emphasis to the umbria-marche region can hardly be reproduced by a model accounting only for external forces such as a rotating or subducting or retreating adria. vol51,2_3,2008 4-03-2009 10:28 pagina 402 403 postseismic deformation and body forces shaping the apennines and adjacent sedimentary basins in umbria-marche acknowledgements this work was done in the framework of the sismaitalian space agency project. we thank rob govers for the use of the fem software gtecton and two anonymous reviewers for their constructive comments. references agostinetti, n.p., f. p. lucente, g. selvaggi and m. di bona (2002): crustal structure and moho geometry beneath the northern apennines (italy), geophys. res. lett., 29. amato, a., r. azzara, c. chiarabba, g.b. cimini, m. cocco, m. di bona, l. margheriti, s. mazza, f. mele, g. selvaggi, a. basili, e. boschi, f. courboulex, a. deschamps, s. gaffet, g. bittarelli, l. chiaraluce, d. piccinini and m. ripepe (1998): the 1997 umbria-marche, italy, earthquake sequence: a first look at the main shocks and aftershoks, geophys. res. lett., 25 (15), 2861-2864. aoudia, a., a.t. ismail-zadeh and f. romanelli (2007): buoyancy-driven deformation and contemporary tectonic stress in the lithosphere beneath central italy, terra nova, 19 (6), 490-495, doi: 10.1111/j.1365-3121.2007.00776.x aoudia, a., a. borghi, r. riva, r. barzaghi, b.a.c. ambrosius, r. sabadini, l.l.a. vermeersen and g.f. panza (2003): postseismic deformation following the 1997 umbria-marche (italy) moderate normal faulting earthquakes, geophys. res. lett., 70, 1390-1393. barba, s., and r. basili (2000): analysis of seismological and geological observations for moderate-size earthquakes: the colfiorito fault system (central apennines, italy), geophys. j. int., 141, 241-252. boncio, p., and g. lavecchia (2000): a geological model for the colfiorito earthquakes (september-october 1997, central italy), journal of seismology, 4, 345-356. boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (eds) (2000): catalogue of strong italian earthquakes, ann. geofis., 43 (4), 268. chiaraluce, l., c. chiarabba, c. collettini, d. piccinini and m. cocco (2007): architecture and mechanics of an active low-angle normal fault: alto tiberina fault, northern apennines, italy, journal of geophysical research, 112, b10310, doi: 10.1029/2007jb005015, chimera, g., a. aoudia, a. sarao and g.f. panza (2003): active tectonics in central italy: constraints from surface wave tomography and source moment tensor inversion, phys. earth planet. inter., 138, 241262. della vedova, b., s. bellani, g. pellis and p. squarci (2000): deep temperatures and surface heat-flow distribution, in anatomy of an orogen: the apennines and adjacent mediterranean basins, edited by g.b. vai and l.p. martini (kluwer academic publishers, dordrecht). deschamps, a., r. scarpa and g. selvaggi (1989): analisi sismologica del settore settentrionale dell’appennino umbro-marchigiano, atti gngts 8th conference, 1, 9-15. feigl, k.l. and e. dupre (1998): rngchn: a program to calculate displacement components from dislocations in an elastic half-space with applications for modeling geodetic measurements of crustal deformation, revised for computers and geosciences. galadini, f., p. galli, i. leschiutta, g. monachesi and m. stucchi (1999): active tectonics and seismicity in the area of the 1997 earthquake sequence in central italy: a short review, journal of seismology, 3 (2), 167-175. govers, r. and m.j.r. wortel (1995): extension of stable continental lithosphere and the initiation of lithosphere scale faults, tectonics, 14 (4), 1041-1055. hansen, f.d. and n.l. carter, (1982): eos trans. agu, 63, 437. meghraoui, m., v. bosi and t. camelbeeck (1999): fault fragment control in the 1997 umbria-marche, central italy, earthquake sequence, geophys. res. lett., 26 (8), 1069-1072. mele, g. and e. sandvol (2003): deep crustal roots beneath the northern apennines inferred from teleseismic receiver functions, earth planet. sci. lett., 211, 69-78. olivieri, m. and g. ekstrom (1999): rupture depths and source processes of the 1997-1998 earthquake sequence in central italy, bull. seism. soc. am., 89, 305-310. pantosti, d., p.m. de martini, p. galli, f. galadini, p. messina, m. moro and a. sposato, (1999): studi paleosismologici attraverso la rottura superficiale prodotta dal terremoto del 14 ottobre 1997 (umbria-marche) atti 18° convegno annuale del gngts, sessione 10 “forti terremoti dell’area mediterranea”. ranalli, g., (1995): rheology of the earth (second edition, kluwer academic publishers, dordrecht), pp. 432. riva, r., a. aoudia, l.l.a. vermeersen, r. sabadini and g.f. panza (2000): crustal versus asthenospheric relaxation and post-seismic deformation for shallow normal faulting earthquakes: the umbria-marche (central italy) case, geophys. j. int., 141, f7-f11. riva, r.e.m., a. borghi, a. aoudia, r. barzaghi, r. sabadini and g. f. panza (2007): viscoelastic relaxation and long-lasting after-slip following the 1997 umbriamarche (central italy) earthquakes, geophys. j. int., 169, 534-546, doi: 10.1111/j.1365-246x.2007.03315.x sabadini, r. and l.l.a. vermeersen (1997): influence of lithospheric adn mantle layering on global post-seismic deformation, geophys. res. lett., 24, 2075-2078. salvi, s., s. stramondo, m. cocco, m. tesauro, i. hunstad, m. anzidei, p. briole, p. baldi, e. sansosti, g. fornaro, r. lanari, f. doumaz, a. pesci and a. galvani (2000): modeling coseismic displacements resulting from sar interferometry and gps measurements during the 1997 umbria-marche seismic sequence, journal of seismology, 4, 479-499. zollo, a., marcucci, s., milana, g. and p. capuano (1999): the 1997 umbria-marche (central italy) earthquake sequence: insights on the mainshock ruptures from near source strong motion records, geophys. j. int., 26 (20), 3165-3168. vol51,2_3,2008 4-03-2009 10:28 pagina 403 599_608 boccolari 30_1.pdf annals of geophysics, vol. 45, n. 5, october 2002 599 gps zenith total delays and precipitable water in comparison with special meteorological observations in verona (italy) during map-sop mauro boccolari (1), slobodan fazlagic′′′′′ (1), paolo frontero (2), luca lombroso (1), sergio pugnaghi (1), renato santangelo (1), stefano corradini (1) and sergio teggi (3) (1) dima osservatorio geofisico (ogmo), università di modena e reggio emilia, italy (2) arpav department of verona province, verona, italy (3) dimec università di modena e reggio emilia, italy abstract continuous meteorological examination of the pre-alpine zones in northern italy (po valley) is important for determination of atmospheric water cycles connected with floods and rainfalls. during a special meteorological observing period (map-sop), radiosounding and other measurements were made in the site of verona (italy). this paper deals with zenith total delay (ztd) and precipitable water (pw) comparisons obtained by gps, radiosounding and other meteorological measurements. pw and ztd from ground-based gps data in comparison with classical techniques (e.g., wvr, radiosounding) from recent literature present an accurate tool for use in meteorology applications (e.g., assimilation in numerical weather prediction (nwp) models on short-range precipitation forecasts). comparison of such ztd for map-sop showed a standard deviation of 16.1 mm and pw comparison showed a standard deviation of 2.7 mm, confirming the accuracy of gps measurements for meteorology applications. in addition, pw data and its time variation are also matched with time series of meteorological situations. those results indicate that changes in pw values could be connected to changes in air masses, i.e. to passages of both cold and warm fronts. there is also a correlation between precipitation, forthcoming increase and the following decrease of pw. a good agreement between oscillation of pw and precipitation and strong cyclonic activities is found. 1. introduction during map-sop (bougeault at al., 2001) field experiments (mesoscale alpine program/ special observing period, september-november mailing address: dr. mauro boccolari, dipartimento di ingegneria dei materiali e dell’ambiente (osservatorio geofisico), università di modena e reggio emilia, vi a vignolese 905, 41100 modena, italy; e-mail: mauro.boccolari@unimo.it key words precipitable water – zenith total delay – radiosounding – gps 1999), the special observation station in verona was set up mainly for inflow probing, and upstream-measurements related to gap-flow studies in connection with neighbouring alpine valleys (the flow over the upstream side of the adige valley should have an important role in precipitation triggering processes, both convective and stratiform). the enhancement of precipitation and related floods during warm and moist south-westerly flow in the pre-alpine zones in northern italy is another process that needs more explanation, both for improving parameterization in nwp and improving the short-range forecasts. meteorological radiosondes 600 mauro boccolari et al. are a fairly accurate but temporally limited classic source of humidity observations. to overcome the time discontinuity of vertical humidity measurements, a gps receiver could be used as a continuous source of atmospheric measurements (businger et al., 1996). atmospheric refraction has a strong influence on gps radio signals. the ionosphere disturbance on gps electromagnetic waves is resolved in a satisfactory manner, thus the main task is to estimate better the neutral atmosphere impact. signals are delayed in the neutral atmosphere because of the refraction, which is a function of temperature, pressure and water content. satellites travel under different elevation angles, but taking a convenient mapping function, the zenith direction is commonly considered. all components of the troposphere contribute to such zenith delay, but it is convenient to study its hydrostatic and wet terms separately. the induced dipole moment of the atmosphere is associated with the zenith hydrostatic delay (zhd). zenith wet delay (zwd, non-hydrostatic) is mainly due to the permanent dipole moment of water vapor, which is highly variable in space and time: the zwd is fully attributed with the content of water vapor along the signal path. gps data may serve for the estimation of integrated water vapor in the atmosphere (bevis et al., 1992), using their zenith total delay (ztd). if the zhd component (after being derived from a surface pressure modelling) is subtracted from ztd, it leaves the zwd, which is associated with the water vapor profile. in the framework of the map/sop campaign, the geophysical observatory of modena university (ogmo) performed a consistent number of radiosounding (98) in verona (italy), on a weather forecast bases, issued by the map operation centre declaring the iop (intensive observation period). during the iop, balloons were launched at six hour intervals or, in case of the extreme events, every three hours, looking for situations with the highest probability of intensive atmospheric processes (fronts, heavy precipitation, foehn conditions, etc.). the limiting fact of radiosounding, having a variable vertical extent of measurements, was considered in this work. surface meteorological data were also permanently recorded at the site. the principal aim of this work was to compare radiosounding-based ztd and pw (precipitable water) with delays and pw obtained from gps analyses. the permanent gps station in padua (igs network, located at 70 km distance from the verona radiosounding site) was used for the comparison. other authors have already presented comparisons of pw between gps data and different measurement techniques: e.g., with wvr (duan et al., 1996) and (emardson et al., 1998), radiosounding and nwp modelling (borbas, 1998) and (vedel et al., 2001), lidars (bock and doerflinger, 2001), sun photometer (pugnaghi et al., 2002). the second aim of the work was to examine variations in pw estimates obtained from gps with respect to the occurrence of significant meteorological events, followed in detail during map-sop, in order to determine possible regularities in meteorological perspective. 2. zenith total delay comparison (radiosounding, gps) 2.1. zenith total delay from radiosounding zenith total delay (mm) is commonly expressed as (2.1) where n, ndry, nwet, are the total, dry and wet refractivity, z is the geometric height. refractivity can be expressed as (thayer, 1974) (2.2) where p d (hpa) is the partial pressure of dry air, e (hpa) is the partial pressure of water vapor, t (k) is the temperature, k1, k2, k3 are empirical constants. in such work, constants derived by bevis (bevis et al., 1994) were used: k 1 = 77.6 ztd ( ) ground toa = =∫ n z dz ( ) ( ) dry ground toa wet ground toa = +∫ ∫n z dz n z dz n n n k p t k e t k e t t d= + = + + dry we 1 2 3 2 ( ) 601 gps zenith total delays and precipitable water in comparison with special meteorological observations in verona (italy) during map-sop k/hpa, k 2 = 70.4, k/hpa, k 3 = 3.739 105 k2/hpa. bevis constants were chosen because they are considered more suitable for deriving delays from meteorological data. using the previous equation, ztd can be expressed in terms of pressure dp as a sum of an hydrostatic part zhd and a non-hydrostatic part znhd (2.3) where q is the specific humidity, r d is the specific constant of dry air, g is the gravitational constant, t is the temperature and ε is the ratio of the water vapor constant to that of dry air. toa stands for top of atmosphere. it should be noted that in literature, znhd is often referred as a wet delay. solving the integral for zhd we obtain that it depends by only the surface pressure p s (2.4) if the barometric pressure is known to 1 hpa, zhd can be estimated with an accuracy of 2.3 mm. the accuracy of barometric sondes used for radiosounding was 1 hpa. specific humidity in eq. (2.3) was calculated from the relative humidity (accuracy was the 3%), measured by radiosounding device, using the goff and gratch formula (wmo, 1975). in the case of the calculus based on radiosounding data, the integration was done from the ground surface to the upper end of the radiosounding. 2.2. zenith total delay modelling (from saastamoinen formula) zenith total delay may be calculated using a tropospheric delay model. for that purpose, we applied the well known saastamoinen model (saastamoinen, 1972) (neglecting the latitude and the station height effects) (2.5) where, again, zhd (m) is the zenith hydrostatic delay, znhd (m) is the zenith non hydrostatic delay, p s is the surface air pressure (hpa), t s (k) is the surface temperature, e s (hpa) is the surface water vapor pressure. it is evident that zhd is the same as that in eq. (2.4). 2.3. vertical correction of radiosounding zenith total delay obviously, ztd calculated by integration (eq. (2.1) for many low-reaching radiosoundings are underestimated. maximum heights reached by balloons of radiosounding during the verona field campaign varied from 2996 m to 19648 m. a solution could be to take into consideration only radiosounding reaching a «chosen» height, but it would involve a very small data set for further analysis. to use all radiosounding measurements for calculation of ztd, a necessary correction for the uncovered part extent of the troposphere is made. for that purpose, we used saastamoinen hydrostatic modelling based on surface meteorological data (eq. (2.5)). looking for ways to overcome the «underestimated» ztd rs , we have already tried two equivalent methods for a vertical correction (boccolari et al., 2001). in this work, the applied method is: zenith non hydrostatic delay from radiosounding (second term of right hand of eq. (2.3), in which integration limits are the ground and the top of radiosounding) is corrected with znhd from the saastamoinen formula for the upper part of atmosphere not covered by radiosounding (znhd saas(upper) ), applying saastamoinen zhd for the whole range ztd zhd znhd toa ground = + = +∫ k r g dpd 1 toa ground −( ) +   ∫ qr g k k k t dpd 2 1 3 ε ε zhd = k r p g d s 1 ztd hd znhd saas saas saas = + = +z p s 0 002277. + + t e s s 0 002277 0 005 1255 . .( ) 602 mauro boccolari et al. (eq. (2.5)) ztd c = znhdrs + znhdsaas(upper) + zhdsaas . (2.6) hereafter, we take ztd c as «corrected». 2.4. zenith total delay from gps data elaboration due to large local radio interference with the gps signal at verona radiosounding site, gpsztd were retrieved for the gps permanent site in padua (italy), which belongs to igs network, as the nearest one to the verona; the distance of approximately 70 km, inside the range of baselines used by other authors, e.g., 50 km (vedel et al., 2001); 100 km (emardson et al., 1998). from the crustal dynamics data information system archive (cddisa) at the nasa goddard space flight center, we retrieved the ztd (combination of tropospheric estimates from all analysis centres by the geoforschungszentrum of potsdam) with a sampling rate of 2 h. owing to the lack of gps measurements in padua for some days, the comparison set was reduced from 98 to 76 cases. the difference between orthometric heights of verona and padua sites was 27.6 m (zpadova = = 39.4 m; zverona = 67.0 m). such vertical offset corresponds roughly to a difference of 3.5 hpa, that is about 8 mm of difference for the zhd. considering that both stations belong to the mainly homogeneous meteorological field (po valley) and 95% of water vapor lies below 5 km, the ztd wet component difference should not be too significant (1-0.5 mm). 2.5. comparison of ztd from gps and (corrected) radiosounding bias and standard deviation of ztd between corrected radiosounding (ztd c ) and zenith total fig. 1. ztd by gps for padua and ztd by radiosounding for verona (mm) versus time for map-sop period (doy 250-320, 07 september 16 november, 1999). 603 gps zenith total delays and precipitable water in comparison with special meteorological observations in verona (italy) during map-sop pwrs = zwdrs lower + znhd saas(upper) upper . (3.4) 3.3. pw comparison the bias and standard deviation of pw calculated from radiosounding and gps (not considering the vertical offset between the two site) is (3.5) the value of 2.7 mm is at the level of the rms indicated by other authors (see table i). 4. gps pw variation in comparison with observed meteorological data surface meteorological data during map-sop were recorded continuously (sampling rate of 10 min) at the experimental (also radiosounding) site at verona. as already seen in relation (3.5), the standard deviation of pwgps and pwrs difference indicated the possibility to proceed with the comparison of continuous gps pw and surface meteorological data. data on precipitation, atmospheric pressure, wind velocity, relative humidity and air temperature are used for the comparison with pw data. in order to express clearly the behaviour of pw we introduce here a symbol of the change of pw, curve derivative coefficient (we call it briefly delay from gps (ztd gps ), without considering the vertical shift between the two sites, are (2.7) a recent comparison between ztd-gps and radiosounding-ztd made by vedel et al. (2001) gives the average offsets of 6.0 mm ± 11.7 mm (see fig. 1). 3. precipitable water (pw) comparison 3.1. pw from gps precipitable water (denoted with pwgps) (mm) estimate from gps data is based on zwd which is nearly linearly proportional to pw (mm). according to bevis et al. (1994) (3.1) where (t m ) is a constant depending on the mean atmospheric temperature t m . the value of 0.15, usually accepted, was taken. zwd is obtained by subtracting zhd (i.e. obtained with saastamoinen formula) from the ztd. 3.2. pw from radiosounding precipitable water (denoted with pw rs ) (mm) is calculated as a sum of the two terms (pwlower + +pwupper). the first term (pwlower) is zwd integrating of nwet (eq. (2.2)) from radiosounding data multiplied for the constant (t m ), that is calculated with the following relation (askne and nordius, 1987): (3.2) where the mean atmospheric temperature t m is given by (davis et al., 1985) (3.3) the second term (pw upper ) takes into account the upper part of atmosphere not covered by radiosounding. it is calculated from znhdsaas(upper) (eq. (2.3)) multiplied for the constant upper calculated using eq. (3.2) for that upper layer. in conclusion pw pw mm .gps rs = ±1 6 2 7. . ztd ztd mm .gps = ±c 5 1 16 1. . pw zwd gps gps = ( )tm t r k t k k m v v m ( ) = + 106 3 2 1 ( ) t e t dz e t dz m = ( ) ( ) 2 . table i. rms (mm) between pw by gps and pw by radiosounding reported by other authors. author(s) rms (mm) borbas (1998) 2.3-2.6 tregoning et al. (1998) 1.5 emardson et al. (1998) 1.8-2.6 baker et al. (2001) 1.2-1.7 604 mauro boccolari et al. fig. 2. precipitable water (pw, mm) and pw curve derivative coefficient sign (∇pw, mm/h) for padua; barometric pressure (hpa); wind velocity (m/s); relative humidity (%) and air temperature at 2 m (°c) for verona versus time for period of doy 265-280, 22 september 07 october, 1999. fig. 3. precipitable water (pw, mm) and pw curve derivative coefficient sign (∇pw, mm/h) for padua; barometric pressure (hpa); wind velocity (m/s); relative humidity (%) and air temperature at 2 m (°c) for verona versus time for period of doy 285-300, 12 october 27 october, 1999. 605 gps zenith total delays and precipitable water in comparison with special meteorological observations in verona (italy) during map-sop fig. 4. precipitable water (pw, mm) and pw curve derivative coefficient sign (∇pw, mm/h) for padua; barometric pressure (hpa); wind velocity (m/s); relative humidity (%) and air temperature at 2m (°c) for verona versus time for period of doy 305-320, 01 november 16 november, 1999. fig. 5. precipitable water (pw, mm) and pw curve derivative coefficient sign (∇pw, mm/h) for padua; barometric pressure (hpa); wind velocity (m/s); relative humidity (%) and air temperature at 2 m (°c) for verona versus time for period of doy 320-335, 16 november 01 december, 1999. 606 mauro boccolari et al. not immediately accompanied by a pw decrease, but at the end of the period, we explain by the constant influence of a strong elevated depression over the area. later, 23rd and 25th october (doy 296 and 298) show intensive oscillation between positive and negative ∇pw after the significant precipitation cases, both connected with strong cyclonic activities (passages of warm fronts). again, a relative maximum of pw values (doy 299) is reported in the situation of strong southern circulation in the upper level. in fig. 4 two significant episodes are distinguished. the first, between 4th and 5th november (doy 308, 309) where weak precipitation was followed with the durable negative ∇pw, caused by the influence of two components – cut off closed in the gulf of genoa and easterly flow from adriatic in surface anticyclonic circulation. the second, opposite case episode (doy 311) demonstrates a pw decrease after the heavy precipitation brought by the formation of classical deep cyclonic circulation in the gulf of genoa and a rapid passage of the cold front. a long constant period of low pw values and absence of significant signs of ∇pw is related to the domination of high pressure field (doy 312317), again broken by the depression with precipitation in doy 318. in fig. 5 the episode with a strong decrease of pw after precipitation (doy 321) should be distinguished. immediately, it follows the next decrease of pw without precipitation, connected to the passage of the deep depression caused by the circulation of polar air mass. low pw values in the high pressure field are interrupted at the end of the period (doy 334) when easterly flow of the adriatic wet air occurred caused by the strengthening of the balcanic anticyclone. to obtain a better correlation between gps detected precipitable water and meteorological events, it would probably be necessary to introduce the elaboration of slant water vapor contribution along the different lines of sight gps receiver-satellites (de haan et al., 2002). 5. conclusions using the map-sop data set in comparison with gps observations, we have confirmed that gradient ∇pw, mm/h). some more intense cases of ∇pw are reported in order to indicate a possible tool for valorisation of ongoing numerical weather forecasts. figures 2 to 5 compare pw (first graph from the top) and pw gradient ∇pw (second graph from the top) with the meteorological parameters. to have the temporal resolution presented in a more convenient manner, the entire map-sop period is divided into four suitable time segments (doy 265-280, doy 285-300, doy 305-320, doy 320-335). generally, several indications are common to all four time segments. changes in pw values could be related to atmospheric processes or to the state of air masses, i.e. to passages of both cold and warm fronts over the gps observing site. also, precipitation cases are usually accompanied by forthcoming increase and subsequent decrease of pw. here we indicate some typical meteorological situations where gps determined pw are confirming the fact to change together with strong weather changes. in fig. 2 the intensive increase in pw which occurred in doy 268 and 269 (25/26 september) could be followed. it corresponds to the southerly flow in both upper and low levels over the northern italy, strengthened even with the anticyclonic circulation over central mediterranean area. after the passage of the cold front and precipitation on 26.09 (third graph from the top), a heavy negative gradient of ∇pw is noted (the second graph from the top). also, other precipitation occurrences in that period were followed with the negative gradient of ∇pw (doy 271, 274, 277), all related to passages of disturbances, indicated with relative minimum of atmospheric pressure (fourth graph from the top). in fig. 3 the pw minimum showed during doy 292 (19 october) coupled with negative ∇pw. a no precipitation situation corresponds to the specific synoptic event with two airflow components – polar dry air circulation in upper level and easterly flow of cold and dry air in low level (indicated with increased wind velocities in fifth graph and decrease in relative humidity – sixth graph from the top). in the map-sop documents that situation is described in details as iop n. 8. for the period of two consecutive days (doy 294, 295) when the low precipitation episode was 607 gps zenith total delays and precipitable water in comparison with special meteorological observations in verona (italy) during map-sop the limitation of the vertical extension of the radiosounding measurements for ztd estimates could be resolved by adding a term from a tropospheric model (saastamoinen). it is also confirmed that for meteorological applications, it is useful to estimate pw by gps elaboration instead of from radiosounding, that occur as spatially and temporally limited. comparing ztd from (corrected) radiosounding with ztd from nearby gps station data the standard deviation error of 16 mm is found, which is a reasonable value to be used in the numerical modelling. comparison of radiosounding pw estimates with gps pw estimates shows standard deviation of 2.7 mm. the bias (considering the vertical shift between two sites) is at the sub-millimetre scale. that standard deviation and bias correspond to the recent results of other authors (table i). as a consequence, precise gps data may be used as an adequate source of integrated vertical humidity field for numerical weather modelling. surface meteorological analysis data during map-sop are used for the comparison with gps pw variations. several indications are detected. changes in pw values could be generally related to atmospheric processes, i.e. to passages of both cold and warm fronts over the gps observing site. there is also a strong correlation between precipitation, forthcoming increase and the following decrease of pw. some intensive oscillations between positive and negative pw variations come after the significant precipitation cases connected to strong cyclonic activities. acknowledgements authors express their gratitude to crustal dynamics data information system archive (cddisa) at the nasa goddard space flight center, geoforschungszentrum of potsdam (gfz) and map data centre eth – zürich for the data used in this work. references askne, j. and h. nordius (1987): estimation of tropospheric delay for microwaves from surface weather data, radio sci., 22, 379-386. baker, h.c., a.h. dodson, n.t. penna, m. higgins and d. offiler (2001): ground-based gps water vapour estimation: potential for meteorogical forecasting, j. atmos. sol. terr. phys., 63, 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(1998): derivation of precipitable water from gps data: an application to meteorology, phys. chem. earth, 23 (1), 87-90. bougeault, p., p. binder, a. buzzi, r. dirks, r. houze, j. kuettner, r.b. smith, r. steinacker and h. volkert (2001): the map special observing period, bull. am. meteorol. soc., 82 (3), 637-650. businger, s., s.r. chiswell, m. bevis, j. duan, r. a. anthes, c. rocken, r.h. ware, m. exner, t. vanhover and f.s. solhein (1996): the promise of gps in atmospheric monitoring, bull. am. meteorol. soc., 77 (1), 5-18. davis, j.l., t.a. herring, i.i. shapiro, a.e. rogers and g. elgered (1985): geodesy by radiointerferometry: effects of atmospheric modelling errors on estimates of baseline length, radio sci., 20, 1593-1607. de hann, s., h. van der marel and s. barlag (2002): comparison of gps slant delay measurements to a numerical model: case study of a cold front passage, phys. chem. earth, 27, 317-322. duan, j., m. bevis, p. fang, y. bock, s. chiswell, s. businger, c. rocken, f. solheim, t. van hove, r. ware, s. mcclusky, t.a. herring and r.w. king (1996): gps meteorology: direct estimation of the absolute value of precipitable water, j. appl. meteorol., 35, 830-838. emardson, t.r., g. elgered and j.m. johansson (1998): three months of continuous monitoring of atmospheric water vapor with a network of global positioning system receivers, j. geophys. res., 103 (d2), 1807-1820. pugnaghi, s., m. boccolari, s. fazlagić, r. pacione, r. santangelo, h. vedel and f. vespe (2002): comparison of independent integrated water vapour estimates from gps and sun photometer measurements and a meteorological model, phys. chem. earth, 27, 355-362. saastamoinen, j. (1972): atmospheric correction for the troposphere and stratosphere in radio ranging of satellites, in the use of artificial satellites for geodesy, 608 mauro boccolari et al. geophys. monogr. ser., am. geophys. un., washington. d.c., 15, 245-251. thayer, d. (1974): an improved equation for the radio refractive index of air, radio sci., 9, 803-807. tregoning, p., r. boers, d. o’brein and m. hendy (1998): accuracy of absolute precipitable water vapor estimates from gps observations, j. geophys. res., 103, 28,701-28,710. vedel, h., k.s. mogensen and y. huang (2001): calculation of zenith delays from meteorological data comparison of nwp model, radiosonde and gps delays, phys. chem. earth (a), 26 (6-8), 497-582. wmo (omm) organisation meteorologique mondiale (1975): tables meteorologique internationales, n. 188, tp94. (received march 4, 2002; accepted october 10, 2002) minor shallow gravitational component on the mt. vettore surface ruptures related to mw 6, 2016 amatrice earthquake annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7299 1 minor shallow gravitational component on the mt. vettore surface ruptures related to mw 6, 2016 amatrice earthquake matteo albano1, michele saroli2,1, marco moro1, emanuela falcucci1, stefano gori1, salvatore stramondo1, fabrizio galadini1, salvatore barba1 1istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma (italy) 2dipartimento di ingegneria civile e meccanica (dicem), università degli studi di cassino e del lazio meridionale, via g. di biasio 43, 03043, cassino (italy). matteo.albano@ingv.it abstract on 24th august 2016 a mw 6.0 earthquake occurred near amatrice (central italy) causing nearly 300 fatalities. the mainshock ruptured a nnw-sse striking, wsw dipping normal fault. the earthquake produced several coseismic effects at ground, including landslides and ground ruptures. in particular, ground surveys identified a 5.2 km long continuous fracture along the mt. vettore flank, both on rock and slope deposits, along one of the active normal fault segments bounding the relief to the west. in this work, we evaluated the contribution of seismically-induced surface instabilities to the observed ground fractures by means of a permanent-displacement approach. the results of a parametric analysis show that the computed seismically-induced gravitational displacements (about 2-10 cm) are not enough to explain field observations, testifying to a mean 20-25 cm vertical offset. thus, the observed ground fractures are the result of primary faulting related to tectonics, combined with gravitational phenomena. i. introduction n august 24th 2016 a mw 6.0 normal faulting earthquake occurred near amatrice (central italy), approximately 50 km nw of l’aquila (figure 1a). almost 300 people were killed and three villages were partially destroyed. about 11500 aftershocks occurred in the first month after the mainshock, including 200 with magnitude between 3 and 4, 14 between 4 and 5 and one of mw 5.4 (figure 1a) [iside working group, 2016]. preliminary inversion models performed with gps, sar and strong-motion data revealed the geometry and the coseismic slip distribution of the mainshock causative fault o annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7299 2 [gruppo di lavoro ingv sul terremoto di amatrice, 2016]. the mainshock produced several coseismic effects at ground, including landslides and ground ruptures along mountain slopes and cultivated fields [emergeo working group, 2016]. in particular, a continuous coseismic rupture, approximately 5.2 km long, was observed on the sw flank of mt. vettore, along the vettore and vettoretto active normalfaults (figure 1b and c). this newly formed fracture was observed both on slope deposits and on the bedrock fault planes. such fractures were initially interpreted as primary surface faulting, although satellite measurements revealed localized displacements in the area, extending for about 0.5 km2 on the mt. vettore flank and bordered upward by the observed ground fractures [gruppo di lavoro irea-cnr & ingv, 2016]. thus, ongoing studies are focusing on investigating the possible role of surface gravitational phenomena to the surface displacement observed along the mt. vettore flank. field observation reveal that, from a geological/geotechnical point of view, the sw flank of mt. vettore is covered in places by a thin layer (lower than 5 meters) of loose scree and talus deposits not exceeding few meters in figure 1: (a) location of the mainshock and aftershocks. the mw 6.0 mainshock and the mw 5.4 aftershock are represented by yellow stars with red and green stroke, respectively. (b) detail of mt. vettore area (white square in panel a). the numbered lithologies are: 1slope deposits; 2alluvial deposits; 3fluvio-lacustrine deposits; 4marly limestones and calcilutites; 5limestones and marly limestones (modified from pierantoni et al. [2013]). (c) pictures of the observed fractures on mt. vettore flank. (d) topographic profiles along sections i, ii and iii in panel b. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7299 3 thickness and with variable grain size. [pierantoni et al., 2013] (figure 1b). very often, the bedrock crops out along the slope, in the hanging wall of the ruptured mt. vettore and vettoretto faults. in this work, we assess the stability of talus sediments during the mw 6.0 earthquake shaking in order to identify the possible contribution of gravitational phenomena on the observed ground fractures. ii. methods seismically-induced slope displacements of the sediment covering the mt. vettore flank were estimated following a permanent-displacement approach [jibson, 2011] with the newmark [1965] sliding-block method. in its simplest form, newmark's method models a landslide as a rigid block that slides on an inclined plane (figure 2a). the block has a known critical acceleration (ac), i.e., the base acceleration that must be exceeded for a landslide block to begin moving relative to its base. a strong-motion record of interest is then selected, and those parts of the record exceeding the critical acceleration are integrated twice to obtain the velocity-time history and the cumulative displacement of the landslide block (δ). the critical acceleration depends on the sliding mechanism and is determined by iteratively conducting pseudostatic limit-equilibrium analyses until a ground acceleration is found yielding a safety factor of 1 (i.e., the instability of the slope). in particular, the expected movement of the debris is essentially translational instead of rotational [varnes, 1978], given the low thickness of the debris with respect to its length and the bedrock immediately beneath. thus, pseudostatic limit-equilibrium analyses are performed with reference to the infinite slope scheme, using the bishop method and considering dry conditions (figure 2b). for an infinite slope, the critical acceleration is given by equation 1. g d c ac       ⋅+ − + ⋅+⋅⋅⋅ = ' ' ' ' tantan1 tantan )tantan1(cos φα αφ φααγ (1) where g is the gravitational acceleration, γ is the unit weight of the soil, d is the thickness of the debris, α is the inclination of the sliding plane (corresponding to the slope angle), and φ' and c’ are the soil friction angle and cohesion. the talus sediments are assumed cohesionless as they are constituted by clast-supported, loose or poorly cemented heterometric carbonate clasts, ranging from very angular to little or moderately rounded [pierantoni et al., 2013]. thus c’=0 in equation (1), hence the critical acceleration depends on the friction angle and slope inclination only. figure 2: analogy between (a) block resting on inclined plane and (b) landslide on an infinite slope scheme. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7299 4 strong-motion records of the mw 6.0 event have been used in the analysis. horizontal acceleration time-histories from the nearest stations to the study area (rqt and nrc in figure 1a) have been checked to select a proper acceleration record. in particular, the horizontal component from the rqt station (figure 3a) has been selected as it presents a lower mean period (tm) and higher arias intensity (ia) and significant duration (d5-95) with respect to the nrc record (table 1). the vertical components were neglected. table 1: selected ground motion records (http://esm.mi.ingv.it.) name comp. pga tm d5-95 ia rqt.hge ew 0.45 0.16 6.9 1.53 nrc.hge ew 0.36 0.44 6.0 1.05 pga = peak ground acceleration (g) tm = mean period (s) d5-95 = significant duration (s) ia = arias intensity (m/sec) laboratory model tests and analyses of earthquake-induced landslides in natural slopes confirm that newmark's method can fairly accurately predict slope displacements if slope geometry, soil properties, and earthquake ground motions are known [jibson, 2011]. however, given the uncertainties on the model parameters, the downslope displacements of the debris have been estimated through a parametric analysis (table 2). in particular, slope inclination varies between 28° and 34° according to the maximum and minimum inclinations of the mt. vettore flank (figure 1d); friction angle varies between 32° and 40° according to literature data on similar material [marsal, 1973; albano et al., 2015]; and the peak acceleration of the rqt record is scaled between 0.2g and 0.5g, according to the pga associated to the ml 6.0 event. (http://shakemap.rm.ingv.it/). table 2: parameters and values adopted in the parametric analysis. parameter range increment name symbol min max slope angle (°) α 28 34 2 friction angle (°) φ 32 40 2 pga (g) ag 0.2 0.5 0.1 iii. results the application of the newmark’s method provides the cumulated downslope displacements δ (parallel to the slope) for a given strong-motion record (figure 3b). the maximum downslope displacements obtained for each combination of the parameters listed in table 2 are displayed in figure 4 against slope angle, friction angle and pga. the expected downslope displacements are on the order of figure 3: example application of the newmark’s method. the rqt strong-motion is scaled at 0.4g, the slope inclination is 30° and the soil friction angle is 36° (ac =0.11g). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7299 5 some tens of centimeters. in particular, downslope displacements decrease with increasing friction angle and increase with increasing pga and slope inclination. the slope is stable for low pga (0.2g) and high friction angles (38° – 40°) only. according to the geomorphological and geotechnical features of the study area, we assumed as representative conditions a slope angle of 30°32°, a friction angle of 36° 38° and a pga between 0.4g and 0.5g. under these assumptions, the modelled downslope displacements are between 2 – 10 cm. iv. discussion and conclusions the performed parametric analyses indicate that probably talus sediments have slipped seismically during the mw 6.0 event. however, the significance of the modeled displacements and their possible effect on the ground must be judged according to the assumed modelling hypotheses. a key assumption of newmark's method is that it treats a landslide as a rigid-plastic body, i.e., the mass does not deform internally and experiences no permanent displacement at accelerations below the critical level. in spite of this strong simplification, the method provides good results for thinner and relatively stiff landslides [jibson, 2011] as in the studied case, then the predicted displacements are reliable. neglecting the vertical component of the seismic input does not dramatically change the results because the vertical component is less relevant in slope stability analysis. moreover, seismically induced ground compaction is not expected on coarse, well-sorted and clastsupported granular materials. finally, the aftershock contribution to the sliding of the debris is negligible because the measured pgas in the study area are much figure 4: results of the parametric analysis expressed as the maximum downslope displacements versus slope angle, friction angle and pga. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7299 6 lower than 0.2g (http://shakemap.rm.ingv.it/). only the mw 5.4 event reached a pga of 0.22g in the study area, thus producing a displacement of about 1 cm (figure 4). shallow landslides commonly are triggered at much lower displacement levels respect to deep landslides, therefore displacements of 210 cm are sufficient to lead macroscopic ground cracking and failure in most soils [jibson, 2011]. however, the observed fractures show a vertical offset up to 30 cm and a horizontal component (opening) up to 20 cm has been also observed in several sectors [emergeo working group, 2016]. thus, the hypothetical plane of the landslide could be at least 45° steep. the modelled displacements cannot explain the field evidence both in magnitude and slope inclination. moreover, it should be considered that the presence of a continuous cover of slope debris is actually contradicted by the field surveys. indeed, at many places where ground cracks have been observed, the carbonate bedrock has been detected both in the footwall and in the hangingwall. in the whole, gathered data suggest that the fractures along the mt. vettore flank are the combination of primary faulting related to tectonics and gravitational phenomena. at this time, it is not possible to quantify the different contributions. further studies currently in progress are need to assess the relative contribution of tectonic and gravitational phenomena on the observed vertical and horizontal throws. references albano, m., g. modoni, p. croce, and g. russo (2015), assessment of the seismic performance of a bituminous faced rockfill dam, soil dyn. earthq. eng., 75, 183–198, doi:10.1016/j.soildyn.2015.04.005. emergeo working group (2016), the 24 august 2016 amatrice earthquake: coseismic effects. gruppo di lavoro ingv sul terremoto di amatrice (2016), secondo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24 agosto 2016 (italia centrale). gruppo di lavoro irea-cnr & ingv (2016), sequenza sismica di amatrice: aggiornamento delle analisi interferometriche satellitari e modelli di sorgente. iside working group (2016), italian seismological instrumental and parametric data-base (iside), , doi:10.13127/iside. jibson, r. w. (2011), methods for assessing the stability of slopes during earthquakesa retrospective, eng. geol., 122(1–2), 43– 50, doi:10.1016/j.enggeo.2010.09.017. marsal, r. j. (1973), mechanical properties of rockfill, in embankment dam engineering, vol. casagrande, pp. 109– 200, john wiley and sons, new york. newmark, n. m. (1965), effects of earthquakes on dams and embankments, geotechnique, 15, 139–159. pierantoni, p., g. deiana, and s. galdenzi (2013), stratigraphic and structural features of the sibillini mountains (umbria-marche apennines, italy), ital. j. geosci., 132(3), 497–520, doi:10.3301/ijg.2013.08. varnes, d. j. (1978), slope movement types and processes, transp. res. board spec. rep., (176), 11–33, doi:in special report 176: landslides: analysis and control, transportation research board, washington, d.c. minor shallow gravitational component on the mt. vettore surface ruptures related to mw 6, 2016 amatrice earthquake matteo albano1, michele saroli2,1, marco moro1, emanuela falcucci1, stefano gori1, salvatore stramondo1, fabrizio galadini1, salvatore barba1* 1istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma (italy) 2dipartimento di ingegneria civile e meccanica (dicem), università degli studi di cassino e del lazio meridionale, via g. di biasio 43, 03043, cassino (italy). abstract table 1: selected ground motion records (http://esm.mi.ingv.it.) table 2: parameters and values adopted in the parametric analysis. s u i c o e f f i c i e n t i d i c o r r e l a z i o n e t r a l e t e m p e r a t u r e m e d i e m e n s i l i g . lovera oggetto di questa b r e v e nota è il c o n f r o n t o dei coefficienti di correla/ione tra l e t e m p e r a t u r e m e d i e di m e s i successivi p e r c i n q u e loc a l i t à : n a p o l i , t o r i n o , v e n e z i a , p r a g a , l u n d , allo scopo di m e t t e r e in evidenza l ' e f f e t t o d e l l a l a t i t u d i n e della stazione s u l l ' e n t i t à e -ull ' a n d a m e n t o a n n u o dei coefficienti -te<si. i c a l c o l i per n a p o l i sono stali eseguiti sui dati da una serie di 6 0 a n n i , dal 1 8 6 6 al 1 9 2 5 , d e f u n t i , p e r l ' i n t e r v a l l o 1 8 6 6 1 8 9 0 , da una m e m o r i a di f . e r e d i a ( ' ) , e p e r l ' i n t e r v a l l o 1 9 0 1 1 9 2 5 da una di s . a u r i n o ( ' ) . i d a t i di l'orino sono q u e l l i già r i p o r t a t i in una prec e d e n t e nota ( ; ; ) . p e r v e n e z i a , m i sono avvalso di una serie di 105 a n n i , dal 1.836 al 1 9 1 0 , i cui d a t i sono c o n t e n u t i in tre m e m o r i e , r i s p e t t i v a m e n t e di a . b e r t i ( 1 8 3 6 1 8 5 5 ) ( '), m . tono ( 1 8 5 6 1 8 7 5 ) <'). e di f . s . z a n o n ( b ) , e , p e r gli a n n i successivi al 1925, nei b o l l e t t i n i d e l l ' o s s e r v a t o r i o g e o f i s i c o del s e m i n a r i o p a t r i a r c a l e p u b b l i c a t i , a cura d e l l o zanon ( c l i c r i n g r a z i o dei dati f o r n i t i m i ) , negli a t t i d e l l ' i s t i l l i t o v e n e t o d i s c i e n z e , l e t t e l e ed a r t i . 1 v a l o r i d i p r a g a s o n o s i a l i t a b e l l a i coeff iriditi (i corrcluz ione l o c a l i t à n a p o l i t o r i n o \ e n e / i a p r a g a 1 .unii l a t i t u d i n e 10,1" l5,():>o 1 5 , 1 " 5 0 , 1 " 5 5 , 7 » m e s i g e n n a i o f e b b r a i o 0 . 1 1 0 . 3 1 0 . 3 6 0 . 3 2 0.33 f e b b r a i o m a r z o 0.21 0.32 0.32 0.33 0 . 5 8 m a r z o a p r i l e (1.27 0 . 2 1 0.21 0 . 2 1 0 . 1 2 a p r i l e m a g g i o 0 . 0 3 0.17 0.01 0.13 0.43 m a g g i o g i u g n o 0.07 o.i a 0 . 0 7 0 . 2 3 0 . 5 7 g i u g n o l u g l i o 0 . 0 1 0.25 0 . 1 0 0.27 0.73 l u g l i o a g o s t o (1.07 0.23 (1.25 0 . 1 0 0 . 6 9 a g o s t o s e t t e m b r e 0 . 1 6 0.25 0 . 0 9 0 . 3 6 0 . 5 2 s e t t e m b r e o t t o b r e 0 . 1 1 0 . 1 9 0 . 1 4 0 . 2 7 0 . 2 6 o t t o b r e n o v e m b r e 0 . 2 3 0 . 2 0 0 . 1 7 0 . 2 2 o.lll no veni! i r e d i c e m b r e 0.16 0 . 1 4 0 . 1 6 (1.30 0.24 d i c e m b r e g e n n a i o 0 . 3 2 0 . 1 9 0 . 2 2 0 . 2 9 — m e d i a 0.17 0 . 2 2 0 . 1 8 0.2!! 0.4 1 m i c o l i r i c i k v n ni c o r r e i , v z i i j m ; t r a i . e t e m i ' k h a t u r k m k d i e m e n s i u 1 t " 4 o •t oc ft e g f m a m 0 l a q f m a n g l a s 5 ondo 0 n d q p c a l c o l a t i >u di una s e r i e di dati d e l l e t e m p e r a t u r e m e d i e m e n s i l i ed a n n u e a b b r a c c i a r n e un p e r i o d o di 165 a n n i , dal 1775 al 1939, p u b b l i c a t a da h l a vàc ( ; ) . i n f i n e , i d a t i di i,iind sono stati a t t i n t i da una t a b e l l a di un m a n u a le di c h a r t i e r ( s ) ; p e r ([iiesta s t a z i o n e m a n c a il c o e f l i c i e n l e di c o r r e l a z i o n e ira d i c e m b r e e g e n n a i o . n e l l a t a b e l l a sono r i p o r t a l i i c o e f f i c i e n t i di c o r r e l a z i o n e per le l o c a lità s u d d e t t e , e le r i s p e t tive l a t i t u d i n i . dal c o n f r o n t o di q u e sti dati a p p a i o n o d e l l e reg o l a r i t à d e g n e di r i l i e v o p e r la l o c a l i t à di napoli c ' è u n ' u n i c a o s c i l l a z i o n e a n n u a del coefficiente di c o r r e l a z i o n e , con valori alti nel s e m e s t r e i n v e r n a le e bassi nel semestrie s t i v o : il l'alio c l i c i eoefli( i e n t i siano s t a l i c a l c o lali -li un p e r i o d o di soii i r r e g o l a r e ncil a n d a m e n t o r i c o n l r a n o un m a s s i m o n e l l e s t a g i o n i di m a s s i m o e s t i v o , m a n c a n t e 61! a n n i dà r a g i o n e di q u a l c h e u m i l i a z i o n e a n n u o . pei io a l l r e q u a t t r o s t a z i o n i , si i n v e r n a l e ed uno e i i v o . s e p a r a t i da m i n i m i t r a n s i z i o n e . e da n o t a r e c h e i napoli e s e c o n d a r i o a i o r i n o e i m p o r t a n z a passando a l l e stazioni c o n s i d e r a z i o n e : a p r a g a esso e già p r e d o m i n a n t e r i s p e l l o a q u e l l o i n v e r n a l e , ed il d i s l a c c o ira i d u e c r e s c e a n c o r a per l u n d . la p o i z i o n c del m i n i m o a s s o l u t o è m e n o c a r a t t e r i s t i c a : c a d e in p r i m a v e r a a n a p o l i . v e n e z i a e p r a g a , iu a u t u n n o a t o r i n o e l u n d . vltra r e g o i v e n e z i a , a c q u i s t a m a n m a n o ' i n e pili a l t a pi esc in ( . . i . o w j i a l a r i l à i n t c r c s a n l e è c l i c i \ alm i dei c o c f f i c i c n t i di c o r r e l a z i o n e in l i n e a di m a s i m a c r e s c o n o con la l a t i t u d i n e , il c l i c a p p a r e con p a r t i c o l a r e e v i d e n z a n e l l e m e d i e a n n u e dei c o e f f i c i e n t i stessi ( s o l a e c c e z i o n e : v e n e z i a r i p e t t o a t o r i n o , c h e -i trova <1 a l t r o n d e p r a t i c a m e n t e a l l a sua l e s s a l a t i t u d i n e ) . q u e t o c o m p o r t a m e n t o è o p r a t u t t o p i c c a l o p e r i m e s i p r i m a v e r i l i ed e s i i v i : in a u t u n n o ed i n v e r n o c ' è un m a g g i o r e l i v e l l a m e n t o ( o a d d i r i t t u r a u n ' i n v e r s i o n e di a n d a m e n t o , c o m e p e r i coefficienti di o t l o h r e n o v e m h r e ) : c i ò è e v i d e n t e in r e l a z i o n e col f a t t o o s s e r v a t o d i a n z i , c h e il m a s s i m o i n v e r n a l e p e r d e p r o g r e s s i v a m e n t e di i m p o t l a n z a l'elativa per le l o c a l i t à a m a g g i o r e l a t i t u d i n e . a n c h e il v a l o r e g l o b a l e a n n u o del c o e f f i c i e n t e di c o r r e l a z i o n e tra due m e s i q u a l i a i , n o n n e c e s s a r i a m e n t e c o n t i g u i , ( c a l c o l a t o s e c o n d o il p r o c e d i m e n t o i n d i c a l o in u n a p r e c e d e n t e n o t a ) ('') p e r ( p i a n t o picc o l o . p r e s e n t a lo s t c ( ) a n d a m e n t o c r e s c e n t e da n a p o l i a ' l . u n d . s a l v o a n c h e q u i i e c c e z i o n e di v e n e z i a r i s p e t t o a t o r i n o : n a p o l i 0 . 0 2 8 : l ' o r i n o 0 . 0 8 0 ; v e n e z i a + 0 . 0 4 7 ; p r a g a 0 . ili : lund + 0 . 2 3 7 . i pi e e n t i r i s u l t a l i c o n c o r d a n o nel c o n v a l i d a r e la c i r c o s t a n z a , c h e le m a n i f e s t a z i o n i l e r m i c h e s i a n o p i ù l e g a t e tra di l o r o a l l e a l l e l a t i t u d i n i . c h e non a l l e b a s s e ; a c o n c l u s i o n i a n a l o g h e si è del resto c o n d o n i a n c h e p e r a l t r i f a t t o r i m e l e o i o l o g i c i . torino istillilo ili fisica dell università giugno 1917. ri issum'o dal confronto dei cocfficicnti di correlazione tra le temperature medie di mesi successi vi. calcolali per cinque località. napoli, torino. i enezia. praga, l.und, risulta che essi crescono, in media, concordemente con la lati i lidi ne. i erigono inoltre messe in rilievo delle regolarità riguardanti i andamento annuo dei medesimi. rllìuogral' i v l ' i l'jìeiiia f . . i.n temperatura in italia. vini. t l ' l ' . c.rntr. met. o r m i . ti.. w x i . ;i (]9(l9i. ( 1 a l k i n g > . . meni. i ii. c c n l r . .mei. g e o ! ' . . i' ( s . 1 1 1 ' . ( : i l l o v e r a g . . alti r . a r e . s e . t o r i n o . tu. 7 6 . | 1 9 i 2 i 3 i . l ' i b e r t i a . . alti i . r . istituto v e n e t o s e . l e t i . arti ( 1 8 5 8 1 . ( s ) t o n o m . . il elima ili i eii"sia. in « l a p r o v i n c i a (li venezia >> ili i.. s o r m a n i moretti ( 1 8 8 8 l ( « ) z a n o n p . f . s . . alti r . l l . v e n e t o s e . i . e l t . a n i . liti (1926-271. c i i i i . v v ve \ .. mei. /.>.. 5 7 . 267 (191(1). c ) c h a r i . i e i i c. v . i . . . i nrlcsiuiaeii iiber die (ìrundziilie der matlieinaliselien slalistile. l u n d . 92 ( 1 9 3 1 ì . ("ì r.overa g.. ani r. a ce. si. torino. 77. 311 (1942i. 055_071 adg v–5 n01.pdf annals of geophysics, vol. 45, n. 1, february 2002 55 geomagnetic storms and the occurrence of phase slips in the reception of gps signals edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov institute of solar-terrestrial physics sd, russian academy of sciences, irkutsk, russia abstract we have investigated a dependence of the relative density of gps phase slips on the geomagnetic disturbance level. the study is based on using internet-available selected data from the global gps network, with the simultaneously handled number of receiving stations ranging from 160 to 323. the analysis used four days from the period 1999-2000, with the values of the geomagnetic field disturbance index dst from 5 to – 300 nt. during strong magnetic storms, the relative density of phase slips on mid-latitudes exceeds that for magnetically quiet days by one-two orders of magnitude as a minimum, and reaches a few percent of the total density of observations. furthermore, the level of phase slips for the gps satellites located on the sunward side of the earth was by a factor of 5-10 larger compared with the opposite side of the earth. the level of slips of l1 phase measurements at the fundamental gps frequency is at least one order of magnitude lower than that in l 1 – l 2 measurements. the slips of l1 – l2 measurements are most likely to be caused by the high level of slips of l2 phase measurements at the auxiliary frequency. as an alternative, we developed and tested a new method for determining tec variations using only data on the pseudo-range and phase measurements at fundamental frequency l1. the standard deviation of the tec variations which were obtained in phase measurements at two frequencies, l 1 – l 2 , and at fundamental frequency l 1 , does not exceed 0.1 tecu, which permits this method to be used in strong disturbance conditions when phase slips at auxiliary frequency l2 are observed. 1. introduction the gps satellite navigation system has become a powerful factor of scientific and technological progress worldwide, and enjoys wide use in a great variety of human activities. in this connection, much attention is given to continuous perfection of the gps system and to the widening of the scope of its application for solving the navigation problems themselves, as well as for developing higher-precision systems for time and accuracy determinations. even greater capabilities are expected in the near future through the combined use of the gps with a similar russian system (glonass). recently, the gps system has also gained widespread acceptance in research in the field of geodynamics, in the physics of the earth’s atmosphere, ionosphere and plasmasphere, etc. (davies and hartmann, 1997; klobuchar, 1997). investigations of this kind are not only of purely scientific interest but are also important for perfection of the gps system itself. to address these problems, a global network of receiving gps stations was set up, which consisted, by august 2001, of no fewer than 900 mailing address: prof. edward l. afraimovich, institute of solar-terrestrial physics sd, russian academy of sciences, p.o. box 4026, irkutsk, 664033, russia; e-mail: afra@iszf.irk.ru key words phase slips – geomagnetic disturbances – total electron content 56 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov points, the data from which are placed on the internet. using two-frequency multichannel receivers of the gps global navigation system, at almost any point on the globe and at any time simultaneously at two coherently-coupled frequencies f1 = 1575.42 mhz and f2 = 1227.60 mhz, highly accurate measurements of the group and phase delays are being made along the line of sight (los) between the receiver on the ground and the transmitters on-board the gps system satellites which are in the zone of reception. these data, converted to values of total electron content (tec), are of considerable current use in the study of the regular ionosphere and of disturbances of natural and technogenic origins (solar eclipses, flares, earthquakes, volcanoes, strong thunderstorms, auroral heating, nuclear explosions, chemical explosion events, launches of rockets). we do not cite here the relevant references for reasons of space, which account for hundreds of publications to date (evans, 1977; davies and hartmann, 1997; klobuchar, 1997). the study of deep, fast variations in tec caused by a strong scattering of satellite signals from intense small-scale irregularities of the ionospheric f2-layer at equatorial and polar latitudes has a special place among ionospheric investigations based on using satellite (including gps) signals (aarons, 1982; yeh and liu, 1982; basu et al., 1988; aarons et al., 1996, 1997; klobuchar, 1997; pi et al., 1997; aarons and lin, 1999; bhattacharrya et al., 2000). the interest in this problem as regards the practical implementation is explained by the fact that as a result of such a scattering, the signal undergoes deep amplitude fadings, which leads to a phase slip at the gps working frequencies (skone and jong, 2000). a limitation of the cited references is the fact that they use, as input data, essentially the values of tec determined from the phase difference l 1 – l 2 (see below). for that reason, fatal phase slips that totally prohibit measurements of continuous tec variations, are excluded from the sample statistics reported in the cited references. in this paper we inquire into the question: what are the statistics of fatal phase slips, and how do they depend on the various geophysical factors (the level of geomagnetic disturbance, the latitudinal and diurnal dependencies)? we used a new technology of a global detector globdet, and relevant software which makes it possible to automate the acquisition, filtering and pretreatment process of the gps data received via the internet (afraimovich, 2000). this technology is being used to detect, on global and regional scales, the ionospheric effects of strong magnetic storms (afraimovich et al., 2000a, 2001b), solar flares (afraimovich, 2000; afraimovich et al., 2001c), solar eclipses (afraimovich et al., 1998a), launches of rockets (afraimovich et al., 2000b), earthquakes (afraimovich et al., 2001a), etc. the experimental geometry and general information on the data base used are presented in section 2. the determination of the relative density of phase slips, and the method of processing the data available from the internet are briefly outlined in section 3. section 4 describes the results obtained for magnetically quiet and disturbed conditions. results are discussed in section 5. 2. experimental geometry and general information on the data base used this study is based on using the data from a global network of gps receiving stations available from the internet (http://sopac.ucsd.edu). for a number of reasons, slightly differing sets of gps stations were chosen for the various events under investigation; however, the experimental geometry for all events was virtually identical. the analysis used a set of stations (from 160 to 323) with a relatively even distribution across the globe. for reasons of space, we do not give here the station coordinates. this information may be obtained from http://sopac.ucsd.edu/cgi-bin/ dbshowarraysitesmap.cgi?array=all&array_ option=sitelist. the set of stations, which we selected out of the part of the global gps network available to us, covers rather densely north america and europe; asia has much poorer coverage. the number of gps stations in the pacific and atlantic oceans is even smaller. however, such 57 geomagnetic storms and the occurrence of phase slips in the reception of gps signals fig. 1a-h. variations of the h-component of the geomagnetic field record at station irkutsk (a); dst (b), thick line, and the dependence of the time derivative d(dst(t))/dt (b), dashed line, during the magnetic storm of april 6, 2000. the ut-dependence of the relative mean l1 – l2 slip density p(t) (d), thick line. for comparison, the panel (c) plots the relative mean l 1 slip density p(t). the same dependencies for a magnetic storm of july 15, 2000 (e,f,g,h). the ssc times 16:42 ut and 14:37 ut are shown by a vertical bar. for comparison, the dashed line in panels (d) and (h) plots the dependencies a(t) of the tec variation intensity obtained for the same set of stations as in the case of p(t). 30°-50° n 200°-300°e 30°-50°n 200°-300°e e f h g a b c d 58 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov coverage over the globe is already sufficient for a global detection of disturbances with spatial accumulation unavailable before. thus, in the western hemisphere, the corresponding number of stations can, already today, reach at least 500, and the number of beams to the satellites no fewer than 2000-3000. the analysis involved four days of the period 1999-2000, with the values of the geomagnetic field disturbance index dst ranging from 0 to −300 nt and k p from 2 to 9. the maximum values of the geomagnetic field disturbance index dst max and k pmax are listed in table i. figure 1a-h presents the measured variations of the h-component of the geomagnetic field at station irkutsk (52.2°n; 104.3°e a,e), and dst (b,f thick line) during major magnetic storms on april 6, and july 15, 2000; a correlative analysis of the data is made in section 4. the ssc times 16:42 ut and 14:37 ut are shown by a vertical bar. the statistics of the data used in this paper for each of the days under examination are characterized by the information in table i about the number of stations used m. the total amount of data exceeds 107 30-s observations. 3. the method of processing the data from the internet the purpose of a preprocessing of the gps data in this paper is to obtain slip density estimates in measuring the phase difference l 1 – l 2 , and slips of phase measurement at the fundamental frequency l1. ascertaining the cause of the increase in slip density was also greatly facilitated by estimating the tec variation intensity for the same stations and time intervals. to this end, we also obtained normalized distributions of los elevations and azimuths, for which slips of measurements of the phase difference l1 – l2, and phase l1 slips were recorded for the time interval of our interest. the gps technology provides the means of estimating tec variations on the basis of phase measurements of tec i in each of the spaced two-frequency gps receivers using the formula (afraimovich et al., 1998a) (3.1) where l1λ1 and l2λ2 are phase path increments of the radio signal, caused by the phase delay in the ionosphere (m); l 1 and l 2 are the number of full phase rotations, and λ 1 and λ 2 are the wavelengths (m) for the frequencies f1 and f2, respectively; const is some unknown initial phase path (m); and nl is the error in determination of the phase path (m). phase measurements in the gps can be made with a high degree of accuracy corresponding to the error of tec determination of at least 1014 m2 when averaged on a 30-s time interval, with some uncertainty of the initial value of tec, however (hofmann-wellenhof et al., 1992). for definiteness, we bring the tec variations into the region of positive values with the minimum value equal to 0 by adjusting the constant term const in eq. (3.1). the tecu (total electron content units), which is equal to 1016 m–2 and is commonly accepted in the literature, will be used throughout the text. i f f f f l l0 1 2 2 2 1 2 2 2 1 1 2 2 1 40 308 = − −( ) +[ . λ λ nl+ + ]const table i. statistics of experiments. n date day number m dstmax, nt kpmax 1 29.07.1999 210 160 −40 3 2 9.01.2000 009 323 −13 − 3 6.04.2000 097 243 −293 8 4 15.07.2000 197 306 −295 9 59 geomagnetic storms and the occurrence of phase slips in the reception of gps signals primary data include series of «oblique» values of tec i0 (t), as well as the corresponding series of elevations θ(t) and azimuths α(t) along los to the satellite calculated using our developed convtec program which converts the gps system standard rinex-files on the internet (gurtner, 1993). series of the values of elevations θ(t) and azimuths α(t) of the beam to the satellite were used to determine the coordinates of subionospheric points, and to convert the «oblique» tec i 0 (t) to the corresponding value of the «vertical» tec i(t) by employing the technique reported by klobuchar (1986). 3.1. the relative density of difference phase l 1 – l 2 slips and of phase l 1 slips we hold fixed a slip of the phase difference l1 – l2 in the case where the modulus of the tec increment for a time interval of 30 s (which is a standard one for the gps data placed on the internet), exceeds the specified threshold of order, for example, 100-200 tecu. a slip of phase l1 is also fixed in a similar manner but with a much larger threshold and with due regard for the time varying distance to the satellite. thus we point out that it is the fatal slips which totally prohibit the determination of the increment of tec from the measures value of the phase difference l1 – l2. at the same time there can be numerous phase slips whose absolute value is smaller than the threshold which we selected. such slips that accompany tec variations of only a few tecu, are caused by ionospheric irregularity effects, and have been thoroughly studied in a large number of publications (basu et al., 1988; aarons et al., 1996, 1997; klobuchar, 1997; pi et al., 1997; aarons and lin, 1999; bhattacharrya et al., 2000). as a result of a pretreatment of the rinexfiles, we have the number s of phase slips within a single selected time interval dt = 5 min, as well as the corresponding number m of observations required for normalizing the data. these data for each of the gps satellites were then averaged for all the stations selected in order to infer the mean density of observations m(t) and the mean density of phase slips s(t). in the middle of the observed satellite pass, the density of observations m(t) averages 10 ± 1 (30-s counts); at the beginning and end of the pass it can decrease because the time intervals of observation of a given satellite at elevations larger than that specified do not coincide at different stations. subsequently, we calculated the mean relative density of phase slips p(t) = s(t)/m(t), %. furthermore, the daily mean value of the relative number of phase slips 〈p〉 that was averaged over all gps satellites and stations was useful for our analysis. as would be expected the mean observation density m(t) for a single satellite exhibits a diurnal variation that is determined by the satellite’s orbit, and varies over the range from 0 to 8. 3.2. estimation of the tec variation intensity we used the series i(t), containing neither slips of the phase difference l1 – l2 nor gaps of counts, to estimate the tec variation intensity for the same sets of stations and time intervals as used in estimating the phase slip density. to exclude the variations of the regular ionosphere, as well as trends introduced by the motion of the satellite, we employ the procedure of removing the linear trend by preliminarily smoothing the initial series with a selected time window of a duration of about 60 min. in a subsequent treatment, we use the standard deviation of the tec variations di(t), thus filtered, as an estimate of the tec variation intensity a (see section 4). figure 2a gives an example of a typical weakly disturbed variation in «vertical» tec i(t) for station wes2 (42.6°n, 288.5°e); satellite number prn10 on july 15, 2000 for the time interval 14:00-16:00 ut, preceding the onset of a geomagnetic disturbance near the wes2 station. for this same series, fig. 2b presents the di(t) variations that were filtered out from the i(t) series (rms of di(t) is smaller then 0.2 tecu). in this paper, we use the term prn (pseudo random noise) to designate the satellite number (hofmann-wellenhof et al., 1992). 60 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov fig. 2a-h. time dependencies of the «vertical» tec i(t), measured at station wes2 (a) prior to the onset of the geomagnetic disturbance of july 15, 2000, and at station wes2 thereafter (e). the di(t) variations filtered from the i(t) series by removing the trend with a 60-min window for this station (b,f). for comparison, the dashed line in panels (b) and (f) plots the di(t) variations calculated from values of the pseudo-range c 1 and of the phase l 1 . distributions p(δ) of the standard deviation δ of the tec variations obtained in phase measurements at two frequencies l 1 – l 2 and at the l 1 only (c,d,g,h). a b c e f d h g 61 geomagnetic storms and the occurrence of phase slips in the reception of gps signals strong variations in tec variation intensity occurred near station wes2 literally within 6 h. figure 2e and fig. 2f present the dependencies i(t) and di(t) for station wes2 for the time interval 20:00-22:00 ut (prn23). as is evident from the figure, the tec variations increased in intensity by a factor of at least 30 when compared with the time interval 14:00-16:00 ut (fig. 2a,b). 3.3. conditions and limitations of data processing slips of phase measurements can be caused by reception conditions for the signal in the neighbourhood of the receiver (interference from thunderstorms, radiointerferences), which is particularly pronounced at low elevations θ. to exclude the influence of the signal reception conditions, in this paper we used only observations with satellite elevations θ larger than 30°. since we are using a global averaging of the number of phase slips for all los and stations, as a consequence of the uneven distribution of stations the proportion of midlatitude stations of north america and, to a lesser extent, of europe is predominant (see above). at the same time, the number of stations in the polar region of the northern hemisphere and in the equatorial zone was found to be quite sufficient for a comparative analysis. to compare the results, we chose 3 latitude ranges: high latitudes 50-80°n; mid-latitudes 30-50°n; and equatorial zone 30°s-30°n. we selected also the data according to the types of two-frequency receivers, with which the gps global network sites are equipped (the relevant information is contained in the initial rinex format). 4. results derived from analyzing the relative density of phase slips 4.1. magnetically quiet days figure 3a-f plots the local time ltdependence of the relative mean slip density p(t) obtained by averaging the data from all satellites in the latitude range 0-360°e irrespective of the type of gps receivers for the magnetically quiet days of july 29, 1999 (at the left) and january 9, 2000 (at the right). the local time for each gps station was calculated, based on the value of its geographic longitude. the number n of satellite passes used to carry out an averaging is marked in all panels. as is evident from fig. 3b, the phase slips on a magnetically quiet day at mid-latitudes have a sporadic character. the daily mean value of the relative density of phase slips 〈p〉, averaged over all gps satellites and stations, was 0.017% for the magnetically quiet day of july 29, 1999. similar data were also obtained for high latitudes (fig. 3a). in the equatorial zone, however, even on a magnetically quiet day, the density of phase slips exceeds the latitudinally mean value of p(t) at least by a factor of 15, and shows a strongly pronounced lt-dependence, with a maximum value of 1.52% (fig. 3c). for the other magnetically quiet day of january 9, 2000, however, the mean value of 〈p〉 at mid-latitudes was already larger 0.06% (fig. 3e). for the diurnal p(t)-dependence on january 9, 2000, one can point out the irregularity of the mean density of phase slips as a function of local time lt. 4.2. magnetic storms of april 6 and july 15, 2000 a totally different picture was observed on april 6, 2000 during a strong magnetic storm with a well-defined ssc. figure 1d (thick line) presents the variations of the ut-dependence of the relative mean slip density p(t) obtained for the territory of north america (200-300°e) at mid-latitudes 30-50°n by averaging the data from all satellites. in this case, for the purpose of achieving a clearer detection of the effect of the magnetic storm ssc influence on the p(t)-dependence, we chose only those gps stations which were on the dayside of the earth at the ssc time (north america region). noteworthy is a well-defined effect of an increase in the density of phase slips that occurred after ssc. 62 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov fig. 3a-f. local time lt-dependence of the relative mean slip density p(t), obtained by averaging the data from all gps satellites in the longitude range 0-360°e irrespective of the type of gps receivers for the magnetically quiet days of july 29, 1999 (at the left), and january 9, 2000 (at the right): a,d) high latitudes 50-80°n; b,e) midlatitudes 30-50°n, and c,f) equatorial zone 30°s-30°n. a b c e f d 30-50°n 30°s-30°n 50-80°n 0-360° e 63 geomagnetic storms and the occurrence of phase slips in the reception of gps signals a maximum mean slip density p max = 2.4% is attained 3-4 h after an ssc. the same values, averaged over all observed satellites and midlatitude stations (0-360°e) but as a function of local time lt, are plotted in fig. 4b. first of all, it should be noted that the relative density of phase slips p(t) exceeds that for magnetically quiet days by one (when compared with january 9, 2000) or even two (when compared with july 29, 1999) orders of magnitude, and reaches a few percent of the total observation density. the mean value of 〈p〉 for this storm is 0.67%, which is by a factor of 40 larger than that of 〈p〉 for july 29, 1999, and by a factor of 10 larger than that for january 9, 2000. it was also found that the averaged (over all satellites) level of phase slips for the gps satellites on the subsolar side of the earth is by a factor of 10 larger than that on the opposite side of the earth (fig. 4b). similar dependencies with a maximum slip density p max = 3.37% and a sharply pronounced diurnal dependence were also obtained for equatorial latitudes (fig. 4c). on the other hand, although the high latitudes show a 10-fold increase of 〈p〉 as against a magnetically quiet day, no ltdependence is observed (fig. 4a). a similar result confirming all of the abovementioned features of the april 6, 2000 storm was also obtained for the other magnetic storm of july 15, 2000 (see the measurements at magnetic observatory irkutsk in fig. 1e, the universal time (ut) dependence in fig. 1h and the local time (lt) dependencies of the relative mean density of phase slips p(t) obtained by averaging the data from all gps satellites, in fig. 4d-f. the mean value of 〈p〉 for this storm at midlatitudes is 0.34%, which is also in appreciable excess of the level of phase slips for magnetically quiet days. the effect of an increase in the density of phase slips after ssc is clearly pronounced for this storm as well (fig. 1h, thick line; see below). 4.3. correlation of the increase in slip density and tec variation intensity equatorial latitudes are characterized by strong scintillations of the transionospheric signal caused by the scattering from f 2 -region ionization irregularities (aarons, 1982; yeh and liu, 1982; basu et al., 1988; aarons et al., 1996, 1997; klobuchar, 1997; pi et al., 1997; aarons and lin, 1999; bhattacharrya et al., 2000). since during the active phase of the magnetic storm the mid-latitude ionosphere becomes increasingly inhomogeneous, it might be anticipated that a similar mechanism is able to cause appreciable scintillations of the gps signal at mid-latitudes as well. to verify this hypothesis, we determined the dependencies a(t) of the tec variation intensity obtained for the same set of stations as in the case of p(t) (see section 3). the dependencies a(t) as a function of ut (fig. 1d,h dashed line) and lt (fig. 4b,e dashed line) are presented below by averaging (over all gps satellites and stations) the standard deviation of the variations di(t) (afraimovich et al., 2001b). in fig. 1h, the dashed line represents the dependence a(t) of the tec variation intensity obtained for all satellites and for the territory of north america at mid-latitudes 30-50°n during the magnetic storm of july 15, 2000. as is apparent from this figure, the dependence a(t) correlates quite well with the ut-dependence of the relative mean slip density p(t) calculated from the same set of stations as in the case of a(t). a similar result on the ut-dependence was also obtained for a major magnetic storm of april 6, 2000 (fig. 1d, dashed line). a correlation of the increase in slip density and in tec variation intensity is shown as clearly by the lt-dependencies a(t) for the magnetic storms of april 6 and july 15, 2000, presented in fig. 4b, e (dashed line). it was found that an increase in the level of geomagnetic activity is accompanied by an increase in total intensity of a(t); however, it correlates not with the absolute level of dst, but with the value of the time derivative of dst (a maximum correlation coefficient reaches – 0.94, fig. 1b,f, dashed line). the derivative d(dst)/dt was obtained from the dependence dst(t) (fig. 1b,f, dashed line) that was smoothed with a 7-h time window. this result is in reasonably good agreement with the conclusions drawn by ho et al. (1998) and afraimovich et al. (2000a, 2001b). 64 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov e fig. 4a-f. same as in fig. 3a-f, but for the magnetic storms on april 6 (at the left) and july 15, 2000 (at the right) at mid latitudes. for comparison, the dashed line in panels (b) and (e) plots the dependencies a(t) of the tec variation intensity obtained for the same set of stations as in the case of p(t). a b c f d 50-80°n 30-50°n 30°s-30°n 30-50°n; 0-360°e 65 geomagnetic storms and the occurrence of phase slips in the reception of gps signals 4.4. the dependence of the slip density of phase measurements l1 – l2 and l1 on the type of gps receivers we carried out an analysis of the dependence of the density of fatal slips on the type of gps receivers; furthermore, we verified slips for which of the channels (l1 or l2) are the cause of slips in measurements of the phase difference l 1 – l 2 . the sample statistics of phase slips for the main types of two-frequency receivers (ashtech, trimble, aoa), installed at the global gps network sites, are presented in fig. 5a-f and fig. 6a-f. an analysis of slips in measuring the phase difference l1 – l2 and of the phase l1 was carried out for the magnetic storms of april 6, and july 15, 2000. figure 5a-f plots the ut-dependencies of the relative mean slip density p(t) of phase measurements of l 1 – l 2 (at the left) and phase measurements of l 1 only (at the right) obtained by averaging the data from all satellites in the longitude range 200-300°e at the mid-latitudes 30-50°n for major magnetic storm on april 6, 2000. the data from fig. 5a suggest that for the ashtech receivers, the slip density of phase measurements at two frequencies l1 – l2 is by a factor of 5-20 smaller than that for the other types of receivers. these estimates are only slightly exceeded by the slip density for the trimble receivers (fig. 5b). the aoa receivers are the most susceptible to slips of l 1 – l 2 measurements (fig. 5c). the mean and maximum values of p(t) exceed the respective values for the ashtech receivers during the magnetic storm of april 6, 2000, at least by a factor of 10-20. figure 6a-f plots the ut-dependencies of the relative mean slip density p(t) of phase measurements of l 1 – l 2 (at the left) and phase measurements of l1 only (at the right) obtained by averaging the data from all satellites in the longitude range 0-360°e at the equatorial zone 30°s-30°n for major magnetic storm on april 6, 2000. the statistics of the l 1 – l 2 slips for the aoa and trimble receivers differ almost not at all from the data for mid-latitudes. for the ashtech receivers, however, the density of slips in the equatorial zone was found to be slightly lower when compared with the aoa receivers. on the other hand, the level of slips of l1 phase measurements at the fundamental gps frequency (fig. 1c,g, fig. 5 a-f and fig. 6a-f, at the right) is at least one order of magnitude lower than that in l 1 – l 2 measurements. the corresponding dependencies p(t) as a function of universal time (ut) on the dayside for the magnetic storms of april 6 and july 15, 2000, are plotted in fig. 1c and g, respectively. although the level of the slips is lower by one order of magnitude (as compared to p(t) for l1 – l2, fig. 1d,h), the fundamental frequency l 1 does show an increase in the relative density of slips in the main phase of the magnetic storm. similar results of a comparison of the level of l 1 – l 2 and l 1 slips for different types of gps receivers were also obtained for the magnetic storm of july 15, 2000. this leads us to conjecture that the slips of l1 – l2 measurements are most likely to be caused by the high level of slips of l 2 phase measurements at the auxiliary frequency. according to our data, these slips are observed at equatorial latitudes under quiet conditions as well, and at mid-latitudes they increase with increasing geomagnetic activity. this means that relatively long-term (about 10 years) data from the global gps network equipped with several hundred older-generation commercial receivers cannot be exploited in full measure to obtain data on tec variations, based on a standard processing of the phase difference l1 – l2. this limitation is legitimate for the period of the main phase of the magnetic storm and of the dayside ionosphere which is characterized by fast, profound tec variations. 4.5. determination of tec variations at the fundamental frequency l 1 some way out of the situation would be by exploiting the method (afraimovich et al., 1998b) for determining tec variations using 66 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov 30-50°n; 200-300°e a b c e f d fig. 5a-f. ut-dependencies of the relative mean slip density p(t) of l 1 – l 2 phase measurements (at the left) and phase measurements of l 1 only (at the right) for a major magnetic storm of april 6, 2000 (a,d ashtech; b,e trimble; and c,f aoa receivers). a p r i l 6 , 2 0 0 0 67 geomagnetic storms and the occurrence of phase slips in the reception of gps signals 30°s-30°n ; 0°-360°e a b c e f d fig. 6a-f. same as in fig. 5a-f, but at the equatorial zone 30°s-30°n. 68 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov only the data on the pseudo-range c and phase measurements at the fundamental frequency l1. in this study the concerned method was tested with the database mentioned in table i. for testing purposes, time intervals were chosen with no data gaps and slips of phase slip measurements of both l1 – l2 and l1. phase changes at frequencies f1 and f2 may be represented as the sum of different components φς = φs + φ + φ0, where φs = 2π c/λ is the fundamental component caused by a change of the distance d to the satellite it follows its orbit; φ = 8.42 ⋅ 10−7 i/f is the ionospheric component proportional to tec at los between the receiver and the satellite (spoelstra and kelder, 1984); and φ0 is the initial phase. to subtract φs, it is possible to use current information on the pseudorange c for each satellite (hofmannwellenhof et al., 1992). for the sake of comparison with the di(t) variations obtained using the standard tec measurement technique described in section 3, from data on the phase difference l 1 – l 2 (fig. 2b, thick lines), dashed lines plot the variations of the «vertical» tec value obtained by measuring the l1 phase and filtered off (as is done for the data on l 1 – l 2 ) by removing the trend with the 60-min time window. it is evident from the figure that the difference of the di(t) dependencies from each other does not exceed in magnitude the value 0.1 tecu, and is more pronounced only for the quiet period 14:0016:00 ut. for the disturbed time interval 20:00-22:00 ut (fig. 2f ), the corresponding dependencies are virtually similar and are not distinguishable. figure 2a-f also presents the distributions p(δ) of the standard deviation δ of tec variations obtained in phase measurements at two frequencies l 1 – l 2 and at the fundamental frequency l 1 . panels c, d, g and h show the time intervals and the number n of satellite passes, over which the averaging was performed. during the quiet period 10:00-12:00 ut on april 6 and july 15, 2000 (fig. 2c,g), the most probable value of δ was 0.04 and 0.05 tecu, and was nearly twice as large as the corresponding value for the same time interval on the magnetically quiet day of july 29, 1999. furthermore, the mean values of δ were by a factor of 1.2-1.5 larger than the most probable ones. for the disturbed period 20:00-22:00 ut (fig. 2d,h), the most probable value of δ remained virtually unchanged; however, the mean values of δ now were larger than the most probable ones by a factor of 2-3. this is indicative of a more significant discrepancy between results of a calculation of the di(t) variations from l1 – l2 and l1. thus the standard deviation of the tec variations obtained in phase measurements at two frequencies l1 – l2 and at the fundamental frequency l1 does not exceed 0.1 tecu, which makes this method useful for strong disturbance conditions where slips at the auxiliary frequency l2 are observed. of course, this method is appropriate for solving only those problems, for which it will suffice to single out the tec variations, and is unsuitable for calculating the absolute value of tec. 5. discussion and conclusions the main results of this study may be summarized as follows: 1) we have detected a dependence of the relative density of phase slips in some gps receivers on the disturbance level of the earth’s magnetosphere during major magnetic storms. 2) during major magnetic storms the relative density of phase slips at mid-latitudes exceeds that for magnetically quiet days at least by one or two orders of magnitude, and reaches a few percent of the total observation density. 3) the level of phase slips for the gps satellites on the sunward side of the earth is by a factor of 5-10 times that on the opposite side of the earth. 4) the level of slips of l 1 phase measurements at the fundamental gps frequency is at least one order of magnitude lower than that in l1 – l2 measurements. the slips of l1 – l2 measurements are most likely to be caused by the high level of slips of l 2 phase measurements at the auxiliary frequency. however, the reason for the slips themselves can include several factors: the influence of additive interferences in the case of a low signal/ noise ratio, the signal scattering from electron 69 geomagnetic storms and the occurrence of phase slips in the reception of gps signals density irregularities, and the inadequate response of gps receivers of some types to fast changes in daytime tec at the auxiliary frequency l 2 . let us analyze these factors separately. 5) the lower signal/noise ratio at l2 is primarily due to the fact that the l2 power at the gps satellite transmitter output is on 6 db of magnitude smaller compared with the fundamental frequency f 1 using c/a code (icd200, 1994; langley, 1998). similar correlations of the effective radiated power of l1 (30 watt) and l 2 (21 watt) signals are also characteristic for the russian glonass system (kharisov et al., 1998). phase slips at l2 can also be caused by the lower signal/noise ratio when use is made of commercial noncoded receivers for the frequency l 2 , with which the global gps network stations are equipped. these receivers have no access to the military «y» code, and have to use the noncoded or semi-noncoded mode of reception. as a consequence, the signal/noise ratio at the frequency l2 is, at best, 13 db lower than the mode of fully coded reception. thus the difference in signal powers at l1 and l 2 for commercial receivers can become larger than 10 db, which can lead to an increase in the level of l2 slips because of the influence of additive interferences. different types of gps receivers respond to this differently; on the whole, however, the picture of the dependence on the local time, latitude range, and on the level of geomagnetic activity remains sufficiently stable. 6) however, the deterioration of the signal/ noise ratio with an increase in the level of geomagnetic disturbance is possible if this is accompanied by an enhancement of the proportion of the signal scattered from ionospheric electron density irregularities. this also involves an increase in the number of phase slips; for a more powerful l1 signal, however, the density of slips is an order of magnitude smaller than that for the less powerful l 2 signal. the high positive correlation of an increase in the density of phase slips p(t) and the intensity of ionospheric irregularities a(t) during geomagnetic disturbances as detected in this study points to the fact that the increase in slips p(t) is caused by the scattering of the gps signal from ionospheric irregularities. nevertheless, the presence of the daytime maximum in the diurnal density distribution of fatal slips and of the tec variation amplitude is inconsistent with existing data on the night-time intensity maximum of equatorial scintillations of transionospheric signals (aarons, 1982; yeh and liu, 1982; aarons et al., 1996, 1997; klobuchar, 1997; pi et al., 1997; aarons and lin, 1999). besides, the level of slips at mid-latitudes was found to be unusually high. this would suggest that the reason for the increase in density of fatal phase slips during geomagnetic disturbances is that the type of gps signal scattering from ionospheric irregularities is different from that causing signal scintillations in the case of the scattering from f region irregularities at the local night-time in the equatorial zone. 7) during magnetic storms, the border between «mid» and «higher» latitudes moves equatorwards which brings scintillation producing ionospheric irregularities into the gps to ground receiver rays. phase slips of the gps signal can be caused by the scattering from the geomagnetic field-aligned ionospheric e-layer small-scale irregularities (smaller than or on the order of the radius of the fresnel zone), the amplitude of which increases with increasing geomagnetic disturbance level (foster and tetenbaum, 1991). such a scattering can lead to strong signal fadings, and even to a particle screening of the «gps satellite-receiver» propagation path. 8) the increase in density of fatal slips during geomagnetic disturbances on the dayside can also be caused by limitations of the design and adjustment of the receivers used in the analysis, rather than the gps signal scattering from ionospheric irregularities. an increase in slip density can be caused in this case by an inadequate response of gps receivers of some types (such as aoa turbo rogue) to fast changes in daytime tec at the auxiliary frequency l 2 . this effect prevents the identification of slips caused by phase fluctuations induced by the scattering from ion density irregularities. unfortunately, we are unaware of any consistent investigation of this 70 edward l. afraimovich, oleg s. lesyuta, igor i. ushakov and sergey v. voeykov kind or published data lending support to such a point of view. 9) as an alternative, we developed and tested a new method for determining tec variations using only data on the pseudo-range and phase measurements at fundamental frequency l1. the standard deviation of the tec variations obtained in phase measurements at two frequencies, l 1 – l 2 , and at fundamental frequency l 1 , does not exceed 0.1 tecu, which permits this method to be used in strong disturbance conditions when phase slips at auxiliary frequency l 2 are observed. of course, phase measurements are more sensitive to equipment failures and to interferences in the gps «satellite-receiver» channel when compared with group delay measurements which are directly used in solving navigation problems. therefore, it is necessary to monitor the errors of determining the position of stationary sites of the global gps network, based on the data in the rinex-format available from the internet, and to analyze these series in conjunction with the data on the conditions of the near-terrestrial space environment. we are aware that this study has revealed only the key averaged patterns of this influence, and we hope that it would give impetus to a wide variety of more detailed investigations. acknowledgements the authors are grateful to v.g. eselevich, v.v. yevstafiev, k.s. palamarchouk, and g.v. popov for their encouraging interest in this study and active participations in discussions. we are also indebted to s.a. nechaev for the data from magnetic observatory irkutsk made available to us. thanks are also due to v.g. mikhalkovsky for his assistance in preparing the english version of the tex-manuscript. we greatly appreciate the referee’s effort to improve the submitted manuscript. this work was done with support from both the russian foundation for basic research (grants 99-0564753 and 00-05-72026) and rfbr grant of leading scientific schools of the russian federation 00-15-98509. references aarons, j. 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(1998): gps for geodesy (springer berlin, heidelberg, new york, barcelona, budapest, hong kong, london, milan, paris, singapore, tokyo), 111-149. pi, x., a.j. mannucci, u.j. lindgwister and c.m. ho (1997): monitoring of global ionospheric irregularities using the worldwide gps network, j. geophys. res. lett., 24, 2283-2286. sko n e, s. and m. d e jo n g (2000): the impact of geomagnetic substorms on gps receiver performance, earth, planets, space, 52, 1067-1071. spoelstra, t.a. th. and h. kelder (1984): effects produced by the ionosphere on radio interferometry, radio sci., 19, 779-788. yeh, k.c. and c.h. liu (1982): radio wave scintillations in the ionosphere, proc. ieee, 70 (4), 324-360. estimating depths and dimensions of gravity sources through optimized support vector classifier (svc) annals of geophysics, 59, 3, 2016, s0319; doi:10.4401/ag-7029 s0319 estimating depths and dimensions of gravity sources through optimized support vector classifier (svc) mohammad ehsan hekmatian1,2,*, vahid ebrahimzadeh ardestani3, mohammad ali riahi3, ayyub memar koucheh bagh2 1 islamic azad university, faculty of basic sciences, science and research branch, tehran, iran 2 nuclear fuel cycle research school, nuclear science and technology research institute (nstri), tehran, iran 3 institute of geophysics, university of tehran, tehran, iran abstract by researching and applying new methods we will be able to improve significantly estimation of shapes, dimensions and depths of gravity sources. after shapes estimation of gravity sources through support vector classifier (svc) in our last research [hekmatian et al. 2015], in this paper svc is applied for estimating depths and dimensions of gravity sources. these estimations give us logical and complete initial guesses regarding shapes, depths and dimensions of gravity sources which are needed in more precise interpretations and inversions of gravity sources. also for better application of svc, we selected more proper features using the technique called feature selection (fs). in this paper, we trained svc with 320 synthetic gravity profiles for estimation of dimensions and depths of gravity sources. we tested the trained svc codes by about 200 other synthetic and some real gravity profiles. the depths and dimensions of a well along with two ore bodies (three real gravity sources) are estimated during the testing process. 1. introduction the dimensions and depths estimation of the gravity sources contains errors. applying new methods can decrease the estimation errors. in this research, we show that svc algorithm is a proper method for the mentioned estimations. support vector classifier (svc) algorithm is one of the pattern recognition (pr) algorithms and is considered as a supervised learning algorithm (that should be trained) of pr [van der heijden et al. 2004]. regarding producing training data in gravity, it should be mentioned that we need to create several synthetic gravity profiles caused by synthetic gravity sources with different definite shapes, depths and dimensions. then, values of proper features from each of synthetic gravity profiles will be extracted (see sections 2.2 and 3). these values are used to train the supervised learning algorithm (svc) and this trained algorithm would be able to estimate dimensions and depths of gravity sources which their approximate shapes was estimated earlier. neural network has been used frequently for gravity interpretation and other geophysical applications [gret and klingele 1998, van der baan and christian 2000, osman et al. 2006] unlike the svc. svc algorithm has very suitable properties such as single global solution and ability to use different kernels. it should be mentioned that one of these kernels is gaussian (or rbf) kernel [see van der heijden et al. 2004]. it is proper for separating the objects of one class article history received april 11, 2016; accepted june 14, 2016. subject classification: gravity sources depths and dimensions, gravity profile, svc, feature, fs. figure 1. the boundary between star shape points and cross shape points shows an example of gaussian (or rbf) kernel of svc. horizontal and vertical axes show 2 features used in the classification. that its features are distributed in feature space in a nonregular condition (see figure 1 as an example). more usually the features of the objects related to each shape of gravity sources are distributed in the mentioned manner in the feature space. so this nonlinear kernel has been applied for shape estimation of gravity sources [hekmatian et al. 2015]. also according to [moran and lavrenko 2011], minkowski kernel (see section 2.1) mimics logical and of variation in feature values, whilst gaussian (or rbf) kernel is closer to logical or. estimating the shapes of gravity sources (separating the class of objects related to each gravity source shape) is a logical or, since each shape is not related to other shapes. on the other hand, for estimation of dimensions or depths of different gravity sources, we are obliged to select two different classes of depths or dimensions. for example, suppose that we want to estimate the depths of gravity sources with spherical shapes with a radius of about 20 m, and we know that these gravity sources are located at a depth between 5 m to 1000 m. in these conditions we have two classes: one class is the spherical gravity sources with depth equal to 5 m, and the other class is the same gravity sources but with depth equal to 1000 m. so we should use a proper training set consisting of these two classes. also we should use a nonlinear kernel of svc which can estimate the depth of the gravity sources by detecting the probability of each of the two mentioned classes for the gravity sources. for example, if one of the gravity sources is related to class 1 (with a depth of 5 m) with a probability equal to 0.2 and the other gravity source is related to class 2 (with a depth of 1000 m) with a probability equal to 0.8, then its estimated depth is (0.2*5) + (0.8*1000) = 801 m. so a kernel that mimics logical and could be a proper one for estimating depths and dimensions of gravity sources. therefore, we applied minkowski kernel for estimations of the depths and dimensions of gravity sources. for better application of svc, we need to optimize svc by selecting proper features. we have done that by applying fs. 2. theories and algorithms 2.1. support vector classifier (svc) algorithm 2.1.1. linear svc prior to linear svc description we should mention that support vector machine (svm) is a concept in computer science for a set of related supervised learning methods that analyse data and recognize patterns used for classification and regression analysis, also the original svm formulations for classification is named svc. svc is clarified briefly in this and the next section according to van der heijden et al. [2004]. linear (and nonlinear) svc is considering only 2 classes of information that can be separated by a linear border. one class will be located on the one side of the linear border and the other will be located on the other side of the linear border. if there are more than 2 classes, all classes except one of them should consider as one class and the remaining one as the other class. we should know that in svc the border between the 2 classes is not a line but a margin and this border line is in the centre of the margin. the linear discriminant function that is used in linear svc is [van der heijden et al. 2004]: g(zn)= w* zn + b, (1) where zn is the matrix of the values of the features (measurement vectors), w is the vector of coefficient and b is a scalar value. the rows of zn are the features of each object (each gravity profile) and the columns of this matrix show the kind of features in all objects. two classes (cn = 1 and cn = −1 are labels of two classes) are defined by van der heijden et al. [2004] as: wtzn+ b≥1 if cn=+1 (2) wtzn+ b≤−1 if cn=−1, (3) where wt means transpose of w. the length of the margin between these two classes is shown in figure 2. for more explanations regarding linear svc, one of proper references is van der heijden et al. [2004]. 2.1.2. nonlinear svc at first some definitions should be mentioned. it should be mentioned that decision function is a function that maps the measurements space (features space) hekmatian et al. 2 figure 2. the linear support vector classifier. the boundary between 2 classes is not only a margin but also is a margin with maximum width and this shows the single global solution of svc [van der heijden et al. 2004]. 3 on to the set of possible classes. it should emphasized that in svc only 2 classes exist. therefore in linear and nonlinear svc, decision function is consist of only 2 discriminant function (g(z)≥1 and g(z)≥−1 and notice to relations 1 to 3). to have nonlinear boundaries in svc, the discriminant function should be defined as [van der heijden et al. 2004]: g(z) = wty(z) =∑ nsn=1 k(z, zn), (4) where k(z, zn) is the kernel (such as polynomial one) and ns is the number of samples in the training set. in relation 4, g(z) = 1 and g(z) = −1 show upper limit and lower limit of the margin of nonlinear svc respectively (see figure 2 for comparing with linear svc). minkowski kernel is a kind of nonlinear kernel [duin et al. 2007]: k(zn,zm) = (∑ ns m=1|zn− zm| p) 1−p , (5) where in equation (5) p is a parameter for increasing or decreasing the power. as we have mentioned earlier, we used this kernel in the depth and dimension estimations. 2.2. the features and features selection (fs) algorithm feature is a kind of measurement value which is obtained from each object. in this paper, object is a gravity profile which is created by a specific gravity source. features that can be used for defining the depths and dimensions of the gravity sources are derivable from a principal gravity profile. gret and klingele [1998] noted that the principal gravity profile is the gravity profile passing through the maximum value along the anomaly and crossing the anomaly lines perpendicularly. gravity profiles in this research mean principal gravity profiles. some examples of the different features used to characterize the gravity anomalies are defined as [gret and klingele 1998]: f1 = xg50/xg75, (6) f2 = xg75/xg50, (7) f3 = xginf/xg75, (8) f4 = (xg50− xg66)/(xg80− xg66), (9) where in the above equations, gy is the value of gravity (g) at y% of maximum value of gravity gmax and xgy is the value of x at gy (for more explanations we should say that in our research x is the horizontal distance in the direction of the profile from the location of gmax in the profile). also xginf is the value of x at ginf and ginf is the value of gravity (g) at inflection point of the gravity profile. in figure 3 an example of a gravity profile and some of the features describing the depths and dimensions of gravity sources are shown. choosing proper features (features selection fs) is very important while dealing with high misclassification in testing our classifier (more proper features are those features that when are used in classification leads to better separation of the classes). the high misclassification means in our selected features space, the different classes are not separate properly. one of the enable fs algorithms is “inter/intra class distance” (iicd) and we have used this algorithm in our research. iicd algorithm like all other fs algorithms, has a criterion for selection of more proper features. this criterion is called performance measure (pm). pm is increased when the suitability of the features for classification is increased and vice versa in other words selection of more proper features is done when their related pm is bigger. pm of iicd is based on euclidean distance between each of the two objects in the training set (all objects used for training svc) in the features space. for more explanation about “inter/intra class distance” (iicd) algorithm, we should mention that as van der heijden et al. [2004] noted ts is a training set with ns samples (objects). the classes wk are represented by subsets tk (subsets tk are subsets of objects of ts), each class having nk samples. measurement vectors in ts without reference to their class are denoted by zn (for each object we measure a depth and dimension estimation using svc figure 3. some of the features (f6, f7, f8 and f9) describing the depths and dimensions of gravity sources [gret and klingele 1998]. set of features, this set of features is called measurement vector; the point location of each object in features space is defined with its measurement vector). measurement vectors in tk (i.e. vectors coming from class wk) are denoted by zk,n. the sample mean of class wk is [van der heijden et al. 2004]: the sample mean of the entire training set is [van der heijden et al. 2004]: we can represent the distances between objects or samples (of the training set) by means of scatter matrices. a scatter matrix gives some information about the dispersion of a population of samples around their mean. for instance, the matrix that describes the scattering of vectors from class wk is [van der heijden et al. 2004]: sk supplies information about the average distance of the scattering in class wk. averaged over all classes, the scatter matrix is [van der heijden et al. 2004]: this matrix is “the within-scatter matrix” as it describes the average scattering within classes. in addition to this is “the between-scatter matrix” sb that describes the scattering of the class-dependent sample means around the overall average [van der heijden et al. 2004]: we know that an individual number is a kind of matrix. so that when “the within-scatter matrix” (sw) is an individual number, it shows the squared average distance of the location of each object in one class from the average location of all objects of that class in features space for all the classes. in this case we name sw as intraclass distance, and when “the between-scatter matrix” (sb) is an individual number, it refers to the squared average distance of the average location of all objects of each class from the average location of all objects of all classes in features space. in this case we name sb as interclass distance. the features that are more suitable for classification are the ones that their interclass distance is greater than their intraclass distance. such that a pm suited to express the separability of classes is the ratio between interclass and intraclass distance. therefore features with greater pm are preferred for classification (according to iicd algorithm). also we should take into consider that the number of possible subsets, q(d), consisting of d features between n features is [van der heijden et al. 2004]: we have used the fs approach as will be described in the following of the paper. 3. the procedure to conduct this research, we needed to program a number of software for producing synthetic gravity profiles, extracting features from them, selecting proper features and training svc for the shape estimation of the gravity sources. there is more explanation in our last research [hekmatian et al. 2015] regarding these software. also the needed svc codes (for the depth or dimension estimation) for this research are implied during the procedure explanations in the following. the overall procedure which we used is as the following: (1) producing more than 500 synthetic gravity profiles for training and testing the svc codes which we produced for estimating the depths and dimensions of gravity sources, having one of the six supposed shapes (in our previous research [hekmatian et al. 2015], we applied svc in estimating the approximate gravity sources shapes between six possible shapes: sphere, horizontal cylinder, vertical cylinder, rectangular prism, syncline and anticline). from these profiles, about 320 were used for training and about 200 were used for testing. (2) extracting 3200 features from the 320 profiles which we used for training. in other word, we extracted 10 features from each of the mentioned 320 profiles. these features are shown in table 1 (for more explanations regarding the features, see section 2.2). these features have good characters: first, they are related to the depth and dimension of the gravity sources and are independent from the shape of the sources; second, they consist of all xgy from xg50 to xg85 systematically; third, they are actually obtainable from common real gravity profiles (for example, in real gravity profiles usually xg50 exists but xg10 does not exist). (3) selecting the most proper 2 features between the mentioned ten features (by inter/intra class distance fs algorithm; see section 2.2). we used the most proper 2 features of the training sets to train our svc codes (for depths or dimensions estimations). according to the shape of gravity sources and also the kind of depth and dimension estimations respectively, the most ( ) ( ) . s s z z ns nk n 1 1 t w k k, n k, n ns nkk k s n n k kk k 1 11 n k n n = = = == t t | || .( ) ( )s n n 1 t b s kk k k k1 n n n n= --= t t t t| ( ) ( ) .z zs n 1 t k, n k, nk k n n k k k1 n k n n= -= t t| ns 1 ns n n 1 s n= =t | .zn1 nk 1 k nk n n k n = =t | .zk, n1 ( ) (q d n d n = ) ! ! ! .d d hekmatian et al. 4 (15) (10) (11) (12) (13) (14) 5 proper 2 features were different. this is shown in table 2. here it should be mentioned that it is possible to select the most proper other number than 2 features (for example the most proper 3 features) between the mentioned ten features by inter/intra class distance fs algorithm. but by selecting the most proper 2 features, we have received good and proper results (as will be showed in the following of this paper). therefore it was not needed to select the most proper other number than 2 features. in any way, we have showed the procedures for selecting any number of features and other researchers can select any number of features in their researches. (4) extracting the most proper 2 features (related to table 2) from each of about 200 synthetic gravity profiles which were produced for testing our trained svc codes (for depths or dimensions estimations). we tested our trained svc codes by the mentioned most proper 2 features extracted from testing sets. using the mentioned most proper 2 features, depth and dimension estimation was done properly. as you see in table 2 the number of different kinds of depth or dimension estimations of the gravity sources is 16. it should be mentioned that for testing each of these 16 different kinds, we produced 10 to 20 synthetic gravity profiles and for training each of the mentioned 16 different kinds, we produced about 20 synthetic gravity profiles too. (5) testing our trained svc codes (for depth or dimension estimations) by some real gravity profiles and the proper features that we extracted from them. 4. results and discussion 4.1. synthetic data (and some other discussions) for depth estimation through svc, we should know the approximate shape and the range or approximate dimensions of the gravity source; it is the same case for dimension estimation. different kinds of depth and dimension estimations that we did using svc are listed in table 2. we show some examples of these different kinds of depth and dimension estimations (with synthetic data in the following). suppose we have estimated the shape of the gravity source to be a vertical cylinder. also regarding its minimum depth using other methods we have reached to a range of 2 m to 9 m and a range of 2 m to 14 m regarding its height. its density difference (with the background) is 2.5 gr/cm3. in this condition, we can train a svc code by proper features extracted from a training set. this training set could consist of 20 synthetic vertical cylinder gravity sources which have density difference (with the background) the same as the gravity source and have the mentioned ranges of minimum depths and heights. the specifications of this training set are shown in table 3. as it has shown in table 2, the best 2 features for training svc code for radius estimation of vertical cylinder gravity sources are feature 1 (xg50) and feature 9 (gmax). depth and dimension estimation using svc number of the extracted features for depth and dimension estimation extracted features 1 xg50 2 xg55 3 xg60 4 xg65 5 xg70 6 xg75 7 xg80 8 xg85 9 gmax 10 integral of g(x) in central 100 m of profile the kind of depth or dimension estimation of the gravity source name or number of 2 proper features (see table 1) radius of sphere 8, 9 minimum depth of sphere 7, 9 radius of horizontal cylinder 9, 10 minimum depth of horizontal cylinder 8, 9 radius of vertical cylinder 1, 9 minimum depth of vertical cylinder 7, 9 height of vertical cylinder 9, 10 minimum depth of rectangular prism 6, 9 vertical extension of rectangular prism 9, 0 horizontal extension of rectangular prism 5, 9 minimum depth of anticline 8, 9 vertical extension of anticline 9, 10 horizontal extension of anticline 9, 10 minimum depth of syncline 1, 9 vertical extension of syncline 9, 10 horizontal extension of syncline 9, 10 table 1. the extracted features and their numbers for depths and dimensions estimations of gravity sources. table 2. the most proper 2 features selected for different kinds of depth or dimension estimations of the gravity sources. for testing the mentioned trained svc code, we used a testing set with 12 synthetic gravity profiles created by vertical cylinder gravity sources which have density difference (with the background) equal to 2.5 gr/cm3 and with minimum depths and heights equal to 5 m and 7 m respectively. table 4 shows the real radii, feature 1 (xg50) and feature 9 (gmax) of the mentioned synthetic gravity profiles and the radii which are estimated by the trained svc code (proper for radius estimation of vertical cylinder gravity sources) and the errors of the radii estimations. if we want to estimate the radius of vertical cylinder gravity sources with other depth, height, density difference and other range of radius, we should use other training set. the results of training the mentioned svc code and testing it in graphic style are shown in figures 4 and 5. the radii of vertical cylinder sources are estimated using minkowski kernel (equation (5) with setting its parameter to 0.1) based on the following formula: ((the probability of having radius equal to 10m calculated by the trained svc)*10) + ((the probability of having radius equal to 1m calculated by the trained svc)*1). we should mention that the range of radius that can be estimated by the mentioned trained svc is from 1 m to 10 m. also, as it has been noted before, estimating the radius of vertical cylinder gravity sources with other range of radius needs using other training set for proper training svc. estimation of the depth and dimension of sources hekmatian et al. 6 numbers of files of the synthetic gravity profiles used as the training set radius (m) minimum depth (m) height (m) 1 1 2 2 2 1 5 5 3 1 2 8 4 1 5 11 5 1 2 14 6 1 4 2 7 1 9 5 8 1 4 8 9 1 9 11 10 1 4 14 11 10 2 2 12 10 5 5 13 10 2 8 14 10 5 11 15 10 2 14 16 10 4 2 17 10 9 5 18 10 4 8 19 10 9 11 20 10 4 14 numbers of files of the synthetic gravity profiles used as the testing set feature 11 (m) feature 19 (mgal) real radius (m) estimated radius (m) real error (absolute of real radius minus estimated radius) (m) relative error (real error/ real radius) 1 6.5 0.06378 3.5 5.14 1.64 0.47 2 6.7 0.07971 4 5.22 1.22 0.31 3 7 0.09629 4.5 5.32 0.82 0.18 4 7.3 0.11323 5 5.41 0.41 0.08 5 7.6 0.13031 5.5 5.50 0.00 0.00 6 7.9 0.14735 6 5.57 0.43 0.07 7 8.3 0.1642 6.5 5.63 0.87 0.13 8 8.6 0.18076 7 5.70 1.30 0.19 9 9 0.19694 7.5 5.79 1.71 0.23 10 9.4 0.21267 8 5.89 2.11 0.26 11 9.7 0.22793 8.5 5.96 2.54 0.30 12 10.1 0.24268 9 6.05 2.95 0.33 table 3. the specifications of the training set which we used for training svc code for radius estimation of vertical cylinder gravity sources. table 4. the real radii, feature 1 (xg50) and feature 9 (gmax) of the synthetic gravity profiles (of the testing set) and the radii which are estimated by the trained svc code (proper for radius estimation of vertical cylinder gravity sources) and the errors of the radii estimations. 7 with other shapes (see table 2) were, in general, in the same manner with little differences. in continuation, three other examples including, minimum depth and height of vertical cylinder source estimation in addition to estimation of minimum depth of rectangular prism source by their related trained svc on their testing sets are presented. regarding the minimum depth of vertical cylinder estimation, the training set includes 20 synthetic gravity profiles which are created by vertical cylinder gravity depth and dimension estimation using svc figure 4. minkowski kernel (of svc) in a 2 dimensional space (horizontal axis is showing xg50 and vertical axis is showing gmax) which is trained by training set of table 3. the cross points are showing the 10 vertical cylinder gravity sources with radius equal to 10 m. and the star points are showing the 10 vertical cylinder gravity sources with radius equal to 1 m. these 20 points are training set. figure 5. this figure have the same parameters and kernl as figure 4, but the minkowski kernel (of svc) separate vertical cylinder gravity sources of the testing set (table 4) according to their radius. the points are showing the 12 vertical cylinder gravity sources of the testing set. numbers of files of the synthetic gravity profiles used as the testing set real minimum depth (m) estimated minimum depth (m) real error (absolute of real minimum depth minus estimated minimum depth) (m) relative error (real error/ real minimum depth) 1 5.5 7.84 2.34 0.43 2 6 7.95 1.95 0.33 3 6.5 8.01 1.51 0.23 4 7 8.15 1.15 0.16 5 7.5 8.29 0.79 0.11 6 8 8.36 0.36 0.05 7 8.5 8.50 0.00 0.00 8 9 8.64 0.36 0.04 9 9.5 8.71 0.79 0.08 10 10 8.85 1.15 0.11 11 10.5 8.99 1.51 0.14 12 11 9.13 1.87 0.17 13 11.5 9.27 2.23 0.19 14 12 9.41 2.59 0.22 15 12.5 9.54 2.96 0.24 16 13 9.60 3.40 0.26 table 5. the real minimum depths of the synthetic gravity profiles of the testing set and the minimum depths estimated by the trained svc code (proper for minimum depths estimations of vertical cylinder gravity sources) and the errors of minimum depths estimations. sources. 10 of these profiles have sources at minimum depth equal to 3 m and 10 other profiles have sources at minimum depth 15 m. the radii of all the 20 profiles are equal to 5 m or 6 m and their height range is 1 m to 36 m. the density difference with background for all of them is equal to 2.5 gr/cm3. with such training set, we would be able to estimate minimum depth of those vertical cylinder gravity sources which their minimum depths are between 3 m and 15 m and have the mentioned ranges or values of height, radius and density difference. the estimated minimum depths of vertical cylinder sources of the testing set are shown in table 5. the training set regarding the height of vertical cylinder sources consists of 20 synthetic gravity profiles with 2 classes. one class is related to height equal to 2m and consists of 10 synthetic gravity profiles created by vertical cylinder sources with height equal to 2 m and the other class is related to height equal to 25 m and consists of 10 other profiles with the height of their sources equal to 25 m. radii of the mentioned 20 profiles are 5 m or 6 m and the range of their depths are between 5 m to 7 m. their density difference with background is 2.5 gr/cm3. the estimated heights of vertical cylinder sources of the testing set are shown in table 6. according to the 3 mentioned testing sets (see table 4 to table 6), if the radii, depths and heights of vertical cylinder sources are about in the middle of the used 2 classes of each of them, then they can be estimated with low errors. for example the lowest error pertaining to radius estimation is related to radius equal to 5 m or 6 m (see table 4). the reason is that as we mentioned in section 1 according to [moran and lavrenko 2011], minkowski kernel (relation 5) mimics logical and of variation in feature values. regarding estimation of minimum depth of rectangular prism sources, the training set consists of 20 rectangular prism sources with density difference with background equal to 2.5 gr/cm3. ten of them have the minimum depths equal to 2 m and depths of other ten are equal to 25 m. horizontal and vertical thickness of all of the 20 rectangular prism sources is between 14 m and 18 m and between 20 m and 30 m respectively. after training by the mentioned training set, we examined the trained svc codes with the testing set. the results are shown in table 7. 4.2. real data we have two sets of real gravity data. the approximate location of these 2 sets is shown in figure 6 by dark spots. the first set of real data belongs to the small grid gravity network at institute of geophysics of university of tehran. for gravity data acquisition, a scintrex (cg3m) gravimeter with 1 micro-gal resolution is used and the coordinates of the points were measured by the total station trimple 5601 with accuracy about 1 cm in x, y and h coordinates. the goal was to detect and model the tunnels of an old buried channel. the grid space of 2*2 square meters is used for measuring the data. in the measured data, by using the gravity corhekmatian et al. 8 numbers of files of the synthetic gravity profiles used as the testing set real height (m) estimated height (m) real error (absolute of real height minus estimated height) (m) relative error (real error/ real height) 1 11 14.87 3.87 0.35 2 12 15.08 3.08 0.26 3 13 15.26 2.26 0.17 4 14 15.42 1.42 0.10 5 15 15.55 0.55 0.04 6 16 15.66 0.34 0.02 7 17 15.77 1.23 0.07 8 18 15.86 2.14 0.12 9 19 15.94 3.06 0.16 10 20 16.01 3.99 0.20 11 21 16.07 4.93 0.23 12 22 16.13 5.87 0.27 13 23 16.19 6.81 0.30 table 6. the real heights of the synthetic gravity profiles of the testing set and the heights estimated by the trained svc code (proper for heights estimations of vertical cylinder gravity sources) and the errors of heights estimations. 9 depth and dimension estimation using svc names of files of the synthetic gravity profiles used as the testing set real minimum depth (m) estimated minimum depth (m) real error (absolute of real minimum depth minus estimated minimum depth) (m) relative error (real error/ real minimum depth) 1 6 8.48 2.48 0.41 2 7 9.38 2.38 0.34 3 8 9.42 1.42 0.18 4 9 9.45 0.45 0.05 5 10 10.81 0.81 0.08 6 11 10.83 0.17 0.02 7 12 10.86 1.14 0.10 8 13 12.29 0.71 0.05 9 14 12.31 1.69 0.12 10 15 12.33 2.67 0.18 11 16 13.79 2.21 0.14 12 17 13.81 3.19 0.19 13 18 15.26 2.74 0.15 14 19 15.27 3.73 0.20 15 20 15.27 4.73 0.24 16 21 16.67 4.33 0.21 17 22 16.67 5.33 0.24 figure 6. the approximate location in iran of the 2 sets of real data is specified by dark spots. the geographic map of iran is extracted from ardestani [2008]. table 7. the real minimum depths of gravity profiles of the testing set and minimum depths estimated by the trained svc code (proper for minimum depths estimations of rectangular prism gravity sources) and the errors of minimum depths estimations. rections due to standard formulas, the bouguer gravity anomalies are computed and the trend effects are removed by polynomial fitting method, and finally the residual anomalies are calculated and shown in figure 7. we should confirm that contour interval of 1 microgal is the default presentation of the software used for producing figure 7 (geosoft) and it is only for better presentation of the figure and does not show the real accuracy of the mentioned figure. a gravity profile (line ab in figure 7) is selected for testing. the mentioned profile is shown in figure 7-1. this profile was collected above a well i.e. the gravity source of this gravity profile was a well full of air. we knew that the well was very similar in shape to a vertical cylinder or a long vertical rectangular prism. the second set of real data belongs to the grid hekmatian et al. 10 figure 7. location of the gravity profile ab over the well. the map of this figure is a residual gravity map of a part of institute of geophysics of university of tehran and the coordinates are the local one [ardestani 2009]. figure 7-1. the gravity profile ab extracted from the map of figure 7 [ardestani 2009]. 11 gravity network carried out in a mining site close to zanjan (a city in iran). the grid space of 10*10 square meters is used for measuring the data. for this set of real data, a scintrex (cg3m) gravimeter with 1 micro-gal resolution is used and the coordinates of the points in the second set of real data were measured by the total station (leika750) with accuracy to a few cm in x, y and h coordinates. after gravity corrections due to standard formulas, the bouguer gravity anomalies are computed. after removing the trend effect by polynomial fitting method, the residual anomalies are computed and for the second set of real data shown in figure 8. three real gravity profiles that we used in this set of real data are presented in figure 8 as the profiles cd, ef and gh. cd, ef and gh profiles are shown in figures 8-1 to 8-3. finally the estimated shapes of the gravity sources of the mentioned real gravity profiles by our different trained svc codes (the shapes estimations have been explained in detail in our last research: hekmatian et al. [2015]) are given in table 8. it has been shown in the last research [hekmatian et al. 2015] that the estimation of the shapes of the real gravity sources with svc is compatible with the reality. after estimating shapes of the real gravity profiles, now we can estimate their minimum depths and dimensions. as we mentioned before, the source of ab profile is a well that can be seen in the surface of the earth, so that we know its depth and dimensions. regarding the radius of the source of ab profile (see figure 7 and table 8), now we know that the shape of this source is depth and dimension estimation using svc figure 8. location of the gravity profiles cd, ef and gh. the map of this figure is a residual gravity map of a mining site near zanjan (a city in iran) and the coordinates are utm. a vertical cylinder. for radius estimation, we should not only know the shape of the source, but also the range of the minimum depth and the height of the vertical cylinder. the minimum depth of this vertical cylinder according to the prior knowledge is less than 2 m (we take this range between 0 m and 2 m). also we knew that the height is less than 8 meters (we take the range of height between 2 m and 7 m). in addition, the source is a well full of air, so that after measuring the density in the field, we take its density difference with the background equal to −2.0 gr/cm3. by these range of possible specifications, we trained the proper svc code for this case. by the mentioned trained svc code, we estimated the radius of the source of ab profile approximately equal to 1.05 m. the real radius of the mentioned source is at maximum 1 m (is between 0.6 m to 1 m and therefore the average radius is about 0.8 m). similarly, for minimum depth estimation of the source of ab profile, we should know shape, radius and height of the source. so far, we estimated shape and radius; we also know the range of height of the source. so we produced proper trained svc by which we estimated the minimum depth approximately equal to 0.3 m which is about the same as real minimum depth (zero meter). finally, we estimated height of the mentioned source approximately equal to 5.8 m. the real height is 5 m. regarding ef profile, the estimated shape of the gravity source is a vertical cylinder (see figure 8 and table 8), and its radius, height an minimum depth are estimated by svc equal to 6 m, 17 m and 6 m respectively. for gh profile, we estimated its shape as a rectangular prism (see figure 8 and table 8), and its minimum depth, horizontal thickness (in the direction of the profile) and vertical thickness are estimated by svc equal to 16 m, 17 m and 18 m respectively. regarding the sources of the profiles ef and gh, we have no real values of their depths and dimensions. but regarding their depths, we should mention that they are consistent with the results of euler estimations (see figure 9). regarding their horizontal thickness (or diameter), they are consistent with the analytic signals map (see figure 10). hekmatian et al. 12 name of real gravity profile shape of gravity source estimated shape of the source by the trained svc codes ab similar to vertical cylinder vertical cylinder cd possibly rectangular prism not interpretable because of non being principal profile ef possibly vertical cylinder or rectangular prism vertical cylinder gh possibly rectangular prism rectangular prism figure 8-1. the gravity profile cd extracted from the map of figure 8. figure 8-2. the gravity profile ef extracted from the map of figure 8. figure 8-3. the gravity profile gh extracted from the map of figure 8. table 8. shapes of gravity sources of the real gravity profiles and the results of their testing by our different trained svc codes. 13 depth and dimension estimation using svc figure 9. location of the gravity profiles ef and gh on euler depth estimations of residual gravity map consists of a mining site near zanjan and the coordinates are utm. figure 10. location of the gravity profiles ef and gh on analytic signals of residual gravity map consists of a mining site near zanjan and the coordinates are utm. 5. conclusion estimations of depths and dimensions of gravity sources through svc are possible. therefore, estimations by svc give us initial guesses regarding shape [hekmatian et al. 2015], depth and dimension of gravity sources which is needed in more precise interpretation and inversion of gravity sources. finally, we propose this approach to be applied for geo-scientific investigations as a new approach for the estimation of the depths and dimensions of the gravity sources in exploration projects. we suggest that the above mentioned approach can be used in other geophysical branches. acknowledgements. we thank all the faculty members who assisted us in preparing this paper. our special thanks goes to dr. hashemi, for introducing us to the pattern recognition laboratory, delft university of technology (whose group produced and designed prtools which we used in this research). also by studying his papers [hashemi et al. 2008, hashemi 2010], we decided to perform this research. we thank prof. duin (pattern recognition laboratory, delft university of technology), because of his helpful guidance at the beginning of this research. we thank the geophysics group of the faculty of basic sciences of science and research branch, islamic azad university, for guiding us so well during this research. also we thank the research council of the university of tehran. references ardestani, v.e. (2008). modelling the karst zones in a dam site through micro-gravity data, explor. geophys., 39, 204-209. ardestani, v.e. (2009). residual gravity map of a part of institute of geophysics of university of tehran, tehran, iran. duin, r.p.w., p. juszczak, p. paclik, e. pekalska, d. de ridder, d.m.j. tax and s. verzakov (2007). prtools4. a matlab toolbox for pattern recognition. version 4.1, delft university of technology, netherlands; http://www.prtools.org/ (accessed june 22, 2011). gret, a.a., and e.e. klingele (1998). application of artificial neural networks for gravity interpretation in two dimensions, institute of geodesy and photogrammetry, swiss federal institute of technology, zurich. hashemi, h., d.m.j. tax, r.p.w. duin, a. javaherian and p. groot (2008). gas chimney detection based on improving the performance of combined multilayer perceptron and support vector classifier, nonlinear proc. geoph., 15, 863-871. hashemi, h. (2010). logical considerations in applying pattern recognition techniques on seismic data: precise ruling, realistic solutions, cseg recorder, 35, 47-50. hekmatian, m.e., v.e. ardestani, m.a. riahi, a. memar koucheh bagh and j. amini (2015). estimating the shapes of gravity sources through optimized support vector classifier (svc), acta geophys., 63 (4), 1000-1024. moran, s., and v. lavrenko (2011). optimal tag sets for automatic image annotation, in: j. hoey, s. mckenna and e. trucco, proceedings of the british machine vision conference, bmva press, september 2011, 1.1-1.11; http://dx.doi.org/10.5244/c.25.1. osman, o., a.m. albora and o.n. ucan (2006). a new approach for residual gravity anomaly profile interpretations: forced neural network (fnn), annals of geophysics, 49 (6), 1201-1208. van der baan, m., and j. christian (2000). neural ne two rks in geophysical applications, geophysics, 65 (4), 1032-1047. van der heijden, f., r.p.w. duin, d. de ridder and d.m.j. tax (2004). classification, parameter estimation and state estimation, john wiley & sons, ltd., chichester, england, 440 p. * corresponding author: mohammad ehsan hekmatian, islamic azad university, faculty of basic sciences, science and research branch, tehran, iran; email: mhekmatian@aeoi.org.ir. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. hekmatian et al. 14 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice i l t e r r e m o t o j o n i c o d e l 2 2 a p r i l e 1 9 4 8 paolo emilio v a l l e allo scopo (li contribuire alla localizzazione dei centri sismici che interessano l'italia e per essere in possesso degli elementi fondamentali necessari ad una successiva ricerca sulla costituzione della crosta terrestre sotto il mediterraneo, sono state calcolate le coordinate epicentrali e il tempo origine del terremoto del 22 aprile 1948. la scossa ha avuto effetti distruttivi nella regione dell'isola di leucade, è stata avvertita a prveza, agrinion, zante, missolonghi, corfù ed ha carattere superficiale. da una stima eseguita mediante i sismogrammi delle stazioni di roma, bologna e salò, la magnitudo (*) è risultata 6%*. in relazione a questo valore della magnitudo, l'energia liberata all'ipocentro si può valutare a circa 1023'r' erg e l'intensità all'epicentro corrispondente al 9" grado della scala mercalli modificata. una prima localizzazione dell'epicentro è stata ottenuta col metodo della proiezione stereografica, applicato ad una decina di stazioni, che ha fornito il seguente risultato: a = 2 0 ° , 5 e cp = 38°,5 n (geografica) [ 1 ] cp' = 38",312 n (geocentrica) in base alle coordinate epicentrali provvisorie [ 1 ] , il tempo origine medio, dedotto dai tempi di arrivo delle onde p, letti direttamente nei sismogrammi o copie degli osservatori elencati nella tabella i, e dai tempi di tragitto delle analoghe onde, calcolati da jeffreys ( 2 ) per centri di perturbazione superficiali, vale h = io1' 42™ 40\8 (t.m.g.) [ 2 ] i valori [ 1 ] e [ 2 ] sono stati presi come valori di partenza per l'applicazione del metodo delle approssimazioni successive (3). se si indica con / il tempo di tragitto delle onde p, c o n ? 0 , cp0' le ('") i l p r o f . g u t e n b e r g m i h a g e n t i l m e n t e c o m u n i c a t o c h e l ' a n a l o g a s t i m a eseg u i t a a p a s a d e n a h a d a t o l o s t e s s o v a l o r e . 2 1 2 paolo k m 11.10 v a l l e coordinate geocentriche dell'epicentro, con h0 il tempo origine, con /; le differenze tra i tempi di arrivo calcolati mediante i valori di partenza e i tempi osservati, si ha per ogni stazione 7 . + n + r = o [ 3 ] 3x; 3 cp'i dove si è posto (<• 1, 2 e = x 0 — x 11=^0'—1qp' t 4 ] t =h0-h e nell'ipotesi che e, e r siano sufficientemente piccoli in modo da poter trascurare i loro prodotti e i loro quadrati, ti indica il numero delle stazioni. le derivate df/dx-, e 3//3cp'i si possono calcolare tenendo presente che si piiò scrivere axi a ai 3xi ' a cp'i 3a; 3cp'; la 3//3a; è stata ottenuta dai tempi di tragitto calcolati da jeffreys e le altre due derivate dalle espressioni 3 a c o s c p ' ( a ; s i n x — b , c o s à ) 3x; ~ sin a; [6] 3a sin cp' («i cos x-f hi sin x)—e, cos qp' 3 c p ' i sin ai nelle quali ait b;, ci sono i coseni direttori della tm" stazione. dato d i e le [ 6 ] non sono indipendenti, ma legate dalla relazione 1 " m ^ y i [ " ] v coscp' 3x; ) \ 3cpi y l a verifica dei calcoli può essere fatta agevolmente. la tabella i contiene i valori dei coefficienti e dei termini noti del sistema di equazioni [ 3 ] , ebe sono stati usati per il calcolo. i valori approssimati più probabili delle incognite e i rispettivi errori medi sono £ = — 0",02±0",04 t) = — 0",01+0°,04 [ 8 ] t = o'.o +0".4 il t e r r e m o t o j o n i c o del 2 2 a p r i l e 1 9 4 8 t a b e l l a i 2 4 3 li il n . s t a z i o n e sh a c p ' i *i s e c . g r a d i 1 sec. g r a d i 1 sec. 1 s o f i a — 4 , 9 — 1 2 , 7 — 0,7 2 r o m a + 9,4 — 7,3 — 0 , 7 3 b u c a r e s t — 6 , 1 — 1 1 , 7 — 1 , 0 4 t r i e s t e + 5 , 9 — 1 1 . 6 + u 5 b u d a p e s t + 1,2 — 1 3 . 7 — 0,7 6 g r a z + 4 , 0 — 1 2 , 8 + 0,6 7 l ' r a g a + 3,3 — 1 2 , 7 + 0,8 8 h e l w a n — 8,0 + 8 , 7 + 0,4 9 z u r i g o + 6,9 — 1 0 , 2 + 0,9 ]() b a s i l e a + 7,0 — 9,8 + 1 , 1 1 1 s t r a s b u r g o + 6,4 — 1 0 , 4 + 0 , 7 1 2 j e n a + 4,2 — 1 2 , 1 — 0 , 1 1 3 g o t t i n g a + 4,5 — 1 1 . 7 — 0,2 1 4 t o r t o s a + 9,8 — 3,3 — 0,9 1 5 p a r i g i + 7,2 — 8,9 — 1 , 0 1 6 u c c i e + 6,1 — 1 0 , 0 — 1 , 1 1 7 l u n d + 2 , 3 — 1 2 , 1 + 0 , 1 1 8 c o p e n h a g e n + 2,5 — 1 2 , 0 + 1-2 1 9 t o l e d o + 8,9 — 2.4 — 0 , 1 2 0 l i s b o n a + 7,6 — 1 , 7 + 1 , 0 2 1 b e r g e n + 2,4 — 9,2 — 0,9 t a b e l l a i i n . s t a z i o n e p li m s s — p sec. 1 a t e n e 1 0 4 3 2 4 , 0 33,2 2 s o f i a 5 5 , 6 5 7 , 7 3 r o m a 4 4 2 7 , 8 8 0 , 2 4 b u c a r e s t 3 1 , 6 8 5 , 0 5 t r i e s t e 5 0 , 0 1 0 3 , 8 6 b u d a p e s t 5 6 , 4 1 0 5 , 6 7 g r a z 5 9 , 1 1 0 8 , 9 8 p r a g a 45 4 0 , 0 1 3 9 , 0 9 z u r i g o 4 1 , 0 1 4 0 , 0 1 0 b a s i l e a 4 9 , 2 1 4 7 , 0 1 1 s t r a s b u r g o 5 7 , 4 1 5 2 , 6 1 2 t o r t o s a 4 6 2 4 , 4 1 7 5 , 6 1 3 p a r i g i 3 7 , 0 1 8 4 , 0 2 4 4 paolo e m i l i o valle questi valori delle correzioni da apportare ai dati di partenza sono sufficientemente piccoli e si rende quindi superflua un'ulteriore approssimazione. pertanto le coordinate definitive dell'epicentro e il tempo origine risultano u = 20",48+0",04 e cp0 = 38",49+0",04 n (geografica! [ 9 ] h0 — 10" 42'" 40s,8+0s,4 ( t.m.g.l si è già accennato al fatto che le registrazioni indicano che il centro di perturbazione è superficiale, tuttavia è sembrata opportuna una verifica del tempo origine con altro metodo indipendente dai precedenti. a tale scopo è stato usato il metodo di v a d a t i (41, il quale si serve, come è noto, dei tempi di arrivo delle onde p e delle differenze s-p. se le stazioni non sono troppo lontane dal centro di perturbazione, risulta approssimativamente s—p 1 p-h0 k — costante [10] la eostante è legata al coefficiente di poisson dalla relazione a — 0,5 — 0,5 a~ [11] 1 + 2 k la tabella ii contiene i tempi di arrivo delle onde p e le differenze s-p espresse in sec, relative alle stazioni usate. i dati di atene sono stati presi dal bollettino di strasburgo. la stazione di helwan è stata esclusa, perché le onde trasversali sembrano arrivare con un sensibile anticipo. questo fenomeno è stato già segnalato in un precedente lavoro (5). i valori approssimati più probabili di h0 e k con i rispettivi errori medi sono k = 1,272 + 0,012 [12] iio = 10" 42'» 42s,3 + l s , 5 (t.m.g.l ii tempo origine si accorda abbastanza bene con quello calcolato precedentemente, anche considerando che il metodo di wadati non consente un'elevata precisione. in base alla [11] il coefficiente di poisson risulta pari a 0,27. questo valore relativamente elevato deriva dal fatto che, sebbene non i l t e r r e m o t o j o n i c o del 2 2 a p r i l e 1 9 1 8 2 4 5 siano stati inclusi nel calcolo i dati delle stazioni eccessivamente distanti dal centro di perturbazione, anche quelle prese in considerazione sono un po' troppo lontane e quindi l'onda si è propagata fino ad una profondità piuttosto elevata. roma — istituto nazionale ili geofisica — gennaio 1951 riassunto allo scopo di contribuire alla localizzazione dei centri sismici che interessano l'italia e per essere in jtossesso dei dati fondamentali per una successiva ricerca, sono state calcolate le coordinate epicentrali e il tempo origine del terremoto jonico del 22 aprile 1948. b i b l i o g r a f i a ( ' ) gutenberg b . a n d f . c . richter: earlhquake magnitude, intensily, energy and acccleration b u l l . s e i s m . s o c . of a m . x x x i i . 1 6 3 1 9 1 ( 1 9 4 2 ) . — di filippo d . e l. marcelli : magnitudo ed energia dei terremoti in italia a n n a l i di g e o f i s i e a i i , 3 3 7 3 4 8 ( 1 9 5 0 ) . <-) jeffreys i l : the time oj p. s and sks, and the velocities of p and s g e o p l i . s u p p l . i v , 4 9 8 5 3 3 ( 1 9 3 9 ) . ( 3 ) geiger l . : herdbestimmung bei erdbeben aus den ankunftszeiten • n a c l i l i c l i t e n d e r k o n i g l i r h e n g e s e l l s e h a f t d e r w i s s e n s c l i a f t e n zu g o t l i n g e n , m a t l i . p l i y s . k l a s s e , 3 3 1 3 1 9 ( 1 9 1 0 ) . — valle f . e . : stilla determinazione delle coordinate epicentrali di un sisma lontano b o l l . s o c . s i s m . i t a l i a n a x l , 73-81 ( 1 9 4 2 ) . < ') wadati k . : tìn the travet time of earlhquake uaves. pari ii • g e o p l i . m a g . 7, 1 1 1 1 1 1 1 ( 1 9 3 3 ) . (•"') valle p . e . : contributo allo studio delle caratteristiche sismiche del mediterraneo centro-orientale a n n a l i di g e o f i s i e a : i, 2 6 6 2 7 8 ( 1 9 4 8 ) . background electromagnetic noise characterization: the role of external and internal earth sources annals of geophysics, 58, 3, 2015, g0330; doi:10.4401/ag-6745 g0330 background electromagnetic noise characterization: the role of external and internal earth sources antonio meloni, cesidio bianchi*, giuliana mele, paolo palangio istituto nazionale di geofisica e vulcanologia, rome, italy abstract the earth is surrounded by the ionosphere and magnetosphere that can roughly be seen schematically as two concentric shells. these two composed and inhomogeneous structured shells around the earth selectively affect electromagnetic (em) waves propagation. both ionosphere and magnetosphere interact also with particles and waves coming from external sources, generating electromagnetic phenomena that in turn might become sources of em waves. conversely, em waves generated inside the ionosphere remain confined at various altitudes in this region, up to a socalled critical frequency limit, depending on frequency, em waves can escape out of the ionosphere and magnetosphere or get through. the em waves generated inside the magnetospheric cavity mainly originate as a result of the electrical activity in the atmosphere. it is well known that also man-made sources, now widely spread on earth, are a fundamental source of em waves; however, excluding certain frequencies employed in power distribution and communication, man-made noise can be dominant only at local scale, near their source. according to recent studies, em waves are also generated in the earth’s lithosphere; these waves were sometimes associated with earthquake activity showing, on the earth’s surface, intensities that are generally orders of magnitude below the background em noise. in this review paper, we illustrate em waves of natural origin and discuss their characterization in order to try discriminate those of lithospheric origin detectable at or near the earth’s surface. 1. introduction the electromagnetic (em) spectrum is subdivided into regions, according to wavelength (table 1). the earth’s atmosphere prevents most of the electromagnetic radiation coming from outer space from reaching the earth’s surface; in fact, only portions of radio and visible light reach the ground. the natural electromagnetic background noise in the earth’s environment originates from different phenomena that involve both earth’s external and internal sources and their possible interaction with the ionospheric and magnetospheric cavities. external sources bring em radiations inside the magnetospheric cavity but they can also be generated inside the cavity, for example by energetic particles acting as forcing mechanisms that perturb and modify the ionospheric and magnetospheric configuration. the em power spectral density (psd) at the earth’s surface was deeply studied and reported since the first extensive measurements were available, as discussed for example by lanzerotti [1978] and lanzerotti et al. [1990]. the most intensive near earth’s sources are associated with the atmospheric electrical activity, while the external sources mainly originate on the sun. relevant information about the external em noise can be found in the ccir/itu (comité consultatif international pour la radio; now also international telecommunication union) radiocommunication sector (itu-r) reports, published in 1964, 1988, and 1990 [ccir/itu 1960, 1988, 1990], as well as in davies [1990], helliwell [2006], and bianchi and meloni [2007]. sudden fluxes of particles that enter the magnetosphere generate magneto-hydro-dynamic (mhd) waves and other relevant phenomena, especially in the polar regions (e.g., auroral displays, magnetic substorms, polar caps absorption) and also wave-particle interactions (whistlers, chorus, hiss). according to lan-zerotti et al. [1990], psd varies of about 16 orders of magnitude in the frequency band 10-5 105 hz, with values ranging from ≈10-1 to 10-17 w m-2 hz-1. for frequencies up to the ghz, the psd falls to ≈10-21 w m-2 hz-1. the spectrum of the em perturbations inside the ionosphere and magnetosphere cavities is mostly studied for the ulf (ultra low frequencies) up to the hf (high frequency) bands. highest frequencies will not be considered in this paper since, starting from hf onwards, i.e. corresponding to waves able to escape the ionosphere, the background noise decreases accordingly. in table 1 acronyms, frequency, wavelength and psd approxiarticle history received january 29, 2015; accepted june 9, 2015. subject classification: radio noise characterization, ionosphere-magnetosphere, background em noise, crustal/lithospheric em sources. mate values of the natural radio noise sources in the magnetosphere and ionosphere, are reported. as briefly reported above, the background em noise becomes less important at higher frequency while the cosmic noise is prevalent starting from vhf-uhf frequency band. at lower frequency a primary source of noise is the interaction between particles and waves coming from outer space and the magnetosphere, while a secondary source originates inside the ionospheric cavity: in this case atmospheric lightning discharges produce several remarkable phenomena such as sferics, tweeks and so on [e.g., wait 1982, helliwell 2006, bianchi and meloni 2007, and references therein]. over the range ≈10-4 to ≈104 hz the amplitude spectra (ft/√hz) fall between the two straight lines f -1.5 and f -1 with an average half line slope that rolls off as f -1.25 (figure 1). as a natural trend, the spectral density decreases considerably from lower to higher frequency, since contributions to background noise from sources internal to the cavities are less important and cosmic noise becomes prevalent up to the millimetric wavelength. the f -1 decay is a natural trend, being a constant when considering a confined environment [füllekrug and fraser-smith 2011]. the purpose of this work is to describe the em background noise in the earth’s environment and to compare the natural external signals with the internal earth’s crust, or more generally lithospheric, sources. these are supposed to be weaker and local and to have peculiar characteristics that can be used to discriminate the internal from the external contributions (see a review by hayakawa and hobara [2010]). em emissions radiated from the lithosphere were at times related to earthquakes occurrence. in particular, many authors have hypothesized that em waves can be generated at the hypocenter during the earthquake or its preparation phase [e.g., hayakawa et al. 1996, johnston 2002, teysseire and ernst 2002, uyeda et al. 2009]; reported em phenomena take place in a wide frequency range [see for example: hayakawa and fujinawa 1994, hayakawa and molchanov 2002]. some reported ionospheric phenomena, were related to earthquake occurrence [pulinets et al. 2007], but were criticized. in fact thomas et al. [2012] had reproduced the same time series results, in which the authors identified an anomalous signal, and found it was normal solar and geomagnetic activity that were, in their opinion, unrelated to the earthquake. in the following, the most relevant sources of natural em waves, in the various frequency ranges, will be treated separately. 2. background noise in the ulf range geomagnetic pulsations, also called micropulsations (or simply pulsations) are the ground signature of ultra low frequency (ulf) hydromagnetic waves propmeloni et al. 2 figure 1. em natural noise psd. in the frequency interval from 10-4 to 104 hz, the noise falls between the two straight lines trending as f -1 and f -1.5 (redrawn after lanzerotti et al. [1990]). sources of natural em radio noise frequency wavelength (m) spectral power (w m-2 hz-1) physical environment resonance in the magnetospheric cavity, interaction with particles of solar origin and radiative pressure with the magnetosphere ulf 0.1 ÷ 3,000 mhz 3·1011 ÷ 108 10-1 ÷ 10-10 magnetosphere resonance in the ionospheric cavity elf 3 ÷ 3,000 hz 108 ÷ 105 10-10 ÷ 10-12 ionosphere propagation in the ionospheric cavity of the radiating atmospheric discharge vlf 3 ÷ 30 khz 105 ÷ 104 10-12 ÷ 10-14 ionosphere atmospheric noise lf-mf-hf 30 khz ÷ 30 mhz 104 ÷ 102 10-14 ÷ 10-17 ionosphere table 1. main physical characteristics of the natural em radio noise sources in the magnetosphere and ionosphere. 3 agating in the earth’s magnetosphere [anderson 1993, 1994]. their period varies from less than 1 s to 10 min, while their amplitude varies from tens to hundreds nt, increasing with increasing period and magnetic latitude. traditionally, micropulsations are classified as continuous (pc) and irregular (pi) as reported in table 2. most of the ulf waves detected at the earth’s surface originate outside the magnetosphere [yumoto 1988]. the solar wind, the magnetospheric foreshock, the magnetospheric bow shock, and the magnetopause, are sources of ulf waves. many of these waves pass through the magnetopause and propagate through the magnetosphere interacting with waveguides, magnetic field lines and the ionosphere generating the pulsations detected at the earth’s surface. externally generated ulf waves detected at the ground level are not identical to waves that enter the magnetosphere through the solar wind, since their energy is transformed and amplified inside the magnetosphere [see for example mcpherron 1991]. schematically, regular pulsations from pc3 to pc5 are due to the so called kelvin-helmholtz instability generated at the magnetopause by the differential flow of the solar wind on either side of the magnetosphere. another mechanism of pulsations generation, mainly in the bands pc3-pc4, is given by the penetration of waves generated upstream from the bow shock region, called ‘upstream waves’ into the magnetosphere. pc1 and pc2 have periods comparable with cyclotronic resonances in the magnetosphere and are associated with the magnetospheric plasma, in particular drift of protons and heavier ions. pulsations amplitude is normally not larger than a few nt in the pc1 and pc2 range and a few tens of nt at longer periods; in exceptional cases pulsations can reach very large amplitudes (100 nt and more). short period pulsations pc1, pc2 and pi1 are travelling waves, while long period pulsations are mainly standing waves inside the magnetospheric cavity. the same mechanism can be observed in the ionospheric cavity for low frequency longitudinal and transversal standing waves. the short period pulsations are related to the kp magnetic activity index; they show both frequency and amplitude modulation and are more sporadic than the longer period pulsations [kangas et al. 1998]. the understanding of the pulsation generation mechanisms requires a detailed knowledge of the magnetosphere structure and dynamics that is beyond the purpose of this work. long period pulsations (pc5 and pc4) are the magnetic signatures of plasma waves with wavelengths comparable to the magnetospheric size. these large-amplitude (up to hundreds of nt), low-frequency continuous pulsations are much stronger in the auroral zone [samson and rostoker 1972]. in the vicinity of the resonance shell, a portion of the energy of these waves is transferred, through a coupling phenomenon, to alfven oscillations and then to pc5 and pc4 pulsation standing waves [kivelson et al. 1984, surkov and hayakawa 2008]. earth’s background em noise characterization continuous pulsations pc1 pc2 pc3 pc4 pc5 period (s) 0.2 5 5 10 10 45 45 180 150 600 frequency (mhz) 5,000 200 200 100 100 22 22 7 7 2 maximum intensity (nt) 1 3 10 < 300 300 source em ion-cyclotron instability in the equatorial magnetosphere em ion-cyclotron instability in the equatorial magnetosphere wave-particle interaction in the bow shock region drift of protons from the night side magnetopause instability impulsive pulsations incoherent noise pi1 pi2 period (s) 1 40 40 150 1 1,000 frequency (mhz) 1,000 25 25 2 1,000 1 maximum intensity (nt) 10 100 source modulation of charged particles sub-storms ionospheric electric currents table 2. iaga classification and standard nomenclature of the most relevant continuous and impulsive pulsations in the ulf band. 3. background noise in elf-vlf range broad-band elf-vlf spectrum measurements made in typical low noise environments, show a uniform structure all over the earth [lanzerotti et al. 1990]; a more intense radio noise is present at high latitude in the frequency band between 0.5 and 1,500 hz, where so called polar hiss/chorus emissions can be relevant [golden et al. 2009]. typical background noise at mid latitude in the vlf-elf band is reported in figure 2 [fraser-smith 2007]. during daytime, the background noise is particularly attenuated in the frequency window 1-5 khz [garcia and jones 2002]. this frequency interval is called “dead band” and is nearly coincident with the transversal resonances of the ionospheric cavity. the em noise background fills the space around the earth interacting in a complex way with the magneto-ionosphere boundary shells at various altitudes and latitudes. the em background noise due the superimposition of numerous sources distributed inside the two cavities [meloni et al. 1992] is the result of all the existing phenomena, such as sferics, tweeks, whistlers and others [e.g., wait 1982, helliwell 2006, bianchi and meloni 2007], and of the complex interaction of the em fields with the approximate reference surfaces delimiting the cavities. in general, as mentioned before, the largest the cavity volume, the lowest the resonant frequencies and the highest the intensity of the fields confined in the cavity. the background noise in the ionospheric cavity is sustained by a range of 60-100 natural lightning electrical discharges per second, especially generated in the tropics, with a wide spectrum ranging from few hz to hundreds of mhz. this spectrum is strongly influenced by the ionospheric cavity shape and, as a result, only signals in the spectral windows ranging from about 8 to 45 hz, survive. these are the schumann longitudinal resonances [bliokh et al. 1980, sentman 1987, sentman and fraser 1991] and related harmonics. all possible em frequencies are given by the relation: where a is the earth’s radius, c is the velocity of light in vacuum and n is any integer number. moreover the conductive layer of the ionosphere, at about 100 km of height, gives the highest amplitude resonant frequency and related harmonics, not on a global scale, in the 9005,000 hz band (transversal resonances). in the frequency band from 5 to 30 khz, the dominant signals are the sferics (also called statics), that are frequently audible in the radio equipments, as well as the tweeks (dispersed sferics) are. taking into consideration a single lightning discharge, the observed spectrum depends on the source-observer distance and is called source spectrum. the convolution of the source function with the cavity transfer function becomes more and more important at increasing distance. as a consequence the nearest observer detects a maximum intensity around 300 khz, the farthest observer experiences a maximum intensity around 7 khz. table 3 shows the main em sources in the magnetosphere and ionosphere. in the frequency range from elf/vlf to lf, the psd varies roughly as the inverse squared frequency [fraser-smith 1995]. 4. possible origin and mechanisms for internal earth generated em waves the description of the ionospheric and magnetospheric signals, in the frequency range from ulf to vlf, above presented, attains to a consolidated investigation field, following several decades of radio wave investigations. in the lasts decades several authors have reported on the existence of internally generated em waves that are reported as weaker and local. detection of em waves of internal earth origin is made with direct observation of electromagnetic emissions from the lithosphere. we should mention however that some authors also operate detecting signals originated in the lithosphere, indirectly. this is done investigating on the propagation anomaly of a pre-existing transmitter signal; the anomaly could be due to a mechanism of lithosphere– atmosphere–ionosphere coupling [see: uyeda et al. 2009, haykawa and hobara 2010, and references therein]. the characterization of em waves generated by internal earth sources, is done in terms of their range of frequency and amplitude, wave attenuation, crustal or subsoil mode of propagation, that allow to detect this kind of em emission. moreover the type of emitting process encompasses a range of uncertainty that is still domir2 a c n n 1n= +f ^^ h meloni et al. 4 figure 2. psd of the elf-vlf background mid-latitude noise. the blue and red lines indicate minima and maxima, respectively. (1) 5 nant in the achieved findings [dea et al. 1993, johnston 2002, teysseire and ernst 2002, palangio et al. 2007]. all authors generally reported that rocks modification or fracture in the earth’s crust, could produce em waves. the fracture in brittle materials, such as solid rocks, can be envisaged as the result of a series of different phases taking place prior to earthquakes, summarized in the so called dilatancy theory [stuart 1974]. this theory is based on the idea that a rock, when stressed, begins to increase its volume or to dilate; this is caused by the opening of micro-cracks and fractures in the rock. this theory was revitalized several times for earthquake prediction, but also severely criticized [geller et al. 1997]. the sequence of events is following: 1) accumulation of stress; 2) micro-fracturing; 3) dilatancy; 4) macroscopic rupture. this last rupture occurs with release of stress and emission of seismic waves. in the earth’s crust the first phase can last tens of years, centuries, or even more, and the stress can increase continuously or with intermittence. the macroscopic rupture of rocks consists in a fracturing at the microscopic scale, radiating elastic energy in a manner analogous to earthquakes [scholz 1968, kato et al. 1994]. when micro-fracturing occurs in certain rocks, it can cause piezoelectric effects that create packets of em waves of different frequency capable of travelling significant distances through the earth’s crust before reaching the surface. this is one of the mechanisms that could account for generation of em waves before earthquakes [johnston 2002, and references therein]. micro-fracturing occurs when a rock is roughly half a way towards its breaking point. several laboratory experiments were undertaken to detect these waves in a controlled environment [e.g., freund 2008, 2009]. the change in the rock size may lead to the uplift of ground surface or to a variation in the groundwater pressure and level. bolt [1988] has shown that, as crustal rocks break up under the tectonic stress preceding an earthquake, the variation in the pore pressure inside the rock causes a temporary decrease of the seismic p waves velocity. unfortunately, the time interval between the p-wave velocity change and the earthquake, might last months or years; the greater the magnitude of the earthquake, the greater the lapse time. much of earthquake predictions was based on the changes that the rocks undergo when they are subject to all these phases. rocks change physically transmitting seismic waves at changing speeds, magnetic properties can be altered and electrical resistance will also vary. famous example is the guam earthquake with magnitude 8.0 occurred in 1993 [hayakawa et al.1996]. thomas et al. [2009] report however, that the anomalous magnetic noise identified by fraser-smith et al. is not related to the loma prieta earthquake but, according to their work, was an artifact of sensor-system malfunction. also in the case of guam 1993 earthquake thomas et al. [2009] reported that the changes in polarization was part of normal global magnetic activity and unrelated to the earthquake. so the above mentioned results and their interpretation are still awaiting for confirmation. for what concerns em waves generation process, piezo-magnetism, but also other physical mechanisms, have been proposed to explain the appearance of magnetic waves prior to earthquakes. other possible em waves generation mechanisms include electro-kinetic [fenoglio et al. 1995, simpson and taflove 2005] and earth’s background em noise characterization (a) magnetospheric sources hz free oscillations in the magnetospheric cavity 3·10-3 102 resonance in the magnetospheric cavity 0.1 10 magneto-hydrodynamic waves in the magnetospheric cavity 101 2·104 resonance wave-particles (transversal and longitudinal) 3·104 2·104 emission due to plasma instabilities 3·10-3 105 (b) ionospheric sources hz emission due to ablation of meteorites 3·102 3·104 sferics 1·103 1·105 sprite, blue jet, red flash, elves 2·103 6·104 longitudinal resonance in the ionospheric cavity 7.8 48 transversal resonance in the ionospheric cavity 8·102 5·103 atmospheric gamma rays 6·103 6·104 table 3. (a) magnetospheric and (b) ionospheric em sources. magneto-hydrodynamic effects [surkov and pilipenko 2014], resulting from fluid circulation through the crust, stress-induced increase in local crustal electrical conductivity [merzer and klemperer 1997], micro-fracturing, thermal magnetization/demagnetization effects and so on [johnston 2002, uyeda et al. 2009]. freund [2008, 2009] has suggested a semiconductor model of rocks to describe unipolar magnetic pulses, as several times was observed prior to earthquakes. the suggested mechanism is based on coupling a semiconductor drift-diffusion model to a magnetic field to describe the electromagnetic effects associated with electrical currents flowing within rocks [see also scoville et al. 2014]. however for fluid saturated samples, according to dahlgren et al. [2014], observation of significant electrical charge buildup is not expected during the observed slow stress accumulation prior to earthquakes, or during any slow precursory stress release that may occur in the region of earthquake nucleation. 5. characterization of internal earth sources for what concerns the earth’s crust, or more generally the lithosphere, among the less known quantities, are the em properties. once the variability range of electrical conductivity (v) and electrical permittivity (f) is established, other features of the em internal signals that enable to discriminate them from external sources, can be inferred within an acceptable level of accuracy. except for frequency and intensity, other signal electromagnetic characteristics like velocity, propagation mode, polarization, impulsiveness, coherence and spectral content, strongly depend on the media electrical properties. most of the published works deal with the ulf and elf part of the electromagnetic spectrum. in fact, electromagnetic signals produced in the seismogenic layer (for example between 10 and 20 km of depth, chiarabba et al. [2005]), that can reach the earth’s surface with little attenuation [molchanov and hayakawa 1995], fall in the ulf-elf bands. in this framework the most probable internal signals should have the following characteristics: frequency range 0.01-1,000 hz magnetic intensity range 1-100 ft depending on v, the diffuse magnetic field velocity range 1,000-10,000 km/s polarization, depending on sources pattern and nature impulsiveness, following estimate of the probability density function coherence, depending on the extent of the source. sources can be magnetic, electric and hybrid, with the spectral content depending on the medium characteristics and the statistical behavior depending on the emission mechanism. in table 4 some of the expected features of the internal sources are listed. depending on distance, the em fields emitted by a source, or scattered by a reflector, are always characterized by an inductive, so-called near field, and a propagating so-called far field. here, ‘near’ and ‘far’, are referred with respect to: a distance d from the source, the wavelength m, the extent and the coherence of the source d physical dimension. through the above quantities, four spatial regions are defined: (2) reactive field region, (3) i fresnel’s region, (4) ii fresnel’s region, and (5) fraunhofer’s region: the em near field contains useful information on the nature of the source, but the way to resolve this matter can be rather complex. in the near field the signal’s power falls as r-5 with the distance r from the source and there is no propagating field except for a small fraction that will sustain the far field. in the above defined condition (a) the e/h ratio gives indication about the magnetic/electric nature of the source [palangio et al. 2008]. in the regions (b), (c) and (d) the power falls as 2 2 4 4 d d d d d d d d 2 2 1 1 1 1 1 2 r m r m m m m m 2 2 2 2 meloni et al. 6 table 4. some expected features of the em wave internal sources. (a) mechanism of emission point source elemental dipole extended elemental dipole point source hybrid multipole extended hybrid multipole (c) statistical behavior gaussian isotropic stationary impulsive sources homogeneous sources (b) spectral content and trend thermic (white) colored complex (2) (3) (4) (5) 7 r-4, r-3 and r-2 respectively, while the wave impedance is defined only in (d). these considerations are essential both for signal measurement purpose and analysis, in order to characterize the observed signal properties. in the terrestrial environment quasi non radiating (qnr) magnetic sources, whose fields remain confined in the same volume of the source, are quite common. typical examples are the toroidal sources e.g., dynamo in the earth’s core and in the ionosphere. these sources are generally complex, with toroidal components that are not observable far from the source, while the system energetic balance cannot be resolved by measurements made externally to the system. only the sources poloidal components can be measured. very extended sources, larger than the dominant observed signal wavelength, emit electric and magnetic fields whose space-time correlation length reflects coherence properties that depend on the nature of the emitting source (see general geomagnetism textbooks, e.g.: backus et al. [1996], campbell [2001]). 6. conclusions in this paper, we started describing the terrestrial natural em noise in the frequency band from 10-5 to 105 hz. sections 2 and 3 were devoted specifically to the main natural sources of em waves in the ionospheric and magnetospheric cavities, in the frequency ranges from ulf to vlf. the following sections dealt with em sources that could be of earth’s internal origin. we have shown some typical characteristics of natural sources in the ionosphere and magnetosphere cavities and some of their relevant properties. we have shown that the em natural background radio noise follows some general physical laws. for example the background noise generated externally to the earth’s surface, shows a psd that follows a typical decreasing trend, as shown in figures 1 and 2, exhibiting a repeating pattern that displays at every scale; this occurrence is typical of fractals, as it happens in many other natural phenomena. we have not dealt here in detail with the em manmade radio noise sources and their characteristics; we know however that this noise can be compared with natural noise, and that only in particular in the elf band, the two contributions can be easily mixed up. moreover in the past decades, the widespread em field measurements eliminated some uncertainty concerning natural noise parameterization in terms of timespace-frequency. electromagnetic signals existing in the ionospheric and magnetospheric cavities are not observed only as primary fields but also as secondary. in fact they can penetrate the earth and propagate at various depths with different attenuations depending on the frequency and on the electric characteristics of the media. this issue is dealt with in magnetotelluric methods that try to characterize the internal earth electrical conductivity by means of a combination of electric and magnetic measurements on the earth’s surface [garcia and jones 2002] and in some cases by means of magnetic field time variations transfer functions only [egbert and booker 1986]. these induced signals can also be reflected at the interfaces and interfere with the primary inducing signals. this leads to a representation of the distribution of electromagnetic fields in the time-space and frequency domain. in case of possible lithospheric sources, the emitted signal’s energy is not yet characterized and is reasonably dependent on the distance. while for ionospheric or magnetospheric sources, the emitted energy is better characterized. it has been reported also that the signal polarization, the ratio of magnetic vertical to horizontal component, is relatively small for the plasma waves coming from the ionosphere/magnetosphere, while this ratio is considerably enhanced, for lithospheric origin emissions. moreover the signal impulsiveness can be important to discriminate the internal sources. in fact impulsiveness is a quality of internal sources while if observed in case of external sources, it should subsist at a large scale, if not worldwide. if a local array network, consisting of at least three stations can be used in the field, the so called ‘direction finding’ radio technique can be used to infer the signal internal origin. the operating method can be goniometric or gradiometric. in the izu and chiba peninsulas ( japan), by measuring the gradient of horizontal and vertical components of the magnetic field at different frequencies, kopytenko et al. [2002] and ismaguilov et al. [2002] have computed the azimuth of the incoming waves from the normal to the observed gradient, leading to an internal source. in case of internal earth generated signals no significant correlation between em signals and geomagnetic activity, expressed also with geomagnetic indices, should be found. possible relations of em waves propagating in the ionospheric-magnetospheric cavity and earthquakes, as we have also mentioned before, cannot be excluded ‘a priori’. we note however that the mechanism of lithosphere–atmosphere–ionosphere coupling, that should be relevant as the final goal of seismo-electromagnetics, still needs to be deeply investigated. as we have reported here the separation between the various external sources of em signals and the earth internal sources, is on the other hand the primary problem to be solved and many questions still remain unanswered on the separation of the two contributions. however some elements earth’s background em noise characterization concerning the internal em source are now considered important, for example the knowledge of electrical conductivity, permittivity, temperature, chemical composition of the crust and its layered structure, with possible travel paths, might play an important role in this research field. moreover the em probable frequency band for these phenomena should fall in the ulf-elf bands. for this reason a detailed monitoring and knowledge of the background em noise is still needed. references anderson, b.j. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice richter_acve_2013_extended_abstract_formatted_131220 ok annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   1   validation strategy for satellite observations of tropospheric reactive gases andreas richter*, mark weber*, john p. burrows*, jean-christopher lambert+, anne van gijsel# *institute of environmental physics, university of bremen +institut d'aéronomie spatiale de belgique (iasb) #royal netherlands meteorological institute (knmi) richter@iup.physik.uni-bremen.de abstract satellite observations of tropospheric reactive gases are an integral part of the earth observing system but require continuous validation by independent measurements. for short-lived tropospheric species, the large variability in space and time results in specific challenges for validation, often combined with the scarcity of appropriate validation data. in this paper, the need for validation is discussed, previous work on validation of satellite observations is briefly reviewed, and the challenges and possible approaches for current and future validation networks are evaluated. i. introduction ver the last two decades, satellite observations of tropospheric composition have become possible using nadir viewing spectrometers operating in the uv, visible, near infrared, and thermal infrared spectral range. using measurements from instruments such as gome, sciamachy, omi, gome-2, iasi, tes, and mopitt, global maps of the spatial distribution of many of the most important tropospheric reactive gases including o3, h2o, co, no2, so2, hcho, chocho and bro can be retrieved. these data for the first time provide a global observational view of tropospheric chemistry, and have led to important insights into the relevance of trace gas emission sources, the transport and chemical transformation of reactive species in the atmosphere and the temporal and spatial scales involved. they have also been applied in studies on air pollution, the assignment of emissions sources and strengths and their changes over time [e.g. martin, 2008, wagner et al., 2008, burrows et al., 2011]. with improving spatial resolution of the sensors and maturity of the retrieval algorithms, applications to chemical weather forecast and routine air quality monitoring will soon become possible. as any remote sensing observation, satellite data on tropospheric species needs to be validated using independent data with known and documented uncertainties, in order to understand and characterise the capabilities and uncertainties of the satellite measurement, assess data quality, and provide users with quality indicators enabling them to judge the fitness of the data for their purpose. for satellite observations of stratospheric composition, in particular for ozone, validation has been performed on a routine o annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   2   basis for several decades, and in many cases, both the independent measurements and the methods applied are close to being mature. however, the situation for tropospheric species in general and in particular for the reactive trace gases is not as advanced, and in fact, many of the relevant data products in this field are still poorly validated. in this manuscript, an attempt is made to discuss the challenges and limitations of validating satellite measurements of reactive tropospheric gases, to review the achievements reached so far, to investigate possibilities to overcome the current limitations, and to formulate recommendations for a validation strategy for the current and future suite of european tropospheric space sensors. the discussion will mainly focus on data products from uv/vis nadir sounders but the concepts are also applicable to tropospheric nir and tir data sets ii.  the challenge when trying to validate satellite observations of tropospheric reactive gases, a number of challenges become apparent that make this a more difficult task than expected. the first characteristic of reactive gases is their large variability in space and time. this is the direct result of their reactivity which leads to a short atmospheric lifetime and this, in combination with often strongly localised sources, results in highly varying atmospheric fields. this is true for both the horizontal and the vertical direction. for instance, species such as no2 are mainly residing in the boundary layer of polluted regions and will not even be well mixed within this layer. a large spatial variability in combination with atmospheric transport also leads to concentrations rapidly changing in time and as such poses a problem for validation as localised validation measurements are not representative for larger areas and time differences between satellite and validation measurement have to be small to ensure comparability. a direct result of the large variability is the presence of strong concentration gradients, which complicate validation as the exact position and time of validation measurement has a large impact on the result. in contrast, the satellite measurements usually average over larger areas, smoothing the gradients and not reproducing the variability of the reference measurement. the short atmospheric lifetime in combination with active photochemistry also leads to diurnal variations of the atmospheric concentrations of some species such as no2, often enhanced by diurnal variations in emissions for example during rush hour or because of the higher probability of thunderstorms and lightning, in the afternoon. again, this necessitates a good temporal coincidence of satellite and validation measurements. in contrast to the stratosphere, the spatial distribution of tropospheric species is strongly influenced by the distribution of emission sources such as cities, power plants, biogenic sources, wild fires, sea ice etc., which are very inhomogeneous and often in regions not well accessible. a validation network well representing the variability of atmospheric conditions therefore has to be wide spread, including also difficult to probe regions such as rain forests, cities or the polar sea ice region. the special characteristics of tropospheric retrievals also have important impacts on the validation needs. in most cases, the sensitivity of the retrieval varies strongly with altitude, usually with lowest values close to the surface where validation measurements are often located. as a result, the retrieval depends critically on a-priori data including the vertical profiles of the species of interest and of temperature, surface reflectance and emissivity, and also the presence of clouds and aerosols. for proper validation, ideally annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   3   these input data will also have to be validated in order to be able to decide if any difference observed is linked to the measurement itself or to the ancillary data used. an additional challenge is the small signal often obtained for tropospheric species, either because their abundances are small or because it is difficult to separate the tropospheric from the stratospheric signals. in many cases, the validation measurements themselves are also not as accurate and precise for these small signals as one would like, adding the uncertainty of the validation data to that of the satellite measurement. considering all the above points, an ideal validation measurement for tropospheric species should provide the vertical profile of the species at different times of the day, for all seasons, at good spatial sampling and covering an area typical for a satellite observation. it should also provide a good coverage of all atmospheric situations, have decent observation statistics and sufficient accuracy (bias, precision) and cover also the quantities needed as additional input in the retrievals. unfortunately, the typical validation measurement falls short in one or even many of these aspects, and in some cases, there exists nearly no independent validation data to compare with. iii. current validation work in spite of the difficulties, many studies have been performed validating tropospheric satellite products. these studies have been documented in the literature, for example in the sciamachy book [gottwald and bovensmann, 2011], the accent-at2 book on remote sensing of tropospheric composition from space [burrows et al., 2011] and the aura validation collection [schoeberl et al., 2008], but also in many individual articles which are too numerous to be referenced here. arguably the best situation exists for tropospheric ozone validation, where data from the ozone sonde network as well as lidar profiles mainly from the ground but also from aircraft and in-situ aircraft measurements can be applied [e.g. verstraeten et al., 2013]. while these data provide vertical profiles at relatively high frequency and good systematic error and precision, they still lack coverage in the southern hemisphere and at low latitudes, in spite of specific programs like shadoz [thompson et al., 2007] which in recent years have increased the number of sonde stations in the tropics. for co, in-situ aircraft observations are the main source of validation which is assisted by ground-based fourier transform spectrometer (fts) observations providing column data in cloud free conditions [e.g. emmons et al., 2009, kerzenmacher et al., 2012]. acquired at a limited number of stations set up initially for stratospheric monitoring, these data sets are sparse and usually do not include pollution hot spots or biomass burning areas. also, regions with challenging retrieval conditions (dark surfaces) are not covered adequately. however, due to the relatively long lifetime of co, coincidence criteria do not have to be very strict and satellite data can be linked to validation observations by using backward trajectories. for tropospheric no2, different validation approaches have been taken. a large number of surface in-situ measurements of no2 are taken within national and local air quality networks. by using assumptions on the vertical distribution of no2, these data can be converted to tropospheric columns to be used for validation of satellite measurements [e.g. ordóñez et al., 2006, boersma et al., 2009]. while providing good statistics at least locally, such comparisons suffer from the large uncertainty introduced by the conversion from surface mixing ratios to columns, from the lack of accuracy of the instruments employed and from sampling issues when using extremely localised road side measurements. tropospheric columns annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   4   (and coarse vertical profiles) can be measured by multi-axis differential optical absorption spectroscopy (max-doas) instruments from the ground, and these data are well suited for validation [e.g. irie et al., 2008]. however, the number of such stations is small and many of them are located in clean air regions. although doas measurements average in the vertical and to a lesser degree also in the horizontal direction, their measurement volume in the troposphere is still much smaller than that of current space instruments. this lack of spatial representativeness and the fairly large uncertainty of individual observations is a problem for validation unless many instruments are distributed over a larger area. the validation of stratospheric columns, that are used to infer the tropospheric column from satellite total column data, can be done using zenith-sky twilight doas measurements [e.g. ionov et al., 2008, peters et al., 2012]. the spatial variability has been addressed by airborne observations using both in-situ and doas remote sensing measurements [e.g. heue et al., 2005, martin et al., 2004] but these are very limited in number. recently, the use of car mounted max-doas has also been demonstrated for validation [shaiganfar et al., 2011], providing spatial coverage at low cost. vertical profile information is currently mainly available from dedicated aircraft flights using in-situ instruments, complemented by max-doas which mainly resolves the lowest layers. for other absorbers such as bro, so2, hcho, chocho and io, even less validation is available, and it is nearly completely limited to a small number of in-situ observations on the ground and in aircraft and to a few active and passive doas and fts observations [e.g. vigouroux et al., 2009, heue et al., 2011, choi et al., 2012, dix et al., 2013]. for so2, a more extensive network of doas instruments has been set-up in the ec project novac [galle et al., 2009] for monitoring volcanic emissions, and this could possibly be used to validate satellite data. iv. strategy for the future any strategy for the future has to address the gaps identified in the current validation activities. the requirements for a good validation strategy are simple – continue acquiring new data, go to the right places, take the right measurements at the right time, accumulate enough data, include validation of ancillary data and facilitate data access. several actions can and should be taken to move into this direction. most importantly, continued operation of existing networks such as the doas, maxdoas, lidar, fts, and ozone sonde networks needs to be secured and maintained. unfortunately, many of the stations in these networks do not have secure funding and the number of measurements taken is currently declining, further limiting our ability to validate tropospheric satellite observations. as many of the stations are in clean air regions as was appropriate for their original purpose of upper atmospheric observations in the context of the network for the detection of atmospheric composition change, ndacc (previously known as network for the detection of stratospheric change, ndsc) or the global atmospheric watch programme of the world meteorological organisation, wmo-gaw, they should be complemented by stations in regions where validation of tropospheric species is needed, for example in pollution hot-spots, biomass burning regions, or areas with large biogenic emissions. also, there is a lack of stations in the southern hemisphere. in some cases, existing networks such as wmo-gaw could be augmented by additional instrumentation such as max-doas instruments to make them (more) useful for validation while making use of existing infrastructure and experience. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   5   one interesting possibility is the establishment of a small number of end-to-end reference sites (or so-called primary sites), dedicated to the validation of tropospheric data and the intermediate steps of their production. these stations should be equipped to provide high quality measurements of all the quantities needed for validation including the input quantities for the retrievals under different conditions. they should be operated having validation in mind by taking year-round measurements at the right times of day and with a perspective of long-term operation. by strategically placing them on different continents (us, europe, asia), such stations could be used to validate both low earth orbit (leo) satellites and the new generation of geostationary (geo) observatories to be launched in the coming years. by ensuring that the same leo satellites are validated by all stations, these validated leo instruments could serve as transfer standards between the three geo satellites which do not have overlapping measurements. while networks of stations provide good statistic and long-term validation, campaign based validation is essential for more detailed analysis. these campaigns should take place in regions close to relevant observations which are not yet validated, for example regions with large spatial gradients, pollution transport, biogenic emissions, shipping emissions, biomass burning, lightning, and bromine explosions. sometimes, experiments of opportunity such as emission reductions for olympic games [e.g. mijling et al., 2009] or changes in legislation affecting pollutant emissions [e.g. kim et al., 2006] can also be performed. often, such campaigns can combine different aims such as addressing a science question (e.g. transbrom campaign [krüger and quack, 2013]), an instrument intercomparison (e.g. cindi [piters et al., 2012]) or a multi-platform experiment such as discover-aq (http://discoveraq.larc.nasa.gov/) with validation, while other campaigns are fully dedicated to validation (e.g. sciavalue [fix et al., 2005]). however, it is essential to ensure that the needs for validation are addressed from the planning to the execution of measurements and finally the analysis of data. in this context, campaigns already planned by other groups can be used for validation purposes by adding instrumentation for example for column measurements or profiling and ensuring that validation aspects are taken into account when designing the measurement programme. in some cases, new developments with potential applications for validation can be supported. examples are the recent construction of no2 sondes [sluis et al., 2010] to be used in a similar way as o3 sondes that can fill an important gap in the atmospheric observation system from ground. the pandora systems of small and flexible remote sensing instruments [herman et al, 2009] could be used to extend max-doas networks. recent developments of highly sensitive cavity enhanced or cavity ring down spectroscopy (crds) instrumentation promise better detection limits for a number of species, and smaller and cheaper lidar and fts instruments could facilitate larger numbers of observations albeit at reduced accuracy. a more radical approach could integrate a large number of small and cheap sensors, usually based on solid-state detectors for crowd measurements with low precision but excellent sampling statistics. a large potential for validation measurements lies in the use of existing platforms such as cars, trains, ships or commercial aircrafts. by mounting small and automated instruments to these vessels, good spatial coverage and statistics can be achieved without having to cover the large costs of vehicle operation. the usability of such platforms for validation measurements is limited by the constraints imposed by the platform operaannals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   6   tors which are often not in line with validation requirements, but as an additional data source, such measurements can provide an important contribution. in addition to existing platforms, unconventional platforms can become very useful for validation measurements, for example ultralight aircraft, unmanned aircraft, zeppelins, tethered balloons or buoys in the ocean and sea ice. all these platforms have the potential to extend the range of validation measurements either vertically or to regions not usually accessible by other observations. one approach to validation which is not linked to independent measurements is the use of chemical data assimilation systems to assess the consistency of a satellite data set. as the data assimilation system implements the chemical and dynamical processes in the atmosphere, it can be used to detect internal biases, spatial offsets and temporal changes in the assimilated data sets. for example, degradation in instrument performance leading to bias in a data product can be pickedup in the quality control of a data assimilation system. this has been successfully used in the past for co observations from mopitt and iasi in the macc assimilation system, and can potentially be extended to other species. in addition, the use of chemical transport models in general and data assimilation systems in particular can increase the number of co-locations usable for validation as the model effectively interpolates in time and space [e.g. klonecki et al., 2012]. such techniques work best for long-lived species which are better constrained by transport and chemistry in the model, limiting the applicability of data assimilation for species such as no2 or bro. finally, comparisons with model data should always be used with care, and can only complement, not replace validation based on real observational data. as important as collecting new data is facilitating access to existing data. this includes data sets assembled for validation purposes as well as data from campaigns and observational networks. ideally, all these data sets should be available for validation through a unified portal, providing a uniform interface and data protocol as well as consistent metadata and formats. such activities are currently pursued in the context of several initiatives and programs including geoms (generic earth observation metadata standard, see http://avdc.gsfc.nasa.gov/geoms), ndacc (http://www.ndacc.org), nors (demonstration network of ground-based remote sensing observations in support of the copernicus atmospheric service, see http://nors.aeronomie.be), the esa and nasa validation data centres (evdc (http://nadir.nilu.no/calval) and avdc (http://avdc.gsfc.nasa.gov)), as well as the ceos cal/val portal (committee on earth observation satellites, see http://calvalportal.ceos.org/). finally, an essential part of a validation strategy is also to ensure the conservation of know-how. as space-borne missions and validation projects come and go, the team of validation scientists is constantly changing, and there is a component of periodic reinvention of the wheel as new generations of scientists encounter the challenges of satellite data validation. it is, therefore, important to provide consistent guidelines to validation groups including information on validation aims, reporting strategies, documentation, fitness for purpose considerations as well as display and interpretation of validation results. in addition, common language and approaches based on metrology should be enforced in validation for the treatment of vertical and horizontal resolution, the treatment of time mismatches, the correct use of appropriate terminology and standards, the differentiation between type a and type b errors, and error reporting in general. annals of geophysics, 56, fast track-1, 2013; 10.4401/ag-6335   7   v. conclusions validation of satellite observations of tropospheric reactive trace gases is a challenging task, both because of the intrinsic variability in the atmospheric fields to be observed and because of the peculiarities of remote sensing of the troposphere. it is an on-going activity that needs continuous support for long-term measurements, campaigns, data analysis, and development of new capabilities. for tropospheric species in particular, it is crucial for validation measurements to have good and adaptive spatial coverage, coverage of the right quantities, and a combination of long-term observations and dedicated campaigns for specific atmospheric events. a multi-tiered approach is needed to improve on the current situation where there are by far not enough validation means available for tropospheric species. this can be only overcome by substantial investment in money and time to establish a more troposphere oriented and more comprehensive validations system. otherwise, we will not have the infrastructure, people, and data needed for tropospheric validation of the many current and upcoming european missions such as gome-2, iasi, sentinel-5p, sentinel-4, and sentinel-5. vi. acknowledgements parts of this work were performed in the framework of the esa funded scilov, multi-taste and valid projects, and as well as the belgian belspo/prodex a3c project and the nl-sciavisie project funded by the dutch government. references [boersma et al., 2009] boersma, k. f., d. j. jacob, m. trainic, y. rudich, i. desmedt, r. dirksen, and h. j. eskes (2009). validation of urban no2 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[wagner et al., 2008] wagner, t., s. beirle, t. deutschmann, e. eigemeier, c. frankenberg, m. grzegorski, c. liu, t. marbach, u. platt, and m. penning de vries (2008). monitoring of atmospheric trace gases, clouds, aerosols and surface properties from uv/vis/nir satellite instruments. j. opt. a pure appl. opt., 10, 104019, doi:10.1088/14644258/10/10/104019. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 1 geoethics in hollyw ood: how can we put more reality into fault hazard zoning? eldon m. gath tan ia gon zález earth con su ltan ts in ter n ation al, san ta an a, ca, usa gath @earth con su ltan ts.com ; tgonzalez@earthconsultants.com abstract the 15-km-long hollywood fault extends through some of the most densely developed and expensive areas in southern california. given that billions of dollars of real estate could be impacted by an earthquake occurring on the fault, and that tens of thousands of people live near it, the seismic hazard posed by the hollywood fault is concerning. but how much so? the city of w est hollywood has required geological fault investigations and building avoidance of at least 15 meters from the fault’s active t race since the mid-1990s, resulting in numerous site investigations. geologic studies in support of the los a ngeles m etro subway and subsequent research found as-yet-unconfirmed equivocal evidence of an early holocene-age (~8 ka) displacement event, and estimated strain rates of 0.3-0.9 mm/yr. in 2014, and following a “not-in-my-backyard” press campaign against the hollywood m illennium project, the california geological survey zoned the eastern hollywood fault as holocene-active under their fault zoning program, requiring geological investigations and building setbacks from holocene-age faults. in the years since the fault was zoned by the city of w est hollywood and the california geological survey, millions of dollars have been invested in geological studies of the fault for building projects. a ll of these dollars have been reluctantly spent by the private sector, and all of this work has been eagerly done by consulting firms. the results? w ith only four exceptions, two in w est hollywood, and two in the city of los a ngeles, and all having opportunities for alternative interpretations, no holocene-age fault displacements have been found. instead, other studies have found definitive evidence that the fault has not ruptured in 10s to 100s of thousands of years. so what is the ethical alternative? should we ignore the hazard posed by the fault? a re we geo-professionals too enamored of the financial gains such regulatory zoning provides that we are unwilling to suggest changes? could we move past the “one size fits all” zoning that requires that single-family homes are held to the same standard as 40-story buildings? in the following sections we present several case studies along the hollywood fault and provide our suggestions for a more progressive program of fault hazard management. 1. in trod uction h e h ollyw ood fau lt is m ap p ed as an eastto n orth east-tren d in g stru ctu re alon g th e sou th ern m argin of th e san ta mon ica mou n tain s (locally referred to as th e h ollyw ood h ills), p lacin g late ju rassic m et am orp h ic an d cretaceou s to miocen e gran itic an d sed im en tary rocks on th e n orth again st th e qu atern ary-aged valley fill of th e h ollyw ood basin . most recen t m ap s gen erally sh ow th e h ollyw ood fau lt exten d in g from th e w est ban k of th e los an geles river w estw ard , th rou gh th e cities of los an geles, west h ollyw oo d an d beverly h ills, a d istan ce of abou t 15 km (fig. 1 an d 2). th e fau lt is con sid ered p art of th e a p p roxim ately 230-km -lon g fau lt system at th e sou th ern ed ge of th e tran sv erse ran ges p h ys iograp h ic p rovin ce of sou th ern californ ia. n earby fau lts th at are also p art of th is system in clu d e th e raym on d an d sierra mad re fau lts to th e east, an d th e san ta mon ica, an d malibu coast fau lts to th e w est . 2. mappin g of the hollywood fault through time as early as 1931, h oots an d kew p rep ared th e first com p reh en sive geologic m ap of th e area, an d in ferred th e san ta mon ica –h ollyw ood t an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 2 fau lts as bu ried stru ctu res at or n ear th e sh arp break in slop e alon g th e sou th ern m argin of th e san ta mon ica mou n tain s. th e fau lt, or se ction s of th e fau lt zon e, w ere also m ap p ed by lam ar (1970), h ill et al. (1978, 1979), weber et al. (1980), crook an d proctor (1992), w ith m ap com p ilation s by dibblee (1991a, b) an d yerkes an d cam p bell (2005). th e m ost d etailed m ap p in g of th e h ollyw ood san ta mon ica fau lt w as d on e by dolan et al. figure 1: location of the hollywood fault at the base of the hollywood hills, forming the northern boundary to the hollywood basin. the fault is considered a short segment of the approximately e-w to n e-w sw system of faults that include the santa m onica, m alibu coast, raymond and sierra m adre faults shown on this figure. a dditional faults that are thought to be part of this system occur farther west and east, respectively, of the mapped image. y ellow square shows the area covered in figures 2 and 3. figure 2: v arious interpretations of the location of the hollywood fault using dibblee’s (1991a, b) geologic maps as a base. n ote that in the east-central portion of the map, most researchers place the fault in almost the same area, whereas to the west and east, the interpreted fault locations vary, sometimes significantly. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 3 (1992, 1997), w h o u tilized u.s. geological su rvey 1920s vin tage 5-foot (~1.5 m ) con tou r top ograp h ic m ap s of th e area to m ap scarp s, fan s an d oth er p oten tially fau lt -related top ograp h ic featu res n ow h id d en by u rban d evelo p m en t. dolan et al. (1992, 1997) id en tified several lin ear scarp s an d faceted sou th -facin g rid ges (see yellow zon es in fig . 2) th at th ey in terp reted as th e n ear-su rface exp ression of th e fau lt. wh ere th e scarp s are broad featu res 15 to 60+ m w id e, th e research ers com bin ed th e geom orp h ic d ata w ith su bsu rface in form ation obtain ed from g eotech n ical an d grou n d w ater stu d ies in an a ttem p t to better con strain th e location of th e m ajor traces of th e h ollyw ood fau lt. h oots (1931) an d oth er early research ers h ad in ferred th e fau lt to be p rim arily a n orth -d ip p in g reverse stru ctu re resp on sible for th e grow th of th e a d jacen t h ollyw ood h ills. h ow ever, based on th e fau lt’s lin ear trace an d steep 80to 90-d egree d ip s observed in d ow n -h ole logged borin gs, dolan et al. (1997, 2000) in te rp reted th e fau lt as p rim arily left-lateral strike-slip w ith a sm all am ou n t of n orth -sid e u p , r everse com p on en t. n otably, in th e west h ollyw ood an d beverly h ills areas, dolan et al. (1997, 2000) in terp reted th e h ollyw ood fau lt sou th of th e m ain break in slop e th at h oots an d kew (1931) an d m ost oth er research ers h ad u sed to in fer th e location of th e fau lt. several fau lt-location stu d ies in th e west h ollyw ood area h ave been con d u cted sin ce th e m id -1990s. abou t a th ird of th ese stu d ies h ave en cou n tered or in terp reted fau lts, an d th e location s of th ese h ave rou gh ly agreed w ith dolan et al.’s (1997, 2000) fau lt scarp s, alth ou gh th e zon e of fau ltin g h as been sh ow n to be w id er an d m ore com p lex, in terp reted to con sist of both n orth -d ip p in g reversesep aration stru ctu res, an d sou th -d ip p in g n orm al fau lts w ith in th e h an gin g w all. sign ifican tly, h ow ever, on ly on e of th ese fau lts h as been in terp reted as h olocen e a ctive, based on th e fin d in gs at th e su n set millen n iu m an d h ou se of blu es sites (on th e far left in fig. 3), w h ere rad iocarbon -d ated sam p les w ere collected from above an d below th e fau lt zon e in d ow n -h ole logged borin gs. several of th ese stu d ies also fou n d th at th e steep step in th e bed rock th at figure 3: part of the zone of required investigation (yellow area) established by the california geological survey (cgs, 2014) for the hollywood fault. a ny property within the yellow zone is required to be investigated for possible active faulting prior to it being developed or re-developed. blue stars denote sites discussed in the text where faults found to be not active were exposed in trenches or were interpreted from transects of cpts and borings; red stars show the only sites where radiocarbon-dated sediments have suggested the occurrence of holocene-active faults. the black lines show the location and extent of transects across the fault zone that were conducted between the 1980s and approximately 2014. since the fault was zoned, several additional studies have been conducted; only a few of these (such as cherokee, hudson, and hollywood m illennium) are included here. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 4 w as p reviou sly in terp reted as a fau lt scarp (h oots an d kew , 1931; dibblee, 1991a, b) is an an cien t beach seacliff. tren ch es an d borin gs h ave exp osed an old m arin e abr asion (w avecu t) p latform an d associated m arin e d ep osits estim ated to be at a m in im u m 400,000 to 900,000 years old (su m m arized in h ern a n d ez an d treim an , 2014; h ern an d ez, 2014). sim ilar m arin e sed im en ts h ave been observed at d ep th both to th e w est an d east, in beverly h ills an d h ollyw ood , resp ectively. th e h ollyw ood fau lt w as in terp reted or o bserved in som e of th e tran sects con d u cted for in frastru ctu re p rojects su ch as th e los an geles (la) metro red lin e alon g cam in o palm ero – martel aven u e, an d la brea aven u e, th e la metrorail p roject alon g cah u en ga bou levard , an d storm d rain tren ch es alon g vista street an d fu ller aven u e (dolan et al., 1997). dolan et al. (2000) revisited th e cam in o palm ero -martel aven u e location an d u sed a series of closely sp aced d ow n -h ole logged borin gs to better locate an d stu d y th e fau lt. th e fin d in gs from th at stu d y su ggested th at th e m ost recen t su rface ru p tu rin g earth qu ake (mre) on th e h ollyw ood fau lt occu rred abou t 7,000 to 9,500 years ago, w ith a p ossible p en u ltim ate even t abou t 22,000 years ago, in d icatin g a lon g in terval betw een earth qu akes, an d an u n con firm ed estim ated strain rate of 0.3-0.9 m m / yr. given its len gth , th e h ollyw ood fau lt is on ly cap able of gen era tin g abou t a mw ~ 6.6 earth qu ake. to p rod u ce a larger-m agn itu d e earth qu ake requ ires th at th e fau lt ru p tu re togeth er w ith oth er fau lts to th e w est an d east, su ch as th e san ta mon ica, malibu coast an d raym on d fau lts. th e existin g p aleoseism ic d ata, h ow ever, sh ow th at each of th ese fau lts h as a sign ifican tly d ifferen t eart h qu ake h istory, w ith th ou san d s of years b etw een even ts on d ifferen t fau lts. th u s, th e available d ata su ggest th at th e h ollyw ood fau lt ru p tu res in d ep en d en tly, h as a slow rate of slip , an d takes th ou san d s of years to reach its brea kin g p oin t. assu m in g th e mre age an d strain rate are correct, th e fau lt h as accu m u lated b etw een 2.1 an d 8.5 m of strain , u n likely h igh valu es for su ch a sh ort fau lt. follow in g th e city of west h ollyw ood exa m p le from th e m id -1990s, th e city of los an geles began requ irin g fau lt stu d ies in th e h ollyw ood area in th e early 2000s. th en in 2014, th at p o rtion of th e h ollyw ood fau lt in th e h ollyw ood qu ad ran gle w as zon ed by th e californ ia ge ological su rvey u n d er th e p rovision s of th e 1972 alqu ist-priolo earth qu ake fau lt zon in g act (figu re 3; h ern an d ez an d treim an , 2014; h e rn an d ez, 2014; cgs, 2014). as a resu lt of th ese requ irem en ts, several stu d ies h ave been co n d u cted in th e h ollyw ood area (su m m arized in h ern an d ez an d treim an , 2014; h ern a n d ez, figure 4a: m ap showing the extensive data set used to evaluate the hollywood m illennium sites, including trenches (in light turquoise), cpts (in red), and continuously sampled borings (in blue). the inferred location of the y ucca street section of the hollywood fault shown by the black dots. two faults (orange lines) were interpreted from the subsurface data; these faults are minor and have not moved in at least 100,000 years. figure 4b: photograph showing a portion of the 9+ m deep trench excavated in the parking lot of the tower records building, which is also the east m illennium site. the footprint of the trenches is shown in light turquoise on the figure to the left. a demonstrably not active fault with about 50 cm of total displacement was exposed near the bottom of the trench. photo courtesy of group delta consultants. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 5 2014). both th e n orth ern an d sou th ern traces sh ow n on th e eastern on e-th ird of th e zon in g m ap h ave been th e su bject of th ese stu d ies. h ow ever, on ly tw o of th ese con su ltin g stu d ies, both on h igh lan d aven u e, h ave in terp reted active (h olocen e) traces of th e h ollyw ood fau lt w ith in th e zon e. 3. site-specific fault stud ies 3.1 hollyw ood millennium sites and hudson avenue th e p rop osed east an d west h ollyw ood millen n iu m sites (far righ t on figu re 3) exten d across th e yu cca street section of th e h ollyw ood fau lt as m ap p ed by cgs on th eir alqu ist-priolo (a-p) m ap . th e geotech n ical con su ltan ts for th is p roject (grou p delta co n su ltan ts) p u sh ed m ore th an 100 con e p en etrom eter test (cpt) p robes, d rilled an d logged m ore th an 30 con tin u ou sly cored borin gs, an d excavated an d logged tw o tren ch es w ith a com bin ed total len gth of m ore th an 120 m (fig . 4). th ese stu d ies h ave sh ow n th at th e area is u n d erlain by ~10 m of h olocen e-aged allu viu m d ep osited w ith in a can yon th at w as carved in to pleistocen e-aged m u d flow d ep osits an d , in th e n orth ern p ortion of th e stu d y area, top an ga form ation (~ 15ma) bed rock an d old er allu vial fan d ep osits th at sign ifican tly p re-d ate th e m u d flow d ep osits. th ese su bsu rface stu d ies su ggest th e p resen ce of at least tw o m in or fau lts u n d er th e area, w ith last m ovem en t on th ese fau lts occu rrin g betw een abou t 100,000 an d 150,000 years ago. th u s, th e area is n ot u n d erlain by active fau lts, con trary to th e cgs’ m ap th at in d icates th e active h ollyw ood fau lt exten d s th rou gh th is site. sim ilarly, cpts an d borin gs d rilled alon g a section of h u d son av en u e en cou n tered a d iscon tin u ity in th e d eep er layers th at is eith er an er osion al su rface or a fau lt. th e featu re is overlain by several u n broken layers of old er allu v iu m m od ified by soilform in g p rocesses. th e d egree of soil d evelop m en t exh ibited by th ese allu vial sed im en ts in d icate th at if th e d iscon tin u ity is fau ltcon trolled , th e fau lt h as n ot m oved in at least 50,000 years. th is estim ate is con sid ered a m in im u m valu e given th at m ost of th e overlyin g u n broken soils w ere tru n cated , in d icatin g sign ifican t erosion al p eriod s. 3.2 cherokee avenue sites to d ate, th ree sep arate stu d ies h ave been co n d u cted off ch erokee aven u e, in h ollyw ood . th e first stu d y fou n d fau lted bed rock w ith n o overlyin g allu vial sed im en ts from w h ich to d eterm in e fau lt activity; th e cgs ch ose to con sid er th e fau lt active (see h ern an d ez et al., 2014). in 2016 w e con d u cted a stu d y im m ed iately to th e sou th . ou r eastern m ost tren ch es exp osed th e h ollyw ood fau lt p lacin g bed rock of th e top an ga form ation (tt) ov er pleistocen e age allu viu m (fig. 5). th e fau lt d ip p ed to th e n orth at abou t 40 d egrees at th e bottom of th e tren ch es, bu t rolled over to n early h orizon tal, form in g th e con tact betw een tw o old er allu vial d ep o sits. th e fau lt tip th ru st qoal2 u n d er qoal3 d u rin g its last su rface-ru p tu rin g earth qu ake. th e fau lt cou ld n ot be traced u p w ard in to th e u p p er section of th e qoal2 u n it, as th is con tact w as in ten sely biotu rbated . in a p arallel tren ch , h ow ever, th e top of th e qoal1 u n it w as clearly n ot broken , so w e are certain th at th e fau lt h as n ot m oved sin ce th e soil in th e qoal1 u n it form ed . ou r age estim ates for th at u n it, based on d egree of soil d evelop m en t, ran ge from abou t 160,000 to 230,000 years, so th is fau lt h as n ot m oved in at least 160,000 years, a fin d in g th at w as later con firm ed by oth ers on an ad jacen t p rop erty. 4. d iscussion desp ite exten sive an d costly in vestigation s, e vid en ce th at th e h ollyw ood fau lt is active r efigure 5: log of southern end of trench eci-1 at the cherokee a venue site showing the fault in red, and the geologic units described in the text. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 6 m ain s elu sive. th e p u blish ed fin d in gs of an early h olocen e d isp lacem en t, an d m ed ia coverage of op p osition to th e h ollyw ood millen n iu m p roject led th e cgs to con sid er th e fau lt su itable for a-p zon in g. th e tw o zon in g criteria u sed by th e cgs are “su fficien tly active” (i.e. h olocen e/ 11,700 ybp ) an d “w ell d efin ed .” if th e few stu d ies su ggestin g h olocen e activity (h ern an d ez an d treim an , 2014), are correct, th e h ollyw ood fau lt m igh t m eet th e extrem e m in im u m con d ition of h olocen e-active, bu t its active trace is certain ly n ot w ell d efin ed , as d em on strated by th e n u m erou s oth er stu d ies attem p tin g, an d failin g, to fin d it. con sid erin g th e m illion s of d ollars in vested by p rivate d evelop ers in th ese stu d ies, can w e n ot d evelop a m ore p rogressive m eth od of ach ievin g eart h qu ake safety alon g th is, an d oth er sim ilarly p oorly d efin ed , low -slip fau lts? californ ia law for active fau lt zon in g sp ecifies avoid an ce of th e fau lt trace as th e on ly p erm itted m itigation altern ative if th e fau lt h as exp erien ced even a sin gle, m in or h olocen e d isp lacem en t. th is bin ary, on e -size-fits-all ch oice h as led m an y ow n ers to n ot u p d ate th eir existin g old er bu ild in gs becau se a fau lt stu d y w ou ld be requ ired before th ey can red evelop . if a h olocen e fau lt is fou n d u n d er th eir site, th e ow n er w ou ld likely be p roh ibited from u n d e rtakin g stru ctu ral m od ern ization for seism ic safety. th u s, on e of th e law ’s u n in ten d ed co n sequ en ces is th e p reserv ation an d con tin u ed u se of stru ctu res th at are seism ically u n safe from h igh -p robability region al earth qu ake even ts becau se of th e low -p robability of su rface fau lt ru p tu re th rou gh th eir site. we p rop ose a m ore p rogressive ap p roach to m itigate th e h azard of su rface fau lt ru p tu re th at takes in to accou n t th e forty years of earth qu ake geology kn ow led ge gain ed sin ce th e law w as en acted . we su ggest th at su rface fau lt ru p tu re m itig ation be in corp orated in to th e bu ild in g cod e, r ath er th an bein g cod ified in to law . in d oin g so, geologists w ill be tasked w ith d eterm in in g th e fau lt’s location an d ru p tu re kin em atics, tim in g of p rior even ts, an d estim ated tim e to th e n ext even t. th en , based on th e typ e of stru ctu re p rop osed , a risk m an agem en t ap p roach w ou ld be em p loyed scaled to th e h azard an d th e risk, sim ilarly to all oth er geologic risk factors (seism ic sh akin g, grou n d failu re, slop e failu re, liqu efaction , flood in g, etc.) th at are alread y con sid ered in th e bu ild in g cod e. th u s, for sin gle-fam ily resid en tial h om es, a fau lt w ou ld be su bject to m itigation requ irem en ts if th e fau lt lies w ith in 500 years of its average recu rren ce in terval. for larger resid en tial an d com m ercial stru ctu res, th e fau lt w ou ld be su bject to m itigation if it lies w ith in 1000 years of its average recu rren ce in terval, an d for critical in frastru ctu re an d facilities, th at in terval cou ld be 1500 or 2500 years. th ese h azard w in d ow s are sim ilar to th ose for seism ic stru ctu ral d esign , liqu efa ction an d lan d slid e geotech n ical stabilization , an d exceed th ose for flood h azard s. mitigation altern atives m igh t in clu d e stru ctu ral isolation or rein forcem en t, soil stren gth en in g, or avoid an ce, d ep en d in g u p on th e an ticip ated fau lt’s d isp lacem en t, site con d ition s, an d econ om ic or social valu e. con tin u in g d ow n th e forty -year-old p ath of strict avoid an ce for an y h olocen e-age fau lt, d esp ite its d isp lacem en t h istory or m agn itu d e, can n o lon ger be d efen d ed as a cost -effective solu tion to im p rove p u blic safety. in telligen t an d p rogressive m itigation of kn ow n an d p rop erly qu an tified h azard s is th e basis of all p erform an ce-based d esign s, an d it is tim e for californ ia’s geologists, an d th e californ ia ge ological su rvey, to m ove on from th e a -p act by eith er m od ern izin g or su n settin g it, an d lettin g th e bu ild in g cod e p rocess h an d le fau lt ru p tu re h azard as ju st an oth er geologic factor in lan d p lan n in g an d d evelop m en t. it is th e eth ical th in g to d o. ackn owledgemen ts th an k you to maria h erzberg of earth co n su ltan ts in tern ation al for p rep arin g figu res 1 th rou gh 3. th e rep ort w as im p roved follow in g com m en ts an d ed its to th e origin al m an u scrip t p rovid ed by um berto fracassi, an an on ym ou s review er, an d giu sep p e di cap u a. th an k you for you r valu able an d h on est feed back . referen ces california geological su rvey (cgs) (2014). earthqu ake fau lt zones of requ ired investigation, h ollyw ood 7.5 minu te qu ad rangle, official m ap released in n ovem ber 6, 2014. californ ia geological su rv ey (cgs) (2018). earth qu ake fau lt zon es of requ ired in ve san n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 7 tigation , beverly h ills 7.5 min u te qu ad ra n gle, revised official m ap released jan u ary 11, 2018. con verse con su ltan ts, earth scien ce associates, an d geo/ resou rce con su ltan ts (1983), seism ological in vestigation an d design cr iteria, for sou th ern californ ia rap id tran sit district, metro rail project, d ated may 15, 1983, parts 1 an d 2 (as referen ced in h e rn an d ez an d treim an , 2014). crook r. jr. an d proctor r.j. (1992). th e san ta mon ica an d h ollyw ood fau lts an d th e sou th ern bou n d ary of th e tran sverse ran ges provin ce. in : pip kin b.w. an d proctor r.j. (ed itors), en gin eerin g geology practice in sou th ern californ ia. association of en gin eerin g geologists, sou th ern californ ia se ction , sp ecial pu blication n o. 4, p p . 231-246 (p u blish ed by star pu blish in g com p an y). dibblee t.w. jr. (1991a). geologic map of th e h ollyw ood an d bu rban k (sou th ½) qu a d ran gles, los an geles cou n ty, californ ia . dibblee geological fou n d ation map #df30, scale: 1:24,000. dibble t.w. jr. (1991b). geologic map of th e beverly h ills an d van n u ys (sou th ½) qu ad ran gles, los an geles cou n ty, califo rn ia. dibblee geological fou n d ation map #df-31, scale: 1:24,000. dolan j.f., sieh k., rockw ell t.k., gu p till p., an d miller g. (1997). active tecton ics, p aleoseism ology, an d seism ic h azard s of th e h ollyw ood fau lt, n orth ern los an geles b asin , californ ia. geological society of am erica bu lletin , vol. 109, n o. 12, p p . 1595-1616. dolan j.f., don ov an s., an d rockw ell t. (2000). paleoseism ic evid en ce for an early to mid h olocen e age of th e most recen t su rface ru p tu re on th e h ollyw ood fau lt, los an g eles, californ ia. bu lletin of th e seism ological society of am erica, vol. 90, n o. 2, p p . 334344. dolan j.f. an d sieh k.e. (1992). stru ctu ral style an d tecton ic geom orp h ology of th e n ort h ern los an geles basin : seism ic h azard s an d kin em atics of recen t fau lt m ovem en t . association of en gin eerin g geologists, procee d in gs of th e 35th an n u al meetin g, p p . 621622. h ern an d ez j.l. (2014). th e h ollyw ood fau lt in th e h ollyw ood 7.5’ qu ad ran gle, los an g eles cou n ty, californ ia. californ ia geological su rvey, fau lt evalu ation rep ort fer 253, su p p lem en t n o. 1, d ated n ovem ber 5, 2014. h ern an d ez j.l. an d treim an j.t. (2014). th e h ollyw ood fau lt in th e h ollyw ood 7.5’ qu ad ran gle, los an geles cou n ty, califo rn ia. californ ia geological su rvey, fau lt evalu ation rep ort fer 253, d ated febru ary 14, 2014. h ill r.l., sp rotte e.c., ben n ett j.h ., real c.r., an d slad e r.c. (1979). location an d activity of th e san ta mon ica fau lt, beverly h ills – h ollyw ood area, californ ia . in : h ill r.l., sp rotte e.c., ch ap m an r.h ., ch ase g.w., ben n ett j.h ., real c.r., slad e r.c., borch ard t g., an d weber f.h . jr. (1979), earth qu ake h azard s associated w ith fau lts in th e greater los an geles area, los an g eles cou n ty, californ ia, in clu d in g fau lts in th e san ta mon ica-raym on d -verd u go-eagle rock an d ben ed ict can yon fau lt zon es. californ ia division of min es an d geology , op en file rep ort 79-16-la (fin al tech n ical rep ort, u.s.g.s. gran t n o. 14-08-0001-g510), p p . b-1 to b-42. h oots h .w. (1931). geology of th e eastern part of th e san ta mon ica mou n tain s, los an g eles cou n ty, californ ia. u.s. geological su rvey profession al pap er 165-c, p p . 83-134, m ap scale: 1:24,000. h oots h .w. an d kew w.s.w. (1931). geologic m ap of th e eastern p art of th e san ta mon ica mou n tain s an d ad jacen t areas, los an geles cou n ty, californ ia. in : h oots h .w., geology of th e eastern part of th e san ta mon ica mou n tain s, los an geles cou n ty, californ ia: u.s. geological su rvey profession al pap er 165-c, plate 16, m ap scale: 1:24,000. lam ar d.l. (1970). geology of th e elysian park rep etto h ills area, los an geles cou n ty, californ ia. californ ia division of min es an d geology sp ecial rep ort n o. 101, 45p . lin d vall s., rockw ell t., kasm an g., an d h elm s j.g. (2001). style, activity, an d u p lift rate of th e h ollyw ood fau lt in h ollyw ood an d west h ollyw ood , californ ia . [abstract] geological society of am erica – cord illeran section 97th an n u al meetin g an d am erican association of petroleu m geologists – pacific section meetin g, ap ril 9-11, 2001, session n o. 12, los an geles, califor n ia. an n als of geoph ysics, 60, fast track 7, 2017; doi: 10.4401/ a g-7541 8 weber f.h . jr., ben n ett j.h ., ch ap m an r.h ., ch ase g.w., an d sau l r.b. (1980). earth qu ake h azard s associated w ith th e verd u go-eagle rock an d ben ed ict can yon fau lt zon es, los an geles cou n ty, californ ia . californ ia division of min es an d geology, op en file rep ort 80-10 la. yerkes r.f. an d cam p bell r.h . (2005). prelim in ary geologic map of th e los an geles 30’x60’ qu ad ran gle, sou th ern californ ia, version 1.0. u.s. geological su rvey op en file rep ort 2005-1019, available from http:/ / pubs.u sgs.gov/ of/ 2005/ 1019 (accessed 15 may 2018). console 723_731.pdf annals of geophysics, vol. 45, n. 6, december 2002 723 probabilistic approach to earthquake prediction rodolfo console, daniela pantosti and giuliana d’addezio istituto nazionale di geofisica e vulcanologia, dept. of seismology and tectonophysics, roma, italy abstract the evaluation of any earthquake forecast hypothesis requires the application of rigorous statistical methods. it implies a univocal definition of the model characterising the concerned anomaly or precursor, so as it can be objectively recognised in any circumstance and by any observer. a valid forecast hypothesis is expected to maximise successes and minimise false alarms. the probability gain associated to a precursor is also a popular way to estimate the quality of the predictions based on such precursor. some scientists make use of a statistical approach based on the computation of the likelihood of an observed realisation of seismic events, and on the comparison of the likelihood obtained under different hypotheses. this method can be extended to algorithms that allow the computation of the density distribution of the conditional probability of earthquake occurrence in space, time and magnitude. whatever method is chosen for building up a new hypothesis, the final assessment of its validity should be carried out by a test on a new and independent set of observations. the implementation of this test could, however, be problematic for seismicity characterised by long-term recurrence intervals. even using the historical record, that may span time windows extremely variable between a few centuries to a few millennia, we have a low probability to catch more than one or two events on the same fault. extending the record of earthquakes of the past back in time up to several millennia, paleoseismology represents a great opportunity to study how earthquakes recur through time and thus provide innovative contributions to time-dependent seismic hazard assessment. sets of paleoseimologically dated earthquakes have been established for some faults in the mediterranean area: the irpinia fault in southern italy, the fucino fault in central italy, the el asnam fault in algeria and the skinos fault in central greece. by using the age of the paleoearthquakes with their associated uncertainty we have computed, through a montecarlo procedure, the probability that the observed inter-event times come from a uniform random distribution (null hypothesis). this probability is estimated approximately equal to 8.4% for the irpinia fault, 0.5% for the fucino fault, 49% for the el asnam fault and 42% for the skinos fault. so, the null poisson hypothesis can be rejected with a confidence level of 99.5% for the fucino fault, but it can be rejected only with a confidence level between 90% and 95% for the irpinia fault, while it cannot be rejected for the other two cases. as discussed in the last section of this paper, whatever the scientific value of any prediction hypothesis, it should be considered effective only after evaluation of the balance between the costs and benefits introduced by its practical implementation. 1. introduction throughout the history of seismology, the problem of earthquake prediction has been characterised by conflicting opinions. these difficulties arise mainly from the lack of a common understanding on how a prediction should be defined. according to the suggestion of the iaspei sub-commission for earthquake prediction, an earthquake precursor is mailing address: dr. rodolfo console, istituto nazionale di geofisica e vulcanologia, dept. of seismology and tectonophysics,via di vigna murata 605, 00143, roma, italy; e-mail: console@ingv.it key words precursors − earthquake forecast − statistical tests − paleoseismology − inter-event time 724 rodolfo console, daniela pantosti and giuliana d’addezio defined as «a quantitatively measurable change in an environmental parameter that occurs before mainshocks, and that is thought to be linked to the preparation process for this mainshock» (wyss, 1991). the set of ideas that forms the basis and lead to the quantitative definition of a precursor is recognisable as an «hypothesis», or a «model». it should be stressed that the hypothesis, or the model, characterising the concerned anomaly or precursor, must be defined in a univocal way, so that it could be objectively recognised and evaluated in any circumstance and by any observer. we refer to rhoades and evison (1989), for a description of the steps of the process leading to the acceptance of a given precursor by the application of a rigorous statistical approach. these steps include a phase during which the parameters of the model are assessed by the analysis of past cases (learning phase), and a subsequent phase where the hypothesis is tested against a new and independent data set (testing phase). non-random time-space distribution of seismicity can be recognised itself as a precursory phenomenon of forthcoming earthquakes. in seismology, two different aspects of non-random occurrence are commonly taken into consideration: earthquake clustering and long-term quasi-periodicity. the first kind of behaviour is frequently observed as a short or medium term interaction among earthquakes, that tend to occur in groups, such as foreshockmainshock-aftershock sequences, multiplets, swarms, etc. the second kind of behaviour is supposed to characterise strong earthquakes, that appear separated by fairly regular time intervals, when they occur on the same seismic sources. both these non-random behaviours, though they appear of opposite nature, can be properly modelled and used for earthquake forecasting. as we shall see later in this paper, while the clustering hypothesis can be straightforwardly tested, because of the abundant data sets collected by the modern seismological networks, testing the recurrence hypothesis requires periods of observations that can exceed even the longest historical records. in this context, data coming from paleoseismology appear a very useful tool. 2. statistical analysis of predictions a simple definition of an earthquake forecasting hypothesis could consist of the identification of particular sub-volumes in the total time-space volume (usually named alarm volumes) within which the probability of occurrence of strong earthquakes is higher than the overall average. the prediction related to the occurrence of a precursor (or a set of precursors) is successful if an earthquake of magnitude equal to or larger than a given threshold magnitude (target event) occurs in the concerned alarm volume. if the observation of a significant number of past cases is available, the number of success (ns), the number of alarms (na) and the total number of earthquakes which occurred in the total time-space volume (ne) can be respectively counted. a good prediction hypothesis should have the capability of maximising both the success rate (the rate at which precursors are followed by target events in the related alarm volume = ns /na) and the alarm rate (the rate at which target events are preceded by precursors = ns /ne). a similar result can be obtained by the trivial way of giving few alarms over very large alarm volumes. this kind of apparent success neglects the fact that non occurrence of events in non-alarm volumes has also to be accounted for as a positive factor (see e.g., rhoades and evison, 1979). in this respect, another parameter, such as the probability gain (the ratio between the rate at which target events occur in the alarm volume and the average rate at which target events occur over the whole target volume = ns /(nava))/(ne /vt)), has been introduced. in principle, a method of prediction can be considered significant if it achieves a probability gain greater than one. a prediction consisting of just the statement that an earthquake of large magnitude will definitely occur within a specific alarm volume may be considered too crude. more frequently, the observation of some precursors allows only an estimation that the probability of occurrence of an earthquake has increased by a certain factor with respect to the past in a given area. so, the conditional probability of occurrence is a parameter that should hopefully supplement the usual time-location-magnitude parameters of the 725 probabilistic approach to earthquake prediction prediction. in this respect, in parallel to what is done for weather forecasting, the language in use refers to synoptic earthquake forecasting, rather than to earthquake prediction. suppose that the target volume is divided into non-overlapping sub-volumes, or cells, that fill it completely. the probability pi of occurrence of at least one target event for each sub-volume i = 1,…, p should be estimated. after a suitable period of observation, a number n of target events will have really occurred in some of the sub-volumes denoted by j = 1,…, n. the log-likelihood of observing that particular realisation of the earthquake process under the hypothesis defining the probabilities pi is (see kagan and jackson, 1995) (2.1) where ci = 0 denotes the cells in which no events have been observed, and ci = 1 those in which at least one event has occurred. the first term in the square brackets derives from the probability of occurrence of a target event in the cell i, the second derives from the probability of nonoccurrence of such event in the same cell. a model will be considered more valid than another, if it attributes higher probabilities to the cells where the earthquakes occur, and if it implies a smaller total number of expected earthquakes. the situation is represented in fig. 1. one can imagine that the integer i represents the time steps in years. according to eq. (2.1), the likelihood of the observed realisation is positively affected by the occurrence of earthquakes in sub-volumes (years) where a probability larger than 0.5 had been estimated (i = 6, 16 and 19), and by the non-occurrence of earthquakes in sub-volumes where a probability smaller than 0.5 had been estimated (i = 2, 3, 4, etc.). on the contrary, it is negatively affected by the occurrence of earthquakes in subvolumes where a probability smaller than 0.5 had been estimated (i = 12), and by the nonoccurrence of earthquakes in sub-volumes where a probability larger than 0.5 had been estimated (i = 1 and 11). if all the probabilities pi are small with respect to the unit, eq. (2.1) can also be written as (2.2) the statistical procedures based on the division of the target volume into discrete regions can be extended to the formulation of the hypothesis of occurrence in terms of continuous variables, by the definition of the occurrence rate density (see console and murru, 2001) (2.3) where x, y, t and m are increments of longitude, latitude, time and magnitude and p x, y, t, m(x, y, t, m) is the probability of occurrence of an event in the volume {x, x + x; y, y + y; t, t + t; m, m + m}. equation (2.2) becomes (2.4) log log log l c p c pi i i i i p = ( )+( ) ( )[ ] = 1 1 1 log log log l c p p p ni i i i p j j n [ ]= = =1 1 ' . x y t m lim p x y t m x y t mx y t m x y t m, , , , , , , , , , , ,( ) = ( ) 0 ln ln l x y t m vj j j j j n = ( )[ ] = , , , 0 1 x y t m dxdydtdm mtyx ( ), , , fig. 1. probability of occurrence for seismic events in non overlapping sub-volumes i = 1,…, n, estimated by a forecasting model (top), and realisation of the process observed in the same sub-volumes (bottom) (after console, 2001). 726 rodolfo console, daniela pantosti and giuliana d’addezio where v0 = ∫x ∫y ∫t ∫m dxdydtdm is a dimensional coefficient. equation (2.4) preserves the same meaning of eq. (2.2), if we consider that the integral in (2.4) gives the total expected number of events according to the concerned hypothesis. 3. long term recurrence testing earthquake forecasting hypotheses in the context of short-term precursors is a fairly easy task, thanks to the abundant data provided by a modern seismological network covering a seismic region. it has been shown, for instance, how the equations seen in the previous section can be applied to clustering analysis (console and murru, 2001). conversely, the understanding of how large earthquakes recur through time, and thus of the seismic cycle, is a more difficult and critical issue. one of the main problems encountered in attempting recurrence models and particularly in verifying them, is the lack of data (i.e. dated earthquakes) due to the short time window within which we can observe how earthquakes recurred in the past. even using the historical record, that may span time windows extremely variable between a few centuries to a few millennia, we have a low probability to catch more than one or two events on the same fault. in fact, large earthquakes rupture the same portion of a fault too infrequently when compared to the time interval spanned by instrumental and even historical seismicity (fig. 2). paleoseismology, that is the geological record of earthquakes of the past, can extend the information on earthquakes back in time up to several millennia representing a great opportunity to study how earthquakes recur through time and thus providing innovative contributions to seismic hazard assessment. in this paper, we present a test carried out using data extracted by the «database of earthquake recurrence data from paleoseismology» developed in the frame of the ilp project «earthquake recurrence through time» (pantosti, 2000). this database includes information on the studied sites (fault, segmentation, location, kinematics, slip rates, etc.) and on the definition of paleoearthquakes: type of observation for event recognition, type of dating, age, size of movement, uncertainties, etc. this test was developed only on data from the mediterranean area. in this area we were able to construct seismic histories with two or more inter-events (i.e. three or more paleoearthquakes) only for four faults: the irpinia fault in southern italy, the fucino fault in central italy, the el asnam fault in algeria, and the skinos fault in central greece (fig. 3). the seismic history for each fault was fig. 2. length of different types of seismic records compared with earthquake recurrence intervals on individual faults in the mediterranean area. note that, because of the relatively long recurrence intervals of large earthquakes typical of this area (1000 or more years), the historical and instrumental records may miss evidence of large earthquakes. in the best cases the historical record may contain evidence for one inter-event interval (i.e. two events on the same fault). 727 probabilistic approach to earthquake prediction reconstructed by integrating results from the different paleoseismological sites investigated along it, following the interpretations proposed by the scientists that performed the investigations (pantosti et al., 1993; meghraoui and doumaz, 1996; collier et al., 1998; galadini and galli, 1999). by using the age of the paleoearthquakes with their associated uncertainty (fig. 4), we tested the null hypothesis that the observed inter-event times come from a uniform random dis-tribution, generally known as the poisson model. for this test we consider the parameter cv, defined as the ratio between the standard deviation σ and the mean inter-event time tr of the observed inter-event times. this parameter is equal to 1 for a uniform random distribution, when the number of observations is very large. cv < 1 denotes a periodical occurrence of events and cv > 1 characterises clustering of events. cv is easily computed for any observed set of inter-events, if all the events are given their occurrence time without any uncertainty. to take into account, the uncertainties we used a monte carlo procedure, computing the average and the standard deviation of cv, for 1000 inter-event sets. these sets were randomly obtained by choosing the occurrence time of each event within the limits of uncertainty provided by the observations (fig. 5). the obtained values are cv = 0.368 ± 0.148 for the irpinia fault, cv = 0.187 ± 0.017 for the fucino fault, cv = 0.806 ± 0.102 for the el asnam fault, cv = 0.656 ± 0.214 for the skinos fault. even if these values, especially that of the fucino fault, appear substantially smaller than 1, we proceeded with the test of the null hypothesis in a more rigorous and quantitative way. in fact, by means of synthetic tests, it was observed that when estimating cv on a small number of inter-event times, it appears systematically biased in defect. in this test, the values of cv obtained experimentally were compared with those obtained by a monte carlo procedure, building up to 1000 synthetic random sets of the same number of events in the same total time covered by the real fig. 3. location of the four faults for which a seismic history was built on the basis of paleoseismological data extracted from the «database of earthquake recurrence data from paleoseismology» of the ilp. 728 rodolfo console, daniela pantosti and giuliana d’addezio fig. 4. ages of paleoearthquakes and associated uncertainty for the four faults used in the test (data from pantosti et al., 1993; meghraoui and doumaz, 1996; collier et al., 1998; galadini and galli, 1999). note that large uncertainties are generally associated with paleoseismological data. paleoseismological data for each fault. in this way we obtained the probability that a value equal to or smaller than each of the observed cv comes by chance from casual fluctuations of a uniform random distribution. as a result of the test, this probability is estimated approximately equal to 8.4% for the irpinia fault, 0.5% for the fucino fault, 49% for the el asnam fault and 42% for the skinos 729 probabilistic approach to earthquake prediction fault. so, the null poisson hypothesis can be rejected with a confidence level of 99.5% for the fucino fault, but it can be rejected only with a confidence level between 90% and 95% for the irpinia fault, while it can not be rejected for the other two cases (fig. 5). 4. economic impact of an earthquake prediction system the choice of the best prediction strategy can be considered an issue for decision makers. different forecasting strategies, even if they are statistically validated, can imply different practical consequences. even if a decision in matter of disaster mitigation is always connected with social and political issues, some statistical analysis can provide elements to indicate a possible strategy. a substantial parameter for the evaluation of different strategies is represented by the cost that each of them would have for the community (vere-jones, 1995). let ctot be the amount of money paid by the community to the earthquakes, including both the cost of prevention measures and the cost of damages c tot = α l (a) + c p n p (a) + c u n u (a) (4.1) fig. 5. comparison between the synthetic random sets of recurrence and the observed cv for each fault. this comparison allows us to derive the probability that the observed intervals are from a random distribution. the average of the coefficient of variation (cv) and of the standard deviation of cv for the fours faults are derived using a monte carlo procedure for 1000 inter-event sets randomly obtained by choosing the occurrence time of each event within the limits of uncertainty provided by the observations. 730 rodolfo console, daniela pantosti and giuliana d’addezio where a represents the level of protection that has to be implemented, α is the cost of maintaining an alarm per unit time, l (a) is the total length of alarms, cp is the cost for recovering the damages after a predicted event, cu is the cost for recovering the damages after an unpredicted event, np(a) is the number of predicted events, nu(a) is the number of unpredicted events. ctot depends, obviously on the threshold of the parameters of the prediction model, to be exceeded before an alarm is declared. in this simple formulation, the cost of implementing and operating the observation system is considered negligible. it seems reasonable to consider cp and cu to depend on the intensity of the earthquakes, and cu larger than cp for any class of intensity. if ne = np(a) + nu(a) is the total number of earthquakes, eq. (4.1) can be written as ctot = cu ne + α l (a) − (cu − cp) np(a) . (4.2) in eq. (4.2), the first term of the right member is the total cost that the community should pay for the earthquakes if no prediction system was operated. the second term is the additional cost to be supported for maintaining the alarms, and the third term is the cost saved by the community because of the successful predictions. the best prediction strategy is the one that minimises ctot. to clarify the meaning of eqs. (4.1) and (4.2) with an example, let us assume that, in arbitrary units, the cost α of maintaining an alarm is 50/ year, the cost cp for recovering the damages of a predicted event is 200 and the cost cu for recovering the damages caused by an unpredicted event is 300. we imagine to apply the warning system to a seismic area characterised, year by year, by the theoretical probabilities pi and the real occurrence c i shown in fig. 1, with i = 1,…, 21. if we let the threshold of probability adopted for declaring an alarm decrease from 1 to 0, the number of alarms changes from 0 to 21. from the values of fig. 1 it is possible to determine, in each case, the number of predicted and non predicted events, and consequently to compute the total cost by means of eq. (4.1) or (4.2). figure 6 shows the results of these computations by three lines, representing respectively the cost of alarms (α l (a)), the cost of earthquakes (cp np(a) + cu nu (a)) and the total cost (ctot ), versus the number of alarms issued. as the duration of each alarm is assumed to be constant, the trend of the cost of alarms is linear. the plot of the cost of earthquakes, obviously not increasing, has some flat interval, in correspondence of false alarms. from the plot of the total cost it is evident that the optimal strategy is achieved by the issue of only two alarms, and therefore with a probability threshold significantly larger than 0.5. moreover, a large fig. 6. cost of alarms, cost of earthquakes, and total cost computed for the realization of the earthquake process shown in fig. 1, versus the number of alarms issued, following the criterion of issuing only alarms for the periods of time characterised by a theoretical probability taken in reverse order. 731 probabilistic approach to earthquake prediction number of alarms gets the total cost even larger than that corresponding to the implementation of no alarm system at all. 5. conclusions statistical analyses are an essential tool for the evaluation of models of earthquake forecasting. their use is conditioned by a rigorous and objective definition of the precursors introduced in the models that have to be compared. in this paper we have dealt mainly with non-random time-space distribution of seismicity, considered as a phenomenon that can allow forecasting the evolution of subsequent seismic activity. we have shown how the evaluation of an earthquake precursor can be carried out comparing the occurrence rate of earthquakes exceeding a given threshold magnitude in the alarm space-time volumes with the average occurrence rate in a seismic area. this estimate can be refined by the addition of the concept of likelihood, computed by the conditional probability of occurrence of earthquakes under a certain hypothesis. a further refinement is also the extension of the concept of likelihood to continuous distributions of the probability density. the evaluation of a forecasting model must be made on a data set independent of the one used in the best fit for the parameters of the model. while this procedure has been demonstrated as feasible for sets of instrumental data collected in a seismic area, it may be hardly implemented for strong earthquakes. this is because the recurrence of strong earthquakes on individual faults implies a very long period of observation to collect statistically significant data sets. paleoseismological data sets, spanning a time interval much longer than that possible for historical data, can provide invaluable information for this purpose. we carried out an analysis on the recurrence of strong earthquakes by selecting from the ilp «database of earthquake recurrence data from paleoseismology» four cases in the mediterranean area. in this way, it has been possible to show that the regularity by which large earthquakes have occurred on the fucino fault is a statistically significant feature (the null poisson hypothesis can be rejected with a confidence level of 99.5%). however, the confidence level is only between 90% and 95% for the irpinia fault, while it is much lower in the other two cases studied: the el asnam and skinos faults. even if a forecasting model is proved to be reliable, its practical application for social and economic purposes can be questionable. by means of elementary formulas and a simple example, it has been demonstrated that the incorrect use of a prediction strategy could lead to costs that exceed the benefits produced by the practical implementation of a prediction system. references collier, r., d. pantosti, g. d’addezio, p.m. de martini, e. masana and d. sakellariou (1998): paleoseismicity of the 1981 corinth earthquake fault: seismic contribution to the extentional strain in central greece and implications for seismic hazard, j. geophys. res., 103, 30,001-30,019. console, r. (2001): testing earthquake forecast hypotheses, tectonophysics, 338, 261-268. console, r. and m. murru (2001): a simple and testable model for earthquake clustering, j. geophys. res., 106, b5, 8699-8711. galadini, f. and p. galli (1999): the holocene paleoearthquakes on the 1915 avezzano earthquake faults (central italy): implications for active tectonics in central apennines, tectonophysics, 308, 142-170. kagan, y.y. and d.d. jackson (1995): new seismic gap hypothesis: five years later, j. geophys. res, 100, 39433959. meghraoui, m. and f. doumaz (1996): earthquake induced flooding and paleoseismicity of the el asnam, algeria, fault-related fold, j. geophys. res., 101, 17,617-17,644. pantosti, d. (2000): earthquake recurrence through time, in proceeding of the hokudan international symposium and school on active faulting, awaji island, hyogo, japan, 17th-26th january 2000, 363-365. pantosti, d., d.p. schwartz and g.valensise (1993): paleoseismicity along the 1980 surface rupture of the irpinia fault: implications for earthquake recurrence in southern apennines, italy, j. geophys. res, 98, 65616577. rhoades, d.a. and f.f. evison (1979): long-range earthquake forecasting based on single predictor, geophys. j. r. astron. soc., 59, 43-56. rhoades, d.a. and f.f. evison, (1989): on the reliability of precursors, phys. earth. planet. int., 58, 137-140. vere-jones, d. (1995): forecasting earthquakes and earthquake risk, int. j. forecasting, 11, 503-538. wyss, m. (editor) (1991): evaluation of proposed earthquake precursors, am. geophys. un., pp. 94. vol. 48, 01, 05ok.qxd 167 annals of geophysics, vol. 48, n. 1, february 2005 key words gas geochemistry – earthquake precursors – greenhouse gases 1. introduction numerous studies on the natural sources of greenhouse gases, such as methane and carbon dioxide, have been carried out in relation to the evolution of the earth atmosphere as well as in the fields of geological and geochemical research. our primary interest lies in the effects of greenhouse gases on the global climate. it is well-known that the greenhouse gas balance is evaluated according to three major sources of emission: abiogenic (volcanoes or thermometamorphism); non-anthropogenic biogenic (vegetation, earth, oceans) and anthropogenic. inorganic co2 emissions form a significant part of the total balance. the origin of this co2 concentration is generally attributed to volcanoes, geochemical cycles and thermometamorphic degassing produced by contact between carbonates and magma. thermometamorphic generation of co2 has been discussed by gianelli (1985), marini and chiodini (1994), rogie et al. (2000), etc. other greenhouse gases, such as abiogenic methane, are generally attributed to deep degassing. chiodini et al. (2000) estimated that thermometamorphic processes are responsible for about 36% of the co2 measured in central italy, while 41% comes from deep sources and 23% is biological. mailing address: dr. giovanni martinelli, arpa – agenzia regionale prevenzione e ambiente dell’emilia-romagna, sezione provinciale di reggio emilia, via amendola 2, 42100 reggio emilia, italy; e-mail: giovanni.martinelli15@tin.it carbon dioxide and methane emissions from calcareous-marly rock under stress: experimental tests results giovanni martinelli (1) and paolo plescia (2) (1) arpa – agenzia regionale prevenzione e ambiente dell’emilia-romagna, sezione provinciale di reggio emilia, italy (2) istituto per lo studio dei materiali nanostrutturati, ismn-cnr, montelibretti (rm), italy abstract the identified emissions of abiogenic carbon dioxide, carbon monoxide and methane are generally attributed to volcanic activity or to geochemical processes associated with thermometamorphic effects. in this paper we show another possible abiogenic source of emission, induced by mechanical, and not thermal, stresses. we investigated the mechanochemical production of carbon dioxide and methane when friction is applied to marly-type rock and studied the mechanisms determining the strong co2 and ch4 emissions observed. a ring mill was used to apply friction and oriented pressure upon a synthetic calcite-clay mixture of varying proportions. we found that the co2 and ch4 release versus the grinding action has a non-linear trend reflecting the behaviour of decreasing crystallinity, which indicates a close link between crystallinity and gas production. for the co2 emission, we propose a release mechanism connected with the friction-induced fractures and the increase in structural disorders induced by creep in the lattice. the ch4 emission could be explained by a sabatier reaction in which co2 and hydrogen are involved to form ch4 and water. 168 giovanni martinelli and paolo plescia another interesting debate on the origin of greenhouse gases concerns the relationship between the greenhouse gas effect and the cretaceous mass extinction. many authors consider that the mass extinction of vertebrates at the end of the cretaceous system was due to a meteorite impacting a carbonate basin; this induced the generation of enormous quantities of co2 through the dissociation of carbonates as an effect of the shockwave (ryder et al., 1996; macleod and koeberl, 2001). however, the studies do not reach agreement on the mechanisms involved. work on the carbonate compressibility by shockwaves shows how the co2 dissociation threshold varies from 10 to 60 gpa (agrinier et al., 2001; skála et al., 2001). according to other studies, carbonate dissociation could occur at much more limited pressures, but under certain stress conditions (dickinson et al., 1991). a recent work demonstrated the possibility of obtaining an intense co2 emission from carbonate rocks with the contribution of mechanical energy by grinding alone (martinelli and plescia, 2004). methane is the subject of particular study, not only because of its environmental implications (greenhouse effect) but also in regard to its possible abiogenic origins of part of the oil reservoirs. a non-biogenic source of methane and more complex carbon composites could radically modify many of our current beliefs concerning the formation of oil and methane reservoirs (beskrovny and tikhomirov, 1968; anders et al., 1973; porfir’ev, 1974; kropotkin and valyaev, 1976, 1984; kropotkin, 1985). another important reason for studying the production/emission mechanisms of carbon dioxide and methane is the monitoring and prediction of seismicity for which there is general agreement on the importance of endogenic derived gases as possible precursors (e.g., irwin and barnes, 1980; geller, 1997; toutain and baubron, 1999). the deformation processes that occurred in central italy in 1997 and 1998 generated geochemical anomalies in the form of gas emissions and thermal springs within a 70 km radius of the epicentre (heinicke et al., 2000; favara et al., 2001; italiano, 2001). these geochemical anomalies can be grouped together as an impressive degassing process due to the variations in crustal permeability generated by deformation. to study the co2 / ch4 emission from dynamic compression, it has been necessary to use a system that can induce a large amount of fracturing through strong compression and friction and with energy sufficiently high as to reduce crystallinity, i.e., replicating what happens in rock during a strong tectonic activity. 2. experimental for laboratory work a ring mill was used, which is the most efficient grinding system available at present for the simulation of friction and mechanical compression over a large surface. thanks to the mill we have (martinelli and plescia, 2004): a high exchange surface; high pressure on the materials; an isochemical ambient with controlled elementary exchange, and a high reaction rate. the ring mill consists of a jar reinforced to contain the material and a grinding body (a steel ring and solid steel roller). a ring mill basically operates through non-hydrostatic compression (impact of the rings on the material particles) and friction (rotation forced by the grinding body on the particles and between the particles and the jar walls). extremely high pressures can be achieved and complete amorphization of minerals with a mohs hardness of less than 3-4 can be obtained within a few minutes (plescia et al., 2003). the exchange surface is very high as the pulverized material has very high specific surfaces and the quantity of material placed in the mill can exceed 10 g. for our experimental trials we used a labtech essa continuous ring mill conceived for continuous production of about 15 kg/h and modified to accommodate diagnostic instruments (fig. 1), i.e., a konimeter to measure gas and dust and an analyzer for the internal temperature. outside the grinding jar, a magnetic pickup connected to a digital oscilloscope provides data on the internal oscillations of the metallic mass, the grinding frequency and the dynamic events occurring during grinding (fig. 2). the milled material consists of a mixture of pure calcium carbonate for analyses (merck) 169 carbon dioxide and methane emissions from calcareous-marly rock under stress: experimental tests results and kaolinitic clay. the clay sample, which was diffraction-analyzed before grinding, was found to be contaminated by quartz and a small quantity of illite. two types of tests were performed: for the first, non-stop grinding was carried out for 30, 60, 120 and 180 min on fixed amounts of 5 g; for the second, fixed-time grinding for samples of 2, 2.5, 5, 10 g was carried out. the trials on samples of different grinding times and/or different weights allowed us to verify the effects of grinding with different amounts of energy per unit weight. the mixtures were analyzed by tga and dta thermoanalyses to determine the amount of co2 released, the temperature and the fig. 1. layout of experimental apparatus. fig. 2. fourier analysis of mill impact frequencies ad different grinding times. table i. experimental conditions. grinding device: labtech essa jar material: steel grinding bodies: one 1 kg internal cylinder, two 1.88 kg rings speed: 1200 rpm ftir gas cell experiment ftir: nicolet avatar 360 sample cell: tga thermostatic transfer line and «100 m» long path cell carrier gas: argon, 99.9% pro analysis x-ray diffraction: rigaku rint 1200 anode: cu, radiation cu kα monochromator: graphite crystal power supply: 30 kv-30 ma differential thermal analyzer: stanton 1500 dta-tga simultaneous analyzer reference α-al2o3 ramp in air from 25 to 1000°c at 5°/min 170 giovanni martinelli and paolo plescia amount of energy utilized in dissociation. the mixtures had a co2 content equal to 32% and an oh content equal to 6% (data obtained by differential thermal analysis). we analyzed the gas using a ftir spectrophotometer with «long path» thermostatic gas cells, similarly to what is used on gc/ftir or tga/ftir systems. the instrument was a nicolet avatar 360. we used multiple acquisition software (nicolet omnic series v.6.1a) to analyse the data in real-time and reproduce the reaction behavior. to analyze the gas in the jar and to avoid contamination from the outside, the jar was flushed with argon at a pressure of 1 bar. the gas flow was 0.5/min, controlled by an analogue flowmeter. for the x-ray diffraction and thermoanalyses, we used, respectively, a rigaku rint 1200 diffractometer and a stanton sta 1000 thermal balance (table i). 3. results practically all the co2 retained in marly rock was emitted in the grinding test. the quantity was slightly lower than that obtained by stoichiometry (about 20% as compared to the 22% obtained thermally), while methane, co and h2o were emitted in measurable quantities. the phenomenon was confirmed by realtime analysis of the gas formed during grinding (figs. 3 and 4). figures 3 and 4 show the behaviour of the ftir spectra in the vibration region of co2, methane and vapor as a function of grinding time for sample of a fixed weight. we can see that the co2 emission curve shows a complex behavior. there is a rapid increase at the beginning of the phenomenon, followed by a progressive decrease. during the experiment, the first two minutes are blank analyses, with the mill off, and no anomalous emission observed. the co2 emission effect is marked by peaks superimposed at a continuous level, while the methane appears after prolonged treatment, together with water (fig. 3). figure 5a,b shows a notable difference between thermal curves of the treated and as-is samples. the latter show three main reactions: weight loss due to evaporation of interstitial water (100-120°c); weight loss from dehydroxylation of clay, between 450-550°c, and weight loss related to the dissociation of co2 from calcite, between 700 and 900°c. after grinding, the dehydroxylation peak, characteristic of the crystalline phase of clay, disappears; the dissociation peak of co2 decreases until it nearly disappears, from 35% for the as-is samples to 6% of the 180′ grinding fig. 3. ftir gas analysis: co2, h2o, ch4 emissions during milling (1 g sample, 180 min grinding). samples, and a crystallization peak appears at very low temperatures. the vitrification temperature, for a mixture calcite/kaolinite clay as is, is around 1300-1400°c. in the grinded samples this temperature is placed at 1100°c. figure 6 shows the x-ray diffraction analysis of grinded samples at different grinding times. the xrd shows how the treatment leads to amorphization of the calcite and clay minerals. in particular, calcite tends to amorphous product, instead of crystallize as aragonite, as observed in previous experiments on calcite single crystal grinding (martinelli and plescia, 2004). in the 120′ sample, it can be observed only the main reflections of quartz, which constitutes the hardest phase of the mixture. 4. discussion of results the most noteworthy data are related to the modality and typology of emission, which can fig. 5a,b. thermal analysis of ground samples: a) tga analysis; b) differential thermal analysis. a b 171 carbon dioxide and methane emissions from calcareous-marly rock under stress: experimental tests results fig. 4. 3d view of ftir spectra (1 g sample). fig. 6. xrd analysis of treated samples at different main grinding time phases: qz – quartz; k – kaolinite; ca – calcite. 172 giovanni martinelli and paolo plescia be attributed to the mechanical, and not the thermal, phenomenon imposed on the marly material. worth noting, firstly of all, are the co2 emissions, which we have explained in terms of two parallel mechanisms: burst emissions, due to the release of co2 molecules from fresh grinding-induced fractures; and continuous emissions, deriving from the bulk, i.e., produced by the material which is not yet fractured, but is undergoing compression and micro-cracking (plastic deformation, slippage of planes). burst emissions seem to be prevalent only during the very early grinding phases, while continuous emissions become dominant after a few minutes’ treatment. the phenomenon of continuous co2 emission brings us to conclude that the deformed lattice of the carbonate releases co2 up to equilibrium. instead water and methane emissions originate in a different scenario. in fact, we suggest that the methane and water derive from the sabatier reaction co h ch h o4 22 2 4 2"+ + . this reaction is moderately exothermic, h° = –165 kj/mol, and could originate when hydrogen molecules become available during grinding. the protonization event coincides with dehydroxylation of the clay during the grinding of the mixture. several authors (aglietti et al., 1986; plescia et al., 2003) have clearly demonstrated that phyllosilicates are particularly sensitive to the mechanochemical action, which easily leads to the elimination of the oh groups and to a completely amorphous structure. it is also well-known that clays, particularly kaolinite, are capable of exchanging protons with the external ambient, but only half of the available protons are exchangeable with a simple high-ph reaction, while acid or neutral ph exchange is not viable. the phenomenon could evolve as follows: during the first grinding phase (or friction action), the material is fractured and the calcium carbonate releases carbon dioxide with spot emission; then, the co2 emission becomes abundant and continuous, while the chemical ambient becomes strongly alkaline; the clays are destroyed by grinding and lose hydroxyls and, hence, protons, some of which recombine with co2 molecules to give rise to methane and water – both observable in the ftir gas spectra; at the end of the grinding, the co2 emission tends to very slowly decrease up to re-equilibrium of the material. to verify our hypothesis, the role of energy during grinding needs to be carefully evaluated. by using the mathematical model developed to study the ring mills (plescia, 2003), we calculated that the energy provided by the mill is about 1240 kj/h (345 j/s). this energy is subdivided by the amount of material put in the mill. the energy generated would be sufficient to complete the reaction, and the chemical ambient is alkaline enough to guarantee de-protonization of the clay. it is also worthwhile recalling a particular mineralogical fact: in some minerals present in certain metapelites, such as cordierite, significant quantities of hydrocarbons derived from methane were revealed by ir (mottana et al., 1983; khomenko and langer, 1999). the presence of methane and related hydrocarbons could be associated with gas formation during metamorphic events. 5. conclusions the data obtained so far allow us to put forward the following hypotheses: in addition to the known mechanisms involved in co2 and methane production, there exists a natural mechanism that works through a mechanochemical-type phenomenon; an abundant amount of co2 is produced during shear stress by mechanochemical molecular dissociation of calcium carbonate; the energies involved are sufficient to dissociate carbonate, so long as there is a very high grinding body/material ratio; the amount of shear stress necessary to obtain the dissociation of co2 and production of ch4 is less then 4 kbar, the threshold value to obtain the crystallization of aragonite. the phenomenon observed in the laboratory could be tentatively extrapolated to a much larger scale so as to give us some idea of what actually happens when carbonate-rock mass undergoes tectonic activity. 173 carbon dioxide and methane emissions from calcareous-marly rock under stress: experimental tests results references aglietti, f., p.j.m. lopez and e. pereira (1986): mechanochemical effects in kaolinite grinding, int. j. min. proc., 16, 135-146. agrinier, p., s.r. boyd, i. martinez, u. scharer, m. javoy and a. deutsch (2001): on the kinetics of cao + co2– – > caco3 and co2 released during impact processes, ann. geophysicae, 13 (suppl. iii) c738 (abstract). anders, e., r. hayatsu and m.h. studier (1973): organic compounds in meteorites, science, 182, 781-790. beskrovny, n.s. and n.i. tikhomirov (1968): bitumens in the hydrothermal deposits of transbaykal, in the genesis of oil and gas (nedra press, moscow), pp. chiodini, g., f. frondini, c. cardellini, f. parello and l. peruzzi (2000): rate of diffuse carbon dioxide earth degassing estimated from carbon balance of regional aquifers: the case of central apennine, italy, j. geophys. res., 105, 8423-8434. dickinson, j.t., l.c. jensen, s.c. langford, p.e. rosenmberg and d.l. blanchard (1991): co2 emission accompanying the fracture of calcite, phys. chem. minerals, 18, 320-325. favara, r., f. italiano and g. martinelli (2001): earthquake-induced chemical changes in the thermal waters of the umbria region during the 1997-1998 seismic swarm, terra nova, 13 (3), 227-233. geller, r.j. (1997): earthquake prediction: a critical review, geophys. j. int., 131, 425-450. gianelli, g. (1985): on the origin of geothermal co2 by metamorphic processes, boll. soc. geol. it., 104, 575-584. heinicke, j., f. italiano, v. lapenna, g. martinelli and p.m. nuccio (2000): coseismic geochemical variations in some gas emissions of umbria region (central italy), phys. chem. earth, 25, 289-293. irwin, w.p. and i. barnes (1980): tectonic relations of carbon dioxide discharges and earthquakes, j. geophys. res., 85, 3115-3121. italiano, f., g. martinelli and p.m. nuccio (2001): anomalies of mantle-derived helium during the 19971998 seismic swarm of umbria-marche, italy, geophys. res. lett., 28, 839-842. khomenko, v.m. and k. langer (1999): aliphatic hydrocarbons in structural channels of cordierite: a first evidence from polarized single-crystal ir absorption spectroscopy, am. min., 84, 1181-1185. kropotkin, p.n. (1985): degassing of the earth and the origin of hydrocarbons, int.. geol. rev., 23, 12611275. kropotkin, p.n. and b.m. valaev (1976): development of a theory of deep-seated (inorganic and mixed) origin of hydrocarbons (goryuchie iskopaemye: problemy geologii i geokhimii naftidov i bituminoznykh porod), edited by n.b. vassoevich, akad. nauk ssr, 133-144 (in russian). macleod, k.g. and c. koeberl (editors) (2001): catastrophic events and mass extinctions: impacts and beyond, geol. soc. am. spec. pap. 356, pp. 749. marini, l. and g. chiodini (1994): the role of co2 in the carbonate-evaporite geothermal systems of tuscany and latium, italy, acta vulcanol., 5, 95-104. martinelli, g. and p. plescia (2004): mechanochemical dissociation of calcium carbonate: laboratory data and relation to natural emissions of co2, phys. earth planet. int., 142 (3/4), 205-214. mottana, a., a. fusi, b. bianchi potenza, r. crespi and g. liborio (1983): hydrocarbon bearing cordierite from the derivo colico road tunnel (como, italy), neues jahrb. mineral. abh., 148, 181-191. plescia, p., d. gizzi, s. benedetti, l. camilucci, c. fanizza and f. paglietti (2003): mechanochemical treatment to recycling asbestos containing waste, waste management, 23, 209-218. porfir’ev, v.b. (1974): inorganic origin of petroleum, am. assoc. petrol. geol. bull., 58, 3-33. rogie, j.d., d.m. kerrick, g. chiodini and f. frondini (2000): flux measurements of non-volcanic co2 emission from some vents in central italy, j. geophys. res., 105, 8435-8445. ryder, g., d. fastowsky and s. gartner (editors) (1996): the cretaceous-tertiary event and other catastrophes in earth history, geol. soc. am. spec. pap. 307, pp. 576. skála, r., j. ederová, p. mateˇjka and f. hörz (2001): mineralogical studies of experimentally shocked dolomite: implications for the outgassing of carbonates, in catastrophic events and mass extinctions: impacts and beyond, edited by macleod k.g. and c. koeberl, geol. soc. am. spec. pap. 356, 571-585. toutain, j.p. and j.c. baubron (1999): gas geochemistry and seismotectonics: a review, tectonophysics, 301, 1-27. s e s s e d e l l a g o d ' o r t a guido pannocchia in q u e s t o l a v o r o -i d e t e r m i n a n o le sesse r i g u a r d a n t i l ' i n t e r o lag', d o r l a con d u e d i f f e r e n t i m e t o d i : q u e l l o di de fa n i e q u e l l o di i l i d a k a r i n v i a n d o a d a t a p o s t e r i o r e e la d e t e r m i n a z i o n e di sesse r i g u a r d a n t i p a r l i di l a g o e l a i n d i v i d u a z i o n e e c o n f r o n t o s p e r i m e n t a l i d e l l e e s s e d e t e r m i n a l e t e o r i c a m e n t e . p e r la p a r t e b i b l i o g r a f i c a i n e r e n t e a l a v o r o e p e r i c h i a r i m e n t i sui m e t o d i u s a t i si fa r i f e r i m e n t o a l l ' a n a logo l a v o r o sul l a g o di g a r d a di p . c a l o i ( ' ) l i m i t a n d o c i q u i a l l e i n d i c a z i o n i e s s e n z i a l i . a ) dati generici relativi al lago d'orta. s e c o n d o p. l a n d i n i ( " ) : s u p e r f i c i e t o t a l e 18 k m " : l a t i t u d i n e n o r d -15" 4 9 ' : l o n g i t u d i n e ovest d i r o m a 1 " , v : l u n g h e z z a s e c o n d o la l i n e a m e d i a n a 13,4 k m ; l a r g h e z z a m a s s i m a 2 . 5 k m : l a r g h e z z a m e d i a 140') m : p r o f o n d i t à m a s s i m a 143 m : q u e s t a si t r o v a a 147 m sul l i s c i l o b) .s'esse nrii-bi e tri-nodale col metodo di defanl. (1 d a t i n u m e r i c i u s a l i sono l u t i r i l e v a t i da u n a c a r t a geografie.-. d e l l ' i s t i t u t o de a g o s t i n i al 5 0 . 0 0 0 ) . ( a m la f o r m u l a di m e r i a n : 2 / t = ~ t t r . i ] i 7 g^o d o v e i i n d i c a la l u n g h e z z a d e l l a g o ( . ' r 13.300 n i ) e /i„ l a p r o f o n d i t à m e d i a (/;. 6 9 , 3 9 m ) si d e t e r m i n a in p r i m a a p p r o s s i m a z i o n e il per i o d o p e r la sessa u n i n o d a l e ; esso r i s u l l a t = 1 6 m , 9 9 ~ f 7 m . s u c c e s s i v a m e n t e , p r a t i c a l e u n e e r t o n u m e r o di sezioni v e r t i c a l i ( 3 2 nel n o s t r o c a s o ) n o r m a l i a l l a l i n e a di v a l l e ( l i n e a c o n g i u n g e n l e i p u n t i di m a s s i m a p r o f o n d i t à ) , d e t e t m i n a t e di esse l e l a r g h e z z e a l l a s u p e r f i c i e del l a g o e le a r c e , d e t e r m i n a t e le a r e e p a r z i a l i del l a g o c o m p r e s e in s u p e r f i c i e t r a u n a s e z i o n e e la s u c c e s s i v a , con l a s e c o n d a d e l l e s e g u e n t i f o r m u l e di d e f a n t : 251 g u i d o p a n n o c c h i a f i g . ] ( 2 x = 2 b a : v ] g r~ i 2r0 ' i 2 ilo fc(-v) dx , ! s(x) j d o v e i s i m b o l i h a n n o i s e g u e n t i s i g n i f i c a l i : 2i}„ a m p i e z z a del m o t o m senso v e r t i c a l e , c u i si a t t r i b u i s c e i n i z i a l m e n t e un v a l o r e a r b i t r a r i o , a d e s e m p i o 1 0 0 ; t„ a m p i e z z a del m o t o i n senso o r i z z o n t a l e ; s(x) a r e a d e l l a s e z i o n e v e r t i c a l e in c o n s i d e r a z i o n e , />(.v) l a r g h e z z a d i d e t t a sez i o n e a l l a s u p e r f i c i e del l a g o ; a.v i n t e r v a l l o f r a una s e z i o n e e l a successiva ( n e l nostro caso o g n i 500 i n , t r a n n e in a l c u n i t r a t t i di l a g o m e n o r e g o l a r i , dove a.v v a l e 250 i n ) : x v a l u t a t o l i m i l o la l i n e a [2] s e s s i ; del l a g o d ' o d i a t a b e l l a \ . 1 'n a 111 s (x) ( * j 1 0 s c m 2 b ( x ) i n b (a) s(x) m 3 io" v(x) p a r z i a l e 1 0 k c m ^ vn ( a ) m i o 3 z a (a a r e a t o t . d e l l a g o ) 1) 1) 0 0 0 0 1 5 0 0 0 , 8 9 6 7 5 8 5 0 7 . 6 2 2 3 7 5 3 2 , 5 0 3 2 5 , 0 0 0 , 0 1 7 9 5 2 1 0 0 0 2 , 0 2 0 0 0 1 4 6 2 , 5 0 2 9 , 5 1 2 5 0 0 4 5 , 0 0 7 7 5 . 0 0 0 . 0 4 2 8 0 :s 1 5 0 0 3 , 1 3 5 0 0 1 3 0 0 4 0 , 7 5 5 0 0 0 6 5 . 0 0 1 1 2 5 . 0 0 0 , 0 7 8 7 0 4 2 0 0 0 3 , 9 9 0 0 0 1 4 6 0 5 8 , 2 5 4 0 0 0 6 7 , 5 0 2 1 0 0 . 0 0 0 . 1 1 5 9 7 5 2 5 0 0 1 . 0 5 0 0 0 1 3 7 5 5 5 , 6 8 7 5 0 0 7 2 , 5 0 2 8 2 5 , 0 0 0 . 1 5 6 0 1 6 3 0 0 0 4 , 3 0 0 0 0 1 5 7 0 6 7 , 5 1 0 0 0 0 6 2 , 5 0 3 4 5 0 , 0 0 0 , 1 9 0 5 3 7 3 5 0 0 9 , 0 5 0 0 0 1 9 5 0 1 7 6 . 4 7 5 0 0 0 9 2 , 5 0 1 3 7 5 , 0 0 0 , 2 4 1 6 1 8 1 0 0 0 1 0 , 7 7 5 0 ( 1 2 2 7 5 2 4 5 . 1 3 1 2 5 0 1 0 0 , 0 0 5 3 7 5 . 0 0 0 . 2 9 6 8 1 9 1250 1 3 , 5 3 7 5 0 2 3 7 5 3 2 1 . 5 1 5 6 2 5 5 4 . 0 0 5 9 1 5 . 0 0 0 , 3 2 6 6 6 10 1 5 0 0 1 6 , 6 2 5 0 0 2 1 6 0 1 ( 1 8 , 9 7 5 0 0 0 6 2 , 5 0 6 5 4 0 . 0 0 0 . 3 6 1 1 8 1 1 175(1 1 6 , 1 5 0 0 0 2 2 4 2 . 5 0 3 6 2 . 1 6 3 7 5 0 6 0 . 0 0 • 7 1 4 0 , 0 0 0 . 3 9 4 3 1 1 2 5 0 0 0 1 1 . 3 5 0 0 ( 1 2 1 2 5 3 0 4 , 9 3 7 5 0 0 5 2 , 0 0 7 6 6 0 . 0 0 0 , 4 2 3 0 3 1 3 5 2 5 0 1 7 . 5 7 5 0 0 2 0 6 2 , 5 0 3 6 2 . 1 8 4 3 7 5 1 5 . 0 0 8 1 1 0 , 0 ( 1 0 . 1 1 7 8 8 1 4 5 5 0 0 1 8 . 8 2 5 0 0 2 0 0 0 3 7 6 , 5 0 0 0 0 0 5 0 , 0 0 8 6 1 0 , 0 0 0 . 4 7 5 4 9 1 5 5 7 5 0 1 7 . 1 7 5 0 0 1 8 2 5 3 1 3 . 1 1 3 7 5 0 1 8 . 7 5 9 0 9 7 , 5 0 0 . 5 0 2 4 2 16 6 0 0 0 1 5 . 4 5 0 0 0 1 5 8 7 , 5 0 2 1 5 . 2 6 8 7 5 0 1 0 . 0 0 9 4 9 7 . 5 0 0 . 5 2 4 5 1 1 7 6 5 0 0 1 1 . 9 0 0 0 0 1 5 5 0 2 3 0 . 9 5 0 0 0 0 7 2 . 5 0 1 0 ' " ' 50 0 . 5 6 1 5 1 18 7 0 0 0 1 3 . 4 2 5 0 0 1 3 7 5 1 8 4 . 5 9 3 7 5 0 7 7 . 5 0 1 0 9 9 7 . 5 0 ( 1 . 6 0 7 3 1 1 9 7 5 0 0 1 1 , 3 7 5 0 0 1 3 5 0 1 5 3 , 5 6 2 5 0 0 6 6 . 2 5 1 1 6 6 0 . 0 0 0 , 6 4 3 9 3 211 7 7 5 0 9 . 1 0 0 0 0 1 2 1 5 1 1 3 . 2 9 5 0 0 0 2 7 . 5 0 1 1 9 3 5 . 0 0 0 , 6 5 9 1 2 2 1 8 0 0 0 9 , 8 5 0 0 0 1 0 8 7 . 5 0 1 0 7 . 1 1 8 7 5 0 3 0 . 0 0 1 2 2 3 5 . 0 0 ( 1 . 6 7 5 6 9 9 2 8 5 0 0 1 3 . 6 0 0 0 0 1 2 1 5 1 6 9 , 3 2 0 0 0 0 5 8 , 7 5 1 2 8 2 2 , 5 0 0 . 7 0 8 1 3 2 3 9 0 0 0 1 1 . 3 7 5 0 0 1 hill 1 5 9 , 2 5 0 0 0 0 6 6 . 2 5 1 3 1 8 5 . 0 0 0.7-1-172 2 4 9 5 0 0 1 5 , 3 0 0 0 0 1 3 7 5 2 1 0 . 3 7 5 0 0 0 6 7 . 2 5 1 1 1 5 7 , 5 0 0 . 7 8 1 8 6 2 5 1 0 0 0 0 1 5 . 5 7 5 0 0 1 4 7 5 2 2 9 , 7 3 1 2 5 0 7 0 . 0 0 1 1 8 5 7 . 5 0 0 . 8 2 0 5 2 9 6 1 0 5 0 0 1 5 . 2 2 5 0 0 111(1 2 1 9 , 2 1 0 0 0 0 7 2 . 5 0 1 5 5 8 2 , 5 0 0 . 8 6 0 5 5 o 7 1 1 0 0 0 1 3 . 4 0 0 0 0 1 3 7 0 1 8 3 . 5 8 0 0 0 0 7 0 . 0 0 1 6 2 8 2 . 5 0 0 . 8 9 9 2 1 1 1 5 0 0 5 , 6 5 0 0 0 9 8 0 5 5 , 3 7 0 0 0 0 6 7 . 5 0 1 6 9 5 7 . 5 0 ( 1 . 9 3 6 4 9 2 9 1 2 0 0 0 1 . 2 0 0 0 0 8 0 5 3 3 . 8 1 0 0 0 0 1 1 . 2 5 ' 1 7 3 7 0 . 0 0 0 . 9 5 9 2 7 3(1 1 2 5 0 0 2 , 7 6 2 5 0 6 7 5 1 8 . 6 4 6 8 7 5 1 0 . 0 0 1 7 7 7 0 . 0 0 0 . 9 8 1 3 6 3 1 1 3 0 0 0 0 , 4 8 0 0 0 120 2 0 . 1 6 0 0 0 0 2 7 . 5 0 1 8 0 4 5 . 0 0 0 . 9 9 6 5 5 3 2 1 3 3 0 0 0 0 6 , 2 5 1 8 1 0 7 . 5 0 1 , 0 0 0 0 0 l e s ( \ l id'i e u l e o l i c o n i] m e i . ili t l i d a k a v e n d o n o s p r i (li v a l l o : a v e n d o scelto pei' il iago in e s a m e c o m e o r i g i n e d e l l e .v 1 estrem o sud del l a g o , -i c a l c o l a il 2 ; „ in q u a n t o è : «l 'l q = i 2 i | 0 b(x1) dx = 2 i ] 0 j h.x. • (/a 2 , ..r:.v, :• o o ( d o v e t'(.v,> l ' a p p r e s e n t a i a r e a p a r z i a l e del l a g o ) la q u a n t i t à d a c q u a c h e t r a n s i t a a t t r a v e r s o la p r i m a s e z i o n e pei' p r o v o c a r e lo s p o s t a m e n t o 251 g u i d o p a n n o c c h i a a s s e g n a l o 2i|0; q u e s t a d i v i s a p e r . s f a ' , ) d à 2'^,. con la p r i m a d e l l e | 2 ] si d e t e r m i n a il 2ai|„ ( v a r i a z i o n e d e l l o s p o s t a m e n t o n e l p a s s a g g i o d a l l a sezione <• a l l a p r i m a s e z i o n e ) ; si p r o s e g u e poi di s e z i o n e in s e z i o n e u s a n d o (li v o l t a i n v o l t a il nuovo 2i|; c h e v i a v i a si d e t e r m i n a . ( n e l p a s s a g g i o da u n a s e z i o n e a l l a s u c c e s s i v a p e r un n u m e r o a b b a s t a n z a e l e v a t o di s e z i o n i i r i t i e n e e l l e l ' e n t i t à d e l l o s p o s t a m e n t o non v a r i i o q u a n t o m e n o a b b i a un a n d a m e n t o l i n e a r e ) . j n c o r r i s p o n d e n z a d e l l ' u l t i m a s e z i o n e ( d i a r c a n u l l a ) se il per i o d o 7' t r o v a t o con la f o r m u l a di m e r i a n ed u s a t o è e s a l t o la q ( q u a n t i t à d ' a c q u a t r a n s i t a n t e ) d o v r e b b e e s s e r e n u l l a . non a v v e n e n d o ciò p e r ( / x ) il t va d i m i n u i t o ; si p r o c e d e i n m o d o a n a l o g o con il nuovo t; se con q u e s t o il v a l o r e d i t c e r c a t o è c o m p r e s o f r a i d u e . •si è o t t e n u t o p e r la u n i n o d a l e il v a l o r e di 7' = 14'",6. si p r o c e d e in m o d o del l u t t o a n a l o g o p e r la b i n o d a l e e t r i n o d a l c ; si -oh,, t r o v a l i i v a l o r i t = 9"',2 p e r la b i n o d a l e ; t = 5 m , 8 p e r la trin o d a l e . n e l l a p r e c e d e n t e t a b e l l a n. 1 si r i p o r t a n o i p r i n c i p a l i d a t i u s a t i con a m b e d u e i m e t o d i n o m i n a l i ; n e l l e t a b e l l e n. 2 , 3 e 4 q u e l l i r e l a t i v i ai c a l c o l i con il m e t o d o d i d e f a n l . e ) .s'esse uni-bi e tri-nodali con il metodo di llidaka ('). n e l l ' e q u a z i o n e eli c b r y s l a l : d'~n 4'j-i„ _ nirl 1 u = 0 [ 3 ] det~g i s i m b o l i h a n n o i s e g u e n t i s i g n i f i c a t i : v a r e a s u p e r f i c i a l e d e l l a p a r t e di l a g o c o m p r e s a tra u n a e s t r e m i t à d i esso ed u n a g e n e r i c a s e z i o n e : 0 < f < a e s s e n d o a l ' a r e a t o t a l e : r r ( i ) , p r o d o t t o d e l l ' a r e a d i u n a gen e r i c a s e z i o n e p e r la sua l a r g h e z z a i n s u p e r f i c i e , p e r c u i a ( o ) -a (</) l i ; 7" p e r i o d o di una s e s s a ; ti f u n z i o n e d i v a v e n t e con l ' a m p i e z z a d e l l a sessa la r e l a z i o n e _ da [4] con l e c o n d i z i o n i ai l i m i l i u.(o) = u(a) = 0. posto = = — e n ( : ) = / i v ( : ) , a l a [ 3 ] d i v e n t a : d'-u }. 4ji3a1 u i) c o n /. = dz ' y \z) tgli e le c o n d i z i o n i a i l i m i l i : i/(o) = 0 . n ( i ) 0 . s e s s e d e l l a g o d oiita 4 ; r t a b e l l a n . 2 ( * ) : t = 14™ 6 = 8 7 6 ' a = a a x 1 0 1 i 2 i l i — r 2 a m) v(x) i o 1 " e m : 1 2 ' ' s (x, i o 3 c m 2 a i l i e m 2 i n 2 a i ) ; c m 0 1 0 0 1 2 6 . 2 2 1 3 2 . 5 — 3 . 6 2 4 — 9 5 0 9 0 . 5 0 2 » 7 3 , 2 — 3 , 6 2 1 9 . 5 0 8 1 0 0 3 » 1 2 5 . 8 — 4 , 0 1 3 1 0 , 5 2 7 0 . 4 8 4 » 1 7 3 . 1 — 4 , 3 46 — 1 1 . 4 0 5 9 . 0 8 5 » 2 1 6 . 2 — 5 . 3 3 8 — 1 4 . 0 0 4 5 . 0 8 6 » 2 4 4 , 4 — 5 . 6 8 1 — 1 4 . 9 0 3 0 . 1 1 ! 7 » 2 7 2 . 3 — 3 , 0 0 9 — 7 . 8 9 2 2 2 9 0 » 2 9 4 . 6 — 2 , 7 3 1 7 . 1 7 1 5 . 1 2 9 1 3 . 1 1 1 3 0 2 . 8 2 . 2 3 7 2 . 9 3 1 2 . 1 9 1 0 » 3 1 0 . 4 — 1 . 8 6 7 — 2 , 4 5 9 . 7 4 1 1 » 3 1 6 . 2 — 1 . 9 5 8 — 2 . 5 7 7 1 7 1 2 » 3 1 9 . 9 — 2 . 2 2 9 — 2 . 9 2 4 2 5 1 3 » 3 2 1 . 8 — 1 . 8 3 1 — 2 , 4 0 1 . 8 5 1 1 » 3 2 2 . 7 — 1 . 7 1 4 — 2 . 2 5 — 0.411 1 5 » 3 2 2 . 5 — 1 , 8 7 8 — 2 . 4 6 2 8 6 1 6 » 3 2 1 . 4 — 2 , 0 8 0 — 2 . 7 3 — 5 . 5 9 i 1 7 2 6 , 2 2 1 3 1 7 . 4 — 2 , 1 3 0 — 5 5 8 1 1 . 1 7 1 8 3 0 8 . 7 — 2 . 2 9 9 — 6 . 0 3 1 7 2 0 1 9 » 2 9 7 , 3 — 2 , 6 1 4 — 6 . 8 5 — 2 4 0 5 2 0 1 3 . 1 1 1 2 9 0 . 7 — 3 . 1 9 1 — 4 . 1 9 9£> 2 4 2 1 i) 2 8 2 . 2 — 2 , 8 6 5 — 3 . 7 6 — 3 2 0 0 2 2 •'6 99,1 2 6 3 . 4 1 . 9 3 7 — 5 0 8 — 3 7 . 0 8 2 3 » 2 3 8 . 8 — 2 . 0 9 9 5 . 5 0 4 2 . 5 8 2 4 i) 2 1 0 . 2 — 1 . 3 7 4 — 3 . 6 0 — 4 6 1 8 2 5 1 7 7 . 9 — 1 . 1 1 2 — 2 . 9 9 4 9 1 7 2 6 l) 1 4 2 , 3 — 0 . 9 3 5 2 . 4 5 — 5 1 6 2 2 7 1 0 6 . 2 — 0 . 7 9 2 2 . 0 8 — 5 3 . 7 0 2 8 7 0 . 0 1 , 2 3 9 — 3 . 2 5 — 5 6 9 5 2 9 » 4 6 . 5 1 . 1 0 7 — 2 . 9 0 5 9 . 8 5 3 0 » 2 2 . 6 — 0 . 8 1 8 — 2 . 1 1 6 1 . 9 9 3 1 » 5 . 6 1 . 1 6 7 — 3 . 0 6 — 6 5 . 0 5 3 2 1 1 . 5 — — sessu uninoilal-e: t 1 4 m , 6 ; l ' u l t i m a c o l o n n a d à l ' a n d a m e n t o d e l l e a m p i e z z e : il n o d o c a d e t r a l a s c z . 13 e 14 e a c a l c o l i f a t t i a l l a d i s t a n z a d i 5155 m d a l l ' e s t r e m o s u d d e l l a g o . [ i l v a l o r e di t u s a t o -i p u ò r i t e n e r e s u f f i c i e n t e m e n t e a p p r o s s i m a t o , s c a r t a m i ' il ([ d i p o c o d a l l o z e r o ] . f * ì s i r i p o r t a n o l e t a b e l l e d e i c a l c o l i s o l o p e r il v a l o r e d i t c h e i è r i t e n u t o su ili c i e n t e m e n t e a p p r o s s i m a t o . 258 ( a i n o p a n n o c c h i \ l i n t e g r a z i o n e d e l l a [ 5 ] f a t t a eoi metodo d e l l e v a r i a z i o n i di r i l z conduce a l l a r e l a z i o n e v i—0 u + 1 1 ( 7 + 1 ) _ li + 2 ) ( / + l ) + ( i : + l ) ( / + 2) ' + 7 + 1 i+j+2 (i + 2)(/+2) ' + 7 + 3 u . j li = 0 dove : v ( l 3 ) s /(s) [6] ( j — 0 1 . 2 » i ) c h e p u ò essere c a l c o l a t o n u m e r i c a m e n t e -e y ( = ) n o t a , cioè se o ( : ) è una f u n z i o n e s e m p l i c e di r . la ruriu normale ottenuta u s a n d o p e r ascisse le v e p e r o r d i n a t e le a( r ) ( f i g . i). nel caso i n e s a m e ha and a m e n t o i r r e g o l a r e : la o(z) non è q u i n d i l'unzione s e m p l i c e di c.ome p e r il lago di g a r d a è q u i n d i n e c e s s a r i a l ' i n t e g r a z i o n e n u m e r i c a dell ' i n t e g r a l e [ 7 ] ( t a b e l l a n . 5). s c r i v e n d o le [<> | d e t t a g l i a t a m e n t e n e l caso rn = 2. si ottengono le tre e q u a z i o n i : u l-f 1 6 1 0 i \ f 0 da cui e l i m i n a n d o l e 1 -i ha l a : j t 1 1 i0i — v 3 6 -— li — li 6 15 u u , + i li 1 i o li 3 5 i 3 3 5 la 1,1 ijll\a.. [ 9 ] e q u a z i o n e di terzo g r a d o in x la cui soluzione d à . p e r mezzo d e l l a seconda d e l l e | 5 ] , il p e r i o d o d e l l a sessa c e r c a t o . in c--;i è /„ = "(1 -zfz" clz ( « = 0 , 1 . 2 , 3 ) . s e s s e del lago u ' o i l t a 2 5 9 t a b e l l a n . 3 : t= 9 " ' . 2 4 i r a = n « u a x '/ ( 2 -li + 2 • '< 2 a i ) i 2 l| i 2 a n « 1 0 i + 2 a v(x) s(x) 1 0 i io1" a i r io'1 c m c m c m (1 100 1 6 . 6 0 1 3 2 . 5 — 3 , 6 2 1 ~ 2 3 . 9 3 7 6 . 0 7 2 » 6 6 . 7 — 3 , 3 0 2 — 2 1 . 8 1 5 4 . 2 6 » 1 0 2 , 0 — 3 . 2 5 4 — 2 1 , 1 9 3 2 . 7 7 1 » 1 2 4 , 1 — 3 . 1 1 0 2 0 . 5 4 1 2 . 2 3 5 » 1 3 3 . 0 — 3 . 2 8 4 — 2 1 . 6 9 9 . 4 6 6 » 1 2 7 , 1 — 2 . 9 5 6 1 9 . 5 2 — 2 8 . 9 8 7 » 1 0 0 , 3 1 . 1 0 8 — 7 . 3 2 — 3 6 . 3 0 ti » 6 4 . 0 — 0 , 5 9 4 3 . 9 2 — 10,22 9 3 . 3 0 2 4 2 , 3 — 0 . 3 1 2 1.03 11.25 111 1 6 , 5 0 , 0 9 9 0.3.3 11.58 | j » — 8 . 4 + 0 . 0 5 2 -f 0 . 1 7 4 1 . 4 1 12 » 2 9 , 9 + 0 . 2 0 8 + 0 . 6 9 10.72 13 » — 4 8 . 2 + 0 . 2 7 1 + 0.9(1 3 9 . 8 2 11 )> 6 8 , 1 + (1.362 11 . 1 9 — 3 8 . 6 3 15 u 8 6 . 9 + 0 , 5 0 6 + 1 . 6 7 — 3 6 . 9 6 1 6 » — 1 0 1 , 7 + 0 . 6 5 8 + 2 . 1 7 — 3 4 . 7 9 17 6 . 6 0 1 1 2 6 . 9 40 . 8 5 2 + 5 . 6 3 2 9 , 1 6 18 » 1 19.5 + 1 . 1 1 1 + 7 . 3 6 2 1 , 8 0 19 » 1 6 3 . 9 + 1 . 1 1 1 + 9 , 5 2 1 2 . 2 8 2(1 3 . 3 0 2 1 6 7 . 3 + 1 , 8 3 8 + 6 , 0 7 — 6 . 2 1 2 1 ii 1 6 9 . 2 : 1,7111 + 5 . 6 7 0 . 5 4 -12 6 . 6 0 1 1 6 9 . 5 + 1 . 2 4 6 + 8 . 2 3 7 . 6 9 23 » 1 6 4 . 4 + 1 . 4 4 5 + 9 . 5 1 1 7 . 2 3 21 » 1 5 2 . 8 + 0 . 9 9 9 + 6.6(1 2 3 . 8 3 25 136.1 + 0 , 8 7 1 45 . 7 7 29.(1(1 2 6 „ 1 1 4 . 6 40 , 7 5 3 + 1.97 3 1 . 5 7 2 7 i> — 9 0 . 1 + o.o75 + 4 . 4 6 3 9 . 0 3 2 8 „ — 6 4 , 1 + 1 . 1 3 5 + 7 . 5 0 16.53 29 » 11.9 + 1 . 0 6 9 47 . 0 6 5 3 . 5 9 3 0 „ 2 3 . 5 + 0 . 8 5 1 + 5 , 6 2 5 9 , 2 1 31 » _ + 1 . 5 0 0 49 . 9 1 6 9 , 1 2 39 2 . 9 sessa binodale: t 9 ' " . 2 : l ' u l t i m a c o l o n n a (1; l ' a n d a m e n t o d •ila a m p i e z z a : i n o d i c a d o n o ira 1 a s e z . 4 . 5 e t r a 2 1 2 2 e a c u l o d i f a l l i a l l e d i s a n z e r i s p e l t m i t e : 2 2 8 0 ni e 8 0 3 2 ili d a l l ' e s t r e m o s u d d e l l a g o . [ i l v a l o r e d i t u s a l o s i p u ò r i t e n e r e s u f l i r i e n l e m e n l e a p p r o s s i m a l o s c a r t a n d o il i| d i p o c o d a l l o z e r o ] n e l l a t a b e l l a 11. 5 si r i p o r t a n o i d a t i n u m e r i c i p e r l ' i n t e g r a z i o n e d e l l a [ 7 ' ] di s e z i o n e i n sezione a v e n d o posto c i o è /i ~ 1. i v a l o r i o t t e n u t i per ì, (i = 0 . 1 , 2 , 3 , 4 ) c o n s e n t o n o l a s o l u z i o n e d e l l a [ 9 j . ( a n i i v a l o r i o t t e n u t i p e r i; la [ 9 ] p u ò s c r i v e r s i : 9 1 1 . 0 7 9 2 1 8 v— 17.102171 1" -r 0 . 0 8 0 4 1 0 9 }.—0.0000952381 0 [ 1 0 ] 2 6 0 ( . 1 1 1 ) 0 p a n n o c c h i a t a b e l l a \ . 4 : 7 ' = 5 m 8 « = 1 (i. \ x '/ : (2)11 r | 9 g. 1 _ \ a i ) i 2 i ] + 2 a n i n ° 10 \ ' l i ) r ( x ) : $ ( x ) 10 io10 cm-1 10" c m c m c m 1) i 1 0 0 1 1 6 . 6 1 5 3 2 . 5 — 3 , 6 2 1 — 6 0 . 2 1 3 9 , 7 9 2 „ 5 0 , 4 — 2 , 4 9 5 — 4 1 , 4 5 1 . 6 6 0 » 19,3 1.573 — 2 6 , 1 4 2 7 . 8 0 1 „ 3 0 . 5 0 , 7 6 4 — 1 2 . 6 9 — 4 0 . 4 9 5 » 1 . 1 — 0 , 0 2 7 0 , 4 5 4 0 . 9 4 6 » — 2 4 , 5 •r 0 . 5 7 0 + 9 , 4 7 — 3 1 . 4 7 7 5 3 . 6 + 0 , 5 9 2 + 9 , 8 4 — 2 1 . 6 3 h » — 7 5 , 2 • 0 . 6 9 8 + 1 1 . 6 0 1 0 . 0 3 9 8 . 3 0 8 — 8 0 . 6 + 0 . 5 9 5 + 4 . 9 4 5 , 0 9 10 « — 8 3 , 8 r 0 , 5 0 4 + 4 . 1 9 — 0 . 9 0 11 » — « 4 . 3 + 0 . 5 2 2 + 4 , 3 4 + 3 . 4 4 12 » — 8 2 . 5 0 . 5 7 5 + 4 . 7 8 + 8 . 2 2 13 » 7 8 . 8 + 0 . 4 4 8 •f 3 . 7 2 + 1 1 . 9 4 11 „ 7 2 . 8 r 0 , 3 8 7 + 3 . 2 2 + 1 5 . 1 6 15 » — 6 5 , 4 + 0 . 3 8 1 + 3 . 1 6 + 1 8 . 3 2 16 » 5 8 . 1 + 0 , 3 7 6 i3 . 1 2 + 2 1 . 4 4 17 1 6 . 6 1 5 12.6 + 0 , 2 8 6 f 4 . 7 5 + 2 6 . 1 9 ih » 2 2 . 3 0 . 1 6 6 h2 . 7 6 + 2 8 . 9 5 19 » — 3 . 1 + 0 , 0 2 7 + 0 , 4 5 + 2 9 , 4 0 211 8 . 3 0 8 5.11 0 . 0 5 5 — 0 , 4 6 + 2 8 . 9 4 2 1 » 1 3 . 7 — 0 . 1 3 9 — 1 , 1 5 j + 2 7 , 7 9 22 1 6 . 6 1 5 3 0 . 0 — 0 , 2 2 0 3 , 6 5 + 2 4 , 1 1 23 » 4 6 . 0 — 0 , 4 0 4 6 . 7 1 + 1 7 , 4 3 24 „ 5 7 , 7 — 0 , 3 7 7 6 . 2 6 + 1 1 , 1 7 25 6 5 . 5 — 0 . 4 2 1 6 . 9 9 + 4 , 1 8 26 „ 6 8 . 5 — 0 , 4 5 0 — 7 . 1 8 — 3 . 3 0 27 „ 6 6 . 2 i — 0 , 4 9 4 8 . 2 1 1 1 , 5 1 28 » 5 8 , 4 1 . 0 3 4 1 7 . 1 8 — 2 8 . 6 9 29 » 1 6 . 6 1.110 1 8 . 4 4 4 7 . 1 3 30 » 2 7 . 8 1 . 0 0 6 1 6 . 7 1 — 6 3 . 8 4 31 » 1 0 . 2 — 2 . 1 2 5 3 5 . 3 1 — 9 9 . 1 5 32 4 . 0 1 1 s e s s o iriu odale ; t = 5 " l ' u l t i m a c o l o n n a d ; l ' a n d a m e n t o ( e l l e a m p i e z z e ; i n o d i c a d o n o t r a 1 ' s e z i o n i 1. 2 ; t r a 1 0 . 11 e ira 25 2 6 e a c a l i o l i f a t t i r i s p e t t i v a m e n t c a l l o d i s t a n z e 9 7 5 n i . 4 5 6 7 111. 1 0 2 7 5 m d a l l ' e s t r e m o s u d d e l l a g o . [ i l v a l o r e d i t u s a t o si p u ò r i t e n e r e m i f l ì c i e m e m e n l e a p p r o s s i m a t o , s c a r t a n d o il (i d i p o c o d a l l o z e r o j . questa e q u a z i o n e si r i s o l v e con il m e t o d o d i newton u s a n d o c o m e p r i m o v a l o r e a p p r o s s i m a t o di (fucilo c l i c si o t t i e n e d a l l a s e c o n d a d e l l e [ 5 ] (/i 1 ) u s a n d o p e r il p e r i o d o d e i r u n i n o d a l e il v a l o r e t r o v a t o c o n il m e t o d o di d e f a n t . s i o t t i e n e cosi p e r à il v a l o r e 0 , 0 0 1 8 2 1 6 s e s s i ; deii.aco d ' o k t a 261 a f i l i c o r r i s p o n d e p e r il p e r i o d o d c i r u n i n o d a l e il v a l o r e |)iii a t t e n d i b i l e t 1 = 1 4 m , 2 r i d n c c n d o poi la [ 1 0 ] ad u n a e q u a z i o n e di secondo g r a d o i n /., p e r m e z z o ad e s e m p i o d e l l a r e g o l a di r u f f i n i , si o t t e n g o n o i n u l t i m o gli a l t r i d u e v a l o r i di a: 0 . 0 0 4 6 7 7 3 à,= 0,01226 d i e d a n n o gli a l t r i d u e v a l o r i t, = 8m,85 t:, = 5m,5 p e r la b i n o d a l e e t r i n o d a l e r i s p e t t i v a m e n t e . determinazione (iella posizione delle linee nodali. •i n o d i si a v r a n n o i n c o r r i s p o n d e n z a di di, essendo m u= v ( 1 z t f a i . i = 0 jnel caso in '2. e s p l i c i t a n d o i i. a , 0 0 e p e r i nodi ' u l = 4 ± a 3 + 3 ( d ± \ s . + 2 ( 1 é ± \ s 1 = 0 [ 1 1 ] dz a0 \ a0 a j \ a0 e q u a z i o n e c u b i c a in z l a c u i s o l u z i o n e d à la p o s i z i o n e z= — dei n o d i . (i a a., i r a p p o r t i -'• e —~ v a n n o d e t e r m i n a t i d a d u e d e l l e e q u a z i o n i [ 8 ] u s a n d o pei l ' u n i b i e t r i n o d a l e r i s p e t t i v a m e n t e i t r e v a l o r i t r o v a t i di /. l ' e q u a z i o n e e u b i c a in ; si r i s o l v e q u i n d i col m e t o d o di n e w t o n , u s a n d o p e r p r i m o v a l o r e a p p r o s s i m a t o d i r q u e l l o c l i c si d e d u c e d a l m e t o d o di d e f a n t . essendo 0 < z < 1 p e r l ' i m i n o d a l c l ' u n i c a r a d i c e di [ 1 1 ] u t i l e s a r à q u e l l a c o m p r e s a f r a 0 e d 1 ; p e r la b i n o d a l e l e d u e c o m p r e s e f r a 0 ed 1 ; p e r la t r i n o d a l e t u t t e e t r e . 251 guido p a n n o c c h i a t a b e j . l . 4 \ . 5 ( d a t i n u m e r i c i p e r l ' e q u a / . 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p e r i c a l c o l i t o r n a c o m o d o u s a r l i c o m e s c r i t t i a l l a r i g a s o t t o s t a n t e . si sono o t t e n u t i i s e g u e n t i r i s u l t a t i : p e r l ' u n i i i o d a l c : 3 = 0 , 4 5 7 3 8 c l i c c o m p o r t a la d i s t a n z a d a l l ' o r i g i n e d e l l u n i n o d o di 5337,82 m ; p e r sf.sse dei. lago ii oiita 2 6 3 la b i n o d a l e : s , = 0 . 1 6 4 6 3 =. = 0 , 7 1 2 7 2 e l l e c o m p o r t a n o le d i s t a n z e d a l l o r i g i n e d e i d u e nodi di 262.1 m e di 8 5 6 0 i n ; p e r l a t r i n o d a l e : = 0 . 0 8 6 7 7 = 0 . 4 5 1 7 3 =:1 = 0.87231 e l l e c o m p o r t a n o l e d i s t a n z e d a l l o r i g i n e dei tre n o d i di 1610 m 5285 in 10515 i n . nella s e g u e n t e t a b e l l a 1). h si b a l l i l o i v a l o r i r i a s s u n t i v i e comp a r a t i v i dei dati o t t e n u t i con i d u e m e t o d i : t a u r i la n 6 t i p o d i s e s s a p e r i o d o e s p r e s s o i n d i l ' . a s e s s a m i n u t i p r i m i d i f f e r e n z a v a l o r i d i s t a n z e d e i n o d i d a l l o e s t r e m o s u d del i a g o i n m d i f f e r e n z e in m t i p o d i s e s s a m c t o d o d i i), i'anfc m et.odo di 1 fi h h l i a ' n u m e r i c i , m e t o d o d i d e i a n t m e t o d o di h i d a k a 1 v a l o r i . ' n u m e r i c i l n i n o d a l e 1 1 , 6 1 1,2 0 , 1 5 4 5 5 5 3 3 8 1 1 7 b i n o d a l e 9 , 2 8 , 8 5 0 , 3 5 2 2 8 0 8 0 3 2 2 6 2 5 8 5 6 0 3 4 5 5 2 8 t r i n o d a l e 5 , 8 5 , 5 0 , 3 9 7 5 4 5 6 7 1 0 2 7 5 1 6 1 0 5 2 8 5 1 0 5 1 5 6 3 5 ì 7 1 8 2 4 0 1 distribuzione delle ampiezze. l a [ 4 ] e s s e n d o i za s c r i v e : d'i a — dz c h e n e l caso m~2 d à : t = a a o p p u r e i —= ; ' + 3 ' <o •io 'o + 2 1 ' 0 w o ' ' o , . [12] (love i v a l o r i dei r a p p o r t i i e d i a0 a„ d e t e r m i n a n o c o m e g i à i n d i c a t o in r e l a z i o n e a i t r e v a l o r i d i a, q u i n d i la [ , 1 2 ] >i c a l c o l a |>er ogni z di c i a s c u n a s e z i o n e e -i h a cosi l ' a n d a m e n t o d e l le a m p i e z z e . la [ 1 2 ] p e r il caso i n e s a m e v a l e : 2 6 1 (.t i n o pannocchia j , 6 6 4 6 3 8 .=:' —3.20049.", = 2 + 3 . 3 0 1 3 4 i r — i 6 , 3 0 2 9 6 7 s 3 — 1 4 , 0 5 2 5 5 1 s 2 + 8 , 2 1 6 8 8 4 s —1 2 9 , 2 1 7 9 2 9 = ; , — 4 f . 2 2 3 5 2 0 ; 2 s i t r a l a s c i a d i r i p o r t a r e l a t a b e l l a dei v a l o r i o t t e n u t i e si r i p o r t a n o i n v e c e n e l l e fig u r e 2 e 3 le c u r v e a n d a m e n t o d e l l e a m p i e z z e in f u n z i o n e di « o t t e n u t e con i d u e m e l o d i , fi c o n f r o n t o n e l l a t a b e l l a n . 6 e n e l l e f i g u r e 2 e 3 f r a i d u e m e t o d i d i c e c h e i r i s u l t a t i o t t e n u t i sono s u f f i c i e n t e m e n t e i n acc o r d o e c h e p r e s u m i b i l m e n t e r i s p o n d o n o a l l a r e a l t à . roma istitìitn nazionale di geofilica dicembre 1947. p e r l ' u n i n o d a l e p e r la b i n o d a l e p e r la t r i n o d a l e s e s s e dee lago d'oitta 2 6 5 riassunto nel presente lavoro si determinano con i metodi di dcfant e di hidaku i periodi delle sesso uni-bi e, trinodale del lago d'orta; si individua la posizione dei nodi e, da ultimo, si ottiene l'andamento <ielle ampiezze di oscillazione. b i b l i o g r a f i a ( ' ) cai.oi i ' . : le sesse ilei ingo ili garda. p a r i e p r i m a . sesse che interessano l'intero ingo. a n n a l i g e o f ì l i c a . 1. 2 1 4 8 ( 1 9 4 8 ) . ( 1 landini p . : il lago d'oria. a p p u n t i d i g e o g r a f i a a n t r o p i c a . b o l l , d e l l a r . s o e . g e o g r a f i c a i t a l i a n a , 4 5 1 ( 1 9 2 3 ) . adg vol5 n02 mor 259_271.pdf annals of geophysics, vol. 45, n. 2, april 2002 259 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) sergio morandi and enrico ceragioli enterprise oil italiana s.p.a., roma, italy abstract val d’agri is a «recent ssw nne graben» located in the middle of the southern apennines thrust belt «chain» and emplaced in plio-pleistocene. the recent sedimentation of the valley represents a local critical geophysical problem. several strong near surface velocity anomalies and scattering degrades seismic data in different ways and compromises the seismic visibility. in 1998, eni and enterprise, with the contribution of the european community (esit r & d project – enhance seismic in thrust belt; eu thermie fund) acquired two «experimental seismic and resistivity lines» across the valley. the purpose of the project was to look for methods able to enhance seismic data quality and optimize the data processing flow for «thrust belt» areas. during the work, it was clear that some part of the seismic data processing flow could be used for the detailed geological interpretation of the near subsurface too. in fact, the integrated interpretation of the near surface tomography velocity/depth seismic section, built for enhancing the resolution of static corrections, with the hr resistivity profile, acquired for enhancing the seismic source coupling, allowed a quite detailed lithological interpretation of the main shallow velocity changes and the 2d reconstruction of the structural setting of the valley. 1. geological framework and geophysical context «val d’ agri» is located in the middle of the southern apennines thrust belt «chain» and in this context the «val d’agri graben» is a typical recent «tectonic valley» controlled by ssw and nne dipping high angle pleistocene faults. this structural setting represents the apennine maximum extension direction associated with the middle pleistocene-holocene normal tectonic regime, where the youngest tectonic elements are normal nw-se trending faults with associated n-s trending left – lateral strike – slip faults, superposed onto the pre-existing apennine fold and thrust structures (hippolyte et al., 1994a,b). the sediments of the valley are essentially plio-quaternary marine, lacustrine and fluvial deposits of highly variable lithology and thickness (0 to several hundred meters). the youngest stratigraphic units of the valley are coarse to fine grained alluvial deposits and coarse slope «breccias». below the «valley» the apennine thrust belt is characterised by «embricate» structures with extreme vertical and later tectonic displacements. mailing address: dr. sergio morandi, enterprise oil italiana s.p.a., via dei due macelli 66, 00187 roma, italy; e-mail: sergio.morandi@rome.entoil.com key words esit project seismic imaging tomostatics integrated interpretation 260 sergio morandi and enrico ceragioli m. hianco e 1 le za 1 t refo t n -1 a r mos nval d'agri 5000 m 10 km pliocene and recent apenn. platform allochtonous basement apulian platform fig. 1. geological cross section across southern apennines. val d' agri fig. 2. example of conventional seismic image across val d’agri. 261 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) in this context the sediments and the tectonic environment of the valley are a sort of recent and thin «roofing» above the apennine complex geological setting (fig. 1). this recent sedimentation is the cause of an important local geophysical problem. in fact, the presence of this recent sedimentation has a negative impact on seismic data quality and degrades the seismic image. – significant shallow velocity variations and anomalies. – heavy wave scattering noise that masks the seismic reflections. and, in particular, – serious problems for optimizing static corrections, – very poor quality of the seismic image, are the usual geophysical consequence on seismic of this «thin roofing» above the «apennine imbricate structures» (fig. 2). 2. 2d resistivity and seismic acquisition tests innovative methodologies have recently been tested in val d’agri for enhancing seismic visibility. in late 1998, eni and enterprise, with the contribution of the european community, acquired two experimental 2d seismic lines associated with two relative high resolution resistivity profiles across the valley (fig. 3). the results obtained on line a (crossing tramutola village, monticello hill and villa d’agri village) are illustrated in this paper. one of the purposes of the test performed was the analysis of methods able to improve seismic data quality by optimising acquisition source coupling and static correction efficiency. the assumptions were that: – the application of correct statics to the seismic data may improve the continuity of pliocene and recent sediments km 10 1998 seismic test lines line a fig. 3. location of the 1998 2d seismic test lines. 262 sergio morandi and enrico ceragioli events, produce a better subsurface image, and give a more appropriate relative position in time domain to each single reflection. – the presence of an hr resistivity profile along the seismic line may aid the individuation of the shallowest «clay sequences» of the valley, that represent the optimal lithology for seismic source coupling during the acquisition of the seismic data. 2.1. hr resistivity survey the equipment used for the hr resistivity survey and the recording parameters were the following: – equipment: agi sting r!; 256 electrods; 1 to 599 ma; 320 to 800 v; reading cycle: 1.2 to 14.4 s. – recording parameters: ab/2 120 to 500 m; dipole length ranges: 5 to 12 m; max number of electrodes deployed at any one time: 56; no. of soundings along the seismic line: 15. the interpretation flow was based on the 2d inversion of each single «automatic true resistivity section» and their «stack» along the line (fig. 4). 2.2. seismic survey the seismic survey was acquired by the «sercel 368 recording unit» using both together «conventional near vertical roll along» and «global offset» combined recording techniques (dell’aversana et al., 1999, 2000). the seismic was recorded using offsets of 3000 to 18 000 m, max cmp fold of 12 000% and source depth and coupling variable; optimised following the hr resistivity data indications. however, the near vertical seismic information was only used for tomostatics and near surface tomography processing. the roll along 2d near vertical seismic recording parameters and layout are schematised in fig. 5. 3. near surface seismic tomography processing the roll along near vertical seismic shots have been only used for near surface tomography issues. with the purpose to enhance the seismic static corrections resolution, «statics» have been calculated by the near surface tomography approach based on the travel time inversion of the near vertical seismic first arrivals (zhu et al., 1992, 1998). tomographic inversion was based on analysis of the automatic and/or manual picking arrival time of the turning waves. this has been done on each sp along the entire near vertical offset (3600 m). there are three distinct steps to perform the above process: 1) divide the model into initial grid cells. 2) ray trace through the model and compute source to receivers travel times. 3) construct an inversion matrix from the raytraced result and solve the linear equations. the commercial «h. russel gli3d software» was used to perform the job. the tomographic inversion method implemented within gli3d uses a new approach that abandons rays and uses a wavefront method in both the forward and inverse processes. this has two advantages: the first is a finitedifference algorithm for the rapid and accuratefig. 4. hr resistivity section – interpretation flow. 263 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) forward modeling of travel times (previously done using ray tracing); the second is the application of a new much faster inversion procedure in order to calculate the back propagation of errors and thereby updating the velocity model (previously done by the solution of linear equations). the inversion process is initiated with an estimate of the true velocity structure (hampson and russell, 1984). the first arrival time is computed by solving the eikonal equation, which calculates the first arrival time from a source location to each point within the model. the method is rapid and accurate, and properly handles the various wave types that comprise a first arrival (fig. 6a). it can also be applied to a heterogeneous medium with moderate velocity variations. the errors between the observed and calculated travel times are then calculated and back propagated to the source and the model velocity is updated during the back propagation. the velocity updates are calculated by using a gradient search technique that minimizes the discrepancy between the observed and calculated travel times. the back propagation algorithm is originally introduced in the field of computer neural networks as a training algorithm. it is a generalization of the least square algorithm. it uses the gradient search technique to minimize the mean square difference between the desired and the actual network output. the inversion is done shot by shot. the iteration «loop» is repeated until an acceptable match is obtained between the observed and calculated travel times (fig. 6b). there are two iterative techniques available in gli3d: art (algebraic reconstruction technique) and sirt (simultaneous iterative reconstruction technique). with art the velocity model is updated for every shot following each iteration, and with sirt the model is updated after all shot records have been modeled. the inversion method implemented within the «hr gli3d software» is calibrated based upon the pick time, such that the error between the observed pick time and the modeled pick time is minimized. eight iterations were executed during the seismic tomography processing. the iterative procedure was stopped when: by "resistivity" information fig. 5. roll-along 2d near vertical seismic recording scheme. 264 sergio morandi and enrico ceragioli – no substantial improvement or modification was obtained in the model by increasing the number of iterations. – the differences between observed and calculated travel times were estimated as minima. the resolution is maximum in the central area of the seismic section and reduces sensibly in function of spatial ray coverage and density. the ray coverage is redundant and constant along the entire section and it usefully investigates the first 400-500 m of the subsurface along the entire profile. the maximum investigation of the subsurface in the final velocity/depth model is around 800 m. 4. imaging improvements the near surface tomography results have been used in two different ways. tomostatics improved the static correction resolution and consequently the conventional seismic time image of the valley. in fig. 7 conventional statics processing and near vertical tomostatics are compared. it is easy to note the improvement of the seismic image obtained using tomostatics relate to the conventional elevation statics based on the sp uphole time correction. however, despite the sensible improvement, any detailed lithological and structural interpretation of valley is possible using this conventional twt representation of the seismic information. near vertical tomography velocity/depth model showed surprising and significant details inside the valley. fig. 6a. automatic picking on lmo corrected shot gathers. fig. 6b. near surface tomography processing flow. 265 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) a new integrated interpretation as for the lithological than for the structural interpretation of the valley was possible using the near surface tomography representation of the seismic information. in fact, the «high resolution shallow velocity/ depth seismic model» displayed by the near surface tomography of the first arrival allowed at the same time (figs. 8 and 9): – an unconventional representation of the shallower part of the seismic information. – a significant illumination of the structural setting inside the «val d’agri graben» to 1000 m at least, as it was never done in the past by conventional seismic images (fig. 10). – a detailed interpretation of the shallower part of the seismic section, by removing noise and time imaging limitations, that are typically the limit of the conventional seismic twt representation. – a new robust 2d seismic image able to contribute to the knowledge of the val d’agri significantly. 5. integrated 2d interpretation of seismic and resistivity images across the «val d’agri graben» the integrated interpretation of the 2d near surface tomography velocity/depth seismic section with the 2d hr resistivity profile sensibly contributed to: – the reconstruction of thicknesses and lithology of the most recent sediments of the valley (figs. 11 and 12). fig. 7. two way time (twt) image of the val d’agri valley using tomostatics or elevation statics processing. 266 sergio morandi and enrico ceragioli figs. 8-9. near surface tomography – initial and final velocity/depth models. 267 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) – the knowledge of the structural setting of the valley (figs. 13 and 14). in fig. 11, the 2d geoelectrical profile is displayed and the lithological interpretation of the main resistivity contrasts is reported. this section was obtained by manual interpolation of 1d hr measurements. the penetration of the resistivity profile is about 200 m. the western border of the valley is associated to the presence of a «carbonate bed rock» > 2000 m (apennine platform outcrop) and its dipping high angle pleistocene fault; while the eastern border of the valley is masked by slope breccias and derbis slope deposits 100-150 m. inside the valley, two coarse alluvial deposits cycles can be separated interpreting the most significant resistivity changes, and the progressive and rapid migration of the agri river to the western border can be underlined. in fig. 12, if the interpretation of the resistivity profile is integrated with the near surface tomography velocity/depth section, a great correspondence is present between shallow resistivity and shallow velocity contrasts, but the resolution and the penetration of the seismic image is sensibly higher than in the resistivity profile. for this reason, on the seismic tomography section it is possible to note the presence of a probable third alluvial deposits cycle (3) plus other 3 or 4 alluvial deeper cycles to about 800 m of depth. the «perfect» correspondence between resistivity and velocity contrasts is probably due to the progressive compaction of the sediments fig. 10. conventional time seismic versus tomography velocity/depth section. 268 sergio morandi and enrico ceragioli fig. 11. hr resistivity versus lithology. agri river 200 m fig. 12. hr resistivity versus lithology. 269 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) fig. 13. near tomography velocity – depth section – structural interpretation of the val d’agri graben. fig. 14. near surface tomography velocity – depth section – structural interpretation of the val d’agri graben in the wider context of the apennine trust setting. 270 sergio morandi and enrico ceragioli and to the presence of a progressive higher coarse deposit facies with the depth. the variation of the sedimentary thickness inside each alluvial cycle could indicate that during the initial phase of the graben opening (alluvial cycle 3) the eastern border of the valley was deepening more quickly than the western one. instead, during the youngest phase of the graben evolution there was an important inversion of the phenomena and the valley seems now deepening more quickly in the western ward. this hypothesis is supported by the seismic reflection twt image of the valley (fig. 10) and by the robust correlation with the hr resistivity model (fig. 11). the high ray coverage and the redundant high data density are in favour of the below lithological interpretation (fig. 12) and exclude to have an apparent multi-layering «artifact because of poor data resolution, or inadequate modeling». in fig. 13, a possible structural interpretation of the seismic tomography image of the valley is represented. the structural setting of the val d’agri graben is quite characteristic. the faulting system of the valley is now imaged while it is still inadequately imaged on the twt conventional seismic (fig. 10). the «graben» is now relatively symmetric and clearly controlled by ssw and nne dipping high angle faults. the actually active faulting is probably located along the western margin of the valley, where this potential active tectonic tilting could also explain the lateral migration of the agri river to west. in the central part of the valley the faulting system can be designed following the distribution of the velocity changes. this faulting system was partially active until now and just sealed by the most recent thin alluvial sediments. from the structural point of view, the val d’agri graben could be interpreted as a «negative flower thrust» due to the transpressive and distensive late apennine tectonic phase, having its depocenter at around 1000 m of depth. in fig.14, this is quite clear when the entire «near surface seismic tomography profile» is displayed and interpreted. here, it is possible to have a more regional 2d view of the «val d’agri graben« and insert it in the wider geological context of the «apennine thrust setting». the graben is young and limited in depth lateral extension. along the entire seismic near surface tomography section it is also possible to note that some of the normal faults of the graben (in particular along its western margin) could interest the deep allochtonous «lagonegro units» too. it is clear that the hypothetical extension of the main thrusts below the resolution limit of the tomography image is purely speculative. 6. conclusions conventional seismic data contain information that can be lost, if they are not processed and displayed properly. this is the case of the val d’agri, where the quality of same seismic data set can be considered average, if processed and displayed by conventionally time wiggle – variable area images; or very detailed and high resolution, if processed differently. this is the case of the conventional seismic first break arrival tomography. near surface tomography of conventional seismic first break arrivals can be used as an innovative interpretation tool, where rough terrain, seismic noise and structural complexity negatively influence the time seismic image and the seismic visibility. it is certainly a very efficient high resolution illumination of the shallower part of the seismic information and it is a very robust representation of the near subsurface, directly in depth. shallow noise, scattering, absorption; static corrections and shallow imaging pitfalls can be very well solved by near surface tomography data processing. the val d’agri subsurface was never illuminated by conventional seismic images in the past. as a consequence, seismic tomography has incidentally given a strong contribution to the knowledge of the structural setting and lithology in «val d’agri graben». a very detailed reconstruction of the val d’agri graben could be made by the near surface tomography of the most part of the large seismic data set present in the area. however, a good near 271 integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy) surface tomography resolution can be obtained where the acquisition offsets are sufficiently long. offsets > 2500-3000 m are absolutely required to allow an efficient near surface tomography processing of seismic first arrivals to about 1000 m of depth. unfortunately this is not the case for a lot of vintage seismic data acquired in the past in the area. hr resistivity information mainly contributed to the lithological identification of the shallower velocity changes in the near surface tomography seismic section. however, the integrated interpretation of geoelectrical and seismic tomography images was crucial to allow the reconstruction of thickness, lithology and alluvial cycles distribution of the most recent sediments of the valley. acknowledgements – eni s.p.a. – agip division, that operated the seismic acquisition and the hr resistivity survey. – enterprise oil italiana s.p.a. – european union – «thermie fund». references cervený, v., i. molotkov and i. psenciik (1977): ray method in seismology, university of karlova, prague, czechoslovakia. dell’aversana, p., s. morandi, l. improta and a. zollo (1999): tomographic images from «global offset» seismic data inversion in southern apennines, ext. abstract 61th eage conference, helsinki, 6-34. dell’aversana, p., e. ceragioli, s. morandi and a. zollo (2000): a simultaneous acquisition test of high density «global offset» seismic in complex geological settings, first break, 18, 87-96. hampson, d. and b. russell (1984): first break interpretation using generalised linear inversion, j. can. soc. explor. geophys., 20 (1), 40-54. herrero, a., l. improta, p. dell’aversana and s. morandi (2000): 2d non linear travel time tomography by multi scale search: imaging an overthrust structure in southern apennines (italy) s7id-08 fall meeting 15-19, 2000 s. francisco, california, supplement to eos, trans., am. geophys. union, 81 (48). hippolyte, j.c., j. angelier and f. roure (1994a): a major geodynamic change revealed by quaternary stress patterns in the southern apennines (italy), tectonophysics, 230, 199-210. hippolyte, j.c., j. angelier, f. roure and p. casero (1994b): piggyback basin development and thrust belt evolution: structural and paleostress analysis in the southern apennines, j. struct. geol., 16 (2), 159-173. zhu, x., d.p. sixta and b.g. angstman (1992): tomostatics: turning ray tomography plus static corrections, the leading edge, 11, 12, 15-23. zhu, x., g.a. burke and d.p. sixta (1998): overthrust imaging with tomo-datuming: a case study, geophysics, 63 (1), 25-36. ˘ ˘ natural radioactivity content in groundwater of mt. etna’s eastern flank and gamma background of surrounding rocks annals of geophysics, 59, 1, 2016, s0103; doi:10.4401/ag-6799 s0103 natural radioactivity content in groundwater of mt. etna’s eastern flank and gamma background of surrounding rocks beata kozłowska1,*, agata walencik-łata1, giuseppina immè2,3, roberto catalano2,3, gabriella mangano2, daniela morelli2,3, salvatore giammanco4 1 university of silesia, institute of physics, department of nuclear physics and its applications, katowice, poland 2 università di catania, dipartimento di fisica e astronomia, catania, italy 3 istituto nazionale di fisica nucleare, sezione di catania, catania, italy 4 istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy abstract waters of mt. etna are the main source of drinking water for the local population and are also distributed in municipal supply systems to neighbouring areas. radioactivity in underground waters and surrounding rocks from the eastern flank of mt. etna was investigated on the basis of 9 water and 8 rock samples from 12 localities altogether. three samples were from water drainage galleries and six from water wells. all water intakes are used for consumption. activity concentration of uranium isotopes 234,238u, radium isotopes 226,228ra and radon 222rn were determined with the use different nuclear spectrometry techniques. the determination of uranium isotopes was carried out with the use of alpha spectrometry. the measurements of radium and radon activity concentration in water were performed with the use of a liquid scintillation technique. additionally, rocks surrounding the intakes were examined with gamma spectrometry. all water samples showed uranium concentration above minimum detectable activity (mda), with the highest total uranium (234u + 238u) activity concentration equal to 149.2±6 mbq/l. conversely, all samples showed radium isotopes activity concentrations below mda. radon activity concentration was within the range from 2.91±0.36 to 21.21±1.10 bq/l, hence these waters can be classified as low-radon waters. gamma natural background of the rocks surrounding the water sampling sites was found on the same levels as other volcanic rocks of italy. 1. introduction there is a growing concern about the health effects produced by natural radioactivity in the environment. natural radionuclides are present in water, air, soil, plants and animals and in consequence in the human diet. in groundwater, their presence is determined by their activity concentration in soil and bedrock. groundwater reacts with the surrounding rocks and releases elements which can be dissolved in it. long-lived isotopes of uranium, radium and potassium are mainly responsible for natural radioactivity content. the most extensively studied natural radionuclide in all types of natural environments is radon since this element was found to be directly or indirectly responsible of about 2% of all death from cancer in europe [darby et al. 2005]. in particular, indoor radon levels derived from groundwater use in the usa are thought to cause 1-7% of fatal lung cancers [cothern et al. 1986]. concerning an active volcanic environment such as mt. etna still few detailed studies exist on the levels of radionuclides other than radon in groundwater. waters of mt. etna are the main source of drinking water for the local population and are also distributed in municipal supply systems to neighbouring areas. moreover, they are used for both agricultural and industrial purposes. a previous study on the determination of the activity concentration of 234,238u, 226,228ra and 222rn in etna’s groundwater intakes was recently carried out [kozłowska et al. 2009] showing values of uranium isotopes generally higher than the minimum detectable activity (mda), radium isotopes generally below mda and low radon activity concentrations. the latter were in the same range as those found by d’alessandro and vita [2003], who performed an extensive survey in mt. etna’s groundwater for the specific determination of dissolved radon contents. in this work we present geochemical data on the activity concentration of uranium isotopes 234,238u, radium isotopes 226,228ra and 222rn in collected unarticle history received may 22, 2015; accepted january 12, 2016. subject classification: groundwater processes, radioactivity and isotopes, chemistry of waters, gases, hydrogeological risk. derground waters from the aquifers of the eastern flank of mt. etna volcano. this area was selected as a good case study because it is characterised by large volumes of groundwater, strong interaction between groundwater, volcanic rocks, volcanic gases and some hydrothermal fluids [aiuppa et al. 2000, brusca et al. 2001, parello et al. 2001, aiuppa et al. 2004]; it is also the area of mt. etna with the highest values of dissolved radon [d’alessandro and vita 2003] and the only area where radium isotopes were detected [kozłowska et al. 2009]. all these facts stimulated the authors to perform detailed studies on natural radioactivity content in groundwater from this area. moreover, gamma spectrometry measurements were performed on five samples from volcanic rocks surrounding water intakes and three samples from volcanic rocks taken close to summit craters, in order to investigate the natural background radioactivity of the area. 1.1. sampling nine groundwater samples from aquifers of the eastern flank of mt. etna were collected for the purposes of this study. we chose sampling sites whose waters are continuously used for drinking purposes by the local population and whose sampling point was not too far from the water well/drainage gallery. there were other waters available for collection but they were either not used for drinking or their closest collection point was too far from the well/drainage gallery, so there was a high chance of losing radon by radioactive decay before collection. sampling was performed three times, in 2011, 2012 and 2013 year. figure 1 presents the investigated area and the sampling sites. three samples were taken from water drainage galleries and six from water wells. in the vicinity of five of these sites rocks samples were collected for the analysis of radioactive isotopes. besides, three more rock samples were collected from the oldkozłowska et al. 2 figure 1. map of study area and sampling sites. green squares represent water drainage galleries; blue open circles represent water wells; yellow open diamonds indicate the location of the three lava rock samples taken near the summit area of the volcano. the three main hydrogeological basins of the volcano (i = eastern basin; ii = western basin; iii = northern basin, according to ferrara [1990]), delimited by thick blue lines, are also shown together with the three main rift zones (ne rift, w rift and s rift, respectively). vb = valle del bove morphological depression. volcano-tectonic data of mt. etna modified from acocella and neri [2003]. 3 est lavas outcropping in the uppermost slopes of the volcano, thus representing the rock types present in the recharge areas of the aquifers that contain the groundwater collected downslope. table 1 presents a description of the intakes and the physical-chemical properties of the investigated waters, i.e. total dissolved solids (tds), ph, electrical conductivity and temperature. the table also shows the values of partial pressure of dissolved radioactivity in waters and rocks of mt. etna sample code sample name sample description sample type rock surrounding water intake ph dapth [m] t (°c) conductibility [ms/cm] tds [mg/l] pco2 [atm] e_1 fleri water from drainage gallery, a village center, public water intake, drinking water gallery not taken 6.51 na 17.5 0.75 360 0.124* e_2 valle san giacomo water from drainage gallery, drinking and irrigation water, local distribution gallery sample from the neighborhood 6.70 na 18.85 1.37 668 0.026** e_3 milo water from drainage gallery, local distribution, drinking water, public intake gallery not taken 7.54 na 17.3 0.81 390 0.002* e_4 primoti water from private well, irrigation water well lava and soil close to the well 6.67 150 18.5 4.52 2340 0.633§ e_5 ilice water mps, local distribution only, drinking water well sample from the neighborhood 6.55 120 18.4 0.74 357 0.124* e_6 macchia di giarre water from well san paolo, drinking water, public intake well not taken 6.86 100 17.8 2.73 1360 0.022** e_7 fornazzo water mps, drinking water, local distribution, private intake well sample from the neighborhood 7.61 >100 17.4 0.81 391 0.010* e_8 linera water used only for irrigation, private intake well not taken 6.39 >100 17.2 2.01 987 0.120** e_9 guardia water for drinking and irrigation, local distribution, public intake well sample from the neighborhood 6.40 >100 19.0 1.54 756 0.156* e_10 la montagnola crater (1763) rock sample e_11 piano del lago rock sample e_12 1792 lava flow rock sample table 1. physical-chemical properties of investigated waters and samples description. na = not available; mps = municipal power supply; * pco2 values from parello et al. [2001]; ** pco2 values from giammanco et al. [1998]; § average pco2 value for this site, from giammanco et al. [2007a]. co2 (pco2) measured in the corresponding sites during previous hydrogeochemical surveys [giammanco et al. 1998, parello et al. 2001, giammanco et al. 2007a]. for radium and uranium determination the samples were collected in 2 and 0.5 volume polyethylene bottles, respectively. samples were acidified to prevent the precipitation of iron as ferric hydroxide in the collecting containers. the initial chemical treatment for radium isotopes was performed at the department of physics and astronomy of the university of catania, italy. further treatment of the samples for radium and uranium determination was performed at the department of nuclear physics and its applications, institute of physics, university of silesia in katowice, poland. for radon determination, samples of the volume of 10 ml were collected by disposable syringes directly at the intakes and immediately put into scintillation vials under 10 ml of scintillation cocktail [suomela 1993a]. the measurements were performed at the department of physics and astronomy of the university of catania. 1.2. hydrogeology and hydrochemistry of the area the investigated area is the eastern flank of mt. etna volcano (figure 1). the volcanic edifice of mt. etna has grown up during the last 500,000 years on the eastern coast of sicily (southern italy) with the alternate superimposition of lava flows and pyroclastic deposits [romano 1982, chester et al. 1985]. during the last 5000 years etna has been almost continuously erupting and has emitted volcanic products essentially made of lavas, ranging in composition from alkali-basalt to hawaiite [chester et al. 1985]. the volcano built up its edifice over a sedimentary substratum with an estimated thickness greater than 15 km, according to cristofolini et al. [1979]. this basement is mostly composed of clays of messinian age to the east and south and of clay, marl and quartz-arenite units, aging from triassic to upper pliocene, to the west and north. in general, etna presents a hydrogeological situation common to many other basaltic volcanoes: fissured and highly permeable lava layers are interbedded with discontinuous layers of scarcely permeable pyroclastics. these volcanic products are superimposed on an impermeable sedimentary substratum, with permeability coefficients ranging from 10-7 to 10-13 cm/s [schilirò 1988]. the base of the main aquifers of etna thus lies at the contact between the volcanic rocks and these sediments [ogniben 1966, ferrara and pappalardo 2008, branca and ferrara 2013]. the volcanic rocks hosting the aquifer under study basically include most of the lithotypes found on mt. etna. they mainly belong to more or less differentiated products of the alkaline (sodic) series [cristofolini and romano 1982, romano 1982, chester et al. 1985, branca and ferrara 2013]. however, the concurrent presence of alkali olivine basalts ranging from “transitional” to sub-alkaline, belonging to the deepest basal lavas, is highly probable [branca and ferrara 2013]. both of these magmatic series have a chemical composition ranging from basic to intermediate. the main mineralogical phases present in such products consist, in order of abundance, of plagioclase (~65%, with a variable composition), pyroxene (~28%, having an average augitic composition), olivine (~3%) and oxides (~3%) such as magnetite and sometimes titanomagnetite and ilmenite. other minerals present to a lesser extent are apatite, alkali feldspars and in some cases nepheline, leucite and biotite [cristofolini and romano 1982, romano 1982, chester et al. 1985]. according to the model first proposed by anzà et al. [1989], etna’s groundwaters are of meteoric origin, based on their deuterium/hydrogen and 18oxigen/16oxigen isotopic ratios. when rain waters enter the aquifer, they become enriched in co2 which, according to the d 13c values already available in literature, is mostly of magmatic origin [allard et al. 1997, aiuppa et al. 2004]. this process is ubiquitous on mt. etna, although it is enhanced in the most fractured and seismically active zones of the volcano (i.e. the sw and the e flanks) and it seems to mask any evolution of water chemistry from recharge to discharge areas [aiuppa et al. 2004, and literature therein cited]. on the basis of structural, geological and geophysical data, three main hydrogeological basins were identified [ogniben 1966, ferrara 1990] (figure 1). they roughly correspond to three sectors of the volcano: i) the eastern one, tributary of the ionian sea; ii) the southwestern one, tributary of the simeto river; iii) the northern one, whose waters feed the alcantara river. the general chemical composition of etna’s groundwater ranges from bicarbonate alkaline-earth to bicarbonate alkaline [anzà et al. 1989, dongarrà et al. 1993, giammanco et al. 1996, aiuppa et al. 2004]. as regards the major species in solution, the relative abundance is generally (na+, mg++) > ca++> k+ among the cations, with magnesium prevailing on sodium in the waters with higher salinity, whereas among the anions bicarbonate (hco3 −) is always the prevailing dissolved species. only very few samples show a chloride-sulphate alkaline-earth composition, normally due to the high concentrations of sulphate ions. values of total dissolved solids (tds) are normally well correlated with the corresponding values of calculated partial pressure of co2 (pco2) in equilibrium with water. this indicates a strong chemical interaction among meteoric water, the host rocks, from kozłowska et al. 4 5 which the ions in solution were extracted by leaching, and a gaseous phase mainly composed of co2. this gas is very abundant both in the ground waters and in the soils of etna, besides its massive emission through the volcanic plume [allard et al. 1991, giammanco et al. 1995, d’alessandro et al. 1997], and its origin is mostly magmatic [anzà et al. 1989, giammanco et al. 1995]. dongarrà et al. [1993] also showed that the concentration of the major species in solution and particularly the calculated values of partial pressure of co2 with which waters equilibrated can be subject to marked variations in time. such variations were found to be caused both by seasonal effects and, more significantly, by changes in the volcanic activity of etna. some minor chemical species are brought into water directly from the rocks or from the volcanic gases that carry them as volatile species. this is the case of radon, that is present both in the volcanic rocks of mt. etna and in the volcanic gases that leak through the flanks of the volcano along tectonic faults [e.g., neri et al. 2011]. the surface of the area investigated by our samplings was about 120 km2. the geological features of this area mainly consist of etna’s volcanic products. the volcanic rocks that outcrop in the study area are essentially represented by lava flows belonging to the so-called stratovolcano supersynthem, whose activity started from 57 ka [branca et al. 2011], although some minor outcrops belong to the timpe supersynthem, that started some 220 ka [branca et al. 2011]. all of the samples of volcanic rocks collected belong to the former. 2. measurement techniques 2.1. water samples analysis since this project is a continuation of the research started in year 2009 the measurement techniques used for water samples studies were already described in the previous publication [kozłowska et al. 2009]. there were two nuclear spectrometry techniques involved: liquid scintillation counting and alpha spectrometry. determination of radon activity concentration was performed at the department of physics and astronomy of university of catania with the use of beckman ls6500 liquid scintillation counter. the radium content in water samples was measured with the use of the winspectral 1414 a/b liquid scintillation counter from wallac at the institute of physics, university of silesia, katowice, poland. the measurements of uranium 234,238u isotopes were carried out with the use of alpha spectrometers 7401vr from camberra–packard also at the institute of physics in the university of silesia. radon activity concentration was determined using a calibrated window for alpha counting. the minimum detectable activity (mda) was calculated according to currie’s publication [currie 1968] and was equal to 1 bq/l at 1800 s counting time. radium was co-precipitated with barium into ba(ra)so4 according to radiochemical procedure based on the polish norm [1989]. the precipitate with radium was transferred into a scintillation vial, mixed with water and gellation scintillator. theoretical calculations were based on bateman’s equations [bateman 1910]. mda was equal to 10 mbq/l for 226ra and 20 mbq/l for 228ra at 3600 s counting time and 3 l sample volume. the separation of uranium from other radionuclides present in water was performed based on the procedure worked out by suomela [1993b]. the a-spectrometry source was prepared by uranium co-precipitation with ndf3 and deposition on polypropylene discs (0.1 µm) according to the procedure worked out by sill [1987]. mda was equal to 0.4 and 0.6 mbq/l for 234,238u isotopes, respectively, for a 2 day counting time. the applied methods were described in detail elsewhere [kozłowska et al. 2009, 2010, 2012]. the methods for isotopes determination in water samples were tested during proficiency tests organized by international authorities [möbius and salonen 2002, iaea 2010]. 2.2. volcanic rocks samples analysis the studies of volcanic rocks samples were carried out both in the department of physics and astronomy of university of catania in italy and in the department of nuclear physics and its applications of the institute of physics at the university of silesia in poland. apart from scientific purposes these studies served as intercomparison measurement in the two laboratories. collected samples were transferred to the laboratory in the department of physics and astronomy of university of catania where they were characterized, dried, crushed and ground to a fraction smaller than 0.2 mm for homogenization. the measurements in both laboratories were carried out in marinelli geometry, in 0.6 dm3 containers, where the rock samples were sealed for 30 days in order to allow secular equilibrium in the uranium and thorium series. the natural radioactivity of 226ra, 228ra(228ac) and 40k in collected samples were determined using hpge gamma spectrometers produced by ortec company. the spectra were analyzed with the use of gennie gamma acquisition program and gannas spectra analysis program and maestro-32 software. the energy calibration and the detector efficiency curves were obtained on the basis of standards prepared from certified materials from the international atomic energy agency from vienna, austria and central laboratory for radiological protection from warsaw, poland. radioactivity in waters and rocks of mt. etna kozłowska et al. 6 sa m p le c o d e 22 2 r n [ b q / l ] 22 2 r n [ b q / l ] (f ro m li te ra tu re ) 22 6 r a [m b q / l ] 22 8 r a [m b q / l ] 23 4 u [ m b q / l ] 23 8 u [ m b q / l ] u [ µ g/ l ] 23 4 u / 23 8 u 22 2 r n / 23 8 u e _1 7. 46 ± 0. 2 (2 01 1 y) < 40 ( 20 11 y ) < 12 0 (2 01 1 y) 4. 8± 1. 3 (2 01 1 y) 4. 9± 1. 3 (2 01 1 y) 0. 4± 0. 1 (2 01 1 y) 1. 0± 0. 4 (2 01 1 y) 15 22 ± 40 6 (2 01 1 y) e _2 2. 91 ± 0. 36 ( 20 11 y ) 10 .1 ( 2) < 40 ( 20 11 y ) < 12 0 (2 01 1 y) 2. 0± 0. 6 (2 01 1 y) 1. 5± 0. 5 (2 01 1 y) 0. 12 ± 0. 04 ( 20 11 y ) 1. 4± 0. 7 (2 01 1 y) 19 40 ± 69 0 (2 01 1 y) e _3 n a 4. 8 (2 ) < 40 ( 20 11 y ) < 12 0 (2 01 1 y) 12 .5 ± 1. 8 (2 01 1 y) 7. 4± 1. 3( 20 11 y ) 0. 6± 0. 1 (2 01 1 y) 1. 7± 0. 4 (2 01 1 y) e _4 12 .5 2± 1. 38 ( 20 11 y ) 43 ( 2) < 40 ( 20 11 y ) < 12 0 (2 01 1 y) 61 .4 ± 4. 9 (2 01 1 y) 64 .5 ± 5. 1 (2 01 1 y) 5. 2± 0. 4 (2 01 1 y) 1. 0± 0. 1 (2 01 1 y) 19 4± 26 ( 20 11 y ) e _5 15 .7 9± 0. 65 ( 20 11 y ) 21 .2 1± 1. 1 (2 01 2 y) 12 .0 ± 0. 6; 28 .1 ( 1. 2 ) < 40 ( 20 11 y ) < 40 ( 20 13 y ) < 12 0 (2 01 1 y) < 12 0 (2 01 3 y) 4. 1± 0. 8 (2 01 1 y) 4. 0± 0. 7 (2 01 2 y) 2. 8± 0. 3 [1 ] 2. 8± 0. 6 (2 01 1 y) 2. 7± 0. 5 (2 01 2 y) 3. 0± 0. 3 [1 ] 0. 23 ± 0. 05 ( 20 11 y ) 0. 22 ± 0. 04 ( 20 12 y ) 0. 25 ± 0. 02 [1 ] 1. 4± 0. 4 (2 01 1 y) 1. 5± 0. 4 (2 01 2 y) 0. 9± 0. 1 [1 ] 56 39 ± 12 31 ( 20 11 y ) 78 56 ± 15 11 ( 20 12 ) e _6 3. 49 ± 0. 76 ( 20 11 y ) 8. 03 ± 0. 6 (2 01 2 y) 14 .8 ( 2) < 40 ( 20 11 y ) < 10 ( 20 12 y ) < 12 0 (2 01 1 y) < 30 ( 20 12 y ) 78 .6 ± 5. 7 (2 01 1 y) 73 .9 ± 5. 9 (2 01 3 y) 70 .1 ± 5. 1 (2 01 1 y) 71 .0 ± 5. 7 (2 01 3 y) 5. 7± 0. 4 (2 01 1 y) 5. 7± 0. 5 (2 01 2 y) 1. 1± 0. 1 (2 01 1 y) 1. 0± 0. 1 (2 01 2 y) 50 ± 11 ( 20 11 y ) 11 3± 12 ( 20 12 y ) e _7 < 1 .4 ( 20 11 y ) 2. 92 ± 0. 41 ( 20 12 y ) 1. 6± 0. 5 (1 ) < 40 ( 20 11 y ) < 10 ( 20 12 y ) < 12 0 (2 01 1 y) < 30 ( 20 12 y ) 8. 0± 1. 5 (2 01 1 y) 8. 3± 1. 0 (2 01 2 y) 6. 4± 0. 4 [1 ] 6. 9± 1. 3 (2 01 1 y) 7. 0± 0. 9 (2 01 2 y) 4. 7± 0. 4 [1 ] 0. 6± 0. 1 (2 01 1 y) 0. 57 ± 0. 07 ( 20 12 y ) 0. 38 ± 0. 03 [1 ] 1. 2± 0. 3 (2 01 1 y) 1. 2± 0. 2 (2 01 2 y) 1. 4± 0. 1 [1 ] 41 7± 79 ( 20 12 y ) e _8 n a < 40 ( 20 11 y ) < 12 0 (2 01 1 y) 33 .8 ± 5. 3( 20 11 y ) 13 .3 ± 1. 2 [1 ] 36 .8 ± 5. 6 (2 01 1 y) 12 .2 ± 1. 1 [1 ] 3. 0± 0. 5 (2 01 1 y) 0. 98 ± 0. 09 [1 ] 0. 9± 0. 2 (2 01 1 y) 1. 1± 0. 1 [1 ] e _9 17 .9 5± 1. 35 ( 20 12 y ) 37 .9 ( 2) < 40 ( 20 11 y ) < 40 ( 20 12 y ) < 12 0 (2 01 1 y) < 12 0 (2 01 2 y) 10 .0 ± 0. 8 (2 01 1 y) 10 .2 ± 1. 3 (2 01 2 y) 9. 8± 0. 8 (2 01 1 y) 8. 8± 1. 2 (2 01 2 y) 0. 79 ± 0. 06 ( 20 11 y ) 0. 71 ± 0. 09 ( 20 12 y ) 1. 0± 0. 1 (2 01 1 y) 1. 2± 0. 2 (2 01 2 y) 20 40 ± 31 8 (2 01 2 y) t ab le 2 .a ct iv it y co nc en tr at io ns o f 22 2 r n [b q/ l ] ( bo th m ea su re d in t hi s w or k an d ta ke n fr om li te ra tu re fo r co m pa ri so n) , 2 26 ,2 28 r a an d 23 8, 23 4 u [m b q/ l ] t ot al u ra ni u m c on te nt [µ g/ l ], 2 34 u /2 38 u a nd 2 22 r n/ 23 8 u is ot op ic r at io s in t he in ve st ig at ed w at er s am pl es fr om m t. e tn a. 7 the activity of 226ra was calculated as the weighted mean of the values obtained directly from the 186.2 kev line and the activities of 214pb (241.9, 295.2, 351.9 kev) and 214bi (609.3, 1120.3 kev) isotopes. the determination of the presence of 228ra and calculation of its activity was based on the following gamma-ray transitions: 212pb (238.6, 300.1 kev), 228ac (338.3, 911.1, 968.9 kev). 40k was calculated from one 1460 kev line. the detection limit for radium measurements was 1 bq/kg and 10 bq/kg for potassium 40k. 3. results and discussion the results of the analyses of groundwater and volcanic rocks collected in the present survey for the detection of radionuclides are described in the following sections relatively to uranium isotopes, radium and radon and reported in table 2 and corresponding figures. 3.1. uranium the activity concentrations for uranium isotopes varied from 1.5±0.5 mbq/l to 71.0±5.7 mbq/l for 238u and from 2.0±0.6 to 78.6±5.7 mbq/l for 234u. a strong positive correlation (pearson’s correlation coefficient r = +0.99) was observed between the concentration of 238u and its daughter 234u. moreover, a highly significant positive correlation (r = +0.85) between the total uranium content in water and tds values was also noted. the highest total uranium (234u + 238u) activity concentration was equal to 149.2±6 mbq/l, that is slightly higher than usual european values [kozłowska et al. 2008, walencik et al. 2012 and references herein]. moreover, comparing the present results with uranium data published for 17 brands of drinking waters collected in the whole italy one can state that they are comparable to the values obtained by jia et al. [2009]. our results confirm those of previous geochemical surveys of mt. etna groundwaters, underlining the important role both of uranium enrichment in water through rock leaching and of the redox state of water in controlling the dissolution of uranium [aiuppa et al. 2000]. this element is actually characterized by high mobility in the aqueous system under oxidized conditions, because it forms soluble oxo-hydroxo complexes in its higher oxidation state (vi). among the most common complexes in equilibrium with mt. etna waters are the uranyl-carbonates [aiuppa et al. 2000]. this process would explain the highest uranium values found in this study. however, under more reduced conditions of waters, uranium can be scavenged by iron minerals and ferric oxyhydroxide, thus resulting to be depleted in those waters. this second process would explain the lowest uranium values found in our samples. interesting results were obtained for the 234u/238u activity ratios in the studied waters (table 2, column 9), as they can be interpreted in terms of possible interference between groundwater and local precipitations (rain/melted snow) that easily percolate through highly permeable lava layers along major faults in the eastern flank of the volcano. the values varied in the range from 0.9±0.2 to 1.7±0.4, with values of arithmetic mean and median equal to 1.2 and 1.1, respectively. the results suggest achievement of secular equilibrium state for uranium isotopes (both arithmetic mean and median values are close to 1), rather than preferential leaching of 234u (vi+) atoms from the mineral lattice of the host rocks, apparent for the disequilibrium state. our results confirm a previous study performed for the mt. etna aquifers, where the highest value of 234u/238u was equal to 1.4±0.1 [kozłowska et al. 2009]. as regards waters studied in the rest of italy [jia et al. 2009], the authors observed a slight disequilibrium between 234u and 238u activity concentration in most of the water samples, with mean isotopic activity ratios of 1.35±0.33. in the literature, groundwater flowing through rocks characterized by heterogeneous structure (e.g. presence of several deep faults and fractures) and where different weather conditions may also play a role, show 234u/238u values up to 30 [osmond et al. 1983]. plotting the 234u/238u ratio versus the inverse of total uranium content 1/u (expressed as l/µg) it is possible to infer processes that occur in waters in time. groundwater characterized by constant 234u/238u values but variable contents of reversed uranium (1/u) in time is likely affected by mixing with other water devoid of uranium, such as, for example, fresh rainwater. figure 2 presents the relation between 234u/238u and 1/u for some of our studied waters (i.e., those for which analyses were repeated twice or three times). the plot indicates only minor mixing between the selected groundwater samples and shallow infiltration waters, probably rainwater or surface runoff produced from melting of snow during early spring. moreover, the variation of 1/u with time seems to be particularly evident only for the waters sampled at higher altitude (i.e., samples e_5 and e_7), thus confirming such hypothesis. besides activity concentrations, table 2 presents also the results of uranium mass expressed in [µg/l], since the consumption limits are given in these units. in 2011, the world health organization (who) proposed a limit for 238u in groundwater equal to 30 µg/l (0.19 bq/l), based on the chemical toxicity of uranium as recommended by epa [2000]. our investigated waters are used for drinking purposes, so the calculation of the effective doses due to uranium absorption seems to be reasonable. assuming a consumption of 2 l of water per day per person, based on the highest uranium activity conradioactivity in waters and rocks of mt. etna centration measured in our samples one can calculate a yearly effective dose of 5.1 µsv, which is much below the proposed limit of 100 µsv/year [who 2011]. 3.2. radium and gamma background all water samples analysed in this study showed activity concentrations of radium isotopes 226,228ra below mda. this is in apparent contrast with the general rule that high tds values, such as those found in the investigated groundwater, correspond to high radium activity concentrations [martín sánchez et al. 1999, dávila rangel et al. 2002, kralik et al. 2003, godoy and godoy 2006, kozłowska et al. 2008]. on the other hand, activity concentrations of radium isotopes in other italian waters [jia et al. 2009] were also mostly below 10 mbq for both isotopes. our previous hypothesis [kozłowska et al. 2009] was that these results may point to a low radium content in the host rocks of etna’s aquifers. we performed gamma spectrometry measurements on rock samples collected in the vicinity of the water intakes sampled in this study. activity concentrations of 226ra varied from 45.5±2.2 bq/kg to 85.2±4.4 bq/kg, with average value of 61.8 bq/kg. this value is almost identical to the median (61.4 bq/kg), which confirms the normal distribution of the data. the activity concentration of 228ra(228ac) varied from 28.8±0.9 bq/kg to 67.7±2.6 bq/kg, with average 53.3 bq/kg and median 47.2 bq/kg. the content of 40k in the studied samples varied from 331±15 bq/kg to 725±35 bq/kg. our measured values, together with average values from other italian magmatic rocks taken from literature for comparison, are presented in figure 3. our study showed that the activity concentrations of 226ra and 228ra (228ac) found in the rocks of mt. etna are on the same levels as the average european values found in bedrock. iarc [1988] comprehensive studies concluded that 226ra concentrations in soils span several orders of magnitude, but they are generally comprised between 10 and 50 bq/kg, with an estimated average concentration of 25 bq/kg. similar values, from 10 to 100 bq/kg, can be found in other publications [cothern and smith 1987, nevissi and bodansky 1987, polański 1988]. an average of 25.3 bq/kg for 226ra and of 24.4 bq/kg for 228ra can be found in soils of poland [radiological atlas of poland 2012]. by comparison, the agency for toxic substances and disease registry reported on its web site concentrations of 226ra in soils of northern italy in the range 3-140 bq/kg, excluding regions with extremely high levels of natural radioactivity. recently, plant et al. [2003] stated that recent volcanic rocks such as those on mt. etna slopes, may be rich in uranium, which should result also in high radium contents. chiozzi et al. [2001] published values of uranium levels in lipari and vulcano islands and chiozzi et al. [2003] did the same for stromboli, salina, filicuidi and panarea islands. these data were based on gamma spectrometric measurements of 214bi line (1.76 mev) or 208tl line (2.62 mev) so these results are representative of 226ra or 228ra radioactive decay, rather than that of uranium or thorium, for which radioactive equilibrium had to be assumed. converting the results of chiozzi et al. [2001, 2003] from ppm to bq/kg, we obtain 226ra values from 32 to 240 bq/kg for lipari and vulcano and from 14 to 117 bq/kg in the other volcanic islands of the aeolian arc. following the same procedure, we obtain 228ra values in the range 37-268 bq/kg kozłowska et al. 8 figure 2. graphical correlation between 234u/238u activity ratios and the inverse of uranium concentration values (1/u) in investigated waters from the eastern flank of mt. etna in three different years of sampling (data for 2007 survey are taken from kozłowska et al. [2009]; data for 2011 and 2012 surveys are from this work). 9 for lipari and vulcano and 15-113 bq/kg for the other aeolian volcanoes. brai et al. [2002], using gamma spectrometry, investigated rock samples from stromboli island obtaining similar results as those of chiozzi et al. [2003] (226ra values in the range 31-112 bq/kg and 228ra (from 228ac line) values from 30 to 106 bq/kg). it seems that the activity concentrations of 226,228ra obtained for mt. etna rock samples in our study are very similar to the average results obtained for stromboli, but slightly lower than those of lipari and vulcano. volcanic products from the latter islands are richer in a wide variety of chemical elements, mostly silica, and hence they may be richer also in radioactive elements from the uranium chain, thus justifying the observed differences with mt. etna. this hypothesis is also supported by the results of d’alessandro and vita [2003], who found a significant positive correlation between median radon content in volcanic groundwater from several active volcanic areas of southern italy (mt. etna, vulcano, pantelleria, ischia) and the median total uranium content in the respective local reservoir rocks, with uranium being in turn more enriched in silica-rich volcanic rocks. as regards the apparent similarities between mt. etna and stromboli, despite the two volcanoes are quite different in terms both of chemical composition of eruptive products and of styles of activity, these could be explained by a similar behaviour of ra enrichment in the erupted fraction of the magmas from the two volcanoes by processes of magma formation, differentiation and eruption, as modelled by capaldi et al. [1976], thus indicating similar pre-eruption histories for the magmas at both volcanoes. in any case, the low radium activity in the groundwater of mt. etna would be explained mostly by the low affinity of this element for the aqueous environment, as radium tends instead to remain in the host rocks of the aquifers [walencik et al. 2010]. furthermore, as for uranium, the behaviour of radium in the aqueous environment is strongly controlled by redox conditions. actually, under the typical eh and ph conditions of groundwater of mt. etna, radium tends to form soluble ra2+ ions. however, in the presence of significant amounts of sulphate and/or carbonate ions in solution, insoluble complexes such as raso4 or raco3 are likely to form, thus removing radium from water. this process could be very efficient in the sampled waters, thus contributing to lower the contents of this element in solution. 3.3. radon the activity concentration of 222rn varied from 2.91±0.36 bq/l to 21.21±1.1 bq/l. obtained radon values are comparable to those of previous ground water surveys at mt. etna [d’alessandro and vita 2003, kozłowska et al. 2009]. the variations in radon activity radioactivity in waters and rocks of mt. etna figure 3. activity concentrations of 226ra (open circles) and 228ra(228ac) (filled triangles) in [bq/kg] in the rocks/soil samples surrounding the water intakes sampled in this work. activity values were obtained from c-spectrometry analyses. for comparison, the range of the results for stromboli is presented [chiozzi et al. 2003], together with average values from other italian magmatic rocks [anpa 2000, arpa piemonte 2006]. observed within the presently analyzed waters cannot be simply explained by the different content of parent radionuclides in the etna volcanic rocks, due to the rather homogeneous chemical composition of the latter, but rather they can be a result of different natural factors related to interaction between volcanic gases, water and volcanic rocks. the first is the enhanced water-rock interaction in the eastern flank of etna that causes a strong release of major ions (described by tds values) and hence of radon as a by-product of radium decay. this factor, however, seems to be effective only for some of the sampled waters. figure 4 shows the correlation between values of dissolved radon and values of tds in all of our samples. as evident from the plot, our samples show two distinct behaviors. in fact, samples e_1, e_5 and e_9 apparently show a much poorer correlation between radon activity in water and the corresponding tds values than the rest of our samples. these three water samples come from the same aquifer, so the plot suggests an additional input of radon in the aquifer represented by these samples and therefore a radon source other than a simple dissolution of volcanic rocks. to study this, we plotted radon activity values versus the corresponding pco2 values in our water samples (figure 5). generally, the figure shows a correlation between the two parameters, indicating that radon activity in our sampled waters increases with increasing contents of dissolved co2. this evidence, therefore, supports the idea that radon is carried by magmatic co2 into the aquifers of the eastern flank of etna, thus enriching water in both of these chemical species, and rules out the possibility of different radon/co2 ratios between the two group of samples highlighted in the plot of figure 4. therefore, a possible explanation for the higher-than-normal radon content in samples e_1, e_5 and e_9 is to assume that radon in these waters derives, if not solely at least in large part, from direct and strong input of magmatic gas in the aquifer in the proximity of these sampling sites. actually, sites e_1 and e_5 are located very close to the fiandaca fracture zone [azzaro et al. 2012, de guidi et al. 2012] and site e_9 is located very close to the santa venerina fracture zone [azzaro et al. 2012, de guidi et al. 2012], both being very active seismogenic faults. therefore, it is reasonable to infer a large, co2-driven, radon release into local groundwater through active shallow rock fracturing along those major structural lines. a specific discussion must also be made for site e_4, due to its peculiar characteristics. according to the above indications, the high amount of radon previously measured by other authors in the waters from this site (222rn = 43 bq/l) [d’alessandro and vita 2003] would stem both from enhanced dissolution of volcanic rocks and from strong direct input of co2 gas into the aquifer, the latter testified by the very high pco2 values calculated for this water. however, it is worth noting from figure 5 that at site e_4 the radon activity value that we measured is much lower than that expected according to the observed general correlation with pco2 kozłowska et al. 10 figure 4. correlation between average radon activity concentration in solution and tds values in the sampled waters of mt. etna. the bestfit lines highlight two populations of samples related to two different mechanisms of radon input into solution (see text for explanation). values of r2 correlation coefficient are also shown. 11 values. actually, if we remove the point corresponding to this site, the general correlation is greatly improved (r = +0.91), thus suggesting some physical mechanism acting in the water from site e_4 that either leads to a removal of radon from solution or that hampers radon dissolution into the ground water at very high values of dissolved co2. the most reasonable explanation for this change in behavior is that the influx of the carrier gas (co2) in the ground water becomes so high that it overwhelms the shallow source of radon and thus dilutes it, so the gas entering the aquifer and dissolving into it has a much lower radon/co2 ratio. actually, a similar process of radon dilution from high co2 flux was already observed by giammanco et al. [2007b] in soil and fumarole gas emissions from mt. etna and by bonfanti et al. [1995] in soil gas samples from south east sicily. in our case, the “dilution” effect would start at values of pco2 higher than 0.2 atm (figure 5). apart from the fact that the mt. etna waters can be treated as low radon activity waters, the values of the activity ratios 222rn/238u in waters (table 2) varied in a wide range from 50±11 to 7856±1511. the obtained results indicated that 222rn isotope was much better transported than 238u isotope. 4. summary and conclusions this paper presents the first full description of radionuclides (234u, 238u, 226ra, 228ra, 222rn) contents in groundwaters of the eastern flank of mt. etna volcano and of radioactivity background (226ra, 228ra) in surrounding rocks. we studied nine groundwater samples from different local aquifers and eight rock samples from twelve localities altogether. three groundwater samples were taken from water drainage galleries and six from water wells. all water intakes are used for drinkable purposes. rock samples were assumed to represent the host rocks of local aquifers. the present results confirmed those of a previous study performed on mt. etna [kozłowska et al. 2009], taking into account a large number of aquifers, but with a much lower spatial density of sampling point in each aquifer. activity concentrations of uranium isotopes 234,238u, radium isotopes 226,228ra and radon 222rn were determined with the use of different nuclear spectrometry techniques. rocks surrounding the intakes were examined with gamma spectrometry. our new study showed that radionuclides levels in waters localized in the eastern flank of mt. etna are not different than the values obtained for the whole area of the volcano [kozłowska et al. 2009]. radionuclides are weakly leached by groundwater from the surrounding reservoir rocks, especially 226,228ra that were at levels below mda. the average 238u and 234u activity concentrations were 21.3 mbq/l and 22.7 mbq/l, respectively, whereas the average 234u/238u activity ratio was equal to 1.2, thus proving achievement of secular equilibrium for uranium isotopes. furthermore, all waters may be classified as low-radon waters, with maximum activity concentrations of 19.2 ± 0.8 bq/l. gamma spectrometry studies of the rock samples radioactivity in waters and rocks of mt. etna figure 5. correlation between radon activity in solution and pco2 values in the sampled waters of mt. etna. the two lines show the main mechanism of radon enrichment and dilution produced by increasing input of volcanic co2 into the aquifers (see text for explanation). the value of the r2 correlation coefficient for the best-fit line relevant to samples affected by radon enrichment is also shown. collected in the vicinity of water intakes showed 226ra and 228ra mean values equal to 61.8 and 53.3 bq/kg, respectively. these values are slightly lower than those of volcanic rocks from other active volcanoes in this geographic area. references acocella, v., and m. neri (2003). what makes flank eruptions? the 2001 etna eruption and its possible triggering mechanisms, b. volcanol., 65, 517-529; doi:10.1007/s00445-003-0280-3. aiuppa, a., p. allard, w. d’alessandro, a. michel, f. parello, m. treuil and m. valenza (2000). mobility and fluxes of major, minor and trace metals during basalt weathering and groundwater transport at mt. etna volcano (sicily), geochim. cosmochim. acta, 64, 1827-1841. aiuppa, a., p. allard, w. d’alessandro, s. giammanco, f. parello and m. valenza (2004). magmatic gas leakage at mount etna (sicily, italy): relationships with the volcano-tectonic structures, the hydrological pattern and the eruptive activity, in: mt. etna: volcano laboratory, a.g.u. geophysical monograph series 143, 129-145; doi:10.1029/143gm09. allard, p., j. carbonelle, d. dajlevic, j. bronce, p. morel, m. robe, j. maurenads, r. faivre-pierret, d. martin, j. sabroux and p. zettwoog (1991). eruptive and diffuse emissions of co2 from mount etna, nature, 351, 387-391. allard, a., p. jean-baptiste, w. d’alessandro, f. parello, b. parisi and c. flehoc (1997). mantle-derived helium and carbon in groundwaters and gases of mount etna, italy, earth planet. sc. lett., 148, 501-516. anpa (2000). il sistema informativo territoriale per la valutazione del potenziale di esalazione di esalazione di radon dal suolo, anpa-rt-00-009, isbn: 88-4480284-8 (in italian). anzà, s., g. dongarrà, s. giammanco, v. gottini, s. hauser and m. valenza (1989). geochimica dei fluidi dell’etna: le acque sotterranee, miner. petrogr. acta, 32, 231-251 (in italian). arpa piemonte (2006). la radioattività ambientale in piemonte rapporto anno 2005, centro regionale radiazioni ionizzanti e non ionizzanti. azzaro, r., s. branca, k. gwinner and m. coltelli (2012). volcano-tectonic map of etna volcano, ital. j. geosci., 131. bateman, h. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 48, 2, 2005defdef 327 annals of geophysics, vol. 48, n. 2, april 2005 key words dynamic rupture – fault constitutive law – fault friction – stress heterogeneities 1. introduction modeling the dynamic propagation of earthquake ruptures requires the adoption of a constitutive relation, which governs the traction evolution within the cohesive zone (ida, 1972; andrews, 1976a; okubo and dieterich, 1984; cocco and bizzarri, 2002). in particular, the rateand state-dependent friction laws (rs) derived from laboratory experiments (dieterich, 1979a,b; ruina, 1983) have been widely used in numerical simulations of earthquake ruptures, both at the laboratory scale (bizzarri and cocco, 2003 and reference therein) and for real-world faults (guatteri et al., 2001, 2003). the frictional stress in rs laws depends on the slip rate and on the state variable. this class of constitutive laws includes an evolution law for the state variable and involves a friction dependence on time. since rs laws account for fault restrengthening after dynamic failure, they can be used to simulate repeated seismic events modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction elisa tinti, andrea bizzarri and massimo cocco istituto nazionale di geofisica e vulcanologia, roma, italy abstract we investigate the effects of non-uniform distribution of constitutive parameters on the dynamic propagation of an earthquake rupture. we use a 2d finite difference numerical method and we assume that the dynamic rupture propagation is governed by a rateand state-dependent constitutive law. we first discuss the results of several numerical experiments performed with different values of the constitutive parameters a (to account for the direct effect of friction), b (controlling the friction evolution) and l (the characteristic length-scale parameter) to simulate the dynamic rupture propagation on homogeneous faults. spontaneous dynamic ruptures can be simulated on velocity weakening (a < b) fault patches: our results point out the dependence of the traction and slip velocity evolution on the adopted constitutive parameters. we therefore model the dynamic rupture propagation on heterogeneous faults. we use in this study the characterization of different frictional regimes proposed by boatwright and cocco (1996) based on different values of the constitutive parameters a, b and l. our numerical simulations show that the heterogeneities of the l parameter affect the dynamic rupture propagation, control the peak slip velocity and weakly modify the dynamic stress drop and the rupture velocity. moreover, a barrier can be simulated through a large contrast of l parameter. the heterogeneity of a and b parameters affects the dynamic rupture propagation in a more complex way. a velocity strengthening area (a > b) can arrest a dynamic rupture, but can be driven to an instability if suddenly loaded by the dynamic rupture front. our simulations provide a picture of the complex interactions between fault patches having different frictional properties and illustrate how the traction and slip velocity evolutions are modified during the propagation on heterogeneous faults. these results involve interesting implications for slip duration and fracture energy. mailing address: dr. elisa tinti, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: tinti@ingv.it 328 elisa tinti, andrea bizzarri and massimo cocco (tse and rice, 1986; rice, 1993). furthermore, they have been applied to model preseismic and postseismic processes, such as earthquake nucleation (dieterich, 1992; lapusta and rice, 2003) and afterslip (marone et al., 1991). another constitutive relation, the slip-weakening law (sw), is also currently used in dynamic modeling of earthquake ruptures; according to this constitutive law the traction depends only on the slip (ida, 1972; andrews, 1976a,b; ohnaka and yamashita, 1989). although these two constitutive formulations are considered alternative (see ohnaka, 2003), they both provide a physically correct description of the dynamic propagation of an earthquake rupture (see bizzarri et al., 2001 and references therein). recent studies have proposed analytical relations to associate the constitutive parameters of these two formulations (cocco and bizzarri, 2002; bizzarri and cocco, 2003). the main difference between these two constitutive laws is that sw prescribes the traction evolution with slip, whereas rs do not and traction spontaneously evolves with time and/or slip driven by the state variable evolution (bizzarri and cocco, 2003; cocco et al., 2004). it has to be pointed out that rs laws have been derived from laboratory experiments at low slip velocity (< 1 cm/s) and are usually assumed (as in the present study) to be valid also at high slip velocities (∼ 1 m/s), such as those observed during large earthquakes. however, other mechanisms might affect fault friction at high slip velocity such as frictional heating (see fialko, 2004 and references therein), thermal pressurization (see andrews, 2002 and references therein) or mechanical lubrication (brodsky and kanamori, 2001). numerous recent investigations have shown that the rupture history imaged on the fault plane during moderate-to-large magnitude earthquakes is quite complex: slip is non-uniformly distributed on the fault and rupture velocity and slip duration can change dramatically during the dynamic rupture propagation. the heterogeneity of slip and the variations of rupture speed are certainly associated with the complexity of fault geometry, the non-uniform distribution of prestress (day, 1982; peyrat et al., 2001) and the variations of frictional properties on the fault plane (boatwright and cocco, 1996). the short slip duration and the healing of slip can also be associated with stress heterogeneity (beroza and mikumo, 1996; day et al., 1998). conversely, in a homogeneous configuration the healing of slip has been modeled by appropriately modifying the constitutive relation (perrin et al., 1995; zheng and rice, 1998; cocco et al., 2004); this allows a sudden traction re-strengthening. however, it is important to point out that, although the constitutive laws which include self-healing of slip are physically reasonable, they have not been corroborated with laboratory experiments. the goal of this study is to model the dynamic propagation of an earthquake rupture on a heterogeneous fault using rs constitutive laws. this implies that we have to describe and represent the frictional heterogeneity in terms of non-uniform distributions of rs constitutive parameters along our 2d fault model. several studies focused on the investigation of the effects of spatial heterotable i. frictional behaviors proposed by boatwright and cocco (1996). a = aσneff and b = bσneff. regime description a and b seismicity strain release velocity strong seismic b >> a main shocks episodic dynamic slip weakening (vw) (s-vw) and some aftershocks weak seismic b − a > 0 interseismic, foreshocks, creep and intermittent (w-vw) b − a ≤ 0.05 mpa main shocks dynamic slip and aftershocks velocity compliant a − b > 0 some aftershocks creep and forced strengthening (vs) (aseismic) a − b ≤ 0.1 mpa dynamic slip viscous a >> b none stable sliding (aseismic) 329 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction geneities of the constitutive properties (see tse and rice, 1986; rice, 1993; boatwright and cocco, 1996 among many others). in particular, boatwright and cocco (1996; bc96 in the following) discussed the frictional control of crustal faulting by using a rs law and a single degree of freedom spring-slider dynamic system. they proposed that the fault response to the dynamic stress perturbations can differ depending on the variability of the constitutive parameters. they proposed four frictional fields separating the velocity weakening regime (vw) into strong and weak fields and the velocity strengthening regime (vs) into compliant and viscous response (see table i). bc96 associated these four frictional regimes with different values of the constitutive parameters and with observed features of seismicity and strain release as depicted in table i. several authors have suggested that fault frictional parameters can vary with depth (blanpied et al., 1991, 1995; rice, 1993; lapusta et al., 2000 among many others). bc96 proposed that these frictional parameters can also change along the fault strike, suggesting that frictional properties may control crustal faulting. in this study we used our 2d finite difference algorithm to investigate the rupture propagation on a fault modeled through the different regimes proposed by bc96. we study how the interaction between velocity weakening and velocity strengthening portions of a 2d fault can affect the dynamic rupture propagation. 2. methodology we solve the fundamental elastodynamic equation neglecting body forces for a 2d inplane shear crack for which the displacement and the shear traction depend on time and one spatial coordinate. we solve the fully dynamic, spontaneous problem by using a finite difference method where the fault is simulated as a plane of split nodes in the grid (the traction-atsplit-nodes – tsn, numerical technique). the approach is described in andrews (1973) and in andrews (1999) and the details of the numerical implementation are in bizzarri et al. (2001). our procedure allows the adoption of different constitutive laws. in this study, we use the rs law with slowness evolution equation derived by dieterich (1986), which has been discussed table ii. model and constitutive parameters used for the simulations on a homogeneous fault. this set of parameters refers to a laboratory-scale experiment. parameter reference case strong seismic regime weak seismic behavior (figs. 1a-d, 2 and 3) (fig. 4) λ = µ 27 gpa 27 gpa 27 gpa vp 5196 m/s 5196 m/s 5196 m/s vs 3000 m/s 3000 m/s 3000 m/s µ* 0.56 0.56 0.56 a 0.012 0.0085 0.015 b 0.016 0.016 0.016 l 1 × 10−5 m 1 × 10−5 m 0.8 × 10−6 m σ neff 100 mpa 100 mpa 100 mpa vinit 1 × 10−5 m/s 1 × 10−5 m/s 1 × 10−5 m/s ψnucl 1 × 10−4 s 1 × 10−4 s 1 × 10−4 s ψoutside the nucleation ψss(vinit) ψss(vinit) ψss(vinit) nucleation region [−1.5 m, 1.5 m] [−1.5 m, 1.5 m] [−2.0 m, 2.0 m] ∆x1 0.01 m 0.01 m 0.01 m fault region [−10 m, 10 m] [−10 m, 10 m] [−10 m, 10 m] 330 elisa tinti, andrea bizzarri and massimo cocco in detail by many authors (gu et al., 1984; tse and rice, 1986; roy and marone, 1996) . ln lna v v b l t ld d v v ψ ψ ψ 1 1 1 * * * n eff = + + + = x n va ak k; e (2.1) in (2.1) v is the slip velocity, ψ is the state variable (which provides a memory of previous slip episodes), µ* and v* are reference values for the friction coefficient and for the slip velocity, respectively. a, b and l are the constitutive parameters: a represents an instantaneous rate sensitivity, that is the direct frictional response to a change in slip velocity, l is the characteristic length that together with b controls the evolution of state variable toward the steady state. σneff is the effective normal stress; in the literature the parameters a = aσneff and b = bσneff are also often used. the length-scale parameter l is different from the characteristic slip-weakening distance dc (see cocco and bizzarri, 2002). we will show the relation between l and dc in the next section. all the parameters used to perform the simulations of dynamic rupture propagation presented in this study are summarized in tables ii and iii: λ and µ are the lamè constants; vp and vs are pand s-wave velocities, respectively; vinit is the initial value of slip velocity; ∆ x is the spatial grid size. the adopted discretization and medium parameters guarantee sufficiently accurate numerical experiments: the simulations presented and discussed in this paper have negligible grid dispersion (i.e. the accuracy frequency in space is 50 khz). moreover, all the stability and convergence conditions (see bizzarri and cocco, 2003) are satisfied. table iii. model and constitutive parameters used for the simulations on a heterogeneous fault. parameter heterogeneity of l barrier-healing finite slip duration complex rupture (figs. 5a,b and 6) (fig. 7a,b) (fig. 8a,b) propagation (fig. 9a,b) λ = µ 27 gpa 27 gpa 27 gpa 27 gpa vp 5196 m/s 5196 m/s 5196 m/s 5196 m/s vs 3000 m/s 3000 m/s 3000 m/s 3000 m/s µ∗ 0.56 0.56 0.56 0.56 a1 0.012 0.012 0.015 0.014 a2 0.012 0.012 0.015 0.015 a3 0.012 0.012 0.012 0.012 b1 0.016 0.016 0.014 0.016 b2 0.016 0.016 0.014 0.013 b3 0.016 0.016 0.016 0.016 l1 = l2 1.5 ×10−5 m 1 ×10−3 m 1 ×10−5 m 1 ×10−5 m l3 0.9 ×10−5 m 1 ×10−5 m 1 ×10−5 m 1×10−5 m x1 5 m 5 m 7 m 6 m x2 5 m 5 m 7 m 4 m σ neff 100 mpa 100 mpa 100 mpa 100 mpa vinit 1 ×10−5 m/s 1 ×10−5 m/s 1 ×10−5 m/s 1 ×10−5 m/s ψnucl 1 ×10−4 s 1 ×10−4 s 1 ×10−4 s 1 ×10−4 s ψoutside the nucleation ψss(vinit) ψss(vinit) ψss(vinit) ψss(vinit) nucleation region [−1.5 m, 1.5 m] [−1.5 m, 1.5 m] [−1.5 m, 1.5 m] [−1.5 m, 1.5 m] ∆x1 0.01 m 0.01 m 0.01 m 0.01 m fault region [−10 m, 10 m] [−10 m, 10 m] [−50 m, 50 m] [−50 m, 50 m] 331 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction 3. dynamic rupture propagation on a homogeneous fault we present in this section the simulation of the dynamic rupture propagation on a homogeneous fault model with a uniform distribution of constitutive parameters. this fault model can be considered as a reference model for the discussion that will be presented in the following of this paper; this reference model was previously described by bizzarri and cocco (2003). the parameters adopted for this simulation are listed in table ii. the model represents a velocity weakening fault for which the value of b – a is 0.004 (i.e. b – a = 0.4 mpa). the results of this simulation are shown in fig. 1a-d and they represent a good example to discuss the typical behavior of a spontaneous dynamic rupture governed by rs. the nucleation strategy is described in details by bizzarri et al. (2001); the nucleation patch is quite small and the nucleation stage is relatively short. figure 1a shows the spatio-temporal evolution of slip velocity, while the bottom panels fig. 1a-d. a) the spatio-temporal evolution of slip velocity during the dynamic rupture propagation on a homogeneous fault. the initial and constitutive parameters used for these calculations are listed in table ii. the bottom panels show the traction evolution as a function of state variable (b), slip velocity (c) and slip (d) for a target point p1. ψinit in (b) represents the initial value of the state variable; v0 in (c) is the slip velocity when the displacement reaches the critical slip weakening distance dceq (d). a b c d 332 elisa tinti, andrea bizzarri and massimo cocco display the traction evolution for a target point p1 as a function of state variable (fig. 1b), slip velocity (fig. 1c) and slip (fig. 1d). we summarize in the following the major features emerging from this simulation: i) the rupture speed increases during the dynamic propagation and remains for this configuration sub-shear (vcrack ∼ 2200 m/s), although in general it might accelerate to a supershear value, asymptotically approaching pwave speed (bizzarri et al., 2001). ii) peak slip velocity increases during the dynamic propagation with an increasing spatial distance from the nucleation; the largest increase occurs when the crack accelerates to higher rupture velocities (figs. 1a and 2). iii) simulations for a homogeneous configuration show no healing of slip: slip velocity does not drop to zero during the rupture propagation (figs. 1a and 2). all points have roughly the same final slip velocity value (v2), which represents the velocity at the new steady state after the dynamic stress release (fig. 2). iv) the dynamic traction evolution shows a characteristic slip-weakening behavior (fig. 1d for a target point p1; see also cocco and bizzarri, 2002; and bizzarri and cocco, 2003). the weakening phase is denoted in this figure by the labels b and d (the latter identifies the point in which dynamic traction reaches the kinetic stress value, τfeq, see fig. 1d). the slip required for traction to drop is called «equivalent» slip weakening distance dceq. v) the phase diagram shown in fig. 1c points out that the shear stress reaches the peak value (the maximum stress value τueq, i.e. the equivalent yield stress, indicated in panel d and characterizing the point b in fig. 1b-d) earlier than slip velocity (label c), in agreement with tinti et al. (2004). vi) figure 1b points out that during dynamic propagation the state variable evolves from the assumed initial steady-state value (ψinit) to a new one (ψss= l / v). this represents the well-known self-damping behavior of the state variable. bizzarri and cocco (2003) and cocco et al. (2004) have shown that it is the state variable evolution (line bc in panel b) that drives the slip acceleration (same line in panel c) and most of the traction drop during the weakening phase (see panel d). vii) the duration of the weakening phase (line bd in panels c and d) defines the breakdown zone duration, as described by ohnaka and yamashita (1989). viii) simulations performed with a homogeneous configuration of rs friction with a slowness evolution law (eq. (2.1)) yield a constant weakening rate (see fig. 1d), provided that the spatial and temporal discretization is accurately selected to resolve the state variable evolution and the fast slip acceleration. the considerations presented above are characteristic of the rupture propagation on a fig. 2. temporal evolution of dynamic traction and slip velocity for several selected points located at different distances from the nucleation. dashed lines represent the envelopes of τ ueq and τ feq values. the constitutive parameters are the same used for the previous figure and are listed in table ii. 333 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction velocity weakening homogeneous fault. it is important to point out that the peak slip velocity, the yield stress, the dynamic stress drop, the rupture velocity, the slip weakening distance are strongly affected by the values of a and b. moreover, the values of τueq and τ feq, whose analytical expressions as a function of rs constitutive parameters are computed by bizzarri and cocco (2003), do not show relevant variations during the rupture propagation: we observe that, as the crack grows, τueq slightly increases while τ feq remains nearly constant (see fig. 2). 3.1. the cohesive zone the concept of cohesive zone was originally introduced by barenblatt (1959a,b) for a tensile crack and subsequently by ida (1972) for shear cracks in order to remove the physically unrealistic singularity of dynamic stress at the crack tip and to avoid an unbounded energy flux at the rupture front. the cohesive zone is the region of shear stress degradation near the crack tip; it is located just behind the rupture front and it is also named breakdown zone (ohnaka and yamashita, 1989). different physical processes can be responsible of the shear stress degradation within the cohesive zone, and we refer to them as breakdown processes. the most important physical quantity characterizing the breakdown process is the characteristic slip-weakening dc. this parameter represents the amount of slip required to drop the dynamic traction from the upper yield stress (τueq) to the kinetic friction level (τ feq) and to absorb fracture energy (see fig. 1d). dc is an input parameter imposed a priori in the well known sw constitutive law (andrews, 1976a,b). the spatial extension of the cohesive zone scales with dc and both these parameters are important to assess the required resolution of numerical experiments (see bizzarri et al., 2001). as briefly mentioned above, numerical simulations performed by adopting rs constitutive laws display a slip-weakening behavior of dynamic traction (see fig. 1d). bizzarri and cocco (2003) refer to this parameter as the equivalent slip-weakening distance (d0 eq, here indicated as dc eq) and suggest that it is related to the characteristic length scale parameter (l) of rs formulation (eq. (2.1)) by the following scaling relation: lnd l v v b lc n u feq eff eq eq 0 init , , v x x b _ l i (3.1) where v0 is the slip velocity value when the displacement reaches dceq (see fig. 1c) and vinit is the initial sliding velocity. this scaling relation was obtained under the assumption that the rupture starts from a steady state value of the state variable, but can be also generalized for an arbitrary initial condition of the state variable (see eq. (9) in bizzarri and cocco, 2003). we show in fig. 3 the slip-weakening curves for a set of neighboring points resulting from the simulation illustrated in figs. 1a-d and 2 (i.e. the reference model). this figure reveals that, in our simulations, dceq does not depend on the distance from the nucleating patch. we infer a value of dceq larger than the adopted l parameter and the ratio dceq/l ranges between 15 and 20, in agreement with cocco and bizzarri (2002), bizzarri and cocco (2003) and lapusta and rice (2003). fig. 3. slip-weakening curves calculated for the reference model shown in the previous figures and listed in table ii. the different traction evolutions as a function of slip have been plotted for different points located at different distances from the nucleation patch. 334 elisa tinti, andrea bizzarri and massimo cocco we observed the slip-weakening curves resulting from several simulations performed by adopting different values of the l parameter (ranging between 8 µm to 15µm): the inferred dc eq value increases for increasing l in agreement with the scaling relation expressed in eq. (3.1). all these simulations were performed with parameter values representative of the laboratory scale (l ∼ 10 µm and dceq ∼ 0.15 µm). the inferred dceq values for actual earthquakes range between several centimeters to meters (see tinti et al., 2004 and references therein), suggesting that dc may be a significant fraction of the maximum slip on the fault. however, the scaling of the constitutive parameters from laboratory to real fault dimension is still an open question and different opinions exist about the reliability of these extremely large values of dc for real earthquakes (see guatteri and spudich, 2000; guatteri et al., 2003). 3.2. the velocity weakening frictional regime because we are interested in modeling the spontaneous dynamic propagation of an earthquake rupture, we first consider the propagation in a velocity weakening frictional regime. this frictional behavior is characterized by (b–a)>0 and relatively small values of the l parameters. we present in this section the results of several simulations performed on a homogeneous fault. the model parameters used for these simulations are listed in table ii. the convergence and stability conditions of numerical experiments are discussed in detail in bizzarri and cocco (2003). we consider a set of parameters representative of the laboratory scale and we assume a constant normal stress σneff (σneff = σn – pfluid; where pfluid is the pore fluid pressure). the velocity weakening (vw) behavior (a < b) represents the unstable regime that causes dynamic slip episodes on the fault plane, dynamic stress drop and the emission of seismic waves. for instance, we can associate vw patches with intermediate depth fault portions, like those located between 3 to 15 km along the san andreas fault. 3.2.1. strong velocity weakening fault we first model a vw regime characterized by a relatively large difference of constitutive parameters (b – a ≥ 0.6 mpa) and, according to bc96, we refer to it as a strong vw regime. the results of the simulations are shown in fig.4a,b and the input and constitutive parameters are listed in table ii. the strength parameter was introduced by das and aki (1977a,b) to quantify how a fault area is unstable and ready to fail [s = (τu – τo)/(τo – τf)]. for a 2d fault governed by a linear sw law, andrews (1976a,b) showed that s expresses also a limit to discriminate if a crack can (s < 1.77) or cannot (s > 1.77) propagate with super-shear rupture velocity. for rs constitutive laws it is possible to define a seq equivalent to s by using the values of parameters τ ueq, τo and τ feq (bizzarri and cocco, 2003). the simulation shown in fig. 4a,b, performed using the rs constitutive laws for a strong seismic behavior (b – a = 0.75 mpa), is associated with a value of equivalent parameter s eq< 1.77. we can clearly see in fig. 4a that the rupture front bifurcates; this was previously obtained by okubo (1989) and bizzarri et al. (2001). the spatio-temporal plot shown in this figure has been drawn with a resolution smaller than that adopted for numerical calculations to better illustrate the increase of peak slip velocity and the jump in rupture front speed. figure 4b shows the temporal evolutions of slip velocity: black lines indicate slip velocity time histories at those fault positions located before the acceleration of the rupture front to super-shear speed; the colored lines show slip velocity at those points where the crack is bifurcated. the increase of peak slip velocity occurs within the cohesive zone and it is more evident for those points located before the rupture front bifurcation. after the bifurcation (x ≥ 4.5 m), peak slip velocity increases during the propagation of the external front (propagating at super-shear rupture speed), but it is nearly constant for the slow internal front. moreover, these simulations illustrate that when the rupture jumps to a super-shear rupture velocity, the slip velocity drops to zero and thus slip heals before to re-accelerate in the internal rupture front. thus, a strong vw regime allows 335 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction the simulation of earthquake ruptures in agreement with theoretical experiments performed with a slip-weakening law (see bizzarri et al., 2001 and references therein) and with laboratory experiments (rosakis et al., 2000). the strong seismic areas have a large breakdown stress drop (defined as the difference between the yield and the frictional stress ∆τb = =τueq – τ feq) and very large coseismic slip: in a strong vw patch, slip and peak slip velocity are three times larger than those obtained for the reference model (fig. 1a-d) at the same distances from the nucleation. the rupture velocity is close to 2.6 km/s immediately after the nucleation and the external front moves at about 4.5 km/s after the bifurcation. the separation between the two fronts increases going far from the nucleation, because of the different rupture velocities. fig. 4a,b. dynamic rupture propagation on a strong velocity weakening fault. the model and constitutive parameters used for these calculations are listed in table ii. panel a) shows the spatio-temporal evolution of slip velocity. we have drawn this plot with a reduced resolution in order to emphasize the increase in peak slip velocity. panel b) shows the time histories of slip velocity in different positions along the fault: black lines identify points located before the point where crack accelerates to a super-shear rupture velocity, while coloured curves identify those points located in the region where the crack has bifurcated. a b 336 elisa tinti, andrea bizzarri and massimo cocco 3.2.2. weak velocity weakening fault according to bc96 we define a weak fault as a fault characterized by a small difference between a and b (b – a < 0.05 mpa). the results extrapolated by comparing figs. 1a-d and 4a,b are representative of simulations characterized by values of b and a parameters selected to have (b – a) > 0.2 mpa. we experienced a severe difficulty in achieving a spontaneous nucleation and then spontaneous dynamic rupture propagation with 0 mpa < b – a < 0.1 mpa performing many numerical experiments. we point out here that decreasing (b – a) we increase the dimension of the nucleation patch, which is defined as (dieterich, 1992) ( )b a l c n eff, = v hn (3.2) where η is a geometric constant that depends on the crack type. therefore, to avoid increasing the size of the nucleation patch we are confined to using a smaller l. we obtained a spontaneous nucleation using (b – a) = 0.1 mpa and l = 0.8 × 10−6 m (see table ii for the whole set of adopted parameters). as expected, under these conditions the slip velocities within the nucleation patch (where the instability is promoted) are much larger than those inferred during the dynamic propagation within the weak fault portion. our modeling results suggest that: i) peak slip velocity in weak regions can be even more than 10 times smaller than that simulated for the reference model shown in fig. 1a-d; ii) the breakdown stress drop is decreased by 30%, although yield stress and kinetic friction level are both decreased; iii) the equivalent slip weakening distance decreases because both the breakdown stress drop and l decrease; iv) the inferred rupture velocity is smaller than that of the reference model and we never observed a crack bifurcation; v) despite the smaller values of the dynamic parameters we still retrieve an evident slip weakening behavior of the traction versus slip curves. we believe that these results and the inferred behavior of a weak fault portion are physically reasonable because, according to bc96, we expect that weak fault zones undergo dynamic instabilities only during micro-earthquakes or when they are loaded by a dynamic rupture front propagating in an adjacent strong weakening area. we will model this latter behavior in the next section. 4. numerical representation of frictional heterogeneities the goal of this section is to model the dynamic rupture propagation on a 2d heterogeneous fault. because in our approach the spontaneous dynamic propagation is governed by a rs friction law, we represent the source heterogeneities in terms of non-uniform distribution of constitutive parameters. as described above, we follow the findings of bc96, who proposed that both the velocity weakening and the velocity strengthening regimes (see scholz, 1990, 1998) are separated into two fields depending on the values of constitutive parameters and the response to external loading (see table i). as shown in the previous section an earthquake rupture can spontaneously nucleate only within a strong velocity weakening area (s-vw) characterized by sufficiently large values of the parameters (b – a) and l. however, a dynamic rupture can also propagate in a weak velocity weakening area (w-vw). nucleation in these regions occurs only for very small earthquakes or when it is forced by a sudden external stress change. we recall here that in their original classification bc96 used the a and b parameters, which in the present study correspond to a = aσneff and b = bσneff. the velocity strengthening (vs) behavior (defined by the condition a > b) models a stable sliding, i.e. an aseismic slip. by definition, it is impossible to obtain spontaneous rupture propagation for a homogeneous case. the vs behavior was proposed to describe several portions of the san andreas fault characterized by creep events (see scholz, 1990). a vs area can be characterized by the thickness of unconsolidated sediments (like southern california) or by unconsolidated gouge within the fault (like in central california; marone et al., 1991). according to bc96 the velocity strengthening areas can contribute to the arrest of an earthquake rupture. in particular, compliant areas are velocity 337 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction strengthening fault regions that slip aseismically but can be driven to instability if they are sufficiently loaded by an abrupt stress increase due to the rupture propagation in an adjacent velocity weakening area (bc96). the vs behavior is characteristic of the rs laws and it cannot be simulated using the sw law. the two «intermediate» fields proposed by bc96, weak and compliant, have frictional velocity dependencies that are close to velocity neutral: they can modulate both tectonic loading and the dynamic rupture process. aftershocks can occur on compliant areas around a high slip patch, but most of the stress is diffused through aseismic slip. fig. 5a,b. dynamic rupture propagation along a heterogeneous fault: the adopted constitutive parameters are listed in table iii. the fault is represented as two patches having different values of the l parameter (l1 and l3 in table iii): the external one has a larger value (1.6 times the inner one). panel a) shows the spatio-temporal evolution of slip velocity. panel b) displays the time histories of slip velocity in different positions along the fault: solid lines identify slip velocity computed for points located in the inner region (low l), while dashed lines identify slip velocity computed for those points located in the external region (high l). a b 338 elisa tinti, andrea bizzarri and massimo cocco in the following sections we will present and discuss the results of different simulations of dynamic crack propagation on a heterogeneous fault representing frictional heterogeneity in terms of a non-uniform distribution of either the l or b and a parameters. 4.1. heterogeneous distribution of the parameter l in this section we will present two numerical experiments in which we consider different values of the characteristic length l along the fault; all models and constitutive parameters are listed in table iii. we recall here that the l parameter controls the state variable evolution and affects the size of nucleation zone (eq. (3.2)). therefore, with the increasing l value, the fault spends a longer time to reach an extension equal to the critical length ,c and to initiate the spontaneous dynamic rupture propagation. in fig. 5a,b we show the results of a numerical experiment in which a fault patch with l = 9 µm is surrounded by a region with a greater l (l = 15 µm). in panel a the spatio-temporal evolution of slip velocity is illustrated. in panel b we present the time histories of slip velocity for different points located along the fault. figure 5a,b points out that the rupture penetrates within the region having a greater l. as previously noted (fig. 1ad), during the propagation the peak slip velocity increases as the rupture front moves far away from the nucleation patch and as soon as the crack tip encounters the region with a greater l, peak slip velocity is immediately reduced, but even in this region it starts to grow again. as in the reference model, v2 (i.e. the velocity at the new steady state) is almost the same for all points. we have plotted in fig. 6 the slip-weakening curves calculated for different points located along the fault strike for the same simulation shown in fig. 5a,b. because the state variable evolution is different for different values of the l parameter, the dynamic traction also displays a quite diverse behavior. when the rupture enters the region characterized by a greater l, the value of dceq and consequently the fracture energy increase. thus, fig. 6 corroborates the linear relation existing between the equivalent slip weakening distance dceq and the l parameter (eq. (3.1)). the kinetic friction level is nearly fig. 6. slip-weakening curves calculated for the simulation shown in the previous figure, whose model and constitutive parameters are listed in table iii. the traction evolutions as a function of slip have been plotted for several points located on the two patches having different values of l parameter at different distances from the nucleation patch. 339 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction the same in the two regions. we emphasize that the slip-weakening curves maintain the expected trend also during the propagation along a heterogeneous fault but the dynamic physical quantities (such as dynamic stress drop or fracture energy) depend on the constitutive parameters. the simulation presented above is characterized by a small contrast of the l parameter in the two regions. we show in fig. 7a,b the results of a numerical experiment in which the variation of l is more pronounced: we simulate the dynamic propagation along a fault where the inner region has l = 10 µm and the external region has a higher l value (l = 1 mm). in this case the rupture is unable to penetrate the external patch. this configuration was named «barrier model» by bizzarri et al. (2001). in fact, we observe that during the propagation within the inner region the rupture behavior is identical to the reference configuration of fig. 1a. when the rupture front approaches the high-l region (l is 1 mm, thus the contrast is 1000), a back-propagating healing front causes the crack arrest, as the rupture does not have enough energy to break the barrier and propagate inside the high-l area. this process is known as a «barrier-healing». slip velocity time histories of different fault points are plotted in fig. 7b: we can clearly observe the increase of the peak as rupture propagates away from the nucleation patch. the back propagating front causes a remarkable decrease of the slip velocity with a consequent healing. in this case our simulation yields a slip velocity time history with a finite duration. 4.2. heterogeneous distribution of (b – a) in this section we will present and discuss some numerical experiments in which a velocity weakening zone is surrounded by velocity strengthening region. in the first simulation we compute the dynamic propagation of an earthquake rupture along a fault on which a velocity weakening patch is adjacent to a velocity strengthening region. the initial and constitutive parameters are listed in table iii. figure 8a,b shows the results of these calculations: the dynamic rupture nucleates and propagates within the velocity weakening zone, thus it penetrates within the velocity strengthening region for a small distance before being arrested. the rupture arrest is gradual and it generates healing of slip only when rupture is stopped inside the vs region. in panel b we plot the slip velocity time histories, which show the expectfig. 7a,b. dynamic rupture propagation along a heterogeneous fault: the adopted constitutive parameters are listed in table iii. the fault is represented as two patches having different values of the l parameter (l1 and l3 in table iii): the external one has a larger value (1000 times the inner one). panel a) shows the spatio-temporal evolution of slip velocity. panel b) displays the time histories of slip velocity in different positions along the fault. a b 340 elisa tinti, andrea bizzarri and massimo cocco ed behavior (similar to that one of fig. 2) until the crack tip penetrates within the vs zone; thus, peak slip velocity gradually decreases and heals. the healing phases (i.e. the way in which slip velocity drops to zero) in fig. 7a,b and fig. 8a,b are evidently different: the first, due to a barrier, is more evident and abrupt than the latter. figure 9a,b shows the results of a numerical experiment in which two vw regions are separated by a vs patch located between them. the initial and constitutive parameters are listed in table iii. as expected, the rupture initially accelerates within the vw region and it partially penetrates within vs area. slip velocity is progressively attenuated and the crack tip decelerfig. 8a,b. dynamic rupture propagation along a heterogeneous fault: the adopted constitutive parameters are listed in table iii. the fault is represented as two patches having different values of the a and b parameters (a1, b1 and a3, b3 in table iii): a velocity strengthening area is adjacent to the velocity weakening patch where the rupture nucleates. panel a) shows the spatio-temporal evolution of slip velocity. panel b) displays the time histories of slip velocity in different positions along the fault. a b 341 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction ates. in this area, the rupture velocity is very low and the crack is almost arrested. because of the small dimension of the vs region, the crack is able to propagate within the vs patch and it re-accelerates again when it reaches the external vw region. figure 9b shows the time histories of the slip velocity. it emerges that slip velocity has a finite duration only during the rupture propagation within the internal vw region, while in the external vw region it does not return to zero (i.e. no healing of slip). these simplistic simulations provide a picture of the complex interactions between fault patches having different frictional properties. our 2d simulations illustrate how the traction and slip velocity evolutions are modified during fig. 9a,b. dynamic rupture propagation along a heterogeneous fault: the adopted constitutive parameters are listed in table iii. a narrow velocity strengthening patch separates two velocity weakening areas (identified by the values of the parameters (a1, b1), (a2, b2) and (a3, b3) in table iii). the rupture nucleates within the inner velocity weakening patch. panel a) shows the spatio-temporal evolution of slip velocity. panel b) displays the time histories of slip velocity in different positions along the fault. a b 342 elisa tinti, andrea bizzarri and massimo cocco the propagation on heterogeneous faults. these calculations propose stimulating implications for slip duration and fracture energy. we will discuss these issues in the following sections. 5. implications for slip duration the simulations presented in the previous sections confirm that the dynamic rupture propagation on a homogeneous fault governed by a rs friction law (the slowness formulation defined in eq. (2.1)) does not show the healing of slip (as clearly shown in figs. 2 and 4a,b). this is in agreement with the results of perrin et al. (1995) and bizzarri et al. (2001). although different regularizations or modifications of the constitutive formulations have been proposed to obtain the healing of slip and/or self-healing pulses (see cocco et al., 2004 and references therein), even source heterogeneities can produce a finite duration of slip velocity and short slip durations (see beroza and mikumo, 1996; day et al., 1998). in the present study we tested two different heterogeneous configurations; the first is based on the variation of the l parameter and the other on the interaction between vw and vs patches represented by changing the values of a and b. a strong contrast of the l parameter represents a barrier and produces a «barrier healing» (see zheng and rice, 1998; bizzarri et al., 2001) similarly to the arrest on the crack-like rupture propagation. figure 7b shows that the slip duration depends on the distance from the barrier and it is shorter for closer distances to the barrier. actually, the slip velocity time histories of the points closest to the barrier have a shorter duration than those located closer to the nucleation region. the interaction between vw and vs still yields finite slip durations (see fig. 8a,b). it is important to emphasize that the time histories of slip velocity simulated for heterogeneous configurations do not resemble a selfhealing pulse, although they have a finite duration. the latter is usually defined as a pulse propagating along the fault with nearly constant slip duration; in this case healing is independent of the crack arrest phase. in other words, while we simulated finite and relatively short slip durations, we are unable to generate self-healing pulses. these results are consistent with the findings of perrin et al. (1995) and corroborate the outcomes of day et al. (1998), who suggested that source heterogeneity yields healing of slip. an important implication emerging from these results is that healing of slip does not require traction re-strengthening: total dynamic traction remains at the kinetic friction level also when slip velocity tends to zero. 6. implications for fracture energy we computed fracture energy for the different simulations performed in this study. the fracture energy is defined as the amount of energy that the crack spends to advance and increase its length. it has been defined as g du 2 1 f 0 = 3+ -x x# _ i (6.1) and is measured as the area below the traction versus slip curve shown in figs. 3 and 6 and above the minimum traction τ f . for a dynamic rupture propagation on a homogeneous fault governed by a sw law the fracture energy g is constant and known a priori over the whole fault surface [g = 1/2(τu− τ f) dc]. this is not the case when rupture propagation is governed by other constitutive laws or when the constitutive parameters are not uniform on the fault. in order to quantify the fracture energy changes during the crack growth, we plot in fig. 10a-d the computed g as a function of the position along the fault for four different configurations investigated in this study. figure 10a shows the resulting values for the homogeneous configuration (the reference model shown in fig. 1a-d), which reveals an increasing trend as the rupture propagates far away from the nucleation patch. this is in agreement with the results shown in fig. 2, which illustrate that the maximum yield stress increases moving far away form the nucleation patch, while τ feq slightly decreases. figure 10b-d displays the resulting values computed for three heterogeneous configurations. in particular, fig. 10b corresponds to the non-uniform distribution of the parameter l: as the rupture penetrates in the region with 343 modeling the dynamic rupture propagation on heterogeneous faults with rateand state-dependent friction higher l, the computed fracture energy increases jumping to a larger constant value; this is also expected looking at the results shown in fig. 6, which displays the increase in dceq. figure 10c refers to the rupture arrest within a vs area shown in fig. 8a,b: after an initial increase of g with increasing distance from the nucleation patch, consistent with fig. 10a, we notice a decrease of g within the vs area. this is due to the decrease of dynamic stress drop and peak slip velocity shown in fig. 6. in the vs patch the total amount of work spent to allow the crack advance is lower than that in the vw patch. finally, fig. 10d shows the fracture energy estimated in different fault positions during the rupture propagation in the heterogeneous fault modeled in fig. 9a,b: the propagation within the vs area produces a sudden drop in g, which is followed by an increase when it starts to propagate again within the external vw region. the calculations summarized in fig. 10a-d point out that frictional heterogeneity explains the variability of fracture energy on the fault plane, which is associated with both the variations of slip and breakdown stress drop. 7. conclusive remarks one of the main goals of this study is to extend the results of bc96 to investigate the dynamic rupture propagation on a 2d fault with a heterogeneous distribution of constitutive parameters. we used the rateand state-dependent formulations to characterize fault heterogeneities following the findings of bc96, who proposed to split the velocity weakening and the velocity strengthening regimes into four distinct frictional fields. our results corroborate the conclusions of bc96 demonstrating that a velocity strengthening area can arrest and can also be driven to a dynamic instability by an earthquake rupture propagating in the adjacent fault patch. we have represented numerically fig. 10a-d. fracture energy values in different positions along the fault for four configurations investigated in this study: a) refers to the reference model shown in fig. 1a-d; b) refers to the heterogeneous l distribution shown in figs. 5a,b and 6; c) and (d) refer to the heterogeneous distribution of the a and b parameters shown in figs. 8a,b and 9a,b, respectively. a b c d 344 elisa tinti, andrea bizzarri and massimo cocco the fault heterogeneity by assigning different values of l or (b − a) parameters along the fault line. our simulations show that the interaction between the propagating dynamic rupture front and the heterogeneous fault patches depends on the values of the constitutive parameters. in particular, we have shown that a variation of the l parameter can modify the peak slip velocity or can arrest the rupture propagation depending on the value of the l contrast. the heterogeneity of the l parameter does not modify the breakdown stress drop nor does it contribute to the variations of rupture velocity if the contrast is smooth. on the contrary, the heterogeneity of the distribution of the difference (b − a) affects the dynamic rupture propagation in a more complex way: dynamic stress drop and strength excess strongly depend on b and a parameters. moreover, rupture can penetrate within a velocity strengthening area and the heterogeneous distributions of b and a yield complex time histories of slip velocity. we propose that frictional heterogeneities can explain the observed complexity of slip distribution and the variability of rupture velocity during earthquakes. our results have important implications for slip durations (i.e. local rise time) and fracture energy. in particular, our results are consistent with the findings of perrin et al. (1995) and bizzarri et al. (2001). a large contrast in the l parameter represents a barrier that produces a crack-like solution with variable but finite slip durations. because the variations of constitutive parameters affect both the critical slip-weakening distance (see bizzarri and cocco, 2003 and references therein) and the breakdown stress drop, the inferred fracture energy varies along the fault. we have shown that the increase in the l parameter results in a fracture energy increase and that heterogeneous distribution of (b − a) yields evident variations of fracture energy along the fault. because we model here a laboratory fault, our estimates (∼103 j/m2) of fracture energy cannot be compared with those inferred for real earthquakes. in this study we mainly focused on the interactions between the propagating dynamic rupture front and the heterogeneous fault patches. according to bc96, the response of fault patches having different frictional properties to a constant tectonic load also controls the pattern of seismicity and the behavior of crustal faults. thus we conclude that fault frictional properties and their variations on the fault plane play an important role in characterizing crustal faulting and the mechanical properties of major fault zones. acknowledgements we thank stefan nielsen for helpful discussions. we thank antonio rovelli, maria elina belardinelli and peter moczo for their detailed reviews of this paper. references andrews, d.j. 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(received december 02, 2004; accepted march 15, 2005) vol50,1,2007 105 annals of geophysics, vol. 50, n. 1, february 2007 key words magnetotelluric inverse problem – neural network – em monitoring 1. introduction in recent years some examples of the neural network (nn) technique applications for solution of the magnetotelluric inverse problems (poulton, 2002) and rapid inverse problems (zhdanov and chernyavskiy, 2004) have been presented. it was shown that this method is effective when the number of the geoelectrical section parameters is about 10 (spichak and popova, 2000; spichak et al., 2002; shimelevich et al., 2002), i.e. the solution is sought within a narrow class of models. an important peculiarity of almost instant nn inversion makes this approach attractive for the real-time monitoring of electromagnetic parameters of the medium. it should be added that the nn can be efficiently applied to the monitoring of a few parameters of the known section using a rarefied set of measurements (shimelevich et al., 2003). in the present paper the nn technique is shown to be effective to reconstruct the conductivity distribution and its time variations in case of hundreds of parameters within a wide class of geoelectrical models. 2. metodology of the nn inversion in a class of the geoelectrical sections the inverse mt problem of the evaluation of the vector of geoelectrical section parameters γ = (γ1, ..., γn) according to the mt data β = = (β1, ..., βm) observed on the earth surface can be reduced to a solution of the non-linear operator equation ak γ = β , γ ∈ γ k ; where γ k its set associated with the given class gk , k is the class number and ak is the mt forward problem operator, defined on the subset γ k . the nn approach is an approximation of the inverse operator sk = ak−1 by a superposition of non-linear functions of the given type. one of the widespread nn approximations is the multilayer perceptron (raiche, 1991). in this rapid neuronet inversion of 2d magnetotelluric data for monitoring of geoelectrical section parameters mikhail i. shimelevich (1), eugeny a. obornev (1) and sergei gavryushov (2) (1) moscow state geoprospecting university, moscow, russia (2) engelhardt institute of molecular biology, moscow, russia abstract the inverse magnetotelluric (mt) operator is approximated by means of the neural network (nn). the methodology of the nn interpretation in classes of the geoelectrical sections described by the hundreds of parameters is proposed. error of the nn inversion and field misfit are evaluated. a rapid nn algorithm solving the inverse problem and detecting changes of time-dependent dynamic parameters of the section is applied to 2d synthetic data. mailing address: dr. mikhail i. shimelevich, moscow state geoprospecting university, 123001, sadovaya kudrinskaya str., 22, building 1, moscow, russia; e-mail: shimelevich@ecc.ru 106 mikhail i. shimelevich, eugeny a. obornev and sergei gavryushov case, a solution of the inverse problem is sought as: γ = f (vf (w β)), where the sigmoid functions are f(x)=1/(1+exp(−x)). to calculate the unknown matrices v, w the method of nn learning with a teacher is used. the nn is learned, using a database of samples that are pairs of vectors {γp, βp}, obeying the equation: ak γp= βp, γp∈ γ k , p = 1, ..., p (p is a number of samples). vector γp is randomly chosen from the set γ k . a numerical implementation of the method based on the error back propagation algorithm (raiche, 1991). to estimate its interpolation properties and quality of the inversion, the independent testing set is used. there are calculated parameter errors ∆ k = ( ∆ k 1, ..., ∆ kn) and the field misfit δk averaged for the testing set. this independent set of samples is used to verify the accuracy of the inversion. this accuracy is increased at growth of the number of samples p (shimelevich et al., 2001). to obtain a practically applicable accuracy of the inversion (about a few percent), the database can include up to hundreds of thousand of examples. parallel algorithms are used to obtain such a database. learned neuronet sk determines an approximate inverse operator ak−1 for the model class gk of the geoelectrical sections, the parameter errors ∆ k = ( ∆ k 1, ..., ∆ kn) and the field misfit δk obtained as the result of the nn testing allow us to estimate the error of the interpretation, and to solve a problem of a correspondence between measured data and medium classes gk (shimelevich and obornev, 1999). to evaluate the impact of the noise, learned neural networks were tested for data with added noise of various kinds and magnitudes. results suggested that relative errors of the inversion led to field deviation not exceeding the norm of the noise (shimelevich et al., 2001). 3. the classes of the parameterized geoelectrical sections in practice, an interpreter can deal with several hypotheses on the conductivity distribution model, so the solution of the inverse problem should be sought in a few classes gk of the medium. in this case, it is necessary to have a set of neuronets s0, s1, ..., corresponding to different classes g0, g1, ... of the parameterized geoelectrical sections. the parameterized class gk is the set of all possible geoelectrical sections which conductivity can be described as: σ( y, z) = fk(y, z, γ1, ..., γn), y, z ∈ ω, γnmin≤ γn≤ ≤ γnmax, n = 1, ..., nk. here ω is a domain of r2, where the electromagnetic problem is formulated; γnmin, γnmax are the limits of the γn parameter variation, the fk is the function of the parameterization of the given class and nk is the number of parameters which depends on the complexity of the class gk. the generalized example of the section parameterization is shown in fig. 1. the parameters of the structure are the layer thicknesses defined at the fixed nodes, the conductivities of the blocks within the layers and the conductivities at the fixed nodes of the rectangular grid. for the forward problem solving the parameterized conductivity of the medium is interpolated between the nodes. the number of sought parameters of such structures is about a few hundreds, whereas the finite-difference scheme dimension of the corresponding boundary value problem is of tens of thousands. we will consider three particular examples of medium classes. fig. 1. the generalised example of the geoelectrical section parameterization. the model can include n layers (1), (2), (3) and 2d conductivity grid. the conductivities at the nodes of the 2d grid, thicknesses and conductivities of the layers in the horizontal direction are varied. 107 rapid neuronet inversion of 2d magnetotelluric data for monitoring of geoelectrical section parameters class g0 (grid) – it is the most general class of the geoelectrical sections. the conductivity can change within a fixed range at all nodes of the 2d grid. between the nodes the conductivity is interpolated by a 2d spline. class g1 (layer+grid) – the upper layer of the section is described explicitly, whereas the rest of the section is parameterized as in class g0. the layer thickness varies within a given range at the fixed nodes in the horizontal direction. between the nodes the layer boundaries are interpolated by a 1d spline. the layer conductivity varies linearly between the centers of the blocks in the horizontal direction. class g2 (3 layers+grid) – the upper three layers of the section are described explicitly. the layer boundaries are parameterized as in class g1. the layer conductivity varies linearly between the centers of the blocks in the horizontal direction. a total number of parameters of the described conductivity classes is from 233 to 336. for these classes neuronets s0, s1, s2 were learned according to the described above principles. 4. nn interpretation of the 2d synthetic data the structure used to generate the synthetic data is shown in fig. 2a. this structure (model 1) includes three layers of the changeable thickness and the underlying stratum. the thickness of the upper layer varies from 100 to 3900 m. its conductivity varies in the horizontal direction from 0.185 to 0.008 s/m. the conductivities of the second, third layers and of the underlying half-space are constant and equal σ 2 = = 0.0004 s/m, σ 3 = 0.2 s/m, σ 4 = 0.0002 s/m. the thickness of the second and third layers vary from 0 to 13.4 km and from 2 to 4.8 km. the apparent resistivities and phases for te and fig. 2a-d. the results of the synthetic data interpretation. a) model 1 used to generate the synthetic data; b) interpretation, step1 (nn s0 is applied); c) interpretation, step2 (nn s1 is applied); d) interpretation, step3 (nn s2 is applied). the thin solid lines are true boundaries of the layers of model 1. a b c d 108 mikhail i. shimelevich, eugeny a. obornev and sergei gavryushov tm modes were calculated at 13 periods from 0.01 to 1000 s in 126 sites along the profile. to interpret the generated synthetic data a set of learned neuronets s = ( s0, s1, s2) was used. at the first stage of the interpretation there is no hypothesis about the model class of the section. so, we have to use neuronet s0 corresponding to the widest class g0 of geoelectrical sections. the result of the first inversion (step 1) is shown in fig. 2b. at this stage of the interpretation, one can draw the only conclusion that there might be a thin superficial conductive layer. as is seen from averaged field misfits given in fig. 3, all results of imaging related to the deeper part of the section are unreliable on this first step of inversion. a relatively low misfit can be observed only at high frequencies, which suggest that one can only trust conductivity distribution close to the surface. taking this conclusion as a starting point, one can use neuronet s1 corresponding to the class (layer+grid). the result of the second inversion (step 2) is shown in fig. 2c. the upper layer is seen better and its lower boundary is practically identical to the true boundary. three layers can be seen under it and the conductivity of the third layer is about 0.1 s/m. at the same time the field misfit has decreased in the whole frequency range (see fig. 3). this proves the correctness of using neuronet s1 on step 2. taking into account this fact and the image of the section in fig. 2c one can try to use neuronet s2 corresponding to the class g2 (3 layers+grid) at the next stage of the interpretation. the result of the inversion (step 3) is shown in fig. 2d. the second conductive layer is seen better and the field misfit at the range t ≤ 25 has dropped to the level of the mean field misfits obtained at the testing of neuronet s2.thus, one can see that the inversion with using neuronet s2 gives a satisfactory variant of the interpretation except for the deepest part of the section. an application of the nn requires knowledge of the model class of the resistivity section. if there is no such information, an interpreter has to invert data using nn s0, s1, which justifies the final application of the nn s2. it is important to note that application of s0 does not required any information since it is based on the most general class g0. the sequence of inversions s0, s1 illustrates an application of the nn at the lack of a priori information about the section. 5. rapid detection of the geoelectrical parameter variations to estimate the resolving power of the nn inverse operator the boundary of the third layer of model 1 was changed in three zones marked by the dashed line in fig. 4. after forward modeling for the modified section, the new synthetic mt data were interpreted using neuronet s2. the difference between the results of the interpretations is shown in fig. 4. as one can see the difference conductivity map localizes the zones of the conductivities changes with a good accuracy. note, that we can apply the same learned neuronet for inversion, since the modified section belongs to the same model class as the initial section. in this case the data interpretation can be done very quickly (several seconds on a single computer). this example shows a high fig. 3. field misfits averaged over three period ranges: t<1 s; 1<t<25 s; t > 25 s at the three nn inversions (solid lines). dotted lines are averaged field misfits obtained at the neuronet testing. 109 rapid neuronet inversion of 2d magnetotelluric data for monitoring of geoelectrical section parameters sensitivity of the nn inverse operators to the geoelectrical parameter variations. it illustrates a possibility of a real-time monitoring of resistivity section parameters if these changes can be detected from the measured fields. 6. conclusions 1) a representative set of the learned neuronets allows us to interpret a 2d geoelectrical sections described by hundreds of parameters and to estimate the accuracy of the interpretation results. 2) due to rapidity of nn inversion and high resolution the neuronet technology could be used for the monitoring and localization of small variations of the conductivity distribution in a real time scale. a clear disadvantage of the method is a large number of the trained nn for different classes of the geoelectrical sections and difficulty to choose a true nn at the lack of information. references poulton, m.m. (2002): neural networks as an intelligence amplification tool: a review of applications, geophysics, 67 (3), 979-993 raiche, a. (1991): a pattern recognition approach to geophysical inversion using neural nets, geophys. j. int., 105, 629-648. shimelevich, m.i. and e.a. obornev (1999): the method of neural network applied to the approximation of the inverse operators in electromagnetic sounding problems, izvestiya vuzov (geol. prospect.), 2, 102-106 (in russian). shimelevich, m.i., e.a. obornev and s.a. gavryushov (2001): a method of designing neural networks for solving multiparametric inverse problems of magnetotelluric sounding, izvestiya vuzov (geol. prospect.), 6, 129-137 (in russian). shimelevich, m.i., e.a. obornev and s.a. gavryushov (2002): neuronet approximation of the inverse mt operators for geoelectrical monitoring, in proceedings of iii international workshop on magnetic, electric and electromagnetic methods in seismology and volcanlogy (meemsv-2002), september 3-6, moscow, 59-62. shimelevich, m.i., e.a. obornev and s.a. gavryushov (2003): application of neuronet approximation to the solution of geo-electromonitoring problems, izvestiya vuzov (geol. prospect.), 4, 70-71 (in russian). spichak, v. and i. popova (2000): artificial neural network inversion of magnetotelluric data in terms of three-dimensional earth macroparameters, geophys. j. int., 142, 15-26. spichak, v., k. fukuoka, t. kabayashi, t. mogi, i. popova and h. shima (2002): ann reconstruction of geoelectrical parameters of the mionou fault zone by scalar csamt data, j. appl. geophys., 49, 75-90. zhdanov, m.s. and a. chernyavskiy (2004): rapid threedimensional inversion of multi-transmitter electromagnetic data using the spectral lanczos decomposition method, inverse problems, 20, s233-s256. fig. 4. the difference between the results of the interpretation of the initial and modified geoelectrical sections. the changed boundaries are marked by the dashed line. the solid lines are the boundaries of model 1. 537_550.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 537 recent seismicity of the «acque albule» travertine basin calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti istituto nazionale di geofisica e vulcanologia, roma, italy abstract the seismic period which occurred in a portion of guidonia montecelio and tivoli territory, two towns situated about twenty kilometres ne of rome, affected four areas with a high density of population: guidonia, collefiorito, villalba and bagni di tivoli. even though the events had a very low magnitude (less than 3.0), local phenomena, like rumbles and strong vibrations of the ground, frightened inhabitants also because some months before (january 26, 2001) in a village named marcellina situated a few kilometres away from guidonia, a large sinkhole (600 m) had appeared without fore signals. the «istituto nazionale di geofisica e vulcanologia» (ingv) installed some digital stations that monitored the seismic phenomena. this paper has the aim to process all the information acquired during the period, both from the micro and macro seismic point of view, in order to explain the phenomenology that involved the area. the synergy of the two methodologies lead us to the conclusion that two different seismotectonic structures originated the events, one in an anti-apenninic direction, the other in a n-s direction. 1. introduction during the year 2001, a seismic sequence felt in the acque albule travertine basin, whose location is shown in figs. 1 and 2, aroused new interest in the area and geophysical studies have been carried out by ingv to explain the phenomenon. from the geological point of view, the mentioned basin represents an interesting evaporitic deposit of continental origin. it is unique, in central italy, for the peculiar geo-mechanical characteristics and for the formation thickness (cipriani et al., 1977). the recent seismic phenomenology represents evidence of the tectonic instability of the area, probably connected to the travertine formation still in progress. another instability factor is probably represented by the presence of some sinkholes (small morphological depressions related to the breakdown of carbonate bedrock), whose origin is not clear yet so it is worth studying them to complete the geo-tectonic sketch of the area. 2. geological and structural settings the areas considered in this study are located in a plain inside the acque albule travertine basin, a few kilometres east of rome, on the margin of the central tyrrhenian sea (figs. 1 and 2). mailing address: dr. calvino gasparini, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: gasparini@ingv.it key words seismicity neotectonic 538 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti fig. 1. schematic map of the latium tyrrhenian margin and their correlation with the acque albule travertine basin. there are also many hydrothermal appearances: a) alunite and kaolin mineralization; b) leakages associated with small springs (with a temperature often up to 20°c); c) groups of sinkholes; d) springs affected by hydrothermal activity (t > 20°c); e) mineralised springs (tds > 0.75 g/l; t < 20°c). from faccenna (1994) modified. 539 recent seismicity of the «acque albule» travertine basin in particular, the sequence shows that the geological history of the acque albule travertine basin is related to the uplift of the apennines. during the opening process of the tyrrhenian sea, about 5 ma ago, the western part of the apennine chain was stretched and sank. such sinking was not homogenous everywhere, some areas remained above the sea level, originating the soratte and the cornicolani mountains that constituted the horsts of the tiberina graben (ogniben et al., 1975). in that deep sea, an argillaceous material was initially deposited, originating the blue clay that crops out inside studies on this area go back to the end of the 19th century (pellenti, 1882). maxia in 1950 conducted the first geologic survey of such travertine formation (maxia, 1950). in fig. 3 the principal lithological sequence of the travertine basin is illustrated from top to bottom: a) whitish calcareous sand locally called testina or cappellaccio; b) ochre coloured travertine; c) greyish coloured ancient travertine; d) blue clay of pliocene; e) marly limestone mesozoic-cenozoic. the stratigraphy has a generic character because the layer thickness is extremely variable across the area. fig. 2. sketch map of acque albule travertine basin and location of the towns involved in this study. 540 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti district located 20 km southeast of rome. this volcanic district was active during the pleistocene from about 0.7 to about 0.01 ma, essentially with pyroclastic eruptions (voltaggio and barbieri, 1995). during the upper pleistocene, the formation of acque albule travertine basin began between guidonia and the aniene river (fig. 2). it is related to the latest magmatic manifestations of the albani hills and tectonic strike-slip activity, however a tectonic structure trending n30°45°e can be observed. such tectonic elements concur to interpret the area of acque albule as a pull-apart basin (kearey and frederik, 1990) characterized by a subsidence area that accommodated a lacustrine zone. sulphur springs, over saturated by caco 3 , spurted out the extensional structures and caused rome (close to the vatican) and in the western part of guidonia. subsequently, as a result of the uplift of this perithyrrenian area, the finest argillaceous material was replaced by yellowish sand, containing an elevated amount of coastal region organisms (cosentino et al., 1990). the strike-slip activity, from pliocene through middle-upper pleistocene, originated a series of n-striking right-lateral en echelon fault segments, with a striking of n5°w in the western part of the acque albule travertine basin (faccenna and funiciello, 1993; faccenna et al., 1994). this tectonic setting starts from the southern sabina, crosses the cornicolani mountains, cuts the aniene valley orthogonally in proximity to pantano locality (figs. 1 and 2) and ends in the northern part of the albani hills, a volcanic fig. 3. lithological sequence of the travertine basin. notice that this scheme is purely indicative of the main lithotypes. moreover the thicknesses is strongly dependent on the point. 541 recent seismicity of the «acque albule» travertine basin the precipitation of carbonate and the travertine formation; this phenomenon is still in progress (maxia, 1950). describing the geomorphological characteristics of the area, we can say that the acque albule travertine basin, with an extension of about 50 km2, spreads out from the shelter of the cornicolani mountains (83 m a.s.l.) to the aniene river (about 46 m a.s.l.). the nw area has an elevation of 100 m a.s.l. with a maximum elongation in the ne-sw direction, and it is connected with the travertine basin through an escarpment with a slope of 7.5%. the morphologic differentiation of this area has a probable tectonic origin. 3. the guidonia seismic district in the second part of the 17th century, a certain area of collefiorito was named valle stregata (enchanted valley) because the inhabitants were perceiving noises from the earth. because of the scarcity of information, seismological characteristics are not well known in the guidonia area, the low seismicity on one side and the recent urbanization on the other, did not allow the correct location of the events, that were put down as from the closest seismic zones. table i shows the most important events about which we have macro-seismic reports, the first one of which felt on may 22, 1972 at 1:15 a.m. (gmt). the administrative manager of the municipal office reported that the event was felt by everybody including sleeping people. the event caused only rubble falling and cracks in the buildings, no victims or injured people, inhabitants heard a loud underground noise. three earthquakes, with an interval of 15-20 min between each other, were reported. on the basis of the description, the main shock was assigned an intensity of vi mcs degree. another seismic sequence, located in the collefiorito area, during january 1989, had been characterized by a north-south migration of the hypocenters and the maximum intensity was v mcs degree, the affected area was small and narrow (0.5 km2 ), that could indicate very shallow depths (meloni and pirro, 1989). on june 27, 1999 at 5:12 a.m. gmt (pirro, 2002), an earthquake was felt only in the eastern part of the old town centre of montecelio with an intensity of iv-v mcs degree. in this case, the ingv centralized network did not detect the event, meaning that it was very shallow. within a range of 20 km from the observed area, we do not find important seismogenetic structures. the aniene valley is characterized by a moderate seismicity that rarely reaches vii mcs degree. in particular, during june 2000, a seismic sequence whose maximum duration magnitude was 4.0, caused damage in some historical towns located in the middle of aniene valley (rocca canterano, gerano, cerreto laziale, canterano). the southern part of guidonia suffers from the effects of the albani hills area, characterized by clustered events with low magnitude (maximum intensity: viii mcs). table i. the most important events on which we have macro-seismic reports. year month day hh:mm lat. (n) long. (e) mcs int. localities (gmt) 1972 may 22 01:15 41.99 12.72 vi guidonia 1989 january 11 03:30 41.98 12.70 iv-v collefiorito 1989 january 12 23:00 41.97 12.69 v collefiorito 1989 january 13 22:30 41.97 12.69 v collefiorito 1989 january 14 12:20 41.97 12.69 v collefiorito 1999 june 27 05:12 42.02 12.73 iv-v montecelio 542 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti lastly, the north-western part of the area is also characterized by low seismicity. the main shock (i = viii mcs) felt on april 24, 1901, injured stazzano and cretone towns (postpischl, 1985). 4. macroseismic investigation and field observations to better characterize the whole phenomenon and the affected area, we performed detailed macroseismic investigations using an appropriate questionnaire, filled in directly by the inhabitants. the first investigation concerned the event which occurred on june 22, 2001 at 4:36 p.m. gmt. figure 4 shows in grey the area with the maximum intensity, about 200 m wide, extended in nne-ssw direction for about 1 km. in this area, whose intensity we attributed the v mcs degree, people heard the rumble, coming from the earth, and felt the quake at the same time. in the northern part, the noise was described as like a thunder coming from the sky. everybody described the rumble as a deep sound and comparable to a collision between two trucks. the larger area, shown in the figure, assessed as iv mcs degree, has an extension of about 2.5 km and about 300 m in width. the maximum elongation is in the same direction as the previous area. in this case, the rumble was described as like a mine blast. the elongated shape of this isoseismal is not dependent on the anthropic structure of the area, but is a propagation effect. it is also clear that the instrumental epicentre locations of the sequence are not comprised in this area, this fact is further evidence that the events are not generated by the same tectonic structure. the macroseismic investigation of the two events occurred on july 12, 2001 at 10:25 p.m. (gmt), and on august 25, 2001 at 10:55 a.m. (gmt), whose intensities were estimated as v mcs and v-vi mcs respectively, did not have a great success. because of summer, most people were on holiday so we were not able to collect a sufficient number of macroseismic forms to draw up a macroseismic map. in any case, on the basis of the information we collected, we can affirm that the locality named pantano was the maximum intensity area for the first event, and the locality named collefiorito was the maximum intensity area for the second. an event which occurred in the same zone, on january 31, 2002 at 10:50 a.m. (gmt) that had a felt area of about 10 km2 (fig. 5). in this case, the isoseismals maximum elongation is in direction nne-ssw. both the localities pantano and collefiorito felt the event remarkably. the scarcity of information in the area between the railway station and pantano and between pantano and san giovanni lake did not clearly define the trend of the isoseismal. in this case also, we could affirm that the attenuation was stronger in the ene direction than on the opposite side. the event which occurred on november 7, 2001 at 3:06 a.m. (gmt) seems related to another seismogenetic structure (fig. 6). in effect, it is located inside the acque albule travertine basin but 2-3 km away from the other events considered in this study. the felt area (7 km2), highly populated, allowed a detailed description of the macroseismic effects. as shown in fig. 6, the isoseismals are elongated in the n-s direction even though in this direction we have less information because it is less populated and, to the south, the presence of the travertine working quarries kept people from feeling the event. the area enclosed in the isoseismal with iv mcs degree is not symmetric with respect to the isoseismal with higher degree (v mcs). from this point of view, we can affirm that the attenuation was stronger on the eastern part than on the western. moreover, due to very local site effects, we found sporadic small areas with intensity up to iv mcs degree randomly distributed. during the evolution of the seismic sequence, a series of natural phenomena, certainly related to a system not in balance, was observed in the area. the first, observed by inhabitants, followed the event of june 22nd. people pointed out the leakage of gas from some points of the subsoil 200 m far from each other. the leakages were odourless, audible and had a consistency of vapour. some operators identified them as co 2 emissions and excluded the presence of sul543 recent seismicity of the «acque albule» travertine basin fig. 4. macroseismic investigation of the event which occurred on june 22, 2001 at 4:36 p.m. gmt, in addition the distribution of the events, occurring during the period may-june 2001 and recorded by the ingv mobile network. 544 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti fig. 5. macroseismic investigation of the event which occurred on november 7, 2001 at 3:06 a.m. (gmt). 545 recent seismicity of the «acque albule» travertine basin fig. 6. macroseismic investigation of the event which occurred on january 31, 2002 at 10:50 a.m. (gmt). phured gas and methane. pantano, that is inside the micro and macroseismic epicentral area, was the locality affected by this phenomenon starting in coincidence with the event and vanishing within one day. the second phenomenon inside the acque albule travertine basin, was observed in coincidence with the july event and was related to a change in the hydrogeological structure. the phenomenon was pointed out by the technical office of bagni di tivoli thermae, that claimed a three quarter loss in the usual flow (about 3.25 m3/s). the new situation caused a water table overloading, but at the occurrence of the august event the usual value of the spring flows of acque albule (regina and colonnelle lakes in figs. 3, 4 and 5) had been slowly re-established. 546 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti 5. data set and processing table ii shows the seven events recorded by the national network in the period between june 2001 and january 2002. it was just the panic created by the frequent noises and shakes felt by the inhabitants and the event recorded by the permanent centralized network (rsnc) on june 22nd that induced ingv to install the local mobile network. in this study, we processed the sequence recorded by the local network during the period between june 28th and july 11th. the whole recorded sequence consists of 95 events concentrated in an area of about 2 km2 named pantano. among the sequences recorded by the mobile network, 65 events have been selected to locate the hypocenter. the criteria we followed for the selection was to consider events with 8 phases at minimum. the hypocenter determination was performed by the routine hypoinverse, the velocity model we adopted takes into account the geology of the area: the stratigraphy of the area was deduced from literature (amato et al., 1986), and from deep drillings and geological survey: the travertine thickness is about 25 m, and is over the blue clay layer that is over the mesozoic-cenozoic limestone bedrock, deeper than 500 m. so the ruptures must have origin in this layer, because the rheology characteristic of the blue clay, that is more recent, cannot justify the ruptures. we could not calculate the local magnitude because the distance function f ( ) (published by richter in 1935 and the one we usually use) does not hold at such short distances between station and hypocenter. for local events, we use the magnitudeduration relation (console et al., 1988) (5.1) where a = 2.0, b = 0.870 and c = 0.082. i is the i-th seismic signal duration and i is the linear distance of the i-th station with respect to the epicenter determination. the md value of a regional event is calculated as the arithmetic average among the single magnitude values, eventually removing the maximum and (/or) the minimum, if they are «very distant» from the average. year month day hh:mm lat. (n) long. (e) mcs int. md localities (gmt) 2001 june 22 16:36 41.96 12.68 v 2.3 guidonia (pantano) 2001 july 12 22:23 41.96 12.68 v-vi 2.7 guidonia (pantano) 2001 august 25 10:55 41.97 12.70 vi 2.6 collefiorito 2001 september 1 19:24 41.96 12.68 v 2.3 pantano 2001 october 8 15:00 41.97 12.70 iii 2.1 collefiorito 2001 november 7 03:06 41.96 12.76 v-vi 2.6 villalba 2002 january 31 09:50 41.96 12.68 v-vi 2.4 pantano (collefiorito) table ii. events recorded by the centralized national network in the period between june 2001 and january 2002. md a c bi i i= +( )log10 thickness velocity (km) (km/s) 0-2 2.2 2-5 4.4 547 recent seismicity of the «acque albule» travertine basin fig. 8. minimum magnitude of completeness mc of the sequence obtained plotting the cumulative number of events versus magnitude and the straight line that fit the data. fig. 7. temporal evolution of the sequence. the figure indicates only the events recorded by the local network with the maximum error. 548 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti the selected sequence has a duration magnitude between – 0.1 and 0.9. figure 7 shows the temporal evolution of the sequence. in the figure only the events recorded by the local network with the maximum error are indicated. the occurrences seem very regular in time, unfortunately the errors on magnitudes do not depend on the magnitude value itself, some of them are very high, some of them are very low. probably it depends on the application of the formula, tested for regional events and not for very local and shallow events. in order to obtain a complete view of the phenomenon, we calculated the minimum magnitude of completeness. starting from the gutenberg-richter (gutenberg-richter, 1944) relationship log n = a bm (5.2) the determination of the minimum magnitude of completeness m c of a data set is obtained plotting log n (cumulative number of events versus magnitude) and the straight line that fits the data. following habermann (1983), m c is the level at which the data fall below the line. in fig. 8 we can identify this value that for md is 0.4. we can write the formula as follows: log n = 4.3 4.5 md . (5.2a) fig. 9. three-dimensional view of the locations with the three axes of the error ellipses, in addition a coloured evaluation of the error ellipse volumes is shown in the horizontal plane. 549 recent seismicity of the «acque albule» travertine basin the correlation coefficient is close to 1, and 0.9 is the expected maximum magnitude for the period under observation. the local network had been dismounted on july 11th because of the lack of significant events, even though the sequence was still in progress. unfortunately, on the july 12th the rsnc recorded an event with md = 2.7, so we do not have events recorded by both the networks (permanent and mobile) and, consequently we cannot perform any calibration of the magnitude. figure 9 shows the three-dimensional view of the locations. in addition, a coloured evaluation of the error ellipse volumes is shown on the horizontal plane. as shown in the figure, they are clustered and seem to lie in a plane with a certain direction. as the hypocenters lie inside the network, the greater errors are not distributed on the border and the major axis of the ellipse has the projection on the depth (z) axis. the result of the regression of the hypocentres on a plane is shown in fig. 10, it has an anti-apennines direction, the coloured horizontal plane indicates the depth of the events, but due to the high error it is not significant. 6. conclusions the recent seismic phenomenology aroused interest in an area that historically was not significant because of the scarcity of information regarding large earthquakes. the interpretation of macroseismic effects of the most significant events presents some difficulties related to the geological knowledge of the urban territory. certainly the events, whose macroseismic epicentres are between guidonia fig. 10. three-dimensional hypocentral locations with the stations of the local network; the horizontal plane indicates the depth of the events. 550 calvino gasparini, rosalba di maro, nicola mauro pagliuca, mario pirro and alessandro marchetti and collefiorito, are characterised by some peculiar factors: the low depth of the hypocentres, the presence of the basin border with consequent surface geological formation heterogeneity and the peculiar morphological formation. it is remarkable that the major effects are in the area where the massive travertine crops out surrounding the area of the obsolete quarries filled by waste material. moreover we can affirm that the seismotectonic structure that originated this events is oriented in anti-apenninic direction. this is also confirmed by the data analysis of the sequence analysed in the study: the events (fig. 9) present a good alignment in a quasi vertical plane. in relation to the event of november 2001 that occurred in the central part of the basin, we can conclude that the seismotectonic structure that originated it, is different. this event presents the maximum elongation of the isoseismals in a n-s direction. this suggests that the seismotectonic structure that originated it, is related to a prevalent strike slip tectonics as reported in a previous paper (faccenna et al., 1994) moreover, even if the magnitude is comparable with the others, the affected area is wider, of course it depends on the depth and on the local attenuation effects. this phenomenology, that is still ongoing, is the response of the seismotectonic system located in the middle of the aniene valley that is not in balance. these studies are important for the characterization of the system and could be helpful to understand similar phenomena that occur in the surrounding area. references amato, a., m. di filippo, m. mattei, p. montone and r. scarpa (1986): interpretazione congiunta di profili gravimetrici e di sismica a rifrazione nella provincia di roma, in atti del lxxiii congresso s.g.i., roma, 57-60. cipriani, n., p. malesani and s. vannucci (1997): i travertini dell’italia centrale, boll. serv. geol. italia, 98, 85-115. console, r., b. de simoni and a. di sanza (1988): riesame della relazione magnitudo-durata, in atti del vii convegno gngts, 30 november 2 december, 51-62. cosentino, d., r. funiciello, p. montone, m. parotto and m. voltaggio (1990): neotettonica nell’area tiburtina, (italia centrale, roma), in convegno «irpinia dieci anni dopo», novembre 1990, napoli, abstract, 209-210. faccenna, c. (1994): structural and hydrogeological features of the pleistocene shear zones in the area of rome (central italy), ann. geofis., 37 (1), 121-133. faccenna, c. and r. funiciello (1993): tettonica pleistocenica tra il monte soratte e i monti cornicolani, il quaternario, 6 (1), 103-118. faccenna, c., r. funiciello, p. montone, m. parotto and m. voltaggio (1994): late pleistocene strike in the acque albule basin (tivoli, latium), memorie descrittive della carta geologica d’italia, 49, 37-50. gasparini, c., a. marchetti, n.m. pagliuca and m. pirro (2001): analisi preliminare del periodo sismico avvenuto nel territorio del comune di guidonia montecelio (rm) tra il maggio e settembre 2001, in xx convegno gngts, cnr, roma (in press). gutenberg-richter, n. (1944): frequency of earthquakes in california, bull. seismol. soc. am., 35, 185-188. kearey, p. and j.v. frederik (1990): global tectonics, london, p. 302. habermann, r.e. (1983): teleseismic detection in the aleutin island arc, j. geophys. res., 88, 5056-5064. maxia, c. (1950): il bacino delle acque albule, contributi di scienze geologiche, cnr, roma, vol. 20, p. 20. meloni, f. and m. pirro (1989): analisi preliminare del periodo sismico del gennaio 1989 in località collefiorito di guidonia (prov. di roma). aspetti neotettonici, amm. com. guidonia-montecelio. ogniben, l., m. parotto, a. praturlon (1975): structural model of italy, quad. ric. sci., cnr, roma, 90, 257-311. pellanti, r. (1882): i travertini della campagna romana, boll. r. com. geol. d’italia, roma, vol. iii, 82-94. pirro, m. (2002): sismicità storica e strumentale dei monti cornicolani e dintorni (lazio), in aspetti naturalistici, geologici e geofisici dei monti cornicolani, edited by m. giardini, amm. prov. di roma, 141-167. postpischl, d. (1985): catalogo dei terremoti italiani dall’anno 1000 al 1980, quad. ric. sci., cnr, roma, 114 (2b), p. 239. voltaggio, m. and m. barbieri (1995): geochronology, in the volcano of the alban hills, rome, edited by r. trigila, 167-192. (received april 8, 2002; accepted july 5, 2002) layout 6 annals of geophysics, 60, supplement to 6, gm672, 2017; doi: 10.4401/ag-7481 a comparative study of sawtooth events and substorm onsets triggered by interplanetary shocks tushar andriyas1,* 1 dsk post-doctoral fellow, nehru science center, university of allahabad, india 211002 article history received july 4, 2017; accepted november 20, 2017. subject classification: interplanetary shocks; geomagnetic substorms; sawteeth events; auroral oval. gm672 abstract a comparison of solar wind conditions, geomagnetic response, and auroral boundary movement, using a similar number of sawtooth events and shock induced substorm triggers is carried out. 81% of the sawtooth onsets were triggered at low latitudes compared to 33% for onsets during shocks. results of superposed epoch analysis indicated that the mean interplanetary magnetic field (imf) bz remained strongly southward during sawtooth events while it was southward during the loading period and turned northward 20 minutes prior to the onset, during the shock triggered events. during both the event types, the mean energy available in the solar wind was found to be above the substorm threshold level. relatively high levels of solar wind density indicated to the magnetosphere being compressed to a larger degree during substorms initiated by shocks. the high latitude indices were elevated during sawtooth events with 23 hr fluctuations was observed in the al and pcn indices but au index after the onset remained at similar levels. sym-h remained below storm time values during shock related onsets but was much stronger and remained above the storm threshold during sawtooth events, that they occurred embedded in a geomagnetic storm. comparatively higher mean values of asym-h indicated to a much stronger asymmetric ring current during sawtooth events. mean boundary locations during sawtooth events were located a few degrees equatorward of those during the shock related onsets. oval in the dawn, dusk, and midnight sectors was much thicker during sawtooth events with clear widening around the onset during both event groups but the thickness in noon sector was similar and remained steady during the epoch window during both the event groups. elevated driving during sawtooth events was also indicative in a greater stretching and relaxation of the magnetic field lines at geosynchronous orbit in the midnight sector. 1. introduction a substorm is a short lived disturbance with a life time of 2-4 hours [tanskanen et al. 2002] in the magnetosphere-ionosphere coupled system that can deposit large quantity of energy into the auroral ionosphere. substorms occur at time scales that are much faster when compared to a geomagnetic storm. three phases have been ascertained as occurring during a substorm [mcpherron et al. 1986]. the growth phase is generally initiated after the southward turning of solar wind interplanetary magnetic field (imf) bz. the expansion phase occurs when instabilities inherent to the magnetotail cause the diversion of the cross-tail current through the polar regions. this is recorded as a fluctuations in the local magnetic field that can reach levels above 1000 nt. the magnetosphere then returns to a steady state during the recovery phase. as such, substorms play an important role in energy circulation through the magnetosphere through dayside reconnection, energy storage in magnetotail, and release of this energy to relax the field lines to a more dipolar shape. interplanetary shocks (ips) are plasma and magnetic field structures that travel through the solar wind and couple efficiently with planetary magnetospheres. increase in the dynamic pressure associated with a shock structure leads to the compression of the magnetosphere indicated by large positive increases in the north-south (h) component of the geomagnetic field. if a geomagnetic storm occurs, such increases are termed as sudden impulses (si) or storm sudden commencement (ssc) [curto et al. 2007, yao et al. 2009]. sources of such structures lie in either coronal mass ejections (cmes) or corotating interaction regions (cirs) with the most of the shocks measured at earth’s orbit occurring due to cmes. cirs are flanked by forward and reverse shocks which only develop sufficiently after the earth’s orbit (1 au) [gosling and pizzo 1999]. shocks induce complex processes in the earth’s magnetosphere and as such are important in understanding the solar wind-magnetospheric coupling. long periods of southward bz that precede an ips can lead to substorm triggering and affect the amount of precipitated power [liou et al. 1998]. ips have been shown to trigger substorms and enhance nightside precipitation with powers reaching substorm levels [meurant et al. 2005]. zhou & tsurutani [2001] found that 43\% of magnetic bays induced by shocks were associated with substorms. using a global magnetohydrodynamic simulation, oliveira and raeder [2014] studied the effects of various types of ips on the nightside magnetosphere. they observed that the frontal perpendicular shock led to a symmetric compression of the magnetotail to trigger a substorm and a large geomagnetic activity. pressure pulses that accompany an ips leads to compression of the magnetosphere on the dayside and can trigger substorms within minutes of arrival [brittnacher et al. 2000]. during a period of southward imf, an increase in the solar wind dynamic pressure can lead to the triggering of substorm expansion phase onset [kokubun et al. 1977]. sawteeth events have been classified as a separate class of activations that can occur during periods of moderately strong and steady solar wind [pulkkinen et al. 2007]. large quasiperiodic oscillations in the energetic particle fluxes, with a period of about 2-4 h [henderson et al. 2006a], particularly in proton channels, at geosynchronous orbits were identified by belian et al. [1995]. rapid increase and slow decrease of the fluxes are a signature of such events. they generally occur during periods of increased solar wind driving with a continuous southward bz. the saw blade like waveform is measured in both the energetic proton and electron fluxes. at geosynchronous orbit, the oscillation is observed as periodic stretching and relaxation of magnetic field lines [m. g. henderson et al. 2006b]. this reconfiguration from a highly stretched to relaxed state has also been observed over a wide range of local time sectors [pulkkinen et al. 2006]. these events also accompany a strongly enhanced partial ring current as measured by the asy-h index. pulkkinen et al. [2007] statistically compared the solar wind conditions and geomagnetic activity during sawtooth and other substorm like electrojet storm time activations. they found that sawtooth events were characterized by a 2-3 hour periodicity, strong stretching of the dusk sector field, and a strongly asymmetric ring current. the level of driving seen during sawtooth events was also found to be similar to that during other types of storm-time activations, the only difference being a weaker auroral (al) response during sawtooth events. partamies et al. (2009) statistically compared sawtooth events with periods of steady magnetospheric convections and isolated substorms. they reported that even though sawtooth events were strongly driven with a strong geomagnetic response, the coupling efficiency was the lowest. solar wind speed also played a part in the type of geomagnetic activity with sawtooth events were triggered during times of elevated geomagnetic activity. given that the passage of ips through the magnetosphere can lead to large increases in geomagnetic activity and that sawtooth events are found to occur during elevated solar wind driving, the similarities and differences between the two event types are investigated. in this paper, shock related substorm onsets and sawtooth events are compared with regards to the solar wind driving and the geomagnetic response. superposed epoch analysis is used to quantify the similarities and differences between the two event groups using multiple in situ and ground based data. the datasets used in the analysis are discussed in section 2. the results report on the superposed epoch analysis of the solar wind parameters in section 3a, the ensuing geomagnetic activity at polar and equatorial latitudes in section 3b, and the auroral boundary movements with the associated magnetic field conditions at geosynchronous orbit in section 3c. the results of the analysis are discussed further with previous literature in section 4 and the conclusions from the study are presented in section 5. 2 data and procedure the solar wind and the subsequent geomagnetic response data at 1 minute (min) cadence, was obtained from http://cdaweb.gsfc.nasa.gov/ [king and papitashvili 2005], during the years 2000 to 2002. in this study, imf bx, by, and bz in nt (in geocentric solar magnetospheric or gsm coordinates), solar wind flow speed (vx in km/s), and plasma ion density, np in particles cm -3 were used. derived parameters such as the flow pressure, p (npa), the imf magnitude, btot=√bx 2+by 2+bz 2 (nt), solar wind electric field (ey in mv/m), and the epsilon parameter (in tw), θ=tan-1(by/bz) being the solar wind clock angle, were calculated from the primary parameters. magnetospheric response was measured through al, au, ae (all in nt), pcn (mv/m), sym, and asym indices (nt). substorm onset timings and locations were derived from the frey et al. [2004] database. sawteeth events were searched for in a list compiled by r. l. mcpherron, who identified the events visually from the particle injections as measured by the los alamos synchronous orbit particle analyzer (sopa) onboard the geostationary 1990-095,1991-080, 1994-084, lanl-97a, lanl-01a, and lanl-02a satelandriyas 2 3 sawtooth and shock related substorm onset comparison lites at multiple magnetic local time sectors [pulkkinen et al. 2007]. the shock parameters were calculated based on high resolution solar wind parameters (as obtained from solar wind electron proton alpha monitor (swepam) instrument [mccomas et al. 1998] and the imf from the magnetic field experiment (mag) [smith et al. 1998] instrument, on board the advanced composition explorer (ace) satellite). the time of the shock interface as measured by the ace satellite was checked with the harvard-smithsonian shock database located at https://www.cfa.harvard.edu/shocks/. rankine-hugonoit equations of conservation were used to determine the speed of the shock along the normal direction [oliveira 2017]. this unit normal vector was calculated based on the abraham-shrauner mixed-mode analysis technique [abraham-shrauner 1972] as given by the following equation: (1) with subscripts 1 and 2 indicating upstream and downstream values and b → and v → representing the solar wind magnetic field and velocity, respectively. shock speed, vs, was calculated using the expression [tsurutani and lin 1985]: (2) n being the plasma density. after calculating the angle θ between the upstream magnetic field and the shock normal, the other parameters such as magnetosonic speed and mach number can be calculated as (3) (4) where va and cs represent the upstream alfvenic and sound speeds, respectively. poleward and equatorward boundary data used to study the oval movement during the onsets was obtained from the british antarctic survey (bas). these boundaries were obtained from the images obtained imager for magnetopause-to-aurora global exploration (image) satellite as described in longden et al. [2010]. single and double gaussians were used to fit to the average intensity profiles in each of the 24 magnetic local time (mlt) bins. non-linear least square fitting yielded parameters that were then used to estimate the equatorward and poleward luminosity boundaries in each mlt sector. the auroral oval boundary locations were further averaged into mlt bins based on the noon, dusk, midnight, dawn sectors. in this study, the noon sector was chosen to be between 1000-1500 mlts, dusk between 1600-2100 mlts, midnight sector between 2000-0300 mlts [sandholt et al. 2014], and dawn sector between 0400-0900 mlts, respectively. additionally, magnetic field at geosynchronous orbit as measured by goes 8 and 10 (active during the years 2000-2002) magnetometers (hp component in the local spacecraft frame), was used to evaluate the amount of magnetotail stretching and relaxation around the substorm onsets [singer et al. 1996]. data from the two satellites were used when they were within the mlts around the midnight sector. 3. results the time at which a shock interface was measured was compared against the substorm onset from the database provided by frey et al. [2004] that occurred in a window of 120 mins after the shock. during the years 2000-2002, 58 unique shocks were observed prior to a substorm onset that could have triggered a substorm. all the shocks measured were found to be fast forward shocks. during the same years, 63 unique sawtooth events had onsets that matched with the frey et al. [2004] onset list. the shocks were either of the fast forward or fast reverse type, with the majority (55) of the onsets triggered by fast forward type shocks. within the shocks, the mean magnetic compression was around 2 while the plasma was compressed by a factor of around 2.3. a mean shock angle (of the magnetic field w.r.t. the shock normal) of 61o indicated that, on average, the shocks were oblique. the onset location (between 50-90o) in the altitude-adjusted corrected geomagnetic coordinates or aagcm [k. b. baker and wing 1989], for both the event classes were plotted. in figure 1, the magnetic latitude and mlt of the onsets associated with shocks and sawtooth events are plotted in blue and red, respectively. as seen in the figure, the onset locations have a preference for premidnight sector. it can be noted that majority (81%) of the sawtooth events occur at low latitudes (below 63o) while only 33% of the shock initiated onsets are triggered at such low latitudes. this should indicate a high solar wind driving associated with sawtooth events as measured by the electrojet indices [grocott et al. 2009]. a superposed epoch analysis was conducted during a time window of 720 mins around the onset with the reported superposed epoch curves being the mean of the individual events, respectively. n̂ = ( → b2 − b1) × ((v2 − v1) × b2) (b2 − b1) × ((v2 − v1) × b2) → → → →→ → → → → vs = n2v2 − n1v1 n2 − n1 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟⋅n̂ → → vms = (va 2 + cs 2) ± (va 2 + cs 2)2 − 4cs 2va 2 cos2 2 mms = vs vms 3.1 mean solar wind conditions superposed epoch analysis is used in this section to study the similarities and differences between the two event types (blue curves for shocks and red for sawtooth events) with regards to the solar wind driving and the geomagnetic response. the mean imf curves resulting from the superposed epoch analysis are shown in figure 2. the values are plotted over an epoch window of 720 mins around the substorm onset (vertical black dashed line) as obtained from the frey et al. [2004] list. the mean imf bx, by, bz, and btot are plotted in blue for shocks and in red line for sawtooth events. the mean bx and by do not exhibit any significant changes during the epoch window with a preference for a zero average value for most of the epoch window, during shock related onsets. however, the mean bx and by during sawtooth events exhibit a parker like sector structure with the imf pointing away from the sun [partamies et al. 2009]. mean bz for the shocks turns southward from a zero average value to around -6 nt around the substorm onset and turns northward around the onset, which may be indicative of magnetotail loading/unloading [d. n. baker et al. 1996]. the total imf strength becomes stronger around 100 mins and strongest 20 mins prior to the onset related to shocks. during sawtooth events, the mean bz remains negative throughout the epoch window (~ -4 nt) and starts to decrease to more negative values 120 mins prior to the onset. it can be seen that there is a reduction in the mean bz values to around -7 nt, 20 mins prior to the substorm onset and a return to less negative values post onset. however, during the sawtooth events, the total field remains much stronger (double) compared to that for the shock related onsets outside a window ±100 mins around the onset. the mean solar wind plasma parameters are plotted in figure 3. the shocks on an average compressed the solar wind by a factor of two, with the density increasing from around 6 particles cm-3 to 13 particles cm-3 around the onset and remained around 10 particles cm-3 through the epoch window after the onset. andriyas 4 0012 06 18 80 70 60 50 figure 1. substorm onset locations associated with the 58 shock and 63 sawtooth events indicated by blue and red filled circles. midnight (00 hrs), dawn (06 hrs), day (12 hrs), and dusk (18 hrs) mlts are indicated along with the magnetic latitude. −2 0 2 epoch [mins] b x shock sawteeth −2 0 2 4 epoch [mins] b y −8 −6 −4 −2 0 2 epoch [mins] b z −300 −200 −100 0 100 200 300 0 5 epoch [mins] b to t figure 2. mean imf components with bx, by, bz, and the magnitude of the imf, btot plotted in blue and red lines for the shocks and sawtooth events, respectively, over an epoch window of 720 minutes around the substorm onset (zero epoch). the vertical and horizontal dashed black lines indicate the zero mean and zero epoch, respectively. 5 sawtooth and shock related substorm onset comparison the mean dynamic pressure, being a function of both the solar wind velocity and density, also increased from around 2 npa to 7 npa around the onset and remained steady at around 6 npa post onset. elevated mean temperature prior to the substorm onset as a result of shock compression can be seen with average temperature increasing from around 1×105 k to around 2.5×105 k around the onset. during the sawtooth events, mean np remained steady at around 6-8 particles cm -3 with a mean dynamic pressure around 3 npa and a mean temperature of around 1×105 k. for the shock related onsets, the mean solar wind velocity, vx, increased across the shock interface before the substorm onset. the mean velocity became faster around the substorm onset from around -410 km/s prior to and -520 km/s after the substorm onset. during the sawtooth events, it increased marginally from -460 km/s to around -480 km/s after the onset. during both the event groups, the epsilon parameter lies below 300 gw, with the shock related onsets experiencing a large increase in the mean epsilon parameter, 90 mins prior to the substorm onset (upper panel in figure 4). as indicated in figure 2, mean imf bz turns southward 90 mins prior to the onset during shock related onsets. as a result, this increased bz contributes to an increased value of the mean epsilon. dur5 10 epoch [mins] n p [ c m − 3 ] 2 4 6 8 epoch [mins] p [ n p a ] 1 2 3 x 10 5 epoch [mins] t [ k ] −300 −200 −100 0 100 200 300 −500 −450 −400 epoch [mins] v x [ k m /s ] figure 3. mean solar wind plasma parameters for the shock (blue) and sawtooth (red) events. mean density (particles cm-3), dynamic pressure (npa), temperature (k), and solar wind velocity (vx km/s) are plotted from top to bottom. 0 0.1 0.2 epoch [mins] ε p a ra m e te r [t w ] −300 −200 −100 0 100 200 300 −2 0 2 4 epoch [mins] e y [ m v /m ] figure 4. mean epsilon parameter (tw) and solar wind electric field (mv/m) in the dawn-dusk direction for the 58 shock (blue) and 63 sawtooth (red) events. ing the sawtooth events, a steady southward imf bz implies that the mean epsilon parameter remains elevated throughout the epoch window. the mean epsilon parameter increases to values around 200 gw close to the substorm onset for the shock related events while it remains between 100-200 gw before increasing to values close to 300 gw around the onset for the sawtooth events. these values are above the substorm loading threshold of 100 gw [akasofu 1981]. however, in both the cases the storm threshold of 1 tw is not exceeded. for the shock related onsets, an increase in both the mean solar wind velocity and more negative bz translated to an increase in the solar wind electric field ey = vx bz), plotted in blue in figure 4. the electric field recovered to preonset values around 80 mins after the onset as seen in the figure. compared to shocks related onsets, the mean electric field during sawtooth events remained elevated throughout the epoch window at values between 1-2 mv/m with a slight increase close to the onset owing to an increase in the solar wind speed and bz becoming more negative. 3.2 magnetospheric response mean geomagnetic activity at high latitudes as measured by the ae, al, au, and pcn indices, are plotted in figure 5. sawteeth events produce a stronger response in the indices the mean ae, al, au, and pcn reaching peak values close to 800 nt, -600 andriyas 6 200 400 600 800 epoch [mins] a e [ n t ] −600 −400 −200 epoch [mins] a l [ n t ] 100 200 300 epoch [mins] a u [ n t ] −300 −200 −100 0 100 200 300 1 2 3 4 epoch [mins] p c n [ m v /m ] −300 −200 −100 0 100 200 300 −100 −50 0 50 100 epoch [mins] m e a n [ n t ] figure 6. mean sym-h (continuous lines) and asym-h (dotted lines) indices during the shock related onsets (indicated in blue) and sawtooth events (indicated in red). figure 5. mean high latitude geomagnetic response as measured by the ae, al, au, and pcn indices (top to bottom) for the shock related (blue) and sawtooth (red) events. 7 nt, 250 nt, and 4 mv/m, when compared to those for the onsets driven by shocks (700 nt, -450 nt, 250 nt, and 3.5 mv/m). expansion phase can be seen to occur for almost the same duration for both the event types (100 mins). a recovery to pre onset values can be seen 120 mins after the onset for both the classes. the mean values of al and as a result ae indices remain elevated during the entire epoch window during the sawtooth events. au index, on the other hand, experiences an almost equal increase after the onset during the two event groups. pcn increases by 1.5 mv/m for the sawtooth events compared to 1 mv/m for the shock related onsets. equatorial activity indices follow the same trend as for the high latitude indices, with much stronger response to the sawtooth related solar wind driving as seen in figure 6. shock related onsets lead to a decrease in the mean sym-h of around -40 nt where during the sawtooth events, the index dips to values close to -70 nt and remains elevated throughout the epoch window. variations in the mean asym-h index are seen to be more quasi-periodic and fairly steady during the sawtooth events compared to the shock related onsets, during which the asym-h experiences a sudden increase from values around 25 nt to 55 nt around the onset as seen in figure 6. higher mean values of asym-h during sawtooth events indicate to a much stronger asymmetric ring current compared to that during shock related onsets. 3.3 auroral oval and field line stretching as indicated in section 2, the auroral oval boundaries were averaged into mlt bins based on the dayside, dusk, midnight, dawn sectors to investigate the differences in auroral movement between the two event groups. figure 7a are plots of the mean equatorward and poleward boundaries in the four sectors, while the plots in figure 7b represent the respective mean oval thickness in the sectors. both the mean boundaries during the sawtooth events are located equatorward of the oval boundaries during shock initiated onsets indicating a stronger stretching of the field lines during sawtooth events. before the substorm onset, an equatorward descent in both the boundaries can be seen in the sectors, with the poleward boundary moving faster than the equatorward boundary in any given sector. a sudden equatorward descent is observed in the mean equatorward boundaries during the shock related onsets, especially in the dawn, dusk, and midnight sectors. due to stronger solar wind forcing during sawtooth events, this equatorward advancement in the mean equatorward boundaries is barely noticeable in the already strongly stretched field lines. the top left panel in figure 7a indicates that in the dusk sector (1600-2100 mlt), the mean equatorward boundary starts to retreat equatorward around 40 mins prior to the onset, during the shock induced onsets (blue continuous line) compared to the sawtooth events, during which the boundary gradually moved equatorwards. the mean boundary recovers to preonset latitudes during sawtooth events but remains at lower latitudes for the shock related onsets. clear preonset equatorward departure and poleward descent after the onset can be seen in the mean poleward boundary for the sawtooth events and is observed to be much stronger compared to shock related onsets. in the noon sector (1000-1500 mlt), both the mean equatorward and poleward boundaries tend to move equatorward at the same rate after the onset, as seen in the top right panel of figure 7a. this trend is similar for both sawtooth events and shock related onsets with the only difference being that the boundaries during the sawtooth events are located equatorward of the mean oval boundaries during the shock related onsets. a similar trend is seen in the mean oval boundaries around the dawn sector (0400-0900 mlt), with equatorward advancement prior to the onset and poleward retreat after the substorm onset. equatorward movement is seen to be much weaker in the equatorward boundary but strong relaxation can be seen in the poleward retreat of the poleward boundary, during the sawtooth events. conversely, the mean equatorward boundary experiences a much stronger equatorward descent in the equatorward boundary and slight variations in the poleward boundary location during the shock related onsets. mean boundaries in the midnight sector (2000-0300 mlt) for the two event groups are plotted in the bottom right panel of figure 7a, which can be seen to have the most dynamics. equatorward movement of the boundaries can be seen prior to the onset caused by the stretching of field lines in the magnetotail. after the onset, the equatorward boundaries gradually move poleward for both the event groups while the poleward departure of the poleward boundaries occurs at a comparatively faster rate. after the onset, the poleward boundary also experienced a large poleward retreat from around 68o to 73o, 30 mins after the onset during sawtooth events while a change from 70oto 73o was observed in 15 mins during shock related onsets. however, during the shock related onsets, the poleward boundary could not recover to its preonset location for the remaining epoch. sawtooth and shock related substorm onset comparison the mean oval thickness (figure 7b) during the sawtooth events is observed to be greater than during the shock related onsets in the dawn, dusk, and midnight sectors. the mean oval around the dusk sector can be seen to be much thicker for the sawtooth events, but the oval around the onset time is more dynamic during the shock related onsets. given that both the respective mean boundaries in both the event groups moved at the same rate in the noon sector, the mean oval thickness was around the same for both the event types with the mean width fluctuating between 6-8o latitude. clear thinning prior to the onset can be seen during the sawtooth events which is absent during the shock related onsets in the dawn sector. rapid widening of the oval can be seen in the dawn sector after the onset compared to the dusk sector, in both the event groups. this thinning prior to the onset and widening post onset for the event groups is attributed due to fast poleward retreat of the mean poleward boundary combined with a relatively gradual poleward retreat of the equatorward boundary. rapid thinning before and widening after the onset can be seen in the oval thickness in the midnight sector during both the event groups. this process is more pronounced for the sawtooth events due to the poleward boundary changing rapidly around the onset (figure 7a) in the midnight sector. in order to study the magnetic field in the midnight sector at geosynchronous orbit during substorm onsets [singer et al. 1996], superposed means for the magnetic field, hp (in the local spacecraft coordinates, measured in nt) were generated for goes 8 (blue and red continuous lines for shocks and sawtooth events, andriyas 8 −200 0 200 60 70 80 epoch [mins] d u s k −200 0 200 65 70 75 80 epoch [mins] n o o n −200 0 200 60 65 70 75 epoch [mins] d a w n −200 0 200 55 60 65 70 75 epoch [mins] m id n ig h t 10 15 epoch [mins] d u s k 6 7 8 9 epoch [mins] n o o n 10 15 epoch [mins] d a w n −300 −200 −100 0 100 200 300 8 10 12 14 16 18 epoch [mins] m id n ig h t figure 7. mean oval boundaries (continuous lines for equatorward locations and dotted lines for poleward locations) during sawtooth events (red) and shock triggered (blue) onsets in figure 7a in the four oval sectors as discussed in the text. figure 7b is the mean oval thickness during the two event groups. (a) (b) 9 respectively) and 10 (blue and red dashed lines for shocks and sawtooth events, respectively). field strength weakening prior to the onset led to the equatorward descent of both the mean boundaries, as noted in figure 7a. mean field values increased after the onset, indicative of a poleward retreat of the boundaries. the mean stretching and relaxation signature is weaker for the onsets triggered by shocks compared to those during sawtooth events. the mean poleward and equatorward boundaries during sawtooth events were found to be equatorward of those found during shock triggered substorms. as seen in figure 8, the mean hp at goes 8 and 10 location was found to be much stronger (dipolar) during shock related onsets as indicated by values between 85 and 95 nt. weak stretching prior to the onset (to values around 80 nt) and relaxation post onset (to values close to 90 nt) was observed in the mean hp during shock related onsets. comparatively, during sawtooth events, the magnetic field was much weaker (or stretched more) prior to the onset with a much stronger relaxation at both goes 8 and 10 locations. this is consistent with the movement of mean poleward boundary during the sawtooth events in the midnight sector (bottom right panel of figure 7a), which retreated poleward to a much greater degree when compared to the onsets triggered by shocks. 4. discussion the response of earth’s magnetosphere to the solar wind driving during substorm onsets related to shocks and sawtooth oscillations is investigated. with almost equal number of events (58 shock related triggers and 63 sawtooth events) in the two groups, a superposed epoch analysis was performed based on merged omni data and goes observations, during 2000-2002. it was found that around 81% of the sawtooth events and 33% of shock driven triggers were at latitudes below 63o latitude. previously, grocott et al. [2009] had reported that onsets at low latitudes lead to intense auroral activity, and a similar observation was made in the case of sawtooth events. superposed epoch analysis was performed on these event groups with onset timing derived from frey et al. [2004], over an epoch window of 720 mins. mean negative values of imf bx and positive values of imf by during sawtooth events indicated to a sector structure with imf pointing away from the sun in the equatorial plane. comparatively, during shock related onsets, the mean imf bx and by did not have any preference for either positive or negative values and averaged to zero over the epoch window. mean imf bz remained strongly negative during sawtooth events (stronger than that during the shock related onsets) throughout the epoch window while it turned southward around a 100 mins before the onset and turned northward 20 mins before the onset. the total field strength was also found to be much stronger for sawtooth events compared to the shock related triggers. for shocks that occurred in a window of 120 mins prior to the onset, the mean velocity jumped from about 400 to 600 km/s. this was accompanied by jumps in particle density (from around 6 to 13 particles cm-3) and a jump in the mean dynamic pressure from 2 to around 7 npa around the onset. the mean solar wind values during sawtooth events were stable around 460 km/s, 7 particles cm-3, and 3 npa during the whole sawtooth and shock related substorm onset comparison −300 −200 −100 0 100 200 300 60 70 80 90 100 s u p e rp o s e d m e a n [ n t ] figure 8. mean goes 8 (continuous lines) and 10 (dashed lines) hp magnetic field during the two event groups. red color indicates the mean values during sawtooth events while plots in blue indicate the mean hp values during shock related onsets. epoch window. the dayside compression was therefore, much stronger during shock triggered onsets. the mean epsilon parameter increased to values around 200 gw around the substorm onset for the shock related events while it remained between 100200 gw before increasing to values close to 300 gw around the onset for the sawtooth events. these values are above the substorm loading threshold of 100 gw [akasofu 1981]. however, in both the cases the storm threshold of 1 tw was not exceeded. mean ey close to 3.5 mv/m around the onset and elevated values during the whole epoch indicated to the sawtooth events being driven strongly. relatively, during the shock related triggers, the mean ey reached peak values close to 2.5 mv/m and did not remain at these values given the nature of mean imf bz which turned northward 20 mins prior to the onset. high latitude indices were also found to be elevated during sawtooth events compared to shock related triggers. given that majority of the sawtooth onsets were triggered at latitudes below 63o, they were associated with a strong electrojet response as is the case with low latitude substorm onsets. mean al index reduced by almost 100 nt more during sawtooth events compared to shock related triggers. although the au index was lower prior to the onset, during shock related triggers, the mean values reached the same levels as found during sawtooth events. this difference in behavior of the al and au indices could be attributed to how au and al indices describe the eastward and westward electrojets [kamide and kokubun 1996, kamide and rostoker 2004]. this was also the case with the pcn index with values during both event groups reaching peak values around 3.5-3.8 mv/m. however, these peak values were reached 20 and 50 mins after the onset during shock related and sawtooth events, respectively. both al and pcn indices indicated to a 2-3 hour (hr) periodicity as also reported in pulkkinen et al. [2007]. sawteeth events occurring usually during geomagnetic storms (dst ≤ -50 nt) [kallio et al. 2000, partamies et al. 2009], can be seen in comparatively large values of the sym-h index. the quasi-periodic fluctuations during sawtooth events in the high latitude indices was also seen as the partial recovery of the sym-h index corresponding to the individual teeth [m. g. henderson et al. 2006]. large quasi-periodic variations, specially prior to the onset were observed in the asym-h index during sawtooth events. the initial decrease and increase in asym-h response to quasiperiodic flux injections at geosynchronous orbit was delayed by around 40 mins compared to that in the sym-h index [pulkkinen et al. 2006]. stronger mean solar wind driving within sawtooth events was also seen in the oval boundary locations and dynamics. the mean auroral boundaries during sawtooth events were found to be located equatorward in all the sectors when compared to the boundary locations during shock triggered substorms. this indicated that the magnetosphere was stretched to a greater degree during sawtooth events (as also seen in the strength of geosynchronous magnetic field measured by the goes satellites) and that the oval was thicker by at least 2-4o in the dawn, dusk, and midnight sectors. the poleward retreat of the mean poleward boundary was much stronger during the sawtooth events in the dawn and midnight sectors. it was observed that the both the mean equatorward and poleward boundaries in the day sector proceeded equatorward at the same rate around the onset and this meant that the the oval thickness during the two event groups was almost the same. equatorward movement of the boundaries seen prior to the onset caused by the stretching of field lines in the magnetotail during the growth phase of a substorm [kozelova and kozelov 2013]. as seen in figure 8, the mean field is much weaker (stretching) during sawtooth events compared to shock related onsets as also indicated in figure 7a, where the oval boundaries, during sawtooth events, were located equatorward of the boundaries during shock related onsets. this is a result of much stronger and persistent southward imf bz during sawtooth events. during sawtooth events, the magnetic field was much weaker (or stretched more) prior to the onset with a much stronger relaxation at both goes 8 and 10 locations. this is consistent with the movement of mean poleward boundary during the sawtooth events in the midnight sector (bottom right panel of figure 7a), which retreated poleward to a much greater degree when compared to the onsets triggered by shocks. 5. conclusions as shocks lead to elevated geomagnetic activity and sawtooth events are found to occur during elevated solar wind driving, a comparative study between sawtooth events and substorm onsets triggered by shocks was carried out. superposed epoch analysis was used to examine the similarities and differences between 58 shock related onsets and 63 sawtooth events during the years 2000-2002. • during both the event types, the mean energy available in the solar wind (as determined by the epsilon parameter) was found to be above the andriyas 10 11 substorm threshold level of 100 gw [akasofu 1981]. during sawtooth events, the mean epsilon parameter was around 300 gw around the onset compared to 200 gw during shock related onsets. the sudden increase in mean epsilon parameter around the onset during shock related events indicates to a result previously found by akasofu and chao [1980], who indicated that a substorm could not only be triggered by a storm sudden commencement but was a direct consequence of increased power of the solar windmagnetosphere dynamo. • both the high latitude indices were elevated during sawtooth events with 2-3 hr fluctuations observed in al and pcn indices. post onset, the au index reached similar levels during sawtooth events and shock initiated onsets. • ring current intensity as measured by the symh was found to be around the storm levels during sawtooth events throughout the epoch window with similar quasi-periodic fluctuations observed in the partial recovery of the index. on average, the asym-h response to quasi-periodic flux injections at geosynchronous orbit was delayed by around 40 mins compared to fluctuations in the sym-h index. • in both the event groups, the latitudinal thickness of the oval in the noon sector did not show any significant variation during the epoch window due to both the boundaries moving equatorward before the onset and poleward after the onset. it was also seen that the oval thickness was around the same during both the event groups. the dawn sector experienced a clear thinning and widening of the oval around the onset during both sawtooth events and shock related onsets. this sequence was however, not observed in the dusk sector and could point to a stronger substorm activity in the dawn mlts compared to dusk mlts [mende et al. 2003]. • sawteeth events were 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(2001). interplanetary shock triggering of nightside geomagnetic activity: substorms, pseudobreakups, and quiescent events. journal of geophysical research: space physics, 106(a9), 18957-18967. h t t p s : / / d o i . o r g / 1 0 . 1 0 2 9 / 2 0 0 0 j a 0 0 3 0 2 8 doi:10.1002/2013eo450001. *corresponding author: tushar andriyas, nehru science center, university of allahabad, india; email: tushar.andriyas@aggiemail.usu.edu © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sawtooth and shock related substorm onset comparison microsoft word 7441-17923-1-rv_williams-et accepted.doc annals of geophysics, 60, fast track 7, 2017; doi :10.4401/ag-7441 1 the role for a large scientific society in addressing harassment and work climate issues billy m. williams christine mcentee brooks hanson randy townsend american geophysical union bwilliams@agu.org abstract the american geophysical union, a scientific society of some 60,000 members worldwide, established in 2011 a set of scientific integrity and professional ethics guidelines to address the actions of its members, the governance of the union in its internal activities, and the operations and participation in its publications, scientific meetings, including its honors and awards programs. however, these guidelines, like those for other societies, did not explicitly address harassment, related workplace issues, or implicit bias in the practice of science. the recent exposure of several issues related to the sciences caused agu to reexamine its role and obligations. in a twelve-month period starting october 2015, more than six high profile cases of harassment in the sciences were publicly disclosed in the u.s., including one case that was brought to the attention of the u.s. congress. a growing body of literature on harassment and other biases in the sciences, including data extracted and reported by agu, has shed new light on this topic. as a result of this assessment, agu now actively helps educate its members on ethics issues in various ways, both broadly as well as specifically around items related to gender bias, harassment and other equity and inclusion topics. new and renewed members are specifically made aware of agu’s ethics policies, and additional efforts are increasingly embedded across publications, awards, meetings, talent pool and other agu member-focused programs. in addition, agu is starting a program focused on addressing harassment and related work-climate matters in the earth and space science community, with an emphasis on the growing need and proposed active roles for scientific societies. 1. introduction: the historic context around harassment and case for change ecent high-profile events in the news have raised awareness of the problem of harassment (including sexual harassment) in science and academia (scoles, 2016). this professional misconduct often preferentially targets women, although men can also be victims. research confirms the extent of harassment in academic environments and especially in disciplines with low diversity, where the lack of established support networks can lead to feelings of vulnerability and professional insecurity. another problem identified by research on harassment is the scarcity of well-defined resources for reporting and responding to inappropriate behavior, including the perceived risk that the victims’ careers may be jeopardized if they speak out (clancy et al., 2014). these events have highlighted the need for support mechanisms for the targets of inappropriate behavior, as well as the need for a suitable institutional response to deter continued misconduct. outright harassment is often part of a broader spectrum of closely-related issues impacting the scientific community, inr annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7441 2 cluding discriminatory practices including conscious and unconscious bias. collectively, harassment, bullying, discrimination, and bias exploit differences in gender identity and presentation, religion, race, ethnic origin, class, ability, citizenship, and sexual orientation. these practices are known to endanger the personal, professional, physical, and emotional well-being of individuals and their communities. such actions factor into decisions to leave science and academia and contributes to under representation in earth and space sciences (ess). bias, even if not perceived, can and does inhibit career evolution and can limit scientific understanding and advancement, which depends on diverse perspectives around problems (rosen, 2017). strong policies, practices and education are important steps in promoting a cultural change where ethics related issues of harassment, bullying and discrimination, and other bias – both explicit and implicit are not only recognized as unacceptable but must be stopped and prevented (st. john et.al., 2016). 2. agu and earth and space science demographics this public surge in harassment in science cases has prompted agu, the american astronomical society (aas), the american geophysical institute (agi), and several agi member societies to reexamine their policies and the role for professional scientific societies in addressing harassment and other work climate issues. the demographics of agu has evolved over the past several years to yield an organization that is more rich in geographic and gender diversity. whereas women represented only 15% of agu in 1975, in 2016, nearly 27% of agu membership were female. agu members under the age of 30 were nearly 50% women. data compiled and reported by agi in 2016 gives a profile of the lower participation rate of women in ess (wilson, 2016). historically, expanding the participation of females and underrepresented groups has presented unique and compounded challenges in ess. in the u.s. women made up 39% of u.s. bachelor’s degrees in the earth, atmospheric, and ocean sciences in 2011, but only 20% of faculty in these fields. of these, black, hispanic, american indian, alaska native, and asian pacific islander women represented only 5% and 7% of bachelor’s degrees and tenure-track faculty, respectively (holmes et al., 2015). the geoscience workforce has a lower proportion of women compared to the general population of the u.s. and compared to that in many other stem fields (science, technology, engineering and mathematics). additional dynamics in ess – such as field research on ships, in remote field stations, and in other isolated environment, and the power structures traditionally involved in these settings, amplify the issues associated with the both the climate for and impact of harassment. this combination of factors in ess is especially troubling. an environment or culture of acceptance of harassment, even if experienced directly by only some, contribute to broader marginalization of individuals within affected groups, and factors into their decisions to leave science and academia or even not to enter a field (glass, 2015). 4. new policy to address harassment the agu scientific integrity and professional ethics policy is a set of principles and practices for professional behavior that governs all agu members, staff, and volunteers, formal agu affiliates, and agu third-party contractual organizations, exhibitors, and sponsors. it was significantly expanded and updated in 2011 and 2012 following several incidents of professional misconduct (gundersen and townsend, 2014). in 2014, members were required to acknowledge the policy as part of joining the society or renewing their membership. before 2015, and the surge of harassment in the science cases, the policy focused primarily on traditional education and enforcement of ethics practices related to scientific publication. for example, during the 3-year period from january 2013, the agu ethics program addressed an average of seven cases each year on a confidential basis. all but one of these cases focused on issues such as authorship, plagiarism, or unauthorized use of data. there were no direct provisions for addressing harassment or misconduct, other than the code of conduct for agu volunteer leaders. annals of geophysics, 60, fast track 7, 2017; doi :10.4401/ag-7441 3 following an agu town hall session in december 2015 forward focused ethics: what is the role of scientific societies in responding to harassment and other workplace climate issues? and extensive discussion at the april 2016 agu board and council meetings, a task force appointed by the agu president in june 2016 to begin an update to the agu ethics policy. the task force was charged with reviewing and making recommendations for new agu ethics policy and practices, with a focus on addressing the issue of harassment and work-climate issues and the needs of agu members. this task force has now completed its work and recommended an updated policy and practices that set new standards for member and scientific workplace expectations, and new resources to support these changes (mcphaden et.al., 2017). this extensive policy update applies to all agu sponsored programs and activities. the updated policy expands the boundaries for the agu ethics program, and for the first time allows coverage of misconduct that takes place outside of agu programs when it impacts an agu member, or could harm the reputation of the organization or the broader scientific community. key provisions of this updated policy include: • an expanded definition of scientific misconduct to include code-of-conduct towards others. • definitions of discrimination, harassment, sexual harassment and bullying as it applies to code-of-conduct towards others. • a higher standard for agu volunteer leader code of conduct. • the extension of agu ethics policy to cover participants in all agu program activities, including honors and awards, and agu governance. • self-reporting requirements for awardees and candidates for agu elected position. • ethical guidelines for publication of scientific research. • a clearly detailed process for reporting and investigating scientific misconduct. • support mechanisms for issues that may not rise to the level of a formal ethics complaint. the revised ethics policy also includes agu leadership affirmations of the international principles that the free, open, and responsible practice of science is fundamental to scientific advancement and human and environmental well-being (icsu, 2014). beyond these agu public statements, specific studies and practices have also been undertaken across agu major programs agu publications, honors and awards, meetings, and talent pool, to help support to achieve the necessary cultural change. in these programs, the focus includes eliminating explicit and implicit bias, the assurance of better and fair recognition for women and underrepresented groups, and in providing the necessary education, training, and resources to impact these issues. at agu, these activities as being linked under the umbrella of ethics, diversity, and inclusion to achieve the broader necessary results. 5. examining bias in publication and data publications are one of main way that scientists receive recognition and career advancement. to help understand potential bias through the peer-review and publication process, and draw attention to the issues, agu conducted a study that looked at both gender and age of authors and reviewers across recent submission and publications (lerback and hanson, 2017). accounting for age is critical to understand potential bias. the analyses showed that women submitted fewer papers on average than men, but were more successful at having these accepted. this result that women submit fewer papers is broadly similar to a larger study across other disciplines (elsevier, 2017). in addition, the study showed that women across all age groups are used as reviewers less than their male counterparts. this is a result of both fewer suggestions by particularly male authors and fewer invitations by particularly male editors, and higher decline rates by women in each age group. as a result, agu is expanding efforts to increase diversity of the journal editoannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7441 4 rial teams, and will be providing explicit reminders to authors and editors about the value of diversity in peer review. 6. examining bias in honors and awards an additional aspect of professional advancement is measured through the accumulation of individual scientific honors and awards. an examination of agu awards data from 20122016 has been undertaken to address the question of whether bias conscious or unconscious is at play (holmes et al., 2017). this examination demonstrates important trends and opportunities. the agu honors program involves receipt of 500-600 nomination packages each year in the selection of awardees for 28 different union wide awards, including the prestigious agu fellows selection process where no more than 0.1% of membership are elected as fellows each year. a promotional campaign to help assure that women scientists are nominated, and educational resources on addressing unconscious bias have been provided to selection committees for each of the past four years. the results from these efforts and the added awareness of issues surrounding diversity and inclusion, are now bearing positive results. for example, examination of agu awards data from 2013-2016 show gender diversity in agu honors and awards is approaching parity with the overall membership demographic. in 2016, the total % of women awarded with union level awards, honors or prizes total 30% of the total awardees. this is a departure from the historically low representation in such activities, where as recently as 2014 the percentage of women awardees was 15% versus the 27% of women in the agu membership population. similar trends are shown for agu fellows selection. the percentage of women scientists currently represented in the total population of nearly 1400 living agu fellows in 10%. from 2007-2012, the percentage of women elected to agu fellowship averaged 12%. however, over the past four years, from 2013 – 2016, the percentage of women elected agu fellow averages 18-20%, representing a step change. whereas additional work and emphasis is still underway, we believe these positive trends are the direct results of promoting diverse nominations, and providing tools and workshops on addressing unconscious bias (mukasa, 2017). 7. addressing harassment at meetings and the safe agu program a recent internet-based survey of meeting attendees sheds new light on the incident rate of harassment at scientific meetings and its impact (marts, 2017). the report highlights harassment as one of several ways that women and other underrepresented minorities encounter conscious and unconscious bias at meetings. although the sample size for the survey was relatively small (221 responses), the survey results point out that harassment at meetings included comments on appearance and other forms of verbal harassment, unwanted touching, stalking, and sexual assault. targets of harassment reported that the experience caused them to think more about their personal safety at meetings, to avoid social events, and to stop attending meetings where harassment occurred. similar stories reported at the agu 2015 town hall and sessions at the 2016 agu annual meeting amplify and are consistent with these results. in addition to the already established agu meetings code of conduct, a recent new program, safeagu, was introduced at the december 2016 agu annual meeting. safeagu was designed to promote awareness of harassment issues, and to ensure that all agu program activities are free from discrimination, bias, or harassment of any type. safeagu offers support to members who may feel harassed, threatened, or unsafe in any way when participating in agu meetings. it also provides additional support and resources related to safe work environments. individuals with concerns or requests for assistance on a harassment or other safety/security issue, including situations that may not rise to an ethics complaint, are provided support on a confidential basis. this program, coupled with the recently updated agu ethics policy, provides a direct address in response to addressing harassment and related work-climate issues. annals of geophysics, 60, fast track 7, 2017; doi :10.4401/ag-7441 5 8. conclusions: the path forward solving the problem of harassment in the sciences including the related ethical issues of diversity and inclusion, involves several critical and interwoven processes on multiple fronts (gundersen, 2017). there are distinct roles for professional societies in (1) facilitating public conversation on the issues involved, (2) providing training and education to help combat the problems, (3) codifying and enforcing policies that help stamp out these problems, and (4) in providing research to better understand the issues involved and support for potential victims. agu is committed to the above strategies that when taken together are designed to build and support a diverse and inclusive scientific community. included in these commitments is an obligation to assure its members facing scientific professional structures such as publications, meetings, and honors and awards are routinely examined to guarantee they reflect the desired diversity, and take steps to avoid unconscious bias. we believe that coupling the above strategies can lead to transformative results. continued attention is needed, but impacts are already being seen. the agu is not alone in its attempt to address the issue of harassment in science. recent keynote statements at an international geoscience conference on behalf of the international association for promoting geoethics provides additional reinforcement: “… a respectful and fruitful working environment is fundamental for maintaining a high level of professionalism and for assuring an ethical conduct while practicing geosciences. therefore, any kind of harassment and discrimination cannot be tolerated and must be denounced. harassment and discrimination offend the dignity of the person, threaten the serenity of the working environment, limit the individual’s freedom of choice, and seriously undermine integrity, quality, and credibility of the geoscience community. these kinds of behavior prevent individuals from taking ethical decisions ...” (peppoloni, 2016). references clancy k.b.h., nelson r.g., rutherford j.n. and hinde k. 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(2016). status of the geoscience workforce, american geoscience institute agi: https://www.monroecc.edu/fileadmin/sit efiles/generalcontent/depts/geology/doc uments/statusgeoscienceworkforce2016.p df, accessed 26 october 2017. vol49_2_2006 569 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words ocean observatory – science requirements 1. introduction since the 1800’s, oceanographers have explored and sampled across two-thirds of earth primarily using ships as observational platforms. this has yielded a series of snapshot views of the oceans which have limited resolution in time. measurements and models from this exploratory, mapping and sampling, phase of oceanography have resulted in growing recognition of the diversity and complexity of processes that operate above, within and beneath the oceans. the questions posed from these efforts increasingly cannot be answered using only the tools of the present, in large part because of a limited ability to resolve temporal change. for this and other reasons, the ocean sciences are beginning a new phase in which scientists will enter the ocean environment and adaptively observe the earth-ocean system. routine, long term access to episodic oceanic processes is crucial to continued growth in the understanding and predictive modeling of complex natural phenomena that are highly variable, spanning many scales of space and time. this new ocean sciences paradigm will be implemented using innovative facilities called ocean observatories which provide unprecedentscience requirements and the design of cabled ocean observatories alan d. chave (1), gene massion (2) and hitoshi mikada (3) (1) deep submergence laboratory, woods hole oceanographic institution, woods hole, ma, u.s.a. (2) monterey bay aquarium research institute, moss landing, ca, u.s.a. (3) department of civil and earth resources engineering, kyoto university, japan abstract the ocean sciences are beginning a new phase in which scientists will enter the ocean environment and adaptively observe the earth-ocean system through remote control of sensors and sensor platforms. this new ocean science paradigm will be implemented using innovative facilities called ocean observatories which provide unprecedented levels of power and communication to access and manipulate real-time sensor networks deployed within many different environments in the ocean basins. most of the principal design drivers for ocean observatories differ from those for commercial submarine telecommunications systems. first, ocean observatories require data to be input and output at one or more seafloor nodes rather than at a few land terminuses. second, ocean observatories must distribute a lot of power to the seafloor at variable and fluctuating rates. third, the seafloor infrastructure for an ocean observatory inherently requires that the wet plant be expandable and reconfigurable. finally, because the wet communications and power infrastructure is comparatively complex, ocean observatory infrastructure must be designed for low life cycle cost rather than zero maintenance. the origin of these differences may be understood by taking a systems engineering approach to ocean observatory design through examining the requirements derived from science and then going through the process of iterative refinement to yield conceptual and physical designs. this is illustrated using the neptune regional cabled observatory power and data communications sub-systems. mailing address: dr. alan d. chave, deep submergence laboratory, woods hole oceanographic institution, woods hole, ma 02543, u.s.a.; e-mail: alan@whoi.edu 570 alan d. chave, gene massion and hitoshi mikada ed levels of power and communication to access and manipulate real-time sensor networks deployed within many different environments in the ocean basins. these facilities, their real time or near-real time information flow, and the data archives associated with them, will empower entirely new approaches to science. in addition, ocean observatories will enable educational-outreach capabilities that can dramatically impact general understanding of, and public attitudes toward, the ocean sciences and science in general. the crucial role ocean observatories will play in 21st century oceanography has received international recognition, and early programs are underway around the world (e.g., beranzoli et al., 1998; momma et al., 1998; delaney et al., 2000; glenn et al., 2000; kasahara et al., 2000; austin et al., 2002; chave et al., 2002; favali et al., 2002; hirata et al., 2002; schofield et al., 2002; petitt et al., 2002; beranzoli et al., 2003; dewey and tunnicliffe, 2003). major ocean observatory infrastructure programs are under consideration in japan (advanced real-time earth monitoring network in the area or arena; shirasaki et al., 2003), europe (the european seafloor observatory network or esonet; http://www.abdn.ac.uk/eco-system/esonet/), and the united states (ocean observatories initiative or ooi; clark and isern, 2003). the largest component of the ooi is a us-canada regional cabled observatory called neptune (northeast pacific time-integrated undersea networked experiment) for which the canadians received c$62m in late 2003. almost all of the cited installations use submarine cables to link land to seafloor, and hence the remainder of this paper will focus on cabled ocean observatories. 2. a generic cabled ocean observatory in an attempt to establish common terminology, a generic cabled ocean observatory structure will be defined. a comprehensive description of the hardware design and implementation of cabled ocean observatories may be found in chave et al. (2004). a software or cyberinfrastructure framework for ocean observatories is described by st. arnaud et al. (2004). figure 1 shows one or more sensors deployed in the water. a suite of sensors is the fundamental measurement device at an observatory, and is ultimately the source of a data stream. sensors are part of, or attached to, instruments. instruments are attached to instrument ports on an observatory node located on the seafloor via an access layer data communications connection (typically, rs232/rs422 serial or10/100baset ethernet). the observatory node also supplies power (typically, 12 or 48 vdc) and may distribute accurate time using standard codes. custom instrument ports may support special instruments with fig. 1. cartoon illustrating the major components of a cabled ocean observatory. see text for discussion. core and pi (principal investigator) instruments represent a preliminary classification of sensors that will be refined in the process of system development. 571 science requirements and the design of cabled ocean observatories unique needs. standard instrument ports are more generic. ocean observatory nodes are connected to each other and to a shore station via a core layer data communications link (typically, high speed serial or gigabit ethernet via a backbone submarine fiber optic cable). the backbone cable also contains an electrical conductor, and provides power to the observatory nodes from shore with a seawater return path. an ocean observatory operations center monitors, maintains, controls, and manages the components of the obsevatory. an observatory instrument control process is used by operators or guest scientists to control node instrument ports, instruments, and sensors. core instruments are managed by the ocean observatory operator or their designees. individual investigator instruments may be one of a kind, and are deployed on behalf of a guest scientist. the instrument data logging process gathers realtime or near real-time data (and sometimes instrument metadata) from instruments and stores it temporarily. this step may happen in the water, on shore, or both. the data archive process gathers/receives data and/or metadata from the instrument logging process or in some cases directly from the instrument itself. the data archive process may extract instrument metadata from the data stream, post-process the data stream, or manage it in some other way. in some ocean observatories, a streaming data process provides subscribers with real-time data from sensors/instruments. in most cases, the ocean observatory is connected to the internet, and an observatory server provides scientists and the public with access to certain observatory services. 3. ocean observatory design requirements the commercial marketplace has driven the design of submarine fiber optic telecommunication systems to very high data rates using a dense wavelength division multiplexed (dwdm) physical layer implemented using comparatively simple, very high reliability submarine equipment. electronic complexity is concentrated at a very small number of shore stations. combined with advances in submarine cable installation and burial, the result is extremely reliable communications infrastructure. many of the principal design drivers for ocean observatories differ from those for conventional submarine telecommunications systems. first, ocean observatories require data to be input and output (i.e., switched and aggregated) at one or more seafloor nodes rather than at a few land terminuses. second, ocean observatories must distribute a lot of power (typically, multiple kw per node) to the seafloor at variable and fluctuating rates to supply both seafloor instruments and the observatory hotel load. third, science requires the delivery of accurate (typically, order 1 µs in an absolute sense) time to seafloor instruments which has no counterpart in the commercial world. fourth, the seafloor infrastructure for an ocean observatory is inherently dynamic, and hence the wet plant has to be expandable and reconfigurable to meet changing science needs. finally, because the wet communications and power infrastructure is comparatively complex, ocean observatory infrastructure must be designed for low cost maintenance and upgradeability. despite these differences, a key design driver in both ocean observatory and commercial telecommunications system design is reliability. a primary reliability measure is the probability that data will be received on shore or at another seafloor instrument or node from a given science instrument on the seafloor, and the least reliable infrastructure components in this path inevitably are the node power and communications electronics. as further discussed by chave et al. (2004), an immediate corollary is that there may be no reliability gain from combining high cost, high reliability submarine telecommunications wet plant with lower reliability node electronic systems. taken together, these points suggest that the overall design of ocean observatories will be fundamentally different from that of submarine telecommunications systems. understanding why this is true is a principal purpose of this paper, and is facilitated by taking a system engineering view of the ocean observatory design process. however, although design requirements differ between telecommunications and science systems, the high quality submarine fiber optic cables, joints, terminations, and wet hardware, as well as the installation and burial expertise of industry, are 572 alan d. chave, gene massion and hitoshi mikada highly relevant to ocean observatory implementation. 4. the system engineering process a system is defined as a collection of functional elements which work together to perform some defined set of functions. it consists of a hierarchy of sub-systems which taken in aggregate comprise the whole system. the observatory node in fig. 1 is one sub-system in the ocean observatory system, and may in turn be divided into a power sub-system, a backbone communications sub-system, an instrument interface sub-system, and so on. system engineering is a management and engineering process which brings a system into being. its development as an engineering discipline grew out of the need to manage and integrate increasingly complex technological projects. recent texts on the subject include eisner (1997), blanchard (1998), and stevens et al. (1998). neither the system engineering process nor its procedures are uniquely defined, but one common implementation is contained in military standard 499b (1991). the main stages of this standard as outlined here are typically included in any well-designed system engineering implementation. figure 2 is a top-level view of the mil std 499b system engineering process. it contains three main stages: 1) requirements analysis; 2) functional analysis/allocation; 3) synthesis; and 4) a continuous, cross-cutting system analysis and control phase. the requirements analysis stage focusses on two types of requirements: user and performance. in the context of an ocean observatory, the user requirements flow from science needs, and are identified through the analysis of a wide range of use scenarios incorporating representative suites of sensors and platforms. the goal is to identify and define the set of functions that the system must do to meet the user requirements, and to place bounds on how well these functions must be carried out. these are called the functional and performance requirements, respectively. in the sequel, these will collectively be referred to as the science requirements. the second system engineering stage is functional analysis/allocation, in which the objectives defined by the science requirements are decomposed into a set of lower-level functions and performance allocations are applied to each of them. the functional interfaces and a candidate functional architecture are defined. this process is iterative, and constant verification that the science requirements are being met must be achieved. the third system engineering stage is synthesis, in which all elements of the functional design are transformed into a physical design. synthesis begins at a concept design level, then passes on to a preliminary design level where risk is mitigated by testing key parts of the hardware and software, and then passes to a detailed design level where full prototypes are constructed. this stage is also iterative, both with the preceding functional analysis/allocation stage to ensure that the required functionality is being provided and with the requirements analysis phase to verify that the science requirements are being met. the cross-cutting phase of the system engineering process is system analysis and control. fig. 2. cartoon illustrating the major stages and phases of the military standard 499b system engineering process. see text for discussion. 573 science requirements and the design of cabled ocean observatories this consists of a set of trade-off studies comparing the feasibility, performance, and cost of alternative technical approaches along with a set of over-arching tasks to manage risk, configuration, and interfaces. ongoing documentation and technical reviews are also a system analysis and control function. the focus in the remainder of this paper will primarily be on the requirements analysis phase and how this leads to functional and physical implementations of an ocean observatory system. as an illustration of the refinement process using trade studies, two selected elements will be examined in detail. 5. science requirements for a cabled ocean observatory in the sequel, selected science requirements for the neptune regional cabled observatory (rco) will illustrate the system engineering process used to derive them. neptune is conceived as a multi-node regional observatory comprising 26 seafloor science nodes with two shore connections covering the juan de fuca plate off northwest north america (delaney et al., 2000). figure 3 presents a notional layout for neptune. the neptune science requirements are broad and generic, and could easily apply to othfig. 3. map illustrating the notional topology of the neptune regional cabled observatory. the letter labels delineate specific nodes, while the numerical labels give the optical link lengths in km. 574 alan d. chave, gene massion and hitoshi mikada er installations such as the planned arena system in japan or components of esonet in europe. in the feasibility study for neptune (neptune consortium, 2000), an initial survey of the range of science that could be accomplished with an rco was carried out, including the development of use scenarios and an assessment of the characteristics of instruments that might be installed. this serves as initial input to definition of the science requirements. a similar procedure was used to define the science requirements for geostar, as documented in thiel et al. (1994). as should be clear from fig. 2, deriving the science requirements is an iterative process. at the outset, functional and performance requirements may be posed by the user community that cannot be met with available technology, or even that may not be consistent with known physics. the inner requirements and design loops combined with the outer verification loop (fig. 2) serve to improve the initial requirements and eliminate such problems. the science requirements presented here are the product of several such stages of iterative refinement, but are still considered to be in draft form until the synthesis stage passes from the concept through the detailed design levels. the science requirements to be discussed are divided into general, power network, and data communications network categories, and are the most mature ones for neptune. there are numerous other categories in the rco, including time distribution, observatory control, science instrument interface, user/observatory interaction, data management and archiving, security, operations, and reliability which will not be discussed. 5.1. design principles the design principles are overarching requirements which apply to all parts of the rco, and serve as guiding principles through the entire design process. there are ten design principles for the rco, each of which is denoted by a keyword. the first and second design principles establish life and cost goals for the rco. the latter is important, as virtually all ocean observatory installations will be cost-capped. life cycle cost is defined as the sum of expenditures for research, development, test, and evaluation (rdt&e), procurement and installation, and operations and maintenance (o&m) over the design life of the system. a.1. lifetime – the rco shall operate, with appropriate maintenance, for a design life of at least 25 years. a.2. cost – the rco shall be designed to minimize the life cycle cost. the third through sixth design principles state that all components of the rco have to be capable of reconfiguration or extension to adapt to a changing mission after installation, and that the basic infrastructure has to be designed to be flexible and able to accommodate changes in technology over time. static designs would not suffice, and technologies which inherently limit the number of nodes that can be implemented over a given part of the rco would not be appropriate. a.3. reconfigurability – the rco shall allow all resources to be dynamically-directed where science needs and priorities dictate. a.4. scalability – the rco shall be expandable, so that additional science nodes which meet the observatory reliability goals can be placed near or at locations of interest that may develop in the future. a.5. extendability – the rco shall support individual instruments or clusters of instruments at sites up to 100 km away from the science nodes with possibly reduced power and communications capability and reliability. a.6. upgradeability – the rco shall be upgradeable to accommodate future technology improvements. the seventh and eighth design principles establish reliability and fault tolerance goals for the rco. these are quantified later in the science requirements, and are intended to establish the manner in which they will be derived. the reliability design principle states that the integral probability of data transmission from seafloor node to shore (or another seafloor node) will be the principal measure of rco reliability. a design which is highly reliable in some parts of the system but offers no improvement 575 science requirements and the design of cabled ocean observatories in the total probability of instrument connectivity is not inherently superior. other criteria, such as life cycle cost, must be applied to compare designs with similar reliability. a.7. robustness – the rco shall utilize fault tolerant design principles for both hardware and software to minimize potential single points of failure, a.8. reliability – the primary measure of rco reliability shall be the probability of being able to send data from any science instrument to shore and/or to other science nodes, exclusive of instrument functionality. the ninth design principle specifies that all components of the rco have to be designed with a forward-looking rather than a status quo perspective in order to meet future science needs. a.9. futurecasting – the rco shall have functionality and performance significantly beyond that required to support current use scenarios so that experiments and instruments that may reasonably be anticipated to develop over the expected life of the facility can be accommodated. the final design principle states that all of the designs for hardware and software elements of the rco shall be public insofar as possible. an ocean observatory should be viewed as an end-to-end scientific instrument, and hence black box elements between instrument and user are not appropriate. a.10. open design principle – the rco hardware designs and specifications shall be freely and openly available, and all software elements shall be based on open standards to the greatest extent possible. 5.2. power network the power network design requirements are specific to the distribution of power to supply both the rco infrastructure and seafloor sensors. where specific numbers appear, these represent requirement and design loop refinement based in particular on the voltage and resistance rating of standard submarine fiber optic cable. the premise is that the design of custom cable would be prohibitively expensive, and the additional weight from adding copper to reduce the intrinsic resistance per unit length of the cable would lead to installation and handling problems. the first and second requirements specify respectively that primary requirements are maximizing the average and peak power to all nodes simultaneously and maintaining the flexibility to direct more power to a small number of selected nodes. designs which either inherently favor some nodes over others or lack flexibility to allocate power would not be appropriate. b.1. the power network shall provide the maximum average continuous user power equally to all nodes, with a goal of 5 kw. b.2. the power network shall provide the maximum peak user power to as many nodes as possible, with a goal of 10 kw. the third requirement states that the power system will always start up in a predictable manner. b.3. the power network shall be in a known state upon power up. the fourth and fifth requirements specify that minimizing the effect of faults (either physical faults on the backbone cable or power system problems) on science and localizing them when they occur are important science needs. b.4. the power network shall be able to detect and localize infrastructure problems, including (but not necessarily limited to) shunt faults or breaks of the backbone cable, high resistance faults, and node power system failures. b.5. the power network shall isolate failed sections of backbone cable or failed nodes such that the remaining operational nodes can function normally. the sixth and seventh science requirements state that monitoring and allocating power at individual science nodes is a critical need. b.6. the power network shall provide voltage and current monitoring functionality adequate to detect and localize infrastructure and user instrument problems, including (but not necessarily limited to) over-current faults in user and infrastructure loads and ground faults. b.7. the power network shall allow power delivery to all nodes and science ports to be scheduled and prioritized. the final design requirement serves to protect both the infrastructure and attached science instruments from damage due to unwanted cur576 alan d. chave, gene massion and hitoshi mikada rent flow through housings and connectors. an additional need to monitor instruments and connectors for ground faults appears elsewhere in the science requirements document. b.8. all pressure cases, including support frames or assemblies electrically connected to them, shall be dc isolated from all signal and power circuits in the rco. additional environmental requirements lead to detailed specifications for power supply ripple, total harmonic distortion, and the like during the design cycle. 5.3. data communications network the data communication network science requirements are specific to the backbone data transport for an rco. specifications for the link between instruments and the infrastructure are stated elsewhere. where specific numbers appear, these represent requirement and design loop refinement based on forward-looking estimates of bandwidth and instrument needs. from use scenarios and investigation of the communications requirements of a wide range of instruments, a preliminary estimate of both the aggregate bandwidth of an rco and the maximum data rate for an individual instrument can be derived. the aggregate bandwidth can then be scaled by a large factor (10 or more) to allow for future growth and technological development. one significant unknown is the maximum instrument data rate; except for high density television (hdtv) operating in an uncompressed mode at a data rate in excess of 1 gb/s, it is difficult to identify anything which requires more than a few times 10 mb/s. since it is unlikely that extensive use of uncompressed hdtv will be required, the total data rate does not reflect this number. the alternative is to scale the bandwidth upward in c.1, significantly raising system costs. note that c.2 states the data rate that must be delivered to the user at all times in the rco life cycle to allow for aging effects. c.1. the data communication network shall support an aggregate backbone data rate of at least 8 gb/s. c.2. each data communication node subsystem shall support an aggregate instrument data rate of at least 1 gb/s exclusive of overhead for system functions such as (but not necessarily limited to) framing or re-transmission due to errors, at all times during the observatory life cycle. the third requirement states that the data communications system will always start up in a predictable manner c.3. the data communication network shall be in a known state upon power up. the fourth science requirements state that the data communications system needs to be automatically fault-tolerant. most standard data communications systems meet this requirement automatically. c.4. each data communications node subsystem shall automatically reconfigure itself to suppress fault propagation, and shall automatically recover from faults. the fifth and sixth science requirements specify a need for remote monitoring and control of the data communications infrastructure. c.5 is met automatically by most modern data transport protocols. c.6 anticipates the standard use of backdoor access into communications hardware, and flows from reliability requirements and the remote deployment of complex hardware on the seafloor. c.5. each data communication node subsystem shall be monitored and controlled over the data communication network using standard protocols. c.6. each data communication node subsystem shall be monitored and controlled over a high reliability, auxiliary channel. the seventh science requirement states that the data communications network must be dynamically changeable to accommodate changing science needs. c.7. the data communication network shall be remotely re-configurable so that data transmission to and from each node can be scheduled and prioritized. the eighth science requirement specifies instrument data rates that must be supported. these are stated in more detail in the instrument interface specifications. c.8. each data communication node subsystem shall support a maximum data rate from each instrument of 100 mb/s, and for at least one instrument or secondary node at 1 gb/s, us577 science requirements and the design of cabled ocean observatories ing standard internet protocols, including (but not necessarily limited to) tcp/ip and standard application layer protocols. the final two data communications science requirements are derived from future-casting the current and emerging state-of-the-art in sensor network development in accordance with a.9. while the initial expectation for neptune was that communications would only occur between numerous seafloor instruments and a few land sites, recent developments in sensor networks suggest that this is very likely to change in the future. the design of distributed, intelligent, selforganizing sensor networks (e.g., «smart dust», «sensor webs») based on low cost, miniaturized (i.e. mems technology) sensors is an exciting and rapidly evolving area of research, and it is reasonable to expect that this will port to the seafloor in the not very distant future. implementation requires minimum latency inter-node and inter-sensor communications paths on the seafloor in addition to links to land. c.9. the data communication network shall facilitate intra-node, peer-to-peer communication by user instruments with the minimum possible latency commensurate with direct inter-instrument propagation delay. c.10. the data communication network shall facilitate inter-node peer-to-peer communication by user instruments with the minimum possible latency commensurate with direct inter-node propagation delay. 6. implementation and trade-study refinement the iterative refinement of the science requirements and concomitant development of the functional and physical designs is complex and multi-faceted, and it is nearly impossible to describe the entire process. instead, two illustrative examples will be given for the power and data communications systems, respectively. 6.1. parallel versus serial power system the technical issues surrounding the choice of serial or parallel power for neptune are described in howe et al. (2002). series connections of terrestrial sources and the fixed loads at seafloor optical amplifiers (along with associated i 2r losses on the submarine cable) with a single seawater return path is always used for standard submarine telecommunications systems. parallel connections of the loads, in which each optical amplifier would have a seawater ground connection, has not been used, and in fact appears to offer no advantages in this application. for a serial power meshed system such as neptune (fig. 3), additional complexity is posed by the need to regenerate the power twice or more at branches and for active reconnection to handle faults. hardware to implement these functions does not currently exist, although there appear to be no overwhelming obstacles to constructing it. perhaps more significantly, because power must be conserved at a branch splitter, there is no efficiency gain to branching with serial power. by contrast, a parallel power system is easy to branch and gains efficiency when there are multiple paths to each load. in addition, a parallel system inherently has a higher power capability because the i2r losses are always lower due to the use of multiple rather than a single ground. however, fault detection on a branched parallel system is inherently more complex due to multiple paths to a break. this can be mitigated by including a capability in the nodes to isolate each section of cable. these observations about ease of branching and power delivery limits, as well as consideration of the voltage and current capabilities of commercial-off-the-shelf submarine fiber optic cable, led to the decision to utilize a parallel rather than a serial power system for neptune. meeting science requirements b.1 and b.2 would be much more difficult if not impossible using a serial power system for a large meshed system like neptune. 6.2. meshed network versus star data communications architecture the neptune data communication design is based on an implementation of gigabit eth578 alan d. chave, gene massion and hitoshi mikada ernet using a dwdm physical layer to provide up to 16 bi-directional gb/s channels over a single pair of optical fibers. the neptune backbone topology (fig. 3) is meshed, and the communications system gains reliability from redundant paths, as is also true for the internet. there are multiple arguments for this design approach, as reviewed by maffei et al. (2003). an alternative approach has also been considered based on a star topology, in which each seafloor node communicates with each shore station using a single pair of wavelengths in a dwdm system, but does not communicate directly with any other node except through a shore station. this network topology was used in the early days of data networking when routing hardware was primitive and costly, but is obsolete and no longer used. the star network would be implemented using point-to-point sonet data transport with custom optical add/drop multiplexing at each node and submarine telecommunications-standard optical amplifiers to provide path gain. the premise was that the extreme reliability of submarine optical amplifiers (typically, of order 10 fits; 1 fit equals 1 failure in 109 h) would produce significantly higher data communications network reliability. a complete comparison of these two approaches is complex, but four simple observations can be used to derive a first order version: 1) to lowest order, the star network is more expensive than the gigabit ethernet system by the cost of 50+ submarine optical amplifiers and the non-recurring engineering required to implement optical add/drop multiplexing, for a total difference of about us$30m. 2) based on statistical modeling, the data network reliability based on the criterion of science requirement a.7 is slightly better for the gigabit ethernet approach, although the difference in barely significant. the much higher backbone reliability of the star network approach does not translate into higher overall reliability because the node electronic reliability is orders of magnitude lower. 3) the failure rate of individual node sonet electronic systems for the star network approach is about 3 times higher than that of high quality gigabit ethernet hardware. this translates into higher o&m costs as more failures will occur per unit time, presuming that all failures are repaired as soon as possible after they occur. given the higher procurement costs of the star network design, the life cycle costs of this approach will be higher, and probably much higher, than the ethernet implementation. 4) the inter-node latency of the star network design will depend on the actual pair of nodes being considered, but will in general be high because of the necessity to transit to shore and back to the seafloor. a typical value will be tens of ms with no tendency to be lower for adjacent than sub-adjacent nodes. by contrast, the adjacent inter-node latency of the ethernet system approaches the goal in criterion c.9, with sub-adjacent node latency being nearly integral multiples of this value. taken in aggregate, these observations mitigate against the star network and in favor of the gigabit ethernet approach in several ways. these examples indicate that the design of ocean observatories is a complex engineering process, and can be facilitated using a systems engineering approach based on requirements derived from science needs and use scenarios. references austin, t.c., j.b. edson, w.r. mcgillis, m. purcell, r.a. petitt jr., m.k. mcelroy, c.w. grant, j. ware and s.k. hurst (2002): a network-based telemetry architecture developed for the martha’s vineyard coastal observatory, ieee j. ocean. eng., 27, 228-234. beranzoli, l., a. de santis, g. etiope, p. favali, f. frugoni, g. smriglio, f. gasparoni and a. marigo (1998): geostar: a geophysical and oceanographic station for abyssal research, phys. earth planet. inter., 108, 175-183. beranzoli, l., t. braun, m. calcara, p. casale, a. de santis, g. d’anna, d. di mauro, g. etiope, p. favali, j.-l. fuda, f. frugoni, f. gamberi, m. marani, c. millot, c. montuori and g. smriglio (2003): mission results from the first geostar observatory (adriatic sea, 1998), earth planets space, 55, 361-374. blanchard, b.s. 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toplis and carroll, 1995). silicate magmas or melts containing significant amounts of fe show considerable dependence in their physical properties on the prevailing redox conditions. e.g., the viscosity of andesitic melt containing 8 wt% feot varies by about 2 orders of magnitude in the high viscosity range depending on the fe oxidation state (fe3+/σfe = 0.2-0.6) (e.g., liebske et al., 2003). due to the significant effect of fe oxidation state on the resulting products of magma genesis it is possible to deduce the redox conditions during magma formation, i.e. in the case of volcanic rocks, the pre-eruptive conditions. the aim of this communication is to give a brief overview on the possible methods for extracting information on the redox conditions of magmas and to point out some of the possible fe in magma – an overview max wilke institut für geowissenschaften, universität potsdam, germany abstract the strong influence of physical conditions during magma formation on fe equilibria offers a large variety of possibilities to deduce these conditions from fe-bearing phases and phase assemblages found in magmatic rocks. conditions of magma genesis and their evolution are of major interest for the understanding of volcanic eruptions. a brief overview on the most common methods used is given together with potential problems and limitations. fe equilibria are not only sensitive to changes in intensive parameters (especially t and f o2) and extensive parameters like composition also have major effects, so that direct application of experimentally calibrated equilibria to natural systems is not always possible. best estimates for pre-eruptive conditions are certainly achieved by studies that relate field observations directly to experimental observations for the composition of interest using as many constraints as possible (phase stability relations, fe-ti oxides, fe partitioning between phases, fe oxidation state in glass etc.). local structural environment of fe in silicate melts is an important parameter that is needed to understand the relationship between melt transport properties and melt structure. assignment of fe co-ordination and its relationship to the oxidation state seems not to be straightforward. in addition, there is considerable evidence that the co-ordination of fe in glass differs from that in the melt, which has to be taken into account when linking melt structure to physical properties of silicate melts at t and p. mailing address: dr. max wilke, institut für geowissenschaften, universität potsdam, postfach 601553, 14415 potsdam, germany; e-mail: max@geo.uni-potsdam.de 610 max wilke sources of error for such redox estimates. in addition, the structural role of fe in silicate glass and melt at high temperature is discussed. 1.1. redox conditions and volatiles knowledge of the redox regime of volcanic systems is a key parameter needed to understand volatile budgets of magmas involved in volcanic activities. the major volatile component affected by the redox regime is sulfur. the sulfur species found in the gas phase is strongly related to the redox conditions. as the solubility of s in the melt is, of course, related to the s species present, a strong dependence on the redox conditions is observed having a minimum at intermediate redox conditions (e.g., carroll and webster, 1994). depending on the oxygen fugacity, s is incorporated into the melt as s2– or so42– species. moreover, at reduced conditions fe and s in the melt directly interact as manifested by the strong correlation of s and fe concentrations in synthetic and natural samples. this led to the hypothesis of fes species being present in the melt (carroll and webster, 1994). when s saturation is reached at reducing conditions fe-sulphide phases precipitate from the melt. at oxidizing conditions, in contrast, especially in the presence of water anhydrite may precipitate from the melt (carroll and rutherford, 1987). 2. fe-redox equilibria exploited for redox estimates 2.1. coexisting fe-ti oxides oxygen thermobarometry using fe-ti oxides is mainly based on the exchange equilibrium fe2tio4 + fe2o3 = fe3o4 + fetio3 which is strongly dependant on temperature and oxygen fugacity (e.g., lindsley, 1991). due to the massive amount of work performed on the stability relations in the fe-ti-o system fe-ti oxides provide pretty accurate estimates of temperature and oxygen fugacity (e.g., andersen and lindsley, 1988; ghiorso and sack, 1991). however, application of this method to volcanic rocks is often restricted by the lack of feti oxides in the eruption products or by uncertainties on the co-existence of oxides found in volcanic rocks. therefore, often only a fragmentary image of the temperature-fo2 history of the magma is achieved (e.g., martel et al., 1999). 2.2. partitioning of fe between melt and plagioclase although plagioclase contains only minor amounts of fe, the incorporation of fe is strongly dependent on oxygen fugacity. fe3+ may easily replace al in the tetrahedral site. fe2+ may be incorporated by replacement of ca on the a site or by a coupled exchange of fe2+ and si versus 2al on the tetrahedral site. the incorporation of fe3+ is favored, so that the fe-partitioning between melt and plagioclase is sensitive to the prevailing redox conditions (sato, 1989; phinney, 1992; wilke and behrens, 1999; sugawara, 2000, 2001). as plagioclase is usually found throughout all volcanic rocks this method can be readily applied in many cases. plagioclase usually shows strong zoning and each zone grew from the melt at a given set of p, t and f o2 (fig. 1). if melt (glass) inclusions are present as well in each of the zones even the evolution of conditions may be deduced. since the partitioning is also influenced by temperature, melt composition and volatile content accurate estimates are only possible if the dependence of partitioning is calibrated for the system of interest. a more rigorous model for the fe-partitioning between melt and plagioclase was suggested by sugawara (2000, 2001). the model was parametrized on partitioning data from several compositions at various p, t and f o2 in the system sio2-al2o3-feo-mgocao-na2o and thus provides a good basis for direct application to similar natural systems. however, sugawara (2001) also showed that the accuracy of the p-t-f o2 estimates can be significantly improved by considering exchange equilibria among more than two phases, fe in magma – an overview e.g., by inclusion of pyroxene or olivine instead of using only melt and plagioclase (sugawara, 2001). 2.3. fe-oxidation state in volcanic glass another possibility to estimate redox conditions is the direct use of the fe oxidation state found in volcanic glass (e.g., helgason et al., 1992a,b). depending on the nature and size of the sample several analytical techniques provide access to the fe oxidation state of volcanic glass. bulk methods like wet chemical analysis and mössbauer provide quite precise data (error of fe3+/σfe ± 0.03−0.1). however, samples have to be homogeneous and free of crystal inclusions which is seldom the case. methods with spatial resolution that also provide insight into the fe oxidation state of volcanic glass in heterogeneous samples include micro-mössbauer spectroscopy (50-100 µm spot size) (mccammon et al., 1991), micro-xanes (x-ray absorption near edge structure, 3-30 µm spot size) (bajt et al., 1994; delaney et al., 1998; bonnin-mosbah et al., 2001; dyar et al., 2002; schmid et al., 2003), eels (electron energy loss spectroscopy, e.g., van aken et al., 1998) or high resolution x-ray emission analysis using the electron microprobe (spotsize ≥ 1 µm) (höfer et al., 1994; fialin et al., 2001). these methods also enable the investigation of glass inclusions within crystals and thus provide insight into the history of redox conditions during magma evolution. while oxidation-state estimates from micro-mössbauer spectroscopy may be as precise as those performed by usual mössbauer spectroscopy provided the fe-content is high enough (approximately 5 wt% feo). estimates from micro-xanes (fe k-edge) usually have larger errors, i.e. an error of fe3+/ /σfe: ± 0.1 − 0.2 if a general calibration as given by bajt et al. (1994) or wilke et al. (2001) is used. using specific calibrations for the glass 611 fig. 1. microscopic image of a chemically zone plagioclase phenocrysts (plg) in a highly vesicular pumice sample from the minoan tuff (santorini, greece). inclusions in the plagioclase host represent quenched melt (gls) trapped at various stages during crystal growth and thus provides information on the compositional development of the erupted magma (large black spot: vesicle filled with resin above plg-crystal). bottom side of image 1.2 mm, crossed polarizers). 612 max wilke composition of interest the error can be substantially improved to an error comparable to the one by mössbauer analysis (berry et al., 2003). the major advantage of this method is that spectra may also be taken at fe-content below 1 wt% if the photon flux is high enough. the use of lα,β -emission lines with the electron microprobe provides information on the oxidation state with the same spatial resolution as for chemical analyses by this technique and a precision of 10-20 % rel. however, the application is usually restricted by the total fe-content (above 3.5 wt%, fialin et al., 2001). the use of eels at the fe ledge may provide data with a precision that may be better than 5% rel. but the volume analysed is only in the 10 nm-range and glasses are potentially not stable under the electron beam which may be overcome by cooling the sample during measurements (liebscher, pers. comm.). to be able to deduce the redox conditions from such data, knowledge is needed, of course, on the relationship between fe oxidation state and temperature, pressure and melt composition. a considerable amount of effort was put into this field to establish the relationship to the above mentioned parameters using wet chemical analysis of fe2+ in combination with electron microprobe analysis for bulk fe content or mössbauer spectroscopy for the determination of the fe oxidation state (e.g., johnston, 1964, 1965; fudali, 1965; sack et al., 1980; mysen et al., 1980, 1985; kilinc et al., 1983; lange and carmichael, 1989; borisov and shapkin, 1990; kress and carmichael, 1991; moore et al., 1995; gaillard et al., 2001; wilke et al., 2002). sack et al. (1980), kilinc et al. (1983) and kress and carmichael (1991) used a rather simple empirical equation parametrized on a large dataset of melts equilibrated at various superliquidus conditions to describe the dependence of fe oxidation state on t, x and p. borisov and shapkin (1990) used a similar approach that differed only in the amount of fitting parameters. as these equations are purely empirical it is not clear to what extent these equations can be extrapolated to melt systems at conditions very different from those used in the parametrization of the equation, especially if they are extrapolated to significantly lower temperatures or to subliquidus conditions. despite the uncertainty inherent in this approach, this equation is used in rather complex thermodynamic models predicting magmatic phase relations, such as the program melts (ghiorso and sack, 1995) or the model used by sugawara (2000, 2001) for fe-partitioning between melt and plagioclase. although these models can certainly be considered major improvements in the modelling of magmatic systems the use of such an empirical equation in such models obviously puts some restrictions on the applicability to systems that strongly differ in either composition or pressure-temperature conditions. another approach to model the relationship of fe oxidation state to intensive and extensive variables only recently introduced to geologically relevant melts is suggested by ottonello and moretti (ottonello et al., 2001; moretti and ottonello, 2003; moretti, 2005). this model is based on an acid-base model for oxide melts (fraser, 1975, 1977) which classifies a given metal oxide in terms of its acidic or basic behavior, which directly affects the polymerization reaction between single bonded, double bonded and free oxygens in silicate melts. basic components produce cationic units whereas acidic components produce anionic units. in the case of fe, fe3+ shows amphoteric behavior whereas fe2+ shows only basic behavior. the disproportion of fe3+ between anionic and cationic matrix in contrast to fe2+ predicts that the ratio of rational activity coefficients fe3+ and fe2+ components cannot be expected to be 1 or even constant. as shown by the authors, this approach describes well fe-oxidation states of published datasets of glasses synthesized at known temperature and oxygen fugacity (ottonello et al., 2001). the effect of water on the fe-oxidation state was studied by several authors (moore et al., 1995; baker and rutherford, 1996; gaillard et al., 2001; wilke et al., 2002). it may be concluded from these studies that water has no direct influence on the oxidation state other than that it influences the prevailing oxygen fugacity. recently the influence of water dissolved in the melt was also included in the model of ottonello and moretti (moretti, 2005). fe in magma – an overview the development of a correct model for predicting the fe-oxidation state of a given melt composition at p, t and f o2 of interest is of major importance, of course. however, additional problems in using the fe-oxidation state of glass to deduce redox conditions arise from the fact that the original oxidation state may not be safely stored in the glass and might be affected by the thermal history of the volcanic glass after the first quench. burkhard (2001) has shown that the fe-oxidation state of a basaltic glass from kilauea, hawaii, may already change during re-heating at relatively low temperatures just above the temperature of glass transformation, well before the onset of crystallization. this reaction is intrinsically controlled and does not depend on any exchange of oxygen with the surrounding environment. such a change may also occur in natural samples if relatively thin lava lobes that have not completely cooled down are re-heated by successive lava flows. burkhard (2003) shows that such a reheating may even lead to complete crystallization of the formerly vitrified parts of the bottom lava lobe. in more silicic melt compositions such effects are certainly less probable as the difference in eruptive style provides few possibilities for alteration by re-heating and thus the pristine oxidation state may well be preserved. the small effect of the quench rate during the first quench after eruption is confirmed by experimental observations. the fe-oxidation state was found not to depend on the usual experimentally accessible quench rates (dyar et al., 1987; wilke et al., 2002). 3. local structural environment of fe in silicate melts although the structural environment of fe in silicate melts is only of minor interest in the light of redox estimation in magmatic systems, it plays a major role in transport properties of silicate melts. the effect of oxidation state on the viscosity described at the beginning indicates that fe3+ may be considered as a network former and fe2+ as a network modifier. although this assignment might well describe the influence of oxidation state on viscosity, it still leaves open the question of the exact local structure around the two possible fespecies. many spectroscopic investigations have been performed to gain further insight into the co-ordination environment of fe in melt and glass and have produced a variable picture on the co-ordination found for fe3+ and fe2+. fe3+ is often assigned to be tetrahedrally co-ordinated although octahedrally co-ordinated is also reported (e.g., dyar, 1985; mysen et al., 1985, virgo and mysen, 1985; hannoyer et al., 1992; galoisy et al., 2001; partzsch et al., 2003). for fe2+, assignments are even more variable ranging from octahedral, over trigonal bipyramidal to tetrahedral co-ordination (waychunas et al., 1988; keppler, 1992; jackson et al., 1993; rossano et al., 1999, 2000). the controversial assignment of co-ordination polyhedra is certainly related to the influence of the bulk composition and/or polymerization of the glasses used, but may be in part also related to the different techniques used. e.g., crystal-field spectra usually show strong clear evidence for fe3+ in tetrahedral co-ordination, whereas evidence for higher co-ordinated fe3+ may be superimposed by bands related to fe2+ (hannoyer et al., 1992). the presence of higher co-ordinated fe3+ may then be cross-checked or more easily detected by the use of mössbauer spectroscopy or x-ray absorption spectroscopy (virgo and mysen, 1985; hannoyer et al., 1992; partzsch et al., 2003; wilke et al., 2004a). spectral features are usually assigned to specific co-ordination polyhedra by comparison of the spectra taken on the glass to spectra of model compounds with fe in well known structural environment, implying that the lack of long-range order has only little effect on the position of spectral bands or features. beside the lack of long-range order, assignments are probably rendered more difficult by the fact that there is no unique co-ordination polyhedron for fe. instead, the co-ordination polyhedra vary from site to site leading to a distribution of the local structure around fe in the glass or melt. this site-to-site distribution is probably best reflected in the distribution of hyperfine parameters that can be determined from mössbauer spectral analysis (alberto et al., 1996; dunlap, 1997; 613 614 max wilke rossano et al., 1999; wilke et al., 2002). thus assignment to distinct co-ordination polyhedra is problematic and would not even describe the situation of fe in glass or melt adequately, so that most spectroscopic analyses provide only information on the average co-ordination. 3.1. difference between melt and glass interpretations on the melt structure based on observations made on glasses assume that the structure found in the glass represents the one that would also be found in the melt. however, the structure found in glass is the one frozen in from the liquid at temperatures around the glass transition, which potentially differs from the melt structure found at higher temperatures. first evidence for differences in the coordination of fe between melt and glass may already be taken from the fact that mössbauer hyperfine parameters vary as a function of quench rate (dyar et al., 1987; wilke et al., 2002). first direct experimental evidence in the case of fe was given by waychunas et al. (1988) and jackson et al. (1993). these authors have shown evidence by collecting xanes and exafs spectra at ambient conditions and high temperature that the co-ordination of fe2+ found in glass differs from the one found in the melt. an increase in the intensity of the pre-edge feature located 10-20 ev before the main edge and a shortening of the measured fe-o distance indicates that the co-ordination of fe2+ at melt conditions is lower than that found in glass. similar observations have been made for other elements like ti and ni (farges and brown, 1996; farges et al., 1996). an in situ raman study on fe-rich sodiumsilicate glass and melt by wang et al. (1993) looking to bands that are related to vibrational modes of the polymeric network has evidenced no changes above the temperature of glass transformation for oxidized samples. the high temperature spectrum of the reduced sample, however, indicates a further anionic species present in the melt and a depolymerization of the melt network, which was related by the authors to the ferrous iron in the melt. new results from further in situ measurements using high resolution x-ray absorption spectroscopy at the fe k-edge yielded similar results to those former studies using this technique (wilke et al., 2004b; wilke et al., unpubl.). xanes and exafs spectra were taken on na2si3o7 glass and melt that was doped with 5 wt% fe2o3 at both reducing and oxidizing conditions. at reducing conditions only a very small increase in the pre-edge intensity is observed in contrast to the results by jackson et al. (1993). the change in the pre-edge intensity is much stronger at oxidizing conditions indicating that fe3+ is affected much more strongly. an increase in the pre-edge intensity may be interpreted by a lowering of the coordination number or a change in site geometry, as the pre-edge intensity of tetrahedrally co-ordinated fe is much higher than the one of octahedrally co-ordinated fe (wilke et al., 2001). so far, this change seems not to be quenchable and thus indicates an immediate re-arrangement of the local structure at tg. although the structural differences between melt and glass are probably unimportant for defining redox equilibria in silicate melts these differences are certainly important for the understanding of transport properties and their relation to the melt structure. 4. conclusions fe equilibria in magmatic systems provide valuable constraints for the understanding of the development of magmas prior to volcanic eruption or sub-volcanic emplacement. each of the methods proposed here, however, has its limitations when applied to natural systems, although they always yield at least some rough estimate on the redox regime, of course. for more precise estimates a combination of techniques or fe equilibria has to be used. as discussed by sugawara (2001) the use of fe-partitioning between three or more phases (e.g., plagioclase, olivine and melt) is probably much more reliable than just partitioning between plagioclase and melt. a very important tool to constrain pre-eruptive conditions for a given volcanic site precisely is probably the experimental determination of phase relations as a function of t, p, fluid and melt composition (e.g., rutherford and devine, fe in magma – an overview 1996; martel et al., 1998; cotrell et al., 1999). beside the determination of phase stability relations such experiments can also be used to calibrate the fe (exchange) equilibria for the bulk composition of interest, i.e. fe partitioning among phases, fe oxidation state in glass, etc. the modelling approaches introduced by ottonello and moretti to the volcanologist/ petrologist community provide an important step forward to profound modelling of redox equilibria in melt systems. such models are certainly needed to model melt-crystal equilibria and to predict the evolution of the composition of magmas during volcanic cycles. an extension of this modelling approach is certainly desirable. for an understanding of the relationship between melt-transport properties and melt structure a better link between composition and melt structure should be established. fe is one further example that the local environment in glass may be significantly different from that in the melt. any model of the melt structure that may be used as an input for the compositional dependence of transport properties should be based on experimental evidence of the molten state. the simple parameter nbo/t (non-bridging oxygen per tetrahedron), often used to approximate changes in the melt polymerization with composition, differentiates the given elements only by the network forming or network modifying role, neglecting completely the potentially large difference in the field strength of the ions. a structural model that also accounts for these differences should provide a better basis for modelling compositional dependence of e.g., viscosity data. acknowledgements the basis for this communication was and still is made possible by collaborations with f. farges, s. rossano, g.m. partzsch, c. schmidt, w. heinrich, h.behrens, f. holtz, d. burkhard, e. welter, k. klementiev and a. erko whose help, commitment and support is highly appreciated. comments by an anonymous reviewer and r. schmid improved the readability of the manuscript. funds provided by the deutsche forschungsgemeinschaft (project wi/2000-1) and by the daad procope program are greatfully acknowledged. references alberto, h.v., j.l. pinto da cunha, b.o. mysen, j.m. gil and n. ayres de campos (1996): analysis of mössbauer spectra of silicate glasses using a two-dimensional gaussian distribution of hyperfine parameters, j. non-cryst. solids, 194, 48-57. andersen, d.j. and d.h. lindsley (1988): internally consisten solution models for fe-mg-mn-ti oxides: fe-ti oxides, am. mineral., 73, 714-726. bajt, s., s.r. sutton and j.s. delaney (1994): x-ray microprobe analysis of iron redox states in silicates and oxides using x-ray absorption near edge structure (xanes), geochim. cosmochim. acta, 58, 5209-5214. baker, l.l. and m.j. rutherford (1996): the effect of dissolved water on the oxidation state of silicic melts, geochim. cosmochim. acta, 60, 2179-2187. berry, a.j., h.s.c. o’neill, k.d. jayasuriya, s.j. cambell and g.j. foran (2003): xanes calibrations for the oxidation state of iron in a silicate glass, am. mineral., 88, 976-977. bonnin-mosbah, m., a.s. simionovici, n. metrich, j.p. duraud, d. massare and p. dillmann (2001): iron oxidation states in silicate glass fragments and glass inclusions with a xanes micro-probe, j. non-cryst. solids, 288, 103-113. borisov a.a. and a.i. shapkin (1990): a new empirical equation rating fe3+/fe2+ in magmas to their composition, oxygen fugacity, and temperature, geokhymiya, 6, 892-897. burkhard, d.j.m. 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(1989): igneous petrogenesis (chapman & hall, london), pp. 466. 617 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 1 students, earthquakes, media: does a seismic crisis make a difference? gemma musacchio*, stefano solarino**, elena eva**and giovanna l. piangiamore*** * istituto nazionale di geofisica e vulcanologia, ac, roma; gemma.musacchio@ingv.it ** istituto nazionale di geofisica e vulcanologia, cnt, sede di genova, genova ***istituto nazionale di geofisica e vulcanologia, roma 2, sede di portovenere, fezzano di portovenere (sp) abstract how do students use the big data flow of information from the internet? what is their opinion and trust in scientists? to what extent catastrophic earthquakes and environmental disasters influence their opinion? in this study we present the results of a poll conducted on high school students (age 13-20) to assess young italian citizens’ trust on geoscientists and their science. the sample of about 700 students refers to areas prone to natural hazards ranging from low to moderate intensity. to allow a fast and easy compilation, held directly in school, the poll included only a very few questions. they investigated and accessed the source where the students retrieve information on catastrophes and natural phenomena, the role of scientists in everyday life and scientists’ ethical integrity. although limited, this is the first poll of this kind and data collected up to now can be used for a rough picture of the present situation, to compare results with recent disasters and to project future results of on-going analysis. all information will also help us in a future analysis to understand if and to what extent a recent earthquake or environmental local crisis can affect the perception. students do not completely trust that scientists are genuinely independent from outer urges. they also believe that media manipulate information with willful misconduct, to hide inconvenient realities or to get economic advantages. however answers from our emilia sample of students were unexpected: they did not show any specific bias after the 2012 seismic sequence. conversely they show less skepticism towards scientists and scientific integrity in comparison to students from other regions. this suggests that the perception towards science and scientists might be driven by cultural and social background and not necessarily affected by recent seismic crisis. in this perspective this on-going study will be challenged as soon as poll after the amatrice 2016 seismic sequence will be available. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 2 i. introduction ecent social research has shown in italy an unprecedented, yet unexpected, level of citizens scientific literacy: only 13% of interviews gave fully wrong answers to basic science questions (pellegrini and saracino, 2016). as citizens' age increases their knowledge in science does the opposite, making young students an interesting mirror of what would be the future society of science and technology. youths are also known as capable of strong criticism towards society: they watch and are judgmental towards actions that have brought to any present situation. they profit from scientific outcome in technology; they watch, with wide-open eyes, risks posed by disregard towards environment, misusage of resources, and wrong application of building codes. they judge societal efforts towards risk mitigation. in a land prone to earthquake hazard, youth may represent the key resource towards risk mitigation. opinion, knowledge and understanding are not mostly built at school. youths are native digital; they surf internet and live in social media, in a world where information seem not to have time or space limits anymore. a few hours after the amatrice mw 6.0 earthquake twitter had reached more than half a million of posts (musacchio and piangiamore, 2016). as speed and wealth of information increase, quality might not follow the same pattern and final users are not always aware of sources reliability. on the other hand, science suffers of intrinsic uncertainty and knowledge often raises more enquires than expected. in this frame alternative hypotheses, not necessarily based on solid scientific ground, might find their way out on the web. furthermore, scientists still underestimate the opportunity the web offers to spread knowledge and increase public understanding of science. how are the young citizens dealing with the unprecedented flow of information provided by internet? what is their opinion and trust in scientists? how far is this affected by the occurrence of a seismic crisis? here we try to answer to some of the above questions investigating on the sources of information on earth science on which high school students (aged 13 to 20 years) rely. the work is unique, as none of the published poll dealing with the impact of science in layman's opinion specifically address geoscience and/or students. ii. data the data analyzed in this study are the responses of italian students to a questionnaire mainly aiming at 1) knowing what sources they use to get informed and updated about natural disasters 2) estimating their level of trust in earth scientists and their science 3) getting their opinion about whether scientists are biased by external pressures (money, politics, career, governmental institutions, media) 4) understanding if and how much a recent earthquake can affect their opinion, in the perspective to compare the results of such analysis with the amatrice case. the questionnaire was made available on the internet to schools by direct contact with teachers. to avoid vandalism, the questionnaire was mainly compiled during school r annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 3 time and a password was provided to each teacher in order to identify fake answers. particular care has been taken that the respondents were not aware of which institution was in charge of the research study to avoid preconceptions in the results. a series of automated scripts were designed to quickly update the statistics of the answers in near real time. the ideal sample would have been data from each one of the italian regions, but this would have required a longer time for collec tion. in order to get prompt results to be used for reference after the amatrice 2016 earthquake we perform our analysis on the available sample, which will be possibly increased in the next future. the data cover almost all northern italy, while only few questionnaires were filled in the central and southern italian peninsula (red circles in fig. 1). in particular, data from the emilia region, recently hit by a major earthquake in 2012 (yellow star in fig. 1) allowed to approach with point 4 of our study. figure 1. left panel. regions where data were collected. most questionnaires were filled in northern italy. the stars are the location of the most recent, destructive earthquakes in italy. right panels. top, distribution of the sample with sex. bottom, age of the respondents. we collected more than 720 questionnaires; after a quality selection to remove unreliable and fake answers, they were reduced to 698. the english version of the questionnaire is available at the web address: annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 4 https://docs.google.com/forms/d/e/1faip qlsevp7zbutolt5rfub_prywrlztb1ksn6g_dtnyf2fsm3sftg/viewfo rm there are four main blocks: the personal data block, the sources block, the scientists in society block and the ethical block. the personal data block allowed to collect information on the age, the region they live, the type of school. in the italian educational system school is compulsory till age 16. there are three levels of high schools for the age 13 to 20 (right – bottom panel in fig. 1) having different targets: the istituto professionale is mainly devoted to form citizens who want to be hired by companies in the tertiary sector or as independent crafts. the istituto tecnico is chosen by students who want to get a career as professionals in many fields or be hired under higher responsibilities positions. although all schools allow to attend the university, the liceo is usually the one having vocation for higher education. the sources block aims at knowing what sources students use to get information on natural phenomena and whether they attend scientific conferences and talks. the question about sources breaks down into several items, including internet and social networks (fig. 2). this distinction allows to have a first feedback on the reliability of the source. as a matter of fact, one can assess the accountability of websites while social networks are platforms more prone to manipulation. on social networks identity and skills of information providers are often not known (solarino 2014 a, b) and information is seldom based on a primary source. figure 2. sources of information. students could choose up to three answers. the scientists in society block aims at estimating the role of scientists in everyday life, by checking if students believe that scientists can really help society to improve and how clear and reliable they are. finally, the last block wants to test the “ethical integrity” of scientists with a couple of questions about the external pressure that scientists might have. the last question is about the manipulation of news and information by the media; in case of a positive answer the questionnaire also provides a list of possible explanations that can be chosen by the respondents. we let some room for intermediate answers by giving five choices for each questions (not at all, a little, fairly, much, very much). to interpret the answers, we decided that the first two scores are negative (not at all and a little equals no), the rest positive. the length of the questionnaire is designed as a good compromise between the attention limits and curiosity that students might have. moreover, since many classes filled in the questionnaire during regular lessons, we https://docs.google.com/forms/d/e/1faipqlsevp7zbutolt5rfub_pr-ywrlztb1ksn6g_dtnyf2fsm3sftg/viewform https://docs.google.com/forms/d/e/1faipqlsevp7zbutolt5rfub_pr-ywrlztb1ksn6g_dtnyf2fsm3sftg/viewform https://docs.google.com/forms/d/e/1faipqlsevp7zbutolt5rfub_pr-ywrlztb1ksn6g_dtnyf2fsm3sftg/viewform https://docs.google.com/forms/d/e/1faipqlsevp7zbutolt5rfub_pr-ywrlztb1ksn6g_dtnyf2fsm3sftg/viewform annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 5 wanted to avoid that in case of subsequent compilation (for example for schools where the number of available computers is much less than the number of potential respondents) the students loose too much school time. the average time for the completion of the questionnaire proved to be of about 1 m 30 sec. iii. results our sample was made of about 56% and 44% male and female students respectively (right – top panel of fig. 1). the two extreme ages, 13 and 20, only represent about 3 and 1.5 %; the intermediate ages are all well balanced (right – bottom panel, fig. 1). it is especially noteworthy that 15 and 18 -beginning and end of high schoolare well represented and reach 20% each. it is interesting to check if and how the schooling or the age changes the students’ minds by comparing the answers from these two peculiar ages. most of our respondents (about 78%) are liceo students while istituto tecnico and professionale are represented only by 17% and 5 % of the sample, respectively. let’s first analyze the answers from a general point of view, beginning from the source of information. the percentage that we computed here is somewhat different from the rest because the respondents in this case are allowed to provide multiple answers, up to three. the estimate is thus calculated from the total number of answers, but it is not directly linked to the sample as any respondent may have answered one to three times. only very few students attended a scientific conference recently, while the majority (57%) never did. however, an overall 43% attended at least once. as expected, conferences, together with books and family, are not the main sources of information about natural phenomena. a fair amount of respondents gets this kind of information from school as about 15% of the answers pointed out that school is a source; we argue that the students' answers meant the scientific part of the phenomena rather than the societal aspects. this percentage is pretty high, but if we consider that it is a value computed on the answers and not on the number of respondents, we can highlight that school may indeed not be the only source. we thus conclude that students are being informed by non-authoritative sources, since internet, social media and tv make up to 78 % of their informants. in particular, if taken as a single (internet or social media), none of the two “digital” sources gave higher results than tv, which is still the primary way to get informed about natural disasters among young people (fig. 2). we also underline that the gradual decline of newspapers and magazines as sources of scientific information (they account for less than 8%) is accompanied, as a cause or a consequence, by the dramatic reduction of the scientific journalists (bauer et al., 2013) and by the proliferation of online scientific authors, often inexperienced or amateur, that simply make “cut and paste” job, flooding the web of rumors (davies, 2008; foresta martin and peppoloni, 2015). this opens a debate on the quality and reliability of information, which will be discussed later on. in regards to the role of scientists in society, the majority of the students believe that scientists are reliable and their studies are annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 6 useful (about 76%); an encouraging 80% believe that scientists are clear in explaining their studies, and the relative results and only less than 20% of the sample thinks that the research does not help the society to face natural disasters. however about 56% of the students believe that the research is biased by politics and the industry, and a similar amount believes that scientists do not disseminate important discoveries or results to accomplish other people or personal interest. finally, the last part of the questionnaire reveals that more than 80% of the students believe that the media are manipulating the information. it must be remarked that studies related to “responsible behavior” of journalists on the whole european community (eurobarometer) provide for italy a slightly different result: according to this study some 54% of the italians thinks that journalists behave responsibly, compared to 80% of our respondents believe the opposite thing. moreover, a significant 42% of the positive answers of our study attribute this behavior to the need of hiding “troublesome” realities. in descending order, 34% attribute the issue to economic interest, 18% to the explanation that false news are more appealing to the society while only 5% believe that journalists and entertainers are not expert. in practice, this view shares the general belief of a “conspiracy” (solarino, 2015) of the media while attributing skills and competence to journalists. this means that our students do not believe to news simply because they think that the provider is cheating, not because it is not a professional on the field. the belief of a “conspiracy” grows for students with age. the trend is very clear that from 14 to 19 years old our respondents believe more (with an increase of 5% on average per year) that scientists depend on non-scientific institutions and rise their tendency to hide important results for personal interests. iv. discussion and conclusions we now make an analysis by decomposing the results according to the sex, age and location of the respondents to see if there is any particular trend. as already discussed, the sample is limited in number of entries and geographical distribution compared to the whole nation, but since there exists no similar analysis on the topic yet, it is worth doing it. for what concerns sex, it must be remarked that the two samples male and female differ of about 90 units, the first being more represented. results must be considered in relative percentage. in general female have more trust than male students on scientists and on their studies; they also believe that researchers behave correctly and are not being biased by personal or economic interests. instead results are very similar for what concerns manipulation: male and female share, with almost identical percentages, the idea that media are manipulating information. we can finally analyze the results with respect to the geography with the aim to understand if a recent earthquake has an im-pact on the opinion of the students towards our questions. in particular, we compare the answers of the students from the emilia region, where the two major shocks of a seismic sequence caused 27 casualties in may 2012, with those in regions where seismic events have not recently occurred. the results may be a reference for further studies annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 7 figure 3. comparison of the answers (top to bottom) relative to the integrity (dependence of scientists from outer urges) of scientists, their tendency to hide information to the society and the manipulation of information from the media. the emilia is among the regions in which students have most trust in scientists. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 8 that will be done in the regions shaken recently by the amatrice earthquake (august 2016). fig. 3 reports such a comparison for some of the questions. from top to bottom of figure 3, the histograms display the answers on the dependence of the scientists from outer factors, their tendency to hide information to get personal gain or to favor other people and the manipulation of news by the media. in the three cases the respondents do not seem to be biased by the occurrence of the earthquake, since the answers from the emilia students are very similar to other regions where a seismic event has not recently happened. even more, by comparing our results with the trend of the whole country as published in the euro-barometer (tns opinion & social, 2013), it looks like the regions on the left part of the histograms are aligned with the opinion of the country, while those on the right part, which are regions from both north and south of the country, disagree strongly with the rest of the italian citizens on these topics. unfortunately we do not have information about the opinion of the emilia students before the earthquake, so we can only propose that the occurrence of an earthquake possibly does not decrease trust in scientists but, conversely, probably decreases the belief that scientists and journalists do no behave in an ethical way. acknowledgements this work profited from the participation of students and teachers of the following schools: istituto statale d'istruzione superiore "j. m. keynes", gazzada (va) (www.iisskeynes.it); liceo scientifico "l. cremona", milano (http://www.iiscremona.gov.it/); istituto superiore "e. fermi", arona (no) (http://www.iisenricofermiarona.it/); licei ed istituti tecnici del "polo scolastico superiore della val tidone", castel san giovanni, borgonovo val tidone (pc) (http://www.polovoltacasali.gov.it/); liceo artistico "ciardo-pellegrino", lecce (http://www.liceociardopellegrinolecce.gov.it/); liceo scientifico “v. sereni”, luino (va) (http://liceoluino.gov.it/); istituto statale d'istruzione superiore, "a. torrente", casoria, napoli (http://www.itctorrente.it/); liceo scientifico "e. torricelli", bolzano (http://www.torricelli.bz.it/); istituto tecnico "e stein", gavirate (va) (http://www.istitutosup-gavirate.net/); liceo scientifico "istituto champagnat", genova (http://www.maristi.it/champagnat/liceo/); liceo scientifico “pacinotti” la spezia (http://www.liceopacinotti.gov.it/); liceo scientifico “pacinotti” levanto (sp) (http://www.liceopacinotti.gov.it/); istituto alberghiero “g. casini” la spezia (http://www.alberghierolaspezia.gov.it/). we are grateful for their help. the manuscript also profited from the comment and suggestions of an anonymous reviewer to whom we are very much indebted. references [bauer et al., 2013] bauer, m. w., s. howard, y.j. romo ramos, l. massarani and l. amorim (2013). global science journalism report: working conditions & practices, professional ethos and future expectations. scidev.net, http://www.iisskeynes.it/ http://www.iiscremona.gov.it/ http://www.iisenricofermiarona.it/ http://www.polovoltacasali.gov.it/ http://www.liceociardopellegrinolecce.gov.it/ http://liceoluino.gov.it/ http://www.itctorrente.it/ http://www.torricelli.bz.it/ http://www.istitutosup-gavirate.net/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7239 9 trident printers, uk. isbn 978-1-909528-024. [davies, 2008] davies, n. (2008). flat earth news: an award-winning reporter exposes falsehood, distortion and propaganda in the global media. chatto and windus, london. isbn-10: 0701181451. [foresta martin and peppoloni, 2015] foresta martin, f. and s. peppoloni (2015). geoethics in communication of science: the relationship between media and geoscientists. poster. egu general assembly 2015, vienna, austria. id.4098 [musacchio and piangiamore, 2016] musacchio, g. and l. g. piangiamore (2016). the 2016 amatrice seismic sequence in the media. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 [pellegrini and saracino, 2016] pellegrini, g. and b. saracino, (2016) annuario scienza tecnologia e società, il mulino, 192. [solarino, 2014a] solarino, s. (2014). geoethics and communications 1. scientists, politicians, journalists, media entertainers, sociologists. can ethics help to establish their exact role in communicating geoscience ? g. lollino et al (eds). engineering geology for society and territory. vol. 7, 55-58. [solarino, 2014b] solarino, s. (2014). geoethics and communications 2. ethics, the notably absent from the internet. g. lollino et al (eds). engineering geology for society and territory. vol. 7, 39-43. [solarino, 2015] solarino s. (2015). how to strengthen public trust in geosciences. peppoloni s. and di capua g. (eds). geoethics: the role and responsibility of geoscientists. geological society, london, special publications. 419, 117-124 [tns opinion & social, 2013] tns opinion & social. (2013). special eurobarometer 401. responsible research and innovation (rri), science and technology, 153. ole_link3 ole_link8 ole_link9 ole_link10 ole_link11 ole_link12 ole_link13 ole_link14 ole_link15 ole_link16 1 annals of geophysics, 65, 3, dm316, 2022; doi:10.4401/ag-8778 o p e n acc e s s the near fault observatory community in europe: a new resource for faulting and hazard studies lauro chiaraluce*,1, gaetano festa2, pascal bernard3, antyonio caracausi1, ivano carluccio1, john f. clinton4, raffaele di stefano1, luca elia2, christos p. evangelidis5, semih ergintav6, ovidiu jianu7, george kaviris8, alexandru marmureanu7, stanka šebela9, efthimios sokos10 (1) istituto nazionale di geofisica e vulcanologia, ingv (italy) (2) università of napoli federico ii, unina (italy) (3) institut de physique du globe de paris ipgp (france) (4) swiss seismological service, eth zürich (switzerland) (5) noa (greece) (6) koeri (turkey) (7) niep (romania) (8) section of geophysics‑geothermics, department of geology and geoenvironment, national and kapodistrian university of athens (9) research centre of the slovenian academy of sciences and arts, zrc sazu (slovenia) (10) upatras (greece) article history: received december 14, 2021; accepted may 26, 2022 abstract the near fault observatories (nfos) community is one of the european plate observing system (epos, http://www.epos‑eu.org) thematic communities, today consisting of six research infra‑ structures that operate in regions characterised by high seismic hazard originating from different tectonic regimes. earthquakes respond to complex natural systems whose mechanical properties evolve over time. thus, in order to understand the multi‑scale, physical/chemical processes responsible for the fault‑ ing that earthquakes occur on, it is required to consider phenomena that intersect different research fields, i.e., to put in place multidisciplinary monitoring. hence, nfos are grounded on modern and multidisciplinary infrastructures, collecting near fault high resolution raw data that allows gener‑ ation of innovative scientific products. the nfos usually complement regional backbone networks with a higher density distribution of seismic, geodetic, geochemical and other geophysical sensors, at surface and sometimes below grade. these dense and modern networks of multi‑parametric sensors are sited at and around active faults, where moderate to large earthquakes have occurred in the past and are expected in the future. they continuously monitor the underlying earth instability processes over a broad time interval. data collected at each nfo results in an exceptionally high degree of knowledge of the geometry and parameters characterizing the local geological faults and their deformation pattern. the novel data produced by the nfo community is aggregated in epos and is made available to a diverse set of stake‑ holders through the nfo federated specific data gateway (fridge). in the broader domain of the solid earth sciences, nfos meet the growing expectations of the learning and communication sectors by hosting a large variety of scientific information about earthquakes as a natural phenomenon and http://www.epos-eu.org lauro chiaraluce et al. 2 a societal issue. it represents the epos concept and objective of aggregating and harmonising the european research infrastructures capabilities to facilitate broader scientific opportunity. the nfos are at the cutting edge of network monitoring. they conduct multidisciplinary experi‑ ments for testing multi‑sensor stations, as well as realise robust and ultra‑low latency, transmis‑ sion systems that can routinely accommodate temporary monitoring densification. the effort to continuously upgrade the technological efficiency of monitoring systems positions the nfo at the centre of marketing opportunities for the european enterprises devoted to new sensor technology. the nfos constitute ideal test beds for generating expertise on data integration, creating tools for the next generation of multidisciplinary research, routine data analysis and data visualization. in particular focus is often on near‑real time tools and triggering alarms at different levels are tested and implemented, strengthening the cooperation with the agencies for risk management. nfos have developed innovative operational actions such as the testing centre for earthquake early warning and source characterisation (crew) and detailed fast ground shaking and damage characterization. complementing the recent growth of modern laboratory and computational models, the nfos can provide interdisciplinary observations of comparable high resolution to describe the behaviour of fault slip over a vast range of spatial and temporal scales and aiding to provide more accurate earthquake hazard characterizations. keywords: near fault observatories; research infrastructures; active faults; multidisciplinary approach; high resolution data products 1. introduction 1.1 mission the near fault observatories in europe target understanding the physics of faulting including the transition between stable and unstable fault slip behaviour and the near‑surface response to earthquake shaking. also crucial is the estimation of the seismic hazard and ultimately the mitigation of risk for exposed populations and infra‑ structures. the idea of performing and implementing interdisciplinary investigations and alert systems, targeting crustal faults, requires consideration of the geological structures as complex natural systems whose physical and chemi‑ cal properties evolve over time. thus, the inference of the multi‑scale processes associated with earthquakes and faulting requires considering phenomena studied in different research fields: this is the road of integration that has led to the birth and development of the nfo community in europe. nfos were identified as required infra‑ structures by epos, the european plate observing system. nfos are innovative, advanced research infrastructures based on dense, state‑of‑the‑art networks, that continuously monitor instability processes developing on under‑ lying faults, over a broad time scale, from seconds to years. nfos collect multi‑parametric near‑fault data and provide advanced scientific products to the earth science community and other stakeholders through epos-related services. some nfos that provide mono-thematic data, with specific, fault-related scientific objectives, have a plan to integrate multi‑disciplinary data in the infrastructure in the near future. nfos complement the existing regional geophysical monitoring networks. generally, these independent back‑ bone networks are characterised by relatively coarse station spacing with similar sensor types (in most cases sev‑ eral tens of kilometres) that are not coordinated. in contrast, as multi-disciplinary densifications, nfos can be considered as on-field laboratories that illuminate underlying active faults by recording and analysing multi-dis‑ ciplinary signals related to the processes that occur beneath our feet, down to very small scales. nfos aim to trace the evolution of fault systems, through accurate detection [ross et al., 2019], location [waldhouser and ellsworth, 2000; chiaraluce et al., 2011] and characterization of micro‑seismicity [abercrombie, 2015; supino et al., 2019], to intercept the aseismic forcing mechanisms, such as creeping, that may influence future rupture development [bouchon et al., 2021] and to identify the diffusive processes associated with fluid migration and fluid-rock interac‑ near fault observatories in europe 3 tion [miller, 2013]. owing to the long‑time scales related to a full single seismic cycle for a large (e.g., tens of kilo‑ metres long) fault, nfos focus on high resolution deformation episodes occurring along small patches of the main faults or along minor faults that pertain to the major system. in this framework, more frequent small events (with m ~3) can be considered as local mainshocks, considerably shortening the seismic cycle. this provides scientists the opportunity to test models and hypotheses on a more robust statistical basis and to follow the pre‑, co‑ and post‑seismic phases of an active fault, even if small (e.g., from hundreds of meters to a few kilometres), with all the related scaling problems. at the same time, the collected information on the geometry and deformation style of the major faults can be used to develop ground shaking scenarios that account for diverse slip distributions and rupture directivity models [evangelista et al., 2017]. 1.2 the nfo community in epos the nfo community in europe originated under the fp7 european project nera – network of european re‑ search infrastructures for earthquake risk assessment and mitigation (2010‑2014), when a networking initiative started to coordinate efforts of different institutions owning dense observatories on faults, culminating in sharing of technology know‑how and data analysis best‑practice. the success of the initiative led to the nfo communi‑ ty becoming a thematic working group of the epos‑pp – preparatory phase (eu project funded under the fp7 programme – 2010‑2014). in the epos‑ip project (epos implementation phase, funded under the horizon 2020 programme – 2015-2019) the nfo community was officially recognized as one of the thematic core service (tcs). during epos-ip, the current foundations for our community were built, through the definition of a consortium agreement that comprised a legal, governance and financial framework. further, services dedicated to nfos we developed to make data, data products and software interoperable and accessible through the epos data distri‑ bution platform, the integrated core service central hub system (ics‑c; https://www.ics‑c.epos‑eu.org/). during the epos‑ip project, the community successfully negotiated the validation phase that required reaching technical, financial and governance targets. the community also received concurrent support from the sera project – seis‑ mology and earthquake engineering research infrastructure for europe – (eu project funded under the infraia figure 1. location of near fault observatories in europe. the map also shows the time derivative of the deformation vec‑ tors with arrows, whose length is associated with the amplitude of the strain rate (serpelloni p.c.). seismicity is also superimposed on the map, with colours associated with event depth. the nfos are in regions with different strain rate and kinematics, featuring shallow and medium depth seismicity. https://www.ics-c.epos-eu.org/ lauro chiaraluce et al. 4 horizon 2020 programme, 2017‑2020) to develop and apply advanced strategies and techniques of data analysis for microseismicity. today, the nfo community is a tcs of the epos delivery framework. the nfos comprise six members and one observer. they span different tectonic regimes, different areas and different space scales over europe (figure 1), all of them at sites of elevated seismic hazard. they include plate boundary systems at the marmara sea and the corinth rift. in mountain settings, nfos monitor the alto tiberina and irpinia faults in the apennine mountain range, the vrancea fault in the carpathian mountains, the valais in the alps and the slovenian karst region in the dinarides. they look at diverse faulting mechanisms (strike‑slip, normal and thrust), high to low angle, shallow to deep faults, as well as regions with fast and slow strain rate accumulation. each fault zone can generate large earthquakes (m > 6) that pose substantial seismic hazard. two of the zones, marmara sea and corinth, include offshore seismic sources that pose an additional, even though low, tsunami hazard. the focus of the observatories varies, ranging from smallto large-scale seismicity and includes the role of different parameters, such as fluids in fault initiation, the geometrical and mechanical structure of fault systems, site effects and secondary phenomena, such as landslides and tsunamis. in response to their specific objectives, the nfos operate a diverse set of instrumentation to monitor the surface and sub‑surface using seismic, deformation, strain, geochemical and electromagnetic equipment. 1.3 the tcs delivery framework the mission of the tcs nfo is to provide a sustainable platform for coordination between each european ob‑ servatory, sharing data and products promoting best practice that ultimately will foster breakthrough research on faulting and earthquakes. his effort is regulated through the consortium agreement (ca), representing the nfo reference framework, specifying the relationship between parties, their rights and obligations, as well as providing organisational, managerial and financial guidelines. the tcs portfolio comprises data access, visualization and processing tools, testing facilities and transnational access. data access from nfos follow existing epos tcs services where they exist for seismic and gnss datasets (raw data and high-level products) and provide new nfo-specific access services otherwise such as for vp/vs time series, high resolution earthquake catalogues and geochemical data. to process multi‑disciplinary time series and intersect information appearing in different records, nfos have developed tools that will help in providing im‑ proved models of transient processes in/around fault zones and in building advanced databases and catalogues of such processes. these tools will be in the future included in the virtual laboratory service. also, the nfo commu‑ nity has developed a testing facility, built on real-time and offline high-resolution data, to foster the development of next generation methodologies and software for real‑time monitoring of faulting processes. for physical access, nfos will open their on-field laboratories to design new experiments, to test new instruments, to validate meth‑ odologies and real-time software, approaching breakthrough scientific questions. data and products delivered by the tcs are intended to provide the earth sciences scientific community with a fundamental dataset that can accelerate to advancement in understanding fault system behaviour. data and high‑resolution data products will provide clues to understand the forcing mechanisms associated to earthquakes (fluid migration, creeping), the preparation phase of large earthquakes, the partition of the slip in seismic and aseismic contributions, as well as markers for changes in the mechanical properties of the faults. the nfos rep‑ resent ideal testbeds for generating expertise on data integration, for creating tools for the next generation of multidisciplinary research, advanced routine data analysis, also grounded on machine learning and artificial intel‑ ligence, and data visualization, for developing new experiments and survey through transnational access. these integrated datasets alongside provision of virtual and physical access to nfos is expected to create positive feed‑ back to solve the puzzle of faulting and associated deformation. the nfo delivery framework also facilitates tech‑ nological and knowledge transfer for scientific and technical training, as well as public outreach, at different levels: general education of public on natural hazards and seismo‑tectonic processes, higher education (summer schools, masters and postgraduate research) on advanced instrumentation techniques, multi‑parameter analysis of crustal processes. bringing together multidisciplinary data and expertise at a single site to be processed and integrated, will put important pieces in the puzzle of earthquake mechanics. finally, besides the monitoring of the faults be‑ fore the occurrence of main events, the dense nfo networks can also improve operational actions conducted in the aftermath of a large earthquake, such as earthquake early warning (eew) and detailed, fast ground shaking and damage characterization during the on‑going rupture on the fault. near fault observatories in europe 5 2. the near fault observatories in europe in the following, we introduce the six nfos that have formally signed the ca. the six nfo infrastructures are indicated in figure 2. nfos are usually operated by a single national organisation but can also be represented by multiple national and european organisations. the complexity beyond national research infrastructures (nris) is managed at the nfo level. nfos, in line with other tcs service providers, fully adopt the epos data policy and permit open and free distribution of their own data and products through the epos services, on behalf of the data owners (suppliers), and in compliance with fair principles. in future, new nfos can be expected to join the community. recently the slovenian karst nfo has joined the consortium as an observer; a status formally recognised by the community, allowing potentially new nfos to par‑ ticipate in the ca board meetings and initiatives. this is an initial step prior to becoming a full member in the future. figure 2. maps of the six european near fault observatories showing details of each infrastructure. the symbols indi‑ cate: yellow squares for seismic stations, blue squares for gnss stations, purple drops for radon stations, green drops for co2 stations. a red circle with a seismogram inside indicates multi sensor sites; in taboo they stand for co-located seismometer, strainmeter, pore pressure gauge and optic fiber cable within boreholes, in irpinia for fibre cable and seismometer, for vrancea infrasound array and seismometer and for corinth strainmeter, borehole water‑level, tide gauge and meteorological sensors. 2.1 the corinth rift laboratory (crl) the western gulf of corinth (wgoc) is a 110‑km‑long rift dominated by active, ~e‑w‑trending normal faults, located in central greece. an extension rate of 15 mm/yr in a n9°w direction has been determined [briole et al., 2021]. it is characterized by significant seismic activity, with the occurrence of destructive earthquakes [mak‑ ropoulos et al., 2012]. in 1995 the ms = 6.2 aigion earthquake occurred, caused by an offshore low‑angle fault [ber‑ nard et al., 1997]. in the beginning of the 21st century, the corinth rift laboratory network (crl) was established (http://crlab.eu/), covering an area of ~30 × 30 km2, centred between patras to the west and aigion to the east [cor‑ net et al., 2004], with main scientific goal to investigate in detail the intense and continuous microseismic activity http://crlab.eu/ lauro chiaraluce et al. 6 and deformation. currently, crl is the only multinational nfo. the european institutions that form crl are the centre national de la recherche scientifique (cnrs, france), the national and kapodistrian university of athens (nkua, greece), the university of patras (upatras, greece) the national observatory of athens (noa, greece) and charles university (cup, czech republic). crl is a dense network of 29 permanent seismological operated by crl (cl network, doi: 10.15778/resif.cl), nkua (ha network, doi: 10.7914/sn/ha), upatras (hp network, doi: 10.7914/sn/hp) and noa (hl network, doi:10.7914/sn/hl). ha, hp and hl networks also belong to the hellenic unified seismological network [husn; evangelidis et al., 2021]. crl is equipped with 16 gnss stations installed by crl and noa [ganas et al., 2008; chousianitis et al., 2021]. furthermore, two multiparameter sites are operated in crl (see figure 2). the first is riza which is equipped with borehole three‑component strainmeter, rainmeter, borehole water‑level and atmo‑ spheric pressure sensors. the second site is moki, where the multparameter instruments are borehole three‑com‑ ponent strainmeter, tide gauge, borehole water‑level and atmospheric pressure sensors. seismological data are available through the european integrated data archive (eida) nodes at resif and noa [evangelidis et al., 2021]. eida has declared the virtual network identifier _nfocrl for all seismic stations within the crl perimeter. crl also distributes scientific products, such as phase and focal mechanism data nkua, noa and upat. gnss data and positioning solutions are now available at the crl portal (https://crlab.eu) and will migrate to the epos data portal. the use of the freely available crl data highlighted that most seismicity is clustered, with the frequent occur‑ rence of swarms, as the 2001 agios ioannis [pacchiani and lyon‑caen, 2010], the 2013 helike [kapetanidis et al., 2015, 2021; kaviris et al., 2017] and the 2015 malamata [de barros et al., 2020] ones. geodetic studies, from per‑ manent and campaign gnss network measurements and insar, identified the deformation sources of the aigion [1995] and efpalio [2010] earthquakes, as well as slow aseismic shallow slip [elias and briole, 2018]. the recent 2020‑2021 seismic crisis was studied by the crl research group [kaviris et al., 2021]. in the latter case, the analysis of the crl multiparametric data significantly contributed to understand the seismogenic process. combined seis‑ mological and geodetic data were considered in order to constrain the geometry and kinematics of the structures that hosted the major events of the 2020-2021 seismic crisis. in addition, the significant role of crl tide-gauge data, analyzed along with seismological and gnss recordings, is proven in ongoing research that aims to identify the rupture process of the largest event of the sequence that occurred on 17 february 2021. the wealth of the multiparametric crl data led to the in‑depth knowledge of the seismogenesis in the west‑ ern gulf of corinth. the dual behaviour of certain fault segments, subject to transient creep and to pore pressure diffusion, has been identified in the wgoc. the detailed analysis of multiparametric crl data also led to the iden‑ tification of a brittle, highly fractured layer in depths between 6 km and 9 km, where the major normal faults of the crl area are rooting [lambotte et al., 2014; duverger et al., 2018]. it is obvious that the free provision of crl data, scientific products and services to the whole community will significantly contribute to the detailed knowledge of the mechanics and kinematics of the area, with a high scientific impact, as it will be possible to apply similar procedures in other regions worldwide. 2.2 the irpinia near fault observatory (info) the irpinia near fault observatory ‑info – (http://isnet.unina.it) is a natural laboratory (see figure 2) that monitors the underlying fault system along the campania‑lucania apennine chain (southern italy), a normal fault environment, frequently struck by destructive earthquakes, with a recurrence period of events with m > 5.5 of about 30 years [cinti et al., 2004], and the last major event in the area being the m = 6.9, 1980 irpinia earthquake. this area undergoes an extensional rate of ~3 mm/yr, as inferred from geodetic measurements [d’agostino et al., 2010]. present‑day low‑magnitude seismicity (m < 3.5) occurs in the shallow portion of the crust, is organized in seismic sequences [festa et al., 2021] and appears spread into a large, fractured volume confined in the graben responsible for the 1980 earthquake. beyond the large seismic hazard of the area, the main scientific questions targeted by this infrastructure is to understand the role of fluid-rock interaction and fluid diffusion water and carbon dioxide [amoruso et al., 2014], in the production of the earthquakes, and the contribution of the charging process of shallow aquifers in affecting the stress of rocks at seismogenic depths [d’agostino et al., 2018]. info is ultimately aimed to catch and track the preparatory phase of large events in the area, through the fine characteri‑ zation and modelling of the seismicity and the deformation. http://resif.cl https://crlab.eu http://isnet.unina.it near fault observatories in europe 7 info is managed by the university of napoli federico ii and operates the irpinia seismic network (isnet), a dense, high‑dynamic range seismic network of 31 stations, covering an area of about 120 × 90 km2, deployed within two concentric ellipses, with the major axis parallel to the apennine chain [iannaccone et al., 2010]. the average inter‑station distance within the inner part of the infrastructure is less than 10 km. all stations are equipped with a strong‑motion accelerometer and a weak motion sensor, the latter being a short‑period, a broadband velocimeter or an accelerometer with lower full‑scale to record both strong and weak motions associated with earthquakes and ambient noise. isnet provides real‑time data with a controlled delay, allowing the infrastructure to be the italian backbone for testing eew systems [satriano et al., 2011]. the seismic network is complemented with 9 additional broadband stations from the national network, 15 gnss stations, managed by ingv, and two seismic arrays of 5 six-component stations at the centre of the network. recently, a 1.1 km fibre optic cable (fibir) has been installed near the emergence of the main fault that generated the 1980 earthquake, and it is now sensed by a das, in the frame of the irpidas survey. in the near future, the area will be also covered with additional gnss and geochemical stations, to better constrain the ground deformation and monitor geochemical fluids. 2.3 the marmara sea observatory the north anatolian fault zone (nafz) is one of the largest plate‑bounding transform faults that separate the anatolia and eurasian plates and extends for 1600 km between eastern anatolian and the northern aegean. the anatolian block is moving westward with respect to the collision zone between the eurasian and arabian plates, at a rate of ~25 mm yr‑1 east of the sea of marmara, activating major strike‑slip and n‑s extensional normal fault‑ ing earthquakes south of the marmara region [armijo et al., 2002]. along the nafz a series of large earthquakes occurred since 1939, starting from erzincan in the eastern anatolia and propagating westward toward istanbul and marmara region located, where the devasting izmit earthquake occurred in 1999. west of the izmit rupture a ~100 km long seismic gap exists along the fault portion placed below the sea of marmara connecting the ganos [1912, mw7.3] and izmit [1999, mw7.4] ruptures; such a segment can generate an earthquake with magnitude ~7.1 [ergintav et al., 2014]. the 30‑year probability for an event m ≥ 7 below the sea of marmara is estimated at 35‑ 70% [parsons, 2014]. there are many multidisciplinary geophysical networks monitoring the heterogenous pattern of interseismic loading of the area, showing both creeping and locked segments [ergintav et al., 2014; schmitbull et al., 2015; klein et al., 2017; lange et al., 2019; yamamot et al., 2019]. the backbone of the monitoring systems has been built by local universities like boğaziçi university (bu-koeri, istanbul university) and governmental organizations like afad, tubitak and general directorate of mapping (gcm). bu‑koeri’s national earthquake monitoring center (nemc) is a 24/7 operational centre comprising 136 broad‑band (bb) and 107 strong motion (sm) sensors at the na‑ tional level. about 200 digital strong motion accelerometers are operated by koeri as dense urban network in and around istanbul (rapid response and early warning system). koeri also operates 5 sea-floor multi-instrument ob‑ servation systems in the sea of marmara, actually deserving renovation, and acts as the national tsunami warning centre for turkey under the icg/neamtws initiative. afad’s earthquake data center system of turkey (tdvms) is a system which pairs up all the earthquake data obtained from seismic stations all around turkey and shares these data via its web portal. it also operates like bu‑koeri and run more than 1000 bb and sm stations. gcm controls more than 150 continuous gnss stations, including other geodesic networks (e.g. gravity, sea‑level monitoring). at the same time, many international efforts have been posed to better define the earthquake hazard pattern of the region. for example, to monitor the offshore microseismic activity in the nafz nearby istanbul, below the çinarcik basin (çb), a permanent seismic array [pires; bohnhoff et al., 2013] was installed on the prince islands in 2006, at a very close proximity to the main fault. jointly operated by german research centre for geosciences pots‑ dam (gfz) and bu‑koeri, pires allows to constrain the spatiotemporal character of the microseismicity based on well‑constrained hypocenters is performed to understand the interaction between fault segments of the nafz along the çb [bohnhoff et al., 2013]. to extend this study, in 2013 the gonaf project [geophysical borehole obser‑ vatory at the north anatolian fault; raub et al., 2016] started under a joint research venture between german gfz and turkish disaster and emergency presidency (afad), turkey. the aim of this project is to monitor earthquake activity at low magnitude‑detection threshold in the istanbul and eastern sea of marmara region where a major (m > 7) earthquake is pending. in this context, also borehole strainmeters have been installed together with us agency unavco [bohnhoff et al., 2010 and martinez-garzon et al., 2019]. https://www.afad.gov.tr/en/ lauro chiaraluce et al. 8 however, in 2014, the marmara region has been designated a “permanent supersite” by the ceos (committee on earth observation satellites) under geo geohazard and natural laboratories initiative (gsnl). this initiative provided the opportunity to measure the surface displacements, in terms of mean deformation velocity maps and corresponding time‑series, affecting the marmara region via the exploitation of sar data acquired by the differ‑ ent satellite systems investigate. the achieved results can be fruitfully compared with the available independent in‑situ measurements (e.g., gps data, strainmeter) providing additional constrains and details [diao et al., 2016; de michele et al., 2017; aslan et al., 2018; martínez‑garzón, 2021]. in this framework, a core study group has been established in the consortium of the european commission funded marsite (new directions in seismic hazard assessment through focused earth observation in the marmara supersite) project (2012‑2016). marsite organized as the demonstrator for integrating satellite and in‑situ observations and for providing integrated access to data until the end of the project. from the end of the marsite project, most of the activities and related data and scientific products, were reorganized and harmonised within the marmara near fault observatory, coordinated under epos support. per‑ manent and temporary sets of data have been standardised following communal open access sharing rules and new web services have been implemented in the epos framework. not all the existing infrastructures have been already harmonised; this is a long process deserving time to be pursued. for sure nfo community represents an optimal opportunity to create a common scientific environment primarily finalised to gain valuable information about the off‑shore unbroken segments of the region to be shared with decision‑makers to improve the hazard maps of the marmara (specially istanbul) and their risk‑plans. at the same time, nfo novel platforms for data exposure and sharing will represent an occasion as well to provide to every kind of users with all the available data and scientific products generated by such an important suite of multidisciplinary monitoring and research infrastructures. 2.4 the alto tiberina near fault observatory (taboo) the alto tiberina fault (atf), located along an extending sector, at a rate of about 3 mm/yr [serpelloni et al., 2006], of the northern apennines (central italy) is a 60 km long very low‑angle normal fault (mean dip 20°) that is the target of taboo (the alto tiberina near fault observatory), a permanent monitoring infrastructure managed by the istituto nazionale di geofisica e vulcanologia [ingv; chiaraluce et al., 2014]. taboo is a state‑of‑the‑art dense network (see figure 2) with mean inter‑distance of about 5 km between mul‑ tidisciplinary sensors, deployed both at surface and within shallow boreholes (< 250 m). stations record and trans‑ mit in real time via dedicated wi‑fi technology; then data is stored in standard formats on open access thematic portals and distributed via web services. while seismic data from taboo reveal release of microseismicity, at a consistently high rate on the atf fault plane, including repeating earthquakes (re), no historical earthquake can be unambiguously associated with the activation of the whole atf [chiaraluce et al., 2007 and references therein]. res [valoroso et al., 2017] together with a steep gradient in crustal velocities [vadacca et al., 2016], measured by gnss, and transient surface motion, lasting for few months and coinciding with seismic swarms [gualandi et al., 2017 and vuan et al., 2020], support the hypothesis that portions of the atf are creeping aseismically. recent studies document that any given patch of a fault can creep, nucleate slow earthquakes, and also host large earthquakes [e.g., iquique earthquake, ruiz et al., 2014; tohoku earthquake, kato et al., 2012 and parkfield, veedu and barbot, 2016]. why a fault patch would switch from one mode of slip to another one runs contrary to the standard theory. thus, these observations are forcing a revolution in our way of thinking about how faults accommodate slip. however, the interaction between creep, slow and regular earthquakes is still poorly documented by observation. taboo is currently collecting unique data needed to address such questions; by illuminating the physics that allows for both seismic and aseismic slip on a single fault patch, will have important implications for seismic hazard and risk assessment globally. 2.5 the valais observatory the nfo valais is a particularly dense multidisciplinary component of the monitoring infrastructure in swit‑ zerland, operated by the swiss seismological service at eth zurich. it is centred around the canton of valais in near fault observatories in europe 9 the se corner of the country and in the high alps. although damaging earthquakes are rare in switzerland when compared to more seismically active regions, such events have occurred and will continue to do so. over the past 700 years, a total of 28 events of magnitude mw ≥ 5.5 are known, twelve of which caused severe damage (intensity of viii or higher). the valais region is the area of greatest seismic hazard in switzerland and has experienced a magnitude 6 or larger event every 100 years (1524, 1584, 1685, 1755, 1855, 1946), with the last magnitude 6.1 earth‑ quake in 1946 close to sion and sierre [fritsche and fah, 2009]. this area and in particular the region of visp holds special interest: on average, the visp region has been struck by damaging earthquakes every 40 years, with the last in 1960 reaching a macroseismic intensity of viii. the visp event of 1855 is the largest in switzerland in the last 300 years. in addition to elevated seismic activity, the extreme topography, with steep unstable slopes and deep soft soil valleys, and extreme climate, with large glaciers and deep snow cover, add to the total hazard level in the valais. the area has experienced great damage from earthquake ground motion and different secondary phenome‑ na, such as liquefaction in the rhone plain, landslides and rock fall [fritsche et al, 2012]. the human impact of the last centuries has further raised the hazard. river regulations, advances in engineering methods and population growth have meant settlements and industries are now sited in seismically vulnerable areas. the particular densification of the seismic monitoring in the valais, as well as the introduction of complementa‑ ry multidisciplinary instrumentation, including those with the potential to provide precursory information, began during the cogear project [fäh et al, 2012] and continues today. nfo valais comprises 16 co-located broadband and strong motion stations and 37 strong motion stations (see figure 2) that are openly available and also inte‑ grated in the backbone national network (swiss seismological service (sed) at eth zurich; 1983). nfo valais sta‑ tions, like all the nfo ones, are updated under eida virtual network code (e.g., _nfovalais). multiple additional temporary stations are also routinely available. gnss and magnetotelluric sensors complement this network. the network features 2 boreholes at visp and collombey that include multiple downhole accelerometers and a string of pore‑water pressure sensors. the seismic network is densest along the rhine valley, with multiple strong motion stations located in the towns of visp and sion that demonstrate the strong and variable site amplifications that can be observed across the heavily populated deep alluvial basin. the network allows the detection and location of microseismicity in high resolution, since in this area swarms are common. the sed/eth team has focused on the real‑time double difference relocations and a weekly updated multi‑event re‑located catalogue, allowing high precision solutions for any sequence. another key feature of this network is the longevity of the stations many of the broadband stations date back over 20 years. the available waveform archive also includes all digitised triggered earthquake records from short period stations dating back as early as 1976, and strong motion stations dating back to 1992. today, the nfo valais seismic stations are all optimised for extreme low-latency data and contribute to the the emerging early warning infrastructure in switzerland [massin et al, 2021]. 2.6 the vrancea observatory vrancea area, located within the bend region of the carpathian orogen and characterised by compressional tec‑ tonic, is one of the most active seismic zones with intermediate depth earthquakes in europe. vrancea seismic zone consists of both intermediate and crustal depth earthquakes (30‑180 km) with the shallower ones (depth <50 km) usually generating moderate events (mw ≤ 5.5), while the deepest (70‑180 km) reaching mw 7.0. vrancea is an observatory composed by state of art networks of multidisciplinary sensors whose backbone con‑ sists of seismic stations together with both infrasound and seismic arrays. all seismic stations are equipped with both short period/broadband and strong motion sensors and are located at the surface or within shallow boreholes. while the infrasound arrays are of course located at surface. then, gnss antennas, radon monitoring system, me‑ teorological stations, electromagnetic stations and atmospheric ionization monitoring systems complement the infrastructure [mărmureanu et al., 2021; toader et al., 2021] such a high‑resolution multidisciplinary monitoring and research infrastructure consents the availability of a large variety of data to better understand the possible extension at depth of the signature of active tectonic pro‑ cesses. however, although deep, the major seismic events occurring in vrancea are not only felt in a very large area but generate massive damage [earthquake intensities up to ix recorded in bucharest for 1977 mw 7.4 event, mărmureanu et al., 2011]. and it is for this reason that starting from 2012 national institute for earth physics designed the nfo seismic network in vrancea area to be fully operational for an efficient early warning system lauro chiaraluce et al. 10 (ews). the seismic infrastructure is (spatially and technically) tailored to detect and locate strong and intermedi‑ ate depth earthquakes. ews main target it is represented by bucharest capital city hosting almost 2 millions of people. until now the existing ews issued more than 50 successful alerts, all with a leading time larger than 25 seconds, to ~450 legal authorities going from emergency response agencies, via sms gateway. 16 additional dedicated ews receivers are located in between romania and bulgaria at various governmental agencies, including a nuclear‑powered research institute in bucharest and vidraru dam. the alerts are also sent to nuclear power plants “kozloduy” in bulgaria and cernavoda [mărmureanu et al., 2021]. having the nfo community the leadership in ews services, furnishes the great opportunity to continuously test and update innovative schemes and benchmarking systems such as the communication freely to users through social platforms, currently counting in romani ~14000 users. 3. fridge: the nfo federated data gateway nfos contribute to the epos enterprise by delivering (long) time‑series of multidisciplinary and high‑reso‑ lution raw data collected near active faults and derived cutting-edge scientific products (named high level data products in the epos taxonomy). for raw data (and station information) repository and distribution, the community benefits as much as possible of the existing services. this is the case of the seismological and geodetic community already possessing format and metadata recognised as standard by the community. thus, these data are delivered and distributed through the existing european nodes organised and managed by the respective thematic communities. in the last years, all nfo seismological (velocimetric and accelerometric) data from both permanent and tem‑ porary (during experiment) stations, have been converted into the standards decided by the seismological com‑ munity and together with the stations (and metadata) information have been moved to the closest european in‑ tegrated data archive (eida) node ready to be distributed [strollo et al., 2021]. in order to facilitate the nfo data identification, discovery and download, together with the seismology tcs, we created the nfo virtual networks (vn), allowing the user an easier and more comprehensive way to discover and download the data. now, all the 6 nfos have a vn code identifying the infrastructure. the same concept is being developed for the geodetic data, working in close collaboration with the gnss tcs. the community is currently inserting/uploading the nfo gnss station information (and metadata) into the nodes of the newly implemented glass open‑source platform. since the nfo also provide additional data and products that are not provided by other tcs, these nfo specific data, defined as raw data and high-level data products originally not having a standard (e.g., format, unit of mea‑ surements, metadata), the nfo community built a dedicated portal named fridge, standing for federated specific data gateway (http://fridge.ingv.it). nfo specific data are the continuous raw geochemical data (e.g., radon counts, co2 flux time series), with associated meteorological parameters. high level data products cover diverse disciplines characterizing the nfos, such as seismology (e.g., vp/vs time series, various generation and typology of earthquake catalogues, velocity and attenuation models, historical earthquakes), geodesy (e.g., strain‑rate time series) and geology (e.g., fault invento‑ ries, geological maps). thus, the process of distributing nfo specific data and products included not only planning fridge from the computer science perspective but also the identification of new standards. an example of this activity comes from the work we carried out and/or currently undergoing, together with the volcanological and seismological communities (always within the specific harmonization groups), in order to standardise geochemical data and seismological data products, respectively. fridge is then a unique and coordinated access for nfo specific data pointed by both the epos-ics-c plat‑ form and services as well as by the indigenous users. from a technical point of view, fridge has been thought as a federator exposing http api for discovering and querying all the available nfo datasets and combining replies for the user (ics-c or anybody) as well as for hosting a virtual laboratory, an engagement and knowledge sharing instrument (see figure 3). the federator is implemented in a modular way through tornado framework, a python web server composed by a main module and different plugins for each web service implemented by the single nfos. this allows an easy development of new future services while reducing its maintenance. the fed‑ erator exposes a set of apis, through which it is possible to query the web services asking for the data provided by the nfos. when a user performs a request based on geographical coordinates, the federator addresses and http://fridge.ingv.it near fault observatories in europe 11 redirects it to the corresponding nfo. the federator exposes their apis also to the fridge web portal that offers an alternative graphical user interface (gui) to search and download data and data products, and view time series and graphical maps. the availability of such a diverse kind of time series gives to the scientific stakeholders the novel opportunity to start looking together at interdisciplinary data and derived scientific products collected in the same area and at the same time. these actions, opening to the possibility of connecting different observations to the same phenomena, can also give the rise to a new generation of scientists capable right from the beginning to consider multidisci‑ plinary data. 4. crew consisting of dense, modern and low‑latency infrastructures, the nfos are also ideal sites to process real‑time data, making them test‑beds for experimenting earthquake early warning (eew) systems. eew systems can be used to activate security actions and mitigate the seismic risk during an occurring earthquake. in this context, the community has developed the platform crew eu testing centre early warning & source characterization to test state‑of‑the‑art systems, software and platforms, to provide a fair comparison of methodologies, based on community‑based performance criteria, providing a complete report of the results. from a technical point of view, crew (figure 4) is structured in virtual machines, such that each system can be fully configured and customized according to its architecture by the relevant experts during the initial set-up and tuning phase that precedes the formal testing. this includes the installation and tuning of the operating system and the installation of support tools that might be needed to achieve its full potential. today, crew compares the eew software presto [satriano et al., 2011] and vs virtual seismologist [behr et al., 2016], on the real time data streaming from isnet [festa et al., 2021]. each virtual guest has been assigned the same amount of (virtual) hardware resources, so that its final performances will only depend on the eew software al‑ gorithms and configuration, given the availability of the same set of recorded waveform data in a timely fashion. inputs and outputs to each eew system are provided in a standardized and widely accessible format. each eew systems, receive data in an identical manner from the seedlink server and produce real‑time alarm messages in case of earthquake detection. a new alarm is issued every time the estimate of a relevant earthquake source figure 3. design of the virtual laboratory in fridge aimed at visualization and combination of multidisciplinary data and products, which are available at the fridge portal. lauro chiaraluce et al. 12 parameter produced by the system is changed (e.g., earthquake location, magnitude) or when a new data input is included in the evaluation (a new p‑ or s‑wave arrival time). the testing centre server hosts the qfeed2 software from eth (written in java), which retrieves the seismic bulletins for the irpinia region, namely the authoritative one from ingv (http://terremoti.ingv.it/) and the bulletin published by info (http://isnet-bulletin.fisica.unina. it/cgi‑bin/isnet‑events/isnet.cgi). the latter might be more complete in terms of lower magnitudes for events well located within the network. the earthquake information from the bulletins is parsed and inserted in a postgresql database, which forms the authoritative source of real earthquakes and source parameters. this event database is also updated when the external agencies revise some of their published information. finally, a gui (http://lccepos.fisica.unina.it) has been implemented that consists of a web application that can list the alarm messages from each system as compared to reference seismic bulletins, plot them on a map and pro‑ vide statistics about the quality and speed of the outputs of each system. scientists can access to crew for testing new eew software or some parts, to compare against state‑of‑the‑art software, through the direct plugin of their modules and the control of parametrization. also, a user can evaluate the performance of existing, state-of-the-art software on their own data, finely tune the system parameters and design tailored decision modules. access to crew may be granted as transnational access [tna; wessels et al., 2022]. beyond scientists, additional stakeholders can be public agencies representatives, and private companies. with‑ in testing eew in crew, stakeholders can design and evaluate the performances of software to be interfaced with actuators for end‑to‑end eew applications: this will include target‑oriented software and output combination and a direct connection of this output to appropriate decision modules, for risk mitigation actions specific to the site to protect. the crew will also act as a flywheel for the nfo, eew and earthquake source communities, allowing to share know‑how and high‑quality data with a broad population of researchers, favouring indeed their enlargement and interaction, and to promote leading‑edge science and technology. finally, the crew may become a proof‑of‑ concept of testing facilities that can be exported to other scientific environments and topics. 5. future perspectives of the european nfos the understanding of the slow and fast deformation within the earth crust associated with fault slip is one of the major challenges in earth sciences. the european nfos plan to integrate state-of-art field observations with novel instruments, such as borehole seismometers and strain meters, fibre optic cables, dense geodetic and geo‑ chemical networks, monitoring several parameters. the resulting data, distributed to the scientific community, will be complemented with geologic studies of fault zone structure, analytical methods and laboratory experiments, to illuminate the geophysical processes at very different scales. the nfo challenge is the achievement of a new expertise able to process and integrate multidisciplinary data and place them in the puzzle of earthquake mechanics, based on state-of-the-art tools (e.g., artificial intelligence). figure 4. the crew workflow and infrastructure. real-time data flow in the server where software runs on different virtual machines to define alerts, that will be compared with authoritative bulletins. http://terremoti.ingv.it/ http://isnet-bulletin.fisica.unina.it/cgi-bin/isnet-events/isnet.cgi http://isnet-bulletin.fisica.unina.it/cgi-bin/isnet-events/isnet.cgi http://lccepos.fisica.unina.it near fault observatories in europe 13 this opening is a great opportunity to train a new generation of scientists with the know‑how of modelling and interpreting long time series of high‑resolution multidisciplinary observations attributable to the same underlying natural phenomena. nfos are also ideal sites for meeting the growing expectations of the learning and communication sectors by hosting a large variety of scientific information about earthquakes as a natural phenomenon and a societal issue. at the same time, by working on the improvement of predictive models for future large earthquakes, based on multi-parameter monitoring of diverse types of transients, nfos contribute to better configure the next genera‑ tion of the monitoring systems and to provide interpretation and communication models for the authorities and public both before and during the occurrence of a large event. within this context, disaster managers and related scientific disseminators are among the main stakeholders. improved monitoring of active faults posing high haz‑ ard to society, as well as progress in the knowledge of seismic faulting, including inspection of different signals, will enhance our common capabilities to face and assess seismic hazard at different spatial and temporal scales. in this perspective, a new generation of near‑real time tools and triggering alarms at different levels can be tested and developed, strengthening the cooperation with the agencies for risk management. a series of various actions like meetings of experts, simulated operations, including educational experiences for the population, can also be tested. however, the processing of continuous and large data flow is expected to progress with time, in particular for the multi‑sensor correlation analysis. these tools will evolve with the improved models of transient processes in/around fault zones. nfo can provide the finest context to exploit this expertise and testing. the nfos are expected to become ideal sites for hosting state-of-the-art experiments for testing in situ both new scientific ideas/models and new geophysical instruments or arrays. a nfo would provide the proper environment in terms of reference instrumentation, data and infrastructures facilities. the accessibility of nfos databases allows for rap‑ idly evaluating and selecting the most appropriate site for the instrument or software testing and/or new scientific hypotheses validation. this will achieve synergies in research, monitoring and tools (e.g., automatic analysis) by sharing technological best practice for implementation (e.g., boreholes instrumentation for multi‑sensor stations) and technological issues regarding instrumentation and software. the nfos are also the natural sites for scientific drilling experiments, as testified by the fact that several sites have already been declared of interest for icdp and iodp. icdp and iodp have been already involved in drilling the rift of corinth. the aig10 borehole of about 1000 m provided cores of the aigion fault, an active normal fault placed at the centre of crl‑nfo, within the deep geo‑ dynamic laboratory (dg-lab project; cornet and vardoulakis, icdp proposal 2003). the objective of the project was a better understanding of the physics of faulting in an extensional tectonic regime, with special attention to interactions between fluids and faulting. the gulf of corinth was also the site of an iodp project dedicated to the seismic sounding and deep sediment coring in the central part of the gulf [mcneill et al., 2018]. icdp is instead currently involved in (e.g. star, standing for a strainmeter array along the alto tiberina fault system, central italy project in taboo‑nfo area; https://www.icdp‑online.org/projects/world/europe/northern‑ apennines-italy/). star is an ongoing project led by ingv in collaboration with unavco us agency, finalised to the instrumentation with strain‑ and seismometers of six 80‑160 m deep vertical boreholes surrounding the creeping portion of the atf. each station, named “small observatory”, is also equipped with surface gps, meteorological instruments, and fibre optic cables (see figure 5). the suite of instruments will enable the collection and calibration of strain records with exquisitely high pre‑ cision, allowing for a quantitative characterization of atf creep (~1 mm over <1 km2), enhanced monitoring of microseismicity (below mc 0.5), and allowing correlation between degassing (co2, rn) measurements and subsur‑ face strain. importantly, the spatiotemporal characteristics of creep on the atf bear directly on the relationship between seismic and aseismic processes, including possible stress triggering of large earthquakes by transient creep events, an issue of global importance in the seismic hazards’ community. this tcs will experience for the first time the technological integration of multidisciplinary observing systems in diverse tectonic environments. shared technical basis, as well as strengthened cooperation between institu‑ tions, will foster exchange of scientific analysis techniques (both strategies and implementations) and will help to lower costs of new scientific achievements. the need for a continuous upgrade in the technological quality and density of multi‑parameter monitoring systems clearly represents a big opportunity of growing market for the european industrial sector for new technologies. at the nfos a new generation of multidisciplinary experiments https://www.icdp-online.org/projects/world/europe/northern-apennines-italy/ https://www.icdp-online.org/projects/world/europe/northern-apennines-italy/ lauro chiaraluce et al. 14 can be established, testing multi‑sensor stations, as well as fast and light transmission systems for temporary densification of monitoring instruments. this environment will allow the nfo community to develop strong syn‑ ergies also with small medium enterprises able to construct state of art sensor prototypes that can be tested in a controlled environment with a large number of external constraints deriving from dense permanent networks. thus, nfos are entirely qualified as a network for transnational education and training. common portals with a large choice of training possibilities may highlight the large variety of the instrumental, scientific and natural contexts offered by the nfos. the nfo community will provide many opportunities for scientific and technical training as well as public outreach, at different levels: general education of public on hazard and natural seismic and tectonic processes, higher education (summer schools, masters and postgraduate research) on advanced in‑ strumentation techniques and multi‑parameter analysis of crustal processes. for these reasons, we expect the implementation of many near fault infrastructures in the coming years, fo‑ cused on the physics of tectonic faulting and able to generate data and scientific products interrogating a broad spectrum of fault slip behaviour [peng and gomberg, 2010]; key ingredient for the next generation of scientists finally able to embrace a real multidisciplinary investigating approach. references abercrombie, r. e. 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tali p e r d i t e di e n e r g i a si r e a l i z z a n o a t t r a v e r s o v a r i p r o c e s s i dei q u a ] i i più r i l e v a n t i s o n o : c o l l i s i o n e e d i r r a g g i a m e n t o . p e r la r a d i a z i o n e c o s m i c a la i o n i z z a z i o n e m i s u r a t a dà n e l l e com u n i e s p e r i e n z e un v a l o r e m e d i a t o -u t u t t o l o s p e t t r o o h e è c o s t i t u i t o , c o m e si s a , da p a r t i c e l l e di m a s s a ed e n e r g i a v a r i a b i l i e n t r o l i m i t i m o l t o l a r g h i : l e e n e r g i e v a n n o d a v a l o r i d e l l ' o r d i n e di 1 0 a i o " cv. l e p a r t i c e l l e <li p i ù b a s s a e n e r g i a . < 1 0 6 cv, v e n g o n o gen e r a l m e n t e t a g l i a t e d a l l e p a r e t i dei c o n t a t o r i i m p i e g a t i n e i d i s p o sitivi s p e r i m e n t a l i , m e n t r e il c o n t r i b u t o a l l a i o n i z z a z i o n e d a t o d a l l o p a r t i c e l l e d o t a t e di e n e r g i a > 1 0 " cv. e s s e n d o q u e s t e p o c o n u m e r o s e ( m e n o del 2 % a l l i v e l l o del m a r e ) ( j ) , p u ò in p r i m a a p p r o s s i m a z i o n e ossero t r a s c u r a t o . l e m i s u r e sia d e l l a i o n i z z a z i o n e p r i m a r i a elio d e l l a i o n i z z a z i o n e t o t a l e ( p r i m a r i a + s e c o n d a r i a ) v e n g o n o r e a l i z z a l e con m e t o d i d i v e r si ( 2 ) ; i r i s u l t a t i n o n s o n o s e m p r e d ' a c c o r d o tra di l o r o ; in p a r t i c o l a r e nel c a s o d e l l a i o n i z z a z i o n e t o t a l e e d e l l a i o n i z z a z i o n e p r o b a b i l e ( ' ) essi s c a r t a n o , q u a l c h e v o l t a a n c h e di p a r e c c h i o ( 5 0 % ) , r i s p e t t o ai v a l o r i p r e v i s t i d a l l a t e o r i a . l a i o n i z z a z i o n e p r i m a r i a si d e t e r m i n a o c o n c a m e r a di w ilson ( ' ) o v v e r o d a m i s u r e s u l l a e f f i c i e n z a dei c o n t a t o r i di g o i g e r m i i l l e r : q u e s t ' u l t i m o m e t o d o i n t r o d o t t o da c o s y n s ed a l t r i ( ' ) . m e n t r e è p i u t t o s t o s e m p l i c e dal l a t o t e c n i c o , p u ò f o r n i r e r i s u l t a t i a b b a s t a n z a p r e c i s i . n e l c o r s o di u n a r i c e r c a s u l l ' a l l e s t i m e n t o e l ' i m p i e g o di c o n i a tori a b a s s o r e n d i m e n t o r i e m p i t i c o n i d r o g e n o p u r o (r >). i n v i s t a d e l i i n t e r e s s e che) p o t e v a p r e s e n t a r e , a b b i a m o e s e g u i t a una m i s u r a d e l l a i o n i z z a z i o n e s p e c i f i c a p r i m a r i a d e i l a r a d i a z i o n e c o s m i c a n e l i . a r i a od;> © i o n i z z a z i o n e s p e c i f i c a p r i m a r i a n e l l a r i a : è -u q u e s t a m i sura c h e ?i r i f e r i s c o n e l l a p r e s e n t e n o t a . il m e t o d o di cui s o p r a c o n s i s t e nel f a r l a v o r a r e un c o n t a t o r e di g . m . in c o n d i z i o n i t a l i c h e una p a r t i c e l l a i o n i z z a n t e a t t r a v e r s a n t e i l c o n t a t o r e a b b i a una p r o b a b i l i t à di i n n e s c a r e la s c a r i c a , ed e s s e r e q u i n d i r e g i s t r a t a , m i n o r e d e l l u n i t a , c p o i c h é t a l e p r o b a b i l i t à r i s u l t a , t e n u t i c o s t a n t i tutti gli a l t r i p a r a m e t r i , f u n z i o n e d e l l a i o n i z z a z i o n e p r i a ( 1 m a r i a j d e l l a p a r t i c e l l a i n c i d e n t e , è c h i a r o c o m e da u n a m i s u r a d e l l a efficienza del c o n t a t o r e : r a p p o r t o del n u m e r o d e l l e p a r t i c e l l e c h e a t t r a v e r s a n o il c o n t a t o r e al n u m e r o 1 d e g l i i m p u l s i r e g i s t r a t i , si possa r i s a l i r e al v a l o r e di ./. d i f a t t i se si a m m e t t e c h e : 1) b a s t a la c r e a z i o n e di u n a sola c o p p i a di ioni p e r c h é il c o n t a t o r e dia l ' i m p u l s o ; 2 ) sia t r a s c u r a b i l e l ' e f f e t t o d e l l e p a r e t i d e l c o n t a t o r e , l ' e f f i c i e n z a , c o m e è s t a t a d e f i n i t a a v a n t i , r i s u l t a p e r p a r t i c e l l e s i n g o l e d a t a d a l l a e s p r e s s i o n e ( 7 ) -ijp/p k=1 — e. [1] d o v e / = c a m m i n o m e d i o d e l l e p a r t i c e l l e n e l l ' i n t e r n o del c o n t a t o r e , p,'p» = r a p p o r t o d e l l a p r e s s i o n e del g a s a q u e l l a n o r m a l e , e ^ i o n i z z a z i o n e s p e c i f i c a p r i m a r i a . \ mv, | c; i r ^ t ^ t ] h inv [ i m v; [ jl < l m v, num.ò in v [ :-]nuiti.2 | fig. 2 dispositivo sperimentale e misure. — l a m i s u r a di k è s t a l a e s e g u i t a c o n il m e t o d o di w o o d w a r d e s t r e e t ( s ) c h e c o n s i s t e essenz i a l m e n t e n e l r e g i s t r a r e c o n t e m p o r a n e a m e n t e l e c o i n c i d e n z e d o p p i e e l e t r i p l e c o n u n t e l e s c o p i o di c o n t a t o r i ( f i g . 1) d e i q u a l i i due e s t r e m i a t u t t i in o t t o n e , e r a n o r i e m p i t i con la s o l i t a m i s c e l a di t r o s t ( a r g o n + a l c o o l ) e q u e l l o c e n t r a l e b i n v e t r o c o n c a t o d o e s t e r n o . c o n t e n e v a a r i a a b a s s a p r e s s i o n e ; l e c o i n c i d e n z e t r i p l e c , e r a n o f a l l e tra i c o n t a t o r i f e li. le d o p p i e ctra i c o n t a t o r i e s t r e m i i• 3 5 6 i o n i z z a z i o n e s p e c i f i c a p r i m a r i a d e l l a r a d i a z i o n e c o s m i c a n e l l ' a r i a i n figura 2 è r a p p r e s e n t a t o s c h e m a t i c a m e n t e il sistema (li r e g i s t r a z i o n e degli i m p u l s i , le fig. 3 e 4 d a n n o i p a r t i c o l a r i dei m u l t i v i b r a t o r i i m p i e g a t i p e r a m p l i f i c a r e ed e q u a l i z z a r e gli i m p u l s i : il p r i m o ( f i g . 3 ) è a s p e g n i m e n t o , es-o v i e n e i m p i e g a t o per il c o n t a t o r e ad a r i a c l i c dà i m p u l s i l e n t i , il s e c o n d o ( f i g . 4 ) è n o r m a l e . i n f i g . 3 fig. 5 è r i p o r t a t o lo s c h e m a di u n a c o m u n e c o i n c i d e n z a a l l a n e w r o s s i . .nella t a b e l l a i sono r a c c o l t i i dati s p e r i m e n t a l i con l ' e r r o r e m e dio p r o b a b i l e , essi si r i f e r i s c o n o a m i s u r e e f f e t t u a l e con il c o n t a t o r e h r i e m p i t o con a r i a a l l a p r e s s i o n e di 9 , 3 i n n i ( q u e s t o v a l o r e è m . s a n t a n c e l o 3 5 7 s t a l o c o r r o t t o p e r la t e m p e r a t u r a ) . nelle c o l o n n e 1 e 5 . a c2 h e a c , li, sono r i p o r t a t e le c o i n c i d e n z e s p u r i e a l l ' o r a , d o p p i o e t r i p l e c a s u a l i , la c u i v a l u t a z i o n e è s t a t a fatta a l l o n t a n a n d o i c o n t a t o r i u n o dall a l t r o di c i r c a 1 m m a n t e n e n d o l i sullo stesso p i a n o . t a b e l l a i p r e s s i o n e in m m c,. h c, h a c-, h a cx li 9 , 3 11,1, 2 , 8 1 7 , 0 . 1 , 4 2 3 , n 1,3 1,0 0 , 0 3 l ' e f f i c i e n z a del c o n i a t o r e b r i s p e l l o ai c o n t a t o r i e s t r e m i è d a t a in c o n s e g u e n z a dal r a p p o r t o q \r i \ = — : — = 0 . 3 0 0 . 0 3 c2 — a c, il c a m m i n o m e d i o d e l l e p a r t i c e l l e i o n i z z a n t i n e l l ' i n t e r n o del c o n t a t o r e è stato c a l c o l a t o i m p o n e n d o d i v e r s e s e m p l i f i c a z i o n i , esso è r i s u l t a l o por la n o s t r a d i s p o s i z i o n e s p e r i m e n t a l e , /,„t.,ii„ = 1 . 6 e m ( ° ) . d a l l a f o r m u l a h ] , s o s t i t u e n d o , si o l l i e n e j = 1 7 , 9 2 , 3 qtie-to v a l o r e va c o r r e t t o p e r le seguenti r a g i o n i : 35 8 m . s a n t a n c e l o j ) p o i c h é r i r n p u l « o f o r n i t o dal i n u l l i v i b r a t o r e a s p e g n i m e n t o lia una d u r a t a di c i r c a 5 . 1 0 s e c . p u ò c a p i t a r e c h e q u a l c h e c o i n c i d e n z a t r i p l a v e n g a p e r d u t a p e r il fatto c h e il c o n t a t o r e c e n t r a l e si t r o v a in uno i a t o di l a t e n z a p e r un t e m p o p i u t t o s t o l u n g o , q u e s t o p o r t a ad una c o r r e z i o n e p o s i t i v a sul v a l o r e del r e n d i m e n t o c h e si p u ò v a l u t a r e del 5 % ; 2 ) la s e c o n d a c o r r e z i o n e , p u r e p o s i t i v a , è dovuta al c o n t r i b u t o d a t o a l l e c o i n c i d e n z e d o p p i e e t r i p l e da p a r t e degli s c i a m i p r e s e n t i n e l l ' a r i a . u n a v a l u t a z i o n e , >ia p u r e g r o s s o l a n a , degli s c i a m i è s t a t a fatta p o s t a n d o i l c o n t a t o r e s u p e r i o r e in m o d o da p o r t a r l o f u o r i d e l l ' a n g o l o solido d e t e r m i n a t o dagli a l t r i d u e . da queste1 m i s u r e r i s u l t a u n a corr e z i o n e di c i r c a il 5 % ; si h a così u n a c o r r e z i o n e t o t a l e sul v a l o r e del r e n d i m e n t o di 1 0 % ; j c o r r e t t o d i v e n t a a l l o r a 7 = 2 0 , 5 . 2 . 3 q u e s t o v a l o r e di j r i s u l t a e s s e r e la m e d i a p e s a l a p e r l e d i v e r s e e n e r g i e d e l l o s p e t t r o d e l l a r a d i a z i o n e c h e i n c i d e sul t e l e s c o p i o dei c o n t a t o r i ; esso è in b u o n o a c c o r d o c o n i v a l o r i t r o v a l i da a l t r i a u t o r i sia con c a m e r e a n e b b i a ( j ) c h e c o n c o n t a t o r i a b a s s a p r e s s i o n e ( " ) . c e r t a m e n t e l e m i s u r e in c a m e r a di w i l s o n si p r e s t a n o a p a r e c c h i e c r i t i c h e s p e c i e q u a n d o si v o g l i a d e t e r m i n a r e la i o n i z z a z i o n e p r i m a r i a , q u e s t o m e t o d o i n v e r o dà la p o s s i b i l i t à d i d e t e r m i n a z i o n i m o l l o p r e c i s e della i o n i z z a z i o n e t o t a l e e d i q u e l l a p r o b a b i l e . p e r q u a n t o r i g u a r d a poi il m e t o d o dei c o n t a t o r i sj p u ò d i r e c h e esso è b a s a l o sulle d u e i p o t e s i già e n u n c i a t e , s u l l a p r i m a d e l l e q u a l i si p u ò sollev a r e q u a l c h e d u b b i o ( ! l ) ; l ' e f f e t t o d e l l e p a r e l i n e l l e n o s t r e m i sure è s t a l o r i d o t t o il m i n i m o c o n l a .sostituzione del c a t o d o met a l l i c o i n t e r n o con i m o e s t e r n o : c u r a p a r t i c o l a r e è s t a t a p o s t a a l l a o p e r a z i o n e d e l d e g a s s a m e n l o c h e , q u a l o r a d i f e t t o s o , p o t r e b b e c a u s a r e la f o r m a z i o n e di i o n i n e g a t i v i l e n t i con c o n s e g u e n t e r i c o m b i n a z i o n e degli i o n i stessi. l ' e f f e t t o d o v u t o a l l a r i c o m b i n a z i o n e nel c a s o d e l l ' a r i a d o v r e b b e essere a l q u a n t o s e n s i b i l e d a t a la f o r t e affinità e l e t t r o n i c a d e l l ' o s s i g e n o , e in ogni m o d o q u e s t o e f f e t t o d o v r e b b e a u m e n t a r e con la pressione i n t e r n a d e l c o n t a t o r e . lina m i s u r a o r i e n t a t i v a e s e g u i t a i n q u e s t o senso, p r o v e r e b b e q u e s t a n o s t r a s u p p o s i z i o n e : n u o v e d e t e r m i n a z i o n i sono n e c e s s a r i e p e r a c c e r t a r e q u a n t o sia e s a t t o p e r u n c o n t a t o r e a basso r e n d i m e n t o , r i e m p i t o con gas e a p r e s s i o n i d i f f e r e n t i , a m m e t t e r e c h e basii u n a sola c o p p i a di i o n i c r e a l a in q u a l s i a s i p u n t o i n t e r n o i o n i z z a z i o n e s p e c i f i c a p i i d i a r i a d e l l a radiazione c o s m i c a n e l l aria 3 5 9 dei c o n t a t o r e p e r a v e r e uno i m p u l s o : è c i ò c l i c ci p r o p o n i a m o di f a r e in p r o s s i m e m i s u r e . roma — istituto nazionale di geofilica — maggio 1948. riassunto con un metodo basalo sulla misura della efficienza di un contatore di gcigcr-miillcr a bassa pressione, si determina la ionizzazione specifica media nell aria della radiazione cosmica al livello del mare. il valore trovato è in accordo con i/uello misuralo da altri autori. sia con lo stesso metodo che con camere di wilson. b i b l i o g r a f i a ( l ) p . m . b i . a c h . e t r . prue. roy. s o c . a. 1 5 9 . 1 < 1 9 3 7 ) . ( ) e . j . w i l l i a m s . proc. roy. soc.. 1 3 5 , 111!; ( 1 9 3 2 1 . r . b . brode, rei. of uo'/i. pliys. 1 1 . 2 2 2 ( 1 9 3 9 ) . < 3 ) e . j . williams. ree. of mod. phys. ( 1 9 3 8 ) . (•') w . l i . ilazen. phys. ree. 6 5 . 2 5 9 ( 1 9 4 1 1 . l ' i m . g . e . cosyns. a mure. 1 9 3 8 . 284. ( 1 9 3 6 ) . w . e . d a n f o r t h a n d w . e . r a m s e y , phys. rei. 4 9 . 8 5 4 ( 1 9 3 6 ) . ('•) g . martelli e m . santangelo. i n c o r s o di s t a m p a s u l nuovo cimento. i 7 ) s . a . koiiff. a n d w . e . danforth. journ. frank. insiit. 2 2 8 . 1 5 9 ( 1 9 3 9 ) . ( 8 l r . h . w o o d w a r d w o d w a r d a n d i . c . s t r e e t , phys. ree. 4 6 . 1 0 3 2 ( 1 9 3 4 ) . ('•') e . j . w i l l i a m s a n d t e r r ò e x , proc. roy. soc. 1 2 6 . 2 8 9 ( 1 9 3 0 ) . c.orson a n d r . b . b r o d e . ì'iiys. rei. 5 3 . 7 7 3 ( 1 9 3 8 1 . w . e . h a z e n . phys. ree. 6 3 . 1 0 7 ( 1 9 1 3 ) . ( 1 0 ) j . d e y r i e s a n d g . 1 . s i z o o , pliysica. v i , 5 9 3 ( 1 9 3 9 ) . vol. 48, 01, 05ok.qxd 85 annals of geophysics, vol. 48, n. 1, february 2005 key words soil radon – water chemistry – volcanic activity – mexico 1. introduction popocatepetl (19°n; 99°′w; 5426 m altitude) is a high-risk active volcano due essentially to its geographical location in central mexico and its eruptive history. it is located in a region marked by the highest population density in the country, 60 km from mexico city, one of the largest cities in the world. the eruptive history of popocatepetl has been documented since 1347. in 1519 an eruptive stage lasted until 1530. in the 20th century multiple eruptive events occurred between 1920 and 1927, having an explosive character of moderate magnitude (de la cruz-reyna et al., 1995; cenapred, 2003). in december, 1994, an eruptive phase started showing stages of different volcanic activity along the last eight years. during this period, the volcano has been subject to intensive studies of radon, seismicity, ejecta composition, etc. (varley and armienta, 1999, 2001; segovia et al., 2001, 2002). the main purpose of this paper is to analyse the effects of the recent volcanic activity (20022003) on soil and groundwater radon and the chemical composition of groundwater. an overall compilation of the volcanic seismicity and the soil radon behaviour from 1994 until 2002 is also shown. 2. experimental approach 2.1. the volcano activity 2002-2003 from february, 2002, to february, 2003, the popocatepetl activity showed an interesting varimailing address: dr. nuria segovia, instituto de geofisica, universidad nacional autónoma de mexico (unam), ciudad universitaria, 04510 méxico d.f., méxico; e-mail: nurina@terra.com.mx radon, water chemistry and pollution check by volatile organic compounds in springs around popocatepetl volcano, mexico nuria segovia (1), pablo peña (2), carlos valdés (1), maría aurora armienta (1), beatriz lopez (2), guadalupe cisniega (2) and manuel mena (1) (1) instituto de geofisica, universidad nacional autónoma de mexico (unam), méxico d.f., méxico (2) instituto nacional de investigaciones nucleares (inin), méxico d.f., méxico abstract popocatepetl volcano is a high-risk active volcano in central mexico where the highest population density in the country is settled. radon in the soil and groundwater together with water chemistry from samples of nearby springs were analysed as a function of the 2002-2003 volcanic activity. the measurements of soil radon indicated fluctuations related to both the meteorological and sporadic explosive events. groundwater radon showed essential differences in concentration due to the specific characteristics of the studied springs. water chemistry showed also stability along the monitoring period. no anthropogenic pollution from volatile organic compounds (vocs) was observed. an overview of the soil radon behaviour as a function of the volcanic activity in the period 1994-2002 is also discussed. 86 nuria segovia, pablo peña, carlos valdés, maría aurora armienta, beatriz lopez, guadalupe cisniega and manuel mena ation characterised by periods of intense seismic activity of short duration. these were occasionally accompanied by the formation of small lava accumulation inside the volcano crater known as lava domes (january 24th, february 19th and april 29th 2002). during other episodes (may 14th, july 23rd and august 15-17th 2002) lava dome formation did not accompany the increase in seismic activity. the dome formation resumed on september 17th and on december 2nd 2002, with a lava dome 40 m tall and 180 m radius. from february 4th to the 23rd 2003, intensive volcanic activity was recorded, marking a significant stage of eruptions, where 13 volcanic explosions destroyed the domes formed during 2002. some of these explosions generated ash clouds of more than 4 km in height, which were transported ne to florida and detected by the goes satellite. 2.2. monitoring sites soil radon monitoring was performed at two fixed stations: tlamacas (4000 m altitude and in a distance 4.7 km from the crater) and paso de fig. 1. location of popocatepetl volcano between mexico city and puebla valleys in central mexico. the soil radon monitoring stations (paso de cortes and tlamacas) and the three monitored springs (axocopan, atlimeyaya and calvario) are also shown. 87 radon at popocatepetl volcano cortes (3400 m altitude and in a distance 7.5 km from the crater). three springs, atlimeyaya, axocopan and calvario, located at the southern flank of the volcano (at altitudes of 2200 m, 1950 m and 2300 m respectively), were systematically monitored during this period recording radon and major chemical compounds (fig. 1). anthropogenic volatile organic compounds (vocs) were also studied in the water samples in order to determine possible contamination paths. 2.3. measuring techniques long term soil radon determination (70 cm depth) was performed with solid state nuclear track detectors (ssntd), lr 115 type ii, from dosirad co. france. the detailed methodology was reported by segovia et al. (2002). the water samples were analysed for dissolved radon with a packard tri-carb 2700tr liquid scintillation detection system and with an alphaguard system from genitron instruments. the liquid scintillation method implies the radon extraction in toluene, while the alphaguard system consists of a ionisation chamber where air is bubbled to extract radon from the sample. in the field, electrical conductivity, temperature and ph-values of the spring waters were also determined. chemical analyses were performed by standard methods, as given in apha-awwawpcf (1995). the ionic charge balance confirmed the accuracy of the analyses. vocs identification was performed with a tekmartm 3000 (gc-ms) equipment using the epa method 524.2. for the majority of the vocs, the detection limit of the method ranges between 0.01 to 0.09 µgl–1 (cisniega et al., 2002). 3. results and discussions soil radon at tlamacas and paso de cortes (fig. 2a), showed significant differences between the two monitoring stations. the soil radon values at tlamacas showed peaks in february and april, 2002, during the period of lava dome formation. afterwards the radon concentration values decreased during the rainy season, between june and october, and smoothly increased from november, 2002, until february, 2003, linked with the activity of this last month. on the contrary, at paso de cortes, higher values corresponded to the rainy season. at this site, no effect on the soil radon concentration was observed linked with the dome formation at the beginning of 2002. however, radon peaks occurred in september and november, 2002, probably related to the dome formation in september and december. at the end of february, 2003, the soil radon at paso de cortes and tlamacas showed similar patterns. the average soil radon concentration values were low at both stations (fig. 2b). however the values at paso de cortes were twice as high as those at tlamacas. the differences in the soil radon behaviour and in average concentration values, are probably related to the specific soil characteristics at each station. tlamacas is located at the cinder cone with scarce vegetation whereas paso de cortes is set in the forest, having an organic rich top soil of up to 70 cm in thickness, and differences in the soil gases evolution are expected. fig. 2a,b. a) soil radon concentration values (kbqm–3) and precipitation (mm) from february, 2002, until february, 2003, at the monitoring stations paso de cortes and tlamacas. b) the average and standard deviation of radon data at the two monitoring stations. b a 88 nuria segovia, pablo peña, carlos valdés, maría aurora armienta, beatriz lopez, guadalupe cisniega and manuel mena the groundwater radon measured by the two methods, liquid scintillation and ionisation chamber (alphaguard), showed differences, but the general results were characterised by a correlation factor of 0.8 (fig. 3). this can be considered a reasonable approximation as the two methods of radon extraction have considerable differences. the average groundwater radon values obtained at the three springs with the two methods, showed different values for each one of the springs (fig. 4). the highest fluctuations in the radon concentration were observed at axocopan, but no link was observed between the groundwater radon fluctuations and the different stages of the volcanic activity during 2002-2003. the water chemistry of atlimeyaya, axocopan and calvario springs (table i and fig. 5) shows low variations for each site and indicates a higher mineralization in axocopan as compared with the two other springs. axocopan has the lowest altitude and the higher temperature of the three studied springs. however, since the chemical fluctuations in the present period of study have been so low, no correlation with the volcanic activity could be observed. volcanic gases emanating from the eruptive stage do not seem to had affected the surrounding aquifers. the analyses of the presence of volatile organic compounds (vocs) in the water samples were performed due to a possible anthropogenic pollution since the sampled springs are utilised for domestic and industrial use. in rural areas, the occurrence of vocs in surface and groundwaters can originate from solvents, gasoline, aromatic hydrocarbons, herbicides and pesticides (cisniega et al., 2002). the results of the study showed that vocs were below the detection limit of the method. an overall compilation of the volcanic activity and the soil radon evolution at popocatepetl volcano along the period from the beginning of the eruptive stage in 1994, until 2003, is shown in fig. 6a-e. panels a and b show soil radon concentration values, in kbqm–3, at tlamacas and paso de cortes respectively. panel c, shows the seismic energy measured as a cumulative measurement of the continuous ground vibration (solid line) and the cumulative energy released by the volcano-tectonic earthquakes (dashed line). the letters «d» and «d» at the bottom of this panel correspond to the formation of small (≤ 1.5 × 106 m3) and large (≥ 1.5 × 106 m3) lava domes, inside the popocatepetl volcano crater. the large energy step observed at the end of 2000, corresponds to the largest volcanic activity between 1994 and 2002. the vertical dashed lines represent different stages of volcanic activfig. 3. plot of groundwater radon data (bql–1) obtained by the liquid scintillation versus alphaguard methods. best fitting line indicates a correlation factor of 0.8. fig. 4. average and standard deviation of groundwater radon values obtained with the two methods (liquid scintillation and alphaguard) at the three springs (atlimeyaya, axocopan and calvario). table i. chemical composition of spring samples from atlimeyaya, axocopan and calvario, from january, 2002, to may, 2003. concentration values in mgl–1, temperature in °c and conductivity (λ) in µscm–1. date ca2+ mg2+ na+ k+ cl– so42– hco3– f– sio2 t °c ph λ atlimeyaya 16.01.2002 10.6 8.4 16.1 2.6 2.9 7.5 97.6 0.6 59.1 14.0 6.6 185.0 13.02.2002 10.7 8.0 15.8 2.7 3.7 8.9 97.6 0.5 68.8 14.0 6.5 183.0 13.03.2002 11.5 8.2 16.5 2.8 1.9 6.2 97.4 0.5 56.7 16.0 6.5 180.0 17.04.2002 12.0 8.5 16.6 2.6 2.9 6.2 97.4 0.5 67.4 17.0 6.3 182.0 15.05.2002 12.1 8.6 17.0 2.3 2.9 6.3 96.6 0.5 53.7 15.0 6.5 185.0 13.06.2002 10.7 7.5 15.3 2.5 2.6 6.5 99.1 0.5 60.6 14.0 6.5 173.0 11.12.2002 13.5 6.1 16.5 2.7 3.4 6.3 99.0 0.6 62.8 13.0 6.6 178.0 15.01.2003 10.0 8.5 14.8 2.7 3.6 6.0 97.1 0.5 61.6 13.0 6.4 180.0 10.02.2003 13.0 6.6 14.7 2.6 3.8 6.7 94.9 0.5 56.4 16.0 6.6 180.0 12.03.2003 11.7 6.1 15.9 2.6 3.1 7.2 103.1 0.6 49.7 16.0 6.5 180.0 09.04.2003 12.3 5.2 15.5 2.6 4.0 7.0 96.9 0.6 55.5 14.0 6.7 180.0 17.05.2003 10.8 7.5 15.5 2.7 3.0 10.2 96.4 0.6 58.7 14.7 6.5 180.0 average 11.6 7.4 15.8 2.6 3.1 7.1 97.8 0.6 59.2 14.7 6.5 180.5 σ 1.1 1.1 0.7 0.1 0.6 1.3 2.0 0.0 5.5 1.3 0.1 3.2 axocopan 16.01.2002 34.8 44.4 57.7 6.4 20.8 41.6 405.8 0.6 89.9 21.0 6.1 747.0 13.02.2002 33.8 44.3 59.6 6.8 19.5 38.6 406.7 0.5 90.1 21.0 6.1 714.0 13.03.2002 34.1 45.0 60.2 6.2 14.1 44.6 407.9 0.5 90.3 21.0 6.9 668.0 17.04.2002 35.1 44.4 60.9 6.3 16.0 39.2 409.9 0.5 90.1 23.0 6.1 678.0 15.05.2002 34.7 44.9 62.5 7.4 18.5 40.8 403.8 0.5 81.7 20.0 5.9 726.0 13.06.2002 35.7 42.6 56.2 6.3 17.6 40.8 403.8 0.5 91.4 20.0 6.0 704.0 11.12.2002 36.9 45.7 61.5 6.5 17.3 39.7 409.7 0.6 95.2 16.0 6.0 708.0 15.01.2003 35.5 41.8 55.4 6.3 16.6 38.5 409.4 0.4 93.5 18.0 6.3 714.0 10.02.2003 35.4 43.6 53.4 5.9 16.6 39.6 405.0 0.6 85.6 18.0 6.3 710.0 12.03.2003 35.4 42.8 57.3 6.1 18.5 39.1 412.6 0.6 75.9 18.0 6.5 700.0 09.04.2003 36.8 42.2 56.9 6.0 19.7 41.4 412.4 0.6 85.4 21.0 6.0 714.0 17.05.2003 35.1 42.1 58.0 6.4 17.5 44.5 409.0 0.5 85.4 20.0 6.0 716.0 average 35.3 43.6 58.3 6.4 17.7 40.7 408.0 0.5 87.8 19.8 6.2 708.3 σ 0.9 1.3 2.7 0.4 1.8 2.1 3.0 0.0 5.4 1.9 0.3 20.5 calvario 16.01.2002 9.0 9.4 12.7 2.2 3.3 24.7 69.3 0.4 66.8 17.0 7.0 182.0 13.02.2002 8.8 9.6 12.7 2.3 4.0 24.8 70.5 0.4 77.0 16.0 6.7 178.0 13.03.2002 9.0 10.3 13.0 2.3 2.2 22.3 69.7 0.4 64.4 17.0 6.7 172.0 17.04.2002 9.0 10.3 13.0 2.2 2.6 22.4 68.5 0.4 64.6 16.0 6.7 171.0 15.05.2002 9.1 9.8 13.5 2.0 3.2 23.4 69.4 0.4 60.3 13.0 5.2 178.0 13.06.2002 9.9 8.2 12.4 2.1 3.5 21.1 69.4 0.4 66.3 16.0 6.5 178.0 11.12.2002 13.5 6.1 13.9 3.3 3.2 22.0 70.2 0.4 68.9 12.0 6.7 174.0 15.01.2003 8.8 9.7 12.7 2.3 4.2 27.2 69.4 0.4 68.4 13.0 7.5 177.0 10.02.2003 9.7 8.4 11.2 3.3 4.4 23.1 71.2 0.4 62.9 13.0 7.0 175.0 12.03.2003 10.5 7.8 12.4 2.2 3.6 22.6 70.7 0.5 55.3 13.0 7.5 175.0 09.04.2003 11.5 6.9 12.4 2.1 4.0 23.5 70.9 0.5 61.2 15.0 6.8 176.0 17.05.2003 10.4 7.7 11.7 2.3 3.4 23.2 69.7 0.4 63.7 15.0 6.8 177.0 average 9.9 8.7 12.6 2.4 3.5 23.4 69.9 0.4 65.0 14.7 6.7 176.1 σ 1.4 1.4 0.7 0.4 0.6 1.6 0.8 0.0 5.3 1.8 0.6 3.0 89 radon at popocatepetl volcano 90 nuria segovia, pablo peña, carlos valdés, maría aurora armienta, beatriz lopez, guadalupe cisniega and manuel mena fig. 5. fluctuations in the physicochemical parameters at the popocatepetl springs, atlimeyaya, axocopan and calvario during 2002-2003. concentration values in mgl–1, temperature in °c and conductivity (e.c.) in µscm–1. ity based on seismic behaviour. panel d correspond to the number of long-period earthquakes per day, associated with the ascending motion of volcanic fluid through cracks or conduits. the letters «e» and «e», located at the bottom of this panel, indicate the occurrence of small and large volcanic explosions, respectively. panel e shows the volcano tectonic earthquakes, which are related to stress that fracture the rock, and are located under the popocatepetl volcano and plotted versus depth, where the cero represents sea level. the size of the symbols is proportional to the magnitude of the earthquakes, which ranges from 1.5 to 3.8. the number at the bottom of panel e corresponds to the number of volcano-tectonic earthquakes during each stage of activity. soil radon had its maximum peaks in correlation with the initial phase of the eruption, i.e. december 1994 at tlamacas (panel a), and also, although not as intense (panel b), at paso de cortes (segovia and mena, 1999). beginning on december, 1997, a large soil radon peak clearly stands out at tlamacas, and it is correlated with the first explosions that partially destroyed a large (≈ 12 × 106 m3) lava dome, that obstructed the crater and probably produced an fig. 6a-e. 1994-2002 soil radon and volcanic activity (see text): a) soil radon at tlamacas; b) soil radon at paso de cortes; c) cumulative seismic energy from continuous ground vibration (solid line); cumulative energy released by the volcano-tectonic earthquakes (dashed line); d) number of long-period earthquakes per day; e) volcano tectonic earthquakes under the popocatepetl volcano versus depth (the cero represents sea level). a b c d e increase in the state of stress below the volcano edifice. paso de cortes did not show a clear correlation in radon with this volcanic event. another episode of volcanic activity, consisting of frequent explosions that destroyed another lava dome, was observed on november-december, 1998, and a small radon increment can be observed at this time in both, tlamacas and paso de cortes sites. the largest volcanic activity observed at popocatepetl since 1994 and up to date, was observed in december 2000, and january 2001, when the largest dome was formed and the most explosive-dome-destruction occurred respectively. these events were preceded by a change in the seismic rate since september, 2000. it is clear that the radon signals at both sites showed an increment by mid 2000, which probably corresponds with the preceding phase of volcanic activity observed months later. it is also clear, that the radon background level has kept a larger value since mid 2000, as compared to the values of the previous years. soil radon measurements at popocatepetl have proved useful together with other volcano measurements, to characterize increments on the volcano activity. acknowledgements the authors acknowledge s. ceballos, d. cruz, f. montes, a. aguayo, n. ceniceros and o. cruz for technical assistance. this study was carried out with partial financial support from conacyt, project 40858. references apha-awwa-wpcf (american public health association-american water works association-water pollution control federation) (1995): standard methods for examination of water and wastewater, edited by m.h. franson and a.e. greenberg (apha-awwawpcf, washington d.c.), 19th edition. cenapred (2003): http://www.prueba.cenapred.unam.mx cisniega, m.g., n. segovia and n. lopez (2002): methodology for volatile organic compounds determination in water from wells and springs in mexico state, mexico, geofís. int., 41, 233-237. de la cruz-reyna, s., j.l. quezada, c. peña, o. zepeda and t. sanchez (1995): historia de la actividad reciente del popocatepetl (1354-1995), in volcan popocatepetl. estudios realizados durante la crisis de 19941995 (cenapred-unam, mexico d.f.), 3-22. segovia, n. and m. mena (1999): soil radon pulses related to the initial phase of volcanic eruptions, nuovo cimento, 22c, 275-279 segovia, n., c. valdes, p. peña, m. mena and e. tamez (2001): soil radon response around an active volcano, radiat. meas., 34 (1), 433-436. segovia, n., m.a. armienta, j.l. seidel, m. monnin, p. peña, m.b.e. lopez, m. mena, c. valdes, e. tamez, r.n. lopez and p. aranda (2002): radon in soil and chemical composition of spring water near popocatepetl volcano, mexico, geofís. int., 41, 399-404. varley, n.r. and m.a. armienta (1999): soil gas measurements as an indicator of volcanic activity at popocatepetl, mexico, nuovo cimento, 22c, 269-274. varley, n.r. and m.a. armienta (2001): the absence of diffuse degassing at popocateptl volcano, mexico, chem. geol., 177, 157-173. 91 radon at popocatepetl volcano vol49_1_2006def 93 annals of geophysics, vol. 49, n. 1, february 2006 key words endmember extraction – hyperspectral mixtures – linear spectral unmixing 1. introduction imaging spectroscopy is the measurement and analysis of spectra acquired as images. the use of hyperspectral imaging sensor data to study the earth’s surface is based on the capability of such sensors to provide hundreds of spectral bands (high resolution spectra), providing one spectrum per pixel, along with the image data. this capability can be used to determine earth’s surface constituent signatures from the spectral information provided by the sensor. the imaging spectroscopy concept can be considered in the general problem of solving unknowns from measurements. unknowns include all the absorbing and scattering components of the atmosphere and the surface that affects the radiance (green et al., 1998). if a problem has more unknowns than measurements, then the problem is underdetermined. if there are more measurements than unknowns then the problem is over determined and solvable. the imaging spectroscopy approach leads to over determined, therefore solvable problems. the problem must be solved taking into account all the unknowns that may affect the expression of interest (there is no magic band for any single material). as the number of equations (bands) increases, the problem is more easily solved. in most cases, the sub-scene represented by each observed pixel is not homogeneous. as a result, the hyperspectral signature collected by the sensor at each pixel is formed by an integration of signatures, associated with the purest portions of the sub-scene. these signatures, which can be considered macroscopically pure, are usually named «endmembers» in hyperspectral analysis terminology (bateson et al., 2000). endmember extraction algorithms from hyperspectral images pablo j. martínez, rosa m. pérez, antonio plaza, pedro l. aguilar, maría c. cantero and javier plaza computer science department, university of extremadura, cáceres, spain abstract during the last years, several high-resolution sensors have been developed for hyperspectral remote sensing applications. some of these sensors are already available on space-borne devices. space-borne sensors are currently acquiring a continual stream of hyperspectral data, and new efficient unsupervised algorithms are required to analyze the great amount of data produced by these instruments. the identification of image endmembers is a crucial task in hyperspectral data exploitation. once the individual endmembers have been identified, several methods can be used to map their spatial distribution, associations and abundances. this paper reviews the pixel purity index (ppi), n-findr and automatic morphological endmember extraction (amee) algorithms developed to accomplish the task of finding appropriate image endmembers by applying them to real hyperspectral data. in order to compare the performance of these methods a metric based on the root mean square error (rmse) between the estimated and reference abundance maps is used. mailing address: dr. pablo j. martínez, computer science department, university of extremadura, avda. de la universidad s/n, 10071 cáceres, spain; e-mail: pablomar@unex.es 94 pablo j. martínez, rosa m. pérez, antonio plaza, pedro l. aguilar, maría c. cantero and javier plaza in this paper, we review some endmember extraction techniques and use a novel approach to compare the accuracy of these methods. the rest of the paper is structured as follows: section 2 provides a description of the concepts of endmember and fully constrained linear spectral unmixing. section 3 discusses the pixel purity index (ppi), n-findr and automatic morphological endmember extraction (amee) techniques. section 4 describes the result achieved after applying the proposed algorithms to a real mineral mapping application, and establishes a comparison between them, using real data collected by the nasa jet propulsion laboratory’s airborne visible infrared imaging spectrometer «aviris». finally, section 5 includes some concluding statements and comments on plausible future research. 2. endmembers in hyperspectral images in order to understand the endmember concept, we will analyze the spectrum composition model, considering that a spectrum is an n-dimensional vector. the radiation source f is characterized by a spectral radiance f(m); this radiation will cross a medium (atmosphere), until it reacts with a sample i; this medium will be characterized by a transmittance, g(m) in such a way that the radiation that reaches the sample will be f(m) g(m). the interaction with the sample will be directed by its spectral reflectance t(m); the radiation emerging the sample will be, therefore, f(m)g(m)t(m) and it will have to suffer a new interaction with the propagation medium, in such a way that the incident radiation over the sensor will be f(m)g(m)t(m)gl(m), agreeing with spectrometer transfer function h(m), the measurement result, will be (2.1) the endmember hyperspectral signature ti, is a reflectance vector obtained from a measurement of a i pure canopy sample xi obtained in the same conditions as the hyperspectral image (detector h, source f and atmospheric conditions g and gl) (2.2) usually the i components present in a non-uniform sample do not participate in this process in the same proportion; if we call ti (m) to the i component reflectance spectrum and ci to the abundance of i component in our sample, the measurement process will be like the one shown in fig. 1. the composed reflectance spectrum t(m) can be expressed as a linear combination of the endmember reflectance signatures ti (m) when each isolated source i stimulates the detector separately, i = 1, ..., k, (where k is the number of endmembers). according to the related notation, the superposition principle can be generalized by (2.3)rc ci i i k 1 = =t t = / ( ) , , ..., ., , ,i i i i n1 2=t m t t t t 6 @ ( ) ( ) ( ) ( ) ( ) ( ) .x f g g h=m m m m m mt l fig. 1. spectrum composition. 95 endmember extraction algorithms from hyperspectral images where r = [t1, t2, ..., tk ] and c = [ c1, c2, ..., ck ]t. for instance, a simple mixture model based on three endmembers has the geometrical interpretation of a triangle whose vertices are the endmembers. cover fractions are determined by the position of spectra within the triangle, and they can be considered relative coordinates in a new reference system given by the endmembers, as shown in fig. 2. although the development of effective devices with new and improved technical capabilities stands out, the evolution in the design of algorithms for data processing is not as positive. many of the techniques currently underway for hyperspectral data analysis and endmember extraction have their origins in classic spectroscopy, and thus focus exclusively on the spectral nature of the data (plaza et al., 2002). available analysis procedures do not usually take into account information related to the spatial context, which can be useful to improve the obtained classification (madhok and langreb, 1999). 2.1. full constraint linear spectral unmixing the objective of linear spectral unmixing (lsu) is to obtain the fractions of components in the mixture. unmixing analysis generally begins with a few basic endmembers or components. endmembers such as green vegetation, soil, non-photosynthetic vegetation, and shade are often used. the spectra of these endmembers may come from laboratory measurements, field measurements or from the imaging spectrometer data set itself. each spectrum is then atmospherically corrected and the imaging spectrometer data set is then decomposed into the fractions of the endmembers that best replicate the measured spectrum. the output of lsu analysis is a series of endmember fraction images. these images report the derived distribution of the endmembers over the image. two constraints that are often considered on this analysis are: that the fractions must sum to 1.0 and negative fractions are not allowed (fclsu) (green et al., 1998). 3. background on endmember extraction techniques a number of algorithms based on the notion of spectral mixture modeling have been proposed to accomplish the complex task of finding appropriate endmembers for spectral unmixing in multi/hyperspectral data. fig. 2. mixture model based on three endmembers. 96 pablo j. martínez, rosa m. pérez, antonio plaza, pedro l. aguilar, maría c. cantero and javier plaza 3.1. ppi one of the most successful approaches has been the pixel purity index or ppi (boardman et al., 1995), which is based on the geometry of convex sets (ifarraguerri and chang, 1999). ppi considers spectral pixels as vectors in a n-dimensional space. a dimensionality reduction is first applied to the original data cube by using the minimum noise fraction. the algorithm proceeds by generating a large number of random n-dimensional vectors, also called «skewers» (theiler et al., 2000), through the dataset. every data point is projected onto each skewer, along which position it is pointed out. the data points which correspond to extreme values in the direction of a skewer are identified and placed on a list. as more skewers are generated, the list grows, and the number of times a given pixel is placed on this list is also tallied. the pixels with the highest tallies are considered the purest ones, since a pixel count provides a «pixel purity index». it is important to emphasize that the ppi algorithm does not identify a final list of endmembers. ppi was conceived not as a solution, but as a guide; in fact, the author proposed comparing the pure pixels with target spectra from a library and successively projecting the data to lower dimensional spaces while endmembers were identified. there are several interactive software tools oriented to perform this task, but the supervised nature of such tools leads to the fact that the analysis of a scene usually requires the intervention of a highly trained analyst. then, interactive solutions may not be effective in situations where large scenes must be analyzed quickly as a matter of routine. in addition, randomness in the selection of skewers has been identified by some authors as a shortcoming of the algorithm. the original implementation of ppi proposes the use of unitary vectors as skewers in random directions of the n-dimensional space. this implementation may be improved by a careful selection of existing vectors to skew the dataset. intelligent selection of skewers may result in a more efficient behavior of the algorithm. some tools based on variations of ppi concepts have been proposed (theiler et al., 2000). 3.2. n-findr the n-findr method (winter, 1999) is an automated approach that finds the set of pixels which define the simplex with the maximum volume, potentially inscribed within the dataset. first, a dimensionality reduction of the original image is accomplished by mnf. next, randomly selected pixels qualify as endmembers, and a trial volume is calculated. in order to refine the initial volume estimation, a trial volume is calculated for every pixel in each endmember position by replacing that endmember and recalculating the volume. if the replacement results in a volume increase, the pixel replaces the endmember. this procedure, which does not require any input parameters, is repeated until there are no replacements of endmembers left. it should be noted that both ppi and n-findr rely on spectral properties of the data alone, neglecting the information related to the spatial arrangement of pixels in the scene. the n-findr method is sensitive to the random selection of an initial set of endmembers. if the initial estimation is appropriate, the algorithm will not require to do many iterations until it reaches the optimum solution. on the contrary, an erroneous initial estimation may lead to a high computational complexity of the algorithm. on the other hand, the algorithm recalculates the volume of the simplex every time a new pixel is incorporated to the endmember set. this property makes the algorithm quite sensitive to noise. 3.3. amee mathematical morphology is the science of shape and structure, based on set-theoretical, topological and geometrical concepts (serra, 1982, 1993). morphology was originally defined for binary images and has been extended to the grayscale (sternberg, 2000) and color (lambert and chanussot, 2000) image cases, but it has seldom been used to process multi/hyperspectral imagery. we propose a new application of mathematical morphology focused on the automated extraction of endmembers from multi-dimension97 endmember extraction algorithms from hyperspectral images al data. this application is fully described in (plaza et al., 2002). in addition, we list key advantages in the use of morphology to perform this task: – a first major point is that the extraction of endmembers is basically a non-linear task. – furthermore, morphology allows the introduction of a local-to-global approach in the search of endmembers by using spatial kernels (structure elements in the morphology jargon). these items define local neighbourhoods around each pixel. this type of processing can be applied to the search of convexities or extremities within a data cloud of spectral points and to the use of the spatial correlation between the data. – as a final main step, morphological operations are implemented by replacing a pixel with a neighbor that satisfies a certain condition. in binary/grayscale morphology, the condition is usually related to the digital value of the pixel, and the two basic morphological operations (dilation and erosion) are respectively based on the replacement of a pixel by the neighbor with the maximum and minimum value. since an endmember is defined as a spectrally pure pixel that describes several mixed pixels in the scene, extended morphological operations can obviously contribute to the location of suitable pixels that replace others in the scene, according to some desired particularity of the pixel, for example, its spectral purity. therefore, we propose a mathematical framework to extend mathematical morphology to the multi-dimensional domain, which results in the definition of a set of spatial/spectral operations that can be used to extract reference spectral signatures. the dilation and erosion operations have been extended to the grayscale image case (sternberg, 2000). in grayscale morphology, images are handled as continuous valued sets. let f(x, y) be a gray level function representing an image, and k another set which comprises a finite number of pixels. this set is usually referred to as a kernel or structuring element in morphology terminology. erosion and dilation of image f(x, y) by structuring element k are respectively written as =( , ) ( , ) ( , )minf k x y f x s y t k s t ( , )s t k 7 + + ! ^ h " , (3.1) where k(s, t) denotes the weight associated to the different elements of the kernel. the main computational task in dealing with grayscale morphological operations is the query for local maxima or minima in a local search area around each image pixel. this area is determined by the size and shape of structuring element k. if a dilation operation using k is applied over a grayscale image, then the local effect of the operation is the selection of the brightest pixel in a 3 × 3-pixel search area around the target pixel. the constraints imposed on the kernel definition cause all pixels lying within the kernel to be handled equally, i.e. no weight is associated with the pixels according to their position along the kernel, and the pixel with maximum digital value is selected. the previous operation is repeated with all the pixels of the image, leading to a new image (with the same dimensions as the original) where lighter zones are developed concerning the size and shape of the structuring element. in contrast, grayscale erosion has the global effect of shrinking the lighter zones of the image. 3.3.1. extending mathematical morphology to multi-spectral images in grayscale morphology, the calculation of maximum and minimum gray values relies on the definition of a simple ordering relation given by the digital value of pixels. in the case of multi-spectral datasets, this natural assumption is not straightforward, since there is no natural means for the total ordering of multivariate pixels. hence, the greatest challenge in the task of extending morphology operations to multispectral images is the definition of a vector ordering relation related to pixel purity that allows us to determine the maximum and minimum elements in any family of n-dimensional vectors (plaza et al., 2002). we have checked one possibility that relies on the calculation of the distance between every spectral point in the kernel and the center of the kernel data cloud. then, a local eccentricity =( , ) ( , ) ( , )maxf k x y f x s y t k s t ( , )s t k 7 + ! ^ h " , 98 pablo j. martínez, rosa m. pérez, antonio plaza, pedro l. aguilar, maría c. cantero and javier plaza measure for a particular n-dimensional point of the kernel f(x, y) can be obtained by calculating the distance dist(f, c) in relation to the center c (dist is a point-wise linear distance function – spectral angle). distance dl= dist(f, c) defines a partial ordering relation in the data cloud that allows the identification of the maximum as that element which is most «eccentric» or distant from the center (fig. 3a), while the minimum is the closest element to the center (fig. 3b). the result of applying an erosion/dilation operation to a multi-spectral dataset is a new data cube, with exactly the same dimensions as the original, where each pixel has been replaced by the maximum/minimum of the kernel neighborhood considered. in order to account for the «eccentricity» of the maximum pixel, we propose the combination of the output provided by dilation and erosion operations. while dilation selects the purest pixel (the maximum is the more extremely placed pixel in the data cloud), erosion selects the most highly mixed pixel (the minimum is the pixel that is more exactly placed in the center of the data cloud) in a certain spatial neighborhood. the distance between the maximum and the minimum (a widely used operation in classic mathematical morp÷hology) provides an eccentricity value for the selected pixel that allows an evaluation of the quality of the pixel in terms of its spectral purity. in order to incorporate this idea, a new quality measure must be introduced. we define the morphological eccentricity index (mei) as the distance between the maximum element (obtained by a dilation) and minimum element (obtained by an erosion) in the kernel. 3.3.2. algorithm description the automated morphological endmember extraction (amee) method (plaza et al., 2002) is described as follows: the input to the method is the full data cube, with no previous dimensionality reduction or pre-processing. parameter l refers to the number of iterations that are performed by the algorithm. parameters smin and smax denote the minimum and maximum kernel sizes that will be considered in the iterative process, respectively. these parameters are interrelated, as the number of iterations depends on the minimum and maximum kernel sizes under scrutiny. firstly, the minimum kernel size smin is considered. the kernel is moved through all the pixels of the image. the spectrally purest pixel and the spectrally most highly mixed pixel are obtained at each kernel neighborhood by multispectral dilation and erosion operations. a morphological eccentricity index (mei) value is associated with the purest pixel by comparing the result of the dilation to the result of erosion. the previously described operation is repeated by using structuring elements of progressively increased size, and the algorithm performs as many iterations as needed until the maximum kernel size smax is achieved. the associated mei value of selected pixels at subsequent iterations is updated by means of newly obtained values, as a larger spatial context is considered, and a final mei image is generated after l iterations. once the competitive endmember selection process is finished, endmember extraction is finally accomplished using a spatial-spectral seeded region growing procedure in which the mei image is used as a reference. here, a threshold parameter t is used to control the process (plaza et al., 2002). 4. results in this section we intend to compare the quality of the endmembers extracted by the ppi, n-findr and amee algorithms when the ground truth information is available and expressed in a set of abundance values. in this exfig. 3a,b. a) maximum and b) minimum in data cloud. a b 99 endmember extraction algorithms from hyperspectral images periment we consider that most of the image pixels are a mixture of endmembers, and some of them are pure pixels. the methodology of comparison is based in the realization of the following steps: 1. the image endmembers are extracted using one of the proposed algorithms. 2. then, the abundance of each one of the endmembers in the image is estimated, using a fclsu procedure (fully constrained linear spectral unmixing) (heinz and chang, 2000). as a result, an abundance map is obtained for each identified spectral signature. 3. the obtained abundance values are compared, on a pixel-by-pixel basis, with reference values estimated by usgs for the minerals in this area, using the rmse error. it is important to emphasize that the reference abundances were obtained using usgs tetracorder algorithm, which produces spectral similarity scores which are consideredin this study as approximated abundance scores. the considered reference maps are available from http://speclab.cr.usgs.gov/. 4. the tests described in this section have been carried out using the image avcup95, taken by aviris, characterized for having ground truth information in the form of maps that contain the abundance of each mineral in each image pixel. this image was acquired in 1995 by aviris sensor over the mining region named cuprite, in the state of nevada, u.s.a. figure 4 shows the placement of the image over an aerial photograph of the zone. the scene comprises some areas composed of minerals, as well as abundant bare soils and an interstate road that crosses the zone in a north to south direction. the image avcup95 consists of 400 × × 350 pixels, each one of which contains 50 reflectance values in the range of 2 to 2.5 nm. this range, placed in the swir-ii region of the spectrum, is characterized by the manifestation of singularities that allow the discrimination of a wide range of calcareous minerals (clark, 1999). each spectral value in the above mentioned range is equivalent to 10 times the percentage of the reflectance in a given wavelength. this values have been obtained as a result of the application of the atrem atmospheric correction fig. 4. location of avcup95 image over an aerial photograph of cuprite mining region, nevada, u.s.a. 100 pablo j. martínez, rosa m. pérez, antonio plaza, pedro l. aguilar, maría c. cantero and javier plaza method (gao et al., 1993), followed by a postprocessing with effort method (boardman, 1998) over the image in radiance units, originally captured by the sensor, suppressing the effects caused by the atmosphere g and the illumination source f. the ground truth information for the image avcup95 has been published by the north american institute of geological studies, or u.s. geological survey (usgs from now on). the availability of this information of ground truth, varied and with high quality, favoured avcup95 as the image used as a reference to test the performance of new algorithms. when analysing this image, we centre our study in the most characteristic minerals of the region, that is, alunite, buddingtonite, calcite and kaolinite. the results regarding the estimation of abundances of alunite mineral are very precise in the three tested methods. the results obtained in the first experiment revealed that the endmember extracted by ppi method for alunite mineral had less spectral similitude with regard to the ground truth than the endmembers extracted by n-findr and amee. however, this fact does not seem to affect the abundances estimation process significantly. with the aim of establishing a quantitative evaluation of the qualitative results previously mentioned, table i shows the rmse total errors obtained when comparing each of the estimated abundance maps with their corresponding ground truth map. the results in table i reveal a very similar behaviour of the three methods, in global terms, for alunite mineral, with an error of 5% in each of the cases. concerning buddingtonite, the calculated error rises in the three cases, being always superior to 10%. this result confirms that there are certain errors in the determination of the spectral shape of the endmember, probably due to atmospheric effects. finally, the estimation of the abundance in the case of the calcite and kaolinite minerals is quite precise in all cases, the ppi algorithm being the one that offers the worst results for calcite mineral, with a global error of 9%, and nfindr offers the worst results for kaolinite mineral, with a global error of 6%. the quantitative comparison in table i is useful to obtain a global vision of the precision of the considered methods when it deals with the estimation of abundances. this study does not reflect the changes caused in the calculated error while the abundance of the component varies. a very interesting question to study in depth is the evaluation of the presented methods in analyzing the magnitude of the calculated error when the material abundance is small and when it is large. 5. conclusions spectral unmixing is only possible when the number of endmembers of the image is lower than the number of channels, so that the system of equations to be solved is over-dimensioned; as the system has a larger number of equations we will be able to determine more endmembers when the spectra have more channels. the best endmembers must be obtained in the same condition as the unmixing spectrum (from the same image) preventing external factors, such as the illumination or elevation angles, affecting the unmixing results. the endmember extraction methods that jointly exploit spatial and spectral information obtain better quality endmembers than those methods which only utilize spectral information. it is necessary to design tools for the evaluation of the quality of endmember extraction algorithms using synthetic images designed for that purpose from a previously known abundance map, as well as real images obtained in table i. global rmse between the estimated abundances and the real ones for every mineral. mineral amee n-findr ppi alunite 0.05 0.05 0.05 buddingtonite 0.12 0.13 0.15 calcite 0.07 0.08 0.09 kaolinite 0.05 0.06 0.04 101 endmember extraction algorithms from hyperspectral images strictly controlled conditions, where it could be possible to obtain the abundance map. acknowledgements this work was supported by regional government junta of extremadura by means of the project 2pr03a061. the authors gratefully acknowledge prof. pedro gómez vilda (universidad politécnica de madrid) for his valuable suggestions and comments on the work described in this paper. references bateson, c.a., g.p. asner and c.a. wessman (2000): endmember bundles: a new approach to incorporating endmember variability into spectral mixture analysis, ieee trans. geosci. remote sensing, 38 (2), 10831094. boardman, j.w (1998): post-atrem polishing of aviris apparent reflectance data using effort: a lesson in accuracy versus precision, in proceedings of the vii nasa/jpl airborne earth science workshop. boardman, j.w., f.a. kruse and r.o. green (1995): mapping target signatures via partial unmixing of aviris data, in summaries of the v jpl airborne earth science workshop. clark, r.n. (1999): spectroscopy of rocks and minerals and principles of spectroscopy, in principles of spectroscopy, manual of remote sensing, edited by a.n. rencz (john wiley and sons, new york), ch. 1, 3-58. gao, b.-c., k.b. heidebrecht and a.f.h. goetz (1993): derivation of scaled surface reflectances from aviris data, remote sensing environ., 44, 145-163. green, r.o., m.l. eastwood, c.m. sarture, t.g. chrien, m. aronsson, b.j. chippendale, j.a. faust, b.e. pavri, c.j. chouit, m. solis, m.r. olah and o. williams (1998): imaging spectroscopy and the airborne visible infrared imaging spectrometer (aviris), remote sensing environ, 65 (3), 227-248. heinz, d. and c.-i. chang (2000): fully constrained least squares linear mixture analysis for material quantification in hyperspectral imagery, ieee trans. geosci. remote sensing, 39, 529-545. ifarraguerri, a. and c.-i. chang (1999): multispectral and hyperspectral image analysis with convex cones, ieee trans. geosci. remote sensing, 37 (2), 756770. lambert, p. and j. chanussot (2000): extending mathematical morphology to color image processing, in first int. conf. on color in graphics and image processing (cgip’2000), october 2000, saint-etienne, france, 158-163. madhok, v. and d. landgrebe (1999): spectral-spatial analysis of remote sensing data: an image model and a procedural design, ph.d. thesis; and school of electrical & computer engineering technical report trece 99-10. plaza, a., p. martínez, r.m. pérez and j. plaza (2002): spatial/spectral endmember extraction by multi-dimensional morphological operations, ieee trans. geosci. remote sensing, 40 (9), 2025-2041. serra, j. (1982): image analysis and mathematical morphology (academic press, london). serra, j. (1993): image analysis and mathematical morphology (academic press, london), vol. 1. sternberg, s.r. (2000): greyscale morphology, computer vision graphics and image processing, 35, 283-305. theiler, j., d.d. lavenier, n.r. harvey, s.j. perkins and j.j. szymanski (2000): using blocks of skewers for faster computation of pixel purity index, spie proc., 4132, 61-71. winter, m.e. (1999): n-findr: an algorithm for fast autonomous spectral end-member determination in hyperspectral data, spie proc., 3753, 266-275. vol48/04/2005def 731 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words phlegraean fields – plinian eruptions – vesicle textures – magma ascent dynamics 1. introduction understanding the dynamics of explosive eruptive events has been the primary goal of the volcanological community for many years, since plinian style volcanic eruptions represent the most powerful and dangerous expression of volcanism on the earth. the development of sophisticated numerical codes as well as the implementation of experimental studies has allowed us to move several steps further in quantifying the complex physico-chemical properties of magmas with different compositions and rheological behaviour (sigurdsson et al., 2000). furthermore, detailed investigations on recently eyewitnessed eruptions have contributed much to highlight the basic eruptive processes (newhall and punongbayan, 1996; druitt and kokeelar, 2002). however, monitoring active volcanoes provides geophysical and geochemical data mostly applying to specific eruptions, and numerical simulations or experimental runs are not yet able to reproduce the full range of natural phenomenologies being observed. textural characterization of pumice clasts has proved to be a valid complementary approach in investigating magmatic processes not directly observable nor constraining the dynamics of volcanic eruptions by characterization of pumice textures margherita polacci istituto nazionale di geofisica e vulcanologia, sede di pisa, italy abstract we have characterized the textures of pumice clasts from phlegraean fields to gain insights into the conduit flow-dynamics of alkaline explosive eruptions. vesicularities, vesicle number densities, and vesicle sizes and shapes were measured to obtain the bulk and groundmass properties of the juvenile fraction of campanian ignimbrite (ci) and agnano monte spina (ams) eruptions. the results report the coexistence of three end-member pumice types in the deposits of both eruptions, 1) microvesicular, 2) tube and 3) expanded, which differ according to clast morphology and the macroto microscopic vesicle texture. vesicularities (0.85-0.94 for ci, 0.51-0.91 for ams) and vesicle number densities (2-4 × 105 cm−2 in ci, 3 × 105-106 cm−2 in ams) span quite a wide range in all the three pumice types. overall, tube pumices exhibit the highest bulk (0.89) and groundmass (ci 0.85, ams 0.82) average vesicle volume fractions but the lowest average vesicle number densities (ci 2 × 105, ams 4 × 105 cm−2). comparison with textures of calc-alkaline pumices has revealed many similarities and points to a common origin and distribution of the products from both magma compositions within the volcanic conduit. in addition, the results of the textural analysis were interpreted in the light of the conduit flow modeling of phlegraean fields eruptions. the comparison of textural observations with results from simulations of conduit magma ascent has exhibited a good agreement between measured and numerically calculated vesicularities for both compositions, helping to constrain the overall dynamics of alkaline versus calc-alkaline eruptions. mailing address: dr. margherita polacci, istituto nazionale di geofisica e vulcanologia, sede di pisa, via della faggiola 32, 56126 pisa, italy; e-mail: polacci@pi.ingv.it now at: istituto nazionale di geofisica e vulcanologia, sezione di catania, piazza roma 2, 95123 catania, italy. 732 margherita polacci recognizable from other studies of volcanic deposits. crystal and vesicle textures have been used as a proxy for the assessment of conditions related to the eruption dynamics (klug and cashman, 1996; hammer et al., 1999; polacci et al., 2001, 2003; klug et al., 2002), and for constraining results from both numerical simulations (polacci et al., 2004) and laboratory experiments of conduit magma flow (mader et al., 1996, 1997; blower et al., 2001). we have conducted detailed textural investigations on pumice clasts from two alkaline explosive eruptions from the phlegraean fields, namely agnano monte spina (ams) and campanian ignimbrite (ci). the dataset on pumice textures was then used to constrain results from numerical modeling of conduit magma ascent. the main goal of this research was to investigate conduit processes occurring during alkaline eruptions, and compare the findings with observations coming from calc-alkaline eruptions typical of subduction volcanoes. results from the investigation of the juvenile fraction of ci fallout deposits have been reported in a recent paper (polacci et al., 2003), and here will be summarized and combined with analyses referring to pumice products of ams to provide a comprehensive picture of the dynamics of alkaline silicic eruptions. 2. methods the samples analyzed in this study are alkali trachytic pumice clasts from the b1 and d1 plinian magmatic fallout layers of ams eruption (di vito et al., 1999). pumices were first classified into different types on the basis of macroscopic features. subsequently, selected clasts from each pumice type were thin-sectioned for qualitative and quantitative investigation of pumice textures. images were acquired via optical and scanning electron microscopy at different magnification (30x, 125x, 250x, 500x, 1000x) to cover the full range of vesicle size, shape, spatial distribution and degree of interconnectivity. backscattered electron images (bse) were then made binary and processed with shareware software for image analysis to obtain bulk and groundmass (only vesicles < 200 µm) crystal-free vesicle measurements. details of the procedure can be found in polacci et al. (2001, 2003). two-dimensional bulk and groundmass vesicularities (area fraction of vesicles), groundmass vesicle number densities (number of vesicles per unit area), average vesicle equivalent diameter (d, the diameter of a circle whose area equals that of the vesicle) and deformation (d=l−b/l+b, where l and b are the major and minor axes of vesicle best fit ellipse) were computed. conversion to volumetric vesicle number densities can easily be obtained dividing the areal number densities by the average vesicle diameter. these values were then compared with measurements made on the campanian ignimbrite pyroclasts (polacci et al., 2003) and on pumice clasts from calc-alkaline eruptions from subduction volcanoes (klug and cashman 1994; polacci et al. 2001; klug et al., 2002; rosi et al., 2004). 3. results 3.1. observation of pumice textures according to the morphological features and vesicle characteristics of the investigated samples, we were able to distinguish different pumice types for which we followed the same classification also applied to the ci juvenile fraction (polacci et al., 2003). 1) «microvesicular» pumices are mostly equidimensional or angular clasts containing heterogeneous vesicles, and they are the most abundant juvenile component of the deposit. 2) fairly elongated, typical «tube» pumice clasts are characterized by alignments of highly stretched, deformed vesicles. evidence of extensively sheared vesicles up to collapse is present in clasts having low aspect ratios (very flattened in the direction parallel to clast elongation). 3) finally, «expanded» pumices include all pumice clasts presenting extensive vesicle expansion features, and span from extremely vesicular reticulites (> 90% interconnected vesicles) to clasts with inflated-like textures (vesicular core and progressively denser outer margins). textures intermediate between the three end-member pumice types are common, and transitional steps among different pumice types can be observed. 733 constraining the dynamics of volcanic eruptions by characterization of pumice textures table i summarizes the observed general macroscopic and microscopic textural characteristics. 3.2. quantification of pumice textures bulk and groundmass textural parameters are reported as minimum and maximum, along with the average value, to provide the textural variability within each pumice type (table ii). owing to the difficulty of decoalescing vesicle textures where extreme interconnectivity occurs, highly expanded pumice clasts were not included in this analysis, and only results pertaining to pumices with inflated core and dense margins are reported in table ii. bulk and groundmass vesicularities, as well as groundmass vesicle number densities, span quite a wide range of values for all three pumice types, with the lowest values applying to inflated pumices (table ii). on average, tube pumices record the highest bulk (0.89) and groundmass (0.82) vesicularities, but the lowest vesicle number densities (4 × 105 cm−2). similar values of vesicle size and deformation are exhibited by all pumice types, with the lowest values pertaining again to inflated pumices (table ii). 4. discussion 4.1. comparison with pumice textures from calc-alkaline eruptions and feed-back with numerical modeling of conduit magma ascent table iii reports data on vesicle measurements related to several well studied sustained table i. pumice types from ams eruption. pumice type clast morphology macroscopic texture microscopic texture microvesicular equidimensional moderately vesicular to vesicular; heterogeneous vesicles, or angular. max vesicle size < 1 mm elongated/deformed vesicles coexisting with spherical vesicles. tube elongated; aspect dense to vesicular, alignments elongated, sheared vesicles. ratio 1:2 to 1:4. of large (> 1 mm) tube vesicles. expanded bread-crusted vesicular to extremely vesicular, highly interconnected vesicles. to reticulites. extensive expansion features. table ii. summary of textural measurements. ams fallout layer b1 and d1 pumice type number *gm vesicularity **bulk vesicularity *gm vesicle ***average average measured range range n density gm d (µm) gm d range (cm−2) m 10136 0.69-0.84 (0.78) 0.80-0.88 (0.84) 32861017.5 0.390 -805330 (488354) t 1871 0.76-0.85 (0.82) 0.85-0.91 (0.89) 29129019.7 0.433 -621637 (402025) e 13024 0.41-0.69 (0.57) 0.51-0.87 (0.70) 275250 12.8 0.367 -1169230 (660697) m, t, and e indicate microvesicular, tube and expanded pumice types (here e refers only to inflated pumices); gm is groundmass (average values in parentheses); * vesicles < 200 µm; ** including vesicles > 200 µm (phenocrystfree); *** calculated as d = 2(φ /naπ)1/2, where φ is vesicularity and na vesicle number density. 734 margherita polacci calc-alkaline explosive eruptions: the 1991 climactic pinatubo eruption (philippines), the 800-year-bp quilotoa eruption (ecuador), the 7700-year-bp mt. mazama eruption (crater lake, oregon), and the 1980 mt. st. helens eruption (washington), respectively. data on the ci eruption are also included to allow full comparison with the textural dataset on alkaline eruptions from phlegraean fields. detailed inspections of the juvenile fraction of ci and ams have revealed that clast morphologies and vesicle textures present close similarities with those exhibited by pumices from the above reported calc-alkaline eruptions, despite their different bulk compositions, volatile and crystal contents. first of all, the bulk of the deposit in eruptions of both magmatic compositions is generally constituted by microvesicular pumice clasts, which contain heterogeneous vesicle populations at various steps in the coalescence process and with varying degrees of deformation (m and white, tables ii and iii). a subordinate pumice type is represented by dense to vesicular tube pumices and/or pumices with foliated surfaces, characterized by alignments of highly elongated/stretched vesicles (t and foliated, tables ii and iii). finally, expanded pumice textures, given by the growth and coalescence of poorly deformed vesicles, are also present in clasts of both magma types (e and gray, tables ii and ii). specific measurements on pumice textures report overlapping ranges of vesicle number densities, and similar vesicle size and deformation. yet vesicularities obtained with image analyses and density-derived vesicularities are higher overall in pumices from ci and ams in comparison to those from the reported calc-alkaline pyroclasts (table iii), with exceptions from mt. mazama and mt. st. helens helium picnometry values that were measured on a limited number of samples and may be not representative of the whole deposit. if pumice clasts do not exhibit evidence of postfragmentation expansion features, vesicularities can be assumed to represent the state of magma at the time of fragmentation, and differences in vesicularities may therefore record different fragmentation conditions owing to the different magma properties characterizing alkaline and calc-alkaline compositions. table iii. vesicle measurements from explosive silicic eruptions. eruption pumice type bulk vesicularity average density-derived average bulk average from image analysis bulk vesicularity vesicle n density (cm−3) gm d *ci m 0.85-0.94 (0.89) 0.78 1×108 0.390 t 0.87-0.92 (0.89) 0.75 1×108 0.591 e 0.81 **pinatubo white 0.76-0.85 (0.81) 0.60 1×108-109 0.371 foliated 0.72-0.82 (0.77) 1×109 0.383 gray 0.68-0.82 (0.74) 0.66 1×108-109 0.234 ***quilotoa white (0.80) 0.66 8.9 ×108 0.413 gray (0.84) 0.71 8.1 ×108 0.274 ^mt. mazama different 0.82 1.0 ×109 types ^^mt. st. helens white (0.80) 0.80 8.2 ×108 gray (0.60) 0.61 2.0 ×109 m, t, and e indicate microvesicular, tube and expanded pumice types; gm is groundmass (in the third column, average values are given in parentheses); * from polacci et al. (2003); ** from polacci et al. (2001), density-derived vesicularities from pallister et al. (1996); *** from rosi et al. (2004); ^ from klug et al. (2002), 0.82 is helium picnometer vesicularity; ^^ from klug and cashman (1994), 0.80 and 0.61 are helium picnometer vesicularities. 735 constraining the dynamics of volcanic eruptions by characterization of pumice textures recent determination of viscosities of phlegraean fields trachytic melts have shown that ci and ams viscosities may be up to two orders of magnitude lower than those of a typical rhyolite at anhydrous conditions (giordano et al., 2004). the lower viscosity of these trachytic melts, together with the higher solubilities (polacci et al., 2004), translate into a different distribution of the flow variables within the conduit and different fragmentation conditions. figure 1 reports an example of the numerical modeling of magma ascent and fragmentation for a typical rhyolite composition and for trachytic compositions corresponding to the ci and ams eruptions. details of the conduit flow model are described elsewhere (papale, 2001). the gas volume fraction distribution of the rhyolitic magma is different from that of the two trachytic compositions. the rhyolitic magma fragments at a much deeper level in the conduit and at lower gas volume fraction (0.70 versus 0.82 and 0.83, in this specific case). a similar behaviour is obtained by changing the water content and/or the conduit diameter. if we take into account vesicularities pertaining to microvesicular pumice clasts, by far the most abundant juvenile component in the deposits from both magma types, differences in calculated trachytic (alkaline) and rhyolitic (calc-alkaline) vesicularities match quite well with results of vesicularities directly measured on natural pumice clasts (table iii). density-derived vesicularities are the best way to statistically represent the vesicularity distribution of the whole deposit (klug et al., 2002; polacci et al., 2003), averaging 0.78 for ci pumice clasts, whereas lower values apply to pinatubo and quilotoa pumices, 0.60 and 0.66 respectively. the higher vesicularities displayed by both mt. mazama and mt. st. helens products may not only be the outcome of a limited number of processed samples but, alternatively, may result from magmatic mixtures whose viscosity is lower than that of a typical rhyolite or dacite, due to specific conditions like very poor crystal content (former case) or lower evolved chemical composition (latter case). by combining the dataset of textural measurements with observations from the numerical fig. 1. calculated distribution of the gas volume fraction along the conduit with the model of papale (2001). input parameters are 5 km conduit length, 122.5 mpa stagnation pressure, 6 wt% water content, 60 m conduit diameter and compositions corresponding to that of ams, ci (romano et al., 2003; giordano et al., 2004, respectively) and a typical rhyolite (innocenti et al., 1982). 736 margherita polacci simulations, we have been able to ascribe differences in pumice vesicularity to the different rheological properties of alkaline versus calcalkaline magmas, which, in turn, may affect the flow property distributions within the volcanic conduit. the lower viscosity of alkaline magmas, together with their higher solubilities, implies that the phlegraean fields trachytic magma needs to run a longer section of the conduit before reaching the conditions necessary to fragment, eventually allowing further gas exsolution and magmatic mixture expansion in comparison to highly viscous rhyolitic to dacitic magmas. 4.2. inferences with eruption dynamics alkaline and calc-alkaline eruptions are characterized by similar eruptive phenomenologies. this indicates that differences in fragmentation conditions do not translate into significantly different flow conditions at the eruptive vent (polacci et al. 2004). indeed, the observed similarities in clast morphologies and textures indicate that the origin of pumices from alkaline and calc-alkaline eruptions may be ascribed to the same magmatic processes. previous studies have interpreted the origin of different pumice types within the same eruptive event as due to changes in the eruptive regime (convective versus buoyant) (klug et al., 2002), or to the development of horizontal gradients of the magma properties within the conduit (polacci et al., 2001; rosi et al., 2004). in agreement with the last hypothesis the following horizontal zonation of flow conditions in volcanic conduits is outlined (fig. 2). assuming a velocity profile, the central part of the conduit is occupied by microvesicular pumice clasts, where vesicles are free to grow only subjected to elongational stresses. pumices with tube and foliated textures are thought to originate in the region between the center and the conduit walls, where, owing to the exerted shear stress and velocity gradient, vesicles are extensively sheared and deformed until they eventually collapse. pumices with expansion features are located at the conduit walls, where the shear stress is maximum and where the local temperature and velocity rise by viscous heating (costa and macedonio, 2003) produces several related effects from crystal resorption (in crystal-rich eruptions as pinatubo and quilotoa) to enhanced volatile exsolution and vesicle growth and coalescence (in low viscosity, high solubility alkaline eruptions as ci and ams) (polacci et al., 2001, 2003). as the three end-member pumice types are related to conduit zones characterized by different magma properties, quantification of their proportion may provide insights into mechanisms of magma exsolution (volatile release from the melt), degassing (volatile separation from the melt), and development of permeability, all of which deeply affect the whole eruption dynamics. tube pumices and pumices with extensive expansion features have the highest vesicularities and degree of vesicle interconnectivity (klug et al., 2002; polacci et al., 2003, and this paper), and their presence in volcanic deposits may be used as a proxy to infer the ability of the magmatic mixture to develop a permeable network of vesicles through which the gas phase may eventually escape non-explosively. the increase or decrease in the proportion of the different pumice types is therefore likely to reflect a change in the eruption dynamics. this simplified scheme may apply to both alkaline and calc-alkaline eruption dynamics and constrain theoretical results of magma fig. 2. zonation of different pumice types within the volcanic conduit, after polacci et al. (2003). not to scale. 737 constraining the dynamics of volcanic eruptions by characterization of pumice textures flow in volcanic conduits with observations on natural pumice clast textures. 5. perspectives we have investigated the dynamics of phlegraean fields eruptions by combining textural characterization of pumice clasts with observations from numerical modeling. comparison with data and observations on pumice clasts from calc-alkaline eruptions has allowed us to provide a general scheme of the rhyolitic versus trachytic eruption dynamics. this interdisciplinary approach applied to other alkaline and calc-alkaline eruption compositions promises to represent a tool to constrain the overall dynamics of explosive eruptions. textural measurements available to date are however limited and do not cover the whole range of crystal and vesicle content present in natural volcanic products. moreover, numerical simulations of conduit magma flow present several restrictions since, for example, they can be applied to specific eruption conditions (steady or sub-steady), and/or do not model the along-the conduit kinetics of crystal and bubble nucleation and growth or the multiphase magma rheology. future efforts should therefore be focused on strengthening the existing textural dataset and on implementing numerical codes aimed at reproducing better the observed phenomenologies. particular emphasis should be placed on measuring textural parameters, like vesicle interconnectivity, specifically linked to the development of magma permeability, which is known to profoundly affect the eruption dynamics (klug and cashman, 1996; papale, 2001). these parameters also represent crucial constraint data for the numerical modeling. acknowledgements this work was supported by gnv project 2000/02/-17. we wish to thank f. colarieti for thin section preparation and sem assistance, and p. pantani for graphic assistance. comments from j. gardner and an anonymous reviewer have been gratefully appreciated. references blower, j.d., j.p. keating, h.m. mader and j.c. phillips (2001): inferring volcanic degassing processes from vesicle size distributions, geophys. res. lett., 28, 347350. costa, a. and g. macedonio (2003): viscous heating in fluids with temperature-dependent viscosity: implications for magma flows, non-linear proc. geophys., 10, 545-555. di vito, m. a., r. isaia, g. orsi, j. southon, s. de vita, m. d’antonio, l. pappalardo and m. piochi (1999): volcanism and deformation since 12000 years at the campi flegrei caldera (italy), j. volcanol. geotherm. res., 91, 221-246. druitt, t.h. and b.p. kokeelar (2002): the eruption of the soufriere hills volcano, montserrat, from 1995 to 1999 (the geological society of london), 1-664. giordano, d., c. romano, p. papale and d.b. dingwell (2004): the viscosity of trachytes, and comparison with basalts, phonolites, and rhyolites, chem. geol., 213, 49-61. klug, c. and k.v. cashman (1994): vesiculation of may 18, 1980, mt. st. helens magma, geology, 22, 468-472. klug, c. and k.v. cashman (1996): permeability development in vesiculating magmas: implications for fragmentation, bull volcanol., 58, 87-100. klug, c., k.v. cashman and c.r. bacon (2002): structure and physical characteristics of pumice from the climactic eruption of mt. mazama (crater lake), oregon, bull. volcanol., 64, 486-501. hammer, j.e., k.v. cashman, r.p. hoblitt and s. newman (1999): degassing and microlite crystallization during pre-climactic events of the 1991 eruption of mt. pinatubo, philippines, bull. volcanol., 60, 355-380 innocenti, f., p. manetti, r. mazzuoli, g. pasquarè and l. villari (1982): anatolia and north-western iran, in andesites, edited by r.s. thorpe (wiley and sons, new york), 327-349. mader, h.m., j.c. phillips and r.s.j. sparks (1996): dynamics of explosive degassing of magma: observations of fragmenting two-phase flow, j. geophys. res., 101, 5547-5560. mader, h. m., e.e. brodsky, d. howard and b. sturtenvant (1997): laboratory simulations of sustained volcanic eruptions, nature, 338, 462-464. newhall, c.g. and r.s. punongbayan (1996): fire and mud: eruptions and lahars of mt. pinatubo, philippines (university of washington press, seattle), 11126. papale, p. (2001): the dynamics of magma flow in volcanic conduits with variable fragmentation efficiency and non-equilibrium pumice degassing, j. geophys. res., 106, 11043-11065. polacci , m., p. papale and m. rosi (2001): textural heterogeneities in pumices from the climactic eruption of mt. pinatubo, 15 june 1991, and implications for magma ascent dynamics, bull. volcanol., 63, 83-97. polacci, m., l. pioli and m. rosi (2003): the plinian phase of the campanian ignimbrite eruption (phlegraean fields, italy): evidence from density measurements and textural characterization of pumice, bull. volcanol., 65, 418-432. 738 margherita polacci polacci, m., p. papale, d. del seppia, d. giordano and c. romano (2004): dynamics of magma ascent and fragmentation in trachytic versus rhyolitic eruptions, j. volcanol. geotherm. res., 131, 93-108. romano, c., d. giordano, v. mincione, k. u. hess, d. dingwell and p. papale (2003): the dry and hydrous viscosities of alcaline melts from vesuvius and phlegraean fields, chem. geol., 202, 23-38. rosi, m., p. landi, m. polacci, a. di muro and d. zandomeneghi (2004): role of conduit shear on ascent of the crystal-rich magma feeding the 800-year-bp plinian eruption of quilotoa volcano (ecuador), bull. volcanol., 66, 307-321, doi: 10.1007/s00445-003-0312-z. sigurdsson, h., b.f. houghton, s.r. mcnutt, h. rymer and j. stix (2000): encyclopedia of volcanoes (academic press, san diego), 1-1417. annals of geophysics, 55 (3), 2012: preface_by_s.seno_layout 6 329 annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-6102 preface it is my distinguished pleasure to introduce this volume devoted to geoethics and edited by silvia peppoloni and giuseppe di capua, for two main reasons. first and foremost, the volume is a collection of papers presented during the ‘geoethics’ scientific session at the 8th italian forum of earth sciences that was held in september 2011, in turin, italy. the forum singled out sustainability – “earth sciences for sustainable development” – as its main theme, thus highlighting the key role of the geosciences with respect to important issues for the present, and in particular, for the future of our society. resources and energy, climate, water, impact reduction of natural hazards, new materials, and protection and conservation of cultural heritage are all aspects of interest where a (geo)ethical approach is strongly needed. public opinion is growing more aware of the critical issues that our society will have to face in the environmental field. unfortunately, neither the perception of virtuous behavior, nor its adoption appear to be keeping pace with this increasing awareness. this gap is common at various levels, from politicians to ordinary citizens, and it has multiple and complex reasons that are sometimes dramatically highlighted by the effects of improper use of the land. admittedly, it is necessary to provide better information about the role that geosciences can have in offering most of the answers to critical questions, providing decision makers not only with the tools they need to face the most daunting challenges – infrastructure safety, water and energy supplies, environmental protection – but also with new cultural and ethical values. the second reason has to do with the careful attention that the federazione italiana di scienze della terra (italian federation of earth sciences) is paying to the sharing and spreading of ethical values, which resulted in the establishment of its own geoethical commission in january 2011. this commission is primarily entrusted with the task of drawing up a list of ethical behavior and commitments that will be submitted for approval by everyone active in the field of the geosciences. we and all those working in the field of environment and natural resources management know only too well that any decisions taken in this context can result in significant, and potentially irreversible, consequences. we therefore have a responsibility to correctly inform the public of any risks involved in excessive and irrational exploitation of our natural resources, and to raise the awareness in each and every one of us, from the early years of public education, that the environment is worth protecting and preserving for future generations. silvio seno, president, federazione italiana di scienze della terra << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice a0332 annals of geophysics, 60, 3, 2017, a0332; doi: 10.4401/ag-7065 analysis of the probable signature of mid-latitude electron density trough at the ionospheric critical frequencies over europe erdinç timoçіn1,*, i̇brahim ünal2 1 i̇nönü university, faculty of science and art, department of physics, malatya, turkey 2 i̇nönü university, faculty of education, department of science teaching, turkey article history received may 29, 2016; accepted march 21, 2017. subject classification: ionosphere, magnetosphere, critical frequency, mid-latitude electron density trough. abstract in this study, probable signature of the mid-latitude electron density trough at the ionospheric critical frequencies ( fof2) is investigated. for this purpose, hourly ionospheric critical frequencies data obtained from ionosonde stations for the year 1972 are used. these stations are situated between 45o-65o invariant magnetic latitudes (λ) of the northern hemisphere. the fof2 data are examined depending on local time (lt), seasons, λ and geomagnetic activity (kp) variation. the results show that the fof2 troughs occur in the nighttime hours between dusk and dawn sector for all situations. the troughs under geomagnetic active (kp>2 +) conditions are more prominent and apparent than that under geomagnetic quiet (kp≤2 +) conditions. under geomagnetic active conditions, the troughs and their minimum positions tend to appear at lower λ for four seasons. also, the average fof2 values of the trough minimum under geomagnetic active conditions have lower values than that under geomagnetic quiet conditions. it is observed that the seasonal variations have important effects on the structure of the fof2 trough, especially for december solstice. the results of this study will contribute a better description and prediction of the mid-latitude electron density trough features for the ionospheric f2 peak heights and better understanding the role of the mid-latitude electron density trough on the ionospheric variability. 1. introduction solar flares and solar wind streams affect the earth’s magnetic field significantly and this interaction causes large and sudden changes in the electron density of the ionosphere. especially, under geomagnetic active conditions, change in the electron density between 40° and 70° λ creates a negative effect on communication and navigation systems. approximately at 550 km height, the ambient electron densities trough observed in magnetic mid-latitudes is called mid-latitude electron density trough [muldrew 1965, calvert and van zandt 1966, sharp 1966, carlson 1968, rishbeth and garriott 1969, tulunay and sayers 1971, kelley 1989]. as a result of subsequent studies were performed using electron density data measured by ariel 3 and ariel 4 satellites about 550 km between the years 19601972, the qualitative identification of the mid-latitude electron density trough has been made and the quantitative criteria has been developed [tulunay and sayers 1971, tulunay 1973, tulunay 1974, tulunay and grebowsky 1975b, tulunay and grebowsky 1978]. the electron density trough is the ionospheric projection of the magnetospheric plasmapause. the trough marks the region where the electron density variation is abrupt over a narrow range of geomagnetic latitudes. the unpredictable variability greatly limits the efficiency of the operations of communication, radar and navigation systems which employ high frequency (hf) radio waves. the electron density depletion in the trough region reduces the maximum usable frequency that can be reflected by the ionosphere along the great circle. therefore, it is essential to understand the role of the trough in the ionospheric variability [tulunay 1973, grebowsky et al. 1974, tulunay and grebowsky 1975a, tulunay and grebowsky 1975b, kersley et al. 1997, tulunay et al. 1998, rothkaehl et al. 2000, tulunay et al. 2003, pryse et al. 2006, mikhailov and leschinskaya 2011]. earlier studies [rothkaehl et al. 2000, tulunay et al. 2003] concluded that (i) it would be advisable to adopt a practical, simple convection model which changes in steps with the planetary geomagnetic activity index (kp) or interplanetary magnetic field (imf) reversals or any other effective event in order to reflect the influence of the trough phenomena on the ionospheric fof2 prediction or forecasting algorithms. (ii) since the fof2 peak can extend above 500 km, the signature of the trough is ti̇moçi̇n and ünal 2 expected to fall in the fof2 variations all over the world. from this point of view it must be essential to include the influence of the trough in fof2 predictions or forecasts for practical applications and modeling. to the knowledge of the authors there has been no final model exists which simulates the trough, based on physical principals to be employed for radio communication applications. in practical applications of the hf radio communications in any model that does not include the trough are not a complete model. the most of previous studies about the mid-latitude electron density trough were performed over the cooperation in scientific and technology-251 (cost-251) area of the europe. cost-251 is area of the europe between latitudes of 35°-70°n and longitude 10° w-60°e. but these studies were limited to only satellite data and were not exactly investigated effect on the electron densities in the ionospheric f2 peak height of the mid-latitude electron density trough considering seasonal, local time and geomagnetic activity changes by using fof2 data. the purpose of our study is to investigate possible signature on fof2 of the structure called the mid-latitude electron density trough depending on local time, seasons, λ and geomagnetic activity changes and is to compare obtained the results from this study with obtained the results from previous studies which were performed for different altitudes. in this way it will establish a better description and prediction of the mid-latitude trough features for fof2. in addition, the results of this work will contribute to better understanding the effect at the ionospheric f2 heights of the mid-latitude electron density trough and the role of the trough on ionospheric variability. 2. data and methods of analysis in this study, hourly fof2 data of year 1972 are used to investigate probable signature of the mid-latitude electron density trough at the ionospheric f2 peak heights over the europe. fof2 data are obtained from ionosonde stations which are situated between 40° and 70° λ. the geographic latitudes, geographic longitudes and λ values of these stations are given in the table 1. we also used the kp index to investigate the effect of geomagnetic activity changes on fof2 values. this data were taken from space physics interactive data resource (spidr) (http://spidr.ngdc.noaa.gov/spidr/). data are analyzed depending on seasons, geomagnetic activity and local time as follows: (1) fof2 data are separated into four seasons as march and september equinoxes and june and december solstices. (2) the hourly kp values are calculated from 3-hr kp values by using linear interpolation method. thus, the effect of geomagnetic activity changes on fof2 is investigated more in detail for each local time. (3) to investigate the geomagnetic activity effect on the fof2 trough, fof2 data are separated into two groups as geomagnetic quiet hours and geomagnetic active hours. (4) because ariel satellite returns around the earth about 1344 times during a season, the satellite measures many times electron density for each local time. the average electron density values for each local time are calculated from this data. then the mid-latitude electron density troughs for each local time are investigated considering variations according to λ of average electron density [tulunay and sayers 1971, tulunay 1973]. therefore we calculated the average fof2 values for severally each local time of all seasons and then we investigated variations according to λ of the average fof2 values for different seasons and different geomagnetic activity conditions. (5). the contours plots of the average fof2 values station names geographic latitude (°n) geographic longitude (°e) invariant magnetic latitude (°n) station names geographic latitude (°n) geographic longitude (°e) invariant magnetic latitude (°n) bekescsaba 46.7 21.2 45.0 uppsala 59.8 17.6 57.0 kiev 50.5 30.5 48.0 lycksele 64.7 18.8 61.0 miedzeszyn 52.2 21.2 50.0 sodankyla 67.4 26.6 64.0 kaliningrad 54.7 20.6 52.0 kiruna 67.8 20.4 65.0 table 1. geographic latitude, geographic longitude and λ values of the ionosonde stations. probable signature of mid-latitude electron density trough 3 are drawn according to λ and local time for different seasons and geomagnetic activity conditions. 3. results and discussions figures 1-4 show the variations according to λ of the average fof2 values for different local times and different geomagnetic active conditions during march and september equinoxes, and june and december solstices of the year 1972. as seen in figures 1 and 3, the average fof2 values during the daytime hours between 05-19 lt gradually decrease with increasing λ under both the geomagnetic quiet and geomagnetic active conditions. the boundaries of this local time region where the average fof2 values gradually decrease with increasing λ is different for june and december solstices. the average fof2 values of local times between 04-00 lt for june solstice gradually decrease with increasing λ, while the average fof2 values of local times between 0615 lt for december solstice gradually decrease with increasing λ. that is, the effect of the mid-latitude electron density trough on fof2 is not observed at all of these local times. in addition, as seen in the figure 2a, the average fof2 values of local times between 0004 lt for june solstice under geomagnetic quite conditions gradually decrease with increasing λ similarly to latitudinal behavior of the average fof2 values of local times between 04-00 lt. that is, under geomagnetic quiet conditions, the effect of the mid-latitude electron density trough on fof2 was not exactly observed for any local times of june solstice. as seen in the figures 1a and 3a, the fof2 troughs figure 1. variations according to invariant magnetic latitudes of the average fof2 values for different local times during 21 march equinox period of 1972 year (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. figure 2. variations according to invariant magnetic latitudes of the average fof2 values for different local times during 21 june solstice period of the year 1972 (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. ti̇moçi̇n and ünal 4 for march and september equinoxes under geomagnetic quite conditions are observed between about 55°62° λ and for the hours between 19-05 lt. the position of the trough minimum are located at about 58° λ for march equinox and about 61° λ for september equinox. for these seasons, the average fof2 value of the trough minimum is about 2.5 mhz. these values are observed for the hours between 03-01 lt for both march equinox and september equinox. as seen in the figure 4a, the fof2 trough for december solstice under geomagnetic quite conditions is observed between about 53°-62° λ and for the hours between 05-18 lt. the position of the trough minimum for this season is located at about 61° λ. the average fof2 value of the trough minimum is about 1.3 mhz for the hours between 20-03 lt. this results show that the effect of the mid-latitude electron density trough on fof2 becomes more prominent during december solstice, but disappear during june solstice. figures 1b-4b show the variations according to λ of the average fof2 values for different local times under geomagnetic active conditions during march and september equinoxes and june and december solstices of the year 1972. as seen in the figure 2a, not exactly determined the fof2 trough for geomagnetic quite conditions of june solstice is clearly observed for 01-03 lt under geomagnetic active conditions. note that the position of the trough minimum shifts equatorward about 2°-5° λ for all seasons under geomagnetic active conditions. that is, the fof2 trough tends to occur at lower λ for all seasons under geomagnetic active conditions. the average fof2 value of the trough minimum reduces about 0.3 mhz with increasing geomagnetic activity for all seasons. in addition, the lofigure 3. variations according to invariant magnetic latitudes of the average fof2 values for different local times during 23 september equinox period of the year 1972 (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. figure 4. variations according to invariant magnetic latitudes of the average fof2 values for different local times during 21 december solstice period of the year 1972 (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. 5 probable signature of mid-latitude electron density trough cal time interval which the trough is observed extends with increasing geomagnetic activity for all seasons, especially for december solstice. that is, under geomagnetic active conditions, the trough has a deeper structure and it is observed for more local times, especially for december solstice. many studies were performed about mid-latitude electron density trough for different altitudes by using satellites, tomography and gps-tec (total electron content) since its discovery at about 550 km altitude in the 1960s. these studies reveal that the mid-latitude electron density trough is a persistent large-scale electron density depletion structure and it forms at the between the mid-latitude ionosphere and the high-latitude auroral. that is, the troughs are located between 40°-70° latitudes of both hemispheres for all seasons. the trough extends from dusk sector to dawn sector, so it can be seen explicitly for the nighttime hours whereas it cannot be seen clearly for the daytime hours of all seasons. the studies also show that geomagnetic activity and seasons have important effect on formation and structure of the trough. the mid-latitude electron density trough becomes prominent during the winter solstice, but weak during the summer solstice for the northern hemisphere. the electron density of the trough minimum reduces with increasing geomagnetic activity, so trough is more evident during the higher geomagnetic activity conditions. the trough minimum position shifts equatorward with increasing geomagnetic activity and this motion is proportional to the strength of geomagnetic activity. in addition, the local time interval which was observed the mid-latitude electron density trough expands with increasing geomagnetic activity [muldrew 1965, tulunay and sayers 1971, tulunay 1972a, tulunay 1972b, tulunay 1973, tulunay 1974, grebowsky et al. 1974, tulunay and grebowsky 1975a, tulunay and grebowsky 1975b, grebowsky et al. 1976, tulunay and grebowsky 1978, kersley et al. 1997, tulunay et al. 1998, rothkaehl et al. 2000, tulunay et al. 2003, pryse et al. 2006, lee et al. 2011, mikhailov and leschinskaya 2011, he et al. 2011]. our results agree with these results from previous studies. we find that the fof2 trough appears between 40°-70° λ for all seasons under both geomagnetic activity conditions. the fof2 trough was not observed during the daytime hours for all seasons under both geomagnetic activity conditions, while it was explicitly observed during night hours for march and september equinoxes and december solstice under both geomagnetic activity conditions. we also find that the seasonal and geomagnetic activity variations have important effects on the structure of the fof2 trough. the fof2 trough is not clearly observed during june solstice for geomagnetic quiet conditions. otherwise december solstice is season to be the most effect of the fof2 trough for both geomagnetic quiet conditions and geomagnetic activity conditions. under geomagnetic active conditions, the minimum position of the fof2 trough is observed at lower λ for all seasons. that is, the fof2 trough tends to occur at lower λ for all seasons under geomagnetic active conditions. in addition, the fof2 value of the trough minimum reduces with increasing geomagnetic activity for all seasons. the local time interval which is observed the fof2 trough extends with increasing geomagnetic activity for all seasons, especially for december solstice. that is, the fof2 trough has a deeper structure and is observed for more local times under geomagnetic active conditions. also the fof2 trough is clearly observed for june solstice with increasing geomagnetic activity. these results demonstrate that the latitudinal behavior of the electron density at the ionospheric f2 peak heights exhibit a similar structure with the latitudinal behavior of the electron density in higher regions than f2 peak. that is, the mid-latitude electron density trough has an important effect on the change of electron density in the ionospheric f2 region. 4. conclusions the results obtained from this study are presented at three main headings as follows: (1) the different behaviors according to λ of average fof2 values for the daytime hours and the nighttime hours are substantially similar to the structure of magnetosphere and plasmosphere in the nighttime hours and the daytime hours. because the region between magnetosphere boundary and plasmosphere boundary is wider during the nighttime hours, the number of magnetic field lines decreases between 40°70° λ. therefore, the plasma density of this region decreases suddenly and sharply during the nighttime hours. this explains why the trough is usually observed during the nighttime hours. in addition to this, because the ionizing effect of ultraviolet and x-rays fill the trough shortly after dawn sector, the trough is not observed during the daytime hours [tulunay 1973, tulunay 1974, tulunay and grebowsky 1978]. (2) the geomagnetic activity increase caused by solar storms occur a magnetospheric electric field. this electric field occurs a drift velocity that represented by . it is oriented towards to the earth in the nighttime hours, while it is oriented outward from exb b2 ti̇moçi̇n and ünal 6 the earth in the daytime hours. this drift velocity is one of the most important physical parameters that determine the width of the plasmosphere borders at night and day sides. because the drift velocity under geomagnetic active hours is greater than that under geomagnetic quiet hour, it causes to more narrowing of plasmosphere border in the nighttime hours and more expansion of plasmosphere border in the daytime hours. this narrowing causes to more decrease of magnetic field lines and electron density between 40°-70° λ. therefore, under the higher geomagnetic activity conditions, the troughs are more prominent and are observed more clearly for all seasons [tulunay 1971, tulunay 1973, tulunay 1974, tulunay and grebowsky 1978]. (3) the formation and structure of trough vary depending to seasonal changes. it is determined that june solstice is season to be least effect on fof2 of the mid-latitude electron density trough, while december solstice is season to be the most effect on fof2 of the mid-latitude electron density trough. this difference is a result of changing at magnetic equator position of the earth according to sun for different seasons and is associated with the relative positions of sub-solar point to the magnetic equator varies considerably for december solstice [tulunay 1971, tulunay 1973, tulunay 1974, tulunay and grebowsky 1978]. the fundamental similar behaviors between the fof2 trough and the mid-latitude electron density trough that are observed at different altitudes can be explained with the effect of diffusion created by the magnetic field gradient and drift. the results also indicate that magnetic structure called as magnetospheric plasma boundary on the mid-latitudes has a significant effect on changes of electron densities at lower altitudes. in addition to this, in order to establish a better description and prediction of the mid-latitude trough features, it is essential to combine the in situ satellite observations with those of the ground based measurements. acknowledgements. the authors are grateful to prof. dr. yurdanur kabasakal tulunay for making this research possible by providing the facilities needed for the study. references calvert, w. and t. e. van zandt (1966). fixed-frequency observation of plasma resonances in the topside ionosphere, j. geophys. res., 71, 1799-1813. carlson, h. c. (1966). ionospheric heating by magnetic conjugate point photoelectrons. j. geophys. res., 71, 195-199. grebowsky, j. m., y. k. tulunay and a. j. chen (1974). temporal variations in the dawn and dusk mid-latitude trough position and plasmapause, planet. space sci., 22, 1089-1099. grebowsky, j. m., n. c. maynard, y. k. tulunay and l. j. lanzerotti (1976). coincident observations of ionospheric troughs and the equatorial plasmapause, planet. space sci., 24, 1177-1185. grebowsky, j. m., n. c. maynard and y. k. tulunay (1976). coincident plasma pause-troughs observations, eos t. am geophys. un., 57:4, 299-299. he, m., l. liu, w. wan and b. zhao (2011). a study on the nighttime midlatitude ionospheric trough, j. geophys. res., 116, a05315, doi: 10.1029/2010ja016252. kelley, m. c. (1989). the earth’s ionosphere (plasma physics and electrodynamics), academic press, 350-354. kersley, l., s. e. pryse, l. k. walker, j. a. t. heaton, c. n. mitchell, m. j. williams and c. a. willson (1997). imaging of electron density troughs by tomographic techniques, radio science, 32-4, 1607-1621. lee, i. t., w. wang, j. y. liu, c. y. chen and c. h. lin (2011). the ionospheric mid-latitude trough observed by formosat-3/cosmic during solar minimum. j. geophys. res., 116, a06311, doi: 10.1029/2010ja015544. mikhailov, a. v. and t. y. leschinskaya (2011). ionospheric altitude profiles in the main ionospheric trough as observed by field-aligned eiscat incoherent scatter radar observations, j. atmosph. terr. phys., 73, 488-498. muldrew, d. b. (1965). f-layer ionisation troughs deduced from alouette data, j. geophys. res., 70, 2635-2650. pryse, s. e., l. kersley, d. malan and g. j. bishop (2006). parameterization of the main ionospheric trough in the european sector, radio sci., 41, rs5s14, doi: 10.1029/2005rs003364. rishbeth, h. and o. k. garriott (1969). introduction to ionospheric physics, academic press, 250-251. rothkaehl h., i. stanislawska, r. leitinger and y. k. tulunay (2000). application of a trough model for telecommunication purposes, physics and chemistry of the earth, 25:4, 315-318. sharp, g. w. (1966). mid-latitude trough in the night ionosphere, j. geophys. res., 71, 1345-1356. tulunay, y. k. (1972a). some topside electron density measurements made by the ariel 3 satellite during the geomagnetic storm of may 25-27 1967, planet. space sci., 20, 1299-1307. tulunay, y. k. (1972b). magnetically symmetrical detection of the mid-latitude electron density trough by the ariel 3 satellite, j. atmosph. terr. phys., 34, 1547-1551. tulunay, y. k. (1973). global electron density distributions from the ariel 3 satellite at mid-latitudes during quiet magnetic periods, j. atmosph. terr. phys., 35, 233-254. tulunay, y. k. (1974). mid-latitude ionosphere as observed by satellites ariel 3 and ariel 4, b. am. meteorolog. soc., 55:6, 650-650. tulunay y. k., h. rothkaehl, g. juchnikowski, y. laletas and i. stanislawska (1998). a comparison between the ariel 4 ambient electron density and ionospheric critical frequency over cost 251 area, proceedings of the 2nd cost 251 workshop, side-antalya, turkey. tulunay, y. k., i. stanislawska and h. rothkaehl (2003). revisiting the ariel trough work for hf telecommunication purposes, cosmic research journal, 41:4, 1-13. tulunay, y. k. and j. sayers (1971). characteristics of mid-latitude trough as determined by the electron density experiments on ariel 3, j. atmosph. terr. phys., 33, 1737-1761. tulunay, y. k. and j. m. grebowsky (1975a). temporal variations in dawn and dusk mid-latitude trough positions, eos t. am. geophys., 56:3, 172-172. tulunay, y. k. and j. m. grebowsky (1975b). temporal variations in the dawn and dusk mid-latitude trough position-measured (ariel 3, ariel 4) and modeling, ann. geophys., 31, 29-38. tulunay, y. k. and j. m. grebowsky (1978). the noon and midnight mid-latitude trough as seen by ariel 4, j. atmosph. terr. phys., 40, 845-855. *corresponding author: erdinç timoçіn i̇nönü university, faculty of science and art, department of physics, 44280 malatya, turkey; email: ertim44@hotmail.com. 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved probable signature of mid-latitude electron density trough 7 earth-prints: log in   istituto nazionale di geofisica e vulcanologia   |earth-prints home page | roma library | bologna library | catania library | milano library | napoli library | palermo library log in to dspace help... new user? click here to register. please enter your e-mail address and password into the form below. e-mail address: password: have you forgotten your password?   ict support, development & maintenance are provided by theaepic team @cilea.powered ondspace software. feedback annals of geophysics [special issue_60_04_2017] monitoring and seismic characterization of archaeological sites and structures. edited by paolo clemente, salomon hailemikael, giulano milana, luciana orlando istituto nazionale di geofi sica e vulcanologia issn 2037-416x annals of geophysics [special issue_60_4_2017] monitoring and seismic characterization of archaeological sites sites and structures. preface paolo clemente, salomon hailemikael, giuliano milana, luciana orlando s0447 articles analysis of the seismic site effects along the ancient via laurentina in rome francesca bozzano, anna buccellato, fulvio coletti, salvatore martino, fabrizio marra, stefano rivellino, chiara varone s0435 geometry and seismic characterization of the subsoil below the amphitheatrum flavium in rome ettore cardarelli, michele cercato, luciana orlando s0436 investigating the foundation of the amphitheatrum flavium through the passage of commodus luciana orlando, giorgio de donno, luca di giambattista, lucia palladini s0437 sub-surface characterization of the amphitheatrum flavium area in rome, through single-station ambient vibration measurements salomon hailemikael, giuliano milana, fabrizio cara, maurizio vassallo, marta pischiutta, sara amorso, paola bordoni, luciana cantore, giuseppe di giulio, deborah di naccio, daniela famiani, alessia mercuri s0438 dynamic characteristics of the amphitheatrum flavium northern wall from traffic-induced vibrations giovanni bongiovanni, giacomo buffarini, paolo clemente, dario rinaldis, fernando saitta s0439 seismic hazard assessment for the protection of cultural heritage in greece: methodological approaches for national and local scale assessment (pilot areas of aighio, kalamata and heraklion) sotiris sboras, john andrew dourakopoulos, evaggelos mouzakiotis, pavlos dafnis, theodoros palantzas, vassilis karastathis, nikos voulgaris, gerasimos-akis tselentis s0440 ambient vibration recording on the maddalena bridge in borgo a mozzano, italy: data analysis riccardo mario azzara, anna de falco, maria girardi, daniele pellegrini s0441 damage identification and seismic vulnerability assessment of a historic masonry chimney maria-giovanna masciotta, luis f. ramos, paulo b. lourenço, marcello vasta s0442 rectangular blocks vs polygonal walls in archaeoseismology klaus-g. hinzen, arnaud montabert s0443 the ruin of the roman temple of kedesh, israel; example of a precariously balanced archaeological structure used as a seismoscope gregor schweppe, klaus-g. hinzen, shmuel marco, sharon k. reamer, moshe fischer s0444 vulnerability assessment of archaeological sites to earthquake hazard: an indicator based method integrating spatial and temporal aspects despina minos-minopoulos, dale dominey-howes, kosmas pavlopoulos s0445 a study of building vibrations induced by weak motions: effects of earthquake excitation, cultural noise and wind speed gaetano riccio, rocco cogliano, giuseppe di giulio, antonio fodarella, stefania pucillo, antonio rovelli s0446 vol49_2_2006 801 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words ocean bottom seismometer – integrated seismic networks – southern tyrrhenian sea – aeolian islands 1. introduction ocean bottom seismometer networks (obsn) are used in different regions of the world to improve the locations of earthquakes occurring off-shore and in coastal areas and to complete data sets from the land-based networks. in the northern japan trench hundreds of seismic events, recorded between july-august 1992, have been relocated by hino et al. (1996) using both obs and land network data, improving focal depth errors to less than 5 km. lawton et al. (1982) studied the seismicity in the area of the kodiak continental shelf (alaska) and reported a mean dislocation of about 12 km of the hypocenters by integrating the obs readings in their land network data set. earthquake relocations, recorded in the central juan de fuca plate (west coast of an improved seismicity picture of the southern tyrrhenian area by the use of obs and land-based networks: the tyde experiment tiziana sgroi (1), thomas braun (2), torsten dahm (3) and francesco frugoni (1) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto nazionale di geofisica e vulcanologia, sede di arezzo, italy (3) institut für geophysik, universität hamburg, hamburg, germany abstract the problem of large location uncertainties for seismicity occurring in the southern tyrrhenian sea have been partially overcame during the implementation of the long-term scientific mission of the tyrrhenian deep-sea experiment (tyde), which allowed the installation of 14 wide-band ocean bottom seismometers (obs) and hydrophones (obh) in the period december 2000-may 2001 on the seafloor around the aeolian and ustica islands. local events recorded at land-stations have also been observed on the seismograms of the ocean bottom seismic network (obsn). moreover, some hundreds of low magnitude events, undetected from the land networks, have been recorded. we combined the readings of body wave arrival times from obs-obh with those from land-stations to locate seismic events. we focused our study on three clusters of events representative of the seismic activity of the area: i) «deep» events, ii) ustica, iii) ne-sicily. the analysis of the integrated data set of the seismicity off-shore and on-shore, obtained from the combined land-obs seismic network (ustica sequence and deep events), has improved locations in terms of rms residuals, azimuthal gap, epicentral and hypocentral errors. moreover, further classes of events have been analysed: the first includes some local events that could be located only by integrating single trigger readings from the few available land-stations with the obsn-data; the second comprises local events that have been detected only by the obs-obh stations. in particular, the last cluster underlines the importance of an obsn in the tyrrhenian deep basin to reveal its unknown intense micro-seismicity, permitting to better understand both the tectonic and geodynamic picture of the area. mailing address: dr. tiziana sgroi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: sgroi@ingv.it 802 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni north america) by a joint land and obs network, provided a more accurate evaluation of the seismicity of the intraplate region (bebel et al., 1992). also the use of obsn allowed other aspects of seismological research to be developed, as an example in tomographic studies performed in the coastal region of northern chile, improving both the hypocenter locations and three-dimensional vp and vp /vs structure (husen et al., 2000). the precise location of off-shore seismicity performed by land-based seismic stations is often difficult. especially in the case of an insular or peninsular seismic network, the observations of local earthquakes occurring in coastal areas show large azimuthal gaps, resulting in a large location error. in italy this problem particularly concerns the southern tyrrhenian basin, one of the most interesting regions of the mediterranean sea considering its intense geodynamic activity linked to tectonic and volcanic processes. up to now only two short-term obss experiments have been performed in the southern tyrrhenian sea: in 1987, a small obs network operated for 11 days close to the calabrian coast, recording microseismicity not detected by permanent and temporary landbased seismic networks (soloviev, 1990); in 1996, a three week passive seismic campaign was realised by a joint italian-japanese collaboration around the aeolian islands and ustica island (aoyagi et al., 1997). the attempt to integrate seismic data from land-based and obs-stations and to improve the epicentre determinations revealed the following problems: i) the use of short-period seismometers (4.5 hz), mainly used for seismic reflection surveys, turned out to be not appropriate for studies of local seismicity in the area. ii) the three week long experiment took place in a seismic quiescent period. iii) the spacing of the obsn (about 40 km) proved to be too large for the low magnitude local seismicity location, therefore only a few seismic events were simultaneously detected by more than 2 obs-stations (beranzoli et al., 1999). fig. 1. location of obs-obh (diamonds) and land-based seismic stations (triangles and squares) used in the present work. each ocean bottom station was equipped with an obh (1 channel), those equipped with an additional 3-component seismic sensor (obs) are represented by underlined numbers. station no. 07 is not reported on the map as it was not recovered. the label 00 reports the position of the geostar multi-parametric observatory, hosting an obh. 803 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks the implementation of the long-term scientific mission of the multi-parameter ocean bottom observatory geostar (beranzoli et al., 2000) in a depth around 1900 m near ustica island (station 00 in fig. 1) was a good opportunity to plan a new obs experiment. between december 2000 and may 2001 the institute of geophysics of the university of hamburg (igh), the research centre for marine science at kiel (geomar) and the geostargroup of the istituto nazionale di geofisica e vulcanologia (ingv) carried out a joint seismological experiment in the southern tyrrhenian sea, the so called tyyrrhenian deep-sea experiment (tyde). a 14 stations obsn was installed on the seafloor around the aeolian islands in the area comprised between latitude 38.4°n-39.6°n and longitude 14.0°n-15.4°n (stations 01-14 in fig. 1), at water depths ranging between 1500 m and 3500 m. each station consisted of a wideband hydrophone (obh), six of them were additionally equipped with a 3-component seismic broadband sensor (obs) (dahm et al., 2002). all the stations except ob07 were recovered. additionally, some temporary broad-band seismic stations were installed in sicily and on the aeolian and ustica islands, to increase the density of the permanent seismic network configuration (braun et al., 2001). the data set collected during the tyde-experiment overcomes the difficulties encountered in the past experiments, because: i) we used broad-band instrumentation, able to record both local seismicity and even teleseismic events; ii) the entire experiment lasted 6 months; iii) obsn was well-spaced (about 30 km) allowing the simultaneous detection of local seismic events at 3 or more obsn-stations. the present paper analyses the hypocentral determinations of the local seismicity as determined by the land-based network and the obsn. we discuss the difficulties concerning the choice of the crustal velocity model, demonstrating that the integration of observations from land-based stations and from the obsn is useful to locate earthquakes in the tyrrhenian basin more accurately. in particular, the integration processes contribute to improve the hypocenter determinations, resulting in less scattered locations associated both to reduced depth and epicentral errors and lower values of azimuthal gap and rms. moreover, we show how only the presence of the obsn placed in the southern tyrrhenian basin allows an intense micro-seismicity linked to both volcanic and tectonic processes to be recorded. 2. structural setting of the southern tyrrhenian sea in addition to high quality seismic data recorded by a suitable network configuration, the accuracy of epicentre locations depends on the reliability of the available crustal velocity model. in the past decades the main structural characteristics of the crust under the southern tyrrhenian sea have been the subject of numerous studies, using mainly data from seismic refraction experiments. to extrapolate velocity models also for depths or areas outside the studied crustal sections, we briefly review the main results of those former studies on the structural setting of the southern tyrrhenian area. the tectonic evolution of the tyrrhenian basin must be considered in a more general geodynamic process of the collision between european and african plates (dewey, 1989). this collision caused the subduction of the ancient tethyan lithosphere under the european plate, with the fragmentation of the european plate margin and the alpine orogenic belt and the opening of the tyrrhenian basin (e.g., malinverno and ryan, 1986; faccenna et al., 1997; argnani, 2000), which started in the miocene (kastens and mascle, 1990). the southern tyrrhenian basin is characterised by the presence of two areas floored by oceanic crust: the vavilov basin of pliocene age and the marsili basin, where opening started less then 2 ma ago (marani and trua, 2002). the marsili basin is surrounded to the south and to the east by the aeolian arc, constituted by volcanic islands and submerged volcanoes of quaternary age (barberi et al., 1974). partly, these volcanoes rest on a crystalline basement of alpine origin which outcrops extensively on the calabro-peloritan sector of the apenninic thrust belt. a clear deep seismicity is present under the marsili basin (anderson and jackson, 1987; gi804 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni ardini and velonà, 1991; selvaggi and chiarabba, 1995), with hypocentral distribution that depicts a very narrow, continuous slab dipping about 70° to the nw down to a depth of about 500 km. tomographic images suggest a connection between the subducted slab and the ionian lithosphere (piromallo and morelli, 1997; cimini, 1999). seismic catalogues (ingv, 2003) report crustal seismicity in the study area both along the coastal areas and in the submerged sector of the apenninic thrust belt and in correspondence to the aeolian islands. off-shore sicily island, this seismicity has been related by neri et al. (1996) to two main faults systems crossing the aeolian islands archipelago (sisifo and vulcano), evidenced by geological studies (frazzetta et al., 1982; ghisetti and vezzani, 1982) and dds surveys (finetti and del ben, 1986). crustal seismic events in the calabrian sector can be related to an extensional tectonic regime. 3. experimental setup for the data analysis described in this paper we created a joint database collecting data recorded by the following seismic networks: obsn – the 14 stations of the ocean bottom seismometer network (obsn; fig. 1) were equipped with wide-band sensors: all the stations included a hydrophone, and six stations were also equipped with a 3-component geophone (fig. 2); the 20 bit digitiser recorded seismic data at a sampling rate of 50 hz. further details about technical aspects of the entire instrumentation pool can be found in dahm et al. (2002). moreover, we used data recorded from the hydrophone mounted on the multi-parameter ocean bottom observatory geostar, deployed near ustica island (gasparoni et al., 2002). rsnc – the ingv operates the national centralised seismic network (rsnc), with more than 100 short-period seismometric stations, mostly vertical-component, installed on the italian territory. in this study, we use data recorded by about 25 stations, located in areas surrounding the southern tyrrhenian sea (fig. 1). these stations are equipped with a s-13 teledyne geotech sensor, acquiring continuous data with a sampling rate of 50 hz. ingv-ct – in eastern sicily operates a local seismic network managed by the ingv section of catania (ingv-ct), consisting of more than 50 short-period stations. they constitute a local network mainly employed in the seismic monitoring of mount etna volcanic edifice, while the seismic stations placed in the messina strait and iblean area permit to locate tectonic earthquakes occurring near the coastal area. in the present study, we used data from eight (fig. 1) 3-component stations (mark l4c), installed in a restricted area surrounding the messina strait, with a 160 hz sampling rate. the time signal for the rsnc and ingv-ct data is set by gps for all the stations, while the fig. 2. organisation of seismic data acquired and used in the framework of the tyde experiment. geomar, uni-hamburg, and geostar are in charge of the obs/obh instruments used during the experiment. istituto nazionale di geofisica e vulcanologia is responsible for the on land networks: for this study we did not use the mednet and temporary networks data set. an obh was mounted on all the off-shore stations; the number of the station is typed in italic style when an obs sensor was mounted, too. in the figure, the characteristics of seismic sensors and the procedures to unify the data format are shown. 805 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks timing for the obsn cannot take advantage of satellite signals when they operate on the seafloor. each obs has an internal clock which was synchronised with the gps clock signal before the deployment (tstart) and just after its recovery (tend). the time delays (tend − tstart) observed at each single station did not exceed ± 0.6 s during the 6months campaign. being the drift of the obs clock signal linear, the correction for this time delay was realised before the data processing. figure 2 sketches the instrumental characteristic, the seismic data processing, and data format conversion. 4. preparatory activities for the event locations the earthquake locations based on data from combined land and obs networks meet several problems not usually encountered by a land network. in the present chapter, we briefly describe all the preparatory work and the problems concerning the integration of different data sets: i) the choice of the location routine; ii) the definition of selection criteria for the local events to be integrated; iii) the choice of velocity models. 4.1. choice of the earthquake location program currently, the hypocenters reported in the bulletins of rsnc and ingv-ct are determined by location routines based on hypo71 (lee and lahr, 1972). in order to integrate seismic data recorded at stations deployed on continental crust (rsnc and ingv-ct) and oceanic crust (obsn), the earthquake location program has to comply with negative station altitudes and with strong crustal inhomogeneities. after balancing carefully the advantages and limits of the different available programs, we decided to use a relatively new location routine, hyposat (schweitzer, 2001). hyposat accepts negative station altitudes, but its main feature of interest for our application is the possibility to use at once different velocity models for the location procedure. preliminary tests with a small cluster of sample events, demonstrated the adequacy of hyposat compared to other location routines. while the determined epicentres were generally well constrained, the focal depths for most events could be determined with minor uncertainties by introducing local velocity models. 4.2. data selection the choice of the local seismic events suitable for the relocation analysis was based on the ingv bulletins (rsnc and ingv-ct). the catalogue data were divided into (a) located earthquakes (§5) and (b) non-located seismic events, where only single station triggers were reported (§6). in the first category (a), the waveforms recorded by seismic land-stations were checked in order to verify the arrival times, the uniformity of the readings, and their relative weights. the final data set consisted of about 800 earthquakes in a magnitude range 1.5<m<3.5, which occurred between december 1st, 2000 and may 18th, 2001. in order to concentrate the present study on the analysis of seismicity in the southern tyrrhenian sea, we made the choice to exclude, a priori, from the analysis earthquakes reported in the ingv-bulletins located in the area of mt. etna, ionian sea, calabria and inner sicily. among the located earthquakes, we selected those where recordings showed a good signal-to-noise-ratio, attributed them to three geographical clusters and used these data for the subsequent integrations with the obsn recordings: cluster 1 – a few deep seismic events in the southern tyrrhenian sea, with reported focal depths below 100 km (13 events). cluster 2 – a small seismic sequence near ustica island (14 events). cluster 3 – a seismic swarm in the area of mts. nebrodi-peloritani system (northern sicily) (20 events). in the second category (b), we included the single stations triggers from rsnc, to be integrated with the obs readings through the event association process. moreover, this category comprehends obs readings relative to events recorded only by the obs array. 806 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni 4.3. choice of the crustal velocity model the complex geodynamic evolution of the southern tyrrhenian basin is confirmed by a pronounced heterogeneity of the crust and of the surrounding emerged regions. the depth of the moho ranges from 10 km in the oceanic-type marsili and vavilov basins (recq et al., 1984; duschenes et al., 1986; kastens and mascle, 1990) to more than 35 km under the apenninic belt in calabria and sicily, where the crust has continental characteristics. moreover, the upper crust presents lateral changes in lithology and in thickness of structural units (aa.vv., 1991). to approximate such pronounced structural variations and taking into account 1d velocity models proposed in literature, we divided the southern tyrrhenian area into 9 sub-zones, each one parameterised by an individual crustal velocity model to be used for location. figure 3 shows the 9 sub-zones and the corresponding pphase velocity 1d models selected from literature. for each zone we also reported the maximum crustal thickness and the main reference used for the definition of the velocity models. for the upper mantle we used the velocity model ak135 (kennett et al., 1995). we started the location procedure using the model of the area where each event has been originally located according to the bulletin. if large errors were associated to the hypocentral parameters, we repeated the location substituting the 1d velocity model with that one of the neighbouring zone, until the result was satisfactory. 5. land-based and integrated earthquake locations forty-seven seismic events have been relocated by integrating land-based stations with the obsn. each earthquake, belonging to one of the three different clusters (1-3), was located following two steps: i) locations based exclusively on the readings of land stations; ii) integration extended to the combined land-obsn observations. all the integrations are characterised by rms residuals smaller than 0.60 s, seismogram records of at least four p-phases and one s-phase, and a station coverage in at least two azimuthal quadrants. figures 4-6 represent the relocations for events of clusters 1-3. grey filled circles depict epi-/hypocenters determined by using time readings of the land based seismic network; black filled circles represent the relocations after integration of the obsn-observations; the dislocations between the old and the new locations are indicated by connecting lines. table i summarises for each cluster the numerical results of the hypocentral determinations, the location errors and the respective relative dislocations. generally the solutions of the relocations are much better constraint in terms of location error, rms and azimuthal gap. this is due to the combination of two effects. on one hand the integration of the obsn data increases the number of observations (phases) and reduces the azimuthal fig. 3. subdivision of the southern tyrrhenian sea in 9 sub-zones (structural map of south italy modified from meletti et al., 2000), each of them characterised by a different velocity model. on the upper part the pattern of p-phase velocity versus the depth for the different models is shown. on the bottom, the table representing the maximum crustal thickness of each velocity model with the relative references is reported. 807 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks fig. 4. cluster 1 – deep events. grey circles represent the located events using the land-stations; black circles are the integrated locations. black lines join the two different locations of the same event. on the right, s-n and w-e sections are presented. fig. 5. cluster 2 – ustica. grey circles represent the located events using the land-stations; black circles are the integrated locations. black lines join the two different locations of the same event. on the right, s-n and w-e sections are presented. 808 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni gap. on the other hand the extension of the observation area to the tyrrhenian sea by integrating the obsn increases the complexity of the velocity structure of the underlying crust, such that an unique 1d velocity model is no more suitable. in this case classical localisation routines, which use a 1d model, do not necessarily improve the quality of the localisations. the choice to apply the hyposat location routine allowed to incorporate an additional local crustal velocity model (fig. 3), depending on the respective analysed epicentral area (cluster). only the combined use of both i) a seismic network extended by integration of the obsn, and ii) the application of an enhanced location routine, that includes different local velocity models, leaded to a significant improvement of the relocation results (table i). 5.1. cluster 1: «deep» seismic events in the southern tyrrhenian sea hypocenters occurring in the upper mantle beneath the southern tyrrhenian basin are routinely mislocated by the ingv-network. the unequal distribution of the land-based stations, as well as the quasi-systematic lack of s-phases determines poor constraints on the depth of deep events and/or on the origin times. for the location of such «deep» seismic events, we used the local velocity models described in the section 4.3. for depths below the moho all models merge into the upper mantle velocity reported in the model ak135 (kennett et al., 1995). the integration of data from the obsn and the land network data resulted always in a more stable location result in terms of rms, horizontal and depths errors, while the depth values of the hypocenters remained stable or increased by using the integrated data set. to solve the problem of a reliable depth of foci, at first we fixed the depth at 100 km for each deep earthquake. afterwards, the depth was increased by 20 km steps and we found the optimum epicentral location at each depth. the final depth of each earthquake is chosen when the solution of the inversion is stable and it represents the solution which minimises the rms residual error. we then relocated the event using the determined optimal depth. fig. 6. cluster 3 – seismic swarm of the nebrodi-peloritani system. grey circles represent the located events using the land-stations; black circles are the integrated locations. on the right, s-n and w-e sections are presented. 809 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks t ab le i. l oc at io n pa ra m et er s fo r th e ev en ts o f cl us te r 1 (d ee p ev en ts ), 2 (u st ic a) a nd 3 ( se is m ic s w ar m o f n eb ro di -p el or it an i sy st em ). t he c om pa ris on b et w ee n la nd -b as ed l oc at io ns a nd i nt eg ra ti on s is r ep or te d. t h e sh if t b et w ee n l an d -b as ed l o ca ti o n s an d i n te g ra ti o n s is r ep o rt ed i n t h e d is lo ca ti o n s co lu m n . c lu st er 1 l an dba se d ne tw or k in te gr at io ns d is lo ca t. t im e l at l on g h e r h e r l at e r l on g r m s g ap n o. l at l on g h e r h e r l at e r l on g r m s g ap n o. (k m ) (k m ) (k m ) (k m ) ph as es (k m ) (k m ) (k m ) ph as es 04 /1 2/ 00 1 1: 13 39 .4 25 14 .8 75 21 2. 79 0 38 .6 90 12 .7 65 53 .2 18 0. 82 0 30 6 6 38 .8 19 15 .5 25 20 5. 08 0 34 .1 20 23 .0 30 15 .4 80 0. 53 0 19 7 11 87 .2 07 /1 2/ 00 1 9: 36 39 .0 88 14 .5 07 0. 00 0 f ix ed 5. 31 7 12 .9 50 1. 30 0 47 14 38 .6 59 15 .8 39 14 5. 79 0 4. 61 0 1. 82 0 3. 18 5 0. 41 0 83 22 12 4. 8 10 /1 2/ 00 0 3: 53 38 .3 61 15 .6 51 11 7. 39 0 3. 96 0 2. 54 2 3. 62 3 0. 68 0 10 3 26 38 .3 68 15 .5 68 11 7. 62 0 5. 18 0 2. 70 0 2. 46 0 0. 53 0 72 37 7. 21 02 /0 1/ 01 2 0: 41 38 .3 05 15 .1 34 14 6. 71 0 4. 95 0 3. 09 7 5. 38 1 0. 47 0 25 1 18 38 .3 12 15 .2 17 15 3. 91 0 4. 46 0 3. 47 4 2. 66 0 0. 43 0 15 3 30 7. 2 04 /0 1/ 01 2 2: 21 38 .7 15 14 .8 62 28 3. 40 0 6. 53 0 1. 51 0 1. 86 4 0. 51 2 10 9 27 38 .7 02 14 .9 10 29 4. 69 0 9. 84 0 4. 35 1 3. 29 9 0. 50 0 51 41 4. 34 09 /0 1/ 01 1 8: 17 38 .7 62 14 .8 47 28 4. 44 0 7. 12 0 4. 87 3 7. 08 8 0. 58 0 17 5 22 38 .6 99 14 .9 04 28 3. 24 0 9. 38 0 3. 75 2 2. 66 9 0. 46 0 51 34 8. 59 20 /0 1/ 01 0 1: 34 38 .8 74 15 .2 07 24 4. 22 0 5. 68 0 1. 47 6 2. 10 9 0. 68 0 14 1 26 38 .8 45 15 .3 22 26 3. 56 0 12 .2 40 4. 38 5 4. 13 9 0. 51 0 52 31 10 .9 20 /0 1/ 01 1 4: 54 38 .4 82 15 .3 32 11 9. 04 0 4. 97 0 5. 71 7 8. 19 0 0. 51 0 23 8 13 38 .3 65 15 .6 48 12 6. 30 0 9. 40 0 4. 86 2 5. 04 9 0. 46 0 79 23 30 .4 5 18 /0 2/ 01 2 0: 29 38 .6 81 15 .5 61 15 4. 86 0 2. 71 0 2. 83 1 6. 47 5 0. 44 9 13 2 24 38 .6 14 15 .7 53 15 8. 96 0 3. 02 0 1. 94 3 2. 19 6 0. 45 0 80 33 18 .2 02 /0 3/ 01 2 3: 47 38 .7 96 15 .6 34 77 .9 70 4. 14 0 1. 51 0 4. 39 3 0. 26 0 20 0 13 38 .7 84 15 .8 03 94 .0 00 4. 39 0 2. 24 2 3. 38 6 0. 40 0 78 26 14 .6 28 /0 3/ 01 1 6: 32 38 .3 00 14 .9 85 10 8. 73 0 1. 56 0 2. 66 4 3. 06 3 0. 27 0 29 3 9 38 .3 01 14 .9 83 10 8. 73 0 1. 55 0 2. 47 5 2. 00 4 0. 27 0 57 18 0. 21 06 /0 4/ 01 1 0: 40 39 .3 76 15 .1 48 28 4. 09 0 6. 88 0 1. 39 9 6. 11 6 0. 87 0 15 9 16 39 .4 34 15 .5 85 33 3. 07 0 28 .3 80 9. 55 7 7. 85 8 0. 42 0 75 26 38 .6 07 /0 4/ 01 1 1: 54 38 .2 96 15 .2 17 91 .8 90 3. 94 0 2. 17 6 2. 26 6 0. 74 3 63 26 38 .1 96 15 .2 03 10 6. 59 0 5. 45 0 2. 68 6 1. 61 9 0. 47 0 75 31 11 .1 c lu st er 2 l an dba se d ne tw or k in te gr at io ns d is lo ca t. t im e l at l on g h e r h e r l at e r l on g r m s g ap n o. l at l on g h e r h e r l at e r l on g r m s g ap n o. (k m ) (k m ) (k m ) (k m ) ph as es (k m ) (k m ) (k m ) ph as es 02 /1 2/ 00 1 1: 09 38 .3 77 13 .8 70 43 .7 40 6. 73 0 5. 18 4 4. 83 9 0. 34 0 18 3 8 38 .5 41 13 .9 95 11 .6 7 2. 06 0 3. 27 5 4. 47 1 0. 57 0 93 14 21 .6 05 /1 2/ 00 1 3: 11 38 .6 63 13 .9 87 8. 83 0 26 .1 20 12 .5 65 8. 48 8 0. 94 0 50 8 38 .6 31 13 .9 56 25 .2 10 4. 35 0 4. 26 0 3. 19 0 0. 54 0 12 5 17 4. 43 09 /1 2/ 00 2 2: 49 n o so lu ti on 38 .5 84 13 .2 99 10 .0 6 4. 99 3. 75 2 1. 57 5 0. 39 11 8 11 16 /1 2/ 00 1 5: 15 38 .7 96 13 .2 84 29 .4 70 14 .1 20 32 .4 01 23 .8 88 2. 04 0 23 8 8 38 .6 73 13 .4 99 3. 98 0 5. 65 0 4. 97 3 1. 59 3 0. 53 0 65 11 23 .7 01 /0 1/ 01 1 2: 17 38 .3 41 12 .5 3 12 .3 8 9. 82 3. 81 8 3. 14 1 0. 63 64 7 38 .3 69 12 .5 43 21 .2 6 1. 98 1. 84 3 2. 88 8 0. 28 20 5 10 3. 3 09 /0 1/ 01 2 1: 05 38 .3 38 12 .6 55 6. 49 0. 19 1. 84 3 3. 26 4 0. 55 19 2 9 38 .3 79 12 .5 63 21 .1 2 2. 26 2. 39 8 5. 98 5 0. 45 67 14 9. 17 13 /0 1/ 01 1 6: 52 38 .1 96 12 .5 01 6. 15 2. 99 2. 59 7 6. 93 0 0. 77 19 5 8 38 .1 85 12 .4 62 6. 27 4. 36 2. 92 0 9. 75 0 0. 49 60 14 3. 56 15 /0 1/ 01 2 1: 57 38 .4 84 12 .3 60 9. 66 0 0. 43 0 1. 19 9 4. 93 5 0. 45 0 24 9 9 38 .4 40 12 .2 74 23 .9 80 4. 48 0 4. 58 4 7. 55 1 0. 59 0 39 13 9. 1 23 /0 1/ 01 1 5: 06 38 .6 39 13 .9 55 10 .8 90 5. 63 0 4. 26 2 1. 82 0 1. 10 2 12 3 23 38 .5 63 13 .9 37 15 .3 10 0. 58 0 3. 70 7 2. 22 3 0. 59 0 55 27 8. 59 810 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni t ab le i (c o n ti n u ed ). c lu st er 2 l an dba se d ne tw or k in te gr at io ns d is lo ca t. t im e l at l on g h e r h e r l at e r l on g r m s g ap n o. l at l on g h e r h e r l at e r l on g r m s g ap n o. (k m ) (k m ) (k m ) (k m ) ph as es (k m ) (k m ) (k m ) ph as es 24 /0 1/ 01 0 0: 07 38 .6 55 13 .9 24 19 .5 7 2. 26 3. 41 9 1. 93 4 0. 8 35 20 38 .5 96 13 .9 54 8. 43 4. 73 3. 65 0 2. 69 0 0. 59 38 28 7. 05 14 /0 2/ 01 2 2: 19 38 .4 31 13 .5 95 13 .0 80 1. 21 0 7. 00 4 4. 28 8 1. 21 2 96 10 38 .3 46 13 .5 52 28 .3 60 4. 42 0 1. 94 3 1. 12 9 0. 45 0 12 9 17 10 .1 27 /0 2/ 01 2 1: 35 n o so lu ti on 9 38 .5 25 13 .9 11 14 .9 70 2. 40 0 3. 18 6 1. 54 0 0. 54 0 12 9 17 27 /0 2/ 01 2 1: 52 38 .5 59 13 .9 15 13 .4 30 f ix ed 4. 05 2 3. 12 4 1. 30 7 16 5 7 38 .5 16 13 .9 28 15 .7 80 1. 87 0 3. 27 5 1. 69 8 0. 45 0 12 6 15 4. 9 04 /0 3/ 01 0 9: 56 38 .4 67 12 .0 43 59 .5 5 10 .2 3 5. 08 4 5. 99 4 0. 71 25 3 8 38 .2 24 12 .0 57 27 .0 1 4. 05 4. 62 0 11 .8 10 0. 37 22 13 26 .8 c lu st er 3 l an dba se d ne tw or k in te gr at io ns d is lo ca t. t im e l at l on g h e r h e r l at e r l on g r m s g ap n o. l at l on g h e r h e r l at e r l on g r m s g ap n o. (k m ) (k m ) (k m ) (k m ) ph as es (k m ) (k m ) (k m ) ph as es 27 /0 3/ 01 1 8: 40 38 .0 73 15 .3 37 29 .6 5 3. 75 8. 12 5 7. 60 4 0. 38 19 0 8 38 .0 34 15 .3 72 27 .1 7 1. 22 2. 91 9 2. 52 0 0. 22 22 4 10 5. 3 27 /0 3/ 01 1 9: 21 38 .1 02 15 .3 19 31 .3 7 3. 48 5. 27 3 3. 71 9 0. 24 15 7 8 38 .1 23 15 .3 05 31 .9 3 3. 19 3. 80 7 2. 63 4 0. 27 13 3 10 2. 58 27 /0 3/ 01 1 9: 42 38 .0 66 15 .3 68 31 .9 8 4. 01 4. 77 3 3. 51 8 0. 31 19 9 9 38 .0 49 15 .3 88 29 .1 3 3. 15 4. 55 1 3. 16 8 0. 38 21 3 11 2. 6 27 /0 3/ 01 1 9: 46 38 .0 74 15 .3 57 29 .3 4 6. 06 7. 01 5 5. 73 1 0. 43 19 1 8 38 .0 57 15 .3 74 26 .7 3 1. 86 3. 18 6 2. 34 5 0. 41 20 7 12 2. 4 27 /0 3/ 01 2 0: 08 38 .0 95 15 .3 64 26 .6 1 2. 91 3. 58 5 3. 01 9 0. 21 15 0 11 38 .0 84 15 .3 85 24 .9 6 1. 31 2. 76 4 1. 56 6 0. 36 16 0 18 2. 18 27 /0 3/ 01 2 0: 09 38 .0 64 15 .3 49 27 .9 3 5. 38 7. 33 7 6. 38 8 0. 5 20 0 8 38 .0 47 15 .3 63 26 .5 1 0. 66 1. 99 8 1. 21 6 0. 33 21 4 11 2. 23 27 /0 3/ 01 2 1: 13 38 .0 83 15 .3 33 30 .2 5 2. 2 3. 37 4 2. 20 5 0. 17 17 9 8 38 .0 66 15 .3 57 25 .0 8 1. 56 3. 94 1 2. 24 9 0. 39 19 8 12 2. 83 27 /0 3/ 01 2 2: 05 38 .0 94 15 .3 23 29 .7 6 2. 12 2. 87 5 2. 73 9 0. 16 16 5 8 38 .0 65 15 .3 67 26 .0 3 1. 24 3. 04 1 1. 89 9 0. 29 20 0 13 5. 02 27 /0 3/ 01 2 2: 29 38 .1 02 15 .3 35 30 .9 1 2. 67 3. 19 7 2. 16 1 0. 19 13 9 10 38 .0 61 15 .3 73 26 .5 5 1. 11 2. 94 2 1. 71 5 0. 36 17 7 12 5. 62 28 /0 3/ 01 0 0: 51 38 .0 91 15 .3 36 30 .9 5 5. 25 7. 44 8 5. 66 1 0. 29 17 2 8 38 .0 84 15 .3 44 30 .2 6 3. 55 4. 97 3 3. 71 9 0. 33 18 1 11 1. 04 28 /0 3/ 01 0 2: 55 38 .1 15 .3 38 31 .8 8 4. 04 5. 03 9 3. 47 4 0. 25 14 1 10 38 .0 69 15 .3 79 26 .6 2 1. 36 2. 98 6 2. 14 4 0. 72 17 1 15 4. 96 28 /0 3/ 01 0 4: 44 38 .1 39 15 .2 91 36 .5 3 2. 72 6. 81 5 4. 71 6 0. 19 21 1 8 38 .0 55 15 .4 07 28 .7 9 1. 39 5. 65 0 3. 39 5 0. 57 20 9 15 13 .7 7 28 /0 3/ 01 0 5: 37 38 .1 25 15 .3 14 28 .0 3 1. 22 3. 88 5 2. 73 0 0. 22 18 8 10 38 .0 95 15 .3 45 23 .3 4 1. 34 2. 77 5 1. 68 9 0. 38 14 7 12 4. 3 28 /0 3/ 01 0 6: 03 38 .0 52 15 .3 98 21 .2 1 6. 81 3. 90 7 2. 80 9 0. 39 18 2 12 38 .1 04 15 .3 73 27 .6 5 1. 49 2. 45 3 1. 64 5 0. 36 13 6 27 6. 18 28 /0 3/ 01 0 6: 53 38 .1 15 .3 45 26 .4 9 2. 55 3. 31 9 2. 30 1 0. 23 14 3 10 38 .1 06 15 .3 39 27 .0 2 1. 87 2. 56 4 1. 72 4 0. 53 13 7 14 0. 85 28 /0 3/ 01 0 7: 36 38 .1 2 15 .3 14 32 .0 8 2. 6 2. 12 0 1. 25 1 0. 24 18 5 9 38 .1 03 15 .3 50 23 .5 9 3. 94 5. 97 2 3. 14 1 0. 53 14 2 12 3. 64 28 /0 3/ 01 1 8: 52 38 .0 78 15 .3 69 26 .8 2 1. 76 1. 63 2 2. 07 4 0. 23 87 16 38 .0 96 15 .3 57 27 .6 7 0. 4 0. 91 0 0. 78 8 0. 47 45 30 2. 25 28 /0 3/ 01 1 9: 00 38 .0 77 15 .3 68 29 .3 9 3. 09 2. 15 3 3. 08 9 0. 26 87 12 8. 08 2 15 .3 57 28 .9 2 1 1. 18 8 1. 06 8 0. 43 79 26 1. 08 29 /0 3/ 01 0 9: 35 38 .0 79 15 .3 58 27 .1 4 1. 29 1. 45 4 1. 61 0 0. 28 96 14 38 .0 69 15 .3 57 27 .1 0. 89 1. 19 9 1. 12 0 0. 34 96 21 1. 1 30 /0 3/ 01 0 6: 48 38 .0 88 15 .3 6 28 .0 9 0. 52 1. 36 5 1. 12 9 0. 22 99 16 38 .0 84 15 .3 82 23 .1 9 1. 36 1. 84 3 1. 40 0 0. 38 47 28 1. 98 811 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks the deepest events occurred in the vicinity of stromboli, panarea and salina islands, according to the scheme of distribution of the foci proposed by blot (1971), with focal depths ranging between 260 km and 330 km. as reported in table i (see column relative to the dislocations) and fig. 4 some of the relocations shifted the original epicentre by even more than 100 km in the southeastern direction. moreover, the events recorded in the tyrrhenian basin are more dislocated than the events located near the coasts of calabria and sicily (fig. 4). 5.2. cluster 2: ustica island throughout the tyde experiment, part of the seismic activity occurred off-shore of northwestern sicily in correspondence of a large area surrounding ustica island. we analysed waveforms of 14 earthquakes, occurring between 2000, december 2-2001, march 4. time readings of rsnc stations have been integrated with readings from obs. the land-based location and integration of these events showed interesting aspects, mainly regarding the choice of the 1d velocity model for the location process. indeed, it was necessary to try different models to locate the events of this cluster and reach a stable solution. this is likely due to the large epicentral area (fig. 5), characterised by different structural sectors. concerning locations with land-based stations data only, no solutions were admitted for two events of the cluster. on the other hand the integration provided acceptable hypocentral solutions. in general, the integrated time readings, using land and obsn stations, improved the quality of the events location, with low rms value, epicentre errors and azimuthal gap (table i). it is noteworthy that hypocenters shifted towards shallower levels confirming the crustal origin of this cluster. figure 5 shows the final integrated epicentre locations and the relative n-s and w-e sections of 14 best located events, compared with those obtained from land-based data. it is important to emphasize, that several tens of small earthquakes were recorded only by the geostar hydrophone, revealing an intense microseismicity close to the ustica island. 5.3. cluster 3: ne-sicily: seismic swarm of mts. nebrodi-peloritani system between 26-30 march, 2001, a small seismic swarm, comprising 53 earthquakes with magnitude md < 3.0, occurred in the area of mts. nebrodi-peloritani system (ne of sicily). twenty of the most energetic events of the swarm have been analysed, comparing locations i) of the land-based network (grey dots in fig. 6) and ii) of the integrated network using the nearest obs-obh (black dots in fig. 6). the addition of a few time readings from obs stations significantly improved the locations in terms of hypocentral errors (table i), while the computed value of gap and rms became slightly higher than for the land-based locations. n-s and w-e sections of the analysed seismic swarm, depict a well confined seismogenic layer in the lower crust, at mean depth of about 27 km. as expected, small differences in hypocenter locations (up to 2 km in focal depth and 6 km in epicentre) are observed when locating the on-shore events with and without obs-obh data, with respect to clusters of seismicity previously shown. 6. locations of not reported earthquakes 6.1. integrated locations from obsn with single station triggers from rsnc some weak seismic events occurred inside the tyrrhenian basin and in the coastal areas, have been recorded only by a one or two stations of the land-based ingv-network, not allowing an hypocentral determination. we extracted the obsn waveforms of 31 single-station triggers reported by ingv bulletin and performed the locations of the integrated data set (table ii). the epicentral distribution of this cluster has been reported in fig. 7. according to the quality of the locations we defined three quality factors: a – number of arrivals ≥8, rms≤ 0.7 s; b – number of arrivals ≥6, 0.7≤rms≤ 1.2 s; c – number of arrivals ≤6, rms< 1.2 s). based on this assumption, the locations of 19 earthquakes are of quality a, 10 locations have quality factor b, while only 2 locations have quality factor c. 812 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni table ii. locations parameters for categories «integrated locations of new events» and «weak local events». integrated locations of new events time lat long h erh (km) erlat (km) erlong (km) rms gap no. phases 07/12/00 02:11 38.37 15.479 115.56 7.13 5.395 4.830 0.47 120 23 09/12/00 08:11 38.773 15.956 49.22 26.09 14.708 21.026 1 261 15 09/12/00 19:50 38.774 13.297 15.16 1 61.949 15.601 0.67 91 9 12/12/00 03:27 38.288 13.919 0 1 3.286 15.418 0.81 219 9 12/12/00 11:04 38.192 14.77 114.84 15.74 13.009 7.446 0.47 140 11 12/12/00 17:09 38.259 14.309 9.31 1.04 4.052 3.281 1.01 145 12 29/12/00 09:35 38.698 13.904 10 1 8.725 13.563 1.35 88 13 07/01/01 22:51 38.417 15.821 37.73 5.95 3.152 3.159 0.69 102 25 08/01/01 03:12 38.378 13.092 33.17 1 15.784 13.816 1.02 57 11 10/01/01 11:43 37.999 15.149 14.32 12.97 14.752 9.835 1.06 203 13 22/01/01 07:42 37.91 14.083 6.96 7.02 12.232 11.253 0.68 174 1 28/01/01 02:02 38.345 14.641 21.68 2.67 5.128 2.345 0.7 218 16 14/02/01 15:34 37.736 14.246 7.08 4.98 9.579 3.194 0.58 265 12 21/02/01 05:12 38.523 14.823 10.93 7.33 4.052 2.048 0.92 155 12 22/02/01 00:49 38.473 14.828 16.18 3.32 9.735 4.524 0.69 222 9 24/02/01 11:32 38.199 15.217 107.94 9.81 8.103 4.078 0.38 186 17 09/03/01 05:48 39.543 14.018 49.25 79.57 23.477 22.409 0.76 271 8 18/03/01 06:59 38.51 15.335 189.46 3.92 3.985 3.159 0.46 76 36 19/03/01 04:59 37.719 15.763 1.75 4.9 6.161 4.025 0.13 278 10 19/03/01 13:44 37.889 14.009 0.1 5.28 6.971 9.310 0.82 277 17 21/03/01 15:30 38.414 15.527 118.61 1.51 2.797 2.406 0.32 116 25 24/03/01 17:38 38.038 14.504 12.37 0.43 3.164 2.039 0.29 129 13 24/03/01 22:02 38.669 12.61 25.53 5.7 3.985 7.831 0.21 289 10 25/03/01 12:51 39.987 16.096 6.9 0.65 3.707 2.879 1.23 69 32 26/03/01 03:07 38.124 14.418 13.64 1 9.335 9.931 0.54 174 12 28/03/01 02:08 38.313 15.526 54.54 19.52 11.477 11.891 0.82 159 14 29/03/01 00:25 38.25 15.055 9.32 1 34.321 14.945 0.91 260 6 29/03/01 05:51 38.052 14.513 12.9 4.75 2.875 2.126 0.36 123 14 31/03/01 19:39 38.028 14.524 11.44 1 4.318 3.255 0.43 131 15 12/04/01 21:55 38.272 14.196 22.49 2.85 3.386 3.964 0.66 125 17 17/04/01 03:34 38.079 15.185 19.56 0.93 2.431 1.251 0.65 230 15 weak local events time lat long h erh (km) erlat (km) erlong (km) rms gap no. phases 07/12/00 09:39 38.519 14.768 20.01 1.13 3.175 1.706 0.36 226 11 11/12/00 01:45 38.515 13.99 2.37 0.66 1.243 2.293 0.19 294 9 21/12/00 03:29 38.574 14.465 0.1 7.36 1.499 1.033 1.13 115 8 26/12/00 15:34 38.238 15.188 44.95 9.7 12.632 5.163 0.58 276 9 03/01/01 07:27 38.44 13.862 5.44 11.8 28.716 19.049 0.66 322 6 05/01/01 08:21 38.499 15.156 13.56 fixed 13.553 11.506 2.92 121 9 11/01/01 20:40 38.46 14.076 9.59 fixed 11.056 9.923 2.05 226 8 12/01/01 11:49 38.524 14.573 15.6 3.56 2.609 3.623 0.53 149 10 26/01/01 17:10 38.542 14.205 14.58 4.65 3.230 3.063 0.6 123 14 27/01/01 12:54 38.521 14.036 6.49 fixed 4.784 3.561 0.84 218 12 06/02/01 11:49 38.504 13.86 14.38 0.08 0.833 2.065 0.34 319 6 06/02/01 17:14 38.754 15.868 39.72 11.01 2.076 6.545 0.27 26 14 07/02/01 15:36 38.205 15.72 68.07 13.55 12.876 14.971 0.54 316 10 12/02/01 05:44 38.469 14.543 18.82 3.62 1.621 1.234 0.25 257 7 17/02/01 21:14 38.468 13.891 3.06 8.68 5.672 13.423 0.58 318 8 813 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks table ii (continued ). weak local events time lat long h erh (km) erlat (km) erlong (km) rms gap no. phases 21/02/01 04:27 38.465 14.822 23.44 3.06 3.041 2.205 0.43 175 16 03/03/01 03:07 38.437 14.407 12.95 3.15 8.236 3.561 0.48 194 13 07/03/01 17:24 37.934 14.861 34.78 52.62 21.534 5.311 0.23 284 9 10/03/01 11:46 38.203 15.073 11.49 4.51 11.411 4.725 0.58 272 9 12/03/01 23:18 38.63 14.924 27.17 1.26 4.706 2.100 0.47 237 8 14/03/01 06:02 38.549 15.258 20.27 3.06 5.461 11.568 0.81 186 7 16/03/01 09:44 38.275 13.416 41.57 1 35.109 5.311 0.91 222 7 24/03/01 22:44 38.448 14.572 14.52 0.23 0.211 0.175 0.03 50 7 26/03/01 17:18 38.206 15.402 63.43 17.87 17.505 14.753 1 297 13 28/03/01 04:02 38.203 14.946 34.87 9.49 5.350 2.546 0.45 259 12 28/03/01 05:57 38.293 14.937 36.01 0.82 15.951 6.781 1 236 9 28/03/01 18:09 39.227 13.918 30.77 fixed 50.605 34.781 1.54 305 8 29/03/01 00:23 38.318 15.213 29.08 fixed 33.999 6.493 1.25 286 8 01/04/01 08:02 38.326 13.787 20.62 fixed 139.638 195.729 2.13 331 6 03/04/01 17:50 38.502 14.18 0 fixed 13.775 9.205 1.35 289 6 09/04/01 02:29 38.365 14.235 1.16 1.57 3.130 0.788 0.27 264 6 10/04/01 14:56 39.063 15.537 34.35 14.08 6.649 20.195 0.91 32 10 12/04/01 10:23 38.573 14.531 18.33 fixed 1.410 1.934 0.63 215 7 15/04/01 21:54 38.135 15.175 11.33 fixed 7.326 5.539 0.93 285 11 18/04/01 10:01 38.577 14.074 0 fixed 2.331 1.295 1.26 207 7 18/04/01 16:00 38.553 14.458 25.53 fixed 3.896 5.040 0.99 157 11 18/04/01 16:03 38.503 14.438 16.7 2.95 1.454 0.665 0.1 204 7 fig. 7. integrated locations of new events. locations of the events and s-n and w-e sections are presented on left and right sides, respectively. 814 tiziana sgroi, thomas braun, torsten dahm and francesco frugoni fig. 8. weak local events. locations of the events and s-n and w-e sections are presented on left and right sides, respectively. 6.2. weak local events recorded by the obsn during the 6-months experiment about 400 low magnitude local events have been detected only by the obsn, with a sufficient number of time arrivals to permit a location. the surprisingly high number of earthquakes recorded only by the obsn emphasizes that seismic activity within the crust in the tyrrhenian sea is systematically underestimated by land networks. the observed waveforms suggest the occurrence both of volcanic and tectonic earthquakes. micro-earthquakes have been roughly located by examining the succession of arrival times at the obsn stations and by the distribution of sp times. the recorded micro-seismicity at the obs05 and obs06 stations seem to be associable to very local shocks (s − p ≤ 3 s), probably linked to the volcanic activity of the stromboli area (dahm et al., 2002). on the western sector of the study area, a large number of small earthquakes were recorded only from the geostar hydrophone. location has been successful for some events with at least three pand one s-phases. the distribution and parameters of 37 earthquakes are presented in fig. 8 and table ii, respectively. the seismic events have been observed mainly in the western part of the aeolian volcanic arc. the located micro-earthquakes have depth values down to around 70 km. 7. conclusions many low magnitude earthquakes occur along the tectonic and volcanic structures of the southern tyrrhenian sea, but location of earthquakes recorded by the conventional land network continue to be less than satisfactory. the use of data recorded from a 6-months obsn, jointly with the recording from land networks, produced an evident improvement in the location of the seismicity that occurred in the southern tyrrhenian basin. the integrated network of obs-obh and land stations recorded about 800 local events, while about 400 weak local events have been detected by the obsn only. the comparison between the locations obtained from the use of land networks only, and 815 an improved seismicity picture of the southern tyrrhenian area by the use of obs and land networks the ones obtained by integrations of on/offshore networks shows strong differences in the epicentre distributions. differences are more pronounced when the events occurred in correspondence of the basin of tyrrhenian sea, and less pronounced when the events occurred near the coastal area of sicily and calabria, where the coverage of on-land stations is better (fig. 4). besides, the integrated locations are greatly improved in terms of rms residuals, azimuthal gap, and epicentral and hypocentral errors (table i). from the joint rsnc and ingv-ct database, we selected some events relative to three spatial clusters of seismicity which are representative of the whole seismic activity of the area. the improvement of the depth estimation of deep events (cluster 1) allowed to better clarify their correlation with the wadati-benioff zone, which characterises the southern tyrrhenian sea in calabria and north-eastern sicily sectors. the second cluster comprises some crustal earthquakes located in the north-western sicily off-shore: this activity can be correlated with the submerged sector of the apenninicmaghrebide chain confining with the oceanic crust of the tyrrhenian basin. after the integration we were able to assign some crustal earthquakes to the deep cluster, and vice versa. the analysis of the on-shore seismic swarm of cluster 3 (in the nebrodi-peloritani area) led to an improvement of locations in terms of depth and epicentral errors. a noteworthy result of our study is the localisation of 68 new events, not previously reported in the bulletin. the association of the single stations triggers from rsnc integrated with the obs-obh readings, allowed for the location of 31 low magnitude events mainly in the coastal region, where the geometry of the on-shore networks prevents the location of this seismicity. among these new events, five of them are part of the deep events cluster and three of the ustica cluster. in addition to the integrated locations of new events, we analysed a cluster of 37 weak local events recorded only by obsn. most of these events occurred in the western sector of the aeolian volcanic arc, while a few of them took place in the coastal area of northern sicily. concerning the depth of the located events, most of them present a crustal origin, some earthquakes occurred in the upper mantle in the 30-100 km depth range, while the deeper ones have focus around 300 km. the distribution of not reported hypocenters (see e-w sections of figs. 7 and 8) put in evidence that the more eastern events lie on the wadati-benioff plane, while a crustal seismicity occurs in the western sector of tyrrhenian basin. the position of intermediate-deep earthquakes (between the benioff plane and the crust) could be related to active processes in the mantle. taking into account the presence of volcanic structures within the study area, some local shocks can be associated to the volcanic seamounts in the south-eastern part of the tyrrhenian sea. the obss tectonic micro-earthquakes recorded both in the mantle and crust beneath the tyrrhenian sea, together with volcanic shocks, may be considered the demonstration of the lively structural frame encompassing the aeolian archipelago. acknowledgements the authors are grateful to christian mueller and one anonymous referee for their useful review. many thanks are due to laura beranzoli, paolo favali, and ernst flueh for conceiving the tyde project. we appreciated the excellent work of jörg reinhard, andré polster, andreas wittwer, martin thorwart, and rolf herber during the survey in the tyrrhenian sea and the data management at the hamburg institute of geophysics. masters and crew of the r/v urania and thetis provided fundamental help during the realisation of obs surveys in the southern tyrrhenian sea. references aa.vv. 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(1994) reported significant variations of q from p waves as a function of depth. in particular, a drop of q values for earthquakes located at depths less than 5 km was observed, supporting the idea that the upper part of the crust and the shallow volcanic layers are characterized by low q. recently a p-wave attenuation tomography study was carried out at mt. etna, down to 15 km of depth, using high quality data recorded in the period 1994-2001 (de gori et al., 2005). results from this first tomography were refined in the shallow layers (around 2 km) of the volcanic edifice by martìnez-arevalo et al. (2005), further confirming significant variations of total q as a function of depth and azimuth. in the frame of the european project named volume (an acronym of «volcanoes understanding of mass movement») aimed to investigate the movement of fluid masses inside mt. etna, a research line has been dedicated to the reconstruction of qp images of the volcano with different techniques. this is because several studies have pointed out that the estimates of q could depend on both the data to be inverted and on the technique used. the attenation parameter q−1 estimated by the inversion of seismic spectra at mt. etna by de gori et al. (2005) and martìnez-arevalo et al. (2005) is the sum of two parameters, qp−1 and qc−1, which take into account two different and concomitant physical phenomena. qp−1 is the intrinsic attenuation of p waves, which is related to the losses of energy due to the anelasticity of the earth. qc−1 is the scattering attenuation of the wave field and is related to the energy content of the secondary wave field, i.e. the wave field generated by the elastic heterogeneities inside the medium travelled by the waves (e.g., mitchell, 1995; del pezzo et al., 2001). it has also been shown (e.g., liu et al., 1994) that the methods based on the inversion of pulse widths of first p waves are more appropriate to estimate the intrinsic qp, in that only the start of the signal, which is relatively free of complicated effects, such as complexities at the source and/or scattering from heterogeneities, is used. based on these reasonings, this article presents the results of a first study aimed to obtain a first estimate of the average intrinsic qp at mt. etna by using the classic rise time method (gladwin and stacey, 1974). the analysis concerns about three-hundred microearthquakes recorded at mt. etna preceeding and accompanying the 2002-2003 mt. etna flank eruption. 2. the mt. etna geological framework mt. etna is located in the complex geodynamic framework of eastern sicily, where major regional structural lineaments play a key role in the dynamic processes of the volcano (e.g., patanè and giampiccolo, 2003). several years of structural and geophysical observations have revealed that the orientation of most of the eruptive systems coincides with two structural trends nnw-sse and ne-sw, observed both in the volcanic area and in the regional context (e.g., azzaro and neri, 1992). these alignments are hypothesized as the main volcanogenetic structures (e.g., gresta et al., 1998) which control the evolution of mt. etna, as their interference establishes a weakness volume along which magma can rise from depth (rasà et al., 1995). over the last 30 years mt. etna has had a high rate of eruptive events and therefore it constitutes one of the most important natural laboratories for the understanding of eruptive processes and lava uprising in basaltic-type volcanic environments. seismic observations allowed us to carry out detailed investigations on major aspects of seismicity. in particular, the most recent effusive eruptions which occurred in 1989, 1991-1993, 1999, 2001 and 2002-2003 have offered good examples for quantitative analysis based on seismic data in digital format. we focus on the 2002-2003 flank eruption which started in the night between october 26 and 27, 2002 with a seismic swarm in the central and upper part of the volcano. fissures on both the ne and s flanks were activated with a huge lava emission and powerful explosive activity from the southern fracture field. the eruption 1217 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence ended on january 28, 2003 after 94 days. more than 800 events were recorded during this period by the permanent seismic network managed by ingv. most of these events have magnitude less than 3.0 and only few earthquakes reached a magnitude duration md = 4.4. 3. technique the rise time method is based on early experimental (gladwin and stacey, 1974) and theoretical (kjartannson, 1979) studies. more recently, wu and lees (1996) showed that the theoretical relationship on which the method is based remains valid if we consider a heterogeneous anelastic structure. the most limiting assumption of this method is that it neglects the directivity effect of the seismic radiation generated by a finite dimension seismic source (zollo and de lorenzo, 2001). on a velocity seismogram the rise time can be defined as the time interval between the onset of the phase and its first zero crossing time (fig. 1). if the spatio-temporal finiteness of the seismic source process can be neglected, the variation of the rise time versus the travel time of the wave will be given by (3.1) where τ0 is the rise time at the source, qp is the quality factor and t is the travel time. the constant c was found to be equal to 0.5 for a constant q attenuation operator (kjartansson, 1979). the methods based on the inversion of the rise times are expected to give the most reliable estimates of the intrinsic attenuation (liu et al., 1994). in fact, since only a very limited portion of the seismogram is used, the effects of multiple waves generated in the thin layers around the recording site are usually minimized. finally, as a consequence of the point-like source assumption, the qp inferred by using eq. (3.1) can be considered as the minimum qp estimated from pulse width inversion (de lorenzo et al., 2004). 4. data analysis we used a data set of about 300 well located events (erx, ery and erz <1 km, gap < 100° and rms < 0.20 s), with magnitude duration ranging between 1.4 and 4.4 and focal depth mainly concentrated in the first 5 km, recorded during an episode of intense seismic activity which preceded and accompanied the 20022003 mt. etna flank eruption. the earthquakes were selected from the catalogue among those re-located by using the most recent 3d velocity model (cocina et al., 2005). data from stations of the mt. etna permanent seismic network run by istituto nazionale di geofisica e vulcanologia of catania (ingv-ct) were analysed. most of the stations considered in this analysis are equipped with one-component short period (1 s) sensors, except 8 equipped with three-component sensors. although other stations were operating during the occurrence of the analysed seismic sequence, data were not as well recorded as required for accurate seismic attenuation analysis in time domain. we discarded the events for which the rise times are available at fewer than four stations. the remaining dataset was then composed of 147 events, all having typical tectonic-earthq c t p 0τ τ= + fig. 1. schematic picture of rise time τ as measured on a velocity seismogram. 1218 fig. 2. epicenters of the events (grey circles) and recording stations (black triangles) considered in this study. table i. the dataset available for this study. md = magnitude duration; id = identification number of the event; date = month-day-hour and minute of occurrence of the event (year: 2002). id# date md id# date md id# date md id# date md id# date md 1 10262135 2.4 1 10262135 2.4 2 10262146 2.3 3 10262155 2.4 4 10262204 2.4 5 10262218 2.1 6 10262225 2.5 7 10262228 2.4 8 10262233 2.4 9 10262240 2.5 10 10262313 2.4 11 10262327 2.4 12 10262346 2.4 13 10262349 2.4 14 10270007 3.5 15 10270010 3.3 16 10270012 2.6 17 10270016 2.7 18 10270021a 2.4 19 10270021b 2.7 20 10270035 3 21 10270036 3.1 22 10270041 3.2 23 10270101 2.7 24 10270107 2.4 25 10270113 3.3 26 10270215 2.5 27 10270218 3.2 28 10270229 3.5 29 10270239 3.3 30 10270242 3.4 31 10270250 4.2 32 10270328 2.8 33 10270417 2.9 34 10270502 2.5 35 10270521 2.6 36 10270531 3.3 37 10270546 3.4 38 10270606 3.4 39 10270626 2.8 40 10270628 2.9 41 10270649 2.9 42 10270732 3.2 43 10271007 2.7 44 10271024 2.7 45 10271210 2.7 46 10271216 2.6 47 10271442 2.5 48 10271456 2.7 49 10271602 2.9 50 10271607 2.6 51 10280912 3.2 52 10281140 3.1 53 10281151 2.8 54 10281627 3 55 10282325 2.8 56 10290131 2.6 57 10290232 2.3 58 10290834 2.9 59 10290913 2.8 60 10290956 2 61 10291002 4.4 62 10291004 3.1 63 10291013 2.8 64 10291017 2.5 65 10291018 2.1 66 10291022 1.7 67 10291025 1.6 68 10291034 1.6 69 10291035 2.9 70 10291059 2.1 71 10291102 4 72 10291122 1.7 73 10291221 2.1 74 10291325 2.7 75 10291639 4 76 10291714 4.1 77 10291907 1.7 78 10292035 2 79 10292224 2.8 80 10300000 3.1 81 10300216 2 82 10300220 2.5 83 10300720 2.5 84 10301005 2.2 salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè 1219 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence quake waveforms, whose localizations, deduced from the ingv catalogue, are shown in fig. 2, together with the position of the recording stations. the origin time, the identification number and magnitude duration of the events used in the study, computed by ingv, are given in table i. a total of 1053 data was available for the study. the rise time was measured on each first arrival p-wave by computing the time interval between the onset of the phase and its first zero crossing time. we discarded all data for which the p signal to noise ratio was lower than 10, and considered only the waveforms on which the onset of p-wave was clearly readable. we did not perform the deconvolution for the instrumental response for the following three reasons: 1) first of all, the technique we are using does not need amplitude information. 2) second, the effect of filtering operated in the deconvolution for the instrumental response could cause the generation of unwanted artificial signals (see mulargia and geller, 2003 and references therein) which could bias the rise time estimate. 3) finally, if we exclude one datum, the observed rise times vary between 0.025 s and 0.3 s (fig. 3), to which correspond an average frequency content ranging from a minimum of about 3 hz to a maximum of about 40 hz. consequently the frequency content of the analyzed signals is always contained in the frequency band (1-50 hz) where the instrument response is flat and cannot distort the duration and shape of the observed signals. only data not affected by multipathing during the first half-cycle of the wave were considered in the analysis. the multipathing effects fig. 3. hystogram of the measured rise times. table i (continued). id# date md id# date md id# date md id# date md id# date md 85 10301006 2.5 86 10301047 2.6 87 10301321 2.8 88 10301525 3.2 89 10302113 1.9 90 10302114 2.5 91 10302117 1.7 92 10302118 1.5 93 10302312 1.6 94 10310640 2.4 95 10310651 1.9 96 10310734 1.9 97 10310919 1.9 98 10311041 3.2 99 10311122 2.4 100 10311150 1.9 101 10311224 1.5 102 10311319 1.9 103 10311325 1.9 104 10312002 1.9 105 10312108 2.3 106 10312209 2.3 107 11010042 2.4 108 11010638 2.1 109 11010857 2.7 110 11010921 3 111 11010929 1.7 112 11011301 1.5 113 11011532 3.1 114 11011814 1.6 115 11012219 1.6 116 11020901 3 117 11021006 2 118 11021033 1.4 119 11021527 1.5 120 11021539 1.7 121 11021709 2.8 122 11022308 2.5 123 11030022 2.3 124 11030535 2.1 125 11030536 2.5 126 11031343 2.2 127 11031344 2.4 128 11040529 1.9 129 11040826 1.9 130 11040847 2.6 131 11040956 1.9 132 11041048 2.4 133 11041052 3 134 11041054 3.1 135 11041117 1.3 136 11041221 1.5 137 11051646 2.6 138 11051900 2.8 139 11060628 2.1 140 11060927 1.9 141 11061640 3.2 142 11061642 2.7 143 11070616 2.7 144 11070618 2 145 11070903 2.3 146 11071507 2.4 147 11071635 2.6 1220 salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè can often be easily recognized as a sharp discontinuity (e.g., de lorenzo and zollo, 2003) which breaks the approximately bipolar shape of the wave on the velocity seismogram (fig. 4) and are generally caused by the presence of thin layers below the recording site, where part of the energy remains trapped and is subjected to multiple reflection. the quality of the onset varies with varying the level of noise of the recorded traces. for this reason we divided the dataset into three categories. a maximum error on rise time equal to 5% was estimated for data which have the highest quality. for data of intermediate quality a maximum error of 10% was estimated. finally, a maximum error of 20% was associated to data having the worst quality. as a consequence an average error on rise time equal to 10 ms was estimated. the plot of the seismic rays under the assumption of a homogeneous velocity model is shown in fig. 5a,b. we infer that the centraleastern part of the array is very well illuminated by the seismic rays, until to a depth of about 3-4 km. the qp estimates we present in the next section will be then particularly representative of the average qp of this volume. 5. results we carried out two different kinds of data inversions. in the first inversion we assumed that the stress drop of the studied earthquakes is a fig. 4. a velocigram of a microearthquake occurred at mt. etna. the discontinuity occurring during the first half-cycle of the first p-wave, due to a secondary arrival, impedes the measurement of the rise time on this seismogram. fig. 5a,b. plot of seismic rays available for this study: a) plot in the horizontal plane; b) plot in the vertical latitude-depth plane a b 1221 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence constant, so that a self-similar behaviour of earthquakes with a different energy content is expected. under this assumption the source rise time τ0 of each event was computed by using the seismic moment versus magnitude duration relationship calibrated for the etnean area (patanè et al., 1993) and the laws which relate source radius, seismic moment, stress drop, rupture velocity and source rise time for a circular crack model. in the second inversion, source rise times were directly estimated from data using the relationship (3.1). 5.1. qp estimates under the assumption of a constant stress drop for each of the 147 considered events, seismic moment m0 was inferred from the magnitude duration md (table i), using the relationship which relates md to m0 for the etnean area (patanè et al., 1993) (5.1) with (5.2) . (5.3) for a circular crack, the source rise time τ0, which represents the time duration of the slipping on the fault, is related to the source radius l and the rupture velocity vr of a circular crack by the equation (brune, 1970) . (5.4) by combining eq. (5.4) with the relationship which combines stress drop ∆σ, m0 and l (keilis-borok, 1959) (5.5) we obtain . (5.6) in order to compute τ0, we need to estimate the rupture velocity vr of the earthquakes. unfortunately, this parameter is poorly known for small v m1 16 7 r 0 03τ σ∆ = l m 16 7 3 0σ∆ = v l r 0τ = . .b 0 9 0 1!= . .a 17 8 1 9!= log m a bm0 d= + magnitude earthquakes, owing to its correlation with the other source parameters (e.g., deichmann, 1997). theoretical and laboratory studies (madariaga, 1976) indicate that vr ranges between 0.6 vs and 0.9 vs. starting from the observation of a great dispersal of rise time versus travel time for the considered events, which is often attributed to significant directivity effects (e.g., de lorenzo et al., 2004), we have assumed a high value of the rupture velocity (vr = 0.9 vs = 2.2 km/s). for the mt. etna ∆σ is known to range from a few mpa to about 100 mpa (e.g., patanè et al., 1994, 1997; patanè and giampiccolo, 2003). for this reason, first of all we carried out five different inversions by using five possible values of ∆σ (∆σ = 1, 5, 10, 50, 100 mpa), and then we compared the results of the inversions. to this aim, for each value of ∆σ, source rise times τ0 of the events were computed using eq. (5.6). then, for each event, the reduced rise times τ − τ0 were inferred from eq. (3.1) and inverted to estimate qp. at the first step of the inversion we assumed a constant q for the entire area. the inversion of the entire data set of rise times was performed using a weighted linear inversion scheme with weigths equal to the inverse of the variance of data. the results are summarized in fig. 6a-c. it is worth noting that about the same quality of the fit is obtained for ∆σ = 10 mpa (an average residual of 42 ms in l1 norm and an average residual of 62 ms in l2 norm) and for ∆σ = 50 mpa (an average residual of 42 ms in l1 norm and an average residual of 61 ms in l2 norm). the coefficient of correlation for the two cases, as computed using the relationship for weighted data (green and margerison, 1978) is also about the same (r2 = 0.24). for this reason, qp was estimated by averaging the two qp estimates for ∆σ = 10 mpa (qp∼33) and ∆σ = 50 mpa (qp∼30), obtaining . (5.7) owing to the difficulties in accurately accounting for both the errors on data and the tradeoff among source parameters, instead of fixing the stress drop to one of the values which produce the comparable fit to data (∆σ = 10 or ∆σ = 50 q 32 2p != 1222 fig. 6a-c. a) results of the inversions of rise times for different values of ∆σ, assuming a constant ∆σ and a homogeneous qp. on the top-left, the plot of the average residual in l2 norm versus ∆σ is shown. on the topright the plot of the average residual in l1 norm versus ∆σ is shown. on the bottom-left the plot of average qp versus ∆σ is shown. on the bottom-right the plot of the squared correlation coefficient versus ∆σ is shown. b) plot of reduced rise times versus travel times for ∆σ=10 mpa and ∆σ=50 mpa. c) plot of rise time residuals versus travel times for ∆σ=10 mpa and for ∆σ=50 mpa under the assumption of a homogeneous qp. a b c salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè 1223 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence mpa), we preferred to consider, in the calculation of qp of each event, both the cases ∆σ = 10 mpa and ∆σ = 50 mpa, choosing, for each event, the qp value which gives rise to the best fit result. results are summarized in table ii. only two events show a negative non physical qp value: event #85, for which we have qp=−37 and event #100, for which we have qp=−100. table ii. qp estimates under the assumption of a constant stress drop. id n data q ∆q l1 residual l2 residual ∆σ id n data q ∆q l1 residual l2 residual ∆σ (s) (s) (mpa) (s) (s) (mpa) 1 14 21 1 43 47 50 2 15 24 1 15 27 10 3 9 30 1 12 14 50 4 7 26 2 18 24 10 5 7 24 1 26 38 10 6 10 25 1 27 33 10 7 14 31 1 28 32 10 8 15 37 3 53 61 50 9 14 31 1 23 27 10 10 19 28 1 18 24 10 11 6 19 1 32 39 50 12 6 28 2 38 55 10 13 14 26 1 14 19 50 14 14 21 1 27 34 50 15 4 14 1 21 33 50 16 7 80 15 21 30 50 17 12 22 1 85 92 10 18 9 27 1 40 44 10 19 4 25 1 7 9 10 20 6 20 2 18 26 50 21 12 1230 100 61 74 50 22 6 14 1 52 66 50 23 4 27 3 15 19 10 24 5 35 4 27 39 10 25 8 35 6 156 180 10 26 4 37 3 63 86 10 27 10 19 1 36 45 50 28 5 19 1 30 52 50 29 6 23 2 76 96 50 30 6 26 3 41 74 50 31 9 96 2 124 173 50 32 7 28 3 21 33 50 33 7 12 1 28 41 10 34 7 18 1 65 88 10 35 7 28 2 20 23 50 36 9 20 2 39 47 10 37 13 25 2 68 89 50 38 9 24 1 67 91 10 39 6 25 1 43 69 50 40 7 15 1 23 32 50 41 5 16 1 172 200 10 42 4 14 1 21 39 50 43 4 −37 5 86 114 10 44 4 30 3 32 55 10 45 5 24 2 30 42 50 46 4 32 5 39 53 50 47 5 33 4 60 82 50 48 8 15 0 85 106 10 49 5 18 1 74 94 10 50 4 69 18 55 73 10 51 4 21 6 56 69 10 52 6 16 1 26 32 50 53 11 38 2 17 23 10 54 17 29 1 65 95 10 55 5 25 1 16 21 50 56 9 28 2 48 63 10 57 6 32 2 10 12 50 58 10 −100 −20 147 170 50 59 11 23 1 31 41 10 60 6 21 1 18 41 10 61 12 29 1 58 70 50 62 14 23 1 77 88 50 63 5 34 2 25 40 10 64 4 35 2 9 12 50 65 11 31 1 17 24 50 66 5 35 2 14 18 50 67 5 41 2 9 13 10 68 4 18 1 17 22 10 1224 table ii (continued). id n data q ∆q l1 residual l2 residual ∆σ id n data q ∆q l1 residual l2 residual ∆σ (s) (s) (mpa) (s) (s) (mpa) 69 11 27 1 11 17 10 70 10 25 1 19 24 10 71 7 16 1 46 62 50 72 8 33 1 7 9 50 73 4 29 1 3 3 50 74 5 23 2 53 77 10 75 8 32 3 100 111 50 76 4 25 1 50 99 50 77 6 43 2 5 7 10 78 7 31 1 37 45 50 79 10 35 1 13 17 50 80 9 12 1 17 26 50 81 7 36 2 21 27 50 82 5 33 4 76 97 10 83 9 18 1 7 9 10 84 5 47 2 32 66 50 85 7 53 4 24.0 30.8 50 86 5 64 12 37.3 44.4 10 87 4 27 2 10.7 16.8 10 88 9 26 2 48.0 58.5 10 89 6 38 3 9.6 15 50 90 13 26 1 59.2 65.8 10 91 4 25 1 6.8 13.5 50 92 5 31 2 9.0 14.4 50 93 6 36 4 8.3 10.6 10 94 6 32 1 14.8 19.3 10 95 6 24 1 7.2 12.1 10 96 5 32 2 9.2 13.2 50 97 6 53 4 13.6 19.1 50 98 13 24 1 51.6 81 10 99 10 32 1 27.1 37 50 100 4 30 2 3.0 5.1 10 101 6 28 1 6.2 8.5 50 102 5 35 3 7.7 11 50 103 5 15 1 37.6 56.1 10 104 5 23 1 9.7 18.7 50 105 4 27 2 21.7 29.9 10 106 7 30 1 7.5 10.3 10 107 5 19 1 9.6 12.5 50 108 5 32 2 12.1 15.2 10 109 7 39 2 7.4 9.3 50 110 11 31 1 15.2 20.1 50 111 4 47 5 34.9 48.6 10 112 4 44 4 21.2 28.8 10 113 5 22 1 28.3 35.9 10 114 5 45 3 5.8 9 10 115 4 25 2 19.0 25.6 50 116 4 20 2 39.1 47.3 50 117 8 44 2 11 15.3 10 118 5 45 2 4.3 6.1 50 119 5 26 2 32.900 38.2 10 120 4 31 3 41.900 50.6 10 121 12 31 1 17.700 23.6 10 122 15 35 1 9.700 12.3 50 123 5 31 2 12.600 20.1 50 124 5 21 1 15.500 23.3 10 125 8 19 1 34.600 49.4 50 126 4 18 2 12.300 20 10 127 4 36 6 57 101 50 128 4 48 4 12 15 50 129 5 23 1 25 31 10 130 8 34 1 10 13 10 131 4 26 5 4 6 10 132 4 30 2 3 4 10 133 12 22 1 15 23 10 134 12 22 1 31 47 10 135 5 33 2 17 24 10 136 6 21 1 45 59 50 137 4 26 5 21 25 10 138 5 19 2 55 90 10 139 4 17 1 12 17 50 140 4 46 5 11 15 10 141 4 9 1 50 63 10 142 5 10 1 36 47 10 143 13 32 1 25 32 10 144 7 29 2 13 15 10 145 5 18 1 11 13 10 146 7 33 2 23 30 10 147 4 16 1 11 14 50 salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè 1225 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence moreover, only event #21 shows a very high value (qp=1230). for the remaining events the qp variations are more limited, with a minimum qp=9 and a maximum qp=96. the weighted average of qp, using the results of all the 147 inversions and as weights the inverse of the residuals in l1 norm is exactly the same as the qp obtained at the first inversion step . (5.8) however, with respect to the previous result, based on the assumption of a homogeneous qp, the assumption of a heterogenous q structure gives rise to a further variance reduction. in fact the average residual is now 34 ms in l1 norm (a residual reduction of 19%) and 53 ms in l2 norm (a residual reduction of about 13%). the q 32 2p != trend of residuals versus travel time is shown in fig. 7a. 5.2. joint estimation of τ0 and qp in another attempt we inverted the rise times to jointly infer τ0 and qp of each event. in a first inversion run, we used a linear weighted inversion scheme, with weights equal to the inverse of the variance of data. unfortunately this approach does not allow us to impose positivity constraints on τ0 and qp . for this reason, after the inversions, only for some events we inferred positive values of both τ0 and qp, reported in table iii. to overcome the problem of negative values of τ0 and/or qp for the remaining events, we used a non linear infig. 7a,b. a) plot of rise time residuals versus travel times under the assumption of a constant ∆σ and a heterogeneous qp. b) plot of rise time residuals versus travel times after the joint inversion of τ0 and qp. a b table iii. τ0 and qp estimates of the events; the lines with bold characters indicates the events for which the simplex method has been used to overcome the problem of negative τ0 and qp values. id n data τ0 ∆τ0 q ∆q l1 residual l2 residual id n data τ0 ∆τ0 q ∆q l1 residual l2 residual (s) (s) (s) (s) (s) (s) (s) (s) 1 14 33 3 22 1 20 28 2 15 51 2 53 9 13 17 3 9 43 3 74 19 12 15 4 7 33 1 23 2 17 19 1226 table iii (continued). id n data τ0 ∆τ0 q ∆q l1 residual l2 residual id n data τ0 ∆τ0 q ∆q l1 residual l2 residual (s) (s) (s) (s) (s) (s) (s) (s) 5 7 69 2 393 36 13 17 6 10 76 4 571 77 18 20 7 14 33 1 25 57 14 21 8 15 83 3 194 99 17 26 9 14 71 3 147 42 14 21 10 19 59 3 77 17 20 25 11 6 12 5 14 1 25 33 12 6 33 4 25 2 19 32 13 14 33 2 31 10 16 22 14 14 83 4 35 4 24 31 15 4 62 1 17 0 34 53 16 7 38 1 100 2 12 16 17 12 19 9 15 3 42 55 18 9 33 3 25 5 22 28 19 4 15 1 16 0 6 8 20 6 59 1 59 5 13 19 21 12 65 2 349 136 12 17 22 6 90 3 42 3 17 25 23 4 32 4 25 1 12 23 24 5 33 6 25 1 30 41 25 8 62 9 20 0 68 84 26 4 36 17 29 1 43 81 27 10 58 7 17 2 30 45 28 5 62 10 22 2 29 54 29 6 62 14 21 2 53 65 30 6 67 36 25 3 48 89 31 9 116 27 100 0 44 64 32 7 44 10 30 5 34 49 33 7 47 13 17 4 74 93 34 7 36 13 17 29 54 71 35 7 38 6 29 1 32 41 36 9 62 6 14 4 29 35 37 13 67 22 22 4 60 78 38 9 67 6 18 0 47 59 39 6 22 15 18 2 35 60 40 7 47 10 17 0 68 81 41 5 47 27 17 1 84 100 42 4 58 19 17 1 70 98 43 4 41 10 50 0 55 96 44 4 41 16 29 0 56 87 45 5 41 19 30 0 58 70 46 4 38 14 41 4 33 44 47 5 36 9 17 1 49 64 48 8 44 7 31 3 35 44 49 5 47 4 17 1 50 66 50 4 38 6 25 24 29 50 51 4 26 2 6 0 10 18 52 6 78 1 112 18 18 38 53 11 30 4 30 3 21 30 54 17 51 8 20 2 49 68 55 5 43 1 55 6 8 10 56 9 53 3 32 3 20 28 57 6 31 2 100 1 17 22 58 10 47 2 50 10 41 57 59 11 44 2 25 5 23 27 60 6 17 2 17 1 25 52 61 12 133 3 100 21 41 53 62 14 63 2 20 1 27 35 63 5 15 2 20 1 15 22 64 4 36 2 100 2 12 17 65 11 9 4 18 1 25 39 66 5 21 6 32 2 17 23 67 5 4 4 24 2 7 14 68 4 51 3 45 5 13 23 69 11 88 2 241 30 11 14 70 10 44 2 37 3 13 23 71 7 10 7 12 0 31 48 72 8 27 1 56 5 7 8 73 4 14 1 22 1 5 6 74 5 41 3 23 5 36 56 75 8 101 11 18 1 71 79 76 4 51 13 26 2 53 103 77 6 21 1 54 22 8 10 78 7 25 3 31 3 14 20 79 10 20 2 28 1 11 13 80 9 160 6 42 9 19 26 81 7 32 4 45 9 21 31 82 5 56 2 20 1 14 21 83 9 60 2 29 3 12 17 84 5 29 25 100 3 42 80 salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè 1227 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence version scheme based on the use of the simplex downhill method (press et al., 1989) which allowed us to impose positivity constraints on both τ0 and qp. to estimate the error on the inferred model parameters we applied a statistical approach based on the use of the random deviates (vasco and johnson, 1998). this consists of computing nrand datasets by adding to each data a random quantity selected in the range of the error affecting them. the inversion results of the nrand datasets can then be used to estimate the average values of the model parameters and table iii (continued). id n data τ0 ∆τ0 q ∆q l1 residual l2 residual id n data τ0 ∆τ0 q ∆q l1 residual l2 residual (s) (s) (s) (s) (s) (s) (s) (s) 85 7 24 2 34 3 14 17 86 5 38 1 50 0 21 29 87 4 24 3 29 2 11 13 88 9 64 1 46 4 31 44 89 6 24 2 30 85 13 18 90 13 17 3 18 1 24 36 91 4 21 5 30 1 17 24 92 5 0 2 19 1 10 14 93 6 19 1 34 0 18 22 94 6 33 5 35 0 21 27 95 6 46 0 162 22 3 4 96 5 24 2 37 0 13 18 97 6 24 2 100 0 19 25 98 13 58 2 25 1 21 30 99 10 20 4 19 1 22 35 100 4 36 1 119 9 4 6 101 6 38 1 101 23 6 8 102 5 24 2 38 3 6 9 103 5 51 1 137 39 12 17 104 5 24 1 20 1 18 21 105 4 31 5 25 6 22 27 106 7 47 2 37 3 9 12 107 5 7 1 14 0 11 22 108 5 27 4 35 0 17 24 109 7 41 1 100 21 7 8 110 11 47 2 75 12 16 19 111 4 21 3 40 10 17 28 112 4 41 2 141 18 11 18 113 5 24 6 25 2 25 40 114 5 19 1 50 1 7 12 115 4 19 1 35 1 14 19 116 4 116 3 213 41 18 34 117 8 25 1 50 1 10 18 118 5 39 2 130 30 5 7 119 5 18 2 18 1 11 17 120 4 21 4 17 2 12 20 121 12 49 1 70 9 19 28 122 15 24 2 26 1 9 14 123 5 31 2 41 0 20 31 124 5 60 1 52 3 7 9 125 8 55 2 43 5 18 32 126 4 53 0 26 1 10 13 127 4 33 2 100 0 46 91 128 4 24 1 36 3 11 13 129 5 24 1 22 6 9 14 130 8 38 1 50 0 18 22 131 4 33 1 29 1 6 9 132 4 23 1 32 1 5 7 133 12 93 2 102 15 10 14 134 12 106 4 129 49 20 26 135 5 39 3 263 112 18 35 136 6 55 3 335 45 22 40 137 4 28 1 17 0 13 18 138 5 44 6 17 31 48 79 139 4 38 1 35 1 12 19 140 4 29 1 57 4 13 18 141 4 27 2 7 0 34 61 142 5 114 1 50 4 29 45 143 13 53 4 79 17 24 31 144 7 25 1 30 1 19 25 145 5 73 2 104 21 9 12 146 7 33 3 42 9 22 33 147 4 72 4 116 32 16 22 1228 salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè their standard deviations. the inversion results are summarized in table iii. after this analysis we inferred qp values ranging from a minimum of 6 to a maximum of 570. the weighted average qp, with weights equal to the inverse of the average residual in l1 norm, was . (5.9) a further variance reduction was obtained with respect to the previous case of a heterogeneous qp but a constant stress drop ∆σ. in fact the average residual in l1 norm is now equal to 23 ms (a residual reduction of 33%) and the average residual in l2 norm is equal to 37 (a residual reduction of 30%). the trend of residuals versus travel times is shown in fig. 7b. 5.3. application of the «occam razor principle» one point that needs to be addressed concerns the evaluation of the statistical significance of the two models used to fit data. since the two models are characterized by a different number of model 68 51qp != parameters it is important establish, for each event, what of them satisfy the so called occam’s razor principle, i.e. realizes the best compromise between the quality of the fit and the simplicity of the model. to solve this problem we used the corrected akaike information criterion (aicc) (cavanaugh, 1997; cavanaugh and shimway, 1998), which represents a modified version of the akaike (1974) criterion, to account for incompleteness of data. this consists of comparing, for the two models, the value of the parameter (5.10) where k is the number of model parameters, n the number of data and l represents the likelihood function, given by . (5.11) the model which gives rise to the minimum aicc has then to be considered the most significant from a statistical point of view. ( ) expl 2 1 2 obs teo ii n i 1 2 2 πσ σ τ τ = − − = ; e% 2 2 ( ) ( ) aicc log l k n k k k 1 1 = − + − − − + table iv. application of the corrected akaike information criterion to the two considered models. aicc1 = value of the akaike information criterion for the model with one model parameter (only qp); aicc2 = value of the akaike information criterion for the model with two model parameters (τ0 and qp); ∆aicc = aicc1-aicc2. id# aicc1 aicc2 ∆aicc id# aicc1 aicc2 ∆aicc id# aicc1 aicc2 ∆aicc id# aicc1 aicc2 ∆aicc event event event event 1 391 18 373 2 −9 −13 4 3 12 56 −45 4 1 −6 7 5 214 3 211 6 196 −6 202 7 339 1 338 8 816 26 789 9 161 −5 167 10 132 92 40 11 119 −16 135 12 59 40 19 13 −30 −24 −6 14 34 46 −12 15 −20 −12 −8 16 35 −19 53 17 2379 137 2242 18 505 177 328 19 −28 −36 9 20 −29 −36 7 21 777 −63 841 22 7 −31 38 23 −23 −31 8 24 9 6 4 25 643 49 594 26 222 22 200 27 56 66 −10 28 −9 −10 1 29 212 69 144 30 −23 −18 −6 31 2653 5832 −3178 32 −31 0 −31 33 −23 30 −53 34 160 87 73 35 −20 40 −60 36 4 −28 32 37 213 229 −17 38 342 66 275 39 39 9 30 40 −11 121 −132 41 1362 195 1167 42 −23 21 −43 43 194 42 153 44 −19 7 −26 45 0 7 −7 46 18 148 −129 47 20 98 −78 48 318 179 139 49 123 34 89 50 115 81 34 51 3 586 −583 52 72 461 −389 53 80 354 −274 54 924 942 −18 55 123 34 89 56 518 935 −417 1229 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence table iv summarizes the comparison between the aicc values for the two considered models. in this way we were able to infer that, for 59 events the above choice of retrieving source rise times assuming a constant stress drop produces a better fit of data, whereas for the remaining 68 events the source rise times are better constrained from the inversion of rise times. by averaging the qp estimates obtained with this analysis we finally obtained . (5.12) figure 8 shows the trend of the qp of each event estimated with the aicc as a function of τ0. it is worth noting that a residual correlation among τ0 and qp is inferred. 4257qp != 6. discussion and conclusions as thoroughly debated in wu and lees (1996) and de lorenzo (1998), the slope c of the straight line interpolating τ versus qp-1 depends on the frequency content of the source; by calibrating eq. (3.1) with different source time functions, wu and lees (1996) showed that c = 0.5 has to be adopted if it is assumed that the the signal generated at the source has a gaussian shape, which is a smooth representation of the unipolar source time function generated by a circular crack. by taking into account that a more realistic source cannot have the same low-frequency content as the gaussian function, wu and lees (1996) showed that the c could be slightly higher. they estimated as table iv (continued). id# aicc1 aicc2 ∆aicc id# aicc1 aicc2 ∆aicc id# aicc1 aicc2 ∆aicc id# aicc1 aicc2 ∆aicc event event event event 57 47 512 −465 58 3974 126 3848 59 189 173 16 60 −41 651 −692 61 1053 494 559 62 1096 252 845 63 7 212 −205 64 18 98 −80 65 14 117 −102 66 16 −1 17 67 −23 561 −584 68 74 75 −1 69 23 449 −426 70 173 93 80 71 −5 1055 −1060 72 45 46 −2 73 −33 203 −236 74 25 −7 32 75 514 246 269 76 −2 −8 6 77 −18 17 −35 78 771 126 645 79 143 22 121 80 −40 −41 1 81 34 41 −6 82 125 −27 152 83 −53 −51 −2 84 −7 71 −78 85 299 58 241 86 25 −4 29 87 −20 −22 2 88 424 94 330 89 −4 30 −34 90 1718 44 1674 91 −33 −4 −29 92 −33 −13 −20 93 −31 −18 −13 94 54 129 −75 95 −38 −55 17 96 −10 76 −85 97 10 100 −90 98 204 −2 206 99 222 90 132 100 −33 −39 6 101 −5 4 −9 102 −31 −38 7 103 200 −10 210 104 −29 4 −32 105 −3 3 −5 106 −47 −44 −2 107 −33 −33 0 108 22 71 −49 109 11 −15 27 110 112 121 −9 111 31 −11 43 112 66 −33 98 113 1 −25 27 114 −37 −32 −5 115 57 61 −4 116 62 −30 92 117 114 112 2 118 −27 −24 −3 119 96 −22 118 120 233 6 227 121 237 81 157 122 −40 −19 −21 123 44 167 −123 124 −23 −28 5 125 46 −31 77 126 −25 −27 2 127 4 −16 19 128 28 −4 32 129 125 −22 147 130 15 123 −109 131 −27 −31 5 132 −30 −34 3 133 −45 −67 22 134 65 −23 88 135 28 3 25 136 303 −17 320 137 −17 −29 13 138 23 9 14 139 13 −26 38 140 −25 −32 7 141 0 −25 26 142 29 −26 55 143 130 187 −56 144 −37 −11 −27 145 1 −30 30 146 68 57 11 147 −12 10 −22 fig. 9. plot of τ0 versus the identification number of the event and of qp versus the identification number of the event. 1230 fig. 8. plot of qp versus τ0 of the studied events after the application of the aicc. the points enclosed in the grey rectangle are mainly responsible for the observed correlation. upper limit the value c = 0.65. if we assume c = 0.65, instead of qp = 57±42 we obtain a slightly higher qp value (qp = 74±55). let us consider now the residual correlation between qp and τ0. first, we note that the qp estimate obtained using the aicc (qp = 57±42) is intermediate between the qp estimate obtained under the assumption of a constant stress drop and that obtained by jointly retrieving τ0 and qp. however, this result does not necessarily imsalvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè 1231 intrinsic qp at mt. etna from the inversion of rise times of 2002 microearthquake sequence ply a departure from the constancy of the energy release per seismic moment, nor is it necessarily caused by a great heterogeneity in the attenuating properties of the area, as one can suppose by taking into account the high value of the standard deviation on qp. a possible interpretation of the correlation between qp and τ0 may be that, in many cases, the data available are not sufficient to correctly estimate the two parameters. as an example, it can be easily demonstratfig. 10. plot of rise time versus travel time for some studied events. on each plot the best fitting straight line, obtained from the inversion of rise times, is superimposed. 1232 salvatore de lorenzo, marilena filippucci, elisabetta giampiccolo and domenico patanè ed that a strong clustering of empirical data in a small range of values for both the considered variables induces a correlation between the regression parameters. thus the uneven and insufficient sampling for many events could be responsible for the observed phenomenon. alternatively, we have to account for the limitations of the hypothesis on which the used technique is based. in fact the adopted technique uses a very simplified source model, i.e. an isotropic seismic source. it is known that, for a finite dimension seismic source, the directivity source effect on the rise time of first p waves can be described through the relationship (zollo and de lorenzo, 2001) (6.1) where θ is the takeoff angle (the angle formed by the tangent to the ray leaving the source and the normal to the fault plane) and vp the p-wave velocity of at the source. from this equation it immediately follows that the variations of the source rise time around the non-directive source rise time given in eq. (5.4) increase with increasing the source dimension l. moreover, for a directive source time function, the equation which describes the relationship between τ and q−1 is non linear, as demonstrated in zollo and de lorenzo (2001) and de lorenzo et al. (2004). in particular zollo and de lorenzo (2001) showed that the higher the source dimension is, the higher will be the difference between the rise times predicted by the directive source model and those predicted by eq. (3.1). in our case, all the data which are responsible for the observed correlation among τ0 and qp (the data shown in the grey square of fig. 8) have been selected by aicc among the model parameters obtained from the joint inversion of τ0 and qp, i.e. without imposing a constant stress drop for the area (fig. 9). this could imply that, above a given threshold (τ0 ~ 30-40 ms) the directivity source effect tends to be so high to produce a non linear trend of rise times versus the travel time, which may result in a bad estimation of the estimated τ0 and qp if one uses the linear interpolating scheme given by eq. (3.1). this result is also supported by the visual sin v l v v 1 r p r 0τ θ= −c m inspection of the quality of the matching of the model to data, as shown in fig. 10 for some events. it is quite evident that, even if, in some cases, an increase in rise time versus travel time can be observed, the dispersal of data around the best fit straight line is significantly higher than the error on data. a possible bias in q estimates could be caused by the different instrumental responses. it has to be noted that twenty-two of the available stations are equipped with mark l4c, having an eigenfrequency of 1 hz. the remaining two stations are equipped with lennartz le3d, having an eigenfrequency of 0.05 hz. for both the two kind of stations the cutoff frequency is around 50 hz. since the average frequency content of the pulses ranges from 3 to 40 hz, i.e. lies in the linear part of the amplitude and phase response of the instrument, we do not expect a significant distortion in amplitude of signals and phase shifting, even if the signals having the higher average frequency content (around 40 hz) could be more sensitive to the non linear part of the instrumental response. however, only a dozen of data (fig. 3) have an average frequency higher than 30 hz and then this problem affects only a very negligible portion of the dataset and cannot affect the obtained average q estimate. therefore we conclude that, for the etnean area and in the case of a limited exploration of travel times, the applicability of the classic linear rise time method has to be limited only to those earthquakes which have a source rise time less or equal to about 35 ms, at which corresponds, using a rupture velocity vr = 0.9 vs = 2.2 km/s, an average source dimension of about 80 m. acknowledgements supported by grants form the european union volume fp6-2004-global-3 and ingv-dpc v3/6 projects. references akaike, h. 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(received january 10, 2006; accepted october 20, 2006) what is the impact of the august 24, 2016 amatrice earthquake on the seismic hazard assessment in central italy? annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ what is the impact of the august 24, 2016 amatrice earthquake on the seismic hazard assessment in central italy? maura murru*, matteo taroni, aybige akinci, giuseppe falcone istituto nazionale di geofisica e vulcanologia, rome, italy * maura.murru@ingv.it abstract the recent amatrice strong event (mw6.0) occurred on august 24, 2016 in central apennines (italy) in a seismic gap zone, motivated us to study and provide better understanding of the seismic hazard assessment in the macro area defined as “central italy”. the area affected by the sequence is placed between the mw6.0 1997 colfiorito sequence to the north (umbria-marche region) the campotosto area hit by the 2009 l’aquila sequence mw6.3 (abruzzo region) to the south. the amatrice earthquake occurred while there was an ongoing effort to update the 2004 seismic hazard map (mps04) for the italian territory , requested in 2015 by the italian civil protection agency to the center for seismic hazard (cps) of the istituto nazionale di geofisica e vulcanologia ingv. therefore, in this study we brought to our attention new earthquake source data and recently developed ground-motion prediction equations (gmpes). our aim was to validate whether the seismic hazard assessment in this area has changed with respect to 2004, year in which the mps04 map was released. in order to understand the impact of the recent earthquakes on the seismic hazard assessment in central italy we compared the annual seismic rates calculated using a smoothed seismicity approach over two different periods; the parametric catalog of the historical italian earthquakes (cpti15) from 1871 to 2003 and the historical and instrumental catalogs from 1871 up to 31 august 2016. results are presented also in terms of peak ground acceleration (pga), using the recent ground-motion prediction equations (gmpes) at amatrice, interested by the 2016 sequence. i. introduction central apennines are one of the most seismically active areas in italy, where a long history of earthquakes has strongly influenced the development of earthquake-resistant structural design. seismicity is mostly characterized by the occurrence of earthquakes which are distributed within a 50-60 km wide mountain belt extending, in nnw-sse apennines direction, from tuscany to abruzzi. during the last century, the region has frequently experienced strong shaking caused by earthquakes. the updated historical/parametric catalog [cpti15, rovida et al., 2016] contains in the macro area of central italy 69 events with magnitude 5.0 or greater, and 7 with magnitude exceeding 6.0, considering a depth 30km. the most relevant of them are those of the 1703 val nerina seismic sequence (mw6.92, i0xi mcs) that struck the whole central italy killing roughly ten thousand people, and the 1915 marsica earthquake (mw7.08, i0xi mcs) about 60 km south of the latest site that killed around 32000 people. more recently, in 1997 the 6.0 magnitude colfiorito earthquake, 50 km to the north, killed 11 people in the umbria and marche regions. the recent august 24, 2016 amatrice earthquake (mw6.0, 01:36 utc), which had an epicenter roughly 10 km southeast of norcia, occurred just over seven years after the april 6, 2009 l'aquila earthquake (mw6.3), killing more than 300 people only about 50 km away. the amatrice area was severely damaged also in 1639 (mw6.2, i0 ix-x mcs) and successively in 1672 (mw5.3, i0 vii-viii mcs). this part of the central apennines has been classified as a first category seismic zone in the current italian probabilistic seismic hazard map (mps04) [gruppo di lavoro mps, 2004]. because the apennines earthquakes are relatively shallow and structurally complex and many of the local towns and cities contain vulnerable buildings, the strong shaking from these earthquakes has the potential to cause major damage and loss of life in urban areas. the implementation of building codes mandating the use of earthquake resistant buildings has been highly useful and successful in annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ mitigating the impact of earthquakes. in fact, the probabilistic seismic hazard (psha) maps have been widely used in building codes to develop design criteria for buildings and for policy makers to implement priorities for earthquake risk reduction. these maps are derived by estimating a variety of parameters for selected models that are used to forecast future seismicity and the resulting shaking. probabilistic seismic hazard maps are revised regularly adding newly observed and studied material to improve the hazard assessments in order to reduce earthquake damages in united states, new zealand and japan, every 6, 7 and 10 years respectively. in italy, following the issuing of the prime minister ordinance n.3519 in 2006, the current probabilistic seismic hazard map (mps04) is released as the official reference for seismic hazard values to be used in engineering applications [gruppo di lavoro, mps04, 2004; stucchi et al., 2011]. the hazard assessment was conducted using the cornell [1968] probabilistic approach and assuming a poisson process, where the predicted occurrence of future earthquakes is independent of previous occurrence on the same source. recently there has been an effort to update the current seismic hazard maps requested in 2015 by the italian civil protection agency to the center for seismic hazard (cps) of the istituto nazionale di geofisica e vulcanologia (ingv). in this study for the seismic hazard calculation in italy, we utilized the smoothed-historical-seismicity method [frankel, 1995] using two different catalog periods from 1871 to 2003 and from 1871 up to august 31, 2016 in order to investigate the effect of the recent seismicity on the seismic hazard maps. results are presented both in terms of the earthquake rates as well as the hazard curves at amatrice, interested by the 2016 sequence. ii. data and analysis the data used for this analysis are drawn from two catalogs: 1) the new cpti15 catalog (release v1.5july 2016) (http://emidius.mi.ingv.it/cpti15‐dbmi15) [rovida et al., 2016] and 2) the italian seismological instrumental and parametric data-base (iside) of ingv. (http://iside.rm.ingv.it/iside/standard/index.jsp) detailed information regarding to data and macro area, considered for the seismic hazard assessment, are reported in the electronic supplementary material as files. we used the declustered cpti catalog and the period of completeness considered for this area as given by the working group, mps16, 2016: the events from 1871 to 2014 with completeness magnitude mw ≥ 4.55, and depth ≤ 30 km. we updated this catalog adding nine recent events provided from the iside catalog for the period between 01/01/2015 and08/31/2016, with magnitude mw ≥ 4.55 and depth ≤ 30 km within the study area. the local magnitude ml of the iside catalog was converted into moment magnitude mw, using the equation suggested by gasperini et al. (2013): mw=1.066∙ml-0.164 (1) in order to remove large fluctuations of seismicity rates in space and time due to aftershock sequences we declustered the iside catalogue using the algorithm developed by gardner and knopoff [1974]. the total number of declustered events was 198. iii. methods to compute the probabilistic seismic hazard analysis (psha) in central italy we used the classical cornell approach [cornell, 1968], which provides an estimate of ground shaking at a given site for each earthquake magnitude and distance, through an earthquake rate model and a ground motion prediction equation (gmpe). the earthquake rate model considers both the spatial and the magnitude distributions of the seismicity. for the spatial distribution we use the classical http://iside.rm.ingv.it/iside/standard/index.jsp annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ time independent smoothed seismicity approach [frankel, 1995], with a fine spatial grid of 0.025° both in latitude and longitude; the spatial rate in each cell of the grid is proportional to: (2) where is the total number of events in the catalog, is the distance between the i-th cell and the j-th event, is the smoothing (or correlation) distance. to obtain the final rate for each cell we normalized the values to preserve the total number of events by using the equation: (3) where is the total number of cells. the larger is the value of , the more smoothed will be the seismicity, but also the larger will be the contamination between different areas. the result is given as number of events per cell size, per total time spanned by the catalog. for each cell i, the smoothed value can be obtained from equation (3), and normalized to preserve the total number of events. the correlation distance,  = 30 km, was determined for “central italy” considering the cpti15 catalog from 1871 to 2014 (mw  4.55 and depth  30km) by maximizing the likelihood of the seismicity contained in half of the catalog, under the time-independent model obtained from the other half of the catalog and vice-versa. for the magnitude distribution we use the tapered gutenber-richter law [kagan and jackson, 2000], that has a cumulative distribution function equal to: (4) where is the seismic moment, is the seismic moment of completeness, is the parameter that controls the distribution in the upper ranges of m (‘upper corner moment’), and represents the slope of the distribution. since this distribution holds for seismic moment, we convert all mw magnitude values with the kanamori [1977] formula for estimating the seismic moment: to estimate , we used the fact that if << [bird and kagan, 2004], as in our case, and is the coefficient of the classical gutenbergrichter law [gutenberg and richter, 1954]; the bvalue was determined by the maximum likelihood method of aki (1965): b= 2.3/(mwmeanmwcompl) (6) where mwmean is the mean magnitude, and mwcompl is the completeness magnitude of the catalog. for the corner magnitude (that once converted with the kanamori formula becomes the corner moment ) we use the maximum observed earthquake in the "central italy" zone in the cpti15 catalog (the marsica earthquake of january 13, 1915 mw7.08). by adopting this simple approach we avoided the estimation of mmax, the maximum possible magnitude, that is a difficult parameter to estimate also with a catalog that lasts centuries [zöller and holschneider, 2015]. we also attempted to include aftershocks and foreshocks into the probabilistic seismic hazard assessment as the non-inclusion may lead to underestimate the seismic risk. in order to incorporate clustered events into psha we used the procedure proposed by marzocchi and taroni [2014] for the new italian seismic hazard map (internal report for mps 2016) by applying a corrective factor for the missing rate due to declustering in the catalogs. first, we calculated the rate using the declustered catalog, then we obtained the new clustered seismicity rates multiplying by a factor in a way that the total seismicity rate matches the whole observed seismicity rate. in this way we assumed that each cell of the grid had similar clustering properties. in the electronic supplementary material we put additional information. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ we calculated the annual rate of exceeding a specified ground motion at a site from a double summation over distance and magnitude, using both the time-independent model and ground motion prediction equations (gmpes). we use two recent gmpes among those selected during the share eu project [delavaud et al., 2012] for active shallow crustal regions to assess ground shaking hazard, without considering the prevalent fault mechanism. the first derives from the italian gmpe model of bindi et al. [2011] (hereafter ita10) and the second one was developed by akkar and bommer [2010] (hereafter ab10) for europe using the mediterranean, and the middle east data. in the electronic supplementary files, we also put the matrix used following the marzocchi and taroni [2014] approach for the rate model and the seismic hazard. iv. results we considered two different time periods of the declustered seismicity to calculate the earthquake rate model in the macro area “central italy”: 1) 1871-2003 (named hereafter “model 2003”) 2) 1871-august 31, 2016 (named hereafter “model 2016”). so that we verified if the latest strong earthquakes (table 1) caused any change to the cumulative annual rate in this study area, with respect to the rate of the first period, when the official map of the seismic hazard for the italian territory (mps04) was released. “model 2003” contains 189 declustered events with mw4.55 and depth30km (bvalue=0.97±0.07) while “model 2016” includes the 9 events (table 1) occurred in the period 20042016 (198 total declustered events with a bvalue=0.94±0.07) (mw4.55 and depth30km). the total cumulative annual .rate and b-values obtained for “model 2003” and “model 2016”, respectively are summarized in table 2. a comparison of model 2016 with model 2003 shows a smaller decrease of the rate, which contributes to a hazard reduction, together with a smaller decrease of the b-value, which, on the other hand, increases the hazard (table 3). table 1 earthquakes occurred in central italy from 01/01/2004 to 08/31/2016 with mw4.55 table 2 cumulative annual rates and b-values of model 2003 and model 2016 figure 1 shows for “model 2003” the annual smoothed seismicity for each spatial cell of 0.025°x0.025°, considering an mw ≥ 4.55. the black dots indicate the nine earthquakes with mw ≥ 4.55 that occurred from 2004 to august 31, 2016, also including the amatrice earthquake (see table 1). id area date (mm/dd/yyyy) coordinates lat(n)/lon(e) m w 1 costa laziale 08/22/2005 41.37/12.45 4.8 2 mugello 03/01/2008 44.06/11.25 4.7 3 parmense 12/23/2008 44.54/10.34 5.4 4 aquilano 04/06/2009 42.31/13.51 6.3 5 garfagnana 01/25/2013 44.16/10.45 5.0 6 valle del liri 02/16/2013 41.71/13.57 4.9 7 lunigiana 06/21/2013 44.09/10.06 5.4 8 costa anconetana 07/21/2013 43.51/13.72 5.1 9 amatrice 08/24/2016 42.7/13.23 6.0 model time period total cumulative annual rate (mw ≥4.55) b-value ± 2003 1871-2003 1.42 0.97±0.07 2016 1871-2016 1.36 0.94±0.07 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ figure 1: cumulative annual earthquake rate for model 2003 (1871-2003), considering the shallow events with mw  4.55 for each spatial (0.025° x 0.025°) cell in a log10 scale. black dots are the locations of nine earthquakes occurred during the period 01/01/2004-08/31/2016 (mw  4.55). the values close to black dots represent the identification number of events as reported in table1. with the arrow is indicated the amatrice earthquake of august 24, 2016 (mw6.0). figure 2 shows the difference in percentage between the two models: “model 2003” and “model 2016”, given as: (rate “model 2016” rate “model 2003”)/ (rate “model 2003”) for each spatial cell. the earthquakes from 1871 to 2003 are indicated with light grey circles and the earthquake from 2004 to 08/31/2016 with black dots. the annual occurrence rate and b-value in the analyzed macro area “central italy” during the period 1871-2016 lowered by 4% and 3%, respectively, compared to the early part of the catalog (1871-2003). the rate in the amatrice zone is slightly decreased (5%), while in the l’aquila area the rate is unchanged. we also verified if the seismic hazard in terms of ground acceleration in the amatrice changed between model 2003 and model 2016. to do so, we computed the hazard curves using seismicity models described with the gmpe derived from the italian strong motion database [bindi et al., 2011] (ita10). for the model 2016 we also computed the hazard curve using the gmpe ab10 [akkar and bommer, 2010], applicable in the european, mediterranean, and middle eastern regions (“model 2016 ab10” in figure 3). figure 3 also reports the hazard curve for amatrice site obtained using the non declustered catalog for model 2016 adopting the procedure provided by marzocchi and taroni (2014) (named as “model 2016 ita10 not declustered” in figure 3). we outline that the marzocchi and taroni [2014] approach works with return periods bigger or equal than 475 years, i.e. annual rate of exceedance lesser than 1/475. figure 3 shows the four hazard curves related to the adopted models, having marked the lines referred to 1/475 and 1/2500 annual rate of exceedance. the following recurrence times, 475 and 2500 years are equivalent to an exceedance probability of 10% and about 2% in 50 years, respectively. table 3 highlights the pga (g) values obtained from the four models, considering the return times of 475 and 2500 years. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ figure 2: percentage difference between the model 2003 and the model 2016; light grey circles are the events from 1871 to 2003, the black dots indicate the 9 events from 2004 to 08/31/2016 (mw  4.55). the values close to black dots represent the identification number of events as reported in table1. figure 3: hazard curves for amatrice: model 2003 green curve, model 2016 red curve, model 2016 not declustered blue curve, model 2016 ab10 purple curve. black dotted line represents the 1/475 annual rate of exceedance in 50 years, while the grey line represents the 1/2500 annual rate of exceedance probability. both x axis (pga in g) and y axis (annual rate of exceedance) are in logarithmic scale. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ table 3 pga values at amatrice for each computed model models with gmpes pga (g) value for rate 1/475 pga (g) value for rate 1/2500 model 2003 ita10 0.190 0.370 model 2016 ita10 0.190 0.375 model 2016 ita10 not declustered 0.240 0.440 model 2016 ab10 0.255 0.455 v. comments and conclusions in this work we calculated the earthquake rate model as well as the seismic hazard in central italy, following the frankel [1995] smoothed seismicity approach and the cornell [1968] one over two different time periods of the catalogs. we explored both the earthquake rate and the ground shaking changes over these two different time periods (1871-2003 and 1871-2016). the choice of these temporal periods was motivated by emulating the periods of time considered in the old (2003) and the ongoing new (2016) italian seismic hazard map. the purpose was to check whether the use of different temporal catalogs could lead to a difference in terms of earthquake rate and pga. as it is seen in figure 1 the 2016 amatrice earthquake occurred in an area where the seismic rate is the highest of central italy. moreover, the percentage change in the earthquake seismicity rate between two models (model 2003 and model 2016) in amatrice is relatively small being around 5% (figure 2). this means that in this area the occurrence of new earthquakes does not significantly change the earthquake rate. we then verified if the use of different gmpes, calibrated (ita10 model) and not calibrated with the italian data (ab10 model), and accounting or not the clustered events in the catalogs could lead to a difference in the seismic hazard in central italy. in particular, we verified the behavior of the seismic hazard curves for the amatrice town obtained from four different models; the first one derived from the model 2003 and gmpe ita10, the second one from model 2016 and gmpe ita10, the third one from model 2016 and gmpe ita10 not declustered and the last one from model 2016 and gmpe ab10. looking at the hazard curves in figure 3, the difference between the two earthquake rate models, model 2003 ita10 and model 2016 ita10 (same gmpe) is negligible. however, the difference becomes evident between model 2016 ita10 and model 2016 ab10, i.e. using different gmpes (pga +34% for the 475 years return time, and +21% for the 2500 years return time). moreover, the seismic hazard calculated using the clustered catalogs leads to increased ground motion. in fact, model 2016 ita10 not declustered compared to model 2016 ita10 has pga +26% for the 475 years return time, and +17% for the 2500 years return time. our results clearly demonstrated that the choice of the gmpe has a major influence to the seismic hazard estimates with respect to that of the earthquake rate models. as known, the standard seismic hazard calculations adopt a poisson earthquake rate model where the earthquake process is assumed to have no memory, i.e. the occurrence of a future earthquake is independent of the occurrence of previous earthquakes from the same source. although this assumption is possibly valid for areal sources, it is physically not solid for individual fault sources, given that the process of stress build up and its release is inherently time dependent. recently, in central italy, there have been several studies on the application of renewal and poisson model in the seismic hazard maps, in terms of expected peak ground acceleration (pga) values in the next 50 years [akinci et al., 2009; akinci et al. 2016; pace et al. 2006]. these studies showed that the contribution of the recently activated source faults vanishes in the overall seismic hazard; the time-dependent pga values are 20% lower than the poissonian ones. on the other hand, some source faults with long elapsed time since the last characteristic event become the most hazardous sites, where the timedependent pga values are about 50% higher than those of the poissonian model. therefore, it is quite important to perceive the impact of earthquake occurrence model on psha estimates. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ the results achieved in this study brought to the following conclusions: 1) the latest activity occurred over the last 13 years in the central apennines does not influence the existing seismic hazard model under the poisson assumption using a smoothed seismicity approach. two maps obtained in two different time periods are quite coherent in terms of earthquake rate activity. the 2016 amatrice mainshock occurred in the most seismically active part of central italy. however, renewal type earthquake rate models need to be taken into consideration for the future seismic hazard assessments in the region. 2) adopting a proper gmpe is a fundamental step for reliable psha calculations, as it is also supported by several previous studies [e.g., visini et al., 2014; akinci et al., 2004]. the uncertainties in ground motion estimation are very large and need to be reduced in the complex gmpes. 3) the seismic hazard calculated using the clustered catalogs leads to increased rates of ground motion at both 10% and 2% probability of pga exceedance in 50 years at amatrice. the declustered seismic hazard maps may lead to underestimate the seismic hazard in the study region. . references akinci a, mueller c, malagnini l, lombardi am (2004) a probabilistic seismic hazard assessment for the alps and apennines (italy) using historical seismicity and new predictive ground-motion relationships. boll geofis teor appl 45(4):285–304 akinci a, galadini f, pantosti d, petersen m, malagnini l, perkins d.(2009). effect of time dependence on probabilistic seismic-hazard maps and deaggregation for the central apennines, italy. bull seismol soc am ;99(2a):585–610. akinci a, perkins d, lombardi am, basili r. (2010). uncertainties in probability of occurrence of strong earthquakes for fault sources in the central apennines, italy. j seismol, doi:10.1007/s10950-008-9142-y. akinci a., vannoli p., falcone g., taroni m., tiberti m. m., murru m., burrato p., and mariucci m. t., 2016. when time and faults matter: towards a time-dependent probabilistic sha in calabria, italy, (submitted to bulletin of earthquake engineering). akkar, s. and bommer, j.j. (2010). empirical equations for the prediction of pga, pgv, and spectral accelerations in europe, the mediterranean, and the middle east. seismol res lett 81(2):195-206. doi: 10.1785/gssrl.81.2.195. bindi d, pacor f, luzi l, puglia r, massa m, ameri g and, paolucci r (2011). ground motion prediction equations derived from the italian strong motion database. b earthq eng. doi: 10.1007/s10518-011-9313-z bird, p., and y. y. kagan (2004). plate-tectonic analysis of shallow seismicity: apparent boundary width, beta, corner magnitude, coupled lithosphere thickness, and coupling in seven tectonic settings, bull. seismol. soc. am., 94(6), 2380-2399, plus electronic supplement, 2004. cornell c. a. (1968). engineering seismic risk analysis, bull seism soc am, 58: 1583-1606. delavaud e, cotton f, akkar s et al (2012) towards a ground-motion logic tree for probabilistic seismic hazard assessment in europe. j seismol 16:451-473. doi:10.1007/s10950012-9281-z frankel a. (1995). mapping seismic hazard in the central and eastern united states, seismol. res. lett., 66, 8-21. gardner, j. k. and knopoff, l. (1974). is the sequence of earthquakes in southern california, with aftershocks removed, poissonian?, seismol. soc. am., 64, 1363–1367, 1974. gasperini p., lolli b. and vannucci g. (2013). empirical calibration of local magnitude data sets annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7209 ___________________________________________________________________________________ versus moment magnitude in italy, bull. seism. soc. am., 103, 2227-2246, 2013. gutenberg, b. and richter c.f. (1944). frequency of earthquakes in california, bull seism soc am, 34, 185-188. kagan, y. y., and d. d. jackson (2000). probabilistic forecasting of earthquakes, geophys. j. int., 143, 438–453. kanamori, h. (1977). the energy release in great earthquakes, j. geophys. res., 82, 2981-2987. marzocchi w. and m. taroni, (2014). some thoughts on declustering in probabilistic seismichazard analysis, bull seism soc am., 104, 18381845, doi:10.1785/0120130300. pace b, peruzza l, lavecchia g, boncio p (2006) layered seismogenic source model and probabilistic seismic hazard analyses in central italy. bull seismol soc am 96:107–132. doi:10.1785/0120040231. rovida a., locati m., camassi r., lolli b., gasperini p. (eds) (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. istituto nazionale di geofisica e vulcanologia, doi:http://doi.org/10.6092/ingv.it‐cpti15 zöller, g., and m. holschneider (2015). the earthquake history in a fault zone tells us almost nothing about mmax, seismol res lett, doi: 10.1785/0220150176. vol. 48, 01, 05ok.qxd 175 annals of geophysics, vol. 48, n. 1, february 2005 key words permeability – radon – soil gas – transfer factor 1. introduction the uniform method, that has been used for radon risk classification in the czech republic since 1994 (barnet, 1994), is based on the assessment of the soil-gas radon (222rn) concentration and of the permeability of soil and rock for gases. both parameters are evaluated in the vertical profile down to the level of assumed building foundations or to the level of assumed contact building-soil. results of detailed radon surveys as well as results obtained from research studies carried out in the czech republic in the period 1994-2000 indicated that the uniform method should be improved. there are many problems connected with the determination of soil permeability itself and some correspond to a variable understanding of what this parameter describes or what it means in fact. due to the original method for classification it is possible to use direct in situ measurements of permeability based on soil gas withdrawal by means of low negative pressure (kašpar et al., 1993; barnet, 1994; neznal et al., 1995) or particle size analyses (the permeability is derived very simply from the weight percentage of fine fraction in the soil sample, barnet, 1992; neznal et al., 1992). these data are completed with the description of changes in vertical profiles with respect to the expected foundation depth of the building. the main disadvantages are given by the fact that in case of particle size analyses other factors influencing the permeability (natural soil moisture, density, effective porosity, etc) are not taken into consideration. furthermore, one sample cannot describe a heterogeneous geological environment (horizontal and vertical changes, influence of human activity). in case of direct measurements the results are strongly dependent on small scale variations of the character of the soil, and the equipment commonly used in the czech republic does not enable us to measure exactly in extremely low and/or high permeable soils (results vary in several orders of magnitude). mailing address: dr. matej neznal, radon v.o.s. corp., novakovych 6, 180 00, prague 8, czech republic; e-mail: neznal@clnet.cz permeability as an important parameter for radon risk classification of foundation soils matej neznal and martin neznal radon v.o.s. corp., prague, czech republic abstract permeability of soils and rocks for gases is one of the main parameters for final radon risk classification of building sites. for the improvement of the method for radon risk classification it is recommended to complete in situ measurements of permeability and/or results derived from grain size analysis by an expert description of parameters of soils and rocks and their changes in vertical profile. 176 matej neznal and martin neznal 2. methods and results at first a wide range of methods for soil-gas permeability determination in radon research was considered for further investigation – mainly single probe measurements (damkjaer and korsbech, 1992), dual probe measurements (garbesi et al., 1993) and derivation of permeability from other parameters – permeability for water (rogers and nielson, 1991), grain size analysis (barnet, 1992; neznal et al., 1992), etc. after comparison of advantages and disadvantages of different systems with respect not only to professional issues, but also to economic ones, three prototypes for direct measurements were prepared. the main goal was to avoid or decrease the disadvantages of radonjok equipment (single probe system that is used in czech republic, kašpar et al., 1993; neznal et al., 1995). radon-jok is a portable piece of equipment which has been developed for in situ measurements of gas permeability of soils. its robustness and simplicity make it very practical for easy, quick and at the same time sufficiently exact in situ investigation. the principle of the radon-jok equipment is based on air withdrawal by means of negative pressure. the air is pumped from the soil under constant pressure through a specially designed probe with a constant surface of contact between the probe head and the soil. the constant active area is created in the head of the probe (driven into the soil to a measured depth) by the extrusion of the tip by means of the punch wire inside the probe by an exact distance. the special rubber sack, with one or two weights, pumps the air from the soil and allows measurements to be made at very low pressures. the permeability is calculated from the equation using the known air flow through the probe. the air flow is defined by the known air volume (= 2000 ccm) in the rubber sack (depression of the bottom of the sack between two notches) and by the pumping time measured. the great advantage of radon-jok is the possibility to perform measurements independently of any source of energy (electricity, compressed air). using the radon-jok system, with the shape factor f = 0.149 m, the feasible range of measured permeabilities is approxitable i. correlation coefficients at the area lysa nad labem. correlation between correlation parameter (depth) coefficient crn (0.8 m) / crn (0.4 m) 0.92 k (0.8 m) / k (0.4 m) 0.83 crn (0.8 m) / k (0.8 m) – 0.68 crn (0.4 m) / k (0.4 m) – 0.60 crn (0.8 m) / w (0.1 m) 0.70 crn (0.8 m) / w (0.25 m) 0.88 k (0.8 m) / w (0.1 m) – 0.52 k (0.8 m) / w (0.25 m) – 0.66 mately from 10–11 to 10–14 m2. the maximum detection limit (measured time about 8 s) is determined by the resistance of the equipment, the minimum detection limit depends on the time spent by any given measurement (usually used near 1.10–14 m2). three prototypes and radon-jok system were tested at several test sites with different geological conditions. the tests confirmed the expected limitations of particular approaches. the range of real gas permeabilities is so large, that it is almost impossible to find a way to measure precisely with the same equipment in a low permeable environment (in case of pressure systems it means higher pressure differences, longer time and lower air flow) and in a high permeable one (just opposite situation). single probe system radon-jok was chosen for testing of temporal and spatial variability due to its simplicity and possibility to compare with previous results. the measurements were performed at four reference areas with respect to various geological conditions each month during a one-year period. permeability of soils and rocks for gasses was measured at 15 points at each reference area. at two reference areas (lysa nad labem, kocanda) the measurements were realized at each point at two depths, 0.4 and 0.8 m, and at two reference areas (svetice, klanovice) at a depth of 0.8 m only. the soil gas samples for soil gas radon concentration measurements (neznal et al., 1996) were collected at the same depths and to the same extent. other important parameters, mainly soil fig. 1a,b. temporal and spatial variations of soil gas radon concentration crn and permeability k in the depth of 0.8 m at the area lysa nad labem – minimal, median and maximal values of 15 measurements. b moisture (kuráž and matoušek, 1997) and/or the mass percentage of fractions, porosity and water saturation were determined with respect to czech national standards. in general, good correlations between the measured parameters were obtained in the areas with homogeneous and high permeable soils. these relations can be illustrated by the correlation coefficients between median values of gas permeability (k), soil gas radon concentration (crn) and soil moisture (w) at the reference area lysa nad labem (table i). as for the geological conditions, the bedrock is formed by cretaceous sediments and the quarternary cover by fluvial sands at the area. the upper soil layers are homogeneous, characterized by low 177 permeability as an important parameter for radon risk classification of foundation soils values of soil gas radon concentration and high permeability (fig. 1a,b). values of the correlation coefficient indicate a positive correlation between the soil gas radon concentration at a depth of 0.8 m and the soil moisture in surface horizons and a negative correlation between the soil gas radon concentration and permeability and between the permeability and soil moisture. positive correlation was confirmed between the soil gas radon concentration determined at two different depths. the same conclusion was valid for the permeability values measured at two different depths. on the other hand a correlation between the soil-gas radon concentration and the permeability, as well as between the permeability, soil moisture and other parameters can be often very weak at areas with medium or low permeable environment. it can be illustrated by the results obtained at the reference area kocanda (see fig. 2a,b). this area lies in the region of proterozoic paragneiss covered by quarternary loess and loess loams. although it can be described as an area with homogeneous upper soil layers, observed spatial and vertical changes in permeability do not allow us to confirm any relation between measured parameters. the single values of direct in situ measurements of permeability as well as soil moisture are substantially affected by the small scale changes in conditions in measured soil volume, especially in case of medium and low permeable environment. although the layers seem to be homogeneous, the in situ measurements characterizing small soil elements result in a large spatial variability. the statistical evaluation of permeability measurements is affected by an occurrence of high permeable outsiders on one hand and of values under the minimum detection limit in case of extremely low permeability on the other. a wide variability of results obtained on small scale conditions makes any evaluation very difficult. a similar conclusion was valid even for the high permeable environment, when relatively high water saturation of upper horizons occurred (reference area klanovice). the changing water content caused large spatial variability of permeability results too. the spatial variability of permeability, expressed as a ratio sd/mean, ranged typically a 178 matej neznal and martin neznal from 50 to 200% at the area kocanda, klanovice, svetice and from 25 to 50% at the area lysa nad labem with high permeable homogeneous environment, respectively. these follow-ups were completed by measurements of permeability and of soil-gas radon concentration in various vertical profiles with substantial changes in permeability to be able to describe the influence of permeability changes on radon concentration with depth and to classify possible errors connected with the application of the uniform depth for permeability measurements. three areas were chosen for these measurements. two of them (kostelec, sedlcanky) with the increasing permeability in the vertical profile with depth (low permeable upper horizon – the depth 0.4 m, medium permeable horizon at the depth 0.8 m and high permeable layer at the depth of 1.1 m). the last area skochovice had the low permeability along a vertical profile with some small changes – a slow decrease with depth. soil-gas radon concentration measurements and in situ permeability measurements were performed at 15 points and at three depths (0.4 m, 0.8 m and 1.1 m) in each area. almost no changes in soil gas radon concentration with depth were observed. this can be illustrated by the variability of soil gas radon concentration (crn) and permeability (k) with depth (range of measured values in the specified depth and median value) at the reference areas (table ii). the increase in radon concentration with depth could be presumed at two areas with substantial changes in permeability in the vertical profile (sedlcanky, kostelec). but this expectation was not confirmed, dramatic changes as well as local anomalies of radon concentration caused by a sealing effect of upper layers with a b fig. 2a,b. temporal and spatial variations of soil gas radon concentration crn and permeability k in the depth of 0.8 m at the area kocanda – minimal, median and maximal values of 15 measurements. table ii. variability of soil gas radon concentration crn and permeability k with depth. depth (m) crn (kbqm–3) median crn (kbqm–3) k (m2) median k (m2) sedlcanky area 0.4 11.7 – 38.0 21.4 9.9⋅10–15 – 7.3⋅10–13 9.9⋅10–15 0.8 14.9 – 36.4 21.9 7.6⋅10–14 – 1.1⋅10–11 6.5⋅10–13 1.1 14.0 – 27.6 18.8 1.0⋅10–13 – 1.6⋅10–11 7.2⋅10–12 kostelec area 0.4 24.7 – 38.7 32.7 9.9⋅10–15 – 1.7⋅10–12 1.2⋅10–13 0.8 26.3 – 39.9 32.7 9.9⋅10–15 – 1.1⋅10–11 5.8⋅10–13 1.1 29.2 – 43.3 32.8 7.2⋅10–13 – 1.1⋅10–11 7.0⋅10–12 skochovice area 0.4 39.3 – 100 66.1 9.9⋅10–15 – 3.9⋅10–12 1.8⋅10–13 0.8 44.4 – 104 74.6 9.9⋅10–15 – 1.1⋅10–11 8.3⋅10–14 1.1 29.8 – 101 74.3 9.9⋅10–15 – 9.1⋅10–12 9.9⋅10–15 179 permeability as an important parameter for radon risk classification of foundation soils lower permeability did not appear. at the area skochovice, similar values of radon concentration were determined in the depths of 0.4 m and 1.1 m, although the upper layers were more permeable and lower values could be therefore expected. the influence of permeability changes on changes of radon concentration with depth seems to be low (neznal et al., 1996). 3. conclusions and recommendations the classification of permeability based only on direct in situ measurements of permeability or on particle size analyses can be misleading. as one sample or one result cannot often describe a heterogeneous geological environment (horizontal and vertical changes, influence of human activity, small scale variations in the character of the soil), it would be necessary to increase substantially the number of measurements (and the costs of classification). but large spatial variability of results could make any statistical evaluation rather difficult and in some cases it could cause the underestimation or overestimation of permeability (changes in soil moisture, significant occurrence macroand micro-fissures, etc.). the expert responsible for the classification should focus not only on those results of laboratory or in situ tests, but also on the following factors influencing the permeability and their changes in vertical profile from surface down to the level of assumed building foundations or to the level of assumed contact building-soil: grain size distribution, soil moisture, water saturation, effective porosity, porosity, density, bulk density, compactness, thickness of quarternary cover, weathering character of the bedrock, modification of soil layers by various antropogeneous activities. to be able to describe the vertical profile, hand drills are usually performed to the desired depth (or to the lowest depth that can be reached by a hand drill, in case of the large amount of coarse fraction or in case of unweathered rock outcrops in the upper layers near the surface). this description, which strongly depends on the expert's personal experience, i.e. on subjective knowledge, should be completed either by a certain number of in situ permeability measurements, or by a subjective evaluation of a resistance during the soil gas sampling for the soil gas radon concentration measurements in all sampling points and by a grain size analysis of representative soil samples. the expert evaluation of permeability based on the subjective expert assessment of the de180 matej neznal and martin neznal scription of changes of those parameters in vertical profile, with the supporting character of results of laboratory or in situ tests, should improve the classification. some problems are connected with the fact that the permeability measured (or derived) during the investigation in an undisturbed environment is just the parameter that can be used for determination of radon potential («risk» point of view). but this permeability would not be the same as the permeability of the environment in the future at the contact between the building and the soil. therefore it cannot be used without correction for the description of the transfer of radon from soils into the buildings-transfer factor, etc. («transfer» point of view). acknowledgements the survey was supported by the state office for nuclear safety, prague. references barnet, i. (1992): permeability and 222rn concentration in soil gas – case studies, in radon investigation in cs, edited by i. barnet, czech geol. surv. prague, 3, 20-23. barnet, i. (1994): radon risk classification for building purposes in the czech republic, in radon investigations in cr, edited by i. barnet and m. neznal, czech geol. surv. prague, 5, 18-24. damkjaer, a. and u. korsbech (1992): a small-diameter probe for in-situ measurements of gas permeability of soils, rad. prot. dos., 45 (1/4), 85-89. garbesi, k., r.g. sextro, w.j. fisk, m.p. modera and k.l. revzan (1993): soil-gas entry into an experimental basement: model measurement comparisons and seasonal effects, environ. sci. tec., 27 (3), 466-473. kašpar, j., p. prokop and m. matolín (1993): fh-1, equipment for in situ permeability measurements, in radon investigation in cs, edited by i. barnet, czech geol. surv. prague, 4, 4-8. kuráž, v. and j. matoušek (1997): water regime of reclamated dumps – monitoring of soil moisture using dielectric method, in proceedings of the international symposium on soil behaviour in time and space, wien 1997, 67-70. matolín, m. and p. prokop (1991): statistical significance of radon determination in soil air, in radon investigation in cs, edited by i. barnet, czech geol. surv. prague, 2, 20-24. neznal, m., m. neznal and j. šmarda (1992): correlation between radon in soil gas and local geology – some remarks, in radon investigation in cs, edited by i. barnet, czech geol. surv. prague, 3, 24-28. neznal, m., m. neznal and j. šmarda (1995): radon risk classification of foundation soils and other radon measurements by private firms, bulletin scientifique l’association ingénieurs électriciens sortis de l’institut montefiore, 3, 31-34. neznal, m., m. neznal and j. šmarda (1996): assessment of radon potential of soils – a five-year experience, environ. int., 22 (suppl. to no. 1), 819-828. rogers, v.c. and k.k. nielson (1991): correlations for predicting air permeabilities and 222rn diffusion coefficients of soils, health phys., 61 (2), 225-230. vol50,6,2007 763 annals of geophysics, vol. 50, n. 6, december 2007 key words seismo-induced effects – liquefaction dataset – magnitude-distance relationships – geologic hazard – sicily 1. introduction liquefaction is one of the most common ground deformation effects of earthquakes and often a major cause of damage and destruction to buildings and infrastructures. liquefaction evidence is considered geological marker of paleoseismicity (seismites), because sites affected by past liquefaction have the potential to liquefy again. numerous and widespread liquefaction phenomena have been triggered by earthquakes in several places of the world and many studies have revealed a strong relationship between earthquake parameters and maximum epicentral or fault distance from the sites in which liquefaction develops. these studies are useful to engineers and urban planners for seismic hazard assessment and the mitigation of seismic risk (russ, 1982; talwani and cox, 1985; saucier, 1989; amick et al., 1990). italian historical records offer several descriptions of seismogeological effects which occurred during the last millennium, such as landslides, liquefaction and ground fracturing. these records were used to compile several catalogues and liquefaction prone area maps. eastern sicily is a seismically active area in which some of the most disastrous italian events, with maw up to 7.4 (maw equivalent moment magnitude according to working group cpti04, 2004), have occurred (e.g., the 1169, a new dataset and empirical relationships between magnitude/intensity and epicentral distance for liquefaction in central-eastern sicily claudia pirrotta, maria serafina barbano, pierpaolo guarnieri and flavia gerardi dipartimento di scienze geologiche, università degli studi di catania, italy abstract strong earthquakes can trigger several phenomena inducing soil deformation, such as liquefaction, ground fracturing and landslides, which can often cause more damage than the seismic shaking itself. a research performed on numerous historical accounts reporting descriptions of seismogeological effects in central-eastern sicily, allowed the authors to update the previous liquefaction datasets. 75 liquefaction-induced phenomena observed in 26 sites, triggered by 14 earthquakes, have been used to define relationships between intensity/magnitude values and epicentral distance from the liquefied sites. the proposed upper bound-curves, at regional scale for central-eastern sicily, are realized by using the updating liquefaction dataset and also the new cpti04 italian earthquake parametric catalogue. these relationships can be useful in hazard assessment to evaluate the minimum energy of an earthquake inducing liquefactions. mailing address: dr. claudia pirrotta, dipartimento di scienze geologiche, università di catania, corso italia 55, 95129 catania, italy; e-mail: c.pirrotta@unict.it 764 claudia pirrotta, maria serafina barbano, pierpaolo guarnieri and flavia gerardi 1693 and 1908 earthquakes) (fig. 1a,b) causing damage, numerous fatalities and triggering several ground failures, as reported by historical sources. this region was also affected by some strong earthquakes occurred in southern calabria, such as the 1783 seismic sequences. geological evidence of liquefactions (fig. 2a), correlated to some of the strongest earthquakes of eastern sicily, were found in the holocene deposits in the mascali area, which extends in the eastern flank of mount etna, and in the catania plain (fig. 1b), both characterized by a continental fluviatile sedimentation environment (guarnieri et al., 2008). the aim of this paper is to revise and update the previous liquefaction dataset, and to define empirical relationships, for central-eastern sicily, between earthquake magnitude/intensity and maximum epicentral distance of liquefied sites, using the earthquake source parameters retrieved from the recent italian earthquake parametric catalogue (working group cpti04, 2004). 2. earthquake-induced liquefaction earthquake-induced liquefaction is a process by which saturated granular sediment loses its strength, due to ground shaking. seismic shear waves generated by strong-motion earthquakes produce inter-particle shear stresses which, in saturated soil, can induce significant increasing of pore-water pressures. the increase induces loss of shear resistance of the sediment and the soil can undergo large viscous deformations. this mechanism typically triggers in sandy deposits, even though cases of liquefaction in gravel-rich deposits are documented (wong et al., 1975; bezerra et al., 2005). the liquefaction occurrence depends on the local site conditions (soil composition, local stratigraphic and topographic amplification) and on earthquake characteristics, such as magnitude and distance, which control shaking duration (i.e. number of cycles and amplitude of imposed shear stress). usually, the minimum earthquake magnitude value for liquefying sand is estimated to be about 5.5-6 (ambraseys, 1991; valera et al., 1994), while for gravel-rich deposits about 7 (valera et al., 1994). earthquake-induced liquefactions commonly produce sedimentary structures such as dikes, sand boils and lateral spreading (obermeier, fig. 1a,b. a) epicentral map of the earthquakes of central-eastern sicily and southern calabria, data from the italian earthquake parametric catalogue (working group cpti04, 2004); the circles are earthquake epicentres with magnitudes higher than 4.8. b) distribution map of the seismogeological effects, classified in table i, retrieved from historical sources (table ii). a b 765 dataset and magnitude/intensity versus epicentral distance relationships for liquefaction in sicily 1996) (fig. 2a). these features generally develop near the epicentral area, more numerous and consistent in the mesoseismic area, decreasing systematically with the distance from the epicentre (obermeier, 1998). the liquefaction occurs underground and the developed structures not always reach the ground surface. in these cases the seismo-induced liquefaction can be revealed by others associated phenomena such as surface deformation, differential compaction, local swelling or collapse (fig. 2b), differential settlement of building (e.g., kuribayashi and tatsuoka, 1975; galli, 2000); moreover, ground fissures with water or fluids emission, can be superficial evidence of liquefaction. 3. the new dataset of historical liquefaction phenomena in central-eastern sicily italian historical bibliography offers numerous accounts describing seismo-induced effects table i. classification of the ground features, associated to liquefaction phenomenon. a class embraces the liquefaction features s.s., b class the ground deformation and c class also includes water, gas and bituminous material emission. a b c sand boils, sand hills ground deformation ground deformation and sand/mud volcano with material emission b1 c1 ground fracturing ground fracturing with gases exhalation b2 c2 ground settlement ground fracturing with hot water, bituminous material and/or fluids emission and/or gases exhalation b3 c3 ground fracturing ground fracturing and settlement and settlement with water and/or gases exhalation fig. 2a,b. examples of liquefaction features in eastern sicily: a) surveyed by means of paleo-seismological analysis in the catania plain (after guarnieri et al., 2008); b) from historical reports, liquefaction in the messina harbour after the 1908 earthquakes (after baratta, 1910). a b 766 claudia pirrotta, maria serafina barbano, pierpaolo guarnieri and flavia gerardi t ab le ii . y ea r, m on th , da y an d ep ic en tr al a re a of e ar th qu ak es f or w hi ch l iq ue fa ct io n ef fe ct s ha ve b ee n ob se rv ed . e ar th qu ak e pa ra m et er s (l at it ud e an d lo ng it ud e of t he e pi ce nt re , ex pr es se d in f ra ct io ns o f de gr ee , i o , m a w an d m a s) , fr om c p t i0 4 (w or ki ng g ro up c p t i0 4, 20 04 ). t he c ol um n r t re po rt s th e so ur ce o f th e m ac ro se is m ic o bs er va ti on s: c f t i (b os ch i et a l. ,2 00 0) ; d o m ( m on ac he si a nd s tu cc hi ,1 99 7) a nd a zz * re fe rs t o a zz ar o et a l. (2 00 7) . s it es w he re l iq ue fa ct io n oc cu rr ed , th ei r la ti tu de a nd l on gi tu de . t he c ol um n r e re po rt s th e ep ic en tr al d is ta nc e of t he s it e. l as t tw o co lu m ns co nt ai n se is m ic al ly i nd uc ed o bs er ve d ph en om en on ( on t he b as is o f ta bl e i cl as si fi ca ti on ) an d th e re la ti ve h is to ri ca l so ur ce s. n ew r ep or ts a nd s ei sm oge ol og ic al e ff ec ts a re s ig ne d by s ta rs . e ar th qu ak e pa ra m et re s r ec or de d ph en om en on n o. y ea r m on th d ay e pi ce nt ra l r t l at l on g i o m a w m a s s it e l at l on g r e o bs er ve d h is to ri ca l ar ea (s ) (s ) (k m ) ph en om en a so ur ce s 1 11 69 2 4 e -s ic il y c f t i 37 .3 2 15 .0 3 10 6. 60 6. 60 c at an ia 37 .3 80 15 .0 50 7. 52 c 2 s am pe ri ( 16 44 ) 2 11 69 2 4 e -s ic il y c f t i 37 .3 2 15 .0 3 10 6. 60 6. 60 l en ti ni 37 .2 84 14 .9 98 5. 38 c 2 s am pe ri ( 16 44 ) 3 11 69 2 4 e -s ic il y c f t i 37 .3 2 15 .0 3 10 6. 60 6. 60 m es si na 38 .1 87 15 .5 29 10 6. 00 b 2* c ro na ca p is an a (1 3t h ce nt .) 4 11 69 2 4 e -s ic il y c f t i 37 .3 2 15 .0 3 10 6. 60 6. 60 s ir ac us a 37 .0 82 15 .2 85 34 .5 0 c 2 s am pe ri ( 16 44 ) 5 11 69 2 4 e -s ic il y c f t i 37 .3 2 15 .0 3 10 6. 60 6. 60 s ir ac us a 37 .0 82 15 .2 85 34 .5 0 b 1 p ri vi te ra ( 18 78 ) 6 11 69 2 4 e -s ic il y c f t i 37 .3 2 15 .0 3 10 6. 60 6. 60 v al d i n ot o 37 .0 70 15 .0 00 27 .7 6 b 1* c ar us o (1 78 1) 7 15 42 12 10 s e -s ic il y c f t i 37 .2 2 14 .9 5 10 6. 62 6. 62 a ug us ta 37 .2 31 15 .2 21 24 .1 0 a * b t jn ( 16 th c en t.) 8 15 42 12 10 s e -s ic il y c f t i 37 .2 2 14 .9 5 10 6. 62 6. 62 s ir ac us a 37 .0 82 15 .2 85 26 .8 8 a b t jn ( 16 th c en t.) 9 16 24 10 3 m in eo c f t i 37 .2 7 14 .7 5 8 5. 57 5. 40 p al ag on ia 37 .3 26 14 .7 45 6. 30 c 2* m on gi to re ( 17 43 ) 10 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 a ug us ta 37 .2 31 15 .2 21 20 .5 3 c 2* a rc hi vi o g en er al d e s im an ca s (1 69 3a ); b ot to ne ( 17 18 ) 11 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 a ug us ta 37 .2 21 15 .2 21 21 .0 7 c 1* b ib l. c om un al e di a ug us ta (1 7t h ce nt .) ; b ot to ne ( 17 18 ) 12 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 a vo la 36 .9 08 15 .1 35 26 .7 0 a * g ub er na le ( 19 10 ) 13 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 c at an ia 37 .5 02 15 .0 87 40 .0 0 b 1* b ot to ne ( 17 18 ) 14 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 c at an ia 37 .5 02 15 .0 87 40 .0 0 a a no ny m ou s (1 69 3) ; b oc co ne ( 16 97 ) 15 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 c at an ia 37 .5 02 15 .0 87 40 .0 0 c 2* b ot to ne ( 17 18 ) 16 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 c at an ia 37 .5 02 15 .0 87 32 .3 0 c 2 b oc co ne ( 16 97 ); p la in b ot to ne ( 17 18 ) 17 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 c at an ia 37 .5 02 15 .0 87 32 .3 0 a b oc co ne ( 16 97 ) p la in 18 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 l en ti ni 37 .2 84 14 .9 98 17 .3 3 b 1 b ot to ne ( 17 18 ) 19 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 l en ti ni 37 .2 84 14 .9 98 17 .3 3 a b oc co ne ( 16 97 ) 20 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 l en ti ni 37 .2 84 14 .9 98 17 .3 3 c 2 b ot to ne ( 17 18 ) 21 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m as ca li 37 .7 57 15 .1 59 70 .7 0 a b ot to ne ( 17 18 ); b oc co ne ( 16 97 ) 767 dataset and magnitude/intensity versus epicentral distance relationships for liquefaction in sicily t ab le ii . (c o n ti n u ed ) e ar th qu ak e pa ra m et re s r ec or de d ph en om en on n o. y ea r m on th d ay e pi ce nt ra l r t l at l on g i o m a w m a s s it e l at l on g r e o bs er ve d h is to ri ca l ar ea (s ) (s ) (k m ) ph en om en a so ur ce s 22 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m as ca li 37 .7 57 15 .1 59 70 .7 0 c 2 b ot to ne ( 17 18 ) 23 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m el il li 37 .1 79 15 .1 28 11 .6 0 c 1* b oc co ne ( 16 97 ) 24 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m es si na 38 .1 87 15 .5 29 12 7. 40 c 1* b ot to ne ( 17 18 ) 25 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m es si na 38 .1 87 15 .5 29 12 7. 40 b 1 a rc hi vi o g en er al d e s im an ca s (1 69 3b ); m on gi to re ( 17 43 ) 26 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m es si na 38 .1 77 15 .5 29 12 8. 10 c 2* b ot to ne ( 17 18 ) 27 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 m in eo 37 .2 66 14 .6 90 32 .8 3 b 1* d el b on o (1 74 5) 28 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 n ot o 36 .9 40 15 .0 23 21 .0 6 b 1* b oc co ne ( 16 97 ) a nt ic a 29 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s ca la 36 .9 00 15 .0 60 25 .8 0 b 1* d el b on o (1 74 5) ; b on ai ut i (1 79 3) 30 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 p at er nò 37 .5 66 14 .9 02 49 .7 0 b 3* b ot to ne ( 17 18 ) 31 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 p ia zz a 37 .3 84 14 .3 68 63 .9 0 c 2* d el b on o (1 74 5) a rm er in a 32 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s ir ac us a 37 .0 82 15 .2 85 24 .4 3 b 1 d el b on o (1 74 5) 33 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s ir ac us a 37 .0 82 15 .2 85 24 .4 3 c 2 b oc co ne ( 16 97 ); b ot to ne ( 17 18 ) 34 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s ir ac us a 37 .0 82 15 .2 85 24 .4 3 a b oc co ne ( 16 97 ) 35 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s ir ac us a 37 .0 82 15 .2 85 24 .4 3 c 1* b ot to ne ( 17 18 ) 36 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s or ti no 37 .1 56 15 .0 27 2. 96 b 2 b oc co ne ( 16 97 ); b on ai ut i (1 79 3) 37 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 s or ti no 37 .1 56 15 .0 27 2. 96 b 1 b on ai ut i (1 79 3) 38 16 93 1 11 e -s ic il y c f t i 37 .1 3 15 .0 2 11 7. 41 7. 41 v al d i n ot o 37 .0 70 15 .0 00 6. 90 b 2 b oc co ne ( 16 97 ) 39 17 80 3 28 n e -s ic il y a zz * 37 .8 6 15 .3 1 8 5. 60 5. 10 f iu m ed in is i 38 .0 20 15 .3 80 18 .7 0 c 2* g al lo ( 17 83 )* 40 17 83 2 5 c al ab ri a c f t i 38 .3 0 15 .9 7 11 6. 91 6. 91 g an zi rr i 38 .2 58 15 .6 11 31 .6 6 b 1 b ar at ta ( 19 10 ) 41 17 83 2 5 c al ab ri a c f t i 38 .3 0 15 .9 7 11 6. 91 6. 91 m es si na 38 .1 87 15 .5 49 40 .5 1 c 2 g al lo ( 17 83 ); l al le m en t (1 87 5) 42 17 83 2 5 c al ab ri a c f t i 38 .3 0 15 .9 7 11 6. 91 6. 91 m es si na 38 .1 87 15 .5 29 40 .8 7 b 2 g al lo ( 17 83 ) 43 17 83 2 5 c al ab ri a c f t i 38 .3 0 15 .9 7 11 6. 91 6. 91 m es si na 38 .1 87 15 .5 29 40 .8 7 b 1 g al lo ( 17 83 ); c or ra o (1 78 4) 44 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 p ar as po lo 37 .4 00 15 .0 80 22 .6 8 b 1 l on go ( 18 18 ) 45 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 p ar as po lo 37 .4 00 15 .0 80 22 .6 8 a l on go ( 18 18 ) 46 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 p ar as po lo 37 .4 00 15 .0 80 22 .6 8 c 2 l on go ( 18 18 ) 768 claudia pirrotta, maria serafina barbano, pierpaolo guarnieri and flavia gerardi t ab le ii . (c o n ti n u ed ) e ar th qu ak e pa ra m et re s r ec or de d ph en om en on n o. y ea r m on th d ay e pi ce nt ra l r t l at l on g i o m a w m a s s it e l at l on g r e o bs er ve d h is to ri ca l ar ea (s ) (s ) (k m ) ph en om en a so ur ce s 47 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 p at er nò 37 .5 66 14 .9 02 20 .4 7 c 2 l on go ( 18 18 ) 48 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 p at er nò 37 .5 66 14 .9 02 20 .4 7 a l on go ( 18 18 ) 49 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 p oz zi ll o 37 .6 60 15 .1 90 8. 84 b 1 l on go ( 18 18 ) 50 18 18 2 20 c at an es e c f t i 37 .6 0 15 .1 3 9 6. 00 6. 00 r am on de tt a 37 .3 50 15 .0 70 28 .2 4 a l on go ( 18 18 ) 51 18 48 1 11 a ug us ta d o m 37 .3 7 15 .1 54 8 5. 48 5. 26 a ug us ta 37 .2 31 15 .2 21 16 .2 3 b 1* f er ru gg ia r us so ( 18 52 )* 52 18 48 1 11 a ug us ta d o m 37 .3 7 15 .1 54 8 5. 48 5. 26 a ug us ta 37 .2 31 15 .2 21 16 .2 3 a * f er ru gg ia r us so ( 18 52 )* 53 18 93 4 22 m . e li co na d o m 38 .0 2 15 .0 1 7 5. 03 4. 60 m on ta lb an o 38 .0 20 15 .0 10 0. 70 b 1* r ic cò ( 18 93 )* 54 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 b ar ce ll on a p. g . 38 .1 46 15 .2 15 58 .5 0 b 1* g io rn al e di s ic il ia ( 18 94 ) 55 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 g an zi rr i 38 .2 48 15 .6 11 22 .7 0 b 1 r ic cò ( 19 07 ); b ar at ta ( 19 10 ) 56 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 g an zi rr i 38 .2 48 15 .6 01 21 .8 0 c 1; c 2 r ic cò ( 19 07 ) 57 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 g an zi rr i 38 .2 48 15 .6 03 22 .0 0 b 2 r ic cò ( 19 07 ) 58 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 m es si na 38 .1 87 15 .5 29 31 .4 1 b 2 r ic cò ( 19 07 ) 59 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 m es si na 38 .1 87 15 .5 29 31 .4 1 b 3 r ic cò ( 19 07 ) 60 18 94 11 16 s -c al ab ri a c f t i 38 .2 8 15 .8 7 9 6. 05 6. 05 t or re f ar o 38 .2 66 15 .6 46 19 .5 0 b 2* r ic cò ( 19 07 ) 61 18 98 11 2 c al ta gi ro ne d o m 37 .2 3 14 .5 1 6 4. 83 4. 30 c . r ac in er i 37 .2 10 14 .4 00 10 .3 3 a c or ri er e di c at an ia ( 18 98 )* 62 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 g an zi rr i 38 .2 58 15 .6 11 13 .5 9 c 2 b ar at ta ( 19 10 ) 63 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 g an zi rr i 38 .2 48 15 .6 11 13 .4 9 b 2* l o g iu di ce ( 19 09 )* ; b ar at ta ( 19 10 ) 64 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 g an zi rr i 38 .2 48 15 .6 13 13 .7 0 a * l o g iu di ce ( 19 09 )* 65 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 g an zi rr i 38 .2 48 15 .6 01 13 .6 5 c 1* l o g iu di ce ( 19 09 )* 66 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 m es si na 38 .1 87 15 .5 49 11 .8 0 b 1 b ar at ta ( 19 10 ) 67 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 m es si na 38 .1 87 15 .5 49 11 .5 9 b 2 f ra nc hi ( 19 09 ); p la ta ni a (1 90 9) ; b ar at ta ( 19 10 ) 68 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 m es si na 38 .1 87 15 .5 49 12 .2 9 a * b ar at ta ( 19 10 ); s au re t an d b ou sq ue t (1 98 4) * 69 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 t or re f ar o 38 .2 66 15 .6 46 13 .2 7 b 1* b ar at ta ( 19 10 ) 70 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 t or re f ar o 38 .2 66 15 .6 46 13 .2 7 b 3* b ar at ta ( 19 10 ) 71 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 t or re f ar o 38 .2 66 15 .6 46 13 .3 4 c 2* b ar at ta ( 19 10 ) 72 19 08 12 28 s -c al ab ri a c f t i 38 .1 5 15 .6 8 11 7. 24 7. 24 t or re f ar o 38 .2 66 15 .6 46 13 .5 0 b 2* b ar at ta ( 19 10 ); p la ta ni a (1 90 9) 73 19 78 4 15 p at ti g ul f c f t i 38 .1 5 14 .9 83 9 6. 06 6. 06 o liv er i 38 .1 24 15 .0 60 7. 37 b 1* g az ze tt a de l s ud ( 19 78 ) 74 19 90 12 13 s e -s ic il y c f t i 37 .2 7 15 .1 21 7 5. 68 5. 26 a ug us ta 37 .2 31 15 .2 21 9. 70 b 1 d e r ub ei s et a l. (1 99 3) 75 19 90 12 13 s e -s ic il y c f t i 37 .2 7 15 .1 21 7 5. 68 5. 26 a ug us ta 37 .2 31 15 .2 25 9. 15 a d e r ub ei s et a l. (1 99 3) 769 dataset and magnitude/intensity versus epicentral distance relationships for liquefaction in sicily which are reported in catalogues (berardi et al., 1991; galli and ferreli, 1995; romeo and delfino, 1997; boschi et al., 2000; galli, 2000; prestininzi and romeo, 2000); these have also been used to draw maps of liquefaction-prone areas of italy (galli and meloni, 1993) and, at local scale, of historical liquefaction-induced phenomena in the catania area (azzaro, 1999). this paper presents an updated dataset of liquefaction phenomena in central-eastern sicily, realized through the revision of historical accounts, retrieved from the aforementioned catalogues and through an original research of historical primary sources. besides liquefaction structures developing at the surface (i.e. sand boils, dikes and mud volcanoes), effects directly connected to the liquefaction mechanism at depth (fig. 2b) have also been reported. soil deformation phenomena related to liquefaction, according to the classification of galli (2000) and to geological and geomorphologic criteria, have been chosen, i.e. ground fissuring, collapse and surface settlements occurred in recent alluvial depositional areas, flat enough to suggest a their deep liquefaction-induced origin. for example bottone (1718) describes the effects observed during the 1693 earthquake in the catania plain as follows: «la terra si aprì in modo spropositato… da questa fenditura fuoriuscì una polla di acqua calda: si osservò che ciò era avvenuto in molti luoghi della pianura» (the ground surface excessively opened… the crack ejected hot water: this phenomenon was observed in several places of the plain). using the same criteria, exhalation of gases and fluids occurred together with sediment liquefaction, as described for 1908, 1894 and 1783 earthquakes by lo giudice (1909) and baratta (1910), have been included. the ground features, associated to liquefaction phenomenon, have been classified into three groups: a class, liquefaction features s.s.; b class, ground deformation; c class, ground deformation with emission of material (table i). the new dataset (table ii) collects 75 liquefaction effects observed in 26 sites triggered by 14 earthquakes occurred in central-eastern sicily and southern calabria, from 1169 a.d. to 1990 a.d., and also contains the new earthquake parameters, location, epicentral intensity io (mcs), magnitude, maw (equivalent moment magnitude) and mas (surface wave magnitude), reported by the cpti04 catalogue (working group cpti04, 2004) and the epicentral distance re, for each liquefied site. the epicentral intensity (io) varies from 6 to 11 mcs (table ii); the magnitude maw from 4.83 to 7.41; and the mas from 4.3 to 7.41. table iii summarises the frequency of occurrence of liquefaction phenomena for magnitude classes. 69% of observed liquefaction features are induced by earthquakes with maw≥ 6.6; while 24% are related to maw ranging from 5.6 to 6.5 and only 7% for maw< 5.5. 4. relationships between magnitude/ intensity versus epicentral distance (re) several studies have been carried out to acquire empirical relationships between earthquake source parameters (magnitude, intensity, etc.) and maximum epicentral or fault distance of liquefied sites at regional scale and worldwide. these relationships are useful tools both in geotechnical applications, such as microzonation studies and hazard assessment at the regional scale, and in seismic application, i.e. for the evaluation of the minimum energy of an earthquake capable to induce liquefaction and of the minimum magnitude of paleo-earthquakes which have caused liquefaction in a given site, finally for the recognition of the probable mesoseismic zone. kuribayashi and tatsuoka (1975) obtained the correlation between maximum epicentral distable iii. number of liquefaction cases for maw interval. column 3 is the number of events that induced liquefaction, to be compared with the total number of events existing in the cpti04 earthquake catalogue (working group cpti04, 2004) for magnitude interval classes at the same epicentral distance. maw interval no. liquefactions no. events events in cpti04 7.0-7.5 40 2 2 6.6-6.9 11 3 4 6.0-6.5 15 3 6 5.6-5.9 3 2 6 5.0-5.5 4 3 75 4.6-4.9 1 1 62 770 claudia pirrotta, maria serafina barbano, pierpaolo guarnieri and flavia gerardi tance of liquefied sites and associated magnitude for strong earthquakes of japan. ambraseys (1991) considered both epicentral and fault distance to compute the relationships between moment-magnitude and distance for 137 liquefaction events scattered around the world. papadopoulos and lefkopoulos (1993) obtained a bounding equation revisiting the worldwide curves proposed by ambraseys (1991), adding their greek data and other liquefaction observations in several places of the world. recently, the magnitude upper bound method was applied by galli (2000) to historical liquefactions induced by 61 earthquakes occurred in italy from 1117 a.d. to 1990 a.d. the author related epicentral intensity io (mcs), magnitude ms and equivalent moment magnitude me, reported in the previous version of the italian parametric catalogue (working group cpti99, 1999), to the epicentral distance re; finally, magnitude ms to the epicentral distance re considering only the instrumentally observed values for the period 1900 a.d.-1990 a.d. prestininzi and romeo (2000) found for the whole italian territory maximum epicentral distances at which ground failures occur as a function of epicentral intensity, distinguishing induced ground effects in topographic changes, liquefaction, landslides and fractures. the regional dataset realized in this work has been used to find local relationships between earthquake source parameters, maw, ms and io from cpti04 (working group cpti04, 2004) and the epicentral distance of the liquefied sites (re). we selected these parameters because most of the historical earthquakes have not instrumental data; maw is the parameter that could be linked to the earthquake energy being computed from io and the extension of the felt area (gasperini et al., 1999). fig. 3. distribution of earthquakes that induced liquefaction effects for the period 1169-1990 in terms of epicentral distances and maw values, mas values and io values. maw, mas and io are from cpti04 (working group cpti04, 2004). the upper bound equations are reported in the text as eq. (4.1), eq. (4.2) and eq. (4.3), respectively. the polygons correspond to the anomalous points: star to the messina site for the 1169 earthquake; square refers to barcellona site for the 1894 earthquake. 771 dataset and magnitude/intensity versus epicentral distance relationships for liquefaction in sicily in the magnitude/intensity-epicentral distance graphs, the point distribution shows the area of occurrence of liquefaction effects (fig. 3), the best-fit of the farthest points gives the upper bound-curves, delimiting liquefactionprone areas, whose equations (1, 2 and 3) are reported below (4.1) (4.2) (4.3) 5. discussion the upper bound curves (maw, ms and io versus re) obtained for central-eastern sicily show a similar trend and highlight that lower intensity earthquakes have caused liquefaction at very close distance from the epicentre (< 10 km); 43% of the observations occurred with lowmedium magnitude (intensity viii-ix mcs) within 50 km from the epicentre. finally the events with maw> 6.6 (intensity x-xi) induced liquefaction at great distances, particularly earthquakes with maw> 7.0 and intensity xi (mcs) can trigger liquefaction as far as ∼130 km from the epicentre (fig. 3). only two points fall out of the upper bound curve, in the area where no liquefaction is predicted. one of these points refers to ground fracturing observed in the messina seaport during the 1169 earthquake and another point refers to a ground fracture observed near the barcellona p.g. village after the 1894 earthquake (fig. 3). these sites are both located in north-eastern sicily. liquefaction at such exceptionally great distance from the epicentres, as observed for the previous two sites (table ii and fig. 3), may be due to local geological characters, but may also be explained with both mislocation and a magnitude/intensity wrong estimate of the 1169 and 1894 earthquakes. ( . . ) ( . ) ( ). ln ri 2 7 0 0 067 32 1 73o e! != + ( . ) ( . . ) ( ). lnm r0 28 1 16 0 061 85as e! != + ( . . ) ( . . ) ( )lnm r2 67 0 04 0 98 0 01aw e! != + moreover, the liquefaction prone areas show a lack of data for 6≤m≤7.4 and 9≤io≤11 and epicentral distance between 40 and 120 km (fig. 3), indicating that the earthquakes with these magnitude intervals (1169, 1542, 1783, 1894, 1908 and 1978 events) could have triggered liquefaction outer from the study area. this lack of data could be due to the inevitable incompleteness of the dataset, assembled from historical accounts, and to the fact that the study region not always embraces the entire mesoseismic area of the analyzed earthquakes. in fact the epicentres of the 1783 and 1894 earthquakes are localized in southern calabria; the 1908 event is located in the messina strait; the 1169 and 1542 sources are close to the southeastern coast of sicily, finally the 1978 epicentre is located in the tyrrhenian sea (fig. 1). comparing ours upper bound curves to previous ones of several regions of the world fig. 4. comparison between different upper bound curves from literature and those proposed in this paper: continuous dark grey line (1) and continuous black line (2) are related to ours maw and mas relationships, respectively; dot black line (5) is related to m values from kuribayashi and tatsuoka (1975); dashed grey line (6) to mw from ambraseys (1991); dot light grey line (7) to ms from papadopulos and lefkopulos (1993); dashed black line (8) to me (equivalent magnitude) values and continuous light grey line (9) to ms ones from galli (2000). 772 claudia pirrotta, maria serafina barbano, pierpaolo guarnieri and flavia gerardi (kuribayashi and tatsuoka, 1975; ambraseys, 1991; papadopoulos and lefkopoulos, 1993) and of the italian country (galli, 2000), it is possible to observe a good correspondence of the curve trends as it regards the surface wave magnitude ms versus re curve (9) from galli (2000) (fig. 4) and between our io versus re curve and galli’s one (fig. 5). mismatch found with the me (equivalent magnitude) versus re curve (8) (fig. 4) can be explained with the different source parameters used by the authors. 6. conclusions the analysis performed on historical sources has enriched previous compilations of liquefaction-induced phenomena that occurred during the last millennium in central-eastern sicily. new data have been introduced and some of the known historical accounts have been reconsidered also including environmental effects directly connected to the liquefaction mechanism, chosen on the basis of geological and geomorphologic criteria. the new dataset contains 75 liquefaction phenomena triggered by 14 earthquakes, with io> 6 (mcs) and maw > 4.6, in 26 sites of central-eastern sicily, and source parameters retrieved from the new italian earthquake catalogue (working group cpti04, 2004). most of the liquefactions are connected to earthquakes with magnitude more than 5.4; only under peculiar site condition, can liquefactions be triggered by earthquakes with maw value less than 5.4, as observed for the 1893 montalbano elicona earthquake (maw= 5.03) and the 1898 caltagirone earthquake (maw= 4.83). the updating of the previous catalogues of liquefaction effects and the use of the data retrieved from the parametric catalogue (cpti04), allowed us to obtain a version, at regional scale, of the upper bound curves, maw, mas and io versus re, for central-eastern sicily. liquefaction effects described at great distance suggest that the studied area is particularly sensitive to these kinds of phenomena, probably due to the seismological characters of the region and to the distribution of holocene deposits. moreover, liquefaction effects that fall out of the bound-curve could reveal mislocation and/or wrong magnitude estimate of historical seismic events such as for the 1169 and 1894 earthquakes. these relationships may be considered a tool for evaluating the minimum energy of an earthquake inducing liquefaction effects, useful in paleoseismic analyses and hazard evaluation, i.e. for engineering applications and for the civil protection department, especially in areas subject to industrial and urban growth. acknowledgements this work was funded by the italian civil defence through ingv-dpc project s2, grants to msb. we are indebted with p.m. de martini and d. pantosti for the helpful discussion. we also thank the anonymous reviewers that with their criticism and suggestion contributed to substantially improve the original manuscript. references ambraseys, n.n. 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(received august 22, 2007; accepted march 11, 2008) 48,06,2005misc 995 annals of geophysics, vol. 48, n. 6, december 2005 key words seismic sequences – stress field evolution – southern apennines – calabrian arc – pollino chain 1. introduction earthquakes play a fundamental role in depicting the ongoing geological processes because they make possible the location of the active structures and the characterization of the stress and strain fields. therefore the occurrence of seismic events becomes the occasion for clarifying tectonic relationships in areas of increased interest. this is especially true when they hit areas of moderate seismicity, where even a magnitude 5.0 event can furnish significant information on the geodynamical regime and in particular where the boundaries between tectonic major domains are not well defined. for these reasons we focus on the seismic sequence that followed an earthquake located on the western slope of the pollino chain (southern italy). this seismic event occurred on 9th september 1998 and caused some damage in several towns and villages located in the mesoseismal area (galli et al., 1998), attaining a maximum intensity of vii mcs. the whole sequence was confined to the upper 15 km of the crust and involved the sedimentary cover of the region where two important contacts exist. at the surface there is the boundary between the apenninic chain and the calabrian arc; more in depth there is the limit between the adriatic and the african plate. these contacts are not yet well understood either at subduction on crustal levels. 2. geodynamic setting from a geodynamic viewpoint, southern italy is divided into two regions. the southernthe 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence ignazio guerra (1), paolo harabaglia (2), anna gervasi (1) (3) and anna b. rosa (2) (1) dipartimento di fisica, università degli studi della calabria, arcavacata di rende (cs), italy (2) dipartimento di strutture, geotecnica e geologia applicata, università della basilicata, potenza, italy (3) istituto nazionale di geofisica e vulcanologia cesis, grottaminarda (av), italy abstract in 1998-1999 a seismic sequence occurred in the southern apennines, after the moderate size (mb = 5.0) 9th september 1998 pollino earthquake. it lasted about 14 months and was clearly localized to the sole north-west area of the main shock epicenter. its peculiarity consisted in sudden changes of activity from a series of normal faults with apenninic (nw-se) trend and transfer, presumably strike slip, faults with antiapenninic (ne-sw) and e-w trend. the complexity of the behavior and the different orientations of the activated systems suggest that the area acts as a hinge between the nw-se trending southern apennines and the locally n-s trending calabrian arc. mailing address: prof. ignazio guerra, dipartimento di fisica, università degli studi della calabria, via p. bucci, cubo 31c, 87036 arcavacata di rende (cs), italy; e-mail: guerra@unical.it 996 ignazio guerra, paolo harabaglia, anna gervasi and anna b. rosa most one, the so-called calabrian arc, is the area where the ionian lithosphere still subducts beneath the tyrrhenian sea; the subduction is characterized by an eastward rollback (e.g., malinverno and ryan, 1986; doglioni et al., 1996). north of the calabrian arc there are the so-called southern apennines that constitute the accretionary prism of the adriatic plate subduction (e.g., doglioni et al., 1996, and references therein); on the basis of geological evidence the hinge of this subduction has been migrating northeastward up to the pliocene; at present it is thought to be quiescent. these two geodynamically separated regions meet in the pollino chain (fig. 1a,b). this latter, broadly shaped as a triangle due to its short e-w extension, is bordered to the south by the sibari plane, to the north-east by the metaponto plane and to the west by the mercure basin. the main structural lineaments shown in fig. 1a,b are taken from valensise and pantosti (2001). it is noteworthy that the tectonic style of the southern apennines is broadly dominated by a ne-sw oriented tensile stress field (amato and montone, 1997) that generates a series of normal fault systems with longitudinal trend. they are usually recognized to dip northeastward and are displaced by shear fault systems with antiapenninic trend. the southernmost normal system that exclusively yields this feature is the agri valley, bordered to the south by the policastro gulf-s. arcangelo line. the extensional systems are characterized by large earthquakes. according to the cpti catalogue (gruppo di lavoro cpti, 1999), the main known seismic event in the agri valley is the 1857 earthquake that caused over 10 000 cafig. 1a,b. a) relative position of the investigated area with respect to the location of the thrust belt. b) main structural lineaments according to valensise and pantosti (2001) and seismic stations in southern italy. blue lines: apenninic features; red lines: other tectonic features; black squares: permanent seismic stations; red squares: temporary seismic stations. a b the 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence sualties (fig. 2). this is the famous «great neapolitan earthquake», that is considered the starting point of the modern seismology, because of the in-depth inspection of its effects and of the subsequent first attempt of a quantitative interpretation of macroseismic observations carried out by mallet (1862) on behalf of the royal society of london. the calabrian arc in its northern portion has an almost north-south trend. it is also apparently characterized by extensional activity oriented along its axis: this is testified by the graben structure of the crati valley (fig. 1a,b) and the fault plane solutions of the two seismic events of february 20, 1980, the strongest ones in the last few decades (cello et al., 1982). in the same area ew trending lineaments exist too, that probably displace the extensional systems in a similar manner to what happens in the southern apennines, even though there is scant geological evidence (finetti and del ben, 1986; moretti and guerra, 1997; moretti, 2000; valensise and pantosti, 2001). the cetraro-rossano line represents the northern boundary of the area where calabrian arc tectonic features are exclusively observed. the calabrian arc showed in historical times earthquakes of such an energetic level that it has to be considered among the regions with highest seismic risk in the world. the pollino chain and the strictly adjacent areas represent a minimum of the seismic activity in terms of moment release along the ridge of the italian peninsula. they are characterized by the occurrence of a relatively large number of events, all however of moderate size. maximum intensities, observed in 1831, 1836, 1894 never attained severity higher than degree viii on the mcs macroseismic scale. from the tectonic point of view, the boundaries of the area can be identified with the policastro gulf-s. arcangelo line to the north and with the cetraro-rossano line to the south: in between these limits both trends, antiapenninic and east-west, coexist generating a complex surface tectonic pattern. one open question 997 fig. 2. historical seismicity (red circles, radius increasing with macroseisimic intensity), main tectonic lineaments (solid red lines), reported active faults on the base of geological evidence (dashed red rectangles – intersection with the surface, solid red). black circles show instrumental locations of the 1982 and 1988 seismic events and their relative dislocation with respect to the macroseismic epicentres. the star refers to the 1998 main event (sources: gruppo di lavoro cpti, 1999; valensise and pantosti, 2001). 998 ignazio guerra, paolo harabaglia, anna gervasi and anna b. rosa is whether they are still active at the present time throughout the whole area. the 1998 seismic sequence followed an earthquake that occurred in the mercure basin, immediately to the northwest of the pollino chain, in an area broadly comprised between the policastro gulf-s. arcangelo line and the verbicaro-s. maria del cedro line. it is noteworthy that aside from the 1831, 1836 and 1894 events, in more recent years the area suffered damaging earthquakes only in 1982, 1988 and 1998 (esposito et al., 1988; gasparini and tertulliani, 1988; galli et al., 1998). it is quite obvious on the basis of the huge shift between the instrumental and macrosesimic epicentres observed in these occasions (fig. 2), that historical locations might be misleading (moretti et al., 1994): they can give a hint on the level of activity in the area but not on the tectonic setting. 3. data acquisition and treatment around the pollino chain several seismic stations belonging to different organizations operate. among these there are the national centralized seismic network of the istituto nazionale di geofisica e vulcanologia, rome (ingv) and the calabria university seismic network. data useful for its seismographic monitoring are also acquired by osservatorio vesuviano (naples), presently a section of ingv. the stations active in 1998 are drawn in fig. 1a,b. soon after the main shock of 9th september, 1998 the above described network was integrated by 14 temporary seismic stations that were installed all around the mesoseismal area. in particular, 6 reftek acquisition systems managed by ingv remained active up to mid october. the other temporary stations, belonging to the university of calabria, remained active up to 20th december 1998, but two are still in activity (lb2 and mm1). these stations were six lennartz mars-88/fd, one locally engineered telemetered digital seismograph, and one analogic sprengnether meq-800 (fig. 3). because of the elongated shape of the italian peninsula, it is often quite difficult to obtain an optimal azimuthal coverage. since the temporary network operated only for a short span fig. 3. event locations throughout the seismic sequence and seismographic stations. blue squares: ingv stations; black squares: university of calabria stations. fig. 5. local velocity model. the 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence of time, it was necessary to verify whether the locations obtained on the basis of the permanent network only could be considered satisfactory enough for an in-depth investigation of the whole sequence. to perform the check, the envisaged procedure was to select a subset of events with azimuthal gap and number of available arrival times respectively below and above given threshold values. the successive step was the comparison of their locations obtained using separately the data from the permanent and the temporary network. as a result, we verified that by using the preferred velocity model, the locations of events with gaps up to 200° and a minimum set of 10 available readings, almost never differed by more than 6 km, with the overwhelming majority falling within a 3 km difference (fig. 4). this procedure finally led to the selection of 881 events from september 1998 to october 1999 (fig. 3). the process of hypocentral location required the treatment of phase arrivals recorded both at the temporary network installed in the aftermath of the main event, and at the regional networks – ingv and university of calabria – that operated in the area (stations within 150 km from the main shock). this imposed a complex procedure to design a velocity model that could satisfy both data sets. the regional model is a 3d model – slab5 – developed at the geophysical laboratory of calabria university (guerra, 1998) that consists of a 30 × 30 km horizontal grid extending down to the depth of 550 km. it proved to yield a good match for the whole calabrian arc and surrounding regions (bruno et al., 1999). this was then integrated with an ad hoc model of the epicentral area that essentially resulted in a 1d description (fig. 5), to be inserted in the 30 × 30 km grid of the regional model. the epicentral area model was initially derived by extrapolation of seismic refraction data available in the surrounding regions (colombi et al., 1973; morelli et al., 1975; milano et al., 1989) and further refined by an iterative procedure of rms minimization. this process resulted in a family of possible models out of which we selected our preferred solution. the criteria we applied were: – depth distribution of the events consistent with the structural asset of the area, located between the nearby tyrrhenian sea and the axis of the apennine mountain chain. – pronounced clustering of the aftershocks. to better focus possible fault planes activated during the sequence, the method proposed by bossu (2000) was applied. it is based on the assumption that being 2σ the 95% confidence 999 fig. 4. histogram of the event frequency with respect to the epicentre displacements as obtained using separately temporary and permanent network data. 1000 ignazio guerra, paolo harabaglia, anna gervasi and anna b. rosa interval of an hypocentral location, it is possible to substitute the location of a given event with that arising by the average locations of all the events that fall within the confidence interval, without any net gain or loss of information. as an example, hypocentral locations of the events that occurred in spring 1999 are reported in fig. 6a,b before and after the application fig. 6a,b. comparison of the surface distribution of the epicentres a) before and b) after the application of the bossu (2001) method. here and in the following figures, colours correspond to different depth ranges. a b the 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence of the bossu method: in both cases hypocenters tend to align along an antiapenninic trending direction but this trend is better evidenced in the second case. 4. the 1998 event and the sequence the main event took place on 9th september 1998 at 11:28:00.9 utc. its localization is 1001 fig. 7. events observed on 9th september 1998. the star refers (in the following figures too) to the main event (at 11.28 utc), larger squares to the foreshocks (with relative utc timing), smaller squares refer to aftershocks locations. fig. 8. event locations in the time interval 10/09/1998-29/09/1998. 1002 ignazio guerra, paolo harabaglia, anna gervasi and anna b. rosa 39.9811°n ± 1.3 km, 16.0499°e± 1.9 km, at the depth of 14.3 ± 4.3 km. magnitude estimates vary widely – e.g., bji (ms = 5.6), isc (ms = 5.2, mb = 5.3), neic (ms = 5.2, mb = 5.2), mos (ms = = 5.3, mb = 5.9), rom (md = 5.0). the mean of the values arising from the records at the stations of the calabria university seismic network is ml = 5.0. it is noteworthy that the harvard cmt catalogue reports a moment release of 3.26∗1024 dyne cm, corresponding to a mw = = 5.6: this high value from teleseismic recordings in association with the lower value from regional ones, suggests a relatively low spectral corner frequency. it is also important to observe that the focal mechanism derived by the cmt inversion yields a normal faulting event with apenninic trend and a rather significant minor to major double couple ratio. the centroid time is set at about 5 s after the nucleation time. this is a quite long delay for a moderate sized earthquake and might indicate a rather complex rupture process. fig. 9. event locations in the time interval 30/09/1998-30/11/1998. fig. 10. fault plane solutions for 41 selected aftershocks in the time interval 12/09/1998-07/10/1998. fps for events in the same day are in an unique column, at height increasing with time. 1003 the 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence the time evolution of the energy release prior to the main shock was also rather remarkable. in the two months preceding the main event, almost no activity was observed in a wide area surrounding the epicenter of the main shock event: the catalogue of the seismic network of the calabria university reports in the area represented in fig. 3 only 4 shocks, all with ml ≤ 2.8. on 9th september 1998 a burst of foreshocks occurred (fig. 7). the first and largest one was an md = 3.7 event at 6:20 utc. other foreshocks occurred at 7.42, 9.32, 10.53 utc. there was another feature that differentiated the event spatial distribution throughout the first day of activity with respect to the rest of the sequence: both the foreshocks and the first few aftershocks were distributed in a rather wider area that embodied the main shock. from 10th september onward, the aftershocks occurred within a narrower area located to the nnw of the main event location (fig. 8). the activity that can be ascribed to the sequence lasted up to october 1999. however after june 1999 the decrement of energy release was significant. as a consequence, the bossu method can be applied only up to the end of may 1999. on this basis, taking into account the time evolution of the hypocentral location, the sequence can be divided into five major phases. the first lasted about 20 days (fig. 8). during this period most of the activity was concentrated along a narrow nnw-sse elongated area at depths between 6 and 9 km. this first phase was therefore compatible with the focal mechanism of the main event. the second phase (fig. 9) went through the end of november 1998. during this period the hypocenters delineate a fault plane that dips to the south and meet the surface in correspondence of the sapri-nocara line. the hypocenters appear to lay along an e-w oriented plane with a low dipping angle that makes hard a straightforward interpretation of the space distribution. the relatively few stable fault plane solutions that could be obtained (fig. 10) by using first arrivals polarities, show a prevalence of extensional focal mechanisms with a significant number of strike slip events: only one reflects a locally compressive stress field. the inhomogeneous orientation of the stress axes suggests that the area behaved as a transfer zone. the third phase (fig. 11) lasted through the end of january 1999 and coincided with the activation of a shear plane dipping southwestfig. 11. event locations in the time interval 01/12/1998-31/01/1999. 1004 ignazio guerra, paolo harabaglia, anna gervasi and anna b. rosa wards. this is in sharp contrast with the geological observation of a northeastward dipping fault plane (valensise and pantosti, 2001). its peculiarity is a sort of activity gap at depths of 7-8 km. the fourth phase (fig. 12) lasted till mid march 1999. it marked a return to the same regime as the second phase, with an e-w trending plane that was dipping southward. the activity level undergone an evident decrease. fig. 12. event locations in the time interval 01/02/1999-15/03/1999. fig. 13. event locations in the time interval 16/03/1999-15/06/1999. 1005 the 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence the fifth and last phase (fig. 13) was characterized by a noticeable increment of seismic activity. the tectonic element activated in this case shows an antiapenninic trend, already envisaged by moretti et al. (1994). its main feature was the southwestward deepening of hypocenters that suggests a coexistence of activity with that of the south dipping fault observed during the second and fourth phase. to characterize the time evolution of the energy and moment release, we show in fig. 14 the cumulative moment obtained by applying the relationship between magnitude and seismic moment proposed by selvaggi et al. (1997). the plot accounts for earthquakes with epicenter in the area represented in the preceding figures according to the catalogue of the geophysical laboratory of calabria university since 1997, excluding however the main shock of 9th september 1998 in order to enhance the variation before and after the begin of the sequence. arrow marks the starting date of the above described last phase. remarkable features are the perfectly flat trend in the years preceding the sequence and the sharp jump corresponding to the fifth phase. 5. conclusions we observe that the major tectonic features along the whole apenninic ridge are directed mostly in directions parallel and perpendicular to the axis of the chain and are referred to as apenninic and antiapenninic elements respectively. analogously, south of the pollino chain, the most evident tectonic features are directed fig. 14. cumulative moment release (dyne·cm∗1020) in the investigated area as resulting by considering all the events with the exclusion of the main shock. md values have been converted to moment applying the relation by selvaggi et al. (1997). arrow marks the starting date of the last phase of the seismic sequence. fig. 15. scheme of the proposed limit of the hinge between the southern apennines and the calabrian arc at crustal levels. 1006 ignazio guerra, paolo harabaglia, anna gervasi and anna b. rosa ns and ew: since in its northern sector the calabrian arc is directed ns, they result respectively parallel and perpendicular to the strike of the arc. therefore we can define them calabrian and anti-calabrian. the seismic sequence started on 9th september 1998 on the western slope of the pollino chain shows the alternate activation of tectonic elements characterized by different strike and dip, that seems to well agree with the location at the contact between different major geological domains, in such way that the characteristics of both them are evidenced: apennic, antiapenninic and anticalabrian, the latter being represented by the sapri-nocara line. the atypical character of the studied area is demonstrated by the southwestward dipping trend of the shear plane activated in the third phase of the seismic sequence, contrasting with that of the major apenninic normal structure that typically deepens northeastward. the peculiar features of the area, classifiable in the typology of transfer zones due to their heterogeneity, are also confirmed by the wide variety of focal mechanisms observed on each lineament. the major features include the e-w trending fault that was activated throughout the second and the fourth phase of the sequence and the antiapenninic fault that was activated throughout the last phase. from the moment release point of view, they seem to be the major transfer structures at the hinge between the southern apennines and the calabrian arc. this is particularly true with regard to the antiapenninic fault, whose activation at the end of the sequence, was characterized by the most intense moment release. its existence had already been suggested by moretti et al. (1994). on the basis of a few dense microearthquake sequences both at sea and on land, these authors identified an antiappeninic trending lineament on the prolonging of that activated in the last phase of the 1998-1999 sequence (fig. 15). at the time, they hypothesized the existence of a major feature that would extend for over 100 km from the submerged palinuro volcano up to the upper sinni valley (s. arcangelo basin). the observations described in the present work seem to confirm this model, so that the lineament can be more correctly called palinuro-s. arcangelo lineament. it is intersected in proximity of palinuro volcano by another major lineament, the palinuro-cetraro line (finetti and del ben, 1986; moretti et al., 1990) and its land expression, the cetraro-rossano line. since this lineament represents the southern limits where apenninic features can be observed, it turns out that the system composed by the palinuro-sant’arcangelo lineament and the palinuro cetraro-rossano line is with highest probability the one that acts as hinge between the southern apennines and the calabrian arc at crustal levels. a hint to further analysis to be carried on in future works arises by the coincidence of this surface lineament with the area where a sharp variation is found in the regime of seismic energy release by intermediate and deep earthquakes in the southern tyrrhenian sea. references amato, a. and p. montone (1997): present day stress-field and active tectonics in southern peninsular italy, geophys. j. int., 130, 519-534. bossu, r. (2000): a simple approach to constrain the position and the geometry of seismogenic structures: application to the karthala volcano (grande comores island, mozambique channel), j. seismol., 4, 41-48. bruno, g., i. guerra, a. moretti and g. neri (1999): space variations of stress along the tyrrhenian wadatibenioff zone, pure appl. geophys., 156, 667-688. cello, g., l. tortorici, e. turco, i. guerra and r. scarpa (1982): geometry of the neotectonic stress field in southern italy: geological and seismological evidence, j. struct. geol., 4 (4), 385-393. colombi, b., p. giese, g. luongo, c. morelli, m. riuscetti, s. scarascia, k.g. schütte, j. strowald and g. de visentini (1973): preliminary report on the seismic refraction profile gargano-salerno-palermopantelleria (1971), boll. geofis. teor. appl., 15, 225254. doglioni, c., p. harabaglia, g. martinelli, f. mongelli and g. zito (1996): a geodynamic model of the southern apennines, terra nova, 8, 540-547. esposito, e., i. guerra, a. marturano, g. luongo and s. porfido (1988): il terremoto dell’8 gennaio 1988 (ml = 4.1) in calabria settentrionale, in atti vii conv. ann. gr. naz. geofis. terra solida, 1637-1644. finetti, i. and a. del ben (1986): geophysical study of the tyrrhenian opening, boll. geofis. teor. appl., 28, 75-156. galli, p., d. molin and g. falzone (1998): terremoto calabro-lucano del 9 settembre 1998: rilievo macro1007 the 1998-1999 pollino (southern apennines, italy) seismic crisis: tomography of a sequence sismico preliminare (on line: http://www.serviziosismico.it/rt/ rrp/980909/terremoto.html). gasparini, c. and a. tertulliani (1988): il terremoto calabro-lucano dell’8 gennaio 1988, geol. tec., 88 (2), 44-47. gruppo di lavoro cpti (1999): catalogo parametrico dei terremoti italiani (ing, gndt, sga, ssn), (ed. compositori, bologna), pp. 92. guerra, i. (1998): programma per il calcolo dei raggi sismici in strati omogenei ad interfacce piane inclinate, tr sism98/01 (università degli studi della calabria). malinverno, a. and w.b.f. ryan (1986): extension in the tyrrhenian sea and shortening in the apennines as a result of arc migration driven by sinking of the lithosphere, tectonics, 5, 227-245. mallet, l. (1862): the great neapolitan earthquake of 1857. the first principles of observational seismology (london). milano, g., f. ferrucci. and i. guerra (1989): interpretazione preliminare di profili dss del tirreno meridionale, in atti viii conv. ann. gngts, cnr, roma, 525-533. morelli, c., p. giese, r. cassinis, b. colombi, i. guerra, g. luongo, s. scarascia and k.g. schütte (1975): crustal structure of southern italy, a seismic refraction profile between puglia-calabria-sicily, boll. geofis. teor. appl., 18, 183-210. moretti, a. (2000): il database delle faglie capaci della calabria: stato attuale delle conoscenze, in le ricerche del gndt nel campo della pericolosità sismica (1996-1999), edited by f. galadini, c. meletti and a. rebez (cnr gruppo nazionale per la difesa dai terremoti, roma), 219-226. moretti, a. and i. guerra (1997): tettonica dal messiniano ad oggi in calabria: implicazioni sulla geodinamica del sistema tirreno-arco calabro, boll. soc. geol. it., 116, 125-142. moretti, a., i. corea and i. guerra (1990): sismicità attuale e sistema di fratture superficiale in calabria, in atti conv. gndt, cnr, pisa, 1, 89-101. moretti, a., m.f. currà and i. guerra (1994): shallow seismic activity in the southeastern tyrrhenian sea and along the calabria-lucania border, southern italy, boll. geofis. teor. appl., 36, 399-409. selvaggi, g., b. castello and a. azzara (1997): spatial distribution of seismic moment release in italy (19831996): seismotectonic implications for the apennines, ann. geofis., xl (6), 1565-1578. valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger than m 5.5 in italy, ann. geofis., 44 (suppl. to 4), pp. 180 (with cd-rom). (received march 9, 2005; accepted june 7, 2005) tartaglione_finalonline:layout 6 annals of geophysics, 57, 5, 2014, a0540; doi:10.4401/ag-6466 a0540 a numerical study of the windstorm klaus: sensitivity to sea surface temperature nazario tartaglione1,2,*, rodrigo caballero1,3 1 university college dublin, school of mathematical sciences, dublin, ireland 2 current affiliation: university of camerino, school of science and technology, camerino, italy 3 current affiliation: stockholm university, dept. of meteorology and bolin center for climate research, stockholm, sweden abstract this article investigates the role of sea surface temperature (sst) as well as the effects of evaporation and moisture convergence on the evolution of cyclone klaus, which occurred on january 23 and 24, 2009. to elucidate the role of sea surface temperature (sst) and air–sea fluxes in the dynamics of the cyclone, ten hydrostatic mesoscale simulations were performed by bologna limited area model (bolam). the first one was a control experiment with european centre for medium-range weather forecasts (ecmwf) sst analysis. the nine following simulations are sensitivity experiments where the sst are obtained by adding a constant value by 1 to 9 k to the ecmwf field. results show that a warmer sea increases the surface latent heat fluxes and the moisture convergence, favoring the development of convection in the storm. convection is affected immediately by the increased sst. later on, drop of mean sea level pressure (mslp) occurs together with increasing of surface winds. the cyclone trajectory is not sensitive to change in sst differently from mslp and convective precipitation. 1. introduction windstorm klaus occurred between january 23 and 24, 2009. when the storm landed on the coast of western europe it caused heavy damage in france and spain [liberato et al. 2011]. analyzing the pressure maps and satellite images we can see that it was similar to cyclone lothar that has been the subject of several papers [e.g. wernli et al. 2002]. klaus belongs to the group of cyclones called meteorological bombs as they deepen by at least 24 hpa in 24 h [sanders and gyakum 1980]. extra-tropical cyclones tend to develop faster over the sea than over land [sanders and gyakum 1980, ulbrich et al. 2009] and because of their rapid development it is challenging to forecast correctly their genesis and evolution [sienkiewicz et al. 2004]. a possible influence of the upstream upper-level baroclinic processes on the explosive cyclogenesis of this kind of cyclones exists, as suggested by uccellini [1986] for instance. most of the meteorological bombs in the atlantic basin form just north of the gulf stream [sanders and gyakum 1980, roebber 1984], showing the fundamental role of the sea surface temperature (sst). lambert [1996] found a positive correlation between the number of northern hemisphere pacific “intense” extra-tropical cyclones and sst gradients and a weak correlation in the atlantic region. a sea surface with high temperature, beneath a cold atmosphere provides an abundant supply of heat and moisture that has a significant effect on the cyclone development [danard 1983, 1986]. the latent heat release associated with condensation of water vapour definitely plays an important role through convection and/or the moist baroclinic processes [e.g. davis and emanuel 1991, fantini 1991]. the impact of moist processes is not limited to cyclone thermodynamics but affects dynamics as well. kuo and low nam [1990] for instance found that latent heat release can promote low level vorticity in meteorological bombs. there is a huge literature on the impact of sst on tropical storms [e.g. emanuel 2005], however the effects of sst on extra-tropical cyclones has been the object of only a few case studies [e.g. danard 1986, giordani and caniaux 2001, ren et al. 2004, booth et al. 2012, ludwig et al. 2014]. indirect assessment of sst impact on extra-tropical cyclone intensity can be found in a few climatological studies, where a moderate positive impact of sst on the number of intense storms is found [semmler et al. 2008a]. climate change research also assesses indirectly the role of sst, but the results seem to be controversial. semmler et al. [2008b] and article history received november 21, 2013; accepted august 29, 2014. subject classification: atmosphere, processes and dynamics, cyclone, windstorm. leckebusch et al. [2006], for example, found a positive increase in the number of extra-tropical cyclones in a warmer climate, whereas the findings of bengtsson et al. [2006] found a reduced number of storms, especially the weaker ones and no change in cyclone intensity. however the number of intense cyclones in semmler et al. [2008b] in their control experiment is very small and the increment observed in a warmer climate was not so high as to be significant. one of the motivations for this study was to understand how possible scenarios of warmer oceans might modify the evolution and the strength of extratropical storms. the earth has already experienced a warmer climate in the past. for instance, in a recent data-model comparison for the early eocene, huber and caballero [2011] found that the best-fit model simulation had a global-mean sea surface temperature of about 31°c, over 15°c warmer than today. in this study the impact of sea surface temperature on the evolution of cyclone klaus is analyzed by means of numerical simulations. changes in surface heat fluxes and moisture convergence will also be investigated in order to evaluate their contribution to the storm’s evolution. although results presented here cannot be extended to all cases, they may give us indications of the behavior of meteorological bombs when an increase of sst is taken into account. the meteorological event together with a brief description of the model and the experimental design of the simulations is presented in section 2. the numerical results are discussed in section 3. the conclusions will be drawn in section 4. 2. synoptic description and experimental set up the data used for the synoptic description are the 6 hourly 0.5° european center for medium range weather forecast (ecmwf) analyses; they were also used for the initial and boundary conditions of the limited area model. the cyclone started as a perturbation of a frontal line that extended from the caribbean to the british isles. this perturbation can already be seen at 00 utc 23 (around 40° west, 40° south, figure 1a). along the temperature front (not shown) high values of total column water vapour (tcwv) characterized this long frontal line (figure 1b). the genesis of the cyclone occurred immediately south of the region with high 300 hpa potential vorticity (pv) values and no preliminary upper level perturbation was present over the cyclogenetic area (figure 1b). however a strong cross front shear was associated with the location of initial cyclogenesis (not shown). sinton and mechoso [1984] suggested that such shears may provide most of the energy in the development of shallow frontal cyclogenesis. figure 2 shows the synoptic situation at 00 utc 24 when the cyclone was over the bay of biscay (figure 2a), the 10 m wind speed reached its maximum intensity when the mean sea level pressure reached its minimum value. the cyclone moved along this frontal region of high tcwv. the cyclone collected water vapour (figure 2b) during its path as indicated by the cyclonic curvature of the tcwv. at that time, the cyclone was embedded in a region of high pv values (figure 2b). the interaction between forcing from upper levels, shown by high values of potential vorticity, and the underlying cyclone might be one cause of the cyclone deepening, as suggested for instance by wernli et al. [2002] for cyclone lothar or by ludwig et al. [2014] for storm xynthia. however, the hypothesis of wernli et al. [2002] was not confirmed by rivière et al. [2010] with their sensitivity experiment on storm lothar. they showed that the absence of upper level perturbation did not inhibit cyclone lothar from turning into a windstorm. the trajectory of klaus occurred along the boundary of the high pv region (figure 2b). however, tartaglione and caballero 2 figure 1. ecmwf analysis of mslp (solid line) [hpa] and 10 m wind speed [m s-1] (shaded) (a) and pv (solid line) at 300 hpa [pv unit], and tcwv (shaded) [kg m-2] (b) at 00 utc 23. 3 as suggested by dacre and gray [2009] the development of some storms are mainly “low-level” forced, thus, the role of pv on meteorological bombs is not clear. the trajectory of klaus and the evolution of its mslp minimum over time, as retrieved by ecmwf data, can be found in liberato et al. [2011]. moreover, liberato et al. [2011] showed that the mslp between 00 utc 23 and 00 utc 24 dropped by 34 hpa. decreasing more than 24 hpa in 24 hours the cyclone can be classified as a meteorological bomb. the system made landfall in france during the early morning of the 24th. after landfall the pressure increased and the strong pressure gradients led to high winds in france and spain. liberato et al. [2011] gives a complete picture of the damage in those countries. the simulations in this study was conducted by using the numerical weather model bolam (bologna limited area model) developed at the institute for the study of atmosphere and climate of the italian national research council [e.g. buzzi and foschini 2000]. a control simulation (sst0) was performed followed by nine other runs in which the sst was increased from 1 k (sst1) to 9 k (sst9). hereafter this increment will be also referred to as an “anomaly”. most of these variations may seem too large and unrealistic even in a warmer climate; however it is our intention to explore how large the effects of such sst variations might be on the development, in terms of mslp and wind speeds, of a meteorological bomb like klaus. the numerical simulations started at 00 utc 23. six-hourly 0.5° ecmwf anayses were provided as initial and boundary conditions and the model was run in hind-cast mode. the model domain was 320×162 grid points with horizontal and vertical resolutions of 30 km and 40 v-levels. even though there was no interest in analyzing the sensitivity of the simulations to the grid resolution, the model was run at two different domains and resolutions, the former with a domain covering western europe with 15 km horizontal resolution and the latter covering most of the north-atlantic basin and the western europe with 30 km horizontal resolution (figure 3). a realistic prediction of this cyclone was only obtained once the initial perturbation was in the domain, i.e. by using the second and larger domain. a numerical study of the windstorm klaus figure 3. bolam domain. figure 2. as figure 1, but at 00 utc 24. the smaller domain with a higher resolution (15 km), but not containing the initial perturbation, gave a mslp minimum of 984 hpa, larger than the 974 hpa obtained by the domain and horizontal resolution (30 km) used in this paper. since our domain is quite extensive, analyses were performed on data of a limited region of 101×101 grid points that moved along with the cyclone. 3. role of sst on storm evolution in this section the simulated mean sea level pressure, precipitation and wind speeds are compared as functions of sst as well as the modeled surface latent heat and moisture convergence. this comparison covers the first 36 hours of the storm evolution, until the cyclone was over france. 3.1. pressure, 10 m wind speed and precipitation: sensitivity to sst figure 4 shows the mean sea level pressure at 00 utc 24, for the control (sst0) and sst9 experiments over the bay of biscay, after 24 hours of simulation. the simulated cyclone trajectories of the 10 experiments (from sst0 to sst9) did not show significant differences, with the cyclone center all ending, at 00 utc 24, over the bay of biscay. this result is different, for example, from that found by giordani and caniaux [2001] where a significant change was observed in the trajectory of another cyclone when compared to the control simulation, in an experiment on warmer sea surfaces. with warmer sst, a change in cyclone dynamics and physics is expected. this change can be analyzed using a few meteorological parameters. figure 5 shows the evolution of the mslp minimum (figure 5a) and the ratio between convective and total precipitation (figure 5b), computed in the 101×101 grid points box that moves with the cyclone. the minimum mslp, at 00 utc 24, simulated by bolam, in the control run, is 10 hpa higher than that retrieved by ecmwf analysis. this might be due to the coarse resolution. however, the aim of this paper is to study sensitivity to sst rather than to provide a perfect simulation of the storm. in the model, sst influenced the convective precipitation from the beginning (figure 5b), whilst it impacted on the mslp only at a later stage in the simulations (figure 5a). the cyclogenesis was due to classic baroclinic instability, but it is very likely that later on airsea-interaction processes played an important role in deepening this meteorological bomb, as water vapour triggered convective instability. this instability is enhanced with warmer sst. the relationship between convection and cyclone deepening was noted, for example, by tracton [1973]. gyakum and barker’s research [1988] on a meteorological bomb attributes the phases of rapid deepening to convection. the relationship between high convective precipitation and the decrease in mslp of some extra-tropical cyclones might tartaglione and caballero 4 figure 5. maximum of 10 m wind speed [m s-1] (a) and ratio convective/total precipitation (b) in a domain following klaus during its evolution as a function of sst anomaly. figure 4. mslp [hpa] for control experiment sst0 (thin line) and experiment sst9 (thick line) at 00 utc 24, when the cyclone is over the bay of biscay. 5 be related to the cisk [danard 1983] or to a similar mechanism like the moist quasi-geostrophic baroclinic instability formulated by mak [1982]. the intense latentheat released during the condensation of water vapour in the convection may further induce cyclone deepening. the deeper pressure minimum and the corresponding increased pressure gradient favor a more intense low-level wind speed and, consequently, sea surface fluxes, inducing a positive feedback [emanuel et al. 1994]. thus, the warmer the sea surface, the more vigorous the feedback mechanism and the deeper the cyclone. figure 5a shows that mslp variation with sst is not linear. during the first 12 hours the impact of sst on the mslp is not significant, but it becomes more important over time. when the temperature is larger than 5 k the mslp drop increases significantly with the sst, suggesting a non-linear effect due to water vapour associated with latent heat released by condensation of water vapour during convection. in fact, the convective precipitation, for anomalies of 5 k and above, is a significant part of the total precipitation (figure 5b). the sea surface is the main source of the water vapour, whose condensation within the cyclone provides the energy necessary to increase the intensity of the cyclone. when the cyclone was over the bay of biscay, the mslp reached the minimum value, whereas over the land the cyclone weakened. figure 6 shows mslp and wind speed over the bay of biscay. the cyclone reached its maximum intensity over the bay of biscay, between 00 utc and 06 utc 24 (figure 6). in this case a fixed box (21×21 grid points) covering the bay of biscay was used to obtain the values shown in figure 6. close to french coastline the roughness difference between sea and land increases the pressure gradient causing strong winds. however wind speeds are higher with a warmer sea because it favors cyclone deepening (figure 6b). 3.2. surface latent heat fluxes air-sea interaction has long been recognized as one of the factors leading to explosive cyclones. the oceans provide heat, through sensible and latent surface heat fluxes, and water vapour through the evapoa numerical study of the windstorm klaus figure 6. minimum of mslp [hpa] (a) and maximum of 10 m wind speed [m s-1] (b) over the bay of biscay as a function of sst anomaly. figure 7. surface latent heat flux [w m-2] at 12 utc 23 for simulations sst0 (a) and sst9 (b). the sign x indicates the cyclone center, the vertical line is for the cross section. ration and convergence of moisture. evaporation is associated with positive surface latent heat flux, i.e. the ocean provides energy to the atmosphere. figure 7 shows the surface latent heat flux for simulations sst0 and sst9 at 00 utc 24. the surface latent heat fluxes in the control simulation are weak (figure 7a) compared to the sst9 experiment (figure 7b). with larger sst anomalies there are high values of surface latent heat fluxes very close to the cyclone (figure 7b), in fact two surface latent heat flux maxima are obvious, one at the south-east, and the other just in the wake of the cyclone in accordance with the results found by petterssen et al. [1962]. the larger the sst anomalies, the higher the surface latent heat fluxes; an unsurprising result, not only because of a warmer sea, but also because of the deeper cyclone and higher wind speed. the highest values of surface latent heat fluxes close to the cyclone center reach about 1200 w m-2 in the sst9 experiment (figure 7b), exceeding those of the control simulation (figure 7a). these values correspond to about 1.7 mm h-1 in terms of evaporation rate. the surface latent heat fluxes are strong in the region of the cold air outbreak behind the atmospheric front. as we will see afterward 1.7 mm h-1 is much less than the contribution of the moisture convergence to the water vapour. figures 8 for sst0 and 9 for sst9 show meridional vertical cross sections along the vertical line in figure 7 at 00 utc 24, showing: potential temperature and vertical velocity (omega) (panel a); wind zonal component (panel b); wind meridional component (panel c); and specific humidity (panel d). after 24 hours of simulation we observe how the adiabatic flow follows the potential temperature contours that come from the south towards the cyclone center. in the sst0 simulation, a region of strong vertical velocity is visible in the center of the cyclone (figure 8a). sst9 shows a warmer atmosphere because of the energy that oceans exchange with the atmosphere. moreover, there are more updrafts in sst9, not only at the center of the cyclone, but also in areas to the north and south of the cyclone (figure 9a). for the sst0 and sst9 the potential temperature shows a cooling region just south of the main updraft, but in the sst9 this cooling is more marked, probably due to more rainfall evaporation. sst0 wind components (figures 8b and 8c) are stronger and gradients larger than sst9 winds (figures 9b and 9c). convergent winds together with moisture advection are favorable conditions for convection. the specific humidity values in sst9 (figure 9d) are larger and extend higher altitudes than sst0 (figure 8d). tartaglione and caballero 6 figure 8. vertical cross section along the meridional direction (see figure 7) of potential temperature (solid line) [k] and vertical velocity (omega) (dashdot line) [pa s-1] (a), wind zonal component (b) and wind meridional component (c) [m s-1] and specific humidity [g kg-1] (d) at 00 utc 24 for the experiment sst0. 7 3.3. moisture convergence there are two mechanisms that provide humidity in the atmospheric boundary layer: evaporation and moisture convergence. evaporation from the sea is related to the surface latent heat flux, as shown in the previous section. moisture convergence may be written as (1) where the vector vf defined as the friction free wind in the atmospheric boundary layer, vf is computed by means of the following formula: where k is the vector along the z direction. the symbol ∇ represents the divergence operator, q represents the specific humidity, f represents the coriolis parameter and g represents gravity acceleration. the first term h on the right-hand side of (1) represents vertically-integrated low-level moisture convergence. integration in the boundary layer is calculated using the pressure p variable. the term w relates to the rotational term of moisture convergence, where the vector x represents surface stress. the moisture convergence measure unit is kg m-2 s-1. it is easy to show that kg m-2 is equivalent to m v q g dp k f q h wc f pbl 0 $ $ #d d x= + = +# v v f k dt dv f g #= + a numerical study of the windstorm klaus figure 9. as figure 8 but for the experiment sst9. figure 10. the contribution of w [mm h-1] to the moisture convergence (equation 1) at 00 utc 24 for experiments sst0 (a) and sst9 (b). mm of water: multiplying and dividing by 3600, kg m-2 s-1 transforms into mm h-1. the use of mm h-1 gives an immediate insight into the effects of moisture convergence on precipitation, although not all the moisture transforms into precipitation. the analysis of both terms w and h shows that the term w is much larger than h. at 00 utc 24 w is about 5 mm h-1 for the sst0 experiment, whereas it reaches 20 mm h-1 for sst9 (figure 10). these values are much larger than those obtained by evaporation (1.7 mm h-1). the term h is an order of magnitude less than w and it has values of 1 mm h-1 at few grid points and only for sst8 and sst9. once more vapour reaches the atmosphere it contributes to the diabatic processes that in cyclones like klaus play an important role. fink et al. [2012] found that, for the windstorm klaus, the contribution of diabatic heating to cyclone deepening was larger than that of baroclinic instability. 4. conclusions in this paper, the sensitivity of cyclone klaus to the sst was studied by mean of limited area model initialized by ecmwf analysis. ten simulations were performed, the first one with ecmwf sst field (sst0 simulation) and nine other simulations using sst analysis raised by 1 to 9 k. the modeled atmospheric parameters: precipitation, mslp and contributions to water vapour were compared. they showed very different behavior with respect to the sst. increasing sst leads to more evaporation, enhancing convection in the cyclone. the surface latent heat fluxes, higher in sst9 than sst0, modify the boundary layer triggering an efficient mechanism for convective destabilization. the latent heat released by condensation of water vapour, during convective processes, has an effect on the drop in mslp. moreover, whereas convection is immediately influenced by sst and precipitation becomes more convective in sst with higher temperatures, the drop in mslp is only affected by higher sst at a later stage. it takes some time for sst to have a significant effect on the mslp (figure 5a). the importance of sst on extra-tropical storms has recently been demonstrated by storm xynthia [ludwig et al. 2014]. increasing sst leads to more moisture in the atmosphere contributing to increase the latent heat released by water vapour condensation. the importance of diabatic processes on storm development is recognized by several studies [e.g. fink et al. 2012, ludwig et al. 2014]. high surface latent heat fluxes, which increase with sst, are necessary for the initial storm intensification, but the contribution of evaporation is less than that of moisture convergence; hence once the storm starts to produce strong winds, these increase water vapour also through moisture convergence. thus, both evaporation and moisture convergence play an important role in cyclone intensification through diabatic processes, especially in experiments with high sst anomalies. acknowledgements. this work has been funded by science foundation ireland under the contract sfi-geof252 “present and future statistics of extreme european storms in a large ensemble of high-resolution atmosphere model simulations”. irish centre for high-end computing (ichec) is acknowledged for the computing time used to run the model. the authors acknowledge three anonymous reviewers for their useful comments and suggestions; a special thank goes to two of reviewers for their significant corrections they made to the manuscript. andrea buzzi and piero malguzzi are acknowledged for bolam. references bengtsson, l., k.i. hodges and e. roeckner (2006). storm tracks and climate change, j. clim., 19, 35183543. booth, j.f., l. thompson, j. patoux and a.k. kathryn (2012). sensitivity of midlatitude storm intensification to perturbations in the sea surface temperature near the gulf stream, mon. wea. rev., 140, 12411256. buzzi, a., and l. foschini (2000). mesoscale meteorological features associated with heavy precipitation in the southern alpine region, meteorol. atmos. phys., 72, 131-146. danard, m.b. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice s u l l e o n d e d i l o v e paolo e m i l i o v a l l e f i ' n o t o f i l e n e l l a f a s e m a s s i m a di u n t e r r e m o t o a p p a i o n o o n d e t r a s v e r s a l i t a n g e n z i a l i . l o v e n e s p i e g ò l ' e s i s t e n z a f o r m u l a n d o u n a t e o r i a c h e si b a s a sull ' i p o t e s i di i m a s t r a t i f i c a z i o n e del m e z z o i n c u i a v v i e n e la p r o p a g a z i o n e . l o v e p e r ò n o n t i e n e c o n t o d e l l e a z i o n i d i s s i p a t i v e , p e r t a n t o b o r i t e n u t o o p p o r t u n o r i f a r e li1 t e o r i a n e l l ' i p o t e s i c l i c il m e z z o s t r a t i f i c a t o p r e s e n t i a t t r i t o i n t e r n o . si c o n s i d e r i i l s o l i l o r i f e r i m e n to c a r t e s i a n o p o s t o c o m e in fig. 1 . u n ' o n d a e l e m e n t a r e p i a n a t r a s v e r s a l e t a n g e n z i a l e si p r o p a g h i nel v e r s o n e g a t i v o d e l l ' a s s e x. d e t t e o , , i i n u ' ( la d e n s i t à e l e c o s t a n t i e l a s t i c h e del p r i m o m e z z o , n 2 , u„, u'o l e a n a l o g h e g r a n d e z z e r e l a t i v e al s e c o n d o m e z nel p r i m o m e z z o d o v r à s o d d i s f a r e a l l ' e q u a z i o n e i , a„ l/'4i i ' , a., v : di ~ : l'i 3te n e l s e c o n d o m e z z o a q u e l l ' a l t r a e q u a z i o n e : ilo a 2 i > + i i ' g — = (_>„ di d2v dio . lo s p o s t a m e n t o v [1] [2] l e c o n d i z i o n i a i c u i d e b b o n o s o d d i s f a r e l o s p o s t a m e n t o e la t e n s i o n e t, p e r s = 0 s a r a n n o : v o l e ! p i i m o m e z z o ) = i ; ( n e l s e c o n d o m e z z o ) t 1 ( n e l p r i m o m e z z o ) = t 1 ( n e l s e c o n d o m e z z o ) [ 3 ] e p e r z~-d. d o v e con d si è i n d i c a t o l o s p e s s o r e d e l l o s t r a t o : t , = 0 [4] s u l l e o n d e di l o v e 3 6 1 l ' e s p r e s s i o n i d e l l a t e n s i o n e t l = o n o : t dv d dv . / = 1 1 (u , n e l p r u n o m e z z o 3= dt dz sv 3 dv 1 , = u„ [— il n e l s e c o n d o m e z z o ' ~ 3 z 3 / 3 = [ 5 ] s i è p r e m e s s o d i p r e n d e r e i n c o n s i d e r a z i o n e o s c i l l a z i o n i l i b e r e e l e m e n t a r i , p o r r e m o q u i n d i : j ( p t + f * ) « = f ( z ) e s o s t i t u e n d o la [ 6 ] n e l l a [ 1 ] , si h a : [6] d• v dzv f qip" 0 ih+j^'ip , e q u i n d i n e l p r i m o m e z z o l o s p o s t a m e n t o a s s u m e l a f o r m a : j j ( p « r r *) ( f — k , ) z — i f-—k/1 z ae + be s o s t i t u e n d o la [ 6 ] n e l l a [ 2 ] i si o t t i e n e : [ 7 ] d-v v i qipdze q u i n d i nel s e c o n d o m e z z o : l ' s + z v s p (f3—k.,) ~z — • f ; k z ce +ee j ( p t + f x ) [8] s u p p o n e n d o s o d d i s f a t t a la c o n d i z i o n e c l i c l a p a r t e r e a l e di (/~—k«) sìa p o s i t i v a , si d e v e p o r r e n e l l a [ 3 ] e — o, se si v u o l e c h e l ' a m p i e z z a d e l l o s p o s t a m e n t o d e c r e s c a c o n l a p r o f o n d i t à . (f-—k2) ir z j(i»-f*) v — ce e p e r t a n t o : n e l l a [ 7 ] e n e l l a [ 8 ' ] si è p o s t o : r i c o r d a n d o c h e : ! ' s + . / v « p [ 8 ' ] [ 9 ] — = v-l3 , — =,v-„s , i i = r, v i s , — = t 2 v-., [10j t'i ciì l'i ( ? 3 3 6 2 paolo e m i l i o v a l l e si h a : a-, = — — — , /, — £ t u ] v ^ a l + j x j ) ) l y j i + j r , p ) i n t r o d u c e n d o la [ 7 ] e la [ 8 " ] n e l l e [ 5 ] e i m p o n e n d o le c o n d i z i o n i [ 3 ] e [-1] -i o t t i e n e il s i s t e m a : \--li c j ( u . f y ' p ' . p ) (/--/'•, p m — b ) = (ilo+yii'op)!/3— [ l ^ + y n ' i p ) (_/s-a-,)' (.-•/ p —b,> ) = 0 [12] s u p p o n e n d o p e r c o m o d i l a 1 = 1 , r i s u l t a : li = e a\, l'i i f' — , { i ) ~ ( f 5 — k , ) 4 d -t ( f — k . ì r r d p p = -, , [ 1 3 ] /. , /••• i .4" — ( f " — ( f — k , ) t £> h [ p 2 ( / ' — a'.,) e — e le p r i m e d u e e q u a z i o n i | ih] c o n s e n t o n o di c a l c o l a r e li e c d a l l a t e r z a , ; c si s u p p o n e d a t o /. si r i c a v a />. c h e in g e n e r a l e «ari c o m p l e s s a e si h a u n ' o n d a s m o r z a t a , m e n t r e = e si s u p j ) o n e d a l a j> < si r i c a v a /. la sua p a r i e r e a l e s a r à p r o p o r z i o n a l e al r e c i p r o c o d e l l ; l u n g h e z z a d ' o n d a , m e n t r e il c o e f f i c i e n t e d e l l ' i m m a g i n a r i o r a p p r e s e n t e r ì il coefficiente ili a s s o r b i m e n t o . si lui d i s p e r s i o n e in e n t r a m b i i ea-i l a [ 1 3 ] sj p u ò s c r i v e r e : ^ i i . / v ' . m l + y r . p ) / d 4 ] 1.1.e o.mii. ni k i m : 3 6 ! s e ora n e l l a | 1 4 ] n o n e t , = r 2 = 0 -i o t t i e n e la e l a m i c a equa/.u di l o v e : h. i >'' _ p v t \ vis t a n g c j ? — _/a\7 [ 1 5 ] • ì» p e r ] ) ( ) . d a l l a [ 1 4 ] e dalla [ 1 5 ] risulta r i s p e t t i v a m e n t e : / ' ' i l , / ' ' = 0 [ | ( , ] i ' g , 2 ( l + y t s p ) r „ s 8 e si ila u n ' o n d a t a n g e n z i a l e trasversale non s m o r z a l a lungo l'anse z. i l c a l c o l o n u m e r i c o d e l l a [ 1 4 ] è t a t o p r o p o s t o a l l ' i s t i t u t o p e r le a p p l i c a z i o n i del c a l c o l o in b a s e a l l e eostanti c l a s t i c h e e ai val o r i p i ù p r o b a b i l i degli spessori d e l l e -i rat i d e a z i o n i d e l l a crosta terr e s t r e . roma — istillilo nazionale eli (,col*.sica — febbraio 1918. ri assunto l' i. estende la teoria delle onde su/terfidali trasversali-tangenziali <di love al caso in cui d mezzo strali ficaio pi esenti nitrito intento. vol51,2_3,2008 461 annals of geophysics, vol. 51, n. 2/3, april/june 2008 key words data inversion – geodesy – coseismic displacement 1. introduction the 1997 umbria-marche sequence includes two moderate magnitude earthquakes (september 26th, mw = 5.8 and mw = 6.0) that occurred near the village of colfiorito followed by a sequence of aftershocks lasted several months. the aftershock activity included a mw = 5.6 earthquake (that occurred near the village of sellano on october 14th) and three events with magnitude larger than 5.2. the colfiorito sequence has been extensively studied using geologic data, strong motion data, teleseismic and regional waveforms, gps, levelling and sar data (see e.g. cinti et al., 1999; capuano et al., 2000; pino et al., 1999; hunstad et al., 1999; lundgren and stramondo, 2002; de martini et al., 2003; crippa et al., 2006). larger shocks share sw strikes and normal fault mechanisms. after the september 26th event, 12 monuments from the igmi95 gps network within 30 km off the epicentre were reoccupied, including foli and colf that were occupied continuously during the survey. coseismic diplacements were obtained by comparing the 1997 survey coordinates with those collected in 1995. two sites (croc, -13.8 cm n and -1.9 cm e; penn, 7.6 cm n and 8.0 cm e) showed inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria antonella amoruso and luca crescentini dipartimento di fisica «e.r. caianiello», università degli studi di salerno, italy abstract the 1997 september-october umbria-marche sequence has been extensively studied in the past by analyzing coseismic displacement data (gps, leveling, sar). here we focus on synthetic data representative of the main event of the 1997 umbria-marche sequence and investigate the effects of a crustal layering proper to the colfiorito area on surface displacements and inferred source features when inverting coseismic geodetic data without taking into account layering. we compare bootstrapping and na-bayes as tools for parameter uncertainty assessment and show how the akaike information criterion can be used to select the model which is most likely to be correct. since sar images offer the most complete coverage of the study area, we use synthetic line-ofsight displacement data. mailing address: dr. antonella amoruso, dipartimento di fisica «e.r. caianiello», università degli studi di salerno, via s. allende, 84081 baronissi (sa), italy; e-mail: antonella.amoruso@sa.infn.it vol51,2_3,2008 4-03-2009 10:28 pagina 461 462 a. amoruso and l. crescentini significant horizontal displacements and one (croc, -24.7 cm) showed significant vertical displacements (hunstad et al., 1999). the epicentral area is crossed by line 21 of the italian first-order levelling network (operated by igm), which runs from foligno, central umbria, to fiumesino, on the marche coast; the first (southernmost) section of line 21 trends almost n-s between foligno and fossato di vico and includes about 40 bench marks. the route was measured in 1951, 1992 and was resurveyed in 1998. the data showed maximum subsidence of 7.8 cm, and maximum uplift of 1.6 cm (de martini et al., 2003). differential interferograms related to the 1997 colfiorito events have been obtained from ers-1 and ers-2. because of the strong topographic relief of the apennines and the significant vegetation cover of the epicentral area, even the best interferograms show large decorrelation areas (salvi et al., 2000). for this reason, even though a large number of sar data are available from july 1993 to october 1997, only few significant interferograms have been obtained, covering different periods of the sequence. the most studied interferogram is the 35-day ers-2 one (e.g. stramondo et al., 1999; crippa et al., 2006) covering the period 7 september to 12 october 1997. geodetic data are usually affected by both correlated and uncorrelated noise. for example, levelling data are obtained by measuring the elevation difference between consecutive bench marks (section height differences). bench mark elevations with respect to the reference bench mark can then be computed, and are often used instead of section height differences for modeling. this operation introduces nondiagonal terms in the covariance matrix of bench mark elevation differences, whose diagonal terms are related to uncorrelated noise (e.g. amoruso and crescentini, 2007). as regards sar data, the unwrapped differential phase is also affected by both uncorrelated and correlated noise. correlated noise is caused by atmospheric disturbances, and can be expressed by an exponential autocorrelation function. this noise can be handled following two different schemes: using a non-diagonal covariance matrix in the data inversion procedure (fukushima et al., 2005) or removing from the interferogram the influence of atmospheric heterogeneities after considering stable areas close to the epicentral area to analyse the spatial autocorrelation (e.g. crosetto et al., 2002). we prefer the former scheme, since, as stressed in crosetto et al. (2002) the removal procedure «suffers for its intrinsic limitation related to the non-stationarity of the analysed signal, which practically confines its use to the vicinity of the stable areas». to give an idea of overall noise in sar data related to the colfiorito events, lundgren and stramondo (2002) assigned a standard deviation of 1 cm to each contoured data point (i.e., points of line-of-sight, los, displacement contours at 28 mm intervals). if both correlated and uncorrelated noise is present, measured data are not statistically independent. however, the data covariance matrix is symmetric and can be reduced to diagonal form by means of a rotation matrix; the same rotation transforms the data to independent form (i.e. a set of statistically independent data, which are linear combinations of the measured ones). the eigenvalues of the covariance matrix give the uncertainties of the transformed independent data. this procedure can also be used in case of non-normal distribution of errors and consequently for robust fitting (amoruso and crescentini, 2007). model parameters are usually obtained from experimental data through the appraisal of a cost function which measures the disagreement between data and model predictions. several different causes contribute to differences between predictions and observations (the residuals of the retrieved model): measurement errors, intrinsic misfits (e.g. misfits due to heterogeneities in slip distribution, non-planar faults, heterogeneities in the elastic properties of the medium, when the adopted model is too simple, like a planar uniform-slipping fault in a homogeneous half-space), local response effects (e.g. soil compaction, landslides, topographic effects). for example, amoruso et al. (2004) generated synthetic geodetic coseismic data (gps, sar, and levellings) in a layered medium and performed inversions of the synthetics, under the assumption of homogeneous half-space. horizontal displacements are more vol51,2_3,2008 4-03-2009 10:28 pagina 462 463 inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria affected than vertical ones by the presence of a superficial soft layer, but differences with synthetics generated in a homogeneous half-space did not show any simple regular behaviour, even if a few features could be identified. consequently, also retrieved parameters of the homogeneous equivalent fault obtained by unconstrained inversion of surface displacements did not show a simple regular behaviour. amoruso et al. (2004) pointed out that the presence of a superficial layer may lead to misestimating several fault parameters both using joint and separate inversions of the three components of synthetic displacement. the effects of the presence of the superficial layer depend on whether all fault parameters are left free in the inversions or some of them are fixed a priori. in the inversion of any kind of coseismic geodetic data, fault size and slip could be largely misestimated, but the seismic potency (product of ruptured area and average slip) is well determined (within a few per cent) in most cases even neglecting layering. of course, the amount of the effect of the superficial low-rigidity layer depend on the contrast of the elastic parameters. for example, battaglia and segall (2004) and crescentini and amoruso (2007) have shown that layering effects are negligible and relevant respectively when considering two different volcanic environments and layering properties (long valley and campi flegrei). assessing the effects of layering is a case-to-case problem. intrinsic misfits and local response effects, as well as possible systematic errors, could be spatially coherent over small or large areas, and are not zero-mean random variables. rigorous handling of data should take into account, and properly treat, all error sources (measurement errors, systematic errors, intrinsic misfits and so on), after a reliable a priori estimate of all the errors, and the determination of how much different error sources effectively affect data. this is quite a hard task, largely case-to-case dependent. in case of levelling data, if measurement errors can be reliably estimated, after inverting the transformed independent data, reduced chi-square of residuals can be computed and used to refine uncorrelated error variance when the number of degrees of freedom is easily computable, e.g. in case of one uniform-slip rectangular fault in a homogeneous half-space (amoruso and crescentini, 2007). the cost function is usually obtained from maximum-likelihood arguments according to the assumed statistical distribution of the residuals. misfit is written in terms of the transformed independent data set, and, usually, either the mean squared deviation of residuals (chi-square fitting, m2, proper for normally distributed errors) or the mean absolute deviation of residuals m1, proper for two-sided-exponentially distributed errors and robust fitting) are used. if possible, it is preferable to compare results obtained using m1 and m2. estimates of the model parameters can be obtained by the best-fitting approach, i.e. the minimization of the cost function. when inverting geodetic data, the cost function depends nonlinearly on parameters and is characterized by a rough landscape and several local minima. monte carlo techniques are the only efficient tool to search for the global minimum, but require computing a large number of forward models (data prediction given a set of model parameters). thus the use of fast codes for forward modelling is preferable if not essential. among the monte carlo global optimization methods (e.g. uniform sampling, genetic algorithms, simulated annealing) we prefer to use adaptive simulating annealing (asa, ingber 1993). generally speaking, each step of any simulated-annealing algorithm replaces the current point in the parameter space by a random «nearby» point, chosen with a probability that depends on the difference between the corresponding costfunction values and on a global parameter (t) that is gradually decreased during the process according to some annealing schedule. asa considers that different parameters might require different annealing schedules. the exponential annealing schedules used by asa ensure ample global searching in the first phases of search and ample quick convergence in the final phases. even if success in finding the global minimum is never guaranteed and inversions from different starting points and/or using different program options can lead to different endpoints, the ability of asa to self optimize its program options recursively makes it a very powerful technique. vol51,2_3,2008 4-03-2009 10:28 pagina 463 464 a. amoruso and l. crescentini the best-fitting approach does not give parameter uncertainties and consequently is of limited significance. a commonly used technique (associated with global minimization) to assess acceptable parameter ranges consists in the use of the bootstrap percentile method. this method applies the best-fit technique to a large number of synthetic data sets, obtained by randomly resampling from the actual data set with replacement and is often used to estimate confidence intervals without making assumptions about the underlying statistics of errors. bootstrapping also estimates correlations between different parameters, by forming a scatter plot of all the estimates for each parameter pair and visualizing the correlations between the parameter pairs (e.g. amoruso et al., 2005). the computed values for the model parameters from the synthetic data sets form an estimate of the sampling distribution of the parameter values, independently from the underlying statistics. this method gives reliable results only if the hypothesis of independent identically-distributed data is verified. as a consequence, in the presence of a non-diagonal data covariance matrix, bootstrapping can only be applied to the independent rotated data, not the measured data. a different approach is used in the neighbourhood algorithm (na-sampler, sambridge, 1999a). na generates ensembles of models which preferentially sample the good data-fitting regions of the parameter space, rather than seeking a single optimal model. the algorithm makes use of only the rank of a data fit criterion rather than the numerical value; in other words, points in the parameter space are listed according to their capability to fit the data (answering the question «does point a give a better fit to the data than point b?») without quantifying the difference in a precise way. with this rank-based approach, the weight of each of the previous sampled points in driving the search depends only on their position in the rank list. the companion code na-bayes (nab, sambridge 1999b) consists of an algorithm for using the entire ensemble of models produced by na, and deriving information from them in the form of bayesian measures of resolution, covariance and marginal probability density functions (pdf) etc. the input ensemble in principle can follow any distribution and be generated by any search method (e.g. the na-sampler algorithm, a genetic algorithm or simulated annealing algorithm). in any case, we assume a priori a physical model to explain the data. in principle, any set of data can be almost perfectly fit by using a sufficiently complicated model, but it could be unrealistic and overmodelling (i.e., using more parameters than necessary) has to be avoided. attempts to find the model that best explains the data with a minimum number of parameters accomplish the parsimony principle. if nested models (the more complicated model includes the simpler one as a particular case) have to be compared, the f-test approach could be used. the f-test is based on traditional hypothesis testing. under the normal distribution hyphothesis, f-test compares the differences in χ2 between nested models with the difference expected by chance, i.e. it quantifies the relationship between the improvement in χ2 and the relative increase in degrees of freedom g. the null hypothesis is that the simpler model is correct, i.e. that none of the additional parameters is significantly correlated with the model. unfortunately, the test is unreliable if residuals depart from the normal distribution even slightly. moreover, the f-test is not valid if the models are not nested or it is difficult to estimate the actual number of g (as in the case of distributed-slip faults, due to the presence of constraints like smoothness or slip positivity). the akaike information criterion (aic, akaike, 1974) is a measure of the goodness of fit of an estimated statistical model. it is based on information theory and does not use hypothesis testing, so there is no conclusion about statistical significance and rejection of a model. aic attempts to find the model that best explains the data with a minimum number of parameters taking into account both the value of the likelihood function and the number of parameters in the model, i.e. trading off the complexity of an estimated model against how well the model fits the data. aic includes a penalty that is an increasing function of the number of estimated parameters. the model with the smallest aic is most likely to be correct. for a small number of observations, a second order vol51,2_3,2008 4-03-2009 10:28 pagina 464 465 inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria correction is added to the general aic; corrected aic (aicc) converges to aic as the number of observations increases (e.g. hurvich and tsai, 1989). in addition, the difficulty of estimating the actual number of degrees of freedom can be surmounted by using the akaike information criterion. even if the results are not automatically generalizable, amoruso and crescentini (2007) have shown, in case of levelling measurements and the one-fault model of the 1908 messina earthquake, that the akaike information criterion is not only an effective tool to discriminate if increasing the number of subfaults in a distributed-slip model really improves the model or does not, but also to estimate uncorrelated errors, which partially reflect model inadequacy. here we investigate the effects of a crustal layering proper to the colfiorito area on surface displacements and inferred source features when inverting synthetic geodetic data representative of the main event of the 1997 umbriamarche sequence. we compare different methods for parameter uncertainty assessment and we show how the aicc can be used to select the distributed-slip model which is the most likely to be correct as a result of the inversion of available data. since sar images offer the most complete coverage of the study area, we use synthetic los displacement data. 2. generation and inversion of synthetic coseismic displacement data 2.1. generation to generate synthetic displacements we use the slip distribution in fig. 1, which resembles the recently published distributed-slip model of the main event in crippa et al. (2006). the source fault (divided into 100-along-strike and 80-along-dip subfaults) is embedded both in a homogeneous half-space (hhs; okada, 1985) 0 1 m 0 1 2 3 4 5 6 7 8 a lo n g − d ip d is ta n ce (k m ) 0 1 2 3 4 5 6 7 8 9 10 along−strike distance (km) fig. 1. slip distribution on the source fault. vol51,2_3,2008 4-03-2009 10:28 pagina 465 466 a. amoruso and l. crescentini and the layered half-space (lhs; wang et al., 2006) detailed in table i (megna, 2007). strike, dip and rake are 138°, 45°, and -75° respectively. we then add uncorrelated random noise. as previously mentioned (see introduction) atmospheric disturbances generate correlated random noise that can be expressed as an exponential covariance function and would require transforming the data to independent form, but treatment of the effects of atmospheric disturbances is beyond the aims of this paper. displacements (northward, eastward, vertical) were computed on a grid spanning 15000 m toward north, step 100 m, and 20000 m toward east, step 100 m. we simulated sar-like unwrapped data through a proper combination of the displacement components taking into account the ers2 los. los displacement (positive toward the satellite) maps for hhs and lhs are in fig. 2a and fig. 2b respectively. figure 2c shows the residuals of the linear least squares regression between lhs and hhs displacements. as expected (e.g. amoruso et al., 2004) residuals are about 15% of the signal, and, being obtained using a linear best fit, are not merely due to a translation or a scale factor. lhs los displacement after the addition of noise (generated using random variates from a normal distribution with zero-mean and 1-cmstandard deviation) is shown in fig. 3. to reduce the number of data in the inversions, the displacement map was decimated (undersampling interval = 3 bins). even if noise added to synthetics was obtained from a normal distribution, model inadequacy is expected to enlarge tails of the residual distribution with respect to the normal distribution and we minimize the mean absolute deviation of residuals m1, commonly used for robust fitting: (2.1) where n is the number of data and vi is the coseismic change estimated for element i of the data set. the data set must be composed of statistically independent data: if they are not (i.e. the covariance matrix is not diagonal) then they have to be transformed accordingly. here experimental data are not rotated because of the absence of correlated noise. model predictions v–i (a) depend on model parameters a, and wi are the uncertainties of the independent data. 2.2. inversion, uniform-slip model at first, we invert noisy lhs los displacements for a uniform-slip rectangular fault embedded both in hhs and in lhs using asa. this model involves a parameter set a which consists of the strike and dip angles of the fault, the rake angle of the slip vector, two geometrical fault dimensions (along-strike length and along-dip width), location of start point of the fault upper side along the strike direction (x, positive northward; y, positive eastward), depth of the upper side of the fault (z, positive downward), and magnitude of the slip. figure 4 shows the two best-fit uniform-slip faults over the distributed-slip source fault. potency of the best-fit faults is the same inside 2% (� 2.65 × 107 m3), about 9% lower than the potency of the source fault (2.89 × 107 m3). the ( )a m w v v 1 1 i i i i n = = r/ table i. multilayered model used in this work (megna, 2007). depth (km) vp (m/s) vs (m/s) density (kg/m3) 0-1 2300 1200 2300 1-2 5400 2500 2600 2-8 6300 3400 2800 >8 5500 3000 2840 vol51,2_3,2008 4-03-2009 10:28 pagina 466 467 inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria only noticeable difference between the two best-fit faults consists in the expected focusing effect in the hhs: while the lhs fault is centered on the slip patch of the source fault, the hhs fault is shifted by about 700 m upward along the dip direction, i.e. 500 m in depth. pdfs of model parameters were estimated using both bootstrapping and nab. strictly speaking, a full optimization procedure should be performed at each resample in the bootstrap procedure, but in our case computation time would be unacceptable. in this work we use a faster method (downhill simplex; press et al., 1992), starting from the asa best-fit model to explore the parameter region surrounding it. as regards nab, the input ensemble has been generated by na. figure 5 shows estimated pdfs as well as best-fit parameter values. the true fig. 2. map of synthetic los displacements generated in the hhs (left) and lhs (middle). right, map of the residuals of the best-fit linear regression between los displacements in the hhs and lhs. fig. 3. map of noisy synthetic los displacements generated in the lhs. horizontal projection of the source fault in black. −29 5 cm 0 5 10 15 km 0 5 10 15 20 km noisy los displacement −26 1 cm 0 5 10 15 km 0 5 10 15 20 km los displacement (homogeneous) −27 2 cm 0 5 10 15 km 0 5 10 15 20 km los displacement (layered) −15 29 mm 0 5 10 15 km 0 5 10 15 20 km los displacement (difference) vol51,2_3,2008 4-03-2009 10:28 pagina 467 468 a. amoruso and l. crescentini source values are also shown when pertinent, i.e. for strike, dip, and rake. the best-fit values and pdfs are in agreement with the true values, but strike is somewhat overestimated, owing to the uniform-slip approximation, as it will be clarified below. pdfs obtained using the two techniques are generally in mutual agreement; apparently larger differences relate to depth of the fault upper side and along-dip width in the lhs case. however, fig. 6 shows that there is a trade-off between them and the trade-off pattern is very consistent between bootstrapping and nab. 2.3. inversion, distributed-slip models as a second step, we consider a small number of coplanar subfaults (2 along strike and 2 along dip, numbered along the strike direction, starting from the shallower part of the fault), which slip independently at the same rake angle, thus adding three additional parameters to the model. figure 7 shows the two best-fit 2 × 2 faults over the distributed-slip source fault. potency of the best-fit faults is the same inside 4% (� 2.6 × 107 m3), about 10% lower than the potency of the source fault (2.89 × 107 m3). as in the case of the uniform-slip model, the only noticeable difference between the two best-fit faults consists in the focusing effect. once again, pdfs of model parameters were estimated using both bootstrapping and nab, as detailed in the case of the uniform-slip fault. figure 8 shows estimated pdfs as well as best-fit parameter values and suggests similar comments with respect to the uniform-slip model. pdfs from nab are more scattered fig. 4. best fit uniform-slip fault in the hhs (thin-line rectangle) and lhs (tick-line rectangle) over the distributed-slip source fault. numbers give slip. 0.84 0.77 0 1 m 0 1 2 3 4 5 6 7 8 9 a lo n g − d ip d is ta n ce (k m ) 0 1 2 3 4 5 6 7 8 9 10 11 along−strike distance (km) vol51,2_3,2008 4-03-2009 10:28 pagina 468 469 inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria than from bootstrapping, especially for subfault slips. this behaviour is probably related to the cost function roughness: on the one side na could be in some trouble in sampling the parameter space efficiently, as suggested by its best-fit misfit (larger than that obtained by asa, using the same number of forward models), on the other, bootstrapping uses simplex, which could be unable to escape from local minima. this problem deserves great attention and deep investigation while inverting real data. as a first simple application of model selection criteria (selecting the model that best explain the data with a minimum number of parameters) we use aic corrected for small sample sizes (hurvich and tsai, 1989) (2.2) to compare the uniform-slip fault model and the 2x2 one. here n is the number of data, k is the number of parameters, and m is the chosen discrepancy (measure of lack of fit, e.g. zucchini, 2000); in this case m = m1. difference in aicc 1 2 lnaicc n k nk n n m= + 20500 21000 21500 x (m) 12000 13000 14000 y (m) 1500 2000 2500 z (m) 5000 5500 6000 6500 along-strike length (km) 5500 6000 6500 along-dip width (km) 138 140 142 144 146 strike (deg) 43 44 45 46 47 dip (deg) -80 -75 -70 rake (deg) 0,7 0,8 0,9 1 slip (m) fig. 5. pdfs of inferred fault parameters in the hhs (thin lines) and lhs (thick lines): dashed lines, bootstrapping; solid lines, nab. vertical dotted lines show asa best-fit values; vertical dash-dotted lines indicate true parameter values. uniform-slip model. vol51,2_3,2008 4-03-2009 10:28 pagina 469 0 1 m 0 1 2 3 4 5 6 7 8 9 a lo n g − d ip d is ta n ce (k m ) 0 1 2 3 4 5 6 7 8 9 10 11 along−strike distance (km) 0.23 0.59 0.46 1.43 0.17 0.57 0.56 1.44 470 a. amoruso and l. crescentini 5200 5600 6000 6400 6800 a lo n g − d ip w id th 1800 2000 2200 2400 z (m) 2d pdf fig. 6. 2d pdf of fault upper-side depth and along-dip width as obtained using nab (linear gray scale) and scatter plot from bootstrapping (circles). lhs uniform-slip model. fig. 7. best fit 2 × 2 fault in the hhs (thin-line rectangles) and lhs (thick-line rectangles) over the distributed-slip source fault. numbers give slip. vol51,2_3,2008 4-03-2009 10:28 pagina 470 471 inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria is about -500 both for the hhs and the lhs, indicating that the uniform-slip model is too simple and requires complex models to be tested. increasing the number of subfaults would disclose the main features of the slip pattern and possible changes in fault geometry while relaxing the assumption of uniform slip (e.g. amoruso et al., 2002), but also increases the difficulty of studying the cost function features. in this work, fault plane geometry and rake angle show very small changes when turning from the uniform-slip fault model to the 2 × 2 one, thus we use both the geometry and mechanism of the fault plane of the best fitting uniform-slip model to estimate the slip distribution, but we increase the along-strike length and the downdip width of the fault. to enlighten the effects of neglecting layering when inverting geodetic data, we consider the hhs case. the fault is divided into an even grid of p-along21000 22000 23000 x (m) 10000 12000 14000 y (m) 1000 1500 2000 2500 3000 z (m) 4000 6000 8000 along-strike length (km) 4000 5000 6000 along-dip width (km) 130 135 140 145 150 strike (deg) 44 46 48 50 52 dip (deg) -85 -80 -75 -70 rake (deg) 0 0,5 1 1,5 2 2,5 slip #1 (m) 0 0,5 1 1,5 2 2,5 slip #2 (m) 0 0,5 1 1,5 2 2,5 slip #3 (m) 0 0,5 1 1,5 2 2,5 slip #4 (m) fig. 8. pdfs of inferred fault parameters in the hhs (thin lines) and lhs (thick lines): dashed lines, bootstrapping; solid lines, nab. vertical dotted lines show asa best-fit values; vertical dash-dotted lines indicate true parameter values. 2 × 2 model. vol51,2_3,2008 4-03-2009 10:28 pagina 471 472 a. amoruso and l. crescentini strike x q-along-dip subfaults, with variable slip magnitude and constant rake. we minimize the cost function (2.3) using the algorithm for the least squares problem with linear inequality constraints of lawson and hanson (1995). here ∇2s is a finite difference approximation to the laplacian of the slip distribution and a is the area of each subfault. it would also be interesting to use misfit m1 of los displacements, but it leads to the prohibitive problem of nonlinear minimization in a huge space. finding the optimal model parameters thus requires the simultaneous optimization of two objective functions, namely the fit to the data 1a w v v fa p q s 1 2 2 2 1 i i i i n i i p q # d + # = = r ] c ] g m g/ / and the roughness, whose relative importance is controlled by the adimensional smoothing parameter f. the choice of f is a basic problem in inverse theory. traditionally, it is selected from a «trade-off» curve which plots the trade-off between the two objective functions when varying the value of f. the trade-off curve is monotonic and asymptotes to the minimum value of the roughness (which is zero) at one end and the minimum value of the fit to the data at the other. choosing a suitable value of f is somewhat arbitrary, but the knee of the trade-off curve is a good compromise between the model complexity and the data fit. cross validation is a more rigorous criterion (e.g., matthews and segall, 1993) but is computationally intensive. tests performed by arnadottir and segall (1994) show that cross validation gives an optimal estimate of f, while the trade-off curve gives a slightly smoother solution. we use the trade-off 1× 10 8 1× 10 9 1× 10 10 3× 10 10 7× 10 10 1× 10 11 2× 10 11 5× 10 8 1× 10 9 1× 10 10 3× 10 10 1× 10 11 5× 10 11 1× 10 9 1× 10 10 3× 10 10 1× 10 11 1× 10 12 1× 10 8 1× 10 9 1× 10 10 3× 10 10 1× 10 11 1× 10 12 3×10 8 1×10 9 1×10 10 3×10 10 1×10 11 1×10 12 1×10 13 5×10 3 6×10 3 7×10 3 8×10 3 9×10 3 0 2×10 -8 4×10 -8 6×10 -8 8×10 -8 6x4 8x6 10x8 15x12 40x32 fig. 9. trade-off curves between the m2 of los displacements and the roughness of the slip distribution, for different p × q-subfault models. labels give the weight f of roughness in the cost function. r o u g h n e ss displacement misfit vol51,2_3,2008 4-03-2009 10:28 pagina 472 0 1 2 3 4 5 6 7 8 9 10 a lo n g − d ip d is ta n ce (k m ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 along−strike distance (km) best−fit slip distribution, hhs 12x10;f=1x108 0 1 2 3 4 5 6 7 8 9 10a lo n g − d ip d is ta n ce (k m ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 along−strike distance (km) 12x10;f=1x1012 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 along−strike distance (km) 12x10;f=3x1010 0 1 2 3 4 5 6 7 8 9 10a lo n g − d ip d is ta n ce (k m ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 6x4;f=3x1010 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 473 inversion of synthetic geodetic data for the 1997 colfiorito events: clues on the effects of layering, assessment of model parameter pdfs, and model selection criteria fig. 10. best fit 40 × 32-subfault slip distribution (f = 3 × 1010 gray contours) over the distributed-slip source fault. fig. 11. gray contours, all plots: best fit 40 × 32-subfault slip distribution (f = 3 × 1010). black contours: top left, best fit 12 × 10-subfault slip distribution (f = 3 × 1010); top right, best fit 6 × 4-subfault slip distribution (f = 3 × 1010); bottom left, best fit 12 × 10-subfault slip distribution (f =108); bottom right, best fit 12 × 10-subfault slip distribution (f = 3 × 1012). vol51,2_3,2008 4-03-2009 10:28 pagina 473 474 a. amoruso and l. crescentini estimation since it is somewhat more conservative in smoothness and computationally less intensive than the cross-validation technique. the trade-off curve between the m2 misfit of los displacements and the roughness of the slip distribution is shown in fig. 9. it is noticeable that f � 3 × 1010 is in the knee region of the tradeoff curve independently of the number of subfaults (as far as the number of subfaults is large enough to introduce roughness) suggesting that f can be chosen using a single trade-off curve (i.e. a single p x q configuration) for each real case. figure 10 shows the retrieved 40 × 32 slip distribution overlying the true one. once again, the most relevant feature consists in the expected focusing effect in the hhs, since the maximum slip patch is shifted by about 1400 m upward along the dip direction, i.e. 1000 m in depth. it is important to realize if so many subfaults (40 × 32) are really necessary to represent the slip distribution or, on the other end, if they are enough. we computed aicc for 6 × 4, 8 × 6, 10 × 8, 12 × 10, 15 × 12, 20 × 16, 40 × 32 subfaults. minimum aicc is obtained for 12 × 10 subfaults. because aicc is on a relative scale, here we give the aicc differences with respect to the 12 × 10 case, i.e. 739 for 6 × 4, 84 for 8 × 6, 10 for 10 × 8, 93 for 15 × 12, 384 for 20 × 16, 3778 for 40 × 32. figure 11, upper plots, shows the capability of aicc to find the model that best explains the data with a minimum of parameters, by comparing contours of the slip distribution for 6 × 4, 12 × 10, and 40 × 32, subfaults. lower plots in fig. 11 show the importance of using the correct weighting factor from trade-off curves when combining different terms in the cost function: if f is too small (left plot) the retrieved slip distribution shows unreal small-scale features, if f is too large (right plot) the retrieved slip distribution is too smooth. the same approach should be followed in joint invertion of different kinds of data (see e.g. amoruso et al., 2008, for edm and levelling data). 3. conclusions inversion of synthetic coseismic displacement data representative of the main event of the 1997 umbria-marche sequence has been used to enlight the effects of a crustal layering proper to the colfiorito area on inferred source features when layering is not taken into account. as a test case we use a recently published distributed-slip model (crippa et al., 2006) and we show that the focusing effect of the fault slip distribution can be as large as 1 km. pdfs of model parameters obtained using bootstrapping and nab are generally in mutual agreement; apparent larger differences relate to the presence of trade-off between selected model parameters (e.g. depth of the fault upper side and along-dip width). pdfs from nab are usually more scattered than from bootstrapping, especially for subfault slips. this behaviour is probably related to the cost function roughness: on one side na could be in some trouble in sampling the parameter space efficiently, on the other we use simplex, which could be unable to escape from local minima, for bootstrapping to reduce computing time. this problem deserves attention in the inversion of real data. when inverting for the slip distribution, we use a roughness weight that appears independent of the number of subfaults, thus suggesting that its optimal value can be assessed by a single trade-off curve (i.e. a single p x q configuration) for each real case. the number of subfaults, that are necessary and sufficient (from the point of view of the information theory) to account for measurements can be obtained from aicc and in this work is as small as 12 × 10. references akaike, h. 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(2000): an introduction to model selection, journal of mathematical psychology, 44, 41-61, doi:10.1006/jmps.1999.1276. vol51,2_3,2008 4-03-2009 10:28 pagina 475 vol49_2_2006 705 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words benthic boundary layer – deep-seafloor – ocean temperature – helium – ocean radioactivity 1. introduction the benthic boundary layer (bbl) is the dynamic interface between the lithosphere (seafloor) and the ocean (seawater) where many physical, geochemical and biological processes play an important role in environmental global changes (boudreau and jorgensen, 2000). geohazards, carbon cycle, heat flow, life generation, climatic oceanography, are only some examples of local or global processes whose understanding is today limited due to the lack of data related to the deep ocean floors (thiel et al., 1994). lithospheric processes at the bbl impact the marine environment at different temporal and spatial scales. earthquakes produce short-term effects (landslides and tsunamis) that threaten the lives and economy of coastal communities, while the emission of greenhouse gases and mineral-rich fluids impact long-term global climates and the formation of economically important mineral resources. the bbl has an important role on carbon cycle being either a potential sinking or transition zone for carbon coming from shallower zones of from the lithosphere. the benthic boundary layer: geochemical and oceanographic data from the geostar-2 observatory giuseppe etiope (1), paolo favali (1), jean-luc fuda (2), francesco italiano (3), matthias laubenstein (4), claude millot (5) and wolfango plastino (1) (6) (1) istituto nazionale di geofisica e vulcanologia, sezione di roma 2, roma, italy (2) centre d’océanologie de marseille (com), cnrs, marseille, france (3) istituto nazionale di geofisica e vulcanologia, sezione di palermo, italy (4) istituto nazionale di fisica nucleare (infn), laboratori nazionali del gran sasso, assergi (aq), italy (5) laboratoire de oceanographie et de geochimie (lob), la seyne-sur-mer, france (6) dipartimento di fisica, università degli studi roma tre, roma, italy abstract geochemical and oceanographic data, acquired throughout 6 months by the geostar-2 benthic observatory in southern tyrrhenian sea, disclosed ocean-lithosphere interactions in the 1900-m deep benthic boundary layer (bbl), distinguishing two water masses with different origin and, possibly, benthic residence time. gas concentration, helium isotopic ratios, radioactivity, temperature, salinity and vertical component of the current converged towards the indication of a bbl characterised by a colder and fresher western water (ww), episodically displaced by the cascading of the warmer and saltier eastern overflow water (eow). the benthic ww has a higher concentration of geochemical tracers diffusing from the seafloor sediments. the data set shows the potential of long-term, continuous and multiparametric monitoring in providing unique information which cannot be acquired by traditional, short-term or single-sensor investigations. mailing address: dr. giuseppe etiope, istituto nazionale di geofisica e vulcanologia, sezione di roma 2, via di vigna murata 605, 00143 roma, italy; e-mail: etiope@ingv.it 706 giuseppe etiope, paolo favali, jean-luc fuda, francesco italiano, matthias laubenstein, claude millot and wolfango plastino oil platforms are sited on the bbl, and cables and pipelines are lain across it. delicate marine ecosystems have evolved in this extreme environment, yet it may be significantly affected by man’s activities. the bbl dynamics however is not completely clear: it is not known how it is perturbed by energy and mass release from the seabed (seismicity, heat flow, gas diffusion, biological productivity, diagenetic reactions), what distances the particles and bottom waves can be transported, what is the evolution and fate of bottom currents and storms. the deep-sea observation and monitoring systems developed in the last decade, in the framework of technological and scientific projects in japan, us, and europe (beranzoli et al., 2002, and references therein) can contribute to cover such a scientific gap. geostar-2 (geophysical and oceanographic station for abyssal research) is the first european deep-sea observatory for geophysical and environmental monitoring at the seabed becoming operative in 2000. it was deployed in september 2000 from the italian r/v urania, in the southern tyrrhenian sea, between the islands of sicily and ustica, at about 1900 m b.s.l. (fig. 1). this area was chosen as a key site for tyrrhenian seismicity and oceanographic studies (beranzoli et al., 1998). the sensors used for this mission (2 magnetometers, a gravitymeter, an hydrophone, a doppler currentmeter, a singlepoint currentmeter, an automatic water sampler for laboratory geochemical analysis, a ctd and a transmissometer) were continuously controlled and managed by a data acquisition and control system able to transmit the data via surface buoy and radio or satellite link to on-shore operators. after 206 days the observatory was recovered in april 2001. more than 4100 h of data were recorded continuously. this mission represented the longest experiment using a complex module, with an intelligent unit, deployed at great depth. this paper presents geochemical and oceanographic data obtained from the automatic water sampler, the ctd and the acoustic doppler current profiler (adcp) to show how multidisciplinary and integrated monitoring at deep bbls may provide various original and unique information, which cannot be acquired by traditional, short-term or single-sensor recordings. specifically, temperature, salinity, light transmission, currents, radionuclides and gases, including helium isotopes, have been examined to trace environmental changes in the benthic boundary seawater. 2. monitoring and data acquisition geochemical data refer to laboratory analyses carried out on 23 water samples collected by the geostar automatic sampler (ras-500 mclane) from 26 september 2000 to 9 april 2001. the sampler, using aluminium bags and a pressure compensation system, was programmed for sampling every 10 days over 240 days. four samples were duplicated and the sequence is composed of 19 time-series data (one sample was lost due to bag rupture). the sampling port was positioned at ∼1 m from the seabed. the seawater samples have been analysed for the following parameters: – gas concentration: co2, ch4, he, ne; – helium isotopic composition: r/ra (3he/ 4he sample/3he/4he atm);fig. 1. location of the geostar-2 deployment site. 707 the benthic boundary layer: geochemical and oceanographic data from the geostar-2 observatory – radioactivity (226ra, 235u, 234mpa, 228ra, 228th, 137cs and 60co). co2 and ch4 were analyzed by gas chromatography with micro-tcd (thermal conductivity detector; etiope, 1997). helium analysis was carried out with a perkin elmer 8500 gaschromatograph (flame ionization detector and hot wire detector; 5 ppmv as detection limit; analytical errors of ± 5%). the 3he/4he isotopic ratio, was determined by a static mass spectrometer (vg5400tft, vg isotopes; typical uncertainties are about ± 1% for 3he/4he ratios in the range of atmospheric values; below ± 0.1% for high-3he samples and below ± 3% for low-3he radiogenic samples). the analysis of radionuclides was carried out by gamma spectrometry with coaxial high purity germanium (hpge) detectors having volumes ranging from 200 to 500 cm3 and a total background rate in the energy range [(60÷2700) kev] varying from (221±2) to (980 ±10) counts/days depending on the detector (arpesella, 1996). each seawater sample was measured for about ten days using a polystyrene box of 70 mm diameter and 30 mm height (plastino et al., 2003). temperature and salinity were recorded by a high accuracy and stability ctd (sbe-16 seacat, sea-bird electronics, inc.; nominal accuracy-stability/month of 5×10−3−2×10−4°c/ 5×10−4−3×10−4 s/m, equipped with a pump and a quartz pressure sensor, resolution 0.01 ppm, repeatability 0.005% fs) positioned at ∼1 m from the seabed. calibrations of temperature and conductivity sensors performed by the manufacturer before and after the mission indicate no significant drifts and guarantee the values accuracy. seawater turbidity was monitored by a 0.25-m-pathlength transmissometer, alphatracka mk ii (chelsea instruments ltd.). the sampling rate was one sample per hour. current magnitude and direction were monitored by a 300 khz acoustic doppler current profiler. it provides ∼ 120-m 3d current profiles, representative of water layers (cells) ∼ 3m thick located at increasing vertical distances from the adcp. the adcp was configured for providing hourly profiles resulting from the average of 100 pings evenly transmitted during 10 min. the first cell was ∼ 7 m above the seafloor. 3. results 3.1. pressure, temperature, salinity and light transmission the pressure sensor was confirmed to be a highly performing tide gauge, being able to sense sea level variations of a few mm at ∼1900 m. in addition to the expected semi-diurnal and diurnal tidal signals, long term variations, as well as what can probably be interpreted as high frequency signals non-correctly resolved (the 1-h sampling was too large in this respect), can be evidenced (fig. 2a). the most remarkable characteristics regarding the temperature and salinity records (fig. 2b,c) resides in the almost regular occurrence, roughly every 2-3 weeks, of sharp peaks deviating from the background (t: 13.05°c, s: 38.51 psu) with values up to 13.45 °c and 38.63 psu, respectively. on the basis of t-s peak height, duration and density variations (fig. 2d) we recognise seven major events occurring on 24 october 2000, 14 and 30 november 2000, 9 and 25 january 2001, 10 and 26 february 2001. these events appear perfectly coherent with the stratification reported in the region 10 years ago (sparnocchia et al., 1999) and characterized, below 1500 m, by t-s gradients of the order of 0.1°c/100 m and 0.02 psu/100 m. these t-s peaks suggest a rapid (hours/days) lowering of the interface separating the relatively warm and saline waters of eastern origin (the socalled eastern overflow water, eow), cascading from the channel of sicily, from the underlying waters of western origin (fuda et al., 2002), sampled most of the time by geostar. sporadic light transmission drops represented by a unique hourly data point were recorded, with a large part of spike-like events; they are expected to be due to isolated large-size particles (fig. 2e). a dramatic few-hour drop down to 3.7 v (i.e. a relative light transmission loss of ∼15 %) occurred on october 24, 2000, following several weaker events during the preceding days and coinciding with the first major t-s peak. other events coincide mainly with t-s peaks of 9 and 25 january and 10 february. however, while most of the t-s peaks can be associated with transmissometer peaks, many transmissometer peaks are not associated with significant t-s ones. 708 giuseppe etiope, paolo favali, jean-luc fuda, francesco italiano, matthias laubenstein, claude millot and wolfango plastino fig. 2a-e. ctd data: a) pressure; b) potential water temperature; c) salinity; d) density; e) light transmission. the major 7 events are numbered over the salinity curve. a b c d e 3.2. currents the adcp provided different information on the deep-water circulation and on its evolution throughout 6 months. the horizontal speed (not shown) displays peaks that are very similar to the t-s ones. for the purpose of this paper, we mention only that significant vertical components of current were recognised associated with the t-s peaks. a noticeable anomaly is the occurrence of permanent downward velocities of the order of a few mm/s at all depths. even though relatively weak, this background residual is a priori abnormal as vertical velocities (either upward or downward) are not expected near the seafloor, except during specific transi709 the benthic boundary layer: geochemical and oceanographic data from the geostar-2 observatory fig. 3. co2 and ch4 variation. vertical bars are the seven main t/s events marked in fig. 2a-e. asw is the air saturated water (equilibrium with the atmosphere calculated at seafloor conditions). checks are the samples where no preservative was used and therefore all ch4 was consumed by biodegradation. tory processes, and except if there is a marked slope of the bottom, or up to a few meters above the seafloor where the observatory was demonstrated to deflect the flow and to induce upward motions (fuda et al., unpublished data). the observed vertical velocities depend on both the current direction and the current speed. for a given current direction, the higher the horizontal speed, the higher the vertical velocity. 3.3. gases and radionuclides figure 3 shows the pattern of dissolved ch4 and co2 throughout the mission. their concentrations are coherently close to the theoretical atmospheric equilibrium level (asw, air saturated water) at the seafloor conditions, suggesting that no significant external gas sources exist. all bottles, except two (as control samples), were initially conditioned by adding 0.1% hgcl2 to prevent bacterial consumption of ch4. as expected, the two non-conditioned samples displayed no ch4 concentration (or it was below the detection limit). figure 4 shows the variation of helium concentration and its isotopic ratio (3he/4he). although he isotopes were measured only in 11 samples, the 3he/4he ratio coupled with he/ne ratio suggested clearly the distinction of two different waters, as shown in fig. 5. five water samples showed a significant enrichment of radiogenic he (low 3he content) and these samples have also higher he mass concentration (mean of 3.2 ppmv) respect to 710 giuseppe etiope, paolo favali, jean-luc fuda, francesco italiano, matthias laubenstein, claude millot and wolfango plastino the others (2 ppmv). the lower 3he content is also indicative of lower tritiugenic 3he, produced by tritium decay within the water column. fig. 5. helium isotope ratio versus he/ne. the plot indicates two different waters probably linked to the different water masses whose interface oscillates producing t/s peaks. the isotope ratio of 1.4×10−6 is that of the atmosphere. samples closer to this ratio indicate major atmospheric signal (shallower water). lower ratios are along the mixing line with crustal (radiogenic) sources, and this can be indicative of bottom water, closer to seafloor, with higher residence time. fig. 4. helium variation. 3he/4he ratio (×10−6) is reported for each data point. underlined ratios are those of the bottom-water group identified in fig. 5. the radioactivity data are coherent with this pattern (figs. 6 and 7). the same group of samples having lower gas content and less radiogenic helium display drops of radioactivity for all radionuclides with typical values of standard seawater (inn et al., 2001). 4. discussion physico-chemical and oceanographic data (one sample per hour) show a fair relationship between temperature/salinity, turbidity peaks and vertical movement and horizontal speed of water masses. the adcp confirms the interpretation of ctd data regarding the existence of two water masses as it sensed the interface motions, with remarkable dynamic features found coincident with the t-s peaks. as also supported by previous local ctd stratification profiles, it is most likely that this correlation is consistent with rapid lowering of the interface separating the warm, saline and more turbid waters of eastern origin (the so-called eastern overflow water, eow), cascading from 400 m in the channel of sicily, from the underlying colder waters of western origin (ww), that are also «quiet» and hence not turbid, sampled most of the time by geostar. 711 the benthic boundary layer: geochemical and oceanographic data from the geostar-2 observatory fig. 6. variation of activity of some radionuclides. lower activities correspond to t/s peaks and are indicative of shallower water mass, as suggested by helium and its isotopic ratio. fig. 7. 235u versus 226ra. a group of lower activity is distinguished, referring to samples closer to t/s peaks, in perfect analogy with fig. 5. the smaller plot shows the clustering in relation to the temporal distance (in days) of water sampling after a t/s event. the main result of geochemical analyses is the sharp distinction of two geochemically different water masses, as suggested by he isotopes (fig. 5) and radionuclides (fig. 7). the main problem is the comparison between geochemical and hydrographic data, due to the limited availability of water samples for laboratory analysis, whose collection followed 712 giuseppe etiope, paolo favali, jean-luc fuda, francesco italiano, matthias laubenstein, claude millot and wolfango plastino a pre-determined schedule; the coincidence of the water sampling (i.e. the geochemical datum) with the t-s peaks is therefore only casual, with shifts variable of several hours. it is not possible therefore to make a point-topoint correlation but from a detailed examination of all the time-series data and as it appears in figs. 3, 4 and 6, it is possible to observe that: – samples relatively far or prior to t-s peaks have higher gas concentration, more he radiogenic and higher radioactivity. – samples taken the same day or just some tens of hours after t-s peaks have gases and he isotopic ratio closer to atmospheric equilibrium and lower radionuclide concentration. only two helium data, 10 december and 8 february, do not respect this pattern displaying atmospheric signature far or before t-s peaks. moreover all geochemical data seem to not be influenced by the t-s peak no. 5, which is, indeed, the shortest event. it is possible to argue that during or close to the major events of t-s variation the gas and radionuclide content decreases and the he isotopic ratio increases. this fact would support the interpretation of the ctd peaks as due to the vertical oscillation of the interface of two different water masses, close to the seafloor (fig. 8). events of lower he and radionuclides, and higher r/ra (closer to the atmospheric ratio) would reflect the sinking of a less deep water (eow). however we can think about this water mass either of having been less deep, although still not in contact with the atmosphere, or as a younger water mass (having been more recently at the surface in its zone of formation). the deeper water (ww), likely with higher age, has higher gas content and radioactivity and a lower he isotopic ratio, with more significant radiogenic component. the enrichment of radionuclides such as 226ra in deep-sea waters due to diffusion from sediments is a wellknown phenomenon (e.g., nozaki, 1986). figures 5 and 7 show clearly the two water masses. this distinction is less marked by co2 and ch4, which may have sources and sinks within the seawater column (and therefore a lower or nil difference between «seabed» signature and oceanic background) and the data can suffer higher biases from sampling to analysis. 5. conclusions the geostar 2 data-set represents the first long-term multidisciplinary monitoring of deep bbl including altogether seawater temperature, salinity, light transmission, 3d current, gas concentration, helium isotopes and radioactivity. the various geochemical and oceanographic data are basically coherent converging towards the indication of a bbl mainly characterised by a colder and fresher western water which is episodically displaced by the cascading of a warmer and saltier eastern water. the colder fresher water has higher concentrations of elements produced in the lithosphere (helium, radionuclides and partially methane and carbon dioxide), likely due also to a higher benthic residence time. the warmer saltier water has geochemical features typical of ocean background. the geochemical data, although potentially affected by sampling and analytical biases, suggest that gas and radionuclides in bbl seawater, deriving from earth degassing and diffusion from sediments, can be useful tracers to distinfig. 8. sketch of the oscillating water mass interface in the bbl. eow – eastern overflow water; ww – western water; bnl – benthic nepheloyd layer; inl – intermediate nepheloyd layer. geostar detected ww for most of the time and events of eow cascading, as suggested by t/s peaks, lower gas and radiogenic helium contents and lower radionuclide activities. 713 the benthic boundary layer: geochemical and oceanographic data from the geostar-2 observatory guish water masses of different origin and benthic residence times. geostar-2 demonstrated the potential of long-term, continuous multiparametric monitoring in providing unique information which cannot be acquired by traditional, short-term or single-sensor investigations. references arpesella, c. 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(1986): 226ra-222rn-210pb systematics in seawater near the bottom of the ocean, earth planet. sci. lett., 80, 36-40. plastino, w., m. laubenstein, g. etiope and p. favali (2003): environmental radioactivity analysis in seawater samples collected by geostar deep-sea observatory (istituto nazionale di fisica nucleare, laboratori nazionali del gran sasso), annual rep. 2002, 201206. sparnocchia, s., g.p. gasparini, m. astraldi, m. borghini and p. pisteck (1999): dynamics and mixing of the eastern mediterranean outflow in the tyrrhenian basin, j. mar. syst., 20, 301-317. thiel, h., k.o. kirstein, c. luth, u. luth, g. luther, l.a. meyer-reil, o. pfannkuche and m. weydert (1994): scientific requirements for an abyssal benthic laboratory, j. mar syst., 4, 421. sensitivities of surface wave velocities to the medium parameters in a radially anisotropic spherical earth and inversion strategies annals of geophysics, 58, 5, 2015, s0545; doi:10.4401/ag-6806 s0545 sensitivities of surface wave velocities to the medium parameters in a radially anisotropic spherical earth and inversion strategies sankar n. bhattacharya lithosphere study centre, new delhi, india abstract sensitivity kernels or partial derivatives of phase velocity (c) and group velocity (u) with respect to medium parameters are useful to interpret a given set of observed surface wave velocity data. in addition to phase velocities, group velocities are also being observed to find the radial anisotropy of the crust and mantle. however, sensitivities of group velocity for a radially anisotropic earth have rarely been studied. here we show sensitivities of group velocity along with those of phase velocity to the medium parameters vsv, vsh, vpv, vph, h and density in a radially anisotropic spherical earth. the peak sensitivities for u are generally twice of those for c; thus u is more efficient than c to explore anisotropic nature of the medium. love waves mainly depends on vsh while rayleigh waves is nearly independent of vsh. the sensitivities show that there are trade-offs among these parameters during inversion and there is a need to reduce the number of parameters to be evaluated independently. it is suggested to use a nonlinear inversion jointly for rayleigh and love waves; in such a nonlinear inversion best solutions are obtained among the model parameters within prescribed limits for each parameter. we first choose vsh, vsv and vph within their corresponding limits; vpv and h can be evaluated from empirical relations among the parameters. the density has small effect on surface wave velocities and it can be considered from other studies or from empirical relation of density to average pwave velocity. 1. introduction a transversely isotropic elastic medium with vertical axis of symmetry or radially anisotropic medium is described by the medium parameters: density t, wave velocity of horizontally and vertically travelling p-waves vph and vpv, vertically and horizontally polarised swaves vsv and vsh and the parameter h as described by takeuchi and saito [1972]. these parameters are linked with 5 elastic moduli as a = tv 2ph, c = tv 2 pv, l = tv 2 sv, n = tv 2sh and f = ht(v 2 ph − 2 v 2 sv). a seismic anisotropy is created by fabric induced from deformation in the crust and mantle; this causes dependence of local wave propagation on the direction and polarization of wave field [chen et al. 2009]. a layer with vsh > vsv associated with horizontal flow such as underneath plates; a vertical flow creates vsh < vsv as underneath spreading centres or subduction zones. thus knowledge of seismic anisotropy allows us to understand the earth’s dynamic evaluation. radial anisotropy in the uppermost mantle was considered to explain the fundamental modes of both rayleigh and love wave phase velocities, particularly, when isotropic structure failed to do so [anderson 1961, mcevilly 1964]. several such results led to incorporation of radial anisotropy in the top 220 km of the mantle in preliminary reference earth model (prem; dziewonski and anderson [1981]). since then surface wave studies have confirmed such anisotropy in many regions [e.g. villasenor et al. 2001, bhattacharya et al. 2013] and might be deeper in some cases [visser et al. 2008]. with the availability of broadband data globally, 3-d global maps of radial anisotropy of uppermost mantle have been prepared [panning and romanowicz 2006, kustowski et al. 2008]. radial anisotropy in the crust has also been observed in himalaya and tibet [shapiro et al. 2004, xie et al. 2013] and se europe [raykova and nikolova 2003]. radial anisotropy in the crust has been noted in the extensional provinces such as, western united states [moschetti et al. 2010] and off-coast of western india [prajapati et al. 2011]. sensitivity study of surface wave velocity to medium parameters is important to interpret a set of observed surface wave velocity data. changes in period in a given mode of spheroidal and toroidal oscillations to medium parameters of a radially anisotropic earth were shown by anderson [1989] and montagner [2007]. sensitivities of phase velocity at different periods to such medium parameters were shown in a few studies [e.g. takeuchi and saito 1972]. muyzert and snieder [2000] compared article history received june 14, 2015; accepted september 23, 2015. subject classification: crust-mantle structure, radial anisotropy, surface waves. phase velocity sensitivities to love parameters [ml] = [a,c,l,n,f, t] and those to anderson parameters [ma] = [vsv, vsh, vpv, vph, h, t]. thus there are a few studies on sensitivities of phase velocities but rarely any study on those of group velocities. in recent years group velocities are also being used to find the radial anisotropy of the crust and mantle [e.g. shapiro and ritzwoller 2002, shapiro et al. 2004, chen et al. 2009]. to fill up the gap we show here the sensitivities of group velocity along with those of phase velocity of fundamental mode to medium parameters [ma] in a spherical earth. we also suggest a few inversion strategies based on these sensitivities. 2. computations of sensitivities for rayleigh wave, bhattacharya and arora [1997] obtained dispersion equation in a radially anisotropic layered spherical earth considering each layer to be heterogeneous as [a, c, l, n, f,t] = [a0, c0, l0, n0, f0,t0 (r/a) -2] (r/a)-2 (1) where a0, c0, l0, n0, f0 and t0 are constants, a is the radius of the earth and r is the radial distance from the centre of the earth. considering the layer thickness to be small compared to the wavelength, the layer behaves nearly like a homogeneous layer [bhattacharya 1976]. advantage of such small heterogeneity is to get solutions of the equations of motions in terms of exponential functions making the dispersion equation as simple as that for a flat layered earth. we call ‘exact flattening transformation’ since we use the exact solutions of equation of motion in terms of exponential functions as in a flat earth. the dispersion equation is obtained from that of a flat layered earth by changing displacementstress matrix [bhattacharya and arora 1997]. for love waves, equation of motion is simple [takeuchi and saito 1972] and its solution is obtained in terms of exponential functions with same heterogeneity as (1) in a layer. the dispersion equation of love waves through exact flattening transformation can be obtained by changing displacement-stress matrix of a flat layered earth as done for an isotropic case [bhattacharya 2009]. the displacement-stress matrix of radially anisotropic spherical earth with flattening transformation will be same as bhattacharya ([2009]; eq. (7) with p = −2) by replacing n0 by l0 and considering ob 2 as let us consider the dispersion equation as f(k,~) = 0, where k is the wave number and ~ is the angular frequency. for a given value of period (t = 2r/~), phase velocity (c = ~/k) is obtained as a root of the dispersion equation through bisection method. the group velocity (u) is obtained as [stein and wysession 2003] where the above partial derivatives in the numerator and denominator at a root are obtained from analytical expressions of f. to compute the sensitivities or partial derivatives of c and u with respect to each model parameter of a given crust-mantle structure, we consider prem, which represents an average earth model. for a particular region such sensitivities can be drawn in a similar manner. prem has 3 km oceanic layer and crust extends down to 24.5 km. between depths 24.5 and 220 km, the model is radially anisotropic; but we have made this zone isotropic with effective isotropic velocities as suggested by dziewonski and anderson [1981]. thus we consider a radially anisotropic prem with simplification that vsv = vsh, vpv = vph, and h = 1 (figure 1). although such simplification makes the model to be isotropic, however the sensitivities each parameter of [ma] will show how c and u changes as each parameter changes from isotropic to radially anisotropic as done for c in muyzert and snieder [2000]; however, t changes show sensitivities for an isotropic prem model. we change each element of [ma] in a layer of 1 km centred at a given depth to get the following dimensionless l l n k a a4 9 4 12 0 2 0 0 0 2 2 2o ~ t =+ + b s x u dk d~ = =/f /f , /f2 /f2 k kvf2 2 f2 2 ~ ~v q q ~v kv bhattacharya 2 figure 1. prem with effective isotropic velocities at 100 s period [dziewonski and anderson 1981]. we change each elements of [ma] = [vsv, vsh, vpv, vph, h, t] to get the corresponding partial derivatives in figures 2 and 3. 3 partial derivatives for a given period t by numerical differentiation where v = either c or u; tm is the density in the mantle just below moho and ct is the phase velocity at the period t. the love wave is independent of vpv, vph and h. we use tm = 3.38 gm/cm 3. we obtain partial derivatives of fundamental mode c and u at periods 20, 50, 100 and 200 s (figures 2 and 3) 3. discussions the sensitivities or partial derivatives shown in figures 2 and 3 allow us to find the depth to which we can explore and find a strategy to improve the exploration with a given set of observed surface wave velocity data. both for rayleigh and love waves, peak partial derivatives of u are normally twice of those of c; this makes u more useful than c. however, measurement of phase velocity is more accurate than that of group velocity in a few cases [suresh et al. 2015]. the partial derivatives of u to velocities have peaks in the positive and negative sides with a deepest zero crossing at a depth about m/2 for rayleigh waves and about m/3 for love waves, where m (= t * ct) is the wavelength; below this depth the partial derivatives are small. the deepest zero crossing depths of sensitivities for all the parameters are close to each other. at zero-crossing depth of a period, the medium parameters are explored by other periods for which sensitivities are non-zero at that depth. thus with a given set of group velocity data, we can approximately explore down to the deepest depth of zero crossing of the highest period available in the set. for rayleigh waves, depth of peak sensitivity increases with period; but for love waves it slightly increases with period but mostly remains in the crust and close to moho. in isotropic case, similar characteristics of sensitivities to vs were noted [levedev et al. 2013]. in an isotropic earth, dahlen and tromp [1998] noted that phase velocities feel perturbations of vs down to ~m/2 for rayleigh waves and ~m/4 for love waves. the large values of sensitivities of rayleigh and love waves to the model parameters of crust and close to moho necessitates us to evaluate parameters of this zone carefully while exploring deeper part [ferreira et al. 2010, panning et al. 2010]. as is well known, love waves do not depend on vpv, vph and h. sensitivities of rayleigh wave velocity to vpv and vph are substantial. these two sensitivities are generally of opposite signs at given depth; thus increase of one and decrease of other affect surface wave velocity as significantly as by change of vsv [anderson 1989]. for an isotropic case, opposite signs of sensitivities cancel approximately and sensitivity of vp is only about 10 per cent of that to vs [suresh et al. 2015]. for love waves, sensitivity to vsh is large but negligible to vsv. thus the velocities of love waves computed in an isotropic medium with vs is nearly same as those computed in a radially anisotropic medium with vsh = vs and vsv may have any value and may be obtained from rayleigh wave inversion. on the other hand, rayleigh wave is nearly indifferent of vsh. hence the velocities of rayleigh waves computed in an isotropic medium with vs and vp will be nearly same as those computed in a radially anisotropic medium with vsv = vs and vsh may be have any value and may be obtained from love wave inversion; here we assume vpv = vph = vp and h = 1. this criteria have been used by a few authors while inverting radial anisotropy [raykova and nikolova 2003, bjarnason and schmeling 2009, prajapati et al. 2011]. although such computation using computational codes of a isotropic medium is nearly correct for love waves, but for rayleigh waves the assumption of an anisotropic medium with vsv ≠ vsh, vpv = vph and h = 1 is not strictly correct [babuska and cara 1991, xie et al. 2013]. in real mineral assemblages, vs anisotropy would be accompanied by vp anisotropy with h ≠ 1. further, assumption of vpv = vph may over estimate of s-wave anisotropy because part of changes in phase/group velocities by changing vsv may be obtained by changing vpv and vph. the sensitivity to the fifth parameter h is also significant. figure 2 shows (^v/^h)/ct; the factor 1/ct has been considered to make the sensitivity dimensionless. density t of the crust mainly influences love wave velocities (c or u) at all periods. sensitivity of rayleigh wave velocity to t is small but significant; depth of peak sensitivity increases with period. however, it is felt that we should restrict the inversion to first five parameters and density can be obtained from gravity studies or from an empirical formula using average p-wave velocity. as stated above, observed love wave velocities can be easily inverted to give vsh. for rayleigh waves, the sensitivities of velocities c and u to vsv, vpv, vph and h show trade-offs; i.e. increase or decrease of one will have nearly same effect on velocities by increase or decrease of the others. so even independent evaluation of all vsv, vpv, vph and h from rayleigh waves may not give correct result. it is suggested that we can explore two parameters: vsv, and either vpv or vph. the remaining two may be obtained from empirical relations of real mineral assemblages such as [xie et al. 2013] f ≈ 0.5c, h ≈ 1.0 − 4.2c , , , , , ,vs v vs v vp v vp v c v c v1 vsv vsh vpv vphph t t m 2 2 2 2 2 2 2 2 2 2 2 2 h t t # & sensitivities of surface wave velocities (2a,b) bhattacharya 4 f ig u re 2 .s en si ti vi ty k er ne ls o r, p ar ti al d er iv at iv es o f r ay le ig h w av e fu nd am en ta l m od e ph as e ve lo ci ty c (l ef t si de ) a nd g ro u p ve lo ci ty u (r ig ht s id e) w it h re sp ec t to m ed iu m p ar am et er s of a r ad ia lly a ni so tr op ic m ed iu m . t he p ar ti al d er iv at iv es a re b as ed o n ef fe ct iv e is ot ro pi c p r e m ( se e te xt ). t m (= 3. 38 g m /c m 3 ) is t he d en si ty in t he m an tl e ju st b el ow m oh o an d c t is t he p ha se v el oc it y at p er io d t . ( a) t = 2 0 s, ( b) t = 50 s , ( c) t = 1 00 s a nd ( d) t = 2 00 s . 5 sensitivities of surface wave velocities f ig u re 3 .s am e as f ig u re 1 fo r l ov e w av es . where c = (vsh − vsv)/vs , f = (vph − vpv)/vp and effective voigt average speed [babuska and cara 1991] vs = √[(2v 2 sv + v 2 sh)/3] and vp = √[(v 2 pv + 4v 2 ph)/5]. we write (3b) as vpv = vph − f vp we first consider values of vsh, vsv and vph the parameter h can be obtained from (2b) after obtaining c from (3a). we preferred first to consider vph rather than vpv because as per equation (4b), we may approximate vp ≈ vph. thus from (5) we get vpv ≈ (1 − f) vph as a first approximation. substituting this expression of vpv in (4b) we get second approximation of vp as vp ≈ (1 − f/5) vph substituting in (5), we get second approximation of vpv as vpv ≈ (1 − f + f 2/5) vph . to evaluate radial anisotropy, love and rayleigh waves need to be inverted jointly with nonlinear inversion where the best solution are obtained among the model parameters within prescribed upper and lower limits for each parameter [suresh et al. 2008, prajapati et al. 2011]. for these model parameters, we first choose vsh, vsv and vph within their corresponding limits and the remaining parameters vpv, h and t can be obtained as discussed above. 4. conclusions (i) love wave velocities in a radially anisotropic medium are close to those in an isotropic medium with vs = vsh. the vs, obtained from inversion of only love wave velocities through isotropic code, is close to vsh, if the medium is radially anisotropic. (ii) however, rayleigh wave velocities in a radially anisotropic medium can only be approximated by an isotropic code with vs = vsv under the assumption that vpv = vph = vp and h = 1; this assumption is not strictly correct if vsv ≠ vsh. further assumption of vpv = vph may cause over estimation of s-wave anisotropy. (iii) sensitivity being greater, group velocity is more efficient than phase velocity to explore anisotropic nature of the medium. the depth limit of inversion is ascertained by the deepest depth of zero crossing of the sensitivities at the highest period available in the data set. this depth is about half the wavelength for rayleigh waves and about one-third of wavelength for love waves (iv) through rayleigh waves, both vpv and vph in radially anisotropic medium are better explored than exploring vp in an isotropic medium. 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bhattacharya, lithosphere study centre, new delhi, india; email: sn_bhattacharya@hotmail.com. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sensitivities of surface wave velocities << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice small_scale variation in polar ionospheric electrical currents fourteen years of geomagnetic daily variation at mario zucchelli station (antarctica) santarelli l., cafarella l., lepidi s., di mauro d., meloni a., palangio p. istituto nazionale di geofisica e vulcanologia, rome, italy abstract during the 1986-87 austral summer a geomagnetic observatory was installed at the italian antarctic base mario zucchelli station. in the first three years continuous time variation monitoring and absolute measurements of the geomagnetic field were carried out only during summer expeditions. starting 1991 an automatic acquisition system, operating through all the year, was put in operation. we present here some peculiarities of the daily variation as observed for fourteen years (1987-2000). the availability of a long series of data has allowed the definition of seasonal, as well as solar cycle effects, on short time variations as observed at a cusp-cap observatory. in particular, contrary to mid latitude behaviour, a clear dependence of the daily variation amplitude on the global geomagnetic kp index was well defined. key words – geomagnetic daily variation, polar areas. introduction it is well known that the interaction of the variable solar wind with the magnetosphere is the main cause of the magnetospheric dynamics and of some special geophysical phenomena such as auroral emission (e.g. russell, 1986; carlson and egeland, 1996). the study of upper atmosphere and magnetosphere phenomena in the polar regions is very important because of the direct connection of polar geomagnetic field lines to the solar wind through the magnetopause and the magnetospheric tail regions. the ground measurements of magnetic field fluctuations in the polar regions are an important tool of investigation for plasma processes (e.g. lanzerotti et al., 1994; villante et al., 1997 and the references therein). important roles, in the generation of magnetic variation, with periods ranging from few seconds to tens of minutes, are played by the incidence of hydromagnetic waves and of density oscillations of the solar wind, the kelvin helmholtz instability on the magnetopause, the interaction with interplanetary shocks, field aligned currents and radiation belt charged particles falling from the magnetosphere onto the upper ionized atmosphere. for longer periods, changes in solar ultra-violet or x-ray fluxes likely play a major role (nishida and kokubun, 1971). in particular, the daily variation (24-hour period and its harmonics), is generated in the polar cap by two different ionospheric current systems: sq 0 (the extrapolation of the mid latitude sq system driven by the ionospheric dynamo) and the polar cap system spq generated by external sources related to processes in the magnetosphere and its interaction with the interplanetary magnetic field (imf) (e.g. nagata and kokubun, 1962; ractliffe, 1972; brekke, 1997). the sq p system is believed to be the primary source for the daily variations, especially during local winter when the southern polar cap is not exposed to solar illumination (e. g. , akasofu et al., 1983; brekke, 1997). the antarctic observatory at mario zucchelli station (formerly terra nova bay; the base name was changed in 2004; international geomagnetic observatory code tnb; geographic coordinates: 74.7°s, 164.1°e; corrected geomagnetic coordinates: 80.0°s, 307.7°e; lt=ut+13) is located in the polar cap. in this paper magnetic field data from 1987 to 2000 are analysed to investigate the influence of solar activity on daily variation and its distribution with respect to local time, season and solar activity. measurements and data analysis the location of mario zucchelli station, together with a statistical pattern of auroral power flux based on data from the total energy detector (ted) on board the noaa-16 satellite (courtesy of the u.s. departement of commerce, noaa, space environment center) over antarctica, is shown in figure 1; the local time at tnb is lt=03. during periods of average magnetic activity, the station is inside the polar cap quite close to the auroral oval. under particular geomagnetic conditions and for a particular range of lt’s it could be situated under the southern polar cusp, and thus the geomagnetic variations in this condition could be heavily influenced by local cusp phenomena, mainly related to field-aligned currents (matsushita and xu, 1982; campbell, 1997). variations in the earth’s magnetic field were measured by means of threeaxis fluxgate magnetometers located at tnb (meloni et al., 1997). the variations are measured along three directions oriented respectively with reference to the local magnetic meridian: the horizontal magnetic field intensity h-component (south-north), the orthogonal-component d in the horizontal plane (west-east, then d is an intensive element, expressed consequently in nt, and not the angular element declination) and the vertical intensity z-component (consequently positive increase inward). we used hourly means for d, h and z after subtracting daily mean values. summaries of the hourly mean values of the deviation from the average of the earth’s magnetic field hand d-components for one year (1993, chosen since it has a good data coverage) are shown in figure 2 as statistical boxplots. for each magnetic element, the hourly average values in 5-day intervals were statistically sorted and binned according to their magnitudes; the centre 50% of the data in each interval is contained in the individual boxes (e.g. kleiner and graedel, 1980). because of the removal of the mean values, the median value tends generally to be close to zero; in each interval the median value is given by an horizontal line across a box. the upper (lower) 25% of the distributions lies within the ranges given by the lines extending up (down) from the individual boxes. the mean daily variations at tnb observatory during years 1991 and 1996, one at the maximum and one at the minimum of solar activity, are shown in figure 3 as box-plot statistics organised as a function of universal time (ut) for three seasons selected according to the lloyd criteria (lloyd, 1861): summer (nov, dec, jan and feb, left panels in the figure); equinoxes (mar, apr, sep. and oct center panels); winter (may, jun, jul and aug, right panels). the plots were obtained by superimposing one day on top of another. in this way a single box represents the variations at each ut hour for the whole period considered. figures 4 a, b, and c show magnetic field hodograms in the horizontal plane sorted by local season (summer, upper panel; equinoxes center panel, winter, lowest panel) respectively, from 1987 to 2000. the d and h median values (one value for each hour) are plotted on the x and y axis, respectively. the 24 values represent the dynamics through the day of the projection of the total field f on the horizontal plane. discussion some general information about solar cycle 22 and 23 can be useful in order to better introduce some features that can be observed in the figures mentioned above. solar cycle 22 is one of the largest amplitude sunspot number cycle ever recorded, continuing the sequence of strong cycles which includes cycle 19 and cycle 21. solar cycle 22 started at the end of 1986 and reached its maximum phase in 1991; then it declined in early 1992 (sunspot number passes from a value of more than 150 at the end of 1991 to less than 100 in 1992). the lowest number of sunspot cycle 22 was officially recorded in may 1996, only after 9.8 years from its beginning. the maximum phase brought some extraordinary intervals of activity. in particular, june 1991 produced the most outstanding solar flare activity of this cycle. in the interval june 1st to june 17th numerous intense solar flares were recorded. cycle 23 started in may 1996 with the monthly sunspot number at 8.0 and peaked in april 2000 at 120.8. the 1993 tnb box-plot statistics in figure 2 (shown as an example) exhibits a very clear seasonal effect in both d and h components (cafarella et al., 1995; cafarella et al. 1998). the magnetic fluctuation level is considerably larger during local austral summer than in austral winter. this feature can be explaned in term of the increase in ionospheric ionization, and then in electrical current intensities, produced by solar em waves irradiation in summer season. there are some exceptions, corresponding to intervals of significant geomagnetic activity during local winter; for example in may 1993 (days 130-134) when the total range of differences in the 5-day bin is comparable to that seen during austral summer conditions. this increase in the range of magnetic variations during periods of high geomagnetic activity should arise from charged particle ionisation in the ionosphere, as well as the intense field-aligned and ionospheric currents which are present in the auroral regions expecially during open magnetospheric conditions (campbell, 1997). additional information on the daily variation can be obtained by examining the magnetic fluctuations at the station as a function of solar activity, local time and season. in figure 3 mean daily variation as box-plot statistics as a function of ut for two different years (max and min solar activity, 1991 and 1996 respectively) is reported. it is clear from the plots that the pattern of diurnal variation, in all the components, does not depend on the season nor the solar activity: in particular, independently on season and solar activity the h component reaches maximum values in the ut afternoon, while the d (z) component shows a broad minimum (maximum) around ut noon. conversely, the variation amplitude strongly depends both on season and solar activity. in general smaller local time variations during austral winter (as would be expected from figure 2) can be observed with respect to summer (figure 3). in any case the most important role determining the variation amplitude is played by the solar cycle: indeed 1991 winter variations are even larger than 1996 summer variations. some peculiarities are evident in both cases: the largest dispersion with respect to the median values occurs in the southern summer and the smallest during the winter. in particular, during summer the largest d day-to-day variations are just before ut midnight and the smallest ones are in early morning and in afternoon. the situation is quite different for the h-component: the largest variations during summer are in ut evening and in the ut early morning, and the smallest variations are just after ut noon, indipendently from solar activity. almost the same situation can be observed during the equinoxes (central panels) and in local winter, exept for a shift of the h component smallest variation at about before ut noon. the vertical z component amplitude variation does not show a definite ut dependence. new information on daily variation under different solar activity can be obtained by means of hodograms. the hodograms shown in figure 4 correspond to local summers, equinoxes and winters, from top to bottom respectively. each data point corresponds to the same hour averaged over the season. the amplitudes of the excursions over 24 hours are shown to be directly dependent on solar activity. the largest excursions occurred in 1989 summer followed respectively (on the same plot) by 1992 and 1991 (almost with the same amplitude) and 1999 and 1994. the smallest excursions occurred, for local summers, in 1997, followed by 1996, 1995 and 1987. for the equinoxes largest amplitude is for 1991 followed by 1992, 1993, 1994 and 1999 while the smallest ones are in 1997, 1996 and 1995. in winter the situation is quite different: largest excursions were in 1991 followed by 1994, 2000; the smallest ones were in 1997, 1996 and 1995. sunspot number, kp index and average excursion of the daily variation for the h, d and z elements for the three seasons are reported in figures 5a, 5b and 5c. in each panel the median daily range (peak to peak) is reported for every year. as is clear from the plots, the strong activity during 1991 is evident, looking at the h and d values for austral winter 1991. the values gradually decrease from the higher values corresponding to the peak of the cycle, to the lowest values, at the end of 1997, approximately one year after the minimum of the solar cycle, and then they grow up again with the new cycle 23. from the same figure it is also evident that the daily excursion amplitude is well related to the kp index. in table 1 the correlation coefficient between h, d and z daily excursions with the kp index is reported. the coefficients indicate a good correlation expecially between horizontal components and kp index. the correlation coefficient with the vertical component in summer and equinoxes is lower than the previous ones and this can be attributed to the influence of the induced electric currents on z component: indeed during winter, when the ionospheric electric currents intensity is lowest, the correlation coefficient with the vertical component is comparable with the other components. h and kp index d and kp index z and kp index local summer 0.83 0.84 0.70 local equinoxes 0.91 0.90 0.53 local winter 0.95 0.94 0.94 table 1: correlation coefficient between h, d and z daily excursions with the kp index. conclusions monitoring of electromagnetic geophysical parameters in polar areas is fundamental in many fields since it can give important information for a better comprehension of the earth's magnetosphere structure and evolution, and the solar control on magnetic activity as observed at earth’s surface. the diurnal variation of geomagnetic field elements is a well known phenomenon that has been extensively studied especially at mid latitudes. an equivalent electric current system is often used to represent the source of the observed field. at middle latitudes the equivalent current system consists of two current vortices, centred at about 40° latitude n and s respectively. a special additional contribution to diurnal variation appeared necessary in order to justify the polar daily variation plots. this additional field was called sdq (solar quiet polar) or dp2 according to various authors ([nishida and kokubun, 1971]; ratcliffe, 1972]). a possible cause of the polar current system is the magnetospheric convection that generates a quasi permanent current system in the polar regions which is steadily oriented with respect to the sun-earth line; the earth rotation underneath this system causes the magnetic field elements daily variation. our analysis, based on box-plot statistics, shows that the pattern of diurnal variation, in all the components, does not depend on the season nor the solar activity; conversely, the variation amplitude strongly depends on both season and solar activity. in fact, for each year a clear seasonal effect is visible, with largest variation during austral summer and middle and low activity during equinoxes and winter respectively. the dependence of the daily current system that causes the s field at the station on solar activity emerges, even more pronounced. looking at the plots, a link between some characteristic of daily variation and magnetic activity was immediately evident: when the planetary magnetic activity is higher the variation amplitudes in all seasons are higher. we can conclude that, at high latitude, contrary to what happens at middle latitude, daily variation depends strongly also on magnetic global activity. this result is consistent with previous findings (lepidi et al., 2003), obtained from the analysis of a very short data set (approximately 1 month) during local summer at cap latitudes. figure captions figure 1: geographical location of mario zucchelli station (tnb) magnetic observatory in antarctica. a statistical pattern of auroral power flux based on data from the total energy detector (ted) on board the noaa-16 satellite (courtesy of the u.s. departement of commerce, noaa, space environment center) is also reported. the red arrow indicates the magnetic local noon. figure 2: hourly horizontal magnetic field intensities shown as box-plot statistics binned in 5-day intervals for the year 1993. figure 3: tnb box plots values of hourly means organised as a function of local time for three seasons in 1991 (high magnetic activity) and 1996 (low magnetic activity). results are given for the magnetic field elements, h, d, and z. figure 4: hodograms (h vs d) for observatory values for local summer (a), equinoxes (b) and winter (c). figure 5: comparison among sunspot number, average kp index and daily variation excursion of h, d and z component for local summer, equinoxes and local winter respectively. references akasofu, s. i., b. h. ahn and g. j. romick (1983): a study of the polar current system using the ims meridian chains of magnetometers, space sci. rev., 36, 337-413. armadillo, e., tabellario g., bozzo e., caneva g. (2003): imaging the deep electrical conductivity structure of the rennik graben (north victoria land, antarctica), geophysical research abstracts, vol. 5, 01171. brekke, a. (1997): physics of the upper polar atmosphere, john wiley & sons ltd in association with praxis publishing ltd, pp. 491. cafarella, l., a. meloni, l.j. lanzerotti and c.g. maclennan (1995): small-scale variations in polar ionospheric electrical currents, antarctic journal of the united states, xxx, 5, pp. 347-349. cafarella, l., meloni a. and palangio p. (1998): solar cycle 22 control on daily geomagnetic variation at terra nova bay (antartica), annali di geofisica, vol. 41, pp. 805-812. campbell, w.h (1997): introduction to geomagnetic fields, cambridge university press, 135-142. carlson, h.c. and egeland a. (1996): the aurora and the auroral ionosphere, in introduction to space physics, m. g. kivelson and c.t. russel eds., cambridge university press, 568. kleiner, b. and graedel t. e. (1980): exploratory data analysis in the geophysical sciences, rev. geophys. and space phys., vol 18, pp. 699-717. lanzerotti, l. j., l. v. meclennan and l. v. medford (1994): inferred quasi-steady ionospheric neutral winds and electrical currents at 79o south latitude in austral summer condition, geoph. res. lett., 21, 217-220. lepidi, s., l. cafarella, p. francia, a. meloni, p. palangio and j.j. schott (2003): low frequency geomagnetic field variations at dome c (antarctica), ann. geophys., vol 21, 923-932. lloyd h. (1861): on earth-currents, and their connexion with the diurnal changes of the horizontal magnetic needle. trans. roy. irish acad., 24, 115141. matsushita, s. and w. xu (1981): imf sector effects on the polar geomagnetic field in winter, j. geophys. res., 86, 7143-7154. matsushita, s and w. xu (1982): equivalent ionospheric current system representing solar daily variations of the polar geomagnetic field, j. geophys. res, 87, 8241-8254. meloni, a., p. palangio, l. cafarella, g. romeo, e. bozzo and g. caneva (1997): the geomagnetic observatory at terra nova bay base. terra antarctica, 1, 181-183. nishida, t. and s. kokubun (1971): new polar magnetic disturbance: sp q, sp, dpc and dp2, rev. geophys. space phys., 9, 2, 417-425. papitashvili, v. o., y. i. feldstein, a. e. levitin, b. a. belov, l. i. gromova and m. valchuk (1990): equivalent ionospheric currents above antarctica during austral summer, antarctic science, 2, 267-276. ratcliffe, j. a. (1972): an introduction to the ionosphere and magnetosphere, com. uni press, cambridge, 256 pp. russell, c. t. (1986): solar wind control of magnetospheric configuration, in “solar wind-magnetosphere coupling”, edited by kamide and slavin, terrapub, tokio, 209-231. villante, u., s. lepidi, p. francia, a. meloni and p. palangio (1997): long period geomagnetic field fluctuations at terra nova bay (antarctica), geoph. res. lett., 24, 1443-1446. figure 1 tnb h ( nt ) -400 -300 -200 -100 0 100 200 300 400 julian days, 1993 d ( nt ) -400 -300 -200 -100 0 100 200 300 400 1 75 15 0 22 5 30 0 figure 2 max min 75% 25% mediana local winter 1991 h ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana equinoxes 1991 h ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median local summer 1991 h ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana d ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana d ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median d ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana ut (hours) z ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana ut (hours) z ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median ut (hours) z ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 a) figure 3a max min 75% 25% mediana local winter 1996 h ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana equinoxes 1996 h ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median local summer 1996 h ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana d ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana d ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median d ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median ut (hours) z ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% mediana ut (hours) z ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 max min 75% 25% median ut (hours) z ( nt ) -500 -400 -300 -200 -100 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 b) figure 3b −100 −50 0 50 100 −100 −50 0 50 100 h ( n t ) d (nt) local summer a) −100 −50 0 50 100 −100 −50 0 50 100 h ( n t ) d (nt) equinoxes b) figure 4 −100 −50 0 50 100 −100 −50 0 50 100 h ( n t ) d (nt) local winter c) figure 5 table 1: correlation coefficient between h, d and z daily excursions with the kp index. conclusions layout 6 1 annals of geophysics, 62, 4, gm456, 2019; doi: 10.4401/ag-7750 “forecasting the 2018 february 12th cme propagation with the p-dbm model: a fast warning procedure „ dario del moro*,1, gianluca napoletano2, roberta forte1, luca giovannelli1, ermanno pietropaolo2, francesco berrilli1 (1) università degli studi di roma tor vergata, italy (2) università degli studi di l'aquila, italy 1. introduction coronal mass ejections (cmes) are expulsion of plasma and magnetic field from the sun with repercus− sions on the whole interplanetary space. most of our knowledge about cmes comes from coronagraphic im− aging of the early stages of the cme propagation in the heliosphere. their plane of sky (pos) speed is among their characterizing attributes that we are able to meas− ure while they are coronograph field of view. during the launch phase, cme are accelerated (from a fraction to several km/s2) near the sun and then move with con− stant speed or slowly decelerate due to the drag force exerted by the ambient medium. their speeds when they leave the coronagraph field of view (at distance of ~30 rs) range from 100 to >3000 km/s [gopalswamy, 2016]. the cmes that have higher speeds are the most energetic phenomena relevant to space weather [gopal− swamy, 2018] and primary space weather consequences of those cmes are large energetic particle fluxes in the article history receveid may 8, 2018; accepted march 5, 2019. subject classification: space and planetary sciences; space weather; solar-terrestrial interaction; interplanetary physics; solar variability and solar wind abstract the forecast of the time of arrival of a coronal mass ejection (cme) to earth is of critical importance for our high−technology society and for the earth’s upper atmosphere status and leo satellites. we realized a procedure based on the drag−based model which uses probability distributions, rather than exact values, as input parameters, and allows the evaluation of the uncertainty on the forecast. we tested this approach using a set of cmes whose transit times are known, obtaining extremely promising results. we realized a real−time implementation of this algorithm which ingests the outputs of automated cme tracking algorithms as inputs to provide early warning for those cme approaching earth. we present the results of this real−time fast warning procedure for the case of the 2018 february 12th cme. near−earth environment and geomagnetic storms [e.g.: gonzalez et al., 1994; tsurutani et al., 1988]. more or less intense storms depend on the interplanetary coro− nal mass ejection (icme) characteristics, such as its velocity, size and the associated magnetic field [gopal− swamy, 2006]. strong geomagnetic storms are more likely when the incoming icme is fast and its magnetic field orientation is opposite with respect to the earth's magnetic field, as this configuration favors magnetic re− connection and the transfer of energy from the icme to the terrestrial magnetosphere [gonzalez et al., 1999; koskinen and huttunen, 2006; tsurutani et al., 2003]. an empirical relation is known to represent how the storm strength, measured by the disturbance storm time (dst) index, is determined by the cme impact ve− locity v and the strength of its magnetic field compo− nent oriented in the direction opposite to the earth’s horizontal magnetic field, bz [gopalswamy, 2009]: (1) our technological society, which strongly relies on in−orbit infrastructures, has become more vulnerable to these events. for this reason, several space−weather centers have been created to continuously monitor the solar activity and possibly forecast its outcomes on earth. the accurate prediction of the arrival time and ve− locity of icmes at earth has always been a primary goal of the space−weather forecasting [e.g.: daglis, 2001; schrijver and siscoe, 2010], and the space−weather centers employ different methods to model and predict the propagation of icmes. among those, the space weather prediction center of the national oceanic and atmospheric administration1, swpc/noaa, in usa, and the met office space weather operations centre2 in uk, employ the wsa−enlil+cone model [odstrcil et al., 2004], while the galileo ionosphere prediction serv− ice [albanese et al., 2018] and the swerto3 regional service [berrilli et al., 2018] in italy employs the prob− abilistic−drag based model [p−dbm − napoletano et al., 2018]. those two icme propagation models differ in ap− proach, assumptions, and complexity. the wsa−enlil+cone model is a 3−d numerical mhd−based model. it starts with a time−dependent simulation of the background solar−wind in the helios− phere where the cme is inserted as a hydrodynamic structure with an initial cone−shaped morphology. to return an accurate description of the icme propagation, wsa−enlil+cone needs a good knowledge of the background solar wind and of several parameters of the cme. in practice, multiple−view observations from the soho and stereo spacecrafts, and from the gong network are used to derive the inputs to the model, with the model becoming less and less accurate when there is no sufficient information to reproduce both a suitable background solar wind description and an appropriate cme replica. the p−dbm belongs to a family of models that apply a somewhat simplified description of the main interac− tion the icme is subject to during its interplanetary journey. the drag−based models (dbms) assume that beyond a certain heliospheric distance, a simple aero− dynamic drag equation [vrsnak et al., 2007] can de− scribe the interaction between the icme and the solar wind environment. namely, the cme acceleration a can be evaluated by considering a drag effect according to: (2) where v is the cme velocity, w is the solar wind velocity and γ is the drag parameter that conceals the details about the cme shape, mass, and in general all the physics re− lated to the effectiveness of the drag effect. the implementation of the various dbms may differ in the description of the cme morphology or other details [vrsnak et al., 2013, 2014, zic et al., 2015], but, due to their simplicity, all run in seconds, and can be made pub− licly available as on−line tools4. notwithstanding the major difference in complexity, the dbms performances are very close to those of the wsa−enlil+cone [vrsnak et al., 2014]. this indicates that, at present, the major limitation to an accurate icme propagation forecast comes from the lack of reliable in− puts, both in the form of large measure errors and of to− tally unknown parameters. taking advantage of the dbm computational light− ness, one can try to compensate for this lack of informa− tion applying an ensemble modeling approach. ensemble modeling incorporate the intrinsic limita− tion of information due to measure errors or due to the del moro et al. 2 1 https://www.swpc.noaa.gov/products/wsa-enlil-solar-wind-prediction 2 https://www.metoffice.gov.uk/public/weather/space-weather/ 3 http://spaceweather.roma2.infn.it/ 4 http://oh.geof.unizg.hr/dbm/dbm.php dst = –0.01v|bz|−32nt a = –γ(v – w)|v – w| 3 forecasting the 2018 february 12th cme lack of measure at all, in form of probability distributions. in practice, instead of a single run to forecast an icme propagation, a set of runs, driven with input parameters extracted from suitable distributions are used to retrieve a distribution of output parameters. a similar approach was used a few years ago by [mays et al., 2015] to evaluate the sensitivity of wsa− enlil+cone to initial cme parameters. their results indicated how the the predictions were heavily dependent on the cme initial parameters and their associated spread. in particular, they reported about the necessity of expanding the parameter space, since their "ensembles did not sample a wide enough spread in cme input parameters". alas, expanding the parameter space was hardly a choice, since the wsa−enlil+cone simulations are com− putationally very intensive and consequently need a rel− atively long time to complete. recently, two ensemble approaches for dbms, the p− dbm and the drag based ensemble model [dbem5 − dumbovič et al., 2018], were independently developed, with the capacity to run thousands of single simulations in seconds and thus exploring thoroughly the parameter space. the model outputs are the most probable icme travel time and velocity at arrival at earth, as well as the associated prediction uncertainties. in this paper, we resume the basic assumptions of the p−dbm and the outcomes of its validation using a dataset of past cmes. then, we focus on its real−time implemen− tation and show the result of an actual application for forecasting the arrival time and speed of a cme at earth. 2. the p-dbm model the p−dbm makes use of probability distributions, rather than exact values, as input parameters, to model the cme propagation in the interplanetary space, allow− ing us to forecast icme arrival times and velocities and to evaluate also the uncertainty on the forecast. details on the implementation and an extended description are in napoletano et al. (2018). in the following, we briefly introduce some concept and present an example of the validation outputs. the model considering constant the solar wind speed w and the drag parameter γ, eq. (2) can be solved explicitly to re− turn both the icme distance and speed as functions of time. consequently, the dbm needs four quantities [r0, v0, w, ] (r0 and v0 are the initial position and speed, re− spectively) to compute the icme distance and speed at any time. with the further simplification of a spherical cme front, we can compute the transit time and the ve− locity at 1au, for any cme that has been launched earthwards and whose [r0, v0, w, γ] are measured or known. however, these quantities are either affected by measure errors or simply not known. whenever those errors are large or that knowledge is lacking, as is the case for real−time icme forecast implementation, the incertitude on the results may be large. the p−dbm addresses this problem by returning a statistical evaluation of the time of arrival and velocity of the icme at a chosen distance from the sun, trans− forming the probability or error distribution functions associated to the input parameters into probability dis− tribution functions (pdfs) for the output results. this al− lows computing the best estimates and the errors for both the time of arrival and the velocity of a given icme. for each icme whose r0 and v0 are measured with an associated uncertainty, we will generate n different [r0, v0, w, γ] initial condition sets, randomly chosen from the relative pdfs, to compute the transit time and the velocity at 1au, for example. this process generates the pdfs associated to t1au and v1au, which can be used to estimate the icme most probable time of arrival and ve− locity and their associated uncertainties at 1au. 2.1 the validation to perform a validation of the p−dbm, we em− ployed the sample of cmes from [shi et al., 2015]. that work provides the travel times from the sun to earth, allow the estimation of the initial position and veloc− ity and the associated errors for 21 cmes, extracted from the larger sample of the gmu cme/icme cata− logue compiled by phillip hess and jie zhang (http://solar.gmu.edu/heliophysics/index.php/gmu_cm e/icme_list), spanning from 2007 to 2017 from soho/lasco [domingo et al., 1995; brueckner et al., 1995] and stereo/secchi [kaiser et al., 2008; howard et al., 2008] data. [shi et al., 2015] selected only those cmes whose front could be unambiguously identified. for each event, they fitted a graduated cylindrical shell [thernisien, 2011] model and employed the j map technique [sheeley et al., 1999] to extract the cme launch parameters and ensuring relatively small errors 3 5 http://oh.geof.unizg.hr/dbem/dbem.php del moro et al. 4 in their estimation. among these cmes, the authors highlighted six that were likely to have undergone in− teractions with non−radial forces (e.g.: a possible cme− cme interactions) and were therefore not suitable to be described by the dbm. the cme arrival times were obtained from the in situ data by wind [acuña et al. 1995], however, in some cases, the travel time had to be corrected, since the au− thors gave the arrival time of the shock preceding the arrival of the icme itself by a few hours. the initial positions and the velocities and their rel− ative errors have been used to generate gaussian pdfs for r0 and v0. the γ pdf has been modeled with a log− normal function (µ = 0.70 and σ = 1.01). the w pdf has been chosen considering the slow or fast solar wind condition measured at 1au in the hours before the icme arrival at earth, using a gaussian function cen− tered at 400 km/s with σ = 33 km/s for the slow solar wind, and a gaussian function centered at 600 km/s with σ = 66 km/s for the fast solar wind. from this data, we realized n=50000 different prop− agations for each icme, we were able to compute the forecast travel times, the velocity at 1au and their as− sociated statistical (±1σ) errors, of the icmes in the sample. in figure 1 we plot the comparison between the forecast travel times and the measured travel times as green dots, with the error bars to represent the error as− sociated to our forecast travel times. in the plot, we shaded in light blue the perfect match zone, where the forecast and measured travel times are the same within the measure errors. for 10 out of 15 cmes of this sample, the absolute difference between measure and forecast travel time is smaller than the forecast error. the average value of this quantity is 8.5±5 hours. since, in principle, we may not know at launch time whether a cme would interact with another cme or not, in this plot we also show (red symbols) those six cmes that were not suitable to be described by the p−dbm, and that were excluded from the whole sample by shi et al. [2015]. considering also those events, the average of the absolute residual times grows to 12±13 hours, largely dominated by those two cmes that arrived with large advance with respect to the forecast. 3. the 2018 february 12th cme to highlight the capabilities of the p−dbm for real− time application, we present one case study, where we compared the forecast provided by the p−dbm fed by soho/lasco data and a simple de−projection program with the measured icme characteristics at its arrival at earth. we stress that in the real−time implementation, only information from sdo and soho is used to esti− mate the cme characteristics. this leads to larger errors in the characteristics estimation than when using also information from stereo, but it is a nice test of the operational capabilities of the p−dbm as forecast tool when stereo data would not be available any more. 3.1 the cme detection a cme launch was detected on the soho/lasco coronagraphic images on february 12th, 2018 (figure 2). the cactus [robbrecht and berghmans, 2004] service estimated its onset time, plane of the sky (pos) angle and width, and pos velocity (as reported in table 1). 3.2 the cme direction assessment to compute the whole 3d propagation vector of the cme, we have to complement the soho/lasco infor− mation with a possible source on the solar surface. with the source on the sun, the hypotheses of a cme onset time: 2018 feb 12 01:25 utc (± 2h) pos angle: 280° pos width: 106° pos velocity: 563±282 km/s table 1. parameters of the 2018 february 12th cme onset as estimated by the cactus service. figure 1. dots with error bars are the observed transit times versus forecast transit times. here we show the result from the whole [shi et al., 2015] dataset: in green the restricted sample and in red those cmes excluded from that sample (see text). the dashed line shows the perfect match expectation, with the travel time sta− tistical error zone shaded in light blue. 5 forecasting the 2018 february 12th cme propagation vector normal to the solar surface and of a conical shape for the cme, we can de−project the ve− locity and estimate the true width. by intersecting the cme cone with the ecliptic, we can estimate the prob− ability of the cme hitting earth (or any other target). to find the most probable cme source on the sun, we developed a source finding algorithm that make use of the heliophysics events knowledgebase [hurlburt et al., 2010] to check which solar features, that may have caused the cme (e.g.: flares, filaments, etc.), are within the area compatible with the cme launch parameters. the algorithm returns the coordinate of the cme source and the line of sight (los) velocity of the cme. this al− gorithm also returns the position of the coronal holes at the cme onset, allowing us to use a fast (cme source close or within a coronal hole) or slow (cme source far from a coronal hole) solar wind pdf for the successive icme propagation computation. figure 3 shows the graphical output of the cme source finding algorithm, visualizing the characteristics of the cme over an sdo/aia [lemen et al., 2012] image of the sun: the green arrow shows the cme pos angle, the yellow sec− tor shows the angle subtended by the cme pos width, the white boxes and the red dots shows the ars and re− cent flare locations, respectively, taken into account to estimate the cme onset location (shown as a black cir− cle). the gray polygons show the position of coronal holes at the time of the cme onset. 3.3 the icme propagation with p-dbm using the estimate of r0 and its associated error from the cactus data, the estimate of v0 and its error from the de−projection procedure, and the solar wind state assessment, we can generate the input pdfs and com− pute the output pdfs at 1au for the icme under con− sideration. also for this real−time implementation, we used n=50000 cme realizations. the pdfs of the com− puted t1au and v1au are shown in figures 4 and 5. in particular, using the pdf of the travel times, we can estimate the most probable t1au and compute the inner solar system planet positions at that moment. figure 6 shows the planet positions at the most figure 3. graphical output of the cme source finding algo− rithm. we visualize several characteristics of the cme over a sdo/aia 0304 image of the sun: the green ar− row shows the cme pos angle, the yellow sector shows the angle subtended by the cme pos width, the white boxes and the red dots shows the ars and re− cent flare locations, respectively, taken into account to estimate the cme onset location (shown as a black circle). the gray polygons show the position of coro− nal holes at the time of the cme onset. heliographic longitude: 13°±1° heliographic latitude: -1°±1° los velocity: 847±367 km/s solar wind: slow table 2. source coordinates and de−projected initial velocity of the 2018 february 12th cme and solar wind conditions as computed by the cme source finding algorithm. figure 2. the 2018 february 12th cme as imaged by the lasco c2 coronagraph on−board soho. the image shown is a contrast enhanced difference of two im− ages acquired few minutes apart. this allows the vi− sualization of the cme front few hours after the on− set time. probable arrival time at 1au of the cme. considering gaussian errors on longitude of the cme origin site on the sun and on the cme width, and the error in the travel time forecast, we define four zones of probabil− ity about the cme position and extension, which are represented by different shades of colors, from red (im− pact probability ~90%), to dark orange (impact proba− bility ~40%), to light orange (impact probability ~10%), to yellow (impact probability ~5%). for this particular cme, the probability of hitting earth was greater than 90%. this forecast was available within one hour from the cme detection by cactus. the measured icme characteristics the icme under investigation actually hit earth. in figure 7, we plot several relevant plasma and magnetic field quantities measured by the magnetometer and faraday cup instruments on−board the dscovr satel− del moro et al. 6 table 3. forecast of impact probability and arrival parameters of the 2018 february 12th cme as estimated by the p−dbm algorithm. earth impact probability: 90% icme arrival date: 2018 feb 15 09:05 (±12h) icme arrival velocity: 480±80 km/s figure 4. probability density function of t1au computed from n=50000 realizations by the p−dbm model for the 2018 february 12th cme. the vertical lines indicate the average travel time (~80h) and the average±rms values (~12h). for this pdf the median value is ~80h. figure 5. probability density function of v1au computed from n=50000 realizations by the p−dbm model for the 2018 february 12th cme. the vertical lines indicate the average velocity (~480km/s) and the average±rms values (~80km/s). for this pdf the median value is ~460km/s. figure 6. graphical output of the cme propagation algorithm. the inner part of the solar system is drawn at the most probable arrival time at 1au of the cme. the estimated cme propagation path is represented by dashed lines, and the cme itself is represented by dif− ferent shades of colors, representing different confi− dence levels of the cme extension: yellow=10% probability, light orange = 50% probability, dark or− ange = 90% probability. inner heliosphere planets are represented by colored symbols: mercury = purple star, venus = green triangle, earth = blue circle, mars = red pentagon. lite [szabo, 2012] at l1 from 06 to 12 utc of the 2018 february 15th. from these data, there is no conclusive signature of the difference between the preceding shock and the actual icme magnetic structure arrivals. there− fore, it has been agreed [cme scoreboard − mullinix, 2016] that the cme arrival time−stamp at earth is: 2018−02−15t07:38 utc, by summing 1 hour of time for the cme to travel from the dscovr satellite position at l1 to earth. it is worth to note that this particular cme had a large pos velocity uncertainty from the initial cactus estimate. this large error propagated in the al− gorithm, leading to a relatively large uncertainty on both t1au and v1au. this notwithstanding, the forecast arrival time differs by a mere 1.5h from the measured arrival time and the icme velocity is partially compatible with the measured value. 4. conclusions we developed a statistical approach for the computation of kinematic propagation of icme, making use of pdf to cope with our lack of knowledge on cme or imf pa− rameters. we tested this approach using a set of cmes whose transit times are known, and obtained extremely promising results. we presented a case study where this approach has been used to forecast in real−time the propagation and arrival at earth of a single cme launched on february 12th, 2018. the forecast was available within one hour from the cme detection and both the forecast arrival time and the velocity at earth are compatible with the measured icme values. 7 forecasting the 2018 february 12th cme table 4. arrival parameters of the 2018 february 12th cme as measured by the instrumentation on−board the dscovr satellite. icme arrival date: 2018 feb 15 07:38 utc icme arrival velocity: ~350 km/s figure 7. relevant plasma and magnetic field quantities measured by the dscovr satellite at l1 from the 6 to 12 utc of the 2018 february 15th. from top to bottom: the total strength (in blue) and the component perpendicular to the ecliptic (in or− ange) of the interplanetary magnetic field; the solar wind proton density; the solar wind proton velocity. the evident dis− continuity (highlighted with the red vertical dashed line) at 06:38 utc marks the arrival of the icme structure at l1. this paper uses data from the cactus cme catalog, generated and maintained by the sidc at the royal observatory of belgium. sdo data is courtesy of nasa sdo/aia and the hmi science teams. soho data supplied courtesy of the soho/mdi and soho/eit consortia. soho is a 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 on amplitudes of p near the shadow zone inge lehmann annali di geofisica, vol. 11, n. 3-4, 1958. annals of geophysics, vol. 53, n. 1, february 2010 abstract 51 short-period vertical north american and canadian records of the samoa earthquake 14 april 1957 were examined. amplitudes of p were very large at the western stations (δ < 84˚) and small at the stations to the east, 8 of which recorded p at distances greater than 100˚. there was a lack of stations at intermediate distances. at the greatest distance, 108˚.2, the p waves were not diffracted waves, for the phase had the same general character as at shorter distances; the period was 2 sec. from 70˚ to about 90˚ the samoa p timedistance curve is steeper than the jeffreysbullen curve. it had been found previously that from about 90˚ epicentral distance onwards the variation ot the p amplitude with distance was not always the same and also that the shadow zone seemed to begin at different distances. it is suggested that this be due to small regional differences of velocity in the deepest layer d" of the mantle. some years ago i made a study [lehmann 1953] in order to find out whether or not there was a sudden drop in the p amplitude at 105˚ or another neighbouring distance marking clearly the beginning of the shadow zone. i made use partly of the i.s.s. data and partly of copenhagen records. it is well known that p is often recorded at distances greater than 105˚, but usually it is small and it is considered to be a diffracted wave. i found, however, that in the i.s.s. there were sometimes series of p entries up to 110˚ or more the residuals of which were consistent enough to show that the onset was sharp and therefore not likely to be due to a diffracted wave. on the other hand i sometimes found that p of an earthquake had considerable uncertainty already at distances of about 95˚ or so and yet was recorded beyond 105˚ as if no decrease of amplitude took place at that distance. using copenhagen records i found also that in some earthquakes p was very clearly recorded up to and considerably beyond 105˚ while in other earthquakes of similar magnitude p became small and insignificant already at distances smaller than 100˚. thus the variation of amplitude with distance is different in different earthquakes. the shadow zone seems to begin at different distances and it is usually not possible to say exactly where it begins; there is no sudden decrease of amplitude to mark its boundary. at the lamont geological observatory the surface waves of the sarno earthquake 14 april 1957 were being studied with the object of finding variations of phase velocity and hence of crustal thickness across the united states and canada. 51 records of short-period vertical instruments had been collected. they were from epicentral distances ranging from 71˚ to 114˚. it was suggested to me by dr. ewing that i examine the records for the greatest distances in order to see how p varied near the shadow zone. it was immediately apparent that there was a considerable drop in amplitude from the nearer to the greatest epicentral distances, but it was clear also that no precise determination of the variation of amplitude with distance could be made, for the instruments varied and their response was not accurately known. also, the distances are subject to errors, an epicentre in the samoan region not being determinable with great precision. however, the two independent determinations of the u.s.c.g.s. and the central bureau at strasbourg differed by less than 1˚. the seismological stations are not evenly distributed over the continent. there are a great many stations in the western part and they were at epicentral distances ranging from 71˚ to 84˚. there were 10 stations in the eastern part at distances greater than 100˚ and there were not many stations in between. p amplitudes were very large at the western stations, the phase being marked by a group, or two groups, of large swings continuing for about a minute. the wave period was two seconds or a little less. rapid city at an epicentral distance of 87˚ still had very large amplitudes though somewhat smaller than the western stations. fayetteville at 90˚.4, however, had considerably smaller amplitudes though the phase was still quite large. columbia at distance 100˚.2 was the nearest of the 10 stations at distances greater than 100˚ and halifax at 114˚.6 the most distant. this station and palisades at 106˚.9 did not record p, but all the others did record it. the cleveland record from epicentral distance 101˚.3 is shown in the figure. it is from a sprengnether short-period vertical instrument. the p amplitudes are here quite small as compared with those at the western stations. where and how quickly the amplitudes fall off cannot be said because of a lack of stations at intermediate distances. the wave period 13 is about 2 sec. as at the smaller distances. the kirkland lake record from a distance of 103˚.4 is almost exactly like the cleveland record. the pattern is the same and the amplitudes are equal. it is also from a sprengnether instrument. chapel hill at 102˚.2 has similar amplitudes but the pattern is different; there is not the same division of the phase. it is somewhat surprising to find that shawinigan falls at an epicentral distance of 108˚.2 has a record similar to the cleveland record and only slightly smaller. the pattern is the same and the waves have period of about 2 sec. the seismograph is a willmore instrument and it may have greater magnification than the sprengnether instrument in the period range in question, but that would be of no avail if the core boundary were grazed by the ray emerging at 105˚. it is obvious that the path to shawinigan falls has the same general character as those to the nearer stations and that the waves recorded are not diffracted waves. the reason why the cleveland record is shown and not the one from shawinigan falls is that the trace of the latter is a little fainter. of the other stations beyond 100˚ columbia at 100˚.2 has a much smaller p phase than the stations mentioned, but the pattern is the same. this is also true for ottawa at 106˚.0. montreal at 107˚.5 has quite a small but distinct p. state college at 103˚.9 has also a clear but small p. we have seen that the p amplitudes are small in the 8˚ range of distance 100˚ to 108˚. no precise determination of the variation of amplitude with distance can be made, not even for the distances of the many western stations, but the decrease of amplitude between 84˚ and 100˚ is very great, undoubtedly far greater than the decrease between 71˚ and 84˚. the strong decrease of amplitude will be due to a marked decrease of velocity gradient at depth. the lack of observations at intermediate distances makes it impossible to say whether the decrease is gradual or abrupt or to determine even roughly the depth at which it occurs. but our finding is in support of the assumption that k. e. bullen’s region d divides itself into two regions of which the inner one, d", is characterised by a velocity gradient that is very small and distinctly smaller than that of d' [bullen 1956]. the fact that the time-curve straightens before the shadow zone is reached has led to this result. the jeffreysbullen p curve straightens at about 90˚ and has very little curvature from there onwards. the samoa p curve runs practically parallel to it at these distances. it does not, however, run parallel to the j.-b. curve all the way from 70˚ to 108˚. taking the time of occurrence of the earthquake to be 19:17:57 as determined by the u.s.c.g.s. we find the following mean residuals: 70˚ – 75˚ 10 observations 0s.96 ± 0s.17 75˚ – 80˚ 8 observations 1s.69 ± 0s.11 100˚ – 108˚.2 8 observations 2s.39 ± 0s.17 there is some scatter of the residuals near 85˚. between 90˚ and 100˚ there are only 4 observations; the mean of their residuals is 2s.23 ± 0s.22. the azimuths of all the observing stations is between 30˚ and 58˚ so it is evident that the samoa earthquake p curve is steeper than the j.-b. curve from 70˚ up to about 90˚. the distances taken were geographic distances, but elipticity corrections were applied to the transmission times. since the amplitude of the direct p wave usually is quite small at the greatest distances where it occurs it will not be easy to decide whether a p phase as recorded is due to a direct or to a diffracted wave to find where the shadow zone begins. there is as a rule no short period waves in distant p's, and this has been taken to indicate diffraction. whether it is a sure criterion it is not known and it is not applicable unless the recording instruments have a good response to short period waves. it does not seem to have been attempted to fix the beginning of the shadow zone of an individual earthquake by a method similar to the one applied here, by direct comparison of records. to obtain definite results it would be necessary to choose earthquakes well recorded at the distances in question by a dense network of stations as on amplitudes of p near the shadow zone 14 samoa earthquake recorded at cleveland, δ = 101˚.3. 15 e. g. that of western north america. stations equipped with short period instruments would be required and it would be highly desirable that the relative response of the instruments as mounted should be known. this latter requirement is not easily fulfilled, but it may be indispensable if we are to succeed in deriving more detailed knowledge about the earth's interior from seismological data. unless the error in the epicentre is unusually large, shawinigan falls is at an epicentral distance greater than 105˚, and the shadow zone of our samoan shock does not begin at this distance. taking the direct p ray to reach out to different epicentral distances in different shocks, and the relative amplitudes to differ, we shall have to look for a possible explanation, and we find that slight regional variations in the composition of the region d" would account for it. the velocity gradient in d" is quite small and a small regional variation would alter slightly the curvature in d" of the rays having their deepest points in the region. the rays are long and therefore a small bending or straightening of them in their deepest portion would give rise to an appreciable change in the epicentral distance at which they emerged. it would alter also the spreading of the rays and therefore affect amplitudes. actually it is not known at present whether the difference in amplitude variation is regionally conditioned, but it would be of great interest to have it investigated. acknowledgements. lamont geological observatory (columbia university), contribution no 319. my thanks are due to dr. maurice ewing by whose kind invitation i worked at the lamont geological observatory and who allowed me to use the records of the samoa earthquake collected there. since the preceding was written the study by a. vogel, «über unregelmässigkeiten der äusseren begrenzung des erdkerns» (doctor's thesis, not printed) has become known to me. observed regional variations in the transmission times of the waves reflected at the core boundary are put down to irregularities in this boundary. undulations in the core boundary would account also for the shadow zone for p beginning at different epicentral distances in different earthquakes, but not for the observed differences in amplitude variation. they seem to require for their explanation the (additional) assumption of regional variation of velocity in d". references bullen, k.e. (1956). seismology and the broad structure of the earth's interior, phys. chem. earth, 1, 68-93. lehmann, i. (1953). on the shadow of the earth's core, bull. seismol. soc. amer., 43, 291-306. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. on amplitudes of p near the shadow zone vol50,4,2007 v annals of geophysics, vol. 50, n. 4, august 2007 preface: fluids and tectonics the 8th international conference on gas geochemistry provided the opportunity for scientists from different countries to meet each other, exchange ideas on the state of the art in gas geochemistry, and discuss advance in fluid geochemistry. the 8th icgg meeting focused on three main geologic environments currently interacting with the human life: volcanoes, earthquakes and hydrocarbons. ninety-four presentations gave participants chance to cover a variety of important research topics on gas geochemistry in geosciences including: gas migration in terrestrial and marine environments, earth degassing and its relation to seismicity, volcanic eruptions, rare gases and application of isotope techniques, measurement and analytical techniques. this special issue of annals of geophysics collects some of the papers presented dealing with the general problem of the relation between fluids and tectonic activity. since virtually all geodynamic processes are related to fluids, the conference constituted a further step to enhance our understanding of the special role of gases in the sesismogenic processes. the scientific sessions held in the ancient castle of the town of milazzo were dedicated to a debate on this problem and were introduced by an invited lecture by g. valensise from ingv, rome entitled «beyond earthquake prediction: gas geochemistry for investigating the structure and evolution of seismogenic areas». he emphasized how fluid geochemistry is a fundamental tool for filling in basic information gaps on where potential earthquake sources are, especially in areas of blind faulting or where the surface evidence for fault activity is limited. the prime target is to establish closer links between the active tectonics and fluid geochemistry communities, aimed at understanding how the earth works from a broader multidisciplinary perspective. the concepts expressed by g. valensise found many links in the talks given by k. notsu, the director of the earthquake chemistry laboratory, tokyo university, on the release of mantle helium related to fault activity and deep tremor, and by d. hilton from san diego, u.s.a. on the helium isotope in seismically-active regions of the san andreas fault zone and the north anatolian fault zone who provided the latest results obtained coupling isotope geochemistry and seismotectonics. other significant examples were given in the presentations by h. woith, from the gfz in vi preface potsdam, on the detection of earthquake-induced pore pressure variations by fluids monitoring, together with that of h. kämpf from the gfz in potsdam on the combined gas-geochemical and geophysical studies on the earthquake swarm area of vogtland-nw-bohemia who provided the most advanced insights into the combined seismological and geochemical approach to studying the seismogenic process. new perspectives on understanding co2 generation by mechanical stress application on carbonate rocks were presented by f. italiano, g. martinelli and p. plescia. the evidence that nowadays all efforts provide useful and new results should push the scientific community to pursue this direction. in the past, the study of seismic events was carried out considering every aspect, as is verifiable on a broader basis in the study of every natural phenomenon, leading to a better grasp of seismic phenomena and the achievement of important results obtained in a few test areas where research projects have been carried out in considerable depth and breadth (heinicke and martinelli, 2005). the activity of geochemical monitoring of the seismic areas carried out lasting recent years is based on modelling fluids circulation and interaction, defining their behaviour in a seismic-prone area. the geodynamic context of an area may strongly influence the fluids composition and behaviour in terms of both chemical and isotopic composition as shallow-originated fluids may mix with fluids coming from different depths of the crust and upper mantle. the mixing proportions may change over time due to both seasonal variations and crustal permeability modifications because of the development of a seismogenic process (stress accumulation, deformation, strain release, etc.). monitoring fluids circulating over a seismic-prone area will address the consequences induced by the development of the seismogenic process that may or may not generate a «seismic shock». following such an approach, geochemical monitoring is not related to the occurrence of an earthquake, but investigates all the modifications related to fault movements through the chemical and isotopic changes shown by the fluid phases rather than «chasing» seismic events. the main targets to achieve are a genetic characterization of the fluids on the basis of their chemical and isotopic features and an evaluation of temporal variations in terms of both chemical and isotopic composition. sampling carried out at regular time intervals over a long time-span allows the assessment of seasonal variations thereby distinguishing the changes induced by development of the seismogenic process. this will disclose «anomalies» in fluid behaviour related to modifications at depth induced by tectonic movements. if prolonged over time, such a methodological approach applied to every single investigated area will determine what and how it changes before, during and after a seismic crisis. vii preface however, in spite of a general consensus among geochemists regarding the methodological approach and the field and laboratory activities, few authors has provided well assessed connections between fluids and changes in local tectonic activity. the main reason is probably the relatively poor knowledge of the earthquake source physics. much attention has focused on «detecting» earthquakes, hence determining where the hypocenter and the epicentre are located (multiplication of denser and denser seismic networks). in the meantime, less scientific effort have been devoted to what happens in porous media and circulating fluids before, during and after seismic shocks have developed. for these reasons, many questions (e.g., how friction between rocks is overcome when slip occurs on a fault? or what happens to hanging walls when fast rate slip generates an earthquake?) remain open, making it clear that a complete understanding of the seismogenic process as a whole is far from complete and only multidisciplinary approaches will provide useful results in the future. the contribution of this special issue to enhancing our knowledge of the seismogenic process moves from an attempt to express the links connecting the geochemical features of fluids to tectonics with some examples from seismic areas of the czech republic, italy, germany and kamchatka. we all know that seismic events are far from being prevented, but we have to improve our basic knowledge on the seismo-tectonic processes occurring beyond the earthquake preparation process that influence the fluid regime. gases and waters are typical fingerprints of crustal deformation processes, and we must remember that monitoring radon emission was the first approach to geochemical monitoring activity. increases in radon concentration were observed in water from a deep well prior to the huge tashkent earthquake (7.5 on the richter scale) of april 26 1966 (ulomov and mavashev, 1971). since then, both, soil gas and ground-water radon concentration changes have been observed and considered potential precursors for earthquakes. radon concentrations in the vicinity of faults have been unusually high and many researchers have monitored the radon content in deep wells as a potential predictor of earthquake activity. as a general result, they found a gradual increase in concentrations until the time of an earthquake, then radon emission decreases rapidly, although some variations can be observed related to earthquake aftershocks. similar behaviour is observed for all circulating fluids. those results yielded very important information for geochemists: hydrogeological and geochemical phenomena related to the earthquake preparation process are matters of controversy for many reasons including: changes in earthquake source characteristics; reviii preface sponse heterogeneity in time and space, and, although fluids are good strain indicators, the physical relation to strain is still not well understood. in the recent past the approach was aimed directly at earthquake forecasting and did not provide useful results in that direction. nowadays major efforts are dedicated to detecting all the fluids released and investigating the possible relations with other chemical and physical phenomena with the aim not to forecast any earthquake but to improve our knowledge of what occurs beyond the earthquake. the paper entitled «investigating correlations of local seismicty with anomalous geoelectrical, hydrogeological and geochemical signals jointly recorded in basilicata region (southern italy)» by g. colangelo et al., describes variations detected coinciding with the local seismicity. four papers in the volume are devoted to various aspects of radon gas, from a detailed revision of the detection techniques «measuring radon in soil gas and groundwaters» by constantin papastefanou, to evidence of rn degassing over three different geologic environments in italy, kamchatka and poland: the papers entitled «indoor and soil radon measurements in the hyblean foreland (south-east sicily)» by c. antoci et al.; «the nexus of soil radon and hydrogen dynamics and seismicity of the northern flank of the kuril kamchatka subduction zone» by p.p. firstov et al. and «radon and its daughter activities in magmatic areas of the karkonosze granite and adjacent volcano-sedimentary intrasudetic basin» by a. solecki et al. three papers present results collected by mid to long-term geochemical monitoring in germany: u. koch and j. heinicke «hydrological influences on long-term gas flow trends at locations in the vogtland/nw bohemian seismic region (germanczech border)» and in italy «the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring» by n. ciancabilla et al. and «anomalous pattern of geochemical data recorded in the seismically active site of pieschi (southern italy)» by g. colangelo et al. earth degassing is discussed with the methane-dominated gas emissions in romania by c. baciu et al. in «mud volcanoes and methane natural emissions in romania». the 8th icgg meeting of milazzo clearly demonstrated that a large and highly scientific community of «earthquake chemists» is actively working with seismologists and geophysicists. traditional cultural barriers have been broken and a new, in some ways pioneering, spirit was felt by participants. challenging tasks for the next icgg9 conference to be held in taipei in october 2007 (tsanyao frank yang organizer) will focus on a better definition of fluidporous media interaction processes, a better statistical evaluation of experimental data, a better definition of earthquake source physics and its associated chemistry. ix preface further attention will be devoted to the assessment of volcanic precursors and the description of geothermal reservoirs and hydrocarbon systems. pioneering novel researches designed to evaluate the earth’s evolution will flank advance in cosmochemistry and contribute to a wider dissemination of the innovative scientific and technologic features character of this kind of meeting. enzo boschi francesco italiano giovanni martinelli references heinicke, j. and g. martinelli (2005): preface: an historical overview, ann. geophysics, 48 (1), vviii. hilton, d., g. de leeuw, n. gulec, h. mutlu, e. furi, j. kulongoski, k. brown and m. tyron (2005): recent helium isotope studies in seismically-active regions: results from the san andreas fault zone, california and the north anatolian fault zone, turkey, in 8th international conference on gas geochemistry, icgg8, 2-8 october 2005, palermo and milazzo, italy, abstr. book, p. 17. italiano, f., g. martinelli and p. plescia (2005): mechanochemical carbon dioxide and methane generation by laboratory simulated fault friction conditions, in 8th international conference on gas geochemistry, icgg8, 2-8 october 2005, palermo and milazzo, italy, abstr. book, p. 34. kämpf, h., w.h. geissler and k. bräuer (2005): combined gas-geochemical and geophysical studies on the vogtland/nw-bohemia nonvolcanic earthquake swarm area, in central europe, in 8th international conference on gas geochemistry, icgg8, 2-8 october 2005, palermo and milazzo, italy, abstr. book, p. 18. notsu, k., t. dogan, h. sumino and k. nagao (2005): release of mantle helium related to fault activity and deep tremor activity, in 8th international conference on gas geochemistry, icgg8, 28 october 2005, palermo and milazzo, italy, abstr. book, p. 19. ulomov, v.i. and b.z. mavashev (1971): 1971. forerunners of the tashkent earthquake, izv. akad. nauk uzb. ssr, 188-200. valensise, g. (2005): beyond earthquake prediction: gas geochemistry for investigating the structure and evolution of seismogenic areas, in 8th international conference on gas geochemistry, icgg8, 2-8 october 2005, palermo and milazzo, italy, abstr. book, p. 24. woith, h., a. pekdeger, c. milkereit, r. wang and j. zschau (2005): earthquake induced pore pressure variations deduced from fluid monitoring, in 8th international conference on gas geochemistry, icgg8, 2-8 october 2005, palermo and milazzo, italy, abstr. book, p. 26. annals 48, 2, 2005defdef 215 annals of geophysics, vol. 48, n. 2, april 2005 key words mt. etna – local magnitude – volcano seismicity 1. introduction in order to know the size of an earthquake without considering the produced effects, richter proposed the definition of magnitude and related it to the maximum amplitude of the ground displacement. the «local magnitude» ml (richter, 1935), is defined with the relationship log logm a al 0= where a is the maximum amplitude peak to peak measured in mm, recorded by a standard woodanderson seismometer with natural period of 0.8 s, magnification 2800 and damping factor 0.8. the quantity «−loga0» is defined empirically with respect to a reference earthquake, which describes the variation of maximum amplitude (a) of the event related to the epicentre distance (∆). geometric spreading, elastic attenuation and scattering of seismic waves, therefore, influence the amplitude decay. richter fixed a0 (∆) level at 1 µm for a distance of 100 km. later, to evaluate magnitude in a more practical approach, principally when the recording of strong earthquakes is clipped in amplitude, empiric relationships were developed using the duration of the seismic event by solov’ev (1965), tsumara (1967) and many other authors. in the last twenty years, earthquake magnitudes were always estimated at mt. etna volcano with the duration of the seismic event using appropriate relationships. caltabiano et al. (1986), used the serra pizzuta calvarina (esp) station of permanent seismic network run by istituto internazionale di vulcanologia (iiv) of the cnr of catania, for the following relationship: . . .log logm 1 367 2 068 0 212d = + +x ∆ local magnitude estimate at mt. etna salvatore d’amico and vincenza maiolino istituto nazionale di geofisica e vulcanologia, sezione di catania, italy abstract in order to verify the duration magnitude md we calculated local magnitude ml values of 288 earthquakes occurring from october 2002 to april 2003 at mt. etna. the analysis was computed at three digital stations of the permanent seismic network of istituto nazionale di geofisica e vulcanologia of catania, using the relationship ml = loga+ alog∆ − b, where a is maximum half-amplitude of the horizontal component of the seismic recording measured in mm and the term «+ alog∆ − b» takes the place of the term «− loga0» of richter relationship. in particular, a = 0.15 for ∆ < 200 km, b = 0.16 for ∆ < 200 km. duration magnitude md values, moment magnitude mw values and other local magnitude values were compared. differences between ml and md were obtained for the strong seismic swarms occurring on october 27, during the onset of 2002-2003 mt. etna eruption, characterized by a high earthquake rate, with very strong events (seismogram results clipped in amplitude on drum recorder trace) and high level of volcanic tremor, which not permit us to estimate the duration of the earthquakes correctly. ml and md relationships were related and therefore a new relationship for md is proposed. cumulative strain release calculated after the eruption using ml values is about 1.75e + 06 j1/2 higher than the one calculated using md values. mailing address: dr. salvatore d’amico, istituto nazionale di geofisica e vulcanologia, sezione di catania, piazza roma 2, 95123 catania, italy; e-mail: damico@ct.ingv.it 216 salvatore d’amico and vincenza maiolino where τ is the duration time of the event in seconds and ∆ is hypocentre distance in km. these authors studied a dataset of 70 earthquakes with hypocentre distance within 11 km for an extremely local relationship. the difference between pand s-waves arrival time was used to estimate the hypocentre distance. istituto nazionale di geofisica e vulcanologia (ingv) supplied the reference magnitude. later, cardaci and privitera (1996) introduced a new relationship to calculate duration magnitude for the permanent seismic network of iiv, based on methodology proposed by real and teng (1973). the dataset analysed by the authors was composed of 198 earthquakes recorded between 1990 and 1994, the reference magnitude, supplied by istituto nazionale di geofisica e vulcanologia, is comprised between 2.0 and 3.5 and was estimated on stations far from mt. etna using the duration of event recording. 2. magnitude of mt. etna earthquakes at present the mt. etna permanent seismic network, of the istituto nazionale di geofisica e vulcanologia, sezione di catania (ingv-ct), consists of 30 stations. the seismic signals are acquired continuously and are transmitted via radio to centro acquisizione dati sismici (cads) of the ingv-ct where they are digitally saved with a sampling rate of 125 hz. as routine, the magnitude of earthquakes recorded by the permanent seismic network of (ingv-ct) is calculated using the duration of the seismic event recorded on a drum recorder using the relationship of caltabiano et al. (1986). the reference station was esp until 1999 and thereafter ema. usually, when an event is «truncated» by the occurrence of another seismic event, the duration is estimated by amplitude decay. recent seismic swarms, which occurred during the opening of the eruptive fractures of the last eruptions (2001 and 2002-2003), were characterized by a high earthquake rate, with very strong events (seismogram results clipped in amplitude on drum recorder trace) and high level of volcanic tremor. figure 1 shows the seismogram of the seismic swarm during the onset of 20022003 mt. etna eruption. from 00:27 gmt of 27.10.2002, it was very difficult to estimate the fig. 1. seismogram at ema station recorded between 26.10.2002 at 23:30 gmt and 27.10.2002 at 00:59 gmt. 217 local magnitude estimate at mt. etna duration of each earthquake and the related magnitude. in order to verify the duration magnitude calculated for the earthquakes of 2002-2003 mt. etna eruption, we worked to simulate a woodanderson seismometer and then computed local magnitude with the relationship (lahr, 1999) log logm a a bl = + -∆ where a is maximum half-amplitude of the horizontal component of the seismic recording measured in mm and the term «+ alog ∆ − b» takes the place of the term «−loga0» of richter relationship. in particular, a = 0.15 for ∆ < 200 km, b = 0.16 for ∆ < 200 km. the approximation for this parametric form is smaller than 0.2 in comparison with correction values for source-receiver distance from richter’s table (di grazia et al., 2001). ∆ is hypocentre distance in km and is calculated by the relationship d h q 2 2 = + +∆ ^ h where d is epicentre distance in km, h is the depth of the earthquake in km b.s.l. and q is the altitude of the station in km a.s.l. 3. the 2002-2003 mt. etna eruption in the night between october 26 and 27, 2002 a seismic swarm occurred in the central upper part of mt. etna. it was the start of a new eruption of etna that formed fissures on both the ne and s flanks of the volcano. on october 27 eruptive fissures opened on the higher flank of the volcano produced high fire fountains, evolving into ash columns (calvari et al., 2004). on october 29, numerous tectonic structures on the eastern flank of the volcano were activated through seismic swarms, causing serious damage to s. venerina village and in the neighbouring areas on mt. etna’s eastern flank (azzaro and mostaccio, 2003; azzaro and scarfì, 2003). the eruption gave rise to a huge lava emission from both fracture fields and powerful explosive activity from the southern one. after 94 days the eruption ended on january 28, 2003. much of the seismicity occurred during the first day of the eruption, while the remarkable clusters of earthquakes on the southeastern flank are largely related to the 29 october seismic crisis. an overall number of 862 earthquakes (md ≥ ≥1) were recorded by the permanent seismic network run by ingv-ct. maximum magnitude observed was 4.4 and 56 earthquakes exceeded md = 3.0. 4. data analysis the dataset used in this work is composed of 288 earthquakes occurring between october 2002 and april 2003 (fig. 2). the magnitude (md) of these earthquakes is between 1.0 and 4.4. the focal depths of the earthquakes are concentrated within the uppermost 5 km below sea level (b.s.l.). to study the relationship for the local magnitude we used the digital stations esp, emv and emg. the first is equipped with a lennartz le-3d/20s seismometer; the others (emv and emg) are equipped with lennartz le-3d/1s sensors. the former is a broadband seismometer with corner frequency ω 0 = 0.05 hz (20 s), output voltage k = 1000 v/m/s and damping h = = 0.707; the le-3d/1s have a corner frequency ω 0 = 1.00 hz, output voltage k = 400 v/m/s and damping h = 0.707. fig. 2. mt. etna map. the grey squares indicate the stations used to calculate the local magnitude (ml) or the duration magnitude (md). the earthquakes epicentres are indicated with crosses; earthquakes occurring on october 26 and 27, 2002 are indicated with asterisks. 218 salvatore d’amico and vincenza maiolino the errors on the epicentre and hypocentre coordinates are smaller than 2 km. 4.1. methodology for each selected earthquake (fig. 3a), we calculated a discrete fourier transformation (dft) on the horizontal components of the seismic record (fig. 3b). the velocity response curve of the seismometer (fig. 3c) is defined by the relationship ~ ih k e 20 2 2 0 2 = + ~ ~ ~ ~ ~ ] ^ g h where k is the sensitivity of the transducer in v/m/s, ω is the angular frequency, ω 0 is the natural period, and h is the damping. this kind of table i. one-dimensional vp velocity model. top layer (km) vp velocity (km/s) 0.00 3.00 0.50 3.59 2.00 4.00 4.00 4.80 6.00 5.59 12.00 6.50 30.00 8.00 fig. 3a-h. the dadisp worksheet used to simulate a seismic signal recorded by standard wood-anderson seismometer. a) velocity seismic signal recorded by geophone; b) dft of velocity seismic signal; c) velocity response curve of geophone; d) displacement response curve of geophone; e) wood-anderson response curve; f) velocity spectrum divided by displacement response curve; g) corrected spectrum multiplied by wood-anderson response curve; h) wood-anderson simulated seismic signal. analytic locations of the earthquakes were performed by hypoellipse routine (lahr, 1999), using a onedimensional vp velocity model with 7 plane-parallel layers (hirn et al., 1991) as described in table i. a b c d e f g h 219 local magnitude estimate at mt. etna sensor has no calibration coil and it is very difficult to know the real technical parameters, for this reason we used the data reported in the factory datasheet. velocity response curve was transformed in displacement response curve (fig. 3d), by multiplying it for the angular velocity ω (bath, 1974) before correcting the velocity spectrum. multiplying the response curve of a standard wood-anderson seismometer (fig. 3e), with static magnification 2800, damping 0.8 and natural period 0.8 s (richter, 1935), with the correct displacement spectrum (fig. 3f) we obtained signal of fig. 3g. the simulated wood-anderson seismogram (fig. 3h) was obtained with dft inverse of fig. 3g. we used the software dadisp 4.0 to analyse the digital signals on the whole seismic record. the half-amplitude a, used to obtain the magnitude, was calculated as a mean of n-s and e-w components. urhammer and collins (1990), verified that the 2800 static magnification is determined by the manufacturer and it is not the static magnification determined from measurements of the natural period and tilt-sensitivity. they suggest using the value of approximately 2080. assuming a static magnification of 2800 (as has been common practice, e.g., bakun et al., 1978; kanamori and jennings, 1978; luco, 1982; del pezzo and petrosino, 2001) will lead to a systematic over estimation of ml by an average of 0.13 ml units (urhammer and collins, 1990). we decided to apply the 2800 static magnification value to this work because it is more widely used in common practice and the calculated ml is comparable with ml calculated from stations of different networks. 5. results 5.1. comparison with duration magnitude figure 4 shows the magnitude values ml calculated at esp compared with the corresponding magnitude values md. the grey squares indicate earthquakes occurring between october 26 and 27. md values are overestimated respect to ml and are scattered for about 1.0 units on md axis below ml = 2.2. this data scattering is due to error on estimate of earthquake duration. moreover, grey squares dataset shows a different trend with respect to white squares dataset. fig. 4. comparison between md and ml at esp station. the grey squares indicate the earthquakes occurring from october 26 at 00:27 gmt and october 27; the thick black line of linear regression and relative equation refers to white squares dataset. 220 salvatore d’amico and vincenza maiolino a linear regression between md and ml values was performed, excluding the october 26 and 27 dataset. the relationship obtained is . .m m0 6668 1 008d l= + with coefficient of variation of the regression r2 = 0.7737. figure 5 shows the magnitude ml (at esp station) with respect to time origin. earthquakes with higher magnitude were recorded fig. 6. difference md − ml (at esp station) with respect to time origin. the grey squares indicate the earthquakes occurring on october 26 at 00:27 gmt and october 27. fig. 5. ml calculated at esp station with respect to time origin. the grey squares indicate the earthquakes occurring on october 26 at 00:27 gmt and october 27. 221 local magnitude estimate at mt. etna during the opening of the eruptive fractures (october 26 and 27, grey squares), while the earthquakes with the smaller magnitude were recorded only at the end of the eruption. we may assume that the estimate of the magnitude md on october 26 and 27 is not perfectly correct. in fact, as mentioned above, when there are many earthquakes in a short time, or when there is higher amplitude of the volcanic tremor, it is more difficult to read the real duration of the earthquake. moreover, the strongest earthquakes recorded on the drum recorder, show clipped amplitude (see fig. 1). for these reasons it is very difficult to estimate the amplitude decay. this amplitude saturation infers only the trace drawn by the drum recorder pen and it does not affect the digitally recorded signal. this theory is in agreement with fig. 6, where the difference md − ml (at esp station) with respect to time origin is shown. the graph again highlights md higher than ml except for earthquakes on october 26 and 27, 2002. figure 7 compares the ml values, at emg and emv, with ml values at esp; there is a good agreement between esp and emg values; the emv values have a good agreement above ml > > 1.5, but are overestimated by about 0.5. figure 8a,b shows velocity and wood-anderson simulated traces of two seismic events. the signal-noise ratio at emv station is smaller than other two stations, both during the eruptive phase (fig. 8a), with a high level of volcanic tremor, and at the end of the eruption (fig. 8b). we think that emv values are affected by site-effect overestimating magnitude values with respect to esp and emg values. this siteeffect is more evident for earthquakes with ml >1.5, while for the other earthquakes the signal-noise ratio at emv station is very small and it is not possible to estimate the maximum amplitude peak to peak clearly. following gasperini (2002), we proceeded to a calibration of a new relation of md on the basis of the dataset of ml magnitudes excluding the october 26 and 27 earthquakes, and of the duration values. in our analysis we computed the coefficients of a linear regression, equivalent to the caltabiano et al. (1986) formula, of ml with both logτ and log∆ as independent variables. this gives ( . . ) ( . . ) ( . . ) log log m 2 477 0 099 0 464 0 172 2 655 0 227 l ! ! ! = + + x ∆ with coefficient of variation of the regression r2 = 0.770. the linear regression computed is nevertheless affected by errors on duration estimate. in order to reduce the errors in the coefficients we performed another linear regression excluding data with residuals obtained from previous refig. 7. relation between ml estimated at emg station (white), at emv station (grey) and at esp station. 222 salvatore d’amico and vincenza maiolino fig. 8a,b. 27.10.2002 at 05:46 gmt (a) and 03.03.2003 at 13:42 gmt (b) events recorded at esp, emv and emg station. left: traces acquired by seismic station in velocity; right: wood-anderson simulated traces. fig. 9. relation between moment magnitude mw (white squares), local magnitude (mbb) at broadband stations of mednet (black squares), duration magnitude md (grey triangles) calculated with the new relation and local magnitude ml at esp station. a b 223 local magnitude estimate at mt. etna gression greater of 0.25 units. the result is ( . . ) ( . . ) ( . . ) log log m 2 494 0 073 0 438 0 131 2 644 0 176 l ! ! ! = + + x ∆ with coefficients of variation of the regression r2 = 0.873. 5.2. comparison with magnitude values calculated using different methods figure 9 plots md values obtained from the new duration-magnitude scale (grey triangles) versus ml values. we also compared the local magnitude values estimated by the broadband stations of mednet seismic network located in sicily (black squares) and the moment magnitude estimated by seismic moment (white squares). the magnitude (mbb) is calculated using the richter relationship (1935) as a mean value from the horizontal components of the aio and vae stations (mednet, 2003). the moment magnitude (mw) is calculated with kanamori’s relationship (lay and wallace, 1995) . . log m m 1 5 10 73w 0 = -c m where m0 (keilis-borok, 1957) is seismic moment obtained by source parameters (brune, 1970) at station esp (giampiccolo et al., 2003). a good agreement is shown between ml values and new md values. moreover, we observe that, with a few exceptions, mbb values agree with ml values, and mw values agree with ml values for ml > 2.7. 6. strain release figure 10 shows the cumulative strain release (joule1/2) calculated by md (thin line) and by ml values (thick line). the strain release value of the earthquake was obtained as the square root of the energy e in erg, which is estimated with richter’s relationship (1958) fig. 10. cumulative energy strain release in the period october 14, 2002-april 5, 2003. thin line represents strain release calculated by md values, while thick line represents strain release calculated by ml values. dark grey area highlights october 27 day and light grey area the eruptive period (october 28, 2002-january 27, 2003). 224 salvatore d’amico and vincenza maiolino . . .log e m m9 9 1 9 0 024 2 = + for m ≤ 4.5 . .log e m11 8 1 5= + for m > 4.5 where m is the magnitude. in the figure two periods are highlighted: the former (dark grey) indicates the seismic swarm occurring on october 27, corresponding to the opening of the eruptive fractures, while the latter (light grey) represents the eruption beginning on october 28 and ending on january 27. a marked difference of cumulative strain release between md and ml series is observable during the seismic sequence of october 27, due to the underestimated md values as seen in figs. 4 and 6. in the later period, where it is easier to estimate the duration of the earthquakes, the strain release values are comparable. after the eruption the overall difference of strain release calculated from ml values and from md values is about 1.75e + 06 j1/2. 7. conclusions the aim of this work was to estimate local magnitude by simulating a wood-anderson seismometer and compare the results with different magnitude scales. ml values calculated at esp and emg are absolutely coherent, while emv values are overestimated by 0.5. ml values compared with mbb and mw values show a good correlation. md values seem to be overestimated. although it is difficult obtaining reliable md values with this dataset for the strongest earthquakes, corresponding to the opening of the eruptive fracture a try to recognise a relationship between ml and md was performed. a new duration-magnitude scale is proposed. the dataset used and the ml values calculated are reported in the appendix. in conclusion, it is remarkable that in environments with high seismic noise, such as mt. etna volcano, the magnitude estimates based on the measurement of the ground amplitude are more reliable and that some care must be taken in using a magnitude scale based on coda duration for low values of magnitude when the noise level is high. as the correct estimate of seismic parameters is important for a quantitative evaluation of volcano dynamics, the wood-anderson magnitude scale should be routinely determined together with duration-magnitude in volcano monitoring. a software program to reach these objectives is in preparation at this time. acknowledgements we are grateful to f. mulargia and s. castellaro for their constructive criticism that strongly improved the early version of this paper. 14.10.2002 03.34.14 2.6 2.0 14.10.2002 03.55.16 2.6 1.8 14.10.2002 04.40.30 2.1 1.9 15.10.2002 21.22.13 2.0 1.8 16.10.2002 10.10.21 1.5 1.2 17.10.2002 19.28.58 2.1 1.5 17.10.2002 19.41.44 1.5 1.6 18.10.2002 04.04.15 2.1 2.1 appendix. dataset used for the analysis and ml values. time origin md ml time origin md ml 18.10.2002 04.06.29 1.9 1.7 18.10.2002 04.47.12 1.7 1.6 22.10.2002 01.39.07 2.1 1.8 22.10.2002 16.29.15 2.5 2.3 26.10.2002 21.35.54 2.4 1.8 26.10.2002 21.46.51 2.3 1.6 26.10.2002 21.55.39 2.4 1.6 26.10.2002 22.04.57 2.4 1.4 225 local magnitude estimate at mt. etna 26.10.2002 22.17.57 2.3 1.6 26.10.2002 22.25.38 2.5 1.8 26.10.2002 22.28.25 2.4 1.9 26.10.2002 22.33.40 2.4 2.2 26.10.2002 22.40.45 2.5 2.2 26.10.2002 22.51.16 2.5 2.0 26.10.2002 23.13.08 2.4 2.2 26.10.2002 23.22.29 2.7 2.7 26.10.2002 23.27.48 2.4 1.8 26.10.2002 23.30.22 2.4 1.7 26.10.2002 23.46.34 2.4 2.0 26.10.2002 23.49.42 2.4 1.8 27.10.2002 00.12.57 2.3 1.4 27.10.2002 00.13.19 2.6 2.6 27.10.2002 00.13.29 3.0 2.8 27.10.2002 00.16.45 2.7 2.9 27.10.2002 00.21.11 2.4 2.1 27.10.2002 00.21.51 2.7 2.5 27.10.2002 00.23.49 2.5 2.4 27.10.2002 00.26.29 2.5 3.0 27.10.2002 00.29.39 3.1 3.4 27.10.2002 00.30.39 3.2 2.8 27.10.2002 00.32.38 2.9 3.7 27.10.2002 00.34.12 3.1 3.4 27.10.2002 00.35.03 3.0 3.3 27.10.2002 00.36.09 3.1 2.4 27.10.2002 00.41.49 3.2 3.9 27.10.2002 01.01.40 2.7 2.9 27.10.2002 01.07.46 2.4 3.1 27.10.2002 01.08.18 2.6 3.3 27.10.2002 01.11.25 3.0 3.5 27.10.2002 01.13.32 3.3 3.9 27.10.2002 01.23.48 3.0 3.8 27.10.2002 01.26.24 3.0 3.3 27.10.2002 01.28.17 3.5 4.1 27.10.2002 01.29.27 2.7 3.1 27.10.2002 01.30.30 2.8 3.3 27.10.2002 01.33.24 2.6 2.7 27.10.2002 01.34.25 2.4 2.6 27.10.2002 01.36.13 2.4 3.0 27.10.2002 01.37.11 2.6 3.0 27.10.2002 01.38.28 2.8 3.4 27.10.2002 01.42.21 2.7 3.3 27.10.2002 01.53.01 2.5 3.2 27.10.2002 02.15.17 2.5 3.4 27.10.2002 02.18.29 3.2 3.9 27.10.2002 02.29.28 3.5 4.0 27.10.2002 02.39.10 3.3 4.0 27.10.2002 02.42.11 3.4 4.2 27.10.2002 03.18.15 2.8 2.2 27.10.2002 03.28.55 2.8 3.5 27.10.2002 03.53.26 2.5 3.3 27.10.2002 04.13.12 2.4 3.1 27.10.2002 04.17.07 2.9 3.8 27.10.2002 05.02.04 2.5 3.1 27.10.2002 05.20.57 2.6 3.2 27.10.2002 05.25.05 2.6 3.5 27.10.2002 05.31.11 3.3 4.2 27.10.2002 05.46.45 3.4 3.9 27.10.2002 06.06.55 3.4 3.4 27.10.2002 06.26.13 2.8 3.3 27.10.2002 06.28.14 2.9 3.8 27.10.2002 06.49.32 2.9 3.8 27.10.2002 07.32.06 3.2 4.4 27.10.2002 08.04.32 2.8 3.4 27.10.2002 10.07.56 2.7 3.3 27.10.2002 10.24.48 2.7 3.4 27.10.2002 11.03.01 2.6 3.2 27.10.2002 12.09.58 2.7 2.9 27.10.2002 12.16.05 2.6 3.0 27.10.2002 12.44.21 2.3 3.1 27.10.2002 13.23.56 2.7 3.3 27.10.2002 13.34.56 2.6 3.3 27.10.2002 14.00.44 2.7 3.8 27.10.2002 14.42.44 2.5 3.1 27.10.2002 14.56.29 2.7 3.4 27.10.2002 15.51.07 3.0 3.8 27.10.2002 15.56.17 2.8 3.2 27.10.2002 16.02.09 2.9 3.2 27.10.2002 16.07.46 2.6 2.9 appendix (continued). time origin md ml time origin md ml 226 salvatore d’amico and vincenza maiolino 27.10.2002 16.47.50 2.7 3.1 27.10.2002 22.04.17 2.6 2.8 28.10.2002 03.01.40 3.2 4.0 28.10.2002 09.12.37 3.2 2.8 28.10.2002 11.40.10 3.1 3.2 28.10.2002 11.51.32 2.8 3.0 28.10.2002 16.27.04 3.0 3.2 28.10.2002 23.25.47 2.0 1.8 29.10.2002 01.31.46 2.6 2.7 29.10.2002 02.32.48 2.3 1.8 29.10.2002 07.22.33 2.6 2.4 29.10.2002 07.27.14 2.6 2.0 29.10.2002 08.34.33 2.9 3.5 29.10.2002 09.56.49 2.0 1.8 29.10.2002 10.02.09 3.0 2.8 29.10.2002 10.02.20 4.4 3.7 29.10.2002 10.04.41 3.1 3.1 29.10.2002 10.12.51 2.4 2.3 29.10.2002 10.13.25 2.8 2.8 29.10.2002 10.17.37 2.5 2.5 29.10.2002 10.18.48 2.1 2.2 29.10.2002 10.34.58 2.9 2.9 29.10.2002 10.56.09 3.6 3.5 29.10.2002 10.59.42 2.1 2.0 29.10.2002 11.02.00 4.0 3.4 29.10.2002 11.20.06 1.9 1.6 29.10.2002 11.22.04 1.7 1.5 29.10.2002 11.32.42 1.9 2.2 29.10.2002 11.51.40 2.7 2.3 29.10.2002 12.21.57 2.1 2.2 29.10.2002 13.25.31 2.7 2.7 29.10.2002 15.49.50 3.8 3.6 29.10.2002 16.39.46 4.0 4.1 29.10.2002 17.14.00 4.1 3.5 29.10.2002 19.07.48 1.7 1.6 29.10.2002 20.35.16 2.0 1.7 29.10.2002 22.24.47 2.8 2.4 30.10.2002 00.00.13 3.1 2.9 30.10.2002 02.16.17 2.0 2.1 30.10.2002 02.20.28 2.5 2.5 30.10.2002 07.20.06 2.5 1.8 30.10.2002 10.05.38 2.2 2.0 30.10.2002 10.06.23 2.5 1.8 30.10.2002 10.47.00 2.6 2.4 30.10.2002 15.25.43 3.2 3.3 30.10.2002 15.38.37 2.6 2.5 30.10.2002 21.13.20 1.9 1.6 30.10.2002 21.14.42 2.5 2.1 30.10.2002 21.15.03 2.1 1.7 30.10.2002 21.17.35 1.7 1.7 30.10.2002 21.18.34 1.5 1.6 31.10.2002 00.44.54 1.8 1.8 31.10.2002 06.51.40 1.9 1.5 31.10.2002 10.41.04 3.2 2.8 31.10.2002 11.22.12 2.4 2.2 31.10.2002 18.07.22 2.7 2.4 31.10.2002 18.50.10 2.1 1.8 31.10.2002 20.22.20 2.0 2.2 01.11.2002 01.29.51 2.5 2.2 01.11.2002 05.16.37 2.7 2.2 01.11.2002 15.32.03 3.1 2.9 02.11.2002 10.06.55 2.0 2.0 02.11.2002 17.09.54 2.8 2.3 02.11.2002 23.08.16 2.5 2.4 03.11.2002 00.22.30 2.3 2.1 03.11.2002 05.32.22 1.4 1.6 03.11.2002 05.35.14 2.1 1.8 03.11.2002 05.36.02 2.5 2.1 03.11.2002 10.21.59 3.5 3.4 03.11.2002 13.03.09 2.7 4.2 03.11.2002 13.43.40 2.2 2.0 04.11.2002 02.49.52 2.7 2.3 04.11.2002 05.29.55 1.9 1.7 04.11.2002 08.47.43 2.5 2.4 04.11.2002 10.52.35 3.0 2.7 04.11.2002 10.54.20 3.1 3.1 05.11.2002 18.54.47 2.7 2.4 05.11.2002 19.00.13 2.8 2.4 07.11.2002 09.03.18 2.3 1.9 07.11.2002 15.07.49 2.4 2.1 appendix (continued). time origin md ml time origin md ml 227 local magnitude estimate at mt. etna 07.11.2002 16.35.30 2.6 2.1 14.11.2002 03.31.10 2.8 2.6 16.11.2002 23.19.43 2.7 2.7 17.11.2002 09.26.21 2.8 2.8 17.11.2002 09.56.18 2.9 2.9 19.11.2002 10.42.03 2.6 2.5 24.11.2002 06.55.55 2.7 3.0 24.11.2002 07.38.29 2.8 2.5 24.11.2002 10.27.36 2.5 2.5 24.11.2002 11.03.36 3.0 2.9 24.11.2002 13.56.23 2.7 2.6 24.11.2002 14.53.02 2.8 3.0 24.11.2002 15.40.12 2.6 1.9 02.12.2002 12.20.38 2.8 2.9 03.12.2002 13.50.25 3.0 2.9 03.12.2002 21.07.00 2.8 3.2 05.12.2002 00.40.34 2.6 2.2 05.01.2003 15.38.56 2.1 1.7 20.01.2003 06.46.43 2.3 1.9 20.01.2003 21.26.50 1.9 2.0 21.01.2003 00.51.30 1.6 1.5 25.01.2003 03.29.18 2.7 2.5 25.01.2003 08.37.13 2.3 1.8 30.01.2003 20.25.06 1.5 0.6 01.02.2003 04.32.25 1.5 1.0 01.02.2003 09.20.03 2.1 1.2 01.02.2003 10.52.08 2.0 1.5 02.02.2003 02.32.51 1.4 1.1 02.02.2003 19.47.32 1.2 0.7 08.02.2003 19.37.09 2.0 0.8 09.02.2003 08.46.10 2.7 1.8 09.02.2003 13.26.52 2.2 1.0 10.02.2003 13.40.07 2.5 1.9 11.02.2003 00.43.44 2.5 1.8 12.02.2003 19.01.28 1.9 1.4 13.02.2003 05.29.10 2.9 2.4 13.02.2003 05.32.41 2.5 2.9 13.02.2003 05.33.01 3.8 3.8 13.02.2003 05.44.35 2.9 2.1 13.02.2003 05.48.00 2.5 1.2 13.02.2003 06.53.43 2.9 2.0 13.02.2003 16.15.32 1.7 0.5 13.02.2003 17.55.36 1.7 0.6 14.02.2003 04.44.11 2.6 1.8 14.02.2003 05.24.03 1.5 0.9 15.02.2003 19.11.33 1.5 0.7 16.02.2003 12.55.09 2.2 1.6 17.02.2003 01.30.43 2.5 1.9 18.02.2003 18.17.36 1.3 0.6 18.02.2003 22.15.06 2.6 1.9 18.02.2003 22.16.14 1.6 0.9 21.02.2003 13.46.23 2.0 0.9 21.02.2003 14.36.11 1.5 0.8 22.02.2003 01.08.39 2.0 1.0 23.02.2003 01.58.45 2.1 1.5 26.02.2003 18.35.02 2.1 1.6 01.03.2003 21.10.35 2.4 1.3 02.03.2003 11.55.07 1.4 1.1 03.03.2003 20.48.44 1.5 1.1 03.03.2003 20.49.23 1.4 0.7 03.03.2003 20.58.14 2.4 2.4 03.03.2003 21.02.48 2.6 2.0 04.03.2003 05.34.33 1.5 0.6 04.03.2003 05.38.06 1.5 0.6 05.03.2003 08.07.42 1.4 0.5 05.03.2003 08.09.53 2.2 1.6 08.03.2003 04.56.12 2.0 1.3 09.03.2003 07.55.59 3.0 3.0 09.03.2003 07.57.25 2.5 2.1 09.03.2003 08.00.05 2.3 1.9 09.03.2003 08.25.36 2.5 2.1 09.03.2003 08.28.48 2.5 2.3 09.03.2003 08.30.27 2.1 1.7 09.03.2003 08.31.31 2.2 1.6 09.03.2003 08.55.21 2.2 1.7 10.03.2003 05.04.53 2.5 1.6 11.03.2003 06.26.44 1.7 0.9 12.03.2003 01.44.39 1.2 1.5 14.03.2003 03.42.31 2.9 2.5 14.03.2003 22.19.26 1.8 1.8 appendix (continued). time origin md ml time origin md ml 228 salvatore d’amico and vincenza maiolino 15.03.2003 00.14.57 1.2 1.0 16.03.2003 17.48.45 1.9 1.0 16.03.2003 21.58.06 2.0 1.3 17.03.2003 20.56.27 1.0 0.5 18.03.2003 01.49.19 1.5 0.9 18.03.2003 10.09.33 1.6 1.0 18.03.2003 18.14.12 2.1 1.7 19.03.2003 04.51.11 1.5 1.0 19.03.2003 15.42.51 2.1 1.6 20.03.2003 05.48.07 1.3 0.7 21.03.2003 15.29.59 1.7 1.1 22.03.2003 14.49.12 2.1 1.5 23.03.2003 04.02.03 1.5 1.1 23.03.2003 20.36.39 2.0 1.4 25.03.2003 03.57.14 2.6 1.7 25.03.2003 07.16.10 2.5 2.3 25.03.2003 08.12.00 2.3 2.1 25.03.2003 12.28.19 2.0 1.2 25.03.2003 13.12.53 2.1 1.6 25.03.2003 14.08.24 1.7 1.2 26.03.2003 08.25.48 2.0 1.3 26.03.2003 10.30.24 1.3 0.5 29.03.2003 14.40.29 1.5 0.9 29.03.2003 16.22.36 2.1 0.7 30.03.2003 13.42.30 1.3 0.8 30.03.2003 13.48.29 1.7 0.7 31.03.2003 12.49.13 2.5 1.9 01.04.2003 13.26.46 3.1 2.9 02.04.2003 17.15.31 1.8 0.9 05.04.2003 19.19.14 2.9 2.3 05.04.2003 19.45.27 2.1 1.3 05.04.2003 20.35.36 2.0 1.8 appendix (continued). time origin md ml time origin md ml references azzaro, r. and a. mostaccio (2003): 1° rapporto preliminare sul terremoto etneo del 29.10.2002 ore 11:02 (loc.), (on line: http://www.ct.ingv.it/report/smmacro20021029.pdf). azzaro, r. and l. scarfì (2003): 1° rapporto preliminare sul terremoto etneo del 29.10.2002 ore 18:15 (loc.), (on line: http://www.ct.ingv.it/report/smmacro20021029c.pdf). bakun, w.h., s.t. houck and w.h.k. lee (1978): a direct comparison of «synthetic» and actual wood-anderson seismograms, bull. seismol. soc. am., 68, 1199-1202. bath, m. (1974): spectral analysis in geophysics (elsevier, amsterdam), pp. 563. brune, j.n. (1970): tectonic stress and the spectra of seismic shear waves from earthquakes, bull. seismol. soc. am., 54, 1875-1888. caltabiano, t., d. condarelli, s. gresta, d. patanè and g. patanè (1986): analisi preliminare dei dati della stazione sismica di serra pizzuta calvarina, cnr iiv open file rep. 10/86. calvari, s., p. del carlo, s. branca, d. andronico, n. bruno, m.r. burton, t. caltabiano, m. cascone, m. coltelli, d. condarelli, r.a. corsaro, a. cristaldi, e. de beni, g. garfì, g. lanzafame, l. lodato, v. longo, s. mangiagli, l. messina, l. miraglia, f. morabito, f. muré, m. neri, e. pecora, m. pompilio, g. salerno, g. sawyer, s. scollo and l. spampinato (2004): the first period of the 2002 etna eruption (27 october-5 november): preliminary results, quad. geofis., 32, 1-10. cardaci, c. and e. privitera (1996): applicazione di metodi per la stima della magnitudo ai terremoti locali registrati dalla rete sismica permanente dell’etna, cnr iiv open file rep. 3/96. del pezzo, e. and s. petrosino (2001): a local-magnitude scale for mt. vesuvius from syntheticwood-anderson seismograms, j. seismol., 5, 207-215. di grazia, g., h. langer, a. ursino, l. scarfì and s. gresta (2001): on the estimate of earthquake magnitude at a local seismic network, ann. geofis., 44 (3), 579-591. gasperini, p. (2002): local magnitude revaluation for recent italian earthquakes (1981-1996), j. seismol., 6, 503-524. giampiccolo, e., s. d’amico, d. patané and s. gresta (2003). attenuation of seismic waves and source parameters of shallow microearthquakes recorded at mt. etna volcano (italy), in egs-agu-eug joint assembly, 06-11 april 2003, nice, france. hirn, a., a. nercessian, m. sapin, f. ferrucci and g. wittlinger (1991): seismic heterogeneity of mt. etna: structure and activity, geophys. j. int., 105, 139-153. kanamori, h. and p.c. jennings (1978): determination of local magnitude, ml, from strong-motion accelerograms, bull. seismol. soc. am., 68, 471-485. keilis-borok, v.i. (1957): on the estimation of the displacement in an earthquake source and of source dimensions, ann. geofis., xii, 205-214. lahr, j.c. (1999): hypoellipse: a computer program for determining local earthquake hypocentral parameters, magnitude, and first motion pattern (y2k compliant), u.s. geol. surv. open file rep. 99-23. lay, t. and t.c. wallace (1995): modern global seismol229 local magnitude estimate at mt. etna ogy (academic press, san diego, ca, u.s.a.). luco, j.e. (1982): a note on near-source estimates of local magnitude, bull seismol. soc. am., 72, 941-958. mednet (2003): mediterranean very broadband sismographic network (on line: http://mednet.ingv.it). real, c.r. and t.l. teng (1973): local richter magnitude and total duration in southern california, bull. seismol. soc. am., 63, 1809-1827. richter, c.f. (1935): an instrumental earthquake magnitude scale, bull. seismol. am., 25, 1-32. richter, c.f. (1958): elementary seismology (freeman, san francisco, california), pp. 768. solov’ev, s.l. (1965): seismicity of sakhalin, bull earthquakes res. inst., tokyo univ., 43, 95-102. tsumara, k. (1967): determination of earthquake magnitude from total duration of oscillations, bull. earthquakes res. inst., tokyo univ., 53, 1-48. urhammer, r.a. and e.h. collins (1990): synthesis of wood-anderson seismograms from broadband digital records, bull. seismol. soc. am., 80, 702-718. (received december 18, 2003; accepted november 17, 2004) vol. 48, 01, 05ok.qxd 93 annals of geophysics, vol. 48, n. 1, february 2005 key words radon – indoor-bedrock relationship – granitoids 1. introduction in the czech republic indoor radon measurements have been performed by the national radiation protection institute (nrpi) since 1990. up to now more than 130 000 track-etch detectors (kodak lr 115) have been distributed within the whole country. the primary random distribution of detectors based on the demands of district and regional authorities was soon changed for goal – directed distribution according to geological radon risk maps in various scales (mikšová and barnet, 2002). the soil gas radon database has been built up since 1990. according to the uniform methodics used in the czech republic the soil gas radon data produced by the czech geological survey and companies joined in the association radon risk are comparable and reproducible and serve as a basis both for radon risk mapping and statistical characterization of geological units and rock types. the content of the database is described in detail in mikšová and barnet (2002). up to now the database comprises the soil gas radon data from more than 8900 test sites (15 measurement each). since 1998 the czech geological survey has vectorised all 214 map sheets of geological maps on a scale 1 : 50 000. regional radon data are also available in austria (friedmann, 2000), united kingdom (appleton and miles, 2002), germany (siehl et al., 2000), slovenia (popit and vaupotic, 2002), luxembourg (kies et al., 1996) and other countries. the aim of the present study is to find a relationship between indoor and bedrock radon in the central bohemian plutonic complex (cbpc), czech republic. 2. experimental part 2.1. geological setting the central bohemian plutonic complex (cbpc) forms the elongated body of 3200 km2 between klatovy (sw) and říčany near prague (ne). according to the subdivision of the variscan orogenetic belt in the bohemian massif mailing address: dr. ivan barnet, czech geological survey, prague 1, klárov 3, 118 21 czech republic; e-mail: barnet@cgu.cz indoor – soil gas radon relationship in the central bohemian plutonic complex ivan barnet (1), jitka mikšová (1) and ivana fojtíková (2) (1) czech geological survey, prague 1, czech republic (2) state office for nuclear safety, prague 1, czech republic abstract the relationship of indoor radon measurements and radon in bedrock was studied in the granitoid central bohemian plutonic complex (cbpc). the indoor data were linked to vectorised geological and radon risk maps using the coordinates of particular dwellings. for each geological unit and rock type it was possible to calculate the statistical characteristics of indoor radon measurements. a clear relationship between indoor radon values and radon in bedrock was confirmed in all 7 districts situated on cbpc, where the study was performed. 94 ivan barnet, jitka mikšová and ivana fojtíková (matte et al., 1990) the cbpc is emplaced between two terranes – the weakly metamorphosed barrandian on the nw (or teplá-barrandian unit) in the sense of other authors – e.g., vrána and štědrá (1997), scheuvens (1999), scheuvens and zulauf (2000), dörr et al. (2002), zulauf et al. (2002) and medium to high grade metamorphosed moldanubian assemblage of precambrian and proterozoic rocks on the se. the tectonothermal history of both mentioned major units has differed since the proterozoic (fig. 1). the different p-t conditions are marked by the presence of largescale shear zones separating both units (west bohemian shear zone and hoher bogen shear zone on sw border and central bohemian shear zone on the se border). the cbpc was emplaced along the central bohemian shear zone during late devonian – early carboniferous. the data from pb-pb zircon evaporation ages, from u-pb zircon dating and 40ar/39ar biotite cooling ages presented by janoušek et al. (2000a) cover the interval between 336 and 351 myr. the relatively long time of magmatic activity imprints the petrogenetic variety of granitoid types and as a consequence has led to newly used terminology – the plutonic complex. a long fig. 1. the major granitoid types and the probable position of the central bohemian shear zone (modified after chlupáč et al., 2002; zulauf et al., 2002). codes of districts: ph – praha-east, bn – benešov, pb – příbram, pi – písek, st – strakonice, pj – plzeň-south, kt – klatovy. 95 indoor – soil gas radon relationship in the central bohemian plutonic complex period of petrogenetic research of the cbpc resulted in distinguishing more than 20 granitoid types. on the contrary, data coming from petrogenetic, geochemical and geochronological similarities simplify the variety of granitoids into four suites (janoušek et al., 2000a,b). the northern part of cbpc is formed by sázava suite comprising the amphibole – biotite diorite, granodiorite and tonalite. říčany intrusion crops out in the northeastern part of cbpc and is characterized by presence of porphyritic k-rich biotite granite. the čertovo břemeno on the south eastern border with moldanubian unit is formed predominantly by porphyritic melagranite and melasyenite (durbachite). the major radiometric difference of čertovo břemeno rocks from other granitoid types is caused by the relatively high concentration of uranium (15-20 ppm u are the frequent values) and presence of accessory minerals, namely zircon (up to 520 ppm – reported by holub, 1997). the geochemical and mineralogical composition of čertovo břemeno suite also causes the major indoor radon problems within all geological units on a countrywide scale. the blatná suite comprised mostly of amphibole – biotite granodiorites forms the central and southwestern part of cbpc. the petrochemical differences of the four distinguished granitoid suites can be observed from afm and k2o/sio2 graphs presented by janoušek et al. (2000a). a long time range of cbpc emplacement, the petrochemic and petrogenetic variety, and trends in tectonic and structural development implicate the importance of the central bohemian shear zone. the southwestern part of the shear zone was thoroughly studied (pitra et al., 1999; zulauf et al., 2002). the advantage of the southwestern part of the shear zone is the clearly observable tectonics separating cbpc from moldanubian and relatively narrow extent of emplaced granitoids. on the other hand, the lateral extent of cbpc is about 40-50 km without distinct patterns or relics of shear zone orientation in the central part. the processes of multiple intrusive activities during the variscan orogeny have covered the surface evidences of deep tectonics but regional geological studies (matte et al., 1990) and seismic profiling help to recover the paleosituation (tomek et al., 1997). the indoor data from 7 former districts extend by their position the range of cbpc’s granitoids. on the se, s and sw of cbpc the bedrock is formed by medium to high grade metamorphosed moldanubian paragneisses, migmatites and orthogneisses. the border with nw tepla – barrandian unit comprises the unmetamorphosed folded palaeozoic sediments (cambrian to devonian shales, limestones and quartzites) and lower proterozoic slightly metamorphosed shales locally with silicites. 2.2. radon measurements after one year exposure time the indoor radon detectors (two for each dwelling) were processed in the national radon reference laboratory in kamenná near příbram. the results of indoor geometric mean radon values for each dwelling were given in equilibrium equivalent concentration (eec). the central bohemian plutonic complex belongs to focused areas (barnet et al., 2002) and the density of indoor measurements exceeds the general average in other areas. the indoor measurements (together 16 145) were selected from 7 former districts praha-east (444), benešov (2985), příbram (6341), písek (2164), strakonice (1779), plzeň-south (1596) and klatovy (836) covering the areal extent of cbpc. for the purpose of this study the coordinates of family houses (digitized by the czech statistical office) from 7 above mentioned districts were selected due to the direct contact of houses with bedrock. the soil gas radon measurements were performed using the scintrex rd 200 radonmeter. the soil gas radon was sampled using the losttip method from 15 probes and the third quartile of the data set was used as a classification parameter together with the permeability of the test site, given in three categories (low-medium-high). this method is fully described in mikšová and barnet (2002) and serves as a uniform method for building site assessment in the czech republic. the mean soil gas radon values for particular rock types were calculated from the radon database of the czech geological survey. 96 ivan barnet, jitka mikšová and ivana fojtíková 3. method of geographical information system (gis) analysis the first phase of data processing was linking the database of centroids of dwellings (czech statistical office) with the database of indoor radon measurements (national radiation protection institute). this part of data processing was performed using the foxpro 3.0 programme and identificators of particular dwellings and municipalities. the resulting database from the area covering the territory of former 7 districts comprised 16 145 dwellings with defined indoor radon mean and maximum value (eec) and x, y coordinates. in the second phase of data processing this database was put into gis based on mge 7.1, oracle and microstation 95 programmes and transformed into the arcview 3.2. programme. the vectorised polygons of geological units and rock types were selected for the area covered by indoor measurements (16 map sheets of bedrock geological maps and radon risk maps on a scale 1 : 50 000). the areal analysis performed using the mge analyst and mapfinisher programmes linked the geological unit and rock type from the uniform legend for each dwelling. as each geological map comprises about 40-90 different rock types according to petrography and mineralogic composition, these rock types were grouped with respect to prevailing radon risk calculated from the soil gas radon database for each rock type. consequently each dwelling was characterised by indoor radon data (eec) and corresponding mean soil gas radon concentration in the underlying rock type and category of radon risk from bedrock. the database of nrpi does not contain data on the technical characteristics of the dwellings (namely the quality of sealing) but a sufficient number of indoor radon measurements enables this load to be neglected. 4. results the mean and maximum indoor rn concentrations in granitoid rock types of the cbpc are given in table i. within each district the mean indoor radon concentration in dwellings situated on the particular rock type was plotted against mean soil gas radon concentration. figure 2 illustrates the indoor – soil gas radon relationship in six districts. mean indoor values are given in eec. for praha – east district the dwellings were situated only on quaternary sediments and granites (insufficient for graphical presentation). from fig. 2 it is obvious that increasing soil gas radon concentration in rock types generally also causes an increase in indoor radon values. in all studied districts the highest indoor and soil gas radon concentrations are observed in durbachites, granites and granodiorites. the other rock types with enhanced indoor and soil gas radon concentrations are silurian black shales and proterozoic metasilicites (both concentrating uranium). the high indoor and soil gas radon concentration observed in ordovician sediments of strakonice district are bound to one village situated on the tectonic zone communicating with underlying granites. figure 3 illustrates the position of dwellings from 7 districts with indoor radon levels exceeding 1000 bqm–3 (eec). most of the houses are situated on the nw border of durbachites of čertable i. mean and maximum indoor rn concentrations in granitoids of cbpc (gr – granite, gd – granodiorite). type of granitoid mean rn max rn (bqm–3) eec (bqm–3) eec gd sedlčany 488.2 572.4 gr čertovo 397.2 463.4 břemeno gd kozárovice 350 400.8 gd benešov 311.5 388.3 gr marginal nw 310.2 377.4 gd blatná270.5 340.8 zvíkov gd červenský 219.8 285.6 gr říčany 167.9 211.4 gd kozlovice157.9 191.7 -maršovice gd pošáry 78.9 91.2 fig. 2. the relationship of indoor and soil gas radon in the rock types of 6 bohemian districts. explanation of rock types: ss – silurian sediments, dr – durbachites (syenites), gr – granites, gd – granodiorites, kr – moldanubian paragneisses, por – palaeozoic volcanites, ms – quartzites, erlanes, gn – moldanubian orthogneisses, ds – devonian sediments, pt – proterozoic metasediments, spr – loess, a – amphibolites, q – quaternary, ga – gabbros, os – ordovician sediments, kas – cambrian sediments, qgravel – quaternary gravel, n – neogene sediments, al – alluvial sediments, pts – proterozoic silicites. 97 indoor – soil gas radon relationship in the central bohemian plutonic complex 98 ivan barnet, jitka mikšová and ivana fojtíková tovo břemeno and sedlčany granodiorite but do not follow the contact of either rock body with other types of granitoids of cbpc. the position of high indoor radon dwellings resembles the orientation of the central bohemian shear zone, especially in the southeastern region of čertovo břemeno syenite and sedlčany granodiorite areas. this feature is not influenced by the density of indoor radon measurements and technical state of dwellings – in all 7 districts both factors are comparable. therefore we suggest that the quasi-linear course of the high indoor radon dwellings position corresponds to the paleocontact of two major terranes – teplá-barrandian and moldanubian. 5. conclusions a large-scale indoor rn and bedrock rn comparison was performed on the area of the central bohemian plutonic complex. the gis analysis based on the coordinates of particular dwellings proved its efficiency in comparing the indoor radon data and corresponding detailed geology. in all seven former districts the distinct trend of increasing indoor radon with increasing soil gas radon was proven in all rock types from precambrian to quaternary in spite of missing data on the technical state of dwellings. this fact supports the prognostic ability of radon risk maps based on the geology and soil gas radon measurements. even if these maps are not used for deriving the radon risk on particular building sites (which is influenced by minor local geological features) they can serve as a useful tool for distributing the track-etch detectors on a wide country scale for setting the priorities of indoor rn measurements. the linear position of extremely high indoor rn dwellings within the cbpc does not follow the local contacts of granitoid rocks or present tectonic network, but corresponds to the supposed course of the central bohemian shear zone. the palaeosuture dividing two variscan terranes was effaced after emplacing the cbpc. the weakened zone of the upper crust in the border area between teplá-barrandian and moldanubian terranes could prepare suitable a environment for radon migration along the deepseated shear palaeozones. the basement patterns fig. 3. the position of dwellings exceeding 1000 bqm–3 (eec) within the cbpc. the reader should compare the framed segment with the course of central bohemian shear zone in fig. 1. 99 indoor – soil gas radon relationship in the central bohemian plutonic complex of the buried variscan fabric were copied into the shallow subsurficial levels. the evidence of the early development of the bohemian part of the variscan orogen can thus be related to the radon distribution on a regional scale. references appleton, d. and j. miles (2002): mapping radon-prone areas using integrated geological and grid square approaches, in radon investigations in the czech republic ix and the sixth international workshop on the geological aspects of radon risk mapping, edited by i. barnet, m. neznal and j. mikšová (cgs praga), 34-43. barnet, i., j. mikšová and i. fojtíková (2002): the gis analysis of indoor radon and soil gas radon in major rock types of the czech republic, in radon investigations in the czech republic ix and the sixth international workshop on the geological aspects of radon risk mapping, edited by i. barnet, m. neznal and j. mikšová (cgs praga), 5-11. chlupáč, i., r. brzobohatý, j. kovanda and j. stráník (2002): geological history of the czech republic (academia prague), pp. 436 (in czech). dörr, w., g. zulauf, j. fiala, w. franke and z. vejnar (2002): neoproterozoic to early cambrian history of an active plate margin in the teplá-barrandian unit – a correlation of u-pb isotopic dilution – tims ages (bohemia, czech republic), tectonophysics, 352, 65-85. friedmann, h. (2000): radon in austria, in radon investigations in the czech republic viii and the fifth international workshop on the geological aspects of radon risk mapping, czech geol. surv., prague, 79-85. holub, f.v. (1997): ultrapotassic plutonic rocks of the durbachite series in the bohemian massif: petrology, geochemistry and petrogenetic interpretation, sb. geol. ved. (j. geol. sci.), ložisková geologie, mineralogie, lg, 31, 5-25. janoušek, v., d.r. bowes, g. rogers, c.m. farrow and e. jelínek (2000a): modelling diverse processes in the petrogenesis of a composite batholith: the central bohemian pluton, central european hercynides, j. petrol., 41 (4), 511-543. janoušek, v., d.r. bowes, c.j.r. braithwaite and g. rogers (2000b): microstructural and mineralogical evidence for limited involve ment of magma mixing in the petrogenesis of a hercynian high – k calc-alkaline intrusion: the kozárovice granodiorite, central bohemian pluron, czech republic, trans. r. soc. edinburgh, earth sci., 91, 15-26. kies, a., a. biell and l. rowlinson (1996): radon survey in the grand-duchy of luxembourg – indoor measurements related to house features, soil, geology and environment, environ int., 22, 805-808. matte, ph., h. maluski, p. rajlich and w. franke (1990): terrane boundaries in the bohemian massif: result of large-scale variscan shearing, tectonophysics, 177, 151-170. mikšová, j. and i. barnet (2002): geological support to the national radon programme (czech republic), bull. czech geol. surv., 77 (1), 13-22. pitra, p., j.p. burg and m. guiraud (1999): late variscan strike-slip tectonics between the teplá-barrandian and moldanubian terranes (czech bohemian massif): petrostructurel evidence, j. geol. soc., 156, 1003-1020. popit, a. and j. vaupotic (2002): indoor radon concentrations in relation to geology in slovenia, environ. geol., 42, 330-337. scheuvens, d. (1999): die tektonometamorphe und kinematische entwicklung im westteil der zentralbohmischen scherzone (böhmische masse) – evidenz für variscischen kollaps, frankfurter geowissenschaftliche arbeiten, ser. a, 18, p. 273. scheuvens, d. and g. zulauf (2000): exhumation, strain localization, and emplacement of granitoids along the western part of the central bohemian shear zone (bohemian massif), int. j. earth sci., 89 (3), 617-630. siehl, a., r. stegemann, r. lehmann and m. valdivia manchego (2000): mapping of the geogenic radon potential in germany – the state of matters, in radon investigations in the czech republic ix and the sixth international workshop on the geological aspects of radon risk mapping, edited by i. barnet, m. neznal and j. mikšová (cgs praga), 92-99. tomek, č., v. dvořaková and s. vrána (1997): geological interpretation of 9 h and 503 m seismic profiles in western bohemia, in geological model of western bohemia related to the ktb borehole in germany, edited by s. vrána and v. štědrá, j. geol. sci., ser. geology (prague), 47, 43-50. vrána, s. and v. štědrá (editors) (1997): geological model of western bohemia related to the ktb borehole in germany, j. geol. sci., ser. geology (prague), 47, pp. 240. zulauf, g., c. bues, w. dörr and z. vejnar (2002): 10 km minimum throw along the west bohemian shear zone: evidence for dramatic crustal thickening and high topography in the bohemian massif (european variscides), int. j. earth sci., 91 (5), 850-864. vol48/04/2005def 775 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words sif4 – vapour crystallisation – silica – degassing 1. introduction silica minerals are frequently found as vesicle fillings in recently erupted volcanic products, and are commonly interpreted as late or postmagmatic secondary phases. in a study of montserrat it was reported that groundmass silica phases occur in slowly cooled (dome) lavas only and not in pumice from explosive eruptions, indicating that crystalline silica was produced by vapour-phase crystallisation and devitrification within the lava dome (baxter et al., 1999). as crystalline silica may pose a serious health threat to humans if it enters the respiratory system, it was suggested that volcanoes with lava domes may pose a greater risk for silicosis than other types of volcanoes (baxter et al., 1999). silica enrichment in hydrothermal aureoles around magma chambers is commonly interpreted as the result of crystallisation from silica-saturated solutions, and leaching of other elements by acidic hydrothermal fluids (e.g., giggenbach, 1984). however, these processes are typical of post-intrusive alteration and it is unlikely that such processes act in erupting lava flows and domes. the importance of f species such as hf and sif4 in mass transport during degassing was demonstrated by white and hochella (1992), who describe the depletion of weathering surfaces of basaltic lava flows in sio2 and enrichment in ca, vapour-phase crystallisation of silica from sif4-bearing volcanic gases jan c.m. de hoog, manfred j. van bergen and michel h.g. jacobs department of earth sciences, utrecht university, utrecht, the netherlands abstract thermodynamic modelling of magmatic gases shows that sif4 may be an important f-bearing species at the high pressures typical of magma reservoirs. upon decompression during degassing, sif4 will react with water vapour to form hf and silica. common magmatic gases of high-t fumaroles seem to contain too little sif4 to be a significant source of silica, except if extremely large amounts of gas percolate through a small volume of rock, as is the case in lava domes. only if fluorine contents of the gases exceed 1 mol% detectable amounts of silica may be formed, but such high fluorine contents have not yet been observed in natural gases. alternatively, silica may be formed by heating of cool sif4-rich gases circulating in cooling lava bodies. we suggest that these mechanisms may be responsible for the deposition of crystalline silica, most probably cristobalite, observed in vesicles in lavas from lewotolo volcano (eastern sunda arc, indonesia). silica occurs as vapour-crystallised patches in vesicles, and is sometimes associated with f-phlogopite, which further supports f-rich conditions during deposition. because of the connection between f-rich conditions and high-k volcanism, we propose that late-stage gaseous transport and deposition of silica may be more widespread in k-rich volcanoes than elsewhere, and long-term exposure to ash from eruptions of such volcanoes could therefore carry an increased risk for respiratory diseases. the dependence of sif4/hf on temperature reported here differs from the current calibration used for temperature measurements of fumarolic gases by remote sensing techniques, and we suggest an updated calibration. mailing address: dr. jan c.m. de hoog, department of earth sciences, gothenburg university, guldhedsgatan 5a, 413 20 göteborg, sweden; e-mail: cees-jan@gvc.gu.se 776 jan c.m. de hoog, manfred j. van bergen and michel h.g. jacobs al, and mg during post-eruptive degassing of cooling magma bodies. here we present thermodynamic calculations that indicate that silica phases may form as a result of decompression of hf-rich gas during ascent of vapour-saturated magma, but also during heating of sif4-bearing gases. sif4 is a highly volatile compound at ambient pressures and temperatures (sublimation point = − 86°c at atmospheric pressure), which at elevated temperatures readily reacts with water vapour to form hf and silica. the reaction is very sensitive to temperature and is shifted in favour of sif4 at low temperatures (< 400ºc; rosenberg, 1973), whereas in high-temperature fumaroles hf is the dominant fluorine-bearing species. the results will be used to explain the presence of small patches of a crystalline silica phase, associated with fluorine-phlogopite, in vesicles in lavas from lewotolo volcano (eastern sunda arc, indonesia). fig. 1. back-scatter electron images of a vesicle filling containing several patches of crystalline silica (cr), inferred to be cristobalite. the dashed lines indicate the approximate outline of the vesicle and interconnecting veins before the vesicle-filling assemblage was formed. note the radial growth texture of the silica patches. the small needles sticking into the open space are f-phlogopite. inset shows detail of one of the silica patches. 777 vapour-phase crystallisation of silica from sif4-bearing volcanic gases 1.1. geological background lewotolo volcano is located at lomblen island (8°27′s, 123°59′e) in the nusa tenggara (lesser sunda) islands region of indonesia. strong fumarolic action at the summit produces a continuous outflow of hot (≤ 490°c) gases rich in h2o, co2, h2s, so2, hcl and hf (poorter et al., 1991), pointing to the presence of a degassing magma chamber at shallow depth. lewotolo has erupted porphyritic, highk calc-alkaline rocks over a wide range of silica contents from basalts to trachyandesites (4762 wt% sio2; stolz et al., 1990; hoogewerff, 1999). high fluorine contents of bulk rock (up to 800 ppm; vroon, 1989), groundmass glass (up to 6000 ppm), apatite and phlogopite (de hoog, 1995) indicate that the lewotolo magmas are relatively rich in fluorine. 1.2. petrography of vesicle fillings a detailed description of the vesicle fillings was presented in de hoog and van bergen (2000), in which the authors describe the presence of the rare mineral zirconolite in these fillings. we will summarize the relevant parts here. vesicles in about half of the lavas from lewotolo volcano are partially to completely filled with an assemblage of mainly alkali feldspar, plagioclase and a silica phase. in addition the vesicles contain small amounts of clinoand orthopyroxene and magnetite, and trace amounts of zirconolite, ilmenite and apatite. these partially filled former vesicles will be referred to as vesicle fillings (fig. 1). the fillings have sieve-like textures and can be distinguished from the groundmass by a coarser grain-size, higher porosity, and the absence of glass. in samples with large amounts of vesicle fillings the whole network of former vesicles is interconnected. if the assemblage fills veinlets, the texture is denser, with a lower porosity close to that of the groundmass. a crystalline silica phase grew as isotropic to low-birefringent patches with a radial structure into voids, or occupies interstitial spaces, e.g., between crystals or near cracks (fig. 1). individual patches can reach sizes up to 200 µm; the total volume in the rock is small, however, and does not exceed 0.02 vol%. optical microscopy did not conclusively distinguish between cristobalite and tridymite, but the common anisotropy and textures typical of shrinkage features point to cristobalite. thin whiskers of phlogopite occasionally intrude into the leftopen space within the former vesicles. the vesicle fillings were probably deposited from a migrating fluid phase, possibly derived from deeper parts of the magma chamber, during a very late stage of magmatic evolution (de hoog and van bergen, 2000). the morphology of later-stage minerals (crystalline silica, phlogopite) suggests that these may have crystallised from a less dense vapour phase. 2. thermodynamic modeling we studied the formation of silica phases as a result of decompression of sif4-rich phases using the equilibrium reaction (2.1) although equilibrium between a vapour phase and silica is a valid assumption for the formation of silica from a sif4-bearing gas, reaction (2.1) is only an approximation of equilibrium between vapour and siliceous melt. in the underlying paper we follow the deductions of white and hochella (1992) that the net free-energy differences between silica and siliceous melt are negligible compared to temperature and pressure effects. in the calculations it is assumed that the rest gases so2, h2s, co2 and hcl do not participate in the equilibrium reaction given by reaction (2.1). however h2s will react with h2o to form so2 and h2 following the reaction (2.2) the equilibrium of the reaction shifts to the right with decreasing pressure, thus consuming h2o. however, because the amount of s in volcanic gases is much smaller than the amount of h2o, the effect of reaction (2.2) on reaction (2.1) will be negligible. ( ) ( ) .g go( ) ( )g gh s 2h so 3h2 2 2 2d+ + .( )goh2+( ) ( ) ( )s g gsio 4hf sif2 4 2d+ 778 jan c.m. de hoog, manfred j. van bergen and michel h.g. jacobs applying the equilibrium condition to reaction (2.1) by setting the gibbs energy of the reactants equal to that of the products results in (2.3) (2.4) where p represents pressure, t thermodynamic temperature, r the gas constant, k the equilibrium constant and the superscript «o» refers to the reference pressure po=1 bar and pj represent the partial pressures of the substances j = sif4, h2o and hf. the gibbs energies of the pure substances, gj o(t), at 1 bar pressure are expressed in terms of the enthalpy of formation, ∆hf, jo , absolute entropies, sjo, and heat capacities cp, jo, as (2.5) the enthalpies of formation and absolute entropies refer to a common reference temperature t0 = 298.15 k and are found in thermodynamic tables (janaf, 1985; barin, 1989). the heat capacity data in these tables were fitted with polynomial functions with an accuracy better than 0.1%. the gibbs energy of solid sio2 at arbitrary pressure and temperature conditions is calculated using (2.6) where j=sio2 and vj, the molar volume of sio2. the molar volume of a solid substance is calculated using a murnaghan equation of state (2.7) ( ) ( )v v k k p p1 ( )k o o o o 1 o = + l l : d s +t $-+h∆=, t + ( )g p c dt t c dt v dp , , , j f j p j t t j p j t t j p p o o o o 0 0 o + * 4 # # # .$t-+h∆ c dt=( )g t s t c dt, , , j f j p j t t j p j t t o o o o o 0 0 +* 4# # +g-g4+( ,g p t)=( ,g p t t t g t2 sio hf o sif o h o o 2 4 2 ∆ ) ] ] ] g g g ( )lnrt k=lnrt=( ,g p t p p p p hf 4 sif h o 2 o4 2∆ ) c m where ko represents the isothermal bulk modulus and (ko)′ its pressure derivative at 1 bar pressure. the molar volume at 1 bar pressure, vo, is calculated using the expression (2.8) where αo represents the thermal expansivity at 1 bar pressure. polynomial expressions for ko, (ko)′ and αo as function of temperature and the molar volume of sio2 were taken from the work of swamy et al. (1994). depending on pressure and temperature, sio2 may exist in the forms α-quartz, β-quartz, tridymite, cristobalite, coesite and stishovite. from these six forms of sio2 we considered only α-quartz, β-quartz and tridymite because our study was limited to a pressure range between 0-2.5 gpa and temperature range between 1100-1400 k. note that cristobalite is not a stable phase at the pressure and temperature conditions of our investigation, although it is often observed to be the most common crystalline silica phase in volcanic rocks (e.g., baxter et al., 1999). the crystallisation of cristobalite, instead of a stable silica form, has a negligible effect on the results of the thermodynamic calculations compared to effects associated with errors in thermodynamic data. the calculations of the partial pressures of hf, sif4 and h2o and the partial pressure of the rest gas at arbitrary pressure and temperature conditions proceeds as follows. when a pressure is applied to the system or when the temperature of the system is changed, the partial pressures of hf, h2o and sif4 change and an amount of sio2 is consumed. at the condition (po,t), the number of moles nohf, nosif4 and n o h2o of each gas phase species is calculated from the measured partial pressures. according to eq. (2.1) at arbitrary conditions (p, t′ ) an amount, m, of sio2 reacts with 4m moles of hf to form m moles of sif4 and 2m moles of h2o. the amount of moles in the gas phase changes from nohf + nosif4 + n o h2o + n o restgas to nohf + nosif4 + n o h2o + n o restgas−m. the partial pressures of the substances in the gas phase are p n n m p p n n m p4 2 hf hf o h o h o o 2 2 = = +] ^g h t t( ) ( ) ( )expv v t dt t t o o o o 0 = af p# 779 vapour-phase crystallisation of silica from sif4-bearing volcanic gases (2.9) inserting these relations into eqs. (2.3) and (2.4) leads to (2.10) the gibbs energy difference ∆g(p,t) is calculated by eq. (2.4). it is constrained by thermodynamic data of the gas phase species and the solid forms of sio2. with nohf, nosif4, n o h2o and norestgas measured at the experimental condition (po,t), the amount m is calculated with eq. (2.10), leading to calculated partial pressures of the gas phase species at arbitrary pressure and temperature conditions. the algorithm of the computational procedure is included in the program xipt, which is a successor of the program txy-calc (jacobs et al., 1996; jacobs and spencer, 1996). ( , .exp n m n m n n n n m n m p p rt g p t 2 4 sif o h o o hf o sif o h o o hf o restgas o o 2 4 4 2 4 2 + + + + + + = ∆ ) ^ ^ ^ ] b h h g h l . p n n m p p n n p n n n n n m sif sif o restgas restgas o hf o sif o h o o restgas o 4 4 4 2 = + = = + + + ^ ^ h h the calculations were performed using the compositions of hot (up to 490°c) summit fumaroles from lewotolo volcano (poorter et al., 1991). equilibrium compositions and temperatures were recalculated by an iterative gasrestoration procedure (poorter et al., 1991) and are given in table i. hcl, hf and h2 values are in the normal range of volcanoes related to subduction zones, but co2 and so2 values and hf/hcl ratios are high compared to worldwide high-temperature fumarole data given by symonds et al. (1994). we note that, because of differences in solubility of the various volatiles, the composition of gases at higher pressures is potentially different from those at atmospheric pressure, but these effects are difficult to quantify and therefore not considered in the paper. 3. results and discussion 3.1. temperature dependence of hf/sif4 the variation of hf/sif4 with t for various partial pressures of hf in the system sio2-hfh2o-sif4 is plotted in fig. 2. note that the x-axis in fig. 2 is a log scale, so that the increase in hf/sif4 between two markers (2 units) corresponds to a 100-fold increase of hf/sif4. the temperature dependence of reaction (2.1) at 1 bar has been documented earlier by rosenberg (1973) and white and hochella (1992), and sif4 and hf are also included in the gasworks program used to calculate equilibrium compositions of volcanic gases (symonds et al., 1992). as shown in fig. 2, our results closely match those of rosenberg (1973), except for a slight deviation at lower temperatures. a few data points published by symonds et al. (1992) are consistent with these trends. however, the calculations of white and hochella (1992) deviate, showing a straight line with a slope which is considerably less steep than the curves predicted by rosenberg (1973) and our data. as largely the same thermodynamic databases were used in all cases, we hypothesize that the trends of white and hochella (1992) were based on models or assumptions that are not documented in their paper, which makes it difficult to verify their calculations. the agreement table i. recalculated compositions of lewotolo gas samples in mol% (after poorter et al., 1991). average 1σ (n = 4) h2o 73.5 4.4 co2 15.9 4.4 so2 7.76 2.5 h2s 0.86 0.51 hcl 0.169 0.106 hf 0.0465 0.0175 n2 1.39 1 h2 0.34 0.355 equilibrium temperatures are 950 ± 50°c; sampling temperatures were ∼ 490°c at 50 cm depth. ar, he and ch4 are present at ppm-level. calculated oxygen fugacities are ± 10−11 bar, slightly higher than the qfm-buffer. 780 jan c.m. de hoog, manfred j. van bergen and michel h.g. jacobs with the data of rosenberg (1973) and symonds et al. (1992) supports that our model is valid for 1 bar, and can be used to predict equilibria at higher pressures (see below). the curves in fig. 2 show the variation of hf/sif4 with t for a fixed value of phf and at a total vapour pressure of 1 bar. as production of sif4 consumes hf, phf is not constant, and the curves do therefore not represent cooling paths. however, hf consumption by production of sif4 is important only if psif4 is close to phf, so that in fig. 2 only the bottom part of the curves would be noticeably influenced. reaction (2.1) is strongly dependent on temperature and on partial hf pressure. for example, a drop in temperature from 1400 to 800 k will decrease hf/sif4 by over 3 log units (∼1250 times). the amount of sif4 will be higher than hf below 500 k if phf = 0.01 bar. therefore, at atmospheric pressure, sif4 does occur only in significant amounts at high partial pressures of hf and low temperatures. dilution of hf-rich gas with hf-poor gas of the same temperature will have a strong effect on sif4, e.g., diluting a gas 10 times will decrease psif4 by a factor 10000. dilution of volcanic with atmospheric gases will lower t and phf, two parameters with opposing effects on sif4 production. modelling shows that conservative mixing of high-t volcanic gases with air will increase the relative amount of sif4, because the temperature effect outweighs the dilution effect. fig. 2. variation of sif4/hf (expressed as log[phf/psif4]) with t (in k) at different partial pressures of hf, and 1 bar total vapour pressure (phf+psif4+ph2o = 1). continuous lines are from our own calculations, whereas dotted lines are data from rosenberg (1973), dashed lines are after white and hochella (1993), and the two circles are data points from symonds et al. (1992) with phf = 10−3.07 bar. uncertainties in our data due to uncertainties in heat of formation and enthalpy of sio2, hf, h2o and sif4 are about 25° and 0.2 log units. note that the relative importance of sif4 increases both with total amount of f in the system (i.e., phf), and with decreasing temperature. white and hochella (1992) trends are clearly offset and predict hf/sif4 variations with temperature that are much larger than other models. 781 vapour-phase crystallisation of silica from sif4-bearing volcanic gases 3.2. pressure dependence of hf/sif4 figure 3 illustrates the variation of phf/psif4 with pressure at p*hf = 10−2.5 and 10−1.0 and temperatures typical of magmatic gases (where p*hf is the partial pressure of hf(phf/ptotal); p*hf = =phf at atmospheric pressure). the ratio phf/ /psif4 increases strongly with decreasing pressure, especially in the low-pressure part of the system. the various curves that were calculated for different temperatures between 1100 and 1400 k are close to parallel. applying a pressure of 860 bar to a system initially at atmospheric pressure will shift the equilibrium by 3 log units, equivalent to a 10-fold increase in phf in fig. 2. therefore, at this pressure the figure is similar to the one at 1 bar, except that the continuous lines are valid at p*hf values that are ten times lower than indicated at 1 bar. even though the relative amount of sif4 increases at high pressure, the absolute quantities are still very low in volcanic gases, as hf concentrations are generally low (phf < 10−2) and the total amount of sif4 is even smaller than hf (as phf/psif4 > 10 5 at magmatic temperatures, see fig. 2). however, if gases would be more hf-rich, sif4 would become a significant component even at moderate pressures (fig. 3). for example, if p*hf = 10−1 then sif4 contains more than 1% of the total amount of fluorine over the whole temperature and pressure interval studied here, except at pressures lower than ∼100 bar, and at 1100 k it is the dominant fluorine species at pressures higher than 500 bar. we fig. 3. variation of log (phf/psif4) with total pressure for various temperatures at p * hf = 10−2.5 and p*hf = 10−1.0, respectively. the importance of sif4 increases with decreasing temperature, increasing phf, and increasing pressure. the circles represent data at 1 bar, and are equivalent to the values in fig. 2 at the same temperature and p*hf . 782 jan c.m. de hoog, manfred j. van bergen and michel h.g. jacobs note, however, that such high partial hf pressures have never been observed in nature. 3.3. formation of sio2 3.3.1. pressure-release induced crystallisation of sio2 because of the temperature and pressure dependence of the sif4/hf ratio, sio2 can be produced in two ways: 1) by an increase of the temperature of the system, or 2) by a decrease of pressure. as pressure decrease is common during magmatic degassing, whereby exsolved vapour rises through a magma column, we will discuss this process in more detail. when magma becomes volatile saturated, e.g., in response to decompression or fractional crystallisation, a separate fluid phase forms in which volatile components, including hf, will be concentrated. hf is a very reactive gas, and it will react with the melt to form sif4, following reaction (2.1), and other less volatile species, such as mgf2, caf2 and alf3 (white and hochella, 1992). this way, appreciable amounts of silica may be entrained in the volatile phase, the exact amount being determined by thermodynamic equilibrium and by the solubility and concentration of fluorine in the system. here we assume that thermodynamic properties during reaction with siliceous melt are not significantly different from reaction with pure sio2 (white and hochella, 1992; see methods). if the magma starts or continues to rise, vapour pressure decreases and reaction 1 will shift to the left resulting in precipitation (sublimation) of silica from the vapour. if we assume a magma which contains a vapour phase with phf = 10−2.6 bar, an isothermal pressure decrease from 1000 to 1 bar at 1400 k would cause a shift in logpsif4 from −8.5 to −11.6, and would result in the production of sio2. however, as the total amount of sif4 in the system is very small, the amount of sio2 formed during pressure release is limited. under the assumed conditions 1 mol sio2 would be formed from ∼794 kmol hf, or, expressed in weight, ∼260 kg hf is needed to form 1 g silica. this amount of hf is equivalent to ∼105 000 kg magmatic vapour. a dynamic model in which large amounts of gas percolate through a relatively small rock volume is therefore needed to deposit significant amounts of silica. in our example from lewotolo, a gas/rock ratio of 1300 is needed to deposit 0.2‰ silica by volume (the estimated amount of silica in some of the samples). however, while such as scenario might work for dome lavas, it is difficult to reconcile with the presence of silica patches within vesicles in lavas on the flanks of lewotolo volcano. if we apply the model to mt. etna (sicily, italy), which is one of the larger gasproducing volcanoes on earth, with an average hf production of 100-300 ton per day (francis et al., 1998; pennisi and le cloarec, 1998), the amount of silica formed during decompression of sif4-bearing gas could be about 1 kg per day, depending on assumptions regarding p*hf at depth. clearly, these amounts are exceedingly small. 3.3.2. the effect of changing temperatures in the above calculations we assumed that the temperature of the magmatic gas phase is constant. because the hf/sif4 ratio is strongly temperature dependent, a temperature decrease during degassing will counteract the formation of sio2 induced by pressure release. thermodynamic calculations on ascending magma-gas mixtures (mastin and ghiorso, 1998) for watersaturated magmas decompressing from 50 mpa to 1 atm suggest that temperature changes may vary between a 50°c decrease to an almost 10°c increase, depending on whether degassing takes place below the fragmentation depth and whether the magma is mafic or silicic. in the case of passive degassing (no fragmentation) the process is isenthalpic for silicic magmas (temperature may increase) and isentropic for basaltic magmas (temperature decreases). however, the temperature effect is small compared to the pressure effect (fig. 3). for example, in order to balance phf/psif4 in a 1400 k vapour during decompression from 1 kbar to atmospheric pressure, a simultaneous temperature decrease of almost 600°c would be needed. we conclude therefore that any temperavapour-phase crystallisation of silica from sif4-bearing volcanic gases tures variations during decompression and degassing have an effect on the formation of sio2 that is negligible compared to that of pressure release. 3.3.3. the effect of phf as almost all sif4 is converted to sio2 during decompression, the amount of sio2 formed is essentially limited by the amount of silicon initially present in the gas phase. at partial pressures of hf common in high-t fumaroles the amount of sio2 formed is extremely small. increasing the amount of sif4 in the gas phase in response to 1) a higher fluorine activity in the vapour, or 2) lower gas temperatures would increase the amount of sio2 formed. a temperature decrease of about 350 k would result in a ten times higher sio2 production, which is still rather insignificant. the effect of p*hf , however, is much stronger: an increase of p*hf from 0.001 to 0.01 results in a decrease of 3 log units of phf/psif4. in other words, while the amount of hf increases by a factor of 10, the amount of sif4 increases by a factor of 10000. if we assume an initial p*hf = 0.05 and t = 1250 k, the amount of sio2 formed during decompression is about 2.5 mol% (or 7.5 wt%) of the amount of hf present in the gas. the validity of such a scenario is difficult to evaluate, as little is known about the concentration of fluorine in late-stage magmatic vapours at high pressures. it is of interest to note that silica patches in the vesicles in lewotolo lavas are often associated with thin whiskers of f-phlogopite that stick out of the silica mass. this suggests a common origin of both phases and a very high fluorine activity during crystallisation (de hoog and van bergen, 2000). although such f-rich compositions have yet to be observed in volcanic gases, the co-existence with f-phlogopite makes crystallisation of silica from f-rich vapour an attractive hypothesis. as an alternative, silica may have been deposited from within the lava flow, i.e., not in response to depressurisation but as a result of the large temperature gradients in the cooling lava. although in cooling gases sif4/hf will increase and therefore the lava surface may become depleted in silica (white and hochella, 1992), the heating of sif4-rich gases is an efficient way of producing silica because of the large sif4/hf ratios of these gases compared to hot, pressurized magmatic vapour. for example, sif4/hf ratios of up to 0.2 and sif4 fluxes of up to 1 kton have been measured in the plume of popocatepetl (love et al., 1998), and because of the steep slope of sif4/hf ratios with temperature at low t (fig. 2), heating of these gases by only 100°c would convert most sif4 to sio2. in a similar manner, silica may be deposited locally by gas circulating in cooling lava bodies. it is of interest to emphasize that lewotolo is a high-k volcano, as k-rich magmatic systems are considered to be often enriched in fluorine as well (e.g., edgar et al., 1996; strecher, 1998). therefore, silica deposited during vapour decompression might be relatively common in volcanoes with high-k characteristics. we discovered silica patches (without f-phlogopite) similar in texture and appearance to those of lewotolo, in lavas from rinjani, a krich volcano on lombok, which supports this supposition. exposure to ash from eruptions of high-k volcanoes could therefore pose an increased risk for respiratory diseases, in a similar manner as eruptions from dome-forming lavas raised concerns about adverse health effects because of their high contents of crystalline silica (baxter et al., 1999). we note that lewotolo does not form lava domes, yet crystalline silica is present in many of its lavas. 3.4. implications for measurements of fumarole temperatures by remote sensing figure 2 shows that the equilibrium temperature can be calculated if phf and psif4 are known. this relation was used by francis et al. (1996) to remotely determine fumarole temperatures at vulcano (italy). the authors calculated that fumarole temperatures were 200-450°c, which was in good agreement with directly measured temperatures of 210-690°c. francis et al. (1996) used the thermodynamic calculations of white and hochella (1992), which, as discussed above, are inconsistent with 783 784 jan c.m. de hoog, manfred j. van bergen and michel h.g. jacobs the results of rosenberg (1973), symonds et al. (1992) and the calculations presented here (fig. 2). our thermodynamic model (cf. reaction (2.3)) predicts the temperature dependence of sif4/hf as follows: (3.1) in which it is assumed that ph2o = 0.8 bar and phf, psif4 << ph2o , which are common conditions in volcanic plumes and fumaroles. repeating the calculations of francis et al. (1996) with our thermodynamic data yields equilibrium temperatures around 100°c, with a maximum of ∼180°c. these values are much lower than the findings of francis et al. (1996) and directly measured temperatures of 250600°c. in addition, the calculated range of temperatures for the different fumaroles is much smaller than observed. we hypothesize that the initial match between remotely measured and observed fumarole temperatures was fortuitous, and suggest that more work is needed to confirm that the remote-sensing approach can be reliably used to estimate fumarole temperatures. the discrepancy might be due to the absence of equilibrium between the gas components (which is more likely for heterogeneous equilibria, in which case one of the phases in the reaction is a solid, here silica). alternatively, in view of our inferred temperature of ∼100°c, the remote-sensing results may represent the temperature of condensing water vapour (steam), as water is the main component of the gases. irrespective of these considerations, the strong influence of phf on calculated temperatures implies that errors in remote-sensing thermometry will be large unless hf can be measured simultaneously with sif4. acknowledgements we thank marie edmonds and particularly mike burton for insightful reviews, and mike carroll and roberto moretti for editorial handling of the manuscript. 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(1992): surface chemistry associated with the cooling and subaerial weathering of recent basalt flows, geochim. cosmochim. acta, 56, 3711-3721. s0329 annals of geophysics, 60, 3, 2017, s0329; doi: 10.4401/ag-7057 a study of radon and thoron concentration in the soil along the active fault of nw himalayas in india gulshan kumar1,4, punam kumari1, arvind kumar2, sangeeta prasher3, mukesh kumar1 1 department of physics, lovely professional university, phagwara, punjab, india 2 national center for research on earthquake engineering, taipei, taiwan 3 department of physics, kanya maha vidyalaya, jalandhar, punjab, india 4 govt. college, sarkaghat, mandi, himachal pradesh, india article history received may 18, 2016; accepted march 01, 2017. subject classification: radon concentration, lithological locations, active fault line, main boundary thrust. abstract the study has been conducted to analyse the radon and thoron flux in the soil of mandi district, himachal pradesh. the detectors have been rooted at seventy one lithological locations in the north-eastern part of the district. the average values of radon concentration has been observed as 4541 bq/m3 with maximum of 19970 bq/m3 at location no. 3 (n 32°00.46': e 76°51.74') and minimum of 867 bq/m3 at location no. 57 (n 31°45.65': e 76°51.56') and the thoron variation ranges from 37 to 6970bq/m3 with an average value of 1778 bq/ m3. the radon liberation at different positions has been correlated to the presence of the active fault to reveal the contributory aspects for abnormal release of radon in the soils. the spatial distribution of radon and thoron gas along the lines passing through the fault zones have unveiled the variances connected to the local tectonic structures. radon exhalation rates, radium contents and porosity of soil samples have been calculated and a correlation factor of 0.64 has been detected for the observed concentrations of thoron and the porosity of the soil. 1. introduction himalayan mountains are one of the most seismically active fault zones in the world. the monitoring of soil gases like hydrogen, helium, radonthoron, methane, carbon dioxide profiles in these zones may provide useful information before the seismic activities. anomalous changes in the subsurface soil gas concentrations may be used as tectonic activities precursor according to the dilitancy-diffusion model for earthquake occurrence [scholz et al. 1973]. moreover the soil gas monitoring has been developed as the effective tool in understanding the gas transportation mechanisms in the seismically active zones and many other fields of geosciences. however, the precursor predictions may not be accurate, but may be helpful to study the under soil activities [han et al. 2014] and is considered as precursor for various deportation processes, such as to discover fault interfaces and uraniumthorium ores [quattrocchi et al. 2000]. improved permeability of soils along active faults customarily favours the gas escape, hydraulic conductivity (for ground water and thermal fluids), however the thermal conductivity of the soil decreases with increase in the porosity and permeability of soil because the pore filling fluids have lower value of coefficient of thermal conductivity [poelchau et al. 1997]. the movement of radon through rocks under the earth largely depends on lithology, compaction, porosity and fractural/tectonic features [choubey et al. 1997, gunderson et al. 1998]. the presence of various fault systems and thrusts in any region provides secondary porosity for upward migration of thermal fluids. the thermal gradient may be diluted if fresh water is mixed in up flow of thermal fluid [sharma 1977, shanker 1988, cinti et al. 2009]. gas anomalies at active faults can be either ‘direct leak anomalies’ where the gas measured corresponds to the deep gas phase or ‘secondary anomalies linked to different mineralogy having only superficial roots like the anomalous distribution of radium [toutain and boubron 1999]. the measurement of various soil gases for earthquake monitoring and prediction of active faults zones has been reported by various researchers. [etiope and lombardi 1995, igarashi et al. 1995, ciotoli et al. 1998, guerra and lombardi 2001, al-tamimi and abumurad 2001, chyi et al. 2005, fu et al. 2005, singh et al. 2005, kumar et al. 2009, 2012, 2013a, 2013b, pereira et al. 2010, singh et al. 2010, sac et al. 2011, yang et al. 2011, li et al. 2013, walia et al. 2013, koike et al. 2014, han et al. 2014, jaishi et al. kumar et al. 2 2014, jashank, 2014, georgy et al. 2015, piersanti et al. 2015]. measurement of natural radon in soil is very important to determine because it helps in monitoring changes in natural background activity with time as a result of any radioactivity release [darko et al. 2015]. the soil gas and water radon has been measured using alpha guards in some areas of punjab and himachal pradesh for health risk assessments [bajwa et al. and walia et al. 2003]. chandrasekharam et al. [2005] and walia et al. [2005] have conducted studies of the himachal pradesh geothermal sub-province mainly on the famous thermal springs of manikaran and kasol along the parvati with the aim to characterize the geothermal resources with respect to their suitability for electric power production. other study by various authors [choubey et al. 1997, 2007, virk and walia 2000, walia et al. 2003] focused on radon monitoring in waters and soils for health hazard assessment and earthquake prediction research. some research papers have reported chemical [gupta 1996] and isotopic data [giggenbach et al. 1983] of the thermal waters. the present studies dealt with radon-thoron measurements in soils, measurement of radon exhalation rates and radium contents of mandi district, himachal pradesh, nw himalaya, india using the passive detectors lr -115 type 2 films and measurement of porosity of soil samples from sampling sites. the technique figure 1. geological map of the study area. soil gas study in nw indian himalayas 3 used is cost effective, easily applicable and less disturbed by different environmental conditions. the statistical variation in measurement of radon concentration is larger in summer than in winter [szabo et al. 2013] keeping in view this fact, study was performed in january and february, 2015 1. geological mapping of study area the study area is mandi (31°13'26"32°04'22" north latitude and 76°36'08" 70°23'26" east longitude) that includes the various thrust and fault systems especially mbt (main boundary thrust), chail thrust, palampur thrust, galma thrust, riwalsar thrust and various fault systems (figure 1). these faults and thrust are formed because of collision of indian and eurasian converging plates [gansser 1964]. this district lies partly on rocks belonging to the central himalayan zone some part of district lies on tertiary shale and sand stone. rocks of area represent the paleoproterozoic period and are strongly foliated with well-developed augen-gneiss, sericitechlorite, carbonaceous slates with lime stone residues, phyllite quartzite, mylonitic gneiss and porphyroblastic biotite gneiss with non-foliated granitoids. these types of geological formations near mct are cause of some geothermal regions in himachal pradesh. the high intensity of thermal energy (>100mw/m2) with temperature gradient of more than 200°c/km have been observed at some places in north west himalaya [shanker 1988]. the areas where aquifers are situated near to earth surface geothermal sources like, manikaran in kullu and tatapani in mandi district formed. whereas micaceous purple clay and silt with intrusive granite are found near mbt. the region under study has a good average rainfall (about 1331.5mm as compare to himachal pradesh’s average of 1251mm) including more than 2000mm in the jogindernagar belt hence a good quantity of the fresh water is seeped to the ground, when this feature is added to high porosity at certain places, unconfined aquifers situation is formed, which causes the elevated ground water levels around mct and mbt in mandi district [walia et al. 2005, chandrasekharam et al. 2008]. these are the oldest rocks exposed in himachal pradesh comprises dominantly of purple coloured arenaceous sediments with argillites and characterized by interstratified basic lava flows of the mandi-darla volcanic [geology and mineral resources of himachal pradesh, 2012]. 2. material and methods 2.1 measurements of the radon and thoron concentration in soil polyvinyl chloride pipe of length 0.25m and diameter of 0.06m with an air tight aluminium caps at the ends has been used as discriminator for the radon-thoron. the detectors lr -115 type -2 films were cut in to the pieces of size of 0.015m × 0.015m and placed at the bottom and top of the discriminator to record alpha particle tracks of thoron and radon and radon, respectively. figure 2 shows the position of the detectors along with different faults and thrust systems in the study area, whereas figure 3 shows the sketch of radon thoron discriminator used at 71 selected sites in the study area. after exposure to standard durations of 15 days the detectors were subjected to chemical processing in a 10 m analytical grade sodium hydroxide solution at (60 ±1)ºc, for 90 min, in a constant temperature water bath to enlarge the latent tracks produced by alpha particles. the washed and dried detectors were observed under an optical microscope (zeiss at 400× magnification) to count the alpha particle tracks. the counted tracks have been converted in to units of radon concentration of bq/m3 using calibration factor [eappen and mayya 2004]. 2.2 measurement of the radon exhalation rates and porosity from soil samples the soil samples collected from 71 different sites of study area (figure 2) have been dried and grinded figure 2. the position of the detectors installed along different faults and thrust system in the study area. kumar et al. 4 l oc at io n n o. a re a ex ha la ti on ra te (b qm -2 h -1 ) m as s ex ha la ti on r at es (b q kg -1 h1 ) r ad iu m c on te n ts (b q kg -1 ) r ad on co n ce n tr at io n (b q/ m 3 ) as m ea su re d in s oi l t ho ro n co n ce n tr at io n (b q/ m 3 ) as m ea su re d in s oi l p or os it y a m ou n t of r ad on a va il ab le fo r tr an sp or t to t he s ur fa ce (r ad on pr od uc ti on r at e pe r un it v ol um e) (b qm -3 h -1 ) 1 1. 14 0 0. 05 0 5. 34 32 73 95 7 0. 36 0. 01 46 2 0. 85 4 0. 03 8 4. 11 43 53 13 87 0. 38 0. 01 15 3 1. 67 0 0. 07 4 7. 98 19 97 0 59 50 0. 39 0. 02 17 4 1. 40 3 0. 06 2 6. 65 16 18 3 41 23 0. 39 0. 01 82 5 1. 32 8 0. 05 9 6. 26 12 74 0 14 90 0. 39 0. 01 76 6 1. 76 9 0. 07 8 8. 20 44 59 24 91 0. 40 0. 02 16 7 1. 50 7 0. 06 6 6. 97 10 67 87 3 0. 32 0. 01 90 8 1. 01 4 0. 04 5 4. 77 17 33 15 27 0. 31 0. 01 17 9 1. 07 9 0. 04 8 5. 28 84 47 34 47 0. 38 0. 01 52 10 1. 72 3 0. 07 6 8. 19 33 37 74 7 0. 36 0. 02 28 11 1. 00 9 0. 04 4 4. 71 93 70 20 48 0. 38 0. 01 22 12 2. 07 6 0. 09 2 9. 33 23 70 16 49 0. 38 0. 02 40 13 2. 11 0 0. 09 3 9. 96 39 40 69 7 0. 36 0. 02 69 14 1. 03 5 0. 04 6 4. 81 58 13 24 63 0. 42 0. 01 27 15 1. 64 3 0. 07 3 7. 41 39 83 36 0 0. 37 0. 01 85 16 0. 86 3 0. 03 8 3. 97 23 53 18 77 0. 41 0. 01 06 17 1. 78 2 0. 07 9 8. 25 89 3 36 7 0. 31 0. 02 25 18 0. 91 4 0. 04 0 4. 37 34 17 29 0 0. 35 0. 01 19 19 1. 97 6 0. 08 7 9. 53 13 40 29 6 0. 33 0. 02 68 20 1. 25 4 0. 05 5 5. 60 34 98 45 3 0. 31 0. 01 41 21 1. 28 5 0. 05 7 6. 37 31 80 37 3 0. 24 0. 01 86 22 1. 04 3 0. 04 6 5. 01 10 40 37 0. 34 0. 01 39 23 0. 97 6 0. 04 3 4. 52 48 30 38 27 0. 43 0. 01 23 24 1. 17 1 0. 05 2 5. 67 10 01 7 77 0. 31 0. 01 63 25 1. 69 8 0. 07 5 8. 35 56 55 20 60 0. 27 0. 02 32 26 1. 78 4 0. 07 9 8. 66 47 40 36 50 0. 31 0. 02 50 27 1. 31 6 0. 05 8 6. 29 56 48 43 66 0. 5 0. 01 79 28 0. 67 5 0. 03 0 3. 18 27 70 16 47 0. 38 0. 00 85 29 1. 65 0 0. 07 3 7. 71 42 77 13 23 0. 41 0. 01 99 30 1. 65 8 0. 07 3 8. 02 36 00 32 47 0. 32 0. 02 29 31 2. 63 5 0. 11 6 12 .4 1 43 23 69 3 0. 39 0. 03 32 32 0. 88 1 0. 03 9 4. 21 41 40 12 90 0. 35 0. 01 19 33 1. 49 4 0. 06 6 7. 04 20 30 60 0 0. 39 0. 01 89 34 2. 52 4 0. 11 1 11 .7 3 35 70 33 37 0. 41 0. 02 97 35 1. 55 6 0. 06 9 7. 12 49 23 33 8 0. 32 0. 01 86 soil gas study in nw indian himalayas 5 36 1. 57 8 0. 07 0 7. 20 16 47 60 0 0. 34 0. 01 87 37 1. 43 3 0. 06 3 6. 63 20 04 52 5 0. 32 0. 01 67 38 1. 35 5 0. 06 0 6. 61 80 17 48 67 0. 45 0. 01 96 39 1. 86 4 0. 08 2 8. 99 46 00 39 23 0. 42 0. 02 54 40 1. 42 0 0. 06 3 6. 88 39 70 29 07 0. 4 0. 01 92 41 1. 62 7 0. 07 2 7. 37 10 05 2 35 3 0. 36 0. 01 86 42 1. 52 1 0. 06 7 6. 94 14 12 3 69 70 0. 5 0. 01 80 43 2. 00 5 0. 08 9 9. 06 19 83 10 50 0. 39 0. 02 37 44 2. 07 5 0. 09 2 9. 57 10 33 6 17 97 0. 4 0. 02 57 45 0. 93 4 0. 04 1 4. 39 10 27 98 0 0. 38 0. 01 17 46 2. 06 1 0. 09 1 9. 44 73 00 20 33 0. 41 0. 02 47 47 1. 53 8 0. 06 8 6. 99 50 03 18 67 0. 4 0. 01 81 48 0. 89 6 0. 04 0 4. 26 24 06 11 80 0. 36 0. 01 18 49 2. 20 4 0. 09 7 10 .4 4 15 16 98 3 0. 37 0. 02 85 50 1. 98 4 0. 08 8 9. 72 14 73 13 53 0. 32 0. 02 76 51 1. 42 2 0. 06 3 6. 74 18 80 18 63 0. 37 0. 01 84 52 1. 25 5 0. 05 5 5. 81 78 07 58 20 0. 48 0. 01 52 53 1. 56 3 0. 06 9 7. 19 29 20 14 40 0. 39 0. 01 92 54 1. 70 5 0. 07 5 7. 88 43 40 38 47 0. 43 0. 02 04 55 1. 44 3 0. 06 4 6. 79 33 53 32 90 0. 4 0. 01 81 56 1. 15 2 0. 05 1 5. 35 30 00 54 0. 29 0. 01 42 57 1. 11 3 0. 04 9 5. 07 86 7 68 4 0. 36 0. 01 31 58 1. 19 7 0. 05 3 5. 59 99 3 16 7 0. 31 0. 01 51 59 0. 77 8 0. 03 4 3. 66 34 30 16 04 0. 39 0. 00 98 60 1. 44 3 0. 06 4 6. 88 26 74 15 72 0. 38 0. 01 95 61 0. 64 4 0. 02 8 3. 11 36 20 93 7 0. 36 0. 00 88 62 1. 36 9 0. 06 0 6. 72 16 80 17 07 0. 39 0. 01 92 63 1. 20 2 0. 05 3 5. 73 15 74 90 7 0. 36 0. 01 62 64 1. 16 0 0. 05 1 5. 46 43 22 12 34 0. 39 0. 01 46 65 1. 42 2 0. 06 3 6. 67 34 25 65 4 0. 4 0. 01 83 66 1. 57 2 0. 06 9 7. 49 22 32 10 87 0. 37 0. 02 10 67 1. 23 9 0. 05 5 5. 71 24 65 15 43 0. 43 0. 01 47 68 3. 31 7 0. 15 15 .4 1 56 78 24 66 0. 41 0. 04 08 69 0. 91 8 0. 04 1 4. 41 53 76 14 89 0. 37 0. 01 23 70 1. 30 4 0. 05 8 6. 27 34 56 98 7 0. 34 0. 01 75 71 1. 11 8 0. 04 9 5. 17 45 66 11 23 0. 38 0. 01 35 t ab le 1 . t he r ad on a nd th or on c on ce nt ra ti on s al on g w it h ex ha la ti on r at es (a re a an d m as s ex ha la ti on r at es ), ra di um c on te nt s, p or os it y an d ra do n pr od uc ti on r at e pe r un it v ol um e at s am pl in g po si ti on s kumar et al. 6 to very fine powder. 100 gm of each powdered samples were placed at bottom of the cylinder of radius 3.5 cm and length 7.5 cm [singh et al. 1997] (figure 4). lr-115 type-ii ssntd (0.015m × 0.015m) were placed at the top of the cylindrical enclosures and the container was sealed tightly for 90 days to establish the equilibrium. the detectors have been etched in 10m naoh at (60±5) ºc, for 90 min, in a constant temperature water bath to enlarge the latent tracks and counted using optical microscope (400 x). the tracks are converted in to radon activity using the calibration factor of 0.02tracks/cm2/day= 1bq/m3 [eappen and mayya 2004]. the area and mass radon exhalation rate has been calculated using the formula [amrani and sr. no. profile porosity (average) average area exhalation rates average radium contents average value (bq/m3 ) standard deviation value(bq/m3 ) radon thoron radon thoron 1 1 to 6 0.39 1.36 6.42 10163 2733 7111 1947 2 7 to 11 0.35 1.27 5.98 4791 1728 3862 1094 3 12 to 16 0.39 1.55 7.10 3692 1409 1431 865 4 17 to 22 0.35 1.46 6.81 2221 303 1267 143 5 23 to 27 0.36 1.39 6.70 6178 2796 2190 1746 6 28 to 34 0.38 1.65 7.76 3530 1734 856 1126 7 35 to 37 0.33 1.52 6.98 2858 488 1797 135 8 38 to 42 0.43 1.56 7.36 8152 3804 4165 2443 9 43 to 47 0.40 1.72 7.89 5130 1545 3826 492 10 48 to 51 0.36 1.63 7.79 1819 1345 432 377 11 52 to 55 0.43 1.49 6.92 4605 3599 2216 1803 12 56 to 60 0.35 1.14 5.31 2193 816 1184 744 13 61 to 63 0.37 1.07 5.19 2291 1184 1152 453 14 64 to 67 0.40 1.35 6.33 3111 1130 958 370 15 68 to 71 0.38 1.66 7.82 4769 1516 993 668 table 2. values of various soil parameters in different profiles (as shown in figure 2). average values along mct average values along mbt sr. no. serial no. of detectors as in figure radon concentration (bq/m3) thoron concentration (bq/m3) serial no. of detectors as in figure radon concentration (bq/m3) thoron concentration (bq/m3) 1 monitoring station no. 5, 9, 14, 21, 25, 62 very near to mct 6253 1923 monitoring station no. 69, 65, 59, 49, 34, 29, 36, 35, 37, 54, 46 very near to mbt 3801 1521 2 above serial no. 1 (towards district kullu) station no. 6, 10, 16, 22, 26, 61 6158 1297 above serial no. 1 (towards district kullu and mct) 68, 64, 60, 48, 28, 53, 45 3113 1502 3 next nearest stations above serial no.2 from chail thrust 2401 1661 4 nearest station from chail thrust (towards mbt) i.e. below mct (station no. 4, 8, 13, 20, 24, 63) 3258 1623 the stations below mbt towards palampur thrust) 70, 66, 56, 50, 30, 55, 47 3159 1698 5 next nearest stations below chail thrust ( towards mbt) from serial no. 4 5217 2186 the stations below mbt towards palampur thrust. station no. 71, 67, 57, 51 2445 1303 table 3. average values of radon-thoron concentration of a relative distance from mbt and mct. soil gas study in nw indian himalayas 7 cherouati 1999]. whereas the porosity η of the soil is calculated using following formula η=1-ρbulk/ρparticle [morgan et al. 2005]. 3. results and discussions the radon and thoron concentration along with exhalation rates, radium contents, porosity and radon production rate per unit volume recorded at 71 locations in the study area are shown in the table 1. the average concentration of radon and thoron gases in the study area has been found to be 4541 bq/m3 and 1778 bq/m3within a range of 867-19970 bq/m3 and 37-6970 bq/m3, respectively. in order to identify possible threshold values of anomalous radon and thoron concentration, various statistical methods have been used by different authors in the past [guerra et al. 2001, walia et al. 2005, fu et al. 2005]. in the present context, statistical threshold values of gas anomalies are fixed at average (µ) plus one standard deviation (σ). figures 5 and 6 shows the variation of radon and thoron concentration (bq/m3) at different sampling locations in comparison to average (µ) and average+standard deviation value (µ+σ). the anomalous value of radon has been observed at 8 locations (3, 4, 5, 11, 24, 41, 42 & 44). the locations 3, 4, 5, 11 & 24 are close to mct and locations 41 & 42 are close to mbt-ii. the location 44 is close to local fault in the study area. the anomalous value of thoron has been observed in 12 locations (3, 4, 9, 23, 26, 27, 34, 38, 39, 42, 52 & 54). the locations 3, 4, 9, 23 & 26 are close to mct and location 27 is close to mbt-i. whereas locations 39 & 42 are close to mbt-ii and locations 34, 38, 52 & 54 are close to the local fault in the study area. anomalies in measurement of radon and thoron concentration are more along and across mct than mbt and any fault system. more anomalies have been found in measurement of thoron concentrations. it may be due to shallower gas source in the study area [yang et al. 2005, kumar et al. 2013b]. the area and mass exhalation rates of radon have been calculated for each site and have been reported in table 1. the average value of area and mass exhalation rates have been found to be 1.46 bq/m2 h and 0.064 bq/kg h with a variation of 0.644bq/m2 h and 0.028 bq/kg h, respectively at location number 61 to 3.317 bq/m2 h and 0.15 bq/kg h respectively at location number 68. the radium content of the soil has been ranged between 3.1115.41 bq/kg with an average value for the area as 6.84 bq/kg. the porosity of the soil has been found to vary with a minimum (0.24) at location 21 to a maximum of 0.5 at two locations 27 and 42 with an average of 0.37 at five locations labelled with numbers 15, 49, 51, 66, 69. the correlation factor of 0.59 has been observed between radon and thoron concentration of the study area and a good correlation (0.64) between thoron and porosity has also been detected. similar kind of correlation has been reported by al jarallah et al. [2005] in a study related to construction materials (especially granite) used in saudi arabia. in this study very less correlation (0.03) between porosity and amount of radon available for transport to the surface has been recorded. however, at the sampling sites 51, 62 and 65 the values of radium contents, porosity and amount of radon available for transport to the surface recorded are 6.74 bq kg-1, 0.37, 0.0184 figure 3. sketch of radon-thoron discriminator (plastic cane with side cap including lr-115 films at the top of the cane and at the bottom of the cane) used in the present study. figure 4. schematic diagram of a container utilized for radon exhalation rate measurements in the present study. kumar et al. 8 bqm-3 h-1, 6.72 bq kg-1, 0.39, 0.0192 bqm-3 h-1 and6.67 bq kg-1, 0.4, 0.0183 bqm-3 h-1 respectively. for sampling sites 17 and 18 these values are 8.25 bq kg-1, 0.31, 0.022 bqm-3 h-15 and 8.02 bq kg-1, 0.32, 0.0229 bqm-3 h-1 and at sampling sites 57 and 71 values are 5.17 bq kg-1,0.38, 0.0135 bqm-3 h-1 and 5.07 bq kg-1,0.36, 0.0131 bqm-3 h-1. these observations shows that if the radium contents of some soil samples are comparable then with the porosity of the soil radon transport factor to surface will increase. also, if there is secondary porosity in any region due to presence of fault systems then permeability/ emanation factor of the soil will increase, which will further increase the radon transport to surface [cinti et al. 2009, ciotoli et al. 2016]. the average value of radon, thoron along with average exhalation rates, average radium contents and average porosity in different profiles (as shown in fig 2) has been reported in table 2. profiling 1, 2, 3, 4, 5 and 13 has been made along and across the main central thrust (mct) and profiling 6, 7, 8, 9, 10, 11, 12, 14 and 15 has been made along and across mbt and local faults in the study area. the radon has been found to vary in the range of 1680-12740 bqm-3 with an average of 6253 bqm-3 along the mct. the concentration of radon and thoron at mbt has varied in the range of 1516-7300 bqm-3 with an average value of 3980 bqm3 and 338-3847 bqm-3 with an average value of 1621 bqm-3, respectively. the values of radon concentration are decreasing by factor of 1.01 to 2.56 on both side of mct and mbt, while thoron concentrations are decreasing by factor of 1.01 to 1.5 on moving distance of 2 to 3 km from mbt and mct, with exceptions at some stations towards palampur thrust, this may be due to the fact that there exist numbers of local faults in between mbt and mct. the radon and thoron concentrations have been detected to increase with a movement from chail thrust (mct) to mbt in the jogindernagar region that may be attributed to the geological stress and strain in this region. the concentrations of radon and thoron along the mbt have been found lesser than that of the mct. this may be due to the reason that mct is under more geological stress and strain as compared to the mbt. the average values of soil radon measured along the mct (main central thrust) in the profiles 1, 2, 3, 4, 5 and 13 have been recorded as 10163 bq/m3, 4791 bq/ m3, 3692 bq/m3, 2221 bq/m3, 6178 bq/m3, 2291 bq/ m3 whereas the average thoron concentration in the same profiles has been observed as 2733 bq/m3, 1728 bq/m3, 1409 bq/m3, 303bq/m3, 2796bq/m3, 1184bq/ m3. the profile 1 has exceptionally high average values because the presence of sling zones along this profile. the radon concentrations have been noticed to decrease from jogindernagar to mandi along mbt-1 due to the decrease in porosity of the soil. the same trend has been observed when the observer has moved along the profile 8, 11, 10 that may be attributed to the cross presence of other faults systems. the decreasing trend of radon and thoron concentrations has been observed along the route followed through the profile 15, 3 and 4 that may be due to the higher radon exhalation rates, radium contents and more or less due to porosity of soil in the area of profile 15 than in soil of profiles 3 and 4. the slight elevation in the concentration along the sampling sites of profile 6 and 7 has been spotted that may have been caused by the presence of secondary fault systems (sundernagar fault)in the vicinity of sundernagar [mahajan et al., 2010] and may be due to high porosity in the soil of profile 6 (0.38) and profile 7 (0.33) and relatively high values of radon exhalation rates1.65 bq/m2 h (in profile 6) and 1.52 bq/m2 h (in profile 7) and high values of radium contents 7.76bq/ kg (in profile 6) and 6.98bq/kg (in profile 7). the results also shows that the mbt and mct of himalayan region is more active than turkish faults as reported by sac et al. [2011] as the values of soil radon concentration measured are more as compared to the values measured near the fault in the western turkey. however the values of soil radon concentration observed in the present study have been found to be lower as compared to the values measured in complex tectonic and seismic tangshan area of northern china (where earthquake ms 7.8 was occurred in 1976) as reported by li et al. [2013]. the average values of radon and thoron concentration in dharamshala near mbt and mct in himachal pradesh were 5992 bq/m3 and thoron values of 901 bq/m3 [kumar et al. 2013b]. the trend in observed value of radon in dharamshala region of himachal pradesh are similar to the present study, however the thoron values in mandi region are found to be almost double than in dharamshala region. the values of radon concentration are decreasing by factor of 1.01 to 2.56 on both side of mct and mbt, while thoron concentrations are decreasing by factor of 1.01 to 1.5 on moving distance of 2 to 3 km from mbt and mct, with exceptions at some stations towards palampur thrust (table 3), this may be due to the fact that there exist numbers of local faults in between mbt and mct. the present study may also be helpful to study soil gas study in nw indian himalayas 9 other natural features of study area like geothermal potential and ground water reservoirs, since high thermal energy flow with good thermal gradient have been observed by various researchers in nw himalaya. this heat flow may be due the melting and reductions of intrusive granites near to mbt and mct in mandi area and presence of uranium, thorium and potassium contents in the soil texture [rao et al. 1976, das et al. 1979, walia et al. 2005]. these reductions in basic strata of area may create faults which are the cause of secondary porosity and hence increase in the gases like radon and thoron along with thermal energy fluids with a path of flow of water to the surface. the beas valley, uhl valley and region from jogindernagar to mandi have potentially good source of ground water. thus elevated levels of radon and thoron gases in seismically active and faulty area may be used detect secondary porosity, so the monitoring of these radio nuclides will further help to study the other gases transport through porous medium of the soil. 4. conclusions the radon-thoron measurement in soil, measurement of radon exhalation rates and radium contents of figure 5. the variation of radon concentration (bq/m3) at different measuring stations in comparison to average (µ) and average+ standard deviation value (µ+σ). figure 6. the variation of thoron concentration (bq/m3) at different measuring stations in comparison to average (µ) and average+ standard deviation value (µ+σ). kumar et al. 10 mandi district, himachal pradesh, nw himalaya, india has been measured using the passive detectors lr-115 type-2 films. the anomalous value of radon-thoron has been reported along and around mct, mbt and local faults in the study area. radon concentration along mct have been found higher than that along mbt, this may be due to the reason that mct is under more geological stress and strain as compared to mbt. more anomalies have been recorded in the measurement of thoron concentration. the thoron concentrations values are very low at some places; this may be due to presence of deeper source in earth at these places. good correlation between porosity and thoron has been recorded in this study, which shows presence of local fault in the area. also it has been found that area exhalation rates, mass exhalation rates depends up on the radium contents of the soil. the porosity and seepage of radon and thoron may be helpful to study ground water potential and its reservoir in any region. the elevated levels of radon and thoron at certain places can be associated to the presence of secondary porosity in the soil texture. secondary porosity due to fracturing of basic strata may provide the easy pathway for upward movement of geothermal fluids. acknowledgements. authors are thankful to the people of the study area for their help and cooperation during the field work. one of the authors (gulshan kumar) is thankful to the principal, govt. college sarkaghat, for his cooperation during the work and providing the lab facilities. we are also thankful to anonymous reviewers for their valuable suggestions for improvement of the manuscript. references al-jarallah m., f. ur-rehman, m.s. musazay, a. aksoy (2005). correlation between radon exhalation and radium contents in granite samples used as construction material in saudiarabia. radiat. meas., 40(2), 625-629. al-tamimi, m.h., k.m. abumurad (2001). radon concentration along faults in north of jordan. radiat. meas., 34, 397-400. amrani, d., d.e. cherouati (1999). radon exhalation rate in building materials using plastic track detectors, j. radioanal. nucl. chem., 242(2), 269-271. bajwa, b.s., n. sharma, v. walia, h.s. virk (2003). measurements of natural radioactivity in some water and soil samples of punjab state, india. indoor and built environment, 12, 357-361. chandrasekharam, d., m.a. alam, a. minissale (2003). geothermal resource potential of himachal pradesh, india. international geothermal conference, reykjavik, sept, 2003 session # 4, 15-20 p. chandrasekharam, d., m.a. alam, a. minissale (2005). geothermal discharge at manikaran himachal pradesh, india. proceedings world geothermal congress, 2005 antalya, turkey, 24-29 april 2005. chandrasekharam, d., v. chandrasekhar (2008). geothermal resources in india: possibilities for direct use in the himalayas. presented at the workshop for decision makers on direct heating use of geothermal resources in asia, organised by unugtp, tblrrem and tbgmed, in tianjin, china, 11-18 may, 2008. chi-yuen, w., s. yao-lin, z. wen-hu (1982). dynamic uplift of the himalaya. nature 298, 553-556. choubey, v.m., k.k. sharma, r.c. ramola (1997). geology of radon occurrence round jari in parvati valley, himachal pradesh, india. j environ. radioact., 34(2), 139-147. choubey, v.m., p.k. mukherjee, b. s. bajwa, v. walia (2007). geological and tectonic influence on water-soil-radon relationship in mandi manali area, himanchal himalaya. environmental geology, 52, 1163-1171. chyi, l.l., t.j. quick, t.f. yang, c.h. chen (2005). soil gas radon spectra and earthquakes, terrestrial, atmospheric and oceanic sciences, 16,763-774. cinti, d., l. pizzine, n. voltattorni, f. quattrocchi, v. walia (2009). geochmistry of thermal fluids along faults segments in the beas and parvati valley (north-west himalaya himachal pradesh) and sohana town(haryana), india. geochemical journal, 43,65-76. ciotoli, g., a. sciarra, l. ruggiero, a., annunziatellis, s. bigi, (2016). soil gas geochemical behaviour across buried and exposed faults during the 24 august 2016 central italy earthquake. annals of geophysics, 59, fast track 5, 2016; 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doi: 10.4401/ag-6111 1. introduction a mw 3.9 foreshock on may 19, 2012, at 23:13 utc, was followed at 02:03 on may 20, 2012, by a mw 5.9 earthquake that hit a densely populated area in the po plain, west of the city of ferrara, italy (figure 1). over the subsequent 13 days, six mw >5 events occurred; of these, the most energetic was a mw 5.8 earthquake on may 29, 2012, 12 km wsw of the main shock. the tragic balance of this sequence was 17 casualties, hundreds of injured, and severe damage to the historical and cultural heritage of the area. from a seismological point of view, the 2012 earthquake was not an outstanding event in its regional context. the same area was hit in 1996 by a mw 5.4 earthquake [selvaggi et al. 2001], and previously in 1986 and in 1967 (dbmi11) [locati et al. 2011]. the most destructive historical event was the 1570, imax 8 event, which struck the town of ferrara [guidoboni et al. 2007, rovida et al. 2011]. the 2012 seismic sequence lasted for several weeks and probably developed on a well-known buried thrust fault [basili et al. 2008, toscani et al. 2009, diss working group 2010], at depths between 2 km and 10-12 km. although the earthquake locations available from the catalog of the italian istituto nazionale di geofisica e vulcanologia (ingv; national institute for geophysics and volcanology) (iside: http://iside.rm.ingv.it) do not image a well-defined fault plane, the most reliable interpretation is that of a south-dipping fault, coinciding with the known buried thrust. quasi real-time moment tensor estimates [scognamiglio et al. 2009] of the most energetic events indicate an analogous deformation style, with the compression axis oriented towards the nne. the moment tensor solution of the foreshock shows a similar reverse faulting style. figure 1 shows the solutions for the may 20, 2012, mw 5.9 mainshock and the foreshock that occurred a few hours before. the mainshock accelerograms were recorded by the network managed by the italian civil protection department, and these indicate a peak ground acceleration of 0.303g on the vertical components at the mrn station, located 13 km west of the epicenter. this study focuses on the analysis of the source mechanism of the may 20, 2012, mainshock. we use data from two distant arrays and four broadband seismic stations surrounding the epicentral area, to infer the characteristics of the finite rupture through multichannel and empirical green's function (egf) analysis, respectively. in particular, by exploiting the multichannel coherency of the array data, we obtain constraints on the duration and the gross shape of the source time function. we then focus our attention on the relative source time functions (rstfs), the time histories and azimuthal variations of which demonstrate a complex pattern of slip and clear directivity effects. 2. array analysis for this part of the study, we use data from two distant clusters of stations. the first array consists of a subnet of the ingv rete sismica nazionale (national seismic network), located in the alta val tiberina, central italy (figure 1). the second array is composed by seven temporary stations that were deployed in the larderello geothermal field (tuscany, italy), in the framework of the gapss (geothermal area passive seismic source) experiment [moretti et al. 2012, this volume]. hereinafter, these two arrays are referred to as the avt and lrd arrays, respectively. the avt array has an aperture of the order of 90 km, and average inter-station spacing of about 20 km. the lrd array has an aperture of about 50 km, and an average station spacing <10 km (figure 1). both of these arrays are homogeneous in terms of sensor types: nanometrics trillium 40s for the avt array, and nanometrics trillium 120s compact for the lrd deployment. despite the large aperture of these arrays, the first few article history received july 17, 2012; accepted august 20, 2012. subject classification: empirical green's functions, array analysis, earthquake source dynamics. 2012 emilia earthquakes piccinini et al. 570 figure 1. map of the study area. red triangles, locations of the four ingv stations used to calculate the relative source time functions; black triangle, location of the mrn accelerometer; black stars, locations of the two arrays, the geometries for which are illustrated in the two insets; black dots, aftershocks located during the first week following the may 20, 2012, mainshock; beach balls, focal solutions for the mainshock and the foreshock, as reported by the time-domain moment tensor catalog [scognamiglio et al. 2009; see also http://cnt.rm.ingv.it/tdmt.html]. the foreshock signal is used as the egf (see text for details). figure 2 (left). (a) vertical-component velocity seismograms from array avt. traces are aligned to the p-wave arrival times, and adjusted using cross-correlation. (b) zlcc as a function of horizontal slowness, for a constant propagation azimuth equal to that measured for the first pulse (151.7˚; see table 1). (c) zlcc as a function of propagation azimuth, for a constant horizontal slowness of 0.12 s/km as measured from the first p-wave arrival (see table 1). dashed lines in (b) and (c), theoretical horizontal slowness and propagation azimuth expected for the one-dimensional velocity structure of li et al. [2007]. figure 3 (right). as for figure 2, from the lrd array. see table 1 for the theoretical and measured propagation parameters. 571 seconds of the p-wave are coherent across the array elements, as depicted in panels (a) of figures 2 and 3. for each array, our analysis begins by aligning to the p-wave onset the 0.1 hz to 1 hz band-pass filtered velocity seismograms. for this procedure, we adjusted the manually picked arrival times using the inter-station time differences estimated via cross-correlation [shearer 1997]. the propagation parameters obtained from these adjusted arrival times (horizontal slowness and propagation azimuth) are listed in table 1, together with the theoretical ones for the one-dimensional earth model of li et al. [2007]. then, we performed a systematic grid search over the azimuth-horizontal slowness plane. we used a grid extending by ±45˚ and ±0.1 s/km around the theoretical azimuth and horizontal slowness, respectively. for each trial azimuth–slowness pair, we aligned the seismograms to the array barycenter according to a plane-wave model [e.g., equation (5) in rost and thomas 2002], and calculated the array-averaged zero-lag correlation coefficient (zlcc) [saccorotti and del pezzo 2000] over a 1.5-s-long time window sliding along the time-shifted seismograms with increments of 0.1 s. after this procedure, the zlcc is expressed as a function of the two propagation parameters. the zlcc maxima will thus correspond to a coherent, plane-wave crossing the array. figures 2 and 3 show the results from this analysis. at both arrays, the most striking feature that emerges from the zlcc maps is the presence of at least two separate, well-correlated pulses that occur within the first 5 s after the p-wave onset. both of these pulses are associated with similar propagation parameters, which are also very close to the theoretical ones (see also table 1). therefore, we infer that these two pulses are representative of the radiation from the source. on this basis, the total duration of the source time function would be of the order of 5 s and 4 s for arrays avt and lrd, respectively. in addition, we observe that at avt, the time separation between the two pulses is larger than that observed at lrd. this evidence appears therefore to suggest an overall westward source directivity, since the apparent duration of the source radiation observed from the se (array avt) is longer than that observed from the sw (array lrd). 3. empirical green's functions egf deconvolution is widely used for the analysis of ground-motion spectra [e.g., mori 2003, and references therein], to retrieve source parameters (seismic moment, source radius, rupture duration, rise time and stress drop). in this method, the path, site and instrument effects are efficiently removed by deconvolving the waveform of a lower magnitude event from the main event waveform, with both recorded at the same site, and when the two events are collocated. the main condition to be satisfied by the mainshock and the associated egf is the similarity of the respective waveforms, which implies closely located hypocenters and a similar focal mechanism. in addition, the egf should represent a point-like source once compared to the main event. to satisfy this condition, the magnitude difference between the two earthquakes has to be at least one unit [velasco et al. 1994]. when the source duration of the egf is sufficiently small as compared to that of the main earthquake, then the former can be approximated to a delta function, and consequently, its waveform approximates the response of the medium between the focus and the measuring site. in principle, the closer the egf source is to the mainshock, the larger is the frequency band which can be resolved by the method [patton 1980]. from the theoretical point of view, the integral of the source time function represents the total seismic moment released by the earthquake. due to rupture directivity, a station located along the fault strike in front of the rupture propagation will record an apparent source time function with a short duration and a large amplitude. conversely, since the scalar moment has to be constant everywhere, the apparent source time function retrieved behind the rupture propagation will have a long duration and a low amplitude. therefore, for mainly unilateral ruptures, the assessment of the fracture propagation direction is quite straightforward, and there is no need for a large number of stations [e.g., pino and mazza 2000], providing these latter stations offer sufficient azimuthal coverage of the focal sphere. on the other hand, well-constrained results can also be obtained in the case of more complex seismic sources, even when just a few rstfs are available [e.g., pino and di luccio 2009]. in our egf deconvolution, we used data from four stations that are located at different source-to-station directions (figure 1), to ensure suitable azimuthal coverage. their distances from the epicenter are between 150 km and 250 km. to efficiently retrieve the best egf, we performed a matched-filter analysis by sliding the mainshock recording along the continuous data streams spanning the may 19-22, 2012, time interval. both time series were first band-pass filtered over the 0.02 hz to 1 hz frequency band. for each temporal position of the template wavelet (i.e., the mainshock ferrara earthquake source complexity table 1. comparisons between the propagation parameters of the first p-wave pulse obtained from array analyses and those predicted from the li et al. [2007] velocity structure. array horizontal slowness (s/km) propagation azimuth (˚ clockwise from n) measured theoretical measured theoretical avt 0.11 0.125 151.74 145.20 lrd 0.14 0.125 185.10 188.18 waveform), we calculated the cross-correlation function, and retained its maximum value. the procedure is iterated for all of the four stations reported in figure 1. the mw 3.9 event that occurred on may 19, 2012, a few hours before the mainshock, scores the maximum cross-correlation index (0.85) and we assume it as the egf for the mainshock. egf deconvolution is performed in the frequency domain, and a simple water-level condition was adopted to stabilize the spectral divisions. the results of this process are shown in figure 4. the rstfs for the four analyzed stations are reported and located at the corresponding azimuth around the mw 5.9 mainshock focal mechanism. 4. discussion and conclusions our preliminary results obtained through two independent datasets and methods show that the may 20, 2012, mw 5.9 event shows a relatively complex pattern of rupture. array analysis clearly shows the presence of at least two separate pulses that are separated by time intervals of the order of 2 s and 1.5 s at the avt and lrd arrays, respectively. although less clear, a third peak is also visible at both arrays some 5 s after the first pulse. two closely spaced pulses marking the early stages of the rupture process are also shown by the rstfs (figure 4, arrows). although small, these 'wiggles' are simultaneously observed at all of the rstfs, thus ruling out the hypothesis that they might result from an artifact of the deconvolution process. the rstf from station maga, located nw of the epicenter, also shows a third peak, the timing of which is consistent with the third pulse observed on the zlcc maps. moving towards the ne (station sabo), the envelope of the first two pulses becomes broader, which indicates an anti-directive direction. the same consideration holds for station asqu, which is located in the se quadrant. at both of these last two sites, the observation of the third pulse is probably inherited by the widening of the rstfs. these preliminary analyses lead us to infer that a great part of the energy was radiated by a source propagating towards the wsw. if we assume that the mainshock was associated with the south-dipping plane, this would correspond to an oblique, down-dip rupture propagation. however, the azimuthal distribution of the rstf amplitudes suggests a more complex pattern of rupture propagation. indeed, the overall amplitude of the rstfs obtained at sabo and asqu is much larger than what would be expected in the case of unilateral rupture propagation towards the wsw. therefore, we hypothesize the development of a secondary rupture front, which propagated towards the east, roughly parallel to the fault strike. dinsar data [salvi et al. 2012, this volume] show a main pattern of fringes that correspond to surface deformation compatible with a shallow slip patch located east of the epicenter. however, salvi et al. [2012] also show a secondary area of ground uplift that is elongated in the wsw direction from the epicenter of the mainshock. although mapping the superimposed effects of several mw >5 events in the sequence, such ground deformation patterns thus appear to confirm our hypothesis of multi-lateral rupture propagation. hopefully, further analyses that include additional earthquakes and stations will provide tighter constraints on the dynamics of the rupture processes involved in the seismic sequence. acknowledgements. the original manuscript benefited from the thoughtful comments by two anonymous reviewers. all the participants to the gapss experiment are sincerely acknowledged for their assistance during the field deployment and data collection procedures. references basili, r., g. valensise, p. vannoli, p. burrato, u. fracassi, s. mariano, m.m. tiberti and e. boschi (2008). the database of individual seismogenic sources (diss), version 3: summarizing 20 years of research on italy's earthquake geology, tectonophysics; doi:10.1016/j.tecto.2007.04.014. diss working group (2010). database of individual seismogenic sources (diss), version 3.1.1: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas, ingv 2010; http://diss.rm. ingv.it/diss/ guidoboni, e., g. ferrari, d. mariotti, a. comastri, g. tarabusi and g. valensise (2007). cfti4med, catalogue of strong earthquakes in italy (461 b.c.-1997) and the mediterranean area (760 b.c.-1500), ingv-sga; http:// storing.ingv.it/cfti4med/ iside working group (2010). italian seismological instrumental and parametric database, http://iside.rm.ingv.it li, h., a. michelini, l. zhu, f. bernardi and m. spada (2007). crustal velocity structure in italy from analysis of regional seismic waveforms, b. seismol. soc. am., 97, 20242039; doi:10.1785/0120070071. piccinini et al. 572 figure 4. relative source time functions calculated through egf deconvolution for the may 20, 2012, mw 5.9 earthquake. arrows, the two primary pulses that appear to correspond to the correlation peaks shown by the zlcc maps of figures 2 and 3. 573 moretti, m., et al. 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aftershock sequence, j. geophys. res., 102, 8269-8283. toscani, g., p. burrato, d. di bucci, s. seno and g. valensise (2009). plio-quaternary tectonic evolution of the northern apennines thrust fronts (bologna-ferrara section, italy): seismotectonic implications, b. soc. geol. ital., 128, 605-661. velasco, a.a., c.j. ammon and t. lay (1994). empirical green function deconvolution of broadband surface waves: rupture directivity of the 1992 landers, california (mw = 7.3), earthquake, b. seismol. soc. am., 84, 735-750. *corresponding author: davide piccinini, istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy; email: piccinini@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ferrara earthquake source complexity << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice an analytical expression for early electromagnetic signals generated by impulsive line-currents in conductive earth crust, with numerical examples annals of geophysics, 59, 2, 2016, g0212; doi:10.4401/ag-6867 g0212 an analytical expression for early electromagnetic signals generated by impulsive line-currents in conductive earth crust, with numerical examples ken’ichi yamazaki miyazaki observatory, disaster prevention research institute, kyoto university, miyazaki, japan abstract changes in the electromagnetic (em) field after an earthquake rupture but before the arrival of seismic waves (“early em signals”) have sometimes been reported. quantitative evaluations are necessary to clarify whether the observed phenomena are accounted for by known theories and to assess whether the phenomenon can be applied to earthquake early warning. therefore, analytical expressions for the magnetic field generated by an impulsive line-current are derived for a conductive half-space model, and for a two-layer model; the somewhat simpler situation of a conductive whole-space is also considered. by analyzing the expressions obtained for the generated em field, some expected features of the early em signals are discussed. first, i verify that an early em signal arrives before the seismic waves unless conductivity is relatively high. second, i show that early em signals are well approximated by the whole-space model when the source is near the ground surface, but not when it is at depth. third, i show that the expected amplitudes of early em signals are within the detection limits of commonly used em sensors, provided that ground conductivity is not very high and that the source current is sufficiently intense. however, this does not mean that the em signals are easily distinguishable, because detector sensitivity does not account for additive noise or false positive detections. 1. introduction seismic fault slip mainly generates ground motion, but electromagnetic (em) variations have been reported during and prior to some earthquakes [e.g., eleman 1966, iyemori et al. 1996, honkura et al. 2002, honkura et al. 2004, abdul azeez et al. 2009, honkura et al. 2009, okubo et al. 2011, gao et al. 2014]. the term “early em signal” means any measurable seismogenic em variation that precedes the first seismic phase arrival. a few reports include early em signals that appear related to earthquakes. for example, iyemori et al. [1996] observed changes in the geomagnetic field at the time of the 1995 mw 7.1 hyogo-ken nanbu (kobe) earthquake at two stations nearly 100 km from the source region. the em variations had maximum amplitudes of 0.6–1.0 nt, and began ~10 s before the pwave arrival (~10 s after the estimated seismic origin time). for the 2007 mw 6.9 iwate-miyagi nairiku earthquake, okubo et al. [2011] observed changes in the geomagnetic field ~3 seconds before the arrival of seismic waves at a hypocentral distance of 27 km; the maximum signal amplitude was ~0.2 nt. the reliability of other previously reported early em signals is disputed, as these studies did not sufficiently discuss discrimination between signal and background geomagnetic fluctuations (i.e., false positive detections). however, if early em signals can be reliably observed, then they could be incorporated into early earthquake warning systems, which currently rely solely on seismic observations [e.g., kanamori 2005]. various mechanisms have been suggested to explain the conversion of mechanical energy to em energy. some examples include the motional induction effect [e.g., eleman 1966, gershenzon et al. 1993], the electrokinetic effect [e.g., ishido and mizutani 1981, pride 1994], the piezoelectric effect [e.g., ogawa and utada 2000a,b, huang 2002], and the piezomagnetic effect [e.g., yamazaki 2011a,b]. to assess the detectability of early em signals, it is essential to estimate expected amplitudes and to determine the distinguishing signal characteristics (if any) associated with each mechanism. the most straightforward way to study early em signals is, of course, to study the measured em fluctuations themselves; however, strictly observational approaches are difficult because large earthquakes and sensitive geomagnetic recording stations are both rare, meaning the undisputed data set is small. most observations of early or coseismic em signals have come from temporary sites designed for em prospecting, including magnetotelluric (mt) instruments [e.g., article history received september 9, 2015; accepted april 4, 2016. subject classification: geomagnetism, earthquake, subsurface electric current, early warning. honkura et al. 2002, abdul azeez et al. 2009, gao et al. 2014]. successful detections of early em signals are rather rare at continuous stations; among the only clear examples are iyemori et al. [1996] and okubo et al. [2011]. one potential way to capture more early em signals is to deploy instruments near the epicenter of a large, shallow earthquake, because many aftershocks are expected [e.g., ujihara et al. 2004, kuriki et al. 2011]. however, because aftershocks are necessarily smaller than the main shock, and early em signals related to the main shock are already near the limits of conventional detectors, aftershocks are difficult to detect in practice. consequently, it is difficult to compile a suitably large data set of early em signals. it is therefore expedient to use theoretical models of em variations to guide observational efforts. in the present work, subsurface electric currents induced by ground motions are examined as possible early em source mechanisms. the situation is schematically shown in figure 1. during an earthquake rupture, electric currents can be induced by the motion of conductive crustal material in the ambient geomagnetic field [e.g., gershenzon et al. 1993, yamazaki 2012, gao et al. 2014]. the objective is to determine the expected temporal variations in the generated em field. an electric current in a vacuum generates a magnetic field that can be easily calculated using the biot–savart law. some earlier numerical calculations were performed by assuming that the medium was a vacuum or a perfect resister [e.g., taira et al. 2009, okubo et al. 2011]. however, an electrical current in a conductive medium should be dealt with by solving the time-dependent maxwell’s equations, to determine, for example, if the conductive medium acts as an insulator against em propagation. the reminder of this paper is organized as follows. in section 2, the governing equations are described for specific situations. in section 3, a set of analytical expressions is derived for the em field generated by an impulsive line-current in a conductive half space. in section 4, some characteristics of early em signals in a conductive medium are discussed, along with some numerical examples. the validity of using a simpler model for evaluating early em signals is also considered. finally, the main conclusions are presented in section 5. 2. definition of the problem in this paper, i derive analytical expressions for the early em signals arising in the two model geometries: a two-layer model comprising a half-space with uniform conductivity, representing the earth’s crust, overlain by a perfect resistor, representing the air (figure 2a) and a whole-space model consisting of a uniformly conductive infinite medium (figure 2b), which is the simplest system that can model the effect of electrical conductivity. a line current is considered an approximation of the electric current induced by the coseismic motion of fault-zone material, which is often more conductive than the surrounding crust, as illustrated in figure 1. considering the situation whereby the fault length is orders of magnitude greater than the spatial scale of conductivity variation across the fault, it is reasonable to assume that the electric current occurs as a line of infinite length, thus reducing the problem to two dimensions. the half-space model (figure 2a) is the simplest realistic approximation of the earth. a realistic model must involve at least two layers: a conductive ground and resistive air. in contrast, the whole-space model (figure 2b) is somewhat unrealistic; however, solving maxwell’s equations in the whole-space model is considerably easier than for the half-space model. for this reason, i also consider the whole-space model throughout the following, so that any situations in which the simplified model is valid can be understood. when the electric current density i is given as a function of location (x, z) and time t, the electric and magnetic fields, e and b respectively, are determined via maxwell’s equations: yamazaki 2 figure 1. a schematic illustration of the subsurface-current generating mechanism analyzed in this paper. seismic motion of a fault block containing a zone with high conductivity subject to the ambient geomagnetic field induces an electric current according to faraday’s law. figure 2. the two models for which formulas are derived in this paper. the direction of the y-axis is chosen such that the x-, yand z-axes form an orthogonal, right-handed, cartesian system. 3 where f and n are, respectively, the electric and magnetic permeabilities. it is assumed that n is constant such that n=n0= 4r× 10-7 h/m. it is also assumed that the electric current density is the sum of an induced current density and a conductive current density; the conductive current density obeys the isotropic ohm’s law (3) where v is the electrical conductivity, and iext is the external current density. additionally, the displacement current term is ignored; that is, the second term of equation (2). this approximation yields (4) introducing a vector potential that satisfies b =∇×a, equations (1) and (4) are combined to form a single expression the two-dimensionality of the problem allows a coordinate system to be chosen such that the y-axis is oriented parallel to the external electric current, and the positive z-direction is vertically downward (see figure 2). in this coordinate system, only the y-component of iext is non-zero; the y-component of iext is denoted as iy ext. the electric current density of an impulsive linecurrent at depth d is therefore given by (6) where d is the dirac delta function. in this situation, we can chose a solution of equation (5) of the form a = (0, a,0), with the function a satisfying the scalar equation (7) the em field is then determined by solving equation (7) under appropriate boundary conditions. the first boundary condition comes from the em field being zero at infinite distance from the source current: i.e., a(x, z → ±∞, t) = const. when considering the two-layer model, conditions on the continuity of a and ∂a/∂z are also imposed at the boundary z = 0 to ensure the continuity of e and b at any instance. the non-zero components of the electric and magnetic fields are then given by ey= ∂a/∂t, bx= −∂a/∂z, and bz= ∂a/∂x. 3. derivation of solutions the governing equations can be simplified by the substitution of regularized variables t*≡n0vt, x *≡n0vx, z*≡n0vz , and d *≡n0vd. using these variables, the source term is written as (8) the differential operators become ∂/∂t =n0v∂/∂t * and ∇=n0v∇ *. the governing equation including the source term can then be rewritten as (9) in a conductive medium (i.e., v> 0), where i0 *≡n0 2vi0, and in a vacuum (i.e., v= 0). to find the solution, consider the integral expression and the corresponding fourier expression of a (12) in these identities, g* and ~* represent the wavenumber and angular frequency, respectively. substituting these expressions into equations (9) and (10), the governing equations for a (g*, z*,~*) are obtained as (13) where solutions to equations (13) and (14) are given as e b t #d 2 2 + ,0= b i#d n= + ,e t2 2 nf ,i e i extv= + b e#d nv= .i indn+ , , , ,, , .a a ix z t x z tx z t t ext2 00d 2 2 n v n=-q q qv v v , , ,vi x z t i x z d texty 0d d d= -q q q qv v v ,v , , , , . a x z t t a x z t i x z d t 2 0 0 0 d 2 2 n v n d d d = q q q q q v v v v v , , .i x x z tz t i dexty 0 3v 0n v d d d= -) ) ) ) ) ) )q q q q qv 3v v v v , , , ,a x z t t a x z t i x z d t 2 0 d 2 2 d d d = ) ) ) ) ) ) ) ) ) ) ) ) q q q q qv v v v v , ,a x z t2d) ) ) )q 0=)v dg d ,cos exp x t g x dg i t 2 1 2 0 # d d r ~ ~ = ) ) ) ) ) ) ) )3 3 3 # q q q v v v 0 # # , , d dg , , .cos exp a x z t dg a g z g x i t 0 #~ ~ ~ = ) ) ) ) ) ) ) ) ) ) ) ) 3 3 3 # q q q v v v# 0 # , ,, , z a g za g z u z d 2 1 2 2 2 22 2 ~ ~ r d=-) ) ) ) ) ) ) ) )q q qv v v , , , , z a gq z a gq zg 022 222 ~ ~=) ) ) ) ) ) ) )gq gqv v , .reu ug i 02/ $~-) ) ) )q v early em signals by line currents (1) (5) (10) (11) (14) and (2) (15) follows. in the case of the whole-space model (figure 2b), which is described solely by equation (13), the solution is given by in the case of the two-layer model (figure 2a), which is described by equation (13) for z > 0 and by equation (14) for z < 0, upwardand downward-propagating terms should be added to equation (16) [e.g., stoyer 1977]. these additional terms are determined such that they satisfy the boundary conditions, and the following solution is obtained: (17) observations of variations in the em field are usually obtained near to the ground surface (i.e., z ≅ 0). because of the continuity of the em field at ground level (z = 0), it is sufficient to consider a either for z > 0 or for z < 0. for this reason, only the time-domain solution for a at z > 0 is considered. to evaluate the integral in equation (12) over ~, i refer to the formulas for the inverse fourier transforms and where u is defined by equation (15), and g is a positive real number. the function erfc represents the complementary error function, the result shown in equation (18) can be derived by changing the integration variable from ~* to u and choosing an appropriate integral path over which to apply the cauchy integral theorem (see figure 3). equation (19) can then be derived from equation (18) after some algebraic manipulation. in the following, i consider phenomena only for t > 0 because, from equations (18) and (19), it is clear that the solution is trivial at negative times. to evaluate the integration of equation (12) over the variable g*, i use the easily derived results formulas: (21) (22) here, erfi is the imaginary error function defined by equation (21) is derived by integration by parts using the identity: (24) using equations (18), (19), (21) and (22), the integration of equation (12) using the function given by equation (17) can be evaluated to yield an analytical expression for the vector potential in the time domain, specifically (25) where z*+= z *± d*. in cases where the source current persists for a finite amount of time, the corresponding vector potential can be determined by integrating a over time t. for example, in the situation where a source electric current exists over the time period 0≤t≤t with constant , , .expa gq z i u u z d 2 1 1 2v 0~ r= ) ) ) ) ) ) ) )gq q rv 2v w , , exp exp exp exp a g z i g u u d g z u u z d g u u u z d z z 2 1 2 1 2 1 0 0 < > 2v 0 #~ r= + + + + ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) s q r q q q q v x 2v w v v v z] [] \ ]] z]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]][] ][]]]]]]]]]]]]]]]]]]]]]]]]]]]]]] \ ]]]] \ e h dexp exp exp exp u u t t t t i t gt 1 0 0 2 1 4 0 > 2 2 # g ~ ~ r r g = g) ) ) ) ) ) ) ) ) ) ) ) ) 3 3 # q q q t v v v y z] [] \ ]]] z]]]]]]]]]]]]]][] ][]]]]]] \ ]]]]]] \ # d erfcfc exp exp exp u u i t gr gq gt t t t t g 1 0 2 0 0 2 > # g ~ ~ gr g g + = + ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 3 3 # gq q gt q v v v y z] [] \ ]]] z]]]]]]]]]]]]]][] ][]]]]]] \ ]]]]]] \ # erfcfc d .expw ww 2 w # 2/ r 3 l lq qv v w # dgexp cos exp t t t t gg x dg x1 2 1 40 # 2 2 r g= -) ) ) ) ) ) ) ) )3 q sv x 0 # erfcfc dg erfcfc exp 4 1 erfifii 1 exp cos im g g g t t g x dg x ix t ixix t x t x 2 1 2 2 0 # 2 2 + + 2 2 g g g g g g + = + + + ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 3 q t t t t v y y y y # &g j 0 # erfifi .expw ww d 2 w 2 0 #/ r l lq v 0 # / .cos im expg g gi xx gi x2 2=) ) ) ) ) )q v , , erfcfc erfifi exp exp im exp exp a x z t i t t x z t t x z x ixix z t z ixix z t x z ixix z i t x z t x 2 1 2 1 4 2 1 4 2 1 2 1 4 4 2 0 2 2 2 2 2 2 2 2 2 2 r = + + + + + + + + + + + ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) + + + + + + + q t t t u t t v y y y z y y # & g j yamazaki 4 (18) (19) (20) (23) and (16) 5 intensity i0, the vector potential, denoted by a step, is calculated as and the corresponding em field is given by derivatives of astep. note that in this calculation evaluated in the next section, we must ensure that the differences between regularized and non-regularized variables (i.e., t and t*) are correctly accounted for. 4. discussion and numerical examples this section considers three points based on the analytical expressions derived for early em signals. first, the onset times of early em signals are considered, in order to impose principal limitations on the detectability of early em signals. second, the validity of using vacuum and conductive whole-space models is considered, in order to assess the viability of further simplifying the calculations. third, the detectability of early em signals is discussed for a few specific cases. the numerical examples below use the following parameters. the current intensity i0 and hypocentral distance d are set to 50 a and 10 km, respectively. for the proposed current generation mechanism (i.e., induction due to coseismic fault motion; figure 1), i0 = |svv × bamb|, where s, v, and bamb represent the cross-sectional area of the conductor, the ground motion velocity, and the ambient geomagnetic field, respectively. an example situation for which i0 = 50 a is |v| = 0.1 ms−1, s = 10 km2, v = 0.1 sm−1, and |b| = 50000 nt, with v perpendicular to b. given the numerical considerations of iyemori et al. [1996], these values might be typical for a mw 7.1 earthquake. however, for simplicity the following calculations are performed by approximating that the electric currents appear on a line, as in equation (6); the discussions below still hold under this approximation. the value of i0 does not become considerably larger, even for earthquakes with much higher moment magnitudes. as long as we assume that induction is due to ground motion (i.e., figure 1), the generated current intensity depends on the ground velocity, not the seismic moment. in addition, early em signals do not scale with seismic moment, but instead scale with seismic moment released at the first stage of the rupture. yet initial seismic moment release is never particularly large, even at mw ≥ 9 [e.g., ammon et al. 2005, koketsu et al. 2011]. therefore, the ampli, , , , d ,a x z t x z t tastep ,m # ax t t t 0 = l lq qqv vvm# early em signals by line currents (26) figure 3. integral paths in the complex ~-plane used to evaluate the integral in equation (18). when t < 0, we can use the path colored green; integration along c– converges to zero for this case. when t > 0, we adopt the black integral contour. integration over c+1 and c+3 converges to zero, and c+2 becomes gauss’s integral in this case. figure 4. an illustration of the two source-receiver geometries used to test the validity of the analytical solutions developed for the two models shown in figure 2. tudes of early em signals are not expected to increase, even for great earthquakes. 4.1. characteristics of the solution an important feature of temporal variations in early em signals is the “delay” of the signal. first, let us analyze the form of the analytical expression for em potential given in equation (23). the arguments of the exponentials and error function terms in equation (23) are proportional to r*2/t*=nvr2/t or its square root, where r represents a value with the same order of magnitude as x and z. therefore, it is expected that equation (23) will have its extremal value at around t*= r*2, which is equivalent to t =nvr2 ≅ 10−6 ×vr2. the time at which the vector potential takes this extremal value is regarded as the delay time of the signal arrival. for the case of r = 10 km = 104 m, for example, the delay time assuming v =10–1, 10–2, and 10–3 sm−1 is of the order of 10, 1, and 0.1 s, respectively. a typical value for the arrival time of a seismic wave at a hypocentral distance of 10 km is about 2 seconds. these results imply that arrivals of early em signals will precede those of seismic waves unless conductivity is rather high (i.e., 10–2 sm−1). 4.2. effect of the ground surface let us consider the temporal variations in the early em signals for the two source geometries shown in figure 4 using the analytical solutions developed above. i consider the case whereby the temporal variation of the source current is given by the heaviside step function h(t), which is zero for t < 0 and unity for t > 0. the intensity of the current i0 and hypocentral distance d are assumed as 50 a and 10 km, respectively. this intensity value is realistic for the proposed current-generation mechanism (i.e., induction due to coseismic fault motion generation; see figure 1), and is approximately equivalent to that expected for a ground motion velocity, fault width, thickness of the conductor around the fault, conductivity of the conductor, and ambient magnetic field intensity of 0.1 ms−1, 10 km, 1 km, 0.1 sm−1, and 50000 nt, respectively. we first consider the case whereby the source current is located directly beneath the observation point (“case 1”, shown in figure 4a). in this situation, the xcomponent of the magnetic field bx can be written as and the z-component is zero. plots of bx for v= 1.0 × 10−1, 10–2, 10–3, and 10–4 sm−1 are shown in figure 5. note that, regardless of the conductivity, this function converges to 1/2rz as expected from the biot–savart law for a steady current. for small values of v(i.e., 10–3 or 10–4 sm−1), bx rapidly converges to its final value bx (0,0,t =∞). in contrast, for large values of v, the convergence is rather slow. these results demonstrate that predictions made using the biot–savart law are good approximations of the time-varying magnetic field for small v, but are poor for large v. it should be noted that magnetic field variations calculated for the whole-space model shown in figure 2b are also considerably different from those calculated for the two-layer model (figure 2a). when we consider the whole-space model, the temporal variation of bx is given by expanding the exponential, we can confirm that its convergence speed is of the order of t –1. in contrast, from equation (25) we see that the convergence speed of bx for the two-layer model is of the order of t –1/2. these differing convergence rates are apparent in figure 6, which compares bx for the two models assuming v= 10−2 sm−1. for comparison, we also consider the situation where both the source current and observation site are located on the boundary of the layer (“case 2”, shown in figure 4b). in this case, the z-component of the magnetic field bz for the two-layer model can be expressed as , , erf ,exp expb t d t t d d t t d d t t d d i d t 0 0 4 4 4 2 4 2 1 4 2 2 x 2 2 2 2 0 2r r + =+ ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) q t s sv y x xg j , , .expb t d d i t 0 0 2 1 4x 2 0r =-) ) ) )q sv x , , .expb t t d i d t 0 0 1 2 4z 2 0 3r = -) ) ) ) )q sv xg j yamazaki 6 (27) figure 5. a comparison of the temporal variation in the horizontal component of the magnetic field for the source-receiver geometry shown in figure 4a for four conductivity values. (28) (29) 7 numerical examples of the temporal evolution of bz are shown in figure 7 for various conductivity values. in contrast to the results obtained for the buried line-source, the whole-space solution provides a reasonable approximation to that of the two-layer model for the situation shown in figure 4b when the source and receiver are located at the same depth. in this geometry, the convergence speed of bz is proportional to t –1 for both the two-layer and whole-space models. this result implies that the temporal variation in bz obtained using the whole-space model can be considered a good approximation to that predicted by the twolayer model, although there are some small differences. the level of agreement between the two models is apparent in figure 8, which compares the temporal evolution of bz computed for half-space, conductive whole-space, and resistive whole-space models assuming v= 10−2 sm−1. 4.3. detectability of early em signals to assess the detectability of early em signals, we examine expected amplitudes and shapes for a few specific cases. for earthquake early warning, the situation demonstrated in case 1 (figure 4a) is most important, because seismic p-waves cannot be detected at observation sites before reaching at the ground surface in this situation. for this reason, we focus primarily on the situation in figure 4a, and specifically on the generated magnetic field bx (figure 5). note, however, that many of the following characteristics are unchanged for the geometry in figure 4b and the resultant bz field in figure 7. the numerical examples show that a high resistivity is required for the signal to be detected. in resistive cases (i.e., v= 10–4 or 10–3 s/m), an early em signal appears almost at the seismic origin time, and the temporal pattern is not considerably different from a step function. recall that this result was obtained by assuming that temporal variations in the source current were also given by a step function. the results for the resistive case therefore imply that the temporal variations in b largely reflect temporal variations in the source current. it was assumed that the source current is generated by early em signals by line currents figure 6. a comparison of the temporal variation in the horizontal component of the magnetic field for the source-receiver geometry shown in figure 4a for a conductive whole-space (solid black line) and two-layer model (dotted line). for reference, the solution for a resistive whole-space (i.e., a vacuum) is also shown. figure 7. a comparison of the temporal variation in the vertical component of the magnetic field for the source-receiver geometry shown in figure 4b for four conductivity values. figure 8. a comparison of the temporal variation in the vertical component of the magnetic field for the source-receiver geometry shown in figure 4b for whole-space (solid black line) and two-layer (dotted line) models. for reference, the solution for a resistive whole-space (i.e., a vacuum) is also shown. motional induction, so that the intensity of the source current is proportional to the slip velocity. therefore, it can be concluded that temporal variations in bx represent temporal variations in the slip velocity (i.e., the earthquake source-time function) when v≤ 10–3 s/m. in contrast, in conductive cases, temporal variations in bx are quite emergent, and appear considerably different from the source-time function. smoothed variations in the magnetic field are rather difficult to distinguish from background geomagnetic variations; hence, detection of early em signals is expected to be difficult in a conductive medium. concerning the amplitudes of early em signals, the present technique yields estimates of ~0.5–1.0 nt. whether or not signals of these amplitudes can be resolved is primarily dependent on the type of instrument used. most devices that have observed a coseismic or pre-seismic em signal are either flux-gate magnetometers [e.g., iyemori et al. 1996] or induction coils [e.g., ujihara et al. 2004, abdul azeez et al. 2009]. their minimum resolutions are 0.1 nt or finer; however, considering fluctuations in (or stability of ) the data, their practical resolution limit is ~1 nt. finer-scale instruments, such as squid magnetometers [e.g., katori et al. 2013], have better resolution; however, these sensors are costly, and dense installations are thus infeasible at present. taking background noise into account, we can estimate the general detection limit of early em signals at ~0.5–1.0 nt. we can therefore tentatively conclude that signals are detectable in principle. numerical examples demonstrate that 0.5–1.0 nt is the expected range of early em signal amplitudes, assuming that the current intensity (i0 = 50 a) is realistic. as mentioned at the beginning of this section, the current intensity (i0 = 50 a) corresponds to a mw 7 earthquake. therefore, a mw 7 or larger earthquake could be detectable before the p arrival if em signals could be observed at distances of ~10 km. it must be noted, however, that the above discussion only provides one possible scenario in which early em arrivals can be detected. this is by no means comprehensive. a wide variety of v, s, and v values are possible in the scenario depicted in figure 1. exact values depend on the electrical structure of the fault and individual earthquake source processes. in this sense, the detection limit is not rigid. the ability to distinguish signal from noise is another limiting factor, though addressing this challenge falls outside the scope of this work. 5. conclusions in this paper, i have derived analytical expressions for the time-varying em field generated by an impulsive line-current in both a two-layer and whole-space model. some quantitative features of the predicted early em signals are demonstrated through analysis of the expressions obtained. concerning the arrival times of early em signals, an important result is that the arrivals of early em signals precede the arrivals of the first seismic waves by a few seconds unless conductivity is high (i.e., 10−2 sm−1). consequently, with a low conductivity, early em signals could be detectable in principle. concerning the viability of the simplified model used in the calculations, the temporal evolution of early em signals is well approximated by a wholespace model when the source is near the surface; 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol52,3,2009 209 annals of geophysics, vol. 52, n. 3/4, june/august 2009 key words ionospheric physics and propagation – cost action 1. background and historical context temporal and spatial changes in the upper atmosphere act to limit and degrade the performance of terrestrial and earth-space radio systems in many different ways. command, control and communication systems involving transionospheric propagation may be disrupted or strongly perturbed, global positioning networks compromised and surveillance (optical and radar) systems affected. in addition, conditions in nearearth space have other adverse effects on modern technology, ranging from the tracking and the lifetimes of satellites to the induction of strong damaging currents in the power grids and transcontinental pipelines. the vulnerabilities of systems to changes in the upper atmosphere incorporate many different aspects like spaceweather effects on communication, navigation and surveillance systems, solar cycle phenomena and their impact on operational systems; modeling, forecasting, and prediction services; ionospheric sounding methods and measurements; applications of ionospheric tomography; transionospheric effects including scintillation; ionospheric propagation for high frequency communication systems; long wave propagation systems and effects; very low frequency propagation for long-range communication with receiver below sea surface; global positioning system (gps) and ionospheric total electron content (tec) studies. the european ionospheric community has long been aware that cooperation research on an international basis is essential to deal with such complex issues. in parcost 296 miers: mitigation of ionospheric effects on radio systems alain bourdillon (1), ljiljana r. cander (2) and bruno zolesi (3) (1) ietr, université de rennes 1, france (2) stfc, rutherford appleton laboratory, chilton, uk (3) istituto nazionale di geofisica e vulcanologia, roma, italy abstract the cost 296 action miers (mitigation of ionospheric effects on radio systems) within the ionospheric community has the objectives, embodied in the memorandum of understanding (mou), to develop an increased knowledge of the effects imposed by the ionosphere on practical radio systems, and the development and implementation of techniques to mitigate the deleterious effects of the ionosphere on such systems. this introductory paper summarizes briefly the background and historical context of cost 296 and outlines the main objectives, working methods and structure. it also lists the participating countries and institutions, the management committee (mc) meetings, the workshops, short-term scientific missions. in addition, the paper discusses the dissemination activities and the collaboration among the participating institutions and researchers, before outlining the content of the final report. mailing address: dr. alain bourdillon, ietr, université de rennes, 1 umr cnrs 6164, 35042 rennes, france; e-mail: alain.bourdillon@univ-rennes1.fr vol52,3,2009 22-09-2009 14:16 pagina 209 210 a. bourdillon, l.r. cander and b. zolesi ticular, international cooperation is required for the collection of data, in both the real-time and in retrospective modes, the development and verification of new methods to improve the performance of both operational and future terrestrial and earth-space communication systems and the exchange of expertise on space plasma effects on global navigation satellite systems (gnss). cost is a european cooperation in the field of scientific and technical research. it is a framework for scientific and technical cooperation, allowing the coordination of national research on a european level for basic and pre competitive research as well as activities of public utility. it is an intergovernmental mechanism that sponsors scientific and technical co-operation and co-ordination between research team and institutional partners from the cost countries and non cost institutions. cost 296 miers on «mitigation of ionospheric effects on radio systems» is an action within the ict «information and communication technologies» domain. the area is managed in the cost office, in close liaison with the 6th fp ist program. it is a four-year project that started in january 2005 to work collaboratively on the topics outlined above. the core of cost 296 has been a dynamic group of researchers from 19 cost countries and 7 institutions from non cost countries and the european space agency (esa) working as a technical community towards a common objective. cost projects are well suited to developing research co-operation in fields where finance is provided at the national level. the success of this approach has been demonstrated in the telecommunications domain in previous cost actions concerned with the prediction of ionospheric propagation characteristics (zolesi and cander, 2008). the earlier cost actions relating to ionospheric radio propagation were particularly useful in creating a critical mass of researchers in projects where the initial emphasis was on long-term ionospheric prediction and retrospective ionospheric modeling over europe. the cost 238 action prime (prediction and retrospective ionospheric modeling over europe) was a four-year research project with objectives to develop regional maps and models over the european ionosphere that are more accurate than internationally available global maps and models (bradley, 1995). it involved the participation of some 72 scientists and engineers from 17 countries. the work encompassed the production of regional monthly median maps and algorithms for generating «instantaneous» snapshot maps of the standard vertical-incidence ionospheric characteristics from coordinated sounding measurements within europe. additional studies involved height profiles of electron density, the vertical total electron content up to an altitude of 1000 km and research leading to an improved understanding and thus potential for better modeling of ionospheric storm morphologies. work conducted within cost 238 was continued in the follow-on project cost 251 iits (improved quality of service in ionospheric telecommunication systems planning and operation) on the application of prime results in the improved quality of service in ionospheric telecommunication systems planning and operation (hanbaba, 1999). both projects yielded significant results, but they also identified further areas where future development was needed. the topics for more research included: hour-to-hour and day-to-day variability prediction capabilities in the topside ionosphere, upgrading of the current models to include scintillation effects, the prediction of the ionospheric and plasmaspheric effects on navigational systems and the development of methods for calculating the reliability and compatibility of hf radio systems using digital modulation techniques and over the horizon hf backscatter radars. the cost 271 action «effects of the upper atmosphere on terrestrial and earth-space communications» was then established to examine a range of relevant research areas within four broad groupings: impact of variability of space environment on communications assessment of space plasma effects for satellite applications ionospheric effects on terrestrial communications space plasma effects on earth-space and satellite-to-satellite communications the major achievements of cost 271 are described in zolesi and cander (2004). vol52,3,2009 20-09-2009 19:05 pagina 210 211 cost 296 miers: mitigation of ionospheric effects on radio systems 2. cost 296 action objectives cost 296 is an action for the promotion, stimulation and co-ordination of european research in ionospheric areas. the memorandum of understanding laid out the main objectives of the cost 296 action as follows: to support and enhance the existing european facilities for historical and real-time ionospheric data collection accessible to all; to exchange information on methods and algorithms to mitigate the effects of ionospheric perturbations and variations on advanced terrestrial and space-based communication services by creating an effective computing infrastructure; to develop an integrated approach to ionospheric modeling; to create the mechanism needed to ingest measured parameters into models, extend and develop suitable mitigation models and define the protocols needed to link models together; to make applicable results available to the itu-r and to promote the research aspects to funding agencies such as esa, esf and the eu; to elaborate inputs for the preparation of the future european space programme (esp); to strengthen the existing areas of expertise by stimulating closer cooperation between scientists and users; to focus the scope of all the previous cost ionospheric related studies to the mitigation of ionospheric effects on specific radio systems. 3. cost 296 action working method and structure the action was organized under three topic headings corresponding to three working groups and associated ten work packages as shown in table i as well their term of references. members of the management committee of the signatory countries, participating institutions from cost and non-cost countries and mc meetings, workshops and short-term scientific mission are listed below. 3.1. working group 1 ionospheric monitoring and modelling • near-earth space plasma monitoring by vertical incidence and oblique sounding networks and gnss techniques (retrospective and real-time). • data ingestion and assimilation into ionospheric models, including data collection and processing, and the adaptation of models to enable data ingestion and assimilation. table i. cost 296 action structure wg1 ionospheric monitoring wg 2 advanced terrestrial wg 3 space-based and modelling systems systems wp 1.1 near earth space wp 2.1 radar and wp 3.1 space plasma plasma monitoring radiolocation effects wp 1.2 data ingestion and wp 2.2 hf/mf wp 3.2 mitigation techniques assimilation in ionospheric models communications wp 1.3 near earth space plasma wp 2.3 spectrum wp 3.3 scintillation modelling and forecasting management monitoring and modelling wp 1.4 climate of the upper atmosphere vol52,3,2009 20-09-2009 19:05 pagina 211 212 a. bourdillon, l.r. cander and b. zolesi • near earth space plasma modelling and forecasting including mitigation of ionospheric perturbations, tomographic imaging for model validation and channel modelling techniques. • climate of the upper atmosphere including long-term ionospheric trends, gravity and planetary wave effects on propagation and ionospheric space weather. 3.2. working group 2 advanced terrestrial systems • radar and radiolocation, including ionospheric effects on surface-wave radar and radiolocation systems, frequency management of ground-wave and sky-wave radars and angle of arrival measurements for sky-wave signals. • hf/mf communications including digital radio systems, wideband propagation modelling and development of a hardware simulator, increased capacity of hf links through mimo techniques (experimental measurements and simulation) and extension of existing wideband hf simulators to the mf band. • spectrum management, including the use of gps to improve hf communications management, adaptive waveform management and the effects of infrasound on radio propagation. 3.3. working group 3 space-based systems • ionospheric effects on space-based remote sensing systems, gravity wave effects on gnss, space plasma media and security implications. • mitigation techniques with emphasis on ionospheric structures, their physical nature and impact on gnss signals, improved accuracy of gnss by better ionospheric correction and errors due to ionospheric perturbations. • scintillation monitoring and modelling with emphasis on scintillation effects, their physical nature and impact on ionospheric radio systems (gnss signals in particular), high-latitude and equatorial scintillation effects (experimental work and modelling). 4. cost 296 participating countries and institutions 4.1. cost 296 management committee 19 cost member countries signed the cost 296 mou as shown in table ii by names of their representatives. some 34 active institutions, listed in table iii, formed the core of the activity. in addition, institutions from seven non-cost member countries from table iv participated and one observer organization (estec-esa). table ii. members of the cost 296 management committee r leitinger, r. warnant, i. kutiev, insitute for geophysics, observatory of belgium, geophysical institute bulgarian astrophysics and meteorology, ave circulaire 3, academy of sciences university of graz, brussels b-1180, ‘acad g bonchev’ str, block 3 universitaetsplatz h.nebdi 1113 sofia a-8010 graz belgium bulgaria w. riedler austria j. lastovicka, e. turunen, a. bourdillon, (chairman) institute of atmospheric physics geophysical observatory university of rennes 1, academy of sciences of sodankyla 35042 rennes, czech republic j.p. luntama y. beniguel bocni ii cp 1401 finland france 14131 praha 4 j. boska czech republic vol52,3,2009 22-09-2009 14:17 pagina 212 213 cost 296 miers: mitigation of ionospheric effects on radio systems table ii. (continued). n. jakowski s. s. kouris p. bencze dlr/dfd fernerkundungsstation department of telecommunications geodetic & geophysical neustrelitz aristotelian university research institute postfach 5123 of thessaloniki hungarian academy of sciences d-17235 neustrelitz thessaloniki h-9401 sopron pob 5 j. bremer a. belehaki g. satori germany greece hungary b. zolesi (vicechairman) i. stanislawska a. m. casimiro istituto nazionale di space research centre polish fct geofisica e vulcanologia academy of sciences universidade do algarve via di vigna murata, 605 bartycka 18a campus de gambelas roma 00143 00-716 warsaw 8000 117 faro p.spalla h. rothkaehl a. de azevedo italy poland portugal romania b. a. de la morena b. forte inta, atmospheric centre for atmospheric research sounding station university of nova gorica ‘el arenosillo’ nova gorica space science division slovenia mazagon (moguer) 21130 huelva d.altadill spain y. tulunay m.warrington lj. r. cander department of aerospace engineer, department of engineering (chairperson for the first year) middle east technical university, university of leicester rutherford appleton laboratory 06531, ankara university road chilton, didcot e.tulunay leicester le1 7rh oxon ox11 0qx turkey h. strangeways united kingdom united kingdom a. vernon (secretary) h. haralambous t. kos rutherford appleton laboratory, frederick university faculty of electrical engineering chilton, didcot po box 24729 and computing, university ox11 0qx 1303 nicosia of zagreb, unska 3 10000 zagreb, united kingdom l. economou croatia cyprus table iii. participating institutions from cost countries institute for geophysics, astrophysics and meteorology university of graz geophysical institute, bulgarian academy of sciences, bulgaria royal observatory of belgium, belgium university of liege, belgium institute of atmospheric physics, academy of sciences of czech republic, czech republic 4.2. participating institutions vol52,3,2009 20-09-2009 19:05 pagina 213 214 a. bourdillon, l.r. cander and b. zolesi table iii. (continued). geophysical observatory, sodankyla, finland university of ouly, finland finish metheorological institute, helsinki, finland dlr/dfd fernerkundungsstation, germany leibniz-institute of atmospheric physics, germany university of zagreb, faculty of electrical engineering and computing, croatia ietr, université de rennes 1, france cnrs, grenoble, france ieea, france istanbul technical university (tü) faculty of aeronautics and astronautics, turkey middle east technical university, ankara, turkey inta, atmospheric sounding station, ‘el arenosillo, spain observatorio del ebro, spain universidad complutense madrid, madrid, spain cefat-uceh, universidade do algarve, portugal university of madeira, portugal institut of geodesy, warnia and masuria, univ. olsztyn space research centre, polish academy of sciences, warsaw, poland istituto nazionale di geofisica e vulcanologia, italy istituto di fisica applicata, cnr, italy international center for theoretical physics «abdus salam», italy department of telecommunications, aristotelian university of thessaloniki, greece national observatory of athens, greece geodetic & geophysical research institute, hungarian academy of sciences, hungary rutherford appleton laboratory, united kingdom qinetiq, united kingdom university of bath, united kingdom university of wales, united kingdom university of leeds, united kingdom university of leicester, united kingdom university of nottingham, united kingdom sheffield university, united kingdom geomagnetic institute, serbia and montenegro frederick university, nicosia, cyprus intercollege, limassol, cyprus vol52,3,2009 20-09-2009 19:05 pagina 214 215 cost 296 miers: mitigation of ionospheric effects on radio systems 5. mc meetings, workshops, short-term scientific missions table iv. participating institutions from noncost countries institute of terrestrial magnetism, ionosphere and radiowave propagation, russia st petersburg state university, st petersburg, russia university of massachusetts lowell, usa polar research institute of china, shanghai, china china research institute of radio wave propagation, beijing, china geomagnetic laboratory, ottawa, canada university kebangsaan (ukm), malaysia table v. management committee meetings 4 february 2005, brussels, belgium, (inaugural meeting) 30 may-1 june 2005, trieste, italy 13 and 16 november 2005, noordwijk, netherlands 27-29 april 2006, neustrelitz, germany 3-7 october 2006, rennes, france 15-17 march 2007, rome, italy 10-14 july 2007, prague, czech republic 13-15 march 2008, limassol, cyprus 7-11 october 2008, rome, italy 20-21 march 2009, warsaw, poland (final meeting) table vi. cost 296 workshops 1st cost 296 workshop during esww2 (2nd european space weather week), noordwijk, netherlands, 1418 november 2005, estec. 2nd cost 296 workshop on «radio systems and ionospheric effects», rennes, france, 3-7 october 2006. papers published on a cd-rom. 3rd cost 296 workshop on «ionosphere, modelling, forcing and telecommunications», 10-14 july 2007, prague, czech republic 4th cost 296 workshop on «general review of cost 296 activities and results» 7-11 october 2008, rome, italy vol52,3,2009 20-09-2009 19:05 pagina 215 216 a. bourdillon, l.r. cander and b. zolesi table viii. cost 296 short-term scientific missions sari m.o. (middle east technical university) to e.m. warrington (university of leicester), 16-20 january 2006. kouba d. (iap, prague, cz) to a. belehaki (noa, athens, greece) 19-26 march 2006. romano v. (ingv rome, italy) to a. bourdillon (university of rennes, france) 9-16 may 2006. krankowski a. (university of warmia and mazury, krakow, poland) to manuel hernandez-pajares (technical university of catalonia, barcelona, spain) 25 june-3 july 2006. tsagouri i. (noa, athens, greece) to j. lastovicka (iap, prague, cz) 26 february-3 march 2007. gunashekar s. (university of leicester) to l. bertel (university of rennes), 2-6 april 2007. kocabas z. (middle east technical university) to l. bertel (university of rennes), 2-7 april 2007. nechutny z. (iap, prague, cz) to e. turunen (sodankyla geophysical observatory, sondakyla, finland) 12-23 march 2007. hoque m.m. (dlr, neustrelitz, germany) to h. starngeways (university of leeds, leeds, uk) 26-30 march 2007. fotiadis d. (university of thessaloiki, thessaloniki, greece) to j. lastovicka (iap, prague, cz) 21-28 april 2007. iñigo blanco alegre (el arenosillo, inta) carried out a short-term mission (stm) at the ingv, instituto nazionale di geofisica e vulcanologia in rome in september 10-17, 2007. bidaine b. (fnrs belgium) to s. radicella (ictp, trieste, italy) 10-21 december 2007. tsagouri i. (noa, athens, greece) to b. zolesi (ingv, instituto nazionale di geofisica e vulcanologia in rome) may 26-30, 2008. kouris s. (university of thessaloniki, thessaloniki, greece) to i. stanilawska (src, warsaw, poland) 25-30 may 2008. rothkhael h. (src warsaw, poland) to e.m. warrington (university of leicester, leicester, uk) 19-24 may 2008. mielich j. (iap, university of rostock, juliusruh, germany) to lj. cander (rutherford appleton laboratory, didcot, uk) 19-24 may 2008. yapici t. and tulunay e. (middle east technical university) to s. radicella (itcp, trieste, italy) 22-27 june 2008. table vii. cost 296 meetings of the working groups 27 april 2005 vienna, austria w.g. leaders meeting during the egu general assembly 4 -5 july 2005, graz, austria seminar on itu-r sg3l issues 27 april 2006, vienna, austria w.g. leaders meeting during the egu general assembly 27 april 2007, vienna, austria w.g. leaders meeting during the egu general assembly 20-22 february 2008, nottingham, uk ionospheric scintillation: scientific aspects, space weather application and services 27 april 2008 vienna, austria w.g. leaders meeting during the egu general assembly 4 may 2008 w.g. leaders meeting during the ies, washington, usa 18 november 2008 w.g. leaders meeting during esww5, brussels, belgium 20 april 2009 vienna, austria w.g. leaders meeting during the egu general assembly vol52,3,2009 20-09-2009 19:05 pagina 216 217 cost 296 miers: mitigation of ionospheric effects on radio systems 6. dissemination of results the target audiences for the dissemination of the cost 296 action results include other scientists, researches and engineers working in the field; other research frameworks and international organization; institutes and academia; standards bodies; industry; european level policy makers; government policy makers, regional planners and policy makers; general public. the dissemination methods used in the course of the cost 296 action have been: (1) posting of general information and working documents on the cost 296 website including set up of an electronic communication network; (2) workshops, seminars, events, schools for young scientist and conferences organised by the mc including contributions to other national and international conferences and symposia; (3) online proceedings, cd proceedings and papers in peer-reviewed related scientific and technical journals. dissemination of results took into consideration the progress of the cost 296 action as well the results of its evaluation in updating the dissemination plan during the course of this action. 6.1. cost 296 action website the website of the cost 296 action has been active since the beginning of the action on the following address: http://www.cost271.rl.ac.uk/. it is maintained at rutherford appleton laboratory, uk and contains the cost 296 action main info documents as the management structure, the minutes of the management committee meetings, the call for papers and other relevant documents for the action in addition to the related web links. 6.2. major events organized/co-organized by mc 1. the 2nd general assembly of the egu 2005 session related to cost 296 action on «ionospheric monitoring and modelling for advanced terrestrial and space-based systems», 24-29 april 2005, vienna, austria. 2. the 2nd european space weather week (joint workshop cost 296, cost 724, esa and e-star); 14-18th november 2005, esaestec, noordwijk, netherlands. 3. the 3rd general assembly of the egu 2006 session related to cost 296 action on «measurements of ionospheric parameters influencing radio systems», 2-april 2006, vienna, austria. 4. international advanced school on space weather at ictp (joint activity with cost 724), 2-19 may 2006, ictp, trieste, italy. 5. 2nd cost 296 workshop «radio systems and ionospheric effects», university of rennes 1, 3-7 october 2006, rennes, france. 6. the 4th general assembly of the egu 2007 session related to cost 296 action on «modelling and measurements of ionospheric parameters influencing radio systems» and working group leaders meeting, 15-20 april 2007, vienna, austria. 7. joint cost 296 and international reference ionosphere workshop on «ionosphere – modelling, forcing, and telecommunications», 10-14 july 2007, prague, czech republic. 8. cost 296 workshop on «ionospheric scintillation: scientific aspects, space weather application and services ‘workshop on ionospheric scintillations, measurements and modelling’», 20-22 february 2008, nottingham university, nottingham, uk. 9. international school of space science, geomagnetism and ionosphere school, 7-12 april 2008, l’aquila, italy. 10. the general assembly of the egu 2008 session related to cost 296 action on «studies of ionospheric parameters influencing radio systems» and working group leaders meeting, 13-18 april 2008, vienna, austria. vol52,3,2009 20-09-2009 19:05 pagina 217 218 a. bourdillon, l.r. cander and b. zolesi 11. the international ionospheric effects symposium session related to results derived from cost-action programs «mitigation of ionospheric and space weather effects on telecommunication and navigation», 13-15 may 2008, city of alexandria, usa. 12. the 4th workshop of the cost 296 action mitigation of ionospheric effects on radio systems (miers), 7-11 october 2008, ingv, rome, italy. 13. the 5th european space weather week (jointly organised by the royal observatory of belgium (rob), esa, the swwt, the cost 724 and cost 296 communities); 16-21 november 2008, brussels, belgium. 14. cost 296 action final meeting, warsaw, 20-21 march 2009. 15. the general assembly of the egu 2009 session st11 related to cost 296 action on «ionosphere and radio systems» and working group leaders meeting, 19-24 april 2009, vienna, austria. 6.3. major publications most of the results reporting activities of the cost 296 action have been published as follows: 1. online proceeding and space weather book from the 2nd european space weather week, 14-18th november 2005, esa-estec, noordwijk, the netherland. 2. 2nd cost 296 workshop cd proceedings on «radio systems and ionospheric effects», university of rennes 1, rennes, france, 3-7 october 2006. 3. special volume of acta geophysica devoted to the selected papers from the 3rd general assembly of the egu 2006 session related to cost 296 action on «measurements of ionospheric parameters influencing radio systems», vol. 55, no. 3, 2007. 4. special volume of journal of atmospheric and solar-terrestrial physics devoted to the selected papers from the 3rd general assembly of the egu 2006 session related to cost 296 action on «measurements of ionospheric parameters influencing radio systems», vol. 70, issue 6, april 2008, doi: 10.1016/j.jastp.2007.02.016. 5. online proceedings of the cost 296 workshop on «ionospheric scintillation: scientific aspects, space weather application and services ‘workshop on ionospheric scintillations, measurements and modelling’», 20-22 february 2008, nottingham university, nottingham, uk. 6. special volume of journal of atmospheric and solar-terrestrial physics devoted to the selected papers from the 4th general assembly of the egu 2007 session related to cost 296 action on «modelling and measurements of ionospheric parameters influencing radio systems», vol. 70, issue 15, december 2008, doi:10.1016/j.jastp.2008.01.016. 7. special volume of advances in space research devoted to the selected papers from the joint cost 296 and international reference ionosphere workshop on «ionosphere – modelling, forcing, and telecommunications», in press 2008. 8. cost 296 final report; supplement to annals of geophysics, 2009, this issue. 7. scientific and technical cooperation the close scientific collaborations established as part of previous cost 238, 251 and 271 actions have led to a number of bilateral and multilateral collaborations successfully continued in the cost 296 action. in addition, participants of this action have been active in different european projects. there is a significant participation of action members from all working groups in esa space weather program, in particular in its space weather working team (swwt), the swenet (space weathvol52,3,2009 22-09-2009 14:17 pagina 218 219 cost 296 miers: mitigation of ionospheric effects on radio systems er european network) scheme (http://www.esaspaceweather.net/swenet) and sweets (the main space weather festival during the european science week 19-24 november 2007) at the schwerin castle/house of parliament in germany). particularly important has been the collaboration between two cost actions: cost 724 devoted to space weather and cost 296 devoted to the study of the ionosphere and its impact on communication and positioning. several colleagues work in the two actions. this resulted is an important input provided by the cost 296 action to cost 724 based on the ionospheric models developed during the last cost actions (cost 238, 251, 271 and 296) for space weather applications (see lilensten et al., 2007 for details). the ursi beacon satellite group closely linked to the work of the working group 3 of this action, provides important links between ionosphere, plasmasphere and upper atmosphere scientists, engineers and users of satellite beacon applications. several cost 296 group members are advisers and observers in the international geodynamics service (igs) and some of its offspring organisations like the gps-iono group. designed to meet the needs of europe, this action also made an impact to the international work of itu-r study group 3l through contributions to recommendations, provision of data for validation of prediction models for europe and by a leading role in working party sg3l (ionospheric propagation). further steps will be taken to make the most applicable results of the cost 296 results available to itu-r. moreover participants in this action have made a considerable contribution to the international heliophysical year (2007-2009) initiatives with the following activities: (a) development and maintenance of cost 296/ihy web site to exchange general information, expertise, know-how, links to the data archive and databases produced by this action among the ihy community (see http://ionos.ingv.it/ihy/ihy_index.html.); (b) coordination of multi-instruments observation campaigns in synergy with the ihy community. the data archive created during the observation periods and/or severe heliogeophysical conditions will also form a baseline collection against which later forecasting efforts may be compared; (c) joint investigations on interconnection between low, middle and high latitude ionosphere and plasmasphere, e.g. pghm campaign (see http://www.bu.edu/cawses). finally, it should be mentioned other international activities of the ionospheric cost community by the organization of the session «mitigation of ionospheric and space weather effects on telecommunication and navigation» during the international ionospheric effects symposium, in alexandria, usa, may 2008 and a long-lasting collaboration with the international reference ionosphere (iri) by organizing the combined iri/cost 296 workshop strongly oriented towards application of ionospheric models and specifically on their effects on radio systems. 8. outline of the final report the cost 296 final report is published here as a supplement of the international journal annals of geophysics. it comprises this introduction that contains the cost 296 action background and historical context, the general objectives and priorities agreed in the memorandum of understanding, the working methods and structure, the participating countries and institutions, information about the mc meetings, workshops and short-term scientific missions, dissemination of results and finally the scientific and technical cooperation. ten scientific papers concerning the main activities and results achieved in the three working groups of cost 296 follow. each paper is in the form of an independent review of one of the activities of the action with an introduction, discussion of the results and references. two scientific papers, in which group of authors review the main achievements obtained in ionospheric cost related actions, make up the final part of this report. the conclusions summarises the main achievements of cost 296 as a whole in the vol52,3,2009 20-09-2009 19:05 pagina 219 220 a. bourdillon, l.r. cander and b. zolesi context of the mou requirements and looks to the way ahead. significant achievements have been made, developing our understanding of the issues and relevance of field and offering new knowledge that will serve to increase economic and social development. in addition, new challenges have been identified and a concrete proposal is outlined for follow-on from cost 296. the cost 296 final report is addressed mainly to those in the mou signatory countries involved in frequency planning, spectrum management and system design; to those who sponsored the work undertaken; to industrial partners concerned with the technology who will learn from the results and to those in national radio regulatory administrations and international planning. in addition, the report is of interest to students of radiowave communication and navigation and others in the propagation research community. references bradley, p.a. (1995): prime (prediction regional ionospheric modelling over europe), cost action 238 final report, (commission of the european communities, brussels). hanbaba, r. (1999): improved quality of service in ionospheric telecommunication systems planning and operation, cost action 251 final report, (space research centre, warsaw). lilensten, j., b. zolesi, a. belehaki, i. stanislawska, and l. perrone (2007): collaboration among cost actions. ionosphere and space weather, cost 724 final report. zolesi, b., and lj. r. cander (2004): cost 271 action «effects of the upper atmosphere on terrestrial and earth-space communications» final report, annals of geophysics, supp. 47 (2/3). zolesi, b. and lj. r. cander (2008): «from cost 238 to cost 296: four european cost. actions on ionospheric physics and radio propagation», radio sounding and plasma physics: 2007 radio plasma imager science team meeting, april 27, 2007, radio sounding and plasma physics symposium, april 29, 2007, xi international digisonde forum, april 30 may 3, 2007, aip conference proceedings, vol. 974, pp. 39-46. vol52,3,2009 20-09-2009 19:05 pagina 220 tiwari_finaonline:layout 6 annals of geophysics, 57, 5, 2014, s0544; doi:10.4401/ag-6513 s0544 analysis of soil radon data in earthquake precursory studies hari prasad jaishi1, sanjay singh1, raghavendra prasad tiwari2, ramesh chandra tiwari1,* 1 mizoram university, department of physics, aizawl, india 2 mizoram university, department of geology, aizawl, india abstract soil radon data were recorded at two selected sites along mat fault in mizoram (india), which lies in the highest seismic zone in india. the study was carried out during july 2011 to may 2013 using lr-115 type ii films. precursory changes in radon concentration were observed prior to some earthquakes that occurred around the measuring sites. positive correlation was found between the measured radon data and the seismic activity in the region. statistical analysis of the radon data together with the meteorological parameters was done using multiple regression method. results obtained show that the method employed was useful for removing the effect of meteorological parameters and to identify radon maxima possibly caused by seismic activity. 1. introduction variation of radon concentration in soil can give evidence on tectonic disturbances in the earth’s crust [king 1986]. however, the changes in radon concentration are not only controlled by seismic activity but also influenced by meteorological parameters like soil moisture, rainfall, temperature, barometric pressure and relative humidity [kraner et al. 1964, pearson 1967, singh et al. 1988, king and minissale 1994, virk et al. 2000, zmazek et al. 2003, walia et al. 2005]. these parameters change the physical characteristics of the soil and thus affect the soil radon variation caused by geophysical processes. therefore, theoretical and empirical algorithm is necessary for removing the effects of meteorological parameters [zmazek et al. 2003, ramola et al. 2008, choubey et al. 2009]. northeast india is seismically one of the six most active regions of the world, the other five being mexico, taiwan, california, japan and turkey. it lies at the junction of himalayan arc to the north and burmese arc to the east [jaishi et al. 2014a, singh et al. 2014]. the high seismicity in this zone is due to the collision tectonics between the indian plate and the eurasian plate in the north and subduction tectonics along the indo-myanmar range (imr) in the east [dewey and bird 1970]. based on the distribution of epicentres, fault plane solutions and geotectonic features, northeast region can be divided into five seismotectonic zones which include eastern himalayan collision zone, indo-myanmar subduction zone, syntaxis zone of himalayan arc and burmese arc (mishmi hills), plate boundary zone of the shillong plateau and assam valley, and bengal basin and plate boundary zone of tripuramizoram fold belt (figure 1). the indomyanmar subduction zone is highly seismic zone extending from the bay of bengal to the mishmi article history received february 19, 2014; accepted september 15, 2014. subject classification: surveys, measurements and monitoring, geochemical data, statistical analysis. figure 1. seismo-tectonic map of north east india showing epicentres of damaging earthquakes (modified after raghukant and dash [2009]). tectonic zones (zones a, b, c, d and e) and major thrusts (mbt-main boundary thrust; mct-main central thrust; nt-naga thrust; dt-disang thrust) are also shown. thick blue line represents the international boundary of north eastern part of india bordering myanmar and bangladesh (not to scale) and red lines represents faults and thrusts. block in a n-s, nne-ssw and ne-sw directions. the main discontinuities in this zone are naga thrust and the disang thrust. the seismic activity in the bengal basin may be related to an intra plate activity whereas tripuramizoram fold belt to plateboundary activity. the main tectonics domains in this region are e-w trending dauki fault, n-e trending sylhet fault, ne-sw hailhakula lineament, nnw tista fault, mat fault and tuipui fault. mat fault is the most prominent fault in mizoram state [jaishi et al. 2014b]. it obliquely cuts across the general north-south trend of the indo-burmese arc. it trends nw-se (figure 2) and is traceable across entire mizoram on the satellite as well as on the geological maps. mat river crosses the mat fault and follows it for considerable distance. most pronounced part of this fault is in serchhip district along serchhip-thenzawl road. hence this part has been selected for monitoring of radon anomaly and their possible correlation with the occurrence of earthquakes. authors have done primary analysis of soil radon and thoron anomalies as a precursor to earthquakes [jaishi et al. 2013, 2014a, 2014b, singh et al. 2014]. the present paper describes the multiple regression method to differentiate those radon anomalies which are solely caused by seismic events and not by meteorological parameters. 2. materials and methods for radon monitoring in soil, we have used lr115(ii) cellulose nitrate detector manufactured by kodak pathe in france, a useful solid state nuclear track detector (ssntd) for registering the alpha tracks generating from radon gas. the lr-115 (ii) films of size 3cm × 3cm was attached in the dosimeter with the sensitive side facing downwards for exposure. the dosimeter has been developed at the bhabha atomic research centre (barc), india. the dosimeters were buried in holes of depth of about 80 cm in the soil (figure 3). the detector films were exposed for a period of 15 days, after that the exposed films were removed and reinstalled with the new one in the dosimeters. the ssntd’s are etched with 10% sodium hydroxide solution in an etching bath at a temperature of 60°c for developing the registered tracks. the etching time was set for 1 hour. the track etched method removes a bulk thickness of 4µm leaving a residual thickness of 8µm. as soon as the etching process is complete, films were washed in cold running water and then kept in distilled water for two hours. the detector films are pre-sparked at a voltage of 900v to fully develop the partially etched track holes using spark counter. the tracks are then counted at a voltage corresponding to plateau region of the counter (around 450v). 3. results and discussion radon concentration in soil gas have been measured after every 15 days using lr-115(ii) films during july 2011 to may 2013 together with the meteorological parameters viz.; rainfall, relative humidity, air temperature and barometric pressure at two locations (mat bridge and tuichang) in serchhip district, mizoram (india). numerous criteria have been used by different authors [rannou 1990, price et al. 1994, king et al. 1996, fu et al. 2005] in order to calculate soil radon gas anomalies which include mean plus ‘n’ (where n = 1, 2, 3,…) standard deviation (sd). fleischer [1981] proposed the empirical relation between the magnitude (m) of the jaishi et al. 2 figure 3. schematic description of the equipment used for radon measurements in soil.figure 2. location map of study area. 3 earthquake and the epicentre distance (d) within which the earthquake prediction zone can be manifested. (1) the details of the earthquakes (www.imd.gov.in) that occurred around the study areas fulfilling equation (1) is summarised in table 1. multiple linear regression (mlr) model multiple regression analysis permits the study of several independent (predictor) variables for a given dependent (criterion) variable. for ‘n’ predictor variables the raw score regression equation is given by (2) where, is the predicted value of the dependent variable, a is the constant term and b1,b2,...,bn represents the regression coefficient of the independent variables x1,x2,...,xn respectively. the drawback of the raw score regression is that the size of the b-coefficients is dependent on the scale of the predictor variables. therefore, they cannot be compared directly with each other to determine which predictor variable has the most influence on the criterion. by standardizing both the coefficients and the variables we could make direct comparison among the coefficients [yount 2006]. the standardized regression equation takes the form (3) where, b’s represents the standardized coefficients and z’s represents the z-scores. in the present investigation we perform mlr taking soil radon concentration (rn) as the dependent variable and the meteorological parameters viz. rainfall (rf), relative humidity (rh), temperature (t) and pressure (p) as independent variables. the data used for analysis is reported in table 2. data analysis at mat bridge at mat bridge rn1 is regressed with rf, rh, t and p (taken from table 2). all the predictor variables mentioned may not be a significant predictor of the dependent variable. to deal with this problem and to build a better model we perform backward regression method using spss statistical package. the backward regression method starts with all the predictor variables, test the significance of each predictor variable using a t-test and then delete the variable (if any) that improves the model the most by being deleted. this process is repeated until the maximum number of steps is reached or no more independent variable qualifies for removal [hocking 1976]. table 3 shows the coefficients of the backward regression output. the backward regression output of the data has four distinct models (model 1, model 2, model 3 and model 4). the first model includes all the predictor variables (rf, rh, t and p). to check the significance of the predictor variables a t-test is performed at 95% confidence interval (i.e., the alpha value is set at 0.05) and its corresponding probability is shown in sig. column of the table (see table 3). if we look at the significance of each predictor variable in model 1, two predictor variables (t and p) are non-significant predictors of the dependent variable as the p-value (sig.) is more than a-value (0.05) in both the cases. the backward regression method eliminates one predictor variable at a time (the predictor variable with the highest p-value is deleted first). so, temperature (t) having the highest p-value (0.757) is removed from model 1 and a second model is constructed with rf, rh and p as independent variables. in model 2, p is a non significant predictor (since the pvalue is more than 0.05). therefore, p is removed from the model and a third model is built with rf and rh as predictor variables. model 3 tells us that rf is a non significant predictor (since the p-value is much more than 0.05). rf is therefore removed from the model and a fourth model is built with rh as a single predictor vari...z z z zy x x xn n1 1 2 2b b b= + + +l ...y a b x b x b xn n1 1 2 2= + + + +l yl . . . exp d m km1 66 10 0 48 = radon in earthquake precursory studies day of occurrence magnitude (m) distance of epicentre (km) from mat bridge and tuichang region of occurrence 65th day 6.8 673 689 sikkim-nepal border region 129th day 5.8 323 320 india-myanmar border region 353rd day 5.8 323 324 phek (nagaland), india 381st day 6.0 147 132 myanmar 486th day 6.7 327 304 myanmar 545th day 5.9 314 312 india-myanmar border region 597th day 5.2 174 187 india-bangladesh border region table 1. details of earthquakes that occurred around the study area during the period of investigation (earthquake data obtained from: www.imd.gov.in). jaishi et al. 4 days radon conc. at mat bridge rn1 (tr.cm -2) radon conc. at tuichang rn2 (tr.cm -2) rf (mm) rh (%) t (°c) p (mbar) 15 814 687 131.7 92.4 23.93 888.29 30 528 476 184 93.7 23.8 888.38 45 682 305 258.5 92.7 23.9 890.78 60 242 215 172 91.8 24.37 889.91 75 342 123 138 90.5 23.66 890.83 90 413 441 58.6 85.6 24.46 893.56 105 126 75 23.8 80.4 22.42 894.69 120 129 72 0 70 21.16 895.81 135 143 88 0 69.5 21.12 896.44 150 154 170 0 72.6 19.01 895.73 165 93 74 0 63.5 19.11 896.01 180 153 204 20.3 80.6 16.49 896.81 195 69 76 0 69.2 16.85 894.5 210 130 102 0 62.6 19.31 895.81 225 65 55 7.3 61.8 21.1 892.67 240 91 55 46.9 69.6 21.15 892.68 255 97 67 0 61.4 24.45 894.36 270 107 81 112.3 74.9 22.09 894.28 285 41 11 152.4 87 22.33 891.61 300 104 48 118.8 81.8 23.07 891.49 315 339 126 56.8 86.5 24.81 889.16 330 590 90 211.1 91.3 23.67 887.36 345 121 84 262.9 93.4 22.79 886.05 360 145 504 158.8 90.6 23.93 887.07 375 666 215 93.6 92.7 23.45 887.09 390 286 192 265.5 93.13 24.23 887.63 405 148 67 200.4 93.12 23.49 889.84 420 241 190 203.7 92.1 23.53 891 435 562 185 160.2 91 23.57 893.07 450 214 208 171.1 89.1 21.84 894.45 465 260 419 0 85.5 22.25 896.27 480 256 304 109.9 88.2 19.68 895.98 495 325 384 6.1 84.4 19.87 895.84 510 180 171 0 77.2 18.37 896.53 525 319 328 0 65.7 17.74 895.3 540 448 447 0 69.8 15.39 895.77 555 198 204 0 70.5 18.02 896.14 570 186 235 0 69 21.03 895.66 585 162 205 3.3 66 21.38 894.42 600 158 157 0 63.6 23.68 894.99 615 377 335 5.6 73.3 23.76 893.35 630 415 371 21.5 74.3 25.19 891.81 645 99 80 39.9 80.2 22.54 892.69 660 101 95 324.4 93.6 20.2 890.18 675 378 307 124 87.7 23.43 888.58 table 2. radon concentrations and the meteorological parameters during the period of observation at mat bridge and tuichang. 5 able. clearly, rh is a significant predictor (p-value <0.05) of rn1. to check how much of the criterion variability is accounted for by the predictor and the viability of the model-as-a-whole we examine the squared multiple r value and a significant f-ratio (p<0.05) respectively, which is reported in table 4. in the model summary of table 4, r-square and adjusted r-square is reported. the adjusted r-square is the measure of the shrinkage if we were to apply this model to another sample (in other words the amount of predictive loss that we would observe). the value of adjusted rsquare in model 4 is 0.198 i.e. 19.8% of the variation in radon concentration is accounted by rh. by reducing the number of predictors from 4 to 1 the adjusted rsquare dropped from 0.240 to 0.198, a change of 0.042 (or a little over 4%) which is good. we do not lose much r-square by dropping 3 of the 4 predictor variables. also the standard error of estimates increase from 162.51 to 166.99 (only 2.68% increase) which is not much. to test the significance of the model-as-awhole an f-ratio (msreg./msres.) is calculated for each model and the probability of the computed f-ratio is displayed in sig. column of table 4. if we compare all the models, f-ratio is largest in model 4 showing better ratio of regression to noise. reducing the number of predictors from 4 to 1 reduces the p-value of f-ratio from 0.004 to 0.001 indicating a more significant and a better model. the raw score regression equation for model 4 can be written as (4). . ( )rn rh371 60 7 8651 =+l radon in earthquake precursory studies model parameters unstandardized coefficients standardized t sig. (pvalue) b std. error coefficients beta 1 (constant) 15506.483 12574.905 1.233 .225 rf -1.069 .513 -.538 -2.086 .043 rh 11.267 3.994 .666 2.821 .007 t 4.115 13.222 .054 .311 .757 p -18.091 13.854 -.309 -1.306 .199 2 (constant) 17546.232 10612.758 1.653 .106 rf -1.090 .503 -.549 -2.169 .036 rh 11.414 3.922 .675 2.910 .006 p -20.287 11.791 -.346 -1.721 .093 3 (constant) -707.813 284.934 -2.484 .017 rf -.708 .461 -.356 -1.535 .132 rh 12.804 3.926 .757 3.261 .002 4 (constant) -371.602 185.026 -2.008 .051 rh 7.865 2.283 .465 3.445 .001 model sum of squares df mean square f sig. r square adjusted r square std. error of estimates 1 regression 473551.25 4 118387.81 4.48 .004 .310 .240 162.512residual 1056409.54 40 26410.23 total 1529960.80 44 2 regression 470993.53 3 156997.84 6.07 .002 .308 .257 160.712residual 1058967.27 41 25828.47 total 1529960.80 44 3 regression 394529.07 2 197264.53 7.29 .002 .258 .223 164.420residual 1135431.72 42 27034.089 total 1529960.80 44 4 regression 330863.50 1 330863.50 11.9 .001 .216 .198 166.990residual 1199097.29 43 27885.98 total 1529960.80 44 table 4. model summary and analysis of variance (mat bridge). table 3. coefficients of the backward regression output at mat bridge. and the corresponding standardized equation would be (5) where zrn'1 is the standardized value (z-score) of rn'1 and zrh is the z-score of rh. we calculate the zscores of the measured radon concentration (zrn1) and relative humidity (zrh) during the entire period of observation and uses equation (5) to find the standardized predicted value (zrn'1). when the difference (i.e. the standardized residuals) between zrn1 and zrn'1 is more than 1sd, we denote it as a possible radon anomaly. the graphical representation of the standardized measured radon concentration and residuals during the observation period is shown in figure 4. the radon maximum observed on 45th day of the observation period (figure 4a) was zrn1 = 2.263 tracks/cm2. the difference (zres.) between the measured and the predicted value was, 1.943 tracks/cm2 (close to 2sd; figure 4b). this radon maximum could be correlated to the earthquake of m6.8 which occurred on 65th day of the observation period (i.e. 20 days after the anomaly was observed). the second radon peak was observed on 330th day. the difference between the measured and the predicted value was recorded to be zres.=1.458 tracks/cm2, which is greater than 1sd. the second peak may be correlated to the earthquake of m5.8 which occurred on 353rd day (23 days after the anomaly was observed). the third anomalous change in radon concentration was recorded on 375th day. the difference between the measured and the predicted value during this period was recorded to be 1.847 tracks/cm2 (much greater than 1sd). an earthquake of m6.0 occurred only 6 days after the anomaly was observed. so, the third radon maximum may be correlated to the mentioned m6.0 quake. the fourth maximum was recorded on 435th day. the difference in the concentration was recorded to be 1.305 tracks/cm2 ( just above 1sd level). though an earthquake of m6.7 occurred several days after the anomaly was observed, it was quite difficult to directly correlate the radon maximum to the mentioned event as the time interval between the anomaly observed and the event was quite large (51 days). in other words, this radon maximum may be treated as a false anomaly . ( )zrn zrh0 4651 =l jaishi et al. 6 model parameters unstandardized coefficients standardized t sig. (pvalue) b std. error coefficients beta 1 (constant) 7180.246 10796.340 .665 .510 rf -1.212 .440 -.753 -2.755 .009 rh 10.783 3.429 .787 3.145 .003 t -3.601 11.351 -.059 -.317 .753 p -8.577 11.894 -.180 -.721 .475 2 (constant) 5395.113 9112.143 .592 .557 rf -1.194 .431 -.742 -2.767 .008 rh 10.654 3.367 .777 3.164 .003 p -6.656 10.123 -.140 -.657 .515 3 (constant) -593.630 237.506 -2.499 .016 rf -1.069 .384 -.664 -2.780 .008 rh 11.110 3.273 .810 3.395 .002 table 5. coefficients of the backward regression output at tuichang. figure 4. variation of (a) standardized measured radon concentration and (b) standardized residuals during the observation period at mat bridge. the solid vertical lines represent the earthquakes along with their magnitudes (m). 7 which may have occurred due to variation in relative humidity. for the radon maximum that occurred on 540th day, the residual was 1.620 tracks/cm2 which is well above the 1sd level. this may be treated as a precursory signal before the event of m5.9 which occurred on 545th day (5 days after the anomaly was observed). data analysis at tuichang as in case of mat bridge we perform backward regression taking rn2 (measured radon concentration at tuichang) as dependent variable and rf, rh, t and p as independent variables (data taken from table 2). the spss print out of the results (coefficients) is shown in table 5. the output of the backward regression method produces three models namely model 1, model 2 and model 3. model 1 includes all the predictor variables. temperature (t) is having a p-value of 0.753 which is greater than the a-value (0.05); therefore t is removed from model 1 to construct model 2 with rf, rh and p as independent variables. it is clear from the sig. column of table 5 that p is a non-significant predictor (since the pvalue >0.05). consequently, p is removed from the model and a third model is established with two predictor variables (rf and rh). both rf and rh are significant predictors since the p-value in both the cases is less than 0.05. the model summary and the analysis of variance are given in table 6. to verify the significance of the model-as-a-whole, first we look at the adjusted rsquare value. the adjusted r-square value is 0.178 (i.e. 17.8 % of the variation in radon concentration is explained by rf and rh). reducing the number of parameters from 4 to 2 does not reduce the adjusted r-square value. in fact, the value increases from 0.148 to 0.178 (i.e. 16.86% increase). second, there is no increment in standard error of estimates by reducing the predictor variables which is a good indication that model 3 is better. third, the f-ratio is largest in model 3 (i.e. a better ratio of regression to noise) and the p-value of f-ratio is least. in conclusion model 3 is the most significant and a better model. the raw score and the z-score regression equations for model 3 can be written as where zrn'2, zrf and zrh are the standardized value of rn'2, rf and rh respectively in units shown in table 2. the plot of standardized measured radon concentration and residuals during the observation period at tuichang is shown in figure 5. the first radon maximum was recorded on 90th day of the observation period (figure 5a). during this period the difference between the 593.63 1.069 ( ) 11.110 ( ) . ( ) . ( ) rn rf rh zrn zrf zrh0 664 0 810 2 2 =+ =+ l l radon in earthquake precursory studies model sum of squares df mean square f sig. r square adjusted r square std. error of estimates 1 regression 226690.92 4 56672.73 2.91 .033 .225 .148 139.526residual 778710.32 40 19467.75 total 1005401.24 44 2 regression 224731.88 3 74910.62 3.93 .015 .224 .167 137.988residual 780669.35 41 19040.71 total 1005401.24 44 3 regression 216501.64 2 108250.82 5.76 .006 .215 .178 137.052residual 788899.60 42 18783.32 total 1005401.24 44 table 6. model summary and analysis of variance (tuichang). figure 5. variation of (a) standardized measured radon concentration and (b) standardized residuals during the observation period at tuichang. the solid vertical lines represent the earthquakes along with their magnitudes (m). (6) (7) measured and the predicted value of radon concentration was, zres. = 1.066 tracks/cm2 ( just above 1sd; figure 5b). this radon maximum was quite difficult to directly correlate to any seismic event (as the time interval between the radon maximum and the closest earthquake m5.8 was 39 days). so, the first radon maximum may be treated as a false anomaly possibly may have caused by variation in meteorological parameters (rf and rh). the next increment in radon concentration was recorded on 360th day. the difference between the measured and the predicted value was recorded to be 1.902 tracks/cm2 (close to 2sd). this peak may be correlated to the event of m6.0 which occurred 21 days after the anomaly was observed. third change in radon concentration was observed on 495th day. but it was quite difficult to correlate this peak to the event of m6.7 as during this period the difference between the measured and the predicted value was quite low (zres. = 0.338 tracks/cm2; less than 1sd). next impulsive change was recorded on 540th day of the observation period. the difference between the measured and predicted value during this period was 1.934 tracks/cm2 (close to 2sd). the mentioned peak may be treated as a precursor signal before the earthquake of m5.9 which occurred 5 days after the signal. 4. conclusion we have performed statistical analysis of the radon data collected from july 2011 to may 2013 at two sites (mat bridge and tuichang) along mat fault, the most prominent fault in mizoram. the multiple regression analysis of the radon data with the meteorological parameters provides useful evidence to extract a radon anomaly possibly caused by seismic event. the results obtained shows that at mat bridge about 57% of the anomaly observed could be correlated to the seismic events whereas at tuichang 28.6% could be correlated, indicating that the radon activity in soil gas is mostly controlled by geophysical process. the precursor time between the observed radon anomalies and the earthquakes varies from 5-23 days. high percentage of correlation between radon anomalies and the earthquakes at mat bridge indicates that the area under investigation is seismically active and would be useful to carry out further studies. several spike-like radon anomalies observed prior to the earthquakes in the present investigation may provide soil radon database and the preliminary trend of their response to earthquake activity in the region. the backward regression method applied gives valuable information about the meteorological parameter(s) that have the strongest effect on radon concentration. however, long term and continuous data recording is necessary to confirm a relation between soil radon activity and earthquakes. acknowledgements. authors are grateful to the ministry of earth sciences, govt. of india, new delhi for their financial support to this work in the form of a major project vide sanction order no. moes/ p.o.(seismo) /1(67)/2009, dated 16-06-2010. data and sharing resources the earthquake catalog used in this study was obtained from: http://www.imd.gov.in/section/seismo/dynamic/wel come.htm references choubey, v.m., n. kumar and b.r. arora (2009). precursory signatures in the radon and geohydrological borehole data for m4.9 kharsali earthquake of gharwal himalaya, sci. tot. environ., 407, 58775883. dewey, j.f., and j.m. bird (1970). mountain belts and the new global tectonics, j. geophys. res., 75, 26252647. fleischer, r.l. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice l e m a r e e a p o r t o m a r g h e r a silvio polli 1. posizione della stazione. — l a stazione m a r e o g r a f i c a è situata nella darsena t e r m i n a l e del canale industriale ovest c h e si a d d e n t r a nella zona degli stabilimenti sino a 1 k m a s di mestre. l a l o c a l i t à è d e n o m i n a t a b o t t e n i g h i e fa p a r t e del p o r t o m a r g h e r a , c h e costituisce o r a il p o r t o i n d u s t r i a l e di venezia. dall'ufficio idrografico del magistrato alle a c q u e di venezia, c h e ne c u r a il f u n z i o n a m e n t o , è i n d i c a t a q u a l e stazione mareografica m a r g h e r a i i , per distinguerla dalla stazione di m a r g h e r a i, situata a l l ' i m b o c c a t u r a del c a n a l e vittorio e m a n u e l e i i i , a fianco d e l l ' e m p o r i o dei sali e t a b a c c h i . n e l complesso l a g u n a r e veneto, il m a r e o g r a f o si t r o v a all'estrem i t à n w alla zona centrale, nel b a c i n o regolato dal c a n a l e p o r t o di l i d o . il sito è distante in linea d'aria 9 k m da v e n e z i a e 16,5 k m d a l l ' i m b o c c o del p o r t o di lido. l e c o o r d i n a t e geografiche della stazione sono : l a t . 45" 2 8 ' 2 2 " n, long. 12" 14' 2 1 " e gr., 0" 1 2 ' 4 7 " w m. mario. dal m a r e a p e r t o vi si giunge lungo i canali di p o r t o di l i d o , s. m a r c o , g i u d e c c a e v i t t o r i o e m a n u e l e i i i . i l p e r c o r s o , dalla b o c c a del p o r t o c a n a l e alla stazione mareografica, è di 2 1 , 5 k m . i l c a n a l e , al p o r t o di l i d o , è largo 8 5 0 m e profondo 11 ni, alla g i u d e c c a h a una larghezza di 2 6 0 ni e una profondità di 10,5 m , dopo il b a c i n o della stazione m a r i t t i m a si r i d u c e a 8 0 m di larghezza ed a 10 ni di p r o f o n d i t à , nel p o r t o industriale la larghezza v a r i a da 8 0 a 100 ili m e n t r e la p r o f o n d i t à si m a n t i e n e quasi costante sui 9 ni. l ' o n d a di m a r e a segue quasi esclusivamente questa successione di vie p u r dilagando nella l a g u n a e p u r usufruendo di altri c a n a l i , c o m e quello delle orfanelle, il c a n a l grande, quello delle tresse ed altri di i m p o r t a n z a t r a s c u r a b i l e . t u t t i questi canali avendo piccole sezioni e a n d a m e n t o tortuoso n o n modificano sostanzialmente la p r o p a g a z i o n e della m a r e a nel c a n a l e principale. il c a l c o l o delle costanti a r m o n i c h e e non a r m o n i c h e per la l o c a l i t à c o n s i d e r a t a , p u r f a c e n d o p a r t e di un vasto e sistematico l a v o r o di ric e r c a sulla p r o p a g a z i o n e della m a r e a nella laguna, assume notevole 3 8 8 s i l v i o p o l l i v a l o r e i n t r i n s e c o , p e r la n e c e s s i t à c h e si h a in un g r a n d e p o r t o di c o n o s c e r e in p r e c e d e n z a il f e n o m e n o d e l l a m a r e a e quelli c o n esso connessi, c o m e l e sesse, le v i b r a z i o n i , le « c o l m e », e le « m a g r e ». 2. il mareografo e le costanti strumentali. — l o s t r u m e n t o regis t r a t o r e è un m a r e o g r a f o « m 4 5 0 » c o s t r u i t o dall'ufficio i d r o g r a f i c o del m a g i s t r a t o alle a c q u e di v e n e z i a . i l r a p p o r t o di r i d u z i o n e è di l e m a r e e a i'orto m a r g h e r a 90 tabella i — costanti armoniche di porto marghera. valori mensili e medie annue. 1 9 4 0 c. m . 5 iv o k k o p m ms 1 9 4 0 c. £ 2 2 i i i i 4 g e n n a i o h 2 4 , 0 1 4 , 3 4 , 1 3 , 9 1 6 , 7 6 , 8 5 , 5 0 , 1 1 , 2 f e b b r a i o » 2 3 , 8 1 4 , 3 3 , 9 3 , 9 2 4 , 8 6 , 8 8 , 2 0 , 7 0 , 8 m a r z o » 2 4 , 4 1 4 , 3 4 , 8 3 , 9 2 0 , 3 " , o 6 , 7 1 , 0 1 , 0 a p r i l e » 2 ; , 9 1 4 , 1 4 , 4 3 , 8 2 0 , 1 6 , 4 6 , 6 1 , 1 1 , 1 m a g g i o » 2 5 , 2 1 3 , 3 4 , 0 3 , 6 1 7 , 9 3 , 4 5 , 9 1 , 2 1,2 g i u g n o » 2 5 , 1 1 4 , 2 3 , 9 3 , 8 1 , 6 6 , 6 5 , 8 1 , 2 1 , 8 l u g l i o » 2 5 , 2 1 5 , 1 5 , 6 4 , 1 1 9 , 5 5 , 4 6 , 4 1 , 2 1 , 6 a g o s t o » 2 5 , 1 1 4 , 7 5 , 2 4 , 0 2 1 , 3 , 9 7 , 2 1 , 1 1 , 0 s e t t e m b r e > 2 4 , 8 1 4 , 7 5 , 2 4 , 0 1 9 , 5 5 , 2 6 , 4 0 , 9 0 , 8 o t t o b r e » 2 3 , 7 1 4 , 0 3 , 8 3 , 8 1 6 , 1 4 , 0 5 , 3 0 , 8 0 , 8 n o v e m b r e » 2 4 , 5 1 3 , 1 3 , 6 3 , 5 13,"4 3 , 3 4 , 4 0 , 8 0 , 8 d i c e m b r e > 2 5 , 1 1 2 , 7 4 , 1 3 , 4 1 5 , 1 7 , 0 5 , 0 0 , 4 0 , 9 m e d i a > 2 4 , 7 1 4 , 1 4 , 4 3 , 8 18,(> 5 , 5 0 , 9 1 , 1 + 0 , 6 0 , 7 0 , 7 0 , 2 3 , 1 1 , 5 1 , 0 0 . 4 0 , 3 g e n n a i o g > 3 2 5 3 4 3 3 3 5 3 4 3 1 0 2 8 4 1 0 2 1 6 0 1 5 8 f e b b r a i o g > 3 1 9 3 3 1 2 9 4 3 3 1 9 2 8 9 2 8 . 1 1 8 m a r z o » 3 2 2 3 3 1 3 2 1 3 3 1 8 o 7 7 8 6 1 1 ? 1 2 9 a p r i l e » 3 2 2 3 3 3 3 1 9 3 3 3 9 0 8 4 9 0 1 2 5 1 4 5 m a g g i o » 3 2 3 3 3 8 3 2 6 3 3 8 9 3 9 8 9 3 1 2 0 1 5 5 g i u g n o » 3 2 3 3 3 6 3 2 8 3 3 6 8 9 9 0 8 9 1 1 1 1 3 9 l u g l i o > 3 2 > 3 4 3 3 2 7 3 4 3 9 9 1 0 2 9 9 1 3 7 1 3 7 a g o s t o » 3 2 4 3 3 3 3 2 0 3 3 3 8 9 6 7 8 9 1 0 8 1 2 2 s e t t e m b r e » 3 2 6 3 3 6 3 2 5 3 3 6 1 1 4 7 7 1 1 4 1 1 9 1 2 7 o t t o b r e > 3 2 / 3 3 6 3 1 5 3 3 6 1 0 8 1 0 6 1 0 8 1 4 2 1 5 0 n o v e m b r e » 3 2 / 3 3 7 3 2 1 3 3 7 9 9 9 8 9 9 1 3 2 1 9 d i c e m b r e » 3 1 9 3 4 0 3 2 9 3 4 0 1 0 5 8 5 1 0 5 1 0 7 1 4 4 m e d i a » 3 2 3 , 5 3 3 « , 4 3-21,7 3 i f i , 4 9 7 , 2 8 7 , 2 9 7 , 2 1 2 2 , t 1 4 1 , 9 ± 2 , 7 4 , 1 1 0 , 2 4 , 1 8 , 8 1 1 , 8 8 , 8 1 9 , 0 1 7 , 3 g e n n a i o ti' 3 2 1 3 3 8 3 3 1 3 3 8 9 9 8 2 9 9 1 5 1 1 4 8 f e b b r a i o » 3 1 5 3 2 6 2 9 0 3 2 6 8 9 7 6 8 9 7 8 1 0 8 m a r z o > 3 1 8 3 2 6 3 1 7 3 2 6 8 3 7 5 8 3 1 0 8 1 1 9 a p r i l e » 3 1 8 3 2 8 3 1 5 3 2 8 ì»7 8 2 8 7 1 1 6 1 3 5 m a g g i o » 3 1 9 3 3 3 3 2 2 3 3 3 9 0 9 6 9 0 1 1 1 1 4 5 g i u g n o » 3 1 9 3 3 1 3 2 4 3 3 1 8 6 8 8 8 6 1 0 2 1 2 9 l u g l i o » 3 2 1 3 3 8 3 2 3 3 3 8 9 6 1 0 0 9 6 1 2 8 1 2 7 a g o s t o » 3 2 0 3 2 8 3 1 6 3 2 8 8 6 6 5 8 6 9 9 1 1 2 s e t t e m b r e » 3 2 2 3 3 1 3 2 1 3 3 1 1 1 1 7 5 1 1 1 1 1 0 1 1 7 o t t o b r e 3 2 3 3 3 1 3 1 1 3 3 1 1 0 5 1 0 4 1 0 5 1 3 3 1 4 0 n o v e m b r e » 3 2 3 3 3 2 3 1 7 3 3 2 9 6 9 o 9 6 1 2 3 1 6 9 d i c e m b r e » 3 1 5 3 3 5 3 2 5 3 3 5 1 0 2 8 3 1 0 2 9 8 1 3 4 mpdin > 3 1 0 , 5 3 3 1 , 4 3 1 7 , 7 3 3 1 , 4 9 4 , 2 8 5 , 2 9 4 , 2 1 1 3 , 1 1 3 1 , 9 1u cuih + 2 , 7 4 , 1 1 0 , 2 4 , 1 8 , 8 1 1 , 8 8 , 8 1 9 , 0 1 7 , 3 1 1 , 8 1 9 , 0 3 9 0 s i l v i o p o l l i l a b o r i o s e o p e r a z i o n i selettive, c o m e q u e l l e dell'analisi p e r i o d a l e , n o n è possibile e s e g u i r e u n a l o r o d e t e r m i n a z i o n e . l'ufficio i d r o g r a f i c o b a c u r a t o l a s i s t e m a z i o n e dello s t r u m e n t o e il suo c o l l e g a m e n t o c o n la rete' a l t i m e t r i c a dello s t a t o . s e c o n d o l a q u o t a z i o n e eseguita n e l 1 9 2 8 , a n n o in cui è s t a t o messo in f u n z i o n e 10 s t r u m e n t o , l e q u o t e dei b u l l o n i di r i f e r i m e n t o della stazione m a r e o grafica sono : b u l l o n e i n t e m o m 2 , 4 7 0 7 sul livello m e d i o m a r i n o , bull o n e e s t e r n o ni 2 , 0 6 8 7 sullo stesso p i a n o di riferimento. 3. metodo di calcolo. — i l v a l o r e delle c o s t a n t i a r m o n i c h e p e r u n d a t o posto è c a r a t t e r i z z a t o d a l l a f o r m a d e l b a c i n o n e l q u a l e a v v i e n e 11 f e n o m e n o della m a r e a . n e i p o r t i c o n a c q u e p r o f o n d e e r i v e a r g i n a t e t a l e f o r m a n o n v a r i a in m a n i e r a a p p r e z z a b i l e col t e m p o , sia esso c o n s i d e r a t o q u a l e d i m e n s i o n e fisica o q u a l e situazione m e t e o r i c a . b e n diverso a p p a r e il f e n o m e n o n e l l a l a g u n a dove, c o n il v a r i a r e della m a r e a , dello s t a t o del t e m p o , e p e r t a n t o delle stagioni, vaste z o n e l a g u n a r i , p e r i n t e r v a l l i più o m e n o l u n g h i , r i m a n g o n o s c o p e r t e o somm e r s e d a l l ' a c q u a . v a r i a d u n q u e n o t e v o l m e n t e la f o r m a del b a c i n o . a questo f a t t o d o v r e b b e c o r r i s p o n d e r e u n a v a r i a z i o n e c o n t i n u a delle cos t a n t i a r m o n i c h e di u n o stesso sito. s t u d i su t a l e a r g o m e n t o non fur o n o a n c o r a eseguiti. e d è a p p u n t o c o n lo s c o p o di p r e p a r a r e il m a t e r i a l e p e r t a l i ricerche c h e il c a l c o l o delle c o s t a n t i a r m o n i c h e è stato eseguito i n d i p e n d e n t e m e n t e p e r ogni singolo m e s e , i n v e c e di consider a r e 1 i n t e r v a l l o a n n u o c o m p l e t o . il m e t o d o c h e più si a d a t t a allo scopo è q u e l l o dell'analisi a r m o n i c a e l a b o r a t o da a . t . d o o d s o n e p u b b l i c a t o n e l l e « t l i e a d m i r a l t y t i d e s t a b l e s », p a r t i l i . dei t r e p r o c e d i m e n t i i n d i c a t i d a l d o o d s o n si seguì quello c h e si a p p l i c a ad u n a successione c o n t i n u a di v a l o r i o r a r i estesa a 6 9 6 o r e ( 2 9 giorni l. f u r o n o analizzati gli i n t e r v a l l i c h e v a n n o dalle o r e 0 del g i o r n o 1 alle o r e 2 3 del g i o r n o 2 9 di c i a s c u n m e s e d e l l ' a n n o 1 9 4 0 . il c a l c o l o s e p a r a t o p e r c i a s c u n p e r i o d o di 2 9 g i o r n i è c o n v e n i e n t e a n c h e p e r il c o n t r o l l o della o m o g e n e i t à e regol a r i t à della serie m e n s i l e e s a m i n a t a e q u a l e c o n t r o l l o di c a l c o l o p e r le singole c o s t a n t i . i n f a t t i , s c o s t a m e n t i f o r t i ( p e r l e c o m p o n e n t i di not e v o l e a m p i e z z a ) sono dovuti o ad e r r o r i di c a l c o l o o ad i r r e g o l a r i t à nel f u n z i o n a m e n t o dello s t r u m e n t o . quali c o s t a n t i a r m o n i c h e del sito e s a m i n a t o si c o n s i d e r a r o n o l e m e d i e a r i t n i e t r i c h e dei v a l o r i c o r r i s p o n d e n t i dei 12 g r u p p i m e n s i l i . i l t e m p o a d o t t a t o è quello m e d i o d e l l ' e u r o p a c e n t r a l e ( t . m . e . c . ) . l e semiampiezze h sono d a t e in c m . l e situazioni adattata g e l e m a r e e a torto marcherà 3 9 1 tabella i i — costanti non armoniche di porto marghera. f alori mensili e medie annue. 1 9 4 0 ti età della m a r e a g i o r n i i n a m ( m h w i ) ivf v m ( m h w i ) a l t a m a r e a imam ( m h w i ) bassa m a r e a i s a m ( h w f & c ) a» c m g e n n a i o 0 , 7 0 1 1 " 0 3 m u n 0 2 " ' 1 1 " 0 4 " ' 1 1 " 1 6 " 2 3 , 1 f e b b r a i o 0,45 10 5 0 10 52 10 4 9 1 0 5 9 13,6 m a r z o 0,33 10 57 10 5 4 10 5 9 11 03 6,5 a p r i l e 0 , 4 1 10 57 10 53 11 01 11 0 4 10,2 m a g g i o 0,57 10 59 10 56 11 02 11 08 2 3 , 1 g i u g n o 0,49 10 5 9 10 58 11 00 11 09 16,0 l u g l i o 0 , 7 0 11 03 10 5 8 11 08 11 15 15,1 a g o s t o 0,33 11 01 i l 01 11 01 11 07 11,8 s e t t e m b r e 0,37 11 05 i l 04 11 0 6 11 12 12,8 o t t o b r e 0,33 11 07 n 03 11 11 11 1 4 2 9 , 1 n o v e m b r e 0,37 11 07 n 07 11 08 11 1 4 28,7 d i c e m b r e 0,82 10 51 10 5 0 10 51 11 04 13,5 m e d i a 0 , 4 9 1 1 " 0 0 m 1g" 5 8 ' " 1 1 " qom 1 1 " 0 9 ° 1 7 , 0 h m o + h s . i 1 m , — h s , 2 h ao 1 9 4 0 c m c m c m c m g e r m a i o 38,3 9,7 76,6 2 3 , 1 f e b b r a i o 38,1 9,5 87,2 13,6 m a r z o 38,7 10,1 8 3 , 4 6,5 a p r i l e 3 9 , 0 10,8 82,5 10,2 m a g g i o 38,5 11,9 75,7 2 3 , 1 g i u g n o 39,3 10,9 8 0 , 0 16,0 l u g l i o 4 0 , 3 10,1 8 4 , 1 15,1 a g o s t o 39,8 1 0 , 4 8 3 , 9 11,8 s e t t e m b r e 39,5 10,1 81,5 12,8 o t t o b r e 37,7 9,7 72,3 2 9 , 1 n o v e m b r e 37,6 11,4 6 7 , 4 28,7 d i c e m b r e 37,8 12,4 73,7 13,5 m e d i a 3 * 7 1 0 , 5 7 9 , 0 1 7 , 0 3 9 2 s i l v i o p o l l i locale y., s o n o e s p r e s s e i n g r a d i s e s s a g e s i m a l i . f u s o o r a r i o = — 1 o r a , a " = 1. p e r i n d i c a r e l e semiampiezze e l e situazioni di u n ' o n d a p a r z i a l e m e t t e r e m o i s i m b o l i h, x , g, d a v a n t i a q u e l l i d e l l e o n d e c o m p o n e n t i c o n s i d e r a t i . c o s ì p e r es. hk\ i n d i c h e r à l a s e m i a m p i e z z a d e l l ' o n d a p a r z i a l e k i l e c o s t a n t i h e g p e r l e o n d e k2 e p i si d e d u s s e r o d a l l e c o m p o n e n t i so e k-2 m e d i a n t e i r a p p o r t i : hk2 = 0 , 2 7 hs2, gk* = gs=; hpx = 0 , 3 3 hku gpj = gku l a situazione locale x si o t t e n n e d a q u e l l a adattata g m e d i a n t e la r e l a z i o n e : x = g + ( p x 0 n ° a h ) , il o v e : /> = suffisso d e l l ' o n d a , x ° = l o n g i t u d i n e d a g r e e n w i e l i , n° = v e l o c i t à o r a r i a d e l l ' o n d a , a h = l o n g i t u d i n e i n o r e del m e r i d i a n o sul q u a l e è r e g o l a t o l ' o r o l o g i o . d a l l e costanti armoniche si c a l c o l a r o n o q u e l l e non armoniche m e d i a n t e l e s e g u e n t i f o r m u l e : età d e l l a m a r e a s e m i d i u r n a i n g i o r n i : xs„ — x l l f . il = £ — 2 4 ° , 4 stabilimento medio del porto o intervallo medio delle alte maree ( i n g l . mean high ivater internai): y.m„ i m a m ( m h w i ) = — 1 jl j t e n e n d o c o n t o d e l l a s o v r a m a r e a di m 2 ' i m a m ( m h w i ) = — ^ + — a r c t g 2 9 ' 2 9 hm„ d o v e il s e g n o — v a l e p e r l e a l t e m a r e e e q u e l l o + p e r l e basse. stabilimento volgare del porto o intervallo sizigiale della alta marea ( i n g l . high ivater full and change): x m j 1 sen (v.m„—nst i s a m ( h w f & c ) = — a r c t g 2 9 3 0 h m z , , . . + c o s ( x 3 / 2 — x s 2 ) hs„ altezza del livello medio o dislivello t r a i l p i a n o di r i d u z i o n e degli s c a n d a g l i e i l l i v e l l o m e d i o d e l m a r e o s e m p l i c e m e n t e livello medio della carta ( i n g l . mean level m l ) . ml — 5 2 c m . l e maree a i'orto marghera 3 9 3 tabella i i i — costanti armoniche di diga sud lido, punta salute, porto marghera. c . m„ s„ a 2 k i o i p i m * msi d i g a sud l i d o . . . h 2 3 , 4 1 3 , 8 3 , 8 5 , 3 1 6 , 0 5 , 2 4 , 3 p u n t a s a l u t e . . . . 2 2 , 6 1 2 , 8 4 , 4 4 , 3 1 6 , 3 5 , 0 4 , 9 0 , 5 0 , 3 p o r t o m a r g h e r a . . . 2 4 , 7 1 4 , 1 4 , 4 3 , 8 1 8 , 6 5 , 5 6 , 1 0 , 9 1 , 1 d i g a sud l i d o . . . 6 ° 2 8 8 2 9 3 2 9 9 2 8 1 7 9 7 0 5 6 — p u n t a s a l u t e . . . . 3 1 6 3 3 1 3 1 6 3 1 6 8 9 7 9 1 0 7 9 0 1 2 1 p o r t o m a r g h e r a . . . 3 2 7 3 3 6 3 2 2 3 3 6 9 7 8 7 9 7 1 2 2 1 4 2 d i g a sud l i d o . . . 54° 2 8 3 2 8 8 2 9 4 2 7 6 7 6 6 7 5 3 — p u n t a salute . . . . 3 1 1 3 2 6 3 1 1 3 1 1 8 6 7 7 1 0 4 8 0 1 1 0 p o r t o m a r g h e r a . . . 3 2 0 3 3 1 3 1 3 3 3 1 9 4 8 5 9 4 1 1 3 1 3 2 tabella i v — rapporti fra le ampiezze delle singole maree. s2 : m» n a : ma k i : m= oi : ma v a l o r e t e o r i c o . . . . 0,465 0 , 1 9 0 0 , 4 0 0 0 , 4 2 0 d i g a sud l i d o . . . . 5 9 0 1 6 0 6 8 0 2 2 0 p u n t a s a l u t e 5 6 6 195 721 2 2 1 p o r t o m a r g h e r a . . . . 5 7 1 178 753 2 2 3 3 9 4 s i l v i o p o l l i media altezza delle alte maree sizigiali, m a m s (ingl. meati high ivater spring, m h w s ) : c o n t a t a d a l livello m e d i o = hm2 hs2; cont a t a d a l p i a n o di r i d u z i o n e degli s c a n d a g l i = ml hm> -(hs2. media altezza delle alte maree alle quadrature, m a m q (ingl. meati high water neap, m h w n ) : c o n t a t a d a l livello m e d i o = h m o — hso; c o n t a t a dal p i a n o di r i d u z i o n e degli s c a n d a g l i : ml -jhm2 — hs->. high ivater neap, m h w n ) : c o n t a t a d a l livello m e d i o — hmo — hs>; i l massimo innalzamento possibile dell'alta marea e la d e p r e s s i o n e m a s s i m a della bassa m a r e a r i s p e t t o al livello m e d i o del m a r e sono cons i d e r a t i quali s o m m e delle a l t e z z e delle n o v e c o m p o n e n t i . a o r a p p r e s e n t a , i n c m , l ' a l t e z z a del livello m e d i o m e n s i l e r i f e r i t o a l livello m e d i o del m a r e d e t e r m i n a t o dall'ufficio i d r o g r a f i c o del magis t r a t o delle a c q u e di v e n e z i a nel 1 9 2 8 . il f a t t o c h e t u t t e le altezze ris u l t a n o positive è dovuto in p a r t e a l l ' a t t u a l e p r o g r e s s i v o a u m e n t o del livello m a r i n o e in p a r t e ad u n l e n t o a b b a s s a m e n t o del t e r r e n o della r e g i o n e l a g u n a r e . 4. le costanti armoniche e non armoniche. — p e r c i a s c u n m e s e d e l l ' a n n o 1 9 4 0 f u r o n o a n a l i z z a t i i p e r i o d i di 6 9 6 o r e c o m p r e s i f r a le o r e 0 del g i o r n o 1 e le o r e 2 3 del g i o r n o 2 9 dello stesso m e s e . p e r ogni v a l o r e m e d i o a n n u o delle c o s t a n t i è s t a t o p u r e d e t e r m i n a t o l'err o r e m e d i o 5. considerazioni e confronti. — l e m a r e e sono di t i p o p r e v a l e n t e m e n t e s e m i d i u r n o . i n v i c i n a n z a alle q u a d r a t u r e a s s u m o n o a n d a m e n t o d i u r n o . r i s u l t a s o d d i s f a t t a la d i s u g u a g l i a n z a è c o m p r e s o f r a i v a l o r i 0 , 2 5 e 1,25. l a m a r e a r i s u l t a di t i p o m i s t o . p r i m a di c o n s i d e r a r e i r a p p o r t i f r a le a m p i e z z e delle c o m p o n e n t i , p r e s e n t i a m o n e l l a tal). 3, p e r u n c o n f r o n t o , i v a l o r i delle c o s t a n t i a r m o n i c h e di diga sud l i d o , p u n t a s a l u t e e p o r t o m a r g h e r a . n e l l a t a b . 4 d i a m o i r a p p o r t i f r a le v a r i e a m p i e z z e o t t e n u t i s e c o n d o le deduzioni t e o r i c h e e s e c o n d o quelle m a r e o g r a f i c h e , p e r le t r e stazioni o r a c o n s i d e r a t e . il r a p p o r t o hs2 : hm2 è s u p e r i o r e a l v a l o r e t e o r i c o , c o m e lo sono hm , — hs2 < hki + hoi . i l r a p p o r t o h(lk1 + 01)ì h ( m 2 + s 2 ) = 0 , 6 2 l e m a r e e a i'orto marghera 3 9 5 tabella v — costanti non armoniche delle maree. imam (mhwi) isam (hwf & c) hivl + hs, hm, hs, 2 h slabilmedio slabil. volg. cm cm cm •diga sud l i d o . . . 911 4 5 m 9h 5 0 37,2 9 , 6 72,8 p u n t a s a l u t e . . . . 10 4 3 10 4 8 35,4 9,8 71,2 p o r t o m a r g h e r a . . . 11 00 11 0 9 38,7 10,5 79,0 tabella v i — costanti armoniche di porto marghera. medie stagionali. 1 9 4 0 c . m„ s n k k o p m ms a 1 9 4 0 c . 2 2 2 i 4 i 4 4 0 i n v e r n o i i 2 4 , 3 1 3 , 8 4 , 0 3 , 7 1 8 , 9 6 , 9 6 , 2 0 , 4 1,0 1 6 , 7 p r i m a v e r a » 2 4 , 8 1 3 , 9 4 , 4 3 , 8 1 9 , 4 5 , 6 6 , 4 1,1 1,1 1 3 , 3 e s t a t e » 2 5 , 1 1 4 , 7 4 , 9 4 , 0 1 9 , 6 5 , 3 6 , 5 1 , 2 in r-t 1 4 , 3 a u t u n n o » 2 4 , 3 1 3 , 9 4 , 2 3 , 8 1 6 , 3 4 , 2 5 , 4 0 , 8 0 , 8 2 3 , 5 i n v e r n o e 3 2 1 , 0 3 3 8 , 0 3 1 9 , 3 3 3 8 , 0 9 9 , 7 8 2 , 3 9 9 , 7 1 1 8 , 0 1 4 0 , 0 1 6 , 7 p r i m a v e r a > 3 2 2 , 3 3 3 4 , 0 3 2 2 , 0 3 3 4 , 0 8 9 , 7 8 6 , 3 8 9 , 7 1 2 0 , 7 1 4 3 , 0 1 3 , 3 e s t a t e > 3 2 4 , 0 3 3 7 , 3 3 2 5 , 0 3 3 7 , 3 9 2 , 3 8 6 , 3 9 2 , 3 1 1 8 , 7 1 3 2 , 7 1 4 , 3 a u t u n n o » 3 2 7 , 7 3 3 6 , 3 3 2 0 , 3 3 3 6 , 3 1 0 7 , 0 9 3 , 7 1 0 7 , 0 1 3 1 , 0 1 5 2 , 0 2 3 , 5 tabella v i i — costanti non armoniche di porto marghera. medie stagionali. i s a m ( h w f & c) i i m „ h s o 2 h l i v . m e d i o 0 h m c m cm cui i n v e r n o 11 0 6 38,1 79,2 16,7 p r i m a v e r a 11 03 38,7 80,5 13,3 e s t a t e 11 10 39,8 82,8 14,3 a u t u n n o 11 13 38,2 73,7 23,5 3 9 6 s i l v i o p o l l i quelli a n a l o g h i di quasi t u t t i i p o r t i d e l l ' a d r i a t i c o s e t t e n t r i o n a l e . e d è m o l t o p r o s s i m o a quello di p u n t a della s a l u t e . i l r a p p o r t o iin1 : hm2 è l e g g e r m e n t e i n f e r i o r e a q u e l l o t e o r i c o ed è c o m p r e s o f r a q u e l l o di p u n t a s a l u t e e diga sud l i d o . p e r gli a l t r i p o r t i d e l l ' a d r i a t i c o v a r i a i n più o in m e n o . quasi d o p p i o d e l v a l o r e t e o r i c o r i s u l t a il r a p p o r t o hk^ : hm2; c i r c a la m e t à q u e l l o hox : hm>. questi d u e v a l o r i s o n o di p o c o diversi da quelli di d i g a s u d l i d o e p u n t a s a l u t e . p e r l a s o v r a m a r e a q u a r t o d i u r n a m4 e p e r l ' o n d a c o m p o s t a di a c q u a bassa ms4 si o t t e n n e r o l e s e m i a m p i e z z e di c m 0 , 9 e 1 , 1 ; v a l o r i l e g g e r m e n t e s u p e r i o r i a quelli n o r m a l i sia p e r l e coste v e n e t e c h e p e r l e r e g i o n i l a g u n a r i p i ù i n t e r n e . l ' a m p i e z z a delle m a r e e è più g r a n d e a p o r t o m a r g h e r a c h e a diga sud l i d o . m e n t r e i n c a n a l i l u n g h i ed a sezione d e c r e s c e n t e l ' a m p i e z z a della m a r e a d i m i n u i s c e n o t e v o l m e n t e con la l u n g h e z z a sino a r i d u r s i quasi a l l a m e t à n e i siti p i ù i n t e r n i della l a g u n a ( a v a l l e d o g à r i s u l t a r i d o t t a ai 4 / 7 ) , p e r p o r t o m a r g h e r a , c o m u n i c a n t e c o n il p o r t o l i d o m e d i a n t e u n s i s t e m a di c a n a l i di a m p i a sezione, si h a l'effetto opposto. l ' o n d a di m a r e a i n c a n a l a n d o s i p r o d u c e u n a p p r e z z a b i l e a u m e n t o d e l l ' a m p i e z z a . r e g o l a r e è i n v e c e il r i t a r d o delle fasi delle singole m a r e e t r a i l p o r t o l i d o e p o r t o m a r g h e r a . l e differenze risultano f a c i l m e n t e d a l l a t a b . 3. p i ù a v a n t i p r e s e n t e r e m o il r i t a r d o espresso in o r e e m i n u t i . c o n s i d e r i a m o la t a b . 5. l ' o n d a di m a r e a risultante i m p i e g a l h 19 n t p e r p r o p a g a r s i d a l l a d i g a s u d l i d o al m a r e o g r a f o di p o r t o m a r g h e r a e p r e c i s a m e n t e 5 8 m i n u t i sino alla p u n t a s a l u t e e 2 1 m i n u t i da q u e sto posto a p o r t o m a r g h e r a . l ' a m p i e z z a della m a r e a c o m p l e s s i v a è l e g g e r m e n t e p i ù g r a n d e a p o r t o m a r g h e r a rispetto a diga sud l i d o , l ' a u m e n t o r i s u l t a del 3 % p e r l ' i n s i e m e delle d u e c o m p o n e n t i m2 e s2, e d e l l ' 8 , 5 % p e r la s o m m a delle n o v e c o m p o n e n t i . 6. variazione stagionale delle costanti armoniche. — c o n f r o n t a n d o l ' a n d a m e n t o dei v a l o r i m e n s i l i delle c o s t a n t i a r m o n i c h e e n o n a r m o n i c h e c o n q u e l l o dei c o r r i s p o n d e n t i v a l o r i dei livelli m e d i m a r i n i ot m e g l i o a n c o r a , e s e g u e n d o il c o n f r o n t o f r a i v a l o r i s t a g i o n a l i , si possono m e t t e r e in e v i d e n z a a l c u n e r e l a z i o n i di d i p e n d e n z a t r a le costanti a r m o n i c h e ed il livello m a r i n o s t a g i o n a l e . n e l l e t a b . 6 e 7 p r e s e n t i a m o i v a l o r i m e d i s t a g i o n a l i delle costanti stesse e dei livelli m a r i n i . i n esse l e stagioni h a n n o il s e g u e n t e , significato : i n v e r n o = die. g e n n . -ff e b b r . , p = m a m, e = l e m a r e e a i'orto m a r g h e r a 3 9 7 — g l a, .4 = s + o + /v. d a l l ' e s a m e flel l o r o a n d a m e n t o o di quello della l o r o r a p p r e s e n t a z i o n e grafica r i s u l t a c h e : l e a m p i e z z e delle m a r e e c o m p o n e n t i e della m a r e a r i s u l t a n t e ass u m o n o v a l o r i m a s s i m i in p r i m a v e r a e d ' e s t a t e , c i o è q u a n d o i livelli m a r i n i sono p i ù bassi. n e l l ' a u t u n n o e n e l l ' i n v e r n o a v v i e n e il c o n t r a r i o , alla m a s s i m a a l t e z z a d e l l ' a c q u a c o r r i s p o n d o n o v a l o r i m i n i m i delle a m p i e z z e . t r a le d u e g r a n d e z z e si h a , in senso l a r g o , p r o p o r z i o n a l i t à inversa. l e s i t u a z i o n i l o c a l e v: e a d a t t a t a g, e gli s t a b i l i m e n t i del p o r t o , m e d i o e v o l g a r e , a s s u m o n o v a l o r i m a s s i m i in a u t u n n o , c i o è c o n a c q u a a l t a , e m i n i m i in p r i m a v e r a e estate, c i o è c o n a c q u a bassa. r i s u l t a n o p e r t a n t o d i r e t t a m e n t e p r o p o r z i o n a l i a l l ' a l t e z z a d e l l ' a c q u a . c o n livello m a r i n o a l t o d i m i n u i s c e l ' a m p i e z z a della m a r e a e aum e n t a il t e m p o di p r o p a g a z i o n e , c o n livello m a r i n o basso a v v i e n e il c o n t r a r i o i c i ò v a l e p e r la z o n a e s a m i n a t a , l e r e l a z i o n i t r o v a t e n o n si possono p e r ò e s t e n d e r e agli altri posti d e l l a l a g u n a , d a t a l a diversità delle situazioni i n c u i essi si t r o v a n o . o c c o r r e r e b b e e s e g u i r e anc h e p e r essi s e r i e d'analisi c o m e si è f a t t o in questo l a v o r o p e r p o r t o m a r g h e r a . trieste — istituto talassografico — maggio 1951. riassunto si calcolano le costanti armoniche e non armoniche delle maree di porto marghera nella laguna di venezia. si segue il procedimento elaborato da t. doodson, applicandolo per un periodo di 12 mesi. le maree sono di tipo prevalentemente semidiurno, in vicinanza alle quadrature diventano diurne. si mette in evidenza una relazione tra il livello medio marino e i valori delle costanti armoniche: a livello alto corrispondono valori minimi delle ampiezze e massimi delle situazioni e stabilimenti del porto, e viceversa per livelli bassi. b i b l i o g r a f i a doodson a . t . , warburg h . d . : the admirulty tides tables. p a r i . i l i , instruc. and tables. h y d r o g r . d e p a r t m . l o n d o n 1 9 3 6 . polli s . : oscillazione annua del mare mediterraneo. a r c h . di o c c a n o g r . e l i m n . v o i . i , 1. m e n i . 2 8 8 c o m i t . t a l a s s . it., 1 9 4 1 . 3 9 8 s i l v i o p o l l i polli s . : il graduale aumento del livello del mure a venezia, trieste e pala. g e o fisica p u r a e a p p l i c a t a , i x , 1-2, 1 9 4 6 , m i l a n o . polli s . : costanti armoniche e non armoniche delle maree di 4 località della laguna di venezia. (paliaga, torcello, torson di sotto, millecampi). a r c h . d i o c e a n o g r . e l i m n o l . v i i ( 1 ) 17-27, 1 9 5 0 . polli s . : determinazione delle costanti armoniche e non armoniche delle maree per i porti di belvedere, cortelazzo, faro rocchetta e cliioggia. a n n . di geofisica, v o i . i i ( 1 9 4 9 ) , n . 3, 4 3 6 4 4 9 . tenani m . : maree e correnti di marea. i s t . i d r o g r . della m a r i n a , 1 9 3 5 , 1 9 4 0 . vercelli f . : ciò che sappiamo delle maree dell'adriatico. l a r i c e r c a s c i e n t e r o m a , 9-10, 1 9 3 2 . warburg h . d . , tenani m . : maree e correnti di marea. i s t . i d r o g r . d e l l a m a r i n a , g e n o v a 1 9 2 5 . vol. 48, 01, 05ok.qxd 195 annals of geophysics, vol. 48, n. 1, february 2005 key words method – radon – soil – convective velocity – radon flux density 1. introduction a knowledge of certain radon transport characteristics (depth distribution function a(z) of the soil gas radon concentration, radon flux density q(z)|z = 0 from the earth’s surface, convective radon flux velocity υ in soil, etc.) is essential in solving a number of radioecological and geophysical problems. radon transport in soil is described by the well-known diffusive-convective equation (nazaroff and nero, 1988). the solution to this equation is an exponential soil radon concentration distribution with depth. the exponential coefficient varies with the physical-geological soil parameters and weather conditions. the latter affect the convective radon flux velocity in soil. it has been verified experimentally (fleischer, 1997; abumurad and al-tamimi, 2001; jönsson, 2001) that the depth distribution of the soil gas radon concentration obeys the exponential law in the case of a relatively homogeneous geological structure and a great depth of occurrence of waterbearing horizons. given the radon concentration distribution function we can readily determine the following parameters: equilibrium soil gas radon concentration a∞, characterizing the radon potential of a given area, depth at which the equilibrium concentration is found, soil gas radon concentration gradient specifying the radon flux density according to fick’s law, and convective velocity. the central problem is to find experimentally the function a(z). reconstruction of the vertical profile of the soil gas radon concentration requires that measurements be performed at different depths. the number of measurements varies with prescribed accuracy. the measurements can be very expensive and difficult to perform. however, the number of radon concentration measurements can be reduced to two measurements, using the properties of the exponential law. the measurements should be performed at shallow depths (≤ 1 m deep), because here the gradient is very step. mailing address: dr. valentina s. yakovleva, tomsk polytechnic university, pr. lenin 30, tomsk, 634050 russia; e-mail: jak@interact.phtd.tpu.edu.ru a theoretical method for estimating the characteristics of radon transport in homogeneous soil valentina s. yakovleva tomsk polytechnic university, tomsk, russia abstract a theoretical method for estimating the characteristics of radon transport in homogeneous soil is developed. the method allows the following characteristics to be estimated: depth distribution function of the soil gas radon concentration, equilibrium radon concentration in the soil air, depth at which the radon concentration reaches its equilibrium value, radon flux density from the earth’s surface, and convective radon transport velocity. the method is based on soil gas radon concentration measurements and is appropriate in the case of relatively uniform geology. 196 valentina s. yakovleva in this work, a method for estimating the radon transport characteristics in soil is developed. the approach under review is based on the above-mentioned diffusive-convective radon transport model and in situ radon concentration measurements at two depths. 2. methodology solving the stationary diffusive-convective radon transport equation in the quasi-homogeneous approximation, we will get a depth distribution of the radon concentration in the soil air (jönsson, 1997) for the z-axis directed downward from the earth’s surface. thus, d d d2 2e e e 3 + + y m y 2 z( )a z a= exp1 j l k kk _b n p o oo i l (2.1) where a(z) is the radon concentration per unit volume of the soil air (bqm–3), υ is the convective radon flux velocity (ms–1), de is the effective radon diffusion coefficient (m2s–1), and λ is the radon decay constant (s–1). the equilibrium soil gas radon concentration depends solely on the physical-geological soil parameters, and we have a k a 1em sra = h t h 3 ^ h (2.2) where kem is the radon emanation coefficient (rel. units), ara is the specific activity of 226ra (bqkg–1), ρs is the solid soil particle density (kgm–3), and η is the soil porosity (rel. units). let us denote the soil gas radon concentration measured at a depth h1 by a1 and that measured at a depth h2 = 2h1 by a2. substituting a1 and a2 into eq. (2.1), we will arrive at the following equation a 1a a a 2 ln h a 1 2 1 1 1 1 1 2 ( ) expa z 1= zj l k k j l k kk j l k k k k k n p o o n p o oo n p o o o o o . (2.3) it is evident from eq. (2.3) that the equilibrium soil gas radon concentration generally found at a great depth and characterizing the soil radon potential (yakovleva, 2002) can be estimated from as few as two measurements near the earth’s surface. thus we obtain (yakovleva and ryzhakova, 2002) a a a a 2 1 2 1= 3 . (2.4) the depth (zeq) at which a∞ is found is determined by introducing the parameter (rel. units) x a z aeq= 3_ i . the parameter specifies the degree to which the soil gas radon concentration approaches its equilibrium value. for example, with x = 0.95, the soil gas radon concentration at the depth sought will be only 5% lower than its equilibrium value. then we can find zeq from the following equation ( ) . ln ln z h a a x 1 1 eq 1 1 2 = c m (2.5) the radon flux density from the earth’s surface is defined by the following relation (ryzhakova and yakovleva, 2002) ( ) ( ) ln q z d z a z d a a a h a a 2 1 1 1 z e e 0 1 2 1 1 1 2 $ $ $ 2 2 = = = h h = j l k k kk ^ n p o o oo h (2.6) and the convective radon flux velocity is expressible as ln ln h d a a a a h 1 1 1 e 1 1 2 1 2 1= + y m j l k k kk c n p o o oo m . (2.7) the radon flux density and convective velocity can be determined by eqs. (2.6) and (2.7). to this end, we need to know the radon diffusion coefficient in addition to two measured values of the soil gas radon concentration. the choice of the diffusion coefficient presents no special 197 a theoretical method for estimating the characteristics of radon transport in homogeneous soil problems. for the majority of sedimentary rocks constituting the surface layer, the diffusion coefficient varies within a small range and is, on average, 0.03 cm2s–1 (durrani and ilić, 1997). the measurements of a1 and a2 should be performed concurrently (by means of any conventional devices and techniques) at two points spaced 0.5 – 1 m apart. there is a limitation on the maximum separation between the two measuring points (∼1 m). this is due to the fact that the soil properties at the measuring points should be the same. a minimum point separation of 0.5 is needed to avoid a possible influence on the results of the two measurements. moreover, measurements for a smaller point separation present some technical problems. it is recommended that both of the measurements should be performed at depths between 0.3 and 1 m for the following reasons: i) the soil gas radon concentration varies comparatively rapidly at these depths, which enables us to reduce the error in determination of the function a(z), ii) the depth h1 should not be smaller than 0.3 m because of a great influence of atmospheric conditions, which reduces the reliability of the results obtained, and iii) an increase in the measurement depth above 1 m would not be economically attractive. the method under review is applicable for areas with a relatively homogeneous geological structure. in the case of radon anomalies (rocks with a high content of uranium, large fractures in the earth’s crust, etc.), the method will be not suitable. 3. preliminary results of practical evaluation of the method under review the method was tested in a small survey area with a homogeneous geological structure (surface soil layer is loam). the area is located in lagernii sad (camp garden) in tomsk (west siberia, russia). two holes spaced 0.5 m apart were drilled by a customized soil auger. one hole was 35 cm deep (h1), and the other was 70 cm deep (2h1). the hole diameter was 5.5 cm. radon radiometers with track etch detectors of lr-115 type iii-b (nikolaev and ilić, 1999) were placed in the holes. then the holes were covered to provide air-tightness and allowed to stay for 72 h. the soil gas radon concentration (a1 and a2) was determined as directed by operating instructions for the aist–tral complex. the etching and track counting methodology are described in nikolaev et al. (1993). the measured soil gas radon concentrations a1 and a2 were 6.8 and 11.4 kbqm–3. the equilibrium radon concentration a∞ calculated by eq. (2.4) was 21.0 kbqm–3. this value is twice as high as the measured value a2 at a depth of 70 cm, which is usually recommended for radon concentration measurements. we have also estimated a∞ by eq. (2.2) to get 20 kbqm–3. to this end, soil samples were taken, and their density, porosity and 226ra specific activity were determined (karataev et al., 2000; yakovleva, 2002). the radon emanation coefficient was taken to be 0.2. the values of a∞ calculated by eqs. (2.2) and (2.4) agree very closely. the depth at which the soil gas radon concentration accounts for 95% of its equilibrium value is 2.7 m. the radon flux density from the earth’s surface is 33.8 kbqm–2s–1, and the convective flux velocity is 1.7·10–4 cms–1. 4. concluding remarks we have developed a method for estimating the radon transport characteristics in soil. the approach under review has the following practical benefits: i) versatility since only two measurements of the soil gas radon concentration are needed to determine a number of radon transport characteristics; ii) validity for any conventional devices and techniques used for measuring the soil gas radon concentration; and iii) low cost since it requires neither a large number of measurements to determine the function a(z) nor detailed information on the physical-geological soil parameters. this method is useful in different fields of applied research such as: radioecology, for improving the reliability of potential soil radon risk estimates and for reducing the weather conditions effect on results of soil radon concentration monitoring; geophysics, for studying the convective gas flow velocity; etc. 198 valentina s. yakovleva references abumurad, k.m. and m. al-tamimi (2001): emanation power of radon and its concentration in soil and rocks, radiat. meas., 34 (1/6), 423-426. durrani, s.a. and r. ilić (editors) (1997): radon measurements by etched track detectors, appl. rad. prot. earth sci. environ. (world scientific, singapore), pp. 387. fleischer, r.l. (1997): radon and earthquake prediction, in radon measurements by etched track detectors, edited by s.a. durrani and r. ilić, appl. rad. prot. earth sci. env. (world scientific, singapore), 285-299. jönsson, g. (1997): soil radon surveys, in radon measurements by etched track detectors, edited by s.a. durrani and r. ilić, appl. rad. prot. earth sci. environ. (world scientific, singapore), 179-188. jönsson, g. (2001): soil radon depth dependence, radiat. meas., 34 (1/6), 415-418. karataev, v.d., v.s. yakovleva and d.e. ergashev (2000): investigation of radioactivity of environmental objects in the territory of tomsk oblast, izv. vyssh. uchebn. zaved., fizika, 43 (4), 105-109 (in russian). nazaroff, w.w. and a.v. nero (editors) (1988): soil as a source of indoor radon: generation, migration and entry, in radon and its decay products in indoor air (wiley-interscience publication), 57-112. nikolaev, v.a. and r. ilić (1999): etched track radiometers in radon measurements: a review, radiat. meas., 30, 1-13. nikolaev, v.a., m.g., buzynniy, i.b. vorobiev, a.v. gromov, a.s. krivokhatskiy, i.p. los, a.v. zelenskiy and yu.a. tomilin (1993): application of the track method for radon measurement in ukraine, nucl. tracks radiat. meas., 21 (3), 433-436. ryzhakova, n.k. and v.s. yakovleva (2003): a technique for determination of the radon flux density from the earth’s surface, rf patent no. 2212688, september 20, 2003 (in russian). yakovleva, v.s. (2002): radon transport processes in nonequilibrium media author’s abstract, cand. phys. math. sci. dissert. (tomsk polytechnic university, tomsk), (in russian). yakovleva, v.s. and n.k. ryzhakova (2003): a measuring technique for the steady-state equilibrium volumetric activity of radon in the soil air, rf patent no. 2212689, september 20, 2003 (in russian). layout 6 1 annals of geophysics, 62, 5, vo565, 2019; doi: 10.4401/ag-7549 “radioactivity of mt. etna volcano and radionuclides transfer to groundwater„ beata kozłowska1, agata walencik-łata1, salvatore giammanco*,2, giuseppina immè3,4, roberto catalano3,4, gabriella mangano3 (1) university of silesia in katowice, institute of physics, department of nuclear physics and its applications, uniwersytecka st. 4, 40 – 007 katowice, poland (2) istituto nazionale di geofisica e vulcanologia, osservatorio etneo, piazza roma, 2, i – 95125 catania, italy (3) department of physics and astronomy, university of catania, via s. sofia, 64, i – 95123 catania, italy (4) national institute for nuclear physics, division of catania, via s. sofia, 64, i – 95123 catania, italy 1. introduction the area under investigation is located in the eastern flank of mt. etna volcano (sicily, italy) (figure1). mt. etna was built up during the last 540.000 years by the alternate superimposition of lava flows and pyroclastic deposits [romano, 1982; chester et al., 1985; branca et al., 2011] and its edifice grew over a sedimentary substratum whose thickness is greater than 15 km [cristofolini et al., 1979]. the source of mt. etna magmatism is presumably connected with voluminous mantle melting, likely resulting from the suction of asthenosphere material induced by backward rolling of the subducting ionian crustal slab [gvirtzman and nur, 1999]. the structure of mt. etna is constituted by several nested strato-volcanoes [condomines et al., 1995; branca et al., 2011] that, as well as many other coeval small eruptive centers, grew on a lava plateau of tholeiitic/transitional composition produced by fissural eruptions some 0.5 ma ago [gillot et al., 1994; corsaro & cristofolini, 1997]. the article history receveid october 10, 2017; accepted april 4, 2019. subject classification: mt. etna volcano; natural radioactivity; volcanic rocks; groundwater; radiological risk. abstract the paper presents the results of a radiometric survey carried out on the eastern flank of mt. etna over an area of approximately 120 km2. activity concentrations of 238,234u, 232,230,228th, 226,228ra, from 238u and 232th decay chains, and potassium 40k were determined using α and γspectrometry techniques. all rock samples presented activity concentrations of u, th and ra isotopes ranging from 20 bq kg-1 to about 90 bq kg-1, and they showed no particular compositional variations over the investigated area. based on their respective elemental concentrations, the isotopic ratios of different elements were studied to check the presence of radioactive equilibrium, or disequilibrium, in the decay chains. moreover, an attempt to calculate radionuclide transfer factors from soil to water was made, and the radiological risk resulting from ingestion of these isotopes contained in drinking water was calculated. the results were compared with current regulations on the quality of drinking water. post-plateau volcanic products, emitted through an almost continuous eruptive activity, show a composition ranging from picritic and alkali basalt to trachytes, with mugearites and hawaiites as dominant products [cristofolini and romano, 1982; d'orazio, 1995]. the predominance of hawaiitic rocks in the recent activity of mt. etna can be explained mainly as a result of fractionation/mixing processes affecting a primary picritic kozłowska et al. 2 figure 1. study area and sampling sites (sites names as in kozłowska et al., 2009). the white area delimits the outcrops of vol− canic products of mt. etna. the three main rift zones (ne rift, w rift and s rift, correspondingly) are also shown. volcano−tectonic data of mt. etna modified from acocella and neri (2003). altitudes in meters above sea level. 3 radioactivity of mt. etna rocks magma in a relatively deep magma reservoir [armienti et al., 1996]. both the existence and the location of this reservoir beneath etna are derived largely from seismological data [sharp et al., 1980; aloisi et al., 2002]. magmatic activity in southern italy has produced huge volumes of volcanic rocks with large concentrations of radioelements that often are characterized by high levels of environmental radioactivity. this was clearly evidenced by specific measurements carried out in the southern tyrrhenian sea and, [chiozzi et al., 2001; 2003] in terms of the radioactive elements content the erupted lavas of mt. etna are instead different from the other tyrrhenian volcanoes, with the exception of stromboli island [capaldi et al., 1976], indicating that the magmas erupted by the two volcanoes share similar pre-eruption histories. on mt. etna, the area that we investigated in this paper showed a rather uniform chemical composition of the outcropping volcanic products. as expected, the lavas found there were mostly alkali-basalts and hawaiites [le maitre, 1989]. the radioactive content in the mt. etna rocks has been little investigated so far, with only a few studies concerning u/th contents [capaldi et al., 1976; condomines et al., 1982, 1987, 1995; villemant et al., 1993]. in a previous work [kozłowska et al., 2016], we determined γ background levels coming from radium isotopes in the rocks surrounding some water intakes. in this paper, we present the results of α– and γ–spectrometry measurements in the rock samples collected near water samplings sites. we focused on the activity concentrations of uranium and thorium and their daughter radium isotopes. in order to carry out our measurements, we applied two different nuclear spectrometry techniques. 2. sampling sites and measurement techniques our investigation consisted in collecting samples of volcanic rocks from the eastern flank of mt. etna, over a surface of approximately 120 km2. the geology of this area is composed exclusively of etna’s volcanic rocks, essentially represented by lava flows belonging to the so-called stratovolcano supersynthem [57 ka to present, according to branca et al., 2011]. nevertheless, some minor volcanic outcrops belong to the timpe supersynthem, which began to develop some 220 ka ago [branca et al., 2011]. it should be noted that all our collected samples of volcanic rocks belong to the former. eight rock samples were collected and analyses of radioactivity were carried out. fig. 1 presents the area under investigation and the sampling sites. the first five samples were collected in proximity of the water intakes from which water samples were taken for a radioactivity performed in earlier studies [kozłowska et al., 2016]. the samples are representative of the average rock type of the aquifers. rock samples e_2, e_7 and in particular e_11 were collected from some of the oldest lavas outcropping in the eastern flank of the volcano [86 ka to 3.9 ka, branca et al., 2011]. the collected rock samples were transferred to the laboratory, where they were characterized, then dried, crushed and reduced to fractions smaller than 0.2 mm for homogenization. from each sample of about 1 kg of dry mass prepared for γ-spectrometry, two separate sub-samples of 5 g each were additionally selected and sent for α‐spectrometric measurements. γ-spectrometry measurements were carried out in two laboratories, in catania and in katowice, in marinelli geometry, using 0.6 dm3 containers, where the rock samples were sealed for 30 days in order to allow for secular equilibrium in the uranium series. activity concentrations of 226ra, 232th(228ac) and additionally 40k in the collected samples were determined using hpge gamma spectrometers (ortec company) and norm (naturally occurring radioactive materials) standards prepared by the central laboratory for radiological protection from warsaw, poland, with known activities of the same isotopes. the details concerning the data acquisition programs and the choice of spectral lines for 226,228ra and 40k analyses are given by [kozłowska et al., 2016]. additionally, using γ-spectrometry technique, the 234th isotope content was determined based on the low energy 63.41 kev spectral line. efficiency calibrations and analyses of spectra were performed based on the reference material iaeargu-1 [iaea, 2002]. this material was prepared by means of dilution of a uranium ore (7.09% u) and a thorium ore (2.89% th, 219 μg u/g) with floated silica powder of similar grain size distribution, respectively. uranium, 226ra and 210pb standard is in radioactive equilibrium. measurements of the samples lasted from two to nine days due to low thorium 234th gamma line absolute emission probability (3.75%). the measurement time was strictly dependent on the activity of each sample. the determination of 234,238u and 232,230,228th was performed with the use of α – spectrometer 7401vr (canberra – packard, usa). the spectrometer was equipped with the passivated implanted planar silicon (pips) detector with a surface area of 300 mm2. for uranium and thorium analyses, a standard of known 232u and 229th activity was added to 5 g samples selected at 3 kozłowska et al. 4 the beginning of each analysis. the wet mineralization of the samples was performed with the use of hot acids: hf, hno3, hcl with h3bo3. uranium and thorium were pre-concentrated with iron and co‐precipitated with ammonia at ph 9. in order to complete the uranium de‐ termination, the precipitate was dissolved in 8m hcl and then transferred to the anion exchange resin dowex 1×8 (cltype, 200-400 mesh) conditioned with 8m hcl. the separation of uranium from other α – radionuclides which could be present in the investigated samples was performed based on a procedure developed by soumela (1993). uranium was eluted with 0.5 m hcl. in order to determine the thorium content, the precipitate was dissolved in 8m hno3 and then transferred to the anion exchange resin dowex 1×8 (cltype, 200-400 mesh) conditioned with the same acid. the thorium fraction was eluted with 10m hcl. thin α sources were prepared from uranium and thorium fractions by coprecipitation with ndf3 (sill, 1987). the sources were measured over a period of 1 – 2 days. minimum detectable activity (mda) was equal to 0.5 mbq l-1 for all uranium and thorium isotopes. the chemical recoveries varied from 50% to 80%. the method for uranium determination was tested during an inter-calibration experiment organized by iaea in 2010. the applied methods for uranium and thorium determination in soil/rock samples were tested using iaea-434 and iaea-375 reference materials (iaea, 2002). it can be noted that the isotopes of interest are the major source of terrestrial norm. starting from uranium 238u decay chain, measurements of 214bi, 214pb involve γ analysis and provide information on the 226ra content, taking account of the fact that radioactive equilibrium is established in this part of the chain after about one month. the parent nuclide 238u cannot be determined with this type of spectrometry, although it is a common practice to assume radioactive equilibrium in the chain in order to assess its content. however, some authors acknowledge that significant disequilibrium in the 238u series is common in rocks younger than 1 million years and results mainly from the relative mobility of 238u itself and its decay products 234u, 226ra, and 222rn [chiozzi, 2001]. performing measurements using separate α-semi-conductor spectrometry of the solid samples may produce precise information on these α-radioactive elements. on the other hand, thorium is an element scarcely soluble in water, thus the 232th series may be with high probability considered in radioactive equilibrium in most geological environments. its daughter 228ra can be evaluated using γ spectrometry by measuring, for instance, the lines of 228ac or 212pb in γ–spectrum. 3. results and discussion 3.1 radioactivity content in volcanic rocks the results of the analyses of the volcanic rocks collected in the present survey for the detection of radionuclides are shown in table 1. the symbols in parenthesis, (α) or (γ), indicate the type of nuclear spectrometry technique adopted to obtain the results. the values of the activity concentrations of all measured radionuclides, except 40k, were in a range from 24±4 bq kg-1 (for 238u) to 91±9 bq kg-1 (for 232th). none of them exceeded 100 bq kg-1. conversely, 40k values ranged from 331±15 bq kg-1 to 725±35 bq kg-1. the results in table 1 are presented in order of appearance of elements in the two radioactive decay chains (i.e. thorium and uranium-radium), respectively. the first three columns present the results for the thorium chain 232th – 228ra(228ac) – 228th, whereas columns 4 – 8 show the results for the uranium-radium chain 238u – 234th – 234u – 230th – 226ra. the specific activity of 232th ranges from 37±6 to 91±9 bq kg-1, while 228ra ranges from 26±2 to 75±2 bq kg-1 and 228th from 37±5 to 85±4 bq kg-1. the results show lower radioactivity than that in the aelian islands rocks investigated by brai et al. [2002], by chiozzi et al. [2001; 2003] and brai [1995]. the concentrations of nuclides of the 238u decay series range from 24±4 to 66±8 bq kg-1 (238u), 20±15 to 51±11 bq kg-1 (234th), 26±4 to 63±8 bq kg-1 (234u), 24±6 to 83±4 bq kg-1 (230th) and 45±2 to 85±4 bq kg-1 (226ra). higher uncertainties in the 234th results obtained for some of the samples were due not only to the small masses of samples available for low-energy γ-spectrometric measurements, but also to the low activity concentrations of the investigated thorium isotope. both uranium and other thorium isotopes were analyzed with the use of α-spectrometry, since in this case 5 g samples were enough to obtain low uncertainty levels even for low-activity samples. radium isotopes analyses were carried out in a previous investigation [kozłowska et al., 2016]. the study revealed that the activity concentrations of 226ra and 228ra (228ac) found in the rocks of the eastern flank of mt. etna were of the same order of magnitude as the european average values found in bedrock, i.e. between 10 and 50 bq kg-1 [iarc, 1988]. moreover, all activity concentrations obtained for the studied isotopes from the uranium-radium chain also point to lower concentrations alkali-basaltic rocks. table 2 presents activity concentration ratios calculated for selected isotopes. in the first column, the 232th/238u ratios of the protoplasts of two radioactive 5 radioactivity of mt. etna rocks series are presented. the obtained values were in the range from 1.1±0.2 (e_2 sample) up to 2.5±0.3 (e_4 sample), with a mean value of 1.5 and median of 1.2. in normal radioactivity magmatic rocks, the th/u ratio varies in a range between 2.5 and 4.5 [plewa and plewa, 1992]. like th/u, the obtained variability of 232th/238u ratios indicate low radioactivity rocks. invariability of th/u ratio is usually related to the similarity of the characteristics of both elements in magmatic environments, i.e. under conditions of high pressure, high temperature and absence of oxygen. under these conditions, both elements are in iv-valence state, have similar ion radius and may replace each other in minerals. conversely, under other environmental conditions, the physical-chemical properties of thorium and uranium are quite different. furthermore, given that the analyzed samples are mainly from rocks that act as groundwater reservoir, the obtained 232th/238u ratios may also indicate that uranium was leached by water, so the equilibrium conditions would be unsettled in the magmatic rocks. however, since different isotopes of both elements are present in the uranium-radium decay series, one may carefully assume that the isotopes in this chain may exist in a disequilibrium state. the results in the 232th decay series are relatively easy to interpret. the values of the activity ratio 228th/232th, ranging from 0.9±0.1 to 1.4±0.5 denote secular equilibrium established between the isotopes. thorium in the lava erupted from the volcano stays in sample code sample characteristics 232th (α) [bq kg-1] 228ra (γ) [bq kg-1] 228th (α) [bq kg-1] 238u (α) [bq kg-1] 234th (γ) [bq kg-1] 234u (α) [bq kg-1] 230th (α) [bq kg-1] 226ra (γ) [bq kg-1] 40k (γ) [bq kg-1] 1 2 3 4 5 6 7 8 9 e_2 rocks from the vicinity of valle s. giacomo intake (age = 86 ka) 75±5 74±2 77±3 66±8 41±5 63±8 69±2 62±3 617±20 e_4 lava and soil close to primoti well (700 ad) 91±9 75±2 85±4 37±4 51±11 36±4 83±4 76±4 716±20 e_5 rocks from the vicinity of ilice well (1634 ad) 49±8 41±2 54±5 26±4 27±5 27±4 48±5 57±4 360±20 e_7 rocks from the vicinity of fornazzo well (age = 15 ka-3.9 ka) 37±6 45±2 37±4 27±4 24±4 26±4 36±4 61±4 445±20 e_9 rocks from the vicinity of guardia well (age = 3.9 ka-2ka) 48±5 68±3 61±3 35±5 28±4 36±6 53±3 85±4 725±35 e_10 lava from la montagnola crater (1763 ad) 41±13 29±1 58±10 25±3 25±5 27±4 24±6 45±2 331±15 e_11 lava from piano del lago (age = 15 ka-3.9 ka) 48±5 48±2 61±3 40±6 28±5 41±6 52±3 46±3 421±15 e_12 1792 ad lava flow 41±8 26±2 37±5 24±4 20±15 27±4 46±5 62±4 558±20 table 1. activity concentrations, in [bq/kg], of radioisotopes representing the beginning of two radioactive decay chains: thorium 232th – 228ra(228ac) – 228th and uranium−radium 238u − 234th − 234u − 230th − 226ra and 40k in the investigated rock/soil samples. the type of spectrometry used is presented in parenthesis. the nomenclature of sampling sites is consistent with that of kozłowska et al. [2016]. the age of the samples is also indicated next to the sample name. the rock and is not leached by water flowing through the ground or on the surface. any 228ra isotope present in this chain does not change the equilibrium state. these results confirm the presence of equilibrium in the thorium series in magmatic rocks noted also by other authors [condomines et al., 1995]. the 238u decay series is, instead, more difficult to interpret. in most cases, the order of abundance of radionuclides is as follows: 226ra>230th>234u=234th=238u. the values of the isotopic ratio 226ra/238u (table 2, column 4) were in the range from 0.9±0.1 (e_2 sample) to 2.6±0.4 (e_12), with mean of 1.93 and median equal to 2.1. considering the isotopes of the radioactive chain one by one, we observed that the 226ra content was slightly higher than that of 230th (mean value of 1.3), and that 230th was on average 1.5 times higher than 234u and 238u. both uranium isotopes were in a state of equilibrium, with isotopic ratio equal to 1 for all samples. moreover, the 230th content was on average 1.7 (median of 1.9) times higher than that of the 234th isotope present in between two uranium isotopes (table 2). based both on previous studies [kozłowska et al., 2016] and the results of the present investigation, it can be concluded that the magmatic rocks of mt. etna have favorable conditions for uranium leaching by water, whereas thorium and radium act as immobile elements, and thus remain in the host rocks of the aquifer. this is in part supported by the general low radium activity in the ground water of mt. etna observed in the previous studies [d’alessandro and vita, 2003; kozłowska et al., 2009]. regrettably, this hypothesis is difficult to test given the fact that mda for 226ra in water determined with the use of liquid scintillation technique in our previous studies was as high as 10 mbq l-1 (or, for smaller sample volumes, even 40 mbq l-1), whereas for 234,238u (analyzed with alpha spectrometry in the present studies) mda was equal to 0.5 mbq l-1. however, one previously analyzed ground water sample from site e_6 seems to support this hypothesis. the radium content in this water sample was <10 mbq l-1 [kozłowska et al. 2016], whereas that of 238u was equal to 70  mbq l-1 (234u = 78  mbq l-1), which may prove the main hypothesis. this assumption is supported by the fact that the 232th/238u ratio in our samples was on average equal to 1.5 (table 2), and in all samples it was higher than the pristine ratio in the magmatic rocks at their formation (which is equal to 1, due to the similarity in the radius of th and u atoms). by considering that the magmatic formation of those elements precede eruption processes, the deficit of uranium in the erupted rocks is likely caused by its removal from the rocks due kozłowska et al. 6 sample code 232th (α)/238u (α) 228th (α)/232th (α) 226ra (γ)/238u (α) 234u (α)/238u (α) 230th (α)/234th (γ) 226ra (γ)/230th (γ) 2 3 4 5 6 7 e_2 1.1±0.2 1.0±0.1 0.9±0.1 1.0±0.2 1.7±0.2 0.9±0.1 e_4 2.5±0.4 0.9±0.1 2.1±0.3 1.0±0.2 1.6±0.4 0.9±0.1 e_5 1.9±0.4 1.1±0.2 2.2±0.3 1.0±0.2 1.8±0.4 1.2±0.2 e_7 1.4±0.3 1.0±0.2 2.3±0.4 1.0±0.2 1.5±0.3 1.7±0.2 e_9 1.4±0.3 1.3±0.2 2.4±0.4 1.0±0.2 1.9±0.3 1.6±0.1 e_10 1.6±0.6 1.4±0.5 1.8±0.3 1.1±0.2 1.0±0.3 1.9±0.5 e_11 1.2±0.2 1.3±0.2 1.2±0.2 1.1±0.2 1.9±0.4 0.9±0.1 e_12 1.7±0.4 0.9±0.2 2.6±0.4 1.1±0.2 2.3±1.7 1.3±0.2 table 2. activity ratios in the investigated rock samples (the nomenclature of sampling sites is consistent with that of kozłowska et al., [2016]. to its leaching by water. the low radium activity in the groundwater of mt. etna was explained by the low affinity of this element for the aqueous environment, as radium tends instead to remain in the host rocks of the aquifers [kozłowska et al., 2016]. these investigations show that the isotopic ratios in the considered radioactive decay chains are often different than 1, and hence the equilibrium state cannot be assumed in our samples from mt. etna. the samples that are several hundred thousand years old show radioactive isotopes compositions that cannot be interpreted without taking into account environmental factors. therefore, the assumption of isotopic equilibrium for old lavas does not hold true and the calculation of the pristine uranium or thorium contents via γ spectrometry based on the radionuclides at the end of the decay chains (i.e., bismuth, polonium or lead isotopes) is not correct. isotopic fractionation could be investigated in young lava samples, which would clearly cause experimental difficulties. 3.2 radionuclides transfer from rocks to groundwater the assessment of radionuclides transfer from rocks to ground water was performed based on the present investigation and on the results of groundwater analyses presented in previous studies. radioactivity of groundwater is mainly influenced by the physical and chemical characteristics of the reservoir rocks. a higher radioactivity level of water 7 radioactivity of mt. etna rocks figure 3. correlation between 238,234u tf values and a,b) tds, c,d) pco2, e,f) ph in the sampled waters of mt. etna. the best-fit lines for each pair of parameters and the respective values of pearson correlation coefficients (r) are also shown. may reflect a higher abundance of radioactive minerals. a transfer factor (tf) can be introduced as a parameter that quantitatively describes radionuclides penetration into the environment, like, for example, from soil to plants, or from soil – through plants – to animals. a large research literature exists on the transfer of radionuclides between different elements of an ecosystem. most often, nonetheless, it refers to radionuclides transfer from soil to plants, mainly regarding 137cs and 90sr, or sometimes transuranium elements [wang et al.l 1996, koehler et al., 2000, uchida et al., 2000, mietelski 2003, baeza & guillen, 2006]. the penetration of cesium and strontium to plants was discussed in detail in a publication of the international atomic energy agency [iaea, 2001]. the quoted tfs obtained for cultivated plants by different laboratories worldwide vary in the range from 10-1 to 10-4, although larger values (up to tf = 22) were also obtained on some occasions [iaea, 2001]. moreover, tf may also be defined as the ratio between the activity concentration in dry mass of crop and the activity concentration of the corresponding dry soil collected from its top (10-20 cm) layer, both activities being likewise expressed in [bq kg-1]. in the studied case, tf measures the penetration of a certain radionuclide from aquifer rocks to ground water. thus, tf may be defined as the ratio between the specific activity of a given isotope in water, awat, and its activity in the reservoir rocks, arock.. tf is dimensionless, as both the activity concentrations are expressed in [bq kg-1]. awat can be expressed as the activity per kg of total dissolved solids (tds), i.e. the activity of the solid mass of material dissolved in one liter of water. so far, data of tfs from aquifer rocks to groundwater were never published. this is presumably due to the fact that in this case tf is not so easy to determine as in the case of soil-to-plant nuclide transfer. the results of the rock-to-water tf values for 238,234u uranium in the investigated area are presented in table 3. activity concentrations of water samples expressed in [bq l-1] were converted into [bq kg-1] of tds dissolved in 1l of water. the values of tfs are from 0.03±0.01 to 0.75±0.10 for 238u and from 0.05±0.02 to 0.79±0.19 for 234u. all tf values obtained were in the order of 10-1 to 10-2, which implies either strong binding of the radioactive elements to the rock minerals, or alternatively, that water is not actively dissolving the material. it can be noted that the reservoir rocks are all volcanic with relatively homogenous elemental content. since the previous studies emphasized that the type of soil is one of the dominant factors influencing soil-to-plant radionuclide movement [twining, 2004], one can apply that conclusion to water as well. evidently, etna's volcanic rocks provide the soil environment with radioactive elements which seem relatively unaffected by leaching from local ground water. table 3 also presents activity concentrations of 238,234u in five rock types surrounding five water intakes, i.e. e_2, e_4, e_5, e_7, e_9. it can be observed that the results of the analyses of our rock samples present radioactive levels in a similar order of magnitude, implying that the types of volcanic rocks in the water reservoirs are not very diverse in their radioactive content. the same observation also applies to water samples whose analytical results are close to each other. the mean 238u activity concentration of five rocks is 38.2±7.3 bq kg-1 while for five water samples it is 13.7±4.3 bq kg-1, with a mean tf value of 0.36±0.13. the average tf value calculated from the tf values of each sample is 0.42±0.13. concerning 234u, the mean activity concentration is 37.6±6.7 bq kg-1, whereas in water samples it is 14.9±4.0 bq kg-1; the mean tf is, therefore, equal to 0.40±0.13 and the average tf value calculated from the single tf values for each sample was equal to 0.47±0.13. these overlapping values indicate the relatively homogenous elemental content of the reservoir rocks. among the factors that may affect the rock-to-water tf, the most important were found to be tds, partial pressure of dissolved co2 (pco2) and ph values of ground water. the relationship between the obtained tf values in some of the sites in the study area (i.e., only those located in the east flank of mt. etna) and the above chemical factors measured in the ground water collected near the same sites [data from kozłowska et al., 2016] is apparent when computing the pearson correlation coefficient (r) among them. figure 2 shows the correlations that we have found in the studied sites, considering either 238u or 234u for the computation of tf. all correlations were positive, with 0.3>r>0.6. in some cases, such as tf(238u) vs. tds, tf(238u) vs. pco2 and tf(234u) vs. ph, the correlations were much higher (0.5>r>0.6). the correlations between tds or pco2 and tfs show that the lower values of those chemical factors correspond to smaller tfs. in the case of the ph tf correlation, lower ph values, that is more acidic water, correspond to a weaker transfer of uranium isotopes from rock to water. although uranium is generally soluble in water more preferably at low ph values, it was found that in the mt. etna ground water this element can be scavenged, for example, by iron oxyhydroxides [aiuppa et al., 2000]. actually, in the ph range between 5 and 8 the uranyl ion (uo2 2+), which is the most common u(vi) species, is strongly adsorbed by iron minerals such as hematite, goethite and amorphous kozłowska et al. 8 ferric oxyhydroxide [langmuir, 1978; hsi and langmuir, 1985]. this process is promoted particularly under reducing water conditions, which are typical of many aquifers of the eastern flank of the volcano [giammanco et al., 1998; aiuppa et al., 2003, 2004]. 3.3 effective dose activity concentrations of water samples collected from intakes around the whole mt. etna volcano were analyzed in the last ten years [kozłowska et al., 2009; kozłowska et al., 2016]. in summary, results for 20 water intakes, some of them re-sampled several times are available from previous studies. only one, collected near our rock sample e_2, exhibits activity concentration of radium isotopes slightly above mda, but that water is not used for human consumption. the activity of 226ra in this water sample is 12±1 mbq l-1 and that of 228ra is 22±9 mbq l-1. the same sample shows also the lowest 234u and238u isotopes, the lowest values (2.0±0.4 mbq l-1 and 1.6±0.3 mbq l-1, respectively). all the other samples showed activity concentrations below mda (10 mbq l-1 for 226ra and 20 mbq l-1 for 228ra). the highest values were obtained at a public water intake on the ne flank of mt. etna (san paolo well): 78.6±5.7 mbq l-1 and 70.1±5.1 mbq l-1 for 234u and 238u, respectively. the average value of 234u, calculated from all the results, is 24.0±5.1 mbq l-1, whereas the median value was equal to 12.9 mbq l-1. for 238u, the results were 23.0±5.0 mbql-1 and 11.0 mbq l-1, respectively. the rather homogenous elemental content of the studied isotopes both in the water samples and reservoir rocks as well as the consequent stable tf values lead to conclude that this level of radioactivity can be expected all around the mt. etna volcano. both the world health organization and the environmental protection agency estimated a consumption of 2 l of water per day per capita [who, 2004], which corresponds to 730 l per year. since the main contribution to the effective radioactive dose comes from uranium isotopes and the mean activity concentration of 234u and 238u in the mt. etna groundwater is 24.0±5.1 mbq l-1 and 23.0±5.0 mbq l-1, respectively, we can infer the annual total effective dose. if we use dose conversion factors of 4.9·10-8 sv bq-1 and of 4.5·10-8 sv bq-1 for 234u and 238u, respectively (who, 2004], the effective doses from 234u and 238u are equal to 0.86 and 0.76 μsv/y, respectively, that is well below the limit prescribed by who guidelines (who, 2017) and by the italian legislation (decreto legislativo del governo del 17 marzo 1995, n. 230: https://www.gazzettaufficiale.it/atto/serie_generale/caricadettaglioatto/originario?atto.datapubblicazionegazzetta=1995-06-13&att o.codiceredazionale=095g0234&elenco30giorni=false). 9 radioactivity of mt. etna rocks rock sample water intake tds* [mg/l] pco2* ph* awat [mbq l-1] awat [bq kg-1] arock [bq kg-1] tf awat [mbq l-1] awat [bq kg-1] arock [bq kg-1] tf 238u* 238u* 238u 238u 234u* 234u* 234u 234u e_2 s. giacomo intake 668 0.026 6.7 1.5±0.5 2.3±0.7 66±8 0.03±0.01 2.0±0.6 3.0±0.9 63±8 0.05±0.02 e_4 primoti well 2340 0.633 6.67 64.5±5.1 27.6±2.2 37±4 0.75±0.10 61.4±4.9 26.2±2.1 36±4 0.73±0.10 e_5 ilice well 357 0.1244 6.55 2.8±0.6 7.8±1.7 26±4 0.30±0.08 4.1±0.8 11.5±2.2 27±4 0.43±0.10 e_7 fornazzo well 391 0.01 7.61 6.9±1.3 17.7±3.3 27±4 0.66±0.16 8.0±1.5 20.5±3.8 26±4 0.79±0.19 e_9 guardia well 756 0.156 6.4 9.8±0.8 13.0±1.1 35±5 0.37±0.06 10.0±0.8 13.2±1.1 36±6 0.37±0.07 *data from [kozłowska et al., 2016] table 3. rock−to−water tf values for 238u and 234u uranium isotopes in the eastern flank of mt. etna. the nomenclature of the sampling sites is consistent with that of kozłowska et al. [2016]. even considering the worst possible case the san paolo well with values of 78.6 mbq l-1 and 70.1 mbq l-1 for 234,238u, respectively, the corresponding calculated dose is equal to 2.8 and 2.3 μsv/y, respectively. 4. conclusions radionuclides from two radioactive decay series were studied in some selected volcanic rocks of mt. etna. the activity ratios among radionuclides in the 232th decay series denoted the attainment of secular equilibrium, whereas in the 238u chain, we observed the following order of abundance: 226ra>230th>234u=234th=238u, thus pointing to isotopic disequilibrium. long – lived isotopes of uranium and thorium decay series are mainly responsible for natural radioactivity content. we also measured 40k activity in the rocks of mt. etna and the resulting values are on the same levels as the average european values found in bedrock. the transfer factor, describing the radionuclides transfer from rocks to ground water, ranges from 0.03±0.01 to 0.75±0.10 for 238u, from 0.05±0.02 to 0.79±0.19 for 234u. as a whole, values varied between 10-1 to 10-2, implying either strong binding of the radioactive elements to the rock minerals or that ground water is not efficiently leaching out the investigated elements from the volcanic rocks of the local aquifers. mean tf values prove the assumption that the reservoir rocks are all volcanic with the relatively homogenous elemental content. finally, the effective radiation doses due to radionuclides ingestion with drinking water were significantly below the limit of 100 μsv/year set by who [2017] and the limit of 1 msv/year set by the italian legislation. samples preparations and γ spectrometry analyses were carried out at the laboratories of the department of physics and astronomy of the university of catania (italy), whereas α-spectrometry analyses were carried out at the department of nuclear physics and its applications of the institute of physics at the university of silesia in katowice (poland). references acocella v., m. neri (2003). what makes flank eruptions? 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drinking water quality, world health organization, switzerland. who (2017). guidelines for drinking water quality, world health organization, switzerland. *corresponding author: salvatore giammanco, istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy; e-mail: salvatore.giammanco@ingv.it © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved kozłowska et al. 12 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 56, 6, 2013, s0670; doi:10.4401/ag-6226 s0670 late quaternary paleoseismic sedimentary archive from deep central gulf of corinth: time distribution of inferred earthquake-induced layers corina campos1,2,*, christian beck2, christian crouzet2, eduardo carrillo3, aurélien van welden4,2, efthymios tripsanas5 1 universidad simón bolívar, departamento de ciencias de la tierra, sartenejas, baruta, venezuela 2 grenoble-alpes university, laboratoire isterre, umr cnrs 5275, cism, le bourget du lac, france 3 universidad central de venezuela, instituto de ciencias de la tierra, caracas, venezuela 4 fugro survey as, skøyen, oslo, norway 5 institute of oceanography, hellenic centre for marine research (hcmr), anavyssos, greece abstract a sedimentary archive corresponding to the last 17 cal kyr bp has been studied by means of a giant piston core retrieved on board r/v marion-dufresne in the north central gulf of corinth. based on previous methodological improvements, grain-size distribution and magnetic susceptibility anisotropy (msa) have been analysed in order to detect earthquake-induced deposits. we indentified 36 specific layers -homogenites+turbidites (hmtu) intercalated within continuous hemipelagictype sediments (biogenic or bio-induced fraction and fine-grained siliciclastic fraction). the whole succession is divided into a non-marine lower half and a marine upper half. the “events” are distributed through the entire core and they are composed of two terms: a coarse-grained lower term and an upper homogeneous fine-grained term, sharply separated. their average time recurrence interval could be estimated for the entire md01-2477 core. the non-marine and the marine sections yielded close estimated values for event recurrence times of around 400 yrs to 500 yrs. 1. introduction following different studies achieved in tectonically active areas, lake and restricted marine basins have demonstrated their potential for the sedimentary recording of seismic shocks [e.g. hempton and dewey 1983, siegenthaler et al. 1987, van loon et al. 1995, syvitski and schafer 1996, mörner 1996, chapron et al. 1999, shiki et al. 2000, de batist et al. 2002]. two major mechanisms may account for this recording: i) in situ disturbances (micro-fracturing, micro-folding; liquefaction, injection; [e.g. rodriguez-pascua et al. 2000, marco and agnon 1995, levi et al. 2006], ii) slope failures and gravity redeposition, associated, or not, to tsunamis or seiche effects. our investigations concern the second case. mass wastings (triggered, or not, by seismic shocks) evolving into gravity flowing (density/turbidity currents) may provide large areas for spreading and produce “classical” turbidites (with the differents terms defined by bouma [1962] or mutti and ricci lucchi [1978]; piper and normark [2009]). conversely, when occurring in more confined (or restricted) basins (lakes or isolated marine basins) re-depositional processes may result into specific complex layers, due to reflections on steep slopes and/or to oscillations of the whole water mass (reflected tsunami, seiche effect). more often, a significant seismo-tectonic activity is responsible for the second type of sedimentary “event”, the reason for which it is particularly searched for paleoseismic purpose and contribution to seismic hazard estimation. the main characteristics of these sedimentary events are: i) a sharp limit at the top of a coarse graded lower part, ii) a structureless highly homogenous finegrained upper part [sturm et al. 1995, siegenthaler et al 1987, chapron et al. 1999, beck et al 2007]. additionally, a mixed term-indicating to-and-fro particle displacements-is often observed in between; it may be directly visible trough x-ray imaging [beck et al. 2007] or be detected with high-resolution grain-size evoluarticle history received november 16, 2012; accepted december 20, 2013. subject classification: homogenite, paleoseismicity, anisotropy of magnetic susceptibility, corinth. special issue: earthquake geology tion (which is proposed for the here-presented examples). the basal layer is coarser-grained and may vary from very thin silty laminae to thick sandy/gravelly normal graded layer as the lower term of a turbidite sensubouma [1962] and mutti & ricci lucchi [1978]; it may consist of several pulses with an overall grain-size decrease. the upper unit is a fine-grained homogeneous term, named “homogenite” (hm) in reference to the concept of kastens and cita [1981]. after the discovery of the mediterranean tsunami-induced finegrained “homogenite” [kastens and cita 1981, cita et al. 1996], similar homogeneous layers have been reported in lakes and closed marines basins and have been associated to subaqueous earthquake-effects [sturm et al. 1995, chapron et al. 1999, beck et al. 2007, bertrand et al. 2008, carrillo et al. 2008, çağatay et al. 2012]. this interpretation (especially the importance of a long-lasting suspension settling under oscillatory conditions at the beginning) has been confirmed by recent observations made immediately after major earthquakes in the cariaco trough [thunell et al. 1999] and north-west haiti [mchugh et al. 2011]. they indicate the presence of a long-lasting “cloud” of fine particles that remained in suspension for several weeks, and which is responsible for the deposition of homogeneous layers. thus, in this work we will focus on the occurrences of such associations homogenite+turbidite (named here hmtu) and their time-distribution along a long core from the gulf of corinth [moretti et al. 2004, lykousis et al. 2007]. 2. geological setting the aegean region is one of the most active extensional continental regions around the world [mckenzie1972, 1978, le pichon et al. 1981]. it is moving up to 30 mm/yr to the southwest with respect to eurasia and progressively migrates to the south [mcclusky et al. 2000, jolivet 2001]. within this framework, the gulf of corinth, a semi-enclosed marine basin located in central greece (figure 1), represents one of the most recent extensional features in the area. it is an active half-graben and is bounded by e-w striking, en echelon faults located onshore and offshore [brooks and ferentinos 1984, armijo et al. 1996, sakellariou et al. 2001]. the focal mechanism solutions indicate a main direction of extension n-s [mckenzie 1978, jackson et al. 1982, bernard et al. 2006]. geodetic data show that the active extension is focused offshore, increasing from 11 mm/yr in the central part of the rift (near xylocastro) to 16 mm/yr in its western part (near aigion) [avallone et al. 2004]. the gulf has a maximum uplift rate of 1.3 mm/yr in the southemargin [armijo et al. 1996]. it is 115 km long, ∼30 km wide, with a 900 m maximum depth (figure 1). it is characterized by steep north and south dipping slopes (8°-25°) [stefatos et al. 2002]. to the west, the gulf is separated from the open marine ionian sea by the rion-antirion strait (2 km wide campos et al. 2 itea rion antirion megara corinth karparelli saro nico s gu lf xylocastro xylocastro f. corinth f. md01-2477 w-ant f. akrata aigion gulf of corinth 200 400 800 600 faults major towns 200 water depth (m) core location 0 m 2500 m eurasian plate north anatolian fault 25 mm/yr anatolian plate a ra bi an p la te aegean sea african plate d ea d s ea f au lt hellenic trench a pulian p late 30 mm/yr 10 mm/yr mediterranean ridge 20 30 40 35 22 00 22 30 23 00 22 00 22 30 23 00 3 20 km figure 1. simplified geodynamic setting of the gulf of corinth with the location of the md01-2477 core. the locations of major faults and the bathymetry are from moretti et al. [2003] (200 m isobaths intervals); onshore relief is an extract from nasa/srtm. motion vectors of the north anatolian fault after le pichon et al. [1995], and of the aegean and african plates relative to eurasia after mcclusky et al. [2000]. 3 and 65 m deep). to the east, it is linked with the saronikos gulf and the western aegean sea through an artificiallydredged channel (the corinth canal), which is 6.3 km long, 21 m wide and 8 m deep. 3. methodology the paleoseismic record in the gulf of corinth was studied using the md01-2477 long piston core, recovered during a r/v marion-dufresnes cruise as part of geosciences-ii program, carried out in october 2001. this calypso piston core was taken in the central part of the gulf of corinth at 38°13.28’n, 22°33.53’e and 867 m water depth. the core is 20.08 m long (figure 2), and it is supposed to represent the last 25.3 cal. kyr bp of sedimentation [moretti et al. 2004, lykousis et al. 2007]. shipboard processing included geotek core logger profiles and splitting; further laboratory samplings and measurements were done on half cores horizontally stored at 4°c. to characterize and discriminate homogenites from other fine-grained deposits, analysis of the textural parameters is the most common tool [chapron et al. 1999, beck et al. 2007, carrillo et al. 2008, bertrand et al. 2008], especially those depending on particles arrays. the combination of grain size, undrained shear stress, and magnetic fabric has been used to get a wellconstrained characterization of the homogenites and their temporal distribution [chapron et al. 1999, carrillo et al. 2008, bertrand et al. 2008, beck 2009]. recent works show the importance of the application of the magnetic susceptibility anisotropy (msa) in the study and identification of post-depositional disturbances related to earthquake triggering [levi et al. 2006, mörner and sun 2008]. when previously settled deposits include hemipelagites, the fine-grained resuspended fraction (incipient homogenite) may be very similar (grain-size and composition) to these hemipelagites. msa appears to be a useful tool to establish this distinction [campos et al. 2011, campos et al. 2013], and, thus, precisely measure the thickness of a hemipelagic interval. the latter represents the time elapsed between two successive reworking events; its duration may thus be considered as an earthquake recurrence time interval [adams 1990, gorsline et al. 2000, goldfinger et al. 2003, huh 2004]. the here-used age/depth curve is based on this assumption: sedimentation rates estimated from hemipelagic thicknesses, zero-time applied to homogenites+turbidites (hmtu) “events” [e.g. goldfinger 2009, mccalpin 2009 and references therein, beck et al. 2012] after visual identification of specific sedimentary “events” (hmtu), a detailed textural characterization was applied to selected ones. two of them (blue rectangle on figure 2) are presented and discussed here: i) the thickest one, which permitted a high resolution analysis with respect to the thickness, ii) a thinner one more representative of the whole “events”. 3.1 grain size and shape analysis the grain size-analyses were conducted using a laser diffraction microgranulometer malverntm mastersizer 2000, which has a 0.02 to 2000 �μm range. measurements were performed in the isterre laboratory, savoie university, france, with sampling intervals varying from 0.5 to 2 cm. the distribution parameters have been calculated following folk and ward [1957]: skewness index (sk), sorting index (so), kurtosis. sk and so were plotted on simple binary diagrams. percentiles 99 (q99=c) and medians (q50=m) were plotted on the cm passega [1964] diagram. both diagrams were used, as proposed in beck [2009], to display and analyse bottom-to-top paths. shape analyses were performed on silt-clay fraction using a sysmex fpia-2100 size and shape particle image analyzer, with a sampling interval of ∼ 50 cm. 3.2 magnetic properties high-resolution bulk magnetic susceptibility data (figure 2) was collected using a bartingtontm ms2 contact sensor every 5 mm. the magnetic susceptibility anisotropy (msa) samples were collected into 8 cm3 non-magnetic plastic boxes and a total of 263 samples were analysed. the measurements were carried out in the cerege magnetism laboratory (aix marseille university, france) using the mfk1-fa kappabridge of agico (spinning specimen method). for each sample, the system reconstructs the ams tensor ellipsoid, defined by three eigenvectors (kmax, kint and kmin) [hrouda 1982, tarling and hrouda 1993]. this susceptibility ellipsoid represents the combined result of the susceptibility anisotropy produced by individual grain shape and/or crystallography in a sample [joseph et al. 1998]. the shape of the msa ellipsoid is illustrated by several parameters defined by jelinek [1981], such as l = kmax/kint and f = kint/kmin. only the major events (6 to 74 cm thick) were sampled for textural (every 5 mm) and msa (every 20 mm) studies; two of them were selected (a and b intervals, position on figure 2) as representative of the whole set. figure 3 and 4, respectively, present the results for a 74 cm thick event (30 cm thick basal tu term and 44 cm thick upper hm term) and a 17 cmthick event (8 cm thick basal tu term and 9 cm thick upper hm term). for the first event, a total of 185 measurements were performed (148 for grain-size dispaleoseismic record of gulf of corinth tribution and 37 for msa parameters). 3.3 chronology two sets of accelerator mass spectroscopy 14c measurements are used: 1) published preliminary dating performed by moretti et al. [2004] and lykousis et al. [2007] in beta analytic laboratory (usa). 2) a new set of radiocarbon ages obtained at poznan radiocarbon laboratory (poland). the measures were performed on plant debris and wood fragments from coarse to fine turbidites (7 measures, table 1) and particulate organic matter collected from organic rich muds (1 measure). radiocarbon ages were calibrated using oxcal-v4 software [bronk ramsey 2001]. as the samples were taken in subaquatic environments (marines and non-marine), we use the marine09 calicampos et al. 4 h m tu lower term tu fine sand to coarse silt mixed term with to-and-fro currents evidences upper term hm homogenite hemipelagic normal deposits 25 c m 1030 cm 1005 cm core depth a 0,1 1 10 100 1000 ms (x10-5 s.i.) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 d ep th ( cm ) md01-2477 263 cm = 3064 2734 cal yr bp 390 cm = 3280 2913 cal yr bp 516 cm = 6052 5706 cal yr bp 1107 cm = 9239 8779 cal yr bp 1633 cm = 14038 13618 cal yr bp 1766 cm = 16928 16218 cal yr bp non-marine to marine transition mud homogeneous turbidite cal. ams 14c age legend hmtu: homogenite (hm)+(tu) turbidite laminate mud h m tu lower term tu fine sand to coarse silt mixed term with to-and-fro currents evidences upper term hm homogenite 1116 cm 1050 cm core depth 66 c m ba b 2008 cm = 25005 24300 cal yr bp ? 650 cm = 13990 13493 cal yr bp figure 2. simplified log of core md01-2477 displaying the distribution of the two types of sedimentary events. t: classical turbidites (grey); ht: association “homogenite + turbidite” (blue). the magnetic susceptibility (ms) profile underlines the difference between marine and nonmarine successions. rectangles refer to the two analyzed layers and their close up (on figure 3 and 4). 5 bration curve [reimer et al. 2009], and the local marine reservoir correction (δr) proposed by reimer and mccormac [2002] for the aegean sea. they proposed a δr =35±50 yr for nafplio, greece (at 70 km from the gulf of corinth) and δr =143±41 yr for piraeus, greece (at 30 km from the gulf of corinth). to use a single value of δr we calculate the average value of both δr with their average errors: δr= 89±58. we applied this δr to the whole sedimentary sequence (marine and non-marine conditions; holocene and late pleistocene). this assumption is probably simplistic, but the lack of data does not allow us to make more precise corrections. results are summarized in table 1 and represented in figure 2. they correspond to 95% of confidence (2σ). 4. results 4.1 sedimentological observations and sedimentary events recognized in the md01-2477 core the md01-2477 sediment infill is holocene-late pleistocene and it represents both marine and non-marine environments. the marine non-marine transition is considered to have occurred between 12 kyr bp [perissoratis et al. 2000, collier et al. 2000] and 13.2 kyr bp [moretti et al. 2004, lykousis et al. 2007]. the studied core is composed of olive grey (5y5/2) to dark yellowish brown (10yr4/2) calcareous muds interlayering with millimetric to decimetric layers of yellowish brown (10yr4/2) sand and silt. in the non-marine section, few centimetres of laminated mud (olive grey, dusky yellow, greyish yellow and brownish black (5y4/1, 5y6/4, 5y8/4, 5yr2/1)) are present. these layers have been interpreted as varves by moretti et al. [2004] and lykousis et al. [2007]. microscopic observations of coarser sediments (binocular) and fine-grained smear slides show sediments principally composed of terrigenous components (carbonates and siliciclastics), being thecarbonates the most abundant. aragonite crystal (abundant in the top of the non-marine section) and traces of micas, glauconite (marine section), pyrite (predominant in the non-marine section) and flakes of organic matter are present. biogenic and bio-induced particles mainly compose the silty/clayey fraction (nannofossil, foraminifers, diatoms and bio-induced micrite). in this core all granulometric fractions have angular to rounded grains with medium sphericity. in the md01-2477 core, two types of deposits linked to density currents and reworking processes have been identified, intercalated within the predominant mud. these depsosits are: 1) turbidites (tu); they are present as 0.5 to 2 cm thick layers, made of silt to very fine sand. they can show sharp (sometimes clearly erosive) bases, slight normal grading typical of classical turbidites defined by bouma [1962], and mutti & ricci lucchi [1978]. otherwise, they locally present inverse gradations; 2) homogenite+turbidite associations (hmtu): they are composed by two main terms (see introducpaleoseismic record of gulf of corinth 0 mean µm 100 200 300 400 500 1120 1110 1100 1090 1080 1070 1060 1050 1040 1030 d ep th ( cm ) 0.8 1.2 1.6 2 2.4 kurtosis 1 1.04 1.08 1.12 foliation -0.6 -0.4 -0.2 0 0.2 skewness 1 1.04 1.08 1.12 lineation hmtu h.d a hmtu tu hm turbidite (tu) homogenite (hm) hmtu.: re-depositional event (homogenite + turbidite)h.d.: hemipelagic deposits 10 100 1000 10000 c =q 99 ( µm ) 2000 500 50 70 1 10 100 1000 m=median (µm) rolling bottom suspension rolling graded suspension pelagic suspension c = m6 1.2 1.6 2 2.4 sorting 2.8 3.2 3.6 1.2 1.6 2 2.4 2.8 3.2 3.6 sorting s ke w ne ss -0.6 -0.4 -0.2 0 0.2 1 2 3 homogenite hemipelagic deposits intermediate term turbidite (1092 1097) turbidite (1097 1102) turbidite (1102 1107) turbidite (1107 1112) turbidite (1112 1116,5) bottom (b) to top (t) hmtu b c b t figure 3. textural characterization of homogenite+turbidite (hmtu) b (1035-1115 cm; location on figure 2). a) main granulometric parameters and msa values; b) sk index vs. so index bottom-to-top path; c) bottom-to-top path on passega [1964]’s diagram. all textural parameters show a sharp break between turbiditic lower part and overlying homogenite. campos et al. 6 tion) with a sharp contact. at this level, oscillation sedimentary structures, that may be direcly eye-visible in between [beck et al. 2007], were detected here through high resolution microgranulometric profiles and msa following the methodology proposed by campos et al. 2013. in the following paragraphs, these textural properties used as criteria to address relationships between sedimentation and seismicity are detailed. 4.2 textural properties of the hmtu events in the md01-2477 core, 36 hmtu events (6 to 74 cm thick) were identified through sedimentological observations. we will focus on the two selected events (see 3.2), detailing first the larger one (event b, figure 3) and then comparing the results with the smaller one’s (event a, figure 4). other events, with intermediate thicknessses, are similar to the here-presented ones, and are not detailed hereafter. for event b, the basal term (tu) has a main grain size which progressively decreases from 500 to 9 μm. the parameters as sorting (2.364± 0.418), skewness (0.369 ± 0.120) and kurtosis (1.321± 0.340) are highly 0 mean µm 50 100 150 200 250 -0.6 -0.4 -0.2 0 skewness 0.8 1 1.2 1.4 kurtosis 1.02 1.04 1.06 foliation 1.02 1.04 1.06 lineation 1 1030 1025 1020 1015 1010 1005 d ep th (c m ) h m tu tu h m 1 1 0 1 0 0 1 0 0 0 m=median (µm) 1 0 1 0 0 1 0 0 0 1 0 0 0 0 c = m c =q 99 ( µm ) h. d.h.d hmtu a turbidite (tu) homogenite (hm)hmtu.: re-depositional event (homogenite + turbidite)h.d.: hemipelagic deposits bottom suspension rolling graded suspension pelagic suspension uniform suspension cb 2 3 4 sorting 0 . 6 0 . 4 0 . 2 0 s ke w ne ss sorting 2 3 41 homogenite hemipelagic deposits turbidite (top) turbidite (medium) turbidite (bottom) turbidite (bottom) turbidite (medium) bottom to top hmtu figure 4. textural characterization of homogenite+turbidite (hmtu) a (1005-1030 cm; location on figure 2). a) main granulometric parameters and msa values; b) sk index vs. so index bottom-to-top path; c) bottom-to-top path on passega [1964]’s diagram. 7 paleoseismic record of gulf of corinth variable. their variability gradually decreases to the top of this unit (figure 3a). contrary to tu, the upper hm term (44 cm thick) have parameters highly constant (mean = 5.486± 0.470 μm, sorting = 2.758 ± 0.987, skewness = -0.150 ± 0.034 and kurtosis = 0.945 ± 0.035). as the hm term, the above hemipelagic deposits show almost constant parameters (mean = 5.860 ± 1.167 μm, sorting = 2.346 ± 0.431, skewness = -0.066 ± 0.043 and kurtosis = 0.964 ± 0.027). the grain-size parameters do not clearly separate the hemipelagic deposits from the upper homogeneous unit; at the opposite, the limit is neat on magnetic foliation profile (see after). the bottom-to-top grain-size distribution displays a three-step evolution both on skewness/sorting and cm (passega’s) diagrams (figure 3-b and 3-c): (1) basal turbidite (dark blue triangles). (2) intermediate term (light blue triangles). (3) homogenite (pink dots). red arrows underline detail evolutions within the three terms. the following has to be underlined: a neat separation between three steps: especially between hm and underlying terms well evidence on both diagrams; a neat separation between tu and intermediate term; complex variations within the tu term the individualisation of the intermediate terms with internal variations corresponding to oscillations; we propose to call it “to-and-fro” interval. an additional remark concerns the hm and the hemipelagites, which are not separated, especially on the cm diagram, and which may indicate similar composition and slow settling from suspension. in the studied section (figure 3a), the msa measurements show a low anisotropy (mostly oblate type ellipsoid), excepted in the homogenite. the magnetic lineation is very low and relatively constant (l= 1.005 ± 0.002). at the difference, magnetic foliation displays significant variations and anomalously high values with respect to expected compaction in the hm term (f= 1.069 ± 0.005) compared to the basal tu term (f= 1.032 ± 0.011). the hemipelagic deposits (f= 1.047 ± 0.009) appear significantly lower than for the hm term, with a neat limit. this difference has been previously underlined by different authors [beck 2009, campos et al. 2013] in marine and lacustrine settings. these authors have shown magnetic foliation in hm up to 1.090, in similar sediments with same age and depth in cores with the same coring device. the second thin-hmtu (event a, figure 4), has been analyzed following the same sampling intervals and measurements as the first thick one. the general bottom-to-top evolution appears similar, although not so complex within the lower term. the intermediate term is present; the fact of not displaying the “to-andtable 1. radiocarbon dating from the md01-2477 core. the sample denoted by the letter “a” was carried out by lykousis et al. [2007]. the 14c ages were calibrated using oxcal software [bronk ramsey 2001], the marine09 calibration curve [reimer et al. 2009]; and applying the local marine reservoir correction (δr) proposed by reimer and mccormac [2002] for aegean sea. samples in italic overestimate the expected age. pdom= plant debris and wood fragments extracted from fine and coarse turbidites; pom= particulate organic matter extracted from organic rich muds. depth in core (m) age age calibrated material in core (m) (yr) error 14c age (2�σ) 2.63 3190 40 3071-2730 pdom 3.9 3355 34 3301-2907 pdom 5.16 5590 40 6082-5701 pdom 6.5 12420 60 13990-13493 pdom 11.07 8500 50 9251-8763 pdom 16.33 12470 60 14040-13614 pdom 17.66 14000 100 16946-15987 pdom 20.08a 21133 110 25005-24300 pom fro” signature may be due either to its actual lack, or to a too low sampling frequency unable to evidence it. the characteristics displayed by the two here-presented hmtu sedimentary “events” may be extended to the 36 hmtu occurring along the md01-2477 core. 5. discussion 5.1 interpretation of the sedimentary hmtu event the analyzed textural parameters show a clear difference between the two main terms which constitute the hmtu deposit. the basal coarse term shows that this unit was deposit by processes of traction and fall-out [passega 1964, lowe 1982, mutti et al. 1999]. this term in the cm diagram (figure 3c, 4c) shows a progressive transformation of the transport mechanism of the sediments. the base is characterized by an intense near-bed transport of coarse particles (coarse sand) forming a traction load, in a density flow [passega 1964, mulder and alexander 2001]. in the middle and upper part of this term, composed mostly by fine sand and silt, the sediments are mainly transported in suspension by turbulent flows [mutti et al. 1999]. furthermore, the high variability in the skewness and sorting (figure 3b) indicate fluctuations in the depositional dynamic. additionally, the presence of two fining-upward sequences within the same basal unit could reflect different pulses of the gravity current, as described by shiki et al. [2000] and nakajima and kanai [2000] in seismo-turbidites from japan. the upper homogeneous term shows a highly constant distribution of the textural parameters (figure 3b, c, figure 4b, c), indicating a stabilization of the energy of the depositional environment, and fallout from a suspended load. three interesting points have to be explained: the sharp separation of the hm term, its thickness and extreme homogeneity, and the intermediate “to-and-fro” term. as previously proposed [chapron et al 1999, beck et al 2007], we explain the latter as fluctuations within a high-density suspension whose relative stability is due to lateral oscillatory displacements of the whole water column; coeval partial settling is inferred. this intermediate episode is considered responsible for an increased “extraction” of the finer-grained fraction, which will be the major component of a long-lasting (almost stable) suspension. based on base-to-top evolutions on the two types of binary diagrams, arnaud et al. [2002] and lignier [2001], in: beck [2009] identified, in a lake infill, two different paths deciphering “flood turbidites” from “slump turbidites”, respectively related to tributary flooding and to collapse of deltaic foresets. the here-obtained paths clearly resemble their “slump turbidite” path, but with a significant difference, the intercalation of the intermediate “to-andfro” term. the msa foliation parameter (f) also confirms the difference between the lower (tu) and the upper (hm) terms; a major difference also appears between the hm term and the overlying hemipelagic deposit. in summary, the here-used textural characterization points out a particular and complex depositional mechanism involving, for each event, density/turbidity current (mass wasting and fluidisation), to-and-fro bottom current (related to water mass oscillation), and quiet long-lasting settling of fine-grained homogenous suspension. seismic shocks and/or subaqueous landslides in closed basins may account for the whole process, especially the whole water column movement (constrained tsunami or seiche effect). this interpretation has been proposed for different marine or lacustrine basins developed in areas with frequent and strong seismic activity [sturm et al. 1995, chapron et .al. 1999, beck et al. 2007, 2012, bertrand et al. 2008, carrillo et al. 2008, çağatay et al. 2012]. 5.2 triggering mechanism of sedimentary hmtu events hmtu associations were found throughout the whole md01-2477 (figure 2). these units, resulting from gravity re-depositional processes, represent a specific evolution of slope failures and mass-wasting. the gulf of corinth is well known for the numerous submarines gravitational mass movements as slides, slumps and debris/mud flows [brooks and ferentinos 1984, papatheodorou and ferentinos 1997, hasiotis et al. 2002, lykousis et al. 2007]. in the gulf, the main mechanisms responsible for these observed mass movements are: (1) the frequent seismic activity in the region, (2) the development of rapid prograding prodeltas associated with high sedimentation rates of the numerous river mouths, (3) the steep slopes and (4) the presence of gas-charged sediments [hasiotis et al. 2002, lykousis et al. 2009]. additionally, historical observations in the gulf of corinth give evidence of earthquake induced nearshore sediment failure. several may be cited, among others: the 373 bc, 1817 and 1861 events [papadopoulos 2003 and references therein], the 1965 event [ambraseys 1967], the 1981, 1989 and 1995 events [perissoratis et al. 1984, papadopoulos 2003, ambraseys and synolakis 2010]. beside, few aseismic submarines gravitational slide have been reported as the 1963 and 1996 events [galanopoulos et al. 1964, campos et al. 8 9 paleoseismic record of gulf of corinth papadopoulos 2003]. despite the presence of sediment failure of aseismic origin, earthquakes remain the main cause of submarine failure. in the gulf of corinth the earthquakes and the sediment failures of seismic or aseismic origen can generate high amplitude tsunami waves [dominey-howes 2002, papadopoulos 2003, ambraseys and synolakis 2010]. these authors have reported a high frequency oscillation (amplitude of 20-30 cm) immediately after the 1981 earthquake, with a gradual attenuation during four days after the earthquake [papadopoulos 2003]. in this study, the gravity-reworked deposits hmtu present in the md01-2477 core occured farther back in time, when no historical data are available. therefore, it is not possible to correlate them with the documented historical earthquakes. despite this, several arguments led us to consider hmtu desposits as earthquake-induced: (1) the drilling site location in the basin plain, avoiding coarser fluxes at tributary river mouths, (2) the high seismic activity in the region, (3) the well known earthquake-induced sediment failures during the last 2000 yrs (4) and the similarities of the hmtu deposits with previous documented earthquake-triggered homogenites+turbidites deposits in marine and lacustrine environments. the specific planar array in the homogenites (high magnetic foliation values), the sharp contacts within hmtu, and the sedimentary structures typical of to-and-fro currents fit with the hypothesis of major earthquakes inducing coeval gravity reworking and seiche effects [chapron et al. 1999, beck 2009 and references therein, campos et al. 2013]. in these cases, the seiche effect is responsible for: (1) the increase of suspended load by the additional extraction of the clayey-silt matrix from the initial flux, (2) the generation of a sharp contact between the two hmtu main terms by oscillatory bottom currents, (3) and the creation of specific settling conditions, producing the specific planar array (phyllosilicates homogeneous orientation) in the homogenites. 6. time distribution of the sedimentary “events” on figure 5, we represent the vertical and temporal distribution of all hmtu events recognized in the md01-2477 core, and the sedimentation rates for the hemipelagic deposits (blue colour). the age vs. depth graph was constructed using six radiocarbon ages, assuming a constant sedimentation rate in between, which is most probably oversimplified. the mean rates of sedimentation and the estimation of the main hmtu recurrence time interval were estimated for the first 17.75m (0 to 17 kyr bp) of the md01-2477 core. the last 2.32m (17.75 to 20.07 m depth) are made of a homogeneous mud displaying highly constant ms, msa, and grain size distribution. this basal section does not show deformation or sedimentary structures. due to its characteristics (high homogeneity and absence of a coarser sediments layer), this section could represent the upper term (hm) of a major hmtu association whose lower part was not retrieved. the measured low values of msa may be related either to the low content of magnetic minerals in this interval (low ms values) or to an actual weak magnetic foliation. for these reasons, this 2.32 m thick interval is not taken in account for the statistical distribution. in the md01-2477 core, the magnetic susceptibility curve and the occurrence of the reworking events (tu, hmtu) are different between the marine and non-marine succession (figure 2). in the non-marine section the values of ms are highly variable compared to the marine section. higher values of ms are present in the non-marine section (>1000 s.i.x10-5). the reworking events are thinner in the non-marine section, compared to the marine one. but the hmtu deposits in both environments represent more or les the same fraction of the total sedimentation (∼32.5%). in the non-marine section the total mean sedimentation rate (hemipelagic + instantaneous deposits) is about 0.68 mm/yr, whereas the hemipelagic deposits have a sedimentation rate between 0.31 and 0.82 mm/yr. in the marine section, the total main rate of sedimentation is higher: about 1.1 mm/yr, showing rates of sedimentation in the hemipelagic deposits between 0.32 and 3.68 mm/yr. in the non-marine section the total mean sedimentation rate (hemipelagic + instantaneous deposits) is about 0.68 mm/yr, varying from 0.31 and 0.82 mm/yr (using hemipelagic deposits only). in the marine section, the mean sedimentation rate is about 1.1 mm/yr, varying from 0.32 and 3.68 mm/yr. this difference in the rates of sedimentation between the marine and non-marine section can be explained by a climatic influence. the non-marine section was developed in a lowstand phase during the last glacial period (70 to 12 kyr bp) [collier et al. 2000, lykousis et al. 2007, bell et al. 2008] characterized, in the mediterranean realm, by a dry and cold climate, with absence of significant tree cover (presence of steppe vegetation) [leeder et al. 1998, collier et al. 2000]. these climatic conditions are inferred to favour higher mechanical erosion and subsequent higher terrigenous material production. however, due to lower transport capacity of the rivers (associated to the dry conditions), these sediments were probably accumucampos et al. 10 lated in the fluvial system or in the margin of the basin as deltas. otherwise, during the transgressive to highstand phases associated to the holocene glacioeustasy (12 to 0 kyr bp) [collier et al. 2000, lykousis et al. 2007, bell et al. 2008], these sediments were remobilized and transported from the fluvial systems to the margin of the basin. it provides a greater amount of sediments available to be transported to the deep basin by seismic destabilization, compared to the last glacial period. similar conditions were reported for post-glacial lakes in the french alps and the andes [beck et al. 1996, carrillo et al. 2008]. despite this, rapid tectonic uplift, especially the southern side of the gulf of corinth (see paragraph 2), has also controlled the sediment supply during the whole quaternary. this basin combined uniform regional uplift of 0.3 mm yr-1 [collier et al. 1992] with 1.3 mm yr-1 south footwall uplift [armijo et al. 1996]. 36 hmtu events are distributed through the md01-2477 core, corresponding to the last 17 cal kyr bp. at the top of the core (0 to 2 m depth) and at the base of the marine section (11.2 to 12.8 m depth) no hmtu were detected, although abundant thin turbidites (0.5 to 1.5 cm thick) are present. the later ones are not thick enough to apply the here-used textural analyses, and no clear evidences for seismic origin can be assumed. to define the recurrence time interval between two hmtu events, an age/depth curve (figure 5) was constructed. in the 17 to 11.7 cal kyr bp period (nonmarine section and in the marine and non-marine transition), 13 events were identified, representing a ∼400 yr average recurrence interval. in the marine section (11 cal kyr bp to present), 22 events were recognized, representing a ∼500 yr average recurrence interval. according to chronological precision, the 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0 1 8 0 0 0 age cal yr bp 263 cm = 3064 2734 cal yr bp 390 cm = 3280 2913 cal yr bp 516 cm = 6052 5706 cal yr bp 1107 cm = 9239 8779 cal yr bp 1633 cm = 14038 13618 cal yr bp 1766 cm = 16928 16218 cal yr bp1 8 0 0 1 5 0 0 1 2 0 0 9 0 0 6 0 0 3 0 0 0 d ep th c m s.r: 0.73 mm/yr s.r: 3.68 mm/yr s.r: 0.93 mm/yr s.r: 0.31 mm/yr s.r: 0.32 mm/yr s.r: 0.82 mm/yr cal. ams 14c age legend hmtu: homogenite (hm)+(tu) turbidite s.r: sedimentation rate figure 5. proposed time distribution of hmtu events based on 14c dating. deduced sedimentation rate is applied to “hemipelagic” constant supply, without the supposed instantaneous deposits. 11 difference between the sections is not significant. these recurrence intervals are compatible (same order) with, or slightly larger than those proposed for the last 2 kyr period concerning the aeghion, skinos and eliki faults using palaeoseismological trenches. the later ones displayed respective recurrence intervals of 360 yr [pantosti et al. 2004], 330 yr [collier et al. 1998], and 200 to 600 yr [mcneill et al. 2005]. using the hmtu events may both represent: an over-estimation of the number of recorded earthquakes, as part of them may be only due to subaqueous lanslides. some historical examples occurred on delta fronts on the southern coast with tsunami impact on the northern coast. in recently retrieved short cores [mortier 2012, beckers et al. 2013] one of these events [1963] displays a clear signature; an under-estimation; first, we did not include classical turbidites in our list; second, some homogenites may be not detected when a basal coarser layer is lacking. as described in the sea of marmara [beck et al. 2007, eriş et al. 2012] or in lake le bourget [chapron et al. 1999] a homogenite may just have a very thin laminae of silt/very fine sand at its base, only visible on a high resolution x-ray picture; the homogenite itself would possibly be characterized with detailed profile of msa. these types of events have not yet been investigated here. with respect to intensities or magnitude of involved paleo-earthquake, previous works [e.g. audemard and de santis 1991, rodrı́guez-pascua et al.2000, mccalpin 2009 and references therein, rodríguez-pascua et al., 2010] indicate that a mw > 5 or 5.5 earthquake is necessary to produce noticeable geologic effects, especially pore water pressure sudden increase (driving force for liquefaction, failures, injections, etc.). additionally, the compilation of historical data in the gulf of corinth [e.g. ambraseys 1967, papadopoulos 2003, ambraseys and synolakis 2010 and references therein] indicates, for the last 2.3 kyr, surface wave magnitudes comprised between ms= 7.0 and ms=6.0. most of them have generated submarine landslides and tsunamis. based on all data, a precise estimate of paleo-magnitudes remains speculative; mw> 6 and reaching 7 may be proposed as a realistic estimate. 7. conclusion based on high resolution analyses of layering, composition, and, overall, textures (magnetic susceptibility, and magnetic susceptibility anisotropy, grain-size distribution), the sedimentary succession sampled through the md01-2477 core is considered as a paleoseismic archive for the last 17 cal kyr bp, for the central part of the gulf of corinth. among used criteria to discuss depositional processes (instantaneous and continuous) msa appeared to be a useful tool to identify the instantaneous homogeneous deposits (hm), and to distinguish them from normal hemipelagic deposits. this textural parameter also permits to precisely measure the time-equivalent of the hemipelagic interval which separates two events inferred as co-seismic (hmtu). in this core, 36 earthquake-induced hmtu (homogenite associated to turbidite) were detected, intercalated within a continuous sedimentary recording. they can be explained by the combination of: 1) a constant seismo-tectonic activity in the gulf of corinth during the late pleistocene-holocene. 2) terrigenous sedimentary feeding of shelves and upper slopes during the same time. these hmtu events have a minimum average recurrence interval between 17 to 11.7 cal kyr bp of ∼400 yr; whereas, the marine sequence, show a minimum average recurrence interval between of ∼500 yr. a 6.0 to 7.0 mw approximate magnitude is considered as responsible for these sedimentary events. in order to improve these paleoseismological studies, future investigations should include in situ geotechnical measurements to assess slope failure potential [strasser et al. 2006, stegmann et al. 2007]. a search of a method to discriminate earthquake-related hmtu and hmtu only due to landslides is also needed. acknowledgements. the here-presented investigations were funded through different grants and facilities: i) university of savoie’s aap-2012-16 agrasm grant, dedicated to asm sedimentological application, ii) cnrs-insu funding through isterre laboratory. iii) cnrs-insu artemis national radiocarbon ams measurement program. these investigations are also associated to french agence nationale pour la recherche (anr)’s siscor project dedicated to seismic hazards related in the gulf of corinth rifting.c. campos’s phd thesis and stay in isterre laboratory is funded through venezuela’s fundayacucho grant n° 20093262. we also acknowledge inqua for the grant which permits her attend the 2nd inqua-icgp 567 international workshop on active tectonics, earthquake geology, archaeology and engineering. authors are grateful to these different institutes and governments, and to two anonymous reviewers who greatly helped to improve the initial manuscript. references adams, j. 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resuspension deposits from a pleistocene proglacial lake (kleszczow graben, central poland), journal of sedimentary research, 65(2), 417-422. *corresponding author: corina campos, universidad simón bolívar, departamento de ciencias de la tierra, sartenejas, baruta, venezuela; email: campossc@usb.ve © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. paleoseismic record of gulf of corinth 609_620 assran20.pdf annals of geophysics, vol. 45, n. 5, october 2002 609 application of ground magnetic and multi-frequency em techniques for the abu-shihat radioactive prospect area, north eastern desert, egypt assran s.m. assran nuclear materials authority, cairo, egypt abstract land magnetic and electromagnetic data enhancement procedures were employed to follow the extensions of the observed anomalies on the surface downwards in the deeper levels and to recognize any possible relations between the localization of uranium mineralization and both the structural and geologic settings. this study revealed that the analytic signal (as) and high pass and low pass filtering approaches significantly improve the interpretability of the measured magnetic data in discriminating the shallow and deep magnetic sources within the abu-shihat prospect area. furthermore, the em survey using varying frequencies, coil separations and station separations was performed in the considered area with the horizontal-loop em equipment. the interpretation of the horizontalloop em data indicated the presence of some conductive zones. these zones are mainly associated with felsite and pegmatite dykes, as well as alteration and fault zones. the target parameters such as the location, width, depth, dip and conductivity thickness were estimated for each conductive zone. combining the ground magnetic and horizontal-loop em surveys with the geological and structural mapping revealed that the mineralizations are concentrated in the eastern and western parts of the study area, and these techniques are considered as valuable exploration tools for uranium associated sulfide mineralization. 1. introduction the abu-shihat prospect area (26°38′n, 33°24′e) is located about 8 km north of qenasafaga road at el-faroukiya station (85 km from qena). the nuclear materials authority (nma) selected it through intensive geological, geochemical and geophysical studies. mailing address: dr. assran s.m. assran, nuclear materials authority, p.o. box 530, maadi road post office, cairo, egypt; e-mail: assran@hotmail.com key words magnetic – analytic signal – wavelength filter – multi-frequency em – geological mapping the geophysical investigation started by aerial radiometric and magnetic survey for the area bounded between latitudes 25°48′n and 26°44′n and longitudes 32°53′e and 33°34′e (ammar, 1973), including the present prospect area (fig. 1). this investigation was followed by ground proving and checking of the recorded anomalies, accompanied by regional geological investigations, followed by semi-detailed and detailed radiometric investigations (el-tahir, 1978; bakhit et al., 1984). the results of the previous radiometric and geochemical studies had identified three anomalous radiometric zones in the study area (eltahir, 1978). each of these locations was excavated with a front-loader. a relative increase in the uranium mineralization was found, mainly 610 assran s.m. assran represented by secondary uranium minerals and some sulfide minerals in the highly fractured granites and granodiorites, as well as the felsitic and pegmatitic dykes. the radioactivity is ranged from 80 to 320 ur at the surface, with a maximum of 480 ur at about 1 m depth. el-tahir (1978) recommended further geophysical and drilling investigations in a trial to discover any subsurface extensions of the uranium mineralizations. magnetic and electromagnetic techniques have historically been the most common geophysical methods used in mineral exploration. both techniques are cost effective and the data can be interpreted quickly in the field. often, the data resulting from the two methods are sufficient to characterize the buried targets (won and keiswetter, 1997). the present work deals with the application of ground magnetic, multifrequency and multi-separation em survey, besides detailed geological mapping for the study area, to follow the extensions of the observed anomalies in the deeper levels, and to recognize any possible relations between the em anomalies and magnetic anomalies, as well as the comparable geologic settings. 2. geologic setting the regional geology, structural set-up and potentialities of uranium in the abu-shihat area have been discussed by many workers (schurmann, 1966; sabet et al., 1972; ghanem et al., 1973; el-tahir, 1978; bakhit et al., 1984). the author carried out a detailed geologic mapping for the prospect area (fig. 2). accordingly, the study area is formed mainly of precambrian basement rocks including quartz diorites, granodiorites and granites. these rocks are injected into the area by numerous pegmatitic, felsitic, quartzitic and basic dykes, and are locally covered by quaternary sediments, which are represented by wadi (valley) sediments. these rocks were exposed to different tectonic events that produced various secondary structural elements as faulting and shearing in the n-s, ne-sw, ene-wsw and nw-se directions, beside the other minor trends. 3. ground magnetic survey 3.1. magnetic data acquisition and correction a total magnetic field intensity survey was performed on a set of parallel n60°w profiles normal to the general strike of the mineralized felsite dykes. the line spacing was 50 m and the measurements were taken at 20 m intervals along the survey profiles. two tie lines were conducted (coded 00 and 300) normal to the observed profiles for controlling the consistency and reliability of the data. two proton magnetometers (model pmg-1, geofyzika, brno, czech republic) were used: one for mobile station readings and the other for the base station readings. base station readings were recorded every 30 s and used to correct for diurnal drift. the corrected values were then reduced to an fig. 1. key map showing the location of the abushihat radioactive prospect area, north eastern desert, egypt. 611 application of ground magnetic and multi-frequency em techniques for the abu-shihat radioactive prospect area, ... arbitrary datum (41 800 nt) and the resulting data used to generate a total intensity magnetic map (fig. 3). 3.2. the total intensity magnetic map according to the visual inspection of the total magnetic map (fig. 3), three magnetic zones of various magnetic characteristics were distinguished on the basis of the differences in the character of the magnetic anomalies i.e. their wavelengths, amplitudes and trend patterns. the first zone (z1) covers the western and northwestern parts of the study area and is associated with felsite, quartz dykes and diorite rocks. it is occupied by broad belts of low frequencies having nearly a nnw-sse direction. this zone decreases gradually in magnitude towards its western part, where its amplitude reaches – 95 nt. the second zone (z2) is situated at the central part of the area and is characterized by a group of relatively strong positive anomalies with a maximum amplitude of about 318 nt. these anomalies are associated with the basic dykes and diorite rocks (fig. 2). they trend in the n-s, nne-ssw and nw-se directions. the third zone (z3) is located at the eastern part of the area and is associated with the pink granites and granodiorites. the northern part of this zone is represented by negative and positive closures of relatively small sizes and varying amplitudes, which may indicate that the causative bodies might be exiting at shallow depths. meanwhile, the southern part is occupied by a negative rectangular belt and is divided into small negative fig. 2. detailed geological and structural map of the abu-shihat radioactive prospect area, north eastern desert, egypt, showing the 3 lines selected for em survey. symbols are: 1 = wadi sediments; 2 = basic dykes; 3 = quartz dykes; 4 = felsite dykes; 5 = pegmatite dykes; 6 = pink granites; 7 = granodiorites; 8 = diorites; 9 = alteration zones, and 10 = fault. 612 assran s.m. assran anomalies trending mainly in the n-s and e-w directions. this may indicate the existence of small separate magnetic bodies resting over a deeper one. 3.3. analytic signal map the analytic signal (as) has been shown to be effective in interpreting the subsurface magnetic contacts (e.g., nabighian, 1972, 1974). this method does not require reduction to pole processing. a geologic contact or fault with significant susceptibility contrast is detected by mapping the maxima of the simple analytic signal, which is composed of two horizontal and one vertical gradients (nabighian, 1972, 1974, 1984; roest et al., 1992). visual inspection of the analytic signal map (fig. 4) exhibits a number of maxima that conform very well with to the structural trends and geologic rock boundaries. the western part of the map is occupied by relatively low frequency anomalies, which may reflect the deep sources at this part and also confirm the results obtained from the total intensity magnetic map (fig. 3). the small highs within this part are associated with felsite and pegmatite dykes, where each peak corresponds to one of the dykes appears. meanwhile, the eastern part is characterized by high anomalies, which generally coincide with pink granites. also, it seems obvious from the southeastern part that, the pink granites and granodiorites have small highs within a larger one. this may reflect a wide aerial extent below thin veneers of the rocks at this part. these high anomalies are separated by relatively low amplitudes and are associated with diorites (fig. 2). therefore, from the above mentioned considerations, it can be concluded that the as is a very good tool that can be used in differentiating the various rock types and delineating the structural trends fig. 3. total intensity magnetic map of the abu-shihat radioactive prospect area, north eastern desert, egypt. � � � � � � � � � � � � � � � � � � � � � � � � � � � ���� ���� ���� ���� ���� ���� � ���� ���� ���� ���� ���� ���� �� ��� �� ����� � � �� � � � � �� � �� � � � �� �� �� �� 613 application of ground magnetic and multi-frequency em techniques for the abu-shihat radioactive prospect area, ... that may be obscured in the total intensity magnetic map. 3.4. regional-residual separation magnetic data observed in geophysical surveys are the sum of magnetic fields produced by all the underground sources. the targets for specific surveys are often small-scale structures that occurred at shallow depths. the magnetic responses of these targets are embedded in a regional field, which arises from magnetic sources usually larger or deeper than these targets. correct estimation and removal of the regional field from the initial effect yields the residual response produced by the target sources. in the present study, the fast fourier transform (fft) was applied on the total intensity magnetic data using geosoft package (1994) to explore the frequency content of these data. as a result of visual inspection of the two-dimensional power spectrum (fig. 5), two linear segments were distinguished and fitted. the first segment lies at the low frequency (long wavelength) with a steep slope, related to the deep and/or broad causative magnetic sources. meanwhile, the second segment is located at high frequency (short wavelength) with a gentle slope and is due to relatively shallow sources. the slope of a line fitted to each of the first and second segments of the spectrum curve is used to estimate the average depths of the regional and residual sources. these depths were recomputed with the power-low correction (fedi et al., 1997). the estimated average depths to the regional and residual sources are 53.1 and 12.4 m, respectively. moreover, the analysis of the power spectrum curve shows that the deepseated magnetic component frequency varies from 0 to 6 cycles/km, while the near-surface magnetic component frequency ranges from 6 fig. 4. analytic signal of the total intensity magnetic map of the abu-shihat radioactive prospect area, north eastern desert, egypt. 614 assran s.m. assran fig. 5. radially averaged power spectrum and depth estimate of the total intensity magnetic map of the abushihat radioactive prospect area, north eastern desert, egypt. � �� �� �� �� � �! � " � �# � �� �� �� �� � �! � " � �# � �� �� �� $��% &���'�& �(����" &���'�& �)*��& +������,�� � -.�# ���&� ( � /�� � ( � ��� � fig. 6. high-pass filtered (frequency = 6-65 cycles/km) magnetic map of the abu-shihat radioactive prospect area, north eastern desert, egypt. 615 application of ground magnetic and multi-frequency em techniques for the abu-shihat radioactive prospect area, ... to 65 cycles/km. these bands of frequency were used through the band-pass filter technique to produce the residual and regional component maps (figs. 6 and 7). 3.4.1. the high-pass (residual) magnetic anomalies the residual magnetic anomalies can be defined as the anomalies that are economically interesting, because they indicate shallow anomalies and are characterized by weaker and more localized anomalies. the high-pass filtered map (fig. 6) reveals that the study area is characterized by small closures with semicircular to circular shapes and relatively high positive and negative amplitudes. the causative bodies of these closures might be cropped out or buried at shallow depths. besides, relatively broad anomalies of deeper origin with low slopes and smooth shapes having relatively low amplitudes were seen. the residual anomalies are frequently preferred in the n-s, nw-se and ne-sw directions and most of them are not shown clearly on the total intensity magnetic map (fig. 3). 3.4.2. the low-pass (regional) magnetic anomalies the regional magnetic anomalies are strong, broad, extend over large areas and usually have high amplitudes and low frequencies. these anomalies are of considerable significance in the regional tectonic studies of the basement complex and of secondary importance in mineral exploration. the low-pass filtered map (fig. 7) shows: 1) a huge zone of high magnetic anomalies at the central part of the study area, bounded from fig. 7. low-pass filtered (frequency = 6-65 cycles/km) magnetic map of the abu-shihat radioactive prospect area, north eastern desert, egypt. 616 assran s.m. assran the west, north and east by relatively steep magnetic gradients. this may suggest the existence of major deep-seated faults bounding this zone from the stated directions. 2) a broad negative magnetic anomaly located at the southeastern corner of the study area, approximately trending in a n-s direction and associated with the pink granites and granodiorites. 3) a negative magnetic belt located at the northwestern part, trending in the nnw-sse direction and associated with a set of felsite and quartz dykes. this may suggest that, these dykes are relatively deep at this part. 4) the regional magnetic anomalies are elongated mainly in the n-s, nne-ssw and e-w directions. 4. horizontal-loop electromagnetic survey the electromagnetic (em) induction method can be used to infer the electrical conductivity of the subsurface at different depths of interest by changing the spacing between transmitter and receiver coils or the frequency of the transmitted field (patra and mallick, 1980; won, 1980; keiswetter and won, 1997). the horizontal-loop electromagnetic (hlem) method is based on the principle of inducing eddy currents in the earth with a transmitter coil (tx), which generates a primary electromagnetic field. the strength of the induced current and consequently that of the secondary electromagnetic field is dependent on the conductivity, shape and size of the target body and its location relative to the transmitter-receiver positions. the in-phase and out-of-phase components of the resulting secondary electromagnetic field are recorded at frequencies 112.5, 337.5, 1012.5 and 3037.5 hz. the coil separation between the transmitter coil (tx) and receiver coil (rx) was either 100 or 120 m. the midpoint of the line joining the tx-rx is used as the measuring point. the hlem measurements were acquired at station spaced 10 and 20 m apart along the survey line, using the tm-2 transmitter and igs-2/em-4 receiver (scintrex limited, canada). by judicious application of the multi-frequency techniques, it is possible to detect the weaker conductors (small width) and to derive detailed information on the parameters of the conductors, from the anomalies obtained with the hlem method. besides the detection of electromagnetic anomalies, the aim of an electromagnetic survey is to estimate the parameters of the causative conductors from details of the gained anomalies. the main characters of the hlem interpretation are, the location, dip (θ ), width (w), depth of burial (h) and conductance (σ t). these parameters can be estimated by using a family of response diagrams and curves, which were designed by nair et al. (1974). the first step in interpreting the hlem data is to establish the background in-phase and out-of-phase values, away from the anomaly to be interpreted. according to the results formerly obtained from the radiometric and geochemical data (el-tahir, 1978), the hlem survey was conducted along three profiles (coded 200, 300 and 1200). the hlem data, corrected for the first-order topographic effects, are shown in figs. 8, 9 and 10. the examination of these profiles led to the following observations. 4.1. line 200 (a-a′) this profile was conducted between stations – 250 and 210 with coil separations 100 and 120 m, and station intervals 20 and 40 m. the hlem data shown on this line (fig. 8) indicate the existence of two em anomalies. the first is close to station – 190 and is associated with the pegmatite body (fig. 2). the width of this anomaly is less than the coil separation and may be due to a thin conductor. the second one has a negative peak for the in-phase and out-of-phase, and extends from about station coded – 40 to about station coded 100. this anomaly coincides with a set of faults and an alteration zone, as shown on the geologic map (fig. 2). the results of quantitative interpretation of the second anomaly are the width that is about 40 m, the depth to the upper edge that attains 52 m and the approximate dip angle that reaches 60° to the south. meanwhile, a considerable increase in the conductance estimates was observed by decreasing the frequency from 12.1 mhos at 3037.5 hz to 47.6 mhos at 112.5 hz. this effect is due to the body thickness and the current 617 application of ground magnetic and multi-frequency em techniques for the abu-shihat radioactive prospect area, ... gathering, a conclusion also supported by the results obtained by betz (1975, 1976) for ground em measurements and by palacky (1978) for airborne em surveys. comparing the estimated depth, dip and conductance measured using the coil separations 100 and 120 m, they are highly similar, but the width estimates increase with increasing spacing. this effect can be explained by the fact that, the values of the in-phase and out-of-phase components are primarily affected by dimensions of the conductor being investigated and its position in relation to the coil positions and coil separation. the first em anomaly coincides with low magnetic amplitudes and high frequencies (figs. 3, 4 and 7). this may indicate that, the causative body lies within shallow depths. also, the second em coincide with low magnetic amplitudes and low frequencies (figs. 3, 4, 6 and 7). this may reveal that, the causative body is relatively deep at this part. the estimated depth of the second anomaly agrees well with the average depths of the deep magnetic anomaly sources. because of the high radiometric values (el-tahir, 1978) in this zone, which is associated with the high conductance and low magnetic values at this part, a more detailed ground survey of this zone is recommended. 4.2. line 300 (b-b′) this profile was carried out between stations – 270 and 90, with coil separation 100 m and station interval 20 m. only one broad conductive zone is indicated by a negative peak that extends from about station – 220 to about station – 60 (fig. 9). this zone corresponds to the extension of a geologically-mapped felsite dyke and the intersecting faults (fig. 2). the shapes of the inphase and out-of-phase components suggest two conductors spaced 60 m apart, as the maximum negative peak values are separated by more than coil spacing of 60 m. the northern shoulder of the in-phase and out-of-phase components is reduced appreciably at higher frequencies, indicating the existence of conductive bodies in this region. this is supported by the magnetic and radiometric data. other researchers observed a reduction of the out-of-phase component in the field and modeled em data (lajoie and west, 1977). the quantitative interpretation of the hlem data exhibits the width of the conductive zone as 60 m, the dip angle as 32° to the north, the depth to the upper edge attains 18 m and the conductivity thickness increases with decreasing frequency. it ranges from 4.8 to 22 mhos. this effect is due to the causative overburdens, which may cause phase rotations of the anomaly, resulting in the estimates of conductivity thickness decreasing with increasing frequency. fig. 8. horizontal-loop em profiles at several frequencies, coil separations and station separations with the corresponding total magnetic field profile along line a-a′ (line 200) and surface geology of the abushihat radioactive prospect area, north eastern desert, egypt. symbols as in fig. 2. 618 assran s.m. assran on the high-pass filtered map (fig. 6), the em conductive zone is represented by two belts of high frequencies and low magnetic amplitudes intruded by a belt of high magnetic amplitudes. also, the low magnetic amplitudes coincide with the negative peaks of the in-phase and out-ofphase components. this may suggest that, two acidic causative bodies lie within the shallow depths. 4.3. line 1200 (c-c′) this profile was conducted between stations – 190 and 490, with coil separations 100 and 120 m, and station intervals 20 and 40 m. the examination of hlem data acquired along this line with coil separation 100 m exhibits a broad conductive zone at the northern part (fig. 10). the shapes of the negative and positive peaks of the two components reveal that this zone is composed of a series of thin conductors over a width greater than 130 m of the coil spacing. on the magnetic maps (figs. 3 and 5), this zone is characterized by a group of relatively low magnetic amplitudes and high frequencies. this may reflect the causative bodies as acidic rocks, that lie within relatively shallow depths. geologically, this zone coincides with a set of faults and felsite dykes (fig. 2). the general parameters of the considered conductive zone (tx-rx = 100 m) show that the width is about 130 m, the depth to the upper edge reaches 26 m, the dip angle is 62° to the south fig. 9. horizontal-loop em profiles at several frequencies, coil separations and station separations with the corresponding total magnetic field profile along line b-b′ (line 300) and surface geology of the abushihat radioactive prospect area, north eastern desert, egypt. symbols as in fig. 2. fig. 10. horizontal-loop em profiles at several frequencies, coil separations and station separations with the corresponding total magnetic field profile along line c-c ′ (line 1200) and surface geology of the abushihat radioactive prospect area, north eastern desert, egypt. symbols as in fig. 2). 619 application of ground magnetic and multi-frequency em techniques for the abu-shihat radioactive prospect area, ... and the conductivity thickness ranges from 3.8 mhos at 3037.5 hz to 34.2 mhos at 112.5 hz. meanwhile, the hlem data with coil separation 120 m shows the existence of a broad and incomplete anomaly, whereas only one positive peak is present and the other one is outside the survey area. besides, some of the anomalies in the in-phase and out-of-phase components are lost. this is often by apparent loss, because the smaller separation discriminates more effectively between the conductivities. also, the positions of the peak values for the two components differ markedly, which may indicate that the observed anomaly represents the combined effect of two or more sources. 5. conclusions the visual inspection and analysis of the total intensity magnetic map and the other filtered magnetic maps revealed that the area concerned is generally characterized by a broad and high magnetic belt at the central part surrounded by low magnetic belts from the east and west. besides, it includes surficial or local anomalies of shallow-seated origin, with narrow semicircular to circular shapes of relatively high amplitudes having n-s, nw-se and ne-sw directions. the average estimated depth for the near-surface magnetic anomaly sources of the area is 12.4 m, while that of the deep-seated anomaly sources is 53.1 m. the investigation of the horizontal-loop electromagnetic profiles reflected the occurrence of many conductive zones. these zones are mainly associated with a set of felsite dykes, alteration and fault zones. the conductive parameters, as estimated from the small coil separation and station interval, revealed widths ranging between 40 and 130 m, shallow depths varying from 18 to 52 m, dip angles ranging between 32° and 62°, and conductivity thickness varying from 22 to 47.6 mhos for the mineralized zones. comparing these values with the estimated values using larger coil separation and station interval, they are highly similar, but the width estimates increase with increasing spacing. this effect can be explained by the condition that the values of the in-phase and out-of-phase components are primarily affected by the dimensions of the conductor being investigated and its position in relation to the coils positions and coil separation. besides, when large spacing is used, some of the anomalies are lost and the resolution of the individual conductor becomes poor, and if more than one conductor occurs, the interpretation of the results becomes difficult. because of the high radiometric anomalous zones, associated with the high conductive and low magnetic values, a more detailed ground survey of these zones and subsequent exploration drilling, based on the geophysical data are recommended to test the depths and grades of the probable mineralizations. acknowledgements i would like to thank professor ahmed a. ammar for fruitful discussions and valuable comments. references ammar, a. a. (1973): application of aerial radiometry to the study of the geology of wadi elgdamiarea, eastern desert (with aeromagnetic application), ph.d. thesis, faculty of science, cairo university, giza, egypt, pp. 424 (unpublished). bakhit, f.s., m.l. meleik and m.a. el-tahir (1984): correlation of the radiometric survey data and the structural analysis with the geology of abu-zawal area, central eastern desert, egypt, geol. rundsc., 73 (2), 833-851. betz, j.e. (1975): test program report with additional comments on the max min ii electromagnetic system, markham, apex parametrics ltd., pp. 34. betz, j.e. (1976): considerations behind the making of a well-rounded exploration system, markham, apex parametrics ltd., pp.14. el-tahir, m.a. (1978): relation between geology and radioactivity of some basement rocks to the north of qenasafaga asphaltic road, eastern desert, egypt, m.sc. thesis, al azhar university, cairo, egypt (unpublished). fedi, m., t. quarta and a. de santis (1997): inherent power-law bahaviour of magnetic field power spectra from a spector and grant ensemble, geophysics, 62, 1143-1150. geosoft package (1994): geosoft mapping and processing system, geosoft inc., toronto. ghanem, m., a.a. dardir, m.h. rancis, a.a. zalata and k.m. abu-zeid (1973): basement rocks in the eastern desert of egypt north of latitude 26°40′n, ann. geol. surv., egypt, 3, 33-38. 620 assran s.m. assran keiswetter, d. and i. won (1997): multi-frequency electromagnetic signature of the cloud chamber, nevada test site, j. environ. eng. geophys., 2, 99-103. lajoie, j.j. and g.f. west (1977): two selected field examples of em anomalies in a conductive environment, geophysics, 42, 655-660. nabighian, m.n. (1972): the analytic signal of twodimensional magnetic bodies with polygonal crosssection: its properties and use for automated anomaly interpretation, geophysics, 37, 507-517. nabighian, m.n. (1974): additional comments on the analytic signal of two-dimensional magnetic bodies with polygonal cross-section, geophysics, 39, 85-92. nabighian, m.n. (1984): toward a three-dimensional automatic interpretation of potential field data via generalized hilbert transforms: fundamental relation, geophysics, 49, 957-966. nair, m.r., s.k. biswas and k. mazumdar (1974): standard curves for the interpretation of horizontalloop electromagnetic anomalies, geol. surv. india, miscellaneous publication, n. 25. palacky, g.j. (1978): selection of a suitable method for quantitative interpretation of towed bird aem measurements, geophysics, 43, 576-587. patra, h. and k. mallick (1980): geosounding principles 2 (elsevier science publishing co., new york). roest, w.r., j. verhoef and m. pilkington (1992): magnetic interpretation using the 3d analytic signal, geophysics, 57, 16-125. sabet, a.h., m.s. el-gaby and a.a. zalata (1972): geology of the basement rocks at the northern parts of elshayib and safaga sheets, eastern desert, egypt, ann. geol. surv., egypt, 2, 111-128. schurmann, h.m.e. (1966): the precambrian along the gulf of suez and the northern part of the red sea, e.j. brill, leiden, the netherlands, pp. 404. won, i. (1980): a wide-band electromagnetic exploration method: some theoretical and experimental results, geophysics, 45, 928-940. won, i.j. and d. keiswetter (1997): comparison of magnetic and electromagnetic anomalies caused by underground structures, j. envir. eng. geophys., 2, 115-125. (received march 15, 2002; accepted october 22, 2002) vol49,4_5,2006 891 annals of geophysics, vol. 49, n. 4/5, august/october 2006 mailing address: dr. miguel a. cabrera, laboratorio de ionósfera, departamento de física, facultad de ciencias exactas y tecnolgía, universidad nacional de tucumán, av. independencia 1800, 4000 san miguel de tucumán, argentina; e-mail: mcabrera@herrera.unt.edu.ar key words total electron content – ionosphere – space weather – satellite – scale height 1. introduction for satellite and aircraft navigation systems or satellite orbit and position determination it is necessary to use radio signals transmitted between the satellite and the ground station (harris et al, 2001; among others). the ionosphere produces several effects on transionospheric radio waves (hargreaves, 1992). these effects are proportional to the number of free electrons encountered by the wave on its passage through the ionosphere (total electron content, tec) (rishbeth and garriot, 1969). tec is a key parameter that describes the major impact of the ionized atmosphere on the propagation of radio waves, which is crucial for terrestrial and earthspace communications. the ionospheric corrections that have to be applied to determine the position accurately are proportional tec along the radar-satellite path (hartmann and leitinger, 1984; lin, 2001; among others). so, for ionospheric corrections, tec measurements are required, or tec predictions from ionospheric models can be a useful tool. different ionospheric models have been developed to predict the electron density (n) distribution in height, which is called n-profile (chiu, 1975; anderson et al, 1987; among others), including the iri model (bilitza, 1990; rawer and bilitza, 1990). with this n-profile the vertical total electron content can be obtained. nevertheless, most of the signal paths are slant paths. slant total electron content for sirio-mortelliccio ray path miguel a. cabrera (1)(3), rodolfo g. ezquer (1)(2)(3) and paolo spalla (4) (1) laboratorio de ionósfera, departamento de física, facultad de ciencias exactas y tecnolgía, universidad nacional de tucumán, san miguel de tucumán, argentina (2) consejo nacional de investigaciones científicas y técnicas (conicet), buenos aires, argentina (3) facultad regional tucumán, universidad tecnológica nacional, san miguel de tucumán, argentina (4) istituto di fisica dell’atmosfera (ifa), cnr, roma, italy abstract the total electron content (tec) is used to indicate the ionisation of the ionosphere. tec is a quantity that concern for predicting space weather effects on telecommunications, improving the accuracy of satellite navigation, fly control vehicles and other systems that use transionospheric signals, because the ionospheric layer affects the mentioned signals. in this work the slant total electron content (stec) was calculated with a technique that uses so-called «auxiliaries stations model», and a chapman layer with scale height equal to atomic oxygen scale height (cho). the validity was checked with stec measurements obtained from geosynchronous satellite signals, for sirio-mortelliccio link considering solstices and equinox, in high solar activity period. in general, the deviations between predictions and measurements were lower than 30% for 16 h per day (average). the results suggest that additional studies for other links and solar activity are required in order to improve the model predictions. 892 miguel a. cabrera, rodolfo g. ezquer and paolo spalla in general to model the ionosphere, the socalled thin layer approximation is adopted (manucci et al, 1999; among others), the stec is related with the vertical total electron content (vtec) through the piercing point with the obliquity factor (ciraolo and spalla, 2002; brunini et al., 2004; among others). the purpose of the present work is to calculate the tec along the ground station-satellite ray path, called slant total electron content (stec). a computation method and ionospheric model are used. we adopted a chapman layer (chapman, 1931) with scale height equal to atomic oxygen scale height, hereafter referred to as cho. 2. method to calculate the stec the length of the slant path is divided into segments of 20 km. the verticals that pass through the ends of these segments intersect the earth’s surface in different points that we call «auxiliaries stations». the coordinates of these stations are determined. with an ionospheric model the electron densities at the points where the slant path intersects the verticals of the auxiliaries stations are calculated and from them the slant n-profile is obtained. then, a numerical integration method and this slant n-profile are used to calculated stec up to 2000 km of altitude (cabrera, 2003). 3. model and discussion the chapman layer offers a simple way to explain the vertical structure of plasma in upper atmosphere (yonezawa, 1955; yonezawa and takahashi, 1960; titheridge, 1993; huang and reisnich, 2001). the chiu (1975) empirical model assumes that, f2 region plasma density can be expressed by two standard chapman profile expressions, one for the bottomside of the f layer and one for the topside. the semiempirical low-latitude ionospheric model (slim) (anderson et al., 1987) assumes two modified chapman expressions to obtain the f region n profile, one for the bottomside and one for the topside. wright (1960) has shown that is possible to apply the following chapman expression to the f region: (3.1) where z= ( h−hm)/h is the normalised height measured from hm in units of the scale height h. he purposed to take h as the scale height of atomic oxygen. in this paper we assume the chapman expression given by eq. (3.1) with h equal to atomic oxygen scale height. nm and hm correspond to peak characteristics and are obtained from ccir global maps (ccir, 1982). h was calculated using h = kt/mg, where k, t, m and g are boltzman’s constant, neutral temperature, atomic oxygen mass and the acceleration due to gravity, respectively. the value of t is obtained from msis-86 model (hedin, 1987). the model predictions were compared with measurements obtained at mortelliccio, a middle latitude station. figure 1 shows the calculated stec for sirio-mortelliccio (42.9°n, 10.7°e) ray path. for the considered link the satellite was placed at (0°n, 345°e). equinoxes and solstices for high solar activity period are considered. it can be seen that, in general, there is a good agreement between cho model and measurements for solstices (june and december), 16 h per day (average). in december the model overestimates stec during 14 h. april picture shows a secondary maximum, at 19 ut, that the model does not describe. the fact that the modelled stec values corresponding to winter are grater than those of summer shows that the model predicts the occurrence of the «winter anomaly» (rishbeth and garriot, 1969). the worst disagreement is observed for august, the model overestimates the measurements since 0 to 5 ut and 20 to 23 ut, with deviations greater than 30%. in general the model represents the stec with good agreement for hours of high ionisation, for the considered period. table i shows the deviations (d) between modelled values and measurements. the white, light grey and dark grey boxes correspond to the cases where d ≤ 30%, 30 < d ≤ 50% and d > 50%, respectively. it can be seen that for ( ) . ( )exp expn z nm z z0 5 1= -6 @" , slant total electron content for sirio-mortelliccio ray path 893 fig. 1. calculated and measured stec for mortelliccio-sirio ray path: median values. 894 miguel a. cabrera, rodolfo g. ezquer and paolo spalla daytime hours a great amount of white boxes are observed. few cases show deviations greater than 50% and they correspond to night time hours. 4. conclusions this work calculated the slant total electron content (stec). a chapman layer with scale height equal to atomic oxygen scale height (cho) was considered. the validity was checked with stec measurements obtained from geosynchronous sirio satellite signals and received at mortelliccio, considering solstices and equinox, in high solar activity period. for the considered cases, the results suggest that cho model shows an adequate performance to predict the stec. in general the deviations between predictions and measurements were lower than 30%, for 16 h per day. the observed disagreements between predictions and measurements could arise because peak characteristics or the shape of n profile, or both are not well predicted. additional studies for other links and conditions are required in order to study the performance of the model to predict stec. references anderson, d.n., m. mendillo and b. herniter (1987): a semi-empirical, low-latitude ionospheric model, report afgl-tr-85-0254 (air force geophysics laboratory, hanscom afb, massachusetts). bilitza, d. (1990): international reference ionosphere, nssdc/wdc-a-r&s 90-22 (maryland, eeuu). brunini, c., a. meza, f. azpilicuata, m.a. van zele, m. gende and a. díaz (2004): a new ionosphere monitoring technology based on gps, astrophys. space sci., 290, 415-429. cabrera, m.a. (2003): ph.d. thesis (universidad nacional de tucumán, argentina). ccir (1982): comité consultatif international des radiocommunications (international telecomunication union, place des nations, switzerland). chiu, y.t. (1975): a improved phenomenological model of ionosphere density, j. atmos. terr. phys., 37, 1536. ciraolo, l. and p. spalla (2002): tec time variability, in proceedings of the iri task force activity 2001, ic/ir/2002/23, trieste. hargreaves, j.k. (editor) (1992): the solar-terrestrial environment (cambridge atmospheric and space science series, cambridge university press), pp. 420. harris, i.l., a.j. mannucci, b.a. iijima, u.j. lindqwister, d. muna, x. pi and b.d. wilson (2001): ionospheric specification algorithms for precise gps-based aircraft navigation, radio sci., 36 (2), 287-298. hartmann, g.k. and r. leitinger (1984): range error due to ionospheric and tropospheric effects for signals frecuencies above 100 mhz, bull. geodin., 58, 109-136. hedin, a.e. (1987): msis-86 thermosphere model, j. geophysics res., 92, 4649. huang, x. and b. reinisch (2001): vertical electron content from ionograms in real time, radio sci., 36 (2), 335-342. lin, lao-sheng (2001): remote sensing of ionosphere using gps measurements, in proceedings asian assosiation on remote sensing, singapore, vol. 1, 69-74. manucci, a.j., b.a. iijima, u.j. lindqwister, x. pi, l. table i. percentual deviations between modelled and measured values. cho mortelliccio date april-82 june-82 august-82 december-81 r12 124 117 109 138 0 ut 35.8 34.2 76.4 12.7 1 27.1 31.5 61.5 31.9 2 10.9 32.4 46.0 28.9 3 31.5 38.0 33.1 33.4 4 29.1 44.1 52.9 39.1 5 43.9 41.9 60.6 42.8 6 14.1 2.4 29.2 41.8 7 9.5 9.7 17.8 19.7 8 25.5 5.7 15.9 1.2 9 25.9 6.8 13.0 38.0 10 28.6 6.4 10.6 39.2 11 22.8 8.0 6.5 16.4 12 16.2 9.4 1.8 19.5 13 14.8 3.0 2.5 22.8 14 16.5 1.8 3.1 25.4 15 18.6 9.1 17.3 17.8 16 27.0 16.4 19.7 20.3 17 39.9 18.3 20.2 27.2 18 21.0 12.7 23.8 51.9 19 4.9 10.1 23.6 28.8 20 10.2 2.8 39.1 28.7 21 16.3 21.8 57.6 30.6 22 20.8 25.2 70.6 12.4 23 44.7 27.4 82.6 0.8 895 slant total electron content for sirio-mortelliccio ray path sparks and b.d. wilson (1999): gps and ionosphere, revised submission to ursi reviews of radio science (jet propulsion laboratory, pasadena, ca). rawer, k. and d. bilitza (1990): international reference ionosphere-plasma densities: status 1988, adv. space res., 10 (8), 5-14. rishbeth, h. and o.k. garriot (1969): an introduction to ionospheric physics (academic press, new york and london), pp. 331. titheridge, j.e. (1993): atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere, j. atmos. terr. phys., 55, 1637-1659. wright, j.w. (1960): a model of de f region above hmaxf2, j. geophys. res., 65 (1), 185-191. yonezawa, t. (1955): on influence of the electron-ion diffusion on the electron density and height of the nocturnal f2 layer, j. radio res. lab., 2 (8), 125-136. yonezawa, t. and h. takahashi (1960): on the electron and ion density distributions from the lower uppermost part of the f region, j. radio res. lab., 7 (32), p. 335. (received september 15, 2005; accepted july 27, 2006) vol48/04/2005def 619 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words alkaline magmas – solubility – chlorine – supercritical fluid 1. introduction chlorine generally occurs as a minor component in most magmas and fumarolic gases. however, it has been observed that the release of cl from volcanoes to the atmosphere during eruptions and quiescent degassing may contribute to thinning of the ozone layer and the production of acid rain (johnston, 1980; devine et al., 1984; symonds et al., 1988; albritton, 1989). in addition, recent developments of spectroscopic methods to measure cl and s emissions at active volcanoes offer the possibility to use measured variations in cl and s emissions to better understand the subsurface movements of magmas and eruption likelihood (e.g., francis et al., 1998; love et al., 1998; duffell et al., 2003; edmonds et al., 2003). however, such applications require adequate knowledge of the factors controlling cl distribution between melt and fluid(s) in decompressing magmas. in recent years the study of volatiles in melt (now glass) inclusions has provided a wealth of data about cl abundance in silicate melts under magmatic conditions (e.g., lowenstern, 1995, and references therein). these data provide information on the amount of cl dissolved in the melt at the time of melt inclusion entrapment. chlorine solubility in evolved alkaline magmas michael r. carroll dipartimento di scienze della terra, università degli studi di camerino, italy abstract experimental studies of cl solubility in trachytic to phonolitic melts provide insights into the capacity of alkaline magmas to transport cl from depth to the earth’s surface and atmosphere, and information on cl solubility variations with pressure, temperature and melt or fluid composition is crucial for understanding the reasons for variations in cl emissions at active volcanoes. this paper provides a brief review of cl solubility experiments conducted on a range of trachytic to phonolitic melt compositions. depending on the experimental conditions the melts studied were in equilibrium with either a cl-bearing aqueous fluid or a subcritical assemblage of lowcl aqueous fluid + cl-rich brine. the nature of the fluid phase(s) was identified by examination of fluid inclusions present in run product glasses and the fluid bulk composition was calculated by mass balance. chlorine concentrations in the glass increase with increasing cl molality in the fluid phase until a plateau in cl concentration is reached when melt coexists with aqueous fluid + brine. with fluids of similar cl molality, higher cl concentrations are observed in peralkaline phonolitic melts compared with peraluminous phonolitic melts; overall the cl concentrations observed in phonolitic and trachytic melts are approximately twice those found in calcalkaline rhyolitic melts under similar conditions. the observed negative pressure dependence of cl solubility implies that cl contents of melts may actually increase during magma decompression if the magma coexists with aqueous fluid and cl-rich brine (assuming melt-vapor equilibrium is maintained). the high cl contents (approaching 1 wt% cl) observed in some melts/glasses from the vesuvius and campi flegrei areas suggest saturation with a cl-rich brine prior to eruption. mailing address: dr. michael r. carroll, dipartimento di scienze della terra, università degli studi di camerino, via gentile iii da varano, 62302 camerino, italy; e-mail: michael.carroll@unicam.it 620 michael r. carroll although the absolute amount of cl in magmas is difficult to estimate, the knowledge of solubility limits is of fundamental importance for interpretation of glass inclusion cl abundance. in many cases, solubility data provide an upper limit on the volatile content of melts and they also provide indirect information about the presence of cl-bearing fluid(s) within the magma body. this paper reviews some recent work involving experimental determination of the solubility and the fluid/melt partitioning of chlorine in hydrous, evolved alkaline magmas at shallow crustal depths (<∼8 km). cl solubility behavior at crustal pressures differs somewhat from that of water and carbon dioxide because in cl-rich systems the silicate melts may also coexist with a cl-rich brine. most experimental studies of cl solubility in hydrous systems have added cl to the experimental samples using aqueous (na, k) cl solutions (shinohara et al., 1989; métrich and rutherford 1992; webster, 1992a,b; student and bodnar 1999; signorelli and carroll, 2000, 2001, 2002). this approach allows description of the behavior of cl-bearing fluid using the well-known system nacl-h2o (sourirajan and kennedy 1962; bodnar et al., 1985; chou, 1987; anderko and pitzer 1993a,b). for h2orich solutions, the binary phase relations (fig. 1) show a single fluid phase of variable composition at high pressures or low cl concentrations. published experimental data for silicate melts coexisting with low-cl fluids, or with supercritical cl-bearing fluids show that cl partitions strongly into aqueous fluids relative to silicate melts by a factor of ∼20 to 300 (e.g., holland, 1972; kilinc and burnham, 1972; webster, 1992a,b; kravchuk and keppler, 1994; student and bodnar, 1999). webster (1992a,b) has also found that cl concentration in rhyolitic melts coexisting with a single aqueous fluid phase with low cl content (<20 wt% cl) decreases as pressure increases from 200 to 800 mpa and the fluid/melt partition coefficient for cl increases with increasing pressure. however, with increasing concentration of cl in the solution at pressures below ∼200 mpa, the binary system h2o-nacl is characterized by an immiscibility gap (subcritical region; see fig. 1) within which a cl-poor, aqueous fluid coexists with a cl-rich, hydrosaline brine. within the subcritical region, the compositions of aqueous fluid and brine are invariant at constant pressure and temperature, which means that cl and h2o concentrations in the fluids and the coexisting silicate melt will be fixed (gibbs’ phase rule). a constant cl content in the glass (flat distribution or plateau) for further addition of cl to the system is the typical expression of the melt being saturated with both a hydrosaline liquid and an h2o-rich aqueous fluid and it is assumed to define the solubility limit for cl in the hydrous silicate melt (malinin et al., 1989; shinohara et al., 1989; métrich and rutherford, 1992; webster, 1992a,b; signorelli and carroll, 2000, 2001, 2002). experiments carried out in this region (coexisting silicate melt + aqueous fluid + brine) for hydrous rhyolitic melts have documented that: 1) chlorine does not partition strongly into the water-rich fluid phase and the aqueous fluid/melt partition coefficient is ∼ 5 at 100 mpa (webster and holloway, 1988; shinohara et al., 1989; métrich and rutherford, 1992; candela and piccoli, 1995; williams, 1995); 2) there is a fig. 1. pressure-xnacl diagram of the system nacl-h2o with 850 and 900°c isotherm (redrawn after anderko and pitzer, 1993a). the polythermal projection of the 3-phase vapor+brine+halite equilibrium onto the pressure-composition section is from bishoff (1991) and anderko and pizer (1993a). experiments with bulk cl contents that fall within the solvus will coexist with both an aqueous fluid phase (containing a small amount of cl) and a salt-rich brine. 621 chlorine solubility in evolved alkaline magmas positive correlation between the fluid/melt distribution coefficient and total pressure (shinohara et al., 1989; malinin et al., 1989); and 3) chlorine solubility, that is the maximum amount of cl in a melt coexisting with a brine + aqueous fluid assemblage, depends strongly on melt composition (e.g., webster and de vivo, 2002) and pressure. the solubility varies from a few thousand ppm to ∼1 wt%, being higher in peralkaline rhyolites (pantellerites) relative to metaluminous compositions (shinohara et al., 1989; métrich and rutherford, 1992) results presented in this review paper extend these observations to consider trachytic and phonolitic melts, which have much higher cl solubilities than typical calcalkaline rhyolitic melt compositions. silicate melts can be saturated with respect to a fluid phase (e.g., h2o, co2, noble gases) or a mineral phase (e.g., anhydrite or pyrrhotite for s-bearing magmas). the solubility of cl is a more complex and ambiguous term since the silicate melt can be saturated with respect to mineral, liquid and fluid phases (e.g. halite, pure liquid nacl or a cl-rich brine, cl2, hcl or cl-bearing aqueous fluids). throughout this paper, the term solubility is used to refer to the maximum amount of cl that can be dissolved in the silicate melt when it is saturated with an immiscible aqueous fluid + brine assemblage at a given set of conditions. the distribution of cl between a melt and an aqueous phase is generally expressed in the literature as nernst-type partition coefficients (dm/aq, cl, where m = melt, aq = aqueous fluid). however, only components that can vary independently in a phase or a system have a real distribution coefficient (candela and piccoli, 1995) and this is not strictly true for cl (although it would be true for components such as nacl, kcl, etc.; e.g., see candela and piccoli, 1995; frank et al., 2003). however, given the lack of knowledge concerning cl speciation in the fluid phase, a simple nernst-style partition coefficient is often used in discussion of melt-fluid partitioning of cl. 2. pressure and cl solubility the solubility of cl in phonolitic melts equilibrated with cl-bearing fluids over a range of pressures and fluid cl contents, as determined by signorelli and carroll (2000), is shown in fig. 2a,b. the melt composition used in these experiments is given in table i and experimental details are given in signorelli and fig. 2a,b. variation of the chlorine molality in the melt with the calculated chlorine molality in a single fluid phase (for brine-saturated conditions this is an «apparent molality» because the melt coexists with both cl-poor aqueous fluid and cl-rich brine). a) chlorine molality (cm, cl) of mb phonolitic melts versus cl molality at p=150 mpa, t=850°c. experiments carried out in the two-phase field (melt + aqueous fluid) are represented as open circles and those in the three-phase field (melt + aqueous fluid +hydrosaline brine) are represented as solid circles. b) summary of variation of the chlorine molality in the melt with the chlorine molality in a single fluid phase (experimental data points not shown). the vertical bars show the two-sigma standard deviations on analyses of chlorine in the silicate melt cm, cl (moles/kg) c m,cl (ppm)/35 450 (data from signorelli and carroll, 2000). b a 622 michael r. carroll carroll (2000); all of the experiments involved equilibration of silicate melt with (na, k) cl solutions of variable initial chloride molality and fluid compositions at the end of the experiments was calculated by mass-balance. these experiments may contain two types of fluid inclusions trapped in the quenched glass: type-i inclusions contain both a vapor bubble and a liquid phase, whereas type-ii fluid inclusions contain a vapor bubble, liquid and one or more daughter minerals (salts). the minerals contained in these inclusions are believed to be alkali chlorides, possibly with other chlorides (e.g., cacl2 and fecl2). only experiments from below 200 mpa are found to contain both typei and type-ii inclusions. experiments above 200 mpa contain only type-ii inclusions. the experiments carried out between 50 and 150 mpa display the common behavior of a positive linear correlation between cl in glass and in the fluid phase, when cl molality in the fluid phase is below ∼2m cl, and then a flat distribution where further increases in the total amount of cl in the system do not increase cl in the glass (fig. 2a). the experiments at 200 and 250 mpa show a parabolic increase in melt cl content with increasing cl in the fluid (i.e., the rate of increase in melt cl content slows in experiments with more cl-rich fluids). figure 2b shows the isobaric trends for all of the data superimposed on the same diagram (t = 850875°c). this comparison clearly shows that as pressure increases there is a decrease in the slope of the straight line (melt coexists with single fluid of variable cl content; positive slope) and a shift of the pivot point between the straight line and the plateau (melt coexists with both aqueous fluid and brine) towards more clrich fluids until the sharp break of slope between them turns into a smooth passage betable i. composition (wt%) on water free basis of experimental starting materials. sample montaña vesuvius phlegraean vesuvius vulsini zikwala seite synthetic blanca fields cidades phonolite mb ves ci vs o er sm d sio2 59.5 57.3 62.1 62.9 59.9 68.3 64.6 58.7 tio2 0.66 0.19 0.44 0.37 0.55 0.53 0.63 al2o3 19.0 21.7 18.60 18.00 17.9 13.8 16.9 24.5 feo* 3.34 2.23 3.30 3.07 4.00 5.81 3.00 mno 0.2 0.13 0.24 0.16 0.12 0.28 0.20 mgo 0.33 0.23 0.35 0.46 1.06 0.16 0.35 cao 0.79 2.94 1.77 2.68 4.34 0.75 0.63 na2o 10.09 5.68 5.64 4.09 2.87 5.90 7.79 11.30 k2o 5.56 9.53 7.32 8.10 8.78 4.11 5.63 5.50 p2o5 0.07 0.04 0.02 0.04 0.17 0.06 0.07 cl 0.35 0.55 0.59 0.55 0.16 0.08 0.21 f 0.09 0.4 0.34 0.19 0.37 0.10 0.17 loi 0.45 3.10 2.50 1.40 2.60 0.57 0.20 [2] ai 1.2 0.91 0.92 0.86 0.79 1.03 1.12 1.0 [3] a/cnk 0.8 0.87 0.91 0.88 0.81 0.89 0.84 1.0 major elements, cl and f for mb have been determined by xrf. feo* = total fe as feo; for ves analysis cl and f determined by potenziometry; loi = loss on ignition; [2] agpaitic index = [(na2o+k2o)/al2o3] molar ratio; [3] molar [(al2o3/cao+na2o+k2o)] ratio (see for details signorelli and carroll, 2000, 2002); alkali exchange and partial crystallization in some experiments resulted in slightly different experimental melt compositions, as documented in signorelli and carroll (2000, 2002). 623 chlorine solubility in evolved alkaline magmas tween 200 and 250 mpa. these results indicate that the melt cl content decreases as pressure increases when the melt is saturated with both an aqueous fluid phase and a brine phase, as shown in fig. 3 for 2 phonolitic compositions (vesuvius 79 a.d. and montaña blanca, teneriffe; supercritical values taken from solubility at 2 molal cl in fluid). the lack of change in melt cl contents for the mb phonolite at pressures below 100 ma is probably due to the fact that this composition begins to crystallize at the lowest pressures investigated and the changes in melt composition result in decreased cl solubilities, as predicted by the model of webster and de vivo (2002) and discussed further below. the cl contents of the brine and aqueous fluid phases coexisting with these melts at any pressure (below the top of the miscibility gap) can be estimated from the h2o-nacl binary fig. 3. chlorine solubility in phonolitic melts saturated with aqueous fluid+brine versus pressure. solid circles and open squares represent mb (montaña blanca, teneriffe) and vesuvius phonolitic glasses (±1σ standard deviation), respectively (data from signorelli and carroll, 2000). fig. 4. chlorine solubility (±1σ standard deviation) in trachytic (signorelli and carroll, 2002) and phonolitic melts (signorelli and carroll, 2000) saturated with aqueous fluid+brine versus pressure. «this study» refers to signorelli and carroll (2002). original melt compositions are given in table i and «rev» refers to reversal experiments. 624 michael r. carroll shown in fig. 1. as indicated in fig. 1, the brine phase actually becomes more cl-rich as pressure decreases and this may explain why the melt also becomes more cl-rich as pressure decreases – if the cl activity in the brine increases as pressure decreases, then the cl activity in the coexisting melt also increases as pressure decreases (see also thermodynamic-molar volumes discussion in signorelli and carroll, 2002). the effect of kcl (and other chlorides) on the field of immiscibility has not been considered due to lack of data but studies at lower temperature (400-600°c) suggest that the addition of kcl tends to enlarge the field of immiscibility but does not change the general geometry of the binary (anderko and pitzer, 1993b). cl solubility in trachytic melts saturated with both an aqueous fluid phase and a brine phase shows variation with pressure very similar to that observed for phonolitic melts, as shown for a variety of trachytic melts in fig. 4 (data from signorelli and carroll, 2002). the compositions of the trachytic melts studied are reported in table i and the variations in cl solubility with melt composition are discussed below. 3. melt composition and cl solubility experimental studies involving hydrous rhyolitic to phonolitic melts equilibrated with an aqueous vapor and a hydrosaline liquid have shown that the behavior of chlorine in the melt is a strong function of the melt composition and in general cl solubilities are higher in more si-poor melts (e.g., métrich and rutherford, 1992; carroll and webster, 1994 and references therein; webster et al., 1999; signorelli and carroll, 2001, 2002; webster and de vivo, 2002). métrich and rutherford (1992) found that the peraluminous-peralkaline character of a silicate melt, expressed as molar ratio of na2o+k2o/al2o3 (= agpaitic index, ai), has a large effect on cl solubility in hydrous rhyolitic melts, as shown in fig. 5. a change of melt composition from metaluminous rhyolite to peralkaline pantellerite yields a more than 3fold increase in cl solubility (fig. 5). on the other hand, signorelli and carroll (2001), working with a peraluminous rhyolitic composition, found that cl solubility is higher in peraluminous rhyolites than in metaluminous rhyolites and the increase of cl solubility with peraluminosity is possibly even more pronounced than the changes observed with increasing peralkalinity (e.g., at 100 mpa, brine + + aqueous fluid saturation, peraluminous rhyolite with ai=0.8 has 0.64 wt% cl). these results suggest a minimum (∼ 3000 ppm) in cl solubility in metaluminous rhyolites and higher cl solubilities in both peralkaline and peraluminous compositions. cl solubility in hydrous phonolites also depends on peralkalinity of the melt (signorelli and carroll, 2000) and the results of signorelli and carroll (2002) for a close-to-metaluminous phonolite (fig. 6) suggest a possible minimum (∼ 0.48 wt%) in cl solubility in metaluminous phonolites. the higher cl solubility in metaluminous phonolite with respect to metaluminous rhyolite is probably due to the lower silica activity of phonolitic melts, as suggested by various previous studies that document increasing cl solubility with decreasing silica activity (kotlova et al., 1960; iwasaki and katsura, 1967; webster, 1997a,b; webster and rebbert, 1998; webster et al., 1999; webster and de vivo, 2002); note that decreasing silica activity is also correlated with increasing activities of network-modifying cations, as discussed further below. in fig. fig. 5. solubility of chlorine at 100 mpa as a function of molar (na+k)/al ratio for natural rhyolites and phonolites saturated with an aqueous fluid + hydrosaline brine (data from metrich and rutherford, 1992; and signorelli and carroll, 2000). 625 chlorine solubility in evolved alkaline magmas 6 the results of signorelli and carroll (2002) for trachytic compositions with variable alkali/alumina are also shown. the most evident difference between the trachytic and phonolitic melts is that the trachytes don’t show any minimum and cl solubility is constant at approximately 0.67 wt% over the range of agpaitic index investigated (0.9-1.08). apparently, the peraluminous-peralkaline character of the trachytic melt has little influence on cl solubility, in contrast to the significant variations seen in phonolitic and rhyolitic compositions. a possible explanation of the lack of variation in cl solubility with peralkalinity comes from the recent study of webster and de vivo (2002) concerning the effects of melt composition on cl solubility in a wide range of silicate melt compositions (41 different compositions, all at 200 mpa, 786-1193°c). webster and de vivo (2002) modelled the effects of melt composition on brine-saturated melts containing up to 3.2 wt% h2o using the idea of association coefficients that express the effect of the molar abundance of each melt component (cations + f −) on cl solubility (positive association coefficients indicate a component increases cl solubility, negative values indicate components that reduce cl solubility). they found that the affinity of individual cations for cl decrease in the order mg ∼ ca > fe > na > k > network-forming al >li∼rb∼cs; f has a positive effect on cl solubility. however, the relation between composition and solubility is more complex in peralkaline and peraluminous melts (because of possible variations in the structural role of al), and the effects of ca, mg and al are greater in mafic melts than in more silica-rich melts. details of this model are explained in webster and de vivo (2002) and the good agreement between their model predictions and measured cl solubilities at 200 mpa for a wide range of melt compositions is shown in fig. 7. the fact that fig. 6. solubility of chlorine (± 1 standard deviation) at 100 mpa, 850-875°c as a function of molar [(na+k)/al] ratio for natural trachytes (open symbols, from signorelli and carroll, 2002), and natural and synthetic phonolites (filled symbols, from signorelli and carroll, 2000) saturated with an aqueous fluid + hydrosaline liquid. 626 michael r. carroll mg, ca and fe have significantly greater effects on cl solubilities than do k and na may help explain why the cl solubilities in trachytes studied by signorelli and carroll (2002) appear to be independent of alkali/alumina ratio. following the results of webster and de vivo (2002), the higher abundances of mg, ca and fe in the trachytes overwhelm the effects of varying alkali/alumina, thus there is no significant variation in cl solubility; such effects might be more apparent in more extremely peralkaline or peraluminous trachytes. extrapolation of the results of webster and de vivo (2002) to other pressures, and consideration of the effects of melt h2o content on cl solubility still need to be investigated. the use of association coefficients to quantify how melt composition variations affect cl solubilities appears promising, but the approach is not completely general because numerous additional corrections are needed depending on the types of melts considered (e.g., peralkaline, peraluminous – see appendix in webster and de vivo, 2002). 4. volcanological implications all studies of cl solubility in hydrous rhyolites, phonolites and trachytes in the presence of subcritical (na, k) cl-h2o fluids have shown that cl solubility decreases regularly as the pressures rises. the higher cl solubility in phonolites and trachytes with respect to rhyolites, however, suggests that evolved alkaline magmas can transport larger amounts of chlorine to the earth’s surface. this reservoir of chlorine may be potentially emitted to the atmosphere if significant cl is lost during cooling, crystallization and degassing of volcanic products. the composition of high-temperature gases released from volcanoes depends on factors such as gas solubility in the silicate melt and vapor/melt partitioning and separation during the generation and rise of magmas. giggenbach (1996) has proposed a model for equilibrium degassing (i.e., closed-system degassing) that takes into account these factors. however, this model requires that volatile solubilities approach henry’s law behavior and that the solubilities decrease with decreasing pressure (as observed for h2o, co2, noble gases). the negative correlation of melt cl concentration with pressure means that cl solubility behavior in brine-saturated magmas cannot be treated as simply as other volatile species, at least when high melt cl contents suggest brine saturation. the solubility of cl may also be complicated by the formation of non-volatile alkali chlorides and hcl° at low pressure. alternatively, various authors have convincingly demonstrated that the potential distribution of cl in magmatic fluids depends on the shape of the immiscibility solvus of nacl-h2o system (or a similar «salt»-h2o system) at low pressures and magmatic temperatures (e.g., hedenquist and lowenstern, 1994; shinohara, 1994; hedenquist, 1995). during decompression and/or crystallisation of a magma at low (< 200 mpa) pressures it may befig. 7. correlation between measured cl solubilities and those predicted by the model of webster and de vivo (2002). all melts are saturated with a clrich brine at 200 mpa pressure and temperatures of 786-1193°c. filled symbols show experiments used to generate the model association coefficients and open symbols show melt compositions for which the cl solubility was calculated using the model association coefficients. typical 1-σ analytical precision for measured cl contents are on the order of 0.05 to 0.1 wt%. 627 chlorine solubility in evolved alkaline magmas come saturated with both a low-density vapor and a high-density brine (first and/or second boiling; see webster and de vivo, 2002). as described by hedenquist and lowenstern (1994), the low density phase will contain components such as h2o, co2, so2, h2s, hcl and the higher density brine phase will contain alkali salts, metals, water and sulphates. the shape of solvus in the nacl-h2o binary (fig. 1) predicts that upon further isothermal decompression, the vapor and the brine will progressively become less and more saline, respectively. once information on cl solubility in silicate melts is available, indirect evidence for immiscibility between silicate melts and h2o-nacl fluids is provided by the study of cl concentrations in glass inclusions present in magmatic phenocrysts. these inclusions are small samples of silicate melt trapped during crystal growth and they represent the most accessible samples of non-degassed magma (e.g., roedder, 1984; lowenstern, 1995, and references therein; signorelli et al., 2001; webster and de vivo, 2002). alternatively, matrix glasses from rapidly erupted and cooled pumices (e.g., from plinian eruptions) may provide useful information on some volatiles (e.g., gardner et al., 1998; signorelli and capaccioni, 1999; signorelli et al., 2001). for example, data collected from the literature on cl contents of phonolitic glass inclusions and matrix glasses are presented in fig. 8. the content of cl in these evolved alkaline melts is quite variable, ranging from 0.18 to 1.03 wt%. the highest abundance of cl occur in volcanic products from vesuvius (1.03 wt%), whereas the lowest pertain to laacher see (0.18 wt%), montaña blanca (∼ 0.35 wt%) and tambora (~ 0.2 wt%). provided that the magma was fluid saturated and the magmatic reservoir was located at pressures below about 200 mpa, the similarity of natural and experimental data for vesuvius suggests that most vesuvian phonolitic melts were saturated with respect to a hydrosaline liquid (brine). if these assumptions are also valid for laacher see, montaña blanca and tambora, the comparison between natural and experimental data suggests that these systems could not have been saturated with a hydrosaline brine prior to eruption. the comparison of cl in glass inclusions and degassed matrix glasses, when possible, suggests that cl may be retained in the silicate melts during the plinian phase of phonolitic eruptions (cl degassing is slow on the eruption timescale). this is not the case of the laacher see and tambora eruption where chlorine has apparently shown a volatile behavior, with cl content in matrix glasses lower than in glass inclusions (devine et al., 1984; harms and schmincke, 2000). there is a sharp gap between the field observation and the laboratory experiment concerning the behavior of chlorine during syn-eruptive decompression. recently, gardner et al. (1998) have shown that at fast decompression rates, cl cannot diffuse out efficiently, although water can. on the contrary, when the erupted material cools more slowly cl is lost, presumably as a result of groundmass crystallization. the observed variations in cl solubility with pressure for brine-saturated melts may be used to estimate pre-eruptive pressures of the most evolved magmas produced during the campanian ignimbrite (ci) eruption (signorelli et al., 2001), assuming that the system was saturated with both brine and aqueous fluid. as shown in fig. 9, experiments at 25 to 200 mpa using an fig. 8. histogram of chlorine content of glass inclusions and matrix glasses from phonolites. the data are from the following volcanic systems: vesuvius (cornell and sigurdsson, 1987; mues-schumacher, 1994 and unpubl.; cioni et al., 1995, 1998; signorelli and capaccioni, 1999; signorelli et al., 1999), tambora (devine et al., 1984), montaña blanca (signorelli, unpubl.) and laacher see (harms and schmincke, 2000). 628 michael r. carroll evolved trachyte from the ci eruption show that the high cl contents of these magmas correspond to minimum pre-eruptive pressures of 50-100 mpa, or depths of only 2-4 km, if the magmas were saturated with both an aqueous fluid phase and a brine prior to eruption, and neglecting possible effects of other components such as co2 (signorelli et al., 2001). in fact, the high cl contents of these magmas can only be explained if the magmas were brine-saturated prior to eruption. higher pressures could be possible if the magmas coexisted with only a brine and not an aqueous fluid phase but preliminary data on water fugacities calculated from the compositions of coexisting biotite-magnetite-sanidine present in the ci rocks also suggest water fugacities of 70-130 mpa, consistent with a relatively shallow, vapor-saturated system (fabbrizio and carroll, unpubl.). 5. conclusions experimental data concerning chlorine solubility in phonolitic to trachytic melts and vapor/melt and hydrosaline brine/melt partitioning of cl under magmatic conditions provide insights concerning the geochemical behavior of cl in evolved alkaline melts. the results summarized in this study are directly applicable to volcano monitoring by allowing better interpretation of the origins of variations in measured halogen emissions from alkaline magmatic systems. in more general terms, the experimental data on cl solubility as a function of magma composition and pressure will provide better constraints on the volcanic degassing of cl and help in the identification of eruptive products preserved in the geologic record which may have been associated with large cl emissions. this will aid studies seeking to correlate the record of climatic variability with the record of volcanic eruptions and will allow better estimation of past volcanic input to the atmosphere. at pressures and temperatures outside the vapor-brine immiscibility field of the nacl-h2o system, the concentration of cl in alkaline melts depends on the activity of cl in the fluid phase. under conditions within the vapor-brine immiscibility field, the concentration of cl in melt is fig. 9. chlorine solubility in trachytic melts saturated with aqueous fluid+brine versus pressure (mpa). solid and open circles represent original and water-free values (± 1σ standard deviation), respectively. shaded areas represent the cl distribution in matrix glasses (most-evolved trachytic magma) and glass inclusions (leastevolved trachytic magma). the arrows indicate the inferred depth of the ci magma chamber (from signorelli et al., 2001). 629 chlorine solubility in evolved alkaline magmas constant and is buffered by the coexistence of aqueous vapor and hydrosaline brine. cl partitioning between hydrosaline liquid and melt and cl solubility exhibit a strong negative dependence on the pressure, whereas cl partitioning between aqueous vapor and melt shows a positive correlation with pressure. these features may be attributed to the shape of the immiscibility field of the nacl-h2o system. when compared with experimental results on rhyolites, the solubility of cl in phonolites and trachytes is approximately twice as high under similar conditions. these results demonstrate that cl solubility in hydrous evolved melts is a strong function of melt composition and higher solubilities can be expected in alkali-rich, low silica activity compositions. the strong dependence of cl solubility on pressure in magmas saturated with aqueous fluid + brine means that melt cl contents might be useful for estimating minimum pre-eruptive pressures, as illustrated for the most evolved (and presumably highest-level) compositions observed in the campanian ignimbrite eruption. additional work is still needed to address the effects of melt composition on cl solubility over a range of pressures and temperatures and to quantify the effects of other volatile components (e.g., f, co2) on cl solubility. the 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(1995): copper and hcl in felsic magmatic systems, ph.d. thesis (university of maryland). t h e e n e r g y o f g r a v i t a t i o n a l d i f f e r e n t i a t i o n o f t h e e a r t h ' s m a n t l e (*) e . n . l y u s t i k h ricevuto il 7 d i c e m b r e i960 1 . i n t r o d u c t i o n . tn general, the e a r t h ' s crust consists of sialic rocks, (i.e. acidic and basic igneous and metamorphic rocks). i t is suggested by many authors (kropotkin 1953, belousov 1954, wilson 1957) t h a t the crust grows gradually on account of permanent income of sial from the e a r t h ' s mantle, which consists of simatic or ultrabasic rocks. (sial is generated due to the differentiation of the mantle: it is lifted up by buoyancy force and emerges in geosynclines). the hypothesis may be proposed t h a t the floating up sial provides with energy all the tectonic processes. thus the question arises whether the output of energy can be sufficient. we may compute all the values to within an order only, since the necessary amount of energy is hardly known even to such exactness. 2 . e n e r g y w h i c h can b e r e l e a s e d . we may assume a flat earth for our computations. the volume of the crust is 1025 cm3 today (poldervaart 1955). the beginning of the forming of the crust may be 3 . 5 x 1 0 ° years ago, as the radioactive ages of rocks have shown. this gives the mean output of emerging sial 3 x 1015 cm3/year. i t is believed t h a t differentiation goes in the upper 700 k m of the mantle only. accepting this supposition we assume the mean depth, sial emerges from, to be as great as 350 km, i.e. 3.5 x10' cm. (*) p a p e r read a t t h e ilelsinky assembly of t h e i . u . g . g . i960. 1 7 0 e . n . l t u s t i k h the density difference between sima and sial is 0.5 g/cm3, or so. gravity in the m a n t l e is 1000 gals to within 2-3 per cent. taking these values we have. e = d(j iiv = 5 x1025 e r g / y e a r , where: e = mean o u t p u t of energy, d — density difference, (j = gravity, h = mean depth sial emerges from v = mean o u t p u t of sial the p a r t of the mantle undergoing differentiation loses light components and so grows heavier. a t last it m a y plunge down and sink to the b o t t o m of the mantle, provided the whole m a n t l e is chemically homogeneous. fresh m a t t e r will t a k e its place, and differentiation m a y be resumed. f o r such " chemical convenction " we have to substitute 2.5 x 10s cm for h. so we get energy o u t p u t e — 4 x 102° erg/year, i. e. one order greater t h a n from the differentiation of t h e upper p a r t of t h e mantle. not all t h e energy released b y the differentiation m a y be useful for tectonic movements. an allowance m u s t be made for the efficiency of the transmission mechanism. the transmission m a y be very effective if sial ascends as a continuous sheet along a planetary r u p t u r e (we suppose eveiy geosyncline is generated b y a great " planetary " inclined r u p t u r e cutting the upper p a r t of t h e mantle). on the contrary, if m a n y isolated balls of sial emerge up through the mantle, t h e efficiency m a y be much less. we estimate presumable the efficiency /? in t h e range 1 > fi > 0.01 , then t h e possible contribution of energy to tectonics t — fie have limits 1027 > j ' > 10 23 e r g / y e a r . 3 . n e c e s s a r y a m o u n t o f e n e r g y . the only way to estimate the a m o u n t of energy necessary for tectonic processes is to s t a r t from t h e average annual energy of all t h e seismic waves on the e a r t h , since in is t h e only geotectonic energy meat h e e n e r g y of g r a v i t a t i o n a l d i f f e r e n t i a t i o n 1 7 1 surable. during 1890-1955 the mean power of seismic waves was 8.4 x1024 erg/year, as it follows from the d a t a of b. gutenberg (1956). i t is doubtful whether this figure m a y be representative for t h e whole period 3 . 5 x 1 0 " years. to be cautious we estimate the seismic energy z in t h e range 10" > z > 1023 erg/year . only a p a r t of energy released in foci t u r n s into wave energy; the another p a r t is immediately converted into heat. taking this fact into considerations we assume for the whole o u t p u t of energy in foci z the limits 1020 > z > 1023 e r g / y e a r . the power of all tectonic processes t m a y be 10 to 1000 times as great as the power of seismic activity z, so 1031 > t > 1024 e r g / y e a r . 4 . c o n c l u s i o n . as you have seen above, the tectonic power needed and t h e tectonic power available m a y coincide in the range 1027 > t > 1024 erg/year. thus t h e mechanism of differentiation can provide geotectonics with energy if the efficiency of this mechanism is high enough, and t h e required amount of energy is not much greater t h a n t h e present energy of seismic waves. summary the earth's crust has been forming owing to the differentiation of the mantle, «sial» being lifted by the forces due to gravity. as a result, the mean energy supply for tectonics might be as great as 10-3-1027 erg /year. we estimate the energy of all tectonic processes at 102i-1031 erg /year. so it may be suggested that the mechanism of differentiation provide geotectonics with energy. 172 e . n . l y u s t i k h riassunto la crosta terrestre si e andata formando a causa della differenziazione del mantello, dato che il « sial » viene sollevato dalle forze dovute alia gravita. di conseguenza il contributo medio di energia per la tettonica potrebbe raggiungere i 10'a-10'" erg/anno. noi calcoliamo venergia di tutti i processi tettonici a lo^-lo31 erg/anno. si pud cost ritenere che il meccanismo della differenziazione fornisea venergia alia geotettonica. r e f e r e n c e belousov v . v . , (eejioycob b. b . ) oaiomibie eonpocti eeomenmoimkn. « tocreojitexn3flat » , mockba ( 1 9 5 4 ) . g u t e n b e r g b . , tr. agtj, 3 7 , n . 5 , ( 1 9 5 6 ) . k r o p o t k i n p . n., (kponotkhh n . h . ) h3eecmuh ah cccp, cepnj[ reojionm., 1 ( 1 9 5 3 ) . lyustikh e . n . , (jiiocthx e. h . ) pheecmun ah cccp, cephh rco())h3hh., 3 ( 1 9 6 0 ) . p o l d e r v a a r t a . , qeol. soc. am., spec. paper 6 2 , ( 1 9 5 5 ) . w i l s o n j . t u z o , nature, 1 7 9 , n o . 4 5 5 3 , ( 1 9 5 7 ) . layout 6 annals of geophysics, 61,1, se108, 2018; doi: 10.4401/ag-7443 earthquake mechanism and characterization of seismogenic zones in south-eastern part of romania mircea radulian1,2,*, andrei bălă1, emilia popescu1, dragos toma-dănilă1 1 national institute for earth physics, măgurele, romania 2 academy of romanian scientists, bucharest, romania article history received may 6, 2017; accepted january 22, 2018. subject classification: earthquake mechanism; seismogenic zone; fault plane; solution diagram. se108 abstract earthquake mechanism information is fundamental to determine the stress field and to define seismogenic zones. at the same time, it is a basic input to compute seismic hazard by deterministic approach. the present paper extends the catalogue of the fault plane solutions for the earthquakes in romania, previously completed until 1997, for 1998 – 2012 time interval. the catalogue is limited geographically to the carpathians orogeny and extra-carpathians area located in the southeastern part of romania because similar investigations cover the rest of the country. the catalogue comprises 259 intermediate-depth seismic events and 90 crustal seismic events, recorded in the considered time interval with acceptably constrained fault plane solutions (minimum 10 reliable polarities, small ratio of rejected polarities versus input polarities and non-zero focal depth). we use specific graphical tools in order to emphasize statistically representative features of the stress field as coming out from our results. the fault plane solutions of the vrancea earthquakes generated in a confined sinking plate in the mantle reflect the dominant geodynamic process in the study region. the typical features revealed by all the previous studies on the subcrustal seismic activity (predominant dip-slip, reverse faulting, characterizing both the weak and strong earthquakes) are reproduced as well by our investigation. as concerns the earthquake activity in the crust, a few new refined aspects are highlighted in the present work: (1) a deficit of the strike-slip component over the entire carpathians foredeep area, (2) different stress field pattern in the făgăraş – câmpulung zone as compared with the moesian platform and pre-dobrogean and bârlad depressions, (3) a larger range for the dip angle of the nodal planes in the vrancea subcrustal source, ~ 40° -70° against ~ 70°, as commonly considered. 1. introduction the present paper is an extension of previous studies dealing with the focal mechanism and related stress characteristics for the earthquakes recorded in romania [e.g., radulian et al. 1999, 2002, bala et al. 2003]. thus, the catalogue of the fault plane solutions built up until 1997 is updated and expanded for the time interval 1998 2012 and subsequently analyzed in correlation with specific cluster of events and active faults. we limit geographically our data set to the carpathians orogeny and extra-carpathians area located in the south-eastern part of romania. the seismogenic zones located in the study area, as defined by radulian et al. [2000] and slightly changed here, are (see figure 1): moesian platform (mo), pre-dobrogean depression (pd), barlad depression (bd), fagaras campulung area (fc) and vrancea intermediate-depth source (vni). similar investigations were carried out by oros et al. (2008a, 2008b), focused on the seismogenic zones located in the western part of romania: danubian area (da) and banat area (ba) (see inset of figure 1). their results (140 earthquakes mechanisms) can be considered as a complement to our work in order to characterize the earthquake mechanism data for the entire romania. an area that remains still uncovered includes the transylvanian depression (td) and crisana maramures (cm) region (north-western romania) where we could not get enough data to compute reliable fault plane solutions. the analysis carried out by sandu and zaicenco [2008] for the time interval 1967 2006 covers roughly the same geographical area (lat. 44° 50°n, lon. 24° 30°e). sandu and zaicenco analyzed 10 different catalogues, identified differences between them and compiled a unified catalogue of the fault plane solutions using the polarities of the p waves reported in isc bulletin as primary data. in the common time interval covered by the two investigations (1998 2006) the number of fault plane solutions in sandu and zaicenco [2008] is significantly smaller than in the present analysis: solutions for 30 events (29 of vrancea intermediate-depth source and a single crustal event) compared to 358 events (out of which 94 crustal events). since their work is based on the isc data, the fault plane solutions are available only for larger events (i.e., mw > 3.6), while the most of the crustal earthquakes considered in this paper are below this magnitude. also, the polarity accuracy is likely to be worse when using isc data, as proved by the greater proportion of inconsistent polarities. the earthquake mechanisms are computed in all cases using the seisan algorithm [havskov and ottemöller 2001] and the polarities of the first p-wave arrivals. the catalogue of fault plane solutions, presented in the appendix a and accessible on line at www.infp.ro, comprises 264 intermediate-depth seismic events and 94 crustal seismic events, recorded in the time interval 1998 2012 (see also table 1). we selected only the solutions with minimum 10 reliable polarities and acceptable stations coverage and with non-zero focal depth: 259 intermediate-depth + 86 crustal events. the location of the events is presented in figure 1. most of the study crustal earthquakes belong to the background seismicity, with magnitudes mw below 4, except the event of 3.10.2004 in the northern dobrogea with mw = 4.9. almost three thirds of earthquakes occurred in the vrancea region (vri) in the mantle range (64 to 167 km depth, table 1). most of them have magnitudes mw below 5, with only 5 earthquakes with magnitudes mw ≥ 5. the largest earthquake was recorded on 27.10.2006 with mw = 6. the crustal earthquakes follow closely the areal distribution of the seismogenic zones, which were set on the basis of seismicity trends as resulting from the entire catalogue of earthquakes in romania and on the basis of geotectonic grounds. histograms of the earthquakes in the vrancea subcrustal region and of the earthquakes in the adjacent crustal region on depth and size are plotted in the figure 2. the number of events are computed on bins of 5/10 km on depth for crustal/subrustal events and 0.4 on magnitude. 2. seismicity distribution seismicity in romania is concentrated at the carpathians arc bend in the vrancea region. here, an isolated lithospheric slab downgoing in the mantle is permanently releasing seismic energy in an extreme narrow volume. in average, three earthquakes with radulian et al. 2 figure 1. tectonic map of the south-eastern part of romania [after sandulescu 1984] and the epicentre location for the earthquakes considered in this paper [in agreement with the romplus catalogue, oncescu et al. 1999, updated on http://www.infp.ro/romplus]. the seismogenic zones for the entire romania are plotted in the top-left inset. 3 earthquake mechanism in south-eastern romania table 1. number of earthquakes recorded between 1998 and 2012, with reliable fault plane solutions, associated with the study seismogenic zones. seismogenic zone no. events mw depth [km] no. stations moesian platform – mo 39 2.3 – 3.3 2 – 38 10 51 pre-dobrogean depression & barlad depression pd-bd 29 2.3 – 4.9 0 – 46 10 25 fagaras – campulung area – fc 18 2.3 – 3.3 0 – 25 10 34 vrancea intermediate-depth source vni 259 2.7 – 6.0 64 – 167 10 60 figure 2. a) histograms of the earthquakes of vni zone (264 events) on depth and magnitude; b) histograms of the crustal earthquakes (94 events) on depth and magnitude magnitude above 7 were reported each century for a time span of six centuries. the origin of intermediate-depth seismicity in the vrancea area is still an ongoing debate. the lithospheric volume which is seismically active can be approximated by a prism vertically oriented between 60 and 170 km depth, with a horizontal cross section of 30 x 70 km2. above 60 km and below 170 km the seismicity is suddenly cut off, although the high-velocity body, as determined by seismic tomography, extends notably beyond these limits [martin et al. 2006, ren et al. 2012]. the seismic activity in the crust is dispersed over the carpathians orogeny and foreland with significant enhancements in several seismogenic zones, as defined first by radulian et al. [2000]: ev the area in the front of the south-eastern carpathians arc bend, fc the făgăraş mountains zone which is prolonging across carpathians towards campulung city, pd-bd pre-dobrogean and barlad depression zones, which belong to the same tectonic unit (scythian platform), ba banat zone, da danubian zone and cm crişana maramureş zone. the crustal earthquakes are commonly small to moderate (mw < 6). only in the fc zone a few shocks of magnitude above 6 (mmax = 6.5) were reported in the romanian catalogue, in about one millennium time interval [oncescu et al. 1999]. the crustal seismicity is generally associated with the basement fracture systems [bala et al. 2015]. in the figure 1 we focus our attention on the seismogenic zones located in the south-eastern part of romania. the positioning of the other seismic areas is represented in the figure 1 after radulian et al. [2000]. given that tectonically and geologically the seismogenic areas situated in front of the carpathians arc, south of the peceneaga-camena fault in the moesian platform, do not differ notably, we prefer to consider in our subsequent analysis a single area (mo) in this region. in the same way, we combined the pre-dobrogean depression and bârlad depression zones into a single area (pd-bd). the vrancea zone (vni) and făgăraş-câmpulung zone (fc) are the same as defined in radulian et al. [2000]. we slightly adjusted the pre-dobrogean zone, which was extended to the north-west in order to cover also the north dobrogea orogeny and to have a common margin with barlad depression zone and to be extended over the southern flank of sf. gheorghe fault (figure 1). the adjustments relative to the previously defined seismogenic zones are made to include all the earthquakes in the catalogue and to keep at the same time their adherence to a specific tectonic province. the eastern sector of the moesian platform, located between the intramoesian fault and peceneagacamena fault, is more seismically active as compared with the sector located west of the intramoesian fault, which is almost aseismic. the seismicity concentrates close to the carpathians arc bend, overlapping to some extend the epicentral area of the vrancea subcrustal earthquakes. quite frequently, the earthquakes are generated in moderate size sequences. typical sequences were recorded in the focşani râmnicu sărat and vrancioaia areas [popescu and radulian 2001, popescu et al. 2012]. many times, the earthquake sequences in front of the carpathians arc bend display alignments parallel to the orogen, probably in connection to a system of buried faults beneath the focşani sedimentary basin [hauser et al. 2001, răileanu et al. 2005]. the intramoesian fault crosses the moesian platform on the se-nw direction separating two distinct sectors with different constitution and structure of the basement. although it is a major fault, which extends from the continental platform of the black sea to the nw under the getic nappe [sandulescu 1984] and which is supposed to reach in depth the lithosphere base [enescu 1992], the associated seismicity is weak and scarce. a deficit in seismicity is noticed to the west of the intramoesian fault which extends over the entire western sector of the moesian platform until it comes into contact with the southern carpathians in the danubian region (da in figure 1). for the seismogenic zones located in the western part of romania, see oros et al. [2008a and 2008b]. the peceneaga-camena fault separates the moesian platform unit from the north dobrogean orogen. the seismic activity to the northern side of the peceneaga-camena fault cover roughly three tectonic units: north dobrogean orogen; pre-dobrogean depression and bârlad depression. the pre-dobrogean depression lies to the north north dobrogean orogen, being separated from it by sf. gheorghe fault. the pre-dobrogean depression is continuing to the north-west into bârlad depression, which is north of trotuş fault [figure 1, after sandulescu, 1984]. the bârlad depression is a subsiding depression on the scythian platform in contact to the north with the southern edging of the east european platform (moldavian platform). as was also noted by other studies [radulian et al. 2002], the seismicity and focal mechanism features in the pd and bd zones are similar, and therefore, we can merge them in a single seismic active area (pd-bd), which includes also the north dobrogean orogen. the făgăraş-câmpulung area is the eastern seismic active segment of the southern carpathians. it is charradulian et al. 4 5 earthquake mechanism in south-eastern romania acterized by strong shocks that reached up to mw ~ 6,5 (the maximum magnitude recorded in romania for crustal earthquakes). the last major seismic event was recorded on 26 january 1916 (mw = 6.4) and it was followed by a significant aftershock activity which lasted several weeks [atanasiu 1961]. the earthquakes generated in the south-eastern corner of this zone could be eventually associated with the north-western edge of the intramoesian fault. a large sequence started on 4 may 1993 in the area and lasted through the entire year [maximum magnitude mw 4.7, estimated using the seismic moment computed by enescu et al., 1996]. the events occurred since 1993 to the present did not exceed magnitude 3.5 [oncescu et al. 1999]. 3. fault plane solutions we follow the convention of aki and richards [1980] to define the nodal plane parameters (strike, dip and rake). the hanging-wall block of the fault is always located to the right, and the footwall block to the left. that means that fault dips always to the right side of the trace when moving along the trace in the strike direction and the rake (which gives the slip direction) is defined as the movement of the hanging wall relative to the footwall block. rake is measured on the fault plane of the fault relative to the fault strike, ±180°. positive rake values indicate thrust or reverse motion on the fault (the hanging wall is moving up), while negative rake values indicate normal motion on the fault (the hanging wall moved down). the fault plane solutions are computed on the basis of the polarities of the first p-wave arrivals using the seisan algorithm [havskov and ottemöller 2001]. the regional velocity model used in the calculations is common for crustal and subcrustal earthquakes. it was estimated from local tomography data (calixto experiment) for vrancea region and surroundings as minimum 1 d model [popa et al. 2001]. it is in a good agreement with the s-velocity model of diehl et al. [2005] derived from the analysis of teleseismic recordings with the receiver function method of the 120 temporarily installed stations during calixto experiment together with the permanent romanian stations. the computed fault plane solutions are plotted in the figure 3. for a better graphical presentation/visualization, we kept for the vrancea subcrustal earthquakes the mechanism solutions only for the larger events (mw > 4.5), which are 22 subcrustal events out of 259 (table 1). several examples representative for crustal events located in the study seismogenic zones are plotted in the figure 4. we can draw some conclusions from a simple visual examination of the figure 3: prevalence of reverse faulting in the vrancea subcrustal source a tendency of the nodal planes for the vrancea subcrustal source to be oriented either parallel or perpendicular to the carpathians arc bend a large variety of fault plane solutions for the crustal events both as faulting type and nodal plane orientation these issues have also been highlighted by previous studies on focal mechanism characterization [see radulian 2014]. a more detailed inspection of the focal mechanisms will be presented in the following chapter. 4. focal mechanism uncertainties the fault plane solutions obtained in this paper are affected inevitably by errors. errors should be significant especially for smaller events due to several factors: greater noise influence, reduced number of reliable stations, larger location errors, and so on. our goal was to investigate particularly the statistical properties of the fault plane solutions and not to look into details on their consistency and reliability. for example, the output from focmec program [snoke et al. 1984] provides a family of possible solutions, given certain input specifications. the main criterium in selecting acceptable solutions is to minimize the number of polarities errors. however, focmec does not provide a quality rank of the members of a family of solutions, as in our case, only polarity data (and not amplitudes) are used. hence, we consider a solution is acceptable when the traces of possible solutions are located relatively close each other on the lower hemisphere. under these circumstances, we simply pick out one of the possible solutions as the ‘true’ solution. a reasonable measure of the solution uncertainty should be the area covered by the family of possible solutions (smaller area meaning better constraint; at limit, we should have a single solution), but, as proved by other studies, the problem of stability of focal mechanism solution is complex and nonlinear and can depend on many factors [see hardebeck and shearer 2002, for example]. thus, the errors in the assumed source location or the seismic-velocity model, which are not taken into account in focmec procedure, can alter the pattern of observations on the focal sphere and therefore can change the best-fitting focal-mechanism solution. so, we are aware that using only p-wave polarities does not assure the accuracy of the fault-plane solution even if a preferred solution relatively well constrained is obtained and mechanisms that are stable with respect to polarity errors may be unstable with respect to small changes in location or velocity model. radulian et al. 6 one way to solve this problem is that proposed by hardebeck and shearer [2002], who determined the stability of focal mechanism solutions by performing repeated trials with different possible event locations and seismic velocity models. in this approach, the preferred mechanism is taken to be the average of the set of acceptable mechanisms generated by each trial and only if the acceptable solutions are tightly clustered around the preferred mechanism the solution is considered stable and well constrained. in our case, the only criteria that we used included the ratio of rejected polarities versus input polarities, station distribution and sensitivity of fault-plane solutions to including/removing key stations. the velocity model by popa et al. [2001] was used in determination of takeoff angles for both vrancea intermediate-depth events figure 3. focal mechanisms for 85 crustal earthquakes (red circles) and 22 intermediate depth earthquakes with mw > 4.5 (blue circles) from vrancea intermediate-depth zone (vni). figure 4. examples of fault plane solutions for representative crustal events (2009/12/06 mw 3.3 mo region left, 2011/01/26 mw 3.2 bdpd region middle, 2010/10/25 mw 3.2 fc region right). 7 earthquake mechanism in south-eastern romania and crustal events. it is a regional model estimated from data recorded in the calixto tomography experiment for the vrancea area and around [wenzel et al. 1998]. in all cases, we consider only clear p-wave polarities (weight assigned to unity), either of emergent or impulsive character, in order to increase the chance of getting meaningful solutions. focal mechanisms can be reliably determined only for events with first-motion observations that adequately sample the focal sphere. the maximum gap in the source-to-station azimuth is required to be < 90°, and the maximum gap in takeoff angle is required to be < 60°, (i.e., the takeoff angles cannot be all near horizontal or all near-vertical). in the present approach we limit the selection criteria to (1) minimum 10 reliable polarities, (2) acceptable ratio of rejected polarities versus input polarities and (3) non-zero focal depth. applying these criteria to our database (appendix a), we remove 13 events assigned as less reliable solutions (lr index in the last colon of appendix a). we are aware that all these measures solve only part of the problem of uncertainties. duputel et al. [2012] stressed, among others, the importance of associating realistic error analyses with any source inversion. they showed that in order to get reliable solutions through inversion, the error estimation should be part of the solution. 5. statistical analysis one of our goals is to determine the main features of the parameters of the fault plane solutions computed in this paper and to compare the results with previous investigations. for this purpose, we used graphical tools able to emphasize statistically representative features in our data set for each seismogenic area considered in the study. in this way, simple polar diagrams were employed to describe strike and dip behavior of the events in each active area. in order to classify the type of faulting, we represent the distribution of the principal axes p, t and b using kaverina et al. [1996] projection, which improves the frohlich and apperson [1992] ternary diagram. a dedicated program is employed for this purpose, which is described in álvarez-gómez [2014 and 2015]. according to the program description [álvarez-gómez 2015], we can classify earthquakes in seven classes represented by specific rupture types: 1) normal; 2) normal strike-slip; 3) strike-slip normal; 4) strike-slip; 5) strike-slip reverse; 6) reverse strike-slip and 7) reverse (figure 5). 5.1 vrancea subcrustal source vni the polar diagrams for the azimuthal distribution of the two nodal planes (a and b) and for the plunge angle are plotted in the figure 6. note the predominance of nodal planes oriented ne-sw, parallel to the carpathians arc bend and the prevalence of plunge angles around 45°. it is worth mentioning that this geometry of the faulting system corresponds with the orientation of the rupture faults for the largest vrancea events [enescu 1980, oncescu 1987; oncescu and trifu 1987; radulian 2014]. taking into account the difference in scale between a moderate earthquake (mw = 4-5) and a major earthquake (mw > 7), it is not trivial such a feature. we may assume that the process of generating moderate earthquakes is controlled largely by the same tectonic forces as for generating the largest shocks. the analysis of the figure 6 reveals also a secondary tendency for the azimuthal orientation of the nodal planes, roughly perpendicular to the carpathians arc bend. this kind of focal mechanism is observed from time to time at moderate-to-large magnitudes (mw 5 to 7). it looks like the rupture orientation and length scale with the seismicity geometry (elongated along ne-sw, see figure 1): the rupture process propagates along ne-sw plane for most of the events, while for the smaller events propagate occasionally along a perpendicular direction (nw-se). similar plots are provided by bala et al. [2003] on the previous catalogue of romanian earthquake mechanism solutions [radulian et al. 2002], but their diagrams are represented on different depth segments (crustal and three subcrustal depth intervals). the diagrams for azimuth and dip angles only for mechanisms in vrancea area are reported also by sandu and zaicenko [2008] on a different catalogue assembled from 10 different international catalogs. the angular diafigure 5. classification diagram. n: normal; n-ss: normal strike-slip; ss-n: strike-slip normal; ss: strike-slip; ss-r: strikeslip reverse; r-ss: reverse strike-slip; r: reverse [after álvarezgómez 2015]. radulian et al. 8 figure 6. angular diagrams for azimuth and dip angles of the nodal planes vrancea subcrustal source (vni): a) first nodal plane; b) second nodal plane. figure 7. angular diagrams for azimuth and dip angles for plane a (a) and plane b (b) for the cmt fault plane solutions of the vrancea earthquakes with 4.7 ≤ mw ≤ 7. recorded between 1977 and 2013. figure 8. diagram for p, t and b principal axes distribution for vrancea zone (vni). a). all the events; b). only events with at least 30 reliable polarities; c). the largest vrancea events (cmt solutions). the dots are proportional to mw magnitude; to the right, the color scale represents the depths of the events in km. 9 earthquake mechanism in south-eastern romania grams for the largest vrancea earthquakes (mw ≥ 4.7) recorded between 1977 and 2013 based on the cmt fault-plane solutions (http://www.globalcmt.org/) are plotted in the figure 7. there are 20 events partly considered in the papers of ganas et al. [2010] and radulian [2014]. they show the same ne-sw tendency of fault plane orientation as for the entire data set of subcrustal earthquakes in figure 6. the distribution of the principal axes p, t and b, is represented in the figure 8 for the entire data set (259 vrancea intermediate-depth earthquakes) and for the fault plane solutions obtained with minimum 30 polarities (31 events) and for the cmt fault solutions (20 events). in order to classify the type of faulting we used the visualization procedure proposed by kaverina et al. [1996]. clearly, the reverse faulting is dominating in the vrancea subcrustal source, independently of depth or magnitude ranges. this is even more evident when plotting only the events with best constraint fault plane solutions (minimum 30 polarities) or the largest vrancea events. practically, none of the events has well-defined normal or strike-slip faulting; two events, located at the bottom edge of the descending seismically active body, are characterized by strike-slip with normal components, and four events by strike-slip with reverse components. whereas, most of the events have pure reverse faulting. 5.2 moesian platform seismogenic zone mo the earthquakes with computed focal mechanism that we consider to fit in the moesian seismogenic zone are spread over a wide area, both to the south-east and south of the vrancea region and covering all the eastern moesian platform (wallachian sector), from peceneaga camena fault to intramoesian fault (figures 1,2). most of them (36) are in connection with the vrancea seismogenic area, while a few of them (6) can be rather associated to the intramoesian fault area. the events are of small-to-moderate magnitude (mw ≤ 3.3) belonging to the background seismicity. four of them are main shocks of local earthquake sequences: 30 april 2004 (15 events), 29 november 3 december 2007 (41 events), 6 30 september 2008 (42 events) and 6 december 2009 (23 events), while the other 38 are single events [tugui et al. 2009, popescu et al. 2011, 2012, romplus earthquake catalogue: http://www.infp.ro/romplus/]. the polar diagrams for the azimuthal distribution of the two nodal planes (a and b) and for the plunge angle are plotted in the figure 9. the azimuthal distribution of the nodal planes does not suggest a preferential orientation, but only a slight tendency to align perpendicularly to the carpathians arc bend. the diagram for p, t and b axes (figure 10) shows an equal distribution among normal and reverse faultings and absence of pure strike-slip faulting. according to our results, reverse and normal faulting are prevalent, with only small strike-slip influence, while pure strike-slip is missing. 5.3 pre-dobrogean depression and bârlad depressions the focal mechanisms were computed for 14 events located in the predobrogean depression and 14 events located in the bârlad depression. the diagram for p, t and b axes (figure 11) shows broadly the same features as for the moesian platform (figure 10): an equal probability for normal and reverse faulting and almost total lack of pure strike-slip faulting. this result suggests that despite the lateral differences among the tectonic units acting in the foredeep area of the carpathians the faulting processes are quite similar, with prevalent subsidence and folding mechanisms. due to the reduced number of earthquakes in this zone and to the fact that the orientation of the p, b, t axes is very much alike the distribution presented in figure 10 for moesian platform zone, the polar diagrams are represented for all the crustal earthquakes considered in the two zones, a total of 68 events (figure 12). the distribution of dip angles in figure 12 is close figure 9. angular diagrams for azimuth and dip angles of the nodal planes moesian platform area (mo): a) first nodal plane; b) second nodal plane. to the distribution computed for 104 events in the same crustal region after the old catalogue of earthquake mechanisms [bala et al. 2003], while the strike angles show the same almost equal distribution to all directions. the corresponding ternary diagram for combined data from mo and pd-bd areas is represented the figure 13. we show comparatively the diagram containing only the mechanisms with 20 or more reliable polarities (right side). despite the poor statistics (17 events), the distribution of data points in this case keeps the same trends as in the complete diagram (left side). 5.4 făgăraş câmpulung seismogenic zone fc the catalogue of earthquakes with computed focal mechanism belonging to the făgăraş-câmpulung seismogenic area includes 18 events, most of them located in the southern side, towards the contact between făgăraş mountains and moesian platform. all the events occurred in the 1998 2012 time interval belong to the radulian et al. 10 figure 10. ternary diagram for p, t and b principal axes distribution for moesian platform seismogenic area (mo). figure 11. diagram for p, t and b principal axes distribution for predobrogean and bârlad depressions. figure 13. ternary diagram for p, t and b principal axes distribution for moesian platform seismogenic area (mo) and pre-dobrogean and bârlad depressions: a). for all events (67 events); b). diagram for the mechanisms with 20 or more reliable data (17 events). figure 12. angular diagrams for azimuth and dip angles of the nodal planes moesian platform area and pd-bd area: a) first nodal plane; b) second nodal plane. 11 background seismicity (magnitudes no greater than 3.3). the polar diagrams for the azimuthal distribution of the two nodal planes (a and b) and for the plunge angle are plotted in the figure 14. the statistics is too low to provide reliable trends in nodal plane orientation. apparently, the fault planes are equally distributed on azimuth, while the dip angles have some three principal directions to 30, 60 and 85°. the picture shown by the diagram for p, t and b axes (figure 15) for the făgăraş câmpulung area is somewhat different from that pointed out for the other seismogenic areas in the crust. it is closer to the typical distribution of the vrancea subcrustal earthquakes, with a particular emphasis on reverse faulting component. this result suggests the prevalence of folding processes as a consequence of the convergent contact between moesian platform and carpathians. 5.5 crustal seismicity in the south-western part of romania to complete the image of focal mechanism characteristics for entire romania, we shall discuss briefly the results obtained for the earthquakes recorded west of the olt river (oltenia region), in the danubian zone (where danube river is crossing the passage between southern carpathians and balkans) and in the banat region. the earthquake mechanisms for these events were described by oros et al. [2008a and 2008b]. the seismicity observed west of olt river is located in the crust and comprises small-to-moderate events. only a few exceptions are reported for events with mw reater than 5.0 occurred in the danubian zone and oltenia [atanasiu 1961, oncescu et al. 1999]: at mehadia-baile herculane (mw = 5.6 in 1991), moldova noua (mw = 5.3, in 1879) and nearby tismana (mw = 5.2 in 1943). a few more events above magnitude 5 have been located in the banat area with the maximum magnitude mw = 5.6 (the event of 12 july 1991 near banloc). some epicenters of moderate events (mw = 3.0-4.9) were located in the neighborhood of targu jiu city. in the south of dolj county a few moderate earthquakes were felt between craiova and caracal cities. the significant earthquakes from oltenia and danubian zone suggested by their focal mechanism a reverse faulting south of turnu severin and nw and se of targu jiu and a normal faulting near baile herculane and south craiova [see oros et al. 2008a and 2008b, for details]. the earthquakes are generated by either the transverse fault systems of the platform or by the e-w fault systems along which the platform sinks underneath the carpathian orogen. other earthquakes could be generated at the contact of getic and danubian domains of southern carpathians, on the timok and jiu faults or in the cerna graben [bala et al. 2015]. banat region has a larger dispersion of epicenters. the earthquake mechanism of the events located in banat area are extensively described by oros [2004], oros et al. [2008a and 2008b]. the earthquakes in the banat area are grouped into two main seismo-active tectonic setting defined by several studies as major seismogenic zones [radulian et al 2000]. thus, in the north-west the banat seismogenic zone (ba) develops corresponding to a depression area (western plain of romania) and in the south-east the danube seismogenic zone (da) overlaps the area of the carpathians mountains. a strike slip faulting combined more or less with thrust faulting is pointed out by the focal mechanisms of the significant events. banat earthquakes occurred at the contact between carpathian and pannonian basement (from timisoara to banloc, and north of timisoara), as well as to the contact between unleveled basement blocks (like horsts and grabens) from sannicolau mare, nadlag-jimbolia, arad-vinga-calacea, and on the timis valley at faget [polonic and malita 1997, bala et al. 2015]. 6. conclusions the catalogue of the focal mechanism for the earthquake mechanism in south-eastern romania figure 14. angular diagrams for azimuth and dip angles of the nodal planes for the events of făgăraş câmpulung area (fc): a) first nodal plane; b) second nodal plane. earthquakes recorded in romania, available until 1997 [radulian et al. 2002, bala et al. 2003] is updated for the period 1998 2012. thus, the fault plane solutions for 259 intermediate-depth earthquakes (vrancea source) and 94 crustal earthquakes (moesian platform, predobrogean depression, bârlad depression and făgăraş câmpulung) are added to the existing catalogue (appendix a). the fault plane solutions are computed in all cases by inverting the polarities of the p-wave first arrivals manually picked up at the seismic stations of romania, republic of moldova, bulgaria and ukraine (according to bilateral protocols, the national institute for earth physics continuously exchanges data with the corresponding seismological institutes in the neighbouring countries). to this aim, the seisan algorithm [havskov and ottemöller 2001] was applied. only the solutions with minimum 10 polarities, acceptable coverage on the lower hemisphere and non-zero focal depth were selected for our analysis. a similar investigation, partly overlapping our study interval, was carried out by sandu and zaicenco [2008], but they used isc polarity data. for this reason, their fault plane solutions are available only for larger events (mw > 3.6), while the present work is based on near-field observations for smaller events which are statistically significant as background seismicity. we draw our attention to the seismic areas located in the south-eastern romania, including the vrancea subcrustal source, făgăraş câmpulung zone, located in the southern carpathians, and the crustal areas laying in the carpathians foredeep region (outward of carpathians arc bend). the papers of oros et al. [2008a, 2008b] have been focused on the other seismogenic areas, located at the western extremity of southern carpathians (danubian area da) and banat area (ba). for that reason, these areas were not considered in our analysis. the seismic activity that took place between 1998 and 2012 was limited to moderate-size events. the largest event was generated on 27 october 2006 (mw = 6.0) in the vrancea subcrustal source. the largest event generated in the crust occurred on 3 oct. 2004 (mw = 4.9) in the pre-dobrogean depression. certainly, the outcomes of this paper refer to a data set which happened to be small-to-moderate earthquakes (no event of magnitude above 6 was recorded within the study time interval 1998 2012). the comparison with largest earthquakes from the vrancea subcrustal source, as shown in figure 8, emphasizes features characterizing strong earthquakes that can be identified in the background activity as well. it remains an open question for future investigations to what extent our conclusions keep valid for stronger earthquakes in the crust, where historical earthquakes of magnitude around 6.5 occurred. we are aware of the critical issue of uncertainties which are inherent in any approach limited to using pwave polarities alone. the main results of our investigation appear to be justified mainly by the number of events, simple statistical considerations. by enlarging our analysis to an extended catalogue of focal mechanisms, a sound base to improve considerably future development of our study will be provided. a summary figure on the statistical analysis of the resulting fault plane solutions carried out for the study seismogenic zones using ternary diagrams for the principal axes is given in the figure 16. for the time interval considered and the related magnitude range, the fault plane solutions of the shallow earthquakes are more likely related to local secondary faults than to the major faults located in the carpathians foredeep region. we can simply explain on this line the variety of solutions in the moesian platform, pre-dobrogean depression and bârlad depression zones. as a general trend which is statistically consistent, note the deficit of strike-slip faulting. this result suggests the prevalence of subsidence and folding processes as stress release mechanisms in the entire carpathians foredeep region, as well as in moesian platform and barlad depression. the hypothesis raised by a few authors in the last century on a trans-current movement along the major faults, crossing the aria situated between the black sea and vrancea [roman 1973, airinei 1977, constantinescu and enescu 1984, enescu and enescu 1993] is not supported by our results. on the other hand, possible transcurrent movement along these major faults, as suggested by some geotectonic studies [e.g. matenco et al. 2007], could be accommodated at small scale by non-strike-slip motions due to lateral variations in the fault tracing or to transferring motion on smaller secondary satellite faults. more than that, it appears that the seismic activity in the moesian platform zone is limited to the east by the danube river, along which an important fault exists, danubian fault, which is presumed to be a crustal tectonic feature. although the tectonic provinces in the central and southern dobrogea belong to the moesian platform, just a few earthquakes occurred here are reported in the romplus catalogue. so, it appears that the danubian fault separates mo seismogenic zone from the more stable central and southern dobrogea zones to the east. the two important peceneaga camena and capidava ovidiu faults go into these zones (as can be clearly seen and monitored at radulian et al. 12 13 surface on several sectors). however, the historic seismic activity along this faults in dobrogea is clearly reduced, if we compare it to the seismic activity along sf. gheorghe fault in northern dobrogea (figure 1). the crustal seismic activity in this region can rather be seen as a response of the intense processes taking place beneath the carpathians arc bend (vrancea area) and materialized by a triple number of earthquakes occurred at intermediate depth in the same time interval, almost the same like in the previous catalogue [radulian et al. 2002]. for the combined seismogenic areas mo and pd-bd (68 events) the distribution of the dip angles is close to the distribution find for 104 events in the same crustal region after the previous catalogue of earthquake mechanisms [bala et al. 2003], while the strike angles show the same almost equal distribution to all directions. crustal earthquakes appear not to reflect the significant trans-current motions between the tectonic units in the region along the main faults, but they rather express the moving and re-positioning of different blocks in the area, delimited by either the main fault system, which runs parallel to the carpathians arc bend or by secondary faults, distributed to a general direction which is roughly perpendicular to the direction of the main fault system. the fault plane solutions of the vrancea earthquakes generated in sinking plate in the mantle are preserving the typical features revealed by all the previous studies on the subcrustal seismic activity in the vrancea region: predominant dip-slip, reverse faulting, characterizing both the weak and strong earthquakes [enescu 1980, oncescu 1987, oncescu and trifu 1987]. t axis close to vertical and p axis close to horizontal tending to be oriented perpendicularly to the carpathians arc. the strong clustering of foci and the high rate of occurrence in time reveals a geodynamical process releasing very efficiently tectonic stresses. the continuous vertical or subvertical extension in the slab, in parallel with nw-se predominant compression, can be associated with quite an extensive series of active models advanced recently: 1. a strong pull down by gravitational sinking into asthenosphere of an oceanic slab [sperner et al. 2004]; 2. an active continental lithospheric delamination [knapp et al. 2005]; 3. dehydration of rocks [ismail-zadeh et al. 2000]; 4. gravitational instability [lorinczi and houseman 2009]; 5. thermal instability [cloetingh et al. 2004, demetrescu et al. 2007]. earthquake mechanism in south-eastern romania figure 1. ternary diagrams for the principal axes for the earthquakes considered in this paper associated to the seismogenic zones in the south-eastern part of romania. tectonic map is after sandulescu [1984]. see above for more explanation on ternary diagrams. 6. unstable triple junction [besutiu 2001]. the focal mechanism patterns obtained in the present study reproduce to a large extent the stress field characterization carried out by radulian et al. [1999, 2000)], using essentially an independent catalogue of fault plane solutions from the period until 1997. however, the considerable increase of fault plane solutions accuracy for the earthquakes recently recorded, due to the significant improvement of the romanian seismic network [neagoe et al. 2014] and of their statistical representativeness, allows us to individualize a few refined aspects not obvious in the previous investigations, such as: (1) deficit of the strike-slip component over the entire carpathians foredeep area, (2) different stress field pattern in the făgăraş câmpulung zone as compared with the moesian platform and pre-dobrogean and bârlad depressions, (3) a larger range for the dip angle of the nodal planes in the vrancea subcrustal source, ~ 40° -70° against ~ 70°, as commonly considered. acknowledgments. the database for this work is organized in appendix a (http://www.infp.ro/) and it is part of this paper. the ternary diagrams in this paper are realized with a dedicated program which was kindly made available by the author and which is described properly in álvarez-gómez [2014]s. the work in this paper was realized in the frame of nucleu program, project no. pn 16.35.01.08 and pn 16.35.01.03 (2016-2017, http://nucleu2016.infp.ro/). we recognize the important contribution of the anonymous reviewers to significantly improving the quality of the paper. references airinei, s. 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(2009). seismotectonics of vrancea (romania) zone: the case of crustal seismicity in the foredeep area, romanian reports in physics, 61, 325-334. wenzel, f., achauer, u., enescu, d., kissling, e., russo, r., mocanu, v. and g. musacchio (1998). detailed look at final stage of plate break-off is target of study in romania, eos, trans. am. geophys. un., 79, 589, 592-594. *corresponding author: mircea radulian, national institute for earth physics, măgurele, romania email: mircea@infp.ro © 2018 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. radulian et al. 16 earthquake forecasting: statistics and information annals of geophysics, 58, 6, 2015, s0658; doi:10.4401/ag-6816 s0658 earthquake forecasting: statistics and information vladimir gertsik1,*, mark kelbert1,2,3, anatoly krichevets4 1 institute of earthquake prediction theory and mathematical geophysics ras, moscow, russia 2 swansea university, department of mathematics, swansea, united kingdom 3 higher school of economics, moscow, russia 4 lomonosov msu, department of psychology, moscow, russia abstract the paper presents a decision rule forming a mathematical basis of earthquake forecasting problem. we develop an axiomatic approach to earthquake forecasting in terms of multicomponent random fields on a lattice. this approach provides a method for constructing point estimates and confidence intervals for conditional probabilities of strong earthquakes under conditions on the levels of precursors. also, it provides an approach for setting a multilevel alarm system and hypothesis testing for binary alarms. we use a method of comparison for different algorithms of earthquake forecasts in terms of the increase of shannon information. ‘forecasting’ (the calculation of the probabilities) and ‘prediction’ (the alarm declaring) of earthquakes are equivalent in this approach. 1. introduction the methodology of selecting and processing of relevant information about the future occurrence of potentially damaging earthquakes has reached a reasonable level of maturity over the recent years. however, the problem as a whole still lacks a comprehensive and generally accepted solution. further efforts for optimization of the methodology of forecasting would be productive and well justified. one of our objectives here is to address some drawbacks of the approach to earthquake prediction based on pattern recognition theory. it is developed by “keilis-borok team” and presented in keilis-borok and soloviev [2003]. the pattern recognition is used in forecast with the hope that ‘training sets’ in which strong earthquakes have higher values of precursors may help to decide whether or not a strong earthquake will happen in the future. as it is demonstrated by below published krichevets’s theorem, this hope has no basis. a comprehensive review of the modern earthquake forecasting state of knowledge and guidelines for utilization can be found in jordan et al. [2011]. note that all methods of evaluating the probabilities of earthquakes are based on a combination of geophysical, geological and probabilistic models and considerations. even the best and very detailed models used in practice are in fact only ‘caricatures’ of immensely complicated real processes. (for example, the equations of the theory of elasticity are valid only for infinitesimal strains, but used even at the edges of the cracks where strains tend to infinity. it is also clear that we do not know about the inhomogeneity of elastic properties in the real medium, and we have no way of knowing about it with sufficient accuracy.) a mathematical toolkit for earthquake forecasting is well presented by harte and vere-jones [2005]. this work is based on the modeling of earthquake sequences in terms of the marked point processes. however, the mathematical technique used is quite sophisticated and does not provide direct practical tools to investigate the relations of the structure of temporal-spatial random fields of precursors to the appearance of strong earthquakes. the use of the multicomponent lattice models (instead of marked point processes) gives a different novel way of investigating these relations by a more elementary way. discretization of space and time allows us to separate the problem in question into two separate tasks. the first task is the selection of relevant precursors, i.e., observable and theoretically explained physical and geological facts which are causally related to a high probability of strong earthquakes. particularly, this task involves the development of models of seismic events and computing probabilities of strong earthquakes in the framework of these models. such probabilities are used as precursors in the second task. the second task is the development of methodolarticle history received june 25, 2015; accepted december 21, 2015. subject classification: earthquake forecasting, lattice, random process, information. ogy of working with these precursors in order to extract the maximum information about the probabilities of strong earthquakes. this is the main topic of this paper. our approach allows us to obtain the following results: estimates of probabilities of strong earthquakes for given values of precursors are calculated in terms of the frequencies of historic data. confidence intervals are also constructed to provide reasonable bounds of precision for point estimates. methods of predictions (i.e., binary alarm announcement [keilis-borok and kossobokov 1990, keilisborok 1996, holliday et al. 2007]) and forecasting (i.e., calculating probabilities of earthquakes [kagan and jackson 2000, harte and vere-jones 2005, wgncep 2007, jordan et al. 2011]) are equivalent in the following sense: the setting of some threshold for the probability of earthquakes allows to update the alarm level. on the other hand, the knowledge of the alarm domain based on historical data allows us to evaluate the probabilities of earthquakes. in a sense, the prediction is equivalent to hypothesis testing as well (see section 11). in our scheme we propose a scalar statistic which is the ratio of the actual increment of information to the maximal possible increment of information. this statistic allows us to linearly order all possible forecasting algorithms. nowadays the final judgement about the quality of earthquake forecasting algorithms is left to experts. this arrangement puts the problem outside the scope of natural sciences which are trying to avoid subjective judgements. the foundation of our proposed scheme is the assumption that the seismic process is random and cannot be described by a purely deterministic model. indeed, if the seismic process is deterministic then the inaccuracy of the forecast could be explained by the non-completeness of our knowledge about the seismic events and non-precision of the available information. this may explain, at least in principle, attacks from the authorities addressed to geophysicists who failed to predict a damaging earthquake. however, these attacks have no grounds if one accepts that the seismic process is random. at the end of the last century (februaryapril 1999) a group of leading seismologists organized a debate via the web to form a collective opinion of the scientific community on the topic: ‘is the reliable prediction of individual earthquakes a realistic scientific goal?’ (see http://www.nature.com/nature/debates/ earthquake/). despite a considerable divergence in peripheral issues all experts taking part in the debate agreed on the following main principles: the deterministic prediction of an individual earthquake, within sufficiently narrow limits to allow a planned evacuation programme, is an unrealistic goal; forecasting of at least some forms of time-dependent seismic hazard can be justified on both physical and observational grounds. the following facts form the basis of our agreement with this point of view. the stringblock burridge-knopov model, generally accepted as a mathematic tool to demonstrate the power-like gutenberg-richter relationship between the magnitude and the number of earthquakes, involves the generators of chaotic behaviour or dynamic stochasticity. in fact, the nonlinearity makes the seismic processes stochastic: a small change in the shift force may lead to completely different consequences. if the force is below the threshold of static friction the block is immovable, if the force exceeds this threshold it starts moving, producing an avalanche of unpredictable size. this mechanism is widespread in the earth. suppose that the front propagation of the earthquake approaches a region of enhanced strength of the rocks. the earthquake magnitude depends on whether this region will be destroyed or remains intact. in the first case the front moves further on, in the second case the earthquake remains localized. so, if the strength of the rocks is below the threshold the first scenario prevails, if it is above the threshold the second scenario is adapted. the whole situation is usually labelled as a butterfly effect: infinitesimally small changes of strength and stress lead to macroscopic consequences which cannot be predicted because this infinitesimal change is below any precision of the measurement. for these reasons determinism of seismic processes looks more doubtful than stochasticity. the only comment we would like to contribute to this discussion is that the forecasting algorithms based exclusively on the empirical data without consistent physical models could hardly be effective in practice (see section 13 for more details). finally, note that our approach may be well-applicable for the space-time forecasting of different extremal events outside the scope of earthquake prediction. 2. critical analisis of the prediction based on pattern recognition the main objective of this paper is to present a mathematical formalism of the earthquake forecasting alternative to the approach based on the pattern recognition theory. so, it is appropriate to summarize the latter approach. an advantage of the formalism of the keilis-borok team [keilis-borok and kossobokov 1990, keilis-borok 1996, keilis-borok and solovoev 2003] is a separation of the forecast into two separate tasks: the selection of relevant precursors and the technique of their processgertsik et al. 2 3 ing. on the other hand, some drawbacks of the scheme should be pointed out. 2.1. pattern recognition the method is relaying on the pattern recognition technique that can provide only empirical but not theoretical foundation. the possibility to base a reliable forcast on the idea of supervised pattern recognition is highly questionable in principle. indeed, the features selection based on the observations only and not related to the physics of earthquakes is not helpful and any hopes for valid ‘training sets’ are not grounded. a successful supervised recognition is possible only if the features has proved causal relation with pattern. this principle is illustrated by a simple but important theorem by a.n. krichevets. theorem. let a be a finite set, b1 ∪ b2 ⊂ a; b1 ∩ b2= ø. we say that b1 and b2 are finite educational samples. let x ∈ a; x ∉ b1∪b2 be a new object. then among all classifications, i.e., subsets {a1, a2} such that b1 ⊂ a1, b2 ⊂ a2, a1∪ a2= a, a1∩ a2 = ø satisfying condition that either b1 ∪ x ⊂ a1 or b2 ∪ x ⊂ a2 exactly a half classifies x as an object of sample b1 and a half classifies x as an object from b2. proof. it is easy to define a one-to-one correspondence between classifications. indeed, if {a1, a2} a1∪ a2= a; is a classification such that b1 ⊂ a1, x ∈ a1, b2 ⊂ a2, one maps it into the unique classification {a'1, a'2} such that b1⊂a'1, x∈a'2, b2⊂ a'2, where a'1= a1\x, a'2= a2∪x. corollary. a supervised pattern recognition is impossible. after the training procedure the probability to classify correctly a new object is the same as before training. this result implies that a clairvoyance is involved in a successful forecast: one should know in advance (or guess correctly) a “relevant” set of precursors. 2.2. voting procedure as a decision rule for a forecast a voting procedure is adopted: an alarm is announced when the number of precursors taking abnormally large values exceeds certain thershold. this procedure is well-justified only if the pattern in question is unique. generically, this is not true: if two or more possible scenarios are present, different precursors may be strongly affected in different scenarious. in this case a combination of abnormal values for precursors related to different scenarios may produce a false alarm. 2.3. alarm domain the values of precursors are computed at the point of some lattice, and are assigned a domain surrounding the point. the above scheme employs a circle with a center at a lattice site and a radius proportional to the minimal magnitude of the forecasted earthquake. however, this choice leads to a contradiction. indeed, suppose we announce an alarm for earthquakes with minimal magnitude 6 in a domain a6. note that outside the alarm domain we expected that the earthquake will not occur. but the announcement of the alarm with minimal magnitude 7 is also correct for this point and the alarm should be announced in the domain a7 as well. by definition a6 ⊂ a7 and we expect an earthquake with a magnitude of at least 7 and do not expect an earthquake with à magnitude of at least 6 in the domain a7\ a6. this is absurd. futhermore, the alarm area is too large with this approach. 2.4. molchan’s error diagram below we demonstrate that the procedure of alarm announcement is equivalent to the classical problem of hypothesis testing. so, the standard error diagram provides the dependence of the second type error (probability of missing a target) from the first type error (probability of false alarm). in line with the general case the well-known molchan’s error diagram [molchan 1990] displays the dependence of the first kind error probability versus the share of alarms. h0: the distribution of the precursor is equal to its unconditional distribution (earthquake “could either happen or not happen”). h1: the distribution of the precursor is equal to its conditional distribution under the condition that the earthquake will happen. note that the rejection of hypothesis h0 leads to absurd results: “earthquake will not happen and will happen”. of course, molchan’s error diagram can be interpreted not in terms of hypothesis testing. then there is no contradiction, but the standard error diagram is different from the molchan’s error diagram by a small value proportional to the share of cells occupied by the strong earthquakes (in our approach the spacetime is divided into disjoint cells). 2.5. the loss function, risk function and the economic criterion for the quality of the forecast the keilis-borok team refers to the economic loss function for the quantitative evaluation of the quality of forecast. first, we calculate the expected loss correctly. a natural economic measure for a quality of binary forecast is the economic risk function (it should not be confused with seismic risk, there is the concept of the theory of statistical decisions) or damage r related to the earthquakes and the necessary protective measures. in mathematical statistics the risk function is earthquake forecasting: statistics and information defined as the expectation of the loss function, which is an expected losses for different outcomes. in our case there are two types of losses: damage and expenses related to protection. for each cell of our grid (in our approach the space-time is divided into disjoint cells) the risk may be specied by the formula r = bpr {no earthquake; alarm} + apr {earthquake; no alarm} + cpr {earthquake; alarm} here the symbol pr {·} denotes a probability, a stands for the average damage from a seismic event, b stands for the average expenses for protection after a seismic alarm is announced, c stands for the average damage after the alarm, c = a + b − d, where d is the damage prevented by the alarm. the coefficient in front of pr {no earthquake; no alarm}, obviously, equals 0, because in the absence both of a seismic event and an alarm there is no loss of any kind. clearly, only the case when d > b is economically justified, i.e., the gain from the prevention measures is positive. obviously, d should be less than a+b, i.e., an earthquake cannot be profitable. taking into account that a, b and c depend on the geographical position of the cell, we write the total risk as the summation over all cells in the region of a given forecast. in the simplest case of the absence of the spacial component, when a single cell represents a region of forecast, the expression for the risk is simplied as follows r = amo+bx+cm(1 −o), where x is a spatiotemporal share of alarm, m is a share of cells occupied by the earthquakes, o is the share of missed targets. in molchan’s theory related to the one-dimensional case, the third component is missing. however, the risk function r, which is very useful for economical considerations and as a basis for an administrative decision, could hardly be used as a criteria for the quality of seismic prediction. first of all, it cannot be computed in a consistent way because the coefficients a, b and c are not known in practice, and hence no effective way of its numerical evaluation is known. the computation of these coefficients is a difficult economic problem and goes far beyond of the competence of scientists. on the other hand, the readiness of the authority to commit resources to solving the problem depends on the quality of the geophysical forecast. this situation leads to a vicious circle. the second drawback of the economic risk as a criterion for the quality of prediction is related to the fact that it depends on many factors which have no relation to geophysics or earthquake prediction. these factors include the density of population, the number and size of industrial enterprises, infrastructure, etc. it also depends on subjective factors such as the willingness of authorities to use resources for prevention of the damage from earthquakes. the natural sciences could hardly accept the criteria for the forecast quality which depend on the type of state organization, priorities of ruling parties, results of the recent elections, etc. in this paper, we introduce a scalar efficiency (information efficiency) forecast that only depends on information about future earthquakes contained in the precursors. 2.6. the measure of an area in molchan and keilis-borok [2008] the area of the alarm domain is defined in terms of a non-homogeneous measure depending on spacial coordinates. specifically, the measure of an area is proportional to the number of earthquake epicenters from a sample catalog. this approach is used to eliminate the decrease of the share of alarmed sites x with the extension of the domain when a purely safe and aseismic territory is included into consideration. it would be well-justied if the quantity x could be accepted as an adequate criterion of the quality of forecast in its own right. on the other hand, it can be demonstrated that the our information efficiency converges to a non-zero value 1 −o when the number of cells with an alarm is fixed but the total number of cells tends to infinity. an inhomogeneous area of the territory under forecast which is proportional to “activity” does not enable us to calculate the informational efficiency. moreover, it possesses a number of unnatural features from the view of evaluating economical damage. a seismic event in the territory of low seismicity is more costly because no precautions are taken to prevent the damage of infrastructure. however, in this inhomogeneous area an alarm announced in an aseismic territory will have a smaller contribution than an alarm in a seismically active territory where the losses would be in fact smaller. we conclude that this approach ‘hides’ the most costly events and does not provide a reasonable estimate of economic damage. next, we build a forecasting method free from this kind of flaws. 3. events and precursors on the lattice in order to define explicitly estimates of probabilities of strong earthquakes we discretize the two-dimensional physical space and time, i.e., introduce a partition of three-dimensional space-time into rectangular cells with the space partition in the shape of squares and time partition in the shape of intervals. these cells should not intersect to avoid an ambiguity in computing the frequencies for each cell. in fact, the space cells should not be perfect squares because of the curvature of the earth’s surface, but this may be neggertsik et al. 4 5 lected if the region of forecasting is not too large. so, we obtain a discrete set xk with n = i × j × k points which is defined as follows. let us select a rectangular domain a of the two-dimensional lattice with i × j points x = (xi, yj), xi = a × i, i = 1, ..., i and yj = a × j, j = 1, ..., j, a is the step of the lattice. a cell in xk takes the shape of parallelepiped of height dt with a square base. clearly, any point in xk has coordinates (xi, yj, tk); tk = t + kdt, k = 0, ..., k. we say that a seismic event happens if an earthquake with magnitude greater than some pre-selected threshold m0 is registered. for any cell in our spacetime grid we define an indicator of an event, i.e., a binary function h (i, j, k). this function takes the value 1 if at least one seismic event is registered in a given cell and 0 otherwise. suppose that for all points (xi, yj, tk) the value of a vector precursor f (i, j, k) = { fq(i, j, k), q = 1, ..., q} is given. the components of the precursor fq(i, j, k), q = 1, ..., q are the scalar statistics constructed on the base of our understanding of the phenomena that precede a seismic event. 4. mathematical assumptions a number of basic assumptions form the foundation of the mathematical technique of earthquake forecasting. in the framework of mathematical theory they can be treated as axioms but are, in fact, an idealization and simplication with respect to the description of the real phenomena. below we summarize the basic assumptions which are routinely used in existing studies of seismicity and algorithms of earthquake forecasting even the authors do not always formulate them explicitly. we accept the following assumptions or axioms of the mathematical theory: (i) the multicomponent random process {h (i, j, k), f (i, j, k)}, describing the joint evolution of the vector precursors and the indicator of seismic events, is stationary. this assumption provides an opportunity to investigate the intrinsic relations between the precursors and the seismic events based on the historical data. in other words, the experience obtained by analysing the series of events in the past, is applicable to the future as the properties of the process do not depend on time. in reality, this assumption holds only approximately and for a restricted time period. indeed, plate tectonics destroys the stationarity for a number of reasons including some purely geometrical considerations. for instance, the movements of the plates leads to their collisions, their partial destruction and also changes their shapes. nevertheless, the seismic process can be treated as a quasi-stationary one for considerable periods of time. at the time when the system changes from one quasistationary regime to another (say, nowadays, many researchers speak about the abrupt climate change) the reliability of any prediction including the forecast of seismic events is severely restricted. (ii) the multicomponent random process {h (i , j, k), f (i, j, k)} is ergodic. any quantitative characteristic of seismicity more representative than a registration of an individual event is, in fact, the result of averaging over time. for instance, the gutenberg-richter law, applied to a given region relates the magnitude with the average number of earthquakes where the averaging is taken over a specific time interval. in order to associate with the time averaging a proper probabilistic characteristic of the process and make a forecast about the future one naturally needs the assumption of ergodicity. this exactly means that any averaging over a time interval [0, t] will converge to the stochastic average when t → ∞. in view of ergodicity one can also construct unbiased and consistent estimates of conditional probabilities of strong earthquakes under the condition that the precursors take values in some intervals. naturally, these estimates are the frequencies of observed earthquakes, i.e., ratios of the number of cells with seismic events and prescribed values of precursors to the total number of cells with the prescribed values of precursors. (recall that an unbiased point estimate î of parameter i satisfies the condition e [i] = î and a consistent estimate converges to the true value i when the sample size tends to infinity). (iii) any statement about the value of the indicator of a seismic event h (i, j, k) in the cell (i, j, k) or its probability should be based on the values of the precursor f (i, j, k) only. this assumption means that the precursor in the given cell accumulates all the relevant information about the past and the information about the local properties of the area that may be used for the forecast of the seismic event in this cell. in other words, the best possible precursor is used (which is not always the case in practice). as in the other cases, this assumption is only an approximation to reality, and the quality of a forecast depends on the quality of the selection and accumulation of relevant information in the precursors. below we present some corollaries and further specifications. (iii-a) for any k the random variables h (i, j, k), i = 1, ..., i, j = 1, ..., j are conditionally independent under the condition that the values of any measurable function u (f (i, j, k)) of the precursors f (i, j, k), i = 1, ..., i, j = 1, ..., j are fixed. in practice this assumption means that the forecast for the time tk = t0+ kdt cannot be affected by the values related to the future time intervals (tk, tk+dt]. in reality earthquake forecasting: statistics and information all of these events may be dependent, but our forecast does not use the information from the future after tk. (iii-b) the conditional distribution of the random variable h at a given cell depends on the values of the precursors f at this cell and is independent of all other variables. (iii-c) the conditional probabilities prij {h|u ( f )} of the indicator of seismic events h in the cell (i, j, k), under condition u ( f ) in this cell do not depend on the position of the cell in space (the time index k related to this probability may be dropped due to the stationarity of the process). in other words, the rule for computing the conditional probability pr based on the values of precursors is the same for all cells, and the space indices of probability pr may be dropped. this condition is widely accepted in constructions of the forecasting algoritms but rarely formulated explicitly. however, the probability of a seismic event depends to a large extent on the local properties of the area. hence, the quality of the forecasting depends on how adequately these properties are summarized in the precursors. this formalism properly describes the space inhomogenuity of the physical space because the stationary joint distribution of prij {h, f ≤ x} for an arbitrary precursor f depends, in general, on the position of the cell in the domain a. below we will use the distributions of precursors and indicators of seismic events in domain a that do not depend on the spatial coordinates and have the following form (iii-d) note that assumption (iii) implies that the conditional probabilities prij {h|u ( f )} are computed via the probabilities pra{h, f ≤ x} only. the properties listed above are sufficient to obtain the point estimates for the conditional probabilities of seismic events under conditions formulated in terms of the values of precursors. however, additional assumptions are required for a testing of the forecasting algorithm: (iv) the random variables f (i, j, k) are conditionally independent under condition that h (i, j, k) = 1. again, these conditions are not exactly true, however they may be treated as a reasonable approximation to reality. indeed, if the threshold m0 is sufficiently high then the strong earthquakes may be treated as rare events, and the cells where they are observed are far apart with a high probability. any two events related to cells separated by the time intervals t are asymptotically independent as t → ∞ because the seismic process has decaying correlations (the mixing property in the language of random processes). the loss of dependence (or decaying memory) is related to the physical phemonema such as healing of the defects in the rocks, relaxation of strength due to viscosity, etc. as usual in physical theories, we accept an idealized model of the real phenomena applying this asymptotic property for large but finite intervals between localizations of seismic events. the independence of strong earthquakes is not a new assumption, in the case of continuous space-time it is equivalent to the assumption that the locations of these events form a poisson random field. (note that the distribution of strong earthquake should be homogeneous in space a priori, because by condition there is no information about the heterogeneity outside precursors. the information about the location of epicenters should be contained in the precursors) the poisson hypothesis is used in many papers, see, e.g. harte and vere-jones [2005]. it is very natural for the analysis of the tails of the gutenberg-richter law for large magnitudes [pisarenko et al. 2008]. summing up, the development of the strict mathematical theory of earthquake forecasting does not require any additional assumption except those routinely accepted in the existing algorithms but usually not formulated explicitly. 5. the standard form for precursors the correct solution of the forecasting problem given the values of precursors f (i, j, k) is provided by the estimate of conditional probability prij{h|f (i, j, k)} of the indicator of a seismic event in the cell (i, j, k). in practice this solution may be difficult to obtain because the number of events in a catalog might be not sufficient. indeed, the range of value of a scalar precursor is usually divided into a number m of intervals, and only a few events are registered for any such interval. for a q-dimensional precursor the number of q-dimensional rectangles, covering the range, is already mq, and majority of them contains zero events. only a small number of such rectangles contains one or more events, that is the precision of such an estimate of conditional probability is usually too low to have a practical value. for this reason one constructs a new scalar precursor in the form of the scalar function of component of the vector precursor, and optimizes its predictive power. this approach leads to additional complication as the units of measurement and the physical sense of different components of a precursor are substantially different. in order to overcome this problem we use some transformation to reduce all the components of the precursor to a standard form with the same sense and range of values. , ,pra pra pra pri pra pri prih f x fe x i j fe x i j h i j h f x x p h 1 1 1 1 1 , , , ra ra ra rijij i j a ra rijij i j a a rijij i j a # # # # # # # / / = = = = = ! ! ! q q q q v v v v e e fe e e fe h h h h h h| | | gertsik et al. 6 7 let us transform all the precursors fq(i, j, k), q = 1, ..., q to variables with the values in [0, 1] providing estimates of conditional probabilities. so, after some transformation f we obtain an estimate of pr {h|u ( f (i, j, k) = 1}, where u is a characteristic function of some interval b, i.e., the probability of event h (i, j, k) = 1 under condition that this precursor takes the value f (i, j, k)∈b. the transformation f of a scalar precursor f (i, j, k) is defined as follows. fix an arbitrary small number f. let l be a number of cells (i, j, k) such that h (i, j, k) = 1, and zl, l = 1, ..., l, be the order statistics, i.e., the values f (i, j, k) in these cells listed in non-decreasing order. define a new sequence zm, m = 1, ..., m, from the ordered statistics zl by the following recursion: z0 = −∞, zm is defined as the first point in the sequence zl, such that zm − zm−1 ≥ f. next, construct the sequence zm* = zm + (zm +1− zm )/2, m = 1, ..., m − 1, and add the auxiliary elements z0 = −∞, zm = 1. define also a sequence nm, m = 1, ..., m, where nm equals to the number of values in the sequence zl, such that zm* −1 ≤ zl < zm*. finally, define the numbers nm, m = 1, ..., m counting all cells such that zm* −1 ≤ f (i, j, k) < zm*, m = 1, ..., m. observe that ∑mm−1 nm= l, ∑ m m−1 nm= n, and use the ratios as estimate of unconditional probability of a seismic event in a given cell the transformation f is defined as follows this definition implies that transformation f replaces the value of a precursor for the frequency, i.e., the ratio of a number of cells containing a seismic event and the values of a precursor from [zm* −1, zm* ) to the number of cells with the value of precursor in this range. these frequencies are the natural estimates of conditional probabilities pr {h (i, j, k) = 1|zm* −1 ≤ f (i, j, k) < zm*}, m = 1, ..., m, computed with respect to stationary distribution pa(x): (this conditional probability can be written as pr {h = 1|u ( f )}, where u is the characteristic function of interval [zm* −1, zm* ). the function g has a stepwise shape, and the length of the step is bounded from below by f. it can be checked that there exists the limit ~g = lim—— f→∞ lim—— k→∞ g = pr {h = 1|u ( f )}. the estimates of conditional probabilities in terms of the function g are quite rough because typically the numbers nm are of the order 1. as a final result we will present below more sharp but less detailed estimates of conditional probabilities and confidence intervals for them. 6. combinations of precursors there are many ways to construct a single scalar precursor based on the vector precursor (f fq, q = 1, ..., q). each such construction inevitably contains a number of parameters or degrees of freedom. these parameters (including the parameters used for construction of the precursors themselves) should be selected in a way to optimize the predictive power of the forecasting algorithm. the optimization procedure will be presented below, its goal is to adapt the parameters of precursors to a given catalog of earthquakes, that is to obtain the best possible retrospective forecast. however, this adaptation procedure creates a “ghost” information related with the specic features of the given catalog but not present in physical propertities of real seismicity. this “ghost” information will not be reproduced if the algorithm is applied to another catalog of earthquakes. it is necessary to increase the volume of the catalog and to reduce the number of free parameters to get rid of this “ghost” information. clearly, the first goal requires the considerable increase of the observation period and may be achieved in the remote future only. so, one concentrates on the reduction of numbers of degrees of freedom. the simplest variant including q − 1 parameters is the linear combination as a strictly monotonic function of a precursor is a precursor itself the log-linear combination is an equally suitable choice here cq, q = 1, ..., q − 1 are free parameters. the result of the procedure has the form g = f f *. 7. alarm levels, point and interval estimations in view of equation (5.3) the precursor g is the set of estimates for probabilities n l m= ; ; ; ; pra pr p h i j k h i j k x dpa x1 1a ra pa / = = = = 3 3 # q q q v v v e f h i # , , if ; ; , , ..., . g f f i j k n n z f i j k z m m1 m m m m1 1# = = = =) )q q v v ; ; ; ; ; ;pr pr h i j k h i j k x dpa x z f i j k z dpa x 1 1 pa m m pa z # z z # z 1 m m m m 1 1 1# = = = = ) ) ) ) ) ) q q q q q v v v v v f f i i z # z # f) f f1 q q f1 2 = +f) = | .c f fqq fq1 2 -| .ln lnf) f f1 c f fqq q q fqf1 1 2 = +f) = q qv v| earthquake forecasting: statistics and information (5.1) (5.2) (6.1) (6.2) (5.3) (5.4) its serious drawback is that typically { z*l−1 ≤ f (i; j; k) < z*l } correspond to single events, and therefore the precision of these estimates is very low (the confidence intervals discussed below may be taken as a convenient measure of precision). in order to increase the precision it is recommended to use the larger groups of cells containing a larger number of events, that is a more coarse covering of the space where the precursor takes its values. in a sense, the precision of the estimation and the resolving power are related by a kind of “uncertainty principle”: the more precise estimate one wants to get the more coarse is the time-space range of their values and vice versa. we adapt the following approach in order to achieve a reasonable compromise. (1) for fixed thresholds as, s = 1, ..., s +1, a1 = 1, as< as+1; as+1= 0, we define s possible alarm levels as+1 ≤ g (i; j; k) < as and subsets xs, s = 1, ..., s , of the set xk corresponding to alarm levels, i.e., xs is a set of cells of xk, such that as+1 ≤ g (i; j; k) < as. there are different ways to choose the number s of alarm levels and the thresholds as, s = 2, ..., s . say, fix s = 5, and select as = 10 −a(s−1). this is a natural choice of the alarm level because at a = 1 it corresponds to decimal places of the estimate of the conditional probability given by the precursor. the problem with s = 2, i.e. two-level alarm, may be reduced to the hypothesis testing and is discussed in more details below. (2) compute the point estimates of probabilities obtained via the distribution px(x) of a precursor g in the same way as in equation (4.4). the property (iv) implies that for any domain xs the binary random variables h (i, j, k) are independent and identically distributed, i.e. and the unbiased estimate of ps takes the form where ns stands for the number of cells in domain xs, and by ms we denote the number of cells in xs containing seismic events. (3) the random variable ms takes integer values between 0 and ns. the probabilities of these values are computed via the well-known bernoulli formula let us specify the confidence interval covering the unknown parameter ps with the confidence level c. in view of the integral mouvre-laplace theorem for ns large enough the statistics is approximately gaussian n (0, 1) with zero mean and unit variance. note that the values ns increase with time. omitting straightforward calculations and replacing the parameter ps by its estimate is we obtain that is − < is < +is +; where and tc is the solution of equation u(tc) = c — 2 . here u stands for the standard gaussian distribution function. (4) as a result of these considerations we introduce ‘the precursor of alarms’ which indicates the alarm level: r ( f (i, j, k) ) = s (i, j, k) . it will be used for calculations of point estimates and the confidence interval in the form this result will be used for a prospective forecasting procedure. 8. the information gain and the precursor quality the construction of a ‘combined’ precursor r involves parameters from formula (6.1) or (6.2) as well as parameters which appear in the definition of each individual precursors fq. it is natural to optimize the forecasting algorithm in such a way that the information gain related to the seismic events is maximal. in a onedimensional case the information gain as a measure of the forecast efficiency was first intoduced by vere-jones [1998]. here we exploit a modification of his ideas in the case of multi-dimensional space-time process. remind the notions of the entropy and information. putting aside the mathematical subtlety (see kelbert and suhov [2013] for details) we follow below an intuitive approach by prohorov and rozanov [1969]. the information containing in a given text is, basically, the length of the shortest compression of this text without the loss of its content. the smallest length s of the sequence of digits 0 and 1 (in a binary code) for counting n different objects satises the relations 0 ≤ s − log2 n ≤ 1. so, ; ; , , ..., .pr h z f i j k z l l f1 1l l1 1#= = ) ) q qv vf i , , , , ..., .pr h a g i j k a s s1 1s s1 1#= =+ q vf i , , ,pr h a g i j k a p1 s s s1 1# /= + q vf i n m s s i= .pr m k n k p p1s s s k s nv ks= = -q s qv x nv p p p n 1 s s s s si q q v v , n t n t 1 1 s s s s s s s s s s i i i i i i i i = = + c c + q q v v , , , ,s i j k s i j1i i q qvf , , ,k s i j k1 i+qv vi gertsik et al. 8 (7.1) 9 the quantity s ~ log2 n characterizes the shortest length of coding the numbers of n objects. consider an experiment that can produce one of n non-intersecting events a1, ..., an with probabilities q1, ..., qn, respectively, q1+ ...+ qn = 1. a message informing about the outcomes of n such independent identical experiments may look as a sequence (ai1, ..., ain), where aik is the outcome of the experiment k. but for long enough series of observations the frequency ni/n of event ai is very close to its probability qi. it means that in our message (ai1, ..., ain) the event ai appears ni times. the number of such messages is by the stirling formula the length of the shortest coding of these messages the quantity sn measures the uncertainty of the given experiment before its start, in our case we are looking for one of the possible outcomes of n independent trials. the specific measure of uncertainty for one trial is known as shannon’s entropy of distribution q1, ..., qn (in physical literature it is also known as a measure of chaos or disorder). after one trial the uncertainty about the future outcomes decreases by the value s = sn− sn−1, this decrement equals to the information gain i = s, obtained as a result of single trial. the quantity is the (unconditional) entropy of distribution for the indicator of the seismic event h in a space-time cell in the absence of any precursors. the conditional entropy s (h|as+1 ≤ g < as) under condition that in the cell (i, j, k) the alarm level s is set up equals the expected conditional entropy sr(h) of indicator of seismic events where the averaging in performed by the distribution of precursors r takes the form we conclude that the knowledge of the precursor values helps to reduce the uncertainty about the future experiment by s (h) − sr(h) which is precisely information i (r,h) obtained from the precursor. taking into account equations (8.1), (8.2) and the fact that we specify the information gain as by analogy with the one-dimensional case [kolmogorov 1965] the quantity i (r,h) may be called the mutual information about the random field h that may be obtained from observations of random field r. it is known that the information i (r,h) is non-negative and equals to 0 if and only if the random fields h and r are independent. this mutual information i (r,h) takes its maximal value s (h) in an idealized case of absolutely exact forecast. the mutual information is the information that the distributions of precursors contribute to that of the indicator of a seismic event. for this reason it may be considered as an adequate scalar estimate for the quality of the forecast. the quantity i (r,h) depends on the cell size, i.e., on the space discretization length a and time interval dt. we need a formal test to compare precursors defined for different size of the discretization cells. for this aim let us introduce the so-called ‘efficiency’ of precursors as the ratio of information gains this efficiency varies between 0 and 1 and serves as a natural estimate of information quality of precursors. it allows comparing different forecasting algorithms and select the best one. a natural estimate of s (h) based on equations (5.1) and (5.2) is defined as follows taking into account equation (8.1) and using an estimate of pa(as+1 ≤ g < as) in the form of ratio xs= ns— n , we construct an estimate of i (r,h) as follows remark. it seems reasonable to introduce a penalty !... ! !n n n n m n1 = ! . .log logs nn n q qn nn i i n i2 1 2. .= | , ..., logn s n s q q q q 1 1 n n n i i n i1 2 1 = == q v | log logs h ppp p 11 aa a a2 2 -= p-q q qv v v . log log s h p p! p p pa a g a11 r s s s s s s p$ a s s 1 2 2 1 1#=+ + = +q q q q v v v v p! p$ | p p pa a g aa s s s pa s s 1 1 1#= = +q v| , . r log log i h p p p# p p p pa a g a1 1 1 a s s s s a s s pa s s 1 2 2 1 1#= + + = + q q q v v v p# & | log logs h a g a ppp p 11s s ss s s1 2 21# -= p-+r q qw v v ,r r r h s h i =q qqv , . h h v v .log logs h 1 12 2m m m m=-tq q qv v v , ,r log logi h 1 1 1 s s s s s s s2 2 1 i m i i m i x= + = tq qv v# &| ; ,r r r h s h i =t t tq qqv , . h h v v earthquake forecasting: statistics and information (8.1) (8.2) (8.3) (8.4) (8.5) related to the number of superficious parameters in evaluating the quality of forecast pointing to the natural analogy with the akaike test [akaike 1974] and similar methods in information theory. in our context the main parameter of importance is r̂ (r; h) and its limit as t →∞. this quantity does not involve the number of parameters directly. probably, the rate of convergence depends on the number of parameters but this dependence is not studied yet. 9. the forecasting procedure the number of time intervals, i.e., the number of observations n used in the construction of estimates increases with the growth of observation time. so, the computation procedure requires constant innovations. on the other hand some computation time is required to ‘adapt’ the model parameters to the updated information about seismic events via an iterative procedure. for these reasons we propose the following forecasting algorithm. (1) given initial parameter values at the moment tk−1= t0 + (k − 2)dt we optimize them to obtain the maximum of efficiencys r̂ (r; h) of a precursor in domain xk−1. for this aim the monte-carlo methods is helpful: one perturbs the current values of parameters randomly and adapts the new values if the efficiency increases. the process continues before the value of efficiency stabilized, this may give a local maximum, so the precedure is repeated for a sufficient number of times. the choice of the initial value on the first step of the optimization procedure is somewhat arbitrary but a reasonable iteration procedure usually leads to consistent results. the optimization procedure takes the period of time tk−1< t ≤ tk. (2) next, we construct the forecast in the following way. at the moment tk the values of precursor ĝ in each cell (i, j, k + 1) is computed with optimized parameters. based on these parameter values the alarm levels, the point estimates and confidence intervals are computed in each cell as well as the values of efficiency of precursors. (3) the estimates of stationary probabilities of seismic events in the cell î(i, j) are defined as follows: they can be used for creation of maps of seismic danger (but not “hazard”, this term is reserved) in the region. 10. retrospective and prospective informativities the efficiency of precursor which is achieved as a result of parameters optimization could be considered as retrospective as it is constructed by the precursors adaptation to the historical catalogs of seismic events. the prospective efficiency for the space-time domain containing the cell in the ‘future’ is based on the forecast. it is computed via formulas (8.3), (8.4), (8.5) with the only difference to the retrospective efficiency, however, approaches this value with time. in principle, the prospective efficiency is an ultimate criterion of the precursors quality and the retrospective efficiency could serve only for the preliminary selection of precursors and their adaptation to the past history of seismic events. 11. testing of the forecasting algorithm the efficiency of precursor could be computed exactly only in an idealized case of infinite observation time. however, its estimate may be obtained based on the observation over a finite time interval. so, if an estimate produces a non-zero value not necessarily the real effect is present. it may be simply a random fluctuation even if the precursor provides no information about the future earthquake. for this reason we would like to check the hypothesis h0 about the independence of a precursor and an event indicator with a reasonable level of confidence. in case the hypothesis is rejected one has additional assurance that the forecasting is real, not just a “ghost”. so, consider the distributions and the function pa(pa −1(y)) = y of variable y = pa(x) provides an uniform distribution f*(y) = pr {p ≤ y} of some random variable p on [0,1]. next, consider a distribution function g(y) = p'a(pa −1(y)) on [0,1], and use a parametric representation for abscissa pa(x) and ordinate p'a(x). if random fields g and h are independent the distribution functions pa(x) = p'a(x) and g(y) are uniform. so, the hypothesis about the absence of forecasting, i.e., about the independence of g and h is equivalent to the hypothesis h0 that the distribution g(y) is uniform. the empirical distribution gl(y) related to g(y) is defined as follows. denote by ul, l = 1, ..., l the values of the function g (i, j, k) sorted in the nondecreasing order and belonging to the cells where h (i, j, k) = 1. let nl be the numbers of cells such that h (i, j, k) = 1, g (i, j, k) = ul. denote by m (ul) the numbers of cells such that g (i, j, k) < ul, and define the empirical distribution gl(y) as a stepwise function with gl(0) = 0 and positive jumps of the size nl— l at points yl = m (ul)—— n . the well known methods of hypothesis testing requires that the function gl(y) has the same shape as for ;i j k 1 k k 1 i = = tq v | ,, ,s i j k 1 i q v| , , ,pripa x g i j ki j x 1 ; pa a rijij i j #= ! q q q v v v e h| , , , , .prpa x i j g i j k x h i j k 1 1 ; pa ijij i j a #= = ! l q q q q v v v v f i| gertsik et al. 10 · · 11 independent trials, i.e., random variables ul, l = 1, ..., l are independent in view of axiom (iv). naturally, we accept the precursors such that the hypothesis h0 is rejected with the reasonable level of confidence. (remind, that the hypothesis is accepted if and only if its logical negation could be rejected based on the available observations. the fact that the hypothesis cannot be rejected does not mean at all that it should be accepted, it only means that the available observations do not contradict this hypothesis. say, the well-known fact that “the sun rises in the east” does not contradict to our hypothesis, however it may not be considered as a ground for its acceptance.) for large values of l the kolmogorov statistics [kolmogorov 1933a] is helpful for this aim with an asymptotic distribution or smirnov’s statistics [smirnov 1939] with asymptotic distribution the asymptotic expressions for these statistics can be used for l > 20 [bolshev and smirnov 1965]. 12. the binary alarm and the hypothesis testing the prediction is the form of forecast when an alarm is announced in a given cell without a preliminary evaluation of the probability of a seismic event. in this case we can estimate the probabilities of events, too. (if the alarm is announced in an arbitrary domain x we set up an alarm if at least half of the cell of our model is occupied by alarm.) let m be the number of cells in x which are in the state of alarm, m0 be the number of cells where the seismic event is present but no alarm was announced (the number of ‘missed targets’). denote by x = m— n the share of the cells with alarm announced, m = l— n the share of the cells with seismic events, and o = m0— m the share of missed targets. let a random variable h (i, j, k) equal 1 if an alarm is announced in the cell (i, j, k) and 0 otherwise. obviously, the estimate of conditional probability pr {h (i, j, k) = 1|h (i; j; k) = 1} of the seismic event under the condition of alarm is m——(1 −o)——— x ; and the estimate of conditional probability pr {h (i, j, k) = 1|h (i; j; k) = 0} of the seismic event under the condition of no alarm is mo——— 1−x . if the alarm is announced according to the procedure described in section 5 the threshold a1 specifying the acceptable domain of values for g (i, j, k) should be treated as a free parameter and selected by maximizing the information efficiency r̂ (h; h). the estimate of information increase for given values of x and o equals the value of h (i; j; k) characterizes the results of checking two mutually exclusive simple hypothesis: h0: the distribution of f *(i, j, k) has the form implying ‘no seismic events’, or h1: the distribution of f *(i, j, k)) has the form implying the presence of seismic event. statistics for checking of these hypothesis is the precursor g (i, j, k), and the critical domain for h0 has the form {g (i, j, k) ≥ a1}. (if usual method of alarm announcement is used the relevant precursor plays the role of statistics and the critical domain is defined by the rule of the alarm announcement). the probability of a first type error it is estimated as x−m——— 1− (1−o) ——— m . the probability of second type error is estimated as o. (note that due to condition (iii) any test used for checking these hypothesis should not depend on the coordinates of the cell). the neyman-pearson theory allows to define the domain of images of all possible criteria: in coordinates (a, b) it is a convex domain with a boundary c which corresponds to the set of uniformly most powerful tests. this family may be defined in terms of the likelihood ratio k(x) = p1a(x)——— p0a(x) under condition that the distribusupd gl l= q yy -v ( ) , ,lim pr l d z e z1 0 l l k k k z2 2 2 2# = "3 3 3 =-f i | sup inf d g d g l l l l = =+ q! ! ,$ ,$y y y y l -q q v v ,$ ,$ , . lim pr lim prl d z l d z e z1 0 l l l l z2 2 2 # #= = = " "3 3 + f fi i , . log log log log i h 1 1 1 1 1 1 1 1 1 1 2 2 2 2 h m o x o mo x o x m o m x x m o x mo x m x mo = + + + + + tq q q q q q q q v v v v v v v v! $ , , , , ,prapa x f ) i j k i j kx h 0pa ra 0 #= =ff )q q qv v vff i , , , , ,prapa x f ) i j k x h i j k 1pa ra 1 #= =ff )q q qv v vff i , , , , ,pr g i j k h i j k1 0a = = =q qv vf i , , , , ,pr g i j k h i j k0 1b= = =q qv vf i earthquake forecasting: statistics and information tions p1a(x) and p 0 a(x) have the densities p 1 a(x) and p0a(x). where ~ denotes the threshold. in gercsik and kelbert [2004] we demonstrated that among all the tests with the images on the boundary c there exists three different best tests. here the term “best” may be understood in three different senses, i.e., maximizing the variational, correlational and informational efficiency. the most relevant criteria is the informational efficiency r̂ (h; h). 13. the choice of precursors we use the term ‘empiric precursor of an earthquake’ for any observable characterisric derived from the catalog only, which provides for this catalog a reasonable retrospective forecast of seismic events, and which is not derived from basic physical concepts of seismicity (say, the periods of relatice calm, deviation of some basic characteristic from a long-time average , etc). in contrast, the physical precursors are derived from some physical processes and characterize physical quantities (stress fields, strength, concentration of cracks, etc.) or well-defined physical processes (i.e., phase transitions, cracks propagations, etc.) in the meteorological forecast the danger of using empirical precursors was highlighted by a. kolmogorov in 1933 [kolmogorov 1933b]. from that time the meteorological forecast relies on the physical precursors which are theoretically justified by the models of atmospheric dynamics. below we will present kolmogorov’s argument adapted to the case of seismic forecast. this demonstrates that the purely empirical precursors work well only for the given catalog from which they are derived. however, their efficiency deteriorates drastically when they are applied to any other independent catalog. consider a group of k empirical precursors used for a forecast and selected from a set of n such groups. according to kolmogorov’s remark the number k is typically rather small. this is related to the fact that a number of strong earthquakes in catalog is unlikely to exceed a few dozen. as the values of precursors are random there exists a small probability p that the efficiency of the forecast exceeds the given threshold c. then the probability of event r̂ (r, h) ≤ c equals 1 − p, and the probability of event r̂ (r, h) > c for at least one collection of precursors equals p = 1 − (1 − p)n and tends to 1 as n → ∞. (according to kolmogorov some arbitrariness of the assumption of independence is compensated by the large number of collections.) summing up, if the number of groups is large enough with probability close to 1 it is possible to find a group giving an effective retrospective forecast for a given catalog. in practice this is always the case as the number of empirical precursors could be increased indefinitely by variation of real parameters used in their construction. it is important to note that for such a group, which is highly efficient for the initial catalog, the probability that the efficiency is greater than c is still equal to p for any other catalog. in other words: the larger the number of the groups of empirical precursors the less reliable the forecast is. so, the collection of a large list of the empirical precursors is counter-productive. much more reliable are the physical precursors intrinsicly connected with the physical processes which preserve their values with the change of sample. the probability to find such a set of precursors by pure empirical choice is negligible because they are very rare in the immense collection of all possible precursors. 14. demonstration of algorithm a preliminary version of the forecast algorithm described above was used in the paper [ghertzik 2008] for califorrernia and the sumatra-andaman earthquake region. these computations serve as a demonstration of the efficiency of the method but their actual results should be taken with a pinch of salt because the selection of precursors does not appear well-justied from the modern point of view: the number of free parameters (34) to be adapted in the precursor “stress indicator” is too large. for this reason the method was not fully exploited. nowadays, the progress is resumed with a particular emphasis on the selection of a new set of precursors strongly related to the critical phenomena in geophysics. the following selection of precursors was used: (1) the stress indicator is a scalar predictor characterizing the local level of the stress field. it is conceivable as a stress tensor component. the predictor increases due to tectonic movements, decreases with viscosity and with increasing fracturing, and is affected by earthquake sources considered as cracks. (2) the damage indicator is a predictor characterizing the stress-induced microfracturing level. the predictor increases with the value of applied stresses and decreases as cracks are healed. (3) the third predictor is the concentration zhurkov-sobolev criterion of seismically active faults based on the calculation of the parameter concentration (density) seismogenic ruptures and modified in such a way that it can be calculated for a local cell. (4) the far-order predictor is the integral of the radial correlation function for seismic energy released in a spatiotemporal cell. its construction is based on the idea of the strong earthquake as a phase transition be, , , , i j k i j k i i1 0 h h = =q q v vg iff x iff x 2 1 ~ ~ k k q q v v gertsik et al. 12 13 tween consolidation and fracture states of a medium. the phase transition is preceded by the appearance of a far order, an abrupt increase in the correlation radius for the energy of shocks released in a cell. for detailed description of precursors we refer to original, for it is only an illustration but not the topic of this work. it is useful, however, to summarize the main results of this paper. california region. the catalog global hypocenter data base cd-rom neic/usgs, denver, co, 1989, together with data from the site neic/usgs pde (ftp://hazard.cr.usgs.gov) for earthquakes with magnitudes m ≥ 4.0 with epicenters between 113°129° of western longitude and 31°43° of northern latitude was used for parameter adaptation. the initial time t0 was selected by subtracting from the time of actual computation, 08.03.2006, an integer number of halfyear intervals such that t0 fits the first half of the year 1936. (the final time 08.03.2006 could be considered as an initial moment for constructing half year forecast forward up to the date of the latest earthquake available in the catalog). during the computation the time interval from the first half of 1936 to the first half of 1976 was used for relaxation of the zero initial data used for precursors. after this date the catalog for the earthquakes with magnitude m ≥ 4.0 was used to estimate the probabilities of strong earthquakes with magnitude m ≥ 6.0 up to the moment of actual forecast. note that the adapted restriction to include into consideration only earthquakes with magnitudes m ≥ 4.0 is a severe restriction. it decreases the precision of precursor computation and therefore, if a prediction is successful, increases the degree of condence in the predictor choice. we choose a = 150 km as a size of the spacial lattice, and dt = 6 months as a time-step. retrospective forecast was performed with 5 alarm levels defined by the thresholds as = 10 −a(5− s), s = 1, ..., 4 and a = 0.75. (due to too short time step no alarms was registered on the lowest level when parameter a = 1 was selected). in order to reduce the influence of the boundary conditions the large square covering all the seismic events in the catalog used in the computations was reduced by two layers of elementary cells from each boundary. as a result of optimization the forecast information efficiency of 0.526 was achieved, i.e., the forecasting algorithm applied to the given catalog extracted from it about 53% of all available information about seismic events. it seems that this result could be only partially explained by a lucky selection of precursors: another contributor to the high efficiency of the algorithm is the adaptation of the parameters to the features of the specific catalog. the influence of this artificial information may be reduced only with the increase of the observation interval. accepting the rule of binary alarm announcement in the cells from group 1 and 2 from 5 levels possible one obtains that the space-time share of alarmed cells is 3.4% and the share of missed targets is 18.2%. this result is comparable with the best forecasts available in the literature and obtained by other methods (in the cases when the quantitative parameters of algorithms are presented in the publications). when the forecast was constructed in the future we obtained that the estimate of probability of a strong earthquake anywhere in the area under study is 0.174, and the maximal point estimate of an event in any individual cell is 0.071. as a whole the seismic situation in california did not look too alarming. indeed, there were no strong earthquakes in california in the next half-year. earthquake forecasting: statistics and information alarm level number of cells number of events left boundary of interval point estimate right boundary of interval 1 43 25 0.4186 0.5814 0.6998 2 155 11 0.0376 0.0710 0.1172 3 457 6 0.0063 0.0131 0.0291 4 1152 2 0.0005 0.001 0.0071 5 4049 0 0 0 0.001 figure 1. mapped areas of five levels of half-year forward forecasting for california on march 8, 2006. table 1. number of events and alarm cells for each level of the retrospective analysis for california. table 1 presents the number of events and the total number of alarm cells for each level of the retrospective analysis, as well as their ratios, which are wellgrounded point estimates of the probability that at least one earthquake with a magnitude m ≥ 6.0 will occur in a cell at a given level of alarm. the 5% condence intervals are also given in the table for each of these probabilities. the true values of event probabilities lie within these intervals with a 95% probability (condence coefficient). in all, 5856 cells participated in the prediction, and 44 of these cells (0.75% of all cells) contained events. sae region. we have conducted a retrospective forecast of strong earthquakes with magnitudes m ≥ 7.0 for the whole region where the sumatra-andaman earthquake (sae) happened on 26.12.2004 with magnitude m = 9.3. the catalog global hypocenter data base cdrom neic/usgs, denver, co, 1989, together with the data from the site neic/usgs pde (ftp://hazard. cr. usgs.gov) for earthquakes with magnitudes m ≥ 5.5 with epicenters between 84.3°128° of eastern longitude and 20°26° of northern latitude was used for the parameters adaptation. the initial moment of time t0 was selected by subtracting from the time of actual computation, 10.11.2004, an integer number of half-year intervals such that t0 fits the first half of the year 1936. (the final time was selected in such a way that the next half-year period covers sae and its powerful aftershock). during the computation the time interval from the first half of 1936 to the first half of 1976 was used for relaxation of the zero initial data used for precursors. after this date the catalog was used to estimate the probabilities of strong earthquakes with magnitude m ≥ 7.0 up to the moment of actual forecast. (for magnitude m ≥ 7.5 the number of seismic events was not sufficient for reliable forecast because the 5%-confidence intervals strongly overlapped). in this case the restriction to include into consideration only earthquakes with magnitudes m ≥ 5.5 was adapted. as before, it decreases the precision of precursor computation and therefore, if the prediction is successful, increases the degree of confidence to the predictor choice. we selected the size of the spacial grid as a = 400 km and the size of time-step dt = 6 months. retrospective analysis was conducted following the same scheme as in the previous case. in order to reduce the influence of the boundary conditions the large square covering all the seismic events in the catalog used in the computations was reduced by two layers of elementary cells from each boundary. as a result of optimization the forecast information efficiency was 0.549, i.e., the forecasting algorithm extracted around 55% of all available information about seismic events when applied to the given catalog. in the case of binary alarm announcement the space-time share of alarmed cells was 3.1% and the share of missed targets was 8.3%. this result is comparable with the best forecasts available in the literature and obtained by the other methods (in the cases when the quantitative parameters of algorithms are presented in the publications). in the case of forward forecast the two most powerful earthquakes, i.e., sae and its major aftershock, happened in the alarm zone of the second level, and two other events with smaller magnitudes in the zone of alarm level 4. the results of the half-year forward prediction beginning from november 10, 2004, and m ≥ 7.0 earthquakes that occurred in this period are plotted in figure 2. as seen from the figure, the two strongest events (the sae and its strongest aftershock) occurred in the level2 alarm zone, while two weaker events occurred in the level-4 alarm zone. it is noteworthy that, if monitoring gertsik et al. 14 figure 2. mapped areas of five levels of half-year forward forecasting for the sae region on november 10, 2004. 15 in this region were conducted according to the proposed scheme, a nine-cell square cluster containing one cell of the level-1 alarm would be regarded as hazardous, giving every reason for anxiety in relation to the impending sae. table 2 presents the number of events and the total number of alarm cells for each level of the retrospective analysis, as well as their ratios, which are wellgrounded point estimates of the probability that at least one earthquake with a magnitude m 7.0 will occur in a cell at a given level of alarm. the 5% condence intervals are also given in the table for each of these probabilities. in all, 8352 cells participated in the forecasting and 36 of these cells (0.43% of all cells) contained events. figure 3 plots the dependence of the fraction of target misses on the spatiotemporal fraction of alarms for points at which the boundaries between alarm levels divide the set of all cells into two classes: union of more anxiety cells and union of less anxiety cells. it is an estimate for molchan’s error diagram. 15. conclusions this article demonstrates that the prediction method based on the pattern recognition does not have sufficient foundation and contains a number of errors. we propoused here the alternative approach that is free of this errors and enhances the computing capability of the forecasting. discretization of spacetime, using elements of the theory of multicomponent random processes and axiomatization of the problem allows to drastically simplify the procedure of earthquake forecasting and to obtain the following results: (1) the method of announcement to declare a multi-level alarm was found. (2) the method of calculating the point estimate of probability of an event for each alarm level was found. (3) the method of calculating interval estimates of event probability for each level of anxiety was found. (4) the equivalence of the announcement of a two-phase alarm and solving the problem of testing hypotheses was shown. (5) the method of testing an algorthim of forecast was found. (6) the scalar criterion for comparison of different algorithms of forecast was found. (7) the equivalence of the method of the announcement of alarms and method assessing the probability of event was shown. it appears to the authors, that a satisfactory mathematical formalism of earthquake forecasting based on arbitrary set of precursors is fully fledged. the most important result we believe is the discovery of the scalar quality criterion of forecast algorithms. the criterion allows to find out which one is the best from existing algorithms and to focus on finding the most effective precursors. acknowledgements. we would like to thank v. pisarenko and g. sobolev for stimulating discussions that gave us the impulse for writing this paper. m. kelbert thanks the hse for the support in the framework of the implementation of the global competitiveness program. earthquake forecasting: statistics and information alarm level number of cells number of events left boundary of interval point estimate right boundary of interval 1 36 18 0.3300 0.500 0.6700 2 222 15 0.0400 0.0676 0.1052 3 220 2 0.0015 0.0091 0.0281 4 602 1 0 0.0017 0.0070 5 7272 0 0 0 0.0006 figure 3. empirical molchan's diagram: the share of missed targets vs the share of alarms. table 2. number of events and alarm cells for each level of the retrospective analysis for sae region. references akaike, h. 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(1990). strategies in strong earthquake prediction, phys. earth planet. in., 61, 84-98. molchan, g.m., and v.i. keilis-borok (2008). earthquake prediction: probabilistic aspect, geophys. j. int., 173, 1012-1017. pisarenko, v.f., a. sornette, d. sornette and m.v. rodkin (2008). new approach to the characterization of mmax and of the tail of the distribution of earthquake magnitudes, pure appl. geophys., 165 (5), 847-888. prohorov, yu.v., and yu.a. rozanov (1969). probability theory, basic concepts. limit theorems, random processes, springer-verlag, 401 p. smirnov, n.v. (1939). on deviations from the empirical distribution curve, mat. sb., 6 (48), 1, 3-24. vere-jones, d. (1998). probability and information gain for earthquake forecasting, in: problems of geodynamics and seismology, computational seismology, moscow, 30, 248-263. wgncep, working group on northern california earthquake potential (2007). the uniform california earthquake rupture forecast, version 3 (ucerf3) project plan: u.s. geological survey open-file report 1-176, 2011. * corresponding author: vladimir gertsik, institute of earthquake prediction theory and mathematical geophysics ras, moscow, rf; email: gercsik@mitp.ru, gertzik@yandex.ru. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. gertsik et al. 16 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol52,3,2009 edited by alain bourdillon, ljiljana r. cander and bruno zolesi a n n a l s of geophysics a n n a l i d i g e o f i s i c a vol52,3,2009 20-09-2009 19:05 pagina iii vol52,3,2009 20-09-2009 19:05 pagina iv v annals of geophysics, vol. 52, n. 3/4, june/august 2009 preface this issue of annals of geophysics contains a series of papers which form the final report of the cost (european cooperation in science and technology) 296 action on the mitigation of ionospheric effects on radio systems (miers). it is important to recognize that cost is one of the longest running european instruments supporting cooperation among scientists and researchers across europe. prior to the cost 296 action, strong collaboration in the field of ionospheric research and applications had already been established under three previous actions on prediction and retrospective ionospheric modelling over europe (prime), on improved quality of service in ionospheric telecommunication systems planning and operation, and effects of the upper atmosphere on terrestrial and earth-space communications. the miers main objective was to broaden our knowledge of the effects imposed by the ionosphere on practical radio systems, and for the development and implementation of techniques to mitigate the deleterious effects of the ionosphere on such systems. as a result of all these activities, more than 130 researchers from about 40 institutions in 26 countries all over the world currently belong to the ionospheric cost community. all the papers in this supplement are in the format employed in the international scientific literature. they describe the principle results achieved in the cost 296 action. in addition to these scientific papers, an introductory paper briefly covers the background, the objectives as defined in the memorandum of understanding, the management structure and architecture of the action, and the technical work programme. an overview of cost 296 activities has been included together with a concluding paper summarising the overall outcomes and looking forward to further investigations in a follow-on action. we would like to thank all the participants of this action, the working group and package leaders, the secretary of the action mrs. angela vernon, and all the local organizers of the different workshops and management committee meetings. lastly, we also wish to acknowledge prof. enzo boschi, president of the istituto nazionale di geofisica e vulcanologia, for supporting the publication of this issue of annals of geophysics. alain bourdillon ljiljana r. cander bruno zolesi vol52,3,2009 22-09-2009 14:16 pagina v annals 48, 3, 2005+app1 439 annals of geophysics, vol. 48, n. 3, june 2005 key words gnss – global positioning system – single point positioning – ionospheric biases correction 1. introduction the global positioning system (gps) is the most developed and extended global navigation satellite system (gnss). its field of application ranges from sub-centimetre level accurate geodynamic studies to decametre level stand-alone positioning. stand-alone positioning, perhaps the most extended use of gps, is normally performed using inexpensive receivers that do not record the observed gps signal measurements, but the coordinates. it is well known that the accuracy of gps positioning is degraded by several biases (seeber, 2003). some of these biases are: delays caused by the atmosphere (ionosphere and troposphere), satellite clock and orbit inaccuracy and spurious signal reflection (multi-path). the main bias is the ionospheric delay, which is typically one order of magnitude greater than any of the other afore mentioned biases (mannucci et al., 1999). in order to mitigate the ionospheric bias effect on positioning, gps receivers use an embedded ionospheric model called ionospheric correction algorithm (ica) (klobuchar, 1987). the gps user community has developed several alternative methods to correct this effect. generally, these methods work in post-processing mode. therefore, the recording of the observed satellite-to-receiver pseudoranges is required. other methods designed for real time positioning require at least a real time gps correction provider and a communication link between the user and the provider. ionospheric biases correction for coordinates derived from gps single point positioning mauricio gende (1)(4), elsa mohíno harris (2), claudio brunini (1), sandro m. radicella (3) and miguel herraiz (2) (1) facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata, argentina (2) departamento de geofísica y meteorología, universidad complutense de madrid, spain (3) aeronomy and radiopropagation laboratory, the abdus salam international centre for theoretical physics (ictp), trieste, italy (4) consejo nacional de investigaciones científicas y técnicas, argentina abstract most gps users employ low cost receivers. these receivers do not allow users to record the pseudorange that they observe, but the computed coordinates. this work presents an original and simple method to correct ionospheric biases introduced in gps signals. the originality of this method is based on the fact that no pseudorange is needed to correct the biases, only the calculated coordinates are used. this distinguishes this method from other classic alternatives. this paper evaluates the efficiency of the method with the use of real data. mailing address: dr. elsa mohíno harris, departamento de geofísica y meteorología, universidad complutense de madrid, avenida complutense s/n, 28040 madrid, spain; e-mail: emohino@fis.ucm.es 440 mauricio gende, elsa mohíno harris, claudio brunini, sandro m. radicella and miguel herraiz this paper will present a post-processing method to mitigate the effect of the ionospheric bias on positioning that does not require the observed pseudorange but only the user coordinates and the pseudo random number (prn) of the observed satellites. the method does not depend on the ionospheric model used and it accepts corrections from any algorithm that models the ionosphere. 2. low precision positioning techniques a detailed review of the gps characteristics, positioning algorithms and different biases affecting the observations can be found in several classical texts (e.g., hofmann-wellenhof et al., 2001). three of the most common single-epoch positioning techniques will be briefly summarized in the following sections. 2.1. stand-alone positioning this is the simplest positioning method in which coordinates are estimated based on range measurements derived from satellite to receiver travel time. satellite positions, i.e. the ephemeris, are known since they are encoded in the broadcasted information. theoretically, having three simultaneous range measurements the problem can be solved by trilateration. however synchronization error due to the receiver clock adds one unknown and consequently at least one extra measurement is needed. in addition to the ephemeris information, the system broadcasts predicted ionospheric parameters in order to reduce the ionospheric error using the ica model. 2.2. precise post-processing point positioning this technique differs form the previous one only by the fact that it includes post processed information. precise ephemeris and satellite clock corrections replace the broadcast ones. an ionospheric delay estimation based on continuously updated global or regional ionospheric models replaces the ica model. as a result of including this improved information coordinates are more accurately determined (héroux and kouba, 1995). the main drawback of this technique is the need to record the observed pseudorange, which is not possible with many inexpensive receivers. 2.3. differential gps positioning (dgps) the basic idea behind differential positioning (blackwell, 1986) is to correct the observed pseudoranges in one gps receiver (rover station) provided that these corrections could be deduced from another receiver placed in a fixed point with well known coordinates (reference station). dgps can work either in post-processing or real time mode. in the first case, two receivers observing and recording pseudorange measurements are needed simultaneously. in the second case, a correction provider and a communication link between the user and the provider are required. 2.4. comparison between the three techniques table i shows typical values for horizontal and vertical errors that result from point positioning, precise point positioning and differential positioning. although these particular values were acquired during only a 24 h period with a sample rate of 30 s, they represent typical errors for each technique. 3. algorithm to mitigate the effect of the ionospheric bias on positioning the idea of the proposed correction algorithm can be summarized as follows: coorditable i. standard deviation error for the different positioning techniques. horizontal vertical error (m) error (m) point positioning 3.5 12.5 precise point positioning 2 3 differential positioning 0.2 0.75 441 ionospheric biases correction for coordinates derived from gps single point positioning nates could be corrected from ionospheric error by a geometric propagation of an ionospheric biases estimation. note that in this process no observed range is needed. 3.1. error propagation into coordinates the basic equation of observation for each range measurement is c t t s r= -t t d d ot ^ h (3.1) tt being the satellite-to-receiver observed pseudorange, ρ the true geometric range, c the speed of light in vacuum, δ t s and δ tr the synchronization errors of the satellite and receiver clocks respectively, ν is the total budget of remaining error that biases the measurements. if d0= +t t t and d rt , cos nad + cos coser r-bd cdo , where cosα, cosβ, cosγ are the director a cosine, (3.1) transforms into . cos cos cos c t n e c t s r r r r 0+ + = + + t t d o ad bd cdo d t (3.2) if there are simultaneous observations to n satellites, using matrix notation (3.2) transforms into a x=t∆ ∆ (3.3) with cos cos cos cos cos cos c c a n n n 1 1 1 f f f f f f f f f f f f = a a b b c c r t s s s s s ss v x w w w w w ww and , , , .n e tx r r r r t = d d do d∆ ^ h this linear system can be solved by least squares x a a a t t 1 = t∆ ∆ ^ h . (3.4) finally, the position vector xt , can be found by iteration using x x x0= + ∆t . where x0 has a first guess value or is to the value obtained in the previous iteration step. it is important to note that eq. (3.4) represents the way that errors are propagated in the coordinate. if t 0s0 = == dt tt , ν = νionosphere and ionosphere=t o∆ ∆ then the error on the coordinates due to the ionosphere can be easily computed using eq. (3.4) x a a a t t 1 ionosphere ionosphere= o∆ ∆ ^ h . (3.5) 3.2. method to correct ionospheric biases at coordinate level ionospheric bias is proportional to slant total electron content (stec). this relation can be expressed as: k stec1ionosphere #=o∆ , where k1 is a constant and stec is defined as the integral of free electrons density along the satellite-receiver ray path. there are several ways to estimate stec values, the present work used global ionospheric maps (gims) produced by the center for orbit determination in europe (code) (schaer et al., 1996). gims provide worldwide grids of vertical total electron content (vtec); they are computed using a global network of dual-frequency gps receivers. code gims are available at <http://www.cx.unibe.ch/aiub/ionosphere.html>. in order to convert vtec values into stec values an appropriate mapping function was used: m k estec 1 2 2cos1 -= vtec , ^ h where k2 is a constant and e is the satellite elevation. having access to stec estimations from gims an ionospheric correction for each pseudorange can be computed as m vtecionosphere code gims #=o∆ -u . (3.6) using eq. (3.5) the impact of those stec estimations on the coordinates is obtained x a a a t t ionosphere ionosphere 1 = o∆ ∆ u u^ h . (3.7) users with a low-cost gps receiver will not have access to each pseudorange measurement, but only to the final biased coordinates xt which can be written as x ionosphere= x + x∆ +t x _other biases+ ∆ where x is the exact or true value, evidently not accessible, and usually >xionosphere∆ > > x _other biases∆> . finally, if xionosphere∆u is provided it can be subtracted from xt and a more accurate position can be found 442 mauricio gende, elsa mohíno harris, claudio brunini, sandro m. radicella and miguel herraiz . x x x x x x x _ improved ionosphere ionosphere ionosphere other biases = + + = + ∆ ∆ ∆ ∆ t t u u (3.8) compared to the methods presented in sections 2.2 and 2.3, the one proposed in this paper does not require access to the pseudorange. this is a clear advantage for situations where pseudoranges are not available, for instance when inexpensive original equipment manufacturer (oem) gps boards are used for vehicle tracking. the algorithm presented in this paper takes it for granted that the satellite geometry contained in matrix a is the same for the user biased coordinates xt , and for the estimated ionospheric correction, xionosphere∆t . if this assumption is wrong the correction will lose precision; thus, caution must be taken to ensure that the satellites involved in the positioning algorithm are the same as those involved in the correction procedure. 4. results 4.1. data set the results shown in this section are based on nearly 360000 samples. two gps stations were used, located one at geomagnetic mid latitude, california, and the other one at geomagnetic low latitude, galapagos islands. two time periods were analysed, the solstice and the equinox. for each period a set of thirty days with non perturbed ionospheric conditions was selected. all data correspond to a high solar activity period, the year 2001. to avoid problems due to poor signal-to-noise ratio, an elevation mask of 10° was imposed. epochs with poor satellite geometry distribution (dilution of precision greater than 6) were not processed. in order to evaluate the performance of the algorithm four positioning strategies were used: 1) stand-alone positioning without any correction (raw); 2) stand-alone positioning applying ica correction (ica); 3) stand-alone positioning using the ionospheric free linear combination (ion free); 4) coordinates correction using a ionospheric model (cc). the first approach represents a situation where the user has no access to any ionospheric correction. the second one corresponds to the user that applies the ionospheric correction provided by the gps system. this should be the standard way of point positioning, but unfortunately not all gps receivers apply the ica corrections. in the third case the user has a double frequency receiver and can eliminate almost all the ionospheric bias (hartmann and leitinger, 1984). note that this latter case is extremely unusual since double frequency receivers are very expensive and usually are not used in point positioning. this situation was included only to show the magnitude of the error that still remains even when more that 99% of the ionospheric effect was eliminated. the last approach uses the method introduced in this paper. in this case gims maps are used in order to estimate vtec. in order to mitigate other sources of biases both precise ephemerides and a tropospheric model were used in the four presented techniques. 4.2. algorithm performance table iia,b presents the accuracy of each positioning method for galapagos and california respectively. results agree with some well known facts about the ionosphere. raw errors clearly show that the ionosphere is more active during the equinox and at low latitudes. ica and code errors demonstrate that, because of its complexity, it is more difficult to estimate the ionosphere during the equinox and at low latitudes. results also agree with the positioning theory. ion free errors are independent of the epoch but depend on the noise of the p code, which is clearly higher in the galapagos receiver. as expected, due to the geometric dilution factor the vertical component is the one that tends to absorb the main part of the ionosphere. it is worth noting that the method works well, in fact, though the aim of the present work does not aim at comparing or assessing the quality of any particular ionospheric model, it provided a better result that the ica corrections. cc columns show similar results compared to ion free ones for the vertical, with the exception of the equinox period in galapagos, probably due to the complexity of the vtec for that region in that period. but larger dis443 ionospheric biases correction for coordinates derived from gps single point positioning crepancy values arise for the horizontal components. this is probably due to the fact that in order to model the horizontal components correctly, good horizontal gradients of vtec must be provided (gende et al., 2003). cc method was also compared against post process precise point positioning. differences are in the order of 10 cm for the horizontal component and in the order of 40 cm for the vertical component. figure 1 shows four hisfig. 1. histograms of vertical errors for galapagos during the solstice. table iia,b. coordinate errors in meters for each positioning technique at galapagos islands (a) and at california (b). solstice equinox raw ica cc ion free raw ica cc ion free north 1.72 1.58 1.30 0.68 3.41 3.56 2.62 0.70 east 1.45 1.24 1.29 0.88 2.12 1.78 1.84 0.98 vertical 10.26 3.89 2.56 2.16 15.52 4.24 4.58 2.21 north 2.32 2.04 0.66 0.70 2.50 1.71 1.21 0.67 east 1.01 0.80 0.54 0.52 1.18 0.82 0.67 0.52 vertical 7.78 2.12 1.30 1.42 11.90 2.89 1.92 1.26 a b 444 mauricio gende, elsa mohíno harris, claudio brunini, sandro m. radicella and miguel herraiz tograms of vertical errors for galapagos station. data correspond to the first week of july 2001 and around 18000 samples are plotted. each graph illustrates epoch by epoch residuals for raw, ica, cc and ion free technique. the first plot clearly illustrates the fact that the ionosphere increases the height of the gps station. the second one shows that the ica model partially corrects this effect. the third plot corresponds to the positioning approach presented in this paper. noticeably better than the first two approaches, it slightly tends to overcorrect the ionospheric effect. the last plot presents a better shape but has some outlier values; these are probably due to the p code signal combination necessary to make ion free solution. 5. conclusions results show that the proposed methodology is capable of improving gps positioning accuracy even when no pseudoranges are recorded. the method presented in this work improves the accuracy of positioning with low cost gps equipment without any extra expense. improvements have the same magnitude as those ones obtained in precise point positioning; approximately 7 m for the horizontal components and 11 m for the vertical. this represents an improvement of 80% if no correction is applied or 40% if ica correction is applied. although the proposed methodology cannot present results as good as dgps, it can be an alternative for users who, needing higher accuracy do not have access to a dgps correction provider and cannot record pseudoranges for precise point positioning. this can be the case for many low-priced original equipment manufacturer (oem) gps boards that are typically used in vehicle tracking. acknowledgements data from the igs (international gps service) permanent network of gps receivers were used in this work. these data were obtained thanks to the university of san diego ftp server (ftp://lox.ucsd.edu/pub/rinex). references blackwell, e. (1986): overview of differential gps methods, navigation red book, vol. iii, 89-100. gende, m., s.m. radicella, b. nava and c. brunini (2003): ionospheric effect in instantaneous positioning, in institute of navigation technical meeting, january 22-24, anaheim, california. hartmann, g.k. and r. leitinger (1984): range errors due to ionospheric and tropospheric effects for signal frequencies above 100 mhz, bull. geod., 58, 109-136. héroux, p. and j. kouba (1995): gps precise point positioning with a difference, presented at geomatics ‘95, june 13-15, 1995, ottawa, ontario, canada. hofmann-wellenhof, b., h. lichtenegger and j. collins (2001): global positioning system: theory and practice (springer verlag), 5th revised edition, pp. 382. klobuchar, j.a. (1987): ionospheric time-delay algorithm for single-frequency gps users, ieee trans. aerosp. electron. syst., aes-23 (3), 325-331. mannucci, a.j., b.a. iijima, u.j. lindqwister, x. pi, l. sparks and b.d. wilson (1999): gps and ionosphere, in reviews of radio science 1996-1999, edited by w.r. stone (ursi), 625-665. schaer s., g. beutler, m. rothacher and t.a. springer. (1996): daily global ionosphere maps based on gps carrier phase data routinely produced by the code analysis enter, in igs analysis center workshop, noaa, silver spring, u.s.a. seeber, g. (2003): satellite geodesy (walter de gruyter), 2nd edition, pp. 589. layout 6 on estimation of the displacement in an earthquake source and of source dimensions vladimir keylis-borok annali di geofisica, vol. 12, n. 2, 1959. annals of geophysics, vol. 53, n. 1, february 2010 abstract an ellipsoid of rotation a round fault plane surrounded by a crushing zone is taken as a source model. the area s of the fault plane and the displacement v along it can be estimated using formulas: where the energy e is known (µ is shear modulus; rs, ls coefficients given in figure 2; p means stress before the earthquake; the value p = 3.107 gives satisfactory results in several cases). these formulas are compared with the well-known fact that the frequency of occurrence of earthquakes is proportional to e0.5– e0.6; it is shown that the weak numerous earthquakes can take a considerable part in the formation of large faults and movement along them, though their total role in releasing seismic energy is negligibly small. 1. introduction the study of an earthquake's mechanism is usually finished by the determination of a fault plane and the direction of displacement along it. in many aspects of geodynamics and seismology it would be very important to estimate also the dimensions of a source and chiefly the magnitude of the displacement in it. this problem can be reduced to the comparing of fields of statical stress before an earthquake and after it. the difference of energies of these two fields depends upon source properties in question; hence it is possible to determine dimensions of the source and the displacement in it if the form of the source and the difference of the energies be known. the energy difference can be found from observations on the seismic waves energy. the idea of such an approach belongs to l. knopoff. he solved the two dimensional problem which deals with the study of very strong earthquakes with long faults. the analogical three – dimensional problem is considered in this paper; it is necessary for studying of more numerous weaker earthquakes and investigation of some contiguous problems. 2. theory the problem can be formally stated as following. before an earthquake a homogeneous field of tangentional tractions exist τxy = p, with corresponding displacement u= cy, v= p/µ– cx, c being arbitrary. we shall call this field an initial field. in the moment of the earthquake tangentional tractions vanish along the fault plane, x² + z² ⩽ b², y = 0. a zone of crushing or intence plastic flows can appear around the fault plane. this zone must be included in the source model because the tangential tractions in it decrease or vanish; that is why we shall consider the source as an ellipsoid. now we shall examine a final field of stress which is defined by two conditions: a) displacements and stress tend to initial ones at infinite distances; b) the source boundary is free from stress except the hydrostatic pressure. we shall suppose the final field to appear after the earthquake with the source (1). i neglect here plastic flows and an inhomogeneity of the initial field which can really exist. besides, the source boundary in the above meaning can be really not sharp. thus idealized problem in its main part can be reduced to this one which is already solved by neuber in the theory of stress concentration. i omit here calculations which are not complicated principally and shall give the final result. the neuber's solution is obtained in elliptical coordinates: it can be represented by the formulas: normal stresses. 17 ; p p e s r v s l 2 s s 3 2 == n n 1 , 0 (1) b x z a y a b 2 2 2 2 2+ + = g g 2 sin cos ( 2 ) 1 2 2 2 2 2 sin cos cos (2a) h p sh u v w p h c q t shu chu ch u chu ch u ch u chu ch u h chu chu ch u v v w 2 2 2 0 2 2 2 2 u 0 0 0 = + + + + + + + + v a ` c j m ; e sin cos cos sin cosx chu v w, y shu v, z chu v w;= = = tangentional stresses. displacements. energy-difference. here σu, τuv are the stress components; δuu are the differences between the displacement in the initial and final fields; δw is the stress energy difference (the last 2/3 π thu0 in (5) represents the energy inside (1)). the maximal tangentional stress, according to neuber is: it is interesting to note, that τmax is always more than ~ 2p (figure 1). the displacements change quaintly from point to point. for the measure of displacement we shall take v – the maximal one on the y-axis, that is in the middle of the source boundary: in particular, it is interesting to consider the displacements at the source boundary (u = u0). if the source is thin (a = 0), we have: here signs « + » and « – » correspond to displacements in upper and lower sides, so that the fault does exist in our model. it can be seen from this that in elastic case the fault plane undergoes also a rotation (δuy≠ 0) which reestablishes the statical equilibrium after the forming of a fault. it is not clear whether this rotation will take place in fact because the reestablishment of the equilibrium can be the result of imperfect elasticity. this will not change the order of quantities studied here. however the rotation will essentially on estimation of the displacement in an earthquake source 18 2 sin cos ( 2 2 ) 1 2 2 2 sin cos cos (2b) h p sh u v w p h c q t sh u chu sh u chu h chu ch u chu v v w 2 2 2 0 2 2 v 0 = + + + + + + v a ` e j o = g 2 sin cos 2 2 cos 2 2 cos 2 2 cos (3a) h p ch u h ch u v w h p c q shu t sh u shu qt ch u ch u shu v tch u shu q v ch u ch u shu h shu ch u ch u w 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 uv 0 0 0 = + + + + + + + x a a ` ^ ` ^ ` ` j h j h j j 8 b 3 1 2 1 2 s 1 2 a sh u sshu t shu ch u ch u sshu t a k s ch u chu a k ch u chu a k s 2 2 2 0 2 2 u v v w = + + + = + + = + = a a a a ^ ^ ^ ` ` h h h j j 8 8 b b 2 ; arc tg ( ); cos ; 1 ; ; 3 1 2 ; ; ( ) 2 . t c shu h sh u v s t shu thu b a q sh u c n u ch u n s shu t shuq 2 2 0 0 0 0 0 2 2 = + + = = + = = = + + = = + a m n m n a -^ h 2 4 2 1 0 x u p b u p u 2 2 x y z = = = ! ! r d n a a t d n a a d + + ^ h 2 3 1 2 2 3 1 3 1 2 3 2 (5) a a a a a a w p b r a b , , r sh u sh u t shu s sh u ch u ch u s sh u chu n thu 2 3 2 0 0 2 0 2 0 0 u u u v v v v w 0 0 = = + + + + + +r r d n a = r r c ` ^ ` m j h j 8 b ' 1 sin sin 2 1 sin sin (3b) h p shu v w h p c qs ch u ch u v w2 2 vw 0 = + + + x a -` j 8 b cos sin 2 (3c) h p chu v w h p c q thu t chu shu ch u ch u2 uw 3 0 = + + + x -` h; e 2 1 sin cos cos (2c) h pc chu ch u ch u v v w 2 3 2 w 0= av +> h , , q b a p q n shu0 0max xy x y z b = = =x x a a= = = ` j ; 2 2 u u v pb l l n chu k s 0= = = + n a ^ h 2 sin cos cos (4a)au p h c v v wu u=d n 2 cos (4b)a au p h c v2v v v=d n + r 2 cos sin (4c)au p c v ww w=d n 19 distort the seismic waves, generated after the formation of a fault; therefore in constructing of the dynamic model of a fault it is necessary to allow for the plasticity near the source. (of course, even in elastic case the above formulae can not be used directly for construction of dynamic model because they represent only its zero-frequency component). 3. interpretation the following formulae can be of the main interest for the interpretation of seismic data: where δw is the difference of energies of the initial and final fields, v is our measure of displacement, s is the fault plane area and ls, rs are plotted on figure 2. by means of these formulae one can try to determine the fault plane area s and the displacement v in the source if the energy of seismic waves e is known. because of energy dissipation e is less than δw. however the ratio e/δw is much more for earthquakes than for explosions and according to byerly e and δw are of the same order. it is better to use these formulae in logarithmic form which corresponds to the accuracy of e – determination. the values of the coefficient rs, ls are given in figure 2 for the case a = 1.5, which corresponds to the poisson coefficient v = 0.25; the variation of v in reasonable limits (0.23-0.27) may be neglected. the dashed line corresponds to the case when the energy inside the source model (ellipsoid (1)) is not included in δw and is considered as dissipated in the plastic flow hear the source. the stress pin the initial field is not known and this is the main difficulty in using formulas (6-8). it may be asserted only that pis less than the strengh limit as it corresponds to an average stress in a great region. h. benioff estimated p for kern county earthquake of 1951 from the formula e = p²/2µ hf (h = thickness of the crust, f – area of· aftershocks' zone); the formula is based on the assumption that the energy e of seismic waves was accumulated in the zone coinciding with the zone of aftershocks. he obtained p = 2.67 dyne/cm². since e is known for this earthquake (~ 5.1022 ergs) p can be estimated using (6). the movement took place along the fault ~ 60 km long. designating the depth of the rupture in kilometres by hwe obtain s = 6h 10¹¹ cm². then with µ = 5.10¹¹ cgs and a = 0 we get h is unlikely to be greater than 35 km (the thickness of the crust in california) and less than 15 km. than p � (5-9).107 cgs. on estimation of the displacement in an earthquake source , (6)s b p r b a w s 2 2 2 3 = =d n a r ` j> h , (7)v p s l b a p w les 2 1 3 = = n a n dc cm m (8)pvs w l r= rd 5 10 5 10 6 10 1.1 p h22 11 2 11 3 2$ $ $ $` j ; .r r l l l lres s 3 2 1 3= = =$r r-` j figures 1 and 2. since the source of this earthquake is greatly elongated if would be more reasonable to use formulas for twodimensional problems. assuming again that h = from 15 to 35 km, we obtain from the formulas of knopoff: p � (1.53.4).107 cgs. the obtained values of p are of the same order; we shall try to use them for the study of other earthquakes. in conclusion it should be noted that in a number of paper the volume v of the region of stress accumulation was estimated from the energy of earthquakes e applying the formula: the value v should not be mixed up with the volume of the model (1): the latter may be equal to zero (with a = 0); although e remains finite. one can see from figure 2 that e relatively little depends upon the volume of the source and is determined mainly by the area of a fault and by the initial stress p. formulas (2), (3) can be used also for the estimating of the influence of earthquakes on the stress field around the source. in a small region around (1) the tangentional stress diminishes near the x-axis and becomes maximal near z-axis (where the screw-deformation occurs). this can stimulate the expansion of the fault or the formation of new faults along zdirection, perpendicular to the motion direction x. but it would be especially important to take into account non-ideal elasticity when studying the stress in the final field. 4. comparison with observations unfortunately the greater part of observational data refers to strong earthquakes with surface faults which are certainly far from the circle (fault planes are unlukly to go much deeper than the earth's crust and their horizontal length amounts to hundreds of kilometres). for the san francisco earthquake of 1906 the length of a fault was l = 435 km; assuming s = lh we obtain with h = 35 km (the thickness of the crust in california) s = 1.5; 1014. assuming a = 0, µ = 3.1011 we obtain from formula (7) v �3 m. this value is of the same order that the actual one. from formula (6) s � 0.7.1014 cm2 which also agrees with actual data. it would be interesting also to compare the analogous computations with the empirical relation between e, v and s communicated by don tocher at the toronto meeting; however he studied the strong earthquakes for which knopoff's formulas are more valid. in conclusion it is interesting to compare formulas (6); (7) with the data on the frequency of occurence of earthquakes. b. gutenberg and c. richter showed, that with the decrease of energy of each earthquake by 100 the number of such earthquakes increases only by 10; therefore the part of weak earthquakes in the total amount of annually released seismic energy is very small. however with the decrease of e by 100 the total area of faults will decrease (after (6)) only by 101/3 and the total movement (according to (7)) will even increase by 101/3. certainly fault areas and movement values cannot be summed up in such direct way. however these estimates show that weak earthquakes can take an essential part in causing great faults or in movements along them. (the frequency of weak earthquakes is determined for short-period ones; each of them can correspond to the movement along a part of a great fault. f. press discovered also the long-period weak shoks, and explained them as a weak movement along a large faults as a whole). acknowledgements. the author is greatly thankful to prof. l. knopoff, who acquainted him with his paper in manuscript, and to g. pavlova for troublesome computations. references benioff, h (1954). mechanism and strain characterist of the white wolf fault as indicated by the aftershock sequence, kern county. cal. earthquake of 1952, bull 171, cal. depart. of mines. byerly, p. (1957). report on x1 ass. uggi, toronto. knopoff, l. (1958). energy release in earthquakes, geophys. jorn., 1, 44-52. lurje, a. (1955). three-dimentimal problems in elastostatics (in russian), moscow. love, a. e. h. (1927). a treatise on the mathematical theory of elasticity, cambridge university press. neuber, h. (1930). kerbspannungslehre, berlin. starr, a. (1928). slip in a cristal and rupture in a solid due to shear, proc. camb. phil. soc., 24. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. on estimation of the displacement in an earthquake source 20 2 .e p v= n annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 1 investigating the effectiveness of rupture directivity during the august 24, 2016 mw 6.0 central italy earthquake. spagnuolo elena*, cirella antonella, akinci aybige istituto nazionale di geofisica e vulcanologia, via di vigma murata 605, 00143 roma (italy) *elena.spagnuolo@ingv.it abstract in this study we investigate directivity effects associated to the august 24, 2016 mw 6, central italy earthquake taking into account the source rupture heterogeneities. we use the directivity predictor proposed by spudich et al. (2004) which is derived from the isochrones theory. the directivity is computed using a source to site geometry and a focal mechanism. for its simplicity it can be computed once that a moment tensor solution is available. we use this technique to validate the real time solutions. moreover, because the directivity predictor depends on the rupture velocity it can be used as a proxy to validate the possible rupture history. for the aforementioned reasons our method revealed fruitful for real time applications and helpful to constrain a few main rupture features for further analysis. i. introduction he 2016 amatrice earthquake (mw=6.0) occurred in the central apennines (italy) on august 24th at 01:36 utc. the hypocenter has been located at 13.238e, 42.704n, at a depth of 8.1 km (http://cnt.rm.ingv.it). the earthquake caused casualties and heavy damages in the amatrice town and in several villages nearby. preliminary analyses have been conducted to retrieve fault and rupture parameters in nearly real time. cirella and piatanesi (2016) investigated the rupture process of the main shock, by inverting strong motion data of 22 stations operated by ran (rete accelerometrica nazionale,) and by ingv. their preliminary source model is characterized by two principal patches (figure1); a largest slip concentration, located at nearly 5 km nw from the nucleation, and a second smaller slip patch, located above the hypocenter. the retrieved front propagation shows a clearly bilateral rupture with heterogeneous rupture velocity. strong motion data registered by the italian strong motion network (ran rete accelerometrica nazionale) which are equipped with kinemetrics episensor (fba-3 200 hz) and with etna 18 bits or k2-makalu 24 bits digitizers, report amplitudes stronger than expected with presumed azimuthal variability (ran.protezionecivile.it). acceleration time series recorded at nearby stations (epicentral distances <50 km) show high variability in both amplitude and duration with pga values ranging from 0.45g to 0.02g. stations located in the n-w region of the source show relatively larger ground accelerations respect to those t annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 2 located to the s-w. it is also observed that four ground motions recorded at amatrice (amt), norcia (nrc), montereale (rm33) and fiastra (mnf) present impulsive characteristics over a multitude of orientations (ingv-reluis report, 2016). directivity could be implicated. directivity has several derivations. here we adopt the concept of directivity due to interfering s waves. waves radiated from points that rupture progressively on the fault plane interfere to modulate the energy in the direction of the site. the directivity effect can have important implications for hazard. however, the validations of directivity model on the real data can also trace-back important information regarding the rupture process on the fault. here we apply a method based on the isochrones directivity predictor by spudich at al. (2004) that shows remarkable similarity with the observed azimuthal variations of the ground motion. three models are proposed to evaluate possible contributions of mechanical heterogeneities of the fault to the observed ground motion amplifications. this study also suggests that a quasi-real time evaluation of the directivity effect can be used to put constraints on a particular fault geometry or provide hints on the rupture process. ii. methods empirical ground motion predictive equations (gmpes) are commonly used to evaluate a ground motion intensity measure (i.e. the spectral acceleration sa) from the source to site distance and magnitude, at first-order. the ground motion predictive equation can be modified to incorporate directivity effects (sadir) using the following equation [spudich and chiou, 2008]: (1) ln (sadir) = ln (sa) +fd where fd is the directivity effect defined as: (2) fd = tapers(rrup, m)*(a + b*idp) a and b are empirical constant generally calibrated respect to the available gmpes, tapers(rrup, m) tapers the fd outside a region which provides the best correlation with the residuals (rrup > 70 km and m < 5.6). the idp is calculated using a close-form analytical solution for the ground displacement for a finite fault in a whole space. it uses a parametric ray form derived from the isochrones theory [spudich et al. 2004]. the idp is calculated considering the source to site geometry, the hypocenter position, a fixed rupture velocity, rupture direction, and focal mechanism: (3) idp = s rri c where rri is the radiation pattern that depends on the focal mechanism and assumes a water level of 0.2, s is a functional form which depends on source to site fault geometry, and c is a functional form of the isochrones velocity c’ defined as: (4) c’ = [1/mach – (rhyp – rrup)/d]-1 mach in eq. (4) is the seismic mach number defined as mach=vr/β where vr is the rupture velocity and β is the s-waves speed. rhyp is the hypocentral distance, rrup is the distance to the fault rupture and d is the distance between the hypocenter and closest point in the sourceto-site direction. for a complete description of the parameters in eq. (4) we refer the reader to the original paper by spudich and chiou, [2008]. we also recall that a new model is recently available and developed for the next generation attenuation (nga) 2 project, annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 3 which unfortunately was not implemented yet for application to the present case. in the present paper first we computed a fast and straightforward directivity evaluation by using the parameters listed in table 1. we adopt the focal mechanism resulting from the nearly real time domain moment tensor solution (tdmt) available at the ingv website (http://cnt.rm.ingv.it) within 6-9 minutes after the occurrence of an earthquake. then we allow for a variable rupture velocity and a depth dependent vs profile to investigate the role of mechanical properties of both fault and propagating media. we avail of the preliminary kinematic inversion model resulting from cirella and piatanesi [2016] (figure 1), which was resampled and processed as explained below. the advantage of this analytical approach is that it is fast and can be computed nearly real time as soon as few information about the seismic source become available. table 1: fault parameters and focal mechanism parameter tdmt mw 6.0 width 17.5 length 30 dip 50 rake -85 rupture velocity variable strike 156 depth to top 0.5 in the present paper we perform a nonconventional calibration of the idp on the real observations of the mainshock. the advantage of this technique is that it is independent from a particular choice of a gmpe. equation (1) models how the observed ground motion deviates from the mean value along a path of constant rrup (the racetrack, spudich at al. 2013). the mean value along the racetrack can be computed using the parametric equation in spudich et al. [2013, appendix e]: (5) idpm = a1 + b1/ (cosh(c1 max(rrup-z1,0)) where a1, b1, c1, and z1 are depth dependent parameters provided in the author’s paper. the idpm represents a general distance dependent model which can be used to remove the non-directive part of the motions from the data. we calculate the residuals t of the observations respect to the idpm using a linear additive model with a constant term (@matlab function regstats). the idpm from eq. (5) was then iteratively shifted in amplitude to minimize the residuals. we thus calibrate the idp on the observations using a linear regression procedure: (6) t= a + b*idp in the regression procedure we use stations with rrup < 140 km in order to include a significant number of observations by doubling the distance tapering in eq. (2). parameters a and b are listed in table 3. we also list in table 3, parameters a and b resulting from the conventional calibration of spudich and chiou [2008] on the abrahamson and silva [2008] (hereafter as08) equation for comparison. we calculated the fd (eq. 2) for two cases, using the fault geometry and focal mechanism listed in table 1. in model 1 we assumed conannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 4 stant rupture velocity and constant vs value (mach number = 0.8). in model 2 we assume both a variable rupture velocity and depth dependent vs profile (table 2). figure 1: inverted rupture model [cirella& piatanesi, 2016] projected on the earth surface with villages up to 20 km distant from the nucleation. colors on the fault plane indicate the slip distribution; black contours represent the position of the propagating rupture at 1 s interval. violet line displays the location of the observed surface breakages (provided by s. gori & e. falcucciemergeo wg). the use of a variable rupture velocity in the idp model is not a standard practice. in this study we propose this implementation to verify possible implications of mechanical heterogeneities along the fault. however, the model resulting from this novel approach is still preliminary and possibly it introduces mathematical singularities that should be considered with care. the rupture velocity model originates from the preliminary inversion results from cirella and piatanesi [2016, figure 1]. we simplify the mach number resulting from the depth dependent velocity profile and the rupture avoiding velocity inversions and supershear, considering only the high mach numbers correlated to a substantial slip, setting maximum and minimum values in a typical range between a water level of 0.7 and a maximum of 0.9. the resulting mach number distribution along fault strike and down dip is shown in figure 2. the grid spacing for slip distribution is 5 km. ground motion results are plotted for a grid of sites uniformly distributed (2 km) around the fault. table 2: crustal vs structure. name depth (km) vs (km/s) 0.0 2.1436 1.5 2.1436 1.5 2.8210 4.5 2.8210 4. 3.4336 7.5 3.4336 7.5 3.1475 14.5 3.1475 14.5 3.3583 29.5 3.3583 29.5 4.0081 35.5 4.0080 35.5 3.9864 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 5 figure 2: along strike and down dip distribution of the mach number=vr/β. the hypocenter position is indicated with a star. the chosen ground motion intensity measure is the pseudo spectral acceleration psa at t=3.0 s. this particular choice of t=3 s is due to: (1) the fact that the effectiveness of a directivity model is larger at period greater than 1s (spagnuolo et al. 2016); (2) fault geometry and details of the rupture process are more important at periods larger than 1s whereas site effects are less important; (3) to allow a direct comparison with the shakemaps (resampled over the same grid of virtual sites), which are readily available after the revised localization of the event. table 3: parameters resulting from the residual regression eq.(6) regression parameters with 95% confidence bounds t=3 seconds a -0.063 (-0.241, 0.114) b 0.189 (0.07623, 0.562) a (as08) -0.1979 b (as08) 0.1319 iii. results figures 3 shows the directivity amplification for the two models proposed. the effect of directivity (in color-bar scale) is expressed by the term [exp(fd) – 1] which represents a relative difference. if the directivity correction for a model ranges from -5% to 45% of the nondirective motion, then the color-bar axis runs from -0.05 to 0.45 [spudich et al. 2013, peer report]. contour lines represent the psa (3s) %g resulting from the shakemap procedure [faenza and michelini, 2010, 2011]. the superposition is here meant to compare the spatial distribution of the real data (absolute values) with the spatial distribution of the directivity effect (relative values). the fd correlates fairly well with the expected azimuthal variations of the ground motion. the constant rupture velocity model 1 (figure 3, top panel) essentially underestimates the ground motion on the ne region and overestimates the ground motion in the area along the surface fault projection. however, it captures peculiar features in ground motion spatial distribution in the south-western fault edge and in the north-eastern region (i.e. bvg recorded psa (3s) = 4.5 %g on the ew component). the variable rupture velocity fd model 2 (figure 3, bottom panel) re-modulates the ground motion which results stronger than model1 towards the sw edge of the fault with broader amplitudes lobes that amplify the ground motion towards amatrice (amt). importantly, the use of a variable rupture model remodulates the up-dip directivity amplifications along the 3 %g contouring of the shakeannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 6 map and better represents the ground shaking observed in the eastern area. figure 3: fd resulting from eq. (2). model 1 (top): constant rupture velocity and constant vs resulting in mach = 0.8; model 2 (bottom): variable rupture velocity and depth variable vs profile (table 2). shakemaps results (from the peak horizontal value of 5% critically damped pseudo-acceleration, %g) are presented in white contouring. it is worth noting that the fd is not symmetric respect to the average idpm. a stronger positive contribution results from the source to site geometry (the hypocentral position in particular) modulated by the radiation pattern (respectively s and c, rri in eq. 3). iv. discussion our results show that the analytic model for directivity can be effectively used in the particular case of the amatrice earthquake to predict the azimuthal variability of the ground motion once that a the fault geometry, the hypocentral position and the focal mechanism are known from preliminary reports [tinti and scognamiglio, 2016; cirella and piatanesi, 2016]. in this study we demonstrated that the variable mach number (the ratio between the rupture velocity and s wave velocity), re modulates the directivity effect along the top surface projection of the fault and introduces features that correlate with a few characteristic aspects of the azimuthal ground motion distribution. moreover, the intervention of structural and geological control is not totally unexpected since variable rupture velocity can originate from the existing lithological contrasts between the synorogenic flysch of the ne portion of the fault (lower seismic velocities) and the carbonatic sequence of the sw portion of the fault [gruppo di lavoro ingv sul terremoto di amatrice, 2016]. though still preliminary, the opportunity of introducing a variable rupture velocity is intriguing as it allows complex features in a rather simple analytical formulation. the relative amplification values in figure 3 are still not able to predict the strong ground motion recordings of the main event especially far from the rupture (rrup > 70 km). this inconsistency is likely due: i) to the fd (eq. 2) itself which best performs for fault dip > 65° and in the near-field [spudich and chiou, 2008]; ii) to the comparison with the shakemap which interpolates the real data using a procedure that differs from the analytical formulation annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 7 proposed here (spagnuolo et al. 2013); iii) to the calibration of eq. (2) that here results from a fast evaluation of the non-directive motion (eq. 5) and could not exhaustively explain regional features. the latter point iii) is related to the fact that first order effects superimpose their features on the directivity pattern (e.g. anelastic attenuation, site effects, topographic effects, rupture features such as the position of the patch of slip) and these effects are not taken into account in the average non-directive model (eq. 5) proposed here. in order to account for such effects, regionally derived gmpes should be used to calibrate eq. (2) for a and b. this strategy was originally proposed for the nga database by spudich and chiu, (2008, see table 3 for comparison) with the purpose of reducing the residuals between the observed (and not explained) azimuthal variability of the ground motion and the regional ground motion predictive equation. the a and b parameters (table 3) are markedly different in the case of as08 (from spudich and chiou [2008]) and the average model proposed here. this difference highlights the relevance of selecting a proper ground motion prediction over which operating the correction for directivity. in the case of the italian database, the ground motion variability shows a better agreement with bindi et al. [2011], and malagnini et al. [2011], than other nga models at almost all distances [pischiutta et al. 2016]. unfortunately, gmeps derived from italian strong motion database do not provide corrective factors for the sc08 and do not include themselves a directivity term. the application to other italian and european gmpes need ad-hoc calibrations to enhance the performance of the idp predictor. in this respect it is also worth stressing out that the directivity model developed by spudich and chiou [2008] provides the average idp over a rich data of both synthetics and empirical recordings. thus, it is likely that the use if the idp for forward modeling of a single event is not able to predict the observations. however, the use of the idp could be still effective in a probabilistic framework when all the possible occurrences of rupture parameters are adequately accounted for [spagnuolo et al. 2012]. although the directivity correction is still not representative of the real contribution to the ground shaking, the azimuthal variations are well explained. given the spatial agreement between the prediction and the observations for this particular earthquake, our results seem to indicate a strong contribution of the directivity effect resulting from radiated waves interfering from the source to the site. the agreement also suggest the predictor can be effectively used in real time to seek the best geometry of the fault, relative position of the hypocenter, focal mechanism and rupture velocity, over a statistical consistent number of simulated directivity scenarios. references abrahamson n., and w. silva, (2008). summary of the abrahamson & silva nga ground-motion relations, earthquake spectra, 24(1), 67–97. bindi d., f. pacor, l. luzi , r. puglia, m. massa, g. ameri, and r. paolucci (2011). ground motion prediction equations derived from the italian strong motion database, bulleannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 8 tin of earthquake engineering. doi 10.1007/s10518-011-9313-z boore, d.m. and g.m., atkinson, (2008). ground-motion prediction equations for the average horizontal component of pga, pgv, and 5 per cent damped psa at spectral periods between 0.01 s and 10.0 s, earthq. spectra, 24, 99–138. chiou, b.s-j., and r. r. youngs, (2008). an nga model for the average horizontal component of peak ground motion and response spectra, earthquake spectra 24, no. 1, 173-215,doi:http:/dx.doi.org/10.1193 /1.2894832 cirella a., and a. piatanesi (2016). source complexity of the 2016 amatrice earthquake from non linear inversion of strong motion data: preliminary results; doi:10.5281/zenodo.153821 faenza, l., and michelini, a., (2010). regression analysis of mcs intensity and ground motion parameters in italy and its application in shakemap. geophys. j. int.,180, 1138–1152. faenza l. and a. michelini, (2011). regression analysis of mcs intensity and ground motion spectral accelerations (sas) in italy. geophysical journal international, vol. 186, pp. 1415–1439. gruppo di lavoro ingv sul terremoto di amatrice (2016). primo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24 agosto 2016 (italia centrale), doi: 10.5281/zenodo.61121 malagnini, l., akinci, a., mayeda, k., munafo, i., herrmann, r.b.,` mercuri, a., 2011. characterization of earthquake-induced ground motion from the l’aquila seismic sequence of 2009, italy. geophys. j. int., 184: 325–337. pischiutta, m., akinci, a., malagnini l., and herrero a., (2016). characteristics of the strong ground motion from the 24th august 2016 amatrice earthquake. annals of geophysics (this issue). reluis-ingv workgroup, (2016). preliminary study of rieti earthquake ground motion records v5, available at http://www.reluis.it. spagnuolo e., herrero a., and g. cultrera (2012). the effect of directivity in a psha framework, geophyisical journal international, 0.1111/j.1365-246x.2012.05630.x spagnuolo e., l. faenza, g. cultrera, a. herrero, and a. michelini (2013). accounting for source effects in the shakemap procedure: the 2000 tottori and the 2008 miyagi earthquakesgeophys. j. int. 194 (3), 1836-1848 2013doi:10.1093/gji/ggt195 spagnuolo e., a. akinci, a. herrero, and s. pucci (2016). implementing the effect of the rupture directivity on psha for the city of istanbul, turkey, bulletin of seismological society of america, vol. 106, no.6, doi:10.1785/0120160020). spudich p., b. s. j. chiou, r. graves, n. collins, and p. sommerville p. (2004). a formulation of directivity for earthquake sources using isochrone theory. usgs open-file report 2004-1268] http://dx.doi.org/10.1193/1.2894832 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7213 9 spudich p. and b.s. chiou, (2008). directivity in nga earthquake ground motions: analysis using isochrone theory, earthquake spectra, 24 (1), 279–298. spudich p., et al. (2013) final report of the nga-west2 directivity working group, usgs peer 2013/09. tinti e. and l. scognamiglio (2016). finite fault rupture history of the 2016/08/24 mw6.0 amatrice earthquake by inverting strong motion data. preliminary results. doi.org/10.5281/zenodo.61370 annals 48, 2, 2005defdef 273 annals of geophysics, vol. 48, n. 2, april 2005 key words earthquake statistics – molise earthquake 1. introduction on october 31 and november 1, 2002, two earthquakes of mw 5.7÷5.9 occurred in the molise region in southern italy. the sequence caused widespread damage and claimed 28 lives in the town of san giuliano, with a maximum intensity equal to vii-ix degrees according to the european macroseismic scale (mucciarelli et al., 2003). just a fraction of the affected area was classified as seismic by the italian building code (pinto et al., 2003). in the public debate following the event, some defined it as a «surprise earthquake»; others noticed that the new proposal for italian seismic zonation (issued before 2002) included all the damaged municipalities, thus the event was expected. as for any other nation, the seismic history of italy is full of events that many think of as «surprise earthquakes». seismologists have been warned to «prepare for the unexpected» (kanamori, 1995). they share the notion that is now impossible to predict when an earthquake is going to happen. however, even the most skeptic about prediction and forecasts agree that the location of future earthquakes should be known thanks to our knowledge of past seismicity or detailed geological studies. thus, when an event occurs in an unexpected location it is considered a «surprise earthquake», not only by laymen or the media, but also by specialists. earthquakes defined as a surprise in the recent literature occurred in various locations around the world: greece (stiros, 1996), turkey (buckthought, 1999), taiwan (lew et al., 2001), australia (crone et al., what is a surprise earthquake? the example of the 2002, san giuliano (italy) event marco mucciarelli dipartimento di strutture, geotecnica, geologia applicata all’ingegneria (disgg), facoltà di ingegneria, università degli studi della basilicata, potenza, italy abstract both in scientific literature and in the mass media, some earthquakes are defined as «surprise earthquakes». based on his own judgment, probably any geologist, seismologist or engineer may have his own list of past «surprise earthquakes». this paper tries to quantify the underlying individual perception that may lead a scientist to apply such a definition to a seismic event. the meaning is different, depending on the disciplinary approach. for geologists, the italian database of seismogenic sources is still too incomplete to allow for a quantitative estimate of the subjective degree of belief. for seismologists, quantification is possible defining the distance between an earthquake and its closest previous neighbor. finally, for engineers, the san giuliano quake could not be considered a surprise, since probabilistic site hazard estimates reveal that the change before and after the earthquake is just 4%. mailing address: dr. marco mucciarelli, dipartimento di strutture, geotecnica, geologia applicata all’ingegneria (disgg), facoltà di ingegneria, università degli studi della basilicata, viale dell’ateno lucano 10, 85100 potenza, italy; e-mail: mucciarelli@unibas.it 274 marco mucciarelli 2003). on the contrary, sometimes it is remarked that an earthquake occurred precisely where it was expected to happen, like the loma prieta, 1989 event (harris, 1998). this paper deals with the the concept of «surprise earthquakes» considering two sides of the problem: i) a quantitative, probabilistic definition is needed, and ii) the different meaning of «surprise» that a geologist, a seismologist or an engineer may have in mind. for a geologist, a surprise earthquake is one occurring on a fault that was unknown or considered a non-active one. for a seismologist, the surprise comes when the event happens far from previously mapped historical or instrumental epicenters. for an engineer the surprise is the occurrence of damage in an area not classified as seismic, or that never experienced high macroseismic intensities. in any case, a simplifying definition is that everyone perceives a «surprise earthquake» when it is «distant» from what is known a priori (be it faults, epicenters, intensity or expected acceleration maps). the problem is to quantify this distance to transform a subjective perception into a numerical probability. it is not easy to define the meaning of probability attached to earthquakes (see stark and freedman, 2003). here the term«probability» is used according to a subjectivist approach (de finetti, 1975) expressing probability as confidence in a given result. to put it simply, a probability is small or great according the amount of money one would bet on that outcome. 2. the geologist’s point of view: analysis of a fault database italy has a recent database of potential sources for earthquakes with magnitude greater than 5.5. (valensise and pantosti, 2001). the lower limit of this database makes the san giuliano event a suitable subject for our discussion. the database comprises two kinds of seismogenic sources: i) faults known from geological or geophysical studies (that may lack association with known historical earthquakes); ii) sources derived from intensity maps with the methodology proposed by gasperini et al. (1999). the latter is a modern representation of macroseismic historical data, but adds no geological information from what is known from historical seismology (i.e. a fault must be there because a large earthquake occurred). the database comprises 60 geological sources and 383 historical sources. correcting for the geological faults without known earthquakes associated with them, it turns out that 85% of past strong earthquakes are not yet associated with a fault mapped by geologists. this is not a bad result, considering that up to 1984 the ruling paradigm stated that due to morphology, climate and low recurrence rate no geological evidence could be found for italian events. figure 1 shows the location of the geological/geophysical faults with the location of the san giuliano event. the quake did not occur on a previously known fault. the database is a work in progress, which at the moment simply contains too few data to permit a statistical estimate of the probability of «surprise earthquakes» according to the geologist’s point of view. 3. the seismologist’s point of view: analysis of seismic catalogues the most recent and official catalogue of italian damaging earthquakes is the cpti catalogue (boschi et al., 1999). it contains 243 events with epicentral intensity equal or greater to vii degree mcs from 217 b.c. to 1984 a.d. figure 2 shows these events with the location of the san giuliano quake. was it a «surprise quake»? as said before, a seismologist may define an earthquake as a surprise if it comes outside the previously known locations. it does not necessarily match another epicentral location, since we tend to be a little indulgent about the possible precision of very old quakes. this concept is easy to quantify: it is sufficient to calculate for any earthquake in the catalogue the distance from all the preceding events and then take the minimum (it is the closest neighbor distance, but just for preceding events, hereafter cpnd). the plot of cpnd versus time is expected to be a monotonically decreasing function. as the catalog completeness increases we know about more and more events, thus increasing the probability that after n events, 275 what is a surprise earthquake? the example of the 2002, san giuliano (italy) event fig. 1. location of the geological/geophysical faults from the database of potential sources for earthquakes with magnitude greater than 5.5. (valensise and pantosti, 2001) with the location of the san giuliano event. fig. 2. events with i ≥ viii mcs from the catalogue of italian damaging earthquakes (cpti catalogue, boschi et al., 1999) with the location of the san giuliano event. circle radius is the 75 percentile of the cpnd distribution (see text for details). fig. 4. cpnd cumulative exceedance frequency (from 1600 to the present). 276 marco mucciarelli n + 1 will occur close to a previously known one. if the hypothesis of space stationarity is correct, given a long enough time, cpnd should reach the zero limit, i.e. all events will occur where at least one was located in the past. figure 3 shows the cpnd function for the cpti catalog , i ≥ viii mcs. the plot starts from 1100 a.d., because the initial part of the catalog was needed to stabilize the statistics shown in the graph. along with individual values of cpnd, fig. 3 reports three percentiles (25%, 50% or median, 75%) calculated from the beginning of the catalog to the n-th quake. as expected, the smoothed cpnd is a decreasing function, but does not reach zero and even in this century some events exceed the threshold of the upper quartile. the function decreased at a lower rate in the past 4 centuries. this confirms the findings of albarello et al. (2001) concerning the completeness of the catalog for most of the italian territory for i ≥ viii mcs after 1600. if we recalculate the cumulative distribution from 1600 to the present, we obtain a possible key for assigning a probability value to personal beliefs about «surprise earthquakes». it is possible to derive the observed non-exceedance probability for three sample quakes: i) the irpina, 1980 earthquake. this event has at least one historical «twin» in 1694, and palaeoseismological observation were able to trace back repetitions on the same fault; for italy, this is the best known example of an event that occurred where expected. ii) the belice, 1968 event. this was a swarm that caused heavy damage in eastern sicily. despite a posteriori geological studies, this is still an earthquake that seems to come fig. 3. individual values of cpnd with three percentiles (25%, 50% or median, 75%) calculated from the beginning of the catalogue to the n-th quake. 277 what is a surprise earthquake? the example of the 2002, san giuliano (italy) event out of the blue; everybody agrees that it would have been impossible to expect it. iii) the san giuliano, 2002 quake, that was a surprise for some but not for all. the relevant cpnd are 6, 12 and 48 km respectively. figure 4 reports the cumulative nonexceedance observed probability from 1600 to the present. the three earthquakes are plotted at their relevant cpnd. the irpinia, 1980 quake is well within the first quartile of the distribution. an earthquake that fully behaved according to our beliefs occurred closer to a previous neighbor than the 80% of all the events with i ≥ viii mcs. the belice, 1968 earthquake, on the contrary, falls well above the last quartile, with just about 10% of events occurring at a greater distance from their previously known neighbors. the san giuliano, 2002 event lies before the median, so it is difficult to call it a surprise when more than half of the catalog has a larger cpnd. 4. the engineer’s point of view: analysis of ground motion probability the last point of view about «surprise earthquakes» is the engineer’s one. it has been mentioned before that the san giuliano municipality was not classified as seismic in 2002. however, a proposal for new classification had already been completed by the date of the quake, including san giuliano among the municipalities where anti-seismic design is mandatory. the new italian zonation become official law in 2003, assigning to san giuliano a pga = 0.25 g. this would have been sufficient for an engineer to expect the san giuliano quake. moreover, the highest intensity observed at the san giuliano was viii-ix mcs, due to a swarm of earthquakes which occurred in the southern apennines in 1456. to quantify how much the hazard estimate has changed before and after the 2002 quake, a direct site hazard estimate can be performed. galli et al. (2003) provided a detailed seismic history for the san giuliano municipality. their site intensity catalog comprises 40 events with i ≥ v mcs from 1125 to the present. for 21 earthquakes a direct observation is available, while for the others intensity is estimated from an attenuation relationship or from isoseismal patterns. to estimate probabilistic seismic hazard at a site we used the non-parametric procedure first proposed by magri et al. (1994) and then refined by albarello and mucciarelli (2002). the estimate was performed using the catalog up to 2001 and then adding the 2002 event. figure 5 shows the average return times estimated separately for intensities ranging from v to viii degree mcs. it is clear that the occurrence of the 2002 quake leaves unchanged the expected average return times for intensities v and vi, while it decreases the estimate for i = vii by a mere 9%. for intensity viii, the occurrence of the 2002 events increases the return time of just 4%, well within the error limits. 5. conclusions the san giuliano, 2002 events gives the opportunity for some thoughts on the meaning of «surprise earthquakes». the meaning is different, depending on the disciplinary approach. for geologists, the italian database of seismogenic sources is still too incomplete to allow for a fig. 5. average return times estimated separately for intensities ranging from v to viii degree mcs using the catalogue up to 2001 and then adding the 2002 event, following the procedure proposed by albarello and mucciarelli (2002). 278 marco mucciarelli quantitative estimate of the subjective degree of belief. for seismologists, quantification is possible defining the distance between an earthquake and its closest previous neighbor. the distribution of cpnd is monotonically decreasing, but not yet equal to zero. this may imply that there is a non-null probability that future earthquakes with i ≥ viii mcs may catch italian seismologists by surprise. the quantification obtained with cpnd seems suitable for describing the degree of expectation relevant to two extreme cases (irpinia, 1980 and belice, 1968). finally, for engineers, the san giuliano quake could not be considered a surprise, since psha maps and maximum observed intensity were not exceeded by this quake. a site hazard estimate reveals that the change before and after the earthquake is just 4%. acknowledgements thanks to francesco mulargia and maria rosaria gallipoli for stimulating discussions and suggestions. references albarello, d. and m. mucciarelli (2002): seismic hazard estimates using ill-defined macroseismic data at site, pure appl. geophys., 159, 1-16. albarello, d., r. camassi and a. rebez (2001): detection of space and time heterogeneity in the completeness of a seismic catalogue by a statistical approach: an application to the italian area, bull. seismol. soc. am., 91 (6), 1694-1703. boschi, e., p. gasperini, g. valensise, r. camassi, v. castelli, m. stucchi, a. rebez, g. monachesi, m.s. barbano, p. albini, e. guidoboni, g. ferrari, d. mariotti, a. comastri and d. molin (1999): catalogo parametrico dei terremoti italiani (ed. compositori), pp. 92 (open file available for download at http://emidius.mi.ingv.it/cpti/home.html). buckthought, k. (1999): selling candu nuclear reactors to turkey earthquake risk, earthquake forecasting inc., internal rep. (available at net.org/nuclear/www.diaspora-buckthought.html). crone, a.j., p.m. de martini, m.n. machette, k. okumura and j.r. prescott (2003): paleoseismicity of two historically quiescent faults in australia: implications for fault behavior in stable continental regions, bull. seismol. soc. am., 93 (5), 1913-1934. de finetti, b. (1975): probability, induction and statistics (wiley, new york). galli, p., d. molin, r. giuliani, v. bosi, m. mattone and the quest wg (2003): ingeneria sismica, special issue on molise earthquake, 1-12 (in italian with english abstract). gasperini, p., f. bernardini, g. valensise and e. boschi (1999): defining seismogenic sources from historical earthquakes felt reports, bull. seismol. soc. am., 89, 94-110. harris, r.a. (1998): forecasts of the 1989 loma prieta, california, earthquake, bull. seismol. soc. am., 88 (4), 898-916 kanamori, h. (1995): preparing for the unexpected, seismol. res. lett., 66, 7-8. lew, m., f. naeim, s.c. huang, h.k. lam and l.d. carpenter (2000): seismological and tectonic setting of the 21 september 1999 chi-chi earthquake, taiwan, struct. des. tall build., 9, 73-87. magri, l., m. mucciarelli and d. albarello (1994): estimates of site seismicity rates using ill-defined macroseismic data, pure appl. geophys., 143 (4), 617-632. mucciarelli, m., a. masi, m. vona, m.r. gallipoli, p. harabaglia, r. caputo, s.piscitelli, e. rizzo, m. picozzi, d. albarello and c. lizza (2003): quick survey of the possible causes of damage enhancement observed in san giuliano after the 2002 molise, italy seismic sequence, j. earthquake eng., 7, 599-614. pinto, a.v., g. tsionis, e. mola and f. taucer (2003): preliminary investigations of the molise (italy) earthquakes of 31 october and 1 november 2002, bull. earthquake eng., 1, 349, 2003. stark, p.b. and d.a. freedman (2003): what is the chance of an earthquake, in earthquake science and seismic risk reduction, edited by f. mulargia and r. geller (kluwer acad. pub.), 201-213 stiros, s. (1996): unexpected shock rocks an «aseismic» area, earth in space, 8 (7), pp. 7. valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger that m 5.5 in italy, ann. geofis., 44 (suppl. to no. 4), pp. 180 (with cd-rom). (received july 15, 2004; accepted october 05, 2004) vol48/04/2005def 739 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words magmatic gas – magma degassing – hydrothermal system – crater lake – meteoric water – scrubbing – reaction path modeling 1. introduction gases are released from magmas both during eruptions (syn-eruptive or active degassing) and in quiescent periods that either may preceed an eruption (pre-eruptive degassing) or may not (passive degassing). studies of pre-eruptive degassing are of uppermost interest since we hope that we are able to interpret the changes in the composition and/or fluxes of volatiles as signals of the evolution of the magmatic system towards an eruptive episode. however, this is not a simple exercise due to the common occurrence of interfering processes, mainly magmatic gas scrubbing by waters and interaction of these aqueous solutions with rocks. the effects of water-rock interaction are enhanced by the high reactivity of most magmatic gases and are similar to what takes place in a giggenbach’s flask during fumarolic gas sampling. in particular, co2(g), h2s(g), hcl(g), and hf(g) are readily neutralised (1.1) (1.2) (1.3) (1.4) whereas the behavior of so2 is unique, in that it may be disproportionate to so42− and hs−, as indicated by the reaction (1.5) so oh so hs h o4 7 3 3( ) ( ) ( ) ( )laq aq aq2 4 2 2( )g + = + + hf oh f h o( ) ( ) ( )laq aq 2( )g + = + hcl oh cl h o( ) ( ) ( )laq aq 2( )g + = + h s oh hs h o( ) ( ) ( )laq aq2 2( )g + = + co oh hco( ) ( )aq aq2 3( )g + = geochemical modeling of magmatic gas scrubbing luigi marini and barbara gambardella laboratorio di geochimica, dipartimento per lo studio del territorio e delle sue risorse (dipteris), università degli studi di genova, italy abstract the eq3/6 software package, version 7.2 was successfully used to model scrubbing of magmatic gas by pure water at 0.1 mpa, in the liquid and liquid-plus-gas regions. some post-calculations were necessary to account for gas separation effects. in these post-calculations, redox potential was considered to be fixed by precipitation of crystalline α-sulfur, a ubiquitous and precocious process. as geochemical modeling is constrained by conservation of enthalpy upon water-gas mixing, the enthalpies of the gas species of interest were reviewed, adopting as reference state the liquid phase at the triple point. our results confirm that significant emissions of highly acidic gas species (so2(g), hcl(g), and hf(g)) are prevented by scrubbing, until dry conditions are established, at least locally. nevertheless important outgassing of hcl(g) can take place from acid, hcl-rich brines. moreover, these findings support the rule of thumb which is generally used to distinguish so2-, hcl-, and hf-bearing magmatic gases from so2-, hcl-, and hf-free hydrothermal gases. mailing address: prof. luigi marini, laboratorio di geochimica, dipartimento per lo studio del territorio e delle sue risorse (dipteris), università degli studi di genova, corso europa 26, 16132 genova, italy; e-mail: lmarini@dipteris.unige.it 740 luigi marini and barbara gambardella but it can also produce many other aqueous sbearing species and elemental sulfur (e.g., giggenbach, 1987, 1997). in natural settings, scrubbing of magmatic gases is carried out by 1) the liquids hosted in the hydrothermal systems frequently interposed between the magma batch stationing at depth and the surface (e.g., reed, 1997; symonds et al., 2001; marini et al., 2003a); 2) the lacustrine waters stored in crater lakes (e.g., rowe et al., 1992; delmelle and bernard, 2000; varekamp et al., 2001; marini et al., 2003b), and 3) the shallow meteoric waters soaking the volcanic edifice (e.g., symonds et al., 2001). interaction of magmatic gases with seawater can also occur, as possibly in the recent crisis of panarea, aeolian islands, italy (chiodini et al., 2002). all these processes can be satisfactorily modeled by means of reaction path modeling, a powerful geochemical tool originally proposed by helgeson (1968), who worked out the thermodynamic relationships needed to evaluate the irreversible gas-water-rock exchanges. the first computer code for reaction path modeling, pathi, was developed by helgeson and coworkers, who presented several applications, including hydrothermal rock alteration and ore deposition, in a series of ground breaking scientific papers (e.g., helgeson et al., 1969). the theoretical evolution of water composition and the changes in secondary mineral assemblages during progressive neutralization of an initially acidic aqueous solution originated by absorption of magmatic gases in meteoric water was modeled by reed (1997), who recognised the appearance/disappearance of a series of intermediate ph buffers until the final, thermodynamically stable mineral paragenesis attains equilibrium with the aqueous solution. although this theoretical model is in line with our present understanding of most hydrothermal systems with close magmatic association (giggenbach, 1997), it cannot be applied throughout. for instance, this model fails to explain the origin of the deep, acidic na-cl-so4 waters of the miravalles geothermal system (costa rica), which are probably generated through interaction of deep, neutral na-cl waters with rocks depleted in alkali and alkali-earth metals and enriched in sio2, al2o3, and sulfate minerals, representing old solfataras buried by recent volcanics, as suggested by bob fournier in an informal communication about 10 years ago. reaction path modeling supports the fournier’s hypothesis (marini et al., 2003a). gas-water-rock interactions in crater lake waters were modeled by marini et al. (2003b) to investigate the reasons for the bimodal ph distribution in crater lake waters, which exhibit an acidic mode at ph 0.5-1.5 and a near neutral mode at ph 6-6.5 (varekamp et al., 2000). the neutralization of acid so4-cl waters with andesite at both low temperature and high temperature, followed by cooling below 100°c, rarely produce aqueous solutions with ph in the 3.5-5 range, in line with available analytical data. reaction path modeling has recently been used to model magmatic gas scrubbing taking place through mixing of magmatic gas with airsaturated water, reaction of a magmatic gas/airsaturated water mixture with rock, and other processes in an excellent paper by symonds et al. (2001). it turns out that scrubbing by water prevents significant so2(g) and most hcl(g) emissions until dry pathways are established, apart from moderate hcl(g) degassing from acidic hydrothermal waters (ph < 0.5). degassing of so2(g) from long-resident boiling hydrothermal systems is also prevented by magmatic gas scrubbing. this paper investigates the irreversible gaswater mass exchanges taking place during addition of magmatic gas to pure water at 0.1 mpa, chiefly by means of the eq3/6 software package, version 7.2 (wolery, 1992; wolery and daveler, 1992). the purpose of this exercise is to test the eq3/6 software package on a rather complex problem, which is similar to case 1 of symonds et al. (2001). they modeled the addition of magmatic gas to air-saturated water instead of pure water as we intend to do. hence, our results are not fully comparable with those of symonds et al. (2001), as we neglect dissolved atmospheric gases (i.e., o2(aq) and n2(aq)) and, consequently, the derived n species (mainly nh4+(aq)) in our computations. it must be stressed that the relatively high concentrations of nh4+(aq) predicted by symonds et al. (2001) are based on the assumption of instantaneous 741 geochemical modeling of magmatic gas scrubbing equilibration, which is rarely attained between n species in abiotic systems. on the other hand, the absence of dissolved o2 in our model determines more reducing conditions, with respect to the model of symonds et al. (2001), in the initial steps of gas-water interactions (ξ <0.4 mol) and, consequently, some differences in the concentrations of s species. 2. setting up the model the model consists in the progressive addition of hot magmatic gas into 1000 g of pure water initially at 25°c. pressure is maintained at 0.1 mpa throughout the process. the magmatic gas is represented by sample 79-2 from merapi volcano, indonesia (le guern et al., 1982), with a discharge temperature of 810°c and an equilibrium temperature of 915°c (symonds et al., 1994). this is the same magmatic gas considered by symonds et al. (2001) and its chemical composition is given in table i. the overall process under investigation determines first the gradual heating of the aqueous solution from 25° to 100°c, second its gradual isothermal evaporation at 100°c, and third gasgas mixing at increasing temperature. the first and second stages of the process, taking place in the pure liquid and liquid-plus-gas regions, can be modeled by means of the eq3/6 software package, provided that some suitable pre-computations and post-calculations are carried out (see below). upon total evaporation of the aqueous solution, the subsequent mixing of the gas phase thus produced with magmatic gas cannot be modeled by means of eq3/6. however, simple mass-balance and equilibrium computations are sufficient to describe this gas-gas mixing process if the number of species considered is reasonably low. reaction path modeling in the pure liquid and liquid-plus-gas regions was performed in reaction path mode, through a purely stoichiometric approach, assuming immediate attainment of chemical equilibrium. the thermodynamic database com was used. this is the most complete database available in the eq3/6 software package and includes the thermodynamic properties of several solids, aqueous species, and gases, mostly derived from supcrt92 (johnson et al., 1992). in eq3/6 runs, activity coefficients of ionic solutes were computed by means of the b-dot equation of helgeson (1969). table i. chemical composition (molar fractions) of the magmatic gas and of the gas produced through total vaporization of the liquid phase. sample 79-2 from merapi volcano, indonesia (le guern et al., 1982) is considered to be representative of the magmatic gas by symonds et al. (2001) and in this study as well. gas species magmatic gas produced through total gas vaporization of the liquid h2o 0.8887 0.8532 co2 0.0707 0.09026 h2 0.0154 1.49e-08 so2 0.0115 4.60e-06 h2s 0.0112 0.00325 hcl 0.0059 0.0528 co 0.0016 4.90e-13 s2 0.0008 8.81e-10 hf 0.0004 0.00055 discharge temperature 810°c equilibrium temperature 915°c 100°c molecular weight 20.59 21.39 742 luigi marini and barbara gambardella 2.1. the enthalpy of liquid water and gas species modeling is also based on the assumption of conservation of enthalpy upon water-gas mixing. the enthalpies tabulated in supcrt92 are apparent standard molal enthalpies of formation from the elements, as defined by benson (1968), and cannot be used for these computations. in steam tables a different reference state is chosen, either liquid water at 0°c (denbigh, 1971) or liquid water at the triple point, i.e., 273.16 k, 0.006113 mpa. the latter reference state, with sh2o(l), triple=0, gh2o(l), triple=0, was adopted by the 5th international conference on the properties of steam (helgeson and kirkham, 1974). for our purpose, the same convention must be adopted not only for h2o, but also for all the gas species of interest. therefore, for all gas species, we obtained the enthalpy value at the triple point consistent with this convention (see table ii for details). the enthalpy of the ith gas species at any temperature t and any pressure p (0.1 mpa in our case) can be computed by means of the relation (e.g., denbigh, 1971) table ii. triple point temperature, enthalpy at the triple point, and coefficients of the empirical power series in the temperature, which were used in this work to fit the molar heat capacity of relevant gas species and liquid water (cp data from chase, 1998). species ttriple htriple a b c d e f g k jmol−1 jmol−1k−1 jmol−1k−2 jmol−1k−3 jmol−1k−4 jmol−1k−5 jmol−1k−6 jmol−1k−7 h2o(l) 273.16 (a) 0.00 (a) 170.293 −9.821e-01 3.833e-03 −6.808e-06 4.755e-09 h2o(g) 273.16 (a) 45063 (a) 33.791 −7.490e-03 2.777e-05 −1.582e-08 3.049e-12 h2(g) 13.95 (b) 831.8 (b) 14.668 1.127e-01 −3.400e-04 5.106e-07 −4.052e-10 1.639e-13 −2.665e-17 ch4(g) 90.69 (c) 7555 (b) 35.277 −4.261e-02 1.946e-04 −1.535e-07 3.854e-11 co(g) 68.13 (c) 5684 (b) 29.803 −8.247e-03 2.382e-05 −1.547e-08 3.243e-12 co2(g) 216.58 (c) 19346 (b) 23.068 5.714e-02 −3.260e-05 6.791e-09 s2(g) 387.15 (d) 34100 (f) 26.747 2.493e-02 −2.029e-05 6.008e-09 h2s(g) 187.6 (c) 16500 (f) 33.765 −1.031e-02 4.991e-05 −3.573e-08 8.182e-12 so2(g) 197.64 (b) 17400 (f) 28.044 4.950e-02 −2.869e-05 5.705e-09 hcl(g) 159.0 (c) 14000 (f) 28.953 2.914e-03 −1.563e-05 3.215e-08 −2.162e-11 4.849e-15 hf(g) 151 (e) 13300 (f) 29.059 1.046e-03 −4.412e-06 6.534e-09 −2.073e-12 n2(g) 63.14 (b) 2011 (b) 28.757 6.667e-03 −4.163e-05 1.066e-07 −1.103e-10 5.186e-14 −9.259e-18 nh3(g) 195.4 (c) 25055 (b) 33.142 −9.534e-03 8.160e-05 −6.472e-08 1.627e-11 (a) after keenan et al. (1978): the reported htriple value of 0.01 j/g for liquid water was set to 0 and the htriple value of steam (2501.3 j/g) was changed accordingly. (b) nist chemistry webbook (http://webbook.nist.gov/). (c) lide (2002). (d) castellan (1971). (e) extrapolated value based on ttriple of hcl, hbr, and hi (from reference (c)) and atomic number of halogens. (f) value computed by means of the equation ∆hvap = 21 × 4.184 × tvap, which is based on the trouton’s rule; this can be applied to non-associated liquids at tvap > 150 k (e.g., castellan, 1971). 743 geochemical modeling of magmatic gas scrubbing (2.1) where cp is the molar heat capacity, v is the molar volume, and α is the coefficient of thermal expansion of the i-th gas species. as usually done in the literature (e.g., denbigh, 1971, and references therein), the heat capacity of gas species was expressed as empirical power series in the temperature (table ii) (2.2) fitting the cp data reported by chase (1998). consequently, the first integral of eq. (2.1) was computed as follows: (2.3) ( ) ( ) ( ) ( ) c dt a t t b t t c t t d t t 2 3 4 p t t triple triple triple triple triple f = + + + + +4 4 2 2 3 3 # c a bt ct dtp 3 f= + + + +2 ta( )h h c dt v dp1t p t p t p triple triple triple = + +# # whereas the second integral of eq. (2.1) was neglected assuming ideal gas behavior. in fact, for ideal gases v = rt/p, and (2.4) it follows that: 1−αt=0 and . this assumption is reasonable in the case under consideration, due to the comparatively small pressure increments from ptriple to 0.1 mpa pressure. computed enthalpies for h2o(l), h2o(g), h2(g), co2(g), co(g), ch4(g), n2(g), and nh3(g) are in excellent agreement with those reported by the nist chemistry webbook (http://webbook.nist.gov/). enthalpies of h2o(l), h2o(g), h2(g), co2(g), co(g), ch4(g), n2(g), nh3(g), s2(g), so2(g), h2s(g), hcl(g), and hf(g) at 0.1 mpa pressure and variable temperature are shown in fig. 1. this plot shows that, at magmatic temperatures (let us say > 800 k): a) the enthalpy of any gas is significantly higher than that of liquid water; and b) h2o(g) has the highest enthalpy and is generally followed by ch4(g), nh3(g), co2(g), so2(g), s2(g), and h2s(g), whereas hcl(g), hf(g), co(g), n2(g), and h2(g) have much lower enthalpy values. from a) it follows that the temperature of liquid water is expected to increase upon input of magmatic gases of any composition. however, considering that the main constituent of magmatic gases is by far h2o(g), followed by co2(g) and so2(g) (e.g., symonds et al., 1994), the enthalpy of magmatic gases is not expected to diverge substantially from that of pure steam. 2.2. isobaric heating of the aqueous solution from 25° to 100°c conservation of enthalpy upon water-gas mixing is conveniently expressed by the following equation: (2.5)h n h xh mw 1000 , , , j j m tm w tw w g j tg= + / ( )v t dp1 0 p triple p =a# . v t v vp r t 1 1 p2 2 / = =a b l fig. 1. enthalpies of h2o(l), h2o(g), h2(g), co2(g), co(g), ch4(g), n2(g), nh3(g), s2(g), so2(g), h2s(g), hcl(g), and hf(g) at 0.1 mpa pressure and variable temperature. 744 luigi marini and barbara gambardella where indices w, g, and m indicate the initial pure water, the magmatic gas, and the aqueous solution formed upon addition of ng moles of magmatic gas to 1000 g of pure water, mww is the molecular weight of water, and xj is the molar fraction of the j-th gas species in the magmatic gas. both the enthalpies of solute species and the enthalpy of mixing are neglected. the temperature of the aqueous solution was derived from the enthalpy computed by eq. (2.5), based on the first relationship in table ii solved with respect to t, i.e. (2.6) then, temperature was related to the reaction progress variable ξ, i.e., the moles of magmatic gas added to 1000 g of pure water, by means of a second-degree polynomial equation. this is (2.7) for the considered example and was used to constrain reaction path modeling by eq6 in the temperature range 25-99°c. it must be noted that, in the system under consideration, the aqueous solution attains saturation with respect to crystalline α-sulfur upon addition of 0.022 mol of magmatic gas. this fact has to be taken into account in post-calculations (see below). for ξ < 0.4 mol, the total fluid pressure, obtained by summing the partial pressures of gas species (mainly h2o, co2, and h2s) given by eq6, is lower than the external pressure of 0.1 mpa and results of eq6 are fully representative. for ξ ≥ 0.4 mol, total fluid pressure exceeds 0.1 mpa, indicating that the aqueous solution experiences gas exsolution (degassing) until total fluid pressure attains 0.1 mpa. single-step degassing was modeled starting from the compositions of the aqueous phase computed by eq6, referring to the following mass balance: (2.8) and assuming equilibrium gas-liquid partitioning of gas species, i.e. f-( ) n n n n f n n 1 j o j g j lh o h o h o2 2 2 = +c c cm m m =( ) . . .ct 25 0 18 696 0 28003 2+ -p pc .. h0033401-. h13 282+.0 07-= 2( )ct , ,m tm m tmc (2.9) where the subscripts o, g, and l indicate the initial undegassed liquid, the separated gas and the degassed liquid phase, respectively, and f is the fraction of separated gas. only co2(g), h2s(g), and h2o(g) are present in significant contents in the separated gas phase. water partial pressure was assumed to be fixed by temperature. the partition coefficients bj of co2 and h2s were computed from gas solubility data (henry’s constants) reported in the com thermodynamic database of eq3/6, kh, j, and ph2o, by means of the equation (2.10) to compute degassing effects, f was tentatively varied until total fluid pressure equalled 0.1 mpa. at this point it was necessary to recompute the composition of the degassed liquid phase, a fact that is complicated by the change in redox conditions caused by h2s loss. however, concurrent precipitation of crystalline αsulfur (see above) constrains fo2, as expressed by the reaction (2.11) oxygen fugacity was, therefore, computed based on the thermodynamic constant of reaction (2.11) and the activity of h2s in the degassed liquid phase, which is assumed to be equal to its molality. this is a reasonable assumption for neutral aqueous species. the composition of the degassed liquid was then recomputed by means of eq3nr, introducing temperature, f o 2, f co 2, f h 2s, and total cl and f molalities, as input data, and forcing ph by the electroneutrality condition. the differences between the ph values thus computed and those of the initial undegassed liquid are in all cases except one less than 0.0004 ph units, probably due to rounding off effects. this virtual ph constancy upon gas loss is not surprising as the aqueous phase is a relatively dilute solution of strong acids (see below). o .s h 1/2o h s( ) ( ) ( ) ( )c gl2 2 2 aq+ = + .b p k ,h j j h o2 = b n n n n h o h o j j l j g 2 2 = c c m m 745 geochemical modeling of magmatic gas scrubbing 2.3. isobaric-isothermal evaporation at 100°c during this second stage of the overall process, temperature is kept constant at 100°c by h2o(l)-h2o(g) coexistence. initially, progressive addition of magmatic gas to water was modeled by means of eq6 neglecting the effects of gas separation. knowing the enthalpy of the aqueous solution (computed by eq. (2.5)), the fraction of separated steam, y = nh2o(g)/(nh2o(g)+ + nh2o(l)), was then calculated assuming isoenthalpic boiling, i.e. (2.12) and equilibrium gas-liquid partitioning of co2 (eq. (2.9)). subscripts o, l, and g refer to the unboiled aqueous solution, the separated (boiled) liquid phase and the separated gas phase. since h2o and co2 represent more than 98% (on molar basis) of the gas species present in the separated gas phase, other gas species were neglected in eq. (2.12). obtained y values were used to distribute the moles of each chemical species computed by eq6 between the separated liquid and gas phases by means of simple mass balances (equivalent to eq. (2.8) with y instead of f), assuming equilibrium partitioning for gas species (eq. (2.9)). again, partition coefficients were computed from the gas solubility data reported in the com thermodynamic database of eq3/6 by means of eq. (2.10) and fo2 was calculated referring to reaction (2.11). then the composition of the boiled liquid was recomputed by means of eq3nr as for the first stage of the process. in this case, the differences between computed ph values and those of the unboiled aqueous solution range from 0.005 to 0.09 ph units, since the change in ph upon boiling cannot be properly accounted for by a simple mass balance like eq. (2.8). 2.4. isobaric gas-gas mixing at 100-915°c for this stage of the process, first we computed the specific enthalpy of the gas mixture hgm, referring to water only, i.e., by the equation h n h n h n$ $ $+ +=ho h o h o h o h o co co( ) ( ) ( ) ( ) ( ) ( )l l g g g g2 2 2 2 2 2 (2.13) in which subscript ts is the saturation temperature (100°c) at the considered pressure (0.1 mpa), ng,ev indicates the moles of magmatic gas consumed to evaporate the aqueous solution completely, and ng are the moles of magmatic gas added to the system after complete vaporization of the liquid. the first term of the numerator refers to the enthalpy of the steam produced through total vaporization of the aqueous solution and the first term of the denominator identifies the corresponding moles of steam. the second term of the numerator represents the enthalpy of the magmatic steam added to the system after complete vaporization of the liquid and the second term of the denominator identifies the corresponding moles of magmatic steam. the temperature of the gas mixture was derived from the enthalpy computed by eq. (2.13), based on the hh2o-t relationship (table ii) solved with respect to t, i.e. (2.14) the molar fraction of magmatic steam in the gas mixture, xs, was readily computed as follows: (2.15) and used to calculate the concentrations of major gas species (h2o(g), co2(g), so2(g), h2s(g), hcl(g), and hf(g)) in the gas mixture by means of simple mass balances, based on the compositions of the magmatic gas (table i) and of the gas produced through total vaporization of the aqueous solution. the concentrations of minor gas species (h2(g), co(g), s2(g)) in the gas mixture were then calculated referring to the following reactions: x mw n x n x n x 1000 , ev h o h o h o s w g g g 2 2 2 $ $ $ = + +b l . h0 1088 $ +-. h38 84 $+.1529 4-=( ) . . ct h6 975 10 m m m 2 5$ $+ 3 c h h n x n x n x h n x mw mw 1000 1000 , , , , ev ev h o h o h o gm t w g w g g t g h o h o h o ( ) ( ) 2 2 g s g g 2 2 2 2 $ $ $ $ $ $ = + + + + + b b l l 746 fig. 2. changes in temperature during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. (3.1) and those constrained by the h2s-so2 magmatic redox buffer (giggenbach, 1987) (3.2) the latter buffer is closely approached upon dry-up of the system, when the so2(g) concentration in the gas phase equals that of h2s(g) (see below). the evolution of the rh parameter against ξ and temperature is shown in figs. 3b and 4b, respectively. this parameter is defined as rh= (giggenbach, 1987) and, therefore, it is directly comparable with the analytical data of fumaroles. 3.3. sulfur precipitation as anticipated, the aqueous solution becomes saturated with respect to crystalline α-sulfur upon addition of 0.022 mol of magmatic gas. in the log (moles of s) logξ plot (fig. 5), the moles of s precipitated experience a very sharp initial in/log f /xf log x,= h hh o h o2 2 2 2 8 . .log logf t f2 657 3 2 o h o2 2= + 2511 .log f t 10 736 25414 o2 = luigi marini and barbara gambardella (2.16) (2.17) (2.18) whose thermodynamic constants were obtained by means of supcrt92 (johnson et al., 1992). 3. addition of magmatic gas to pure water at 0.1 mpa: modeling results apart from minor differences arising from the choice of modeling magmatic gas scrubbing by pure water rather than by air-saturated water, our results are very similar to those of symonds et al. (2001). 3.1. temperature the plot of temperature versus the reaction progress variable ξ (fig. 2) shows a comparatively smooth increase from the initial temperature of liquid water, 25°c, to the saturation temperature at 0.1 mpa pressure (100°c). however, the transition from the single liquid phase region to the biphase liquid-plus-gas region occurs at ξ of 0.4 mol (see above), corresponding to a temperature of 32.4°c only. the temperature of 100°c is attained through addition of 4.29 mol of magmatic gas and it is kept constant up to ξ of 79.3 moles by coexistence of h2o(l) and h2o(g). upon dry-up of the system, temperature immediately experiences a very sharp increase. then it flattens for temperatures approaching that of the magmatic gas, 915°c. 3.2. redox potential the diagram of log f o2 versus ξ (fig. 3a) is similar in shape to fig. 2, due to the marked temperature dependence of log f o2. as shown by fig. 4a, log f o2 values are initially intermediate between those fixed by the feo-feo1.5 hydrothermal redox buffer (giggenbach, 1987, temperature in kelvin) 1 hh s s 2( ) ( ) ( )g g g2 2 2 = + h oco h co( ) ( ) ( ) ( )g g g g2 2 2+ = + h s h ho so 322 ( ) ( ) ( ) ( )g g g g2 2 2= ++ 747 geochemical modeling of magmatic gas scrubbing crease, but for ξ > 0.14 they follow a linear trend which is parallel to that of the moles of s added to the system. this means that for ξ > 0.14 mol the ratio between the amount of s removed from the system and the amount of s added to the system is constant. since this ratio is 0.81, it can be concluded that most of the s added to the system is precipitated as crystalline α-sulfur, as already observed by symonds et al. (2001). 3.4. chemistry of the liquid phase the ph of the aqueous solution decreases almost linearly with logξ, up to ξ of ∼ 40 mol (fig. 6), whereas the decrease in ph becomes dramatic above this threshold, when the liquid phase is essentially a very concentrated aqueous solution of hydrogen chloride. in particular, the «last drop» of liquid (for ξ of 79.3 moles fig. 3a,b. variations in a) oxygen fugacity and b) during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. fig. 4a,b. plot of a) log fo2 and b) rh versus temperature showing the expected changes in these variables during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water, initially at 25°c (line labelled simulation). pressure was kept at 0.1 mpa throughout the simulation. also shown are the feo-feo1.5 hydrothermal redox buffer and the h2s-so2, magmatic redox buffer of giggenbach (1987). the latter was computed for the same fh2o of the computer simulation. / /log logr f f x xh h h o h h o2 2 2 2,= 3 4 a a b b 748 fig. 5. moles of sulfur both added to the system and precipitated as crystalline α-sulfur during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. fig. 6. changes in the ph of the aqueous solution during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. fig. 7. changes in dissolved carbonate species during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. fig. 8. changes in dissolved sulfur species during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. luigi marini and barbara gambardella 749 geochemical modeling of magmatic gas scrubbing fig. 9. changes in dissolved chloride and fluoride species during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. fig. 10. weight percentage of gas in the system during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. and y of 0.993) is dominated by hcl(aq), with a molality of 20.2, followed by h+ and cl−, which are present with equal molalities of 7.96. since activity coefficients of ionic solutes were computed by means of the b-dot equation of helgeson (1969), which is expected to be accurate only in dilute aqueous solutions (i.e., for ionic strength lower than 1 molal; wolery, 1992), these values are highly uncertain. on the one hand, the concentrations of dissolved co2 and h2s (which enter preferentially the gas phase, with bi at 100°c of 1405 and 29.8, respectively) increase with ξ in the single liquid phase but experience a marked decrease with ξ upon transition from the single liquid region to the biphase liquid-plus-gas region (figs. 7 and 8). the corresponding dissociation products, i.e., hco3− and hs− ions, show variations similar to those of co2 and h2s. on the other hand, the concentrations of dissolved hcl and hf (which preferentially remain into the liquid phase, with bi at 100°c of 0.00121 and 0.00221, respectively) increase continuously with ξ, both in the liquid region and in the liquid-plus-gas region (fig. 750 fig. 11. variations in the molar fractions of h2o(g), co2(g), h2s(g), so2(g), co(g), and s2(g), during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. fig. 12. variations in the molar fractions of h2(g), hcl(g), and hf(g) during the progressive addition of hot (915°c) magmatic gas into 1000 g of pure water initially at 25°c. pressure was kept at 0.1 mpa throughout the simulation. luigi marini and barbara gambardella 751 geochemical modeling of magmatic gas scrubbing 9). changes in the concentration of dissolved hcl and hf with ξ are controlled by the dissociation reactions (3.3) and (3.4) whose pk values at 100°c are −0.62 and 3.85, respectively. these pk values correspond to the iso-activity ph’s, where the activity of hx(aq) equals that of x−. since ph decreases regularly with ξ, the iso-activity condition is attained much earlier for hf than for hcl. the distribution of hf and f− is further complicated by hf dimerization (3.5) and acid dissociation of the dimer (3.6) the evolution of dissolved s species (fig. 8) is rather complicated due to the occurrence of several association/dissociation and redox reactions (symonds et al., 2001). dissolved hydrogen sulfide is the dominant aqueous s species for ξ ≤ 3 moles, whereas hso4− ion prevails above this threshold, indicating an increase in fo2. concurrently, the concentration of aqueous so2 increases, although it remains a subordinate s species throughout the process. similar to what was observed for ph and cl species, also some s species (e.g., hso4− and so4 2−) experience dramatic changes approaching dry-up of the system. 3.5. chemistry of the gas phase in the biphase liquid-plus-gas region, the weight percentage of gases in the system considered increases almost linearly with ξ in the log-log plot of fig. 10, attaining 100% (dry-up of the system) for ξ of 79.3 moles. the gas phase is initially dominated by co2(g), but h2o(g) becomes the prevailing gas species for ξ ≥ 4 .h f hf h2 2 2= + + hf h f2 2 2= h fhf(aq) = + + h clhcl(aq) = ++ moles (fig. 11). hydrogen sulfide is the third major gas species through most of the biphase liquid-plus-gas region. approaching dry-up of the system, hf(g) first and hcl(g) and so2(g) afterwards appear in the gas phase and their concentrations increase progressively with ξ. in particular, hcl(g) experiences a large increase soon before dry-up and becomes the third major gas species, due to the high concentration of hcl(aq) in the «last drop» of liquid. in the single gas region, h2o(g) and co2(g) are the two main constituents, initially followed by hcl(g), whose molar fraction decreases with ξ. on the contrary, the concentrations of so2(g), h2(g), s2(g), and co(g) increase by several orders of magnitude, and h2(g) becomes more important than hcl(g) for ξ > 1600 mol. gas phases relatively rich in the magmatic component are characterised by similar concentrations of so2(g), h2s(g), h2(g), and hcl(g), whereas co(g), s2(g), and hf(g) are present in lower amounts. as already recalled, the similarity in h2s(g) and so2(g) molar fractions explains why redox conditions are consistent with the h2s-so2 magmatic redox buffer of giggenbach (1987). 4. conclusions the eq3/6 software package, version 7.2, can be effectively used to compute the irreversible gas-water mass exchanges taking place during addition of magmatic gas to pure water at 0.1 mpa, in the liquid and liquid-plus-gas regions, similar to what was done by symonds et al. (2001) by means of the software code chiller. however some post-calculations are needed to take in due account the effects of gas separation. in this specific case, these post-calculations were simplified by concurrent precipitation of crystalline α-sulfur, which fixes the redox potential. post-calculations might be not as simple as in this case if precipitation of elemental sulfur does not take place. luckily, this process is ubiquitous and precocious, considering that it begins upon addition of 0.45 g of magmatic gas only, at least in the case considered. since reaction path modeling is largely based on the assumption of conservation of enthalpy upon water-gas mixing, we reviewed the 752 luigi marini and barbara gambardella enthalpies of the gas species of interest, adopting as reference state the liquid phase at the triple point. used data are presented (table ii) for comparison with other possible test cases. our results of geochemical modeling are very similar to those obtained by symonds et al. (2001), apart from minor differences due to our choice of modeling magmatic gas scrubbing by pure water rather than by air-saturated water, as done by symonds et al. (2001). as already shown by symonds et al. (2001), results of reaction path modeling carried out in this investigation indicate that scrubbing by water prevents significant emissions of strongly acidic gas species (so2(g), hcl(g), and hf(g)) until dry conditions are established, at least locally. however substantial hcl(g) degassing can occur from acidic brines rich in hcl(aq). these results are also consistent with the rule of thumb which is generally used to distinguish so2-, hcl-, and hf-bearing magmatic gases from so2-, hcl-, and hf-free hydrothermal gases (giggenbach, 1980, 1987; chiodini et al., 1993). finally, we would like to emphasize once more the power of reaction path modeling, which is also highly recommended for the correct interpretation of isotope data, especially multivalent elements, e.g., sulfur (e.g., gambardella, 2001; marini et al., 2002). acknowledgements we thank roberto moretti, paolo papale, and guido ventura for the excellent organisation of the esf lesc exploratory workshop «gases in magmatic evolution: from depth to atmosphere, from micro to macro-scale, from calculation to observation», held in rome on 11-13 may 2003. we express our appreciation to giovanni chiodini, johan varekamp, and ünsal gemici for their constructive comments on the first version of the manuscript. our contribution was inspired by the outstanding 2001 paper by robert symonds, terrence gerlach, and mark reed, who really did a great job. we do not intend to rival with them but to pay a tribute to them, stressing once more the importance of geochemical modeling to understand natural processes. references benson, s.w. 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(1997): hydrothermal alteration and its relationship to ore fluid composition, in geochemistry of hydrothermal ore deposits, edited by h.l. barnes (john wiley & sons, new york), 3rd edition, 303-366. rowe, g.l. jr., s. ohsawa, b. takano, s.l. brantley, j.f. fernandez and j. barquero (1992): using crater lake chemistry to predict volcanic activity at poás volcano, costa rica, bull. volcanol., 54, 494-503. symonds, r.b., w.i. rose, g.j.s. bluth and t.m. gerlach (1994): volcanic-gas studies: methods, results, and applications, in volatiles in magmas, edited by m.r. carroll and j.r. holloway, rev. mineral., 30, 1-66. symonds, r.b., t.m. gerlach and m.h. reed (2001): magmatic gas scrubbing: implications for volcano monitoring, j. volcanol. geotherm. res., 108, 303-341. varekamp, j.c., g.b. pasternack and g.l. jr. rowe (2000): volcanic lake systematics, ii. chemical constraints, in crater lakes, edited by j.c. varekamp and g.l. rowe jr., j. volcanol. geotherm. res., 97, 161179. varekamp, j.c., a.p. ouimette, s.w. herman, a. bermúdez and d. delpino (2001): hydrothermal element fluxes from copahue, argentina: a «beehive» volcano in turmoil, geology, 29, 1059-1062. wolery, t.j. (1992): eq3nr, a computer program for geochemical aqueous speciation-solubility calculations: theoretical manual, user’s guide and related documentation (version 7.0), rep. ucrl-ma-110662 pt iii (lawrence livermore national laboratory, livermore). wolery, t.j. and s.a. daveler (1992): eq6, a computer program for reaction path modeling of aqueous geochemical systems: theoretical manual, user’s guide, and related documentation (version 7.0), rep. ucrlma-110662 pt iv (lawrence livermore national laboratory, livermore). s u l l ' o r i g i n e d e l c a m p o m a g n e t i c o t e r r e s t r e e s u l l e t e m p e s t e m a g n e t i c h e (:::> enrico m i ni in una noia p u b b l i c a l a negli « \niiali di geofisica » ( a . i. '•'•>). h o e s p o s t o una t e o r i a c h e p o t r e b b e d a r e r a g i o n e d e l l ' e s i s t e n z a ilei c a m p o m a g n e t i c o della t e r r a . r i n v i a n d o l ' e s a m e c r i t i c o elei p a r t i c o l a r i d e l l a t e o r i a a successive n o i e , m i l i m i l o a t r a t t e g g i a r n e o r a le l i n e e g e n e r a l i . lina c o r o n a di e l e t troni n e g a t i v i c i r c o n d a la t e r r a : gli e l e t t r o n i p e r c o r r o n o l ' o r b i t a d e l l a c o r o n a di r a g g i o r. con la v e l o c i t à v, n e l senso stesso di r o t a z i o n e del p i a n e t a . e s s i c o s t i t u i s c o n o una c o r r e n t e e l e t t r i c a , c l i c p r o d u c e un c a m po m a g n e t i c o p r i m a r i o i i d i r e t t o , n e l l ' i n t e r n o d e l l a c o r o n a , nel senso c h e va dal p o l o g e o g r a f i c o nord a q u e l l o s u d . a t t r i b u e n d o al p i a n e t a una suscettività m a g n e t i c a m e d i a '/. l ' a z i o n e del c a m p o p r i m a r i o ii dà luogo ad u n a i n t e n s i t à di m a g n e t i z z a z i o n e m e d i a i : n e r i s u l t a un m o m e n t o m a g n e t i c o c o m p l e s s i v o m . i l c a m p o da noi o s s e r v a l o a l l a s u p e r f i c i e d e l l a t e r r a è d o v u t o a l l a p r e s e n z a di t a l e m o m e n t o t o t a l e . l a c o n d i z i o n e di e q u i l i b r i o d e l l e c a r i c h e l u n g o l ' o r b i t a d e l l a c o r o n a è dovuto alla e g u a g l i a n z a fra la forza di l o r e n t z , c h e p i e g a gli e l e t t r o n i verso il e e n t r o e la f o r z a di r e p u l s i o n e e l e t t r o s t a t i c a , dov u t a al la c a r i c a n e g a t i v a d e l l a t e r r a , -lilla cui s u p e r f i c i e noi m i u r i a m o il c a m p o e l e t t r i c o e: ,, i <r r ev e qe . . . . /• i r r — 1 = /',. — fu raggio del p i a n e t a ) . 3 li c r" 1 e 4, q v l 'ò r c 2 ; cr si ot ; iene il v a l o r e del p r o d o t t o d e l l a c o n c e n t r a z i o n e n degli c l e l ( * ) simili di lilla comunicazione esposta al congresso internazionale dell i g . g . i . di oslo del 28 agosto l'mij. d a l l a c o n d i z i o n e i i e i — n e s v emi ino medi t r o n i ( n u m e r o di essi per c e n t , c u b o ) p e r l ' a r e a s d e l l a sezione d e l l a c o r o n a 2 p ° 1 . 7 1 1 0 3 ee 7 p o i c h é le c a r i c h e e l e t t r i c h e n e g a t i v e t e n d o n o a d i f f o n d e r a i n e l l o s p a z i o , in v i r t ù d e l l a r e p u l s i o n e e l e t t r o s t a t i c a ed e l e t t r o m a g n e t i c a , si possono s t a b i l i r e v a r i e i p o t e s i p e r g a r a n t i r e la s t a b i l i t à del l'ascio e l e t t r o n i c o . f r a q u e s t e si p u ò f o r m u l a r e l 'ipotesi di c a r i c h e p o s i t i v e c l i c r u o t a n o n e l l o stesso p e n n e l l o e l e t t r o n i c o , m a con v e l o c i t à m o l l o m i n o r e . i n l a i c a s o le f o r z e e l e t t r o s t a t i c h e ed e l e t t r o m a g n e t i c h e e s i s t e n t i f r a le p a r t i c e l l e , t e n g o n o q u e s l c u n i t e , m e n t r e il c a m p o m a g n e t i c o l'i r i m a n e , p r a t i c a m e n t e , c o m e d o v u t o ai soli e l e t t r o n i n e g a t i v i , d a t a la l o r o m o l l o m a g g i o r e v e l o c i t à . p e r s p i e g a r e la genesi del f e n o m e n o sono in c o r s o di p u b b l i c a z i o n e a l t r e n o i e ; è p o s s i b i l e p e r ò fin da o r a i n t r a v e d e r e c o m e , p e r un a l t r o q u a l s i a s i p l a n c i a . s'ul p o s s i b i l e la r e a l i z z a z i o n e di c o n d i z i o n i s i m i l i , la cui causa i n i z i a l e può e s s e r e c o s t i t u i t a dal passaggio di fasci di c i e l i rolli n e l l e p r o s s i m i t à di esso. v a r i e c o n s e g u e n z e si p o s s o n o d e d u r r e d a l l a t e o r i a : fra q u e s t e n o t i a m o la s e g u e n t e p o s s i b i l e i n t e r p r e t a z i o n e d e l l ' o r i g i n e d e l l e t e m peste m a g n e t i c h e . i / a r e a ^ d e l l a c o r o n a e l e t t r o n i c a è a t t r a v e r s a t a dal (lusso del c a m p o m a g n e t i c o s o l a r e . s e il m o m e n t o m a g n e t i c o del sole è u, e la disianza t e r r a s o l e /), il llu--o r i s u l l a <!• = « . j l / ? 8 e lì. 1(1 ' tp s e — j p r o d u c e u n a v a r i a z i o n e del c a m p o s o l a r e ( d o v u t a a f e n o m e n i c h e h a n n o o r i g i n e sul sole stesso) si g e n e r a l u n g o la c o r o n a , p e r i n d u z i o n e e l e t t r o m a g n e t i c a , un c a m p o d e l i n c o e 0 , /»'2,ì( f , e0xdl=2 jz r en da cui e li <• si j 2 c 3i s e gli e l e t t r o n i h a n n o v e l o c i t à v. il l a v o r o c o m p i u t o dal c a m p o e l e t t r i c o i n d o t t o e„. -u c i a s c u n o di e ì nel l e m p o di è : dl — p en v d l c h e c o r r i s p o n d e ad una v a r i a z i o n e d e l l ' e n e r g i a c i n e t i c a e q u i n d i della v e l o c i t à d e l l e p a r t i c e l l e data da sri.i.'ohicine dei. i m i o magnetico tehhestre ììl v d — = e /'.0 v d l m v d v = e e0 v di d r l r di l a v a r i a z i o n e di' ili v e l o c i t à d e g l i e l e t t r o n i p r o d u c e una v a r i a z i o n e dh del c a m p o m a g n e t i c o p r i m a r i o «lilla t e r r a d l l = 2nnes dv cr l n c n d o i n s i e m e le v a r i e e s p r e s s i o n i t r o v a l e e i n l e a r a n d o , ,r /' 2 j t n e s e i r c b. , 7111 se~ c li. , dh— / — di = — / dt ! cr m 2 e di ' m r~ ci ( r i t e n e n d o , d a t o il v a l o r e d e l l e v e l o c i t à , t r a s c u r a b i l e l a c o r r e z i o n e r e l a t i v i s t i c a p e r la m a s s a ) , si lui ii. il, t — ~ i / c 11., i \ li k a/i. d a l l a t e o r i a s o p r a e s p o s t a si b a c l i c il v a l o r e ilei p r o d o t t o è : ns . 1 , 7 1 . i o 1 ' p e r c i ò : h\ il 1 , 5 3 1 0 " ' ( / l s — « , , ) s e : (b.--bl)~ 2 . 1 0 s i l u i : (li., i l , • <-»•• 3 . 1 0 -•• r . p e r u n a v a r i a z i o n e del c a m p o s o l a r e ( a l l a d i s t a n z a s o l e t e r r a ) p a r i a c i r c a 1111 c e n t e s i m o del v a l o r e m e d i o si p r o d u c e s u l l a t e r r a u n a p e r t u r b a z i o n e sul c a m p o e s t e r n o p r i m a r i o p a r i a c i r c a t r e c e n t o l'anima. il s i s t e m a si c o m p o r t a c o m e 1111 c o l o s s a l e betatrone cosmico. f u n z i o n a n t e a t t r a v e r s o la m u t u a i n d u z i o n e s o l e t e r r a . s i m i l i f e n o m e n i i p o s s o n o p e n s a r e a n c h e p e r a l t r i c o r p i c e l e s t i , p e r la s p i e g a z i o n e d e l l a g e n e s i di p a r t i c e l l e di e l e v a t a e n e r g i a . la t e o r i a o f f r e la p o s s i b i l i t à di i n t e r p r e t a r e a l t r i v a r i a s p e t t i del 5 1 2 enrico m e d i m a g n e t i s m o t e r r e s t r e ( v a i i a z i o n i s e c o l a r i , e c c . ) e d à la p o s s i b i l i t à di col l e g a r e diversi f e n o m e n i geofisici in una u n i c a v i s i o n e . palermo — istillilo di fisica dell'università — agosto 1948. riassl \to la nota è mi sunto di una comunicazione esposta dall' autore al recente (congresso internazionale dell'u.g.g.i. di oslo del 28 agosto 1948. si richiamano le linee generali di una teoria, già formulala in una nota precedente, che potrebbe dare ragione, dell' esistenza del cani po magnetico della terra come di qualsiasi altro pianeta. i iene successi ramerete sviliippata, nel quadro della stessa teoria, una possibile interpretazione dell'origine delle tempeste magnetiche. polarization pattern of low-frequency geomagnetic field fluctuations (0.8-3.6 mhz) at high and low latitude 1 low frequency geomagnetic field fluctuations at cap and low latitude during october 29-31, 2003 s. lepidi, l. cafarella, l. santarelli istituto nazionale di geofisica e vulcanologia, rome, italy abstract on october-november 2003 complex interplanetary structures, originated by a series of solar eruptions, hit the earth, triggering violent sun-earth connection events. in this paper we analyze the low frequency geomagnetic field fluctuations detected on the ground during oct. 29-31, 2003, a time period characterized by extremely high solar wind speed values and by out-of-ecliptic interplanetary magnetic field orientation for intervals of several hours. we analyze geomagnetic field measurements at four high latitude stations located in the polar cap, three in the southern and one in the northern hemisphere. from a comparison with simultaneous measurements at low latitude, we address the question of the global character of the observed phenomena. the results show, for selected time intervals, the occurrence of simultaneous fluctuations at all the stations, with high coherence even between high and low latitude; it is interesting that these fluctuations are detected during open magnetospheric conditions, when the high latitude stations are situated well within the polar cap, i.e. far from closed field lines. key words: magnetospheric ulf waves, sun-earth connections, polar cap. 2 introduction several studies analyzed the interaction between coronal ejecta, characterized by long periods of out-of-ecliptic interplanetary magnetic field (imf) orientation, and the earth’s magnetosphere. while southward imf conditions are associated with major geomagnetic storms (farrugia et al., 1996; lepping et al., 1991; gopalswamy et al., 2005), during northward imf conditions solar wind (sw) density enhancements can compress the magnetosphere and trigger geomagnetic field fluctuations (francia et al., 1999). in previous papers (villante et al., 1998; lepidi et al., 1999) we analyzed the geomagnetic field fluctuations at low and cap latitude triggered by magnetic clouds. in addition to storm signatures, we found at both sites fluctuations following sw pressure variations; we also found, during closed magnetospheric conditions, fluctuations at discrete frequencies (few mhz) with high coherence between the stations. these results have been interpreted in terms of global cavity/waveguide modes of the whole magnetosphere which acts as a resonant cavity when excited by external stimulations, such as sw pressure pulses or kelvin-helmholtz instability on the magnetopause (walker et al., 1992; harrold and samson, 1992). evidence for such global magnetospheric oscillations had already been found at auroral latitudes (samson et al., 1992; ziesolleck and mcdiarmid, 1995), at low latitude (ziesolleck and chamalaun, 1993) and even within the polar cap (villante et al., 1997). in the auroral region, i.e. the region separating open and closed field lines, intense ionospheric currents can give rise to local variations of the geomagnetic field (campbell, 1997); around midnight these features are mostly related to substorm occurrence (olson, 1986). during daytime, typical polar cusp signals are observed in the pc5 frequency band but they find no correspondence in higher latitude observations (lanzerotti et al., 1999). recent studies (yagova et al., 2002, 2004) have shown the occurrence of geomagnetic pulsations with frequency of few mhz specific to the polar cap; these cap pulsations are decoupled from simultaneous auroral activity in the same magnetic local time (mlt) sector. in this study we focus on the low frequency geomagnetic field fluctuations (~1-5 mhz) observed at high latitude on october 29-31, 2003 when complex interplanetary structures, with anomalous imf and sw conditions, hit the earth. we analyze the geomagnetic field variations at the three antarctic stations mario zucchelli station (formerly terra nova bay, international geomagnetic observatory code tnb), scott base (sba) and dumont d'urville (drv), at the same geomagnetic latitude but different mlt, and at the canadian station cambridge bay (cbb), at the same mlt and almost 3 opposite corrected geomagnetic latitude as tnb (table 1). the antarctic stations are located in the southern polar cap, at the footprint of open geomagnetic field lines and around local geomagnetic noon they approach the polar cusp. the canadian station is located at a slightly equatorward latitude, so during daytime hours it is typically under the polar cusp and in particular magnetospheric conditions it could be even situated at the footprint of closed field lines (zhou et al., 2000). in order to ascertain the possible global character of the pulsation trains simultaneously observed at the antarctic stations, for selected time periods we extended the analysis also to a latitudinal chain of european low latitude stations: gibilmanna (gib), l'aquila (aqu), castello tesino (cts) and furstenfeldenbruck (fur), located at the footprint of closed geomagnetic field lines (table 1). the analysis is based on 1-min values of the horizontal h component. stations tnb, gib, aqu and cts are run by ingv; data from sba, drv, cbb and fur have been downloaded from intermagnet web site. interplanetary data are from ace spacecraft; regarding sw measurements, standard swi mode ion data, collected every 64 sec, were recorded only from oct. 31, 0051ut; for preceding period, only sti mode ion data, collected approximately every 33 min, were available (skoug et al., 2004). experimental observations and discussion in fig. 1 we show interplanetary data on oct. 29-31, 2003, together with dst index. it can be seen that in the time interval of interest there are several periods with strongly southward imf: approximately 06-09 ut and 14-03 ut on oct. 29-30 and 17-01 ut on oct. 30-31; these periods correspond to strong geomagnetic storms, with dst index reaching ~-180 nt, 360 nt and -400 nt, respectively. there are also periods with strongly northward imf: approximately 03-10 ut on oct. 30 and 01-11 ut on oct. 31. plasma data show exceptionally high sw speed; indeed, it mostly exceeds 1000 km/s and, after the interplanetary shocks on oct. 29, 0558 ut and on oct. 31, 1619 ut (skoug et al., 2004), it reaches values around 1700 km/s. the sw density shows several variations, especially during oct. 31. the variation of the geomagnetic field h component at the eight stations in the time interval of interest is shown in fig. 2. it is evident that the geomagnetic activity is intense, especially during the main phase of the geomagnetic storms. it is also evident that the 4 geomagnetic variations are always very similar between the low latitude stations; in some time intervals there is a strict similarity between the geomagnetic variations at tnb and at the closest station sba and sometimes also between tnb and drv. fig. 3 shows, in the upper panels, the dynamic power spectra at the four high-latitude stations and at aqu, taken as reference for the low latitude stations; the spectra are computed from the differenced data (in order to make more evident higher frequency variations) by means of maximum entropy method, at order 20 of the prediction error filter, over 2-hour intervals with a time shift of 30 min. the figure also shows, in the lower panels, the dynamic coherence between couple of stations, computed over 2-hour intervals (with 8 degrees of freedom) with a time shift of 30 min; in the plots the coherence is shown only where the spectral power at high latitude exceeds 10 4 nt 2 /hz. from the dynamic spectra is evident a strong similarity at the high latitude stations in the time sequence of the broadband major power enhancements, i.e. for several hours from ~06 ut on oct. 29 and from ~17 ut on oct. 30, corresponding to the onset of geomagnetic storms; however, in both cases the fluctuations are not coherent, even between the two closest stations tnb and sba. at low latitude, only the onset of the first storm, occurring during local daytime hours, emerges from the power spectrum. the power spectra show also fluctuations which are observed only at some of the stations, and then can be considered as local phenomena. for example, we note the power peaks at discrete frequencies at 20-21 ut on oct. 29, only at antarctic stations and not at the northern high latitude station cbb nor at low latitude, and the broader power enhancement around 18 ut on oct. 31, very evident only at cbb. more interestingly, there are also fluctuations which are simultaneously observed over a wide spatial separation, with high coherence between stations, and then are not just local phenomena. in this sense, we selected for a more detailed analysis three fluctuation events. two of them, around 23 ut on oct. 29 and 20 ut on oct. 30, are observed during southward imf conditions, in the main phase of strong geomagnetic storms, in correspondence to dst~350 nt and dst~-300 nt, respectively. the last one, around 11 ut on oct. 31, is observed during northward imf conditions, at the end of the recovery phase of a strong geomagnetic storm, and can be related to the sw pressure pulses observed from ace around 1030 ut. fig. 4 shows the analysis of the pulsation event around 23 ut on oct. 29: the filtered (2.5-5 mhz) data (left panels), the power spectra from differenced data (center panels) and the coherence between selected couple of stations (right panels). this event occurs during southward imf conditions, in the main phase of a strong geomagnetic storm (dst~-350 nt), 5 and during high sw speed conditions (v~1000 km/s). stations tnb, sba and cbb are in the early magnetic local afternoon sector, drv in the prenoon sector and the european stations around magnetic midnight. in the filtered data emerge at all stations a main wave packet between ~2250-2315 ut (less clear at cbb) and a strong damping at ~0015 ut. from the spectral and coherence analysis we can see that a major power peak emerges at all stations around 2.9-3 mhz; the corresponding fluctuations are highly coherent all over antarctica and between tnb and low latitude, while at cbb they are completely decoupled. observations at cbb are coherent with other observatories only at frequencies higher than 3.5-4 mhz, but in this frequency range there is a power peak common only to the high latitude, and not also to the european, stations. also the event around 20 ut on oct. 30 (fig. 5) is observed during southward imf conditions, in the main phase of a strong geomagnetic storm (dst~-300 nt), and during extremely high sw speed conditions (v~1500-1300 km/s); it could be related to the small sw pressure pulse observed by ace at ~1930 ut. tnb, sba and cbb are around magnetic local noon, drv in the morning and the european stations in the evening sector. the filtered data show at all stations a signal intensification just before 20 ut, a phase jump around 2020 ut, a strong damping around 2045 ut and a smaller signal intensification just before 21 ut; the peak-to-peak amplitude of these pulsations exceeds 100 nt at the antarctic stations and reaches 30-40 nt at the low latitude stations the spectral and coherence analysis shows that two major power peaks emerge at all stations, around 3.2 and 4.2 mhz. the corresponding fluctuations are highly coherent between high latitude stations and also between high and low latitude. the event around 11 ut on oct. 31 (fig. 6) is observed during northward imf and high sw speed conditions, at the end of the recovery phase of the preceding strong geomagnetic storm; it could be related to the strong sw pressure pulse observed by ace at ~1030 ut. tnb, sba and cbb are in the early magnetic morning, drv in the premidnight sector and the european stations just after local noon. it is interesting to note that in this case the pulsation amplitude at low latitude is comparable, even higher, than at high latitude. at the antarctic and low latitude stations a main wave packet starts at 1100 ut; in antarctica it stops at 1130 ut, while at low latitude it continues, with increased frequency, for further 15 min. the common antarctic and low latitude waves between 11-12 ut (fig. 6, solid lines in the spectral and coherence analysis plots), at ~2.8 mhz, are highly coherent all over antarctica but less coherent between tnb and low latitude. the time sequence of pulsations at cbb is 6 different: a main wave packet appears at 1130-1145 ut; it seems to correspond to the final part of the pulsation train at low latitude: indeed, between 11-12 ut there is at all northern hemisphere stations a broader power enhancement with frequency ~3.5 mhz, and the corresponding coherence between cbb and aqu is very high. in the following hour, between 12-13 ut, a similarity between the wave packets observed at low latitude and cbb emerges; also the power spectra are very similar, with clear power peaks at ~2.5 and 3.6 mhz, and the corresponding coherence is very high (fig. 6, dashed lines in the spectral and coherence analysis plots); conversely, in this time period fluctuations in antarctica are completely decoupled not only from those the northern hemisphere, but also all over antarctica. summary in this paper we analyze the low frequency geomagnetic field fluctuations detected during oct. 29-31, 2003 at four high latitude stations within the polar cap, three in the southern and one in the northern hemisphere. from a comparison with simultaneous measurements at low latitude, we address the question of the global character of the observed phenomena. the time period of interest is characterized by extreme values of the sw speed and by several rotations of the imf, which is strongly southward or strongly northward during time intervals of several hours. fluctuations with different characteristics are observed: broadband fluctuations at the onset of strong geomagnetic storms, which are observed at all high latitude stations but are not spatially coherent; fluctuations observed only at some of the stations, corresponding to local phenomena; fluctuations simultaneously observed at all stations which are spatially coherent, even between high and low latitude. we focus on the spatially coherent fluctuations, selecting three wave packets: two occurring during southward imf conditions, in the main phase of strong storms, and one during northward imf conditions, in the recovery phase of a storm. for both pulsation events occurring during geomagnetic storms, in correspondence to open magnetospheric conditions, the high latitude stations are in the dayside sector, between 07 mlt and 16 mlt. we found that these events are characterized by simultaneous wave packets at discrete frequencies, the same at all the stations; moreover the pulsations are generally highly coherent not only between antarctic stations, but also between high and low latitude. this result is interesting in that for previous pulsation events triggered by magnetic 7 clouds, we had found that the coherence between tnb and aqu attains high values only during closed magnetospheric conditions and when tnb is located close to the local geomagnetic noon (villante et al., 1998; lepidi et al., 1999); we had speculated that in a similar situation tnb, which is usually located in the polar cap, reaches minimum distances from closed field lines (carbary and meng, 1986), and this is the most favorable condition for a cap station to observe the same phenomena as a low latitude station. conversely in this particular case, which is characterized by extreme sw speed values, we found evidence for oscillations extending to a major portion of the earth’s magnetosphere, even deep in the polar cap, during open magnetospheric conditions. on the other hand, some differences emerge, for these two pulsation events occurring during open magnetospheric conditions, in the observations at cbb, which is located close to the northern dayside cusp; in this sense, it is well known that, expecially during open magnetospheric conditions, intense ionospheric currents are present in the auroral oval, giving rise to local geomagnetic field variations (campbell, 1997). in the event occurring during closed geomagnetic conditions, the fluctuations are still highly coherent among the antarctic stations, but not between high latitude opposite hemispheres. in this case, the observations at european stations are definitely more coherent with cbb than with antarctic stations. we note that in this case all the high latitude stations are far from the local geomagnetic noon, then the antarctic stations are located deep in the polar cap. as to cbb, taking into account that the latitude of the auroral oval moves poleward for closed magnetospheric conditions, it could be located in a different magnetospheric region with respect to the antarctic stations, being surely closer to the auroral oval and to closed field lines. in this sense, our result indicates the occurrence of phenomena extending, in the northern hemisphere, from auroral to low latitudes and the simultaneous occurrence of different phenomena deep in the southern polar cap. acknowledgements. the research activity at tnb is supported by italian pnra. authors thank dr. john steinberg (lanl, usa) for providing sti mode ion data from ace. 8 references campbell, w.h. 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the coherence is shown only when the spectral power at high latitude exceeds 10 4 nt 2 /hz. figure 4. analysis of the pulsation event around 23 ut on oct. 29. upper panels show the observations at high latitude, lower panels at low latitude. from left: filtered (2.5-5 mhz) data, power spectra from differenced data (2230-0030 ut) and coherence (2230-0030 ut). the coherence in the upper panel is between tnb and the other high latitude stations; in the lower panel between tnb-aqu and cbb-aqu. figure 5. the same as fig. 4, for the pulsation train observed around 20 ut on oct. 30. power spectra and coherence are computed between 1930-2130 ut. figure 6. the same as fig. 4, for the pulsation train observed around 11 ut on oct. 31. power spectra and coherence are computed between 11-12 ut (solid lines) and between 12-13 ut (dashed lines); 12-13 ut power spectra have been down shifted by two decades. 12 figure 1 13 figure 2. 14 figure 3. 15 figure 4. 16 figure 5. 17 figure 6. vol49_2_2006 625 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words deep-sea borehole observatory – broadband seismometer – seismic noise level – long-term observation 1. introduction tomographic studies using earthquake waves propagating through the earth’s interior have revolutionized our understanding of mantle structure and dynamics. one of the most important limitations on existing tomographic analyses and images of the earth’s interior is the uneven distribution of seismic stations, specifically the lack of stations situated on oceanic crust or in oceanic basins. the scientific importance of establishing long-term geophysical stations at deep ocean sites has been acknowledged by the earth science communities and is detailed in various reports (e.g., ion/odp international workshop, 1995). the objective of tomographic studies is to understand the structure and dynamics of the tectonic system from a regional to global scale by imaging the earth’s interior with seismic waves. unfortunately, few seismometers are sited on the 71% of the earth’s surface covered by oceans. deep-sea borehole seismological observatories in the western pacific: temporal variation of seismic noise level and event detection masanao shinohara (1), ei’ichiro araki (2), toshihiko kanazawa (1), kiyoshi suyehiro (2), masashi mochizuki (3), tomoaki yamada (1), kazuo nakahigashi (1), yuka kaiho (2) and yoshio fukao (1) (2) (1) earthquake research institute, university of tokyo, japan (2) japan agency for marine-earth science and technology, yokosuka, japan (3) institute of industrial science, university of tokyo, japan abstract seismological networks provide critical data for better understanding the dynamics of the earth; however, a great limitation on existing networks is the uneven distribution of stations. in order to achieve a more uniform distribution of seismic stations, observatories must be constructed in marine areas. the best configuration for oceanic seismic observatories is thought to be placement of seismometers in deep boreholes. two deep-sea borehole seismological observatories (wp-1 and wp-2) were constructed in the western pacific and form the initial installations of a 1000 km span network. at present, seismic records of more than 400 total days were retrieved from both the wp-1 and wp-2. long-term variations in broadband seismic noise spectra (3 mhz-10 hz) in the western pacific were revealed from these records, and the data showed that ambient seismic noise levels in borehole observatories are comparable to those of the quietest land seismic stations. in addition, there is little temporal variation of noise levels in periods greater than 10 s. due to this low seismic noise environment, many teleseismic events with magnitudes greater than 5 were recorded. it is confirmed that seismic observation in deepsea borehole gives the best environment for earthquake observation in marine areas. mailing address: dr. masanao shinohara, earthquake research institute, university of tokyo, tokyo 113-0032, japan; e-mail: mshino@eri.u-tokyo.ac.jp 626 masanao shinohara et al. the asymmetry and non-uniformity of the seismic station distribution makes high-resolution imaging of some parts of the mantle nearly impossible. many ocean bottom geophysical sensors, whose locations have been carefully selected to optimize imaging, are needed around the world to attain the goals of the various international geoscience programs. the japanese ocean hemisphere network project (ohp: 1996-2002, http://eri-ndc.eri.utokyo.ac.jp/en/ohp/index.html) selected the western pacific area for installation of ocean-bottom sensors because it is ideal for addressing problems related to plate subduction. based on the results of previous studies using borehole seismometers on the seafloor, it is becoming clear that emplacement of seismometers into the igneous crust using boreholes can provide low noise environments (kanazawa et al., 1992; suyehiro et al., 1992; montagner et al., 1994; beauduin and montagner, 1996; romanowicz et al., 1998; stephen et al., 1999; collins et al., 2001; araki et al., 2004). however there are no observational records covering a span of more than a half year in any deep-sea borehole observatory. the ohp installed four borehole geophysical stations at three sites covering a period from 1999 to 2001 (fig. 1). in 1999, two borehole stations (jt-1 and jt-2) were installed during odp leg 186 (sacks et al., 2000) and are located immediately above the inter-plate earthquake generation zone on the landward side of the japan trench. the jt-1 and jt-2 stations were installed approximately 50 km apart and have two broadband seismometers, a strainmeter and a tiltmeter for each station. in august 2000, during odp leg 191, the borehole seismological observatory wp-2 was successfully installed at odp site 1179 in the northwestern pacific basin (kanazawa et al., 2001). borehole seismic observatory wp-1 was installed in the west philippine basin during odp leg 195 in april 2001 (salisbury et al., 2002). the wp-1 and wp-2 observatories each have two identical broadband seismometers. together, these observatories can cover a large expanse of the northwestern pacific in areas that comprise major gaps in the coverage provided by pre-existing global seismological observatories. at present, the data from these four borehole observatories are retrieved by rov visits at an interval of about 0.5-1.5 year (suyehiro et al., 2002). araki et al. (2004) have already reported on the jt-1 and jt-2 observatories and the characteristics of the geophysical data obtained from the jt-1 observatory. in this paper, we describe the characteristics, layout, and system parameters of the wp-1 and wp-2 seismological observatories, and the characteristics of the seismic data retrieved from them, specifically the ambient seismic noise levels, temporal noise variations, and performance of the observatories. 2. deep-sea borehole seismological observatories wp-1 and wp-2 2.1. system the wp-1 and wp-2 seismic observatories were designed to work for more than five years as stand-alone systems. because the characteristics of the wp-1 and wp-2 observatories have already been described in detail (kanazawa et al., 2001; salisbury et al., 2002), we explain their fig. 1. location map of seismic station coverage in the northwest pacific. small black and light gray circles indicate land seismic stations, dark gray circles are seafloor borehole observatories. the wp-1 and wp-2 observatories were installed in august 2000 and april 2001, respectively. note that a few borehole stations can effectively complement and expand the existing network. 627 deep-sea borehole seismological observatories in the western pacific f ig . 2. s ch em at ic b lo ck d ia gr am o f th e w p -1 s ei sm ic o bs er va to ry . t w o id en ti ca l se ns or s ba ck u p ea ch o th er . t he s ig na ls f ro m t he s en so r ar e di gi ti ze d at t he b ot to m o f th e bo re ho le a nd a re r ec or de d to h ar d di sk s on t he s ea fl oo r. f ig . 3. s ch em at ic b lo ck d ia gr am o f th e w p -2 s ei sm ol og ic al o bs er va to ry . t he p ow er f or t he e nt ir e sy st em i s su pp li ed f ro m t he s ea w at er b at te ry an d ba ck up l it hi um b at te ri es . t he s en so rs w er e gr ou te d at d ep th s of 4 60 m b el ow t he s ea fl oo r in b as al ti c ba se m en t. 2 3 628 masanao shinohara et al. design and layout briefly here. both systems are basically identical except for the power supply (figs. 2 and 3). there is a retired co-axial transoceanic telephone cable near the wp-1 observatory that will be utilized in the future for data recovery and power supply. presently, however, both observatories work as stand-alone systems with their own batteries and recorders. two broadband seismometers (guralp cmg-1td) were placed near the bottom of the borehole in igneous basement. the seismometers were emplaced in the borehole permanently; they were cemented into the hole in order to assure good coupling with the surrounding crust. the signals are digitized in the sensor packages and sent in digital form to the seafloor data-recording packages. the seafloor package serves to combine the digital data from the two seismometers to a single serial data stream. the data are stored to a data recorder in a digital format via a rs-232c link. because the recorder unit can be recovered by an rov, the recorder has been improved in concert with the improved electronics packages developed over the past few years. at present, the recorder has four 30 gb scsi hard disks equivalent to more than 6 channels of one-year continuous data of 24 bit dynamic range at a 100 hz sampling rate. the recorder unit also provides a communication link to the borehole system. the status of the observatory can be checked via an «rov interface» that enables rs-232c communication between the recorder and the rov during an rov visit. the major difference between the wp-1 observatory and the wp-2 observatory is in the electric power supply system. for the wp-1 observatory, power is supplied from a lithium battery system (lbs) consisting of two units, each comprising sixteen lithium cells. eight of these cells are connected serially and two series are connected in parallel, and each unit has a capacity of 2600 ah. each battery unit is contained in two titanium spheres with a diameter of 65 cm. the battery system should, based on system specifications, provide power to the observatory for ∼2.5 years. electric power for the wp-2 observatory is supplied from a sea water battery (swb) (hasvold et al., 1997), which can supply up to ∼24 w with more than 400 kwh capacity. the power is derived from electrolytic dissolution of a magnesium anode in seawater, and power generation is dependent on seawater conditions (salinity, temperature). because the performance of the swb depends on the condition of seawater, additional lithium batteries are used as a backup power supply. all the seafloor instruments and the magnesium anode of the swb are designed to be replaceable by rov. 2.2. installation installation of the seismometer systems for the wp-1 and wp-2 observatories was performed in april, 2001 and august, 2000, respectively. each of the boreholes was drilled through sedimentary rock into basaltic oceanic basement, and the seismometers were installed in the basaltic sections of the boreholes. the seismometers were supported on 4.5m diameter casing pipe hanging on the base of a permanent reentry cone mounted on the mouth of the borehole at the seafloor (fig. 4). this pipe provided a basis for instrument insertion and a conduit for cement pumping for anchoring of the seismometers. grouting cement was pumped through the casing pipe, and then up around the seismometers. the seismometers were grouted at depths of 561 m below seafloor (mbsf) and 460 mbsf in basement rock 19 m and 15 m above the bottom of the boreholes for wp1 and wp-2, respectively. after cementing, the drill pipe from the ship was decoupled and withdrawn leaving the casing pipe in the hole. the power and data cables attached to the seismometer and leading to the data recording and power supply were protected from wear and chafing by being anchored to the casing pipe and are protected from wall contact by centralizers. the cable itself is designed to be almost neutrally buoyant in seawater, further minimizing long term stress on the cables. after cementing of the seismometers was complete, assembly of the battery/recorder unit was accomplished onboard, and the battery/recorder unit was lowered through the moon pool on drilling deck of the joides resolution and mounted on the reentry cone, after which time the batteryrecorder unit was released from the ship via acoustic signal and the drill string was recovered. 629 deep-sea borehole seismological observatories in the western pacific 2.3. system activation and data recovery in march 2002, kaiko, a full-ocean depth rov belonging to the japan marine science and technology center (jamstec), visited the wp1 site to activate the observatory, and long-term observation was started using the first lbs unit. both the seismometers at the wp-1 were config. 4. installation procedure of the deep-sea borehole broadband seismic observatory. after completion of drilling, the sensor string assembly was lowered into the hole. because a tv camera was needed for the re-entry of the sensor string assembly, the sensors could not be connected to the batteries and data recorder on the seafloor. the cement was displaced down hole for fixing of the sensors. the battery/recorder unit was lowered along the drill string and was released by an acoustic command. after installation, an rov was used to activate the system. 630 masanao shinohara et al. firmed to work properly from the r/v kairei via an rs-232c link, one seismometer was deactivated to decrease power consumption. in october 2002, the kaiko re-visited the wp-1 site and we obtained ∼6 months of continuous data (mar 26october 6, 2002) (fig. 5). the kaiko visited the wp-1 site again in may 2003 and ∼ 7 months of continuous data (october 7, 2002-may 7, 2003) were retrieved. during the third visit, the kaiko changed an electric connection and the second lbs unit was put on line to supply the power to the system. at present, seismic records of 408 days in total have been obtained from the wp-1 observatory. the wp-2 observatory was activated in october 2000 using the rov kaiko. since the performance of sea water batteries (swb) depends on the environment around the deployed position, there was a possibility that swb could not generate enough power for the whole system. therefore, we decided to operate wp-2 using lithium batteries during the first observation period as the preliminary observation and had developed a lithium battery system, which supplies the power to the system. the lithium battery system consists of five lithium batteries with a total capacity of 1300 ah. only one seismometer was activated in order to minimize power consumption, similar to the case for wp-1. in august 2001, the kaiko re-visited the wp-2 site. approximately threemonths of continuous data (october 29, 2000january 21, 2001) were recovered. during the second visit, it was confirmed that the swb system continued working properly, and long-term observation using both the swb and backup table i. summary of observation period of the wp1 and wp-2 observatories. wp-1 observatory wp-2 observatory installation odp leg 195 odp leg 191 april, 2001 august, 2000 activation march 26, 2002 october 29, 2000 data periods march 26, 2002october 29, 2000october 6, 2002 january 21, 2001 october 7, 2002august 3, 2001may 7, 2003 june 30, 2002 total 408 days 417 days status in operation in operation fig. 5. an rov view of the wp-1 station after the maintenance of the system (october 6, 2002). the power is supplied from lithium batteries in the titanium spheres (lithium battery system unit). top of the battery frame is also used as an rov landing pad. the data recorder package can be seen on top of the battery/recorder unit. fig. 6. installation of the titanium sphere containing the data recorder and the lithium batteries as backup at the top of the platform for long-term observation for the wp-2 station (august 5, 2001). the power is mainly supplied from the sea water battery system. the swb system has four swb1200 cells. the cell is a magnesium and oxygen battery based on a magnesium anode that uses seawater as the electrolyte. when voltage from the swb is dropped, the lithium batteries in the sphere supply backup power. 631 deep-sea borehole seismological observatories in the western pacific lithium battery system was initiated (fig. 6). when the swb generates enough electric power for the system, the system is supplied the power from the swb. if the swb cannot generate the power, the backup lithium battery system supplies the power to the system. the backup lithium battery system can supplies the power for approximately three months. the kaiko visited the wp-2 site again in june 2002 and ∼11 months of continuous data (august 3, 2001-july 29, 2002) were retrieved for the second observation period. the kaiko also recovered the monitoring data for the swb system, confirming that the swb system worked continuously for almost one year. the operation details of the wp-1 and wp-2 observatories until the end of 2003 are summarized in table i. 3. seismic records from the wp-1 and wp-2 observatories 3.1. background seismic noises in deep-sea boreholes the seismic data from the wp-1 and wp-2 seismological observatories enable the study of broadband seismic noise in the frequency range from 3 mhz to 50 hz in the northwestern pacific over a period covering more than one year of continuous recording. temporal changes in ambient seismic noise within deep-sea borehole can be analyzed in depth using this data. figure 7a-d shows the noise level as a function of time and period for vertical and horizontal components of the wp-1 and the wp-2 observafig. 7a. running spectra of the records from the deep-sea borehole observatories. each spectrum is calculated with a moving time window for a total of one hour record. here the spectra are for the vertical component of the wp-1 observatory. there is little change in noise levels above 10 s period. many earthquakes are visible on the running spectra in periods greater than 10 s. small temporal variations (maximum fluctuation is 10 db) of noise levels for periods around a few seconds are seen. 632 masanao shinohara et al. fig. 7b,c. running spectra of the records from the deep-sea borehole observatories. each spectrum is calculated with a moving time window for a total of one hour record. b) spectra for the horizontal component of wp-1. a total of 408 days of continuous data were obtained form the wp-1 observatory. c) spectra for the vertical component of the wp-2 observatory. a total of 417 days of continuous data were retrieved from wp-2. there is little change in noise levels above 10 s period. many earthquakes are visible on the running spectra in periods greater than 10 s. small temporal variations (maximum fluctuation is 10 db) of noise levels for periods around a few seconds are seen. b c 633 deep-sea borehole seismological observatories in the western pacific tories. each spectrum is calculated with a time window of about four minutes shifted every two minutes and is averaged for a total of one hour record. all spectra for one hour are plotted as a function of time. the major advantage of borehole observatories is that there are little or no temporal changes in noise levels for periods longer than 10 s. vertical high-level bars in the running spectra indicate earthquakes. in contrast, small temporal variations (maximum fluctuations of 10 db) in noise levels for periods around a few seconds are seen. all power spectrum densities calculated using the moving window for every one hour are plotted as a function of periods (fig. 8a-d), and the spectra are grouped by seasons. it is confirmed from fig. 8a-d that the noise levels for both vertical and horizontal components of the both the wp-1 and wp-2 observatories for periods of greater than 10 s are stable year round. it is found that the noise levels in the vertical component at the wp-1 reach −180 db for time-periods between 10 s and 100 s. on the other hand, small temporal changes of noise levels in periods around a few seconds are seen. the wp-1 observatory recorded relatively large noise levels in summer and fall. the vertical component of the wp-2 observatory has minimum noise levels about −145 db. the noise levels in the horizontal component in the wp-2 seismometer are −160 db for time-periods between 10 s and 100 s and are lower than that of the vertical component. the records from the wp-2 observatory reveal high noise levels in a period range of a few seconds during winter. for both the vertical and the horizontal components of both the observatories, the noise levels are close to the low noise model (peterson, 1993) near the 10 s period. in general, the peak in noise spectrum near a period of 100 s in measurements taken at the seafloor or in a shallow-sea borehole is due to deformation of the seafloor by ocean gravity waves (webb, 1998). fig. 7d. running spectra of the records from the deep-sea borehole observatories. each spectrum is calculated with a moving time window for a total of one hour record. here the spectra are for the horizontal component of wp-2. a total of 417 days of continuous data were retrieved from wp-2. there is little change in noise levels above 10 s period. many earthquakes are visible on the running spectra in periods greater than 10 s. small temporal variations (maximum fluctuation is 10 db) of noise levels for periods around a few seconds are seen. 634 masanao shinohara et al. fig. 8a,b. power spectra of the deep-sea borehole seismic observatories. power spectra estimated using onehour records for the all observation period are plotted. hnm and lnm indicate the high noise model and the low noise model by peterson (1993), respectively. the power spectra are grouped by seasons: a) for vertical component of wp-1; b) for horizontal component of wp-1. the horizontal component records of the wp-1 observatory have larger noises in periods longer than 20 s. the records of the wp-1 have larger noise levels near a few seconds period in summer. araki et al. (2004) reported that installation of a seismometer in the basement rocks minimize the effect of the ocean gravity wave. because all the seismometers of the wp-1 and wp-2 observatories were installed into basaltic basement, there is no peak due to the ocean gravity waves. the horizontal component records of the wp1 observatory have larger noise levels than the a b 635 deep-sea borehole seismological observatories in the western pacific vertical component records for periods longer than 20 s. there is a possibility that the horizontal sensors in the wp-1 seismometer have a coupling problem. in addition, the noise level of the vertical component for period longer than 10 s in the wp-2 observatory is larger than those of the horizontal component. because the vertical sensor of the wp-2 observatory may have some fig. 8c,d. power spectra of the deep-sea borehole seismic observatories. power spectra estimated using one-hour records for the all observation period are plotted. hnm and lnm indicate the high noise model and the low noise model by peterson (1993), respectively. the power spectra are grouped by seasons: c) for vertical component of wp2. the vertical component records of the wp-2 observatory have also larger noise levels in periods longer than 10 s. d) for horizontal component of wp-2. the wp-2 observatory has quiet noise environment in summer. c d 636 masanao shinohara et al. damage, we switched the sensor to the alternative in june 2002. at present, we have not obtained records from the alternative sensor in the wp-2 observatory. 3.2. comparison with meteorological data wind speeds on the earth surface are considered to increase seismic noise level at high frequency (> 0.1 hz) with increasing velocity (wilcock et al., 1999; collins et al., 2001). a small island (okinotori-shima) lies about 150 km northeast from the wp-1 observatory. a meteorological observatory is being operated at the okinotorishima by jamstec, and average wind speed, maximum wind speed, wind direction, air temperature, humidity, air pressure, insolation, and radiation are observed every 30 min. comparison of the noise spectra of the vertical component of the wp-1 seismometer with average wind speed and atmospheric pressure (fig. 9a,b) indicates that wind speed seems to be related to noise levels for periods around a few seconds. specifically, typhoon 200225 passed near the okinotori-shima island on november 23, 2002. large noise levels for a few second periods at wp-1 were observed around november 23, 2002. for the wp-2 observatory, the high seismic noise level in winter is observed. although there is no meteorological station near the wp-2, it is known that the northwestern pacific has strong winter winds due to dominant low-pressure weather systems. we infer that wind speed is also related with the seismic noise level of the wp-2. 3.3. teleseismic events and performance of event detection the wp-1 and wp-2 deep-sea borehole seismic observatories reside in a quiet environment fig. 9a. comparison between the noise spectra of vertical component of the wp-1 and the meteorological data (average wind speeds and air pressure) observed at okinotori-shima, which lies about 150 km northeast of wp-1: from april to september 2002. rapid decreases of air pressure indicates typhoon passing. the wind speed appears to be related to noise levels in periods around a few second. 637 deep-sea borehole seismological observatories in the western pacific suitable for earthquake observation. specifically, stable ambient seismic noise levels in marine areas enables us to record many teleseismic events uniformly in time. figure 10 shows three component seismograms of the teleseismic event that occurred on november 15, 2002 in the region of south sandwich island (ms= 6.6) and was recorded at the wp-1 observatory. the seismogram clearly shows minorand major-arc love and rayleigh waves. at present, we have data obtained simultaneously from both the wp-1 and wp-2 observatories for events that occurred between march 26 and june 30, 2002. for example, an earthquake that occurred in the mariana islands region at a depth of 86 km on april 26, 2002 (mb = 6.6) was observed at both the wp-1 and the wp-2 observatories (fig. 11). simultaneous observation and data recording by both the wp-1 and wp-2 observatories continues to the present. to investigate the performance of the deepsea borehole seismic observatory, we plotted all vertical seismograms obtained at the wp-2 observatory from october 29, 2000 to january 21, 2001 and manually picked events from the plot. the seismograms were filtered between periods of 20 and 500 s. a list of recorded events by the wp-2 observatories was compared to a list published by the usgs (the quick epicenter determinations). as a result, the deep-sea borehole observatory proved to record all the teleseismic events with magnitudes greater than 5 at this frequency band (fig. 12). because there is little temporal variation of ambient seismic noise, we can obtain more than one seismic event per a day from the deep-sea borehole seismic observatories. the wp-2 observatory records many events that are not included in the list of the qed, usgs. most of these events were identified by the japanese seismic network and their magnitudes are less than 4. the epicentral distances of the regional events with magnitudes less than 4 are less than 2000 km. on the other hand, there is no obfig. 9b. comparison between the noise spectra of vertical component of the wp-1 and the meteorological data (average wind speeds and air pressure) observed at okinotori-shima, which lies about 150 km northeast of wp-1: from october 2002 to january 2003. rapid decreases of air pressure indicates typhoon passing. the wind speed appears to be related to noise levels in periods around a few second. 638 masanao shinohara et al. fig. 10. example of observed earthquake at the wp-1 observatory. all the components are shown. the records were band-pass filtered at periods from 50 s to 5 s. the earthquake occurred in the region of south sandwich islands on november 15, 2002, and had a magnitude of 6.6. epicenteral distance is about 143°. minorand major-arc love and rayleigh waves are clearly seen. servation of local earthquake whose s-p time is less than 10 s for both the wp-1 and the wp-2 observatories, because both the wp-1 and the wp-2 are placed in inactive deep-sea basins. 3.5. comparison with previous deep-sea borehole observations prior to our observation, several deployments of broadband seismometers into boreholes on the seafloor had been carried out and ambient seismic noise spectra were obtained in the atlantic ocean and the pacific ocean off hawaii (montagner et al., 1994; beauduin and montagner, 1996; stephen et al., 1999; collins et al., 2001). we compare the ambient seismic noise spectra from the wp-1 and the wp-2 observatories with those in previous deep-sea borehole observations (fig. 13a,b). for both vertical and horizontal components, noise levels in periods smaller than a few seconds are similar, however there are some differences for periods greater than 10 s. for the vertical component, the wp-2 observatory has larger noise level than the other deep-sea borehole observatories in periods between 10 and 100 s. on the other hand, the wp-1 observatory and the osn-1 observatory, the latter of which was installed in the pacific ocean off hawaii, are quiet in this frequency band for the vertical component. noise levels of both observatories are comparable to those of quietest seismic station on land at periods between 10 s and 30 s. there is a possibility that at least one of the sensors for ver639 deep-sea borehole seismological observatories in the western pacific fig. 11. the records of earthquakes observed simultaneously by both the wp-1 (upper) and the wp2 (lower) observatories. a one hour records of all the component from 16:00:00, april 26, 2002 are shown. no filter is applied. the earthquake occurred in mariana region and its magnitude was 6.6. fig. 12. magnitude distribution of earthquakes recorded by the wp-2 observatory from october 29, 2000 to january 21, 2001 with epicentral distance (large symbols). plus signs and dots refer to body wave magnitude determined by the usgs. solid circles and squares relate to surface wave magnitude. small dots and squares denote the magnitude of earthquakes by the usgs, which were not detected by the wp-2 observatory. almost all events with magnitudes greater than 5 are observed in the borehole observatories. tical component of the wp-2 observatory was damaged during installation. although the jt-1 observatory has a peak in noise levels near a period of 100 s in the horizontal component, the wp-2 observatory has no such peak. this peak is explained by the ocean gravity wave, and we infer that wp-2 escapes from the noise due to installation into the basement deep below the seafloor (araki et al., 2004). the noise levels recorded by the horizontal component for periods greater than 10 s in the wp-1 observatory are larger than those in the other observatories. sensor coupling or mechanical problems may ac11 12 640 masanao shinohara et al. count for these results from the wp-1 observatory. generally, the noise levels of the wp-1 and the wp-2 observatories are comparable with those of previous observations in seafloor boreholes and land seismic stations. we confirm that seismic observation in deep-sea boreholes gives the best environment for earthquake observation in marine areas. 4. conclusions in 2000 and 2001, the seafloor borehole seismological observatories wp-1 and wp-2 were successfully installed in the northwestern pacific. the wp-1 site is in the west philippine basin west of the kyushu-palau ridge. the wp-2 observatory is situated on normal oceanic mesozoic crust in the northwestern pacific basin. both observatories fill important observational gaps in the worldwide seismic network, since no other land site can replace these sites. each observatory has two identical broadband seismometers, which are cemented near the bottom of the hole. the wp-1 observatory was activated in march 2002. at present, seismic records of 408 days in total have been obtained from the wp-1 observatory. the wp-2 observatory was activated in october 2000. approximately fourteen-months of data were retrieved from the wp-2 observatory. the long-term variations of broadband seismic noise spectra (3 mhz-10 hz) in the northwestern pacific basin were revealed. the noise level above a period of about 10 s is stable in time. the vertical component of wp-2 has noise levels of about −145 db. noise levels for the horizontal component in wp-2 are lower than that of the vertical component (−160 db in periods greater than 100 s). because there is a possibility that this vertical sensor was damaged, we activated an alternative seismometer and shut down the sensor that had been used during the third rov visit. it is found that the noise levels of the vertical component at wp-1 reach −180 db in periods between 10 s and 100 s. on the other hand, small temporal variations (maximum fluctuation is 10 db) of noise levels in periods around a few seconds are seen; the wp-1 station and the wp2 observatory have higher noise levels in summer and in winter, respectively. from comparison with meteorological data observed at a small island near the wp-1 observatory, wind speed fig. 13a,b. comparison of ambient seismic noises between the station in this study and those in previous studies: a) for vertical component; b) for horizontal component. ofp and osn-1 indicate borehole observatories in north atlantic ocean (montagner, 1994) and the pacific ocean off hawaii (stephen et al., 1999), respectively. at present, observations at both these stations have been discontinued. hnm and lnm indicate the high noise model and the low noise model by peterson (1993), respectively. the noise levels of the wp-1 and wp-2 observatories are comparable with those of the previous studies. there is no horizontal record for the ofp. a b 641 deep-sea borehole seismological observatories in the western pacific appears to be related to the noise levels in periods around a few seconds. due to the low seismic noise environment, many events were recorded in the records from both the deep-sea borehole observatories. it is possible to record teleseismic earthquakes with magnitude greater than 4.5 within an epicentral distance of 45°. we confirm that seismic observation in deep-sea borehole gives the best environment for long-term, low-noise, high-resolution earthquake observation in marine areas. acknowledgements the success of the observations was made possible by the active co-operation of many scientists, engineers, technicians, ship officers and crew, and rov operators from various institutions in the world. in particular, we express thanks to drs. william w. sager, matthew h. salisbury, carlota escutia, carl richter, mr. mike storms and many people from odp who led successful installations of the borehole observatories. the skillful maneuvers of the rov by the kaiko team, jamstec are acknowledged. we thank drs. ralph a. stephen, hajime shiobara, hitoshi mikada, kenji hirata, kimihiro mochizuki, messrs. michio arisaka, masakazu ito for fruitful discussions and active support on this study. we would like to thank two anonymous reviewers for their thoughtful reviews and helpful comments. the comments by dr. daniel curewitz were also helpful for improving the english of this manuscript. this study was partly supported by the ocean hemisphere project from the ministry of education, culture, sports, science and technology of japan. most of the figures were created using gmt (wessel and smith, 1998). references araki, e., m. shinohara, i.s. sacks, a. linde, t. kanazawa, h. shiobara, h. mikada and k. suyehiro (2004): improvement of seismic observation in the ocean by use of seafloor boreholes, bull. seismol. soc. am., 94, 678-690. beauduin, r. and j.p. montagner (1996): time evolution of broadband seismic noise during the french pilot experiment ofm/sismobs, geophys. res. lett., 23 (21) 2995-2998. collins, j.a., f.l. vernon, j.a. orcutt, r.a. stephen, k.r. peal, f.b. wooding, f.n. spiess and j.a. hildbrand (2001): broadband seismology in the oceans: lessons from the ocean seismic network pilot eexperiment, geophys. res. lett., 28 (1), 49-52. hasvold, ø., h. henriksen, e. melvær, g. citi, b.ø. johensen, t. kjønigsen and r. galetti (1997): sea-water battery for subsea control systems, j. power sources, 65, 253-261. ion/odp international workshop (1995): multidisciplinary observatories on the deep seafloor, marseille (france). kanazawa, t., k. suyehiro, n. hirata and m. shinohara (1992): performance of the ocean broadband downhole seismometer at site 794, proc. odp, sci. results, 127/ 128, pt2, 1157-1171. kanazawa, t., w.w. sager, c. escutia and shipboard scientific party (2001): northwest pacific seismic observatory and hammer drill tests, sites 1179-1182, proc. odp, init. rep., 191 (cd-rom). montagner, j.p., j.f. karczewski, b. romanowicz, s. bouaricha, p. lognonné, g. roult, e. stutzmann, j.l. thirot, j. brion, b. dole, d. fouassier, j.c. koenig, j. savary, l. floury, j. dupond, a. echardour and h. floc’h (1994): the french pilot experiment ofm-sismobs: first scientific results on noise level and event detection, physics earth planet. inter., 84, 321-336. peterson, j. (1993): observations and modeling of seismic background noise, us dep. inter. geol. surv., open-file rep. 93-322. romanowicz, b., d. stakes, j.p. montagner, p. tarits, r. uhrhammer, m. begnaud, e. stutzmann, m. pasyanos, j.f. karczewski, s. etchemendy and d. neuhauser (1998): moise: a pilot experiment towards long term seafloor geophysical observatories, earth planets space, 50, 927-937. sacks, i.s., k. suyehiro, g.d. acton and shipboard scientific party (2000): western pacific geophysical observatories, sites 1150 and 1151, proc. odp, init. rep., 186 (cd-rom). salisbury, m.h., m. shinohara, c. richter and shipboard scientific party (2002): seafloor observatories and the kuroshio current, proc. odp, init. rep., 195 (cd-rom). stephen, r.a., j.a. collins and k.r. peal (1999): seafloor seismic stations perform well in study, eos, trans. am. geophys. un., 80, 592. suyehiro, k., t. kanazawa, n. hirata, m. shinohara and h. kinoshita (1992): broadband downhole digital seismometer experiment at site 794: a techinical paper, proc. odp, sci. results, 127/128, pt2, 1061-1073. suyehiro, k., e. araki, m. shinohara and t. kanazawa (2002): deep-sea borehole observatories ready and capturing seismic waves in the western pacific, eos, trans. am. geophys. un., 83 (621), 624-625. webb, s.c. (1998): broadband seismology and noise under the ocean, rev. geophys., 36, 105-142. wessel, p. and w.h.f. smith (1998): new improved version of the generic mapping tools released, eos, trans. am. geophys. un., 79, 579. wilcock, w.s.d., s.c. webb and i.t. bjarnason (1999): the effect of local wind on seismic noise near 1 hz at the melt site and on iceland, bull. seismol. soc. am., 89, 1543-1557. vol52,6,2009new 549 annals of geophysics, vol. 52, n. 6, december 2009 key words historical archives – istituto nazionale di geofisica – geophysical research – guglielmo marconi – antonino lo surdo 1. introduction the reordering of the historical archives of the istituto nazionale di geofisica (ing, national institute of geophysics), promoted by the istituto nazionale di geofisica e vulcanologia (ingv, national institute of geophysics and volcanology) and recently completed in accordance with legislation enacted regarding the archives of public bodies – legislative decree october 29, 1999 n. 490 – was carried out to safeguard the acquired heritage of historical documents and, at the same time, render these documents more easily accessible. these archives include all the documents produced by the institute during its operating years from 1945 to 1965. 2. the foundation the ing was actually founded in 1936 as an institute of the consiglio nazionale delle ricerche (cnr, national research council) and the documents produced from this date until 1944 are therefore considered to belong to the cnr and are stored in the archivio centrale dello stato (acs, central archives of the state). the idea to create a geophysics institute came about during the 1930’s and can be attributed to the cnr’s top management. until this time, the ufficio centrale di meteorologia e geofisica (central office of meteorology and geophysics, so called since 1923) was responsible for geophysics activity. several observatories and stations, public as well as private, were distributed throughout the territory (kingdom of italy). however, they worked and carried out their geophysical prospecting research and observation campaigns independently of one another. the istituto nazionale di geofisica and its historical archives giuseppina calcara istituto nazionale di geofisica e vulcanologia, roma, italy abstract herewith are presented the contents of the ing (istituto nazionale di geofisica) historical archives, recently reorganized to preserve and make available to scholars documents concerning the scientific activity and management of the institute. the documents date from 1945, when the institute became an independent board after separating it from the cnr (consiglio nazionale delle ricerche) to 1965. it is a decisive period for the ing, which had been founded in 1936 by guglielmo marconi, president of the cnr at the time and by the physicist antonino lo surdo, who became its first director until his death in 1949. these valuable papers document how the ing rose from the war destruction, expanded its seismic monitoring network and developed experimental and theoretical research in all the main fields of the earth physics. mailing address: dr. giuseppina calcara, istituto nazionale di geofisica e vulcanologia (ingv), via di vigna murata 605, 00143 roma, italy; temporarily attached to archivio centrale dello stato, piazzale degli archivi 27, 00144 roma, italy; e-mail: giuseppina.calcara@ingv.it; giuseppina.calcara@beniculturali.it vol52,6,2009new 2-12-2009 12:39 pagina 549 550 g. calcara the discoveries made in fundamental physics in the early decades of the 1930’s led to an increase in more in-depth research in several branches of geophysics-physics of the atmosphere, cosmic ray, and terrestrial magnetism. these areas had, until then, each been researched separately and still laboured under the yoke of their original naturalistic imprint (dominici, 1986). the study of geophysics, according to antonino lo surdo, professor in charge of earth physics studies at the university of rome at the time, could not be separated from a «coordinated functioning of a homogeneous group of laboratories and observatories» (ing, 1945) or from the systematic gathering of data carried out by qualified personnel in dedicated institutes. in fact, lo surdo firmly believed in the practical applications of research results in fields such as communications, defence, public works, and mining activities. it is therefore evident that the reasons behind the need to reorganize geophysical research and services under a single, centralized, national leadership were manifold. an initial draft for the soon-to-be-founded geophysics institute was presented by gino cassinis (full professor of topography and geodesy at the milan polytechnic, and also secretary of cnr commettee of geodesy and geophysics) on may 8, 1936 during a meeting of the cnr executive board, which in those days was composed of guglielmo marconi (president), ugo frascherelli (general secretary) and vincenzo azzolini (managing director). some time later, antonino lo surdo announced his own project draft in which he outlined an institute divided into five divisions, each specialized in a different area of geophysics: seismology, terrestrial and atmospheric electricity, atmospheric optics and radiation, meteorology and terrestrial magnetism. these divisions were to include a network of stations, laboratories and workshops (cnr, 1936a). after having examined both these project drafts, the executive board decided that the two were not that dissimilar and charged its secretary general, ugo frascherelli, with the task of integrating them (cnr, 1936b). so, in november 1936, the papers creating the national institute of geophysics were officially signed into an act and antonino lo surdo was appointed director (fig. 1). the funds necessary for setting up an initial installation were authorized and headquarters were established within the offices of the higher institute of physics at the «la sapienza» university of rome. 3. the national geophysical network among the various research activities performed by the institute, seismology studies represented the principal line of inquiry to which all the geophysicists of the staff (pietro caloi, guido pannocchia, francesco peronaci, ezio rosini) dedicated their efforts, carrying out studies on the composition of the earth, the characteristics of superficial and inner stratifications, the propagation of seismic waves, and on methods for determining the absolute intensity of macro-seismic events (lo surdo, 1940). september 1, 1938 marked the beginning of recording activity at the «experimental seismic station«, albeit using a limited number of instruments installed in dedicated areas of the headquarters. this was to become the foundation of the budding national seismic network. as far as other research fields were concerned – ionosphere, cosmic rays, atmospheric electricity – the institute also availed itself of the collaboration of other university physicists who, under contract, carried out research work for the institute in their field of specialization. for example, studies regarding the ionosphere were entrusted to ivo ranzi who, in 1938, came up with the first instrument for atmospheric radio sounding; one of the few of its kind in the world at that time. the cosmic ray research group was made up of gilberto bernardini, bernardo n. cacciapuoti, marcello conversi, mario ageno and oreste piccioni. lo surdo, together with guglielmo zanotelli, conducted scientific inquiries into microwave physics, while enrico medi was involved in atmospheric electricity studies. among the papers deposited in the archives, there is also a testimony regarding applied research carried out for the military, performed by lo surdo himself and commissioned by the war, navy and air ministries. vol52,6,2009new 2-12-2009 12:39 pagina 550 551 the istituto nazionale di geofisica and its historical archives one of the most important tasks of the institute was that assigned by parliamentary act january 5, 1939 no. 18, whereby the responsibility for geophysics services was transferred from the ufficio centrale di meteorologia to the cnr and therefore on to the ing. this reorganization of services involved the creation of a nationwide geophysics network made up of a seismic network and several stations for the study of the ionosphere, atmospheric electricity and geomagnetism. the draft presented by lo surdo for the organization of the network and discussed in may 1940 read that «with the exception of explicit changes made», this network was to be made up of «main stations»: in rome, pavia, rocca di papa, and messina; and of «minor stations»: in salò, padua, udine, genoa, bologna, florence, pisa, bari, foggia, cuglieri, and in calabria. «special stations» were also provided for: in naples, including the clinometric station on vesuvius; in catania, including the clinometric station on mount etna; a station for atmospheric electricity in rome at st. alessio; and in trieste (cnr, 1940). the distinction between observatory and station was for all practical purposes established. the observatories were to be for the purpose of detecting as many observable natural phenomena as possible via the installation of various types of apparatus, and for carrying out experimental studies and research which would mark them as study centres; each would have a resident geophysicist as station director along with a support staff of technicians and assistants (ing, 1954). the stations were to be of a smaller size, with the installation of specialized instruments regarding, among others, seismic, meteorological, and electricity studies. the stations, for the most part, were run by a director, often a professor and in any case an expert in the field at hand, along with a technician. several of the buildings in which the observatories and stations were located were the property of the institute others were set up under contract in schools, universities and religious institutes (see the inventory, series «x istituti, osservatori e stazioni», for more informations). notwithstanding the difficulties created by the ongoing war, in 1942 saw the seismic observatories in catania, genoa, and padua, along with the seismic stations in rome, bologna and salo’ became operative. twelve kilometres from rome, on the via ardeatina at st. alessio, an observatory for studies on atmospheric electricity phenomena and terrestrial and solar radiation was running (cnr, 1943 ca.). correspondence between the heads of the observatories and stations in rome provide an insight into how the setting up and running of the network continued non-stop until the spring of 1943: slowing down after the summer of that year as the conflict became more intense following the landing of the allied troops. 4. the ing autonomy the ing officially became an autonomous, non-profit organization under the supervision of the ministry of education by delegated defig. 1. resolution establishing the istituto nazionale di geofisica (nov. 15, 1936). vol52,6,2009new 2-12-2009 12:39 pagina 551 552 g. calcara cree «luogotenenziale» march 3, 1945 no. 82. when the decree became effective, the ing’s statute was drawn up defining its institutional obligations (decree of temporary head of state december 13, 1946 n. 731), its administrative and management organigram, along with their responsibilities, norms and regulations regarding the staff, which was to include forty persons. the institute was organized with a director (chairman of the board), a board of directors, an administrative board, an advisory committee, and an audit board. once a legal entity in its own right, the institute had to face the issue of financing. in 1945 the ministry of the treasury assigned interim financing of 2,030,000 liras; 80% of the funds went to cover expenses for personnel. lo surdo requested a substantial increase in these funds in order to be able to finance the scientific activity and the functioning of the institute itself. parliamentary act october 16, 1947 no. 1293 allotted an annual sum of 26,000,000 liras, along with a lump sum of 13,460,000 liras to allow the institute to face its most pressing problem of starting up activities again after the war. among these were: rebuilding and repairs to structures damaged during the conflict (rocca di papa, st. alessio, genoa, salò, bologna, florence, naples); completion of the new equipment for the seismic network that would include stations in bari, palermo and cagliari; the construction of new ionosphere apparatus and four stations for atmospheric electricity studies; continued research into the best sites for the magnetism observatory and its network. a paper filed in june 1948 reports on the situation: the seismic network had at that time fourteen stations and observatories – university of rome, trieste, padua, salò, pavia, genoa, bologna, florence, rocca di papa, naples (two stations), catania and messina –, along with a seismic station in merano. the atmospheric electricity division had its main observatory in rome at st. alessio, the radiations and atmospheric optics division had its offices, equipment and instruments in the observatory at rocca di fig. 2. galitzin seismographs installed at roma seismic station (1939). vol52,6,2009new 2-12-2009 12:39 pagina 552 553 the istituto nazionale di geofisica and its historical archives papa. the institute was charged with the task of setting up a nationwide network for magnetism studies made up of a central, main observatory and several peripheral stations (ing, 1948a). as for the institute’s scientific production, twelve years of scientific research saw 150 monographs and published papers «regarding original work in geophysics carried out within the institute» (ing, 1948b). moreover, the ing’s director had founded a journal entitled annali di geofisica (first published in 1948) to which many illustrious foreign seismologists contributed: among these j.p. rothe’, k.e. bullen, l. mintrop, b. gutenberg, v. keylisborok, e. teysseyre. 5. post-war organization when lo surdo died (rome, june 7, 1949), enrico medi became the institute’s director, inheriting a well consolidated and organized research body and an already operating geophysics network. undoubtedly, there was still much to be done: complete the seismic network, set up new stations for atmospheric electricity studies, institute a nationwide magnetism observatory. an increase in funding, amounting to 76,000,000 liras annually (parliamentary act december 28, 1950 no. 1138), made it possible for the new director to complete the tasks undertaken and increase financing to the laboratories, stations and observatories. cost for personnel accounted for one third of the government’s allotment and it was therefore possible to hire new staff via state examinations. throughout the early 1950’s, seismology continued to be the institute’s most important branch of activity, enjoying international recognition for the studies carried out by pietro caloi (caloi, 1954). a special issue of the annali di geofisica, printed in 1954 for the x iugg (international union of geodesy and geophysics) congress held in rome, contains a list of observatories and stations which made up the national geophysics network at that time (observatories: oropa, genoa, pavia, salo’, merano, pieve di cadore, trieste, monte cimone, corinaldo, rocca di papa, rome st. alessio, rome university, l’aquila, reggio calabria, gibilmanna, messina, cuglieri, vesuvius; stations: asiago, padua, bologna, florence, catania, palermo, tormezzo, siena). every two weeks the seismic division published a provisional draft bulletin and every month, a final version containing the results of the elaborations of the network data. the ionosphere division had two stations set up in st. alessio. data recorded there was elaborated and sent in monthly bulletins to international data collection centres. the importance of ionosphere data had grown considerably in those years and as a consequence the institute had assumed the role of providing ionosphere sounding services for the entire national territory (experimental studies began in 1956). scientific inquiries regarding terrestrial magnetism surged forward in the 1950’s; experimental studies began with the recording of magnetic events via instrumentation installed in the marches (corinaldo) and in asiago and headquarters for the main magnetism observatory were established at the end of the decade in masseria calore, near l’aquila. between the end of the 1950’s and the beginning of the 1960’s, profound changes occurred in the field of «observational geophysics» (dominici, 1986) influencing the very way this science was perceived and as a consequence deeply affecting the development of the institute itself. most importantly, the interdisciplinary nature of earth physics studies was recognized – up until that time, each area had worked and operated independently. advances in the field of «solid state electronic devices» provided a new system, telemetry, which vastly improved observation quality, making it possible to select sites that were more remote and therefore less disturbed by anthropic noise and then connecting these sites to operators via telephone cables or radio link. these scientific innovations widened the horizons of geophysics research and the institute found it absolutely necessary to overhaul its organizational structure and technical hardware. papers and documents, minutes of meetings of the board of directors, illustrate this concern as well as the acknowledgement of the need in 1957 to operate vol52,6,2009new 2-12-2009 12:39 pagina 553 554 g. calcara a reform that will bring the institute up to date in accordance with the demands imposed by «developments in the field of geophysics in recent years» (ing, 1957) – two years later, the ministry of education was officially involved in this process. in the meantime, the director proposed to set up a commission to study a revision of the statute (which changed several years later by decree of the president of the republic december 8, 1971 n. 1425, in vigour may 1972, with the first internal regulation approved by interdepartmental decree march 21, 1973). this commission, whose findings were published in 1961, suggested a considerable increase in personnel and financing. the emphasis placed on an increase in numbers rather than on an improvement in quality probably came about as a result of an underestimation of the substantial and important changes that a reorganization of technology would have brought about – had the funds available been invested in instrumentation for the head offices and in a renewal of network apparatus, there would have been a net decrease in the need for personnel. from an analysis of the financial resources available to the institute, it can be seen that the contribution it received in 1962 was exactly the same as that accorded the previous decade. however, the fixed and reflected charges had increased by 80%, leaving a margin of only 20% for research costs (surveying, maintenance of the observatories, scientific equipment and instruments). in fact, the last investment increase for the network can be traced back to the early 1950’s and the structural upkeep of the observatories was possible only because of contributions by local bodies or by special government funding (the same occurred for gibilmanna, castel tesino and l’aquila). investments in research and scholarship programs were proposed by the institute’s director at the beginning of his term of office, but they remained only proposals. no new hiring through state exams was undertaken, even though there were posts for geophysicists to be filled. in an effort to find a solution to its financial problems, a series of appeals was addressed to the ministry in 1961 requesting a revision of the annual allowance. in the meanwhile, in march of 1958 medi, already a member of parliament and university professor, was nominated vice-president of euratom in brussels and he, in turn, empowered guglielmo zanotelli, his close collaborator and board advisor, to sign administrative documents and payments. the management of the direction of the institute began to be conducted via correspondence (ing, 1958) and, as a consequence, all the decisionmaking and its enactment became more and more protracted in time. the board’s appeals for increased funding fell for the most part on deaf ears, with the exception of a series of special budget appropriations: one for 20,500,00 liras parliamentary act march 5, 1961 no. 158 for the purchase of equipment and apparatus for the main offices and for l’aquila and gibilmanna; an additional 2,500,000 liras to supplement the annual budget; another for 12,500,000 liras in order to improve salary levels. the financing accorded by parliamentary act july 24, 1962 no. 1073, amounting to 75,000,000 liras annually for a period of three years, saved the institute in extremis from total paralysis. 6. archives contents the ing archives are stored in the central offices of the istituto nazionale di geofisica e vulcanologia in rome. it is important to note that the documents and papers produced by the ing were discovered a few years ago in the offices of the observatory at rocca di papa where they had been transferred prior to 1980 (see the inventory introduction for more detailed information and for the reordering criteria). they were in complete disarray, grouped in stacks without binders or envelopes of any kind, but nonetheless in fairly good condition. various types of documents and papers were found: ranging from minutes of meetings, resolutions, agreements, contracts, personnel files and administrative-accounting records – ledgers and orders; to correspondence, station logs, scientific-technical reports, maps, photographic and printed material. files regarding ex-cnr personnel transferred to the ing were assigned to the safekeeping of the institute in 1945 at the signing of vol52,6,2009new 2-12-2009 12:39 pagina 554 555 the istituto nazionale di geofisica and its historical archives those agreements which stipulated the presentday relationship between the newly founded body and the cnr. without any doubt, some records have gone astray and it has not been possible to trace when or how they were lost or misplaced. this collection of documentation, records and papers offers ample opportunity for study for those interested in the history of the first italian institute dedicated to geophysics, both from an institutional and administrative historical view point and for the retracing of the steps taken by italian geophysics research between the first and second half of the twentieth century. references archive sources cnr (1936a): istituto nazionale di geofisica, project by a. lo surdo, rome, archivio centrale dello stato (acs), cnr, istituto nazionale di geofisica, box. 1, file 2. cnr (1936b): summary of the minutes of the executive board meeting, held july 1, 1936, acs, cnr, istituto nazionale di geofisica, box 2, file 2. cnr (1940): minutes of the executive board meeting held may 27, 1940, acs, cnr, istituto nazionale di geofisica, box 1, file 1. cnr (1943 ca.): ing activity during 1941-1942, report by a. lo surdo, 1943 ca., acs, cnr, series istituti e centri di ricerca pos. 10 pg, box 2006 (temp. reference number), file «10 pg. attività. istituto nazionale di geofisica» (in arrangement). ing (1945): ing activity, report by a. lo surdo, 1945, rome, istituto nazionale di geofisica e vulcanologia (ingv), ing, box 12, file 4. ing (1948a): minutes of the board of directors meeting held october 28, 1948, ingv, ing, box 5, file 1. ing (1948b): minutes of the board of directors meeting held june 24, 1948, ingv, ing, box 5, file 1. ing (1957): minutes of the board of directors meeting held november 11,1957, ingv, ing, box 5, file 1. ing (1958): delega.corrispondenza col sig. direttore. 1958. zanotelli, ingv, ing, box 11, file 6. bibliography caloi, p. (1954): l’astenosfera come canale-guida dell’energia sismica, in annali di geofisica, 7, 491-501. dominici, p., l’istituto nazionale di geofisica dalla sua costituzione all’attuale assetto statutario 1936-1983, speech given at the fiftieth anniversary celebration of the institute, roma, december 12, 1986; courtesy of dr. guido dominici (ingv). istituto nazionale di geofisica (ingv) (1954): supplement to annali di geofisica, 1954, pp. 5-86. lo surdo, a. (1940): la registrazione e lo studio dei fenomeni sismici nell’istituto nazionale di geofisica del cnr, la ricerca scientifica , xi (10), 1-23. vol52,6,2009new 2-12-2009 12:39 pagina 555 annals 47, 1, 2004, 01/07def 1 annals of geophysics, vol. 47, n. 1, february 2004 key words self-potentials – extreme events – earthquakes – cross-correlation 1. introduction in recent years, seismic research has involved the scientific community in debates concerning the predictability/unpredictability of earthquakes (i.e. geller, 1996; evans, 1997). the most important key point is to perform indepth experimental research aimed to develop i) well-based geophysical monitoring activities, ii) investigating correlations between earthquakes and extreme events in self-potential data recorded in a seismic area of southestern appennine chain (italy) luciano telesca (1), marianna balasco (2), gerardo colangelo (1) and vincenzo lapenna (1) (1) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy (2) dipartimento di geologia e geofisica, università degli studi di bari, italy abstract the normalized wavelet cross-correlation function (nwccf) was used to study correlations between the series of extreme events in self-potential data and earthquakes, both modelled as stochastic point processes. this method gives objective results, robust to the presence of nonstationarities that often affect observational time series. furthermore, the nwccf identifies the timescales involved in the cross-correlated behaviour between two point processes. in particular, we analyzed the cross-correlation between the sequence of extreme events in selfpotential data measured at the monitoring station tito, located in a seismic area of southern italy, and the series of earthquakes which occurred in the same area during 2001. to evaluate the influence of rain on the dynamics of geoelectrical variations, we applied the same approach between the selected extreme values and the rain data. we find that the anomalous geoelectrical values seem to cross-correlate with the rain at short and intermediate timescales (τ < 500 h), while they significantly cross-correlate only with earthquakes (m ≥ 2.5) at long timescales (τ > 500 h). mailing address: dr. luciano telesca, istituto di metodologie per l’analisi ambientale (imaa), cnr area della ricerca di potenza, contrada s. loja, 85050 tito scalo (pz), italy; e-mail: ltelesca@imaa.cnr.it quantitative methods for the analysis of precursory signals and iii) physical models able to explain the generation mechanism of anomalous geophysical signals in earthquake preparation zones (wyss, 1997). geophysical anomalous temporal patterns, interpreted as seismic precursors, have been observed and attributed to stress and strain changes which were followed by earthquakes (i.e. rikitake, 1988; park, 1996), but the physics underlying the source mechanisms of geophysical precursory phenomena is still difficult to understand (nur, 1972; scholtz et al., 1973; scholtz, 1990). therefore, it is necessary to objectively define criteria to evaluate the reliability of shortterm predictions based on this type of precursory signals (i.e. wyss, 1991; geller, 1996). in this research field, the mean weak points are the sporadic and irregular monitoring activi2 luciano telesca, marianna balasco, gerardo colangelo and vincenzo lapenna ties in active seismic areas and the absence of robust statistical methods able to discriminate extreme events from random noise in precursory time series. currently, there is no scientific consensus for a significant correlation between the reported anomalies and the occurrence of earthquakes. precursor research is required to demonstrate quantitatively that its data are anomalous and are causally related to earthquakes. this work tackles the problem of the crosscorrelation between the series of extreme events identified in geoelectrical time series and earthquake activity. after identifying anomalies in self-potential data by means of objective methods (run theory or crossing theory) (cuomo et al., 1996, 1997, 1998), the fundamental question to be solved is the correlation with the seismic events which occurred in the investigation area. rather than simply observing the concomitantly occurring self-potential extreme events and earthquakes by visual inspection, the cross-correlation between the two series of events has been quantitatively stated by means of the so-called normalized wavelet crosscorrelation function (nwccf). this function not only identifies the presence of significant cross-correlations between two sequences of events, but also detects the timescales mainly involved in such cross-correlation. this statistical measure was used in lowen et al. (2000) concerning the investigation of cross-correlated behaviour in two spike trains of neural discharges in cat rgc and lgn cells. 2. geological and seismological settings the measuring station is located on the southern apennine chain whose framework consists of a pile of thrust sheets forming a complex system orogenically transported over the flexured south-western margin of the apulia foreland. it is the result of a complex sequence of tectonic events associated with the collision between africa and europe (pantosti and valensise, 1990). starting in the middle miocene up to the upper pliocene, five or more compressional tectonic phases (patacca et al., 1988) caused progressive thrusting and piling of different tectonic units corresponding to different paleogeographic domains toward the stable external domains of the apulo-adriatic foreland. during the quaternary, the southern apennines were affected by an important extensional tectonic phase, with ne-sw extensional trend, that caused further chain fragmentation into several isolated blocks (doglioni et al., 1996). the southern apennine chain is one of the most active areas of the mediterranean region. in this area, on november 23, 1980 (ms = 6.9), a large normal-faulting earthquake occurred (pantosti and valensise, 1990). one of the most historically important events, the december 16, 1857 normal-faulting earthquake (mallet, 1862), occurred in val d’agri. seismic activity occurred after the 1980 event consisting of medium intensity events (m < 5.0) located close to the border between campania and basilicata regions. the may 5, 1990 (md = = 5.0, ing-istituto nazionale di geofisica) and the may 26, 1991 (md = 4.7) earthquakes may be considered the strongest events after the irpinia 1980 earthquake (tertulliani et al., 1992) which occurred in this area. these events were followed by aftershock sequences that identify a fault structure located near potenza town. seismological analysis of the above events demonstrated that such earthquakes were generated by a strike-slip fault in we direction, perpendicularly oriented toward the apennine chain (ekström, 1994). this fault lies north of potenza town and is located in such a way as to limit toward north and south two great seismogenetic faults that caused the 1857 val d’agri and 1980 irpinia earthquakes respectively. the fault area outlined by the aftershocks extends approximately 20 km in length and 10 km in depth, making it significantly larger than expected for a ml = 5.0 earthquake. the aftershocks were concentrated between 15 and 25 km depth, which is deeper than over well determined focal depth in the central and southern apennines (ekström, 1994). the geological and seismological settings allow us to consider the investigated area an ideal outdoor laboratory to study the possible correlations between tectonic activity and anomalous patterns in the geoelectrical signals. 3 correlations between earthquakes and sp extreme events in southern italy 3. data analysis and discussion in this work we analyze the time series of hourly means recorded at station tito during 2001. figure 1 shows the location of the station and the epicenters of the earthquakes which occurred in a circular area of 100 km radius from the measuring station. technically, a geoelectrical or self-potential time series is a sequence of voltage differences measured with a selected sampling interval using a receiving electrode array. during the geoelectrical soundings, where a current is injected into the ground, the self-potential represents the noise (lapenna et al., 1994). on the other hand, when we record using a passive measurement technique (i.e. without an energizing system), we measure the signal. to avoid some self polarizing effects, we used ceramic electrodes made of ceramic vessels filled with a saturated solution of copper sulphate. the time series obtained with the different probes are constantly checked to remove anomalous patterns related to polarization effects (di bello et al., 1994). the problem of the identification of extreme events in self-potential data, that are electrical values above or below a 15° 16° 17° 15° 16° 17° 40° 41° 40° 41° apulia calabria ionian seatyrrhenian sea 0 10 20 30 40 km potenza irpinia basilicata adriatic sea campania n tito fig. 1. location of the geoelectrical monitoring station tito in southern apennine chain. the epicentral distribution of the events occurring during the measurement period is also shown. 4 luciano telesca, marianna balasco, gerardo colangelo and vincenzo lapenna fixed threshold, has already been approached in previous works by lapenna et al. (1994) and cuomo et al. (1996, 1997, 1998), to which we refer the reader. in this paper we deal with the cross-correlation between the set of seismic events and the series of geoelectrical extreme values. both the sequences of earthquakes and extreme events were modelled by means of stochastic point processes s t a t tn n n = -d!^ ^h h (3.1) where an is a coefficient proportional to the magnitude of the earthquake or to the value of the geoelectrical extreme event. fixing a threshold for the coefficient an, we obtain the spike train representing the seismic sequence and that describing the sequence of the extreme events. figure 2a-c shows the time variation of the «nor malized» (obtained subtracting the mean and dividing by the standard deviation) geoelectrical signal; the horizontal dotted lines indicate the thresholds t (1.5 σ, 2.0 σ, 2.5 σ, 3.0 σ). an extreme event is defined as a value s of the signal so that |s| ≥ t. furthermore, the earthquakes (m ≥ ≥ 2.5) and the rain, recorded during the observation period, are also shown in the figure. the earthquakes, extracted from the instrumental seismic catalogue of the istituto nazionale di geofisica e vulcanologia, were selected as those contained in the irpinia-basilicata seismogenetic zone, in which the geoelectrical station was installed. 2.4 2.6 2.8 3.0 3.2 3.4 3.6 m a g n itu d e -6 -4 -2 0 2 4 s p ( m v ) 0 2000 4000 6000 8000 0 2 4 6 8 10 12 14 16 18 ra in ( m m ) t (hour) fig. 2a-c. a) normalized self-potential data recorded at tito station during the 2001. the horizontal dotted lines indicates the thresholds t = ± 1.5 σ, ± 2.0 σ, ± 2.5 σ, and ± 3.0 σ. the extreme event (ee) is defined if |s| ≥ t. b) earthquakes (eq) with magnitude m ≥ 2.5 which occurred in the investigation area during the 2001. c) rain data (rr) recorded for the tito site. a b c 5 correlations between earthquakes and sp extreme events in southern italy one way by which the information in an experimental sequence of events can be made more digestible is to reduce the data into statistics that emphasize a particular aspect of the data. the nwccf is a haar-wavelet-based version of the correlation function and is therefore insensitive to linear trends and nonstationarities. to calculate the nwccf at a particular timescale τ, the two spike trains first are divided into contiguous windows of duration τ. the number of spikes z1, n falling within the n-th window are recorded for all indices n corresponding to windows lying entirely within the first spike-train data set. this process is repeated for the second spike train, producing z2, n. the difference between the count numbers in a given window in the first spike train (z1, n) and the one after it (z1, n+1) is then computed for all n, with a similar procedure followed for the second spike train. the nwccf is defined as 1 1+ + = e z e z e z z z z 2 , , , , , , n n n n n n 1 2 1 1 2 2 2 1 x x x x x x x nwccf =^ ^ ^ ^ ^ ^ ^ h h h h h h h 8 8 8 8 b b b b & & 0 0 (3.2) where e {...} indicates the average value. the normalization has two salutary properties: 1) it is symmetric in the two spike trains, and 2) when the same homogeneous poisson point process is used for both spike trains the normalized wavelet cross-correlation function assumes a value of unity for all counting times τ. a nwccf ∼ 0 indicates that the processes are independent of each other. to determine the significance of a particular value for the normalized wavelet cross-correlation function at a certain timescale τ, we compared it with the value at the same timescale of nwccf calculated for synthetic data sets, that are shuffled versions of the original data sets (same interevent intervals but in random order). figure 3 shows the nwcff(τ) ∼ τ relation between geoelectrical extreme events (ee) (|s| ≥ ≥ 1.5 σ) and the earthquakes (eq) (m ≥ 2.5), and the nwcff(τ) ∼ τ relation between the same series of ee an the rain (rr). in order to evaluate the significance of the results, we also generated one hundred synthetic series of the three sequences.we generated the synthetic series by means of the random shuffling technique, well suited to test the significance of correlation in point processes (viswanatham et al., 1997; duarte and zatsiorsky, 2001; pavlov et al., 2001). the technique works as follows: first we generate a series of a normally distributed random numbers r ( j), j = 1,..., n, where n is the number of the events (ee or eq or rr). next the series r ( j) is rewritten in the order of increasing values, so we obtain the random sequence of indexes. the values in the original series are shuffled according to this sequence. the advantage of this method is that the synthetic series has the same mean, variance and probability density function of the inter-event times as the original one. therefore, we calculated for each synthetic pair the nwccf, then, we averaged all the synthetic nwccfs. figure 3 shows the mean synthetic nwccf (thin line) and its 1σs range (dotted lines). we see that the mean surrogate nwccf is approximately zero for almost all time-scales τ, as expected for independent processes. the original nwccf (bold line) can be considered at a particular timescale significantly different from zero if its value at that timescale is external to the surrogate 1-σs range. in this case, we observe that the ee-eq curve varies within the 1-σs range of the shuffles, indicating that the ee with t = 1.5 σ are approximately independent of the eq with magnitude m ≥ 2.5. the correlations between ee and rr seem to be clearer at small time-scales (up to ∼ 50 h), indicating that the rain influences the generation of ees, whose values exceed 1.5 σ. at long timescales, the rain does not seem to be correlated with the ee series, since the nwccf does not lie outside the shuffle standard deviation range. figure 4 shows the nwccf for ee-eq and ee-rr pairs, with ee threshold t = 2.0 σ. we observe that the correlations between ee and eq involve the intermediate timescales ∼ 100-300 h, becoming rather evident at longer timescales. the ee series seems to cross-correlate with the rr series at small timescales, as observed in fig. 3; furthermore, intermediate timescales (up to approximately 500 h) also seem to be involved in the cross-correlated behaviour. figure 5 shows the nwccf 6 luciano telesca, marianna balasco, gerardo colangelo and vincenzo lapenna 0 200 400 600 800 1000 -2 0 2 extreme events rain x (hour) 0 200 400 600 800 1000 -4 -2 0 2 extreme events earthquakes t=2.0v n w c c f (x ) t=2.0v n w c c f (x ) fig. 4. nwccf(τ) ∼ τ relation for ee-eq and ee-rr pairs, with t = 2.0 σ. 0 200 400 600 800 1000 -0.4 -0.2 0.0 0.2 0.4 0.6 extreme events rain t=1.5v x (s) n w c c f (x ) 0 200 400 600 800 1000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 extreme events earthquakes t=1.5v n w c c f (x ) fig. 3. nwccf(τ) ∼ τ relation for ee-eq and ee-rr pairs, with t = 1.5 σ. 7 correlations between earthquakes and sp extreme events in southern italy 0 200 400 600 800 1000 -2 0 2 4 extreme events rain x (hour) 0 200 400 600 800 1000 -4 -2 0 2 4 extreme events earthquakes t=2.5v n w c c f (x ) t=2.5v n w c c f (x ) fig. 5. nwccf(τ) ∼ τ relation for ee-eq and ee-rr pairs, with t = 2.5 σ. 0 200 400 600 800 1000 -4 -2 0 2 4 extreme events rain x (hour) 0 200 400 600 800 1000 -4 -2 0 2 4 extreme events earthquakes n w c c f (x ) t=3.0v t=3.0v n w c c f (x ) fig. 6. nwccf(τ) ∼ τ relation for ee-eq and ee-rr pairs, with t = 3.0 σ. 8 luciano telesca, marianna balasco, gerardo colangelo and vincenzo lapenna variations with a higher value for the threshold t = 2.5 σ; both in the ee-eq and ee-rr curves the cross-correlation is very clear. we can see that the original ee-eq nwccf is significantly negative at small timescales (from 100 to almost 300 h), and positive for approximately τ > > 400 h. furthermore, negative ee-rr nwccf characterizes the cross-correlation from approximately 50 to 100 h. positive nwccf is displayed at intermediate timescales, up to ∼ 500 h, although in the range 300-400 h the rain and the geoelectrical data seem to be independent of each other. for timescales τ > 500 h the rain does not seem to cross-correlate with the series of self-potential anomalous values, which, instead, show a significant correlation with the earthquakes. figure 6 shows the nwccf for the pairs ee-eq and ee-rr, for the threshold t = 3.0 σ: the results are approximately similar to those obtained for 2.5 σ threshold. the results obtained by the nwccf analysis suggest that: i) the rain could influence the generation of extreme events in geoelectrical signals at small and intermediate timescales (< 500 h), while for longer timescales the processes seem to be independent of each other. ii) the earthquakes seem to be correlated with the geoelectrical anomalous values for several timescale ranges. iii) the cross-correlated behaviours seem to be independent of the threshold of selection of the geoelectrical anomalous values, especially when the threshold is high. iii) excluding the timescale ranges, characterized by the correlation between rain and selfpotential data (that cannot allow us to draw firm conclusions on the correlation between self-potentials and earthquakes), our findings can allow us to objectively establish that earthquakes could be responsible for the generation of geoelectrical extreme values at long timescale ranges, suggesting a transfer of information between the geophysical processes governing them. 5. conclusions in this work we performed a correlation analysis by means of the nwccf between the series of extreme events in the time series of hourly self-potential means, recorded at monitoring geoelectrical station tito (southern italy) and earthquakes which occurred in the investigation area during 2001. we found significant correlations between geoelectrical anomalous values and seismic events in long timescale ranges (> 500 h), indicating correlated mechanisms of generation of both anomalous self-potential data and earthquakes. longer data sets can improve the analysis, especially in investigating the presence of a relationship between the values of the extreme events and the magnitudes of the earthquakes, or between the intensity of the self-potential anomaly and the distance of the earthquake from the station location. the results presented in this paper, although still preliminary disclose the potential of the nwccf, and similar investigations performed over other geophysical data sets measured in seismic areas can give a contribution to the earthquake prediction problem. references cuomo, v., g. di bello, v. lapenna, m. macchiato and c. serio (1996): parametric time series analysis of extreme events in electrical earthquake precursors, tectonophysics, 262 (1-4), 159-172. cuomo, v., v. lapenna, m. macchiato and c. serio (1997): autoregressive models as a tool to discriminate chaos from randomness in geoelectrical time series: an application to earthquake prediction, ann. geofis., xl (2), 385-400. cuomo, v., g. di bello, v. lapenna, m. macchiato, c. serio and l. telesca (1998): linear and nonlinear dynamics in electrical precursory time series: implications for earthquake prediction, tectonophysics, 287 (1-4), 279-298. di bello, g., v. lapenna, c. satriano and v. tramutoli (1994): self-potential time series analysis in a seismic area of the southern apennines: preliminary results, ann. geofis., xxxvii (suppl. to n. 5), 1137-1148. doglioni, c., p. harabaglia, g. martinelli, f. mongelli and g. zito (1996): a geodynamic model of the southern apennines accretionary prism, terra nova, 8, 540-547. duarte, m. and v.m. zatsiorsky (2001): long-range correlations in human standing, phys. lett. a, 283, 124-128. ekström, g. 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(1988): earthquake prediction: an empirical approach, tectonophysics, 148, 195-210. scholz, c.h. (1990): the mechanics of earthquakes and faulting (cambridge university press), pp. 439. scholz, c.h., l.r. syches and y.p. aggarwal (1973): earthquake prediction: a physical basis, science, 181, 803-810. tertulliani, a., m. anzidei, a. maramai, m. murru and f. riguzzi (1992): macroseismic study of the potenza (southern italy) earthquake of 5 may 1990, natural hazards, 6, 25-38. viswanathan, g.m., c.-k. peng, h.e. stanley and a.l. goldberger (1997): deviations from uniform power law scaling in nonstationary time series, phys. rev. e, 55, 845-849. wyss, m. (editor) (1991): evaluation of proposed earthquake precursors (american geophysical union, washington), pp. 94. wyss, m. (1997): cannot earthquakes be predicted?, science, 278, 487-488. sagir_fc_ok:layout 6 annals of geophysics, 57, 2, 2014, a0216; doi:10.4401/ag-6469 a0216 the characterization of diffusion tensor for mid-latitude ionospheric plasma selcuk sagir1,*, ali yesil2, gulay sanac2, ibrahim unal3 1 mus alparslan university, department of physics, mus, turkey 2 fırat university, department of physics, elazig, turkey 3 inonu university, department of science teaching, malatya, turkey abstract in this study, the relationship between diffusion and the electrical conductivity tensor is investigated by using the real geometry of the earth’s magnetic field in the northern hemisphere for mid-latitudes. this relationship is derived from equation. only in the elements of the diagonal of the tensor is an equal relationship found between the electrical conductivity called the longitudinal conductivity and the diffusion coefficient. the diagonal elements are equal to each other and do not depend on the earth’s magnetic field; however, the other elements of the tensor strongly depend on the earth’s magnetic field and they have characteristic of bohm diffusion became a semi-empirical model (db= (1/16)kt/eb) but they do not depend on the electrical conductivity or the classical diffusion coefficient. 1. introduction the ionosphere is the layer of the earth’s atmosphere that extends from 50 to 1000 km. the ionosphere significantly affects the propagation of radio waves so the variability of the ionosphere is important for ionospheric physics and radio communications. the ionosphere can be characterized by its electron density distribution, which is a complex function of spatial and temporal variations and geomagnetic and solar activity [rishbeth 1965, rishbeth and garriott 1969, whitten and poppoff 1971, banks and kockarts 1973]. usually, the ionosphere with respect to its electron density distribution is separated into five independent regions, sometimes called layers. the bottom one, from 50 to 80 km, is called the d layer, from 80 to 130 km is the e layer, and above 150 km is the f layer. the f region is usually separated at the f1 layer, from 130 to 200-250 km, and the f2 layer is above 250 km [rishbeth and garriott 1969, whitten and poppoff 1971, banks and kockarts 1973, rishbeth 1977]. the ionosphere is a natural plasma. many plasma phenomena can be analyzed using macroscopic transport equations, either by considering the plasma as a multi-constituent fluid or by treating the plasma as a single conducting fluid [rishbeth 1975, st.-maurice and schunk 1977]. in some cases, however, a satisfactory description can be obtained only through the use of kinetic theory. the presence of a pressure gradient term in the momentum transport equation provides a force that tends to smooth out any inhomogeneities in the plasma density [zhilinskiĭ and tsendin 1980]. ion electrons diffuse under the influence of partial pressure gradients and gravity. the diffusion of the particles in a plasma results from this pressure gradient force. to deduce the expression for the electron diffusion coefficient for a warm weakly ionized element, the momentum transport equations for the electrons are used with a constant electron-neutral collision frequency in the ionospheric plasma [banks and kockarts 1973, st.maurice and schunk 1977, abbas et al. 2011]. the electrons have far lower masses than ions, and they have far higher typical speeds at fixed temperature and are much more easily accelerated (i.e., they are much more mobile). as a result, it is the motion of the electrons, not the ions, that is responsible for the transport of heat and charge through plasma. transport properties can now be computed, including the electric conductivity and thermal conductivity, on the presumption that it is coulomb collisions that determine the electron mean free paths and that magnetic fields are unimportant in providing a more serious impediment to charge and heat flow than coulomb collisions and thus dominate the conductivities [rishbeth and garriott 1969, whitten and poppoff 1971, banks and kockarts 1973]. article history received november 22, 2013; accepted march 6, 2014. subject classification: ionosphere, diffusion tensor, electrical conductivity. this motion is hindered by collisions with the neutral air. the equation for the diffusion of plasma is obtained by adding the ion and electron force equations. the gravity term for electrons is omitted as it is quite negligible [banks and kockarts 1973]. the diffusion coefficient, called diffusivity, is an important parameter indicative of the diffusion mobility. the diffusion coefficient is not only encountered in fick’s law, but is found in numerous other equations of physics and chemistry. the diffusion coefficient is generally prescribed for a given pair of species. for a multi-component system, it is prescribed for each species in the ionospheric plasma. the higher the diffusivity, the faster the diffusion. the diffusion coefficient is proportionally constant between the diffusion flux and gradient in the concentration of the diffusing species, and it depends on both temperature and pressure [rishbeth and garriott 1969, whitten and poppoff 1971, banks and kockarts 1973]. 2. the relationship between diffusion and electrical conductivity tensor the behavior of the ionospheric plasma and the neutral thermosphere is subject to the equation of state (perfect gas law) and the general conservation equations for mass, momentum and energy [rishbeth 1965, bittencourt 1995]: a) continuity equation: density change=production–loss–transport b) equation of motion: acceleration=force–drag–transport c)temperature change=heating–cooling–conduction. if it does not have an impact outside, the transport of particles in the ionosphere plasma from place to place results from the pressure-gradient (∇p). this force occurs in any part of the plasma density to eliminate inhomogeneity. if b≠0, the medium is called the anisotropic [banks and kockarts 1973]. due to this, the ionospheric plasma is anisotropic. the real geometry of the earth’s magnetic field for the northern hemisphere is shown in figure 1, where bx = bcosisind, by = bcosicosd, and bz = −bsini. i is the magnetic dip angle and d is the magnetic declination angle. if u and m are the velocity and mass of the electron, respectively, then the force acting on the electron is as follows: where and, are the electron-ion and electron-neutral collision frequencies, respectively. it is assumed that the velocity and fields vary as ei(k · r−~t), where ~ is angular wave frequency, and ~c is electron angular gyro frequency. its vector components are: the current density can be obtained as follows: in which is the longitudinal conductivity. we assumed that the zaxis of the coordinate system with its origin located on the ground is vertical upwards. the x-and y-axes are geographic eastward and northward in the northern hemisphere [aydoğdu et al. 2004]. when the real geometry of the earth’s magnetic field is used for the steadystate (∂/∂t=0, that is, ~=0), the conductivity tensor of the solution of equation (2) is obtained as follows: with all of the tensor elements of equation (4) are real if equation (1) is used for the diffusion coefficient, depending on the real geometry of the earth: where, m dt d e mu u b e u# o=+ -^ h ei eno o o= + n . n t t 5.4 10 n t m.k.s 10 m.k.s andlog59 4 18ei e en n e e 3 16 21 6 3 2 # #o o = + = -; 6 6 e @ @ c m eb ebm , m eb and m .cx x cy c y z z ~ ~ ~= = = m i ej e j b0 #v o ~ = -^ h m i ne 0 2 v o ~ = -^ h j ev= c cx cx cy cz cx cz cy cx cy cz cy cy cz cx cx cz cy cy cz cx cz 2 2 0 2 2 2 2 2 2 v o ~ v o ~ ~ ~ ~ o ~ ~ ~ o ~ ~ ~ o o ~ ~ ~ ~ o ~ ~ ~ o ~ ~ ~ o o ~ = + + + + + + + -> h n nb e d# dnc c= + -^ h m e , d m k t and nubn o o c= = =^ h sagir et al. 2 figure 1. the geometry of the earth’s magnetic field (for the northern hemisphere). (1) (2) (3) (4) (5) 3 are respectively: the electron mobility, the electron diffusion coefficient and the flux of density. the flux of density in terms of the current density is as follows: from here, the electric field vector that depends on the flux of density could be written as follows: if this equation (7) is used equation (5), equation (8) is obtained as follows: where i is the unit matrix and v is the conductivity tensor. from here, the diffusion tensor depends on both the classical diffusion coefficient [abbas et al. 2011] and the electrical conductivity: where, with respect to equation (9), the diffusion tensor is given by: the diagonal elements of tensor (equation 10) are equal to each other and they only depend on the classical diffusion coefficient (d) and the electrical conductivity (v0). they do not depend on the earth’s magnetic field. however, the tensor is symmetric and they have characteristic of bohm diffusion became a semi-empirical model (db= (1/16)kt/eb). this diffusion coefficient does not depend on the density but it depends on magnetic field (that is, dbxy= −dbyx, dbxz= −dbzx, and dbyz= −dbzy). these results are interesting because the tensor (equation 10) is a mixture of the electrical conductivity, the classical diffusion coefficient, and the diffusion depending on the magnetic field. 3. numerical analysis and results the general system of transport equations is applied to the mid-latitude topside ionosphere. the restriction to this region of the ionosphere enables us to make several simplifying assumptions that significantly reduce the general system of transport equations. first is fully ionized plasma composed of two major ions, electrons, and a number of minors. next, the steady state and the species temperature and flow velocity differences are small in the mid-latitude topside ionosphere. this enables us to neglect stress and nonlinear acceleration terms [rishbeth 1975]. we wondered what the relationship between the electron diffusion and conductivity became in the ionospheric plasma by using the real geometry of the earth’s magnetic field. this relationship is given by equation (10). according to equation (10), only in the elements of the diagonal of the tensor is the relationship found between the electrical conductivity called the longitudinal conductivity and the diffusion coefficient; the diagonal elements are equal and thus do not depend on the earth’s magnetic field. however, the other elements of the tensor strongly depend on the earth’s magnetic field, but they do not depend on the electrical conductivity or the classical diffusion. in this study, the relationship between the diffusion tensor and the electrical conductivity is investigated by using the real geometry of the earth’s magnetic field in the northern hemisphere for mid-latitudes ionosphere plasma. the results obtained: i (dip angle)= 55.6°, d (declination) = 3°, r=10, by using equations (4)-(10). the ionospheric parameters used for calculations are obtained using the iri (international reference ionosphere) model. for a given location, time, and date, the iri model describes the electron density, electron temperature, ion temperature, and ion composition in the altitude range from about 50 to about 2000 km; it also describes the electron content. the modelprovides monthly averages in the non-auroral ionosphere for magnetically quiet conditions. the diagonal elements of equation (10) are equal and only depend on the classic diffusion coefficient and longitudinal conductivity (v0). the seasonal diagram of the diagonal element for mid-latitude in ionospheric plasma is given in figure 1 for local time (12.00-24.00). according to figure 1, the diagonal element that depends on both the classical diffusion coefficient and electric conductivity decreases with altitude in mid-latitudes ionospheric plasma at 12.00 lt (local time) and for every season. it takes the minimum value and is negative at around n and enu j u evc= == ee v c= e id n n b#d v n nc c= + ^ h` j n n n d 1 d d d d d d d d 1 1 1 1 1 1 1 1 bz cz b c bz cz b c by cy b c x x y z y y x x x x y z 0 0 0 $ d d d v o ~ o ~ o ~ v o ~ o ~ o ~ v c c c + + + + + + = ^ ^ ^ h h h r t s s s s s s s > > v x w w w w w w w h h c c m m d eb k t , d eb k t and d eb k t .bz z b b b b b y y x x = = = d 1 d d d d d d d d d 1 1 1 1 1 1 1 1 bz cz b c bz cz b c by cy b c b y y x x x x 0 0 0 v o ~ o ~ o ~ v o ~ o ~ o ~ v + + + + + + = r t s s s s s s s v x w w w w w w wc c m m diffusion tensor in the ionosphere (6) (7) (8) (9) (10) 225-400 km for march 21 and september 23 in general. however, for the same seasons, it suddenly decreases to north latitude (30°n) and is approximately −1.2 × 10-6 for march 21 and −7.0 × 10-6 at 300 km. for june 21 and december 21, it takes two times minimum values for south latitudes with an altitude (200-250 km; 275-400 km) except for the north latitudes. in addition, the diagonal elements of equation (10) increased about 300 km after sagir et al. 4 figure 2. the seasonal diagram of diagonal elements in equation (10) for mid-latitude in ionospheric plasma. figure 3. the seasonal diagram of dbxy (m 2/sn) in equation (10) for mid-latitude in ionospheric plasma. altitude (km) altitude (km) 5 every season and latitude, but are negative. for 24.00 lt, the trend in change of altitude of the diagonal elements of equation (10) is similar to the change of in 12.00 lt for every season. it is also the minimum altitude (about 300 km) but its magnitude has decreased with respect to 12.00 lt. the seasonal diagram of dbxy(m 2/sn) is given for 12.00-14.00 lt in figure 2. dbxy(m 2/sn) is positive both for 12.00 lt and 24.00 lt in mid-latitude ionosdiffusion tensor in the ionosphere figure 4. the seasonal diagram of dbyz(m 2/sn) in equation (10) for mid-latitude in ionospheric plasma. figure 5. the seasonal diagram of dbxz(m 2/sn) in equation (10) for mid-latitude in ionospheric plasma. altitude (km) altitude (km) pheric plasma for the northern hemisphere and all seasons, but is negative for the southern hemisphere. the magnitude of dbxy(m 2/sn) is bigger at 12.00 lt than 24.00 lt and seasonal. the change in altitude of dbyz(m 2/sn) is the same as both the local time and the magnitude in figure 2; however, the decrease in south latitudes and increase in north latitudes is exponential. the change in altitude of dbxz(m 2/sn) is shown in figure 4. dbxz(m 2/sn) decreases with altitude for both seasonal and mid-latitudes (12.00-24.00lt) but the most noticeable results occur at 30° south latitude. dbxz(m 2/sn) quickly decreases with altitude for both season and latitude. dbxz(m 2/sn) is approximately maximum between 300 and 450 km depending on the season 30° south latitude. the relationship between the magnitudes is dbxz(m 2/sn) >dbyz(m 2/sn)>dbxy(m 2/sn). 4. conclusion this article has reviewed the diffusion tensor for electrons in the mid-latitude ionospheric plasma and investigated whether any relationship between the diffusion and electrical conductivity exists. the findings indicate that the diffusion coefficient depends only on 0 (longitudinal conductivity) in equation (10). however, the other conductivities, such as pedersen and hall conductivity, do not affect the diffusion in ionosopheric plasma. the diagonal elements of tensor (equation 10) are equal to each other and they only depend on the classical diffusion coefficient (d) and the electrical conductivity (v0). they do not depend on the earth’s magnetic field. however, the tensor is symmetric and they have the characteristic of bohm diffusion became a semi-empirical model (db=(1/16)kt/eb). this diffusion coefficient does not depend on the density but it depends on magnetic field. (that is, dbxy= –dbyx,dbxz= –dbzx, and dbyz= –dbzy). these results are interesting because the tensor (equation 10) is a mixture of the electrical conductivity, the classical diffusion coefficient, and the diffusion depending on the magnetic field. based on our results, the diagonal elements in equation (10) take the minimum value around 200-400 km as both seasonal and latitudes are negative in general. this altitude (200-400 km) is the peak height of the region and has generally the maximum electron density in this region for every season. the tensorial elements of the diffusion expected for the diagonal element in equation (10) do not depend on electrical conductivity, but the tensorial elements relate to the electron temperature and collision frequency due to a magnetic field constant in mid-latitudes ionospheric plasma. the diffusion coefficients with respect to all diagrams change to linear with electron temperatures for mid-latitudes ionospheric plasma. our procedure may be useful in assessing ionospheric variables such as conductivity and it can be used for ionospheric plasma density measurements. this study could become important in mid-latitude stations for ionosonde measurements because the electron concentration of the medium determines the behavior of the medium against any external influence. references abbas, q.a., f.y. hadi and s.s. al-awadi (2011). modified bohm diffusione quation in q-machine, baghdad sci. j., 8, 339-344. aydoğdu, m., a. yeşil and e. güzel (2004). the group refractive indicies of hf waves in the ionosphere and departure from magnitude without collisions, j. atmos. sol.-terr. phy., 66, 343-348. baker, w.g., and d.f. martyn (1953). electric currents in the ionosphere. i. the conductivity, phil. trans. r. soc. lond. a., 246, 281-294. banks, p., and g. kockarts (1973). aeronomy, part b, academic press, new york, 430 pp. bittencourt, j.a. (1995). fundamentals of plasma physics, co-edition fapesp, 676 pp. rishbeth, h. (1965). a review of ionospheric f region theory, proceedings of the ieee, 55, 16-35. rishbeth, h., and o.k. garriott (1969). introduction to ionospheric physics, academic press, amsterdam, 331 pp. rishbeth, h. (1975). on the theory of diffusion in the ionosphere, geophys. j. roy. astr. s., 41, 311-317. rishbeth, h. (1997). the ionospheric e-layer and flayer dynamos a tutorial review, j. atmos. sol.-terr. phy., 59, 1873-1880. st.-maurice, j-p., and r.w. schunk (1977). diffusion and heat flow equations for the mid-latitude topside ionosphere, planet. space sci., 25, 907-920. whitten, r.c., and i.g. poppoff (1971). fundamentals of aeronomy, john wiley & sons, new york, 462 pp. zhilinskiĭ, a.p., and l.d. tsendin (1980). collisional diffusion of a partly-ionized plasma in magnetic field, sov. phys. uspekhi, 131, 343-385. *corresponding author: selcuk sagir, mus alparslan university, department of physics, mus, turkey; email: s.sagir@alparslan.edu.tr or sagirselcuk@hotmail.com. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sagir et al. 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol48/04/2005def 787 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words volcanology – geochemistry – soil degassing – monitoring – risk 1. introduction the azores archipelago comprises nine volcanic islands in the atlantic ocean where the american, african and eurasian lithospheric plates meet (searle, 1980; fig. 1). the midatlantic ridge clearly defines the northern and southern branches of the azores triple junction (lourenço et al., 1997) but the location and geodynamic relations of the boundary between the eurasian and african plates, corresponding to the so-called terceira rift (machado, 1959), is still controversial (madeira and ribeiro, 1990). the complexity of this geological setting, reflected by the seismic and volcanic activity, may bear on the inferred existence of a mantle plume (white et al., 1976). during more than 500 years of history about thirty important destructive earthquakes and twenty-eight volcanic eruptions have been reported in the archipelago, causing thousands of monitoring of fumarole discharge and co2 soil degassing in the azores: contribution to volcanic surveillance and public health risk assessment teresa ferreira (1), joão luís gaspar (1), fátima viveiros (1), márcio marcos (1), carlos faria (1) and fernando sousa (2) (1) centro de vulcanologia e avaliação de riscos geológicos, observatório vulcanológico, universidade dos açores, ponta delgada, açores, portugal (2) delegação regional do instituto de meteorologia, relvão, ponta delgada, açores, portugal abstract fluid geochemistry monitoring in the azores involves the regular sampling and analysis of gas discharges from fumaroles and measurements of co2 diffuse soil gas emissions. main degassing areas under monitoring are associated with hydrothermal systems of active central volcanoes in s. miguel, terceira and graciosa islands. fumarole discharge analysis since 1991 show that apart from steam these gas emissions are co2 dominated with h2s, h2, ch4 and n2 in minor amounts. mapping of co2 diffuse soil emissions in s. miguel island lead to the conclusion that some inhabited areas are located within hazard-zones. at furnas village, inside furnas volcano caldera, about 62% of the 896 houses are within the co2 anomaly, 5% being in areas of moderate to high risk. at ribeira seca, on the north flank of fogo volcano, few family houses were evacuated when co2 concentrations in the air reached 8 mol%. to assess and analyse the co2 soil flux emissions, continuous monitoring stations were installed in s. miguel (2), terceira and graciosa islands. the statistical analysis of the data showed that some meteorological parameters influence the co2 flux. the average of co2 flux in s. miguel stations ranges from 250 g/m2/d at furnas volcano to 530 g/m2/d at fogo volcano. at terceira island it is about 330 g/m2/d and at graciosa 4400 g/m2/d. mailing address: dr. teresa ferreira, centro de vulcanologia e avaliação de riscos geológicos, observatório vulcanológico da universidade dos açores, complexo científico, ala sul, 3º piso, 9501-801 ponta delgada, açores, portugal; e-mail: tf@notes.uac.pt 788 teresa ferreira, joão luís gaspar, fátima viveiros, márcio marcos, carlos faria and fernando sousa deaths and severe damage. the july 9, 1998 faial earthquake (senos et al., 1998) and the 1998-2001 submarine eruption west of terceira island (gaspar et al., 2003) were the most recent events. present-day volcanic activity in the azores is marked by highly active fumarolic fields, hot springs and soils diffuse degassing phenomena. this paper reports the fluid geochemistry monitoring programme that is being undertaken for volcanic surveillance and public health risk assessment in the region. special attention is given to the co2 diffuse emissions whose significance in the total degassing of volcanic areas is of major importance (allard et al., 1991). 2. degassing areas degassing areas in the azores are related to hydrothermal systems (ferreira, 1994). the most important zones of degassing are located in s. miguel, terceira and graciosa islands and at some submarine volcano-tectonic structures along the terceira rift. less significant degassing fields exist in faial, pico, s. jorge and flores islands (ferreira, 1994; viveiros, 2003). there are no reported gas emanations in santa maria or corvo islands. the monitored degassing areas in s. miguel island are associated with the furnas and fogo active central volcanoes. furnas village (mfv) and furnas lake (m-fl) fumarolic fields are located inside the furnas caldera (fig. 2) defining a wnw-ese path compatible with a major regional tectonic trend that traverse the island (gaspar et al., 1995). pico vermelho (mpv), caldeira velha (m-cv) and caldeiras da ribeira grande (m-crg) fumaroles are on the northern flank of the fogo volcano associated with the nw-se ribeira grande graben (fig. 3). in terceira island monitoring is focussed in the furnas do enxofre (t-fe) fumarolic field (fig. 4). the area covers a part of the galhardo trachytic lava dome and extends beyond its crater being located in the intersection of nw-se, e-w and nne-ssw faults (lloyd and collis, 1981). at graciosa island the main hydrothermal area under surveillance is within the furna do enxofre lava cave (g-fe). this volcanic structure was formed at the end of an intracaldera lava lake episode and is part of the main conduit of the caldera volcano, controlled by ne-sw and nw-se faults (gaspar, 1996; fig. 5). fig. 1. geographic location of the azores archipelago. 789 monitoring of fumarole discharge and co2 soil degassing in the azores fig. 2. furnas volcano, s. miguel island, fumarolic fields (m-fl – furnas lake; m-fv – furnas village). fig. 3. fogo volcano, s. miguel island, fumarolic fields (m-pv – pico vermelho; m-crg – caldeiras da ribeira grande; m-cv – caldeira velha). 790 fig. 4. furnas do enxofre fumarolic field at terceira island (t-fe – furnas do enxofre). fig. 5. furnas do enxofre fumarolic field at graciosa island (g-fe – furnas do enxofre). teresa ferreira, joão luís gaspar, fátima viveiros, márcio marcos, carlos faria and fernando sousa 791 monitoring of fumarole discharge and co2 soil degassing in the azores 3. fumaroles regular monitoring programme 3.1. sampling and analytical processes the procedures for fumarole gas sampling are described in ferreira (1994) and ferreira and oskarsson (1999) and follow the so-called giggenbach methodology (giggenbach and gogel, 1989) using a known volume of a strong koh solution (12 m) for adsorption of the acid gases. the samples are collected by placing a funnel, connected to a silicone tubing sampling train, over the steam vents. gas flowing through the tubing system is collected into an evacuated borosilicate sample-bottle containing a measured weight of the koh solution. co2 and h2s are adsorbed in this solution, which becomes diluted with the condensed steam, while the fixed gases h2, ar, ch4, n2 and o2 fill the remaining space. after being equilibrated to atmospheric pressure, gases in the sample-bottle are analysed with a gas chromatograph (perkin-elmer) with thermal conductivity detector, automated fixed sample loop, molecular sieve column and he as carrier gas. ar is analysed in a separate run after washing the gas sample with basic pyrogallol solution. co2 and h2s dissolved in the basic solution are quantified by titrimetric methods. co2 is determined by automated titration with hydrochloric acid and h2s is titrated with mercury acetate using dithizone for end point detection. a small volume of the basic solution is weighted for density determination, needed for mass balance involved in total gas analysis quantification. 3.2. results occasional monitoring of s. miguel (m-fl, m-fv, m-cv, m-cgr), terceira (t-fe) and graciosa (g-fe) fumaroles started in 1991 but regular measurements were started within the present programme. the available data show that co2 is the main constituent of the total gas in the steam discharges, representing 94 to 99.6 mol%. h2s and h2 are the other major components contributing for 0.1 to 3 mol% of the total gas. nitrogen concentration is below 1.5 mol% in all samples, the highest values being found at t-fe. ch4 values are in general lower than 0.08 mol% except at t-fe where they can be 10 times higher. all monitored fumaroles have discharges of broadly similar composition. however, on terfig. 6. h2/ch4 relation for the fumaroles under monitoring at the azores archipelago. references as in text. 792 teresa ferreira, joão luís gaspar, fátima viveiros, márcio marcos, carlos faria and fernando sousa ceira island fumaroles are richer in h2 and ch4. the differences between gas compositions are better displayed using the ratio h2/ch4. in fact these gases show the most regular abundances of all the analysed components allowing a discrimination between the fumarolic fields (fig. 6). based on equilibrium relations of h2 and ch4 and assuming a deep origin for these gases a simple model was applied to s. miguel fumarole discharges considering a binary carbon dioxide-steam system (ferreira and oskarsson, 1999). according to this model these gases are derived from supercritical conditions, presumably equilibrated within hot rocks below the hydrothermal systems. 4. mapping of co2 soil concentrations 4.1. methodology analysis of co2 soil concentrations at the azores follows the methodology applied by baubron et al. (1994) and ferreira and gaspar (1997). an iron probe 1 m long is buried in the soil at a depth of 50-70 cm and the gas is pumped into a portable co2 analyzer with an infrared detector. the instrument is calibrated to measure concentrations between 0 and 100 mol%. soil temperature is measured at the same sites as the co2 using a digital thermometer with 0.1ºc precision. the distance between sampling sites depends on the observed values, being shorter, in the order of a few meters, where the concentrations are above the ground noise level (0 to 1.5 mol%). the collected data are inserted in a gis and the anomaly grid maps are produced for co2 and temperature by means of the kriging statistical method. 4.2. results co2 soil concentration maps have been produced in the azores for hazard and risk assessment. baubron et al. (1994) showed the existence of an important anomaly in furnas village, within the furnas caldera in s. miguel island, and baxter et al. (1999) discussed the public health risk associated with such hydrothermal area. fig. 7. co2 soil concentrations at furnas village, s. miguel island (modified from sousa, 2003). 793 monitoring of fumarole discharge and co2 soil degassing in the azores a new survey recently performed in the same zone led to the conclusion that the general shape and magnitude of the anomaly remain almost identical 10 years later (fig. 7). soil co2 concentrations reach 95 mol% in some areas and soil temperatures can be as high as 100ºc. these observations indicate that on a large temporal scale the furnas village anomaly map can be used as a reference level concerning furnas volcano surveillance. with respect to public health risk assessment, sousa (2003) calculate that 62% of the 896 houses at furnas village are within this geochemical anomaly, 5% being located in areas of moderate to high risk according to the classification of baubron et al. (1994). public health risk at s. miguel island was also reported by ferreira and gaspar (1997) at ribeira seca village, about 3 km nw of the pico vermelho geothermal area. in april 1997 a co2 geochemical anomaly affecting an area of approximately 250 m2 was detected but regular observations until the end of 1997 revealed that the anomaly pattern remained unchanged. at that time the maximum co2 soil concentration was about 13.5 mol% and measured soil temperatures reached 50°c. in march 1998 a new survey carried out in the zone showed that the anomaly increased to an area of about 10000 m2 (fig. 8). co2 soil concentration levels of 24 mol% and soil temperatures of 50°c were then detected and four families were evacuated from their residences because co2 concentrations inside houses reached values of 8 mol% (gaspar et al., 1998). regular monitoring surveys done since then show a slight decrease of the anomaly area but the maximum observed values remain similar. co2 soil concentration maps are also useful for identification of major active tectonic faults (sugisaki et al., 1983; giammanco et al., 1998; baubron et al., 2002). in the azores, this technique has proved to be very efficient considering that young pyroclastic deposits often bury the active fault areas of the main trachytic central volcanoes (faria et al., 2003; marcos et al., 2003). fig. 8. co2 soil concentrations at ribeira seca in 1998, s. miguel island (modified from ferreira, 2000). 794 teresa ferreira, joão luís gaspar, fátima viveiros, márcio marcos, carlos faria and fernando sousa 5. co2 soil flux continuous monitoring 5.1. monitoring network and sampling continuous monitoring of co2 soil diffuse degassing at the azores started in october 2001. the first permanent station was installed on s. miguel island in a large degassing area that extends into furnas village (station gfur1) inside the caldera of the furnas volcano. a second station was set up on the island on february 2002 in the northern flank of the fogo volcano, within the pico vermelho geothermal area (gfog1). on december 2002 two other stations were installed at terceira island, in furna do enxofre fumarolic field (gter1), and at graciosa island, inside the furna do enxofre lava cave (ggrc1). all co2 soil flux continuous monitoring stations are based on the accumulation chamber method (chiodini et al., 1998). the station ggrc1 is also equipped with a sensor for h2s flux measurements. all the stations include additional sensors to record information on meteorological and environmental parameters such as barometric pressure, air temperature, air humidity, wind speed and direction, rainfall, soil water content and soil temperature. flux stations are configured to read data every hour. each co2 flux measurement is made during a time period of 1-3 min and the value is registered at the same time as the data from all the above-mentioned sensors. additional information on the operating status of the equipment is also available. data transmission links from the field stations to the volcanological observatory of the azores university, on s. miguel island, were established using gsm (gfur1), freewave (gfog1 and gter1) and uhf plus freewave (ggrc1). 5.2. results the azores climate is typically oceanic such that low pressure areas are followed by increased wind speed and increased precipitation that strongly affect the soil degassing. knowledge of how meteorological parameters interfere with diffuse soil degassing at any particular monitoring site is crucial to recognise changes due to deep processes. the influence of external factors on co2 flux is gauged by applying multiple regression analysis (draper and smith, 1981) and time series analysis modelling (box and jenkins, 1976). for s. miguel island the results obtained show that the main influencing parameters do not act in the same way on each monitoring site (viveiros et al., 2003). at gfur1 barometric pressure, rainfall, soil water content, air temperature and soil temperature are the parameters that have significant statistical connection to co2 flux fluctuations. at gfog1 station wind speed, barometric pressure, soil temperature, soil water content and rainfall are the statistically relevant influencing parameters (viveiros, 2003). the average of co2 flux at gfur1 is near 250 g/m2/d while at gfog1 it is around 530 g/m2/d. the recent co2 flux data of terceira and graciosa islands are still under analysis. however, the preliminary study of the obtained data showed that the average co2 flux is about 330 g/m2/d at gter1 and 4400 g/m2/d at ggrc1. in a general way, the co2 flux averages observed in the above mentioned hydrothermal zones are lower than the ones reported for other volcanic areas, such as solfatara and vesuvio (granieri et al., 2003). the furna do enxofre soil degassing area at graciosa island is the exception. 6. future developments the volcanological observatory of the azores university is responsible for the seismic and volcanic monitoring of the azores region using geophysical, geodetic and geochemical techniques. geophysical monitoring includes the management of a seismological network of more than 40 seismic stations, run in collaboration with the meteorological institute. geodetic monitoring is based on a gps network of several permanent receivers and complemented by regular gps field surveys. as discussed, the actual geochemistry monitoring programme involves the regular sampling and analysis of gas discharges in fumaroles and the study of co2 795 monitoring of fumarole discharge and co2 soil degassing in the azores diffuse soil gas emissions. to a minor extent, hot springs, geothermal and water wells are also under observation. the development of the azores seismovolcanic network comprises the set up of warning and alert systems based on the integration of all the monitoring data. this strategy implies the extension of the monitoring programme to all the islands and will benefit from the data transmission system that is being installed in the archipelago for civil protection activities. the recognition of hazardous zones due to the permanent diffuse soil degassing in several inhabited or tourist places requires the installation of alarm sensors in order to reduce the public health risk. such preventive measure is already being implemented at furna do enxofre lava cave, where two people died in 1992 due to an increase in co2 in the atmospheric air (ferreira et al., 1993; gaspar, 1996). acknowledgements this work was performed in the scope of the project «alerta-interreg iiib – reducing volcanic risks in islands» supported by the european community and the project «simova2 – risk assessment and emergency planning in the azores archipelago» supported by the regional government through the srpcba. references allard, p., j. carbonnelle, d. dajlevic, j. le bronec, p. morel, m.c. robe, j.m. maurenas, r. faivrepierret, d. martin, j.c. sabroux and p. zettwoog (1991): eruptive and 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(2003): cartografia de anomalias geoquímicas associadas a processos de desgaseificação difusa no vulcão das furnas (s. miguel, açores): contribuição para a avaliação de riscos vulcânicos, tese de mestrado no ramo de geologia especialidade vulcanologia e riscos geológicos (universidade dos açores), pp. 106. sugisaki, r., m. ido, h. takeda, y. isobe, y. hayashi, n. nakamura, h. satake and y. mizutani (1983): origin of hydrogen and carbon dioxide in fault gases and its relation to fault activity, j. geol., 91 (3), 239-258. viveiros, f. (2003): contribuição para o estudo dos processos de desgaseificação difusa nos açores no âmbito da monitorização sismovulcânica e da avaliação do risco: discriminação de factores que influenciam a variação do fluxo de co2, tese de mestrado no ramo de geologia especialidade vulcanologia e riscos geológicos (universidade dos açores), pp. 140. viveiros, f., t. ferreira, c. vieira and j.l. gaspar (2003): the influence of environmental parameters on co2 soil diffuse degassing in s. miguel island (azores), in egsagu-eug joint assembly, 6-11 april, nice, france (poster). white, m., j. schilling and s. hart (1976): evidence for the azores mantle plume from strontium isotope geochemistry of the central north atlantic, nature, 263, 659-663. geomagnetic coast effect at two croatian repeat stations annals of geophysics, 59, 6, 2016, g0652; doi:10.4401/ag-6765 g0652 geomagnetic coast effect at two croatian repeat stations eugen vujić*, mario brkić university of zagreb, faculty of geodesy, zagreb, croatia abstract knowledge of inductive effects is important for the reliability of geomagnetic surveys as well as reduction of measurements, and hence for the accuracy of models and maps of the earth’s magnetic field. detection of anomalous induced fields, due to the geomagnetic coast effect, was carried out by the transfer function method to estimate the induction arrows indicating areas of anomalous induced currents. to determine the transfer function at the two coastal croatian repeat stations used in this study, the so-called geomagnetic plane-wave events from july 2010 were used. analysis of transfer functions for krbavsko polje and sinjsko polje first order repeat stations, using observatories grocka and tihany as references, revealed the existence of the adriatic coastal effect on periods of 10-65 minutes. 1. introduction following the establishment of the croatian geomagnetic repeat station network, a series of geomagnetic surveys has been performed from 2004 to 2010, with the aim of obtaining reliable geomagnetic data over croatia [brkić et al. 2013]. the reliability of such data, as well as derived products and models such as maps relies, among other factors, on the consistency between observations made at geomagnetic stations on one hand, and those made at reference observatories on the other hand. a local cause of inconsistencies is given by electromagnetic effects caused by different electric conductivity values of the lithosphere below stations and observatories. such differences can be explored with simultaneous measurements of geomagnetic field variations in the context of deep geomagnetic sounding [banks 1973, armadillo et al. 2001]. geomagnetic sounding methods are used to model subsurface electrical conductivity and, on a regional scale, they allow to determine lateral conductivity contrasts in the earth’s crust and upper mantle. such contrasts produce anomalous induced magnetic fields, which need to be separated from the sum of the primary external magnetic fields produced in the earth’s magnetosphere and ionosphere and of those they induce in the earth’s interior (secondary) [banks 1973, gough et al. 1973, hitchman et al. 2000]. an example of such anomalous induced fields, are those arising in proximity of coasts because of the contrast of electrical conductivity at the interface continent-ocean. in this case, time variations of the vertical component of the earth’s magnetic field have enlarged amplitudes, and correlate positively with the horizontal component at the close overland positions [parkinson and jones 1979]. this phenomenon is called “geomagnetic coast effect“. in some cases, there are coastal regions where tectonic anomalies are spatially large and this effect is absent [parkinson and jones 1979, and references therein]. this effect is very intense during geomagnetic storms and substorms, but can occur also under quiet geomagnetic conditions. its maximum intensity is achieved on periods between 30 and 90 minutes, and its influence can be measured on distances larger than 150 km inland from a coast [parkinson and jones 1979, viljanen et al. 1995, srivastava et al. 2001]. the models that explain the coastal effect are based on the electromagnetic induction (by the time-varying ionospheric and magnetospheric magnetic fields) contrasts: in the crust and upper mantle under continents and oceans (due to their prominently different electrical conductivities), and/or in the oceans and continents (ocean water has much higher electrical conductivity with respect to the land masses). an electromagnetic induction in the oceans could be also due to their motion across the earth’s magnetic field originating in the core and in the lithosphere [parkinson and jones 1979]. as part of a bilateral project “joint croatian-hungarian geomagnetic survey and model” (2009-2012) a number of absolute measurements and a dataset consisting of measurements of didd variometer (with overhauser effect proton sensor) were gathered at article history received march 11, 2015; accepted november 22, 2016. subject classification: geomagnetic repeat station, geomagnetic transfer function, inductive arrows, geomagnetic coast effect. three croatian repeat stations. csontos et al. [2012] indicated the possible existence of geomagnetic effect of the coast in front of the adriatic sea, and this result was the reason for further investigation of inductive effects at the coastal croatian repeat stations, here described and discussed. 2. transfer function method and data the transfer function method for detection and separation of anomalous induced fields due to conductivity contrasts from the normal induced and primary external fields, derives widely from the work of parkinson [1959] and wiese [1962]. in this context, let the so-called normal magnetic field bn= (xn, yn, zn) be defined as the vector sum of the primary external field (produced by electric currents flowing in the ionosphere and magnetosphere) and of the secondary internal magnetic field caused by currents in a laterally homogeneous, vertically stratified earth’s interior, which are induced by the external magnetic field variations. the local field b = (x, y, z) at any point on the earth surface is then given by the vector sum of bn and a local anomaly field bia= (xia, yia, zia) caused by induced currents related to local anomalies/conductivity contrasts of the underground electric conductivity. in the case of the vertically stratified earth’s interior, the transfer function approach assumes that a linear relationship exists between bia and bn in the frequency domain, i.e. between the fourier transforms of bia and bn [banks 1973]. it is assumed that the local field bia is induced by the spatially homogenous field bn, and due to linearity of the maxwell’s equations, there has to be a linear relationship between inducing (bn) and induced (bia) fields [schmucker 1970, lilley and bennett 1973]. this relationship can be written in matrix form as [banks 1973]: (1) where the coefficients wxx, wxy, ..., wzz of the matrix w denote the frequency-dependent transfer function between bn and bia, and the asterisk * denotes the complex fourier transform. now the problem given by the above equations can be reduced onto estimation of the components of the normal external field at each measurement point. the transfer function can be obtained from equation (1) after some simplifying assumptions, based on the generally valid observation that the magnitude of zn* is small compared to z* at mid latitudes, which is generally satisfied in the case of the vertically stratified interior and when the primary external field has relatively large wavelengths [banks 1973, and references therein, viljanen et al. 1995]. this means that the products of zn* with wxz, wyz and wzz can be neglected. further, it is assumed that xn* and yn* are independent of zn*, and that they can be replaced by x* and y*, respectively [banks 1973]. in this case, equation (1) reduces to [banks 1973, viljanen et al. 1995, hitchman et al. 2000, srivastava et al. 2001]: (2) where zia* is the difference between z* measurements at an anomalous (coastal) point and at a reference geomagnetic observatory, a and b are coefficients of the simplified transfer function, and f is the residual of zia* that is not correlated to x* and y*, i.e. instrumental noise and the neglected correlation with zn*. all the quantities from equation (2) can be obtained from the time series at the anomalous (coastal) point and at the normal (reference) point, as explained in the following. also, the transfer functions given by the other two equations in (1), i.e. those for xia* and yia*, can not be determined from a single station analysis of this kind [schmucker 1970, banks 1973, lilley and bennett 1973]. a normal point is regarded to be the location where a variometer measures variations of the earth’s magnetic field, without influence of anomalous induced electric currents. generally a geomagnetic observatory satisfies this requirement. magnetic fields produced by induced currents below normal points will be independent of the direction of the time varying external field, so that no persistent correlations exists between zia* and the horizontal field (i.e., a = b = 0 in equation (2)). it is assumed that under the geomagnetic observatories is a conductivity structure characterised by a lateral homogeneity and vertical stratification. on the other hand, a variometer station located close to a conductivity anomaly will measure vector sum of the normal field bn and an anomaly field bia induced by the horizontal components of bn according to equation (2) [banks 1973, hitchman et al. 2000, srivastava et al. 2001]. if lateral variations of bn over the distance separating an anomalous point from the corresponding reference observatory are negligible in comparison to the induced anomaly field itself, zia* in equation (2) can be identified with the difference between the vertical components of the total field measured with at the variometer station and the observatory, respectively. the complex geomagnetic transfer function can then be calculated by solving equation (2) with respect to the coefficients a and b. stable solutions are given by [schmucker 1970, banks 1973, gough et al. 1973]: x yi z wx w wz wx w wz wx w wz x y z ia yia ia n n n wxxxx yxyx wzx wxy yy wzy wxz yzyz wzz = ) ) ) ) ) ) pq r q pqqqqqqqqqqqqqqqqqqqqqqqq r pq r qq pqqqqqqqqqqqqqqqqqqqqq r qqqq r pq r q pqqqqqqqqqqqqqqqqqqqqqqqq r tu v u tuuuuuuuuuuuuuuuuuuuuuuuu v tu v uu tuuuuuuuuuuuuuuuuuuuuu v uuuu v tu v u tuuuuuuuuuuuuuuuuuuuuuuuu v z ax byia = + ) ) y f+) vujić and brkić 2 · 3 where sxx, sxy , etc. denote the cross-spectra calculated from the horizontal components x and y of the total field measured at the anomalous point, e.g. sxx = x x−*/t0, sxy = x y − */t0, where t0 is the duration of the time series, and x−*, y−* denote the complex conjugates of x* and y*, respectively. the complex transfer coefficients a and b obtained in this manner define a so-called induction arrow [banks 1973], whose real part describes the in-phase response to variations of the horizontal field, with amplitude and azimuth given by equations (4a) and (4b), respectively: the quadrature response is defined in an analogous manner using the imaginary components of a and b. the induction arrow rotated by 180° points towards the cause of bia [banks 1973, viljanen et al. 1995]. magnetic storms or substorms are commonly used for finding the transfer function at a given location. to these phenomena correspond time variations of the external magnetic field that are produced by independent currents systems with acceptable levels of strength in a wide range of frequencies. equation (2) was obtained under the above assumptions, and implies homogeneity of time variation of the horizontal external field, over areas where normal and possibly anomalous magnetic fields are monitored by variometers [banks 1973, viljanen et al. 1995, srivastava et al. 2001]. in addition to the abovementioned intervals of increased geomagnetic activity, it is possible to use so-called plane wave events [viljanen et al. 1995]. such events are associated with laterally homogeneous electromagnetic plane waves traveling in the underground, in which case a vertical bia is generated where the wave crosses regions with lateral electrical conductivity heterogeneities. necessary condition for using planar wave events is the existence of a good linear correlation between the horizontal field components at the reference point and at the point where the transfer function is calculated. furthermore, the parameter r given by equation (5), expressing the relative difference between field variations at the variometer site and the reference station [viljanen et al. 1995], should be relatively small (possibly below 25%): where r is expressed in percent, e is the time variation a s s s s s s s s s s s s s s b xxxx yyyy yxyx zx yyyy zy yxyx xx yy yxyx z zy xxxx x xy 2 2= = . re re arctan re re ampmp a b az a b r r 2 2= + = q q q qv v v v! !$ $ a d m r e 100 v =q v max e e e rerefef rerefefv -q r v w geomagnetic coast effect at two croatian repeat stations (3a) (4a) (5) (3b) (4b) gck thy time interval (utc) r(x)/% r(y)/% r(x)/% r(y)/% krbp 3.9 13.2 9.3 9.6 15:31-17:39 4.0 12.6 8.2 10.0 15:39-17:47 4.7 11.4 6.9 10.4 15:55-18:03 6.2 11.7 7.8 8.4 16:10-18:18 5.1 8.4 9.9 6.2 16:31-18:39 3.2 5.6 9.1 5.7 16:51-18:59 3.1 23.5 11.8 11.2 14:07-16:15 sinp 14.7 17.9 20.5 22.6 08:09-10:17 9.9 16.9 19.2 25.3 08:34-10:42 10.8 14.2 18.5 23.0 08:37-10:45 11.0 11.9 18.2 19.1 08:39-10:47 7.3 16.0 10.8 11.4 17:27-19:35 6.6 9.4 9.4 8.9 17:42-19:50 6.2 9.7 9.0 8.3 17:46-19:54 table 1. parameters r(x) and r(y) between the observatories gck and thy and repeat stations krbp and sinp, and selected time intervals of plane wave events on july 21 and 22, 2010, for krbp, and july 26, 2010, for sinp. · (with respect to the quiet-night value) of geomagnetic components x or y at the point for which the transfer function is calculated, the subscript ref refers to the reference point, and v is the standard deviation. the possible existence of inductive effects in croatian repeat stations was postulated already after the 2010 survey. at that time, the survey at krbavsko polje (krbp) and sinjsko polje (sinp) was performed according to the iaga standards [newitt et al. 1996] and magnete recommendations (http://www-app3.gfzpotsdam.de) using the hungarian nonmagnetic theodolite zeiss theo 020a with fluxgate sensor and dmi d&i electronic unit, and gemsys gms-19g overhauser effect proton precession magnetometer, as well as overhauser effect didd variometer. in that occasion, it was noticed that the spatial gradient of total field intensity was correlated with the spatial gradient of the eastern component of the field (y), whereby the phenomenon was more strongly expressed near the coast, leading to the hypothesis that it was caused by the conductivity contrast between adriatic sea and mainland [csontos et al. 2012]. because only one substorm occurred during the whole 2010 survey [gjerloev 2012], plane wave events were instead used to calculate the transfer functions at krbp and sinp. suitable events (table 1) have been selected from geomagnetic time series of a repeat station and the corresponding reference observatory, using a sliding window in the time domain to search for 128minutes-long intervals that satisfy the abovementioned plane wave conditions. two observatories, grocka (gck) in serbia and tihany (thy) in hungary, have been used as reference points for the calculation of the induction arrows at sinp and krbp (figure 1). the main objective was to confirm whether there is a geomagnetic effect of the adriatic coast at the considered repeat stations, whose distances from the adriatic sea coast are of about 53 km (krbp) and 22 km (sinp). 3. results and discussion using plane wave events from table 1 and the equations (3)-(4), the simplified transfer functions between the horizontal and vertical fourier components were calculated for periods comprised between 2 and 128 minutes, considering gck and thy as references (table 2). the cross-spectra of geomagnetic components have been calculated using the fast fourier transform (fft) of the time series corresponding to the plane wave events listed in table 1. in the time domain the adjustable tukey window (tapered cosine window) was applied. multiplication of the signal with a suitable window in the time domain previous to the fft is necessary to avoid high-frequency artifacts. windows are smoothing functions that peak in the middle frequencies and decrease to zero at the edges, thus reducing the effects of the discontinuities as a result of finite duration [bendat and piersol 2000]. further, each cross-spectra used in equation (3) must be a mean value from a number of variation events [schmucker 1970, gough et al. 1973], and in this case they were averaged over the events given in table 1 for krbp and sinp, respectively. seven plane wave events each of 128 min duration were selected (table 1). the periods used for induction arrow calculations have been chosen to be evenly distributed, on a logarithmic scale, between the sampling interval of 1 minute and the total event duration of 128 vujić and brkić 4 station ampr azr/° v15-60 ampi azi/° ampr azr/° v15-60 ampi azi/° gck thy krbp 0.30 ±0.07 266 ±17 ±0.08 0.12 ±0.08 176 ±113 0.35 ±0.07 259 ±10 ±0.07 0.11 ±0.06 197 ±92 sinp 0.37 ±0.09 233 ±8 ±0.05 0.08 ±0.06 123 ±98 0.34 ±0.14 234 ±8 ±0.13 0.12 ±0.07 142 ±125 table 2. results of the transfer functions analysis for krbp and sinp, using observatories gck and thy as a reference point. all entries are written as mean ± standard deviation. amp and az are the lengths and azimuths of induction arrows, indices (r) and (i) apply to the real and imaginary arrows, and v15-60 is the standard deviation of the length of the real arrows in the period interval of 15-60 min. the values refer to the average and the standard deviation of data obtained from periods of (10.7 min, 12.8 min, 18.3 min, 25.6 min, 32.0 min and 64.0 min) for krbp, and of (12.8 min, 18.3 min, 21.3 min, 42.7 min and 64.0 min) for sinp. figure 1. the positions of two croatian repeat stations krbp and sinp, together with two reference observatories (gck and thy). in detail, latitude, longitude and elevation of krbp are 44.7° n, 15.6° e, 648 m, for sinp geographical coordinates are 43.6° n, 16.7° e, 296 m. 5 minutes. figure 2 shows the power spectrum of x, y and z components of the two selected plane wave events, as an example. the powers of components generally increase with period, and have the highest values for x component. r values calculated with equation (5) were <12% for x and <24% for y in krbp, and <21% and <26% for x and y in sinp. furthermore, correlation coefficients between the horizontal field components measured with the variometer and at the reference observatory were >0.97 for krbp and >0.93 for sinp. these results confirm the validity of the plane wave assumption and thus the suitability of the chosen time series intervals for the calculation of induction arrows. the real (i.e. in-phase) induction arrows are characterized by relatively small amplitudes (<0.4, see table 2) and point consistently to the west at krbp and to the south west at sinp over the whole frequency range. the imaginary (quadrature) induction arrows, on the other hand, are smaller in amplitude (<40% of the real arrows) and are characterized by much more scattered directions, depending on the chosen period. these directions are, on average, almost opposite to those of the real induction arrows. the angular scatter of the real induction arrows is in the range of 8-17°, while the scatter of the imaginary arrows is much higher, i.e. in the range of 92-125°. further, the lower dispersion of real induction arrows azimuths at sinp is observed, compared to krbp when using different reference points. the scatter of the real induction arrows is random for both stations, i.e. it does not show a systematic correlation with the corresponding period. it appears that the real induction arrows scatter at krbp is much smaller if referred to thy instead of gck, and one could presume that thy is a more representative reference station for krbp. a similar reasoning does not hold for sinp, where the scatter is the same for both reference stations. the general observation is that bn is reconstructed from data collected at a different place while being attributed to the same location as far as the equations are concerned. deviations of the results for amplitudes and azimuths of induction arrows at the same periods, when gck and thy are used separately at any point, could be caused by the fact that the selected events do not fully correspond to laterally homogeneous plane wave model. it should not be forgotten that the field generated in the ionosphere and magnetosphere can have some degree of heterogeneity over the distances between variometers and reference stations, which are in this case much larger than the thickness of the ionosphere. moreover, a presence of anomalous field in horizontal components of the inducing field is possible [armadillo et al. 2001], so the reference observatories could also not be the ideal normal points. finally, the mentioned deviations can be attributed also to the effect of spatial electric conductivity gradients on the mainland. at krbp repeat station inductive coast effects were found at six periods, and at sinp at five periods (table 2). figures 3a and 3b show the corresponding real and imaginary induction arrows for both repeat stations, respectively, with respect to the reference observatories. a relatively small value of the standard deviation of the amplitudes of the real induction arrows, for periods of 15-60 min (table 2), can be one of the indicators of the geomagnetic coast effect [parkinson and jones 1979]. additional indicators of this effect can be: amplitude of the imaginary induction arrows should, at the same pegeomagnetic coast effect at two croatian repeat stations figure 2. left: power spectrum density (psd) as a function of period (t), of x and y components from thy, and of the difference of z component at krbp and thy, for time interval on july 21, 2010, 16:51-18:59 utc. right: the same as for krbp, but for another selected time interval at sinp ( july 26, 2010, 17:46 -19:54 utc). riods, be 0.25-0.5 times lower than that of the real ones; in addition, the imaginary arrows have a greater range of azimuths, i.e. they are more random and are more susceptible to the period. figure 4 illustrates an example of real induction arrows at the period t = 18.3 min for krbp (amplitude of 0.24) and sinp (amplitude of 0.50), with respect to a reference point thy. at the repeat stations the real induction arrows are directed toward the coast of the adriatic sea. 4. conclusion coastal inductive effects at the krbavsko polje and sinjsko polje repeat stations have been investigated with the geomagnetic transfer function method during plane wave events, using grocka and tihany observatories as reference. the real induction arrows at the repeat stations point towards the adriatic sea, where the anomalous induced currents are more intense. the geomagnetic coastal effects were found in both repeat stations and for periods from 10.7 min to 64 min. this work confirmed geomagnetic coastal effects near the eastern coast of adriatic for the first time; the effects found near the western adriatic coast were described in armadillo et al. [2001]. a practical consequence is that data reduction at the stations near the eastern coast of adriatic has to be performed with proper caution. it is advisable to use the recordings from on-site triaxial variometer for data reduction, i.e. to choose the appropriate time intervals as free as possible from the anomalous induced effects. it is also recommended to apply the other methods in order to investigate inductive effects at sites of the basic geomagnetic network of the republic of croatia. the vujić and brkić 6 figure 3. (a) real (black) and imaginary (red) induction arrows for krbp, with gck and thy as reference points. these arrows have been obtained for periods of 10.7 min, 12.8 min, 18.3 min, 25.6 min, 32.0 min and 64.0 min. (b) real (black) and imaginary (red) induction arrows for sinp, with gck and thy as reference points. these arrows have been obtained for periods of 12.8 min, 18.3 min, 21.3 min, 42.7 min and 64.0 min. figure 4. real induction arrows at period t = 18.3 min for krbp (amplitude of 0.24) and sinp (amplitude of 0.50), with respect to the reference point thy. 7 method for the transfer functions estimation in frequency-domain used in this work is based on a standard least squares procedure [schmucker 1970]. the methods based on a robust estimation of the transfer functions in frequency-domain can also be used, see e.g. hitchman et al. [2000] and references therein. additionally, the estimations of induction arrows can be performed in the time-domain; an example is given in viljanen et al. [1995] and references therein. finally, the method used in this work cannot be applied to points beyond a certain critical distance (up to several hundreds of km at mid latitudes) from the closest reference observatory. the two reference observatories used here were selected because they were the closest to the repeat stations in different directions (closer than 410 km), and far away from the sea coast. in the future, it can be expected that the data from croatian observatory lonjsko polje will be used in such studies, since this observatory is closer to the near-coastal repeat stations than the other observatories, and still far away from the adriatic sea. acknowledgements. data used in this paper are product of projects funded by the ministry of science, education and sports, state geodetic administration, as well as ministry of defence of the republic of croatia. the results presented in this paper rely on data collected at magnetic observatories. we thank the national institutes that support them and intermagnet for promoting high standards of magnetic observatory practice. the authors are very thankful to dr. r. egli and anonymous reviewers for their constructive comments and suggestions that significantly improved the manuscript. data and sharing resources the one-minute gck and thy data were downloaded from intermagnet web site (www.intermagnet. org). references armadillo, e., e. bozzo, v. cerv, a. de santis, d. di mauro, m. gambetta, a. meloni, j. pek and f. speranza (2001). geomagnetic depth sounding in the northern apennines (italy), earth planets space, 53, 385-396. banks, r. (1973). data processing and interpretation in geomagnetic deep sounding, phys. earth planet. int., 7, 339-348. bendat, j.s., and a.g. piersol (2000). random data: analysis and measurement procedures, john wiley & sons inc., usa. brkić, m., e. vujić, d. šugar, e. jungwirth, d. markovinović, m. rezo, m. pavasović, o. bjelotomić, m. šljivarić, m. varga and v. poslončec-petrić (2013). basic geomagnetic network of republic of croatia 20042012, with the geomagnetic field maps at 2009.5 epoch, ed.: m. brkić, croatian geodetic administration, zagreb, isbn: 978-953-293-521-9, (in croatian). csontos, a., d. šugar, m. brkić, p. kovács and l. hegymegi (2012). how to control a temporary didd based observatory in the field?, annals of geophysics, 55 (6), 1085-1094. gough, d.i., j.h. de beer and j.s.v. van zijl (1973). a magnetometer array study in southern africa, geophys. j. r. astr. soc., 34, 421-433. gjerloev, j.w. (2012). the supermag data processing technique, j. geophys. res., 117, a09213, doi:10.10 29/2012ja017683. hitchman, a.p., f.e.m. lilley and p.r. milligan (2000). induction arrows from offshore floating magnetometers using land reference data, geophys. j. int., 140, 442-452. lilley, f.e.m., and d.j. bennett (1973). linear relationships in geomagnetic variation studies, phys. earth planet. in., 7, 9-14. newitt, l.r., c.e. barton and j. bitterly (1996). guide for magnetic repeat station surveys, iaga, boulder, sad. parkinson, w.d. (1959). directions of rapid geomagnetic fluctuations, geophys. j. r. astr. soc., 2 (1), 1-14. parkinson, w.d., and f.w. jones (1979). the geomagnetic coast effect, rev. geophys. space phys., 17 (8), 1999-2015. schmucker, u. (1970). anomalies of geomagnetic variations in the southwestern united states, bull. scripps inst. oceanogr., univ. calif. san diego, 13, 1-165. srivastava, b.j., abbas habiba, t. ramagopal, d.r.k. rao and b.m. pathan (2001). geomagnetic coast and other effects deduced from the new observatory at visakhapatnam, india, geophys. j. int., 146, 827-832. viljanen, a., k. kauristie and k. pajunpää (1995). on induction effects at eiscat and image magnetometer stations, geophys. j. int., 121, 893-906. wiese, h. (1962). geomagnetische tiefentellurik, teil ii: die streichrichtung der untergrundstrukturen des elektrischen widerstandes, erschlossen aus geomagnetischen variationen, geofis. pura e appl., 52, 83-103. corresponding author: eugen vujić, external associate of faculty of geodesy, university of zagreb, croatia; email: eugvujic@gmail.com. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geomagnetic coast effect at two croatian repeat stations << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 statistical analysis of automatically detected ion density variations recorded by demeter and their relation to seismic activity michel parrot laboratoire de physique et chimie de l’environnement et de l’espace, université d’orléans, cnrs, orléans, france annals of geophysics, 55, 1, 2012; doi: 10.4401/5270 abstract many examples of ionospheric perturbations observed during large seismic events were recorded by the low-altitude satellite demeter. however, there are also ionospheric variations without seismic activity. the present study is devoted to a statistical analysis of the night-time ion density variations. software was implemented to detect variations in the data before earthquakes world-wide. earthquakes with magnitudes >4.8 were selected and classified according to their magnitudes, depths and locations (land, close to the coast, or below the sea). for each earthquake, an automatic search for ion density variations was conducted from 15 days before the earthquake, when the track of the satellite orbit was at less than 1,500 km from the earthquake epicenter. the result of this first step provided the variations relative to the background in the vicinity of the epicenter for each 15 days before each earthquake. in the second step, comparisons were carried out between the largest variations over the 15 days and the earthquake magnitudes. the statistical analysis is based on calculation of the median values as a function of the various seismic parameters (magnitude, depth, location). a comparison was also carried out with two other databases, where on the one hand, the locations of the epicenters were randomly modified, and on the other hand, the longitudes of the epicenters were shifted. the results show that the intensities of the ionospheric perturbations are larger prior to the earthquakes than prior to random events, and that the perturbations increase with the earthquake magnitudes. 1. introduction a substantial number of effects in the earth’s atmosphere and the ionosphere that are possibly associated with earthquakes have been revealed over the past 20 years. ionospheric phenomena and their correlation with seismic events have been examined in numerous experimental studies. the seismo-electromagnetic effects include: electromagnetic emissions in a large frequency range, perturbations of ionospheric layers, anomalies in the records of very low frequency (vlf) transmitter signals, night airglow observations, and heating. the hypotheses of the generation mechanisms of precursors to earthquakes are mainly related to: wave production by rock compression, diffusion of water in the epicentral area, and redistribution of electric charges at the surface of the earth and then in the earth’s atmospheric system. these hypotheses and modeling have already been published, and they can be found for example in monographs published by hayakawa and fujinawa [1994], hayakawa [1999] and hayakawa and molchanov [2002], in books by gokhberg et al. [1995] and pulinets and boyarchuk [2004], in special issues of journals [parrot and johnston 1989, 1993, hayakawa 2002, hayakawa et al. 2004], and in reviews by molchanov [1993], hayakawa [1997], freund et al. [2006], tronin [2006], ouzounov et al. [2007], pulinets [2009] and harrison et al. [2010], and in references therein. the demeter (detection of electro-magnetic emissions transmitted from earthquake regions) satellite was the first satellite with a complete payload that was specifically dedicated to this scientific objective [parrot 2006]. with its data, many ionospheric perturbations have been observed in relation to earthquakes, and examples can be found in parrot et al. [2006], ouyang et al. [2008], zhu et al. [2008], zeng et al. [2009], zhang et al. [2009a,b,c, 2010a,b,c), akhoondzadeh et al. [2010], bankov et al. [2010], he et al. [2010], yufei et al. [2010], zlotnicki et al. [2010] and píša et al. [2011]. however, it needs to be kept in mind that all of the measured parameters also show variations in the absence of seismic activity, as the mid-latitude and equatorial ionosphere is affected by a number of other sources of perturbations, and primarily by solar activity. therefore, only a statistical study covering many events will be able to show the general behavior of these ionospheric perturbations, and will help us to define a signature of ionospheric perturbations prior to earthquakes. this is possible with the demeter data, because the lifetime of the mission is more than 6 years. the demeter payload is briefly described in section 2. in section 3 the events that occurred before the main special issue: earthquake precursors 149 article history received july 4, 2011; accepted september 26, 2011. subject classification: ionosphere, satellite, earthquake interactions and probability. shocks of earthquakes are shown. the statistical analysis and the results are presented in section 4. discussion and conclusions are provided in section 5. 2. the demeter satellite demeter is a low-altitude satellite (710 km) that was launched in june 2004 into a polar and circular orbit. it can measure the electromagnetic waves and plasma parameters all around the globe, except in the auroral zones [parrot 2006]. the altitude of the satellite was decreased to 660 km in december 2005. the scientific mission of demeter came to an end in december 2010. due to technical reasons, the data were only recorded at invariant latitudes of less than 65˚. the orbit of demeter is nearly sun-synchronous and the up-going half-orbits correspond to night-time (22.30 lt), whereas the down-going half-orbits correspond to day-time (10.30 lt). as nearly sun-synchronous, this means that everyday the satellite did not return to exactly above the same point, but to above the same area (it could be at more than 1,000 km from the point it flew over in the previous days). the payload of demeter included an instrument known as a thermal plasma analyzer (iap; instrument analyseur de plasma). the aim of the iap was to measure the main parameters of the thermal ion population; i.e. the densities of the major ionospheric ions, h+, he+ and o+, their temperature, and the ion flow velocity from the earth’s frame of reference. the iap provided ion densities with a 4s time resolution. details of the iap experiment can be found in berthelier et al. [2006]. 3. examples of ionospheric perturbations figures 1 and 2 show a remarkable event that was recorded by demeter before the m 8 samoa earthquake that occurred on september 29, 2009, at 17.48.11 ut (location 15.51˚s, 187.97˚e). figure 1 corresponds to data recorded on september 22, 2009 (7 days before the earthquake) whereas data in figure 2 have been recorded on september 28, 2009 (1 day before the earthquake). both of these figures have the same presentation. the top panels in figures 1 and 2 show the o+ ion density, and the bottom panels show the earthquake occurrences along the satellite orbit. these bottom panels also show the satellite closest approach to past and future earthquake epicenters that are within 2,000 km of the demeter orbit. the y-axis represents the distances d between the epicenters and the satellite, from 750 km up to 2,000 km. the symbols are filled green squares for postseismic events and filled red triangles for pre-seismic events. the color scale on the right of these bottom panels represents the time interval between the earthquakes and the demeter orbit, with color gradation from >30 days down to a 0-6 h interval. the symbol sizes correspond to earthquakes of magnitudes 5-6, 6-7, and >7. the red triangles indicate the closest approach to the samoa earthquake and the many aftershocks, with their elongated positions indicating that the demeter orbit was almost parallel to the rupture fault. the density presents a clear fluctuation when the satellite arrives above the future epicenter. other ionospheric perturbations for this earthquake were published by akhoondzadeh et al. [2010]. by examining the three 150 parrot figure 1. data recorded on september 22, 2010, between 09.29.00 ut and 09.35.00 ut, 7 days before a powerful earthquake in the samoa islands. top panel, ion density, where due to the scale, only the density of o+ ions is seen, as the majority. bottom panel, distance and magnitudes of forthcoming earthquakes, as a function of time. red triangles, future main shocks and aftershocks; green symbols, past earthquakes (see text for details). below the panels, details of the observations (during night-time along the rupture zone of the earthquakes). 151 preceding months, they showed that these perturbations were very uncommon in this area at that time. many other ionospheric perturbations were recorded by demeter, and even for earthquakes of smaller magnitudes. these were generally observed a few hours before the moderate earthquakes. an example is given in figure 3. all of these observations were performed during the night-time. density variations with seismic activity figure 2.data recorded on september 28, 2010, between 09.14.00 ut and 09.20.00 ut, 1 day before a powerful earthquake in the samoa islands. see figure 1 for details. figure 3. data recorded on november 27, 2007, between 11:05:00 ut and 11:11:00 ut, a few tens of minutes before a m 6.6 earthquake, which occurred at 11:50:02 ut (latitude, –11.02˚s; longitude, 162.17˚e; depth, 41.6 km). see figure 1 for details. 4. the statistical analysis it must be said that the variations in the ionospheric parameters are not only due to earthquakes. there are numerous possibilities of ionospheric perturbations that can come from other sources (solar activity, acoustic gravity waves, traveling ionospheric disturbances, plasma dynamics, and large meteorological phenomena). in the examples that have been given in the past, great care was taken to show that these ionospheric variations that are associated with seismic activity are uncommon at these locations and at the times when they were observed. on the other hand, with satellite data, we do not find ionospheric perturbations for all earthquakes. this might be due to the crust composition and soil configuration. however, we also do not expect to have continuous ionospheric perturbations, and with a single satellite we are ‘above’ a given future epicenter for only 3 min per day (nighttime half-orbit). here, the term ‘above’ refers to a distance of less than 1,500 km. for these two above-mentioned reasons, we preferred to search for possible influences of seismic activity on the ionosphere using a statistical analysis. automatic software to detect density fluctuations was developed. the inputs are the earthquake list and the demeter data. earthquakes with magnitudes >4.8 were selected, and for each of these, we associated a parameter related to the position of the epicenter: below the sea or inland, and close to the coast. during the period considered (august 2004 to october 2009), there were 17,366 such earthquakes. the software searched for the data of the orbits that were close to the epicenters (less than 1,500 km) between 0 and 15 days before each of the earthquakes. it then retains one night-time half-orbit per day, whereby if there were two orbits, it retains the closest one. it is possible that for a given earthquake there was no half-orbit for that given day because the data were not recorded or because the half-orbit was too far away. to detect variations, the software considers the demeter ion density data over the 3 min around the closest approach to the future epicenter of an earthquake. the data are smoothed and variations (crests or troughs) are searched using change in the derivative sign. at the end, the software produces a final database with the results. for each earthquake, the first line of the database gives the information about its date, time, latitude, longitude, magnitude, depth and position. the following 15 lines give the information on the data recorded along the half-orbits. the second line relates to the 24 h before the earthquake, then the third line to the –48 h to –24 h period, and so on. the information provided includes the half-orbit number, the date and time where the half-orbit was closest to the epicenter, the date, time and position when the automatic software detected any variation in the ion densities, and parameter a, which gives the percentage of these variations relative to the background. the software could also detect no variations. to evaluate the results of the automatic detection software working with the earthquake list, we considered two other lists with random data. first, we took the list of the earthquakes and randomly changed their latitudes and longitudes (keeping the same times). the resulting database after the software application is called rand1. secondly, we took the same list of earthquakes and only shifted their longitudes 25˚ to the west (keeping the same latitudes and times). after the software application, this third database is called rand2. this shift of the longitude is because, on the one hand, most of the earthquakes were concentrated around the equator, and on the other hand, it is known that during the night-time, natural occurrences of ionospheric perturbations are also more concentrated around the equator. then with rand2 we have a database that fits more with the reality concerning the positions of the earthquakes and the possible ionospheric perturbations due to usual geophysical activities. to reduce the effects of solar activity, we eliminated the data when the kp index was >3+. we also do not take into account the aftershock data when the time of the aftershock is too close to the time of the main shock, in order not to mix pre-seismic and post-seismic effects. it is known that at the exact time of an earthquake, you can have propagation of an acoustic gravity wave that can perturb the ionosphere [see for example, garcia et al. 2005]. then, if n is the number of days between the main shock and the aftershock, and if n <15, then we have only considered the aftershock data for n–1 days before this aftershock. this automatic software to detect variations is certainly not perfect, because the shapes of any ionospheric perturbations might be very different. we have examples with very sharp peaks or very smooth peaks in the ion density. however, we ran the software in the same way on the three lists to search for ionospheric perturbations and to produce three databases (earthquakes, rand1 and rand2). the largest values of a were selected in each data bases (earthquake and random) from the 15 days before each earthquake (each event for the random databases). a distinction was made regarding the positions of the earthquakes (all epicenters, with epicenters below the sea, with epicenters inland or close to the coast). to check whether the ionospheric perturbations depend on the earthquake magnitudes, the results are displayed as functions of several magnitude intervals ([4.8, 5.0], ]5.0, 5.5], ]5.5, 6.0], and ]6.0, 9.0]). this selection of intervals was designed to have enough events in each case for the statistical analysis. even if it has no meaning, this was also done for the random databases, to have several draws in these databases. the characteristics of the statistical distributions of these largest values of a were evaluated using the kurtosis and the skewness parameters. it parrot 152 153 appears that the kurtosis is always much less than 3 and the skewness is not close to 0 (see tables 1 and 2). this means that the distributions of these data do not follow a normal law and that it is better to use a median value than a mean value of parameter a. table 1 gives the results obtained with the rand1 and rand2 databases. as expected, larger perturbations (median values) are observed with the rand2 database, and in the following, all of the comparisons are carried out with this rand2 database. to obtain a reference median value, 20 draws were made in the rand2 database (varying the false magnitudes and depths). all of these median values were averaged and a standard deviation was calculated. table 1 gives the events that occurred everywhere, although similar calculations were carried out for events below the sea and for inland events. for the rand2 database, the final averaged median values are 7.82 ± 0.61 (all events), 8.21 ± 1.23 (events below the sea), and 7.46 ± 0.34 (inland events). these values can be compared with the median values calculated from the real earthquake database, which are given in table 2. this table 2 gives for each magnitude interval and for each epicenter position, the median values of each distribution. it can be seen that the maximum perturbations increase with the magnitude, as was expected, and that they are more intense for earthquakes that occurred below the sea. these results are significant if they are larger than the average median value for random events plus the variance (8.43 for all events, 9.44 for sea events, 7.80 for inland events). this is true for powerful earthquakes (table 2, last column) and all inland earthquakes. the influence of the depth was checked for powerful earthquakes (]6.0, 9.0]), and these results are shown in table 3. table 3 can be compared with the last column of table 2 (identical values in the tables are just by chance). as expected, the perturbations are not so important for deep earthquakes (>40 km). 5. discussion and conclusions a few examples of ionospheric perturbations prior to seismic activity have been presented in this study. these perturbations occur in close vicinity of the earthquake epicenters, and a few hours to a few days before the shocks. however, many other phenomena can perturb the ionosphere, and as people might doubt the relationships between the ionosphere and the earth’s crust despite the various mechanisms briefly mentioned in the introduction, a statistical analysis was performed while considering a huge number of events. these statistics show that ionospheric perturbations are more important close to the epicenters of future inland density variations with seismic activity statistic magnitude range ]4.8, 5.0] ]5.0, 5.5] ]5.5, 6.0] ]6.0, 9.0] rand1 number of events 7576 4347 1259 570 median 7.49 7.52 7.89 7.12 kurtosis 1.548 1.565 1.669 2.456 skewness 1.145 1.160 1.124 1.373 rand2 number of events 6218 3460 987 415 median 7.68 7.77 8.03 7.19 kurtosis 0.719 0.843 0.805 0.346 skewness 0.931 0.966 0.897 0.878 table 1. statistics relating to the maximum intensities a of perturbations (see text for explanations) according to the rand1 and rand2 random databases with events occurring everywhere. statistic magnitude ]4.8, 5.0] ]5.0, 5.5] ]5.5, 6.0] ]6.0, 9.0] all earthquakes number of earthquakes 6332 3506 1032 420 median 8.16 8.23 8.47 9.27 kurtosis 0.437 0.598 0.003 -0.032 skewness 0.812 0.809 0.686 0.696 sea earthquakes number of earthquakes 1734 1084 328 98 median 8.66 8.69 8.33 10.48 kurtosis 0.628 0.457 0.021 -0.582 skewness 0.830 0.777 0.650 0.474 inland earthquakes number of earthquakes 4598 2422 704 322 median 7.94 7.93 8.53 8.96 kurtosis 0.373 0.656 -0.086 0.201 skewness 0.812 0.809 0.687 0.774 table 2. statistics relating to the maximum intensities a of perturbations for the earthquake database (see text for explanations). the results are given as functions of the earthquake epicentre positions and magnitudes. statistic depth ≤40 km >40 km all earthquakes number of earthquakes 313 107 median 9.35 8.64 sea earthquakes number of earthquakes 75 23 median 10.84 8.64 inland earthquakes number of earthquakes 238 84 median 8.96 8.84 table 3. statistics relating to the maximum intensities a of the perturbations for powerful earthquakes (]6.0, 9.0]) as a function of their locations and depths. earthquakes than prior to events at random positions. it also indicates that the intensities of these perturbations is more important when the magnitudes of the earthquakes are greater. one surprising result is that there are more intense perturbations for earthquakes with an epicenter below the sea. however, the comparison with the random database indicates that this variation is inside the normal fluctuation range (mean ± standard deviation). this can be linked to a possible mechanism of generation of these natural ionospheric perturbations. for instance, it is known that the electrical conductivity is larger above the sea. finally, deep earthquakes (>40 km) do not produce ionospheric perturbations. the presented statistical analysis agrees with previous studies that have reported local increases in particle densities in time and positions close to forthcoming earthquakes [see for example, liu et al. 2009, kon et al. 2011]. acknowledgements. this study was supported by the centre national d’etudes spatiales. it is based on observations with the plasma analyzer on demeter. the author thanks j.j. berthelier, the pi of this instrument, for the 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doi: 10.1016/j.asr.2010.11.001. zhu, r., d.-m. yang, f. jing, j.-y. yang and x.-y. ouyang (2008). ionospheric perturbations before pu’er earthquake observed on demeter, acta seismol. sinica, 21, 77-81; doi: 10.1007/s11589-008-0077-8. zlotnicki, j., f. li and m. parrot (2010). signals recorded by demeter satellite over active volcanoes during the period 2004 august – 2007 december, geophys. j. int., 183, 1332-1347. *corresponding author: michel parrot université d’orléans, cnrs, laboratoire de physique et chimie de l’environnement et de l’espace, orléans, france; e-mail: mparrot@cnrs-orleans.fr © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. density variations with seismic activity annals of geophysics, 60, 2, 2017, g0219, doi: 10.4401/ag-7062 the italian magnetic repeat station network: results from the 2012.5 ‘reduced network’ completion guido dominici1,*, antonio meloni1 1 istituto nazionale di geofisica e vulcanologia, rome, italy article history received october 26, 2016; accepted december 7, 2016. subject classification: earth’s magnetic field, geomagnetic map, magnetic surveys, magnetic repeat stations, secular variation, global and regional models abstract in italy ingv (istituto nazionale di geofisica e vulcanologia) has systematically undertaken the task of making periodic measurements of the earth’s magnetic field on a network of more than 110 points (repeat stations), with an average spacing around 55-60 km. measurements are repeated regularly every 5 years and the last published data reports and magnetic maps, refer to 2010.0, are available in dominici et al, 2012. at that time the report referred to a survey of 131 repeat stations (including 2 observatories, 11 stations in albania, 3 stations in corsica and 1 in malta) carried out between 2009 and 2010, also with the purpose of updating the national magnetic cartography. at the epoch 2012.5 a selection of stations, from the first order italian magnetic network, selected on the basis of the lowest amplitude values of anomaly with respect to a ‘normal’ field, was repeated. the number of the selected stations amounts to 25, distributed according to an almost uniform national geographical coverage, with an average spacing around 100 km. secular variation and analytical expressions, such as second order polynomials, in latitude and longitude for all field elements, were determined and coefficients were obtained for the spatial field variation and secular variation. we describe here the characteristics of this reduced network with the data elaboration procedure, normal fields and maps, and compare the results with other magnetic field models 1. introduction magnetic surveys are normally undertaken for magnetic maps compilation, and associated data elaborations: in particular for main field and magnetic anomalies representation, for observation of geomagnetic field elements secular variation and for the computation of regional magnetic models. all surveys are based on two typical magnetic networks, usually called first order (repeat stations) and second order. repeat stations are devoted to main field representation; they are constituted of accurately located points at the earth’s surface, where the three components of the geomagnetic field are regularly measured to the highest possible accuracy [see for example: newitt et al. 1996, lanza and meloni 2006]. regional magnetic models, also called normal fields, [see haines et al. 1990] are computed on the basis of repeat stations to represent the main geomagnetic field elements pattern over a certain area. second order networks are denser measurement point networks that are generally finalized to improve crustal magnetic anomalies representation. the magnetic characteristics of the italian territory have been the subject of several investigations. a brief history of italian magnetic surveys dates back to the seventeenth century and includes the first survey made in 1640, when declination (d) was measured over 21 stations by fathers borri and martini. in more recent times the magnetic survey of 1881-1892 by chistoni and palazzo can be considered the first ‘modern’ 3-component national magnetic survey in the unified kingdom of italy. at that time d, i (inclination) and h (horizontal intensity) were measured on 284 stations by the ufficio centrale di meteorologia e geodinamica (ucmg). more detailed information on this and older magnetic measurement compilations, can be found in cafarella et al. [1992a], [1992b]. for what concerns recent magnetic networks in italy a fundamental starting point was the 1979.0 survey. in that occasion a repeat station network of about 120 data points was assessed [molina et al. 1985] and moreover a full second order magnetic data network, with a higher spatial density of measurements, was deployed. this last one consists of horizontal intensity (h), vertical intensity (z) and total field (f), with measurements taken in the period 1977-1981 on 2552 stations within the framework of the finalized project geodinamica (pfg-cnr), see molina et al. [1985]. the 1979.0 network (all above mentioned measurements were reduced to this central epoch) has geg0219 dominici et al. nerated a full representation of magnetic elements, a magnetic maps compilation and a set of coefficients for normal field regional models to represent the main magnetic field elements pattern over italy. for what concerns declination, this was measured over 1529 points and measurements were, undertaken in the 30s of nineteen hundred by the military geographical institute (igm), for military demands. this set of declination measurements were successively updated for a publication released in 1973.0. information on this network can be found in talamo [1975]. in later years national magnetic surveys were all dedicated to the repetition of the first order network of about 120 repeat stations including 2 observatories. from 1994 onwards surveys also included 11 stations in albania, from 2005 3 stations in corsica and from 2010 1 in malta, as shown in figure 1 that reports situation at 2010.0. the main goal of these studies was the determination of secular variation and consequently the main field generated in the earth’s core. these most recent surveys have been described in meloni et al. [1988, 1994], coticchia et al. [2001] and for the years after 2005.0 dominici et al. [2007, 2012], that can be considered the latest surveys at the national level. all magnetic stations, constituting the so-called second order network, have been used since then, only updated on the basis of the first order network (repeat stations) results in order to bring up to date the magnetic cartography. this was done for all the latest years at 2000.0, 2005.0, 2010.0. [de santis et al. 2003, dominici et al. 2012]. if we compare italian repeat station density (an average of 1 station over 3000 km2) with similar european magnetic networks, we find that other nations usually rely on a sensibly smaller density of repeat stations. for example france relies on about 1 station over 20,000 km2 [mandea 2004], spain on 1 station over 12,500 km2. several workshops in the european magnetic operators community were convened to discuss problems regarding magnetic measurements, in order to exchange experiences and coordinate the individual efforts, and also to discuss what measurements density would be considered optimal for a national undertaking. one successful initiative at the european level is known as magnete (magnetic network of europe) and was supported by a resolution of iaga (international association of geomagnetism and aeronomy). magnete workshops were organised every two years, in order to ensure continued cooperation between the participants from about 20 european countries. the latest workshops were held in may 2011 in rome, in prague in 2013 and in budapest in 2015. in this paper we present the results of a recent magnetic survey undertaken in 2011-2012 on a selection of the above mentioned repeat stations of the first order italian magnetic network. the number of these selected stations amounts to 25, compared to the about 120 stations typically measured in the last 40 years or so. in the next paragraphs we will describe this ‘reduced’ network, how points were selected, results of the measurements, and we will discuss advantages and disadvantages of this reduced density magnetic network. 2. magnetic repeat station distribution on a ‘reduced’ network geomagnetic repeat station surveys are mainly devoted to secular variation knowledge (from the short time scale 2-5 years onwards) and the understanding of its pattern over the interested area, usually starting with the national area. with this information available, local magnetic models and magnetic maps can be updated. the average distance of magnetic stations, the time interval for their reoccupation and all the procedures needed for a full measurement of magnetic field elements, are the basis of the newitt et al report [newitt et al. 1996] where all details for magnetic repeat stations surveys are described. this report and subsequent discussions and agreements, have brought to the suggestion that an average distance between stations of 125 km and a 2 year time interval for reoccupation, are considered satisfactory in almost all cafigure 1. the original location of 131 repeat stations in italy including 2 observatories. the latest full survey undertaken at 2010.0 also included 11 stations in albania, 3 stations in corsica and 1 in malta. circles indicate observatory and variometric station areas of influence. 2 italian reduced magnetic network ses; these parameters are also considered to be a good compromise between costs related to the survey operations and value of the results. the same would also be true if we consider the survey results accuracy. in fact a good survey should not be completed in a too long time window, since secular variation keeps going on during the whole effort. it must be considered that the time window should be kept as short as possible in order to make minimum correction for secular variation; furthermore scientific institutions generally can rely only on one or two teams in the field, so one year total measurement time is considered acceptable. in the ‘reduced’ network described here we started taking in consideration that first order magnetic surveys must rely on stations whose magnetic elements, once measured, should report the magnetic value representative of the main field for that determined region. this is because magnetic measurements provide not only a measurement of the earth’s core field (at large the main field) but also an insight into the magnetic structure of the earth’s crust, that is closely connected to its geological structure. for this reason the station selection should be done in order to include only stations representative of the main field. the need to cover almost uniformly all the national area was also a constrain. following above said criteria at the end the number of the selected stations amounts to 25, distributed according to a national geographical coverage, with an average spacing around 100 km (figure 2). for what concerns field instrumentations, also in 2012.5 survey a dim (declination inclination magnetometer) a portable theodolite fitted with a fluxgate magnetometer, was used for the angular measurements of the earth’s magnetic field (therefore declination d and inclination i, with 0.1 minute of arc accuracy) and a proton precession magnetometer for the measurement of the field intensity f (0.01 nt accuracy). all other magnetic elements can be derived. survey was repeated on known repeat stations and in the general case azimuth were already determined and known. however in a few cases, stations could not be reoccupied (i.e. a station mark was lost) and a new mark was manufactured onsite. in these cases azimuth determination was made anew; this was also done on those stations in which the visibility of the old azimuth marks was limited or absent. a gyroscopic theodolite was used in all these cases. this instrument was preferred to global positioning system (gps) instruments because it can also be used for measurements in tunnels or roof covered places. moreover even where, in presence of bushes of tall stem, gps could show problems for the satellites limited visibility. in addition the two instruments show comparable precisions of tenth of minutes. for each repeat station a monograph is realized. in the monograph the site is described, instructions are given to easily reach it, what land marks are useful for the approach, what azimuths are visible and all information that could be useful for identification. 3. magnetic measurement reduction procedures, normal fields and maps magnetic elements observed at the repeat stations are reduced firstly to 02 ut using diurnal variation correction from digital data obtained at magnetic observatories working at the time of measurements in the field. for what concerns the 2012.5 survey these were duronia (dur) and castello tesino observatory (cts). portable variometric stations were also used for rapid magnetic variation corrections, when stations were considered far away from the two indicated observatories; in this case a portable time variation fluxgate station was installed (all variometric stations are reported in figure 2). the value of a generic element e (i.e., d,i or f) figure 2. the geographical distribution of the 25 reduced network stations as described in this paper for the 2012.5 reduced network. colored circles indicate observatory and variometric station areas of influence. pink refers to cts observatory area of influence: yellow refers to dur observatory area of influence. green refers to variometric ott station area of influence; light blue refers to variometric gib station area of influence. in case of overlapping regions the two variometric records were considered. 3 dominici et al. at station s, i.e. reduced, at 02ut was calculated following the: es(02ut)d = evar (02ut)d+ [es(t) d evar(t)d] where: es(02ut)d = value of element e at station s reduced at time 02ut of day of measure d evar (02ut)d = value of element e at variometer station s at time 02ut of day of measure d es(t) d = value of element e observed at station s at time t at that day d evar(t)d = value of element e at variometer station at time t at that day d secondly data are reduced to castello tesino obs. for the fixed epoch, i.e. 2012.5 following the: es (2012.5) = eobs (2012.5)+ [es (02ut) d eobs (02ut)d] where: es (2012.5) = value of element e at station s reduced at epoch 2012.5 eobs (2012.5) = mean value of element e at observatory at epoch 2012.5 eobs (02ut)d = value of element e measured at observatory at time 02ut of day of measurement d. the classical technique of sha (spherical harmonic analysis) represents mathematically the configuration of the earth’s magnetic field in terms of spherical harmonics. this can be applied only to the entire earth’s globe and the characteristic minimum wavelength associated with this technique, used for example in igrf (international geomagnetic reference field), when maximum degree is 12, is around 4000 km [iaga 2010]. this is the reason why, on a limited part of the globe, regional magnetic models are normally used. in the italian area different techniques were used during the years. in many cases simple polynomial second order, together with other methods, for example scha (spherical cap harmonic analysis) see de santis et al. 1997, and other related techniques, were applied at the local scale. in this paper a regional magnetic model was constructed using a mathematical second order polynomial, that we call ‘normal field’. at the scale of the case here discussed we expect that the normal field includes almost all the contribution from the earth’s core (equivalent to a sha with maximum degree 12-14). however it cannot be excluded that a deep lithospheric contribution could be involved and represented in the normal field coefficients [see also korte and lusur 2012]. the normal field was computed independently for each element and results, for a generic element e, from the expression: e(ϕ,λ) = ao + a1ϕ + a2λ + a3ϕ 2 + a5ϕλ ϕ and λ are latitude and longitude respectively and ai are the coefficients. these coefficients were computed by means of a least squares fit over the observational values. when the total number of repeat stations was used, as for all latest surveys (for example 2010.0, 2005.0, etc…) in order to avoid contamination from stations located in anomalous areas, the chauvenet rejection criterion was generally used for removing anomalous values from the computation of the least squares fit coefficients. essentially the mean residual of each individual station was calculated using the expression σ =± υυ[ ] / (n−6) where [υυ] is the sum of the square differences between observed and computed values, n is the number of stations used in the inversion and 6 is the number of coefficients. stations with residuals greater than 2σ were neglected and the calculation repeated for the remainder. this table 1. coefficients of the normal geomagnetic field in italy for 2012.5; ϕ = (lat. – 42°) in minutes λ = (long. – 12°) in minutes d, i in minutes f, h, z in nt d (‘) = 145.28 + 0.03227 ϕ + 0.19034 λ 0.00007 ϕ² 0.00005 λ² + 0.00014 λϕ f (nt) = 46273.7 + 5.70875 ϕ + 1.17744 λ 0.00191 ϕ² + 0.00055 λ² 0.00020 λϕ h (nt) = 24390.3 9.46341 ϕ 0.26936 λ 0.00017 ϕ² + 0.00070 λ² + 0.00061 λϕ z (nt) = 39319.2 + 12.65370 ϕ + 1.56750 λ 0.00480 ϕ² + 0.00027 λ² 0.00090 λϕ i (‘) = 3490.91 + 1.09820 ϕ + 0.07938 λ 0.00031 ϕ² 0.00003 λ² 0.00010 λϕ 4 italian reduced magnetic network procedure was repeated until no residuals greater than 2σ were left. for the 2010.0 survey the stations neglected and the mean residual σ for all the remaining stations were 39 and σ = 2.535’ for d, 31 stations and σ = 25.1nt for f, 25 stations and σ = 22.18nt for h, 28 stations and σ = 27.1nt for z. in the case of the reduced network (at 2012.5), reported in this paper, this criterion was not applied. this point was taken in consideration since the chauvenet criterion is a means of assessing whether one experimental data (an outlier) from a set of observations, is likely to be spurious; we considered that this should not be applied on an objectively more limited data set. it was considered that deletion of outlier data should not be done especially in small sets where a normal distribution cannot be assumed. coefficients of the normal geomagnetic field in italy for 2012.5 are reported in table 1. in order to test the validity of a reduced network approach, at least in the case of the italian area, we made a comparison of the normal polynomial field interpolation, called here ingrf with the igrf for total intensity f. these comparisons are reported in figure 3. for this test we had first compared the normal field for the epoch 2010 (ingrf), when all repeat stations were available (panel a) and afterwards using only the 25 stations (ingrf_red), that were successively used for the 2012.5 reduced network (panel b), both models were compared to igrf. for the same epoch (2010.0) we found that the use of a limited number of stations (that were afterwards used for the reduced network) had not introduced sensibly larger differences with respect to the use of the full set of available repeat stations data. in panel c the only available 25 reduced network station were used for a new polynomial normal field (ingrf_red) at 2012.5. this last panel shows almost same patterns with respect to igrf 2012.5 similarly to the 2010.0 case. all panels show larger differences at the edges and better agreements at the center of the considered areas. this effect can be interpreted as a larger ability of polynomial models to represent the earth’s core field at the map’s center than at the edges. we conclude that a comparison of the two 2010.0 models, obtained from the original and reduced networks, shows a moderate difference across all the italian area. 4. conclusions in this paper we presented the results of a magnetic survey undertaken in 2012 on a selection of the italian network of magnetic repeat stations (first order network). the original number of stations for the previous full 2010 survey was 131 including 2 observatories including 11 stations in albania, 3 stations in corsica and 1 in malta, as shown in figure 1. from this network some stations were selected for the 2012.5 that was designed as a ‘reduced’ survey. we have described how the reduced network stations were chosen and we stressed that all reduced network points were selected on the basis of their minimum deviation from the expected earth’s core magnetic field, and considering the geographical distribution in order to cover uniformly almost all the italian area. at the end the number of selected stations amounts to 25, distributed over the italian area with an average spacing around 100 km (figure 2). regional modeling of magnetic data from a repefigure 3. at 2010.0 difference represented with a colour scale between f normal field polynomial interpolation from ingrf on all standard repeat stations, and 2010.0 f from igrf (panel a). at 2010.0 f normal field polynomial interpolation ingrf_red computed on only 25 stations that were selected later for the 2012.5 survey and igrf (panel b). finally in panel c the comparison made at 2012.5 between igrf and ingrf_red from 2012.5 measurements. 5 dominici et al. at station network was also the mathematical technique used in the case of this reduced network at 2012.5, in order to represent the main field (essentially the core field) as accurately as possible at the local scale. in this study we used only simple polynomials that were considered to give an adequate amount of information to represent the main field. an empirical rule given by bullard [1967] relates the number of coefficients of a global sh model with the minimum wavelength that a global model can represent and gives the details represented by the regional model. according to bullard’s rule such relation brings to a minimum wavelength of about 800 km for the second order degree polynomial representation for the area here considered. in figure 3 all comparisons have shown that the use of a limited number of stations (that were afterwards used for the reduced network) had not introduced sensibly larger differences with respect to the use of the full set of available repeat stations data when compared to igrf. references bullard, e.c. (1967): the removal of trend from magnetic surveys, earth planet. sci. lett., 2, 293-300. coticchia, a., a. de santis, a. di ponzio, g.dominici, a.meloni, m. pierozzi, m. sperti, (2001) la rete magnetica italiana e la carta magnetica d’italia al 2000.0, bollettino di geodesia e scienze affini, 4, 261-291. cafarella, l., a. de santis and a. meloni (1992a): secular variation from historical geomagnetic field measurements, phys. earth planet. inter., 73, 206-221. cafarella, l., a. de santis and a. meloni (1992b): il catalogo geomagnetico storico italiano, pubblicazioni dell’istituto nazionale di geofisica vol. 7, 160 pp. de santis, a., m. chiappini, g. dominici e a. meloni (1997): regional geomagnetic field modelling: the contribution of the istituto nazionale di geofisica, annali di geofisica, xl, 5, 1161-1169. de santis, a., l. gaya-piqué, g. dominici, a. meloni, j.m. torta, and r. tozzi (2003): italian geomagnetic reference field itgrf: update for 2000 and secular variation model up to 2005 by autoregressive forecasting, annals of geophysics, 46, 3, 491-500. dominici, g., a. meloni, m. miconi, m. pierozzi and m. sperti (2007): la rete magnetica italiana e la carta magnetica d’italia al 2005.0, bollettino di geodesia e scienze affini,1,1-47. dominici, g., a. meloni, a. di ponzio, m. miconi (2012): italian magnetic network and magnetic reference fields at 2010.0, annals of geophysics, 55, 6, doi: 10.4401/ag-5411. haines, g.v. (1990): regional magnetic field modelling: a review, j. geomag. geoel.,42, 1001-1018. iaga (international association of geomagnetism and aeronomy) working group v-mod (2010): international geomagnetic reference field: the eleventh generation, geophysical journal international, 183, 1216-1230. korte, m. and v. lesur (2012): repeat station data compared to a global geomagnetic field model, annals of geophysics, 55, 1101-1111. lanza, r., and a. meloni (2006): the earth’s magnetism: an introduction for geologists, springer, 290 pp. mandea, m. (2004): reseau magnétique de répétition de la france 2002, bureau central de magnetisme terrestre, paris, bullettin n. 20. meloni, a., o. battelli, g. dominici, s. arca and a. marchetta (1988): italian magnetic network at 1985.0, bollettino di geodesia e scienze affini, 4, 339-350. meloni, a., battelli, o., de santis, a. and dominici, g. (1994): the 1990.0 magnetic repeat station survey and normal reference fields for italy, annali di geofisica, vol. xxxvii, n. 5, 949-967. molina, f., e. armando, r. balia, o. battelli, e. bozzo, g. budetta, g. caneva, m. ciminale, n. de florentiis, a. de santis, g. dominici, m. donnaloia, a. elena, v. iliceto, r. lanza, m. loddo, a. meloni, e. pinna, g. santarato, r. zambrano (1985): geomagnetic survey of italy. repeat stations network and magnetic maps: a short report, annales geophysicae, 3 (3), 365-368. newitt, l.r., barton c.e. and j. bitterly (1996): guide for magnetic repeat station surveys, iaga working group, 115. shepard, d. (1968): a two-dimensional interpolation function for irregularly-spaced data, in: proceedings 23rd national conference acm, acm, 517-524. talamo r (1975): le carte magnetiche d’italia delle isodinamiche della h e delle isogone dell’istituto militare e loro aggiornamento al 1973.0, bollettino di geodesia e scienze affini, vol. xxxiv, n.1,p.34. ___________ *corresponding author: guido dominici istituto nazionale di geofisica e vulcanologia, roma, italy; tel. +39-6-51860312 fax +39-6-51860397 email: guido.dominici@ingv.it 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved 6 vol50,4,2007 513 annals of geophysics, vol. 50, n. 4, august 2007 key words geochemical monitoring – seismicity – tectonic lines 1. introduction the thermal springs of porretta are located in the northern apennines over an area crossed by several tectonic lineaments (fig. 1). the thermal waters have been locally utilized for medical purposes since the etruscan age. a large scientific literature on the therapeutic characteristics of porretta thermal waters has been written during the past five centuries and many observational data were recorded. in particular capponi (1608) noted that «after five local seismic events water flow-rate increased up to four times more than usual and their temperature was too hot to touch with the hands». post seismic thermal and flow rate anomalies were also detected on april 20, 1765, april 11, 1787, june 10, 1904, december 28, 1908, june 5, 1910 and july 24, 1911 together with anomalies in methane gas bubbling and in local methane dry gas emissions (facci et al., 1995). demetrio lorenzini, a local chemist who utilized cold groundwaters and thermal waters for chemical preparations, observed water flow rate and temperature anomalies in concomitance with a local seismic event in 1872 (lorenzini, 1898). lorenzini monitored water level fluctuations in a drilled well close to the thermal springs in the period 1873-1882 to study possible seismic precursors in cooperation with m.s. de rossi (de rossi, 1875). he observed that in concomitance with local seismic events a water level decrease in a local well accompanied a flow rate increase of the thermal springs. a detailed statistical processing of the available data recorded by lorenzini was carried out by albarello et al. (1991) revealing the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring nicola ciancabilla (1), manuela ditta (2), francesco italiano (2) and giovanni martinelli (3) (1) agenzia regionale prevenzione e ambiente (arpa) dell’emilia romagna, sezione di bologna, italy (2) istituto nazionale di geofisica e vulcanologia, sezione palermo, italy (3) agenzia regionale prevenzione e ambiente (arpa) dell’emilia romagna, sezione di reggio emilia, italy abstract the thermal springs of porretta are located on a seismically active area of the northern apennines. in 19th century a chemist identified anomalous behaviour of the thermal waters in concomitance with local seismic events. recent studies assessed the geochemical features of the circulating fluids (e.g., waters carry a dissolved ch4-dominated gas phase with a radiogenic signature of the helium isotopic ratio) and observed anomalous hydrologic and geochemical signals possibly related to crustal strain phenomena due to local seismic events. long-term geochemical monitoring was carried out from 2001 to 2006 with the aim of detecting the behaviour of the circulating fluids possibly coinciding with seismic activity. the collected data reveal a sensitivity of the thermal waters to the activity of the main fault crossing the village of porretta and identify a «seismogenic» structure crossing the village. mailing address: dr. manuela ditta, istituto nazionale di geofisica e vulcanologia, sezione palermo, via ugo la malfa 153, palermo, italy; e-mail: m.ditta@pa.ingv.it 514 nicola ciancabilla, manuela ditta, francesco italiano and giovanni martinelli fig. 1. location of the investigated area on the northern apennines and the recognized tectonic structures (after biblio). the earthquakes that hit the area from 1980 to 2002 (castello et al., 2005) and the recent 2003-2005 events are also plotted. it is easy to observe an high density of events on the tectonic line crossing porretta terme. 515 the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring possible precursory beaviours of water level fluctuations. after a local mw = 5.3 seismic event in 2003 (seismic bulletin, 2007) whose epicentre was located close to the village of monghidoro about 25 km to the east of porretta terme, significant anomalies in temperature, flow rate and bubbling gas emissions were recorded both by visual observations and direct measurements. to better understand possible relations among local seismicity and hydrogeochemical patterns of porretta thermal springs a geochemical prospection and a continuous monitoring program were carried out. this paper deals with the main results obtained from the collected geochemical data focussing on the relationship between fluids and tectonic structures. 2. study sites and geological background the thermal manifestations located in the porretta area are caracterized by spontaneous spillage of about ten springs inside the town (fig. 1). among those, five springs named bove, marte, donzelle, leone and sale, are located to the southwest where the old hotel of the thermal baths is located (fig. 2). the water temperature ranges from 34°c to 37°c; they are salty waters enriched in bromium and iodine ions and characterized by a methane-dominated dissolved gas phase. the other springs named puzzola 1, puzzola 2, porretta nuova, porretta vecchia and majocchi, are spread in proximity of the reno river 1 km from the thermal baths. they are also salsobromoiodine waters with bubbling and dissolved ch4-dominated gases, with lower temperature (20-26°c) and salinity compared to the above mentioned springs (ciancabilla et al., 2004). water and gas samples were collected at porretta nuova, puzzola 1, bove and sale springs to investigate the porretta thermal system; the selected springs are considered the most representative of the whole thermal system. fig. 2. sampling sites at porretta thermal area. the thermal waters are close to the old and new thermal plants respectively located on the rio maggiore and reno rivers. 516 nicola ciancabilla, manuela ditta, francesco italiano and giovanni martinelli 2.1. geology and tectonic setting the northern apennines are a ne-verging thrust and fold belt characterized by a crustal extension of about 2.5 mm/yr and by moderate and large normal faulting seismic events (hunstad et al., 2003; chiaraluce et al., 2004). the extension along nw-trending faults is balanced by compressive trends recognized in the nearby po valley area (montone et al., 2004). tectonic processes are accomodated by complex mechanisms on pre-existing structures. while the extension along the apenninic belt is related to the adriatic lithosphere retreat, compressive trends are probably related to subordinate processes (piccinini et al., 2006). no significant historical earthquakes are reported in the porretta terme area but evidence for widespread low-slip quaternary faulting along the entire length of the reno river were recently obtained by detailed strata terraces analysis (bierma et al., 2005). local seismicity is characterized by relatively frequent low magnitude events consistent with a low-slip quaternary faulting. the village of porretta terme is located at the confluence of the rio maggiore and the reno rivers, over a land that is mostly made up of a formation of so-called scaly clays (palombini shales), close to a large formation of marly-arenaceus and calcarenitic turbidite. the geologic setting in the high reno valley area, where the thermal sources of porretta are located, is characterized by two main rock formations: the tuscan units, characterized by litic arenaceus silt formations (porretta, macigno and cervarola formations) and the ligurian units, made by clays and strongly fossilized shales with rocky mass characterized by different nature and size. the geologic structure of the over said units is to be tied up to the contraction (middle cretaceous) and to the closing (middle eocene) of the piemonte-ligurian ocean (located between the african and the european plate). the compressive stress pushed and crossed over (oligo-miocene) the cretaceous eocene formations (ligurian facies) on the triassic-oligocene formations (tuscan facies), then everything was pushed as a wave through east shaping folds and overlaps (duchi et al., 2005). from a geologic point of view the porretfig. 3. schematic section showing the recharge area/thermal springs relative positions and the main structures governing the groundwater circulation through the different lithotypes. 517 the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring ta area consists of an anticlinal fold with the nucleus made by oligo-miocene «macigno» sediment. the northern side of the fold is vertical and the southern side goes down toward sw through a series of fold and faults with anti appennine direction and other one with sub-vertical immersion and orthogonal to the first (bencini and duchi, 1980). the turbidite formations (tuscan units) represent the recharge basin for the investigated springs (cianciabilla et al., 2004 and references therein). in particular the water discharge in porretta baths crosses the porretta formation (a sand-silt grain stone entirely similar to the rock in feeding area) and rises to the surface through a deep regional tectonic dislocation (fig. 3). the porretta formation acts as a natural draining wedge in a low permeability setting (sandstone unities). it connects the warm overpressure waters accumulated in depth to the external environment with its stratification joints and fractures. 3. sampling and analytical procedures samples of thermal waters, dissolved and bubbling gases were collected to be analyzed for their chemical and isotopic composition. water temperature, ph value and electrical conductivity were measured directly in the field. the water samples were analyzed for major and minor elements by ion chromatography (dio-nex dx-120 equipped by dionex cs12a and as14a columns, reproducibility within ±2%). the hco3 amount was measured by volumetric titration with 0.1 n hcl. mass spectrometric analyses were carried out by a finnigan deltaplus xp and by an analitical precision 2003 mass spectrometers respectively for d/h and for the δ18o isotopic ratios. d/h isotopic analyses of water were performed using the kendall and coplen (1995) technique (reaction with zn at 450°c), while 18o/16o analysis were carried out by the co2-watable ii. analytical results for dissolved and bubbling gases. dissolved gas data in cc/stp per litre; bubbling gas data in vol% for o2, n2, ch4 and co2, in ppm for he and h2. dissolved gas date he h2 o2 n2 co ch4 co2 r/ra he/ne sample puzzola1 02/03/2005 < < 0.05 1.7 8.68e−05 3.52e+01 10.7 n.a. n.a. bove 02/03/2005 < < 0.07 1.2 1.53e−04 4.94e+01 29.6 n.a. n.a. porretta nuova 02/03/2005 < 2.0e−03 0.05 2.1 3.74e−05 3.15e+01 6.2 n.a. n.a. sale 02/03/2005 < < 0.04 2.4 7.49e−05 2.80e+01 22.9 n.a. n.a. bubbling gas date he h2 o2 n2 co ch4 co2 r/ra he/ne sample porretta nuova 02/03/2005 25 < < 11.1 < 88.0 0.9 0.04 8.28 sale 02/03/2005 < 605 < 1.2 < 94.5 4.2 n.a. n.a. table i. chemical composition of the sampled thermal waters. concentrations in meq/l; tds in mg/l. ec= = electrical conductivity in ms/cm at 20°c. the isotopic composition of deuterium and oxygen are reported in δ unit per mil versus v-smow standard. sample date t°c ph ec li na k mg ca f cl br so4 hco3 tds d18o dd puzzola 102/03/05 21.1 7.54 3,85 0.224 36.66 0.89 0.56 1.68 0.15 29.34 0.012 0.54 10.9 1987 -7.33 -50 porretta 02/03/05 29.1 7.69 2,66 0.150 26.78 0.61 0.28 0.37 0.16 17.88 0.008 0.48 10.5 1310 -6.88 -52 nuova sale 02/03/05 35.3 7.25 8,81 0.499 79.67 1.91 0.73 1.23 0.23 70.32 0.000 0.11 16.9 4442 -6.80 -51 bove 02/03/05 35.5 7.28 8,65 0.480 84.73 1.95 0.89 1.70 0.21 70.04 0.030 0.11 17.0 4561 -6.88 -52 518 nicola ciancabilla, manuela ditta, francesco italiano and giovanni martinelli ter equilibration technique (epstein and mayeda, 1953). the results are reported in ‰ per mil units versus the v-smow standard. the standard deviations of measurements are approximately ±1% for d/h and ±0.2% for the 18o/16o. the chemical composition of the dissolved and bubbling gases was determined by gaschromatography by a perkin elmer autosystem xl gas chromatograph equipped with a 4m carboxen 1000 column, using ar carrier and a double detector (hwd and fid detectors). the detection limits are 2 ppmv for he and h2, 1 ppmv for ch4 and co2 and 500 ppm vol. for n2 and o2. the analytical error is ±5% for he, and ±3% for the other gaseous species. the dissolved gases were extracted following the method proposed by capasso and inguaggiato (1998). helium was purified in an ultra high vacuum line for helium isotope analyses following standard procedures (italiano et al., 2004). the he to ne ratio was measured by an in-line quadrupole mass spectrometer (qms, vg quartz). the isotopic analyses of the purified helium fraction were performed by a modified static vacuum mass spectrometer (vg5400tft, vg isotopes) that allows for the concomitant detection of 3he and 4he-ion beams, reducing the 3he/4he measurement error to very low values. typical uncertainties in the range of low-3he (radiogenic) samples are below ±5%. the analytical results are listed in tables i-ii. 3.1. continuous monitoring following tales and visual observations reported by local inhabitants, we found out that the thermal springs of porretta terme have undergone changes coinciding with seismic activity. variations in water level, gas bubbling, turbidity etc. are the most common reported features. to better understand the possible effects of seismicity on the porretta terme thermal waters, a multiparameter device (green-span ctd 350 sensor) was installed at the puzzola1 spring (in 2001, in 2004 and in 2006 respectively) and one more probe was installed in the bove thermal spring (t≅35°c) to monitor warmer water probably of deeper origin (from july 2004 to march 2005). the probes provided conductivity, temperature and pressure data with an accuracy of ± 0.2% fs, ± 0.1°c, 0,02% fs, respectively (fs = full scale). the aim of the continuous monitoring activity was also to detect the possible man-induced noise to discriminate natural-induced signals from artifacts. 4. results and discussion 4.1. chemical composition of the thermal waters all the sampled waters show a cl-alkaly and a na-dominated chemical signature, however a remarkable difference in salinity, 1300 and 4600mg/l and conductivity, 2600 µs and 8600 µs, marked the two groups of thermal springs from the new and old thermal plants respectively (fig. 2). the triangular diagrams in fig. 4 display the alkali enrichment of the water springs feeding the porretta terme system. the lower cl and na contents of porretta nuova and puzzola 1 samples (fig. 5) are interpreted as a dilution process by parent waters of which the sale and bove springs represent the high salinity components. it may be the result of a mixing process between shallow meteoric waters and groundwaters which rise up through the main lithological contact or through tectonic alignments. the δ18o values reveal a meteoric origin of thermal springs (minissale et al., 2000), infiltrating through local arenaceous rocks of «macigno» and «porretta» formations and temperatures consistent with a source of the main aquifer at a depth of about 1 km. figure 5 shows the clear na and cl enrichment that induces na/cl ratios well above the stoichiometric ratio of sea-water. a groundwaters halite interaction process cannot explain a na abundance as high as 1950 mg/l. since the springs are also enriched in other alkaline elements (fig. 6), an interaction with clay layers where large cationic na-mg exchange occurs can be considered (shoeller, 1955). according to nanni and zuppi (1986), salty waters are the expression of a mixing process between waters of meteoric origin and trapped brines inside the neogenic marine sediments underlyng the lig519 the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring urean allochthonous flysh. the brines would be squeezed and pushed toward the surface through the overlapping surfaces of the ligurian units. ch4 gas phase, always present in the springs, further facilitates the brines to rise and favour ionic exchange processes between clays and salty waters. as a result (fig. 7) the waters catch na and release mg and subordinately ca. although bencini and duchi (1980) found an interaction of the porretta springs with soluble evaporitic deposits (see ca/so42ratio in fig. 8), the puzzola1, sale and bove springs show a large increase in ca concentration not balanced by the so42ions, which do not support the interpretation of bencini and duchi (1980). furthermore, the waters are undersaturated with respect to the minerals anhydrite, gypsum, and halite (table iii), confirming that the waters do not emerge from evaporitic rocks. the dolomite saturation index approaches zero showing the occurrence of equifig. 4a,b. ternary plot a) ca-mg-alkali and b) hco3-cl-so4. the triangular diagrams display the composition of four main springs of porretta terme plant. all the samples are enriched in alkali element with high cl content. fig. 5. meq/l na versus meq/l cl diagram. the figure distinguishes two water groups: puzzola 1 and porretta nuova, marked by lower salinity and bove and sale springs characterized by higher concentrations. the porretta nuova and puzzola1 samples represent the more diluted terms of a hypothetical mixing between a meteoric and a groundwater terms (dashed line) in the porretta thermal system. all the samples are na-enriched with respect to the stochiometric na/cl ratio and the na/cl ratio of seawater (black and dash-dot line respectively). fig. 6. meq/l k versus meq/l cl and meq/l li versus meq/l cl concentration at the porretta terme springs showing the enrichment in alkaline elements with respect to sea water. 5 6 fig. 9. n2-co2-ch4 diagram showing a ch4-dominated gas phase for both dissolved and bubbling gases. 520 nicola ciancabilla, manuela ditta, francesco italiano and giovanni martinelli librium with dolomitic rocks for water circulating at relatively shallow levels. during their uprising towards the surface, they mix with waters of deeper origin (brines) hosted in pliocene sediments thus reaching their final na-cl connotation. 4.2. chemical and isotopic features of dissolved and venting gas the chemical composition of the gas phase of both bubbling and dissolved gases is displayed in table ii. the dissolved gases exhibit a ch4 amount up to five orders of magnitude higher than that of air saturated water. the n2co2-ch4 diagram (fig. 9) displays the chemical composition of the bubbling gases collected at porretta nuova and sale springs, besides the chemical composition of the dissolved gases fig. 8. ca versus so4 contents for the collected water samples showing a clear ca enrichment.only porretta nuova falls on the stoichiometric ca/so4 ratio line. table iii. saturation indexes (si) calculated for the main mineralogical phases in equilibrium with the thermal waters. it is evident how aragonite and calcite are close to the saturation showing the attainment of the equilibrium for the less saline waters (puzzola 1 and porretta nuova, new thermal plant), while more concentrated waters (sale and bove, old thermal plant) are equilibrated with calcite and dolomite. si calculated by the wateqp software (appelo, 1988) and expressed as the logarithm of the ion activity ratio pertinent to a mineralogical phase, and the equilibrium constant of its solubility product. phase puzzola 1 porretta nuova sale bove anhydrite −3 −3.6 −3.98 −3.86 aragonite 0.03 −0.34 −0.13 0.04 calcite 0.17 −0.2 0.01 0.18 dolomite −0.04 −0.34 0.04 0.32 fluorite −0.6 −1.24 −0.7 −0.64 gypsum −2.77 −3.4 −3.82 −3.69 halite −4.7 −5.05 −4.07 −4.05 fig. 7. meq/l na versus meq/l mg diagram. the values for the sampled thermal springs show a high na content suggesting an interaction with na-rich clay minerals. 521 the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring fig. 10. temporal variations of conductivity and water level at the puzzola spring recorded during november 2001-january 2002.the vertical black line marks the md = 3.1 seismic event that occurred on december 6, 2001. fig. 11. temporal variations in water temperature (21-23°c) and water level (0.5-3 m) in the puzzola spring from august 2003 to february 2005. the earthquakes that occurred within a 20 km-radius area centered on porretta terme town are shown as vertical grey lines. the rainfalls are reported as daily mean value for the entire monitoring period (0-16 cm). the rain-gauge is located close to the porretta thermal center. 522 nicola ciancabilla, manuela ditta, francesco italiano and giovanni martinelli collected at all the sampling sites. the graph exhibits their very similar composition. the he concentration measured in the free gas collected at porretta nuova spring is distinctly higher than the atmosphere. on the other hand the high he/ne ratio highlights a negligible atmospheric contribution. the isotopic he composition as low as 0.04 r/ra, reveals a clear radiogenic signature. this information rules out any release of mantle-derived components over the study area. according to carbon and hydrogen isotopic data of gases collected over the emiliano appennine (duchi et al., 2005), the ch4 gas phase would represent the extreme stadium of the thermal degradation of organic substance trapped in the sediments. 4.3. long-term automatic monitoring during the 2001-2004 observation period many variations in the water level and conductivity were recorded (figs. 10-11). the data were analyzed besides the shocks occurred over a 20 kmradius area (table iv). a slight anomaly in conductivity and water level at the «puzzola 1» 06/12/2001 44.091 11.076 3.1 08/06/2002 44.274 10.618 4 09/06/2002 44.386 10.692 2.5 11/06/2002 44.395 10.693 2.6 13/06/2002 44.391 10.692 2.7 18/06/2002 44.396 10.683 4.2 05/07/2002 44.396 10.702 3.1 08/07/2002 44.386 10.711 2.7 09/07/2002 44.397 10.696 3.3 13/08/2002 44.396 10.689 2.5 06/09/2002 44.063 11.286 2.5 11/10/2002 44.397 10.688 2.7 18/11/2002 44.221 10.778 2.5 10/02/2003 44.137 10.812 2.5 30/03/2003 44.256 10.793 2.5 06/04/2003 44.291 10.981 2.7 06/04/2003 44.266 10.987 3 07/04/2003 44.306 10.991 2.7 11/04/2003 44.264 10.982 2.6 11/04/2003 44.268 10.970 2.7 21/04/2003 44.324 10.701 2.5 24/04/2003 44.268 10.965 2.7 01/06/2003 44.344 10.708 2.6 11/07/2003 44.376 10.606 2.7 04/08/2003 43.978 10.991 2.5 18/08/2003 44.362 10.749 2.9 18/08/2003 44.364 10.717 2.5 18/08/2003 44.379 10.723 2.8 15/09/2003 43.936 11.299 2.5 table iv. md ≥ 1.8 seismic events within a 20 km-radius area centered on porretta terme town during the 20012006 observation period. date latitude longitude magnitude date latitude longitude magnitude 06/11/2003 44.389 10.628 2.9 11/12/2003 44.326 11.055 3.2 12/12/2003 44.288 11.019 2.8 13/12/2003 44.297 11.038 2.8 04/01/2004 44.273 10.946 2.9 05/01/2004 44.169 10.783 2.5 23/02/2004 44.396 10.789 2.5 16/03/2004 44.248 10.959 2.6 04/05/2004 44.295 10.957 2.6 10/06/2004 44.021 11.002 2.1 22/07/2004 44.258 11.076 1.9 23/07/2004 44.155 10.820 2.6 30/07/2004 44.221 10.620 2.6 22/08/2004 44.349 10.738 2.5 10/09/2004 44.277 10.738 2.5 09/10/2004 44.352 10.828 2.7 26/12/2004 44.046 11.109 2.6 11/01/2005 44.092 10.823 1.8 01/03/2005 44.272 11.080 2.2 29/01/2006 44.324 11.409 2.0 02/02/2006 44.239 11.470 2.0 05/02/2006 44.301 11.491 2 03/04/2006 44.155 11.393 2 05/06/2006 44.259 11.275 2 17/08/2006 44.282 11.550 2 21/06/2006 44.319 11.451 2 14/06/2006 44.272 11.449 2 21/11/2006 44.200 11.580 2.0 523 the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring spring followed the seismic shock occurred on december 6th, 2001 lasting also as post-seismic effect (fig. 10), when the water level increased by a factor of two. after two months, in january 2002, the continuous monitoring stopped, however the physico-chemical parameters were still above the pre-seismic values. the trend shown by the electric conductivity might highlight that higher values than those previously measured lasted for a long time span after the seismic shock as a consequence of a slow deformation process as already observed along the apennines chain (italiano et al., 2004). the increases in the water level observed in 2003 at the «puzzola 1» site, (when the continuous monitoring had started again) were accompanied by sharp temperature drops probably due to changes in the mixing proportions between a shallow cold water component and a deeper thermal one. figure 11 displays that the above-mentioned modifications have occurred coinciding to other natural phenomena: seismicity and rainfalls. both of them are well fig. 12. temporal variations in conductivity at the bove spring during june 2004-february 2005 observative interval. the earthquakes occurred within a 20km-radius area centered on porretta terme town are shown as vertical black lines. only the july 2004 shocks caused temporary modifications in the electrical conductivity. the recorded spikes are a consequence of the «anthropic disturbances» due to chlorine addition to the water. 524 nicola ciancabilla, manuela ditta, francesco italiano and giovanni martinelli known to be able to induce modifications on the groundwaters, however, the absence of carstic systems in the area does not allow fast water circulation that, on the other hand, can be enhanced by the opening of microfractures due to seismic shocks. as a matter of fact, the collected data make it hard to discriminate which, between seismicity and rainfall, is responsible for the observed modifications. a possible model can be as follows: a long-term deformation process is active along that sector of the apennines chain; sometimes ruptures occur generating shocks of generally low magnitude; the intrusion of rain water along the fault lines acts as a lubricant and allows small movements of the fault planes under stress accumulation. the concomitant occurrence of low-energy seimic events, rainfalls and modifications to the mixing proportions of different water bodies is the consequence of a complex. these observations stress the role of fault movement on the mixing ratios changes at depth that can modify, for example, the electrical conductivity without changes in the water temperature. coinciding with the shocks in june and july 2004 respectively, we observed a slight water level increase not associated with any temperature variation. the same shocks induced modifications at the bove site (fig. 12) where the following macroscopic observations fit with the recorded data: i) an anomalous gas bubbling activity was observed inside the bove spring from the beginning of june 2004; ii) the electrical conductivity probe recorded a variation that increased the background value from less than 8300 to about 9000 µs. such a variation, which lasted more than a month, started about one month before the seismic sequence of july 2230th, when the electrical conductivity reached its maximum value. similar behavior was never observed even when earthquakes with equivalent magnitudes occurred around porretta. the modifications recorded during the 2006 monitoring period highlighted how deformation can be considered responsible for geochemical modifications in the absence of seismic activity. although the variation in the water level (from 60 to more than 80 cm) started together with the low-energy shock which occurred on 21st november 2006 (fig. 13), it is hard to accept that a low-energy shock which occurred 20 km away from the observation site induced similar modifications to the groundwater circulation. however, following the above mentioned model, the deformative effect may be responsible for the observed phenomena as the low energy shock was a rupture episode that occurred at a distance but was generated by the same apennines deformative activity. 5. conclusions the thermal springs of porretta represent an interesting study site to better understand the relation between fluids and seismogenic structures. in this sense the results we obtained are of general interest even though the study area is small and of local interest. the discharged thermal waters are fed by meteoric waters which reached saturation with carbonatic rocks, mixed to brines coming from pliocene sediments. a methane-dominated gas phase is dissolved at all the thermal springs that sometimes become gas-oversaturated. fig. 13. temporal variations of the water level at the bove spring during october-december 2006 observative interval. one earthquake occurred within a 20 km-radius area, however because of the low energy is not considered as responsible for the observed modifications (see text). 525 the porretta thermal springs (northern apennines): seismogenic structures and long-term geochemical monitoring the temporal variations of the geochemical features of both gas and thermal waters were not investigated (e.g., by a monthly sampling) but long-term continuous monitoring investigated water temperature, electrical conductivity and water level even though with much datamissing (greenspan probes from 2001 to 2006). the interpretation of the geochemical variations highlights the importance of the fluid monitoring in a seismogenetic structure. some of the recorded modifications occurred in concomitance with both earthquakes located on the tectonic lineament crossing the town of porretta, and rainfalls. the model we propose interprets the modifications of the water composition as a consequence of local stress accumulation, where the infiltrating rain water allows local fault-planes under stress to slide generating low-magnitude earthquakes. the obtained results are in full agreement with previous observations showing how geochemical modification to the circulating fluids can be induced by crustal deformation besides crustal ruptures (earthquakes). as a consequence, the seismogenic behaviour of a tectonic line can be recognized from the modifications occurring to the uprising fluids due to local stress accumulation. acknowledgements the authors thank h. woith and f. yang whose criticisms and suggestions greatly improved the paper. the research work was supported by the grant imont-2002 «monitoraggio chimico-fisico e valutazione delle possibili correlazioni con eventi sismici in alcune sorgenti idrotermali dell’appennino tosco emiliano romagnolo: approcci innovativi per la valutazione delle caratteristiche di permeabilità degli acquiferi» and by the project «gestione idrogeologica e monitoraggio ambientale 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(ingv-cnt, roma). chiaraluce, l., a. amato, m. cocco, c. chiarabba, g. selvaggi, m. di bona, d. piccini, a deschamps, l. margheriti, f. courboulex and m. rippe (2004): complex normal faulting in the apennines thrust-andfold belt: the 1997 seismic sequence in central italy, bull. seismol. soc. am., 94, 99-116. ciancabilla, n., g.c. borgia, r. bruni, f. ciancabilla, s. palmieri and l. vicari (2004): le sorgenti dell’alta valle del reno (appennino bolognese): nuovi elementi per approfondire la genesi dei movimenti gravitativi profondi nei terreni argillitici caoticizzati dell’appennino tosco-emiliano, il geologo dell’emilia-romagna, 18, 5-14. de rossi, m.s. (1875): quadro generale statistico topografico giornaliero dei terremoti avvenuti in italia nell’anno meteorico 1874 col computo di alcuni altri fenomeni, atti acc. pont. nuovi lincei, 28, 514-536. duchi, v., g. venturelli, i. boccasavia, f. bonicolini, c. ferrari and d. poli (2005): studio geochimico dei fluidi dell’appennino tosco-emiliano-romagnolo, boll. soc. geol. it., 124, 475-491. epstein, s. and t. mayeda (1953): variation of 18o content of water from natural sources, geochim. cosmochim. acta, 4, 213-224. facci, m., a. guidanti and r. zagnoni (1995): le terme di porretta nella storia e nella medicina (nuèter editions), vol. 2, pp. 649. hunstad, i., g. selvaggi, n. d’agostino, p. england, p. clarke and m. pierozzi (2003): geodetic strain in peninsular italy between 1875 and 2001, geophys. res. lett., 30 (1181), doi: 10.1029/ 2002gl016447. italiano, f., g. martinelli and a. rizzo (2004): seismogenic-induced variations in the dissolved gases of the thermal waters of the umbria region (central apennines, italy) during and after the 1997-98 seismic swarm, geochem. geophys. geosyst., 5 (11), q11001, doi: 10.1029/2004gc000720. kendall, c. and t.b. coplen (1985): multisample conversion of water to hydrogen by zinc for stable isotope determination, anal. chem., 57, 1437-1440. lorenzini, d. 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(1955a): geochemie des eaux souterraines, masson. schoeller, h. (1955b): le eaux souterraines, masson. seismic bulletin (2007): ingv-cnt seismic bulletin (available on line at: http://www.ingv.it/~roma/reti/rms/ bollettino/). vol50,4,2007 27 vol50,4,2007 28 vol50,4,2007 29 vol50,4,2007 30 vol50,4,2007 31 vol50,4,2007 32 vol50,4,2007 33 vol50,4,2007 34 vol50,4,2007 35 vol50,4,2007 36 vol50,4,2007 37 vol50,4,2007 38 vol50,4,2007 39 vol50,4,2007 40 caporali_corretto_layout 6 767 analysis of the displacement of geodetic stations during the emilia seismic sequence of may 2012 alessandro caporali*, luca ostini università di padova, dipartimento di geoscienze, padova, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6115 1. introduction the two events of mw 6.11 and 5.96 [emergeo working group 2012, this volume] that took place on may 20 and 29, 2012, respectively, in the po plain, and the associated seismic sequence, represent the response of the brittle upper crust (hypocentral depth ca. 10 km) to the compressive stress in the outer apennine arc. kinematically, the motion of the apennines that has led to a compressional regime in the po plain can be represented as a counterclockwise rotation, as demonstrated by the shortening of the transalpine baselines, with a rate that decreases going west [caporali and martin 2000]. the compressive stress field is known from borehole breakout data [pierdominici and heidbach 2012] and it agrees with the principal directions of the strain rate derived from global positioning system (gps) data. the geodetic strain rate for seismic zone 912 (dorsale ferrarese, according to meletti et al. [2008]) is 92.86 ±0.04 nstrain/yr, which is a relatively high value [caporali et al. 2011]. on a more local scale, the mirandola fault is described in the database of individual seismic sources (diss; http://diss.rm.ingv.it/) under the id itis107 as a possible individual source, and it has a position, strike, dip, size and expected maximum magnitude [burrato et al. 2003, carminati et al. 2010, scrocca et al. 2007] that are very close to the main events of the 2012 sequence. several permanent gps stations were in activity in the area. using ultrarapid international global navigation satellite systems (gnss) service (igs) orbits and the bernese bsw 5.0 software [dach et al. 2007] at our computing facility, a fast solution for the coseismic displacement was published on the may 21, 2012, and on the may 30, 2012, for the events of may 20 and 29, 2012, respectively. the largest signal was about 2 cm in the north direction at the station sgip (san giovanni in persiceto), which was relatively consistent with the fault-plane solution [devoti 2012, this volume]. in this study, we present more detailed estimates of the coseismic offsets at several other stations in the area, including a kinematic solution at 30 s temporal resolution. the coseismic displacements are inverted using the okada [1985] model and a preliminary model of the two faults is obtained for each of the two main events, taking into account the available synthetic aperture radar (sar) data. 2. gps data we compute weekly network solutions for some 150 italian permanent gps stations using igs/euref processing standards (http://www.epncb.oma.be/_documentation/ guidelines/guidelines_analysis_centres.pdf ). the normal equations that have been available since 1999 are then stacked, minimum constraints are imposed on euref permanent network (epn) class a reference stations, and the time series are generated. the velocity map is updated weekly, which excludes stations with insufficiently long tracking logs [http://147.162.229.63/scidata/velocity_s.pdf]. this systematic procedure allows the deformation field in the national territory, in the sense of the average regional strain rate, to be continuously monitored and updated. in the case of seismic events, the procedure yields an immediate detection of coseismic offsets. an ad-hoc kinematical solution for selected sites is then run to investigate displacements at high frequency. figure 1 shows the vertical and horizontal coseismic offsets obtained for the may 20 and 29, 2012, events, and the corresponding time series. in both cases it clearly appears that the stations located south of the epicenters moved north (e.g. mops, sgip), and those north of the epicenter moved south (e.g. legn), as would be expected for a reverse fault with strike nearly ew. based on the routine daily analysis using double differences, we reprocessed selected baselines in kinematic mode, with output at 30 s. the tropospheric parameters and the ambiguities obtained in the daily processing were back-substituted into the kinematic solution, and 30 s estimates of the coordinates of selected stations were obtained in a time window centered on the events of may 20 and 29, 2012. to minimize effects of multipath and satellite geometry, the computation was repeated for the article history received july 19, 2012; accepted august 22, 2012. subject classification: crustal deformations, gps geodesy, insar, coseismic deformation. 2012 emilia earthquakes caporali and ostini 768 f ig u re 1 .t op : c os ei sm ic d is pl ac em en ts (l ef t: h or iz on ta l; ri gh t: v er ti ca l) o f th e pe rm an en t g p s st at io ns fo r th e m ay 2 0, 2 01 2 (b lu e ar ro w ) a nd m ay 2 9, 2 01 2 (r ed a rr ow ) e ve nt s. p a d o (r ed d ot ) w as o ne o f th e e p n c la ss a p er m an en t g p s st at io ns u se d as r ef er en ce in th e le as t s qu ar es a dj u st m en t. t he lo ca ti on o f th e ep ic en te rs is b as ed o n th e pr el im in ar y so lu ti on b y g fz (h tt p: // ge of on .g fz -p ot sd am .d e/ eq in fo /l is t. ph p) . b ot to m : t im e se ri es a t da ily in te rv al s of t he s ta ti on s sg ip a nd l e g n , w hi ch a re o n op po si te s id es o f th e fa u lt s an d w er e m os tl y af fe ct ed b y th e co se is m ic d is pl ac em en ts . t he a ss oc ia te d di sc on ti nu it ie s fo r th e m ay 20 a nd 2 9, 2 01 2, e ve nt s ar e m ar ke d by a g re en li ne . 769 earthquake geodesy of the emilia 2012 sequence f ig u re 2 . t im e se ri es o f l e g n a nd s g ip a t 30 -s t im e re so lu ti on , f or t he m ay 2 0 an d 29 , 2 01 2, e ve nt s. t he a rr ow s in t he s g ip t im e se ri es m ar k po ss ib le c re ep p ha se s of s om e 40 m in d u ra ti on b ef or e th e sh oc k. t he x ax is is in h ou rs u t c s in ce m id ni gh t of t he r es pe ct iv e da y. same time window one day earlier, with the same baselines and satellites, which generated a reference time series that was not affected by the seismic motion, but with very similar systematic effects related to the satellite geometry and multipath [bock 1991, choi et al. 2004]. these time series, shifted by 3 min 56 s, were subtracted from the time series centered at the epochs of the two seismic events, which resulted in a mitigation of high and low frequency noise (sidereal filter). figure 2 shows the time series of stations sgip and legn after the removal of the common noise. the high frequency changes of the nominal coordinates that were visible in the unfiltered time series are now removed, but it is as yet too early to conclude whether they are a geophysical signal or they are noise. the ramp in the north component of station sgip on may 20 and 29, 2012, is very clearly resolved. we speculate a sort of creep movement that initiated roughly 40 min before the respective shocks, but evidence based upon such small amounts of data cannot be claimed. 3. modeling to model the gps data, we used the coulomb 3.20 software developed by the u.s. geological survey [lin and stein 2004, toda et al. 2005], complemented with a module for numerical computation of partial derivatives, set-up for the normal equations and parameter adjustment by weighted least squares. the data available for the inversion are unfortunately few. the first check we did was to verify that a source model was relatively compatible with the observed horizontal displacements of the gps stations. this was based on the published fault-plane solution, on the empirical relations of wells and coppersmith [1994], and on the data published in the diss for the mirandola fault. we tested the option of north-verging faults, although it appears most logical from the geological sections that southverging faults are more likely. the fault in both events was initially considered a rectangle of length (l) × width (w) of 12 × 9 km, with an in-plane slip of 0.5 m, mostly in the reverse (up-dip) direction. this yielded a seismic moment of 1.62 × 1018 nm, or mw 6.07, assuming (µ = 30 gpa). however, the publication of the insar data based on the radarsat 1 and, in part, the cosmo skymed satellites [salvi et al. 2012, this volume] made it clear that the structures of both faults, and particularly the may 29, 2012, event, were somewhat different from that implied by the seismological data. in addition, a rather large uplift of some 0.12 m was reported in the central parts of the faults, which would have been incompatible with the original model, and in particular with a slip of only 0.5 m. we constrained the seismic moment to be equal to the seismologically derived value and started to investigate an alternative source model, with smaller l and w, and a slip in the range of 0.7 m to 0.9 m. the result is described in figure 3 (right), which shows that the gps data that is intended as the sum of the displacements of the may 20 and 29, 2012, events, still fits this new model. the vertical uplift along the strike of the two faults is consistent with that obtained from the insar data. the epicenters of the two main events, however, do not coincide with the centers of the faults, but are slightly offset to the east (may 20) and to the west (may 29). the measured gps displacements are systematically larger than the computed ones, perhaps due to monument instability, although in the same direction. figure 3 (right) also shows how the reverse faulting of the may 2012 events is consistent with the deformation regime implied by long-term gps data. the solutions independently presented in this volume by bignami et al. [2012] and serpelloni et al. [2012] appear qualitatively consistent with our proposed, preliminary solution. 4. conclusion the geodetic data available for the emilia 2012 seismic sequence are few but valuable, in that they can discriminate displacements attributable to the individual events of may 20 and may 29, 2012. the insar data are also of moderate quality. the radarsat 1 interferogram refers to the combined events, and the cosmo skymed only to the event of may 29. merging the gps and sar datasets appears to fill the mutual gaps and enables a reasonable source model to be outlined. taking into account the available information on the insar and cmt solutions, we conclude that the data are consistent with the elastic model defined in table 1. caporali and ostini 770 coordinates slip dip strike rectangular fault mw date longitude (˚e) latitude (˚n) right lateral (m) reverse (m) (˚) (˚) depth of lower corner (km) length (km) width (km) (ddmmyyyy) 11.35 44.83 0.19 0.85 35 118 12 10 6 6.06 20052012 11.20 44.83 0.12 0.70 35 91 11 9 6 5.97 29052012 table 1. central coordinates and characteristics of the rectangular faults, with implied moment magnitude for the two main events, assuming a shear modulus µ = 30 gpa. the 1 formal uncertainties are 0.05˚ for the geographical coordinates, 0.05 m for the slip vector, 3˚= for the dip and strike angles, 2 km for the depth, length and width. 771 earthquake geodesy of the emilia 2012 sequence f ig u re 3 .l ef t: b es tfit m od el o f th e g p s an d r ad ar sa t 1 (d es ce nd in g or bi t) d at a fo r th e to ta l d is pl ac em en t as so ci at ed w it h th e tw o m ai n ev en ts . c on to u rs (m m ) a re p re di ct ed fr om t he e la st ic m od el a nd s ca le d to ac co u nt f or t he li ne -o fsi gh t an gl e, a nd a re c om pa re d w it h th e pa tt er n of t he in te rf er om et ri c fr in ge s re su lt in g fr om t he p as se s of m ay 1 2 an d ju ne 5 , 2 01 2. r ed /b lu e ar ro w s re pr es en t co m pu te d/ m ea su re d di spl ac em en ts ( cu m u la ti ve f or t he m ay 2 0 an d 29 e ve nt s) o f th e pe rm an en t g p s st at io ns . d is pl ac em en t m ap c ou rt es y of t r e t el er ile va m en to e u ro pa . r ig ht : p ri nc ip al d ir ec ti on s of t he g eo de ti c st ra in r at e co m pu te d fo r ea ch o f th e 36 s ei sm ic z on es o f m el et ti e t al . [ 20 08 ] [ fr om c ap or al i e t al . 2 01 1] . c om pr es si on is in r ed a nd e xt en si o n in b lu e. t he r ec ta ng le s ho w s th e st u dy a re a fo r th e 20 12 s ei sm ic s eq u en ce in e m ili a. c m t s ol u ti on s ar e ta ke n fr om t he i n g v q u ic k c m t c at al og . acknowledgements. this study is supported by a research grant from the university of padova entitled monitoring from space and interpreting surface deformations in seismic areas. we are grateful to ufficio cartografico of regione veneto for the support to the stations of the permanent gps network used in this study. we thank dr. a. ferretti and ms. s. del conte for kindly making available the georeferenced insar displacement maps of radarsat 1. we thank dr. a. ganas and an anonymous reviewer for valuable comments which helped to improve the first draft of the manuscript. references bignami, c., p. burrato, v. cannelli, m. chini, e. falcucci, a. ferretti, s. gori, c. kyriakopoulos, d. melini, m. moro, f. novali, m. saroli, s. stramondo, g. valensise and p. vannoli (2012). coseismic deformation pattern of the emilia 2012 seismic sequence imaged by radarsat-1 interferometry, annals of geophysics, 55 (4); doi:10.4401/ag-6157. bock, y. (1991). continuous monitoring of crustal deformation, gps world, 2 (6), 40-47. burrato, p., f. ciucci and g. valensise (2003). an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting, annals of geophysics, 46 (5), 865-882. caporali, a., and s. martin (2000). first results from gps measurements on present day alpine kinematics, j. geodyn., 30 (1-2), 275-283 caporali, a., s. barba, m.m.c. carafa, r. devoti,g. pietrantonio and f. riguzzi (2011). static stress drop as determined from geodetic strain rates and statistical seismicity, j. geophys. res., 116, b02410; doi:10.1029/2010jb007671. carminati, e., d. scrocca and c. doglioni (2010). compaction-induced stress variations with depth in an active anticline: northern apennines, italy. j. geophys. res., 115, b02401; doi:10.1029/2009jb006395. choi, k., a. bilich, k.m. larson and p. axelrad (2004). modified sidereal filtering: implications for high-rate gps positioning, geophys. res. lett., 31, l22608; doi:10.1029/ 2004gl021621. dach, r., u. hugenthobler, p. friedez and m. meindl (2007). bernese gps software version 5.0, astronomical institute, university of bern, 612 pp. devoti, r. (2012). combination of coseismic displacement fields: a geodetic perspective, annals of geophysics, 55 (4); doi:10.4401/ag-6119. emergeo working group (2012). technologies and new approaches used by the ingv emergeo working group for real-time data sourcing and processing during the emilia romagna (northern italy) 2012 earthquake sequence, annals of geophysics, 55 (4); doi:10.4401/ag-6117. lin, j., and r.s. stein (2004). stress triggering in thrust and subduction earthquakes, and stress interaction between the southern san andreas and nearby thrust and strikeslip faults, j. geophys. res., 109, b02303; doi:10.1029/ 2003jb002607. meletti, c., f. galadini, g. valensise, m. stucchi, r. basili, s. barba, g. vannucci and e. boschi (2008). a seismic source model for the seismic hazard assessment of the italian territory, tectonophysics, 450, 85-108. okada, y. (1985). surface deformation due to shear and tensile faults in a half-space, b. seismol. soc. am., 75, 11351154. pierdominici, s., and h. heidbach (2012). stress field of italy — mean stress orientation at different depths and wavelength of the stress pattern, tectonophysics, 532-535, 301311; doi:10.1016/j.tecto.2012.02.018. salvi, s., c. tolomei, j.p. merryman boncori, g. pezzo, s. atzori, a. antonioli, e. trasatti, r. giuliani, s. zoffoli and a. coletta (2012). activation of the sigris monitoring system for ground deformation mapping during the 2012 emilia seismic sequence, using cosmo-skymed insar data, annals of geophysics 55 (4); doi:10.4401/ag-6181. scrocca, d., e. carminati, c. doglioni and d. marcantoni (2007). slab retreat and active shortening along the central-northern apennines, in: o. lacombe, j. lavé, f. roure and j. verges (eds.), thrust belts and foreland basins. from fold kinematics to hydrocarbon systems, frontiers in earth sciences, springer, 471-487. serpelloni, e., l. anderlini, a. avallone, v. cannelli, a. cavaliere, d. cheloni, c. d'ambrosio, e. d'anastasio, a. esposito, g. pietrantonio, a.r. pisani, m. anzidei, g. cecere, n. d'agostino, s. del mese, r. devoti, a. galvani, a. massucci, d. melini, f. riguzzi, g. selvaggi and v. sepe (2012). gps observations of coseismic deformation following the may 20 and 29, 2012, emilia seismic events (northern italy): data, analysis and preliminary models, annals of geophysics, 55 (4); doi:10.4401/ag-6168. toda, s., r.s. stein, k. richards-dinger and s. bozkurt (2005). forecasting the evolution of seismicity in southern california: animations built on earthquake stress transfer, j. geophys. res., 110, b05s16; doi:10.1029/2004jb003415. wells, d.l., and k.j. coppersmith (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, b. seismol. soc. am., 84, 974-1002. *corresponding author: alessandro caporali, università di padova, dipartimento di geoscienze, padova, italy; email: alessandro.caporali@unipd.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. caporali and ostini 772 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true /maxsubsetpct 100 /optimize false /opm 1 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol. 48, 01, 05ok.qxd 33 annals of geophysics, vol. 48, n. 1, february 2005 key words poas volcano – geochemistry – radon – precursor 1. introduction the historical activity of the poas volcano (costa rica) was observed and reported throughout the last century, showing periods of strombolian activity in 1910 and 1953-1955. phreatic eruptions of geyser type, with plumes of water, gravel, mud and blocks originating from the principal crater lake were also reported (barquero et al., 1981; casertano et al., 1983). since radon-in-soil fluctuations and chemical variations observed in water bodies associated with active volcanoes may give information concerning the activity changes (matsuo et al., 1982; de la cruz reyna et al., 1985, 1989; notsu et al., 1991; brondi and dall’aglio, 1991; d’alessandro and parello, 1994; dall’aglio et al., 1994; tedesco and pece, 1996), a radon-insoil survey was initiated in 1981 and a systematic monitoring of the chemical composition of the hot water lake has also been implemented since 1983. in the present paper, we report the radonin-soil behaviour together with the geochemical data obtained from the water sampling as a function of the volcanic activity and of its environmental effects during the 1981-1994 period. 2. poas volcanic activity the poas volcano (10.20°n, 84.23°w) is one of the main active volcanoes of the central range of costa-rica, rising up to 2708 m a.s.l. (fig. 1a). it is located in the n.e. part of the central valley where the most populated cities of the country are also found. the morphology of the summit presents two calderic structures with several cones such as von frantzius to the north and botos to the south, where a cold freshwater lake is found. prominent features of the active crater include a plain of eroded pyroclastic material to the south and a 30 m high cone formed during the 19531954 eruptions. the core of the cone consists of mailing address: dr. michel monnin, hydrosciences, université montpellier 2 (um2) cnrs, 34095 montpellier cedex 05, france; e-mail: monnin@msem.univmontp2.fr water chemistry and soil radon survey at the poas volcano (costa rica) jorge barquero (1), erick fernandez (1), michel monnin (2), jean luc seidel (2) and nuria segovia (3) (1) observatorio vulcanológico y sismológico de costa rica, universidad nacional (ovsicori-una), heredia, costa rica (2) hydrosciences, université montpellier 2 (um2) cnrs, montpellier, france (3) instituto nacional de investigaciones nucleares (inin), méxico d.f., méxico abstract radon-in-soil monitoring at the poas volcano (costa rica) has been performed together with water chemistry from the hot crater lake since 1981 and 1983 respectively. the results are discussed as a function of the eruptive evolution of the volcano over a 13 years period (1981-1994). it is shown that no definitely clear precursory radon signals have been recorded. on the contrary, ionic species concentrations are likely to be considered good precursors, together with the temperature variations of the crater lake water. 34 jorge barquero, erick fernandez, michel monnin, jean luc seidel and nuria segovia fractured lava of basaltic-andesitic composition; the northern portion of the cone is truncated by a pit crater which contains the acidic crater lake (rowe et al., 1992a,b). the activity of the volcano during the in-soil radon monitoring period showed a quiescence phase since 1979 which lasted for several years. in june 1987, phreatic eruptions started and continued during the remaining part of the year, characterising a new activity phase of the volcano. the eruptions originated from the central and western part of the crater lake, ejecting water, sediments and gas columns. the water temperature rose from 58°c in january 1988 to 70°c in june. fig. 1a,b. location map of poas volcano; radon network. a the lake level decreased by 5 m during 1987. in april 1988, the ejecta column reached more than one kilometre height and the ejected materials affected particularly the southern flank. the lake level decreased by almost 40 m and the temperature fluctuated between 65 and 85°c. fumaroles were also observed at the lake bottom with temperatures ranging from 75 to 98°c. gas emission increased and the acidic rain affected areas of local economic importance such as coffee, grass, and other plantations. the sporadic phreatic activity continued during the year at a lower degree. from april 1989, the activity intensified and the formation of a cone in the crater took b 35 water chemistry and soil radon survey at the poas volcano (costa rica) place with the ejected material. the crater lake disappeared at the end of the same month. on april 22th an eruption projected solid material 600 m away from the crater while on april 28th the sediments ejected from the lake bottom reached 2.5 km along the southern flank. on april 30th ash eruptions started and by may 1st the ash column reached one kilometre height. the crater lake recuperated during the rainy season. a phreatic eruption occurred at the end of may destroying the lake cone. from june 1989 to early 1990, the gas emanations were permanent with sporadic phreatic eruptions. the crater lake disappeared again in april 1990. eruptive activity was less intense than during the previous year, although ash falls reached 11 km on the southern flank around may 1990, affecting the cities of san miguel, san luis, grecia and trojas. gas venting was observed at different locations of the crater with temperatures ranging from 90°c to 550°c. from 1990 to 1994 the activity continued at a lower rate, with the hot lake almost disappearing during the dry seasons. 3. experimental 3.1. radon measurements originally a radon-in-soil network consisting of 7 monitoring stations was set up in 1981, while two new stations were added in 1986 (fig. 1b). the monitoring has been carried out using solid state nuclear tracks detectors, lr-115 type 2 from dosirad co., france. the technique for exposure and detectors processing has been previously described (seidel et al., 1988). however, in brief, the set-up consists of a 1m long pvc pipe inserted in the ground which received a shorter 30 cm tube with an open bottom. its upper bottom is closed and holds inside itself the polymeric rn detector facing down. this shorter tube plays the role of a diffusion chamber. the remaining 70 cm upper part of the pvc tube is thermally insulated with rockwool and closed. by this means one measures actually the local radon flux accross the earth, which results after a while in a specific concentration within the diffusion chamber. hence, data are expressed in kbqm–3. the detectors were recovered essentialtable i. average radon-in-soil concentration values (x+2σ/n) and uranium content in soil at the 9 monitoring stations. station radon uranium rn/u (kbqm–3) (ppm) a.u. s1 7.8 ± 0.4 7 1.11 s2 1.5 ± 0.1 6 0.25 s3 0.96 ± 0.1 10 0.10 s4 1.9 ± 0.2 10 0.19 s5 1.0 ± 0.2 10 0.10 s6 1.4 ± 0.1 10 0.14 s7 1.5 ± 0.1 7 0.21 s8 1.6 ± 0.3 10 0.16 s9 3.1 ± 0.3 – – ly on a fortnightly basis. however, due to logistic problems, e.g., temporary inaccessibility, some of the detectors were recovered after different periods of exposure. the detectors were processed by batches in a naoh solution and read by means of a jumping spark counter. 3.2. chemical and physical data since 1983, temperature and ph measurements of the hot water lake have been performed monthly, together with water samples collection. water samples were analysed for sulphates, chlorides and fluorides. chlorides were quantified with a combination electrode (model 93-17, orion) with a liquid membrane. the fluoride determination was performed with a specific electrode (model 96-07, orion) and a tissab buffer solution. sulphates were determined by a modified turbidimetric technique from the american public health association. the concentrations of the specific ions were quantified using analytical standard curves and the results are reported in ppm. the ph was measured with a beckman 39820 combination electrode. 4. results and discussions table i presents the average radon value and the 95% confidence limit (2σ/n), during the mon36 jorge barquero, erick fernandez, michel monnin, jean luc seidel and nuria segovia itoring period, and the uranium content of the soil at each station. regarding stations 2, 3, 4, 5, 6, 7, and 8, the average radon concentrations show small variations and their ratio to the local uranium contents (expressed in arbitrary units) keep within a rather narrow range (0.10 to 0.25), indicating that the measured radon is either supported by the uranium in soil content or that the additional rn flux is, on average, similar in intensity from point to point. this fact does not mean that rn variations are not correlated with the activity of the volcano since it is well known that even when rn signals are due to some deep phenomenon they show the modulation of in-soil rn which originates from a small volume around the measuring site and is not from deep origin. the average rn concentration as measured on these 7 stations is lower than 2 kbqm–3. fig. 2. raw radon time series and rainfalls from 1981 to 1994 for the whole stations. the two vertical lines indicate the periods of hot lake disappearence. 37 water chemistry and soil radon survey at the poas volcano (costa rica) on the contrary, on stations 1 and 9 the average is higher than 3 kbqm–3. in addition, for station 1 located at the edge of the crater, the rn/u ratio is substantially higher (1.11), indicating that an additional outgassing occurred on this site. u content on station 9, which is located in an altered zone, has not been measured. the crude data recorded on the nine stations are exhibited in fig. 2, together with monthly rainfall recorded during the same period of time. at first site it seems that a strong peak appears particularly on station 1 in correlation with the 1989 eruption. however, before being able to make any data analysis some corrections must be made. it has been observed by several authors (e.g., fleischer, 1997) that the efficiency of the type of detectors used in this survey may vary, especially with the exposure time (ageing of the detectors and moisture). in order to account for this behaviour, on every station, the detectors were separated in several groups according to exposure time: 8 to 14 days (the most abundant) taken as reference; 1 to 7 days, 15 to 21 days and longer than 21 days. the average radon concentrations measured on every site for the above mentioned periods of times during 13 years (which legitimates the procedure) were calculated for a given period of time, namely 7 days. they have then be compared and related to those measured during the 8-14 exposure times. the corresponding multiplicating correction factors are indicated in table ii. the data have been corrected accordingly, keeping in mind that the effect of ventilation (when changing the detector) is negligible with respect to the loss of detector efficiency. only relative variations as a function of time have significance as possible precursory signals of an eruption. accordingly, all the data were transformed, for the sake of comparison, under the form of centred and reduced values according to the well established formula (e.g., bry, 1995) zz z zi= v ) ^ h where < z> is the mean value and σz the standard deviation of a given set of zi values of the rn concentration. the same treatment was applied to rainfall data. the resulting rn concentration and rainfall curves are exhibited in fig. 3. the importance of the corrections carried out is clearly observed by comparing the two sets of data (figs. 2 and 3). on each set of curves, the 2σ horizontal line (which is commonly accepted as the criterion for an «anomalous» deviation) has been drawn. the two vertical lines show the time of the two lake disappearances in 1989 and 1990. for instance, if one focuses on the period of quiescence before 1989, several rn peaks above the 2σ line are present. among them, one is particularly «strong» around the end of 85-beginning of 86, on stations 3 and 4. after performing the correction, the «anomaly» on station 3 has completely disappeared. another possible correlation can be observed in december 1984 at stations 2, 3, 4, 5, 6, 7. on all stations the anomalous peaks are still present afer corrrection. yet, they all occur at a time when the volcano was particularly quiet according to the observations of the volcanological and seismological observatory of costa rica (barquero ed., 1983/1987) and our own temperature and chemistry measurements (see below). over most of the sites atmospheric temperature and pressure variations do influence the in-soil rn concentration. before going further in the data analysis and discussion, it must be pointed out that it is not likely in this case since the environmental temperature (19.2 ± 0.8°c) and the atmospheric pressure (779 ± 0.66 mm hg) are quite constant during the year, with very small night-to-day variations. a fast fourier transform analysis of the table ii. correction coefficient calculated for each station, in function of the detectors exposure time. exposure s1 s2 s3 s4 s5 s6 s7 s8 s9 1-7 days 0.31 0.58 0.34 0.34 0.23 0.38 0.38 0.49 0.43 8-14 days 1 1 1 1 1 1 1 1 1 15-21 days 1.22 1.20 1.32 1.41 1.20 1.20 1.13 1.06 1.19 38 jorge barquero, erick fernandez, michel monnin, jean luc seidel and nuria segovia radon time series shows a 365 days periodicity peak that corresponds to the precipitation pattern of the zone (rainy season from may to november and dry season from january to april). in general the rainy season exhibits the lowest radon-in-soil concentrations due to the water presence in the pores of the upper part of the soil. a radon increase occurs during the dry season for all the stations. the conclusions that can be drawn from this data analysis are: i) most probably, none of the «anomalous peaks» recorded during the present survey have any relation with volcanic activity. ii) rn data from lr-115 type ssntd detectors must be corrected for detection efficiency. iii) rn concentrations measured on a longterm basis are mostly governed by rainfall. iv) if rn precursory signals are to be found on such volcanic edifices, they must be sought using a short term rn measuring probe such as the fig. 3. corrected radon time series and rainfalls from 1981 to 1994 for the whole stations. 39 water chemistry and soil radon survey at the poas volcano (costa rica) one devised by monnin et al. (1998). as a matter of fact such a device takes care of secondary effects such as external atmospheric parameters and it also detects short-term rn outbursts that are likely to occur on volcanoes (garcia et al., 2000). nevertheless, if one focuses on the period when the volcano entered an active phase until it went into a quieter period, strong anomalous peaks can be observed at stations 1, 2, 3, 4, 5, 6, 7, 8 and 9. these peaks could be governed by rainfall but we would like to point out that all stations responded to the increase in activity of the volcano and also that the amplitude of the peaks are substantially higher than «dry-season» induced peaks observed previously. it could be said that both phenomena, the drying of the soil and the volcanic activity both contributed to the excess rn release. figure 4 shows the evolution of the hot lake water temperature during the 1983-1993 period. from january 1983 to june 1986, the temperature was quite stable (around 49°c). from july 1986, the temperature increased, reaching a maximum (94°c) in july 1990. seismicity also correlates with the eruptive pattern, increasing and reaching a maximum during the 1989-1991 period. the variations of so42–, cl– and f– concentrations of the hot crater lake water samples are shown in fig. 5. the results obtained by other fig. 5. variations of so42 –, cl – and f – concentrations. fig. 4. temporal variations of crater lake temperature (°c) and local seismicity. 40 jorge barquero, erick fernandez, michel monnin, jean luc seidel and nuria segovia investigators (rowe et al., 1992a,b) are also shown in the same figure. having such a double set of data covering the same period (november 1984-september 1990) and extending sideways for our data to january 1983 and to march 1993 allows us to draw some conclusions. from the end of 1984 to the beginning of 1988, the two sets of data are remarkably similar regarding in both concentration values and the behaviour of the ions. the two surveys confirm each other. so42–, cl–, f– and temperature measurements exhibit a stable behaviour from january 1983 to the end of 1986, with a lag time of 6 months with respect to the temperature increase which started in march 1987. a sharp increase then occurred by mid-1986 for all the ionic species. this behaviour is equally observed in rowe and co-workers’ report and in the present work. the highest concentrations were measured during the first lake disappearance in april 1989. then if the general behaviour is similar to rowe and co-workers’ report and ours, large discrepancies are observed in concentrations values. in other words, our chemical analysis data are affected by a large dispersion. these differences can be attributed to the mere fact that at this time the distribution of the ionic species in the lake was perfig. 6. so42 –/cl – ratio and f –/cl – ratio. turbed and also because sampling was not performed exactly at the same place, or at the same times by the teams of investigators. what remains nevertheless and, as a common finding, is that: – regarding sulphates: there is an increase that reaches its maximum before the april 1989 eruption and lake disappearance; followed by a general decrease till the beginning of 1991 i.e. 6 months after the second eruption and lake disappearance. our data continue and show another increase starting in march 1991, reaching a maximum in february 1992 to decrease to a very low value at the end of the survey at the beginning of 1993. – regarding chlorides: there is an increase, more marked at our samples than at those of rowe and his co-workers, but at the same time, with its maximum by mid-1988, followed in both cases by a decrease reaching its lowest values right before the 1989 eruption. then again the chloride concentration increases, all the way after the second eruption. then, our data show a slow decline of the cl concentration till 1993. – regarding fluorides: we again observe values that are higher than those of rowe and his co-workers but that exhibit the same behaviour i.e. an increase reaching maximum values between the two eruptions, followed by an erratic 41 water chemistry and soil radon survey at the poas volcano (costa rica) decrease, then, as in the case of chlorides, a maximum but finally ending in an erratic distribution up to 1993. out of these results it could be said that the eruptive phase of the poas volcano started in depth, when the temperature of the lake water started to increase 3 years before the actual eruption. the ionic activities in the lake confirm the start of the eruptive phase, the ion species being brought up to the level of the lake, as was suggested by rowe et al. (1992, 1995), by the circulation of a hydrothermal fluid convection. however, when one looks at the chloride based element ratios, it seems that it cannot be concluded that a so42 – peak occurred before the 1989 eruption. figure 6 displays the so42 –/cl– ratio values obtained by the two teams are displayed. the peak observed by rowe and his coworkers does not show on our data, or at least is barely marked (one point that looks more as being out of the statistics that having a genuine meaning). regarding the f–/cl– ratio (fig. 6), the behaviour is more in agreement in the two surveys, yet with a difference of two months at least for the time of maximum value. however, we observe a comparably high ratio long after the second eruption, in 1992 i.e., 2 years later after a time when the volcano was going into a period of quiescence. in the specific case of the poas volcano and for this very eruptive phase, it is tempting to conclude that the precursory signal of the volcanic activity is the increased concentration of the ionic species, which is enhanced by the decrease of the lake volume, as rowe et al. pointed out. this decrease of the lake volume is due itself (brown et al., 1989) to the evaporation from the lake surface caused by an increase in the power output attributed to the emplacement of a small magma intrusion inferred from microgravity measurements (rymer and brown, 1989). however, because of the large dispersion of the data, one cannot really rely on the chloride based ratios to estimate the imminence of the eruption. 5. conclusions the radon survey reported in this paper shows that radon data should be taken with great care. after appropriate corrections, this survey neither shows nor allows us to conclude positively that any precursory signal was observed at the measuring stations prior to the eruptive phase 1989-1990. it could have been expected that some rn excess could show at the stations located within the area of acidic rain since radon and hcl have similar properties regarding their solubility and water/air fraction ratios. it is not the case. rn activities are similar in all the stations including station 6 which was located out of this area and which does not show any rn depletion with respect to the other stations that were included within the area of acidic rain. it seems that long exposure rn measurement by means of ssntd is not suitable for this type of survey and that automatic short term rn measurements should be preferred. the hydrochemical survey shows that an increase in all the species measured is definitely related to the volcanic activity but that the chloride-based ratios should be taken with great care because of the large dispersion of the data. acknowledgements part of this project was performed under iaea and cnrs (france)-conacyt (mexico) scientific agreements. references barquero, j., e. malavassi and a. araya (1981): the poas volcano activity during the 1978-1980, bol. vulcanol. (escuela de ciencias geograficas, univ. nacional), 11,10-12 (in spanish). brondi, m. and m. dall’aglio (1991): evolution of mercury, arsenic, antimony, radon and helium contents in ground waters and fumaroles since 1983 through 1989 at volcano island (southern italy), volcanol. acta, 1, 233-241. brown, g.c., h. rymer, j. dowden, p. kapadia, d. stevenson, j. barquero and l.d. morales (1989): energy budget analysis for poas crater lake: implications for predicting volcanic activity, nature, 339, 370-373. bry, x. (1995): analyse factorielle simple (economica ed., paris), 1-112. casertano, l., a. borgia and c. cigolini (1983): the poas volcano, costa rica: chronology and activity, geofis. int., 22 (3), 215-236 (in spanish). d’alessandro, w. and f. parello (1994): soil degassing from the lower flanks of mt. etna, environ. geochem. health, 16 (suppl.), 189-201. 42 jorge barquero, erick fernandez, michel monnin, jean luc seidel and nuria segovia dall’aglio, m., f. pagani and f. quattrocchi (1994): geochemistry of the groundwaters in the etna region before and after the paroxysmal phase of the eruption of december 1991: implications for the geochemical surveillance of mt. etna, acta vulcanol., 4, 149-156. de la cruz reyna, s., m. mena, n. segovia, j.f. chalot, j-l. seidel and m. monnin (1985): radon emanometry in soil gases and activity in ashes from el chichon volcano (mexico), pure appl. geophys., 123, 410-421. de la cruz reyna, s., m.a. armienta, v. zamora and f. juarez (1989): chemical changes in spring waters at tacana volcano, chiapas, mexico: a possible precursor of the may 1986 seismic crisis and phreatic explosion, j. volcanol. geotherm. res., 38, 345-353. fleischer, r.l. (1997): radon and earthquake prediction, in radon measurements by etched track detectors, (world scientific press, singapore), 285-300. garcia, r., g. natale, m. monnin and j.l. seidel (2000): shock wave radon surface signals associated with the upsurge of t-p solitons in volcanic systems, j. volcanol. geotherm. res., 96, 15-24. matsuo, s., j. ossaka, j. hirabayashi, t. ozawa and k. kimishima (1982): chemical nature of volcanic gases of the usu volcano in japan, bull. volcanol., 45 (3), 261-264. monnin, m.m. and j.l. seidel (1998): an automatic radon probe for earth sciences studies, j. appl. geophys., 39, 209-220. notsu, k., h. wakita, g. igarashi and t. sato (1991): hydrological and geochemical changes related to the 1989 seismic and volcanic activities of the izu peninsula, j. phys. earth, 39, 245-254. rymer, h. and g.c. brown (1989): gravity changes as a precursor to volcanic eruptions at poas volcano, costa rica, nature, 342, 902-905. rowe, g.l, s.l brantley, m. fernandez, j.f. fernandez, a. borgia and j. barquero (1992a): fluid-volcano interaction in an active stratovolcano: the crater lake system of poas volcano, costa rica, j. volcanol. geotherm. res., 49, 23-51. rowe, g.l., s. ohsawa, b. takano, s.l. brantley, j. f. fernandez and j. barquero (1992b): using crater lake chemistry to predict volcanic activity at poas volcano, costa rica, bull. volcanol., 54, 494-503. rowe, g.l., s.l. brantley, j.f. fernandez and a. borgia (1995): fluid flow and mass transfer involving an acid crater lake at poas volcano, costa rica, j. volcanol. geotherm. res., 64, 233-267. seidel, j.l., m. monnin, a. bonneville, h. delorme, jf. lenat and n. segovia (1988): radon in soil measurements related to the activity of the piton de la fournaise volcano (reunion), nucl. tracks rad. meas., 15, 567-570. tedesco, d., r. pece and r. avino (1996): radon, ph and temperature monitoring in water wells at campi flegrei caldera (southern italy), geochem. j., 30 (2), 131-138. c o n v e g n i e c o n g r e s s i c o n v e g n o d e l l a « c o m m i s s i o n s é i s m o l o g i q u e e u r o p é e n n e » ( s t o c c a r d a 22-27 s e t t e m b r e 1952) d a l 22 al 27 s e t t e m b r e 1952 e b b e luogo, in s t o c c a r d a , presso il wiirtt. e r d b e b e n d i e n s t , u n convegno di sismologi e u r o p e i . la r e l a z i o n e sullo s v o l g i m e n t o e i r i s u l t a t i del convegno sarà a suo t e m p o p u b b l i c a t a a c u r a della « commission séismologique e u r o p é e n n e ». qui si r i t i e n e o p p o r t u n o p u b b l i c a r e s u b i t o , allo scopo di d a r l o r o la p i ù a m p i a d i v u l g a z i o n e , le 5 risoluzioni p r e s e a c h i u s u r a del convegno. eccole, n e l l ' o r d i n e resolution 1 c o n s t a t a n t q u e la m é t h o d e la plus s u r e p o u r la d é t e r m i n a t i o n précise de la p r o f o n d e u r des f o y e r s c o m p o r t e l ' u t i l i s a t i o n d ' e n r e g i s t r e m e n t s o b t e n u s au voisinage m é m e des é p i c e n t r e s , la c o m m i s s i o n séism o l o g i q u e e u r o p é e n n e r e c o m m a n d e la c r é a t i o n de n o u v e l l e s stations m u n i e s d ' a p p a r e i l s m o d e r n e s , p e r m e t t a n t la d é t e r m i n a t i o n des teinps d ' a r r i v é e s avec u n e précision d ' a u m o i n s u n d i x i è m e de seconde et installées en p r o x i m i t é i m m é d i a t e des zones d ' a c t i v i t é s é i s m i q u e . resolution 2 la c o m m i s s i o n s é i s m o l o g i q u e e u r o p é e n n e r e c o m m a n d e la constit u t i o n de calalogues n a t i o n a u x q u i c o m p o r t e r a i e n t p o u r c h a q u e séisme : il le c o o r d o n n é e s g é o g r a p h i q u e s de l ' é p i c e n t r e el les précisions avec lesquelles ces c o o r d o n n é e s sont d é t e r m i n é e s ; 2) l ' i n t e n s i t é m a c r o s é i s m i q u e m a x i m u m o b s e r v é e ; 3 | la s u r f a c e m a c r o s é i s m i q u e ; 4) p o u r les séismes du xxe siècle u n e i n d i c a t i o n de la magnit u d e du séisme ou à d é f a u t de la classe d ' a p r è s la définition de gut e n b e r g (classes d , e); u n e classe / p o u v e n t i n d i q u e r u n séisme inscrit p a r u n e seule s t a t i o n r a p p r o c h é e . d a n s c h a q u e pays on devra c h e r c h e r à é t a b l i r u n e c o r r e s p o n d a n c e e n t r e la m a g n i t u d e d é f i n i e d a n s l ' é c h e l l e gutenl)erg et r i c h t e r et les d o n n é e s microet m a c r o s é i s m i q u e s . 6 1 1 c o n v e g n i e c o n g r e s s i resoli!tion 3 c o n s t a t a n t q u ' o n p e u t a t t e i n d r e u n e m e i l l e u r e c o n n a i s s a n c e de la s t r u e t u r e du c o n t i n e n t e u r o p é e n p a r l ' i n s c r i p t i o n des reflexions sur les couclies p r o f o n d e s de l e c o r c e , la c o m m i s s i o n s é i s m o l o g i q u e e u r o p é e n n e r e c o n u n a n d e q u e des e x p é r i e n c e s m e t t a n t e n j e u des c h a r g e s explosives i m p o r l a n t e s s e r a i e n t organisées et e n r e g i s t r é e s d a n s des régions p a r l i e u l i è r e m e n t i n t é r e s s a n t e s , p a r e x e m p l e axe g r a v i m è l r i q u e négatif des alpes, h o h e t a u e r n , axe d ' a n o m a l i e m a g n é t i q u e du bassin de p a r i s eie.... u n e d é p e n s e t o t a l e de l ' o r d r e de 300.000 dollars serait n é c e s s a i r e p o u r r é a l i s e r u n e d o u z a i n e d ' e x p l o s i o n s ; la c o m m i s s i o n s é i s m o l o g i q u e e u r o p é e n n e d e m a n d e aux g o u v e r n e m e n t s d e c o n t r i b u e r à c e l t e é t u d e . resolutioii 4 la c o m m i s s i o n s é i s m o l o g i q u e e u r o p é e n n e r e c o m m a n d e l'institut i o n d ' u n service de d é l e r m i n a t i o n r a p i d e des é p i c e n l r e des séismes se p r o d u i s a n t en e u r o p e et d a n s les régions voisines. c b a q u e fois q u ' u n séisme p r o c b e i m p o r t a n t est e n r e g i s t r é à u n e d i s t a n c e é p i c e n t r a l e inf é r i e u r e à 5000 k i l o m è t r e s , les s t a t i o n s e u r o p é e n n e s et c i r c u i n médit e r r a n é e n n e s sont invitées à e n v o y e r p a r le i n o y e n la p l u s r a p i d e (tél é g r a m m e , poste a é r i e n n e ) a u b u r e a u c e n t r a l i n t e r n a t i o n a l de séismologie à s t r a s b o u r g le d é p o u i l l e m e n t p r o v i s o i r e de l e u r s séismogramines ou au m o i n s l ' h e u r e e x a c t e du p r e m i e r i m p é t u s avec la précision du d i x i è i n e de seconde si possible. le b u r e a u c e n t r a l d i f f u s e r a le p l u s r a p i d e m e n t possible la dél e r m i n a t i o n d ' é p i c e n t r e . resoliition 5 la c o m m i s s i o n s é i s m o l o g i q u e e u r o p é e n n e r e c o m m a n d e express é m e n t a u x s t a t i o n s séismologiques de n e pas d é t r u i r e les a n c i e n s enr e g i s t r e m e n t s , m é m e s'ils n e s e m b l e n t pas p r é s e n t e r d ' i n t é r é t a c t u e l . e l l e s p r e n d r o n t soin de c o n s e r v e r l e u r collection c o m p l è t e . si r i t i e n e di p a r t i c o l a r e interesse la r i s o l u z i o n e n. 3 ( p. c.ì. c o n v e g n i e c o n g r e s s i 615 a s s o c i a z i o n e g e o f i s i c a i t a l i a n a c o n v e g n o a n n u a l e p e r il 10-11 g i u g n o ii g i o r n o 13 f e b b r a i o 1953 presso la sede d e l l ' i s t i t u t o n a z i o n a l e d i geofisica in r o m a si è r i u n i t o il consiglio d i p r e s i d e n z a dell'a.g.l. t r a le a l t r e q u e s t i o n i sono state discusse a n c b e le s e g u e n t i : a) stato del b i l a n c i o ; b i d o m a n d e di n u o v i soci; c) p u b b l i c a z i o n e d e l l e m e m o r i e scientifiche p r e s e n t a t e al convegno d e l l ' a s s o c i a z i o n e del giugno 1952; d | convegno p e r la c o m m e m o r a z i o n e del p r o f . f. v e r c e l l i ; e) a t t i v i t à scientifica da s v i l u p p a r e n e l c o r r e n t e a n n o . i n r i g u a r d o a l l ' a t t i v i t à scientifica del cori-ente a n n o , il consiglio b a deciso di fissare p e r il 10 e 11 g i u g n o prossimo la d a t a del convegno a n n u a l e , con il d u p l i c e scopo d i c o m m e m o r a r e s o l e n n e m e n t e il c o m p i a n t o p r e s i d e n t e p r o f . f r a n c e s c o v e r c e l l i e di svolgere u n a riun i o n e p e r la t r a t t a z i o n e di p r o b l e m i scientifici. il consiglio, s t a n t e la b r e v i t à del t e m p o d i s p o n i b i l e e p e r il f a t t o c h e la s c o m p a r s a del p r o f . v e r c e l l i n o n b a permesso di p o t e r svolgere u n a a d e g u a t a p r e p a r a z i o n e , ha deciso di n o n fissare p e r il prossimo convegno scientifico u n p a r t i c o l a r e a r g o m e n t o da t r a t t a r e m a di lasciare a m p i a l i b e r t à ai soci d i p r e s e n t a r e e v e n t u a l i c o m u n i c a z i o n i s c i e n t i f i c h e su qualsiasi a r g o m e n t o di geofisica. è s t a t o q u i n d i fissato il s e g u e n t e o r d i n e del g i o r n o p e r il c o n v e g n o c h e si t e r r à in r o m a presso la sede d e l l ' i s t i t u t o n a z i o n a l e di geofisica. 10 giugno 1953 ore 9 consiglio di p r e s i d e n z a , o r e 10 a) e l e z i o n e del p r e s i d e n t e . b l e l e z i o n e dei d u e consiglieri, ci a m m i s s i o n e dei n u o v i soci, o r e i i a s s e m b l e a c o m m e m o r a t i v a del p r o f . f r a n c e s c o vercelli, ore 12 r i u n i o n e del n u o v o consiglio di p r e s i d e n z a p e r la delib e r a z i o n e sull'attività scientifica dell'associazione d a p r o p o r r e all'assemblea, ore 16 c o m u n i c a z i o n i scientifiche dei soci e relativa discussione. 11 g i u g n o 1953 ore 9 c o m u n i c a z i o n i s c i e n t i f i c h e dei soci, ore 11,30 oì r e l a z i o n e del s e g r e t a r i o sul b i l a n c i o finanziario dell'associazione. b i a p p r o v a z i o n e delle p r o p o s t e del consiglio eli presi6 1 6 c o n v e g n i e c o n g r e s s i denza r e l a t i v e a l l ' a t t i v i t à scientifica dell'associazione p e r l ' a n n o 1953-1954. c) v a r i e , e v e n t u a l i . ora da d e s t i n a r s i : g i t a sociale. sono i n v i t a t i a p a r t e c i p a r e al c o n v e g n o t u t t i i soci dell'a.g.i. e t u t t e l e p e r s o n e che, i n v i a n d o la l o r o adesione, r i c e v e r a n n o appositoi n v i t o dal consiglio di p r e s i d e n z a . il segretario p r o f . maurizio giorgi vol52,3,2009 417 annals of geophysics, vol. 52, n. 3/4, june/august 2009 key words ionospheric model – model assessment – model uses 1. introduction the nequick ionospheric model is based on a model (dgr from now on) introduced by di giovanni and radicella (1990). the original dgr model uses a sum of epstein layers to reproduce the electron density distribution in the ionosphere analytically. its formulation is such that the function and its 1st derivative are always continuous. the construction of the analytical function is based on «anchor» points related to the ionospheric characteristics routinely scaled from ionograms (fof2, m(3000)f2, fof1, foe). for this reason the dgr model is essentially a «profiler». the original dgr «profiler» can be used with experimental or modeled data of the ionospheric characteristics. an improved version of the original dgr introduced by radicella and zhang (1995) describes the topside f2 region by introducing a constant shape factor k that modifies the thickness parameter for that region and can be derived empirically by comparison with experimental vertical tec data. the improved version of the dgr profiler calculates vertical tec values. the modeling effort described above was part of the activities done under the scheme of cost 238 (prediction and retrospective ionospheric modeling over europe, prime). the improved dgr «profiler» was adopted by the action as part of its final product. a new family of electron density models, differing in complexity and whith different but related application areas, based on the dgr «profiler» concept has been developed in collaboration with the university of graz, austria. these models are particularly suited for the study of trans-ionospheric radiopropagation effects of interest to satellite navigation and positioning (hochegger et al., 2000; radicella and leitinger, 2001 ). the models are: nequick – a quick-run model for transionospheric applications; costprof – a model suited for ionospheric and plasmaspheric satellite to ground applications; the nequick model genesis, uses and evolution sandro m. radicella arpl, the abdus salam ictp, trieste, italy abstract the genesis and evolution of the nequick model is reviewed from the initial ionospheric efforts made in the framework of the european cost actions on ionospheric issues to the last version of the model (nequick 2). attention is given to the uses of the model particularly by the european satellite navigation and positioning systems egnos and galileo. recent assessment studies on the performance of nequick 2 are also reviewed. mailing address: dr. sandro m. radicella, arpl, the abdus salam ictp, strada costiera 11, 34151 trieste, italy: e-mail: rsandro@ictp.it vol52,3,2009 20-09-2009 19:06 pagina 417 418 s.m. radicella neuog-plas – a model suited for assessment studies involving satellite-to satellite propagation of radio waves. all three models give electron density as a function of height, geographic latitude, geographic longitude, solar activity (specified by the sunspot number or by the 10.7 cm solar radio flux), season (month) and time (universal time ut or local time lt). the models, like the original dgr, are continuous in all spatial first derivatives, particularly needed in applications like ray tracing and location finding. they also allow the calculation of electron density along arbitrarily chosen ray paths and the corresponding total electron content (tec). cost prof model was adopted in the final product of cost 251 action (improved quality of service in ionospheric telecommunication systems planning and operation). 1.2. the nequick model and its uses particularly successful was the development of the nequick model. to describe the electron density of the ionosphere above 100 km and up to the f2 layer peak this model uses a modified dgr profile formulation. a semi-epstein layer represents the electron density distribution in the topside with a height dependent thickness parameter empirically determined. the model has been adopted by the international telecommunication union, radiocommunication sector (itu-r) recommendation p. 531-6, now superseded by p. 531-9, (itu, 2007) as a suitable method for tec modeling. the nequick (fortran 77) source code is available at: http://www.itu.int/itu-r/software/studygroups/rsg3/databanks/ionosph/. the basic inputs of the nequick model code are: position, time and solar flux (or sunspot number); the output is the electron concentration at the given location in space and time. in addition the nequick package includes specific routines to evaluate the electron density along any ray-path and the corresponding tec by numerical integration. the original version of the nequick model has been used by the european geostationary navigation overlay service (egnos) of the european space agency (esa) for system assessment analysis. for such objective a series of ionospheric scenarios have been created under esa contracts to simulate realistic ionospheric conditions under disturbed conditions. these scenarios were done in collaboration with the university of graz. some of them were done also in collaboration with the rutherford-appleton laboratory of the uk. a series of additional uses of the original version of the model are known. it has been adapted by the rutherford-appleton laboratory of the uk to forecast vertical tec from forecasted values of fof2 and muf(3000)f2 (cander, 2003). it has been integrated also in the simulation toolkit developed in australia to conduct a qualitative assessment of the performance characteristics of the future gnss infrastructure (seynat et al., 2004). the model is used to calculate ionospheric delays in the raw data generation (rdg) capability of the galileo system simulation facility (gssf) (zimmerman et al., 2005). it was implemented in the global ionospheric scintillation model (gism) to calculate the background ionosphere (beniguel and ieea, 2004). a very important use of the nequick model is, without doubt, its adoption as the model for ionospheric corrections in the single frequency operation of the european galileo satellite navigation system (arbesser-rastburg, 2006). for its utilization by the galileo system the model will be driven by an «effective ionisation level» az, defined as follows: az(µ) = a0 + a1µ + a2µ2 where µ is the modip, a geomagnetic coordinate introduced by rawer (1983), and the coefficients a0, a1, a2 are broadcast to the user to allow az calculation at any wanted location. at system level, a set of worldwide distributed monitoring stations will be used to evaluate the slant tec needed to compute the values of the world wide parameters a0, a1, a2 which describe the global behavior of az for a given day. nequick model driven by the parameters broadcast during the following day is applied at user level to compute the slant tec along any ray-path satellite-receiver. vol52,3,2009 20-09-2009 19:06 pagina 418 419 the nequick model genesis, uses and evolution 2. nequick recent evolution the performance of the nequick model has been tested intensively and a series of improvements have been introduced in its original version. in «mapping» applications of nequick (construction of electron density grids at fixed heights below the f2 peak) strong gradients and strange structures appear at e and f1-layer heights in some cases. to eliminate these problems an elaborate revision of the original dgr modeling approach for the f1 and e regions was necessary (leitinger et al., 2005). the ionosphere topside profile in the original version of the nequick model is based on an empirically derived thickness parameter. two formulas are used, each of them valid for six months of the year. these formulae involve f2 peak parameters, bottom-side thickness and solar activity and were given in radicella and zhang (1995). to estimate empirically the thickness parameter formulation for the topside a large series of tec data derived from geostationary satellite radio signals recorded at florence and ionosonde data from rome were used in that early work. the possibility to derive the thickness parameter empirical formulation using newly available topside sounder data from the isis 2 satellite allowed introducing a substantial improvement in the topside profile of nequick. the profile is now characterized by a single empirical formula for the thickness parameter. such an improvement had important consequences also for the evolution of the international reference ionosphere (iri) electron density model. taking into account an outcome like that indicated in coïsson et al. (2006) and results like those shown in fig. 1 the iri in its last version, iri2007, adopted the nequick new formulation as the default option for its topside (http://omniweb.gsfc.nasa.gov/vitmo/iri_vitmo.html). bilitza and reinisch (2008) consider the new nequick topside electron density profile «the most mature of the different proposals for the iri topside». the improvements to the nequick model mentioned above and made during the cost 296 action, plus others related to the structure of the computational code itself, made possible to issue a new version of it: nequick 2. a full description of this version of the model and of the main differences with the previous version is given in nava et al. (2008). figure 2 shows an example of the electron density profile along the ray slant produced by the nequick 2 model. on-line nequick 2 calculation tool is available for demonstration purposes (http://arpl.ictp.trieste.it/nq-online/index.html). a detailed study done under contract with the european gnss supervisory authority (gsa) through france developpement conseil, have shown that, for gnss single frequency operation, nequick in its two versions performs better than the equivalent ica model used for gps operations. parameters a0, a1, a2 for a given day were computed from experimental slant tec values obtained in a set of worldwide distributed fig. 1. examples of comparisons among electron density topside profiles. red curves correspond to isis 2 experimentally derived profiles. green corresponds to iri-2000 profiles and light-blue are nequick 2 profiles. vol52,3,2009 20-09-2009 19:06 pagina 419 420 s.m. radicella «monitoring» stations. these parameters were used during the following day to drive the nequick model and compute the slant tec along any satellite to receiver ray-path. a specific nequick program adaptation was devised for that purpose. in the case of ica, the gps broadcast coefficients have been used to compute the slant tec on the same satellite to receiver links. to perform a global analysis of the model’s performance, a set of 27 test stations homogeneously distributed around the world has been chosen to cover all modip regions. the statistical analysis was performed through cumulative distributions of the absolute values of the mis-modelling, defined as the difference between an experimental and its corresponding modelled slant tec. it appears that in almost all cases nequick mis-modelling are smaller than the ica ones, reflecting the more realistic global behavior of nequick. figure 3 shows the daily values of the 95 percentile of the mis-modelling for the nequick 2 and the ica models from the cumulative distribution for the year 2000 of high solar activity. each daily value corresponds to the full set of 27 test stations. 3. independent evaluation of nequick 2 performance nequick 2 code has been requested by a large number of researchers from all over the world and its performance is being evaluated. a few examples of these evaluations follow. bidaine and warnant (2008) made done a comparative study of the two versions of the nequick model using high solar activity data from three ionospheric soundings stations in fig. 2. example of nequick 2 profiles and tec along ray-paths. different colors correspond to different path elevation angles. 90º means vertical profile and tec. vol52,3,2009 20-09-2009 19:06 pagina 420 421 the nequick model genesis, uses and evolution europe (dourbes, roquetes and el arenosillo) and vertical tec derived from gps receivers located near those stations. they concluded that the new formulation of the topside introduced in nequick 2 give rise to a clear homogenization of the statistics and to an improvement of the performance of the model. the paper shows that using the data obtained from the ionospheric soundings about 75% of total vertical tec correspond to the topside of the ionosphere, making evident the importance of the improvement in the topside formulation of the model. orus et al. (2007) analyzed the performance of the two versions of the nequick model when it is used in the galileo single frequency operations algorithm for ionospheric corrections. these authors assessed the model performance on a global scale for the whole year 2000 using different sets of experimental data for comparison. they concluded that the nequick 2 version of the model improves the mis-modelling in almost all latitudes. moreover, the new nequick version presents major reduction of the mis-modelling global bias, which leaves the bias for the whole year 2000 below 1 tecu (1 tecu = 1016 m2). in preparation for the deployment of the galileo system, the european space agency (esa) initiated the development of an overall galileo system test bed (gstb-v2) including two galileo in-orbit validation element (giove) satellites: giove-a (launched on 28th december 2005) and giove-b (launched 27th april 2008) with several objectives including ionospheric issues aspects. in this framework, orus and prieto-cerdeira (2008) made an assessment study of the galileo single frequency algorithms based on the nequick model using data from the giove-a experimentation campaign. the authors concluded that nequick 2 version of the model improves the ionospheric corrections calculation speed (due to the improvements on the computer code of the model) and reduces some of the mis-modelling in the topside ionosphere indicating again the value of the improvements introduced in the topside model formulation. fig. 3. 95 percentile (95% probability) of the cumulative distribution for global daily mis-modelling (difference between an experimental and its corresponding modelled slant tec) for the ica model (gray) and nequick 2 model (black). in the top part of the figure the corresponding samples used in each daily calculation is given. vol52,3,2009 20-09-2009 19:06 pagina 421 422 s.m. radicella 4. conclusions the groups of the abdus salam ictp aeronomy and radiopropagation laboratory and the university of graz institute of geophysics, astrophysics and meteorology have made important ionosphere modeling efforts in the framework of different ionospheric related european cost actions and specifically in cost 296 action. one of these efforts, nequick model, is at present widely recognized and used internationally. in particular, it has been utilized for assessment studies by the european egnos project and is also adopted in the single frequency operation algorithm for ionospheric corrections for the european galileo system for satellite navigation and positioning. references arbesser-rastburg, b. (2006): the galileo single frequency ionospheric correction algorithm, (3rd european space weather week, brussels). beniguel, y. and ieea (2004): global ionospheric scintillation model (gism) technical manual, v 5.1, itu-r rp 257 2004-10-03. bidaine, b. and r. warnant (2008): modelling the ionosphere over europe: investigation of nequick formulation, presented at the 5º european space weather week, (brussels). bilitza, d. and b. reinisch (2008): international reference ionosphere 2007: improvements and new parameters, adv. space res., 42 (4), 599-609. cander, lj. r. (2003): towards forecasting and mapping ionospheric space weather under cost actions, adv. space res., 31 (4), 957-964. coïsson, p., s.m. radicella, r. leitinger and b. nava (2006): topside electron density in iri and nequick: features and limitations, adv. space res., 37, 937-942. di giovanni, g. and s.m. radicella (1990): an analytical model of the electron density profile in the ionosphere, adv. space res., 10 (11), 27-30. hochegger, g., b. nava, s.m. radicella and r. leitinger (2000): a family of ionospheric models for different uses, physics and chemistry of the earth, part c: solar, terrestrial & planetary science, 25 (4), 307-310. itu-r (2007): recommendation itu-r p 531-9. leitinger, r., m.l. zhang and s.m. radicella (2005): an improved bottomside for the ionospheric electron density model nequick, annals of geophysics, 48 (3), 525-534. nava, b., p. coïsson and s.m. radicella (2008): a new version of the nequick ionosphere electron density model, journal of atmospheric and solar-terrestrial physics, doi:10.1016/j.jastp.2008.01.015. orus, r., b. arbesser-rastburg, r. prieto-cerdeira, m. hernandez-pajares, j.m. juan and j. sanz (2007): performance of different ionospheric models for single frequency navigation receivers, ibss-07 proceedings, (boston, 2007). orus, r. and r. prieto-cerdeira (2008): giove-a experimentation campaign: ionospheric related analysis, presented at navitec, (noordwijk). radicella, s.m. and m.l. zhang (1995): the improved dgr analytical model of electron density height profile and total electron content in the ionosphere, annali di geofisica, xxxviii (1), 35-41. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27 (1), 35-40. seynat, c., a. kealy and k. zhang (2004): a performance analysis of future global navigation satellite systems, journal of global positioning systems, 3 (12), 232-241. zimmermann, f., t. haak, e. steindl, s. vardarajulu and o. kalden (2005): generating galileo raw data – approach and application, presented at data systems in aerospace (dasia), (edinburgh, scotland). vol52,3,2009 20-09-2009 19:06 pagina 422 404 not found s u l l a s t r u t t u r a d e l l a c r o s t a t e r r e s t r e n e l m e d i t e r r a n e o c e n t r o o c c i d e n t a l e e n e l l ' a d r i a t i c o paolo emilio v a l l e premessa. — in ima n o t a p r e c e d e n t e (-1) è stato segnalato c h e la velocità di g r u p p o m e d i a delle o n d e superficiali m c h e si p r o p a g a n o a t t r a v e r s o il m e r i t e r r a n e o c e n t r o o c c i d e n t a l e è, a p a r i t à di p e r i o d o , sensibilmente più elevata della velocità di g r u p p o m e d i a delle anal o g h e o n d e c h e si p r o p a g a n o attraverso il c o n t i n e n t e europeo. p e r i n t e r p r e t a r e cpiesto f a t t o è stato r i t e n u t o sufficientemente app r o s s i m a t o il m o d e l l o di un mezzo ad u n a semplice stratificazione. sotto questa ipotesi ed assumendo la velocità m e d i a delle o n d e traversali nel p r i m o strato pari a 3,3 k m s e c 1 , la c u r v a di dispersione delle onde superficiali m c a l c o l a t a da jeffreys ( 2 ) , h a c o n d o t t o ad a t t r i b u i r e allo spessore medio dello s t r a t o s u p e r i o r e della crosta terr e s t r e o « s t r a t o del g r a n i t o », un v a l o r e di 14-15 k m nel m e d i t e r r a n e o c e n t r o o c c i d e n t a l e ed un v a l o r e c h e si aggira i n t o r n o ai 2 5 k m , in corrispondenza del c o n t i n e n t e europeo. i dati sperimentali da cui derivano questi risultati sono p e r ò alq u a n t o grossolani e t r o p p o scarsi, e quindi è stata f a t t a l a riserva di c o n d u r r e u l t e r i o r i r i c e r c h e . i n questa n o t a viene r i p o r t a t o uno studio sulla p r o p a g a z i o n e delle o n d e ' superficiali, o r i g i n a t e dal t e r r e m o t o j o n i c o del 22 aprile 1 9 4 8 , lungo tragitti m e d i t e r r a n e i e continentali. v i e n e t e n t a t a inoltre u n a i n t e r p r e t a z i o n e dei risultati, la quale è in buon a c c o r d o con lo studio p r e l i m i n a r e a cui si è a c c e n n a t o . la misura della velocità di gruppo delle onde superficiali. — l a m i s u r a della velocità di gruppo delle onde superficiali, r e l a t i v a ad un c e r t o periodo, viene eseguita dividendo la distanza e p i c e n t r a l e p e r un t e m p o t. r e c e n t e m e n t e l ' a u t o r e h a sviluppato u n a t e o r i a , c h e consente di d e d u r r e rigorosamente il t e m p o t dalla conoscenza dell'and a m e n t o della registrazione, della p e r t u r b a z i o n e iniziale e delle costanti strumentali. n o n è qui il caso di r i p o r t a r e l a t e o r i a , la quale p e r a l t r o è stata s u l l a s t r u t t u r a della crosta t e r r e s t r e nel mediterraneo centro-occ. 4 0 1 g i à esposta a b b a s t a n z a d e t t a g l i a t a m e n t e ( 3 ) , b a s t e r à r i c o r d a r e c h e se si a p p r o s s i m a il s i s m o g r a m m a c o n sinusoidi, si o t t e n g o n o notevoli s e m p l i f i c a z i o n i . è f a c i l e v e d e r e in questa a p p r o s s i m a z i o n e c h e , se il g r u p p o delle o n d e superficiali n o n è e c c e s s i v a m e n t e disperso e c i si l i m i t a alla m i s u r a della v e l o c i t à di g r u p p o r e l a t i v a a l p e r i o d o della sinusoide p e r il q u a l e il p r o d o t t o del p e r i o d o stesso p e r l ' a m p i e z z a è s e n s i b i l m e n t e p i ù g r a n d e di t u t t i gli a n a l o g h i p r o d o t t i c h e si possono e s e g u i r e n e l g r u p p o , il t e m p o t si o t t i e n e , c o n b u o n a a p p r o s s i m a z i o n e , s o t t r a e n d o il t e m p o o r i g i n e a l t e m p o di a r r i v o del c e n t r o della sinusoide. se poi il g r u p p o si p u ò a p p r o s s i m a r e c o n più sinusoidi dello stesso p e r i o d o e a m p i e z z a , il t e m p o t è d a t o d a l t e m p o di a r r i v o d e l e e n t r o del g r u p p o d i m i n u i t o del t e m p o o r i g i n e . a l m e r i a h m s l f i g . 2 ciò v a l e s o l t a n t o se si t r a s c u r a l ' a z i o n e d e l l o s t r u m e n t o e se l a d u r a t a della p e r t u r b a z i o n e i n i z i a l e è t r a s c u r a b i l e r i s p e t t o alla d u r a t a del passaggio d e l l ' o n d a p e r il p u n t o in cui v i e n e e f f e t t u a t a l a r e g i s t r a z i o n e ; in c a s o c o n t r a r i o s a r à n e c e s s a r i o a p p o r t a r e l e d o v u t e c o r r e zioni. i n g e n e r e p e r ò tali c o r r e z i o n i sono t r a s c u r a b i l i . velocità di gruppo media delle onde l nel mediterraneo centrooccidentale, nell'europa e nel mar adriatico, in corrispondenza del periodo t = : 2 0 s . — l o s t u d i o della p r o p a g a z i o n e delle o n d e superfic i a l i del t e r r e m o t o j o n i c o del 2 2 a p r i l e 1 9 4 8 , è s t a t o l i m i t a t o alle o n d e t r a s v e r s a l i t a n g e n z i a l i l, l e quali, d a t a la l o r o n a t u r a , r i c h i e d o n o , p e r essere i n d i v i d u a t e , b u o n e r e g i s t r a z i o n i delle sole c o m p o n e n t i orizzont a l i dello s p o s t a m e n t o . n e l l a quasi t o t a l i t à dei s i s m o g r a m m i u s a t i , q u e s t e o n d e si sono s u l l a s t r u t t u r a della crosta t e r r e s t r e n e l mediterraneo centro-occ. 105 p r e s e n t a t e n o t e v o l m e n t e s v i l u p p a t e e c o n l e c a r a t t e r i s t i c h e r i c h i e s t e p e r l a m i s u r a sufficientemente c o r r e t t a e s p e d i t a d e l t e m p o t, i n corr i s p o n d e n z a d e l p e r i o d o t = 20". l e figg. 2 e 3 m o s t r a n o due esempi di r e g i s t r a z i o n i i n osservat o r i c o n a z i m u t , r i s p e t t o a l l ' e p i c e n t r o , p r o s s i m o a j t / 2 . d a t o c h e l e c o s t a n t i s t r u m e n t a l i delle c o m p o n e n t i o r i z z o n t a l i sono q u a s i u g u a l i , l a p o l a r i z z a z i o n e è e v i d e n t e . i n u n a p r e c e d e n t e n o t a ( 4 ) è s t a t o a c c e n n a t o c h e il c e n t r o di u ? p s a l a 10h 5 0 m 0 2 » g f i g . 3 p e r t u r b a z i o n e è p o c o p r o f o n d o e sono s t a t e c a l c o l a t e l e c o o r d i n a t e e p i c e n t r a l i e il t e m p o o r i g i n e , p e r v e n e n d o a l s e g u e n t e r i s u l t a t o : lo = 20°, 4 8 + 0 ° , 0 4 q?o = 3 8 ° , 4 9 + 0 ° , 0 4 ( g e o g r a f i c a ) h0 = 1 0 " 4 2 m 4 0 s , 8 + 0 , 4 ( t . m . g . ) i n base a questi e l e m e n t i e ai t e m p i tj m i s u r a t i n e i s i s m o g r a m m i , sono s t a t e d e d o t t e l e v e l o c i t à di g r u p p o m e d i e c ; i n 1 4 stazioni. i val o r i o t t e n u t i sono r i p o r t a t i n e l l a t a b e l l a i, m e n t r e la fig. 1 m o s t r a l a d i s t r i b u z i o n e geografica delle stazioni e le g e o d e t i c h e c h e le c o l l e g a n o a l l ' e p i c e n t r o . p e r c a l c o l a r e da questi p r i m i dati la v e l o c i t à m e d i a di g r u p p o n e l m e d i t e r r a n e o c e n t r o o c c i d e n t a l e e n e l l ' e u r o p a , sono s t a t e v a l u t a t e l e p e r c e n t u a l i d e i t r a g i t t i nelle d u e zone, t e n e n d o c o n t o c h e p a r t e dei t r a g i t t i i n t e r e s s a n o il m a r e a d r i a t i c o e, in m i s u r a p e r ò m o l t o m i n o r e , il m a r e del n o r d . 4 0 4 paolo e m i l i o valle p e r t a n t o sono s t a t e c o n s i d e r a t e t r e z o n e : a ) m e d i t e r r a n e o c e n t r o o c c i d e n t a l e ; b) e u r o p a ; c ) m a r e a d r i a t i c o ( e m a r e d e l n o r d ) . l a p a r t e d e l t r a g i t t o r e l a t i v o alla s t a z i o n e di p a r i g i , c h e i n t e r e s s a l ' i t a l i a c e n t r o m e r i d i o n a l e è s t a t a assegnata alla zona c. s e si i n d i c a n o p e r ogni s t a z i o n e c o n p„ q„ ed rj l e p e r c e n t u a l i dei t r a g i t t i r i s p e t t i v a m e n t e n e l l e z o n e a, b, c, con ca, cb, ce, le relat i v e v e l o c i t à m e d i e di g r u p p o e c o n n il n u m e r o delle stazioni, si h a pl + 3l + j 1 [1] ('a cb ce e r i s u l t a 2 > i + < 7 i + r i = l ( i = 1, 2, ..., n) è c o n v e n i e n t e a s s u m e r e c o m e i n c o g n i t e v 1 1 1 a = y— —7— ca ' ( b ' ce in m o d o c h e le [ 1 ] d i v e n g o n o p.x+q.y+r-j^^[ 2 ] i l sistema di e q u a z i o n i [ 2 ] è s t a t o risolto col m e t o d o dei m i n i m i q u a d r a t i , in hase ai v a l o r i dei coefficienti p ; qit r-, c o n t e n u t i nella t a b e l l a i. i v a l o r i a p p r o s s i m a t i più p r o b a b i l i delle i n c o g n i t e , c o n i rispettivi e r r o r i m e d i , s o n o i s e g u e n t i x = 0 , 2 7 5 6 ± 0 , 0 0 1 5 y = 0 , 3 1 2 5 + 0 , 0 0 1 5 [ 3 ] z = 0 , 3 2 3 2 + 0 , 0 0 2 4 p e r t a n t o le v e l o c i t à m e d i e di g r u p p o delle o n d e l , r e l a t i v e al p e r i o d o t — 20% v a l g o n o : m e d i t e r r a n e o c e n t r o o c c i d e n t a l e ca =z 3 , 6 3 + 0 , 0 2 k m s e c — 1 e u r o p a cb = 3 , 2 0 ± 0 , 0 1 5 k m s e c " 1 m a r e a d r i a t i c o ( e m a r e del n o r d ) ce = 3 , 0 9 + 0 , 0 2 k m s e c " 1 l ' e f f e t t o della r i f r a z i o n e sui v a l o r i delle g r a n d e z z e ci, p,. q-, ed r, si p u ò r i t e n e r e t r a s c u r a b i l e . i n f a t t i p e r i t r a g i t t i a t t r a v e r s o l ' a d r i a t i c o , il c o n t i n e n t e e il m a r e del n o r d , l ' i n d i c e di r i f r a z i o n e è m o l t o p r o s s i m o a l l ' u n i t à , c o m e p u ò verificarsi c o n i dati del p r o s s i m o p a r a g r a f o , m e n t r e p e r i tra» s u l l a s t r u t t u r a della crosta t e r r e s t r e nel mediterraneo centro-occ. 107 g i t t i a t t r a v e r s o il m e d i t e r r a n e o e il c o n t i n e n t e , s e b b e n e l ' i n d i c e di rif r a z i o n e sia m e n o p r o s s i m o a l l ' u n i t à , gli angoli d ' i n c i d e n z a n o n sono t r o p p o g r a n d i e la p e r c e n t u a l e del t r a g i t t o m a r i n o p r e v a l e in m e d i a l a r g a m e n t e sulla p e r c e n t u a l e del t r a g i t t o c o n t i n e n t a l e . tentativo di prospezione profonda. — i v a l o r i della v e l o c i t à di f i g . 4 g r u p p o delle o n d e l nelle t r e zone, m o s t r a n o c h e la r e l a t i v a costituz i o n e d e l l a c r o s t a t e r r e s t r e è a b b a s t a n z a differente. v i è u n a c e r t a a m b i g u i t à n e l l ' i n t e r p r e t a z i o n e di q u e s t o f a t t o , anc h e p e r c h é n o n sono n o t e l e v e l o c i t à delle o n d e t r a s v e r s a l i spaziali sg e s* n e l l e due z o n e m a r i n e . si p u ò s u p p o r r e c h e gli s t r a t i s u p e r i o r i a b b i a n o spessori diversi nelle t r e zone, o p p u r e c h e a b b i a n o i n s i e m e spessori e c a r a t t e r i s t i c h e e l a s t i c h e diverse. p e r q u a n t o r i g u a r d a il m e d i t e r r a n e o c e n t r o o c c i d e n t a l e , è o p p o r t u n o r i l e v a r e c h e l e a l i q u o t e m e d i t e r r a n e e dei t r a g i t t i sono s t a t e val u t a t e in superficie, m e n t r e , c o m e m o s t r a n o l e sezioni b a t i m e t r i c h e a p p r o s s i m a t e i n fig. 4, le zolle c o n t i n e n t a l i si e s t e n d o n o s e n s i b i l m e n t e a n c h e a l d i s o t t o della superficie del m a r e . s e m b r a p e r c i ò p l a u s i b i l e 404 paolo e m i l i o valle a t t r i b u i r e l ' a u m e n t o m e d i o della v e l o c i t à delle o n d e l ad u n assottig l i a m e n t o m e d i o del p r i m o s t r a t o . p i ù d e t t a g l i a t e i n f o r m a z i o n i si s a r e b b e r o p o t u t e a v e r e m i s u r a n d o l a v e l o c i t à di g r u p p o i n f u n z i o n e d e l l a p r o f o n d i t à m e d i a dei v a r i t r a t t i dei t r a g i t t i delle o n d e superficiali n e l m a r e , m a u n a m i s u r a del g e n e r e a v r e b b e r i c h i e s t o u n e l e v a t o n u m e r o di r e g i s t r a z i o n i r e l a t i v e a l l a pen i s o l a i b e r i c a . p e r la z o n a a d r i a t i c a la differenza della v e l o c i t à è q u a s i c e r t a m e n t e d o v u t a a n c h e a v a r i a z i o n i delle c a r a t t e r i s t i c h e e l a s t i c h e d e l l o s t r a t o s u p e r i o r e . il m a r e a d r i a t i c o è p o c o p r o f o n d o , c o m e m o s t r a la sezioni bat i m e t r i c a l o n g i t u d i n a l e i n fìg. 4 . s e m b r a q u i n d i a b b a s t a n z a r a g i o n e v o l e r i t e n e r e c h e l e c a r a t t e r i s t i c h e e l a s t i c h e m e d i e d e l l a c r o s t a a l disotto del suo f o n d o , n o n siano m o l t o diverse d a l l e a n a l o g h e c a r a t t e r i s t i c h e r e l a t i v e a l l ' i t a l i a e alla j u g o s l a v i a . o r a è b e n n o t o (5~7) c h e in i t a l i a l a v e l o c i t à d e l l e o n d e t r a s v e r sali n e l l o s t r a t o s u p e r i o r e , a sud dell'asse della v a l l e p a d a n n a , è not e v o l m e n t e più bassa c h e n e l c o n t i n e n t e . l a c o s t i t u z i o n e della c r o s t a in e u r o p a n o n è a n c o r a sufficientem e n t e p r e c i s a t a , sebbene sia s t a t a o g g e t t o di n u m e r o s e r i c e r c h e , a l c u n e delle quali sono c i t a t e n e l l a b i b l i o g r a f i a ( 8 ~ 1 3 ) . u n a q u e s t i o n e a b b a s t a n z a c o n t r o v e r s a è c o s t i t u i t a dall'esistenza del s e c o n d o s t r a t o , c h e t a l u n i a u t o r i sostengono, m e n t r e a l t r i la neg a n o . u n r e c e n t e l a v o r o ( 1 3 ) , b a s a t o sulla d i s p e r s i o n e d e l l e o n d e l e sui d a t i d e d o t t i dai t e r r e m o t i p r o f o n d i , p o r t a alla c o n c l u s i o n e c h e n e l l ' e u r a s i a lo spessore del p r i m o s t r a t o è c i r c a 3 3 + 3 k m , m e n t r e lo spessore del s e c o n d o s t r a t o è p r e s s o c h é n u l l o . t e n e n d o c o n t o delle c o n s i d e r a z i o n i c h e sono s t a t e esposte e nell'ipotesi c h e l ' a z i o n e del s e c o n d o s t r a t o sulla p r o p a g a z i o n e delle o n d e l sia t r a s c u r a b i l e , si p u ò v a l u t a r e l o spessore del p r i m o s t r a t o n e l l e t r e zone. è chiaro che il risultato della valutazione avrà soltanto un carattere indicativo. p e r il c o n t i n e n t e ed il m e d i t e r r a n e o c o n v i e n e a s s u m e r e c o m e vel o c i t à delle o n d e sg il v a l o r e m e d i o n e l p r i m o s t r a t o , t r o v a t o p e r l ' e u r o p a c e n t r a l e ( n ) , ossia 3 , 3 3 k m s e c " 1 , il c u i e r r o r e m e d i o è m o l t o p r o b a b i l m e n t e i n f e r i o r e a + 0 , 0 5 k m s e c ~ p e r l ' a d r i a t i c o il v a l o r e 3 , 0 1 + 0 , 0 2 k m s e c — c h e c o s t i t u i s c e il r i s u l t a t o della p i ù r e c e n t e m i s u r a n e l l ' i t a l i a c e n t r a l e ( 1 4 ) . m e d i a n t e la c u r v a di d i s p e r s i o n e , c a l c o l a t a c o n l ' i p o t e s i c h e il r a p p o r t o t r a l e densità dello s t r a t o s u p e r i o r e e del m e z z o s o t t o s t a n t e s u l l a s t r u t t u r a della crosta t e r r e s t r e n e l mediterraneo centro-occ. 109 sia 4 / 5 e il r a p p o r t o delle r i g i d i t à p a r i a 9 / 2 0 ( i 5 " 1 6 ) , si o t t e n g o n o p e r lo spessore d dello s t r a t o s u p e r i o r e n e l l e t r e z o n e i v a l o r i c o n t e n u t i n e l l a t a b e l l a i i . t a b e l l a i i c d z o n e vsg k m m e d i t e r r a n e o c e n t r o o c c i d e n t a l e . 1 , 0 9 0 + 0 , 0 1 7 1 5 + 1 e u r o p a 0 , 9 6 1 + 0 , 0 1 5 ( 3 0 ) a d r i a t i c o 1 , 0 2 7 + 0 , 0 1 0 1 7 + 1 il v a l o r e r e l a t i v o al c o n t i n e n t e è m o l t o i n c e r t o p e r c h é il r a p p o r t o cb/vsg c o n d u c e in p r o s s i m i t à del m i n i m o della c u r v a di dispersione, c o m e si v e d e n e l l a fig. 5. gli a l t r i due v a l o r i sono i n v e c e affetti da u n e r r o r e m e d i o p i u t t o s t o p i c c o l o . t vsi) 404 paolo e m i l i o valle n e l q u a d r o del m o d e l l o assunto e d e i v a l o r i delle g r a n d e z z e fisiche c h e lo definiscono, è possibile a v e r e u n ' i d e a dell'effetto d e l l o s t r a t o s e d i m e n t a r i o . t a l e effetto si d o v r à r i t e n e r e t a n t o p i ù grande» q u a n t o m a g g i o r e è la p e r c e n t u a l e d e l l a densità d e l l ' e n e r g i a a s s o c i a t a a l l ' o n d a , in p r o s s i m i t à d e l p e r i o d o t =r 2 0 s e p e r gli spessori calcolati^ c h e si p r o p a g a n e l l o s t r a t o s u p e r i o r e . s o s t i t u e n d o i v a l o r i dei r a p p o r t i f r a l e v e l o c i t à di g r u p p o e l e v e l o c i t à delle o n d e t r a s v e r s a l i d e l l a t a b e l l a i i n e l l e r e l a z i o n i c o n t e n u t e in u n a t e o r i a p r e c e d e n t e m e n t e s v i l u p p a t a d a l l ' a u t o r e ( 1 7 ) , r i s u l t a c h e la p e r c e n t u a l e s u d d e t t a è c i r c a il 4 0 % p e r il m e d i t e r r a n e o , l ' 8 6 % p e r il c o n t i n e n t e e il 5 6 % p e r l ' a d r i a t i c o . p e r t a n t o è da r i t e n e r e c h e p e r la z o n a a d r i a t i c a e m e d i t e r r a n e a l'effetto d e l l o s t r a t o s e d i m e n t a r i o n o n sia s t a t o sensibile. u n a m i s u r a della v e l o c i t à di g r u p p o e s e g u i t a su o n d e di p e r i o d o m a g g i o r e , s a r e b b e s t a t a c e r t o più c o n v e n i e n t e , m a l e d i m e n s i o n i delle t r e z o n e s o n o t r o p p o l i m i t a t e p e r c h é la d i s p e r s i o n e e l ' a s s o r b i m e n t o possano mett e r e in e v i d e n z a o n d e di p e r i o d o m o l t o l u n g o . è o p p o r t u n o r i l e v a r e infine, senza p e r a l t r o e n t r a r e in u n a dettag l i a t a discussione, la q u a l e p o t r à e v e n t u a l m e n t e essere f a t t a in a l t r a sede, c h e l ' a s s o t t i g l i a m e n t o m e d i o dello s t r a t o m ° n o d e n s o n e l medi» t e r r a n e o c e n t r o o c c i d e n t a l e , d o v r e b b e c o n d u r r e a l l ' o s s e r v a z i o n e di anom a l i e positive della g r a v i t à , t e n u t o c o n t o della sua p r o f o n d i t à m e d i a e delle densità m e d i e del sial e del s i m a . e f f e t t i v a m e n t e le m i s u r e g r a v i m e t r i c h e s e m b r a n o c o n f e r m a r e questa p r e v i s i o n e ( 1 8 " s 0 ) . roma — istituto nazionale di geofisica — agofto 1951. riassunto mediante una misura della velocità media di gruppo delle onde l relativa al periodo di 2 0 s , si pone in evidenza il fatto che tale velocità è notevolmente più elevata nel mediterraneo centro-occidentale che nell'europa, mentre nel mare adriatico diminuisce leggermente. un tentativo di prospezione profonda nel mediterraneo centro-occidentale e nell'adriatico, mostra il notevole assottigliamento, rispetto all'europa, dello spessore medio del primo strato della crosta terrestre. b i b l i o g r a f i a ( j ) festa c . e valle p . e . : una valutazione dello spessore dello «strato del granito » nel mediterraneo centro occidentale. a n n a l i di geofisica i , 6 1 6 6 2 0 ( 1 9 4 8 ) . ( 2 ) jeffreys h . : the surface jfaves of earthquakes. g e o p h y s . s u p p l . i l i , 2 5 3 2 6 1 ( 1 9 3 5 ) . s u l l a s t r u t t u r a della crosta t e r r e s t r e n e l m e d i t e r r a n e o centro-occ. 111 ( 3 ) valle p . e . : sitila misura della velocità di gruppo delle onde sismiche superficiali. a n n a l i di geofisica i i , 3 7 0 3 7 6 ( 1 9 4 9 ) . ( 4 ) valle p . e . : il terremoto ionico del 22 aprile 1948. a n n a l i di g e o f i s i c a i v , 2 4 1 2 4 6 ( 1 9 5 1 ) . ( 5 ) caloi p . : caratteristiche sismiche dell'appennino tosco-romagnolo. r i c . s c i e n t . i , 2 1 8 2 3 4 ( 1 9 4 0 ) . ( 6 ) rosini e . : il terremoto della garfagnana del 15 ottobre 1939. r i c . s c i e n t . 7-8, 4 9 6 5 1 7 ( 1 9 4 0 ) . ( 7 ) giorgi m . e valle p . e . : contributo allo studio delle onde «ma. a n n a l i d i geofisica i , 87-102 ( 1 9 4 8 ) . ( 8 ) gutenrerc b . a n d richter c . f . : structures oj the crust. continents and oceans. p h y s i c s o f t h e e a r t h . v i i ( 1 9 3 9 ) . ( , j ) gutenberg b . : seismologictd evidence jor roots of mountains. b u l l . g e o l . s o c . a m . 5 4 , 4 7 3 4 9 8 ( 1 9 4 3 ) . ( 1 0 ) jeffreys h . : a jurther study oj near earthquake. g e o p h y s . s u p p l . i v , 196-225 ( 1 9 3 7 ) . ( h ) caloi p . : caratteristiche sismologiche fondamentali dell'europa centrale quali risultano dallo studio di 17 terremoti centro-europei. b o l l . s o c . s i s m . i t a l . x i , 41-72 ( 1 9 4 2 ) . ( 1 2 ) caloi p . : sullo spessore dello strato delle onde pg nell'europa centrale. r i c . s c i e n t . i , 3 3 4 3 3 8 ( 1 9 3 8 ) . ( 1 3 ) stoneley r . : the continental layers of europe. b u l l . s e i s m o l . s o c . a m . 3 8 , 2 6 3 2 7 4 ( 1 9 4 8 ) . ( 1 4 ) d i filippo d . e marcelli l . : uno studio sul terremoto del gran sasso d'italia del 5 settembre 1950. a n n a l i di geofisica i v , 2 1 3 2 4 0 ( 1 9 5 1 ) . (lr>) jeffreys h . : on the surface waves of earthquukes. g e o p h y s . s u p p l . i , 282-292 ( 1 9 2 5 ) . ( 1 ( t ) valle p . e . : effetti dell'attrito interno sulla propagazione delle onde di love. a n n a l i di geofisica i i i , 47-62 ( 1 9 5 0 ) . ( 1 7 ) valle p . e . : contributo allo studio delle onde di love. a n n a l i d i g e o fisica i i i , 2 3 1 2 5 0 ( 1 9 5 0 ) . ( 1 5 ) cassinis g . : la crociera gravimetrica del sommergibile « des geneys ». p u b b l . i s t . g e o d e s i a n . 47 ( 1 9 3 5 ) . ( 1 b ) coster h . p . : the gravity of the western and central mediterranean. ( 1 9 4 5 ) . ( 2 0 ) morelli c . : la rete geo fìsica e geodetica in italia nel suo stato attuale e nei suoi rapporti con la struttura geologica superficiale e profonda. p u b b l . i s t . n a z . g e o f . n. 121 ( 1 9 4 6 ) . an n als of geoph ysics, 60, fast track 7, 2017; d oi: 10.4401/ ag-7553 1 the cape tow n statement on geoethics giuseppe di capua, silvia peppolon i istitu to n azion ale d i geofisica e vu lcan ologia, italy iapg – in tern ation al association for prom otin g geoeth ics giu sep p e.d icap u a@in gv.it, silvia.p ep p olon i@in gv.it peter t. bobrowsky geological su rvey of can ad a – n atu r al resou rces can ad a iapg – in tern ation al association for prom otin g geoeth ics peter.bobrow sky@can ad a.ca abstract recently the interest by geoscientists in (geo)ethical aspects of geoscience knowledge, education, research, practice and communication has grown considerably. today the topic of geoethics has gained a significant visibility within the scientific community. the ia pg – international a ssociation for promoting geoethics (http://www.geoethics.org), founded in 2012, has worked to widen the discussion and create awareness about issues of ethics as applied to the geosciences. thanks to continuous voluntary work, the respectful exchange, and fruitful sharing of ideas, the ia pg community has produced a conceptual substratum on which to base the future development of geoethics, by clarifying the meaning of the word “geoethics”, formalizing its definition, and better identifying a framework of reference values on which the geoscience community can base more effective codes of conduct and guidance. the members of the ia pg community have pu blished various books and articles in peer-reviewed international journals, and organized numerous scientific sessions to bring geoethics to the most important geoscience conferences. geoethical issues have been also included in the european project en v ri-plus, which is dedicated to the environmental and solid earth research infrastructures. the tangible result of these efforts is that, now, many prestigious geoscience organizations recognize geoethics as a fundamental issue, worthy of attention. this result was confirmed by the high quality of content and the large participation of scientists in the six technical sessions and a panel session on geoethics organized by ia pg at the 35th igc – international geological congress, held in 2016 in cape town (south a frica). largely successful due to the cooperative work of different geoscience organizations (iugs-tggp – task group on global geoscience professionalism; gsl geological society of london; efg european federation of geologists; egs eurogeosurveys; a gi – a merican geosciences institute; a gu – a merican geophysical union, and a a w g – a frican a ssociation of w omen in geosciences). the ia pg considers the 35th igc as the scientific event that opened a new phase for furthering the concept of geoethics. in order to mark this milestone, the "cape town statement on geoethics" (ctsg) was tabled by the ia pg and reviewed in an international effort. it shall focus the attention of geoscientists on the development of shared values, policies, guidelines, strategies and tools, with the long-range goal of fostering the regular adoption of ethical values and practices within the geoscience community. the document summarizes the values, concepts, and contents developed by ia pg so far, providing a perspective for the future development of geoethical thinking. this paper addresses in detail the content of the “cape town statement on geoethics”, which is now supported officially by several geoscience organizations. 1. in trod uction n recen t years, th e geoscien ce com m u n ity h as sh ow n a grow in g in terest for geoeth ics. th e w ork carried ou t by th e iapg in its first five years of activity h as brou gh t forw ard ta n gible resu lts. th ey h ave facilitated th e grow th of th is em ergin g field of sch olarly in qu iry. th is in clu d es th e p u blication of n u m erou s books an d articles w ritten u n d er a p eer -review p rocess (lollino et al., 2014; pep poloni and di capu a, 2012 and 2015; pep poloni et al., 2017; wyss i an n als of geoph ysics, 60, fast track 7, 2017; d oi: 10.4401/ ag-7553 2 and pep poloni, 2014) and m ore than 70 events (scientific sessions, conferences, w orkshop s, sem inars, etc.) that have been organized w orld w id e (http:/ / w w w .geoethics.org/ events, accessed 30 october 2017. th e con sequ en ce is th at m ore g eoscien tists are n ow d iscu ssin g th e eth ical an d social im p lication s of th eir w ork (man su r et al., 2017; meller et al., 2017; stew art an d gill, 2017; stew art an d lew is, 2017; vascon celos et al., 2016) an d recogn ize geoeth ics as a tran sversal su bject in geoscien ces (bobrow sky et al., 2017). th an ks to th e in volvem en t of m an y geoscien tists, it h as been p ossible to p rovid e geoeth ics w ith a m ore solid con cep tu al su bstratu m (for a su m m ary see pep p olon i an d di cap u a, 2014 an d 2017; bobrow sky et al., 2017). startin g from th e etym ological an alysis of th e w ord s “geo” an d “eth ics”, a d efin ition of geoeth ics h as been elaborated , several geoscien ce issu es u p on w h ich geoeth ics can ap p ly its an alyses h ave been id en tified , an d a set of geoeth ical valu es h ave been p rop osed . th e latter offer a sort of com m on grou n d of reason in g to be u sed by geoscien tists com in g from variou s cou n tries, h avin g often h u ge d ifferen ces in th eir cu ltu ral exp erien ces, view s an d econ om ic p ersp ectives, alth ou gh n eed in g to sh are a com m on geoscien ce vision . after years of activity, w e are n ow able to trace a coh eren t p ath in th e th eoretical an d p ractical d evelop m en t of geoeth ics th at can stren gth en its cred ibility in th e geoscien ce com m u n ity. th e cap e tow n statem en t on geoeth ics (ctsg), released officially on october 2016, is a d ocu m en t th at d efin es a con cep tu al fram ew ork for th e stu d y of geoeth ics, an d p rovid es a first step to start to exp lore w h eth er geoeth ics cou ld evolve in to a n ew d iscip lin e, eith er w ith in ge oscien ces or w ith in eth ical scien ces. 2. the con text the ctsg w as prep ared d u ring the 35th intern ational geological congress, held in cape tow n (sou th africa) in au gu st-septem ber 2016, w here iapg organized six technical sessions and a p anel session on geoethics, w ith a large nu m ber of speakers and a d iverse and active au d ience (http:/ / w w w .geoethics.org/ 35th-igc, accessed 30 october 2017). th e d ocu m en t is th e p rod u ct of an in tern ation al effort, th e resu lt of a coop er ative w ork th at h as in volved geoscien tists from all over th e w orld d u rin g th e p eriod 2012 to 2016. th e ctsg’s aim is to focu s th e atten tion of geoscien tists on th e d evelop m en t of sh ared p olicies, gu id elin es, strategies an d tools, w ith th e lon g-ran ge goal of fosterin g th e w id esp read ad op tion of eth ical p ractices in th e ge oscien ce com m u n ity. discu ssion s d u rin g scien tific m ee tin gs, ed itorial p rojects an d oth er activities ca rried ou t u n d er th e u m brella of iapg h ave p r ovid ed th e fertile grou n d to circu late id eas an d to m ake m ore colleagu es aw are abou t th e n ecessity to give geoeth ics a coh eren t th eoretical fram ew ork stru ctu re. 3. why this document was n ecessary eth ical an d social im p lication s in geoscien ce research an d p ractice are becom in g key -asp ects in th e geoscien ce p rofession . geoeth ics sh arp en categories to face th e m an y issu es th at in volve both th e scien tific com m u n ities an d societies: in tellectu al freed om an d h on esty, resp on sibility, resp ect, p reven tion , ad ap tation , an d su stain ability (pep p olon i et al., 2015; pep p olon i an d di cap u a, 2016). in ord er to fram e th e ge oeth ics d iscu ssion , it w as im p ortan t to fix seve ral essen tial resu lts com in g from a coop erative effort in ren ew ed th in kin g abou t geoscien ce’s role in itself an d for society. th e p restigiou s in tern ation al geological con gress (igc), th at sin ce 1878 h as attracted geoscien tists from all over th e w orld , seem ed th e best even t to lau n ch a statem en t on geoeth ics as a p rod u ct of a scien tific com m u n ity w ith a h igh level tech n ical-scien tific kn ow led ge, bu t n ow m ore aw are also of its eth ical an d social resp on sibility in p racticin g geoscien ces. sin ce th e 34th in tern ation al geological co n gress, geoeth ics h as reach ed a m u ch h igh er level of visibility an d stan d in g. moreover, th e com m on sp ace of a broad in tern ation al n e tw ork of colleagu es an d organ ization s in volved in th e d iscu ssion on geoeth ical issu es an d th e h igh qu ality con ten ts in books an d article s u rged to p rop ose a su m m ary d ocu m en t, w h ich gives to th e geoscien ce com m u n ity a fram ew ork of referen ce on geoeth ics, ou tlin es ach ievem en ts, an d orien tates th e fu tu re d eve lop m en t of th e eth ical an d social d iscu ssion . an n als of geoph ysics, 60, fast track 7, 2017; d oi: 10.4401/ ag-7553 3 4. the ctsg structure th e ctsg is stru ctu red in to several section s (see ap p en d ix a). th e “pream ble” clearly in d icates th e overarch in g objective of th e en tire d ocu m en t. th e “in trod u ction ” ou tlin es both , th e con text in w h ich geoscien tists are n ow o p eratin g, an d th e p rem ises for follow in g a ge oeth ical ap p roach . th e section “defin ition of geoeth ics” d etails th e iapg form u la (origin ally a con tribu tion in wyss an d pep p olon i, 2014). th e form u la is based on th e term “eth ics” as in trod u ced by aristotle (384-322 bc), an d w as d efin ed w ith th e in volvem en t of sociologists an d p h ilosop h ers. th e section “pu rp ose” d elin eates th e scop e of geoeth ics, w h y it is esse n tial for th e geoscien ce com m u n ity an d society. th e p u rp ose to em brace geoeth ics is to im p rove th e qu ality of research an d p rofession al w ork an d th e cred ibility of geoscien tists, foster excellen ce in geoscien ces, gu aran tee su stain able ben efits for com m u n ities, p rotect local an d global en viron m en ts, an d en su re th e con d ition s for a h ealth y an d p rosp erou s d evelop m en t of fu tu re gen eration s. in th e section “fu n d am en tal valu es of geoeth ics” th e ctsg lists valu es to be con sid ered for an eth ical ap p roach to tech n ical an d scien tific p roblem s related to g eoscien ces. th ey regard both th e in d ivid u al an d th e social sp h ere of geoscien tists, in p articu lar th e fou r d im en sion s to con sid er in geoeth ical an alysis (self, colleagu es, society, an d en v iron m en t). th e keyw ord is “resp on sibility”, on e th at sh ou ld becom e th e fou n d ation of an y a ction of geoscien tists, in all th e d ifferen t field s of th eir activities (e.g. p rofession al, acad em ic an d research field s, ed u cation activity, p u blic e m p loym en t, in d u stry) (pep p olon i an d di cap u a, 2016). th e section “geoeth ical prom ise” co n tain s a sligh tly revised version of th e geoeth ical prom ise, an h ip p ocratic-like oath for geoscien tists, origin ally p rop osed by matteu cci et al. (2014). th e geoeth ical prom ise is on e w ay to em p h asize th e valu es th at you n g scien tists m u st keep in m in d d u rin g th eir geoscien ce career. it is also a tool for sen ior geoscien tists to re-con sid er id eals th at d irect th eir activities. th e section “fin al statem en t” closes th e d ocu m en t, u n d erlyin g fu tu re en viron m en tal an d social ch allen ges, en cou ragin g geoscien tists to raise th eir resp on sibility, an d stren gth en th e cred ibility of geoscien ces in ord er to secu re s ocietal tru st. th e fin al statem en t stresses th e n ecessity of th e d ocu m en t for th e geoscien ce com m u n ity in th e ligh t of global ch allen ges (en ergy, w ater, m in erals, georisks, p ollu tion , lan d d egrad ation ) th at geoscien tists face by s ociety. th e fin al version of th e ctsg, im p roved w ith com m en ts an d su ggestion s of review ers, w as p resen ted at th e 35th igc on 29 au gu st 2016, d iscu ssed at th e 1st iapg gen eral assem bly in cap e tow n on 31st au gu st 2016, th en ratified by th e iapg execu tive cou n cil on 26th october 2016 an d released officially in th e iapg w ebsite. th e ctsg is a referen ce d ocu m en t an d it is su p p orted by 18 geoscien ce organ ization s th u s far (h ttp :/ / w w w .geoeth ics.org/ ctsg, accessed 30 october 2017). in ord er to in crease its global im p act, th e statem en t w ill be tran slat ed in to several lan gu ages, th an ks to th e coop eration of d ozen s of colleagu es, as alread y d on e for th e mon treal statem en t on research in tegrity (pep p olon i, 2015). provid in g access in a n u m ber of lan gu ages m ean s w e can en h an ce th e cu ltu ral d iversity as an elem en t of u n ion , th rou gh w h ich all scien tists can sh are u n iversal valu es an d stren gth en th eir com m on id en tity in social d iversity. 5. con clusion th e ctsg sh all en cou rage a m ore resp on sible beh aviou r of p ractice in geoscien ces. it a d d resses firstly th e geoscien ce com m u n ity, bu t h as im p ortan t sign ifican ce also to society as a w h ole, as it syn th esizes som e valu es th at m u st gu id e h u m an action s w ith resp ect to th e earth system . th rou gh th e ctsg several im p ortan t m atters h ave been recom m en d ed th at m ay e n cou rage colleagu es to fu rth er th eir eth ical an d social role, to d u tifu lly foster excellen t scien ce an d a p rofession al service to th e society. man y colleagu es p rom ote a resp on sible w ay to ap p roach p roblem s in volvin g geoscien ce kn ow led ge an d to fin d reason able solu t ion s, based on accep table com p rom ises if n eed ed . re-d iscoverin g also an id eal d im en sion in th e geoscien ce p rofession m ean s to w ork m ore e ffectively for th e com m on good , goin g so b eyon d th e sim p le p erson al su ccess. an n als of geoph ysics, 60, fast track 7, 2017; d oi: 10.4401/ ag-7553 4 referen ces bobrow sky p., cron in v.s., di cap u a g., kieffer s.w., pep p olon i s. (2017). th e em ergin g field of geoeth ics. in : scien tific in tegrity an d eth ics w ith ap p lication s to th e geoscien ces, ed ited by l.c. gu n d ersen . sp ecial pu blication am erican geop h ysical un ion , joh n wiley an d son s, in c. lollin o g., arattan o m., giard in o m., oliveira r., pep p olon i s. (2014). en gin eerin g geology for society an d territory volu m e 7 “ed u cation , profession al eth ics an d pu blic recogn ition of en gin eerin g geology”, xvii, 274 p ., sp rin ger, isbn : 978-3319093024. man su r k.l., pon cian o l.c.m.o., de castro a.r.s.f. (2017). con tribu tion s to a brazilian cod e of con d u ct for field w ork in geology: an ap p roach based on geocon servation an d geoeth ics. an ais d a acad em ia brasileira d e ciên cias (an n als of th e brazilian acad em y of scien ces), vol. 89, n o. 1, su p l. 0, 431-444. matteu cci r., gosso g., pep p olon i s., piacen te s., wasow ski j. (2014). th e “geoeth ical prom ise”: a prop osal. ep isod es, vol. 37, n o. 3, p p . 190-191. meller c., schill e., bremer j., kolditz o., bleicher a., benighaus c., chavot p., gross m., pellizzone a., renn o., schilling f., kohl t. (2017). accep tability of geothermal installations: a geoethical concept for geolab. geoethermics, available online 14 august 2017: http:/ / www.sciencedirect.com/ science/ article / pii/ s0375650517300858, accessed 4 october 2017. pep p olon i s. (2015). sharing ethical principles through cultural diversity. translations of the montreal statement on research integrity in cross boundary research collaborations. iapg international association for promoting geoethics: http:/ / www.geoethics.org/ translationsms, accessed 30 october 2017. pep p olon i s., bobrow sky p., di cap u a g. (2015). geoeth ics: a ch allen ge for research in tegrity in geoscien ces, p p . 287-294, d oi: 10.1142/ 9789814632393_0035. in : sten eck n ., an d erson m., klein ert s., mayer t. (ed s.), in tegrity in th e global research ar en a, 336 p p ., world scien tific pu blish in g co, isbn : 9789814632386. pep p olon i s. an d di cap u a g. (2012). geoeth ics an d geological cu ltu re reflection s from th e geoitalia conference 2011. annals of geophysics, vol. 55, no 3, p.163: http:/ / www.annalsofgeophysics.eu/ index.php / annals/ issue/ view/ 482, accessed 30 october 2017. pep p olon i s. an d di cap u a g. (2014). th e mean in g of geoeth ics. in : wyss m. an d pep p olon i s. (ed s), geoet h ics: eth ical ch allen ges an d case stu d ies in earth scien ce, 450 p ., elsevier, walth am , massach u setts, isbn : 9780127999357. pep p olon i s. an d di cap u a g. (2015). geoeth ics: th e role an d resp on sibility of geoscie n tists. geological society, lon d on , sp ecial pu blication s, 419, isbn : 9781862397262. pep p olon i s. an d di cap u a, g. (2016). geoet h ics: eth ical, social, an d cu ltu ral valu es in geoscien ces research , p ractice, an d ed u cation , p p . 17-21, d oi: 10.1130/ 2016.2520(03). in : wessel g.r. an d green berg j.k. (ed s), geoscien ce for th e pu blic good an d global develop m en t: tow ard a su stain able fu tu re: geological society of am erica sp ecial pap er 520, 2016, isbn : 9780813725208. pep p olon i s. an d di cap u a g. (2017). geoeth ics: eth ical, social an d cu ltu ral im p lication s in geoscien ces. an n als of geop h ysics, 60, fast track 7, d oi: 10.4401/ ag -7473. pep p olon i s., di capua g., bobrowsky p.t., cronin v. (2017). geoethics at the heart of all geoscience. annals of geophysics, vol 60, fast track 7: http:/ / www.annalsofgeophysics.eu/ index.php / annals/ issue/ view/ 537, accessed 30 october 2017. stew art i.s. an d gill j.c. (2017). social geology in tegratin g su stain ability con cep ts in to earth scien ces. proceed in g of th e geologists’ association , volu m e 128, issu e 2, pages 165–172. stew art i.s. an d lew is d. (2017). com m u n icatin g con tested geoscien ce to th e p u blic: movin g from ‘m atters of fact’ to ‘m atters of con cern ’. earth -scien ce review s, volu m e 174, pages 122-133. vascon celos c., torres j., vascon celos l., mou tin h o s. (2016). su stain able develo p m en t an d its con n ection to teach in g ge oeth ics. ep isod es, vol. 39, n . 3, p p . 509-517. wyss m. an d pep p olon i s. (2014). geoeth ics, eth ical ch allen ges an d case stu d ies in earth scien ces. p . 450, elsevier, isbn : 9780127999357. an n als of geoph ysics, 60, fast track 7, 2017; d oi: 10.4401/ ag-7553 5 appen d ix a the cape town statemen t on geoethics preamble th e con cep ts, valu es an d view s on in d ivid u al resp on sibilities of geoscien tists, exp ressed in th e “cap e tow n statem en t on geoeth ics” r eflect an in tern ation al con sen su s. th e statem en t aim s to cap tu re th e atten tion of geoscien tists an d organ ization s, an d to stim u late th em to im p rove th eir sh ared p olicies, gu id elin es, stra tegies an d tools to en su re th ey con sciou sly e m brace (geo)eth ical p rofession al con d u ct in th eir w ork. introduction geoscien ces h ave m ajor im p acts on th e fu n ction in g an d kn ow led ge-base of m od ern societies. geoscien tists h ave sp ecific kn ow led ge an d skills, w h ich are requ ired to in vestigate, m a n age an d in terven e in variou s com p on en ts of th e earth system to su p p ort h u m an life an d w ellbein g, to d efen d p eop le again st geoh az ard s an d to en su re n atu ral resou rces are m an aged an d u sed su stain ably. th is en tails eth ical obligation s. th erefore, geoscien tists m u st em brace eth ical valu es in ord er best to serve th e p u blic good . geoeth ics is an em ergin g su bject, w h ich p r om otes a w ay of th in kin g an d p racticin g geoscien ces, w ith in th e w id er con text of th e roles of geoscien tists in teractin g w ith colleagu es, society an d th e p lan et. on ly by gu aran teein g th e in tellectu al freed om of research ers an d p ractition ers to exp lore an d d iscover in th e earth system , is it p ossible for geoscien tists to follow eth ical ap p roach es in th eir w ork. likew ise, on ly by in creasin g r esearch ers’ an d p ractition ers’ aw aren ess of th e eth ical im p lication s of th eir w ork is it p ossible to d evelop excellen t geoscien ce to ser ve society an d to red u ce th e h u m an im p act on th e en v iron m en t. d efinition of geoethics geoeth ics con sists of research an d reflection on th e valu es w h ich u n d erp in ap p rop riate beh a viou rs an d p ractices, w h erever h u m an activities in teract w ith th e earth syste m . geoeth ics d eals w ith th e eth ical, social an d cu ltu ral im p lication s of geoscien ces kn ow led ge, ed u cation , research , p ractice an d com m u n ication , an d w ith th e social role an d resp on sibility of geoscien tists in con d u ctin g th eir activities. purpose em bracin g geoeth ics is essen tial: to im p rove both th e qu ality of p rofession al w ork an d th e cred ibility of geoscien tists, to foster excellen ce in geoscien ces, to assu re su stain able ben efits for com m u n ities, as w ell as to p rotect local an d global en viron m en ts; all w ith th e aim of creatin g an d m ain tain in g th e con d ition s for th e h ealth y an d p rosp erou s d evelop m en t of fu tu re gen eration s. fundamental values of geoethics  h on esty, in tegrity, tran sp aren cy an d reliability of th e geoscien tist, in clu d in g strict ad h eren ce to scien tific m eth od s;  com p eten ce, in clu d in g regu lar train in g an d life-lon g learn in g;  sh arin g kn ow led ge at all levels as a valu able activity, w h ich im p lies com m u n icatin g scien ce an d resu lts, w h ile takin g in to a ccou n t in trin sic lim itation s su ch as p robabilities an d u n certain ties;  verifyin g th e sou rces of in form ation an d d ata, an d ap p lyin g objective, u n biased p eer-review p rocesses to tech n ical an d scien tific p u blication s;  workin g w ith a sp irit of coop eration an d recip rocity, w h ich in volves u n d erstan d in g an d resp ect for d ifferen t id eas an d h yp oth eses;  resp ectin g n atu ral p rocesses an d p h en o m en a, w h ere p ossible, w h en p lan n in g an d im p lem en tin g in terven tion s in th e en v iron m en t;  protectin g geod iversity as an essen tial a sp ect of th e d evelop m en t of life an d biod iversity, cu ltu ral an d social d iversity, an d th e su stain able d evelop m en t of com m u n ities;  en h an cin g geoh eritage, w h ich brin gs t ogeth er scien tific an d cu ltu ral factors th at h ave in trin sic social an d econ om ic valu e, to stren gth en th e sen se of b elon gin g of p eop le for th eir en viron m en t; an n als of geoph ysics, 60, fast track 7, 2017; d oi: 10.4401/ ag-7553 6  en su rin g su stain ability of econ om ic an d social activities in ord er to assu re fu tu re ge n eration s’ su p p ly of en ergy an d oth er n at u ral resou rces.  prom otin g geo-ed u cation an d ou treach for all, to fu rth er su stain able econ om ic d eve lop m en t, geoh az ard p reven tion an d m itig ation , en viron m en tal p rotection , an d in creased societal resilien ce an d w ell-bein g. geoethical promise th e ad op tion of th e follow in g h ip p ocratic-like oath (th e “geoeth ical prom ise”) by early -career geoscien tists is p rop osed , to p rom ote resp ect for geoeth ics v alu es in geoscien ce research an d p ractice: i promise… … i will practice geosciences being fully aware of the societal implications, and i will do my best for the protection of the earth system for the benefit of humankind. … i understand my responsibilities towards society, future generations and the earth for sustainable development. … i will put the interest of society foremost in my work. … i will never misuse my geoscience knowledge, resisting constraint or coercion. … i will always be ready to provide my professional assistance when needed, and will be impartial in making my expertise available to decision makers. … i will continue lifelong development of my geoscientific knowledge. … i will always maintain intellectual honesty in my work, being aware of the limits of my competencies and skills. … i will act to foster progress in the geoscien ces, the sharing of geoscientific knowledge, and the dissemination of the geoethical approach. … i will always be fully respectful of earth processes in my work as a geoscientist. i promise! final statement it is essen tial to en rich th e roles an d resp on s ibilities of geoscien tists tow ard s com m u n ities an d th e en viron m en ts in w h ich th ey d w ell, as w ell as p ayin g atten tion to each scien tist’s in d ivid u al con scien ce an d relation sh ip s w ith co lleagu es. h u m an com m u n ities w ill face great en viron m en tal ch allen ges in th e fu tu re. geoscien tists h ave kn ow -h ow th at is essen tial to or ien tate societies tow ard s m ore su stain able p ra ctices in ou r con sciou s in ter action s w ith th e earth system . ap p lyin g a w id er kn ow led ge base th an n atu ral scien ces, geoscien tists n eed to take m u ltid iscip lin ary ap p roach es to econ om ic an d en viron m en tal p roblem s, em bracin g (geo)eth ical an d social p ersp ectives. geoscie n tists are p rim arily at th e service of society. th is is th e d eep er p u rp ose of th eir activity. in th e com in g years, esp ecially w h en ad d ressin g m atters like en ergy su p p ly, u se of geo resou rces, lan d m an agem en t, p ollu tion abat em en t, m itigation of geo -risks, an d clim ate ch an ge ad ap tation an d m itigation , eth ical an d social issu es w ill be cen tral in scien tific d iscu ssion an d in p u blic d ebate. in ad d ition , h an d lin g large qu an tities of d ata, scien ce an d risk co m m u n ication , ed u cation strategies, issu es of r esearch in tegrity, an ti-h arassm en t an d an tid iscrim in ation p olicies, gen d er balan ce an d in clu sion of th ose livin g w ith d isabilities w ill be m ajor top ics for geoscien tists. raisin g th e (geo)eth ical aw aren ess an d com p eten ces of th e m em bers of th e geoscien ce co m m u n ity is essen tial, also to in crease tru st an d cred ibility am on g th e p u blic. th is can best be ach ieved in th e n ear fu tu re by tw o m ean s: by p rom otin g m ore effectively existin g gu id an ce su ch as cod es of eth ics/ con d u ct an d research in tegrity statem en ts; an d by in trod u cin g ge oeth ics in to geoscien ce cu rricu la, to m ake geoeth ics a basic featu re of th e train in g an d p rofe ssion al activity of geoscien tists. n ote: the cape town statement on geoethics was prepared during the 35th igc – international geological congress in cape town, south a frica (27 a ugust – 4 september 2016). drafting committee: giuseppe di capua, silvia peppoloni, peter t. bobrowsky. w ith the contribution of: n ic bilham, m artin bohle, a ndy clay, emilia hermelinda lopera-parejas, david m ogk. a pproved by the ia pg executive council on 26 october 2016. w ebsite: http://www.geoethics.org/ctsg vol51,2_3,2008 477 annals of geophysics, vol. 51, n. 2/3, april/june 2008 key words dinsar – gps – strong-motion – teleseismic data – seismic source 1. introduction during the 1997-1998 umbria–marche earthquake sequence more than 2000 shocks were recorded, three of which had moment magnitudes larger than 5.7: the september 26, 1997 at 00:33 gmt (event 1, mw=5.7) and 09:40 gmt (event 2, mw=6.0) in the colfiorito area and the october 14, 1997 at 15:23 (event 3, mw=5.6) in the sellano area (fig. 1). a reference value of the scalar seismic moment and focal mechanism of the ruptures is given by the moment tensor analysis of long period seismological data from the mednet network (ekstrom et al., 1998, cmt solution hereinafter). this analysis suggests normal faulting with a ne-sw tension axis and the presumed fault plane dipping towards sw. several data sets concerning the coseismic displacement field were available after the three events. the assessment of the properties of extended seismic sources is a fundamental requirement for further studies aimed at relating the source properties to damage, computing accurate stress transfers and characterizing hazard related to a given seismic province. when this result is obtained through inversion of available data, it is said that a finite-fault inversion is perextended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data maria elina belardinelli dipartimento di fisica-settore di geofisica, università degli studi di bologna, italy abstract the three largest events of the 1997 umbria-marche (italy) sequence occurred on september 26, 1997 at 00:33 gmt (event 1, mw=5.7) and 09:40 gmt (event 2, mw=6.0) in the colfiorito area and on the october 14, 1997 at 15:23 (event 3, mw=5.6) in the sellano area. the availability of different sets of geodetic and seismological data allowed several studies to characterize the extended sources of events 1-3. in this work, i review some of the studies that obtain the properties of the seismic sources by inversion of available data. generally these studies assume the seismic sources as dislocations or distributions of equivalent point sources in elastic half-spaces. following their chronological order, they model increasing complexities of the sources by using an increasing number of data. some of the differences between results obtained, such as the top edge depth estimates, are shown to be due to the different approaches used. commonly a 1-d crustal model is used in inverting strongmotion data. instead homogeneous elastic half-spaces are mainly assumed in inverting geodetic data to obtain the three main sources of the 1997 umbria-marche sequence. assuming the same crustal structure is important to make comparable results obtained analyzing seismological data or geodetic data separately, as it has been done till now for this sequence. mailing address: prof. maria elina belardinelli, università di bologna, dipartimento di fisica, settore di geofisica, viale berti pichat 8, 40127 bologna, italy; e-mail: mariaelina.belardinelli@unibo.it vol51,2_3,2008 4-03-2009 10:28 pagina 477 478 m. e. belardinelli formed. we review here the studies that characterize the three seismic sources of events 1-3 by means of inversions of two kinds of data: geodetic data (e.g. lundgren and stramondo, 2002; belardinelli et al.., 2003; de martini et al., 2003; hernandez et al., 2004; santini et al., 2004; crippa et al., 2006; dalla via et al., 2007) and seismological data (e.g. capuano et al., 2000; pino and mazza, 2000; hernandez et al., 2004). we briefly indicate as umi studies (umbria-marche inversion studies) the finitefault inversions of the three main events of the 1997 umbria-marche sequence that will be discussed here. after an introduction of the data used, this paper will present generalities of the inversion methods and the specific methodological approach followed by studies dealing with geodetic data or seismological data separately. then the main results achieved on the three main events of the umbria-marche sequence will be presented. some topics about umi studies versus the state of the art of finite-fault inversions will be discussed in the last section. it is worth noting that for this sequence no study to date has inverted the source parameters using both seismological and geodetic data jointly, whereas there are several examples of such a kind of inversions for subsequent earthquakes. for instance the 1999 mw 7.5-7.6 izmit, turkey mainshock is studied by delouis et al. (2002) by inverting gps, dinsar, strong-motion and teleseismic data. the 2000 mw 6.6 western tottori, japan earthquake is studied by piatanesi et al. (2007) by inverting gps and strong-motion data. 2. data selection the moderate magnitudes of events 1-3 of the umbria marche sequence impose measurements with the largest «signal to noise» ratio within the ensemble of available data. permanent displacements induced by the three events were measured through the analysis of dinsar data. during the colfiorito sequence several interferometric images were acquired by satellites ers1 and ers2, but few of them can be organized in pairs of images revealing enough coherence in the area of interest (lundgren and stramondo, 2002; crippa et al., 2006). the difference between the images taken on 1997 september 7 and 1997 october 12 by satellite ers2 will be called «colfiorito interferogram» hereinafter. it regards the displacement (in the satellite line of sight or slant range direction) caused by events 1 and 2 and intervening smaller events. another pair of images taken by ers2 and ers1 at 1997 august 9 and 1997 october 17, respectively, also contains the coseismic displacement caused by event 3 and will be referred to as the «colfiorito-sellano interferogram». finally a pair of images taken by ers2 on 1997 october 12 and 1997 november 16 basically contains the deformation due to event 3 and will be referred as the «sellano interferogram» (lundgren and stramondo, 2002). measures of permanent coseismic displacement as a 3-d vector were obtained at gps monuments by differentiation of the data collected in two surveys (e.g. belardinelli et al., 2003). the first survey was taken in 1995 by istituto geografico militare (igm) and the second survey, between 1997 october 7 and 10, by 12˚30' 12˚45' 42˚45' 43˚00' 13˚00' colfiorito sellano 1 2 3 8˚ 12˚ 16˚ 40˚ 44˚ aqu tri vsl cerreto cesi nocera umbra assisi 0 10 20 km fig. 1. the epicentres (red stars) of three largest events in the umbria-marche (1997-1998) sequence. strong-motion stations are indicated by black triangles. the inset shows the area in the figure (red rectangle) and the location of broad-band mednet stations used in umi studies (black triangles). vol51,2_3,2008 4-03-2009 10:28 pagina 478 479 extended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data istituto nazionale di geofisica at 13 monuments of the first survey. of the latter, only four monuments (capa, croc, foli and penn, fig. 2) showed significant horizontal displacement caused by events 1 and 2 and intervening smaller events and only two stations, croc and colf, were subjected to a vertical displacement greater in absolute value than the accuracy. unfortunately, the coseismic displacement errors are affected by the significantly lower accuracy of the first survey compared to the second one. accordingly, the accuracy of gps data are estimated as 2.4 cm and 4.0 cm for the horizontal and vertical components of displacement, respectively (belardinelli et. al., 2003). after event 3, no major coseismic deformation at gps sites was observed (hernandez et al., 2004). the elevation changes caused mainly by event 2 were also measured along a levelling line surveyed in 1992 and 1998 by igm (de martini et al., 2003). seismological data consist of measurements of ground acceleration, velocity or displacement as a function of time at the locations of seismic stations. records of 3-d ground acceleration were provided by strong-motion accelerometers deployed by servizio sismico nazionale (ssn), national energy agency (enea) and electric national board (enel). in order to provide the highest level of detail on the rupture process, umi studies consider the records taken by accelerometers located at distances less than 30 km from the focal area and having a good azimuthal coverage relative to the fault orientations (taken from the cmt solution). moreover only the accelerometers providing the simplest waveforms (with the least site-effects) are considered (capuano et al., 2000) or weighted more than other stations (hernandez et al., 2004). for events 1 and 2 these accelerometers are located at the stations of assisi and cerreto (fig. 1). for event 3, they are located at the stations of cerreto and cesi. the latter station, not considered by hernandez et al. (2004), has been operating only since 1997 october 3. during events 1-3, three broad-band seismic stations of the mednet regional network, at distances less than 500 km from the focal area (aqu, tri and vsl, fig. 1), recorded good quality (high signal-to-noise km 0 10 20 12° 45’ 13° 00’ 43° 00’ 43° 15’ capa penn colf foli data model 5 cm croc fig. 2. geometry assumed by hernandez et al. (2004) for the sources of events 1 and 2. gps data of horizontal displacement recorded in near field stations are compared to model results (best model according to both dinsar data and gps data). the figure shows only five out of twelve gps stations used in the inversion (see text). the epicentres of events 1 and 2 are indicated with black stars. the fault discretization used is also shown in map view. the solid lines parallel to fault strikes represent the intersections of the prolongation of the fault planes with the earth surface. the top edge depth of the fault of event 2 is fixed to 700 m. the crustal model assumed is reported in table i. figure modified from hernandez et al. (2004). ratio) waveforms of 3-d ground displacement as a function of time (e.g. pino and mazza, 2000). station aqu is also conveniently placed to show directivity effects, having an azimuth close to the strike of the fault planes involved in the seismic ruptures (figs. 1 and 2). 3. methodological approaches 3.1. generalities we may schematize an inversion problem as it follows. let m be a vector containing the parameters to be inverted (or free parameters), c the vector of parameters fixed or assumed as known during the inversion. the inversion devol51,2_3,2008 4-03-2009 10:28 pagina 479 480 m. e. belardinelli termines the parameter m by comparing the available data organized in a vector d with their estimates dmod provided by a model equation dmod = g(m;c) (3.1) where dmod is the vector containing the model estimates. the inversion is performed by minimizing a misfit or cost function that, following the approach of belardinelli et al. (2003), can be expressed as ε(m) = (dmod – d)t c-1(dmod – d) (3.2) where c is the covariance matrix whose elements contain data uncertainties. the vector of free parameters retrieved from the inversion, mi, also called «inverted model», is such that: ε(mi) = min �ε(m)�, with varying m in the parameter space. in order to find the minimum value of the misfit function, the exploration of the parameter space is performed starting from a given set of parameters ms, the so-called starting model. if the model equation is linear in m, i.e. in (3.1) g(m;c) = g(c)m, where g is a matrix, then the inversion is linear and mi satisfies the so called «least squares» criterion. in this case, the retrieved parameters can be expressed analytically as it follows mi = (g t c-1 g)-1 gt c-1 d) (3.3) when the model equation is nonlinear in m the inversion is said non linear and the misfit function can be characterized by several local minima. in this case, a local exploration of the parameter space, such as a trial and error exploration, may determine estimates of parameter values mi which depend on the starting model. this occurs when mi is incorrectly identified with the point in the parameter space realizing the local minimum of the misfit function that is closest to the starting model, ms. to avoid this, it is better to perform a global exploration of the parameter space, by means of genetic algorithms, for instance. the latter algorithm is used by hernandez et al. (2004) to generate different models of slip distribution of events 1 and 2 that are in agreement with geodetic observations within the data accuracy (i.e. they represent models that are consistent with data). the resulting model population is used to evaluate the resolution of the inverted model along the fault planes, as will be discussed later in section 4. also simulated annealing algorithms are suitable to escape local minima of ε and to find the absolute minima in nonlinear inversions. inversions using simulated annealing (belardinelli et al., 2003 for event 2, lundgren and stramondo, 2002, for events 1-3) are a particular case of monte carlo inversions, where trial sets of parameters m are generated randomly (santini et al., 2004, for events 1-3). in order to improve the resolution and the stability of the inversion obtained with a single data set, it is possible to perform a joint inversion of different kinds of data. in a joint inversion, following the notation of belardinelli et al. (2003), the cost function is obtained by assigning a different weight to different data by means of a weight matrix w, as it follows ε = (dmod – d)t wc-1(dmod – d) (3.4) the weight matrix is diagonal and it should be chosen in such a way that the cumulative contribution to the cost function of each data set is comparable, regardless of the number of data contained in the different data sets. 3.2. parameter selection and data treatment inversions of geodetic data allow the determination of parameters that characterize the fault from a static point of view. these are the fault geometry (location, focal mechanism, maximum length and width) and the final distribution of the slip vector over the fault plane. in addition to the parameters mentioned above, inversions of seismological data determine parameters that characterize the fault from a kinematic point of view, such as the distributions over the fault plane of the rupture time (or the rupture velocity) and the rise time. in the latter case, generally the same source time function over the whole fault is assumed. the larger number of parameters that can be inverted from seismological data is justified because, for each component of ground displacement at every vol51,2_3,2008 4-03-2009 10:28 pagina 480 481 extended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data seismic station, seismological data are in the form of a time series instead of the single value provided by geodetic data. we shall see that, given the restricted number of good-quality available data discussed in section 2, in order to limit the number of free parameters, most of umi studies further restrict the number of free parameters with respect to what is stated above, by making a priori explicit assumptions. for instance, apart from lundgren and stramondo (2002) who also invert for the direction of the slip vector in each point of the extended sources (fig. 3), most umi studies assumed a seismic source with a homogeneous rake angle. in so doing, the slip distribution is meant as the slip amplitude distribution over the fault plane. geodetic data are used to invert the slip amplitude distributions by hernandez et al. (2004); santini et al. (2004), crippa et al. (2006) and dalla via et al. (2007). it is worth recalling that in principle, this kind of inversion is linear, since the model displacement, dmod, is linked to the vector m of slip values on the fault plane, through a linear green operator, g. apart from hernandez et al. (2004) who also invert for the rupture time distribution of events 2 and 3, umi studies dealing with seismological data assume a homogeneous rupture velocity vector over the fault plane, to be determined from the inversion. all umi studies dealing with seismological data assume a homogeneous rise time value, eventually vanishing (pino and mazza, 2000; capuano et al., 2000). some umi studies perform a joint inversion of different kinds of geodetic data. the colfiorito interferogram and gps data are jointly inverted by belardinelli et al. (2003), for event 2 and hernandez et al. (2004), for events 1 and 2. for events 1-3, lundgren and stramondo (2002) perform a joint inversion of different dinsar image pairs and gps data in order to better resolve horizontal components of the slip vector (fig. 3). 3.2.1. inversion of geodetic data unlike seismological data, geodetic data refer to temporal windows encompassing more than one seismic event of the umbria-marche sequence (see section 2). in principle these data view a displacement field caused by events 1-3 and the smaller events, intervening between the times of the first and the second gps survey (or between sar images). however the contribution of minor events to the observed displacement is much smaller than that of events 1-3. the minor intervening events are either neglected (lundgren and stramondo, 2002; hernandez et al., 2004; crippa et al., 2006; dalla via et al., 2007) or re0 6 12 18 048d is ta n ce a lo n g d ip (k m ) distance along strike (km) events 1 and 2 slip (m) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.1 -7 -5 -3 -1 d ie p th (k m ) nw se distance along strike (km) 0486 120 d is ta n ce a lo n g d ip (k m ) event 3 -6 -4 -2 d ie p th (k m ) senw 1 2 3 fig. 3. joint inversion of gps data and dinsar data performed by lundgren and stramondo (2002) for the slip distribution of events 1 and 2 (top) and event 3 (bottom). the inverted direction of slip (normalized vectors) is shown in the patches of slip larger than 1 m (green arrows). black stars represent vertical projections of the observed epicentres on the inverted fault planes. events 1 and 2 are assumed on the same fault plane (see section 3.2.1.). faults are assumed as rectangular dislocations in an elastic homogeneous half-space. vol51,2_3,2008 4-03-2009 10:28 pagina 481 482 m. e. belardinelli solved as a part of the inversion for events 1-3 (santini et al., 2004 under the assumption that minor events roughly belong to the same fault zone of events 1-3) or explicitly taken into account as fixed parameters, c, together with event 1 parameters. the latter procedure is followed by umi studies inverting for event 2 only (belardinelli et al., 2003; de martini et al., 2003), that choose c in agreement with previous studies. forward modelling of events 1 and 2 is made by hunstad et al. (1999) in order to reproduce gps data, in particular their study focuses on the heterogeneous slip distribution, the top edge depth and the rake of event 2. details concerning the slip distribution of events 1and 2 are provided by inversions of the colfiorito interferogram (lundgren and stramondo, 2002; hernandez et al., 2004; santini et al., 2004; crippa et al., 2006; dalla via et al., 2007). in these cases, either a fixed geometry of the fault (location, focal mechanism and maximum dimensions) is assumed in agreement with previous studies or some of the fault parameters are chosen as it follows. trial values of these parameters are put in the vector of «fixed» parameters c and an inversion with respect to the remaining free parameters m is performed for each set of trial values. then the set of trial values that produces the inversion with the smallest cost function is chosen. even if not inverted simultaneously as the free parameters m are, the parameters c retrieved in this way can be considered as «inferred» by trial and error. similar studies are performed for event 3 by considering the sellano interferogram (lundgren and stramondo, 2002; santini et al., 2004) or the colfiorito-sellano interferogram (lundgren and stramondo, 2002; hernandez et al., 2004). belardinelli et al. (2003) invert the fault mechanism, the location of the fault with respect to the observed hypocenter, the depth of the fault and a simplified slip distribution (fig. 4a), largely inspired to findings of hunstad et al. (1999), using both gps data and the colfiorito interferogram. both gps data, the colfiorito interferogram and the colfiorito-sellano interferogram concern the cumulative coseismic displacement of events 1 and 2, which occurred very close to each other in space and time and have a similar focal mechanism, according to the cmt solutions. for this reason some umi studies dealing with geodetic data assume that events 1 and 2 occurred on the same fault plane (e.g. fig. 3, lundgren and stramondo, 2002, santini et al., 2004). this hypothesis was recently confirmed by high-resolution studies assessing the contia 0 5 10 distance along strike (km) 05d is ta n ce a lo n g d ip ( k m ) 10-3 -6 d e p th ( k m ) event 2nw se slip (m) 0.2 0.4 0.6 0.8 b nw se distance along strike (km) -4 -6 -8 d is ta n ce a lo n g d ip ( k m ) d e p th ( k m ) event 2 0 5 10 05fig. 4a,b. simplified slip distribution of event 2 obtained by a) joint inversion of dinsar and gps data (belardinelli et al., 2003, model a in their table ii) and b) inversion of strong-motion data (capuano et al., 2000). an elastic homogenous half-space is assumed for inversion of geodetic data and a layered half-space (table ii) for strong motion inversion. in each panel the black star represents the location of the hypocenter on the fault plane and the arrow represents the inverted direction of slip, assumed as homogeneous on the whole fault plane. in panel a slip is obtained by linear interpolation of the inverted solution outside the patch with the largest slip (53 cm). figure in panel b is redrawn after emolo and zollo (2001). vol51,2_3,2008 4-03-2009 10:28 pagina 482 483 extended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data nuity of the slip distribution across the two almost parallel faults of event 1 and 2 (dalla via et al. 2007). at first, umi studies using the colfiorito interferograms assumed it to consist of nine fringes of equal slant range displacement. the latter are discretized into 258 sites of known slant range displacement (fig. 5) in belardinelli et al. (2003). similarly the colfioritosellano and sellano interferograms are assumed to consist of a discrete number of fringes of equal slant range displacement, subsequently digitized in a discrete number of data points (e.g. lundgren and stramondo, 2002). later, from the colfiorito interferogram crippa et al. (2006) estimated slant-range displacement values in a 2-d grid containing more than 8900 data points. this denser data set allows a greater resolution in the inverted slip distribution compared to previous studies, but it also requires smoothing algorithms in order to reduce the roughness (the average gradient of the slip over the rupture area) of the inferred slip distribution (dalla via et al., 2007). in order to limit the parameter space explored during the inversion, all the theoretically-based a priori constraints on m should be used, mainly in cases of nonlinear inversions. for instance, while inverting for the source locations, the fault planes should be forced to pass through the instrumental hypocenters or to intersect the volume spanned by their hypocenter, taking into account the uncertainties in its location. in order to determine the inverted model of event 2, belardinelli et al. (2003) consider the large error in its hypocentral depth (amato et al., 1998) and vary the fault location in such a way that the vertical projection of the observed epicentre on the fault plane is located within the depth interval of uncertainty of the observed hypocenter, [3.7 km, 7.7 km], (fig. 4a). the same is true for the inverted model obtained by lundgren and stramondo (2002) for event 2 (fig. 3, top), but not for events 1 and 3. as we can note in fig. 3, for these events the vertical projections of the epicentres on the inverted fault planes locate at depths that are smaller than the minimum estimates of hypocentral depth provided by seismology (5 km for event 1 and 6.2 km for event 3, amato et al., 1998). in order to perform an inversion it is necessary to refer to a particular crustal model. if the inverted parameters include the slip distribution over the fault plane, then the scalar seismic moment can be derived from the inverted solution, mi, and the crustal model used. thus in this case, the scalar seismic moment cannot be considered an additional free parameter to be determined by inversion, unlike the approach followed by lundgren and stramondo (2002). to this aim, penn capa colf croc 4 cm data model fig. 5. geometry used by belardinelli et al. (2003) for the sources of events 1 and 2 and the other two minor events occurred on 3 and 6 october 1997 (grey boxes), whose epicentres are represented by black open stars. gps data of horizontal displacement in the four stations located within 15 km from the mainshock epicentre are compared to model results. results of the best model for event 2 according to joint inversion of gps (at the four stations indicated) and dinsar data are shown (model a in table ii of the mentioned paper). slant range displacement seen by dinsar data is assumed to be known in 258 «data points» (black dots). dot size scales with the logarithm of the squared residual between model and dinsar datum, as evaluated in the dot location. for the fault of event 2, the inverted value of the top edge depth is 70 m. faults are assumed as rectangular dislocations in an elastic homogeneous half-space. vol51,2_3,2008 4-03-2009 10:28 pagina 483 484 m. e. belardinelli both belardinelli et al. (2003) for event 2 and santini et al. (2003) for events 1-3 determine by inversion of geodetic data a slip distribution fulfilling the a priori constraint of a scalar seismic moment equal to the cmt solution. similarly, for event 2, de martini et al. (2003) inverted levelling data by grid search, assuming the scalar seismic moment of the cmt solution as a constraint to determine the fault length, width, top depth and a slip distribution homogeneous along dip, largely inspired to pino and mazza (2000) results (see next subsection). 3.2.2. inversion of seismic data for events 1-3, pino and mazza (2000) obtain the apparent source time function (stf) from waveforms recorded at three broadband stations (see section 2), using the method of the empirical green functions (egf) by deconvolution in the frequency domain. for each of the three major events, the authors select a suitable egf (as a smaller event with a similar focal mechanism located close to the studied event). the stf is low-pass filtered below 1 hz in order to remove small-scale source complexities of the egf. the stf is also averaged along the three coordinate axes for each station and each event considered. for each event, the maximum amplitude and the duration of the obtained stf at the three stations are the data d that can be compared with model results. the latter are developed using the haskell source model and approximating the stf with a triangle having a width equal to the observed duration. the model estimates of the stf amplitude and duration (dmod) depend on the following source parameters (m): the fault length, the horizontal component of the rupture velocity vector (assumed as homogeneous and unilateral) and the lp moment ratio (the ratio between the scalar seismic moment of the event studied and that of the egf) for a fixed value of the average wave velocity (c). the free parameters, m, are inverted by trial and error. assuming fixed values of crustal rigidity and fault width, it is also possible to determine the maximum slip on the fault and its position along strike as a part of the solution. for event 2, a bimodal stf is suggested by aqu data, yielding a slip distribution with two maxima along strike. capuano et al. (2000) perform a trial and error inversion of s-waveforms recorded by the strong-motion accelerometers to infer the source geometry (focal mechanism, length and width), the location of the hypocenters on the fault plane, the rupture velocity vector (assumed as homogeneous), and a simplified slip distribution for events 1-3. unlike the other events, for event 2 an inhomogeneous distribution of slip is required by data, with a gaussianlike peak located at about 3-4 km up-dip and nw of the nucleation point (fig. 3b). only the acceleration field associated with the direct swave motion is computed, assuming that it largely dominates in amplitude with respect to direct p-wave and converted/reflected waves in the near-source distance. the authors filter the signals of accelerometers in the range 1-5 hz and integrate them to obtain ground velocities. the low frequency limit is imposed by the assumptions made by the authors in modelling the seismic radiation using the asymptotic theory (frauhnofer approximation: wavelengths much smaller than distance from the source, comparable with the fault dimension in near field). the parameters of the starting model are inferred from previous analyses of the same authors. the latter estimate the stf and isochrones of events 1-3 at the considered seismic stations (see section 2) and study the swave polarization of measured waveforms. hernandez et al. (2004) instead low pass the frequency content of accelerometer signals (assuming a 0.1-1.5 hz filter) and integrate them twice (to obtain ground displacement), owing to difficulties in modelling the high frequency content of waveforms. the obtained data set is used to invert the slip distributions and the rupture histories of events 2 and 3 by means of non linear iterative inversion in the frequency domain, where each step is linearized. in this case, data, d, are represented by the waveform spectra for each frequency and component of displacement at each station. free parameters, m, are the rupture time and slip distributions over the fault plane, assuming fixed the fault geometry and a homogeneous source time function with a rise time equal to 1 s. vol51,2_3,2008 4-03-2009 10:28 pagina 484 485 extended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data 4. results inversion studies using only one kind of data or a part of the available data set should in principle check the consistency of their results with the observational evidences not directly taken into account during the inversion. to this aim, several umi studies verify if the «misfit a posteriori» with observations not used for the inversion is small, where the misfit a posteriori is obtained by means of a forward model using the inverted source parameters mi. for instance, this is the case of santini et al. (2004) who compare inversion results obtained for events 1 and 2 with vertical gps data and with the colfiorito-sellano interferogram (not used in the inversion). for events 13, hernandez et al. (2004) check the inversion results obtained from geodetic data with strong-motion data, assuming the inverted slip model, a homogeneous rupture velocity and 1 s rise time. capuano et al. (2000) check their inverted final model for events 1 and 2 comparing the waveforms obtained by forward modelling at the station of nocera umbra (not used in their inversion) with recorded data at the same station. moreover, umi studies not using the a priori constraint of the total scalar seismic moment (see section 3.2.1.) verify a posteriori the consistency of their estimate of the seismic moment with the cmt solution. the estimates of the inverted parameters, mi, are affected by uncertainties that can be related to errors in data or parameters fixed in the inversion. uncertainty is also intrinsic to the inversion method, taking into account the limitations concerning the resolving power of the data used and the modelling appropriateness. capuano et al. (2000) and pino and mazza (2000) estimate an uncertainty of 10-20% in each component of mi by determining the maximum and minimum values of each of the free parameters producing model results dmod compatible with the reading errors of data. for event 2, belardinelli et al. (2003) define as «acceptable» a model m that reproduces on average the data within the measurement error (having ε< εmax). an ensemble of acceptable models, mja , j = 1,2, …, na, is built by selecting a subset of the inverted models mii generated by varying randomly the starting models mis, i = 1, 2, …, 500. the authors provide estimates of the uncertainty of each free parameter by analyzing the distribution of that parameter within the ensemble of acceptable models. some umi studies demonstrate that the resolution of the inversion of geodetic data decreases with increasing the source depth. in particular in the deepest parts of the seismic sources of events 1 and 2 (depth> 3 km), the resolution gets significantly worse. for these events, both the uncertainty of each inverted parameter (hernandez et al., 2004) and the «minimum resolvable area of the fault» (with respect to the slip distribution, crippa et al., 2006) increase significantly at depths larger than 3 km. the minimum resolvable area of the fault is defined as the area of a fault patch producing on surface measure points an average displacement of the order of data uncertainty, for 1 m of homogeneous slip. the following state of knowledge of the three seismic sources of events 1-3 can be outlined on the basis of umi studies. all three sources are recognized as nw-se trending and sw dipping (e.g. fig. 2). however the values fixed, or inferred (in the sense explained in section 3.2.1.), or inverted for the strike of event 3 belong to a quite large interval 123°160°. the hypocenter is located near the border of the faults of event 2 (se border, e.g. figs. 2, 4 and 5) and event 3 (nw border, e.g. figs. 3 and 6). the rupture of events 2 and 3 propagated from the hypocenters with an average rupture velocity of about 2.6-3 km/s (capuano et al., 2000; pino and mazza, 2000). in most of the studies, the hypocenter of event 1 is assumed to be located near the nw border of the fault, suggesting a unilateral rupture (e.g. figs. 2 and 5), however capuano et al., 2000 attribute a bilateral rupture mechanism to event 1. the most studied source is that of event 2, characterized by a fault length of about 12-15 km (e.g. capuano et al., 2000; pino and mazza, 2000, see e.g. fig. 4). for event 2, gps data, in particular the large horizontal displacement recorded at the station penn (hunstad et al., 1999), require the top of the fault to be vol51,2_3,2008 4-03-2009 10:28 pagina 485 486 m. e. belardinelli shallower (0.04-0.7 km) than using levelling data (3.4 km, de martini et al., 2003), sar (1-2 km) or strong motion data (0.7-3.38 km). except for the capuano et al. (2000) solution (fig. 4b), the inverted value of the rake angle of event 2 entails a small left lateral component superimposed to the dominant normal component (figs. 3 and 4a) in agreement with the cmt solution. even if less resolved by gps and dinsar data, the inferred value of the dip angle of event 2 is around 40 degrees (belardinelli et al., 2003; hernandez et al., 2004, de martini et al. 2003; capuano et al., 2000) in agreement with the cmt solution, showing an anomalously small value for a normal fault. on the average, the top of the fault of event 2 is found to be shallower than that of events 1 and 3 (e.g. fig. 6). the inverted slip distributions are heterogeneous and characterized by patches containing the maximum values near the hypocenter locations (figs. 3, 4 and 6). for event 2, this patch has about 5 km extent along strike and concentrates below about 2 km depth (lundgren and stramondo, 2002, belardinelli et al., 2003, santini et al.; 2003, hernandez et al. 2004; crippa et al., 2006). for event 2, another smaller patch of slip located close to nw end of the fault can be inferred on the basis of some of the inversion studies (pino et al., 1999; lundgren and stramondo, 2002; hernandez et al., 2004; de martini et al., 2003; crippa et al., 2006; dalla via et al., 2007, see figs. 3 and 6). outside and around the two high-slip patches, the distribution of aftershocks recorded in the first three months after event 2 shows a larger density (de martini et al., 2004) suggesting the presence of an asperity. slip (m) 0.2 0.4 0.6 0.8 05-2 -3 -4 -5 -6 0 5 event 1nw se d is ta n ce a lo n g d ip ( k m ) d e p th ( k m ) distance along strike (km) -1 -2 -3 -4 -5 0 5 10 event 2 05d is ta n ce a lo n g d ip ( k m ) d e p th ( k m ) distance along strike (km) nw se 05-3 -4 -5 -6 0 5 event 3nw se d e p th ( k m ) distance along strike (km) fig. 6. best fit model for slip distribution of events 1-3 from inversion of geodetic data (hernandez et al., 2004). a joint inversion of dinsar and gps data is performed in case of events 1 and 2 (see fig. 3), while event 3 rupture is inverted from dinsar data alone. the open white star indicates the hypocenter on the fault plane of event 3. the crustal model used is reported in table i. figure redrawn after hernandez et al. (2004). vol51,2_3,2008 4-03-2009 10:28 pagina 486 487 extended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data 5. discussion and conclusive remarks this paper reviews several studies of the extended sources of the three major events of the umbria-marche sequence that are performed through inversion of observed coseismic displacements (umi studies). comparing the methods and the results obtained by each umi study we see that: 1) the best resolved source is that of the largest event (event 2: september 26, 1997 at 09:40 gmt, mw=6.0); 2) only some studies estimate the uncertainties of the inverted parameters and these studies are not necessarily the most recent ones; 3) in general, only one kind of data set is inverted and joint inversions of different data sets are performed only using different kinds of geodetic data (gps and dinsar). if point 1) is quite obvious given the moderate size of seismicity recorded during the 1997 the umbria-marche sequence, points 2 and 3 are worth discussing in the following part of this section, guided by other inversion studies of extended sources of recent earthquakes occurred in the world. piatanesi et al. (2007) point out that most of computational effort in finite-fault inversions seems to be devoted to finding the model which yields the minimum cost function and that only few studies deal with a posterior error analysis, while the latter may have important implications, for instance, for providing ground motion scenarios. then they propose a technique to estimate the variability of rupture models that are consistent with data. referring the results to each of the inferred parameters, the authors consider well resolved a parameter in a certain point of the fault if the ratio between its variability range and the value of the inverted parameter is low (�0.5). the regions of the fault where this occurs are stably inverted with respect to that parameter. for the umbria-marche sequence a similar analysis was made only for events 1 and 2 by hernandez et al. (2004) when they perform a joint inversion of dinsar and gps data (see sections 2 and 4). however, synthetic tests shown by piatanesi et al. (2007) point out that the variability of one parameter can correctly estimate the error bar of the same parameter only if the modelling is consistent and in particular if a realistic crustal model is used to evaluate the field of coseismic displacement. this is due to the well known fact that uncertainties in the crustal model represent a main source of noise in inversion studies of extended seismic sources (e.g. wald and graves, 2001). generally, umi studies assume the seismic sources to be described by dislocations or point sources in an elastic homogeneous half-space, except for studies considering strong-motion data (e.g. capuano et al., 2000 and hernandez et al., 2004), where a layered half-space is considered with rigidity values smaller than 30 gpa (average value for the crust) in the first 78 km depth (see tables i and ii). in general, if the layering of the crust is taken into account, the top edges of the three faults turn out to be deeper (1.5-3.5 km , 0.7-3.4 km, 2.4-4.8 km for events 1, 2 and 3 respectively) than the values obtained using half-space models (0.04-2 km, 0.04-1 km, 0.64-1.15 km, for events 1, 2 and 3 respectively). alternatively, since in umi studies different data sets are considered, one could explain the different estimates of the top edge depth referring to the different «importance» (e.g. tarantola, 1987) of a particular data set in allowing the resolution of slip at depth. however synthetic tests inverting geodetic data and strong motion data separately (delouis et al., 2002) do not seem to show that strong motion data are more important or more sensitive to final slip at depth than geodetic data are. neglecting low rigidity layers near the surface leads to underestimate mainly the horizontal components of displacement at the surface. this is confirmed by several studies (e.g. wald and grave 2001; megna et al., 2008). in the framework of inversion studies, for the same displacement observed at the surface, overestimating the rigidity of near surface layers leads to slip overestimates near the surface (e.g. wald and graves, 2001; simons et al., 2002; piatanesi et al., 2007) or to underestimate the top depth of the dislocation, in agreement with theoretical modelling of dislocations in layered media (savage, 1998). however piatanesi et al. (2007) show that the along strike slip distribution can be affected by uncertainties in the crustal model too. for event 2, we can note that the amplitudes of horizontal displacement at gps sites are compavol51,2_3,2008 4-03-2009 10:28 pagina 487 488 m. e. belardinelli rable if either a 1-d crustal model (fig. 2) or a half-space model (fig. 5) is used, despite of the fact that in fig. 2 the fault top is much deeper than in fig. 5. in both figures the amplitudes of horizontal displacement are underestimated with respect to data. this is particularly true for the station of penn where secondary effects such as deep-seated gravitational slope deformations are likely to have amplified the observed horizontal displacement with respect to estimates of coseismic displacement due to dislocations in elastic media (moro et al., 2007). in principle these effects should be taken into account before inverting geodetic data using elasto-static dislocation models. neglecting these secondary effects at penn might lead to underestimate the fault top depth of event 2 in umi studies that invert geodetic data alone. the joint inversion of different kinds of data increases in a robust way both the information carried by the inverted model (because different data can have different importance for each model parameter) and its resolving power (because the number of data of comparable importance to estimate the same parameter increases). usually, the single data set is reproduced by the model resulting from a joint inversion worse than it is when that kind of data is inverted separately, however the stability of the joint inversion is generally greater. for instance, joint inversions of gps data and dinsar data allow to estimate horizontal components of the slip vector better than using dinsar data alone (e.g. lundgren and stramondo, 2002; belardinelli et al., 2003) since gps data are most precise in horizontal deformations (pedersen et al., 2003). on the other hand, slip inversions using seismological data only are likely to be affected by trade off between rupture times and slip location (delouis et al., 2002). to conclude, the lesson learnt from the analysis of umi studies suggests that separate inversions of geodetic and seismological data can lead to fault models that are not strictly comparable, also due to the different crustal models that are used in evaluating static and dynamic displacement values. finite-fault studies dealing with recent earthquakes that occurred in the world are in favour of joint inversions assuming at least 1-d earth structures (wald and graves, 2001). the uncertainties related to the choice of table i. crustal model used by hernandez et al. (2004). the rigidity modulus is indicated with µ. top depth (m) vp (m/s) vs (m/s) ρ(kg/m3) µ(gpa) 0 4800 2666 2600 18.5 4000 5500 3055 2800 26.1 7000 6300 3500 2900 35.5 30000 8000 4444 3100 61.2 table ii. crustal model reported by emolo and zollo (2001) and used by capuano et al. (2000). the rigidity modulus is indicated with µ. top depth (m) vp (m/s) vs (m/s) ρ(kg/m3) µ(gpa) 0 2000 1100 2000 2.4 200 4400 2400 2400 13.8 1800 5900 3200 2400 24.6 8000 6250 3500 2500 30.6 vol51,2_3,2008 4-03-2009 10:28 pagina 488 489 extended sources of the main events of the umbria-marche (1997) seismic sequence inverted from geophysical data the crust structure could be faced by comparing the results of inversions obtained with all the crustal models available for a given region. acknowledgements i want to thank laura sandri and paolo baldi for introducing me to the subject of joint inversion of geodetic data and the anonymous reviewers for constructive comments. i am also grateful to antonella megna for having provided material in advance of publication and salvatore stramondo and paul lundgren for having provided slip models according to their joint inversion of dinsar and gps data. references amato, a., r. azzara, c. chiarabba, g. b. cimini, m. cocco, m. di bona, l. margheriti, s. mazza, f. mele, g. selvaggi, a. basili, e. boschi, f. courboulex, a. deschamps, s. gaffet, g. bittarelli, l. chiaraluce, d. piccinini and m. ripepe (1998): the 1997 umbria-marche, italy, earthquake sequence: a first look at the main shocks and aftershocks, geophys. res. lett., 25 (15), 2861-2864. belardinelli, m. e., l. sandri and p. baldi (2003): the major event of the 1997 umbria-marche (italy) sequence; what could we learn from dinsar and gps data?, geophys. j. int., 153, 1, 242-252. capuano, p., a. zollo, a. emolo, s. marcucci and g. milana (2000): rupture mechanism and source parameters of umbria-marche mainshocks from strong motion data, j. seismol., 4, 463-478. crippa, b., m. crosetto, e. biescas, c. troise, f. pingue and g. de natale (2006): an advanced slip model for the umbria-marche earthquake sequence: coseismic displacements observed by sar interferometry and model inversion, geophys. j. int., 164 (1), 36-45. dalla via, g., b. crippa, e. m. toraldo serra, g. giacomuzzi and r. sabadini (2007): exploitation of high-density dinsar data points of the umbria-marche (italy) 1997 seismic sequence for fault characteristics, geophys. res. lett., 34, l17301, doi: 10.1029/2007gl030718. delouis, b., d. giardini, p. lundgren and j. salichon (2002): joint inversion of insar, gps, teleseismic and strong-motion data for the spatial and temporal distribution of earthquake slip: application to the 1999 izmit mainshock, bull. seismol. soc. am., 92 (1), 278-299. de martini, p. m., n. a. pino, g. valensise and s. mazza (2003): geodetic and seismologic evidence for slip variability along a blind normal fault in the umbriamarche 1997-1998 earthquakes (central italy), geophys. j. int., 155, 819-829. emolo, a. and a. zollo (2001): accelerometric radiation simulation for the september 26, 1997 umbria-marche (central italy) main shocks, annals geophys., 44 (3), 605-617. ekström, g., a. morelli, e. boschi and a. m. dziewonski (1998): moment tensor analysis of the umbriamarche earthquake sequence of september-october 1997, geophys. res. lett., 25, 11, 1971-1974. hernandez, b., m. cocco, f. cotton, s. stramondo, o. scotti, f. courboulex and m. campillo (2004): rupture history of the 1997 umbria-marche (central italy) main shocks from the inversion of gps, dinsar and near field strong motion data, annals geophys., 47 (4), 1355-1376. hunstad, i., m. anzidei, m. cocco, p. baldi, a. galvani and a. pesci (1999): modelling coseismic displacements during the 1997 umbria-marche earthquake (central italy), geophys. j. int., 139, 2, 283-295 lundgren, p. and s. stramondo (2002): slip distribution of the 1997 umbria-marche earthquake sequence: joint inversion of gps and synthetic aperture radar interferometry data, j. geophys. res., 107 (b11), 2316, doi:10.1029/2000jb000103. megna, a., s. barba, s. santini and m. dragoni (2008): effect of geological complexities on coseismic displacement: hints from 2d modelling, terra nova, 20 (3) , 173-179, doi:10.1111/j.1365-3121.2008.00800.x. moro, m., m. saroli, s. salvi, s. stramondo and f. doumaz (2007): the relationship between seismic deformation and deep-seated gravitational movements during the umbria-marche (central italy) earthquakes, geomorphology, 89, 297-307. pedersen, r., s. jónsson, t. árnadóttir, f. freysteinn and k. feigl (2003): fault slip distribution of two june 2000 mw6.5 earthquakes in south iceland estimated from joint inversion of insar and gps measurements, earth planet. sci. lett., 213, 487-502. piatanesi, a., a. cirella, p. spudich and m. cocco (2007): a global search inversion for earthquake kinematic rupture history: application to the 2000 western tottori, japan earthquake, j. geophys. res., 112, b07314, doi:10.1029/2006jb004821. pino, n.a. and s. mazza (2000): the umbria-marche (central italy) earthquakes: relation between rupture directivity and sequence evolution for the mw >5 shocks, j. seismol., 4, 451-461. santini, s., p. baldi, m. dragoni, a. piombo, s. salvi, g. spada and s. stramondo (2004): monte carlo inversion of dinsar data for dislocation modeling: application to the 1997 umbria-marche seismic sequence (central italy), pure appl. geophys., 161, 817-838. savage, j.c. (1998): displacement field for an edge dislocation in a layered half-space, j. geophys. res., 103 (b2), 2439-2446. simons, m., y. fialko and l. rivera (2002): co-seismic deformation from the 1999 mw 7.1 hector mine, california, earthquake as inferred from insar and gps observations, bull. seism. soc. am., 92, 1390-1402. tarantola, a. (1987): inverse problem theory: methods for data fitting and model parameter estimation (elsevier), pp. 63-76. wald, d. j. and r. w. graves (2001): resolution analysis of finite source inversion using oneand three-dimensional green’s functions: 2. combining seismic and geodetic data, j. geophys. res., 106 (b5), 8767-8788. vol51,2_3,2008 4-03-2009 10:28 pagina 489 a n d a m e n t o t e r m i c o d e l l e f u m a r o l e l a v i c h i ' d e l l ' e t n a ( * ) s . ctjcuzza sit.y estui c . ponte l e osservazioni e o i i l i n u a l i v c s u l l ' a n d a m e n t o t e r m i c o delle itimar o l e d e l l ' e r u z i o n e e i u e a del 19-17 ci h a n n o l a t t o s v e l a r e a l c u n i fenom e n i di p a r t i c o l a r e i n t e r e s s e c l i c e s p o n i a m o in q u e s t a n o t a . n e i p o s t i d e l l a c o l a t a p r i n c i p a l e ove l ' a m m a s s a m e n t o lamico era n o t e v o l e si m a n i f e s t a r o n o , a l c u n i giorni d o p o c e s s a l a 1 e r u z i o n e , d e l l e f u m a r o l e a c l o r u r o a m i n o n i c o a l c u n e d e l l e q u a l i sono t u t t o r a a t t i v e . l e p i ù p e r s i s t e n t i si t r o v a n o verso q u o t a 1 0 5 0 e -u s e l l e di es«e. non m o l t o d i s t a n t i l ' i m a d a l l ' a l t r a , a h h i a m o f e r m a l a la n o s t r a m a g g i o r e a t t e n z i o n e . l a c o l a t a , in questo t r a t t o , è l a r g a c i r c a 120 m e d i t a 1 8 0 0 m e t r i dal f r o n t e p r i n c i p a l e c h e si a r r e s t ò d o p o 8 g i o r n i di eruz i o n e ( ' ). in a l t r i posti, più a m o n t e , c i o è p i ù v i c i n o a l l a b o c c a e l l u s i v a . ove la lava si e r a a c c u m u l a t a in m i n o r q u a n t i t à p e r c h é più f l u i d a e su t e r r e n o a f o r t e p e n d e n z a , le f u m a r o l e e b b e r o b r e v e d u r a l a . l a d e t e r m i n a z i o n e della t e m p e r a t u r a è stata l a t t a c o n un p i r o m e t r o t e r m o e l e t t r i c o c o n q u a d r a n t e t a r a l o da 0 " a 600'v,. l e d e t e r m i n a z i o n i sono s t a l e f a t t e ad i n t e r v a l l i da o l i o a d o d i c i g i o r n i , a p a i t i r e dal m e s e di a p r i l e , c i o è un m e s e d o p o c e s s a l a l ' e r u z i o n e d e l 1917. a b b i a m o a vu to c u r a d ' i n t r o d u r r e la p i n z a t e r m o e l e t t r i c a s e m p r e a d a «tessa p r o f o n d i t à d e n t r o ogni f u m a r o l a , b a d a n d o di non m o d i f i c a l i ' i a p e r t u r a o n d e n o n t u r b a r e l ' a n d a m e n t o t e r m i c o , c o m e q u a l c h e v o l t a è a v v e n u t o . 1 r i s u l t a l i sono i s e g u e n t i : a b b i a m o r i s c o n t r a l o la m a s s i m a temp e r a t u r a i n i z i a l e di 3 0 5 ° n e l l a f u m a r o l a g, « i l u a l a v e r s o il c e n t r o d e l l a c o l a t a . i v i l ' a t t i v i t à a n d ò affievolendosi fino a r a g g i u n g e r e 4 0 ' d o p o c i r c a sei m e s i . nella f u m a r o l a e s i t u a l a ove la lava si a c c u m u l ò m a g g i o r m e n t e , a b b i a m o r i s c o n t r a l o una t e m p e r a t u r a i n i z i a l e «li 2 9 5 " . q u e s t a f u m a r o l a si p r e s e n t a t u t t o r a a t t i v i s s i m a . ee f u m a r o l e li e r i s p e t t i v a m e n t e c o n t e m p e r a t u r a i n i z i a l e di 2 9 5 " e di 2 9 0 " , ( * ) r i c e r c h e eseguite con il contributo del consiglio nazionale delle r i c e r c h e . i 1 ) g . ponti:. i.'eruzione dell'etna del febbraio-marzo 1947. annali di geofisii a. i . 1 ( 1 9 4 8 ) . s . c u c l ' z z l s i l v e s t r i ' c . ponte p u r esse l u i l o r a a l l i v c , si sono f o r m a l e : la b a c i r c a m 2(1 d a l l a a e d a l l a d piii in b a s s o , verso il f r o n t e , in un posto ove la lava t r o v a s i m a g g i o r m e n t e a m m a s s a t a . l e a l t r e f u m a r o l e h a n n o p r e s e n t a t o l e seguenti t e m p e r a t u r e m a s s i m e i n i z i a l i : 2 7 0 " la e, c h e h a a v u l o b r e v i s sima d u r a t a : 1 6 0 " la c e la f. di d u r a l a non m o l t o l u n g a . l ' a b b a s s a m e n t o di t e m p e r a t u r a n e l l e f u m a r o l e c, e, f e g p r o c e d e t t e p i u t t o s t o r e g o l a r m e n t e fino al c o m p l e t o r a f f r e d d a m e n t o . l e f u m a r o l e ./. b e d. d e l l e c u i v a r i a z i o n i t e r m i c h e r i p o r t i a m o i grafici. p r e s e n t a r o n o un p a r t i c o l a r e a n d a m e n t o c h e è s t a l o oggetto spec i a l e del n o s t r o s t u d i o . nella f u m a r o l a l a l l ' i n i z i o , c i o è il 10 a p r i l e 1 9 4 7 , l a t e m p e r a tura r i s u l t o di 2 9 5 " : scese poi g r a d a t a m e n t e e d o p o c i r c a d u e m e s i , il .'50 m a g g i o , raggiunse 130". nella successiva m i s u r a z i o n e , fatta d i c c i giorni d o p o , n o t a m m o c h e la t e m p e r a t u r a aveva f a t t o un s a l t o di 65° elevandosi b r u s c a m e n t e a 195". nei successivi m e s i l ' a b b a s s a m e n t o d e l l a t e m p e r a t u r a p r o c e d e t t e con a n d a m e n t o r e g o l a r e fino al g i o r n o 2 9 l u g l i o 1947 s c e n d e n d o a 1 0 5 ' . s u b i l o d o p o , il g i o r n o 8 a g o s t o , n o t a m m o una n u o v a b r u s c a e l e v a z i o n e : la t e m p e r a t u r a si e r a p o r tala i 2 8 0 " . f a c e n d o un saiio di 175". d a q u e l g i o r n o l ' a b b a s s a m e n t o andamento t e r m i c o delle f u m a r o l e laviche dell'etna d e l l a t e m p e r a t u r a p r o c e d e t t e g r a d a t a m e n t e e l e n t a m e n t e , latito c l i c d o p o n o v e m e s i , il 1 4 m a g g i o 1 9 1 8 , i l p i r o m e t r o segnava 195". s i m i l i a n o m a l i e p r e s e n t a r o n o le f u m a r o l e b e d. nella 15 m n o t ò u n a r e g o l a r e d i m i n u z i o n e a p a r t i r e dal 10 a p r i l e 1 9 4 7 ( 2 9 5 " ) fino al 9 g i u g n o q u a n d o la t e m p e r a t u r a r i s u l t ò di 1 3 5 " . n e l l a d e c a d e successiva n o t a m m o u n r a p i d o «allo p o s i t i v o di 1 3 2 " , g i a c c h é la t e m p e r a l i n a si e l e v ò a 2 6 7 " . i n seguito la f u m a r o l a p r e s e n t ò u n a r e g o l a r e f u m a r o l a b t e m p e r a t u r a d e c r e s c e n t e c h e il 17 s e t t e m b r e si p o r t ò a 1 6 7 " . nella d e c a d e s e g u e n t e si e b b e a n c o r a un a l i t o salto di 9 0 " , e s s e n d o la t e m p e r a t u r a s a l i t a r a p i d a m e n t e a 2 7 5 " . i n s e g u i t o l ' a n d a m e n t o d e c r e s c e n t e d e l l a t e m p e r a t u r a si p r e s e n t ò r e g o l a r e ed il 14 m a g g i o 1 9 4 8 t r o v a m m o 175". la f u m a r o l a d p r e s e n t ò un solo a l i o : da 1 2 5 " m i s u r a t i il 2 2 agosto la t e m p e r a t u r a si e l e v ò a 180" il g i o r n o 2 7 e poi a n d ò g r a d a t a m e n t e d e c r e s c e n d o , tanto c h e il 1 4 m a g g i o e r a scesa a 8 5 ' . r i a s s u m e n d o : gli a b b a s s a m e n t i d e l l a t e m p e r a t u r a n e l l e f u m a r o l e s o p r a d e s c r i t t e sono a v v e n u t i p i u t t o s t o g r a d u a l m e n t e a n c h e nel p e r i o d o i n v e r n a l e d e l l e p i o g g e ; g l ' i n n a l z a m e n t i , i n v e c e , sono -tati semp r e b r u s c h i , e p o i c h é sono a v v e n u t i a n c h e nel p e r i o d o di s i c c i t à , con s . cu» uzza s i l v e s t r i g. ponte salii c o n s i d e r e v o l i , c r e d i a m o a t t e n d i b i l e c l i c la s o p r a e l e v a z i o n e b r u sca di t e m p e r a t i l i a s i a dovuta alla f o r m a z i o n e di n u o v e c o n t r a z i o n i s i n c l a s i c h e n e l l a p a r t e d e l l a lava in v i a d i c o n s o l i d a m e n t o . l e f u m a r o l e v u l c a n i c h e , c o m e a b b i a m o d i m o s t r a t o i n a l t r e p u b b l i c a z i o n i ( ' ) . sono d o v u t e a s p a c c a t u r e c l i c si f o r m a n o n e l l e c o l a l e e n e i c r a t e r i ove l ' a r i a p e n e t r a n d o v i e n e a c o n t a t t o c o n la p a r t e p i ù c a l d a d e l l a lava. r i t e n i a m o p e r t a n t o c l i c , fin q u a n d o nessun t u r b a m e n t o a v v i e n e n e l l ' i n t e r n o d e l l a f u m a r o l a l a v i c a , la t e m p e r a t u r a -i p r e s e n t i r e g o l a r m e n t e d e c r e s c e n t e ; c i ò m v e d e c h i a r a m e n t e nel grafico d e l l a f u m a r o l a g. q u a n d o nel l ' i n t e r n o d e l l a f u m a r o l a a v v i e n e un t u r b a m e n t o , c h e p u ò a v e r luogo p e r la f o r m a z i o n e di u n a n u o v a s p a c c a t u r a dovuta a c o n t r a z i o n e del m a g m a l a v i c o i n v i a d i solidific a z i o n e , a l l o r a si r i a t t i v a u n a pili p r o f o n d a c i r c o l a z i o n e d ' a r i a n e l l a p a r i e p i ù r o v e n t e con i l c o n s e g u e n t e a u m e n t o d e l l a t e m p e r a t u r a . t a l e f e n o m e n o h a luogo s o l t a n t o ove l ' a m m a s s a m e n t o di lava in via di r a f f r e d d a i , i c n l o è p i ù n o t e v o l e . n e l l ' a t t o in c u i si f o r m a n o le c o n trazioni i n c l a s i c h e si a v v e r t o n o s o v e n t e dei c u p i r u m o r i , m a n o n così l 1 ) g . ponte. fumarole vuleanieue. r e m i . \<r. lincei, x v i i s . v i . gemi. 1913. f u m a r o l a d v f i g . 3 andamento te1ìm1co d1.li.l elmahole laviche del1.'|i-na r.i: l'orli c o m e q u e l l i c l i c si o d o n o , a n c l i c a d i s t a n z a , d u r a n t e la f o r m a z i o n e dei c r e p a c c i dei g h i a c c i a i . r i p e t i a m o c h e «inora n o n t r o v i a m o a l t r a s p i e g a z i o n e p i ù conv i n c e n t e sulla o r i g i n e del f e n o m e n o degli i m p r o v v i s i s a l t i di t e m p e r a t u r a n e l l e t r e f u m a r o l e a. b, e d s o p r a d e s c r i t t e . a g g i u n g i a m o o r a i r i s u l t a l i di a l c u n e n o s t r e r i c e r c h e c h i m i c h e f a t t e su q u c s i o g r u p p o di f u m a r o l e l a v i c h e : la f u m a r o l a g , con t e m p e r a t u r a i n i z i a l e di 3 0 5 ° , la piìi e l e v a t a rlcl g r u p p o , c l i c nel magiiio scese a 160° e n e l s e t t e m b r e d e l 1 9 4 7 a 4 0 " , si p r e s e n t ò s e m p r e senza c l o r u r o a m m o n i c o m a con t r a c c e di a c i d o c l o r i d r i c o e v a p o r e a c q u e o , tanto c h e le r o c c e c i r c o s t a n t i n e f u r o n o a t t a c c a l e con f o r m a z i o n e p r e v a l e n t e di m o l i s i t e . n e l l e a l t r e f u m a r o l e a b b o n d a v a n o i c r i s t a l l i di c l o r u r o di a m m o n i c o e si p r c s e n l a r o n o i n t o r b i d a t i da t r a c c e di c l o r u r o f e r r i c o ( m o l i s i t e ) . a b b i a m o p u r e p r o c e d u t o al l a r i c e r c a s i s t e m a t i c a d e i gas ed abb i a m o s e m p r e r i s c o n t r a t o u n a n o t e v o l e a c i d i t à p e r la p r e s e n z a di a c i d o c l o r i d r i c o , m e n t r e non a b b i a m o m a i c o n s t a t a t o m i n i m e t r a c c e n é di a n i d r i d e s o l f o r o s a , n é di i d r o g e n o s o l f o r a t o . il v a p o r e a c q u e o n o n è m a n c a t o in t u l l e le f u m a r o l e . a b b i a m o v o l u t o s e g u i r e l ' a n d a m e n t o d e l l e r e a z i o n i c h i m i c h e d e l l e s u d d e t t e fus . cucuzza s i l v e s t r i ( ; . ponte c o l a l a e i n e a del 1s47 & a m i a m i . m i i termico dei.i.e f u m a r o l e h m c h e di l i . ' ) 1n » m a r c i e p e r s t u d i a r e la g e n e i d e l c l o r u r o a m m o n i c o e dai d a t i ricavati r i t e n i a m o c h e non si possa v e r i f i c a r e la r e a z i o n e del m c l s e n s inv o c a l a dal d e v i l l e , p e r c h é a v r e m m o dovuto r i s c o n t r a r e n e l l e f u m a r o l e la p r e s e n z a di so-2 c o m e u l t i m o p r o d o t t o d e l l a r e a z i o n e : h,s + 2h.,0 + 211 ci. v, • 2mixx + so, d a l l r a p a r t e gli i n t o r b i d a m e n t i r i s c o n t r a l i n e i c r i s t a l l i di c l o r u r o a m m o n i c o c h e si s o s p e t t a v a n o dovuti a sostanze c a t r a m o s e , p r o v e n i e n t i d a l l a c a r b o n i z z a z i o n e d e l l e p i a n t e e p o l t e sotto la l a v a , sono r i s u l t a t i d e i c l o r u r i di f e r r o m i s t i a p o l v e r e s o t t i l e . p e r t a n t o eselud i a m o l ' o r i g i n e o r g a n i c a del c l o r u r o a m m o n i c o p r o s p e t t a t a dal b i n i seli a n c h e p e r il f a t t o c h e a b b i a m o r i s c o n t r a t o il c l o r u r o a m m o n i c o nel c r a t e r e s u b t e r m i n a l e ove non vi è stala m a i v e g e t a z i o n e . catania — istillilo vulcanologico l'ittico deu'vniversità — 1 9 1 7 . hi tssl \ l () da alcuni' osseti azioni continuali re stili andamento termico ili un gruppo di fumarole laviche dell eruzione etnea dei 1947 sono risultati bruschi innalzamenti di lem perni tira, mollo probabilmente, dovuti a spaccature di contrazione (sinclasi ) che si formano mano a mano che la lava consolida. ricerche chimiche eseguite sulle fumarole a cloruro ammonico fanno escludere l'ipotesi della loro origine organica. annals 48, 3, 2005+app1 445 annals of geophysics, vol. 48, n. 3, june 2005 key words ionosonde – automatic ionogram scaling – ionospheric data – vertical soundings 1. introduction ionospheric observations contributed to the knowledge of physical phenomena such as the radio propagation in ionised media, physical and chemical processes in upper atmosphere, ionosphere and magnetosphere coupling, solarterrestrial relations, etc. nowadays the scientific interest remains but the ionospheric observations are much more focused on radio propagation forecasting. to achieve this objective the new ionospheric sounders should have some distinctive characteristics especially oriented towards routine service with network link and automatic scaling of the ionograms. like other recent sounders, ais-ingv ionosonde is practically built around a pc that constitutes the most important part (fig. 1). this ionosonde has been designed to fulfil certain physical characteristics such as the power reduction (around 200 w against several kilowatts of traditional systems) and consequently weight, size, power consumption and hardware complexity. it exploits the computer resources to manage the sounding, real time signal processing, data storing and sharing; it also has the capability to be remotely programmed. the basic work of this prototype is to generate an ionogram from which virtual heights and critical frequencies can be scaled. this basic system allows future expansions including polarization measurement and doppler analysis. this first prototype has been installed at ingv gibilmanna ionospheric observatory located in the centre of the mediterranean area where no systematic ionospheric observations have been performed. in recent years the growing interest in real time mapping and short term forecasts produced efforts to achieve real time scalnew low power pulse compressed ionosonde at gibilmanna ionospheric observatory baskaradas james arokiasamy (2), cesidio bianchi (1), michael pezzopane (1), vincenzo romano (1), umberto sciacca (1), carlo scotto (1), giuseppe tutone (1) and enrico zuccheretti (1) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) tril fellow, the abdus salam international centre for theoretical physics (ictp), trieste, italy abstract a digital low power pulse compressed ionosonde was developed at the istituto nazionale di geofisica e vulcanologia (ingv), rome, italy. the aim of this advanced ionospheric sounder, ais-ingv, is to reduce the transmitted power and, consequently, weight, size, power consumption and hardware complexity. to compensate the power reduction the most advanced hf radar techniques such as the pulse compression and a phase coherent integration are used. the ionosonde is completely programmable and a pc supports the data acquisition, control, storage and on-line processing. the first prototype was installed at gibilmanna ionospheric observatory (sicily), an interesting location in the center of mediterranean area. the new ionosonde will contribute to ionospheric database and real time knowledge of south european ionospheric conditions for space weather applications. in this work the first results (ionograms and autoscaled characteristics) are presented and briefly discussed. mailing address: dr. enrico zuccheretti, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: zuccheretti@ingv.it 446 baskaradas james arokiasamy et al. ing of ionograms which led to many softwares (e.g., reinisch and huang, 1983; fox and blundell, 1989; igi et al., 1993; tsai and berkey, 2000). among these, widely used and tested (gilbert and smith, 1988) is the artist system, developed at the university of lowell, center for atmospheric research. the ingv ionospheric laboratory designed and developed software to scale ionospheric parameters fof2 and muf(3000)f2 automatically within a few minutes after every sounding. to date, autoscaled characteristics as well as ionograms are available real time at the site <http://eskimo.ingv.it>. fig. 1. block diagram of the ais-ingv ionosonde: red lines are digital signals while blue lines are analog. 447 new low power pulse compressed ionosonde at gibilmanna ionospheric observatory 2. description of the basic principle and system characteristics the specifications of the new system are reported in zuccheretti et al. (2003) and arokiasamy et al. (2002). the new ionosonde was designed on the base of radar systems theory (skolnik, 1980, 1997) applied to the study of the ionosphere (hunsucker, 1991). we exploited the information of the transmitted code, as in other phase-coded hf-vhf radars, to perform the pulse compression and the coherent integration (a more detailed description of the mathematical processes is given in bianchi et al., 2003). according to a general radar equation (le chevalier, 2002) the received power pr is p r p g l g 4 1 4r t t p r 2 2 $ $= r r m (2.1) where pt is transmitted power, r is the range, gt and gr are the transmitting and receiving antenna gain, λ is the wavelength, and lp represents all the losses. the bandwidth of the receiver, being less than the thermal noise, can be neglected with respect to the environmental noise n. the minimum s/n useful for detection is /s n n p g gr a process$ $ = (2.2) where ga includes all the analog gain in the receiver chain and gprocess is the processing gain. to achieve the desired s/n, the processing gain gprocess must be greater than 20-30 db, because of various factors like poor antenna gain (less than 2 dbi), the reduced transmitted power and the maximum required range. this constraint imposes limits on the system parameters like the pulse repetition frequency (prf), pulse width t, subpulse number m, phase-code and modulation characteristics (bianchi et al., 2003). a 16 chips bi-phase complementary code has been employed (golay, 1961); this particular code theoretically eliminates the side lobes as they are opposite in phase (fig. 2). the processing gain after the pulse compression, expressed as s/n, is given by 10log(m) that for m = 16 is approximately 12 db. a further contribution to the gain comes from the integration process based on the ionospheric coherence. the process of the phase coherent integration, depending on the ionospheric variability, can be performed till the phase of echoes sequence differs less than π / 2, which limits the number n of the integrations. at gibilmanna ionospheric obfig. 2. correlation results for code 1, code 2 and their addition. on x axis distance of an echo in km is indicated, while on y axes an arbitrary scale is used. table i. processing and phase code features. code specifications type bi-phase complementary code length t 480 µs subpulse number m 16 subpulse length τ 30 µs pulse repetition 30 hz frequency (prf) processing gain due 12 db to the correlation m processing gain due 8-18 db to the integration n 448 baskaradas james arokiasamy et al. servatory different tests suggested a maximum number of integrations of 30. table i summarizes the main parameters of the phase code and processing gain; other parameters have been chosen on the basis of the radar techniques. assuming light speed as the radio wave velocity and 750 km as the range, the maximum prf should be around 200 hz, but processing time limits this parameter to 30 hz; this value of prf still makes the integration process advantageous. other design parameters are related to spatial resolution and minimum detectable distance from the ground (arokiasamy et al., 2002). a pulse width of 480 µs and 16 chips code with a chip length of 30 µs lead to a minimum range of 72 km and a radar resolution of around 5 km (table ii). 3. the ingv software for automatic scaling of ionograms the ingv software is based on a technique of image recognition and is able to work without polarization information. hence it can be used table ii. ais-ingv programming parameters. parameter requirement height range (90–750) km distance resolution 5 km peak transmitted power 200 w (5 ∼ 10 w) (medium power) receiver sensitivity ∼ –85 dbm for 0 db s/n input dynamic range ∼ 80 db frequency range (1–20) mhz frequency resolution 25 khz, 50 khz, 100 khz scan duration 3 min (1.5 mhz-20 mhz, 50 khz step) acquisition sampling rate 100 khz acquisition quantization 8 bit storage data rate (max) 60 kbytes (with 50 khz step) fig. 3a-c. a) a typical ais-ingv ionogram. b) selected element of the family of functions superposed on the ionogram. the automatically detected traces are reported (in red the ordinary trace and in yellow the extraordinary trace). c) vertical asymptote and tangent transmission curve of the automatically ordinary trace detected in correspondence of which fof2 and muf(3000)f2 are calculated. a b c 449 new low power pulse compressed ionosonde at gibilmanna ionospheric observatory with both single antenna system and crossed antenna system. a family of empirical functions having the typical shape of the f2 trace is considered. a particular element of this family is selected by a maximum contrast technique and it is assumed as representative of the f2-layer trace. the vertical asymptote of the selected function corresponds to the critical frequency fof2; the muf(3000)f2 is calculated numerically finding the transmission curve tangent to the selected function (fig. 3a-c). using different families of empirical functions this method can in principle be applied for the identification of the other ionospheric layers. for radiopropagation purposes the real time scaling of e sporadic and f1layers would also be important. with respect to the previous version of ingv software for automatic scaling of ionograms (scotto and pezzopane, 2002), two main improvements have been introduced: 1) the parameterization of the girofrequency that makes this version able to scale ionograms recorded in any location. 2) the capability to identify ionograms with sufficient information, to make them properly scaled. if the ionogram is identified by the software to have insufficient information it is discarded by the program and neither the fof2 nor the muf(3000)f2 are given as an output. to test the software a comparison between the values scaled by an operator and by the program was performed considering 1124 ionograms recorded by the ais-ingv installed at gibilmanna ionospheric observatory in 2002 from december 1st to december 15th. 3.1. quantitative estimation of the software’s capability to identify ionograms with sufficient information to test the capability of the software to properly identify the ionograms with sufficient information, the processed ionograms were divided into two subsets: subset s, that contains the ionograms considered by the program with sufficient information, hence scaled, and subset n containing ionograms considered by the program with insufficient information, hence discarded. for each subset we considered: a) the number of ionograms for which the operator was able to scale neither the fof2 nor the muf(3000)f2; b) the number of ionograms for which the operator was able to scale fof2 only; c) the number of ionograms for which the operator was able to scale muf(3000)f2 only; d) the number of ionograms for which the operator was able to scale both fof2 and muf(3000)f2. the results of this analysis are reported in table iii. it can be observed that 9 ionograms are considered with sufficient information without having it, while 7 ionograms are improperly considered with insufficient information. this shows that the error percentage in recognising the trace of the ionogram by the software is 1.4% (16 out of 1124). with reference to this it is important to emphasize that for the software the lack of trace near the f2 region asymptote makes the choice of a particular element of the family of functions more difficult. this lack of trace sometimes leads the software to discard the ionogram even if an operator could scale muf(3000)f2. on the contrary if the trace near the f2 region asymptote is clearly visible the program scales the ionogram for both while an operator could scale only fof2. we can therefore conclude that the software capability to identify ionograms with sufficient information is quite good. 3.2. test of accuracy and acceptability of the automatically scaled parameters in this work an accurate value is considered to lie within ± 0.1 mhz of the value obtained by the operator for fof2 and ± 0.5 mhz for muf(3000)f2. an acceptable value is considered to lie within ± 0.5 mhz for fof2 and ± 2.5 mhz for muf(3000)f2. limits of acceptability have been adopted in accordance with the ursi limits of ± 5 ∆ (∆ is the reading accuracy). with reference to the ionograms scaled by the ingv software, the following three subsets have been considered (see table iii): subset a, containing the ionograms in which the operator was able to scale both the critical frequency fof2 and the muf(3000)f2; subset b, containing the ionograms in which the operator was 450 baskaradas james arokiasamy et al. able to scale the muf(3000)f2 only; subset c, containing the ionograms in which the operator was able to scale the fof2 only. a quantitative comparison between the values scaled automatically and manually has been performed and the results of the comparison are reported in table iv. 4. observatory and data management the software performs all the system operations and data management. sounding parameters, number of integrations and sounding time scheduling, can be remotely changed at any time writing a new configuration file downloadable by the ionosonde, so remote control is also possible. the system is completed with a device able to turn the amplifier on and to connect the antenna system just during the sounding time, in order to use less energy and to protect the system against esd due to storms. at present the ionosonde performs soundings every fifteen minutes using a frequency range from 1.5 mhz to 13.0 mhz with a step of 50 khz. at the end of every sounding the acquired data file is immediately available on <ftp://relay.ingv.it>, while processed ionograms and autoscaled data are available at the web site <http://eskimo.ingv.it> within few minutes as gif file and txt file respectively. the research team makes also a manual validation of the hourly data that can be shared on demand. 5. conclusions nowadays ionosondes are intended to be ionospheric conditions monitoring systems, rather than being just scientific instruments for studying the physics of the ionosphere. in fact short-term ionospheric forecasts are based on real-time data from vertical sounders and best results could be obtained connecting several ionosondes in a net covering an area and obtaining data from it. this means that a modern efficient ionosonde has to be low cost, easy to install and remotely programmable; in addition it should have the possibility to autotable iv. subset a, subset b and ubset c: acceptability and accuracy percentages of autoscaled parameters. subset a subset b subset c fof2 muf(3000)f2 muf(3000)f2 fof2 n° [%] n° [%] n° [%] n° [%] total 847 100.0 847 100.0 182 100.0 12 100.0 acceptable 833 98.3 847 100.0 179 98.5 9 75.0 accurate 638 75.3 827 97.5 170 93.5 6 50.0 table iii. test results of the software capability in identifying ionograms with sufficient information. no. of cases scaled no. of cases discarded by the ingv software by the ingv software (subset s) (subset n) the operator scaled neither fof2 nor muf(3000)f2 9 46 the operator scaled both fof2 and muf(3000)f2 847 (subset a) 7 the operator scaled muf(3000)f2 only 182 (subset b) 21 the operator scaled fof2 only 12 (subset c) 0 total 1050 74 451 new low power pulse compressed ionosonde at gibilmanna ionospheric observatory matically scale the main characteristics giving a contribution to have reliable forecasts within few minutes after sounding. the ais-ingv ionosonde has been designed taking into account simplicity and flexibility to be installed even in a remote observatory. moreover the automatic scaling of fof2 and muf(3000)f2 suggests that the system can be a node of a possible net for space weather purposes. references arokiasamy, b.j., c. bianchi, u. sciacca, g. tutone and e. zuccheretti (2002): the new ais-ingv digital ionosonde design report, ingv internal tech. rep. no. 12. bianchi, c., u. sciacca, a. zirizzotti, e. zuccheretti and b.j. arokiasamy (2003): signal processing for phasecoded hf-vhf radars, ann. geophysics, 46 (4), 697-705. fox, m.w. and c. blundell (1989): automatic scaling of digital ionograms, radio sci., 24, 747-761. gilbert, j.d. and r.w. smith (1988), a comparison between automatic ionogram scaling system artist and the standard manual method, radio sci., 23, 968-974. golay, m.j.e. (1961): complementary series, ire trans. inf. theory, it-7, n.y. institute of radio engineers, 82-87. hunsucker, r.d. (1991): radio technique for probing the terrestrial ionosphere (springer-verlag n.y.), pp. 293. igi, s., k. nozaki, m. nagayama, a. ohtani, h. kato and k. igarashi (1993): automatic ionogram processing systems in japan, in proceedings of session g4 at the xxivth general assembly of the international union of radio science (ursi), august 25-september 2, 1993, kyoto, japan. le chevalier, f. (2002): principles of radar and sonar signal processing (artech house), pp. 418. reinisch, b.w. and x. huang (1983): automatic calculation of electron density profiles from digital ionograms 3. processing of bottomside ionograms, radio sci., 18, 477-492. scotto, c. and m. pezzopane (2002): a software for automatic scaling of fof2 and muf(3000)f2 from ionograms, in proceedings of ursi 2002, 17-24 august 2002, maastricht. skolnik, m.i. (1980): introduction to radar systems (mcgraw-hill n.y.), pp. 581. skolnik, m.i. (1997): radar handbook (mcgraw-hill n.y.). tsai, l.c. and f.t. berkey (2000): ionogram analysis using fuzzy segmentation and connectedness techniques, radio sci., 35, 1173-1186. zuccheretti, e., c. bianchi, u. sciacca, g. tutone, and b.j. arokiasamy (2003): the new ais-ingv digital ionosonde, ann. geophysics, 46 (4), 647-659. ag_57.04.14_dominici-meloni_finalonline_layout 6 annals of geophysics, 57, 4, 2014, g0433; doi:10.4401/ag-6525 g0433 revision of the italian magnetic database for the albegna basin (south tuscany, italy) guido dominici*, antonio meloni istituto nazionale di geofisica e vulcanologia, sezione roma 2, rome, italy abstract a comparison between ground level total magnetic field intensity anomaly map (f) of italy and the total intensity aeromagnetic map by eni/agip, had shown that an anomaly pattern for the albegna basin (south tuscany), quite evident from ground measurements, doesn’t show in the aeromagnetic map. ligurian units, made of ophiolite blocks (metagabbros, basalts, serpentinites), intrusives and subordinate volcanic products, all able to trigger a strong magnetic signal, could not be excluded in the area, and for this reason the magnetic anomaly estimated by ground level measurements was not considered unreasonable. in this paper the result of a magnetic survey finalized to verify the authentic existence of such a large magnetic total intensity anomaly in the albegna basin, is reported. on the basis of the new result, the suspected ground level total intensity anomaly in the albegna basin, was demonstrated to be non-existent and then the italian magnetic database corrected accordingly. measurements and procedures that brought to the magnetic elements elaboration and new anomaly maps for albegna basin, are shown here. 1. ground level magnetic maps and the sea level magnetic anomaly map of italy magnetic field measurements are undertaken in different ways and for different reasons. observatories monitor time variations of the field, while measurements carried out on land and sea by ships, helicopters, planes, and also onboard satellites at low orbital altitudes, provide information concerning the spatial structure of the field and then give information on the main earth’s magnetic field and on that of crustal origin. in particular, for crustal magnetism studies, ground measurements are widely used for their immediate proximity to the magnetic sources. information on earth’s magnetism can be found, for example, in campbell [2003], lanza and meloni [2006], encyclopedia of geomagnetism and paleomagnetism [2007]. in many nations, ground magnetic measurements are organized in networks of survey points. the socalled first order networks are devoted mainly to secular variation monitoring, while second order networks, with higher spatial detail, are dedicated to geomagnetic elements map compilation and to crustal magnetism studies. as shown in figure 1, the first order italian magnetic network has an average stations spacing of about 100 km, while the second order network has an average spacing of about 5-10 km. every five years, for their institutional roles, ingv (istituto nazionale di geofisica e vulcanologia) and igm (istituto geografico militare) carry out measurements (total intensity f, horizontal component h, vertical component z, declination d and inclination i) over the about 120 repeat stations of the first order magnetic network (figure 1). the aim of this periodic effort is to replicate magnetic measurements at repeat stations in order to monitor geomagnetic field secular variation and to compute updated coefficients of regional models for representing the main geomagnetic patterns over italy, at the various epochs. in italy ground level magnetic measurements on the second order network were performed in an organized national effort only once between 1977 and 1981, within the framework of the progetto finalizzato geodinamica of the consiglio nazionale delle ricerche (pfg-cnr). all the work was coordinated by the then ing (istituto nazionale di geofisica). at that time a database of 2552 magnetic data (total intensity f, horizontal component h, vertical component z) was assembled with the objective to produce magnetic field maps of italy for intensive elements and to successively compile a magnetic crustal field anomaly map for italy [molina et al. 1986]. magnetic anomaly maps are produced starting from field measurements, after the earth’s core field and magnetic time variations, are removed. the removal of the core main field is generally undertaken by subtraction of igrf (international geomagnetic reference field; finlay et al. [2010]) or by the removal of article history received march 6, 2014; accepted june 26, 2014. subject classification: geomagnetism, magnetic survey, reference field, secular variation, magnetic cartography. an equivalent regional magnetic field that can be derived directly from measured data. removal of time variations of earth’s magnetic field is done by using a magnetic observatory time measurements. in our case all field measurements are reduced to nighttime level (generally 02.00 ut) and corrected for diurnal and possible perturbations in the field. magnetic field maps of italy for intensive elements (f, h and z), from second order network data, were published for the land areas [molina et al. 1985] and also a total field anomaly magnetic map was produced and published, initially for the land area only, excluding the surrounding seas [molina et al. 1994]. one important result of the five year surveys on the first order network, is to provide an analytical formula able to represent, at large, the earth’s core magnetic field over a certain area, this is the so-called regional magnetic field model. regional fields can be formulated as a second order degree polynomial expression of the form e(z,m) = a'0 + a1z + a2m + a3z 2 + a4m 2 + a5zm where e is the generic geomagnetic element, m the longitude, z the latitude (both expressed in degrees or minutes of arc), a'0 is the base level (in nt) for each element, an(n=1.5) are the coefficients, expressed in nt/degree or nt/minutes of arc in longitude and latitude. even if regional field models can include some contribution of deep crustal origin, superimposed to the core field, it has been shown that at the spatial scale of italy they are in a good fit to igrf [finlay et al. 2010] and effectively represent the core field (see for example, de santis et al. [1997]). in italy the regional polynomial expression called itgrf (italian geomagnetic reference field) is regularly updated by using the five years repeat station measurements: this model avoids the boundary effects that are commonly present in the regional fields and makes use of the secular variation as determined by observatory measurements (for details, see de santis et al. [2003]). once regional models are computed at five year intervals, they are used to update all measurements of the italian area, like for example second order network data points. every five years the national maps of magnetic elements at the specific epoch are then redrawn and the magnetic database is updated. for the various procedures on this point see dominici et al. [2007], and the last compilation for year 2010.0 in dominici et al. [2012]. between 1965-1972, 28,833 offshore total field intensity data were acquired by oceanographic ships by the osservatorio geofisico sperimentale (ogs) in the seas surrounding italy [morelli et al. 1969, morelli 1970a, 1970b]. at the end of the 90s of last century, onshore and offshore total field data were merged in order to obtain a single, large total field data set. this data set was then reprocessed and all data were reduced to the common epoch 1979.0, using the itgrf as main regional reference field. all data (2552 onshore and 28,833 offshore) elaborated by ingv, constitute the italian magnetic database. finally, data of the italian database wase integrated for the compilation of a sea level shaded relief magnetic anomaly map of total intensity magnetic field for italy and the surrounding seas, for the geomagnetic epoch 1979.0 [chiappini et al. 2000]. the published map was considered the final product of a long time effort, allowing an unprecedented view of magnetic anomalies in italy and showing their relations to the major tectonic elements in their regional setting, confirming a good correlation between known structural geology and magnetic anomalies. 2. aeromagnetic anomaly map of italy for what concerns aeromagnetic investigations, the italian national oil company agip (azienda generale italiana petroli, now part of eni, ente nazionale idrocarburi) commissioned an aeromagnetic survey of italy to a private company in the early 70s of last century. agip collected all total field magnetic data at vardominici and meloni 2 not surveyed heavily anomalous areas first order network second order network figure 1. italian magnetic measurements network. red stars refer to first order network (repeat stations) while black crosses refer to the second order network points, as undertaken by pfg-cnr for elements f, h and z. all second order network data points were measured between 1977-1981, while only first order network points are repeated every five years for secular variation monitoring. 3 ious altitudes, and drew a first aeromagnetic anomaly map for the italian territory in 1981 and a later contour version superimposed to regional geological features in 1994 [agip 1981, cassano et al. 1986, servizio geologico nazionale 1994]. an inspection of the two maps, the one produced by ingv at ground level, and the one produced by agip, has shown differences that could not be attributed to measurement problems. this was noticeable especially in the long wavelength magnetic anomaly features. it was realized that the discrepancy could be attributed to an incorrect removal of the reference field from measurements in the agip aeromagnetic compilation map; in fact, neither the igrf or other regional magnetic models were used for reference. only in 2005, after a revised version of the aeromagnetic map of italy and surrounding seas was undertaken (also including the addition of some new surveys), agip company allowed a re-processing of all the aeromagnetic data set. the result of this work was a new map that used igrf as main reference field [caratori tontini et al. 2004]. the detailed reprocessing of aeromagnetic data at the national scale, the inclusion of the new data, the subtraction of an appropriate reference level, are the main reasons for the tangible enhancement of the anomaly definition emerging from this new aeromagnetic anomaly compilation. finally the two anomaly maps, ground level and aeromagnetic, could be fruitfully used for magnetic crustal studies. the two maps are in fact comparable, provided that ingv map is referred to the sea level and the agip-eni map to an average elevation of 2500 m and, consequently, the resulting anomalies are enhanced or flatted in the two maps. figure 2 shows the maps from italian database (panel a) and eni/agip database (panel b). one of the local striking features that comes to the eye in the comparison of the two maps at the local scale, is the case of the albegna basin (south tuscany), that is the area delimited by the red square. in fact, the two maps display a different anomaly structure in this area. in detail, a strong magnetic anomaly circularly shaped that can be seen in the ground level map (panel a), does not show in the aeromagnetic map (panel b). 3. albegna basin: geological characteristics and magnetic implications the study area of interest for this paper (figure 3) is located in the neogene albegna basin, a region characterized by a complex tectonic and sedimentary evolution. this area is delimited to the north by the volcanic complex of the mount amiata and to the east by the volcanic complex of the vulsini mountains. in the compressive phase (ended in the early miocene), the crust thickening resulted in an emplacement of tectonic units originating by different paleogeographic domains. various authors [bettelli 1980, bonazzi et al. 1992, bonciani et al. 2005] worked out a dynamic preand syn-collisional tectonic model for the area and a post-collisional extension event framework. albegna basin magnetic data revision -450 -350 -250 -150 -50 50 150 250 350 450 550 650 750 850 a b m a g n e ti c a n o m a ly [ n t ] m a g n e ti c a n o m a ly [ n t ] figure 2. total intensity magnetic anomaly maps: a) ground level from italian database; b) aeromagnetic from agip/eni database. in both maps, the red rectangles show the area referred to in this paper. from the bottom to the top, the nappe (nappe is a large sheetlike rock that has been moved from original position for tectonic movements) superposition is represented by: the metamorphic units of the tuscan basement that crop out in the south-east margin of the area with the “verrucano formation”; the tuscan nappe unit, a sedimentary succession, that ranges from the evaporites of “anidriti di burano formation” to the sandstone and siltstone of “macigno formation”, with wide outcrop in this area; the ligurian unit, a tectonic unit derived from external oceanic domain (the “canetolo formation”, the “pietraforte formation”) up to the internal oceanic ligurian domain of “ofioliti formation”. then, in the extensional events, the sedimentation is represented by continental-lacustrine deposits controlled by a system of normal faults nnw-sse with crustal thinning and after, in the lower pliocene, by a transgressive sequence where an unconformity sedimentation of shallow-marine deposits controlled by a different system sw-ne takes place; the sedimentary evolution ended with pleistocenic sequences. the presence, in the ligurian units, of ophiolite blocks (metagabbros, basalts, serpentinites) which trigger a strong magnetic signal, is a possible source of magnetic crustal anomaly. ophiolites are not detected in the study area (they are certainly present only at north) but the hypothetical occurrence of ophiolite blocks below the sedimentary successions which filled the albegna basin, could not be excluded. another important magnetic inference is given by the magmatic activity (7.0-4.0 ma) related to the extensional process [carmignani et al. 1994, jolivet et al. 1998, acocella et al. 2002], mainly through anatectic intrusives and subordinate volcanic products, as in the near tuscan magmatic province of mount amiata, or the younger (1.7 ma-present) potassium alkaline volcanism of the roman province, as in the near volcanic complex of the vulsini mountains. moreover, a possible uplift of a magmatic batholith can also be a cause of magnetic anomaly, although not supported by other data. for all reasons above reported the magnetic anomaly found in the ground level map in the albegna basin, was considered reasonable and only after the final compilation of the new aeromagnetic map, the area was taken in consideration for deeper investigation. 4. magnetic survey and elaboration figure 4 shows the second order network onshore data (red crosses) and the contour values of the total field magnetic anomaly for the region of interest. the mean distance between the stations is of about 9 km. the definitive answer to the question whether the anomaly under investigation was real or not could come only from a new set of measurements. for this reason a new dedicated magnetic survey was performed in area: in figure 4 the 19 stations of the new survey are marked in blue points. the distribution of the new measurement points dominici and meloni 4 42.4 42.5 42.6 42.7 11.2 11.4 11.6 longitude e la ti tu d e n figure 3. geological sketch map of albegna basin. legend: 1=tuscan units; 2=ligurian units; 3=mt. amiata and vulsini volcanic units; 4=miocene clastic deposits; 5=pliocene-holocene sedimentary units; 6=normal faults; the rectangle on the map refers to the area referred in this paper. 5 was focused to obtain a concentric geometry and the doubling of the density of stations around the anomaly centre. some stations of the new survey are very near to the stations of the old network, i.e. stations number 1, 13, 16, 17, 18, 19. at these stations the total field f was measured with an overhauser magnetometer, and also the magnetic inclination i with a dim, declination inclination magnetometer. at the other stations only the total field f was measured. all new measurements were reduced to a common day-time, at 02ut hour (considered as the magnetic quite time level) and, using l’aquila observatory data (aq), to a mean epoch. the general procedure requires that the value of element e (i.e., f, h or z) at station s, reduced at 02ut of day d (for this survey the day is september 3rd, 2007), is calculated following the formula: es (02ut)d = eaq (02ut)d + ⎣ es (t)d − es (t)d ⎦ where es (02ut)d is the value of e at station s reduced at time 02ut of day of measure d, eaq (02ut)d is the value of e at observatory at time 02ut of day of measure d, es (t)d is the value of element e observed at station s at time t at that day d, eaq (t)d is the value of element e at observatory at time t at that day d. in table 1 the geographic coordinates of the stations with the values at the ground level of es (02ut)09/03/2007, where e is the magnetic element, are reported. the element e at station s computed at epoch 2005.0 ( january 1st, 2005) is obtained according to es(2005.0) = eaq(2005.0) + ⎣ es(02ut)d − eaq(02ut)d ⎦, where es(2005.0) is the value of e at station s reduced at epoch 2005.0, eaq(2005.0) is the average value of e at albegna basin magnetic data revision 11 11.2 11.4 11.6 42.4 42.5 42.6 42.7 7-27 2 -2 -10 -16 -15 9 -13 364 -43 -25 -47 -33 3 -745 -15-12 -15 -1 -73 -36 -23 -16 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 nt longitude e la ti tu d e n figure 4. total intensity anomaly map over albegna basin: crosses show the onshore stations from italian database with the corresponding values of anomaly, and blue stars indicate the new measurements. station latitude longitude elevation fs(02ut)09/03/2007 hs(02ut)09/03/2007 zs(02ut)09/03/2007 1 42.53350 11.36221 128 46251.4 2 42.52823 11.36488 122 46262.9 3 42.52625 11.36918 105 46258.1 24040.5 39520.4 4 42.56002 11.39479 105 46266.5 5 42.56148 11.33409 64 46260.4 6 42.50813 11.42970 109 46260.8 7 42.46215 11.51502 190 46291.3 8 42.51749 11.46162 88 46332.7 9 42.51301 11.47297 110 46279.3 10 42.54005 11.30206 12 46261.6 11 42.51400 11.32638 16 46236.0 12 42.49473 11.37053 34 46301.2 13 42.46282 11.39454 45 46199.0 24026.6 39459.7 14 42.58813 11.41664 89 46281.5 15 42.59515 11.30521 66 46267.8 16 42.54303 11.25712 21 46254.3 24002.2 39539.3 17 42.49373 11.28276 71 46222.8 24021.3 39490.8 18 42.62627 11.49241 224 46288.9 23946.9 39613.2 19 42.55345 11.51112 235 46265.7 23986.4 39562.2 table 1. coordinates of stations with the values of magnetic elements at 02ut hours of day september 3rd, 2007. l’aquila observatory at epoch 2005.0, eaq(02ut)d is the value of e at l’aquila observatory at time 02ut of the day of measure d, es(02ut)d is the value of e at station s at time 02ut. considering that each station f value had its own elevation above sea level, a reduction to the sea level for all stations data has been made considering only the earth’s centered dipole contribution, according to the equation δe=3eh/r, where e is the generic geomagnetic element, h is the station elevation a.s.l., r is the average earth radius. finally, in order to make this survey exploitable for a comparison with the published anomaly maps, all data were reduced at the epoch 1979.0, using the secular variation computed by the regional main reference field itgrf. in table 2 the expressions with the coeffidominici and meloni 6 f (nt) = 46063.1 +5.72335 { +1.22044 m −0.00184 {2 −0.00003 m2 −0.00047 {m 2005.0 f (nt) = 45388.4 +5.709 { +1.111 m −0.00153 {2 +0.00049 m2 −0.00068 {m 1979.0 h (nt) = 24283.9 −9.38524 { −0.09122 m +0.00009 {2 −0.00008 m2 +0.00002 {m 2005.0 h (nt) = 24104.2 -9.043 { +0.110 m +0.00036 {2 +0.00004 m2 −0.00042 {m 1979.0 z (nt) = 39133.1 +12.68257 { +1.60346 m −0.00493 {2 −0.00016 m2 −0.00122 {m 2005.0 z (nt) = 38451.7 +12.467 { +1.259 m −0.00444 {2 +0.00060 m2 −0.00069 {m 1979.0 table 3. values of magnetic element at 1979.0: eingv(1979.0) = values of element e at 1979.0 from italian database; ° = values of element e on station s reduced at 1979.0 with the itgrf; °° = values of anomaly of the element e on station s respect itgrf; °°° = values of anomaly of the element e on italian database with respect to itgrf. ° ; °° ; °°° . table 2. coefficients of the italian geomagnetic reference field (itgrf) for the epochs indicated at the end of each row, for the magnetic elements f, h, z. latitude { and longitude m are referred to 42°n and 12°e, respectively, with values expressed in degrees. f, h, z are in nt. st at io n f i n g v (1 97 9. 0) h in g v (1 97 9. 0) z in g v (1 97 9. 0) 1 45490.8 −21 2 45502.3 −8 3 45891.0 45497.5 −12 381 24538.0 23863.2 8 −317 38780.0 38747.9 −9 23 4 45505.8 −17 5 45499.8 −20 6 45500.1 −8 7 45530.5 34 8 45571.9 58 9 45518.5 6 10 45501.1 −11 11 45475.5 −29 12 45540.6 40 13 45457.0 45438.4 −53 −34 23906.0 23848.7 −42 16 38663.0 38687.3 −26 −50 14 45520.7 −14 15 45507.3 −22 16 45511.0 45493.9 −16 2 23883.0 23823.1 −23 37 38741.0 38768.3 6 −21 17 45465.0 45462.5 −31 −29 23902.0 23841.9 −31 29 38675.0 38719.8 −7 −52 18 45549.0 45527.9 −21 0 23796.0 23773.5 −27 −4 38839.0 38838.3 1 2 19 45537.0 45504.7 −21 11 23863.0 23812.1 −28 23 38784.0 38787.6 3 −1 a n z in g v it g r f ^ h 1 2 3 44 44 19 79 .0 z s it g r f ^ h 1 2 3 44 44 a n h in g v it g r f ^ h 1 2 3 44 44 19 79 .0 h s it g r f ^ h 1 2 3 44 44 a n f in g v it g r f ^ h 1 2 3 44 44 19 79 .0 f s it g r f ^ h 1 2 3 44 44 ( . )e 1979 0s itgrf 1 2 344 44 ( )an e itgrf ingv1 2 344 44 7 cients of the reference field itgrf are listed. itgrf 1979.0 epoch values are calculated by linear interpolation between neighboring sets 1975.0 and 1980.0. the value of the generic element e at the epoch 1979.0 is deduced by es(1979.0) = es(2005.0) − ⎣eitgrf (2005.0) − eitgrf (1979.0)⎦ where eitgrf (epoch) is the value of element e calculated with the expressions of table 2 at the two epochs, and ⎣eitgrf (2005.0) − eitgrf (1979.0)⎦ represents the value of secular variation between the two epochs. in table 3 all values of measurements reduced at the epoch 1979.0 with the secular variation calculated by itgrf and the anomalies values with respect the itgrf, both for this survey and for italian database are reported. 5. conclusions in this paper we have considered two total magnetic anomaly field maps compiled for italy in the last years. the first is the sea level shaded relief magnetic anomaly map of total intensity of the earth’s magnetic field for italy and the surrounding seas, for the geomagnetic epoch 1979.0 [chiappini et al. 2000]. the second is the aeromagnetic map [caratori tontini et al. 2004] compilation obtained after a revision of an older version of the aeromagnetic map of italy, and surrounding seas originally undertaken by agip, and referred to the same epoch. one of the striking features that emerged from the comparison of the two maps, was a discrepancy in the area of the albegna basin (south tuscany). in figure 2 the dissimilar anomaly structure in the area is delimited by red squares. a strong magnetic anomaly, circularly shaped, can be clearly seen in the ground level map but not in the aeromagnetic map. albegna basin magnetic data revision 42.4 42.5 42.6 42.7 7-27 2 -2 -10 -16 -15 9 -14 -14 -30 -25 -51 -33 3 -745 -15-12 -21 -19 -73 -36 -23 -16 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 nt longitude e la ti tu d e n 43900 44100 44300 44500 44700 44900 45100 45300 45500 45700 45900 46100 46300 46500 46700 46900 47100 47300 47500 47700 47900 nt 46217 46223 46200 46190 46198 46178 46559 46132 46136 46125 46108 46149 4612046167 46218 46206 46137 46142 46124 46129 43900 44100 44300 44500 44700 44900 45100 45300 45500 45700 45900 46100 46300 46500 46700 46900 47100 47300 47500 47700 47900 nt 46217 46223 46200 46190 46198 46171 46175 46140 46136 46116 46108 46149 4612046167 46207 46183 46137 46142 46124 46129 figure 5. new total intensity anomaly map over albegna basin: crosses indicate the onshore stations by italian database now corrected. figure 6. two maps for comparison. left: map of total field f at 2005.0 by italian database with the enlargement of albegna basin and values. right: map of f at 2005.0 by italian database corrected after new data from survey described in this paper were included (red points in the enlargement). a magnetic survey made on purpose in 2007, with the data reduction to 1979.0 for a common reference epoch with the two above mentioned maps, was undertaken. by means of an independent new data set, a new evaluation of the situation is now possible. the new survey shows clearly that that there is no inconsistency between the two sets of ground level data except for the large anomaly value reported in the chiappini et al. [2000] map, and not at all present in the new survey results. this holds true not only for total field f but also for horizontal intensity h (not shown here), computed after also new inclination data i were used in this new elaboration. with this last consideration we assume that the old ground level measurements set included a wrong measurement value for f. after this revision of the total field data set, the incorrect station value in the italian database was corrected and, as reported in figures 5 and 6 the total intensity anomaly map for this area and the geomagnetic field map were redrawn. in the albegna area possible sources of magnetic crustal anomaly are actually present, ligurian units of ophiolite blocks (metagabbros, basalts, serpentinites), subordinate volcanic products, as in the near tuscan magmatic province of mount amiata, or the younger (1.7 ma-actual) potassium alkaline volcanism of roman province, as in the near volcanic complex of the vulsini mountains. for this reason the magnetic anomaly detected in the albegna area was initially considered feasible. however, after the new magnetic survey and following data elaboration, as shown in this paper, we can conclude that no strong magnetic anomalies are present in the area. aknowledgements. we would like to express our sincere gratitude to m. miconi for his help in the survey work. references acocella, v., v. pascucci and g. dominici (2000). basin deformation due to the laccolith emplacement at radicofani (southern tuscany, italy), b. soc. geol. ital., special issue, 1, 749-756. agip spa (1981). italia, carta magnetica: anomalie del campo magnetico residuo, scale 1:500,000, san donato milanese, italy. bettelli, g. 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(1970a). physiography, gravity and magnetism in the tyrrhenian sea, b. geofis. teor. appl., 12 (48), 275-309. morelli, c. (1970b). bathimetry, gravity and magnetism in the strait of sicily, saclantcen conference, proc. no. 7, la spezia. servizio geologico nazionale (1994). carta aeromagnetica d’italia. *corresponding author: guido dominici, istituto nazionale di geofisica e vulcanologia, sezione roma 2, rome, italy; email: guido.dominici@ingv.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. albegna basin magnetic data revision << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice o n t h e s e d i m e n t a r y s t r u c t u r e i n s o m e p o r t i o n s o f t h e p a c i f i c o c e a n f r o m d a t a o n s e i s m i c r e f l e c t e d w a v e s (*) s . m . z v e r e v r i c e v u t o il 7 dicembre 1960. 1. during 1957-1958 the institute for physics of the e a r t h of the ac. sci. ussr carried out investigations by deep seismic sounding (dss) of the earth's crust in the north-western part of the pacific ocean and in the adjoining aquatoria (l-2). the investigation technique consisted mainly in following on the seismograms the seismic waves coming from the deep boundaries in the crust. at the same tune data were -also collected on the structure of the upper layer consisting of ocean sediments. 2. simultaneously with the seismic waves coming from the deep crustal layers, vertical reflections from the bottom surface and from the boundaries in the sediments were also observed in order to investigate the structure of the sediments. the observations of the reflected waves were carried out by two methods. when heavy charges (above 100 kgr) were fired for dss observations, a unichannel set on the moving ship was used for recording reflected waves. a piezocrystal secured to the underwater part of the ship's hull served as a receiver. the preamplified signal was recorded in a wide frequency range and then passed through a narrow band-pass filter. the frequency band was so chosen as to increase the effective sensitivity of the channel. i t was known from experience t h a t good records of reflected waves were obtained in the 50-70 c/sec frequency range. a sample seismogram is shown in fig. 1. on this and similar seismograms not only reflections from subbottom boundaries are seen, b u t also pulses due to secondary shocks and possibly also to reflections from the water surface a t the shooting point. (*) p a p e r r e a d a t t h e helsinky assembly of t h e i . u . g . g . 1960. 1 8 8 s . m. z v e r e v b y this method reliable records of waves reflected from boundaries in the sediments were obtained a t ocean depths of 5-6 km. i n the region of the deep-water through the intensity of the reflected waves decreased and this method proved not sensitive enough to produce reliable records. the conditions for firing the charges and for reception were sufficiently uniform, so t h a t we were able to compare t h e behaviour of t h e reflecting boundaries over large areas. fig. 1 a s a m p l e record of waves reflected f r o m t h e b o t t o m s u r f a c e (b) a n d f r o m t h e b o u n d a r i e s in t h e s e d i m e n t s (1,2,3), o b t a i n e d on t h e m o v i n g ship. traces 4 a n d 5 a t t h e t o p are r e c o r d s of d i f f e r e n t l y amplified signals. 3. during t h e short stops of t h e moving ship recording was m a d e on a floating multichannel set similar to those used in prospecting for oil. i n t h e observations of 1958 a set consisting of six hydrophones distanced at 50 metres was used. their stretching into line was left to the ship's wind drift. the seismic observations of the reflected waves were carried out simultaneously with oceanologic observations. charges of 1-5 kgr suspended on rubber floats were used. the amplifiers chosen were similar to those used in prospecting. beflected waves having frequencies of 30-35 c/sec were recorded. a sample seismogram is given in fig. 2. since our reciever contained no volume control nor mixer, we were able to m a k e use of the dynamic characteristics of t h e records. because of different ways in which the charges were fired and records made, the absolute times of arrival of waves reflected f r o m t h e same b o u n d a r y obtained from both sets differed considerably. a f t e r corrections had been introduced for the m o v e m e n t of the ship and for mr-wsi js153 mob hp 9-0 k pi w / w a w / w a w / w a \a. wo pig. 2 a s a m p l e record of reflections f r o m t h e b o t t o m (b) a n d f r o m t h e b o u n d a r i e s in t h e s e d i m e n t s (1, 2, 3) o b t a i n e d on t h e m u l t i c h a n n e l set. traces 1-5 a t t h e t o p — are records f r o m different h y d r o p h o nes. traces 6-9 r e p r e s e n t , records f r o m t h e 1-t, 3-d a n d 5 t h h y d r o p h o n s h a v i n g a lower s e n s i t i v i t y . o a h b ' h co m o h h > » h( u1 h » a o h « w is 3 <» o w © w h o 2! 00 o h b •h > o o o a h > 188 s . m . z v e r e v different shooting depths this difference decreased. however, even t h e n the times of arrival of t h e b o t t o m reflections obtained from the multichannel set exceeded by 0,01-0,07 seconds those f r o m the records b y the unichannel set. this might have been connected with different depths of penetration into sediments of waves of different frequencies (lowfrequency waves p e n e t r a t e deeper). using t h e multichannel set, we were able to record waves reflected from the boundaries in the sediments in t h e regions of the ocean bed, of the deep-water trough and also in some places on the shelf. 5. i t should be noticed t h a t sections constructed from experimental d a t a on vertical reflections might be distorted by multiple reflections of waves in crustal layers, by interference of waves whose wavelengths are comensurable with t h e thickness of the layers involved and b y other complicating phenomena we are usually confronted with in seismic prospecting. i n our case when studying thin sedimentary layers underlying a deep water layer, with the time-distance curves extending not far enough, discrimination of interfering phenomena becomes even more difficult t h a n in usual prospecting. t h a t is why the here given sections should be considered as a simplest interpretation depicting to some extent t h e real situation. 6. vertical reflections were observed on all profiles m a d e in 1958. let us consider the d a t a obtained on t h e profiles running close t o t h e k a m c h a t k a peninsula and the k o m a n d o r islands profiles nono 7 and 9 crossing t h e continental shelf near t h e k a m c h a t k a peninsula and the k u r i l e k a m c h a t k a trough, ran into the deep-water bed of t h e pacific ocean extending a t a distance of 500 k m from t h e shore. profile no 8 crossed t h e shelf near the k o m a n d o r islands and also t h e aleutian deepwater trough, ending within t h e boundaries of the pacific border swell a t a distance of 150 k m f r o m the flat p a r t of the ocean bed. sections through these profiles are shown in fig. 3, 4, 5. the thin lines whose lengths are proportional to the intensities of the reflections taken from t h e seismograms illustrate the reflecting power of t h e boundary under consideration. the d a t a obtained f r o m the multichannel set are marked out. 7. the sections t h r o u g h the sediments are here represented in times of arrival instead of in thickness. this was done because with the methods used for observations of refracted waves (dss) a n d of vertical reflections p i g . 3 t i m e s of a r r i v a l of v e r t i c a l reflections on profile no 9 crossing t h e k u r i l e k a m c h a t k a d e e p w a t e r t r o u g h beginning f r o m t h e s o u t h e r n e x t r e m i t y of t h e k a m c h a t k a p e n i n s u l a . 1) t i m e s of a r r i v a l o b t a i n e d f r o m o b s e r v a t i o n s of reflections m a d e on t h e m o v i n g ship; 2) t i m e s of a r r i v a l o b t a i n e d f r o m t h e m u l t i c h a n n e l s e t ; 3) places on t h e profile w h e r e a s h a r p change in t h e t i m e s of arrival of reflections f r o m t h e b o t t o m a n d f r o m t h e b o u n d a r i e s in t h e s e d i m e n t s w a s observed. <00 150 200 250 j 00 350 400 450 5 0 0 550 p i g . 4 times of a r r i v a l of v e r t i c a l reflections o b t a i n e d on profile no 7 crossing t h e k u r i l e k a m c h a t k a deep-water t r o u g h f r o m t h e region of p e t r o p a v l o v s k k a m c h a t s k i . d e s i g n a t i o n s t h e s a m e as in fig. 3. o n t h e s e d i m e n t a r y s t r u c t u r e i n s o m e p o r t i o n s o f t h e p a c i f i c o c e a n 1 9 1 the determination of velocities in the sediments becomes extremely difficult. although some d a t a on the velocities were obtained, this problem cannot be considered as finally solved. however, the here given d a t a can be used for a comparative geologic interpretation. moreover, t h e velocity range in the sediments might be indicated and the thickness of separate layers roughly calculated. 2 0 0 1 7 5 1 3 0 i 2 j 0 0 7 j j o fig. 5 times of a r r i v a l of vertical reflections on profile no 8 crossing t h e a l e u t i a n d e e p w a t e r t r o u g h f r o m t h e k o m a d o r islands. 8. in evaluating t h e velocities in t h e sediments use was made of t h e multiple reflections in t h e water layer picked out from the t)ss seismograms and also of the reflection coefficient calculated from different vertical reflections. f r o m an analysis of the multiply reflected w a t e r waves, including the regions of growing amplitudes connected with the critical angles (3), on one of the profiles considered below running above t h e ocean bed, the following values of velocities were obtained: in t h e subbottom layer — 1,62-1,67 km/sec (*) and in the deeper layer — 1,89-1,91 km/sec. the way in which t h e experimental d a t a agree with (*) here a n d below t h e t h i n layer (5-14 m t h i c k ) covering t h e sea bott o m is n o t t a k e n i n t o consideration. w e a k reflections f r o m s t h i s layer can be seen on t h e seismograms of fig. 2. 188 s . m . z v e r e v theory (fig. 6) gives reasons to consider t h e velocity 1,89-1,91 km/sec as belonging t o transversal waves. if so, t h e n the velocity of t h e longitudinal waves in t h e deeper layer becomes 3,2-3,5 km/sec. f r o m t h e relation between the amplitudes of the first and double vertical reflections t h e following average reflection coefficients were obtained: for t h e b o t t o m surface — 0,17; for the sharpest b o u n d a r y — 0,58 a n d for t h e deepest reflecting b o u n d a r y — 0,34. calculations of t h e vertical reflection coefficients b y means of r a y leigh's formula have shown t h a t there is a satisfactory agreement between t h e above given d a t a and t h e following values of velocities v and densities o in t h e sediments illustrating their variation with d e p t h : a) ocean b o t t o m sediments (unconsolitated) v = 1,65 km/sec as will be seen below, the thickness of t h e layer with a velocity of 1,65 km/sec is of t h e order of 300-600 m and t h a t of t h e layer with a velocity of 3,5 km/sec a b o u t 150-300 m. this gives for t h e average veloci t y t h r o u g h o u t the entire sedimentary layer a b o u t 2 km/sec. all t h e above given velocities are very approximately estimated. i t should be noticed though, t h a t all t h e obtained d a t a are not yet used and in particular, t h e existence of a thin layer of consolidated sediments having a velocity of 3,5 km/sec should not be considered as finally proved. f u r t h e r investigations are needed to determine more accurately t h e distribution of velocities in the sediments. 9. w e shall now consider t h e sections t h r o u g h the profiles made. on all three of t h e m a sharp change in the n a t u r e of sediments is observed when crossing t h e deepwater trough. on t h e eastern slope of t h e trough a distinct and relatively stable layering of t h e sediments is observed for long stretches of t h e profile. the a m p l i t u d e of the b o t t o m reflection is usually 2-4 times smaller t h a n t h a t of t h e later reflections. this indicates t h a t t h e sea b o t t o m is a less sharp b o u n d a r y t h a n t h e deeper boundaries in t h e sediments. on t h e west slope of t h e trough facing the land t h e layering of t h e sediments is less obvious. the reflectc) b o t t o m of sediments (crystalline rocks) b) deeper sediments (consolitated) q = 1,35 gr/cm3 v = 3,50 km/sec o = 2,50 gr/cm3 v = 6,40 km/sec ~g = 2,80 gr/cm3 o n t h e s e d i m e n t a r y s t r u c t u r e i n s o m e p o r t i o n s of t h e p a c i f i c o c e a n 1 9 3 ing power of t h e sea b o t t o m is very variable and on the whole 2-3 times larger t h a t of the ocean bed region. this means high densities and velocities of t h e rocks of the sea b o t t o m whose composition is rapidly carying. 10. on profile no 9 running to south-east of the southern extremity of t h e k a m c h a t k a peninsula, on t h e western slope of the deep-water trough 3 reflecting boundaries were observed. the difference in times of arrival (at) of waves reflected f r o m the first and third boundaries was 0,2-04, second (fig. 3). fig. 6 a comparison of e x p e r i m e n t a l a n d calculated amplit u d e curves for w a t e r w a v e s t h r i c e reflected in t h e ocean h a v i n g for an object t h e d e t e r m i n a t i o n of velocities in t h e s e d i m e n t s : 1) e x p e r i m e n t a l d a t a ; 2) d a t a calculated for t h e first critical angle ( t h e second m a x i m u m in t h e c u r v e corresponds t o a velocity of 1,65 km/sec in t h e s u b b o t t o m l a y e r ; a n d t h e first m a x i m u m — t o t h e velocity 1,90 km/sec of longitudinal waves in t h e deeper s e d i m e n t a r y l a y e r ; (3) d a t a calculated for t h e second critical angle (the fiist m a x i m u m corresponds t o a velocity 1,90 km/sec of t h e t r a n s v e r s a l waves in t h e deep s e d i m e n t a r y layer). i n t h e region of the deep-water trough 5-6 reflecting boundaries were found, at between successive arrivals having been 1,5-1,8 seconds. within t h e boundaries of t h e oceanic swell, bordering the southeastern slope of the deep-water trough, 2-5 reflecting boundaries were observed. near the trough at varied considerably from 0,2 to 0,9 seconds thus indicating a complex relief of the surface underlying the sediments. at a distance of 130 km from t h e axis of the trough a sharp change in the 20 10 0 o 1 • 2 + 3 188 s . m . z v e r e v relief of t h e b o t t o m a n d a displacement of t h e reflecting elements was found indicating a faulted surface of t h e rocks underlying t h e sediments. at, longer distances f r o m t h e axis of the trough 3-4 reflecting boundaries were detected. w i t h moving away f a r t h e r f r o m t h e axis the relief of t h e reflecting surface became more uniform a n d t h e time interval between successive reflections decreased, reaching 0,4-0,7 seconds. assuming and average velocity in t h e sediments of 2 km/sec we obtain t h e following values for t h e thickness of t h e layered sediments: in t h e region of t h e trough — 1,5-1,8 k m ; in the ocean bed elose to the trough — 0,20,9 k m : along t h e south-eastern p a r t of t h e profile — 0,4-0,7 k m . the thickness of t h e entire sediments within t h e trough might h a v e been larger t h a n 2 k m since to all appearance no reflections from t h e b o t t o m of t h e sediments were obtained. w i t h i n the boundaries of the north-western slope of t h e trough the s u m m a r y thickness of the sediments down to the " granitic " layer could not be obtained f r o m the here considered d a t a . i t might be only stated t h a t t h e thickness of t h e almost horizontally layered sediments, giving appreciable seismic reflections, does not exceed 100-300 metres. underlying the sediments, according the dss d a t a , are rocks also having small velocities of seismic waves. the absence of reflections shows t h a t these rocks are of a complex nature. 11. the section through profile no 7 (fig. 4) is basically similar to t h a t of profile no 9. the relief is more uniform and sediments are f o u n d near t h e south-eastern slope of t h e trough. here at varies between 0,5 and 0,9 second (and t h e thickness f r o m 0,5 to 0,9 km). at a distance of 250 k m f r o m t h e axis of t h e trough a f a u l t was detected cutting all t h e sedimentary layers and appearing in t h e relief as a ledge with an amplitude of a b o u t 800 m. a f a u l t of a smaller amplitude was found a t a distance of 200 k m f r o m the axis of t h e trough. a t t h e south-eastern end of t h e profile t h e thickness of t h e sediments was 0,4-0,7 k m . 12. let us discuss now t h e d a t a obtained on t h e profile runningsouth-west of t h e k o m a n d o r islands through the aleutian trough (fig. 5). these d a t a are less reliable because of b a d weather which prevented us f r o m using t h e multichannel set. on t h e side facing t h e shelf 2-4 reflecting boundaries were f o u n d corresponding to at not exceeding 0,5 second. i n t h e region of the trough and to t h e south of it the n u m b e r of reflecting boundaries increased sharply reaching 8 and even more. the time interval at between successive o n t h e s e d i m e n t a r y s t r u c t u r e i n s o m e p o r t i o n s o f t h e p a c i f i c o c e a n 1 9 5 reflections was 2.0-2,5 seconds, i.e. 2-3 times those observed in the ocean bed on profiles nono 7 and 9. from these figures follows t h a t in t h e region of the pacific ocean border swell near t h e aleutian trough t h e thickness of the sediments was no less t h a n 2,0-2,5 km. c o n c l u s i o n s . 1. the deep water trough divides the horizontally layened sediments of t h e ocean bed only, relatively slightly varying in thickness, from the variabfe sediments of t h e shelf ha ving a complex structure. the thickness of t h e sedimentary rocks of the ocean bed decreases with distance f r o m t h e trough. 2. within the boundaries of the ocean swell, bordering the kurilek a m c h a t k a trough, the surface of the rocks underlying the ocean bed sediments has a complex relief mainly levelled out by sediments. when moving away from the trough the relief of t h e sediments b o t t o m becomes more uniform. a t distances of 150-250 km from t h e axis of the trough faults of large amplitudes are found catting all the sedimentary layers and showing in the relief of the bottom. these faults had been probably produced b y relatively recent tectonic movements occuring on t h e border of t h e ocean bed near the deep-water trough. 3. w i t h i n t h e boundaries of the pacific border swell near t h e aleutian deep-water trough, the thickness of the sediments is 2-3 times t h a t of t h e sediments in t h e ocean bed, close to the kurile-kamchatka trough. this indicates t h a t the conditions for accumulation of sediments in t h e outer p a r t s of t h e kurile-kamchatka and of the aleutian deep-water troughs are essentially different. summary the special observation of vertical reflected waves, made during the i g y simultaneously with deep seismic sounding in the pacific ocean, allowed to study the structures of sediments, covered the ocean bed near kurilo-kamchatlca and aleutian trenches. the profiles of observations crossed some great faults, disturbing sedimentary complex on the whole its depth. 1 9 6 s . m . z v e r e v sommario la particolare osservazione delle onde verticali riflesse, effettuata durante va.g.i., contemporaneamente al sondaggio sismico profondo nell'oceano pacifico, ha permesso di studiare le strutture dei sedimenti, ricoperte dal fondo delvoeeano presso kurilo-kamchatka e le trincee aleutine. i profili delle osservazioni hanno incontrato alcune grosse faglie, che disturbano il complesso sedimentario in tutta la sua profonditd. r e f e r e n c e s t1) g a l p e r i n , e . i . , g o r j a c h e v a . v . , z v e r e v s . m . , investigation of the earth's crust in the zone of transition from the asiatic continent to the pacific ocean. p u b l . h o u s e of ac. sci. u s s r , m. (1958). (2) vasiloev v. g. a n d otli., investigation of the earth's crust in the zone of transition from the asiatic continent to the pacific ocean. s y m p . " seismic i n v e s t i g a t i o n s d u r i n g t h e i g y " 4, p u b l . h o u s e of ac. sci. u s s r , ( 1 9 6 0 ) . (3) zverev s. m., multiply reflected water waves and their use for the determinaion of velocities in the sediments. t r a n s . ac. sci. u s s r , g e o p h . s e r . 1 . ( 1 9 6 0 ) . vol49,4_5,2006 945 annals of geophysics, vol. 49, n. 4/5, august/october 2006 key words ionospheric variability – radio wave propagation – space and satellite communications 1. introduction perturbations of ionization density at the heights of the ionospheric f-region are permanent features. therefore, meaningful longer term predictions can only have statistical character, i.e. specify probabilities (e.g., rawer, 1957, 1993). systematic studies on the day-today and hour-to-hour variability of the critical frequency of the f-region, fof2, and of the total electron content, tec, have shown that these perturbations, with negative and positive response, depend on local time, season and location (kouris et al., 1998, 1999, 2002, 2006; rawer et al., 2003; fotiadis et al., 2004). regarding variations of fof2 in the time interval of an hour have been reported in different papers (kouris et al., 2000; fotiadis et al., 2001; zolesi et al., 2001; buresova and laštovička, 2001) showing that a variation of fof2 around 12% (positive or negative) with respect to the hourly daily value measured at the standard-hour time is always present. this ionospheric variability «within-the-hour» or otherwise «ionospheric density noise» can be estimated from the variation of the relative deviations of 5-min daily fof2 measurements with respect to the standard hour daily value of fof2 measured at the corresponding standard hour. the diurnal variation of these 5-min relative deviations shows clearly that in certain hours and under particular circumstances, values of variability greater than 12% (in absolute value) of the corresponding hourly daily value can occur (kouris et al., 2000; buresova and laštovička, 2001; fotiadis et al., 2001). moreover, variations in tec in the time interval of an hour within-the-hour variability: levels and their probabilities stamatios s. kouris (1), kostas v. polimeris (1), vincenzo romano (2), bruno zolesi (2) and ljiljana r. cander (3) (1) division of telecommunications, department of electrical engineering, faculty of engineering, aristotle university of thessaloniki, greece (2) istituto nazionale di geofisica e vulcanologia, roma, italy (3) radio communication research unit, rutherford appleton laboratory, chilton, oxon, u.k. abstract the study of fof2 data measured every 5-min and of tec measurements made every 10-min shows that the within-the-hour variability is different in the two parameters. deciles of this variability for fof2 and for tec are determined together with the probabilities of exceeding a given level of variability. furthermore, considering hourly values, it is found that the variability in tec is like an «intrinsic noise» throughout the day of the order of less than 5% of the hourly value; but at sunrise and often at sunset large values take place. a seasonal dependence is evident. besides, a within-the-hour variability in fof2 is always present with large values at sunrise or sunset depending on the season, and also during disturbed ionospheric conditions. mailing address: prof. stamatios s. kouris, division of telecommunications, department of electrical engineering, faculty of engineering, aristotle university of thessaloniki, box 435, gr-54 124 thessaloniki, greece; email: kouris@vergina.eng.auth.gr 946 stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander were reported as preliminary results in a previous paper (polimeris et al., 2004) considering a limited number of data. it was then shown that the variability in tec is rather negligible, except at sunrise and sunset where values up to 20% higher or lower of the corresponding daily standard hour value have been observed. in the present work we investigate the variability within-the-hour in tec at different european locations and, for comparison, in fof2 at the location of rome. the statistical study of their variations in the interval of an hour allow us to determine bounds of the within-the-hour variability in fof2 and in tec for all hours of the day in each month and season. 2. data and analysis we use fof2 5-min data measured at rome (41.9°n, 12.5°e) during the years 1997-2001 and 10-min tec data obtained from gps measurements made mainly at matera (40°n, 16°e) during the years 1993 to 1999, and at hailsham (50.9°n, 0.3°e) during 1998 to 2004. we have also examined tec data measured every 10min during august to december in 2002 to 2004 at the locations of brussels (50.8°n, 4.3°e) and nicosia (35.1°n, 33.2°e). first we evaluate the 10-min relative deviations of tec from the measured every 10-min daily values in each interval time of an hour (half an hour before and after each standard hour) of each day/month/year/location using the expression (2.1) where dt is the 10-min relative deviation, t10 is the measured value of tec every 10-min and th is the hourly measured value corresponding at the standard hour of the examined one hour interval time. a similar expression is used to calculate the variability within-the-hour of fof2 (fotiadis et al., 2001) that is (2.2) with df the 5-min relative deviation, f5 the value of fof2 measured every 5-min and fh the corred f f f f h h5 = − d t t t t h h 10= − sponding hourly daily value of fof2 in the considered one hour interval. in this analysis for each ionospheric parameter we deal with the relative deviations with respect to the standard hour value in the interval time of one hour, in order to remove the regular daily variation and to avoid any effect of the variability from hour-to-hour and from day-today on the obtained 5-min/10-min relative deviation, and so we may consider the totality of relative deviations as a whole. since we also compare the variability of within-the-hour with the variability from day-today, that is their deciles. hourly tec data and monthly median are also used from measurements made at the above referred locations and during the same periods. then, the hourly relative deviation is estimated using the expression (2.3) where dt is now the hourly relative deviation, th is the hourly measured value and tm is the corresponding monthly median value. evidently, the variation of dt from day-to-day describes the day-to-day variability of tec at the given hour. of course a similar procedure is used in the case of fof2. 3. results and discussion the counted relative deviations are first studied month by month and then by season to investigate any annual or seasonal dependence. to inquire whether deviations as high as 20% of the corresponding standard hour value are related to magnetic disturbances, the calculated df and dt values have been studied day by day. variability in tec – the analysis shows that the within-the-hour variability in tec has a different behavior from that in fof2. it is found that a variation in tec of 10% with respect to the standard hour value has a probability much less than 10% if not zero, contrary to what is found for fof2. indeed, only an «intrinsic» variability of the order of less than 5% is found, comparative to the one found for all other ionospheric characteristics, e.g. foe (kouris and fotiadis, 2002). table i reports probabilities d t t t th m m= − within-the-hour variability: levels and their probabilities 947 table i. probability the level of variability within-the-hour in tec exceeds the value of 0.05 (a), 0.10 (b), 0.15 (c), 0.20 (d). data measured at hailsham during 2002 (80 < r 12 < 120). a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 6.7 12 7.3 6 15 22 16 98 100 85 53 14 1.3 2 6 23 53 81 91 81 49 24 17 13 f 1.9 13 16 13 22 14 50 99 98 77 32 2.2 1.5 0 0 6.7 21 56 63 68 60 44 27 10 m 3.3 6.7 11 12 27 26 95 89 73 52 21 1.3 0 0 0.7 2.7 6 11 32 57 59 45 29 17 a 3.4 14 18 17 30 72 68 46 41 32 20 6.2 1.4 1.4 2.1 2.1 4.1 4.8 9.7 29 44 46 26 15 m 12 11 17 18 63 59 33 18 11 6 4 6 1.3 2 2.7 0.7 1.3 2 8.7 6.7 17 21 23 10 j 3.4 15 1.4 17 50 33 9 4.1 2.8 2.8 0.7 0 0 0.7 0 0 0 0.7 0.7 1.4 9.7 3.4 2.8 3.4 j 15 12 15 9.3 61 51 13 9.3 5.3 2.7 0 0 1.3 2.7 1.3 2 0.7 2 1.3 0 4.7 10 16 15 a 12 27 14 16 38 82 56 41 28 7.7 4.5 2.6 0.6 0.6 0 2 0.6 0.6 1.3 8.4 32 54 42 23 s 5 9.3 21 17 27 72 91 65 43 35 9.3 5.3 2.7 0 1.3 2 4 4 27 53 56 50 25 13 o 37 23 17 21 35 28 97 89 66 56 43 11 2.7 1.3 3.3 7.3 25 52 64 77 62 33 16 17 n 31 22 22 28 18 21 55 100 97 72 46 8.3 4.1 2.8 7.6 35 61 78 87 75 43 28 32 40 d 9.7 29 16 11 19 31 21 97 100 89 41 7.6 3.4 4.8 22 43 70 93 83 59 27 24 35 21 b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 3.3 2 0 1.3 0.7 0.7 1.3 87 93 48 9.3 0 0 0 0 0.7 12 41 54 38 6.7 3.3 2.7 0.7 f 0 1.5 0.7 1.5 0 2.2 30 91 66 35 0 0 0 0 0 0 0.7 6.7 19 17 10 4.4 0.7 0 m 0 0 0 1.3 2.7 13 69 55 33 9.3 1.3 0 0 0 0 0 0 0.7 2 9.3 13 0 0.7 2 a 3.4 4.1 3.4 5.5 14 35 28 17 9 6.9 2.8 1.4 0 0 0 0.7 0.7 0 0.7 3.4 5.5 5.5 4.8 4.8 m 5 1.3 4 4 23 15 4.7 3.3 1.3 3.3 0 1.3 0 0 0.7 0 0 1.3 1.3 0 0.7 2.7 1.3 0.7 j 0 0 0 0 12 4.1 0 0 0 0.7 0 0 0 0 0 0 0 0 0 0 0.7 0 0 0 j 0 0 2 0 21 6 1.3 0 0 0 0 0 0 1.3 0 0 0 0 0 0 0 0 1.3 0 a 1.6 3.2 1.3 3.2 16 42 20 5.2 1.3 0 0 0 0 0 0 0 0 0 0 0 3.9 15 6 1.4 s 0 1.3 2 2 4.7 42 53 26 11 1.3 0.7 0 0 0 0 0 0.7 0 3.3 8 14 8.7 6 2.7 o 25 9.3 7.3 7.3 6.7 12 71 56 27 21 7.3 0 0 0 0 1.3 5.3 17 19 38 23 2.7 0 4 n 14 4.8 2.8 7.6 4.1 3.4 29 93 61 29 9 0 0 0 0.7 4.1 20 39 50 32 7.6 4.1 11 17 d 0 2.6 1.3 3.9 5.2 9.7 5.2 74 85 51 5.5 1.4 0 0 2.8 4.8 29 57 44 21 9.7 9.7 7.6 4.1 c 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 0 0 0 0 0 0 0 58 63 20 0.7 0 0 0 0 0 1.3 17 25 8 0.7 0.7 2.7 0 f 0 0 0 0.7 0 0 18 66 47 7.4 0 0 0 0 0 0 0 0 1.5 2.2 0.7 0 0 0 m 0 0 0 0.7 1.3 8.7 46 36 4.7 0.7 0 0 0 0 0 0 0 0 0 0.7 0.7 0 0 0 a 1.7 2.8 2.8 3.4 6.2 15 10 3.4 0 0.7 1.4 0 0 0 0 0 0 0 0 0 2.8 2.1 0.7 4.1 m 3.3 0 2 2 8.7 2 0.7 2 1.3 2 0 0 0 0 0 0 0 0 0 0 0 0.7 0 0 j 0 0 0 0 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0 0 0 6.7 1.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a 1.6 1.3 0 1.3 5.8 14 3.9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.6 2 0 s 0 0 0 0 0 26 25 7.3 1.3 0 0 0 0 0 0 0 0 0 0 2 2 1.3 0 1.3 o 13 2.7 4 0.7 1.3 5.3 45 31 8.7 5.3 0 0 0 0 0 0.7 2 4 2.7 13 2.7 0.7 0 2 n 8.6 0.7 0 0.7 1.4 0.7 19 70 46 4.1 0.7 0 0 0 0 0 0.7 12 23 10 0 1.4 2.8 10 d 0 0 0 0.6 1.9 3.9 1.9 52 62 19 0 0 0 0 0 0 11 26 23 2.8 3.4 5.5 2.8 2.1 948 stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander table i (continued). d 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 0 0 0 0 0 0 0 43 55 6 0 0 0 0 0 0 0 2.7 6 0.7 0 0.7 2.7 0 f 0 0 0 0 0 0 12 52 24 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 m 0 0 0 0 0.7 4.7 29 17 2 0.7 0 0 0 0 0 0 0 0 0 0 0.7 0 0 0 a 1.7 1.4 2.8 2.8 3.4 6.9 1.4 0 0 0 0.7 0 0 0 0 0 0 0 0 0 2.1 0.7 0 2.8 m 3.3 0 1.3 2 3.3 0 0 1.3 1.3 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a 1.6 0.6 0 0.6 1.9 3.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0 s 0 0 0 0 0 13 8.7 1.3 0 0 0 0 0 0 0 0 0 0 0 0.7 1.3 0.7 0 0.7 o 6.7 2 0 0 0.7 2 32 14 2.7 1.3 0 0 0 0 0 0.7 1.3 0.7 0 4 0.7 0 0 1.3 n 3.4 0.7 0 0 0.7 0 12 56 19 0 0 0 0 0 0 0 0 2.1 9.7 0.7 0 0.7 2.1 5.5 d 0 0 0 0 0 1.9 0 40 43 3.3 0 0 0 0 0 0 2.1 6.9 5.5 0 4.1 4.1 0.7 0 table ii. probability the variability in tec may exceeds the bounds 0.05 (a), 0.10 (b) or 0.20 (c). data from matera 1996 (r 12 < 10.5). a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 16 19 22 28 25 27 80 45 37 39 17 8.4 14 7.1 29 34 31 29 11 14 12 13 18 2.7 f 30 24 22 20 31 39 94 29 18 13 11 7.1 6.4 6.4 8.6 25 45 26 16 19 5.7 5.7 2.1 4.3 m 38 20 30 14 18 87 70 42 28 18 12 8 11 6.5 14 14 9.7 34 48 39 19 9 9.2 7.6 a 22 16 15 25 68 75 42 31 27 16 9.7 2.9 6.2 9.7 7.6 4.8 6.2 6.9 28 51 47 21 13 18 m 28 25 18 33 99 76 33 17 8 6 3.3 6 2.7 0 4.7 2 6 10 5.3 13 32 43 41 19 j 11 42 44 57 92 75 33 11 7.3 7.4 3.4 2.7 0.7 3.3 4.1 4.1 3.3 11 7.3 19 36 27 22 20 j 50 26 40 42 96 63 44 19 18 19 9.2 8.6 11 6.4 9.3 4.3 7.1 8.6 7.1 30 58 41 19 20 a 54 57 36 43 82 74 43 41 36 20 14 15 6 6 8 1.3 4.7 2.7 6.7 35 62 42 24 22 s 50 23 35 21 52 85 49 41 30 9.5 19 16 11 13 8.7 11 5.4 6.4 22 43 55 33 21 12 o 62 26 12 43 31 95 82 60 22 28 19 6.7 23 13 18 26 40 64 43 31 31 53 58 46 n 13 20 28 35 36 41 100 40 24 14 11 11 15 15 25 41 52 23 11 7.7 7.7 1.5 3.1 6.2 d 18 15 16 15 15 32 82 47 47 25 14 10 4 13 23 49 49 12 12 9.3 6.7 10 6.7 6.7 b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 5.2 4.5 2.6 4.5 2.6 7.7 48 12 9 8.7 0.6 0.6 0.7 1.9 3.9 6.7 6.7 3.3 1.3 5.2 2.6 1.3 2.7 0.7 f 3.6 5.7 5 3.6 7.9 21 58 5.1 2.1 1.4 0.7 0.7 0 0 0.7 5 7.1 5 2.1 2.1 0 0 0 0 m 18 7.1 12 5.2 0 50 25 7.7 2.6 2.6 0 0.7 1.3 0 2.6 1.3 1.9 9 11 8.4 0 0 1.3 2.1 a 6.7 4.7 1.3 6 41 37 11 4 1.3 0.7 0.7 0 0 2.1 0.7 0.7 0 0 5.5 19 13 1.4 2.2 3.6 m 12 3.9 4.1 13 60 33 5.3 0 0.7 0.7 0 0 0 0 0 0 0 0 0 2.7 8.8 13 9 1.4 j 5.6 13 14 38 55 30 4.9 0 0.7 1.4 0 0 0 0 0 0 0 0.7 0.7 4 6 5.3 4.1 3.5 j 24 9.6 13 26 64 22 11 5.8 3.3 8.2 3.1 2.1 2.9 1.4 1.4 0 0 0.7 0 7.9 19 8.6 2.3 4.4 a 31 29 14 18 56 37 16 18 6 5.3 3.3 0.7 0 0 0 0 0.7 0 0 9.5 18 10 4.4 2.9 s 32 4.4 15 6.1 28 46 12 7.7 7.6 0 4.5 6.1 4.3 3.5 2.6 4.3 1.8 1.8 7.3 16 22 5.5 3.8 2 o 21 6.7 1 9.5 7.6 63 43 15 3 6 3 1 5.7 5.7 5.9 8 20 36 25 15 18 30 52 34 within-the-hour variability: levels and their probabilities 949 table iii. probability the within-the-hour variability in tec be greater than 0.10 (a, c) or 0.20 (b, d). data measured in 1999; matera (a, b) and hailsham (c, d). high solar activity (80 < r 12 < 120). a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 10 7.1 8.4 5.2 3.9 4.5 76 39 12 6 0 0 0.6 0.6 3.9 17 31 21 5.2 1.9 3.2 0 0.6 0 f 6.3 2.4 0 0 0 26 82 15 5.7 2.1 0 0 0 0 0 4.3 20 29 8.6 8.6 0.7 0 0.7 1.4 m 0 0.7 0 2.2 3 69 40 19 7.2 4.7 0 0 0.6 1.4 0 0.7 4.5 12 28 14 2.6 0.6 0 3.2 a 0 0.9 0.9 6.1 40 39 18 9 6.2 0 0 0.7 0 0 0 0 0 0 12 13 4.4 2.2 0 0 m 0 0 0.7 7.1 50 11 2.7 0 0 0.7 0 0 0 0 0 0 0 0 0 3.3 1.4 0 0 0 j 1.7 0 0 16 30 2.1 0 0 0 0 0 0 0 0.7 0 0 0 0 0 0 0 0.7 0 0 j 0 0.7 2 4 25 3.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 2 2 0 a 1.7 0.7 0.7 0.7 40 26 3.4 0 0.7 0 0 0 0 0 0 0.7 0 0 0 4.8 4.1 2.1 0 0 s 0 0.7 1.4 0.7 23 52 12 6.5 4.3 0.7 0.7 0 0 0 0.7 0.7 0 5 8.6 2.1 2.9 0 0 0 o 0 0.8 0 0 1.6 73 57 18 1.6 0.8 0 0 0 0 0 3.2 19 26 10 2.4 2.3 0 0 2.3 n 0 0 0.7 2.1 0 41 81 43 11 0 0.7 0 0 0 1.3 17 38 24 8 2.7 0 0 0 0.7 d 1.7 1.3 0.7 1.3 0 15 86 39 21 4.7 2.1 0 0 0 2 22 42 19 6.7 2.7 2 0 0 0 b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 0 2.6 1.3 0 0 0.6 34 3.2 0 0 0 0 0 0 0 0 0 0.6 0 0 0 0 0 0 f 0 0.8 0 0 0 11 39 0.8 0 0 0 0 0 0 0 0 0.7 0 0 0 0 0 0.7 0 m 0 0 0 0 0 28 7.4 2.1 0 0 0 0 0 0 0 0 0 1.3 0.6 1.3 0 0 0 0 a 0 0 0 0 11 6.1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 m 0 0 0 0 7.1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0 0 0.7 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 table ii (continued). b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 n 0 1.3 2.5 16 11 21 60 10 7.5 1.3 2.5 0 3.8 6.3 9.1 10 11 1.5 1.5 0 1.5 0 0 0 d 3.3 4.7 4 3.3 2.7 10 41 11 15 3.3 0 2 0 0.7 3.3 15 13 0.7 0.7 0.7 0.7 1.3 0.7 0.7 c 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 5.2 1.3 0.7 0 0 1.9 19 1.3 0.6 0.7 0 0 0 0 0 0 0 0 0 3.2 0.6 0 0 0 f 0 0 0.7 0 1.4 4.3 14 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 m 5.4 0 1.9 1.3 0 16 0.6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a 0 0.7 0 0 15 5.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0 1.4 0.7 m 1.7 0 0 4.7 18 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0 0 j 3.7 2.1 0.7 17 16 2.1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.3 0 0 0 j 9.5 3.5 1.8 6.7 24 1 7.9 0 0 6 0 0 0 0 0 0 0 0 0 0 3.6 0 0 1.5 a 12 7.9 11 6.8 26 5.3 6 6 0.7 1.3 0 0 0 0 0 0 0 0 0 0.7 1.4 0 0 1.4 s 7.1 0 5.2 1 7 13 1 0 0 0 3.6 3.5 0.9 0 0 3.5 1.8 0.9 0.9 2.7 0 0.9 2.9 0 o 4.8 1 0 1.9 3.8 23 2.9 0 0 1 0 0 1 1.9 2 2 4 14 12 5.9 2.5 18 42 21 n 0 0 0 7.5 5 10 13 0 1.3 0 0 0 0 1.3 6.5 4.4 0 0 0 0 0 0 0 0 d 0 1.3 0.7 0 0 0 8 0.7 0.7 0 0 2 0 0 0.7 1.3 0 0 0 0 0 0 0 0 950 stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander that the level of variability in tec exceeds given bounds during 2002, a year of high solar activity (80 < r12 < 120) at hailsham while table ii reports probabilities from data measured at matera during 1996, a year of low solar activity (r12 < 10.5). it can be seen that the variability exceeds the limit of 5% mostly during the night-time hours (table i) and also under disturbed ionospheric conditions (table ii). thus, we may deduce that the variability in tec withtable iii (continued). b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 0 0 0 0 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a 0 0 0 0 13 1.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.4 0 0 s 0 0 0 0 4.3 9.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 o 0 0 0 0 0 32 8.8 0 0 0 0 0 0 0 0 0 0.8 0 0 0 0.8 0 0 0.8 n 0 0 0 0 0 20 38 2.1 0 0 0 0 0 0 0 0 1.3 0 0 0 0 0 0 0 d 0 0 0.7 0 0 2 40 2 0 0 0 0 0 0 0 1.3 2 0.7 0 0 0 0 0 0 c 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 3.3 0 2.7 0 1.3 0.7 0 58 73 38 0 0 0 0 0 3.3 15 34 37 12 4.9 1.4 0 3.6 f 0 0 0 1.6 1.6 2.4 26 78 52 15 3.2 0.8 0 0 0.8 6.8 6.8 5.8 23 27 18 3.3 0 0.8 m 0 0.7 0.7 0 2.8 15 70 47 19 12 2 0 0 0 0 0 0.7 2 6 18 24 18 2 2 a 0 0 0.7 3 8.9 51 32 13 1.5 1.5 0 0 0 0 0 0 1.5 0.7 0 0.7 5.9 12 3.7 2.2 m 0 0 0 2 22 17 2.7 0.7 0 0 0 0 0 0 0 0 0 0 0 0.7 0.7 0.7 0 0 j 0 0 0 0 10 6.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0.7 2 1.3 13 12 0 0 0 0 0.7 0 0 0 0 0 0 0 0 0 0.7 1.3 0.7 2.7 a 5.4 5.7 6.2 5.5 20 43 13 9.3 2 0 0 0 0 0 0 0 0 0 0 0 1.4 8.3 3.4 4.1 s 7.4 4.4 2.2 5.2 2.2 39 50 19 3 5.2 0.7 0 0 0 0.7 0 2.2 0 0.8 7.7 16 11 3.1 0 o 10 2.7 0.7 4.7 4.7 13 71 58 29 13 2.7 0 0 0 0 1.3 2 8.7 15 33 25 7.3 1.3 0 n 8.6 0 3.4 0 2.1 0 28 88 64 37 12 0 0 0 0 4.8 21 42 46 23 2.8 0.7 0.7 0 d 0 5.3 0.7 0 0 0 0.7 62 78 43 2.7 0 0 0 0.7 11 29 39 38 18 1.3 2 5.4 9.7 d 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 0 0 0 0 0 0 0 32 30 1.3 0 0 0 0 0 0.7 2 2 2.7 2.7 1.4 0 0 1.4 f 0 0 0 0 0 0 12 36 4 0.8 0 0 0 0 0 0 0 0 2.5 0.8 0 0 0 0 m 0 0 0 0 0 2 31 6 2 2 0 0 0 0 0 0 0 0 1.3 0 0 0 0 0.7 a 0 0 0 0 0 16 2.2 1.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0 0 m 0 0 0 0 0.7 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 j 0 0 0.7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a 0 0 1.4 0 5.4 11 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.4 s 1.9 0 0 0 0 13 8.9 1.5 0 0 0 0 0 0 0 0 0 0 0 0.8 1.5 0 0 0 o 3.3 0 0 1.3 0 4 33 15 1.3 0 0 0 0 0 0 0 0 0 0.7 2.7 3.3 0 0 0 n 1.7 0 0 0 0 0 9.7 48 19 1.4 0 0 0 0 0 0 0.7 2.8 3.4 0 0 0 0 0 d 0 2 0 0 0 0 0 32 35 0.7 0 0 0 0 0 0 0.7 1.3 2.7 0 0 0 2 2.8 951 within-the-hour variability: levels and their probabilities in the interval of an hour, if any, is very low except around the sunrise hours and often around sunset too. the latter usually occurs in winter and equinoxes as can be seen from table iii, where the probabilities that the within-the-hour variability in tec be greater than given levels at two locations of different latitude and during 1999 a year of rather high solar activity (80< r12 < 115) are shown. at sunrise and sunset the probability of value of variability withinthe-hour greater than 10% and less than 30% is very high reaching values up to 60%-80%. these high values of variability are observed during winter months and even during local disturbed conditions (tables i and ii). figure 1 illustrates from measurements made at hailsham the probability level that the variability within-the-hour be greater in abfig. 1. probability the variability within-the-hour in tec, positive (right) or negative (left), be greater than 0.10, 0.15, 0.20 or 0.30, in absolute value. data from hailsham. 952 stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander solute value than 0.10 or 0.15 or 0.20 or 0.30 with respect to the hourly value of the corresponding standard hour. it can be seen that high values of probability of a variability greater than 10% are found around sunrise and also at sunset during winter and equinoxes, otherwise the variability can be assumed to be an «intrinsic noise» during all the other hours of the day. figure 2 shows the probability the variability within-the-hour, positive or negative, be greater than a given level at different seasons. the data used are those measured at matera during the period from 1993 to 1999. the difference with season, regarding the time of appearance of high probabilities and the value of probability is evident. clearly, the variability is rather negligible during day-time hours. figure 3 illustrates the upper and lower deciles of the variability in tec from hourly measurements and from 10-min measurements made at matera and hailsham. they are determined for each of the two locations from the respective relative deviations counted according to eq. (2.1) and (2.3) and from all the avaliable data measured at each one location, respectively. figure 3 clearly shows the difference between bounds of variability from day-to-day and those of the within-the-hour variability. in this last case the variability is limited within a range of values less than ± 0.05 except at sunrise and also at sunset in winter or during local disturbed conditions (see also tables i and ii). it can be stated that the present analysis shows clearly that the within-the-hour variability in tec is very low, practically negligible, except at sunrise and sunset. we may therefore assume that tec is stationary in the interval time of one hour contrary to fof2, which seems to have a stationarity of less than 10 min. variability in fof2 – the analysis points out that the within-the-hour variability in fof2 may reach values greater, in absolute value, than 20% fig. 2. probability the variability within-the-hour in tec, positive (right) or negative (left), be greater than 0.10, 0.15, 0.20 or 0.30, in absolute value. data measured at matera 1993-1999. 953 within-the-hour variability: levels and their probabilities fig. 3. deciles of tec from hourly daily values and deciles from tec measured every 10-min at matera (left) and hailsham (right). table iv. probability the variability within-the-hour in fof2 be greater than 0.10 (a), 0.15 (b), 020 (c). data from rome measured during 1997 (november, december), 1999 (june, july, august, september), 2001 (january, february, march, april). black lines are due to missing measurements in may and october. a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 9.8 9.4 9.1 20 23 20 68 39 4.6 24 16 16 16 12 12 12 35 35 24 24 33 17 22 12 f 8.1 17 16 14 14 17 68 19 5.6 5.9 11 19 15 18 6.9 8.7 16 17 8.1 23 24 25 24 18 m 13 11 12 12 11 37 30 7 8.6 12 14 15 29 13 15 6.6 12 15 27 12 20 22 16 16 a 14 11 15 7.7 10 14 7 8.4 11 14 11 12 9.2 11 11 2.4 7.6 10 27 17 12 13 16 17 m j 24 13 12 14 7.8 10 6 6.4 7.3 14 12 13 13 6.8 8.5 9.6 8.8 4.4 7.6 18 20 16 25 21 j 16 11 13 8.2 8.8 9.7 13 9.9 12 8.8 5.2 6.1 8.7 6.5 8 9.6 8.4 13 7 18 19 13 15 12 a 26 9.6 22 20 12 22 11 10 10 7.3 6.6 9.1 9.1 1.9 4.8 1.3 0.6 6.2 6.7 21 20 34 20 14 s 13 19 18 9.8 8 39 18 13 5.7 4.4 2.2 1.3 5.5 6.6 4.9 1.6 7.1 14 9.4 11 15 15 13 14 o n 13 10 16 20 28 22 71 16 11 9.5 9.5 15 17 8.2 15 27 58 39 24 26 21 20 2.9 7.2 d 6.4 10 3.5 16 25 32 70 26 28 24 17 19 18 20 21 24 42 29 23 7.7 11 20 15 15 954 stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander and less than around 30% of the corresponding hourly value measured at the standard hour during disturbed conditions or around sunrise and sunset (table iv). it is also clear that a withinthe-hour variability around 12%, positive or negative, is always present throughout the day. figure 4a,b reports the curves of probabilities which show that the variability within-thehour could exceed a given value. it can be seen that the probability of high values of variability exist during winter and equinoxes. they are more frequent from sunset up to sunrise and seem to be related to disturbed conditions (buresova and laštovička, 2001). on the other hand, the probability of variability values greater than 0.30 (in absolute value) of the corresponding hourly value is very small except at summer and usually after sunset up to midnight. figure 5 illustrates the probabilities in each season that the variability in fof2 exceeds a given value. this figure confirms the above mentioned statements. it is evident that the variability within-the-hour in fof2 is bounded between a positive and a negative level of 0.10 to 0.15. values of variability greater than 0.30 may appear mostly in summer after sunset, but the probability is small, less than 10%. figure 6 shows the upper and lower deciles of variability in fof2 from hourly measurements and from 5-min made in rome. they are calcutable iv (continued). b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 3.8 4.4 1.5 10 6 9.1 48 20 0.7 6.3 7.3 3.3 8 3.7 1.8 3.2 19 24 12 11 14 6 8.4 4.9 f 3.7 9.1 9.2 9.4 4.2 9.2 49 4.9 0.4 1.5 4.5 8.6 1.6 6.5 1.7 5.2 6.6 6.1 2.1 6.6 13 13 12 9.6 m 6.5 8.3 7.6 5.3 5 22 13 0.9 3.2 3.5 4 4.2 11 3.2 6.6 0.3 5.3 4.7 9.7 7.3 7.9 11 9.4 8.8 a 11 9.2 12 5.2 7.1 6.1 2.9 3 4.3 6.2 3.4 5.2 1 2.5 1.6 0.9 0.3 0.9 9.7 7 7.1 6.1 15 12 m j 17 11 7.3 11 2.9 6.5 1.4 0.8 0.8 3.8 3.8 2 3.2 1.1 4.8 5 7.7 2.2 3 16 14 10 19 13 j 12 9.2 6.8 3.4 3.4 3.8 9.7 7.4 3.1 4.4 2.6 0 6 3.2 3.8 2.6 4.8 9.9 4 15 17 10 9.7 9 a 22 6.8 17 15 5 7.6 5 5.6 3.4 3.7 2.6 2.9 2.8 0.4 0.7 0.3 0 2.5 3.2 16 14 22 14 9.4 s 7.2 15 12 4.4 3.5 22 2.3 1.5 2.7 2.6 0.4 0.4 0.7 4.5 1 0 1.3 6.7 4.5 5.3 7 3.8 8.3 7.1 o n 5.5 3.7 11 9.4 8.3 8.4 47 5.5 2.3 3.6 1.5 3.1 4.1 0 11 7.2 36 21 8.2 11 8.1 12 1.4 4.1 d 0 6.4 1.8 5.8 11 17 52 11 14 12 5.8 7.5 8.7 6.6 12 9 27 14 16 3.8 2.7 8.8 8.9 8.3 c 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 j 3.3 3.2 0 6.2 3 1.8 34 8 0.3 1.9 1.5 2.2 3.8 0.7 0.7 1.2 12 15 5.2 4.9 6.2 2 2.3 2.5 f 1.9 6.7 4.9 5.5 1.6 3.6 30 1.5 0 0 1.2 1.6 0 2 0 3.6 4.5 3.6 0.4 1.6 5.7 8.8 5.6 6.2 m 5.9 5.3 4.7 2.6 2.6 11 4.4 0.6 1.6 1.7 1.8 1.4 3.8 0.4 4.6 0 3.2 2.4 2.7 3.5 4.4 7.6 5 5.9 a 10 7.3 9.7 3.1 6.2 2.8 1.3 2 1.4 2.4 1.6 1.3 0.3 0.6 0.6 0 0.3 0 5.5 3.6 5.5 3.7 13 5.2 m j 9.8 9.8 4 8.9 2 3.9 1.1 0 0 1 1.4 0.4 0.7 0 2.6 3.7 6.4 1.3 2 15 13 5.9 13 10 j 9.2 7.2 2.1 2.4 1 3.1 7.4 5.4 0.5 1.3 1.5 0 4.1 0.5 1.3 1.7 1.3 8.3 3.7 13 13 7.5 9 5.8 a 15 5.6 13 12 2.4 3.4 3.8 3.3 1.5 2 0.4 0.4 0.4 0.4 0.3 0.3 0 1.2 1.3 13 9.6 14 11 7.9 s 4.8 11 6.6 3.7 1.7 9.7 0 0.7 0.4 0.4 0 0 0.4 3.8 0 0 0 3.9 1 1.7 0.6 1 7.3 3.4 o n 3.6 0.9 4.5 1 0.9 3.2 26 2.3 0 2.2 0.7 0.8 0.8 0 2.5 2.2 23 13 2.2 1.7 0 2.9 0 1 d 0 0.6 0 3.5 5.7 7.6 37 4 8.6 4.7 1.7 0.5 6.7 2.7 2.2 3.4 13 4.1 6.7 0.6 1.8 3.5 8.1 5.1 955 within-the-hour variability: levels and their probabilities f ig . 4a ,b . p ro ba bi li ty t he v ar ia bi li ty w it hi nth eho ur i n fo f 2, po si ti ve ( ri g h t) o r ne ga ti ve ( le ft ), be g re at er t ha n 0. 10 , 0. 15 , 0. 20 o r 0. 30 , in a bs ol ut e va lu e. d at a m ea su re d at r om e: a) 1 99 819 99 ; b) 2 00 020 01 . a b 956 stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander lated from all the available measurements using eq. (2.2) and a similar one for the hourly deciles (fotiadis et al., 2001). figure 6 shows clearly the existing difference between day-to-day and within-the-hour variability. the bounds of variability for the 90% of the time are around 20% in absolute value in the case of the day-to-day variability (kouris and fotiadis, 2002) whereas for fig. 5. probability the variability within-the-hour in fof2, positive (right) or negative (left), be greater than 0.10, 0.15, 0.20 or 0.30, in absolute value, using all the available data from rome. fig. 6. deciles of fof2 from hourly daily values and deciles from fof2 5-min measurements made in rome during 1997-2001. 957 within-the-hour variability: levels and their probabilities the within-the-hour these bounds are around 10% in absolute value with respect to the standard hour value. exceptions occur again at sunrise in winter and equinoxes, and around sunset in summer (fig. 6) where these limits are more pronounced. variability and disturbances – buresova and laštovička (2001) selecting strong geomagnetic storms found that during ionospheric disturbances there is a considerable daytime within-the-hour variability in fof2. the results reported here confirm the strong variability. indeed, although table iv refers to monthly conditions, it clearly shows that a substantial variability exists throughout the day, being more pronounced around sunrise and sunset. on the contrary, the within-the-hour variability in tec is around 5% of the corresponding hourly daily value except around sunrise and sunset, and during local disturbed conditions where values of the order of 20%-30% are reached. however, it is known that ionospheric disturbances may follow magnetic disturbances (fig. 7) but there is no simple direct relationship between them. indeed, we have observed that when kp is much greater than 2 the level of the within-thehour variability in fof2 could be that of the «quiet variability» (fig. 8), that is about 0.12 or less, which is of the same order of the variations observed in the critical frequency of the e-layer (kouris and fotiadis, 2002). on the other hand, the variability in tec is around 5% or less, except at sunrise (figs. 7 and 8). figure 9 shows clearly that although the kp index is less than 2, the variability in fof2 is quite substantial, reaching values around 0.20; whereas that in tec is negligible apart from around sunrise. we may therefore state that there is nearly always a substantial variability in fof2 in the interval time of an hour, whereas the withfig. 7. relative deviations of 5-min fof2 measurements (rome, 22-23 september 1999) and 10-min relative deviations of tec measurements (matera, 22-23 september 1999). values of kp index. 958 fig. 9. relative deviations of 5-min fof2 measurements from rome (upper) and 10-min tec measurements from matera (upper middle) and hailsham (low middle); values of kp index (bottom). fig. 8. relative deviations of fof2 from measurements made in rome (upper) and of tec made at matera (upper middle) and hailsham (low middle); values of kp index (bottom). stamatios s. kouris, kostas v. polimeris, vincenzo romano, bruno zolesi and ljiljana r. cander 959 within-the-hour variability: levels and their probabilities in-the-hour variability in tec is about 5% or less apart from during local disturbed conditions (table ii) and at sunrise and sunset. in these last cases the pronounced variations in tec are simply due to its rapid increase or decrease during its diurnal normal variation, that is at sunrise and sunset when rapid changes in the atmospheric conditions occur. the present study also points out that ionospheric disturbed conditions are more marked in fof2 rather than in tec and this is in agreement with previous findings regarding other ionospheric parameters (kouris and fotiadis, 2002). 4. conclusions the obtained results lead to the conclusion that the deciles of the within-the-hour variability in fof2 are 12% higher/lower than the corresponding standard hourly value of fof2, at least regarding the region of rome. variations higher, that is of the order of 20%-30%, are usually observed at sunrise and sunset due to changes in the atmospheric conditions. moreover, large variations (positive/negative) in the interval time of one hour of the same order (0.20 to 0.30) due to disturbed conditions are also observed, but their probability of appearance is less than 5% per month. the variations in tec for 90% of the time are usually less than ±5% of the corresponding hourly value. however, at sunrise and also at sunset in winter variations of the order of 20%30% are observed and should be attributed to the rapid increase/decrease of tec during its diurnal normal variation. they depend strongly on season.the analysis shows also that a stationarity in tec of about one hour may be accepted. in addition ionospheric disturbances affect the f2 layer maximum electron density rather than tec in the interval time of one hour. acknowledgements this work is part of a joint greek-italian research project supported by the cultural collaboration (7th protocol) between the two countries. references buresova, d. and j. laštovička (2001): changes in the f1 region electron density during geomagnetic storms at low solar activity, j. atmos. solar-terr. phys., 63, 537-544. fotiadis, d.n., s.s. kouris and b. zolesi (2001): preliminary results on the within-the-hour ionospheric variability, phys. chem. earth (c), 26 (5), 315-318. fotiadis, d.n., s.s. kouris, v. romano and b. zolesi (2004): climatology of ionospheric f-region disturbances, ann. geophysics, 47 (4), 1311-1323. kouris, s.s. and d.n. fotiadis (2002): ionospheric variability: a comparative statistical study, adv. space res. 29 (6), 977-985. kouris, s.s., d.n. fotiadis and t.d. xenos (1998): on the day-to-day variation of fof2 and m(3000)f2, adv. space res., 22 (6), 872-876. kouris, s.s., d.n. fotiadis and b. zolesi (1999): specifications of the f-region variations for quiet and disturbed conditions, phys. chem. earth (c), 24 (4), 321-327. kouris, s.s., b. zolesi, d.n. fotiadis and c. bianchi (2000): on the variability within-the-hour and from hour-to-hour of the f-region characteristics above rome, phys. chem. earth (c), 25 (4), 347-351. kouris, s.s., k.v. polimeris and lj.r. cander (2006): specifications of tec variability, adv. space res., 37 (5), 983-1004. polimeris, k.v., v. romano, s.s. kouris and b. zolesi (2004): the within-the-hour variability in tec: some preliminary results, bulg. geophys. j., 30 (1-4). rawer, k. (1957): the ionosphere (f. ungar, new york). rawer, k. (1993): wave propagation in the ionosphere, (kluwer publ. co., dordrecht nl). rawer, k., s.s. kouris and d.n. fotiadis (2003): variability of f2 parameters depending on modip, adv. space res., 31 (3), 537-541. zolesi, b., s.s. kouris, d.n. fotiadis and c. scotto (2001): evaluation of the ionospheric noise and variability within-the-hour by 5 min ionospheric soundings, phys. chem. earth (c), 26 (5), 359-362. (received october 19, 2005; accepted june 20, 2006) layout 6 high-precision gravity measurements using absolute and relative gravimeters at mount etna (sicily, italy) antonio pistorio1,2,*, filippo greco1, gilda currenti1, rosalba napoli1, antonino sicali1, ciro del negro1, luigi fortuna2 1 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy 2 università di catania, dipartimento di ingegneria elettrica, elettronica e informatica (dieei), catania, italy annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5348 abstract accurate detection of time gravity changes attributable to the dynamics of volcanoes requires high-precision gravity measurements. with the aim of improving the quality of data from the mount etna gravity network, we used both absolute and relative gravimeters in a hybrid method. in this report, some of the techniques for gravity surveys are reviewed, and the results related to each method are compared. we show how the total uncertainty estimated for the gravity measurements performed with this combined use of absolute and relative gravimeters is roughly comparable to that calculated when the measurements are acquired using only relative gravimeters (the traditional method). however, the data highlight how the hybrid approach improves the measurement capabilities for surveying the mount etna volcanic area. this approach enhances the accuracy of the data, and then of the four-dimensional surveying, which minimizes ambiguities inherent in the gravity measurements. as a case study, we refer to two gravity datasets acquired in 2005 and 2010 from the western part of the etna volcano, which included five absolute and 13 relative stations of the etna gravity network. 1. introduction high-precision gravity measurements can obtain the value of the acceleration of gravity with a precision of 1 µgal (10−9 g; microgravimetry). they thus represent a useful tool for any volcano monitoring strategy, as they are essential for the detection of underground mass movements due to volcanic activity that might trigger a pre-eruptive state [battaglia et al. 2003, carbone et al. 2003, hautmann et al. 2010, greco et al. 2010, williams-jones and rymer 2002]. detection of clear gravity signals associated with the renewal of volcanic activity has led to increased applications for the four-dimensional gravity approach. since the 1980s, we have used high-precision relative gravity measurements to determine significant correlations between the eruptive activity of the mt. etna volcano and temporal changes in the gravity field, which can occur with different patterns [budetta et al. 1999, branca et al. 2003, carbone et al. 2003, bonforte et al. 2007, carbone and greco 2007, greco et al. 2010, bonaccorso et al. 2011]. conventionally, gravity measurements have been carried out using relative spring gravimeters, which measure spatio-temporal gravity changes with respect to a fixed reference site. it is well known that the accuracy of these relative spring gravimeters is largely limited by instrumental drift. together with influences arising from temperature and pressure variations, this instrumental effect can make the detection of volcanic source effects more difficult. this can hinder the detection of small time gravity changes that can be useful for an understanding of the geophysical processes preceding and accompanying volcanic unrest. in the framework of a project with the italian energy company eni s.p.a., we had the opportunity to test a fg5#238 absolute gravimeter from micro-g lacoste (owned by eni s.p.a.) at mt. etna. as well as presenting here the performance of this instrument in a volcanic environment, we present the advantages of the use of a combined approach of absolute and relative gravimeter measurements performed at mt. etna volcano (hybrid gravimetry). the state of art clearly shows that the hybrid approach is a well-known advance and many applications have already been performed [berrino 2000, furuya et al. 2003, d'agostino et al. 2008, ferguson et al. 2008]. we demonstrate that the inclusion of repeated absolute stations within the existing gravity network for relative measurements represents an important step towards the improvement of the quality of the data acquired, which has obvious consequences for volcanic monitoring. 2. gravity network surveying mt. etna volcano the mt. etna gravity network was set up in 1986, and to reach its present configuration, it has been under continuous development over the intervening years. it consists of 71 article history received april 26, 2010; accepted july 26, 2011. subject classification: absolute and relative gravimeters, uncertainty, microgravimetry, etna volcano, volcano monitoring. 500 special issue: v3-lava project relative stations that are located between 0.5 km and 4 km apart, which cover an area of about 400 km2 and are arranged in four array subsets that are in circular profiles around the volcano and radial profiles along its slopes (figure 1). these subarrays refer to a reference station that is not affected by volcanically induced gravity changes, and they differ from one another in station density, access (determined by snow cover), and time required to collect the gravity measurements. each subarray can be occupied independently, which optimizes the flexibility in the data collection, to accommodate variations in activity and accessibility of the volcano. measurements over the entire mt. etna gravity network are generally conducted once a year, although some of the arrays are occupied more frequently. since 1994, discrete gravity measurements have been performed using a cg-3m#9310234 scintrex gravimeter (figure 2a). even when this instrument is used under the unfavorable conditions encountered on mt. etna (e.g. rough unpaved roads, marked differences in altitude), it provides high-precision measurements due to its low sensitivity to both shocks and external temperature changes. a calibration line (see figure 1) was also established in february 1995, to monitor the systematic variations in instrumental calibration factors with time [budetta and carbone 1997]. to better constrain the gravity measurements acquired with relative gravimeters, we also started to collect absolute gravity data. we combined the use of absolute and relative gravimeters as a hybrid method to extend the potential of gravity measurements for the surveying of the mt. etna volcanic area. at present, the gravity network for absolute measurements is composed of 13 stations that are distributed around the volcano edifice at altitudes ranging between 1,250 m and 2,900 m a.s.l. (figure 1). pistorio et al. 501 figure 1. sketch map of mt etna showing the current gravity network for absolute and relative measurements. white triangles and circles indicate, respectively, the absolute and relative stations in which the measurements reported in this study were made in 2005 and 2010. at the bottom left, the calibration line for the relative gravimeters and the cta absolute station are also indicated. 502 in this study, we focus on two different gravity surveys that were carried out for the western part of the mt. etna volcano (figure 1). the first survey was performed in 2005 at 13 stations of the gravity network, and only a relative gravimeter was used (scintrex cg-3m#9310234). the second survey was carried out in 2010, and as well as the same relative stations, five absolute stations were added, with measurements using a fg5#238 gravimeter as the reference points for the relative gravimeter (the hybrid approach). as the different array subsets of the mt. etna gravity network have approximately the same characteristics (e.g. nonasphalted roads, stations 2-5 km apart, large altitude differences between pairs of stations, volcanic noise), even if we consider a restricted number of absolute and relative stations in a single sector of the volcano, the data from this case study can be considered representative for the entire network. 3. absolute gravity data absolute gravity meters are based on the reconstruction of the trajectory followed by a test body subjected to the gravity field in a vacuum chamber. the ballistic absolute fg5#238 gravimeter (figure 2b) uses the freely falling method, where an object is dropped downwards [niebauer et al. 1995]. the motion of the free-falling test body with an optical reflector is measured using a michelson interferometer. an optical-fiber-connected, frequency-stabilized, he-ne laser (red light, 633 nm) is used to illuminate the interferometer where the input laser beam is split into the test and reference beams. the test beam bounces on the reflective test body, while the reference beam travels straight through the interferometer. these beams are recombined and their interference signals (fringes) are used to track the trajectory of the test body. the time intervals between the occurrence of each interference fringe are measured by the time interval measuring system, with a rb oscillator as a reference. a model equation derived from the law of motion of freefalling objects is fitted to the data. after removing the geophysical corrections (due to the earth tides, ocean loading, polar motions) and instrumental effects (e.g. diffraction correction), the free-fall acceleration at the observation point is evaluated as the mean of all of the freefall accelerations obtained for each drop of the test body during a measurement session. with the fg5#238 gravimeter, a total of 700 timeposition points are recorded over the 20-cm-length trajectory of each drop. usually, the repetition rate of each drop is 10 s, even if drops can be produced every 2 s. measurements typically consist of one or two sets per hour, with the mean of several sets (usually 12 to 48) providing a final gravity value. the software supplied by the micro-g lacoste was used for the data acquisition and analysis. this software provides an immediate value for the local gravity; it also includes a full-featured post-processor that allows the data analysis procedures to be varied and that calculates the statistical uncertainty (dstat ), as given by the standard deviations of the absolute gravity values obtained for each set (vset ) divided by the square root of the number of sets, nset : (1) the total uncertainty (dabs ) for the final gravity value is given by: (2) where dsys is the systematic uncertainty that is due to different components of the measurements that can be grouped into high precision gravity measurements figure 2. gravimeters used at mt. etna volcano: (a) cg-3m#9310234 scintrex relative gravimeter; (b) fg5#238 micro-g lacoste absolute gravimeter; (c) cg5#8664.196 scintrex relative gravimeter. nstat set set=d v 2 2 abs stat sys= +d d d four separate areas: (i) modeling; (ii) system; (iii) environment; and (iv) set-up. the systematic uncertainty of the fg5#238 gravimeter is about 2 µgal, as reported by micro-g lacoste. the measurements of the absolute gravity with the total uncertainty (dabs ) gathered in 2010 in five selected stations of the mt. etna gravity network (figure 1) are given in table 1. considering the difficulties of running such measurements under non-laboratory conditions and in an environment where the observation sites are affected by severe ambient conditions, the absolute gravity values (adjusted for earth tides, ocean loading, and local atmospheric and polar-motion effects) show a total uncertainty ranging from 2.7 µgal to 7.5 µgal (see table 1). data scattering due to the strong floor vibrations, together with strong temperature and humidity fluctuations [van camp et al. 2005], were the most significant limitations to the uncertainty of the absolute gravity values measured at mt. etna. 3.1. test measurements using the fg5#238 gravimeter the absolute data presented here refer to the stations located at altitudes between 1,400 m and 1,900 m a.s.l. at elevations higher than 2,000 m a.s.l. it was impossible to take the measurements using the fg5#238 gravimeter because the laser beam was not generated. to determine the possible cause of this problem, various considerations were taken into account, including the power-supply system, and the humidity, temperature and pressure of the sites where the tests were carried out. on mt. etna we used the same gasoline generator at all of the absolute stations without any problems; therefore, we can exclude that the problem was related to the power generator. we also excluded the humidity, as we had measured g at lower-altitude sites but under wetter conditions. for the ambient temperature, to ensure an operative temperature in the stations, we used a gas heater that faces the laser. although the temperature had reached about 20 ˚c, the fg5#238 gravimeter still did not work. after these tests were performed, we hypothesized that the problem was due to the high altitudes of the sites (which correspond to low atmospheric pressures) where the fg5#238 gravimeter was used. gravity measurements are affected by the simultaneous actions of various physical parameters that act both on the local gravity field and on the behavior of the gravimeter. a scheme of a generalized configuration that indicates the significant input-output relationships for all measuring apparatus is shown in figure 3 [doebelin 1990]. the input quantities are classified into three categories: the desired pistorio et al. 503 station latitude (˚n) longitude (˚e) elevation (m a.s.l.) gabs (mgal) h (mm) �dabs (µgal) fag ±�dfag (µgal/m) *sln 37.694 14.973 1730 979641363.5 1292.7 2.7 336.5 ±2.8 *gal 37.732 14.950 1875 979600598.0 1293.7 7.5 279.6 ±3.3 *msc 37.770 14.950 1720 979632850.7 1289.7 4.0 282.1 ±3.0 *msp 37.823 14.961 1450 979693569.4 1291.7 3.0 297.5 ±2.2 *gpl 37.827 15.028 1570 979667690.6 1296.2 2.8 310.8 ±3.5 figure 3.scheme of a generalized configuration that highlights the significant input-output relationships present in all measuring apparatus (after doebelin [1990]); see text for details. table 1. results of absolute gravity measurements acquired in 2010 using a fg5#238 gravimeter. the first four columns give the acronyms and the coordinates of the absolute gravity stations (the locations of the sites are mapped in figure 1). the last four columns give their measured gravity values (gabs) referred to the height (h), the total uncertainty (dabs), and the free-air vertical gradient (fag) with the uncertainty (dfag). the symbol * indicates the absolute reference point. 504 inputs, the interfering inputs, and the modifying inputs. the desired inputs represent the quantities that the instrument is specifically intended to measure. the interfering inputs represent the quantities to which the instrument is unintentionally sensitive. fd and fi are the symbolized inputoutput relations for these desired and interfering inputs, respectively. the modifying inputs are the quantities that cause a change in the input-output relations for the desired and interfering inputs. the symbols fm,i and fm,d represent the specific manners in which im affects fi and fd, respectively. basically, for both absolute and relative gravimeters, the main interfering inputs are temperature and humidity, while the pressure is considered to be a modifying input (at least with modern gravimeters). changes in atmospheric pressure can affect the gravity measurements in two ways: directly, through gravitational effects; and indirectly, although to a lesser extent, through surface deformation of the earth due to the weight of the atmosphere [warburton and goodkind 1977]. the effects produced in measuring instruments that are not related to a gravity-field variation are considered to be negligible. however, for the fg5#238 gravimeter, we assumed that the pressure is both a modifying input and an interfering input. to demonstrate this hypothesis, we tried to reproduce the same atmospheric-pressure conditions at the high altitude sites, such as in the catania laboratory (50 m a.s.l.). for this, we developed a hyperbaric chamber where the laser was isolated (see figure 4a,b). the hyperbaric chamber was obtained from a pvc pipe that was sealed at both ends, in which cavities were made to allow the passage of the cables and the optical fiber; all of the cavities were made watertight. in addition to the laser, an absolute pressure sensor (ptx 1400), a temperature sensor (lm 35), and a humidity sensor (hih 3605) were also placed inside the chamber. the temperature was also monitored outside the chamber through another temperature sensor similar to that inside the chamber. all of the signals were sampled at 1-min intervals and stored by a datalogger (cr10x). the experiment was performed at the pdn observatory (2,800 m a.s.l.; see figure 1). with the compression within the chamber, the pressure value increased from 707 mbar (corresponding to the pressure at an altitude of 2,800 m a.s.l.) up to 900 mbar. at about 900 mbar, the laser suddenly began to work (see figure 5). a threshold value for the pressure was determined as just above 900 mbar, and an automatic system using a schmitt trigger circuit (with a window 65 mbar) to manage a 12 v compressor (see figure 4c) was installed for automatic adjustments around this pressure threshold. using this unusual solution, we were able to take measures also at stations with altitudes greater than 2,000 m a.s.l. this clearly showed a net dependence of the laser of the fg5#238 absolute gravimeter on atmospheric pressure. accordingly, a dependence between changes in the laser frequency and variations in the atmospheric pressure has already been shown [chartier 1987]. we hypothesized that there were two reasons for this stopping of the generation of the laser at low atmospheric pressure that can be checked: it might have been a misalignment of the optical components of the laser, or a change in the refraction index of the air. unfortunately, neither of these effects have been demonstrated previously. 4. relative gravity data the high-precision gravity measurements were carried out using relative gravimeters, which measure spatial changes with respect to a fixed reference site. basically, with relative gravimeters, the gravity differences (dg) are measured between a pair of stations, and according to the study objectives and field conditions, the ‘step method’, ‘star method’ and/or ‘profile method’ can be used [watermann 1957]. to measure the gravity network of mt. etna, the step method is mainly used. following the multiple occupation sequence in the various stations, as required by this method (eg. a-b, b-a, a-b, b-c, …), and starting from a primary reference point, each station was occupied at least three high precision gravity measurements figure 4. the hyperbaric chamber made from a pvc pipe. the laser, together with pressure (ptx 1400), temperature (lm 35) and humidity (hih 3605) sensors, were placed inside the chamber (a, b). all of the signals were sampled at 1-min intervals and stored by a datalogger (cr10x). a 12 v compressor was installed for automatic adjustment around the operative pressure values threshold (c). times. in this way, the final dg values between the pairs of adjacent stations can be obtained from the mean of the three dg calculated, and the uncertainty of the link (ddg ) to assign to this value will be the square root of the sum of the squares of the individual uncertainties attributed to each dg. table 2 shows the dg with the uncertainty ddg between the pairs of adjacent stations as collected in 2005 for the 13 selected stations of the relative gravity network (figure 1). these were adjusted for tidal effects using a local-tide model, corrected for the instrumental drift, and referred to a single pistorio et al. 505 station link dg (mgal) ddg (µgal) gadr (mgal) drel (µgal) ifo ifo-mdz −10.288 8 650.856 6 mdz mdz-mfo −32.148 10 640.568 7 mfo mfo-mpa −22.502 8 608.442 8 mpa mpa-mnz 29.085 5 585.940 8 mnz mnz-bch 32.148 10 615.025 8 bch bch-rln 33.901 6 647.173 8 rln rln-msp 12.925 6 681.074 8 msp msp-l81 16.658 6 693.999 9 l81 l81-msm −49.171 3 710.657 7 msm msm-gla −25.376 7 661.486 7 gla gla-gpa 20.633 3 636.110 6 gpa gpa-mro 13.474 3 656.743 9 mro mro-mar 29.392 5 670.317 6 figure 5. waveforms of the internal pressure of the chamber (a), the signal to drive the 12 v compressor (b), and the activation of the laser (c). table 2. results of the traditional gravity measurements acquired in 2005 for the 13 selected relative gravity stations of the mt. etna gravity network. the columns indicate, respectively, the station acronyms, the links between two adjacent relative stations, the gravity differences (dg) between each pair of adjacent relative stations; the uncertainties (ddg ) associated to each �dg; the gravity values (gadr) at each station obtained with the least-squares method, as described in the text and referred to adr station, and the associated errors (drel ). 506 reference station (adr; figure 1). the distances between the adjacent stations ranges from 1 km to 3 km, and each station can be reached on unpaved roads. considering all of the gravity stations as part of a loop, to evaluate the gravity and the associated total uncertainty at each station, we applied a strict compensation procedure that was based on a least squares adjustment method, by solving a linear system that consisted of observation equations (the method of indirect observations). errors in the gravity measurements can occur both for the measuring point and during the transfer from one station to another. obviously, these errors will have an accidental and a systematic component. the errors that can accumulate at the measurement point are those that arise from incorrect readings or imperfect leveling of the instrument, or errors induced by the external environment (e.g. mechanical vibrations, microseisms, magnetic effects). relevant effects are also caused by temperature gradients and pressure. during the transfer from one station to another, the mechanical vibrations that can cause changes to the behavior or properties of the gravimeters are particularly critical. the most important systematic errors originate between two stations; to eliminate this, observations are treated as gravity differences between pairs of stations occupied consecutively. the differences between the accidental errors of the two stations are considered to have a stochastic distribution. the systematic components of the errors are included in the observation equation, which takes the general form of: (3) where pn is the observation weight, gi and gj are the unknown gravity values at the i-th and j-th stations, km is the unknown scale factor for the m-th link, dgij is the measured gravity difference between the i-th and j-th stations, and fij is the residual. the solution with the least squares method for the linear system of observation equations, the number of which corresponds exactly to the measured gravity differences, leads to the adjusted g values. to describe the mathematical procedure, considering the pn term of equation (3) equal to 1, the equation simplifies to: (4) and starting from the known terms gi 0, gj 0 and km 0: (5) (6) (7) where xi, xj and xm are the unknown corrections to be made to the known terms therefore equation (4) becomes: (8) for the known terms : (9) equation (4) can be further simplified as: (10) or in matrix notation, as: (11) where a is the n×r matrix of the r unknowns (xi, xj and xm) due to the n measured gravity difference: the coefficients of the matrix are +1 for xi, −1 for xj, and −dgij for xm; x is the vector of the r unknown corrections, l is the vector of the n errors calculated with the known terms , and v is the vector of the n residuals. to satisfy the least-squares method, the following relationship must exist: (12) and by defining the following matrix: (13) the unknowns xi, xj and xm are obtained by the expression: (14) with the residuals fij: (15) therefore, through equation (14), we can calculate the corrections to be included in equations (5), (6) and (7), to obtain the values of the gravity. the determination of the gravity values at each station (see table 2) is achieved through iterative methods: after each iteration, the new approximated values are calculated and used for the next iteration. all of the links are rejected where the residuals fij exceed the limit of 3v0, where v0 is the standard deviation of the unit weight. the value of v0 is related to the theoretical variance of the unit weight from the equation: (16) where n is the number of accepted links and r is the number high precision gravity measurements p g g k gn i j m ij ij= fd-^ h g g k gi j m ij ij= fdg g x0i i i= + g g x0j j j= + k k x0m m m= + g x g x k x g0 0 0i i j j m m ij ij+ + + = fd-^ ^ ^h h h g g k g l0 0 0i j m ij ij=dx x x g li j m ij ij ij+ = fdax l v+ = v v mint = r a at= x r a l1 t= v ax l= + v v n r0 0 2 t = =v v / , andg g .k0 0 0i j m , andg g k0 0 0i j m , andg g k0 0 0i j m 0 2v of unknowns; in this way, a small number of links with major errors can be separated from others that have a normal error distribution. to calculate the total uncertainty drel for each station (see table 2), the rii diagonal elements of the r matrix is used, with the following relationship applied: (17) applying the procedure described above to the data acquired in 2005 for the 13 selected gravity stations, the total uncertainty is between 6 µgal and 9 µgal (see table 2). although the uncertainty ddg between some of the pairs of stations is small enough (3 µgal in some cases), at the end of the compensation procedure, a larger total uncertainty at each measurement is assigned to all of the values. consequently, the final gravity value at each station will depend both on the uncertainty ddg and on the closure error of the loop formed by the stations. 5. hybrid gravity data the accuracy of the gravity measurements obtained using an absolute gravimeter is very high compared to the accuracy in measurements obtained with a relative gravimeter, although the sensitivity and complexity of the absolute gravimeters makes it very difficult to use them in any location other than under laboratory conditions. only recently have field-usable absolute gravimeters been developed, making high-precision gravity observations possible using absolute readings [ferguson et al. 2008]. even though movable, the sensitivity and fragility of a field-usable absolute gravimeter complicates its use. these factors have the effect of limiting the number of points where the gravity can be measured within a specific amount of time. at present, the best way to assess the temporal and spatial gravity variations over a gravity network is to combine data collected with absolute ballistic and relative spring gravimeters in a hybrid approach [berrino 2000, furuya et al. 2003, d'agostino et al. 2008, ferguson et al. 2008]. the accuracy of relative gravimeters is known to be limited by the instrumental drift that can increase when these instruments are subjected to mechanical and thermal shocks during transportation. in addition, the measurement schedule for a relative gravity survey that occupies at least one reference point usually represents a huge amount of work for the operators. it is worth noting that if an absolute gravimeter is used at different points of a relative gravity network, many reference stations would be available, which allows loops between stations separated by very long distances to be avoided (the mt. etna reference stations that are not affected by volcano-related gravity changes are about 20 km from the summit zone). thus the time required to accomplish discrete surveys would be drastically decreased, and the reliability of discrete data would be improved. to reduce the inconvenience due to the use of only relative gravimeters, and to exploit instead the advantages offered by the absolute gravimeter, for the hybrid procedure, pistorio et al. 507 station link dg (mgal) ddg (µgal) gabs (mgal) dhyb (µgal) ifo *sln-ifo 9.158 3 650.837 5 mdz *gal-mdz 39.736 4 640.626 9 mfo *gal-mfo 7.570 2 608.460 8 mpa *gal-mpa 14.894 3 585.996 9 mnz *msc-mnz −18.120 5 615.024 7 bch *msc-bch 14.009 7 647.153 9 rln *msp-rln −12.868 5 681.011 6 msp *msp-msp 0.062 4 693.942 5 l81 *msp-l81 16.550 6 710.430 7 msm *gpl-msm 25.362 4 661.430 6 gla *gpl-gla −31.949 2 636.067 5 gpa *gpl-gpa −11.231 5 656.785 7 mro *gpl-mro 2.281 6 670.296 8 table 3. results of the hybrid gravity measurements acquired in 2010 for the 13 selected relative gravity stations and the five absolute gravity reference points of the mt. etna gravity network. the columns indicate, respectively, the relative stations acronym, the links between the relative stations and an absolute reference point, the gravity differences (dg) between them and their associated uncertainty (ddg), the absolute gravity values acquired at the reference point by the fg5#238 absolute gravimeter (gabs), and the associated uncertainties (dhyb). each relative station was linked with the closest absolute reference point, as indicated with the symbol “*”. r0 2 rel ii= $d v 508 we use the absolute stations that are located on mt. etna volcano as references for the relative measurements acquired in the different sectors of the relative gravity network (figure 1). in brief, we connect three or four relative gravity stations at each absolute site (used as a reference). the high-precision gravity values at all of the stations are then determined by adding the difference (dg) measured between each absolute station with the relative stations. all of these absolute values are transferred to 250 mm from the ground through the vertical gravity gradient acquired at each station, using a cg5#8664.196 scintrex relative gravimeter (figure 2c). we chose this height to compare the absolute and relative measurements at the same height from the ground. following the above hybrid procedure, table 3 shows the data acquired in 2010 for the western part of the volcano from the 13 relative stations (the same as those used in 2005, to evaluate the uncertainty of the relative data; see section 4) and at the five absolute stations located in the same area of the volcano and used as references for the relative measurements. as stated before, each relative station was linked with the closest absolute station, as labeled with the symbol * in table 3. following this hybrid approach, and considering that the free-air vertical gravity gradients are measured at each absolute station (table 1), we estimate the total uncertainty at each station according to the equation: (18) where ddg is the uncertainty calculated for all of the differences (dg) necessary to connect a relative gravity station to an absolute site, dfag is the uncertainty of the vertical gravity gradient, and dabs is the total uncertainty for each absolute measurement, calculated as described in the section 3. the uncertainties attributed to each dg (ddg) are calculated as described in section 4. the results highlight that the hybrid method that was applied to collect data in the western part of the mt. etna volcano includes 13 relative and five absolute stations, which allowed gravity data to be obtained with a total uncertainty, as calculated by equation (18), of 5 µgal to 9 µgal (see table 3). the results indicate that the final total uncertainty depends strongly on the ddg uncertainty calculated between each single pair of stations, and the propagation errors are not included. 6. discussion and conclusions in this study, we compared high-precision gravity data collected at mt. etna volcano with different methods. as a case study, we considered first a gravity dataset that was acquired from the western side of the mt. etna volcano in 2005, at 13 stations using only a relative gravimeter. the data were processed through the least-squares adjustment method, to provide gravity values and their related uncertainties. the second dataset was acquired in 2010 in the same area of the mt. etna volcano, and as well as the same relative gravity stations, another five absolute points were added and used as references for the relative measurements (the hybrid method). the results highlight that the uncertainty calculated with data acquired by the hybrid method is comparable with the uncertainty that affects the gravity data when a single relative gravimeter is used (see tables 2 and 3). obviously, this extraordinary result depends greatly on the ability of the operators to perform gravity measurements with both of these gravimeters in hostile environments. moreover, in the latter case, the gravity values and the final total uncertainty at each station depend heavily on a compensation procedures. this means that if several wrong data are included in the analysis procedure, the final gravity values are adjusted and high uncertainty values are equally assigned to each gravity value. this provides ambiguous results and prevents an unequivocal interpretation of the anomalies observed. in addition, we must consider that the errors of the measurements increase with the number n of the consecutive differences, and after the n-th difference, the measurement error is multiplied by . the final result also depends on the total number of stations included in the loop. conversely, the hybrid approach allows the obtaining of the gravity value at each relative station through a direct link between the absolute station and the relative station. in this case, the final total uncertainty strongly depends on the ddg uncertainty that is calculated between each single pair of stations and the ambiguity in the final gravity values, which is not included due to the propagation errors during both the field measurements and the data processing. in summary, we conclude that the hybrid approach provides the following advantages: a) it allows the optimization of the traditional techniques and strategies of gravity measurements of the mt. etna network, thus enabling a drastic reduction in the time required to accomplish discrete surveys and ensuring improvements to the quality of the data; b) it obtains gravity information with an accuracy that is comparable to the absolute gravity information on an extensive gravity network, which thus reduces the typical ambiguity that is inherent in the singlecomponent method; and c) it also allows the investigation of low amplitude gravity variations of the order of a few µgal, which have never been considered previously for mt. etna, and which can provide useful information towards our better understanding of the geophysical processes that precede and accompany volcanic phenomena. acknowledgements. this study was developed within the framework of the tecnolab, the laboratory for the technological advance in volcano geophysics, organized by ingv-ct and dieeiunict. thanks are due to the e&p division of the aesi department of eni s.p.a. for making the fg5#238 absolute gravity meter available. high precision gravity measurements 2 2 2 hyb g fag abs= + +d d d dd n references battaglia, m., p. segall and c. roberts (2003). the mechanics of unrest at long valley caldera, california. 2. constraining the nature of the source using geodetic and micro-gravity data, j. volcanol. geoth. res., 127, 219-245; doi: 10.1016/s0377-0273(03)00171-9. berrino, g. 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(1957). über systematische fehler bei gravimetermessungen. deutsche geodätische kommission, dgk c21, münchen. williams-jones, g., and h. rymer (2002). detecting volcanic eruption precursors: a new method using gravity and deformation measurements, j. volcanol. geoth. res., 113, 379-389; doi: 10.1016/s0377-0273(01)00272-4. *corresponding author: antonio pistorio, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy; email: antonio.pistorio@ct.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. pistorio et al. 509 s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e d e l g o l f o d i cagliari p . c a l o i m . 0 . s p a d e a r i c e v u t o il 1 f e b b r a i o 1962 proseguendo lo studio sull'idrodinamica dei golfi interessanti l'italia, abbiamo questa volta preso in esame il golfo di cagliari (fig. 1). il golfo di cagliari sito nella parte meridionale della sardegna, si apre, con un'ampia bocca che dal capo pula va al capo carbonara lungo una linea ideale di 45 km circa, sul mar mediterraneo. caratteristica di questo golfo è la eccezionale larghezza della sua bocca rispetto alla sua lunghezza, che risulta essere di 22,5 km. 1. oggetto della nostra nota è stato quello di determinare il periodo dell'uninodale dell'intero golfo. si è suddiviso, a tale scopo, il golfo suddetto in 22 sezioni; di queste si sono calcolate le aree verticali e in superficie (fig. 2). ottenuti questi dati, ci è stato possibile applicare il metodo di goldberg, per la descrizione del quale rimandiamo ad altra nota (1). dopo vari tentativi si è trovato, per il periodo dell'uninodale, un valore di t = 40"',2 valore che conduce ad un residuo trascurabile. poiché, come abbiamo già detto, l'ampiezza della bocca del golfo è molto rilevante, ci si doveva attendere una correzione molto elevata, conformemente del resto alla teoria (2); infatti il fattore più rispondente alle osservazioni è risultato essere quello di 1,5: ne consegue per il periodo corretto un valore di t = 60m = 1" in accordo con quello osservato sui mareogrammi, ottenuti dal mareografo sito presso la capitaneria del porto di cagliari. è da aggiungere inoltre che, nelle osservazioni jnareografiche, fra le altre, sono state isolate e messe in evidenza numerose oscillazioni con un 9 6 p . c a l 0 i m . c . s p a d e a periodo di circa 20m, periodo questo che dovrebbe corrispondere a quello della .binodale dell'intero golfo. nella tabella i sono riportati i dati o t t e n u t i per l'applicazione del metodo di gohlberg. t a b e l l a i ses(x)v av(x)v tv vv mv zioni m io8 m a 10» m 2 m ni io12 m 3 0 12.157 0 82 257 0 1,000 1 1.000 11.146 45,25 89 629 56,8396 0,999 2 » 9,488 42,40 104 975 118,7732 0,996 3 » 8.203 42,18 120 931 191,3109 0,992 4 » 6.556 42,69 150 396 274,8742 0,986 5 » 5,477 39,54 178 747 378,7978 0,979 6 » 4 , 8 7 8 32,87 199 057 502,3120 0 , 9 7 1 7 » 3,864 39,51 247 930 639,8604 0 , 9 5 8 8 » 3,554 28,62 266 179 811,1800 0,946 9 n 3,371 27,58 276 476 995,1097 0,932 10 » 2,226 29,40 411 051 1.186,1546 0,915 11 » 1,294 29,21 690 881 1.470,1908 0,894 12 » 0,996 27,49 870 482 1.947,5896 0,867 13 » 0,889 25,12 939 258 2.549,0927 0,835 14 » 0,706 27,64 1 120 397 3.198,1200 0 , 7 9 1 15 » 0,646 25,02 1 147 059 3.972,3143 0,741 16 » 0,468 27,25 1 444 444 4.764,9321 0,676 17 » 0,383 26,90 1 561 358 5.763,0429 0,598 18 » 0 , 3 4 3 25,70 1 486 880 6.841,9413 0,510 19 » 0,271 35,67 1 365 314 7.869,3754 0,370 20 » 0,228 22,14 1 144 737 9.812,8074 0 , 2 6 1 21 » 0,136 28,77 794 118 10.603,8207 0,108 22 750 17,29 11.015,3719 0 , 0 1 3 2. al fine di selezionare le oscillazioni dovute esclusivamente al golfo, nella sua totalità, da quelle proprie del porto di cagliari e delle sue suddivisioni, sarebbe necessario poter registrare anche ad oriente di capo s. elia. le registrazioni ottenute dal mareografo sito presso la capitaneria del porto di cagliari, contengono le oscillazioni libere non solo dell'intero golfo e dello zoccolo dell'isola, m a anche quelle derivanti dai bacini f o r m a t i dai numerosi moli che dividono il porto di cagliari propriamente detto in t a n t e parti a n i m a t e da oscillazioni proprie. d a t a la complessità di queste suddivisioni, alcune delle quali entrano come p a r t e integrante di quelle, più estese, riesce molto diffìcile v a l u t a r n e le caratteristiche dinamiche singole dal p u n t o di vista teorico. il problema è reso più arduo dai bassi fondali, che consentono un'azione ritardatrice della viscosità (3), non agevolmente valutabile. 5 v 1 9 5 7 6 v 20 11 t* 2 * fi a io il u 16 tb io ti pig. 4 e s e m p i o di oscillazioni libere u n i e b i n o d a l i d e l l ' i n t e r o golfo e di sue p a r t i ( r i d o t t i a circa 1/15 dell'ampiezza n a t u r a l e ) s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e d e l g o l f o d i c a g l i a r i 9 7 e v e n t u a l i oscillazioni multinodali interessanti l'intero golfo, possono quindi essere mascherate da oscillazioni associate dai bacini del porto. di qui la grande utilità — come si è detto — di poter usufruire di registrazioni o t t e n u t e al di fuori del porto stesso. solo in questo modo potrebbe essere agevolato il compito di una più attendibile interpretazione dei molteplici periodi figuranti f r a le registrazioni mareografiche di cagliari. per ora, pertanto, la nostra indagine può essere limitata soltanto a quelle oscillazioni per le quali l'interpretazione sembra accettabile anche perché, per il suo completamento, si renderà necessario il confronto con i contemporanei elementi meteorologici. 3. se si v a l u t a in 13 km la lunghezza medio-massima dello zoccolo sommerso, ne deriva, per la corrispondente oscillazione libera (« shelfseiche .») dello strato liquido sovrastante (il cui spessore medio può essere v a l u t a t o sui 15-20 metri), un periodo proprio di 65m ca. tale periodo quindi, viene praticamente a coincidere con il periodo fondamentale dell'intero golfo. la persistenza di oscillazioni superanti di poco l'ora può essere spiegata da questa concomitanza di periodi e da eccitazioni prossime alla risonanza. p e r quanto si riferisce all'ampiezza, tenuto conto del poco materiale a disposizione, possiamo aggiungere che l'uninodale viene registrata a cagliari con un'ampiezza medio-massima di 10 cm. i n alcuni dei casi osservati, le binodali, meno frequenti come registrazione, possono superare n e t t a m e n t e questo valore: il giorno 1 giugno 1957, p. e. la semiampiezza della binodale ha superato i 30 cm., e si è presentata sovrapposta a chiari esempi di uninodale (fig. 5). riassunto si espongono i risultati di una indagine sulle oscillazioni libere del golfo di cagliari e del suo porto. l'uninodale dell'intero golfo lia un periodo di l'l, a conferma del valore calcolato per via teorica. sia l'uninodale che la binodale (t = 20m ca.) presentano sovente sensibili ampiezze; per quanto concerne l'uninodale, la sua persistenza può essere agevolata dal fatto che l'oscillazione libera relativa allo zoccolo sommerso presenta un analogo periodo. un'elaborazione più dettagliata sarà possibile soltanto dopo più estese osservazioni. 9 8 p . c a l o i m . c . s p a d e a su m mary the results of a first research on the free oscillations in the gulf of cagliari and its port are exposed. the uninodal of the entire gulf has a lh period, and so confirms the theoretically calculated value. the uninodal and the binodal (t = 20m ca.) often show considerale amplitudes: for the uninodal, its persistence could be facilitated by the fact that the shelf-seich shows a similar period. a more detailed elaboration will only be possible after more extensive observations. b i b l i o g r a f i a i1) c a l d i p . , le sesse del lago di garda, p a r t e i i , « a n n a l i di geoli.sica », 1, 2, (1948). ( 2 ) c a l o i p . , s p a d e a m. c . , le sesse del lago di garda. p a r t e i v , « a n n a l i di geofisica », v i , 4, (1953). ( 3 ) c a l o i p . , s p a d e a m . c . , le oscillazioni ubere del golfo di civitavecchia e l'azione della viscosità cinematica, « a n n a l i di geofisica », x i i , 2, (1959). ag_56.03.13_choudhury_correctedok:layout 6 annals of geophysics, 56, 3, 2013, r0331; doi:10.4401/ag-6235 r0331 a statistical study on precursory effects of earthquakes observed through the atmospheric vertical electric field in northeast india abhijit choudhury, anirban guha*, barin kumar de, rakesh roy tripura university, department of physics, suryamaninagar, agartala, india abstract the study of anomalous variations in the near-surface atmospheric vertical electric field (vef) that have the form of bay-like depressions in strength have been used as precursors of earthquakes in various studies. we present here the first statistical report from an earthquake-prone zone in northeast india from july 2009 to july 2012. the 10 days that were meteorologically fair and with earthquake occurrences were selected for the present analysis. the average vef bay durations and depths were ca. 50 min to 70 min, with the corresponding magnitudes of 500 vm-1 to 800 vm-1. anomalous variation in vef before 7 to 12 hour of the impending earthquake has been observed. there was a 31% probability that a vef bay would show as an earthquake precursor. the positive correlation coefficient was 0.72 between the vef bay depth and the ratio of earthquake magnitude to depth, while the negative correlation coefficient of 0.82 was calculated between vef bay duration and the ratio of earthquake magnitude to depth. there was moderate correlation for distance of the earthquake epicenter to the observation point with both vef bay depth and vef bay duration. the correlation of the time difference of vef variations and earthquakes with vef bay depth was good, whereas the correlation of the time difference of vef variations and earthquakes with vef bay duration was too low to be considered. 1. introduction the atmospheric vertical electric field (vef) is generated by thunderstorm discharges that can create a potential difference between the ground and the ionosphere in regions with fair weather [price 2002, guha et al. 2010]. the vef is normally directed downwards and its intensity close to the ground surface is of the order of 100 vm-1 to 200 vm-1 [bennett and harrison 2007, mikhailov et al. 2002]. electric field measurements at the ground surface have been performed for a long time, to gather cloud charge distributions, and to study the number, intensity and polarity of thunderstorm discharges [pawar and kamra 2002]. it is relatively difficult to forecast the severity of thunderstorms due to nonlinearity in the cloud dynamics and the small temporal and spatial extensions [litta and mohanty 2008]. however, as the atmospheric vef is a contributor to the global electric circuit (gec), it is one of the important parameters for further improvement of the gec model [fullekrug 2004]. moreover, the study of the gec and atmospheric electricity provide information on the solar–terrestrial weather relationships, as well as on the global temperature and climate change, which link studies of the vef in fair-weather regions [bering 1995, adlerman and williams 1996, rycroft et al. 2000]. the fair-weather electricity is defined as the electric field induced by the space charge from an excess of positive ions in the atmosphere, and excluding other phenomena such as local cloud electrification and dust storms [stow 1969]. during these fair-weather days, there have been several scientific reports of uncharacteristic variations in the atmospheric vef with respect to fair weather vef variation prior to earthquakes in various seismo-active regions of the world, as has been shown in reports from countries like russia, japan and china [kondo 1968, pierce 1976, rulenko 2000, hao et al. 2000, smirnov 2001, mikhailov et al. 2006, zhang-hui et al. 2011]. interestingly, all of these reports have indicated negative variations in the vef. these vef variations usually occur at intervals of a few hours to a few days before an earthquake main shock, and they are seen as either bay-like field intensity decreases or oscillatory trains that last a few hours [mikhailov et al. 2004]. these anomalous variations are usually classified as short-term earthquake precursors [pulinets and ouzounov 2011]. the mechanism of this anomalous electric field generation was described by pulinets and boyarchuk [2004]. the nature of these effects is possibly related to variations in the stress–strain phenomena in the upper lithosphere that are a result of the preparation of the earthquake source [mikhailov et al. 2002]. several possible mechanisms of lithosphere–ionosphere interactions have been suggested in the literature. article history received october 16, 2012; accepted may 20, 2013. subject classification: vertical electric field, earthquake, negative bay anomaly, lithosphere–atmosphere–ionosphere coupling. surface air ionization caused by radon emanation into the atmosphere before earthquakes has also been investigated as one of the major sources of electric field variation [pulinets et al. 1997, liperovsky et al. 2008, harrison et al. 2010]. moreover, it is evident from the study of pulinets et al. [1998] that there is a dependence of ionospheric electron density due to seismic variations on local time as well. studies of low-latitude seismic regions have shown this to be explainable through zonal electric field generation due to equatorial anomalies (pulinets 2012), hence making the study of vefs more important from our location, which is at a latitude of 23 °n. thus, even simply from the enumeration of the sources of the atmospheric electric field variations, it is clear that the problem of separating out earthquake effects is still very complex. in this regard, the present study was designed to provide further insight into forecasting earthquakes with the help of these vef anomalies. at the low latitude station of the department of physics, tripura university, india (latitude, 23.75 °n; longitude, 91.25 °e; 42 m a.s.l.), we have been recording the data from atmospheric vefs around the clock, from july 2009 to date. this report discusses the preliminary results of the morphological structure of the variations of the vef with the magnitude of the negative anomaly and with the anomaly duration, and the dependence of these parameters on earthquake magnitude and on the distance between the observation point and the earthquake epicenter. only negative anomalies were considered; i.e., negative deviations of the vef from the positive diurnal behavior, in the period from 24 h before an earthquake occurrence. 2. experimental set-up to measure the atmospheric vef, a calibrated boltek efm–100 atmospheric electric field monitor (efm) was installed on the roof of the department of physics, tripura university (india), at 15 m above ground level. the efm has six sensor plates, each with dimensions of 17 cm, which are arranged symmetrically around the rotating axis. six electronically controlled, mechanically grounded, conductive ‘choppers’ are used to alternately shield and expose the six sensor plates to the atmospheric direct current electric field. alternate exposure and shielding of the sensor electrodes generates a to-and-fro motion of charge between the ground and the sensor plates through a high resistance resistor. this moving charge promotes an alternating current, which appears as an alternating current voltage across the sensor resistor. the magnitude of this alternating current and the corresponding alternating current voltage is proportional to the strength of the direct current atmospheric electric field. this voltage is amplified and fed into an analog-to-digital converter. the data is stored in a computer with a sampling rate of 1 datum/s. to maintain time synchronization, the internal clock of the computer is synchronized with a gps receiver. the maximum electric field that can be measured by such an efm is ±20 kvm-1. however, after the correct calibration and scaling under fair weather conditions, a scaling factor of 0.3 was determined, which restricted the maximum field variation to ca. ±6 kvm-1 at a height of 15 m from the ground, where the efm was installed. the efm detected electric field variations due to cloud activity within a radius of 30 km. 3. analysis of data and observational results the diurnal variation of the vef is continuously affected by the presence of clouds, thunderstorms (both local and global), precipitation, geomagnetic storms, and aerosol concentrations, among other factors [rycroft et al. 2000, harrison and aplin 2002, bennett and harrison 2007]. thus, it is necessary to distinguish between any seismic effects on the vef measurements and various local or global perturbations. our geographical location is at the juncture point of three tectonic plates: the eurasian, indian and australian plates (figure 1). therefore, it is a strategically significant place to perform a statistical study on precursory variations in vef measurements before major local earthquakes. three years of vef data from july 2009 to july 2012 were selected for the present analysis. first of all, the fair-weather days were selected from among the whole range of data. we defined fair-weather conditions as a maximum wind choudhury et al. 2 figure 1. map of the geographical positions of all of the 10 earthquakes for which the precursor variations were observed (triangles) along with location of the vef instrument (open circle). the remaining 22 of the 32 earthquakes, where precursors were not observed, are also shown (rectangles). dashed lines demonstrate the limit of eurasian, indian and australian plates. 3 speed of 4 m/s, less than 3 octa cloud cover, and no cumulonimbus clouds visible in the sky from the observation site [latha 2003], and also with no precipitation. the confirmation of fair-weather conditions was also verified according to the data from an automatic weather station (set-up at our receiver position), and from lightning imaging satellites and meteorological and oceanographic satellites for this region. however, due to the adverse geographical location of our receiver, which is located at the foot hills of the himalayas and also in one of the regions with the highest rainfall in the world (cherrapunji, with an average yearly rainfall of ca. 12,000 mm, atmospheric pg as earthquake precursor eq dates magnitude (m) depth (d) m/d dt * (hr:min:sec) vef depth (kv/m) vef duration (hr:min:sec) dist. from epicenter (km) preparation zone (km) 26.07.2009 5.2 10 0.52 08:20:59 0.714 01:01:34 1460 172 12.12.2009 5.0 10 0.50 05:15:26 0.548 00:53:32 1924 141 13.12.2009 5.1 10 0.51 11:15:47 0.633 00:51:36 140 156 01.05.2010 4.6 12 0.38 10:06:40 0.834 00:53:33 1110 95 12.06.2010 7.8 10 0.78 14:05:08 1.385 00:40:42 1763 2260 10.09.2010 4.8 15 0.32 11:22:56 0.804 01:10:22 133 116 12.12.2010 4.8 15 0.32 06:05:27 0.457 01:04:50 269 116 12.02.2011 4.0 10 0.40 07:42:30 0.515 00:54:40 70 53 24.04.2012 5.5 10 0.55 12:16:08 0.684 00:59:36 1580 232 11.05.2012 5.4 20 0.27 12:39:22 0.562 01:17:35 370 210 date eq time (ist) lat long depth (d) magnitude (m) m/d dist. from epicenter (km) preparation zone (km) 2012-03-27 03:24:54.38 33.284 95.605 11.4 4.1 0.36 1167 58 2012-03-04 22:41:45.44 30.167 101.767 10.0 4.3 0.43 1217 71 2012-02-20 19:48:03.82 35.840 79.835 10.0 5.0 0.50 1757 141 2012-01-01 10:49:34.93 10.632 91.737 10.0 4.6 0.46 1431 95 2011-12-14 11:09:25.15 39.463 94.384 10.0 4.7 0.47 1798 105 2011-11-30 01:12:34.43 7.802 93.858 17.0 5.6 0.33 1766 256 2011-11-13 11:59:11.66 36.045 81.066 10.0 4.1 0.41 1703 58 2011-09-24 09:38:58.84 36.377 82.526 10.0 4.3 0.43 1650 71 2011-08-01 01:10:53.48 33.739 87.574 13.6 5.1 0.37 1192 156 2011-01-23 21:16:19.83 23.034 101.689 10.0 4.2 0.42 1066 64 2010-11-28 10:03:47.10 7.588 91.875 15.8 4.2 0.26 1770 64 2010-11-06 07:42:43.64 36.636 87.506 10.0 5.2 0.52 1503 172 2010-09-08 07:28:10.63 33.315 96.365 10.0 4.2 0.42 1199 64 2010-07-25 14:27:03.95 16.345 94.611 10.0 4.6 0.46 869 95 2010-06-22 04:44:10.84 29.872 80.428 16.3 5.2 0.32 1285 172 2010-04-27 01:52:27.72 30.432 101.424 10.0 5.1 0.51 1267 156 2010-03-10 20:02:21.77 36.426 86.966 10.0 5.1 0.51 1493 156 2010-03-03 20:05: 3.33 32.821 105.211 13.1 4.3 0.33 1713 71 2009-12-18 10:49:30.08 32.641 92.841 10.0 4.6 0.46 1027 95 2009-12-01 16:53:28.43 27.272 91.455 10.0 4.2 0.42 419 64 2009-11-20 04:24:19.36 37.157 101.978 10.0 4.2 0.42 1829 64 2009-08-09 09:32:07.30 35.578 81.647 10.0 5.1 0.51 1630 156 table 1. details of the 10 earthquake days where the precursors were observed. dt *, time difference between occurrence of bay-like variation in the vef and the earthquake. table 2. details of the 22 earthquake days where no precursors were observed despite the fair-weather conditions. is within 350 km of the observational site), ideally clear meteorological days were rare. next, we looked for local earthquakes that occurred in the vicinity of about 2000 km from the receiver position, but only during these meteorologically fair days. a total of 32 such earthquakes were reported over the 3 years of the observational period. among these, there were only 10 cases of vef variations in the form of bay-like depressions in the vef magnitudes that were observed as precursors to these earthquakes (as listed in table 1). figure 1 shows the geographical positions of these 10 earthquakes for which the precursor variations were observed (figure 1, triangles), along with the location of the vef receiver (figure 1, open circle). the remaining 22 of the 32 earthquakes, where precursors were not observed, are indicated with gray rectangles in figure 1, and the full details are provided in table 2. in the present analysis, the earthquakes with earthquake magnitude to depth ratios >0.25 were only considered. the reason behind the selecting of this ratio was to introduce both the earthquake magnitude as well as depth into the analysis. also, if two successive earthquakes occurred within an interval of 24 h, they were considered as a single event. only the negative anomalies in vef that preceded an earthquake by no more than 24 h were considered in the statistical analysis. figure 2 shows the typical diurnal variation in the vef for a meteorologically fair weather day (figure 2a), a day with local lightning and precipitation (figure 2b), and a day with fair weather and an anomalous vef variation that preceded an earthquake (figure 2c). in figure 2c, the bay-like variation is highlighted by the circle, and the time of the earthquake is indicated by an upward arrow. we also checked the selected 10 days for any solar and geomagnetic storm events. nine of choudhury et al. 4 figure 2. typical diurnal variations in the vef for a meteorologically fair-weather day (a), a day with local lightning and precipitation (b), and a day with fair weather and an anomalous vef variation that preceded an earthquake (c). arrow, time of the main strike of the earthquake. figure 3. diurnal variations in the vef along with the earthquake for the 10 cases where a precursor was observed. arrow to right/left, anomalous vef variations; arrow up, time of strike of the earthquake plus ratio of earthquake magnitude to depth. 5 these 10 days were very quiet days, while one, namely april 24, 2012, was associated with a minor geomagnetic storm of kp = 5. figure 3 shows all of the 10 days of the bay-like vef variations, along with the earthquake strike times. the anomalous bay-like variations in the vef are shown by sidewise arrows, while the times of the associated earthquakes and the corresponding ratios of the magnitude to the depth are shown by upward arrows (figure 3). to determine if there was any dependency of earthquake class on the vef variations, we selected the ratio of the magnitude to the depth of the epicenter of each earthquake as a parameter to define all 10 of the earthquakes. there was a positive correlation coefficient of 0.72, with the associated 95% confidence interval from 0.16 to 0.93 (www.psyctc.org/stats/r/ci_ correln1.html), for the plot between the ratio of the magnitude to the depth and the vef bay depth, as shown in figure 4. figure 5 shows a further plot between the ratio of the magnitude to the depth and the vef bay duration, which shows a negative correlation coefficient of -0.82, with a 95% confidence interval from -0.96 to -0.39. we then determined if there was any dependence between the distance of the earthquake epicenter to the observation point and the vef bay depth, and also the vef bay duration (figure 6). the computed correlation coefficients were 0.45 and 0.47, with 95% confidence intervals from -0.25 to 0.84 and from -0.85 to 0.23, respectively. this indicates that there might be moderate dependence of the bay depth and the bay duration on the distance of the epicenters. figure 7 shows the time difference (dt) between the ocatmospheric pg as earthquake precursor figure 7. correlation plot between the time duration of the observed precursor of the earthquake and the vef bay depth (top) and vef bay duration (bottom). figure 6. correlation plot between the distance of the earthquake epicenter to the receiver and the vef bay depth (top) and vef bay duration (bottom). figure 4. correlation plot between the ratio of magnitude to depth and the vef bay depth. figure 5. correlation plot between the ratio of magnitude by depth and the vef bay duration. currence of the bay-like variations in the vef and the earthquakes, according to the bay depth and bay duration. for the vef bay depth, there was a positive correlation of 0.65, with a 95% confidence interval from 0.03 to 0.91, although the vef bay duration plot was a scattered graph which showed very low correlation (figure 7). a box plot (figure 8) was plotted between the 10 cases where the precursors were observed (group a) and the 22 cases (i.e., of the total of 32 fairweather days with earthquakes) without precursors (group b), to compare these two groups. here, the distribution was well scattered for group a, with a mean ratio of 0.5 and a maximum ratio of 0.78 for the earthquake magnitude to depth ratio. for group b the distribution was relatively suppressed for the earthquake magnitude to depth ratio, with a mean of 0.43 and a maximum of 0.5, from which it is hard to quantify a specific range for earthquake precursors. from the investigations of the ten earthquakes, we found the mean vef bay-depth variation to be ca. 500 vm-1 to 800 vm-1, with a maximum variation of around 1385 vm-1. here, the mean vef bay duration was within the range of 50 min to 70 min, with the maximum variation as 77 minutes. the most probable vef precursors were seen 7 h to 12 h before an impending earthquake, whereas the maximum variation was ca. 14 h and 5 min (figure 9). one of the most interesting things about all 10 of these earthquakes was that all of them were shallow; i.e., within 10 km to 20 km in depth only. from a total of 32 earthquakes reported during the time period of our data analysis, only 10 showed vef variations, indicating a 31% probability of vef earthquake precursors from our data, considering within 24 h before an impending earthquake, and only during the meteorologically clear days. 4. discussion the atmospheric vef near the earth surface exists under all weather conditions. under fair-weather conditions, these vefs arise through the gec, due to thunderstorms occurring in meteorologically disturbed regions around the globe. in the fair weather regions, the vef is around 100 vm-1, and it shows an identifiable global diurnal variation, known as the carnegie variation. this is named after the sailing vessel on which the original marine measurements were performed and named [parkinson and torreson 1931, guha et al. 2010]. generally for atmospheric electricity, fair-weather electric fields are considered as negative [pawar and kamra 2002] in a spherical polar coordinate system, although in our analysis here we took the fairweather electric fields to be positive, as per convention of fair-weather atmospheric electricity. among all of the scientific reports on variations in atmospheric electric field before earthquakes, pierce [1976] made the first observations of a 30% decrease in the electric field from its ambient value. thereafter, several reports across the scientific literature were presented that showed quasistatic anomalous electric field variations before earthquakes [kondo 1968, rulenko et al. 1992, hao et al. 2000, smirnov 2005]. the exact variation is something like a negative bay-like anomaly in the vef, with a sharp leading front and subsequent smooth recovery to the background level over a period of time, which has been observed as a few hours before several earthquakes reported from kamchatka (russia), japan, taiwan and china [mikhailov et al. 2003, mikhailov et al. 2006, smirnov 2001, kamogawa et al. 2004, zhang-hui et al. 2011]. smirnov [2008] carried out a statistical study at kamchatka from 1997 to 2002 over 103 cases of bay-like choudhury et al. 6 figure 8. box plot comparing the 10 days with precursors observed (group a) and the 22 days without precursors observed (group b) with the ratio of earthquake magnitude to depth. figure 9. histograms showing time difference of vef variation (top) and earthquake and vef bay duration (bottom). 7 depressions in the strength of the near earth atmospheric electric field (ez) and found the most probable bay duration to be 40 min to 60 min. the most probable negative variations in ez are around 106 vm -1 to 300 vm-1. no dependence of these values on the earthquake class or the distance to the epicenter was found. the probability of earthquake forecasting over a 24 h period before an earthquake based on the ez anomaly has been shown to be around 36%. another statistical study was reported from lisbon (portugal) for the period from 1955 to 1991, which examined the influence of seismic activity on the vef, to define an exploratory method that involved the selection of events for which the distance from the atmospheric electrical field sensor to the earthquake epicenter was smaller than the preparation radius of the event [silva et al. 2012]. in our statistical investigation, for northeast india, we found that the mean vef bay-depth variations for earthquake precursors to be ca. 500 vm-1 to 800 vm-1, whereas the average vef bay duration was about 50 min to 70 minutes. the most probable vef precursors were observed 7 h to 12 h prior to earthquakes. processes like cosmic rays, solar radiation, lightning discharge, natural ground radioactivity and aerosol concentration all contribute to the air ionization and consequent latent heat release due to water vapor condensation on the newly formed ions, and hence the resulting changes in the vef [pudovkin and raspopov 1992, svensmark and friis-christensen 1997, rulenko 2000, freund et al. 2009]. thus it is necessary to be sure enough to identify the precursor effects on the vef from among all of these effects. accordingly, at our location, the fair-weather vef graph looks something like that shown in the top panel of figure 2, which shows a quiet background level of about 120 vm-1 to 170 vm-1 at local morning and noon hours, with a further increase at the evening up to 250 vm-1 to 300 vm1, the dynamic range of which is consistent with other land observations [mikhailov et al. 2002, 2003, smirnov 2005]. when a cloud passes over our receiver and with no precipitation, the vef reading is about 4000 vm-1 to 4500 vm-1, while with lightning and precipitation the vef shows relatively chaotic positive and negative deviations from the mean diurnal behavior of about 6000 vm-1. in contrast, the anomalous variations of vef for these earthquake precursors during fair-weather days is about 500 vm-1 to 800 vm-1. thus the observed vef variation is well distinguishable from local cloud passing over the sensor, and from the local lightning and thunderstorm activity. as a result, in the total duration of our analysis, we found a total of 32 cases of earthquakes during fair-weather days and among these only 10 showed vef variations in the form of bay-like depressions in the vef magnitude, which were seen as a precursory for an earthquake. a positive dependency was found between earthquake class and vef bay depth, which is shown in figure 4. this signifies that with larger earthquake magnitudes (since the depths were almost equal in our observations; see table 1) the vef bay depth will be more. whereas a negative correlation was observed between the earthquake class and the vef bay duration, which highlights that as the earthquake magnitude decreases, the duration of the vef bay variation increases, and vice versa. our observations here contrast with those of smirnov [2008], who monitored the kamchatka seismicity, where there was no dependence of bay duration and bay strength on earthquake class or distance from the epicenter. however, we also observed a low dependency between vef bay duration and depth with the distance of the earthquake epicenter from the observation point. the probable cause might be due to some difference between the seismicity of kamchatka and northeast india, which is located at the juncture of three tectonic plates: namely, the indian, eurasian and australian (figure 1). among the various models proposed in the literature, radon emanation is viewed as a significant cause of such vef variations [smirnov 2008]. radon is a product of uranium decay series that has a short half-life of 3.8 days, and due to this, radon shows poor intrinsic mobility [singh et al. 2010]. as a consequence of radon emanation, long-living ion complexes of opposite signs are formed in the near ground layer of the atmosphere. ions of different signs have different mobilities; generally the mobility of the negative ions is 1.3-1.4-fold more than for positive ones. under the actions of the natural atmospheric electric field, the positive ions would tend to move to the surface of the earth, where they would recombine, but because of their low mobility, after some time, a spatial layer of positive ions will be formed at the surface, whereas the negative ions will move vertically upwards. in such a way, at the near ground, an ‘electrode layer’ can be formed, along with the local electric field, which diminishes the natural atmospheric electric field, which is known as the ‘electrode effect’ [hoppel 1967, pulinets and boyarchuk 2004]. it is thus expected that the changes in surface radon emission would modify the surface atmospheric electricity conditions and potentially lower the ionospheric properties, via the weak fairweather conduction current that carries the negative charge upwards throughout the troposphere and stratosphere [harrison et al. 2010]. moreover, more recently, freund et al. [2009] showed that the charge generated in stressed rock can travel over distances of several kilometers, to reach the surface and to enhance the air ionization, which consequently induces atmospheric electrical atmospheric pg as earthquake precursor perturbations. however, air ionization by increased radon release before earthquakes might be only a small part of the total ionization balance, and the identification of the atmospheric processes that are initiated by earthquake preparation processes is part of short-term earthquake prediction [pulinets and ouzounov 2011]. the release of radon into the atmosphere is also thought to be closely related to the state of deformation processes in the surface layers of the earth during the earthquake preparation phase [rulenko et al. 1992, rulenko 2000]. among the few such models proposed, morgounov and maltsev [2003] suggested a model that generates a quasistatic electric field in the atmosphere by considering the formation of polarization charges on the walls of rock fractures. whereas alekseev and aksenov [2003] pointed out that electrical conductivity between the air and the earth depends on the conductivity of the fluids that fill the rock pores and fractures, which ultimately have effects on the atmospheric electric field. more importantly, at the lower latitude regions, under relatively geophysical conditions, the equatorial anomaly or appleton anomaly formation occurs as a regular daytime phenomena, and might lead to generation of an anomalous zonal electric field [pulinets 2012], thus separating our location from the higher latitude regions. this might be a reason as to why in spite having some fair-weather days, we did not observed any precursors from the china region, which is above 30° north; i.e. outside the equatorial anomaly region. sobolev [1993] stated that these vef variations are short-term predictors and cannot be used over the long term. so generally in this type of vef, the variation does not follow the empirical formulae of 100.43m for earthquake preparation zones. this has also been observed in our records as well. from table 1, it can be seen that 8 of the 10 earthquakes showed precursor effects even outside the earthquake preparation zone. morgounov [2004] provided an explanation of this by means of nonuniformity of the stress and strain processes in the earth crust before impending earthquakes. another point to be noted from figure 1 is that the positions of almost all of the 31 of the 32 earthquakes under study, both with and without precursors, occurred more or less near the tectonic plate boundaries, although there was an exception in one case where the precursor was being observed, and its position was situated far to the west, near mumbai, which is good distant from any tectonic plates (figure 1). however the probable cause of this is still a matter of further investigation. these pre-seismic variations on the atmospheric electric field could be due to a surface ionospheric coupling mechanism [kamogawa et al. 2004]. kamogawa [2006] and hayakawa [2006] summarized three possible coupling mechanisms. they highlighted that the gases released from the ground during plate motion before major earthquake shocks modulate the properties of the entire atmosphere. secondly, ground motions might also excite atmospheric gravity waves that propagate upwards. lastly, electromagnetic radiation produced by processes acting in the ground before earthquakes could also initiate ionospheric effects. liperovsky et al. [2008] highlighted that different physical mechanisms acting together are necessary to explain the same precursor event as well. but pulinets and ouzounov [2011] raised questions against this, and they introduced a model known as the lithosphere–atmosphere–ionosphere coupling (laic) model. this model discusses a mechanism that exists between different layers of the atmosphere, which might explain the linkage of events occurring simultaneously before an earthquake, between the ground surface, the atmosphere and the ionosphere. so, from the above discussions, it is perhaps reasonable to suggest that radon emanation might not be the sole source of vef variations before earthquakes. although from figure 8, for the ratio of earthquake magnitude to depth above 0.5, there is a possibility of getting a precursor; however, this is still hard to confirm, and it might not always be a governing factor. a more complex mechanism deep inside the earth crust could be responsible for all of these variations in the vef, and this link might map its effects on the entire atmosphere before an impending earthquake. nevertheless, the main objective of this study was only to demonstrate the vefs from the earthquake prone zone of northeast india, and discuss the probable causes of these perturbations. as we do not have any radon-monitoring devices in the vicinity of the receiver to potentially correlate this with our observed results, we cannot at present verify this hypothesis. furthermore, from figure 3, it appears that 90% (i.e., 9 out of these 10 earthquakes) of the precursors appear before 14:00 ist and 70% (7 out of 10) earthquakes strike in between 16:00 and 24:00 ist. this might not be mere coincidence, as pulinets et al. [1998] showed dependence of the local time on the seismic ionospheric variations. however, at this stage our main objective was the ratio of earthquake magnitude to the depth, so these further aspects are beyond the scope of the present study. 5. concluding remarks the present study is the first preliminary statistical observation made from a region of the globe where no similar previous studies have been made. from the present analysis, good correlation was found between the vef bay depth and bay duration with the ratio of the choudhury et al. 8 9 earthquake magnitude to the depth, while low dependence was found for both the distance of the earthquake epicenter to the observation point, and the time of the observed precursor of the earthquake, for both the vef bay depth and vef bay duration. various sources have indicated that these changes in ionization are due to radon emanation or might be due to seismo-ionosphere coupling; we might have observed the decrease of vef before earthquakes but no consensus has been reached. this makes the subject very interesting, but nevertheless complex, which the need for extensive studies here. we believe that longer duration vef data along with continuous radon monitoring have good spatial distributions in strategic locations, and further investigations carried out in a coherent manner are required to provide more convincing data with regard to this potential precursory phenomenon of earthquakes. acknowledgements. the authors are thankful to department of science and technology, government of india, for providing financial support to conduct the research through department of physics, tripura university, suryamaninagar, india, under the inspire fellowship programme. references adlerman, e.j., and e.r. williams (1996). seasonal variation of the global electrical circuit, j. geophys. res., 101, 679-688. alekseev, a.s., and v.v. aksenov (2003). on an electric field in an earthquake epicenter, dokl. akad. nauk., 392, 106-110. bennett, a.j., and r.g. harrison (2007). atmospheric electricity in different weather conditions, weather, 62, 277-283. bering, e.a. 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(2001). anomalous behavior of the electric field strength before earthquakes observed at paratunka magnetic observatory in 1997-2000, solarterrestrial coupling and electromagnetic precursors of earthquakes, petropavlovsk-kamchatskii, russia, 48-49. smirnov, c.e. (2005). characteristics of negative anomalies in the quasistatic electric field in the near-earth atmosphere on kamchatka, geomagn. aeron., 45, 265-269. smirnov, s. (2008). association of the negative anomalies of the quasistatic electric field in atmosphere with kamchatka seismicity, nat. hazards earth syst. sci., 8, 745-749. sobolev, g.a. (1993). fundamentals of earthquake prediction (in russian), nauka, moscow, 313. stow, c.d. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 attenuation of electromagnetic waves at the frequency ~1.7 khz in the upper ionosphere observed by the demeter satellite in the vicinity of earthquakes david píša1,2,3, františek němec2, michel parrot1, ondřej santolík3,2 1 laboratoire de physique et chimie de l’environnement et de l’espace, université d’orléans, cnrs, orléans, france 2 charles university in prague, faculty of mathematics and physics, prague, czech republic 3 institute of atmospheric physics, academy of science of the czech republic, prague, czech republic annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5276 abstract the demeter satellite was the first satellite specifically dedicated to the recording of electromagnetic phenomena connected with seismic activity. almost 6.5 years of measurements provide good opportunities to analyze a unique dataset with global earth coverage. we present the results of a statistical study of the intensity of very low frequency electromagnetic waves recorded in the upper ionosphere. robust two-step data processing has been used. the expected unperturbed distribution of the power spectral densities of electromagnetic emissions was calculated first. then, the power spectral densities measured in the vicinities of earthquakes are compared with the unperturbed distribution and are examined for the presence of uncommon effects related to seismic activity. the statistical significance of the observed effects is evaluated. we confirm the previously reported results of a very small, but statistically significant, decrease in wave intensities a few hours before times of main shocks using this much larger dataset. the wave intensity decrease at a frequency of about 1.7 khz is observed only during the night and only for shallow earthquakes. this can potentially be explained by increases in the cut-off frequency of the earth ionosphere waveguide caused by imminent earthquakes. 1. introduction over the past two decades, considerable progress has been made in the field of seismo-electromagnetic effects. the main aim of this study is to find possible short-term precursors of earthquakes that would allow us to predict these devastating events. although various types of possible precursors have been reported (e.g. changes in temperature or conductivity), electromagnetic perturbations are probably one of the most promising candidates. these have been observed both using ground-based measurements and satellites, and they are believed to occur across a wide range of frequencies, from direct current to very high frequency [parrot and mogilevsky 1989, larkina et al. 1989, tate and daily 1989, serebryakova et al. 1992, molchanov et al. 1993, parrot 1994, asada et al. 2001, bortnik et al. 2008, hobara et al. 2005, němec et al. 2008, 2009]. however, it remains very difficult to identify these effects in the ionosphere, because they are most certainly particularly weak when compared to other electromagnetic effects due to other sources (e.g. lightning, geomagnetic storms, man-made noise). consequently, there is still controversy concerning the actual existence of these seismo-electromagnetic effects, and there are quite a few studies that dispute their existence [henderson et al. 1993, rodger et al. 1996, clilverd et al. 1999]. there are many theories that try to describe the processes before upcoming earthquakes and to explain why precursors should be observed; e.g. motion of the positive holes [freund 2007, 2011], microfracturing electrification [gershenzon et al. 1989, molchanov and hayakawa 1998, molchanov et al. 2006], or a release of radon gas [sorokin et al. 2001]. nevertheless, widely accepted and demonstrated short-term precursors are still missing. the advantage of the robust statistical study that we present here is that we are able to exclude the electromagnetic variations due to other than seismic sources. a similar statistical study that used a dataset obtained by the demeter (detection of electro-magnetic emissions transmitted from earthquake regions) satellite was performed by němec et al. [2008]. they systematically investigated 2.5 years of the demeter data, and they showed that during the night there was a small, but statistically significant, decrease in wave intensities in the vicinity of earthquakes shortly before (0-4 h) the time of the main shock. in the present study, we applied the method of němec et al. [2008] to the complete dataset acquired by demeter special issue: earthquake precursors 157 article history received july 7, 2011; accepted september 19, 2011. subject classification: surveys, measurements, and monitoring, waves and wave analysis, wave propagation. (~6.5 years). the dataset that we used is described in section 2. section 3 presents the data processing method that we applied. the results that were obtained are presented in section 4, and discussed in section 5. finally, section 6 gives a brief summary. 2. the dataset demeter was a low-altitude satellite (altitude, 710 km) that was launched in june 2004, on a circular polar orbit. it measured electromagnetic waves and plasma parameters all around the globe, except in the auroral zones [parrot 2006]. the altitude of the satellite was decreased to 660 km in december 2005. the satellite science mission came to an end in december 2010, providing a total of about 6.5 years of data. the orbit of the demeter satellite was nearly sunsynchronous. the up-going half-orbits corresponded to night-time (22.30 lt), whereas the down-going half-orbits corresponded to day-time (10.30 lt). the electric field measurements were carried out by an electric field instrument known as an ice (instrument champ electrique), which operated from direct current up to about 3 mhz. there were two different modes of operation of the satellite, called ‘burst’ and ‘survey’. during the burst mode, more detailed data were collected. however, this was active only above limited areas, and therefore it is not very suitable for the intended statistical study. in the survey mode and the very low frequency (vlf) range (20 hz to 20 khz) that we have used, the frequency spectra of one electric field component calculated on board with a frequency resolution of about 19.5 hz and a time resolution of either 2.048 s or 0.512 s are available. a more detailed description of the ice instrument can be found in berthelier et al. [2006]. we limited our analysis to the frequency range below 10 khz, to avoid the frequencies of terrestrial vlf transmitters. following němec et al. [2008], we selected 16 frequency bands (117 hz each) in such a way that they omit spacecraft interferences and cover the entire studied frequency range as uniformly as possible. we also limited our analysis to the geomagnetic latitudes between –65 and +65 degrees, because there were no data acquired by demeter at larger geomagnetic latitudes. all of the appropriate data recorded during the lifetime of the demeter mission were included in the present study; i.e. from september 2004 to december 2010. we tried to identify and remove all of the time intervals when the data might be corrupted. we used a list of the single event upset events [ziegler and lanford 1979], which is available on the project website (http://demeter.cnrsorleans.fr). the single event upsets are the results of effects on the microelectronic device (e.g. memory) of electromagnetic radiation caused by, e.g., an energetic particle. this can cause a change in state that occurs in or close to an important node of a logic element, which can consequently destroy the data measured at that given time. moreover, we excluded the time intervals with possible nonnatural events; e.g. interference of the langmuir probe, monochromatic radiation, and diffusive noise, among others. a complete list of these events, along with their descriptions and examples, can be obtained from the website of the lpc2e laboratory in orléans, france (ftp://lpc2e.cnrsorleans.fr/projects/demeter/pub/seti/). for the earthquake data, we used the us geological survey earthquake catalog (http://earthquake.usgs.gov). altogether, about 9,000 earthquakes with magnitudes ≥5.0 and depths ≤40 km occurred all over the world during the period analyzed. all of these were included in the study. 3. processing methodology we used the data processing method developed and described by němec et al. [2008, 2009]. as the method is described in detail in these studies, we will only present a brief overview. the basic idea of the method is to collect all of the available data observed in the vicinity of earthquakes and to interpret these in the context of long-term distributions obtained for similar ionospheric conditions. in the first step of the data processing, it is therefore necessary to describe the distribution of the intensity of electromagnetic waves observed by demeter using all of the available data. this long-term distribution of intensities is calculated separately for each combination of the parameters used to characterize the state of the ionosphere. the parameters that we selected and the number of bins used for each of these are the same as those used by němec et al. [2008], namely: frequency (16), geomagnetic latitude (66), geomagnetic longitude (36), magnetic local time (2), geomagnetic activity expressed by kp index (3), and season of the year (2). we constructed a multidimensional array with the number of dimensions equal to the number of parameters. in each bin of this array, we stored a histogram of the wave intensities observed during the appropriate ionospheric conditions. the histograms were then used to construct the experimental cumulative distribution function (cdf) of the wave intensity for each of the bins. in the second step of the data processing, the data related to the earthquakes (i.e. the data acquired close to earthquake epicenters both in space and time) are considered. these data were evaluated using the cdfs obtained in the first step of the data processing, and it was determined whether they were different from the expected distributions that were or were not seismically perturbed. to do so, we organized the data related to earthquakes into a grid, as a function of the frequency, the time relative to the main shock, and the distance from the epicenter. moreover, the day-time and the night-time data were treated separately. values that would be used more than once (typically the main shock and aftershocks) were excluded from our 158 píša et al. 159 analysis, to avoid the mixing of pre-seismic and post-seismic activity. otherwise, a single measurement can be used more than once, and it would be impossible say to which earthquake the observed data can be attributed. this would result in greater uncertainty of the statistics. for each measurement, the values of the corresponding cdf were evaluated. afterwards, we calculate the mean values of the cdfs in each bin of the grid (the ‘probabilistic intensity’; 0.5 is subtracted to get the mean value equal to 0). the resulting mean values were then normalized to have the mean value equal to 0 and the standard deviation equal to 1 (the ‘normalized probabilistic intensity’). a lower estimate of the standard deviation used for normalization was obtained by taking into account the uniform distribution of the probabilistic intensity values, and by assuming that all of these values are independent. as this is not exactly the case, an additional factor is introduced, expressing the relative fraction of the data that was measured during the same halforbit that can be considered as independent [see němec et al. 2008, 2009]. 4. results the main purpose of the present study is to apply the method of němec et al. [2008] to the whole dataset acquired by the demeter satellite, and to determine whether the results obtained by němec et al. [2008] using the first few years of data hold. to enable a direct comparison with the previously obtained results, exactly the same data processing method was used, with the same selection of frequency intervals and binning of individual parameters. following their results, we have focused only on the night-time measurements in the vicinity of earthquakes with magnitudes ≥5.0 and with shallow depths ≤40 km. figure 1 shows the frequency-time dependence of the normalized probabilistic intensity obtained for distances lower than 330 km from the epicenter. it can be seen that the main observed feature is a decrease in the normalized probabilistic intensity at the frequency of about 1.7 khz shortly before (04 h) the time of the main shocks. the value of this decrease corresponds to about 2.74 standard deviations. this effect is based on 3,920 values of cdfs collected during 115 different orbits. the mean value of the cdfs in this bin is about 0.419, which corresponds to a decrease in the wave intensity equal to about 2 db. the night-time data in the frequency band centered at 1.7 khz measured all over the world during all of the period analyzed were used as the referent distribution of wave intensities for the calculation of decreases in db [see němec et al. 2009, figure a1]. from the same histogram, variations corresponding to one standard deviation from the mean are equal to about ± 7.5 db. figure 2 shows the same dependence obtained for distances up to 440 km from the epicenter of earthquakes. the statistical significance of the effect is stronger when this extended area is considered. the normalized probabilistic intensity decreases by about 3 standard deviations. this effect is based on 6,790 values of cdfs collected during 153 different orbits. the mean value of the cdfs in this bin is 0.429, corresponding to a decrease in the wave intensity of about 1.8 db. figure 3 shows the distance-time dependence of the normalized probabilistic intensity in the frequency interval 1,640 hz to 1,740 hz. this shows that the affected area is limited to distances ≤440 km from the epicenter. the effect wave attenuation due to seismic activity figure 1. frequency-time dependence of the normalized probabilistic intensity (see text) obtained from the night-time electric field data measured within 330 km from the epicenters of earthquakes with magnitudes ≥5.0 and depth ≤40 km. figure 2. frequency-time dependence of the normalized probabilistic intensity (see text) obtained from the night-time electric field data measured within 440 km from the epicenters of earthquakes with magnitudes ≥5.0 and depth ≤40 km. in this plot appears to be stronger at greater distances from the epicenter. however, as the amount of data included in the individual bins is not constant, but increases with the distance from the epicenter, a certain awareness is needed when interpreting the normalized results. the idea becomes clear when it is considered that even relatively small changes in the wave intensity can be statistically significant if they appear systematically in the large amount of data. on the other hand, even rather huge decreases in wave intensity would not be statistically significant if they were observed only in a couple of orbits. that this intuitive explanation is valid for our situation is demonstrated in figure 4. this represents the same dependence as in figure 3, but this time for the results before the normalization; i.e. the values of the probabilistic intensity. it can be seen that the largest decrease in the probabilistic intensity is observed at distances <110 km from the epicenter, and the magnitude of the effect gradually decreases with increasing distance. it can be seen that an increase is clearly seen right after the time of the shocks. this is not a surprise, as it is known that the shock itself induces acoustic gravity waves that propagate upward [see for example, blanc 1985]. to analyze the frequency-time interval that corresponds to the decrease in figure 2 in more detail (i.e. the frequency range 1640 hz to 1740 hz, and the time interval 0 h to 4 h before the time of the main shock), we analyzed the distribution of the appropriate values of cdfs. only the orbits with at least 12 values contributing to the effects were selected. a single value was obtained for each of these as a mean of the 12 cdf values closest to the epicenter. this was done as the data recorded within one orbit cannot be considered as independent. using this procedure, we obtained a number of values that are lower than the number of orbits that contributed to the decrease in figure 2. a histogram of these values is shown in figure 5. it can be seen that this histogram is shifted towards the lower values, which corresponds to a decrease in the normalized probabilistic intensity. the mean value of the distribution is marked by the vertical dashed red line in figure 5, which is about –0.069. we can evaluate the statistical significance of this shift; i.e. we can determine the probability that it might occur randomly. two different methods were used. first, we used the classical t-test [press et al. 1992, ch. 14.2]. this is a statistical hypothesis test designed to assess whether the means of two normally distributed populations are equal. we have tested the píša et al. 160 figure 3. distance-time dependence of the normalized probabilistic intensity (see text) obtained from the night-time electric field data measured at a frequency of about 1.7 khz close to earthquakes with magnitude ≥5.0 and depth ≤40 km. figure 4. distance-time dependence of the probabilistic intensity (without normalization; see text) obtained for the night-time electric field data measured at a frequency of about 1.7 khz close to earthquakes with magnitude ≥5.0 and depth ≤40 km. figure 5. distribution of the mean values of the cdfs that contribute to the observed wave intensity decrease. vertical dashed red line, mean cdf. 161 possibility that the observed deviation of the mean from 0 is random (0 is the expected mean if no seismic-related effects were present). the resulting probability is about 0.1%, which means that the observed deviation of the mean value from 0 is unlikely to be random. the second method that we used to estimate the statistical significance of the effects was to determine the number of values corresponding to the decrease (97) and the number of values corresponding to the increase (47). the probability that such a situation might occur randomly was then evaluated based on the binomial distribution with the anticipated probability of a decrease being equal to the anticipated probability of an increase. the resulting probability is lower than 0.1%, which means again that the observed shift toward the lower values is unlikely to be random. 5. discussion the existence of the previously reported decrease in the wave intensity shortly before the time of main earthquake shocks [němec et al. 2008, 2009] has been confirmed here using the complete demeter dataset. however, the amplitudes of the observed effect – with mean and median cdfs of 0.419 and 0.375, respectively – are approximately the same as those reported in these previous studies. they reported mean and median cdfs as 0.417 and 0.374, respectively [němec et al. 2009], and 0.354 and 0.324, respectively [němec et al. 2008]. the differences might be caused by using different emission maps describing a longer time period. these can hide possible small time-scale effects, which can have a role in the final results. it can be expected that any potential effect should be very close to the epicenter. however, němec et al. [2009] showed a shift of the effect by 2 degrees westwards. this is not shown by our study, as we used a different analysis method than němec et al. [2009]. we cannot therefore exclude that a small shift in the affected area from the vertical projection of the epicenter is hidden in statistical fluctuations of the results shown in figure 4. this is also suggested by the normalized values shown in figure 3. the distribution of the mean values of the cdfs obtained for the data forming the effect shown in figure 5 represents an independent test of the statistical significance of the effect. moreover, it can be used to exclude the possibility that the decrease is caused by a few orbits of exceptionally low wave intensity. we used two independent statistical tests. the one sample t-test is designed specifically to test if a mean of a distribution is equal to a given value or not. however, it assumes that the distribution is normal, which is not exactly the case for the distribution studied here. consequently, we used the independent and rougher test based on the binomial distribution. according to both of these tests applied, the decrease is statistically very significant. as noted by němec et al. [2008], the frequency of about 1.7 khz where the decrease is observed corresponds approximately to the cut-off frequency of the earthionosphere waveguide during the night-time [budden 1961]. the lower frequency cut-off of this waveguide is inversely proportional to the height of the ionosphere. taking into account that during the night-time the electromagnetic waves generated due to thunderstorm activity were a crucial source of vlf radiation observed by demeter [němec et al. 2010], it can be expected that the exact frequency of this cut-off can significantly affect the power spectrum of the electromagnetic waves observed by demeter at frequencies close to 1.7 khz. this is illustrated well in figure 6, which presents an example of the frequency-time spectrogram of the power spectral densities of electric field fluctuations observed during one half orbit. the frequency cut-off at a frequency of about 1.7 khz can be clearly identified as the frequency below which the power spectral density of the electric field fluctuations decreases drastically. wave attenuation due to seismic activity figure 6. frequency-time spectrogram of the power spectral density of the electric field fluctuations recorded by demeter along a complete half-orbit on december 30, 2009. note the clear cut-off frequency of the whistlers (vertical lines) at about 1,700 hz. the cut-off frequency remains relatively constant over all of the sampled latitudes. an increase in this cut-off frequency would therefore necessarily lead to a decrease in the power spectral density of electric field fluctuations observed by demeter in the appropriate frequency range. such an increase of the cut-off frequency would correspond to a decrease in the height of the ionosphere. our results could therefore indicate that the height of the ionosphere is statistically lower above epicenters of imminent earthquakes. we have visually checked the earthquake orbits for the presence of such changes in the cut-off frequency. however, no clear change in the cut-off frequency was identified. this might be explained by the fluctuations of the background intensity in the dynamic range of about ±7.5 db, as compared to the 2 db change that corresponds to the attenuation effect. a more detailed analysis of the changes in the cut-off frequencies is beyond the scope of the present study, and this will be investigated in the future. according to a recent study by harrison et al. [2010], this effect might result from an increase in the electric conductivity in the lower troposphere due to an increase in radon emanation prior to major earthquakes. the corresponding variations in the vertical atmospheric current that flows between the ionosphere and the surface of the earth would modify the lower d-region electron density profile, and then the cut-off frequency of the earth–ionosphere waveguide. 6. conclusions we have studied previously reported decreases in the intensities of electromagnetic waves at frequencies of about 1.7 khz observed in the vicinity of imminent earthquakes [němec et al. 2008, 2009]. using the very large demeter dataset of ~6.5 years of measurements, we have confirmed that during the night there was a very small, but statistically significant, decrease in the wave intensity observed by the satellite close to earthquakes with magnitudes >=5.0 and depths <= 40 km shortly before the time of the main shocks (0-4 h). however, it is important to note that this effect is very small (a decrease of ~2 db) as compared to the usual variations in the background activity (± 7.5 db), and therefore it is very difficult to observe it directly in separate cases. these results are in agreement with an explanation suggested by harrison et al. [2010] based on changing the properties of the earth-ionosphere waveguide. attenuation of a certain type of waves (e.g. 0+ whistlers) has not been visually detected. acknowledgements. this study was supported by the centre national d’etudes spatiales. it is based on observations with the electric field experiment embarked on demeter. the authors thank j.j. berthelier, the pi of this instrument, for the data supply. a part of the studies leading to these results was also funded from the european community seventh framework programme (fp7/2007-2013), under grant agreement n° 262005. we acknowledge additional support from grants me09107, gacr 205-09-1253, and gauk 403511. references asada, t., h. baba, m. kawazoe and m. sugiura (2001). an attempt to delineate very low frequency electromagnetic signals associated with earthquakes, earth planet. space, 53, 55-62. berthelier, j.j., m. godefroy, f. leblanc, m. malingre, m. menvielle, d. lagoutte, j.y. brochot, f. colin, f. elie, c. legendre, p. zamora, d. benoist, y. chapuis and j. artru (2006). ice, the electric field experiment on demeter, planet. space sci., 54, 456-471. blanc, e. 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(2006). first results of the demeter microsatellite, special issue of planet. space sci., 54, 411-558. press, w.h., s.a. teukolsky, w.t. vetterling and b.p. flannery (1992). numerical recipes in c. the art of scientific computing, 2nd edition, chapter 14.2. rodger, c.j., n.r. thomson and r.l. dowden (1996). a search for elf/vlf activity associated with earthquakes using isis satellite data, j. geophys. res., 101, 13369-13378. serebryakova, o.n., s.v. bilichenko, v.m. chmyrev, m. parrot, l. rauch, f. lefeuvre and o.a. pokhotelov (1992). electrodynamic elf radiation from earthquake regions as observed by low-altitude satellites, geophys. res. lett., 19, 91-94. sorokin, v.m., v.m. chmyrev and a.k. yaschenko (2001). electrodynamic model of the lower atmosphere and the ionosphere coupling, j. atmos. sol. terr. phys., 63, 1681-1691. tate, j. and w. daily (1989). evidence of electro-seismic phenomena, phys. earth planet. inter., 57, 1-10. ziegler, j.f. and w.a. lanford (1979). effect of cosmic rays on computer memories, science, 206, 776-788. *corresponding author: david píša institute of atmospheric physics, academy of science of the czech republic, prague, czech republic; e-mail: david.pisa@cnrs-orleans.fr © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. wave attenuation due to seismic activity annals 47, 1, 2004, 01/07def 11 annals of geophysics, vol. 47, n. 1, february 2004 key words self-potential signals – multifractal formalism 1. introduction the dynamics underlying tectonic processes could be directly revealed by the investigation of the temporal fluctuations of self-potential signals, which may be useful to monitor and understand many seemingly complex phenomena linked to seismic activity (johnston, 1997; park, 1997). self-potential field variability may be induced by stress and fluid flow field variability (scholz, 1990). therefore, the analysis of these induced fluctuations could yield information on the geophysical mechanisms governing normal as well as intense seismic activity. in this context, this work investigated the dynamic properties of geoelectrical signals, as they can be detected from observational time series. self-potential signals are the result of the interaction among very heterogeneous and not well known mechanisms, which can be influenced by the particular structure of the monitored zone (patella et al., 1997). this means that local features can be mixed with the general thereby increasing the difficulty of rightly characterizing and interpreting the signal time variations. in addition, as occurs for many environmental signals, observational data are made even more erratic by the presence of anthropic phenomena: electrical signals coming from anthropic sources may be added, e.g., to the natural signal, hindering its dynamical characterization (cuomo et al., 1997; pham et al., 1998). in a previous paper, cuomo et al. (1998) analyzed the geoelectrical daily means to give information on the statistical features of the geoelectrical background noise and the inner dynamics of geophysical processes producing the electrical phenomena observed on earth surface in seismic areas. they discussed the statistical analysis of dynamic systems based on the estimation of their a preliminary study of the site-dependence of the multifractal features of geoelectric measurements luciano telesca (1), gerardo colangelo (1), vincenzo lapenna (1) and maria macchiato (2) (1) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy (2) dipartimento di scienze fisiche, università degli studi di napoli «federico ii», italy abstract multifractal analysis was performed to characterize the fluctuations in dynamics of the hourly time variability of self-potential signals measured from january 2001 to september 2002 by three stations installed in the basilicata region (southern italy). two stations (giuliano and tito) are located in a seismic area, and one (laterza) in an aseismic area. multifractal formalism leads to the identification of a set of parameters derived from the shape of the multifractal spectrum (the maximum α0, the asymmetry b and the width w ) and measuring the «complexity» of the signals. furthermore, the multifractal parameters seem to discriminate self-potential signals measured in seismic areas from those recorded in aseismic areas. mailing address: dr. luciano telesca, istituto di metodologie per l’analisi ambientale (imaa), cnr area della ricerca di potenza, contrada s. loja, 85050 tito scalo (pz), italy; e-mail: ltelesca@imaa.cnr.it 12 luciano telesca, gerardo colangelo, vincenzo lapenna and maria macchiato degree of predictability, distinguishing randomness from chaos and providing a parsimonious representation in terms of autoregressive models of observations, by means of the only information coming from the time series itself. in the study of seemingly complex phenomena, like those generating self-potential signals, methodologies able to capture the dynamic peculiarities in observational time series are particularly useful tools to obtain information on the features and causes of signal time variability. in particular, fractal techniques, developed to extract qualitative and quantitative information from time series, have been applied recently to the study of a large variety of irregular, erratic signals and have proved very useful to reveal deep dynamic features. cuomo et al. (2001) detected scaling behaviour in the power spectra of geoelectrical time series, revealing the antipersistent character of the self-potential fluctuations. telesca et al. (2001) proposed a new approach to investigate correlations between geoelectrical signals and earthquakes, analyzing the time variations of the fractal parameters, characterizing their dynamics. balasco et al. (2002) found that self-potential measurements seem to be featured by long-range correlations with scaling exponents that indicate that the underlying geophysical process is characterized by stabilizing mechanisms. in all the previous works, monofractal analyses were performed leading to the estimation of only one scaling exponent. monofractals are homogeneous objects, in the sense that they have the same scaling properties characterized by a single singularity exponent (stanley et al., 1999; and references therein). the need for more than one scaling exponent to describe the scaling properties of the process uniquely indicates that the process is not a monofractal but could be a multifractal. a multifractal object requires many indices to characterize its scaling properties. multifractals can be decomposed into many – possibly infinitely many – sub-sets characterized by different scaling exponents. thus multifractals are intrinsically more complex and inhomogeneous than monofractals (stanley et al., 1999), and characterize systems featured by very irregular dynamics, with sudden and intense bursts of high frequency fluctuations (davis et al., 1994). the most adequate manner to investigate multifractals is to analyze their fractality or singularity spectra. the singularity spectrum quantifies the fractal dimension of the sub-set characterized by a particular exponent, that is gives information on the relative dominance of various fractal exponents present in the process. in particular, the maximum of the spectra furnishes the dominant fractal exponent and the width of the spectrum denotes the range of the fractal exponents. therefore, the multifractal formalism appears to furnish more deep information on the complexity of a time series. in the present work, we investigate the temporal fluctuations of self-potential data, measured in southern italy from january 2001 to september 2002, using multifractal formalism, to disclose typical dynamic features. 2. data our data consist in eight geoelectrical time series recorded at three monitoring stations: giuliano (giul1 and giul2), tito (tito1, tito2, tito3 and tito4) and laterza (lat1 and lat2). the first two stations are located in seismic sites, the third in an aseismic one. figure 1 shows the locations of the monitoring stations. as far as the technical features of the experimental equipment are concerned, we refer the reader to cuomo et al. (1997) and for the results of monoand multiparametric preliminary statistical analysis of the monitored variables to di bello et al. (1994). figure 2 shows the time variations of the self-potential signals measured. 3. methods the concept of multifractal object was developed by mandelbrot (1974) to investigate several features in the intermittency of turbulence (meneveau and sreenivasan, 1991). many authors have applied multifractality to several fields of scientific research. multifractal formalism is based on the definition of the so-called partition function z (q, ε) i 1= , .z q ( ) i q n boxes =f n f f !_ ^i h8 b (3.1) 13 a preliminary study of the site-dependence of the multifractal features of geoelectric measurements the quantity µi (ε) is a measure and it depends on ε, the size or scale of the boxes used to cover the sample. the boxes are labelled by the index i and nboxes (ε) indicates the number of boxes of size ε needed to cover the sample. the exponent q is a real parameter, giving the order of the moment of the measure. the choice of the functional form of the measure µi (ε) is arbitrary, provided that the most restrictive condition µi (ε) ≥ 0 is satisfied. the parameter q can be considered a powerful microscope, able to enhance the smallest differences of two very similar maps (diego et al., 1999). furthermore, q represents a selective parameter: high values of q enhance boxes with relatively high values for µi (ε); while low values of q favour boxes with relatively low values of µi (ε). the box size ε can be considered a filter, so that large values of the size are equivalent to apply a large scale filter to the map. changing the size ε, one explores the sample at different scales. therefore, the partition function z (q, ε) furnishes information at different scales and moments. the generalized dimensions are defined by the following equation: fig. 1. location of the geoelectrical monitoring stations giuliano, laterza and tito in southern italy. 14 luciano telesca, gerardo colangelo, vincenzo lapenna and maria macchiato -30 -20 -10 0 10 20 30 40 giul1 t (hour) -30 -20 -10 0 10 20 lat2 s p ( m v ) t (hour) -50 -40 -30 -20 -10 0 10 lat1 t (hour) -70 -60 -50 -40 -30 -20 -10 0 10 tito1 s p ( m v ) t (hour) -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 tito3 s p ( m v ) t (hour) -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 s p ( m v ) s p ( m v ) s p ( m v ) -30 -20 -10 0 10 20 30 40 giul2 t (hour) s p ( m v ) -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 tito2 t (hour) s p ( m v ) 0.0 5.0x10 3 1.0x10 4 1.5x10 4 0.0 5.0x10 3 1.0x10 4 1.5x10 4 0.0 5.0x10 3 1.0x10 4 1.5x10 4 0.0 5.0x10 3 1.0x10 4 1.5x10 4 t (hour) 0.0 5.0x10 3 1.0x10 4 1.5x10 4 0.0 5.0x10 3 1.0x10 4 1.5x10 4 0.0 5.0x10 3 1.0x10 4 1.5x10 4 0.0 5.0x10 3 1.0x10 4 1.5x10 4 fig. 2. hourly variability of the 8 geoelectrical signals recorded at stations giuliano, laterza and tito. 15 a preliminary study of the site-dependence of the multifractal features of geoelectric measurements 0 , .lim ln ln d q q z q 1 1 = f f "f _ _ i i (3.2) d (0) is the capacity dimension; d (1) is the information dimension, and d (2) is the correlation dimension. an object is called monofractal if d (q) is constant for all values of q, otherwise is called multifractal. in most practical applications the limit in eq. (3.2) cannot be calculated, because we do not have information at small scales, or because below a minimum physical length no scaling can exist at all (diego et al., 1999). generally, a scaling region is found, where a power-law can be fitted to the partition function, which in that scaling range behaves as . ( )q ,z q "f fx_ i (3.3) the slope τ (q) is related to the generalized dimension by the following equation: .q q d q1= -x _ _ _i i i (3.4) a usual measure in characterizing multifractals is given by the singularity spectrum or legendre spectrum f (α) that is defined as follows. if for a certain box j the measure scales as j j"n f fa^ h (3.5) the exponent α, which depends on the box j, is called hölder exponent. if all boxes have the same scaling with the same exponent α, the sample is monofractal. the multifractal is given if different boxes scale with different exponents α, corresponding to different strength of the measure. denoting as sα the subset formed by the boxes with the same value of α, and indicating as nα (ε) the cardinality of sα, for a multifractal the following relation holds: . ( )a n f"f f-a^ h (3.6) by means of the legendre transform the quantities α and f (α) can be related to q and τ (q) q dq d q =a x _ _ i i (3.7) ( ) ( ).f q q q= -a a x^ h (3.8) the curve f (a) is a single-humped function for a multifractal, while it reduces to a point for a monofractal. in order to quantitatively recognize possible differences in legendre spectra stemming from different signals, it is possible to fit the spectra to a quadratic function around the position of their maxima at α 0 by a least square method (shimizu et al., 2002) .f a b c0 2 0= + +a a a a a^ _ _h i i (3.9) parameter b measures the asymmetry of the curve, which is zero for symmetric shapes, positive or negative for left-skewed or right-skewed shapes respectively. another parameter is the width of the spectrum, that estimates the range of α where f (α) > > 0, obtained extrapolating the fitted curve to zero; thus the width is defined as w max min= -a a (3.10) where f (α max) = f (α min) = 0. these three parameters serve to describe the «complexity» of the signal. if α 0 is low, the signal is correlated and the underlying process «loses fine structure», becoming more regular in appearance (shimizu et al., 2002). the width w measures the length of the range of fractal exponents in the signal. therefore, the wider the range, the «richer» the signal in structure. the asymmetry parameter b informs us about the dominance of low or high fractal exponents with respect to the other. a right-skewed spectrum denotes relatively strongly weighted high fractal exponents, corresponding to fine structures, and low ones (more smooth-looking) for left-skewed spectra. 4. results and discussion we performed multifractal analysis, calculating the legendre spectra by means of the software fraclab, developed at inria and available at the internet site http://www.rocq.inria.fr. since the data present gaps, we considered for each signal the longest segments with16 luciano telesca, gerardo colangelo, vincenzo lapenna and maria macchiato 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.0 0.2 0.4 0.6 0.8 1.0 a giul2 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 a lat2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 tito2 a 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 a tito4 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 giul1 a 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 a lat1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 a tito1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.0 0.2 0.4 0.6 0.8 1.0 a tito3 f( a ) f( a ) f( a ) f( a ) f( a ) f( a ) f( a ) f( a ) fig. 3. legendre spectra of the signals measured in southern italy. for each signal, we selected the 3-4 longest segments without gaps, whose order of magnitude of length is 103. all the spectra show a single-humped shape, typical of multifractal signals. 17 a preliminary study of the site-dependence of the multifractal features of geoelectric measurements out data missing. the order of the magnitude of the length of each segment is about 103, thus yielding reliable estimates of the singularity spectrum and multifractal parameters. figure 3 shows the legendre spectra for the selected segments for each signal. all the spectra present the typical single-humped shape, which characterizes multifractal signals. figure 4 shows the average spectra. the differences among the spectra are very clear. thus, after the determination of the maxima α0, we fitted each average spectrum by eq. (3.9), thus estimating the asymmetry b and the width w. the results are summarized in fig. 5a-c. the time series lat1 and lat2 measured in station laterza located in an aseismic area present the lowest value for the maximum α 0 (fig. 5a), indicating a more regular process governing the time variability of such signals that seem to be characterized by more «coarse» structures. the smooth-looking behaviour displayed by lat1 and lat2 is also recognized in fig. 5b, where the asymmetry b assumes the highest positive value for such signals. furthermore, the lowest values for the width w indicate that a narrower range of fractal exponents characterizes lat1 and lat2 signals. the signals measured by station tito (tito1, tito2, tito3 and tito4) and giuliano (giul1 and giul2) located in a seismic area appear more complex because they present large maximum α 0 values, large width w values and relatively low value for the asymmetry b. in fig. 6 the geoelectric signals are represented as points in the space (α 0, b, w) of the multifractal parameters: the signals lat1 and lat2, measured in aseismic sites, are well discriminated from the other signals (tito and giuliano), recorded in seismic areas. the complex geophysical phenomenon underlying the geoelectrical variations is influenced by the geological and tectonic environment outlined in the previous sections. the use of multifractal methods to investigate the temporal fluctuations of geoelectrical signals can 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 giul1 giul2 lat1 lat2 tito1 tito2 tito3 tito4f( α ) α fig. 4. average legendre spectra, calculated, for each signal, averaging the segment multifractal spectra. 18 luciano telesca, gerardo colangelo, vincenzo lapenna and maria macchiato lead to a better understanding of such complexity. the development of the southern apennines fold-and-thrust belt has been attributed to the subduction of the ionian lithosphere beneath the adriatic plate (doglioni et al., 1999; bonini et al., 2000). this part of the apennines comprises a stack of east-verging tectonic units representing an accretionary wedge composed of mesozoic-cenozoic sediments originally deposited in different paleogeographic domains, both basinal (lagonegro) and shallow-water which were thrust onto the apulian foreland to the east (mostardini and merlini, 1986; pescatore, 1988; schiattarella 1998) (fig. 7). the monitoring network area extends along the southern apennine chain where only laterza station is out of an active seismic zone. in fact laterza station is located in puglia region, this area is part of the apulian foreland which represents the plio-pleistocene foreland of the southern apennines orogenic system («avampaese apulo» of selli, 1962). the apenninic foreland basins not yet incorporated into the orogenic wedge is characterized by a 6 km thick mesozoic carbonate succession (apulia carbonate platform – d’argenio, 1974; richetti, 1980). the uplift of the apulian foreland (middle-late pleistocene) has induced a low to giul1 giul2 lat1 lat2 tito1 tito2 tito3 tito4 0.50 0.55 0.60 0.65 0.70 0.75 m a xi m u m a 0 station giul1 giul2 lat1 lat2 tito1 tito2 tito3 tito4 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 a sy m m e tr y b station giul1 giul2 lat1 lat2 tito1 tito2 tito3 tito4 0.9 1.0 1.1 1.2 1.3 1.4 w id th w station fig. 5a-c. variation with the station of the multifractal parameters: a) maximum α 0, b) asymmetry b and (c) width w. b c -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.9 1.0 1.1 1.2 1.3 1.4 0.55 0.60 0.65 0.70 lat2lat1 tito4 giul1 tito3 tito2 tito1 giul2 m a xi m u m a 0 wid th w asymmetry b fig. 6. 3d plot of the multifractal parameters: the signals lat1 and lat2, recorded in an aseismic area, are well discriminated from the signals (tito and giul) measured in seismic areas. a 19 a preliminary study of the site-dependence of the multifractal features of geoelectric measurements moderate-energy seismicity documented by paleoliquefaction features (moretti, 2000) and historical and instrumental recorded seismic events (pieri et al., 1997): it is considered a poorly tectonized area in the apenninic foreland (doglioni et al., 1995). on the other hand, giuliano and tito stations are located in the campania-basilicata area, one of the most active seismic zones of the southern apennines. large destructive earthquakes occurred both in historical and recent times in this region, which was struck in 1980 by the strongest event (ms = 6.9) of the past century in the southern apennines (improta et al., 2003). the highest number of disastrous events with i ≥ x mcs took place in the irpinia area in historical times. such events (1964, 1930) appear to be concentrated in the same zone; only one strong event, the december 16, 1857 earthquake affected the southern sector of this area (agri valley – basilicata) and 1550 and 1561 earthquakes have been located in the west of the area, in the vallo di diano (campania) (alessio et al., 1995). ten year after the devasting irpinia earthquake, a moderate event on may 1990 (ml = 5.2) and may 1991 (mb = 5.1) occurred approximately 40 km east of the southern end of the 1980 aftershock zone, causing damage in the nearby town of potenza (ekström, 1994). the normal faults affecting the internal side of the fold-and-thrust belt are commonly related to this extensional regime, which followed the progressive migration of the compressive thrust-front toward the apulian foreland (bonini et al., 2000). 5. conclusions the determination of the multifractal parameters of geoelectrical time series recorded in southern italy was performed by means of the calculation of the legendre spectrum. we derived three parameters, the maximum α 0 of the spectrum, the asymmetry b and the width w of the curve. the time series lat1 and lat2, recorded in an aseismic site, are characterized by low maximum α 0, high positive asymmetry b and low width w. these features qualify these time series as less complex than the others (tito1-4 and giul1-2), which were measured in seismic sites. therefore, this set of multifractal parameters seem to well discriminate signals measured in seismic from signal recorded in aseismic sites. references alessio, g., e. esposito, a. gorini and s. porfido (1995): detailed study of the potentino seismic zone in the southern apennines, italy, tectonophysics, 250, 113-134. balasco, m., v. lapenna and l. telesca (2002): 1/f α fluctuations in geoelectrical signals observed in a fig. 7. schematic section of the shallow structure of the southern apennines across the irpinia region: a – pliopleistocene deposits of the bradano trough; 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(1990): the mechanics of earthquakes and faulting (cambridge university press), pp. 439. selli, r. (1962): il paleogene nel quadro della geologia dell’italia meridionale, mem. soc. geol. ital., 3, 737-789. shimizu, y., s. thurner and k. ehrenberger (2002): multifractal spectra as a measure of complexity in human posture, fractals, 10, 103-116. stanley, h.e., l.a.n. amaral, a.l. goldberger, s. havlin, p.ch. ivanov and c.-k. peng (1999): statistical physics and physiology: monofractal and multifractal approaches, physica a, 270, 309-324. vol52,6,2009new 557 annals of geophysics, vol. 52, n. 6, december 2009 key words geological map of italy – international geological congresses – central office of meteorology – geodynamic service – seismological observatory of ischia 1. scientific competencies of the maic and correspondence in the earth sciences preserved in the archive decree no. 4192, 5th july 1860, which established the competencies of the ministry of agriculture industry and commerce (maic), also introduced an activity of experimentation and study aimed at perfecting methods and techniques in the various economic sectors and at increasing agricultural and industrial education (caracciolo, 1960). according to the prevalent opinion at the time, a good administration, «without taking the place of private forces», was supposed to «illuminate» and uphold the productive world by assuming the burden of informing and diffusing technical and scientific knowledge (de cesare, 1868). a division for general statistics is known to have been created under the maic since 1861. the meteorological, geological and seismic services for the new unitary state, instead, were initiated between the seventies and the eighties under the department for agriculture, which became general directorate in 1883 (eramo, 1995); hydrographical surveys pertaining to the whole national territory were alscientific correspondence in the archive of the general directorate of agriculture under the ministry of agriculture industry and commerce nella eramo archivio centrale dello stato, roma, italy abstract the archive, which testifies the activity of the offices dedicated to geological studies and to the collection of meteorological investigations from 1870 to 1895, contains important files of scientific correspondence. the latter includes letters from scientists, members of the councils, committees and research commissions established under the ministry of agriculture industry and commerce, as well as correspondence of technicians who were charged with particular duties by the administration, like nicola maraglia, general director for agriculture, and felice giordano, director of the service of the mines. they are rich in news, opinions, observations and reports portraying scientific events and personalities of the time. as an example, some documents concerning the following topics are illustrated: the origins of the central office of meteorology and the role of giovanni cantoni; the establishment of a service for the systematic collection of data about seismic phenomena, by michele stefano de rossi; the activity of the geodynamic commission; the project by giulio grablowitz for the construction of the observatory of ischia. mailing address: dr. nella eramo, archivio centrale dello stato, piazzale degli archivi 27, 00144 roma, italy; e.mail: neramo@archivi.beniculturali.it vol52,6,2009new 2-12-2009 12:39 pagina 557 558 n. eramo so instituted (eramo, 1996). therefore, as well as files concerning mainly agricultural subjects such as «agrarian stations and laboratories», «practical and specialised schools of agriculture», «vegetal pathology», «zoology and veterinary», etc., the general directorate archive containes documents related to the activity of offices involved in hydraulics, land reclamation and irrigation studies, meteorology, mineralogy, geology, volcanology and geodynamics. the chronological range of the fonds extends from 1860 to the end of the century; although, more ancient records are present, produced by the different pre-unitary administrations. the correspondence available in the archive shows the role of scientists, professors and technicians during the establishment of the ministry’s scientific services, as well as during their later development, when directly responsible for the administration and when members of councils, committees and research commissions. such documents are not often known because they have remained hidden inside ponderous files of an essentially administrative nature. they are important, however, for their potential to enlighten the reasons, conditioning and motivations for the choices made and the directions followed. on one hand, the letters of scientists addressed to the minister, the directors, the inspectors and the presidents of committees tell us about a whole activity aimed at informing, convincing, educating and supporting the administration’s work; on the other hand they show the willingness and the awareness of some of those in charge of the services towards the occurring needs. still to be better analysed, but absolutely fundamental, is the role of nicola miraglia, director of agriculture for over thirty years (from the end of the sixties to the nineties of the 19th century), who ensured the continuity of the ministry’s policy and was able to effectively mediate between the proposals of scientists and the ministerial decisions, often conditioned by budget constraints. 2. geology classified as «geology» are all the files contained in more than fifty folders and related to: general affairs, congresses held in italy and abroad, exhibitions, the geological map, geological offices, personnel, geological and mining education. the documents include: the minutes from the advisory committee charged to establish the methods and rules for the geological map; the correspondence with the geological committee, which was entrusted with the high scientific direction of the surveys; all work progress reports from the geological engineers; etc. they can offer important information about the origins of the service and its actual running till 1895. as an example, i quote a document from the beginning of the unitary state, a note by felice giordano, chief inspector of the corps of mines, addressed to the minister of agriculture, tommaso corsi, on 19th october 1860, about the «geological map of the country» (1). giordano, mining engineer in the ministry of agriculture and commerce from the early fifties, later in the ministry of public works of the kingdom of sardinia, was nominated chief of the inspectorate of the mines. afterwards, from december 1866, he became a member of the geological committee, as well as director of the service of the mines and of the surveying activities for the geological map until 1892, the year of his death (2). this report synthesises the events that led, with the royal decree of 28th july 1861, to the establishment of the advisory committee. it proves giordano’s belief, already mature in 1860, that the new state required a scientific instrument fundamental also for the development of productive activities. aware that «there are issues which, although implying a long term solution, already require a direction to define the most adequate elements for the purpose» (3), he underlines the importance, «for a progressing state» (4), of a process which, at that time, other nations in europe and america had already accomplished or started: «marking in different colours, on each region’s map, the masses of the different grounds constituting its framework or subsoil, sorted by age of formation, is of a high scientific interest as it refers to the chronology of their subsequent formation. it is, in fact, of a highly practical value because the genesis of lands and their underground structures are essential elements for all industries, vol52,6,2009new 2-12-2009 12:39 pagina 558 559 scientific correspondence in the archive of the general directorate of agriculture under the ministry of agriculture industry and commerce which directly or indirectly depend on the mineral realm. among these, stands the agricultural industry for which it is important to know the layout and the extension of lands of different formation and nature […]. it is also pertinent to all the operations related to sewers, irrigation canals, water sources and, more generally, the safe accomplishment of public works» (5). news and references to the work and the positions of scientists from other countries can be found in the correspondence of engineers and professors, members either of the international committee for the geological map of europe or of the international commission for the unification of the geological nomenclature; as well as in the reports written for the minister by the participants in international conferences. worthy of note, for example, is the file on the international conference of geology in london (6), held from 17th to 22nd september 1888, which includes, among other things, the reports sent by giordano and giovanni capellini, director of the reale istituto geologico e paleontologico of the university of bologna and president of the international commission for the unification of the geological nomenclature. on a note written on 21st november 1888, the minister bernardino grimaldi expressed doubt about the benefit of the italian participation in the geological conferences, as in london no significant decision had been taken about the items on the agenda, which concerned the «classification of the cambrian and the silurian and the limits of the tertiary and quaternary». in his reply, on 27th november, capellini recalled how, in the conference of manchester of 1887, the american geologists had proposed, to their english speaking colleagues, to create an association permanently based in london, aimed at dealing with all the issues related to geological nomenclature. their intention was clearly to bring about the cancellation of the iv international conference, due in london the following year. capellini explains: «if i had not taken part in the discussion in manchester, where such a proposal had to be evaluated, it would have probably been accepted, with great disadvantage to the geological studies in general and with sacrifice of our national self-respect in particular. the importance of the international geological conference became clear during the second session in bologna in 1885, and as it is now widely acknowledged, it’s our duty to stand our ground and fight» (7). another file, named «international commission for the unification of geological nomenclature» (8), contains a letter written on 23rd september 1887, where capellini reports even more clearly what happened during the meeting in 1887. in manchester he had managed to foil the attempt to institute a great association of great britain and all the depending colonies, in fact of all the english-speaking countries, including the united states of america, so that anything concerning the geological classification and nomenclature would be decided by this association rather than by the international conference. the aim was to force all the remaining nations to accept the conclusions and the rules agreed in that association. under the pretext that geologists of the other european nations could not reach an agreement, while it would be extremely simple for english speakers to do, without any apparent hostility, a deadly blow would have occurred for the international conference, supposed to meet in london the following year. 3. meteorology the archive also holds numerous and copious files on meteorology, in particular related to the central office of meteorology, from its institution in 1876 until near the end of the century. it is undoubtedly worth carefully going through the abundant and regular correspondence between the director of agriculture and pietro tacchini, director of the central office of meteorology between 1879 and 1896, the last documented year in the archive. worthy of note are also: the reports on the beginning of a systematic series of studies in the field of forestale meteorology, and, in particular, on the surveys conducted by tacchini and elia millosevich in the woods of vallombrosa and camaldoli in 1881 (9); the correspondence between tacchini and angelo celli about the installation of a network for the parallel measurement of nosographic and meteorological data, aimed at betvol52,6,2009new 2-12-2009 12:39 pagina 559 560 n. eramo ter identifying the connections between climatic-environmental conditions and malaria (10). as other contributions in the conference are dedicated to pietro tacchini and to the period of his directorship of the central office, i would rather consider in more detail some files of the years 1873-74, when the meteorological office was taking its first steps and the surveys and measurements were still under the division of statistics, directed by luigi bodio. the technical direction of the service was then entrusted to the physician giovanni cantoni, who worked to rationalise and standardise the meteorological stations. the correspondence from that period shows a restless activity orientated to create a national central service. in a report for the ministry on 30th december 1874 (11), cantoni refers to the annual activity of the meteorological office. based on what had been agreed in the international conference of vienna in 1873, about the need to introduce shared protocols for the measurement and the publication of data from the different european countries, a decadal bulletin was launched, where «in as many as twenty stations, spread on the surface of the peninsula, the parameters of the main meteorological events are compared; thus, even with a quick glance, it is possible to examine the general trend of the great perturbations and the daily variations that occurred all over italy; then they can be easily compared to the ones registered in other regions of europe» (12). cantoni also declares that, compared to the previous year, thirteen more stations, out of a total of eighty, regularly send their meteorological notes to the office. he points out to luigi bodio that, within the maic, the hydrographical commission is collecting the measurements from pluviometrical stations installed in the major rivers’ basins, so he proposes that the different offices systematically share their data. he also highlights how, in the same ministry, an increasing number of agricultural stations are provided with rain and meteorological measurements, which could be published on the monthly bulletin with an increased benefit for both agriculture and meteorology. in the same way, all the data telegraphed by the harbour offices to the ministry of the navy should be shared as well. using cantoni’s own words, he constantly underlines the necessity «to unify and revitalise the meteorological services, which lie unorganised and wasted under several ministries, by creating one central meteorological institute based on the dutch, danish and swedish models; it should aim at coordinating the data measured by the numerous meteorological, maritime and hydrometrical stations present in our country» (13). 4. geodynamics the central office of meteorology, instituted by the royal decree 26th november 1876 and directed by tacchini from 1879, later acquired also the directorship of the geodynamic service (14). some documents on this service’s events have been located. in particular, they concern: the role of the observatory and central geodynamic archive, founded in 1882 under the royal geological committee and directed by michele stefano de rossi, from the beginning to its drastic reduction, due to the unification of the geodynamic service and the central office of meteorology in 1887; the work by the royal geodynamic commission, established by the royal decree 20th december 1883; the design of the observatory of casamicciola. the first steps of the service, based in via santa susanna, are reported by de rossi himself in a letter (15), whose subject was «attribuzioni dell’archivio geodinamico», addressed to the general director miraglia on 12th june 1888: «first of all i think i should remind the ministry that in 1882 i was asked, directly by the minister, to move the geodynamic archive, that i created, under the royal geological committee, in order to enhance it and to make it the centre of a geodynamic service, as well as the well known publication of the italian volcanism bulletin. […] i also recall how, in order to propose the final configuration for the new scientific service, the royal geodynamic commission, later established, recognised the personal property of such an archive and of the associated geodynamic library, asking the ministry to purchase them; such a proposal only involved the part preceding the year 1883, when the collection of news and docuvol52,6,2009new 2-12-2009 12:39 pagina 560 561 scientific correspondence in the archive of the general directorate of agriculture under the ministry of agriculture industry and commerce ments started to depend on the small office whose direction was given to me by the ministry» (16). the correspondence between the mentioned central archive, directed by de rossi, and the prefects of the kingdom is available. on 20th april 1883 a circular was sent to the prefects to ask for the systematic transmission of any news related to seismic events occurring in the various provinces, «[…] even those events whose low intensity wouldn’t normally attract interest beyond the boundaries of the city or the place where they occurred» (17). the prefects immediately started to communicate important information to the office and proposed the installation of new measurement stations (27 new stations during 1883); they also signalled people and institutes that would organise direct and scientifically rigorous surveys for free, providing they were given the right means. that is what de rossi states in his letter to miraglia on 30th december 1883 (18), adding that the new stations, together with the other 39 major ones, would produce a very effective network of geodynamic analysis. in the note de rossi wrote on 16th december 1883 for giordano, director of the geological service, he highlighted the necessity for the observatory of rome and for the other provincial ones to be provided with appropriate equipment. a detailed report, attached to the note, describes the first steps taken to institute the new service. it reads: «among the other requirements of the geodynamic service, there is a need for mobile as well as fixed observatories. «mobile observatories» are a set of portable instruments that can be used either for scientific excursions or to create temporary measuring stations, ready to be installed in case of sudden seismic activities. such a need has been partially fulfilled, despite the lack of means, thanks to the availability of a limited number of instruments; during this year, mobile instruments have been working in cascia in palazzuolo, in albano, in serrara fontana and in ischia. while considering the geodynamic service’s work, it is fundamental to mention the network of measuring stations, supplied with some seismographs, installed on the slopes of etna; a central observatory has also been established in catania, promoted by the director of the meteorological office, tacchini. tacchini also provided the most valuable observatories with some seismoscopic detectors, for the analysis of more intense earthquakes» (19). during the debate in parliament on a law to define the institutional aid for the people affected by the earthquake of ischia, on which occasion the ministry of agriculture was involved for the installation of a geodynamic observatory on the island, de rossi wrote a letter (3th february 1884) (20) to the minister domenico berti, «[…] in order to make the use of some essential terminology clearer» (21). his purpose was to avoid confusion and to clarify the main objective of seismic studies and observations; he explained: «the study of the internal activity of the earth, that we call geodynamics, is now made of continuous observations and based on special instruments, because this has been proved to be the only method to investigate its rules and to discover its causes. even though the forecast of dangerous phenomena is not the direct purpose of this study, it could represent its final result; for this reason it is worth conducting such observations in all those places where seismic activity is most intense» (22). he added that science cannot be expected to foresee the place and time of an earthquake or of an eruption, however «the experimental method based on scientifically regular observations, conducted in appropriate observatories, has proved the existence of precursory phenomena. when the course of such phenomena will be better understood, even the scientific prediction of the greatest phenomena will be possible» (23). so, regarding the observatory of ischia, «it must not be used to inform the public daily about the dynamic configuration of the earth, but it will record all the different phenomena detected on the island. when an abnormal accumulation of these phenomena will occur, as happened before the earthquake of ischia on 28th july, it will be possible to warn the population about the expectation of a similarly extraordinary event» (24). it is also interesting to examine what appears from the documents concerning the geodynamic commission, which had the task to propose the final configuration of the geodynamic service. the commission members, bevol52,6,2009new 2-12-2009 12:39 pagina 561 562 n. eramo sides de rossi, tacchini and giordano, were: luigi palmieri, director of the vesuvian observatory; cerasio silvestri, director of the geodynamic observatory of catania; galileo ferraris, director of the laboratory of physics of the industrial museum of torino; francesco denza, director of the meteorological observatory of moncalieri; giovanni cantoni, member of the council of meteorology; torquato taramelli, director of the museum of mineralogy and geology of the university of pavia; giulio grablowitz, director of the geodynamic observatory of casamicciola. the correspondence between the president of the commission, pietro blaserna (at that time, director of the department of physics in the university of rome) and the general directorate of agriculture is particularly relevant, as it contains information and comments on the main subjects discussed in the commission meetings. for example, in his letter dated 17th june 1886 (25), blaserna writes: «the commission considers neither useful nor appropriate to suggest to the government the establishment of a central geodynamic office. since an extended network of meteorological stations already exists, and because there are many analogies between the meteorological and the geodynamic service, they could be joined in a single central office, under the competent guide of tacchini» (26). the geodynamic commission also debated about the most suitable instruments and methods for the investigations. the contribution of the commission members on this subject is highly criticised by blaserna, who describes the reports by palmieri and silvestri as insufficient and consideres de rossi’s superficial description of the available instruments as a sign that he is underestimating the matter. blaserna also adds: «unfortunately we must admit that we have been overtaken by other countries and, if we had continued following our old ways, italy wouldn’t have been able to conduct strictly scientific studies on ground movements» (27). commenting on the report by grablowitz about the scientific instruments, whose publication he highly recommends (28), blaserna refers to the commission’s decision about it: «the commission holds that the future of seismic studies in italy is based on the availability of recording instruments able to register the three components of ground movement. for this reason, the commission suggests that the main observatories collect all the instruments listed in the report, in order to test and select them» (29). the archive of the general directorate of agriculture also preserves important documentation on the history of the observatory of casamicciola. several files concern the building of the observatory, in particular: the initial inspections performed by de rossi, tacchini, and grablowitz; the preliminary design by grablowitz (december 1884); the comments by grablowitz about the feasible project drafted by the ufficio speciale del genio civile. worthy of note, as an example, are two reports by grablowitz (30): the first one, written on 18th december 1884 and addressed to the president of the geodynamic commission, is about the place and position of the observatory, the subsoil steadiness and the stability of the building, which are very important factors for the installation of the seismic instruments; the second one, sent to the ministry of agriculture industry and commerce on 28th september 1886 (when grablowitz was the director of the observatory), illustrates the basic characteristics of the construction, like the optimum surface area, the perfectly square shape, and the dimensions of the central pillar. notes (1) archivio centrale dello stato (acs), ministero di agricoltura industria e commercio (maic), direzione generale dell’agricoltura (dga), versamento v, b. 951, fasc. 4193 «86/1. carta geologica. direzione scientifica. giunta consultiva per la formazione della carta geologica», 1860-1865. (2) acs, maic, dga, versamento iv, b. 770, fasc. 4365 «84/1. giordano comm.re felice», 1852-1892. (3) «esistono questioni le quali, benché di lunga soluzione, esigono tuttavia sin d’ora una direzione che ne predisponga gli elementi allo scopo migliori». vol52,6,2009new 2-12-2009 12:39 pagina 562 563 scientific correspondence in the archive of the general directorate of agriculture under the ministry of agriculture industry and commerce (4) «per uno stato progrediente». (5) «il segnare sulla mappa delle diverse regioni con colori diversi le masse dei terreni che ne costituiscono l’ossatura o sottosuolo, distinte per l’età della loro formazione, è d’interesse scientifico in quanto si riferisce alla cronologia della successiva loro formazione, ed è però nel fatto di somma utilità pratica, stante che il modo della genesi dei terreni, la loro disposizione sotterranea sono gli essenziali elementi di tutte le industrie che dipendono in modo diretto e indiretto dal regno minerale. fra queste primeggia l’agricola, per la quale è importante conoscere la disposizione ed estensione delle zone dei terreni di varia formazione e natura […]. ne dipendono anche tutte le operazioni, oggidì capitali, della fognatura, del tracciamento dei canali irrigatori, dell’allacciatura delle sorgenti, e in generale tutto il ramo dell’idrografia sotterranea». (6) acs, maic, dga, versamento v, b. 950, fasc. 4189 «85/1. congresso geologico internazionale a londra», 1888-1889. (7) «se io non avessi preso parte alla discussione a manchester ove si doveva deliberare su quella proposta, essa sarebbe forse stata accolta favorevolmente con grande svantaggio degli studi geologici in generale e con sacrifizio del nostro amor proprio nazionale in particolare. l’importanza del congresso geologico internazionale emerse nella seconda sessione in bologna nel 1885 e, poiché oggi ciò è ammesso da tutti, a noi incombe l’obbligo di restare sulla breccia e lottare». (8) acs, maic, dga, versamento v, b. 950, fasc. 4185, anni 1886-1888. (9) acs, maic, dga, versamento iv, b. 633, fasc. 3670 «32/8. meteorologia forestale. camaldoli», 1873-1886. (10) acs, maic, dga, versamento v, b. 885, fasc. 3918 «42/6. malaria. complessiva», 1886-1891. (11) acs, maic, dga, versamento ii, b.198, fasc. 1422 «32. meteorologia. complessiva. dal 1873 al 1878», 1873-1878. (12) «per ben venti stazioni, ripartite sulla faccia della penisola, si espongono in forma comparativa, le vicende decadiche dei principali elementi meteorici, tantochè, quasi a colpo d’occhio, può argomentarsi l’andamento generale delle grandi perturbazioni e delle variazioni diurne occorse in tutte le parti d’italia, e lo si può di poi facilmente paragonare a quello contemporaneamente verificatosi nelle altre regioni d’europa». (13) «[…] di dare unità e vita ai servizi meteorici che fra noi giacciono sperperati e malfermi sotto vari ministeri, creando un istituto meteorico centrale, sul tipo di quelli dell’olanda, della danimarca e della svezia, il quale valga a ridurre ordinate e feconde le tante osservazioni che si van facendo nelle numerosissime stazioni meteoriche, marittime, idrometriche, che sono nel regno nostro». (14) royal decree 9th june 1887, n. 4636. (15) acs, maic, dga, versamento v, b. 965, fasc. 4242 «90/1. osservatori geodinamici. roma. osservatorio e archivio centrale geodinamico», 1881-1892. (16) «parmi innanzi tutto dover ricordare ciò che è ben noto a codesto ministero, essere cioè io stato senza mia richiesta direttamente invitato nel 1882 da s.e. il ministro a recare presso il regio comitato geologico l’archivio geodinamico da me formato, per continuarlo e farne il centro di un servizio geodinamico, unitamente alla ben nota pubblicazione del bollettino del vulcanismo italiano. […]. ricordo inoltre come la regia commissione geodinamica poscia nominata, per proporre l’assetto definitivo del nuovo servizio scientifico, riconoscesse la proprietà personale di detto archivio con l’annesso nucleo di biblioteca geodinamica, proponendo a codesto ministero l’acquisto di esso per la parte anteriore al 1883; dal quale anno cominciava poi la raccolta delle notizie e documenti fatta per mezzo del piccolo ufficio allora fondato dal ministero sotto la direzione dello scrivente». (17) «[…] anche quelli la cui conoscenza, per la loro tenue gravità, non esce dai confini della città o abitazione nella quale sono avvenuti». (18) acs, maic, dga, versamento v, b. 962, fasc. 4230 «90. vulcanologia. affari generali. disposizioni generali», 1883-1888. (19) «fra i bisogni del servizio geodinamico vi è quello degli osservatori volanti, oltre quello degli osservatori stabili. chiamiamo “osservatori volanti” un corredo di istrumenti portatili, i quali servono non solo nelle escursioni ed esplorazioni, ma anche per organizzare piccoli osservatori temporanei dove se ne manifesti il bisogno, in seguito alla comparsa di un periodo sismico o d’altro genere di manifestazione endodinamica. anche a questo bisogno, malgrado la mancanza di mezzi, si è provveduto con qualche istrumento; e nel corrente anno funzionano di tali osservatori temporanei in cascia in palazzuolo, presso albano, in serrara fontana e in ischia. parlando dell’operato del servizio geodinamico, devesi accennare all’istituzione di una rete di stazioni fornite di qualche sismografo, distribuita nelle falde dell’etna, con un osservatorio regionale centrale in catania, mercé le cure del direttore dell’ufficio meteorologico comm. tacchini. anzi il medesimo, per quanto lo scopo del suo servizio meteorologico lo comportava, ha procurato anche fornire, ad alcuni più meritevoli fra gli osservatori meteorologici, un qualche avvisatore sismoscopico per lo studio dei terremoti di una certa intensità». (20) acs, maic, dga, versamento v, b. 967, fasc. 4249 «90/1. casamicciola. osservatorio geodinamico», 1884-1891. vol52,6,2009new 2-12-2009 12:39 pagina 563 564 n. eramo (21) «[…] per far conoscere come debbano essere definiti e intesi alcuni concetti essenziali nella discussione». (22) «lo studio dell’attività interna della terra, che chiamiamo geodinamica, è ora organizzato nella forma di osservazioni continue e con appositi istrumenti, perchè si è visto essere questo il solo metodo per indagarne le leggi e scoprirne le cause. la previsione dei fenomeni pericolosi non è lo scopo diretto di questo studio ma potrà discenderne come risultato. interessa perciò principalmente l’organizzare tali osservazioni nei luoghi ove esistono le maggiori manifestazioni dell’attività interna». (23) «il metodo sperimentale delle osservazioni scientificamente regolari fatte in appositi osservatori ha dimostrato vera l’esistenza finora empiricamente conosciuta di fenomeni precursori. conosciuto che sia in avvenire l’andamento di detti fenomeni, diverrà possibile anche una scientifica previsione dei fenomeni maggiori». (24) «non si deve credere che debba avere l’ufficio di informare giornalmente il pubblico dello stato dinamico della terra, ma poiché in esso si dovrà tenere conto esatto di tutti i fenomeni diversi notati nell’isola intera, allorché si verifichi il complesso di uno straordinario cumulo di tali fenomeni, quale sembra essere avvenuto prima del 28 luglio, si avrà ragione di sospettare e anche di avvertire la probabilità di un fenomeno parimenti straordinario». (25) acs, maic, dga, versamento v, b. 963, fasc. 4233 «90. commissione reale per il riordinamento del servizio geodinamico», 1883-1887. (26) «la commissione è partita dal concetto che nelle attuali circostanze non sarebbe né utile né opportuno di consigliare al governo l’impianto di un vero e proprio ufficio centrale geodinamico. […] il servizio di meteorologia e quello di geodimamica hanno tra loro grandi analogie: si occupano entrambi di un ramo della fisica terrestre ed entrambi si esplicano per via di osservazione. trovandosi quindi impiantata un’estesa rete di osservatori per le osservazioni meteoriche, evidente si chiariva la convenienza di trarre profitto da essi per stabilirvi anche le osservazioni sismiche. si poteva in tal modo non solamente realizzare una rilevante economia, ma dare agli studi e alle osservazioni un indirizzo unico. fu perciò pensato di sottoporre i due servizi a un’unica alta direzione e riunirli sotto un solo ufficio centrale. […] mercé la grande attività e l’autorità del tacchini, il servizio geodinamico acquisterà il grande sviluppo che tutti riconoscono alla meteorologia italiana». (27) «purtroppo bisogna confessare che siamo stati scavalcati da altri paesi e che, continuando con l’andazzo finora seguito, l’italia non sarebbe mai stata nel caso di fare degli studi strettamente scientifici sui movimenti del suolo». (28) acs, maic, dga, versamento v, b. 963, fasc. 4234 «90. vulcanologia. affari generali. pubblicazioni estranee. lavori scientifici inviati al ministero e proposte di acquisto», 1883-1889. (29) «la commissione esprime parere che l’avvenire degli studi sismici in italia sia fondato sulla possibilità d’impiegare strumenti registratori i quali siano atti a segnare le tre componenti dei moti del suolo, in modo possibilmente indipendente dai moti propri dello strumento e colla maggior possibile sensibilità. sotto questo punto di vista essa raccomanda ai direttori degli osservatori principali di fornirsi al più presto possibile degli strumenti proposti nella relazione grablowitz e di sottoporli a un esame pratico, onde constatare quali di questi corrispondano meglio allo scopo sopracitato». (30) acs, maic, dga, versamento v, b. 967, fasc. 4249 «90/1. casamicciola. osservatorio geodinamico», 1884-1891. references caracciolo, a. (1960): stato e società civile. problemi dell’unificazione italiana (einaudi, torino), pp. 34-53. de cesare, c. (1868): l’amministrazione dell’agricoltura, dell’industria e del commercio durante l’anno 1868 (g. pellas, firenze). eramo, n. (1995): fonti per la storia della scienza e della tecnica negli archivi del maic, in gli archivi per la storia della scienza e della tecnica, international meeting, desenzano del garda, italy, 4-8 giugno 1991 (ministero per i beni culturali e ambientali ufficio centrale per i beni archivistici, rome), pp. 1019-1039. eramo, n. (1996): fonti per la storia delle acque di roma e del lazio, rivista storica del lazio, 4, 163-193. vol52,6,2009new 2-12-2009 12:39 pagina 564 vol48/04/2005def 633 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words co2 – h2o – solubility – mixed fluid – silicate melt – experimental data 1. introduction the knowledge of the distribution of volatile species between silicate melts and gases (or fluids) is crucial to understand degassing processes in magmatic systems. although natural fluids are complex multicomponent phases and may contain several mol% so2, h2s, hcl, hf and others, co2 and h2o are commonly the major fluid species exsolving from degassing magmatic melts and the system c-o-h is often solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models roman botcharnikov, marcus freise, francois holtz and harald behrens institut für mineralogie, uni hannover, germany abstract the effect of pressure, temperature, and melt composition on co2 and h2o solubilities in aluminosilicate melts, coexisting with co2-h2o fluids, is discussed on the basis of previously published and new experimental data. the datasets have been chosen so that co2 and h2o are the main fluid components and the conclusions are only valid for relatively oxidizing conditions. the most important parameters controlling the solubilities of h2o and co2 are pressure and composition of melt and fluid. on the other hand, the effect of temperature on volatile solubilities is relatively small. at pressures up to 200 mpa, intermediate compositions such as dacite, in which both molecular co2 and carbonate species can be dissolved, show higher volatile solubilities than rhyolite and basalt. at higher pressures (0.5 to 1 gpa), basaltic melts can incorporate higher amounts of carbon dioxide (by a factor of 2 to 3) than rhyolitic and dacitic melts. henrian behavior is observed only for co2 solubility in equilibrium with h2o-co2 fluids at pressures < 100 mpa, whereas at higher pressures co2 solubility varies nonlinearly with co2 fugacity. the positive deviation from linearity with almost constant co2 solubility at low water activity indicates that dissolved water strongly enhances the solubility of co2. water always shows non-henrian solubility behavior because of its complex dissolution mechanism (incorporation of oh-groups and h2o molecules in the melt). the model of newman and lowenstern (2002), in which ideal mixing between volatiles in both fluid and melt phases is assumed, reproduces adequately the experimental data for rhyolitic and basaltic compositions at pressures below 200 mpa but shows noticeable disagreement at higher pressures, especially for basalt. the empirical model of liu et al. (2004) is applicable to rhyolitic melts in a wide range of pressure (0-500 mpa) and temperature (7001200°c) but cannot be used for other melt compositions. the thermodynamic approach of papale (1999) allows to calculate the effect of melt composition on volatile solubilities but needs an update to account for more recent experimental data. a disadvantage of this model is that it is not available as a program code. the review indicates a crucial need of new experimental data for scarcely investigated field of pressures and fluid compositions and new models describing evident non-ideality of h-c-o fluid solubility in silicate melts at high pressures. mailing address: dr. roman botcharnikov, institut für mineralogie, uni hannover, callinstr. 3, d-30167 hannover, germany; e-mail: r.botcharnikov@mineralogie.unihannover.de 634 roman botcharnikov, marcus freise, francois holtz and harald behrens taken as an analogue for natural mixed fluids. however, even in this simplified system, the solubilities of volatile components in melts remain difficult to predict for natural aluminosilicate melts and controversial datasets can be found in the literature. apart from experimental or analytical problems, two main reasons explain the difficulty to elaborate a general model from the available experimental database: 1) the relative abundance of molecular species (e.g., co, co2, h2o, ch4, h2) in the fluids and their fugacities depend upon pressure, temperature and oxygen fugacity, which is notoriously difficult to control in high pressure and high temperature experiments; 2) the solubility mechanisms of volatiles in silicate melts depend on bulk composition of the melt. for example, carbon is mainly dissolved as molecular co2 in rhyolites but as carbonate in basalts, and both species are present in intermediate systems (e.g., holloway and blank, 1994). in this study we use the available experimental datasets and new results, presented here, to work out the possible effects of bulk compositions (rhyolite to basalt), pressure and temperature on the solubility of c-o-h species in common natural silicate melts. thereby, we consider only data from studies in which co2 and h2o have been the dominant species in the coexisting fluid phase. thus, the discussion is limited to f o2 conditions varying from oxidizing to moderately reducing. for example, at 1 gpa, 1400°c, co2 should be the dominant carbon species down to a log f o2 of delta nno-1 (holloway and blank, 1994). based on pioneering studies on water and co2 solubilities in silicate melts (e.g., burnham and davis 1971, 1974; holloway, 1987; stolper, 1982; silver and stolper, 1985; fogel and rutherford, 1990; blank et al., 1993; dixon et al., 1995), models for predicting co2 and h2o solubilities and equilibrium fluid compositions have been elaborated by holloway and blank (1994), dixon (1997), papale (1999) and newman and lowenstern (2002). the merit of the volatilecalc model proposed by newman and lowenstern (2002) is that the software is directly available and that it can be used easily to model magmatic processes in basaltic and rhyolitic systems. the authors recommend using the model up to 500 mpa (but not above) and examples of applications are given by lowenstern (2000, 2001). in addition, an empirical model was proposed by liu et al. (2004) to predict co2 and h2o solubilities in rhyolitic melts. applicability and limitations of the more recent models (papale, 1999; newman and lowenstern, 2002; liu et al., 2004) are discussed in the light of new experimental data. 2. generalities the solubility of volatiles in melts in equilibrium with a c-o-h-bearing fluid is commonly represented as shown in fig. 1 for rhyolitic melts. the xand y-axes in fig. 1 correspond to the amount of dissolved o-h species (expressed as wt% h2o) and of dissolved c-o species (expressed as wt% co2) in the silicate melt, respectively. the thick curves in fig. 1 represent the maximum amount of h2o and co2 which can be dissolved concomitantly in melts coexisting with c-o-h-bearing fluids. the intersections of the solubility curves with the xand y-axes of fig. 1 correspond to the solubility of pure h2o or co2, respectively. the composition of the fluid coexisting with the melt is given by the thin lines (for fixed mole fraction of water, x fh2o ). at a given pressure (p) and temperature (t), melts in equilibrium with water-rich fluids contain high water amounts and can only incorporate little co2 (the water activity in such systems is high) whereas melts equilibrated with co2-rich fluids contain less water and can incorporate higher co2 amounts (the water activity is low). the thick curves in fig. 1 show that volatile solubility is strongly dependent on pressure. the nonlinear shape, especially marked at high pressure (500 mpa) is the result of non-ideal solubility behavior of mixed c-o-h fluids in the silicate melt. in pioneering experiments performed at high pressure (up to 2 gpa), the addition of small amounts of water to a co2bearing fluid was found to increase the solubility of co2 (mysen et al., 1976). this cannot be observed in rhyolite melts up to 500 mpa (fig. 1) but does not necessarily disagree with the high pressure studies in which the amounts of 635 solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models dissolved c-o-h species are much higher (see discussion in holloway and blank, 1994). it must be noted also that water has a similar positive effect on solubility of noble gases in rhyolitic and basaltic melts as found experimentally by paonita et al. (2000) and modeled by nuccio and paonita (2000). the authors showed that the solubility of noble gases in silicate melts at 100 to 200 mpa and 1130 to 1160°c increases with h2o content of the melt and becomes almost constant when water concentration is higher than 3 wt%. the qualitative explanation of the noble gas solubility enhancement has been that new sites for noble gas atoms are created due to depolymerization of the silicate melt structure by dissolved h2o. it can be expected that a reactive molecule such as co2 shows an even more pronounced dependence on dissolved water in the melt. however, the discussion of incorporation mechanisms of h2o and co2 in silicate melt is beyond the scope of this paper which is restricted mainly to a review of the existing experimental and modeled data on the solubility of h2o and co2 in silicate melts. examples for using diagrams of the same type than fig. 1 for various melt compositions are given by holloway and blank (1994) and dixon et al. (1995). the curves in fig. 1 can be used to determine the p-t conditions at which magma starts to degas (provided that the amount of dissolved c-o-h species is known), to determine the partitioning of co2 and h2o between coexisting melts and fluids, and therefore the evolution of fluid compositions during degassing processes in open or closed systems (e.g., dixon and stolper, 1995). the recent model of newman and lowenstern (2002) allows us to calculate directly the evolution trends for volatile concentrations in melt and fluid phases. experimental datasets on volatile partitioning between fluids and melts at high t and high p are scarce. identification and quantification of equilibrium volatile species in fluids from quenched products are difficult. hence, most studies have been concentrated on the determination of the solubility curves only (the analysis of volatile concentrations in quenched glasses is less problematic). thus, we focused this review on the effects of p, t, and fig. 1. typical diagram illustrating solubility of volatiles in aluminosilicate melts in equilibrium with c-o-h fluids. the curves are based on the experimental datasets obtained by blank et al. (1993) at t = 850°c and p = 75 mpa and tamic et al. (2001) at t = 1100°c and 200 and 500 mpa for rhyolitic melts. 636 roman botcharnikov, marcus freise, francois holtz and harald behrens melt composition on the solubility of c-o-h species in silicate melts (thick lines in fig. 1). 3. solubilities of h2o and co2 as a function of temperature experimental datasets on solubilities of both carbon dioxide and water in rhyolitic silicate melts equilibrated with c-h-o fluids are available from the studies of blank et al. (1993) and of tamic et al. (2001). blank et al. (1993) performed experiments at 850°c and 75 mpa and tamic et al. (2001) at higher pressures of 200 and 500 mpa and temperatures of 1100°c and 800°c (figs. 1 and 2). in both studies, the co2 concentration was determined by infrared spectroscopy using the absorption coefficient of 1066 l·mol−1·cm−1 from blank (1993) for the absorption band at 2348 cm−1. recently, behrens et al. (2004a) proposed a new absorption coefficient (1214 l·mol−1·cm−1) for the molecular co2 band in hydrous rhyolitic glasses. thus, the reported data of tamic et al. (2001) and blank et al. (1993) have been corrected and are slightly lower than previously published by 12% relative. the best fits of the corrected experimental data are presented in fig. 2. at low pressure (75 mpa), the co2 content varies almost linearly with h2o content of the melt. with increasing pressure, deviation from linearity becomes more and more pronounced. temperature has no significant effect on the h2o-co2 solubility curve at 200 mpa but shifts the curve towards higher values at 500 mpa, at least in water-rich systems. to test the ability of the volatilecalc model of newmann and lowenstern (2002; further in the text referred to as n&l) to reproduce the experimental data, the calculated h2o and co2 concentrations in the rhyolitic melts are shown in fig. 2 as gray lines. the calculations have been carried out for the same temperatures and pressures as reported for the experimental data. fig. 2. temperature dependence of h2o and co2 solubility in rhyolitic melt at 200 and 500 mpa after tamic et al. (2001). the data of blank et al. (1993) are shown for comparison. all data have been corrected using the ir absorption coefficient for molecular co2 of 1216 l·mol−1·cm−1 after behrens et al. (2004a). gray lines are the calculated volatile solubilities by the model of newman and lowenstern (2002). note the opposite temperature effects observed in experimental and modeled solubility curves at 500 mpa. the predictions of the empirical model of liu et al. (2004) coincide with the fits of experimental data and not presented in the diagram (for detail see fig. 6 in the work of liu et al., 2004). 637 solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models the model of n&l always predicts a negative effect of temperature on volatile solubilities at pressures 200 and 500 mpa and this effect increases with pressure. this trend is in agreement with the experimental data at 200 mpa but it contrasts with the experimental findings for water-rich conditions at 500 mpa. since the model is based on experiments performed with pure h2o and co2 fluid phases (silver, 1988; silver et al., 1990; fogel and rutherford, 1990), and on the low-pressure data of blank et al. (1993), it reproduces data for mixed fluids with a good precision only at low pressures (deviations are almost in the range of the error bars). it is noteworthy that at 500 mpa, the model of n&l predicts almost linear solubility curves in the h2o-co2 solubility field and does not reproduce the non-linear solubility behavior of co2 and h2o in the c-h-o-rhyolite system. this is due to the assumption of an ideal behavior of volatiles and independence of h2o solubility on co2 concentration in the melt and vice versa in the n&l’s model. the empirical model of liu et al. (2004) gives a much better prediction of solubility trends, especially at high p. the calculated solubility curves coincide with the fitted experimental data (fig. 2). a rough estimation of the temperature dependence of h2o-co2 solubility in basaltic melts can be derived comparing the results of dixon et al. (1995) and jendrzejewski et al. (1997) obtained at 1200 and 1300°c, respectively. since solubility of volatiles is a strong function of pressure, it is possible to compare only a few experimental data obtained at 50 mpa. in these two studies, different absorption coefficients were used to calculate the amount of dissolved co2 from the peak height of the mid infrared carbonate band at 1522 cm−1. applying the same absorption coefficient for both studies (398 l·mol−1·cm−1, jendrzejewski et al., 1997) and using samples with similar water content (0.35 to 0.4 wt% h2o), the co2 solubility is 210 ppm at 1200°c (dixon et al., 1995) and 257 ppm at 1300°c (jendrzejewski et al., 1997). this suggests that temperature may have a small positive effect on co2 solubility at 50 mpa in basaltic melts. however, this conclusion is based on data from different laboratories and may be an artifact of different experimental conditions. in contrast, pan et al. (1991) noted a negative temperature dependence of pure co2 solubility at higher pressures (1.0 and 1.5 gpa). however, the variation of co2 solubility is small and remains constant within error over a temperature range from 1300 to 1600°c. it can be noted that, in the pressure range 0.5 to 3.5 gpa, a compilation of all available data for co2 solubilities in other silicate liquids coexisting with pure co2 confirms a general negative temperature effect on co2 solubility in a variety of compositions (ca-rich leucitite: thibault and holloway, 1994; albite: stolper et al., 1987, melilitite: brey, 1976; diopside: rai et al., 1983; ca-melilitite, mg-melilitite, phonolite, andesite: brooker et al., 2001; haplo-phonolite: morizet et al., 2002). 4. the effect of melt composition on h2o and co2 volatile solubilities it is well known that the solubilities of water and co2 in silicate melts are strongly dependent on the melt composition. in general, at given p and t, the h2o solubility increases whereas the co2 solubility decreases with sio2 content of the melt (e.g., blank and brooker, 1994; holloway and blank, 1994; brooker et al., 2001; king and holloway, 2002). to understand the compositional effects of the melt on the solubility of h2o and co2, experimental data obtained at identical pressures have to be compared. identical temperatures may be not strictly required because of the small temperature dependence of volatile solubilities. only few experimental datasets for silicate melts of different compositions at same pressure are available in the literature. the pressure range in which comparisons are possible is 75 mpa to 1 gpa. the existing data are mostly restricted to the rhyolitic and basaltic compositions (e.g., blank et al., 1993; dixon et al., 1995; tamic et al., 2001). one study illustrates volatile solubility in icelandite (composition close to andesite, jakobsson, 1997) at high temperature and pressure. in addition, king and holloway (2002) studied experimentally the solubility of h2o and co2 in water-poor (< 3.5 wt% h2o) andesitic melt at 1300°c and 1 gpa. recently, behrens 638 roman botcharnikov, marcus freise, francois holtz and harald behrens et al. (2004b) presented a dataset for dacitic melts investigated at 1250°c and 100, 200 and 500 mpa. in our comparison, we also used new experimental data for basaltic melts obtained at 1150°c, 500 mpa and 1200°c, 200 mpa which are presented in more detail in the appendix a. systematic datasets at same p and t are missing for pressures below 100 mpa and, hence, the direct examination of the difference in h2o and co2 solubility between silicic and mafic melt compositions is difficult. rough estimations are possible only at 75 mpa based on the data of blank et al. (1993) for rhyolite at 850°c and one sample of jendrzejewski et al. (1997) for basalt at 1300°c. the results show that the concentrations of dissolved co2 are higher in rhyolite than in basalt (450 ppm for rhyolite, corrected value, and 370 ppm for basalt), neglecting the temperature effect (which should be small at this pressure). the experiments of dixon et al. (1995), performed at 72 mpa and 1200°c, also indicate lower solubility of volatiles in basalt (290 ppm co2, value corrected). at 100 mpa, compositional trends can be extracted from three studies (fig. 3): two for basalt (pawley et al., 1992; jendrzejewski et al., 1997) and one for dacite (behrens et al., 2004b). note that we present only the data of pawley et al. (1992) for relatively oxidizing conditions assuming that co2 and h2o are the main fluid species. for low water contents, the data for basaltic melts indicate co2 solubility in the range 500-650 ppm. the solubility of both co2 and h2o is slightly higher in dacitic melt than in basaltic melt (fig. 3) in the co2-rich as well as in the h2o-rich part of the diagram. for comparison, the modeled solubilities (n&l) of c-o-h species in rhyolite and basalt (calculated for sio2 = 49wt%) are shown as gray lines in fig. 3. the model is within the experimental error for basaltic melts but tends to underestimate slightly the volatile solubilities. assuming that volatile solubilities in rhyolite and dacite do not differ strongly at 100 fig. 3. solubilities of h2o and co2 in basaltic (dots) and dacitic (dotted line) melts at 100 mpa. the dataset for dacitic melt and the experimental procedure is described in detail by behrens et al. (2004b). gray lines show modeled solubilities of volatiles in rhyolitic (dashed line) and basaltic (solid line) melt compositions. 639 solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models mpa (which is at least the case for pure water solubility), this underestimation is more pronounced for dacite and rhyolite melts. the available experimental data for different melt compositions at 200 mpa are summarized in fig. 4. assuming that the temperature effect is low, the solubilities of h2o and co2 in rhyolite and basalt do not differ significantly, except for water-rich compositions. at high water activities, water solubility is known to be higher in rhyolitic than in basaltic melts. the volatile solubility curves, calculated with the n&l model (basaltic system is modeled again for sio2 = 49wt%), predict lower values than the experimental data. it should be noted that dacitic melt shows a higher ability to dissolve co2 and h2o when compared to basalt and higher ability to dissolve co2 when compared to rhyolite. this difference may be related to the presence of both molecular co2 and carbonate species in dacitic melt. at 500 mpa total pressure, a similar co2 solubility gap between rhyolite and dacite can be observed (fig. 5). however, in contrast to lower pressures, basaltic melts have a much higher capacity to dissolve c-o species compared to rhyolitic and dacitic melts (figs. 3, 4 and 5). the increase in co2 concentration in basaltic melts is very pronounced with the first addition of co2 to the fluid. with a further increase in the mole fraction of co2 in the fluid (x f co2), the co2 solubility remains almost constant. the strong deviation from an ideal behavior in basalt melt suggests that water dissolved in basaltic melt may influence the solubility mechanism of co2 and stabilize carbonate groups (e.g., king and holloway, 2002). the co2 solubility in basalt melts for high x fco2 shown in fig. 5 is approximately two times higher than values extrapolated from data of pan et al. (1991). this may be related to the effect of oxygen fugacity influencing the fluid phase composition and particularly the fig. 4. the effect of silicate melt composition on solubilities of water and carbon dioxide at 200 mpa in the temperature range 1100-1250°c. the black lines are the best polynomial (2nd order) fits of experimental data. basalt composition (sc1, see table a.i. in appendix a) was investigated at 1200°c and mno-mn3o4 oxygen buffer (to vary mole fraction of h2o in the fluid phase, co2 was added as silver oxalate source, ag2c2o4). the description of experimental and analytical technique is presented in the appendix a. gray lines are modeled solubilities for rhyolite and basalt after newman and lowenstern (2002). 640 fig. 5. the solubility of h2o and co2 in melts of different compositions at 500 mpa. the experimental strategy used for the basaltic system (ob93-190) and description of the lines are given in appendix a and fig. 4. note the significant increase in co2 solubility in basalt (by a factor of 2 to 3) when compared with rhyolitic and dacitic compositions. fig. 6. available experimental data on h2o-co2 solubility at 1 gpa for melt compositions close to andesite. solid and dashed lines are the best fits of the data points obtained by jakobsson (1997) at 1400°c for icelandite (54.5 wt% sio2) and by king and holloway (2002) at 1300°c for andesite (59-60 wt% sio2). remarkable is that the solubility of co2 in icelandite at low x f h2o is almost independent on h2o content of the melt (or on water activity), which is similar to the observed solubility behavior of co2 in basaltic melt at 500 mpa and 1150°c (see fig. 5). roman botcharnikov, marcus freise, francois holtz and harald behrens 641 solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models co2/co ratio (e.g., pawley et al., 1992). the experiments plotted in figs. 4 and 5 were at strongly oxidizing conditions (log f o2 = nno + 3 if ah2o = 1.0) and those of pan et al. (1991) at more reducing conditions close to nno oxygen buffer (note however, that pan et al., 1991, estimated the molar ratio co2 /(co2+co) to be 0.93). experimental datasets obtained at identical conditions but different f o2 conditions over a wide range of fluid phase composition (x fh2o varying from 0 to 1) would help to understand volatile solubility laws in basaltic melts. the calculated volatile solubilities after newman and lowenstern (2002) are presented as gray lines in fig 5. the predictions are closer to the experimental data for rhyolite than for basalt. experimental datasets for andesitic melts obtained at 1 gpa and 1400°c (jakobsson, 1997; icelandite) and 1300°c (king and holloway, 2002) are plotted in fig. 6. despite the fact that the temperatures of the experiments are similar, the solubilities of h2o and co2 are found to be much higher in icelandite than in andesite. king and holloway (2002) attributed this difference to the amount of non-bridging oxygens in both melts, emphasizing that icelandite with 54.5 wt% sio2 has a higher nbo/t content (0.40) than andesite (60 wt% sio2) with a nbo/t of 0.30. it is also interesting to note that the solubility of co2 in icelandite at 1 gpa is approximately constant over a wide range of h2o content of the melt (at high x fco2), as was observed for basaltic melt at 0.5 gpa (see fig. 5). 5. pressure effect on h2o and co2 volatile solubilities the pressure effect on h2o-co2 solubilities in rhyolite, dacite and basalt melts can be estimated from the comparison of datasets shown on figs. 3-6. in general, pressure has a large positive effect on both h2o and co2 solubility. in detail, the dependence of co2 solubility on p is more pronounced in basalt (containing carbonates) than in dacite (containing molecular co2 and carbonates) and in rhyolite (containing only molecular co2). this is consistent with higher reaction volumes when co2 is dissolved in molecular form compared to its dissolution as carbonate in silicate melts (holloway and blank, 1994; behrens et al., 2004b). 6. limitations for using co2-h2o solubility models the comparison between calculated h2oco2 solubilities using the model of n&l and experimental results (fluids containing mainly co2 and h2o) shows that the model predicts solubilities within 10% relative at low pressure (at least up to 200 mpa). however, at higher pressure, the error of the model can be more than 20%, especially at low temperatures. furthermore, the temperature effect predicted for rhyolitic melts at 500 mpa is not consistent with the experimental data (fig. 2). in addition, the model does not reproduce the data for basaltic melt at 500 mpa. although experimental data are missing to test the model at pressures between 200 and 500 mpa, we suppose that the deviation of calculated data from realistic values increases with pressure. the experimental datasets obtained at 500 mpa and above show increasing non-ideal solubility behavior of c-oh species with increasing pressure (illustrated by the pronounced curvature of the solubility curves, figs. 5 and 6). the change in shape of the solubility curves with pressure is not reproduced in the model of n&l. this is a further indication that the ideal-mixing model of n&l is difficult to apply at pressures above 200 mpa. it can be noted that the model of papale (1999) is better able to reproduce the experimental data at 500 mpa and above, at least for rhyolitic melts (e.g., see fig. 6 in tamic et al., 2001). this can be attributed to the fact that the model does not assume ideal mixing in the fluid and melt phases but considers interaction between components. furthermore, using the recently published experimental data, the thermodynamic approach used by papale (1999) can be improved to predict accurately c-o-h volatile solubilities in silicate melts. in particular, there is a crucial need for solubility data above 200 mpa for intermediate and mafic melt compositions to calibrate the model of papale (1999). however, a disadvantage of this model is that it is not available as a program code. 642 given temperatures of 800 and 700°c. this diagram demonstrates clearly that the discussion of co2 and h2o solubilities in rhyolitic melt at 675°c in the range 100-400 mpa (as done by lowenstern, 2000, 2001) has no physical meaning because at those conditions rhyolitic melts exist only in a very small field of the diagram (p > 200 mpa; x fh2o > 0.8 and melt h2o content > 6 wt%). on the other hand, if cooling or decompression processes are relatively rapid and crystal nucleation and growth are relatively slow, the melt can be metastably preserved and local equilibria can be reached between gas bubbles and small batches of the melt. in this case, the low-temperature solubility models can be applied for such metastable phases but only on the local scale. acknowledgements we very much appreciated the reviews of n. metrich and a. paonita that greatly improved the quality of this paper. p. papale is acknowledged for editorial work. fig. 7. limitations for application of solubility models imposed by properties of rhyolitic systems. the curves of h2o-co2 solubility (thick solid lines) are from tamic et al. (2001) obtained at 800°c. the hatched area represents the subsolidus region of the eutectic composition in ab-or-qz system at 800°c. in this field, water activity is too low for melts to be stable (100% crystallization). the solidus as a function of p and x fh2o at 800°c has been drawn based on the data of johannes and holtz (1996). the area marked by the gray parallel lines and hatched area show the subsolidus field at 700°c. roman botcharnikov, marcus freise, francois holtz and harald behrens 7. limits for applications of the models recent studies have applied h2o-co2 solubility models to discuss the role of volatiles in the evolution of physical and chemical properties of ascending and crystallizing magmas. for instance, model solubility plots for h2oco2-rhyolitic melt system at 675°c and different pressures are shown and discussed by lowenstern (2000: fig. 2; and 2001: fig. 3). however, the use of such models requires a careful definition of the prevailing conditions and, in particular, kinetic aspects of cooling or decompression in silicic systems. it is well known that solidus temperatures in aluminosilicate systems depend upon water activity. at 800°c, 200 mpa and equilibrium conditions, rhyolitic melts with compositions close to the thermal minimum in the ternary system qzab-or crystallize if the mole fraction of water in the fluid phase (x fh2o) is below 0.4. this also explains why the data of tamic et al. (2001) obtained at 800°c are restricted to the water-rich part of the diagram in fig. 2. figure 7 shows the fields in which rhyolitic melts can be stable at table a.i. starting compositions (wt%). microprobe analysis of the starting glasses (a) sc1 (b) σ ob93-190 (c) σ sio2 48.34 0.29 48.84 0.54 tio2 2.86 0.05 2.75 0.07 al2o3 14.61 0.13 16.14 0.26 feotot 12.91 0.28 11.85 0.32 mno 0.17 0.05 mgo 6.40 0.11 5.86 0.13 cao 10.87 0.15 9.76 0.16 na2o 2.60 0.11 3.12 0.15 k2o 0.30 0.03 1.12 0.07 total 98.89 99.60 (a) glass compositions are average values from 10 measurements. (b) synthetic analogue of parental liquid of the skaergaard layered intrusion (brooks and nielsen, 1978). (c) natural alkali basalt from the mont crozier on the kerguelen archipelago. 643 solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models appendix a. experimental setup. a.1. starting materials and preparation of charges in this study, two basaltic compositions were used for experiments on the solubility of h-c-o mixtures in silicate melt. one starting composition (sc1) is a synthetic analogue of a ferrobasalt, which is assumed to be a parental magma of the skaergaard intrusion (dike c, brooks and nielsen, 1978). the starting powder was prepared from mixture of oxides (sio2, tio2, al2o3, fe2o3, mgo) and carbonates (caco3, na2co3, k2co3). the other sample is a natural alkali basalt (ob93-190) from the mont crozier on the courbet peninsula of the kerguelen island. this basalt represents an evolved lip magma derived from the kerguelen plume. a detailed description of the sample ob93190 can be found in damasceno et al. (2002). both starting materials were first ground in a ball mill to < 100 µm, then loaded in a pt crucible and fused twice (with grinding in between) for 1 h at 1600°c and 1 atm. the major element compositions of the obtained homogeneous and crystal-free glasses were determined by electron microprobe and are given in table a.i. the powder was sieved to the grain sizes of < 100 µm and 100-200 µm, and two fractions were mixed together in a ratio ∼1:1 to minimize the free volume between grains when powder is charged into the capsules. capsules were cleaned in acetone and annealed for 1 h at 1000°c at 1 atm before use. the capsules were welded shut at one side with an electric arc and filled with glass powder (ca. 20 mg), h2o and/or ag2c2o4. the purity of ag2c2o4 was tested using a gravimetric determination. the observed weight loss was 90% of the expected value. different h2o/co2 ratios were adjusted by varying the amounts of h2o and ag2c2o4. the total amount of fluid in the experiments was always higher than the expected fluid-saturation values of the melt. the filled capsules were cooled in liquid nitrogen, welded shut and heated in a drying furnace for 120 min at 200°c to decompose the ag2c2o4 to ag and co2 and to test for possible leakage. 644 table a.ii. experimental results. run xh2ofin h2o [wt%] (a) σ co2 [ppm] (b) σ ferrobasalt (sc1)/200 mpa/1200°c b17 1.00 4.67 0.10 b21 0.55 2.90 0.08 814 115 b10 0.44 2.43 0.11 782 64 b19 0.22 1.85 0.13 794 147 b6 0.24 2.00 0.12 992 109 b9 0.07 1.14 0.05 1033 66 b22 0.06 0.94 0.06 1006 52 b7 0.08 0.97 0.05 976 174 b8 0.05 0.82 0.05 1061 75 b20 0.02 0.72 0.05 1059 59 alkali basalt (ob93-190)/500 mpa/1150°c 148 1.00 9.54 0.48 149 0.62 6.47 0.32 6530 457 150 0.24 3.34 0.16 6610 462 151 0.09 2.36 0.11 7120 498 (a) h2o content of the glasses is determined by nir spectroscopy using the linear absorption coefficient ε= =0.56 l·mol−1·cm−1 for both ohand h2o bands at 4471 cm−1 and 5195 cm−1 after ohlhorst et al. (2001). (b) co2 content of the glasses is determined by mir spectroscopy using the linear absorption coefficient ε= =398 l·mol−1·cm−1 for the co32− bands at 1522 cm−1 after jendrzejewski et al. (1997). roman botcharnikov, marcus freise, francois holtz and harald behrens a.2. experimental equipment and run procedure all crystallization experiments were conducted in an internally heated pressure vessel (ihpv) pressurized with ar as pressure medium. a detailed description of the vessel can be found in berndt et al. (2002). the temperature in the ihpv was recorded with four unsheathed s-type thermocouples with a temperature gradient along the sample of ± 3°c. total pressure was recorded continuously with a burster type 8221 transducer (pressure uncertainty ± 1 mpa). all pressure and temperature data were logged automatically by a labview© monitoring system. each experiment consists of a set of 4 capsules which was brought directly to run temperature. the h2o/co2 solubility was investigated under intrinsic conditions of the ihpv (log f o2 ∼ qfm + 4) at pressures 200 and 500 mpa and temperatures 1200°c and 1150°c for sc1 and ob93-190 basalts, respectively (see table a.ii). the run duration was about 24 h for the alkali basalt (ob93-190) and 1-1.5 h for the ferrobasalt (sc1). after rapid quench, each capsule was weighted to check for leaks. a.3. analytical methods a conventional weight-loss method was applied to determine the mole fraction of water in the fluid phase (x h2o fl ): 1) the capsule was weighed; 2) the fluid phase was frozen by placing the capsule in a liquid nitrogen; 3) the capsule was pierced with a needle; 4) after warming to room temperature, the capsule was weighed again to determine the mass of co2 in the fluid and 5) the capsule was in silicate melts at high pressures, contrib. mineral. petrol., 57, 215-221. brooker, r.a., s.c. kohn, j.r. holloway and p.f. mcmillan (2001): structural controls on the solubility of co2 in silicate melts, part i. bulk solubility data, chem. geol., 174, 225-239. brooks, c.k. and t.f.d. nielsen (1978): early stages in the differentiation of the skaergaard magma as revealed by a closely related suite of dike rocks, lithos, 11, 1-14. burnham, c.w. and n.f. davis (1971): the role of h2o in silicate melts, i. p-v-t relations in the system naalsi3o8-h2o to 10 kilobar and 1000°c, am. j. sci., 270, 45-79. burnham, c.w. and n.f. davis (1974): the role of h2o in silicate melts, ii. thermodynamic and phase relations in the system naalsi3o8-h2o to 10 kilobar, 700° to 1000°c, am. j. sci., 274, 902-940. damasceno, d., j.s. scoates, d. weis, f.a. frey and a. giret (2002): mineral chemistry of mildly alkalic basalts from the 25 myr mont crozier section, kerguelen archipelago: constraints on phenocryst crystallisation environments, j. petrol., 43, 1389-1413. dixon, j.e. (1997): degassing of alkalic basalts, am. mineral., 82, 368-378. dixon, j.e. and e.m. stolper (1995): an experimental study of water and carbon dioxide solubilities in midocean ridge basaltic liquids, part ii. applications to degassing, j. petrol., 36, 1633-1646. 645 solubility of c-o-h mixtures in natural melts: new experimental data and application range of recent models placed in a drying oven at 110°c for 3-5 min and subsequently weighed to measure the mass of h2o lost from the capsule. an entrapment of atmospheric nitrogen in the experimental charge during preparation of the capsules was estimated to be quite low (tamic et al., 2001) and was not considered in the calculations. h2o and co2 contents in basaltic glasses were determined using infrared spectroscopy. volatilebearing glass slabs were ground and polished at both sides to a thickness of 150-200 µm for near-infrared (nir) and of 30-40 µm for mid-infrared (mir) spectroscopic measurements. the sample thickness was determined with an accuracy of ± 2 µm using a digital micrometer. ir absorption spectra were recorded using a bruker ifs 88 spectrometer equipped with an ir-scope ii microscope and an insb-mct sandwich detector (local resolution of 100 ×100 µm was adjusted with a slit aperture). dried air was measured as reference and 100 scans for background and sample measurement were collected. nir spectra in the range 6000-4000 cm−1 were recorded with a spectral resolution of 4 cm−1, using a tungsten lamp (nir) and a caf2 beam splitter. a globar light source and a kbr beam splitter were used for measuring mir spectra in the range 4000-1200 cm−1 with a spectral resolution of 2 cm−1. the h2o concentrations in the basaltic glasses were determined using the absorption bands at 5200 cm−1 and 4500 cm−1 for molecular h2o and oh-groups, respectively. absorbances were determined using linear background corrections for each peak. the total h2o concentration was calculated using lambert beer’s law as described by ohlhorst et al. (2001). the co2 concentration of the samples was measured using the band system at 1300-1600 cm−1 due to distorted carbonate groups. the peak height at 1522 cm−1 was determined after subtraction of a reference spectrum of a volatile-free sample normalized to same thickness. calculation of co2 concentration follows jendrzejewski et al. (1997). the results of analytical measurements are summarized in table a.ii and presented in figs. 4 (sc1; 200 mpa, 1200°c) and 5 (ob93-190; 500 mpa, 1150°c). references behrens, h., n. tamic and f. holtz (2004a): determination of molar absorption coefficient for the ir absorption band of co2 in rhyolitic glasses, am. mineral., 89, 301-306. behrens, h., s. ohlhorst, f. holtz and m. champenois (2004b): co2 solubility in dacitic melts equilibrated with h2o-co2 fluids – implications for modelling the solubility of co2 in silicic melts, geochim. cosmochim. acta, 68, 4687-4703. berndt, j., c. liebske, f. holtz, m. freise, m. nowak, d. ziegenbein, w. hurkuck and j. koepke (2002): a combined rapid-quench and h2-membrane setup for internally heated pressure vessels: description and application for water solubility in basaltic melts, am. mineral., 87, 1717-1730. blank, j.g. (1993) an experimental investigation of the behaviour of carbon dioxide in rhyolitic melt, ph.d. dissertation (california institute of technology, pasadena, ca). blank, j.g. and r.a. brooker (1994): experimental studies of carbon dioxide in silicate melts: solubility, speciation and stable isotope behavior, rev. mineral., 30, 157-186. blank, j.g., e.m. stolper and m.r. carroll (1993): solubilities of carbon dioxide and water in rhyolitic melt at 850°c and 750 bar, earth planet. sci. lett., 119, 2736. brey, g. (1976): co2 solubility and solubility mechanisms 646 roman botcharnikov, marcus freise, francois holtz and harald behrens dixon, j.e., e.m. stolper and j.r. holloway (1995): an experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids, part i. calibration and solubility models, j. petrol., 36, 1607-1631. fogel, r.a. and m.j. rutherford (1990): the solubility of carbon dioxide in rhyolitic melts: a quantitative ftir study, am. mineral., 75, 1311-1326. holloway, j.r. (1987): igneous fluids, rev. mineral., 17, 211-232. holloway, j.r. and j.g. blank (1994): application of experimental results to c-o-h species in natural melts, rev. mineral., 30, 187-230. jakobsson, s. (1997): solubility of water and carbon dioxide in an icelandite at 1400°c and 10 kilobar, contrib. mineral. petrol., 127, 129-135. jendrzejewski, n., t.w. trull, f. pineau and m. javoy (1997): carbon solubility in mid-ocean ridge basaltic melt at low pressures (250-1950 bar), chem. geol., 138, 81-92. johannes, w. and f. holtz (1996): petrogenesis and experimental petrology of granitic rocks (springer verlag), pp. 335. king, p.l. and j.r. holloway (2002): co2 solubility and speciation in intermediate (andesitic) melts: the role of h2o and composition, geochim. cosmochim. acta, 66, 1627-1640. liu, y., y. zhang and h. behrens (2004): solubility of h2o in rhyolitic melts at low pressures and a new empirical model to predict h2o and co2 solubility in rhyolitic melts, j. volcanol. geotherm. res., 143, 219-235. lowenstern, j.b. (2000): a review of the contrasting behavior of two magmatic volatiles: chlorine and carbon dioxide, j. geochem. explor., 69-70, 287-290. lowenstern, j.b. (2001): carbon dioxide in magmas and implications for hydrothermal systems, mineral. deposita, 36, 490-502. morizet, y., r.a. brooker and s.c. kohn (2002): co2 in haplophonolitic melt: solubility, speciation and carbonate complexation, geochim. cosmochim. acta, 66, 1809-1820. mysen, b.o., d.h. eggler, m.g. seitz and j.r. holloway (1976): carbon dioxide solubility in silicate melts and crystals, part i. solubility measurements, am. j. sci., 276, 455-479. newman, s. and j.b. lowenstern (2002): volatilecalc: a silicate melt-h2o-co2 solution model written in visual basic for excel, comput. geosc., 28, 597-604. nuccio, p.m. and a. paonita (2000): investigation of noble gas solubility in h2o-co2 bearing silicate liquids at moderate pressure, ii. the extended ionic porosity (eip) model, earth planet. sci. lett., 183, 499-512. ohlhorst, s., h. behrens and f. holtz (2001): compositional dependence of molar absorptivities of near-infrared ohand h2o bands in rhyolitic to basaltic glasses, chem. geol., 174, 5-20. pan, v., j.r. holloway and r.l. hervig (1991): the pressure and temperature dependence of carbon dioxide solubility in tholeiitic basalt melts, geochim. cosmochim. acta, 55, 1587-1595. paonita, a., g. gigli, d. gozzi, p.m. nuccio and r. triglia (2000): investigation of the he solubility in h2o-co2 bearing silicate liquids at moderate pressure: a new experimental method, earth planet. sci. lett., 181, 595-604. papale, p. (1999): modeling of the solubility of a two-component h2o+co2 fluid in silicate liquids, am. mineral., 84, 477-492. pawley, a.r., j.r. holloway and p.f. mcmillan (1992): the effect of oxygen fugacity on the solubility of carbon-oxygen fluids in basaltic melt, earth planet sci. lett., 110, 213-225. rai, c.s., s.k. sharma, d.w. meunow, d.w. matson and c.d. byers (1983): temperature dependence of co2 solubility in high pressure quenched glass of diopside composition, geochim. cosmochim. acta, 47, 953-958. silver, l.a. (1988): water in silicate glasses, ph.d. thesis (california inst. tech., pasadena, ca). silver, l.a. and e. stolper (1985): a thermodynamic model for hydrous silicate melts, j. geol., 93, 161-177. silver, l.a., p.d. ihinger and e. stolper (1990): the influence of bulk composition on the speciation of water in silicate glasses, contrib. mineral. petrol., 104, 142162. stolper, e. (1982): water in silicate glasses: an infrared spectroscopic study, contrib. mineral. petrol., 81, 1-17. stolper, e., g. fine, t. johnson and s. newman (1987): solubility of carbon dioxide in albitic melt, am. mineral., 72, 1071-1085. tamic, n., h. behrens and f. holtz (2001): the solubility of h2o and co2 in rhyolitic melts in equilibrium with a mixed co2-h2o fluid phase, chem. geol., 174, 333-347. thibault, y. and j.r. holloway (1994): solubility of co2 in a ca-rich leucitite: effects of pressure, temperature and oxygen fugacity, contrib. mineral. petrol., 116, 216-224. vol48/04/2005def 797 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words hydrothermal fluids – faults – cracks – fluid pressure – volcanic/hydrothermal areas – seismicity – vesuvius – campi flegrei 1. introduction faults and fractures play a major role in the localization and evolution of hydrothermal flow (sibson, 1996; curewitz and karson, 1997; riedel et al., 2001). this follows from the nearly ubiquitous close association of faults and hydrothermal outflows sites, which include hot springs, geysers, fumaroles, and diffuse degassing areas (chiodini et al., 2001a; craw et al., 2002; minissale, 2003; molin et al., 2003). similar evidence for fault control on hydrothermal activity is the close association between faults and mineral deposits (e.g., gypsum, anhydrite, quartz) arranged in veins or aligned spots (hannington et al., 2001; singlenton and criss, 2002; clark and james, 2003). fluids may also play a major role in triggering seismicity (sibson, 2000; collettini, 2002; tenthorey et al., 2003). as faults propagate, interact and link, hydrothermal circulation may evolve from scattered migrating outflow sites to stable discharge sites affecting a fault or the whole fault system. changes in the behavior of hydrothermal outflows near faults may reflect the evolution of the fault zone or of the hydrothermal systems (chiodini et al., 2001a; molin, 2003). then, the estimate of the fluid estimates of fluid pressure and tectonic stress in hydrothermal/volcanic areas: a methodological approach guido ventura (1) and giuseppe vilardo (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli, italy abstract an analytical approach to estimate the relative contribution of the fluid pressure and tectonic stress in hydrothermal/volcanic areas is proposed assuming a coulomb criterion of failure. the analytical procedure requires the coefficient of internal friction, cohesion, rock density, and thickness of overburden to be known from geological data. in addition, the orientation of the principal stress axes and the stress ratio must be determined from the inversion of fault-slip or seismic data (focal mechanisms). at first, the stress magnitude is calculated assuming that faulting occurs in ‘dry’ conditions (fluid pressure = 0). in a second step, the fluid pressure is introduced performing a grid search over the orientation of 1) fault planes that slip by shear failure or 2) cracks that open under different values of fluid pressure and calculating the consistency with the observed fault planes (i.e. strike and dip of faults, cracks, nodal planes from focal mechanisms). the analytical method is applied using fault-slip data from the solfatara volcano (campi flegrei, italy) and seismic data (focal mechanisms) from the vesuvius volcano (italy). in these areas, the fluid pressure required to activate faults (shear fractures) and cracks (open fractures) is calculated. at solfatara, the ratio between the fluid pressure and the vertical stress λ is very low for faults (λ = 0.16) and relatively high for cracks (λ = 0.5). at vesuvius, λ = 0.6. limits and uncertainties of the method are also discussed. mailing address: dr. guido ventura, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: ventura@ingv.it 798 guido ventura and giuseppe vilardo pressure required to activate a fault or a crack and of the tectonic stress field acting on a fault is of primary importance for the study of 1) active hydrothermal systems or paleo-flow systems and b) the analysis of the hydraulic structure of fault zones. this also has implications for the study of the hazard related to the seismic, volcanic, and hydrothermal activity (sibson, 1998; saccorotti et al., 2001; finizola et al., 2002). in this study, we propose an analytical method to obtain semi-quantitative estimates of the magnitude of tectonic stress and fluid pressure in fault zones. the method is based on the collection and elaboration (stress inversion) of structural data and/or data from focal mechanisms and allow us to estimate the magnitude of the principal tectonic stress (σ1, σ2, and σ3) and fluid pressure pf in active hydrothermal/tectonic areas. examples of application to areas characterized by extensional (solfatara volcano, campi flegrei, italy) and strike-slip tectonics (vesuvius, italy) are provided and the results are discussed. 2. analytical method jaeger and cook (1979) present an equation that shows the criteria for coulomb shear failure along a plane. rewriting their equation in terms of θ, the angle that the greatest principal stress σ1 makes with the plane of weakness gives (fournier, 1996) (2.1) where µ is the coefficient of internal friction and c is the cohesion, note that eq. (2.1) is valid if the fluid pressure pf = 0. if pf ≠ 0, σ1, σ2, σ3 may be determined assuming a coulomb criterion of failure and resolving the system of equations (2.2) (normal stress regime), or (2.3a) (strike-slip regime), or (2.3b)gh2 =v t gh1 =v t ( ) ( ) ( ) tan sin c 1 2 2 2v 1 3 3 = + v v n i i n v mv 6 7 @ a ( [ ] ) ( )tan sin c2 2 1 2 31 3 + = v v nv n i i6 @ (compressive regime) (2.3c) (2.4) (2.5) where ρ is the rock density, g is the acceleration of gravity, h is the thickness of overburden. this system may be resolved if a) ρ and h are known from geological data; b) r and the orientation of the principal stress axes are known from inversion of fault-slip data or focal mechanisms of earthquakes; and c) θ is determined from the angular relationships between the maximum principal stress and the preferred rupture plane(s). the analytical procedure to solve the system consists of three main steps. in a first step, the stress magnitude is calculated setting λ = 0 and c = 0, and assuming that faulting occurs on pre-existing planes of weakness. in a second step, pf is introduced performing a grid search over the orientation of fault planes that slip by shear failure under different values of λ and calculating the consistency with the observed fault planes (e.g., strike and dip of nodal planes from focal mechanisms) following the procedure of morris et al. (1996) and phillips et al. (1997). in this step and in the next step, c and µ are unchanged. in a third step, pf is varied performing a grid search over the orientation of planes that shear under different values of λ and calculating the consistency with the measured (faults) or determined (focal mechanisms) rupture planes. here below, this method is applied using structural data from the solfatara crater (campi flegrei, italy) and seismic data (focal mechanisms) from the vesuvius volcano (italy). in these areas, the fluid pressure required to activate faults (shear fractures) and cracks (open fractures) is calculated. 3. application 3.1. solfatara crater (campi flegrei) the campi flegrei is a nested caldera resulting from two large collapses related to the cam; tan p 2 1 v f = =n i m v ( ) ( ) r 3 2 1 3 = v v v v gh3 =v t 799 estimates of fluid pressure and tectonic stress in hydrothermal/volcanic areas: a methodological approach panian ignimbrite (39 kyr) and to the neapolitan yellow tuff (12 kyr) eruptions (fig. 1a; orsi et al., 1996). the campi flegrei magmatic system is still active and the last eruption occurred in 1538 a.d. at monte nuovo. faults affecting the campi flegrei caldera follow two preferred strikes, nw-se and ne-sw, which are the same as the faults affecting the campanian plain and the inner sectors of the apennine belt (hippolyte et al., 1994; orsi et al., 1996). the solfatara volcano (180 m a.s.l.; fig. 1b) is a 3.8 to 4.1 kyr old tuff-cone located ne of pozzuoli. the solfatara crater is affected by two main faults striking nw-se. outside the crater area, two nw-se striking faults cut the eastern sector of the tuff cone (fig. 1b). the solfatara nw-se striking fault segments show a maximum measured vertical offset equal to 40 m on the sw corner of the crater. in the crater area, ne-sw and nw-se striking cracks and fluid-filled veins occur (fig. 1b). results of a mesostructural study carried out to characterize the solfatara deformation pattern show that the faults have normal slips with pitch ≥ 75°. they move in response to a normal stress field characterized by a sub-vertical σl = σv (strike 264°, plunge 79°) and a ne-sw striking subhorizontal σ3 (strike 45°, plunge 8°) (fig. 1b). σ2 (strike 316°, plunge 7°) parallels the nw-se fault strike and r = 0.18 (chiodini et al., 2001a). this stress configuration is consistent with that obtained by zuppetta and sava (1993) from the analysis of focal mechanisms of earthquakes which occurred in 1982-1984. to verify if the fluid pressure plays a role in the kinematics of the solfatara faults, the above fig. 1a-d. a) sketch map of the campi flegrei caldera (from orsi et al., 1996). b) structural map of the solfatara area and summary of the mesostructural measurement in the solfatara area. fault-slip data and orientation of the stress axes are reported right down (schmidt net, lower hemisphere projection; data from chiodini et al., 2001). the azimuthal distribution of the strike of cracks in different selected sites is plotted on rose diagrams. each measurement site does not exceed 40 m2. c, d) results of the stress field analysis on the solfatara faults (c; λ = 0.16) and cracks (d; λ = 0.5). poles to measured fault planes (upper panel) and crack walls (lower panel) are reported as dots (schmidt net, lower hemisphere). a b d c 800 guido ventura and giuseppe vilardo described procedure is applied. in a first step, we set λ = 0 and ρ = 2000 kg/m3 (average density of the flegrei pyroclastics; barberi et al., 1991). µ was determined measuring the angle θ between the fault planes and the orientation of σ1 determined from the inversion of fault-slip data, i.e. θ∼30°. c was set to zero assuming that faulting occurs on pre-existing planes of weakness. this assumption is justified by the fact that the solfatara faults follow the same trend as the main structural discontinuity affecting the campi flegrei caldera. in a second step, we introduce pf performing a grid search over the orientation of fault planes that slip by shear failure under different values of λ and calculated the consistency with the nw-se fault planes following the method of morris et al. (1996). both c and µ are unchanged. the obtained results (fig. 1c) indicate that 1) at pf = 0, the variation of the stress magnitude with depth is ∆σ1= 20 mpa/km, ∆σ2= 9 mpa/km and ∆σ3= 6.7 mpa/km; 2) the differential stress is about 13 mpa at 1 km of depth; and 3) pf required to activate the nw-se faults is 3.3 mpa/km (fig. 1c). as a result, λ = 0.16. the effective stress magnitude (σ1eff = σ1−pf; σ2eff = σ2−pf; σ3eff =σ3−pf) are: σ1eff = 17 mpa/km, σ2eff = 6 mpa/km and σ3eff = = 3.7 mpa/km. we conclude that fluids play a minor role in the kinematics of the solfatara faults. however, both ne-sw and nw-se cracks and fluid-filled veins occur within the crater area. then, we vary pf performing a grid search over the orientation of planes that open (cracks) under different values of λ and calculating the consistency with the measured crack parameters (strike and dip of crack wall). results indicate that pf required to open both the nw-se and ne-sw cracks is of about 10 mpa/km (fig. 1d). the calculated λ value is 0.5. in summary, the obtained results indicate that pf < σ3 is required to activate the ne-sw faults by shear, whereas σ2∼pf >σ3 is required to open the ne-sw and nw-se striking cracks. 3.2. vesuvius volcano vesuvius volcano has been in a quiescent stage since 1944 and is characterized by a low temperature fumarolic activity (tmax=100°c). the vesuvius cone is located within a caldera formed during both sub-plinian (e.g. 1631; rosi et al., 1993) and plininan eruptions (e.g. 79 a.d.; santacroce, 1987). two main fault systems striking nw-se and ne-sw affect the volcanic edifice. vesuvius seismicity (about 200 events/yr; −0.4 ≤ md ≤ 3.6) concentrates within the caldera volume and is characterized by swarm type sequences. the hypocentral depths range from the top of the volcanic edifice up to about 6 km b.s.l. (fig. 2a). maximum event clustering occurs between 2 and 3 km b.s.l., at the interface between the sedimentary basement of the volcano (limestones and dolomites) and the intra-caldera volcanic rocks (fig. 2a; cassano and la torre, 1987), and where the present-day hydrothermal system is located (chiodini et al., 2001b). available focal mechanisms show prevailing strike-slip solutions (vilardo et al., 1996). correlations between the annual pattern exhibited by the time series of the events number and the seismic energy release indicate that pore pressure variations due to diffusion processes within the hydrothermal system are responsible for the lubrification of pre-existing fractures, so triggering seismicity (vilardo et al., 1999; saccorotti et al., 2002). the stress field acting within the vesuvius volcano was determined by bianco et al. (1998) and vilardo et al. (1999) using focal mechanisms from 1996-1997 time period. the inversion of 60 focal mechanisms of events located at depth between +1 and −6 km a.s.l. and md between 1 and 3.2 show that the vesuvius stress field is hetereogeneous. however, the majority of the solutions are consistent with a prevailing strike-slip stress field characterized by subhorizontal wnw-ese σ1, nne-ssw σ3 and subvertical σ2. according to bianco et al. (1998), this configuration reflects the action of the regional stress field on the volcano. here, the 35 available focal mechanisms of events with md ≥ ≥ 1.5 which occurred between 2 and 5 km b.s.l. from the data set of vilardo et al. (1999), which refers to 1996-1997 seismicity, are selected. the fault plane solutions are characterized by subvertical nodal planes following two main strikes: nnw-sse and ne-sw (fig. 2b). the inversion (sperner et al., 1993) of the selected 801 estimates of fluid pressure and tectonic stress in hydrothermal/volcanic areas: a methodological approach fault plane solutions is consistent with a strikeslip stress configuration characterized by σ1= 91° and 7° (trend and plunge), σ2 = 219° and 79°, σ3= 360° and 8° (fig. 2b), and r = (σ2+ −σ3)/(σ1−σ3) = 0.24. in a first step, we calculate stress magnitude in depth by setting σv = σ2 (strike-slip configuration), λ = 0 and ρ = 2600 kg/m3 (mean density of the vesuvius lavas; fig. 2a-c. a) schematic cross sections of the vesuvius volcano (form cassano and la torre, 1987, modified) with circles representing the hypocentral location of the 1986-1997 earthquakes. number of events versus depth is also reported. b) azimuthal distribution of angular parameters of nodal planes from 35 focal mechanisms of earthquakes with md ≥ 1.5 occurred in 1996-1997 period, and distribution of p and t axes and of the principal stress axes. c) results of the stress field analysis on the nodal planes from earthquakes (λ = 0.6). a b c 802 guido ventura and giuseppe vilardo cassano and la torre, 1987). µ was determined measuring the angle θ between the strike of nodal planes and the orientation of σ1. c was set to zero assuming that faulting occurs on preexisting planes of weakness. this assumption is justified by the fact that the nodal planes follow the same trends as the main structural discontinuity affecting the volcano basement. in a second step, we introduced pf performing a grid search over the orientation and slip direction of planes which slipped under different values of λ and calculating the consistency with the nodal planes following the procedure of morris et al. (1996) and phillips et al. (1997). the results obtained (fig. 2c) are: µ = 0.18 and λ = 0.6. effective principal stresses (σ1eff = σ1−pf ; σ2eff = = σ2−pf ; σ3eff = σ3−pf ) vary with depth with the following gradients: ∆σ1eff = 17.3 mpa/km, ∆σ2eff = 10 mpa/km and ∆σ3eff = 7.6 mpa/km. we conclude that fluid pressure plays a major role in triggering vesuvius seismicity. 4. discussion and conclusions the proposed analytical method is based on the assumption that some geotechnical data (e.g., cohesion; density) of the rocks are known. in the above reported examples, we assume that faulting occurs on pre-existing zones of discontinuity and set c = 0. however, this may be not true in hydrothermal zones, where self-sealing processes in fractures are common and mineralfilled fractures (veins) may act as barriers to fluid flow (e.g., thentorey et al., 2003). in addition, we assume no variations of density with depth. in the cases of vesuvius and campi flegrei, this is a reasonable assumption because significant variations in density with depth are lacking at least in the first 2-3 km below the volcano axis (cassano and la torre, 1987; rosi and sbrana, 1987). however, this may be an over-simplification in many hydrothermal and/or volcanic areas because of the occurrence of strong layering within both the shallow and deeper parts of the crust. an additional problem concerns the occurrence of areas subjected to heterogeneous stress sources. this is the case of volcanoes, whose structural features generally result from the superimposition of gravity, tectonic and magmatic induced stress. in this picture, a procedure of data selection must be implemented to select brittle structures related to tectonic and fluid-induced stresses alone, without the contribution of gravity. taking into account the above reported limitations, the results of analysis presented here may give useful information on the relative contribution of tectonic stress and fluid pressure in triggering faulting and cracking in volcanic/hydrothermal areas. examples from solfatara and vesuvius show that hydrothermal fluids play a major role in triggering vesuvius seismicity and in opening of the solfatara cracks, whereas they play a minor role in the dynamics of the solfatara faults. in particular, the solfatara cracks may open only by fluid overpressure, i.e. by hydrofracturing. these results are consistent with the conclusions from independent geochemical and seismological studies on solfatara and vesuvius (chiodini et al., 2001a,b; saccorotti et al., 2001, 2002). the analytical method proposed here may be applied to other non-volcanic areas where there is geophysical/geological evidence of fluid involvement (collettini, 2002). acknowledgements we are grateful to giovanni chiodini, gilberto saccorotti and edoardo del pezzo for the numerous discussions about the problems of hydraulic fracturing and tectonic stress in hydrothermal/volcanic areas. we also thank laura crispini for review of an early version of the manuscript and roberto moretti for the editorial handling. references barberi, f., e. cassano, p. la torre and a. sbrana (1991): structural evolution of campi flegrei caldera in light of volcanological and geophysical data, j. volcanol. geotherm. res., 48, 33-49. bianco, f., m. castellano, g. milano, g. ventura and g. vilardo (1998): the somma-vesuvius stress field induced by regional tectonics: evidences from seismological and mesostructural data, j. volcanol. geotherm. res., 82, 199-218. cassano, e. and p. la torre (1987): geophysics, in somma-vesuvius, edited by r. santacroce, quad. ric. sci., 114, 175-195. 803 estimates of fluid pressure and tectonic stress in hydrothermal/volcanic areas: a methodological approach chiodini, g., f. frondini, c. cardellini, d. granieri, l. marini and g. ventura (2001a): co2 degassing and energy release at solfatara volcano, campi flegrei, italy, j. geophys. res., 106 (b8), 16,213-16,222. chiodini, g., l. marini and m. russo (2001b): geochemical evidence for the existence of high-temperature hydrothermal brines at vesuvio volcano, italy, geochim. cosmochim. acta, 65, 2129-2147 clark, c. and p. james (2003): hydrothermal brecciation due to fluid pressure fluctuations: examples from the olary domain, south australia, tectonophysics, 366, 187-206 collettini, c. 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(editor) (1987): somma-vesuvius, quad. ric. sci., 114 (8), 1-251. sibson, r.h. (1996): structural permeability of fluid-driven fault-fracture meshes, j. struct. geol., 18, 1031-1042. sibson, r.h. (1998): brittle failure mode plots for compressional and extensional tectonic regimes, j. struct. geol., 20, 655-660. sibson, r.h. 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a. and a. sava (1993): stress pattern at campi flegrei from focal mechanisms of the 1982-1984 earthquakes (southern italy), j. volcanol. geotherm. res., 48, 127-137. annals 48, 2, 2005defdef 301 annals of geophysics, vol. 48, n. 2, april 2005 key words paleomagnetism – sardinia – non-sulfide ores – calamine 1. introduction non-sulfide ores, mainly the supergeneformed types, are rapidly becoming an important source of metallic zinc as well as lead (large, 2001; boni, 2003; hitzman et al., 2003). however, time constraints for the deposition of these ores are still unclear, due to multiple oxidation events through time, which control their silicates/carbonates paragenesis. because the timing and evolution of weathering profiles might have important implications for the exploration of non-sulfide deposits, it is a priority to obtain a reliable estimate for the age of the oxidation phenomena, either by use of radiogenic isotope systems, or by other unconventional methods. in this paper, we report on a first attempt made to use paleomagnetism for constraining the age of the non-sulfide zn-pb ore in the iglesientesulcis district (sw sardinia, italy), where the oxidation of primary sulfides has been related to paleoweathering episodes dating back to cenozoic and even mesozoic times (boni et al., 2003). 2. paleomagnetism and ores in the last twenty years, a major effort has been made in dating stratabound, disseminated and massive sulfide ore deposits, as well as their paleomagnetic dating of non-sulfide zn-pb ores in sw sardinia (italy): a first attempt maria boni (1), jaume dinarès-turell (2) and leonardo sagnotti (2) (1) dipartimento di geofisica e vulcanologia, università degli studi di napoli «federico ii», napoli, italy (2) istituto nazionale di geofisica e vulcanologia, roma, italy abstract a first paleomagnetic investigation aimed at constraining the age of the non-sulfide zn-pb ore deposits in the iglesiente district (sw sardinia, italy) was carried out. in these ores, the oxidation of primary sulfides, hosted in cambrian carbonate rocks, was related to several paleoweathering episodes spanning from the mesozoic onward. paleomagnetic analyses were performed on 43 cores from 4 different localities, containing: a) non-oxidized primary sulfides and host rock, b) oxidized fe-rich hydrothermal dolomites and (c) supergene oxidation ore («calamine»). reliable data were obtained from 18 samples; the others show uninterpretable results due to low magnetic intensity or to scattered demagnetization trajectories. three of them show a scattered characteristic remanent magnetization (chrm), likely carried by the original (i.e. paleozoic) magnetic iron sulfides. the remaining 15 samples show a well defined and coherent chrm, carried by high-coercivity minerals, acquired after the last phase of counterclockwise rotation of sardinia (that is after 16 myr), in a time interval long enough to span at least one reversal of the geomagnetic field. hematite is the main magnetic carrier in the limestone, whereas weathered hydrothermal dolomite contains goethite or a mixture of both. the results suggest that paleomagnetism can be used to constrain the timing of oxidation in supergene-enriched ores. mailing address: dr. leonardo sagnotti, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: sagnotti@ingv.it 302 maria boni, jaume dinarès-turell and leonardo sagnotti host rocks with paleomagnetic methods (symons et al., 1996, 2002; symons and stratakos, 2000; leach et al., 2001; bradley and leach, 2003). these authors claim, though not without debate (kesler et al., 2004), that a characteristic remanent magnetization (chrm), acquired during the ore minerals forming processes, can be recognized in most ore deposits. in these studies, magnetic susceptibility, demagnetization of the natural remanent magnetization, acquisition of artificial remanences and other magnetic measurements were performed on several thousand specimens of mainly mississippi valley type (mvt) mineralization and gangue, in order to identify their magnetic carriers. thermal demagnetization and specific rock magnetic analyses show that the chrm in most deposits, as well as in monomineralic specimens, is carried by single or pseudosingle domain pyrrhotite and/or magnetite. almost all the paleomagnetic studies used several stability tests (conglomerate, fold, contact and reversals test) to constrain the acquisition age of ore remanence (symons et al., 1996). the main conclusion is that the paleomagnetic ages of most sulfide deposits seem to be coeval with the later stages of major orogenic events in the same region. few investigations have been undertaken so far to date non-sulfide deposits, with the exception of few economic laterites and bauxites (mainly aimed to broader geodynamic and/or paleoclimatic studies) and of proterozoic manganese deposits (evans et al., 2001). 3. geological setting of sw sardinia and the non-sulfide zinc-lead deposits 3.1. geological setting the iglesiente-sulcis area in sw sardinia (fig. 1) is one of the oldest mining districts in the world, where exploitation started in pre-roman times, initially for silver-lead-copper and later for zinc and barium deposits. due to increasing economic difficulties, the last producing mine in the area was closed in 1998. sw sardinia is dominated by paleozoic lithotypes, of sedimentary as well as igneous origin, followed by cenozoic and quaternary volcanics and sedimentary rocks. among the paleozoic rocks, cambro-ordovician lithologies (limestones and dolomites of the gonnesa group) predominate (bechstädt and boni, 1994). the cambrian sediments are the preferential host for both primary and secondary (oxidated) zn-pb ores. most of the primary ores are stratabound (sedex + mvt): they consist of zn-pb sulfides locally associated with barite, with hydrothermal alteration limited to local dolomitization and silicification (boni, 1985; bechstädt and boni, 1994; boni et al., 1996). a strongly pervasive hydrothermal dolomitization («geodic dolomite»: boni et al., 2000) of possible permo-triassic age affected large volumes of the cambrian carbonate rocks across the whole district (more than 500 km2). these dolomites are fe-rich, both replacive and saddleshaped, frequently with zebra structures. the late variscan uplifts in sardinia were followed by the inset of small porphyry stocks and several pulses of extensional tectonics, which caused repeated circulation of hydrothermal fluids during the mesozoic (boni et al., 1992, 2002), accompanied by deep karstification of the cambrian carbonate rocks, which continued during the cenozoic. the development of karstic networks in the cambrian limestone, was favored in all periods by the aggressive character of the circulating waters and by the high sulfide content of the carbonate rocks undergoing dissolution. a phase of widespread tensional tectonics in the middle oligocene produced a large rift system locally accompanied by the deposition of continental sediments. this was followed by a major episode of oligocene-miocene volcanism, correlated to a drift-related ∼50° counterclockwise rotation of the corsica-sardinia microplate (gattacceca, 2001). the spreading in the liguro-provençal basin and the rotation of corsica-sardinia, ended no earlier than 16 myr (early langhian; see speranza et al., 2002) in response to the retreat of a westward dipping subducting slab (i.e. faccenna et al., 2004). during miocene and pliocene, sw sardinia was subjected to several emersion-erosion phases, alternating with marine transgressions, dramatic climatic changes (messinian evaporites) and volcanic episodes of calc-alkaline to alkaline basalts. a last plio-pleistocene tensional 303 paleomagnetic dating of non-sulfide zn-pb ores in sw sardinia (italy): a first attempt tectonic phase was responsible for the differentiated uplift of the paleozoic basement, which was fragmented into a series of stepped fault blocks in both the iglesiente and sulcis areas. 3.2. non-sulfide deposits the «calamine»-type ore is considered to be the result of supergene oxidation of primary carbonate-hosted sulfide mineralization, with subsequent remobilisation of zinc and partly of lead, followed by redeposition of secondary oxidized minerals either in situ as gossans, or as reaction products within the hosting carbonates. in the latter case, they not only replace substantial areas of the fractured and karstified carbonate rocks, but fill as internal sediments their dissolution vugs and karst cavities, producing secondary enrichments also at greater distances from the primary sulfide bodies (large, 2001; hitzman et al., 2003). historically, the «calamines», consisting of a mixture of zinc carbonates and hydroxy-carbonates and silicates (with minor pb), capping in the whole area the primary sulfide bodies, were the principal source of zinc of the iglesiente-sulcis district (moore, 1972; boni et al., 2003). the most important mines (fig. 1), where non-sulfide ores have been exploited for several hundred of meters in depth from the present surface, were: san giovanni, monteponi and campo pisano in the iglesias valley, as well as the nebida-masua mineralized complex along the western coast. smithsonite, hydrozincite, and hemimorphite were the principal zinc-bearing economic minerals. cerussite, anglesite and the rare mineral phosgenite were also common, fig. 1. geological sketch map of the iglesiente mining district, with the location of the sampled sites (modified from boni et al., 2003). abbreviations: 1 – overthrust; 2 – normal fault; 3 – cenozoic; 4 – mesozoic; 5 – variscan granites; 6 – palaeozoic (allochthonous); 7 – ordovician to devonian succession; 8 – iglesias group (middle cambrian-lower ordovician); 9 – gonnesa group (lower cambrian); 10 – nebida group (lower cambrian). 304 maria boni, jaume dinarès-turell and leonardo sagnotti generally associated with nodules and lenses of residual galena and a range of fairly exotic species (billows, 1941; moore, 1972; stara et al., 1996; aversa et al., 2002). a complex assemblage of iron and manganese oxy-hydroxides characteristic for its red-brown staining (goethite, lepidocrocite, hematite) and residual clay minerals accompany the non-sulfide ore. an interesting parallel effect of the oxidation of the sulfide ore, is the supergene alteration of the ferriferous hydrothermal dolomites («geodic dolomite»), which are the host rock of most sulfide deposits in the iglesiente district. in fact, these dolomites, are not only fe-rich (some of them approach ankerite composition), but contain small pyrite grains. when affected by supergene processes, the «geodic dolomite» turns reddish-brown, its disseminated pyrites alter to goethite and hematite and a patchy de-dolomitization process starts locally. these effects in the dolomite host rocks can be followed deep down, at least until the same levels in the mines where the «calamine» mineralization has been exploited (e.g., level −100 of the monteponi mine). we emphasize that we consider the weathering of the «geodic dolomite» not related to recent surficial weathering only, but contemporary with the formation of «calamine» ore. when investigating the timing of the nonsulfide deposits in a given area, one must consider the ages of the various stages of tectonic uplift and peneplanation. to document the major constraints on the oxidation and preservation of the deposits, it is also necessary to relate them to climatic oscillations both at a global and local scale. what makes this task difficult in sw sardinia, is the lack of precise geological constraints on the development of the numerous karstification phases that are closely related to formation of the «calamine» ores. karstic dissolution related to uplift of the paleozoic carbonates, occurred repeatedly: 1) during permo-mesozoic, 2) several times during cenozoic, and 3) after the plio-pleistocene block tectonics until the present. however, the most reliable time span we can envisage from the limited geological constraints, in which both tectonic and climatic conditions were favorable for the formation of the non-sulfide deposits ranges from middle eocene to plio-pleistocene. the latter was the time when the differentiated uplift of distinct sectors of the paleozoic basement displaced at very different levels the base of the weathering profiles in the various areas of the iglesiente-sulcis. our aim was to use paleomagnetism to provide constraints on the age(s) of the oxidation process, evaluating the characteristic remanent magnetization carried by various iron minerals, like hematite, goethite and, in case, of the remaining as well as of newly formed magnetic iron sulfides. sardinia has been the subject of several paleomagnetic studies since the 1960’s and a reliable dataset of paleomagnetic results is available (see van der voo, 1993; speranza, 1999; speranza et al., 2002 and references therein), referring to a wide age range (paleozoic to present), to provide a suitable reference for comparison with the new data. 4. sampling strategy samples were collected using a portable gasoline-powered drill and were oriented in situ with a compass mounted on an inclinometer. 43 oriented cores were collected from 4 sites in the surroundings of iglesias, either on outcrops (sites ig01 and ig04) or inside two abandoned mines (sites ig02 and ig03). site ig01 weathered «geodic dolomite» along the «strada camionabile» (nebida) – the sampling took place shortly east of the village of nebida, in a small valley cut in the carbonates (39°20′n-8°27′e), where patchy outcrops of hydrothermal dolomite («geodic dolomite») on the cambrian limestone are clearly exposed. the contacts with non-dolomitized host rock are sharp, locally marked by a slight silicification. vertical schistosity planes mostly control the dolomite bands; the control is rarely made by fractures. the crystals are typically saddleshaped, lining concretionary voids and small horizontal cavities. the weathered hydrothermal dolomite is brown-reddish due to its high fe-oxide content. locally, the dolomitization affects a postvariscan breccia, whose carbonate clasts show schistosity and traces of primary sulfide crys305 paleomagnetic dating of non-sulfide zn-pb ores in sw sardinia (italy): a first attempt tals, that have also undergone weathering and oxidation processes. however, some of the breccia clasts have not been dolomitized and still consist of white cambrian limestone. 21 cores were sampled at this site, in varying lithologies: 6 samples represent the cambrian limestone, with oxidized sulfides, 9 samples were collected from the weathered «geodic dolomite» and 6 samples were drilled in the carbonate clasts, with traces of primary sulfides crystals, composing a brecciated patch. site ig02 «zona ossidati piombiferi», level + 150, san giovanni mine (iglesias) – the sampling was performed underground in the oxidation area of idina-sant’anna of the san giovanni mine (39°15′n8°27′e). these are the upper levels of the mine and contain only local remnants of sulfides (galena), prevailing fe-oxideshydroxydes, clay-infillings and limited concentrations of znand pb-carbonates. it is probable that in this area of the mine, the primary orebodies consisted not only of the stratabound zn-rich pre-variscan mineralizations, but also of the post-variscan (ag)-galena bodies in paleokarst breccias. host rocks are the cambrian limestones and the weathered hydrothermal dolomites. 7 cores were sampled at this site: 5 of them represent the mineralized dolomite breccias, 1 was drilled in a clast of cambrian limestone and 1 in the weathered hydrothermal dolomite. site ig03 zona «calaminari», level +92, nebida mine (nebida) – the sampling has been performed underground at the +92 level of the nebida mine (39°25′n-8°25′e). in this level, situated well above the present water table, there are almost no sulfides left, whereas in the lower levels, a continuous transition to zn-pb sulfides takes place. the direction of the oxidized ore bodies, which replace the primary sulfides as well as part of the carbonate host rocks, is n-s, which is the general tectonic trend of the whole area. the carbonate host rock is heavily dolomitized («geodic dolomite» replacing hydrothermally cambrian limestone), but also karstified and altered to a mixture of de-carbonated material, zn-carbonates and hydrocarbonates, zn-silicates, pb-carbonates and sulfates, and fe oxides/hydroxides. beyond the described occurrences of non-sulfide mineralization at the level +92, allochthonous concentrations of ferruginous, «earthy» smithsonite and hemimorphiterich clays, fill a maze of interconnecting karst cavities and open conduits in this, as well as in the other upper levels of the mine. 9 cores were sampled at this site, all in the dolomite hosting the non-sulfide zn ores. site ig04 – san giovanni mine (iglesias) – in this site, behind the old building of the mine direction (39°15′-8°27′), only the white limestone belonging to the upper stratigraphic part of the lower cambrian succession (gonnesa group) was sampled. in this outcrop the limestone is strongly deformed by variscan tectonics (penetrative schistosity and recrystallization), not dolomitized neither mineralized. in this site 6 cores were sampled. 5. methods in the laboratory two to three standard cylindrical specimens of 2.54 cm in diameter and 2.20 cm length were cut from each sample. natural remanent magnetization (nrm) was measured on a 2g enterprises dc squid cryogenic magnetometer operating in a magnetically shielded room at the istituto nazionale di geofisica e vulcanologia (ingv) in rome, italy. a pyrox oven located in the shielded room was used for thermal demagnetization. alternating field (af) demagnetization was performed with three orthogonal coils installed inline with the cryogenic magnetometer. we first analysed the behaviour of 10 couples of pilot «sister» specimens (i.e. standard paleomagnetic cylinders cut from the same core). of each couple, one specimen was subjected to stepwise af demagnetization up to 100 mt, the other to thermal demagnetization up to 650ºc. on the basis of the pilot study results, the remaining specimens followed an hybrid af and thermal demagnetization protocol. in this case, stepwise af demagnetization was first applied and consisted in 9 steps at increments of 5 mt in the interval 5 to 30 mt and then increments of 10 mt up to 60 mt. samples were subsequently demagnetized at 60°c, 80°c, 100°c, 120°c, 200ºc, 300°c, 400°c, 500°c, 306 maria boni, jaume dinarès-turell and leonardo sagnotti 580°c, 610°c and 640°c. magnetic susceptibility was measured after each heating step in order to monitor alteration and/or neoformation of magnetic minerals during the thermal treatment. the af followed by thermal demagnetization protocol was designed as the most efficient demagnetization treatment, which could also provide information on magnetic mineralogy, so that the chrm components carried by high-coercivity minerals having low (goethite) and high (hematite) unblocking temperatures could be easily identified. low-coercivity minerals like magnetite and magnetic iron sulfides (pyrrhotite, greigite), instead, would have been mostly demagnetized below 60 mt upon the af routine. orthogonal vector demagnetization plots (zijderveld, 1967) were used to represent demagnetization data, and principal component analysis (kirschvink, 1980) was used to identify the characteristic remanent magnetization. the mean directions were calculated using fisher (1953) statistics. the reversal test of mcfadden and mcelhinny (1990) was applied to assess the antipodality of the normal and reverse means. 6. results the af demagnetization behaviour indicated that for most samples the main magnetic carriers are high-coercivity minerals only (i.e., the nrm intensity shows very low to null decrease in af up to 100-150 mt). for such samples, therefore, magnetic components were retrieved after thermal demagnetization (figs. 2a-c to 5a-c). the measurements of magnetic susceptibility after fig. 2a-c. stepwise demagnetization data for sample ig01-01a (non-dolomitized brecciated limestone, nebida «camionabile»). from left to right: a) normalized intensity plot, the intensity of the natural remanent magnetization (nrm) at the beginning of the treatment is also indicated; b) equal area projection in geographic coordinates (open circles – upper hemisphere, reverse polarity); c) orthogonal vector diagrams in geographic coordinates (open circles – vertical projection; full circles – horizontal projection). the af demagnetization treatment (upper diagrams) essentially did not affect the nrm of the sample, whereas the sharp drop in nrm intensity during the thermal treatment between 610°c and 640°c (lower diagrams) indicate that the characteristic component of the nrm is of reverse polarity and is carried by hematite. a b c c a b 307 paleomagnetic dating of non-sulfide zn-pb ores in sw sardinia (italy): a first attempt fig. 3a-c. stepwise demagnetization data for sample ig01-03b (non-dolomitized brecciated limestone, nebida «camionabile»). plots and symbols as in fig. 2a-c (full circles in the equal area projection (b) indicate projection on the lower hemisphere, normal polarity). the sample shows essentially the same behavior of sample ig01-01 (see fig. 2a-c), but its characteristic component of the nrm is of normal polarity. fig. 4a-c. stepwise demagnetization data for sample ig02-07a (san giovanni mine, level +150). plots and symbols as in fig. 2a-c. the nrm of the sample is essentially not affected by af treatment and shows a sharp drop in intensity just after the first heating step (60°c) and is almost fully demagnetized at 120°c, indicating that the characteristic component of the nrm is of reverse polarity and is carried by goethite. a a b b c c a b c a b c 308 maria boni, jaume dinarès-turell and leonardo sagnotti each heating step indicate that neoformation of magnetic minerals occurs above 500°c. in any case, no significant change in paleomagnetic direction was detected in association to such thermal alteration. reliable paleomagnetic data were obtained from 18 samples out of a total of 43 (table i). no tectonic correction was applied to the paleomagnetic data. the white-coloured, non-mineralized limestones (samples 13-18 taken in the carbonate clasts with traces of primary sulfides crystals at site ig01-nebida and all samples from site ig04-san giovanni), show very low nrm intensities (of the order of a few 10–5 a/m) and irregular behaviour during progressive demagnetization. no reliable paleomagnetic data could be obtained from these samples. the weathered hydrothermal dolomites sampled at nebida, site ig01, are generally reddish and show demagnetization behaviours indicating that their main magnetic carriers are high-coercivity minerals only (the nrm intensity shows very low to null decrease in af up to 150 mt). the behaviour during thermal demagnetization indicates that dolomite samples contain a mixture of goethite (indicated by a significant loss of the nrm intensity at 80-120°c; i.e. dekkers, 1988, 1989) and prevailing hematite (as indicated by complete demagnetization at temperatures of 640-680°c; i.e. dunlop, 1971). the prevalence of hematite is evident in the non-dolomitized limestones containing weathered crystals of former pyrite, sampled in the same locality (i.e. ig01-01 and ig01-03, figs. 2a-c and 3a-c). the narrow range shown by the unblocking temperature of hematite is possibly related to its pseudomorphic nature on precursor pyrite crystals. the mineralized limestones lacking sulfides sampled in both mines (sites ig02 and ig03) are also characterized by high-coercivity magnetic fig. 5a-c. stepwise demagnetization data for sample ig02-02a (san giovanni mine, level +150). plots and symbols as in fig. 2a-c. the nrm of the sample is reduced at ca. 50% of its initial value after af treatment in peak field up to 100 mt. the following thermal treatment pointed out a sharp drop in intensity between 300°c and 400°c, after which the sample is essentially demagnetized, indicating that the characteristic component of the nrm is likely carried by intermediate-coercivity magnetic iron sulfides (pyrrhotite ?). the direction of the characteristic remanence is however aberrant. a b a c cb 309 paleomagnetic dating of non-sulfide zn-pb ores in sw sardinia (italy): a first attempt table i. summary of reliable paleomagnetic data. specimen lithology decl chrm incl chrm mad mag. min. (°) (°) (°) ig01-01 cambrian limestone 179.6 − 53.9 4.9 hematite ig01-02 cambrian limestone 12.9 44.5 3.1 hematite ig01-03 cambrian limestone 3.1 50.5 2.8 hematite ig01-04 cambrian limestone 17.6 47.5 4.0 hematite ig01-05 cambrian limestone 356.8 50.7 5.8 hematite ig01-07 weathered dolomite 347.7 48.2 7.0 magnetite ig01-09 weathered dolomite 182.9 − 67.9 3.2 hematite ig01-12 weathered dolomite 3.4 52.0 8.5 goethite + hematite ig01-20 weathered dolomite 352.4 51.1 5.3 goethite + hematite ig01-21 weathered dolomite 8.7 33.1 4.4 goethite ig02-01* mineralized dolomite breccia 160.9 − 26.1 5.9 ? fe-sulfides ig02-02* mineralized dolomite breccia 222.1 47.3 3.3 ? fe-sulfides ig02-03 mineralized dolomite breccia 169.7 − 58.8 2.7 goethite ig02-04 mineralized dolomite breccia 169.6 − 46.4 6.3 goethite ig02-06* cambrian limestone 228.8 50.6 6.0 ? fe-sulfides ig02-07 weathered dolomite 170.2 − 44.5 4.3 goethite ig03-02 weathered dolomite 0.9 52.3 6.8 hematite ig03-06 weathered dolomite 357.0 41.4 7.1 hematite mean (n == 15) 359.7 49.9 α 95 = 4.7 decl – declination; incl – inclination; chrm – characteristic remanent magnetization; mad – maximum angular deviation from principal component analysis (kirschvink, 1980); mag. min. – main magnetic minerals carrying the chrm (see text); * – discarded samples, whose chrm is likely carried by magnetic iron sulfides; α 95 – half-angle of the 95% confidence circle about the mean direction (fisher, 1953). carriers, with a very low to null nrm intensity loss up to 150 mt af. their behaviour during thermal demagnetization indicates that their main magnetic minerals are hematite (that is, most of the remanence is lost between 580°c and 640°c) and/or goethite (i.e. sample ig0207, fig. 4a-c). generally, however, it has been observed that the prevailing magnetic carrier is goethite at site ig02 (san giovanni mine) and hematite at site ig03 (nebida mine) (table i). three samples among those cored at the site ig02 (san giovanni mine, level +150) show a characteristic remanent magnetization (chrm) likely carried by magnetic iron sulfides (thermal demagnetization achieved at 200-350°c; i.e. sample ig02-02, fig. 5a-c), whose directions were not coherent among the 3 samples (fig. 6 and table i). the «anomalous» paleomagnetic directions observed in such sulfides-dominated samples are likely due to their acquisition before tilting of the host-rocks (i.e. magnetic ironsulfides are of paleozoic age). structural complexity of the area and lack of precise elements to define a simple tectonic correction to apply to the data prevent further speculation about paleomagnetic data from those three samples. paleomagnetic data from all the remaining 15 samples, indicate that the high-coercivity minerals provided a well defined and coherent chrm that was acquired after the rotation of sardinia (that is after 16 myr) and in a time interval long enough to span at least one reversal of the geomagnetic field. in fact, 10 samples display a normal polarity chrm and 5 specimens a reverse polarity chrm (fig. 6). assuming that, by chance, the oxidation event started just be310 maria boni, jaume dinarès-turell and leonardo sagnotti fore a reversal and ended soon after it, a lower limit of the order of 5-10 kyr can be placed for the duration of such oxidation event. no upper limit can be set in view of the paleomagnetic data. the mean value calculated on such high-coercivity chrm, after transforming all polarities to normal is: declination = 359.7°, inclination = 49.9°, α 95 = 4.7°, k = 68.2, n = 15 (fig. 7 and table i). the data pass the reversal test of mcfadden and mcelhinny (1990), with a classification of «b» (i.e. the observed angle between the mean of the normal polarity set and the mean of the reverse polarity set is 8.9° and the critical angle for the reversal test is of 9.3°). such chrm was recognized as carried by hematite (or a mixture between goethite and prevailing hematite) in 10 samples, by only goethite in 4 samples (3 of which with reverse polarity, all from the san giovanni mine, site ig02, where hematite was not recognized as a main magnetic carrier) and by magnetite in 1 sample (from the reddish dolomites at site ig01). 7. conclusions despite the limited dataset due to the scarcity of accessible «exposures» or mining sites, fig. 6. equal area projection in geographic coordinates (open circles – upper hemisphere, reverse polarity; full circles – lower hemisphere, normal polarity,) of reliable paleomagnetic data for sites ig01, ig02 and ig03. ellipses around individual directions indicate the maximum angular dispersion on the mean paleomagnetic direction computed by principal component analysis (kirschvink, 1980) on the demagnetization data. at site ig01 the characteristic remanent magnetization (chrm) are carried by hematite and goethite and define two antipodal clusters of normal and reverse polarity along the n-s axis. at site ig02 the chrm are carried either by goethite (3 samples) or magnetic iron sulfides (3 samples). the first group defines a cluster of reverse polarity close to the n-s directions, the chrm of the second group are dispersed and far from the n-s direction. at site ig03 only two reliable chrm were obtained, which are similar and of normal polarity close to the n-s axis. fig. 7. equal area projection and statistical parameters (fisher, 1953) of all the reliable paleomagnetic data carried by high-coercivity oxi-hydroxides for sites ig01-ig03. geographic coordinates, all data flipped to normal polarity. ellipses around the individual directions indicate maximum angular dispersion on the mean paleomagnetic direction computed by principal component analysis (kirschvink, 1980) on the demagnetization data. the star indicates the direction of the geocentric axial dipole field in the study area. 311 paleomagnetic dating of non-sulfide zn-pb ores in sw sardinia (italy): a first attempt some interesting conclusions can be drawn from the paleomagnetic data obtained in this study. first of all, in both the non-sulfide ore bodies and in their host rocks consisting of deeply weathered ferroan dolomites, a complex magnetic mineralogy could be detected. the most common magnetic carrier is hematite, followed by goethite; in a few samples it was even possible to detect primary (?) iron sulfides. these different magnetic mineralogical phases could be related to distinct weathering episodes, which took place repeatedly during the geological evolution of the area. however, the last measurable oxidation event happened for sure after the rotation of the corsica-sardinia continental block (early miocene, ca. 16 myr), as indicated by the clustering of the paleomagnetic directions on the present-day geocentric axial dipole field. because some of the measured samples show inverse polarity, the last oxidation episode must have started before ca. 780 kyr (age of the brunhes-matuyama boundary; see tauxe et al., 1996). however, it could have been possible that the last oxidation episode lasted much longer in time and encompassed one or more polarity reversals of the geomagnetic field. based on geological reasoning only, we can argue that the most probable age for the bulk of supergene non-sulfide zn-pb ore in sw sardinia, as well as for the weathering and «reddening» of the hydrothermal «geodic dolomite» host rocks, was comprised in a time span between middle eocene and plio-pleistocene (boni et al., 2003). by paleomagnetic methods, we could not narrow this interval, but only confirm that at least part of the oxidation episodes were younger than the early miocene (early langhian) and likely extending back quite far from the present. the observed reactivation of the alteration profiles in the whole mining district, which caused the replacement of earlierdeposited supergene mineral phases by others adapted to new hydro-geochemical conditions, could also have caused the precipitation of several generations of magnetic minerals presently coexisting in the same oxidized lithotype. furthermore, the recognition that the goethite-dominated samples from site ig02 show reverse polarity implies that the host-rocks were not significantly heated (the 120°c neél temperature of goethite drops radically in goethites substituted with diamagnetic elements, see mathé et al., 1999) during at least the last 780 kyr. even though a precise age determination could not be reached for the genesis of non-sulfide zn-pb ores in the iglesiente district, the paleomagnetic method, together with radiogenic isotope systems and fission tracks, could be a powerful tool to establish the timing of the weathering profiles (théveniaut et al., 2002) associated to non-sulfides in other mineralized areas with a less complex geological history than sw sardinia. acknowledgements the authors wish to thank the igea company (iglesias, sardinia) for having granted access to their properties and r. sarritzu for his help during the sampling campaign. we thank f. rodeghiero for his review of the manuscript. we also acknowledge comments from j. gattacceca and an anonymous reviewer, that improved the manuscript both in substance and style. references aversa, g., g. balassone, m. boni and c. amalfitano (2002): the mineralogy of the «calamine» ores in sw sardinia (italy): preliminary results, period. mineral., 71 (3), 1-18. bechstädt, th. and m. boni (editors) (1994): sedimentological, stratigraphical and ore deposits field guide of the autochthonous cambro-ordovician of southwestern sardinia, memorie descrittive carta geologica d’italia, xlviii (servizio geologico d’italia), p. 434. billows, e. 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(received september 09, 2004; accepted february 22, 2005) vol48/04/2005def 755 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words aeolian islands – panarea – submarine fumaroles – gas chemistry – geochemical monitoring 1. introduction on november 3 2002 at least three large and whitish plumes suddenly appeared at the sea surface about 3 km eastward of panarea island (aeolian archipelago, southern italy), in an area characterized by a relatively shallow sea (10-15 m deep) and by the presence of emerging reefs of different size, roughly forming an elliptical shape (fig. 1). the phenomenon proved to be the surface expression of huge gas columns emerging from the sea floor as a mixthe november 2002 degassing event at panarea island (italy): five months of geochemical monitoring bruno capaccioni (1) (6), franco tassi (2)(4), orlando vaselli (2)(4), dario tedesco (3)(5) and piermaria luigi rossi (6) (1) istituto di vulcanologia e geochimica, università degli studi di urbino, italy (2) dipartimento di scienze della terra, università degli studi di firenze, italy (3) dipartimento di scienze ambientali, seconda università degli studi di napoli, caserta, italy (4) istituto di geoscienze e georisorse, cnr, firenze, italy (5) istituto di geologia ambientale e geo-ingegneria, cnr, roma, italy (6) dipartimento di scienze della terra e geologico-ambientali, università degli studi di bologna, italy abstract on 3rd november 2002, at about 3 km off-shore of panarea island (aeolian islands, southern italy), a series of gas vents suddenly and violently opened from the seafloor at the depth of 10-15 m, with an unusually high gas flux and superimposing on the already existing submarine fumarolic field. starting from the 12th november 2002 a discontinuous geochemical monitoring program was carried out. the emissions consisted in an emulsion whose liquid phase derived from condensation of an uprising vapor phase occurring close to the fluid outlets without significant contamination by seawater. the whole composition of the fluids was basically h2oand co2-dominated, with minor amounts of typical «hydrothermal» components (such as h2s, h2, co and light hydrocarbons), atmospheric-related compounds, and characterized by the occurrence of a significant magmatic gas fraction (mostly represented by so2, hcl and hf). according to the observed temporal variability of the fluid compositions, between november and december 2002 the hydrothermal feeding system was controlled by oxidizing conditions due to the input of magmatic gases. the magmatic degassing phenomena showed a transient nature, as testified by the almost complete disappearance of the magmatic markers in a couple of months and by the restoration, since january 2003, of the chemical features of the existing hydrothermal system. the most striking feature of the evolution of the «panarea degassing event» was the relatively rapid restoration of the typical reducing conditions of a stationary hydrothermal system, in which the feo/fe1.5o redox pair of the rock mineral phases has turned to be the dominating redox controlling system. mailing address: dr. bruno capaccioni, istituto di vulcanologia e geochimica, università degli studi di urbino, loc. crocicchia, 61029 urbino, italy; e-mail: b.capaccioni@uniurb.it 756 bruno capaccioni, franco tassi, orlando vaselli, dario tedesco and piermaria luigi rossi ture of gas, fine-grained sediments and colloidal sulphur, in an area previously characterized by a gentle bubbling sub-marine fumarolic field (gabbianelli et al., 1990; italiano and nuccio, 1991; calanchi et al., 1995). a particularly large plume occurred a few tens of meters se of bottaro reef (hereafter named bottaro 1). according to the size of the «crater» depression produced (about 20×14 m wide and 10 m deep), the event was probably the result of a sort of submarine «gas blast». in order to define the nature of these new gas emissions and their temporal evolution, a first gas sampling campaign was carried out starting on november 12 and followed by three samplings, almost on a monthly basis, until march 2003. 2. geological and volcanological setting panarea is the smallest island (3.3 km2) of the aeolian archipelago (i.e. the subaereal portion of the aeolian volcanic district – avd). the avd is a ring-like volcanic arc consisting of 7 islands and 10 seamounts, and it constitutes about 200 km of the inner side of the peloritanian-calabrian orogenic belt (boccaletti and manetti, 1978; beccaluva et al., 1982, 1985; gabbianelli et al., 1990; calanchi et al., 2002). the dynamics of this arc (which is located along regional ns-, ewand ne-sw-oriented fault systems) is determined by tectonics through some faults that are still active (gasparini et al., 1982; lanzafame and rossi, 1984). the avd is characterized by a subduction-related magmatism that ranges from calcalkaline to shoshonite products. the volcanic activity has taken place almost entirely since the quaternary, 400 kyr up to the present (calanchi et al., 2002). the morphology of the sea floor, as defined by the 100 m isobaths, resembles that of a volcanic complex characterized by a multi-phase evolution (gabbianelli et al., 1986, 1990). after the growth of a central apparatus, centered in the island and in the submarine surroundings, volcanic activity occurred within the eastern area through a ne-sw fissure system. according to radiometric ages, the subaereal portion grew in a short period of time (150100 kyr). after 50 kyr of quiescence, the activity resumed with the emplacement of the endogenous dome of basiluzzo, located ne of panarea, along the panarea-stromboli tectonic line (ne-sw fault system). between basiluzzo and panarea, some small islets (dattilo, bottaro, lisca bianca, panarelli and lisca nera) (calanchi et al., 1999a,b) are arranged along a circular rim about 1 km in diameter. the area defined by the islets has a crater-like shape, with a maximum depth of 30 m, and is characterized by an important gravimetric anomaly fig. 1. map of panarea island with the locations of calcara, bottaro 1, frattura, bottaro 2 and bottaro 3 gas discharges. 757 the november 2002 degassing event at panarea island (italy): five months of geochemical monitoring t ab le i. c he m ic al c om po si ti on o f su bm ar in e an d su ba er ea l (c al ca ra ) ga se s. g as c om po un d co nt en ts a re i n µ m ol /m ol . s am pl e d at e t °c c o 2 h c l h f s o 2 h 2s s h 2o n 2 c h 4 a r o 2 n e h 2 h e c o c al ca ra n ov em be r 20 02 10 1 60 60 4 61 .0 1. 94 1. 72 26 1 0. 05 93 8 22 5 39 5 31 1 3. 00 51 .5 0. 00 3 82 .9 0. 18 0. 00 9 c al ca ra d ec em be r 20 02 99 48 24 5 24 .8 0. 74 0. 58 21 3 0. 05 95 1 23 9 10 8 12 2 0. 66 4. 41 0. 00 1 41 .6 0. 09 0. 00 4 c al ca ra ja nu ar y 20 03 98 52 78 3 6. 66 < 0. 01 < 0. 01 25 4 < 0. 01 94 6 47 6 25 2 12 6 2. 17 46 .8 0. 00 1 53 .2 0. 06 0. 00 7 c al ca ra m ar ch 2 00 3 99 41 80 6 7. 48 < 0. 01 < 0. 01 18 6 < 0. 01 95 7 80 6 96 .7 66 .4 1. 44 2. 73 0. 00 1 27 .5 0. 04 0. 00 2 b ot ta ro 1 n ov em be r 20 02 47 28 0 04 5 6. 23 1 25 .0 25 6 1. 60 9 2. 11 70 9 99 8 1. 29 1 1. 11 30 .3 29 5 0. 02 9 21 7 1. 52 1. 12 2 b ot ta ro 1 d ec em be r 20 02 46 33 6 49 4 23 6 0. 47 24 .2 4. 05 8 1. 18 65 7 32 0 1. 17 7 1. 62 24 .2 32 0 0. 02 2 33 5 2. 08 1. 73 7 b ot ta ro 1 d ec em be r 20 02 46 38 0 33 6 63 0 0. 58 86 .4 5. 19 4 1. 25 61 0 14 4 2. 51 8 4. 35 42 .2 50 1 0. 04 0 52 8 4. 74 3. 13 0 b ot ta ro 1 d ec em be r 20 02 46 32 9 21 5 23 2 0. 50 25 .5 5. 81 7 1. 43 66 3 25 5 92 1 1. 62 18 .7 17 2 0. 01 6 33 2 1. 21 1. 72 1 b ot ta ro 1 ja nu ar y 20 03 37 46 2 00 6 15 0 < 0. 01 6. 89 4. 69 7 0. 18 53 1 57 9 1. 02 6 0. 87 16 .0 53 .5 0. 01 4 46 2 1. 64 1. 67 4 b ot ta ro 1 ja nu ar y 20 03 37 44 4 83 3 16 3 < 0. 01 9. 25 4. 78 9 0. 19 54 7 89 4 1. 54 7 1. 36 25 .1 36 0 0. 02 1 37 4 1. 22 2. 23 7 b ot ta ro 1 m ar ch 2 00 3 35 74 0 79 1 34 0 < 0. 01 10 .7 18 .5 48 0. 39 23 6 00 2 2. 79 4 3. 22 46 .0 34 5 0. 03 9 1. 11 3 2. 27 4. 16 4 b ot ta ro n ov em be r 20 02 n. d. 29 4 56 9 12 .9 78 16 9 89 9 1. 87 3 0. 95 68 7 35 8 1. 54 0 4. 41 19 .2 30 3 0. 01 7 28 3 2. 13 1. 67 5 f ra tt ur a b ot ta ro d ec em be r 20 02 n. d. 34 9 36 9 36 3 0. 58 38 .5 2. 61 4 1. 25 64 5 72 5 1. 42 4 7. 83 20 .8 32 2 0. 01 7 10 6 1. 64 1. 13 2 f ra tt ur a b ot ta ro ja nu ar y 20 03 n. d. 44 4 82 7 22 6 < 0. 01 8. 48 4. 81 5 0. 29 54 6 73 2 2. 18 8 4. 97 37 .6 51 3 0. 03 0 64 1 2. 26 2. 77 0 f ra tt ur a b ot ta ro m ar ch 2 00 3 n. d. 52 4 36 9 18 9 < 0. 01 4. 43 6. 64 1 0. 16 46 5 45 9 2. 09 7 2. 73 34 .5 55 8 0. 03 0 63 8 2. 29 4. 32 2 f ra tt ur a b ot ta ro 2 n ov em be r 20 02 n. d. 64 8 28 3 2. 03 1 20 .2 11 5 4. 54 5 5. 12 34 1 37 6 3. 06 2 0. 76 28 .8 45 7 0. 02 5 76 .5 4. 72 0. 20 3 b ot ta ro 2 d ec em be r 20 02 51 38 1 13 3 2. 62 2 22 .4 10 5 5. 40 7 4. 02 60 8 30 3 1. 83 5 0. 91 24 .3 46 2 0. 01 9 77 .1 3. 48 0. 09 1 b ot ta ro 2 ja nu ar y 20 03 50 49 2 94 0 14 3 < 0. 01 6. 09 7. 19 7 0. 22 49 3 53 8 4. 98 7 0. 86 74 .7 1. 10 7 0. 06 2 3. 95 1. 78 0. 15 4 b ot ta ro 2 ja nu ar y 20 03 50 46 2 50 9 20 1 < 0. 01 7. 13 6. 88 7 0. 22 52 6 87 8 2. 57 4 0. 98 40 .6 89 8 0. 03 5 1. 24 2. 23 0. 26 7 b ot ta ro 2 m ar ch 2 00 3 49 42 3 57 8 11 2 < 0. 01 < 0. 01 8. 67 0 0. 17 56 6 32 7 1. 28 6 0. 37 18 .2 1. 46 0. 01 4 4. 63 1. 88 0. 03 3 b ot ta ro 2 m ar ch 2 00 3 49 39 8 03 4 14 9 < 0. 01 < 0. 01 7. 81 0 0. 16 59 2 66 3 1. 16 9 0. 31 17 .1 15 5 0. 01 3 1. 19 1. 66 0. 06 8 b ot ta ro 3 n ov em be r 20 02 n. d. 37 4 64 4 3. 95 2 19 .2 22 2 1. 73 0 1. 55 61 0 31 0 6. 09 3 2. 75 81 .9 1. 60 3 0. 07 3 1. 33 2 4. 54 5. 55 1 b ot ta ro 3 d ec em be r 20 02 n. d. 24 2 40 9 31 7 0. 63 20 .6 3. 52 2 1. 13 75 2 49 5 90 7 3. 76 12 .9 16 2 0. 01 0 14 1 0. 75 1. 14 7 b ot ta ro 3 d ec em be r 20 02 n. d. 42 9 96 3 32 2 0. 65 23 .0 3. 55 7 1. 18 56 3 73 3 1. 70 3 5. 68 23 .5 33 8 0. 01 9 32 1 1. 39 2. 37 9 b ot ta ro 3 ja nu ar y 20 03 18 52 0 93 9 98 .4 < 0. 01 3. 50 7. 55 4 0. 21 46 7 42 1 3. 18 0 2. 47 47 .2 74 9 0. 04 0 1. 81 2. 37 0. 37 8 b ot ta ro 3 ja nu ar y 20 03 18 52 1 06 0 91 .0 < 0. 01 3. 62 7. 68 9 0. 22 46 8 97 1 1. 70 6 1. 11 26 .5 44 7 0. 02 4 2. 78 1. 99 0. 18 5 b ot ta ro 3 m ar ch 2 00 3 n. d. 27 7 90 4 56 .6 < 0. 01 < 0. 01 2. 44 4 0. 12 71 6 95 7 2. 35 5 0. 87 35 .0 24 3 0. 02 9 2. 45 1. 33 0. 04 0 b ot ta ro 3 m ar ch 2 00 3 n. d. 30 2 60 4 12 4 < 0. 01 < 0. 01 2. 54 6 0. 30 69 3 13 0 1. 42 2 1. 08 21 .1 14 7 0. 01 7 2. 03 1. 45 0. 13 5 758 bruno capaccioni, franco tassi, orlando vaselli, dario tedesco and piermaria luigi rossi (bonasia et al., 1973; calanchi et al., 1999b), whose interpretation is still a matter of debate. this area is also affected by a strong exhalative and hydrothermal activity known since the roman age. the oldest detailed descriptions of such activities date back to the 18th century (de dolomieu, 1783). on the other hand, the first modern descriptions of this hydrothermal system were given by gabbianelli et al. (1990), italiano and nuccio (1991) and calanchi et al. (1995). 3. sampling and analytical methods fluids were collected four times between november 2002 and march 2003 (table i) from four different sites (fig. 1): a) bottaro 1, which includes both bottaro 1 and bottaro frattura samples, located a few tens of meters from the western margin of the bottaro islet and consisting of a central huge 15 m deep emission discharging from a crater-shaped depression on the seafloor and minor diffuse bubbling all around; b) bottaro 2 is a group of gas discharges close to the nw corner of bottaro islet at a depth of 712 m; c) bottaro 3 consists of diffuse gas bubbling with relatively low flux and discharging se of bottaro islet in an area of 100-200 m2; d) calcara is a pre-existing, weak fumarolic discharge located at about 10 m inland close to the eastern shore of panarea island. the gas emissions from bottaro 1 vent have the largest flux of the entire fumarolic field, although a correct estimation of the total flux rate (not carried out in the present study), significantly decreased since december 2002, is difficult to computed. samples were collected in pre-weighted and pre-evacuated 50-ml thorion-tapped glass tubes, partially filled with 20 ml of a solution 0.15m cd(oh)2 and 4m naoh, connected to a plastic funnel positioned up-side-down over the uprising bubbles. to avoid contamination by seawater, the silicon tube between the funnel and the collecting glass tube was filled with milli-q water and isolated from seawater by a plastic plug (fig. 2). the plug was removed only after the complete evacuation of the seawater contained in the funnel by gases. acidic gases (co2, so2, h2s, hcl, hf) and water were trapped in the alkaline solution. during sampling, elemental sulphur precipitates, so2 dissolves in the alkaline solution, whereas h2s reacts with cd2+ to form insoluble cds. inert gases (n2, o2, co, h2, he, ar, ne, ch4), collected in the head-space, were analyzed with a gas-chromatograph (shimadzu 15a), equipped with a thermal conductivity detector. during the analysis, separation of h2, he and ne peaks was obtained by using a 9 m long molecular sieve column, at a temperature of 30°c. to allow a complete separation of aro2 peaks, the temperature was lowered to 0°c by means of cryogenic equipment (shimadzu crg-15) fed by liquid co2. after analysis of the inert gases, the solution was separated by centrifugation from the solid precipitates and oxidized with h2o2 to convert so2 to so42− that was then analyzed using a dionex dx100 ion-chromatograph equipped with an ionpac as9-hc column. cds in the solid phase was dissolved and oxidized with h2o2 and then analyzed by ion-chromatografig. 2. hcl/10-hf∗10-so2 ternary diagram. black circles: bottaro 1 and frattura gas discharges; open squares: calcara gas discharge; open circles: bottaro 2 gas discharges; open triangles: bottaro 3 gas discharges. 759 the november 2002 degassing event at panarea island (italy): five months of geochemical monitoring phy as so42−. s0 was extracted from residual precipitate with ccl4 and oxidized to s2i2 with the addition of ki. finally, sulphur of s2i2 was oxidized to so42− by kbro3 and analyzed by ion chromatography (montegrossi et al., 2001). co2 in the caustic solution was analyzed by acidimetric titration with 0.5n hcl solution. f− and cl−, in the alkaline solution, were also analyzed by ion-chromatography with the ionpac as9-hc column. to permit the complete separation of f− and oh− peaks a 1 µmol na2co3 solution was used as eluent phase instead of the 10 µmol na2co3 solution, the latter being commonly utilized for ion-chromatographic analysis of anions. analytical precision was <1% for major gas components and <5% for minor and trace compounds. 4. volatile acid compounds or contamination by seawater? some critical considerations before providing any interpretation of the collected data it is useful to discuss what they really represent. as will be discussed below, the most impressive features of the gases collected on november and december 2002 are the presence of chloride, fluoride, and sulphite, and the relatively high water contents, exceeding what would be expected in fluids at 50°c and 2.5 bar (i.e. the values of temperature and pressure during sampling). a possible persistence of acid species (so2, hf and hcl; table i) in gases discharging from the sea bottom clearly conflicts with the high solubility in seawater of these compounds. this would imply a severe internal disequilibrium of the emerging gases that can only be explained by the huge gas flux, particularly at bottaro 1 vent. despite the specific sampling procedures adopted to avoid seawater contamination, the problem of introducing seawater droplets or micro-droplets has been considered and discussed accordingly. the measured so2/hcl ratios of the collected gas samples range from 0.08 to 0.19 and from 0.06 to 0.37 on november 2002 and december 2002, respectively. since the so2/hcl ratio of normal seawater is 0.14, the reasonable suspicion of the occurrence of seawater contamination instead of the introduction of volatile acid compounds into the sampling device cannot be ruled out. nevertheless, volcanic gases with so2/hcl ratios < 1 are not so uncommon. some crater emissions at el chichon volcano (tassi et al., 2003) have so2/hcl ratios in the range of 0.070.64, whereas up to 0.007 so2/hcl ratios have been measured at usu volcano (symonds et al., 1996). moreover, the observed significant positive correlation between the two gas compounds is not a critical factor since it can be due to either seawater contamination or input of acid species into the sampling device. however, according to the relatively high br− content in seawater (65 mg/l), in the case of seawater contamination the collected naoh solutions of bottaro 1 samples collected on december 2002 should have at least 1 mg/l of br−, well above its detection limit (0.01 mg/l; table i). in addition, together with the absolute absence of br−, it is noteworthy to point out the occurrence of high f− contents in spite of its relatively low content in seawater (1 mg/l). a further critical factor is provided by sulphur speciation analytically observed in the naoh solution of gas samples. besides the h2s fraction, precipitated as cds, and the elemental sulphur, collected as residual precipitate (see above), so32−, that can only be explained by a direct collection of gaseous so2, is the prevailing sulphur compound before the addition of h2o2 to the naoh solution. this seems to definitively exclude contamination by seawater. the dramatic drop of seawater ph (see below), the corrosion effect on shells of foraminifers in the surrounding areas (panieri et al., 2003), other than skin irritation reported by the scuba-divers represent convincing clues of the presence of highly acidic species in thermal fluid discharges. the excess of water clearly conflicts with the expected «instantaneous» condensation of water vapour at 50°c and 2.5 bar. it seems reasonable to assume that water vapour condensed close to the sampling point and the condensation could have been transported by the gas flux as micro-droplets into the sampling device, together with acid species partially or totally dissolved in it (as suggested by chiodini, pers. comm.). thus, the detected chloride, fluoride and sulphite contents could derive, at least par760 bruno capaccioni, franco tassi, orlando vaselli, dario tedesco and piermaria luigi rossi tially, from the condensed phase collected during sampling instead of directly from the gas phase. hence, we conclude that the acid gas species, characterizing the chemical composition of submarine gases discharged close to panarea island on november and december 2002, were mostly dissolved in the condensed phase before the introduction into the sampling device, without any significant contamination by normal or chemically modified seawater. 5. the november 2002 gas composition following the above mentioned considerations, the presence of variable amounts of water-soluble acid species (hf, hcl and so2) in sampled gas discharges requires prevailing dry conditions of the fluids up to shallow depths, other than a severe disequilibrium between the submarine gas discharges and seawater. as already stated, this can only be explained by the observed huge gas flux, able to minimize chemical and physical interactions between the emerging fluids and the surrounding environment. analytical data for major, minor and trace gas compounds are listed in table i. the highest outlet temperature, measured as close as possible to the gas vents and probably representing a weighted mean between seawater and fluid temperature, was 50°c, with ph values ranging between 5.0 and 5.5 (three units lower with respect to normal seawater; table i). simultaneous measurements at la calcara gas discharge (fig. 1) revealed a temperature range of 9599°c, in agreement with what was observed by calanchi et al. (1995). on november 2002, besides of the occurrence of water soluble acids, the gas composition of the submarine gas discharges was characterized by the prevalence of h2o (up to 710000 µmol/mol), co2 (up to 650000 µmol/mol), n2 and h2s (up to 6100 and 4500 µmol/mol, respectively). h2 and co contents have a large compositional variability in terms of space, ranging from 0.0076 to 0.13 µmol/mol and from 0.2 to 5.6 µmol/mol, respectively. concerning the soluble acids, the already mentioned prevalence of hcl over so2 with respect to the typical compositions of the acid-bearing subaereal gases from vulcano crater (fig. 2) could be related to the «stripping» of hcl from an acidified chlorinerich seawater (symonds et al., 2001). simple thermodynamic calculations based on the following equilibrium reactions: and allow us to obtain a phcl in the range of those measured (4-12 mbar) by stripping hcl from seawater at 100°c and ph = 2, conditions becoming less severe if we consider halite saturated solutions and/or temperature in excess of 100°c. the relative contents of o2-ar-ne (fig. 3) clearly point to an atmospheric origin for these compounds through degassing of air saturated and heated seawater. moreover, the trend tohcl hclg aq= hcl cl haq = + + fig. 3. ar-o2/10-ne*1000 ternary diagram. air and air saturated water (asw) compositions are reported. black circles: bottaro 1 and frattura gas discharges; open squares: calcara gas discharge; open circles: bottaro 2 gas discharges; open triangles: bottaro 3 gas discharges. 761 the november 2002 degassing event at panarea island (italy): five months of geochemical monitoring wards the ar-ne side suggests a variable o2 consumption that seems to increase from november 2002 to march 2003. 6. temporal variations between november 2002 and march 2003 the monitored submarine gas exhalations displayed a complex combination of temporal and spatial changes of their chemical compositions. the temporal variations of h2o and co2 contents measured at bottaro 1 and the h2o and co2 values calculated according to the method of chiodini and cioni (1989) are reported in fig 4. the h2o/co2 ratios progressively decreased from november 2002 to march 2003: after january 2003 the gas mixture became co2-dominated. the comparison between calculated and measured contents of h2o and co2 suggests that in november and, partly, in december 2002, the relative amounts of collected water was not so different from those calculated at the re-equilibration zone, while a significant removal of the condensed phase seems have occurred from january 2003. between november and december 2002, water-soluble acid species (hcl, hf and so2) also underwent a sharp decrease (fig. 5), closely approaching the analytical detection limits. such a behaviour recorded at bottaro 1 area is in agreement with the expected trends for a progressive increase in the seawater/gas ratio along the whole fracture feeding system, as suggested by symonds et al. (2001). the progressive disappearance of water-soluble acid species and collected water correspond to an increase in vapour phase condensation, possibly related to with the perceptible decrease of the gas flux at bottaro 1 vent. binary diagrams in fig. 6a,b report the expected trends (calculated according to the method suggested by chiodini et al., 1996) for h2 and so2 versus he at various degrees of gas dissolution in water, starting from the mean composition of the bottaro 1 gas sample collected in november 2002. both h2 and he (fig. 6a), due to their very low solubility on seawater, should display an infig. 4. temporal variation of co2 and h2o contents of bottaro 1 and frattura gas discharges (mean values) between november 2002 and march 2003. calculated co2 and h2o concentrations (chiodini and cioni, 1989) are reported. fig. 5. temporal variation of hcl, hf and so2 contents of bottaro 1 and frattura gas discharges (mean values) between november 2002 and march 2003. 762 bruno capaccioni, franco tassi, orlando vaselli, dario tedesco and piermaria luigi rossi crease as dissolution proceeds. although the mean compositions of bottaro 1 fluid discharge (closed circles; fig. 6a) sampled in november and december 2002 actually followed the expected dissolution trend, samples collected after january 2003 dramatically shifted towards an h2-rich and he-poor end-member. similarly, so2 versus he contents (fig. 6b) at bottaro 1 seem to be controlled by dissolution and liquid separation only in november and december 2002, while, in the following months, the trend moved towards an apparent so2-poor end-member. moreover, the composition of the whole gas discharges collected in december 2002 (open circles; fig. 6a), able to describe the spatial evolution of the chemical characteristics of the system, followed the same temporal pattern shown by bottaro 1 in the period january-march 2003. thus, the distribution of samples in fig. 6a,b seems to define a two-component mixing process consisting of: i) a h2-rich/ /he-poor (hydrothermal) and ii) a h2-poor/ /he-rich (residual magmatic?) end-member. this seems to represent the main controlling mechanism also in terms of spatial variability. figure 7 provides further insights into the temporal evolution of low water-soluble, redoxand temperature-dependent chemicals at bottaro 1 in the period november 2002-march 2003. in march 2003, h2 content was almost an order of magnitude higher with respect to the values measured in november and december 2002. also co showed a concomitant increase in its contents (1-2 to 4-5 µmol/mol; table i). these compositional changes cannot only be explained in terms of a simple balance between the two different end-members. temporal changes in the fig. 6a,b. h2 versus he (a) and so2 versus he (b) diagrams. expected trends for simple solubilization process are reported. black circles: mean values of gas samples collected in november 2002, december 2002, january 2003 and march 2003 at bottaro 1 and frattura gas discharges; open circles: gases sampled in december 2002 at bottaro 1, frattura, bottaro 2 and bottaro 3 gas discharges. a b fig. 7. temporal variation of h2 and co contents of bottaro 1 and frattura gas discharges (mean values) within the period november 2002-march 2003. 763 the november 2002 degassing event at panarea island (italy): five months of geochemical monitoring chemical-physical properties of the hydrothermal reservoir feeding the submarine discharge system, such as an increase in temperature and/or reducing conditions, must be invoked. figure 8 plots co and h2 concentrations versus h2s/ar ratios. h2s is a temperatureand redox-dependent chemical species that forms a redox pair with so2 and equilibrates with the h2/h2o pair typically at magmatic conditions (giggenbach, 1987, 1996; giggenbach and glover, 1992). due to both the chemical inertia and the very low water solubility of ar, the abrupt increase in h2s/ar ratios measured in march 2003 cannot be explained by simple passive enrichment during dissolution processes. since h2s is favoured by decreasing temperatures and increasing reducing conditions, its positive correlation with co and h2 (favoured by the increase of both temperature and reducing conditions of the system) suggests that redox conditions between january and march 2003 had a fundamental role to control the chemical variations recorded at the surface. 7. concluding remarks the november 2002 degassing event occurring close to panarea island was supplied by an uprising so2-bearing fluid of possible magmatic origin, variously mixed with a «transient», rapidly evolving hydrothermal fluid. in november and partly in december 2002, both fluids were chemically recognizable. the whole emitted fluid was possibly a sort of emulsion, made up of a mixture of gas and micro-droplets of condensation containing ions from acid species dissolution, dragged along the ascending conduits by the extremely high gas flux. according to the temporal variation of the fluid compositions recorded between november 2002 and march 2003 two main steps on the evolution of the feeding system can be recognized. the first step, occurring between november and december 2003, was dominated by a selective dissolution of the chemical species, mostly affecting the «magmatic» components, leading to a passive enrichment of the less soluble compounds. the second step, that took place after january 2003, appears to be almost completely dominated by the hydrothermal fig. 8. h2 versus h2s/ar and co versus h2s/ar diagrams of bottaro 1 and frattura gas discharges (mean values). fig. 9. log(co/co2) versus log(h2/h2o) diagram of bottaro 1 and frattura gas discharges. rock (hm, fm and gt) and gas (so2/h2s) redox buffers are reported. 764 bruno capaccioni, franco tassi, orlando vaselli, dario tedesco and piermaria luigi rossi component which undergoes a rapid chemical evolution of its feeding system. in the bottaro 1 area this chemical evolution can be well summarized by the h2/h2o versus co/co2 binary diagram (fig. 9). the samples appear to be arranged along a trend which from november 2002 to march 2003 moved towards a lower temperature (from 300 to 200°c) and towards increasing reducing conditions. as previously discussed, in the period between november and december 2002 the partial condensation of water vapour did not significantly modify the gas/water ratios observed, while it played a crucial role after january 2003. in any case, it seems that in november 2002 the feeding hydrothermal system was characterized by prevailing oxidizing conditions closer to «magmatic» ones (characterized by the influence of the h2s/so2 redox buffer), shifting towards more reducing conditions starting from january 2003 and reaching a final rock-dominated redox state typical of a «stabilized» hydrothermal system starting from march 2003. acknowledgements this work was financially supported by gnv-ingv (gruppo nazionale vulcanologia). many thanks are due to mario cosenza for his help during the diving campaigns at panarea. giovanni chiodini is thanked for the stimulating discussions. references beccaluva, l., p.l. rossi and g. serri (1982): neogene and recent volcanism of the southern tyrrhenian-sicilian area: implications for the geodynamics evolution of the calabrian arc, earth evol. sci., 3, 222-238. beccaluva, l., g. gabbianelli, f. lucchini, p.l. rossi and c. savelli (1985): petrology and k/ar ages of volcanoes dredged from the eolian seamounths: implications for geodynamic evolution of the southern tyrrhenian basin, earth planet. sci. lett., 74, 187-208. boccaletti, m. and p. manetti (1978): the tyrrhenian sea and adjoining areas, in the ocean basins and margins, edited by a.e.m. nairn, w.h. kanes and f.g. stehli (plenuum press, new york), vol. 4a, 149-151. bonasia, v., g. luongo and s. montagna (1976): a land gravity survey of the aeolian islands, bull. volcanol., 37, 134-176. calanchi, n., b. capaccioni, m. martini, f. tassi and l. valentini (1995): submarine gas emissions from panarea (aeolian archipelago): distribution of inorganic and organic compounds and inference about source conditions, acta vulcanol., 7 (1), 43-48. calanchi, n., c.a. tranne, f. lucchini and p.l. rossi (1999a): geological map of the island of panarea and basiluzzo (aeolian islands), 1:10000 scale (gnv, siena). calanchi, n., c.a. tranne, f. lucchini, p.l. rossi and i.m. villa (1999b): explanatory notes to the geological map (1:10000) of panarea and basiluzzo islands (aeolian arc, italy), acta vulcanol., 11 (2), 223-243. calanchi, n., a. peccerillo, c.a. tranne, f. lucchini, p.l. rossi, m. kempton, m. barbieri and t.w. wu (2002): petrology and geochemistry of volcanic rocks from the island of panarea: implications for mantle evolution beneath the aeolian island arc (southern tyrrhenian sea), j. volcanol. geotherm. res., 115, 367-395. chiodini, g. and r. cioni (1989): gas geobarometry for hydrothermal systems and its application 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(1783): voyage aux iles de lipari. fait en 1781, registres de l’académie des sciences. gabbianelli, g., p.v. gillot, g. lanzafame, m. ligi, d. postpischl and p.l. rossi (1986): controllo strutturale nell’evoluzione vulcanica di panarea (isole eolie), iiv-cnr, catania, 239. gabbianelli, g., p.v. gillot, g. lanzafame, c. romagnoli and p.l. rossi (1990): tectonic and volcanic evolution of panarea (aeolian islands, italy), mar. geol., 92, 313-326. gasparini, p., g. iannaccone, p. scandone and r. scarpa (1982): the seismotectonics of the calabrian arc, tectonophysics, 84, 267-286. giggenbach, w.f. (1987): redox processes governing the chemistry of fumarolic gas discharges from white island, new zealand, appl. geochem., 2, 143-161. giggenbach, w.f. (1996): chemical composition of volcanic gases, in monitoring and mitigation of volcano hazards, edited by r. scarpa and r.i. tilling (springer verlag, berlin), 222-256. giggenbach, w.f. and r.b. glover (1992): tectonic regime and major processes governing the chemistry of water and gas discharges from the rotorua geothermal field, new zealand, geothermics, 21, 121-140. italiano, f. and p.m. nuccio (1991): geochemical investigations of submarine volcanic exhalations to the east of panarea, aeolian islands, italy, j. volcanol. geotherm. res., 46, 125-141. lanzafame, g. and p.l. rossi (1984): evidenze di attività tettonica recente a panarea (eolie): implicazioni vulcanologiche, geol. rom., 23, 131-139. montegrossi, g., f. tassi, o. vaselli, a. buccianti and k. garofalo (2001): sulfur species in volcanic gases, anal. chem., 73, 3709-3715. panieri, g., r. barbieri, f. gamberi and m. marani (2003): hydrothermal vents in the aeolian arc: effects on benthic foraminifera, in egs agu eug joint assembly, 6-11 april, nice (france). 765 the november 2002 degassing event at panarea island (italy): five months of geochemical monitoring tassi, f., o. vaselli, b. capaccioni, j.l. macias, a. nencetti, g. montegrossi and g. magro (2003): chemical composition of fumarolic gases and spring discharges from el chichon volcano, mexico: causes and implications of the changes detected over the period 1998-2000, j. volcanol. geotherm. res., 123, 105121. symonds, r.b., t.m. gerlach and m.h. reed (2001): magmatic gas scrubbing: implications for volcano monitoring, j. volcanol. geochim. res., 108, 303-341. symonds, r.b., y. mizutani and p.h. briggs (1996): long-term geochemical surveillance of fumaroles at showa-shinzan dome, usu volcano, japan, j. volcanol. geotherm. res., 73, 177-211. p o l a r i s a t i o n o f t h e s p h a s e o f s e i s m o g r a m s (*) r . stoneley 11 lias l)een o b s c r v e d b y f. n e u m a n n ( 1 ) , p . b y e r l y ( ) a n d i. l e l i m a n n ( ! ) t l i a t in s e i s m o g r a n i s of e a r t b q u a k e s r e c o r d e d a l m o d e r a l e d i s t a n c e s , lite s p h a s e a p p e a r s first as sii, f o l l o w e d s o m e 101 4 s e c o n d s l a t e r b y sv. miss l e h m a n n a t t r i b u t e s t b i s p b e n o m e n o n l o t h e s m a l l a n g l e t b a t t l i e s — o s c i l l a t i o n s m a k e w i t l i t b e h o r i z o n t a l , w h i l c p r o f . b y e r l y suggests t b a t s o m e k i n d of d o u b l é r e f r a c t i o n m a y b e t h e c a u s e . b o t l i a g r e e t b a t t h e i n t e r r a i d o e s n o t s e e m t o d e p e n d o n e p i c e n t r a l d i s t a n c e . e v i d e n t l y m o r e o b s e r v a t i o n a l e v i d e n c e i s n e e d e d , b u t a p r e l i m i n a r y e x a m i n a t i o n can b e m a d e of t h e p o s s i b i l i t y t b a t t h e p b e n o m e n o n a r i s e s b e c a u s e t h e o u t e r p a r t of t h e e a r t h ' s m a n t l e is n o t e l a s t i c a l l y i s o t r o p i e . t h e s i m p l e s t h y p o t h e s i s w e can m a k e to c o n f o r m t o t b i s a s s u m p t i o n is p e r h a p s t h a t ali d i r e c t i o n s a t r i g h t a n g l e s to t h e v e r t i c a l a r e e q u i v a l e n t ; t b i s t y p e of s o l i d is t e r m e d « t r a n s v e r s e l y i s o t r o p i e ». t h e a s s u m e d t y p e of a n i s o t r o p y is s u f f i c i e n t l v g e n e r a l to g i v e an i n d i c a t i o n of t h e o r d e r of m a g n i t u d e of t h e a n o m a l y to b e exp e c t e d . f u r t l i e r , i t is u n l i k e l y t h a t t h e o u t e r l a y e r s a r e v e r y m a r k e d l y a n i s o t r o p i e , e l s e t li is d e p a r t u r e f r o m i s o t r o p y w o u l d p r o b a b l y b a v e b e e n n o t e d l o n g ago. i t s l i o u l d suflìce t b e n , t o t a k e as a s t a n d a r d of c o m p a r i s o n a t r a n s v e r s e l y i s o t r o p i e m a t e r i a l w l i o s e e l a s t i c c o n s t a n t s a r e k n o w n : s u c h a c r y s t a l is b e r v i , t b e c o n s t a n t s of w l i i c l i w e r e d e t e r m i n e d b y v o i g t ('). t h e s e i s m o l o g i c a l i m p l i c a t i o n s of a t r a n s v e r s e l y i s o t r o p i e m e d i u m b a v e b e e n i n v e s t i g a t e c i , a n d o n e i m p o r t a n t f a c t is t h a t if a l i d i r e c t i o n s p e r p e n d i c u l a r to t h e v e r t i c a l a r e e q u i v a l e n t t b e n ( n e g l e c t i n g t h e c u r v a t u r e of t h e e a r t h ) t h e v e l o c i l i e s of sii a n d sv a r e d i f f e r e n t , a n d t h e y d e p e n d on tlie a n g l e of i n c i d e n c e of t h e w a v e . t h e d i f i e r e n c e i n t l i e l i m e of p r o p a g a t i o n t h r o u g l i a s u r f a c e l a y e r of t l i i c k n e s s li c a n e a s i l y b e c a l c u l a t e d . l e t suffixes h, v r e f e r l o sii, sv r e s p e c t i v e l y ; l e t c h e t h e w a v e v e l o c i t y i n a n y m e d i u m a n d i tlie a n g l e of i n c i d e n c e ( * ) l a t r a d u z i o n e in l i n g u a i t a l i a n a d e l l a p r e s e n t e n o t a è r i p o r t a t a a p a g . 1 1 3 . 4 11. s t o n e l e y i n t l i a t m e d i u m . if r is t h e r a d i a i d i s t a n c e f r o n t t l i e c e n t r e of t h e e a r t l i , t h e n f o r a n y r a y , i n t h e s t a n d a r d n o t a t i o n , r s i n i dt p ~ c ~~<ìk s u p p o s e t l i a t a d i s t u r h a n c e o r i g i n a t i n g a t a g e n e r a t e s sh a n d sv w a v e s a r r i v i n g a t b a l o n g p a t h s pq„b a n d pqvb. l c t i\m a n d qvn h e p e r p e n d i c n l a r t o p „ m a n d q„ n r e s p e c t i v e l y , w h e r e r e f r a c t i o n is s u p p o s e d t o o c c u r a t t l i e b a s e of a s u r f a c e l a y e r . t l i e n f r o m t h e d i a g r a m p„m = nqn—li ( t a n iv — t a n j„ì s i n i, so t l i a t t h e d i f f e r e n c e of t h e t i m e of t r a n s m i s s i o n of s a l o n g t h e p a t h s pyqy a n d puq„ i n t h e l o w e r m a t e r i a l , s u p p o s e d i s o t r o p i e , is 2(/i/c) s i n i ( t a n i v — t a n i„) — 2dtt s a y . a l t e r n a t i v e l y , if 0 is t l i e c e n t r e of t h e e a r t l i a n d r t h e r a d i u s a t t h e d i s c o n t i n u i t y , w h i l e t h e a n g l e qv oq„\zd a , t h e n qvq„ — r. d \ — r(da/dti) dt\=c dri\ s i n i, g i v i n g t h e s a n i e r e s u l t as h e f o r e . t l i u s , i n a l i , t h e d i f f e r e n c e ili t h e t i m e s of t r a n s m i s s i o n of sh a n d sv t t 2h 2,1 • 2h • • u • , • i i „ — 1 y = s e c i„ s e c i v s i n i (tan iy — t a n t„) ch cv c w h e r e c , i r e f e r t o t h e m e d i u m i m n i c d i a t e l y h e l o w t h e j t i n c t i o n . t o s e e t h e t y p e of v a r i a t i o n of c v a n d c „ w i t l i i n c l i n a t i o n 0° t o t h e v e r t i e a l , i t w i l l s u f l ì c e t o w o r k w i t l i t h e d e n s i t y a n d e l a s t i c c o n s t a n t s of h e r y l , a n d t o s c a l e d o w n t h e r e s u l l s so as t o c o r r e s p o n d t o a v e l o c i t y 3 . 4 0 k m / s e c . i n t h e l a y e r f o r sv a t tì = 0 " a n d i l = 9 0 " . i n l'oi.,\risatlo'' of the: s • l'ibsf. of sf.is~io!;ra~is 5 the undedying material we take c=4.38 km'see.; thus the modcl eorresponds to a granitic eontinent rchting on ultrahasic matei·ial. the i·erluisite fonllulae for c h ami c v are found in thc paper alreariy cited (h). for a transversely isotropie hody in whieh the stmin.encrgy function w is given ily 2 w =a(é 11+ e\y}+ c e 2 .. + 2f (eu +f>yy) e .. + +2 (a-21\') e n e n -1l (e~y"+e2 .. )+ ne ~~y thc velocity c of a wave eorrehponding to hs is given ily where n = eoh ù; l=sin ù. for waves of sv type 2, c' . ~al'+c n'-ii (a-l)l'-(cl)n' l' +41'1'.' l! in whieh j=f +l. voigt's values, in dynesi cm", are a=2.fi94xi0 i \ c=2.363xi01~j a-2n=0.961xi01~; f=0.661xi0 1\ l=0.6;)3xl01~; n=o.866xioi~. from the foregoing formulae, the value8 of cii and cv, sealed so that they correspond to matel"ial in which c~ll and c~ ,. foi· i) = 0" are 11.56 x 10 10 , are given in the following tahle: t anle i ,')0 ch x 1o"~ c,. x 10-~ o 3.40 3.40 10 3.41 3.48 20 3.46 3.68 30 3.54 i 3.905 40 3.62 4.0,1 50 3.71 4.02 fio 3.79 3.855 70 3.86 3.64 80 i 3.90 3.47 90 i 3.92 3.'10 1he riiffel"ing trends of ch ami c,· with u dre evident fi"om this tahle. in what follows the layer wili he tl·eated as thin, so that for propal e f i • ** 6 r . s t o n e l e y g a t i o n i n t h i s l a y e r t l i e e a r t l i m a y h o c o n s i d e r e d as p i a n e . t h e n ih a n d ;'v a r e g i v e n b y sin i s i n ih s i n i v c ch c v w h e r e c = 4 . 3 8 k m / s e c . a n d (sin i)/c=(dt/d a0)/110.7 s i n c e dt/d a is k n o w n f r o m s e i s m o l o g i c a l t a b l e s , t h e a n g l e i c a n h e c o m p u t e d f o r v a r i o u s e p i c e n t r a l d i s t a n c e s a. t o c o m p l e t e t h e c a l c u l a t i o n s w e first c o n s i d e r sii, a n d p u t {) = in iri t a h l e i. t h u s ( s i n in )/c c a n h e c o m p u t e d as a f u n c t i o n of i , a n d f r o m t h e f o r e g o i n g e q u a t i o n /„ is f o u n d as a f u n c t i o n of a . t h e c a l c u l a t i o n s f o r s f a r e m u d i l o n g e r , b u t t h e g e n e r a l p r i n c i p l e is t h e s a n i e . w e t h u s o b t a i n t a b l e i i . i t w i l l h e s e e n t l i a t t h e d i l f e r e n c e s h e t w e e n /',, a n d i v a r e n o t l a r g e . t h e v a i n e of ( t h — t v ) / 2 / i c a n n o w h e c o m p u t e d d i r e c t l y . t a b l e i i a dta »h iy i d e g r e e s sec/deg d e g r e e s d e g r e e s d e g r e e s 1 0 2 4 . 4 5 6 . 3 5 8 . 7 7 4 . 5 2 0 1 9 . 4 s d 3 9 . 3 4 5 . 0 5 0 . 0 2 0 1 8 . 7 s r 3 8 . 0 4-3.0 4 7 . 7 3 0 1 5 . 8 3 0 . 0 3 4 . 8 3 8 . 8 4 0 1 5 . 0 2 8 . 5 3 2 . 3 3 6 . 4 5 0 1 3 . 9 2 6 2 9 . 4 3 3 . 5 6 0 1 2 . 8 2 3 . 5 2 6 . 4 3 0 . 5 7 0 1 1 . 8 2 1 . 5 2 3 . 0 2 7 . 8 8 0 1 0 . 5 1 9 . 0 2 0 . 7 2 4 . 5 9 0 9 . 0 1 6 . 2 1 7 . 2 2 0 . 9 1 0 0 9 . 4 1 5 . 0 1 6 . 0 1 9 . 2 s i n c e t h e o b j e c t of t h i s i n v e s t i g a t i o n is l o a s c e r t a i n o n l y t h e o r d e r of m a g n i t u d e of t h e e f f e c t of d e p a r t u r e f r o m i s o t r o p y , i t s u f f i c e s t o c o n s i d e r t h r e e v a l u e s of ( t i , — t v ) / 2 h . f o r a = 2 0 ° ( s r ) it is 0 . 0 3 9 , f o r a = 5 0 ° i t is 0 . 0 3 5 , a n d f o r a = 8 0 " i t i s 0 . 0 2 5 . t h u s , t h e v a r i a t i o n w i t l i e p i c e n t r a l d i s t a n c e is n o t l a r g e . h o w e v e r , t h e t i i r e d i f f e r e n c e is q u i t e s m a l l , e.g. f o r / i = 3 3 k m . , a b o u t t h e m a x i m u m p e r m i s s i b l e t h i c k n e s s of t h e g r a n i t i c l a y e r , t „ — t v a m o u n t s o n l y t o a b o u t 1 . 6 sec. t h u s t h e e x p l a n a t i o n i n t e r m s of d e p a r t u r e f r o m isopoi.arisation of t h e s p h a s e of s e i s m o c r a m s 7 t r o p y i n t h e o u t e r l a y e r s s e e m s to f a i ] , f o r w i t l i a t y p i e a l c r y s t a l l i n e m e d i u m (« a d j u s t e d » t o f i t l l i e v e l o c i t y of sg), i t w o u l d n e e d a t l i i c k n e s s of a n i s o t r o p i e r o c k of t h e o r d e r of 3 0 0 k m to a c c o u n t f o r a d i f f e r e n c e of 1 4 s e c o n d s in t h e t i m e s of a r r i v a i of sii a n d sv a t a n e p i c e n t r a l d i s t a n e e of 80". w h i l e o n e c a n n o t e n t i r e l y r i d e o u t t l i e p o s s i h i l i t y of a p r e f e r r e d o r i e n t a t i o n in t l i e o u t e r c r y s t a l l i n e p a r t of t h e m a n t l e ( r o u g l i l y t h e p a r t a b o v e t h e 20" d i s c o n t i n u i t y ) it is t o h e r e m e m h e r e d t h a t t h e m o d e l c h o s e n is o n e in w l i i c h t h e a x e s of s y m m e t r y of ali t h e c o n s t i t u e n t c r y s t a l s a r e r a d i a i . a c e o r d i n g l y , u n l e s s d e c i s i v e e v i d e n c e of a n i s o t r o p y i n t l i e e a r t l i ' s c r u s t is f o r t h e o m i n g f r o m o t h e r s o u r c e s , t l i i s w i l l n o t h e a n a d e q u a t e e x p l a n a t i o n of t h e o h s e r v e d l i m e d i f f e r e n c e t „ — t v . p r o v i s i o n a l l y , t h e n , m i s s l e h m a n n ' s e x p l a n a t i o n s e e m s t h e m o r e l i k e l y . comunicazione presentala al convegno per la costituzione della f.s.e., tenutosi a verona dal 12 al 16 settembre 1950 (v. « annali di geofìsica », voi. i l i , pag. 579). summary in earthquakes recorded at moderate distances it lias been ohserved that s phase appears first as sii, folloived some 10 to 14 seconds later by sv. the object of tliis paper is to try to decide ichether doublé refraction is likely to be the explanation of tliis jìlwnomenon. a simpie model to consider ivould be a « transversely isotropie » material, symmetrical about the radiai direction. formulae for the velocities of sii and s v waves are available; tliese velocities depend on the angle that the ray makes ivi t li the norma!. it is unlikely that the eartli could be as markedly anisotropie as the minerai beryl, which is transversely isotropie; aceordingly, this material, of ivhich the fi ve clastic constants are knoivn is taken as an extreme example, and the velocities of sh and sv for different angles of incidence are « scaled down » so as to match the velocity of distortional ivaves in granite. it is then possible to calcitiate the difference in the timo taken by ivaves from one point of the surface of the earth to anotlier point on the surface according as the s wave in the surface layer is of sh or sv type. it is found that, even in this extreme case, a layer of anisotropie rock some 30 km thick tvould account only for a time difference of 8 r . s t o n e l e y about 1 y, seconds. thus, if the earth ivere as strongly anisotropie as beryl (which is unlikelyj oue would need tlie layer of preferred orientatimi to extend to a depth of about 300 km. this is difficult to believe, and thus no great credence can be attaclied to ari explanation in ternis of doublé refraction. b i b l i o g r a p h y ( ! ) f . neumann, bull. seis. soc. amer. 20, 1 5 2 0 ( 1 9 3 0 ) . ( 2 ) p . byerly, bull. seis. soc. amer. 28, 1 2 ( 1 9 3 8 ) . ( 3 ) i . leumann, pubi. bur. cent. séism. n . 1 2 , 1 9 ( 1 9 3 5 ) . ( ' ) w . v o i c t , lehrbuck der kristaliphysik ( 1 9 2 8 ) . ( s ) r . stoneley, geophys. supp. 5, 3 4 3 , 1 9 4 9 , e.g. k . e . b u l l e n , t h e o r e t i c a l s e i s m o l o g y , p . 1 0 9 . ( f i ) r . stoneley, loc. cit. e q u a t i o n s ( 1 1 ) o ( 1 1 a ) . marzorati_et_al_corretto_layout 6 575 automated control procedures and first results from the temporary seismic monitoring of the 2012 emilia sequence simone marzorati1, simona carannante1,*, marco cattaneo1, ezio d'alema1, massimo frapiccini1, chiara ladina1, giancarlo monachesi1, daniele spallarossa2 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, ancona, italy 2 università di genova, dipartimento per lo studio del territorio e delle sue risorse, genova, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6116 1. introduction after moderate to strong earthquakes in italy or in the surrounding areas, the istituto nazionale di geofisica e vulcanologia (ingv; national institute for geophysics and volcanology) activates a temporary seismic network infrastructure. this is devoted to integration with the italian national seismic network (rsn) [delladio 2011] in the epicentral area, thus improving the localization of the aftershocks distribution after a mainshock. this infrastructure is composed of a stand-alone, locally recording part (re.mo.) [moretti et al. 2010] and a real-time telemetered part (re.mo.tel.) [abruzzese et al. 2011a, 2011b] that can stream data to the acquisition centers in rome and grottaminarda. after the may 20, 2012, ml 5.9 earthquake in the emilia region (northern italy), the temporary network was deployed in the epicentral area; in particular, 10 telemetered and 12 stand-alone stations were installed [moretti et al. 2012, this volume]. using the dedicated connection between the acquisition center in rome and the ancona acquisition sub-center [cattaneo et al. 2011], the signals of the real-time telemetered stations were acquired also in this sub-center. these were used for preliminary quality control, by adopting the standard procedures in use here (see next paragraph, and monachesi et al. [2011]). the main purpose of the present study is a first report on this quality check, which should be taken into account for the correct use of these data. in a second step, the data recorded by the real-time temporary network were integrated with data from the 12 stand-alone stations and from 27 stations of the rsn (up to a distance of ca. 120 km). the use of automatic detectors and phase pickers allowed the recognition and locating of more than 2000 earthquakes in the first 12 days of the sequence. these data were used for preliminary determination of a three-dimensional (3-d) propagation model, which was used, in turn, for a re-location of the events. this led to a first reconstruction of the seismogenic structures that were activated during the first part of the sequence. 2. quality control of the data the continuous stream of data from the telemetered stations is subjected to control procedures that are mainly devoted to the monitoring of the data availability and the quality of the seismic signals. data availability is checked by means of scripts contained in the passcal package (http://www.passcal.nmt. edu/content/software-resources), the outputs of which are organized to allow a graphical representation, which is useful for quick recovery of possible critical points. indeed, the real-time stations use different transmission channels: satellite, and ultrahigh frequency + satellite [abruzzese et al. 2011a, 2011b], or universal mobile telecommunications system [govoni et al. 2012]. a comparison between the time occurrence of data gaps during the day at the different stations allows restriction of the possible causes of data loss, and thus to drive the intervention of the field teams in case of critical data-flow interruptions. for example, universal mobile telecommunications system station t0822 showed occasional data losses during the day, while the night transmission was perfect, which suggested a saturation of the phone cells in case of higher usage; a change of the phone operator completely solved the problem. on the other hand, the quality of seismic signals is monitored by means of the pqlx software [mcnamara and buland 2004], which is aimed at monitoring the noise levels in the signals. for each channel of each station, it is possible to compute power spectral densities (psds), using one-hour time windows with an overlap of 50%, which are the basis for the evaluation of probability density functions (pdfs). the comparisons of the pdfs between different article history received july 19, 2012; accepted august 14, 2012. subject classification: data quality control, emilia sequence 2012, automatic earthquake location, 3d tomographic model. 2012 emilia earthquakes stations, and with respect to reference terms, like the new high-noise model (nhnm) [peterson 1993], allow possible imperfections in the sensor installation to be quick ly recognized (e.g., leveling, coupling). it should be remembered that in these cases, the installations are often performed in relatively critical situations. fortunately, with this installation to date, the sensors of the real-time stations have not shown any particular critical situations, apart from a leveling loss at t0800, which was solved at the first station control in the field. the availability of pdf statistics also allows the resolution capability of the stations to be recognized; this resolution is determined from one side by the instrumental characteristics, and from the other side by the environmental noise. in this area, the first instrumental aspect appears to be negligible compared to the environmental aspect, at least at short periods. figure 1 shows the comparison between the pdf statistics at station t0800, as measured from a velocimetric broad-band sensor (nanometrics trillium t120 compact; figure 1, left side) and a strong-motion accelerometer (kinemetrics episensor; full scale 2g; figure 1, right side). in the accelerometer plot, the theoretical resolution limit of the coupling sensor-digitizer is also reported (figure 1, thick black line). in this case, it is evident that the resolution limit does not influence the acquisition, because the spectra of the recorded signals are above this limit. in particular, in the short period part of the spectrum (below 1 s), the pdf distributions are in good agreement, which confirms that the accelerometer can correctly record the ground motion related to weak events that are detected by the velocimeter. this is common to all of the stations installed in the epicentral area, and it is not surprising: the mean noise curves are in agreement with results of previous analyses in the po plain. marzorati and bindi [2006] showed that the noise levels often reach the limit of the peterson's nhnm. also, the observed variations in the noise levels between day and night, which suggest a mainly anthropic source, are similar to values already reported [marzorati and bindi 2006], and are between 5 db and 15 db [marzorati et al. 2012]. 3. quality control through data use one of the best controls of the performances of the infrastructure of a seismic network that is composed of a locally recording part and a real-time telemetered part is to use its data to locate earthquakes. for this purpose, to obtain a quick but reliable response on the effectiveness of the integration between the temporary and permanent networks operating in the area, we adopted a semi-automated approach. the first step was the construction of an integrated dataset: we selected the stations of the national network located at distances of less than 120 km from the barycenter of the sequence. their continuous recordings were merged with the recordings of the temporary network. in this first analysis, the time span from 20 to 31 may, 2012, was considered. the continuous recordings were searched for events, using a simple short-time average (sta)/long-time average (lta) algorithm on band-pass filtered signals between 2 hz and 15 hz, and a coincidence criterion. for this preliminary analysis, relatively restrictive criteria were adopted (e.g., signal-to-noise ratio threshold of 10, coincidence of 4 stations within 20 s): in this way, 2365 time windows were selected, each of 65 s long. these time windows were used for an automatic picking procedure [spallarossa et al. 2011]. the proposed automatic phase picker includes envelope calculation, band-pass filtering, an akaike information criterion (aic) detector for both p-arrivals and s-arrivals, checking for impulsive arrivals, evaluation of expected s onset on the basis of a preliminary location derived from the p-arrival times, and quality assessment. the advantage of this technique is that it provides consistent arrival-time picks for p-waves and s-waves, and provides reliable automatic earthquake locations. more insight into the marzorati et al. 576 (s) (s) figure 1. comparison between the pdf statistics at station t0800. left: velocimetric broad-band sensor; right: strong-motion accelerometer. thick black line, theoretical resolution limit (−128 db) of the coupling sensor-digitizer. both panels: upper gray curve, reference new high-noise model; lower gray curve, new low-noise model [peterson 1993]. 577 adopted procedure can be found in spallarossa et al. [2012]. the picker furnishes arrival times of p-phases and (if recognized) s-phases, and an estimate of uncertainty, classified in the standard hypo-family quality class (0, most reliable; 3, less reliable), based on an analysis of the signal-to-noise ratio. the automatic picker recognized 30467 p-phases and 22689 s-phases; these phases allowed the localization of 2300 earthquakes. in figure 2a, the number of automatic pickings for each station is reported on the map. it is worth noting that the effectiveness of stations in furnishing a high number of pickings is mainly related to the mean distance with respect to the epicenter distributions, while the kind of sensor, e.g., velocimeter (blue bar) or accelerometer (yellow bar), does not appear to influence the detection. as already stated, this is due to the relatively high mean noise level below 1 s that in this area hides the loss of resolution of the strong-motion sensors. the strong asymmetry in the picking numbers between alpine stations (figure 2a, northemilia 2012: data quality and first results figure 2. number of automatic p pickings for each station. (a) whole area; (b) zoom on the epicentral area, as shown by the black rectangle in (a). the color of the bars represents the two kinds of sensors: velocimeter, blue; accelerometer, yellow. marzorati et al. 578 f ig u re 3 . d iff er en ce s be tw ee n a u to m at ic a n d h an dm ad e pi ck in g fo r th e p ( le ft ) an d s (r ig h t) p h as es , re sp ec ti ve ly . b o tt o m , r es u lt s fo r th e be st -q u al it y pi ck in g (c la ss es 0 a n d 1) ; t o p, t h e le ss r el ia bl e pi ck in g (c la ss es 2 a n d 3) . (f ig u re c re at ed w it h g en er ic m ap pi n g t o o ls : w w w .s o es t. h aw ai i.e du /g m t; w es se l an d sm it h [ 19 98 ]) . f ig u re 4 .m ap o f th e p -w av e ve lo ci ti es o f th e sh al lo w er la ye rs , e xp re ss ed a s pe rc en ta ge v ar ia ti o n s w it h re sp ec t to t h e 1d i n it ia l re fe re n ce m o de l in t h e ep ic en tr al a re a. y el lo w t ri an gl es , st at io n l o ca ti o n s. (f ig u re c re at ed w it h g en er ic m ap pi n g t o o ls : w w w .s o es t. h aw ai i.e du /g m t; w es se l a n d sm it h [ 19 98 ]) . 579 ern part) and apennine stations (figure 2a, southern part) is also noticeable, due to a higher signal-to-noise ratio. to control and tune the picker parameters, about 200 events were also picked by an expert analyst, and used for a comparison. in spite of the previously noted high levels of ambient noise, the results are very encouraging (figure 3): the difference between the automatic and the hand-made pickings is quite narrow, at around the zero, with 77.7% of data below 0.1 s for the best-quality picking (class 0 and 1) for p-waves, and 84.8% below 0.2 s for s-waves. the less reliable picking (classes 2 and 3) show differences that are obviously higher; nevertheless, 74.1% of the differences are below 0.4 s for p-waves, and 81.3% below 0.8 s for s-waves. the distribution of differences shows an asymmetry towards positive values (i.e., the automatic picking is delayed with respect to the hand-made one). this asymmetry is limited to the tail of the distribution for p-phases, and it is more enhanced for the low-quality pickings: indeed, for classes 0 and 1 the mean value is 0.016 s if we take into account just the differences below 0.2 s, and it increases to 0.050 s on the whole dataset. for classes 2 and 3, the mean values are 0.040 s (below 0.2 s) and 0.161 s (whole dataset), respectively. for s-phases, the mean values are higher, mainly due to the less reliable pickings; indeed, for classes 0 and 1, the mean values are emilia 2012: data quality and first results figure 5. relocated events in the 3-d structure. full black color, the most reliable locations (at least 20 phases with nonzero weight, final residual rms <0.2 s). gray tones, remaining locations, with less drastic constraints (at least 8 phases, rms <0.4 s). red points, the most reliable events with local magnitudes >4.0 (light red points, those with less drastic constraints). stars, the two mainshocks. (figure created with generic mapping tools: www.soest.hawaii.edu/gmt; wessel and smith [1998]). 0.048 s (below 0.2 s) and 0.037 s (whole dataset), while for classes 2 and 3 we obtain 0.072 s (below 0.2 s) and 0.125 s (whole dataset), respectively. in our opinion, these very small asymmetries confirm the good performances of the automatic picker, and should lead to a reduced impact on the location procedure. to take into account the strong heterogeneity of the propagation medium, both in the epicentral zone and in the surrounding areas, a preliminary 3-d model was evaluated using a joint inversion for earthquake locations and propagation anomalies, through the simulps14 code [thurber 1983, 1993]. for this, the best located events of the dataset were selected on the basis of the number of used phases (30), of the maximum azimuthal gap (150˚) and of the final rms of the residuals (0.3 s): in this way, 499 earthquakes were selected. the joint inversion led to the reconstruction of 3-d pwave and s-wave propagation models that are relatively detailed in the epicentral area. the main characteristics of the p-wave velocity miodel are shown in figure 4. although a complete discussion of this model is beyond the scope of this preliminary study, it is worth noting the good correlation between the high-velocity anomalies of the shallower layers and the well-known structural highs buried beneath the eastern po plain [bigi et al. 1992]. for the deeper layer (figure 4, z = 16 km), the main feature is the evident mean velocity of the alpine crust (propagations towards the north), with respect to the apennine one (towards the south). the 3-d p-wave and s-wave propagation models were used to locate all of the events of the dataset; in figure 5, a selection of the locations is plotted, as map and crosssections perpendicular to the strike of the mainshock, as inferred by the time-domain moment tensor mechanism computed by the ingv (http://cnt.rm.ingv.it/tdmt.html). the most reliable locations (i.e., at least 20 phases with nonzero weight, final residual rms <0.2 s) are reported in full color (908 events). for the remaining ones that are plotted in gray tones in figure 5, less drastic constraints were adopted (i.e., at least 8 phases, rms <0.4 s), with 1104 events selected, for a gross total of 2012 reported events (87.5% of the initial events). the events with local magnitudes larger than 4.0 are plotted in red in figure 5, while the stars demonstrate the two mainshocks (2012/05/20, 2h03', ml 5.9, and 2012/05/29, 7h00', ml 5.8). from the map view, the misalignment of the eastern sector of the sequence is quite evident, where the first mainshock occurred, and the western sector, which was mainly active in the last part of the period analyzed. the easternmost cross-sections (figure 5, g-h, i-j and k-l) show clear deepening of the hypocenters towards the south at an angle of about 45˚, which is comparable with one of the focal planes obtained by the time-domain moment tensor mechanism. the central sections show a more complex distribution, which might be related to the superposition of different structures, or maybe to nonplanar structures. only the westernmost section appears to depict a simple south-dipping plane again. a more complete analysis of the 3-d propagation model and of its effects on the location procedure will be the object of a more detailed study, which is already in progress, and which also uses a more complete dataset. references abruzzese, l., g. de luca, m. cattaneo, g. cecere, v. cardinale, a. castagnozzi, c. d'ambrosio, a. delladio, m. de martin, l. falco, d. franceschi, a. govoni, a. mammolo, f. migliari, f. minichiello, m. moretti, m. moschillo, m. pignone, g. selvaggi and l. zarrilli (2011a). la rete sismica mobile in telemetria satellitare (re.mo.tel.), rapporti tecnici ingv, 177, issn 20397941 (in italian). abruzzese, l., g. cecere, g. de luca and l. zarrilli (2011b). re.mo.tel.: l'esperienza durante l’emergenza "l'aquila 2009", in: m. cattaneo and m. moretti (eds.), monitoraggio sismico del territorio nazionale: stato dell'arte e sviluppo delle reti di monitoraggio sismico, miscellanea ingv, 10, 55-56 issn 2039-6651 (in italian). bigi, g., d. cosentino, m. parotto, r. sartori and p. scandone (1992). structural model of italy. scala 1:500,000. c.n.r., progetto finalizzato geodinamica, sottoprogetto: modello strutturale tridimensionale, quaderni della ricerca scientifica, 114, 3. cattaneo, m., e. d'alema, m. frapiccini, s. marzorati and g. monachesi (2011). acquisizione presso la sede di ancona, in: m. cattaneo and m. moretti (eds.), monitoraggio sismico del territorio nazionale: stato dell'arte e sviluppo delle reti di monitoraggio sismico, miscellanea ingv, 10, 91-93, issn 2039-6651 (in italian). delladio, a. (2011). monitoraggio sismico del territorio nazionale, in: m. cattaneo and m. moretti (eds.), monitoraggio sismico del territorio nazionale: stato dell'arte e sviluppo delle reti di monitoraggio sismico, miscellanea ingv, 10, 11-18, issn 2039-6651 (in italian). govoni, a., g. monachesi, m. cattaneo, m. moretti, a. delladio and m. frapiccini (2012). la sequenza sismica del 2010 nelle province di macerata e fermo e la sperimentazione della trasmissione dei dati in tempo reale via umts, quaderni di geofisica, 102, issn 1590-2595 (in italian). marzorati, s., and d. bindi (2006). ambient noise levels in north central italy, geochem. geophys. geosyst., 7, q09010; doi:10.1029/2006gc001256. marzorati, s., s. carannante, m. cattaneo, e. d'alema, m. frapiccini, c. ladina and g. monachesi (2012). emergenza sismica 2012 in emilia romagna: attività sperimentali di supporto alla rete sismica mobile ingv svolte dal personale della sede di ancona, rapporti tecnici ingv, 236, issn 2039-7941 (in italian). marzorati et al. 580 581 mcnamara, d.e., and r.p. buland (2004). ambient noise levels in the continental united states, b. seismol. soc. am., 94, 1517-1527. monachesi, g., m. cattaneo, e. d'alema, m. frapiccini, s. marzorati and m. ferretti (2011). sistemi di controllo in uso al centro di acquisizione della sede di ancona, in: m. cattaneo and m. moretti (eds.), monitoraggio sismico del territorio nazionale: stato dell'arte e sviluppo delle reti di monitoraggio sismico, miscellanea ingv, 10, 103-107. issn 2039-6651 (in italian). moretti, m., a. govoni, g. colasanti, m. silvestri, e. giandomenico, s. silvestri, f. criscuoli, l. giovani, a. basili, c. chiarabba and a. delladio (2010). la rete sismica mobile del centro nazionale terremoti. rapporti tecnici ingv, 137. issn 2039-7941 (in italian). moretti, m., et al. (2012). rapid response to the earthquake emergency of may 2012 in the po plain, northern italy, annals of geophysics, 55 (4); doi:10.4401/ag-6152. peterson, j. (1993). observations and modelling of background seismic noise, u.s. geological survey open file rep., 93-322. spallarossa, d., g. ferretti, d. scafidi and m. pasta (2011). picking automatico nella rete sismica dell'italia nordoccidentale (rsni), in: m. cattaneo and m. moretti (eds.), monitoraggio sismico del territorio nazionale: stato dell'arte e sviluppo delle reti di monitoraggio sismico, miscellanea ingv, 10, 141-146. issn 2039-6651 (in italian). spallarossa, d., l. tiberi and g. costa (2012). comparison of manual and automatic onset time picking for local earthquake in north-eastern italy, egu general assembly, april 22-27, vienna. thurber, c.h. (1983). earthquake locations and three-dimensional crustal structure in the coyote lake area, central california, j. geophys. res., 88, 8226-8236. thurber, c.h. (1993). local earthquake tomography: velocities and vp/vs – theory, in: h.m. iyer and k. hirahara (eds.), seismic tomography: theory and practice, chapman & hall, 842 pp. *corresponding author: simona carannante, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, ancona, italy; email: simona.carannante@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. emilia 2012: data quality and first results << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol52,3,2009 221 annals of geophysics, vol. 52, n. 3/4, june/august 2009 key words ionosphere – monitoring – data validation – monitoring techniques – campaigns – dissemination 1. introduction the ionosphere is a rather complex and highly variable medium, and to be able to predict and when necessary mitigate the influence of the ionosphere on radio wave propagation including signals from the global navigation satellite systems (gnss), ionospheric monitoring measurements are crucial as a necessary basis for studying and understanding the ionosphere. real-time data are necessary for operational ionospheric mitigation. ionospheric monitoring in the european area is done by a network of ionosondes and, in recent years, also by a network of high-accuracy global positioning systems receivers (gps), which work primarily for near earth space plasma monitoring under cost 296 david altadill (1), josef boska (2), ljiljana r. cander (3), tamara gulyaeva (4) (10), bodo w. reinisch (5), vincenzo romano (6), andrzej krankowski (7), jürgen bremer (8), anna belehaki (9), iwona stanislawska (10), norbert jakowski (11) and carlo scotto (6) (1) observatori de l’ebre, universitat ramon llull – csic, spain (2) institute of atmospheric physics, ascr, czech republic (3) stfc, rutherford appleton laboratory, chilton, uk. (4) izmiran , troitsk, moscow region, russia (5) center for atmospheric research, umass lowell, usa (6) istituto nazionale di geofisica e vulcanologia, roma, italy (7) university of warmia and mazury, poland (8) leibnitz-institute of atmospheric physics, kühlungsborn, germany (9) national observatory of athens, athens, greece (10) space research centre pas, warsaw, poland (11) dlr, institute of communications and navigation, neustrelitz, germany abstract this review paper presents the main achievements of the near earth space plasma monitoring under cost 296 action. the outputs of the cost 296 community making data, historical and real-time, standardized and available to the ionospheric community for their research, applications and modeling purposes are presented. the contribution of cost 296 with the added value of the validated data made possible a trusted ionospheric monitoring for research and modeling purposes, and it served for testing and improving the algorithms producing real-time data and providing data users measurement uncertainties. these value added data also served for calibration and validation of space-borne sensors. new techniques and parameters have been developed for monitoring the near earth space plasma, as time dependent 2d maps of vertical total electron content (vtec), other key ionospheric parameters and activity indices for distinguishing disturbed ionospheric conditions, as well as a technique for improving the discrepancies of different mapping services. the dissemination of the above products has been developed by cost 296 participants throughout the websites making them available on-line for real-time applications. mailing address: dr. david altadill, observatori de l’ebre, universitat ramon llull – csic, crta. de l’observatori, no. 8, e43520 roquetes, spain; e-mail daltadill@obsebre.es vol52,3,2009 20-09-2009 19:05 pagina 221 222 d. altadill, j. boska, l.r. cander, t. gulyaeva, b.w. reinisch, v. romano, a. krankowski, j. bremer, a. belehaki, i. stanislawska, n. jakowski and c. scotto geodesy and positioning. the cost 296 action joins scientists from all the european institutions operating ionosonde systems (vertical ionospheric sounding instruments, essentially radars but the newest being a combination of radar and doppler interferometer). these systems work in a continuous monitoring regime, covering essentially the whole of europe, including scandinavia’s sodankyla station. data from a network of gps stations have also been used in cost 296 in recent years, as well as radio occultation gnss measurements. the aim of this paper is to review the main results achieved by cost 296 working group wg-1«ionospheric monitoring and modeling» under the working package wp1.1 «near earth space plasma monitoring». the wp1.1 encompassed the ionospheric measurements, data collecting, archiving and dissemination, and development of new methods of ionospheric measurements, monitoring, and conditions characterization. the wp1.1 created an observational basis for the project cost 296 and its work was defined by four terms of reference: 1) maintaining and extending the flow of real-time and retrospective ionospheric monitoring data to databases; 2) supporting and developing internet sites and protocols for disseminating data products; 3) validating the quality and consistency of monitoring data, particularly those collected in real time; and 4) developing monitoring techniques and parameters describing the state of the ionospheric plasma, to include ground-based and space based techniques. the paper is organized according to these terms of reference and related work. section 2 summarizes the main efforts for archiving real time and historical databases. section 3 describes the main results for validating the monitoring data. section 4 reviews the monitoring techniques and parameters describing the state of the ionospheric plasma developed within the project. section 5 highlights the main efforts for disseminating data and added value products. finally, section 6 presents results from the specific cost 296 campaigns. the paper ends with a summary. 2. real-time and historical databases the european ionospheric community has been working for long time on several cost actions; cost 238, 251 and 271 (e.g. zolesi et al., 2007; zolesi and cander, 2008). this joint work started on the early nineties and ever since then this community has worked hard to create a large and homogeneous ionospheric data base. the data base has continuously increased and has mainly been fed by data deduced from the ionograms obtained by vertical incidence (vi) soundings. the ionograms provide the critical frequencies and heights of the ionospheric layers over the measuring station and appropriate ionogram inversion tools provide the vertical electron density profiles. the main reason for this data base is to make data standardized and available to the ionospheric community for their purposes, e.g. research, telecommunication applications, etc. therefore, having data available is of capital importance for our community and this task has continued in the cost 296 action. this section discusses the task carried out by the cost 296 community to archive ionospheric data. the main data bases dedicated to this task will also be presented. at present, the stations producing ionospheric data linked to the cost 296 community keep information on their measurements and most of them make the measurements available on their own websites. the links to those stations can be found at the cost 296 web site (http://www.cost296.rl.ac.uk/). in addition, these stations also provide data to data bases (db) maintained by cost 296 members. most of db keeps the acquired historical data, these being increased by received real-time data. the real-time data of modern ionosondes is deduced by intelligent algorithms (galkin et al., 2008a) providing the so called «auto-scaled data». though known, it is worth noting the importance of real-time data accessibility for monitoring, forecasting and nowcasting the state of the ionosphere. the cost prompt ionospheric database at the rutherford appleton laboratory (ral), uk (http://www.wdc.rl.ac.uk/cgi-bin/digisondes/cost_database.pl) continues to receive, catvol52,3,2009 20-09-2009 19:05 pagina 222 223 near earth space plasma monitoring under cost 296 alogue and archive auto-scaled vi data on a real time basis from ionospheric sounders across europe. this data set includes 11 contributing instruments in europe, at athens, chilton, dourbes, el arenosillo, juliusruh, lycksele, pruhonice, rome, ebro, tromsø and warsaw (cander, 2008). the dias project (http://www.iono.noa.gr/dias/), a pan-european distributed information server providing information on the ionospheric conditions over europe, is supporting the real-time acquisition and archiving of the ionospheric observations currently obtained from eight european ionospheric stations operating in athens, rome, juliusruh, chilton, ebro, pruhonice, lycksele and el arenosillo (belehaki et al., 2005, 2006). the ingv (istituto nazionale di geofisica e vulcanologia), rome, carried out the eswua project aimed to realize a hardware-software system for db to standardize historical and real time observations for different instruments installed by the upper atmosphere group of ingv (romano et al., 2008). the eswua db related to the rome digisonde and the 4 gistm (gps ionospheric scintillation and tec monitor) measurements. a dynamic website (www.eswua.ingv.it) has been opened to the community for real time access to raw and processed data. the space research centre (src) poland constructed a db for vi data with standard format for ionospheric data exchange iiwg (http://rwc.cbk.waw.pl/iiwg) which is being distributed via the international space environment service (ises) network and archiving data distributed worldwide. src also makes available daily reviews on solar, magnetic and ionospheric activity also. the center for atmospheric research of the university of massachusetts at lowell (umlcar) continued to archive in the didbase all digisonde data available in real time via internet (http://umlcar.uml.edu/didbase/) including the european cost 296 network. umlcar has proposed a new xmlbased ionosonde data exchange format allowing addition of measurement uncertainties to the data user, thus making the ionosonde auto-scaled data acceptable to the modern assimilation models based on the kalman filter method, such as the global assimilation ionospheric model, gaim, (galkin et al., 2008b). all of the above db handle groundbased ionospheric data, giving information on the bottomside ionosphere. the german aerospace center (dlr) automatically retrieves and archives of ionospheric radio occultation measurements onboard champ satellite, giving not only bottomside but also topside ionospheric information. electron density profiles from champ can be accessed via the new swaci service (jakowski et al., 2006) at http://w3swaci.dlr.de/. in addition to providing the real-time data, it is worth mentioning here the availability of data validated by experts. this type of data has an added value compared to the auto-scaled data, with a double application: they serve for testing the auto-scaling algorithms and a continuous feedback makes possible improvements to the above algorithms and a trusted ionospheric monitoring for research and modeling purposes. accordingly, the didbase archives edited ionogram data together with the autoscaled values for some stations. the edited data files include the name of the expert, and the didbase user can select the files with the «most trusted» editor. the institute of terrestrial magnetism, ionosphere and radio wave propagation (izmiran) makes available validated data of 14 ionospheric characteristics recorded with the ionosonde system «parus» at moscow (http://www.izmiran.ru/services/iweather/daily). the eswua db provides the validated data of ionospheric observatories at rome and gibilmanna, italy. the aforementioned cost prompt ionospheric database provides on-line validated data from the chilton ionosonde station, uk with one hour resolution. other individual ionospheric stations make available their edited data through their websites. the ebro observatory, spain makes available a catalogue of their validated data availability and a data request form (http://www.obsebre.es/php/ionosfera.php). most of these data have a one hour resolution, but 15 minute sampling data is available from may 2004 date. there are also several campaigns at 5 minute sampling available also, these being performed for detailed analyses of ionospheric behavior under specific events (solar eclipses, satellite validation data, etc). vol52,3,2009 20-09-2009 19:05 pagina 223 224 d. altadill, j. boska, l.r. cander, t. gulyaeva, b.w. reinisch, v. romano, a. krankowski, j. bremer, a. belehaki, i. stanislawska, n. jakowski and c. scotto 3. validation of monitoring data most of the research and work carried out in the past by the ionospheric community have been done using historical data validated by experts. the current situation has significantly changed mainly due to the facilities of the information society. a large amount of data is now available in real-time providing products to users soon after the data have been produced. however, the real-time data and related products should contain warnings to users noting their quality and consistency. this section discusses the task carried out by the cost 296 community to validate the quality and consistency of monitoring data and products, particularly those collected in real time. a large amount of ionospheric data produced and used by the cost 296 community has been recorded by vi sounders, in particular the digisondes built by the umlcar (http://ulcar.uml.edu/digisonde.html). this system is supplied with software of automatic scaling of ionograms (artist), providing ionospheric data in quasi-real time. sometimes, however, the autoscaled data are not good enough and data users should be warned. a significant effort has been made to improve the autoscaled data by the artist software (galkin et al., 2008a) and uncertainties of the data have been added for user information. this has been done in part thanks to efforts of the experts validating data and subsequent comparisons and statistics. though validated data are recommended for research purposes and for monitoring fine details of the ionospheric structure, most monitoring ionosonde stations now rely on automatic processing rather than edited processing to provide ionospheric characteristics. a systematic assessment of the quality of all the key ionospheric characteristics scaled automatically was made from hourly ionograms from the midlatitude chilton ionosonde in the united kingdom, by comparing them with the definitive values produced by manual scaling (bamford et al., 2008). the above comparisons clearly showed the improvement the goodness of the autoscaling software from the late 1990s to date (fig.1). fig. 1. time evolution of the correlation coefficients calculated for years 1996-2004 between manually scaled and autoscaled values of the indicated ionospheric characteristics. results are obtained for chilton station. adapted from bamford et al. (2008). vol52,3,2009 20-09-2009 19:05 pagina 224 225 near earth space plasma monitoring under cost 296 the investigation has been the first comprehensive examination of the performance of automatic scaling without any data pre-selection over an extended period covering the majority of solar cycle 23. the accuracy of autoscaled values during storm periods was examined against the global storm index dst for the whole 9-year data set. geomagnetic conditions were found to have only a small effect on autoscaling performance, with the most important identifiable cause of error being the truncation of automatic layer traces due to broadcast interference. overall, the performance of the autoscaling algorithms was found to be acceptable, with the characteristics fof2, h’e, m(3000)f2, and muf(3000)f2 within defined error bounds for more than 90% of the time and all characteristics within these bounds more than 80% of the time. also, the performance of the artist algorithm used in digisondes for n(h) profiles and doppler system measurements (common volume measurements) were tested for pruhonice station (buresova et al., 2007). it was found there that the results of artist, as for height determination in n(h)-profiles, are essentially of good quality and reliable under quiet geomagnetic conditions and in the absence of the sporadic-e layer, but rather unreliable during moderate and strong geomagnetic storms or in the presence of a well-developed sporadic-e layer. the discrepancies could be related to the uncertainties of the observational inputs and to the interpretation of the digisonde data. the groundbased data generated by digisondes served for calibration and validation of space-borne sensors, as the guvi instrument onboard the timed spacecraft (demajistre et al., 2007) and calval campaign for uv sensor onboard dmsp f-16 spacecraft and cosmic constellation radio occultation profiles. the cvalval was supported by the real-time work of 34 digisondes (10 of them located in europe). the cosmic (constellation observing system for meteorology, ionosphere, and climate) campaign in december 2006 was supported by 41 digisondes around the globe collecting electron density profiles at a 5-min cadence, including nine of the european cost stations. in addition to the above, the cost 296 community contributed to testing and validating other data and techniques. the changing state of the ionosphere is generally monitored by networks of vertical ionosondes that provide us with regular ionospheric sounding. many ionospheric applications require determination of the true-height electron density profiles n(h). therefore, ionograms must be further inverted into n(h). the consistency of two different methods of obtaining electron density profiles from inversion of ionograms polan (titheridge, 1985) and nhpc (huang and reinish, 1996; reinish et al., 2005), widely used by the ionospheric research community was analyzed. the analyses were carried out for two midlatitude ionospheric stations (sauli et al., 2007) and the results show significant systematic differences between n(h) calculated by these two inversion methods. nhpc provides smoother time variation than polan does. polan systematically obtains heights lower than nhpc for nighttime profiles, the opposite being true for daytime profiles. the best agreement of these two methods is seen for two-layer profiles. location of the largest difference between profiles corresponds to the f1 region and transition region between f1 and f2 regions. the results are consistent at two distant observatories and they remain the same through changing solar and geomagnetic activity. as the validation of vtec maps refers, the accuracy of the european ral vtec maps under stormy conditions of 17-21 january 2005 were assessed by applying the tests used to validate the international gnss service (igs) vtec (orus et al., 2007). the results show discrepancies between the ral vtec and igs maps which lead to significant rms and bias values regarding to the self-consistency and altimeter test of several tec units during storm conditions. a kriging technique applied in this work gave a relative improvement up to 26% in the highest storm conditions. 4. development of ionospheric monitoring techniques and parameters the main monitoring techniques and parameters describing the state of the ionospheric plasma developed by the cost 296 communivol52,3,2009 20-09-2009 19:05 pagina 225 226 d. altadill, j. boska, l.r. cander, t. gulyaeva, b.w. reinisch, v. romano, a. krankowski, j. bremer, a. belehaki, i. stanislawska, n. jakowski and c. scotto ty, including ground-based and space based techniques, are discussed in this section. the variability of the critical frequency of the f2 layer, fof2, over ionospheric station grocka (44.48°n, 20.31°e) was studied during the declining phase of the solar cycle 23 from 2004 to 2006 (mitic and cander, 2008). the variability index was introduced to identify the daily and seasonal patterns characterizing the local mid-latitude ionosphere during quiet and disturbed geomagnetic conditions. in addition, the behavior of the vtec values, obtained from gps measurements in the surrounding area under these conditions is reported. the analysis shows a number of interesting features representative of the ionospheric variability relevant for ionospheric modeling as well as ionospheric propagation applications based on a single station approach. an ionospheric activity index was developed in the dias project co-funded by the econtent program of the european union (belehaki et al., 2005). it proposed a new ionospheric activity index derived from automatically scaled online data from several european ionosonde stations (bremer et al., 2006). the index describes the ionospheric disturbance level at different european ionosonde stations and it is derived on-line from automatically scaled ionosonde observations. these indices are used to distinguish between normal ionospheric conditions expected from prevailing solar activity and ionospheric disturbances caused by specific solar and atmospheric events (flares, coronal mass ejections, atmospheric waves, etc.). the most reliable indices are derived from the maximum electron density of the ionospheric f2-layer expressed by the maximum critical frequency fof2. similar indices derived from ionospheric m(3000)f2 values show a markedly lower variability indicating that the changes in altitude of the f2-layer maximum are proportionally smaller than those estimated from the maximum electron density in the f2-layer. by using the ionospheric activity indices for several stations the ionospheric disturbance level over a substantial part of europe (34°n-60°n; 5°w-40°e) can now be displayed online. a planetary index of the ionospheric changes does not exist so far because it can be difficult to immediately deduce it from a mixture of increases and decreases of plasma density and electron content on the globe. this task is resolved using the numerical gps-ionex maps of the vtec, available daily since 1998 (gulyaeva and stanislawska, 2008). the vtec values are extracted at 600 grid points of the map at latitudes 60ºn to 60ºs with a step of 5º and longitudes 0º to 345ºe with a step of 15º providing hourly values for 0 to 23 h of local time for a representative set of conditions during a period 1999-2008. the local effects of solar radiative energy are filtered out. the derivation of the planetary perturbation index is a two-step process. the first step consists of generating a w-index map, a degree of perturbation computed as the logarithm of vtec relative to quiet reference taken as daily-hourly median for 27 days prior the day of observation. the planetary wp index is obtained in the second step from the above w-index map as the latitudinal average of the ranges between maximum positive index and minimum negative index which are weighted by the latitude-longitude extent of the extreme index values on the map. the single-sign wp index varies from 2 to 16 units providing the solar cycle variation of the planetary ionospheric storms. the wp index shows equinoctial maxima, these being correlated with the dst index. the planetary wp index discloses the disturbances which may belong either to magnetosphere or the ionosphere-plasmasphere system or both providing broader proxy index driving the space weather than the geomagnetic indices alone. instantaneous mapping techniques have been applied to monitor european, asian and japanese region as well as a 24 hours ahead forecast for the above regions and at some ionospheric stations (stanislawska et al., 2000). the ingv has developed a computer program (autoscala) for the automatic scaling of the critical frequency fof2 and muf(3000)f2 from ionograms (pezzopane and scotto, 2007) that has been extended for obtaining the sporadic-e layer (scotto and pezzopane, 2007) and the f1 layer (pezzopane and scotto, 2008). autoscala was designed to scale automatically the ionospheric parameters from the ionograms vol52,3,2009 20-09-2009 19:05 pagina 226 227 near earth space plasma monitoring under cost 296 recorded by the ais (advanced ionospheric sounder) developed at the ingv but it can be easily applied to scale the ionograms obtained by any kind of ionosonde. the ingv has recently developed the above software related to the ais with a routine for the real time computation of the electron density profile, which is essential for space weather applications. the electron density profile is computed with a model (called adaptive ionospheric profiler, aip) with 12 free parameters (6 related to the e-region and 6 to f2-f1 layers). the parameters defining the profile are initially estimated considering the helio-geophysical conditions and the ionospheric characteristics obtained automatically from the ionograms by autoscala. then the candidate profile is inverted into trace and is adapted to a final computed profile by minimizing the root mean square error between the trace restored from the candidate profile and the recorded one. the model is capable of describing a wide set of ionospheric profiles and the related research has been accepted for publication in advances in space research. the standard dda method of drift velocity evaluation (reinisch et al., 2005) has been improved by skymap-points selection in three steps as a quality control and improvement (kouba et al., 2008): (i) robust height range selection, (ii) setting limits on doppler frequency shift, and (iii) setting limits on the echo arrival angle. such approach requires a sufficiently large amount of data points in skymap. raw drift data from pruhonice observatory for period jan-may 2006 were recalculated using this method. preliminary results show the behavior of the f-region drift: velocity components diurnal variability during quiet geomagnetic conditions and seasonal trends of daily characteristics. as significant decrease in the daily-maximal horizontal component from winter to summer 2006 was found within the analyzed data. this method excludes multiple and/or too oblique reflections. the above method enables e-region and f-region drifts to be separated (this is not routinely done by any other ionospheric sounding station), characteristics of which are substantially different. since may 2005, the pruhonice digisonde measures e-region drifts every 15 minutes, using four fixed frequencies between 2.0 and 2.6 mhz. unlike the autodrift setting, e-region sounding frequencies do not depend on critical frequency, they are set and fixed for all measurements. during summer 2006 the first special campaign for monitoring drifts in es-layer was performed. drift-measurement on a higher sounding-frequency window 3.2-4.7 mhz was run every 15 minutes in addition to the standard eregion drift measurement. e-region drift measurement with a two frequency-window setting represents an important source of information on the dynamics of the e-region ionosphere and brings new pieces of information on sporadic e layer formation and its behavior. differences of the plasma motion confirm the different dynamics of e and es layers. the new phase-difference technique for digisondes was evaluated for monitoring precise heights records of ionospheric layers. the program settings for this technique were provided by the umlcar. the technique, which analyzes the phase differences between signals at slightly different frequencies, allows measurement of the reflection range, i.e., the virtual height h’(f) for vertical sounding, with accuracies better than one kilometer (reinisch et al., 2008). first results of measurements carried out at millstone hill demonstrate the robustness and reliability of the developed technique, and show the potential of the method for routine application. the technique has been applied in a specific campaign of precise measurements of virtual height of the e-layer carried out under the international heliophysical year (ihy) initiatives of the cost 296 whose preliminary results will be discussed later. the mapping techniques developed by krankowski et al. (2007) provide regular vtec monitoring over europe. the vtec maps are created from gps observations collected at igs and euref permanent network (epn). the large number of stations in europe provides good data coverage yelds high-accuracy vtec maps with an error at a level of 1-3 tec units. the vtec maps are available with a spatial resolution of 100-300 km and a time resolution of 5 minutes. figure 2 shows examples of the above tec mapping technique above european area for particular time intervals. vol52,3,2009 20-09-2009 19:05 pagina 227 228 d. altadill, j. boska, l.r. cander, t. gulyaeva, b.w. reinisch, v. romano, a. krankowski, j. bremer, a. belehaki, i. stanislawska, n. jakowski and c. scotto 5. dissemination of data and products most of the above ionospheric data and products have been set on-line and disseminated to the e research community and users. as already mentioned above, most institutions participating in the cost 296 action have their own websites making available their data and products. this section highlights, the main tasks carried out by the cost 296 community to support and develop internet sites and protocols for disseminating data products. the space weather web facilities for radio communications users of the ral has continued to maintain, support and improve (http://ionosphere.rcru.rl.ac.uk/). this facility is based on the contributions of a number of cost 296 participating institutions and it is 24/7 on-line service that includes the following products: (1) interactive forecast maps of fof2, muf(3000)f2 and itu-r nequick modelled tec values over europe based on ionosonde measurements. (2) near real-time dynamic system for monitoring ionospheric propagation conditions over europe. (3) near real-time tec maps over europe and 24 hours single station plots based on tec evaluation from igs gps measurements. (4) near real-time solar-terrestrial and ionospheric indices and warning messages so that ionospheric and trans-ionospheric propagation conditions are known to worldwide users. (5) archive of all data and images. the service has also been extended to enable users to extract ionospheric profiles from standard archive output files (sao) as calculated by the nhpc algorithm applied by umlcar digisondes, in addition to those calculated by polan. the server at regional warning centre (rwc) of the src, poland, fed with ursigrams of cost 296 stations and others provides some ionospheric characteristics in quasi-real time and from the previous 2 months (http://cbk.waw.pl/rwc and ftp://www.cbk.waw.pl/idce/). in addition, it provides 24 hours ahead forecast maps of ionospheric characteristics for european, east asia and australia regions and the forecast of some ionospheric characteristics for individual stations. in particular, the web services of the ionospheric dispatch centre in eufig. 2. the detailed tec maps over europe with 5-minute resolution during the eclipse day on 3 october 2005 (a) and one day after the solar eclipse on 4 october 2005. vol52,3,2009 20-09-2009 19:05 pagina 228 229 near earth space plasma monitoring under cost 296 rope (src) provide on line access to data base of the critical frequency of f2 ionospheric layer and m(3000)f2 forecast for all available sites. the ionosonde data are available from 1983 up to the latest measurements from all over the world in iiwg format. the gps measurements from warsaw station are also available. daily plots for 30 stations from all over the world are presented along with their digital version. it also makes available daily reviews on solar, magnetic and ionospheric activity. catalogues of the quiet and disturbed days and of ionospheric disturbed periods lasting three hours or longer are compiled and presented for european stations (these catalogues are available at the izmiran web site also). finally, the links to the websites of cost 296 related topics are available on the web navigator (http://rwc.cbk.waw. pl/cost296/ ). the izmiran team has set up a website on ionospheric weather (http://www.izmiran.ru/services/iweather/). data of 30 ionospheric stations (fof2, hmf2 and their deliverables) are presented therein for the current month. the past data are available in the archive for two years proceding the current month. missed data are reconstructed by cloning data of other stations so the data sets are complete for the daily-hourly grids (gulyaeva et al., 2008). the ionospheric disturbances are being indexed in comparison with the running median of the preceding 27 days calibrated with itur prediction of the seasonal trend. a proxy for fof2 critical frequency has been created by applying a technique for reducing the fof2 critical frequency as a function of the solar zenith angle (gulyaeva, 2009). the technique improves the correlation between the data of different stations and the inter-seasonal correlations at a given location, and it is applied for reconstruction of missed observations of fof2 at 30 stations worldwide, thereby increasing the number of stations data at the ionospheric weather web site, including some stations at low and high latitudes in both north and south hemispheres. a dynamic website has been opened to the community for real time access to raw and processed data within the eswua project of the ingv (www.eswua.ingv.it), also reported in section 2. the site is based on measurements performed by all the instruments installed by the upper atmosphere group of ingv (romano et al., 2008). this interactive website supports a well organized data base aiming to be a powerful tool for the scientific and technological community in the field of telecommunications and space weather. the eswua is contributing to the projects in the frame of international collaborations where the interoperability of the system and effective data access are necessary requirements (virtual observatories, www.egy.org, and interhemispheric conjugacy effects in solarterrestrial and aeronomy research, icestar, http://www.scar-icestar.org). the eswua allows the user to explore and download validated vertical sounding data since 1976, view the ionograms and access the automatic scaled data. specific tools to survey the ionospheric behaviour both for real and historic time have been developed and are already accessible by web. 6. cost 296 campaigns using the potential of the different observing and monitoring systems belonging to the participating institutions in cost 296, two campaigns have been carried out, a campaign to follow ionospheric effects of the annular eclipse occurred over europe on 3 october 2005 and a campaign contributing to the objectives of the third cawses global tidal campaign from 1 june to 14 august 2007. on the occasion of the annular eclipse, which occurred over europe in the morning hours of 3 october 2005, the cost 296 community prepared a specific campaign for monitoring its effects on the ionosphere to obtain a comprehensive view of the eclipse by utilizing different observation techniques. the well-defined obscuration function of solar radiation during the eclipse provided a good opportunity to study the ionospheric/thermospheric response to solar radiation changes during the eclipse (jakowski et al., 2008). since the peak electron density behavior of the ionospheric f2 layer follows the local balance of plasma production, loss and transport, the ionospheric plasma redistribution processes significantly affect the shape of the electron density profile. these processes have been discussed based on vol52,3,2009 20-09-2009 19:05 pagina 229 230 d. altadill, j. boska, l.r. cander, t. gulyaeva, b.w. reinisch, v. romano, a. krankowski, j. bremer, a. belehaki, i. stanislawska, n. jakowski and c. scotto a comparison of vertical incidence sounding (vs) and vertical total electron content (tec) data above-selected ionosonde stations in europe. the equivalent slab thickness, derived with a time resolution of 10 min, provides relatively good information on the variation of the electron density profile during the eclipse. the computations reveal an increased width of the ionosphere around the maximum phase (fig. 3). as indicated by the available measurements over spain, the photo production is significantly reduced during the event leading to a slower increase in the total ionization in comparison with the neighboring days. the supersonic motion of the moon’s cool shadow through the atmosphere possibly generated atmospheric waves that were detected at ionospheric heights above the spanish station (fig. 4). high-frequency (hf) doppler shift spectrograms were recorded during the eclipse showing a distinct disturbance along the eclipse path. although ionosonde and hf doppler measurements show enhanced wave activity, the tec data analysis does not, which is an indication that the wave amplitudes are too small for detecting them via this interpolation method. more about wave activity is in bremer et al. (this issue). the total ionization decreases up to about 30% during the event. a comparison with similar observations from the total solar eclipse of 11 august 1999 revealed a remarkably different ionospheric behavior at different latitudes. a detailed analysis based on gps observations from euref were used to observe the response of tec (total electron content) to the total solar eclipse on october 3, 2005 (krankowski et al., 2008). the effect of the eclipse was detected in diurnal variations and more distinctly in the variations of tec along individual satellite passes. the trough-like variations with a gradual decrease and followed by an increase in tec at the time of the eclipse were observed over a large region. the depression of tec amounted to 3-4 tecu. the maximum depression was observed over all stations located at the maximum path of the fig. 3. variations of the ionospheric slab-thickness over the ebro, spain (left) and juliusruh, germany (right) stations during the solar eclipse on 3 october 2005. after jakowski et al. (2008). vol52,3,2009 20-09-2009 19:05 pagina 230 231 near earth space plasma monitoring under cost 296 solar eclipse. the delay of a minimum level of tec with respect to the maximum phase of the eclipse was about 20-30 min. the two-dimensional tec maps constructed with high temporal resolution (5-min interval) show that the eclipse produced remarkable changes in the structure of the ionosphere. these tec maps demonstrate also that the depression of tec reached 20-30% compared to a quiet day (october 4, 2005). the cost 296 community had performed a coordinated e-layer precision group height measurement (pghm) campaign to contribute to the objectives of the third cawses global tidal campaign run from 1 june to 14 august 2007. the campaign was based on the advantage of high doppler resolution mode technique embedded in the digisondes which makes able to measure the virtual heights h’(f) of the e-layer with an accuracy better than 1 km (reinisch et al., 2008). eight european ionospheric stations participated to this initiative; tromso (69.6°n, 19.2°e), juliusruh (54.6°n, 13.4°e), chilton (51.6°n, 358.7°e), pruhonice (50.0°n, 14.6°e), rome (41.8°n, 12.5°e), ebro (40.8°n, 0.5°e), athens (38.0°n, 23.6°e), and arenosillo (37.1°n, 353.3°e). ebro and arenosillo had stopped the campaign on july 14 due to noisy and unreliable measurements. athens and rome recorded good nighttime data, making possible analyses for nighttime sporadic e-layer height changes. the other stations recorded high quality measurements clearly showing the two different regimes for nighttime and daytime elayer height variations. an example of the campaign measurements recorded at chilton station is depicted in fig. 5. a number of interesting effects on the daily variations of the e-region virtual heights were observed: day-to-day variability of the e-region heights, repetitive «hooks» in the height records (inter-diurnal variations) and a distinct diurnal variation. preliminary ideas on the observed effects suggest that day-to-day variability of the e-region height variations could be related to planetary wave modulation of metallic ion transport while inter-diurnal variations may be caused by tidal/gravity wave activity in that region. although not shown here, spectral analysis revealed other interesting phenomena: station-to-station variations in the diurnal harmonfig. 4. contour of the electron density above ebro station, spain as function of time and height for october 3, 2005. note the train of height oscillations starting at about 08:00 ut and further developed from 10:00 to 14:00 ut. after jakowski et al. (2008). vol52,3,2009 20-09-2009 19:05 pagina 231 232 d. altadill, j. boska, l.r. cander, t. gulyaeva, b.w. reinisch, v. romano, a. krankowski, j. bremer, a. belehaki, i. stanislawska, n. jakowski and c. scotto ics (24, 12, 8 hours), and possible evidence for the coupling of tidal (24 h) and long period oscillations (120 h). results of the data analysis demonstrated great potential offered by the ionospheric sounding with enhanced range resolution. though results presented are focused on the e-region, the technique may be applied to other ionospheric regions. 7. summary and concluding remarks the main achievements of the near earth space plasma monitoring under cost 296 may be summarized as follows. the historical and real-time data bases of the soundings of the ionosphere have been maintained and increased with continuous flowing of monitoring data. the cost 296 community succeeded in making data standardized and available to the ionospheric community for their purposes as research, modeling and telecommunication applications, this being possible throughout the websites of the stations managed by cost 296 participants. many validated data have also been made available. the added value of the validated data served for testing the auto-scaling algorithms and a continuous feedback makes possible improvements to the above algorithms, and a trusted ionospheric monitoring for research and modeling purposes. the quality of the realtime data quality was systematically assessed for key ionospheric parameters, serving to provide data users measurement uncertainties and fig. 5. e-region precision group height measurements at chilton, uk as function of time for june 2007. the horizontal axes represent time (universal time) and the vertical axes represent virtual height (km). the grey shaded areas indicate night-time measurements and the arrows point out significant inter-diurnal height variations as hook shaped or trains of height oscillations. vol52,3,2009 20-09-2009 19:05 pagina 232 233 near earth space plasma monitoring under cost 296 to improve automatic scaling algorithms, and so making the real-time data acceptable to modern assimilation models. the availability of the edited data also served for calibration and validation of space-borne sensors. moreover, a technique for improving the discrepancies of different mapping services under stormy conditions has been developed. further techniques and parameters have been developed for monitoring the near earth space plasma. time dependent 2d maps of tec and key ionospheric parameters are available on-line throughout the websites of the cost 296 participants and activity indices for distinguishing disturbed ionospheric conditions have been obtained for real-time applications. most of the above achievements have been obtained for ground vi data or for vtec derived from gnss signals. the potential of the different observing systems for ionospheric monitoring has been combined to obtain a comprehensive view of the effects of a solar eclipse on the ionosphere. thanks to the cooperation for the last 4 years in the frame of the cost 296 action, significant progress has been achieved for data availability and validation, 2d monitoring products, and dissemination of products. however, a step forward is needed in the future, especially to combine different ionospheric observing systems for monitoring. the development of 4d techniques of ionospheric monitoring (time dependent 3d products) would be the goal of further cooperation actions. acknowledgements the authors wish to thank cost for support and to acknowledge the work of many observers and technicians at european ionospheric stations; their work is quite crucial for ionospheric research. j.b. thanks the ministry of education, youth and sports of the czech republic for support of czech participation in cost 296 through grant oc-091. the contribution of d.a. has been partly supported by project cgl2006-12437-c0202/ant of the ministry of education and science of spain. references bamford, r.a., r. stamper and l.r. cander (2008): cost 271 action effects of the upper atmosphere on terrestrial and earth-space communications: introduction, radio sci., 43, rs1001, doi:10.1029/2005rs003401915-925. belehaki, a., l.r. cander, b. zolesi, j. bremer, c. juren, i. stanislawska, d. dialetis and m. hatzopoulos (2005): dias project: the establishment of a european digital upper atmosphere server, j. atmos. solar-terr. phys., 67, 1092-1099. belehaki, a., l.r. cander, b. zolesi, j. bremer, c. juren, i. stanislawska, d. dialetis and m. hatzopoulos (2006): monitoring and forecasting the ionosphere over europe: the dias project, space weather, 4, s12002, doi:10.1029/2006sw000270, 2006. bremer, j., l.r. cander, j. mielich and r. stamper (2006): derivation and test of ionospheric activity indices from real-time ionosonde observations in the european region, j. atmos. solar-terr. phys., 68, 2075-2090. bremer, j., j. lastovicka, a.v. mikhailov, d. altadill, p. bencze, d. buresova, g. de franceschi, c. jacobi, s. kouris, l. perrone and e. turunen (2009): climate of the upper atmosphere, (this issue). buresova, d., v.m. krasnov, y.v. drobzheva, j. lastovicka, j. chum and f. hruska (2007): a method for ionogram interpretation quality using the hf doppler technique, ann. geophysicae, 25, 895-204. cander, l.r. 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(1985): ionogram analysis with the generalised program polan, uag report-93, (http://www.ips.gov.au/ipshosted/inag/uag_93/uag_ 93.html). zolesi, b., l.r. cander and d. altadill (2007): from cost 271 to 296 eu actions on ionospheric monitoring and modelling for terrestrial and earth–space radio systems, adv. space res., 39, 899-903. zolesi, b., and l.r. cander (2008): from cost 238 to cost 296: four european cost actions on ionospheric physics and radio propagation, in radio sounding and plasma physics, aip conf. proc., 974, 39-46. vol52,3,2009 20-09-2009 19:05 pagina 234 vol50,5,2007 615 annals of geophysics, vol. 50, n. 5, october 2007 key words equatorial f-region – ionospheric variability – ionospheric modelling – ionosonde observations – ionospheric dynamics 1. introduction the nature of ionospheric plasma motions in the f-region at the magnetic equator is such that they are driven by the neutral air wind along the earth’s magnetic field and by the electric field in the plane perpendicular to it. accordingly, measurements of vertical plasma drifts in the equatorial f-region are a valuable means of deriving information on the electric field. the plasma drifts, however, in the equatorial zone significantly affect the morphology of the equatorial f-region, the evolution of the appleton anomaly, the latitudinal variation of thermospheric neutral winds, and the low-latitude protonospheric ion composition (fejer and scherliess, 1997). furthermore, equatorial e×b drift velocities are significant input parameters that go into many ionospheric models, because they help describe vertical plasma motion near the magnetic equator. in the evening sector, vertical drifts are of particular significance as they are the major drivers for the generation of equatorial spread f (esf) (fejer et al., 1999; martinis et al., 2005). the quiet time low-latitude plasma drifts are driven by a complex interaction of eand f-region electrodynamic processes (richmond, 1995). the relative efficiency of these dynamo mechanisms varies significantly with the time of day, from day to day, and also with season, solar cycle, and longitude. theoretical and numerical morphology of f-region vertical e××b drifts in the african sector using ionosonde measurements oyedemi s. oyekola and cornelius c. oluwafemi department of physics, college of science and technology, covenant university, canaanland, ota, ogun state, nigeria abstract f-region vertical velocities are derived from the ground-based ionosonde data for ibadan (7.4°n, 3.9°e; dip 6°s: an equatorial station in the african zone), to study the general characteristics of electrodynamics of equatorial ionosphere, such as their variation with season, solar cycle, and magnetic activity at different local time sectors. the results show profound seasonal and geomagnetic effects. except for equinoctial period, there is an excellent consistency in the magnitudes (nearly 20 m/s) and patterns of upward daytime f-region drifts at low and high solar activity periods. evening f-region exhibits strong motion with absolute mean value for quiet-time (15 m/s) greater than on disturbed-time (10 m/s). the average downward quiet midnight-early morning hours sector value is well below than 10 m/s. the evening reversal time is earliest and latest during solstitial periods. prereversal peak is season dependent and varies strongly with magnetic activity. we show that prereversal peak, daytime, and nighttime maximum drifts saturate at particular values of f10.7 cm solar radio flux index, effects not noticed with corresponding sunspot number. our observations confirm several previous results from other equatorial sites utilizing different experimental techniques. mailing address: dr. oyedemi s. oyekola, department of physics, college of science and technology, covenant university, canaanland, ota, ogun state, nigeria; e-mail: osoyekola@yahoo.com 616 oyedemi s. oyekola and cornelius c. oluwafemi models that explain the low-latitude dynamo electric fields and currents are plentiful (rishbeth, 1971; heelis et al.,1974; richmond et al., 1976; anderson and mendillo, 1983; takeda and maeda, 1983; farley et al., 1986; takeda and yamada, 1987; kelley, 1989; haerendel and eccles, 1992; eccles, 1998). during geomagnetic active conditions magnetospheric and disturbance dynamo effects can dramatically affect the low-latitude drifts (fejer and scherliess, 1997, and references therein; scherliess and fejer, 1997; kelley et al., 2003). a prereversal peak in the upward velocity in the dusk sector is the most common feature during solar cycle maximum years (woodman, 1970). this phenomenon occurs just before the f-region vertical plasma drift reverses from upward (positive) to downward (negative) direction. prereversal velocity enhancement is believed to be caused by the very rapid decrease in the e-region conductivity after sunset compared to that of the f-region (farley et al., 1986). the evening upward velocity enhancement is responsible for the rapid rise of the equatorial f-layer after sunset, which plays an important role in the generation of eand f-region plasma instabilities (fejer and kelley, 1980; fesen et al., 2000). nevertheless, on the basis of extensive data from various experimental observation techniques, climatological regional models of the equatorial plasma drifts in the peruvian, brazilian, and indian sectors have been developed. global empirical models for the quiet time fregion equatorial vertical drifts have been reported (richmond et al., 1980; scherliess and fejer, 1999). additionally, a study carried out by anderson et al. (2002) showed that daytime measurement of the difference between the horizontal component of the magnetic field at an equatorial and a slightly off-equatorial station can be used to infer the equatorial vertical e×b drift (eastward electric field). thus, equatorial and low-latitude studies indicate that under quiet magnetic conditions, to first order, the f-region electrodynamic plasma drifts are upward and westward during the day and downward and eastward at night. however, these probing techniques for investigating the bottomside ionospheric plasma give generally consistent equatorial vertical drift velocities, but often contradictory patterns are reported. unfortunately, most of these experimental observations of f-region plasma drifts described above are available only from a few locations at limited times in the african sector of the equatorial ionosphere. this remarkably sparse database in the african region is a major deficiency in global low latitude ionospheric modeling studies. this study used ground-based ionosonde data obtained near the magnetic equatorial station in africa (7.4°n, 3.9°e, dip. lat. 6°s) to investigate the general characteristics of f-region vertical plasma drifts, such as variation with local time, season, solar cycle, and magnetic activity. 2. date sets and method of analysis the first data set consists of a half-hourly data record (called ionogram) obtained during the daytime from 07:30-18:30 lt sector. five international quiet day ionograms from each month of the year were picked for the investigation. we ensured that the data chosen were good enough to be scaled, and magnetic storm days were avoided. the records were manually analysed using a ten-point kelso (1952) method to obtain electron density profiles at half a hour intervals. results of comparisons between automatically and manually scaled data have been reported to be in excellent agreement (renisch and huang, 1983; pezzopane and scotto, 2004, 2005). already, iheonu and oyekola (2006) have proposed a model description, obtained from a simplified electron continuity equation, for estimating daytime vertical e×b drift velocities in the equatorial f-region ionosphere. the model calculation is valid away from the peak of the f-layer, the formula is expressed as (2.1) where hp(h) represents plasma scale height, a function of height, q(h, χ) represents the rate of ionization production per unit volume and so is a function of both height and solar zenith distance, χ. the production rate is estimated from the ( , ) ( ) ( , ) ( ) ( )v n t h h h q h h h hz e p p χ β= − 617 morphology of f-region vertical e×b drifts in the african sector using ionosonde measurements «chapman theory» (chapman, 1931). ne(t, h) represents electron density at altitude, h and time t, obtained from ne(h) profiles. β(h) is the loss coefficients, a function of height, calculated from the expression given by ratcliffe and weeks (1960). the product hp(h) β(h) is the apparent vertical drift velocity due to chemical recombination. thus, we use eq. (2.1) to compute the vertical velocities during the daytime. accordingly, the vertical plasma drifts are divided into 2 solar activity, and 3 seasonal divisions under quiet geomagnetic condition during the daytime. january through december 1958 (era of international geophysical year, igy) comprises solar maximum with the averaged yearly f10.7 radio flux value of 208 (in units of 10−22 w/m2/hz) and the mean yearly sunspot number of 185. january to december 1964 comprises solar minimum with averaged yearly decimentric solar flux of 65 and the mean annual sunspot number of 9.3. f-region vertical drifts are also estimated from the h’f variations for the equatorial station ibadan from one year of data during 1958 period, which comprises the second dataset. this method has been used for several years to calculate vertical motion of the equatorial ionosphere in the peruvian, brazilian, and indian sectors (e.g., abdu et al., 1981; bittencourt, 1986; sastri et al., 1995), but such estimate has not been carried out at ibadan longitude sector. the technique is often limited to the evening hours when the f-region bottomside altitude rises beyond 300 km (fejer et al., 1989; jayachandran et al., 1993; subbarao and krishna murthy, 1994). in this case, our data are divided, according to 4 seasonal and 2 geomagnetic activity divisions at solar maximum. the months of the year are grouped as follows: march equinox (january, february, march), june solstice (april, may, june), september equinox (july, august, september), and december solstice (october, november, december). kp≤3 comprises quiet geomagnetic conditions and kp>3 comprises disturbed condition. 3. results seasonal averages of the vertical component of the equatorial f-region plasma drifts as derived from ionosonde data at ibadan during january to december 1964 (solar minimum), and january to december 1958 (solar maximum) under quiet magnetic conditions are plotted in fig. 1. standard deviations are shown as error bars on the two data sets in each panel. f10.7 index (the solar radio flux at a wavelength of 10.7 cm at earth’s orbit) are in the range 62-65, and 184-224 (in flux units) during low and high solar epoch, respectively. figure 1 indicates the classical behaviour for the vertical component of the equatorial f-region plasma drift velocity. it is upward during the daytime for low and high solar activity periods. as can be seen, except for equinoctial periods, there is an excellent consistency in the variability pattern of upward daytime f-region drifts during low and high solar flux conditions. the average value of upward daytime f-region plasma drifts for solar minimum equinox (top panel) (∼18.6 m/s) is smaller than that of solar maximum equinox fig. 1. average daytime f-region vertical plasma drifts obtained for three different seasons from ibadan ionosonde data during 1964 low solar activity (right verical axis) and 1958 high solar activity (left vertical axis) under quiet magnetic condition. the vertical bar indicates the standard deviation about the mean. 618 f ig . 2. e xa m pl es o f di ur na l an d se as on al v ar ia ti on s of v er ti ca l vi rt ua l ve lo ci ty d ed uc ed f ro m i on os on de d at a at i ba da n in t he f -r eg io n du ri ng d ay ti m e hi gh s ol ar f lu x co nd it io ns f or q ui et a nd d is tu rb ed p er io ds . 19 57 /1 95 8 in te rn at io na l g eo ph ys ic al y ea r (i g y ) da ta w as u se d. t he d as he d cu rv e (r ig ht v er ic al a xi s) g iv es t he m ea n va ri at io n of t he r ef le ct in g (h ’f ) la ye r. f ig . 3. ib ad an e ve ni ng f -r eg io n av er ag e ve rt ic al p la sm a dr if ts d er iv ed f ro m i on os on de o bs er va ti on s fo r th e th re em on th s ea so na l pe ri od s du ri ng un di st ur be d an d di st ur be d hi gh s ol ar a ct iv it y co nd it io ns . 1 95 7/ 19 58 i nt er na ti on al g eo ph ys ic al y ea r (i g y ) da ta a re u se d. t he d as he d cu rv e (r ig ht v er ic al a xi s) g iv es t he m ea n va ri at io n of t he r ef le ct in g (h ’f ) la ye r. 2 3 oyedemi s. oyekola and cornelius c. oluwafemi 619 morphology of f-region vertical e×b drifts in the african sector using ionosonde measurements (∼23.3 m/s). solar minimum winter data (middle panel) (∼15 m/s) is also notably lower than solar maximum winter value (∼19.1 m/s). on the other hand, low solar flux summer (bottom panel) magnitude (∼13.8 m/s) is much lower than high solar flux summer (∼20.9 m/s). the average daytime upward vertical drift velocity is nearly 16 m/s and 20 m/s during low and high solar activity, respectively. figure 2 shows 3-month seasonal periods of daytime f-region averaged virtual vertical velocities deduced from the virtual height changes in the time during quiet and disturbed geomagnetic conditions. the dashed curve on the right vertical axis gives the mean variation of the height of reflecting layer. the figures show that except for disturbed september equinox drift, which is about zero drift (∼0 m/s) throughout the daytime and short fluctuations between 11:00-14:00 lt in the corresponding quiet september equinox vertical velocity; the average virtual vertical velocities exhibit similar morphological pattern of variability. the variation appears as negative velocities (downward) in the morning sector and increases gradually until the drift reverses direction gently at about 13:00 lt in all seasons. notice that the velocity does not exceed 5 m/s either downward or upward from 08:00-16:00 lt. these weak velocities were observed by blanc and houngninou (1998) during the international equatorial electrojet year (1993-1994) in africa (korhogo: 9.24°n, 5.4°w, dip. 4°s), ivory-coast, using high resolution multi-frequency hf doppler radar measurements in the upper f-region during daytime on 21, 27, 30 may 1993. the results on diurnal, seasonal, and magnetic variations in the vertical drifts are presented in fig. 3. the dashed curve on the right vertical axis gives the mean variation of reflecting layer. again, apart from disturbed september equinox vertical drifts, which ∼0 m/s throughout the entire period; the trends for both datasets in each season are similar. figure 3 shows that the vertical drifts on all the seasons are characterized by upward enhancement followed by a downward reversal. the enhancements have peak velocity in the range 25-40 m/s and occur at around 19:00 lt during quiet period. the average quiet evening prereversal peak e×b drift is typically 30 m/s and occurs at 19:00 lt. conversely, the values of prereversal peak velocity during disturbed march equinox and june solstice are 26 m/s and 23 m/s, respectively; whereas for september equinox, it is absolutely insignificant, while december solstice it is much smaller than 20 m/s. figure 3 also indicates that geomagnetic activity effect (as determined by the kp index) on the vertical plasma drifts show a noticeable season dependency. the late afternoon to evening sector seasonal average values for magnetically quiet periods are consistently larger than on disturbed periods. the estimated mean magnitude of the drifts for quiet periods are ∼17.4 m/s, ∼11.5 m/s, ∼13.7 m/s, and ∼16.4 m/s for march equinox, june solstice, september equinox, and december solstice, respectively, while the absolute average values for disturbed march equinox, june solstice, september equinox, and december solstice drifts are 12.4 m/s, 10.6 m/s, 3.5 m/s, and 14.2 m/s, in that order. the reversal time is latest during the june solstice months for quiet and disturbed drifts and earliest in all other seasons. the most regular feature of all the curves is the gradual reversal of drifts. it is noteworthy to see that the vertical drifts reverse direction during the periods of high f-layer heights. the average virtual height of the bottomside f-layer (h’f) is lifted as high as 412 km at 19:00 lt (march equinox), 461 km at 21:00 lt (june solstice), 459 km at 20:00 lt (september equinox), and 450 km at 19:00 lt (december solstice). certainly, we can say that the postsunset heights observed here appear to be meaningful threshold values needed to seed esf. figure 4 illustrates the behaviors of vertical drifts data in the midnight to early morning sector. the vertical bars are calculated standard deviations about the mean. the dashed curve yet again (right vertical axis) gives the mean variation of the height of reflecting layer. figure 4 indicates noticeable variability in the occurrence patterns of vertical plasma drifts even between equinoctial and solstitial seasonal periods. generally, quiet and disturbed vertical drift velocities are characterized by variations in the downward direction during the entire period until the drifts reverse to upward direction at about 05:00 lt, except june solstice drift data, which reverse upward at 23:00 lt followed by 620 oyedemi s. oyekola and cornelius c. oluwafemi a gentle downward reversal at midnight. magnetic activity effects are also apparent on the vertical plasma drift velocity, particularly between midnight to 02:00 lt in june solstice. the average downward nighttime drift velocity is as low as ∼4 m/s (march equinox) and as high as ∼12 m/s (june solstice) during quiet period. evening f-region prereversal upward velocity peaks as a function of f10.7 cm solar flux index for both quiet and disturbed magnetic activity is displayed in fig. 5. the best fits to the data are also shown. the drifts values correspond to monthly averaged values of f10.7 cm. the slopes relating prereversal peak velocity to f10.7 during quiet period are about 36% smaller than during disturbed magnetic activity, but the correlation coefficient value is also approximately 52% smaller during quiet period than disturbed time. furthermore, the prereversal peak velocity appears to saturate around 185, 205, and 220 (in solar flux units). figure 6 further illustrates the dependence of prereversal peak e×b drift on solar and magnetic activity. here, the slopes connecting the evening f-region prereversal upward fig. 4. ibadan nighttime f-region average vertical plasma drifts derived from ionosonde observations for the three-month seasonal periods during undisturbed and disturbed high solar activity periods. the vertical bars are calculated absolute standard deviations about the mean. the dashed curve (right vertical axis) gives the mean variation of reflecting layer. 1957/1958 igy data are used. 621 morphology of f-region vertical e×b drifts in the african sector using ionosonde measurements fig. 5. evening f-region prereversal upward velocity peaks as a function of solar flux for both quiet and disturbed magnetic activity. the monthly values of velocity peaks correspond to the monthly mean values of solar flux. linear fits to the data are shown. fig. 6. evening f-region prereversal upward velocity peaks as a function of sunspot numbers for both quiet and disturbed magnetic activity using the same format as in fig. 5. velocity peaks to monthly averaged sunspot number (r12) during magnetically quiet period is just about 27% smaller than during magnetically active period, while the regression coefficient is nearly 49% lower in value during quiet time than disturbed time. figure 7 compares peak upward daytime (upper panel) and peak downward nighttime (bottom panel) vertical velocity patterns versus f10.7 solar flux index during quiet magnetic conditions. notice that the monthly values of maximum velocities correspond to the monthly average values of solar index. the first feature to note in fig. 7 is that the vertical drifts saturate around the f10.7 values mentioned for fig. 5 during both the daytime and nighttime periods. the slopes of the lines are different by about 90%. such a much larger discrepancy clearly points to a reasonable confirmation of solar influence on maximum upward daytime f-region plasma drifts variability. maximum upward daytime and downward nighttime vertical drifts versus sunspot numbers using the format as in fig. 7 are shown in fig. 8. the situation is rather different, however, from the variability pattern of maximum vertical drifts obtained with solar flux. vertical plasma drifts vary around their monthly averaged upward daytime value of approximately 28 m/s (±9.76 m/s) (the upper panel), while peak downward nighttime drifts vary about their monthly mean magnitude of nearly 18.7 m/s (±5 m/s) (the bottom panel), where the values in the brackets indicate standard deviations of the mean. the slopes relating maximum vertical drifts with sunspot numbers during daytime and nighttime periods are consistent at ∼0.1. 4. discussion the data presented above clearly described the general patterns of f-region vertical plasma drifts at different local time sectors, seasons, so5 6 622 oyedemi s. oyekola and cornelius c. oluwafemi lar activity, and geomagnetic activity periods. the study considered vertical drifts as the key parameter determining the dynamics of ionospheric f-region, and of course the occurrence of esf. indeed, earlier results cited in section 1 have pointed out a clear relation between the magnitude of the prereversal enhancement and occurrence of postsunset esf: irregularities are more likely to develop when a strong prereversal enhancement is present. the strength will depend on the prevailing geophysical conditions, e.g., at solar maximum, a threshold level of ∼50 m/s is required, while during solar minimum it is reduced to ∼20 m/s (fejer et al., 1999). subbarao and krishna murthy (1994) using coherent hf radar data covering 28 days of observation spread over the period 1973-1976 during the late afternoon-early evening sector in the india equatorial region, reported that the mean value of peak velocity on nights with spread f (34.4 m/s) is rather greater than on nights without spread f (29.2 m/s). in their study, solar activity (f10.7 cm flux) was in the range 75-107 (in solar flux units), representative of low solar activity period. in the present study, a vertical plasma drift of ∼30 m/s seems to be a meaningful threshold value needed to trigger esf. a careful comparison of values of the enhancement peak drifts quoted above, instantly discloses that solar activity appears not to have important effect on the threshold value of vertical plasma drift required for the onset of esf. moreover, during geomagnetically quiet periods, the equatorial vertical plasma drifts result from the combined effects of eand f-region magnetic field line-integrated thermospheric winds weighted by the integrated pedersen conductivity. richmond (1994) pointed out that quifig. 7. dependence of the peak upward daytime (upper panel) and peak downward nighttime (bottom panel) vertical velocity on the 10.7 cm solar flux during quiet magnetic condition. notice that the monthly values of velocity peaks correspond to the monthly mean. fig. 8. maximum upward daytime and downward nighttime vertical drifts versus sunspot numbers using the same format as in fig. 7. 7 8 623 morphology of f-region vertical e×b drifts in the african sector using ionosonde measurements et-time ionospheric variability is probably associated with irregular day-to-day variation due to short term changes in tidal forcing, the effects of planetary waves and irregular winds in the dynamo region, and changes in the dynamic conditions at the base of the thermosphere. different types of upward coupling from neutral atmosphere dynamics are perhaps the major sources yet unknown of quiet-time f-region vertical drift variability. however, at magnetically disturbed times, two major sources of drift perturbations are found in the magnetospheric and ionospheric disturbance dynamo. electrodynamical mechanisms of magnetospheric dynamo processes are found in the work of martinis et al. (2005, and the references therein). the ionospheric disturbance dynamo, on the other hand, results from thermospheric disturbance wind generated by joule heating at auroral latitudes during periods of high magnetic activity (blanc and richmond, 1980; richmond et al., 2003). these disturbance winds affect the lowlatitude region several hours after the increase in magnetic activity. these winds are equatoward of region 2 current and any electric fields they generate are not shielded from the low-latitude ionosphere. they produce changes in the ionospheric dynamo (long lived effects) that are more persistent than those associated with the magnetospheric dynamo (prompt effects). the high latitude electric field should penetrate most efficiently into the equatorial ionosphere near dusk and in the post midnight to early morning hours (fejer et al., 1989). the most frequent and largest equatorial drift perturbations are downward during the day and upward at night (gonzales et al., 1979; fejer 1986). at equatorial latitudes, gravity waves have been found to play an important role in the formation of the equatorial spread f (hysell et al., 1990). blanc and houngninou (1998), using high-resolution fh radar during the 1993-1994 international equatorial electrojet year (ieey), found that the amplitude of gravity wave is variable from day-to-day and does not depend on the magnetic conditions. the wave activity was even strong on magnetic quiet conditions. they reported that the wave characteristics are periods in the range 5-30 min, and a downward phase velocity (during the daytime) of the order of 80 m/s at altitudes of 240-280 km, appearing as a phase delay at lower radar frequencies, hence at lower altitudes. millard et al. (2001), using a coupled thermosphere-ionosphere-plasmasphere (ctip) model showed the importance of lower thermospheric tidal forcing on the daytime vertical ion drifts. 5. conclusions f-region vertical plasma drift velocities derived from ionosonde observations at ibadan have been utilized to examine the morphology of vertical plasma drifts with season, solar cycle, and magnetic activity at various local time sectors. the f-region drifts show the largest variations in the evening and nighttime periods. the daytime vertical plasma drift occurrence patterns during high and low solar activity periods were essentially identical, except for the equinox season. the evening reversal time from upward daytime to downward nighttime is earliest and latest during solstitial seasonal periods. our results suggest that the daytime f-region drifts and, therefore, the corresponding e-region electric fields and neutral winds are nearly independent of solar variability. the evening prereversal enhancement, daytime and nighttime peaks drifts seem to saturate for large solar intensity values. finally, some results (e.g., fejer et al., 1989, 1991; balan et al., 1992; hari and krishna murthy, 1995; ramesh and sastri, 1995) obtained for the equatorial region in the india and south american sectors were confirmed in this study. acknowledgements the author would like to thank the reviewer for the helpful comments on the original drafts of this paper. references abdu, m.a., j.a. bittencourt and i.s. batista (1981): magnetic declination control of the equatorial f-region dynamo electric field development and spread f, j. geophys. res., 86, 11443-11446. anderson, d.n. and m. mendillo (1983): ionospheric 624 oyedemi s. oyekola and cornelius c. oluwafemi conditions affecting the evolution of equatorial plasma depletion, geophys. res. lett., 10, 541-544. anderson, d., a. anghel, k. yumoto, m. ishitsuka and e. kudeki (2002): estimating daytime vertical e×b drift velocities in the equatorial f-region using groundbased magnetometer observations, geophys. res. lett., 29 (12), 1596, doi: 10. 1029/2001gl014562. balan, n., b. jayachandran, r.b. nair, s.p. namboothiri, g.j. bailey and p.b. rao (1992): hf doppler observations of vector plasma drifts in the evening f-region at the magnetic equator, j. atmos. terr. phys., 54, 15451554. bittencourt, j.a. 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(received may 8, 2007; accepted november 5, 2007) sugar_final:layout 6 comparison of the reference mark azimuth determination methods danijel šugar*, mario brkić, drago špoljarić university of zagreb, faculty of geodesy, zagreb, croatia annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5405 abstract the knowledge of the azimuth of the reference mark is of crucial importance in the determination of the declination which is defined as the ellipsoidal (geodetic) azimuth of the geomagnetic meridian. the accuracy of the azimuth determination has direct impact on the accuracy of the declination. the orientation of the declination-inclination magnetometer is usually carried out by sighting the reference mark in two telescope faces in order to improve the reliability of the observations and eliminate some instrumental errors. in this paper, different coordinate as well as azimuth determination methods using gnss (global navigation satellite system) observation techniques within vpps (high-precision positioning service) and gpps (geodetic-precision positioning service) services of the cropos (croatian positioning system) system were explained. the azimuth determination by the observation of the polaris was exposed and it was subsequently compared with the observation of the sun using hourangle and zenith-distance method. the procedure of the calculation of the geodetic azimuth from the astronomic azimuth was explained. the azimuth results obtained by different methods were compared and the recommendations on the minimal distance between repeat station and azimuth mark were given. the results shown in this paper were based on the observations taken on the poku_sv repeat station. 1. introduction the knowledge of the azimuth of the reference mark is of crucial importance in the determination of the geomagnetic declination. according to the definition, the geomagnetic declination is the angle between the geographic north direction and the magnetic meridian [jankowski and sucksdorff 1996] i.e. the ellipsoidal (geodetic) azimuth of the magnetic meridian. there are several azimuth determination methods: it can be calculated upon known geodetic coordinates of the station and target point (reference mark), by methods of geodetic astronomy, by usage of gyro-theodolite as well as by modern gnss (global navigation satellite system) methods. in this paper is given an overview and comparison of the determination of azimuth by gnss methods within the cropos (croatian positioning system) system as well as by the astronomic methods. the determination of azimuth by gyro-theodolite hasn't been discussed in detail because such an instrument wasn't available for the survey on the field. 2. azimuth determination by gnss methods the gnss today includes different global satellite navigation and positioning systems: american gps (global positioning system), russian glonass (globalnaya navigatsionnaya sputnikovaya sistema), european galileo and in future many other systems. generally, gnss uses two basic positioning methods: absolute and relative. absolute method provides the coordinates with lower precision and therefore it is used in navigation, while the relative methods provide higher precision of the coordinates and are used for positioning. furthermore, positioning methods are classified as static or kinematic depending whether the gnss receiver during the observations was stationary or roving. such two positioning methods (static relative and kinematic relative) were applied in this paper for the determination of the coordinates and subsequent calculation of the ellipsoidal azimuth. both methods require at least two gnss receivers simultaneously observing the same satellites in order to allow the computation of the baseline vectors between receivers (stations). using the static relative method the baseline vectors are obtained in post-processing, while using the relative kinematic method with ambiguity resolution in real time (rtk, real time kinematic) the coordinates are determined directly on the field. the unknown coordinates of the station can be determined by adding the processed baseline vectors to the known coordinates of the reference station. if at several stations with known coordinates the baseline vectors towards an unknown station were processed then the coordinates of the unknown station can be determined by the least-squares adjustment procedure [hofmann-wellenhof et al. 2008]. 2.1. gnss observations during the cgrsn setup during the setup and survey of the cgrsn (croatian geomagnetic repeat stations network) in 2004, the coordinates were determined by static relative positioning method [brkić et al. 2006]. three trimble 4000 ssi gps receivers article history received september 30, 2011; accepted april 18, 2012. subject classification: satellite geodesy, geodetic (ellipsoidal) azimuth, astronomic azimuth, repeat station, reference mark, gnss, cropos. 1071 magnete workshop 2011 were simultaneously used for gps observations: two receivers at reference stations with known coordinates, one receiver at new geomagnetic repeat station and corresponding reference marks. upon the baseline vectors processing and network adjustment the coordinates were determined in the reference system of the known coordinates (etrs89). the european terrestrial reference system 1989 (etrs89) is an ecef (earth-centered, earth-fixed) geodetic cartesian reference frame, in which the eurasian plate as a whole is considered static. the coordinates and maps in europe based on etrs89 are not subject to change due to the continental drift (http://etrs89.ensg.ign.fr/). at the time of the cgrsn setup and survey the etrs89 was not the officially accepted national reference system, so the coordinates were transformed into the croatian state coordinate system (cscs). the model of seven-parameter helmert's transformation with parameters valid for the territory of the zagreb county was used for the transformation of the coordinates. in order to improve the transformation accuracy, the geoid surface was used [bašić 2009]. after the coordinate transformation from etrs89 and associated ellipsoid grs80 (geodetic reference system 1980) to cscs and associated bessel-1841 ellipsoid, the plane coordinates were computed in transverse mercator projection. the main property of that projection is its conformity meaning that the projection of the rotational ellipsoid into the plane is free from angle deformation. the grid azimuth from the repeat station to the reference mark was calculated from the plane coordinates. in addition to the grid azimuth, the convergence of the meridian was calculated for the repeat station and by adding it to the grid azimuth, the ellipsoidal (geodetic) azimuth was obtained. in the ellipsoidal azimuth computation the correction for curvature of the geodesic was neglected because of its small amount. of course, the ellipsoidal azimuth could be computed from the ellipsoidal coordinates of the repeat station and reference mark (for example using the clarke method), but the computations from plane coordinates were much easier and quicker. the ellipsoidal azimuth calculated from plane coordinates was used for dim (declination-inclination magnetometer) orientation and the subsequent observations of geomagnetic declination. the transverse mercator projection plane coordinates of the repeat station as well as the reference mark at pokupsko location, along with grid azimuth, convergence of the meridian and ellipsoidal azimuth are given in table 1. 2.2. gnss observations today – cropos system in august 2004 the croatian government introduced the new official geodetic datums. as the new positional geodetic reference system the etrs89 was adopted i.e. its realization etrf89. the european commission has recommended the adoption of etrs89 as the geodetic datum for geo-referenced information and has promoted the use of etrs89 within its member states (http://www.euref.eu/euref_egrs.html). in comparison to the older csrs which is a local geodetic system, the etrs89 is a global system facilitating the application of the gnss in geodetic practice in croatia. a further important step in gnss application took place at the end of 2008 with the establishment of the cropos (croatian positioning system) system. cropos is the state gnss reference network consisting of 30 cors (continuously operation reference station) stations evenly distributed over the national territory at the average distance of 70 km. at each reference station the gps and glonass satellites are observed, observations are sent to the computing centre where the data are checked, processed, modeled and adjusted in order to get a networked solution [marjanović 2010]. the coordinates of the cors stations were precisely determined in itrf2005 (international terrestrial reference frame) and subsequently transformed in etrf2000 (r05) (http://www.cropos.hr/ šugar et al. 1072 station y (m) x (m) grid azimuth (dms) convergence of meridian (dms) ellipsoidal azimuth (dms) poku_sv 5576879.548 5036993.908 359°00'11.7'' 0° 42' 04.0'' 359°42'15.7'' poku_got2 5576869.417 5037576.209 cropos service solution method transfer of data accuracy data format gpps post-processing internet (ftp, e-mail) 1 cm (2d, 3d) rinex vpps network solution of phase measurements in real time wireless internet (gprs/umts), ntrip protocol, gsm 2 cm (2d); 4 cm (3d) rtcm table 2. main characteristics of the cropos gpps and vpps services (http://www.cropos.hr/index.php?option=com_content&view=article&id=14& itemid=9&lang=en). table 1. plane coordinates (y,x) and the ellipsoidal azimuth of the poku_sv repeat station and the reference mark poku_got2 (y-north direction, x-east direction). 1073 index.php?option=com_content&view=section&layout= blog&id=1&itemid=2&lang=en) which is today the conventional frame of the etrs89 system (http://etrs89.ensg. ign.fr/memo-v8.pdf ). the cropos system provides three services, but only two services used in azimuth determination were considered: gpps (geodetic precise positioning service) and vpps (highly precise positioning service). the gpps service is used for static relative positioning providing 1-cm positional accuracy, while the vpps service is used for rtk applications providing 2-cm positional accuracy. the main characteristics of both services are given in table 2. 2.2.1. the application of gpps service in azimuth determination with gpps service there is no need for several gnss receivers observing at stations with known coordinates, only one receiver is required for the observations on the station with unknown coordinates. the data files with satellite observations at cors stations can be downloaded in the rinex (receiver independent exchange format) format from the cropos web server (http://195.29.118.122/). in addition to cors station data, the data can be downloaded for any vrs (virtual reference station), preferably in the vicinity of the station which coordinates are to be determined. the usage of vrs data enables considerably shorter station occupation with insignificant loss of precision in comparison to the usage of data from cors stations [šugar et al. 2009]. three vrs stations were selected at the average distance 1.5 km from the poku repeat station and poku_got2 reference mark. the duration of conventional static occupation was determined depending on the baseline vector length according to the rule 20 min + 2 min/km [hoffmann-wellenhof et al. 2008]. considering the baseline vector length of approximately 1.5 km, the duration of occupation was 23 minutes. the survey with one gnss receiver trimble r8 took place on april 16, 2011 (figure 1, right). according to the rules on the fundamental geodetic works performance (http://www.dgu.hr/ userdocsimages/pravilnik_o_nacinu_izvodjenja_osnovnih _geodetskih_radova.pdf ) the gnss survey of the 3rd order network has to be performed with: 5 second observation interval, elevation mask 10˚, the network adjustment with at least three reference (known) stations. the baseline vectors processing and network adjustment was carried out in trimble business center (tbc) software, ver. 2.20. for baseline vectors processing the gps and glonass data were used along with the broadcast ephemeris. the baselines were processed with precise ephemeris too, but due to short baselines the difference between solutions was negligible and consequently the solution with broadcast ephemeris was retained. the network was adjusted with precision confidence level 95% and three reference stations with fixed coordinates. the network configuration with fixed and newly determined coordinates along with error ellipses (graphical representation of the magnitude and direction of the error) is shown in figure 1 (left). the adjusted ellipsoidal (grs80) coordinates with their precision (confidence level 95%) determined with gpps service of the cropos system are given in table 3. azimuth determinations in geomagnetism station latitude (dms) northing precision (m) longitude (dms) easting precision (m) poku_sv 45°28'22.91664" 0.005 15°58'42.66654" 0.005 poku_got2 45°28'41.78152" 0.005 15°58'42.52674" 0.004 table 3. ellipsoidal coordinates with their precisions (gpps). figure 1. gnss network with the error ellipses (left); gnss receiver observing on the reference mark (right). the ellipsoidal azimuth computed from coordinates in table 3 is referred to the azimuth of the direct normal section from the repeat station poku_sv to the reference mark poku_got2: an (gpps) = 359˚ 42' 04.7''. reducing the precision of the coordinates in table 3 to the confidence level 68% (1.00 v) and taking into consideration the worst case where the longitudes on both stations are burdened with maximal easting errors in opposite directions, the difference in azimuth is estimated to 1.6''. in this case, where the line of sight is oriented in n-s direction, mostly easting (e-w) errors have influence on the error in azimuth. in the general case, the influence of coordinate errors on both stations on the error in azimuth should be analyzed by taking into account the coordinate errors perpendicular to the line of sight between stations. the coordinate errors perpendicular to the line of sight can be estimated from the error ellipses on both stations. the difference between the direct normal section and geodesic is calculated according to expression: (1) where ag is the azimuth of the geodesic, an azimuth of the direct normal section, e is the eccentricity, a is the ellipsoidal semimajor axis, s is the length of the geodesic [torge 2001]. for the repeat station poku_sv and parameters of the grs80 ellipsoid the difference (ag − an) = 5 · 10 −9 ". 2.2.2. the application of vpps service in azimuth determination with vpps service the coordinates were obtained in real time during the survey. according to the rules on the fundamental geodetic works performance (http://www.dgu.hr/user docsimages/pravilnik_o_nacinu_izvodjenja_osnovnih_ geodetskih_radova.pdf ), the stations of the supplemental gnss network (reference network of 3rd order) have to be observed in two independent repetitions (one repetition with three consecutive coordinate determinations, each one with 30 second duration after the successful ambiguity initialization). the second repetition has to be observed two hours after the first one with elevation mask 15˚ in both repetitions. the coordinates were determined from altogether six repetitions, the average ellipsoidal coordinates along with standard deviations are given in table 4. the azimuth computed from the coordinates in table 4 was: an (vpps) = 359˚ 42' 03.3''. since the station coordinates were determined six times, the computation of 36 azimuth values was enabled. the average azimuth value along with maximal and minimal values as well as the scatter defined as scatter = max [abs(ave-min), abs(ave-max)] are given in table 5. the comparison of the azimuth values computed from coordinates determined by gpps service with those determined by vpps service has given a difference an(gpps) − an(vpps) = 1.4". after the coordinates transformation in csrs using the transformation parameters valid for the territory of the zagreb county, the transverse mercator plane coordinates were computed along with grid azimuth, convergence of meridian and ellipsoidal azimuth referred to ellipsoid bessel-1841. the comparison of the coordinates from table 1 and table 6 has led to the coordinate difference dy = 0.022 m and dx = 0.021 m for the station poku_sv, and dy = 0.009 m šugar et al. 1074 station latitude (dms) st. dev. (m) longitude (dms) st. dev. (m) poku_sv 45°28'22.91643" 0.007 15°58'42.66675" 0.005 poku_got2 45°28'41.78176" 0.015 15°58'42.52677" 0.007 azimuth (dms) stdev ('') max (dms) min (dms) scatter ('') 359° 42' 03.3'' 2.7'' 359° 42' 08.1'' 359° 41' 58.4'' 4.9'' station y (m) x (m) grid azimuth (dms) convergence of meridian (dms) ellipsoidal azimuth (dms) poku_sv 5576879.570 5036993.929 359°00'07.2'' 0°42'04.5'' 359°42'11.7'' poku_got2 5576869.426 5037576.250 2cos sina e s12g n n2 2 2 2$ $a a { a=table 6. transverse mercator plane coordinates computed from vpps. table 5. azimuth average value from 36 solutions (vpps). table 4. ellipsoidal coordinates and their precision (vpps). 1075 and dx = 0.041 m for the station poku_got2 as well as the difference in azimuth dan = 4.0''. these differences are likely to be a consequence of the differences within the positioning methods performed in 2004 and 2011. the ellipsoidal coordinates from table 3 (gpps) were transformed into csrs using transformation parameters valid for the territory of the zagreb county. the azimuth computed from transformed coordinates was an(csrs) = 359˚ 42' 12.5'', compared to the azimuth an(gpps) = 359˚ 42' 04.7'' gave the azimuth difference an(csrs) − an(gpps) = 7.8". it can be inferred that the transformation of coordinates from etrf2000 reference frame to the csrs in the area surrounding poku_sv repeat station has led to such a difference in azimuth values. considering such a difference of azimuth values, it is recommended to adopt the azimuth and thus the geomagnetic declination in the global reference frame (etrf2000). 2.3. the dependence of the distance between stations and the accuracy of the azimuth the comparison of the azimuth values computed from gpps (which was considered 'absolutely' accurate within this analysis) and vpps service yielded the difference of 1.4" at the distance between stations of 582.42 m. for the sake of further analysis, the difference in the azimuth values was rounded to 2" (conservative approach) because of the resolution of the circle readings in one-second theodolites (e.g. zeiss theo 010b) widely used for geomagnetic surveys. if we assume the homogeneous positional accuracy along the line of sight between the station and the reference mark, the difference in azimuth values depending on the distance is presented in table 7. from the analysis of the values in table 7, it can be concluded that at distances between stations less than 200 m, the assumed homogeneous positional accuracy causes the difference in azimuth greater than 6". this value should be considered as a threshold in the consideration of the minimal distance between stations because in one-minute optical theodolites the readings of the circles are estimated to 0.1' = 6". therefore, it is confirmed to choose the reference mark at least 200 m away from the repeat station, like it was recommended earlier already [newitt et al. 1996]. on the other hand, reference marks that are too far away (several km) may be difficult to sight in hot conditions due to atmospheric refraction or in hazy and misty conditions [newitt et al. 1996]. at short distances special attention should be paid to the setup (centering and levelling) of the mark (label) over the reference mark as well as the theodolite over the repeat station. the azimuth error of 1" at a distance of 200 m causes the arc length of 1 mm, similarly at a distance of 500 m the arc length rises to 5 mm. the reference mark sighting is usually performed in two theodolite faces due to the elimination of some instrumental errors. for example, the average reading from sighting the reference mark with the optical theodolite (with two diametrically opposite readings) is free from the collimation error, the error of the horizontal axis, and eccentricity of the alidade [macarol 1978]. special attention should be paid to the levelling of the theodolite because the influence of the error of the vertical axis on the horizontal circle readings cannot be eliminated by taking the average of readings in both theodolite faces [benčić 1990]. this is especially true for a steep line of sight. the error of the horizontal direction due to the error of the vertical axis is given by the expression [benčić 1990]: (v) = v · sin a · ctg z (2) where v is the angle between the vertical axis of the theodolite and the plumb line, a is the angle in horizontal circle plane between the line of sight and the intersection of the plane of the horizontal circle and the true horizontal plane, z is the zenith distance of the line of sight. according to the expression (2), the horizontal direction error due to the error of the vertical axis is minimized for zenith distance readings z ≈ 90˚. this is the reason why the reference mark should be chosen on the horizon around the repeat station. 3. astronomic azimuth determination methods the astronomic azimuth is determined by means of the two most popular methods [terzić 1988]: – hour-angle method, – zenith-distance method. by hour-angle method the azimuth is calculated from the known astronomic latitude u of the station, known declination d as well as the hour angle t of the observed celestial body. by zenith-distance method the azimuth is calculated from know astronomic latitude u of the station, known declination d as well as the zenith distance z of the observed celestial body (figure 4, left). azimuth determinations in geomagnetism distance between stations (m) da (") 1000 1.2 600 1.9 500 2.3 400 2.9 300 3.9 250 4.7 200 5.8 150 7.8 100 11.7 table 7. dependence of the azimuth accuracy and the distance between stations. obviously, in both methods the astronomic latitude u have to be known, hence in the sequel the astronomic coordinate determination will be considered. 3.1. simultaneous determination of the astronomic coordinates in the theory of astrometry there are methods for individual as well as for simultaneous determination of the astronomic station coordinates (latitude u, longitude k), but in astrogeodetic practice mostly simultaneous methods are used. simultaneous determination of the coordinates from the measurements of the time needed for the stars to pass through the same almucantar by means of astrolabe with compensator meets a few basic conditions: accuracy, light (portable) observation equipment and significantly shorter observation period [špoljarić and solarić 2010]. the determination of zenith distances of the celestial body is burdened with instrumental errors as well as refraction influence. in the 1808 c.f. gauss suggested to use the time needed for the three stars to pass through the same almucantar (known in the literature as the problem of three heights) instead of measuring the zenith distance for the simultaneous determination of the astronomic coordinates of the station. from the sphere triangle (figure 2, figure 4-right) celestial pole (p) zenith (z) celestial body (r1), with known apparent coordinates of the stars ri (right ascension ai , declination di) and sideral time of the star's upper transit at greenwich meridian gasti (greenwich apparent sideral time), the zenith distance of the celestial body is obtained according to expression [schödlbauer 2000]: cos z = sin u sin di + cos u cos di cos ti (3) there, ti = gasti + k − ai is the hour-angle of the celestial body. in practice, usually twenty or more stars evenly distributed over the almucantar of constant zenith distance z are observed. the observation of additional stars (more than three) yields to redundant measurements so the unknowns are determined through the least-squares adjustment procedure. the automated measurement system for simultaneous determination of the astronomic coordinates of the station consists of an astrolabe zeiss ni2, garmin gps 35 hvs receiver for accurate time signals and notebook computer with software for the processing of the measurements (figure 3). with this system the clock correction determination has been eliminated, the selection of the stars to be observed is done on the computer screen, the measurements are shorter and could be processed directly on the field [špoljarić and solarić 2010]. the astronomic coordinates of the repeat station poku_sv were determined upon the observations performed on the evening of 5 may 2011. altogether 25 stars were observed, but after the astronomic coordinates calculation by least-squares adjustment, one observation was rejected due to significantly higher residuals. the final solution was calculated with 24 stars, the astronomic coordinates with their precision are given in table 8. šugar et al. 1076 astronomic coordinates (dms) standard deviation (") u 45°28'23.8" ± 0.7" k 15°58'41.1" ± 0.5" table 8. astronomic coordinates of the poku_sv repeat station. figure 2 (above). the problem of three heights. figure 3 (right). automated system for the simultaneous (u,k) determination. 1077 3.2. the hour-angle method the astronomic azimuth ad of the reference mark on the surface of the earth is determined from the azimuth a* of the celestial body (star or a sun) at the moment of observation and the measurement of the horizontal angle between the reference mark and the celestial body (figure 4, left). the azimuth of the celestial body a* can be computed from the known hour angle t and declination d of the celestial body, and astronomic latitude u of the station according to the expression [schödlbauer 2000]: (4) differencing the expression (4) by the variables a*, u, t and d and after some rearrangement the next expression is obtained (q is the angle between the hour circle and the vertical circle of the observed celestial body) [terzić 1988]: (5) upon analysis of the expression (5) can be inferred that: – the error of the astronomic latitude du has no impact on the precision of the azimuth if – the celestial body was observed in the vicinity of the meridian where a* ≈ 180˚, – the error of the hour angle dt has no impact on the precision of the azimuth if the celestial body was observed in the vicinity of the celestial pole where d ≈ 90˚, – the error of the hour angle dt has no impact on the precision of the azimuth if the celestial body was observed in the maximal elongation (q = 90˚ or q = 270˚). all these conditions are best fulfilled for the observation of the star polaris (a ursae minoris). if the clock correction was well determined i.e. dt was a small value, the astronomic azimuth can be well determined from the observation of the sun close to the sunrise or the sunset where z ≈ 90˚. if the sun was observed symmetrically to the local meridian during the morning and evening sets, the errors du and dd canceled out. 3.2.1. determination of the astronomic azimuth on the poku repeat station the observations for the determination of the astronomic azimuth on the poku_sv repeat station were carried out in the evening on 26 may 2011. the astronomic azimuth was determined by the hour-angle method and observations to the reference mark poku_got2 and polaris which is a bright star with apparent magnitude 2.02. polaris can be sighted starting from 3 hours before the sunset and ending at 3 hours after the sunrise. the automated measurement system for the determination of the astronomic azimuth consists of: electronic theodolite kern e2, notebook computer for the registration of the horizontal directions and zenith distances to the reference mark and celestial body as well as for the recording of the time of star observation (figure 5, left). the electronic theodolite kern e2 has the resolution of the horizontal and vertical circle readings of 0.1 mgon (0.324") and the dual-axis compensator with ± 0.1 mgon accuracy and ± 50 mgon of operating range [benčić 1990]. the reference mark was labeled with an illuminated signal customized for the sighting in the darkness (figure 5, right). the observations were carried out in both theodolite faces. the compensator (automatic vertical index) was turned on for the readings of the horizontal and vertical circles, so the observed horizontal directions and zenith distances were almost free from the nonverticality error of the instrumental vertical axis. due to some problems concerning the ephemeris computation within the automated system, the time and theodolite circles reading were recorded by the software, but the further azimuth calculations were carried out in the office azimuth determinations in geomagnetism figure 4. relationship between the astronomic azimuth of the celestial body and the reference mark (left). astronomic azimuth on the celestial sphere (right). .sin cos cos sintga t tg t du u = -) ctg .sin sin sin sin cos cos a z q a z z q t$ $ $ d d d d du d= +) ) according to the procedure described in sequel. from the known astronomic latitude u, measured horizontal angle between the reference mark d and the star * (polaris) with apparent coordinates (a, d) and known the time of observation t, the instantaneous hour angle of the observed star was obtained from the expression [špoljarić 2010]: t = last – a (6) last = [t + czt – dzt + (ut1-utc)] · · 1.002737909 + s0 + k (7) where t is the time reading of the clock synchronized to zt = cet (zonal time = central european time), czt is the difference between zt and utc (during the summertime cet – utc = 2h), dzt is the clock correction to the zonal time, (ut1-utc) is the difference taken from iers bulletin-a at http://maia.usno.navy.mil/ser7/ser7.dat, coefficient 1.002737909 is used for the conversion of the mean sun time to mean sideral time, s0 is the sideral time at 0h ut at greenwich meridian (s0 = gast) obtained from astronomisches rechen – institut, database for the apparent places of fundamental stars at http://www.ari.uni-heidelberg. de/datenbanken/ariapfs/query.html. once the instantaneous hour angle t of the star has been determined, the astronomic azimuth of the star is calculated by expression (4). astronomic coordinates (a, d) of polaris were downloaded from the same web site of the astronomisches rechen – institut. the coordinates are based on the fk6 (fundamental catalog), and are given for every day at the time of upper transit at the greenwich meridian as well as at 0h ut. the apparent coordinates of the star were given considering the precession-nutation model iau 20002006, including pop3 precession theory (http://www.ari. uni-heidelberg.de/datenbanken/ariapfs/intro.html). for the astronomic azimuth calculation (4), the apparent coordinates were interpolated for the time of observation by means of the bessel interpolation function. the calculated astronomic azimuth of the star was corrected for the daily aberration according to da = − 0.32'' cos u cos a cosec z. (8) during the observation of polaris, its azimuth was a ≈ 180˚ ± 1˚ (calculation based on astronomic latitude of the poku station as well as on the star declination d for the day of observation) i.e. cos a ≈ −1, taking into account that for polaris was z ≈ d − u then the correction for the daily aberration was da = + 0.32''. the astronomic azimuth is reckoned clockwise from the south point s of the horizon (figure 4, left). in order to obtain the azimuth reckoned from the north point n of the horizon ± 180˚ has to be added to the astronomic azimuth a*. from the astronomic azimuth of the star a*, the astronomic azimuth of the reference mark is calculated by expression where hd and h* are the horizontal directions to the reference mark and the observed star respectively: ad = a* + (hd − h*) ± 180˚. (9) the final solution of the astronomic azimuth was calculated from 8 sets with average value a = 359˚ 42' 04.4" and standard deviation ± 2.6". 3.3. the zenith-distance method the azimuth of the celestial body a* can be computed from the known zenith distance z and declination d of the celestial body and astronomic latitude u of the station acšugar et al. 1078 figure 5. automated measurement system (kern e2 + notebook computer) (left). illuminated signal on the reference mark for the observation in the darkness (right). 1079 cording to the expression [terzić 1988]: (10) differencing the expression (10) by the variables a*, u, z and d and after some rearrangement the next expression is obtained (q is the angle between the hour circle and the vertical circle of the observed celestial body) [terzić 1988]: (11) upon analysis of the expression (11) can be inferred that: – if the celestial body was observed in the vicinity of the meridian, the influence of the errors dz, du and dd to the error in azimuth become too high, – if the celestial body was observed in the maximal elongation (q = 90˚ or q = 270˚), the error dz has no influence on the azimuth precision, – if the celestial body was observed when t = 90˚ or t = 270˚ the latitude error du has no effect on the azimuth precision. at mid-latitudes, by the zenith-distance method good precision can be obtained if polaris was observed in the maximal elongations. the astronomic azimuth determination by the hour-angle method provides high precision at all the times, hence zenith-angle method has no practical importance. during the observations by the zenith-distance method the time readings are rounded to one minute, because the time readings are needed only for the interpolation of the star coordinates. the apparent zenith angle z' observed with theodolite must be corrected for refraction r in order to get the true zenith angle of the celestial body [terzić 1990]: z = z' + r. (12) according to laplace's theory of refraction, at the temperature 10 ˚c and air pressure 1013.25 mbar, the refraction for the celestial body at the zenith distance 0˚-79˚ is calculated by expression [terzić 1990]: r = 58.12'' tg z' − 0.067'' tg3 z' + 0.0002 tg5 z' − 0.000001 tg7 z' the advantage of an astronomic azimuth determination by observation of the sun is that the measurements are performed during the day. the drawback is in the lower precision. the astronomic azimuth determination by the zenith-distance method provides lower precision due to simultaneous registration of the horizontal and vertical readings as well as possible errors in refraction at certain zenith angles [terzić 1988]. the influence of the daily aberration on azimuth determination by observation of the polaris is practically the same (independently on the chosen azimuth determination method). usually, the sun is observed in prime vertical where, according to (11), da = 0. 3.4. comparison of the hour-angle and zenith-distance method the automated measurement system for determination of the astronomic azimuth was tested by observing the sun using the zenith-distance as well as hour-angle method [solarić and špoljarić 1988]. using the zenith-distance method the sun was observed almost during whole day (except the short period around the local noon) in 219 observation sets. the analysis of the azimuth results has shown that the optimal observation periods for azimuth determination were in the morning (7 till 9) and afternoon (15 till 17), but not around the local noon (figure 6, left). furthermore, it was found out that the observations within the optimal period should not be done in more than 5 to 7 sets because the observations from additional sets haven't led to the improvement of the accuracy. the mean value of the azimuth was obtained with standard deviation of 2.6'' [solarić and špoljarić 1988]. using the hour-angle method, the sun was observed in 198 sets during the whole day (without the pause at the noon). the analysis of the measurements has indicated that this method should be used during whole the day but the best preazimuth determinations in geomagnetism .cos cos sin sin sin cosa z z $ $d u u =) ctg .cos sin cos cos cos sina t q z t t$ $ d d u d u du u d = +) figure 6. daily changes of the standard deviations from individual sets of the sun observations using the zenith-distance (left) and hour-angle method (right). (ltst = local true solar time) [solarić and špoljarić 1988]. cision is obtained in the morning and afternoon (figure 6, right). it was concluded that the azimuth should not be observed in more than 3 to 5 sets during the optimum period. similarly, as stated for the zenith-distance method, the increasing of the number of sets has not led to the improvement of the accuracy. the mean value of the azimuth was obtained with standard deviation of 1.9'' [solarić and špoljarić 1988]. the zenith-distance method of the astronomic azimuth determination by observing the stars was tested too [solarić et al. 1990]. four stars were observed in the vicinity of the digression (maximal elongation), in the prime vertical and significantly apart from the prime vertical. during ten nights 347 sets were observed. the analysis of the measurements has indicated that no more than 7 sets should be observed using this method. the average standard deviation of the azimuth determination (from 6 sets) from the observation of the star in digression was 1.4'', and from the star in the prime vertical was 1.9''. the azimuth was determined from the observation of polaris by the hour-angle method too [solarić and špoljarić 1992]. the observations were performed simultaneously by two observers and two theodolites kern e2 in the series of 10 sets. altogether 41 series i.e. 460 sets were observed during seven nights. using all sets observed in a single evening, the standard deviation was 0.88''. the average standard deviation in all series (seven-day period) was 0.5'' (figure 7, left). the analysis of the results has indicated that the azimuth should not be observed in more than 5 sets because the further increase of the number of sets n does not significantly change the standard deviation amount (figure 7, right). 3.5. reduction to the ellipsoid the azimuth a determined by astronomic observation is referred to the plumb line, in order to obtain the azimuth an which is referred to the ellipsoid normal, the effect of the deflection of the vertical has to be taken into account according to laplace's equation of orientation [torge 2001]: (14) where g is the ellipsoidal zenith-distance to the reference mark, h and p are the components of the deflection of the vertical: p = u − { h = (k − m) · cos {. (15) from the astronomic coordinates (u, k) (table 8) and the ellipsoidal coordinates ({, m) (table 3) of the station poku_sv, the components of the deflection of the vertical were computed according to the equation (15): p = 0.9", h = −1.1". the astronomic latitude, deflection of the vertical and zenith distance to the reference mark (g ≈ z = 89˚ 53' 23.7") yielded the difference (an − ad) = 1.1". the astronomic azimuth reduced to the ellipsoid according to (14) was: an (astro) = 359˚ 42' 05.5" (15) and compared to the ellipsoidal (geodetic) azimuth obtained from gpps service yielded the difference dan = an(astro) − an(gpps)= + 0.8". this difference was expected since the standard deviation of the astronomic azimuth was ± 2.6", proving that both azimuth determination methods have led to correct results. the gyro-theodolites have been developed for the determination of the astronomic azimuth [torge 2001]. in order to reduce the astronomic azimuth to the ellipsoid, the components of the deflection of the vertical have to be known. 4. some practical aspects of the azimuth determinations as shown in previous chapters, the azimuth from the repeat station poku_sv toward the reference mark poku_got2 was determined with three independent methods: gpps and vpps services of the cropos system as well as with the hour-angle method observing the star polaris. within the vpps service, the coordinates of the repeat station as well as the coordinates of the reference mark were determined with one trimble r8 gnns receiver enabled to receive the network rtk corrections from cropos system via a wireless internet. the plane coordinates were obtained šugar et al. 1080 figure 7. average standard deviation in all series, on the horizontal axis h is the hour angle of polaris (left), dependence of the average standard deviation on the number of sets n and time needed for the observations t (right) [solarić and špoljarić 1992]. tan tan cos sin an $ $ $ a h { g h a p a =+ d 1081 in real time during the survey with standard deviation within 2 cm which is consistent with the specifications given in table 2. for the coordinates determination with conventional rtk method, at least two gnss receivers are required (base and rover), one observing at the station with known coordinates (reference receiver) and broadcasting the corrections through the radio link or a cellular network to the rover receiver. the coordinates of the rover receiver station are determined from the satellite observations and the received corrections. the drawback of the conventional rtk method is the need for an additional gnss receiver (reference receiver) as well as the dependence of the accuracy of the rover station coordinates on the distance to the reference receiver [premužić et al. 2010]. for the coordinate determination with gpps service of the cropos system, just one gnss receiver is needed taking observations on the repeat station and reference mark. the station occupation is determined depending on the baseline vectors length to the selected vrs points with known coordinates. for the conventional static relative method (particularly for baselines up to some 20 km), the station occupation for dual frequency (l1+l2) observations is determined according to the rule: 20 min + 2 min/km of the baseline vector length. these values are based on the visibility of four satellites, good geometry and normal atmospheric conditions [hofmann-wellenhof et al. 2008]. the plane coordinates of the stations were determined with the accuracy better than 1 cm (table 3) which is in accordance with the characteristics of gpps service (table 2). if the conventional static relative gnss method were used for the coordinate determination, then at least two gnss receivers would be needed, one observing at the unknown station, the other(s) at the station(s) with known coordinates. in that case, of course, the use of an additional or more gnss receivers would extend the duration of the observation, and thus increase the cost of the entire procedure. the basic prerequisite for the use of network solutions (e.g. gpps and vpps service) is the existence of the cors network. in many european countries (e.g. germany, austria, switzerland, hungary, great britain etc.) such a network providing similar services is already established. in the countries where the cors network has not been already established, the coordinates could be determined by the conventional static relative gnss method or by the conventional rtk method provided that there are stations of the geodetic network. at high geodetic latitudes (polar region), the gnss methods provide slightly worse results due to satellite constellation. in order to determine the azimuth by methods of geodetic astronomy, the astronomic station coordinates have to be known. the astronomic coordinates of the repeat station poku_sv were determined with the automated measurement system which includes the astrolabe zeiss ni2, the gps receiver for the time signals and notebook computer with the appropriate software for measurement processing. for the determination of the astronomic azimuth, the automated measurement system was used which consists of the electronic theodolite kern e2, notebook computer with the appropriate software and the illuminated signal for the observations in the darkness. the nonmagnetic theodolite zeiss 010b which is usually used for the declination/inclination observations is not suitable for the astronomic azimuth determinations within the automated system. the hour-angle azimuth determination method by observation of the star polaris, at the mid-latitudes provides the azimuth with precision better than 1". at high latitudes on the northern hemisphere (polar region) the line of sight to the polaris is steep whereas on the lower latitudes (equatorial regions) its sighting is burdened with the refraction affecting the accuracy of the azimuth determination. similar circumstances apply to the southern hemisphere where, instead of the polaris, another star near the southern celestial pole could be observed. the mass of the trimble r8 gnss receiver which was used within the gpps and vpps service coordinate determination method (including the carrying case, controller, tribrach and tripod) was estimated to approximately 10 kg. the mass of the automated measurement system for the determination of the astronomic coordinates (including astrolabe zeiss ni2, tripod and the notebook computer) was estimated to approximately 7 kg, whereas the mass of the equipment for the astronomic azimuth determination (electronic theodolite kern e2, tripod and the additional tripod with the illuminated signal) was estimated to 15 kg. the main characteristics of methods used in the azimuth determination are summarized in table 9. 5. conclusions the accuracy of the azimuth determination has direct impact on the accuracy of the geomagnetic declination that follows from its definition as the ellipsoid (geodetic) azimuth of the magnetic meridian. the azimuth determination methods using gpps and vpps services of the cropos system were tested and compared at the poku_sv repeat station. the ellipsoidal azimuth calculated from coordinates determined from the static relative gnns positioning and gpps service was considered as 'absolutely' accurate and it was compared to the azimuth calculated from the coordinates obtained with the vpps service. the difference between azimuth obtained from the gpps and vpps service was 1.4" at the distance (repeat station – reference mark) of 582.42 m. if the homogenous positioning accuracy was assumed, the reference mark has to be chosen at a minimal distance of 200 m away from the repeat station as it was already known. at that distance the assumed positioning accuracy led to an azimuth error of 6" which was considered as the threshold value of the azimuth error. the advantage of azimuth deazimuth determinations in geomagnetism termination with vpps service was that the coordinates were obtained during the survey, but for the improvement of the coordinate accuracy the observations have to be performed in two independent repetitions, each one with three coordinate determinations. the most reliable coordinates were determined by static relative gnss observations, but the azimuth computation was done in the post-processing. the ellipsoidal azimuth calculation from coordinates obtained from the gpps service before transformation (grs80 ellipsoid) and after transformation (bessel 1841) gave the azimuth difference of 7.8". that was the proof that the azimuth and thus the geomagnetic declination have to be referred to a global etrf2000 reference frame of the etrs89 coordinate system, instead of being referred to a local system. that should be a step toward the unification of declination determination all over the european continent. the azimuth obtained from the gpps service was compared to the azimuth value computed during the poku_sv repeat station setup in 2004: the difference of 4.0" can be attributed to the difference in positioning methods. the astronomic coordinates of the poku_sv repeat station were simultaneously determined with the automated measuring system. the components of the deflection of the vertical were computed from the ellipsoidal and astronomic coordinates of the station and subsequently used for the reduction of the astronomic to the ellipsoidal azimuth. the astronomic azimuth was determined with hour-angle method observing the star polaris. the azimuth was calculated as average from 8 sets with standard deviation 2.6". after the reduction to the ellipsoid, the comparison with the azimuth obtained by the gpps service yielded the difference +0.8". independent azimuth determination methods (gpps, vpps, hour-angle + polaris) tested on the poku_sv repeat station yielded the azimuth value with small differences proving that the azimuth was accurately determined. the comparison of the azimuth determination methods by observing the sun pointed out that the hour-angle method can be used almost whole the day, but better results were obtained from morning and afternoon optimal periods. generally, astronomic azimuth determination methods by observing the stars led to better results in comparison to the result yielded by observing the sun. at mid-latitudes observing the polaris by hourangle method has shown to be the optimal astronomic azimuth determination method. the accurate time required for the azimuth determination by hour-angle method is achievable today by gps receivers within the automated measuring system. considering the required equipment and time needed for the azimuth determination, as well as the achievable accuracy, the vpps method can be recommended for the determination of the azimuth adhering to the specific observation rules. acknowledgements. the gnss receiver trimble r8 used for the preparation of this paper was borrowed by the state geodetic administration of the republic of croatia, sector for state survey, department for basic geodetic works. thanks to the astronomisches rechen-institut (ari) of the university of heidelberg, germany, for providing the data of the apparent places of fundamental stars (apfs). references bašić, t. (2009). unique transformation model and new geoid model of the republic of croatia, reports on the scholarly projects in the period 2006-2008, state geodetic administration of the republic of croatia, zagreb, croatia, 5-21 (in croatian). benčić, d. (1990). geodetic instruments, školska knjiga zagreb, croatia (in croatian). brkić, m., d. šugar, m. rezo, d. markovinović and t. bašić (2006). croatian geomagnetic repeat station network, šugar et al. 1082 method gpps service vpps service automated measurement system (u, k) automated measurement system (a) coordinate accuracy < 1 cm < 2 cm < 1'' (u) ; < 1'' (k) ----azimuth accuracy ------------< 1'' observation duration depending on the baseline vectors length 2 × 5 minutes with 2 hours time span 1.5 hours 45 minutes post processing/ real time post processing real time real time real time target station to be occupied yes yes no no mass of the equipment ca. 10 kg ca. 10 kg ca. 7 kg ca. 15 kg table 9. main characteristics of the used azimuth determination methods. 1083 "geomagnetics for aeronautical safety: a case study in and around the balkans", nato security throug science series, proceedings of the nato advanced research worksshop on new data for the magnetic field in the former yugoslav republic of macedonia for enhanced flying and airport safety, ohrid, 18-22 may 2005, springer, dordrecht, the neatherlands, 137-143. hofmann-wellenhof, b., h. lichtenegger and e. wasle (2008). gnss – global navigation satellite systems, springer-verlag, wien/new york. jankowski, j., and c. sucksdorff (1996). guide for magnetic measurements and observatory practice, iaga, boulder, usa, 15. macarol, s. (1978). practical geodesy, tehnička knjiga, zagreb, croatia (in croatian). marjanović, m. (2010). cropos – croatian positioning system, ekscentar, student journal of the faculty of geodesy, student organization, faculty of geodesy, zagreb, croatia, 28-34 (in croatian). newitt, l.r., c.e. barton and j. bitterly (1996). guide for magnetic repeat station surveys, iaga, buolder, usa, 22. premužić, m., d. šugar and m. marjanović (2010). concepts of network rtk solutions, proceeding of the iii symposium of the chartered geodetic engineers, croatian chamber of chartered geodetic engineers, opatija, 2223 october 2010, croatia, 174-186 (in croatian). schödlbauer, a. (2000). geodetic astronomy, walter de gruyter berlin, new york, usa (in german). solarić, n., and d. špoljarić (1988). accuracy of the automatic grid azimuth determination by observing the sun using kern e2 theodolite, surveying and mapping, usa, 48 (1), 19-28. solarić, n., d. špoljarić and č. nogić (1990). analysis of the accuracy of the automatic azimuth determination by measuring zenith distances of star with electronic theodolite kern e2, hvar obs. bull. 14, 1990, 1-14 solarić, n., and d. špoljarić, (1992). accuracy of the automatic astronomical azimuth determination by polaris with leica-kern e2 electronic theodolite, surveying and land information system, 2, 80-85. špoljarić, d. (2010). the basics of geodetic astronomy, lecture materials, university of zagreb, faculty of geodesy, zagreb, croatia (in croatian). špoljarić, d., and n. solarić (2010). automation of astrogeodetic measurements, proceedings of the 10th international multidisciplinary scientific geoconference sgem 2010, i, 837-844. šugar, d., m. marjanović and ž. bačić (2009). the implementation of the cropos system in education at the faculty of geodesy, university of zagreb, proceedings of the 1st cropos conference, croatian geodetic society, state geodetic administration, zagreb, 87-104 (in croatian). terzić, p. (1988). geodetic astronomy ii, university of zagreb, faculty of geodesy, zagreb, croatia, 114-122 (in croatian). terzić, p. (1990). spherical astronomy, university of zagreb, faculty of geodesy, zagreb, croatia, 314-327 (in croatian). torge, w. (2001). geodesy, walter de gruyter, berlin/new york. *corresponding author: danijel šugar, university of zagreb, faculty of geodesy, zagreb, croatia; email: dsugar@geof.hr. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. azimuth determinations in geomagnetism << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol49_2_2006 607 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words ocean bottom seismic observatory – broadband seismology 1. introduction two-thirds of the earth’s surface is covered by oceans, and this represents a significant challenge for the investigation of global scale dynamic processes in the earth’s interior, as well as tectonic processes at ocean-continent boundaries. in particular, the need for long term ocean floor seismic observatories has now been widely recognized, and several national and international efforts are underway to resolve the technological and logistical issues associated with such deployments (e.g., cosod ii, 1987; purdy and dziewonski, 1988; purdy, 1995; forsyth et al., 1995; montagner and lancelot, 1995). following pioneering efforts in the 1960’s (sutton et al., 1965), a number of pilot projects have been conducted in the last ten years, coordinated internationally by ion (international ocean network, suyehiro et al., 1995), to test technological solutions and demonstrate the feasibility of seafloor seismic observatories either in boreholes or on the ocean floor (e.g., suyehiro et al., 1992; beranzoli et al., 1998). in particular, in 1992, a french experiment involving the manned underwater vessel «nautile» inthe monterey bay broadband ocean bottom seismic observatory barbara romanowicz (1), debra stakes (2), david dolenc (1), douglas neuhauser (1), paul mcgill (2), robert uhrhammer (1) and tony ramirez (2) (1) seismological laboratory, university of california, berkeley, ca, u.s.a. (2) monterey bay aquarium research institute, moss landing, ca, u.s.a. abstract we report on the installation of a long-term buried ocean-floor broadband seismic station (mobb) in monterey bay, california (u.s.a.), 40 km off-shore, at a water depth of 1000 m. the station was installed in april 2002 using a ship and rov, in a collaborative effort between the monterey bay aquarium research institute (mbari) and the berkeley seismological laboratory (bsl). the station is located on the western side of the san gregorio fault, a major fault in the san andreas plate boundary fault system. in addition to a 3-component cmg-1t seismometer package, the station comprises a current meter and differential pressure gauge, both sampled at high-enough frequency (1 hz) to allow the study of relations between background noise on the seismometers and ocean waves and currents. the proximity of several land-based broadband seismic stations of the berkeley digital seismic network allows insightful comparisons of land/ocean background seismic noise at periods relevant to regional and teleseismic studies. the station is currently autonomous. recording and battery packages are exchanged every 3 months during scheduled one day dives. ultimately, this station will be linked to shore using continuous telemetry (cable and/or buoy) and will contribute to the earthquake notification system in northern california. we present examples of earthquake and noise data recorded during the first 6 months of operation of mobb. lessons learned from these and continued recordings will help understand the nature and character of background noise in regional off-shore environments and provide a reference for the installation of future off-shore temporary and permanent broadband seismic stations. mailing address: dr. barbara romanowicz, seismological laboratory, university of california, 215 mccone hall #4760, berkeley, ca 94720-4760, u.s.a.; e-mail: barbara@seismo.berkeley.edu 608 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez stalled two sets of 3-component broadband seismometers in the north-equatorial mid-atlantic, one directly on the seafloor, and the other, using the ifremer re-entry vessel nadia, at 300 m depth inside odp hole 396b (montagner et al., 1994a,b). the data comparison between the two systems seemed to indicate that the ocean-floor installation was quieter at long periods (beauduin et al., 1996), however, this remained controversial, as only 10 days of data were acquired in this experiment, and water circulation may have increased the noise in the borehole. since then, several other buried seafloor installations have been deployed in the deep oceans, some of them making use of abandoned submarine telecommunications cable (e.g., kasahara et al., 1998; butler et al., 2000). during the osn1 experiment in 1998 (e.g., collins et al., 2001) 3 broadband systems were installed 225 km southwest of oahu (hawaii), at a water depth of 4407 m, one on the seafloor, one buried, and the third one at 248 m below the seafloor, in a borehole drilled by the odp in 1992 for this purpose (dziewonski et al., 1992). data were collected for 4 months and demonstrated the importance of burying the seismometer package below the seafloor to obtain good coupling with the ground and ensure good quality of data at long periods. the osn1 experiment also demonstrated that a borehole installation can be quieter at teleseismic body wave periods than a buried or seafloor deployment, because it avoids signal-generated noise due to reverberations in the near surface sediment layers. this was confirmed by the long-term japanese nereid observatory deployment, which also documented that a properly cemented oceanfloor borehole in basement rock can be very quiet at long periods as well (suyehiro et al., 2002; araki et al., 2004). long-term ocean floor observations are also necessary to better constrain regional tectonics, such as on the western margin of north america, where tectonics and seismic activity do not stop at the continental edge. for example, in california, the zone with most abundant seismicity is associated with the mendocino triple junction, and is mostly off-shore. much effort has been expended to deploy networks of seismic stations in the western us, most recently broadband stations, with multiple goals of monitoring the background seismicity, understanding modes of strain release, documenting seismic hazards and providing constraints on crustal and upper-mantle structure. however, because there are very few off-shore islands in central and northern california, practically all stations are located on the continent. as a consequence, the study of plateboundary processes, as afforded by regional seismological investigations, is heavily squewed on the continental side of the san andreas fault (saf) system. off-shore seismicity is poorly constrained, both in location and in mechanisms, as is crustal structure at the continental edge. while consensus seems to have been reached that permanent, borehole installations are best for seafloor deployments of broadband seismometers, they are very expensive: the spatial resolution required for regional studies, either off-shore or in the middle of the oceanic plates, may not be achieved for many years to come. it is therefore important to conduct pilot studies to determine how to optimally deploy ocean floor broadband systems, and in particular how to minimize the strong perturbing environmental effects, both through improved installation procedures, and through a posteriori deconvolution of ocean current, tide, pressure, temperature and other such signals that can be recorded simultaneously. in the summer of 1997, the international moise experiment (monterey ocean bottom international seismic experiment) allowed us to collect 3 months of broadband seismic data from a seafloor system installed 40 km off-shore in monterey bay, in a cooperative experiment between mbari, ipg (paris france) and uc berkeley (e.g., romanowicz et al., 1998; stakes et al., 1998). during this experiment, the feasibility of performing under-water electrical and data cable connections between instruments, using an rov operated from a ship, were successfully illustrated for the first time. the moise experiment also demonstrated the sensitivity of ocean floor systems to sea currents at long periods and the importance of simultaneous recording of current velocity and direction, at a sampling rate sufficient for quantitative comparisons with seismic data: the conventional current meter sampling rate used by oceanographers (4 sample points once every 4 min), was too low to correct 609 the monterey bay broadband ocean bottom seismic observatory the moise seismic data for noise generated by currents, although this is theoretically possible (e.g., stutzmann et al., 2001). the monterey bay ocean broad band observatory (mobb, mcgill et al., 2002; uhrhammer et al., 2002; romanowicz et al., 2003) was installed in april 2002. it is a direct follow-up of moise, and capitalizes on the lessons learned during that pilot experiment. the ultimate goal of this collaborative project between mbari and the berkeley seismological laboratory (bsl) is to link the mobb station by continuous telemetry to the shore, so that mobb becomes part of the berkeley digital seismic network (bdsn, romanowicz et al., 1994). the data can then be contributed to the real-time earthquake monitoring system in northern california (gee et al., 2003). the opportunity to do so awaits the installation of the mars cable (monterey accelerated research system; http://www.mbari.org/mars). in the meantime, data are recorded on-site and retrieved every 3 months using mbari’s ship and rov, and data analysis is focused on understanding sources of background noise at long periods in this relatively near-shore environment. we view mobb as the first step towards extending the on-shore broadband seismic network in northern california to the seaside of the northamerica/pacific plate boundary, providing better azimuthal coverage for regional earthquake and structure studies. in what follows we describe this observatory and discuss some of the data recorded during the last 18 months. 2. location, instrument packages and deployment the mobb station is located at a water depth of 1000 m, 40 km off-shore in monterey bay, in an area called «smooth ridge» on the western side of the san gregorio fault, and closer to it fig. 1. location of the mobb and moise stations in monterey bay, california, against seafloor and land topography. fault lines are from the california division of mines and geology database. mobb is located at a water depth of 1000 m. 610 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez than was moise (fig. 1). the planned mars cable route is down the center of smooth ridge and with a termination near the mobb site. the san gregorio fault (sgf) splays from the saf at the golden gate and extends south past the san francisco peninsula and santa cruz mountains, mostly off-shore. it is the principal active fault west of the saf in central coastal california, yet it remains the largest known fault whose seismogenic potential is not well characterized in this region. however, begnaud and stakes (2000) and begnaud et al. (2000) used a temporary off-shore seismic network to demonstrate the unusually high seismicity levels of the northern sgf, dominated by compressional mechanisms as well as an east-dipping focal plane (simila et al., 1998). the sgf is thought to be capable of m > 6 earthquakes, making the mobb site particularly interesting from the tectonic and seismic hazards point of view. using refined crustal velocities based on the results of begnaud et al. (2000), simila et al. (1998) relocated the 1926 m > 6 doublet to show that the first event occurred on the northern sgf followed by the second on the adjacent monterey bay fault zone. the coseismic geodetic slip on the saf during the 1906 earthquake and the late holocene geologic slip rate on the san francisco peninsula and southward are about 50-70% of their values north of san francisco (thatcher et al., 1997; schwartz et al., 1998). review by the working group on northern california earthquake potential (1996) suggests that the slip rate on the saf in the santa cruz mountains is about 14 mm/yr, which is 58% of the slip rate north of san francisco, so the rest of the slip must be accommodated by other faults, both on-land and off-shore. this slip gradient reflects partitioning of the plate boundary slip onto the san gregorio, sargent and other faults south of the golden gate. because of the limited on-shore extent, few detailed geologic studies have been conducted to evaluate the style and rate of late quaternary deformation along this complex fault zone. the ocean-bottom mobb station currently comprises a three-component seismometer package, a current meter, a differential pressure gauge fig. 2a,b. a) photo of a guralp cmg-1td seismometer in the byerly vault (bks). shown are the various circuit boards on the sides and the top of the sensor package. three of the nine batteries used by the leveling system are on the left front and the system clock is on the circuit board on the right. the seismometers are in the µ metal shielded container mounted on leveling gimbals in the center. b) photo of the titanium pressure vessel containing the cmg-1td and resting on a ~ 1 cm thick bed of kiln dried fine sand on the concrete pier at bks, for the purpose of noise comparisons (uhrhammer et al., in prep.). a b 611 the monterey bay broadband ocean bottom seismic observatory (dpg) and a recording and battery package. the data logger, battery, and dpg are contained in a modular frame which is removed and replaced when the rov services the system. this configuration permits hardware and software upgrades to take place as required. for instance, the dpg (cox et al., 1984) was not present in the initial deployments, but was added later during a data retrieval dive in september, 2002. the seismic package contains a low-power (2.2 w), three-component cmg-1t broadband seismometer system, built by guralp inc., with a 24-bit digitizer, a leveling system, and a precision clock (fig. 2a). the seismometer package is mounted in a cylindrical titanium pressure vessel 54 cm in height and 41 cm in diameter (fig. 2b), custom built by the mbari team and outfitted for underwater connection. the component design of the instruments permit the sensor, datalogger, and current meter to be carried to the seafloor separately, then tested and connected in situ. this component design permits us to update software, change batteries and replace instruments without disturbing the sensor package. the system has been designed to permit a gps time mark, applied during an rov visit, to establish the offset and drift rate of the guralp clock. these errors are recorded and the timing of the seismic data is corrected in post-processing. the clock is not adjusted in situ to prevent abrupt jumps in the time marking of the data. establishing these corrections on the seafloor and after the system has reached thermal equilibrium is a critically important feature for longterm autonomous deployments. because of the extreme sensitivity of the seismometer, air movement within the pressure vessel must be minimized. in order to achieve this, after extensive testing at bsl, the top of the pressure vessel was thermally isolated with two inches of insulating foam and reflective mylar. the sides were then insulated with multiple layers of reflective mylar space blanket, and the vessel was filled with argon gas (fig. 2b). the low thermal conductivity of argon allows better thermal insulation. this resulted in significant noise reduction on the 3 components, in the 10-100 s period range (fig. 3a,b). fig. 3a,b. comparison of typical background noise psd levels observed by the cmg-1td and the co-sited sts-1’s in the bks vault, a) before and b) after it had been installed in the titanium pressure vessel, appropriately insulated and purged with argon gas. the large dashed line and the solid lines are the z component and horizontal component psd’s, respectively, and the small dashed lines are the sts-1 psd’s. note that the cmg1td psd levels are within ∼ 5 db of the sts-1 psd levels at long periods after insulation (uhrhammer et al., in prep.). a b 612 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez near-bottom water currents are measured by a falmouth scientific 2d-acm acoustic current meter. it is held by a small standalone fixture and measures the current speed and direction about one meter above the seafloor. the recording system is a geosense lp1 data logger with custom software designed to acquire and record digital data from the guralp sensor and from the current meter over rs-232 serial interfaces, as well as analog data from the dpg. the seismic data are sampled at 20 hz and the current meter and dpg are sampled at 1 hz. data are stored on a 6-gb, 2.5-inch disk drive. all the electronics, including the seismometer, current meter, and dpg, are powered by a single 10 kwh lithium battery. all installations were done using the mbari ship point lobos and the rov ventana. prior to the instrumentation deployment, the mbari team manufactured and deployed a 1181 kg galvanized steel trawl-resistant bottom mount to house the recording and power systems, and installed a 53 cm diameter by 61 cm deep cylindrical pvc caisson to house the seismometer pressure vessel. the bottom mount for the recording system was placed about 11 m away from the caisson to allow the future exchange of the recording and battery package without disturbing the seismometer. the seismometer package was tested extensively at bsl, then brought to mbari where its internal clock drift was calibrated against gps time in an environmental test chamber at seafloor temperature. the actual deployment occurred over 3 days (04/09-11/2002). on the first dive, the seismometer package was lowered into the pvc caisson (fig. 4a), and its connection cable brought to the site of the recording unit. on the second dive, the fig. 4a-c. mobb installation snapshots. a) the seismometer package is being lowered into the hole bounded by the pvc pipe, held by the arm of the rov ventana. b) the recording and battery package is being installed inside the trawl-resistant mount. c) the rov arm (at front) is connecting the current meter cable to the recording system. the connector of the seismometer package on the right is already in place. a b c 613 the monterey bay broadband ocean bottom seismic observatory recording package was emplaced in its trawl-resistant mount (fig. 4b), and connected to the seismometer package (fig. 4c). tiny (0.8 mm diameter) glass beads were poured into the caisson until the seismometer was completely covered, to further isolate it from water circulation, as dictated by lessons learned from previous experiments (e.g., sutton et al., 1992; duennebier and sutton, 1995). the seismometer package is now buried at least 10 cm beneath the seafloor. on the third dive, the rov immobilized the cable between the seismometer and recording package with steel «wickets» inserted into the sediment. it then connected the seismometer to the recording system, leveled and recentered the seismometer, and verified that everything was operational. finally, the current meter was installed and connected to the recording system. on april 22nd, 2002, the rov returned to the mobb site to check the functioning of the seismometer and recording system. some slight settling of the seismometer pressure vessel had occurred, and so the seismometer was recentered electronically. over 3 mb of data were then downloaded from the recording system over a period of about two and a half hours, using a 9600 bps fig. 5. mass position data, for portions of the time period 11/04/2002-07/01/2003, showing the progressive settling of the seismometers. at periods longer than the free period of the seismic sensor, the mass position is is proportional to tilt on the horizontal components, and to perturbations in gravity on the vertical component. the mass position has therefore been converted to acceleration (vertical component, mmz) and tilt (horizontal components (mmn, mme). the large steps on the horizontal components are associated with: 1) installation (day 100); 2) re-centering (day 112, day 263). there is a smaller step on day 134, associated with a local mw 4.95 earthquake. the vertical component data have been detrended by subtracting a running 36 h average (±18 h) from each 1/2 h duration smoothed data sample, to bring out the clearly visible tide signal. 614 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez serial data connection through the rov. these data included the recordings of two regional earthquakes in california and two teleseismic events that occurred in guerrero, mexico and in northern chile. the site was revisited two months later, on june 27th, to check the functioning of the system and replace the data recording and battery module, in the first of a series of such dives planned for the next 3 years. the following functions were performed: 1) disconnected the current meter and seismometer from old datalogger. 2) removed old datalogger frame with datalogger and batteries from the trawl-resistant mount. 3) installed new datalogger frame in the trawl-resistant mount. 4) connected the current meter to the new datalogger. 5) connected the rov to the new datalogger and verified that the datalogger was operational. 6) connected the seismometer to the new datalogger, and monitored its reboot. 7) centered the seismometer. 8) re-centered the seismometer. 9) verified that the guralp was receiving the gps clock signals from the rov (nmea time messages and pulse per second), and recorded the clock offset. during this dive, the guralp clock was not resynchronized to gps time. 10) brought the old datalogger frame with datalogger and batteries back to the ship. figure 5 shows the evolution of the tilt signal obtained from the seismometer mass position channels (mmz, mme, mmn), for the first 7 months of deployment. these data indicate that the seismometer package has been experiencing an exponentially decaying tilt in a south-southwesterly direction, which is also the down slope direction (e.g., fig. 1). the large step on day 112 (22/04/2002) was caused by re-centering, when the instrument was checked 12 days after installation. the small step on day 134 (14/05/2002) is coincident with the occurrence of a mw 4.96 earthquake which occurred 55 km n59w of mobb on the san andreas fault near the town of gilroy. the instrument was recentered again on day 178 (27/06/2002). the slow drift rate of the horizontal component mass positions in the latter half of the plot (day 263 onward) indicates that the obs pressure vessel stabilized in the ocean floor sediments after about two and a half months after deployment. on the mmz component, the semidiurnal gravitational tide is visible, riding on the tilt signal. as with the horizontal components, the largest signals are associated with rapid changes in the second derivative of the tilt, caused by recentering or by significant ground shaking (day 134). during the first two months of recording, many regional and teleseismic events were recorded, as described later. the site has been revisited regularly every three months since. during each visit, the datalogger and battery packages are changed, the seismometers are recentered, and the clock re-synchronized to gps time. due to multiple datalogger problems (hardware and software) encountered in the first half of 2003, the best data available so far span the time period april-december 2002, as illustrated below. 3. examples of data and preliminary analysis figure 6 shows the location of mobb with respect to the nearby bdsn stations. notably, we will be discussing comparisons between recordings on the ocean floor (mobb), in the noisy farallon island environment (farb) and on the continent (sao, jrsc). figure 7a-c shows power density spectra for two different time periods, comparing background noise at mobb and three land stations of the bdsn network. day 143 (23/05/2002) is a «quiet» day, as assessed from the ocean wave data recorded on the noaa buoy in monterey bay, whereas day 350 (16/12/2002) is a «stormy» day (spectral ocean wave density is an order of magnitude higher at around 30 s). increased noise level for periods between 20 and 500 s, due to ocean currents and infragravity waves, is observed at mobb on all 3 components on the stormy day, but only on the vertical component on the quiet day. the spectral width of the noisy long-period band is larger on the stormy day, and, interestingly, it also corresponds to a band of increased noise at the island site farb, where it 615 the monterey bay broadband ocean bottom seismic observatory fig. 6. location of mobb (and mois) with respect to nearby broadband stations of the berkeley digital seismic network (bdsn). farb is located on the farallon islands. mobb is located just west of the san gregorio fault. fig. 7a. comparison of noise recorded at mobb and 2 other stations of the bdsn network, on two days in 2002 when no significant earthquake signals were recorded: a «quiet day» (143), and a «stormy» day (350), as assessed by the mean wave height recordings at a nearby noaa buoy, located in monterey bay. the usgs highand low-noise models for land stations are shown in black. increased noise level for periods between 20 and 500 s, due to ocean currents and infragravity waves, is observed at mobb, as well as at the island station farb. the noise level at mobb between 10 and 20 s is comparable to the land station ybh, one of the quietest stations of the bdsn. see fig. 6 for farb and sao locations. station ybh is 560 km north of mobb. here it is shown the vertical component. 616 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez is largest on the east component. this most likely indicates loading of shallow water around the small farallon island by gravity waves, similarly to what is observed in hawaii (stephen et al., 2003). we are currently investigating the source of this noise in more detail. in particular, it should be correlated with the dpg data, if it is indeed due to gravity waves generated by breaking waves at the coastline (webb et al., 1991). the bell shape of this noise peak is in agreement with theoretical calculations by araki et al. (2004). on the other hand, the noise level at mobb between 30 and 100 s on a quiet day is comparable to the noise level at the island station farb on a stormy day. the «low noise notch» (webb, 1998) is very narrow at mobb (10-30 s), and contains the single-frequency micro-seismic peak (12 s), fig. 7b,c. comparison of noise recorded at mobb and 2 other stations of the bdsn network, on two days in 2002 when no significant earthquake signals were recorded: a «quiet day» (143), and a «stormy» day (350), as assessed by the mean wave height recordings at a nearby noaa buoy, located in monterey bay. the usgs highand low-noise models for land stations are shown in black. increased noise level for periods between 20 and 500 s, due to ocean currents and infragravity waves, is observed at mobb, as well as at the island station farb. the noise level at mobb between 10 and 20 s is comparable to the land station ybh, one of the quietest stations of the bdsn. see fig. 6 for farb and sao locations. station ybh is 560 km north of mobb. b) north component; c) east component. b c but the level of noise is comparable to the land station ybh, one of the quietest bdsn stations. the corresponding «double-frequency» microseismic peak around 6 s is visible at all stations most of the time, but on day 350, it is hidden by higher amplitudes between 2 and 4 s at mobb. on day 143, two additional narrow-band microseismic peaks are clearly resolved at mobb (around 2.5 and 4 s). these could be related to a combination of local sea-state and distant storms (e.g., bro-mirski and duennebier, 2002). they are clearly distinct in frequency from those observed in the open sea (i.e., stephen et al., 2003). noise levels at frequencies higher than 2 hz and lower than 200 s are comparable to those observed at some of the land stations. farb (island site) is sometimes noisier at long periods, but the 617 the monterey bay broadband ocean bottom seismic observatory fig. 8. comparison of instrument responses of the cmg-1t at mobb and the cmg-3t at farb. fig. 9. spectrum of current speed [units are (mm/s)/ hz] based on a 78-day period of current meter data. the four dominant group of peaks coincide precisely with the frequencies of the diurnal, semidiurnal, 8 h and 6 h components of the gravitational tides. fig. 10. distribution of current velocity data as a function of azimuth for the 78-day period shown in fig. 9. the contour label units are fractions of the average density distribution of the current velocity. the two dominant maxima (centered at 60° and 240°, i.e. orthogonal to the continental shelf) are associated with the semi-diurnal tidal currents. the third directional peak is roughly parallel to the coastline and appears to be related to the dominant ocean circulation. seismometer there is a guralp cmg-3 with a shorter period high-pass corner (100 s) than mobb (360 s) (fig. 8). some of the longer period background noise is clearly correlated with currents that are associated with tidal flows, as was documented from moise (romanowicz et al., 1998; stutzmann et al., 2001). figure 9 shows a spectrum of the ocean current speed at mobb computed using a 78 day time series, illustrating the dominant effects of tides on the bottom currents. this is also illustrated in fig. 10, which shows the corresponding distribution of current direction and velocity as a function of azimuth. with the current meter and dpg data sampled at 1 s, it will now be possible to deconvolve the tide and current related noise from the mobb data. unfortunately, the dpg dataset is still too small (recording system problems since 01/01/2003) to assess the effectiveness of such a deconvolution. 618 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez figures 11-13 illustrate observations that have been recorded at mobb during the two months period 10/04/2002-28/06/2002, and which were retrieved during the first datalogger exchange, on 29/06/2002. figure 11 shows a component-by-component comparison of the recording of the 26/04/2002 mw 7.1 teleseism in the mariana islands at mobb and 3 nearby bdsn stations (see location on fig. 6). the comparison shows consistency between the recordings of mobb and nearby stations. on the horizontal components, there appears to be some signal-generated noise following the s-waves and the love wave, which is associated with ringing in the shallow mud layers as we will show below. in fig. 12, such ringing is clearly demonstrated for a deep earthquake (17/11/2002, depth = 459 km, mw = 7.3). only the vertical component pwave portion of the seismograms is displayed, filtered in two pass-bands. signal-generated noise is very apparent in the p-waves in the 0.03-0.3 hz pass-band, where the p-wave at mobb displays a 3 min long coda. the ringing is narrow-band and disappears at frequencies lower than 0.1 hz. such observations should be helpful in understanding the triggering of submarine landslides in strong motion events, and may be relevant for ocean floor structures such as oil platforms and pipelines. on the other fig. 11. comparison of vertical, n and e component records of the 26/04/2002 mw 7.1 mariana earthquake (depth = 85.7 km, distance = 85.2°, azimuth = 283° from mobb), at mobb and 3 stations of the bdsn. the records have been band-pass filtered between 10-100 s. fig. 12. comparison of vertical component records at stations farb, jrsc, mobb and sao for the deep kurile island earthquake of 17/11/2002 (mw = 7.3; depth = 459 km; distance to mobb = 65°). the data are shown in two pass-bands: 0.03-0.1 hz and 0.03-0.3 hz to emphasize the narrow-band character of the ringing in the mobb p-wave data. clearly visible in the lower frequency band are the p, pp and sp arrivals. 11 12 619 the monterey bay broadband ocean bottom seismic observatory seismic events and could be similarly removed. on the vertical component, the water reflection of the p-wave is clearly seen 1.3 s after the pwave. in spite of these strong site effects, these data can be used in moment tensor studies, as illustrated in fig. 15, which shows the results of a moment tensor inversion using a time domain whole waveform methodology (dreger and romanowicz, 1994). a robust solution is obtained using data from 5 stations, including data from mobb. the waveform fit at mobb is outstanding with a 92.8% variance reduction. the addition of mobb to the existing bdsn stations in this particular case provides an additional sh lobe but, since 3 sh lobes are already sampled by other bdsn stations, this particular strike-slip mechanism is well constrained in any case. however, this example serves to show that the mobb data are well calibrated and have potential for providing valuable constraints in moment tensor studies of events of hand, this type of noise may be unavoidable in a shallow buried installation. we are currently evaluating ways to eliminate this signal-generated noise by post-processing. one possibility is to design an «observational» transfer-function, using data from near-by land stations that do not show the ringing. this is illustrated in fig. 13, where we show a comparison of original p-wave train at mobb (blue) and jrsc (green) and «cleaned» mobb data (red) after removal of the corresponding transfer function. we are working on combining this type of processing with direct modelling of the ringing effect, by computing theoretical transfer functions based on simple sediment layer models (uhrhammer et al., 2003; dolenc et al., 2006) that can be obtained from local studies (e.g., begnaud et al., 2000). figure 14 shows the records, deconvolved to ground velocity, for a mw 3.63 regional event which occurred on 23/04/2002 on the saf at a distance of 53.4 km from mobb. the very large s-wave pulse on the horizontal components as well as the subsequent ringing are likely due to site response, as observed for the telefig. 13. raw vertical component data (p-wave and depth phases) observed on the vertical component at stations mobb (blue) and jrsc (green) for the 17/11/2002 deep kurile island earthquake. clearly seen is the ringing due to the soft sediment layer in monterey bay. the red trace shows the mobb data after removal of the transfer function constructed using jrsc data. fig. 14. deconvolved ground velocity records at mobb of the 23/04/2002 m 3.63 san andreas fault event (latitute = 36.866; longitude = −121.61; depth = = 9 km). 620 barbara romanowicz, debra stakes, david dolenc, douglas neuhauser, paul mcgill, robert uhrhammer and tony ramirez fig. 15. results of moment tensor inversions for the m 3.63 regional event shown in fig. 14. top: inversion using 4 stations of the bdsn and mobb (bdm, bks, cmb are bandpass filtered between 0.02 and 0.05 hz; mhc and mobb, between 0.05 and 0.10 hz). bottom: results of inversion using only mobb, showing the good fits of the single station solution to the other bdsn waveform data. 621 the monterey bay broadband ocean bottom seismic observatory other types, such as reverse fault events in the coast ranges or strike slip events on faults closer to the shore or off-shore. to further demonstrate the consistency of the mobb data, we also show the results of a single station moment tensor inversion using only mobb, and the comparison of the corresponding synthetic predictions with the actual data at the four other bdsn stations. the single station solution results in a nearly identical focal mechanism, but a slightly larger clvd component and scalar moment, which is not unlike other single station inversions. 4. discussion and future work data collected at the mobb site can be used for several purposes. they provide complementary constraints for regional crustal and upper mantle structure to land-based broad band stations, as well as for the study of earthquakes along the san andreas fault system. currently, they are noisier than records from land-based stations, so that only relatively large events can be successfully analyzed. as we have seen, there are two sources of increased noise: signal-generated noise due to reverberation in the sediment pile, and background noise generated by currents as well as local and distant waves in the ocean. the first type of noise can be dealt with by designing appropriate deconvolution filters. in particular, the vicinity of high quality broadband land stations of the bdsn provides a helpful reference. the second type of noise is complex and time variable, however, the combination of mobb and bdsn seismic data, current meter and dpg data, sampled at sufficiently high rates, as well as local and regional buoy data, provides a promising dataset to try to reduce the mobb background noise – at least in the infragravity wave band. ultimately, the level of success that we may reach in this endeavor will provide a reference for what can be expected of data from shallow-buried broadband ocean bottom systems. finally, the collection of these different types of data and their geographical distribution also provides an opportunity to further study the generation of such infragravity waves, at least in the particular setting of a relatively shallow ocean bay, as was pioneered by bromirski and duennebier (2002). acknowledgements the mobb station instrumentation and deployment was supported by funds to mbari from the lucile and david packard foundation, the nsf (grant oce-9911392) and uc berkeley funds to bsl. we thank doug dreger for performing the test moment tensor inversion and providing fig. 15. this is bsl contribution 04-11. references araki, e., m. shinohara, s. sacks, a. linde, t. kanazawa and k. suyehiro (2004): improvement of seismic observation in the ocean by use of seafloor boreholes, bull. seismol. soc. am., 94, 678-690. beauduin, r., j.-p. montagner and j.-f. karczewski (1996): time evolution of broadband seismic noise during the french pilot experiment ofm/sismobs, geophys. res. lett., 23, 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(2003): mobb: data analysis from an ocean floor broadband seismic observatory, eos, trans. am. geophys. un., 84 (46), abstr. s52d-0162. uhrhammer, r., b. romanowicz, d. neuhauser, d. stakes, p. mcgill and t. ramirez (2002): instrument testing and first results from the mobb observatory, eos, trans. am. geophys. un., 83, f1008. webb, s.c. (1998): broadband seismology and noise under the ocean, rev. geophys. space phys., 36, 105-142. webb, s.c., x. zhang and w. crawford (1991): infragravity waves in the deep ocean, j. geophys. res., 96, 2723-2736. working group on northern california earthquake potential (1996): database of potential sources for earthquakes larger than magnitude 6 in northern california, us geol. surv. open file rep. 96-705, pp. 40. o n d i ; s u p e r f i c i a l i a s s o c i a t e a l l e o n d e s . s s . n e l t e r r e m o t o d e l t u r k e s t a n d e l 2 n o v e m b r e 1 9 4 6 p . cat.oi e . peroraci i n xin r e c e n t e l a v o r o ( x ) . a b b i a m o d e t e r m i n a t o , c o n u n nuovo m e t o d o di c a l c o l o , l e c o o r d i n a t e i p o c e n t r a l i del f o r t e t e r r e m o t o del t u r k e s t a n del 2 n o v e m b r e 1 9 4 6 . e c c o i r i s u l t a l i d e i n o s t r i c a l c o l i : (p — 4 1 " 5 2 , . 3 f v . v , 2 v x = 71° 4 2 ' , 2 9 + 2 ' , 9 e h • 73 2 3 k m 7 / = 18b 2 8 m 4 0 + 2 s , 3 u n o d e g l i s c o p i di q u e s t o l a v o r o f u q u e l l o di p r o c u r a r c i gli e l e m e n t i necessari p e r lo studio d e i c i l i a r i e s e m p i di o n d e c. o t t e n u t i in p a r e c c h i e stazioni i n o c c a s i o n e di d e t t o t e r r e m o t o . e ' già s t a l a discussa ( j ) la p r o b a b i l e o r i g i n e di q u e s t o p a r t i c o l a r i s s i m o l i p o d i o n d e s u p e r f i c i a l i . p e r q u a n l o si r i f e r i s c e al t e r r e m o t o in e s a m e , p a r t i c o l a r m e n t e c h i a r i f u r o n o gli e s e m p i di o n d e c 1 m ( d e t c r m i n a t i da o n d e ,s i n c i denti solto l ' a n g o l o l i m i t e a d i s i a n z e d e l l ' o r d i n e di 4 . 0 0 0 k m e con p e r i o d i d e l l ' o r d i n e d i 3 3 s — 3 6 s ) . l a t a b e l l a dà i l e m p i di r e g i s t r a z i o n e d e l l e o n d e c o s s e r v a t e . l e c c o m i n c i a n o a c o m p a r i r e c h i a r a m e n t e a b e l g r a d o . g l i e s e m p i p i ù n o t e v o l i sono q u e l l i o t t e n u t i a u p p s a l a , u c c i e , s t o c c a r d a , z u r i g o . c o i r à e x e u c b à t e l ( i n q u e s t e i r e u l t i m e s t a z i o n i , n o n o s t a n t e il b r e vissimo p e r i o d o p r o p r i o degli s t r u m e n t i ) e c o s t i t u i s c o n o le o n d e a p i ù lungo p e r i o d o di lutto il s i s m o g r a m m a . e ' eia n o t a r e il f a t t o c h e n e l l e s l a z i o n i p o g g i a n t i sul suolo c o m p a t t o , l ' o n d a .s a p p a r e di i n c e r t a reg i s t r a z i o n e , m e n t r e in stazioni su strati a l l u v i o n a l i , ( k ò b c n h a v n , s t r a s b u r g o , p a v i a , r o m a , e c c . ) l ' o n d a c r i s u l t a m a s c h e r a l a da oscilla/ioni a più g r e v e p e r i o d o , p r o b a b i l m e n t e d e s t a l e dal t e r r e m o t o n e l l a c o l t r e s u p e r f i c i a l e . l ' o n d a c 1 ? l n o r m a l m e n t e , v i e n e r e g i s t r a t a fino a d i s t a n z e dell ' o r d i n e di 7 . 0 0 0 k m , a l l e (piali s u b e n t r a l ' a v v e n t o d e l l ' o n d a c 0 , i n e l t e r r e m o t o in e s a m e , la s t a z i o n e s i s m i c a e u r o p e a a m a s s i m a d i s i a n z a e p i c c n l r a l c , di cui a b b i a m o p o t u t o a v e r e i s i s m o g r a m m i , f u c o i r à ? xi 1 9 ^ 6 x ( + 6 2 ' w , 7 0 = 2 , 7 ) 3 z t y | » y : n 2 b " e . t„=2'.7] 1 8 " 4 3 " 4 4 * 4 6 " z ( 70= l',7 n e u c h a t l l c , , n s + ^ py ^ z ^'vv^'uyw'^-'-v, r \ r \ / > . w ì a ' w v 7 ^ ^ l/v v v vr ^ t ' w • z kew j1'' i ii « c o m p . n 5 c „ 1 1 t i il i! h i l u w i uno, • i> li c o m p e w i z u r i g o ~ fu c o m p . n • 5 c , r ^ li comp. e • w 5 2 2 p . caloi f . peronaci c a r t u j a (z) = 6 2 6 2 k m ) , q u a s i a l l i m i t e s u p e r i o r e d i r e c e z i o n e d e l l a cimi c h e vi a p p a r e a n c o r a s e n s i b i l e . l ' o c e a n o a t l a n t i c o s ' i n t e r p o n e p o i f r a le s t a z i o n i e u r o p e e e a m e r i c a n e . si c h e l ' o n d a c , m e b b e p o c h e p r o b a b i l i t à di e s s e r e r e g i s t r a t a , se non v e r s o l ' e s t r e m o s u p e r i o r e d e l =uo i n t e r v a l l o s p a z i a l e di r e g i s t r a z i o n e . l a d i m i n u i t a i n t e n s i t à del t e r r e m o t o e gli a p p a r e c c h i c o n p e r i o d i p r o p r i m o l t o b a s s i , f e c e r o ì c h e d e t t a r e s i d u a p o s s i b i l i t à n o n j i o t é r e a l i z z a r s i . f u d e t e r m i n a t a la d r o m o c r o n a r e l a t i v a a l l e c u l . e s s a è r e t t a d a l l ' e q u a z i o n e : 0 , 1 3 3 6 a + 2 3 5 , 3 5 = ts l a v e l o c i t à a p p a r e n t e di p r o p a g a z i o n e è r i s u l t a t a p e r t a n t o di 7 , 5 k m / s c c . f u r o n o o t t e n u t i p u r e c h i a r i e s e m p i di o n d e c i , 2 , in n u m e r o p e r ò t r o p p o l i m i t a t o p e r c o n s e n t i r n e la d e t e r m i n a z i o n e , sia p u r e a p p r o s s i m a l a , d e l l a d r o m o c r o n a . a d h u a n c a y o ( d = 1 5 . 4 0 2 k m ) , s t a z i o n e più l o n t a n a di cui abb i a m o p o t u t o a v e r e l e r e g i s t r a z i o n i , si e b b e u n a c h i a r a r e g i s t r a z i o n e di o n d a c„. = ( t = 4 8 ), onda c h e si r i t r o v a p u r e fra i s i s m o g r a m m i di c h r i s i c l i u r c h . u c c l e comp. n 5 p r a g a comp. e w " v ^ u p 5 a l a | j c , , i comp verticale comp. e w f i z . 2 «h v i* v w m " r i i l e c o m p o n e n t i v e r t i c a l i in f u n z i o n e n o n sono m o l t e e t u t t e a p e r i o d i p i c c o l i o p i c c o l i s s i m i . u g u a l m e n t e , in p a r e c c h i e s t a z i o n i la r e g i s t r a z i o n e d e l i e c è s t a t a a p p r e z z a b i l e a n c h e sulle c o m p o n e n t i vert i c a l i , il c l i c ci ha p e r m e s s o di v e r i f i c a r e la l o n g i t u d i n a l i t à d e l l e o n d e a l l ' e s a m e , c i ò c h e c o s t i t u i s c e u n ' u l t e r i o r e c o n f e r m a a l l ' i p o t e s i ni ti i n a m e n t e emessa c i r c a la l o r o o r i g i n e ( " ) . c e r t o , un p i ù a c c u r a t o e s a m e di d e l l e o n d e r i c h i e d e r e b b e u n ' c onde s u p e r f i c i a l i associate alle onde s . s s . . . nel terremoto del t u r k e s t a n 5 2 3 q u a d i s l r i b u z i o n e d i s i s m o g r a f i e d e l e v a t o p e r i o d o n e l l e v a r i e r e g i o n i d e l p i a n e t a . l a f i g . 2 ( n e l s u o p r i m o s i s m o g r a m m a , r i d o t t o a 1. <> d e l r o r i g i n a l e ) m o s t r a q u a n t o p i ù n e t t a e d i n c o n f o n d i b i l e a v v e n g a l a r e g i s t r a z i o n e d e l l e o n d e c c o n s t r u m e n t i a l u n g o p e r i o d o . s a r e m m o p e r t a n t o g r a t i a i n o s t r i c o l l e g l l i s e . n e l l i m i l e d e l p o s s i b i l e , p o t e s s e r o o v v i a r e a l l a l a m e n t a l a l a c u n a n e l s e n s o s u a c c e n n a t o . roma — istituto nazionale di geofilica — setnendìrc 1918. t e m p i d i r e g i s t r a z i o n e d e l l e c j v o t e s t a z i o n i a t e m p i regi tr. i r k o u s t 24° 14\92 iksara 29 10.13 .— e l s i n k i 33 36.12 — h e l w a n 33 36.12 — a t e n e 36 31.53 — b e l g r a d o 36 4 5 . 8 8 18" -111" 4us u p p s a l a 37 1 1 . 7 2 -11 43 z a g a b r i a 39 33.74 42 15 p r a g a 39 37.-13 42 20 k o b e n l i a v n 39 4 3 . 9 0 4 2 2 9 : t a r a n t o 40 1 8 . 3 3 42 29 t r i e s t e 11 0 6 , 4 0 42 3 7 1 c a t a n i a 42 5 9 . 6 0 ' r o m a 43 0 8 . 6 3 43 22 s t o c c a r d a 43 1-1.27 4 3 06 f i r e n z e ( a r c e t r i l 43 2 2 . 5 5 4 3 2(1 p r a t o 43 2 6 . 6 5 — c o i r à 43 33.20 -13 1 5 1 . : j 2 s 31 32 33 3-1-36 35 34 36 c o m p a r o s u l l e t r e c o m p o n e n t i . f a s e p a r t i c o l a r m e n t e vistosa sulla e . w . n e t t a m e n t e sulla e w . d e b o l e sulla n s e sulla, verticale. f a s e p a r t i c o l a r m e n t e vistosa sulla e . w . i d e m . l a presenza di i m p u l s i n o n p e r m e i l e di i n d i v i d u a r e la e v e n t u a l e onda c . f a s e poco a m p i a m a scherata da altri i m p u l l ' i n i z i o della s è (pianto m a i i n c e r t o , essa è quasi non r e g i s t r a t a . c h i a r i s s i m e e vistose l e c . s u l l e i r e c o m p o n e n t i . 5 2 1 p . c u o i f . peronaci (segue) t e m p i d i r e g i s t r a z i o n e d e l l e c stazioni a t e m p i rcgislr. t note zurigo 44" o r 10 43 24 35 ben visibile su tutte e ire l e componenti piacenza 44 01.15 .— — 11 groviglio di fasi d i o seguono la s maschera una eventuale registrazione della c. pavia 44 18.55 — — idem. strasburgo 44 10,97 •13 5 0 : 30-32 poco ampie ma nettebasilea 44 36.95 43 32 — compare su tutte l e componenti. de hill 44 51-09 43 52 35 neuelnìtel 45 11,19 18 13 41 35s sulle tre componenti. uccie 45 41,50 41 12 33 t r e onde di eccez. ampiezza. su tulle le componenti. parigi 47 26,48 44 21 36 su tutte e ire le componenti. k e w 48 17.46 44 26 — torlosa 51 50.62 45 16 37 cartuja 56 21.43 46 27 — s . ciillege 92 51.62 59 48 chicago 94 16,65 59 58 32 c u 2 19 01 28 28 pasadena 103 24 4 3 c-2,3 19 02 i i sulle tre componenti. tucson 105 50,63 c-2,3 06 14 24-26 1 t a e u b a j a 118 08,47 19 10 30 33 cristchurch 124 11.35 c„,2 19 07 11 10 19 20 2 6 33 huancayo 138 37.15 09 44 48 c „ s 15 25 38-36 poco ampie. ^213 18 30 -j 35 b i b l i o g r a f i a ( ! ) caioi p . . peronaci f . : il terrcnwlo del turkestan del 2 novembre 1046. « annali ili geofilica », i , 2, 1948. ( 2 ) c u o i p . : sull'origine delle onde superficiali associate alle onde >'. .ss, sss, « a n n a l i di g e o f i l i c a » , i , 3, 1918. l e s e s s e d e l l a g o d i v i c o g . pannocchia nella o r m a i a m p i a serie di studi di sesse (*) dei laghi italiani iniziata e guidata da p . caloi p e r l'istituto n a z i o n a l e di geofisica, si può i n s e r i r e a n c h e quello relativo al lago di v i c o . a n c h e p e r questo lago, c o m e già per gli altri, si fa riserva di d e t e r m i n a r e s p e r i m e n t a l m e n t e i valori delle sesse e di c o n f r o n t a r l i c o n i valori c h e conseguono dai calcoli teorici, e a n c h e p e r questo, 4 1 2 c . pannocchia 80 1 0 0 1 2 0 v ( x ) i c r c o m e p e r gli altri, non è da escludere, d a t a la f o r m a piuttosto complessa (fig. 1), c h e f r a i valori calcolati e i valori m i s u r a t i ci sia u n a c e r t a divergenza. il lago di v i c o si t r o v a a c i r c a 12" 10' e . g. e 42° 2 0 ' n ad u n a altezza s. 1. m. di 507 m . ; esso r i m a n e p o c o a n w del n o t o c e n t r o di r o n c i g l i o n e . i l suo specchio d ' a c q u a ha una superficie di k m 2 11.9 ( a t l a n t e dei laghi i t a l i a n i g. de agostini tav. x v ) ; ha u n a profonc , dita m a s s i m a di 4 9 , 5 m e m e d i a di 1 9 , 9 0 ili; una lunghezza secondo la linea 1 (linea di valle) di 4 9 5 0 m ; il suo volume d ' a c q u a è di c i r c a 2 6 8 . 0 0 0 ni3. il c a l c o l o delle sesse è stato eseguito con i due m e t o d i , già a l t r e volte usati e noti, di defant e di h i d a k a ; di questi non si f a r à un r i c h i a m o essendo superfluo per l'esistenza dei numerosi analoghi lavori, m a si not e r a n n o i risultati essenziali; e con il metodo di n e u m a n n ( 3 ) di cui si d a r à un breve ragguaglio e il risultato. nella fig. 1 sono indicate, o l t r e il cont o r n o dello specchio d ' a c q u a , o l t r e le i s o b a t i m e t r i c h e , le t r a c c e delle sezioni verticali c h e si i m m a g i n a di aver p r a t i c a t o (la l o r o distanza risulta dalla s c a l a ) ; p e r origine delle x si è preso il punto 0 ; in fig. 2 è riportata l a curva normale del lago secondo i calcoli. con il m e t o d o di defant si sono ottenuti i seguenti valori per l e sesse uni-bi-e tri-nodale e relative distanze dei nodi dall'origine 0 : sessa uni-nodale : t = 8 m ,3 = 4 9 8 s m 2 2 5 0 distanza del i n o d o ; 520 b(x)5[x) 1 0 6 m 5 f i g . 2 c u r v a n o r m a l e d e l l a g o di v i c o l e s e s s e del laco di vico 4 1 3 sessa b i n o d a l e : t = 5 m , l l = 3 0 6 s , 6 ili 1 0 0 0 distanza del i n o d o 111 3 2 5 0 distanza del i i n o d o ; sessa t r i n o d a l e : t = 3 m , 6 7 = 2 2 0 s , 2 ni 4 6 0 distanza del i n o d o ni 2 0 0 0 distanza del i i n o d o ni 3 7 1 2 , 5 distanza del i i i n o d o . con il m e t o d o di h i d a k a si sono o t t e n u t i i seguenti v a l o r i : sessa u n i n o d a l e : t = 7'",68 = 4 6 0 s , 6 m 2 3 4 2 , 4 4 distanza del i n o d o ; sessa b i n o d a l e : t = 4"',71 = 2 8 2 s , 9 m 1 3 9 3 , 3 5 distanza del i n o d o ni 3 4 8 0 , 2 0 distanza del i i n o d o ; sessa t r i n o d a l e : t = 3™,14 = 1 8 8 s , 6 ni 1 1 7 3 , 4 6 distanza del i n o d o in 2 5 4 7 , 5 8 distanza del i i n o d o m 4 1 1 9 , 5 6 distanza del i i i n o d o . n e r i s u l t a n o le differenze p e r i p e r i o d i d i : 0 m , 6 2 u n i n o d a l e ; 0 m , 4 0 b i n o d a l e ; 0 r a ,53 t r i n o d a l e e p e r le d i s t a n z e dei n o d i : m 92 u n i n o d o m 3 9 3 i bi-nodo m 2 3 0 i i bi-nodo ni 7 1 3 , 5 i t r i n o d o ni 5 4 7 , 6 i i t r i n o d o in 4 0 7 , 0 i i i t r i n o d o . n e l l e figg. 3 e 4 s o n o t r a c c i a t e le c u r v e c b e r a p p r e s e n t a n o l'and a m e n t o delle a m p i e z z e di s p o s t a m e n t o della m a s s a d ' a c q u a n e l c a s o dell'uni-bi e t r i n o d a l e d e s u n t e dai v a l o r i c a l c o l a t i p e r i d u e m e t o d i c i t a t i ( c u r v a c o n t i n u a m e t o d o di d e f a n t e t r a t t e g g i a t a m e t o d o di h i d a k a ) . c o m e si è d e t t o all'inizio, si è f a t t o u s o a n c l i e del m e t o d o di n e u m a n n p e r d e t e r m i n a r e la sessa u n i n o d a l e e q u e s t o p e r c h é si è r i t e n u t o c h e la f o r m a del l a g o si prestasse a l l a a p p l i c a z i o n e di t a l e m e t o d o . n e u m a n n i n t r o d u c e il c o n c e t t o di i m p e d e n z a , già u s a t o in acustica e nei c i r c u i t i oscillanti e l e t t r o m a g n e t i c i , p e r i sistemi c o n t i n u i , 4 1 4 c . pannocchia p„ a m p i e z z a d i p r e s s i o n e / . — • s (3to/3t)max s u p e r f i c i e x a m p i e z z a di v e l o c i t à [1] e n e c a l c o l a il v a l o r e a t t r a v e r s o l e e q u a z i o n i i d r o d i n a m i c h e p e r i c a s i p i ù c o m u n i . si r i p o r t a q u i s o l o il v a l o r e c h e essa a s s u m e , s e c o n d o il 0,200 0 0 , 2 0 0 £ -0,600 -1,000 • 1 , 4 0 0 • 1.000 0.600 0,200 0 -0,200 -0,600 f i g . 3 a p p l i c a n d o l o q u i n d i alla t e o r i a d e l l e sesse. 11 n e u m a n n t i e n c o n t o d e l l ' i n f l u e n z a r e c i p r o c a di b a c i n i c o m u n i c a n t i p e r m e z z o di c a n a l i s u l p e r i o d o c o m p l e s s i v o d ' o s c i l l a z i o n e ; o di q u e l l a di v a r i r a m i d i u n o stesso b a c i no. p e r i m p e n d e n z a id r o d i n a m i c a n e l l e os c i l l a z i o n i di m a s s e di a c q u a e g l i d e f i n i s c e il r a p p o r t o : z f i g . 4 l e s e s s e del laco di vico 4 1 5 n e u m a n n , i n c a s o eli b a c i n i c h i u s i (la u n solo l a t o dalla superficie t r a sversale s = bh (b — l a r g h e z z a , h = p r o f o n d i t à ) : z = _ ì ^ l c o t g . ^ , l = x / 4 [ 2 ] .5 c 2 km d o v e — 1 , c=]gh, c o = — , l — l u n g h e z z a del b a c i n o , l u n g h e z z a d ' o n d a , q = densità d e l l ' a c q u a . s e m p r e p e r n e u m a n n p e r u n l a g o i, simile al c a s o d e l l a g o di v i c o , c h e si r a m i f i c h i in ii e iii, i d u e r a m i ii e iii v e n g o n o a t r o v a r s i i n p a r a l l e l o d o p o il r a m o i di i m p e d e n z a z. s i c c h é a n a l o g a m e n t e ai c i r c u i t i e l e t t r o m a g n e t i c i l ' i m p e d e n z a c o m p l e s s i v a di ii e iii sarà : t a b e l l a i e u • e b " a m o -o o t s del u t3 » lago 3 — " v 7 "o 9 8 . 5 8 7 . 5 7 6 . 5 1 6 a 5 . 5 5 4 . 5 4 2 1 7 (> 5 i l a 4 3 2 1 1 2 i l i 3 4 5 6 nerea d e l l e sezioni v e r t i c a l i 3 8 . 8 0 0 5 8 . 2 t 0 8 3 . 0 0 0 8 3 . 3 0 9 7 . 9 5 0 9 7 . 5 " 0 9 1 . 0 ' 0 8 2 . 1 5 0 9 2 . 0 0 0 7 2 . 0 0 6 6 . 8 5 0 5 4 . 4 0 0 3 6 . 5 0 0 1 0 . 4 : 0 = ( . 8 . 8 7 5 1 1 . 2 0 0 8 . 2 5 0 7 . 7 5 0 7 . 4 5 0 " „ „ = 6 5 7 8 . 5 7 6 . 8 0 0 3 . 8 5 0 7 : 0 3 0 . 6 0 3 9 . 0 5 0 _ 3 5 . 2 0 0 a = 2 4 . 8 8 3 2 2 . 1 0 ! 1 4 . 5 0 0 7 . 8 0 0 l a r g h e z z a alla s u p e r l i c i e d e l l a s e z i o n e in ni. 17 5 2 2 2 5 2 6 8 5 2 7 7 5 2 9 5 0 3 0 5 3 1 5 0 17 3 1 7 5 3 1 7 5 2 8 2 5 2 7 5 2100 1 7 5 0 9 1 0 = 2 5 4 8 . 9 3 1 0 0 0 8 7 5 9 2 5 — 1 0 5 0 1»ju = 9 3 9 . 2 9 1 0 7 5 1 2 7 5 3 7 5 8 5 0 1 5 0 0 1 4 7 5 1 1 7 5 1 0 0 0 9 7 5 = 1 1 6 2 . 5 distanza p r o g r e s s i v a d a l l ' o r i g i n e in ni. d e l l a sezione 4 5 0 7 0 0 9 5 0 1 2 0 0 1 4 5 0 1 7 0 0 1 9 . : 0 2200 2 4 0 2 7 0 0 2 7 7 5 282". 3 0 7 5 3 3 0 0 = 3 3 0 0 2 " 0 5c0 50 1 = 1 9 : 0 a u i l a 1 0 0 0 1 2 5 0 l i 0 0 1 9 5 0 1 5 4 2 5 _ 6 7 5 i = 1 6 7 5 9 2 5 ' " " 1 1 5 1 4 2 5 1 6 7 5 p r o f o n d i t à m a x d e l l a sez. in in. 3 0 3 4 4 1 4 2 4 2 4 2 4 5 ti = 2 7 , 0 2 4 3 " 4 3 4 4 4 3 4 5 4 3 2 5 1 5 1 6 1 5 1 4 11 5 2 = 27,co 7 , 0 0 4 0 4 0 3 5 26 20 13 h = 2 1 . 4 0 i m i = 1 6 2 8 . 0 8 5 5 c m . s e c . = 8 2 8 , 6 7 4 c m . s e c . = 1 4 4 8 . 9 1 c m . s e c . l e s e s s e del laco di vico 4 1 7 j _ p 1 /„ d a c u i e d e s s e n d o il r a m o i in s e r i e c o n ( / / ( / / / ) (fig. 6 ) : 7 7 zl+p=zl+d a c u i c o n s e g u e l a c o n d i z i o n e p e r l a f r e q u e n z a p r o p r i a d e l l ' i n s i e m e : i _ + j _ + j _ = o zl z„ z 3 elle p e r l a [ 2 ] m o l t i p l i c a t a p e r g : i w r t , 7 wl, , ml, . r „, b i c i »g + k c , tg — +bsc3 tg 3 = 0 [ 3 ] cj c 2 c3 l ' e q u a z i o n e , n o t a n e u m a n n , c o i n c i d e c o n q u e l l a d e d o t t a d a zei< l o n . l ' e q u a z i o n e [ 3 ] t r a s c e n d e n t e si è c i m e n t a t a d a p p r i m a a l l a g o s u d d i v i s o c o m e d a l l a fig. 1 ( r a m i i. ii. ili, o r i g i n e d e l l e x u xo, x 3 i n 0 , 0 ' , 0 " u s a n d o s u c c e s s i v a m e n t e p e r co i v a l o r i d e d o t t i d a i v a l o r i d i t = 8 m , 3 ; t — 7 m , 6 8 ; t = 7 m ; t= 9r a e p e r bit c„ ( i = l ° , 2 ° , 3 ° ) i v a l o r i d e d u c i b i l i d a l l a c a r t a fig. 1 ; n o n si è m a i o t t e n u t o di p o t e r l a s o d d i s f a r e ; si è q u i n d i c i m e n t a t a s u d d i v i d e n d o il l a g o n e i r a m i /„, / / „ , / / / „ fig. 5 ( o r i g i n e d e l l e xu x2, x3 i n 0 , 0 ' 0 " ) . n e l l a t a b e l l a i s o n o r i p o r t a t i i v a l o r i u s a t i ( m i s u r a t i o c a l c o l a t i ) p e r bit c ; , ( i = 1°, 2 ° , 3 ° ) . p e r co = 0 , 0 1 1 0 8 1 , c o n s e g u e n t e d a t = 9 m , 4 5 = 567% l ' e q u a z i o n e [ 3 ] r i s u l t a p r a t i c a m e n t e s o d d i s f a t t a . y, ^ -v si p u ò c o n c l u d e r e per— ^ ' t a n t o c h e c o n q u e s t o m e t o d o il p e r i o d o d e l l a u n i n o d a l e v a l e : r t = 9 m , 4 5 b, h r lj f i g . 6 bz h i £ d e i t r e p e r i o d i p e r l a l i n i n o d a l e t r o v a t i p e r v i a t e o r i c a n o n si p u ò d i r e a p r i o r i q u a l e sia il p i ù p r o s s i m o a l l a r e a l t à ; p e r a l t r o si r i t i e n e c h e q u e s t ' u l t i m o sia il p i ù a t t e n d i b i l e s e m b r a n d o m o l t o s u a s i v e l e i n d u z i o n i e a n c h e l e m i s u r e e c a l c o l i f a t t i d a l n e u m a n n ; p e r a l t r o l a m i s u r a d i r e t t a d i r à l ' u l t i m a p a r o l a . roma — istituto nazionale di geofisica giugno 1950. u w 4 1 8 c . p a n n o c c h i a riassunto nel presente lavoro si determinano con i metodi di defant e di hidaka i periodi delle sesse nni-bi e tri-nodale del lago di vico, le relative distanze dei nodi e le curve rappresentanti l'andamento delle ampiezze di oscillazione con i metodi detti. con il metodo indicato da neumann si determina di nuovo il periodo della sessa uni-nodale. b i b l i o g r a f i a ( ! ) caloi p . : le aesse del lago di garda. p a r t e p r i m a . ses«e che interessano l'intero lago. a n n . g e o ! . , i , 2 4 4 8 ( 1 9 4 8 ) . caloi p . : le sesse del lago di garda. p a r t e s e c o n d a . a n n . g e o f . , i , 1 7 5 1 9 9 ( 1 9 4 8 ) . ( 2 ) perrone: carta idrografica d'italia. v o i . t e v e r e , 2 5 6 2 5 8 . ( 3 ) neumann g . : beitrage zur theorie der seiches in zusammengesetzten seebeckensystemen. a n n . g e o f . , i l i , 15-24 o p p . 11-116 ( 1 9 5 0 ) . vol50,3,2007 411 annals of geophysics, vol. 50, n. 3, june 2007 key words underwater acoustics – signal-to-noise ratio – shallow water – high frequency – gulf of naples 1. introduction over the last ten years, there has been a rapid extension in the use of digital underwater acoustic communication from mainly military purposes to commercial sues to satisfy the growing needs for underwater acoustic wireless communication systems in real time and oversee the submerged instrumentation without the direct intervention of man. underwater acoustic wireless sensor networks are an emerging topic of research and are envisioned to enable applications on the surveillance and monitoring of sea areas (adams et al., 2000; sozer et al., 2000; akyildiz et al., 2005; benson et al., 2006; jaffe et al., 2006). there is, in fact, significant interest in monitoring aquatic environments for scientific, environmental, commercial, safety and military reasons such as detect tectonic movements, incoming tsunamis, water pollution, global warming, distributed tactical surveillance, mine reconnaissance, and many other facts that are bound to affect our lives (badia et al., 2006; pompili et al., 2006). the main problem of underwater acoustic communication is the complexity of the marine underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission tony alfredo stabile (1), aldo zollo (1), maurizio vassallo (1) and giovanni iannaccone (2) (1) dipartimento di scienze fisiche, università degli studi di napoli federico ii, napoli (2) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli abstract in this paper we studied the physical properties of the gulf of naples (southern italy) for its use as a communication channel for the acoustic transmission of digital data acquired by seismic instruments on the seafloor to a moored buoy. the acoustic link will be assured by high frequency acoustic modems operating with a central frequency of 100 khz and a band pass of 10 khz. the main operational requirements of data transmission concern the near horizontal acoustic link, the maximum depth of the sea being about 300 m and the planned horizontal distance between seismic instruments and buoy 2 km. this study constructs the signal-to-noise ratio maps to understand the limits beyond which the clarity of the transmission is no longer considered reliable. using raytheory, we compute the amplitudes of a transmitted signal at a grid of 21×12 receivers to calculate the transmission loss at each receiver. the signal-to-noise ratio is finally computed for each receiver knowing also the transmitter source level and the acoustic noise level in the gulf of naples. the results show that the multipath effects predominate over the effects produced by the sound velocity gradient in the sea in the summer period. in the case of omnidirectional transmitters with a source level (sl) of 165 db and a baud rate of 2.4 kbit/s, the results also show that distances of 1400-1600 m can be reached throughout the year for transmitter-receiver connections below 50 m depth in the underwater acoustic channel. mailing address: dr. tony alfredo stabile, dipartimento di scienze fisiche, università degli studi di napoli federico ii, napoli, italy; e-mail: stabile@na.infn.it checking copy 412 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone acoustic channel: background noise from the environment and from human activities; unevenness in the water caused by the presence of solid and gaseous particles; the presence of spatial and temporal sound velocity gradients that can arise from salinity, pressure and temperature variations; multipath effects caused by multiple reflections on the sea surface and bottom; reverberations caused by the agitation of the sea and the topography of the sea bottom; and marine currents. in particular, with a digital communication system, the multipath that occurs with the propagation of signals results in intersymbol interference (isi), which limits the capacity of the underwater acoustic communication channel. all of these phenomena are magnified in the situation of acoustic transmission in shallow water with respect to transmission in deep water, due to the depth of the sea and the vicinity to the coast (cazzolato et al., 2001; smith et al., 2003), so the shallow water acoustic communication channel is characterized by strong signal degradation. the use of high frequencies in acoustic communication improves the velocity of digital transmission of data, but also causes a greater attenuation of the signal (thorp, 1967; fisher and simmons, 1977), with the consequent decrease in the range of distance, i.e. in the ability to transmit data over distance. acoustic communication has been progressively improving over the years, both in terms of the distances and the data transmission capacity, due to the development of new techniques of signal modulation. in more recent years, the feasibility of modulation techniques for underwater communication and telemetry have been demonstrated (baggeroer, 1984; catipovic, 1990; coates, 1993; stojanovic et al., 1993, 1994, 1995; stojanovic, 1996), minimizing the isi to improve the capacity of the transmission channel. internationally, the systems of acoustic links that are now most advanced use frequencies above 20 khz (ultrasound), reaching bit rates of 10 kbit/s (stojanovic, 2004). to be able to complete any underwater acoustic communication system, it is necessary to know the physical properties of the transmission channel to be able to understand what the limits on signal transmission are and how its complexity can influence the quality of the information transmission. in the gulf of naples (southern italy) a feasibility test is ongoing which provides for the installation of seismic instruments on the seafloor to extent offshore the geophysical monitoring of the neapolitan volcanoes, mt. vesuvius and campi flegrei caldera. the seismic data loggers will be acoustically linked to a moored buoy using a two-way communication modem, then a radio apparatus on the buoy will telemeter data on land. here, we studied the physical properties of the water layer of the gulf of naples for its use as an acoustic communication channel for highfrequency digital transmissions (around 100 khz), and analyzed the influence of the seasonal variations in its physical parameters on underwater transmission and the limits that the channel forces upon this communication. for this purpose we are interested to understand the effects produced by the underwater acoustic channel on a high frequency digital transmission to choose the best configuration of the communication system. we do not take into account the effects of bubble scattering and absorption because the layer that contains bubbles is near the surface (preisig, 2006) and, as our models use wind speeds less than 6 m/s, bubbles cannot significantly attenuate propagating signals in this layer (dahl, 2004). we also consider the sea as a steady propagation environment with smooth sea surface and seafloor during the transmission of data. the first section provides a description of the physical properties of the gulf of naples as a one-dimensional (1d) communication channel in shallow water, considering that the average depth of this water layer is less than 300 m. in the second section, the potential noise sources are described, and the noise levels (nl) expected at the 100 khz frequency are calculated. in the third section, we proceed to the simulation of underwater acoustic propagation, describing the calculation method carried out to determine the signal-to-noise ratio (snr). finally, in the fourth section, the results we obtained for a standard piece of apparatus for digital acoustic transmission of information are reported, evaluating the limits of high fre413 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission quency acoustic transmission in the gulf of naples, starting from the snr calculated from the sea surface to the bottom, and up to a distance of 2 km. 2. one-dimensional velocity model of the gulf of naples the gulf of naples is located along the south-western coast of italy, between 40°35l and 40°50l latitude north, and between 14°00l and 14°30l longitude east. the depth of the sea in the area of interest is not deeper than 300 m, except for the northern part of the dohrn canyon (fig. 1), in which the sea depth reaches 400 m. since the gulf of naples is not deeper than 300 m, it can be considered a shallow water acoustic communication channel. this implies a greater complexity on the sound propagation because in shallow water the surface, volume, and bottom properties are all important. moreover the seasonal variation in soundspeed structure is significant with winter condition being nearly isospeed (jensen et al., 2000). the essential characteristic of sea water is, in fact, the velocity at which the sound can be propagated through it. the square of the velocity of the sound depends on the ratio between the density and the bulk modulus of the propagation medium. in particular, in the sea the density is a function of the pressure, temperature and salinity of the sea, while the bulk modulus is a function of the pressure and temperature of the sea. for this reason, the propagation velocity of acoustic energy in the sea can be described fig. 1. bathymetric and topographic image of the gulf of naples. a bright ellipse indicates the area of interest for our studies, where the researchers of the anton dohrn zoological station carried out measurements of the physical properties of the seawater in the period from february 2002 to february 2003. 414 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone as a function of three physical parameters: pressure, temperature and salinity (jensen et al., 2000). it is not only necessary to know the mean values of these three physical parameters for the sea, but it is also important to know their spatial and temporal variabilities, as these variations significantly modify the propagation velocity of the acoustic waves in the sea. over the years, various formulae have been suggested that link the sound velocity in the sea to the three parameters of pressure, temperature and salinity (kinsler et al., 2000; mackenzie, 1981; coppens, 1981), and in particular, the unesco algorithm (chen and millero, 1977; wong and zhu, 1995) and the equation of del grosso (del grosso, 1974) are commonly used. if the sea is considered to be a resting fluid with a constant density ρ, the pressure is solely a function of the depth. there are also some very accurate, empirical relationships between sea depth and pressure. the sea temperature and salinity, however, depend upon the ambient conditions of the location, such as the solar irradiation, evaporation, the atmospheric conditions, the water circulation, the wind, the presence of rivers, and the geographical position. the characterization of the gulf of naples starts, therefore, from the knowledge of the values and variabilities of these parameters in space and time. for this, the temperature and salinity data were analyzed from measurements carried out by the researchers of the anton dohrn zoological station (ribera, personal communication, 2005) in the gulf of naples from the sea surface to a depth of 300 m, in the period from february 2002 to february 2003. the sound velocity data, calculated by the same with the equation of del grosso (del grosso, 1974) from these salinity and temperature measurements, were analyzed. figure 2 shows the salinity profiles of the sea between the surface and the bottom at 300 m in depth, with a sampling rate of 0.1 m, across 15 different periods of the year. the salinity went through small variations throughout the year, with a value of about 38 ppm. it can be seen from fig. 2 that there was an increase in the salinity with depth of about 0.5 ppm, from the surface to the bottom, and also rapid oscillations in the salinity values in the first 100 m in the non-winter periods due to surface marine currents, where at some depths the salinity can change of about 2 ppm from the mean value. in the winter periods, the strong agitation of the sea causes a mixing of the water down to great depth, resulting in stability of the salinity values. figure 3 shows the temperature profiles measured at the same time as the salinity at the same depths. the temperature varies with time, and on large spatial scales, in concomitance with the daily and seasonal variations of solar irradiation; fine spatio-temporal variations are determined locally by turbulent movements of the seawater. the variability in the temperature of the sea depends on the variability of the thermal content of the water as a consequence of the heat transfer between the surface and the adjacent atmosphere (de dominicis rotondi, 1995). from fig. 3 it can be seen that at the surface the higher temperatures are reached in the summer months, with maximum values of 27°c in august, while the lowest temperatures are reached in the winter months, with minimum values of 13°c in february; in the deepest water, the mean temperature is lower and the temperature variations between the summer and winter seasons are greatly reduced. in summer, water mixings hardly occur and results in a large negative temperature gradient in the first 75 m of depth, where the temperature can drop from 27 °c at the surface to 14°c at 75 m in depth. in autumn and winter, the mixing of the water is very intense, which generates isothermic surface layers that reach down to a depth of 50 m in october to 160 m in february. for depths greater than 160 m the mean temperature is 14°c and its seasonal variation rarely reaches 0.1°c; at this depth the temperature is no longer influenced by the outside meteorological conditions. figure 4 shows the sound velocity profiles calculated with the equation of del grosso (del grosso, 1974), starting from the salinity and temperature data. it can be inferred from this figure that the highest variations in the sound velocity are at the surface, and are between 1508 m/s and 1541 m/s, equivalent to a percent variation of ±1% of the mean velocity value. at increasing depths, the seasonal variations decrease until 415 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission they become lesser than 5 m/s below 100 m and negligible below 160 m. a comparison between the sound velocity profiles with those of temperature reveal an overall correlation at all depths for all the periods of the year, except for a positive linear trend followed by the sound velocity profiles for increasing depths, caused by a rise in pressure with growing depth. from 160 m to 300 m in depth, where the temperature variations are negligible, the velocity only depends upon the fig. 2. salinity profiles in the gulf of naples from the sea surface to the bottom, at 300 m in depth, measured across 15 different periods between february 2002 and february 2003. 416 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone pressure variations, increasing by around 2 m/s. the comparison between the salinity and sound velocity profiles shows that the rapid salinity oscillations in the hottest months, and for depths less than 100 m, do not correspond to appreciable variations in the sound velocity profiles, since the latter are less then 0.5 m/s (that is 0.03% of the mean sound velocity in the seawater). consefig. 3. temperature profiles in the gulf of naples from the sea surface to the bottom, at 300 m in depth, measured across 15 different periods between february 2002 and february 2003. the main temperature variations are above 50 m depth. for depths greater than 160 m the seasonal temperature variation rarely reaches 0.1°c. 417 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission quently, the rapid variations in salinity have not been considered in the construction of a sound velocity model of the gulf of naples. knowledge of the effects of the three physical parameters of temperature, pressure and salinity on the sound velocity profiles has allowed the construction of a multiparameter function of depth z that simulates the sound velocity profiles throughout the year and for depths from 0-300 m in the gulf of naples, asfig. 4. velocity profiles in the gulf of naples calculated from the salinity and temperature data obtained from the measurements carried out between february 2002 and february 2003. 418 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone suming that the variations seen over the sampling period are stationary. the parameters of the function were obtained through the inversion of the 15 sound velocity profiles using the non-linear simplex global optimization algorithm (nelder and mead, 1965; press et al., 1992) and creating a function of time for each parameter that interpolates the values obtained from the inversion. the chosen multiparameter model is described by the following function: . (2.1) the linear dependence with depth (βz) expresses the increase in the mechanical pressure. the parameter α represents the value of the sound velocity in sea water at a temperature of 15 °c and a pressure of 1 atm (depth of z = 0 m); the parameter v0 represents the variation with respect to α in the sound velocity at the sea surface for a different temperature. the parameter zc gives information on the thickness of the surface isothermic layer. the adimensional parameter γ amplifies the z/zc ratio and determines the negative slope of the function v(z) for z ≈ zc. 3. noise sources in the underwater acoustic channel in the gulf of naples the noise signal of an underwater acoustic sensor is composed of electronic and acoustic noise, generated by the pressure fluctuations in the sea water due to the action of ambient sources (de dominicis rotondi, 1996). here, we consider only acoustic noise, and in particular we calculate the noise level around a frequency of 100 khz in the gulf of naples. analytically, the noise level (nl) will be expressed in decibels (db) as the ratio between the noise pressure (or intensity) revealed with omnidirectional hydrophones and a reference pressure (or intensity) of a plane acoustic wave in the same frequency bandwidth. typically, the reference pressure level used in underwater acoustics is 1 µpa. in the sea, there are various noise sources that are both artificial and natural, which have intensities that vary in time, space and frequency. the natural sources have physical origins (molecular ( ) ( / )v z v z z z1 c0 α β= + + +γ6 @ thermal agitation, sea tides and turbulence), meteorological (wind, rain, hail, storms, wave backwash, glaciers), terrestrial (earthquakes, volcanoes, distant storms) and biological (cetaceans, crustaceans, fish); the artificial sources are due to human activities like seaside traffic, coastal industrial works, ports, boreholes for research into energy sources, and underwater acoustic communications besides those of interest. this noise covers a vast frequency range that goes from frequencies lower than 1 hz to frequencies higher than hundreds of khz. for the various frequency ranges, the trend of the spectrum shows particular characteristics as a consequence of the peculiarities of the various sources: each source has its own dominant frequency band in which it has a predominant aspect with respect to the other noise sources; sometimes, different sources can predominate in the same frequency band. the range going from a few khz to 100 khz is of interest in our study and is dominated by the noise generated by the impact on the sea surface of drops and water spray that arise from the breaking of the waves. a classical representation of the spectral distribution in the range from 100 hz to 25 khz comes from the historic knudsen curves (knudsen et al., 1948), which came from a vast measurement campaign carried out during the second world war by a group of researchers guided by v.o. knudsen, along the coastal waters of various ocean regions. the curve, parameterized according to the condition of the sea or equivalently as a function of the wind velocity (in knots), shows a linear trend on a logarithmic frequency scale with a slope of around −20 db/dec. a series of subsequent investigations in the knudsen range, carried out by wenz (1962) and urick (1986), established a correlation between wind speed and the ambient noise spectrum level. using the empirical formulae given by coates (coates, 1990; stojanovic, 2006), it is possible to calculate the noise level (nl), from 1 hz to 1 mhz, as a function of frequency (in khz), wind speed w (in m/s) and shipping activity factor s, whose value ranges between 0 and 1 for low and high activity, respectively. from these formulae it is possible to infer that the shipping activity factor s is negligible at frequencies of about 100 khz. 419 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission the calculation of the noise level of the gulf of naples for a frequency of 100 khz, was carried out using coates formulae and the mean wind velocity in the gulf of naples for both the summer and winter months. the wind velocities at 10 m above the sea surface were considered with the ready program starting from the fnl model (the model and program are available online on the website of the national oceanic and atmospheric administration) they were sampled every six hours from february 2002 to february 2003, the period in which the measurements of the physical parameters of the sea were taken. the mean wind velocity calculated for the summer months was 3.9 m/s, while for the winter months it was 5.6 m/s. the noise levels in the gulf of naples at the frequency of f = 100 khz are then 28 db/µpa in summer and 30 db/µpa in winter. 4. efficiency of the acoustic link for standard digital data transmission apparatus knowledge of the physical parameters and the use of the function designed to simulate the sound velocity variations with depth and time in the gulf of naples has allowed us to carry out various simulations of high frequency acoustic propagation in the gulf of naples. using eq. (2.1), a mean velocity profile was constructed for the summer and winter months, with the aim of studying the behavior of the acoustic propagation according to the variations in the sound velocity in the sea; the two velocity profiles are shown in fig. 5. we also calculated (fig. 5) the residuals for the summer and winter months as the difference between the values of the mean velocity profiles constructed using eq. (2.1) and those calculated using the profiles of fig. 4. the residuals are less than 0.5 m/s for both summer and winter mean velocity profiles, except for the first 50 m depth in summer profile where the residuals can reach a maximum value of 1.8 m/s (only 0.1% of the sound velocity in the water). below, we describe the calculation method used for the simulation of the signal propagation and for the calculation of the signal-to-noise ratio, considering a standard digital data transmission apparatus. this considers an omnidirectional transmitter that delivers sinusoidal acoustic signals for the transmission of a digital binary sequence, with a velocity of 2.4 kbit/s at a dominant frequency of 100 khz, in an operative band of 10 khz. the power delivered to the antenna was assumed to be 0.4 w with a electroacoustic efficiency of 50%. 4.1. description of the calculation method for simulation of signal propagation the propagation medium is described assuming a sea water layer (as previously described) and two solid layers (sea bottom sediments). the sediments samples taken from the first few meters of the sea bottom in the gulf of naples show that it is composed of clayey silt sediments in the first five meters and sand-siltclay sediments below, because of lower percentage of silt. from hamilton studies on physical properties of marine sediments (hamilton, 1978, 1979, 1980) we can therefore define a geoacoustic model of the seafloor assigning, for each solid layer, the thickness, the sound velocity (p-waves), the vp/vs ratio, and the density. the values of the physical parameters of the propagation medium used for the simulation are given in table i. we also consider a variation of the vp/vs ratio for both the solid layers to understand if it influences the propagation. the discontinuity surfaces between the layers are flat and horizontal and no rough sea surtable i. physical values assigned to the propagation medium for the simulation: the medium is described according to a liquid layer (sea water) and two solid layers (clayey silt sediments above sand-silt-clay sediments). stratum thickness sound vp/vs density typology (m) velocity (g/cm3) vp (m/s) sea water 300 1510-1534 ∞ 1.03 clayey silt 5 1550 13.0±2 1.40 sand-silt-clay ∞ 1650 9.4±2 1.80 420 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone face and seafloor are considered in this paper. within the water layer, the sound velocity is assumed to vary continuously with depth, following the trend given in fig. 5. the model describing the characteristics of the propagation medium in the summer months essentially differs from the one relative to the winter months in terms of the sound velocity values in the water layer above the cut-off depth of about 50 m. table ii gives the sound velocity values for the two models at increasing depths. the simulations were carried out with the 3d dynamic ray-tracing code (farra and madariaga, 1987) with modifications to allow for the anelastic attenuation in the sea, which is considered by multiplying for each phase (direct p, multiple p reflections in the water) the amplitude of the nonattenuated wave a0 by a negative exponential (4.1) where α is the attenuation coefficient and has the dimensions of inverse length and x is the length of the ray-path. from the empirical thorp’s formula (thorp, 1967; fisher et al., 1977; jensen et al., 2000) ( )expa a x0 α= − table ii. sound velocities of the summer and winter models for increasing depths. the main differences between the two models are above 50 m depth. depth (m) 0 10 20 30 40 50 60 80 100 160 300 summer model vp (m/s) 1534 1533 1527 1523 1517 1514 1512 1511 1510 1511 1513 winter model vp (m/s) 1514 1514 1514 1514 1514 1514 1513 1512 1511 1511 1513 fig. 5. mean velocity profiles (left) for the summer and winter seasons calculated from eq. (2.1) and residuals (right) evaluated respect to the mean velocity profiles computed using the 15 velocity profiles obtained from data. below 50 m depth the residuals are lesser than 0.5 m/s for both summer and winter mean velocity profiles. barbara evidenziato barbara casella di testo oppure fisher and simmons, 1977 (vedi bibliografia)? prego verificare. grazie 421 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission (4.2) the attenuation is expressed as a loss in decibels per unit of distance as a function of the frequency f (in khz) of the wave that propagates in the sea and it is indicated by αl (jensen et al., 2000), instead of α. from the eq. (4.2) and considering a frequency of 100 khz we find that αl is equal to 34.3 db/km. taking into consideration eq. (4.1), we can obtain α from αl as follow: (4.3) to study the reliability of the acoustic communication, we simulated the propagation from the transmitter to the receivers of a signal consisting of a sinusoidal transient of a 1 s duration and with a frequency of 100 khz. the transmitter, i.e. the source of the signal, is omnidirectional and was positioned at the three different depths of 1 m, 190 m and 299 m from the sea surface; for each transmitter position, the reception of the signal was simulated on a grid of 21×12 receivers, arranged in a vertical plane inside the area of interest showed in fig. 1 by a bright ellipse. the horizontal distance between the receivers is ∆x = 100 m from the vertical axis through the source for up to 2 km in distance, while the vertical distance between the receivers is ∆z = 25 m, from 5 m in depth to 280 m in depth. for each receiver, the particle displacement velocity v=(vx, vy, vz) was simulated, from which it is possible to deduce the intensity of the signal, information that is useful for the determination of transmission losses. indeed, the instantaneous acoustic intensity i of field produced by force f that acts normally on a unitary surface a is i = fv/a= pv, where p is the pressure and v is the particle displacement velocity along the normal to the surface a. considering that the timescale of oceanographic changes is an order of magnitude greater than the time-scale of acoustic propagation, it can be assumed that the . km3 95& .α = . . logx a a x20 8 686 8 686 0 1 &.α α α = − − l l c m 44 ( ) 3.3 10 . . db/km f f f f f 1 0 11 4100 3 3 10 3 4 $ $ .α + + + + + + − − 2 2 2 2 2 l density ρ and the sound velocity c are independent of time. with this assumption, both the pressure p and the velocity potential φ(v=∇φ) satisfy the wave equation, and using the linearized euler equation (jensen et al., 2000), this gives (4.4) where v is the particle displacement velocity along the direction of acoustic wave propagation. the simulated signal at each receiver is perturbed by the multipath effects, arising from the multiple reflections from the surface and the bottom of the sea. because of this effect, with the transmission of digital data, the coding signals applied to the preceding bit are superimposed on the coding signals of the succeeding bit upon reception, causing an increase in the intersymbol interference. 4.2. calculation of the signal-to-noise ratio (snr) an object that radiates with a source level (sl) will be registered by a receiver with a lower signal level (s) due to the transmission loss (tl) that it undergoes in its passage through the propagation medium. the signal-to-noise ratio (snr) for this receiver is obtained by subtracting the noise level (nl) from the signal level (s), such that: (4.5) where all of the quantities under consideration are expressed in db. to determine the limits beyond which the transmission is no longer reliable, using eq. (4.5) we can construct the snr maps for a determined source level (sl) starting from the knowledge of the transmission loss (tl) at each receiver and the noise level (nl) for the gulf of naples. the noise level (nl) was discussed above in the second section, so we now consider the determination of the source level (sl) and calculate the transmission loss (tl) at the frequency of 100 khz on the whole grid of 21×12 receivers. the source level is determined according to the relationship (de dominicis rotondi, 1990): snr (db s nl sl tl nl) = − = − − p cv i cv2&ρ ρ= = 422 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone (4.6) where we represents the electrical power supplied to the antenna, η the electroacoustic efficiency, and di the directivity index. considering a realistic example where the transmitter has an efficiency of 50% and the current supplied to the antenna is 185 ma with a tension of 3.24 v, thus giving it an electrical power of 0.6 w, from eq. (4.6) the source level will be sl= =165 db. the transmission loss undergone by the signal at any specific receiver are obtained instead from the ratio between the mean intensity i of the signal simulated at each receiver and the mean intensity i0 of the signal simulated at one meter distance from the transmitter, giving the value in db according to the relationship (4.7) which, taking into account eq. (4.4), becomes: (4.8) using eq. (4.8), it is therefore possible to calculate for each receiver the transmission loss knowing the mean value of the particle displacement velocity v = (vx, vy, vz). the mean value is calculated across a period of time equal to the duration of the signal sent by the transmitter. 5. results obtained starting from the knowledge of the transmission loss in the acoustic communication channel at the frequency of 100 khz, and from the knowledge of the noise level in the gulf of naples, and considering a source with a 165 db level, using eq. (4.5) we have constructed the maps that give the signal-to-noise ratio (snr) for the summer and winter months, for three different transmitter positions, considering both 10 10 tl (db) log log x x y z y z0 2 0 2 0 2 2 2 2 0 2 2 ν ν ν ν ν ν ν ν = − − + + + + =d f n p 10tl (db) log i i 0 = − d n di+10 ( )log η+10 ( )log we+170.5=snr (db) the multiple reflections and the direct phase only. if it is assumed that the band pass of the acoustic communication is 10 khz around the frequency of 100 khz, and that the transmission velocity of the data from the emitter is 2.4 kbit/s, from shannon’s capacity theorem for a communication channel (shannon, 1948) the signal-to-noise ratio must be greater than 22 db for reliable information transmission. figures 6, 7, and 8 indicate in black the regions in the section of water in which the communication is not reliable, as in those zones of the channel in which the signal-to-noise ratio is less than 22 db. where the transmission should be reliable (snr >22 db) a different color has been assigned to each snr value calculated, in decreasing order from red to violet. figure 6 shows the signal-to-noise ratio for a transmitter positioned at 1 m in depth. a comparison between the summer and winter maps relative to the direct phase only shows that if the source is positioned at a low depth, the effect of the negative velocity gradient that is seen in summer does not favor surface communication for increasing distances. if, however, a comparison is made between the summer and winter maps that takes into account multiple reflections between the surface and the bottom, we see that the multipath effect predominates over the velocity gradient effect. figures 7 and 8 show the signal-to-noise ratio for transmitters positioned at 190 m and 299 m in depth, respectively. from a comparison of the summer and winter maps, relative to the direct phase only, it can be seen that for these source depths the negative velocity gradient of the summer months no longer influences communication. comparing figs. 6, 7, and 8 for the cases in which multiple reflections are taken into account, it can be seen that if the source is a long way from the bottom and from the surface of the sea, the communication is less disturbed for up to 1.0 to 1.2 km, because of the lesser influence of the surfaces that delimit the communication channel. above this distance, the effects produced by the bottom and the free surfaces are similar, independently of the position of the source. the vp/vs ratio does not influence the communication. in fact changing the vp/vs ratio in the range illustrated in table i we obtain the 423 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission f ig . 6. t he s ig na lto -n oi se r at io f or t he s um m er a nd w in te r m on th s in t he g ul f of n ap le s w it h th e tr an sm it te r po si ti on ed a t a de pt h of 1 m . t he r ece iv er g ri d is p lo tt ed w it h w hi te d ot s. a c om pa ri so n be tw ee n th e su m m er a nd w in te r m ap s re la ti ve t o th e di re ct p ha se o nl y sh ow s th at i n su m m er t he ef fe ct o f th e ne ga ti ve v el oc it y gr ad ie nt d oe s no t fa vo r su rf ac e co m m un ic at io n fo r in cr ea si ng d is ta nc es . c on si de ri ng a ls o m ul ti pl e re fl ec ti on s, th e m ul ti pa th e ff ec t pr ed om in at es o ve r th e ve lo ci ty g ra di en t ef fe ct . f ig . 7. t he s ig na lto -n oi se r at io f or t he s um m er a nd w in te r m on th s in t he g ul f of n ap le s w it h th e tr an sm it te r po si ti on ed a t a de pt h of 1 90 m . t he re ce iv er g ri d is p lo tt ed w it h w hi te d ot s. f or t hi s so ur ce d ep th t he n eg at iv e ve lo ci ty g ra di en t of t he s um m er m on th s no l on ge r in fl ue nc es c om m un ic ati on ; m or eo ve r th e co m m un ic at io n is l es s di st ur be d fo r up t o 1. 0 to 1 .2 k m b ec au se o f th e le ss er i nf lu en ce o f bo th t he s ea a nd b ot to m s ur fa ce s, si nc e th ey a re f ar f ro m t he s ou rc e. 6 7 424 tony alfredo stabile, aldo zollo, maurizio vassallo and giovanni iannaccone same results as reported in figs. 6, 7, and 8. moreover, from these three figures (6, 7, and 8), it can be seen that the transmission is slightly favored in the summer period due to a lower noise level from agitation of the sea. it is nevertheless important to bear in mind that on average the difference in the noise levels between the summer and winter months due to the conditions of the sea is only 4 db in the gulf of naples. finally considering the information of fig. 5 we can see that below 50 m depth the mean sound velocity profiles for summer and winter season are similar, so also the snr is stable during the year for transmitter-receiver connections below this depth. 6. conclusions here, we have simulated the propagation of sinusoidal acoustic signals sent by an omnidirectional transmitter at the frequency of 100 fig. 8. the signal-to-noise ratio for the summer and winter months in the gulf of naples with the transmitter positioned at a depth of 299 m. the receiver grid is plotted with white dots. for this source depth the negative velocity gradient of the summer months no longer influences communication, but the influence of the seafloor is magnified. 425 underwater acoustic channel properties in the gulf of naples and their effects on digital data transmission khz for the binary sequential digital transmission of information, in a propagation medium with the physical characteristics of the gulf of naples. three different transmitter positions have been considered, at 1 m, 190 m and 299 m (1 m above the bottom) in depth. the reception of the transmitted signal was simulated, for all three cases, by a 21×12 receivers grid in the sea of 300 m in depth, for a range of 2 km; from the simulation, the snr maps have been constructed for all of the cases under consideration. considering a transmitter with a source level of 165 db and a velocity of 2.4 kbit/s, for a band pass of 10 khz, it is seen that the transmission of information is reliable up to distances of 1400-1600 m between source and receiver below 50 m depth and in all seasons of the year, because the mean sound velocity profiles for summer and winter months are similar below this depth. the summer seasonal effects on the sound velocity cannot be overlooked when the transmitter is positioned at the surface or at a low depth. the presence of the sea surface and bottom produce, moreover, important multipath effects that result in errors in the decoding process of the bit sequence of the transmitted information, particularly if the transmitter is positioned near the surfaces limiting the acoustic channel. although the sea surface and bottom produce multipath effects, the vp/vs ratio of the sedimentary layers does not influence the communication. if the transmitter is supplied with directionality, the source level can then be increased up to 40 db, and in this case it is possible to reach a distance of 2000 m while minimizing the multipath effects due to the multiple reflections. figures 6, 7, and 8 that are relative to the direct phase only give indications as to the snr that would be present in the case in which the transmitter is directional; these values must then be increased by the factor of directionality (di) of the source, which in the case examined in this article is equal to zero, because it involves a non-directional source. this study represents a feasibility test of using the acoustic communication to transmit data acquired on the seafloor of the gulf of naples. in this region a complex geophysical monitoring system operates for the surveillance of the neapolitan volcanoes (mt.vesuvius and campi flegrei caldera). all the instruments are installed inland, while on the seafloor only temporary experiments have been carried out. this study offers a new perspective on the future developments of the monitoring system by the installation of geophysical stations on the seafloor with data transmission by acoustic link to a moored buoy that provides connectivity back to the land by a radio link. acknowledgements the authors would like to thank the engineers of the whitehead alenia sistemi subacquei spa of monteruscello (na) for providing the technical specifications of the underwater acoustic transmitters and of the modalities of the underwater acoustic transmission of digital data. we also thank dr. m. ribera for providing us with the data on the physical characteristics of the water channel in the gulf of naples after the measurements carried out by a. dohrn zoological station staff in the period february 2002-february 2003. this work was funded by miur (ministry of education, university and research) ponsisma project. references adams, a., o. hinton, b. sharif, g. salles, n. orr and c. tsiminedes (2000): an experiment in sub-sea networks the lotus sea trials, in proceedings of the ecua’00, lyon, france. akyildiz, i.f., d. pompili and t. melodia (2005): underwater acoustic sensor networks: research challenges, ad hoc networks, 3 (3), 257-279. badia, l., m. mastrogiovanni, c. petrioli, s. stefanakos and m. zorzi (2006): an optimization framework for joint sensor deployment, link scheduling and routing in underwater sensor networks, in proceedings of the first acm international workshop on underwater networks (wuwnet’06), los angeles, california, u.s.a., 56-63. baggeroer, a. 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(received .............; accepted ................) barbara evidenziato barbara casella di testo questo riferimento sembra non essere citato nel testo. prego verificare. grazie a l c u n e e s p e r i e n z e su f o r m a z i o n i a c q u i f e r e c o n i m e t o d i d e l l a p o l a r i z z a z i o n e i n d o t t a e d e l l a r e s i s t e n z a a d o p e r a n d o u n d i s p o s i t i v o di m i s u r a b i p o l a r e g . p e t r u c c i e s . c o p p o l i n o ( * ) r i c e v u t o il 7 giugno 1961 il metodo di prospezione della polarizzazione indotta è in fase di sviluppo. molti sono i geofìsici che si occupano dell'argomento, sia dal punto di vista teorico che dal punto di vista sperimentale. . s. coppolino (23) ha pubblicato una nota compilativa nella quale riassume i principali risultati ottenuti con tale metodo da alcuni autori. l. v. patella (27), recentemente, ha fatto il punto sull'argomento, rivendicando i precedenti europei ed italiani su tale nuovo metodo di prospezione. in italia, a. belluigi si è occupato estesamente del problema, sia dal punto di vista strumentale che dal punto di vista teorico (!• 2> 4> 5' 6> 8). nella bibliografìa sono riportate le memorie venute fuori in questi ultimi anni. la possibilità della prospezione di strati acquiferi è stata messa in vista da yacquier v,. holmes c. e., kintzinger p. e., prische e . h., lavergue m. nei due lavori pubblicati in « geophysics » nel 1956(12) e nel 1957 (14), e da a. belluigi (6) nel 1956. in questo lavoro viene mostrata la possibilità della prospezione degli strati acquiferi per mezzo di un nuovo metodo messo a punto da g. petrucci ed n. orilia, descritto in « geofìsica pura ed applicata » (13' 15' 16). oltre alle prove di sondaggi elettrochimici (p. p.) dell'acqua, sono stati fatti altri rilievi nelle formazioni argillose, nelle alluvioni e nei tufi (*) i s t i t u t o di geodesia u n i v e r s i t à di p a l e r m o via arcliirafi. 2 5 6 g. petrucci s. coppolino calcarei, in alcune località della provincia di palermo, tendenti a rilevare le curve: cioè le curve della f. e m. di polarizzazione in funzione della corrente polarizzante i per lo stesso tempo t di carica. tali curve, che per piccoli valori di i — cioè lontani dai valori di saturazione — sono delle rette, permettono una misura della costante di polarizzazione della formazione (12), infatti è: quindi il rapporto direttivo delle rette ottenute sperimentalmente per ciascun mezzo dà la costante di polarizzazione del mezzo stesso. la costante di polarizzazione pertanto può servire a differenziare le formazioni geologiche del sottosuolo. i risultati di queste ultime prove sono stati pubblicati in « geofìsica e meteorologia » (1960) (28). a t t r e z z a t u r a s t r u m e n t a l e e d i s p o s i z i o n e d e l l e s o n d e d i m i s u r a . l'attrezzatura strumentale impiegata è stata analoga a quella riportata nella pubblicazione (13), apportandovi delle modifiche per aumentarne la precisione di misura. le misure sono state f a t t e usando i due elettrodi di corrente anche come elettrodi potenziometrici. come è noto, v. vacquier e c (u), p . sumi (24) ed altri adoperano due sonde di corrente e due potenziometriche distinte, usando una disposizione tipo wenner come quella per l'esecuzione dei s. e. col metodo della resistività. la ragione della nostra preferenza per la prima disposizione è dipesa dal fatto che proprio in corrispondenza delle sonde di corrente si ha la massima f . e. m. di polarizzazione; basta spostarsi di poco da tali sonde per avere una forte diminuzione della f. e. m. di polarizzazione. naturalmente il campo che viene a stabilirsi nel sottosuolo è quello relativo alla differenza di potenziale applicata alle sonde di corrente (dipolo) e pertanto la polarizzazione sarà una funzione di tale distribuzione, il cui effetto massimo si manifesterà proprio su di esse. i n questo ep = / (i) per t = eostante ep = ki per t = costante alcune esperienze su formazioni acquifere, ecc. 2(31 caso, inoltre, si ha un effetto di prospezione in profondità più grande che nel caso della disposizione wenner. le misure sono state eseguite con impulsi di corrente continua di frequenza 16 h. s o n d a g g i col m e t o d o d e l l a p o l a r i z z a z i o n e i n d o t t a p e r l a p r o s p e z i o n e d e g l i s t r a t i a c q u i f e r i d e l s o t t o s u o l o . sono stati scelti dei casi conosciuti dal punto di vista geoidrologico per l'esistenza di pozzi o perchè già studiati col metodo della resistenza; precisamente un caso in cui gli strati acquiferi si trovano in terreni alluvionali in vicinanza di un torrente e due casi in cui gli strati acquiferi si trovano in seno a dei tufi calcarei del quaternario. un sondaggio è stato f a t t o in corrispondenza di una formazione di argille eoceniche affioranti. tutti i sondaggi elettrochimici sono stati eseguiti mantenendo cocostante l'intensità della corrente polarizzante (40 ma) e spostando una sonda di corrente, mantenendo l'altra fissa. la durata della corrente polarizzante è stata per ogni misura di 1'. le variazioni della f. e. m. di polarizzazione, dovute ai cambiamenti locali, superficiali del terreno, in direzione della sonda mobile, sono state eliminate con misure rispetto ad una terza sonda posta a distanza relativamente grande (25) e pertanto le variazioni riscontrate nelle misure sono da attribuire alle variazioni di costituzione degli strati in profondità. operando in questo modo vengono valutate ed eliminate anche le polarizzazioni parassite che insorgono nelle sonde stesse e nel terreno vicino ad esse assieme a quelle da mettere in vista, delle quali giustamente si è preoccupato a. belluigi fin dal primi lavori pubblicati sull'argomento. s o n d a g g i o e l e t t r o c h i m i c o p . p . i n u n a f o r m a z i o n e a r g i l l o s a . il sondaggio elettrochimico è stato eseguito in corrispondenza di una formazione di argille eoceniche affioranti, esistenti nel territorio di misilmeri (palermo). tale formazione è molto estesa a nord, ad est e a sud di misibneri. il diagramma del sondaggio elettrico è stato riportato nella fig. 1. esso mostra un piccolo massimo in corrispondenza della profondità di 1 m dovuto all'esistenza di infiltrazioni acquifere in seno al terreno vegetale, constatate attraverso l'esistenza di un pozzetto superficiale. 2 5 8 g. petrucci s. coppolino il diagramma mostra' poi un andamento parallelo all'asse delle ascisse denotando un andamento costante della polarizzazione da attribuire alla formazione di argille eoceniche. o 1 3 4 5 s 7 8 9 10 11 12 m. f i g . 1. d i a g r a m m a di sondaggio col m e t o d o della polarizzazione i n d o t t a in argille eoceniche affior a n t i , misilmeri, ( p a l e r m o ) . s t r a t i a c q u i f e r i i n t e r r e n i a l l u v i o n a l i p o g g i a n t i s u a r g i l l e e o c e n i c h e . il sondaggio elettrochimico è stato eseguito in contrada ballestrero, proprietà aggiusti, misilmeri (palermo), in una valletta. i n tale località era stata f a t t a una ricerca col metodo diretto della resistenza apparente di g. petrucci (26) ed era stato individuato uno strato acquifero dell'ordine di 3 litri/sec., e riscontrato con un pozzo eseguito dopo la prospezione. nella fìg. 2 sono stati riportati sia il diagramma relativo al metodo diretto della resistenza (diagramma in basso), sia il digramma relativo al sondaggio col metodo della polarizzazione provocata p. p. (diagramma in alto). il diagramma relativo alla p. p. presenta un andamento che corrisponde a quello della resistenza. i n f a t t i presenta un primo tratto, fino a circa 5 m di profondità, con valori costanti della f. e. m. di p. p . attribuibile a strati alluvionali asciutti e poi si hanno due massimi successivi in corrispondenza delle due variazioni del digramma della resistenza, cioè in corrispondenza dello strato acquifero. dalla profondità di 11 m e fino a 16 m, la p. p. si mantiene elevata con oscillazioni intorno ad un certo valore medio attribuibile alle argille; alcune esperienze su formazioni acquifere, ecc. 2(31 infatti il diagramma della resistenza a partire da 11 m indica, col suo andamento, l'esistenza della formazione di argille plioceniche (cioè di una formazione a resistenza specifica bassa) (26). fig. 2. diagrammi di sondaggi con i metodi della polarizzazione indotta e della resistenza in terreni alluvionali acquiferi, contrada ballestrero, misihneri, (palermo). s t r a t i a c q u i f e r i i n s e n o a i t u f i c a l c a r e i d e l l a p i a n a d i p a l e r m o , p o g g i a n t i s u a r g i l l e (ginolfo). il caso riportato è stato rilevato nella proprietà spanò a romagnolo (palermo). gli strati del sottosuolo sono costituiti da qualche metro di terreno vegetale e da tufi calcarei quaternari della piana di palermo, poggianti su ginolfo. entro i tufi calcarei scorrono degli strati acquiferi dell'ordine di 12 litri/sec., accertati mediante pozzo. lo spessore degli strati di tufo acquiferi va da 7 a 16 m di profondità. 2 6 0 g. petrucci s. coppolino nella fig. 3 sono stati riportati il diagramma rilevato col metodo della polarizzazione indotta (sopra) e quello della resistenza (sotto). il diagramma elettrochimico presenta un ampio intervallo di massimo propig. 3. d i a g r a m m i di s o n d a g g i oon i m e t o d i della polarizzazione i n d o t t a e della r e s i s t e n z a in t u f i calcarei a c q u i f e r i . r o m a g n o l o , p a l e r m o . prio in corrispondenza della zona acquifera. anche le piccole variazioni lungo il diagramma si possono, grosso modo, mettere in relazione con quelle esistenti nel diagramma della resistenza in corrispondenza degli stessi strati acquiferi. anche in questo caso, a partire da 16 m., cioè dall'inizio del ginolfo, la f. e. m. di p. p . si mantiene elevata, ma più piccola del valore massimo raggiunto in corrispondenza dello strato acquifero. s t r a t i a c q u i f e r i i n s e n o a i t u f i c a l c a r e i d e l l a zona d i t e r r a s i n i (palermo) p o g g i a n t i s u a r g i l l e (ginolfo). i diagrammi riportati sono relativi ad una formazione di tufi, calcarei quaternari poggianti su una estesa formazione di ginolfo. a e p polarizzazione r p r o f o n d i t à ' esplorata 1 2 3 4 6 8 10 12 14 16 18 20 22 24 26 m. a l c u n e e s p e r i e n z e su formazioni a c q u i f e r e , ecc. 2(31 in seno ai tufi esistono degli importanti strati acquiferi provenienti da monte, ove esistono dei rilievi montuosi di calcari mesozoici che costituiscono un grande bacino di raccolta. 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 m f i g . 4. d i a g r a m m i di sondaggi con i m e t o d i della polarizzazione i n d o t t a e della resistenza in t u f i calcarei a c q u i f e r i . t e r r a s i n i , p a l e r m o . il diagramma della resistenza, fig. 4, presenta, dalla profondità di circa 23 m e fino alla profondità di circa 38 m, delle variazioni dovute agli strati acquiferi esistenti nella formazione di tufi calcarei. i n corrispondenza di tale intervallo il diagramma delle f. e. m. di p. p. presenta una ampia zona di massimo con un minimo molto stretto intorno alla profondità di 34 m. lateralmente alla zona di massimo si hanno delle f. e. m. di polarizzazione più basse e però il valore medio a profondità elevata risulta più grande di quello a profondità piccole, come negli altri casi. aep polarizzflzione r c 2 6 2 g. p e t r u c c i s . coppolino in tale zona sono state rilevate due curve: ep — f (i) per t = costante una delle quali, per una distanza fra le sonde di misura di 4 m, in maniera che la corrente polarizzante attraversasse i tufi asciutti superficiali, ed un'altra, per una distanza di 68 m, in maniera clìe la corrente polarizzante attraversasse i tufi acquiferi, posti a circa 34 m di profondità. fig. 5. curve di polarizzazione i n d o t t a in f u n z i o n e della i n t e n s i t à della c o r r e n t e di polarizzazione in t u f i calcarei sterili ed in t u f i calcarei a c q u i f e r i . i risultati delle misure sono riportati nella fig. 5. i rapporti direttivi delle due rette mostrano che i tufi acquiferi presentano una polarizzazione più elevata di quelli sterili (tufi asciutti 23,87 volt/amp.; tufi acquiferi 31,66 volt/amp.). ciò conferma i risultati dei sondaggi, cioè la possibilità di individuare gli strati acquiferi in seno ai tufi calcarei per mezzo della polarizzazione indotta. alcune e s p e r i e n z e su formazioni a c q u i f e r e , ecc. 2(31 c o n c l u s i o n i . da quanto precede risulta che col metodo della polarizzazione indotta gli strati acquiferi vengono messi in vista per mezzo di massimi della f. e. m. di p. p . apparente. fig. 6 d i a g r a m m i r i l e v a t i d a v a c q u i e r su u n profilo t r a s v e r s a l e alla valle rio g r a n d e socorro n e w mexico. i diagrammi riportati mostrano una correlazione fra le variazioni riscontrate nei sondaggi eseguiti col metodo della polarizzazione provocata e quelle riscontrate col metodo della resistenza. appare anche che col metodo delle p. p. si hanno maggiori ampiezze di variazione nei diagrammi rispetto a quelle ottenute col metodo della resistenza. ciò lascia intravedere delle ottime possibilità d'impiego del metodo della p. p. nel campo delle ricerche idriche. naturalmente occorre ancora provare in corrispondenza di altre situazioni idrologiche, per avere una idea delle effettive possibilità del metodo. nella fig. 6 è riportata una serie di diagrammi rilevati da vacquier e collaboratori (12'14) su un profilo trasversale alla valle rio grande socorro new mexico, dal fiume ad un pozzo profondo 500 piedi. nella parte sinistra della figura sono riportate le sezioni del terreno e lo schema di misura tipo wenner. nella parte destra sono riportati i 2 6 4 g. p e t r u c c i s . coppolino diagrammi rilevati; quelli punteggiati si riferiscono al metodo della resistività e quelli a tratto e punto al metodo delle p. p. per ciascun diagramma è segnata la profondità dello strato acquifero nel punto corrispondente. l'effetto dell'acqua è messo in vista con l'aumento della polarizzabilità a partire da una profondità vicina a quella dell'acqua. tutti i diagrammi mostrano che la polarizzabilità aumenta con la profondità fino a notevoli valori di essa e ciò mostra che attraverso tali diagrammi non viene messo in vista lo spessore dello strato acquifero. tale spessore invece viene messo in vista col metodo bipolare attraverso l'intervallo di profondità entro cui cade la zona di massimo della f. e. m. di p . p., fig. 2, 3, 4. un confronto sulle possibilità pratiche fra il metodo quadripolare ed il metodo bipolare, allo stato attuale delle applicazioni, non si può fare, ma in linea teorica il metodo bipolare presenta il vantaggio di misurare nella sua interezza la risposta alle sollecitazioni della corrente polarizzante, mentre col metodo quadripolare viene misurata solo una piccola frazione di essa. infatti, yacquier, per avere un effetto sensibile, adopera nelle sue esperienze correnti di 4 + 5 amp. invece nelle esperienze riportate in questo lavoro, le correnti impiegate sono state dell'ordine dei milliampere. il metodo bipolare ha un effetto di prospezione in profondità circa y2 della distanza elettrodica f r a le sonde di corrente, mentre nel metodo quadripolare col sistema wenner l'effetto in profondità è circa % di essa. col metodo bipolare e con l'applicazione della terza sonda (metodo petrucci) (25) è possibile eliminare le numerose polarizzazioni parassite agli elettrodi e le variazioni di f. e. m. di p. p. dovute alle inomogeneità superficiali del terreno in cui essi vengono infissi. riassunto vengono eseguiti dei sondaggi col metodo della misura della resistenza apparente del terreno e col metodo della polarizzazione indotta, secondo lo schema di g. petrucci ed n. orilia (13), i quali confermano la possibilità della prospezione degli strati acquiferi col metodo della p. p. messa in vista da altri autori (12>14> 16 ). in ultimo viene fatto un confronto fra il metodo che sfrutta per le misure di p. p. la disposizione quadripolare e quello che sfrutta la disposizione bipolare. a l c u n e e s p e r i e n z e su formazioni a c q u i f e r e , ecc. 2(31 summaby tests are carried aut by the method of the measure of the apparent resistance of the soil and by the method of induced polarization, according to the scheme of g. petrucci & n. orilia (13), which confirm the possibility of prospecting water-bearing strata already pointed out by other authors (i2, i4, i6)_ finali]/ a comparison is made between the method which uses for the measure of p. p. the quadripolar disposition and that which uses the bipolar disposition. b i b l i o g r a f i a (' ) belluigi a., fortsclirifte in elelctrischen aufsuchen voti petroleum. w o r l d p e t r o l e u m congress, l o n d o n , (1933). (2) — the need of revising the apparatus employed in tlie measurement o j an electrochemical effeet of the ground. b e i t r u g e z. a n g . geophisik, b d . 9, (1940). (3) — sul metodo « elflex » per la ricerea diretta del petrolio. « boll. serv. geol. d ' i t a l i a » , r o m a , (1955). (4) — sulla ricerca geoelettrica diretta del petrolio. « geofisica e meteorologia », 3, g e n o v a , (1955). (6) — sull'effetto e. m. diretto di emittori alternativi in un terreno omogeneo. « a n n a l i di geofisica», 7, r o m a , (1954). (6) — ricerche elettro-idrologiche con i. p. « a n n a l i di geofisica » 9, r o m a , (1959). (7) — la ricerca geofisica dell'acqua tellurica. convegno d e l l ' a p p r o v v i g i o n a m e n t o idrico della capitale, r o m a , maggio 1957. (8) — das skalare fiir einen bechteckigen impuls in einem homogenen medium erzeugte potential. « zeit geophysik » h . 6, (1958). (0) petrucci g., possibilità applicative del metodo di prospezione elettrochimico (polarizzazione provocata). « a t t i del congresso minerario i t a liano » (1948). (10) petrucci g., panfalona v., sulla polarizzazione provocata in alcuni campioni di rocce e di minerali con particolare riguardo agli strati delle serie gessosa zolfifera della sicilia. « r e n d i c o n t i dell'associazione miner a r i a s a r d a » , 6, (1952). (11) petrucci g., orilia n., il comportamento di alcuni campioni di minerali dell'inglesiente rispetto alla polarizzazione elettrica provocata. « rendiconti dell'associazione mineraria s a r d a » , i, (1955). ( 1 2 ) v a c q u i e r v . , k i n t z i n g e r p . r . , h o l m e s c . r . , f r i s c h e r . i i . , prospecting for ground water by induced electrical polarization. new mexico i n s t i t u t e of mining a n d technology r e s e a r c h a n d d e v e l o p m e n t division, f e b b r a i o (1956). 2 6 6 g. p e t r u c c i s . c o p p o l i n o (13) petrucci g., orilia n . , su una attrezzatura adatta alla prospezione del sottosuolo col metodo elettrochimico. « geofisica p u r a e a p p l i c a t a » 33, (1956). ( 1 4 ) v a c q u i e r v . , h o l m e s c . r . , k i n t z i n g e r p . r . , l a v e r g u e m . , prospecting for ground water by indueed electrical polarization. « g e o p h y s i c s » 22, 3, (1957). (15) petrucci g., orilia n . , la polarizzazione provocata nel cemento consolidato e nell'argilla. « geofisica e meteorologia » 1/2, (1957). (16) — — lo studio della presa ed indurimento dei cementi per mezzo del fenomeno della polarizzazione provocata. « r i v i s t a min. siciliana », 46, 47 (1957). (17) frisciie r . h . , a n d von buttlar h . , a teoretieal study of indueed electrical polarization. « geopliysics » 22, 3 (1957). (18) wait j a m e s r . , frisciie r i c h a r d h . , von buttlar a d h a r o , duscussion on « a theoretical study of indueed electrical polarization« «geophisics » 23, i, (1958). (10) von henkel h . , some theoretical consideration on indueed polarization. « geophysics », 23, (1958). (2u) sciiufle j . a., cation exchang and indueed electrical polarization. «geop h y s i c s » , 24, 1, (1959). (21) seigel g a r o l d 0 . , mathematical formulation and type eurves for indueed polarization. « geophisics » 24, 3, (1959). (22) donald y . , marschall a n d m a d l e n t h e o d o r e r . , indueed polarization, a study of its causes, « geophysics », 24, 4 (1959). (23) coppolino s., sulle modalità e possibilità d'impiego del metodo di prospezione elettrochimico (polarizzazione provocata). « l ' i n d u s t r i a miner a r i a » marzo (1959). (24) sumi f . , geophysical esploration in mininy by indueed polarization, « geophisical p r o s p e c t i n g », 3, (1959). (25) petrucci g., atti del congresso mondiale del petrolio. « p r o c e d i n g s » sez. 1, 573, r o m a , (1955). (26) — i sondaggi elettrici col metodo diretto di misura della resistenza elettrica del terreno. teoria, applicazioni pratiche. « boll, del servizio geologico d ' i t a l i a » , 4, 5, (1959). (27) patella l . u., i precedenti europei ed italiani sui nuovi metodi di prospezione geofisica. « overvoltage. « meteorologia e geofisica» (1960). (28) petrucci g., coppolino s., la determinazione della costante di polarizzazione provocata e la sua applicazione alla prospezione delle formazioni geologiche. « m e t e o r o l o g i a e geofisica» (1960). s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e d e l g o l f o di g e n o v a (*) p . c a l o i m . c . s p a d e a ricevuto i ' l l novembre 1960 1. proseguendo neb'indagine concernente l'idrodmamica dei porti, dei golfl e dei mari comunque interessanti la penisola itabana, abbiamo portato la nostra attenzione sul mar ligure e sulla possibibta che la distesa d'acqua che va sotto questo nome, possa essere aniniata — quando se ne presentano le condizioni idonee — da movimenti proprii. la possibibta di t a b movimenti, in linea di massima poteva senz'altro essere affermata, attesa la particolare situazione geograflca del mar ligure. dal punto di vista idrodinamico, questo infatti pu6 essere approssimato ad un grande golfo, aperto verso il mediterraneo occidentale. la bocca di tale gob'o pu6 essere rappresentata da una bnea che va da calvi, in corsica, a nizza, mentre nel suo lato di sud-est pu6 ritenersi bmitato fra capo corso, l'isola d'elba e piombino. i venti, le tempeste e, soprattutto, il transito di ampie aree di disturbi microbarici, possono senz'altro essere invocati come cause di perturbazioni capaci di interessare l'intero mar ligure e di provocarne le oscillazioni proprie. 2. ci siamo proposti di determinare le caratteristicbe fondamentab debe eventuab oscillazioni proprie del mar ligure, considerato appunto come golfo osciuante. a tale scopo, come e ben noto, necessita l'uso di carte batimetriche accurate, che riproducano, con buona approssimazione, l'andamento del fondo per il tratto di mare interessato dal fenomeno sopra accennato. malauguratamente, almeno bno ab'epoca in cui questo studio e stato iniziato, non esistevano carte del genere, e quebe che siamo riusciti a trovare portavano dati non molto dettagbati e che per di piu si arresta(*) questo lavoro e stato condotto con contributi del consiglio nazionale delle rieerehe. 50 p . c a l o i m. c. s p a d e a vano all'isobata di 2000 metri. per tale motivo, ci siamo visti costretti a limitare i calcoli ad una zona in cui l'estrapolazione oltre la profondita di 2000 metri era, sia pure grossolanamente, eonsentita. essa e riprod o t t a nella fig. 1. t a b e l l a i 104 m 106 m 2 a v (»)„ 106 m 2 t = 2ii 4 q m sezioni 104 m 106 m 2 a v (»)„ 106 m 2 hx m „ rl(x)v m m v 1013 „,3 0 274 28 0 36 459 0 1.000 1 1,0 257 20 1650 38 764 40 7247 0.997 2 i) 217 42 1507.5 45.442 84 0241 0.988 3 » 215 04 1450 45 201 134 7828 0.972 4 » 197 72 1555 47 896 185 2723 0.947 5 » 196 66 1890 46 273 238 7721 0.910 6 » 159 70 2068.2 53 538 290 4590 0.855 7 » 127 47 2165 61 191 350 2609 0.780 8 i) 124 79 2210 55 694 418 6112 0.695 9 i> 88 60 2177.5 67 381 480 8214 0.597 10 i> 53 08 2077.5 92 125 556 0860 0.489 11 » 30 22 1822.5 125 083 658 9896 0.378 12 » 6 12 1482.5 441 176 798 7073 0.270 13 » 1 06 813.5 1.745 283 1291 5009 0.185 14 » 0 516.5 0 3240.9820 0.068 era sottinteso che i valori forniti dal calcolo, in tali condizioni, non potevano che essere approssimativi. sicche — almeno in questo primo lavoro — ci si doveva accontentare dell'ordine di grandezza dei possibili periodi del mar ligure. fortunatamente — ma soltanto quando il lavoro di calcolo era gia stato ultimate — il direttore dell'ufhcio idrografico di genova ci fece cortesemente pervenire una carta batimetrica del mar ligure estremamente dettagliata. questa carta, f r u t t o di un lavoro delicate e minuzioso degno del massimo elogio, formera la base per una prossima ricerca piu accurata dei movimenti del tratto di mare in questione. 3. ci6 premesso, riassumiamo i risultati ottenuti in questa prima parte dell'indagine, che trova i suoi limiti nelle deficienze sopra accennate. 9° 10* 44 43° 44° 43° 8* 10 fig. 1. 3.341 j > 1 1 »5 2 p . caloi m. c. s p a d e a la tabella i porta i dati necessari all'applicazione del metodo di goldberg, suba base debe sezioni pratieate come da fig. 1. riteniamo superfluo esporre il suddetto metodo, gia sunteggiato altrove come estensione del metodo di defant (1). ii risultato deb'appbcazione lia portato per il periodo deb'uninodale al valore di t = in 40», valore cbe soddisfa appuuto le condizioni del metodo di goldberg. tale valore naturabnente, e suscettibbe di un aumento, come conseguenza della correzione di bocca. quest'ultima, per i motivi accennati al n° 1, non potra cbe essere approssimativa. ad ogni modo, prendendo in via provvisoria, un fattore di correzione dell'ordme di 1,5, il periodo reale del mar ligure, come golfo oscibante, risulta dell'ordine di t corr = 2 h,55. una determinazione accurata di detto periodo quale risulta dabe osservazioni, ci riserviamo di effettuarla nel successivo lavoro sub'argomento. ad ogni modo, suba base degb esempi registrati a genova e livorno, il periodo approssbnativo dell'uninodale del mar ligure risulta deb'ordine di 3h, 0. 4. una testimonianza suba reale esistenza di oscillazioni bbere interessanti il mar ligure, poteva trarsi dabe registrazioni mareograficbe ottenute in porti interessati dabe acque del mare stesso. i n altri termini, se l'interpretazione idrodinamica era esatta, si dovevano avere registrazioni contemporanee di dette oscillazioni, e di ampiezza comparabile, aba base del golfo stesso cioe fra genova e livorno, mentre per quanto concerne le ampiezze, doveva constatarsi una graduale dbninuzione di esse, a mano a mano cbe si procede verso la bocca del gobo stesso, cioe verso nizza. in eft'etti, nel porto di imperia l'ampiezza di t a b oscibazioni e gia notevolmente ridotta, mentre a genova e a livorno si lianno registrazioni dell'uninodale contemporanee e di ampiezza appunto comparabile, (fig. 2). riassunto si da notizia dell'esistenza di un'oscillazione interessante vintero mar ligure. essa viene registrata, sovrapposta alia marea, da genova a livorno, e — con minore ampiezza — a imperia, che si trova infatti s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e d e l g 0 l f 0 d i g e n o v a 5 3 in prossimita della bocca del mar ligure, considerato come ampio golfo aperto sul mediterraneo occidentale. un esame piii accurato delle caratteristiche di tale oscillazione sard compiuto sulla base della nuova carta batimetrica del golfo stesso pubblicata dall' ufficio idrografico della marina di genova. summary notice is given of the existence of an oscillatory mouvement interesting the whole ligure sea. this mouvement is registered, overimposed to the tide, from genova to livorno and, to a minor extent, at imperia, that in fact is in proximity of the ligure sea, which is considered as alarge opened gulf on the western mediterranean sea. a more accurate examination of the characteristics of this oscillatory mouvement ivill be completed on the basis of the new batimetric chart of the gulf itself, published by the ufficio idrografico della marina of genova. b i b l i o g r a f i a ( l ) c a l o i p., le sesse del lago di garda, p a r t e i i , «annali di geofisica», i , 2, 1948. song_finalonline:layout 6 annals of geophysics, 57, 5, 2014, a0542; doi:10.4401/ag-6474 a0542 acoustic ray tracing in the atmosphere: with gravitational effect and attenuation considered yang song*, yuannong zhang, chen zhou, zhengyu zhao wuhan university, department of space physics, school of electronic information, wuhan, china abstract an acoustic ray tracing model is developed to take into account the impacts of gravitational field and realistic atmospheric attenuation. ray tracing equations are deduced from the real part of the dissipative dispersion relation, while the acoustic attenuation coefficient and growth rate in a stratified moving atmosphere are deduced from the imaginary part of the dispersion relation. to account for the non-isothermal effect and realistic attenuation, the buoyancy frequency and the cut-off frequency are substituted by the values in the slowly varying atmosphere, and the attenuation coefficient is corrected by the realistic absorption. in the validation by numerical experiment, the ray trajectory obtained by this ray tracing model agrees well with the result calculated by the fdtd method. it is shown that the acoustic trajectory can be accurately predicted by this ray tracing model. the numerical results for 5 hz acoustic waves show that in the stratospheric ducting the gravitational effect plays a leading role while the attenuation effect could be neglected. but for the thermospheric ducting, the contribution of the absorption becomes more important. 1. introduction it is widely considered that the acoustic wave is one of the essential forms of energy transportation in the atmosphere. depending on the profiles of the temperature and wind, waves could be ducted through the troposphere, stratosphere, and thermosphere [drob et al. 2003]. this phenomenon, called long range sound propagation (lrsp) [kulichkov 2004], is frequently observed at infrasonic frequencies because of the low attenuation in the lower atmosphere [bedard and georges 2000]. consequently, natural and artificial sources could be detected and monitored through the network of infrasonic stations, such as the international monitoring system [le pichon et al. 2009]. in addition, as the propagation is controlled by the background profiles, it is also possible to study the properties of the atmosphere by analyzing infrasonic signals, such as probing the high altitude wind [le pichon et al. 2005]. therefore, it is important to comprehensively understand the propagation properties of acoustic waves. ray tracing is an efficient way to study the propagation of acoustic waves in the atmosphere. it can be used to calculate the trajectories of acoustic waves as well as a number of propagation parameters including travel time and arrival angle. with the ray path available, the location and intensity of the fluctuations can be predicted [pilger and bittner 2009]. since the early model developed by barry [1963], enormous progresses have been achieved in ray tracing studies. there are three classical ray tracing models, namely harpa, wasp-3d and tau-p, all of which have been widely applied to study the sound propagation [e.g., le pichon et al. 2010]. harpa is derived from the dispersion relation in a stratified moving atmosphere on the basis of hamilton equations [jones et al. 1986]. wasp-3d [dessa et al. 2005] is also a hamiltonian solution extended from 2d cartesian ones [virieux et al. 2004], but it is deduced from the eikonal equation. tau-p model [garcés et al. 1998] is developed for computation of travel times as a reformulation of the original method presented by buland and chapman [1983]. all of these three models are developed to study the atmospheric wave propagation through a windy, non-isothermal, non-dissipative atmosphere. theoretically, the high frequency approximation adopted by the above three models confines the ray tracing modeling to acoustic signals above 5 hz for which the gravity influence is neglected. moreover, the attenuation of acoustic wave is not considered well in the three models. however, in the atmosphere acoustic waves can extend down to much lower frequencies ~0.02 hz [drob et al. 2003]. in addition, there are many types of signals in the 0.02-5 hz frequency band, due to the radiation by some natural or artificial sources. for instance, signals with frequencies from 0.5 to 5 hz article history received december 10, 2013; accepted september 2, 2014. subject classification: acoustic ray tracing, dissipative dispersion relation, gravity effect, attenuation. were observed in the cases of large bolides [hedlin et al. 2010], meteorites [le pichon et al. 2013], volcano [kanamori et al. 1994, evers and haak 2005] and explosions [stevens et al. 2002, mckenna et al. 2007]. under the influence of gravity, the propagation of acoustic waves becomes more dispersive and anisotropic as frequency decreases. for modeling the propagation of acoustic waves below 5 hz, the high frequency approximation is no longer valid and the effect of gravity needs to be taken into account. damping of acoustic waves in the realistic atmosphere is another important ingredient of acoustic propagation modeling [de groothedlin et al. 2011], which however is often ignored in ray tracing modeling. based on the calculation of the speed and attenuation at altitudes up to 160 km developed by sutherland and bass [2004], the attenuation effects on the propagation of infrasound is studied without considering gravitational effect [bass et al. 2007]. the damping effect of low frequency waves is not significant near the ground, but it increases remarkably at higher altitudes. in addition, the accumulation of attenuation effects along the long-range ray path is nonnegligible. therefore, in order to model the ray path of acoustic waves precisely, the wave attenuation also needs to be included. ray theory for lossy media has been discussed in radio wave propagation by jones [1970]. to take losses into account, the complex phase refractive index instead of the real index defined in the ideal media is used, so that the ray path can be calculated using the extended snell’s law or haselgrove’s equations. the expression of such a path is in complex space, so only those rays ended in real space are physically significant. another way is developed by extending hamilton’s equations for lossy media under the constraint that the space, time and group velocity should be real [sonnenschein et al. 1997]. when studying the acoustic wave propagation, especially the infrasonic propagation, both the effects of attenuation and the gravitational dispersion should be taken into account. other than using the complex index, it could be a better way to obtain the ray equation through the hamilton’s equations by employing the dissipative dispersion relation as an extension. in the present study, an acoustic ray tracing model in the realistic atmosphere is developed on the basis of analyzing both imaginary part and real part of the dissipative dispersion relation. in section 2.1, the dissipative dispersion relation is derived from the governing equations of an isothermal dissipative atmosphere. in sections 2.2~2.4, the acoustic attenuation coefficient and growth rate in the moving atmosphere are deduced from the imaginary part of the dispersion relation, while ray tracing equations are obtained from the real part of the dissipative dispersion relation. modifications for the parameters are presented in section 3. the buoyancy frequency and the cut-off frequency are substituted by the values in the slowly varying atmosphere. the acoustic attenuation coefficient is corrected by the theory of absorption in the atmosphere up to 160 km [sutherland and bass 2004, 2006], aiming to include the non-isothermal effect and the realistic atmospheric attenuation. this ray tracing model is validated by the comparison with fdtd solution in section 4. numerical examples are shown in section 5 to study the influences of the gravitational effect and attenuation, followed by the conclusion in section 6. 2. inclusion of gravity and absorption into ray tracing equations in order to introduce gravity influence and dissipation effects into acoustic ray tracing theory, we start with the compressible fluid equations with molecular viscosity and thermal diffusivity in three dimensional cartesian systems [beer 1974]. (1) where t, p, s and t are particle density, pressure, entropy, and temperature respectively, v = (u,v,w) is the wind velocity, is the stokes derivative, g is the gravitational acceleration, h and p are shear and bulk viscosity. for the convenience of formula derivation, the viscous term is neglected in following calculations. with the knowledge of the thermodynamic relationship, the entropy and temperature in the atmosphere can be given by (2) in equations (2), c = cp/cv is the ratio between specific heat at constant pressure and constant volume, and r is the universal gas constant. it is assumed that the basic quantities such as t, p and v can be expressed as the sum of the ambient value (with subscript 0) and the perturbation caused by the passage of a sound wave (with subscript 1), i.e. (3) as the acoustic wave in the atmosphere is an irrotational wave, the variable v satisfies ∇∇·v1=∇ 2v1. vdt d t $2 2 d= + v v g vv t dt d p t dt d s 3 0 0 2 $ $ 2 2 d d dd d t t t t p h t h + + = + = = + ^ h z [ \ ] ]] ] ]] ` j const ,lns c p t r p v yt t = + =c m , , v v vp p p0 1 0 1 0 1t t t= + = + = + song et al. 2 3 with the approximation of a stratified atmosphere, equation (1) can be simplified and linearized to yield where v0 = (u0,v0,0) is the wind profile of the atmosphere, is the adiabatic sound speed with t0 as the ambient temperature. 2.1. dissipative dispersion relation assume that the acoustic wave has a plane wave solution in the form (5) where p, r, x, y and z are polarization terms, ~ and k=(kx,ky,kz) are angular frequency and wave number components, respectively. substitution into equation (4) yields a set of fifth-order algebraic equations for (u1,v1,w1,t1/t0,p1/p0), which can be expressed in the matrix form as (6) in the coefficient matrix of equation (6), x = ~ − v0 · k, the doppler-shifted frequency, is the coefficient of kinematic viscosity of air, h = c2/cg is the scale height. according to the cramer’s rule, for a non-trivial solution of equation (6), the determinant of the coefficient matrix must vanish. therefore, the dissipative dispersion relation becomes (7) equation (7) is complex because the inhomogeneity and dissipation of the atmosphere results in complex wave number components. obviously, if the absorption is neglected, equation (7) reduces to the dispersion relation of acoustic gravity waves [yeh and liu 1974]. since the wave vector k is complex, it can be written as (8) where k' = (k'x,k'y,k'z) is the real part of the wave vector, known as the wave vector in an ideal medium, and k" = (k"x,k"y,k"z) is the imaginary part of the wave vector that can characterize the attenuation and growth of wave amplitude in the inhomogeneous atmosphere. subsequently, the dispersion relation equation (7) can be decomposed into two equations, i.e., the imaginary part, equation (9) and the real part, equation (10). (9) (10) the first term on the left side of the dissipative dispersion relation, g2 (c − 1)(k2x + k 2 y), plays an important role in the real part of the dispersion relation, as it represents the influence of the gravity that generates gravity dispersion. this term is discarded in the derivation of equation (9), because the imaginary part of the dissipative dispersion relation is mainly associated with the attenuation relation between frequency and imaginary wave vector k". however imaginary wave vector k" represents the changes of wave amplitude, we keep it in the derivation of equation (10). the imaginary wave vector k" can be derived from equation (9) in order to calculate the amplitude variations along a ray path. by substituting k" into equation (10), the ray equations can be determined on the basis of hamilton equations. 2.2. the attenuation and growth of acoustic waves as acoustic waves propagate in the atmosphere, the wave amplitude usually drops due to the attenuation effect. growth in the wave amplitude may occur because of the atmospheric density gradient in the vertical direction. thus, in the dissipative stratified atmosphere, the imaginary wave vector can be considered as a combination of omnidirectional attenuation and growth in the vertical direction. assuming the attenuation efficient and growth factor as a and b respectively, we obtain that (11) by substituting equation (11) into equation (9) and (10), the imaginary part and real part of the dissipative dispersion relation can be rewritten as 0 dt d u x p dt d p dt d p g dt d x z w dt d p c dt d z p c z w u v y v w z w u y v z w 3 4 3 4 3 4 0 0 0 1 1 0 0 1 1 2 1 0 0 1 1 1 2 1 0 1 0 1 0 1 0 1 0 1 0 1 0 2 0 1 0 2 0 1 0 2 12 2 2 2 d 2 2 d 2 2 2 2 2 2 2 2 2 2 2 2 dt p h t p h t t p h t t t t t t t + = + + = + + + = + + + + + = + = z [ \ ] ] ] ] ]] ] ] ] ] ]] ` ` ` c j j j m exp p p p r x u y v z w a i t k y k y k zx y z 0 1 0 1 1 1 1 t t ~ = = = = = = -^ h6 @ ,vdt d t 0 0 $2 d= + 3 4 0g p h t= +` j 1 0 g k ik i k c k ik i gk yx z 2 22 2 2 2 2 2 c g g c x x x x + + + = ^ ^ ^ ^ h h h h k k ki= +l m 2 0k k kc gkz0 2 2$ g cx+ =l m l l yx1 2 2 0k k k k k k g k gk k c z 2 22 0 2 2 2 2 $ $c g g c x x x + + + = l l l m l m l m m ^ ^ ^ ^ h h h h6 @ , ,k k k k k k k k x x y z z a a a b= = = + yk m l l m l l m l l c rt0 0c= acoustic ray tracing in the atmosphere (4) 1 i k ik i k ik i k ik h h g i i ighk ighk ighk g i u v w p p 0 0 0 0 0 0 0 0 1 0 0 0 0 x x y y z z 2 2 2 2 1 0 1 g g g c c t t x x x x x x + + + = p r q q q q q q qq p r q q q q q q q q t v u u u u u u uu t v u u u u u u u u (12) the attenuation coefficient and growth rate can then be solved from equation (12) as (14) (15) in equation (14), minus sign means the decrease of wave amplitude; doppler-shifted frequency x represents the impact of wind on the attenuation by changing the propagation distance in a unit time. in the windless atmosphere, x reduces to ~, and this coefficient becomes the classical absorption coefficient [beer 1974]. equation (15) indicates that an exponential amplification or decay with height can be caused by the inhomogeneity of the atmosphere in the vertical direction. 2.3. final form of dissipative dispersion relation substituting the solved attenuation coefficient a and growth factor b into equation (13), the real part of the dissipative dispersion relation becomes the final form of dispersion, written as (16) the term , which is the contribution of the gravitational effect, where ~2g = (c− 1)g2/c2, ~g is the isothermal vaisala-brunt frequency; and ~a = c/(2h) is the acoustic cut-off frequency; the term is the contribution of attenuation. if both gravitational effect and attenuation effect are neglected, the dispersion will become the classic acoustic dispersion relation (17) according to the theory of atmospheric waves, the waves whose frequency is greater than acoustic cut-off frequency are identified as acoustic waves, and the waves whose frequency is less than vaisala-brunt frequency are the internal gravity waves. the dispersion relation equation (16) can be employed to describe both acoustic waves and internal gravity waves, but this work mainly focuses on the propagation of the acoustic wave, the internal gravity waves will not be discussed. actually, when the dispersion is employed to obtain the ray tracing equations, the accuracy of the ray tracing equations depend on the dispersion relation equation. so the gravitational term gr and attenuation term at are of great importance to the dispersion relation. it can be discussed in two limiting cases that are low frequency case and high frequency case. for the acoustic wave with low frequency which is close to the acoustic cut-off frequency (~ → ~a), the dissipative effect is so small that could be neglected, meanwhile the gravitational effect is enhanced, so the dispersion relation is transformed as (18) equation (18) is the classic dispersion relation of acoustic gravity wave. as to the acoustic waves with high frequency (~ >> ~a), the dispersion equation becomes (19) different from the classic acoustic dispersion relation, equation (17), the attenuation term at is taken into consideration, because as the frequency becomes higher, the attenuation becomes stronger. in the high frequency approximation, the gravitational dispersion term , but the scale height of a mean density variation still exists. the term ~2a can be taken as the contribution of the inhomogeneity of atmosphere density. both limiting cases demonstrate that the inclusion of the gravity and attenuation are reasonable enrichment for the classic acoustic dispersion relation. the effect caused by gravity and attenuation can be studied by the group velocity which can be derived as where u0 and v0 represent zonal wind and meridional wind respectively. if both gravitational effect and attenuation are neglected, equation (20) would be sim2c k k k gk 0z0 2 2 2a b g cx+ + =l l l l^ h x y 2 g k k c k k k g k k c k k k 1 4 2 0 z z z 2 2 2 4 0 2 2 2 2 2 2 3 2 2 c a b ab c a b a b x x x x x + + + + + =l l l l l l l l l ^ ^ ^ ^ h h h h c c m m c k 2 0 2a g x =l c g h2 2 1 0 2b c = = c k gr at 04 2 2 2x x+ + =l x ygr c k kg a 2 2 2 2 2 2~ ~x= + -l l^ h 7 2at k k c c kh k 4 z2 2 2 2 2 2 2 23g gx x x x= -+l l l l^ h` j 0c k2 2 2x =l c k gr 02 2 2x + =l 0c k ata 2 2 2 2~x + =l x yc k k 0g 2 2 2 2 2 "~ x+l l^ h x y x y x y k c k k at c k c k at u k c k k at c k c k at v k c k k at c k at 4 2 1 2 1 4 2 1 2 1 4 2 1 2 1 x g x g x k y g y g y k z g z k 3 2 2 2 2 2 2 2 2 0 3 2 2 2 2 2 2 2 2 0 3 2 2 2 2 2 x y z 2 2 2 2 2 2 2 2 2 2 2 2 ~ ~ ~ ~ ~ ~ ~ ~ x x x x x x x x =+ + + + + =+ + + + + =+ + + x x x l l l l l l l l l l l l l l ^ ^ ^ h h h z [ \ ] ] ] ] ] ]] ] ] ] ] ] ]] song et al. 4 (13) (20) 5 plified as the group velocity derived from the classic acoustic dispersion relation. in equation (20), we can find that the terms which mainly controlled by attenuation modify the valve of the group velocity. the gravity term gr brings in the gravitational dispersion and makes the sound propagation become inhomogeneous. taking the gravity and attenuation into consideration, the profile of group velocity in the atmosphere can be predicted more accurately. 2.4. ray tracing equations equation (16) is a real variable equation. on the basis of equation (16), ray equations can be derived from the hamilton equations, expressed as (21) where r is a point on the ray path, ∇r and ∇k are the gradient operators in r and k space, respectively, and h is the dispersion relation [yeh and liu 1974].by substituting equation (16) into (21), the ray equations can be derived as (22.a) (22.b) for the components of the wave number, (23) in equations (22) and (23), (24.a) (24.b) (24.c) from equations (22-24), we can observe that the parameter g and play important roles in the ray tracing equation. as mentioned in section 2.3, with attenuation considered, the group velocity would be modified. what’s more, the gradient of kinematic viscosity also can enhance the gradient of the group velocity. both terms demonstrate that the acoustic trajectory is influenced by the effect due to the attenuation in a dissipative atmosphere. for given atmospheric parameters including the wind profile, temperature field and the profile of viscosity g, the ray trajectory in the dissipative atmosphere can be determined by numerically solving equations (22-24). 3. parameter modifications in the realistic atmosphere derivation of the dissipative dispersion is very complex, because a fifth-order determinant has to be solved. in order to simplify the derivation, two assumptions are employed. firstly, the atmosphere is assumed isothermal so that the gradients of pressure and temperature can be described in a simple form in terms of the scale height. secondly, only the viscous dissipation in the momentum equation is considered. in the realistic atmosphere, the temperature varies with height, and other dissipation mechanisms, such as heat conduction and molecular relaxation losses, also play a role. provided that the atmospheric temperature varies very slowly and the high order derivatives of scale height can be ignored, modifications of ~a and ~g can be obtained as (25) in equation (25), ~an and ~b are the acoustic cutoff frequency and vaisala-brunt frequency in the non-isothermal atmosphere, respectively [beer 1974, matsumura et al. 2012]. by using the temperature and wind profiles given by the msise-00 [picone et al. 2002] and hwm-93 [hedin et al. 1996] empirical reference models, the typical values of acoustic cut-off frequency and vaisala-brunt frequency in the realistic atmosphere are shown in figure 1. varying with altitude, the acoustic cut-off frequency reaches the extreme value of about 0.0039 hz and 0.005 hz at the height of the stratoandat atk2 2xl ,dt dr h dt d hkk rd d= = z d d z x ck h g k k ak bu 2 2z x 3 2 2 0 g c g x x = + + + l l l l ^ h z d d z y ck h g k k ak bv 2 2z y 3 2 2 0 g c g x x = + + + l l l l ^ h z d dk k d d 0, dz z z ck h g k k d e 2 2 x y x z 3 2 2g c gx x = = = = + + + dk l l l^ h 2 4 2 a c c k g k 2 7 g 2 2 2 2 2 4 2 2 2 2 ~ g g gc x x x x = + + -l l ^ h 4 2 2 7 2 c k c k k k c k b h 6 a 2 2 2 2 2 2 2 2 2 2 3 3 4 ~ g g g x x x x x x = + + + l l l l l^ h 2 2 7 4c c c k 2 2 2 2 2 2 2 4g gx x x=+ l yx 4 d z c k z z k c k k u k v c k2 a z a g x z y 2 2 2 2 0 2 2 2 2 2 0 2 2 2 2 2 2 2 2 2~ ~ ~ x x x = + + + + + l l l l ^ ^ ^ ^ ^ ^ h h h h h h ; 6 e @ 7 7 2 2 e c z c k z k k c k z z c k k u k c k k h k h k c k z h k c k h k h c h v 8 7 6 2 4 x z z z z z y z 3 2 4 2 2 2 2 2 2 2 2 0 0 2 2 2 3 2 4 2 2 2 3 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 g g g g g g g g x x x x x x x x x x = + + + + + + + l l l l l l l l l l l l ^ ^ h h ` ` c j j m z2 g z2 g ,t g z t t g z t an a b g 2 2 2 2 2 2 ~ ~ ~ ~= + = + acoustic ray tracing in the atmosphere (24.d) (24.e) sphere and thermosphere, respectively. the prediction of acoustic attenuation in the realistic atmosphere should consider the contribution of different dissipation mechanisms including viscosity, diffusion and molecular relaxation [brown and hall 1978]. the coefficients for different kinds of dissipation mechanism can be combined as one net attenuation coefficient in terms of linear superposition. in this study, kinematic viscosity g is modified by the value of the net attenuation coefficient, so that we can account for the realistic attenuation effects into dissipative dispersion relation. according to the algorithms developed by sutherland and bass [2004, 2006], which can precisely estimate the acoustic absorption up to 160 km, the net attenuation coefficient at (np/m), is written in the form of linear summation (26) where acr is the attenuation coefficient associated with the combination of classical and rotational relaxation losses, adiff is the attenuation coefficient due to diffusion loss, and amax,i is the maximum loss per wavelength for the ith molecular vibration relaxation component with a relaxation frequency fvib,i. in the realistic atmosphere, four primary gas components, o2, n2, co2, and o3 are considered. typical profiles of attenuation coefficient at frequencies of 0.05, 0.5, 5 and 8hz are shown in figure 2. clearly, the total attenuation coefficients for all considered frequencies increase significantly in the thermosphere. from equation (14), in the windless conditions, the parameter g that describes the acoustic attenuation can be given by (27) combining equation (26) with equation (27), we understand the parameter g as an artificial viscosity, which is determined as a function of frequency, temperature, pressure and relative humidity. using the modified values of ~a, ~g, and g, the ray tracing model can compute the trajectory of acoustic waves more accurately, since the contributions of both temperature gradient and acoustic attenuation are taken into account. 4. comparison with fdtd solution to validate this ray tracing model, we simulate the propagation of a gaussian infrasound wave packet by fdtd method, and compare the trajectory obtained by fdtd solution with the result calculated by the ray tracing model. by using a finite-difference time-domain method [wochner et al. 2005] consisting a dispersion relation preserving scheme in space and a runge-kutta scheme in time, this fdtd model is built on the basis of the governing equations of acoustic waves, equation (4). in the fdtd experiment, the gaussian infrasound wave packet at the frequency of 0.5 hz is set to be emitted eastward with shooting angle of 15° at the height of 10 km. the pressure perturbation of the packet has the following form: (28) where pc = 100 pa; xc = 8 km; zc = 10 km; kx and kz are the horizontal and vertical component of the wave number vector respectively; and vx and vz are the half width of the wave packet in the horizontal and vertical direction respectively. in the simulation, vx = vz = 1.6 km. according to the background profiles, the parameter kx and kz are initially set by using the dispersion equation (16). the other initial quantities such as u1, w1 and t1 / / / a c f f f f12 ,maxt cr diff ii vib vib 22 # # a a a= + + +^ ^^h h h 6 6 @ @ " , / 2 .k c t 2 g ~ a = ,p x z k x x k z zc x x z z x c z c 2 2x c z c 2 2 2 2 = + -v v sinp e e ^ ^ ^ ^ ^ h h h h h 6 @ song et al. 6 figure 1. profiles of acoustic cut-off frequency (solid) and vaisalabrunt frequency (dashed), calculated using the msise-00 and hwm-93 empirical model. figure 2. total attenuation coefficient as a function of altitude for frequencies of 0.05, 0.5, 5 and 8hz (from left to right). 7 are derived from the polarization equations of acoustic waves in the atmosphere [beer 1974]. the background profiles of density, temperature shown by figure 3a are provided by the msise-00 [picone et al. 2002], and the wind profile shown by figure 3b is provided by hwm-93 [hedin et al. 1996]. the attenuation coefficient profile is derived by the algorithms founded by sutherland and bass [2004, 2006] which is already exhibited in figure 2. for the msise and hwm models, the time we set is the 300th day of the year at 9:00 ut; while the geographic location of the source is (39.92°, 116.46°) and the parameter f10.7 index is 150. the simulation results are represented by the acoustic intensity of the packet. continuous capture of the infrasound packet every 30s during the propagation are shown by figure 3c. as seen in figure 3c, the position and altitude of the packet are changed due to the refraction controlled by the atmospheric profile. because of the effects of attenuation and geometric spreading, the intensity of the infrasound packet becomes weaker as the packet gets higher. in this case, the trajectory of the infrasound is the wave energy propagation path which can be represented by the positions of the wave packet’s energy center. the propagation path of the packet obtained from the fdtd solution and the ray path predicted by the ray tracing model are compared in figure 4. the comparison shows that the ray path predicted by the ray tracing model is equal to the propagation path of the infrasound packet obtained from the fdtd solution within the computational precision. based on the results shown in figure 4, we can conclude that this ray tracing can be proven accurate and reliable. 5. numerical results to investigate the impacts of gravity dispersion and acoustic dissipation, based on the background models of the msise-00 and hwm-93, three cases of acoustic ray tracing in the atmosphere figure 4. comparison between the trajectory obtained by the fdtd solution and ray solution. the fdtd solution is denoted by ‘+’, while the ray solution is represented by the solid line. figure 3. (a) background profiles of density and temperature; (b) background profiles of zonal wind and meridional wind; (c) the acoustic intensity of the infrasound packet in the form of continuous capture every 30s. a b c numerical results are presented. the first is calculated using the acoustic ray tracing model for the realistic atmosphere, as described in section 2, in which both gravity dispersion and acoustic dissipation are considered. the second is calculated using the lossless ray tracing model that considers only the gravity dispersion, which can also be called “the acoustic gravity wave ray tracing model” [yeh and liu 1974]. the last is obtained from the classical ray tracing model in which either gravity dispersion or acoustic dissipation is neglected [jones et al. 1986]. the numerical results for the above three cases are compared with each other under the condition of long range sound propagation (lrsp). 5.1. trajectories in this calculation, the input parameters of msise00 and hwm-93 are set as same as the validation experiment in section 4. under the background profiles of density, temperature and wind shown by figure 3a and figure 3b, we assume that the acoustic wave with a frequency of 5 hz is emitted eastward by an infrasonic source on the ground. shooting angles of the rays vary from 3° to 43°, with an increment of 8°. figure 5 shows the wave trajectories for the three cases. the solid, dotted, and dashed lines denote the wave trajectories obtained using the acoustic ray tracing model for the realistic atmosphere, the acoustic gravity wave ray tracing model, and the classical ray tracing model, respectively. two channels can be identified from these modeled trajectories. one is the reflection from the height less than 50 km, which is called “stratospheric ducting”, and the other is “thermospheric ducting” that occurs at higher altitudes from the height of thermosphere. the effect of acoustic dissipation can be registered by comparing the solid and dotted curves, and the effect due to the gravity can be evaluated by comparing the dotted and dashed curves. we first discuss the effect of acoustic dissipation and then the gravitational effect. for the rays with shooting angles of 3°-19°, which are in the channel of stratospheric ducting, the two sets of trajectories coincide well. but for the other rays with shooting angles of 27°-43° that are in the channel of thermospheric ducting, there are obvious differences between the two sets of trajectories. it indicates that the dissipation of acoustic waves has an effect on the trajectory of 5 hz acoustic waves. the influence due to the dissipation is quite significant for thermospheric ducting but less important to stratospheric ducting. it is mainly because the attenuation coefficient varies with altitude. as shown in figure 2, the net attenuation coefficient for 5 hz waves reaches a peak valve (~0.03 db/km) at the altitude of 10km in the troposphere and stratosphere. as expressed by equation (18), the dispersion relation would be reduced to the traditional dispersion of acoustic gravity wave when the attenuation could be neglected. as the altitude increases, the attenuation coefficient increases, reaching ~400 db/km in the thermosphere. because the attenuation coefficient is very small (< 0.03 db/km), the effect of dissipation has little influence on the rays in the stratospheric ducting. for thermospheric ducting, in the height range 100 km < z <140 km, the turning points of ray trajectories with the realistic attenuation are at an altitude lower than that of rays in the lossless atmosphere. obviously, the refraction is enhanced by the strong acoustic attenuation in the thermosphere. by comparing the dotted and dashed lines, discrepancies occur in both stratospheric ducting and thermospheric ducting. for the trajectory with shooting angle 3°, due to the gravitational effect, the dotted lines are below the dashed line in the region z <160 km. after the intersection of the two types of lines, the dotted lines are above the dashed lines. however, this phenomenon does not occur for all rays. take the trajectory with shooting angle 19° for an example. the dotted lines are always above the dashed lines. since comparisons between the trajectories are complicated, we will describe the discrepancies of the three kinds of trajectories via some concrete parameters such as turning point and propagation distance. 5.2. turning point and propagation distance turning point and propagation distance of the three kinds of trajectories as a function of elevation are shown in figure 6. with the same range of shooting angle but a smaller step of 1°, a clearer view of the insong et al. 8 figure 5. ray paths estimated based on three ray tracing models. the solid and dotted lines show the results calculated using our acoustic ray tracing model and the acoustic gravity wave ray tracing model respectively, which both consider the effect of gravity dispersion. the dashed lines represent the results calculated using the classical ray racing model that ignores both gravity dispersion and acoustic dissipation. shooting angles of 5 hz acoustic waves vary from 3 to 43° with a step of 8°. 9 fluences associated with the gravity dispersion and dissipation can be obtained. a turning point is an altitude that characterizes the transformation of wave rays between the two channels, i.e., stratospheric ducting and thermospheric ducting. in figure 6a, for the solid and dotted lines, the elevation range of stratosphere ducting is from 3° to 19°, while the elevation range of stratosphere ducting is 20°~43°. for the dashed lines, the elevation range of stratospheric ducting and thermospheric ducting are 3°~21° and 22°~43°, respectively. due to the gravitational effect, the rays with shoot angles 20° and 21° no longer belong to the stratospheric ducting but the thermospheric ducting, suggesting that the gravitational effect play an important role in determining the channel of acoustic ray trace. by comparing the dotted line and the dashed line in figure 6a, the calculation deviation of the turning point is about 1.5 km in stratospheric ducting, and it will rise to about 8 km (at the initial elevation 31°) in the propagation channel of thermospheric ducting. as indicated in figure 5, little difference is observed between solid line and dotted lines in stratospheric ducting in both figure 6a and figure 6b. this difference illustrates that the deviation between the solid line and dashed line in stratospheric ducting is caused by the gravity. in thermospheric ducting, deviation of the turning point between solid line and dotted lines could reach about 15 km, as indicate in figure 6a. controlled by the deviation of the turning point, deviation of the propagation distance between solid line and dotted lines increases along with the initial elevation, and reaches the peak value of about 160 km, then decreases to the level of approximately 60 km (figure 6b). these great deviations between solid line and the other lines shown in figure 5 indicate that the strong attenuation is a dominant factor for the ducts in the thermosphere. as seen in figure 2, the profile of acoustic attenuation varies with frequency. the attenuation decreases when frequency goes down, so for the propagation with the frequency smaller than 5 hz, the influence due to attenuation in the thermospheric ducting will become weaker. 6. conclusions an acoustic ray tracing model including the effects of gravity and attenuation is developed in the present study. the propagation of acoustic waves in nonisothermal lossy atmosphere can be described by this model. by comparing the ray path estimated by this model with the fdtd solution, the ray tracing model has been proven accurate and reliable. the influences due to absorption and gravity are investigated by comparing the numerical results obtained using this acoustic ray tracing model with the other two models, i.e., acoustic gravity wave ray tracing model and the classic ray tracing model that ignores the two effects. the results indicate that in stratospheric ducting, the gravitational effect plays a leading role while the effect associated with the dissipation is weak. because of the gravity, some rays ducted in the stratosphere as a lossless medium can merge into the channel of thermospheric ducting in the realistic atmosphere. for thermospheric ducting, the refraction is enhanced by the strong absorption. this results in the dominance of dissipative effect over gravitational effect in thermospheric ducting. to conclude, by taking the gravity into consideration, the acoustic frequency in our model is no longer constrained by the high frequency approximation. instead, it is available to accurately predict the propagation of infrasound with frequencies lower than 5 hz. consideration of the dissipation effect in our model can also improve the computation accuracy of ray trajectories and the efficiency of wave amplitude evaluation. acoustic ray tracing in the atmosphere figure 6. comparison of the turning height (a) and propagation distance (b) as a function of elevation angle. the solid and dotted lines show the results calculated using our acoustic ray tracing model and the acoustic gravity wave ray tracing model respectively, which both consider the effect of gravity dispersion. the dashed lines represent the results calculated using the classical ray racing model that ignores both gravity dispersion and acoustic dissipation. shooting angles vary from 3~43° with a step of 1°. a b references barry, g. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 6, rs642, 2022; doi:10.4401/ag-8823 o p e n a c c e s s the fastvrp automatic platform for the thermal monitoring of volcanic activity using viirs and slstr sensors federica torrisi*,1,2, eleonora amato1,3, claudia corradino1, ciro del negro1 (1) istituto nazionale di geofisica e vulcanologia, osservatorio etneo – sezione di catania, piazza roma 2, 95125 catania, italy. (2) department of electrical, electronic and computer engineering, university of catania, viale andrea doria 6, 95125 catania, italy (3) department of mathematics and computer science, university of palermo, via archirafi 34, 90123 palermo, italy article history: received march 23, 2022; accepted october 29, 2022 abstract satellite thermal remote sensing is widely used to detect and quantify the high-temperature volcanic features produced during an eruption, e.g. released radiative power. some space agencies provide fire radiative power (frp) products to characterize any thermal anomaly around the world. in particular, level-2 frp products of the visible infrared imaging radiometer suite (viirs) and the sea and land surface temperature radiometer (slstr) are freely available online and they allow to monitor high-temperature volcanic features related to the dynamics of volcanic activity. here, we propose the fastvrp platform developed in google colab to process automatically the frp products provided by the national aeronautics and space administration (nasa) and the european space agency (esa) space agencies. fastvrp was designed to monitor the volcanic radiative power (vrp) related to eruptive activity of mt. etna (sicily, italy). we compared the quality of these frp products during a number of recent paroxysmal lava fountains occurred at etna volcano between february and march 2021. we highlighted the advantages and the limits of each sensor in monitoring intense volcanic eruptions lasting a few hours. furthermore, we combine the mid-high spatial/ low temporal resolution viirs and slstr with the low spatial-high temporal resolution seviri (spinning enhanced visible and infrared radiometer imager) to improve estimates of the energies released from each paroxysmal episode. in particular, we propose a fitting approach to enhance the accuracy of seviri low spatial-high temporal resolution measurements exploiting the few acquisitions from viirs and slstr high spatial-low temporal resolution during lava fountain cooling phase. we validated the radiative power values forecasted from viirs and slstr with the radiative power values retrieved using modis (moderate resolution imaging spectroradiometer) sensor. keywords: volcanic thermal anomalies; satellite remote sensing; volcanic radiative power (vrp); level-2 product; geostationary and polar satellites federica torrisi et al. 2 1. introduction satellite remote sensing thermal images have been used to observe volcanoes worldwide for several decades and have been proved to be a valuable and effective tool to detect and quantify thermal anomalies (also called hotspots) associated with volcanic activity. infrared radiometers on-board of geostationary and polar satellites orbiting around the earth are used to measure radiance from high-temperature volcanic features [harris, 2013; corradino et al., 2019], whose released energetic content is quantified by the radiative power. firstly, hotspot pixels, i.e. thermally active pixels indicating the presence of a thermal anomaly, are detected by means of different techniques, e.g. spatial-based (or contextual) algorithm [harris et al., 1995; higgins & harris, 1997; ganci et al., 2012], temporal-based algorithm [tramutoli, 1998; di bello et al., 2004; pergola et al., 2004], spectral-based (or fixed threshold) algorithm [wright et al., 2002; flynn et al., 2002], machine learning techniques [piscini & lombardo, 2014; corradino et al., 2022]. then, the radiative power released by thermal anomalies can be estimated using the middle infrared (mir) radiance technique proposed by wooster et al. [2003], according to which the energy emitted by a hot surface, i.e. temperature greater than 500k, is directly related to its satellite measured radiance at about 4 μm. numerous volcanic hotspot monitoring platform were developed for the near real-time monitoring of thermal anomalies using satellite data, such as modvolc [wright et al., 2004], hotvolc [gouhier et al., 2016], firms [davies et al., 2019], mirova [coppola et al., 2020] and lav@hazard [vicari et al., 2011]. in particular, the last two platforms are able to provide a complementary information because of the different temporal and spatial resolutions of sensors used, namely modis (moderate resolution imaging spectroradiometer) in mirova and seviri (spinning enhanced visible and infrared radiometer imager) in lav@hazard. modis, onboard aqua and terra polar satellites, provides data four times per day with a spatial resolution from 250 to 1000 m, but a low temporal resolution (about 2 acquisitions per day); on the other hand, seviri, onboard the geostationary meteosat second generation (msg) satellites, has a coarse spatial resolution (about 3 km at the equator), but this is balanced by its high temporal resolution (up to one sample every five minutes in rapid scan mode for north africa and central europe). mirova uses modis data to detect, locate and quantify thermal anomalies in near real-time (https://www.mirovaweb.it/). the hotspot detection algorithm implemented in mirova exploits both spatial and spectral information, in particular the mir bands at 3.959 μm and thermal infrared (tir) bands at 12.02 μm are used to detect the presence of hot objects inside the pixels. once the hot pixels have been identified, the radiative power is calculated using the mir radiance technique. lav@hazard is a web-gis platform for volcanic hazard assessment, based on the analysis of infrared data acquired by seviri. lav@hazard uses a contextual algorithm to detect volcano thermal anomalies and, for each hotspot pixel, the radiative power is calculated using the mir radiance technique. seviri enables the monitoring of short and rapidly evolving eruptive phenomena, e.g., lava fountains which have a duration of a few hours [hirn et al., 2008; lombardo et al., 2019]. generally, lava fountains feed several-km high eruptive columns and ash plumes, as well as short-lived lava flows. the thermal waveform during a lava fountain is made by an initial phase of sharp increase in heat fluxes, which reach a peak corresponding to the main fountaining activity, and a subsequent phase characterized by waning heat flux associated with the slow cooling of the lava overflowed from the crater rim. this thermal behavior is described by the so-called cooling curve [ganci et al. 2012]. high temporal resolution thermal data, like seviri data, are well suited to characterize these cooling curves, which follow a fountaining event and decay within a few hours. as it is discussed in ganci et al. [2012], fitting the general shape of a cooling curve and applying an algorithm of best fit, determined by minimizing the mean squared error between the modelled radiative heat fluxes and the measured heat flux curve, it is possible to obtain an estimation of the area covered by the lava flow. sensors used for a more accurate estimate of the lava flow area are those having a good spatial resolution, like the multispectral instrument (msi, 10-m bands) on board of sentinel-2 satellite, the operational land imager (oli, with 30-m bands) and the thermal infrared sensor (tirs, with 100-m bands) on board landsat-8 satellite, or the advanced spaceborne thermal emission and reflection radiometer (aster, 15-m to 90-m bands), onboard terra satellite [li et al., 2017; corradino et al., 2019; corradino et al., 2021; amato et al., 2021]. the development of algorithms exploiting multi-sensor data for monitoring and quantifying thermal volcanic activity has allowed to get more accurate results over time [ganci et al., 2016; marchese et al., 2019; valade et al., 2019; massimetti et al., 2020]. nowadays, space agencies make available processed satellite data free of charge. in particular, specific geophysical parameter values are usually derived using models and auxiliary datasets, these are named level-2 products. among the level-2 products, fire radiative power (frp) measures the rate at which a thermally active area is emitting radiative energy. in particular, the national aeronautics and space administration (nasa) and the european space https://www.mirovaweb.it/ fastvrp to monitor volcanic radiative power 3 agency (esa) space agencies provide frp level-2 products derived respectively from the visible infrared imaging radiometer suite (viirs) and the sea and land surface temperature radiometer (slstr). only recently viirs frp product has been used to monitor volcanic activity because of its improved spatial resolution, up to 375 m [gunda et al., 2021; campus et al., 2022], while slstr has not been used yet for this purposes even though it has a comparable temporal resolution with respect to modis but a higher saturation temperature at mir and tir bands. here, we propose a new platform in google colab, called fastvrp, to analyze changes of volcanic radiative power (vrp) emitted by active volcanoes, using level-2 frp products. we explore the potentiality of these products retrieved from viirs and slstr data, in detecting and quantifying the thermal output from the etna volcano (sicily, italy) during the sequence of intense lava fountains occurred between february and april 2021. moreover, a novel approach to forecast the thermal behavior of a lava fountain and then to reconstruct its cooling curve from the low temporal resolution vrp values is proposed. this procedure is performed outside the google colab platform, but exploits the data provided by fastvrp. it is based on the combination of the temporal features provided by the geostationary satellite sensor seviri and the accurate spatial features provided by the two polar satellite sensors, viirs and slstr. we validated this approach exploiting the radiative power estimated from modis, having features similar to those of viirs and slstr sensors. 2. materials 2.1 record of the lava fountain events since the beginning of the new millennium, the eruptive activity of mt. etna (figure 1) is dominated by sequences of lava fountain events, mostly occurring from the south-east crater (sec) located in the summit area. generally, paroxysmal episodes taking place close in time are impulsive and characterized by rapid waxing and waning phases compared to the episodes more distant in time showing a slower pattern [andronico et al., 2021]. the first relevant sequence occurred in 2000, when the sec produced 64 episodes [alparone et al., 2003], followed by explosive phases occurred in 2001 [behncke & neri, 2003], 2002‑2003 [andronico et al., 2008], and 2011‑2015 [ganci et al. 2018]. the last explosive phase started on 13 december 2020 and ended on 21 february 2022, giving rise to 66 paroxysmal lava fountain episodes from sec [calvari & nunnari, 2022]. lava fountains were significantly more intense and frequent between mid-february and early april 2021. this short period of time was characterized by a sequence of 17 paroxysms of short duration (a few hours) and high intensity, occurred with a certain regular frequency (every about 30 hours from each other). these events are listed in table 1 as identified by the ingv weekly bulletins published at www.ct.ingv.it. most of these lava fountains were associated with several km high ash plumes (up to 9-10 km above sea level), generating serious concern among the population living in towns located around figure 1. mt. etna volcano in sicily, and relative summit area (red box), with four summit craters, the voragine (vor), the bocca nuova (bn), the northeast crater (nec), and the southeast crater (sec), and the valle del bove. http://www.ct.ingv.it federica torrisi et al. 4 the volcano [torrisi, 2022]. the impact of ash fallout was quite strong on the air traffic, on the stability of roofs, and on the health of the population living near the volcano [andronico & del carlo, 2016; torrisi et al., 2022]. in addition, pyroclasts and the small lava flows erupted during the lava fountains represented the greatest hazards to the tourist facilities on the upper flanks of etna [corradino et al., 2021; amato, 2022]. number event start time end time time duration max height lava fountains 1 16 february 2021 17:05 utc 18:00 utc 45 min 1500 m 2 17-18 february 2021 22:30 utc 00:40 utc 35 min 1200 m 3 19 february 2021 07:55 utc 10:10 utc 65 min 1300 m 4 20-21 february 2021 20:00 utc 01:20 utc 180 min 1500 m 5 22-23 february 2021 20:30 utc 10:38 utc 50 min 3600 m 6 24-25 february 2021 16:00 utc 22:20 utc 140 min 1800 m 7 28 february 2021 08:10 utc 09:33 utc 54 min 3600 m 8 2-3 march 2021 10:45 utc 14:50 utc 120 min 600 m 9 4 march 2021 00:20 utc 08:30 utc 130 min 3200 m 10 7 march 2021 00:00 utc 14:00 utc 60 min 4000 m 11 9-10 march 2021 18:00 utc 03:30 utc 163 min 1860 m 12 12 march 2021 03:20 utc 10:00 utc 175 min 2400 m 13 14-15 march 2021 20:10 utc 02:50 utc 170 min 1300 m 14 17-18 march 2021 00:30 utc 06:00 utc 170 min 1500 m 15 19 march 2021 06:34 utc 11:20 utc 120 min 600 m 16 23-24 march 2021 19:05 utc 11:55 utc 480 min 1300 m 17 31 march – 1 april 2021 23:00 utc 08:00 utc 540 min 630 m table 1. paroxysms occurred at mt. etna between february and april 2021. 2.2 satellite data sources the vrp values retrieved in this study are based on the outputs of the level-2 active frp products from viirs and slstr, and radiometric measurements from seviri and modis, derived from lav@hazard. seviri sensor is the main payload of the current msg geostationary satellite operated by eumetsat (european organization for the exploitation of meteorological satellites). it has the capacity to observe the earth in 12 spectral channels covering the visible to the infrared spectral region, with a baseline repeat cycle of 15 minutes (four full disk images per hour) and a 3 x 3 km nominal spatial resolution at the sub-satellite point. modis is an instrument aboard the terra and aqua satellites, which view the entire earth’s surface every 1 to 2 days. it acquires data in 36 spectral bands and has a spatial resolution from 250 m to 1 km. 2.2.1 level‑2 frp products the viirs sensor is aboard two satellites: suomi national polar-orbiting partnership (suomi npp) and national oceanic and atmospheric administration-20 (noaa-20). suomi npp and noaa-20 were respectively the first and the second in a series of four joint polar satellite system (jpss) satellites, a collaborative mission between noaa and fastvrp to monitor volcanic radiative power 5 nasa. they are satellites with a sun-synchronous near-circular polar orbit, located at an altitude of about 833 km. noaa-20 has the same orbit of suomi npp, but is separated in time and space by 50 minutes, providing additional viirs coverage [lp daac-viirs overview]. suomi npp crosses the equator at about 1:30 a.m. and 1:30 p.m., whereas noaa-20 crossing at 12:40 a.m. and 12:40 p.m. the 3000 km swath width of viirs allows a coverage with no gaps near the equator, with a scan angle range of ±56.28° from nadir [cao et al., 2013]. viirs provides 22 spectral bands at two spatial resolutions: 16 moderateresolution bands (m-bands) with a spatial resolution of 750 m at nadir, 5 imaging resolution bands (i-bands) with a 375 m spatial resolution at nadir, and one panchromatic day-night band (dnb), with a near constant 750 m spatial resolution throughout the scan. generally, the m-bands have a better signal-to-noise ratio than the i-bands. the viirs level-2 active fire products are distributed by lance/firms (land atmosphere near-real-time capability for eos / fire information for resource management system) platform (https://firms.modaps.eosdis.nasa.gov/). the attribute fields for viirs active fire products include for each active pixel its latitude and longitude, the brightness temperature of the mir bands, the acquisition time and acquisition date, the satellite, the scan, the track, the confidence, and the fire radiative power in mw. the active fire detection algorithm uses a multi-spectral contextual algorithm to identify thermal anomalies [schroeder et al., 2014]. it is based on the complementary use of the i-bands and m-bands. the improved spatial resolution of the i-bands (375 m) increases the capability of the sensor to detect thermally active areas of relatively small size. the bands used for the hotspot detection are the mir band i4 (spectral range: 3.55-3.93 μm) and tir band i5 (spectral range: 10.5-12.4 μm). in presence of i4-band saturation, the i-bands are replaced by the m-bands, in particular the mir band m13 (spectral range: 3.97-4.13 μm) and tir band m15 (spectral range: 10.26-11.26 μm). the slstr sensor is an instrument carried by the european sentinel-3 satellites, onboard sentinel-3a and sentinel-3b, which are part of the copernicus programme operated by esa and eumetsat. each sentinel-3 satellite has a near-polar and sun-synchronous orbit, with an altitude of about 814 km, and crosses the equator at 10:00 a.m. and 10:00 p.m. [sentinel-3 – mission summary]. the slstr uses two independent scan chains, including a separate scan mirror. this configuration is employed to provide a dual-view of the same position of the earth, “near nadir” and “oblique” [xu et al., 2021]. the oblique view has a local zenith angle close to 55° and a width of the nominal image swath equal to 740 km, while the nadir view is almost vertical at the sub-satellite point and has a wider swath width than the oblique view, equal to 1400 km. slstr has 11 bands: 3 bands in the visible and near-infrared (vnir) and 3 bands in the short-wave infrared (swir) with a spatial resolution of 500 m at nadir, and 5 bands in the thermal infrared (tir) with a spatial resolution of 1000 m at nadir. the slstr level-2 active fire products are distributed by copernicus open access hub (https://scihub.copernicus.eu/). the attribute fields for slstr active fire products include for each active pixel its latitude and longitude, the brightness temperature of the mir bands, the detection time, the confidence, and the fire radiative power in mw. the slstr active fire product is derived from data collected in the slstr near-nadir view swath since this offers finer spatial detail and a wider swath width than the oblique view. the active fire detection algorithm uses a contextual algorithm to identify thermal anomalies [wooster et al., 2012; xu et al., 2020]. the two mir channels extremely sensitive to the presence of fire exploited are the s7 channel (central wavelength: 3.74 μm) and the low-gain “fire channel” f1 (central wavelength: 3.74 μm). f1 channel is used when the s7 channel saturates, in fact it is able to measure brightness temperatures in excess of 450 k. furthermore, the tir bands used for the detection of thermal anomalies are the s8 channel (central wavelength: 10.8 μm) and the f2 channel (central wavelength: 10.8 μm), which is able to measure higher brightness temperature than the standard s8 channel, although this last band saturates rarely. the saturation temperature of the s8 channel is 321 k, while for the f2 channel is 400 k. table 2 shows the comparison between viirs and slstr in sensing geometry and in the algorithm used for active fire characterization. the viirs and slstr level-2 frp products depend on the characteristics of the sensors themselves (i.e. spatial resolution, scan angle, minimum detection limit, overpass time), the techniques used to detect the thermal anomalies and the algorithms implemented to derive the frp values. these sensors are onboard polar satellites and acquire about two images per day, but these two sensors do not have a similar overpassing time. the pixel areas of the viirs’s bands (375 m i-bands and 750 m m-bands) are smaller than that of slstr (with a spatial resolution of 1000 m), therefore this means that viirs has an improved active fire detection capability because it is able to find smaller thermal anomalies [zhang et al., 2017]. in both cases, thermal anomalies are identified using a contextual algorithm which exploits mir and tir bands. a hotspot detection algorithm relies on three different steps, i.e. detection of potential active pixels, background characterization and active https://firms.modaps.eosdis.nasa.gov/ https://scihub.copernicus.eu/ federica torrisi et al. 6 pixel confirmation. they are common to the two sensors but differ in how the thresholds are set and in the size of the background windows. finally, the vrp of each hotspot pixel is calculated using the mir radiance approach of wooster et al. [2003]: (1) sensor parameters viirs slstr orbit altitude ~ 830 km ~ 814 km equator crossing time suomi npp: 1:30 a.m. (descending node) 1.30 p.m. (ascending node) noaa-20: 12:40 a.m. (descending node) 12.40 p.m. (ascending node) sentinel-3a & sentinel-3b: 10:00 a.m. (descending node) 10:00 p.m. (ascending node) scan angle ±56.28° 55° (oblique view) 0° (nadir view) swath width ~ 3000 km ~ 740 km (oblique view) ~ 1400 km (nadir view) fire detection bands and saturation temperature mir i4 (3.55-3.93 μm) saturation: 367 k spatial resolution: 375 m s7 (3.74 μm) saturation: 311 k spatial resolution: 1000 m m13 (3.97-4.13 μm) saturation: 634 k spatial resolution: 750 m f1 (3.74 μm) saturation: 500 k spatial resolution: 1000 m tir i5 (10.5-12.4 μm) saturation: 380 k spatial resolution: 375 m s8 (10.8 μm) saturation: 321 k spatial resolution: 1000 m m15 (10.26-11.26 μm) saturation: 363 k spatial resolution: 750 m f2 (10.8 μm) saturation: 400 k spatial resolution: 1000 m active fire detection algorithm viirs active fire detection algorithm [schroeder et al., 2014] slstr active fire detection algorithm [wooster 2012, xu et al., 2020] frp calculation based on the mir radiance approach of wooster et al. [2003] viirs-i α = 3.21∙10–9 w∙m–2∙sr–1∙μm–1∙k–4 viirs-m α = 2.87∙10–9 w∙m–2∙sr–1∙μm–1∙k–4 [zhang et al., 2017] α = 3.30∙10–9 w∙m–2∙sr–1∙μm–1∙k–4 [xu et al., 2021] table 2. comparison between viirs and slstr in sensing geometry and the algorithm used for active fire characterization. fastvrp to monitor volcanic radiative power 7 where lmir and lmir,bg are respectively the spectral radiance (w∙m–2∙sr–1∙μm–1) of the hot pixels and the nonfire background pixels recorded in the mir band, apixel is the pixel area (km2), σ is the stefan-boltzmann constant (5.67∙10–8 w∙m–2∙k–4), τmir is the atmospheric transmittance in the mir band, ε is the emissivity, εmir the surface spectral emissivity in the mir band, and finally α (w∙m–2∙sr–1∙μm–1∙k–4) is a constant. α is determined from empirical best-fit relationships between the blackbody temperature and emitted spectral radiance at 4 μm, based on the approximation of the stefan’s law with the fourth power of the emitter temperature [wooster et al., 2003]. this constant α depends on the specific sensor employed and the different values for each sensor is reported in table 2. 3. methods 3.1 fastvrp platform fastvrp is an entry-level, cloud-based platform that leads to an automatic and fast retrieve and process of level-2 frp products to detect and measure the heat radiation released during volcanic activity (figure 2). the fastvrp platform is based on google colaboratory (colab for short), a free cloud computing platform that allows multiple users to easily share and execute python code through the browser. this environment runs entirely in cloud, providing many pre-installed libraries and dedicated graphical processing units (gpus) that accelerate the execution of the code. vrp values are retrieved by using level-2 frp products derived from viirs and slstr. in particular, viirs active fire data are accessed from earthdata lance (https://earthdata.nasa.gov/earth-observation-data/near-realtime), which provide a variety of data products within 3 hours from satellite observation. lance distributes near real-time (nrt) active fire data from viirs. slstr active fire data, instead, are downloaded from the copernicus open access hub, a portal developed in collaboration with the esa, which allows access to all users for the sentinel data products (https://www.copernicus.eu/en). the process of retrieving data from these space agency portals is quite slow and time-consuming, because the user has to download the products manually, selecting them one by one. by using google colab, fastvrp speeds up and automates the data downloading processes. we have used application programming interface (apis) to do the request to remote web server for retrieving active fire data. earthdata apis are used to access data stored in nasa’s lance; moreover, the fastvrp platform exploits the python package “sentinelsat” that makes searching, figure 2. scheme of the fastvrp platform developed in google colab, with the graphic interface, the main functions implemented to download automatically the level-2 frp products and the results section, characterized by a file excel and a graphic plot. https://earthdata.nasa.gov/earth-observation-data/near-real-time https://earthdata.nasa.gov/earth-observation-data/near-real-time https://www.copernicus.eu/en federica torrisi et al. 8 downloading and retrieving the metadata of sentinel satellite images from the copernicus open access hub easy, thanks to a powerful python api (https://sentinelsat.readthedocs.io/en/stable/). fastvrp has a graphic interface, which gives to the user the possibility to choose the time period to investigate, to set the coordinates of the region of interest, to decide what types of products to download by ticking the appropriate check boxes and to set the name of the file in which the downloaded data will be stored. once these input parameters are set, fastvrp sends a request to the specific server to download the level-2 frp products, which are automatically saved in the user google drive account. each viirs and slstr product contains the information related to all the thermal anomalies detected, including the latitude, the longitude, the acquisition time, and the fire radiative power (in mw). these data are filtered according to the time period and the coordinates chosen by the user, and the fire radiative power of each pixel of the selected area is summed in order to obtain a final and unique value of vrp for each acquisition. as result, fastvrp generates a data file with all the vrp values related to the sensors selected at the beginning, arranged in chronological order, and a graph in which these radiative power estimates are plotted. 3.2 forecasting of thermal behavior of lava fountains viirs and slstr sensors are onboard polar satellites and therefore they acquire images about two times a day. for this reason, vrp values derived from these sensors are not enough to follow accurately the temporal evolution of lava fountain events (lasting few hours). we propose a procedure to forecast missing values by exploiting seviri data at higher temporal resolution. the seviri radiative power values are able to follow comprehensively the thermal behavior of lava fountains, as shown in figure 3, thanks to the high sampling rate of the sensor (15 minutes). since the cooling phase of lava fountains is characterized by an exponential decay of vrp values, we fit with an exponential function the seviri radiative power values in order to estimate the time constant associated with the exponential decay of the measured curve, that is characteristic of each monitored lava fountain event. starting from the assumption that the time constant characteristic of the lava fountain is not dependent on the sensor but on the physical event, we reconstruct the cooling curve for the low-temporal resolution satellite sensors, i.e. viirs and slstr, by fitting the few vrp values available. the exponential function used for the fitting is the following: (2) figure 3. radiative power estimated from seviri data during the lava fountain that occurred on 28 february 2021. the green line indicates the cooling curve obtained fitting the seviri radiative power values, while dotted gray lines specify the peak (on the left) and the end of the eruptive activity (on the right). https://sentinelsat.readthedocs.io/en/stable/ fastvrp to monitor volcanic radiative power 9 where f0 is the initial value (at time t=0) while τ is the exponential decay time constant. therefore, from the exponential fitting of seviri data, it is possible to obtain the time constant τ related to each paroxysm occurred in the period analyzed (figure 3). to reconstruct the cooling curves related to viirs and slstr vrp values, we need the exponential time constant τ and the initial value f0, which can be estimated starting from the knowledge of τ and taking into account the few vrp values available for each sensor. generally, two vrp values are used to calculate the initial value f0, but in cases where (a) only a value is acquired, (b) the second acquisition is too far from the peak of the activity (more than two-thirds of the eruptive interval time range) or (c) falls within the first half of the interval time range but takes too low values (<0.5 gw), only the first vrp value is considered to estimate f0. the advantage of using seviri lies in the fact that the forecasting procedure can always be applied, as long as there is at least one viirs or slstr acquisition. it does not matter if the acquisitions are close to the peak of the activity or far away. if they occur far from the peak of the activity and during the cooling phase, it seems difficult to forecast the thermal activity of the initial phase of the lava fountain. but thanks to the high accuracy of the temporal features provided by seviri, it is however possible to reconstruct of the viirs and slstr cooling curves precisely. taking two vrp values related to the viirs sensor (𝑓(1,𝑉𝐼𝐼𝑅𝑆), 𝑓(2,𝑉𝐼𝐼𝑅𝑆)), the instants they are acquired (𝑡(1,𝑉𝐼𝐼𝑅𝑆), 𝑡(2,𝑉𝐼𝐼𝑅𝑆)) and the exponential decay factor τ, we can estimate the initial value 𝑓(0,𝑉𝐼𝐼𝑅𝑆) using eq. (3): (3) the same procedure is applied to estimate the initial value 𝑓(0,𝑆𝐿𝑆𝑇𝑅) for the slstr data: (4) moreover, given the technical similarity between the viirs and slstr sensors, in order to increase the number of data points to fit, the cooling curve of the lava fountains is also reconstructed considering a viirs value and a slstr value; in this case, we take into account the first viirs and the first slstr acquisition and we calculate the initial value 𝑓(0,𝑉𝐼𝐼𝑅𝑆&𝑆𝐿𝑆𝑇𝑅) as follows: (5) the reconstruction of the cooling curves allows us to have an estimation of the radiative power for the entire duration of the eruptive event also for sensors like viirs and slstr that acquire only 2 data per day, and then to forecast the thermal behavior of lava fountains with higher spatial accuracy exploiting the temporal information of sensors like seviri. to validate the proposed approach for the reconstruction of cooling curves from viirs and slstr data, we use the radiative power estimated from modis data and retrieved from lav@hazard. the finer spatial resolution (i.e. 1 km pixels), the good spectral resolution and the high signal to noise ratio of modis enable the identification of less intense thermal anomalies and to locate them with more detail. the modis radiative power values for the events analyzed are compared with the radiative power extracted from the cooling curves related to viirs and slstr at the same observation time of modis. what we expect is that if the modis point falls in the proximity of the viirs and slstr cooling curves (or even if it is located above the curve), this means that the physical phenomenon of cooling has been successfully modeled. the steps for the forecasting of thermal behavior of lava fountains are summarized below: – fitting of seviri radiative power values and calculation of the exponential decay time constant τ; – reconstruction of the cooling curves related to viirs and slstr vrp values exploiting the constant τ and two (or one) vrp values for each sensor; – validation of the procedure comparing the results with modis radiative power values. federica torrisi et al. 10 4. results to match the vrp time series retrieved at mt. etna during the entire 2021 from viirs (vrpviirs) and slstr (vrpslstr), we calculated for each dataset the weekly mean. results are shown in figure 4a, on a logarithmic scale. moreover, the two weekly datasets have been correlated using a linear regression analysis (figure 4b). the obtained best-fit coefficient m = 0.81 indicates a good correlation between the time series of vrpviirs and vrpslstr. the temporal trend of radiative power recorded at mt. etna during the time span from february to april 2021 is shown in figure 5. the vrp values from the viirs and slstr level-2 products were retrieved with the fastvrp platform, and the radiative power estimated using seviri data were retrieved from lav@hazard. we forecasted the thermal behavior of the 17 paroxysms occurred at mt. etna between february and april 2021. all the paroxysms were characterized by lava fountains of short duration and high intensity. however, for some events it was not possible to measure the temporal thermal behavior because the cooling curves were not well defined due to missing data. this was caused either by the fact that the cooling process lasted only a few hours or by the presence of adverse weather conditions during the paroxysmal episode that did not allow to process data correctly. thus, the four paroxysmal events occurred on 10, 15, 17 and 19 march 2021 were not investigated. the results obtained for the other thirteen paroxysmal events analyzed are collected in table 3. it contains the time interval used for the fitting of seviri radiative power values, the exponential decay factor τ and the initial values 𝑓(0,𝑆𝐸𝑉𝐼𝑅𝐼) obtained as results of the fitting procedure and the initial values 𝑓(0,𝑉𝐼𝐼𝑅𝑆), 𝑓(0,𝑆𝐿𝑆𝑇𝑅), and 𝑓(0,𝑉𝐼𝐼𝑅𝑆&𝑆𝐿𝑆𝑇𝑅) estimated from the knowledge of τ. the symbol (*) indicates that the estimation of f0 was conducted using only a unique vrp value. as case studies, we considered the three events occurred on 16 february 2021, 20-21 february 2021 and 24-25 february 2021. the reconstructed cooling curves related to viirs, slstr and combining viirs and slstr data (viirs&slstr) are shown in figures 6-8. for all cases, the initial time, which corresponds to the peak of the figure 4. (a) time series of weekly mean of vrp values from viirs (vrpviirs) and slstr (vrpslstr), in logarithmic scale, indicated respectively by crosses and circles. (b) linear regression analyses between the weekly mean of vrpviirs and vrpslstr in logarithmic scale. the best-fit coefficient m = 0.81 indicates a good correlation between the time series of vrpviirs and vrpslstr. for both the graphs, the colors in the range from blue to red indicate the values between january and december 2021, in chromatic order. fastvrp to monitor volcanic radiative power 11 figure 5. temporal trend of vrp values derived from the level-2 products of viirs and slstr and estimated using seviri data at 15 minutes’ temporal resolution over the period 10 february – 4 april 2021 at mt. etna, during which 17 paroxysms occurred. events interval time fitting seviri [h] τ from fitting seviri (∙104) 𝒇(0,seviri) from fitting seviri [gw] estimation of 𝒇(0,viirs) [gw] estimation of 𝒇(0,slstr) [gw] estimation of 𝒇(0,viirs&slstr) [gw] 16 february 2021 17.50 1.13 22.85 8.96 (*) 5.28 5.39 17-18 february 2021 21.00 1.56 23.88 9.26 9.77 9.77 19 february 2021 22.25 1.21 28.82 9.32 14.23 (*) 9.27 20-21 february 2021 24.25 1.37 19.98 11.33 (*) 10.44 9.9 22-23 february 2021 24.00 1.66 20.01 12.72 10.82 12.62 24-25 february 2021 23.00 1.39 18.18 7.52 (*) 11.79 (*) 7.95 28 february 2021 24.00 1.33 16.38 3.89 (*) 4.21 4.01 2-3 march 2021 16.50 1.66 12.77 7.57 (*) 4.41 (*) 5.34 4 march 2021 18.75 1.13 29.59 13.67 (*) 4.64 13.62 7 march 2021 24.25 0.95 31.26 2.78 (*) 5.78 4.30 12 march 2021 22.50 1.21 30.37 28.14 (*) 17.47 (*) 27.63 23-24 march 2021 20.25 2.02 13.08 6.20 3.92 (*) 5.67 31 march – 01 april 2021 26.25 3.72 10.48 6.92 (*) 4.99 5.71 table 3. results obtained applying the procedure for the forecasting of thermal behavior of a lava fountain to thirteen paroxysmal events, occurred at mt. etna between february and april 2021. federica torrisi et al. 12 figure 7. reconstruction of the cooling curve from low temporal resolution vrp values, considering viirs data (7a), slstr data (7b) and together viirs and slstr data (7c) for the 20-21 february 2021 paroxysmal event. in magenta color, modis radiative power values used for the validation step. figure 8. reconstruction of the cooling curve from low temporal resolution vrp values, considering viirs data (8a), slstr data (8b) and together viirs and slstr data (8c) for the 24-25 february 2021 paroxysmal event. in magenta color, modis radiative power values used for the validation step. figure 6. reconstruction of the cooling curve from low temporal resolution vrp values, considering viirs data (6a), slstr data (6b) and together viirs and slstr data (6c) for the 16 february 2021 paroxysmal event. in magenta color, modis radiative power values used for the validation step. fastvrp to monitor volcanic radiative power 13 volcanic activity, and the interval time chosen are the same adopted for seviri fitting. in figures 6-8, the asterisk points (blue for viirs and red for slstr) indicate the values taken into account to calculate the initial value. generally, two vrp values are used in order to make the calculation more robust, but if one of the two acquisitions is too far from the peak activity (like in figure 8b for slstr) or assumes underestimated values near the peak of the activity (like in figure 6a, figure 7a and figure 8a for viirs), only one vrp point is considered to estimate the initial value. figures 6-8 show the reconstructed cooling curve for each investigated event and a shaded area corresponding to the 95% confidence interval (ci) representing the uncertainty bound (in light blue for viirs, in yellow for slstr and in gray for viirs&slstr). to validate the reconstructed synthetic data points, we compared them with the radiative power values from modis, retrieved from lav@hazard. in particular, the modis radiative power values of each paroxysmal event analyzed were compared to temporally coincident radiative power values retrieved from the reconstructed cooling curve related to viirs, slstr and viirs&slstr. if these values are very close, it means that the process of events modis viirs slstr viirs&slstr acquisition time vrp value [gw] vrp reconstructed value [gw] vrp difference with modis [gw] vrp reconstructed value [gw] vrp difference with modis [gw] vrp reconstructed value [gw] vrp difference with modis [gw] 16 february 2021 16/02/2021 20:15 3.5 3.44 0.06 2.03 1.47 2.07 1.43 17-18 february 2021 18/02/2021 09:00 1.39 1.46 0.07 1.54 0.15 1.54 0.15 19 february 2021 19/02/2021 20:50 0.81 0.41 0.41 0.62 0.19 0.4 0.41 20-21 february 2021 21/02/2021 09:30 1.36 1.57 0.2 1.45 0.08 1.37 0.01 22-23 february 2021 23/02/2021 09:20 0.64 1.61 0.98 1.37 0.73 1.6 0.96 24-25 february 2021 25/02/2021 01:20 4.89 3.04 1.84 4.77 0.12 3.22 1.67 28 february 2021 28/02/2021 09:40 5.38 2.59 2.79 2.8 2.58 2.67 2.71 2-3 march 2021 03/03/2021 00:45 2.11 1.13 0.98 0.66 1.45 0.8 1.31 4 march 2021 04/03/2021 20:15 1.39 0.45 0.94 0.14 1.24 0.44 0.94 7 march 2021 07/03/2021 09:45 1.39 0.98 0.41 2.03 0.64 1.51 0.12 12 march 2021 12/03/2021 21:05 0.59 0.73 0.14 0.45 0.14 0.72 0.13 23-24 march 2021 24/03/2021 21:30 0.8 1.09 0.29 0.69 0.11 1 0.19 31 march – 01 april 2021 01/04/2021 12:55 6.24 4.59 1.65 3.31 2.93 3.78 2.46 table 4. comparison between the modis radiative power values of each paroxysmal event analyzed and the temporally coincident radiative power values retrieved from the reconstructed cooling curve related to viirs, slstr and viirs&slstr. federica torrisi et al. 14 reconstruction of the cooling curves works well, since it can estimate radiative power even when the sensor does not acquire data. in figures 6-8, modis radiative power values considered for the validation step, respectively for the events of 16 february 2021, 20-21 february 2021, and 24-25 february 2021, are those represented in magenta color. in table 4, the results of the validation step are reported. in particular, for each modis acquisition time is shown the corresponding estimated modis radiative power value, the radiative power values retrieved from the reconstructed cooling curve for viirs, slstr and viirs&slstr and the difference between these values and the modis radiative power value. 5. discussion the synergic use of viirs and slstr acquisitions allows to obtain a higher number of data with a good spatial resolution for the monitoring of thermal anomalies. the information content provided by the two sensors is in fact comparable as shown in figure 4. the best-fit coefficient m equal to 0.81 confirms the strong correlation between the two datasets. the main difference lies in the increased ability of viirs to detect thermal anomalies even in periods of less intense volcanic activity, like the period september-december 2021, which was characterized just by 4 paroxysms (21 september 2021, 23 october 2021, 4 december 2021 and 15 december 2021). in this same period, slstr vrp values are less than those of viirs and are related mainly to the paroxysmal activity. on the other hand, since slstr has higher saturation temperature than viirs-i channel, i.e. higher radiative power, its trend is higher than viirs during most of the analyzed time interval. however, the high best-fit coefficient demonstrates that viirs and slstr are actually two comparable sensors with a similar sensitivity in the detection and quantification of high-temperature anomalies. furthermore, the difference in the acquisition time allows to monitor volcanic events otherwise missed when using only one of the two satellite sensors. this is the case of the events occurred in may-june 2021 that have been monitored by slstr but missed by viirs. the advantages of using fastvrp to monitor vrp are shown in figure 5, in fact each lava fountain occurred on etna from february to april 2021 is well described by combining information coming from viirs and slstr. figure 5 shows that viirs provides relevant information especially about the paroxysm of 20-21 february and 12 march 2021, due to satellite overpass times matching to the time of the events. results retrieved from viirs indicate that the 12 march 2021 paroxysms was the most intense in terms of radiative power, with values estimated around 17 gw (viirs image of 12 march 2021 at 10:48 utc). this latter event was one of the most powerful and peculiar, because was accompanied by the formation of lava fountains which fed a very high eruptive column and an extensive ash plume [calvari et al., 2021; torrisi et al., 2022]. marchese et al. [2021] investigated the mt. etna paroxysms of february-april 2021 and for the 12 march 2021 paroxysm indicated a lower value of radiative power, about 7 gw from modis observations. this difference is probably due to the fact that viirs provides a superior capability to detect smaller thermal anomalies respect to modis, with no notable dependence on view zenith angle [li et al., 2018, 2020]. sometimes viirs vrp showed lower values than modis vrp and this may be due to the presence of plume above the crater and the atmospheric absorption of carbon dioxide at viirs m13 band, used for vrp calculation [fu et al., 2020]. during the 31 march – 1 april paroxysm, vrp retrieved from viirs data (~ 5.5 gw, viirs image of 1 april at 11:12 utc) appeared underestimated compared to the temporally coincident modis observations (~ 14 gw, modis image of 1 april at 09:33 utc – 14 gw), retrieved from marchese et al. [2021]. this impact on viirs estimates could be caused by the volcanic plume and partial cloud cover above the crater. some synthetic initial vrp values f0 from viirs and slstr appear particularly different and the main reason can be related to the smaller pixel area of viirs i-bands. viirs which is able to discriminate thermal anomalies with a lower vrp than those identifiable with slstr [xu et al., 2020]. moreover, it is important to highlight that the two sensors observe the scenes at slightly different times and this means different conditions of the observed thermal anomaly and of the cloud coverage. this situation arises if we consider the 4 march and 12 march 2021 events. in these cases, the vrp values used for the forecasted procedure from viirs are higher than that from slstr, and therefore the synthetic initial vrp values f0 from viirs (13.67 gw for the 4 march 2021 and 28.14 gw for the 12 march 2021) appear much greater than that of slstr (4.64 gw for the 4 march 2021 and 17.47 gw for the 12 march 2021). the use of the fitting procedure allowed to exploit the temporal information provided by seviri while keeping the higher accuracy given by viirs and slstr. this helps us dealing with the missing data points during fountain fastvrp to monitor volcanic radiative power 15 cooling phases as shown in figures 6-8. the main advantages of using seviri data is the possibility of promptly detecting eruptive events. however, the low spatial resolution of seviri could lead to omission errors, like the inability to detect thermal anomalies below a certain size and intensity [freeborn et al., 2014]. other factors impact on estimates of the radiative power from seviri data, such as the atmospheric effects and the variation in retrieved thermal anomalies related to the subpixel location of the thermal anomaly with respect to the sensor instantaneous field of view (ifov). these effects lead to a general underestimation of seviri radiative power compared to the temporally coincident modis observations [wooster et al., 2005; roberts et al., 2005]. moreover, the geometric resampling and the digital filter applied to seviri raw data before to be available from eumetsat can underestimate the seviri estimates of vrp [wooster et al., 2015]. nevertheless, seviri could overestimate vrp due to the impact of point spread function (psf) on active fire detection. this effect is so large that even a small lava flow within a single pixel can generate significant effects on the measured radiance [calle et al., 2009; barnie & oppenheimer, 2015]. we decided to combine polar and geostationary satellite sensor data to overcome the limits of each of them and exploit their strengths. viirs and slstr offer mid-high spatial resolution with which it is possible to spatially characterize eruptive phenomena and to retrieve accurate spatial information such as the areal extent of the volcanic thermal anomaly. on contrast, seviri offers high temporal resolution capabilities and allows to get accurate temporal information of the monitored volcanic phenomena, such as the exponential decay factor of a lava fountain cooling curve. the combined use of these sensors is advantageous because it allows us to get more accurate radiative power estimation. reconstructed synthetic data points are coherent with the modis values that has been never used to reconstruct cooling curves but that fall inside the uncertainty band of the reconstructed curve. this provides a first proof of the validity of our approach for the cooling curve reconstruction. it is noticeable that the distance between the modis radiative power values and the corresponding radiative power values on the reconstructed cooling curves is always lower than the uncertainty value (see table 4). it means that modis radiative power values are always within the 95% ci. in addition, there are cases where the distance is negligible, that is, the modis point is exactly on the cooling curves (e.g. 20-21 february 2021 paroxysm). these results highlight the correctness of the procedure for the reconstruction of the cooling curves and for the retrieval of the radiative power values related to viirs and slstr. although viirs and slstr acquire few points for each event, they show improvements in terms of vrp accuracy and sensitivity. in particular, they may either detect earlier the starting of a lava fountain event such as for the 20-21 february 2021 event (figure 9a), i.e. viirs detects a peak on 21 february at 00:24 utc due to the start of less intense lava fountain episodes from 00:00 utc as documented in ingv weekly bulletins published at www.ct.ingv.it., or detect earlier subtle increases in vrp intensity such as for the 2 march 2021 eruption (figure 9b), i.e. viirs detection occurs on 1 march at 23:24 utc while seviri on 2 march at 00:42 utc. the reconstruction procedure considering both viirs and slstr polar sensors permitted to forecast the cooling curves of the overall sequence of the lava fountains occurred at etna between february-april 2021. viirs and slstr figure 9. vrp monitored by viirs, slstr and seviri for the events (a) 21 february 2021 and (b) 2 march 2021. http://www.ct.ingv.it federica torrisi et al. 16 vrp values retrieved by fastvrp are exploited together with the seviri data to apply the fitting procedure and then to reconstruct the cooling curves of the lava fountains sequence thanks to the retrieved synthetic data. figure 10 shows the vrp values retrieved by fastvrp and reconstructed synthetic data using viirs and slstr (viirs&slstr). although the acquisition frequency of viirs and slstr is low, it was possible to follow the thermal behavior of the lava fountains thanks to the temporal information provided by seviri, maintaining the mid-high spatial accuracy of the two polar sensors. figure 10. vrp values retrieved by fastvrp (markers) and reconstructed synthetic data using viirs and slstr (dashed line). 6. conclusions the fastvrp platform streamlines the retrievals of level-2 frp products related to viirs and slstr sensors onboard polar satellites, provided respectively by nasa and esa space agencies. fastvrp was implemented in google colab and allows to download automatically and process quickly both frp products, which can be used to monitor the volcanic thermal activity. moreover, we proposed a novel approach to forecast the thermal behavior of a lava fountain with the synergic use of multi-sensor satellite data. this procedure is performed outside the google colab platform, but exploits the data provided by fastvrp. in particular, we combined the accurate spatial features provided by the polar satellite sensors viirs and slstr and the temporal features provided by the geostationary satellite sensor seviri. this approach wants to improve and make more accurate the quality of the radiative power estimates. this procedure was applied to the sequence of lava fountains occurred at mt. etna between february and april 2021. the viirs and slstr data were retrieved by the fastvrp platform, while seviri radiative power estimates from lav@hazard. the forecasting of thermal behavior of these lava fountains produced satisfactory results. the synthetic data obtained from viirs and slstr were used to reconstruct the cooling curves of each lava fountain, starting from the seviri exponential decay calculated for each of these. the forecasted radiative values retrieved from the viirs and slstr cooling curves are close to the modis radiative power values (retrieved from lav@ fastvrp to monitor volcanic radiative power 17 hazard), used as validation since modis has technical characteristics similar to those of the two sensors of interest. we have demonstrated that modis radiative power values are always within the 95% ci, therefore the vrp differences between the synthetic data and modis is lower than the uncertainty value (2.9 gw). despite the few vrp values obtained from viirs and slstr due to their long revisit time, it was possible to monitor the thermal activity of the lava fountains even in the instants of time in which the two polar sensors do not acquire data, simply exploiting the temporal information provided by seviri. furthermore, we evaluated the performance of viirs and slstr frp products. the linear regression analysis between the viirs and slstr vrp data confirms the correlation of the two datasets for the monitoring of volcanic thermal anomalies (best-fit coefficient m=0.81). thus, we demonstrated that viirs and slstr frp products can be exploited to derive information about the thermal activity associated with the volcanic eruptions. the fastvrp platform can be a useful tool for near real-time monitoring of a volcanic activity. moreover, the combined use of mid-high spatial resolution data offered by the viirs and slstr frp products and of high temporal resolution data provided by geostationary sensors like seviri give us the possibility to forecast the thermal behavior of a lava fountain and then to get more accurate radiative power estimates. funding. this research was funded by [athos research programme] (ob.fu. 0867.010) and by [project first – fore‑ casting eruptive activity at stromboli volcano: timing, eruptive style, size, intensity, and duration, ingv – progetto strategico dipartimento vulcani 2019] (delibera n. 144/2020). acknowledgments. this work was developed within the framework of the laboratory of technologies for volcanology (technolab) at the ingv in catania (italy). slstr and viirs level‑2 frp products used in this work are provided respectively by the european space agency and by nasa. note: google colaboratory™ is a trademark of google llc – ©2018 google llc all rights reserved. references alparone, s., d. andronico, l. lodato and t. sgroi (2003). relationship between tremor and volcanic activity during the southeast crater eruption on mount etna in early 2000, j. geophys. res.: solid earth, 108, b5, 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e. pilger (2002). automated volcanic eruption detection using modis, remote sens. environ., 82, 1, 135‑155, https://doi.org/10.1016/s0034-4257(02)00030-5. wright, r., l. p. flynn, h. garbeil, a. j. l. harris and e. pilger (2004). modvolc: near-real-time thermal monitoring of global volcanism, j. volcanol. geotherm. res., 135, 1, 29‑49, https://doi.org/10.1016/j.jvolgeores.2003.12.008. xu, w., m. j. wooster, j. he and t. zhang (2020). first study of sentinel-3 slstr active fire detection and frp retrieval: night-time algorithm enhancements and global intercomparison to modis and viirs af products, remote sens. environ., 248, 111947, https://doi.org/10.1016/j.rse.2020.111947. xu, w., m. j. wooster, e. polehampton, r. yemelyanova and t. zhang (2021). sentinel-3 active fire detection and frp product performance – impact of scan angle and slstr middle infrared channel selection, remote sens. environ., 261, 112460, https://doi.org/10.1016/j.rse.2021.112460. zhang, t., m. j. wooster and w. xu (2017). approaches for synergistically exploiting viirs iand m-band data in regional active fire detection and frp assessment: a demonstration with respect to agricultural residue burning in eastern china, remote sens. environ., 198, 407‑424, https://doi.org/10.1016/j.rse.2017.06.028. *co r r e s po n d i n g a u t h o r: federica to r r i s i, istituto nazionale di geofisica e vulcanologia, osservatorio etneo – sezione di catania, catania, italy and department of electrical, electronic and computer engineering, university of catania,catania, italy e‑mail: federica.torrisi@ingv.it https://doi.org/10.1393/ncc/i2022-22081-0 https://doi.org/10.1117/12.332714 https://doi.org/10.3390/rs11131528 https://doi.org/10.4401/ag-5347 https://doi.org/10.5194/acp-15-13217-2015 https://doi.org/10.1029/2005jd006318 https://doi.org/10.1029/2005jd006318 https://doi.org/10.1016/j.rse.2011.09.033 https://doi.org/10.1016/s0034-4257(03)00070-1 https://doi.org/10.1016/s0034-4257(02)00030-5 https://doi.org/10.1016/j.jvolgeores.2003.12.008 https://doi.org/10.1016/j.rse.2020.111947 https://doi.org/10.1016/j.rse.2021.112460 https://doi.org/10.1016/j.rse.2017.06.028 mailto:federica.torrisi@ingv.it vol52,2,2009 107 annals of geophysics, vol. 52, n. 2, april 2009 relationship between geomagnetic classes’ activity phases and their occurrence during the sunspot cycle frédéric ouattara ecole normale supérieure, université de koudougou, burkina faso, africa abstract four well known geomagnetic classes of activity such as quiet days activity, fluctuating activity, recurrent activity and shock activity time occurrences have been determined not only by using time profile of sunspot number rz but also by using aa index values. we show that recurrent wind stream activity and fluctuating activity occur in opposite phase and slow solar wind activity during minimum phase and shock activity at the maximum phase. it emerges from this study that fluctuating activity precedes the sunspot cycle by π/2 and the latter also precedes recurrent activity by π/2. thus in the majority the activities do not happen at random; the sunspot cycle starts with quiet days activity, continues with fluctuating activity and during its maximum phase arrives shock activity. the descending phase is characterized by the manifestation of recurrent wind stream activity. key words classes of geomagnetic activity – phases of geomagnetic classes – sunspot cycle 1. introduction the long term aa and storm sudden commencement (ssc) data series (mayaud, 1971; 1972; 1980) provide continuous information on geomagnetic activity from 1868 onwards. using aa indices values and ssc data series from 1868 to 1978 (legrand and simon, 1989) classified geomagnetic activity into four classes. recently ouattara and amory mazaudier (2008) reported the percentage of their events for the period 1968-2005. their work prolongs the previous work of legrand and simon (1989) from 1989 to 2005. richardson et al. (2000) and richardson and cane (2002) have also classified geomagnetic activity into four classes by using aa values and in situ solar wind velocity. ouattara and amory mazaudier (2008) showed that the conclusions obtained on the one hand by legrand and simon (1989) and on the other hand by richardson and colleagues (2000) and richardson and cane (2002) are the same. to determine geomagnetic classes of activity legrand and simon (1989) based their classification on 2 facts: 1) the contribution of shock wave to geomagnetic activity; this contribution is called shock activity. 2) the strong correlation between the aa index and the solar wind (svalgaard, 1977). this correlation permits them to divide solar wind speed into three types: low speed, fluctuating and high speed. each type generates one class of activity named respectively quiet activity, fluctuating activity and recurrent activity. the criteria to determine the four classes of geomagnetic activity are given by ouattara and amory mazaudier (2008) and must be expressed as: mailing address: dr. frédéric ouattara, ecole normale supérieure, bp 376 burkina faso; e-mail: ouattarafred@yahoo.fr; fojals@yahoo.fr vol52,2,2009 17-06-2009 19:02 pagina 107 108 1) quiet days activity is defined as the slow solar wind (v<450 km/s) flowing continuously past the magnetosphere activity. its concerns the days with daily aa < 20 nt. the three other classes (shock activity, recurrent activity and fluctuating activity) constitute the disturbed geomagnetic activity classes and are obtained by taking into account the days with daily aa > = 20 nt. these classes are distinguished as follows: 2) recurrent (stream) activity presents a continuing evolution during one solar rotation as well as during the following solar rotation. there is no ssc during the main phase. this class corresponds to high speed solar wind; 3) shock activity is determined by the special storms which arise by random bursts and without a recurrence of 27 days during 2, 3 or 4 rotations. 4) fluctuating activity is determined by subtracting from the disturbed activity (aa > =20 nt) the shock activity and the recurrent activity. the two last activities constitute the only transient activity among the four classes. shock activity is related to very active spots on the solar disk and fluctuating activity is due to the existence of fluctuating solar wind jets. it is important to underline that each class of activity determines solar wind structure and expresses solar magnetic field topology behaviour. thus, physically different classes differ from others. here, the key role is attributed to daily aa value for determining the class of activity not to the summarized daily aa values. the latter value is used to determine only the whole level of each class of activity (ouattara and amory mazaudier, 2008). in the following each geomagnetic class of activity will be expressed as: quiet days activity (qa), fluctuating activity (fa), shock activity (sa) and recurrent activity (ra). to study the long-term variability of different solar classes of activity, we will use not only sunspot number (rz) which has been largely used to infer the long-term variability of the solar activity with annual averages available since 1700 (eddy, 1976) but also the aa activity index computed by mayaud (1971; 1972; 1973). in this paper the long-term variability of the four classes of geomagnetic activity for the period 1868-2005 are compared each others in order to determine not only their time profiles which are well known (legrand and simon, 1989; ouattara et al., 2008; ouattara and amory mazaudier, 2008) but also to show their phases and to determine the relationship between them. the present study is quite different from our work submitted to journal des sciences (ouattara, 2008) where after separating the two components of solar magnetic fields by using aa values we try to find the relationship between each solar magnetic field component according to their time evolution and in term of their amplitude. we show that solar magnetic field components are in opposite phase and the ratio between their amplitudes is one half. the present paper starts by analyzing the relationship between geomagnetic activity index aa and sunspot number (rz) and after determining geomagnetic classes activity phases and their occurrences in reference of rz time profiles. a comparison among them will be also made. 2. data sets sunspot number and aa geomagnetic indices values used in this study are provided by cetp (centre d’etude des phénomènes terrestres et planétaires) data base and ngdc (national geophysical data center). 3. methodology our methodology consists in indicating the process used to obtain the sum per year of aa values for each class of activity. the sum per year of aa values for each class of activity is obtained by adding the daily aa value per class of activity for a given year. this assumes that the daily class of activity is determined before adding the values. for that, first we will indicate the method used to obtain the daily aa values for each class of activity and secondly we will show how to sum the values obtained. f. ouattara vol52,2,2009 17-06-2009 19:02 pagina 108 109 relationship between geomagnetic classes’ activity phases and their occurrence during the sunspot cycle 3.1. determination of each daily class of activity to determine the daily class of activity we organized the daily aa values by means of a pixel diagram. the pixel diagram is a colour diagram used to select the geomagnetic data as a function of the solar activity as described by solar rotation (27 days). this method was developed and used by legrand and simon (1989) for the determination of the four geomagnetic classes of activity for the period 1868-1978. this method was clearly exposed for the year 2003 and validated by ouattara and amory mazaudier (2008): «each pixel diagram has thirty one rows corresponding to the maximum day of the month. to obtain the thirty one rows the first four rows have been repeated at the end. the diagram is started by reading from the third row and finished at the 29th row. the corresponding day of the third row has mentioned on the left of the diagram and the corresponding year on the top of the diagram» (ouattara and amory mazaudier, 2008). in a pixel diagram the dates of sscs are indicated to permit the determination of sa. each yearly pixel diagram is characterized by its colour repartition as a function of a daily aa values. the colour code repartition is expressed as: white colour for aa < 10 nt. this colour concerns a very quiet days periods; blue colour for 10 nt <= aa < 20 nt. all the white and blue colours characterize quiet days periods and correspond to aa < 20 nt. the disturbed periods determined by aa >= 20 nt are characterized by the following colours according to the different values of aa. we have: green colour for 20 <= aa < 30 nt; yellow colour for 30 nt < aa <= 40 nt; orange color for 40 nt <= aa < 60 nt; red colour for 60 nt <= aa < 100 nt and alive red for aa >= 100 nt. for determining geomagnetic activity, pixel diagrams have been made for all values of aa since 1868, year after year until 2006. the continuum description of the geomagnetic activity as a function of solar activity is given by the picture. for more details on the pixel diagram see ouattara and amory mazaudier (2008). the daily qa is determined by considering the days with aa < 20 nt. in a pixel diagram this determination is easy. one must choose the days with white colour (very quiet days: aa < 10 nt) and the days with blue colour (10 nt <= aa < 20 nt). the daily qa can be obtained without using the pixel diagram. ra is due to the corotating solar wind streams. this class of activity is the class of recurrent (stream) activity from coronal holes: it presents a continuing evolution during one solar rotation as well as during the following rotation. there is no ssc during the main phase. this class corresponds to high speed solar wind. in a pixel diagram this condition corresponds to 40 nt < aa < 100 nt with, 40 nt <= aa < 60 nt: orange colour and 60 nt <= aa < 100 nt: red colour. it is important to note that to improve the determination of this class of activity by using in situ data measurements (solar wind parameters measured on board spacecraft) ouattara and amory mazaudier (2008) added a supplementary condition determined by recurrent yellow colour in a pixel diagram and connected to aa by 30 nt <= aa < 40 nt. according to these authors this supplementary condition corresponds to the class of associated corotating interactions regions activity. sa corresponds to shock event. it is estimated in the pixel diagram by taking into account 2 or 3 disturbed days after no recurrence ssc date. to improve this class determination ouattara and amory mazaudier (2008) show that this shock event is related to the interplanetary coronal mass ejections (icmes). thus they added transient forward shocks generated ahead of fast icmes and related postshock flows determined in he pixel diagram by isolated aa > 40 nt (orange, red and alive red colours) without ssc. fa is expressed by the other daily values of aa > = 20 nt which do not contribute to sa and ra. 3.2. determination of the sum per year of the level of each class of activity aa index can be expressed as: aa = aa (quiet) + aa (disturbed) with aa (disturbed) = aa (recurrent) + aa (shock) + aa (fluctuating). vol52,2,2009 17-06-2009 19:02 pagina 109 110 f. ouattara each year activity is characterized by four components: aa = aaq + aar + aas, + aaf where aaq σ aa (quiet)) is the sum of the daily aa indices for all the day with aa< 20 nt, aar is the sum of the daily aa indices of days with ra (σ aa (recurrent)), aas is the sum of the daily aas indices of days with sa (σ aa (shock)) and aaf is the sum of the daily aa indices of days with fa (σ aa (fluctuating)). it is important to note that there is another way of obtaining the sum of fa for a given year. this must be obtained by removing from the sum of the whole value of aa for a given year the sum of aa values which have contributed to qa, the sum of aa values of ra and the sum of aa values of sa. this classification gives for each year a level of geomagnetic activity for the four classes of activity. 4. results and discussion figure 1 gives the time evolution of rz and the sum per year of geomagnetic activity i.e. the sum of aa during a year. it clearly appears from this figure what is well known: geomagnetic activity presents not only two peaks during sunspot cycle with the first peak which is related to sunspot activity and the second which is out phase but also 11-year periodicity as rz (e.g. echer et al., 2004; hady, 2006). one must conclude that there is no correlation between rz and geomagnetic activity expressed here by aa index. geomagnetic indices must be divided into three intervals which are (1) aa < 10 nt for very quiet days activity, (2) aa < 20 nt for qa and (3) aa>= 20 nt for disturbed days activity. the disturbed days activity to gather together fa, ra and sa (see legrand, 1984, legrand and simon, 1989, ouattara and amory mazaudier, 2008; ouattara et al., 2008). figure 2 gives the activities of very quiet days and sunspot. one can see that both activities are in opposite phase. by taking that into account that: 1) sunspot activity expresses toroidal magnetic activity, 2) a linear relationship exists between very quiet days activity and quiet days activity (ouattara et al., 2008) and 3) quiet days activity really fig. 1. time profiles of rz and sum per year of geomagnetic activity for the period 1868-2005. vol52,2,2009 17-06-2009 19:02 pagina 110 111 relationship between geomagnetic classes’ activity phases and their occurrence during the sunspot cycle characterizes the signature of poloidal magnetic field activity (legrand and simon, 1989; simon and legrand, 1989), one must conclude that both fields are in opposite phase. this result has been also observed by other authors (legrand and simon, 1989; luhmann et al., 2002; ouattara, 2008). it also emerges from fig. 2 that the amplitudes of qa and rz exhibit reverse development. thus the year of greater (respectively smaller) rz corresponds to the year of smaller (respectively greater) qa. thus the knowledge of rz amplitude infers a rough estimate of the amplitude of qa. figure 3 expresses the relationship between fa and sunspot activity. minimums of sunspot arrive during the maximum of fa and the maximums of sunspot occur during the manifestation of the second peak or in the course of declining phase of fa. it can be concluded that fa precedes sunspot activity by π/2. the second peak in fa graph expresses that some fa arrive at rz maximum and consequently some geomagnetic activities with greater aa can contribute to fa. it can be also seen that the amplitudes of rz and fa develop together. thus the greater (respectively smaller) rz corresponds to the greater (respectively smaller) amplitude of fa. thus when rz amplitude is known it is possible to give a rough estimate of fa amplitude. the comparative analysis of fig. 2 and fig. 3 shows on the one hand that greater (respectively smaller) fa corresponds to smaller (respectively greater) qa and on the other hand that through the solar cycle some intense fa with intense qa occur during rz minimum. this comparative analysis shows that qa occurs at rz minimum qa and fa occurs during rz ascending phase. figure 4 shows the time series of rz and ra. it emerges that there is no ra during the maximums of rz and the maximums of ra occur during the declining phases of rz. this kind of evolution shows that rz precedes ra by π/2. from fig. 4 one can see that during cycle 22 there is no ra. the addition of the absence of ra during rz maximum phase to the above observation, allowed us to conclude that rz cycle and ra cycle are not managed by the same solar phenomenon. 0 200 400 600 800 1000 1200 1400 1600 1868 1885 1902 1919 1936 1953 1970 1987 2004 years s u m p e r y e a r o f v e ry q u ie t d a y s a c ti v it y ( n t ) 0 20 40 60 80 100 120 140 160 180 200 r z very quiet days activity sunspot number fig. 2. similar to fig. 1 but for rz and very quiet activity. vol52,2,2009 17-06-2009 19:02 pagina 111 112 from the comparison between qa, fa and ra occurrences through the solar cycle it follows that ra happens during the declining phase while the formers occur respectively during minimum phase and increased phase. figure 5 shows the time series of sa and rz. it can be seen that most sa comes at sunspot maximum. this indicates that sa and 0 20 40 60 80 100 120 140 160 180 200 1868 1885 1902 1919 1936 1953 1970 1987 2004 years r z 0 2000 4000 6000 8000 10000 12000 s u m p e r y e a r o f fl u c tu a ti n g a c ti v it y ( n t ) sunspot number fluctuating activity fig. 3. similar to fig. 1 but for rz and fluctuating activity. 0 20 40 60 80 100 120 140 160 180 200 1868 1885 1902 1919 1936 1953 1970 1987 2004 years r z 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 s u m p e r y e a r o f re c u rr e n t a c ti v it y ( n t ) sunspot number recurrent activity fig. 4. similar to fig. 1 but for rz and recurrent activity. f. ouattara vol52,2,2009 17-06-2009 19:02 pagina 112 113 relationship between geomagnetic classes’ activity phases and their occurrence during the sunspot cycle rz are in phase and most sa is due to cmes (coronal mass ejections) as is well known. few sa are out of phase of rz and precede it. this observation can be attributed to the error coming from the method of sa determination. because of the occurrence of fa through the solar cycle (see fig. 4) and by reference to the classes activity determination method (see ouattara and amory mazaudier, 2008) the few sa out of phase of rz are the contribution of fa in sa. figure 5 shows that the major fa which contributed to sa occurred before the 20th century. this highlights that the methods used for the determination of sa are more and more precise. as a consequence sa during the 20th century they are exclusively due to cmes. when we plot the major activity due to aa > 100 nt and compare its activity with that of rz (fig. 6) we confirm what has been previously said and can affirm that the major sa is produced by intense storms generated by cmes. the comparison between fig. 5 and fig. 6 shows that sa out of phase of rz is not due to major activity. this observation permits us to conclude that sa is due to major storms during the last century. such result points out the change in solar activity. this has been already pointed out by 1) ouattara et al. (2008) in terms of decreasing of the number of quiet days, by 2) rouillard et al. (2007) in terms of change in the dipole magnetic field and by 3) svalgaard and cliver (2007) in terms of the increase in solar wind speed. ouattara and colleagues (2008) show that each term of change contributes to show the decreasing of the number of quiet days. figure 7 expresses the time evolution of ra and fa. we find that both activities are in opposite phase. the minimum of one arrives at the maximum of the other. this opposite phase was previously noted by legrand and simon (1989) and ouattara (2008). this time evolution shows that through the solar cycle fa occurs during increased phase while ra happens during decreased phase. 0 20 40 60 80 100 120 140 160 180 200 1868 1885 1902 1919 1936 1953 1970 1987 2004 ye ars s u m p e r y e a r o f s h o c k a c ti v it y (n t ) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 r z sunspot number shock activity fig. 5. similar to fig. 1 but for rz and shock activity. vol52,2,2009 17-06-2009 19:02 pagina 113 114 0 500 1000 1500 2000 2500 1868 1885 1902 1919 1936 1953 1970 1987 2004 years s u m p e r y e a r o f a a > 1 0 0 n t 0 20 40 60 80 100 120 140 160 180 200 r z aa > 100 nt sunspot number fig. 6. similar to fig. 1 but for rz and aa > 100 nt. 0 2000 4000 6000 8000 10000 12000 1868 1885 1902 1919 1936 1953 1970 1987 2004 ye ars s u m p e r y e a r o f re c u rr e n t a c ti v it y ( n t ) 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 s u m p e r y e a r o f fl u c tu a ti n g a c ti v it y ( n t ) fluctuating activity recurrent activity fig. 7. similar to fig. 1 but for fluctuating activity and recurrent activity. f. ouattara vol52,2,2009 17-06-2009 19:02 pagina 114 115 relationship between geomagnetic classes’ activity phases and their occurrence during the sunspot cycle summarizing, it emerges that the different time occurrences of the different activities permit us to indicate their position in the time profile of rz. the minimum of rz coincides with the manifestation of slow solar wind activity (qa) and during its ascending phase there are the events due to fa. in rz maximum cmes produce sa and when the declining arrives it is the manifestation of ra. the division of the different activities shown here was reported by ouattara (2008) analysing the results shown by legrand and simon (1989) according to the relationship between the solar cycle and geomagnetic activity cycle. the different time profiles of the four classes of geomagnetic activity let us conclude that the yearly frequencies of each activity differ and the contribution of each class of activity to the whole geomagnetic activity is also different and changes through time. ouattara and amory mazaudier (2008) show that from 1868 to 2004 geomagnetic activity level is constituted by 14% of sa, 14% of ra, 40% of fa and 32% of qa. the yearly frequencies of each class have been given by legrand and simon (1989) for the period from 1868 to 1978. overthe time during this period yearly frequency occurrences are 8.5% for sa, 7% for ra, 17.5% for fa and 67% for qa. 5. conclusions the different classes of geomagnetic activity do not occur on average at random. fluctuating activity produced by fluctuating solar wind stream precedes the sunspot cycle while the latter goes ahead on shock activity which is due to cmes. recurrent wind stream activity generated by high solar wind coming from coronal holes arrives after shock activity. thus the sunspot cycle begins with slow solar wind activity continues with fluctuating wind stream activity and during its maximum arrive shock activity produced in majority by cmes. the cycle finishes its evolution in descending phase by the manifestation of recurrent high solar wind speed coming from coronal holes. through the cycle the contribution of each class of activity to the whole geomagnetic activity differs from one class to another. the contribution of each class to geomagnetic activity is not constant in time. moreover the yearly frequency occurrence of each class of activity also differs from one class to another class and per class, varies through the time. acknowledgments we thank all data providers such as cetp (centre d’etude des phénomènes terrestres et planétaires) and ngdc (national geophysical data center) for respectively aa data and rz data. we thank here legrand jean pierre, simon paul and christine amory mazaudier for their help and encouragement which have permitted us to understand solar phenomena and their link to terrestrial events. we also remember simon paul who died on january 20, 2008. references echer, e., w.d gonzalez, a.l.c. gonzales, a. prestes, l.e.a. vieira, a. dal lago, f.l. guarnieri and n.j. schuch (2004): long-term correlation between solar and geomagnetic activity, journal of atmospheric and solar-terrestrial physics, 66, 1019-1025. eddy, j.a. 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(received september 22, 2008; accepted november 12, 2008) vol52,2,2009 17-06-2009 19:02 pagina 116 annals 48, 3, 2005+app1 453 annals of geophysics, vol. 48, n. 3, june 2005 key words ionosphere – ionospheric nowcasting – ionospheric forecasting – space weather – digisonde – ionospheric monitoring networks – ionogram scaling 1. introduction knowledge of the state of the upper atmosphere is very important in several applications. the space effects on radio frequency (rf) communications and satellite positioning and navigation applications are determined by the ionospheric electron density structure and the total electron content (tec). ionosphericspace weather effects can also cause time-varying ionospheric currents causing problems in ground systems such as systems for power generation and supply, oil and gas pipeline distribution, aerial surveying for minerals oil and gas, drilling for oil and gas, railways, especially at the higher latitudes. lately there has been an increasing interest of the seismic hazards research community in identifying electromagnetic phenomena in the ionosphere as predecessors of seismic events. systematic real-time measurements of the ionosphere in europe are obtained by ground digisondes (reinisch, 1986) suitable for research and commercial use. the geographic distribution of these digisondes provides almost full coverage of the ionosphere over europe. these digisondes have the capability of automatically scaling and transmitting in real-time all-important parameters characterising the state of the upper atmosphere and the propagation of radio waves in the ionosphere. at the real time monitoring for nowcasting and forecasting ionospheric space weather in europe with ground digisondes anna belehaki ionospheric group, institute for space applications and remote sensing, national observatory of athens, palaia penteli, greece abstract the earth’s ionosphere largely determines space weather effects on radio wave communications, navigation and surveillance systems. lately there has been an increasing demand for ionospheric nowcast and accurate forecast services by various groups of users, including european industry. the paper reviews research activities in europe based on the exploitation of real-time ground digisondes for the provision of nowcasting and forecasting ionospheric space weather information and useful products and services to support operational applications. during the last few years, important progress in databasing, modelling and forecasting ionospheric disturbances based on real-time data from ground digisondes was achieved in the frames of cost action 271 «effects of the upper atmosphere on terrestrial and earth-space communications». further developments are expected to be deployed with the new cost action 724 on «developing the basis for monitoring, modelling and predicting space weather», as well as through the space weather pilot project of the european space agency and through projects funded by the european commission programmes. mailing address: dr. anna belehaki, ionospheric group, institute for space applications and remote sensing, national observatory of athens, metaxa and vas. pavlou str., 15236 palaia penteli, greece; e-mail: belehaki@space.noa.gr 454 anna belehaki fig. 1a,b. ionograms from athens digisonde station (a) for the case of undisturbed ionosphere and (b) for a case of spread f appearance. moment these digisondes operate independently, and knowledge on the state of the ionosphere is generated for a limited area around each station only. this independent operation creates several barriers for transforming this information into usable data, products and services. considering the increasing demand for upperatmosphere nowcast and forecast services by the scientific community and various commercial users, the need to develop a european network is pressing, especially since similar systems already exist in the us and australia. ground digisondes perform the following operations: – automatic scaling of ionograms (with artist software). – real-time derivation and transmission of ionospheric parameters (densities and heights of e-, es-, f1-, f2-layers peaks) – real-time derivation of electron density profile up to 1000 km and estimation of the ionospheric electron content (itec). – real-time derivation of muf for specific distances. – real-time derivation of spread f status. the performance of the artist automatic scaling software (reinisch and huang, 1983), which is integrated to the digisonde computer system, is the key for the exploitation of realtime autoscaling characteristics. the results of the autoscaling program are presented in fig. 1a for the case of undisturbed ionosphere and in fig. 1b for the case of spread f appearance. the artist performance is very accurate in both cases. to better support the previous result, the fof2 values derived from automatic scaled ionograms from athens digisonde are compared with manual validated fof2 values from the corresponding ionograms for a time period of 11 days (fig. 2a) during which a geomagnetic storm occurred on 24 september 2001 and lasted for three days. it is obvious that the fof2 values from the automatic scaling follow the manually validated data very closely. figure 2b shows the differences between the values of fof2 as scaled by artist and by the manual method from 2019 ionograms recorded at athens during 20-30 september 2001 as histograms. for the period under study, 97.6% of the comparisons for fof2 lay within ± 0.5 mhz. this agrees with the comparison published by gilbert and smith (1988). the distribution is weighted in favor of negative values. this feature results from the effect of interference on the echo trace in the vicinity of fof2, which often causes premature termination of the artist trace. the same effect appears in the line plot of a b 455 real time monitoring for nowcasting and forecasting ionospheric space weather in europe with ground digisondes the fof2 (fig. 2a) as sharp peaks in the automatic scaled fof2. this effect was also reported by gilbert and smith (1988) in their comparison of artist and manual scaled ionospheric parameters observed at slough. a new version of artist was recently released from the center of atmospheric research of the university of massachusetts lowell, and the first results are very promising for successful performance on premature terminated traces (reinisch, 2003, 2004, pers. comm.). the accuracy of real-time data, the satisfactory geographic coverage of the european realtime digisondes, and the availability of long term series of historical manual validated data that in some cases cover more than six decades provide the possibility for the development of a system for high quality ionospheric space weather services, for the european region. currently two major research projects are implemented by the european scientific community to study space weather and develop models and forecasting techniques: – cost271 «effects of the upper atmosphere on terrestrial and earth space communications» (http://www.cost271.rl.ac.uk), which puts the emphasis on space weather effects on telecommunications (zolesi and cander, 2004). – cost724 «developing the scientific basis for monitoring, modelling and predicting space fig. 2a,b. a) comparison of automatic scaled (gray line) and manually scaled validated (black line) fof2 from athens digisonde station for a time period of 11 days. b) differences between the values of fof2, as scaled by artist and by manual method from 2019 ionograms recorded at athens during the period 20-30 september 2001. a b 456 anna belehaki weather» (http://cost724.obs.ujf-granoble.fr), which addresses wider issues in modelling the sun-earth system (lilensten et al., 2004). while cost724 action is in its initial phase, cost271 is in progress and major efforts have been reported in the development of a comprehensive database of prompt ionospheric parameters for nowcasting and forecasting purposes and of forecasting algorithms to predict the ionospheric and space plasma effects on communications up to a few days ahead of the present. 2. nowcasting ionospheric space weather one of the most important results of cost271 action is the development of the prompt ionospheric database at rutherford appleton laboratory (stamper, 2003). this database receives prompt data on regular basis from the european digisondes. developments can be followed at the web site of the database <http://www.wdc.rl.ac.uk/cgi-bin/digisondes/ cost_database.pl>. this tool can be used by researchers to access prompt and past ionospheric data from several european stations. besides this basic tool, noticeable progress has been reported during the last few years, developed mainly in the frames of cost271 action and of the esa pilot space weather project, aiming to map the ionospheric characteristics in realtime over europe and to monitor and forecast the space environment including the ionosphere and plasmasphere. 2.1. ionospheric mapping the effective use of automatic scaled ionospheric parameters derived from ground digisondes was demonstrated in the frames of cost271 action and the esa pilot space weather project with the development of a method for real-time mapping of the state of the ionosphere over europe, proposed by zolesi et al. (2003). the objective of the proposed method is to update the simplified ionospheric regional model (sirmup) in real-time using automatic scaled ionospheric data from the four european digisondes operated in athens, rome, juliusruh and chilton. the achieved result is monitoring the state of the ionosphere over europe with sufficient accuracy to be of use from earth-space communications and navigation systems (as galileo). sirm (zolesi et al., 1993) provides ionospheric parameters that represent monthly median values and vary as a function of geographic location, local time, and r12. the updated sirm (sirmup) takes as input the real-time automatic scaled values from the four digisondes to compute an effective sunspot number (reff) that the method is using instead of r12. reff is the sunspot number for which minn fof fof 1 2 2 i n obs calci i 2 1 = = !^ h (2.1) where n is the number of the reference stations, fof2obsi is the observed fof2 at the reference station i, fof2calci is the calculated fof2 from sirm at the coordinated of the reference station i. to test the reliability of the method, the relative errors (houminer et al., 1993) between the observed values (manually validated) at some test stations and the calculated fof2 values by the model, were determined using the following equations: e fof fof fof 1 2 2 2 obs obs sirm = (2.2) e fof fof fof 2 2 2 2 obs obs sirmup = (2.3) where fof2obs is the observed fof2 at the test station, fof2sirm is the sirm calculated fof2 at the test station using the observed r12 sunspot number, and fof2sirmup is the sirmup calculated fof2 at the test station using reff. according to the above description, when the criterion e2 < e1 (2.4) is valid, the method of real-time updating is successful in the sense that the resulting map in the specific area is more representative of the real ionosphere than the corresponding map resulting from the use of monthly median values. 457 real time monitoring for nowcasting and forecasting ionospheric space weather in europe with ground digisondes the performance of sirmup method was tested by zolesi et al. (2004) under all possible ionospheric conditions, including quiet intervals, largescale disturbances such as storm negative phases and daytime positive effects of long duration as well as small-scale effects such as travelling atmospheric disturbances (tads) and nighttime positive effects, by presenting simulation results for several time intervals. an indicative interval is presented in fig. 3 for the storm period of 16 to 24 august 2001. the storm started with an initial compression phase in dst observed on 17 august 2001 at 12:00 ut. the european region was already in the noon sector and therefore was affected by tads, causing the strong daytime positive effect, which was observed from sofia during local afternoon hours. at 22:00 ut, dst reached its minimum and consequently the recovery phase started, although considerable substorm activity was recorded by the ae-index (not shown) until midnight. it is possible that this substorm activity caused the generation of a new composition disturbance zone (tsagouri et al., 2000), producing the strong negative phase in the early hours of the 18 august. consequently, the adjacent tads generated in the daylight sector probably caused the noticeable positive effect in the morning sector. as the geomagnetic activity recovered in the following days, there was a remaining ionospheric disturbance recorded in sofia of small magnitude. an intense substorm activity recorded by the aeindex from the morning of 21 august until noon of 23 august produced tad’s on the 21 and 22 august in the daylight sector and a noticeable negative phase on 23 august. most of the time the quantity e1-e2 was positive indicating that the performance of sirmup was particularly successful during periods of large scale ionospheric disturbances causing the negative effects recorded on the 18 and 23 august 2001, as well as small scale ionospheric disturbances generated by tads on 17 august 2001. in the following the performance of the sirmup model is presented during the relatively fig. 3. simulation results for the storm period 16-24 august 2001 at the test station in sofia. 458 anna belehaki quiet geomagnetic interval from 6 to 10 december 2001. the simulation results are presented in fig. 4 for sofia test station. during this period a different type of ionospheric activity was observed, caused probably by changes in the global wind circulation (prölss, 1995), which resulted in the considerable increase in the fof2 value during day hours, for the first three days of the interval. although the dst-index did not present any disturbance, the source fig. 5. the scatter plots of e2 versus e1 for the two simulated time periods. the best-fit line and its equation are also shown. fig. 4. simulation results of the quiet period 6-10 december 2001 at the test station in sofia. 459 real time monitoring for nowcasting and forecasting ionospheric space weather in europe with ground digisondes of the observed ionospheric disturbance is the continuous substorm activity recorded by the ae-index. from the inspection of the differences between the relative errors (e1-e2) plotted in the bottom of fig. 4 it is concluded that sirmup performance is particularly successful during large-scale positive effects observed on the first three days of this interval. during nighttime positive effects the performance is worse but still better than sirm. to obtain a quantitative estimate of the sirmup performance, the scatter plots of e2 versus e1 are shown in fig. 5 for the two simulated time periods. the best-fit line is overplotted in each diagram. in both cases the criterion e2 < e1 is statistically satisfied. especially for the disturbed period (august 2001) a significant improvement in the mapping performance is noted since e2 is much smaller than e1. nevertheless this is not the case for the quiet period (december 2001) as well, since the high correlation between the two error values e1 and e2 indicate an almost equivalent performance of the two mapping techniques. overall, the results presented by zolesi et al. (2003) show a significant improvement of sirmup performance comparing to sirm, during quiet intervals and also during large-scale ionospheric disturbances. a marginal improvement during localised ionospheric disturbances is also reported. this very detailed analysis demonstrates that the use of real-time automatic data can improve existing mapping techniques in order to be useful for operational applications. 2.2. use of itec parameter for nowcasting the ionospheric and plasmaspheric electron content another service that could be generated from the exploitation of real-time digisondes is the real-time availability of the ionospheric electron content (itec). total electron content (tec) is a key parameter that describes the major impact of the ionized atmosphere on the propagation of radio waves, which is crucial for terrestrial and earth-space communications including navigation satellite systems such as gps, glonass, and the future galileo system. a standard technique to determine tec is from dual frequency gps (global positioning system) measurements. this technique measures the electron content along a slant signal path, from which a vertical tec is estimated by simple geometric corrections (see for example jakowski, 1996). as the orbit altitude of gps satellites is ∼20 000 km, gps derived tec corresponds to the total electron content (bottomside and topside ionosphere, and plasmasphere) and it is sensitive to topside ionospheric and plasmaspheric processes. recently, reinisch and huang (2001) proposed a new technique to determine the vertical ionospheric electron content from groundbased ionogram measurements. the ionogram provides the information to directly calculate the vertical electron density profile up to the peak of the f2-layer. the profile above the peak is approximated by an α-chapman function with a constant scale height ht derived from the bottomside profile shape at the f2 peak, according to the equation zexpn nmf z e2 2 1 1t = -^ h; e (2.5) where z h hmf h2 t= -^ h , nmf2 is the maximum electron density and hmf2 the height of the f2-layer peak. the ionosonde tec, or itec, is then calculated as an integral from 0 to ∼1000 km over the entire profile. from the above definition it is obvious that itec is not equivalent to the gps derived tec parameter (gpstec). since the itec parameter could develop into a very useful tool for nowcasting ionospheric space weather, it is important to determine with accuracy the relationship between itec and tec. a preliminary itec validation based on case studies was performed by comparing itec with tec values derived from gps, incoherent scatter radar, and geostationary satellite beacon measurements at middle latitudes and with topex measurement at the equator (reinisch and huang, 2001; belehaki and tsagouri, 2002a). it was shown that itec is generally within ∼10% of the satellite tec. as a first approach the difference between gpstec and itec may be interpreted as the plasmaspheric content, but this interpretation needs to 460 anna belehaki be verified on a statistical basis (reinisch et al., 2001; belehaki and jakowski, 2002). recently, belehaki et al. (2003) presented a systematic analysis of the itec values from digisonde measurements at athens and of gpstec data from tec maps produced by the german aerospace center (dlr/ikn) for europe using gps measurements, for athens coordinates. it was shown that the residuals of the two independent quantities, ∆tec, provide qualitative information on the plasmaspheric dynamics as deduced from their diurnal and seasonal behavior and their variation during geomagnetic storms and this conclusion is based on three findings: a) in general, the daily variations of electron content above 1000 km for athens have the same qualitative characteristics as the plasmaspheric content daily variation (fig. 6). there is some diurnal interchange between the ionosphere and plasmasphere with fig. 6. the daily variation of the monthly median values of ∆tec over four months. a morning minimum and an evening maximum characterize the daily variation of ∆tec. fig. 7. superposed epoch analysis on 13 geomagnetic storms of the daily averages of the difference between gpstec and itec as a function of number of days since storm initiation. 461 real time monitoring for nowcasting and forecasting ionospheric space weather in europe with ground digisondes fig. 8. the daily variation pattern of the slab thickness parameters τt (black-dotted line) and τi (black-solid line) at athens, extracted from the monthly median values, for the four seasons. downward diffusion from the latter helping maintain the nighttime f2-layer (lunt et al., 1999; belehaki and tsagouri, 2002b) and daytime refilling. the evening peak in ∆tec residuals, which is the striking feature in ∆tec variation over athens and is also observed over chilton, is attributed to the evening plasmaspheric bulge. b) the effect of geomagnetic activity in ∆tec residuals, presented in fig. 7, is qualitatively the same as the effect on the plasmasphere, showing a drastic depletion immediately after the start of the storm and a consequent replenishment that lasts for 9 days. c) the daily variation of the ionospheric slab thickness (τi = itec/nm f2) is compatible with the variation of the daytime thermospheric temperature (fig. 8). there is, however, a significant nighttime increase in the total slab thickness (τt = = gpstec/nm f2), which is most prominent in winter and is due to the lowering of the o+/h+ transition height. the study demonstrates that the itec parameter is a qualitative measure of the ionospheric electron content and that comparing the ionosonde derived tec (itec) with gps derived tec (gpstec) at a given location it is possible to estimate experimentally the effect of the plasmasphere on gps measurements. as a next step, in order to determine the quantitative relation between the itec parameter and the total electron content a statistical study was recently presented by belehaki and kersley (2003) using incoherent scatter radar (isr) profiles from malvern site (52.1°n, 2.3°w). the tec estimates from isr profiles were compared with itec estimates, extracted from the same profiles, computed by extrapolating the bottomside isr electron density profile to the topside according to the method proposed by reinisch and huang (2001). a case showing the results of the topside extrapolation of the bottomside isr ionograms is presented in fig. 9. the first results from the comparison study between tec (from isr profiles) and itec (computed from the extrapolated isr profiles using the h-r method) are presented in fig. 10. here the statistics are limited to 317 cases of daytime profiles and 368 cases of nighttime profiles. in summary, the results show a very good agreement especially during nighttime hours and support the original derivation of the itec as a measure of the ionospheric electron con462 anna belehaki tent. statistical analysis over the whole sample of 4000 profiles is needed before extracting final conclusions. 3. forecasting ionospheric space weather systematic ionospheric space weather forecasts are provided by the rutherford appleton laboratory (http://ionosphere.rcru.rl.ac.uk) using the short-term ionospheric forecasting (stif) tool for the european region based on continuous monitoring of the ionosphere (cander, 2003). this operational system provides forecasting maps for up to 24 h ahead and archive measurement maps of the critical frequency of ionospheric f2-layer fof2, the maximum usable frequency for a 3000 km range muf(3000)f2, nequick modelled vertical total electron content (tec) and frequency of optimum traffic (fot) for the area of interest at each ut hour. accuracy of forecast of fof2, muf(3000)f2 and tec has been studied through several statistical comparisons between measured and forecast values of fof2 and muf(3000)f2. these results clearly suggest that stif tool is a very reliable forecasting technique in relatively quiet geomagnetic conditions. although these conditions prevail in the normal earth’s ionosphere, the conditions during the geomagnetic storms are of more importance for current and future radio communications services as well as space weather operational requirements and scientific studies and under such conditions stif has been proved inadequate (cander et al., 2002). a new approach to forecast ionospheric space weather is attempted based on the idea that solar wind disturbances can determine the timing and the magnitude of ionospheric disturbances (belehaki and tsagouri, 2002b), with the primary goal to develop the understanding and the means to forecast how the ionospheric f2-layer will respond to abruptly and dramatically changing solar and geomagnetic conditions. real-time measurements of the critical frequency of the f2-layer, fof2, and the propagation factor for a 3000 km range, m(3000)f2 from four european digisondes operating in athens, rome, chilton and juliusruh and being available from wdc-c1 at ral (http://www.wdc.rl.ac.uk/cgifig. 9. comparison between the isr profile (gray line) and the extrapolated topside profile after the huang and reinisch (2001) method. the resulted total electron content is also noted. fig. 10. statistical results from the comparison between tec (from isr profiles) and itec (computed from the extrapolated isr profiles using the h-r method) for daytime cases (upper panel) and nighttime cases (bottom panel). 463 real time monitoring for nowcasting and forecasting ionospheric space weather in europe with ground digisondes bin/digisondes/cost_database.pl) and the bz-component of the interplanetary magnetic field, bzimf, from nasa advanced composition explorer (ace) spacecraft mission (http://sec.noaa.gov/ data) are combined for the development of a real-time dynamic system, oriented to monitor the ionospheric propagation conditions over europe. currently such an automated data analysis at 30 min intervals is operational at ral <http://ionosphere.rcru.rl.ac.uk>. the latest charts show the modified bz-imf rate of change over the preceding 30 min interval, with the corresponding percentage deviation from the median of fof2, dfof2 (%), and percentage deviation from the median of m(3000)f2, dmf2 (%), for juliusruh, chilton, rome and athens ionosonde stations. a very representative example is shown in fig. 11, where after a very strong disturbance in the bz component of the imf at midnight, a strong ionospheric depletion is observed over athens with a time delay of 3 h. the presented facility is available for both operational purposes and scientific studies. future developments will include the introduction of other solar wind parameters from ace such as the rate of change of solar wind number density (dn/dt), and the rate of change in solar wind bulk speed velocity (du/dt) to fully monitor solar-terrestrial conditions in purpose to issue a preliminary warning. moreover, the statistical analysis of the future results could help in modelling and forecasting the ionospheric response during storm conditions few hours ahead, which is of crucial importance for the reliable performance of technologically advanced radio communications systems under disturbed propagation conditions (johnson et al., 1997), since the ionospheric forecasting models currently in operation have shown a high degree of reliability during quiet conditions, but have been proved inadequate during storm events (cander et al., 2002). 4. conclusions significant progress has been reported during the last few years by the european ionospheric community towards the development of tools and models for the nowcasting and forecasting ionospheric space weather based on real-time measurements acquired from european digisondes. most of this progress is part of the cost271 action and of the esa space weather pilot project. the new staring cost724 acfig. 11. bz-imf rate of change over the preceding 30 min interval, with the corresponding percentage deviation from the median of fof2, dfof2 (%), and percentage deviation from the median of m(3000)f2, dmf2 (%), for athens digisonde. 464 anna belehaki tion will contribute in this direction, improving our basic knowledge on space weather modelling and forecast. europe has the major advantage of possessing modern ionospheric stations capable of providing in real-time accurate automatic scaled ionospheric parameters that characterize the propagation of radiowaves (cander et al., 2004). the existing digisondes are reliable, operate on a continuous basis and their geographic distribution provides an almost full coverage of the european region. the systematic exploitation of automatic validated ionospheric data from the network of real-time digisondes in europe can i) lead to the development of space weather, communications, navigation models; ii) improve our understanding of the physical processes involved in the ionosphere and plasmasphere; iii) lead to the development of an operational system for nowcasting and forecasting the state of the ionosphere over europe. an important step in this direction can be achieved through the successful implementation of dias project (european digital upper atmosphere server). dias is a project co-funded by the european commission econtent programme, and has as the main objective to develop a pan-european digital data collection on the state of the upper atmosphere, based on real-time information and historical data collections provided by most operating ionospheric stations in europe. dias system will distribute the information required by various groups of users for the specification of upper atmospheric conditions over europe suitable for nowcasting and forecasting purposes. the successful operation of dias system will lead to the development of new european added-value products and services and to the effective use of observational data in operational applications. the participating institutes working on the acquisition, processing, storage and dissemination of data and services and on the development of the technological system are the national observatory of athens, the rutherford appleton laboratory, the istituto nazionale di geofisica e vulcanologia, the swedish institute of space physics, the leibniz institute of atmospheric physics, the space research centre of the polish academy of science, and the department of informatics and telecommunications of the university of athens. the official web site can be accessed in the address <http://www.iono.noa.gr/dias>. finally it is our belief that the successful operation of the dias server planned to start in february 2006, will contribute, as a europe-wide effort, to the development of a facility for the extensive use of ground-based ionospheric observations. acknowledgements the author of this paper would like to express her gratitude to the conveners of the egs2003 session st17 «effects of the ionosphere on terrestrial and earth-space communications», dr. bruno zolesi and dr. ljiljana cander for their kind invitation to present this review talk. this work is part of the european cost271 action and 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(2003): the cost271 action – databasing and validating prompt ionospheric parameters, third workshop of the cost271 action, 23-27 september 2003, spetses, greece. tsagouri, i., a. belehaki, g. moraitis and h. mavromichalaki (2000), positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms, geophys. res. lett., 27, 3579-3582. zolesi, b. and lj.r. cander (2004): cost271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction, ann. geophysics, 47 (suppl. to no. 2/3), 915-925. zolesi, b., lj.r. cander and g. de franceschi (1993): simplified ionospheric regional model for telecommunication applications, radio sci., 28 (4), 603-612. zolesi, b., a. belehaki, i. tsagouri and lj.r. cander (2004): real-time updating of the simplified ionospheric regional model for operational applications, radio sci., 39 (2), rs2011, 10.1029/2003rs002936. c:/documents and settings/elisa/documenti/lavori/cf annali/cf gravita review/cf_revised.dvi gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy e. trasatti istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy (elisa.trasatti@ingv.it) m. bonafede università degli studi di bologna, viale berti pichat 8, 40127 bologna, italy maurizio.bonafede@unibo.it abstract employing 3d finite element method, we develop an algorithm to calculate gravity changes due to pressurized sources of any shape in elastic and inelastic heterogeneous media. we consider different source models, such as sphere, spheroid and sill, dilating in elastic media (homogeneous and heterogeneous) and in elasto-plastic media. the models are oriented to reproduce the gravity changes and the surface deformation observed at campi flegrei caldera (italy), during the 1982-84 unrest episode. the source shape and the characteristics of the medium have great influence in the calculated gravity changes, leading to very different values for the source densities. indeed, the gravity residual strongly depends upon the shape of the source. non negligible contributions also come from density and rigidity heterogeneities within the medium. furthermore, if the caldera is elasto-plastic, the resulting gravity changes exhibit a pattern similar to that provided by a low effective rigidity. even if the variation of the preprint submitted to annals of geophysics 3 september 2007 source volumes is quite similar for most of the models considered, the density inferred for the source ranges from ∼ 400 kg/m3 (supercritical water) to ∼ 3300 kg/m3 (higher than trachytic basalts), with drastically different implications for risk assessment. key words: campi flegrei, deformation, gravity, finite element method, heterogeneous medium 1 introduction volcanic activity produces deformation and gravity changes that may be used as precursors of future eruptions. monitoring of active areas, together with interpretation of these changes, may reveal the physics and characteristics of the magma reservoir. indeed, the detection of ground deformation allows to constrain location and type of deep reservoirs, while gravity monitoring is recognized as a valuable tool for mapping subsurface density distributions. both of them contribute to quantify the change in subsurface mass. several models have been developed in order to interpret geodetic and gravity signals in active volcanic areas. walsh and rice (1979) developed a method to calculate the gravity changes as due to subsurface mass redistribution with respect to the gravimeters at surface. the calculation is suitable for any dislocation source in the half-space; however the stress change in a dislocation-free half-space must be known. bonafede and mazzanti (1998) suggested the separation of density variations, affecting the computation of gravity changes, into 3 different contributions: the first contribution comes from the input of new mass from remote distance, the remaining two from the linearized continuity equation of the material already present in the region. fernandez and rundle (1994) developed a method to compute gravity variations due to point magma intrusion in a horizontally layered elastic-gravitational media. the solutions were extended and generalized to point-sources in multi-layered viscoelastic-gravitational media 2 (fernandez et al., 2001). a comprehensive tool for evaluating deformation and gravity changes due to dislocation in horizontally layered viscoelastic media was also provided recently by wang et al. (2006). the approaches described above are able to simulate only partially the complex geology of volcanic regions, since they model point-sources in horizontally layered media. furthermore, biases in estimate of source density may arise from oversimplified models. trasatti et al. (2005) developed finite element deformation models taking into account the structural and rheological characteristics of campi flegrei. the introduction of rigidity layering and plastic rheology in the local crust causes a shrinking of the deformation shape and enhances the maximum values within the caldera. this behavior allows to estimate a source depth of 5 km (plastic medium) instead of 3 km (elastic medium), adopting the same overpressure and in accordance with informations from groundwater studies and petrological data. recently, currenti et al. (2007) calculated the gravity changes in fe models characterized by an ellipsoidal source expanding in a heterogeneous medium with real topography of mt. etna. they found that gravity estimates may be biased in terms of mass gain/loss if medium complexities are neglected. in the present paper we attempt to model both the deformation and the gravity. we develop a technique to calculate gravity variations due to deep pressurized sources in heterogeneous media. the method consists in two separated steps: (i) the displacement and strain fields due to the source are computed by fe modelling; (ii) the results are employed in a procedure to integrate numerically the gravity variations from the displacements and strain fields. the great advantage of this technique is that the fe models may include several complexities: arbitrary source shape, lateral heterogeneities of the medium, non linear rheological properties and so on. we reproduce known solutions to test the method and then we furnish new calculations of models which cannot be solved analytically. we consider spherical and non spherical (sill and spheroid) sources; 3 furthermore, different medium properties are included, such as density contrasts, elastic inhomogeneities and rheological variations. the models are discussed as a preliminary approach to reproduce the gravity changes observed at campi flegrei during the 198284 unrest episode. 2 the campi flegrei caldera the campi flegrei caldera, hereafter cf, near naples (italy), is a complex volcanic system with several, mostly monogenic, explosive vents (fig. 1) in a densely populated area of some 80 km2. the cf is characterized by the outer caldera rim, about 12 km in diameter (open triangles in fig. 1) and the inner caldera of 6-8 km in diameter (full triangles). slow and remarkable ground movements are typical of this area, as recorded since roman times. in 1970-1972 and 1982-1984, a cumulative uplift of about 3.5 m was observed, followed by changes in the shallow hydrothermal system (chiodini et al., 2003) and an increased rate of shallow seismicity (see de natale et al., 1991, for an overview). following january 1985, the ground began to subside at a much slower rate than during the uplift. the deformation pattern, both during the uplift and the subsidence, exhibits a dominant axial symmetry, concentrated in the inner caldera, with the maximum uplift located at the caldera center (the city of pozzuoli). gravity measurements at cf have been carried out since 1981 and the largest gravity variations are also observed at pozzuoli. the gravity changes are well correlated with the elevations changes (stars) as reported in fig. 2a both during the 1982-84 uplift and the 1985-91 deflation phase. in particular, during the unrest phase, at serapeo (a roman market in the center of pozzuoli) the gravity change normalized to the uplift was -216±7 µgal/m, in good agreement with the average of all the stations -213±6 µgal/m. this value is comparable to those measured during inflation episodes at long valley and 4 rabaul calderas. however, during the deflation phase, a ratio between gravity change and uplift of -224±24 µgal/m (similar to that computed during uplift) was observed only at serapeo (the gravity benchmark closest to the point of maximum uplift), while more scattered ratios were inferred at other stations (e.g. solfatara and la pietra), possibly due to local effects (berrino, 1994). the direct measurement of the free-air vertical gradient at cf is γ = -290±5 µgal/m (berrino et al., 1984), about 20 µgal/m smaller (in absolute value) than the reference value (fig. 2b). the residual gravity ∆gr (the difference between the observed ∆g and the free-air gravity ∆gf a) increased with the uplift by 75±12 µgal/m during 1982-84 (fig. 2c). the 1982-84 crises was interpreted as due to increase of pressure of a deep magma chamber, while geochemical evidences suggested thermodynamic disturbances of shallow aquifers as a response to increased heat flow from below (bullettin volcanologique, 1984). the bell shape of the vertical surface displacement suggests to model the data by means of mogi isotropic pressure sources. gottsmann et al. (2006) presented inversions of deformation data adopting different source geometries: point-source, sill and spheroidal cavity. all the sources result to be located at about 3 km below sea level, and they suggested that the spheroidal source may depict an envelope around a hybrid of both magmatic and hydrothermal sources. battaglia et al. (2006) performed joint inversions of gravity and deformation data, founding that a sill-like source is preferred to interpret the inflation phase, while a shallower spheroid source is more suitable for the deflation phase. the works mentioned above assume the medium to be elastic and homogeneous. these approximations only provide limited insights into subsurface dynamics at areas such as cf, where intense faulting and inelastic deformations are likely to take place. 5 3 method of gravity calculation by fe 3.1 gravity contributions in general, the total gravity change observed at a benchmark can be separated into the term depending on the elevation change (removed by free-air correction) and the term due to mass redistribution: ∆g = ∆gf a + ∆gr (1) where ∆gf a = γuz (γ is the free-air gradient and uz is the vertical displacement of the benchmark). as suggested by many authors, the data is corrected using the measured free-air vertical gradient (e.g. berrino, 1994). the residual gravity change is given by: ∆gr = g ∫ v ∆ρ cos θ r2 dv (2) where g is the gravitational constant, v is the volume over which the density variations ∆ρ do not vanish, r is the vector between dv and the observation point and θ is the angle between r and the vertical (fig. 3). according to sasai (1986) and bonafede and mazzanti (1998), ∆ρ in eq. (2) accounts for three contributions: ∆ρ = δρs − ρǫkk − u · ∇ρ (3) where δρs is the density change related to the introduction of new mass from remote distance, ρ is the material density in the reference configuration, ǫij is the strain tensor and u is the displacement field. the first term accounts for the new mass intruded; the second term is the relative density change arising from the compressibility of the medium; the third one is due to density variations within the medium. considering the 6 three density changes indicated in eq. (3), ∆gr can be written as (fig. 3): ∆gr = ∆gs + ∆gm with ∆gm = ∆gv + ∆gl (4) where ∆gs depends on δρs (vanishing for a massless source) and ∆gm is the contribution due to deformation of the medium surrounding the source. the term ∆gv is expressed by ∆gv = −g ∫ v ρǫkk cos θ r2 dv (5) and depends on the finite compressibility of rocks; it vanishes if the medium is incompressible. the ∆gl term depends on u · ∇ρ and it accounts for the bouguer correction (expressing the contribution of the lifted mass above the free surface) and the source inflation/deflation (from the displacement of source boundaries). furthermore, it accounts for the displacement of volumes with different density if the medium is not homogeneous. if ρ varies continuously, it is given by ∆gl = −g ∫ v u · ∇ρ cos θ r2 dv (6) if ρ is discontinuous across a surface s within v , the exceeding mass depends on the displacement of the boundary between densities ρin and ρout as (ρout − ρin)u · ds. note that the scalar product between the orientation of the density interface ds and its displacement u accounts for contributions of any orientation of the surface. therefore, the contribution due to the density discontinuities can be expressed as: ∆gl = g(ρ out − ρin) ∫ s cos θ r2 u · ds (7) where s is the surface over which the density changes. it is important to note that ∆gl may include the contribution of the new mass within the source, if the density gradient 7 is chosen properly. however, we separate the effect of the displacement of the source boundary (within ∆gl term) from the effect of the new mass (∆gs) because the density contrast between inside/outside the source is unknown, being one of the main target of gravity modelling. 3.2 numerical integration of gravity within fe models the subdivision in single contributions of eq. (4) is a suitable method to compute gravity changes from fe models of deformation. we consider massless cavities since the source term ∆gs is treated separately in sec. 5. the gravity changes are calculated in two separate steps: (1) fe calculation of displacement u and strain ǫij fields in the medium, due to the source inflation; (2) computation of gravity terms ∆gl and ∆gv from displacements and strains obtained at (1). since the gravity terms ∆gl and ∆gv are integrals over the total volume, they are computed as a sum of contributions within each finite element: ∫ v f (x, y, z)dv = n ∑ i=1 ∫ vi f (x, y, z)dv (8) where n is the total number of elements, vi is the volume of the i-th element of the grid and f is a generic integrand function. the gravity term ∆gv expressed in eq. (5) can be easily evaluated within the fe domain as ∆gv = −g n ∑ i=1 ρi ∫ vi ǫkk cos θ r2 dv (9) 8 where ρi is the density of the i-th element. the term ∆gl due to the density variation is complicated by the integration over the faces of elements. indeed, for each face of the i-th brick element we must consider the density variations within the adjacent element. the term ∆gl from eq. (7) is expressed by: ∆gl = g n ∑ i=1 6 ∑ j=1 (ρout − ρin) ∫ sij cos θ r2 u · ds (10) where the surface integral is performed over each j-th face of the i-th element sij and ρ in is the density of the element considered, while ρout is the density of the adjacent element sharing the face sij. note that this contribution must be evaluated only once. for the elements forming the free surface, this contribution is the bouguer correction (since the faces lying on the free surface have confining density equal to 0). if a density contrast is present within two or more elements of the medium, ∆gl accounts for this contribution. the great advantage of this integration is that non planar interfaces between layers can be considered. each element may have arbitrarily shape and may be characterized by different density, elastic parameters or rheology independently from the rest of the model. the integrals over single elements are evaluated numerically using the gauss quadrature formula. we give a brief overview of this technique: (1) the gauss integration is computed by mapping each arbitrarily distorted brick element, the (x, y, z) space, to a trilinear hexahedral element of side 2l, the (x′, y′, z′) space. the transformation applies to local cells only (fig. 4a). the volume integration for an element becomes: ∫ v f (x, y, z)dv = l ∫ −l l ∫ −l l ∫ −l f [x(x′, y′, z′), y(x′, y′, z′), z(x′, y′, z′)]j(x′, y′, z′)dv ′(11) where j is the jacobian determinant. 9 (2) the volume integral is evaluated as the sum of the integrand function calculated at gauss points. we adopt the two-points integration (for each dimension), which requires to approximate the integrand function at 8 gauss points: (x′g, y ′ g, z ′ g) = (± l√ 3 , ± l√ 3 , ± l√ 3 ). the volume integral becomes the sum of the values: l ∫ −l l ∫ −l l ∫ −l f (x′, y′, z′)dv ′ = 8 ∑ 1 f (x′g, y ′ g, z ′ g) (12) since the gauss integration is the technique adopted by fe to calculate many derived fields such as strain and stress, these values are directly available from our code (marc , 1994, version 2005) at gauss points without any further approximations. the term ∆gv in eq. (9) can be computed from eq. (12). however, computation of ∆gl in eq. (10) involves surface integrals over each element face. following the development outlined in greenberg (1978), each face is locally transformed in space (x′, y′, z′), defining the vectors ds1 and ds2 tangent to the plane considered. in the example shown in fig. 4b, the face 4-3-7-8 is mapped into the x′-constant plane. the vector ds1 is defined to be along y′-constant curve and ds2 is along z ′-constant curve. the elemental area vector ds, denoted by the shaded parallelogram, can be computed as the vector product between ds2 and ds1 (positive pointing out of the cell). the surface integral becomes ∫ s f u · ds = ∫ s f u · (ds2 × ds1) (13) and it is computed by applying the two-points gauss rule in two-dimensions. indeed, 4 gauss points are determined over the mapped plane, e.g. (± l√ 3 , ± l√ 3 , l), and the integrand function is approximated at these points following the procedure outlined above. 10 4 cases of study we develop a fe model characterized by about 75000 8-nodes brick elements, extending 160 × 160 × 80 km3 (fig. 5). the domain is large enough to avoid bias from boundaries over which displacement and stress fields are imposed to vanish. furthermore, the bottom boundary is kept fixed. the grid is characterized by the presence of a flat interface at 3 km depth which may be used to represent a shallow layer, and a cylindrical boundary of radius 3 km and depth 3 km lying at the center of the model, to approximate the cf caldera. the element size is about 350 m side in the central part of the grid, while it increases with distance up to 10 km close to the domain boundaries. we show examples of gravity computations for pressurized sources of various shapes in homogeneous/inhomogeneous media. in all the cases shown in this section, the sources are considered empty so that only ∆gm is computed. a brief outline of the main characteristics of the models are reported in table 1. unless changes due to specific model configurations, the medium is considered as homogeneous, elastic and isotropic with rigidity µ0 = 1 gpa and poisson ratio ν = 0.25. all the sources are placed at 5 km depth, at the center of the model, co-axial with the caldera and undergo to overpressure ∆p = 50 mpa. when the density is homogeneous, its value is ρ0 = 2500 kg/m 3. the models named mogi-n are characterized by a spherical source of radius a = 1 km; in particular the case with homogeneous and elastic medium is mogi-1. density contrasts are considered in models mogi-2 and mogi-3, characterized by a density equal to ρ1 = 1800 kg/m3 within the shallow layer and the caldera (respectively); the density of the remaining medium is ρ0. in model mogi-4 the caldera has heterogeneous rigidity with respect to the remaining medium µ1 = 0.1µ0, while the density ρ0 is constant anywhere. model mogi-5 contains both the last characteristics: the caldera has heterogeneous rigidity µ1 and density ρ1, while the rest of the medium has rigidity µ0 and 11 density ρ0. we consider also two inelastic models, assuming a plastic rheology of the medium to describe the highly fractured rocks of the cf area. the medium behaves plastically when the maximum shear stress is greater than a specified yield stress. the plastic rheology is only treatable by means of numerical methods, and easily implementable in fe models. in model mogi-6 the whole medium behaves plastically with yield stress σy = 30 mpa, while mogi-7 is characterized by plastic caldera (σy = 0.5 mpa) and remaining elastic medium, to evidence the highly deformable inner caldera. two further models are considered, with different shape of the source: spheroid and sill. the spheroid has the same volume of the sphere and aspect ratio equal to 0.4 with resulting semi-major axis a = 1850 m and semi-minor axes b = c = 740 m; it is vertically elongated. the sill is a horizontal square crack of side l = 2 km; both sources act in a homogeneous and elastic medium, to consider the effects of the different shape only. fig. 6 reports the gravity patterns from the source axis to 10 km distance along the x axis; the solid line is the analytical solution while the squares are the results from numerical integration. the figure shows that the gravity change ∆gm due to a mogi source in a homogeneous and elastic half-space is null and the only contribution observable at surface would be the free-air effect. this agrees with analytical results by walsh and rice (1979), sasai (1986), bonafede and mazzanti (1998). our numerical “zero” is 0.5 µgal, well below instrumental error, confirming the robustness of the gauss integration within the fe mesh and the good approximation of the half-space by the numerical domain. different results are obtained for models spheroid and sill, dilating in a homogeneous medium (fig. 7). we report the computed vertical displacement (the thin solid line) which is representative of ∆gf a. from now on the gravity changes will be shown normalized to the uplift above the source center (coinciding with the maximum uplift for sphere and sill models). in the case of the spheroid, the gravity variation ∆gm 12 is negative (fig. 7a, thick solid line). therefore, if the gravity signal generated by a spheroid is modeled by a spherical source, it may lead to an overestimation of the density within the source. this result is also confirmed by battaglia and segall (2004) that performed inversions of geodetic and gravity data generated by a spheroid, by means of a spherical source. they found that the isotropic point-source approximation leads to an overestimation of depth, mass and density of the intrusion. fig. 7b shows the opposite effect due to the sill: the total contribution is positive, without new mass input. note that in this case the contribution ∆gv (dotted line) is almost null. this behavior arises from the strain field generated by the sill in the half-space, causing contraction above it and lateral dilatation. we compare these results with the analytical ones provided by sasai (1986), finding good agreement (gilda currenti personal communication). in a recent paper battaglia et al. (2006) implemented the analytical solutions of the spheroid (yang et al., 1988) within the gravity calculation method developed by walsh and rice (1979). they found that the gravity change due to the medium deformation is negligible for a spheroidal cavity while it is noticeable for a sill. good agreement is found from the comparison between our and their results for the sill. instead, our results for the spheroidal cavity is not in agreement with their findings, as shown in fig. 7a. however, since comparisons between our models and the analytical solutions (when available, like models mogi-1, mogi-2 and sill) show very good agreement, and since the method of gravity computation shown here is independent on the source shape and properties of the medium, we are confident with the robustness of the present results. in fig. 8 the results are reported for two models characterized respectively by density contrast of 30% at 3 km depth (mogi-2) and within the caldera rim (mogi-3). in the first case (fig. 8a), solutions are in agreement with those by bonafede and mazzanti (1998) (not shown here). a larger effect is visible when the low density is restricted to the caldera (fig. 8b). in this case, ∆gl accounts for the density discontinuity at the 13 circular boundary of the cylinder also. the density jump reflects into the step of the ∆gl pattern, while the volumetric and free-air (∝ uz) terms are continuous. indeed, if we compare the two models, we find a similar pattern of ∆gv because the density contrast has little effect in the strain field, while ∆gl is more sensitive to this discontinuity. these models show that this kind of inhomogeneities, even if very localized (like within the caldera), must be taken into account to perform gravity modelling. results similar to mogi-2 were obtained by battaglia and segall (2004) for a massless cavity in a layered medium. they found a small (but different from 0) contribution to the residual gravity due to a density contrast of 13% at long valley caldera. the contribution of heterogeneous rigidities within the medium is emphasized in model mogi-4. indeed, as long as the rigidity ratios are small, the contribution to ∆gm /uz is of the order of some µgal/m. in fig. 9a the results is shown for an extreme ratio of µ1/µ0 = 0.1 between caldera and remaining medium. the low rigidity of the caldera amplifies the displacement field and hence the terms linked to this parameter (∆gf a and ∆gl), while the volumetric term ∆gv is almost unchanged from the homogeneous case in fig. 6. a very low effective rigidity could be suitable to describe long term deformation for a standard linear solid rheology; this rheology may apply to volcanic areas such as cf, due to the high temperature, the presence of erupted products, incoherent materials and hydrothermal activity. a further model, mogi-5, accounts for both the characteristics of mogi-4 and mogi-3, having the caldera with low rigidity and low density (fig. 9b). since the displacement generated from this model is equal to mogi-4, ∆gv is very similar to that of panel (a). the density contrast between the caldera and the remaining medium causes a lower ∆gl and a small step in its pattern. we consider also two cases of inelastic medium, described by plastic rheology. in the case of uniform yield stress within the medium, the deformation is approximately radial around the spherical source and the total effect is null (fig. 10a), similarly to the 14 elastic homogeneous model. the sill is also modelled (not shown here) in a medium homogeneously plastic, finding that ∆gm /uz is unchanged with respect to the elastic case, as observed for the spherical source. indeed, the plastic medium contributes by amplifying both uz and the gravity terms, leaving unchanged the ratio between them. in the case of model mogi-7, a rheologic discontinuity is present, since the caldera is plastic while the rest of the medium is elastic. it is evident from fig. 10b that when the yield stress is not uniform, the gravity change ∆gm is different from 0. indeed, the total contribution is positive within the caldera while it becomes negative out of the caldera rim, vanishing at 7-8 km from the center of the model. this model is useful to describe localized rheology discontinuities, likely to be present in volcanic areas. it is interesting to note that the gravity terms are very similar to those shown in fig. 9a, indicating that both the low rigidity and the plastic rheology act to enhance the deformation within the caldera, allowing for larger ∆gl terms while ∆gv is similar to the elastic value. indeed, ∆gl is sensitive to the deviatoric strain (mostly plastic) while ∆gv is sensitive only to the isotropic strain (elastic). this kind of behavior was also figured out by trasatti et al. (2005), showing that the elasto-plastic rheology induces larger deformations within the caldera. 5 contribution of the inflating source in the previous section the reservoirs are considered massless, i.e. they are cavities within which the overpressure ∆p . whether the expansion of cavities is due to new mass input, internal processes or pore pressure and temperature migration, the gravity change due to the source at a benchmark can be computed as ∆gs = g    ∫ v ′ s ρ′s cos θ′ r′2 dv ′ − ∫ vs ρs cos θ r2 dv    (14) 15 where v ′s, vs are the source volumes after and before the deformation, respectively, and all primed variables refer to the deformed configuration. we assume that the main process determining the ∆gs is the new mass entering into the reservoir. this process has several consequences: volume variation, density growth due the increased internal pressure and the displacement of the center of mass of the reservoir due to the asymmetric deformation of the medium in proximity of the free surface. for simplicity, we assume ρs, ρ ′ s to be mean internal densities, and we approximate the mass at the source center (due to the source depth, assumed as 5 km, much larger than the extension). the mass change is ∆m = ρ∆v + v ∆ρ and the density after deformation is expressed by ρ′s = (m + ∆m )/(v + ∆v ), at the first order. it results that: ρ′s ∼ m v ( 1 − ∆v v ) + ∆m v ∼ ρs ( 1 − ∆v v ) + ρs ∆v v + ∆ρ = ρs + ∆ρ (15) the ratio ∆ρ/ρs = ∆p/k ∼ 10 −3, where ∆p is the overpressure and k is the isothermal incompressibility, so that ρ′s ∼ ρs and the main effect on the gravity change is the volume increase of the magma reservoir (franchini, 2005). in conclusion, the resulting ∆gs is expressed by: ∆gs = gρs∆v cos θ r2 (16) the volume variation of a cavity with arbitrary shape is obtained by integrating the normal displacement over the boundary: ∆v = ∫ s u · ds (17) within the fe grid the integral is performed adopting the procedure outlined for the gravity term ∆gl (see eqs. (7) and (10)), using eq. (13). the observed gravity change at cf is 75±12 µgal/m, which must be equal to ∆gr = ∆gs + ∆gm . from eq. (16) and 16 the values of ∆gm computed for the sources in table 1 we obtain the source densities ρs. volume variations and density estimates are reported in fig. 11. the density value calculated for the sphere in the homogeneous medium (mogi-1) is very similar to that usually found in literature for magmatic sources. indeed, berrino et al. (1984) and berrino (1994) estimated 2500 kg/m3, suggesting the hypothesis of silicate melts entering a reservoir rather than hydrothermal fluids within a confined aquifer. all the models (except mogi-6) show similar volume variation, while the densities inferred range from ∼400 to ∼3300 kg/m3. the contribution of the medium, when homogeneously plastic (mogi-6), is null and the inferred density is comparable to the elastic models, ∼2000 kg/m3. however, the volume variation is very large, due to the enhanced deformation allowed in the medium. the density inferred in models characterized by density layering (mogi-2 and mogi-3) is greater than that inferred in a homogeneous model. indeed, since the gravity contribution ∆gm is negative, the observed positive gravity change must be due to a larger density for the new mass. also mogi-5 (low rigidity and low density caldera) requires high density in spite of the positive ∆gm value, because of the larger uplift. it is interesting to note that models mogi-4 (low rigidity caldera), mogi-5 and mogi-7 (plastic caldera) produce very similar ∆gm value (fig. 9 and 10), but intrusion density differ by ± 25%. due to the compensation of the negative value computed for ∆gm (fig. 7a), the density inferred for the spheroid is quite high, being about 3300 kg/m3. this value is much higher than pertinent to trachytic basalts at cf (∼ 2300-2500 kg/m3). on the other side, the density inferred for the sill model is very low, 410 kg/m3. since supercritical fluid densities are of the order of hundreds of kg/m3, this value would indicate a hydrothermal source instead of a magmatic source, as suggested by battaglia et al. (2006) for a penny shaped crack. 17 computations for mass input ∆m = ρ∆v are reported in table 2. based on the evaluation of gravity residuals and the assumption of a pressurized point-source, berrino et al. (1984) and berrino (1994) suggested that a subsurface mass increase of 2·1011 kg took place between 1982-84. results from models with spherical sources are typically 2-3 times greater than this value. the all plastic model, mogi-6, has a very high volume variation due to the enhanced deformation of the plastic medium. the computed density is ∼2000 kg/m3 while the mass input is 3-4 times the estimated value. on the other side, the mass calculated for the sill model is a factor 4 less than the reference value. 6 conclusions we develop a numerical technique to calculate gravity changes due to deep inflating sources using fe modelling. even if the models presented are too simple for realistic application to the cf caldera, the method is very general and allows to account for many characteristics of the studied area: various source shapes, density and rigidity discontinuities, rheological heterogeneities. very few analytical solutions are available in literature (e.g. sphere, sill, spheroid in elastic media and point-sources in viscoelastic media), while the fe models may be characterized arbitrarily. the gravity calculations distinguish between the contribution of the medium and of the mass intrusion. very different contributions come from the deformation of the medium, according to source shape. we separate the effect of the displacement of layers interfaces (∆gl) and of the medium compressibility (∆gv ). we consider the gravity changes observed at cf caldera during the 1982-84 unrest episode, where a free-air corrected gravity residual of 75±12 µgal/m was observed. the fe models developed includes heterogeneities in density, rigidity and rheological properties in order to point out which may be important in 18 affecting the observations. it must be noted that the large deformation is well outside the elastic limit of any rock material. we show that non-spherical sources such as sill and spheroid yield positive or negative gravity changes (respectively), without input of new mass. the density values computed for these models are very different, changing by a factor 8 between extreme end models. the results clearly point out the importance of the source shape and of the characteristics of the medium in gravity calculation and source density estimation. the proposed models, even if not oriented yet to data inversions, give hints in understanding the effects of considering realistic structure of the studied area in gravity computations. we fit the uplift and the gravity above the source but do not yet attempt to reproduce the spatial pattern away from the axis. this will be done after developing a realistic model taking into account the characteristics of the area. density and elastic parameters can be extracted, in principle, from seismic tomography studies and from empirical relationships between seismic velocity and density (brocher , 2005). however, long term deformation in volcanic areas may be largely affected by anelastic relaxation and drainage conditions. the large deformations which took place at cf are better described in terms of plastic rather than elastic constitutive relationships. laboratory studies on samples extracted from deep drilling seem the only plausible way to obtain information on the rheological properties of the medium. the source shape is also very important, but only direct modelling has been attempted up to now. inversion procedure to retrieve the shape of a point-like deformation source in laterally heterogeneous and inelastic media have been proposed recently (trasatti et al., 2007), and this techniques should be further developed to include modeling of gravity changes. from direct modeling we know that vertical and horizontal deformation patterns are necessary to distinguish among different source shapes (e.g. dieterich and decker , 1975): including gravity data gives further constraints to restrict the range of acceptable models. 19 acknowledgments: this work was supported by funding from italian civil protection ingv-dpc v3-2 campi flegrei project. we thank c. giunchi for the useful discussions and hints and g. berrino for providing deformation and gravity data. useful suggestions from frank roth and gilda currenti are greatly acknowledged. references battaglia, m., and p. segall (2004), the interpretation of gravity changes and crustal deformation in active volcanic areas, pure appl. geophys., doi:10.1007/s00024-0042514-5. battaglia, m., c. troise, f. obrizzo, f. pingue, and g. de natale (2006), evidence for fluid migration as the source of deformation at campi flegrei caldera, (italy), geophys. res. lett., doi:10.1029/2005gl024904. berrino, g. (1994), gravity changes induced by height-mass variations at campi flegrei caldera, j. volcanol. geotherm. res., 61, 293–309. berrino, g., g. corrado, g. luongo, and b. toro (1984), ground deformation and gravity change accompanying the 1984 pozzuoli uplift, bull. volcanol., 47, 187–200. bonafede, m., and m. mazzanti (1998), modelling gravity variations consistent with ground deformation in the campi flegrei caldera (italy), j. volcanol. geotherm. res., 81, 137–157. brocher, t. m. (2005), empirical relations between elastic wavespeeds and density in the earth’s crust, bull. seism. soc. am., 95, 2081–2092. bullettin volcanologique (1984), the 1982-1984 bradyseismic crisis at campi flegrei, italy, special issue, 47. chiodini, g., m. todesco, s. caliro, c. del gaudio, g. macedonio, and m. russo (2003), magma degassing as a trigger of bradyseismic event: the case of phlegrean fields (italy), geophys. res. lett., 30, doi:10.1029/2002gl016790. 20 currenti, g., c. del negro, and g. ganci (2007), modelling of ground deformation and gravity fields using finite element method: an application to etna volcano, geophys. j. int., 169, 775–786, doi:10.1111/j.1365-246x.2007.03380.x. de natale, g., f. pingue, p. allard, and a. zollo (1991), geophysical and geochemical modelling of the 1982-1984 unrest phenomena at campi flegrei caldera (southern italy), j. volcanol. geotherm. res., 48, 199–222. dieterich, j. h., and r. w. decker (1975), finite element modelling of surface deformation associated with volcanism, j. geophys. res., 80, 4094–4102. fernandez, j., and j. b. rundle (1994), gravity changes and deformation due to a magmatic intrusion in a two layered crustal model, j. geophys. res., 99, 2737–2746. fernandez, j., k. f. tiampo, and j. b. rundle (2001), viscoelastic displacement and gravity changes due to point magmatic intrusions in a gravitational layered solid earth, geophys. j. int., 146, 155–170. franchini, l. (2005), deformazioni e variazioni della gravità in aree vulcaniche: influenza della struttura reologica, in italian, università degli studi di bologna, corso di laurea specialistica in fisica. gottsmann, j., h. rymer, and g. berrino (2006), unrest at the campi flegrei caldera (italy): a critical evaluation of source parameters from geodetic data inversion, j. volcanol. geotherm. res., 150, 132–145. greenberg, m. d. (1978), foundations of applied mathematics, 456 pp., prentice-hall, inc., englewood cliffs, new jersey. marc (1994), analysis research corporation, palo alto, ca. sasai, y. (1986), multiple tension-crack model for dilatancy: surface displacement, gravity and magnetic change, bull. earth. res. inst., 61, 429–473. trasatti, e., c. giunchi, and m. bonafede (2005), structural and rheological constraints on source depth and overpressure estimates at the campi flegrei caldera, italy, j. volcanol. geotherm. res., 144, 105–118. 21 trasatti, e., c. giunchi, and n. p. agostinetti (2007), numerical inversion of deformation by pressure sources: application to mount etna (italy), submitted to geophys. j. int. walsh, j. b., and j. r. rice (1979), local changes in gravity resulting from deformation, j. geophys. res., 84, 165–170. wang, r. f., f. l. mart́ın, and f. roth (2006), psgrn/pscmp a new code for calculating coand post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory, computers & geosciences, 32, 527–541. yang, x., p. m. davis, and j. h. dieterich (1988), deformation from inflation of a dipping finite prolate spheroid in an elastic halfspace as a model for volcanic stressing, j. geophys. res., 93, 4249–4257. 22 table 1. list of models of which we calculate the gravity variations. the overpressure within the sources is ∆p = 50 mpa and all the sources are placed at depth d = 5 km. the layer has horizontal interface and a height of 3 km. the caldera has a cylindrical shape of 3 km radius and 3 km height; it lays at the center of the model, co-axial with the sources. the parameters are explained in the text. table 2. volume variation and density calculated for each model (characteristics reported in table 1) due to input of new mass within the cavity. fig. 1. map of campi flegrei caldera showing the outer rim (open triangles) and the inner caldera (full triangles); the leveling lines (dotted) were surveyed during 1982-84 unrest episode, while the squares represents the gravity stations active in that period. fig. 2. (a) gravity variations and elevation change observed during 1981-2001 at serapeo roman market (pozzuoli), at the center of cf; (b) gravity due to the ground uplift (freeair correction); (c) residual gravity due to the difference between the observed and the free-air corrected gravity. fig. 3. schematic representation of the gravity contributions: ∆gf a is the free-air change proportional to the uplift; ∆gl is the change caused by the lifted portion of the ground (bouguer anomaly) and of interfaces between density layers; ∆gv is the gravity field due to the medium compressibility; ∆gs is the contribution of the material filling the expanding part of the source. in the right hand side of the figure is indicated the geometry of the gravity integration of elementary volume dv , its displacement u, the observation point p , the vector r between dv and p , and the angle θ between them. fig. 4. (a) the gauss integration is computed by mapping each arbitrarily distorted brick element (x-space) to a trilinear hexahedral element (x′-space). the two-points gauss quadrature is performed by calculating the integrand function at the 8 gauss points 23 indicated by “+” symbol. (b) the surface integral within element faces is performed by mapping x into x′. the vectors ds1 and ds2 are defined to be tangent to the surface, defining the unit area shaded (see text for details). fig. 5. perspective view of the fe model. the caldera is a cylinder of radius 3 km and height 3 km, surrounded by a layer of similar thickness on top of the halfspace. the mogi source is shown at a depth of 5 km, with a radius of 1 km. fig. 6. contributions and medium gravity change due to an inflating spherical pressure source in a homogeneous and elastic half-space. comparison between analytical solutions (solid line) and numerical (squares). the contribution due to the inflation of a massless sphere ∆gm = ∆gl + ∆gv is null. fig. 7. gravity terms normalized to the maximum uplift umaxz , and vertical displacement in a homogeneous medium due to (a) vertically elongated spheroidal source, (b) horizontal sill (in this case ∆gv ≃ 0 and ∆gm overlaps ∆gl). fig. 8. gravity terms normalized to the maximum uplift umaxz , and vertical displacement due to an inflating spherical pressure source in a medium characterized by density contrasts of 30% in: (a) shallow layer of height 3 km; (b) cylindrical caldera. fig. 9. gravity terms normalized to the maximum uplift umaxz , and vertical displacement due to an inflating spherical pressure source in a medium characterized by: (a) heterogeneous caldera µ1/µ0 = 0.1; (b) caldera with heterogeneous rigidity µ1 and density ρ1. fig. 10. gravity terms normalized to the maximum uplift umaxz , and vertical displacement in an inelastic medium: (a) all plastic, (b) plastic caldera and remaining elastic. fig. 11. density calculations versus volume variations of the proposed models. 24 model source medium characteristics parameters mogi-1 sphere homogeneous & elastic µ0 = 1 gpa; ρ0 = 2500 kg/m 3; a = 1 km mogi-2 ” heterogeneous density layer ρ1 = 1800 kg/m 3; ρ0 = 2500 kg/m 3 mogi-3 ” heterogeneous density caldera ” mogi-4 ” heterogeneous rigidity caldera µ1 = 0.1µ0 mogi-5 ” heterog. rigidity & density cal. µ1=0.1µ0; ρ1=1800kg/m 3; ρ0=2500kg/m 3 mogi-6 ” all plastic σy = 30 mpa mogi-7 ” plastic caldera σy = 0.5 mpa spheroid spheroid homogeneous & elastic v sphere = v spheroid; a=1850 m; b=c=0.4a sill sill ” l = 2 km table 1 25 model ∆v (106 m3) ρs (kg/m 3) ∆m (1011 kg) mogi-1 155 2690 4.2 mogi-2 155 2907 3.8 mogi-3 155 3194 3.4 mogi-4 159 2523 4.0 mogi-5 159 3328 5.3 mogi-6 354 2033 7.2 mogi-7 159 2031 3.2 spheroid 174 3271 5.7 sill 125 410 0.5 table 2 26 baia 0 1 2 km n leveling route gravity stations quarto napoli nisida miseno m. nuovo solfatara pozzuoli (serapeo) la pietra fig. 1. 27 −300 −200 −100 0 ∆ g o b se rv e d (µ g a l) 0.0 0.5 1.0 1.5 u p li ft u z ( m ) −500 −400 −300 −200 −100 0 100 ∆ g f a (µ g a l) −300 −200 −100 0 100 200 300 ∆ g r (µ g a l) 1980 1985 1990 1995 2000 ∆g uz (b) (a) (c) year fig. 2. 28 ∆g v ∆g fa ∆g l p. u r θ ∆g s dv ∆g r = ∆g m + ∆g s ∆g m = ∆g l + ∆g v fig. 3. 29 ++ ++ ++ ++ 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 x = (x, y, z) x’ = (x’, y’, z’) 3 4 7 8 3 4 7 8 n ds1 ds2 (a)(a) (b) fig. 4. 30 fig. 5. 31 ∆ g (µ g a l) x (km) mogi-1 −500 −400 −300 −200 −100 0 100 0 5 10 15 ∆gfa ∆gl ∆gv ∆gm fig. 6. 32 ∆ g / u zm a x (µ g a l / m ) ∆ g / u zm a x (µ g a l / m ) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) ∆g v ∆gl ∆gm uz (a)spheroid ∆g v ∆gl ∆gm uz (b)sill fig. 7. 33 ∆ g / u zm a x (µ g a l / m ) ∆ g / u zm a x (µ g a l / m ) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) ∆g v ∆gl ∆gm uz (a)mogi-2 ∆g v ∆gl ∆gm uz (b)mogi-3 fig. 8. 34 ∆ g / u zm a x (µ g a l / m ) ∆ g / u zm a x (µ g a l / m ) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) ∆g v ∆gl ∆gm uz (a)mogi-4 ∆g v ∆gl ∆gm uz (b)mogi-5 fig. 9. 35 ∆ g / u zm a x (µ g a l / m ) ∆ g / u zm a x (µ g a l / m ) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) 0.0 0.5 1.0 1.5 2.0 2.5 u p li ft (m ) −150 −100 −50 0 50 100 150 0 5 10 x (km) ∆g v ∆gl ∆gm uz (a)mogi-6 ∆g v ∆gl ∆gm uz (b)mogi-7 fig. 10. 36 ρ s (k g /m 3 ) 0 100 200 300 400 ∆ v (106m3) mogi-1 mogi-4mogi-2 mogi-3 mogi-7 sill mogi-6 spheroidmogi-5 0 500 1000 1500 2000 2500 3000 3500 fig. 11. 37 layout 6 structural analysis of the eruptive fissures at mount etna (italy) marco neri1,*, valerio acocella2, boris behncke1, salvatore giammanco1, francesco mazzarini3, derek rust4 1 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy 2 università roma tre, dipartimento di scienze geologiche, rome, italy 3 istituto nazionale di geofisica e vulcanologia, sezione di pisa, pisa, italy 4 university of portsmouth, school of earth and environmental sciences, portsmouth, united kingdom annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5332 abstract mount etna produces frequent eruptions from its summit craters and from fissures on its flanks. the flank fissures trend approximately radially to the summit, and are mainly concentrated in three rift zones that are located on the ne, s and w flanks. many flank eruptions result from lateral magma transfer from the central conduit into fractures intersecting the flanks, although some eruptions are fed through newly formed conduits that are not directly linked to the central conduit. we analyzed the structural features of eruptions from 1900 to the present, one of the most active periods in the documented eruptive history of etna, which comprised 35 summit and 33 flank events. except for a small eruption on the w flank in 1974, all of the flank eruptions in this interval occurred on or near the ne and s rifts. eruptions in the ne sector were generally shorter, but their fissure systems developed more rapidly and were longer than those in the s sector. in contrast, summit eruptions had longer mean durations, but generally lower effusion rates (excluding paroxysmal events characterized by very high effusion rates that lasted only a few hours). this database was examined considering the main parameters (frequency and strike) of the eruptive fissures that were active over the last ~2 ka. the distribution in time and space of summit and flank eruptions appears to be closely linked to the dynamics of the unstable e to s flank sector of etna, which is undergoing periodic displacements induced by subvolcanic magma accumulation and gravitational pull. in this framework, magma accumulation below etna exerts pressure against the unbuttressed e and s flanks, which have moved away from the rest of the volcano. this has caused an extension to the detachment zones, and has facilitated magma transfer from the central conduit into the flanks. 1. introduction mount etna (italy) is one of the most active volcanoes on earth. although it is nearly continuous, the summit activity does not pose a significant threat to human life and property. on the contrary, flank eruptions represent the most dangerous type of eruptive activity, since these occur closer to vulnerable areas, such as towns, villages, lifelines, and cultivated land [crisci et al. 2010]. etna is located at the edge of the appenine-maghrebian chain [cristofolini et al. 1979, lanzafame et al. 1997], along the n-s trending extensional malta escarpment (figure 1, inset), and it lies on pliocene-pleistocene foredeep deposits [lentini 1982]. its tectonic setting is characterized by an overall e-w extension direction [bousquet and lanzafame 1986, monaco and tortorici 2000]. volcanism at etna evolved from sporadic, predominantly subaerial and submarine, activity along fissures (up to 200 ka ago), with the development of several vents that are dispersed over a wide area. finally (200 ka ago – today) there was the construction of the present-day stratovolcano [romano and sturiale 1982, corsaro et al. 2002, tanguy et al. 2003, branca et al. 2004]. central stratovolcanoes like etna are often characterized by summit and flank eruptions. summit eruptions are the consequence of the ascent of magma from a reservoir through a central conduit. flank eruptions are commonly characterized by multiple aligned vents that radiate from the summit of the volcano. most of the observed flank eruptions at etna originated from the central conduit: here, while the magma rises to often feed summit eruptions, at times it propagates laterally and downslope, to feed radial fissures and flank eruptions [i.e. rubin and pollard 1987]. sometimes, flank eruptions are not fed through the central conduit, but they are directly fed through new conduits from a reservoir beneath the volcano [acocella and neri 2003, and references therein]. in this report, we illustrate the modalities of the flank eruptions at etna over approximately the last 2000 years, with particular attention to the last 110 years of activity, for which a detailed structural database is shown and discussed. article history received november 26, 2010; accepted april 30, 2011. subject classification: eruptive fracture, dike, magmas, tectonics, structural geology. 464 special issue: v3-lava project the main goal is the construction of an interpretative model of the transfer of magma within the volcanic edifice, to define the main structural parameters, and possibly, the role of the magmatic intrusions in the framework of the instability of the volcano flanks. 2. types and frequency of mount etna eruptions mount etna is the highest active volcano in europe (at 3,329 m above sea level [asl] in summer 2007) [neri et al. 2008b], and it has an open central conduit that feeds four summit craters that are located close to the w rim of the valle del bove depression: voragine (vor; active since 1945 inside the former central crater), the ne crater (active since 1911), bocca nuova (bn; active since 1968), and the se crater (sec; active since 1971) (figure 1). the volcanic activity is mainly focused at the summit of the volcano, and it generates gas emissions, strombolian to paroxysmal (hawaiian, violent strombolian to sub-plinian) activity, and lava overflows (figure 2) from one of these summit craters, producing so-called "summit eruptions". at times, the volcanic activity occurs along radial fissures that form flank eruptions from three main "rift zones" [rittmann 1973, corazzato and tibaldi 2006]: the ne rift, the s rift and the w rift (figure 1). in these cases, magma moves upwards through the central conduit, and at shallow levels (a few hundreds of meters, up to 1-3 km) it propagates laterally, penetrating, in most cases, into the rift zones (figure 3a). thus, these eruptions are fed by the central conduit rather than directly from the magma reservoir [mcguire and pullen 1989, tanguy and kieffer 1993, bousquet and lanzafame 2001]. this type of eruption was named as "central-lateral" by acocella and neri [2003] neri et al. 465 figure 1.simplified structural map of mount etna (a), showing the distribution of etnean volcanics, eruptive fissures and major pyroclastic cones produced by flank eruptions, significant fault systems, and the sector of the volcano that is affected by flank instability-induced displacement. arrows indicate directions of movement for different portions of the mobile flank sector. inset at top: lidar-derived digital elevation model of the summit crater complex [neri et al. 2008b]. cc, central craters; pfs, pernicana fault system; rfs, ragalna fault system; tf, trecastagni fault; tmf, tremestieri fault. the pre‐quaternary sedimentary rocks include the amc and cgf deposits. the regional tectonic context is shown in inset (b); amc, appenine‐maghrebian chain; cgf, gela‐catania foredeep; hf, hyblean foreland; me, malta escarpment; cof, compressional front; ef, extensional front; the area outlined by the broken line is the area enlarged in (a). 466 the eruptive fissures of mt. etna figure 2.aerial view of a typical lava overflow from one of the etna summit craters, the bocca nuova, in october 1999. such overflows are directly fed through the central conduit system, which leads up to the four summit craters of etna. figure 3. conceptual model of the two types of flank eruptions at etna, central-lateral (a) versus eccentric (or peripheral) (b), as illustrated by the two phases of the 2002-2003 eruption. (a) lateral transfer of magma from the central conduit (the ne crater) into the ne rift leads to a typical central-lateral eruption, with numerous vents aligned along a downslope-propagating eruptive fissure system of more than 5 km long (october 27 – november 5, 2002). (b) eruptive activity on the upper s flank is fed through a new, eccentric conduit, and restricted to a relatively short fissure with fewer vents. the activity here is characterized by much more enhanced explosivity compared to the central-lateral activity on the ne rift (october 28, 2002 – january 28, 2003). both central-lateral and eccentric activities occurred simultaneously during the 2001 and 2002-2003 eruptions. and neri et al. [2005]. the same mechanism of intrusion has been commonly observed at hawaii [holcomb 1987, lockwood et al. 1987], stromboli [neri et al. 2008a], vesuvius [acocella et al. 2009, and references therein], nyiragongo [tedesco et al. 2007], miyakejima [geshi et al. 2002, 2010], izu-oshima [sumner 1998], taveuni [cronin et al. 2001], huaynaputina [adams et al. 2001], and piton de la fournaise [mcclelland et al. 1989, peltier et al. 2008]. a few flank eruptions have been triggered by magmatic intrusions that are not fed through the central conduit, as shown by any lack of continuity between the flank eruptions and the summit craters of the volcano (in terms of seismicity, surface deformation, and petro-chemical data). these eruptions are possibly directly fed by the reservoir beneath the volcano, and they are here named as "eccentric" or "peripheral" eruptions [rittman 1973, acocella and neri 2003] (figure 3b). this type of eruption was first described for lamongan volcano [kemmerling 1922] and has also been observed at miyakejima [mcclelland et al. 1989] and fernandina [jonsson et al. 1999] volcanoes. stress fields controlled by regional tectonics have been commonly invoked to explain the dynamics of the lateral flow of magma from reservoirs, and thus the occurrence of peripheral eruptions [gudmundsson 1987, 1998]. nevertheless, all of these mentioned peripheral eruptions, except for lamongan, occurred within volcanic edifices at least 2,000 m high. this suggests that as an alternative to a regional component, peripheral volcanic activity can also be influenced by a component of stress due to the load of the volcano [acocella and neri 2009]. while the summit activity is close to continuous at etna, flank eruptions (both central-lateral and eccentric) occur at irregular intervals, in a range of less than one year to several decades, and they appear to be organized in cycles. indeed, behncke and neri [2003] suggested that the cycles occurred on two main time scales: in the long-term, and in the shortterm. a long-term cycle lasts several centuries and consists of three phases: a first phase characterized by low levels of activity, mainly at the summit, and two subsequent phases during which flank eruptions become more frequent and more voluminous, especially during the third culminative phase [wadge et al. 1975 and guest and murray 1979]. conversely, short-term eruptive cycles last several decades. in the recent history of etna, short-term cycles have been common since 1865, and consist of three distinct phases: (a) a period of eruptive quiescence that lasts <3.5 years; (b) a period of more or less continuous summit activity that lasts from 6 to 16 years; and (c) a series of flank eruptions that often have intervening summit activity, and that extend from 9 to 22 years. the last eruptive cycle of etna began after the 1991-1993 flank eruption, and it has developed into 8 years of progressively more frequent summit activity, followed by a series of flank and summit eruptions. there have been four important flank eruptions since 2001 (figure 4) [allard et al. 2006, neri et al. 2009]. here we review the eruptive history of the etna volcano over the last 110 years, as organized into a database (table 1). furthermore, to expand the number of structural elements that can be analyzed as much as possible, we consider also the main parameters (frequency and strike) of the eruptive fissures up to ~2 ka bp that are still visible on the flanks of the volcano, or that have been covered by recent lava, but are neri et al. 467 figure 4.cumulative magma output at mount etna from 1900 to 2010, showing how higher mean output rates and larger individual events are linked to periods when there are clusters of flank eruptions (marked with gray shading). these periods are also marked by enhanced movement of the unstable flank sectors [see behncke and neri 2003, walter et al. 2005, allard et al. 2006, neri et al. 2004, neri et al. 2009, solaro et al. 2010]. dashed arrows indicate three periods (1908-1949, 1950-1970, 1071-2010) that were characterized by different outputs (numbers on the arrows give the average eruption rate of each period). 468 the eruptive fissures of mt. etna t ab le 1 .m ai n fe at u re s of t he e tn a er u pt io ns s in ce 1 90 0 (m od ifi ed fr om a co ce lla a nd n er i [ 20 03 ] a nd b eh nc ke e t al . [ 20 05 ]) . 1 29 /0 4/ 19 08 30 /0 4/ 19 08 0. 75 f v b 28 00 -2 27 5 42 50 13 5; 1 70 b oc ch e de l 1 90 8 4. 45 1 2. 00 0. 15 2. 15 33 .0 0 2 23 /0 3/ 19 10 18 /0 4/ 19 10 27 f s r ift 23 00 19 00 51 90 18 0; 2 25 ; 2 65 15 0. 10 14 10 m on ti r ic cò 11 .0 0 4. 92 65 .0 0 0. 20 65 .2 0 28 .0 0 3 27 /0 5/ 19 11 10 /0 7/ 19 11 45 s n e c ra te r 31 60 b ir th o f n e c , p re ce de d by c c a ct iv it y 4 10 /0 9/ 19 11 22 /0 9/ 19 11 13 f n e r ift 25 50 16 25 47 10 30 ; 4 5 15 0. 09 c om pl ex s ys te m o f er u pt iv e fr ac tu re s 8. 50 5. 54 55 .4 0 1. 62 57 .0 2 50 .8 0 5 24 /0 6/ 19 17 24 /0 6/ 19 17 < 1 s n e c ra te r st ro ng f ir efo u nt ai ni ng e pi so de a nd r he om or ph ic la va fl ow s 6 30 /1 1/ 19 18 01 /1 2/ 19 18 2 f n e r ift 31 00 20 25 17 80 18 0; 3 25 d ry fr ac tu re s on u pp er s fl an k 1. 40 0. 27 1. 22 1. 22 7. 00 7 17 /0 6/ 19 23 18 /0 7/ 19 23 31 f n e r ift 25 00 18 00 42 75 30 ; 4 0 25 .5 0. 05 m on te c or ba ra 11 .5 0 6, 48 78 .0 0 0. 50 78 .5 0 29 .0 0 8 02 /1 1/ 19 28 20 /1 1/ 19 28 18 f n e fl an k 27 00 12 00 48 85 70 56 0. 04 b oc ch e de l 1 92 8, d es tr u ct io n of m as ca li 9. 15 5 40 .0 0 2. 50 42 .5 0 27 .3 0 9 ?? /? ?/ 19 36 29 /0 6/ 19 42 s c c in te rm it te nt e xp lo si ve a nd in tr ac ra te r ef fu si ve ac ti vi ty fr om c c 10 30 /0 6/ 19 42 01 /0 7/ 19 42 0. 54 f sw fl an k 28 00 25 00 21 00 18 5; 2 35 5 0. 12 60 v en ts c lo se t o m . f ru m en to s u pi no 4. 10 0. 83 1. 66 0. 10 1. 76 37 .6 0 11 30 /0 6/ 19 42 05 /0 7/ 19 42 6 s c c p ow er fu l e xp lo si ve a ct iv it y fr om c c 12 ?? /1 1/ 19 45 ?? /1 1/ 19 45 s c c b ir th o f v o r w it hi n c c 13 05 /0 2/ 19 47 23 /0 2/ 19 47 18 s c c ; n e c ra te r se ve ra l l av a fo u nt ai ns 14 24 /0 2/ 19 47 10 /0 3/ 19 47 15 f n e r ift 30 50 22 00 52 00 35 5; 0 ; 2 0; 4 0 18 0. 08 15 0 n o ap pr ec ia bl e se is m ic it y 6. 60 1. 98 11 .8 8 0. 03 11 .9 1 9. 20 15 02 /1 2/ 19 49 05 /1 2/ 19 49 3 s+ f su m m it a re a; n w fl an k 32 40 19 90 30 95 21 0; 3 40 ; 3 30 13 0. 07 b i-r ad ia l e ru pt io n 4. 60 2. 87 10 .0 0 0. 20 10 .2 0 39 .3 0 16 25 /1 1/ 19 50 01 /1 2/ 19 51 37 2 f v b 28 50 -2 20 0 18 50 90 2 0. 26 40 0 m on te g ae ta no p la ta ni a 9. 70 10 .8 8 15 1. 36 0. 63 15 1. 99 4. 73 17 18 /0 7/ 19 55 02 /1 2/ 19 55 13 6 s n e c ra te r 32 00 l av a ov er flo w p re ce de d by in tr ac ra te r la va e ffu si on a nd c on e gr ow th s in ce 8 ju ly 2. 4 1. 45 3. 63 0. 60 4. 23 3. 60 18 28 /0 2/ 19 56 02 /0 3/ 19 56 4 s+ f c c ; v b 33 00 27 00 15 6 13 0 l av a ov er flo w s fr om c c a nd s u bt er m in al f is su re in u pp er v db 2. 6 0. 31 0. 90 0. 21 1. 11 3. 30 19 02 /0 4/ 19 56 08 /0 4/ 19 56 6 s c c l av a ov er flo w s fr om c c 20 16 /0 4/ 19 57 03 /0 5/ 19 57 18 s n e c ra te r l av a ov er flo w s fr om n e c 21 ?? /0 8/ 19 57 ?? /0 7/ 19 60 ~ 10 60 s n e c ra te r g ro w th o f an e xt en si ve la va fl ow -f ie ld t o n an d n e o f n e c 22 17 /0 7/ 19 60 05 /0 8/ 19 60 19 s c c t hr ee e pi so de s of s tr on g ex pl os iv e ac ti vi ty fr om c c 23 30 /1 2/ 19 60 ?? /0 1/ 19 64 ~ 11 00 s c c ; n e c ra te r g ro w th o f an e xt en si ve la va fl ow -f ie ld t o n an d n e o f n e c ; s tr on g ex pl os iv e ac ti vi ty w it h la va o ve rf lo w s fr om c c 1 213 m ay 1 96 1 24 01 /0 2/ 19 64 05 /0 7/ 19 64 15 5 s+ f c c ; v b 32 00 29 00 28 95 in te ns e, o ft en e pi so di c ex pl os iv e ac ti vi ty a nd la va o ve rf lo w s fr om c c ; l av a ef fu si on fr om fi ss u re o n e s id e of c c in f eb ru ar y 3. 8 1. 84 18 .9 0 5. 00 23 .9 0 1. 78 n˚ on se t dd /m m /a aa a en d dd /m m /a aa a du ra ti on (d ay s) ty pe o f er u pt io n lo ca ti on m ax -m in al ti tu de (m a .s .l. ) le ng th (m ) az im u th (˚ ) du ra ti on o f pr op ag at io n (h ) m ea n sp ee d of pr op ag at io n (m /s ) m ax w id th o f th e fr ac tu re fi el d (m ) no te s an d/ or n ew a pp ar at u s m ax le ng th (k m ) ar ea (1 06 m 2 ) vo lu m e (1 06 m 3 ) vo lu m e (1 06 m 3 ) to ta l vo lu m e (1 06 m 3 ) m ea n er up tio n ra te (m 3 s − 1 ) ve nt la va fl ow s te ph ra neri et al. 469 t ab le 1 (c on ti nu ed fr om p re vi ou s pa ge ). n˚ on se t dd /m m /a aa a en d dd /m m /a aa a du ra ti on (d ay s) ty pe o f er u pt io n lo ca ti on m ax -m in al ti tu de (m a .s .l. ) le ng th (m ) az im u th (˚ ) du ra ti on o f pr op ag at io n (h ) m ea n sp ee d of pr op ag at io n (m /s ) m ax w id th o f th e fr ac tu re fi el d (m ) no te s an d/ or n ew a pp ar at u s m ax le ng th (k m ) ar ea (1 06 m 2 ) vo lu m e (1 06 m 3 ) vo lu m e (1 06 m 3 ) to ta l vo lu m e (1 06 m 3 ) m ea n er up tio n ra te (m 3 s − 1 ) ve nt la va fl ow s te ph ra 25 13 /0 1/ 19 66 01 /0 4/ 19 71 ~ 19 00 s+ f n e c ra te r 32 50 30 70 41 5 0; 9 0 g ro w th o f an e xt en si ve la va fl ow -f ie ld t o n an d n e o f n e c 2. 5 2 3. 80 3. 80 0. 12 26 07 /0 1/ 19 68 04 /0 5/ 19 68 11 9 s+ f v b 30 50 26 00 13 0; 7 0 su bt er m in al e ff u si ve a ct iv it y pr ob ab ly r el at ed to p er si st en t n e c a ct iv it y; b ir th o f b n t o th e w o f v o r o n 9 ju ne 1 96 8 2. 4 0. 4 1. 00 1. 00 0. 09 27 05 /0 4/ 19 71 12 /0 6/ 19 71 69 s+ f se c ra te r; v b ; s c 30 50 18 00 51 72 65 ; 8 0; 1 45 ; 17 0 11 0 0. 01 50 0 m t. p on te ; b ir th o f se c 7. 25 7. 6 45 .2 0 2. 00 47 .2 0 7. 92 28 30 /0 1/ 19 74 17 /0 2/ 19 74 17 e w r ift 16 70 27 0 m on te d e fi or e i 1. 5 0. 3 2. 40 2. 03 4. 43 3. 00 29 11 /0 3/ 19 74 29 /0 3/ 19 74 18 e w r ift 16 50 27 0 m on te d e fi or e i 1. 3 0. 2 2. 10 1. 08 3. 18 1. 00 30 10 /1 0/ 19 74 25 /0 2/ 19 75 13 8 s n e c ra te r 32 00 in tr ac ra te r ex pl os iv e ac ti vi ty b eg in s 29 s ep te m be r 19 74 , f ol lo w ed b y op en in g of f is su re on o u te r fla nk o f n e c ; l av a flo w s in d iff er en t di re ct io ns 1. 81 10 .9 0 1. 00 11 .9 0 1. 00 31 24 /0 2/ 19 75 29 /0 8/ 19 75 18 7 f n e r ift 26 25 20 ; 3 0 g ro w th o f se ve ra l h or ni to s 1. 75 1. 07 6. 00 6. 00 0. 37 32 12 /0 9/ 19 75 28 /1 1/ 19 75 77 s n e c ra te r 32 00 l av a em is si on fr om fl an ks o f n e c c on e 6. 70 6. 70 1. 00 33 29 /1 1/ 19 75 08 /0 1/ 19 77 40 6 f n e c ra te r; n e r ift : p u nt a l u ci a 31 00 28 50 11 20 34 0 m on te c u m in 8. 00 4. 83 29 .4 0 29 .4 0 1. 00 34 16 /0 7/ 19 77 27 /0 3/ 19 78 25 4 s n e c ra te r 32 00 ~ 20 la va fo u nt ai ns a cc om pa ni ed b y vo lu m ino u s la va e m is si on ; s ig ni fi ca nt g ro w th o f n e c c on e 7 3. 33 9. 66 5. 00 14 .6 6 35 29 /0 4/ 19 78 05 /0 6/ 19 78 37 f se c ra te r; v b 30 00 26 00 18 65 14 0; 1 50 20 0. 03 fi rs t m ag m at ic e ru pt io n of s e c 4. 00 2. 3 27 .5 0 27 .5 0 8. 60 36 25 /0 8/ 19 78 30 /0 8/ 19 78 5 s+ f se c ra te r; v b 30 50 23 50 18 65 50 ; 1 20 48 0. 01 a cc om pa ni ed b y la va fo u nt ai ns a t se c 2. 70 1 4. 00 4. 00 7. 70 37 23 /1 1/ 19 78 30 /1 1/ 19 78 8 f se c ra te r; v b 30 50 16 75 26 00 11 0; 1 60 48 0. 02 p re ce de d by s tr om bo lia n ac ti vi ty a t se c fr om 1 8 n ov em be r 4. 70 3. 4 11 .0 0 11 .0 0 10 .6 0 38 03 /0 8/ 19 79 09 /0 8/ 19 79 6 f se c ra te r; v b , s c 29 50 17 00 38 25 13 0; 1 00 ; 6 5 72 0. 01 25 0 p re ce de d by s tr om bo lia n ac ti vi ty a t se c a nd v o r fr om 1 6 ju ly t o 3 a u gu st ; c om pl ex e ru pti ve f is su re s ys te m s an d st ro ng e xp lo si ve a ct ivi ty a t se c 6. 00 2. 5 7. 50 7. 50 14 .5 0 39 01 /0 9/ 19 80 26 /0 9/ 19 80 25 s n e c ra te r 32 00 t hr ee la va fo u nt ai ns fr om n e c 4 0. 33 0. 99 0. 99 40 05 /0 2/ 19 81 07 /0 2/ 19 81 2 s n e c ra te r 32 00 l av a fo u nt ai ns a nd o ve rf lo w s fr om n e c ; p re ce de d by e ar th qu ak e an d gr ou nd fr ac tu ri ng at t he p fs o n 8 ja nu ar y 2 0. 1 0. 30 0. 30 41 17 /0 3/ 19 81 23 /0 3/ 19 81 6 f n w fl an k 25 50 11 40 80 45 35 5; 3 30 ; 3 20 33 0. 07 pe ak e ff u si on r at es o f 30 0 m 3 s1 7. 50 7. 4 18 .0 0 3. 30 21 .3 0 34 .7 2 42 28 /0 3/ 19 83 06 /0 8/ 19 83 13 1 f s r ift ; p r 26 80 22 50 25 80 18 5; 2 00 12 0. 06 c iv il d ef en ce c ar ri es o u t bl as ti ng a nd c on st ru ct io n of b ar ri er s to d iv er t la va fl ow 7. 50 6 79 .0 0 0. 02 79 .0 2 86 .9 8 43 28 /0 4/ 19 84 17 /1 0/ 19 84 17 2 s se c ra te r 30 50 c on st ru ct io n of a c on e w it hi n se c ; a ct iv it y ac co m pa ni ed a nd fo llo w ed b y in te ns e se is m ic ac ti vi ty o n e tn a' s fla nk s 3 1. 3 10 .0 0 0. 22 10 .2 2 0. 70 44 10 /0 3/ 19 85 13 /0 7/ 19 85 12 5 f se c ra te r, s r ift : p r 30 50 -2 48 0 10 95 19 0; 1 95 6 0. 05 e ru pt io n be g in s w it h la va fo u nt ai n an d ov er flo w fr om s e c 3. 80 2. 7 30 .0 0 0. 03 30 .0 3 2. 80 45 25 /1 2/ 19 85 31 /1 2/ 19 85 5 f v b 27 50 24 20 94 0 10 5; 1 10 50 0. 01 18 0 e ru pt io n pr ed ed ed b y st ro ng e ar th qu ak e an d gr ou nd fr ac tu ri ng a t p fs c au si ng d es tr u ct io n an d on e de at h at p ia no p ro ve nz an a 3. 50 0. 9 0. 80 0. 09 0. 89 2. 10 46 13 /0 9/ 19 86 24 /0 9/ 19 86 11 s n e c ra te r 32 40 st ro m bo lia n ac ti vi ty a nd la va o ve rf lo w fo llo w ed b y a vi ol en t ex pl os iv e ep is od e 1. 3 0. 16 0. 40 7. 00 7. 40 7. 79 470 the eruptive fissures of mt. etna t ab le 1 (c on ti nu ed fr om p re vi ou s pa ge ). n˚ on se t dd /m m /a aa a en d dd /m m /a aa a du ra ti on (d ay s) ty pe o f er u pt io n lo ca ti on m ax -m in al ti tu de (m a .s .l. ) le ng th (m ) az im u th (˚ ) du ra ti on o f pr op ag at io n (h ) m ea n sp ee d of pr op ag at io n (m /s ) m ax w id th o f th e fr ac tu re fi el d (m ) no te s an d/ or n ew a pp ar at u s m ax le ng th (k m ) ar ea (1 06 m 2 ) vo lu m e (1 06 m 3 ) vo lu m e (1 06 m 3 ) to ta l vo lu m e (1 06 m 3 ) m ea n er up tio n ra te (m 3 s − 1 ) ve nt la va fl ow s te ph ra 47 30 /1 0/ 19 86 01 /0 3/ 19 87 12 2 f se c ra te r; v b 30 50 21 80 25 75 75 ; 9 0 12 0. 06 30 0 m on te r it tm an n, p re ce de d by o ne d ay b y st ro ng e ar th qu ak e an d gr ou nd fr ac tu ri ng a t p fs ; l av a fo u nt ai n at s e c o n 31 o ct ob er 5. 00 6. 1 60 .0 0 60 .0 0 5. 70 48 ?? /0 5/ 19 89 10 /0 9/ 19 89 ~ 12 0 s v or ag in e l av a fo u nt ai ns a t v o r ( 4 m ay , 2 9 a u gu st , 4 an d 10 s ep te m be r) 49 11 /0 9/ 19 89 27 /0 9/ 19 89 16 s se c ra te r 30 50 14 la va fo u nt ai ns a t se c 3. 00 3. 75 12 .2 0 12 .2 0 12 7. 27 50 27 /0 9/ 19 89 09 /1 0/ 19 89 12 f v l 30 50 25 50 87 40 65 ; 1 55 19 2 0. 01 60 0 d ry fr ac tu re s ys te m d ev el op s fr om s e c o ve r 6. 3 km t ow ar d se ( p ia no d el v es co vo a re a) 8. 00 2. 56 26 .2 0 26 .2 0 24 .3 0 51 04 /0 1/ 19 90 02 /0 2/ 19 90 29 s se c ra te r 30 50 fo u r la va fo u nt ai ns fr om s e c 3 0. 73 2. 15 2. 15 52 14 /1 2/ 19 91 31 /0 3/ 19 93 47 3 f se c ra te r; v l 31 00 -2 40 0 28 65 15 0; 1 65 22 0. 04 50 0 l av a flo w t hr ea te ni ng z af fe ra na is d iv er te d by e xp lo si ve s in m ay 1 99 2 8. 75 7. 65 23 5. 00 23 5. 00 5. 75 53 30 /0 7/ 19 95 30 /0 8/ 19 96 39 5 s n e c ra te r 32 00 in cl u de s 10 la va fo u nt ai ns 1. 3 1. 40 5. 00 6. 40 0. 19 54 06 /1 1/ 19 96 26 /0 7/ 19 98 62 7 s se c ra te r 31 00 pe rs is te nt m ild s tr om bo lia nef fu si ve a ct iv it y 0. 74 0. 25 1. 50 1. 50 0. 02 55 15 /0 7/ 19 97 15 /1 0/ 19 99 82 2 s v or ag in e 33 00 in cl u de s se ve ra l l av a fo u nt ai ns a nd 2 s u bpl ini an e ru pt io ns 1. 07 0. 31 2. 39 4. 47 6. 86 56 15 /0 9/ 19 98 04 /0 2/ 19 99 14 2 s se c ra te r 31 00 23 la va fo u nt ai ns 1 0. 39 1. 55 2. 86 4. 41 19 .2 4 57 04 /0 2/ 19 99 14 /1 1/ 19 99 28 2 s+ f se c ra te r 32 00 29 20 40 0 15 0; 1 10 2 0. 06 40 in cl u de s 1 la va fo u nt ai n 2. 8 1. 45 24 .3 7 24 .3 7 1. 00 58 05 /1 0/ 19 99 05 /1 1/ 19 99 31 s b oc ca n u ov a 33 00 in cl u de s 14 la va fo u nt ai ns 4. 1 3 16 .5 0 16 .5 0 6. 16 59 26 /0 1/ 20 00 28 /0 8/ 20 00 21 5 s se c ra te r 31 00 66 la va fo u nt ai ns 3. 2 2. 9 37 .1 0 9. 66 46 .7 6 83 .0 0 60 20 /0 1/ 20 01 17 /0 7/ 20 01 17 8 s se c ra te r 31 00 in cl u de s 16 la va fo u nt ai ns 2. 4 1. 12 9. 20 2. 20 11 .4 0 13 .4 0 61 17 /0 7/ 20 01 08 /0 8/ 20 01 23 f se c ra te r, v l 30 00 -2 60 0 47 00 17 0; 6 0; 4 0 79 0. 02 50 0 c en tr al -la te ra l p or ti on o f 20 01 e ru pt io n 3. 8 2. 18 6. 16 0. 30 6. 46 3. 10 62 18 /0 7/ 20 01 09 /0 8/ 20 01 23 e s r ift 25 70 -2 10 0 21 00 0 41 0. 01 50 0 p ia no d el l ag o co ne ; e cc en tr ic p or ti on o f 20 01 e ru pt io n 6. 9 3. 2 19 .1 5 7. 50 26 .6 5 13 .4 1 63 27 /1 0/ 20 02 05 /1 1/ 20 02 9 f n e r ift 30 10 -1 89 0 40 00 20 ; 6 5 27 0. 04 13 00 c en tr al -la te ra l p or ti on o f 20 02 -2 00 3 er u pt io n 6. 4 2 10 .0 0 1. 00 11 .0 0 3. 22 64 27 /1 0/ 20 02 28 /0 1/ 20 03 94 e s r ift 28 50 -2 60 0 10 00 18 0 4 0. 07 15 0 e cc en tr ic p or ti on o f 20 02 -2 00 3 er u pt io n; t w o la rg e py ro cl as ti c co ne s 4. 5 2. 64 29 .5 0 45 .0 0 74 .5 0 9. 17 65 07 /0 9/ 20 04 08 /0 3/ 20 05 18 2 f v b 30 00 -2 35 0 18 40 12 5 14 7 0. 00 3 10 0 a bs en ce o f pr ec u rs or y ph en om en a 4. 7 2. 34 40 .0 0 40 .0 0 2. 50 66 14 /0 7/ 20 06 24 /0 7/ 20 06 10 s+ f se c ra te r, v b 30 60 -2 94 0 15 5 12 0 2 0. 02 50 in cl u de s 1 la va fo u nt ai n 3. 5 0. 7 3. 50 0. 20 3. 70 4. 28 67 31 /0 8/ 20 06 14 /1 2/ 20 06 10 6 s+ f se c ra te r, v b 32 80 -2 80 0 10 00 11 0; 1 75 in cl u de s se ve ra l l av a fo u nt ai ns 1. 6 2. 19 34 .5 0 34 .5 0 3. 77 68 26 /1 0/ 20 06 25 /1 1/ 20 06 30 s+ f se c ra te r 32 80 -3 09 0 50 0 20 0; 2 25 ; 2 50 3 0. 05 11 0 g ro w th o f an h or ni to 2. 94 0. 87 4. 50 0. 01 4. 51 1. 74 69 29 /0 3/ 20 07 24 /1 1/ 20 07 24 0 s se c ra te r 32 00 in cl u de s 6 la va fo u nt ai ns 4. 7 3. 41 8. 53 8. 53 70 10 /0 5/ 20 08 10 /0 5/ 20 08 1 s se c ra te r 32 00 v io le nt la va fo u nt ai n 6. 2 1. 9 4. 75 4. 75 71 13 /0 5/ 20 08 06 /0 7/ 20 09 41 7 f v l ; v b 30 50 -2 62 0 38 00 14 0; 1 20 ; 0 5 0. 21 80 0 d ry fr ac tu re s ys te m d ev el op s fr om th e er up tiv e fi ss ur e ov er 2 .5 k m to w ar d n (n e r ift a re a) 6. 1 5. 5 77 .0 0 0. 50 77 .5 0 2. 15 n ot e: p r = p ic co lo r if u gi o; s c = s er ra d el le c on ca zz e; v b = v al le d el b ov e; v l = v al le d el l eo ne ; f = f la nk ; e = e cc en tr ic ; s = s u m m it . a .s .l. = a bo ve s ea le ve l; c c = c en tr al c ra te rs ; v o r = v or ag in e; b n = b oc ca n u ov a; n e c = n e c ra te r; s e c = s e c ra te r; p fs = p er ni ca na f au lt s ys te m . neri et al. known from the literature [romano and sturiale 1982, behncke et al. 2005]. all of these eruptive fissures are mapped together to improve our understanding of the dynamics of the etna summit and flank eruptions, to better evaluate the roles of tectonics and topography. possible links with the structural instability that affects the volcano flanks are also analyzed. 3. the eruptive fissures in the present study, we consider information on 330 eruptive fissures tha occurred over approximately the last 2 ka. the reliability of the sources on past eruptions at etna can be considered adequate only from the beginning of the 17th century [behncke et al. 2005, branca and del carlo 2005]. fissures that formed before 1600 are usually quite uncertain figure 5.historically active eruption fissures at etna, grouped into periods: pre-1600 (a), 17th and 18th centuries (b), 19th century (c), 1900-2010 (d), and all of the fissures together (e). 471 472 the eruptive fissures of mt. etna in age. therefore, here we refer to the pre-1600 fissures as a single group (figure 5a). this group includes 200 fissures that have a largely s-ward (43%), ne-ward (23%) and w-ward (20.5%) direction, which is mainly along the rifts. these fissures are in general poorly known, and they have been studied previously mainly through morpho-structural analysis using light detection and ranging (lidar) [neri et al. 2008b, favalli et al. 2009], detailed topographic mapping, and previous geological studies [romano and sturiale 1982]. many of the fissures are only partly cropping out, as they have been, to lesser or greater degrees, buried by more recent eruptive products (in particular the fissures above 2,000 m asl) or by vegetation. for this reason, we consider only the frequency and the strike of these pre-1600 fissures, knowing that a significant part of their information might be lacking. for the period between 1600-2010, we have identified a total of 76 eruptive events in the summit area, and 68 on the flanks. the first part of this period (1600-1669) was caracterized by a very high average output rate (~1.4 m3 s−1) [behncke and neri 2003, behncke et al. 2005], and in one occasion (in 1669) by a devasting eruption that partially destroyed the town of catania and numerous villages located on the s flank of the volcano [crisci et al. 2003]. for a century after 1669, the eruption frequency dramatically dropped, and then it started to rise again in the second half of the 18th century. a clear increase in the eruption frequency and output rate has occurred since 1950 (0.71 m3 s−1 on the average output in the period 1050-1970), and especially since the late 1970s (1.26 m3 s−1, one eruption every 1.5 years on average; see also figure 4). analysing the dataset by centuries, we observe that seven summit and 11 flank events occurred during the 17th century, and 14 summit and eight flank events during the 18th century (figure 5b); 13 summit and flank events occurred during the 19th century (figure 5c), and 32 summit and 27 flank events occurred during the 20th century (figure 5d). during the first decade of the 21st century, we must add other summit events (in 2000-2001, 2006, 2007-2008), three summit eruptions which occasionally fed eruptive fissures close to the summit area (2006, 2007), and six flank events between 2001 and 2010 (figure 5). for the 1900-2010 period, the dataset allows us to construct a detailed chronology and structural description of the eruptive fissures, which is shown in table 1. the total time that etna was in eruption is ~13,300 days (36% of the analyzed time); 71 eruptions (31 of which were summit eruptions) produced 81 eruptive fissures on the volcano flanks. indeed, in some cases, the same eruption produced two or more eruptive systems with different orientations, such as the 2001 and 2002-2003 eruptions. these both consisted of a central-lateral event located near the se crater, in the valle del leone (in 2001), and on the ne rift (20022003), as well as eccentric events on the s flank [neri et al. 2004]. the independence of the eccentric portions of these eruptions from the central conduit was suggested by the distribution of the seismic activity [acocella et al. 2003, neri et al. 2005, walter et al. 2005], by the composition of the eruptive products [corsaro et al. 2007], and by the geometry and timing of the opening of the eruptive fractures [billi et al. 2003, lanzafame et al. 2003]. with the exception of the twin 1974 eccentric eruptions in the w rift zone [corsaro et al. 2009], the other flank eruptive systems were equally distributed in the n (19 fissures) and s (19) sectors. the flank eruptions lasted between less than one day (e.g., 1908 and 1942) and 473 days (in 1991-1993), with a median of 23 days. the velocity of propagation of the fractures was up to ~1 ms−1 (during the initial phase of the fracturing sequence), similar to that inferred for iceland [gudmundsson 1995], with mean values between 0.26 ms−1 and 0.01 ms−1; the time required to develop a steady deformation pattern is of the order of a few tens of hours (median, 20 h), up to a few hundreds of hours (tables 1 and 2). in the n sector, the eruptions did not last quite as long table 2. median volcanological and structural parameters of etna eruptions, displayed for all of the post-1900 eruptions, summit and flank eruptions, and for each of the n, s and w sectors. type of eruption and sectors duration (days) max altitude (m a.s.l.) min altitude (m a.s.l.) length (m) duration of propagation (h) mean speed of propagation (m/s) width of the fracture field (m) max length (km) area (106m2) volume (106m3) volume (106m3) total volume (106m3) eruption rate (m3s−1) all eruptions 30.5 4 2 10 1 11 5.7 summit eruption 77 3 1.3 4.8 3.7 6.9 4.9 all 23 3030 2375 2338 20 0.05 300 4.6 2.3 18.5 0.5 22.6 5.7 n sector 21 3010 2200 3825 22.8 0.053 450 6.1 3 18 1 21 7 s sector 30 3050 2420 1840 15 0.04 150 4.1 2 19 0 24 5.8 w sector 18 1660 1.4 0.3 2.3 1.6 3.8 2fl an k er u pt io ns vents lava flows tephra note: a.s.l. above sea level. neri et al. 473 (median, 21 days), developed faster (0.053 ms−1), and produced longer (3,825 m) fissures. in the s sector, the eruptions were frequently longer in duration (median, 30 days), but they were characterized by more slowly propagating (0.04 ms−1) and shorter (1,840 m) fissures. conversely, the summit eruptions were usually longer (median, 77 day) and had moderate eruption rates (<5 m3 s−1), although they were also frequently characterized by violent paroxysms that lasted for a few tens of minutes at very high effusive rates (50−>200 m3 s−1) [see behncke et al. 2005, 2006]. several correlations have been derived from the data given in table 1, and these are shown in figure 6. there is a positive correlation between the volume of the erupted products and the duration of the eruptions (figure 6a), in a similar fashion to the positive correlation between the erupted volumes and the length of the lava flows (figure 6i), as well as the area covered by the lava flows (figure 6l). in some cases, there is a moderate correlation (r2 = 0.56) between the length of the eruptive fissures and the maximum width of the fracture field (figure 6b). moreover, the longer the eruptive fissures, the longer the related lava flows are (figure 6c), and the higher the eruption rates are (figure 6g). it is also evident that higher eruption rates characterize shorter eruptions (figure 6d). correlations between the velocities of the propagation of the fissures and the remaining parameters considered in table 1 appear less-well defined. only the erupted volumes can somehow be correlated to this velocity, which suggests that the largest eruptions were produced by rapid propagation of the eruptive fissures. 4. discussion the eruptions over the last century show a common trend of development: initially strombolian activity from the summit craters that lasts for several months and is often punctuated by brief, violent paroxysms; this activity precedes, and occasionally accompanies, the flank eruptions. these last occur through fractures associated with the lateral propagation of radial dikes from the summit of the volcano (i.e. the central craters). thus, very often, radial dikes transfer magma from the central conduit (and from the summit craters) towards the lower portions of the volcano, along sub-horizontal flow paths [murray and pullen 1984, ferrari et al. 1991, bousquet and lanzafame 2001, acocella et al. 2009]. summit activity typically ends shortly after the beginning of such flank eruptions. during dike propagation, the magmatic activity and the related eruptive fracture field migrate downslope. this migration generally lasts less than two days, at speeds of the order of ~0.1-1 ms−1 (except in very rare cases like in 2004, when the propagation lasted ~6 days) [neri and acocella 2006], after which the fracture field is complete and the eruptive vents are stabilized in the lower portion. the fracture field consists of tension fractures and normal faults, with vertical displacements of <2-3 m; the cumulative extension along each fissure, as measured directly in the field, is usually <6 m. the faults are typically arranged in grabens, with a width between 10 m and 1,400 m [acocella and neri 2003, billi et al. 2003, neri and acocella 2006]. the formation of tension fractures or normal faults at the surface usually depends upon the depth of the dike [bousquet and lanzafame 2001]. a shallow dike, under lower lithostatic pressures, can induce tension fractures and minor grabens (up to ~500 m wide). conversely, a deeper dike, at higher confinement pressures, can form shear fractures [pollard et al. 1983], and thus grabens at the surface, which might become strongly asymmetric if the fractures propagate along dip scarps (as happened in the valle del leone area during the 2001 eruption) [billi et al. 2003]. above 2,000 m asl, the eruptive fissures have a scattered orientation, although they also show a preferential n-s trend, especially close to the summit crater area. at lower altitudes, so below 2,000 m asl, the fissures mainly propagate towards the ne, s-se and w, which coincides with the rift zones of the volcano (figure 1). this structural arrangement is more evident when looking at the distribution of the older eruptive fissures (pre-1600; figure 5a), which were widely concentrated along the rift zones and (partly) aligned accordingly to several faults (figure 1). the fissures propagating towards the ne are mainly concentrated in the homonymous ne rift, which consists a network of n-to-ne-striking, closely spaced, subparallel eruptive fractures that can be traced down to the mount rosso area, at 1,500 m asl. the width of the rift is 0.5 km, and its length exceeds 7 km (figure 1). a secondary zone that is affected by less numerous, but concentrated, ne-ene figure 6 (next page). comparisons of different parameters for eruptions and eruptive fissures, updated and extended from acocella and neri [2003] and behncke et al. [2005]. (a) eruption volume versus duration, showing generally higher lava volumes for longer eruptions, although there are wide variations, especially for eruptions lasting <1 yr. (b) fracture field width versus length, indicating that longer fracture fields tend to also be wider. (c) length of the lava flow-field versus length of the eruptive fissure system, showing that longer fissures produce longer flows. (d) eruption duration versus mean eruption rate, where it is clear that shorter eruptions have higher mean eruption rates. (e) eruptive fracture length versus altitude, indicating that fissures tend to be longer on the lower flanks. (f) mean eruption rate versus elevation of the lowest vents, showing that eruption rates tend to be higher from low-lying vents and fissures. (g) length of eruptive fissure systems versus eruption rates, where a broadly positive correlation is evident. (h) eruptive fissure propagation speed versus total erupted volume, indicating that larger volumes are produced from fast-propagating fissure systems. (i) maximum length of the lava flow-field versus total erupted volume, showing that in most cases longer lava flow-fields are also more voluminous. (j) lava flow-field area versus total volume are clearly positively correlated. the eruptive fissures of mt. etna 474 figure 6 (see previous page). neri et al. 475 aligned eruptive fissures is located a couple of kilometers to the e of the ne rift, where the vents reach minimum altitudes of about 850 m asl (figure 1). all these vents are associated with fractures with left lateral components of motion [neri et al. 2004]. the ne-ene aligned fissures represent a secondary rift zone, which indicates a buried laterally-propagated dike system that filled a buried graben. the magmatic and tectonic activities of this secondary rift zone can be related to the development of an e-dipping rotational detachment that is confined by the ne rift and the pernicana fault [ruch et al. 2010]. the s sector of etna hosts the s rift, which occupies a wider (~10 km) and longer (>16 km) area, with respect to thene rift. here, the eruptive vents propagate mainly to the s (above 1,300 m asl; see rose diagram in figure 5e) and to the se (below 1,300 m), and they are associated with fractures with right-lateral motion [acocella and neri 2003]. these latter often strike parallel (or close to parallel) to several faults that cut the lower se flank of the volcano (figure 1, tf and tmf). these faults are frequently invoked as margins of blocks collapsing in slow motion towards the ionian sea [falsaperla et al. 2010, solaro et al. 2010, and references therein], which suggests the existence of a link between these eruptive fractures and the local tectonic framework. more in general, the kinematics characterizing the eruptive fractures cropping out on the n and s flanks is consistent with that of the pernicana fault system (to the ne), the tardaria-mascalucia fault system (to the se), and the ragalna fault system (to the sw), confining the mobile e and s sectors of the volcano (figure 1) [rust et al. 2005, solaro et al. 2010, and references therein]. these observations suggest a lack of shallow magma storage beneath the ne and s rifts [mcguire and pullen 1989]. moreover, most dykes that crop out in the valle del bove (figure 1) show sub-horizontal flow directions [ferrari et al. 1991, acocella et al. 2009]. this leads to the proposal of a similar lateral mechanism of magma emplacement during the last approximately 45 ka, the age of the current main feeding system [branca et al. 2004]. etnean rifts appear to be influenced by the regional tectonic stress field. kieffer [1985] suggested that the rifts are the w boundary of a fault system that separates the sicilian continental crust and the ionian oceanic crust. mcguire and pullen [1989] argued that only the ne rift is related to the regional stress field, because it is parallel to the maximum compressive stress. borgia et al. [1992] and lo giudice and rasà [1992] interpreted the ne and s rifts as disjunction zones that separate sectors of the volcano that are characterised by different kinematics. in this context, the magma fills the rift zones because of the collapse of the portion of the volcano that is delimited by the pernicana fault system to the n, and by the ragalna fault system to the sw (figure 1). even though the ne and s rifts may have a link with the spreading phenomena affecting the e to s flanks of etna [see acocella et al. 2003], the role of the w rift remains unclear. the seismicity of this area is often deeper (>5-10 km) if compared to that of the e flank [patanè et al. 2002, mattia et al. 2007]. in several cases, earthquakes in this sector heralded and marked the rise of magmatic intrusions, as in 1974 [corsaro et al. 2009], 1991 [ferrucci and patanè 1993], and 1998-1999 [bonaccorso and patanè 2001, patanè et al. 2003]. this suggests that for the w flank the possible existence of a storage area under the w rift, or a preferential path used by magmas to reach the surface. as noted by several studies [wadge et al. 1975, behncke and neri 2003, allard et al. 2006], the activity of mount etna (in terms of output rate and flank eruption frequency) undergoes significant fluctuations over time. these fluctuations do not occur in a random manner, but represent various stages of the eruptive cycles on a scale of decades and centuries [behncke and neri 2003]. as shown in section 2, short-term cycles last 20-40 years and consist of three distinct phases: (i) a period of rest; (ii) a period of summit activity; and (iii) a series of flank eruptions that occur at intervals of a few years, with frequent summit activity during the intervals between the flank eruptions. the timing, frequency, and clustering of the flank eruptions appear to be related to the instability of the e to s sector of etna, which is expressed in episodic slippage of portions of the unstable sector. during such events, displacement on a scale varying from centimeters to meters can affect one or more distinct slide blocks [solaro et al. 2010, and reference therein], and this is commonly accompanied by shallow seismicity along the faults confining the slide blocks [acocella et al. 2003, neri et al. 2005, walter et al. 2005]. in general, the longer the eruptive fracture systems, the faster and wider the displacement of the sliding blocks is [neri et al. 2004]. this means that the most dangerous eruptions (characterized by vents at lower elevation, feeding large lava volumes at a high rate; see figure 6c, e, g, i) appear to be related to periods of high dynamics of flank movement. we thus envisage the following scenario for a feedback mechanism between magma accumulation, flank instability and slippage, and eruptions: (a) magma accumulation occurs about 5-10 km below the sea level [murru et al 1999, palano et al. 2009], just below the depth of the presumed basal décollement plane of the unstable, slipping flank sector [bonforte et al. 2008, ruch et al. 2010]; this causes inflation of the volcanic edifice; (b) swelling of the volcanic edifice leads to destabilization (at first slow spreading and fault creep, followed by instantaneous flank slip accompanied by shallow seismicity) of the unbuttressed e to s sector of the volcano; (c) extension at the head of the active slide block(s) facilitates magma migration from the central conduit system and/or shallow eccentric reservoirs into radial flank fracture systems (and into the rift zones), usually leading to lateral the eruptive fissures of mt. etna flank eruptions. the observed patterns over the recent eruptive history of etna appear therefore to be controlled by the shorterterm cycles, which might be involved in an effective interaction with the shallow structure of the volcano and its unstable portions. 5. conclusions in the present study, we have investigated the structural elements of eruptions since 1900 at etna, with the integration of the database with the main parameters (frequency and strike) of older eruptive fissures (up to approximately 2 ka bp). this approach has allowed us to improve our understanding of the dynamics of the summit and flank eruptions, with an analysis of the possible links with the structural instability that affects the volcano flanks. at high altitudes (above 2,000 m asl), the eruptive fissures have a scattered orientation, while at lower altitudes, they mainly propagate towards ne, s-se and w, marking three rift zones. over the last 110 years, which represents our most complete volcano-tectonic dataset, the flank eruptions lasted a median of 23 days, with eruption rates >5.7 m3 s−1. the mean interval required to develop a steady deformation pattern is 20 h, with fissures that propagate at an average speed of 0.05 ms−1. in the n sector, the eruptive fissures developed slightly quicker (0.053 ms−1) and grew for longer (3,825 m), with respect to the fissures that have affected the s sector (velocity, 0.04; length 1,840 m). conversely, the summit eruptions were usually longer (77 days), with lower eruption rates (1-2 m3 s−1), even if they were occasionally characterized by short-lived violent paroxysms (eruption rate, >50-100 m3 s−1). longer eruptive fractures tend to produce higher eruptive rates, greater lava volumes, and longer lava flows. the longest eruptive fractures, moreover, develop wider fracture fields. the observed patterns in the recent eruptive history of etna also confirm the existence of a feedback mechanism between magma accumulation, flank instability and eruptions, which consists of: (a) magma accumulation at shallow crustal levels (5-10 km bsl); (b) swelling of the volcanic edifice, leading to the destabilization of the unbuttressed sectors of the volcano; and (c) extension at the head of the active sliding block(s), which facilitates magma migration from the central conduit system into radial flank fracture systems, thus feeding flank eruptions. the most dangerous eruptions for human activities (characterized by vents at lower elevation, i.e. close to inhabited areas, feeding large lava volumes at high rates) appear to be related to periods of intense dynamics of flank movements, such as in the 1980s and at the beginning of the 21st century. in this framework, continuous monitoring of the volcano-tectonic structures that affect the etna volcano is an essential tool to better define the relationships between regional tectonics, flank dynamics and flank volcanic activity, and thus to better understand the modalities (duration, extrusion rates) of flank eruptions, which represent the most dangerous type of eruptive activity at etna. acknowledgements. this work was sponsored by the italian national civil defence department and ingv (istituto nazionale di geofisica e vulcanologia), project v3-lava (ru01–team 01c). the manuscript benefited from the useful reviews by s. calvari and r. sulpizio. references allard, p., b. behncke, s. d'amico, m. neri and s gambino (2006). mount etna 1993-2005: anatomy of an evolving eruptive cycle, earth-sci. rev., 78, 85-114; 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doi: 10.1029/2005jb00 3688. *corresponding author: marco neri, istituto nazionale di geofisica e vulcanologia, sezione di catania, catania, italy; email: marco.neri@ct.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. layout 6 annals of geophysics, 56, 6, 2013, s0669; doi:10.4401/ag-6236 s0669 combined geophysical techniques for detailed groundwater flow investigation in tectonically deformed fractured rocks john alexopoulos1, emmanuel vassilakis1, spyridon dilalos1 1 national and kapodistrian university of athens, university of athens, faculty of geology and geoenvironment, kapodistrian, panepistimiopolis, greece abstract in this paper we present a combination of several near surface geophysical investigation techniques with high resolution remote sensing image interpretations, in order to define the groundwater flow paths and whether they can be affected by future seismic events. a seasonal spring (amvrakia) located at the foot of meteora pillars near the village of kastraki (greece) was chosen as a test site. the meteora conglomeratic formations crop out throughout the study area and are characterized by large discontinuities caused by post miocene till present tectonic deformation [ferriere et al. 2011, royden and papanikolaou 2011]. a network of groundwater pathways has been developed above the impermeable marls underlying the conglomeratic strata. our research aims to define these water pathways in order to investigate and understand the exact mechanism of the spring by mapping the exposed discontinuity network with classic field mapping and remote sensing image interpretation and define their underground continuity with the contribution of near surface geophysical techniques. five very low frequency (vlf) profiles were conducted with different directions around the spring aiming to detect possible conductive zones in the conglomeratic formations that the study area consists of. moreover, two electrical resistivity tomography (ert) sections of a total length of 140m were carried out parallel to the vlf profiles for cross-checking and verifying the geophysical information. both techniques revealed important conductive zones (<200 ohm m) within the conglomerate strata, which we interpret as discontinuities filled with water supplying the spring, which are quite vulnerable to displacements as the hydraulic connections between them might be easily disturbed after a future seismic event. 1. introduction the research area is located at the southwest foot of meteora pillars area and more specifically southeast of the kastraki village (in greece), where amvrakia spring is found. due to the fact that during the summer the spring suspends its function, our primary aim was to define its water capacity and its hydrogeological sensitivity to ground displacements since the area is not far (less than 20 km) from historical and contemporary earthquake epicenters [papadimitriou and karakostas 2003]. the study area is surrounded by almost vertical bluffs consisted of the meteora molassic conglomer-atic forarticle history received november 16, 2012; december 20, 2013. subject classification: magnetic and electrical methods, instruments and techniques, groundwater processes, downhole, radioactivity, remote sensing and other methods. figure 1. a simplified geological map of the surrounding area (marked at the inset map of greece) draped on a shaded relief showing the impressive contrast of the topography is displayed, with the molassic conglomeratic formations of tsotili (2), pentalofos (3) and eptachori (4) cropping out and surrounded by quaternary deposits (1). the location of the study area (continuous line, see figure 4), the direction of the photograph at figure 2 as well as the extent of figure 3 (with the dashed line) are also marked. special issue: earthquake geology mations (figure 1) and several difficulties had to be overcome for establishing the equipment and applying the described geophysical techniques. the dense vegetation at the foot of the hills, where the spring is located, the increased arduous accessibility and the relatively intense relief were the most significant problems that needed to be solved during fieldwork. taking into account that the spring’s supplying mechanism would probably take advantage of the dense network of the semi-open discontinuities existing in the conglomeratic strata, we had to define the orientation of these water pathways. the geophysical investigation aimed to focus on the detection of conductive zones and therefore geoelectrical and electromagnetic methods such as ert and vlf that are the most indicative geophysical techniques for such environments [caputo et al. 2003, sharma and baranwal 2005, papadopoulos et al. 2008, 2010] were planned on several directions. 2. field observations and remote sensing data interpretation the meteora bluffs (figure 2) are located at the southern margin of the oligo-miocene meso-hellenic molassic basin and are consisting mainly of the meteora conglomerates.the geomorphology of the area is rather unique and therefore remote sensing datasets can prove to be very helpful in conjunction with field observations and structural measurements. [ori and roveri 1987] mention that the meso-hellenic basin is consisted of gilberttype delta deposits and deep chan-nel fills. they describe in detail the geometry of the sedimentary sequences of meteora conglomerate, consisted mainly of transferred pebbles probably due to the erosion of several units composing the alpine orogene of the hellenides [papanikolaou 2009, ferriere et al. 2011]. at the area of the bluffs, three out of the five units of the meso-hellenic molassic basin can be identified [brunn 1956, ferriere et al. 2004], while the stratigraphy of the study area is comprised of 3 different formations. (1) the deepest formation is part of the transgressive succession of eptachorion unit consisting of impermeable grey-blue marls of upper oligocene age (600 m thickness). (2) above these marls, massive conglomerate (700 m thickness) appears normally overlying [brunn 1956, papanikolaou et al. 1988], consisting of pebbles up to 20cm in diameter and originated from ophiolithes, marbles, limestones and metamorphic rocks in a sandy matrix. these massive crossed layered conglomerates have been deposited during aquitanian [brunn 1956] and are considered to be the southern outcrops of pentalofos unit (which can be found further north of the study area). it is the material that the impressive pillars are comprised of and is known as the meteora conglomeratic strata, which are gently dipping westwards [ori and roveri 1987]. the uppermost part of the formation consists of well-bedded sandstones. (3) the upper formation of the area consists of disorganized conglomerates (100 m thickness) and is deposited above the meteora conglomeratic strata through an angular unconformity [ferriere et al. 2004, 2011]. it is quite often for these outcrops to be found at the highest elevations of the pillars and it is the lowest member of tsotyli unit (of burdigalian age) also known as the upper conglomerate of meteora [ori and roveri 1987]. several generations of structural discontinuities have been observed throughout the area of meteora and are attributed to a large number of tectonic episodes during and after the meso-hellenic molassic basin evolution, which took place between upper eocene and lower miocene [brunn 1956, savoyat et al. 1972, ferriere et al. 2004]. the general trending orientation of these almost vertical and open discontinuities is between n040e and n080e. their hydrogeological significance is that they usually allow the water flow [alexopoulos et al. 2011] but in many cases the sudden motion along them is responsible for groundwater level changes especially after seismic events [marcaccio and martinelli 2012, gosar 2012]. the underlying impermeable marls of eptachorion unit do not further allow the vertical infiltration of the groundwater since the flow happens within the discontinuities of the overlying conglomerates. a high spatial resolution (1 meter) pansharpened, multispectral ikonos-2 satellite image dataset acquired during 2007 was combined with large scale topographic maps (1/5000), which were used as a base map. a digital elevation model of 5 meter spatial resolution was used for ortho-rectifying the remote sensing dataset in order to use it for detailed mapping the surface exposure of the discontinuities and high accuracy measurements. various image interpretation techniques, including spectral band combination and band ratios, led to the construction of a lineament map with morpho-lineaments [vassilakis 2006], which refer to linear alexopoulos et al. 2 figure 2. an aspect of the geomorphological features developed on the molassic strata is shown. the visible discontinuities of tectonic origin are crossing the stratigraphic bedding and allow the groundwater flow through selective pathways. 3 features that are related either to the surface expression of tectonic structures or geomorphic features. in most cases they are relevant to post deposition tectonic activity. the 4,2,3 (r,g,b) band combination of the ikonos image along with image enhancement techniques proved to be the most useful one (figure 3), as different types of vegetation were highlighted and some of them delineate tectonic structures most of which were verified during field work. in this interpretation taller trees with high chlorophyll content are represented as red and create high contrast with shrub vegetation (brown), which in most cases in this area is located above the existence of groundwater. therefore mapping this type of vegetation with multi-spectral remote sensing images can be very useful for detecting features that are related to structures of tectonic origin. the statistical interpretation of these features showed two main trends (ne and nw) that are in agreement with the field observations during which we measured several fault planes at the wider surrounding area. the measured throw of the faults varied from centimeters to tenths of meters. most of the displacements can be estimated as post-aquitanian and in many cases post-burdigalian, since the uppermost formation strata were also displaced by some of these faults or even more recent as the deep incision throughout the study area implies high uplift rates and consequently neotectonic activity, even though no clear marginal fault scarps were identified [e.g. burbank and pinter 1999 and references within]. therefore, it is rather possible to observe displacements along these discontinuity planes in conjunction to a number of secondary phenomena (rockfalls, changes in groundwater circulation etc.) after a future seismic sequence, as quite a few earthquakes with m≥6.0 have occurred the last five centuries at the surrounding area [papadimitriou and karakostas 2003]. 3. electrical resistivity tomography (ert) the morpho-lineament map and the field observations led to the definition of a plan for the initial directions for ert sections. the dense vegetation slightly altered the desired orientation with no particular impact on the final results (figure 4). two ert sections trending normally to each other were carried out with a total length of 140 m. a 41 electrode array was established for applying the wenner array configuration (190 measurement points of apparent resistivity) with electrode spacing up to 2 m. topographic leveling measurements along the ert sections were also carried out with high accuracy real time kinematics global positioning system (rtkgps) equipment, since it is rather important to record the morphology as precisely as possible along the three axis (x,y,z). the overall methodology includes high precision measured elevation (+/0.01 meters) for each electrode since this is a great contribution to minimize the errors during the signal interpretation especially for groundwater investigations [johnson et al. 2012]. the ert measurements were processed with the res2dinv software of geotomo. the raw resistivity data along with the topographic profile measurements for each transverse were imported into the software since the topographic correction was an important factor due to the intense relief of the study area. the inverse 2d model resistivity sections, derived from this interpretation and the processing results are quite impressive and detailed. the first section (ert-1) was chosen to be established between the two conglomeratic pillars, which are possibly supply the spring and at slightly higher elevations. the center of the electrode array was placed just uphill the spring location (figure 5). the initial thought was to capture the main groundwater pathway and record the exact resistivity valgeophysical and remote sensing techniques figure 3. (a) photogrammetric interpretation of an ikonos-2 satellite image (4,2,3 r,g,b) led to a photo-lineament map of the surrounding bluffs hosting the studied spring (amvrakia). the inset rose-diagram is a result of the statistical interpretation of the lineaments’ strike frequency. (b) most of the lineaments are surface expressions of faults such as the one shown in this figure, displacing the strata of the meteora conglomerates for tenths of centimeters. ues. after the interpretation a conductive (<200 ohm m) curvy area has been revealed in the center of the section (25-35 m), beneath the spring, which is compatible with an overflow spring mechanism. a smaller conductive zone of the same resistivity values seems to exist further to the sw at the distance of 20 m from the spring, but its hydraulic connection to the main supplying pathway is not clear. therefore, a second electrode array was established (ert-2) trending normally to the first one and uphill from the spring, aiming to capture several discontinuities that are used by the groundwater as pathways to the spring. two parallel conductive zones (<200 ohm m) at several depths have been detected after the interpretation (figure 6). the topographic correction of the section was very significant as it became alexopoulos et al. 4 figure 4. a 3-dimensional perspective representation of the study area on which the ert and the vlf geophysical transverses are indicated. a 2-dimensional map of the same area along with the interpreted conductive zones and the groundwater flow paths is displayed below. 5 rather clear that both of the zones are gently dipping towards the spring and this could be interpreted as water-bearing discontinuities of the conglomerates. 4. very low frequency electromagnetic survey (vlf) the vlf technique was performed in order to validate the conductive zones, detected by ert measurements in higher detail. in many cases vlf measurements are proved to be ideal for detecting vertical to sub-vertical conductive zones or possible karst structures mainly for hydrogeological investigation purposes [monteiro-santos 2006, papadopoulos et al. 2008, sharma and baranwal 2005, dilalos 2009, alexopoulos et al. 2011]. by applying this methodology, the lateral and vertical resistivity distribution is investigated. based on the field mapping, the statistical analysis of the morpho-lineaments and the orientation of the exposed fractures, five profiles were conducted (figures 6, 7). two of them (vlf1 and vlf3) were designed to partially cover the ert sections and consequently validate those results by com-bining the measurements from both geophysical methods. the rest of them were conducted along several directions around the spring’s area, aiming to either detect the extension of the already mapped conductive zones or reveal new unexposed water-bearing discontinuities at the conglomeratic strata. the spacing of the measurement stations along the section was 2 meters, as a high detailed investigation needed to be carried out. a main vlf source frequency of 23.4 khz was used, due to the good signal and alignment, towards the direction of the expected anomalies (westwards inclination). the processing of the vlf profiles included smoothing of the raw data and topographic corrections, according to baker and myers [1980] and eberle [1981]. afterwards, we applied the fraser [1969] and karous-hjelt filters [karous 1979, karous and hjelt 1983], both of which spotlight the anomalies. karous-hjelt filter provides information of relative current density distribution with depth by producing the pseudo-sections (semi-quantity interpretation) (figure 7). at vlf 1 pseudo-section, a main conductive zone is located be-tween 30-75 m after the starting point, probably due to the spring (located at 65 m) and a smaller one at 125-140 m. the interpretation of vlf 3 pseudo-section reveals a conductive zone at 25-55 m, whilst at vlf 6 pseudo-section a main congeophysical and remote sensing techniques figure 7. the karous-hjelt pseudo-sections of the selected profiles vlf 1, vlf 3 and vlf 6 are represented. the respective resistivity section derived from inversion with inv2dvlf software is also included (initial resistivity 500 ohm.m with 20 iterations. rmsvlf 1= 1.16%, rmsvlf 3=1.82%, rmsvlf 6=0.95%). frequency: 23.4 khz. figure 5. the first geoelectrical transverse was established uphill the spring location (above) and the interpreted tomography section, including topographic relief (below) revealed several conductive zones of relatively low resistivity values (ert-1: 10th iteration, rms: 2.38%). see figure 6 for scale values. figure 6. comparison of resistivity sections generated from the inversion of the ert-2 data values (11th iteration, rms: 2.74%) and the vlf-3 data inversion with inv2dvlf. it is clear that both the techniques have produced similar results. ductive zone between 40-70 m and a smaller one at 1530 m are indicated (figure 4, 7). additionally, the vlf measurements were processed with inv2dvlf inversion software presented by monteiro-santos [2006, 2007]. the software is in general an algorithm that inverts normalized vlf data with the method of smoothed least-squares, based on the scalar tipper originated from the relation of vertical and horizontal component of the magnetic field. the result of this procedure is the subsurface distribution of the resistivity and has proved to give reliable results similar to those of detailed geoelectrical measurements [monteiro-santos et al. 2006, dilalos 2009]. the resistivity profiles extracted by this procedure are illustrated in figure 7 below each karoushjelt pseudo-section, representing the same conductive zones (<200 ohm m), highlighted according to the color scale. the initial resistivity chosen for the in-version (20 iterations according to the software’s author) was 500 ohm m, based on the dominant resistivity of the con-glomerates [alexopoulos et al. 2005]. 5. discussion conclusions both of the geophysical techniques applied for this study proved to be ideal for indicating conductive zones. the results of the comparison between the two techniques along the same transverse are represented at figure 6. the resistivity section generated by the inversion of the ert-2 data compared to the resistivity section generated by the vlf-3 data inversion using the inv2dvlf algorithm are surprisingly very well related. the ert section, obviously illustrates the conductive zones in higher detail than the vlf measurements, but it is rather clear that along these two sections, where we applied both ert and vlf measurements, we had the opportunity to detect the exact same conductive zones, confirming and validating the results. high resolution remote sensing data interpretation and ground truth field observations proved to be very useful in planning the preferable geophysical transverse orientation, considering the vegetation density as well. since the area is comprised of massive conglomeratic strata, which are formations of high resistivity (>500 ohm m) [alexopoulos et al. 2005], we were able to identify the conductive zones (<200 ohm m) as the most probable underground water pathways and relate them to the observed discontinuities, trending towards the amvrakia spring. at the ert-2 section gentle dipping of these underground pathways to north-west is indicated (better shown on ert section figure 6) towards the location of the spring, whilst at the section of ert-1 a local concavity is represented, which verifies the mechanism of an overflow spring. the conductive zones along with the delineated water pathways (figure 4) leading towards the spring are in full correspondence to the discontinuities of the arising meteora conglomerate pillars (figure 4). it is noteworthy that the groundwater pathways that supply the spring are quite many but not very extensive throughout the uphill area. additionally, the geophysics interpretations show that the crucial for the spring operation conductive zones are detected at relatively shallow depths, although the hosting discontinuities should extend much deeper than that. on top of that, at this rather sparse discontinuity network we observed displacement at several of them and a sig-nificant number of these fault surfaces seem to have the potential to be easily reactivated, especially as a secondary phenomenon after a future seismic event at a relatively close epicenter area. during this case scenario the water supply might suffer changes that could reduce either the supply or the spring operation time space. acknowledgements. the authors would like to thank professor fernando monteiro-santos for kindly providing the use of inv2dvlf soft-ware. the critical and constructive reviews of two anonymous reviewers as well as the comments of the managing guest editor dr ioannis papanikolaou were highly appreciated and helped improve the manuscript. references alexopoulos, j., s. dilalos and e. vassilakis (2011). adumbration of amvrakia’s spring water pathways, based on de-tailed geophysical data (kastraki-meteora). in: n. lambrakis, g. stournaras and k. katsanou (editors), advances in the research of aquatic environment. springer, 105-112. alexopoulos, j.d., t. papadopoulos and a. mastroyannis (2005). investigation of hydrogeological factors affecting the potable water wells of kalampaka urban by using geophysical techniques. proc. of the 7th hellenic hydrogeological conference, 19-30. baker, h.a., and j.o. myers (1980). a topographic correction for the vlf-em profiles based on model studies. geoex-ploration 18, 135-144. brunn, j.h. (1956). etude geologique du pinde septentrional de la macedoine occidentale. ann. geol. pays. hellen. 7, 1-358. burbank, d. w., and n. pinter (1999). landscape evolution: the interactions of tectonics and surface processes. basin research, 11, 1-6. caputo, r., s. piscitelli, a. oliveto, e. rizzo and v. lapenna (2003). the use of electrical resistivity tomographies in active tectonics: examples from the tyrnavos basin, greece. journal of geodynamics 36, 19-35. dilalos, s. (2009). exploration of hydrogeological and alexopoulos et al. 6 7 environmental conditions with geophysical techniques, at selected sites in chios island (greece) m.sc thesis. national and kapodistrian university of athens, faculty of geology and geoenvironment. athens. eberle, d. (1981). a method of reducing terrain relief effects from vlf-em data. geoexploration 19, 103-114. ferriere, j., f. chanier, j.y. reynaud, a. pavlopoulos, p. ditbanjong, g. migiros, i. coutand, a. stais and j. bailleul (2011). tectonic control of the meteora conglomeratic formations (mesohellenic basin, greece). bull. soc. geol. fr. 182, 437-450. ferriere, j., j. reynaud, a. pavlopoulos, m. bonneau, g. migiros, f. chanier, j. proust and s. gradin (2004). geologic evolution and geodynamic controls of the tertiary intramontagne piggyback meso-hellenic basin, greece. bull. soc. geol. fr. 175/4, 361-381. ferriere, j., f. chanier, j.y. reynaud, a. pavlopoulos, p. ditbanjong, g. migiros, i. coutand, a. stais and j. bailleul (2011). tectonic control of the meteora conglomeratic formations (mesohellenic basin, greece). bull. soc. geol. fr. 182, 437-450. fraser, d.c. (1969) contouring of vlf-em data. geophysics 34/6, 958-967. gosar, a. (2012). application of environmental seismic intensity scale (esi 2007) to krn mountains 1998 mw =5.6 earthquake (nw slovenia) with emphasis on rockfalls. nat. hazards earth syst. sci. 12, 1659-1670. johnson, t. c., l.d. slater, d. ntarlagiannis, f. d. daylewis and m. elwaseif (2012). monitoring groundwater-surface water interaction using time-series and time-frequency analysis of transient three-dimensional electrical re-sistivity changes. water resources research, 48(7), w07506. karous, m. (1979). effect of relief in em methods with very distant source. geoexploration 17, 33-42. karous, m., and s.e. hjelt (1983). linear filtering of vlf dip-angle measurements. geophysical prospecting 31, 782-794. marcaccio, m., and g. martinelli (2012). effects on the groundwater levels of the may-june 2012 emilia seismic sequence, annals of geophysics, 55(4). monteiro-santos, f.a. 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(2007). new features prepvlf and inv2dvlf. version 1.1. lisboa. monteiro-santos, f.a., almeida e.p., gomes m., and a. pina (2006). hydrogeological investigation in santiago island (cabo verde) using magnetotellurics and vlf methods. journal of african earth sciences 45, 421-430. monteiro-santos, f.a., a. mateus, j. figueiras and m.a. concalves (2006). mapping groundwater contamination around a landfill facility using the vlf-em method-a case study. journal of applied geophysics 60, 115-125. ori, g.g., and m. roveri (1987). geometries of gilberttype deltas and large channels in the meteora conglomerate, meso-hellenic basin (oligo-miocene), central greece. sedimentology 34, 845-859. papadimitriou, e.e, and v.g. karakostas (2003) episodic occurrence of strong (mw≥6.2) earthquakes in thessalia area (central greece). earth and planetary science letters 215, 395-409. papadopoulos, t.d., j.d. alexopoulos, s. dilalos and m.j. pippidis (2008). resistivity and vlf measurements for spring mechanism determination at ne chios isl. proc. of the 8th international hydrogeological congress of greece, 337-346. papadopoulos, t.d., g. stournaras and j.d. alexopoulos (2010). geophysical investigations for aquifer detection in fis-sured rocks of volcanic origin. a case history. journal of the balkan geophysical society 13/2, 1-8. papanikolaou, d. (2009). timing of tectonic emplacement of the ophiolites and terrane paleogeography in the helleni-des. lithos 108, 262-280. papanikolaou, d.j, e.l. lekkas, i.d. mariolakos and r.m mirkou (1988). contribution to the geodynamic evolution of the mesohellenic basin (in greek). bull. geol. soc. greece 20, 17-36. royden, l.h., and d.j. papanikolaou (2011). slab segmentation and late cenozoic disruption of the hellenic arc. geo-chem. geophys. geosyst. 12, q03010. sharma, s.p., and v.c. baranwal (2005). delineation of groundwater-bearing fracture zones in a hard rock area integrat-ing very low frequency electromagnetic and resistivity data. journal of applied geophysics 57, 155-166. savoyat, e., n. lalechos, n. philippakis and g. bizon (1972). geological map of greece, scale 1:50.000, kalambaka sheet. inst. geol. and miner. explor., athens. vassilakis, e. (2006). study of the tectonic structure of messara basin, central crete, with the aid of remote sensing techniques and g.i.s. phd thesis, national & kapodestrian university of athens, athens, 546 pp. *corresponding author: john alexopoulos, national and kapodistrian university of athens, faculty of geology and geoenvironment, , panepistimiopolis, greece; email: jalexopoulos@geol.uoa.gr © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geophysical and remote sensing techniques layout 6 a new assessment of the elastic thickness (te) structure of the indian shield, and its implications deshraj trivedi1, yudhvir singh2, tanmay k. maji1, suresh ch. kandpal1, rajesh r. nair2,* 1 iit kharagpur, department of geology and geophysics, kharagpur, india 2 iit madras, department of ocean engineering, madras, india annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-5172 abstract the elastic thickness (te) of continents is a matter of much debate. recent studies have shown that a number of factors control the continental te, including age, heat flow, and lithospheric thickness. here, we estimate the te structure of the whole indian shield using an improved isotropic fan wavelet land ocean deconvolution methodology, and we compare these results with the global published te estimates in the archean, proterozoic and younger geological provinces. our study reveals low (0-45 km/0-35 km), intermediate (45-70 km) and high (70-100 km) te values in the archean/quaternary, the proterozoic, and the tertiary provinces, respectively, of the indian shield. this is in contrast with global estimates of te in similar geological provinces. in the absence of any correlation of te with any controlling parameters, we propose that the mantle properties, rather than the tectonic history, are responsible for influences on the te values within the indian shield. the global positioning system horizontal velocity vectors yielded a locking depth of ca. 20 ±4 km, and the aseismic creep beyond correlates well with the high strength of ca. 70 km to 100 km in the central himalayan foreland. 1. introduction although different methods yield different absolute values, it has been convincingly shown that the elastic thickness (te) in cratons is always controlled by age and heat flow [pérezgussinyé and watts 2005, simons et al. 2000]. several studyies have suggested that the te of continents cannot be described by any relationship with only a single parameter [watts and burov 2003, mcnutt et al. 1988, watts 1992]. earlier investigations around the globe using spectral analysis have suggested that the te of cratonic regions is greater than ca. 60 km; e.g., for africa [djomani et al. 1995], south america [pérez-gussinyé et al. 2009], canada [wang and mareschal 1999], and australia [simons et al. 2000]. recently, it was argued that the recovery of high te values is dependent on the window sizes [pérez-gussinyé and watts 2005]. using several window sizes, as 400 km2 × 400 km2, 600 km2 × 600 km2, 800 km2 × 800 km2 and 1,000 km2 × 1,000 km2, pérez-gussinyé and watts [2005] showed that smaller window sizes improve the resolution, but may not be large enough to recover the maximum value of te. the first attempts to estimate the te in the indian craton were made by karner and watts [1983] and lyoncaen and molnar [1985]. using forward modeling between the bouguer anomaly and the topography they obtained te values of 80 km to 110 km in the ganges basin. free-air admittance by mckenzie and fairhead [1997] yielded the lower te value of 24 km, which was found to be correlated with seismogenic thickness (ts). however, subsequent studies by handy and brun [2003] suggested that the te can easily exceed the ts as well. using multitaper spectral analysis, rajesh et al. [2003] characterized the relative variations in the te in the india-eurasia collision zones. in another analysis, rajesh and mishra [2004] used the transitional coherence wavelength to characterize the tectonic provinces. jordan and watts [2005] used both forward and inverse flexural and gravity modeling techniques, and they obtained spatially varying te structures that varied from 0 km to 125 km in the india-eurasia collision zones. however, none of these studies could demonstrate any convincing correlations between the te values and any of the controlling parameters, such as age and temperature. one possible reason for this might be that most of the forward modeling techniques were only along some one-dimensional profiles, and hence they failed to capture the spatial variation of the te. hence, a reappraisal of the spatial variation of the te with an improved methodology would add new dimensions to the study of the lithospheric strength within the indian shield. recent studies have shown that the fan wavelet method has been relatively successful in deriving the te structure in continental areas; e.g., in the australian shield [kirby and swain 2011], the canadian shield [audet and mareschal 2004], and the south american shield [tassara et al. 2007]. highresolution, spatially varying te maps can resolve regional article history received may 19, 2011; accepted january 25, 2012. subject classification: elastic thickness, indian shield, seismic zoning, himalayas. 273 research articles scale features that can be correlated to surface geological structures. in the present study, we look at the discrepancies in the estimation of the te within the indian shield using the isotropic fan wavelet, which uses large window sizes and estimates the te by merging the databases of the land [rajesh et al. 2003, rajesh and mishra 2004] and the ocean [anderson and knudsen 1998, anderson et al. 2008]. 2. the tectonic settings the indian subcontinent has a complex geological and tectonic setting that consists of precambrian cratons of archean age and rift zones filled with proterozoic and phanerozoic sediments [e.g. biswas 1999]. a geological map of the whole indian shield, along with the major tectonic features, is shown in figure 1. the geological structure of the indian shield can be subdivided into three main units [krishnan 2006]: (a) the himalayan front in the north, which results from the mesozoic subduction and the collision between the indian and eurasian plates. (b) the indo-gangetic plains, which are located between the abruptly rising himalayas in the north and the indian peninsula in the south, and which extend from east to west. (c) the indian peninsula in the south, which comprises the indian shield with the deccan traps and the dharwar cratons. the precambrian rock in india [e.g. naqvi 2005, sharma 2009] is subdivided into two categories: the archean system, and the dharwar system. the archean rock is mainly found in the dharwar, singbhum, bundelkhand, and bastar cratons. the aravalli and eastern ghat cratons are of late proterozoic age. the indo-gangetic plains are mostly filled with quaternary sediments that come from the erosion of the himalayas [e.g. bir singh 1996]. an important feature in the southern indian peninsula, the deccan traps, evolved as flood basalts in the late mesozoic and early tertiary period. the origin of these traps is still not unanimous; hot-spot volcanism and mantle plumes are the most discussed theories at present [e.g. sen 2001, sheth 2007]. 3. the data the topography, bathymetry and bouguer anomaly data are shown in figure 2. the bathymetry data were extracted from the digital atlas of the general bathymetric chart of the oceans (gebco; http://www.gebco.net/), which was published by the british oceanographic data centre on behalf of the international oceanographic commission of unesco (http://ioc-unesco.org/) and the international hydrographic organization (http://www.iho. int/srv1/). the gebco data were made available by the national oceanic and atmospheric administration [2003]. we merged the gravity data for the land and the ocean trivedi et al. 274 figure 1. geotectonic map of the whole indian shield, indicating the ages of the different zones. the map is modified after rajesh and mishra [2004]. abbreviations: bc, bhandara craton; bhc, bundelkhand complex; cb, cudappah basin; dafb, delhi aravalli fold belt; dvp, deccan volcanic province; edc, eastern dharwar craton; egmb, eastern ghat mountain belt; grp, godavari rift province; sc, singbhum craton; sgt, southern granulite terrain; wdc, western dharwar craton. 275 using the land–ocean deconvolution scheme. the gravity and topography data source for the land is the same as that used by rajesh et al. [2003], rajesh and mishra [2004], and rajesh [2004]. the cumulative error in the bouguer gravity field including the errors in the elevation is estimated to be 1 mgal to 2 mgal [rajesh and mishra 2004].the free-air anomaly data for the oceanic regions come from the global marine gravity field from the ers-1 and geosat geodetic mission altimetry of anderson and knudsen [1998] and anderson et al. [2008]. the marine free-air gravity anomaly data, dgf, is converted to the bouguer gravity anomaly, dgb, using the formula: (1) where dt = 1,820 kg.m—3 is the density contrast between the surface rock and the water, h is the bathymetry (in m), and g is the gravitational constant. the crustal thickness data used was taken from the crust 2 model [bassin et al. 2000]. 4. methodology the fan wavelet technique [kirby and swain 2004, audet and mareschal 2007, kirby and swain 2011] uses the superposition of the rotated two-dimensional (2d) morlet wavelets arranged in a 'fan' geometry to obtain the wavelet coherence or admittance, by computing the co-spectra and cross-spectra of the gravity and topography data. the continuous wavelet transform of a 2d spatially distributed signal g(x) is estimated by taking the convolution of the signal with the complex conjugate of a wavelet, given by: (2) where is the complex wavelet coefficient, s is the width of the wavelet, i is the rotation parameter, k is the 2d wavenumber, f—1 is the inverse 2d fourier transform, is the 2d fourier transform of the signal g(x), is the complex conjugate of , where: (3) is the 2d fourier transform of the 'daughter'wavelets derived by dilating, translating and rotating the mother wavelet, and x(i) is the rotation matrix. for the calculation of isotropic coherence function, the wavelet coefficients must be complex quantities. the problem with real wavelets is that they are not complex, although they are isotropic, whereas complex wavelets are not in general isotropic. to overcome this problem, superposition of morlet wavelets is used, which produces isotropic and complex wavelet coefficients. the 2d wavelet coherence can then be estimated by summing the wavelet co-spectra and cross-spectra over different azimuths: (4) where and are the complex wavelet coefficients of the bouguer anomaly and topography, respectively. the wavelet method has advantages over the windowed fourier transform method, such as the multitaper method, because unlike the windowed fourier transform, where the window size is the same for all scales, the wavelet method uses an optimal window size for each scale. the only drawback is that the spatial localization is not good for large-scale wavelets. this results in a decrease in the mapping quality of the te for thicker plates, compared to thinner plates, as higher te values correspond to higher transitional wavelengths. 5. results we computed the te for the whole indian shield and the adjoining regions using the isotropic fan wavelet method. to avoid edge effects, we used the land–ocean deconvolution elastic thickness structure of the indian shield ! 7! 7! 8! 8! 9! 9! 5! 1 ! 1 ! 2 ! 2 ! 3 ! ! ! ! 0! 10! 20! 30! (mgal ! bouguer ! 7! 7! 8! 8! 9! 9! 5! 1 ! 1 ! 2 ! 2 ! 3 ! !!! 0! 150! 300! 450! topography ! figure 2. the indian shield and the adjoining oceanic regions. (a) topography. (b) bouguer anomaly. g g 2 ghb f= + r td d d , ,x f k kg s g1 ,s=i } ) i -^ ^ ^h h h" ,j t w , ,xg s i^ hj kg^ ht k,s} ) i^ hw k,s} i^ hw k ks s, 1 s =} } ix ) i -^ ^^h h hw w , ,xg s i^ hj ( , )xs g g h h g h 2 2 w sx sx sx =c ) ) i i i i i i sx sx sx i i i ^ ^ ^ h h h6 @ j j j j j j gsxi j hsxij technique. as the main aim of this study is to discuss the spatial variations of the te within the indian shield and not in the adjoining oceanic regions, we blanked out the portions outside the indian shield. however to explain the global positioning system velocity vectors in the nepal himalaya, we only considered the te values for that particular region outside india. we ensured that the window size was large enough to capture the flexural signatures. the te values obtained are quite low for peninsular india (<50 km), except for the central himalayan foreland, where they are 70 km to 100 km (figure 3a). moreover the geological provinces of the different ages have different te values i.e., the archean (0 to 45 km), proterozoic (45 to 70 km), and tertiary and quaternary provinces (70 to 100 km). thin te values (0-45 km) are unusual for continental regimes of generally low heat flow and other indicators of low temperature, such as old geologic age. the transitional wavelength is shown in figure 3b. we also computed the flexural loading ratio (figure 4a), which shows a value within 0-9. the error in the estimation of the te is shown in figure 4b. the estimated average errors for the archean, proterozoic, tertiary and quaternary regions are 6 km, 8 km, 9 km and 5 km, respectively. 6. discussion the te values of continental cratons are normally high (>60 km). in table 1, we have listed the results of recent studies of the te in continental shields. in most cases, the te is greater than the depth of the crustal thickness, which suggests that the strength resides in the upper mantle as well. moreover, there is no clear correlation of te with geological age (table 1). the correlations between the te and its controlling factors have always been complex and nonuniform. although several studies have computed the te values for the indian shield, none of these, except jordan and watts [2005], captured the spatial variation of the te. jordan and watts [2005] used gravity and flexural modeling, and they reported high values of the te (80125 km) in the central himalayan foreland. our estimated te values are in good agreement with those of jordan and watts [2005]. we found that the te values within the indian shield vary for the different geological ages: 0 km to 45 km for trivedi et al. 276 ! 7! 7! 8! 8! 9! 9! 5! 1 ! 1! 2! 2! 3! 8 ! 10 ! 12 ! 14 ! 16 ! 18 ! 20 ! 22 ! transitiona w avelength ! 7! 7! 8! 8! 9! 9! 5! 1 ! 1! 2! 2! 3! 1! 2! 3! 4! 5! 6! 7! 8! 9! 10! te ! figure 3. (a) te map of the indian shield and the adjoining oceanic regions. (b) transitional wavelength (km). figure 4. (a) flexural loading ratio. (b) error in the estimation of the te. 277 archean, 45 km to 70 km for proterozoic, 70 km to 100 km for tertiary, and 0 km to 35 km for quaternary. our rigidity structure is shown in figure 5, and it indicates that the older sections of the indian shield are indeed weaker, with respect to the younger ones. the te patterns demonstrate a nucleus of variable high strength (50-100 km) along the north-central part of the indian shield. in the north-eastern part, the rigidity structure is uniform. the southern shield demarcates a variable low-strength variation (0-50 km). the reconstructions of the lithospheric geotherm carried out by artemieva and mooney [2001] and artemieva [2006] showed that the southern indian shield (chiefly the archean) is colder than the north indian region. these above-mentioned results are in contrast to our present estimates of the long-term lithospheric strength using the rheological profiles for the indian region. manglik and singh [1999] showed that the stronger zones are mostly located in southern india, and are characterized by the lowest surface heat flow. taking into account the above considerations, our present results suggest the possibility of the coupling/decoupling of the crust–mantle interface that can considerably increase or decrease the elastic strength [burov and diament 1995]. this possible causal elastic thickness structure of the indian shield region investigators method age te (km) average tc (km) canadian shield wang and mareschal [1999] bouguer coherence(mem) archean >80 35-40 audet and mareschal [2004] bouguer coherence(mtm) archean ~50 35-40 paleo-proterozoic 35-40 audet and mareschal [2007] bouguer coherence(wavelet) archean 30-120 35-40 paleo-proterozoic 35-40 european shield perez-gussinye and watts [2005] bouguer coherence(wavelet) archean >60 40-45 proterozoic >120 40-45 free-air admittance archean <25 40-45 tiley et al. [2003] admittance (free-air anomaly) ---5 ±2 40-45 australian shield simons et al. [2000] bouguer coherence (mtm) archean ~50 35-40 proterozoic ~35 35-40 south american shield petez-gussinye et al.[2009] bouguer coherence (land ocean deconvolution scheme: isotropic fan wavelet) archean and early/middle proterozoic >70 35-40 phenorozoic 20-30 40-60 african shield djomani et al. [1995] bouguer coherence archean >40 37-43 indian shield mckenzie and fairhead [1997] free-air admittance tertiary 24-42 45-50 rajesh and mishra [2004] bouguer coherence (mtm) archean 7-22 34-36 proterozoic 15-30 34-36 jordan and watts [2005] flexural modeling archean 0-50 34-36 proterozoic 50-70 34-36 tertiary 100-125 45-50 quaternary 0-40 55-65 stephen et al. [2005] bouguer coherence(mtm) archean 12-15 34-36 proterozoic 19-25 34-36 hetenyi et al. [2006] thermal mechanical modeling (bouguer anomaly) tertiary 60-80 35 quaternary 20-30 70 present study [trivedi et al. 2012] bouguer coherence (land ocean deconvolution scheme: isotropic fan wavelet) archean 0-45 34-36 proterozoic 45-70 34-36 tertiary 80-100 45-50 quaternary 0-30 45-55 table 1. estimation of the elastic thickness (te) at the continental cratons and the average crustal compensation (tc). relationship needs to be investigated further to improve our understanding of the results obtained for the indian shield. burvo and diamond [1995] provided several pieces of evidence around the globe that suggest that the lower crust of most continental plates has a low temperature activation rheology that can result in crust–mantle decoupling. our present data show that the decoupling of the crust and the mantle might be one of the important properties that determine the present apparent values of the te. the decoupling can be attributed to variations in the composition and thickness in the crust, due to a low-temperature-activated lower crust or a thick crust. in contrast, coupling can be due to basic high-temperatureactivated crust with mantle lithosphere. gupta et al. [2003] used receiver function analysis at several sites of archean and proterozoic terrains in south india and they obtained anomalous crustal thicknesses of 42 km to 51 km beneath the mid-archean. these reported high crustal thicknesses favor the possibility of the crust–mantle decoupling mechanism. to check the plausibility of the te results thus obtained, we conducted several tests with other approaches, to estimate the te and the loading ratio simultaneously at each point. these tests ensure that the present te estimates are real features and not artifacts due to the methodologies used. in the present fan-wavelet method used, the computation is performed at each grid point of an initial surface and subsurface loads. these tests confirmed that the method is robust and the pattern of the te variation is realistic and interpretable. the spatial variations in the te values thus obtained were retrieved using continuous wavelet transform instead of the fourier approach. the problem of using smaller window sizes in the fourier approach is thus bypassed by using larger window sizes in our present estimation. tassara et al. [2007] used a similar approach based on fanwavelet and obtained a causal relationship of rigidity structure with respect to the seismogenic zone along the seduction fault in south america. the flexural loading ratio (figure 4a) varies from 0 to 9. the loading ratio is <1 for the region with the high te values, whereas it has a high value for regions with low te values, contrary to the observations of tassara et al. [2007], who reported a loading ratio >1 for the te <15 km. this correlation between the te and the loading ratio explains the subsurface mass distribution. the transitional wavelength (figure 3b) is the wavelength where the coherence between the bouguer anomaly and the topography changes from high to low, or the coherence approaches the value of 0.5. thus, it is a quality check for the estimation of the te and it is seen to correlate well with our te values obtained. the transitional wavelength is high (ca. 200-220 km) for the regions with high te, whereas it is low trivedi et al. 278 figure 5. spatial variation of the te within the indian shield shown with the tectonic features in india, with some of the major active faults and lineaments [dasgupta et al. 2000]. abbreviations: mdf, mahendergarh-dehradun fault; nsl, narmada son lineament; tl, tapti lineament; ssf, shan-shagaing fault; mbt, main boundary thrust; kmf, kutch mainland fault. epicenters of the different earthquakes with magnitude mw ≥4 are shown. their hypocentral depths are projected onto the vertical plane as longitude versus depth for three windows (w1, w2, w3), as shown on the right-hand side. the seismicity of the region was taken from neic (http://earthquake.usgs.gov/regional/neic) and gcmt (http://www.globalcmt.org/). the geodetic velocity vectors for the indian subcontinent are shown with respect to eurasia (arrows) [calais et al. 2006, meade 2007, thatcher 2007]. (k m ) (k m ) (k m ) 279 (ca. 75-90 km) for the small pockets with low te values. the north-south strip that shows a moderate te value through central india (ca. 45 km) also finds its resemblance in the map of the transitional wavelength. the error in calculating the te values is within acceptable limits (figure 4b), and hence supports the te values obtained. 7. te versus ts: a seismotectonic map is shown in figure 5, with the earthquake epicenters and some of the major faults and lineaments shown. in figure 5, the spatial te variation is shown, and for three windows (figure 5, boxes w1, w2 and w3) the hypocenters are projected onto the vertical planes. for windows 1, 2 and 3, the majority of the earthquakes are within 50 km, 40 km and 100 km in depth, respectably. from the analysis of these windows and their corresponding te values, we can see that for windows with greater te values, this means having more ts. hence, from the analysis, we can conclude that te is proportional to ts. for windows 1 and 2, most of the events occurred within the moho. however in window 3, the crust and the upper mantle are seismically active, as subduction is still active. thus, there is an obvious correlation between te and ts (te ≈ ts), except for a few isolated events in these regions. these data agree well with those obtained by mckenzie and fairhead [1997], who on account of their free-air admittance method, obtained te~ts and suggested that the strength of the lithosphere resides in the seismogenic layer only. maggi et al. [2000] obtained te<ts, and therefore suggested that the strength resides in the seismogenic layer, while the lower part of the seismogenic layer does not contribute to the strength. however, watts and burov [2003] suggested that te>>ts for the continental lithosphere, especially in the case of cratons, convergent zones, and rifts. they attributed this result to the multilayer rheology of the continental crust below cratons. from the global positioning system velocity vector in the himalayan region, bilham et al. [1997] suggested a locking depth of 20 ±4 km and aseismic creep beyond that depth. in figure 5, the geodetic velocity vectors for the india–eurasia collision zone are shown [calais et al. 2006, meade 2007, thatcher 2007]. one possible interpretation of our data might be that the aseismic part of the te has the major role in defining the lithospheric strength, whereas the upper seismogenic layer is weak over a geological time scale. this result appears to be quite reasonable if we consider the ts to be indicative of the frictional instability, which leads to the release of strain energy rather than strength. this also dismisses the concept of a weak mantle in active subduction zones in continental regions. 8. conclusions our results infer that the cratonic te of the indian shield is significantly lower than the global averages. a north–south high te is hypothesized, due to the effects of the indian shield that ploughed into the mantle during its movement. this enhanced travel might be due to the high root strength of the indian shield. we propose that the perfect coupling and high root strength at the central himalayan zone is due to competent rheology with the absence of wet crustal rheology. a large accumulation of stress connected with a network of faults might result in localization of the stress and incompetent layering, with a wet crustal rheology interpreted for the indian peninsular shield and therefore relatively weak, although a craton. in addition, the presence of a crustal root in the peninsular shield without expression of the topography might have resulted in reduced strength. although the indian plate had an eventful tectonic history, including its movement over various hot spots, the present te structure is suggestive of the dominant signatures of the last major tectonic episode of the formation of the himalayas, and as a result, of crust–mantle coupling and decoupling. these results also support the inverse correlation between geological age and te values. acknowledgements. we thank dst and incois for the necessary funding. profs. swain and kirby are gratefully acknowledged for providing the software code for the computation. c. s. vijayalakshmi is thanked for the data preparation. references andersen, o.b., and p. knudsen (1998). global marine gravity field from ers-1 and geosat geodetic mission altimetry, j. geophys. res., 103, 8129-8137. andersen, o.b., p. knudsen, p. berry, j. freeman, n. pavlis and s. kenyon (2008). the dnsc07 ocean-wide altimetry-derived gravity field, in: egu proceedings, session g1, general assembly, vienna, austria, april 14-18. artemieva, i.m., and w.d. mooney (2001). thermal structure and evolution of precambrian lithosphere: a global study, j. geophys. res., 106, 16387-16414. artemieva, i.m. 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(1992). the effective elastic thickness of the lithosphere and the evolution of foreland basins, basin res., 4, 169-178. watts, a.b., and e. burov (2003). lithospheric strength and its relationship to the elastic and seismogenic layer thickness, earth planet. sci. lett., 213, 113-131. data sources gcmt, global centroid moment tensor project: http://www.globalcmt.org/ neic, national earthquake information centre, usgs: http://earthquake.usgs.gov/regional/neic *corresponding author: rajesh r. nair, iit madras, department of ocean engineering, madras, india; email: rajeshnair@iitm.ac.in. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. elastic thickness structure of the indian shield layout 6 new constraints for site-effect characterization from seismic noise analysis in southern italy. san fele case study margherita corciulo1,2 *, paola cusano3, simona petrosino3 1analisi e monitoraggio del rischio ambientale (amra) s.c.ar.l., napoli, italy 2 laboratoire de géophysique interne et tectonophysique, université j. fourier, grenoble, france 3 istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli, italy annals of geophysics, 53, 4, 2010; doi: 10.4401/ag-4667 abstract in the framework of ground-motion amplification analysis for southern italy, the main target of this study is to provide new constraints on onedimensional, shallow-velocity profiles for a site in the san fele area near the city of potenza (southern italy) where a permanent irpinia seismic network (isnet) seismic station is installed. ambient noise vibrations were recorded during a seismic survey in san fele, and the data acquired were used to define the shallow shear-wave velocity profiles and thicknesses of the shallow soil layers, through analysis of the dispersion characteristics of the surface waves. single station and array techniques were used to obtain robust results, which show relatively flat curves of the h/v spectral ratios and variations in shearwave velocities confined to the first 50 m in depth. on the basis of these results for the san fele site, the present study aims to delineate a standard procedure that can be systematically applied to all of the other isnet stations to improve site characterization. this will allow more accurate evaluation of peak ground-motion quantities (e.g. peak ground acceleration, peak ground velocity) at rock sites for use in shakemap analysis. introduction in areas with high seismic-hazard levels, the correct evaluation of site amplification effects will allow the production of accurate ground-shaking maps that can reproduce the actual pattern of damage suffered by areas hit by an earthquake. the most common methods to analyze and quantify site effects are based on the shear-wave velocity (vs) profile, the knowledge of which in the first 30 m (vs30) has an important role in classification of the soil according to international schemes, such as for the national earthquake hazards reduction program [nehrp 1997; eurocode 8, 2003]. in recent years different techniques have been used to evaluate vs profiles because knowledge of these allows evaluation of the site transfer function. many studies have proposed the use of the geological characteristics to assign mean shear-wave velocity values to studied areas [joyner and boore 1981]. one of the main problems related to this technique is that detailed knowledge of the subsurface geology is required. to overcome this difficulty, several studies have proposed the use of the average of the shearwave velocities measured to a depth of 30 m (vs30), in agreement with the typical depth reached by a drill rig in a single day [joyner and fumal 1985, borcherdt et al. 1991]. in vertically heterogeneous media, surface waves are dispersive; i.e. their velocity depends on the frequency. this in turn controls their penetration depth [aki and richards 2002], and therefore a wider range of frequency allows a wider range of investigation depth. different studies have been based on surface-wave dispersion to define shear-wave velocity profiles using active seismic sources (e.g. spectral analysis and multi-channel analysis of surface waves) [nazarian et al. 1983, park et al. 1999] and/or ambient noise vibrations. the main advantage of using ambient noise is in the wider range of frequencies available. therefore, these methods represent useful, non-invasive and low-cost tools to determine the shear-wave velocities and thicknesses of soil layers, under the hypothesis that the noise wavefield is dominated by surface waves. the spatial auto-correlation (spac) technique introduced by aki [1957] is based on the assumption that seismic noise can be viewed as the sum of waves propagating in a horizontal plane in different directions with different powers, but with the same phase velocity for a given frequency [aki 1965]. the phase velocity can be computed by estimating the space-correlation function for a fixed frequency without knowledge of the directionality of the waves. then, varying the frequency, the phase velocity can be determined as a function of frequency. article history received march 23, 2010; accepted september 7, 2010. subject classification: seismic risk, seismic methods, data processing, waves and wave analysis, instruments and techniques. 59 the original method of phase-velocity determination requires only a set of data acquired from a circular array with one seismometer placed at the center. in urban areas, and in a lot of other cases, this geometry is difficult to lay out. this led to some studies that improved on the original spac method, with the definition of the modified spac method (mspac) [bettig et al. 2001]. two further modifications were proposed by cho et al. [2004, 2006] and tada et al. [2006]: the centerless circular array (cca) and the two-radius (tr) circular array methods, respectively. in the present study, we follow the same approach of di giulio et al. [2006], which consists of application of both single-station [nakamura 1989] and multi-channel techniques to seismic noise for complete site characterization of the test area of san fele. moreover, we applied the nonlinear methodology implemented by wathelet [2005] and based on the neighborhood algorithm of sambridge [1999], to invert the rayleigh-wave dispersion curves and obtain the s-wave velocity profiles. the test site was chosen in the framework of groundmotion amplification analysis for the campania-lucania region because it hosts one of the stations of the permanent irpinia seismic network (isnet) [weber et al. 2007]. our aim was to carefully evaluate the results obtained for the san fele site to determine the applicability of the method to all of the other sites hosting isnet stations. this will provide new constraints to compute site-specific coefficients for correct estimates of peak ground-motion quantities (e.g. peak ground acceleration, peak ground velocity, among others) at rock sites [convertito et al. 2009]. the consequent accurate definition of ground-shaking maps will indeed be useful for the civil protection authorities to plan emergency actions in case of the occurrence of earthquakes. geological framework the campania-lucania region (southern apennines) has experienced numerous disastrous earthquakes that have made it one of the highest seismic-risk areas in italy [cinti et al. 2004]. this area therefore represents a natural laboratory to implement and test methodologies for the evaluation of site amplification and for investigation of the characteristics of seismic earthquakes. a quaternary–volcanic–tertiary–mesozoic (qvtm) site-condition map for the campania-lucania region was proposed recently [cantore 2008]. this map is based on the geological units that outcrop in this area and on the vs profiles available from the database of the national strong motion network (ran). the accuracy of this map depends on the availability of vs profiles, and as stated by the authors, the accuracy increases as more vs profiles become available. the structural geology of the san fele area is characterized by an antiformal that involves different layers of the lagonegro unit superimposed tectonically. this unit is characterized by complex stratification of siliceous claystones, cherty limestones, and dolomites, down to a depth of about 5,000 m, as inferred by the san fele well that was drilled in the area [improta et al. 2003, patacca 2007]. acquisition layout the data analyzed in this study were collected by a group of researchers of the analisi e monitoraggio del rischio ambientale (amra) s.c. a r.l. society of naples and of the istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano (ingv department in naples), who installed three temporary seismic arrays to measure ambient noise in the area of san fele, near the city of potenza (southern italy) (figure 1). the instrumentation comprised four portable site effect constraints from seismic noise 60 figure 1. (a) survey area. arrow, location of the sfl3 shelter. (b) the sfl3 isnet station shelter. arrow, position of the temporary seismic station (a2) used during the noise acquisition survey. (c) the complete portable station: lennartz marslite, battery and buried seismometer. (d) the le3d 1-hz seismometer in the hole. 61 digital lennartz marslite stations equipped with 1-hz le3dlite seismometers, which were located close to the sfl3 isnet permanent station at 15.5782 ˚e and 40.7889 ˚n. the sampling rate and gain were set to 125 hz and 8 nv/count, respectively. all of the seismometers were buried in 20-cmdeep holes and oriented to geographic north. the map in figure 2 shows the location of the area of acquisition (red circle, upper-right corner panel). the gray triangle corresponds to the location of the sfl3 permanent isnet station. figure 2 clearly shows the triangular geometry of the arrays used during the experiment. three stations were deployed in three concentric triangular configurations, with rays of about 10 m, 40 m and 80 m; the remaining station was fixed at the center during the entire experiment. this particular configuration was chosen to obtain the widest azimuthal coverage; therefore, the stations were deployed with a distance of 120˚ each from the others. the data were collected in three times, with the timetable shown in table 1. figure 3 shows an example of the data that was acquired for the outer array, with the seismograms acquired for the three components shown on the left, and the relative spectra plotted on the right. the experiment was carried out from 27 to 28 october, 2008. data analysis the ambient vibration data recorded at san fele were analyzed using both single-station and array techniques implemented in the geopsy software (geophysical signal database for noise array processing), which is distributed in the framework of the site effects assessment using ambient excitations (sesame) project (2001–2004) [sesame wp05, 2002]. the surface-wave-dispersion curve obtained has been inverted, to define a velocity profile for the area studied. single station analysis (h/v ratio) the data were analyzed using the method of nakamura [1989, 2000], which was applied to the recordings from each single station to compute the spectral ratio between the horizontal and the vertical components (h/v ratio). this method allows the identification of the fundamental resonance frequency for each site when the impedance contrast between the surface and the deep material is high [bard 1998]. the importance of h/v peak estimation in the framework of the s-wave velocity profile definition has been analyzed in different studies [e.g. bonnefoy-claudet 2004] that have shown that the resonance frequency for a site is close to the h/v peak and depends on the layer thickness and the s-wave velocity. this means that the h/v peak gives additional constraint to the s-wave velocity profile estimation. the results obtained through applying the h/v method to each station of the array are shown in figure 4. these results were obtained by using all of the data recorded. the horizontal component considered in the computation was the squared average of the east-west and north-south components. to study the stability and the reliability of the site effect constraints from seismic noise table 1. timetable and resolution limits (kmin and kmax) for each array. resolution limits are the minimum and the maximum resolvable wavenumbers. figure 2. acquisition geometry. crosses, locations of the temporary stations. gray triangle, the sfl3 station location. the different colours correspond to the different arrays. dotted lines, the three triangular configurations used. red, inner array; green, middle array; blue, outer array. array name starting time ending time kmin kmax a 2008-10-28 10:59:08 2008-10-28 11:55:24 0.234 0.5310 b 2008-10-28 09:11:12 2008-10-28 10.48:24 0.064 0.1450 c 2008-10-27 14:52:56 2008-10-27 16:01:08 0.0316 0.0708 site effect constraints from seismic noise 62 figure 3. example of the data acquired. left: seismic noise records acquired for the outer array for the ew (top), ns (middle) and vertical (bottom) components. right: relative computed spectra for the noise acquired. figure 4. mean h/v (continuous lines) and standard deviation (dashed lines) functions computed for each array. from left: the inner, middle and outer arrays. 63 results obtained, several tests have been carried out, through changing the parameters used, as well as working on the raw and filtered signals. this included, time windows ranging from 5 s to 50 s, smoothing values ranging from 10 to 40, and butterworth band-pass filtering between 1 hz and 15 hz. all of the results obtained showed the same peaks, with little change in the amplitudes and the widths. figure 4 shows the best mean h/v functions obtained for each station using a long-term average/short-term average (lta/sta) antitrigger algorithm on the raw signals, with the lta fixed a 30 s and the sta fixed at 1 s, a time window of 5 s, and the konno and ohmachi [1998] smoothing procedure with a smoothing value fixed at 40. a rapid analysis of figure 4 allows it to be stated that except for the a0 station, there is no evidence of a resonant frequency for the san fele site. analyzing the data thoroughly for the a0 station, i.e. for both of the two days of acquisition, a peak appears at a frequency of 11 hz. as it has been indicated from several studies [fabbrocino, personal communication; cantore 2008], this peak is due to the presence of the shelter, the structure hosting the permanent station (figure 1, black arrow). the other peaks in the plots are also of no significance, as they do not respect the criteria for a «clear peak» proposed in the guidelines published in the framework of the sesame project [sesame wp12, 2004]. these peaks are very wide (figure 4, a1, b1, b2, c1) or their sharp decrease is evident only on one side (b3). these features mean that these peaks are not reliable for the resonance analysis. array analysis the array data analysis was performed using the modified mspac method to define the dispersion curve for each array [bettig et al. 2001]. the dispersion curves retrieved were inverted to obtain the s-velocity profile for the san fele site. for each array, the array transfer function (atf) was computed, as defined as the array response for a vertically incident impulsive signal. the atf is commonly used to determine the resolution of array configurations and to determine the dependence of the aliasing on the azimuth, in the wavenumber domain [woods and lintz 1973]. we computed the atf for a frequency band defined from the spectral analysis (figure 3) and ranging from 0.5 hz to 20 hz. as an example, figure 5 shows the results obtained for the smallest array (array a), with the results for vertical sections of the atf amplitude versus wavenumber on the left. the right side of figure 5 shows the resolution limits retrieved. in particular, these curves, and those obtained for the others two arrays, were used in the mspac analysis to validate the retrieved dispersion curves. the resolution limits are defined by the kmin and kmax values that are the minimum and maximum wavenumbers, respectively, that can be solved with the configuration used. the lowest wavenumber (kmin) is defined as the value of the main peak that reaches the atf magnitude of 0.5 (figure 5, black horizontal line). the maximum wavenumber (kmax) is defined, instead, as the wavenumber of the secondary peak (the closest to the principal one) that exceeds the atf magnitude of 0.5 [di giulio et al. 2006]. table 1 shows the values retrieved for each of the array configurations. the mspac method computes the averages of the spatial auto-correlation ratio on the rings, including all of the possible pairs of stations defined, starting from the location of each of the stations of the array used. the concept of the ring has been introduced to provide the imperfectly shaped array (as required in the spac method), both in terms of rays and angles (r, {). using the concept of the rings, only the relative locations of the station pairs must be taken into account in the computation of the correlation values. in other words, the mspac analysis computes the averages of the spatial auto-correlation ratios in the plane (r, {) on the rings with rays r1 ;r2, where r1 <r<r2. in its general formulation, the spac method allows computation of the spatial auto-correlation ratio for each frequency, using the following equation: site effect constraints from seismic noise figure 5. array transfer function (atf) for array a. left: azimuthal projection of the atf amplitude as a function of the wavenumber k. right: the vs versus frequency curves corresponding to the minimum (continuous line) and maximum (dashed line) resolvable wavelengths (0.234 and 0.5310, respectively). (1) where is the spatial auto-correlation ratio computed using the vertical component of the noise; j0 is the bessel function of zero order; ~ is the angular frequency, and r is the radius. finally cr (~) is the phase velocity of the dispersive rayleigh waves. knowing at several frequencies, the phase velocity profile can be estimated. however, equation (1) holds true if the data have been acquired using a perfect circular array geometry. in the case of irregular arrays, the average of the spatial auto-correlation ratio must be computed, so the mspac formulation must be used. using the rings concept, a given array can be divided into several semicircular sub-arrays k, defined by a pair of stations (i, j) having a spacing rij such that rk1 < rij < rk2. for each sub-array, the average of the spatial auto-correlation ratio can be computed using: (2) where {ij is the azimuth between the stations (i, j). thus, knowing the average of the spatial auto-correlation ratio, the cr value of equation (1) can be computed for a given frequency. in other words, knowing , the dispersion curves can be estimated. dispersion curves the vertical components of the data acquired were used to compute the dispersion curves for each array. the data were processed using a classical procedure to synchronize all of the signals and to remove the mean and the trend using sac algorithms [goldstein et al. 2003]. table 1 gives the timetable for each array configuration. to compute the theoretical depth of investigation for each array, we used the tokimatsu [1997] criteria, known as the m/3 criteria: (3) where kmin is the minimum wavenumber retrieved computing the atf function. a theoretical depth of about 90 m was computed for the largest aperture array (array c). the use of several aperture arrays allows to solve a wider range of frequencies. therefore, by combining together the results for the three arrays, we were able to solve a bandwidth from 1 hz to 20 hz. figure 6 shows the mean dispersion curves retrieved, with the colors used to identify the arrays used. in figure 6, we represent together the mean dispersion curves retrieved (in bold) and the resolution curves retrieved with the atf analysis for all of the three arrays. for each of the dispersion curves, we also plotted the computed error bars that give information on the reliability of the curve. the red curve site effect constraints from seismic noise 64 figure 6. dispersion curves (continous lines) obtained from the correlation coefficients for the three arrays. vertical bars, errors computed for each dispersion curve. dashed lines, minimum and maximum resolvable wavelengths retrieved for each array. blue, outer array; green, middle array; red, inner array. , rr j c0z r =t ~ ~ ~^ ^h hc m ,rzt ~^ h ,rzt ~^ h 1 , ,r < < k ij ij r r r ij 1 2k ij k =t ~ r t { ~ {d^ ^h h/ ijkt ~^ h 3 3 2d kmax max min = = m r 65 in figure 6 appears to be better constrained, which is due to the high-density solution retrieved in the frequency–velocity grid obtained after seeking all of the possible solutions for the data [wathelet et al. 2005]. starting from these three curves, we computed a single mean dispersion curve that was used to obtain the s-wave velocity profile. inversion the mean dispersion curve computed for the data acquired was inverted to obtain the s-wave velocity profile using the implementation of the neighborhood algorithm [sambridge 1999] performed by wathelet et al. [2004]. this implementation allows the generation of random velocity models, which are then used to compute the theoretical dispersion curves. to find the best velocity model, the theoretical dispersion curves are compared to the experimental ones, and the misfit value is computed. the solution of the inversion, i.e. the final velocity model, is that which has the lower misfit value. as in all classical inversion problems, we had to define the data to invert and the parameters to retrieve. in our case, the data were represented by the mean dispersion curve computed for the vertical component of the ambient noise. the parameters were, instead, the s-wave velocity and the thickness of the soil layer. to explore all of the parameter space of the velocity models and to choose the best one, we used a trial-and-error procedure. as the geological model known for the area, which comes from well data, is available only for the p-wave, we cannot use this as the starting model in our procedure. nonetheless, we can use information about the structure and density values. using this little information, we assumed a velocity model as a two-layered structure: a sedimentary layer over a bedrock. by studying the best-fit model retrieved, as the inversion proceeded, we were able to modify our parameters (s-wave velocity and thickness range). starting from this simple two-layered model, we moved towards a more complicated sedimentary layer, increasing the number of its sub-layers (table 2). finally, the best model retrieved had 1,000 sub-layers in the sedimentary layer over a bedrock. this high number of sub-layers allows us to assume a gradient feature for the sedimentary layer, in agreement with the p-wave velocity profile known for the area. to test the robustness of the final model retrieved, and to randomly explore all of the parameter space, we ran 3 to 4 inversion processes for each set of parameters (table 2) using different seeding (i.e. starting model) and exploring about 7,000 models for each run. figure 7 shows the one-dimensional s-wave velocity model obtained, and the corresponding dispersion curves. the vs model shows a velocity gradient for the sedimentary layer ranging from 200 m/s to 800 m/s down to about 50 m in depth. each model explored in figure 7 is plotted using a color that corresponds to the computed misfit value. all of the models approach the best-fit model (figure 7, red). this agreement is evident down to 40 m, while instead a broader convergence appears for the bedrock velocity. the variability in the range of a 40-50 m depth does not have great relevance. the misfit values computed for the profiles plotted in pink in figure 7 are the highest that were retrieved in the entire inversion process, and this means that these models are those that are badly constrained. the fundamental rayleigh-wave dispersion curves computed for all of the models retrieved are also plotted in figure 7, and they are compared with the experimental model (figure 7, black solid line). figure 7 also shows the convergence of the velocity profiles. all of the theoretical dispersion curves fit the uncertainty of the mean dispersion curve very well, as indicated by the error bars, which confirms the robustness of the result obtained. the amplitude of the error bars is larger at low frequency, which corresponds to greater depth of exploration. this can be related to the increasing uncertainty of the phase-velocity estimate at lower frequency, and to the decrease in the sensitivity with depth. conclusions in this study, we have shown the results of the seismic noise analysis performed on data acquired at the san fele site site effect constraints from seismic noise of sub-layer min max vmin vmax vmin vmax vmin vmax 1 100 150 3500 1000 4500 1 1000 50 2500 1000 4500 2 1 500 50 500 150 1000 1000 4 1 100 50 350 300 800 1000 3000 10 1 100 50 1000 500 2000 500 2000 1000 1 90 50 500 150 1500 50 3500 number thickness (m) top (m/s) bottom (m/s) sedimentary layer bedrock table 2. velocity and thickness parameters used in the inversion procedure. (southern apennines). the aim of this study was to investigate the feasibility of the mspac methodology to obtain s-wave velocity profiles from ambient noise vibration data also in very complex areas, such as the southern apennines. we used both single-station and array data analysis, obtaining results that are in agreement. the single station analysis, which were performed using the h/v methodology, show that no resonance frequency characterizes the san fele site. this might mean that there is no high-velocity contrast between the soil-layer and the bedrock at the site. this result is confirmed by that obtained using the mspac methodology. indeed, figure 7 shows that a gradient s-wave velocity profile is obtained down to about 50 m, and that there is no abrupt change in the velocity of the underlying layer. furthermore, the procedure used has allowed simulation of quite a complex model in the sedimentary layer, i.e. a velocity gradient, using a simple two-layered model. we assume that the gradient velocity profile retrieved is probably due to the presence of a single tectonic unit, for which the velocity variation gives information as to the different degrees of alteration and/or different sizes of the grain. this is supported by the data from the san fele well, which was drilled down to a depth of about 5,000 m, and encountered a pile of imbricate deposits that are referable to the lagonegro unit [patacca 2007, improta et al. 2003]. the data available for this well show that down to about 40 m in depth the lithology is referred to as marly shale deposits that overlap shale and siliceous shale deposits. these data appear to explain the broader convergence of the s-wave velocity profile that is obtained at the depth of 40-50 m (figure 7). with reference to the nehrp classification, the area of san fele can be classified in the c class, as dense soil and unconsolidated rocks due to the high number of fractures. the present study shows once again that the combined use of h/v and mspac methodologies provides robustness for the results obtained. moreover, the use of ambientnoise vibration data is a very useful, non-invasive and low-cost method to obtain shear-wave velocity profiles and thicknesses of soil-layers. finally, the vs profile is one of the most important pieces of information for the assessing of site effects in earthquake engineering. for this reason, we believe that the methodology applied to the san fele site is very useful in the seismic-risk assessment for the southern apennines. acknowledgements. this study was supported financially by amra s.c. a r.l. 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(2005). array recordings of ambient vibrations: surface-waves inversion, phd thesis, university of liege, belgium, 177 pp. wathelet, m., d. jongmans and m. ohrnberger (2005). direct inversion of spatial autocorrelation curves with the neighborhood algorithm, b. seismol. soc. am., 95, 1787-1800. weber, e., v. convertito, g. iannaccone, a. zollo, a. bobbio, l. cantore, m. corciulo, m. di crosta, l. elia, c. martino, a. romeo and c. satriano (2007). an advanced seismic network in southern apennines (italy) for seismicity investigations and experimentation with earthquake early warning, seismol. res. lett., 78, 622-634. woods, j.w., and p.r. lintz (1973). plane waves at small arrays, geophysics, 38, 1023-1041. *corresponding author: margherita corciulo, laboratoire de géophysique interne et tectonophysique (lgit), université j. fourier, grenoble, france; e-mail: margherita.corciulo@obs.ujf-grenoble.fr. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. site effect constraints from seismic noise 68 c a m p o m a g n e t i c o s o l a r e e c o r r e n t f t e l l u r i c h e enrico medi nel q u a d r o di due p r e c e d e n t i n o t e p u b b l i c a t e n e g l i « a n n a l i di c e o f ì i c a » ( ' ) n e l l e q u a l i è t r a t t a t a u n a n u o v a i p o l e s i s u l l ' o r i g i n e del c a m p o m a g n e t i c o d e l l a t e r r a e di a l c u n i f e n o m e n i ad esso c o l l e g a l i è p o s t a la e s p o s i z i o n e q u i p r e s e n t e . e s s a m e l l e in e v i d e n z a u n f a t t o fisico c h e p o t r e b b e c o s t i t u i r e u n a d e l l e c a u s e d e l l ' e s i s t e n z a d e l l e c o r r e n t i t e l l u r i c h e . si f a c c i a n o le s e g u e n t i c o n s i d e r a z i o n i . il sole p o s s i e d e un m o m e n t o m a g n e t i c o c h e al l a d i s t a n z a m e d i a a l l a q u a l e si trova la t e r r a ( 1 , 5 . io1 3 c m ) può e s s e r e v a l u t a l o c i r c a 2 . i o la t e r r a nel suo m o t o di r i v o l u z i o n e i n t o r n o al sole si sposta in q u e s t o c a m p o m a g n e t i c o ed è di c o n s e g u e n z a soggetta a l l a f o r z a di l o r e n t z in tutti i s u o i p u n t i . p e r t r o v a r e l ' o r d i n e di g r a n d e z z a d e l l e torze e l e t t r o m o t r i c i c h e n e r i s u l t a n o si s u p p o n g a u n a s b a r r a c o n d u t t r i c e c h e si m u o v a p e r p e n d i c o l a r m e n t e al c a m p o m a g n e t i c o s o l a r e con la v e l o c i t à m e d i a di r i v o l u z i o n e del p i a n e t a ( 3 . 1 0 ° c m / s e c ) . agli e s t r e m i d e l l a s b a r r a si g e n e r a u n a forza e l e t t r o m o t r i c e ind o t t a f a c i l m e n t e c a l c o l a b i l e bev_ = p f d v = e x d l = bvdl r e c m e t t e n d o i v a l o r i n u m e r i c i e r i d u c e n d o in v o l t si h a p e r u n a s b a r r a a v e n t e la l u n g h e z z a di uri c h i l o m e t r o d i f o r z a e l e t t r o m o t r i c e fra gli e s t r e m i , di i r e m i l l i v o l l . t a l e v a l o r e è d e l l o stesso o r d i n e di g r a n d e z z a del v a l o r e m e d i o t r o v a l o s p e r i m e n t a l m e n t e n e l l e i n d a g i n i s u l l e c o r r e n t i t e l l u r i c h e . i . o s t u d i o p a r t i c o l a r e g g i a t o della i p o t e s i s a r à p r e s e n t a t o s u c c e s s i v a m e n t e ; in ogni caso s e m b r a e v i d e n t e l ' e s i s t e n z a di u n t a l e e f f e t t o , m a è c h i a r o c h e l e c o r r e n t i c h e ne r i s u l t a n o sono e n o r m e m e n t e c o m p l i c a t e n e l l o r o a n d a m e n t o d a l l a diversa c o n d u c i b i l i t à d e i m a t e r i a l i c h e c o s t i t u i scono il p i a n e t a e da a l t r e m o l t e p l i c i c a u s e . e f f e t t i a n a l o g h i si d e b b o n o p r o d u r r e negli s t r a t i i o n i z z a l i del1 alta a t m o s f e r a dove si possono p r e v e d e r e c o r r e n t i i o n i c h e dovute a c a m p o m a g n e t i c o s o l a r e e c o r r e n t i t e l l u r i c h e 5 6 0 t a l e c a u s a e i n f l u e n z a n t i la v a r i a z i o n e d i u r n a del c a m p o m a g n e t i c o t e r r e s t r e . a p p l i c a n d o t a l i c o n s i d e r a z i o n i a l l a t e o r i a g e n e r a l e d e l l a c o r o n a e l e t t r o n i c a ( v e d i n o t e p r e c e d e n t i ) si p o s s o n o i n t r a v e d e r e a l c u n e c o n s e g u e n z e s u l l a v a r i a z i o n e d i u r n a del c a m p o m a g n e t i c o t e r r e s t r e . palermo •— istituto di fisica dell'università — ottobre 1918. riassunto nel quadro di ilue precedenti note su una nuova teoria circa l origine del campo magnetico terrestre, si formula una ipotesi sulle cause dell'esistenza delle correnti, telluriche e della variazione diurna del campo magnetico terrestre. b i b l i o g r a f i a (1) medi s . , sull'origine del campo magnetico terrestre. « a n n . di geofìsica », i. 3. 1948. medi s . , stili'orìgine, del campo magnetico terrestre e sulle tempeste magnetiche. « a n n . di geofisica », 1. 4, 1918. hongwei_corrected_layout 6 a signal-strength model for gravity-field spectrum components in a local space liu hongwei1,*, wang zhaokui2, zhang yulin1,2 1 institute of aerospace science and engineering, national university of defence technology, changsha, china 2 school of aerospace, tsinghua university, beijing, china annals of geophysics, 55, 5, 2012; doi: 10.4401/ag-5459 abstract in the earth gravity-field model, potential coefficients are projections of the earth gravitational potential on the corresponding spherical harmonics by degree and order. these coefficients reflect the spectral composition of the earth gravitational potential in the whole space, but they cannot reflect the spectral composition in a local area. in this study, the earth gravitational potential was projected on spherical harmonics in a local area using a window function introduced to facilitate the mathematical expressions. then a model for the spectral structure of the earth gravitational potential in the local area was established. the model can reflect the signal strength of any gravity-field spectrum component in the local area, which adds information for the description of the gravity field of the earth, and which has great significance for earth-sciences research and satellite gravity measurements. taking the third-degree coefficients, for example, the signal-strength distributions of potential coefficients were computed. the data show that the signal-strength distributions of thirddegree coefficients on the sphere surface are independent of the longitude. among the third-degree coefficients, the zero-order, first-order and secondorder coefficients are stronger near the two poles of the earth, while the third-order coefficient is stronger near the equator. 1. introduction the earth gravity field is one of basic physical fields of the earth, and it reflects information relating to matter distribution inside the earth and affects physical events on the earth and in the neighboring space. therefore, research into the earth gravity field is always a hot issue in geodesy [jinsheng 2001, 2002]. the gravity field outside the earth is caused by matter inside the earth. as well as the shape of the earth affecting the earth gravity potential, this is also affected by the asymmetry of the matter density distribution inside the earth [jian 1994]. researches into the earth shape and density are two basic missions in earth gravity research [xiaoguang et al. 2000]. due to anomalies in the shape of the earth and to the asymmetry of the matter density distribution inside the earth, the earth gravity potential cannot be expressed as a simple analytical form. the earth gravity field is usually approximated by a spherical harmonics series, and the set of all the gravitational potential coefficients in an infinite series expansion of spherical harmonics represents the earth gravity field model. the idea that the earth gravity potential can be expressed as an infinite series of spherical harmonics has been widely used in geoscience research, and it is a basic method in earth sciences and gravity satellite research. the literature contains conclusions about the relationships between disturbing potential coefficients and matter density anomalies inside the earth [jian 1994]. it was believed that lower degree potential coefficients reflect deep earth matter density anomalies, while higher degree potential coefficients reflect shallow earth matter density anomalies. by using the grace time-varying gravity-field model, global water storage changes were analyzed in xuhua et al. [2006], antarctic ice-sheet mass changes were studied in guangbin et al. [2009], and application of a gravity-field model in tectonic detection was analyzed in leyang et al. [2007]. at the same time, the spherical harmonics infinite series expansion of the earth gravitational potential has been widely used in satellite gravity measurements. satellites can measure the earth gravity field, such as champ, grace and goce, which were designed for recovering gravity potential coefficients in special bands. among these three satellites, champ has measured the earth gravity field using orbit perturbation theory, which has helped to detect lower degree potential coefficients [reigber et al. 2003]. grace measured the earth gravity field by satellite-to-satellite tracking in a low earth orbit, which helped in the detection of the lowerand middle-degree potential coefficients [reigber et al. 2005, zhao et al. 2011]. goce has used gravity gradient measurements that have helped to detect middleand higher-degree potential coefficients [muzi and allasio 2003]. in the earth gravity field model, gravitational potential coefficients are projections of gravitational potential on the article history received october 28, 2011; accepted june 11, 2012. subject classification: earth gravitational potential, local area, spectrum structure, spherical cap harmonic analysis, signal strength. 1017 research articles corresponding spherical harmonics by degree and order in the whole space, and they reflect the spectral composition of the gravitational potential in the whole space. however, gravitational potential coefficients cannot reflect the spectral composition of gravitational potential in a local space, which is a shortcoming of conventional earth gravity-field models. although spherical cap harmonic analysis (scha) can give the spectral composition of gravitational potential on a spherical cap [de santis et al. 1999], because of less gravity potential coefficients, the spectral structure of the spherical cap area reflected by a scha model is limited. in the present study, based on the results in slepian and pollak [1961], the earth gravitational potential was projected onto spherical harmonic functions in a local area, and by appropriate mathematical expressions, the signal-strength model for the gravity potential spectral component in a local space was established. this model will be helpful for earth-science research and gravity satellite measurements. 2. a signal-strength model for gravity-field spectrum components in a local space we establish a spherical coordinate system with the center of the earth as the origin, and we take the z-axis and the earth rotation axis in the same direction, as is shown in figure 1. the spherical harmonic expansion of earth's gravitational potential is: (1) where the definition of the spherical coordinates (t,i,m) is shown in figure 1, and g is the gravitational constant, m is the earth mass, a is the semi-major axis of the earth ellipsoid, are the gravitational potential coeff icients, and is the fully normalized associated legendre polynomial. the relationship between and the associated legendre polynomial, pnk (cosi), is: (2) the nonspherical earth perturbation potential function can be given from equation (1): (3) by the orthogonality of spherical harmonics functions, the gravitational potential coefficients can be derived as: (4) (5) where x, the whole spherical surface, is the integral area. x can be divided into n parts, (6) where, xi (i=1,2,...,n) are the local areas on the spherical surface, as shown in figure 2. xi can be described by a certain longitude and latitude range, and its center is of interest. in equations (4) and (5), if the integral area x is changed into xi, then projections of the nonspherical earth perturbation potential r(t,i,m) on n-degree and k-monics in the local area xi can be obtained: (7) (8) then, (9) it is known that reflect projections of the nonspherical earth perturbation potential on n-degree and k-order spherical harmonics {pnk (cos i)cos km, pnk (cos i)sin km} in the local areas xi, which represent the signal strengths of n-degree and k-order spherical harmonic components in xi, while reflect the sum of these projections in the liu hongwei et al. 1018 , , , v cos sin cosgm a c k s k p1 nn n k2 0 t i m t t m m i = + + 3 = = nk nk nk ^ ^ ^ h h h; e` j// ! , , ! , . cos cos p n k k n n k p 2 0 1 2 1 2 k k nk i d i d + = = = + nk k 0! ^ ^ ^ ^ ^ h h h h h ' , , sin .cos cos r c s k pgm a k k n n n 02 t i m t t m m i = + 3 == nknk nk ^ ^ ^ h h h` j// 2 ! ! , , sin cos d d ,kcos c gm a n n k n k r p 2 1 2 1n k nk m m r t t d t i m i i i = + + x nk ^ ^ ^ ^ h h h h ` j ## 2 ! ! , , sin cos d d ,kcos c gm a n n k n k r p 2 1 2 1 ,nk i n k nk i m m r t t d t i m i i i = + + x ^ ^ ^ ^ h h h h ` j ## 2 ! ! , , sin d d ,sinkcos s gm a n k n n k r p 2 1 2 1n nk m m r t t t i m i i i = + + x nk ^ ^ ^ ^ ^ h h h h h ` j ## 2 ! ! , , sin d d .sinkcos s gm a n k n n k r p 2 1 2 1 ,nk i n nk i m m r t t t i m i i i = + + x ^ ^ ^ ^ ^ h h h h h ` j ## ... ,n1 2x x x x= " , , .c c s snk i n nk i n 1 1 = = = = , ,nk i nk i/ / figure 1. spherical coordinate system with the earth center as its origin. c , snk nk" , c , snk nk" , c , snk nk" , cosp ink^ h cosp ink^ h c , snk, nk,i i" , 1019 whole area x, and represent the total signal strength of n-degree and k-order spherical harmonic components in the whole area. for purposes of satellite gravity field measurements, if the recovery frequency bands involved contain n-degree potential coefficient, sampling data at the places where the signal of the n-degree spectral component is stronger helps to increase the signal-to-noise ratio of n-degree spectrum components and increase the recovery accuracy of n-degree potential coefficients. the signal strength of n-degree and k-order spectrum component in the local areas xi can be computed by equations (7) and (8), which is approximately equivalent to the computing of the data with perturbation potentials in equations (4) and (5), multiplied by a window function. the window function takes the maximum in xi, and the farther the distance away from xi it is, the smaller the window function will be. here, on the sphere surface of fixed radius t0, the signal strength of n-degree and k-order spectrum components in the local area x0 will be computed, where the center of x0 is (t0,i0,m0). the unit vector from the sphere center to the point (t0,i0,m0) is: (10) the unit vector from the sphere center to any point, (t0,i,m), on the sphere surface is: (11) assuming that the angle between two unit vectors in the above is }, then: (12) thus, (13) on the sphere surface of radius t0, making a window function with the point (i0,m0) as its center, (14) as } is the spherical distance from (t0,i,m) to (t0,i0,m0), the window function satisfies 0<h≤1. the window function is 0 only for the point (i0,m0), and the longer the spherical distance from (i,m) to (i0,m0) is, the smaller the h is. the attenuation factor satisfies p>0. the greater p is, the narrower h is, and the smaller p is, the wider h is. multiplying equations (4) and (5) by the window function in equation (14) and substituting equation (3) gets the sine and cosine signal strength of n-degree and k-order spectrum components in the local area x0 where the center is (t0,i0,m0), (15) (16) then the signal strength of n-degree and k-order spectrum components in the local area x0 is: (17) 3. comparison among spherical harmonic analysis, spherical cap harmonic analysis, and translated origin spherical cap harmonic analysis from spherical harmonics expansion, it is known that the gravitational potential can be seen as the superposition of an infinite number of spherical harmonic waves, and potential coefficients are just the vibration amplitudes of these spherical harmonic waves, which reflect the energy of corresponding spherical harmonic waves. by projecting gravitational potential on spherical harmonic functions in the global area, i.e., using spherical harmonic analysis (sha), a conventional gravity model can be obtained, and one potential coefficient reflects the total energy of the corresponding spherical harmonic in the global area. for the projection of gravitational potential on spherical harmonic functions in a local area, i.e., the method investigated here, we get the energy of the spherical harmonics in the local area. this is the relationship between sha and our method. spherical cap harmonic analysis (scha) is a method for the gravitational potential on the spherical cap, which was first proposed by haines, and the effectiveness of this method has been shown by geomagnetic field modeling [haines 1985]. scha was first applied in geodesy by jiancheng, with an scha model for a local gravity field given gravity field spectrum in local area .sin cos sin sin cose 0 0 0 0 0i m i m i=0 ^ h .sin cos sin sin cose i m i m i=^ h .cos sin sin cos cos cose e 0 00$} i i m m i i= = +0 ^ h arc cos cos cos .sin sin cos0 0 0} i i m m i i= +^ h6 @ , .h e,0 0 2 i m =i m p}-^ h ) 2 ! ! , sin cos d dk cos sin cos cos c a n n k n k a h c k s k p p 2 1 2 1 , n k m l l l lm lm lm nk 02 0 0 m m r t d t i m m m i i i i = + + + 3 i m x == nk ^ ^ ^ ^ ^ ^ h h h h h h ` `j j// ## ) 2 ! ! , sin d d .k cos sin cos cos sin s a n k n n k a h c k s k p p 2 1 2 1 , n m l l l lm lm lm nk 02 0 0 m m r t t i m m m i i i i = + + + 3 i m x == nk ^ ^ ^ ^ ^ ^ ^ h h h h h h h ` `j j// ## ) ) .e c snk 2 2= +nk nk^ ^h h figure 2. sketch map of the local region xi on the spherical surface. [jiancheng et al. 1995, de santis and torta 1997]. the basic functions in scha are composed of noninteger degree associated legendre polynomials and trigonometric functions, with orthogonality and completeness, and they can approximate the earth gravitational potential function on the spherical cap. the nonspherical perturbation potential on the spherical cap can be expressed as: (18) where pnkm (cos i) is the noninteger degree associated legendre polynomial. from a mathematical point of view, scha is the method that the potential on a spherical cap is projected on spherical cap harmonic functions. compared to the sha model, when the same spatial resolution for a local gravity field is achieved, fewer gravitational potential coefficients are required in the scha model [fuqing and jinhai 2000], which means that the scha model contains less spectrum components than sha model. at the same time, with scha, the nonorthogonality of odd basis functions (k-m = odd) and even basis functions (k-m = even) can lead to computation instability of potential coefficients and deterioration in the scha model at the edge of the cap when the data distribution is not uniform over the region [de santis 1991]. for the computation instability of potential coefficients in scha, de santis [1991] proposed translated origin spherical cap harmonic analysis (tosca), which was mathematically constructed by moving the new origin in the direction of the cap region and applying scha in the new reference system. tosca can greatly improve the detail in the central regions and can smooth the observations at the edge of the cap. in addition, for the computation of more complex legendre functions with non integer harmonic degrees in scha, de santis proposed adjusted sha (asha), in which the co-latitude interval was adjusted to a hemisphere, and then conventional spherical harmonics could be used [de santis 1992]. just like scha and tosca, the method in the present study can be used for analysis of a potential spectrum in a local area. however, in our method, the gravitational potential in a local area is directly projected on the spherical harmonic functions, not on the spherical cap harmonic functions, which compared to scha, can obtain more potential coefficients and consequently a more comprehensive spectrum structure for the local area, although the computation cost will increase. to facilitate the mathematical expressions, the product of gravitational potential and an exponential function called the window function, which decreases with distance from the center of the local area involved, is projected on the spherical harmonic functions in the global region, which indirectly obtains the projection of the gravitational potential in the local region; i.e., equations (15) and (16). it is in the global region for the projection of gravitational potential that the observation data scope increases and the detail in the central regions is greatly improved, which is similar to de santis 1991, and helps to reduce errors. in de santis [1991], the details in the central regions were greatly improved by using more observation data under a new coordinate system that was obtained by moving the origin of the old coordinates upwards from the center of the earth. the relationships between sha, scha, tosca and asha and the method in the present study are shown in figure 3. liu hongwei et al. 1020 figure 3. the relationships between sha, scha, tosca, asha and the method in the present study. , , ,cos sin cos r mgm a c m s p m k k n km km n m 02 k k t i m t t m m i = + 3 x == ^ ^ ^ h h h` j// 1021 4 numerical computations take the first 10 degrees in the earth gravitational model 1996 (egm96) as a simulation of the earth gravity field. the attenuation factor p determines the size of local area selected, and a right attenuation factor can be obtained by computational results. according to equations (15), (16) and (17), in the case of p = 0~10, the computed signal strength of the thirddegree potential coefficient along the longitude m = r/4 at the altitude of 200 km is shown in figures 4-7. from figures 4-7, it is known that: (1) when attenuation factor p = 0, the signal strength of each spherical harmonic component does not change with the co-latitudes. the reason for this is that p = 0 means the local area is just the entire spherical surface, and their projections on spherical harmonics are certainly the same, making the signal strength of each spherical harmonic component equal in any area of the spherical surface. (2) with the increase in the attenuation factor, local features gradually appear in the signal strength of each spherical harmonic component. with different attenuation factors, the sizes of selected local areas are different, and consequently there will be some differences among the signal strengths of each spherical harmonic component, although their tendencies with the co-latitudes are consistent. gravity field spectrum in local area figure 4. the signal strength of third-degree and zero-order spherical harmonics at different co-latitudes (longitude, m = r/4). figure 5. the signal strength of third-degree and first-order spherical harmonics at different co-latitudes (longitude, m = r/4). according to figures 4-7, when p = 5, the change tendency for the signal strength of each spherical harmonic component with co-latitudes can be shown clearly. take the attenuation factor of the window function as 5, as shown in figure 8, and analyze the distribution law for the signal strength of the third-degree spherical harmonic component along the whole sphere surface. from equations (15), (16) and (17), the signal strength distributions of the third-degree potential coefficients on the sphere surface at the altitude of 200 km are shown in figures 9 to 12. figures 9 to 12 show that the signal strength distributions of the third-degree potential coefficients on the sphere surface are independent of the longitude. the zero-order, first-order and second-order components are stronger near the two poles of the earth, and weaker near the equator, while the third-order is weaker near the two poles and stronger near the equator. from equation (3), it is known that when the longitude m is unchanged, the associated legendre polynomial, pnk (cosi), reflects the change tendency of the n-degree and k-order spherical harmonic with the co-latitude. this represents the signal strength change tendency of n-degree spherical harmonics in the conventional gravity model, such as the egm96 liu hongwei et al. 1022 figure 6. the signal strength of third-degree and second-order spherical harmonics at different co-latitudes (longitude, m = r/4). figure 7. the signal strength of third-degree and third-order spherical harmonics at different co-latitudes (longitude, m = r/4). 1023 model. figure 13 shows the changes of p30 (cosi), p31 (cosi), p32 (cosi) and p33 (cosi) with the co-latitudes, and figure 14 shows the signal strengths of third-degree spherical harmonic components along the longitude m= r/4, with attenuation factor p= 5. from the comparison of figures 13 and 14, it is known that for the change tendency of first-order and third-order spherical harmonic components with the co-latitudes, the results given by the egm96 model and the method in the present study are the same, while for the zero-order and third-order, the results from the egm96 model and the present method are different. this shows that the method in the present study can give a more sophisticated structure for the gravitational potential, and increases the information describing the earth gravity field, which cannot be seen from the conventional gravity model. 5. conclusions here, by projecting the earth gravitational potential on the spherical harmonics in a local area, the signal strength model for each spectral component in the earth gravity field model in a local area was established. compared to the traditional earth gravity-field model, i.e., sha model, this can provide a more sophisticated gravity field spectrum structure in gravity field spectrum in local area figure 8. the window function with an attenuation factor p = 5 on the earth surface. figure 9. the distribution of on the sphere surface.c s2 2+) ) 30 30 the local area and increase the information that describes the earth gravity field, which cannot be seen from the conventional gravity model. compared to scha, the present model can reflect more spectral components in the local area, which helps to design a more reasonable satellite orbit and sampling area for the given gravity field measurement band, and thus we can strengthen the gravity satellite sample data at the places where the signal of the potential coefficients is stronger, and improves the accuracy of the gravity field measurement. in the end, the signal strength distributions for third-degree coefficients were computed. these results show that the signal strength distributions of third-degree coefficients are independent of the longitude, and the zero-order, first-order and second-order coefficients have stronger signals nearer the two poles of the earth, while the third-order coefficient has a stronger signal near the equator. liu hongwei et al. 1024 figure 10. the distribution of on the sphere surface. figure 11. the distribution of on the sphere surface. c s2 2+) ) 31 31 c s2 2+) ) 32 32 1025 acknowledgements. this study is supported by the national natural science foundation of china (grant no. 11002076) and national defence pre-research (grant no. 51320010201). the author would like to thank zou xiancai, associate professor in the school of surveying and mapping, wuhan university, china, for useful discussions. the author gratefully acknowledges helpful suggestions and comments from prof. angelo de santis, istituto nazionale di geofisica e vulcanologia (ingv), rome, italy. references de santis, a. 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(2002). the satellite gravity gravity surveying technology and research of earth's gravity field, journal of geodesy and geodynamics, 22 (1), 1-5 (in chinese). leyang, w., z. jianjun and z. ziqiang (2007). applicable study of eigen-grace02s gravity model on detection of earth's crust tectonics, journal of geodesy and geodynamics, 27, 31-34 (in chinese). muzi, d., and a. allasio (2003). goce: the first core earth explorer of esa's earth observation programme, acta astronautica, 54, 167-175. reigber, ch., p. schwintzer, k.h. neumayer, f. barthelmes, r. könig, ch. förste, g. balmino, r. biancale, j.m. lemoine, s. loyer, s. bruinsma, f. perosanz and t. fayard (2003). the champ-only earth gravity field model eigen-2, advances in space research, 31 (8), 1883-1888. reigber, ch., r. schmidt, f. flechtner, r. könig, u. meyer, k.-h. neumayer, p. schwintzer and s.y. zhu (2005). an earth gravity field model complete to degree and order 150 from grace: eigen-grace02s, journal of geodynamics, 39, 1-10. slepian, d., and h.o. pollak (1961). prolate spheroidal wave functions, fourier analysis and uncertainty — i, the bell system technical journal, 40, 43-64. xiaoguang, h., x. houze and l. dajie (2000). earth's density flat rate and the hypothesis of latitudinal normal density, science in china (series d), 30 (4), 436-441 (in chinese). xuhua, z., w. bin, p. bibo, x. houze (2006). detection of global water storage variation using grace, chinese j . geophys., 49 (6), 1644-1650 (in chinese). zhao, q., j. guo, z. hu, ch. shi, j. liu, h. cai and x. liu (2011). grace gravity field modeling with an investigation on correlation between nuisance parameters and gravity field coefficients, advances in space research, 47, 1833-1850. *corresponding author: liu hongwei, institute of aerospace science and engineering, national university of defence technology, changsha, china; email: liuhw05@163.com. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. liu hongwei et 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 1, 2004, 01/07def 21 annals of geophysics, vol. 47, n. 1, february 2004 key words pressure stimulated currents (psc)piezo stimulated current – rocks – marble-electric precursors 1. introduction transient electric phenomena in the lithosphere have been observed for a long time (varotsos and alexopoulos, 1984a,b; fujinawa and takahashi, 1990; nomicos and vallianatos, 1997; hayakawa, 1999). during the last decade, interest in transient electric signals has been growing and observation networks have been extended in many countries of the world (see park et al., 1993; hayakawa, 1999; kopytenko et al., 2001; hayakawa and molchanov, 2002). many models have been suggested to explain such transient electric phenomena accompanied by fracture. piezoelectric effects constitute one of several factors for modeling (e.g., finkelstein et al., 1973). however the proposed mechanism cannot explain why non-piezoelectric minerals or rocks generate electric phenomena. moreover, electrokinetic effects (mizutani et al., 1976) are also limited to the cases of water-saturated rocks or water flowing through rocks. since electric phenomena are also observed during fracturing of dried rocks it is evident that both piezoelectric and electrokinetic effects may not be the main factors for modeling such phenomena. many researchers accept that the transient electric phenomena are related to crack generation and propagation in the earth’s crust (e.g., molchanov and hayakawa, 1995, 1998; vallianatos and tzanis, 1998, 1999a,b; tzanis and vallianatos, 2002). in order to understand the mechanisms that produce these electric signals, many fracture tests in the laboratory have been conducted using various kinds of minerals and rocks under pressure stimulated currents (psc) in marble samples cimon anastasiadis (1), dimos triantis (1), ilias stavrakas (1) and filippos vallianatos (2) (1) department of electronics, technological educational institution of athens, greece (2) department of natural resources engineering, technological educational institute of crete, greece abstract the electrical behaviour of marble samples from penteli mountain was studied while they were subjected to uniaxial stress. the application of consecutive impulsive variations of uniaxial stress to thirty connatural samples produced pressure stimulated currents (psc). the linear relationship between the recorded psc and the applied variation rate was investigated. the main results are the following: as far as the samples were under pressure corresponding to their elastic region, the maximum psc value obeyed a linear law with respect to pressure variation. in the plastic region deviations were observed which were due to variations of young’s modulus. furthermore, a special burst form of psc recordings during failure is presented. the latter is emitted when irregular longitudinal splitting is observed during failure. mailing address: dr. cimon anastasiadis, department of electronics, technological educational institution of athens, athens-egaleo 12210, greece; e-mail: cimon@teiath.gr 22 cimon anastasiadis, dimos triantis, ilias stavrakas and filippos vallianatos both dry and saturated conditions (e.g., nitsan, 1977; ogawa et al., 1985; brady and rowell, 1986; cress et al., 1987; yamada et al., 1989; enomoto and hashimoto, 1990; hadjicontis and mavromatou, 1994,1995; o’keefe and thiel, 1995; freund, 2000; takeuchi and nagahama, 2001; stavrakas et al., 2003). since transient electric phenomena are promising candidates as earthquake precursors, a series of laboratory experiments of uniaxial compression of marble samples were carried out to understand the underlying physical mechanisms of electric signal generation. in the first set of experiments, marble samples were subjected to a time-varying uniaxial compressional stress at both variable and constant stress rates, not exceeding the elasticity limit (stavrakas et al., 2003; vallianatos et al., 2004). the applied stress henceforth in the experiments is uniaxial compressional stress. the technique used to measure the current emitted from rock samples while applying stress at various rates will henceforth be referred to as pressure stimulated current technique. the experimental results support the validity of the moving charged dislocation (mcd) model (see vallianatos and tzanis 1998, 1999a,b; tzanis and vallianatos, 2002). in the present paper, we show experimental results obtained in the case of applying stress that produced deformations in the plastic range up to fracture. the results suggest that the proportionality factor γ between the emitted current (i) and the stress rate (ds/dt) changes as we pass to the plastic region, in consistency with the mcd model. furthermore, an experimental attempt to understand an electrical activity (i.e a series of short pulses) observed approaching failure is given. 2. sample and experimental description marble belongs to the class of metamorphic rocks. its structural inhomogeneities are due to either natural or man-made causes such as the application of mechanical stress or chemical processing. in the described experiment, thirty dionysos marbles (see table i) collected from mt. penteli, attica were used. the dionysos marble, which has been typically used since ancient times for the construction of artifacts and monuments, is mainly composed of calcite (98%) and other minerals, depending on the variety of marble, such as muscovite, sericite and chlorite (kleftakis et al., 2000). its content in quartz is very low, about 0.2%. its density is 2.7 g/cm3 and its porosity is approximately 0.4%. calcite crystals are polygonic, mainly equisized, sometimes exhibiting twinning and their texture may be characterized as quasi-homoblastic. the rock is white with a few thin parallel ash-green coloured veins containing silver-shaded areas due to the existence of chlorite and muscovite. matrix rocks were intentionally selected to be quasi single-grained. the experiment was conducted in a faraday shield to prevent electric noise. the noise-protected system comprised a uniaxial hydraulic load machine (enerpacrc106) that applied compressional stress to the sample, which was placed on a stainless steel base. the marble sample was placed between two thin teflon plates in the direction of stress to provide electrical insulation. the values of the externally applied stress were recorded using a manometer. a pair of electrodes was attached to the marble sample using conductive paste. the electrodes were attached in a directable i. information table containing the samples used during the described experiments. sample code dimensions experimental technique applied marble dionysos (mm) md001 to md010 50 × 50 × 50 psc technique applied on sequential stress variations md011to md030 50 × 50 × 45 psc measurements closely to the failure with constant stress rate 23 pressure stimulated currents (psc) in marble samples tion perpendicular to the axis of the applied stress (see fig. 1). for electrical measurements, a sensitive programmable electrometer keithley 617 was used, (current range from 0.1 fa to 20 ma). 3. experimental results and discussion in a set of previously conducted psc experiments on marble samples (stavrakas et al., 2003; vallianatos et al., 2004), the samples were subjected to uniaxial stress in the elastic range of the material. in the present set of experiments, multiple incremental stress variations were applied to the sample to pass progressively from the elastic into the plastic range. figure 2a-c shows the measured time series of the applied stress s (fig. 2a), the stress rate ds/dt (fig. 2b) and the current emission (fig. 2c) which is of the order of pa. the recording described corresponds to a stress range that the material behaves elastically. our experimental data obey a scaling law relating the emitted current (i) and the stress rate ds/dt, (see hadjicontis and mavromatou, 1994; vallianatos and tzanis, 1998, 1999a; stavrakas et al., 2003). fig. 1. experimental setup. fig. 2a-c. time records of (a) stepwise applied stress to penteli marble sample (md007), b) the corresponding stress rate (ds/dt) and (c) the emitted psc. recordings of the currents emitted due to successive abrupt changes of the applied stress both in regions where the material behaves elastically and in regions where the material is in the plastic range are shown in fig. 3a-c. figure 3a depicts the sequence of steps of incremental stress variations. we note that the secondary axis was graded in values of normalized stress s/smax where smax is the maximum applied stress on the material close to failure. figure 3b shows the stress rate ds/dt with respect to time. figure 3c is the emitted current (i) with respect to time. we proceed now to the study of the relation between the emitted current (i) and the stress rate ds/dt, when the applied stress s takes values in both the elastic and plastic ranges. in a b c 24 cimon anastasiadis, dimos triantis, ilias stavrakas and filippos vallianatos fig. 3a-c. time recordings of (a) successive abruptly applied stresses onto the sample (md007), b) the corresponding stress rates and (c) psc. fig. 4. scaling factor γ with respect to the normalized stress. fig. 5. normalized experimental stress-strain diagram from a penteli marble sample. b c a 25 pressure stimulated currents (psc) in marble samples previous papers, based on the mcd model, vallianatos and tzanis (1998, 1999a) propose a scaling between the emitted current and the stress rate ds/dt, when the material is uniaxially compressed i dt ds = c (3.1) where γ is a scaling factor which has a reciprocal dependence to the young’s modulus y (i.e. γ ∼∼ ∼∼ 1/y) of the material. since we study the behaviour of marble samples in both the elastic and plastic ranges we may estimate the dependence of the scaling factor γ on stress. figure 4 demonstrates the dependence of factor γ on the normalized stress (s/smax). the scaling factor γ was calculated using the experimental data according to the relationship dt ds i max max =c b l where imax is the maximum value of the emitted current during the application of uniaxial stress (s) and (ds/dt)max is the corresponding maximum stress rate. in the calculation of the quantity s/smax, the stress s corresponds to its averfig. 6a-c. time records of two psc peaks taken from a marble sample (md014) at fracture: a) curves, b) depict stress and (c) stress rate respectively. a b c 26 cimon anastasiadis, dimos triantis, ilias stavrakas and filippos vallianatos fig. 7. sample photo after fracture to show the fracture planes (sample: md027). fig 8a,b. fracture modes of geomaterials: a) planes parallel to the direction of stress, b) planes diagonal to the direction of stress. age value during each stress step. this is practically equal to the instantaneous stress on the sample at the time when the maximum value of the stress rate (ds/dt)max is exerted. it becomes clear in the diagram of fig. 4 that when the applied stress is less than 0.5 smax, the value of the factor γ remains practically constant. noticeable is the fact that as far as s/smax < 0.5 the material behaves elastically and young’s modulus remains constant. this becomes evident in the normalized stress-strain diagram (fig. 5). the diagram was constructed using data of marble samples from mt. penteli, attica (kleftakis et al., 2000). when the ratio s/smax > 0.5 the material exits the elastic range and gradually enters the plastic range thus young’s modulus y is continuously decreasing. according to the mcd model, the scaling factor γ is proportional to 1/y. this is consistent with the experimental result indicating that as the values of normalized stress s/smax increase, the factor γ increases too. the latter becomes evident if the calculated values of γ that correspond to the plastic range for 0.6 < s/smax < 0.7 are considered (fig. 4). from the microphysical point of view we note that applying stresses in the plastic range, structural changes are introduced into the samples depending on the stress state. according to hallbauer et al. (1973), when the sample is stressed uniaxially with stress beyond 0.55 smax up to 0.65 smax then microcracks appear. these cracks are the most dominant factor of all heterogeneities that govern the failure nucleation process in rock samples (lei et al., 2000) and are the sources of macrocracks that will appear when stress exceeds 0.85 smax and increases up to failure. we proceed now to study psc near the failure range. figure 6a-c shows psc emission when the applied stress was greater than 0.95 smax. the continuously increasing stress on the sample and the corresponding stress rate ds/dt are depicted in fig. 6a and 6b respectively. the maximum recorded stress smax on the sample is recorded at t = 334 s, accompanied by a short abrupt decrease of the stress value. simultaneously, the first current peak is recorded (fig. 6c). at the time interval between 340 s and 350 s stress was kept approximately constant and was followed by sample fracture accompanied by a second more intense current peak. a photograph of the status of the specimen after the experiment is shown in fig. 7. the two main fracture planes (which were created while stress was instantaneously decreased) can be seen to lie along the direction of stress (point a in fig. 6a). such fractures could be the result of a large number of microcracks that had already been generated when the sample had suffered a stress between 0.55 smax and 0.85 smax (jaeger and cook, 1979). systematic laboratory study of the psc emitted by marble samples due to stress slightly before fracture suggests that when irregular longitudinal splitting is observed during the failure process (fig. 8a), then for each fracture plane corresponding to a macrocrack a psc peak is observed. thus, the number of psc peaks appearing just before dynamic failure is a b 27 pressure stimulated currents (psc) in marble samples fig. 9a-c. multiple psc peaks of a marble sample (md016) in the time interval of the appearance of microcracks (parallel to the direction of the applied stress) and fracture time: a) stress, b) stress rate and (c) psc. fig. 7) gave multiple psc peaks (see fig. 9a-c). on the other hand, when the failure of the sample forms a shear fracture, then a «single» psc peak is observed (see fig. 10). the latter experimental results could possibly be related to the two types of electric earthquake precursors (i.e. single signals and electrical activities) reported by varotsos and lazaridou (1991). 4. concluding remarks in this paper, pressure stimulated currents (psc) were studied on a typical geomaterial (penteli marble). we first established a correlation between the emitted psc and the applied stress rate. when the material was stressed within its elastic range, a linear relation between psc and stress rate (ds/dt) was observed. deviation from linearity exists when the applied stress on the geomaterial is driven to the plastic range. this is due to the dependence of the scaling factor between psc and stress rate on young’s modulus. we have shown that slightly before fracture, psc emissions were detected associated with the fracture mode of the geomaterial. when the failure of the sample forms a shear fracture, then a «single» psc peak is detected. when irregular longitudinal splitting is observed during the failure process then a psc sequence is recorded which may suggest that each fracture plane corresponding to a macrocrack activates an electrical process. references brady, b.t. and g.a. rowell (1986): laboratory investigation of the electrodynamics of rock fracture, nature, 321, 488-492. cress, g.o., b.t. brady and g.a. rowell (1987): sources of electromagnetic radiation from fracture of rock samples in laboratory, geophys. res. lett., 14, 331-334. enomoto, j. and h. hashimoto (1990): emission of charged particles from indentation fracture of rocks, nature, 346, 641-643. finkelstein, d., r.d. hill and j.r. powell (1973): the piezoelectric theory of earthquake lightning, j. geophys. res., 78, 992-993. freund, f. (2000): time-resolved study of charge generafig. 10. psc peak at diagonal fracture of a marble sample (md018). curve (a) corresponds to stress changes on a normalized axis; curve (b) corresponds to psc. associated with the number of macrocracks created in directions nearly parallel to the direction of the applied stress. the experimental stressing of marble samples with multiple failure planes (i.e. similar to the one depicted in a b c 28 cimon anastasiadis, dimos triantis, ilias stavrakas and filippos vallianatos tion and propagation in igneous rocks, j. geophys. res., 105 (b5), 11001-11019. fujinawa, y. and k. takahashi (1990): emission of electromagnetic radiation preceding the ito seismic swarm of 1989, nature, 347, 376-378. hadjicontis, v. and c. mavromatou (1994): transient electric signals prior to rock failure under uniaxial compression, geophys. res. lett., 21, 1687-1690. hadjicontis, v. and c. mavromatou (1995): electric signals recorded during uniaxial compression of rock samples: their possible correlation with preseismic electric signals, acta geophys. polon., 43, 49-61. hallbauer, d.k. h. wagner and n.g.w. cook (1973): some observations concerning the microscopic and mechanical behaviour of quartzite specimens in stiff, triaxial compression tests, int. j. rock mech. min sci. geomech. abstr., 10, 713-726. hayakawa, m. 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doi:10.4401/ag-7063 s0541 slip of a one-body dynamical spring-slider model in the presence of slip-weakening friction and viscosity jeen-hwa wang institute of earth sciences, academia sinica, taipei, taiwan abstract this study is focused on analytic study at small displacements and numerical simulations of slip of a one-body dynamical slider-slider model in the presence of slip-weakening friction and viscosity. analytic results with numerical computations show that the displacement of the slider is controlled by the decreasing rate, c, of friction force with slip and viscosity, h, of fault-zone material. the natural period of the system with slipweakening friction and viscosity is longer than that of the system without the two factors. there is a solution regime for h and c to make the slider slip steadily without strong attenuation. the viscous effect is stronger than the frictional effect. meanwhile, a change of h results in a larger effect on the slip of the slider than a change of c. numerical simulations are made for a one-body dynamical slider-slider model in the presence of three slip-weakening friction laws, i.e., the thermal-pressurization (tp) friction law, the softening-hardening (sh) friction law, and a simple slip-weakening (sw) friction law, and viscosity. results show that slip-weakening friction and viscosity remarkably affect slip of the slider. the tp and sw friction laws cause very similar results. the results caused by the sh friction law are quite different from those by the other two. for the cases in study, the fixed points are not an attractor. 1. introduction essentially, the rupture processes of an earthquake consist of three steps: nucleation (or initiation), dynamical propagation, and arrest. it is necessary to study the mechanisms controlling the whole rupture processes. such processes are very complicated, and cannot be completely solved just using a simple model. several factors, including brittle-ductile fracture rheology [jeffreys 1942; scholz 1990], normal stress [fang et al. 2011], re-distribution of stresses after fracture, the geometry of faults [fang et al. 2011], friction [nur 1978; dieterich 1979; ruina 1983], distribution of frictional strengths [wang and hwang 2001; wang 2008], healing from dynamic to static friction after an earthquake [wang 1997], pore fluid pressure [scholz 1990; wang 2009, 2011]; elastohydromechanic lubrication [brodsky and kanamori 2001; garagash and germanovich 2012]; thermal pressurization [rice 2006; bizzarri 2011a, 2011b], stress corrosion [anderson and grew 1977; atkinson 1984], and metamorphic dehydration [brantut et al. 2011], will control earthquake ruptures. friction is one of the most important factors in controlling the rupture processes of an earthquake [nur 1978; dieterich 1979; ruina 1983; cao and aki 1986; knopoff et al. 1992; rice 1993; wang 1996, 1997, 2007, 2012; rubin and ampuero 2005; ampuero and rubin 2008; bizzarri 2011c; bhattacharya and rubin 2014]. the friction coefficient, f, is defined as the ratio of shear stress, x, to the effective normal stress, veff , on the fault plane. laboratory rock sliding experiments [byerlee 1978] show that for most rock types, the friction coefficient at which slip initiates is about 0.6 0.8. at lithostatic normal stress and hydrostatic pore pressure, observations suggest that the shear strength of faults exceeds ~100 mpa at seismogenic depths on continental strikeslip faults. the frictional force between two contact planes is classically considered to drop from static one to dynamic one after the two planes move relatively. indeed, the friction law that has been mainly inferred from laboratory experiments is quite complicated and not completely understood, especially for that on natural faults due to a lack of observational constraints. this makes the proper constitutive law for fault friction an elusive mathematical formulation. dieterich [1972] first found time-dependent static frictional strength of rocks in laboratory experiments. dieterich [1979] and shimamoto [1986] observed velocity-dependent frictional strengths. dieterich [1979] and ruina [1983] proposed empirical, velocityand state-dependent friction laws. there are two state evolution laws, i.e., “aging” and “slip” versions. essentially, velocityand state-dependent friction includes two different processes: the velocity-weakening process and the velocity-hardening article history received may 27, 2016; accepted september 6, 2016. subject classification: one-body spring-slider model, displacement, slip-weakening friction, viscosity. one. in fact, a large debate related to the friction laws governing earthquake ruptures has been made for a long time. although this problem is important, it is out of the scope of this article and thus will not be explained in details. a detailed description of the generalized velocityand state-dependent friction law and the debates can be found in several articles [marone 1998; wang 2002; bizzarri and cocco 2006c; bizzari 2011c]. several simple friction laws have been taken into account by some researchers. burridge and knopoff [1967] first considered a velocity-dependent, weakeninghardening friction law. carlson and langer [1989] proposed a purely nonlinearly velocity-weakening friction law which was also used by others [carlson 1991; carlson et al. 1991; beeler et al. 2008] to theoretically model earthquakes. wang [1995, 1996, 2012] considered a piecewise, linearly velocity-dependent weakeninghardening friction, which is simplified from the friction law proposed by burridge and knopoff [1967]. the decreasing (weakening) and increasing (hardening) rates of dynamic friction with sliding velocity are two main parameters of this friction law. cao and aki [1984/85] took a displacement softening-hardening friction law. cochard and madariaga [1994] and madariaga and cochard [1994] assumed that purely velocity-dependent friction models can lead to unphysical phenomena or mathematically ill-posed problems. this means that the velocity-dependent friction law is very unstable at low velocities both during the passage of the rupture front and during the possible slip arrest phase. moreover, ohnaka [2003] stressed that purely velocity-dependent friction is in contrast with laboratory evidence, that is, the friction law is not a one-valued function of velocity. bizzarri [2011c] deeply discussed this point. brune [1979] showed that frictional sliding at x≈10 mpa would produce a heat flow anomaly that is not observed in field data adjacent to the san andreas fault [lachenbruch 1980]. temperature measurements in boreholes drilled across faults shortly after earthquakes also indicate low shear traction during seismic slip [kano et al. 2006; wang 2006, 2011]. since lachenbruch [1980] pointed out the heat-flux paradox, numerous studies have been placed on the thermal properties of the fault-zone rocks to address the importance of thermal effect during earthquake ruptures [chester and higgs 1992; fialko 2004; bizzarri and cocco 2006a, 2006b; wang 2006, 2007, 2009, 2011; bizzarri 2011a, 2011b; bizzarri and crupi 2013]. sibson [2003] also stressed the presence of pseudotachylytes [sibson 1975] due to heating during earthquake ruptures. numerous studies have conducted on modeling the frictional heat produced during seismic sliding. theoretical studies on the spontaneous propagation of earthquake ruptures on 3d faults by bizzarri and cocco [2006a, 2006b] revealed that melting of rocks and fault gouge is likely to occur even with the inclusion of the thermal pressurization of pore fluids. moreover, the dramatic fault weakening at high slip rates predicted by the flash heating of micro-asperity contacts is not able to avert melting [bizzarri 2009]. when fluids are present in faults, thermal pressurization can play a significant role on earthquake rupture and also result in resistance on the fault plane [sibson 1973; fialko 2004; bizzari and cocco 2006a, 2006b; rice 2006; wang 2009, 2011, 2013; bizzarri 2010; bizzarri 2011a, 2011b]. rice [2006] proposed two endmembers models for thermal pressurization: the adiabatic-undrained-deformation (aud) model and slip-ona-plane (sop) model. he also obtained the shear stressslip functions caused by the two models. in spite of a large number of fault-governing friction laws, the only constitutive law able to avoid the melting is a slipand velocity-weakening friction law [sone and shimamoto 2009; bizzarri 2010], for which the fault weakening is so dramatic that it cannot be counterbalanced by the resulting enhanced slip velocities. however, both thermal pressurization of pore fluids and flash heating predict not only a very dramatic stress drop, but also a very high peak in fault slip velocity, so that the final result is that melting temperature is very often exceeded, unless the slipping zone (where the deformation is concentrated) is extremely large [bizzarri and cocco 2006b; bizzarri 2009]. bizzarri [2011a] stressed that when melting occurs, the rheological behavior of the fault zone no longer obeys the coulomb– amonton–mohr formulation, in that a viscous rheology is needed to describe the traction evolution during the ruptures. jeffreys [1942] first emphasized the influence of viscosity on faulting. viscosity can also be controlled by the presence of frictional melts in fault systems [byerlee 1968]. temperature, pressure, water content, etc., will influence viscosity [turcotte and schubert 1982]. scholz [1990] suggested that the residual strength of fault-generated friction melts would be high and so present significant viscous resistance to shear. this inhibits continued slip. on the other hand, spray [1993, 1995] stated that most pseudotachylytes are partial melts possessing low viscosity, and capable of generating a sufficient melt volume to reduce the effective normal stress. thus, friction melts can act as fault lubricants during co-seismic slip [spray 2005]. his results show that viscosity remarkably decreases with increasing temperature. rice et al. [2001] discussed the physical basis of velocityand state-dependent friction, including the direct effect in thermally activated processes aljeen-hwa wang 2 3 lowing creep slippage at asperity contacts on the fault surface. wang [2007] stressed the viscous effect on earthquake ruptures from the comparison between simulated waveforms and seismograms recorded near the chelungpu fault of the 1999 chi-chi, taiwan, earthquake. wang [2011] assumed that quartz plasticity could be formed in the main slip zone of the 1999 chichi, taiwan, earthquake when t >300°c after the fault ruptured. the shear zone with quartz plasticity would be localized in a 5-mm thick heated layer. quartz plasticity could lubricate the fault plane at higher t and yield viscous stresses to resist slip at lower t. on the other hand, several researchers [knopoff et al. 1973; cohen 1979; xu and knopoff 1994; knopoff and ni 2001; dragoni and santini 2015] took the viscous effect as a factor in causing seismic radiation to reduce energy during earthquake ruptures. dragoni and santini [2015] considered with twodegrees-of-freedom dynamical spring-slider model to approach two asperities on a fault. except for the coupling between two sliders, the equation of motion of a slider in their model is essentially the same as that used in this study. hence, their studies are explained in details more or less here. they introduced a term proportional to slip rate in the equations of motion to represent seismic radiation during the slipping modes. they gave a complete analytical solution of the four dynamic modes of the system. any seismic event can be expressed as a sequence of modes, for which the moment rate, the spectrum and the total seismic moment can be calculated. they also considered the energy budget of the event and calculated its seismic efficiency. seismic radiation might change the evolution of the system from a given state, since it moves the boundaries between the different subsets of the sticking region. in addition, the slip amplitude in a seismic event is smaller, while the slip duration is longer in the presence of radiation (or viscosity). the shape of the moment rate function depends on the seismic efficiency and the seismic moment decreases with increasing efficiency at constant radiated energy (or constant viscosity). since the ingredients of an ideal model are only partly understood, a set of equations to describe comprehensively fault dynamics has not yet been established. nevertheless, some models, for instance the crack model and dynamical lattice model, have been developed to approach fault dynamics for a long time. although the frictional effect on earthquake ruptures has been widely studied as mentioned above, the studies of viscous effect on earthquake ruptures are few. the viscous effect mentioned in rice et al. [2001] was just an implicit factor included in the direct effect of friction law. in this work, i will explore the effects of slip-weakening friction, especially due to thermal pressurization, and viscosity on earthquake ruptures on the basis of a one-body spring-slider model, which is used to approach an earthquake fault. the viscous effect is represented by an explicit parameter. results will be applied to understand earthquake ruptures. 2. model one-body spring-slider model the equation of motion of the one-body dynamical spring-slider model (see figure 1) is: md2u/dt2 = −k(u − vpt) −f (u,v) −u(v), (1) where m is the mass of the slider, u is the displacement of the slider, k is the spring constant, vp is the speed of the driving force, v = du/dt is the velocity of the slider, f is the frictional force between the slider and the background and could be a function of u and v, and u is the viscous force between the slider and the background and is a function of v. the slider is pulled by a leaf spring of strength, k, with a constant velocity, vp, which represents the speed of a moving plate. when the driving force, kvpt is slightly larger than the static frictional force, fo, friction changes from static friction strength to dynamic one. viscosity for deformed materials, there are two components, i.e., elastic component and viscous component, when the viscous effect is present. the elastic component can be modeled as a spring with an elastic constant e, given by the formula: v=ef, where v, e, and f are, respectively, the stress, the elastic modulus of the material, and the strain that occurs under the given stress. the viscous component can be modeled as a dashpot such that the stress–strain rate relationship can be given as, v=y(df/dt) where y is the viscosity of the material. there are two models to describe the viscous materials (cf. hudson [1980]). the first one is the maxwell model which can be represented by a purely viscous damper (denoted by “d”) and a purely elastic spring (denoted by “s”) connected in series, as shown in figure 1. the model can be represented by the following equation: df/dt=dfd/dt+dfs/dt=v/y+e -1dv/dt. if the material is put under a constant strain, the stresses gradually relax. when a material is put under a constant stress, the strain has two components. first, an elastic component occurs instantaneously, corresponding to the spring, and relaxes immediately upon release of the stress. the second is a viscous component that grows with time as long as the stress is applied. the maxwell slip of one-body spring-slider model model predicts that stress decays exponentially with time. one limitation of this model is that it does not predict creep accurately. the second one is the kelvin-voigt model, also known as the voigt model, consists of a newtonian damper and hookean elastic spring connected in parallel, as shown in figure 1. it is used to explain the creep behavior of materials. the constitutive relation is expressed as: v(t) = ef(t) + vdf(t)/dt, which represents a solid undergoing reversible, viscoelastic strain. upon application of a constant stress, the material deforms at a decreasing rate, asymptotically approaching the steady-state strain. when the stress is released, the material gradually relaxes to its un-deformed state. at constant stress (creep), the model is quite realistic as it predicts strain to tend to v/e as time continues to infinity. the model is extremely good with modeling creep in materials, but with regards to relaxation the model is much less accurate. although viscosity varies with temperature, pressure, and water content, only a constant viscosity for each segment is considered below. the newtonian viscous force is described by a dash-pot shown in figure 2 specified with viscosity y between the slider and the moving plate, and, thus, the viscous force at the slider is represented by −yv where v is the velocity of the slider. for the kelvin-voigt model, the stress is a function of both strain and strain rate and thus can be applied to the seismological problems [hudson 1980]. however, it is not easy to directly implement viscosity in a dynamical system as used in this study. hence, viscosity is here represented in an alternative way. viscosity leads to the damping of oscillations of a body. the damping coefficient is usually proportional to viscosity and is controlled by the linear dimension of the body in a viscous fluid. for example, according to stokes’ law, the damping coefficient h of a sphere of radius r in a fluid of viscosity o is given by h= 6rry (cf. kittel et al. [1968]). in order to simplify the problem, the damping coefficient is regarded as viscosity hereafter. hence, the viscous force is u=hv. noted that the unit of h is n/(m/s). friction due to thermal pressurization equation (1) exhibits that the motion of the slider is controlled by the frictional force, f (u,v). as mentioned above, friction can also be produced from thermal pressurization. on a fault plane with an area of a and an average displacement u–, the frictional energy caused by the dynamic friction stress, xd, is ef = xdu –a which could result in a temperature rise, dt. frictional heat can conduct outwards from the slipping zone to wall rocks. theoretical analyses [fialko 2004; bizzari and cocco 2006a, 2006b] show that t is described by an error function of distance and decays outwards from the fault plane. under thermal pressurization, the energy and fluid mass conservation equations in a 1-d fault plane, in which the xand y-axes denote the directions along and normal to the fault plane, respectively, can be found in rice [2006]. rice [2006] proposed two end-members models for thermal pressurization: the adiabatic-undraineddeformation (aud) model and slip-on-a-plane (sop) model. the first model corresponds to a homogeneous simple shear strain at a constant normal stress vn on a spatial scale of the sheared layer that is broad enough to effectively preclude heat or fluid transfer. the second model shows that all sliding is on the plane with x(0) = f (vn− po) where po is the pore fluid pressure on the sliding plane (y = 0). for this second model, heat is transferred outwards from the fault plane. the shear stressslip functions, x(u), caused by thermal pressurization [rice 2006] are: (a) xaud(u) = f (vn− po)exp(−u/uc) for the aud model; and (b) xsop(u) = f (vn− po) exp (u/l*) erfc (u/l*) 1/2 for the sop model, the two parameters uc and l* are the respective characteristic displacements, which are in terms of physical properties of the fault-zone materials, to control the shear stress-slip functions of the individual endmember models. for the aud model, uc is associated with the thickness of fault zone and a characteristic displacement of the shear stress-slip function. for the sop model, rice [2006] addressed that there is not a charjeen-hwa wang 4 figure 2. the two types of viscous materials: (a) for the kelvin– voigt model and (b) for the maxwell model. (l= spring constant and y= coefficient of viscosity). figure 1. one-body spring-slider model. in the figure, u, k, h, vp, n, and f denote, respectively, the displacement, the spring constant, the coefficient of viscosity, the velocity of the driving force, the normal force, and the frictional force. 5 acteristic displacement for the shear stress-slip function. the stress xaud(u) displays exponentially slip-weakening friction. indeed, the stress xsop(u) also shows slip-weakening [wang 2009]. since the sop model is based on a constant velocity, it is not appropriate in this study. in order to perform the analytical study of the frictional effect on earthquake ruptures based on the onebody spring-slider model, a simplified linearly slipweakening friction law in the form of f (u) = fo−cu [wang 2016], where c is the weakening rate, is considered hereafter. this leads to the marginal analyses of slip of one-body spring-slider model in the presence of friction, and the results could work for slow earthquakes. for numerical simulations of the dynamical model, a slip-weakening friction law (denoted by the tp friction law hereafter): f (u) = fo exp(−u/uc), where uc is a characteristic displacement, from the stress-slip function of the aud model, is first taken into account. in addition, the displacement softening-hardening friction law (denoted by the sh friction law hereafter): f (u) = fo exp[−(u 2−uc 2 )/c2 ], where fo , uo, and c are constants, used by cao and aki [1984/85] is considered. for the purpose of comparison, a slip-weakening friction law (denoted by the sw friction law hereafter): f (u) = fo/(1+u/uc) similar to the velocity-weakening friction law: f (u) = fo/(1+v/vc), where fo is the static frictional force and vc is the characteristic velocity, proposed by carlson and langer [1989] is also used. the functions of normalized friction force versus displacement for the three slip-weakening friction laws, with uc= 0.1 m and c = 0.3 m, are displayed in figure 3. obviously, the sh friction law that increases slightly with slip and then decreases with increasing slip is quite different from the other two. for the given values of uc and c, the value of sh friction is higher than those of sw and tp friction when the displacement is shorter than a certain value. the value of sw friction is higher than that that of tp friction and the difference between them increases with displacement. obviously, the tp friction law leads to a faster drop of friction and a higher stress drop than the other two. 3. analytical studies substituting the simplified slip-weakening friction law, i.e., f (u) = fo−cu, and viscous force, i.e., u(v) =hv, into equation (1) leads to md2u/dt2 = −k(u−vpt) − fo+ cu − hv. (2) when the driving force, kvpt is slightly larger than fo, the frictional force changes from static one to dynamic one and thus the slider moves. since the duration time of an earthquake rupture is usually short and vp (≈10 −11 m/s) is also very small, the value of kvpt during an earthquake can be ignored. hence, the equation of motion becomes: md2u/dt2 +hdu/dt + (k−c)u = 0. (3) inserting the trail solution ei t into equation (3) leads to ma2 − iha− (k−c) = 0. (4) the solutions of equation (4) are a= (ih/2m) ± [k/m − (h2 + 4mc)/4m2]1/2. (5) inserting equation (5) into eiat leads to e−ht/2mexp {±i [(k/m) − (h2 + 4mc)/4m2]1/2 t}. the first term shows attenuation of slip with time and the second one represents the slip of the slider. the slip remarkably decreases when viscosity is high or the mass of the slider is small. let ~o=(k/m) 1/2 be the natural angular frequency of the one-body spring-slider system without friction and viscosity. the natural period is to= 2r/~o= 2r(m/k)1/2. the natural angular frequency of the present system with friction and viscosity is ~n= [~o 2 − (h2 + 4mc)/4m2]1/2. (6) equation (6) indicates that ~n is an imaginary number when ~o<(h 2 + 4mc)1/2/2m. together with the term e−ht/2m, this inequality results in thorough attenuation of slip. hence, the condition of ~o>(h 2 + 4mc)1/2/2m is necessary for the existence of stable slippage of the slider. under this condition, ~n is lower than ~o. in other words, the natural period of the system, i.e., tn= slip of one-body spring-slider model figure 3. three slip-weakening friction laws: the solid line for f(u) = 1/(1+u/uc) with uc= 0.1 m; the dashed line for f(u) = exp[−(u 2−uc 2)/c2] with uc= 0.1 m and c = 0.3 m; and the dotted line for f(u) = 1/(1+u/uc) with uc= 0.1 m. 2r/~n, is tn= to/[1 − to 2 (h2 + 4mc)/(4rm)2]1/2. (7) obviously, tn is longer than to and increases with h and c, thus indicating that friction and viscosity both lengthen the natural period of the system. in order to further describe the condition: ~o> (h2 + 4mc)1/2/2m, this inequality is re-written as mk > (h/2)2 + mc. figure 4 displays the curve of parabolic equation: h2 + 4mc= 4mk. this gives c= k when h= 0 and h2= 2(mk)1/2 when c= 0. since the four parameters are all positive, the values of h and c that lead to a real value of ~n for stable slippage of the slider must be inside the range (called the solution regime) below the curve. let the general solution of equation (3) be u(t) = c1exp(ia1t) + c2exp(ia2t) with a1= ih/2m +~n and a2= ih/2m −~n. this gives u(t) = e −ht/2m [c1exp(i~nt) + c2exp (−i~nt)]. the velocity of the slider is v(t) = (−h/2m) e−ht/2m[c1exp(i~nt) + c2exp(−i~nt)]+i~ne −ht/2m ×[c1exp (i~nt) + c2exp(−i~nt)]. in order to solve the problem, the initial conditions are: (1) the displacement of the slider is null at t = 0, i.e., u(0) = 0; and (2) the initial velocity is vo at t = 0, i.e., v(0) = vo. the lower bound of vo should be vp because the plate motion is always in operation. this gives c1+ c2= 0; (8a) (−h/2m + i~n)c1− (h/2m + i~n)c2= vo. (8b) the solutions of equation (8) are c1= vo/2i~n and c2= −vo/2i~n. inserting the solutions with c1 and c2 into equation (3) leads to u(t) = e−ht/2m (vo/~n)[exp(i~nt) −exp(−i~nt)]/2i. this gives u(t) = (vo/~n) e −ht/2m sin (~nt), (9) where the ratio vo/~n denotes the amplitude of slip function and varies with four model parameters. the value of u(t) increases with time, reaches the peak value and then decreases with time. the peak value of u(t) with v(t) = 0 occurs at time tp = tan −1(2m~n/h). in order to schematically demonstrate equation (9) in terms of c and h, a few computational examples for the normalized displacement, i.e., u(t)/(vo/~n), with k = 10 nt/m and m = 10 kg, which lead to ~o= 1 hz or to= 2r sec, are given below. based on figure 3, c and h must be smaller than 10 nt/m and 20 nt/(m/s), respectively, when k = 10 nt/m and m = 10 kg. figure 5 exhibits u(t)/(vo/~n) for five values of c, i.e., 0, 2, 4, 6, and 8 nt/m, when h= 0 nt/(m/s). since the displacement decays very fast with h, only u(t)/(vo/~n) for five small values of h, i.e., 0.0, 0.2, 0.4, 0.6, and 0.8 nt/(m/s), when c= 1 nt/m are displayed in figure 6. in figures 5 and 6, jeen-hwa wang 6 figure 4. the solution regime of h and c based on k described by the parabolic equation: h2 + 4mc= 4mk for the one-body springslider model. figure 5. the slip of the slider for five values of c, i.e., 0, 2, 4, 6, 8 n/m (from left to right), when k = 10 n/m, m = 10 kg, and h = 0. figure 6. the slip of the slider for five values of h, i.e., 0.0, 0.2, 0.4, 0.6, 0.8 n/(m/s) (upside down), when k = 10 n/m, m = 10 kg, and c= 1 n/m. 7 the normalized displacements for t > tp are displayed by dashed lines, because the slider stops when v(tp) = 0. it is noted that since ~n decreases with h and c, the actual amplitude should increase with both h and c. 4. numerical simulations in numerical simulations, above-mentioned nonlinear friction laws with linear viscous law are taken into account. substituting the tp friction law and the viscous law into equation (3) leads to md2u/dt2 = −k(u−vpt) − foexp(−u/uc) − hv. (10) in order to easily perform numerical computations, equation (10) must be normalized. the normalization parameters are: do = fo/k, u = u/do, uc= uc/do~o, and vp= vp/do~o and x=(k/m) 1/2 t = ~ot. this gives dx= ~odt, du/dt = [fo/(mk) 1/2] du/dx, d2u/dt2 = (fo/mk) d2u/dx2, a = d2u/dx2, and v = du/dx. in addition, h/(mk)1/2 is simply denoted by h below. clearly, all normalized parameters are dimensionless. hence, equation (10) becomes: d2u/dx2 = −u − hdu/dx− exp(−u/uc) +vpx. (11) let y1= u and y2= du/dx. equation (11) can be rewritten as two first-order differential equations: dy1/dx= y2 (12a) dy2/dx= −y1− hy2− exp(−y1/uc) + vpx. (12b) for the sh friction law, equation (11) can be normalized and re-written as two first-order differential equations with the normalized parameters: dy1/dx= y2 (13a) dy2/dx= −y1− hy2− exp[−(y1 2 −uc 2)/|2] +vpx. (13b) where |= c/do. for the sw friction law, equation (11) can be normalized and re-written as two first-order differential equations with the normalized parameters: slip of one-body spring-slider model figure 7. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized cumulative displacement ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by tp friction of f(u) = exp(−u/uc), for h= 10 (solid line), 20 (dashed line), and 30 (dotted line) when uc = 1. dy1/dx= y2 (14a) dy2/dx= −y1− hy2− 1/(y1+ uc) +vpx. (14b) equations (12) (14) will be numerically solved using the fourth-order runge-kutta method (cf. press et al. [1986]). it is noted that only the positive displacement is considered in this study, because the negative values of slip are usually not admitted in seismological applications. for each friction law, four diagrams are produced from numerical simulations: the time variations in normalized acceleration, a/amax, the time variations in normalized velocity, v/vmax, the time variations in normalized cumulative displacement, ∑u/(∑u)max, where ∑u denotes the cumulative displacement, and the phase portrait of v/vmax versus u/umax. a phase portrait, denoted by y = f (x), is a plot of a physical quantity versus another of an object in a dynamical system [thompson and stewart 1986]. the intersection point of the bisection line, i.e., y = x, and f (x) is called the fixed point, that is, f (x) = x. if the function f (x) is continuously differentiable in an open domain near a fixed point xf and |f ’(xf )|<1, attraction is generated. in other words, an attractive fixed point is a fixed point xf of a function f (x) such that for any value of x in the domain that is close enough to xf, the iterated function sequences, i.e., x, f (x), f 2(x), f 3(x),…, converges to xf . an attractive fixed point is a special case of a wider mathematical concept of attractors. chaos can be generated at some attractors. the details can be seen in thompson and stewart [1986] or other nonlinear literatures. figure 7 displays the time variations in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax with h=10 (solid line), 20 (dashed line), and 30 (dotted line) when uc= 0.1 for the tp friction law. this figure shows the viscous effect. figure 8 displays the time variations in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax for the tp friction law with uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when h=10. figure 9 displays the time variations in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax for the sw friction law with uc= 0.1 (solid jeen-hwa wang 8 figure 8. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized cumulative displacement ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by tp friction of f(u) = exp(−u/uc), for uc = 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when h= 10. 9 line), 0.3 (dashed line), and 0.5 (dotted line) when h=10. figure 10 displays the time variations in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax for the sh friction law with uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when |= 0.3 and h=10. figures 8-10 exhibit the frictional effect. 5. discussion analytic results at low velocities leads to equation (9) which reveals the influences on slip of the slider by friction and viscosity. when c= 0, equation (9) demonstrates the slip of the slider specified by an exponentially damped sinusoidal function. friction only lengthens the natural period of the system, tn; while viscosity not only lengthens tn but also makes the wave attenuated. when h= 0, equation (9) expresses a purely sinusoidal wave. figure 5 shows that the time functions of displacements exhibit a sinusoidal function because of h= 0 and change with c. since the normalized displacement and h= 0 are considered, the peak amplitude does not change with c. actually, the displacement itself depends upon h and ~n which are a function of c, h, k, and m. the occurrence time of peak amplitude and the predominant period increase with c. the initial changing rates of displacement with time are almost the same for the five values of c. results suggest that slipweakening friction can influence slip of the slider. figure 6 exhibits that the time functions of displacements show a sinusoidal function (displayed by the upmost curve) when h= 0, and departs from a sinusoidal function when h> 0. the amplitude decreases with increasing h. meanwhile, the peak amplitude appears earlier when h becomes larger. this might be due to a fact that attenuation of slip wave remarkably increases with h. based on equation (7), the predominant period should increase with h. this point cannot be clearly viewed from figure 6 due to strong attenuation. unlike figure 5, the initial changing rate of displacement with time slightly decreases with increasing h. results suggest that viscosity plays a significant role on slip of the slider. a comparison between figure 5 and figure 6 reveals that a change of h will result in a larger slip of one-body spring-slider model figure 9. the time variations in normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized cummulative displacement ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by sw friction of f(u) = 1/(u + uc), for uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when h= 10. effect on slip of the slider than a change of c. the viscous effect on slip of the slider in the presence of tw friction is displayed in figure 7 which shows the time functions of a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider for h=10 (solid line), 20 (dashed line), and 30 (dotted line) when uc= 0.1. figure 7a shows that the amplitude of a/amax does not change with h, but the occurrence time of the amplitude increases with h. the predominant period of the time function of a/amax increases with . figure 7b shows that the amplitude of v/vmax does not change with h, but the occurrence times of the amplitude increase with h. the predominant period of the time function of v/vmax increases with uc. figure 7c shows that the value of ∑u/(∑u)max decreases with increasing h. figure 7d shows the phase portrait of v/vmax versus u/umax. the three portraits are almost coincided. in the plot, the intersection points of the bisection line (denoted by a thin solid line) with the three curves are the fixed points. although the slope is not calculated for each fixed point, the value can be estimated from the plot. the absolute values of slope at the fixed points are almost the same and are likely all larger than 1, and thus the fixed points are not an attractor. the frictional effects caused by three different slipweakening friction laws in consideration are displayed in figures 8-10. figure 8 shows the time functions in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by tw friction, for uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when h=10. figure 8a shows that the amplitude of a/amax obviously change with uc, and the occurrence time of the amplitude increases with uc. the predominant period of the time function of a/amax increases with uc. figure 8b shows that the amplitude of v/vmax does not change with uc, but the occurrence time of the amplitude increases with uc. the predominant period of the time function of v/vmax increases with uc. figure 8c shows that the value of ∑u/(∑u)max decreases with increasing uc. figure 8d shows the phase portrait of v/vmax versus u/umax. in the plot, the intersection point of the bisection line (denoted by a thin solid line) with each of the three curves jeen-hwa wang 10 figure 10. the time variations in normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized cumulative displacement ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by sw friction of f(u) = exp[−(u 2−uc 2)/|2], for uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when |= 0.3 and h= 10. 11 is the fixed point. although the slope is not calculated for each fixed point, the value can be estimated from the plot. the absolute values of slope at the fixed points increases with uc and are all larger than 1. hence, the fixed points are not an attractor. figure 9 shows the time functions in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by sw friction, for uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when h=10. figure 9a shows that the amplitude of a/amax changes with uc, and the occurrence time of the amplitude increases with uc. the predominant period of the time function of a/amax increases with uc. figure 9b shows that the amplitude of v/vmax does not change with uc, and the occurrence time of the amplitude increases with uc. the predominant period of the time function of v/vmax increases with uc. figure 9c shows that the value of ∑u/(∑u)max decreases with increasing uc. figure 9d shows the phase portrait of v/vmax versus u/umax. in the plot, the intersection point of the bisection line (denoted by a thin solid line) with each of the three curves is the fixed point. although the slope is not calculated for each fixed point, the value can be estimated from the plot. the absolute values of slope at the fixed points increases with uc and are all larger than 1. hence, the fixed points are not an attractor. a comparison between figure 8 and figure 9 suggests that the tp and sw friction laws likely make similar effects on earthquake ruptures. this might be due to similar variations in friction force with slip for the two friction laws as displayed in figure 3. figure 10 shows the time functions in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider, acted by sw friction, for uc= 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) when h=10. unlike figures 7-9, figure 10a shows that a/amax drops suddenly from an initial value to zero and almost does not change with uc. figure 10b shows that the time functions of v/vmax and the occurrence times of the peak value of v/vmax do not change with uc. figure 10c shows that the time functions of ∑u/(∑u)max only slightly increases with uc. figure 10d shows that the phase portraits of v/vmax versus u/umax for the three values of uc are almost the same and form a line intersecting the bisection line. at the respective fixed points. the three fixed points are almost the same. although the slope is not calculated for each fixed point, the value can be estimated from the plot. the absolute values of slope at the fixed points are likely all slightly larger than 1, and thus the fixed points are not an attractor. in addition, the plots in figure 10 are quite different from those in figures 8-9. this might be due to a fact that the sw friction law is quite different from the other two as displayed in figure 3. 6. conclusions in order to study the effects on earthquake ruptures caused by slip-weakening friction and viscosity, the slip of a one-body dynamical slider-slider model is theoretically analyzed and numerically simulated when the two factors are present. analytic results with numerical computations show that the displacement of the slider is controlled by the decreasing rate, c, of friction force with slip and viscosity, h, of fault-zone material. the natural period of the system with slip-weakening friction and viscosity is longer than that of the system without the two factors. there is a solution regime controlled by the parabolic equation of h2 + 4mc= 4mk to result in stable slippage of the slider. meanwhile, a change of h will result in a larger effect on slip of the slider than a change of c. numerical simulations lead to the time functions in a/amax, v/vmax, and ∑u/(∑u)max and the phase portrait of v/vmax versus u/umax of the slider in the presence of three slip-weakening friction laws, i.e., the thermalpressurization (tp) friction law, the softening-hardening (sh) friction law, and a simple slip-weakening (sw) friction law, and viscosity. results show that slip-weakening friction and viscosity remarkably affect the three time functions and phase portrait. the tp and sw friction laws cause very similar results. the results caused by the sh friction laws are quite different from those by the other 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(2016). a dynamical study of frictional effect on scaling of earthquake source displacement spectra, annals of geophysics, 59 (2), s0210, 1-14; doi: 10.4401/ag-6974. xu, h.j., and l. knopoff (1994). periodicity and chaos in a one-dimensional dynamical model of earthquakes, phys. rev. e, 50 (5), 3577-3581. corresponding author: jeen-hwa wang, institute of earth sciences, academia sinica, taipei, taiwan; email: jhwang@earth.sinica.edu.tw. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. jeen-hwa wang 14 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 48, 2, 2005defdef 313 annals of geophysics, vol. 48, n. 2, april 2005 key words ionospheric data ingestion – electron density model – total electron content 1. introduction like other empirical ionospheric models (e.g., international reference ionosphere iri: bilitza, 2001) nequick has been conceived to reproduce median values of the ionosphere electron density. in order to improve its capabilities to describe the ionosphere for actual conditions, a technique based on experimental total electron content (tec) data ingestion into the model has been developed. the technique considers global vertical tec maps to determine the corresponding global grids of the nequick driving parameter az. when the az grid is computed, nequick model is able to calculate the 3d electron density of the ionosphere and therefore the total electron content value for any given ray-path can be calculated by means of numerical integration. the emphasis has been put on tec because it is directly related to the range delay in trans-ionospheric propagation and therefore such kind of reconstruction method could have an application related to satellite navigation systems. it must be noted that other techniques to reconstruct the electron density of the ionosphere have been developed. they are of different complexity and can rely on several kinds of models. the global assimilative ionospheric model (wang et al., 2004), for example, is based on assimilation of data originating from different sources and implies the use of first a model assisted ionospheric electron density reconstruction method based on vertical tec data ingestion bruno nava (1), pierdavide coïsson (1), gloria miró amarante (1), francisco azpilicueta (2) and sandro m. radicella (1) (1) the abdus salam international centre for theoretical physics (ictp), trieste, italy (2) facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata, argentina abstract a technique to reconstruct the electron density of the ionosphere starting from total electron content values has been developed using the nequick ionospheric electron density model driven by its effective ionization parameter az. the technique is based on the computation of az values for a suitable worldwide grid of points. a simple way to obtain relevant az grids is to use global vertical total electron content (tec) maps to define for each grid point as az value, the one that minimizes the difference between the experimental and the modeled vertical tec. having a global grid of az values it is possible to compute the electron density at any point in the ionosphere using nequick. as a consequence, slant tec values for specific ground station to satellite links or ionosphere peak parameter values at any location can be calculated. the results of the comparisons between experimental and reconstructed slant tec as well as experimental and reconstructed peak parameters values indicate that the proposed reconstruction method can be used to reproduce the observed ionosphere in a realistic way. mailing address: dr. bruno nava, the abdus salam international centre for theoretical physics (ictp), strada costiera 11, 34014 trieste, italy; e-mail: bnava@ictp.it 314 bruno nava, pierdavide coïsson, gloria miró amarante, francisco azpilicueta and sandro m. radicella principle models. on the contrary, the proposed reconstruction method aims to be simpler, being based on the use of an empirical electron density model and it foresees the ingestion of only tec data that, for instance, are the most widely available ionospheric data. 2. nequick model the nequick model is an ionospheric electron density model developed at the aeronomy and radiopropagation laboratory of the abdus salam international centre for theoretical physics (ictp), trieste, italy and at the institute for geophysics, astrophysics and meteorology of the university of graz, austria, in the framework of the european commission cost (co-operation in the field of scientific and technical research) action 251. it is based on the original «profiler» proposed by di giovanni and radicella (dgr, di giovanni and radicella, 1990). nequick is able to give the electron concentration distribution on both the bottomside and topside of the ionosphere and it is a quick-run model particularly tailored for trans-ionospheric applications. the model has been used by the european space agency egnos (european geostationary navigation overlay service) project for assessment analysis and has been proposed for single-frequency positioning operations in the framework of the european galileo project. it has been adopted by the international telecommunication union, radiocommunication sector (itu-r) recommendation p.531-6 (now superseded by p.5317; itu, 2001) as a suitable method for tec modeling. to describe the electron density of the ionosphere above 100 km and up to the f2 layer peak this model uses a modified dgr profile formulation (radicella and leitinger, 2001) which includes five semi-epstein layers with modeled thickness parameters and is based on anchor points defined by foe, fof1, fof2 and m(3000)f2 values (hochegger et al., 2000). these values can be modeled or experimentally derived. the model topside is represented by a semi-epstein layer with a height dependent thickness parameter empirically determined. the basic inputs of nequick electron density model are: position, time and solar flux (or sunspot number); the output is the electron concentration at the given location and time. in addition the nequick package includes specific routines to evaluate the electron density along any ray-path and the corresponding tec by numerical integration. nequick (fortran 77) source code is available at http://www.itu.int/itu-r/software/studygroups/rsg3/databanks/ionosph/ 2.1. reconstruction technique for the purpose of the present work, nequick has to be regarded as a «profiler» driven by its local ionization level index that plays the role of an effective 10.7 cm solar radio flux and determines the complete vertical electron density profile at a given epoch and for a given location. being the vertical tec calculated with nequick a monotonic function of this index alone, it is possible to define as az the ionization level value that minimizes the difference between an experimental vertical tec and the corresponding modeled vertical tec obtained by integration of the vertical electron density profile. applying this concept to all vertical tec values of a global experimental vertical tec map, it is possible to retrieve an az grid that drives nequick to generate a 3d representation of the ionosphere over the globe and reproduces the starting vertical tec map with the requested accuracy. the same approach was previously used in nava et al. (2003), while similar concepts have been adopted in komjathy et al. (1998). in order to apply the described procedure, nequick code has been adapted to use global az grids as input. the grid points are spaced 2.5° in latitude and 5° in longitude and are interpolated to retrieve az values at any wanted geographic location. under suitable conditions, az grids for different epochs can also be interpolated in time to obtain new grids for epochs when experimental data are not available. an operational application of this reconstruction method was implemented using available experimental vertical tec maps (nava et al., 2003). several institutions around the world pro315 a model assisted ionospheric electron density reconstruction method based on vertical tec data ingestion duce such maps using global positioning system (gps) derived slant tec data: the network of the international gps service (igs) stations in fact provides measurements that can be used to obtain information on ionospheric electron content along the ray-paths from satellites to ground receivers. for example the centre for orbits determinations (code), university of berne, switzerland, produces global bi-hourly maps (schaer et al., 1996) based on spherical harmonics expansion of vertical tec. these maps are given on a daily basis and made available through the internet (http://www.cx.unibe.ch/aiub/ionosphere.html). for the purpose of the present study the vertical tec maps produced by the satellite and geodesy group of the national university of la plata, argentina, have been used as reference for the minimization process. la plata vertical tec maps are computed using gps data collected from an average of about 130 igs stations distributed worldwide. la plata maps are based on a spherical harmonic expansion of degree and order 15 for vertical tec and the daily solution consists of 24 sets of coefficients and one instrumental delay value for each station and for each satellite included in the computation (brunini, 1998). la plata maps have been chosen because they are computed on hourly basis and they are built using the sun fixed/modip reference frame, modip µ being defined by tan cos =n z z with φ the true magnetic dip in the ionosphere (usually at 300 km) and φ the geographic latitude (rawer, 1963). these features improve temporal and spatial resolution of the vertical tec maps and then slightly better results are expected when nequick is used together with the reconstruction technique to reproduce the electron density of the ionosphere. an example of equivalent vertical tec for the day 27 march 2000 at 17:00 ut is shown in fig. 1. 2.2. test of the technique in order to evaluate the effectiveness of the reconstruction technique, slant tec and vertical incidence experimental data have been used for comparison with the corresponding reconstructed quantities. fig. 1. example of global vertical tec map produced with la plata technique for the day 27 march 2000, 17:00 ut. 316 bruno nava, pierdavide coïsson, gloria miró amarante, francisco azpilicueta and sandro m. radicella the day 27 march 2000 (a quiet day in a period of high solar activity) was chosen as reference day for the test and hourly la plata maps were used to generate the set of 24 az grids needed to drive the nequick model. 2.3. slant tec for consistency reasons, the slant tec data used to validate the reconstruction technique were computed from rinex files recorded at fig. 2. cumulative distributions of the absolute values of the differences between experimental and reconstructed slant tec data for stations lying in different modip intervals. the statistics are computed using slant tec data at 5 min time intervals for the day 27 march 2000. 317 a model assisted ionospheric electron density reconstruction method based on vertical tec data ingestion the same igs stations used to obtain the vertical tec maps. an elevation mask of 10° to all available satellites was applied and for practical purposes slant tec data every 5 min were used. the experimental slant tec data were compared with the corresponding slant tec values computed with nequick driven by az grids derived from la plata vertical tec maps. therefore az values at 5 min time interval were calculated by interpolation of consecutive hourly az values. defining as error the difference between a reconstructed and the corresponding experimental slant tec, for each igs station the data analysis was based on the computation of cumulative distributions of the absolute values of the errors obtained with 24 h of data. in order to have a global view of the reconstruction technique capabilities, cumulative distributions of the absolute values of the errors were computed taking into account data for stations lying in specific modip intervals. these results are shown in fig. 2, where the cumulative distributions of the absolute values of the errors are plotted together with the lines corresponding to 50 and 95 percentiles. the statistics are summarized in table i, where the number of stations considered and the value in tecu (1 tecu = 1016m−2) corresponding to the 95 percentile for each modip interval are indicated. the analyzed data show a general asymmetry of errors behavior in the two hemispheres of table i. statistical analysis summary for the day 27 march 2000: the first and second columns indicate the modip interval with the corresponding number of igs stations whose data were used to compute the cumulative distribution of the absolute value of the errors; the third column indicates the 95 percentile of the distribution of the absolute value of the errors. modip interval number of stations 95 percentile (−90°, −60°) 2 31.6 tecu (−60°, −30°) 25 27.0 tecu (−30°, 0°) 5 37.4 tecu (0°, 30°) 4 37.4 tecu (30°, 60°) 72 19.6 tecu (60°, 90°) 24 17.9 tecu fig. 3. location of the igs stations homogeneously distributed used to perform a concise analysis of the reconstruction technique performances for the day 27 march 2000. modip isolines at −60°, −30°, 0°, 30°, 60° are indicated. fig. 4. cumulative distributions of the absolute values of the differences between experimental and reconstructed slant tec data corresponding to 27 igs stations homogeneously distributed in modip, for the day 27 march 2000. in the first case (left panel) the modeled slant tec were reconstructed using nequick driven by az grids derived from la plata vertical tec maps. in the second case (right panel) the modeled slant tec data were calculated using nequick driven by the solar flux of the day. 318 bruno nava, pierdavide coïsson, gloria miró amarante, francisco azpilicueta and sandro m. radicella the earth, whereas in the same hemisphere a latitudinal dependence of the errors magnitude can be observed: the agreement between experimental and reconstructed slant tec is worse for stations with low modip than for the other regions. this would indicate a lower effectiveness of the reconstruction method at low latitudes, as could be expected because of the complexity and high variability of the ionosphere at low latitudes. nevertheless it must be noted that a part of this performance reduction could be related to a poor data coverage and lower experimental data quality for low-latitude regions. as a final step, in order to have a concise indication of the reconstruction technique performances at global level, 27 igs stations homogeneously distributed in modip (see fig. 3) were selected and the corresponding experimental and reconstructed slant tec data (using nequick driven by az grids derived from la plata vertical tec maps) were used to compute the cumulative distribution of the absolute value of the errors for the reference day. the results are shown in the left panel of fig. 4, where the cumulative distribution of the absolute value of the errors is plotted together with the lines corresponding to 50 and 95 percentiles. in this case the 95 percentile of 28.6 tecu represents the global performances of the reconstruction technique for the reference day. the same experimental data set was used with modeled slant tec obtained using nequick driven by the 10.7 cm solar radio flux corresponding to the day 27 march 2000 as given by the space environment center (http://www.sec.noaa.gov). the corresponding cumulative distribution of the absolute value of the errors was computed and is shown in the right panel of fig. 4. in this case, the 95 percentile is 56.1 tecu and represents the global performances of nequick model for the reference day, without any adaptations derived from tec data ingestion. therefore the comparison of the cumulative distributions in fig. 4 gives an indication of the potentiality of the reconstruction technique based on model adaptation through actual data. for the reference day, the slant tec errors at 95 percent level are reduced to a half of those obtained with the standard use of the nequick model. 2.4. vertical incidence parameters experimental peak parameters obtained from worldwide distributed ionosondes were used to test the capability of reconstruction method to reproduce peak parameters values on the basis of az grid calculation. an exhaustive analysis is given in (miró amarante et al., 319 a model assisted ionospheric electron density reconstruction method based on vertical tec data ingestion 2003) and, as a further example, the comparison between hourly fof2 measured at the locations indicated in table ii on 27 march 2000 and the same values reconstructed with nequick model driven by az grids derived from la plata maps is presented in fig. 5. as can be seen from the linear fit shown in the plot, there is a good agreement between experimental and reconfig. 5. measured and reconstructed fof2 using nequick model driven by az grids derived from la plata vertical tec maps for the day 27 march 2000. the best fit-line is also indicated. fig. 6. example of global fof2 map reconstructed using nequick model driven by az grids derived from la plata vertical tec maps for the day 27 march 2000, 17:00 ut. table ii. ionosondes used to validate the electron density reconstruction technique based on nequick model driven by az grids derived from la plata vertical tec maps for the day 27 march 2000. ionosonde ionosonde ionosonde lat n (°) long e (°) bermuda 32.4 – 64.7 dyess 32.5 – 99.7 millstone hill 42.6 – 71.5 puerto rico (ramey) 18.5 – 67.1 wallops island 37.9 – 75.5 fairford 51.7 – 1.5 goose bay 53.3 – 60.3 narssarssuaq 61.2 – 45.4 sondrestrom 67.0 – 51.0 qaanaaq 77.5 – 69.4 structed fof2. therefore, even if the same degree of accuracy cannot be expected at all geographical locations, an attempt to reconstruct global fof2 maps using only gps derived tec 320 bruno nava, pierdavide coïsson, gloria miró amarante, francisco azpilicueta and sandro m. radicella data has been made. as an example, fig. 6 illustrates a reconstructed worldwide fof2 map for the day 27 march 2000, 17:00 ut. 3. conclusions a model assisted reconstruction technique, able to provide a realistic 3d representation of the electron density of the ionosphere under given conditions, has been developed. it is based on experimental vertical tec data ingestion into the nequick model: vertical tec maps are used to retrieve worldwide grids for the model driving parameter az. these az grids are computed at any epoch for which a vertical tec map is available and, under particular conditions, the az grids can be interpolated in time to provide new grids for epochs when vertical tec maps are not available. the computation of an az grid allows to calculate the electron density of the ionosphere all over the world with the nequick model. as a consequence it is possible to calculate the tec along any ground-to-satellite ray-path or to reconstruct global fof2 maps. the reconstruction technique itself improves the model performances at global level when nequick is used to reconstruct experimental slant tec values as it is demonstrated by the reduction to a half of the slant tec errors at 95 percent level. the tests performed and the results obtained encourage further developments of the technique. 4. future developments at present the described reconstruction method is based on pre-calculated vertical tec maps used to retrieve the az grid values that can drive nequick. it is important to observe that az grids can be calculated in several ways. a possible method to construct az grids, starting from experimental data, could be based on direct ingestion of slant tec data. in this case a new approach should be followed in order to derive global az grid, starting from scattered slant tec measurements. in addition, due to the flexibility of the nequick model, a reconstruction method based on a combined use of az grids and experimental peak parameters values could be implemented. references bilitza, d. (2001): international reference ionosphere 2000, radio sci., 36 (2), 261-275. brunini, c. a. (1998): global ionospheric model from gps measurements, ph.d. thesis (facultad de ciencias astronómicas y geofísicas, la plata national university). di giovanni, g. and s.m. radicella (1990): an analytical model of the electron density profile in the ionosphere, adv. space res., 10 (11), 27-30. hochegger, g., b. nava, s.m. radicella and r. leitinger (2000): a family of ionospheric models for different uses, phys. chem. earth, 25 (4), 307-310. itu (2001): ionospheric propagation data and prediction methods required for the design of satellite services and systems ionosphere characteristics, recommendation p.531-6, approved in 2001-02, managed by itu-r study group sg3 (version 6 superseded in 2004 by version 7). komjathy, a., r.b. langley and d. bilitza (1998): ingesting gps-derived tec data into the international reference ionosphere for single frequency radar altimeter ionospheric delay corrections, adv. space res., 22 (6), 793-801. miró amarante, g., s.m. radicella, b. nava and p. coïsson (2003): validation of a new method for ionospheric electron density reconstruction by means of vertical incidence data, in proceedings of ‘atmospheric remote sensing using satellite navigation systems’, special symposium of the ursi joint working group, matera (fg), italy, 335-342. nava, b., p. coïsson, g. miró amarante and s.m. radicella (2003): a new model assisted method for ionosphere electron density reconstruction, in proceedings of ‘atmospheric remote sensing using satellite navigation systems’, special symposium of the ursi joint working group, matera (fg), italy, 343-348. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27, 35-40. rawer, k. (1963): propagation of decameter waves (hfband), in meteorological and astronomical influences on radio wave propagation, edited by b. landmark (pergamon press, new york), 221-250. schaer, s., g. beutler, m. rothacher and t.a. springer (1996): daily global ionosphere maps based on gps carrier phase data routinely produced by the code analysis center, in proceedings of the igs ac workshop, silver spring md, u.s.a. wang, c., g. hajj, x. pi, i.g. rosen and b. wilson (2004): development of the global assimilative ionospheric model, radio sci., 39, rs1s06, doi: 10.1029/ 2002rs002854. (received october 15, 2004; accepted march 09, 2005) e m a n o m e t r i s c h e m e s s u n g e n v o n r a d o n u n d t l i o r o n in b o d e n l u f t tt. i s r a e l s . b j o r n s s o n s . s t i l l e r ( * ) r i c e v u t o il 12 marzo 1962 i . e i n l e i t u n g . die direkte messteehnisclie trennung von radon und tlioron in l u f t ist bisher nur dann sieher moglich, wenn die zu untersuchende l u f t ausser den beiden radioaktiven edelgasen weder induktionen noch (geladene und ungeladene) suspensionen entha.lt. da dies nur selten — so z.b. bei luftproben, die aus den bodenkapillaren abgesaugt werden -— der fall ist, ist die unmittelbare emanometrische trennung von radon und thoron bisher nur bei bodenluft in einfacher weise moglich geworden. bei der methode von j . satterly (x) wird die zu untersuchende l u f t probe zu einem delìnierten zeitpunkt moglichst rasch in ein lonisat.ionsgefàss eingesaugt. wàhrend der ersten minuten setzt sich dann der ionisationsstrom zusammen aus dem durch radon bedingten langsam ansteigenden strom (kurve 1 in abb. 1 oben) und dem mit der halbwertszeit von 54,5 sec abklingenden anteil, der durch tlioron bedingt wird (kurve 2 in abb. 1 oben). j . c. sanderson (2) saugt einen bodenluftstrom bestimmter geschwindigkeit durch einen kondensator. der strom steigt ebenso wie im normalem lonisationsgefàss in dem masse an, wie sich die induktionen r a a und rac in ihm ablagern. wird zu einem bestimmten zeitpunkt der luftstrom gestoppt, so klingt der im apparat enthaltene thoronantp.il mit 54,5 sec halbwertszeit ab; der strom sinkt also in der in abb. 1 u n t e n skizzierten weise und steigt bei erneutem durchsaugen in gleicher weise wieder an. (*) rheinisch-westfalische technische hochscliule aachen. 1 1 6 h . i s r a e l s . b j ò r n s s o n s . s t i l l e l t in dei' l i t e r a t u r linden sieh ausser den messungen von j . satterly (*) und j . c. sanderson (2) bisher n u r vereinzelt angaben ùber den thorongehalt in bodenluft. so beriehten h . israel und f . becker (3) ùber messungen in bad nauheim, die nach den genannten methoden von satterly und sanderson in tiefen von 0,5 m, .1 m u n d 2 m ausgefùhrt wurden. h . norinder, a. metnieks und r . siksna (4) (5) haben nach der methode von satterly in tjppsala messungen in 90 cm tiefe durchgefuhrt. r j t l , i i 1 0 2 4 6 8 itìt in min. 0 2 a 6 8 io 1 2 zeit in min. a b b . 1. s e l i e m a t i s c h e d a r s t e l l u n g d e r m e s s t e c h n i s c l i e n t r e n n u n g v o n r a d o n u n d t h o r o n n a c h j. satterly ( o b e n ) u n d j. g. sanderson ( u n t e n ) . i m l'olgenden soli ùber eine verfeinerung und messtechnische e r weiterung der satterly-methode und einige bisher d a m i t erzielt-e ergebnisse berichtet werden. t i . m e s s m b t h o d e . die darstellung der satterly-methode in abb. 1 oben ist idealisiert und entspricht nieht ganz der wirklichkeit. denn es ist dabei angenommen, dass das ionisationsgefàss zum z e i t p u n k t t = 0 so rascli mit der zu untersuchenden l u f t gefullt werden k a n n , dass in dieselzeit kcin zerfall des thorons stattfindet. dies ist eine praktisch unerfùllbare forderung. bei der kurzen lebensdauer des thorons wird wahrend der e m a n o m e t r i s c h e m e s s u n g e n von r a d o n u n d t h o r o n i n b o d e n l u f t 1 li) fùllzeit immer ein gewisser verlust eintreten, dem rechnung getragen werden muss. ferner muss beim einsaugen der luftprobe in das vorher teilweise evakuierte lonisationsgefàss die abhàngigkeit der ionenbildung vom druck beriicksichtigung tìnden. beides zusammen bedingt einen stromverlauf im lonisationsgefàss, der von dem schemabild der abb. t oben erheblich abweieht. zum elektrom et e r ter p a b b . 2. m e s s a n o r d n u n g ( s c h e m a t i s c h ) abb. 2 zeigt die versuchsanordnung: fiir die messungen wurde ein e m a n o m e t e r nach h . israel (6)(*) mit zwei ionisationskammern von je etwa 20 1 i n h a l t verwandt. die aufnahme der bodenluft erfolgte nach der soe. « sondenmethode » (7) durch ansaugen der l u f t durch ein bis in die gewiinschte tiefe in den boden eingeschlagenes diinnes rolir. zur trocknung und befreiung von ionen, staub und induktionen wurde die l u f t durch ein cacl2-rohr und ein watte-, glasoder schwebstofffilter gel ci te t. abb. 3 zeigt eine mit dieser anordnung ani 9.5.1959 in aachen gewonnene messung. dargestellt ist der zeitliche verlauf des lonisationsstromes wahrend der ersten 15 minuten vom beginn des lufteinstromens an. der druckausgleich des vorher auf 400 m m h g evakuierten gefàsses erfolgte dabei mit etwa 20 sec halbwertszeit. das gefass war also nach etwa 100 sec wieder zu 98,5% gefiillt. — der strom steigt (*) h e r s t e l l e r f i r m a s p i n d l e r & h o y e r , g ò t t i n g e n . 131 h . i s r a e l -s . b j ò r n s s o n s . s t i l l e l t wàhrend der ersten i y 2 minuten rasch an. dieses ansteigen ist bedingt durch die z u n a h m e des radons und thorons ini gefass und durch die zunahme des ionisierungsvermògens der a-strahlung bei der druckzunahme im gefass. zeit in minuten a b b . 3. zeitlicher verlauf iles i o n i s a t i o n s s t r o m e s w à h r e n d der ersten 20 m i n u t e n (messung am 9 . 5 . 1 9 5 9 ) . e r s t nach beendetem einstròmen zeigt dann der stromverlauf das nach abb. 1 oben zu erwartende yerhalten. i i i . a n a l y s e . die analytische zerlegung des gemessenen stromverlaufes in den von radon und den von thoron herriilirenden anteil erfolgt entweder in nàlierung unter alleiniger benutzung der nach der gefàssfullung gewonnenen messwerte, anwendung der satterly-analyse gemàss abb. 1 oben auf diese und zuruekextrapolation auf den zeitpunkt t = 0 (vergi. abb. 4) oder exakter u n t e r genauer rechnerischer behandlung des einstromungsvorganges und der dabei dera gefass zugefiihrten thoronmengen. der naherungswert ist, wie m a n sich leicht iiberzeugt. grosser als der wahre w e r t und iibertrifft diesen umso melir, je langsamer das einstròmen erfolgt. e m a n o m e t r i s c h e m e s s u n g e n von r a d o n u n d t h o r o n in b o d e n l u f t 1 l i ) bei deiumrechnung der stromwerte des tlioron-anteiles in curie ist zunàehst zu beachten, dass das erste tjmwandlungsprodukt tha selir kurzlebig ist (0,16 sec halbwertszeit), wahrend der nàcliste a-strahler (tlic) nur selir langsam iiber das langlebige tlib (10,6 li halbwertszeit) entsteht. man braucht deshalb zur umrechnung nur die a-ionisation von tlioron und tha zu berùcksichtigen. \ \ • \ • \ \ • \ \ • \ 0 2 i 6 zeit m minuten a b b . 4. t h o r o n s t r o m a n t e i l bei d e r in a b b . 3 d a r g e s t e l l t e n messung. der rasche zerfall des tha h a t weiter zur polge, dass den im lonisationsgefàss entstehenden tha-atomen keine zeit bleibt, sich ihrer positiven l a d u n g entsprechend (*) zur negativen elektrode liin zu bewegen. sie zerfallen vielmehr praktisch an der gleichen stelle, wie ihre mutteratome. wir kònnen also fui' jeden thoron-zerfall 2 a-strahlen a n n e h m e n . setzen wir nun fiir die a-strahlung des thorons den w e r t von 6,4 mev und tur tha den wert von 6,9 mev an (8) und berechnen daraus u n t e r der aunahme, dass zur bildung eines ionenpaares 35 ey benòtigt werden, ihre ionisationswirkung zu 1,83-io5 fiir thoron bzw. 1,97-io5 ionenpaare je a-strahl fiir tha, so errechnet sich das stromàquivalent st n fiir 1 curie thoron (tn) zu st n = 3,7 • io 10 • (1,83 + 1,97) • io6 • 1,8 • io-10 = 6,75 • io6 aes (**) |1] =-• (2,25 • io"3 a) (*) w i r n e h m e n diese f i i r d a s r a a b e k a n n t e e r f a h r u n g aneli fiir t h a a n . (**) e l e k t r o s t a t i s c l i e s t r o m e i n h e i t e n . 1 2 0 h . i s r a e l s . b j ò r n s s o n s . s t i l l e l t s 1 tì w ss > t> o a co h cs <5 n £ 5 r! 2 s s 1 h ^ ea c i o 00 w . 2 « h h eh 2 a £ 6 o t> o •j o „ y & <1 w w o m e» b h 3 fi g fc w « c5 <3 h 2, 0 m eh p3 3 ,2 4 2, 68 2, 62 2, 81 il 00 in 2, 0 m eh to o a co io 2, 0 m 1 in co <n io i< •!• il [ i l co i 1, 55 m r n /t n 2, 05 1, 54 1, 87 1, 54 1, 48 1, 55 m eh © © -t co io in m m <n in il in 1, 55 m — o o o i i -t -+ il co" -t 1, 0 m r n /t n 1 , 07 1, 84 ! ,2 5 1 ,2 4 1 , 35 1, 35 1, 0 m e h ci <n i in <n | co ! co co a in 1, 0 m ci io o in co in co ' i -t i " 00 co 0, 5 m r n /t n 0, 43 0, 53 0, 59 0, 39 il © 0, 5 m è © in co co co in co co 0, 5 m co l> co co io d at u m 19 61 3. 6 5. 6 10 . 7 11 . 7 12 . 7 14 . 7 18 . 7 19 . 7 24 . 7 26 . 7 m it te l tabelle 2 g l e i c i i z e i t i g e r a d o n u n d t i i o r o n b e s t i m m u n g e n i n b o d e n l u f t i n v e r s c h i e d e n e n t i e f e n i m g a r t e n d e s m e t e o r o l o g i s c i i e n o b s e r v a t o r i u m s a a c i i e n , p i p i n s t r a s s e 1 2 . ( 1 9 6 1 ) . w e r t e i n i o 1 3 curie/cm3 ( u n s i c h e r e w e r t e i n k l a m m e r n ) . 0,5 m 1,0 m 1,5 ni 2,3 m 4,0 ni d a t u i n bri tu bn/tn bn tn bn/tn bn tn bn/tn bn tn bn/tn bn tn bn/tn 1961 31. 7 0 , 9 2,2 0,41 1,1 (0,7) ----2 , " 2 , 3 1,18 6 , 2 4 , 2 1,47 8 , 8 4 , 4 2,00 1. 8 0,8 2,9 0,27 1,0 1,2 0,83 2,5 2 , 5 1,00 6,2 3,8 1,64 8 , 3 5,1 1 ,63 2. 8 1,0 2,3 0,44 — — — — — — — — — — 3. 8 1,0 2 , 5 0,40 1,1 1,2 0,84 — — 6,0 (6,1) — — — — 4. 8 0 , 8 2,4 0,33 1,0 1 ,2 0,83 2 , 5 2 2 1, 14 6 , 3 3,5 1,80 8,7 3 , 9 2,22 7. 8 0,7 2,1 0,33 1,0 1,2 0,83 2 , 5 2,4 1 ,04 5,9 3 , 8 1 ,55 8,1 4 , 7 1 ,73 8. 8 0,6 2 , 3 0,26 1,0 (0,9) — — — 5 , 9 3,9 1 ,51 8,9 3,7 2,31 mittel (*) 0,8 2,4 0,35 1,0 1,2 0,83 2,6 2,4 1,09 6,1 3,9 1,60 1 8,6 4 , 4 1,98 (*) oline die eingeklemmerten werte. • a o g h h o d a! ffl h ss <1 o te! e o a; d a a h o 1 2 2 h. i s r a e l s. b j ò r n s s o n s . s t i l l e l t i v . e r g f . b n i k s e . nach den beschriebenen messverfahren wurden in aachen eine reihe von thoron-bestimmungen in bodenluft durchgefiihrt. geinessen wurde in verschiedenen bodentiefen in den sandboden ini garten des hauses chlodwigstrasse 8 (dienstràume des lehrgebietes geophysik u n d tabelle 3 r a d o n u n d t i i o r o n w e r t e i n b o d e n l u f t in v e r s c h i e d e n e n t i e f e n in b a d n a u h e i m ( 3 ) . w e r t e i n 1 0 1 3 ^ c u r i e / c m . 0,5 in 1,0 ni 2,0 m d a t u m d a t u m bn tn bn/tn bn tn bn/tn rn tn bn/tn 1934 16. 10 — — 4 , 7 6 , 6 — — — 18. 10 ì — 1,3(*) — — — — 4 , 5 — 19. 10 — 3,0(*) — — — — — 25. io . f c " ' . ' 10,6 — — — — 26. 10 — 4 , 8 — 30. 10 — 4 , 7 — — — — mittel 28(**) 3 , 0 9 , 3 57** 7 , 3 7,8 107** 4 , 7 22,8 (*) 0,4 m tiefe. (**) m i t t e l w e r t e a u s einer gròsseren / a h i v o n die zu a n d e r e r zeit a u s g e f i i h r t w u r d e n . e i n z e l m e s s u n g e n , meteorologie der technischen hochschule) und in der nàhe des meteorologischen observatoriums aachen auf dem wingertsberg im stadtpark aachen. die auswertung erfolgte in der hegel nach dem verfahren der genauen rechnerischen beriicksichtigung der einstromungsverhàltnisse. weiter wurden die bei der radon-emanometrie iiblichen korrekturen fiir das verwandte e m a n o m e t e r (") aucli auf die thoron-messung m i t diesem iibertragen. die ergebnisse sind in den obigen tabellen 1 und 2 zusammengestellt. die 1. kolonne e n t h a l t jeweils den radongehalt, die 2. den thoron-gehalt, die 3. das verhàltnis radon/thoron e m a n o m e t r i s c h e m e s s u n g e n von r a d o n u n d t h o r o n i n b o d e n l u f t 1 li) fiir die betreffende tiefe. ma n erkennt, dass si eli die einzelnen gròssen an den beiden messstellen verschieden verhalten. der radon-gelialt steigt an beiden orten rasch mit der tiefe an. der thoron-gelialt niinnit einmal (tabelle 1) mit der tiefe ab, ini anderen fall (tabelle 2) dagegen zu. das verhàltnis zwischen radon und thoron steigt in beiden fallen mit deitiefe an, wenn auch in versehiedenem masse. tn der weiteren tabelle 3 sind zum vergleich die in rad nauheim gewonnenen werte (3) angegeben. die werte sind aus der seinerzeit v e r w a n d t e n einheit in curie umgereclinet. (*). die radon-und thoronwerte sind dabei niclit gleichzeitig bestimmt, die ersten sind mittelwerte aus einer gròsseren zahl von einzelnen messungen an der gleiclien messstelle. das material ist zwar noeli viel zu klein, uni eingehendere schlussfolgerungen zuzulassen, immerhin aber kann man ihm einige llinweise entnehnien : 1. nach theoretischer ttberlegung (») ist zu envarten, dass in einem homogenen boden, der in den hier in frage kommenden tiefen von einigen metern gleiche zusammensetzung, gleiche aktivitàt und gleiche emanierungsfàhigkeit besitzt, der radonund thoron-gelialt der bodenluft exponentiell mit der tiefe zunimmt. ihre konzentrationen naliern sicli dabei mit einer ilalbwertstiefe von 105,5 cm fiir radon und 1,35 cm fiir thoron ihrem konstanten tiefenwert. der thoron-gelialt solite also von etwa 10 cm tiefe an nach unten konstant bleiben, das verhàltnis r.adon/thoron also von dieseltiefe an parallel zum radon-gehalt ansteigen. die tabellen zeigen in dem raschen anstieg des radon-gehalt.es u n d der ini ganzen zu erkennenden zunahme des badon/thoron-verhàltnisses mit der tiefe ein verhalten, das qualitativ in der erwarteten riclit u n g liegt. (*) die seinerzeit als tlioron-einheit gewàhlte « 0 . r . e i n h e i t » (die v e r a l l g e m e i n e r t e definition des curie existierte d a m a l s noch nicht) war deiìniert w o r d e n als die tlioroninenge, die mit 1 g t h o r i u m im r a d i o a k t i v e n grleichgewicht stelit. llir s t r o m à q u i v a l e n t ergab sich zu 59,5 x 10-2 aes. z u s a m m e n mit gleicliung i folgt d a r a u s die u m r e c h n u n g 1 0 . r . e i n h e i t e n t s p r i c h t 59,5 • 10-2 6,75 • 106 = 8 ' 8 ' 10 8 curie l2] 1 2 4 h. i s r a e l s . b j ò r n s s o n s. s t i l l e l t 2. "nach dem mittleren w e r t des uranu n d thorium-gehaltes voi) gesteinen und bòden (10) von etwa 3 x io-6 g u prò g gestein u n d io-5 g th prò g gestein folgt u n t e r der annahme, dass der radonund thoron-gehalt dem nach diesen zahlen zu erwartenden gleichgewichtswert entspricht, fiir mehrere meter tiet'e einem e r w a r t u n g s w e r t fiir das radon/thoronverhaltnis von 0,93. tatsàchlieh liegen aber die w e r t e an alien 3 messorten durchweg hòher. eine erklàrung kònnte vielleicht darin liegen, dass an diesen 3 messstellen das verhàltnis von uran/thorium ein anderes ist, als es dem allgemeinen erwartungswert entspricht. auch kònnte mòglicherweise das emanierungsvermògen bei den beiden e m a n a t i o n e n r a d o n und thoron ein verschiedenes sein. 3. die resultate von b a d n a u h e i m sind trotz ihres geringen umfanges dadurch interessant, dass bei wesentlich erhohten radongehalt der thoron-gehalt nicht entsprechend vergròssert ist. man k a n n aus dieser tatsache den hinweis entnehmen, dass offenbar der fiir die bad nauheimer verhàltnisse anzunehmende yorgang ( u ) der radonanreichung in der bodenluft, fiir die ein aus der e quellenspalte >> (taunusrand-yerwerfung) nach oben dringender c0 2 strom als triiger verantwortlich gemacht wurde, sich im thoron-gehalt wegen des selir viel rascheren zerfalls des thoron nicht auswirken k a n n . die arbeiten werden fortgesetzt. z usa mmuffa ss uno mittels eines fmanometers nacli h. israel wurde im sommer 1961 der gehalt der bodenluft an rn und tn an zwei plàtzen in verschiedener tiefe nach der satterly-methode bestimmt. die ergebnisse werden mit àhnlichen messungen von h. israel und f. becker in bad nauheim verglichen. — die rasche zunahme des rn-gehaltes und des verhàltnisses von rn-gehalt zu tn-gehalt mit der tiefe entspricht der theoretischen erwartung. die ergebnisse werden im, einzelnen dislcutiert. e m a n o m e t r i s c h e m e s s u n g e n v o n r a d o n u n d t h o r o n i n b o d e n l u f t 1 li) riassunto durante l'estate 1961 sono state misurate, in due località e a varie profondità, le concentrazioni di rn e tn in terreno aereato. le misure sono state eseguite con remanometro di israel usando il metodo satterly. i risultati sono paragonati a misure analoghe fatte da e. israel e f. becker a bad nauheim. il rapido aumento di crn e del rapporto cr„ictn rispetto alla profondità in un terreno omogeneo, soddisfa qualitativamente all'aumento esponenziale ottenuto in teoria. vengono discussi i risultati. su m mary the concentrations of rn and tn in soil-air were measured at two places and various depths during the summer 1961. the measurements were done with the emanometer of israel using the satterly-method. the results are compared with similar measurements mode by h. israel and f. becker in bad bauheim. the rapid increase of crn and of the ratio cr„ict,> does qualitatively agree with the theoretical exponential increase with depili in a homogeneous soil. the results are discussed. l i t e r a t u r ( ' ) s a t t e r l y j . , the quantities of radium and thorium emanations contained in the air of certain soils « proc. cambridge phil. soo. » 16, 514, 19101912. ( 2 ) s a n d e r s o n j . c . , the probable influence of the soil on locai atmospheric radioactivity. « a m e r . j . sci.» 32, 169-184.(1911). (deutsche ubersetz u n g s. « pliys. z.» 13, 142 ft'„ (1912). ( 3 ) i s r a e l i i . u n d b e c k e r f . , die bodenemanation in der umgebung der bad naulieimer quelleiispalte. «gerì. beitr. geopliys. » 44, 40-55, (1935). ( 4 ) n o r i n d e r ii., m e t n i e k s a. u n d s i k s n a r., radon content of the air in the soil at uppsala. « a r k i v fòr geofysik », i , 571-579. (1952). ( 5 ) n o r i n d e r h . , s i k s n a r. u n d m e t n i e k s a., badon and thoron content of the soil-air at almunge. « geol. f o r e n . stockholm f o r h », 74. ii 4, 450-454, (1952). 120 h . i s r a e l s . b j o r n s s o n s . s t i l l e r ( 6 ) i s r a e l h . , zur methodik kumatologischer emanationsmessungen i: ùber ein neues emanometer. « balneologe », 1, 318-327. (1934). ( 7 ) i s r a e l h . , radioaktive messmethoden. i n : « t a s c h e n b u c h der a n g e w a n d t e n g e o p h y s i k » ( h e r a u s g e g e b e n v o n r. reicli u n d r . v. zwerger), 373-385, leipzig, (1943). ( 8 ) d a n i e l i l , die vier radioaktiven zerfallsreihen. « e r g e b n . d. e x a k t . n a t u r w i s s », 32, 188-197. (1959). ( " ) i s r a e l h . , die natiirliche eadioaktivitàt in boden wasser und luft. « b e i t r . z. pliys. d. a t m o s p h . » 30, 177-188. (1958). ( 1 0 ) l a n d o l t b o r n s t e i n , zahlenwerte und funktionen, 6 aufl., b d . 3, 307 if. ( u ) i s r a e l i l , der diffusionskoeffizient des radons in der bodenluft. (bem e r k u n g zu der a r b e i t von e. budde: « b e s t i m m u n g des bewegliclikeitskofflzienten der r a d i u m e m a n a t i o n in l o e k e r g e s t e i n e n ») « ztselir. f. geophys. », 25, 104-108. (1959). —• der diffusionskoeffizient des radons in bodenluft. (abscliliessende s t e l l u n g n a l i m e zur diskussion init e . b u d d e ) . « zeitsebr. f. geopliys. », 2 7 , 13-17, (1961). marchenko_final:layout 6 ukrainian geomagnetic repeat station network and results of the field work reduced to the epoch 2005.5 valentyn maksymchuk1,*, myxailo orlyuk2, viktor tregybenko3, yurij horodyskyy1, dmytro marchenko1 1 institute of geophysics nas of ukraine, carpathian branch, lviv, ukraine 2 institute of geophysics nas of ukraine, kyiv, ukraine 3 ukrainian state geological prospecting institute (ukrdgri), kyiv, ukraine annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5406 abstract the results of geomagnetic f ield components of the renewed ukrainian repeat stations (rs) network are presented. the methods of absolute geomagnetic and astro-geodetic measurements are described. the reduction of geomagnetic field components is carried out to the 2005.5 epoch and a catalogue of rs is created. maps of magnetic declination for the ukraine are constructed and compared with results calculated by the igrf-2005 model. 1. introduction investigation of the temporal and spatial structure of the earth's magnetic field (emf) is important to solve some fundamental problems in geophysics. one problem is the separation of the earth magnetic field into contribution from sources of internal and external origin, and the separation of the internal sources into the main (normal) field, arising from the dynamo process in the liquid earth core and a lithospheric field. to solve this problem it is necessary to have precise data of the absolute values of the magnetic field components and their time changes. high quality data of the magnetic field components at the earth's surface can be obtained from measurements at magnetic observatories (mo) as well as from the periodic measurements (2-5 years) at repeat stations (rs) or from space by satellites. these measurements are extremely important for the following tasks: – estimation and analysis of the time-spatial dynamics of the emf, its separation into the various parts and investigations of their nature; – modeling of the earth magnetic field (for example the igrf) [finlay et al. 2010]; – reduction of magnetic surveys and development of maps of the anomalous (lithospheric) magnetic field. for a temporal reduction of the rs measurements a continuous record of the earth's magnetic field vector bx, by, bz, bf, bh, d, i from a mo is needed. repeated measurements of the earth magnetism elements at the rs network allow to map the secular variations (sv) of the geomagnetic field. many organizations from the geology, geophysics, geodesy and military claim for this type of data. 2. historical background first general magnetic survey in the ukraine was accomplished during the 1930s and 1940s and the second field campaign in 1969-1972 with a density of 1 point per 400 km2. at the beginning of 1970 a new repeat station network with 39 points was installed in the ukrainian region [krutikhovskaya et al. 1973]. absolute values of the field strength bf were measured by the proton magnetometer, bh by the quartz h-magnetometer and declination was measured with the fluxgate magnetometer on top of the demagnetized theodolite 3t2k. mean-square errors of the measurements at this rs network for bf component ±2.4 nt, bh ±4.0 nt, bz ±2.5 nt, d 1.3 minutes of arc. the next magnetic survey at the rs network was done during 1974 and 1975. the components of the geomagnetic field were reduced to 1974.5 by using the data of the magnetic observatories data "lviv", "kiev" and "odessa". this work was used for the mapping of all components of the geomagnetic field for the territory of the ukraine and the ussr and for the mapping of the main field for the epoch 1975 year [krutikhovskaya et al. 1982]. some of these repeat stations were repeated until the year 1985. after 30 years from the last magnetic survey many points of this network were lost. for this reason, the goal of this work is to analyze the condition, reviving and increasing the existing rs network in ukraine. in the year 2003 the carpathian branch of the subbotine institute of geophysics nas of ukraine (cbigf) and subbotine institute of geophysics nas of ukraine (igf) started this work. article history received september 30, 2011; accepted july 31, 2012. subject classification: earth's magnetic field, repeat station, secular variation, magnetic declination. 1161 magnete workshop 2011 3. ukrainian rs network the rs network in ukraine was renewed and increased during the years 2005-2006. most of the new points were reinstalled at the locations of the destroyed points. but reinstallation can't give us the same positions of the points as in 1972 to 1973. for this reason measured data at the new ukrainian rs network during 2006 and 2007 cannot be used for the determination of the secular variations since 1974. this new rs network consists of 52 first order points with a distance between the points of approximately 100-150 km. the density of the rs network is one point per 10,000 km2 with the even distribution in the ukrainian area. for the astronomical gauge azimuth determination, or in general for the bringing of the theodolite horizontal scale to the geographic meridian several methods, we use the classical method of the horary angle of the celestial body [pandul 1983]. as it was mentioned for the determination of all components of the earth magnetic field, it is necessary to determined three independent components. flux-gate magnetometers lemi-203 [see the website of the lviv center of institute for space research: http://www.isr.lviv.ua/products.htm#flux-gate magnetometers], installed on top of demagnetized theodolite 3t2k, can be used for the determination of the angles d and i. module of the complete vector bf of the magnetic field was measured by the proton magnetometers minimag and mb-01 (proton magnetometers with achieved accuracy 0.01 and 0.1 nt, developed by "geologorazvedka", st. petersburg, russia). in general all recommendations of iaga was take into account [newitt et al. 1996] 4. results the results of the second cycle measurements at ukrainian rs network were reduced to the epoch 2005.5 [maksymchuk et al. 2010]. the reduction [newitt et al. 1996] was based on the data from the magnetic observatories "kyiv", "belsk" and "lviv". in table 1 all reduced components are shown. in general, comparison of measured values with the modeled ones show us that differences between the linear components (bx, by, bz) can reach several hundred nt. differences in the magnetic declination angles at various regions vary from one minute to half degree. if the analytical model field is very flat without any local (regional) gradients (figure 1), whereas the real measured field composed of big gradient in various area of ukrainian region. especially this is visible at the d component. magnetic declination d in the ukrainian region fluctuates from 4˚ at the west to 8˚ at the east with the sufficiently complicated spatial structure (figure 2). strong regional anomalies can be divided to the three parts: western part of ukraine, in the region of the mo "lviv"; southern part of ukraine near the maksymchuk et al. 1162 figure 1. ukrainian rs network and magnetic declination d (isogonic counter lines in degree) for the epoch 2005.5. black stars: rs network; black squares: magnetic observatories. 1163 ukrainian rs network. epoch 2005.5 nr. rs name {, nº m, eº bx (nt) by (nt) bz (nt) dº 1 balaklia 49.603 36.500 20465 2839 46377 7.884 2 bashtanka 47.510 32.107 20977 2257 44673 6.128 3 berduchiv 50.066 28.590 19788 1769 45647 5.105 4 beregove 48.234 22.648 20587 1542 44223 4.280 5 beregomet 48.197 25.378 20700 1610 44515 4.440 6 bila cerkva 49.819 29.774 19829 2179 45613 6.265 7 borynja 49.055 23.059 20337 1665 44540 4.677 8 verhnjodniprovsk 48.523 34.421 20390 2275 45550 6.362 9 volnovakha 47.627 37.622 20617 2473 46000 6.829 10 gadjach 50.476 33.985 19249 2275 46455 6.740 11 goluba zatoka 44.422 33.982 22691 2223 42993 5.572 12 horokhiv 50.580 24.522 19513 1453 45747 4.259 13 guljaipole 47.778 36.146 20775 2809 45581 7.688 14 deljatyn 48.549 24.634 20566 1738 44598 4.827 15 dzergynsk 48.399 37.764 20512 2479 45549 6.883 16 dymer 50.806 30.274 19242 2103 46308 6.236 17 dykanka 49.840 34.625 19728 2273 46134 6.571 18 zhashkiv 49.118 30.015 19763 1900 44764 5.490 19 zavadiv 50.064 23.390 19771 1716 45381 4.959 20 kalna 48.953 23.814 20423 1738 44661 4.861 21 kamjanec 48.578 26.696 20480 1998 44848 5.568 22 kaniv 49.739 31.386 19704 1841 45433 5.336 23 kiverci 50.971 25.533 19017 1788 45904 5.374 24 kirovograd 48.564 32.019 20233 2199 45031 6.198 25 kovel 51.449 24.758 18818 1532 45811 4.658 26 kodyma 48.064 29.075 20567 1590 45109 4.412 27 kurortne 45.904 30.268 22122 2055 43851 5.280 28 lugansk 48.559 39.696 20253 2645 46353 7.431 29 luboml 51.267 24.008 18753 1430 45661 4.360 30 nemuriv 49.008 28.667 20044 2146 44674 6.106 31 nyzhne selyshe 48.198 23.456 20703 1597 44083 4.407 32 nyzhni sirogozy 46.836 34.411 21333 2047 44529 5.468 33 ovruch 51.099 28.712 18931 1829 45642 5.521 34 okunivka 45.364 32.751 22110 2071 43446 5.338 35 orelka 48.956 35.794 19956 2419 45901 6.905 36 orlovka 46.661 36.106 21531 2405 44877 6.355 37 pervomaisk 48.041 30.747 20635 1623 44637 4.487 38 pryluki 50.589 32.146 19263 2330 46169 6.895 39 reni 45.338 28.441 22307 1932 43032 4.925 40 rogatyn 49.454 24.589 20111 1580 45199 4.492 41 sarnu 51.237 26.759 18959 2012 46164 6.062 42 starobelsk 49.118 38.868 20242 2638 46608 7.418 43 stepanivka 46.777 30.900 21246 1326 44222 3.554 44 sumy 51.000 35.101 19063 2814 46419 8.397 45 urzuf 46.886 37.061 21354 2374 44513 6.328 46 kherson 46.777 33.116 21344 2080 44475 5.562 47 cherkasy 49.406 32.670 19943 2261 45457 6.465 48 chernigiv 51.512 31.443 18571 1829 46767 5.628 49 chortkiv 48.999 25.927 20424 1697 45023 4.741 50 shepetivka 49.938 27.247 20003 1709 45270 4.881 51 shostka 51.803 33.402 18676 2769 46800 8.435 52 jastrubivka 45.516 34.127 22097 2202 43647 5.688 table 1. rs catalogue of components of the geomagnetic field for the epoch 2005.5. maksymchuk et al. 1164 figure 2. magnetic declination d (degree) based on igrf-2005 model. figure 3. anomalous magnetic field in ukrainian area (nt) [orlyuk and romenets 2005]. 1165 mo "odessa" and eastern part near the ukrainian–russian border. the structure of the magnetic declination in general agreed with the anomalous magnetic field (figure 3). anomalous values of the declination in the western ukraine coincide with the lviv regional magnetic anomaly, in the south with the odessa regional magnetic anomaly, and in the east with the western fork of the kursk magnetic anomaly. this demonstrates their common nature. 5. conclusions during 2003 and 2004, 52 repeat stations were renewed and installed at the ukrainian rs network taking into account the condition of the rs network after 1972. these points are located almost uniformly on the ukrainian area. comparison of measured components with modeled ones from igrf model demonstrates important differences; it is especially the case of magnetic declination d. to estimate with higher accuracy the real field feature and to improve existing maps of the anomalous magnetic field is necessary to densify rs network during next cycle of repeat measurements. results of geomagnetic measurements will be applied to the adjustment of normal magnetic field model for the ukrainian territory and for future research of secular variations of the earth magnetic field. acknowledgements. the authors would like to thank the anonymous reviewers and the editor, dr. gerald duma, for their significant comments which led to improvements in the presentation and discussion of these results. references finlay, c.c., s. maus, c.d. beggan, t.n. bondar, a. chambodut, t.a. chernova, a. chulliat, v.p. golovkov, b. hamilton, m. hamoudi, r. holme, g. hulot, w. kuang, b. langlais, v. lesur, f. j. lowes, h. lühr, s. macmillan, m. mandea, s. mclean, c. manoj, m. menvielle, i. michaelis, n. olsen, j. rauberg, m. rother, t. j. sabaka, a. tangborn, l. tøffner-clausen, e. thébault, a.w.p. thomson, i. wardinski, z. wei and t.i. zvereva (2010). international geomagnetic reference field: the eleventh generation, geophysical journal international, 183, 1216-1230. krutikhovskaya, z.a., i.k. pashkevich and i.m. silina (1982). magnetic model and the structure of the earth crust of the ukrainian shield, kyiv, naykova dumka, 216 pp. (in russian). krutixovskaya, z.a., i.k. pashkevich, o.m. rusakov and v.d. solovjov (1973). ground absolute magnetic survey of the ukrainian ssr, visnyk an ursr, 9. 101-102 (in ukrainian). maksymchuk, v.yu., m.i. orlyuk, v.i. tregybenko, yu.m. horodyskyy, v.p. mjasoedov and e.f. nakalov (2010). ground absolute magnetic survey at the ukrainian rs network for the epoch 2005, geophysical journal, kyiv, 32, (5), 102-166 (in ukrainian). newitt, l.r., c.e. barton and j.u. bitterly (1996). guide for magnetic repeat station surveys, iaga, boulder, 112. orlyuk, m.i., and a.a. romenets (2005). new criterion of the earth's magnetic field disturbance and some aspects of its use, geophysical journal, kyiv, 27, (6), 1012-1023 (in russian). pandul, i.s. (1983). astro-geodetic determinations on sun for geographers, geologists and surveyors, moscow, nedra, 128. *corresponding author: valentyn maksymchuk, institute of geophysics nas of ukraine, carpathian branch, lviv, ukraine; email: vmaksymchuk@cb-igph.lviv.ua. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ukrainian rs network. epoch 2005.5 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 2, dm209, 2022; doi:10.4401/ag-8786 o p e n acc e s s long-term sustainability of a distributed ri: the epos case kauzar saleh contell*,1,2, karin karlzén3, massimo cocco4, helle annette pedersen3, kuvvet atakan5, daniele bailo5, otto lange6, daniela mercurio4,2, giovanna maracchia4, agata sangianantoni4, diana piras2, maria fredella2 and carmela freda2 (1) eth zurich, switzerland (2) epos eric, italy (3) cnrs, university of grenoble, france (4) ingv, istituto nazionale di geofisica e vulcanologia, italy (5) university of bergen, norway (6) utrecht university, the netherlands article history: received december 28, 2021; accepted april 2, 2022 abstract the european plate observing system (epos) is a distributed research infrastructure (ri) with the mission to establish and maintain sustainable and long-term access to solid earth science data and services by integrating the diverse national research infrastructures under a common federated framework governed by epos eric (european research infrastructure consortium). this paper presents the epos approach to ensure financial viability and to tackle the challenge of long-term sustainability of the ri during its operational phase. the epos approach to sustainable operation considers the scientific impact and the promotion of scientific research as the preconditions to achieve long-term sustainability. enabling scientific excellence implies that high-quality data and services are provided reliably and continuously to establish the ri as the enabler of investigations to solid earth scientists. the strategic approach and the solutions adopted by epos eric to address the long-term sustainability of a pan-european distributed ri are discussed in this paper focusing on the governance structure, considered as the qualifying dimension that gathers and connects the financial, legal and technical dimensions. the governance and the financial models are discussed to delineate the legal framework necessary to operate the epos ri relying on the implemented technical solutions. a sufficiently stable investment environment is necessary to allow the ri to concentrate on providing high quality services for their user communities. this paper discusses the current actions and challenges to be addressed for achieving this goal. keywords: research infrastructure; sustainability; solid earth science; open science; eric kauzar saleh contell et al. 2 1. introduction research infrastructures play a key role in the scientific research process through the generation of high-quality data and new technologies, often necessary to trigger innovation and scientific progress. research infrastructures (hereinafter ris) are facilities, scientific equipment and observing systems (sets of instruments and networks) of a unique nature, identified by the scientific communities to conduct research and promote innovation in their fields [eu regulation no. 1291, 2013]. ris also include knowledge-based resources, i.e., collections, archives and repositories of scientific data, and e-infrastructures, i.e., data and computing systems and communication networks [esfri roadmap, 2018]. moreover, ris bring together and attract large numbers of developers and users, fostering both domain-specific and cross-disciplinary investigations. in addition, ris are identified as key enablers of socioeconomic impact [oecd, 2019; van belle et al., 2018]. for their implementation, different organizational models exist, typically single-sited ris, such as large research plants in one single or a few fully dedicated sites (e.g., astronomical observatories, accelerator-based sources, nuclear reactors or extreme laser sources) or distributed ris composed of geographically distributed observing systems and/or laboratories, remotely accessible computational resources and data repositories. ris can offer their users physical access [see philippin et al., 2019] and/or virtual access for remote usage of scientific data and related services [petzold et al., 2019]. ris are nowadays assuming an increasingly central role in supporting open science by fostering the sharing of scientific data and products. this is particularly relevant for environmental sciences [kutsch et al., 2016; sorvari et al., 2016; petzold et al., 2019], but also for transdisciplinary initiatives like the european open science cloud (https://eosc.eu). the key requirements for a useful and meaningful sharing of data are findability, accessibility, interoperability and reusability, which are combined in the so-called fair (findable, accessible, interoperable, reusable) principles (wilkinson et al. 2016, 2018; mons et al., 2017; boeckhout et al., 2018). fair data management is currently an objective for ris in europe and a global challenge requiring well-defined principles and shared practices. the role of ris is evolving since the original stand-alone undertakings are more and more becoming part of a connected system forming a unique resource for advanced research and interdisciplinary analysis of complex scientific problems [esfri roadmap, 2021]. for instance, national ris are encouraged to network with international ris, to deliver the best solutions to the national research communities [oecd, 2020]. the european strategy forum on research infrastructures (esfri) is coordinating the european landscape of ris. esfri is a strategic body established in 2002 by the council of the european union to support a coherent and strategy-led approach to policy making on ris in europe. esfri periodically updates its roadmap as mandated by the competitiveness council of the european union to provide a coherent and strategic vision ensuring that europe has excellent ris in all fields of science and innovation. esfri has completed roadmaps in 2006, 2008, 2010, 2016, 2018 and launched the last one in 2021 (https://www.esfri.eu). the esfri roadmap contains a description of the european ris and their progress status, based on a thorough evaluation and selection procedure. the roadmap combines esfri projects, which are new ris in progress towards implementation, and esfri landmarks, which are successfully implemented ris. the esfri roadmap also describes the broader landscape of research in europe which is an important element of the esfri methodology [esfri, 2019]. esfri ris apply a lifecycle approach coherent and consistent with ri funding under the eu framework programs for research and innovation. moreover, the lifecycle of a ri is a reference to understand the needs and targets of the ri at a given time and at various stages [esfri roadmap, 2021]. the lifecycle starts with the conception phase, which typically arises bottom-up from the scientific communities clustering around well identified scientific needs and goals and continues with the design-, preparatoryand implementation phases (see figure 1). the preparatory phase is dedicated to the proof of the scientific concept and technical feasibility of the ri, including the architecture as well as the governance and financial models defining the organizational structure. next, the implementation phase is dedicated to the development of the functional architecture and to the governance and management structure (figure 1). for distributed ris, the creation of a successful governance and financial structure may be of higher complexity than for single-sited ris, requiring intense negotiations with national authorities and research organizations as both the central hub and the national nodes require specific and shared commitments to operate. the analysis of the potential user community, both science and innovation oriented, continues throughout the preparatory and the implementation phases. in this paper, we present and discuss the concept development, design, preparation and implementation of the european plate observing system [epos, www.epos-eu.org, cocco et al., 2022, this volume], a distributed ri dealing with solid earth science in europe, with the goal of discussing the approach adopted for long-term sushttps://eosc.eu https://www.esfri.eu http://www.epos-eu.org long-term sustainability of a distributed ri 3 tainability. solid earth science brings together many diverse disciplines, such as geology, seismology, geodesy, volcanology and geomagnetism, as well as chemistry and physics as they all apply to the workings of the earth. the epos mission is to establish and underpin sustainable and long-term access to solid earth science data and services integrating diverse european research infrastructures under a common federated framework. to that end, epos grants open access to solid earth science data through new e-science solutions to foster multidisciplinary scientific research in the diverse solid earth disciplines. in addition, epos promotes physical access to research facilities in solid earth science across europe. epos entered the esfri roadmap in 2008 and became a landmark in 2018. the european commission granted epos the eric status in 2018 cocco et al., 2022, this volume]. figure 2 shows the current lifecycle status of the epos ri and illustrates the different stages successfully achieved on its way to sustainable operations. indeed, the long path towards the construction of the epos ri explains the “longterm” vision anticipated in the epos mission statement. currently, the so-called pilot operational phase (epos pop, 2020-2022) is aimed at testing altogether the different frameworks (governance, legal, technical, financial) developed so far to ensure access to the interoperable epos central hub (ics-c). legal in this context also includes the regulatory framework specifically developed for the implementation of epos. another key objective of the pop is to bring epos from financial viability to long-term sustainability, a common challenge for infrastructures transitioning from the establishment/construction stage into operations (oecd, 2017). the construction stage should be more explicitly identified in the esfri lifecycle model (figure 1), since it is a necessary and costly step to move forward the ri operation. the latter objective is also addressed through the epos sp project (epos sustainability phase), funded by the european commission within the horizon 2020 framework program (h2020-infradev-2019-2 call, project n° 871121), since sustainability is the overarching goal of the epos pop. figure 1. esfri lifecycle approach to ris (esfri 2021) https://www.esfri.eu/sites/default/files/esfri_roadmap2021_ public_guide.pdf. figure 2. epos timeline during the ri lifecycle and current stage epos pilot operational phase (pop) https://www.esfri.eu/sites/default/files/esfri_roadmap2021_public_guide.pdf https://www.esfri.eu/sites/default/files/esfri_roadmap2021_public_guide.pdf kauzar saleh contell et al. 4 the issue of long-term sustainability of ris is addressed in this paper by discussing the epos federated approach to integrating scientific data and fostering fair data management in solid earth science [cocco et al., 2019; bailo et al., 2020; 2022, this volume]. as already introduced, ris are long-term enterprises requiring decades to progress from the conception and design to the implementation and construction phases and designed to remain operational for several decades or more [see for instance elixir, 2019]. moreover, ris represent strategic investments which are indispensable for enabling and developing research in all scientific domains and require careful planning as well as continuous support, not limited to financial aspects, to maximise their socio-economic impact [van belle et al., 2018; elixir, 2019]. developing and maintaining the sustainability of a pan-european distributed ri like epos is a complex endeavour, which has to be considered a target during the entire ri lifecycle. indeed, the de-centralised nature of epos is also reflected in its approach to sustainability, as specific elements, not present in single-sited infrastructures, need to be considered [oecd, 2014]. in the following, we present the approach developed to ensure the long-term operation of epos, in full awareness of the important strategic investments made in the past and with the ambition to make epos resilient to the rapidly evolving financial and political contexts. 2. the epos architecture since its conception phase, epos is managing its complexity relying on a shared and effective roadmap towards operations in which the scientific concept and the architecture are evolving consistently with the original epos vision and mission and with the ambition to ensure sustainability. figure 3 depicts the epos functional architecture, characterised by an epos eric perimeter and a delivery framework perimeter. the epos eric perimeter contains the components managed under the epos eric governance as assets of the ri (eco, ics-c, further explained in the following). the epos delivery framework perimeter contains other essential components for the delivery of the epos mission (tcs, ics-d, also further explained in this section), whose governance is shared with the participating research communities. there are three distinctive elements in the epos functional architecture (see figure 3): data generation; qualification and quality-management; integration and provision. each element can be seen as a distinctive layer. the first element, data generation, is represented by the national research infrastructures (nris) characterising the framework in which data are generated from multiple locations, processed, analysed and archived. this includes scientific data like measurements from different observing systems and sensors, but also information about laboratories or observatories accessible to researchers for physical access [see wessels et al., 2022, this volume]. the second element, data qualification and quality management, is represented by the thematic core services (tcs), established in epos since the implementation phase (2015-2019) to ensure an effective and suitable management of the available high-quality data over the entire data lifecycle for a given domain (e.g., seismology). the tcs1 represent the community-specific integration and the governance framework to ensure the data and service provision within epos. the third element, integration and provision, is represented by the ics (integrated core services), which is the platform where data and services are integrated and made interoperable, providing user access through the novel epos data portal2 [bailo et al. 2022, this volume]. the ics is composed of the ics central hub (ics-c) and the ics distributed (ics-d). the latter represents the virtual research environment, where access is provided to services for the analysis of aggregated datasets and for developing computational earth science. the epos eric legal seat is hosted in italy (ingv rome), where the executive director, supported by the executive coordination office (eco), ensures the governance of the epos ri and the coordination of the epos delivery framework. the ics central hub (ics-c) is hosted in france (brgm) and the united kingdom (ukri-bgs). the eco and the ics-c interact through the ics-c office, composed of members of epos eric and of the ics-c hosting organisations. the ics-c office is led by the ics-c director who reports to the epos eric executive director. the rationale behind this architecture is to guarantee an effective engagement of the research communities providing the required skills for the data and service provision. at the same time, it ensures the coherency with and the effectiveness of epos’s legal, governance and technical framework [see cocco et al, 2022, this volume for a detailed description of the epos delivery framework]. 1 https://www.epos-eu.org/tcs 2 https://www.epos-eu.org/integrated-core-services, https://www.epos-eu.org/dataportal https://www.epos-eu.org/tcs https://www.epos-eu.org/integrated-core-services https://www.epos-eu.org/dataportal long-term sustainability of a distributed ri 5 figure 3. main components of the epos architecture: nris, tcs and ics (ics-c & ics-d) form the epos functional architecture, coordinated by epos eric, and designed to ensure the epos data and service provision. the integrated core services central hub (ics-c) and the executive coordination office (eco) belong to the epos eric legal subject. 3. the epos science case the main mission of ris is to serve and facilitate scientific research. according to esfri, landmarks have been presented as “pan‑european hubs of scientific excellence, generating new ideas and pushing the boundaries of science and technology” and “important pillars of european research and innovation for the next decade” (esfri roadmap, 2016). scientific excellence has been included by esfri as one of the main recommendations for tackling the challenge of long-term sustainability of ris [esfri scripta, 2017]. therefore, impact on science can be considered as the prerequisite and main driver for ri sustainability. for a distributed ri such as epos, enabling excellence in scientific research implies that high-quality data and services are provided reliably and continuously to establish the ri as the enabler of investigations to solid earth scientists. this also requires the effective engagement of the user community, as well as access mechanisms for researchers to use the ri when addressing the most challenging problems in solid earth science. for these reasons, the epos approach to long-term sustainability considers the science case as the essential requirement motivating the establishment of the ri, confirming its original vision and mission statements. there are specific reasons why in epos we refer to scientific impact and perspectives rather than scientific excellence, as will be discussed in this paper. the epos science case is fundamentally represented by the multidisciplinary contents of its delivery framework. data and services provided by epos can contribute effectively to answering grand challenges and big scientific questions as well as to maintain earth a safe, prosperous, and habitable planet. for instance, accessible data can be used to forecast volcanic ash emission and related hazards by bringing together observations from volcanology, petrology, rock physics and geochemistry, meteorology and atmospheric sciences. epos can also promote multidisciplinary investigations of anthropogenic deformation for a safe and secure exploitation of geo-resources. seismological data and observations can be used for calibrating ground motion prediction equations (i.e., measuring the ground shaking during earthquakes and its distribution in space) for seismic hazard assessment and risk mitigation. the growing ability of scientists to make accurate predictions about natural phenomena and to improve our understanding of how planet earth works can be strengthened by facilitating access to multidisciplinary solid earth data enabling re-use and serendipity. this of course requires the access to high quality data and products in a way in which they can be analysed, processed and modelled to derive new products for science and innovation. this can be done through the epos ics (see figure 4). this is, indeed, the epos goal and the long-term scientific perspective. kauzar saleh contell et al. 6 figure 4. the tcs-ics system representing the epos delivery framework. by enabling scientists to access and use its data portal, epos can promote the discovery of the excellent opportunities for earth science. the epos delivery framework foresees continuous developments deriving from the promotion of the use of the available data and services also associated with the generation of new scientific products that can be shared and made accessible by epos after quality-control and standardization. this corresponds to transforming users into data product providers, fostering open science and the culture of data sharing. the community building achieved by epos and the enhanced integrated contributions to society that will be available by using epos services, further corroborate the epos impact on science and its added value for society. a shared scientific perspective and a proved scientific impact represents the framework in which sustainability can be addressed in the long term. in the following section we will discuss the epos approach to sustainability, intended as the sustainable operation of a science-oriented distributed ri. 4. a multidimensional approach to sustainability the epos ri successfully concluded its implementation phase in 2019 and is currently facing the transition to the operational phase. this transition represents a specific stage in the lifecycle of epos, named epos pilot operational phase (epos pop), and it is governed by a strategic plan (2020-2022) with the overarching objective of building and operating the epos delivery framework (figures 2 and 3) in a sustainable way. the epos sp activity plan represents a specific objective of the current epos eric strategic plan, dealing with the financial viability and tackling the challenge of long-term sustainability. the first strategic action has been performing a so-called landscape analysis of the epos delivery framework. the landscape analysis has been designed to contribute to the implementation and the adoption by epos eric of the long-term sustainability plan under development. specifically, the landscape analysis consists of the detailed mapping of the components included in the epos delivery framework, e.g. ics, tcs. we consider that each component shapes the construction and operation of the ri, thus affecting the overall sustainability of epos. by analysing each component through qualifying features or dimensions, we are able to create awareness about the sustainable operation of the epos ri and its delivery framework. extending the landscape analysis to the external framework is presently envisioned as a subsequent step to move forward in the path to sustainable operation. long-term sustainability of a distributed ri 7 in addition to the scientific framework already discussed, the following dimensions can be considered as key for ensuring sustainable operation: i. governance dimension ii. financial dimension iii. legal dimension iv. technical dimension v. users’ environment vi. other stakeholders’ environment (private sector, society) vii. global connections. indeed, the last three dimensions contribute to the exploitation of the ri, thus essentially affecting the scientific perspectives discussed in the previous section. the inclusion of the users’ and stakeholders’ dimensions within the landscape analysis is motivated by the fact that they can influence and be influenced by the way in which we are building and operating the epos delivery framework. it is also within these two dimensions that communication is tackled as an essential aspect for the exploitation of the ri. as an example, epos eric has a dedicated vocabulary task force dedicated to harmonise and improve the vocabularies present in the future epos data portal. in addition, the epos approach to sustainability relies on the definition and the implementation of suitable models for the governance and financial dimensions as well as of effective and coherent legal and technical solutions. these models have been designed and implemented during the previous and current stages of the ri lifecycle. the governance dimension of the epos sustainability depends on the effectiveness of the adopted governance model. the epos governance model has been designed in full awareness that a distributed research infrastructure must rely on key components that are under the control of the eric. the model has been implemented in order to enable the epos eric management and coordination, relying on authoritativeness and trust, as well as to federate data and service providers with epos eric and ics-c, through tcs community coordination. to this task, the implemented governance structure considers the executive coordination office (eco) and the ics central hub (ics-c) as the components under the governance of epos eric. this is graphically represented by their inclusion in the epos eric perimeter (see figure 3). the tcs are located outside the epos eric perimeter, but within the perimeter of the epos delivery framework (see figure 3). this is because the tcs have their own governance structure operated by the involved research organizations and scientific communities. this translates the concept of a federated approach into a governance model suitable for the coordination of the epos delivery framework by the eric. the building of the adopted governance structure requires appropriate financial and legal frameworks. they have been implemented through dedicated solutions, such as host premium provision by eric members (hosting countries) aimed at financially supporting the operation of the components located inside the epos eric perimeter as well as the signature of consortium agreements by research organizations to join and support the tcs. these solutions will be discussed in the next section. here, we anticipate that the legal framework has been implemented to define: (i) the contents of the partnership agreements between epos eric and ics-c hosting organisations necessary to host the ics-c in countries different from that of the eric legal seat; (ii) tcs consortium agreements between research organisations recognizing roles and commitments to implement the tcs (including data policy, access rules, service provision, supply letters for data provision); (iii) collaboration agreements between research organisations and epos eric to federate tcs, sharing resources to support sustainable governance as well as data and service provision, as well as (iv) other legal agreements to support training and the community’s scientific and outreach initiatives. the financial framework has been implemented according to the ri architecture (see figures 3 and 4) and to the governance structure. the epos financial model is illustrated in figure 5. this figure shows the four founding components of the epos architecture, namely the national ris (nri), the eco, the ics and tcs and the three envisioned funding sources. these are i) national funding in the form of epos eric membership fees in cash (eric fees in figure 5), ii) national funding provided as host contributions (eco, ics) or in-kind contributions (tcs) to epos eric, as well as directly to research organisations for the operation and development of tcs and/or for data generation and maintenance of national research infrastructure (all labelled under direct funding in figure 5), and iii) eu funds for activities devoted to further development and integration. the rationale behind this financial model is twofold: on the one hand, including in the epos eric budget cash contributions, host contributions and in-kind contributions (ikc) for supporting operational costs and, on the other hand, using european and distinct national funds for development and for the sustainability of the nris. the financial model also relies on the implekauzar saleh contell et al. 8 mentation of a tcs cost book to manage the sustainability of tcs and the data and service provision, by quantifying the resources provided in-kind by national and international research organizations. although tcs are outside the epos eric perimeter, the financial model contemplates cash contributions to tcs for the establishment of collaboration agreements with epos eric for tcs governance and coordination, training and outreach, virtual access to data and service provision and physical access to research facilities. these are small contributions compared to the resources required at national level and are aimed at leveraging national support to tcs sustainability. the estimated costs of the epos delivery framework are nearly 15% of the total operational costs spent by national governments to operate the national ris federated in epos. this might represent a feasible starting condition to address financial viability. to take full advantage of the major investments in national ris, the data they produce need to be made openly and easily available to researchers across a broad span of fields in sustainable environments. this introduces the technical dimension of the sustainable epos operation. figure 5. the epos financial model. dashed boxes show the elements and the funding sources included inside (black) and kept outside (red) the epos eric budget. the technical framework is represented by the implementation of the epos functional architecture (figure 3), which has been designed to respond to the increasing amounts of data generated by solid earth national ris. this requires harmonizing and integrating the standardized data and metadata provided by the communities through service providers belonging to tcs, ensuring interoperability with the ics-c, as well as to operate the epos data portal ensuring integrated access and findability to users and stakeholders. the technical dimension of sustainable operation of the whole epos delivery framework has direct implications with the user engagement and stakeholder framework. the governance, legal, technical and financial dimensions identified to characterize the sustainability concept are interdependent, as evidenced by the previous discussion. however, we consider the governance dimension as the qualifying dimension that connects the other three dimensions, establishing the epos sustainability framework. in the next section, we will further discuss the epos approach to sustainable operations with the goal of presenting adopted solutions and challenges to address the long-term sustainability of the epos ri. 5. solutions and challenges epos eric has been designed to govern and operate a pan-european distributed ri. the epos governance structure relies on the shared decision, taken during the preparatory phase, to set up an eric in order to establish and operate the research infrastructure. this choice has characterized the preparatory and implementation long-term sustainability of a distributed ri 9 phases influencing the governance and financial models. the epos governance model is illustrated in figure 6. the general assembly is the epos eric decision-making body, and it is composed of representatives from national authorities of the countries having joined the eric. fourteen out of the 25 countries involved in epos have joined epos eric so far. according to the eric regulation [eu regulation no. 723/2009], the statutes of an eric shall contain the rights and obligations of the members, including the obligation to make contributions to a balanced budget. in the epos eric statutes [2018], members and permanent observers, directly or through representing entities, shall provide annual membership fees in cash, host contributions (if applicable), and may also provide other contributions in kind. the eric members providing host contributions are identified as hosting countries. according to the epos architecture (figure 3) and the governance and financial models (figures 5 and 6), host contributions are foreseen for hosting and operating the two core components of epos eric, namely the eco and the ics-c. italy is the host country for hosting the eco and the legal seat. france and united kingdom are the two hosting countries for hosting and operating the ics-c. it is therefore evident that the host contributions play a key role in the epos eric governance and financial models (as shown in figure 5). figure 6. details of the epos governance structure (arrows representing governance relationships): the general assembly is composed of representatives from national authorities of those countries that joined the eric is the decision body of epos eric; the executive director (ed) and the eco oversee the managing of the eric as execution subjects. the service coordination committee is composed of representatives from the tcs, ics and the eco and it is chaired by the ed. the executive committee is composed of the chair of the scc, the scientific, community building and it officers (the latter chairs the it board) and the ics-c director. ec is chaired by the ed. the ics-c office is composed of staff members coordinating the operation of the ics central hub. a tcs board is established in each tcs to coordinate the community and govern the data and service provision. the host contribution solution is common to several other distributed ris, being considered an effective way to share resources for operating core components of the eric coherently with the governance structure. its implementation relies on direct funding from eric members, which is straightforward when the financial contribution is provided as cash. in this case, the contribution is part of the epos eric budget, and as such reported in the financial books of the eric. if the host contribution is provided by a research organization on behalf of the eric member, a mandate letter is necessary to delegate the research organization and include the financial contribution in the eric kauzar saleh contell et al. 10 budget, confirming the commitment taken by the eric member in the general assembly (ga). the necessity of the mandate letter has been thoroughly discussed among eric members in the ga. it may pose a challenge, in that it may trigger decisions at national level taking time to be implemented, yet they are important to reinforce the political commitment to hosting a key component and to fully implement the adopted governance structure. this situation might be common to other distributed ris. a further challenge is the inclusion of in-kind contributions (ikc) in the epos eric financial books, especially when the ikc refer to already existing e-infrastructures or facilities and not fully dedicated to the exclusive use of the eric. to address this challenge in epos, we have distinguished the host contributions associated with resources dedicated to host and operate the core components located inside the epos eric perimeter, from in-kind contributions dedicated to support activities for key components located outside the eric perimeter but within the epos delivery framework perimeter (see figure 3 and 6). the epos eric perimeter identifies the assets to be kept under eric governance (the eco and the ics-c); consequently, they are part of the epos eric budget and are reported in the financial books. this is also needed to ensure that efforts and investments can be sustained over long periods to develop the next-generation technology that provides a competitive edge and can help driving innovation. the epos delivery framework perimeter contains the tcs in charge of key actions such as tcs governance and coordination, data and service provision, outreach and community building. the consortium agreements have been implemented to engage national and international research organizations in tcs activities, including the data and service provision. the creation of tcs consortia was a key component in stabilising the epos delivery framework because they ensure accountability, representativeness and transparency with all the involved organisations and their personnel and give a clear mandate to the tcs coordinator to interact with epos eric. the pan-european dimension of the epos federated delivery framework is corroborated by the number of countries and research organizations participating to the data and service provision and to the tcs governance: 25 countries are involved in epos through nearly 140 research organizations. among them, 65 research organizations and 4 international organizations are formally engaged in the tcs consortia, belonging to 21 countries (out of the 25 involved in epos). the data and service provision are currently ensured by 35 service providers, which make available nearly 197 validated data, data-products, and services provided through 236 web-services. an important challenge for epos is to harmonize the pan-european integration with national strategies as well as to keep the national research organizations committed to sharing resources for the operation of the epos delivery framework [atakan et al., 2022, this volume]. most of the national research infrastructures engaged in epos are deployed for the monitoring of areas prone to geo-hazards and the surveillance of national territory including areas used for exploiting geo-resources. they have not been deployed for the specific epos mission. epos is integrating the data, scientific products and services generated by these national ris to create new services for the integrated use of multidisciplinary aggregated sets of data. for this reason, a portion of the total costs for the tcs operation (inventoried in the tcs cost book) is ensured by the national research organizations committed to maintaining the operation of their ris. this is considered as in-kind resources, a background information in the tcs cost book, not reported in the epos eric budget and annual activity plan, as these resources are external to epos eric. at the same time, epos eric is committed to ensuring the financial viability and the sustainable operation of the epos delivery framework. to this task, epos is encouraging the provision of national funds for support of the participation to tcs and their data and service provision, as envisioned in the financial model shown in figure 5. these funds are considered as in-kind contributions (ikc) in the tcs cost book. indeed, epos eric strives to improve the tcs sustainability through increased ikc. for example, by financially contributing to the tcs governance and coordination, epos eric promotes the active search of mechanisms to strengthen sustainability, as this is an activity included in the collaboration agreements with epos eric. at tcs level, this mechanism favours the sharing of approaches across different research organisations. also, epos eric disseminates the model of epos national consortia put in place in several countries for more efficient interactions with ministries and sponsoring or funding agencies. within the epos sp project, we are exploring the suitability of this approach to ensure financial viability, provided that the ikc are recognized by both the committed research organizations and the epos eric general assembly. under these conditions, these ikc can be annotated in the eric financial books and included in the sustainability plan. the ongoing landscape analysis mentioned above represents a direct contribution to the monitoring of financial viability as well as a tool to assess values and gaps in operating the epos ri. the sustainable operation of the epos delivery framework also requires the implementation of a suitable legal framework. a set of different legal agreements have been implemented in order to establish appropriate and effective connections with research organizations depending on their role, responsibility and commitments. partnership agreements have been implemented to recognize the governance, financial and technical frameworks long-term sustainability of a distributed ri 11 established to host the core components located inside the epos eric perimeter. in this case, research organizations signing the partnership agreements with epos eric are recognized as hosting organizations and representing entities in the epos eric statutes. this legal agreement regulates the hosting and the operation of assets belonging to epos eric and included in its budget and financial books. multi-year collaboration agreements have been set up to share resources between epos eric and the research organizations committed to coordinating the tcs, as well as with those acting as service providers. tcs coordinators and service providers are identified in the consortium agreements signed to establish the tcs. collaboration agreements also ensure the it development and the tcs-ics interactions and can support sponsored research activities. preliminary results emerging from the landscape analysis indicate that epos eric will sign a high number of legal agreements to coordinate and operate the epos delivery framework. this is an important aspect of the sustainability dealing with the legal dimension that needs to be analysed in order to adapt suitable practices and transform viability into long-term sustainability. the legal dimension of sustainability also concerns the data and service provision and requires the adoption of practices to sustain access and use of data and scientific products. epos integrates data from different data providers through shared policies and access rules, which requires the provision of supply letters from data providers to service providers to enable data redistribution on their behalf. there is a legal framework to be built in order to make the epos federated approach to data and service integration operational, coherently with the governance and financial models and with the technical developments. in other words, the generation and exploitation of digital data, products (including software) and services require a suitable legal framework. this challenge is common to all ris, e-infrastructures and data infrastructures designed to contribute to open science through the management and the sharing of fair data. the legal framework is also affecting the technical dimension, since effective policies are required both for redistributing data and/or scientific products and managing users’ information to access the epos data portal (i.e., gdpr, authentication, authorization). the technical dimension of sustainability concerns the operation of the tcs-ics system, the key component of the epos delivery framework enabling data and service provision. in accordance with the functional architecture and the governance structure, tcs-ics interaction is considered a permanent activity in epos, necessary to ensure both operation and it development. for this reason, this strategic action is coordinated by the university of bergen (norway) and associated activities are included in the epos activity plan. this exemplifies the need for distributed ris to adopt an organizational structure functional to the governance and technical operation and capable to engage partners and share responsibilities. the maintenance of the data and service provision also requires automated procedures to implement data and metadata for ensuring interoperability and fair data management. this is a further essential element for the sustainable operation of the epos ri. to ensure the provision of high-quality data and services reliably and continuously through decades requires human resources and skills. the long-term sustainable operation of the epos distributed ri involves continuous interactions among service providers, tcs communities and ics-c operators, that have to be managed to ensure adequate performance, engage users and follow the users’ and stakeholders’ expectations. this challenge is motivating the current stage of epos, namely the pilot operational phase, and corroborating the need of testing and implementing the interface between tcs and ics from a technical, governance and legal point of view. to tackle this challenge, dedicated surveys have been envisioned in the activity plan of the epos sp project in order to interact with data and service providers. this strategic action is currently on-going and will be further discussed in the next section. 6. discussion the number of countries (25) and research organizations (≈140) participating in the epos integration plan corroborates its pan-european dimension. this is surely a good starting point and represents an added value for finding solutions to share resources and strengthen epos sustainability. however, this requires the adoption of an effective governance and a suitable approach to harmonizing national strategies and priorities with the epos needs and requirements [see atakan et al, 2022, this volume]. the recognition of the epos role by the national authorities (eric members or observers) and the confirmation of their commitments, not limited to financial contributions to epos eric are essential to tackle the challenge of ensuring long-term sustainability [see also cocco et al. 2022, this volume]. in this framework, the current epos challenge is to ensure the financial viability of the epos operational phase, also relying on national funds to support data and service provision, as well as to create the conditions for addressing long-term sustainability. kauzar saleh contell et al. 12 according to esfri, a robust long-term vision is essential to successfully develop, construct and operate research infrastructures [esfri roadmap, 2018]. the epos ri has been conceived with a long-term vision to develop and operate its delivery framework. esfri set up an ad hoc working group (wg) in 2016 with the mandate to provide a set of recommendations to strengthen the long-term sustainability of ris contributing to the dedicated action plan of the european commission. the working group delivered an exhaustive report going well beyond the mere economic analysis [esfri scripta, 2017]. the report proposed seven main recommendations covering the key aspects of long-term sustainability of research infrastructures. these recommendations were expanded into 35 specific points ranging over securing highly qualified and motivated human resources, realizing a robust transfer of information to society from the research infrastructure via a quality-controlled e-infrastructure, building an effective interface between ris and innovation activities, understanding the actual value of the diverse benefits to society, addressing the optimal governance and management structure and promoting coordination among ris [esfri scripta, 2017]. epos has developed its approach to long-term sustainability trying to adopt practices capable to address these recommendations coherently with its scientific mission. according to the esfri report, scientific excellence is the necessary condition for sustainability throughout the entire ri lifecycle and its persistence is crucial in the long-term operational phase. excellence in science and outstanding quality services to the users are imperative for the successful performance of each ri, and they are addressed in the monitoring and reviewing processes by esfri. this justifies the epos decision to consider the scientific impact and perspectives as the overarching framework to drive the sustainable operation of the ri during the entire lifecycle through the definition of suitable practices for each of the main qualifying dimensions of sustainability (i.e., governance, financial, technical and legal dimensions). as anticipated before, in epos it has been intentionally decided to ensure and assess the scientific impact resulting from using the data and the services made available to users, rather than focusing on establishing excellence. in other words, epos has acknowledged the esfri’s indication to establish and maintain excellence but adapted it to its vision and mission. epos is a science-driven ri dedicated to unlocking the innovation potential of exploiting multidisciplinary data and services generated by the involved thematic communities (tcs) in solid earth science. the epos functional architecture, relying on the tcs-ics system, has been designed to fully exploit the ri’s potential to enable scientific research, promoting innovation in all scientific domains while fostering cross-disciplinary investigations in earth science. this is the scientific perspective, and the epos scientific impact will be monitored to preserve the long-term exploitation of data and services by scientists and other stakeholders. the epos access rules and data policy have been designed and implemented to be coherent with open access and fair data management. this implies that user access mode to the epos platform (i.e., the tcs-ics system and the epos data portal) is open and not limited to users from those countries participating to epos eric. this has implications on the expectation to establish scientific excellence. the european commission in 2016 published a document concerning principles and guidelines for access to ris and related services [european charter for access to research infrastructures, 2016]. they considered three access modes for ris: excellence-driven access, market-driven access and wide access. the excellence-driven access allows users to get access to the best facilities, resources and services wherever located after peer review evaluation conducted by internal or external experts. the market-driven access mode applies when access is defined through an agreement between the user and the ri, involving the payment of a fee for the access, that may remain confidential. the wide access mode guarantees the broadest possible access to scientific data and services provided by the ri to users wherever they are based. epos ri has adopted the latter mode in order to maximise findability, accessibility and re-use of the provided data and services. epos excluded peer-review and pricing of the data because its federated approach to integrating data and services relies on joint efforts to share data which are not owned by epos eric. this has implications on the sustainable operation and on the future epos eric long-term sustainability plan. because this situation is common to several other ris in environmental science, it would have been reasonable to expect that the access mode adopted by these ris would have greater visibility and consideration by the european commission, rather than to be included in the generic wide access mode. as explained before, epos is currently testing the tcs-ics system and the capacity to maintain the data and service provision operational. these tasks are characterizing the epos pilot operational phase [epos pop, 2020-2022] together with the implementation of a sustainability plan. the latter is the overarching objective of the epos sp project. the successful completion of the epos pop is essential to demonstrate that the epos ri works and is capable to enter in its fully operational phase. this is an extremely important message to be delivered in 2022 to the national authorities and epos funders. to this goal, the landscape analysis of the epos delivery long-term sustainability of a distributed ri 13 framework is a crucial element to adopt suitable metrics for analysing the qualifying dimensions identified in epos to define sustainability. this requires the assessment of the effective governance, supported by a suitable legal framework, as well as the appropriate management of the epos ri. following the esfri wg indications, the sustainable operation of an ri requires a human resources plan ensuring skills and experiences to have the right people at the right place and at the right time. this does not solely concern the staff members of the epos eric executive coordination office (eco), but also the ics-c and the tcs implying an effective tcs governance and harmonization between epos eric needs and contributions by participating research organizations. the longterm sustainable operation of the epos distributed ri, through its federated approach to data and service integration and provision, requires an effective plan to engage human resources and skills to be included through time in the governance and organizational structures. this challenge is common to both epos eric and the tcs, as well as more generally to other erics as highlighted by the egeric expert group (2021). as soon as epos will start its operational phase in 2023, the assessment of the exploitation of data and services by users and stakeholders will be an important factor to corroborate its impact and to confirm the appropriateness of previous investments. this will be an important verification of the technical dimension of epos. in parallel with the evaluation of the exploitation of the epos data and services, the assessment of the epos impact in general, and the scientific impact in particular, will be conducted with appropriate key performance indicators and qualitative impact indicators. they have already been identified and the adopted metrics is currently under discussion. this includes independent evaluation ensured by the epos eric scientific advisory board and by the ethics board, created to impartially advise the eric general assembly on scientific and ethics-related topics. following the recommendations of the esfri wg, demonstrating the economic value and the wider benefit to society is vital for ris. this implies that sufficient resources have to be dedicated both to evaluating the epos value to the economy and society at large and to communicate this to targeted audiences, from the national authorities to policy makers and research organizations, in order to confirm their commitments toward the long-term sustainable operation of the epos ri [marti et al., 2022, this volume]. the abovementioned actions are strategic to discuss the long-term funding with national authorities and research organizations, while communicating epos’s achievements in fostering multi-disciplinary and cross-disciplinary research addressing the societal challenges related to geohazard and to the sustainable exploitation of geo-resources. epos has currently ensured the financial viability of the epos pop. the financial viability and the long-term sustainability of the epos delivery framework require further resources and the increase of the financial contributions by eric members in terms of both membership fees and in-kind contributions, as well as through the engagement of new eric members. long-term sustainability will also depend on national investments in ris and, in particular, in national funds dedicated to support service providers (national nodes) and data providers. this further national funding will be likely represented by direct support to national nodes, reported in the tcs cost book and in the epos eric financial books as in-kind contributions. to achieve this goal, the coordination with national authorities and national research organizations has been implemented in the epos sp project to develop and maintain a dialogue with epos contributors and funders. this will be a delicate and vital stage of the consolidation of epos sustainability, which requires effective communication with all potential stakeholders (eric members, other national authorities, research organizations) to explain demand and impact and this needs the adoption of efficient communication strategies. 7. conclusions epos is a distributed ri with the mission to establish a sustainable access to solid earth science data and services, integrating national research infrastructures under a common federated framework governed by epos eric. to accomplish its mission, epos has been implemented to ensure scientific impact and innovation, enabling access and use of multidisciplinary data and products in solid earth science. for this reason, scientific impact and novel perspectives emerging from using the provided data and scientific products is considered in epos as the overarching frame to undertake a sustainable operation of the ri. in other words, this is the context in which enabling scientific excellence through the governance of a stable and effective exploitation of the ri while addressing long-term sustainability. the european plate observing system (epos) has successfully completed its implementation phase and it is currently facing the transition to its fully operational phase. this transition is not sharp and immediate; rather it kauzar saleh contell et al. 14 requires time and a dedicated stage in the epos lifecycle aimed to construct and further develop the ri together with the testing of the operational performances. this stage is represented by the currently ongoing epos pilot operational phase, which is dedicated to consolidating the epos delivery framework (from governance, legal, financial and technical points of view), testing the tcs-ics system, and addressing the sustainable operation of the epos ri. this paper has presented and discussed the epos approach to ensuring financial viability and tackling the challenge of long-term sustainability of the ri during its full operational phase. the epos experience in addressing financial viability and ri sustainability confirms that substantial funding and support will be necessary in the coming years to reach full operation and ensure long-term sustainability. in order to ensure an adequate funding level, epos eric has to demonstrate to funders and contributors (national authorities and research organizations) that the ri construction has been successfully completed, exploitation is warranted, and impact is routinely assessed. the epos pop has been designed through a dedicated strategic plan to accomplish these tasks and, as shown in this paper, a three years’ time window is necessary. the epos approach to sustainability relies on the definition and the implementation of suitable models for each of the four qualifying dimensions (namely, governance, financial, legal, and technical dimensions) identified to guarantee the operation of the ri, considering the scientific innovation and impact as the overarching frame. this paper shows that these four dimensions are interdependent, although governance is considered as the qualifying dimension that gathers and connects the other three dimensions. indeed, a suitable governance is necessary to manage the pan-european ri through the designed federated approach as well as to share resources and optimize investments also through the harmonization of national roadmaps and dedicated national ri budgets to support tcs and related epos services. the concept of host contribution and the role of hosting countries is considered essential to ensure long-term commitments for operating the core components located inside the epos eric perimeter. the solutions identified to accomplish the sustainable operation of epos will be monitored through the landscape analysis designed to assess values and identify gaps in operating the epos ri. this will allow the implementation of the epos eric long-term sustainability plan, which is a key deliverable of the epos sp project. the implemented governance and financial models as well as the epos functional architecture corroborate that the operation of the epos ri is viable. however, the long-term sustainability of a pan-european research infrastructure like epos requires long-term commitments by national authorities and funding agencies, whose lifetime goes well beyond the cycles of updating national roadmaps to support the eric. even if the solutions adopted by epos eric to tackle the long-term sustainability challenge will be proved workable, a sufficient stable investment environment will be necessary to allow the ri to concentrate on providing high quality services for their user communities instead of continuously looking for funding even for basic operations. acknowledgements. we wish to thank the people involved in the epos eric service coordination committee, composed of tcs and ics representatives, as well as those in the it board for their valuable efforts to make the epos ri operational ensuring progress toward operation. special thanks to all the epos supporters who worked in past years or are still working within thematic communities and ics to build the epos ri sharing its vision and mission. we wish to thank barbara angioni for her support in the graphic design 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(2016). envri cluster – a community-driven platform of european environmental research infrastructures for providing common solution for science and society, geophysical research abstracts, 18, egu2016-15226, https://meetingorganizer.copernicus.org/egu2016/ egu2016-15226.pdf. van belle j., j. van barneveld, b. v. bastiaanssen, a. buitenhuis, l. saes and g. van veen (2018). icos impact assessment report, technopolis group august 2018, https://www.icos-cp.eu/sites/default/files/2018-09/ icos_impact_assessment_report_2018.pdf wilkinson, m. d., m. dumontie, ij. j. aalbersberg, g. appleto, m. axton a. baak, n. blomberg, j.-w. boiten, l. b. da silva santos, p. e. bourne, j. bouwman, a. j. brookes, t. clark, m. crosas., i. dillo, o. dumon, s. edmunds, c. t. evelo, r. finkers, … b. mon (2016). the fair guiding principles for scientific data management and stewardship. sci. data, 3, 160018, https://doi.org/10.1038/sdata.2016.18 wessels r., g. ter maat, e. del bello, l. cacciola, f. corbi, g. festa, f. funiciello, g. kaviris, o. lange, j. lauterjung, r. pijnenburg, g. puglisi, d. reitano, c. rønnevik, p. scarlato, l. spampinato (2022). transnational access to facilities for solid earth sciences: a service to promote 1 multi-domain research in europe. ann. geophys., this volume. wilkinson m.d., s.a. sansone e. schultes, p. doorn, l.o.b. da silva santos and m. dumontier (2018). comment: a design framework and exemplar metrics for fairness, sci. data, 5, doi:10.1038/sdata.2018.118. *co r r e s po n d i n g a u t h o r: kauzar s a l e h co n t e l l, eth zurich, switzerland and epos eric, rome, italy, email: kauzar.saleh@epos-eric.eu https://doi.org/10.1109/escience.2019.00038 https://doi.org/10.1109/escience.2019.00038 https://mediawiki.envri.eu/images/c/c8/d10.4._ri_strategy_for_for_flexibility_and_sustainability.pdf https://mediawiki.envri.eu/images/c/c8/d10.4._ri_strategy_for_for_flexibility_and_sustainability.pdf https://meetingorganizer.copernicus.org/egu2016/egu2016-15226.pdf https://meetingorganizer.copernicus.org/egu2016/egu2016-15226.pdf https://www.icos-cp.eu/sites/default/files/2018-09/icos_impact_assessment_report_2018.pdf https://www.icos-cp.eu/sites/default/files/2018-09/icos_impact_assessment_report_2018.pdf https://doi.org/10.1038/sdata.2016.18 mailto:kauzar.saleh@erdw.ethz.ch cellura_corretto_layout 6 annals of geophysics, 57, 3, 2014, s0329; doi:10.4401/ag-6476 s0329 diffuse soil co2 degassing from linosa island dario cellura1, vincenzo stagno2, marco camarda3,*, mariano valenza1 1 università degli studi di palermo, dipartimento di scienze della terra e del mare (dstm), palermo, italy 2 carnegie institution of washington, geophysical laboratory, washington, dc, usa 3 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy abstract herein, we present and discuss the result of 148 measurements of soil co2 flux performed for the first time in linosa island (sicily channel, italy), a plio-pleistocene volcanic complex no longer active but still of interest owing to its location within a seismically active portion of the sicily channel rift system. the main purpose of this survey was to assess the occurrence of co2 soil degassing, and compare flux estimations from this island with data of soil degassing from worldwide active volcanic as well as non-volcanic areas. to this aim soil co2 fluxes were measured over a surface of about 4.2 km2 covering ~80% of the island. the soil co2 degassing was observed to be mainly concentrated in the eastern part of the island likely due to volcano-tectonic lineaments, the presence of which is in good agreement with the known predominant regional faults system. then, the collected data were interpreted using sequential gaussian simulation that allowed estimating the total co2 emissions of the island. results show low levels of co2 emissions from the soil of the island (~55 ton d-1) compared with co2 emissions of currently active volcanic areas, such as miyakejima (japan) and vulcano (italy). results from this study suggest that soil degassing in linosa is mainly fed by superficial organic activity with a moderate contribution of a deep co2 likely driven by nwse trending active tectonic structures in the eastern part of the island. 1. introduction the release of gases such as co2 from the soil is a common process that occurs in both tectonic and volcano-tectonic areas. in active volcanic areas, large amounts of co2 are released from ascending magmas or generated by crystallization of deep magma bodies [harris and rose 1996, camarda et al. 2012]. part of this co2 is released into the atmosphere through summit craters, while another part is emitted from the soil along the flanks of volcanoes. in non-volcanic environments, moderate to strong emission of co2 are usually encountered in areas with the occurrence of active tectonic structures, such as faults and fractures, which constitutes highly permeable preferential pathways for natural emissions of deep gases. because tectonic lineaments are often buried by soil and/or sediments that act as a gas diffuser, measurements of gas emissions from soil can provide useful information regarding the presence of hidden and/or active faults [giammanco et al. 1997, ciotoli et al. 1998, lewicki and brantley 2000, guerra and lombardi 2001, de gregorio et al. 2002, aiuppa et al. 2004, giammanco et al. 2006], as well as changes in the in feeding system of active volcanic areas [giammanco et al. 1995, federico et al. 2011, camarda et al. 2012]. here we discuss the results of a soil co2 flux survey performed in linosa island. the island is located in the south-western part of the mediterranean sea (see figure 1), and with pantelleria island constitutes part of the sicily channel rift system (scrs). volcanism occurred at linosa island in the late plio-pleistocene [lodolo et al. 2012, and references therein], while there is no evidence of current volcanic activity, such as fumaroles or thermal waters. only few submarine exhalative manifestations were identified near the top of a submerged relict volcanic structure located in the sw offshore of the island [lanti et al. 1988]. the main aim of this study was to investigate the soil co2 flux in a volcanic island with no signs of a recent volcanic activity but located in a currently active seismic area. the results of this study can contribute to extend our knowledge of the global co2 emissions within volcanic and non-volcanic areas [burton et al. 2013]. 2. regional and local tectonic setting the sicily channel rift system (figure 1) formed as consequence of the continental rifting process that affected the northern margin of the african plate since the late miocene in response to the opening of the tyrrhenian sea [boccaletti et al. 1987]. it is characterized by three main tectonic depressions known as panarticle history received december 12, 2013; accepted may 21, 2014. subject classification: fluid geochemistry, co2, soil degassing, volcano monitoring, instruments and techniques. telleria, linosa and malta grabens with depths of 1350, 1580 and 1720 m below sea level (b.s.l.), respectively [grasso et al. 1991, civile et al. 2010]. the uprising of the asthenosphere to 60 km in depth [della vedova et al. 1989], and the thinning of the african foreland continental crust [scarascia et al. 2000] resulted in the formation of widespread volcanism that characterized the sicily channel. the formation of the short-lived island of ferdinandea in 1831 from the graham bank area [gemmellaro 1831] and the submarine eruption of the volcano foerstner in 1891 [washington 1909] sited 5 km nw of pantelleria island, were described as the last two volcanic events responsible of a wide range of products from basalt to trachytic/pantelleritic composition with different geochemical affinity, such as tholeiitic, alkaline, and peralkaline [rotolo et al. 2006]. thermal and bubbling springs, mofettes and fumaroles are all present in pantelleria island. in addition, the actual seismic activity [schorlemmer et al. 2010] is recorded principally near the axial zone of the sicily channel (linosa and pantelleria grabens). located about 160 km south of the southern sicilian coast in the pelagian archipelago (figure 1), linosa island (about 5.2 km2; 196 m a.s.l.) represents the subaerial portion of a larger volcanic complex rising from a depth of 800 m b.s.l. near the south-west sector of the homonymous graben [calanchi et al. 1989]. the influence of the regional tectonic trends (nw-se and nnwsse) on the volcanic evolution of the island can be inferred both by the distribution of the inland and offshore volcanic centers (i.e. the ancient submerged volcanic center of secca di tramontana and a shoal in the se margin), and by the maximum axis of elongation of some principal edifices, such as montagna rossa and pozzo salito (figure 2). these edifices represent the most ancient sub-aerial volcanic complexes in the scrs dated 1.06 ± 0.10 and 0.53 ± 0.07 ma [lanzafame et al. 1994]. 3. soil co2 flux measurements and sampling method 3.1. materials and methods we performed 148 measurements of soil co2 flux during the summer in 2006. the measurement points were distributed over a surface of about 4.2 km2 (80% of the entire area) passing through all the main eruptive centers of the island (monte vulcano, montagna rossa, monte nero, timpone, monte bancarella, fossa cappellano and pozzo salito; see figure 2), with the exception of some lava fields located along the nw and sw side of the island (caletta and arena bianca). in these last areas the soil was found to be totally absent or not thick enough to perform co2 flux measurement with the method described hereinafter. the soil gas investigation was carried out with a mean distance of 150 m between adjacent measurement points, chosen as a good compromise between the detail of the survey and the extent of the investigated area. it is known that the diffuse degassing from soil is influenced by atmospheric conditions [diliberto et al. 2002, granieri et al. 2003, rinaldi et al. 2012]. to minimize the effect of cellura et al. 2 figure 1. geographic sketch of the sicily channel rift with the location of linosa island (fully black). the main regional volcanic manifestations are also reported (heavy grey); seamounts: an = anfitrite; cm = cimotoe; fs = foerstner; gb = graham bank; gl = galatea; lns = linosa-seamounts; nb = nameless bank; td = tetide. 3 the variability of atmospheric conditions the measurements were conducted within few hours during the day, implying a negligible temperature variation. measurements of co2 diffuse degassing were performed using the dynamic concentration method [gurrieri and valenza 1988, camarda et al. 2006b]. this method consists of measuring the co2 concentration in a mixture of air and soil gas that has been generated in a specifically designed probe inserted into the soil to a depth of ~50 cm. the gas mixture is then obtained by producing a very small negative pressure in the probe using a pump at a constant flux of 0.8 l min-1. the mixture is subsequently analyzed using an infrared (ir) gascard ii spectrophotometer provided by edinburgh instruments ltd. after a given time of pump activation (generally less than 1 minute), a constant co2 concentration is measured in the gas mixture, that is the dynamic concentration cd [camarda et al. 2006b]. the relationship used to convert dynamic concentration values to co2 flux was calibrated after laboratory measurements during which several measurements of cd were collected in a soil layer fed by known and constant co2 fluxes [camarda et al. 2006b]. several tests were performed using soils with different values of gas permeability and porosity in order to evaluate the influence of soil permeability on the calculated co2 fluxes. these results revealed that the propagated error in the flux calculation is generally less than 5% for soils with gas permeability varying between 0.36 and 123 darcy [camarda et al. 2006b]. due to the absence of values of gas permeability for the soils of linosa, we considered a value of soil gas permeability of 37 darcy used to calculate the co2 flux from all the 148 measurement points with dynamic method. this value corresponds to the average value of gas permeability estimated for the soils in vulcano island, sicily [camarda et al. 2006a]. the soil gas permeability in this island ranges between 5 darcy (fine sand) and 80 darcy (coarse sand). other values of gas permeability of volcanic soils were reported by moldrup et al. [2003], where the air permeability of some volcanic ash soils were determined from three locations in japan. in this case the reported values ranged from 1.8 to 168 darcy with a mean value of 31 darcy. additional data available in literature generally refer to water permeability and cannot be used to accurately assess the value of gas permeability of volcanic soils due to the known klinkenberg effect [klinkenberg 1941]. therefore, our value of 37 darcy can be considered representative for gas permeability of soils in linosa island. further uncertainties in measurements of co2 from soil using the dynamic concentration method might be introduced by pressure variations during the field measurements. however, owing to the almost flat topography of the island (max altitude of ~190 meters), possible errors can be considered negligible (about ±3% of range). during the survey three gaseous samples were collected in those sites with the highest co2 fluxes to determine their carbon isotopic composition. gases were diffuse soil co2 degassing from linosa island figure 2. simplified representation of the island of linosa with the main tectonic lineaments (black lines) and volcanic centers (grey areas and dotted lines) (modified after lanzafame et al. [1994]). black points indicate the location of measurements for the soil co2 flux. sampled from the soil at a depth of 50 cm using a teflon tube of 5 mm in diameter connected to a syringe, and then stored in glass flasks equipped with vacuum stopcocks. the carbon isotopic composition of co2 was determined using a finnigan mat delta plus mass spectrometer. measured isotopic values are expressed as d13c(co2) versus the pee dee belemnite (pdb) standard and characterized by uncertainties of ±0.2‰. 3.2. sequential gaussian simulation and estimation of total co2 flux sequential gaussian simulation (sgsim) was performed with the aim to map the spatial variation of soil degassing and calculate the total co2 output of the island. the (sgsim) was performed using the stanford geostatistical modeling software (sgems), an opensource package developed by the stanford center for reservoir forecasting for solving problems involving spatially related variable [remy 2005]. to perform sgsim the distribution of data must be gaussian. therefore, as first step, all fluxes were normalized in order to obtain a distribution with mean and variance being equal to 0 and 1, respectively. then, the semivariogram of normalized data was determined in order to examine the spatial variability of the measured soil co2 fluxes. further technical details of the sgsim consisted of, (1) definition of the grid of the simulation; (2) definition of a random path visiting each node of the grid area; (3) random choice of the node in the sampled grid; (4) estimation at each node of the mean value and the variance of the local conditional cumulative distribution function (ccdf) by using simple kriging algorithm considering the semivariogram of the normalized data; (5) drawing of a value from that gaussian ccdf and addition of the simulated value to the data set; (6) proceeding to the next node in the path and repeating the (4) and (5) steps until all nodes have been visited. the total co2 output emitted from the soils of the island of linosa and its relative uncertainty were obtained as the mean and the standard deviation of the total soil co2 flux estimated for each of the 100 simulations. these flux values were calculated for each simulation integrating over the grid area the average value of the soil co2 flux given by the sgsim. 4. results and discussion 4.1. statistical analysis of the soil co2 fluxes the data relative to the measured dynamic concentrations (cd) and the calculated fluxes are shown in table s1 along with the coordinates of each measurement point (see supplementary files). the values of soil co2 flux collected during the survey showed a small variation between 12 and 78 g m-2 d-1, with a mean value of 11 g m-2 d-1. this range of fluxes is lower than that observed in active volcanic areas, such as the island of vulcano and mount etna (italy), where the measured soil co2 fluxes generally display values up to three orders of magnitude [chiodini et al. 1998, camarda et al. 2012]. the high variability of soil co2 fluxes observed in active volcanic areas can be manly explained as due to mixing between co2 with different origin such as (a) microbial decomposition of soil organic matter and root respiration, and (b) emissions from a deep degassing source [camarda et al. 2007, chiodini et al. 2008]. as a consequence, the resulting statistical distribution is expected to be, at least, bimodal. in this study, although the observed low variability of soil co2 fluxes, we constructed a normal probability plot (pp) of the soil co2 fluxes at the aim to discriminate populations of data ascribable to different sources [sinclair 1974, chiodini et al. 2008]. this type of plot shows the cumulative frequencies (x axis) of the measured soil co2 fluxes (y axis; figure 3) on a probability scale. data that are normally distributed will appear in this plot as a straight line. the presence of multiple straight lines with different slopes indicates, therefore, the presence of various populations of data with normal distribution but with diverse oricellura et al. 4 table 1. summary statistics for the portioned co2 flux populations. co2 flux populations mean co2 flux (g m-2 d-1) standard deviation proportion i 9 1 80 ii 13 2 14 iii 43 21 6 figure 3. probability plot of soil co2 fluxes measured on the island. the red dotted curve shows the theoretical distribution produced by combining the populations in the proportions indicated by the threshold values according to the equation reported at the top of the graph. solid lines indicate the populations reconstructed according to the procedure illustrated by sinclair [1974]. 5 gin of the co2. the pp of all field measurements performed for this study displayed this feature. figure 3 shows the pp of the soil co2 flux data measured in linosa island during august 2006. the pp shows two inflection points reflecting three overlapping populations of data with normal distributions, a low-flux group (population i), an intermediate-flux group (population ii), and a high-flux group (population iii). the main statistics of these populations are listed in table 1. to test the validity of the three-population model we also compared the theoretical distribution resulting from the combination of the portioned populations (dotted line in figure 3) with the actual data distribution (empty circles in figure 3). a good agreement between the real and ideal distributions was found, as proof of the high goodness of the three-population model. in particular, the population i is characterized by low fluxes with a mean value of 9 g m-2 d-1 and a standard deviation of 1 g m-2 d-1. these low values are consistent with typical values reported for superficial soil respiration (3.6–13.7 g m-2 d-1, monteith et al. [1964]; 6–11 g m-2 d-1, brown and rosenberg [1971]). the population iii is characterized by a mean value of 43 g m-2 d-1 and standard deviation of 21 g m-2 d-1, and can be considered as the anomalous population, likely suggesting the possible contribution of a deeper source of co2. the intermediate population ii shows a mean value of 13 g m-2 d-1 with a standard deviation of 2 g m-2 d-1. this flux group can be explained assuming mixing of shallower biogenic co2 and co2 of deep origin [chiodini et al. 2008]. alternatively, the population ii can also be ascribed to the biogenic superficial activity, being its flux consistent with those values typically reported for soil respiration. in this case, we would have two different background populations (i and ii) that, for instance, may be related to the presence of different types of vegetation in the island. 4.2. mapping soil co2 flux and total co2 output the experimental semivariogram of the normalized data (figure 4) has been computed and fitted by the following standardized exponential model, expc c c a h1 30 0c = + -^ h8 b` j diffuse soil co2 degassing from linosa island figure 4. variogram of normal scores of the soil co2 flux. figure 5. maps of soil co2 flux. from the top to the bottom, the map relative to the realization giving the lowest value of the soil co2 flux output, the map relative to the realization giving the mean value of the soil co2 flux output, and the map relative to the realization giving the highest value of the soil co2 flux output. a b c the values assumed by the nugget (c0), the sill (c) and the range (a) are reported in figure 4 and are here used in the sgems software to adjust the model of the sgsim. figure 5 shows three examples of maps representative of low, intermediate and high flux, respectively (a, b and c). the spatial distribution of soil co2 flux is significantly different for the three cases. areas with high flux resulting from one simulation may have lower flux resulting from another simulation. however, the different simulations are equiprobable and “honor” the experimental data in the sense described by journel and alabert [1989]. indeed, each simulation has the same univariate statistics and variogram as the experimental data, and furthermore retains the measured flux value unchanged at the original measurement sites. therefore, we cannot use a single simulation to trace degassing tectonic structures in linosa island. in order to show a more suitable map and define the extent of the anomalous degassing areas of linosa island, as demonstrated by cardellini et al. [2003], we elaborated a probability map that displays the probability that the soil co2 flux exceeds a threshold value at each node of the grid. figure 6 shows a map with probability exceeding 18 g m-2 d-1. this value was selected by inspecting the probability plot of figure 3, and it was assumed as cutoff value separating the intermediate co2 flux from anomalous values. as discussed by cardellini et al. [2003] the map in figure 6 represents the probability that each location belongs to diffuse degassing structures (dds). these are well-defined regions of a surveyed area characterized by high levels of soil co2 emission and they are generally related to structural discontinuities acting as preferential pathways for gas migration towards the surface [chiodini et al. 2001, cardellini et al. 2003]. the map shows that in linosa island, dds lie manly in the eastern sector of the island and have generally a preferential nwse direction of elongation, corresponding to the main direction of the faults system in the sicily channel. the distribution of the low-energy seismic events along the axial portion of the rift system or in correspondence of the graben borders [agius and galea 2011], would confirm the activity of these faults. hence, the persistent seismic activity in the scrs would contribute to maintain a high permeability of soils driving deep fluids towards the surface. the carbon isotopic composition of the co2 of three gas samples collected at 50 cm deep in the soil from two different areas showed d13c(co2) values versus pdb of −17.3, −14 and −8.4 ‰, respectively. due to the low values of the soil co2 flux (< 80 g m -2 d-1), the kinetic isotopic fractionation can be considered negligible in any case [camarda et al. 2007]. the measured d13c values fall within the typical range of co2 with organic origin (−30 to −10 ‰ vs. pdb; hoefs [1980], o’learly [1988]), except for the sample with d13c(co2) of −8.4 ‰. there are two possible explanations to justify the observed isotopic values. as first, the observed variability can be ascribed to different types of plants present in the island. for example, c3 plants are characterized cellura et al. 6 figure 6. map that probability exceeds the threshold value of 18 g m-2 d-1, discriminating background from anomalous co2 flux. the main volcano-tectonic lineaments (red lines) by lanzafame et al. [1994] are also reported. 7 by d13c(co2) of −27 ‰ whereas c4 plants have value of −13 ‰ [cheng 1996]. an alternative explanation that can explain the observed data is that the observed shift toward less negative values of d13c might be due to contribution of co2 from a deep (mantle) source that reaches the surface through the mapped dds [frezzotti et al. 2009]. however, our data do not allow us to rule out with accuracy one of these two mechanisms, and both might also coexist. table 2 reports the total co2 flux output calculated as explained above (section 3.2). in the same table we reported for comparison the total co2 output estimated from other authors from some active volcanoes, tectonic and non volcanic areas in the world. as exdiffuse soil co2 degassing from linosa island table 2. total co2 flux output estimated for the island of linosa compared with those of some active volcanoes and those of tectonic, hydrothermal or inactive volcanic areas; data are sorted based on co2 output in ton km -2 d-1. area (km2) co2 flux co2 flux ref. (ton d-1) (ton km-2 d-1) cuicocha caldera, ecuador 13.3 106 ± 5 8 padròn et al. [2008] pululahua caldera, ecuador 27.6 270 ± 19 10 padròn et al. [2008] linosa 5 55 ± 0.5 11 this work satsuma-iwojma volcano, japan 2.5 80 32 shimoike et al. [2002] iwojima volcano, japan 22 760 35 notsu et al. [2005] vesuvio volcano, italy 5.5 193.8 ± 26.4 35 frondini et al. [2004] mt. epomeo, italy 0.86 32.6 ± 2.6 38 chiodini et al. [2004] nisyros caldera, greece 2 84 ± 3.8 42 cardellini et al. [2003] yanbajain geothermal field, china 3.2 138 43 chiodini et al. [1998] nea kameni island, greece 0.28 15.4 55 chiodini et al. [1998] reykjanes geothermal area, iceland 0.22 13.5 ± 1.7 61 fridriksson et al. [2006] taupo volcanic zone, new zealand 8.9 620 ± 37 70 werner and cardellini [2006] vulcano (italy) 5.4 453 84 inguaggiato et al. [2012] miyakejima (summit caldera), september 1998 1.1 98 89 hernández et al. [2001] ustica 4.5 712 158 etiope et al. [1999] furnas caldera 5.9 959 ± 84 164 viveiros et al. [2010] hakkoda region, japan 0.58 127 219 hernández et al. [2003] miyakejima (summit caldera), may 1998 0.62 146 235 hernández et al. [2001] methana volcanic system, greece 0.01 2.59 259 d’alessandro et al. [2008] poggio dell’olivo, italy 0.82 233.5 ± 27.9 285 cardellini et al. [2003] stromboli (italy) 1.1 397 361 inguaggiato et al. [2013] hot spring basin, yellowstone 0.16 63 ± 20 393 werner et al. [2008] mud volcano, yellowstone 3.5 1730 494 werner et al. [2000] ribeira quente village 0.35 243 ± 43 694 viveiros et al. [2010] teide volcano (summit), spain 0.53 380 717 hernández et al. [1998] liu-huang-ku, taiwan 0.03 22.4 747 lan et al. [2007] horseshoe lake 0.13 104.3 ± 5 802 cardellini et al. [2003] mammoth muntain (usa), september 1997 0.145 130 897 gerlach et al. [1998] albani hills 0.06 74 1233 chiodini and frondini [2001] solfatara volcano, italy 1 1500 1500 chiodini et al. [2001] mammoth muntain (usa), august 1995 0.145 350 2414 gerlach et al. [1998] cerro negro volcano, nicaragua 0.58 2800 4828 salazar et al. [2001] pected, the total co2 flux estimated from this study for linosa island is low as compared with active volcanic areas, also considering the much wider areas under investigation. for instance, the flux calculated by inguaggiato et al. [2012] in the island of vulcano over a comparable surface, resulted to be one order of magnitude higher than the value found in linosa. indeed, as shown here, soil co2 fluxes in linosa are only fed by superficial organic activity and degassing of mantle co2 through the recognized dds. no co2 contribution from a hydrothermal systems, such as that identified in vulcano island [nuccio et al. 1999], can be claimed in case of linosa island. furthermore, by inspecting the co2 emission standardized for area (fourth column in table 2), linosa resulted to be one of the areas with the lowest co2 emission in the world. nevertheless, his standardized value is comparable with the co2 emitted from the cuicocha and pululahua caldera in equador [padròn et al. 2008]. 5. conclusions a survey of soil co2 diffuse degassing in linosa island was conducted during summer 2006, over ~80% of the total surface. the island represents the oldest sub-aerial volcanic centre in the sicily channel (1.06 to 0.53 ma), and to date direct evidences of related volcanic manifestations (fumaroles, thermal springs) are lacking. the results from soil co2 flux measurements would confirm this current state of the island. the sequential gaussian simulation allowed estimating total co2 output from the island of linosa, which appears very low in comparison to active volcanoes and other inactive volcanic areas. in conclusion, our data showed that the actual soil co2 degassing in linosa is primarily fed by co2 with superficial organic (biogenic) origin with a moderate contribution limited to some areas where the origin of co2 fluxes can be likely referred to deeper sources. by using a map of the probability that co2 flux would exceed a cutoff value we identified the location and the extent of the diffuse degassing structures in the eastern sector of the island. carbon isotopic composition of co2 from samples collected in relatively high-flux zones would suggest a moderate contribution of co2 with mantle origin. however, we do not exclude the possibility of a more rigorous geochemical characterization of these gas emissions in future aimed to confirm the possibility of a degassing mantle source beneath the island of linosa. acknowledgements. the project was funded by the dipartimento della protezione civile (dpc-ingv agreement 2004-2006, projectv5). critical comments and helpful suggestions by the associate editor, francesco frondini, an anonymous reviewer and domenico granieri greatly improved the manuscript. references agius, m.r., and p. galea (2011). a single-station automated earthquake location system at wied dalam station, malta, seismol. res. lett., 82, 545-559. aiuppa, a., a. caleca, c. federico, s. gurrieri and m. valenza (2004). diffuse degassing of carbon dioxide at somma-vesuvius volcanic complex (southern italy) and its relation with regional tectonics, j. volcanol. geoth. res., 133, 55-79. boccaletti, m., g. cello and l. tortorici (1987). transtensional tectonics in the sicily channel, j. struct. geol., 9, 869-876. brown, k.w., and n.j. rosenberg (1971). energy and co2 balance of an irrigated sugar beet (beta vulgaris) field in the great plains, agron. j., 63, 207-213. burton, m.r., g.m. sawyer and d. granieri (2013). deep carbon emissions from volcanoes, rev. mineral. geochem., 75 (1), 323-354. calanchi, n., p. colantoni, p.l. rossi, m. saitta and g. serri (1989). the strait of sicily continental rift system: physiography and petrochemistry of the submarine volcanic centers, mar. geol., 87, 55-83. camarda, m., s. gurrieri and m. valenza (2006a). in situ permeability measurements based on a radial gas advection model: relationships between soil permeability and diffuse co2 degassing in volcanic areas, pure appl. geophys., 163, 897-914. camarda, m., s. gurrieri and m. valenza (2006b). co2 flux measurements in volcanic areas using the dynamic concentration method: influence of soil permeability, j. geophys. res., 111, b05202; doi:10.10 29/2005jb003898. camarda, m., s. de gregorio, r. favara and s. gurrieri (2007). evaluation of carbon isotope fractionation of soil co2 under an advective–diffusive regimen: a tool for computing the isotopic composition of unfractionated deep source, geochim. cosmochim. acta, 71, 3016-3027. camarda, m., s. de gregorio and s. gurrieri (2012). magma-ascent processes during 2005-2009 at mt. etna inferred by soil co2 emissions in peripheral areas of the volcano, chem. geol., 330-331, 218-227; doi:10.1016/j.chemgeo.2012.08.024. cardellini, c., g. chiodini and f. frondini (2003). application of stochastic simulation to co2 flux from soil: mapping and quantification of gas release, j. geophys. res., 108 (b9), 2425-2437. cheng, w. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 64, 5, vo548, 2021; doi:10.4401/ag-8670 appendix to historical sources of the roman age. formation of a new volcanic island (or tephra platform) in 183 bc marco manni1, mauro rosi2 (1).istituto nazionale geofisica e vulcanologia sezione di catania osservatorio geofisico di lipari, lipari, italy (2).università degli studi di pisa, dipartimento di scienze della terra; pisa, italy strabonis, geographia vi 2, 10-11, c.275-277 news on the creation of a new volcanic island in the aeolian sea dates back to the 2 century bc. strabo mentions posidonius, a historian and geographer living between c. 135 and 50 bc, who narrates that in his time (to be understood as several decades earlier as expressed below): “towards the summer solstice, at dawn we saw the sea, in the stretch between iera and evonimo, rise to an extraordinary height, stay high for a certain time, under the pressure of a continuous eruption, and then retreat again … many days later, a patch of mud appeared on the sea and everywhere flames and smoke and sooty steam were released; then the mud solidified and the mass took on a colour similar to the grindstone. the governor of sicily tito flaminio (229-174 bc) gave news of the incident to the senate, who sent a deputation to make a sacrifice on the islet and lipari, to the underworld and marine gods”. greek-italian translation by biffi n. [pagliara, 1995]. posidonius also speaks of a large tidal wave that formed at the time of the emergence of the islet [giustolisi, 2005]. titi livi, annales xxi 49, 1-3; xxxix 56,6 titus livy (59 bc-17 ad) specifies the exact period in which the event took place: “not far from the sicilian coast: in the year of hannibal's death” i.e. 183 bc [giustolisi, 2005]. c. plinii secundi, naturalis historia ii 89. pliny the second (about 61-113 a.d.) also recalls its creation: “before our time, near italy, one arose in the middle of the aeolian islands…” latin-italian translation by barchesi a. et al. [conte et al., 1982]. strabonis vi 2, 10-11, c. 275-277. “about halfway between lipara and sicily is termessa, which now bears the name of iera di hephaestus, all rocky, deserted and on fire, with three openings which, in a certain way, correspond to as many craters. the largest, amidst the flames, erupts large incandescent masses that have now filled a large part of that stretch of sea”. greek-italian trans. by: libertini g. [1921], biffi n. [pagliara,1995]. sources of the 2nd 1st century bc. other submarine eruptions in 126 and 91-88 bc between lipari and vulcano iulii obsequentis, prodigiorum liber 29 “during the consulate of marco emilio and lucio aurelio (126 bc) near the lipari islands, the sea boiled and caused the death of a large number of sailors, the ships having been burned by the steam, moreover, it transported here and there a huge quantity of lifeless fish, of which the inhabitants of lipari too greedily went in search to eat and died from contamination of the belly, as if the islands were devastated by an extraordinary pestilence…” [pagliara,1995]. c. plinii secundi, naturalis historia ii 238 pliny the elder adds that, during the roman social war fought from 91 to 88 bc, powerful eruptions occurred in vulcano both on land and at sea: “in the middle of the sea the aeolian islands of iera (vulcano) and lipari, near italy, burned together with the same sea for a few days at the time of the social war until a delegation of the senate made propitiatory sacrifices”. latin-italian trans. by barchiesi et al. [pagliara,1995]. virgilio, aeneid, viii the poet virgil (70-19 bc) indicates in his verses the position of the island of vulcano between lipari and sicily, without noting vulcanello: “a small island lies between sicania on the one hand / and lipari on the other / which comes out of the waves high up and smokes”. latin-italian trans. by caro a. [castellino and peloso, 1967]. sources of the first millennium. between misleading reconstructions and lengthy silences eutropio, breviarium historiae romanae, book iv flavio eutropio, a roman politician and historian born in 387 in bordeaux, france, narrates that: "in sicily, the island of vulcano suddenly came out of the sea to everyone's amazement and still stands today" [ zagami, 2006]. pauli orosi, historiae adversus paganos, iv 20 paolo orosio, a priest born in 390 in tarragona (spain), writes: “at that time during the consulate of m. claudio marcello and q. fabio labeone (= 183 bc), near the coast of sicily it suddenly rose up from the sea, to everyone's astonishment, the island of vulcano, which did not exist before and continues to this day”. latin-italian trans. by bartalucci-chiarini [pagliara, 2005]. cassiodorius, variarium, iii epist. 47 senator cassiodorius (485-585 ad) wonders about the perennial nature of the flames of the island of vulcano, which emerged as is handed down in the year of hannibal's death (183 bc) and in his time it became a place of confinement for the curial iovino, by theodoric [giustolisi, 2005]. ravennatis anonymi, cosmographia et guidonis geographica the islands of the archipelago were presented with the same name in the 7th century, still without vulcanello: “erculis, lipparis, stroile, pagnarea, vulcana, didimi, enicodes, basilidin, erigodes” (erculis, lipari, stromboli, panarea, vulcano, salina, filicudi, basiluzzo, alicudi) [bernabo brea, 1988; pagliara, 1995]. enkomion, san teodoro studita san teodoro studita (758-826 ad) tells of the legendary arrival of the remains of san bartolomeo in lipari at the end of the 3rd century ad, and of the consequent removal of mount vulcano from the largest island of the aeolian islands: “-because the mountain called vulcano arose too close to the island (of lipari), it caused constant damage to those living in the surroundings. then at night, with an imperceptible motion of skidding, it detached itself for almost seven stages suspended over the water, so that even today the stretch covered by the fire when it retreats is evident to the viewer. today we see the outline of a fiery island that seems to be still in flight” [ bernabò brea, 1988]. almas-ῡdī, gold meadows and gem mines “of this (island, sicily) it will be spoken of further on, touching the gabal 'al burkan (mount of the volcano), the one that sends fire, mixed with (other) materials and large bodies. burkan is the atîmah (crater) that erupts igneous bodies resembling headless men; whose bodies rise into the air at night, and fall back into the sea they remain afloat. this is the stone that is used to scrape the sheep-paper scrolls. it has the appearance of a honeycomb or a small wasp's nest”. biblioteca arabo-sicula [amari, 1880]. almas-ῡdī “tanbih…” (the warning and the review) “i also saw the island of sicily and the adjacent mountain of ‘al burkan (vulcano), from which the source of the fire emerges…” [amari, 1880]. sources of the second millennium: geographical and toponymic evidence on medieval vulcanello al-bakrī, kitāb al-masālik wa al-mamālik “the great volcano is located on two islands to the north of sicily” [amari, 1880]. ‘abû abd ‘allȃh muhammad ibn ‘alī, detto ibn sabbat “in sicily is the great volcano, the likes of which nothing more terrifying in appearance, nor more singular in substance is known in the world. this is the case of the two islands in the north of sicily” [amari, 1880]. al-idrīsī (edrisi), "bliss for those who delight in traveling the world", alias “il libro di ruggiero”. al-idrīsī, the arab geographer who settled in palermo in 1139 at the invitation of ruggiero ii, was formally commissioned by him in 1154 to create a collection of maps and geographical information entitled "il libro di ruggero”. describing the large and small islands of the mediterranean, including vulcanello for the first time: "'gabal' al burkān (mount of volcano), 'gazīrat' al burkān (island of the volcano), 'gazīrat' al lībar (island of lipari), etc. [amari, 1880]. ‘abû ‘al husayn ‘ibn gugayr, rhalat ‘al kinȃnî (kinani trip) the spanish pilgrim ibn jubayr on 18 december 1184, during the return journey, sailed from the strait of messina with his companions to the northern coasts of sicily. “on the right hand, nine islets (the aeolian islands) can be seen in the sea, rising like ghosts (or mountains) not far from the land of sicily, two of which (vulcano and vulcanello) are constantly emitting fire. in fact, we saw the smoke that came out of both: and at night a red fire came out of it that threw [many] tongues [of flame] into the air. this is the well-known [phenomenon of the] volcano. we knew that the fire comes out of two mountains of said (islands) through certain openings, from which a fiery puff bursts violently and ignites. often a large stone is thrown into the middle of the fire, which the force of the said puff causes it to rise into the air and prevents it from stopping and falling to the bottom: this is one of the most wonderful facts that are told…" [amari, 1880]. arch. capitolare di patti, de fondationibus i, f.211 below is a short extract from an original inventory of the paetensis ecclesiae dated 2 july 1247 “isola di vulcano, mining of alum and sulphur, as well as offering possibilities for good rabbit hunting. island of vulcanelli offers possibilities of rabbit hunting” [catalioto, 1996; iacolino, 2001]. la embajada de enrique iii al talmorlàn the travel diary of the ambassadors of henry iii on their way to samarkand provides interesting information on the state of activity of the aeolian volcanoes at the beginning of 15th century. “july 16, 1403. the following monday in the morning they passed between some unpopulated islands one of which is called sallinas (salina), one strangoly (stromboli), another bolcanin (vulcanello) and a further bolcan (vulcano), and from them came out much smoke and they made a great din. and they also passed by two other islands that are unpopulated, one called panarca (panarea) and the other panarin (basiluzzo?). -17 july 1403. ... and the storm continued on tuesday and wednesday during two hours of the night and the aforementioned mouths, in particular those of stromboli and vulcano with the strong wind launched mighty flames of fire and smoke with great noise, and during the storm the patron had the litanies sung, and everyone asked for god's mercy” [lopez estrada, 1996; iacolino, 2007]. tabula regio novem a manuscript dating back to around the middle of the 15th century, kept at the aragonese castle of ischia, the subject of a recent linguistic study, contains a large folding page divided into a three-bythree format, therefore nine zones, hence its name tabula regio novem (nine-region panel). each includes a round illustration with numerous annotations and other components that unite the nine zones into a cohesive whole. the islands of lipari, vulcano, during its eruption between 4 and 5 february in 1444, and vulcanello have been recognized in it, separated by a short stretch of sea crossed by those on the nearby island who were taken by surprise by the aforementioned eruption [cheshire, 2018]. vulcanello in the modern age fazello, le due deche dell'historia di sicilia fazello (1498-1570), a sicilian presbyter and historian, on describing the aeolian islands recalls that: “the third island is called vulcanello, which is divided from vulcania by a very narrow stretch of sea, and this island is also seen sometimes launching fire. this breast, or arm of the sea up to my time was navigable, and gave a trusted port, when it was needed by the ships, but now it has been filled with ashes and stones, which are hurled from the island of vulcania”. “this island (vulcano) was not formed like the others at the beginning of the world, but it was created, and came about in an instant, from the wind excited by the fire, as eusebius writes, out of the water, at the time that spu. postumius albinius and quintus fabio labeone were consuls in rome, five hundred and fifty years after its construction, as pliny affirms in the second book in chapter 90. isidoro writes it again in 14. paolo orosio states it in the fourth book, 92 in the chapter 19. and eutropius confirms it in book 4”. latin-italian translation by nannini, r. [fazello, 1563]. hieronimo maurando, itinéraire d’antibes in constantinople hieronimo maurando, a french priest, writer and traveller, in his itinéraire d’antibes in constantinople documented the siege of lipari in 1544 alongside the army of the saracen pirate ariadeno barbarossa. on 2 july, he landed in vulcanello in order to reach the gran cratere. “i mentioned the island of vulcano, which is half a mile east of lipari, uninhabited and very large; what needs to be known is that this island was once divided into two islands, one called vulcano and the other vulcanello, but today they are but one, because the abundant ash that escaped from vulcano blocked the channel that divided them and now thanks to these abundant ashes a good port has arisen where, on july 2 [..] we landed with the rialle galley and then we climbed to the top of the volcano and saw where the perpetual fumes came from its summit emitted by the volcano ". latin-italian trans. by nannini, r. [fazello, 1563]. annals of geophysics, 64, 5, vo548, 2021; doi:10.4401/ag-8670 appendix to origins of vulcanello based on the re-examination of historical sources (vulcano, aeolian islands) marco manni1, mauro rosi2 (1).istituto nazionale geofisica e vulcanologia sezione di catania osservatorio geofisico di lipari, lipari, italy (2).università degli studi di pisa, dipartimento di scienze della terra; pisa, italy kircher athanasius, mundu subterraneum. in xii libros digeftu "it is said that about 60 years earlier (in 178 bc) a liparian volcano had thrown up such a quantity of ashes and lapilli, that vulcanello formed close to it in the middle of the sea, which for this reason is called after the son generated by his father” [kircher, 1678]. friedrich munter, nachrichten von neapel und sizilien auf einer reise in den jahren 1785 und 1786 gesammelt another precious testimony is that of friedrich munter of february 1786: “i did not derive any other advantage from my return, than to review the delightful strait of sicily (messina)… and particularly vulcano and vulcanello which i was allowed to consider distinctly. these two islands smoke, and vulcano sometimes still threw fire at night. i myself was enchanted when three flame throwing mountains appeared at the same time: that is, these two, and etna, whose boundless summit with an eternal snow cover ascended high in the clouds "[ansla, 1995; la greca, 2009, b]. henri-rené-albert-guy de maupassant, la vie errante “and here is the cone of vulcan before reaching it, another islet, vulcanello, suddenly emerged around 200 bc, and attached to its older brother by a narrow strip of land, swept by the waves on stormy days”. translation french-italian by rota, c. [palermo, 2001]. luigi salvatore d'asburgo-lorena, die liparischen inseln salvatore luigi d’asburgo-lorena (archduke of austria, prince of tuscany) frequenter of the aeolian islands, visited the island of vulcano several times from 1867. based on what the author himself writes, it is plausible to attribute the following description of the isthmus of vulcanello to 1891, the year of his last visit, immediately after the last eruption of the great crater dated 1888-90. “vurcanu is connected to vurcaneddu by a flat, sandy strip of land that forms, on either side, the ports of punenti and livanti. on the west side, the waves often break violently and pushed by the winds they sometimes reach the centre of the strip of land which, cultivated with rushes, has a marshy appearance” [paino, 1987]. annals 48, 3, 2005+app1 467 annals of geophysics, vol. 48, n. 3, june 2005 key words inospheric forecasting − ionospheric modelling − ionospheric storms 1. introduction access to real-time information on ionospheric conditions over a certain area (e.g., europe), as a requirement for high frequency communication, satellite-to-ground links, and solarterrestrial research, requires: a) a network of vertical ionosondes covering as large an area as possible with real-time data access (hanbaba, 1999); b) short-term forecasting algorithms to be developed for fof2, m(3000)f2, and tec to predict their values up to 24 h ahead (kutiev et al., 1999; hochegger et al., 2000); c) a mapping algorithm for interpolation between the stations in the network to be implemented by using one of the available mapping procedure (samardjiev et al., 1993); d) validation of the mapping and forecasting algorithms. as a response to these needs, an operational short-term ionospheric forecasting (stif) tool for the european region based on continuous monitoring of the ionosphere has been developed and is available on the world wide web for interactive use (http://ionosphere.rcru.rl.ac.uk) (cander, 2003, and references therein). it provides forecasting maps for up to 24 h ahead and archive measurement maps of the critical frequency fof2 of the ionospheric f2-layer, the maximum usable frequency for a a dynamic system to forecast ionospheric storm disturbances based on solar wind conditions ioanna tsagouri (1), anna belehaki (1) and ljiljana r. cander (2) (1) ionospheric group, institute for space applications and remote sensing, national observatory of athens, palaia penteli, greece (2) rutherford appleton laboratory, chilton, didcot, oxon, u.k. abstract for the reliable performance of technologically advanced radio communications systems under geomagnetically disturbed conditions, the forecast and modelling of the ionospheric response during storms is a high priority. the ionospheric storm forecasting models that are currently in operation have shown a high degree of reliability during quiet conditions, but they have proved inadequate during storm events. to improve their prediction accuracy, we have to take advantage of the deeper understanding in ionospheric storm dynamics that is currently available, indicating a correlation between the interplanetary magnetic field (imf) disturbances and the qualitative signature of ionospheric storm disturbances at middle latitude stations. in this paper we analyse observations of the fof2 critical frequency parameter from one mid-latitude european ionospheric station (chilton) in conjunction with observations of imf parameters (total magnitude, bt and bz-imf component) from the ace spacecraft mission for eight storm events. the determination of the time delay in the ionospheric response to the interplanetary medium disturbances leads to significant results concerning the forecast of the ionospheric storms onset and their development during the first 24 h. in this way the real-time ace observations of the solar wind parameters may be used in the development of a real-time dynamic ionospheric storm model with adequate accuracy. mailing address: dr. ioanna tsagouri, ionospheric group, institute for space applications and remote sensing, national observatory of athens, metaxa and vas. pavlou str., 15236 palaia penteli, greece; e-mail: tsagouri@space.noa.gr 468 ioanna tsagouri, anna belehaki and ljiljana r. cander 3000 km range muf(3000)f2, nequick modelled vertical total electron content (tec), and frequency of optimum traffic (fot) for the area of interest at each ut hour. a similar technique can be applied in any other areas, like u.s.a., australia, china, where enough data are available in timely form. accuracy of forecast of fof2, muf(3000)f2 and tec has been studied through several statistical comparisons between measured and forecast values of fof2 and muf(3000)f2 (cander et al., 2003). the forecast values selected were those deduced for one day ahead of the measured values. the principal results concern: 1) comparisons at each ut hour during 10 geomagnetically quiet days of each month in two years of maximum solar activity in the current solar cycle 2000 and 2001; and 2) comparisons at each ut hour during 5 geomagnetically disturbed days of each month in the same years. these gave the root mean square error (rmse) of approximately 0.76 mhz for fof2 and 2.51 mhz for muf(3000)f2 respectively and normalised rmse (nrmse) of 0.17 for both in the case of the geomagnetically quiet ionosphere. figure 1 shows an example of the typical stif fof2 and muf(3000)f2 results and measurements during seven quiet days of december 2001. global statistical comparisons for the 5 most disturbed days in 2000 and 2001 gave rmse for fof2 and muf(3000)f2 of 1.48 mhz and 4.38 mhz, respectively and nrmse = 0.67 for both. it should be noted that individual hourly rmse in muf(3000)f2 are generally about three times those of fof2. this is to be expected as m(3000)f2 propagation factors are typically about 3 though additional errors can be introduced by the measurements of the m(3000)f2 factor. figure 2 shows an example of the typical stif fof2 and muf(3000)f2 results and measurements during seven extremely disturbed days of march and april 2001. these results clearly suggest that the stif tool provides a very reliable forecasting technique in relatively quiet geomagnetic conditions. although these conditions prevail in the normal earth’s ionosphere, the conditions during the geomagnetic storms are of high importance for current and future radio communications services as well as space weather operational requirements and scientific studies, and under such conditions stif has been proved inadequate. stif weakness concerns the forecast of the ionospheric storms onset and their development during the first 24 h. to improve its prediction accuracy we have to include recent scientific and technological advances. recent studies on ionospheric dynamics during geomagnetic storm events in the light of the global solar wind-magnetosphere-ionosphere interaction suggest that the ionospheric response depends on the solar wind conditions leading to geomagnetic storms and on the intensity of the storm. for example, belehaki and tsagouri (2002), in an investigation of the nighttime ionospheric response at middle latitudes, related the recording of positive storm effects from ground ionosondes at night with the type of the storm. geomagnetic storms with initial phases, followed by short in duration but very fast evolving main phases, produce nighttime ionization depletion as a global effect at middle latitudes, independent of the storm intensity. during gradual driven storms, positive storm effects are frequently observed at middle to low latitudes. given that geomagnetic storms are gradually evolving, the observation of positive effects at night depends also on the storm intensity and the latitude of the observation point. the critical point in the explanation of the two different types of ionospheric response to different types of storms is the expansion of the neutral composition zone (belehaki and tsagouri, 2002). the sudden orientation of the imf to a very large southward field severely disturbs the magnetosphere-ionosphere system and changes the effective input energy to the auroral ionosphere, causing the very fast expansion of the deep negative phase equatorward. in contrast, during gradually evolving storms the neutral composition disturbance zone remains restricted to high latitudes, and the middle to low latitudes ionosphere is affected by other competing mechanisms. in the light of these findings, one could argue that the forecast of the type of the storm and of storm intensity could lead to a prediction of the ionospheric response at middle latitudes. the crucial feature that determines the storm type is the arrival of a shock wave in the earth vicinity, which can be identified by the rate of change of bz-imf (dbz/dt), which is also indicative of the rate of 469 a dynamic system to forecast ionospheric storm disturbances based on solar wind conditions fig. 1. comparison between measured and stif forecast fof2 (blue and red line, respectively) and muf(3000)f2 (dark magenta and cyan line, respectively) values at st. petersburg ionosonde station with ap values (black line) as an indicator of the low geomagnetic activity. fig. 2. comparison between measured and stif forecast fof2 (blue and red line, respectively) and muf(3000)f2 (dark magenta and cyan line, respectively) values at st. petersburg ionosonde station with ap values (black lines) as an indicator of the high geomagnetic activity. 470 ioanna tsagouri, anna belehaki and ljiljana r. cander the solar wind energy entering the magnetosphere-ionosphere system. the nasa advanced composition explorer (ace) spacecraft at the vantage l1 point performs measurements over a wide range of energies and nuclear masses, under all solar wind flow conditions, and during both large and small particle events including solar flares. the data are fundamental to enabling the formulation of highly accurate forecasting techniques and the subsequent issue of alerts and warnings of incoming major geomagnetic disturbances and their possible effects on the structures in ionospheric layers variations (kappenman, 1998). the time delay for the solar wind to travel from the l1 point where ace is to the earth is about one hour, giving forecaster the advanced knowledge of the storm type one hour before its consequences are detectable in the earth’s environment. therefore, the data from ace could be a useful tool in real time ionospheric space weather forecasting. this contribution is an investigation of the ionospheric response to interplanetary magnetic field disturbances with the primary goal of developing the understanding and the means to forecast how the ionospheric f2-layer will respond to abruptly and dramatically changing solar and geomagnetic conditions. it is an attempt to use the real-time solar wind (rtsw) data from the ace spacecraft mission to derive criteria for issuing alerts for forthcoming major geomagnetic storms, as one of the dominant space weather events, and their effects on the ionospheric critical frequency fof2 in the first 24 h. 2. the effect of solar wind conditions in ionospheric storm development one of the most popular and complete descriptions of the ionospheric storm-induced effects is the prölss phenomenological model (prölss, 1993), which was extended by fullerrowell et al. (1994, 1996). this model captures most of the basic aspects of ionospheric storms, such as the long-lived negative storm effects and daytime positive effects. negative storm effects are attributed to the formation and evolution of the neutral composition bulge. the magnitude and the distribution of the negative ionospheric phase depend on the magnitude and the position of the neutral composition bulge. since the expansion of the neutral composition zone has been related to the solar wind conditions under which the global solar wind-magnetosphere-ionosphere coupling occurs (belehaki and tsagouri, 2002), it is reasonable to investigate, as a next step, the temporal correlation between the occurrence of imf disturbances and the recording of negative storm phases at middle latitudes. for this purpose we analyse hourly observations of the fof2 critical frequency parameter from chilton station (http://www.wdc.rl.ac.uk) in conjunction with hourly observations of interplanetary magnetic field parameters (total magnitude and bz-imf component) from the ace table i. list of the storms used in this study. start and main days dst (nt) chosen onset sector onset of imf quiet days (ut) disturbances (ut) 21-22 october 1999 – 237 07 october 1999 midnight 23:00 06-07 april 2000 – 288 22 april 2000 noon 14:00 15-16 july 2000 – 301 08 july 2000 noon 13:00 11-12 august 2000 – 235 19 august 2000 midnight 22:00 30-31 march 2001 – 358 11 march 2000 midnight 23:00 11-12 april 2001 – 256 26 april 2001 noon 12:00 05-06 november 2001 – 277 12 november 2001 midnight 23:00 07-08 september 2002 – 170 01 september 2001 noon 12:00 471 a dynamic system to forecast ionospheric storm disturbances based on solar wind conditions spacecraft mission (http://cdaweb.gsfc.nasa.gov/ cdaweb/istp_public) and hourly records of the dst-index (http://swdcdb.kugi.kyoto-u.ac.jp/wdc) for eight intense storm events. table i presents a list of the storm events used in this study, together with the minimum dst values. the ionospheric response was considered in terms of the ratio fof2observed / fof2quiet, where fof2quiet represents the diurnal quiet time behaviour for chilton. the latter was formed by the records of the chosen quiet day for each month. the selection of the quietest day of the month was based on the inspection of the dst values and it is also given in table i. in an attempt to classify the storm events, we note the onset sector for each event in the fourth column of table i, referring to the sector where chilton was when the onset of the imf disturbances occurred. the onset of the imf disturbances was determined as the time of sudden increase in the solar wind magnetic field bt, which denotes an abrupt increase in the solar wind energy that enters the earth’s magnetosfig. 3. the results of the superposed epoch analysis on bt, bz, fof2/fq and dst for the four storm events with near midnight onset. the vertical bars represent the standard deviation computed from the whole stack of superposed traces. 472 ioanna tsagouri, anna belehaki and ljiljana r. cander phere. it is obvious that the eight storm events may be separated into two groups: the first includes the 4 storms with a near midnight onset, while the second includes the 4 storms with onset near noon. then, a superposed epoch analysis was applied on the bt, bz, fof2 / fq, and dst for both groups in order to obtain an overall perspective of the observations time sequence. the results are given in figs. 3 and 4, respectively. figure 3 shows the results of the superposed epoch analysis on bt, bz, fof2/fq and dst for the four storm events with near midnight onset. a rapid increase in bt was followed by southward turning of the bz-imf component, indicating that the solar wind energy entering in the magnetosphere-ionosphere system increases rapidly. the bz-imf southward turning triggered the onset of the geomagnetic storm, as this was recorded by the dst-index with no significant time delay. at the same time the middle latitude ionosphere responded almost immediately to drastic changes in imf parameters with significant negative storm effects and a deep negative phase recorded in chilton. fig. 4. the results of the superposed epoch analysis on bt, bz, fof2/fq and dst for the four storm events with onset near noon. the vertical bars represent the standard deviation computed from the whole stack of superposed traces. 473 a dynamic system to forecast ionospheric storm disturbances based on solar wind conditions figure 4 shows the results of the superposed epoch analysis on bt, bz, fof2/fq and dst for the four storm events with onset near noon. the rapid increase in bt was recorded simultaneously with the southward turning of bz-imf component that triggered the onset of the geomagnetic storm and consequently the observed decrease of the dst-index with a time delay of 2 h. significant negative storm effects in the middle latitude ionosphere were recorded 6 h later and when the chilton station had been rotated in the evening sector. this latter observation is also consistent with prölss’s phenomenological scenario. two interesting features are extracted from the above preliminary analysis: in the first case when the imf disturbances occurred near midnight, the evolution of dst-index and the ionospheric response in chilton occurred simultaneously. this means that the dst-index cannot be used as a prediction tool for ionospheric forecasting. on the other hand, the time delay between the imf disturbances and the ionospheric response seems to depend on the onset time sector. but even near midnight when the ionospheric response is expected to be direct we can count on the quite safe time delay of one hour required for the solar wind disturbance, once it is detected by ace, to reach the earth. of course, statistical studies based on a large sample of events are needed to establish these results but in general, they reveal a unique opportunity to predict the f2-layer response to geomagnetic storm by using the ace data in near real time mode. to work further on the idea of developing a real time dynamic system to forecast ionospheric storm effects in middle latitudes in operational form, we calculated the bz rate of change, dbzmod (dbzmod = 100*dbz/dt) over the preceding 20 min time interval and the percentage deviation of hourly fof2 from median values, dfof2 (%), for chilton for all the events listed in table i. here we present an example of the impulse type of geomagnetic storm on 30 march 2001 with initial compressive phase and very fast evolving main phase, triggered by an interplanetary shock wave in fig. 5. figure 5 shows the dfof2 (%) immediate response to the sudden and significant changes of fig. 5. bz rate of change in 20 min interval with corresponding fof2 response for march 30, 2001 storm as seen at chilton ionosonde station (51.5°n, – 1.3°e). 474 ioanna tsagouri, anna belehaki and ljiljana r. cander the bz expressed by dbzmod. minutes after dbzmod changes from + 70 to − 100, the dfof2 (%) revealed a huge negative response of more than 50% followed by a decrease of up to − 70% during the main phase of the storm. the negative storm effect over chilton was dominant for more than 36 h. this example illustrates the knowledge required to the development of a successful realtime dynamic ionospheric storm forecasting algorithm. in that context a time delay between measurements can have a significant role and will be the subject of further studies. 3. discussion the ionospheric critical frequency of the f2layer is highly variable on time-scales ranging from decades to seconds with the occurrence of ionospheric disturbances associated with geomagnetic storms. many attempts have been made recently in europe to study the space environmental disturbances frequently referred to as space weather (cander, 2003). in these studies the focus is on forecasting the geomagnetic storm effect on the main ionospheric parameters to enable extreme conditions to be quantified so that particularly for telecommunications planning likely variability bounds can be defined. in general, forecasts are made mostly on the basis of persistence and recurrence that are not always strong. the current existing 24 h forecasting service (http://ionosphere.rcru.rl.ac.uk) relies heavily on an extrapolation of past and prevailing ionospheric conditions. in practice this requires an automation of the data collection and processing and on-line forecasting message distribution. there has been ample evidence from different national as well as international space weather research projects that true forecasts of ionospheric disturbances are needed with lead times of up to 24 h of the present. very recently sufficient real-time data on interplanetary conditions (e.g., solar-wind parameters) have been obtained to do this effectively (kamide, 2000). however, f-region storm morphology has a complex spatial and temporal structure that requires continuous monitoring with high time resolution. in this paper we examined the solar-terrestrial conditions and ionospheric f2-layer responses associated with eight geomagnetic storms to illustrate what knowledge needs to be included in a successful real-time dynamic ionospheric storm forecasting algorithm. such forecasting requires automatic daily retrieval of rtsw data from the ace spacecraft mission at <http://sec.noaa.gov/data> and the ionospheric characteristics from ionospheric stations operating in real-time in europe from wdc-c1 at ral (http://www.wdc.rl.ac.uk/ cgi-bin/digisondes/ cost_database.pl). currently an automated real-time dynamic system for monitoring ionospheric propagation conditions over europe is operational at ral (http://ionosphere.rcru.rl.ac.uk) in collaboration with national observatory of athens (cander et al., 2004). acknowledgements this work is part of the european action cost271 and it was realised during a cost271 short-term mission of one of the authors i.t. in rutherford appleton laboratory in the u.k. references belehaki, a. and i. tsagouri (2002): on the occurrence of storm induced nighttime ionisation enhancements at ionospheric middle latitudes, j. geophys. res., 107 (8), 23,1-23,19. cander, lj.r. (2003): toward forecasting and mapping ionospheric space weather under the cost actions, adv. space res., 31 (4), 957-964. cander, lj.r, r. bamford and j.g. hickford (2003): nowcasting and forecasting the fof2, muf(3000)f2 and tec based on empirical models and real-time data, iee conf. proc., 491 (1), 139-142. cander, lj.r, j.g. hickford, i. tsagouri and a. belehaki (2004): real-time dynamic system for monitoring ionospheric propagation conditions over europe, electron. lett., 40 (4), 224-226. fuller-rowell, t.j., m.v. codrescu, r.j. moffett and s. quegan (1994): response of the thermosphere and ionosphere to geomagnetic storms, j. geophys. res., 99, 3893-3914. fuller-rowell, t.j., m.v. codrescu, h. rishbeth, r.j. moffett and s. quegan (1996): on the seasonal response of the thermosphere and ionosphere to geomagnetic storms, j. geophys. res., 101, 2343-2353. hanbaba, r. (1999): improved quality of service in ionospheric telecommunication systems planning and operation, in cost action 251 final report, space research centre, warsaw, 102-103. hochegger, g., b. nava, s.m. radicella and r. leitinger 475 a dynamic system to forecast ionospheric storm disturbances based on solar wind conditions (2000): a family of ionospheric models for different uses, phys. chem. earth (c), 25, 307-310. kamide, y. (2000): from discovery to prediction of magnetospheric processes, j. atmos. sol.-terr. phys., 62, 16591668. kappenman, j.g. (1998): geomagnetic storm forecasting mitigates power system impacts, ieee power eng. rev., 18 (11) (available on line at: http://www.spectrum.ieee.org/pubs/mags/9902/kapp.html). kutiev, i., p. muhtarov, lj.r. cander and m. levy (1999): short-term prediction of ionospheric parameters based on autocorrelation analysis, ann. geofis., 42 (1), 121-127. prölss, g.w. (1993): on explaining the local time variation of ionospheric storm effects, ann. geophysicae, 11, 1-9. samardjiev, t., p.a. bradley, lj.r. cander and m.i. dick (1993): ionospheric mapping by computer contouring techniques, electron. lett., 29 (20), 1974-1975. n u m e r i caratteristici d e i flocculi d ' i d r o g e n o e di c a l c i o e d e i filamenti d ' i d r o g e n o p e r l ' a n n o 1 9 6 0 g . g o d o l i r i c e v u t o il 4 agosto 1961 proseguendo la pubblicazione annuale dei numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno (1), vengono riportati nelle seguenti tre tabelle di immediata interpretazione i valori giornalieri per il 1960. i numeri caratteristici sono stati stimati sugli spettroeliogrammi eseguiti alla torre solare di arcetri con l'immagine di 63 mm; le caratteristiche strumentali e la tecnica delle osservazioni sono state descritte in altra sede (2). durante tutto il 1960 sono stati eseguiti quotidianamente tre spettroeliogrammi successivi nella riga ha3 dell'idrogeno e tre spettroeliogrammi successivi nella riga k2i3i2 del calcio ionizzato. l'esecuzione di spettroeliogrammi successivi ha lo scopo di mettere in evidenza le variazioni di intensità delle zone più attive. tali variazioni di intensità, interessantissime nello studio dell'evoluzione dei fenomeni, non hanno però alcuna influenza nella stima dei numeri caratteristici. per questo abbiamo continuato a stimare i numeri caratteristici su di un solo spettroeliogramma in hog ed uno solo in k2j3j2. gli spettroeliogrammi in ha3 esaminati erano generalmente i primi eseguiti. solo in qualche raro caso, quando il primo risultava difettoso, veniva esaminato il secondo od il terzo spettroeliogramma. gli spettroeliogrammi in k 2 ) 3 2 esaminati erano quelli ridotti in sede di programma cgi. come è noto l'osservatorio astrofisico di arcetri è stato, durante l'anno geofisico internazionale 1957-1958 e la collaborazione geofisica internazionale 1959, centro mondiale c principale (p wdc c) per la 3 2 0 g . g o d o l i raccolta e l'elaborazione dei dati relativi ai flocculi di calcio (3). dal 1960 l'osservatorio astrofisico di arcetri è centro mondiale c nell'ambito del comitato internazionale di geofisica per la raccolta e l'elaborazione dei dati relativi ai flocculi di calcio. per questo, dal 1° gennaio 1957, l'attività dei flocculi di calcio viene seguita rilevando area e posizione dei singoli fenomeni. sebbene questo metodo di determinare l'attività dei flocculi sia più informativo della stima del numero caratteristico e sebbene il numero caratteristico sia in media una funzione nota dell'area (4), tuttavia abbiamo continuato a stimare i numeri caratteristici dei flocculi di calcio: sembra infatti che in fasi peculiari del ciclo solare, i due diversi metodi diano informazioni di significato diverso (5). dall'ottobre 1958 è stato iniziato ad arcetri un programma di osservazione fotografica al monocromatore halle in h a che già era in funzione per l'osservazione visuale dall'agosto 1957 (6). vengono giornalmente eseguiti diversi fotogrammi formato 24 x 36 mm ad intervalli di 10-15 minuti. se vengono osservati visualmente fenomeni a rapida evoluzione quali brillamenti o protuberanze eruttive, la frequenza dei fotogrammi viene aumentata adeguandola alla variabilità del fenomeno. l'osservazione fotografica al monocromatore è molto più rapida che allo spettroeliografo cosicché in molti giorni misti in cui non è possibile eseguire spettroeliogrammi alla torre solare, è invece possibile effettuare filtrogrammi al monocromatore. un filtrogramma per ognuno di tali giorni è stato ingrandito a 63 mm di diametro ed esaminato per la determinazione dei numeri caratteristici dei flocculi e filamenti in iia3 i numeri caratteristici così dedotti sono individuati nelle tabelle la ed l e con un asterisco. sono stati esaminati complessivamente 151 spettroeliogrammi e 84 filtrogrammi in ha3 e 129 spettroeliogrammi in k2i3)2. la stima veniva eseguita, come per il passato, attenendosi ai modelli distribuiti dall'osservatorio di mt. wilson. per evidenti ragioni di omogeneità e costanza di giudizio, è stata determinata prima t u t t a la serie dei numeri caratteristici per i flocculi d'idrogeno; quindi t u t t a la serie per i flocculi di calcio ed infine t u t t a la serie per i filamenti d'idrogeno. le stime sono state eseguite per due volte seguendo un diverso ordine di esame. lo scarto delle singole determinazioni si è sempre mantenuto inferiore a 0.2 unità. nei pochi casi in cui veniva riscontrato uno scarto di maggiore entità, un riesame degli spettroeliogrammi metteva in evidenza un errore di stima. t a b e l l a l a n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d ' i d r o g e n o p e r l ' a n n o 1 9 6 0 giorno g e n n a i o f e b b r a i o m a r z o aprile maggio giugno luglio a g o s t o s e t t e m b . o t t o b r e n o v e m b . d i c e m b . 1 2 . 0 1 . 8 1 . 4 * 1 . 8 1 . 7 1 . 1 1 . 5 _ 1 . 3 2 — 2 . 0 * 1 . 6 * 2 . 0 1 . 7 — 1 . 3 1 . 0 1 . 6 — 1 . 3 * 1 . 3 3 — 2 . 1 1 . 7 1 . 9 * 1 . 7 1 . 7 — 0 . 9 1 . 4 0 . 6 1 . 5 1 . 3 * 4 1 . 3 — 1 . 6 * 2 . 0 * — — 1 . 5 0 . 8 1 . 3 * — — 1.8* 5 1 . 3 — 1 . 5 1 . 8 — — 1 . 4 0 . 4 — 0 . 5 — 1 . 8 6 1 . 3 * 2 . 0 1 .4* 1 . 9 1 . 6 1 . 8 1 . 3 0 . 4 1 . 5 * — 1.3* 7 1 . 4 1 . 9 * 1 . 2 1 . 7 1 . 6 * 1 . 7 — . 0 . 4 * 1 . 6 — — — 8 1 . 5 * 1 .9 1 . 3 1 . 8 1 .7* 1 . 6 — 0 . 5 — — — 9 — 1.9* 1 . 6 * 1 . 6 1 . 7 1 . 7 — 0 . 6 * 1.8 — 1 . 7 * 1 . 7 * 10 1 . 3 * — — 1 . 7 * 1 . 6 1 . 6 — 0 . 7 * 1 .9 — 1 .2 11 — — 1 . 5 * 1 . 6 1 . 6 1 . 7 1 . 2 1 . 1 2 . 0 * — — 12 — 1 . 6 1 . 4 1 . 6 * 1 . 5 1 . 8 * — 1 . 7 2 . 1 — .— 13 — 1 . 8 * 1 . 5 * 1 . 6 1 . 4 1 . 7 1 . 3 — 2 . 1 1 . 0 — — 14 1 . 2 * — — 1 . 7 1 . 3 1 . 8 1 . 4 2 . 0 * 1 . 8 — 1 .6 1 . 6 15 — 1 . 9 * — • 1 . 8 1 . 3 * 1 . 8 * 1 . 5 2 . 2 * 1 .9 — — 1 .4* 16 1 . 2 * 1 . 5 1 . 3 — 1 . 2 * 1 . 6 * 1 . 5 2 . 4 * 1 .9* — — — 17 — — — — 1 . 2 1 . 4 — 2 . 6 2 . 0 * 1 . 2 — 18 1 . 8 — 1 . 4 — — 1 . 5 1 . 3 2 . 8 — . 1 . 2 19 — — — — 1 . 2 * 1 .4* 1 . 3 2 . 5 — 0 . 9 — 20 — — — 1 . 8 * 1 . 2 1 . 4 1 . 3 2 . 6 1 . 6 * — 1 . 6 * 1 . 3 * 21 — 1 . 0 * 1 . 4 1 . 8 — 1.4 1 .2 2 . 5 * 1 . 8 * 1 . 3 22 1 . 4 * — 1 . 9 1 . 8 — • 1 . 3 1 . 2 2 . 4 — 1 . 2 * — 23 — • 1 . 6 * 1 . 8 1 . 8 1 . 3 1 . 2 — 2 . 3 2 . 1 1 . 4 * — 0 . 9 * 24 — 1 .7* 1 . 6 — 1 . 3 1 . 2 * 1 . 3 * 2 . 1 2 . 1 1 . 3 1 . 1 * 25 — — 1 . 5* 1 . 6 * 1 .2 1 . 2 * 1 . 1 2 . 4 — — 1.3 26 1 . 5 * 1 .7 — 1 . 4 1 .3* — 1 . 2 2 . 4 2 . 0 1 . 1 * 27 — 1 .7* 1 . 6 * — 1.3 1 . 3 — 2 . 4 — — — 28 — . 1 . 5 * — 1 . 4 * 1 . 3 1 . 4 1 . 0 1 . 8 * — — . 29 — — . — 1 . 5 1.4* 1 .6* 1 .0 1 . 2 0 . 7 * 1 . 2 0 . 9 30 2 . 0 — 1 . 7 — 1 .4* 1 . 8 * 1 . 1 1 . 4 — 1.3* 1 . 4 1 .4* 31 2 . 1 * — 1 . 8 — 1 . 8 — 1 . 2 * 1 . 5 — — 1 . 2 media 1 . 5 1 . 8 1 . 5 1 . 7 1 . 4 1 . 6 1 . 3 1 . 6 1 . 7 1 . 1 1.4 1 . 3 media 1. 58 | 47 s e m e s t r a l e 1. 58 47 media 2 53 a n n u a l e t a b e l l a l b n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d i calcio p e r l ' a n n o 1 9 6 0 giorno g e n n a i o f e b b r a i o m a r z o aprile maggio giugno luglio agosto s e t t e m b . o t t o b r e n o v e i n b . d i c e m b . 1 _ 2 . 2 2 . 6 _ 2 . 3 2 . 9 2 . 3 1 . 7 2 . 0 2 — — — 2 . 6 2 . 2 — 2 . 8 2 . 2 — — — 2 . 1 3 — 2 . 2 2 . 0 — 2 . 1 2 . 3 — 2 . 2 1 . 7 1 . 8 2 . 0 — 4 2 . 0 — — 2 . 9 2 . 2 — — — — 5 2 . 1 — 1 . 9 2 . 5 — — — 2 . 0 — — — — 6 — 2 . 1 — 2 . 4 — 2 . 3 — — — — — — 7 1 . 9 — 1 . 8 2 . 4 — 2 . 3 — — 1 . 9 — — — 8 — 2 . 1 1 . 7 2 . 4 — 2 . 2 — 1 . 9 — — — — 9 — — — 2 . 3 2 . 3 2 . 3 — — 2 . 5 — — — 10 — — — — 2 . 3 2 . 1 — — 3 . 1 — — — 11 — — — 2 . 4 2 . 1 2 . 3 2 . 6 2 . 7 — — — — 12 2 . 1 — — 2 . 2 — — 3 . 3 3 . 1 — — — 13 — — — 2 . 5 2 . 2 2 . 2 2 . 3 — 3 . 1 2 . 2 — • — 14 — — — 2 . 6 2 . 2 2 . 2 2 . 2 — — — — 2 . 2 15 16 17 — — 1 . 8 2 . 7 — — 2 . 0 — — — — — 15 16 17 z z 1 . 8 2 . 6 2 . 2 4 . 1 2 . 0 18 1 . 8 — 1 . 7 — — 2 . 2 2 . 6 4 . 0 — 2 . 0 — — 19 — — — — — 2 . 6 3 . 8 — 2 . 1 — — 20 — — — — . 2 . 4 2 . 1 3 . 0 3 . 8 — — — — 21 — — 1 . 9 2 . 3 — 2 . 0 3 . 1 — — — — — 22 — — 1 . 9 2 . 2 — 2 . 0 2 . 9 3 . 3 — — — — 23 — — 2 . 4 2 . 2 2 . 2 2 . 0 — 3 . 2 — — — — 24 25 26 — — 2 . 3 — 2 . 2 — 3 . 0 2 . 8 2 . 7 2 . 8 — — — 24 25 26 2 . 1 2 . 8 3 . 0 2 . 8 2 . 7 2 . 6 27 — — — . — 2 . 1 2 . 1 — 2 . 7 — — — — 28 — — — — 2 . 3 2 . 2 2 . 9 — — — — — 29 — — — 2 . 0 — — 2 . 7 2 . 2 — — — — 30 2 . 3 — 2 . 4 — _ — 2 . 6 2 . 0 — — 2 . 0 — 31 — — 2 . 7 — 2 . 2 — — 1 . 8 — — — 1 . 7 media 2 . 0 2 . 1 2 . 0 2 . 4 2 . 2 2 . 2 2 . 7 2 . 8 2 . 5 2 . 0 2 . 0 2 . 0 media s e m e s t r a l e 2 . 20 2 . 56 media a n n u a l e 2 . 36 t a b e l l a l e n u m e r i c a r a t t e r i s t i c i d e i f i l a m e n t i d ' i d r o g e n o p e r l ' a n n o 1 9 6 0 giorno g e n n a i o f e b b r a i o m a r z o aprile maggio g i u g n o luglio a g o s t o s e t t e m b . o t t o b r e n o v e m b . d i c e m b . 1 1 . 5 1 . 9 1 . 0 * 1 . 8 1 . 8 1 . 5 2 . 4 _ _ 2 . 4 2 — 1 . 6 * 1 . 6 * 2 . 1 1 . 4 — 1 . 2 1 . 2 2 . 7 — 1 . 5 * 2 . 3 3 — 1 . 9 1 . 7 1 . 8 * 1 . 6 1 . 6 — 1 . 2 2 . 7 2 . 2 2 . 0 2 . 4 * 4 2 . 3 — 1 . 6 * 1 . 9 * — — 2 . 2 1 . 5 2 . 7 * — — 2 . 6 * 5 2 . 2 — 1 . 5 1 . 9 — — 2 . 1 1 . 9 __ 1 . 6 — 2 . 8 6 2 . 3 * 1 . 5 1 . 4 * 2 . 1 1 . 7 1 . 8 2 . 2 2 . 3 2 . 9 * — 1 . 7 * 7 2 . 4 1 . 3 * 1 . 2 2 . 2 1 . 6 * 1 . 7 — 2 . 1 * 3 . 2 — — 8 2 . 5 * 1 . 0 1 . 3 2 . 3 1 . 6 * 1 . 9 — 2 . 0 .— — — — 9 — 1 . 2 * 1 . 4 * 2 . 1 1 .7 2 . 2 — 1 . 0 * 3 . 1 — 2 . 0 * 1 . 9 * 10 2 . 4 * — — 1 . 8 * 1 . 5 2 . 4 — 1 . 0 * 2 . 6 — 1 . 6 11 — — 1 . 5 * 1 . 9 1 . 3 2 . 5 2 . 5 1 . 2 2 . 6 * — 12 — 1 . 3 1 . 6 1 . 6 * 1 . 4 2 . 4 * — 1 . 4 2 . 6 — — 13 — 1 . 4 * 1 . 5 * 1 . 8 1 . 5 2 . 4 2 . 7 — 2 . 4 2 . 6 — — 14 2 . 3 * — — 1 . 9 1 . 4 2 . 4 2 . 8 1 . 4 * 2 . 4 — • 1 . 8 3 . 0 15 — 1 . 6 * — 1 . 7 1 . 5 * 2 . 0 * 2 . 5 1 . 4 * 2 . 3 — — 2 . 5 * 16 2 . 0 * 1 . 8 2 . 2 — 1 . 5 * 1 . 8 * 3 . 2 1 . 5 * 2 . 0 * — — — 17 — — — — 1 . 5 1 . 8 — 1 . 8 2 . 1 * 1 . 9 — — 18 2 . 0 — 3 . 4 — — 1 . 9 2 . 2 1 . 7 — 1 . 8 — — 19 — — — — 2 . 2 * 1 . 6 * 2 . 2 2 . 2 — 1 . 8 — — 20 — — — 1 . 8 * 2 . 4 1 . 5 2 . 0 2 . 1 1.9* — 1 . 7 * 1 . 6 * 21 — 1 . 4 * 3 . 2 2 . 0 — 1 . 8 1 . 9 1 . 9 * 1 . 8 * 1 . 8 — — 22 2 . 1 * — 3 . 1 1 . 9 — 1 . 7 1 . 9 2 . 0 — 1 . 4 * — — 23 — 1 . 4 * 2 . 4 1 . 8 2 . 3 1 . 5 — 2 . 0 1 . 9 1 . 4 * — 1 . 4 * 24 — 1 . 5 * 2 . 0 — 2 . 5 1 . 3 * 1 . 7 * 1 . 8 2 . 0 — 2 . 4 1 . 6 * 25 — — 1 . 8 * 1 . 6 * 2 . 4 1 . 2 * 1 . 3 2 . 2 — — 2 . 4 — 26 2 . 1 * 1 . 8 — 1 . 5 2 . 3 * — 1 . 3 2 . 3 2 . 1 — — 1 . 5 * 27 — 1 . 9 * 1 . 9 * — 2 . 2 1 . 0 — 2 . 3 . — — — . 28 — 1 . 9 * — 1 . 0 * 2 . 2 1 . 0 1 . 3 1 . 8 * — — — — 29 — — — 1 . 0 1 . 8 * 1 . 0 * 1 . 4 1 . 7 2 . 1 * — 2 . 2 1 . 2 30 2 . 0 — 2 . 0 — 1 . 6 * 1 . 0 * 1 . 2 1 . 8 — 1 . 8 * 2 . 6 1 . 7 * 31 1 . 5 * — 2 . 2 — 1 . 7 — 1 . 3 * 2 . 2 — — — 1 . 7 media 2 . 2 1 . 5 1 . 9 1 . 8 1 . 8 1 . 7 2 . 0 1 . 7 2 . 4 1 . 8 2 . 0 2 . 0 media s e m e s t r a l e 1. 80 1 . 9 9 media a n n u a l e l.é 3 2 4 g . g o d o l i 71 contributo dei singoli osservatori alla raccolta delle lastre ridotte è stato il seguente: o s s e r v a t o r i h « ^2,3,2 t o t a l e m. c. ballario 105 91 196 b. r o m a g n o l i 46 38 84 il contributo dei singoli osservatori alla raccolta dei fotogrammi ridotti è stato invece il seguente: o s s e r v a t o r i g. aiazzi . 1 f . d r a g o . . . . . . . . 3 g. godoli . 5 a. l e p r i . 57 m. meco . 6 g. noci . 7 m. r i g u t t i la distribuzione mensile degli spettroeliogrammi e flltrogrammi ridotti è la seguente: mese a ^ ^ ^ 1 2 3 4 5 6 7 8 9 10 11 12 s . e . g . 5 7 13 16 17 18 20 22 13 1 7 6 h a filtr. 8 10 8 7 9 8 2 8 8 3 4 9 k2,3,2 5 5 12 16 15 18 17 21 9 5 2 4 n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d ' i d r o g e n o , e c c . 3 2 5 riassunto si dànno i numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per l'anno i960, secondo le osservazioni eseguite alla torre solare ed al monocromatore halle dell'osservatorio di arcetri. su m mary the character figures of ca briglit flocculi and ha brighi and darle flocculi for the 1960 are given according to the observations carried out at the solar tower and ha fllter of the arcetri observatory. b i b l i o g r a f i a (*) godoli, g., numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per gli anni 1950-51-52, «ann. geof. », 6, 199, (]953). i d e m per l ' a n n o 1953, 1. e. 7, 135, (1954). i d e m p e r l ' a n n o 1954, 1. o. 8, 325, (1955). i d e m p e r l ' a n n o 1955, 1. e. 9, 299, (1956) i d e m p e r l ' a n n o 1956, 1. e. 10, 133, (1957). i d e m per l ' a n n o 1957, 1. e. l i , 235, (1957). i d e m p e r l ' a n n o 1958, 1. o. 12, 197, (1959). i d e m p e r l ' a n n o 1959, 1. e. 13, 231, (1960). (2) righini, g. godoli, g., riduzione del materiale spettroeliografico raccolto alla torre solare di arcetri nel periodo 1932-1949 « meni. s a i t », 21, 333, (1949). (3) godoli, g., relazione preliminare sull'attività del « principal world data center agi» ad arcetri, « l a ricerca scientifica», 29. 1366, (1959). (4) righini, g. godoli, g., sui numeri caratteristici dell'attività solare, « a n n . geof. » 3, 501, (1950). — — the physicai meaning of the character figures of solar plienomena, « j . of geopliys. res. », 55, 415, (1950). (5) godoli, g., attività dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per il periodo 1° luglio 1957-30 giugno 1958, « boll, di geod. e scienze affini», 17, 487, (1958). (6) rigutti, m., il filtro interferenziale dell'osservatorio astrofisico di arcetri, « l a ricerca scientifica», 29, 1159, (1959). vol52,6,2009new 605 annals of geophysics, vol. 52, n. 6, december 2009 key words geology – mineralogy – volcanology – correspondence – uniformitarianism vienne, le 7.3.90 «mon cher maître! acceptez, je vous prie, mes renseignements sincères pour le nouvel envoi de mémoires que je viens de recevoir et qui m’apprend, que votre activité est encore tout à fait celle de la jeunesse, dirigée toutefois par l’expérience et la sagacité que nous admirons tout à vous, mon cher maître». e. suess (fig. 1). it was with these words that, in march 1890, the famous austrian geologist edward suess (1) addressed arcangelo scacchi (1810-1893), who was then in his eighties and had been teaching mineralogy at the university of naples for over fifty years, during which time he had also been dean (2). amongst his other roles, he was also the curator of the royal mineralogical museum, which, under his guidance enjoyed a period of unparalleled success. scacchi, the «brilliant crystallographer», was «the first italian mineralogist to win international recognition for his work», (3) and to achieve the honour of having a mineral – scacchite – named after him. scacchi began by studying vesuvius and the geology of the campania region but, from the beginning of the 1830s, broadened his research into the fields of palaeontology, geology, chemistry and mineralogy. although today his name is seldom mentioned in histories of italian science in the nineteenth century (4), even a brief look at his life and career illustrates how eminent a figure he was in the scientific community of his day. scacchi’s vast correspondence reveals how he occupied a position right at the centre of a network of contacts made up of some of the most important italian and international scientists of the age. the south of italy was full of places of interest – naples and vesuvius, sicily and etna – for naturalists, who flocked there on field trips. scacchi was a point of reference for the visiting scientists and he frequently accompanied them on their expeditions. an extraordinary rapport seems to have developed between northern scithe scientific correspondence of arcangelo scacchi rossella de ceglie seminario di storia della scienza, università degli studi di bari, italy abstract the scientist arcangelo scacchi (1810-1893), is today rarely mentioned in histories of italian science in the nineteenth century. even a brief consideration of his career, however, reveals that his work was of great importance to the scientific community of his age. for more than fifty years he was professor of mineralogy at the university of naples and curator of the royal mineralogical museum, which under his guidance enjoyed a period of unprecedented success. the as yet unpublished scacchi papers shed interesting light on the world of this italian naturalist. his correspondence reveals much about scacchi’s role in the scientific community both in italy and abroad, and illustrate the extent to which he was involved in contemporary debates and research in the fields of geology, mineralogy, volcanology and crystallography. mailing address: dr. rossella de ceglie, seminario di storia della scienza, università degli studi di bari, palazzo ateneo, p.zza umberto i, bari, italy; e-mail: deceglie@filosofia.uniba.it vol52,6,2009new 2-12-2009 12:39 pagina 605 606 r. de ceglie entists, who came down to naples and the surrounding countryside on geological and volcanological field trips, and leading local scholars: besides scacchi, other noteworthy academics included oronzio gabriele costa and the gemmellaro brothers (5). the area became a kind of «promised land» where geologists from all over europe could meet established colleagues. scacchi’s letters, most of which are in the keeping of the pomarici santomasi foundation in gravina in puglia, his birth place (6), provide important clues on the environment in which the naturalist grew up and worked. they also illustrate the key role scacchi played in the scientific community both in italy and abroad in the nineteenth century, and in research in the fields of geology, mineralogy, volcanology and crystallography. the many letters written to scacchi by various italian and foreign scholars cover a long period of time, from approximately 1830 until the death of the naturalist in 1893; they reveal how scacchi was at the centre of an international network of scholars and underline how highly he was regarded by the leading academics of the day, who saw him as a valuable point of reference for their own research. the letters also reflect the diversity of scacchi’s interests which ranged freely across disciplines as varied as zoology, palaeontology, chemistry, geology, volcanology, mineralogy and crystallography. the letters dating from the 1830s confirm that initially scacchi’s main focus was on zoology, which would subsequently become of secondary interest to him, as he devoted an increasing amount of time and energy to geology and mineralogy. it is also evident from the correspondence in this early period of his career that he was already well-known both in italy and abroad (7). foreign scholars at leading research centres were interested in scacchi’s work and followed his career closely, asking him for materials and specimens as well as for his scientific opinions (8). a case in point is that of the french ichthyologist valenciennes (9) from the muséum d’histoire naturelle in paris, who was interested in scacchi’s malacological studies and in the clavagella specimens sent to scacchi by brongniart (10). in belgium too, scacchi’s work on shells was well-known and his fame as a mineralogist was beginning to spread; as early as 1839, andrè hubert dumont (11), a geologist at lieges university, expressed interest in exchanging mineral specimens with his italian colleague (12). scacchi’s fame – as well as his collections – grew over the following ten years as he expanded his research. again, his correspondence shows the extent of his involvement with some of the most important national and international scholars of the day, and with prestigious research centres. scacchi’s correspondence with his fellow italian, filippo corridi (13), offers evidence of the size and quality of the collection he assembled (14), as do letters written by the geologist giovanni capellini (15): capellini provided the geological museum in bologna with mineral samples from vesuvius that he had been given by scacfig. 1. letters from edward suess to scacchi, vienna, 7 march 1890, biblioteca fondazione pomarici santomasi, gravina in puglia (bari, italia), cart. scacchi, b4, f 11, n 131. vol52,6,2009new 2-12-2009 12:39 pagina 606 607 the scientific correspondence of arcangelo scacchi chi, whose work in the scientific field he considered to be without equal (16). exchanges of books and minerals also took place between scacchi and angelo sismonda (17), the director of the mineralogical and geological museum in turin. scacchi enjoyed contact with numerous scientists from abroad. in september 1847, the chemist friedrich wöhler (18) visited naples and, along with scacchi and piria (19), went on an excursion to vesuvius, while in the autumn of 1850 scacchi accompanied three scientists from berlin, eilhard mitscherlich (20), gustav rose (21) and justus roth (22), to solfatara, monte nuovo and vesuvius. after the visit scacchi wrote to mitscherlich, thanking him for «extremely pleasant and instructive conversations» (23), a sentiment no doubt shared by the german scholars too, as rose and mitscherlich both recalled their stay in naples with their friend scacchi «with the greatest pleasure» (24). the high esteem they held him in led to an enduring friendship, and to their receiving mineral samples and papers from him. according to rose, scacchi’s work on humite (25) was remarkable (26). rose himself checked roth’s german translation of scacchi’s work and proof-read the final text and illustrations, which were published in poggendorff’s annalen der physik in 1853 (27). scacchi was in contact with various german scientists, such as the mineralologist paul groth (28), who included scacchi’s name in the list of contributors to his journal of crystallography and mineralogy («zeitschrift fur krystallographie und mineralogie»), which had been founded in 1877 (29). scacchi’s work was well-known outside the kingdom of naples thanks to correspondence between scientists: his scientific publications were read and circulated abroad and some publications were translated and published in specialist journals (30). scacchi’s contacts were not limited to europe: from 1853 to 1890 he corresponded with the american geologist and zoologist james dwight dana (31). scacchi himself closely followed the work published by other scholars; he often received papers written by academics who had consulted him in the course of their research, as in the case of wilhelm körner (32). many of those who corresponded with scacchi actually visited him personally in naples whilst in italy on research trips. on these occasions scacchi – often together with his colleague luigi palmieri (33) and his student guglielmo guiscardi (34) – usually accompanied the scientists on their expeditions around the region. one such visitor was the scottish geologist, charles lyell (35) who, after a first, crucial trip to italy in 1828, returned in 1858 and visited monte nuovo, vesuvius and pompei with scacchi. the correspondence reveals how the two scientists worked together. it is well-known that lyell is regarded as an important figure in the history of geological studies on account of his revolutionary views (36) about how events from the past can be explained in terms of phenomena present today, and his rejection of theories of unknown or cataclysmic causes. lyell’s work would in fact help to demolish catastrophe theories. he introduced into geology the concepts of unlimited time and natural forces, and the theory that the physical and geological phenomena at work today function with the same intensity as in the past. the earth’s past could be explained by making reference to present phenomena, such as the movement of the earth’s crust due to volcanic activity, earthquakes or erosion due to wind or rain. the development of lyell’s new point of view was significantly affected by his direct study of the tertiary and volcanoes while on his visit to italy in 1828 (37). through direct observation of volcanoes, lyell intended to elaborate on scrope’s theory (38) that volcanoes were formed as a consequence of the accumulation of lava and detritus around eruptive channels and could not therefore be explained by von buch’s theory of craters (39). for lyell, not only volcanoes but all geological phenomena could be explained in terms of processes still functioning today. in this context, lyell’s study of the tertiary was of particular importance: the observation of significant changes in the most recent strata (the tertiary) was fundamental to his project. his observations were made while he was in italy, first in naples, visiting vesuvius, monte nuovo and the temple of serapis, and afterwards on ischia, where he was struck by his discovery at an altitude of 600 m of shell fossils of extant species. vol52,6,2009new 2-12-2009 12:39 pagina 607 608 r. de ceglie there is no specific evidence of any meeting between lyell and scacchi during the former’s first visit in 1828, although it is known that scacchi was already in naples by that date. what is certain, however, is that over the following years the two wrote to each other regularly and certainly met during lyell’s second visit to naples in 1857. the two scientists enjoyed a friendship based on cooperation and mutual respect. before his 1857 visit, lyell wrote to scacchi thanking him for sending him his precious work on the geology of terrestrial volcanoes in the naples area and expressing keen interest in reading other work by the naturalist: «monsieur, vous avez eu la bonté de m’envoyer vos mémories très precieux sur la géologie des terreau volcaniques des environs de naples, que j’ai cité dans mon mémoire publié dans le journal de la soc. geol.que de londres […] avez-vous continué vos publications comme vous avez promis? je suis très curieus de les voir […]» (fig. 2). in the «memoires trés precieux», mentioned here by lyell and referred to in the quarterly journal of the geological society in 1849 (40), scacchi dealt with the subject of the volcanic regions of campania (41) and rejected the theory of crater upheaval for astroni, monte nuovo and other neighbouring cones. lyell agreed completely with scacchi: monte nuovo could only have been formed by an eruption. scacchi had also mentioned that the ancient temple to apollo or serapis (near pozzuoli), the walls of which are perfectly perpendicular, could not have remained so if the monte nuovo cone had been formed through a process of upheaval. «since writing the above i have received a memoir on the volcanic region of campania by signor arcangelo scacchi, published in the memoirs of the royal academy of naples for 1849, in which he entirely concurs with me in rejecting the theory of upheaval for astroni, monte nuovo, and other cones of that district.» (42). scacchi’s observations about monte nuovo and the temple of serapis, which underlined the unsustainability of a theory of crater upheaval, were taken up by lyell in his principles of geology (fig. 3) (43). scacchi’s opposition to fig. 2. letters from charles lyell to scacchi, london, 27 may 1852 (biblioteca fondazione pomarici santomasi, gravina in puglia (bari, italy), cart. scacchi, b 4, f 11, n 30. this theory is mentioned in his letter to monsignor lavinio de medici spada (44) in which, on the subject of the geologist giacinto provana di collegno (45), he says that, while respecting and liking the man as a friend, he was however forced to take scholarly distance from him owing to his becoming an italian apologist for the theory of crater upheaval. in a letter of 1852, lyell asked scacchi for copies of his most recent publications, not only because of his general interest in his colleague’s work, but, more specifically, because he desired information regarding the current condition of the temple of serapis, as he was due to give a lesson on it. he was also preparing a new edition of principles in which ample space was to be devoted to the subject (46). vol52,6,2009new 2-12-2009 12:39 pagina 608 609 the scientific correspondence of arcangelo scacchi lyell considered the temple of serapis highly important, and in fact an illustration of it appeared on the cover of principles of geology as a kind of emblematic representation of his theory that past events could be attributed to ongoing phenomena and not to any catastrophe theory. the columns of the temple bore evidence of a series of upheavals: marine mollusc imprints indicated that the edifice – built during the roman period on dry land – had later sunk into the sea; subsequently, the temple ruins were thrust upwards to their present position above sea level. the temple suggested to lyell how a succession of small changes could, over a given length of time, have tremendous effects, such as the creation of mountain chains or the sinking of continents. lyell’s theory integrated human history with geological history, the past with the present, and used a man-made monument (the temple) as a witness to geological change (47). the temple of serapis, like monte nuovo (formed in 1538), provided tangible evidence of the effects of geological processes through history (48). a significant paragraph in lyell’s principles is devoted to the temple (49); it describes the changes in the land and sea levels in the area around pozzuoli from the beginning of the christian era, which consisted of downwardsinking and upward-thrusting movements in the bay of bays, to the north and south of pozzuoli, including monte nuovo and solfatara (50). in response to lyell’s request (51), scacchi examined the temple and deduced that for several years there had been practically no further sinking. lyell himself had made a careful examination of the temple and noted that at high tide almost two feet of water entered it. a letter dated march 1855 (52) from lyell to leonard horner (53) mentions scacchi’s opinions and work. lyell notes (54) how scacchi’s extremely interesting observations on the 1850 eruption of vesuvius and on all that had occurred from 1845 to 1850 – observations which had been passed on to him by eilhard mitscherlich (55), who was also in contact with scacchi – completely undermined the theories proposed by von buch and dufresnoy (56). in expressing surprise at the influence von buch’s theories had enjoyed, the scottish geologist wrote: «in one paper scacchi is so astonished at dufresnoy’s assertions, who never walked once over one whole side of somma, that he says in so many words, ‘that if the french savant was ever at naples, he could never have gone out of his house, or only night’» (57). during his stay in naples in the autumn of 1857, lyell wrote to george hartung (58) to update him on the latest interesting observations made in the area. he was particularly interested in the movement and varying speed of the lava flows, which led him to elaborate new theories. lyell noticed that all the lava flows described by scacchi between 1850 and 1855, had been covered up by new, steeper flows. he examined them and the walls of monte somma, once again agreeing with scacchi: «i begin seriously to doubt whether any of the rents, which are now dikes, imply distension as de beaumont assumes. scacchi’s observations seem to show that they are rather the effect of a failure of support, and attended with a partial sinking. but i will explain more when i have devoted more days to the examination of this region» (59). scacchi made sure that his young student-assistant guiscardi was available to accompany lyell on his various expeditions around the area. lyell mentions his «good fortune» in having such a companion – «no one possesses a more fig. 3. temple of serapis, pozzuoli (neaples), on the cover of lyell’s principles of geology, 1830-33. vol52,6,2009new 2-12-2009 12:39 pagina 609 610 r. de ceglie thorough knowledge of the structure and composition of both vesuvius and monte somma» (60). several months later, when the irish engineer robert mallet (61) visited the kingdom of naples to examine the effects the earthquake had had on the provinces (62), he brought with him a letter from lyell to scacchi, in which the scotsman asked scacchi to give mallet advice, which would be «de la plus haute importance» (63). scacchi was always there, always ready to work together with his italian and foreign colleagues, and to show them around the land that he knew so well and would continue to study until almost the end of the century. in 1891, during the celebrations organized for his fifty years of teaching, he was described quite simply as having «contributed so much to the progress of geology and mineralogy.» (64). notes (1) edward suess (1831-1914), the austrian geologist and palaeontologist, was professor of geology at vienna from 1857. after working initially in the field of palaeontology, he subsequently devoted himself to plate tectonics and the study of strata. his masterpiece, das antlitz der erde, (wien, preg, tempsky; leipzig, freytag, 1885-1909), was a history of geological studies. in his studies of the alps, he focused primarily on orogeny. he was the first to introduce the concept of gondwana. (2) in 1844 scacchi held the chair of mineralogy at the university of naples, and was curator of the real museo mineralogico. in 1865 he was elected dean; he was re-elected in 1875. (3) millosevich, f. (1936), p. 991. translations mine. (4) apart from obituary notices and anniversary or commemoration tributes, which date to the end of the 19th and beginning of the 20th century. more recently, a. mottana has studied scacchi and his work in various fields (e.g. mottana, a., 1993; 2001). (5) oronzio gabriele costa (1787-1867), a zoologist and palaeontologist from naples, offered charles lyell precious help in the identification of shell fossils in the kingdom of naples. of the gemmellaro brothers, who were working in sicily, carlo (1787-1866), a naturalist and volcanologist, was the one who travelled the most and was most influenced by english culture; giuseppe (1788-1876), a volcanologist, often accompanied the geologist charles lyell on his excursions to etna; mario (1773-1839), built the first refuge on etna, which was later enlarged with funding from england (it was this refuge that lyell described as the english house). (6) the scacchi archive, in the biblioteca della fondazione pomarici santomasi, gravina in puglia (in the province of bari), consists of numerous letters sent to scacchi (some 200 correspondents have been listed so far, but a more precise figure is expected to emerge in the future). the collection also includes private papers regarding his work and qualifications, awards, budget, household affairs, diaries, notes and sonnets, funeral notices and telegrams. there are also a series of letters housed in other libraries in italy and abroad, including: accademia delle scienze, turin, italy; biblioteca dell’archiginnasio, bologna, italy; biblioteca comunale saffi, forlì, italy; biblioteca comunale, palermo, italy; biblioteca nazionale centrale, florence, italy; bayerische staatsbibliothek, monaco, germany; biblioteca universitaria, pisa, italy; deutsches museum, monaco, germany; museo civico di storia naturale, milan, italy; staatsbibliothek, berlin, germany. (7) see, for example, the letter from carlo passerini to scacchi, florence, 31 march 1838, biblioteca fondazione pomarici santomasi, gravina (hereafter: bpsg), cart. scacchi, b4, f 12, n 75. (8) scacchi kept a shell collection made up of «local and exotic» specimens which, in 1837, was bought by the zoological museum at the university of naples (mss neapolitan state archives, min. pubbl. istr., fasc. 284, ii): the sale of this collection marked a significant change in the direction of scacchi’s research: from a general interest in zoology and palaeontology he switched to more specific geological pursuits. after graduating in 1832, scacchi tried (without success) to win the chair of zoology left vacant after the death of luigi petagna. then, in 1836, when the bourbon government awarded the chair to oronzio gabriele costa, scacchi decided to devote himself to mineralogy. (9) the naturalist achille valenciennes (1794-1865) was considered the leading french ichthyologist of the nineteenth century. a disciple of cuvier, he worked with the latter on histoire naturelle des poisson (1828-1849), a project he continued after the death of cuvier. in 1832 he took over the chair of invertebrate zoology at the muséum d’histoire naturelle in paris. (10) alexandre brongniart (1770-1847), a geologist and palaeontologist renowned for his ability to identify the age of fossils in rock strata, was the first to study the trilobite. he worked with cuvier and conducted research vol52,6,2009new 2-12-2009 12:39 pagina 610 611 the scientific correspondence of arcangelo scacchi in the north of italy, comparing the organic remains in the tertiary formations around vicenza with those in the area around paris. (11) andrè hubert dumont (1809-1875), the belgian geologist and professor of geology at the university of lieges, was primarily interested in the geology and palaeontology of belgium. he organised geological surveys of the country and was also responsible for the drawing up of a geological map of belgium. in addition, he wrote numerous interesting papers on the mineral resources of his native country. dumonite, a yellow-ochre rhombic mineral, basic phosphate uranium hydroxide and lead, is named after him. (12) letter from edmond de selys longchamps to scacchi, lieges, 12 january 1839, bpsg, cart. scacchi, b 4, f 11, n 6. (13) filippo corridi (1806-1877), a mathematician from livorno, taught differential calculus at the university of pisa. from 1850 he was the principal of the scuole tecniche delle arti e manifatture. he wrote la geometria: libri dieci, florence, piatti, 1843. (14) letter from scacchi to corridi, naples, 13 december 1852; 23 august 1853, biblioteca universitaria, pisa, ms 922, fasc. 175. (15) the geologist giovanni capellini (1833-1922) studied at pisa. after a period spent in paris, he became a teacher of natural history in genoa and in 1861 obtained a professorship in bologna. he was the curator of the geological museum in bologna. he organized conferences, founded the società geologica italiana and travelled extensively in america and europe. (16) letter from capellini to scacchi, bologna, 28 july 1889, bpsg. letter from capellini to scacchi (18731890), bpsg, cart. scacchi, b4, f11, n 95; letter from scacchi to capellini (1873-1890), biblioteca dell’archiginnasio, bologna, fondo capellini, cxxx. (17) angelo sismonda (1807-1878), a geologist and palaeontologist from conegliano d’alba. from 1832 he was professor of mineralogy at the university of turin and curator of the mineralogical and geological museum. particularly interested in the alps, his best-known works include: notizie e schiarimenti sulla costituzione delle alpi piemontesi, turin, stamp. reale, 1845. see letters from sismonda to scacchi (1844-1875), bpsg, b 4, f 11, n 14. (18) letter from wolfang sartorius von waltershausen to scacchi, gothenberg, 27 august 1847; bpsg, b 4, f 11, n 22. friedrich wöhler (1800-1882), a german chemist, developed the first organic synthesis (urea). invited by berzelius to work in his laboratory in stockholm, he later returned to germany. in 1832 he visited cassel with liebig; in london he met faraday; in 1836 he won the chair in gothenberg. he worked with liebig and poggendorff on the first six volumes of the dictionary of pure and applied chemistry. (19) raffaele piria (1814-1865) was one of the leading italian chemists of the nineteenth century. he studied in paris, where he met macedonio melloni, with whom he became friends. in 1839 he returned to naples, where he taught a private course in chemistry. in 1842 he won the chair in chemistry at pisa and in 1857 moved to the university of turin. he studied salicin. stanislao cannizzaro was a student of his. piria wrote trattato elementare di chimica inorganica, florence, le monnier, 1851. (20) eilhard mitscherlich (1794-1863), a chemist and mineralogist from berlin, studied crystallography, first with link and later with berzelius in stockholm. in his later career he focused on geology and, in particular, volcanic regions. he was a brilliant writer of scientific manuals. his research included work on the relation between crystal shapes and their chemical composition (he discovered isomorphism). mitscherlichite, a blue-coloured, tetragonal mineral discovered inside vesuvius and composed of copper chloride and potassium hydrate – the product of a process of sublimation – was named after him. (21) gustav rose (1798-1873), mineralogist, professor of mineralogy at the university of berlin (from 1826) and curator of the local mineralogical museum (from 1856). he studied the relation between the shape and chemical composition of minerals and between crystal shapes and their electrical properties. in 1829, together with von humboldt, he embarked on a long journey to the urals and central asia where, notably in russia, he collected an impressive amount of mineralogical material. (22) justus ludwig adolph roth (1818-1892), a geologist and petrographer, was professor of petrography and geology at the university of berlin (from 1867). he was one of the first scientists to apply chemico-microscopic methods, which had only recently been introduced into petrological studies, to the study of the rocks of vesuvius, monte somma and the island of ponza. he contributed to the geological map surveys of lower silesia. (23) letter from scacchi to mitscherlich, naples, november 1850, deutsches museum munich, hs 00 884. translation mine. (24) letter from rose to scacchi, berlin, 12 august 1851, bpsg, b 4, f 11, n 23. translation mine. (25) scacchi, a., (1851). (26) letter from rose to scacchi, berlin, 12 august 1851, bpsg; see also letter from rose to scacchi, berlin, 14 august 1852, bpsg («cependant l’humite est devenue une substance si intéressante par vos recherches remarquables, que je voudrai bien en avoir d’autres»). vol52,6,2009new 2-12-2009 12:39 pagina 611 612 r. de ceglie (27) erganzungsband iii, leipzing, 1853, pp. 161-167. (28) letter from scacchi to paul groth, 1876-1883, bayerische staatsbibliothek, munich, grothiana x scacchi. paul groth (1843-1927), mineralogist and crystallographer, taught first in berlin (1870) and then from 1872 in strasburg, where he established a mineralogical collection with a catalogue (ed. 1878) which was considered a model for the whole period. in 1883 he was awarded the chair of mineralogy in munich and the curatorship of the bavarian state collection, to which he added samples from mineral deposits in the alps. his scientific research was conducted principally in the fields of structural and microscopic crystallography and crystal chemistry. the mineral grothite was named after him. (29) he remained as editor of this journal for all 25 volumes, until 1920. (30) poggendorff’s annalen der physic, annales des mines, the american journal of science and art, gelehrte anzeiger, erddmann’s journal für praktische chemie, zeitschrift der deutschen geologiche gesellschaft, neues jahrbuch für mineralogie geologie geognosie und petrefakunde. (31) james dwight dana (1813-1895), the famous american geologist and zoologist, was professor of geology at yale and foreign correspondent to the lincei. during his visit to italy in 1833-34, he observed the volcanic activity of vesuvius and, in the american journal of science, published: on the condition of vesuvius in july 1834. in 1836 he became silliman’s assistant at yale. from 1838 to 1842 he participated as the resident geologist in the largest american oceanic expedition of the nineteenth century, under wilkes. author of the very successful a system of mineralogy, new haven, durre & peck and harrick & noyes, 1837, and the manual of geology (philadelphia, bliss, 1869), he was also involved in important geological and palaeontological studies of, among other things, the appalachian mountains. he supported the theory of geosyncline and the role of orogenesis. the letters written to scacchi cover the period 1853 to 1890 (and do not date from 1845, as is erroneously stated in the gravina collection and in raguso, f., d’agostino, m. (1993). dana most probably met scacchi during his visit to naples in 1833-34. (32) he wrote to scacchi: «further to the communication i received from prof. cannizzaro on your behalf, i am sending you without delay the substances you requested, and at the same time i would like to express my gratitude to you for the assistance you have been so kind as to offer me in my research.» translation mine. palermo, october 9, 1869, bpsg, b 4, f 12, n 57. wilhelm körner (1839-1925), a german organic chemist, worked with kekulé at gand and in bonn; for health reasons, he moved to italy, palermo, where he worked with cannizzaro. körner took part in the project which led to the development of the theory of aromatic compounds. (33) luigi palmieri (1807-1896), physicist, volcanologist and philosopher. he directed the osservatorio vesuviano from 1850 and the specola universitaria from 1860; he was a correspondent for the lincei (1871). in 1860 he won the chair of earth physics. his work in physics dealt with questions concerning induced electrical currents (in particular telluric currents), atmospheric electricity and volcanology. his physical-chemical and spectroscopic work on volcanoes and lava was of particular importance; he also developed the method for determining the liquidity of lava. in 1856 he invented a seismograph which was set up on vesuvius to record earthquakes preceding eruptions. he was a great friend of scacchi, with whom on occasions he worked. together they wrote the paper on vulture (1852). he is also the author of: sull’origine dell’elettricità atmosferica, naples, fibreno, 1862. (34) guglielmo guiscardi (1821-1885), was scacchi’s student and assistant. he accompanied both lyell and mallet on their neapolitan excursions and his help was greatly appreciated by both scientists. he was the curator of the geological museum in naples from 1866 (the year it was founded) to 1885. the geological museum at trinity college, dublin, houses, amongst other things, the entire remains of the guiscardi-haughton collection (see wyse jackson, p.n. (1998). (35) charles lyell (1797-1875), the scottish geologist, was a proponent of the theory of uniformity and the author of principles of geology, (1830-33) which had a significant impact on the history of geology, influencing the work of charles darwin. he was the first to suggest the subdivision of the cenozoic era into the eocene, miocene, pliocene and pleistocene. he studied the formation and structure of volcanoes and recognised the correlation between volcanology, seismic activity and tectonics. he was professor of geology at king’s college, london from 1831 and was elected president of the royal geological society in 1835. (36) with the publication of principles of geology, london, murray, 1830-1833. (37) in the sense that, as rudwick points out, when lyell left, the general outline of his book was already clear and he was aware of having formulated a revolutionary theory; the aim of his visit was to gain first hand experience of phenomena that might be used as persuasive examples for his argument. see rudwick, m.j.s.(1969), cf. p. 289. (38) in 1825 the first treatise on volcanoes was published (considerations on volcanoes: the probable causes of their phenomena, the laws which determine their march, the disposition of their products and their connexion with the present state and past history of the globe: leading to the establishment of a new theory of the earth, london, phillips, yard, 1825) had been published by george poulet scrope (1797-1876) who, taking into convol52,6,2009new 2-12-2009 12:39 pagina 612 613 the scientific correspondence of arcangelo scacchi sideration the work of italian scholars, proved the doubtful nature of the theory of crater upheaval in the formation of volcanoes. (39) according to this theory, the birth of a volcano, like that of a mountain, is due to endogenic upheaval and not to rock accumulation after an eruption. leopold von buch (1774-1853) «links volcanic activity to sudden, violent upward movements of the ground. volcanoes are formed by the accumulation of scoriae and lava: the pressure of incandescent underground masses pushes the ground upwards, thus causing the previously horizontal strata to form reliefs; the peak then explodes, leading to the formation of an eruptive cone inside the crater. vesuvius,» von buch wrote, «emerged from the mound of earth that sprang up in 79 bc» la vergata, a. (1989), p. 567. (40) lyell, c. (1849). (41) scacchi, a. (1849). (42) lyell, c. (1849), cit., p. 226. (43) on the subject of the formation of monte nuovo (in 1538), the temple of serapis (where he cites scacchi’s opinion as being exact), and the eruption of vesuvius (1850). (44) letter from scacchi to de medici spada, 16 october 1845, biblioteca comunale saffi, forlì, collezione piancastelli. lavinio de medici spada (1801-1863), man of letters, politician and mineralogist. he was interested in botany, but is best known for his mineral collection, which was bought by the vatican in 1851 for the university of rome and which today still forms the nucleus of the mineralogical museum. (45) giacinto provana di collegno (1794-1856), a nobleman from turin, an officer in the armed forces and a member of the carboneria, he had a direct role in the revolution of march 1821. in exile in genoa and marseilles he devoted himself first to botany and then to the study of geology. (46) letter from lyell to scacchi, london, 27 may 1852, bpsg, b 4, f 11, n 30. (47) see rudwick, m. (1996), p. 282. (48) rudwick, m. (1969), cit., in particular, p. 291. (49) lyell, c. (1875), vol. ii, cap. xxx. (50) quart. journ. geol. soc., vol. iii, p. 237. (51) see letter from lyell to scacchi, 1852. (52) lyell, k.m.(ed.) (1881), berlin, march 24, 1855, pp. 203-204. (53) leonard horner (1785-1864), geologist, founder of the edinburgh review (1802), president of the royal geological society (1845-47 and 1860-62). one of darwin’s correspondents. (54) scacchi, a. (1850). (55) see note 20. (56) the geologist and mineralogist ours-pierre-armand-petit dufrénoy (1792-1857) began work in 1823 on a geological map survey of france with e. beaumont, which was published in 1836. he published work on the geology of the massif central, on southern france, and on the formation of monte somma and vesuvius. (57) lyell, k. m. (ed.) (1881), cit., p. 203 letter to horner. (58) ivi, letter to george hartung, naples, october 16, 1857, pp. 273275. (59) ivi, p. 274. (60) lyell c. (1875), cit., vol. i, p. 635. (61) robert mallet (1810-1881), irish engineer and seismologist. he was involved in several public works projects and in the field of industrial construction. his work as a civil engineer led him to study questions regarding the earth’s surface and related dynamics; his work on the statistics and dynamics of earthquakes and on seismic wave velocity assured him of recognition as one of the founders of seismology, a term he himself coined. (62) december 16, 1857. (63) letter from lyell to scacchi, london, january 25, 1858, bpsg, b 4, f11. n 30. the observation work carried out in the naples area led to the publication of great neapolitan earthquake of 1857, a work which was to become a key text in the history of seismology. in the introduction, mallet acknowledges the enormous help he received from his italian colleagues scacchi, palmieri and guiscardi. (64) the tribute was from johnston-lavis, h.s. (the representative of the london geological society and the british mineralogical society) (1891). on his eighty-first birthday, february 8, 1891, scacchi received a medal engraved with his own portrait, as well as awards from other research establishments, at home and abroad, for his scholarly contribution to science. vol52,6,2009new 2-12-2009 12:39 pagina 613 614 r. de ceglie references johnston-lavis, h.s. (1891): cinquantesimo anniversario dell’insegnamento di arcangelo scacchi, (tipografia della reale università, napoli). la vergata, a. (1989): geologia e paleontologia fra la metà del settecento e la metà dell’ottocento, in storia della scienza moderna e contemporanea, vol. ii, edited by p. rossi, (utet, torino). lyell, c. (1830-1833): principles of geology, (murray, london). lyell, c. (1849): on craters of denudation, with observations on the structure and growth of volcanic cones, the quarterly journal of the geological society of london, (proceedings of the geological society, december 19). lyell, c. (1875): principles of geology, (murray, london). lyell, k.m. (ed.) (1881): life, letters and journal of sir charles lyell, (murray, london). mallet, r. (1862): great neapolitan earthquake of 1857. the first principles of observational seismology, vol. ii, (chapman and hall, london) (rist. anast., ing and sga, 1987). millosevich, f. (1936): scacchi arcangelo, in enciclopedia italiana, (treccani, roma), vol. xxx. mottana, a. (1993): ricordo di arcangelo scacchi a cent’anni dalla morte, in rend. accad. ital. sc., detta dei xl, mem. sc. fis. nat., s. 5, vol. 17, pp. 45-74. mottana, a. (2001): arcangelo scacchi: massimo scopritore di minerali al vesuvio, in atti del bicentenario real museo mineralogico, edited by m.r. ghiara and c. petti, (mediasitalia, napoli). palmieri, l. and a. scacchi (1852): della regione vulcanica del monte vulture e del tremuoto ivi avvenuto nel dì 14 agosto 1851, (relazione fatta per incarico della reale accademia delle scienze, napoli, nobile). raguso, f., m. d’agostino (1993): arcangelo scacchi. pietra miliare. cristallografia, mineralogia, vulcanologia, (teleservice, gravina). rudwick, m.j.s. (1969): lyell on etna, and the antiquity of the earth, in toward a history of geology, edited by c. j. schneer, (the m.i.t. press, cambridge). rudwick, m. (1996): mineral, strata and fossil, in cultures of natural history, edited by n. jardine, j.a. secord, e.c. spary (cambridge university press, cambridge). scacchi, a. (1849): memorie geologiche della campania. memoria i, rend. acc. sc. fis. e mat., 8 [43], pp. 4165, memoria ii, 8 [44-45], pp.115-140, memoria iii, 8 [48], pp. 317-335, 9 [50], pp. 84-114, napoli. scacchi, a. relazione dell’incendio accaduto nel vesuvio nel mese di febbraio del 1850, seguita dai giornalieri cambiamenti osservati in questo vulcano dal 1840 sin ora, in rend. r. acc. delle scienze fis. e nat., 9, 49, naples, 1850, pp. 13-48; trans. into french by damour and published in «annales des mines», de la derniére eruption du vésuve arrivée en fevrier 1850…, 4° serie, t. xvii, paris, 1850, pp. 323-380. scacchi a. (1851): della humite e del peridoto del vesuvio, in atti della r. acc. delle scienze di napoli, 6, pp. 241-273. wyse jackson, p.n. (1998): hibernian research on italian volcanic rocks. the mallet and guiscardi/haughton collections in trinity college, dublin, in volcanoes and history, edited by m. morello, (inhigeo, genova, brigati). vol52,6,2009new 2-12-2009 12:39 pagina 614 d e t e r m i n i n g e n e r g y o f elastic w a v e s caused by e a r t h q u a k e (*) v . l . b e l o t e l o v , n . "v. k o n d o r s k a y a , e . t h . s a v a r e n s k y rieevuto il 7 dicembre 1960 the recent achievements in seismology and the development of the ussr seismic station network make it possible to determine the absolute value of the energy of elastic oscillations radiated by a source, on the basis of the method suggested by b. b. galitzin in 1915 (l) and developed by h. jeffreys (2). to solve this problem, it is necessary to determine the changes in the density of elastic wave energy with distance, and to work out methods for determining the energy of nonstationary oscillations. the present communication represents the contents of two articles (3'4), connected with the question of determination the absolute value of the energy of longitudinal and transverse waves. the value of the energy radiated by the source is that of the energy of body waves in the period range of 2 to 10 sec. where standard equipment of d. p. kirnos type has constant magnification. in estimating elastic energy radiated by a source we shall proceed from the following assumptions: 1. the flux of the energy transmitted from the source to the earth surface is directed along the rays. 2. the variations in the energy flux due to reflection and refraction at the intermediate underground boundaries are of the same order as the errors in determining the oscillation energy, and only the effect of the bottom of the earth's crust and the earth surface is taken into account. 3. the duration of elastic oscillations in p and s phases radiated by the source does not change in propagating towards the earth surface. (*) p a p e r read a t the helsinky assembly of the i . u . g . g . , 1960. 5 6 v . l. b e l o t e l o v , n . v . k o n d o r s k a y a , e . til. s a v a r e n s k y in this paper the term " earthquake energy " will signify the energy of elastic body waves radiated by the earthquake source. this energy we represent as the sum of energies of longitudinal (ep) and transverse (es) waves radiated from the source. e = ep es the calculation of the energy of p or s waves from the data of individual stations has been carried out by means of the following formulas: 4 n r2 sin 6 sin e0 ( e0 \ e = —— e ) • p o f(e,a) cos e de dd qc e0 = oc d an y / da,: dt ) + \ dt k 2 v dajv dt ) k/ dt, where 0 = epicentral distance, e = angle of emergence of the seismic ray from the earthquake source, k — coefficient of energy absorption in the earth, c = velocity of the incident p or 8 wave near the earth surface, o = rock density in the vicinity of the seismic stations, e0 = density of oscillation energy in the incident wave at the point of observation, e0 = angle of emergence of the seismic ray to the earth surface, ak, ae, az = components of true ground displacement, at the earth surface itv, kz = coefficients of reflection at the free interface for the horizontal and the vertical component respectively, / (e, a) function taking into account the mechanism of the earthquake source. energy determinations were carried out for 11 earthquakes recorded by seismic stations of the usse. the co-ordinates of their epicentres were determined on the basis of the time of arrival of 8 and p waves at soviet and foreign seismic stations. the basic data on the sources are given in table 1. the geographical distribution of the epicentres is presented in fig. 1. 112 seismograms with the most distinct recordings were selected, which served as basic material for the determination of p and 8 wave energy. these seismograms represented the records obtained by soviet instruments " ctk " and " cbk " (records made by b. b. galitzintype instrument were used only in two cases). in calculating the oscillation energy we first selected the oscillation groups corresponding to p and 8 waves in the seismograms. we cald e t e r m i n i n g e n e r g y ok e l a s t i c w a v e s c a u s e d b y e a r t h q u a k e 5 7 ciliated the energy at the point of observation for the time interval corresponding to these selected groups. the arrival of the respective wave was assumed to be the beginning of the group, its duration being determined by the time interval from the arrival to the appearance of new visible arrivals. in our cases this average duration was equal to t a b l e 1 nn d a t a time of origin 7t m s koordinates of epicenter m number station of determining nn d a t a time of origin 7t m s <p° > hkm m m ige 1 26-x-1952 19 19 18 39.4 143 3 30 6 2 15 13 2 27-x-1952 03 17 15 39.6 143 3 30 6 2 28 12 3 28-iii-1954 20 36 24 50.9 175 9 50 6 4 22 11 4 1-iv-1954 18 18 47 46.8 153 5 60 6 0 24 9 5 18-vii-1954 09 07 41 36.0 141 0 40 6 2 15 10 6 9-vi1i-1954 19 16 51 53.3 160 7 60 6 2 26 8 7 30-viii-1954 07 57 24 43.9 147 4 60 5 9 18 9 8 6-ix-1954 18 30 52 51.7 157 9 60 6 2 30 10 9 23-ix-1954 21 43 37 48.6 156 8 60 6 3 21 13 10 18-viii-1957 21 42 36 50.0 156 5 40 6 4 15 13 11 3-1-1957 12 48 29 44.0 130 0 560 6 4 17 34 the doubled time of amplitude increment after arrival. i t proved to be equal on the average to about 20 sec for p and 10 sec for s waves. the abovementioned intervals were selected after a detailed consideration of many records. i n calculating the energy of the selected p and s groups, we assume t h a t we determine the energy of p and 8 waves within the given frequency range. since waves of different frequencies have different attenuation with distance, it was necessary to estimate the frequency spectrum of p and s waves as a function of the cbstance. the average periods in the p and s phases were measured. the dependence of the average period on the distance in the selected groups is shown in fig. 2. as may be seen, t h e average periods in the groups investigated are very similar and do not change with a 5 8 v . l . b e l o t e l o v , n . v . k o n d o r s k a y a , e . til. s a v a r e n s k y variation in the epieentral distance (on the average, tp is 4-5 sec and ts — 6-8 sec.). one may assume that starting with d = 20° and up p and s waves propagate without substantial distortions or shape variations. our data on energy values are somewhat understated owing to the limited range of the periods under consideration. r / ( d a \2 i — j-j dt we departed from the usual o scheme representing oscillations as sinusoid curves. the calculation of these integrals was carried out by means of a special device. the operating principle of this device consists in the following. the seismogram is placed on the drum of a station recorder which rotates at a constant speed. a rheostat having an indicator on its slide is fixed parallel to the drum generatrix. a permanent voltage is applied at the ends of the rheostat, which is proportional to the amplitude on the seismogram at each moment of time. further, the electronic device differentiates the oscillation curve, squares and then integrates it (5). the integrals were measured repeatedly, the results being continuously d e t e r m i n i n g e n e r g y ok e l a s t i c w a v e s c a u s e d b y e a r t h q u a k e 5 9 compared with the calibration sinusoid curves for which the integral value was determined numerically. the average error of individual measurements turned out to be about 20%. for calculation the amplitudes of true displacement on the earth surface, the normal magnification of the instruments was used as a divider; it is characterized by the magnification of stationary sinusoidal oscillations. the paper by d. p. kirnos and n. v. kondorskaya (6) shows t h a t in the case of non-stationary oscillations the instrument amplification may differ considerably from normal. an estimate was made of the calfox pwuuvj foi 3 u/xx irej 10 t s e c * < s • • •50 ' j o ® pig. 2. dependence of the average period in the p a n d s phases on t h e epicentral distance (6). . . . — for p waves xxx for 8 waves culation error due to the adoption of stationary normal magnification. i t proved to be up to 20%. the reflection coefficients at the free interface and the refraction coefficients at the intermediate boundaries have been taken from papers by b. gutenberg (') and s. d. kogan (8). sin 0 ' sin. o the function — 0 was calculated on the basis of hodgson's cos e de do tables (9) and the numerical differentiation with respect to the tabulated values of e. values of this function for different depths from 0 to 600 km were plotted. we defined / (e, a) as the amount of energy per unit solid angle in the direction e, a as related to the average energy of the source in a unit solid angle. to calculate this, it is necessary to known the direction of the forces in the source, and the position of the seismic stations which 6 0 v . l. b e l o t e l o v , n . v . k o n d o r s k a y a , e . til. s a v a r e n s k y is determined by the angles e and a. for the calculations of / (e, a) we employed the equations for displacements in p and s waves suggested by keibs-borok (10). the source mechanism was studied specially for one of the earthquakes of the series under review, namely t h a t of j a nuary 3, 1957, on the basis of distribution of displacement signs upon the arrival of p.sv, sh and pcp waves at the seismic stations. the force system of this earthquake is modelled by a double force with a t a b l e 2 d a t a hkm igep iges lg ep+s m igeui) a ige (igep+sige(m)) 1 26-x-52 197; 30 20 9 20.7 21 1 6 2 21 1 0 2 27-x-52 03/i 30 20 8 20.6 21 0 6 2 21 1 — 0.1 3 28-111-54 20 h 50 20 9 20.9 21 2 6 4 21 4 — 0.2 4 l-iv-54 1 8h 60 21 0 20.5 21 1 6 0 20 8 + 0.3 5 18-vti-54 09h 40 20 8 20.7 21 0 6 2 21 1 — 0.1 6 9-viii-54 197; 60 20 5 20.9 21 1 6 2 21 1 0.0 7 30-viii-54 07 h 60 20 7 20.7 21 0 5 9 20 7 + 0.3 8 6-ix-54 18 h 60 20 7 20.9 21 1 6 2 21 1 0.0 9 23-ix-54 21h 60 20 6 20.8 21 0 6 3 21 3 — 0.3 10 18-viii-57 21 h 40 20 3 20.3 20 6 6 4 21 4 — 0.8 11 3-1-57 127i 560 20.45 20.65 20 9 6 4 21 4 — 0.5 moment, the forces forming a plane extending at 303° and inclined at 10°. the movement in the rupture plane is nearly horizontal. the determination of the direction of forces in the source of this earthquake was helpful in determining / (e, a) which varied between 0 and 1 for the system of the stations used. i t turned out that the scatter of e values for individual stations exceeded / (e, a) variations. for the other 10 earthquakes we assumed / (e, a) = 1. i n calculating the average e value for each earthquake, we determmed the average geometrical of all values calculated for individual seismic stations. e — | ex • e2 en , where n = number of stations whose data served to determine the energy. d e t e r m i n i n g e n e r g y ok e l a s t i c w a v e s c a u s e d b y e a r t h q u a k e 6 1 in this case e = \ el • e2 • e3... jqn • ek(di+ez+d3 + where e = ee k° , k = 0.00012a.,,,-1 (gutenberg's value). at, 3, • -t v n • oo* • • a <1 • • m. • oo . 1 fig. 3. experimental d a t a on a lg ep and a lg es a lg ep = lg ep* (individual) lg ep (average) a ]g es = lg es* (individual) lg es (average) as a function of the epic.entral distance (0). . . . experimental d a t a on earthquakes no. 1-10 ooo experimental d a t a on e a r t h q u a k e no. 11 taking into account all the values indicated, we obtained the values of the energy of p and s waves reduced to the source. they are presented in table 2. the data of the table enable the following conclusions to be drawn: 1. the average values of lg ep and lg es are very similar. 6 2 v . l. b e l o t e l o v , n . v . k o n d o r s k a y a , e. til. s a v a r e n s k y 2. all the earthquakes under consideration have nearly identical values of m. the energy values obtained are also rather close. 3. if the relationship of the m value and the energy as obtained by gutenberg and richter ( u ) is assumed to be lg e = 11.8 + 1.5 m, then the values of ig e derived agree with the energy values obtained by our method. 4. the values of a lg fjp and a lg es were considered, which represent the deviations from the average values of lg ep and lg es for each station within individual regions. one can observe a regularity of a lg ep and a lg es values for individual stations. these deviations may be regarded as corrections which should be introduced when making determinations according to the data from individual stations. i t is noteworthy that these corrections agree with the features of the hodograph which were previously noted by n. y. kondorskaya (12). i t was shown that the travel times of seismic waves to individual stations within the region exhibit a regularity with respect to the averaged jeffreys-buben hodograph: the travel times to some stations are always less than to others. the deviation signs of a lg ep and a lg es for individual stations agree with deviation signs of travel times to these stations from the jeffreys-buben hodograph. the most probable cause of these deviations obtained evidently bes in the peculiarities of the geological structure in the region of each seismic station. to analyse ab the obtained data statistically, a single graph was used for plotting the deviations of individual values of lg e * for individual stations from the average value of lg e for each earthquake. such e* plotting enables to obtain the dependence lg j = a \ge = f (0) for earthquakes of different intensities. the respective dependences a ige = f (6) for p and s waves are given in fig. 3. in spite of the considerable scatter of individual values, the general trend is toward an increase in a ig e with the epicentral distance which may be due to the decrease in the absorption coefficient of energy with depth. summary the energy of 11 earthquakes in the far east was examined by using observations of seismic stations of the ussr with epicentral distances from 20° up. determining energy ok elastic waves caused by earthquake 63 the absolute value of energy of elastic ivaves (p and s) ivas determined on the basis of development of galitzin method. the results agree with the value of energy calculated on the basis of gutenberg and richter formula. ige = 11.8 + 1.5 m. the dependence of the value of lg e with epicentral distance was noticed. it is suggested that this tvay be due to the decrease with absorption coefficient of energy with depth. riassunto e stata esaminata i'energia di 11 terremoti nclv estremo oriente utilizzando le osservazioni delle stazioni sismiche dell'u.r.s.s. con distanze dalvepicentro da 200 in poi. ii valore assoluto delvenergia delle onde elastiche (p ed e) e stato determinato sulla base dello sviluppo del metodo di galitzin. i risultati concordano con il valore delvenergia calcolato sulla base della formula di gutenberg e richter lg e = 11,8 + 1,5 m r e f e r e n c e s (1) e. b. rojimihhj o 3emjiempjicehuu 18/11/1911 r., « h3bcthh pocc. a h , » c e p . 6, 1915 r . (2) jeffreys h . , the pamir earthquake of 1911 february 18 in relation to depth of earthquake foci. « m o n . n o t . r o y . a s t r o n o m . s o c . g e o p l i y s . n , suppl. 1, 2, (1923). (3) e. <e>. cabapehckhh, h . b. kohaopckah, b. ji. bejiotejiobb, 06 onpede.ieuuu shepeuu ynpyzux eo.au, nopomcdaemux 3em>iempncehuem, h3becthh ah cccp, cepn reo(j)h3hmeci<ah, n° 5, 1960. (4) b. ji. bejiotejiob, h . b. kohaopckah, k eonpocy onpebejieuuh zuepiuuu 3emaempncehun, h3becthh a h cccp, ccpj-ih reocf)h3., _na 12, i960. (5) b. ji. bejiotejiob, ycmauoeka d/w sbihucjieuuh snepeiui ynpyzux eojih no ceucmozpammum, h3becthh ah cccp oth, jng 6, 1959. (6) ji. n. khphoc, h . b. kohaopckah, o ebikhucaenuu ucmuuhozo 3hanehuh nepeoii amuaumydbi deuoiceuuh noueu npu ecmynjieuuu ceuc.mmeckou eojiuu, « h3b. » ah cccp, cep. reo<j)h3. jn° 12, 1958. 6 4 v . l . b e l o t e l o v , n . v . k o n d o r s k a y a , e . t h . s a v a r e n s k y ( 7 ) g u t e n b e r g b . , energy ratio of reflected and refracted seismic waves. « bull, of t h e seismol. soe. of america », 34. 2, (1944). (8) c. xik o r a h , ilapamempbi ouaeoe !ay6oi<o(fiokychux 3em.iemphceituu, « tpyaw reo(j)h3h<ieckoro hhcthtyta », .ns 30 (157), 1955. ( 9 ) h o d g s o n i l , s t o r e y r . , tables extending byerly's fault plane techniques to earthquakes of any focal depth. « bull. seism. soc. am. », 43, (1953). ( 1 0 ) b . h . kehjihc-eopok, rpatfiuneckue memodu pacuema duha.huueckiix napamempoe onaea 3emaempncehun, h3uecthh ah cccp, cep. reotj)h3. we 6, 1951. ( u ) g u t e n b e r g b . , r i c h t e r c . f . , magnitude and energy of earthquakes « a n n . geofls. », r o m a , 9, 1, (1956). ( 1 2 ) h . b . kohflopckah, tio noeody pezuohajibhbix ocobenhocmeu epe.uen npo6eea ceucmuueckux 80ah, h^bectjih ah cccp, cep. reo<j>h3., n° 7, 1957. vol52,3,2009 437 annals of geophysics, vol. 52, n. 3/4, june/august 2009 1. introduction the need for more reliable and efficient communications services, especially those involving ionospheric hf communications and navigational systems, imposes increasing demand for a better knowledge of the effects imposed by the earth’s upper atmosphere and ways to mitigate disturbing effects. temporal and spatial changes in the upper atmosphere act to limit and degrade the performance of terrestrial and earth-space radio systems in many different ways and this is why mitigation activities must involve several topics like ionospheric monitoring and modeling, development of new hardware for communication systems and new propagation simulator, measurements and modeling of ionospheric total electron content (tec) and ionospheric scintillations, using in particular the global positioning system (gps). the european ionospheric community has long been aware that cooperation research on an international basis is essential to deal with such complex issues. in particular, international cooperation is required for the collection of data, in both the real-time and in retrospective modes, the development and verification of new methods to improve the performance of both operational and future terrestrial and earth-space communication systems and the exchange of expertise on space plasma effects on global navigation satellite systems (gnss). in this context the cost 296 action miers on the «mitigation of ionospheric effects on radio systems» has made a significant impact in a number of areas. 2. the major achievements of cost 296 a) a study dedicated to the improvement of the nequick model for galileo users has been performed to identify the weaknesses of the model at mid-latitudes. b) neuro-fuzzy modelling based methods to predict fof2 1-24 hours in advance have been presented. c) an analytical model has been developed for mid-scale ionospheric inhomogeneities occurring either as patches in the high-latitude ionosphere or bubbles in the low latitude/equatorial ionosphere in order to investigate and predict their scintillation effects. d) the day-to-day variability in tec is greater than in fof2 and exhibits its higher values during the nighttime hours. the slab thickness variability shows a similar pattern with that of tec or of fof2 variability. e) the prompt ionospheric database has been improved (http://www.wdc.rl.ac.uk/cgicost 296 miers: conclusion alain bourdillon (1), ljiljana r. cander (2) and bruno zolesi (3) (1) ietr, université de rennes 1, france (2) stfc, rutherford appleton laboratory, chilton, uk (3) istituto nazionale di geofisica e vulcanologia, roma, italy a brief summary is given here of the major achievements of the cost 296 action miers. mailing address: dr. alain bourdillon, ietr, université de rennes 1, 35042 rennes, france; e-mail: alain.bourdillon@univ-rennes1.fr vol52,3,2009 20-09-2009 19:06 pagina 437 438 a. bourdillon, l.r. cander and bruno zolesi bin/digisondes/cost_database.pl). this is 24/7 on-line service that is based on the contributions of a number of cost 296 participating institutions. f) the rwc-warsaw/idce web service (http://www.cbk.waw.pl/rwc/idce and ftp:// www.cbk.waw.pl/idce) provides on line access to the 24-hours ahead forecast of fof2 and m(3000)f2 for quiet as well as disturbed conditions based on data of continuous monitoring of regional ionospheric conditions from all available sites. the list of available forecasting models. g) the structure of a heterogeneous array has been proposed for hf direction finding purpose with advantages resulting from its sensibility to the incident polarization. h) research has been directed at exploiting mimo techniques for long-range communications within the hf band. several hf-mimo measurement campaigns were performed using different antenna array configurations at both the transmitter and receiver ends of the link. i) time-of-flight measurements (tof) over the radio link between uppsala (sweden) and bruntingthorpe (u.k.) have been performed and compared with the tof provided by the long-term prediction models sirm. l) a long-term interference prediction model for the hf spectrum is being developed. this model could be used in the absence of system capability to assess interference background in real time or near real time. m) an operational service for space-plasma and space-weather monitoring has been developed (space weather applications center ionosphere, http://w3swaci.dlr.de/). n) the role of higher order ionosphere errors (the term in f-3 due to the effect of the geomagnetic field; the three f-4 terms (i) due to the curvature of the path in the plane of incidence, (ii) curvature perpendicular to the plane of incidence and (iii) due to the actual distribution of electron density along the path) is under consideration. o) the monitoring of ionospheric scintillation activity has been continued in different parts of the world. the database has been analyzed and the development of forecasting models has been initiated 3. proposal for a follow-on action since 1991, the cost 238, cost 251, cost 271 and cost 296 actions relating to ionospheric radio propagation have been particularly useful in creating a critical mass of researchers in projects where the initial emphasis was on long-term ionospheric prediction and retrospective ionospheric modeling over europe. the cost 238 action prime (prediction and retrospective ionospheric modeling over europe) was a four year research project with objectives to develop regional maps and models over the european ionosphere that are more accurate than internationally available global maps and models. it involved the participation of some 72 scientists and engineers from 17 countries. the work encompassed the production of regional monthly median maps and algorithms for generating «instantaneous» snapshot maps of the standard vertical-incidence ionospheric characteristics from coordinated sounding measurements within europe. additional studies involved height profiles of electron density, the vertical total electron content up to an altitude of 1000 km and research leading to an improved understanding and thus potential for better modeling of ionospheric storm morphologies. work conducted within cost 238 was continued in the follow-on project cost 251 iits (improved quality of service in ionospheric telecommunication systems planning and operation) on the application of prime results in the improved quality of service in ionospheric telecommunication systems planning and operation. both projects yielded significant results, but they also identified further areas where future development was needed. the topics for more research included: hour-to-hour and day-to-day variability prediction capabilities in the topside ionosphere, upgrading of the current models to include scintillation effects, the prediction of the ionospheric and plasmaspheric effects on navigational systems and the development of methods for calculating the reliability and compatibility of hf radio systems using digital modulation techniques and over the horizon hf backscatter radars. vol52,3,2009 20-09-2009 19:06 pagina 438 439 cost 296 miers: conclusion the cost 271 action «effects of the upper atmosphere on terrestrial and earth-space communications» was then established to examine a range of relevant research areas within four broad groupings by considering the impact of variability of space environment on communications, to assess space plasma effects for satellite applications, to consider the ionospheric effects on terrestrial communications and the space plasma effects on earth-space and satellite-to-satellite communications. the cost 296 action miers is now finishing and it is clear that with the development of gps applications, the coming galileo system and the more frequent use of hf communication systems with commercial aircraft, on polar routes for instance, there is a strong requirement for improved understanding of ionospheric effects on such systems and better modeling of the ionosphere itself. this is the reason why the ionospheric cost community envisages proposing a new action. 4. conclusion the cost 296 action has been very successful in bringing together in collaborative studies researchers who represent most of the available european expertise on the effects and mitigation of ionospheric effects on radio systems. the significant results and advances in knowledge obtained during cost 296 have been summarized here. in addition a new proposal is currently under consideration. vol52,3,2009 20-09-2009 19:06 pagina 439 vol49_2_2006 681 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words geomagnetic field – marine magnetometers – seafloor observations – skin depth 1. introduction the earth’s magnetism has its origin mainly within the fluid outer core by a dynamo mechanism that produces an essentially dipolar geomagnetic field (95% of the total field), whose axis is slightly inclined with respect to the rotation axis by about 10°. in detail around 98% of the total field measured at the earth’s surface is of internal origin and 1% of the total field is due to crustal magnetisation characterising the different tectonic plates. the measured geomagnetic field also includes external fields whose sources are in the ionosphere and magnetosphere that, together with their crustal induced counterparts, contribute about 1% of the total field. finally, the oceans also make a small contribution through the motions of seawater in the geomagnetic field. the geomagnetic field changes with time. among its variations we recall, listed from longer to shorter characteristic periods: a) the secular variation, which is the slow (annual, decadal, and secular) trend of the field; b) solar diurnal variation, which is the daily oscillation due to the solar ionisation of the high atmosphere; c) bays, substorms and storms (range: from a few minutes to 1-2 days), the latter representing the most rapid variations with significant amplitude recorded by geomagnetic observatories on the earth’s surface. since land covers only around 1/3 of the earth’s surface, to better understand the morphology of the geomagnetic field it is necessary to extend magnetic observations to the seafloors. measuring the geomagnetic field beneath the sea has the characteristic that the external rapidly varying fields are partially screened by seawater (average conductivity of 3-4 s/m depending on salinity), while slowly varying fields, which are of internal origin, are practically unperturbed. the simultaneous magnetic variations recorded on the seafloor and at different locations on land are also related to the presence of electrical conductive layers or deep seafloor magnetic observations under geostar project angelo de santis, domenico di mauro, lili cafarella, roberto d’anna, luis r. gaya-piqué, paolo palangio, giovanni romeo and roberta tozzi istituto nazionale di geofisica e vulcanologia, roma, italy abstract performing good quality magnetic observations is not an easy task; making them in the extreme marine environment is even more challenging. the european funded geostar project succeeded in reaching this difficult goal. after the shallow seawater test experiment performed in the adriatic sea in 1998, the main aims of the geostar project were achieved two years later during the six-month deep seafloor mission in the tyrrhenian sea at around 2 km depth. details and results of the shallow seawater mission in the adriatic sea were published in previous articles. this paper is concerned with the deep seafloor mission in the tyrrhenian sea close to ustica island and presents some results related to the geomagnetic recordings. mailing address: dr. angelo de santis, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: desantisag@ingv.it 682 angelo de santis et al. of lateral electrical discontinuities in the oceanic crust and upper mantle on a global scale and to the presence of local patterns in restricted areas. the under-sea environment presents evident constraints on any kind of measurements. the pressure increases by 1 atm for every 10 m depth increment; hence at a depth of 2 km we have around 200 atm. this high pressure requires that the instrumentation must be contained in strong, non-collapsible pressure cases. corrosion is another adverse factor affecting many components of instruments or sensors, therefore proper protection must be adopted. the most important aspect for seafloor magnetic measurements is the seawater electrical conductivity. salinity and pressure variations do not really influence this quantity significantly but temperature does. seawater temperature decreases with depth: from 20-30°c (typical temperatures at middle temperate latitudes) at the surface to few degrees at the seafloor (e.g., around 3°c at −3 km) with very high stability. correspondingly, the seawater electrical conductivity decreases with depth (from 6 s/m at the surface to 3.5 s/m at around −1 km and to 3.3 at around −2 km). below a depth of 2 km the conductivity has an almost constant value of 3.3 s/m; for the oceans this latter value can be considered as the average conductivity. smaller seas such as the mediterranean can present higher conductivity values that, in any case, are not higher than 4 s/m. the relatively high conductivity of seawater screens the propagation of electromagnetic (em) variations. this can be easily understood by introducing the so-called skin depth δ (expressed in meters), of an em variation of period t in a medium of conductivity σ as the propagation distance in a homogeneous conductor over which em waves are attenuated by a factor of e−1, i.e.: δ= 503 (t/σ)1/2 with t in seconds and σ in s/m (filloux, 1987). from the fourier power spectrum of natural magnetic variations in open oceans, it can be deduced that there is no significant attenuation for periods longer than 30 min. at periods shorter than 20 min, seafloor magnetic variations undergo considerable attenuation. if one needs a very high accuracy in studying magnetic variations between 1 day and 1 h, water motions must be taken into account. however for many general purposes they can be neglected since they are one or two orders of magnitude smaller than natural variations; otherwise they should be monitored and then removed. in summer 1998, the benthic station of the european project geostar (geophysical and oceanographic station for abyssal research; a project in the framework of the marine science and technology program, mast iii) was deployed for the first time in the adriatic sea, running for around 1 month and the magnetometers provided a good set of magnetic data (e.g., de santis et al., 1999). the geostar deep-sea mission was then performed from september 25, 2000 till march 16, 2001 close to ustica island (38°32l24mn, 12°46l30me; sicily, italy). details on the geostar project and its deployment and observational system can be found in some recent literature (e.g., beranzoli et al., 1998, 2000, 2003; gasparoni et al., 1998, 2002; marvaldi et al., 1998, 2002; de santis et al., 1999; favali et al., 2002), therefore here we present only a brief description of the magnetic experiment of the deep-sea mission and some preliminary analysis of the magnetic data. the next section introduces the aims of the project and some information about magnetic measurements within the geostar project. the next section is dedicated to the calibration and estimation of the orientation of the magnetometers. two sections then describe the magnetic data together with some spectral analysis; another section shows some results obtained from the method of geomagnetic deep sounding. finally the conclusions assess the most important results and findings. 2. seafloor magnetic measurements within geostar in terms of the magnetic experiment, the geostar project had the following objectives: – contribution towards achieving an even distribution of observatories around the world in order to improve the reliability of global 683 deep seafloor magnetic observations under geostar project models (such as the international geomagnetic reference field, igrf), and regional (mediterranean) models of the magnetic field. – study of geomagnetic temporal variations: a) long period (e.g., secular variation); b) short period (studies of solar diurnal variation, geomagnetic storms and substorms at mid latitudes). concerning long period variations, particular attention should be paid to the study of some peculiar aspects of the secular variation such as, for instance, the so-called jerks. jerks are sudden changes in the rate of the secular variation that occur on a time scale of 1-2 years (e.g. courtillot and le mouël, 1984). although they are among the most investigated subjects of geomagnetism, their origin and their nature are still a matter of debate: seafloor magnetic field measurements might contribute to the elucidation of this intriguing phenomenon when compared with simultaneous ground data. – study of earth’s conductivity structure: contemporary magnetovariational investigations on the seafloor and at the surface allow, in principle, the detection of conductive layers in the oceanic crust and upper mantle; this kind of information is very important since electrical conductivity is closely related to temperature, phase and composition of materials. – coincidence with oersted (1999-present) and champ (2000-present) satellite missions. since the study of the radial variation of the magnetic field has always been one of the most interesting aspects of geomagnetism, it is very relevant to have contemporary magnetic measurements taken at three such different altitudes, i.e. from 700 to 300 km (satellite altitude), around 0 km (i.e. at the earth’s surface), a few km under the sea level (seafloor observations). two magnetometers were installed on the geostar benthic station for the tyrrhenian deep-sea mission: a scalar overhauser gsm19l proton magnetometer by gem system inc. (canada) and a three-axis suspended magnetometer, designed and built in the laboratories of istituto nazionale di geofisica e vulcanologia (rome and l’aquila, italy). both sensors were protected from high pressure by two suitable benthospheres. the omnidirectional sensor of the scalar magnetometer provided the total intensity of the geomagnetic field with a nominal resolution of 0.1 nt, an absolute accuracy of 1 nt and a power consumption of 1 w at a sampling rate of 1 value per minute. the fluxgate three-component sensor was suspended to ensure its verticality. its resolufig. 1. october 5, 2000 – l’aquila geomagnetic observatory (thick line) and geostar (thin line) x, y, z components. x, y, z are north, east and vertically downward magnetic components, respectively. an appropriate baseline must be added to each component. 684 angelo de santis et al. tion was 0.1 nt, with an absolute accuracy of 510 nt and a power consumption of 2 w at a sampling rate of 6 values per minute. its analog outputs were digitised by a 16 bits a/d converter, achieving a quantisation step size of 1.4 nt in order to cover all possible ranges of the geomagnetic field. the geostar deep-sea mission of 20002001 provided 4123 h of geomagnetic data. a sudden failure in the electronic device of the scalar magnetometer occurred at the very beginning of the mission and caused a reduction of the planned sampling rate of 1 value per minute and the achievement of a rate of only 1 value every twelve minutes. in figs. 1 and 2 we show a comparison between geostar and l’aquila observatory (42°23ln, 13°19le, 0.7 km altitude) vector magnetic components and total intensity of the magnetic field for october 5, 2000 (characterised by moderate magnetic activity) and march 15, 2001, respectively. in order to have a reference station closer than l’aquila and in close proximity to the geofig. 3. location of geostar site, ustica and gibilmanna (sicily, italy). fig. 2. march 15, 2001 – l’aquila geomagnetic observatory (continuous line) and geostar (crosses) total field intensity, f. fig. 4. electronic equipment (top) and the three-axis fluxgate sensor (bottom) for the land stations. 685 deep seafloor magnetic observations under geostar project star site, a land station was installed at ustica island (38°42ln, 13°11le, 0.2 km altitude) and another at gibilmanna (37°59ln, 14°01le, 1 km altitude) (see fig. 3). each station was based on a vector fluxgate sensor, a control unit and a data logger (fig. 4), using a solar panel for power supply. moreover, at the beginning and at the end of the mission, absolute (static) measurements of the magnetic declination and inclination at gibilmanna and at ustica island were performed. such measurements were necessary to calibrate and validate the recordings made by the variographs. 3. calibration and orientation of geostar frame two important considerations must be taken into account when analysing the geostar dataset: firstly the calibration and compensation for magnetic materials in the other instruments, and in the supporting structure of the module; secondly the orientation offset of the structure during the mission with respect to the true magnetic north. for convenience, a dedicated calibration was carried out in the italian magnetic observatory of l’aquila (central italy) some days after the recovery of the benthic station (fig. 5). a comparison between the simultaneous recordings of the magnetic field components from l’aquila and from the magnetometers mounted at the end of the booms on the submarine module, gives a quantification of the artificial offset due to geostar. the compensation model used can be expressed as follows: bref = bgeos − bperm − δ⋅bgeos where bref is the magnetic field as recorded by the reference observatory, bgeos is the magnetic field as observed by geostar, bperm is the permanent field due to the station frame’s metal parts, and δ is a 3×3 matrix of constant coefficients characterising the induced field (seen by geostar). δ and bperm are unknowns with nine and three parameters, respectively. for instance, for the x direction, we have (3.1) where bx, by and bz are the cartesian components of the geomagnetic field in the different coordinate references. +bxby$+ δbx bx= $bx bz ref geos perm geos geos geos xx xy xz $+ δ δ 6 @ fig. 5. geostar submarine module (frame) at l’aquila during calibration and comparison with magnetic data from observatory. magnetometers are mounted at the extremity of the oppositely extended booms. 686 angelo de santis et al. first we noticed that the original bygeos and bxgeos had to be exchanged; then, the sign of bzgeos had to be reversed. from now on we will denote the cartesian components observed by geostar as those obtained after these changes. we see that eq. (3.1) has two separate contributions, the permanent and induced parts, respectively. inverting eq. (3.1) for the parameters δ when considering different orientations of the station with respect with true north, we found that the induced part is practically negligible; for what concerns the permanent part, we estimated the following values: bxperm = 1075.4 nt, byperm = 448.2 nt, and bzperm = 1077.6 nt. apart from its intermittent difficulties, the scalar magnetometer provided values that need a correction of −295 nt. an estimated orientation of the whole station frame during the final release at a depth of 2000 m under the sea surface was known from the compass included in the 3d-acm (threecomponent acoustic current meter), which gave an angle of +242º (counter-clockwise from north). an in-depth evaluation of the geostar frame orientation was made comparing data recorded by geostar with data from the two land stations of ustica island and gibilmanna and from l’aquila observatory. the comparison was made using least squares regression to discriminate between small or large angle of rotation, as shown in fig. 6a,b respectively. to check the method we compared the horizontal x, y components as recorded at l’aquila and castello tesino (46°03ln, 11°39le, 1.2 km altitude) observatories for a day of moderate magnetic activity (february 15, 2001) and then we calculated the rotation offset between l’aquila observatory-geostar and ustica station-geostar. the compensation angle between the corresponding orthogonal sensor frames had a median value of –1.9º, which is a small deviation, as expected for reference systems of observatories. fig. 7. results of the orientation analysis between l’aquila observatory and geostar. fig. 6a,b. two possible orientations of the geostar frame. a b 687 deep seafloor magnetic observations under geostar project for the same day we calculated the median (and the mean) values for the couples l’aquilageostar (fig. 7) and ustica-geostar, obtaining respectively −120.64º (mean −120.98º) and −119.97º (mean −123.73º) which confirm, within a few degrees, the values provided by the compass in the 3d-acm. 4. data description data validation for the whole geostar dataset was achieved by reducing any manipulation of the original recordings as much as possible. each component was reconstructed applying the rotation and calibration corrections as described in the previous section to the vector magnetometer recordings. the six values per minute were averaged to produce 1-min values after applying a spike remover filter which acted on 6 to 15 points for each day. a list of all missing data segments (for both vector and scalar recordings) is given in table i. the scalar magnetometer provided a total of 20 615 available data. the leakage of data corresponds to the ratio of available data over the table i. missing data segments of the geostar deep-sea mission. data starting time ending time 27/09/2000 13:00 13:25 30/09/2000 13:00 14:25 03/10/2000 03:00 04:25 08/10/2000 12:00 13:25 09/10/2000 08:00 08:25 20/10/2000 20:00 21:25 26/10/2000 05:00 05:25 04/11/2000 07:00 07:25 16/11/2000 02:00 03:25 02/12/2000 00:00 01:25 02/12/2000 20:00 20:25 03/12/2000 00:00 00:49 20/12/2000 22:00 22:25 06/01/2001 01:00 01:25 25/01/2001 16:00 16:25 28/01/2001 16:00 16:25 22/02/2001 09:00 10:25 26/02/2001 05:00 05:25 28/02/2001 13:00 13:25 03/03/2001 01:00 02:25 fig. 8. six months of seafloor recordings. cartesian components and derived f are displayed. daily variations and some magnetically active periods are clear in all curves. 688 angelo de santis et al. fig. 9. comparison between derived and measured f and their difference. parameters of linear fits on their smoothed curves are also reported. fig. 10. total geomagnetic field f: comparison between geostar and l’aquila observatory. dataset time windows: september 25, 2000 at 11:00 – march 16, 2001 at 00:59 – tick mark at 23:59 of the day. total duration (in minutes) of the mission. for the scalar data we have about 91.6% of missing data. the vector magnetometer provided a total of 1 484 282 available data but between 6 and 15 spikes are present for each 24-h segment of recording. in this case we have about 0.2% of missing data. the total intensity f of the magnetic field was then derived as the square root of the sum of the squares of the three re-oriented and calibrated components. 689 deep seafloor magnetic observations under geostar project the three geomagnetic components together with the f values obtained in this way are displayed in fig. 8. the total intensity directly measured by the scalar magnetometer mounted on geostar is displayed in fig. 9 (third horizontal panel) as single points, since the instrumentation collected only 1 data point every 12 min. the difference between derived and measured f values is shown as scattered points in the lower part of the upper panel of fig. 9. from this panel an adjustment of the system in the first 10 days is clear, after which the value of difference is quite stable with an average value of 294.88 nt (sd = 1.15 nt). a greater spread of points in the initial three months was due to strong magnetic activity occurring at that time. this reflects the instability conditions during strong magnetic field activities. besides the good agreement between the derived and measured f daily smoothed curves in fig. 9, we observe that the daily variations completely vanish in the linear fit applied to the smoothed curves. this shows the good agreement between the sixmonth increases in the two datasets: that derived at the geostar site and that measured at l’aquila observatory (fig. 10) with a similar value of secular variation of around 42 nt/year. 5. data analysis in order to faithfully compare data recorded at l’aquila observatory with those recorded by geostar in terms of the energy and dynamics of the corresponding time series, we chose three days characterised by different geomagnetic activity and evaluated their power spectra. the k activity index computed from l’aquila observatory geomagnetic recordings allows us to make an appropriate selection of data segments to be used for both power spectra analysis and electromagnetic induction studies (fig. 11). for our purposes we considered the daily sum of the corresponding eight k-indices, calling this value the daily total k-index. then we selected three days with different levels of magnetic activity. fig. 11. comparison between total magnetic f and l’aquila observatory daily total k-index. 690 angelo de santis et al. fig. 12a-d. spectral analysis of geostar (a, c) and l’aquila (b, d) total intensity f for october 5, 2000 (a, b) and january 01, 2001 (c, d). from left: temporal plot, spectrum with frequency and period at the abscissa. the corresponding daily total k-index was 43 (a, b) and 3 (c, d). a b c d 691 deep seafloor magnetic observations under geostar project the selected days are october 5, 2000 (daily total k-index = 43; strong magnetic activity), january 1, 2001 (daily total k-index = 3; low magnetic activity), and january 22, 2001 (daily total k-index = 21; moderate magnetic activity). the analysis of each daily segment of data (1440 data points) provides suitable spectral information over the frequency range from 0 to 0.5 cycle/min. the cases examined show in particular a difference between the l’aquila and geostar power spectra, which becomes less and less evident with decreasing total k-index (fig. 12 a-f), i.e. with decreasing geomagnetic activity. this difference consists in a larger content of high frequency energy at l’aquila than at the geostar site in the case of high geomagnetic activity, on the contrary the low frequency energy content remains of the same order of magnitude for any activity level. this behaviour can be explained by the screening effect of the seawater, especially on short period components and it is more marked when the total k-index has large values as in the case of october 5, 2000 when it was 43. in this case the high frequency portion of the power spectrum of geostar data asymptotically approaches values that are at least two orders of magnitude lower than the analogous spectrum obtained for the signal simultaneously recorded at l’aquila. on the contrary low frequency variations affect measurements taken at l’aquila and those taken at the geostar site practically in the same way. the seawater screening effect is also confirmed in the following description of how the induction mechanism dominates at longer periods. 6. preliminary analysis of em induction and electrical conductivity a combination of the vertical and horizontal components of the fluxgate magnetometer dataset allows the estimation of the so-called parkinson arrows (or induction arrows) of the gefig. 12e,f. spectral analysis of geostar (e) and l’aquila (f) total intensity f for january 22, 2001. from left: temporal plot, spectrum with frequency and period at the abscissa. the corresponding daily total k-index was 21. e f 692 angelo de santis et al. omagnetic depth sounding (gds) technique (e.g., armadillo et al., 2001). they provide both quantitative and qualitative indications of the presence of conductivity contrasts near the point of measurement from the complex ratios (i.e. the so-called transfer functions) of the fourier transforms of the horizontal and the vertical magnetic components at different periods (the longer the periods the deeper the corresponding sounded layer). in the graphic representation, the real parts of such arrows are directed towards regions with higher conductivity. this technique can be successfully applied also in the band limited seafloor environment. we applied the gds technique to the time variations of the geostar magnetic data for a preliminary selection of 12 events (duration: 512 min) from 6 magnetically undisturbed days (not shown here). in general, most of the arrows become larger as the period increases. this confirms that at the geostar site (i.e. at a depth of 2000 m) the bulk of electromagnetic induction generated by variations in the geomagnetic field contains mainly long period components. moreover, most of the arrows point toward north or north-east, indicating that a higher electrical conductivity contrast is located along these directions with respect to ustica island. this anomaly could be possibly due to the asthenospheric upweeling of the tyrrhenian basin. we expect that this aspect will be better investigated by analysing the whole dataset. 7. conclusions the magnetic experiment of the geostar project succeeded in providing reliable magnetic data for most of the period of the deep water mission. techniques were applied in order to calibrate and orientate the geostar magnetometers. comparison with ground stations (gibilmanna and ustica sites) and observatories (l’aquila and castello tesino) showed a good agreement, confirming the quality of the data. power spectral analysis was performed on selected data segments (characterized by the daily total k-index). the results showed that the energy in the power spectra at short periods is larger at l’aquila than at the geostar site especially when the magnetic activity is high. on the contrary, the energy involved at longer periods is of the same order of magnitude at the geostar site and at l’aquila for any level of geomagnetic activity. finally, induction studies showed that a higher electrical conductivity contrast is located north-north-east of the ustica island. in the future, more detailed analyses will be needed to better evaluate the data, together with a careful comparison with the corresponding satellite data from the oersted and champ missions (e.g., reigber et al., 2005). acknowledgements the geostar project was funded by the ec under the marine science and technology programme (contract n. mas3-ct95-0007). we thank all the people who were involved directly or indirectly in the geostar project. among them, giuseppe («pino» to his friends) smriglio merits special mention for his great patience, strength and humanity expressed to everybody during this time. his premature departure has left a great emptiness in all of us. we thank david barraclough, mioara mandea and antonio meloni for providing important comments for improving the original version of the paper. references armadillo, e., e. bozzo, v. cerv, a. de santis, d. di mauro, m. gambetta, a. meloni, j. pek and f. speranza (1998): geomagnetic depth sounding in the northern apennines (italy), earth planets space, 53, 385-396. beranzoli, l., a. de santis, g. etiope, p. favali, f. frugoni, g. smriglio, f. gasparoni and a. marigo (1998): geostar: a geophysical and oceanographic station for abyssal research, phys. earth planet. inter., 108 (2), 175-183. beranzoli, l., t. braun, m. calcara, d. calore, r. campaci, j.-m. coudeville, a. de santis, d. di mauro, g. etiope, p. favali, f. frugoni, j.-l. fuda, f. gamberi, f. gasparoni, h.w. gerber, m. marani, j. marvaldi, c. millot, p. palangio, g. romeo and g. smriglio (2000): geostar: the first european long term seafloor observatory, eos, trans. am. geophys. un., 81 (5), 45-49. beranzoli, l., t. braun, m. calcara, p. casale, a. de 693 deep seafloor magnetic observations under geostar project santis, g. d’anna, d. di mauro, g. etiope, p. favali, j.-l. fuda, f. frugoni, f. gamberi, m. marani, c. millot, c. montuori and g. smriglio (2003): mission results from the first geostar observatory (adriatic sea, 1998), earth planets space, 55 (7), 361373. courtillot, v. and j.l. le mouël (1984): geomagnetic secular variation impulses, nature, 311, 709-716. de santis, a., d. di mauro, p. favali, p. palangio, g. romeo and g. smriglio (1999): geostar project: the performed seafloor mission in the adriatic sea, in proceedings of marelec ‘99 conference, brest, france, 219-230. favali, p., g. smriglio, l. beranzoli, t. braun, m. calcara, g. d’anna, a. de santis, d. di mauro, g. etiope, f. frugoni, v. iafolla, s. monna, c. montuori, s. bozzoli, p. palangio and g. romeo (2002): towards a permanent deep-sea observatory: the geostar european experiment, in science-technology synergy for research in marine environment: challenges for the xxi century, edited by l. beranzoli, p. favali and g. smriglio, development in marine technology series (elsevier, amsterdam), 12, 111-120. filloux, j.h. (1987): instrumentation and experimental methods for oceanic studies, in geomagnetism, edited j. jacobs (academic press), vol. 1. gasparoni, f., d. calore, r. campaci and a. marigo (1998): geostar – development and test of an innovative benthic station for long-term observations at abyssal depths, in proceedings of the ieee conference oceans ’98 (on cd-rom). gasparoni, f., d. calore and r. campaci (2002): from abel to geostar: development of the first european deep-sea scientific observatory, in science-technology synergy for research in marine environment: challenges for the xxi century, edited by l. beranzoli, p. favali and g. smriglio, development in marine technology series (elsevier, amsterdam), 12, 143-159. marvaldi, j., j. blandin, ch. podeur, j.-m. coudeville, j. antoine, d. barbot, d. fellmann and d. rhodes (1998): geostar – development and test of a communication system for deep-sea benthic stations, in proceedings of the ieee conference oceans ‘98 (on cd-rom) . marvaldi, j., y. aoustin, g. ayela, d. batbot, j. blandin, j.-m. coudeville, d. fellman, g. loaëc, c. podeur and a. priou (2002): design and realisation of communication systems for the geostar project, in science-technology synergy for research in marine environment: challenges for the xxi century, edited by l. beranzoli, p. favali and g. smriglio, development in marine technology series (elsevier, amsterdam), 12, 161-181. reigber, c., h. luhr, p. scwintzer and j. wickert (editors) (2005): earth observation with champ: results from three years in orbit (springer, berlin), pp. 628. devoti_corretto_layout 6 781 combination of coseismic displacement fields: a geodetic perspective roberto devoti istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6119 1. introduction this study provides the mathematical framework for the rigorous combination of coseismic offsets observed by a global positioning system (gps) network and investigates the results obtained on the occasion of the recent emilia earthquakes (italy). this seismic sequence that affected northern italy from may 20, 2012, allowed two offset fields to be computed, one with reference to the mainshock (m 5.9, followed by two other m 5.1 events on the same day), and a second with reference to the replicas that occurred on may 29, 2012 (m 5.8, m 5.3 and m 5.2; iside data archive, http://iside.rm. ingv.it). the final displacement field is basically the result of a comparison and validation process with repeated feedback between the different analysis groups at the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology) that was established to obtain prompt coseismic displacement solutions, as precise as possible, and in the first days after an event. this is important for early seismic-source evaluation as it represents the most complete and validated dataset at the very early stage of a seismic crisis, and it is also extremely useful in reducing random and systematic errors in the estimated parameters. this study is the result of a cooperative effort that involved different research groups at ingv, with the sharing of all of the collected gps data. the intention was to compare these results and thus reducing sources of error associated with individual processing strategies, to allow the final combination of the different displacement fields into a single consensus solution. the process assessed the robustness of each single gps result, thus minimizing erroneous interpretations of individual solutions. for more information about the interpretation of the coseismic displacements, see serpelloni et al. [2012, this volume]. the combination procedure can be easily generalized to other geodetic parameters, and might be useful whenever a validated dataset is desirable or is intrinsically required (i.e., near real-time observations of unknown surface processes, tectonic motion of large geographic areas, angular momentum observations of the whole earth, coseismic surface deformation patterns). other important advantages of the combination process include efficient outlier detection via cross-checking of independent solutions, and realistic estimates of the displacement uncertainties. it is important to emphasize here that in this kind of activity, the cooperative behavior between individual researchers and research groups should prevail over competitive and self-assertive behaviors, because the individuals (input solutions) become part of the same objective (validation). on the other hand, the validation process should not overlap or replace the research activity; i.e., there should always be space for individual initiatives. in this respect, the cooperative effort spent in constructing a prompt and validated product is a distinctive activity that involves both technological and scientific aspects, and it certainly stimulated the individual research activities. methods for comparison and combination of geodetic network solutions were introduced and developed in several studies in the early 1990's [heflin et al. 1992, boucher and altamimi 1993, davies and blewitt 2000, and references therein]. it is a particular least-squares task in which the input data types are the same as the output parameter types and the covariance matrices of the input datasets are fully populated (because each dataset is the output parameter set of an earlier least-squares analysis). the parameters that can be combined can be three-dimensional (3-d) station positions at a stated epoch, constant 3-d station velocities, earth-orientation parameter time series, or coseismic station offsets. in principle, each of the aforementioned parameter types, or a subset of them, can be treated in the same mathematical framework and combined into a unique average solution with only slight modifications to the formulation given below. 2. coseismic displacement estimation each analysis group analyzed the gps data using its own procedures and models. in our case, they provided three time series obtained with the bernese (http://www. bernese.unibe.ch/), gamit (http://www-gpsg.mit.edu/ ~simon/gtgk/), and gipsy/oasis (https://gipsy-oasis.jpl. nasa.gov/) software. time series of daily station positions article history received july 19, 2012; accepted august 30, 2012. subject classification: crustal deformation, data processing, gps coseismic displacement, combination. 2012 emilia earthquakes devoti 782 10 oe 11 oe 12 oe 44 on 20' 40' 45 on 20' 40' 46 on bass blgn bolc bolg bolo bor2 borr bras brea bris btac cafv carp codi coll conc darf faez ferr fera firefosc foza grzm guas imol itim legn lero maga mant medi mo05 mode mops msel mtrz parm parr pers pto1 rams rave rebo regg rovrsalo sbpo schi serm sgip sir2 teol tre3 verg vero bobb cast crem ldns pado pia1 rovi taro vice vr02 asia bosc brix cgia citd crea gari itrn lasp odez port rmn2 ronc rove sdna tgpo tvso valx velo ven1 10 mm 10 oe 11 oe 12 oe 44 on 20' 40' 45 on 20' 40' 46 on bass blgn bolc bolgbolo bor2 borr bras brea bris btac cafv carp codi coll conc darf faez ferr fera firefosc foza grzm guas imol itim legn lero maga mant medi mo05 mode mops msel mtrz parm parr pers pto1 rams rave rebo regg rovr salo sbpo schi serm sgip sir2 teol tre3 verg vero bobb cast crem ldns pado pia1 rovi taro vice vr02 asia bosc brix cgia citd crea gari itrnlasp odez port rmn2 ronc rove sdna tgpo tvso valx velo ven1 sir2 10 mm figure 1. combined (red arrows) and individual solution offsets of the horizontal (top) and vertical (bottom) components associated with the may 20, 2012, seismic events. the individual solutions are represented in green (bernese), cyan (gipsy) and blue (gamit). stars, epicenters of the m >5 events; circle, delimitation of the 50-km range from the mainshock (m 5.9); error ellipses, one-sigma confidence regions. 783 combination of coseismic displacements 10 oe 11 oe 12 oe 44 on 20' 40' 45 on 20' 40' 46 on 5603 5901 5902 6903 bass bolc bolgbolo bor2 borr bras brea btac cafv carp cast codicoll conc darf faez ferr fera fire foza grzm itim legn lero maga mant medi mo05 mode msel mtrz parm parr pers pto1 rams rebo regg rovr sbpo schi serm sgip sir2 teol verg vero 5904 blgn crem guas ldns mops pado rave rovi vice vr02 bosc brix cgia citd gari imol pala pia1 piac salo tgpo 10 mm 10 oe 11 oe 12 oe 44 on 20' 40' 45 on 20' 40' 46 on 5603 5901 5902 6903 bass bolc bolg bolo bor2 borr bras brea btac cafv carp cast codi coll conc darf faez ferr fera fire foza grzm itim legn lero maga mant medi mo05 mode msel mtrz parm parr pers pto1 rams rebo regg rovr sbpo schi serm sgip sir2 teol verg vero 5904 blgn crem guas ldns mops pado rave rovi vice vr02 bosc brix cgia citd gari imol pala pia1 piac salo tgpo 10 mm figure 2. combined (red arrows) and individual solution offsets of the horizontal (top) and vertical (bottom) components associated with the may 29, 2012, seismic events. the individual solutions are represented in green (bernese), cyan (gipsy) and blue (gamit). stars, epicenters of the m >5 events; circle, delimitation of the 50-km range from the main event (m 5.8); error ellipses, one-sigma confidence regions. were computed in the international terrestrial reference frame (itrf)2008 reference framework [altamimi et al. 2011], which provided three distinct time series of a regional gps network located around the epicenter, in north-eastern italy (see figures 1, 2, for maps of the gps network). to estimate the coseismic offsets we considered only a single day of observations after the event, to minimize the effects of early post-seismic displacements, which can be eventually caused by afterslip or poroelastic rebound [e.g., peltzer et al. 1996, mahsas et al. 2008]. fast exponential decays in the very early phase of the seismic cycle are commonly observed in a wide range of seismic events that cause deformations of up to several centimeters, depending on the magnitude and the driving mechanism [segal et al. 2000, yu et al. 2003, devoti et al. 2012a]. therefore, the coseismic offsets were estimated using 15 days before the epoch of the earthquake (whenever possible), and one day after the event, with the following days being used only to check the consistency of the coseismic displacement at the given station. we checked for outliers in the time series using the detrended 3-d position residuals, using different criteria for before and after the event. before the earthquake, a point was rejected if its residual exceeded the weighted-root-mean-squared (wrms) of all of the residuals by three times; after the event instead, if the difference between consecutive points exceeded 3 times the wrms, we computed the weighted average between them. this latter condition was applied to only a few sites in the near field: conc, msel and sbpo. all of the offsets were estimated directly in the same least-squares inversion, to obtain the estimates of the vector and the associated network covariance matrix for each of the three processing schemes. figures 3 and 4 show the residuals of the estimated station offsets (namely the wrms of the residuals between pairs of solutions) in the near field (<50 km from the epicenters). this parameter measures the repeatability of the estimates across independent solutions, and it is extremely useful for the isolating of systematic errors of the individual solutions. for the may 20, 2012, event, the overall average wrms was 2.1 mm in the horizontal plane and 4.2 mm in the vertical direction; for the may 29, 2012, event, the wrms was 2.6 mm in both components. the worst case was observed for station mo05 (finale emilia), where the gipsy solution differed on average by 15 mm from the other two, in the vertical direction. nevertheless, given the high uncertainties of the vertical gps estimates, this difference can still be considered acceptable. 3. displacement field combinations consider the linearized observation equation: (1) where y is the column vector of the observations, a is the design matrix, x is the column vector of the functional model devoti 784 0 2 4 6 8 1 0 1 2 1 4 solution residuals (mm) m a y, 2 0 blgn bolg bolo btac conc ferr fera guas legn mant mo05 mode mops pers regg sbpo serm sgip ldns h o riz v e rt 0 1 2 3 4 5 6 m a y, 2 9 solution residuals (mm) blgn bolg bolo btac codi conc ferr fera guas ldns legn mant mo05 mode mops pers regg sbpo serm sgip 5603 5901 5902 5904 6903 h o riz v e rt figure 3. cumulative wrms of the station offset differences between the individual may 20, 2012, solutions (bernese, gamit and gipsy) for the horizontal (gray) and vertical (open bar) components. figure 4. cumulative wrms of the station offset differences between the individual may 29, 2012, solutions (bernese, gamit and gipsy) for the horizontal (gray) and vertical (open bar) components. ay x= + o 785 parameters, and o is a column vector of random errors. in our case, the observations and the model parameters are of the same nature, and therefore the linearized observation equations are simply: (2) where xs and xc are the offsets (observations) of the solution s and the combined parameters c (estimates), respectively, while the design matrix is a simple reordering matrix, as observed by davies and blewitt [2000], which reparameterizes the parameter list from the combined offsets to the solution offsets (i.e., the component aij = 1 if element i of list c is the same as element j of list s). a notable property of the design matrix is that: (3) i.e., transposing the reordering matrix from list c to list s corresponds to reversing the ordering from list s to list c. to simplify the mathematical formulation, we adopt the convention that ; i.e., the design matrix of the i-th solution (the reordering matrix from solution s = i to the combined solution c) is denoted simply as ai. the combination of the individual solutions is obtained by solving for the unknowns xc in a least-squares sense, and a useful recursive solution can be easily derived: (4). from each input solution (usually in sinex format), we can extract the estimate parameter vector xi and its covariance matrix ci, compute the reordering matrix ai, and finally cumulate the normal matrix and vector, as the first and second factors, respectively, of the right-hand side of equation (4). the relative weight of the input solutions is imposed by the condition that the contribution to the total |2 of each individual solution is equally balanced [devoti et al. 2012b]: (5). from the first preliminary combination, a variance factor fi is estimated for each solution, and then applied to the covariance matrix of the corresponding solution in an iterative way, until the global |2 reaches unity. this procedure assures that each solution contributes equally to the global |2, which avoids an individual solution from dominating or any suffering from the combination of the individual solutions (i.e., over-weighted or down-weighted solutions). it might happen that the gps stations are geographically clustered, especially in urban areas where there are different installations over inter-distances of a few kilometers. in this case, it is useful to constrain the clustered offsets, estimating them only once. if the inter-distances of the gps stations are small with respect to the coseismic deformation pattern, i.e., of the order of 1 km, then clustered observations will rather bias any inverse solution because repeated information at the same site might artificially over-weight that particular site, thus providing biased solutions. this can be handled during the combination estimation process by imposing the constraint: (6) whenever sites i and j have to be estimated together. the constraints were imposed using the classical lagrange multipliers technique [e.g., tarantola 2005] that provides the constrained least-squares solution with a fully populated covariance matrix of the combined parameters. 4. results for the emila earthquake figures 1 and 2 show the combined offsets (red arrows) of the two seismic events (the may 20 and 29, 2012, events, respectively) projected in the local horizontal and vertical directions. the maximum horizontal offset was observed at mo05, the closest site to the mainshock epicenter (horizontal, 30 mm; vertical, 73 mm). the offsets caused by the may 29, 2012, events were generally smaller than the offsets caused by the mainshock, although it has also to be noted that the available gps network did not correctly sample the expected surface deformation, and therefore the measured offsets might be aliased. the individual solutions are also shown with different colors in figures 1 and 2, to have a first glance of their repeatability. apart from any apparently random differences, we will show that systematic bias might also characterize the individual solutions. a first validation of the individual solutions was carried out by plotting the offsets measured outside an area of 50-km radius from the epicenter. ideally, the far-field offsets should be randomly distributed (if not affected by the earthquake), and they represent a good test case to determine any systematic errors. figures 5 and 6 show the histograms of the far-field offsets for the two events, where the most notable features are non-zero medians for the individual solutions; e.g. for may 20, 2012, a median vertical component of -2.1 mm (gamit), 3.0 mm (gipsy) and north component of 2.1 mm (gipsy), and less pronounced bias for the may 29, 2012, event. the combined offsets show instead a general bias reduction of the far-field offsets, indicating that the combined solution is in fact the best unbiased representation of the displacement field. the combined solution also shows a significant reduction in the formal errors (table 1): both events show (on average) 1.2 mm one-sigma error in the horizontal plane, and about 3.1 mm in the vertical direction, whereas the individual solutions show (on average) a 50% higher sigma. combination of coseismic displacements x a xs cc s= + o" a c s" a a c s t cs = " "^ h a ai i c= " ... ... x a c a a c a a c x a c x 1 1 1 1 2 2 1 2 1 1 1 1 2 2 1 2 c t t t t 1= + + + + + --^ ^h h x x c x x x x c x x f f 1 1 c i t i i c i c j t j j c j = = ^ ^ ^ ^ ^ ^ h h h h h h x xi j= devoti 786 0 2 4 6 8 10 0 2 4 6 8 10 -10 -5 0 5 10 0 2 4 6 8 10 combined 0 2 4 6 8 10 u p 0 5 10 15 e a st -10 -5 0 5 10 0 2 4 6 8 n o rt h gamit 0 1 2 3 4 5 0 1 2 3 4 -10 -5 0 5 10 0 1 2 3 4 5 residuals (mm) gipsy 0 1 2 3 0 1 2 3 4 5 -10 -5 0 5 10 0 1 2 3 bernese 0 2 4 6 8 10 u p 0 2 4 6 8 10 e a st -10 -5 0 5 10 0 2 4 6 8 10 12 n o rt h gamit 0 2 4 6 8 0 2 4 6 8 -10 -5 0 5 10 0 2 4 6 8 gipsy residuals (mm) 0 2 4 6 0 1 2 3 4 -10 -5 0 5 10 0 1 2 3 4 bernese 0 5 10 15 0 5 10 -10 -5 0 5 10 0 5 10 15 combined figure 5. distribution of the far-field offsets for the individual (bernese, gamit and gipsy) and combined may 20, 2012, solutions. red line, bestfit gaussian curve. the median (m) and dispersion ( ) are computed for the three spatial directions (up, east, north). figure 6. distribution of the far-field offsets for the individual (bernese, gamit and gipsy) and combined may 29, 2012, solutions. red line, bestfit gaussian curve. the median (m) and dispersion (v) are computed for the three spatial directions (up, east, north). event bernese gamit gipsy combined may 20, 2012 1.8 / 5.9 1.6 / 4.0 2.1 / 5.1 1.2 / 3.1 may 29, 2012 1.8 / 6.1 1.7 / 4.5 1.8 / 4.4 1.2 / 3.2 table 1. average formal errors (1-sigma) of the estimated coseismic offsets. the two values represent the horizontal and vertical components, respectively, of the uncertainties. 787 acknowledgements. i would like to offer my special thanks to all of those who helped in carrying out the gps analysis at the ingv, to all who archived and provided gps observations: ring, euref, asi, irealp, regione veneto, igm, italpos leica geosystems, fondazione geometri e geometri laureati dell'emilia romagna, department of earth sciences of the university of siena, and department of physics of the university of bologna, for providing data from the mo05 station, and geotop s.r.l. for providing data from the conc station. finally, my thanks also go to all of those who participated in the post-seismic gps campaign and the field assistance. references altamimi, z., x. collilieux and l. métivier (2011). itrf2008: an improved solution of the international terrestrial reference frame, j. geodesy, 85, 457-473; 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(b. soc. geol. ital.), 131 (3), in press. devoti, r., e. flammini, g. pietrantonio, f. riguzzi and e. serpelloni (2012b). toward a dense italian gps velocity field: data analysis strategies and quality assessment, in: n. sneeuw et al. (eds.), vii hotine-marussi symposium on mathematical geodesy, international association of geodesy symposia, 137, doi:10.1007/978-3-64222078-4_51. heflin, m., et al. (1992). global geodesy using gps without fiducial sites, geophys. res. lett., 19 (2), 131-134; doi: 10.1029/91gl02933. mahsas, a., k. lammali, k. yelles, e. calais, a.m. freed and p. briole (2008). shallow afterslip following the 2003 may 21, mw = 6.9 boumerdes earthquake, algeria, geophys. j. int., 172, 155-166; doi: 10.1111/j.1365-246x.2007.03594.x. peltzer, g., p. rosen, f. rogez and k. hudnut (1996). postseismic rebound in fault step-overs caused by pore fluid flow, science, 273, 1202-1204. segall, p., r. bürgmann and m. matthews (2000). time-dependent triggered afterslip following the 1989 loma prieta earthquake, j. geoph. res., 105, 5615-5634. serpelloni, e., l. anderlini, a. avallone, v. cannelli, a. cavaliere, d. cheloni, c. d'ambrosio, e. d'anastasio, a. esposito, g. pietrantonio, a.r. pisani, m. anzidei, g. cecere, n. d'agostino, s. del mese, r. devoti, a. galvani, a. massucci, d. melini, f. riguzzi, g. selvaggi, and v. sepe (2012). gps observations of coseismic deformation following the may 20 and 29, 2012, emilia seismic events (northern italy): data, analysis and preliminary models, annals of geophysics, 55 (4); doi:10.4401/ag-6168. tarantola, a. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vertical movements along the northwestern side of messina straits, in the 1967-1970 period v . a c h i l l i * * f . b r o c c i o * f . mulargia * received on april 8th, 1982 a b s t r a c t t h e r e s u l t s of a comparison of the spirit levelling data made by i.g.m. in 1967 a n d in 1970, a r e p r e s e n t e d . an analysis of these d a t a shows that the altitude v a r i a t i o n s of b e n c h m a r k s in the considered triennial period seem to originate from t h e c o l l e c t i v e r o t a t i o n of an inclined plane, which intersects the geoid surface with a n a z i m u t h of 43° to the n o r t h . t h i s is in a c c o r d a n c e with the orientation of the fault pattern given by local g e o l o g y . t h e fact t h a t the existing bench marks were sited along the coastline, and the l a c k of a c o m p a r i s o n with d a t a relative to motion on more compact geological s t r u c t u r e s , lead necessarily to provisional conclusions. r i a s s u n t o v e n g o n o r i p o r t a t i i risultati ottenuti da un confronto di livellazioni effettuate d a l l ì . g . m . dal 1967 al 1970. un'analisi di questi dati mostra che le variazioni di a l t e z z a dei c a p o s a l d i , nel triennio preso in esame, s e m b r a n o essere originate da una r o t a z i o n e collettiva di un piano inclinato che interseca la superficie del geoide con u n a z i m u t di 43° n. ciò è in accordo con l'orientamento del modello di faglia dato d a l l a geologia locale. il fatto che i caposaldi esistenti siano situati lungo una linea c o s t i e r a e la m a n c a n z a di un confronto con i dati relativi al moto su s t r u t t u r e g e o l o g i c h e più c o m p a t t e , p o r t a necessariamente a conclusioni provvisorie. * i s t i t u t o geofisico e geodetico, università di messina, italy. ** i s t i t u t o nazionale di geofisica, roma, italy. 9 6 v. achilli f. broccio f. mulargia i n t r o d u c t i o n t h e s t r a i t s of messina is one of the most i m p o r t a n t areas, f r o m t h e geological point of view, of the whole mediterranean. d i s a s t r o u s e a r t h q u a k e s repeatedly occurred in this zone (baratta, 1901), r e a c h i n g a m a x i m u m on the 28th december 1908, with a m a g n i t u d e 7.2 e a r t h q u a k e which completely destroyed the cities of m e s s i n a a n d reggio calabria a n d caused over 120,000 victims. c o n s i d e r i n g just one of the relatively recent periods of strong s e i s m i c activity, i.e., the period beginning in 1783, which consisted of s e v e r a l e v e n t s reaching a m a g n i t u d e m > 7.0, we can infer an a l o n g a t e d elliptical mesoseismic area, with m a j o r axis in the n e s w d i r e c t i o n (baratta, 1901). this a r e a extended its bounda r i e s f r o m nicotera to the lower p a r t of aspromonte, while some of t h e following great shocks seem to have origin in the area of m e s s i n a s t r a i t s . b e c a u s e of the complex seismicity of the area, it is very d i f f i c u l t to estabilish precisely which are the seismogenetic s t r u c t u r e s , especially since for most of the events i n s t r u m e n t a l r e c o r d s a r e not available. for t h e greatest e a r t h q u a k e in this area, the 28,h december 1908 one, s o m e i n s t r u m e n t a l records are available. m a n y r e s e a r c h e r s studied this e a r t h q u a k e (schick, 1977), s t a r t i n g f r o m the d e t e r m i n a t i o n of focal p a r a m e t e r s a n d concluded t h a t it is r e l a t e d to the sicilian and calabrian tectonics structures, s h o w i n g a c l e a r correlation with a fault that s t a r t i n g from comiso a n d p a s s i n g t h r o u g h e t n a and messina, reaches s. eufemia (see fig. 1). t h e focal m e c h a n i s m was found to be a n o r m a l fault with d i p p i n g of 70° a n d a z i m u t h aligned on a ne-sw direction. a m a p p i n g of the e a r t h q u a k e s recorded in the last two decades with e p i c e n t e r in this a r e a (see fig. 1), shows a clear correlation with t h e m a i n tectonic a l i g n m e n t s (bottari, 1971), while the focal m e c h a n i s m s of these events show a typical s t r u c t u r e with n o r m a l a n d vertical f a u l t i n g . 2 . g e o l o g i c a l pattern of t h e m e s s i n a s t r a i t s a r e a t h e geological l i t e r a t u r e concerning sicily and calabria has, 15° 10' 20' 30' 40' 50' 16° fig. 1 t h e e p i c e n t e r s of the seismic events instrumentally located in the messina s t r a i t s area in the last 20 years (3). o n a v a r i e t y of ways, supported an overthrust model which has t a k e n t h e p l a c e (bottari et al., 1975) of an earlier model based on t o t a l a u t o c h t o n y given by cortese (1882, 1895); he was the first to r e c o g n i z e t h e messina s t r a i t s fault and to set its origin in the l o w e r to u p p e r pliocene. t r e v i s a n (1954) in a study of quaternary tectonic movements, a r g u e s t h a t these are the continuation of previous movements, and d i v i d e s sicily into four separate units, two of which are the p e l o r i t a n i m o u n t a i n s a n d the ragusa plateau, and the other two 9 8 v. achilli f. broccio f. mulargia a r e s e p a r a t e d by a ne-sw strike faults system in agreement with s e i s m i c activity d a t a . vecchia (1956) considers the fault system in t h e s t r a i t s as active, and, given the continuity of positive isostatic a n o m a l o u s zones, both in the peloritani a n d calabrian areas, f a v o u r s t h e idea of vertical movements along a large fault-system. g l a n g e a u d (1952a, 1952b) formulates a geodynamic model of t h e m e d i t e r r a n e a n area, by supporting the hypothesis of the e x i s t e n c e in the mesozoic of two continental blocks, one african a n d t h e o t h e r e u r o p e a n , divided by sial-free-zone. in t h i s tectonic scheme, sicily and calabria are clockwise t u r n e d a n d s h i f t e d to the south, because they are p a r t of the edge of t h e african c o n t i n e n t a l block. o g n i b e n (1960, 1966, 1971, 1973) taking u p limanowski's i d e a s (limanowski, 1913), develops an alpine type structural m o d e l f o r the centre-southern apennines a n d for sicily. on the b a s i s of t h e g r a v i m e t r i c m a p s and the positions of the axes of i s o s t a t i c u p l i f t , he emphasizes the p r o b a b l e pleistocenic n a t u r e of t h e g r e a t t r a n s c u r r e n t fault of the gela — messina — s. eufemia f a u l t i n g zone, with anticlockwise t r a n s c u r r e n t movement. in the last years, the overthrust models have been given f u r t h e r s u p p o r t by global tectonics. following the a p p r o a c h of the t w o c o n t i n e n t a l blocks of europe a n d africa (including sicily), a s u b c o n d u c t i o n of the oceanic crust began a n d continued u p to the m i d miocene, when the collision of the two blocks w a s acc o m p a n i e d by the f o r m a t i o n of the peloritani m o u n t a i n s (barberi et a l „ 1973; bottari a n d lo giudice, 1975b; caputo et al., 1970; f i n e t t i a n d morelli, 1972; ritsema, 1971). t h e s u b c o n d u c t i o n u n d e r the calabrian arc seems to be c o n t i n u i n g still. the impact of the two continental blocks occurred in d i f f e r e n t phases, progressing in an anticlockwise direction from west t o e a s t . in the lower miocene, e a s t e r n sicily w a s the limit b e t w e e n t h e zone of collision of the continental blocks and the z o n e of s u b c o n d u c t i o n of the oceanic crust u n d e r the calabrian a r c . sicily, therefore, w a s involved in a local distensive tectonics p e r p e n d i c u l a r to the direction of movements of the oceanic crust. t h e origin of the messina abyssal plain a n d the messina cone, w h i c h b e g i n s in the south of the straits, is therefore based on this vertical movements along, ecc. 9 9 d i s t e n s i v e tectonics, opening up in an anticlockwise direction ( b o t t a r i et al., 1975). i n t e r e s t i n g results are also available from geohpysical studies. f r o m t h e o c e a n o g r a p h i c seismic reflection explorations (finetti a n d morelli, 1972), it seems that the african continental shelf c o n t i n u e s b e y o n d the messina s t r a i t s and the ionian sea up to the s o u t h e r n edge of the calabrian arc, and includes the ragusa m a s s i f . f r o m these explorations it is also clear that the african c o n t i n e n t a l shelf, due to a system of subvertical direct faults along t h e e a s t e r n edge of sicily, is still subsiding u n d e r the ionian sea. t h e first result of geodimetric measurements carried out b e t w e e n t h e t w o shores of the messina straits (caputo et al., 1974) gives p a r t i a l s u p p o r t to the idea of a n o r t h w a r d drift of sicily and a n a n t i c l o c k w i s e r o t a t i o n of the calabrian arc. 3 . g r o u n d deformation measurements. in o r d e r to detect vertical movements, we can utilize the v a r i a t i o n in a l t i t u d e of fixed bench-marks, i.e. the variation of the h e i g h t of fixed points on the ground with respect to the geoid s u r f a c e . levelling, which consists of measuring differences in level b e t w e e n t w o or m o r e fixed points, is well suited to investigate s l o w a n d c o n t i n u o u s e a r t h crustal movements. in particular spirit l e v e l l i n g w h i c h is the most accurate levelling technique appears b e s t s u i t e d to this purpose. in t h e 1908, the " i s t i t u t o geográfico militare" (i.g.m.), has b e e n in c h a r g e of a precision geometric levelling in the region m o r e violently shaken by 1908 earthquake. the results of this s u r v e y (loperfido, 1909), are particularly interesting because the p r e v i o u s levelling survey had been completed just a few days b e f o r e t h e d i s a s t r o u s event. unfortunately the latter levelling c o u l d n o t b e referred to the mean sea level because the earthquake d a m a g e d t h e tide gauges. it w a s therefore referred to a reference plane which was a s s u m e d as fixed. 1 0 0 v. achilli f. broccio f. mulargia t h e r e s u l t s of these levelling surveys were corrected in order to set t h e gravity level surface ortometric. dynamic corrections w e r e also m a d e according the clairaut's hypothesis, in order to t a k e i n t o a c c o u n t the defective parallelism of the level surface. t h e r e s u l t s showed a m a x i m u m a l t i t u d e variation of about 71 c m n e a r the h a r b o u r office; variations of the same order of m a g n i t u d e w e r e found along the coastline. with regards to these m a x i m u m a l t i t u d e variations, we must take into account superfic i a l l a n d s l i d e s which took place and were observed along the m e s s i n a coastline (loperfido, 1909). after these, no f u r t h e r survey w a s m a d e , until 1967-70 when i.g.m. carried out two high p r e c i s i o n levelling surveys on two different lines but having some c o m m o n p a t h in the messina s t r a i t s area (istituto geografico m i l i t a r e , 1972). as it will be a p p a r e n t in the following, the results of t h i s survey are highly interesting. 4 . d i s c u s s i o n and r e s u l t s . t h e a n a l y s i s of the geological situation of the area, together at 1908 levelling d a t a , shows a clear tendency to a sinking of the c o a s t w i t h respect to the peloritani m o u n t a i n s . the same tendency is c l e a r i n g a p p a r e n t in the 1967-1970 d a t a . fig. 2 shows the variations of the considered bench-marks d i f f e r e n c e of levels, in the 1967-70 period (table 1). we will see t h a t f u r t h e r m o r e these variations can be ascribed to the pivoting c o l l e c t i v e m o t i o n of a rigid plane sinking on one side. if y i n d i c a t e s the reference plane respect to which we measure t h e v a r i a t i o n s of the a l t i t u d e differences detected in the considered t r i e n n i a l p e r i o d , let 8 indicate the inclined plane produced by t h e s e v a r i a t i o n s , then elementary trigonometric considerations give a n expression that relates the a l t i t u d e variation of a benchm a r k as a f u n c t i o n of a and <p, which are respectively the angle t h a t oa f o r m s w i t h the nodal line of the planes y and 8, and the p i v o t i n g a n g l e between the planes y and 8 (see fig. 3). in p a r t i c u l a r we have: aa' = oa sen a sen ? vertical movements along, ecc. 1 0 1 t a b l e i t h e d i f f e r e n c e in level between the 1967 and 1970 surveys. note that there is a d i f f e r e n c e in the absolute reference level between them since the 1967 d a t a w e r e referred to the mareograph of catania while the 1970 was r e f e r r e d to a t r i g o n o m e t r i c survey vertix of the i.g.m. b e n c h m a r k s siting 1967 1970 1 stazione * 5.0941 4.9359 2 duomo * 7.1442 6.9896 3 municipio * 4.8920 4.7367 4 fiera * 2.9011 2.7460 5 v. libertà 3.5313 3.3797 6 paradiso 3.2126 3.0600 7 villa prestopino 5.0064 4.8557 8 porticatello 6.4898 6.3406 9 grotta 4.3040 4.1547 10 guardia 6.1851 6.0326 11 s. agata 11.4666 11.3138 12 lavalle 2.0999 1.9468 13 rocca 1.2737 1.1194 14 granatari 3.0473 2.8914 15 villa de bernardis 6.3993 6.2456 16 muro sostegno 13.7917 13.6363 17 timpazzi 14.0933 13.9464 18 casablanca gemelli 16.3348 16.1817 19 casabianca agrillo 13.9021 13.7506 20 tono 13.2598 13.1065 * in t h e m e t r o p o l i t a n a r e a of messina. 1 0 2 v. achilli f. broccio f. mulargia 2 0 km fig. 2 differences in level from the 1967-70 i.g.m. survey. the d i f f e r e n c e s in level a r e referred to bench-mark 1 (stazione di messina). w h e r e aa' r e p r e s e n t s the difference of level variation in the c o n s i d e r e d interval of time, when we proceed f r o m " o " to "a" b e n c h m a r k s . if (j is t h e a z i m u t h w i t h respect to the north of the line ef, w h i c h is given f r o m the intersection of t and 8 planes, letting aa' s e n = k, we h a v e = k sen ( (j a ) and therefore oa' aa' = k (sen p cos a cos fj sen a ) [ 1 ] oa we c an easily solve equation [1] by taking couples of d a t a a n d c o n s i d e r i n g the a z i m u t h of the connecting path together with t h e i r d i f f e r e n c e in level. vertical movements along, ecc. 1 0 3 f fig. 3 the pivoting plane geometry. ef is the pivoting axis, w h i l e 9 is t h e pivoting angle. 0, a' and a are points on the g r o u n d ( b e n c h m a r k s ) in the 1967 while a. is the a z i m u t h to plane strike. if t h e solution of (3 , relative to different couples, are similar w e will c o n c l u d e t h a t the measured vertical movements possibly o c c u r a s a consequence of the collective pivoting of an inclined p l a n e , w h i c h intersect the reference plane with an azimuth (3 . i n o r d e r to c a u t i o n from local level anomalies, we use the fact t h a t s o m e b e n c h m a r k s are consecutively aligned and therefore by a l i n e a r regression we find the best fit line a h = (a, + b{d) a =cost. t h e l a t t e r expression allows to calculate the kilometric trends, i.e. t h e d i f f e r e n c e in level per kilometer along the considered direct i o n s . in o u r analysis we have considered six sets of equations o b t a i n i n g six solutions for (3 with average values of [3 = 43° a n d a p e r c e n t s t a n d a r d deviation of 10,9%. a s i m i l a r analysis performed for the 1907-1909 data shows t w o a l i g n m e n t s (scala — km 5 and zona militare — castanea) w h i c h do not show variations in their differences of level. the d i f f e r e n c e in a z i m u t h of these two alignments is very small (circa 10°) a n d t h e fact t h a t differences in level are found going 1 0 4 v. achilli f. broccio f. mulargia w e s t w a r d suggests t h e m as representative of the pivoting axis f o u n d w i t h the above technique. this see m s c o n f i r m e d also by the 1967-1970 d a t a relative to t h e t i m p a z z i t o n o . if it is a s s u m e d that the 1907-09 equal level points lie a p p r o x i m a t e l y on the pivoting axis, then two plate collective fig. 4 t h e t w o pivoting planes. both show a sinking rotation of t h e i r e n d s . the n u m b e r s indicate the position of the bench-marks as indicated in table i. vertical m o v e m e n t s along, ecc. 1 0 5 r o t a t i o n s a r e f o u n d . the first, which has been already described is o n t h e east side of the axis and shows a sinking (clockwise) r o t a t i o n (see fig. 4); the second one is found on the left of the axis, a n d , a l t h o u g h the evidence is based on fewer points, shows as well a s i n k i n g (anticlockwise) rotation (see fig. 4). c o n c l u s i o n s . t h e a n a l y s i s of the results of high precision levelling surveys p e r f o r m e d by i.g.m. in 1967 and 1970 shows a trend of crustal v e r t i c a l s t r a i n s in agreement with the coseismic movements of the 1908 messina e a r t h q u a k e . the movement found consists of two p l a n e s , one on the western and the other on the eastern side of a p i v o t i n g axis w i t h a z i m u t h strike p = 43° to the the north; both t h e p l a n e s show a sinking rotation of their ends. on the other h a n d t h e sitting of the existing bench marks is mainly along the c o a s t l i n e a n d d a t a on compact geological structures are sparse. t h i s led t h e a u t h o r s to start a high precision levelling survey ( p r e s e n t l y in progress) which, together with a trigonometric — t r i l a t e r a t i o n survey, will hopefully help to settle more precisely t h e n a t u r e a n d the entity of the movements in the whole messina s t r a i t s a r e a . a c k n o w l e d g e m e n t s . we t h a n k prof. e. boschi and prof. p. baldi for helpful advise, d i s c u s s i o n a n d c o n t i n u o u s support. 1 0 6 v . a c h i l l i f . b r o c c i o f . m u l a r g i a r e f e r e n c e s b a r a t t a m., 1901 1 terremoti d'italia, arnaldo forni editore. b a r b e r i f . , g a s p a r i n i p . , i n n o c e n t i f . , v i l l a r i l . , 1 9 7 3 volcanism of^southern tyrrhenian sea and its geodynamic implications. " j . g . r . " , 78, p p . 5 2 2 1 5 2 3 2 . b o t t a r i a., 1971 l'attività sismica nello stretto di messina nel ventennio 1950-1969. "annali di geofisica", 24, pp. 103-133. b o t t a r i a., b r o c c i o f., l o g i u d i c e e., 1975 some seismological results and geostructural suggestions from a study of the reggio calabria earthquake of 16 january, 1975. "annali di geofisica", 28, p p . 219-239. b o t t a r i a. a n d lo g i u d i c e e., 1975b il terremoto di reggio calabria del 16 gennaio 1975. "ann. geofisic.", 28, 2-3. c a p u t o m . , p a n z a g . f . , p o s t p i s c h l d . , 1970 deep structure of the mediterranean basin. " j . g . r . " , 75, p p . 4919-4923. c a p u t o m., f o l l o n i g., p i e r i l . , u n g u e n d o l i m., 1 9 7 4 geodimetric control across the strait of messina. "g.j.r. astr. soc", 38, p p . 1-8. c o r t e s e e., 18^2 sulla formazione dello stretto di messina. "boll. reg. c o m . i t a l . " , 3, p p . 4-29. c o r t e s e e . , 1 8 9 5 geotettonica e sismologia della descrizione geologica della calabria " m e m . descr. carta geol. d ' i t a l i a " , 9, p p . 30-62. f i n e t t i i . , m o r e l l i c . , 1 9 7 2 wide scale digital seismic exploration of the mediterranean sea. "boll. geofis. teor. appi.", 14, p p . 2 9 1 3 4 2 . g l a n g e a u d l . , 1952a les phénomènes géophysiques et l'évolution de la méditerranée occidentale. "ann. geophys.", 8, 1, pp. 112-132. g l a n g e a u d l . , 1952b interprétation téctonophysique des caractères structuraux et paléogéographiques de la méditerranée occidentale. " b u l l . soc. géol. f r a n c e " , 6, 1, p p . 735-762. i s t i t u t o g e o g r a f i c o m i l i t a r e , 1 9 7 2 controllo geodetico periodico dello stretto di messina "i.g.m.", firenze. l i m a n o w s k i m., 1913 die grosse kalabrische decke. "bull. int, acad. se. cracovie", ci sc. m a t h . nat., s.a., 6, a, p p . 370-385. l o p e r f i d o a., 1 9 0 9 livellazione geometrica di precisione. relazione della c o m m i s s i o n e reale, r o m a , allegato d, p p . 146-152. o g n i b e n l . , 1 9 6 0 nota illustrativa dello schema geologico della sicilia nordorientale. " r i v . miner. sicil. p a l e r m o " , 11, p p . 183-212. o g n i b e n l., 1966 lineamenti idrogeologici dell'etna. " r i v . miner. sicil. p a l e r m o " , 17, p p . 151-174. v e r t i c a l m o v e m e n t s a l o n g , e c c . 1 0 7 o g n i b e n l., 1971 tettonica della sicilia e della calabria. " b o l l . acc. g i o e n i a se. n a t . " , catania, ser. iv, 11, pp. 14-26. o g n i b e n l . , 1 9 7 3 schema geologico della calabria in base ai dati odierni. " g e o l o g i a r o m a n a " , 12, pp. 243-585. r i t s e m a a . r . , 1971 notes on plate tectonics and movements in the mediterranean region. "proc. e u r . seism. comm.", luxemburg. s c h i c k r . , 1977 studio sismotettonico del terremoto di messina dell'anno 1908. " a n n u a r i o geologico", e 11, p p . 3-74. t r e v i s a n l . r 1 9 5 4 les mouvements tectoniques récent en sicile. hypothèses et p r o b l è m e s , "geol. rend.", 43, p p . 2 0 7 2 2 1 . v e c c h i a o . , 1 9 5 6 la sicilia e le aree circostanti, lineamenti geofisici e geologia profonda. "boll. soc. geol. ital", 7 5 , pp. 6 1 8 7 . 1 station lat. (°) lon. (°) h (km) 𝛋 anq 30.6 117.0 30.3 1.79 bas 31.5 117.4 31.5 1.77 beb 32.9 117.3 32.0 1.75 bzy 31.4 116.2 36.1 1.76 chz 32.3 118.3 30.5 1.77 dyn 32.6 117.5 33.0 1.76 fyt 32.9 115.8 33.8 1.81 fzl 31.3 116.3 34.4 1.83 hbe 34.0 116.8 33.1 1.84 hef 31.8 117.1 32.6 1.82 hsh 31.8 118.1 29.5 1.88 hus 29.7 118.4 32.7 1.77 jas 32.8 118.3 28.6 1.83 jix 30.7 118.4 31.7 1.82 jza 31.7 115.9 34.4 1.73 lan 31.7 116.5 33.9 1.77 lna 31.3 116.1 35.6 1.86 mas 31.7 118.6 30.9 1.75 mcg 33.4 116.5 31.5 1.75 station lat. (°) lon. (°) h (km) 𝛋 sch 31.3 117 32.1 1.82 six 33.6 117.9 31.3 1.78 sjh 31.4 116.1 34.5 1.80 cttz 25.8 116.4 31.0 1.78 dsxp 23.7 117.4 26.9 1.80 fdqy 27.1 120.3 29.1 1.86 fqdz 25.7 119.3 28.6 1.85 fzcm 26.0 119.4 30.0 1.87 gtsk 26.4 118.7 29.9 1.79 hahf 25.0 117.5 31.0 1.73 hajf 25.0 119.0 27.2 1.79 ljtl 26.4 119.9 30.1 1.80 lyxp 25.1 117.0 30.4 1.80 mqdq 26.4 118.9 30.9 1.81 mxxf 26.4 117.2 31.0 1.80 ndzw 26.7 119.6 30.7 1.81 npdk 26.6 118.2 30.1 1.73 pcnp 27.9 118.5 32.2 1.78 phsg 24.4 117.3 28.4 1.84 annals of geophysics, 66, 2023; doi:10.4401/ag-8825 appendix to azimuthal anisotropy of receiver functions in the central south china block and its tectonic implications yutao shi1, yuan gao1, ziqi zhang2, yongqian zhang3, guohui li1 1 key laboratory of earthquake prediction, institute of earthquake forecasting, china earthquake administration, beijing, 100036, china 2 department of earth and environmental sciences, university of rochester, rochester, ny, usa 3 china deep exploration center-sinoprobe center, china geological survey & chinese academy of geological sciences, beijing 100037, china table s1. measured crustal thickness and vp/vs ratio by the h-κstacking of the receiver function beneath each station in the study region. the first three columns are the station name, latitude, and longitude of the station; the fourth and fifth column indicates the crustal thickness average and vp/vs ratio in the crust. 2 station lat. (°) lon. (°) h (km) 𝛋 ptaq 25.5 119.9 28.6 1.82 ptlc 25.5 119.0 27.8 1.85 ptnr 25.2 119.5 27.8 1.82 pttc 25.5 119.8 28.6 1.82 qzh 24.9 118.6 28.0 1.80 shlc 25.0 116.5 30.6 1.74 snqy 27.4 119.5 31.5 1.78 swdt 27.3 117.5 30.3 1.74 dji 32.6 111.5 33.7 1.74 dwu 31.5 114.1 34.3 1.74 hme 30.1 115.9 34.3 1.73 jgs 29.4 114.7 34.1 1.78 jme 31.1 112.2 32.5 1.80 jyu 29.8 113.8 36.0 1.74 mch 31.1 115.2 32.6 1.76 nzh 31.4 111.6 34.0 1.78 ssh 29.6 112.7 28.8 1.74 sya 32.6 110.7 34.1 1.83 szh 31.6 113.4 35.3 1.72 wha 30.5 114.5 35.0 1.73 whn 30.5 114.4 30.6 1.78 xfa 32.0 112.0 33.1 1.70 xni 29.7 114.4 36.3 1.74 ych 30.8 111.3 36.9 1.69 ydu 30.2 111.2 32.7 1.77 ynx 30.0 115.1 33.6 1.73 yxi 33.0 110.4 37.3 1.69 zhx 31.2 112.7 34.9 1.73 zsh 32.2 110.2 35.9 1.79 chl 26.8 113.5 31.0 1.65 chz 25.8 113.0 31.2 1.78 cns 28.2 112.9 30.2 1.74 hey 26.9 112.5 28.7 1.74 liy 28.2 113.6 31.3 1.68 lod 27.7 112.0 30.8 1.76 mil 28.8 113.1 30.7 1.74 nix 28.0 112.3 29.3 1.71 shy 27.2 111.5 33.0 1.73 jgs 29.4 111.9 31.5 1.74 station lat. (°) lon. (°) h (km) 𝛋 jis 28.1 109.7 33.8 1.80 yiy 28.6 112.3 30.9 1.74 yoz 26.2 111.6 32.6 1.71 ah201 31.8 117.2 33.2 1.78 ah202 31.7 117.5 31.7 1.79 ah203 31.5 118.2 31.5 1.77 ah204 31.7 118.6 30.5 1.77 ah205 31.1 119.2 29.8 1.87 ah206 31.6 116.4 32.9 1.8 ah207 31.3 117.0 32.1 1.82 ah208 31.2 117.8 30.1 1.82 ah209 31.1 118.2 31.0 1.84 ah210 31.0 118.9 29.3 1.81 ah211 31.1 116.3 35.5 1.82 ah212 30.9 117.4 33.2 1.81 ah213 30.7 118.4 31.8 1.81 ah214 30.7 119.0 32.2 1.73 ah215 30.8 119.3 29.4 1.86 ah216 30.5 116.2 35.4 1.74 ah217 31.1 116.9 32.5 1.77 ah218 30.6 117.0 30.1 1.83 ah219 30.3 118.5 33.1 1.74 c001 29.2 115.4 30.4 1.73 c002 28.7 115.4 29.7 1.74 c003 28.3 115.4 29.1 1.74 c004 26.9 115.9 28.4 1.80 c005 26.6 116.1 30.9 1.70 c006 28.3 115.9 28.7 1.75 c007 27.8 116.0 28.7 1.83 c008 27.4 116.1 30.4 1.81 c009 27.1 116.5 31.8 1.76 c010 30.3 115.5 30.3 1.82 c011 29.7 115.7 31.6 1.76 c012 27.7 115.6 29.7 1.75 c013 29.2 115.8 28.1 1.71 c014 28.8 116.2 28.2 1.73 c015 28.2 116.6 29.3 1.8 c016 27.9 116.7 29.2 1.78 c017 27.5 116.9 31.7 1.74 3 station lat. (°) lon. (°) h (km) 𝛋 c019 30.3 116.1 33.7 1.85 c020 29.1 116.7 33.8 1.80 c022 27.9 117.2 32.1 1.77 c023 28.8 117.3 30.8 1.79 c029 29.8 116.9 33.6 1.74 c030 29.9 116.9 32.8 1.74 c031 30.0 116.8 33.0 1.72 c035 30.1 117.3 34.9 1.75 c036 30.4 117.9 33.0 1.75 c037 29.7 117.6 33.4 1.78 c038 30.1 118.0 32.4 1.82 c039 29.7 118.1 32.5 1.73 c040 30.0 118.5 31.6 1.78 c041 29.3 117.9 31.2 1.76 c042 29.7 118.8 34.3 1.77 c043 30.2 119.5 33.0 1.79 c044 29.9 119.8 30.4 1.84 c045 29.7 119.4 33.9 1.76 c046 29.6 120.0 32.0 1.80 c047 29.4 119.6 30.1 1.88 c048 29.2 120.1 32.7 1.75 c049 29.0 119.9 31.7 1.8 c050 29.9 119.1 32.4 1.83 c051 29.1 118.2 33.5 1.73 c052 29.2 118.9 31.8 1.81 c053 28.9 118.4 33.7 1.74 c054 28.7 119.2 32.7 1.76 c055 28.6 120.1 33.3 1.70 c056 28.4 118.7 32.2 1.86 c057 28.2 119.1 31.5 1.82 c058 28.2 119.8 30.3 1.84 c059 27.9 119.2 32.5 1.74 c060 27.7 119.5 33.3 1.75 c062 27.5 118.1 31.8 1.74 c063 27.5 118.8 31.2 1.78 c064 27.0 117.1 31.1 1.74 c065 27.0 117.6 29.0 1.78 c066 27.0 118.2 30.0 1.77 c067 27.1 119.1 30.7 1.81 station lat. (°) lon. (°) h (km) 𝛋 c069 26.6 117.6 29.5 1.77 c070 26.2 117.6 30.9 1.74 c071 25.9 116.7 31.2 1.74 c072 25.6 117.6 30.9 1.76 c073 28.2 117.7 30.6 1.81 c074 27.9 118.4 32.1 1.79 c075 28.0 118.3 32.6 1.79 c076 28.1 118.1 32.1 1.78 c077 28.3 118.1 32.4 1.76 c079 28.5 118 32.5 1.74 c080 28.6 117.9 31.9 1.79 c081 28.7 117.8 33.9 1.74 c082 28.8 117.7 34.4 1.73 c083 29.0 117.7 33.6 1.72 c084 29.1 117.6 32.4 1.74 c201 25.3 114.8 28.0 1.72 ha01 25.5 112.1 31.3 1.71 ha03 25.5 112.3 31.3 1.7 ha04 25.5 112.4 32.0 1.71 ha05 25.5 112.6 30.5 1.79 ha06 22.4 112.7 31.1 1.77 ha07 25.4 112.8 30.8 1.74 ha08 25.4 112.9 29.7 1.78 hj01 26.0 112.6 29.7 1.72 hj02 25.9 113.3 30.3 1.73 hj03 25.9 113.8 29.6 1.76 hj04 26.4 112.4 29.0 1.77 hj05 26.4 113.2 29.7 1.74 hj06 26.3 113.6 31.2 1.72 hj07 26.8 113.5 30.8 1.65 hj08 27.1 113.0 28.7 1.71 hj09 27.2 113.4 31.3 1.77 hj10 27.4 112.4 29.6 1.69 hj11 27.5 112.9 28.5 1.75 hj12 27.5 113.4 29.5 1.74 hj13 27.9 112.8 29.2 1.74 hj14 28.1 113.4 30.4 1.71 hj15 28.2 113.9 30.6 1.72 hj16 28.2 114.4 28.9 1.84 4 station lat. (°) lon. (°) h (km) 𝛋 hj17 28.1 115.0 29.6 1.67 hj18 27.9 115.0 28.8 1.74 hj19 27.6 115.2 28.6 1.70 hj20 27.4 115.3 30.0 1.67 hj21 27.5 114.6 29.0 1.74 hj22 27.5 113.9 32.4 1.72 hj23 27.3 114.9 28.0 1.73 hj24 27.1 114.5 28.3 1.75 hj25 27.0 114.0 30.2 1.74 hj26 26.6 114.5 27.1 1.77 hj27 26.6 115.0 28.4 1.81 hj28 26.8 115.4 30.1 1.73 hj29 26.2 114.0 30.4 1.80 hj30 26.2 114.4 31.2 1.79 hj31 25.9 114.2 29.4 1.77 hj32 26.3 115.7 30.6 1.72 hn01 32.0 112.8 34.0 1.81 hn02 32.1 113.4 33.8 1.75 hn04 32.1 115.1 32.7 1.76 hn05 32.1 115.8 31.7 1.79 hn06 31.5 112.2 32.3 1.77 hn07 31.6 112.9 34.4 1.76 hn08 31.6 113.7 36.2 1.74 hn09 31.6 114.4 32.7 1.73 hn10 31.6 115.0 32.6 1.79 hn11 31.5 113.3 32.6 1.80 hn12 31.1 113.8 35.2 1.80 hn14 31.0 114.9 32.9 1.80 hn15 31.1 115.5 35.0 1.75 hn16 30.9 112.2 32.2 1.80 hn17 30.8 113.0 34.7 1.72 hn18 30.4 112.2 29.7 1.84 hn19 30.6 113.6 31.0 1.78 hn20 30.7 114.8 32.1 1.77 hn21 30.6 115.2 32.8 1.72 hn22 29.8 112.2 30.9 1.70 hn23 30.2 113.2 31.3 1.70 hn24 30.1 114.1 32.8 1.75 hn25 30.1 114.7 35.0 1.75 station lat. (°) lon. (°) h (km) 𝛋 hn26 29.7 113.2 31.9 1.74 hn27 29.7 114.1 35.0 1.84 hn28 29.6 114.8 35.0 1.71 hn29 29.7 115.2 32.9 1.77 hn30 29.2 112.1 32.0 1.73 hn31 29.1 112.7 31.0 1.76 hn32 29.1 113.3 31.4 1.78 hn33 29.2 113.8 33.4 1.81 hn34 29.3 114.4 34.9 1.66 hn35 29.2 114.9 34.2 1.70 hn36 28.6 112.9 32.7 1.68 hn37 28.6 113.3 31.8 1.73 hn38 28.7 113.7 33.9 1.72 hn39 28.7 114.3 32.6 1.71 hn40 28.8 115.0 33.2 1.70 any 25.1 115.4 30.1 1.75 day 25.4 114.4 29.3 1.75 duc 29.3 116.2 29.6 1.71 fec 27.9 115.7 29.0 1.74 gaa 28.4 115.4 29.6 1.75 gaz 25.8 115.0 27.7 1.73 huc 25.6 115.8 29.5 1.72 jdz 29.3 117.3 30.5 1.68 jgs 26.6 114.2 30.1 1.72 jia 27.1 115.0 28.3 1.74 jij 29.6 116.0 33.2 1.72 jix 28.2 116.2 28.7 1.74 lea 27.4 115.8 28.5 1.79 lon 24.8 114.8 28.4 1.81 nac 27.5 116.6 31.9 1.76 nnc 28.8 115.8 29.2 1.68 shc 26.3 116.3 31.2 1.75 shr 28.4 118.0 30.3 1.82 waa 26.4 114.8 29.7 1.77 xis 29.0 114.6 33.1 1.70 xuw 25.0 115.6 31.6 1.75 yic 27.8 114.4 32.3 1.72 yox 29.1 115.6 32.4 1.73 yug 28.8 116.6 30.4 1.70 5 station lat. (°) lon. (°) h (km) 𝛋 dax 31.6 121.5 30.6 1.77 dys 30.6 122.1 30.4 1.77 huh 30.4 119.8 32.8 1.76 jiz 31.1 120.9 32.3 1.65 nah 31.0 121.8 32.7 1.65 qhs 30.8 121.3 30.5 1.74 shx 31.2 121.4 32.2 1.69 sse 31.1 121.2 31.2 1.73 tms 31.1 121.1 32.6 1.73 tps 31.3 120.5 32.9 1.71 xks 31.1 121.1 31.7 1.75 zhy 31.2 121.5 32.8 1.69 bel 29.9 121.8 31.3 1.83 cha 29.5 118.4 31.4 1.82 chx 31.1 119.7 31.7 1.79 hay 30.4 120.8 28.5 1.78 haz 30.3 120.1 31.0 1.80 huz 30.8 120.1 30.5 1.78 station lat. (°) lon. (°) h (km) 𝛋 jax 30.8 120.9 30.6 1.73 jin 28.0 119.6 31.9 1.76 lia 30.1 119.0 35.2 1.70 nib 30.0 121.5 29.9 1.78 nih 29.2 121.7 31.5 1.78 njd 27.5 121.1 31.6 1.79 qiy 27.6 119.1 33.2 1.71 shs 30.7 122.5 31.8 1.81 soy 28.5 119.5 31.5 1.81 wez 27.9 120.7 31.7 1.79 wxj 28.7 118.8 34.0 1.77 xaj 29.5 119.3 35.3 1.74 xic 29.5 120.9 31.6 1.75 yix 29.8 121.3 31.2 1.79 yok 29.0 120.2 32.4 1.75 yuq 28.4 121.1 31.6 1.75 yuy 30.0 121.1 31.1 1.76 zhs 30.0 122.1 32.0 1.73 table s2. estimated fast polarization direction and splitting time beneath each station in the study region. the first three columns are the station name, latitude, and longitude of the station; the fourth shows the event number; the fifth and sixth column indicates the fast polarization orientation (clockwise from north) and the standard deviation of the fast orientation; the seventh and eighth column indicates splitting time (s) and the standard deviation of the splitting time. station lat. (°) lon. (°) no. rfs polarization (°) sd of the polarization splitting time (s) sd of the splitting time anq 30.6 117.0 159 70.2 6.37 0.25 0.08 any 25.1 115.4 127 48 6.73 0.36 0.06 bas 31.5 117.4 122 –28.6 5.58 0.36 0.08 bzy 31.4 116.2 219 –42.7 5.31 0.35 0.06 cha 29.5 118.4 143 34.2 6.87 0.31 0.07 chz 25.8 113.0 101 66 6.43 0.31 0.09 chz 32.3 118.3 101 44.5 13.47 0.27 0.06 cns 28.2 112.9 156 64.7 4.7 0.37 0.08 day 25.4 114.4 133 77.6 6.44 0.29 0.08 dji 32.6 111.5 250 –55.4 6.49 0.32 0.05 dsxp 23.7 117.4 115 69.5 4.72 0.4 0.12 duc 29.3 116.2 112 –58.9 7.55 0.36 0.06 6 station lat. (°) lon. (°) no. rfs polarization (°) sd of the polarization splitting time (s) sd of the splitting time dwu 31.5 114.1 208 –29 6.91 0.26 0.07 dyn 32.6 117.5 184 –61.9 6.54 0.31 0.06 fdqy 27.1 120.3 117 13.5 7.2 0.3 0.07 fec 27.9 115.7 130 85 3.9 0.47 0.09 fxi 31.9 110.7 215 74.6 2.13 0.6 0.08 fzl 31.3 116.3 202 88.6 3.45 0.5 0.07 gaa 28.4 115.4 108 65.7 4.28 0.19 0.07 gtsk 26.4 118.7 132 –17.1 4.61 0.38 0.09 hahf 25.0 117.5 126 8.9 7.31 0.3 0.08 hajf 25.0 119.0 133 63.7 4.44 0.43 0.1 hay 30.4 120.8 122 37.8 14.11 0.17 0.06 hef 31.8 117.1 79 –60.8 15.81 0.19 0.06 hey 26.9 112.5 144 –24.9 7.49 0.27 0.09 hna 32.6 116.8 224 –80.9 6.38 0.31 0.07 hsh 31.8 118.1 125 39.8 5.69 0.41 0.06 huc 25.6 115.8 137 –88.6 6.43 0.29 0.08 huh 30.4 119.8 164 –69.4 10.91 0.23 0.06 hur 29.5 112.6 27 –19.8 5.47 0.39 0.11 hus 29.7 118.4 130 64.6 3.87 0.46 0.08 huz 30.8 120.1 141 81.2 5.63 0.3 0.08 jdz 29.3 117.3 136 88.4 7.4 0.25 0.09 jgs 29.4 114.7 93 76.9 5.85 0.3 0.09 jia 27.1 115.0 141 47.3 5.09 0.44 0.07 jij 29.6 116.0 156 –88.4 4.7 0.35 0.08 jin 28.0 119.6 67 –76.6 9.91 0.25 0.08 jix 30.7 118.4 149 38.8 5.44 0.41 0.06 jix 28.2 116.2 143 –24.9 8.38 0.21 0.08 jme 31.1 112.2 200 –43.1 8.83 0.21 0.07 jyu 29.8 113.8 175 –19.7 3.45 0.5 0.07 jza 31.7 115.9 194 –21.2 7.01 0.24 0.08 lan 31.7 116.5 167 –72.8 8.86 0.27 0.07 lea 27.4 115.8 82 72.4 8.49 0.27 0.15 lia 30.1 119.0 135 –89 2.84 0.56 0.08 liy 28.2 113.6 45 32.3 6.91 0.41 0.08 ljtl 26.4 119.9 106 –17.9 3.65 0.45 0.08 lna 31.3 116.1 208 –46.9 5.85 0.36 0.06 lon 24.8 114.8 116 55 7.12 0.4 0.1 lyxp 25.1 117.0 90 –6.7 4.75 0.35 0.1 7 station lat. (°) lon. (°) no. rfs polarization (°) sd of the polarization splitting time (s) sd of the splitting time mas 31.7 118.6 141 6.1 4.78 0.4 0.07 mcg 33.4 116.5 231 –29.1 4.95 0.35 0.06 mch 31.1 115.2 191 46.1 7.95 0.27 0.06 mqdq 26.4 118.9 130 20.9 6.65 0.43 0.08 mf 26.4 117.2 119 1.8 4.62 0.39 0.09 ndzw 26.7 119.6 122 54.9 5.98 0.4 0.07 nib 30.0 121.5 125 29.6 12.26 0.16 0.08 nnc 28.8 115.8 155 –7.9 3.56 0.47 0.08 pcnp 27.9 118.5 128 –88.4 7.3 0.27 0.09 phsg 24.4 117.3 121 –65.6 13.56 0.2 0.07 qhs 30.8 121.3 132 65.1 9.08 0.25 0.06 qiy 27.6 119.1 115 –8.2 6.81 0.25 0.09 qzh 24.9 118.6 141 38 10.37 0.28 0.06 shc 26.3 116.3 129 –87.5 7.72 0.27 0.11 shr 28.4 118.0 128 –30.7 9.3 0.24 0.08 shs 30.7 122.5 109 62.1 5.04 0.38 0.08 shx 31.2 121.4 71 11.9 7.04 0.29 0.08 shy 27.2 111.5 124 –23.8 6.57 0.3 0.08 six 33.6 117.9 175 –38.4 5.75 0.36 0.07 sjh 31.4 116.1 173 –84.7 3.28 0.62 0.07 soy 28.5 119.5 59 68.9 8.36 0.24 0.09 ssh 29.6 112.7 174 –23.7 4.86 0.35 0.08 ah202 31.7 117.5 71 20.5 5.63 0.35 0.06 ah204 31.7 118.6 85 15.9 5.67 0.37 0.08 ah206 31.6 116.4 51 –83.5 13.71 0.16 0.1 ah207 31.3 117.0 119 –77.9 8.89 0.22 0.07 ah213 30.7 118.4 101 53.8 3.41 0.51 0.07 c005 26.6 116.1 23 68.7 5.41 0.41 0.12 c023 28.8 117.3 73 49 6.97 0.31 0.07 c030 29.9 116.9 58 –15.6 4.06 0.44 0.08 c031 30.0 116.8 30 3.7 2.59 0.42 0.14 c035 30.1 117.3 50 –13.9 2.21 0.45 0.12 c040 30.0 118.5 56 73.2 4.36 0.47 0.1 c041 29.3 117.9 67 69.2 6.16 0.35 0.11 c047 29.4 119.6 69 20.7 5.92 0.39 0.06 c049 29.0 119.9 90 86.8 8.84 0.19 0.08 c053 28.9 118.4 91 30.6 12.51 0.27 0.08 c059 27.9 119.2 77 –58.7 11.45 0.2 0.06 8 station lat. (°) lon. (°) no. rfs polarization (°) sd of the polarization splitting time (s) sd of the splitting time c065 27.0 117.6 69 –37.8 5.39 0.37 0.07 c070 26.2 117.6 85 75 9.85 0.3 0.08 c071 25.9 116.7 78 81 9.35 0.25 0.08 c073 28.2 117.7 64 –83.8 7.09 0.3 0.08 c077 28.3 118.1 47 –86.9 5.14 0.43 0.09 hj06 26.3 113.6 76 66.6 5.66 0.42 0.07 hj13 27.9 112.8 78 14.7 3.86 0.43 0.08 hj14 28.1 113.4 74 57.8 4.15 0.42 0.09 hj16 28.2 114.4 74 47.8 4.14 0.51 0.08 hj28 26.8 115.4 64 76.8 4.52 0.41 0.09 hj29 26.2 114.0 64 66.5 4.84 0.49 0.1 hn02 32.1 113.4 46 –67.7 4.98 0.51 0.07 hn05 32.1 115.8 52 –61.5 8.46 0.33 0.07 hn06 31.5 112.2 56 –17.2 5.72 0.32 0.11 hn10 31.6 115.0 65 –22.9 3.03 0.54 0.1 hn11 31.5 113.3 90 –40.7 4.11 0.54 0.07 hn34 29.3 114.4 49 77.9 4.05 0.46 0.12 anq 30.6 117.0 159 70.2 6.37 0.25 0.08 vol50,3,2007 301 annals of geophysics, vol. 50, n. 3, june 2007 key words resonances – elf observations – ionospheres 1. introduction schumann resonance oscillations are the most remarkable natural electromagnetic phenomena in the lower frequency side of elf range. as pointed out by several studies (raemer, 1961; balser and wagner, 1962a; pierce, 1963; ogawa et al., 1969; polk, 1969; galejs, 1972; clayton and polk, 1977; sentman, 1996; nickolaenko, 2002) the intensities of the schumann resonances reflect the totality of global thunderstorm activity exciting the zeroth-order transverse magnetic (tm0) normal modes of the earth-ionosphere cavity. moreover the excitation of the earth-ionosphere cavity can be the effect of hydromagnetic waves propagation through the ionosphere (abbas, 1968; nickolaenko, 2002). the space separating the earth and the ionosphere indeed forms a cavity, which can support electromagnetic standing waves with wavelengths comparable to planetary dimensions. large electromagnetic transients, such as lightning, radiate broadband electromagnetic impulses that spread radially into the cavity. the low frequency impulse components can circumnavigate the globe several times before suffering serious degradation, and so produce a resonant line spectrum, by the phase addition and cancellation of waves that have traversed the global circumference several times along multiple paths. the resulting waves are quasi-transverse electromagnetic (quasi-tem) normal modes of the earthionosphere cavity. the total resonant spectrum is the incoherent superposition of the effects from the totality of global lightning. these resonances, called schumann resonances, can, in principle, be detected from any investigations on diurnal and seasonal variations of schumann resonance intensities in the auroral region claudia rossi, paolo palangio and franco rispoli istituto nazionale di geofisica e vulcanologia, osservatorio geofisico di l’aquila, italy abstract measurements of the magnetic component of the schumann resonance in the frequency range 6-14 hz were performed at high latitude location (tnb antarctica; geographic coordinates: 74.7°s, 164.1°e; geomagnetic coordinates: 80.0°s, 307.7°e; lt = ut+13; mlt = ut – 8; altitude= 28 m a.s.l.), during the two years 1996-1997. tnb is a particularly important observation site located in a region characterised by a high electromagnetic activity in the elf and vlf bands. moreover its remote location in antarctica provides the important advantage that electromagnetic background noise is not corrupted by anthropogenic noise and that the continental lightning activity is very low. the combination of low additional anthropogenic electromagnetic radiation and low atmospheric noise in this area allows detailed investigations into wave generation and amplification in the polar ionosphere and magnetosphere not possible anywhere else in the world. this paper reports the study of the magnetic power of the 8 hz schumann resonance mode. for both the years considered diurnal and long-term seasonal variations were observed. mailing address: dr. claudia rossi, istituto nazionale di geofisica e vulcanologia, osservatorio geofisico di l’aquila, castello cinquecentesco, via castello, 67100 l'aquila, italy; e-mail: rossi@ingv.it 302 claudia rossi, paolo palangio and franco rispoli place on the planet, and, away from thunderstorms and artificial electromagnetic noise sources, constitute the principal component of the natural background of the electromagnetic spectrum over the frequency range 6-50 hz. resonance proprieties of the earth-ionosphere cavity were first predicted and discussed theoretically by schumann (1952) and the earliest experimental detection of the resonances was made by schumann and könig (1954). the first spectral representation showing the resonance lines was presented by balser and wagner (1960). during the five years following these initial results, experimental measurements of increasing sophistication were performed (raemer, 1961; balser and wagner, 1962a,b, 1964; pierce, 1963; rycroft, 1965). at the same time several theoretical descriptions of the resonances were advanced to take into account realistic proprieties of the ionosphere (wait, 1992). following these initial reports, there were about two decades of active research on the entire subject of elf propagation. although study of the schumann resonances formed a part of this research, the primary emphasis in these studies was on frequencies above 45 hz. the general electromagnetic theory for vlf and elf waves in the earth-ionosphere system is contained in the books by galejs (1972), bliokh et al. (1980), and polk (1982). several subsequent research and theoretical studies have also be important to the schumann resonance description (nickolaenko and rabinovich, 1982; sentman, 1987, 1990, 1996; sentman and fraser, 1991; burke and jones, 1992; wait, 1992; füllekrug, 1995; füllekrug and frasersmith, 1996; märcz, 1997; morente et al., 2003). the most recent comprehensive reference works on schumann resonances are the books by sentman (1995), and nickolaenko (2003). many published works show that the schumann resonance intensities undergo a diurnal modulation in both the vertical electric field and horizontal magnetic field intensities at particular locations (keefe et al., 1964; polk, 1969; clayton, 1977; sentman and fraiser, 1991; sátori and zieger, 1996; märcz et al., 1997). keefe et al. (1964) and sentman and fraiser (1991) have shown that there may be a local time effect in the diurnal intensity variation. in particular they observed that the schumann resonance intensities maximize near local noon at each location. märcz et al. (1997) measurements have evidenced annual and seasonal variations in the relative amplitudes of schumann resonances’ vertical electric component in the first three modes (meloni et al., 1992). the diurnal and seasonal variations of the background noise may be the result of changes in both the propagation conditions (frequency and amplitude response of the earth-ionosphere cavity), and the source characteristics (intensity and distribution of thunderstorm). in particular one of the most important features in the propagation of lightning signals may be the variation of the ionosphere morphology from day to night. the dayside ionosphere indeed is denser and lower compared to the nightside. the tnb station is far from lightning sources, so we expect that the main contribution to background noise variation at this location is the ionospheric one. this paper presents the diurnal variation of the magnetic component of the 8 hz schumann resonance for the 1996 and the 1997 years. section 2 describes the instrumentation and the observation site. data analysis is described in section 3, where we show that two principal peaks are present in the diurnal variation and we illustrate their seasonal variation. 2. instrumentation within the framework of the italiantartide project, continuous geomagnetic measurements were performed in the frequency range 5-11 hz during 1996-1997 at tnb antarctica (tnb antarctica; geographic coordinates: 74.7°s, 164.1°e; geomagnetic coordinates: 80.0°s, 307.7°e; lt= =ut+13; mlt=ut−8; altitude=28 m a.s.l.). the tnb observation site is located in a region characterised by a high electromagnetic activity in the elf and vlf bands. tnb indeed lie in the auroral region, which is connected by lines of force that reach the magnetopause in the daylight side and the plasmasheet of the magnetotail in the nightside. this consents to look extreme regions of the magnetosphere. moreover its remote locafig. 2. magnetic power of the 8 hz schumann resonance mode over the two time interval january 24-29, 1996, and july 1-6, 1996. marked diurnal variations, characterised by two principal peaks are evident. the two peaks profiles seem to vary from day-to-day. 303 investigations on diurnal and seasonal variations of schumann resonance intensities in the auroral region tion in antarctica provides the important advantage that electromagnetic background noise is not corrupted by anthropogenic noise an that the continental lightning activity is very low. the combination of low additional anthropogenic electromagnetic radiation and low atmospheric noise in this area allows detailed investigations on the wave generation and amplification in polar ionosphere and magnetosphere not possible anywhere else in the world. the observations reported in this work were derived from data obtained from a wide band elf monitoring. the measuring equipment includes a three search coil sensors, which were fig. 1. block diagrams of the whole tnb station: x, y, and z signals were processed, and then stored in a large memory. 304 claudia rossi, paolo palangio and franco rispoli used in order to obtain measurements of the directional characteristics of the noise (palangio, 1993). a single sensor is composed of 8 coils of 50 000 turns each (fig. 1). two of the sensors were oriented in the horizontal plane, one in the magnetic north-south (x) and the other in the magnetic east-west (y), the third is oriented in the local vertical direction (z). the magnetic declination mean value at the time of measurements was 137°. the station is powered by battery and solar cells, and includes a microprocessor, filters and data collecting system. data recording electronics consists of a microcomputer interface, which performs sampling, a/d conversion, and storing of x, y and z signal amplitudes in a small memory. at the end of each measurement the data stored in the interface memory were transferred into the microcomputer for processing. the results were stored in large memory (fig. 1). the station is therefore able to record data for long time in autonomous way. a block diagram of the whole experimental system is presented in fig. 1. the station can perform wide band geomagnetic measurements from 0.0005 hz to 1000 hz. to obtain the measurements considered in the present paper, signals were sampled at a frequency of 32 hz, and then double integrated in the frequency range f = f0 ± ∆ f /2 (where f0 = 8 hz and ∆ f = 6 hz ), and in a time window of 120 s. the measurements are expressed in spectral density unit ( f t 2/hz). 3. data analysis to motivate the analysis procedure we show in fig. 2 the plot of the magnetic power the horizontal components of the 8 hz schumann resonance mode versus the local time (lt), for two time intervals 5 days long, selected respectively in january and july 1996. for each day two principal peaks are evident: a first one (m1) lying in the time interval 8-12 lt, and a second one (m2) approximately at 20-21 lt. the two peak profile varies from day-to-day. we performed a data analysis to study the seasonal variation of the hour of appearance and amplitude of the two peaks. for each day we computed hourly means (lt) of the magnetic power. then we determined peaks time positions and amplitudes using a program based on the derivative method. 3.1. m1 peak annual variation figure 3 shows the hour of occurrence (lt) of the m1 peak with respect to the day for the horizontal components of the 8 hz schumann magnetic power in the 1996 and 1997 years. in order to reduce day-to-day fluctuations, m1 hourly position values were averaged over 15 days. observed trends are similar for the two years. for both the x and y components m1 peak appear at about 8.30 lt at the end of january (austral summer), it moves toward the noon from january to june, and it appear at ∼12÷12.40 about the austral winter solstice. then it moves back to reach again its position at about 8.30 lt in november. the m1 peak intensity seasonal variation for 1996 and 1997 is shown in fig. 4. intensity values are averaged over 15 days. in the 1996 trend three different regions are visible for both the x and y components: – in the first region, extending from the end of january to about the 20th april, the intensity decrease from its maximum value to its minimum value. the decrease trend is compatible with the linearity. – in the second region extending from about the 20th april to about the 10th august, the intensity grows, reach a relative maximum and then decrease to its minimum. – in the third region extending from about the 10th august to the end of october, the intensity grows from its minimum value to its maximum value. the 1997 intensity trend is quite similar to the 1996 one, but the second region is anticipated by about 50 days (fig. 4). 3.2. m2 peak annual variation the seasonal variation of the m2 peak hour of occurrence (lt) is shown in fig. 5. m2 peak values were averaged over 15 days to reduce statistical fluctuations. the m2 peak time of occurrence does not seem to vary in valuable 305 investigations on diurnal and seasonal variations of schumann resonance intensities in the auroral region fig. 4. plot of the intensity of the m1 peak versus the day for the horizontal components of the magnetic power of the 8 hz schumann resonance mode in the years 1996-1997. equinoxes and austral winter solstice are showed. in 1996 the intensity is maximum in austral summer, and reach a relative maximum approximately in correspondence of the winter solstice. in 1997 the relative maximum is anticipated by about 50 days. fig. 3. plot of the hour of occurrence (lt) of the m1 maximum versus the day for the horizontal components of the magnetic power of the 8 hz schumann resonance mode in the years 1996-1997. m1 appear early in the morning (~8.30 lt) in the austral summer and lightly after the noon (12.30 ÷13.00) about the austral winter solstice. 306 fig. 5. plot of the hour of occurrence (lt) of the m2 maximum versus the day for the horizontal components of the magnetic power of the 8 hz schumann resonance mode in the years 1996-1997. m2 peak position does not show a significant seasonal variation. claudia rossi, paolo palangio and franco rispoli fig. 6. statistical distribution of the m2 peak position with respect to the hour (lt). computed weighted mean values are also shown. way during the year. figure 6 shows the statistical distributions of the m2 hour of occurrence, and the computed weighted mean values are reported. the m2 peak intensity as a function of the day is shown in fig. 7. the m2 intensity decreases from january (austral summer) to june (austral winter), and grows from august to october. as outlined before, the m2 peak analysis is complicated because m2 is missing in some days. the work to explain this phenomenology is still in progress. 3.3. polarization analysis the statistical analysis of the polarization indicate that during quiet geomagnetic conditions the waves are always elliptically polarized, the ellipticities falling mainly in the range from 0.1 to 0.3. preliminary results from this analysis show also that sometimes the waves are linearly polarised, the degree of polarization of the signals is equal to 1, the field components are mutually coherent and the wave field is polarized in a single plane. this is consistent with the presence of a single plane wave in a single wave mode. the occurrence frequency of this event appears to increase with increasing ulf activity, it is predominantly a dayside phenomenon with a broad maximum within four hours of local noon (mlt). the duration of this events varies approximately from 50 to 1000 s. we have looked at a few individual cases, and conducted a superposed epoch analysis to determine the average behavior for all the events. the results are shown in fig. 8. as can be seen, there are five plots: the coherency, the degree of polarization, the angle of polarization, the ellipticity and the sense of polarization. the ellipticity of the magnetic field polarization is estimated from the eigenvector associated to the nonnull eigenvalue of the magnetic spectral matrix. the ellipticity is near zero which implies a linear polarization. the determination of the sense of polarization in the plane perpendicular to z is based on the sign of the imaginary part of the cross-spectrum between the x and y components. figure 9a shows the spectral shape of linearly polarized signals, as can be seen the shape 307 investigations on diurnal and seasonal variations of schumann resonance intensities in the auroral region fig. 7. plot of the intensity of the m2 peak versus the day for the horizontal components of the magnetic power of the 8 hz schumann resonance mode in the years 1996-1997. equinoxes and austral winter solstice are showed. the intensity is maximum in austral summer, and minimum in austral winter. 308 fig. 8. average behaviour of coherency, degree of polarization, angle of polarization, ellipticity and sense of polarization. claudia rossi, paolo palangio and franco rispoli fig. 9a,b. spectral shape of a) linearly polarized signals, b) signals due to the traditional generation mechanism. a b 309 investigations on diurnal and seasonal variations of schumann resonance intensities in the auroral region is much harper than that appearing in fig. 9b which represents the spectral power density of the signal due to the traditional generation mechanism. the shape of the spectra is strongly influenced by the number of averaged fft used to get the spectral analysis. this influence is, however, not of great practical importance since we compare values obtained by the same procedure. 3.4. discussion the analysis performed has evidenced an annual variation in the m1 peakhour of appearance. in their works keefe et al. (1964), and sentman and fraiser (1991) observed that the intensities of the horizontal components of the schumann resonances maximize near local noon at the locations of their measurements (rhode island, germany, california and western australia). they attributed the observed modulation to the variation of the height of the d-region of the ionosphere. due to the high latitude of our station, a direct comparison with our measurements is quite difficult. we think that plasma structures in the f layer may play a role as wave-guides, which could canalise the sr waves along the geomagnetic field. for frequencies in the sr band the wavelengths are comparable with the thickness of the ionosphere, and the waves could be confined into the region around the alfven minimum. such structures above the polar regions enhance the probability that the magnetospheric signals exit from the earth-ionosphere waveguide. satellites measurements have shown that low frequency waves are generated in the magnetosphere by different types of instability, and propagate along the lines of force of the geomagnetic field to reach the lower ionosphere in the polar regions where signals from low and middle latitude strokes are weak. moreover the observations from satellites show elf emissions at high latitudes, which maximize in the interval of magnetic latitude (ml) 73°-80°. cerenkov radiation involving a longitudinal resonance and low energy electrons connected to physical processes in the magnetosphere might produce them. waves linearly polarized could originate in the magnetosphere, when observed from the ground, they appear to originate from currents in the ionosphere. the signals observed on the ground can be interpreted as a convolution of a source function, the intensity of the waves in the magnetosphere, and the propagation function, which takes account of the effects of ionospheric conductivity inhomogeneities on the mode waves as they travel through the ionosphere, guided along the magnetic field. the intensity of both m1 and m2 peaks undergoes an annual variation characterised by a maximum value in the austral summer. sátori and zieger (1996), and märcz et al. (1997), performed measurements of the vertical electric field component in the frequency range of schumann resonance at nagycenk observatory in hungary. they found an annual variation in the vertical electric field intensity. in particular they observed a maximum in the northern hemisphere summer. märcz et al. (1997) correlated the schumann resonances amplitudes to simultaneous measurements of the atmospheric electric potential gradient. they explained their results in terms of correlation between surface air temperature and parameters of the atmospheric electric global circuit. in particular the observed summer maximum in the schumann resonances amplitudes is considered partly a response to the intrinsic global source (i.e. to the increased global thunderstorm activity in this season), and partly the response to the source proximity effect. our observations agree with those of sátori and zieger (1996), and märcz et al. (1997), however, due to the tnb location, they are not affected by source proximity effect. in future extended statistical analysis we intend to study the possible connection between pc3-4 pulsations and events linearly polarized, because most of this signals were observed with magnetic pulsations in the pc3-4 range. 4. summary measurements of the magnetic power of the 8 hz sr mode were performed at tnb during the years 1996-1997. diurnal and seasonal vari310 claudia rossi, paolo palangio and franco rispoli ability were observed in the horizontal components of the magnetic power. in the diurnal trend two principal peaks were found: a first one (m1) lying in the time interval 8-12 lt, and a second one (m2) approximately at 20-21 lt. for the m1 peak we observe a seasonal variation in both hours of appearance and amplitude. the study of the m2 peak is more difficult, because m2 is missing on some days. we notice a seasonal variation of its amplitude, while its hour of appearance does not seem to vary during the year. the observed elf in the auroral zone is different from that at middle or low latitudes. we think that plasma structures in the f layer may play a role as wave-guides, which could canalise the sr waves along the geomagnetic field. this is compatible with satellite measurements. moreover in the auroral regions the f layer density has a maximum in the hours of the m2 peak appearance. this preliminary analysis represents a reliable starting point for further studies to make use of long data series to identify a possible magnetosphere-ionosphere influence superimposed on the earth-ionosphere cavity mode. these studies will be the subject of a separate paper. references abbas, m. (1968): hydromagnetic wave propagation and excitation of schumann resonances, planet. space sci., 16, 831-844. balser, m. and c.a. wagner (1960): observations of earthionosphere cavity resonances, nature, 188, p. 638. balser, m. and c.a. wagner (1962a): diurnal power variations in the earth-ionosphere cavity modes and their relationship to world-wide thunderstorm activity, j. geophys. res., 67, p. 619. balser, m. and c.a. wagner (1962b): on frequency variations of the earth-ionosphere cavity modes, j. geophys. res., 67, p. 4081. balser, m. and c.a. wagner (1964): thunderstorm excitation of the earth-ionosphere cavity, in propagation of radio waves at frequencies below 300 kc/s, edited by w.t. blackband (pergamon press), pp. 257. bliokh, p.v., a.p. nikolaenko and yu.f. filippov (1980): schumann resonances in the earth-ionosphere cavity (peter perigrinus, london), pp. 168. burke, c.p. and d.l. jones (1992): an experimental investigation of elf attenuation rates in the earth-ionosphere duct, j. atmos. terr. phys., 54, 243-250. clayton, m.d. and c. polk (1977): diurnal variation and absolute intensity of world-wide lightning activity, september 1970 to may 1971, in electrical processes in atmospheres, edited by h. dolezalek and r. reiter (verlag, darmstadt, germany), p. 440. füllekrug, m. (1995): schumann resonances in magnetic field components, j. atmos. terr. phys., 57 (5), 479-484. füllekrug, m. and a.c. fraser-smith (1996): further evidence for a global correlation of the earth-ionosphere cavity resonances, geophys. res. lett., 23 (20), 27732776. galejs, j. 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(received december 18, 2006; accepted april 16, 2007) layout 6 the april 6, 2009, mw 6.3, l'aquila sequence: weak-motion and strong-motion data recorded by the rais temporary stations ezio d'alema1, simone marzorati1, marco massa1, gianlorenzo franceschina1, paolo augliera1,* 1 istituto nazionale di geofisica e vulcanologia, sezione di milano-pavia, milano, italy annals of geophysics, 53, 2, april 2010; doi: 10.4401/ag-4565 abstract the aim of this study is the sharing of waveforms recorded by several istituto nazionale di geofisica e vulcanologia (ingv) temporary stations (managed by the milan-pavia section; ingv mi-pv). these stations were installed after the april 6, 2009, mw 6.3, l'aquila earthquake (central italy). the work synthesizes the activities conducted in the field by the ingv mi-pv working group over the three months following the mainshock. the field activities were developed in four different phases that were defined according to their time periods. starting from april 7, 2009, for the first phase, the temporary stations were installed in correspondence with the more damaged areas. the scope was to record the strongest aftershocks in the days that followed the mainshock. in this phase, the stations were composed of a sixcomponent acquisition system that was coupled with both a weak-motion and a strong-motion sensor. after the first month, the last three phases of installation investigated the seismic responses of sites located in the epicentral area, involving villages within a radius of about 20 km from the epicenter of the april 6 mainshock. in this way, over four specific time-period phases, the stations were installed in sites with different lithological and geomorphological conditions. the instruments worked from april 7 to july 14, 2009; in this period, 9,155 aftershocks (134,262 accelerometric waveforms and 133,242 velocimetric waveforms), with ml ≤5.3 were recorded. this study describes the dataset of these earthquake waveforms recorded with both velocity and acceleration transducers. selected waveforms are available through ftp://ftp.mi.ingv.it/download/rais-ts_rel01/, with their corresponding information concerning instrumental characteristics, installation sites, and earthquakes recorded. introduction on april 6, 2009, a mw 6.3 normal faulting event struck close to the city of l'aquila in the abruzzo region, central italy [ameri et al. 2009, anzidei et al. 2009, cirella et al. 2009]. soon after the mainshock, the first emergency structure of the centro nazionale terremoti (national earthquake centre) of the istituto nazionale di geofisica e vulcanologia (ingv-cnt; http://cnt.rm.ingv.it/) was activated, for the installation of a real-time temporary seismic network in the epicentral area [chiarabba et al. 2009], with the aim of capturing the migration of the hypocenters of the aftershocks. in the meantime, the ingv milan-pavia section (ingv mi-pv) made available the mobile stations of the rete accelerometrica in italia settentrionale (rais; the strong-motion network of northern italy, http://rais.mi.ingv.it) [augliera et al. 2009], to set up a highdensity temporary network of stations (the rais-ts). in this way, ingv mi-pv operated together with the ingv rome1 section (http://www.roma1.ingv.it/) and the helmholtz centre, potsdam, germany (http://www.gfzpotsdam.de/portal/gfz/home) in the framework of a coordinated experiment [cultrera et al. 2009]. a relevant number of stations were installed in the area surrounding the town of l'aquila (in the upper aterno valley), near to or inside the most damaged villages [galli et al. 2009] and where co-seismic surface effects have been observed [emergeo working group 2010]. in the first month after the april 6, 2009, mw 6.3, l'aquila mainshock (first phase), the rais-ts were mainly installed corresponding to villages that had experienced the greatest damage. in this phase, the rais-ts were installed also with the aim of recording the more energetic aftershocks and to evaluate the presence of possibile nonlinear effects; in particular, accelerometric sensors were used to avoid signal saturation. in the last three phases, the rais-ts were moved to several areas up to 20 km from the town of l'aquila (see figure 1), to evaluate the local site responses using ambient noise and/or earthquake recordings. these phases of installation was carried out in agreement with the research groups operating in the area, with the aim of investigating a relevant number of areas, while avoiding superimposition of recording stations. the present study describes the features of the data collected by rais-ts in the epicentral area of the l'aquila mainshock (mw 6.3) during these four phases over the three months following the april 6, 2009, mainshock. the rais-ts recorded more then 267,000 waveforms that article history received february 11, 2010; accepted july 14, 2010. subject classification: surveys, measurements and monitoring, seismological data, l'aquila earthquake, aftershocks. 101 l'aquila temporary network 102 figure 1. the rais-ts stations. the coloured triangles indicate the locations of the temporary stations installed in the different periods (as indicated). event id data origin time (gmt) latitude (° n) longitude (° e) depth (km) ml 20090407092628 2009-04-07 09:26:28 42.342 13.388 10.2 4.7 20090407174737 2009-04-07 17:47:37 42.275 13.464 15.1 5.3 20090407213429 2009-04-07 21:34:29 42.380 13.376 7.4 4.2 20090408225650 2009-04-08 22:56:50 42.507 13.364 10.2 4.3 20090409005259 2009-04-09 00:52:59 42484 13.343 15.4 5.1 20090409031452 2009-04-09 03:14:52 42.338 13.437 18.0 4.2 20090409043244 2009-04-09 04:32:44 42.445 13.420 8.1 4.0 20090409193816 2009-04-09 19:38:16 42.501 13.356 17.2 4.9 20090413211424 2009-04-13 21:14:24 42.504 13.363 7.5 4.9 20090414201727 2009-04-14 20:17:27 42.530 13.288 10.4 4.1 20090423151408 2009-04-23 15:14:08 42.247 13.492 9.9 4.0 20090423214900 2009-04-23 21:49:00 42.233 13.479 9.3 4.0 20090622205840 2009-06-22 20:58:40 42.446 13.356 14.2 4.5 20090703110307 2009-07-03 11:03:07 42.409 13.387 8.8 4.1 20090712083851 2009-07-12 08:38:51 42.338 13.378 10.8 4.0 table 1. earthquake list for the present study (ml ≥4.0). the focal parameters come from the iside bulletin (http://iside.rm.ingv.it/iside/standard/index.jsp; last accessed on december 4, 2009). 103 l'aquila temporary network table 2. the rais temporary stations. in the last column a preliminary soil (s) classification is reported (r = rock and s = soft). v. sens. and a. sens. indicate velocimetric and accelerometric sensors respectively. s. time is the starting time, e. time is the ending time. lon, latand el indicate longitude, latitude and elevation. code locality lon (° e) lat (° n) el (m) v. sens. a. sens. s. time ( d/m/y hh:min) e. time ( d/m/y hh:min ) s mi01 pescomaggiore 13.5098 42.3580 740 le3d-lite fba es-t 07/04/09 07:14 27/04/09 11:14 r mi02 paganica 13.4743 42.3545 648 le3d-5s fba es-t 07/04/09 08:38 29/04/09 07:38 s mi03 onna 13.4757 42.3274 581 le3d-5s fba es-t 07/04/09 11:35 09/06/09 09:00 s mi04 fossa 13.4814 42.3031 612 le3d-lite 07/04/09 15:03 27/04/09 09:14 r mi05 s. eusanio forconese 13.5252 42.2895 585 le3d-5s fba es-t 07/04/09 16:49 28/04/09 08:05 s mi06 camarda 13.4979 42.3894 807 le3d-lite fba es-t 27/04/09 14:18 09/06/09 08:48 s mi07 bazzano 13.4459 42.3362 662 le3d-lite fba es-t 27/04/09 17:14 09/06/09 08:14 s mi08 sant'antonio 13.3713 42.3574 650 le3d-lite fba es-t 28/04/09 13.12 09/06/09 10:12 s mi09 san giacomo 13.4144 42.3658 782 fba es-t 28/04/09 15.20 09/06/09 08:20 s mi10 sant'elia 1 13.4307 42.3368 607 le3d-5s fba es-t 29/04/09 11:32 08/06/09 14:53 s mi11 sassa zona polivalente 13.3183 42.3659 657 le3d-5s 29/04/09 15:58 08/05/09 11:00 s mi12 cese di preturo 13.2910 42.3716 672 fba es-t 29/04/09 17:00 09/06/09 11:49 s mi13 sassa zona polivalente 13.3183 42.3659 657 fba es-t 08/05/09 15:12 08/06/09 13:12 s mi14 villa sant'angelo 13.5365 42.2706 587 le3d-5s 09/06/09 15:15 24/06/09 07:15 s mi15 vallecupa 13.5740 42.2532 680 le3d-5s fba es-t 10/06/09 15:46 22/06/09 15:46 s mi16 pedicciano 13.5916 42.2411 749 le3d-5s 10/06/09 17:52 25/06/09 09:08 s mi18 fossa 13.4904 42.2970 622 le3d-5s 11/06/09 08:14 22/06/09 13.14 s mi19 casentino 13.5114 42.2806 579 fba es-t 11/06/09 09:56 24/06/09 08:56 s mi20 stiffe 13.5464 42.2567 609 fba es-t 11/06/09 11:24 24/06/09 08:24 s mi21 fossa 13.4928 42.2827 741 le3d-5s 11/06/09 15:54 22/06/09 13:54 r mi22 vallecupa 13.5767 42.2534 692 le3d-5s fba es-t 12/06/09 17:20 25/06/09 07:55 r mi23 arischia 13.3370 42.4185 862 le3d-5s fba es-t 23/06/09 16:30 14/07/09 09:30 s mi24 arischia 13.3498 42.4228 1052 le3d-5s 23/06/09 13:40 14/07/09 08:30 r mi25 arischia 13.3437 42.4146 810 le3d-5s 23/06/09 10:10 14/07/09 09:15 s mi26 s. demetrio ne' vestini 13.5630 42.2914 697 fba es-t 24/06/09 12:06 14/07/09 11:06 s mi27 san pio delle camere 13.6552 42.2819 807 fba es-t 24/06/09 14:29 14/07/09 12:29 s mi28 poggio picenze 13.5370 42.3186 715 le3d-5s 24/06/09 16:14 13/07/09 08:14 s mi29 barisciano 13.6035 42.3354 1191 le3d-5s fba es-t 25/06/09 12:50 14/07/09 11:50 r mi30 barisciano 13.5944 42.3205 899 le3d-5s 25/06/09 14:50 13/07/09 15:50 s related to 9,155 aftershocks. in general, the selected waveforms refer to earthquakes with epicentral distances up to 65 km, 93% of which were recorded at distances <30 km. in particular, 15 aftershocks with magnitudes >4.0 were recorded in the first month, two of which had magnitudes >5.0 (table 1). the aftershocks were recorded by 29 sites, corresponding to 25 localities inside the province of l'aquila (table 2). the main scope of the present study is the dissemination of the data for these collected waveforms, to stimulate further research associated with the april 6, 2009, l'aquila earthquake, and with the sincere belief that it will be useful for both the safety and awareness of the population. data acquisition starting from april 7, 2009, several instruments of the rais-ts (figure 1) were installed in the epicentral area of the april 6, 2009, l'aquila mainshock (mw 6.3), to achieve the above-mentioned tasks regarding both seismic monitoring and local site-effect investigations. due to the limited number of instruments available, a particular fourphase installation strategy was adopted to set up different network configurations during the evolution of the sequence. the stations were working globally from april 7 to july 14, 2009, and were installed in sites with different lithological and geomorphological conditions, according to four different phases of network configurations that corresponded to the following time periods: april 7-26, 2009: in this first phase, five rais-ts stations were installed corresponding to the more damaged villages [galli et al. 2009]. four six-component stations (with three-component velocimetric and three-component accelerometric transducers) and one three-component station (with a threecomponent velocimetric sensor) were installed near the municipalities of pescomaggiore, paganica, onna, fossa, sant'eusanio forconese (figure 1). april 27 june 8, 2009: during this second phase, the rais-ts were increased to seven stations (four with six-component and three with three-component instruments), which were installed in selected areas located in the municipality of l'aquila (near to the source): camarda, bazzano, sant’antonio, san giacomo, sant'elia, sassa zona polivalente, and cese di preturo (figure 1). the new configuration of the network was set up to investigate the seismic responses of these areas, assessing the soil resonance frequencies through empirical analysis of the events recorded: the horizontal to vertical spectral ratios (h/v) of earthquakes [lermo and chavez-garcia 1993] and receiver functions [borcherdt 1970]. june 9-22, 2009: in this third phase, the network was composed of eight stations (two with six-component and six with threecomponent instruments) that were installed in areas of the aterno valley, between 10 km and 20 km se of the town of l'aquila, between the municipalities of fossa and pedicciano (figure 1). the instrumentation was installed inside the municipalities of fossa (two stations), fagnano, casentino, villa s. angelo, stiffe, vallecupa, and pedicciano. june 23 july 14, 2009: in this last period, the rais-ts were moved in both nw and se directions, to complete the geophysical characterization of the aterno valley. this last phase included a total of eight installations, five of which were in the municipalities of poggio picenze, san demetrio ne' vestini, barisciano (two stations) and san pio delle camere (sites located se of l'aquila), and three corresponding to the municipality of arischia, located about 10 km nw of l'aquila (figure 1). in general the seismic stations (see table 3) were equipped with 5-s or 1-s lennartz velocimetric sensors and kinemetrics episensors (strong-motion sensors), coupled l'aquila temporary network 104 seismic sensors type name natural frequency (hz) sensitivity poles zeros velocimeter lennartz le-3d/5s 0.2 400 v/m/s -0.888 + 0.888j -0.888 0.888j -0.220 + 0.000j triple zero at the origin velocimeter lennartz le-3dlite 1.0 400 v/m/s -4.444 + 4.444j -4.444 – 4.444j -1.083 + 0.000j triple zero at the origin accelerometer kinemetrics episensor fba es-t 200 10 v/g s seismic recorder reftek 130-01 3 or 6 channel recording resolution 24 bit clock: gps sampling 100 hz table 3. instrumentation. 105 with 24-bit reftek 130 digital recorders (of both three and six channels). the time code adopted was based on the global positioning system (gps), and the electricity supplies were assured by solar panels with service batteries. where possible, the sensors were buried 30 cm below the soil. the seismic data were recorded on local memory cards and the instruments were sheltered in watertight boxes. figure 2 shows the seismicity recorded by the rais-ts over the whole of the above-mentioned periods. data selection all of the stations recorded in continuous mode (the data were stored in 1-hour-long data files), so it was possible to recover the time series related to each event from continuous signals. to extract the waveforms recorded from the rais-ts over the whole period considered, we refer to the hypocenters reported in the ingv official seismic bulletin (italian seismological instrumental and parametric database; iside; http://iside.rm.ingv.it/iside/standard/index.jsp; last accessed, december 4, 2009). the iside reference location file is composed of quasireal-time precise locations, data that came from the italian national seismic surveillance, an advanced real-time system for seismic data analysis operated by the ingv-cnt in rome, italy. this provides the first location estimate of any italian earthquake within 40 s of its origin time, starting from a minimum ml of 1.8, along with a definitive location within 5 min. although all of the events detected by the italian national seismic network are located automatically, the iside only reports the list of locations as revised by the seismologist who is in charge of the seismic surveillance activity when the earthquake occurs. this localization is the best evaluation of the hypocenter parameters during the evolution of a seismic sequence. l'aquila temporary network figure 2. the rais-ts recorded events. each panel shows the seismicity and the station distribution for each period considered (as indicated). to make the recorded waveforms of the l'aquila sequence available in a short period, we extracted the data from april 7, 2009 at 07:10 (gmt) to july 14, 2009 at 15:50 using the iside. we considered all of the events, without any magnitude selection, that occurred within a rectangular area limited between 42.15˚ and 42.70˚ north in latitude and between 13.00˚ and 13.80˚ east in longitude. for all of the events, the reference time was set to the origin time of the earthquake. we considered a pre-event window for data extraction of 30 s, and a post-event window with a duration that depended on the magnitude of the event (see table 4). the final events listed include a total of 9,155 earthquakes with ml ≤5.3 (figure 3). table 1 reports an example of the event list for the earthquakes with magnitude >4.0. the complete list of the extracted dataset is available on the ftp site. the dataset is composed of a total of 267,504 waveforms, 134,262 of which are strong-motion waveforms, and 133,242 of which are velocimetric data. figure 4 shows the magnitude versus epicentral distance distribution of the whole dataset. the selected earthquakes were recorded by a variable number of stations, depending on the network configuration at the time. the coincidence list in table 5 shows an example for the events with magnitudes >4.0. even if in the present study we operated a selection based on iside, since the waveforms were recorded in continuous mode, further time series can, however, be recovered on demand from continuous recordings. dataset structure and data format the earthquake waveforms were at first extracted from continuous signals, and then converted into little-endian sac binary files (http://www.iris.washington.edu/software/sac/) [tapley and tull 1992]. each time series was individually written into the sac file data section as an evenly sampled data sequence. consequently, the sac header variables leven and iftype were set to true and itime, respectively. all of the data were sampled at 100 sps and the data values are expressed in counts. ground motion units can be obtained through the sac header variables user0 and user1, respectively set as sensitivity (v/m/s and v/g for velocimetric and accelerometric sensors, respectively), and digitization constant (v/count) and kuser0 (gain). the extracted waveforms were baseline corrected using the sac command rtrend, which removes any linear trend from l'aquila temporary network 106 table 4 (left). selection criteria for waveform extraction. the increasing post-event times are related to the increasing ml. figure 3 (right). all the selected earthquakes. the magnitude distribution (circles) and cumulative number of events according to magnitude (stars). figure 4. magnitude versus epicentral distance distribution of the events recorded. top panel: records from the strong-motion sensors. bottom panel: records from the weak-motion sensors. in both panels, the data located above and below the dashed lines indicate records with low signalto-noise ratio (top panel, for the accelerometers) and possible signal saturation (bottom panel, for velocimeters). magnitude class pre-event time (s) post-event time (s) number of events m >= 4.0 30 240 15 3.0 <= m < 4.0 30 150 131 2.0 <= m < 3.0 30 90 1505 1.0 <= m < 2.0 30 60 6984 m < 1.0 30 30 520 n u m b e r o f e v e n ts epicentraldistance(km) 107 the data and media removed using the sac command rmean. the waveforms were not corrected for the instrument response and were not filtered. the sac header parameters were generally set according to the conventions adopted by the sac users manual (http://www.iris.edu/manuals/sac/manual.html), with a certain number of variables customized for the present dataset. the complete description of the sac header variables is reported in the appendix. the waveforms datasets were organised following an event code, which was defined by the origin time of the earthquake (expressed as year, month, day, hour, minute and second), as reported in iside. the dataset was structured into directories with names that correspond to the event codes. each directory contains sac files with the waveforms related to the stations that recorded the corresponding earthquake. the file names are composed of four different sections: extraction time (year, month, day, hour, minute and second, defined according to the above-mentioned pre-event time, which was selected to be 30 s before the origin time of the earthquake); station code (four characters); channel (according to the seed-format conventions; see below); and file type (e.g. .sac). the seed-format convention for the channel naming adopts three characters, each of which describes a single aspect of the data. the first character specifies the general sampling rate and the response frequency band of the instrument: for example e stands for «extremely short period instrument with a sample rate in the range of 80-250 hz and a corner period <10 s». the second character specifies the sensor family: h stands for «high-gain seismometer» and n for accelerometer data. the third character specifies the orientation code (z, n, e for traditional vertical, north-south, east-west). for more information, please refer to appendix a of the seed reference manual (incorporated research institute for seismology; iris, 2009; http://www.iris.washington.edu/manuals/seed_appa.htm). for example, following the iside origin time, an event that occurred on april 9, 2009 (at 19:38:16) and recorded by station mi01 (equipped with a lennartz le3d-lite velocimetric sensor, corner period 1 s and sampling 100 sps) was extracted at 19:37:46: in this case the name of the related file (included in the folder /20090409193816/) will be 20090409193746_mi01.ehz.sac. all of the extracted sac waveforms are included in the section /waveforms/ of the dataset in which the directories are grouped according to four magnitude classes that correspond to the different earthquakes: 3.0 ≤ml <5.5 (146 events); 2.0 ≤ml <3.0 (1,505 events); 1.5 ≤ml <2.0 (3,673 events); 0.0 ≤ml <1.5 (3,831 events). each class corresponds to a uncompressed tar archive file, which also includes the information files of the whole dataset. the information files are subdivided into sections /maps/ and into /files/, the former of which includes kml files to display the stations and epicentres in an earth browser such as google earth (http://earth.google.com). the section /files/ includes bulletin files, coincidence files, station books, and instrument information. the complete structure of the datasets is summarized in figure 5. conclusions the dataset presented in the present study consists of more than 267,000 waveforms related to 9,155 aftershocks that occurred after the april 6, 2009, mw 6.3, l'aquila earthquake, and were recorded by the rais-ts over the period of april, 7 july 14, 2009. the selected waveforms are l'aquila temporary network table 5. coincidence list of the dataset events for m ≥4.0. station: a = accelometer; v = velocimeter. component: n = north-south; e = east-west; z = up-down. for each event and for each station the existing components are marked as «+». all available in sac format, and they correspond to events with a maximum epicentral distance of 65 km. in particular, the dataset includes 15 events with ml >4, two of which have an ml >5, which were recorded at epicentral distances of <30 km (table 1). the waveforms have been extracted from signals recorded in continuous mode after selection of the corresponding time windows, based on the origin times reported in the iside bulletin. the localizations used for this dataset corresponded to the best evaluations of hypocenter parameters available in december, 2009. minimal pre-processing was applied to all of the records, which basically consisted of linear-trend removing. the location parameters and other information about the recorded events, station characteristics and instrumental settings are included in the sac file headers. it is worth noting that in the case of energetic events recorded by stations close to the epicenter, the weak-motion channels (velocimetric sensor) show saturated waveforms (see figures 4 and 6, dashed lines in the bottom panels). in contrast, the strong-motion stations that recorded all of the strongest aftershocks can show low-quality data (in terms of signal-to-noise ratios) related to weak motions (ml ≤1.5), especially if they were recorded at distances greater than about 20 km. in any case, the database described in the present study includes all of the records. it will be the responsability of the final users to focus their attention on these considerations. the maximum recorded peak ground acceleration was 0.69 g at 5.4 km of epicentral distance (station mi05) during the april 7, 2009, ml 5.3, earthquake (figure 7). as is usual in seismic sequences, the first period of activity was characterized by a high seismic rate, with a large number of events over short time periods. the waveforms belonging to the different earthquakes often overlapped within the same time windows (figure 8). the 267,504 selected waveforms and the corresponding information relating to the instruments, installation sites and recorded earthquakes are all downloadable from ftp://ftp.mi.ingv.it/download/rais-ts_rel01/ acknowledgements. the authors would like to thank all of the people that helped in the fieldwork and analysed the waveforms. there were a lot of people involved, and they should all be noted. particular thanks go to our colleagues of ingv mi-pv: gabriele ameri, chiara ladina, sara lovati, lucia luzi, mariano maistrello, francesca pacor, carlo piccarreda, davide piccarreda, rodolfo puglia and fabrizio galadini. thanks are also due to our colleagues of gfz and ingv-rome1 for their collaboration during the fieldwork. l'aquila temporary network 108 figure 5. dataset structure (see text for explanation). figure 6. april 7, 2009, ml = 5.3, aftershock recorded at station mi05 by a six-component acquisition system equipped with both strong-motion (left panel; hne, hnn and hnz for east-west, north-south and vertical components) and weak-motion (right panel; ehe, ehn and ehz for east-west, northsouth and vertical components) sensors. it is worth noting that the strong-motion sensor (time sequences on the left) recorded the event without saturation. 109 l'aquila temporary network figure 8. waveforms of a ml = 1.5 earthquake recorded at station mi01 by a six-component acquisition system equipped with both strong-motion (left panel) and weak-motion (right panel) sensors. at least two of these events overlap between 30 s and 40 s, and a small event occurred before the selected earthquake was detected (between 10 s and 20 s). figure 7. april 7, 2009, ml = 5.3, aftershock recorded at strong-motion stations mi01, mi02, mi03 and mi05 (top panel, east-west; middle panel, northsouth; and bottom panel, vertical components) in the near source (epicentral distances <10 km). references ameri, g., m. massa, d. bindi, e. d'alema, a. gorini, l. luzi, m. marzorati, f. pacor, r. paolucci, r. puglia and c. smerzini (2009). the 6 april 2009, mw 6.3, l'aquila (central italy) earthquake: strong-motion observations, seism. res. lett., 80., 951-966. anzidei, m., e. boschi, v. cannelli, r. devoti, a. esposito, a. galvani, d. melini, g. pietrantonio, f. riguzzi, v. sepe and e. serpelloni (2009). coseismic deformation of the destructive april 6, 2009 l'aquila earthquake (central italy) from gps data, geophys. res. lett., 36, l17307. augliera, p., e. d'alema, s. marzorati and m. massa (2009). a strong-motion network in northern italy: detection capabilities and first analyses, bull. earthquake eng., doi: 10.1007/s10518-009-9165-y. borcherdt, r.d. (1970). effects of local geology on ground motion near san francisco bay, bull. seism. soc. am., 60, 29-61. chiarabba, c., a. amato, m. anselmi, p. baccheschi, i. bianchi, m. cattaneo, g. cecere, l. chiaraluce, m.g. ciaccio, p. de gori, g. de luca, m. di bona, r. di stefano, l. faenza, a. govoni, l. improta, f.p. lucente, a. marchetti, l. margheriti, f. mele, a. michelini, g. monachesi, m. moretti, m. pastori, n. piana agostinetti, d. piccinini, p. roselli, d. seccia and l. valoroso (2009). the 2009 l'aquila (central italy) mw 6.3 earthquake: main shock and aftershocks, geophys. res. lett., 36, l18308. cirella, a., a. piatanesi, m. cocco, e. tinti, l. scognamiglio, a. michelini, a. lomax and e. boschi (2009). rupture history of the 2009 l'aquila (italy) earthquake from nonlinear joint inversion of strong motion and gps data, geophys. res. lett., 36, l19304. cultrera, g. and l. luzi with g. ameri, p. augliera, r.m. azzara, f. bergamaschi, e. bertrand, p. bordoni, f. cara, r. cogliano, e. d'alema, d. di giacomo, g. di giulio, a.m. duval, a. fodarella, g. franceschina, m.r. gallipoli, p. harabaglia, c. ladina, s. lovati, s. marzorati, m. massa, g. milana, m. mucciarelli, f. pacor, s. parolai, m. picozzi, m. pilz, r. puglia, s. pucillo, j. régnier, g. riccio, j. salichon and m. sobiesiak (2009). valutazione della risposta sismica locale di alcuni siti dell’alta e media valle dell’aterno, progettazione sismica, 3, 69-73. emergeo working group (2010). evidence for surface rupture associated with the mw 6.3 l'aquila earthquake sequence of april 2009 (central italy), terra nova, 22, 43-51. galli, p., r. camassi, r. azzaro, f. bernardini, s. castenetto, d. molin, e. peronace, a. rossi, m. vecchi and a. tertulliani (2009). terremoto de l'aquila del 6 aprile 2009: distribuzione delle intensità macrosismiche ed implicazioni sismotettoniche, il quaternario, 22, 235-246. lermo, j. and f.j. chavez-garcia (1993). site effect evaluation using spectral ratio with only one station, bull. seism. soc. am., 83, 1574-1594. tapley, w.c. and j.e. tull (1992). sac – seismic analysis code, users manual, rev.4., university of california (usa). corresponding author: dr. paolo augliera, istituto nazionale di geofisica e vulcanologia, sezione di milano-pavia, milano, italy; e-mail: paolo.augliera@mi.ingv.it © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. l'aquila temporary network 110 111 appendix l'aquila temporary network sac header example file: 20090408175805_mi02.hnz.sac variable description possible values rais-ts field (*) typical value npts number of points per data component. 18000 b beginning value of the independent variable 0.000000e+00 a ending value of the independent variable 1.800000e+02 iftype type of file = itime {time series file} = irlim {spectral file---real and imaginary} = iamph {spectral file--amplitude and phase} = ixy {general x versus y data} = ixyz {general xyz (3-d) file} time series file leven true if data is evenly spaced true delta increment between evenly spaced samples 1.000000e-02 depmin minimum value of dependent variable -6.655318e+04 depmax maximum value of dependent variable 4.807683e+04 depmen mean value of dependent variable 2.723649e-06 o event origin time (seconds relative to reference time.) 30 ko event origin time identification. (t0) kzdate alphanumeric form of gmt reference date apr 08 (098), 2009 kztime alphanumeric form of gmt reference time 17:58:05.000 iztype reference time equivalence = iunkn (unknown) = ib (begin time) = iday (midnight of reference gmt day) = io (event origin time) = ia (first arrival time) = itn (user defined time pick n, n=0,9) begin time kinst generic name of recording instrument. x rtf130 kstnm station name x mi02 cmpaz component azimuth (degrees clockwise from north) x 0.000000e+00 cmpinc component incident angle (degrees from vertical). x 0.000000e+00 stla station latitude (degrees, north positive) x 4.235449e+01 stlo station longitude (degrees, east positive). x 1.347428e+01 stel station elevation (meters). x 6.480000e+02 kevnm event name x abruzzo x on=1; off=0 x (1=butterworth filter on; 0= b l'aquila temporary network 112 variable description possible values rais-ts field (*) typical value x m x evla event latitude (degrees, north positive) x 4.236400e+01 evlo event longitude (degrees, east positive). x 1.339600e+01 evdp event elevation (meters) x 8.800000e+00 dist station to event distance (km). 6.535192e+00 az event to station azimuth (degrees). 9.927607e+01 baz station to event azimuth (degrees). 2.793286e+02 gcarc station to event great circle arc length (degrees). 5.879549e-02 lovrok true if it is okay to overwrite this file on disk. true user0 sensitivity velocimetric lennartz sensor=400; accelerometer kinemetrics sensor=10 x 1.000000e+01 user1 digitalization constant v/c 1.58997e-06 x 1.589970e-06 user2 trend removal (1=rtrend on; 0=rtrend off) x 1.000000e+00 user3 tapering on=1; off=0 x 0.000000e+00 user4 mean removal (1=rmean on; 0=rmean off) x 1.000000e+00 user5 filtering procedure (1=butterworth filter on; 0= butterworth filter off) x 0.000000e+00 kuser0 sensitivity unit # gain velocimeter sensor=v/m/s accelerometric sensor=v/g gain=1 or 32 x v/g#1 kuser1 trace unit count x count kuser2 sensor model episens=kinemetric episensor; le3d-1s=lennartz lite 1 second; le3d5s=lennartz 5 seconds x episens nvhdr header version number. current value is the integer 6. older version data (nvhdr < 6) are automatically updated when read into sac 6 norid origin id 0 nevid event id 0 lpspol true if station components have a positive polarity (lefthand rule). false lcalda true if dist, az, baz, and gcarc are to be calculated from station and event coordinates. true kcmpnm component name hnz= accelerometric vertical component (up-down) hnn= accelerometris orizzontal component (northsouth) hne= accelerometris orizzontal component (eastwest) ehz=velocimetric vertical component (up-down) ehn= velocimetric orizzontal component (north-south) ehe=velocimetric orizzontal component (east-west); seed convention x hnz knetwk name of seismic network. x rais x 3 113 l'aquila temporary network variable description possible values rais-ts field (*) typical value x l s x mag event magnitude. x 3.200000e+00 imagtyp magnitude type = imb (bodywave magnitude) = ims (surfacewave magnitude) = iml (local magnitude) = imw (moment magnitude) = imd (duration magnitude) = imx (user defined magnitude) x local magnitude (*) the user defined parameters of the rais-ts network are labeled with “x”. r i c e r c h e g e o m a g n e t i c h e sopra u n s u p p o s t o d i a p i r o 0 . b a t t e l l i a . francaviglia m . giorgi f . m o l i n a r i c e v u t o il 26 luglio 1962 nella prima edizione della carta geologica della sicilia, e precisamente nel foglio 268 « caltanissetta », nella tavoletta « xirbi » (268 iv so), poco a nord della stazione ferroviaria di caltanissetta xirbi, allora denominata s. caterina xirbi, è segnato uno spuntone di calcare riferito all'eocene superiore, formante la collinetta di cozzo dei disi, attraversato da una intrusione di roccia basica. questo spuntone è stato descritto da baldacci (*) come formato da calcare nummulitico e marne a fucoidi riferibile all'eocene superiore, « attraversato da un piccolo dicco di basalto, ormai quasi distrutto, ma la cui presenza si riconosce bene per l'alterazione prodotta nei calcari stessi e per l'abbondanza di zeoliti che si trovano sparse in quella località ». occorre ricordare che, secondo baldacci, nella zona considerata esiste una lacuna stratigrafica fra l'eocene superiore e il miocene medio. egli, infatti, dice che i terreni fondamentali fra s. caterina villarmosa e caltanissetta sono costituiti da argille mioceniche con strati di sabbie più o meno cementate, che a volte diventano predominanti; ci informa, inoltre, dell'esistenza di alcune faglie, con direzione predominante neso, le quali mettono in vista i terreni inferiori, rappresentati da larghe zone di argille scagliose dell'eocene medio. una delle faglie più importanti è quella che ha inizio a sud presso la mass. chiapperia, si dirige verso la c.da garistoppa, passando per l'anzidetta collina di cozzo dei disi. poiché baldacci ricorda anche dei calcari a rudiste presso la mass. pescazzi, ubicata lungo la faglia, e degli scisti marnosi bruni riferiti all'eocene medio-inferiore, risulta evidente che egli ritiene che nella zona esista una serie stratigrafica normale e continua dal cretaceo all'eocene superiore, alla quale seguono gli strati miocenici separati dalla lacuna stratigrafica alla quale abbiamo accennato. in conseguenza, i calcari della collinetta di cozzo dei disi, al pari di quelli 22h o . b a t t e l l i a . f k a n c a v i g l i a m. g i o r g i f . m o l i n a non lontani della mass. chiapperia e del m. marcassita, si trovano in serie regolare e debbono essere considerati in posto. successivamente behrmann (2) lia pubblicato i risultati di un suo rilevamento geologico della sicilia centro-meridionale; anche egli riporta nella sua carta l'affioramento del calcare di cozzo dei disi attraversato dal dicco di basalto. benché questo a. abbia considerato molte delle argille scagliose di baldacci come argille venute in posto per dei particolari fenomeni di diapirismo e abbia considerato t u t t i gli spuntoni di rocce calcaree emergenti dalle argille circostanti come dei microdiapiri, tuttavia considera come diapiri soltanto gli spuntoni di calcari mesozoici e non quelli eocenici, che perciò debbono essere considerati come rocce in posto. nel 1948 floridia (3) ha pubblicato una nota sopra un preteso fenomeno diapirico nei dintorni di leonforte (enna), descrivendo un affioramento di un calcare triassico, attraversato da un dicco di basalto. secondo beneo (4), invece, si t r a t t a di uno « spuntone diabasico di discreta mole, sopportante una placca di calcari scuri a lastre con intercalari di argilla nera, forse del trias. f r a i calcari e il diabase esiste un'aureola di contatto rappresentata da una piccola fascia di roccia silicea stratiforme che potrebbe essere cornubianite. intorno allo spuntone si vedono dei banchi di arenarie saccaroidi, di tipo oligocenico, che rappresentano i resti di una catena smembrata i cui elementi sono letteralmente piantati nelle argille circostanti che costituiscono la formazione più rappresentata ». beneo aggiunge di aver visto una cosa analoga nei pressi di lercara friddi (palermo), e ritiene che t u t t i i «microdiapiri » di behrmann debbano essere considerati come schegge più o meno grandi di autoctono siciliano o extra siciliano inglobate e trascinate dalla massa di argille scagliose considerate come facenti parte di una grande coltre di ricoprimento. signorini (5), che ha rilevato la tavoletta « calascibetta » (268 i no) nella quale si trova il supposto diapiro, lo considera come un esotico inglobato nella colata gravitativa delle argille scagliose. francaviglia (6) ha considerato come « esotici » t u t t i i lembi di rocce rigide, ad esclusione di quelle della serie gessoso-solfìfera e delle arenarie associate alle argille del tortoniano, che nel foglio di caltanissetta sembrano emergere dalla massa argillosa. infatti, nei campioni prelevati al di sotto dei calcari marnosi a fucoidi del m. marcassita, nella tavoletta « imera » (268 iv se), e di t u t t i gli altri spuntoni analoghi che formano come una ghirlanda fra il sudetto monte e le masse calcaree della star i c e r c h e g e o m a g n e t i c h e . s o p r a u n s u p p o s t o d i a p i r o 119 zione eli villarosa, è stata riscontrata una microfauna di foraminiferi riferibile senza dubbio al tortoniano. i n particolare, alla roccia basica di cozzo dei disi sono associati dei tufi pieni zeppi di piccole concrezioni di calcite fibrosa, che sono state scambiate per zeoliti. avendo considerato come esotici i calcari di cozzo dei disi e del microdiapiro di leonforte, ne consegue che i dicchi intrusi in essi non possono essere radicati, se non ammettendo un vulcanismo post tortoniano, del quale non si hanno tracce in alcun posto della sicilia centro-meridionale. la questione aveva una grande importanza, perchè se le masse calcaree in parola erano degli esotici, bisognava ammettere la alloctonia delle argille scagliose che la inglobano, come ha sostenuto beneo, il che si ripercuoteva nella interpretazione della tettonica regionale, e rendeva necessaria la formulazione di nuove ipotesi di lavoro nella ricerca degli idrocarburi e degli zolfi stessi. per avere una conferma della giacitura non radicata dei dicchi in questione, il servizio geologico d'italia, d'accordo con l'ente zolfi italiani, ha dato l'incarico all'istituto nazionale di geofisica di eseguire un rilevamento magnetico nelle due zone, inteso ad accertare se i due dicchi si estendessero o no in profondità. la questione è stata affrontata determinando l'estensione orizzontale delle anomalie magnetiche; infatti, se l'affioramento di roccia basica è costituito da una semplice massa isolata, le anomalie saranno limitate ad una ristretta area attorno alla massa stessa, mentre se essa fa parte di una massa magnetica molto più estesa e profonda, esse saranno sensibili in una regione molto più vasta. il rilevamento è stato effettuato nel giugno 1955 mediante due bilance magnetiche askania gf6 per la componente verticale, e completato nel maggio 1960 mediante una bilancia ruska. le caratteristiche degli strumenti erano: per la bilancia askania n. 509998, costante di scala 10,0 y/div., coefficiente di temperatura — 0,1 y/grado; per la bilancia askania n. 509999, coefficiente di temperatura + 1,6 y/grado, costante di scala 11,4 y/div.; per la bilancia ruska, costante di scala 9,8 y/div. i coefficienti di temperatura sono stati determinati durante la campagna stessa mediante la ripetizione di stazioni fisse a temperatura diversa; le differenze di temperatura, dovute alla escursione diurna, raggiungevano i 10°. per individuare il « campo normale », ci siamo serviti di una stazione sicuramente non anomala, e precisamente della stazione fondamentale 22h o. b a t t e l l i a . f k a n c a v i g l i a m. g i o r g i f . m o l i n a dell'istituto geografico militare n 1294 (7), situata in contrada misteci, a sufi di caltanissetta, stazione che faceva parte anche della rete dell'istituto nazionale di geofìsica nel rilevamento della sicilia centrosettentrionale del 1952 (8). la distanza di tale stazione da cozzo dei disi è di circa 10 km verso sud; si è dovuto perciò determinare la correzione per la latitudine. sono stati all'uopo eseguiti due lunghi profili in direzione n-s: il primo fra villarosa e eiesi, che nella parte più settentrionale si trova alla stessa latitudine di cozzo dei disi e a circa 10 km verso est, nonché poco a sud del dicco di leonforte; il secondo dall'incrocio delle strade statali 121 e 122 bis, circa 10 km a nord di caltanissetta (barriera noce), fino alla stazione fondamentale di contrada misteci, circa 6 km a sud della città, profilo che attraversa la zona di cozzo dei disi. sono stati ottenuti i seguenti valori del gradiente di latitudine della componente verticale: per il primo profilo + 7,7 y/km, per il secondo + 6,4 y/km; tra le due stazioni più settentrionali del primo profilo, la cui latitudine media è uguale a quella della zona oggetto di studio, il gradiente è risultato di + 6,7 y/km, in ottimo accordo con il valore del secondo profilo. nei computi abbiamo scelto quest'ultimo valore, in quanto adatto per la latitudine e non affetto da eventuali influenze delle anomalie da rilevare. la correzione per la variazione diurna della componente verticale del campo magnetico terrestre è stata apportata sulla scorta delle registrazioni magnetiche dell'osservatorio di gibilmanna, presso cefalù, dell'istituto nazionale di geofìsica. la precisione delle singole misure, dedotta dalle differenze dei valori nelle stazioni ripetute è risultata di i 3y. sulla collina dell'affioramento di cozzo dei disi e nella zona immediatamente circostante sono state eseguite un centinaio di stazioni, di cui 73 distribuite in sei profili attraverso l'affioramento stesso, distanziate in ciascun profilo di circa 10 m l'una dall'altra. nella fig. 1 sono rappresentate l'altimetria della zona, le stazioni eseguite con i valori delle anomalie rispetto alla stazione fondamentale di contrada misteci, e le curve isoanomale, costruite con l'approssimazione consentita dalla distribuzione delle stazioni. la figura mostra una rapida diminuzione delle anomalie fino a valori quasi normali già a distanza dell'ordine dei 50 o 60 metri dall'affioramento. nella fig. 2 sono riportate le stazioni eseguite a maggiore distanza dall'affioramento, il quale è rappresentato dalla zona tratteggiata; i valori di queste stazioni coincidono, entro una diecina di gamma, con i valori del campo normale. 9 0 0 3 0 0 2 5 0 200 150 o. b a t t e l l i a . f r a n o a v i g l i a m . g i o r g i p . m o l i n a 100 50 0 t 9 0 0 8 5 0 800 7 5 0 .-10 7 0 0 6 5 0 6 0 0 h 8 5 0 5 5 0 8 0 0 h 7 5 0 7 0 0 a n o m a l i e p o s i t i v e a n o m a l i e n e g a t i v e 6 5 0 6 0 0 5 5 0 3 0 0 2 5 0 2 0 0 150 100 50 0 fig. 1 le coordinate espresse in metri sono riferite ad una stazione ubicata vicino al km 11 della strada caltanissetta-s. caterina villarmosa r i c e r c h e g e o m a g n e t i c h e . s o p r a u n s u p p o s t o d i a p i r o 2 2 9 le isoanomale della fig. 1 mostrano l'esistenza di due sistemi magnetici distinti. sono stati ricostruiti due profili pressoché rettilinei in l o o o m 0 s c a i a l 2 k m . — r i m u j m h j " ^ " v 1 fig. 2 direzione so-ne attraversanti i sistemi di anomalie e indicati in figura 1 dalle rette a e b. i valori delle anomalie su questi due profili sono riportati nella fig. 3; le due curve possono esser interpretate come dovute: per il profilo a ad un dipolo orizzontale avente polarizzazione 22h o. b a t t e l l i a . f k a n c a v i g l i a m . g i o r g i f . m o l i n a coerente con l'attuale campo magnetico terrestre, per il profilo b ad un dipolo nel medesimo verso del precedente, ma inclinato di circa 20° t i 700 -t0 10 20 30 40 50 metri fig. 3 verso il basso. naturalmente tale interpretazione è da accogliersi con cautela, in quanto alcune stazioni sono state effettuate direttamente sull'affioramento, e di conseguenza i valori delle anomalie risultano in esse esaltate. i risultati suddetti mostrano che anomalie magnetiche sensibili si hanno soltanto nelle immediate vicinanze dell'affioramento; nella regione circostante, fino a distanza di vari chilometri, non sono state rilevate anomalie apprezzabili; in altre parole, il campo, tranne che nella ristretta zona dell'affioramento, appare « normale ». r i c e r c h e g e o m a g n e t i c h e . s o p r a u n s u p p o s t o d i a p i r o 2 3 1 questo risultato ci porta a ritenere improbabile l'esistenza in posto di una massa magnetica profonda ed estesa della quale l'affioramento faccia parte. per conseguenza, la bancata di calcare nella quale è intrusa la roccia basica non può avere radici profonde, e perciò deve essere considerata come una scheggia inglobata nelle argille sottostanti. sul presunto « microdiapiro » di leonforte sono stati eseguiti alcuni profili a titolo orientativo. siccome le anomalie riscontrate avevano le stesse caratteristiche di quelle di cozzo dei disi, non si è creduto necessario procedere ad un rilevamento di dettaglio. anche per questo caso si ritengono applicabili le conclusioni dedotte dallo studio della zona di cozzo dei disi. si ringrazia il servizio geologico d'italia per avere suggerito il presente studio e fornito le diverse ipotesi di lavoro, l'ente zolfi italiani per il contributo finanziario che ha permesso l'effettuazione del rilevamento magnetico, e il rev. dr. di martino per il prezioso aiuto prestatoci durante la prima parte del rilevamento fornendoci tempestivamente le registrazioni magnetiche dell'osservatorio di gibitmanna a lui affidato, e per la collaborazione in campagna durante la seconda parte. r o m a , i s t i t u t o nazionale di geofisica, giugno 1962. riassunto gli a a. riferiscono sopra un rilevamento magnetico di una zona della sicilia centrale, inteso a stabilire se un dicco di roccia basica intruso in una bancata calcarea affiorante fra masse argillose potesse far parte di una massa magnetica profonda ed estesa. il risultato porta a ritenere improbabile questa ipotesi. su m mary the aa. report on a magnetic survey of an area of central sicily, meant to establish ivhetlier a dilce of basic rock inserted in a calcareous layer outcropping among masses of clay, may be part of a deep and wide magnetic mass. the result makes this hypothesis seem improbable. 2 3 2 o . b a t t e l l i a . f r a n c a v i g l i a m . g i o r g i f . m o l i n a b i b l i o g r a f i a i1) b a l d a c c i l., descrizione geologica dell'isola di sicilia. mera, descr. c a r t a geologica d ' i t a l i a , v. 1, ( r o m a , 1886). ( 2 ) b e h r m a n n r . b . , appunti sulla geologia della sicilia centro-meridionale. r a p p o r t o al min. corporazioni, y a c u u m oil co. in deposito presso il serv. geol. d ' i t . , ( r o m a , 1938). ( 3 ) f l o k i d i a g. b., di un interessante fenomeno diapirico nei dintorni di leonforte (enna). « giorn. se. n a t . e c . », 45, sez. 1, ( p a l e r m o , 1948). ( j ) b e n e o e . , sul « microdiapiro » di leonforte e su quelli a nord di gela (sicilia centrale e meridionale). r e n d . acc. lincei, ci. se. fis. m a t . n a t . , s. v i l i , 7, ( r o m a , 1949). ( 6 ) s i g n o r i n i r., note descrittive del rilevamento geologico delle tavolette « villarosa », « calascibetta » e « leonforte ». « boll. ser. geol. d ' i t . », l x x v i i , f. 2, 3, ( r o m a , 1955). ( 6 ) f r a n c a v i g l i a a., relazione sul rilevamento delle tavolette « xirbi » e « s. caterina villarmosa ». relazione i n e d i t a a l l ' è . z . i . , ( r o m a , 1953). ( ' ) m o r e l l i c., la rete geofisica e geodetica in italia nel suo stato attuale e nei suoi rapporti con la struttura geologica superficiale e profonda. p u b blicazione i s t . n a z . geof. 121, (trieste, 1946). ( 8 ) g i o r g i m . , m e d i e . , m o l i n a f . , rilievo magnetico della sicilia centrosettentrionale. « a n n a l i di geofisica», v i i , 4, ( r o m a , 1954). vol52,2,2009 117 annals of geophysics, vol. 52, n. 2, april 2009 microseismic feasibility study: detection of small magnitude events (ml<0.0) for mapping active faults in the betic cordillera (spain) martin häge and manfred joswig institute for geophysics, universität stuttgart, germany abstract we present the results of the first application of the newly developed concept «nanoseismic monitoring» on active faults in the region close to murcia, spain. the aim of this microseismic feasibility study is to test if it is possible to record small magnitude events (ml<0.0) within a short period of time with surface installations and to investigate if these events are related to the regional catalog in terms of amount of events. the seismic monitoring was performed with one small array called the seismic navigating system. it consists of one central three component and three one component seismometers arranged tripartitely around the central station. in the measurement period of two nights at two different sites we were able to detect 19 microearthquakes down to ml = -2.6. the results correlate well with the frequency-magnitude distribution of the regional bulletin. this in turn will allow for estimation of monitoring rates before actual field measurements just from bulletin data. given an activity rate of 5 to 10 events per night one may map active fault zones within just a few weeks of field campaign. mailing address: dr. martin häge, institute for geophysics, universität stuttgart, azenbergstrasse 16, 70174 stuttgart, germany; e-mail: haege@geophys.uni-stuttgart.de key words betic cordillera – active faults – microseismicity – gutenberg-richer law – nanoseismic monitoring 1. introduction characterizing recent seismicity and mapping active fault segments must be based on the compilation of seismological bulletins. the fundamental data collection by semi-permanent seismic networks is a time-consuming and costly task. only a few studies deal with the investigation of small magnitude events (ml<0.0) (e.g., abercrombie, 1995; von seggern et al., 2003; ruiz et al., 2006), whereby most researchers investigate aftershocks and not the background seismicity or use borehole sensors for event detection. butler (2003) suggests the term «nanoearthquakes» for events with ml<0.0. the concept of nanoseismic monitoring (joswig, 2008), a technique developed to detect and characterize small magnitude sources, is tested as a short-term alternative for semi-permanent seismic networks to reduce network recording time. it is successfully tested for onsite-inspections of the preparatory commission for the comprehensive nuclear-test-ban treaty organization and was applied to detect and characterize small magnitude events triggered by material impacts in sinkholes along the western dead sea shores (wust-bloch and joswig, 2006). nanoseismic monitoring as a kind of seismological microscope shall finally help to shed light on small earthquake trigger mechanisms. fault weakness can be caused by increased fluid pressure that reduces the effective normal stress (hainzl et al., 2006). in this vol52,2,2009 17-06-2009 19:02 pagina 117 118 m. häge and m. joswig model, stress and pore pressure redistribution after large earthquakes come along with higher permeability for fluids causing possible new nucleation points and a characteristic migration scheme (cox, 1995; miller et al., 2004). the detection and location of small magnitude seismicity may support this model of fluid transport and a shear stress behavior driven by porosity reduction (johnson and mcevilly, 1995; miller et al., 1996). for the study, a seismically active section of the betic cordillera (near murcia in spain) with favorable signal-to-noise conditions was selected. since we started to build up the system at that time one recording unit was available for field use. the small magnitude seismicity detected and partly localized by the single, smallaperture tripartite array within two successive monitoring nights, at two different sites, is compared with the regional 1984-2003 bulletin of the instituto geográfico national, madrid (ign). brune and allen (1967) have shown along the san andreas fault system that usually a two-day measurement of seismic activity is sufficient to make an approximate estimation about the local rate of microseismicity. 2. geological and tectonic setting the betic cordillera, which is situated in the southern part of spain, is a collision zone generated by the nearby african-eurasian plate boundary. this boundary is defined by a high seismicity which is distributed over several hundreds of kilometers (calvert et al., 2000). the area selected for the feasibility study lies within the subbetic zone which, together with the prebetic zone, represents the external zone of the betic cordillera. the thickness of the crust beneath the betic is 25-39 km (banda et al., 1993). focal depths of regional events is restricted to the top 40 km (fig. 1) with moderate magnitudes generally less than 5.5 (buforn et fig. 1. spatial distribution of 1040 earthquakes from 1984-2003 (source: ign, 2004). the two measuring sites (capres and burete) are indicated by triangles pointing up (not in scale). our located events are marked by black dots and the two co-detected events by triangles pointing down, connected with lines. the dashed black line sketches the trace of the crevillente fault zone (cfz). vol52,2,2009 17-06-2009 19:02 pagina 118 119 detection of small magnitude events (ml<0.0) for mapping active faults in the betic cordillera (spain) al., 2004). the shallow seismicity is associated with the dense spread of fractures in this region (sanz de galdeano et al., 1995). nanoseismic monitoring was carried out at two different locations in the vicinity of the crevillente fault zone (cfz): one near capres, at the fringe of the fortuna basin; the other near burete, in the east of the sierra de espuna (fig. 1). the cfz strikes ne-sw parallel to the axis of the subbetic zone and extends laterally over 600 km. the main activity of the cfz was during late miocene (alfaro et al., 2002), and it is still active. focal mechanisms indicate that the cfz is a right-lateral strike slip fault which can also be observed on geological features (buforn et al., 1988). estimates for the total displacement along the cfz range between 75-100 km (nieto and rey, 2004) and 400 km (de smet, 1984). the subbetic zone consists of deposits situated far from the south iberian margin (ruano et al., 2004) which comprise parautochthonous to allochthonous, non-metamorphic sediments. the main geological units in these areas beside quaternary unconsolidated sediments and miocene alluvial fan deposits include triassic marls, claystones, gypsum, dolomites as well as jurassic and cretaceous limestones and marls. to consider potential site effects, the thickness of sediments to the top of the basement at each of the two monitoring sites has been estimated on the basis of available data and field evidence: near capres the thickness of sediments reaches 100 m (ign, 1972a), near burete it is about 50 m (ign, 1972b; poisson and lukowski, 1990). 3. data acquisition and processing data was acquired by one seismic navigation system (sns). this six-channel sns is a portable, sparse array consisting of four shortperiod sensors: a central, three-component instrument and three one-component seismometers arranged as a tripartite array. high resolution and coherency of microseismic events are attained by utilizing a small aperture of 200 m and 400 hz sampling rate. nanoseismic monitoring was performed at night to reduce the effect of anthropogenic noise sources. event detection and location were carried out by sonodet and hypoline modules of the sparsenet software (joswig, 1999; 2008). measurements were performed during four nights. due to the first operation of the system some adaptations to the equipment had to be made in the field. two of the four nights were successfully completed and were taken for analysis. during this period, a total of 19 seismic events in the magnitude range -2.6 � ml � 1.5 could be detected and discriminated from noise bursts by sonogram analysis (joswig, 1990) (see table i). figure 2 shows two examples of table i, events nos. 5 and 11, demonstrating the usefulness of sonograms for event detection. hypocentral locations could be estimated for 15 events. four other weak events did not present clear pand s-phase onsets. however, ml magnitudes could be estimated for all 19 events as maximum amplitude and distance from ts-tp time differences could be estimated with confidence by sonogram analysis (catalog a of table ii). the apparent velocities of most of the events, derived from array analysis, were not in accordance with much faster velocities of the standard velocity model for spain (dãnobeitia et al., 1998). location residuals reduced significantly using a data-adapted half-space model with velocities ranging from vp = 1 to 5 km/s for 0.3 to 14 km depth. figure 1 shows the location of the 15 events (solid black dots) on the background of the regional seismicity from 1984-2003 (open gray circles). both positions of the sns deployments are marked with triangles pointing up (not in scale). most of the events south of burete are aftershocks and were generated by the 2002 bullas (ml = 5.0) earthquake (ign, 2004) that occurred 607 days before our measurement. event locations were calculated with one single array which results in a large location error of a few kilometers. depths were estimated with the intersection of hyperboloids by tp-tp information. two events were co-detected by the local network (nos. 3 and 12 in table i) which are displayed as triangles pointing down in fig. 1. for these two events, the mean horizontal location difference is about 20 km and the mean vol52,2,2009 17-06-2009 19:02 pagina 119 120 m. häge and m. joswig magnitude variation 0.4. figure 3 plots magnitudes versus distances for all 19 events and the distance-correction curve used for ml calculation fitted empirically to the data (note: slantdistance instead of epicentral distance for <10 km). additionally, the two co-detected events are shown with triangles pointing down. the observed detection threshold indicates that signal-to-noise conditions were better at burete than at capres. the overall sensitivity limit was about ml = -1.0 at 10 km, and ml = -2.0 at 2.5 km. 4. characterization of regional seismicity the 1984-2003 regional seismic catalog (ign, 2004) was used to assess the performance of our measurement. magnitudes of both datasets, are equivalent. by fig. 3, and as will be shown later in fig. 5, our magnitude of completeness is ml = -1.0 in about 10 km distance. however, the analysis of regional seismicity within a 10 km radius presents several challenges. first, the local bulletin (1984-2003) intable i. parameters of the recorded events. number date origin [utc] longitude latitude depth [km] magnitude [ml] 1 3-04-2004 20:45:45 -1.8359 37.9567 0.3 -2.5 2 3-04-2004 21:31:27 -0.8 3 3-04-2004 23:25:30 -1.4090 37.8050 12.0 1.5 4 3-04-2004 23:54:15 -1.8369 37.9136 2.0 -1.1 5 4-04-2004 1:00:58 -0.8 6 4-04-2004 1:17:28 -1.8267 37.9615 1.4 -1.9 7 4-04-2004 1:40:53 -1.8209 37.9430 0.9 -2.6 8 4-04-2004 1:48:20 -1.6941 37.9436 11.0 -0.3 9 4-04-2004 3:38:51 -1.8248 37.9560 1.8 -2.0 10 4-04-2004 5:05:15 -1.8135 37.9614 2.0 -2.1 11 4-04-2004 21:33:18 -1.1682 38.2121 2.4 -1.7 12 5-04-2004 0:41:02 -1.0790 38.0490 11.0 0.6 13 5-04-2004 1:46:05 -1.1147 38.1096 7.5 0.0 14 5-04-2004 2:20:08 -1.4 15 5-04-2004 2:34:35 -1.1714 38.2146 1.2 -1.4 16 5-04-2004 2:42:33 -1.1264 38.2077 2.7 -1.6 17 5-04-2004 2:50:22 -1.1213 38.2133 1.2 -1.5 18 5-04-2004 3:18:59 -1.1718 38.2375 2.7 -0.9 19 5-04-2004 5:34:59 -0.9 vol52,2,2009 17-06-2009 19:02 pagina 120 121 detection of small magnitude events (ml<0.0) for mapping active faults in the betic cordillera (spain) cludes very few events whose source is located within 10 km radius of the two recording sites (catalog b of table ii). consequently, a much larger reference area (about 13,620 km2) with 1040 events was selected for increased statistical stability (catalog c of table ii). second, homogeneous seismicity in time and space is a prerequisite when comparing source regions of different size. artifacts like modifications in network geometry and density, station hardware and processing software result in a rather heterogeneous distribution of seismicity, both in space (fig. 1), and in time. figure 4 shows catalog c of table ii with 1040 events as open gray circles (right vertical axis) and the annual event frequency by the black curve (left vertical axis). the annual event frequency and the detection level increase with time. fig. 2. seismograms and the corresponding sonograms of the four vertical components for two events. seismograms are filtered between 3 and 30 hz (optimized filter setting). p and s onsets are indicated with arrows. the event in a) corresponds to no. 5, the event in b) to no. 11 in table i. table ii. data parameters of seismic catalogs a, b and c. catalog a includes 19 events recorded during this measurement campaign. catalog b includes events within a 10 km radius of our two measurement sites in the period 1984-2003. catalog c covers a larger area for a statistical robust b-value calculation in the period 1984-2003. catalog a b c description nanoseismic local regional monitoring capres burete size of area [km2] 630 300 300 13620 number of events 19 34 175 1040 max ml 1.5 3.4 5 5 min ml -2.6 1 0.4 0.4 vol52,2,2009 17-06-2009 19:02 pagina 121 122 numerous investigations deal with catalog completeness and its statistical fluctuations (e.g., rydelek and sacks, 1989; zúñiga and wiemer, 1999; woessner and wiemer, 2005). the first change took place in 1997 when a digital recording system was installed; the second jump occurred after 2000 when another two stations (etob and emur) were added. an additional increase in number of events is caused by the aftershock activity of the 2002 bullas ml = 5.0 earthquake. between 1984 and 1998, there is a constant magnitude of completeness (mc) of 2.6, calculated with the entire-magnitude-range method (woessner and wiemer, 2005). it has decreased since 1998. further investigations have shown that there is no catalog contamination by quarry blasts. 5. gutenberg-richter relationship: regional seismicity and nanoseismic monitoring we make two assumptions in order to compare the gutenberg-richter relationship estimated from the regional seismicity (catalog c of table ii) with the same relationship based on nanoseismic data (catalog a of table ii). first, catalog c characterizes a representative b-value for the whole region. this assumption is rooted in fig. 4 which shows an average constant seismicity without any high seismicity cycles above mc = 2.6. second, catalog a can be obtained by concatenating the data recorded over two nights near capres and near burete without loss of statistical significance. fig. 3. magnitude-distance relationship for the 19 events detected at burete and at capres as well as the distance correction for ml. the two co-detected events are shown with triangles pointing down, linked to the corresponding events by gray lines. m. häge and m. joswig vol52,2,2009 17-06-2009 19:02 pagina 122 123 detection of small magnitude events (ml<0.0) for mapping active faults in the betic cordillera (spain) fig. 5. frequency-magnitude distributions of catalogs a, b and c of table ii. the cumulative distributions are marked in black, the normal distributions in grey. note that catalogs b and c are normalized to the measurement period of two nights. the solid black line shows the b-value for catalog c that is applied to catalog b (dashed black line). fig. 4. time-event (left vertical axis) and time-magnitude distribution (right vertical axis) of catalog c of table ii. vol52,2,2009 17-06-2009 19:02 pagina 123 124 m. häge and m. joswig figure 5 compares the frequency-magnitude relationships for the different catalogs of table ii. note that catalogs b and c of table ii are downscaled to the measurement period of this feasibility study of two nights. due to the small number of events of catalogs a and b the bvalue for the larger area, catalog c, was determined and applied to our study area, catalog a. analyzing catalog c (black dots) a b-value (solid black line) of 1.16±0.07 was derived according to the formula of aki (1965) with mc =2.6. this result corroborates the b-value of 1.1±0.1 estimated by lópez casado et al. (1995) for magnitudes larger than 3.5 in the murcia region between 1930-1992. in hazard assessment, the gutenberg-richter relation, log n = a – bm, (5.1) with n the cumulative number of earthquakes of magnitude m or greater and a and b constants (ishimoto and iida, 1939; gutenberg and richter, 1944), is used to predict the frequency of occurrence of large earthquakes on the basis of smaller events. inversely, few studies investigate the extrapolations made from the stronger to the weaker events. studies concerned with the detection of very small earthquakes (e.g., iio, 1991; piccinini et al., 2003) or with the constancy of the b-value to lower magnitudes and self-similarity of seismic events (von seggern et al., 2003) failed to show a b-value decrease towards small magnitudes. abercrombie and brune (1994) verified that there was no significant decrease down to magnitude 0.0 on three major fault zones in california. furthermore there is good agreement between b-values extrapolated from regional bulletins and those of microseismic activity (abercrombie, 1995; 1996). although an investigation of induced seismicity (trifu et al., 1993) showed a non-similar frequency-magnitude distribution between magnitudes -0.5 and 0.0, there is no reason why a decrease in b-value should be expected for natural seismicity at local distance. in conclusion, the b-value of 1.16 from catalog b was extrapolated to small magnitudes. although it is not possible to calculate a b-value for our events and the existence of high uncertainties in the statistical estimation, i.e. the normalization of the frequency of events to the measurement period of two nights, the extrapolated b-value fits remarkably well with catalog a. there might also be a slight shift in magnitudes due to the different applied velocity models which are used for location and the resulting variation in epicentral distances. however, the agreement of catalog b with a is obvious, even with an assumed maximum magnitude deviation of 0.4. this result suggests that it might be possible to infer on the basis of bulletin data on the expected amount of events in a certain area prior to a field campaign. a similar observation was made by brune and allen (1967) who found out that the amount of microseismicity could be approximately predicted by extrapolation of frequency-magnitude curves from 29year records of larger earthquakes. 6. conclusions in this feasibility study, the new concept of nanoseismic monitoring was applied to characterize small magnitude natural seismicity in the betic cordillera (spain). a total of 19 events (-2.6 � ml � 1.5) were recorded and detected within an observation period of two nights, indicating the high sensitivity of nanoseismic monitoring. the analysis of the frequency-magnitude distributions shows a good approximation between the amounts of recorded events with those extracted from local catalogs. however, it must be further proven by investigations in other geological and tectonic settings if the amount of small magnitude seismicity can be anticipated from existing catalogs by linear extrapolation. the performance of nanoseismic monitoring of about ml = -1.0 at 10 km and ml = -2.0 at 2.5 km demonstrates its potential for a costeffective technique for active fault mapping. fault mapping could be realized at high resolution within weeks instead of years. due to the use of only one small array, there is a high location error of a few kilometers. a further challenge was the discrepancy between the standard velocity model for spain and our observations. therefore it was not possible to identify specific fault segments. at least two small arrays vol52,2,2009 17-06-2009 19:02 pagina 124 125 detection of small magnitude events (ml<0.0) for mapping active faults in the betic cordillera (spain) must be deployed to reach this aim. both small arrays can be combined as a kind of network to reduce the azimuthal gap and hence to increase the location accuracy. cross bearing and the intersection of two ts-tp circles provide additional location constraints and support the determination of an appropriate velocity model. it can then be tested if relative location methods might be applicable for further improvement of the location results. acknowledgments the authors are grateful to the instituto geográfico national, madrid, spain, especially to arancha izquierdo álvarez and resurrección antón for the allocation of the regional bulletin data used in this work and for their kindly support. hillel wust-bloch suggested many improvements of the manuscript. the map in fig. 1 was partly created using gmt software (wessel and smith, 1991). references abercrombie, r.e. and j.n. brune (1994): evidence for a constant b-value above magnitude 0 in the southern san andreas, san jacinto and san miguel fault zones, and at the long valley caldera, california, geophys. res. lett., 21 (15), 1647-1650. abercrombie, r.e. 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(received august 15, 2008; accepted december 11, 2008) vol52,2,2009 17-06-2009 19:02 pagina 126 n o n linear laminar flow of fluid into an open bottom well s . k . j a i n * received on february 19th, 1982 a b s t r a c t in s t e a d y s t a t e condition, non linear l a m i n a r flow of fluid into an open b o t t o m well j u s t p e n e t r a t i n g the semi-infinite porous aquifer is considered. the i n f l u e n c e of non-linear l a m i n a r flow on discharge and its dependance on related p h y s i c a l q u a n t i t i e s is examined. it is found that an open bottom well actually b e h a v e s like a h e m i s p h e r i c a l well, which is an obvious practical phenomenon. r i a s s u n t o si c o n s i d e r a il flusso l a m i n a r e non-lineare di un fluido in un pozzo a fondo a p e r t o c h e p e n e t r i un acquifero poroso semi infinito. si e s a m i n a poi l'influenza del flusso laminare non-lineare sulla discarica e la s u a d i p e n d e n z a da q u a n t i t à fisiche correlate. si trova che un pozzo a fondo aperto si c o m p o r t a c o m e un pozzo emisferico che è un fenomeno pratico ovvio. 1 . i n t r o d u c t i o n in t h e s t u d y of flow through porous media, there exists a large n u m b e r of investigations seeking steady state solutions of darcy's * d e p a r t m e n t of mathematics, rivers s t a t e college of education, rumuolum e n i , p.m.b. 5047, port harcourt (nigeria). 8 6 s.k. jain l a w w i t h respect to certain applications. one of the most import a n t a p p l i c a t i o n is the study of flow of fluid from s t r a t a into wells. m a n y i n v e s t i g a t o r s obtained a series of such solutions (cf. [1], [2], [3]). t h e well p e n e t r a t i n g fully the fluid bearing s t r a t a of finite t h i c k n e s s is t e r m e d as fully p e n e t r a t i n g well otherwise partially p e n e t r a t i n g well. but in a semi-infinite s t r a t u m , a well is always c o n s i d e r e d to be p a r t i a l l y penetrating. in such cases, the flow in t h e re g ion of sa n d not p e n e t r a t e d by the well will have an u p w a r d c o m p o n e n t of velocity tending to bring the fluid into the well, w h i l e t h e flow in the u p p e r portion of the sand will mainly be r a d i a l as it will have a small negligible vertical component. the m o s t i n t e r e s t i n g limiting case of a partially penetrating well is an ' o p e n b o t t o m well', which corresponds to a well just tapping a u n i f o r m s a n d of great thickness [1]. beside t h e n a t u r e of a well the other i m p o r t a n t aspect is the t y p e of flow which mainly depends upon the fluid velocity and the s t r u c t u r a l constitution of the porous m a t r i x through which it f l o w s . on the basis of fluid velocity, however, the flow [3] can be c h a r a c t e r i s e d into three different regimes — l a m i n a r , nonlinear l a m i n a r a n d t u r b u l e n t , reynolds n u m b e r being the criteria for s u c h d e m a r c a t i o n . but, the intricacy of the n a t u r e of porous media d o e s n o t a l w a y s justify the n a t u r a l flow of fluid to be purely l a m i n a r a n d it a p p e a r s more desirable to be either non-linear l a m i n a r or t u r b u l e n t . consequently, uchida, 1952, e n g e l u n d , 1953, a n a n d k r i s h a n a n d varadarajulu, 1 9 6 3 , w r i g h t , 1 9 6 8 , ahmad a n d s u n a d a , 1 9 6 9 , k h a n a n d raza, 1 9 7 2 , jain a n d upadhyay, 1 9 7 6 , upadhyay, 1977a b, a n d others obtained solutions of certain specific n o n l i n e a r l a m i n a r flow problems. in the p r e s e n t paper, we consider non-linear l a m i n a r radially s y m m e t r i c a l flow of an incompressible fluid into an open bottom well in s t e a d y s t a t e condition. the influence of non-linear laminar f l o w on d i s c h a r g e a n d its d e p e n d a n c e on related physical q u a n t i t i e s is e x a m i n e d . it is found that an open bottom well a c t u a l l y b e h a v e s like a hemispherical well, as in the former case w h e r e t h e well just p e n e t r a t e s the u p p e r surface of semi-infinite a q u i f e r , a n h e m i s p h e r i c a l cavity is automatically formed to prov i d e a s u r f a c e for discharge. this is an obvious practical phenomen o n . non-linear laminar flow, ecc. 8 7 2 . b a s i c equations of fluid flow in porous medium t h e d a r c y ' s law governing the l a m i n a r flow of fluid in porous m e d i a is [2] v = k j " , [1] d h v, k a n d —— d e n o t e the seepage velocity, seepage coefficient and d s h y d r a u l i c g r a d i e n t respectively; flow being in the opposite direct i o n of i n c r e a s i n g h. i n c a s e of l a m i n a r flow, the total head hc at an infinitely large d i s t a n c e f r o m t h e axis of the circular perforation of radius rw in t h e p l a n e i m p e r v i o u s boundary of semi-infinite homogeneous a q u i f e r is h c = « „ + p i w h e r e q d e n o t e s the flow r a t e and ht0 the head at the well surface. besides, relations [1] a n d [2], the law for non-linear laminar flow is [2] dh = av + bv\ [3] d s a a n d b b e i n g c o n s t a n t s , which according to engelund are 2 0 0 0 j l b , pgd*' gd' where p and |jl being density and viscosity of the fluid respectively and d the grain size of the medium. 8 8 s.k. jain fig. 1 3 . s t a t e m e n t of t h e problem in s t e a d y s t a t e condition, we consider radially symmetrical flow of a n incompressible fluid into an open b o t t o m well, which is a c i r c u l a r p e r f o r a t i o n w i t h radius r w of an impervious boundary of s e m i i n f i n i t e p o r o u s aquifer (fig. 1). the aquifer is assumed to be h o m o g e n e o u s a n d isotropic. the pressure at the contour of well a n d a t t h e c o n t o u r of intake are prescribed as pw and pc respect i v e l y . let r be the r a d i a l distance measured f r o m the axis of well. it is a s s u m e d t h a t the flow is (i) non-linear l a m i n a r within the c i r c u l a r zone t̂j ^ y vt, and (ii) l a m i n a r in the region r, < y ^ oo. let p, be t h e p r e s s u r e at the transition boundary r = r,. t h e p r o b l e m is to examine the influence of non-linear l a m i n a r f l o w on discharge, its dependance on the grain size of the m e d i u m a n d t h e viscosity of the fluid. non-linear laminar flow, ecc. 8 9 4 . s o l u t i o n s i n c e p = pgh, we write [2] as qpg _ o p g pc pos = 4 k r„ i d p = = c 4 k q pg 4 k q p g 4 k r ' ( 1 1 ). [ 5 ] [6] [ 7 ] w h e r e c is the c o n s t a n t of integration determinable by boundary c o n d i t i o n s . thus, equation [7] describes the pressure distribution p f o r a n y a r b i t r a r y r a d i u s r (rw r ») in the system for which e q u a t i o n [2] holds good. h e n c e , f r o m [7] the expression for pressure distribution at a r a d i a l d i s t a n c e r in the l a m i n a r zone r, < r is obtainable in t h e f o r m p, = pc — q p g 4 k r , for r a d i a l l y s y m m e t r i c a l flow, equation [1] becomes k dp [8] v = jg dr [ 9 ] s u b s t i t u t i n g t h e value of d p d r from [7], equation [9] reduces to v = 4 r 2 [ 1 0 ] w h e r e v is t h e seepage velocity at a radial distance r. thus, v is i n d e p e n d e n t of the well radius rw and z — the vertical coordinate. 9 0 s.k. jain using p = p gh in equation [3) and then combining it with r e l a t i o n [10], the expression for pressure distribution in the n o n l i n e a r l a m i n a r zone is obtainable in the form dp n / aq b q2 \ —r— = pg —— + —— , « r<r, 11 d r 8 u r 2 16 r 4 / w 1 1 i n t e g r a t i n g [11] and evaluating the constant of integration w i t h t h e h e l p of b o u n d a r y conditions p = w e o b t a i n po> a t r = rto. [ 1 2 ] p, at r ~ rt, í aq ( \ 1 \ b q1 / 1 1 \] [131 at t h e b o u n d a r y of transition f r o m l a m i n a r to non-linear l a m i n a r flow, the relation between critical reynold's n u m b e r = 0.07 a n d critical velocity vc is given by " " t v t * 1141 d h s i n c e a t t h i s b o u n d a r y — — a s given [1] a n d [3] yield the same d s v a l u e , it follows f r o m [14] that 1.07 a at = 1 [15] u s i n g [15] in [8] a n d then equating it to [13], we get \±q ( 1 + 0 . 0 7 1 ) + b-91 (1 1)1 | 1 6 , pg l 4 \ r w r, ) 48 i tf. r > ) j 1 1 0 1 non-linear laminar flow, ecc. 9 1 c o m b i n i n g e q u a t i o n s [4 a, b] a n d [14] w i t h [16], we obtain ?d>(pc-pj _ l^2 = 8 0 0 0 ( i l ) [ ü + 0 . 0 2 3 3 ( ü ) > + 0 . 0 4 6 7 1 [ 1 7 ] rw l rw r(ii j w h e r e r, = y / u j 1 6 (jl if w e a s s u m e p u r e l y l a m i n a r flow in the region r t h e n t h e f l o w r a t e q]am is o b t a i n a b l e f r o m [7] in the form n 4 k r r ï q | a m = t v ^ " [18] t [ 1 9 ] h e n c e f r o m [14] a n d [19], we o b t a i n the r a t i o 0 qlan = 8 5 6 0 m-2 ^ p d } (p, p w l (-ï—)2 [ 2 0 ] i n t r o d u c i n g d i m e n s i o n l e s s q u a n t i t y x a n d r a t i o 7 such t h a t [21 a, b] x = pd>(pc-pj f = 1¿2 'lam a n d c o m b i n i n g [20] w i t h [17], we o b t a i n an implicit relation x = 8000 i + 0 . 0 2 3 3 ( ^ ) 2 + 0 . 0 4 6 7 ( — ) v 4 1 ( 2 2 ] l 8 5 6 0 8 5 6 0 8 5 6 0 j t h i s e x p r e s s i o n is the s a m e as t h a t o b t a i n e d by the a u t h o r [ j a i n , 1 9 8 1 ] , w h i c h describes the n o n l i n e a r l a m i n a r flow into a h e m i s p h e r i c a l w e l l . 9 2 s.k. jain 5 . c o n c l u s i o n it m a y t h u s be concluded that an open b o t t o m well actually b e h a v e s like a h e m i s p h e r i c a l well, as in the f o r m e r case where the well j u s t p e n e t r a t e s the u p p e r surface of a semi-infinite porous a q u i f e r , a h e m i s p h e r i c a l cavity is automatically formed to provide a s u r f a c e for discharge. this is an obvious practical p h e n o m e n o n , a s o u r conclusion based on theoretical discussion justifies the s t a t e m e n t : "... t h a t is a practical case when well just taps the s a n d , t h e well s u r f a c e is really a hemisphere'.'. n o n l i n e a r l a m i n a r f l o w , e c c . 9 3 r e f e r e n c e s a h m a d , n. a n d s u n a d a , d.k., 1969 non linear laminar flow in porous media, "j. hy. d n . " , proc. a s c e , 95, p p . 1847-1857. a n a n d k r i s h a n , m. varadara-julu , g.h. 1963 laminar and turbulent flow of water through sand. " j . soil mech.", proc. asce, 89, 1, p. 15. bear, j., 1972 dynamics of fluids in porous media (american elsevier publishing co. inc., new york 1972). e n g e l u n d , f., 1953 on laminar and turbulent flow of ground water in homogeneous sand. " t r a n s . dan. acad. tech. sei.", 3, 1, p. 105. j a i n , s . k . a n d u p a d h y a y , k . k . , 1 9 7 6 non-linear laminar flow of fluid into a partially penetrating well. " g e r l a n d s beiträge zur geophysik", 8 5 , pp. 1 5 1 1 5 6 . j a i n , s . k . , 1 9 8 1 non linear laminar flow of fluid into a hemispherical well just penetrating the semi-infinite porous aquifer. "gerlands beiträge zur geophysik". khan, i.h. a n d raza, s.a., 1972 a study of non-linear flow through porous media by electrical analogy method. " j . inst, of engg. india", c.e. dn., 53, pp. 57-60. m u s k a t , m . , 1946 the flow of homogeneous fluids through porous media (j.w. e d w a r d s inc., ann arbor michingan 1 9 4 6 ) . p d l u b a r i n o v a k o c h i n a , p. ya., 1 9 6 2 theory of ground water movement (princeton u n i v e r s i t y press, princeton, new jersey 1 9 6 2 ) . u c h i d a , s . , 1 9 5 2 on the non-linear percolation at high reynolds number. nat. c o m m i t t e e for theor. appl. mech., pp. 4 3 7 4 4 2 . u p a d h y a y , k . k . , 1 9 7 5 non linear laminar flow into eccentrically placed well. " a n n a l i di geofísica", xxviii, pp. 311-319. u p a d h y a y , k . k . 1 9 7 7 non-linear laminar flow into well partially penetrating a porous aquifer of finite thichness. " p u r e and applied geophysics", 1 1 5 , pp. 6 3 1 6 3 8 . u p a d h y a y , k . k . 1 9 7 7 non-linear laminar flow of fluid into a fully penetrating cylindrical well. " i n d . j o u r , theor. physics", 2 5 , pp. 1 4 9 1 5 6 . w r i g h t , d.e., 1 9 6 8 non-linear flow through granular media. "j. hy. dn.", proc. asce, 9 4 , p p . 8 5 1 8 7 2 . annals 48, 3, 2005+app1 505 annals of geophysics, vol. 48, n. 3, june 2005 key words ionospheric scintillations − scintillation models − gps − sbas 1. introduction when traversing drifting ionospheric irregularities, a radio wave experiences fluctuations in its amplitude and phase: the fluctuation characteristics depend on the radio frequency, magnetic and solar activity, time of day, season of the year and magnetic latitude of the observation point in the case of satellite to ground links. nowadays, one of the most important fields where as complete as possible knowledge of ionospheric scintillation effects is needed is that of satellite based navigation systems. ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of navigation systems. for this reason several type of analysis of ionospheric scintillation effects on gps signals have been carried out. in principle, scintillation data can be measured by means of gps scintillation monitors, in order to directly assess the effect of ionospheric scintillation on gps signals and thus on satellite navigation operations. the actual effect of ionospheric scintillation on satellite navigation systems cannot be assessed by analysing experimental data (even if measured by gps scintillation monitors) in a «classical» way. the classical method used to analyze scintillation data deals with average scintillation activity varying with geophysical conditions such as season and solar activity. such an approach does not take into account any particular navigation system characteristics and is only able to generate from experimental data global morphology of scintillation activity, as observed at a given site (or in a given area) by means of a particular gps receiver. using such an approach the actual effect of ionospheric plasma irregularities on satellite navigation operations can be assessed only to a limited extent. for satellite navigation systems two levels should be considered at least: user level and comparison of ionospheric scintillation models with experimental data for satellite navigation applications biagio forte and sandro m. radicella the abdus salam international centre for theoretical physics (ictp), trieste, italy abstract a comparison between two of the most used scintillation models and experimental data is presented. the experimental data have been derived from a gps scintillation monitor developed at cornell university and placed in tucuman (argentina), under the peak of the anomaly. the models used (gism and wbmod) have been run for the geophysical conditions corresponding to the measurements. the comparison is done by subdividing the information on the basis of an ionospheric grid of 5° × 5° surface square boxes. the comparison has been performed for several local times, from 18 lt until 04 lt. here, only a few cases of particular interest are shown. the goal is to understand if the models are able to forecast actual scintillation morphology (from the satellite navigation systems point of view) and if they could be used to yield an estimate of scintillation effects on satellite navigation systems. mailing address: dr. biagio forte, the abdus salam international centre for theoretical physics (ictp), strada costiera 11, 34100 trieste, italy; e-mail: bforte@ictp.trieste.it 506 biagio forte and sandro m. radicella monitoring station level (forte et al., 2002). the impact of ionospheric scintillation on each level is, in principle, different and needs to be properly described. in this work a different approach to the analysis of scintillation data will be presented. this method of analysis is conceived as system oriented since it considers the main operations made by satellite navigation systems at the two aforementioned levels. the next step is to consider data coming from different sites and measured simultaneously, in order to assess scintilation effects on a regional scale. one tool used for planning satellite systems robust against ionospheric scintillation is modeling. several modeling efforts have been attempted in order to forecast scintillation activity under given geophysical conditions as well as its effect on satellite navigation systems. this work compares two of the most used scintillation models (gism and wbmod) with experimental data. the goal is to understand if the models are able to forecast actual scintillation morphology (from the satellite navigation systems point of view) and if they could be used to have an give an estimate of scintillation effects on satellite navigation systems. a validation of wbmod has been attempted in the southeast asian sector (cervera et al., 2001). 2. the models used several modeling efforts have previously been made to describe the physical mechanism giving rise to scintillation as well as the statistical distribution of ionospheric electron density irregularities. the most important goal is the forecast of scintillation activity. this is required in the evaluation of ionospheric scintillation impact on important applications of radio wave propagation, as satellite based navigation and positioning. 2.1. wbmod the wbmod model describes a worldwide climatology of the ionospheric plasma density irregularities which can produce scintillation of radio waves traversing the ionosphere. wbmod is a climatological model for ionospheric electron density irregularities coupled with a propagation model able to describe the effects of ionospheric plasma irregularities on transionospheric radio signals (secan et al., 1995). the parameters of ionospheric irregularities are modeled on the basis of experimental data (secan et al., 1995), while the propagation model is essentially a phase screen model (rino, 1979). the model provides the intensity scintillation index s4 and the phase scintillation index σφ , computed by means of the propagation model under the pre-specified geophysical conditions. the model can compute the percentage of time that those indices would exceed a specified threshold level for given geophysical conditions. it is also able to compute the si scintillation index, which has engineering applications (secan et al., 1995). 2.2. gism the global ionospheric scintillation model (gism) provides the statistical characteristics of the transmitted signals, in particular scintillation indices, fade duration, and cumulative probability of the signal, permitting the determination of the margins to be included in a link budget. maps of scintillation indices can easily be obtained. the model is composed of two submodels. the first model provides the mean errors and is based on a solution of the ray equation for a background propagation plasma. the refractive index is calculated from an accurate value of the electron density inside the medium, which is provided by the nequick model (radicella and leitinger, 2001). the second model is based on the multiple phase screen technique and provides second order propagation effects, namely radio wave scintillation. inputs to the model consist of geophysical paramaters, inhomogeneity data, and the operating data (e.g., carrier frequency, link geometry, local time, etc.). the inhomogeneity data are essentially deduced from measurements. intensity and phase scintillation indices are provided as output (beniguel, 2002). 507 comparison of ionospheric scintillation models with experimental data for satellite navigation applications 3. the dataset used the data set used for this study was collected at the institute of physics of the national university of tucuman (argentina), by means of a scintillation monitor developed and provided by cornell university (beach and kintner, 1999). s4 scintillation indices averaged over one minute periods are computed. data were collected over 11 months; the measurements time interval is 12 h during nighttime. the ionosphere over the tucuman sky is assumed to be approximated by a thin shell at 350 km altitude. this modeled ionosphere is then interfaced with a grid divided into 5° × 5° surface square boxes. the use of an ionospheric grid at 350 km is suitable to spatially and temporally divide both experimental and modeled information. thus, an ionospheric grid of 5°×5° surface square boxes is adopted. in this way a clear geographical distribution of the information can be displayed. this method, repeated for several hours, gives at the end a full spatial-temporal view of the observed and modeled phenomena, enabling not only a careful comparison between models and experimental data, but also determination of the impact of ionospheric scintillation on gps positioning and navigation operations. the reason for such an approach is to test the use of scintillation models for their application to operational systems like space based augmentation systems (sbas). sbas are gps augmentation systems which will be used for precision air navigation procedures. three systems are planned: a) the us wide area augmentation system (waas); b) the european geostationary navigation overaly system (egnos), and c) the japanese mtsat (multifunctional transport satellite) satellite based augmentation system (msas). basic gps service cannot meet the accuracy, availability and integrity required by the air navigation precision approach. sbas are safety critical navigation systems that should provide a quality of positioning information never available to the aviation community before. it should improve accuracy, integrity and availability of the basic gps service. sbas are based on a network of ranging and integrity monitoring stations (rims). signals from gps satellites are received by rims which determine the actual ionospheric error for a particular area. the corresponding correction to be applied to a single frequency positioning determination is computed and broadcast to a user inside a given area via a geostationary satellite. the analysis presented hereafter has been carried out in the framework of the egnos project and its goal is to estimate actual effects of ionospheric scintillation on gps positioning and on the rims calculation of the ionospheric error in a given area. thus, the excercise of comparing such experimental data with models’ predictions has been done to assess whether theoretical models (at least those available) can be used in defining the impact of scintillation on these systems, instead of using actual and not widely available experimental data. penetration points are considered at ionospheric grid height, namely at 350 km of altitude. experimental and modeled scintillation indices are analyzed in each ionospheric grid box. for each ionospheric grid box, percentages of occurrence of s4 greater than given thresholds (i.e. 0.5, 0.8, and 0.9) are computed, in order to assess whether scintillation may have a critical impact on rims operations and, thus, on sbas performance (forte et al., 2002). 4. discussion the comparison between data and wbmod is made on the basis of percentages of occurrence of an s4 greater than a given threshold (i.e. 0.5). the comparison between data and gism simulations has to be taken into account with caution, because while gism is directly computing the most probable s4 within a given ionospheric box and during particular geophysical conditions, the experimental data are in the form of percentages of occurrence of an s4 greater than a given thresholds. this kind of comparison, even if not complete, is in any case significant: high percentages of occurrence of an s4 > 0.5, for instance, indicate a high probability that the average s4 in that particular box 508 biagio forte and sandro m. radicella is greater than 0.5 and from here the comparison with gism predictions is feasible. the experimental data are used in two ways: 1) in a climatological way, where all the measurements taken during a given month and at a particular hour are put together to compute percentages of occurrences; 2) in a «single spot» way, where the measurements taken during just one night and at particular local times are compared with the corresponding model output. the comparison is considered for several hours, ranging from 18 lt to 04 lt, for both climatological and spot comparisons, and for the months of september 1998, october 1998, february 1999, march 1999, september 1999, and october 1999. here, the representative cases of september 1998, october 1998, september 1999, and october 1999 are presented. several limitations arise from such a comparison. first of all, the models predict scintillation activity only in a fixed time interval at low latitudes. wbmod predicts scintillation activity from 18 lt to 02 lt, while gism predicts scintillation activity from 20 lt to 02 lt. as can be deduced from experimental data (going from 18 lt to 04 lt) ionospheric scintillations can occur on gps signals even outside these local time intervals. this happens because usually the models are calibrated using particular data sets and do not take into account the proper physical mechanism giving rise to the onset of ionospheric small scale irregularities producing scintillation of radiowaves. most of the times, this calibration is made on the basis of the percentage of occurrence of scintillation activity in a given hour. of course, low percentages of occurrence do not mean total absence of scintillation activity. a given model, even if based on experimental data, should produce consistent predictions in other conditions, different from those of the data set used. figures 1a-f to 4a-f show the comparison between experimental data and models output for different months at 20 lt and 22 lt, in fig. 1a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 20 lt (september and october 1998). a b c d e f 509 comparison of ionospheric scintillation models with experimental data for satellite navigation applications fig. 2a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 20 lt (september and october 1999). fig. 3a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 22 lt (september and october 1998). a b c d e f a b c d e f 510 biagio forte and sandro m. radicella terms of percentages of occurrence of an s4 greater than 0.5. another limitation arising from the comparison of experimental data and theoretical models, is that the models fail to describe the patchy irregularity structure. gps signals with different link geometries can encounter several regions containing small scale irregularities able to produce radiowaves scintillation. this is a typical feature of ionospheric irregularities: the patchy structure depends on several factors. at low latitudes, different mechanisms can produce radiowaves scintillation: irregularities forming in the bottomside (which produce bottomside spread f) and those associated with fully developed bubbles. the dynamics of the small scale irregularities embedded in the bubbles is extremely difficult to model and the presence or absence of an irregularity able to produce radiowaves scintillation depends on the electron density deviation from the surrounding background electron density and on the fresnel scale. these factors strongly depend on the particular geophysical conditions during which such irregularities develop. both wbmod and gism depict a smoothed situation. wbmod predicts onset of small scale irregularities giving rise to lband scintillation only in a belt located southward of the observing site. gism predicts scintillation activity with strongly geographic latitudinal dependence: the peak appears to be located in a wide region northward of the observing site. this limitation clearly arises from figs. 5a-f to 7a-f, where model predictions are compared to single day experimental data (i.e. the day 15 of each month) at 20 lt. particular cases are shown, where single day measurements are compared to model predictions. in fig. 7a-f, a case of no experimental s4 indices greater than 0.5 occurred at 23 lt on 15 september 1999, while the models predict some scintillation events in different ways. these difficulties lead to modeling results that have to be interpreted in probabilistic fig. 4a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 22 lt (september and october 1999). a b c d e f 511 comparison of ionospheric scintillation models with experimental data for satellite navigation applications fig. 5a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 20 lt during the day 15 of each month (september and october 1998). fig. 6a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 20 lt during the day 15 of each month (september and october 1999). a b c d e f a b c d e f 512 biagio forte and sandro m. radicella terms. gism predicts scintillation activity in a wide area over the observing site, with scintillation intensity decreasing with latitude: the patchy structure of ionospheric irregularities giving rise to l-band scintillation (mostly related to bubbles) is completely absent. gism predicts the same behaviour for scintillation activity at different local times, changing just the scintillation intensity, but not its morphology. wbmod predictions appear to be more sensitive to changes in local times. wbmod predicts scintillation activity in a belt southward of the observing site, with peaks in scintillation intensity moving from east to west, for local times going from 18 lt to 02 lt. this description in general seems to be much more realistic than gism because it takes into account the drift of the ionized plasma (with electron density irregularities embedded in it), as well as the formation and decay of large scale structures (like bubbles in this case) which depend on the local times. this mechanism could be responsible for an increase in scintillation activity in some sectors of the ionosphere above the observing site, namely eastward of the observing site where the onset of irregularities caused by background ionization decay is most likely to occur. then peaks in scintillation activity move along the east-west direction, corresponding to the east-west drift of ionization decay and of electron density irregularities which form in the evening hours (just after sunset), last for some hours and then decay allowing re-distribution of electron density. in wbmod, as well as in gism, the patchy structure of irregularities producing radiowaves scintillation is absent. as already mentioned this patchy structure seems really very hard to model. the scintillation models here analyzed appear to be limited in their ability to estimate the impact of ionospheric small scale irregularities on satellite positioning and navigation systems, like gps and galileo, for a series of reasons. as previoulsy mentioned, the patchiness feature of those irregularities giving rise to lband scintillation (as clearly visible from exfig. 7a-f. percentages of occurrence of s4 > 0.5 for experimental data and wbmod and predicted s4 values by gism are shown at 03 lt during 15.02.1999 and 23 lt during 15.09.1999. a b c d e f 513 comparison of ionospheric scintillation models with experimental data for satellite navigation applications perimental data here presented) is not properly modelled. this feature is completely absent from model predictions, even if the models are compared with «climatological» data because experimental data show that the occurrence of patchy irregularities is much more structured (figs. 1a-f to 4a-f). radiowaves scintillation is just an effect of radiowaves propagation through electron density irregularities. this effect strongly depends on the receiver used and, in principle, different receivers could react in different ways to the same scintillating conditions. scintillation models just try to predict the occurrence of electron density irregularities and, in a probabilistic way, try to give an estimate of the scintillation indices for a given transionospheric link (by specifying satellite and receiver positions as well as the transmission frequency). the models do not take into account that the effect of scintillation is strongly dependent on the receiver used: for instance, while an s4 = 0.8 could mean loss of lock for a particular receiver, the same s4 value could not affect signal tracking in another receiver. this concept is a very important one and indicates that the estimate of the actual impact of scintillation on the performance of a given satellite system has to be considered. in the latter case, the engineeristic approach appears to be extremely relevant. scintillation models are climatological models. they of course fail when compared to «single spot» situations (for instance, when compared to single measurements nights), where patchy structures are present and where the receiver response is critically dominant. as climatological models both wbmod and gism work quite well since they are able to identify, more or less, the area and the local times affected by scintillation problems. of course, they fail to resolve the details, because they are just theoretical models calibrated on the global morphology of scintillation activity (figs. 5a-f and 6a-f). the most important limitation in the use of scintillation models for scintillation effects assessment is the following. those models are based on the weak scattering theory and, even modeling an irregularity on a given link in the best way, they are limited to low scintillation indices values. this is an intrinsic limitation of the models, since the weak scattering approximation works well only when the electron density fluctuations are not very high (i.e. of the order of few tens of percentage). low scintillation indices at l-band and in the case of gps operations do not create significant problems for them. the most severe limitation in the comparison of scintillation models with gps derived experimental data is focused on very high scintillation activity (i.e. scintillation indices) which is responsible for loss of signal lock and consequently degrading of gps positioning and navigation operations. this limitation also justifies the interpretation of such a comparison in probabilistic terms. because of the weak scattering approximation and validity of models, the comparison of time series of experimental and modeled scintillation indices has absolutely no meaning. what is still consistent, even in the presence of weak scattering approximation in the propagation models, is the modeling of small scale irregularities (and their spatial-temporal features) able to produce radiowaves scintillation at l-band, always in a probabilistic and climatological way. 5. conclusions wbmod and gism models have been analyzed against gps derived scintillation data. these models (as probably any other model in general) are able to reproduce the global morphology of small scale irregularities, including their onset and decay. most of the time, the models fail to forecast scintillation indices on a given gps link. as visible from climatological comparison (figs. 1a-f to 4a-f), the experimental s4 indices are distributed in areas not predicted by the models, which describe a sort of smoothed area (southward of the receiver for wbmod and along northsouth direction for gism) where the onset of small scale irregularities is likely to occur. when compared to single day data, the models clearly show a lack of precision in the detailed resolution. the single day data can be distributed in localized spots, clearly indicating patchy structures. the models, on the contrary, 514 biagio forte and sandro m. radicella are not able to reproduce the patchy feature of small scale irregularities, since they predict probabilistic belts (north-south elongated for gism and east-west elongated for wbmod) of occurrence of small scale irregularities rather than localised or patchy irregularities. these models reproduce well enough the global morphology of ionospheric scintillations, being calibrated with data sets that do not include gps derived data. in the case of gps links the geometry is always changing and different from geostationary or polar orbiting satellite radio links. this leads to another limitation of scintillation models when applied to gps cases. from the analysis of the comparison made the use of scintillation models to estimate ionospheric scintillation effects on single gps links and thus on gps operations does not appear to be feasible. in addition it should be noticed that ionospheric scintillation effects on gps operations are strongly and critically dependent on the particular receiver used and on the satellite navigation system requirements. different receivers can have different responses in the same scintillating environment. the use of scintillation models, then, should be limited to the description of global scintillation morphology and to particular cases as for beacon satellites, taking into account all the aforementioned limitations for satellite navigation and positioning systems. references beach, t.l. and p.m. kintner (1999): simultaneous global positioning system observations of equatorial scintillations and total electron content fluctuations, j. geophys. res., 104 (a10), 22553-22565. beniguel, y. (2002): gism: a propagation model for scintillations of transmitted signals, radio sci., 37 (3), 10.129/2000rs002393. cervera, m.a., r.m. thomas, k.m. groves and a.g. ramli (2001): effendy, validation of wbmod in the southeast asian region, radio sci., 36 (6), 10.1029/ 2000rs002520. forte, b., s.m. radicella and r.g. ezquer (2002): a different approach to the analysis of gps scintillation data, ann. geophysics, 45 (3-4), 551-561. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27, 35-40. rino, c.l. (1979): a power law phase screen model for ionospheric scintillation, 1. weak scatter, radio sci., 14 (6), 1135-1145. secan, j.a., r.m. bussey and e.j. fremouw (1995): an improved model of equatorial scintillation, radio sci., 30 (3), 607-617. layout 6 effects of solar and geomagnetic activities on the sub-ionospheric very low frequency transmitter signals received by the demeter micro-satellite mohammed yahia boudjada1, konrad schwingenschuh1, emad al-haddad2, michel parrot3, patrick h.m. galopeau4, bruno besser1, günter stangl5, wolfgang voller1 1 institut für weltraumforschung der österreichischen akademie der wissenschaften, graz, austria 2 akademischer lehrgang software engineering an der fh campus 02, graz, austria 3 laboratoire de physique et chimie de l’environnement et de l’espace, université d’orléans, cnrs, orléans, france 4 laboratoire atmosphères, milieux, observations spatiales, cnrs, ipsl, guyancourt, france 5 bundesamt für eichund vermessungswesen, vienna, austria annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5463 abstract in the framework of seismic precursor electromagnetic investigations, we analyzed the very low frequency (vlf) amplitude signals recorded by the instrument champ electrique (ice) experiment on board the demeter micro-satellite. the sun-synchronous orbits of the micro-satellite allowed us to cover an invariant latitude of between –65˚ and +65˚ in a time interval of about 40 min. we considered four transmitter signals emitted by stations in europe (france, ftu, 18.3 khz; germany, dfy, 16.58 khz), asia (japan, jp, 17.8 khz) and australia (australia, nwc, 19.8 khz). we studied the variations of these vlf signals, taking into consideration: the signal-to-noise ratio, sunspots, and the geomagnetic activity. we show that the degree of correlation in periods of high geomagnetic and solar activities is, on average, about 40%. such effects can be fully neglected in the period of weak activity. we also find that the solar activity can have a more important effect on the vlf transmitter signal than the geomagnetic activity. our data are combined with models where the coupling between the lithosphere, atmosphere and ionosphere is essential to explain how ionospheric disturbances scatter the vlf transmitter signal. 1. introduction electromagnetic phenomena over a wide frequency range have been recognized as precursors to earthquakes. precursor emissions have been found to cover a large frequency spectrum, as reported by several studies: ultra-low frequency (ulf) [fraser-smith et al. 1990, molchanov et al. 1992, hayakawa et al. 1996], extremely low frequency/ very low frequency (elf/vlf) [gokhberg et al. 1982, parrot and lefeuvre 1985], low frequency (lf) [biagi and hayakawa 2002], and high frequency (hf) [warwick et al. 1982]. these first investigations were supported by laboratory experiments, where the stress of the rock was shown to release free electrons and electric charges, as reviewed by parrot [1995], hayakawa and molchanov [2002], and molchanov and hayakawa [2008]. 1.1. very low frequency transmitter signals over seismic regions vlf transmitters are primarily devoted to navigation and communication with military submarines. most of the energy radiated by such transmitters is trapped between the ground and the lower ionosphere, which forms the earth-ionosphere waveguide. the remote sensing of the ionosphere using different techniques, like the ground vertical sounding [liperovskaya et al. 2008, liperovsky 2008a, b, pulinets et al. 2002], has shown anomalies in the electron density of the ionosphere above seismic regions. the complementarity of ground-based and space observations allows us local and spatial cover of earthquake regions. the demeter micro-satellite was devoted to the study of electromagnetic pre-seismic emissions using a series of experiments that provide a more complete view of these phenomena [parrot et al. 2006]. the use of electric and magnetic field measurements on board the demeter microsatellite have demonstrated that the intensity levels of transmitter signals decrease a few days before a seismic event occurrence [molchanov et al. 2006]. molchanov et al. [2006] estimated the variations in the vlf signals emitted by transmitters in australia (code: nwc), france (code: ftu), germany (code: dfy), and japan (code: jp). these showed drops in their vlf signals a few days before the occurrence of large earthquakes (magnitudes >5.5) in europe and in asia. later, several investigations [rozhnoi et al. 2007, boudjada et al. 2008, muto et al. 2008, slominska et al. 2008] confirmed special issue: earthquake precursors 49 article history received july 15, 2011; accepted november 14, 2011. subject classification: earthquakes prediction, ionospheric disturbances, solar and geomagnetic activities, pre-seismic electromagnetic emissions. this drop in vlf intensity. it is important to note that the decrease in the vlf intensity also concerns the natural whistler emissions, as reported by boudjada et al. [2010] for the l’aquila earthquake (april 6, 2009). hence, it was seen that the signal of the dfy transmitter was ‘damped’ from 5 days before to a minimum of one day before the earthquake. also, the vlf whistler emission, which is generated mainly in the magnetosphere, decreased from 7 days before and attained a minimum on the day of the l’aquila seismic event. the disturbance of both of these vlf signals (i.e. transmitter and whistler emission) might largely be due to the earthquake preparation zone. this indicates that not only the lower part of the ionosphere is disturbed, but also the higher part, at the altitudes of the demeter satellite. 1.2. effects of solar and geomagnetic activities on the earth ionosphere the influence of geomagnetic activity, and also solar activity, cannot be neglected in studies of pre-seismic anomalies related to the earth ionosphere. several parameters, like the kp-index, ap-index, dst-index and the sunspot numbers, are taken into consideration to separate ionospheric anomalies that are associated with earthquakes from others that are principally linked to geomagnetic activity. in general, the selected earthquake events are analyzed during periods of relatively quiet geomagnetic activity. rozhnoi et al. [2007] suggested that the effects of geomagnetic activity (described by the dst-index) can be neglected, because such effects were absent in vlf transmitter signals in the hawaii islands (code npm) and in australia (code nwc) for japanese seismic events (november to december 2004, august 2005). boudjada et al. (2008, 2010) considered the kp-index, and showed that the geomagnetic activity was weak for the european earthquakes that occurred in italy (december 2004, april 2009). however, the general behavior of the geomagnetic activity is similar to the vlf intensity variations. 2. data investigation in the following subsections, we describe the steps in the analysis of the relationships between the solar/geomagnetic activities and the vlf transmitter signals. first, we consider the vlf observations recorded by the instrument champ electrique (ice) experiment above the vlf transmitter ground-based stations. after this, we estimate the signal-tonoise ratio, and we fill the gaps in the data by using the interpolation technique. this allows us to give an averaged estimation of the vlf intensity and the corresponding standard deviation. then we select specific periods, between august 8, 2004, and december 31, 2006, when the sunspot number, or the daily ap-index, was large. in the last step, we proceed to the correlation of the solar/geomagnetic activities and the vlf averaged signals. 2.1. methods of analysis the ice experiment [berthelier et al. 2006] provides a survey from ulf to hf for each half-orbit. the survey consists of a dynamic spectrum that shows the intensity level variations versus the frequency ranges and the observation times. each dynamic spectrum comprises 980 sequential spectra, each of which consists of 1024 frequency channels. the high spectral resolution of the experiment (pixel size, 50 boudjada et al. transmitter frequency latitude longitude ice: closest frequency code (khz) (°) (°) (khz) nwc 19.8 -21.5 114 19.8047 ftu 18.3 +46.4 1.05 18.3008 jp 17.8 +32.0 130 17.7930 dfy 16.56 +52.5 13.0 16.5625 table 1. main features of the vlf stations and closest frequencies to the transmitter. figure 1. daily ap-index (upper panel) and sunspot number (lower panel) variations versus doy (first day: january 1, 2004). the period investigated starts from august 8, 2004 (doy 221) and goes to december 31, 2006 (doy 1098). the blue and yellow vertical lines indicate the periods of high geomagnetic and solar activities. 51 2.048 s × 19.51 hz) allows the recording of ionospheric components like the hiss, chorus, and whistlers, and the transmitter artificial signals. we use the so-called survey mode, which allows the recording of low bit-rate data all around the earth at invariant latitudes <65˚. we center our attention on the vlf signals associated with four transmitters: (a) the nwc transmitter (australia: latitude = –21.5˚, longitude = 114˚), the ftu transmitter (france: latitude = +46.4˚, longitude = 1.05˚), the jp transmitter (japan: latitude = +32˚, longitude = 130˚) and the dfy transmitter (germany: latitude = +52.5˚, longitude = 13.5˚). we select the demeter orbits during periods where this micro-satellite was above the transmitter station. a ‘rectangle’ area centered on the vlf transmitter station is selected, with latitude and longitude ranges of about 10˚. this allows daily collected measurements of the ice experiment for frequencies close to the vlf transmitter frequency. table 1 lists the main features of the vlf stations and the closest frequencies to the transmitter. 2.2. solar and geomagnetic activities figure 1 shows the variations of the daily ap-index (upper panel) and the daily sunspot number (lower panel) versus the day of the year (doy; starting from january 1, 2004). in each panel of figure 1, the beginning and the end of the time intervals where the high activity was recorded are indicated by colored vertical dashed lines. this time interval corresponds to about several days, on average. table 2 and table 3 list the sunspot numbers associated with the solar activity, and the daily ap-index linked to the geomagnetic activity, respectively. for each maximum of activity, table 2 and table 3 give: the observation date, the corresponding doy, the ap-index or the sunspot number, and the probable common event. it is clear from figure 1 that the decrease in the solar activity (minimum in june 2006) is followed by a decrease in the geomagnetic activity. hence, events (a), (b), (c) and (d), as reported in table 2 and table 3, are probably correlated with a time delay of about 17 days, 51 days, 8 days and 42 days, respectively. the other events show first the growth of the geomagnetic activity, followed by the increase in the solar activity. selected periods are used to study the presence, or not, of any correlation between these activities and the vlf flux density linked to the ground-based transmitter signal. 2.3. correlations between the geomagnetic and solar activities and the very low frequency transmitter signals figure 2 shows the variations in the vlf transmitter signals as recorded by the ice experiment above the corresponding stations. we consider the period beginning on august 11, 2004, to december 31, 2006. the maximum vlf intensity is about 8 ×105 mv2 m−2 hz−1 for the nwc transmitter, 9 ×104 mv2 m−2 hz−1 for the ftu transmitter, 8 ×103 mv2 m−2 hz−1 for the jp transmitter, and 3 ×103 mv2 m−2 hz−1 for the dfy transmitter. the nwc transmitter is remarkably intense when it is compared to the others, e.g. it is two orders of magnitude more intense than the dfy transmitter. we note the presence of gaps in the data, because the ice experiment was not operating or the transmitter was stopped. for example, the common gap of about 23 days for all of the stations between september 27, 2005 (doy 616 in figure 2) and october 19, 2005 (doy 659 in figure 2) is due to the absence of observations for the demeter satellite. sometimes the transmitter was not in operation, as seen in figure 2, where the vlf signals from the other transmitters were recorded. we correlated the vlf transmitter signals with the selected periods of solar and geomagnetic activities, as reported in table 2 and table 3, respectively. for this, we used a routine written in the interactive data language (idl) software, that allows the estimation of the degree of correlation and the corresponding lag. the formula is: this function pxy(l) computes the cross-correlation of two sample populations x and y as a function of the lag l. in this case, x–and y–are the means of the sample populations x= = (x0, x1, x2, ... , xn–1) and y = (y0, y1, y2, ... , yn–1), respectively ( ) ( ) ( ) ( ) ( ) for 0 ( ) ( ) ( ) ( ) for 0 p l x x x x y y l x x y y x x y y l y y xy k k n k k n k l k k n l k k n k k n k k l k n l 2 0 1 2 0 1 0 2 0 1 2 0 1 0 1 1 1 $ = = = + = = = + = r r r r r r r r ; ; ; ; e e e e z [ \ ] ] ] ] ] ] ] ] ]] / / / / / / sub-ionospheric vlf transmitter signals date day of year daily sunspots significance 2004 10 24 0298 099 a 2005 01 16 0382 065 2005 05 11 0497 082 2005 07 04 0551 107 b 2005 12 03 0703 060 2006 04 06 0827 054 c 2006 11 03 1038 044 d table 2. sunspot numbers associated with the solar activity. date day of year daily ap index significance 2004 11 10 0315 161 a 2005 01 10 0376 084 2005 05 10 0494 091 2005 08 10 0602 102 b 2006 04 14 0835 065 c 2006 12 10 1080 094 d table 3. daily ap-index linked to the geomagnetic activity. [fuller 1976]. figure 3 shows the procedure to estimate the degree of correlation. we used two sets of data, one from the jp transmitter, and the other from the daily ap-index, for the same period from august 24, 2004 (doy 237) to december 26, 2004 (doy 306). figure 3 shows the averaged transmitter signal of the jp signal (first panel), the daily apindex (second panel), and the degree of correlation (third panel). in this example, the degree of correlation is about boudjada et al. 52 figure 2. flux density variations versus the doy for the vlf transmitter stations in australia (nwc, top panel), france (ftu, second panel), japan (jp, third panel), and germany (dfy, bottom panel). figure 3. the vlf transmitter signals (top panel) and the daily ap-index (second panel) and their estimated degree of correlation (bottom panel). the degree of correlation is about 15%, with a lag of <1 day. 53 15%, with a time lag of less than one day. in the correlation processes, we consider three different signal-to-noise ratios of 1.0, 10 and 100 [schroeder 2000]. the aim is to determine the dependence, or not, of the correlation on the signal-tonoise ratio. the results are shown in figure 4, where the green and red indicate the correlation of the vlf signal with the geomagnetic and solar activities, respectively. first, the correlation is, on average, <40% for all of the stations. secondly, the solar activity might have a more important effect on the vlf transmitter signal than the geomagnetic activity. thirdly, the variations in the correlation coefficients versus the doy are similar for all of the stations. hence a decrease in the correlation from august 2004 (coefficient of correlation of about 50%) to november 2005 (coefficient of correlation of about 20%) is evident when combining all of the stations. fourthly, only in one case is the degree of correlation in the order of 80%, as shown in the fourth panel of figure 4 (i.e. for the dfy transmitter). 3. discussion and conclusions we studied the variations in the vlf transmitter signals recorded by the ice experiment on the demeter microsatellite. the orbital features allowed us to cover a time interval of about 40 min for all longitudes and the invariant latitude range between –65˚ and +65˚. the subsequent analysis investigates the influence of the solar and geomagnetic activities on the reception of the vlf transmitter by the demeter micro-satellite. in the following, we discuss our main results: (a) the influence of the solar and geomagnetic activities on the detection, or not, of precursor electromagnetic emissions; and (b) the way to explain how the vlf signal is disturbed along its path to the satellite. 3.1. effects of the solar and geomagnetic activities on the transmitter signals we considered here a period of investigation over more than two years. this allows us to select different time intervals where we can note enhancements of the geomagnetic and solar activities. we have estimated the degree of correlation for four vlf transmitter stations. the correlation was, on average, about 40%. as shown in figure 1, the ap index and the sunspot number reached over 100 during these strong geomagnetic and solar activities. the effects of these activities can be considered in the analysis of the pre-seismic precursors, although it is still not dominant. this effect can be neglected when the daily ap-index or sunspot number is <20. also, we see that the control of the solar activity is usually dominant when it is compared with the geomagnetic activity. this result is interesting, because it shows that ionization of the dand e-layers of the ionosphere disturbs the path where the vlf transmitter signals propagate in particular under the effects of the sun. sub-ionospheric vlf transmitter signals figure 4. variations in the degree of correlations versus the doy. the red/green circles correspond to the degrees of correlations between the sunspot/ap-index and the vlf transmitter signals, for the australia (nwc, top panel), france (ftu, second panel), japan (jp, third panel) and germany (dfy, bottom panel) in the literature, the investigations of pre-seismic ionospheric anomalies mainly report parameters that are related to the geomagnetic activity (like the ap-index or dst-index), and only a few investigations have analyzed solar activity effects [e.g. liperovskaya et al. 2008]. however, we show in our studies that both can be considered to estimate their effects on the ionosphere. 3.2. lithosphere-atmosphere-ionosphere coupling the low degree of correlation (<40%) allows us to conclude that the drop in the vlf transmitter over seismic regions, as reported in several studies, might be related to the preparatory zone. the model proposed by liperovsky et al. [2000] and molchanov [2004] might explain how the gas– water release from the earthquake preparatory zone can disturb the ionosphere. these studies showed that an upward energy flux of atmospheric gravity waves can disturb the ionospheric electron density above seismic regions. a recent study of stangl et al. [2011] found that for the l’aquila earthquake, there was anomaly enhancements in the total electron content (tec) measurements. in a quasi-similar time interval, a drop in the intensity of the vlf electric field occurs. this study concluded that the paths of the vlf signals are deviated during their propagation in the turbulent ionosphere. the vlf signals drop because they only occasionally reach the demeter satellite. other models have suggested that the ionospheric disturbances are linked to the changes in the atmospheric electricity (conductivity and vertical field) due to air ionization produced by radon released from an active tectonic fault before an earthquake. space observations have reported electric fields of about 3 mv/m to 7 mv/m [yokoyama et al. 2002, chmyrev et al. 1989]. however, liperovsky et al. [2005, 2008b] showed an electric field of the order of 1000 v/m. recent investigations of ampferer et al. [2010] and denisenko et al. [2008] proposed much lower fields at the ionosphere level. this suggests that a further comparison of models and observations is recommended for the prediction of preseismic electromagnetic phenomena. acknowledgements. the authors acknowledge centre national d’etudes spatiales (cnes) for the use of the demeter data, and they thank jean-jacques berthelier, who provided the data from the electric field experiment (instrument champ electrique – ice). references ampferer, m., v.v. denisenko, w. hausleitner, s. krauss, g. stangl, m. y. boudjada and h. k. biernat (2010). decrease of the electric field penetration into the ionosphere due to low conductivity at the near ground atmospheric layer, ann. geophys., 28, 779-787. berthelier, j.j., m. godefroy, f. leblanc, m. malingre, m. menvielle, d. lagoutte, j. y. brochot, f. colin, f. elie, c. legendre, p. zamora, d. benoist, y. chapuis, j. artru and r. pfaff (2006). ice, the electric field experiment on demeter, planet. space sci., 54, 456-471. biagi, p.f. and m. hayakawa (2002). possible premonitory behaviour of lf radiowaves on the occasion of the slovenia earthquakes (m = 5.2-6.0-5.1) occurred on march-may 1998, in: seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov, terrapub, tokyo, 249-253. boudjada, m.y., k. schwingenschuh, r. döller, a. rozhnoi, m.parrot, p.f. biagi, p.h. m. galopeau, m. solovieva, o. molchanov, h.k. biernat, g. stangl, h. lammer, i. moldovan, w. voller and m. ampferer (2010). decrease of vlf transmitter signal and choruswhistler waves before l’aquila earthquake occurrence, nat. hazards earth sys., 10, 1487-1494; doi: 10.5194/ nhess-10-1487-2010. boudjada, m.y., k. schwingenschuh, h.k. biernat, j.j. berthelier, j. blecki, m. parrot, m. stachel, ö. aydogar, g. stangl and j. weingrill (2008). similar behaviors of natural elf/vlf ionospheric emissions and transmitter signals over seismic adriatic regions, nat. hazards earth sys., 8, 1229-1236; doi:10.5194/nhess-8-12292008. chmyrev, v.m., n.v. isaev, s.v. bilichenko and g. stanev (1989). observation by space-borne detectors of electric fields and hydro-magnetic waves in the ionosphere over an earthquake centre, phys. earth planet., 57, 110114. denisenko, v.v., m.y. boudjada, m. horn, e.v. pomozov, h.k. biernat, k. schwingenschuh, h. lammer, g. prattes and e. cristea (2008). ionospheric conductivity effects on electrostatic field penetration into the ionosphere, nat. hazards earth sys., 8, 1009-1017,. fraser-smith, a.c., a. bernardy, p.r. mcgill, m.e. ladd, r.a. helliwell and o.g. villard jr. 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(2004). on the origin of lowand middlelatitude ionospheric turbulence, phys. chem. earth, 29, 559-567, 2004. molchanov, o.a., a. rozhnoi, m. solovieva, o. akentieva, j.j. berthelier, m. parrot, f. lefeuvre, p.f. biagi, l. castellana and m. hayakawa (2006). global diagnostics of the ionospheric perturbations related to the seismic activity using the vlf radio signals collected on the demeter satellite, nat. hazards earth sys., 6, 745-753, molchanov, o.a. and m. hayakawa (2008). seismo-electromagnetics and related phenomena: history and latest results, terrapub, tokyo. muto, f., m. yoshida, t. horie, m. hayakawa, m. parrot and o.a. molchanov (2008). detection of ionospheric perturbations associated with japanese earthquakes on the basis of reception of lf transmitter signals on the satellite demeter, nat. hazards earth sys., 8, 135141; doi: 10.5194/nhess-8-135-2008. parrot, m. (1995). electromagnetic noise due to earthquakes, in: handbook of atmospheric electrodynamics, edited by h. volland, crc press, boca raton, fl, vol. ii, 95-116. parrot, m. and f. lefeuvre (1985). correlation between geos vlf emissions and earthquakes, ann. geophys., 3, 737-748, http://www.ann-geophys.net/3/737/1985/. parrot, m., d. benoist, jj. berthelier, j. blecki, y. chapuis, f. colin, f. elie, p. fergeau, d. lagoutte, f. lefeuvre, c. legendre, m. leveque, j.l. pincon, b. poirier, h.c. serana and p. zamora (2006). the magnetic field experiment imsc and its data processing onboard demeter: scientific objectives, description and first results, planet. space sci., 54, 441-455. pulinets, s.a., k.a. boyarchuk, v.v. hegai and a.v. karelin (2002). conception and model of seismo-ionospheremagnetosphere coupling, in: seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o.a. molchanov, terrapub, tokyo, 353-361. rozhnoi, a., o.a. molchanov, m. solovieva, v. gladyshev, o. akentieva, j.j. berthelier, m. parrot, f. lefeuvre, m. hayakawa, l. castellana and p.f. biagi (2007). possible seismo-ionosphere perturbations revealed by vlf signals collected on ground and on a satellite, nat. hazards earth sys., 7, 617-624, http://www.nat-hazards-earthsyst-ci.net/7/617/2007/. schroeder, d.j. (2000). astronomical optics, academic press, london. slominska, e., j. blecki, m. parrot and j. slominski (2008). satellite study of vlf ground-based transmitter signals during seismic activity in honshu island, physics and chemistry of the earth, phys. chem. earth, parts a/b/c; doi: 10.1016/j.pce.2008.06.016. stangl, g., m.y. boudjada, p.f. biagi, s. krauss, a. maier, k. schwingenschuh, e. al-haddad, m. parrot and w. voller (2011). investigation of tec and vlf space measurements associated to l’aquila (italy) earthquakes,nat. hazards earth sys., 11, 1019-1024. yokoyama, t., m. yamamoto, r.f. pfaff, s. fukao and n. iwagami (2002). seek-2 campaign measurement of the electric field in the e-region and its association with the qp echoes, in: abstracts for the 112th sgepss fall meeting, university of electro-communications, tokyo, 12-13. warwick, j.w., c. stoker and t.r. meyer (1982). radio emission associated with rock fracture: possible application to the great chilean earthquake of 22 may 1960, j. geophys. res., 87, 2851-2859. *corresponding author: mohammed yahia boudjada, space research institute, austrian academy of sciences, graz, austria; email: mohammed.boudjada@oeaw.ac.at. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sub-ionospheric vlf transmitter signals ag_56.05.13_shun_finalonline:layout 6 annals of geophysics, 56, 5, 2013, a0567; doi:10.4401/ag-6318 a0567 a method for separating o-wave and x-wave and its application in digital ionosonde wang shun*, chen ziwei, zhang feng, gong zhaoqian, li jutao, fang guangyou key lab. of electromagnetic radiation & sensing technology, inst. of electronics, chinese academy of sciences, beijing, china abstract separation for o wave and x wave is a very important job in interpretation of ionograms, which is premise for automatic scaling. in this paper, a new digital method for separating o wave and x wave is presented, based on a numerical synthesizing technique, which is different from using image recognition to separate trace o and trace x in the ionograms, and from using the electrical method to synthesize and detect circularly polarized waves. by replacing analog phase shifters and switches in existing ionosonde with digital phase shifters with different initial phase, 0°, +90°, −90°, circularly polarized waves are synthesized digitally within the range of 1-30 mhz, which eliminates the nonlinearity and expands the bandwidth of the ionosonde, and there is no need to switch the analog switches continuously. the new method has been successfully applied to cas-dis ionosonde and testing results show that the new digital method is capable of separating o wave and x wave well. 1. introduction vertical sounding of the ionosphere is a standard technique to study the electron density profile in the lower ionosphere. by scanning the transmitted frequency from 1 mhz to 30 mhz and measuring the time delay of any echoes, a vertically transmitting sounder can provide a profile of electron density vs. height. this is possible because the relative refractive index of the ionospheric plasma is dependent on the density of the free electrons ne. this simplified principle is somewhat more complex in practice since the group velocity of the radio wave is slower in the ionosphere so the height obtained using the time delay of the echo is an overestimation of the actual height of reflection. according to the magnetoionic theory, the propagation of radio waves through the ionosphere is significantly affected by the presence of the earth’s magnetic field. with regard to this, the appleton equation shows that the refractive index can assume two values, this results in the splitting of wave incident into the ionosphere in two components, known as the ordinary wave (o-wave) and extraordinary wave (x-wave). the o and x waves propagate with different velocity and therefore appear as two distinct echoes in ionograms; it is then necessary to distinguish the ordinary mode of reflection for implementing the true height analysis as the one performed by the polan program [titheridge 1998] or the artist system [huang and reinisch 1996]. currently, there are mainly two kinds of method used for separating the o-wave and x-wave, using image recognition technique and using antenna arrangements. image recognition technique for the separation of o-wave and x-wave has no requirement to the antenna of the ionosonde, such methods extract the trace o and trace x in ionograms by digital image processing technology. a more detailed review of these methods can be found in pezzopane and scotto [2007], scotto [2009] and krasheninnikov et al. [2010]. the separation of o-wave and x-wave can be done in an alternative way, electrically combining signals from perpendicularly orientated receiving antennas, in such a way that the two modes of the echoes can be determined. a typical representative of such an equipment is dps-4 digisonde produced by the university of massachusetts, lowell [reinisch et al. 2009] or the vipir ionosonde developed by scion associates, supported by the us air force research laboratory and their small business innovative research program [bullett et al. 2010]. in dps-4, a cross antenna system, an analog ±90° phase shifter (2-32 mhz) and a series of analog switches are used to synthesize broadband circularly polarized waves, and consequently recognize the polarization of the ordinary and extraordinary wave. anyway, the ultimate effect depends heavily on the linearity and bandwidth limitation of analog phase shifters, still, this method needs to switch analog switches between consecutive observations. article history received march 22, 2013; accepted june 5, 2013. subject classification: ionosonde, ionogram, trace o, trace x, circularly polarized antenna, phase shifters. in this paper, a new digital method for separating o-wave and x-wave is presented. just as it is in dps-4, two orthogonal antenna arrangements are used for transmitting and receiving, but removing phase shifters and switches in analog domain and introducing ±90° phase shifters in digital domain, implemented in digital down convertor process. circularly polarized waves are synthesized within the range of 1-30 mhz, which eliminates the nonlinearity and expands the bandwidth of the whole system. also there is no need to switch the analog switches continuously. currently, the method has been successfully applied to the chinese academy of sciences-digital ionosonde system (cas-dis) [shun et al. 2013] and testing results show that the new digital method can separate o-wave and x-wave well. 2. principle for separating o-wave and x-wave 2.1. a-h equation in the presence of the earth’s magnetic field, as regards the propagation of high-frequency radio waves, the ionosphere behaves as a bi-refractive medium. on the assumption that the collisions between electrons and heavy particles are negligible, the corresponding refractive index n is given by the well-known appletonhartree (ah) equation: (1-a) where: (1-b) fp is the plasma frequency, f is the radio wave frequency, fh is the electron gyrofrequency, and i is the angle between the earth’s magnetic field and the propagation vector of radio wave. a wave incident vertically into the ionosphere will be reflected when n = 0, and according to equation (1-a) this happens when the following relations are satisfied: (2-a) (2-b) the relation (2-a) represents the reflection condition of the o mode of propagation, the relation (2-b) represents the reflection conditions of the x mode of propagation. 2.2. separating o-wave and x-wave ionospheric echoes usually consist of the superposition of two elliptically polarized waves, corresponding to the cited ordinary and extraordinary waves. at the magnetic dip equator the electric field of the ordinary wave is north-south and electric field of the extraordinary wave is east-west so that the waves can be separated by the use of two linear antennas oriented north-south and east-west. in sufficiently high latitudes, or on sufficiently high radio frequencies, the polarization ellipses are essentially circles [davies 1990]. the two circularly polarized waves can be separated by two orthogonal dipoles oriented in the x and y directions. consider two oppositely rotating waves of amplitude eo and ee incident on the antennas. the outputs ex and ey are (3-a) (3-b) we now introduce a phase shift of ±90° into one of the signals, take ex for example, (4-a) (4-b) addition of ex2 and ey gives ordinary wave, and addition of ex1 and ey gives extraordinary wave. (5-a) (5-b) the addition of the signals can be done numerically, when the data are recorded digitally, or electrically. a numeric method is employed in cas-dis, different from the electrical method used in dps-4. 3. application of the method in digital ionosonde 3.1. the digital ionosonde of cas-dis a new ionosonde, called cas-dis is developed for the sake of testing the new method for separating owave and x-wave. the new design of the ionosonde is based on the thinking of software defined radios, which can be divided into five parts. specifically, these parts are timing & control unit, transmitting path, receiving path, antenna system and display terminal. in order to shun et al. 2 , x y y x y x1 1 2 1 4 1 t t l 2 2 2 4 2! n = + -^ ^h h , ,x f f y f fp h 2 2 = = andcos siny y y yl ti i= = ,x 1= .x y1 != cos cos cos cos e e t e t e t e t x o e o e ~ ~ ~ ~ = + = = + ^ h sin sin sin sin e e t e t e t e t y o e o e ~ ~ ~ ~ = + = = ^ h cos cos sin sin e e t e t e t e t 2 2x o e o e 1 ~ r ~ r ~ ~ = + + + = =^ ^h h cos cos sin sin e e t e t e t e t 2 2x o e o e 2 ~ r ~ r ~ ~ = + = = + ^ ^h h 2 sine e e e to x y o2 ~= + = sine e e e t2x yx e1 ~= + =3 realize the function of polarization separation, two transmitting channels, two receiving channels and three ddcs with different initial phase are included in cas-dis. figure 1 shows the block diagram of the whole system. the whole system of cas-dis work as follows, the configuration parameters are downloaded to parameter ram and a corresponding reset signal is generated after power on. once the timing & control unit detects the reset signal, the control logic will generate the timing and control signals according to the configuration parameters. the dds module of the transmitting path generates the carrier signal, and the phase encoded signals are fed to two orthogonal transmitting antennas by two parallel coaxial cable after d/a convertor, low pass filter and amplifier. echo signals received by the two orthogonal receiving antennas are fed to the receiving path: after digitizing and digital down converting processing in three ddcs with different initial phases, a baseband signal is obtained; the coherent integration and the pulse compression processing are implemented in data processor unit, and eventually the ionogram is produced by the display terminal. cas-dis system works in the frequency range of 1-26 mhz with step of 50 khz; it can detect layers within the range from 90 km to 990 km with the range resolution of 5 km. the 16 bit complementary code pair used in cas-dis is modulated onto the oddnumbered pulse and even-numbered pulse, and the maximum transmitted power is about 600 w. the specifications of cas-dis are shown in table 1. 3.2. the timing for separating o-wave and x-wave just as we have discussed above, when the frequency of the transmitted wave is sufficiently high the polarization ellipses of the echoes are essentially circles and two circularly polarized waves can be separated by two orthogonal dipoles. in cas-dis, there is a couple of orthogonal antennas for transmission and another couple for reception. according to equations of (3), (4) and (5), when a left-handed circularly polarized wave is transmitted the o-wave can be separated from the echoes by adding the two antenna signals after the signal captured by one of the receiving antennas is phase shifted by −90°. similarly, when a right-handed circularly polarized wave is transmitted the x-wave can be separated from the echoes by adding the two antenna signals after one antenna signal is shifted by 90°. transmitting two pulses, alternatively with left and right circular polarization, it is possible to separate o-wave and x-wave correctly, provided that the interval between the two transmitted pulses is small enough with respect to the typical timescale of ionospheric variations. a method for separating o-wave and x-wave figure 1. block diagram of cas-dis ionosonde. symbols description frequency range: 1-26 mhz height range: 90 km~990 km range resolution: 5 km tx power: 600 w (max. peak) frequency point num: 500 (max) code type: 16 bits complementary adc sampling rate: 60 mhz adc bit number: 14 bits receiver sensitivity: −120 dbm @ 150 khz (the useful bandwidth of the baseband signal) antenna: crossed delta antenna table 1. cas-dis specifications. figure 2 shows the timing of the separation process in cas-dis. the prf (pulse repetition frequency) is 20 hz, corresponding to the pulse repetition interval, pri, is 50 ms, of which 40 ms is used for transmitting and data sampling, and 10 ms is used for data transferring. for the purpose of separating for o wave and x wave, there are five operating modes switching in a pri in cas-dis system. 3.3. transmitting path the transmitting path contains dds unit, code generator unit, phase encoding unit, dacs unit, low pass filters and dual channel amplifier. in cas-dis, the dds unit is implemented by the logic resources within the fpga, which generates the sine and cosine signals from 1 mhz to 26 mhz, and the frequency of the dds can be tuned to a fixed frequency or be modified real time by the parameters. the initial phase of the dds should be fixed, for example, 0°, 90°or 270°(which is used to substitute −90°), and this is the premise for separating o-wave and x-wave. the phase encoding unit generates the 16 bits complementary code to modulate the carrier signal generated by the dds unit. after filtered by the lowpass filter with a band of 27 mhz, the modulated signal is converted to analog signals by two dacs. the outputs of dacs can be amplified to 600 w when fed to antennas by the dual channel transmitters in figure 1. in order to get good performance in separator o-wave and x-wave, the dual channel transmitters should have the same characteristics in amplitude and phase response. 3.4. receiving path the receiving path is comprised of adcs, ddcs and data processor, as shown in figure 1. in cas-dis, two high performance 14 bits adcs are used, with 60 mhz sampling rate. testing results show that the sensitivity of the digital receiver is −120 dbm. it should be noted that the receiver sensitivity is measured in the baseband and the useful bandwidth with −3db attenuation is 150 khz. because of the higher sampling rate, a process of ddc is applied before pulse compression processing. the four parts of the ddc are digital mixers (m1, m2), the cascaded integrator-comb filter (cic), compensation filter (cfir) and programmable filter (pfir). figure 3 shows the block diagram of ddc. for the sake of improving the snr of the echo signal, the technique of coherent pulse integration is used in the new system design, the times of integration is in a range of 1-128, which can provide an improvement of more than 20db to the snr of the received signal. also, the digital pulse compression technique is used, adding additional 12 db to the previous gain, and shun et al. 4 figure 2. the timing for separating o wave and x wave in cas-dis. figure 3. block diagram of ddc in receiving path. 5 reaching more than 30 db for the overall gain in the cas-dis system. 3.5. antenna system in cas-dis system, the orthogonal delta antennas or orthogonal dipoles can be used as transmitting or receiving antennas, the loads with the value of 600 ohm are installed on the top or center of antennas, and each has a balun used to match the balanced antennas with the unbalanced coaxial rf cable. 3.6. implementation for separating o-wave and x-wave let sew be the analog signal received by the antenna in ew (east-west) direction after quantization of adc1, and ssn be the analog signal received by the antenna in sn (south-north) direction after quantization of adc2, as shown in figures 4 and 5. the process of separation of o wave in cas-dis ionosonde is shown in figure 4. in the process of separation of o wave, the left-hand circular polarization signals are transmitted. in this case, the components of sin and cos from the dds unit are fed to transmitting antennas in ew direction and sn direction, respectively. the received signals of sew and ssn are fed to ddcs with initial phases of 0 and −90°, being the ddc process linear, the phases added by the ddcs are the same for the two channels, so there is no influence on the output. suppose signals after ddc process are s'ew and s'sn, then (6-a) (6-b) a method for separating o-wave and x-wave s i jq° °ew 0 0= +l s i jq° °sn 90 90= +-l figure 4. block diagram for separating o wave in cas-dis. figure 5. block diagram for separating x wave in cas-dis. in equation (6), i0°, q0°, i−90°, q−90° are the real and imaginary parts of the signals received by the antenna in ew direction and sn direction after ddc. let so be the o wave, then (7) figure 5 shows the process for separation of x wave in cas-dis. when the right-hand circular polarization signals are transmitted, the components of sin and cos from the dds unit fed to transmitting antennas are interchanged, and the receiving signals of sew and ssn are fed to ddcs with initial phases of 0° and 90°, respectively. adding the results of the two ddcs, the x wave can be separated from the received signals. let sx be the x wave, (8) in equation (8), i0°, q0°, i90°, q90° are the real and imaginary parts of the signals received by the antenna in ew direction and sn direction after ddc. to illustrate the process of separation of o and x waves in cas-dis, table 2 shows a summary to the relationship within different parts of ionosonde, which are transmitting and receiving antennas, adcs, ddcs and the polarization of transmitted and received signals. 4. testing result and conclusion 4.1. testing result in order to validate the new design of separating o-wave and x-wave, two experiments have been made made in china, at ji’an in jiangxi province and wuhan in hubei province last year. figure 6 shows an ionogram observed in ji’an in the south part of china. in the ionogram, the curve with green is the trace of o wave and red is x wave, the start frequency of e layer is about 2 mhz and the cut-off frequency of the f2 layer is about 18.5 mhz for o-wave. since the station is located in southern china, the cutoff frequency of the f2 layer is much higher than the ionogram observed in other stations, especially in northern china. the transmitting antennas used in the experiment are two orthogonal delta antennas with the height of 10 meters. the receiving antennas are two orthogonal dipole antennas with the length of 20 meters and are about 2 meters high from the ground. figure 7 shows an ionogram observed in wuhan in the center part of china, in which the trace of owave and x-wave are separated clearly. the start frequency of e layer is about 1.8 mhz and the cut-off frequency of the f2 layer is about 10.6 mhz for owave. the transmitting and receiving antennas used in the experiment are orthogonal delta antennas, with the height of 30 meters and 17 meters, respectively. shun et al. 6 categories antennas signals adcs/dacs ddcs initial phase separation for o wave (left-hand circular polarization signals are transmitted) transmitting path (ew) sin dac1 —— transmitting path (sn) cos dac2 —— receiving path (ew) —— adc1 0° receiving path (sn) —— adc2 −90° separation for x wave (right-hand circular polarization signals are transmitted) transmitting path (ew) cos dac1 —— transmitting path (sn) sin dac2 —— receiving path (ew) —— adc1 0° receiving path (sn) —— adc2 90° table 2. the relationship within parts of ionosode for separation of o wave and x wave in cas-dis. s s s i jq i jq° ° ° °o ew sn 0 0 90 90= + = + + +-l l s s s i jq i jq° ° ° °x ew sn 0 0 90 90= + = + + +l l 7 4.2. conclusion the method for separating o wave and x wave in cas-dis presented in this paper is on the basis of digital technology, which is different from the analog method or by using image processing method. by using digital technology to implement phase shifter of ±90° and synthesizing of circularly polarized waves, the new method eliminates the nonlinearity and expands the bandwidth of phase shifter, and there is no need to switch the analog switch continuously, which improves the reliability of the whole system of the ionosonde. to verify the effectiveness of the new method, an ionosonde called cas-dis is developed and experiments have been made in 2012, the testing results show that the new method for separating o wave and x wave is effective and reliability in practice. acknowledgements. this study was supported by the national high technology research and development program (863 program) (no.2012aa121005). a method for separating o-wave and x-wave figure 6. ionogram of cas-dis on april 23, 2012, in station of ji’an (lt). figure 7. ionogram of cas-dis on november 5, 2012, in station of wuhan (ut). references bullett, t., a. malagnini, m. pezzopane and c. scotto (2010). application of autoscala to ionograms recorded by the vipir ionosonde, advances in space research, 45 (9), 1156-1172. davies, 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 1, 2004, 01/07def 29 annals of geophysics, vol. 47, n. 1, february 2004 key words tectonomagnetism – seismomagnetic network – seismic activity – vlf – central italy 1. introduction magnetic and electromagnetic field natural signals are monitored in seismogenic areas in the search for possible correlations with seismic activity. this field of investigation covers phenomena that vary on a very wide range of frequencies from dc to vhf. this paper reports the results of the analysis of data coming from two subsets of electromagnetic investigations, the so-called tectonomagnetic field observations and vlf natural electromagnetic field waves observation. as concerns tectonomagnetic field observations, we refer to changes in the physical properties of the crustal materials that could lead to local anomalous variations of the earth’s magnetic field. magnetic changes due to stress field variations in the uppermost crustal layers have been predicted and observed historically and the term tectonomagnetism was introduced for this research field. some indirect effects of the electrokinetic type caused by fluid flows have also been invoked as a possible cause for local magnetic anomalies. the term seismomagnetism is in particular referred to magnetic field changes associated with earthquake occurrence. following the report that tectonic events, like earthquakes and volcanic eruptions, can produce observable variations in the absolute value of the geomagnetic field intensity, many groups have started such investigations. in the case of tectonomagnetic and seismomagnetic effects the scientific literature has many examples. parrot and johnston (1993), hayakawa and fujinawa (1994), gasparini et al. (1997), johnston (1997), johnston and parrot (1997), mueller and johnston (1998), meloni et al. (2001) are dedicated to such topics. seismomagnetic effects are detected with sensitive magnetometric instrumentectonomagnetic and vlf electromagnetic signals in central italy antonio meloni (1), domenico di mauro (1), stefania lepidi (2), giuliana mele (1) and paolo palangio (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) osservatorio geofisico di l’aquila, istituto nazionale di geofisica e vulcanologia, l’aquila, italy abstract tectonomagnetic field observations from absolute magnetic field level measurements were undertaken in central italy in an area extending between latitude 41°n and 43°n and between longitude 13°e and 15°e. moreover, natural electromagnetic signals from a system of two vlf search coil wide-band antennas were collected at the geomagnetic observatory of l’aquila (42º23′n, 13º19′e). the analysis of these data allowed the investigation of the electromagnetic properties of the study area at different time and spatial lengthscales. tectonomagnetic field observations were obtained comparing data simultaneously recorded at three magnetometer stations using l’aquila observatory as a reference for differentiation. we report on the time evolution of magnetic and electromagnetic indicators related to local and regional seismic activity. mailing address: dr. antonio meloni, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: meloni@ingv.it 30 antonio meloni, domenico di mauro, stefania lepidi, giuliana mele and paolo palangio tation in the near field of earthquakes only when magniude m is greater than 6 with an amplitude of about 1 nt or so (johnston, 2002). according to many investigations, electromagnetic phenomena in a wide range of frequencies ranging from ulf to vhf that take place in the atmosphere and ionosphere also show anomalous emissions in association with earthquakes. we will refer in this paper to measurements in the vlf band only. in this band transient signals and noise are generated by various natural and artificial phenomena of which radio atmospherics (or sferics for short and sometime statics) are one kind of typical impulsive electromagnetic signals that can propagate in the atmosphere for thousand kilometers. generated by atmospheric lightning strokes, radio atmospherics travel in the earthionosphere wave guide as a vlf wave with power concentrated mainly in the vlf range, from 0.1 to 10 khz (see for a recent review barr et al., 2000). gokhberg et al. (1989) were among the first to report perturbations on vlf waves amplitude in association with earthquakes. other reports, by hayakawa and fujinawa (1994), morgunov et al. (1994) and parrot (1995), followed. anomalous vlf signals were also detected in the case of the 1995 kobe earthquake (see nagao et al., 2002). electromagnetic waves at elf and vlf frequencies related to earthquakes were also detected from satellite instrumentation, see for example molchanov et al. (1993) and parrot (1994). 2. area of investigation and geodynamic framework the study area extends between latitudes 41°n and 43°n and longitudes 12°e and 16°e (fig. 1). this area has been investigated in the last twelve years starting in july 1989 with the fig. 1. topography map of the study area showing l’aquila observatory and seismomagnetic stations used in this work; epicenters (white dots) of magnitude m ≥ 4.0 earthquakes occurred from 1989 to 2001 are also reported (see list in table i). 31 tectonomagnetic and vlf electromagnetic signals in central italy intent of detecting electromagnetic and magnetic natural signals related to local seismic activity. such studies used magnetotelluric stations, magnetometer total field stations and vlf natural wave detection systems (mele et al., 1994; meloni et al., 1996, 2001; ernst et al., 1997). central italy has a long history of moderateto-intense seismicity (boschi et al., 1997; ingv, 2003) that makes this area suitable for studies of possible relations between electromagnetic natural phenomena and seismic activity. even though rock magnetization in the table i. earthquakes with m ≥ 4.0 occurring in central italy in the study period. # date origin time lat. n long. e depth m d.m.y ut km 1 06.10.1989 17:38:27 42.17 15.59 5 4.1 2 22.12.1989 06:48:15 43.02 12.77 5 4.0 3 01.02.1990 06:24:15 42.13 15.58 5 4.1 4 16.07.1992 05:38:55 42.34 14.21 5 4.0 5 05.06.1993 19:16:18 43.13 12.67 5 4.3 6 02.06.1994 17:38:15 42.44 13.21 5 4.0 7 07.08.1994 06:31:12 41.95 13.37 5 4.0 8 30.09.1995 10:14:34 41.80 15.90 22 4.4 9 20.10.1996 19:06:56 42.56 13.16 5 4.0 10 19.03.1997 23:10:49 41.38 14.63 9 4.1 11 12.05.1997 13:50:16 42.83 12.54 5 4.6 12 26.09.1997 00:33:11 43.02 12.91 8 5.6 13 26.09.1997 09:40:24 43.02 12.93 5 5.8 14 03.10.1997 08:55:20 43.02 12.90 5 5.0 15 04.10.1997 15:07:19 42.92 12.98 4 4.1 16 04.10.1997 16:13:31 42.92 12.98 4 4.3 17 04.10.1997 18:47:46 42.92 12.97 5 4.3 18 12.10.1997 11:08:35 42.90 13.01 5 4.7 19 13.10.1997 11:01:44 42.88 13.05 3 4.0 20 13.10.1997 13:09:19 42.86 13.00 4 4.0 21 14.10.1997 15:23:09 42.91 12.97 5 5.5 22 15.10.1997 22:53:09 42.92 12.00 6 4.1 23 21.03.1998 16:45:09 42.98 12.90 5 4.1 24 15.08.1998 05:18:10 42.44 13.04 5 4.5 25 10.10.1999 15:35:52 42.69 13.13 5 4.0 26 11.03.2000 10:35:28 42.03 13.17 5 4.3 27 27.06.2000 07:32:32 41.00 13.04 5 4.0 28 16.12.2000 07:31:06 42.53 12.62 8 4.0 32 antonio meloni, domenico di mauro, stefania lepidi, giuliana mele and paolo palangio study area is moderate to low (molina et al., 1994; chiappini et al., 2000), due to the presence of carbonates and marls along with flysch deposits, the generally very low level of artificial electromagnetic noise, makes it suitable for natural magnetic and electromagnetic field investigations. earthquake distribution in peninsular italy mainly follows the nw-se axis of the apennine mountain belt. focal mechanisms indicate that this chain is currently dominated by extensional deformation (e.g., anderson and jackson, 1987; amato et al., 1998) overimposed on older compressional features of paleogene age. in the central apennines, where the study area is located, a m = 6.9 earthquake occurred in 1915 (e.g., ward and valensise, 1989) causing more than 32000 fatalities (boschi et al., 1997). moderate-magnitude events also occurred in recent years: the m = 5.4 val comino earthquake in 1984 (console et al., 1989) and the 19971998 umbria-marche seismic sequence, with the six largest earthquakes of magnitude ranging between 5.0 and 6.0 (e.g., amato et al., 1998). in table i all m ≥ 4.0 earthquakes occurred from 1989 to 2001 are reported; epicenters are shown in fig. 1. many studies have shown that in the phases of preparation of a seismic event and also in case of coseismic observations, patterns of time variations of electromagnetic or magnetic parameters vary generally greatly in shape, time scales, phases and extent around the hypocentral area. moreover, quantitatively defining an ‘anomaly’ is sometimes very difficult. for such reasons, besides the identification of magnetic and vlf wave signals and their relations to seismic activity, our investigation was also finalized to improve the knowledge of the natural background electromagnetic phenomena in this region. 3. data analysis 3.1. observations by proton procession magnetometer network proton procession magnetometers (ppms) and optical pumping magnetometers are generally used for reliable tectonomagnetic studies (parsons and wiatr, 1982; mueller and johnston, 1998). the installation of a seismomagnetic network in the area of study started in middle 1989. data are gathered by means of ppms installed in a network with station separation of a few tens of kilometers. magnetometers simultaneously measure the total magnetic field intensity with a sampling rate that has been variable in the last ten years (4-10-15 min) and is now set to 15 min. the present network consists of three stations shown in fig. 1 with their acronyms. measured values are averaged and differentiated with respect to l’aquila geomagnetic observatory to filter out the contributions from external sources and from regional inhomogeneities of the earth’s magnetic field. after processing, the total magnetic field data are plotted and searched for possible local field changes in order to relate them to earthquake occurences in a defined area around the instruments magnetic network. 3.2. observations by vlf antennas in l’aquila time variability of natural signals in vlf band is monitored in the l’aquila observatory by means of two search coil wide band antennas operating in the range 15-40 khz. the system has 4 band pass filters centered at 15, 20, 30 and 40 khz respectively. radiometer sensitivity is 200 ft at 40 hz and data are sampled at 1 hz and stored with a 16 bit resolution, after averaging every 60 s. the main purpose of this monitoring procedure is to investigate the time behaviour of recorded natural radio signals in the vlf band to study its time evolution and stability. the time plots, here produced only for 20 and 40 khz, were also related to seismic activity as recorded in the defined area around l’aquila observatory. some authors have pointed out that vlf waves monitoring could be more advantageous if an artificial source is used (see molchanov and hayakawa, 1998; saraev et al., 1999). however in this area the overall low artificial electromagnetic noise in this band has facilitated the vlf natural waves data acquisition. 33 tectonomagnetic and vlf electromagnetic signals in central italy 3.3. results figure 2 reports the daily mean values for the earth’s magnetic field intensity from aqu and the seismomagnetic stations in their time evolution. a steady increasing secular variation of about 25 nt/yr is observed at all stations; synchronous spikes are due to solar activity, some interruptions are caused at the seismomagnetic stations because of instrumental failure (this figure is an update of meloni et al., 2001). figure 3 shows five-day means of simple differences for the total geomagnetic field between the aqu reference station and rit, mdm, and cvt stations. vertical lines in the plot indicate the occurrence of the most intense earthquakes (md ≥ 4.0); see also table i for the complete list. the sequence of september-october 1997, where 4 earthquakes with md 5.0 to 6.0 took place in the umbria-marche region, on fig. 2. time variation of total magnetic field f (daily means) at l’aquila observatory (aqu) and the seismomagnetic stations (rti, mdm and cvt; see also fig. 1 for map location). 34 antonio meloni, domenico di mauro, stefania lepidi, giuliana mele and paolo palangio f ig . 3. f iv e da ys a ve ra ge s of d if fe re nc es b et w ee n l ’a qu il a g eo m ag ne ti c o bs er va to ry ( a q u ) an d th e se is m om ag ne ti c st at io ns c v t ,m d m a nd r t i fr om 1 98 9 to 2 00 1. v er ti ca l ba rs i nd ic at e ea rt hq ua ke s w it h m ≥ 4. 0. a t hi ck v er ti ca l li ne r ep or ts t he s ei sm ic s eq ue nc e of s ep te m be ro ct ob er 1 99 7 th at t oo k pl ac e in u m br ia -m ar ch e re gi on s n w o f l ’a qu il a. 35 tectonomagnetic and vlf electromagnetic signals in central italy the nw sector of the monitored area, is reported as a gray band. in january-february 1990 and only at that time, an anomalous variation was clearly detected at mdm station. figure 4 reports the time variability of natural electromagnetic signals, measured at l’aquila observatory in the vlf band by means of two search coil wide band antennas, for 20 and 40 khz. this variability is shown for the year 2001 by means of 1 min data from march to september with an interruption caused by technical failure for twenty days in july and august. 4. discussion and conclusions the plots of magnetic field differences between seismomagnetic stations and l’aquila observatory show a scatter around the mean difference values; the scatter amplitude increases with the distance from l’aquila observatory showing the limits in the removal of external magnetic field variation gradients. although a moderate seismicity occurred in the study period and sometimes anomalies have been detected from this network, there is no clear evidence of a one-to-one relation between magnetic anomalies and seismic events (see also meloni et al., 1998). some variations of moderate amplitude occur in correspondence with the umbriamarche in 1997 while no other significant effects appear in the following years here reported up to 2001. the weak evidence of magnetic field changes during the observational period indicates that magnetic long term crustal origin effects do not frequently occur in this area. furthermore, the weak to moderate energy seismic activity that occurred in this observation period fig. 4. vlf 20 khz and 40 khz one minute data for part of the year 2001. 36 antonio meloni, domenico di mauro, stefania lepidi, giuliana mele and paolo palangio (1989-2001) is probably one of the causes that did not allow us to find an evident correlation between local magnetic field variations and earthquake occurrence. vlf wave characteristics recorded at any station are mainly influenced by ionospheric parameters. among these, the ionospheric height and consequently the profile of electron density in the lower d layer are connected primarily to solar radiation. of course many other possible external sources of ionization can also contribute (cosmic rays, whistler induced particle precipitation and other). an anomalous enhancement of vlf waves amplitude in l’aquila was reported in meloni et al. (2001) for september and october 1997. since at that time no other ionospheric effects were detected at the ionospheric station of rome (only about 100 km away) an external cause for the anomaly that should operate at a larger scale was reasonably excluded. the anomaly consisting in a significant increase in signal amplitude and daily range of the order of a factor three above the preceding quiet level, was instead observed in correspondence with the umbria-marche sequence ca. 75 km from the l’aquila observatory lasting for about two months. for this reason, meloni et al. (2001) considered an influence from the umbria-marche earthquakes possible. a qualitative explanation of such effect may be related to rock creeping that takes place in the earthquake focal area and the intensive strain rate in the surrounding crust, that can give a mechanical background for the generation of electromagnetic anomalous fields. as concerns vlf signals, since more evidence is necessary before final conclusions can be reached, a continuous monitoring was set up to have a better assessment in case other possible earthquake electromagnetic related phenomena in this area were recorded. figure 4 shows the time plots for 20 and 40 khz by their 1 min values, here shown only for part of 2001 as an example. no other sinificant variations comparable to those recorded in the 1997 events (see meloni et al., 2001) were recorded and at the same time no other relevant earthquakes in the area were occurred. in conclusion, seismomagnetic and vlf data gathered in recent years were used to obtain a better knowledge of the natural tectonomagnetic and electromagnetic phenomena observed in this region. data were also analyzed to evaluate if results could be used to search for possible correlations to seismic activity. no evidence of time variations of crustal magnetic and earthquake occurrence was clearly found. significant enhancements in correspondence to earthquakes occurrence were found for vlf waves in the case of the 1997 umbria-marche seismic sequence. data for the subsequent years have not shown that those strong variations are frequently recorded at l’aquila and at the same time no other significant earthquakes were recorded. more data are needed to reach convincing conclusions but continuous monitoring of vlf time variations will be continued for their promising role in the assessment of earthquake electromagnetic related phenomena in central italy. references amato, a., r. azzara, c. chiarabba, g.b. cimini, c. cocco, m. di bona, l. margheriti, s. mazza, f. mele, g. selvaggi, a. basili, e. boschi, f. courboulex, a. deschamps, s. gaffet, g. bittarelli, l. chiaraluce, d. piccinini and m. ripepe (1998): the colfiorito, umbria-marche earthquake sequence in central italy (september-november, 1997): a first look to mainshocks and aftershocks, geophys. res. lett., 25 (15), 2861-2864. anderson, h. and j. jackson (1987): active tectonics of the adriatic region, geophys. j. r. astron. soc., 91, 937-983, boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990 (ing, roma), pp. 644. barr, r., d. llanwyn jones and c.j. rodger (2000): elf and vlf radio waves, j. atmos. solar terr. phys., 62, 1689-1718. chiappini, m., a. meloni, e. boschi, o. faggioni, n. beverini, c. carmisciano and i. marson (2000): shaded relief magnetic anomaly map of italy and surrounding marine areas, ann. geofis., 43 (5), 983-989. console, r., r. di giovanbattista, p. favali, g. mattietti and g. smriglio (1989): sismicità recente e tettonica dell’area val comino-matese, in atti del viii convegno gngts (esa, roma), 3-7. ernst, t., r. teisseyre, a. meloni, p. palangio and m. marchetti (1997): magnetic variation analysis for the june 1993 seismic events in central italy, ann. geofis., xl (2), 539-545. gasparini, p., a. meloni and d. patella (editors) (1997): magnetic, electric and electromagnetic methods in seismology and volcanology, ann. geofis., xl (2), pp. 368. 37 tectonomagnetic and vlf electromagnetic signals in central italy gokhberg, m.b., i.l. gufeld, a.a. rozhnoy, v.f. marenko, v.s. yampolsky and e.a. ponomarev (1989): study of seismic inuence on the ionosphere by super long-wave probing of the earth-ionosphere wave-guide, phys. earth planet. int., 57, 64-67. hayakawa, m. and y. fujinawa (editors) (1994): electromagnetic phenomena related to earthquake prediction (terra scientific publishing co., tokyo), pp. 677. ingv (istituto nazionale di geofisica e vulcanologia) (2003): seismic bulletin (roma). johnston, m. 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(1994): statistical study of elf/vlf emission recorded by a low altitude satellite during seismic events, j. geophys. res., 99 (23), 339-349. parrot, m. (1995): electromagnetic noise due to earthquakes, in handbook of atmospheric electrodynamics, edited by h. volland (crc press, boca raton, fl), vol. 2, 2nd edition, 95-116. parrot, m. and m. johnston (1993): seismo-electromagnetic effects, phys. earth planet. inter., 77, 65-83. parsons, l.w. and z.m. wiatr (1982): rubidium vapour magnetometer, j. sci. instrum., 39, 292-299. saraev, a.k., m.i. pertel, p.a. parfent’ev, v.e. prokof’ev and m.m. kharlamov (1999): experimental study of the electromagnetic field from a vlf radio set for the purposes of monitoring seismic activity in the north caucasus, phys. solid earth, 35 (2), 101-108. ward, s. and g. valensise (1989): fault parameters and slip distribution of the 1915 avezzano, italy, earthquake derived from geodetic observations, bull. seismol. soc. am., 79, 690-710. layout 6 the flank eruption history of etna (1610-2006) as a constraint on lava flow hazard cristina proietti*, emanuela de beni, mauro coltelli, stefano branca istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5333 abstract data of the flank eruptions of etna from over the last 400 years were extracted from the new geological map for the lava flow extensions and vent positions, and from the catalogs of historical eruptions for the eruption durations and lava volumes. the partially or widely hidden lava fields on the new geological map were retrieved from older geological maps. the distributions of the eruption durations and lava volumes were analyzed, with the definition of six eruptive classes for use in numerical simulations. the threshold values for the eruption durations and lava volumes were set at 45 days and at 35 × 106 m3 and 100 × 106 m3, respectively. a global analysis was performed on the whole volcano to evaluate the recurrence of the classes, and to estimate for each class the ranges, means and standard deviations of the durations, volumes and elevations of the main vent. the same analysis was repeated by subdividing the volcano into three sectors, which were defined on the basis of the distribution of the eruptive fissures over the last 15 ka. the classes have different recurrences across these various sectors, and different distributions of volumes, durations and elevations of the main vent. finally, a lava flow resurfacing map that counts the number of lava flows on each given area of the volcano over the last 400 years was compiled and then normalized. 1. introduction the volcanic threat is the combination of the hazards (the dangerous or destructive natural phenomena produced by a volcano) and the exposure (the people and property at risk from the volcanic phenomena). volcanic hazard is the probability that given areas will be affected by potentially destructive volcanic processes [fournier 1979]. evaluation of volcanic hazard is generally based on the past eruptive behavior, on the assumption that the previous activity is representative of that in the future [guest and murray 1979, behncke et al. 2005]. hazard assessment can be supported by the use of computer simulation codes, which are becoming valuable tools for better understanding and forecasting of lava flow emplacement [vicari et al. 2006]. mount etna is a basaltic composite stratovolcano that formed along the ionian coast of eastern sicily as a consequence of the complex eruptive history over the last 500 ka [branca et al. 2008]. as etna volcano is located in a densely inhabited area, assessing the hazard posed by its eruptions is particularly important, and this can be based on the long record of its activity. the key information for analyzing its flank activity comes from recent revisions of the catalog of its eruptive history [branca and del carlo 2004, 2005] that provide the durations and volumes of the recorded eruptions. moreover, geometrical and geographical information on the lava flow fields can be found in eight different datasets: three geological maps: the new geological map (ngm) of etna volcano, which was updated to 2007 [branca et al. 2011], and the previous geological maps compiled by waltershausen [1845-59] and romano et al. [cnr, 1979]; orthophotographs extracted from aerial surveys in 1994, 2001 [coltelli et al. 2007], 2004 [baldi et al. 2006] and 2005 [gwinner et al. 2006]; lava flow maps reported in azzaro and neri [1992]. eruptions of etna consist of quasi-continuous activity at summit craters, and quite frequent events along fissures on its flanks [branca and del carlo 2005]. strombolian activity and periodic lava fountaining episodes, which are often associated with lava flows, frequently occur at the summit craters [guest 1982, coltelli et al. 1998, coltelli et al. 2000, calvari et al. 2002, alparone et al. 2003]. in contrast, the flank eruptions take place at intervals of years, and they produce lava effusion that is commonly associated with weak explosive activity [branca and del carlo 2005]. about three millennia of etna eruptions are documented in historical sources, which represent a unique record for an active volcano [tanguy 1981, branca and del carlo 2004]. however, the catalog of flank eruptions is complete and accurate only after the mid-17th century article history received november 5, 2010; accepted june 30, 2011. subject classification: etna, effusive flank eruptions, last 400 y geodatabase, volcanic risk. 480 special issue: v3-lava project [branca and del carlo 2005], although a few events (e.g. 1702, 1755) are poorly documented, so that the lava flows and eruptive systems that occurred are today irretrievably lost for a valuable study [tanguy et al. 2007]. the catalog of summit activity is reliable only since the late 19th century [salvi et al. 2006], although the summit activity poses relatively minor problems in terms of hazard to inhabited areas when compared to the flank eruptions [duncan et al. 1981, wadge et al. 1994, behncke et al. 2005]. furthermore, the high frequency of summit eruptions implies that most of their products are soon covered by successive volcanics, thus making it difficult to identify and delimit each flow field. fifty-one summit effusive eruptions over the last 400 years have been retrieved from the historical catalogs [branca and del carlo 2004, 2005], although only 30 lava flow fields can be identified on the three geological maps, and no more than four of these are totally outcropping. as summit eruptions are generally not dangerous for inhabited areas, the evaluation of the hazard at etna can be limited to flank eruptions, and can be based on the running of several numerical simulations of the lava flow paths, provided that the past eruptive behavior has been analyzed. this study was devoted to the collection and analysis of all of the available data regarding flank eruptions of etna that have occurred over the last 400 years (1610-2006). it has allowed the characterization of the distributions of duration and volume, as well as the spatial distributions of these past flank eruptions. these analyses are necessary to define the input of the numerical simulations to be used to compile an etna hazard map. 2. etna flank eruptions over the past 400 years this study of the eruptive activity that occurred along the etna flanks from 1610 to 2006 (figure 1) was firstly based on an analysis of the historical catalogs [romano and sturiale 1982, branca and del carlo 2004, andronico and lodato 2005, behncke et al. 2005, branca and del carlo 2005, tanguy et al. 2007]. these catalogs furnished information on the eruption durations and on the lava volumes of 67 effusive events, with the volumes of the 2001 and 2006 lava flows retrieved from two studies in the literature [coltelli et al. 2007, behncke et al. 2009]. then, a geodatabase, i.e. a database designed to store, query, and manipulate geographic information and spatial data, was implemented with the arcgis software (www.esri.com), to store all of the data available for an analysis of the geometry and the location of proietti et al. 481 figure 1. etna flank lava flows over the last 400 years mapped from the ngm (orange) and etna volcanics (beige) inset: location of mount etna (red dot) with respect to sicily and italy. 482 the lava flows during the 67 eruptions (an example of a geodatabase implemented for monitoring etna eruptions can be found in de beni and proietti [2010]). in particular, the two geological maps of waltershausen [1845-59] and romano [cnr, 1979], as well as the maps reported in azzaro and neri [1992], were digitized and compared with the ngm, to reduce distortions that might arise from the map georeferencing and orthorectifying. three layers, as polygonal, punctual and linear, were also created inside the geodatabase, for collecting the lava flow fields, their main vents, and the eruptive fissures, respectively. the analysis of the ngm allowed the identification of the limits and locations of the lava flows during the 67 selected eruptions. nevertheless, it should be noted that a single eruptive event can produce several lava flows that can be fed by different vents along the same fissure, e.g. the 20022003 eruption on the north-east flank of etna [andronico et al. 2005]. in such cases, only the main flows, which are those having the longest duration and that covered the greatest distances from the vent and/or widest areas, were generally considered. minor lava flows were excluded because they pose little concern for risk reduction purposes, with respect to the main lava flow, and thus they are not specifically relevant when evaluating the spatial distribution of the lava flow fields. in a few cases, the vents that fed lava flows that covered small areas were included, if they threatened inhabited areas and infrastructure, as during the 2001 eruption [scifoni et al. 2010]. instead, multiple lava flows were considered when they were on different flanks of the volcano, e.g. during the 1879, 2001 and 2002-03 eruptions [branca and del carlo 2004, coltelli et al. 2007, andronico et al. 2005]. the polygons that define the geometry of the selected lava flow fields were collected in the geodatabase, and the covered area was measured for each polygon and associated to the corresponding record, together with its outcropping area. this parameter qualitatively classifies the present outcrop with respect to the area that each flow field would have originally covered. indeed, older lava flows might have been covered by more recent eruptions, and thus their limits appear incomplete or they are missing on the ngm although they have a greater outcrop in previous maps. the flow fields can therefore be distinguished as: totally or almost totally outcropping, and partially, widely or totally hidden. auxiliary information (e.g. eruption year and flow name, eruption style, duration and area, geological map from which the limits were traced, outcropping area, volume, outcropping fracture length, main vent location) is also stored in the attribute table of the polygonal layer of the geodatabase that contains the main lava flows. different layers of the geodatabase were also defined for collecting the main vents associated to each lava flow, as well as the eruptive fissures. the attributes associated to the last layer are the fissure length and considerations on their present outcrop, likewise for the lava flow fields. a single vent (called the main vent) was associated to the main lava flows identified on the ngm, and its cartographic coordinates and elevation were measured. the main vent was located along the eruptive fissure, generally where most of the lava was emitted, or at the fissure center, when the lava was uniformly distributed along it. the spatial distribution of the vent relative to the etna flank eruptions over the last 400 years was necessary for computing the etna hazard map, as reported in cappello et al. [2011]. 3. data analysis as the evaluation of volcanic hazard is generally based on the past eruptive behavior, the data available for these 67 etna flank eruptions over the last 400 years were analyzed. in particular, the distributions of durations and volumes, and the positions of the main vent were investigated, which allowed the definition of a set of eruptive classes that can be used to carry out a number of simulations for the implementing of the etna hazard map [cappello et al. 2011]. to increase the information on the spatial distributions of the lava flows as much as possible, all of the available maps were analyzed together with the ngm. in particular, the waltershausen [1845-59] and romano [cnr, 1979] geological maps were used to retrieve the lava fields during the 19th century and the 19th and 20th centuries, respectively. the maps reported in azzaro and neri [1992] show the lavaflow fields between 1971 and 1991. finally, the orthophotographs allowed the complete mapping of the 1991-1993 and 2004-2005 lava flows in the valle del bove, as well as the 2001 lava flows. ten effusive events among the 67 were excluded because their records are not complete, for different reasons. in particular, five lava fields cannot be located: four (1643, 1702, 1755 and 1918) do not outcrop in any of the maps, and one (1975) cannot be distinguished from another flow field (197577) that occurred soon after in the same area. moreover, three events (1956, 1964 and 1975-77) have unknown volumes, and two (1682 and 1689) have unknown durations and volumes. nine eruptions were also excluded because they represent exceptional events, due to their extraordinarily brief (1869, 1908 and 1942) or long (1651-54 and 1614-24) duration, as well as their extremely low (1883, 1968, and december 1985) or high (1651-54 and 1669) volumes. duncan et al. [1981] have already indicated that the 1669 eruption was a feature of the activity at the peak over the last 400 years and that its inclusion in the calculations might distort any general predictive model based solely on more normal flank activity. forty-eight flank eruptions were therefore available for the analysis. to analyze the spatial distributions of these 48 lava fields, the geometry of those that are partially, widely or totally hidden on the ngm was updated. in particular, their etna history constrains lava flow hazard limits were modified on the basis of the additional data sources, starting from what was still outcropping on the ngm. for example, the 1811 and 1819 lava flow fields (figure 2) in the valle del bove were buried by several eruptions, and so their original geometries were retrieved from the waltershausen [1845-59] map. their outcropping areas measured on the ngm are about 0.26 × 106 m2 and 0.13×106 m2, and respectively corresponds to 5.5% and 2.2% of the values measured in the waltershausen [1845-59] map (4.77×106 m2 and 5.98×106 m2, respectively). after analysis of all the available data sources, the parameters defining the flow outcrop were updated (figure 3), showing that two hidden flows were retrieved, and that the definition of the planar expansion of the lava flows was improved. indeed, the percentage of partially, widely or totally hidden flows decreased from 52% to 17%. the distributions of the durations and volumes were analyzed through two histograms (figure 4). the duration histogram (figure 4a) shows that 26 of the events (~54% of the whole dataset) were shorter than 45 days, while the remaining records are quite scattered. the volume proietti et al. 483 figure 2. example of delimitation on the waltershausen [1845-59] geological map of lava flow fields that are widely hidden on the ngm. the limits of the 1811 and 1819 flow fields defined on the waltershausen [1845-59] map are drawn as blue and red lines, respectively; the red and blue filled areas represent their portions outcropping on the ngm. green lines and filled area define the 1811 scoria cone on the waltershausen [1845-59] map and on the ngm, respectively. figure 3. classification of the lava flow fields over the last 400 year as: totally or almost totally outcropping (to or ato), and partially, widely or totally hidden (ph, wh or th). green bars refer to the flows defined only on the ngm, while gray bars refer to all of the datasets. the percentages associated with each group are given above the corresponding bar. 484 distribution (figure 4b) shows that 23 of the events (~48%) had a volume (v) lower than 35 × 106 m3. a second group can be identified by events with 35 × 106 m3 <v < 100 × 106 m3, and contains 17 records (~35%), with the remaining eight records quite scattered. by combining these considerations of the distributions of the durations and volumes, six eruptive classes (figure 5a) can be defined: 1) brief eruptions that emitted low lava volumes (durations d <45 days, volume v <35×106 m3); 2) brief eruptions that emitted medium lava volumes (d <45 days, 35×106 m3 <v <100×106 m3); 3) brief eruptions that emitted high lava volumes (d< <45 days, v >100×106 m3); 4) long eruptions that emitted low lava volumes (d >45 days, v <100×106 m3); 5) long eruptions that emitted medium lava volumes (d >45 days, 35×106 m3 <v <100×106 m3); 6) long eruptions that emitted high lava volumes (d > >45 days, v >100×106 m3). etna history constrains lava flow hazard figure 4. distributions of the durations (a) and volumes (b) of the 48 selected flank eruptions over the last 400 years. the bar widths are 5 days and 5 × 106 m3, respectively. figure 5. a) distribution of durations (d) and volumes (v) of the 48 lava flows, and their subdivision into six classes (class 1: d <45 days, v <35 × 106 m3; class 2: d <45 days, 35 ×106 m3 <v <100 ×106 m3; class 3: d <45 days, v >100×106 m3; class 4: d >45 days, v <35×106 m3; class 5: d >45 days, 35 × 106 m3 <v <100 × 106 m3; class 6: d >45 days, v >100 × 106 m3). the dots of the different classes are colored according to the legend reported in (b). b) pie diagram showing the percentages of the eruptive events belonging to each class. the recurrence of each of the six classes can be seen in the pie diagram in figure 5b, which shows that class 1 is the most recurrent (39.6%), followed by class 5 (20.8%), class 6 (16.7%) and class 2 (14.6%); class 4 contained the lowest number of events (8.3%), although finally, class 3 is empty. the ranges, means and standard deviations of the durations, volumes and elevations of the main vent were also evaluated, for the whole volcano and for the events according to each class (table 1). a same analysis was performed by dividing the volcano into three radial sectors that are centered on the summit craters (figure 6a). the zero and minima of the distribution of fissure orientation, relative to the last 15 ka [azzaro et al., in press], were considered as delimiting angles of each sector (355˚-115˚ for sector 1; 115˚-225˚ for sector 2; 225˚-355˚ for sector 3), according to the duncan et al. [1981] analysis. these authors observed that the sectorial distribution of all of the adventive cones over the last few thousand years is very similar to that of the flank eruptions between 1537 and 1974, thus strongly implying that the distribution remains proietti et al. 485 class number of events d range (days) dmean (days) �vd (days) v range (106 m3) vmean (106 m3) �vv (106 m3) h range (m) hmean (m) vh (m) all classes 48 3-555 101 142 2-231 55.8 54.3 1207-2900 2112 393 d <45 days v <35 × 106 m3 19 3-32 15 8 2-30 12.2 9.2 1207-2826 2035 436 d <45 days v = 35-100 × 106 m3 7 11-31 19 8 46-78 61.5 10.4 1637-2471 2027 319 d <45 days v >100 × 106 m3 0 ------------------d >45 days v <35 × 106 m3 4 60-125 87 29 29-34 31.1 2.0 1921-2772 2326 396 d >45 days v = 35-100 × 106 m3 10 69-380 146 92 40-100 64.5 24.2 1724-2900 2299 386 d >45 days v >100 × 106 m3 8 58-555 331 184 117-231 155.8 44.1 1509-2377 2019 290 1 2 3 4 5 6 table 1. number of events, ranges, means and standard deviations (v) of the durations (d), volumes (v) and elevations of the main vent (h) for each class. figure 6. a) subdivisions of the etna volcano edifice into three sectors, and classification of the main vents on the basis of the six eruptive classes. b) pie diagram showing the percentages of the vents located in each sector. 486 fairly constant over a time scale of hundreds to thousands of years. the sectorial analysis presented here shows that most of the flank eruptions over the last 400 years occurred in sectors 2 and 1 (figure 6b), which include the south and north-east rifts, respectively. the frequency of the six classes on the three sectors was analyzed and is shown in figure 7, which allows the identification of the most recurrent class, which was different for the various sectors. class 1, i.e. that containing brief eruptions that emitted low lava volumes, was confirmed as having the highest number of events in sectors 1 and 3, where it represents 50.0% and 55.6% of the events, respectively (figure 7b, d). class 5, i.e. long eruptions that emitted medium lava volume, was the most recurrent (33.4%) in sector 2 (figure 7c). it should also be noted that in sector 2, which coincides with the densest inhabited slope of mount etna, class 6 events (long eruptions that emitted large lava volumes) have a recurrence of 23.8%, as does class 1. the ranges, means and standard deviations of the durations, volumes, and main vent elevations were then evaluated for each sector, for all of the events, and for the most recurrent classes (table 2). this analysis shows that class 1 has different ranges and means in sectors 1 and 3. the same behavior can be seen when looking at the results of all of the events in each sector. 4. resurfacing map of the flank lava flows a resurfacing map was also constructed that shows how many times each point of the surface of etna (in practice each pixel of the dem adopted for the simulations) has been covered by a lava flow field over the last 400 years. to retrieve as much information as possible, the smaller lava flows that occurred during events that produced multiple flows and that were excluded when defining the vent positions were now taken into account, giving a total of 97 flow field limits. the resurfacing map (figure 8a) shows that the more frequently covered areas are the valle del bove, as well as the south flank, and that the north-east rifts were relatively active. then the map was normalized by dividing etna history constrains lava flow hazard figure 7. a) distributions of the six classes in the three sectors into which etna was divided. the legend refers to all of the panels. (b-d) pie diagrams showing the distributions of the six classes, as: sector 1 (b), sector 2 (c) and sector 3 (d). it by 97, the total number of flow fields identified. the normalized resurfacing map (figure 8b) counts how many times a pixel was covered with respect to the total number of lava flows, and it represents the only quantitative information available for the description of the spatiotemporal evolution of these etna flank eruptions. it should therefore be considered in the validation processes of lava flow hazard maps that are implemented during the lava project. 5. discussion and conclusions this study analyses the etna flank eruptions over the last 400 years on the basis of data collected from historical catalogs and geological maps. a geodatabase was implemented for the collection of all of the information available for each eruption; e.g. the georeferenced flow limits (geographic records) and the geological maps on which they were drawn, the definition of the present flow outcrop, the flow areas and the information collected from historical proietti et al. 487 all --18 3-372 62 94 2-158 47.1 47.0 1207-2826 2082 380 1 d <45 days v <35 × 106 m3 9 3-18 12 5 2-29.4 13.2 8.8 1207-2826 2065 457 all --21 10-555 145 156 4-231 72.6 62.6 1509-2900 2258 379 1 d <45 days v <35 × 106 m3 5 10-23 18 7 4-30 13.4 11.7 1829-2754 2289 368 5 d >45 days v = 35-100 × 106 m3 7 69-380 144 111 40-100 64.3 27.4 1908-2900 2420 369 6 d >45 days v >100 × 106 m3 5 173-555 335 171 121-231 164 50.0 1509-2293 1949 296 all --9 3-540 79 174 2-117 33.9 37.4 1390-2900 1950 519 1 d <45 days v <35 × 106 m3 5 3-328 15.2 11.5 2-19 9.4 8.7 1390-2243 1727 314 sector class number of events d range (days) d mean (days) �vd (days) v range (106 m3) v mean (106 m3) �vv (106 m3) h range (m) h mean (m) �vh (m) 1 2 3 table 2. number of events, ranges, means and standard deviations (v) of durations (d), volumes (v) and elevations of the main vent (h) for all the events and the most recurrent classes in each sector. figure 8. a) resurfacing map of all of the minor and main lateral lava flows that occurred between 1610 and 2006. b) normalized resurfacing map. 488 catalogs; i.e. eruption durations and lava volumes. the connection of the information derived from different data sources has facilitated the carrying out of the main tasks of this study; namely, the definition of the eruptive classes and the evaluation of the etna resurfacing map. a classification based on the durations-volumes of the main lava flows over the last 400 years is presented, with 45 days and 35× 106 m3 and 100× 106 m3 set as the thresholds of the durations and volumes, respectively. nevertheless, the analysis shows that five classes are sufficient to include all of the eruptions considered because class 3 (short durations and high volume eruptions) is empty, while class 4 (long durations and low volume eruptions) is poorly represented. to explain the results obtained for classes 3 and 4, it should be taken into account that two effects control the rates and durations of magma discharges: the balance between the driving and the lithostatic pressures, and the thermal evolution of the magma and the host rock. as an eruption continues, the driving pressure gradually decreases, and it can reduce the flow rate [bruce and huppert 1990]. the effect of the driving pressure, and in particular of the de-pressurization of the magma source, is mainly due to relaxation of the reservoir (elastic contraction of the magma body), which was first studied by wadge [1981]. he analyzed effusion rate trends for various volcanoes, among which there was mount etna, and he identified a two-phase trend: "waxing", as characterized by a rapidly increasing flow rate, followed by "waning", a longer period in which the flow rate decreases exponentially. he also observed that the maximum effusion rate is directly proportional to the magma overpressure. when looking at the effusive events analyzed here, a long waning phase might have been responsible for high volume eruptions (v >100×106 m3), and indeed, these generally had durations longer than 170 days, which corresponds to mean effusion rates lower than 6 m3/s. this quite low value suggests that most of the eruption could have been characterized by stable and low discharge that was related to a long equilibrium phase between the internal pressure of the dike and the lithostatic pressure which, together with the lava cooling, tend to close the dike. otherwise, the early end of brief eruptions (<45 days) was probably due to internal dike pressure that was not sufficient to keep the dike open. moreover, an eruption shorter than 45 days would have a lava volume higher than 100 × 106 m3 only if its mean effusion rate was higher than 26 m3/s. such a high mean effusion rate (total volume divided by total duration) was observed only for eruptions with durations of less than 31 days, which are thus characterized by relatively short waning phases. finally, as the peak effusion rate is proportional to the initial driving pressure of the reservoir [wadge, 1981], and as the effusion rate decreases rapidly during the waning phase, a lava volume as high as 100×106 m3 cannot be emitted during brief eruptions (d <45 days). for these reasons, over the last 400 years etna did not produce eruptive events that belong to class 3 (short durations and high volumes eruptions). class 4 (long durations and low volume eruptions) is poorly represented for a similar reason: to have a long duration, the internal lava pressure should be high enough to keep the dike open, and this more commonly resulted in medium-high lava volumes, and thus to eruptive events that belong to classes 5 or 6. after defining the eruptive classes, two different analyses were performed: a global analysis in the whole volcano, and a sectorial one that considered three radial sectors centered on the summit craters. the recurrence of each class was analyzed through pie diagrams (figure 7), which show that for the whole volcano, most of the eruptions considered (39.6%) belong to class 1 (short durations and low volumes). this was also confirmed when looking at the sectorial analysis in sectors 1 and 3. sector 2, coinciding with the southern etna flank, mainly contains (33.4%) class 5 events (long durations and medium volumes). this sector also has a higher frequency (23.8%) of class 6 events (long durations and high volumes) with respect to the whole volcano (16.7%). moreover, class 6 has the same frequency as class 1. taking into account that sector 2 coincides with the most inhabited area of the volcano, the corresponding hazard for this sector should be more carefully assessed. the results obtained also suggest that the south-east rift is the area that has most frequently been affected by magma intrusions since 1610. the ranges, means and standard deviations of the volumes, durations and elevations of the main vent were then evaluated for each class, for the whole volcano (table 1), as well as for the most recurrent classes, on the three sectors (table 2). the means can be considered as the representative values to be assigned to each class, while the standard deviations show the data dispersions. by assuming that the future behavior of etna will be the same as in the recent past, this analysis furnishes the input parameters (duration and volume) to be used in simulations for compiling an etna hazard map. different representative values should be considered in the three sectors, because each class has different ranges, means and standard deviations of the volumes, durations and elevations of the main vent. finally, the resurfacing maps (figure 8) allow the evaluation that the areas that have been mostly invaded by lava flows are the valle del bove and the south flank. as a probabilistic representation of etna flank eruptions, the normalized resurfacing map can be compared with the hazard maps produced during the lava project, as support in their 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(1845-59). atlas des aetna, berlin, göttingen, weimar. *corresponding author: cristina proietti, istituto nazionale di geofisica e vulcanologia, sezione di catania, catania, italy; email: cristina.proietti@ct.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. etna history constrains lava flow hazard layout 6 usgs “did you feel it?” internet-based macroseismic intensity maps david j. wald1,*, vincent quitoriano1, bruce worden2, margaret hopper1, james w. dewey1 1 u.s. geological survey, national earthquake information center, golden, colorado, usa 2 synergetics, inc., fort collins, colorado, usa annals of geophysics, 54, 6, 2011; doi: 10.4401/ag-5354 abstract the u.s. geological survey (usgs) “did you feel it?” (dyfi) system is an automated approach for rapidly collecting macroseismic intensity data from internet users’ shaking and damage reports and generating intensity maps immediately following earthquakes; it has been operating for over a decade (1999-2011). dyfi-based intensity maps made rapidly available through the dyfi system fundamentally depart from more traditional maps made available in the past. the maps are made more quickly, provide more complete coverage and higher resolution, provide for citizen input and interaction, and allow data collection at rates and quantities never before considered. these aspects of internet data collection, in turn, allow for data analyses, graphics, and ways to communicate with the public, opportunities not possible with traditional data-collection approaches. yet web-based contributions also pose considerable challenges, as discussed herein. after a decade of operational experience with the dyfi system and users, we document refinements to the processing and algorithmic procedures since dyfi was first conceived. we also describe a number of automatic post-processing tools, operations, applications, and research directions, all of which utilize the extensive dyfi intensity datasets now gathered in near-real time. dyfi can be found online at the website http://earthquake.usgs.gov/dyfi/. introduction over the past decade, the u.s. geological survey’s “did you feel it?®” (dyfi) system has automatically collected shaking and damage reports from internet users immediately following earthquakes. dyfi is now a rapid and vast source of macroseismic data, providing quantitative and qualitative information about shaking intensities for earthquakes in the usa and around the globe. our systematic collection of citizen-provided data preceded the use of the formal concept of ‘crowdsourcing’ by more than a decade. dyfi has become vital for automatically collecting macroseismic data for all felt seismic events in the united states; it is also one of the most popular, interactive websites within the federal government. for earthquakes occurring outside the usa, the worldwide dyfi data rapidly indicate or confirm earthquake occurrence for seismic analysts and scientists at the usgs national earthquake information center, giving a quick indication of the extent and nature of shaking effects. the global intensity data from dyfi, and in some cases its international counterparts, can be automatically used as constraints in our global shakemap system (gsm) (figure 1) [wald et al. 2006a], which is the hazard input for the usgs prompt assessment of global earthquakes for response (pager) system [wald et al. 2006a]. we first provide background on the dyfi system, and then discuss how internet-based data collection has changed the approach, coverage, and usefulness of intensity observations. we then promote the advantages and note the limitations of online intensity data collection. dyfi is fundamentally a citizen-based science endeavor, and this affords opportunities to both educate and analyze societal response and earthquake awareness. we then discuss how user feedback and scientific considerations have led to a number of important, iterative, and continuing improvements to the dyfi system, and we document these changes. finally, we describe recently developed tools and analyses of the dyfi data that yield other benefits from online macroseismic intensity-data collection. intensity-data gathering in the internet age the community internet intensity map (ciim, more commonly known as “did you feel it?”) system went online informally in california in 1997, and was first fully described by wald et al. [1999a]. in the subsequent years, significant refinements to the system and its use have ensued. dyfi expanded to the entire usa in 2000, and global macroseismic data collection via dyfi began in 2004. the ciim system was an adaptation of a system to process questionnaires collected by telephone interviews, developed by dengler and moley [1994] and dengler and dewey [1998]. in an effort to process larger quantities of article history received august 5, 2011; accepted september 12, 2011. subject classification: seismology, macroseismology, ground motions, internet seismology. 688 citizen empowered seismology/special section edited by r. bossu and p.s. earle questionnaires, these researchers recognized the need to assign intensities in a quantitative manner, eliminating the need for exhaustive and subjective assignments of individual intensity reports. they did this by assigning numerical values to answers of individual questions based on the modified mercalli intensity (mmi) questionnaire, and then relating the cumulative sum of the numerical values (weighted differently within varying shaking indicator categories) to independently assigned mmi values. for a specified areal extent (normally, a postal zip code), the intensity assignment was done by averaging the numerical values associated with the answers to each question on the questionnaire and weighting different indicated categories to determine a “community weighted sum” (cws) [dengler and dewey 1998]. the cws value is related to traditional mmi numerically through linear regression and is assigned a community decimal intensity (cdi). once dengler and co-workers had related cws values to traditional usgs mmi [stover and coffman 1993], intensities could be assigned objectively and numerically from their telephone-based macroseismic questionnaires. dengler and dewey [1998] referred to the resulting intensities as community decimal intensities (cdis), to distinguish them from traditional mmis. following closely on their work, automating the data collection and processing system to take advantage of the growing popularity of the internet was the natural course of evolution. wald et al. [1999a] refined dengler’s approach by regressing additional questionnaire and historical macroseismic intensity data, expanding the range of applicable intensities to lower and higher values. they also began to automatically and rapidly compute, map, and update the cdi values in what they referred to as a community internet intensity map (ciim). subsequent analyses confirmed the consistency and compatibility of cdi and mmi values (discussed later), and over time ciim gave way to the more popular reference “dyfi”. the authors initially preferred the use of “ciim” over “dyfi”, given that data would be collected not just for felt events but also for earthquake disasters, but the dyfi nomenclature appears to be more successful at attracting contributions from non-seismologists. of particular note, the dengler and dewey [1998] strategy, and the wald et al. [1999a] internet-based implementation of it, resulted in decimal rather than ordinal values of assigned intensities. the authors find significant advantages in the use of decimal intensities, though we acknowledge that real-valued representation of intensities also has potential for misuse [e.g., richter 1958, musson and cecić 2002]. we do not know of another group that uses a similar decimal intensity strategy to wald et al. [1999a], and the alternative approaches discussed below for numerically assigning intensities at a variety of institutions all use logicbased strategies and integer intensity assigments rather than numerically regressed intensity values. this distinction will become evident in subsequent analyses. national and international systems the evolution from manual/postal, to emailed, to web-based macroseismic surveys has been accomplished in many regions of the world. many countries maintain the manual approach as either the primary strategy, or reserve the option to augment their web-based approaches with traditional assignments. several very successful internet-based macroseismic survey systems are now implemented in several countries and regions, and a non-comprehensive survey of a few of these systems follows for comparitive purposes. in new zealand, coppola et al. [2010] describe the online system at gns science (http://www.geonet.org.nz) available since 2004. the gns web interface, part of the geonet project, is an interactive and zoomable interface showing both instrumental and macroseismic assignments. gns uses what coppola et al. [2010] describe as a logic-based pyramid strategy to assign intensities, with subsequently higher intensities requiring more ‘positive’ answers than lower values. intensities are integers, employing new zealand’s customization of the mmi scale [dowrick 1996]. the gns automatic system only allows intensity assignments up to intensity viii, although dowrick’s version of the mmi scale goes up to xii. gns allows web users to retrieve felt reports via interactive queries based on event or date and time. sbarra et al. [2010] summarize the istituto nazionale di geofisica e vulcanologia (ingv) web-based macroseismic collection system for italy (“did you feel the quake?”/“hai sentito il terremoto”: http://www.haisentitoilterremoto.it), which has been online since 2007. their 2010 analyses of the web-contributed data in comparison to their traditional macroseismic surveys indicated the reliability of the former strategy. individual entries are assigned the most probable macroseismic degree (an integer) by statistical analyses of the likelihood of the collection of responses being associated with any intensity. for any entry, they also use the variance associated to the weighted mean intensity in their database to cull entries that appear inconsistent [sbarra et al. 2010]. in addition, as done with dyfi, entries are removed if their value is outside preset bounds about a selected intensity prediction equation as a function of magnitude and distance; sbarra et al. remove entries more than three intensity units above or below the prediction. assigned intensities are averaged for each town or village, unlike the usgs system, for which the community decimal intensity (cdi) results from the average response to each questionnaire question (community weighted sum, see above, and electronic appendices iii and iv). the ingv system supports assignments of both mercalli-cancani-sieberg (mcs) [seiberg 1930] scale and european macroseismic scale (ems) [grünthal 1998] intensities. as an effort to try to avoid wald et al. 689 690 sampling bias, ingv is also building a large group of spatially distributed volunteers who are alerted to the occurrence of an earthquake; their group is rapidly growing given recent significant earthquakes in italy. the european-mediterranean seismological centre (emsc) has embarked on a project to cover its region with an online questionnaire (http://www.emsc.eu/earthquake/ felt.php). the system has been operating since 2004, but has not settled on a permanent solution for automatic intensity assignments (r. bossu, personal communication, 2010); their logic-based algorithm, written by roger musson at the british geological survey (bgs), is being analyzed and modified by sebastian gilles at emsc. the emsc is multilingual, translated into 32 languages. currently, it provides integer ems assignments for cities with a minimum of five responses [mazet-roux and bossu 2010]. additional systems are in place at the british geological survey, le bureau central sismologique français, natural resources of canada, the swiss seismological service, and the royal observatory of belgium, among several others. these systems employ a variety of approaches, but both the canadian and belgian approaches employ the same questionnaire as dyfi (wald et al. [1999a]; appendices i-iv, this report). as can be inferred from the above descriptions of various national systems, there is very little uniformity among different groups’ choices of the macroseismic scales, the nature of the questionnaires, and in the approach for assignments of intensities from these forms. (for a good comparison of macroseismic scales, see the review by musson et al. [2010], and its supplemental materials.) recognizing the importance of uniform data collection, the esc working group for internet macroseismology was established at the 2008 general assembly of the esc in heronissos, crete. the working group was charged with developing common methods for collecting and disseminating macroseismic data using online methods [musson 2010]. a primary goal of that group, to which the present authors contribute, is to determine a suite of internationally agreeable questionnaires and an exchange usgs “did you feel it?” figure 1. felt area and distribution of dyfi reported intensities about two days after the february 27, 2008, m 8.8 maule, chile, earthquake. the red polygon indicates the surface projection of the approximate rupture area used in the usgs shakemap. intensities do not exceed viii probably because they are areal averages over the extent of each city rather than over postal codes or districts within cities, and because the subduction rupture surface is over 20 km distant at its closest proximity to coastal cities. the inset shows a smaller-scale map, showing more observations from low-intensity regions. ! 75˚w 70˚w 65˚w 40˚s 40˚s 38˚s 38˚s 36˚s 36˚s 34˚s 34˚s 32˚s 32˚s offshore maule, chile usgs community internet intensity map feb 27 2010 03:34:14 local 35.8464s 72.7189w m8.8 depth: 35 km id:us2010tfan processed: mon mar 1 17:16:41 2010 santiago cordoba mendoza san juan neuquen vina del mar talcahuano temuco rancagua talca rio cuarto los angeles o valdivia curico san rafael mercedes santa rosa coronel linares general pico rio terce angol villarica parral victoria santa cruz la ligua illapel 1071 responses in 106 cities (max cdi = viii) city size 2m+ 100,000+ 10,000+ <10,000 75˚w 70˚w 65˚w 40˚s 40˚s 38˚s 38˚s 36˚s 36˚s 34˚s 34˚s 32˚s 32˚s 100 km damage shaking intensity none none none very light light moderate moderate/heavy heavy v. heavy not felt weak light moderate strong very strong severe violent extreme i ii-iii iv v vi vii viii ix x+ buenos aires sao santiago curitiba cordoba porto alegre rosario santa cruz tucuman mar del plata uberlan salta florianop talcahuano temuco iquique catamarca puerto montt luque comodoro rivadavia tarija curico trelew rio gallegos viedma ilo azul concepcion castro jardim miranda chuy format for distribution. it is anticipated that processing of the responses can still be left to different strategies unless a global macroseismic scale (gms) is established. advantages of online macroseismic data collection traditionally, intensities are assigned by a classification process and are assigned integer values (or, less frequently, a range of integer values such as “6-7” or “>6” for less certain assignments; see grünthal [1998], section 4.5). the ciim process of assigning numerical values to macroseismic observations and then calculating real-valued intensities thus represents a philosophical as well as a procedural departure from traditional intensity-assigning methods. over the years, analyses of the dyfi data strongly suggest that reporting intensities to higher precision is warranted [e.g., atkinson and wald 2007, wald et al. 2006b, worden et al. 2011]. usgs now carries a single decimal place, and the discrete (i to x) intensity scale is replaced with a continuous, real-valued, scale. when the dyfi and similar decimal intensities for widelyobserved earthquakes are plotted as a function of distance, the retention of information to tenths of an intensity unit results in lower scatter than is the case when intensities are assigned by classification and reported to the nearest integer [e.g., dengler and dewey 1998, dewey et al. wald et al. 691 ! !figure 2. comparison of the usgs national seismic hazards map (nshm) [petersen et al. 2008], top, with a decade of dyfi responses (1999-2011), bottom. for each postal zip code, the maximum intensity reported during that time period is shown. dyfi intensity color coding is the standard intensity palette used by usgs for shakemap and dyfi; for nshm the 10% probability of exceedence of peak acceleration in 50 yrs ground motions is scaled approximately to the dyfi color palette. during this period there were over 1.6 million individual responses in over 25,000 zip code areas. 692 2002, wald et al. 2006b]. for map representation of decimal intensities, a continuous palette of colors is chosen using algorithms that automatically interpolate between discrete color values, using a similar technique to the one used by shakemap [wald et al. 1999c]. this allows us to present more subtle variations of intensity than previously achievable. designed to work in conjunction with rapid epicenter and magnitude determinations that are provided by regional and national seismic networks, the dyfi system is now triggered automatically; individuals can respond to and view maps for a particular earthquake within minutes and watch as maps are continuously updated with new data (every 1-2 minutes). within the usa, intensity observations are grouped, averaged, and plotted according to postal codes. postal zip code polygons are color-coded according to their computed intensity values. we can now also automatically geocode the users’ locations if they correctly provide their street addresses (most do), and we can thus provide refined, spatially aggregated intensity assignments as needed. starting in 2004, we implemented the dyfi system for international data collection, with observations grouped, averaged, and plotted by city. outside of the usa, we enable users to select their country and city from pull-down menus. thus, the resolution for automatic intensity assignment outside of the usa is at the level of individual cities, which we color code to the intensity value and map as a circle (figure 1). currently we have approximately 140,000 cities in our database, which was culled from the open-source geonames database [geonames 2011]. as of this writing, we are augmenting the global city-based system with usergeocoded locations. rate of responses data for widely felt earthquakes come in at a rate that allows confirmation of the earthquake’s occurrence within minutes. the dyfi maps evolve rapidly to informative macro-seismic intensity distributions that are useful to the public, media, scientists, and even emergency managers typically within tens of minutes. statistics attest to the abundance and rapid availability of these internet-based macroseismic data: nearly two million entries have been amassed over the decade (figures 1 and 2); there are 33 events each with more than 10,000 responses; 250 events have over 1,000 entries (table 1). the greatest number of responses for an earthquake is more than 77,000 for the april 2010, m 7.2 baja california, mexico, event. table 1 summarizes some of the notable statistics associated with dyfi data collection. outside the usa, dyfi has gathered over 145,000 entries in 7,600 cities covering 192 countries since its global usgs “did you feel it?” within the usa and territories top 3 earthquakes by number of entries 77,758 entries for m 7.2 near baja california (april 4, 2010) 58,362 entries for m 5.0 near ontario-quebec border (june 23, 2010) 47,763 entries for m 4.7 near los angeles, ca (may 18, 2009) number of us postal zip codes with entries 25,909 (out of a total of 41,558) total number of individual entries 1,603,100 largest rate of responses 73/s for m 5.7 southern california (june 15, 2010) 51,624/h for m 5.6 northern california (oct 31, 2007) outside of the usa number of cities with entries 7,603 (out of 145,286) total number of individual entries 140,263 number of countries with entries 192 (out of 234) top 3 cities by number of entries santiago, chile, 6,690; toronto, canada, 5,755; melbourne, australia, 3,250 top 3 countries by number of entries canada, 25,859; chile, 14,839; japan, 10,566 top 3 earthquakes by number of entries (outside n. america) m 5.0 england, u.k., (february 27, 2008), 19,044 m 4.7 australia, (march 6, 2009), 3,794 m 5.5 near spain (december 17, 2009), 3,298. table 1. sample usgs “did you feel it?” statistics (as of july 2011). inception in late 2004 (figure 3). the rapid intensity data are automatically used in the global shakemap system (gsm) [wald et al. 2006a], providing intensity constraints near population centers (figure 1) and in places without strongmotion instrument coverage (most of the world), and allowing for bias correction to the empirical prediction equations employed in shakemap. in practice, we automatically incorporate dyfi intensities into gsm at present for a minimum number of responses per location (currently set at three or more). shakemap has also been recently refined to automatically use macroseismic input data in their native form, and treat uncertainties rigorously in concert with the more standard-use recorded ground-motion data (shakemap version 3.5) [worden et al. 2010]. dyfi contributions to gsm have two important aspects. first, they provide ground truth intensity assignments, predominantly at sites with significant populations (such as cities). second, with 5-10 intensity assignments in the near-source area, these data can allow gsm to compute a bias correction term to be applied to the ground-motion-prediction equations, effectively removing the inter-event variability, or correcting for an incorrect initial magnitude calculation [wald et al. 2006a, worden et al. 2010, worden et al. 2011]. the internet makes it possible to rapidly gather larger, wald et al. 693 ! ! figure 3.comparison of the global seismic hazards map (gshap) [giardini et al. 1999], top, with six years of dyfi responses (late 2004-2011), bottom. for each city or postal zip code, the maximum intensity reported during that time period is shown. dyfi intensity color coding is the standard intensity palette used by usgs for shakemap and dyfi; for gshap the 2% probability of exceedence of peak acceleration in 50 yrs ground motions is scaled approximately to the dyfi color palette. earthquake dates and epicenters are shown for selected significant events. during this period there were 140,000 individual responses in over 7,000 cities outside of the usa. 694 more comprehensive datasets than ever before, and at minimal cost. prior to this system, intensity maps were rarely made for us earthquakes of magnitude less than about 5.5; now intensities are routinely reported for the smallest felt earthquakes nationwide (figure 2). in addition, thousands of reports are available for moderate to large events, often tens of thousands for those in densely populated areas. the greatly expanded datasets allow for post-processing and analysis in ways that were not before possible. to date, over 250 earthquakes having over 1,000 entries present a substantial data resource for portraying shaking distributions and for quantitative analyses. figure 2 shows the intensity of shaking reported over the entire usa for more than a decade (figure 2, bottom panel). as far as we are aware, nearly every felt earthquake is or can be reported now (although separating contributed reports for multiple shocks occurring close together remains problematic). thus, this map represents something we have never been able to show before: the actual distribution of shaking intensity over the entire nation for a decade. this is actually quite an extraordinary contribution of the dyfi system. in comparison with usgs national seismic hazard map (figure 2, top panel), one can analyze the overall consistency in many areas, and identify those that have not experienced events in a decade despite high longer-term probabilities of shaking. in the comparison in figure 2, we have scaled the thirty-year, 2% exceedance probability in terms of peak acceleration to the same range of intensities (colors) as for the mmi scale used for dyfi maps. while these maps should not be compared quantitatively, the qualitative comparison essentially implies that the range of likelihoods of ground motions span the range of intensities. as discussed below, such maps provide significant earthquake awareness and educational opportunities. figure 3 provides the analogous comparison for the global dyfi data over a six-year period (2005-2011) with the global seismic hazard map (gshap) [giardini et al. 1999]. rates and completeness of responses, as well as the total time period, are not as impressive as for the usa, but nonetheless the opportunity for comparison with a long-term hazard maps is a good one, and it will improve with time. the impressive rate of responses and feedback from users prompted us to routinely plot entries contributed as a function of time (figure 4). questionnaire response rates have reached 62,000 per hour (ca. 1,000 per min; figure 4), requiring substantial web resource allocation and capacity (see appendix 1). these plots are provided online for each event, and they show logical patterns of immediate postearthquake surges followed by decays; late-night lulls followed by morning surges. occasionally, aftershocks and media attention result in belated surges. continuous plots of the entry rates allow operators to track system performance and gauge future bandwidth requirements. the data quality and quantity depend primarily on population density and prevalence of internet access, but not necessarily on earthquake awareness or the overall hazard of the region. surprisingly, events in the eastern and western usa have comparable response rates, despite significantly different historical rates of earthquake occurrence in the two regions (e.g., examples in figures 5, 6, 7). usgs “did you feel it?” ! 0 10000 20000 30000 40000 50000 60000 70000 80000 n um be r o f r es po ns es 0 4 8 12 16 20 24 time since earthquake (hours) responses vs. time plot (id ci14607652) processed wed mar 2 04:55:18 2011 figure 4.plot of individual questionnaire responses versus time for the april 2010, m 7.2 baja california, mexico, event. over 62,000 entries were received in the first hour, or roughly 1,000 per minute. similar response plots are generated for every event. the earthquake occurred at 15:40 local time. wald et al. 695 !!!!!!!!!!!!!!!! figure 6. figure 5. 696 accuracy of responses given that dyfi intensities are assigned by responses to questionnaires by the general public, one might expect them to be subjective, or even unreliable. however, the large numbers of responses from most communities make the resulting intensity approximations surprisingly robust. qualitatively, for the most part, dyfi intensity maps show smooth variations of intensity in areas from which there are many observations (e.g., figures 6, 7, 8). dyfi macroseismic intensity maps also generally agree with instrumental intensity maps (shakemaps) that are based solely on seismographically measured peak ground motions, at least in areas where both can be made with sufficient datasampling density (e.g., california). examples of earthquakes with both human-and instrumentally generated intensity maps abound on the usgs shakemap and dyfi websites. there is a wide variety of potential sampling biases associated with any macroseismic survey. with the dyfi system, we know from comparison with the traditional usgs mmi, that at the low end of the scale (i-iii), internetbased questionnaires actually reach a greater potential sample than postal surveys made in parallel, and thus felt reports can be more widely distributed than traditionally sampled mmi [dewey et al. 2002]. at middle range of intensities (iv-vii) we see of no obvious differences between traditional mmi and dyfi. dyfi limitations and sampling biases at the high end of the scale are discussed in a later section. more quantitatively, worden et al. [2011] used the large volumes of dyfi data available in california to show that the standard deviation for ten responses at a given location is about 0.25 intensity units (figure 8); additional responses lower repeatability differences to less than 0.10 units. since adjacent integer intensity levels correspond to roughly a factor of two increase in peak ground motion [wald et al. 1999b, worden et al. 2011], the fact that maps show intensities that vary smoothly between integer values means that a usgs “did you feel it?” ! figure 5 (top of previous page). felt area and distribution of dyfi reported intensities for the april 18, 2008, m 5.2 illinois earthquake. the inset shows the decay of intensities with hypocentral distance: green dots are zip code intensities, red circles are median intensities in log-distance bins, and the green line is the prediction equation of atkinson and wald [2007]. the prominent inflection at a distance of about 100 km is typical of most central and eastern usa earthquakes and results from amplification from post-critical reflections from the mohorovičić discontinuity. figure 6 (bottom of previous page). felt area and distribution of dyfi reported intensities for the april 29, 2003, m 4.6 alabama earthquake. inset figures show smooth variations of intensities near the epicenter for the rectangle on the main map (top), and smooth attenuation with distance (green curve, bottom), consistent with atkinson and wald’s [2007] predictions; red dots are distance bin averages of individual entries (green dots). there were 16,941 individual responses for this earthquake in 1,500 zip code areas. the prominent inflection at a distance of about 100 km is typical of most central and eastern usa earthquakes, and results from amplification from post-critical reflections from the mohorovičić discontinuity. figure 7. comparison of felt area and intensity–distance attenuation for the december 9, 2003, m 4.2 columbia, virginia, earthquake (left) with the august 2, 2006, m 4.4 santa rosa, california, earthquake (right). note the dramatic difference in the overall felt area and difference in epicentral intensity. maps scales are approximately the same. the inset plot shows dyfi intensities as a function of distance for the two events. symbols show mean and standard deviation. slight offset of distance values used for california (ca) dataset for plotting clarity [after atkinson and wald 2007]. wald et al. 697 figure 8. standard deviation of bootstrapped dyfi responses as a function of the number of responses within an area of 2 km radius about a ground motion observation. the mean within each bin (binned by number of responses) is shown, along with an exponential function fit to the means. from worden et al. [2011]. figure 9.the pga vs. mmi bilinear total least squares fit to the 50% probability picks (shown) is overlain upon the pga/mmi pairs. the wald et al. [1999b] and atkinson and kaka [2007] relations are also shown. from worden et al. [2011]. 698 ‘community’, at least on average, is capable of distinguishing rather small differences in ground motions, potentially within about ±20%. from a seismological perspective, this observation is rather remarkable: a consensus opinion of the general public (e.g., human perceptions) on ground shaking, as determined from average responses to a series of simple questions, implies the assignment of the absolute level of shaking to considerably better than a factor of two, and repeatedly to within about 20% with sufficient numbers of contributors. worden et al. [2011] detail the process and uncertainties associated with directly transforming dyfi intensity to peak ground motion estimates, but based on the repeatability seen with these data, we consider that process to be particularly informative for a number of real-time (shakemap and magnitude determinations) and other seismological analyses. the uncertainties that we estimate for macroseismic intensities may seem surprising. for traditionally assigned intensities, disagreements about integer assignments can occur due to different experts’ subjective evaluations, differences in the observations included at a particular location, the assumed extent of an area used for the assignment, and many other reasons. for example, ems-98 [grünthal 1998], the most comprehensive macroseismic scale to date, can produce fundamental ambiguities in integer assignments if damage grades for two different structure types indicate conflicting values. given the limitations of intensity assignments (summarizing multiple observations over an unspecific area), the numerical consistencies and low scatter of dyfi assignments found by worden et al. [2010] and seen in the smooth intensity gradients on the maps shown herein (figures 5, 6, 7), are rather remarkable. that said, there is now general agreement within the loss-modeling communities that highly detailed damage assessments for modern earthquakes require not only aggregated shaking and impact assignments implied by macroseismic data, but also comprehensive, detailed geospatial collections of individual structures, their structural descriptions, and their damage states [e.g., coburn and spence 2002]. in addition to uncertainties in intensity assignment, there is also the uncertainty in the spatial location and extent of our users’ observations. dyfi users are asked to provide their zip code (or city, for non-us locations), and also their street address for geocoding (see “geolocation” below). zip code extents could be less than a kilometer across (for densely populated cities) or tens of kilometers for sparsely populated areas. for non-us events, a user’s “city” can be an area of several kilometers or more. on the other hand, geocoded locations (when available) are accurate down to the scale of city blocks. as discussed previously, dyfi intensities are an aggregate of user responses over a certain area, rather than point observations. thus the spatial resolution of our intensity maps and the scale at which they are meaningful depend on the spatial precision of the underlying data. for widely felt events outside the usa, we make maps with intensity assignments at the city level; for small events felt within populated us urban areas, we can create maps with neighborhood-scale intensity levels, if there is a sufficient number of responses. catching missed events dyfi has proven to be of particular value for the usgs’s national earthquake information center (neic) when earthquakes occur in areas of the usa that have sparse seismographic stations. some earthquakes that are identified with dyfi macroseismic data often have not been automatically detected by domestic seismographic networks, yet their locations can subsequently be confirmed by analysts with scrutiny (and the timing and location guidance provided by dyfi) and thus their magnitudes determined with seismographic data from several stations. likewise, the dyfi system routinely provides an immediate heads-up for destructive events around the globe within minutes of the event. unlike more informal information flowing from social media outlets, which are difficult to quantify, the rapid dyfi reports provide data on location, time, and valid macroseismic values; these data can be used both qualitatively and quantitatively. remarkably, first-arriving intensity observations from earthquakes often precede the seismic network automatic determination of earthquake location and magnitude. we refer to these earliest, as-yet unassociated responses as “unknown event” responses, since at the time we receive them we do not have other information on the events that produced the reports. we monitor the unknown event reports by email and algorithmically. when the number of reports from a region exceeds 20 in a five-minute period, we alert operators and seismic analysts of a likely event (times, locations, and intensities). such a rate of unknown event reports is almost inevitably associated with an earthquake from which we have not yet received seismograph-based source information. once the event is located, the earlier unassociated reports are associated with it based on appropriate space-time windows. with dyfi the usgs is now capable of monitoring and automatically collecting intensity data for all felt earthquakes in the united states. that is a remarkable statement. events of magnitude less than 2.0, well below routine reporting level for most u.s. domestic seismic networks, are not uncommonly reported on the dyfi website. what’s more, dyfi can capture felt reports for non-earthquake-related shaking; dyfi maps are routinely made for sonic booms from supersonic aircraft, bolides, and even mining and other explosions. thus, dyfi allows comprehensive, quantitative analyses of macroseismic events that would otherwise go usgs “did you feel it?” uncollected, and had for decades prior. for example, some very widely felt events, such as the unusual magnitude 6.0 event of 10 september 2006 in the gulf of mexico (responsible for many of the felt reports in the gulf coast region, figure 2), would not have been canvased in the past due to the overall low intensities of shaking. with dyfi, over 6,000 entries were tallied, providing unique intensity data for this normally seismically quiet region. in that particular case, shaking likely exceeded the 50-year shaking probabilities shown by the usgs national seismic hazard map (figure 2a). likewise, the number of dyfi responses generated by a descending space shuttle passing over los angeles warranted a separate event webpage dedicated to the reports, and the reports were sufficient to map out the re-entry trajectory. sonic booms from aircraft, particularly in earthquake country, rattle residents and rile nerves, and yet prior to dyfi, most could not be confirmed, since seismic networks do not routinely locate and report sonic events. dyfi reports of sonic booms provide quantitative data for long-observed characteristics that are diagnostic of the phenomena: spatially and temporally correlated lowintensity reports from a rather broad geographic area, the lack of an associated instrumentally recorded earthquake that is large enough to be felt over the area from which reports are received, and the nature of user entries: rattled windows are noted nearly ubiquitously in the response or comments provided. usgs’s ability to document and map the occurrence of such events serves a public service. uniformity and flexibility one long-term objective of the usgs in gathering internet-based macroseismic data was to do so uniformly thoughout the usa. this was particularly important as regional networks and universities began collecting email notifications of earthquake effects by employing a wide variety of questions that only approximated those required for systematic and official intensity assignments. now, almost all internet-based macroseismic data collection in the usa is done via the usgs dyfi portal, providing uniformity of intensity assignments and, critically, a single collection point and long-term archive of these data. regional network operators linking their felt reports to dyfi benefit directly by not having to support local macroseismic data collection and processing efforts, yet can receive dyfi data as it is generated by usgs. we also customize event triggering and provide automatic notifications of felt events for the regional seismic network operators. interactivity and citizen-based science millions of annual web visitors and hundreds of thousands of individual questionnaire entries are a testimonial to the outreach potential and benefit of dyfi. dyfi also provides a unique opportunity for citizen-based science. in some scientific disciplines, data collection has long depended on the citizenry, e.g., backyard rain and temperature measurements for the us national weather service, and the national audubon society’s hundred-year plus annual christmas bird count; both have provided long-trending, spatially distributed data collection not possible by groups of scientists or instruments in the field. more recently, a number of other systems have been developed with similar ‘citizen science’ applications, allowing scientists to obtain vast data collections in ways not previously or otherwise possible. one example is cornell university’s bird-surveying program project feeder-watch (http://birds.cornell.edu/pfw/), in which citizens contribute via internet a wealth of observations that help constrain migratory bird habits, quantify variations in population density, and establish species ranges. another example is the cocorahs, a grassroots aggregator of backyard weather observations, which allows citizen to report rain, hail, and snowfall data online (http://www.cocorahs.org/). originally serving colorado, over 5,000 cocorahs observers now report valuable observations daily from all 50 us states. such observations would not be possible without this informal organization of thousands of volunteer ‘citizen scientists’. the internet facilitates the rapid and extensive collection of such datasets. what’s more, automatic data collection and processing provides instant feedback to the participants, or ‘field assistants’, further connecting science to citizen and creating a feedback loop for further participation. participatory science not only expands the observational base for data collection, an obvious advantage for intensity observations, but it also empowers the community to take ownership and allow better understanding of important scientific issues of the day [e.g., trumbull et al. 2000]. the dyfi system takes full advantage of online data collection by being fully interactive, providing users’ intensity assignments instantaneously, and by showing the effects of their entries on the updated intensity maps. an important subtlety with respect to dyfi contributors is that they are not trained for the task, and as such they are not ‘citizen scientists’; rather, dyfi is more accurately described as ‘citizen-based’ science. other citizen science systems do, in fact, require substantial expertise on the part of the contributor, for example, being able to recognize the species of birds that their feeder attracts for project feederwatch, or knowing the phenophases of flowering plants for the usgs phenology program (http://www.pwrc.usgs.gov/ bpp/). yet the well-established nature of macroseismic collection, with exception of the highest intensities, has been reliant on casual observers for many decades; only of late has the internet facilitated the collection of these data more rapidly and systematically. the dyfi system also educates the public on oftmisunderstood seismological concepts like the spatial wald et al. 699 700 variations of shaking intensity, and it provides a basis for clearing up confusion of macroseismic intensity with instrumental magnitude. as described in a later section, we have built educational tools that further this goal. greater awareness of these concepts allows for more rational decision-making for both long-term mitigation and emergency response in the immediate aftermath of an earthquake [e.g., goltz 2003]. dyfi also provides an important human perspective on earthquakes, providing sociological documentation of the way people behave and respond, and how they perceive risk [e.g., celsi et al. 2005]. finally, and perhaps most rewarding to the authors, dyfi seems to provide emotional help to citizens who have just had a frightening or even traumatic experience. by allowing citizens to share their experiences and by enabling them to contribute their observations towards a general public understanding of the phenomenon they have experienced, dyfi provides many with a form of catharsis at an opportune time, and in a rapid fashion. often users describe the desire to confirm their experience with others, or in the case of dyfi, with the collective community. in short, the dyfi website provides a two-way information conduit, since citizens coming to the usgs for information are empowered to become data providers themselves by contributing valuable observations that benefit the usgs as well as the observers, their local communities, and earthquake responders. challenges and limitations of online macroseismic data collection the voluntary nature of macroseismic questionnaires collected online and the open-environment character of the internet itself, pose problems in data collection that force us to continually modify aspects our dyfi processing system. likewise, we must also retain realistic expectations of the performance of the dyfi system in the immediate aftermath of moderate magnitude events in areas with poor internet coverage, and different expectations for severely destructive earthquakes. limited access to the dyfi system due to damage and assignments of high intensities with dyfi both require special consideration. with dyfi, the highest contribution rates to date have come from events with a combination of dense population, ubiquitous internet access, and a lack of significant damage, power or internet outages, or otherwise disrupting influences. it is for these events that we have slowly grown processing capacity at the usgs. in contrast, and without specifics, several countries’ webpages have had nearimmediate loss of service even after relatively minor but widely-felt events, and systems like dyfi can certainly be largely culpable in contributing to that traffic spike without proper precautions. several mitigating strategies are outlined in the section below on robustness and system specifications. for very widespread damaging events, which to date have not been experienced in the usa (where response rates are normally the highest) since dyfi’s inception, we anticipate that response-related traffic will be limited by some of the disruptive factors mentioned above. in addition, providing maps for emergency response was never the intended main goal or service of the dyfi system. rather, where risks dictate and resources permit, the procedure of choice for more robust post-earthquake hazard evaluation is the shakemap system [wald et al. 1999c]. shakemap, in conjunction with the related, downstream systems like shakecast and pager [e.g., wald and bausch 2009], provides situational awareness to inform and initiate appropriate response activities. as a tool for earthquake response, dyfi has several severe limitations immediately after a major earthquake. in the high-intensity regions we cannot expect internet and power connections to be available; equipment may be damaged, and there is no expectation that getting online to report to the usgs should or will be a priority for citizens. in contrast, shakemap depends on hardened (yet not overly robust) seismic networks, often with redundant or alternate communication channels; it does not rely on the availability of internet users. for regions with sparse strong-motion seismographic coverage, dyfi does provide a partial substitute for instrumentally generated shakemaps, again with the caveat of a lack of robustness for serious events. that said, dyfi macroseismic maps are more representative of earthquake felt areas and impacts than shakemaps. shakemap, with intensity derived from conversion of recorded ground motions [e.g., wald et al. 1999c, worden et al. 2011], serves only as a proxy for what macroseismic effects actually occurred; dyfi directly reports them and thus defines the macroseismic intensities. outliers automated data collection from the public via the internet inevitably results in data outliers, some unintentional and others deliberate. we have developed two automated filters that remove the bulk of such outliers. no data are discarded in the process of filtering; rather, outliers are flagged as “suspect” and bypassed in the processing of results. the first filter works on checking the self-consistency of each individual entry. we reject responses that have contradictory answers, such as a response of “not felt” and “frightened”. in particular, the damage portion of the questionnaire (see appendix i) is the one most likely to be falsified; for some reason, checking all possible damage options is not uncommon, yet it is an invalid selection. the second filter is based on the individual intensity computed from each entry and the distance of that user from the epicenter. in the past, this was compared to a simple, empirically derived intensity-distance curve that was usgs “did you feel it?” based on the event’s magnitude. entries whose computed intensity was more than two intensity units above the empirical curve were rejected. now, we use intensity prediction equations or ipes [e.g., atkinson and wald 2007], which are regionally dependent and were derived using archived dyfi data explicitly. aside from deliberate false entries, we also get simple user errors, like typos. most users show genuine concern over the potential impact of their errors, often sending email to the operators when they realize that they have inadvertently caused an error (e.g., entering their home rather than their work zip code). users occasionally attempt to correct an error by sending in additional reports; since the last entry is normally the one with which they are satisfied, we automatically filter out duplicate entries and keep the latest. another common error is a mistyped zip code, which could place a user hundreds of kilometers away and is easily caught by the intensity-distance filter. we save the time that users submit their forms and the time they indicated they believe the earthquake was felt for later sorting, filtering, and analyses. often the users’ reported times aid in determining if the response was for a mainshock or an aftershock and help to resolve other ambiguities. conveniently, most errors and intentional mischief are obvious and can thus be filtered or sorted out. on the other hand, more subtle attempts at pranks have little impact on our results due to the overwhelming majority of quality reports. it is notable that mischievous responses tend to occur in times without earthquakes; in the post-earthquake time period the vast majority of users are responsible and the numbers of valid responses overwhelm any potential pranksters. the result is high-quality data when it is most needed. finally, the dyfi operators maintain ultimate data quality control over the data and results of the system. analysts can flag suspected entries based on obvious errors, specific comments, or other indicators. finally, usgs also reserves the option to assign intensities at specific locations as described below. limitations at high macroseismic intensities higher macroseismic intensity values (typically, viii and greater) primarily describe observed structural damage to buildings [e.g., musson 2010]. given the lack of engineering expertise among the usual dyfi contributor, one could argue that assigning intensities to intensity viii and higher should formally be done by professionals. that said, dyfi has been calibrated to achieve a maximum of mmi ix by relating community responses to the traditional mmi questionnaire. in comparison with mmi data for high-intensity earthquakes in california and elsewhere, the consistency of communitybased intensity viii and ix assignments is quite convincing [e.g., dewey et al. 2000, 2002]. for this reason, we assume equivalence of dyfi and mmi assignments unless there is specific evidence to the contrary. nonetheless, for destructive earthquakes, it is necessary to have the capability to accommodate reliable alternative macroseismic observations in addition to dyfi as well as to minimize the influence of errors in web questionnaire responses. dewey et al. [2002] introduced the notion of a reviewed community internet intensity (rcii) for just this purpose. rcii entails three elements that go beyond completely automated online data collection. initially, by default, rcii is the cii computed at a two-week cutoff time, so that the cii value is not allowed to continue changing with additional later responses. second, rcii data are more thoroughly reviewed for errors or inconsistencies. third, rcii may be adjusted from cii or assigned to communities lacking ciis on the basis of other types of macroseismic observations such as engineering reports, press reports, and field reconnaissance (see dewey et al. [2002] for more details). hence, professional review by seismologists and field-based macroseismic observations will continue to be important for augmenting higher intensity dyfi observations in the future to enable seismologists to calibrate and fully document what the dyfi data represent. in such cases we can assign a zip code intensity based on the independent observations, overwriting the community decimal intensity. to date, this has only been done in rare cases. as previously noted, the authors feel that there are diminishing returns to highly detailed, field-based macroseismic surveys which assign integer macroseismic values to locales, since gis-based geospatial archiving of damage distributions has become the norm and the expectation following significant earthquake disasters. operational robustness, web traffic, and system specifications a natural concern with a system like dyfi is the challenge of accommodating the post-earthquake deluge of input data and web traffic. as dyfi has grown in popularity, we have continuously improved capacity by making both hardware and software improvements in order to handle the spike in internet traffic following a widely felt earthquake (an issue known as the “slashdot effect”, see a decade-old description at http://pasadena.wr.usgs.gov/office/stans/slashdot.html). to date, the largest such spike is 78 questionnaires submitted in one second recorded for a m 5.7 event in southern california,and there have been four other events that exceeded 50 questionnaires per second; sustained rates at greater than 1,000 forms per minute are likely in the future. in order to handle this load, we currently have two separate, redundant back-end servers for triggering, event processing, and map creation. three additional public servers are dedicated to serving the questionnaire itself and handling user input. we cache all dyfi web content, relegating webpage generation to another layer of servers maintained by the earthquake hazards program web team. currently, wald et al. 701 702 there are two intermediate product servers and four web servers that store content from the entire earthquake hazards program. for robustness, these servers are stored in different usgs locations: golden, co; denver, co; pasadena, ca; menlo park, ca; and reston, va. finally, we commercially contract to a web delivery service provider (level3 communications), which redistributes our cached webpage content to thousands of servers distributed globally. in spite of these precautions, we have no grand expectations for the performance of the dyfi system for the areas hard-hit by damaging ground motions. it is possible, if not likely, that power outages, damage to users’ networks and computers, and limited internet access will lead to significant data gaps. thus, the low likelihood of dyfi reliability for retrieving data from the most heavily damaged regions in the immediate aftermaths of destructive earthquakes necessitates a separate, robust post-earthquake response tool like shakemap [wald et al. 1999c]. unlike dyfi, shakemap does not depend on internet-based human input to place ground motion and intensity maps online immediately. on the other hand, the dyfi approach does not require expensive, high quality real-time seismic stations – it can be implemented anywhere in the world where there are people with internet connections, and dyfi data provide direct intensity observations as opposed to intensity values inferred from peak ground motions as in the shakemap system [wald et al. 1999c]. moreover, we hope the dyfi system will perform well for much of the affected region even for damaging events, and that data will later be available from areas that were not able to respond in the immediate aftermath. in conjunction with shakemap and abundant seismic recordings, dyfi provides a constantly improving database for calibrating relations from recorded ground motions to intensities. for these and other noted reasons, the two very different dyfi and shakemap approaches to rapidly mapping ground shaking and intensities naturally complement each other: a “man versus machine” challenge. new and ongoing developments with internet intensity data after several years of dyfi data collection and processing, and from beneficial advice from our contributors, data users, and other scientists, we have made several modifications to the dyfi system and developed numerous post-processing tools and products for use with the dyfi data. all dyfi data collected to date are now in an archival database; this greatly facilitates research by streamlining the selection, organization, and exportation of data. the dyfi system includes a graphical user interface that allows seismic analysts to perform common functions, including map triggering and resizing, as well as sorting, searching, geocoding, and flagging entries. new web-based geolocation and geocoding services are being incorporated into dyfi for improving the accuracy of the users’ locations. all these improvements are leading to additional ways to utilize the vast quantities of dyfi data. dyfi database and graphical user interface in a recent advancement over command-line interaction, we now allow dyfi to be operated by the neic staff of seismic analysts. we have developed a web-based graphical user interface (gui) that allows our seismic analysts to trigger, delete, resize, or re-center maps, as well as view, supplement, or flag intensity observations. easy interactive searching and manipulation of the dyfi database allow for additional manual quality control, including flagging obvious outliers or suspected entries and then regenerating maps and webpages for that event. the mysql database for dyfi contains approximately 77,000 events, with a total of nearly two million individual entries; it supports common queries allowing database reports for annual summaries and use statistics. all completed entries and summary data (zip code averaged intensities) are permanently archived. in order to facilitate the further use of the dyfi data for research, analysis, and visualization, we also provide summaries of dyfi data in a variety of formats online. one simple form of the data available is a tab-delimited summary of the zip (geocode) intensity, zip code centroid latitude and longitude, epicentral distance, and the number of responses contributed to that zip code. due to privacy considerations, we cannot redistribute personal information or users’ comments. however, upon request, we can provide a sanitized version for research purposes, stripped of identifying data, yet allowing more detailed analyses of the nature of individual responses. we have also begun to produce google kml files of city and zip code intensities for visualization in google earth and google maps. since systems comparable or complementary to dyfi now operate in several countries, collaborative efforts to uniformly collect and exchange data in near-real time are underway [musson and the esc working group 2009, musson 2010]. to contribute to this effort, usgs now generates and exports xml-formatted versions of each report, ultimately for near-real time exchange of data with emsc and other global partners. systems comparable or complementary to dyfi now operate in several countries, and collaborative efforts to uniformly collect and exchange data in near-real time, particularly in a standardized xml schema, are underway [musson and esc working group 2009, musson 2010]. currently, dyfi is served and collected in both english and spanish, the choice of the majority of our domestic constituents. web-based translators provide some additional access to other users, and these capabilities will undoubtedly usgs “did you feel it?” be enhanced in the future. however, since the majority of usgs users are english speakers, and due to limited development resources, we have not expanded language options at this time. the emsc system does provide more extensive multilingual options. geolocation for data collected within the usa, we can manually trigger a dyfi-specific geocoding algorithm, which turns street addresses into latitude and longitude coordinates with precision enough to distinguish adjacent street blocks. even though we require that users supply only their postal zip codes in order for their observations to be processed, up to 90% of respondents to the usgs dyfi website questionnaire volunteer their street addresses. since intensity assignments optimally make use of numerous entries for robustness [e.g., worden et al. 2011], we select spatial domains over which to average responses rather than using the zip code polygons. to date this has been achieved by subdividing the map extent into a grid of 25 × 25 or 50 × 50 equally spaced boxes and averaging entries in each box. averaging is done as described earlier and in wald et al. [1999a], where community averages for each question are determined, rather than simply averaging intensities. in the future we are considering using coordinates of the national grid domestically and the universal transverse mercator coordinate system internationally rather than mapcentric boxes, in order to subdivide our geocoded maps on standard, repeatable domains. from experience, we have found the additional geocoding processing to be unnecessary in populated areas where zip codes sizes are small, but the spatial refinement provided by geocoding is important where zip code areas are larger. for this reason, geocoding is manually triggered as deemed necessary. since not all respondents provide an address, some information is lost, and there is an inherent trade-off between spatial resolution and the number of responses available for geocoded maps. examples of the geocoded maps and data can be found online for many of the larger magnitude us events for which thousands of responses have been received. we are currently augmenting the global city-based system with user-supplied, geocoded locations, allowing us to apply our geocoding tools for global events. dyfi promotes web form access via mobile web devices, and we are facilitating mobile use via the development of smart-phone apps; however, automatic locations provided by such devices can complicate processing since the user’s instantaneous position is not necessarily the site where the earthquake was experienced. dyfi tools and products in addition to community intensity maps, we produce several complementary products for each dyfi event. we publish a list of the aggregated intensities by zip code or city (see figure 10), sortable by location or intensity, and downloadable in csv or xml format. in addition, it is convenient to portray these intensity data as a function of distance, thereby showing at a glance the attenuation of intensity from the hypocenter. we now systematically generate and update plots of intensity versus distance as the dyfi data are processed (e.g., figure 5). following bakun and wentworth [1997], the individual intensities are plotted wald et al. 703 figure 10. search of the dyfi achive webpage allows queries based on event names, number of responses, magnitude, and intensity ranges. sortable columns allow listing by any field. 704 as well as the average intensities calculated for bins in increments of distance. the curves recovered from the dyfi data generally follow regional intensity attenuation functions well, and in fact, an obvious systematic offset of the observed data from previous attenuation curves has provided an independent incentive to re-examine the seismic network magnitudes and/or location assignments for individual events. the intensity–distance plots show clear differences between east and west within the usa: the central and eastern usa typically show higher epicentral intensities for a given magnitude, slower rates of attenuation with distance, and clear indications of the amplifying effects of post-critical reflections from the mohorovičićdiscontinuity (e.g., figures 5, 6, 7; also, see atkinson and wald [2007]). finally, as discussed above, we publish plots of the incoming dyfi responses as a function of time after the earthquake origin (see figure 4). this feature provides the system operators the ability to see various effects on the rate of user responses, such as time of day or news coverage. also, network slowdowns or other problems can be easily inferred from interruptions in the time history. research utilizing dyfi data an important goal in seismology is rapidly estimating shaking and damage after an earthquake. such impact assessments require calibration against past damage and associated ground motions. oftentimes only macroseismic data are available to characterize the shaking, and it is of great benefit to be able to infer ground motion peak amplitudes from such data. given the dyfi data for an event, we have developed tools to estimate ground motions in the absence of data from seismic instruments. with such available conversion equations, the global dyfi data are also now automatically used as constraints in our global predictive shakemap system, which in turn is used as the hazard input for the usgs prototype prompt assessment of global earthquakes for response (pager) system [wald et al. 2006a]. while numerous relations now relate peak ground motions to intensities [e.g., wald et al. 1999b, atkinson and kaka 2007], these equations suffer two limitations. first, they can only be used to estimate intensity from recorded peak ground motions, or vice versa, but not both; second, they were typically regressed with integer intensity values. worden et al. [2011] fully utilize dyfi data to derive new relations among peak ground motion parameters and intensity data. the development of these relations sets a new standard for ground-motion-to-intensity relations in that the dyfi intensity data used are decimal intensities and reverse relations are provided; prior regressions were limited to the use of ordinal intensity values, which complicate the formulation and limit the potential resolution and were regressed only in the direction of estimating intensity from ground motions. similarly, dyfi data have recently been used to derive direct intensity prediction equations or ipes [e.g., atkinson and wald 2007, allen et al. 2011]; these equations are important for robust shakemap generation [worden et al. 2010] and hazard evaluations [e.g., cua et al. 2010], as well as for improved dyfi filtering. we are also testing the use of unassociated responses: those to compute earthquake magnitude and location in the case when the instrumentally derived centroid is delayed or unavailable. bakun and wentworth [1997] developed a method for deriving these parameters from the historical intensity data. they perform a grid search centered on the area with the highest intensity responses, treat each grid node as a “trial epicenter,” and determine the magnitude and intensity centroid that best fits the dyfi observation points according to a region-dependent intensity-distance attenuation relation. more sophisticated systems have been developed since (e.g., boxer) [gasperini et al. 2010]. we will test these algorithms with the new relations derived specifically for dyfi-based ipes to continuously solve for earthquake magnitude and intensity centroid as unassociated dyfi data are received. the intensity centroid and ground motions determined from the dyfi data correlate well with instrumentally derived parameters. with further efforts at calibrating regional variations of intensity attenuation, this approach could be used to automatically determine location and magnitude globally, independently from seismic network operations, with the added capability of doing so for events below many regional seismic networks’ reporting thresholds. education and outreach dyfi has been recognized by a number of educators as a natural format and opportunity for earthquake-hazard education. for example, the usgs produced an educational exercise using the northridge earthquake dyfi map as a children’s coloring map to help explain the difference between magnitude and intensity. this exercise was later adopted by the national geographic society for their educational webpages and is used routinely to help explain a related question recently added to the state of california education standards which requires an understanding of the difference between magnitude and intensity (l. wald, usgs, personal communication, 2005). independently, haase and park [2006] held a series of grade-school-teacher workshops following a widely felt indiana earthquake, training teachers to use dyfi for explaining magnitude and intensity, as well as getting students to do online submissions (thus further improving the map for that event). the authors are aware of several elementary schools that routinely use dyfi for a ‘teachable moment’, right after students experience an earthquake firsthand at home or at school. dyfi maps greatly facilitate communication of earthquake hazards by allowing concrete examples of seismic usgs “did you feel it?” intensity. a long-standing limitation in communicating hazards to the public has often been the difficult challenge of explaining hazard in terms of earthquake magnitude. the resurgence of the use of macroseismic intensity for describing hazard is a welcome reversal. shakemap, and more so dyfi, intensity maps have contributed to these discussions. wald and dewey [2005] published a public-service-based usgs fact sheet describing dyfi, which was originally printed with 10,000 copies. it was later printed at 50,000 additional copies, primarily to promote citizen-based science during earth science week in 2005 in the usa. one of the fact sheet figures, an earlier version of distribution of felt intensities over the nation for the past decade shown in figure 2, has been found to be a much more intuitive and effective way of communicating the extent of the earthquake problem throughout the country. it is much easier to remind the public about the national earthquake hazard by showing them what actually happened than by telling them what shaking level has a 10% probability of being exceeded in 50 years (e.g., the usgs national seismic hazard map) [petersen et al. 2008]. similar dyfi maps have been used for two congressional briefings, and the usgs and many related organizations recognize the advantages of communicating with maps of actual felt reports rather than with probabilistic seismic hazard maps. discussion and conclusions the u.s. geological survey’s “did you feel it?” (dyfi) system, relying on internet data collection after earthquakes, has significant advantages over earlier macroseismic intensity data collection approaches, yet there are some notable limitations arising from its web dependence. awareness of these limitations reduces potential detrimental impacts, and we are continuing to improve the system as new tools and approaches become apparent. dyfi has always been an evolving system. among the recent developments, we have described a number of post-processing tools, applications, and studies that make use of the extensive intensity data sets now gathered, including automatic location and magnitude determination, estimating ground motions from the intensity observations, automatic geocoding to allow for more refined intensity localization, recovering higher-precision decimal intensities rather than limiting intensities to integer values, and socialscience analyses of earthquake response and risk perception. we have also expanded dyfi data collection from usa zip codes to the rest of the globe, and all indications show the usefulness of the global data. we nonetheless see potential for improving the current international dyfi through new automatic geocoding tools. the dyfi procedure also has limitations, as discussed. it is strongly conditioned by us traditions of macroseismic data interpretation; we envision collaboration with non-us macroseismologists to make the product more useful in nonus contexts. questionnaires in the native language of the source region would clearly facilitate collection of data globally. ultimately, we expect the global intensity database will prove useful for regional attenuation as well as other seismological studies. data flow rates are extraordinary, and precautions have been put in place through the use of redundant, hardened, high-capacity processing systems. nonetheless, data flow after major damaging earthquakes may be limited by power outages, excessive internet traffic, infrastructure damage, and the more important priorities of users. from our experience with dyfi, essential components of an internet-based citizen-science portal include: i) easyto-use forms, ii) instant feedback so that users may see their contributions (validating their experiences), iii) open space for first-person accounts and discussion of effects not covered in the questionnaire, and iv) routinely addressing user comments and questions. in addition, online user-friendly tools now include common searches, statistics, sorting of responses, time-entry histories, comparisons of data with empirical intensity estimates, and data that are easily downloadable for researchers (figure 10). a major advantage of the dyfi system is that its contributors do not need to be trained for the task, and as such they are not ‘citizen scientists’; rather, dyfi is ‘citizenbased’ science. other citizen-science and crowd-sourced systems do, in fact, require substantial expertise on the part of the contributor. many new web-based data aggregators face rather daunting challenges in that data quality is more directly tied to or limited by the level of expertise of the available ‘crowd’. the macrointensity maps shown in figures 1-7 fundamentally depart from those produced with more conventional means in the past. we have described the limitations of internet-based data collection in the macroseismic realm. yet, despite the limitations of data collection via the internet outline, the advantages are both numerous and remarkable: 1. unprecedented macroseismic data collection rates and consistency provide usgs with the capacity to rapidly map out intensity distribution for all felt earthquakes in the united states and its territories. the cost per observation is orders of magnitude lower than traditional macroseismic data collection efforts. 2. global data collection provides usgs national earthquake information center with rapid first indications of earthquake occurrence and the potential degree and extent of impact, providing constraints for the global shakemap and thus the pager system. 3. macroseismic data quality is sufficient for: i) portraying and making quantitative use decimal intensities for response as well as scientific purposes, ii) revising otherwise poorly constrained earthquake locations and wald et al. 705 706 magnitude, and iii) distinguishing seismic events from other phenomena (such as sonic booms). 4. millions of macroseismic observations are available for social-science and seismological analyses. 5. the citizen-based science of the dyfi portal provides an unmatched opportunity for interaction between the scientists of a government agency and the community that they serve. dyfi provides a two-way flow of post-earthquake information providing the usgs with quality macroseismic data, as well as an avenue of information for concerned citizens, and a form of reassurance for those who experienced frightening ground shaking. dyfi maps also greatly facilitate usgs communication about earthquake hazards. data and sharing resources the dyfi data we used in this study are freely available for downloading from the archive at http://earthquake.usgs. gov/dyfi/. geocoded locations can be acquired for many well-reported events online or by contacting the authors, and aggregated datasets can be acquired from the dyfi operators. acknowledgements. we are thankful for helpful reviews from daniel amorese and roger musson. greg smoczyk produced the us and global maps of intensity and hazards; other maps were made using gmt [wessel and smith 1995]. dyfi fundamentally relies on the capabilities put into place on and beneath the hood of the usgs earthquake program webpages; for this we thank the earthquake program web team. references allen, t.i., d.j. wald and c.b. worden (2011). prediction of macroseismic intensities for global active crustal earthquakes, submitted to journal of seismology, in press. atkinson, g.m. and s.i. kaka (2007). relationships between felt intensity and instrumental ground motion in the central united states and california, b. seismol. soc. am., 97, 497-510. atkinson, g.m. and d.j. wald (2007). “did you feel it?” intensity data: a surprisingly good measure of earthquake ground motion, seismol. res. lett., 78, 362-368. bakun, w.h. and c.m. wentworth (1997). estimating earthquake location and magnitude from seismic intensity data, b. seismol. soc. am., 87, 1502-1521. celsi, r., m. wolfinbarger and d.j. wald (2005). the effects of earthquake measurement concepts and magnitude anchoring on individuals’ perceptions of earthquake risk, earthq. spectra, 21 (4), 987-1008. coburn, a.w. and r.j.s. spence (2002). earthquake protection, 2nd edition, chichester, john wiley & sons ltd. coppola, j.m., x. lorena, x. cowan, g.l. downes, k.f. fenaughty, p. grimwood, p. leach and e. de robertson (2010). felt earthquake reporting via the internet in new zealand, seismol. res. lett., 81 (6), 984-991. cua, g., d.j. wald, t.i. allen, d. garcia, c.b. worden, m. gerstenberger, k. lin and k. marano (2010). “best practices” for using macroseismic intensity and ground motion to intensity conversion equations for hazard and loss models, gem technical report 2010-4, report series, 69 pp.; http://www.globalquakemodel.org/node/747. dengler, l.a. and k. moley (1994). toward a quantitative, rapid response estimation of intensities, seismol. res. lett., 65, p. 48. dengler, l.a. and j.w. dewey (1998). an intensity survey of households affected by the northridge, california, earthquake of 17 january 1994, b. seismol. soc. am., 88, 441-462. dewey, j., d. wald and l. dengler (2000). relating conventional usgs modified mercalli intensities to intensities assigned with data collected via the internet, seismol. res. lett., 71, p. 264. dewey, j.w., m.g. hopper, d.j. wald, v. quitoriano and e.r. adams (2002) intensity distribution and isoseismal maps for the nisqually, wa, earthquake of 28 february 2001, usgs open-file report 02-0346, 57 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(1996). the modified mercalli earthquake intensity scale; revisions arising from recent studies of new zealand earthquakes, bulletin of the new zealand national society for earthquake engineering, 29, 92-106. gasperini, g., d. vannucci, d. tripone and e. boschi (2010). the location and sizing of historical earthquakes using the attenuation of macroseismic intensity with distance, b. seismol. soc. am.,100 (5a), 2035-2066. geonames (2011): www.geonames.org (accessed july 6, 2011). giardini, d., g. grünthal, k.m. shedlock and p. zhang (1999). the gshap global seismic hazard map, annali di geofisica, 42 (6), 1225-1230. goltz, j. (2003). science can save us: outreach as necessity and strategy, seismol. res. lett., 74, 491-493. grünthal, g., ed. (1998). european macroseismic scale 1998 (ems-98), cahiers du centre européen de géodynamique et de séismologie 15, centre européen de géodynamique et de séismologie, luxembourg, 99 pp. haase, j. and c.h. park (2006). mapping amplification of earthquake ground motion in indiana, series “lessons learned over time”, vol. vi-a, earthquake engineering research institute (eeri), oakland, ca, usa, 36 pp. mazet-roux, g. and r. bossu (2010). emsc real time earthquake information services, european-mediterranean seismological centre (emsc) report, 31 pp.; available at: www.emsc-csem.org/files/news/emsc/ emsc_rtei_services.pdf. musson, r.m.w. and i. cecić (2002). macroseismology, in: w.h.k. lee, h. kanamori, p.c. jennings and c. kisslinger (eds.), international handbook of earthquake and engineering seismology, academic press, amsterdam, 807-822. musson, r. (2010). draft standard internet macroseisimic usgs “did you feel it?” questionnaire, in: proceedings of the european seismological commission (esc2010), p. 65. musson, r.m.w., g. grünthal and m. stucchi (2010). the comparison of macroseismic intensity scales, j. seismol., 14, 413-428; doi: 10.1007/s10950-009-9172-0. musson, r. and the esc working group “internet macroseismology” (2009). towards a standard internet macroseismic questionnaire, report to esc, 35 pp.; http:// seismologist.co.uk/resources/wg_discussion_v4.doc. petersen, m., a. frankel, s. harmsen, c. mueller, k. haller, r. wheeler, r. wesson, y. zeng, o. boyd, d. perkins, n. luco, e. field, c. wills and k. rukstales (2008). documentation for the 2008 update of the united states national seismic hazard maps, u.s. geological survey open-file report 2008-1128, 61 pp. richter, c.f. (1958). elementary seismology, freeman, san francisco, 768 pp. sbarra, p., p. tosi and v. de rubeis (2010). web-based macroseismic survey in italy: method validation and results, nat. hazards, 54, 563-581. sieberg, a. (1930). scala mcs (mercalli-cancani-sieberg), geologie der erdbeben, handbuch der geophysik, 2, 552-555. stover, c.w. and j.l. coffman (1993). seismicity of the united states, 1568-1989 (revised), u.s. geological survey professional paper 1527, 418 pp. trumbull, d., r. bonney, d. bascom and a. cabral (2000). thinking scientifically during participation in a citizenscience project, science education, 84 (2), 265-275. wald, d.j., v. quitoriano, l. dengler and j.w. dewey (1999a). utilization of the internet for rapid community intensity maps, seismol. res. letters, 70 (6), 680-697. wald, d.j., v. quitoriano, t.h. heaton and h. kanamori (1999b). relationships between peak ground acceleration, peak ground velocity, and modified mercalli intensity in california, earthq. spectra, 15, 557-564. wald, d.j., v. quitoriano, t.h. heaton, h. kanamori, c.w. scrivner and c.b. worden (1999c). trinet “shakemaps”: rapid generation of peak ground motion and intensity maps for earthquakes in southern california, earthq. spectra, 15, 537-555. wald, d.j. and j.w. dewey (2005). did you feel it? citizens contribute to earthquake science, u.s. geological survey fact sheet, 4 pp. wald, d.j., p.s. earle, k. lin, v. quitoriano and b. worden (2006a). challenges in rapid ground motion estimation for the prompt assessment of global urban earthquakes, b. earthq. res. i. tokyo, 81 (3/4), 273-281. wald, d.j., v. quitoriano and j.w. dewey (2006b). usgs “did you feel it?” community internet intensity maps: macroseismic data collection via the internet, in: first european conference on earthquake engineering and seismology (a joint event of the 13th ecee & 30th general assembly of the esc; geneva, switzerland, september 3-8, 2006), 10 pp. wald, d.j. and d. bausch (2009). new research and tools lead to improved earthquake alerting systems, cusec journal (central united states earthquake consortium), 13 (5), 6 pp. wessel, p. and w.h.f. smith (1995). new version of the generic mapping tools released, eos trans. agu, 76 (33), p. 329. worden, c.b., d.j. wald, k. lin, g. cua and d. garcia (2010). a revised ground-motion and intensity interpolation scheme for shakemap, b. seismol. soc. am., 100, 30833096. worden, c.b., m.c. gerstenberger, d.a. rhoades and d.j. wald (2011). probabilistic relationships between peak ground motion and modified mercalli intensity, b. seismol. soc. am., 101. *corresponding author: david j. wald, u.s. geological survey, national earthquake information center, golden, colorado, usa; email: wald@usgs.gov. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. wald et al. 707 abstract introduction intensity-data gathering in the internet age national and international systems advantages of online macroseismic data collection rate of responses accuracy of responses catching missed events uniformity and flexibility interactivity and citizen-based science challenges and limitationsof online macroseismic data collection outliers limitations at high macroseismic intensities operational robustness, web traffic, and system specifications new and ongoing developments with internet intensity data dyfi database and graphical user interface geolocation dyfi tools and products research utilizing dyfi data education and outreach discussion and conclusions data and sharing resources references 1 annals of geophysics, 66, 2, 2023; doi:10.4401/ag-8829 appendix to shear-wave splitting patterns in perachora (eastern gulf of corinth, greece) ioannis spingos1, vasilis kapetanidis1, georgios michas1, george kaviris1, filippos vallianatos1,2 1 section of geophysics – geothermics, department of geology and geoenvironment, national and kapodistrian university of athens, athens, greece 2 institute of physics of earth’s interior and geohazards, unesco chair on solid earth physics and geohazards risk reduction, hellenic mediterranean university research center, crete, greece appendix a. template-matching to increase the number of events available for shear-wave splitting parameter determination, we applied a templatematching technique, by employing the open-source eqcorrscan code [chamberlain et al., 2018]. this method takes advantage of the waveform similarity between groups of seismic events with identical source parameters to detect them, by scanning continuous waveform records, as well as pick their pand s-wave arrival-times automatically. we used the initial solutions for 828 manually analyzed events as templates. the preparation of data for the template matching method includes down-sampling to 50 hz (from the original 100 hz sampling rate), to reduce processing time, de‑trending and filtering in a frequency band between 2 and 15 hz, to reduce noise, while ensuring enough waveform complexity to distinguish between different multiplets. the p-waves are cropped on the vertical channel, while the s-waves are cropped on the two horizontal channels of each station with available data and positioned in a template by retaining their relative time delays, according to the differences in their arrival times. during the detection procedure, a normalized correlation coefficient is calculated for all available channels, considering these time delays. we used the median absolute deviation (mad) as a measure of similarity, with a threshold value of 8. when the mad value surpasses the threshold and is then maximized, the corresponding time is registered as a “detection” and the p and s arrival‑times of the templates are imposed at the respective time of the best fit on the continuous records. in a next step, these detected arrival times are fine‑tuned, by performing cross‑correlation for each p or s‑wave on each station separately, within a small interval of time‑lags around the initial detection time. this allows for some flexibility, as the detected event’s hypocenter may be slightly offset from that of the template, causing small differences in the respective travel-times. the hypocenter of the template is imposed as a solution for each detection made using it. accordingly, the origin time is estimated by subtracting the pand s-wave travel-times of the template from the respective arrival-times of the detection. finally, a “relative magnitude” is determined, in the form of a magnitude difference 𝛥𝑚, from the ratio of waveform amplitudes between a template and an associated detection, following the formula in eq. (a1), similar to the one proposed by schaff and richards [2014]: (a1) where 𝑠𝑡𝑑 is the standard deviation of the waveform amplitudes, 𝑡𝑟1 and 𝑡𝑟2 are the traces of the template and detection, respectively, with 𝑠𝑛𝑟1 and 𝑠𝑛𝑟2 the corresponding signal-to-noise ratio for template and detection, and 𝐶𝐶 is the correlation 2 coefficient. the difference 𝛥𝑚 is subtracted from the known magnitude of the template to estimate the “relative” magnitude of the detected event. finally, a search for duplicate detections is performed, as the same signal could be recognized by different templates belonging to the same multiplet. in such cases, the detection that provided the higher mad value is retained. from the above procedure, a total of 23,000 detections were made. in the context of the current study, template-matching was used as a means of increasing candidate event-station pairs for shear-wave splitting. therefore, our main goal was to acquire arrivals even in single stations. however, due to the sparsity of the network, we identified cases of clear and impulsive detections at a single station (e.g. hp.ltk) that were noisy, missing or contaminated in others (i.e. in ha.lout and, especially, in ha.ltrs, which is equipped with a less sensitive accelerometer). as determining enough phases for locating the additional low‑amplitude events anew was impossible, the fixed (relocated) hypocenters of the templates were used as the focal parameters. to select arrivals suitable for sws determination, we developed a station-centric and arrival-independent approach. signal windows were cut 10 s before and 30 s after each detection. then, after detrending and applying a 2-15 hz bandpass filter, a modified snr value was estimated according to eq. (a2): (a2) where 𝐴𝑠 and 𝐴𝑛 the normalized amplitudes in the signal and noise windows, respectively. the start of the noise window was set 5 s before the detection and the end of the signal window 15 s after. then, the detections and their corresponding template were plotted and visually inspected to determine a satisfying modified snr cutoff (fig. a1). finally, only detections with a modified snr of at least 20 were considered for the sws analysis, leading to 4,908 additional events. for figure a1. plot of a template (top row, shaded traces) and corresponding detections (origin times shown at the left of each subplot), for the vertical (left column), north‑south (middle) and east‑west (right) channels at station hp.ltk. the modified snr is shown at the bottom-right corner of each east-west diagram. 3 each detection, we used the travel time obtained from the related template to estimate the p‑ and s‑ arrivals. the fixed (relocated) hypocenters of the templates are used as the focal parameters of the associated detections, as their relative location errors are expected to be much smaller. appendix b. shear-wave splitting analysis the parameterization used in our study is provided in table b1. the travel times for p and s waves were obtained from the catalog, with the exception of the automatically picked cases. the windowing scheme during the processing for splitting involves the automatic determination of the dominant period in the arriving shear-waves. the left end, before the arrival, of each candidate window is given by the ts – tp time and a fixed value, while the right side of the windows are based on the estimated shear‑wave period and a given value. trial frequencies for the filters were decided after visually inspecting the spectra of a sample of recordings. we preferred wide ranges to avoid oversimplifying the signals and risk cycle-skipping. parameter value maximum time-delay 350 ms minimum snr 1.5 minimum ts 0.1 s maximum ts 2.0 s maximum ts – tp 1.5 s period factor (ct) 3.0 startpoints of signal windows between (ts – tp)/2 and 0.1 s, before the shear-wave arrival (step of 0.05 s) endpoints of signal windows between 0.1 s and ts * ct after the shear-wave arrival ccritical 3.2 maximum number of clusters 10 minimum number of points per cluster 5 linkage criterion ward lower frequency filter corners (hz) 0.5, 0.8, 1.0 upper frequency filter corners (hz) 8.0, 10.0, 20.0 table b1. parameterization for the shear-wave splitting analysis. ts is the shear-wave period estimated by the software and tp and ts are the travel times for p and s waves, respectively. in fig. b1, an example of the processed event-station pairs is shown. the successful removal of anisotropy leads to a linearized particle motion diagram. 4 figure b1. shear-wave splitting measurement with the eigenvalue (ev) method, on a window selected with cluster analysis (see fig. b2). (a) initial recordings of the horizontal channels (top), rotated to an axial system defined by 𝜑 (f) and its perpendicular direction (s) after correcting the time-delay (middle) and corrected for anisotropy (bottom), (b) particle motion diagram before (top) and after (bottom) correcting for anisotropy and (c) contour plot of the second eigenvalue 𝜆2 dependent of each splitting parameter pair. basic information about the event-station pair is shown at the top. baz; the backazimuth, ain; the angle of incidence, epi; the epicentral distance (in km), mag; the magnitude, flt; the filter range used. selection of the window shown in fig. b1 was carried out by the cluster analysis [teanby et al., 2004] shown in fig. b2. the algorithm recognized sub‑clusters that would not be specified visually by the user. the parameters used for the automatically grading of the splitting results are presented in table b2. the null criterion refers to the difference between 𝜑 and the final polarization p obtained from the waveforms, after correcting for anisotropy. in other words, if the sfast polarization is either (sub)parallel or (sub)perpendicular to the corrected particle motion, we are unable to reliably assess anisotropy, therefore the measurement is considered as a null. grading scores are the upper threshold for each qualitative score. observations with a score higher than that of “d”, are marked as “e”. for more information about the grading algorithm and its modular nature, see spingos et al. [2020]. 5 parameter value maximum accepted 𝜑 error 10° maximum accepted td error 10.0 ms correlation coefficient in the fs system 0.60 correlation coefficient in the ne system 0.60 null criterion 10° or 0 ms maximum “a” score 0.25 maximum “b” score 0.50 maximum “c” score 0.75 maximum “d” score 1.00 table b2. grading parameters. figure b2. example of cluster analysis results, based on which the signal window used in fig. b1 was selected. (a) initial space defined by 𝜑 and td, (b) initial clustering, (c) selection of the optimal cluster, (d) variation of 𝜑 per trial signal window and (e) variation of td per signal window. 6 appendix c. null measurements a total of 40 observations were characterized as null, i.e. with 𝜑 being either sub-parallel or sub-perpendicular to the corrected shear-wave polarization p [wüstefeld and bokelmann, 2007]. we also checked for cases of null time-delays, but there were none in the dataset (even though pytheas tries for td = 0 ms when performing the grid search in the eigenvalue method). all observations were made at hp.ltk, with an average 𝜑 of n70°e and a standard deviation of 27°. distribution of 𝜑 shows significant scatter, with a preferential ne‑sw orientation and a modest e‑w concentration (fig. c1a). time-delays ranged between 70 and 350 ms (the latter being the boundary imposed by user criteria), with most being well over 100 ms (fig. c1b). the average td from null observations is 207 ms with a standard deviation of 73 ms. only 7 null measurements belong in the 2008-2019 era, with the majority (33) of them being determined by events belonging to 2020. figure c1. (a) rose diagrams for null measurements at hp.ltk. angles are binned every 5°. f indicates the interval of the grid. the black line shows the mean direction of 𝜑 at each station, as acquired from accepted measurements (graded a, b or c). (b) distribution of td obtained from null measurements at hp.ltk. on magnitude d e t e r m i n a t i o n by using macroseismic data (second paper) a . g . g a l a n o p o u l o s ( * ) r i c e v u t o il 27 n o v e m b r e 1 9 6 1 i n t h e first paper (galanopoulos, 1961), it was assumed t h a t t h e r e is a linear relation of t h e earthquake intensity to t h e acceleratimi a t t h e epicenter. on this assumption t h e magnitude formula: m = oi log i0r2 + c2 [1] was derived f r o m t h e empiri cai equation: ara„ r2 = ge [2] found b y gutenberg a n d ricliter (1942). the formula [1] was proved to be appropriate for m-determination from macroseismic d a t a . b y using t h e basic relation: i = p log a + q [3] applied in ali macroseismic computations, t h e magnitude formula: m = kx log r2 + ~ + k3 [4] -/t2 is derived f r o m t h e equation [2] on t h e sanie assumptions. the results obtained f r o m t h e equations [1] and [4] with t h e proper parameters derived from shocks in greece (n = 124) and from californian shocks (at = 36) b y the least-square method are presented in t h e table below. f o r reasons of comparison the results obtained from t h e corresponding equations set up by gutenberg and richter (1956) are given in t h e same table. d a t a used are tliose contained in the tables (4) and (5) of t h e first paper. notation is t h e same. (*) acknowledgment. t h e a u t l i o r is m u c h i n d e b t e d t o d r . v . k a r n i k f o r s u g g e s t i n g t h i s i n v e s t i g a t i o n . 4 0 2 a . g . g a l a n o p o u l o s the comparison shows t h a t there is no difference in t h e j f d e t e r m i nation if a linear or logarithmie relation of t h e intensity to t h e aeeeleration a t t h e epieenter is assumed. the same is t r u e if a linear or logarithmie relation of the e a r t h q u a k e m a g n i t u d e to the epicentral intensity is applied for determining t h e magnitude. t a b l e 1 d i f f e r e n c e s b e t w e e n m a c r o s e i s m i c m a g n i t u d e s c o m p u t e d f r o m t h e c o r r e s p o n d i n g e q u a t i o n s a n d i n s t r u m e n t a i . m a g n i t u d e s m* a s s i g n e d b y g u t e n b e r g a n d r l c i i t e r ( g r ) , m . b a t h ( b ) a n d v . k a r n i k ( k ) . m a g n i t u d e li n s s. e . s. d . m = 1.38 l o g i„ r 2 — 1.63 n, (n), > n 124 — 0 . 0 1 ± 0 . 0 4 ± 0 . 4 0 m = = 0 . 8 3 l o g r 2 + — + 0.14 n, (n), >n 124 — 0 . 0 2 ± 0 . 0 4 ± 0 . 4 2 3.62 n, (n), >n ± 0 . 0 4 m = 1.58 log r 2 — 1.38 11, (11), >11 124 — 0 . 0 1 ± 0 . 0 4 ± 0 . 4 8 m = 1.79 log i 0 r 2 — 3.97 n 36 — 0 . 0 2 ± 0 . 0 5 ± 0 . 2 8 m 1.02 log r 2 + i o 1.45 n 36 — 0 . 0 3 ± 0 . 0 5 ± 0 . 3 1 2.92 ± 0 . 0 5 m = 2.12 log r 2 — 3.98 n 36 — 0 . 0 3 ± 0 . 0 5 ± 0 . 3 2 m = — 3.0 + 3.8 l o g r ( g r ) n 36 — 0 . 0 5 ± 0 . 0 5 ± 0 . 2 9 m = 1 + f i » ( g r ) n 36 + 0.05 ± 0 . 0 8 ± 0 . 5 0 m = 0.7 i 0 + 0.7 n 36 + 0.01 ± 0 . 0 9 ± 0 . 5 2 m = 12.53 l o g t 0 — 4 . 8 9 n 36 + 0 . 0 3 ± 0 . 0 9 ± 0 . 5 7 li = d e p t h of f o c i of e a r t h q u a k e s u s e d ( n = n o r m a l , (n) = s l i g h t l y b e l o w n o r m a l , > n — d e e p ) ; n . = n u m b e r s of e a r t h q u a k e s u s e d ; = m e a n d i f f e r e n c e ; s. e . = s t a n d a r d e r r o r of t h e m e a n ; s. d . = s t a n d a r d d e v i a t i o n of a s i n g l e o b s e r v a t i o n . more i m p o r t a n t is t h e f a e t t h a t in either relationship t h e epicentral intensity in itself alone is r a t h e r unreliable for determining earthquake magnitudes. this is due to t h e f a c t t h a t t h e relation of t h e intensity to t h e aeeeleration a t t h e epieenter is neither logarithmie nor linear. moreover, in estimating t h e e a r t h q u a k e intensity t h e personal faotor of t h e field investigator is inevitably involved. on m a g n i t u d e d e t e r m i n a t i o n b y u s i n g m a c r o s e i s m i c d a t a 4 0 3 m m* ( g r . b . k ) m 1 . 3 8 l o g l 0 r 2 _ 1 6 3 •1 m * ( g r j 1 m * ( b ) m * ( k ) a oo o + + (sd & + + + * ©eoo -il-o * 04 o o 0 + -h-—©-0-* + +o oo o® + 0 4fo + oo o * + '2eoo + o + q a q a & o o o oo + • 1 . 3 8 l o g lo r> . 1 6 3 -10 0 8 -06 0 * -03 ioo * 0 2 » 0 4 * 0 6 * 0 8 «1.0 • 0 8 + 0.6 * 0 * • 0.2 1 0.0 -0.2 -0* -0.6 -0.8 1.0 o + m m ' ( g r ) m m * ( b ) m-m* (k) o o a o + o® o o ©0 a o + o o + a + + a o o o * + a _ f f a odo o o o o oo© o + o + + «a o©0 o a a a o o o o * * o o \ -12 -io -08 -q6 -0.« -02 1q0 »02 »0« *q6 *0.8 *1.0 m m * (gr.b.k) m = 1 5 8 l o g r 2 1 . 3 8 + m m * ( g r ) o m m ' ( b ) a m m * (k) o (jd® * o + a © o » + od o « w t * _ o—©—oo oo +•-+ •*•_+ ® 0 offi o 4 * -oo b o o o o ocjd 0 + ù. a aqu • a •• + o © a o + © * o o «. + m = 1 soiogr'.l 38 / oob f i g . 1 v a r i a t i o n s w i t l i m * of d i f f e r e n c e s b e t w e e n m a c r o s e i s m i c m a g u i t u d e s c o m p u t e d f r o m t h e c o r r e s p o n d i g e q u a t i o n s v a l i d f o r s h o c k s i n g r e e c e a n d i n s t r u m e n t a i m a g n i t u d e s m* ( g r , b , k ) . i n s e r t , f r e q u e n c y d i s t r i b u t i o n s of d i f f e r e n c e s b e t w e e n m a n d 31* ( g l i , b , k ) . 4 0 4 a . g . g a l a n o p o u l o s m m ' ( g r > i • o e + 0 6 + 04 • 0 . 2 t 0.0 : 0 2 0.4 0.0 0 6 m1.79logri„r2-3 97 + + + + + m . 1 7 9 l o g l 0 r > 3 9 7 > 0 6 0 4 0 2 1 0 0 '0.2 *0.4 . 0 6 .0.6 • 0 4 .0 2 0 . 4 -0.6 m 1 . 0 2 l o g r ' . j o _ _ v 4 5 2 9 2 + ++-+ 6 . 0 — » m " m = 1 0 2 l o q r i + j 2 t 4 s 2 . 9 2 0 8 0 6 0 4 -02 to.o *0.2 +04 + 0 6 j i 1 i i 1 1— 4.s 5x1 56 6 0 6 5 7.0 7.5 m » 2 . 1 2 l o g r 2 _ 3 . 9 8 20 m = 2.12 logr3-3.98 + + + io 44 .4+ + 5 44f 4t r t -+ + + -h4 4 44 + 4 + + 4 4 + 4+ i i i i -ofl -05 -0.4 + i -02 too *02 «-0.4 *0j6 +0b 4 5 5 0 5 5 6 0 6 5 7 0 7 5 « 0 8 5 — m ' f i g . 2 v a r i a t i o n s w i t h 31* of d i f f e r e n c e s b e t w e e n m a c r o s e i s m i c m a g n i t u d e s c o m p u t e d f r o m t h e c o r r e s p o n d i n g e q u a t i o n s v a l i d f o r c a l i f o r n i a s h o c k a a n d i n s t r u m e n t a i m a g n i t u d e s m * ( g r ) . i n s e r t , f r e q u e n c y d i s t r i b u t i o n s of d i f f e r e n c e s b e t w e e n m a n d 31* ( g r ) . o n m a g n i t u d e d e t e r m i n a t i o n b y u s i n g m a c r o s e i s m i c d a t a 4 0 5 on t h e other hand, t h e relationsliip of the m a g n i t u d e to the radius of perceptibility appears to be most reliable for the determination of magnitudes with t h e same focal depth. the determination of the radius m-m'(gr) t « 0 . 2 ± 0 . 0 0 . 2 0 . 4 0 . 6 o a m » 3 0 + 3.8 logr m-_3.o +3.8 logp •#• + « ffl j |_ i 1 1 l . 0 8 q 6 0 4 0 2 1 0 0 +02 + 0 * 0 6 0 6 -j l« 6 6 0 6.6 6.0 6.5 7.0 7.6 6 0 8 5 — » m " m—m" ( gr ) f m « 1 + 2 1 0 -i? -io -oa -06 -0.4 -02 100 -0.2 *0.4 »0.6 *0.a -1.0 -12 f i g . 3 v a r i a t i o n s w i t h ì l i * of d i f f e r e n c e s b e t w e e n m a c r o s e i s m i c m a g n i t u d e s c o m p u t e d f r o m t h e c o r r e s p o n d i n g e q u a t i o n s v a l i d f o r c a l i f o r n i a s h o c k s a n d i n s t r u m e n t a i m a g n i t u d e s m * ( g r ) . i n s e r t , f r e q u e n c y d i s t r i b u t i o n s of d i f f e r e n c e s b e t w e e n m a n d m* ( g r ) . of perceptibility is not influenced appreciably by personal factors, and t h e errors involved are negligeable. variations in differences between macroseismic and instrumentai magnitudes and percentage frequency distributions of t h e various ma4 0 6 a . g . g a l a n o p o u l o s gnitude differences versus instrumentai magnitude, m*. presented in figures 1 to 4, show clearly the relative superiority of the m m ' ( s r ) t * 0 . 4 + 0 . 2 1 0 . 0 0 2 0 4 m o . 7 i 0 + o.7 m 0.7 lo +0.7 . ì l l l l e i h j ^ l 1 . 4 1 2 1 0 -os 0 . 6 0 4 q 2 ±00 + 0 2 + 0 4 + 0 . 6 + 0 . 8 1 . 0 +12 _ j 1 i i i m 12.53log i 0 4 8 9 + -+ + 4+ m = 12 53logl04.89 . e 3 1 6 1 4 . 1 2 1.0 o s 0 . 6 0 4 0 2 too '02 0 . 4 0 . 6 + 0 . 8 + 1 . 0 4 . 5 5 . 0 5 5 6 . 0 6 5 7 . 0 7.5 8 0 8.5 — » m * f i g . 4 v a r i a t i o n s w i t h m * of d i f f e r e n c e s b e t w e e n m a o r o s e i s m i c m a g n i t u d e s c o m p u t e d f r o m t h e c o r r e s p o n d i n g e q u a t i o n s v a l i d f o r c a l i f o r n i a s h o c k s a n d i n s t r u m e n t a i m a g n i t u d e s m* ( g r ) . i n s e r t , f r e q u e n c y d i s t r i b u t i o n s of d i f f e r e n c e s b e t w e e n m a n d m* ( g r ) . formula [1] for jlf-determmation of earthquakes originating a t any focal depth. the differences in coeffìcients c and k for the two sets of data are due primarily to t h e difference in r used. i t was stated in the first on m a g n i t u d e d e t e r m i n a t i o n b y u s i n g m a c r o s e i s m i c d a t a 4 0 7 paper t h a t r denotes t h e m a x i m u m radius of perceptibility for t h e shocks in greece and t h e mean radius for t h e california shocks. owing to t h e existing differences in t h e building-civilization in t h e two countries, t h e same picture of damages does not correspond to the same earthquakeforce. as another reason we might mention t h e f a c t t h a t t h e california d a t a are generally the result of a more detailed investigation t h a n in the case of d a t a for shocks occurred in greece. most of t h e epicenters of shocks in greece are offshore and the p a r t of t h e shaken area corresponding to t h e open sea is unknown. ali t h e california shocks are of nearly the same focal depth. the list of shocks in greece includes shocks of normal and intermediate focal depth. the difference in number of observations and in t h e range of magnitudes must also be allowed for the differences in coefficients g and k for the two sets of d a t a . su m mary this is a supplementary to paper 1 (galanopoulos, 1961). it is shown that tliere is no difference in the m-determination if a linear or logarithmic relation of the intensity to the acceleration at the epicenter is assumed, and that the relation of the earthquake magnitude to the radius of perceptibility is much more appropriate for the m-determination of earthquakes with the same focal depth than that of the earthquake magnitude to the epicentral intensity used tlius far. z usammenfass ung es wird gezeigt, dass keine merkliche differenz in der magnitudenbestimmung existiert, falls eine lineare oder logarithmische beziehung zwischen der intensitàt und der beschleunigung angenommen wird, und dass die beziehung der bebenmagnitude zu der makroseismischen reichweite viel melir geeignet ist fior die magnitudenbestimmung von erdbeben derselben herdtiefe als diejenige zwischen der bebenmagnitude und der gróssten stàrke im scjiiittergebiet, die bisher dafilr benutzt wird. riassunto questa nota fa seguito alla prima pubblicata nella rivista « annali di geofìsica » voi. xiv, fase. 3 (anno 1961). 4 0 8 a. g. g a l a n o p o u l o s l'autore dimostra che nella determinazione della magnitudo dei terremoti, non esiste alcuna differenza se, anziché usare una relazione lineare fra intensità e accelerazione all' epicentro, ci si serve di una relazione logaritmica. e inóltre molto più appropriato, sempre per la determinazione della magnitudo di terremoti con la stessa profondità ipocentrale, servirsi della relazione fra la magnitudo del terremoto ed il raggio di percettibilità macrosismica dello stesso, che usare la relazione fra magnitudo del terremoto e la sua massima intensità epicentrale, finora applicata. r e f e r e n c e s g a l a n o p o u l o s , a . , on magnitude determination by using macroseismic data, i n " a n n . d i g e o f . " , 1 4 , 3 , ( 1 9 6 1 ) . g u t e n b e r g b . r i c h t e r c . , earthquake magnitude, intensity energy and aeeeleration, i n " b u l l . s e i s m . s o c . a m . 3 2 , 3 , 1 6 3 1 9 1 , ( 1 9 4 2 ) . — — earthquake magnitude, intensity, energy, and aeeeleration, i n " b u l l . s e i s m . s o c . a m . " , 4 6 , 2 , 1 0 5 1 4 5 , ( 1 9 5 6 ) . untitled 119 annals of geophysics, vol. 51, n. 1, february 2008 key words campi flegrei – deformation – gravity – finite element method – heterogeneous medium 1. introduction volcanic activity produces deformation and gravity changes that may be used as precursors of future eruptions. monitoring of active areas, together with interpretation of these changes, may reveal the physics and characteristics of the magma reservoir. indeed, the detection of ground deformation will constrain the location and type of deep reservoirs, while gravity monitoring is recognized as a valuable tool for mapping subsurface density distributions. both of them contribute to quantify the change in subsurface mass. several models have been developed to interpret geodetic and gravity signals in active volcanic areas. walsh and rice (1979) developed a method to calculate the gravity changes as due to subsurface mass redistribution with respect to the gravimeters at surface. the calculation is suitable for any dislocation source in the half-space; however the stress change in a dislocation-free half-space must be known. bonafede and mazzanti (1998) suggested the separation of density variations affecting the computation of gravity changes into 3 different contributions: the first contribution comes from the input of new mass from remote distance, the remaining two from the linearized continuity equation of the material already present in the region. fernandez and rundle gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy elisa trasatti (1) and maurizio bonafede (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) università degli studi di bologna, bologna, italy abstract employing a 3d finite element method, we develop an algorithm to calculate gravity changes due to pressurized sources of any shape in elastic and inelastic heterogeneous media. we consider different source models, such as sphere, spheroid and sill, dilating in elastic media (homogeneous and heterogeneous) and in elasto-plastic media. the models are oriented to reproduce the gravity changes and the surface deformation observed at campi flegrei caldera (italy), during the 1982-1984 unrest episode. the source shape and the characteristics of the medium have great influence on the calculated gravity changes, leading to very different values for the source densities. indeed, the gravity residual strongly depends upon the shape of the source. non negligible contributions also come from density and rigidity heterogeneities within the medium. furthermore, if the caldera is elasto-plastic, the resulting gravity changes exhibit a pattern similar to that provided by a low effective rigidity. even if the variation of the source volumes is quite similar for most of the models considered, the density inferred for the source ranges from ∼400 kg/m3 (super critical water) to ∼3300 kg/m3 (higher than trachytic basalts), with drastically different implications for risk assessment. mailing address: dr. elisa trasatti,istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: elisa.trasatti@ingv.it vol51,1,2008_delnegro 5-02-2009 10:28 pagina 119 120 e. trasatti and m. bonafede (1994) developed a method to compute gravity variations due to point magma intrusion in a horizontally layered elastic-gravitational media. the solutions were extended and generalized to point-sources in multi-layered viscoelastic-gravitational media (fernandez et al., 2001). a comprehensive tool for evaluating deformation and gravity changes due to dislocation in horizontally layered viscoelastic media was also provided recently by wang et al. (2006). the approaches described above are only partially able to simulate the complex geology of volcanic regions, since they model pointsources in horizontally layered media. furthermore, biases in estimate of source density may arise from oversimplified models. trasatti et al. (2005) developed finite element deformation models taking into account the structural and rheological characteristics of campi flegrei. the introduction of rigidity layering and plastic rheology in the local crust causes a shrinking of the deformation shape and enhances the maximum values within the caldera. this behavior allows us to estimate a source depth of 5 km (plastic medium) instead of 3 km (elastic medium), adopting the same overpressure and in accordance with information from groundwater studies and petrological data. recently, currenti et al. (2007) calculated the gravity changes in fe models characterized by an ellipsoidal source expanding in a heterogeneous medium with real topography of mt. etna. they found that gravity estimates may be biased in terms of mass gain/loss if medium complexities are neglected. in the present paper we attempt to model both the deformation and the gravity. we develop a technique to calculate gravity variations due to deep pressurized sources in heterogeneous media. the method consists in two separated steps: i) the displacement and strain fields due to the source are computed by fe modelfig. 1. map of campi flegrei caldera showing the outer rim (open triangles) and the inner caldera (full triangles), the leveling lines (dotted) were surveyed during 1982-1984 unrest episode, while the squares represents the gravity stations active in that period. vol51,1,2008_delnegro 5-02-2009 10:28 pagina 120 121 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy ling; ii) the results are employed in a procedure to integrate numerically the gravity variations from the displacements and strain fields. the great advantage of this technique is that the fe models may include several complexities: arbitrary source shape, lateral heterogeneities of the medium, non linear rheological properties and so on. we reproduce known solutions to test the method and then we furnish new calculations of models which cannot be solved analytically. we consider spherical and non spherical (sill and spheroid) sources. furthermore, different medium properties are included, such as density contrasts, elastic inhomogeneities and rheological variations. the models are discussed as a preliminary approach to reproduce the gravity changes observed at campi flegrei during the 1982-1984 unrest episode. 2. the campi flegrei caldera the campi flegrei caldera, hereafter cf, near naples (italy), is a complex volcanic system with several, mostly monogenic, explosive vents (fig. 1) in a densely populated area of some 80 km2. the cf is characterized by an outer caldera rim, about 12 km in diameter (open triangles in fig. 1) and an inner caldera of 6-8 km in diameter (full triangles). slow and remarkable ground movements are typical of this area, as recorded since roman times. in 19701972 and 1982-1984, a cumulative uplift of about 3.5 m was observed, followed by changes in the shallow hydrothermal system (chiodini et al., 2003) and an increased rate of shallow seismicity (see de natale et al., 1991, for an overview). following january 1985, the ground began to subside at a much slower rate than during the uplift. the deformation pattern, both during the uplift and the subsidence, exhibits a dominant axial symmetry, concentrated in the inner caldera, with the maximum uplift located at the caldera center (the city of pozzuoli). gravity measurements at cf have been carried out since 1981 and the largest gravity variations are also observed at pozzuoli. the gravity changes are well correlated with the elevations changes (stars) as reported in fig. 2a both during the 1982-1984 uplift and the 1985-91 deflafig. 2a-c. a) gravity variations and elevation change observed during 1981-2001 at serapeo roman market (pozzuoli), at the center of cf; b) gravity due to the ground uplift (free-air correction); c) residual gravity due to the difference between the observed and the free-air corrected gravity. tion phase. in particular, during the unrest phase, at serapeo (a roman market in the center of pozzuoli) the gravity change normalized to the uplift was −216±7 µgal/m, in good agreement with the average of all the stations −213±6 µgal/m. this value is comparable to those measured during inflation episodes at long valley and rabaul calderas. however, during the deflation phase, a ratio between gravity change and uplift of −224±24 µgal/m (similar to that computed during uplift) was observed only at serapeo (the gravity benchmark closest to the point of maximum uplift), while more vol51,1,2008_delnegro 5-02-2009 10:28 pagina 121 122 e. trasatti and m. bonafede scattered ratios were inferred at other stations (e.g., solfatara and la pietra), possibly due to local effects (berrino, 1994). the direct measurement of the free-air vertical gradient at cf is γ=−290±5 µgal/m (berrino et al., 1984), about 20 µgal/m smaller (in absolute value) than the reference value (fig. 2b). the residual gravity ∆gr (the difference between the observed ∆g and the free-air gravity ∆gfa) increased with the uplift by 75±12 µgal/m during 1982-1984 (fig. 2c). the 1982-1984 crises were interpreted as due to an increase in pressure of a deep magma chamber, while geochemical evidence suggested thermodynamic disturbances of shallow aquifers as a response to increased heat flow from below (bullettin volcanologique, 1984). the bell shape of the vertical surface displacement suggests modelling the data by means of mogi isotropic pressure sources. gottsmann et al. (2006) presented inversions of deformation data adopting different source geometries: point-source, sill and spheroidal cavity. all the sources were located at about 3 km below sea level, and they suggested that the spheroidal source may depict an envelope around a hybrid of both magmatic and hydrothermal sources. battaglia et al. (2006) performed joint inversions of gravity and deformation data, finding that a sill-like source is preferred to interpret the inflation phase, while a shallower spheroid source is more suitable for the deflation phase. the works mentioned above assume the medium to be elastic and homogeneous. these approximations only provide limited insights into subsurface dynamics at areas such as cf, where intense faulting and inelastic deformations are likely to take place. 3. method of gravity calculation by fe 3.1. gravity contributions in general, the total gravity change observed at a benchmark can be separated into the term depending on the elevation change (removed by free-air correction) and the term due to mass redistribution: (3.1)g gg fa r∆ ∆∆ += where ∆gfa=γuz (γ is the free-air gradient and uz is the vertical displacement of the benchmark). as suggested by many authors, the data is corrected using the measured free-air vertical gradient (e.g., berrino, 1994). the residual gravity change is given by (3.2) where g is the gravitational constant, v is the volume over which the density variations ∆ρ do not vanish, rr is the vector between dv and the observation point and θ is the angle between rr and the vertical (fig. 3). according to sasai (1986) and bonafede and mazzanti (1998), ∆ρ in eq. (3.2) accounts for three contributions (3.3) where δρs is the density change related to the introduction of new mass from remote distance, ρ is the material density in the reference configuration, εij is the strain tensor and u is the displacement field. the first term accounts for the new mass intruded; the second term is the relative density change arising from the compressibility of the medium; the third one is due to density variations within the medium. considering the three density changes indicated in eq. (3.3), ∆gr can be written as (fig. 3): (3.4) where ∆gs depends on δρs (vanishing for a massless source) and ∆gm is the contribution due to deformation of the medium surrounding the source. the term ∆gv is expressed by (3.5) and depends on the finite compressibility of rocks; it vanishes if the medium is incompressible. the ∆gl term depends on u⋅∇ρ and it accounts for the bouguer correction (expressing the contribution of the lifted mass above the free surface) and the source inflation/deflation (from the displacement of source boundaries). furthermore, it accounts for the displacement cos g g r dvkk v v 2ρε θ ∆ = − # withg g g g g gr s m m v l∆ ∆ ∆ ∆ ∆ ∆= + = + us kk $4ρ δρ ρε ρ∆ = − − cos g g r dvr v 2ρ θ ∆ ∆= # vol51,1,2008_delnegro 5-02-2009 10:29 pagina 122 123 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy of volumes with different density if the medium is not homogeneous. if ρ varies continuously, ∆gl is given by (3.6) if ρ is discontinuous across a surface s within v, the exceeding mass depends on the displacement of the boundary between densities ρin and ρout as (ρout−ρin) u⋅ds. note that the scalar product between the orientation of the density interface ds and its displacement u accounts for contributions of any orientation of the surface. therefore, the contribution due to the density discontinuities can be expressed as (3.7) where s is the surface over which the density changes. it is important to note that ∆gl may include the contribution of the new mass within the source, if the density gradient is chosen properly. however, we separate the effect of the ( ) cos g g r du sout inl s 2 $ρ ρ θ ∆ = − # cos g g r dvu v l 2$4ρ θ ∆ = # displacement of the source boundary (within ∆gl term) from the effect of the new mass (∆gs) because the density contrast between inside/ outside the source is unknown, being one of the main target of gravity modelling. 3.2. numerical integration of gravity within fe models the subdivision in single contributions of eq. (3.4) is a suitable method to compute gravity changes from fe models of deformation. we consider massless cavities since the source term ∆gs is treated separately in section 5. the gravity changes are calculated in two separate steps: 1) fe calculation of displacement u and strain εij fields in the medium, due to the source inflation; 2) computation of gravity terms ∆gl and ∆gv from displacements and strains obtained at 1). since the gravity terms ∆gl and ∆gv are integrals over the total volume, they are computed as a sum of contributions within each finite element: (3.8) where n is the total number of elements, vi is the volume of the i-th element of the grid and f is a generic integrand function. the gravity term ∆gv expressed in eq. (3.5) can be easily evaluated within the fe domain as (3.9) where ρi is the density of the i-th element. the term ∆gl due to the density variation is complicated by the integration over the faces of elements. indeed, for each face of the i-th brick element we must consider the density variations within the adjacent element. the term ∆gl from eq. (3.7) is expressed by (3.10) where the surface integral is performed over each j-th face of the i-th element sij and ρin is the density of the element considered, while ρout is ( ) cos g r du sout inl ji n s 1 6 1 2 ij $ρ ρ θ ∆ = == // # cos g g r dvv i i n kk v 1 2 i ρ ε θ ∆ = = / # ( , , ) ( , , )f x y z dv f x y z dv vi i n v 1 = = / ## fig. 3. schematic representation of the gravity contributions: ∆gfa is the free-air change proportional to the uplift; ∆gl is the change caused by the lifted portion of the ground (bouguer anomaly) and of interfaces between density layers; ∆gv is the gravity field due to the medium compressibility; ∆gs is the contribution of the material filling the expanding part of the source. in the right hand side of the figure is indicated the geometry of the gravity integration of elementary volume dv, its displacement u, the observation point p, the vector r between dv and p, and the angle θ between them. vol51,1,2008_delnegro 5-02-2009 10:29 pagina 123 124 e. trasatti and m. bonafede the density of the adjacent element sharing the face sij. note that this contribution must be evaluated only once. for the elements forming the free surface, this contribution is the bouguer correction (since the faces lying on the free surface have confining density equal to 0). if a density contrast is present within two or more elements of the medium, ∆gl accounts for this contribution. the great advantage of this integration is that non planar interfaces between layers can be considered. each element may have arbitrarily shape and may be characterized by different density, elastic parameters or rheology independently from the rest of the model. the integrals over single elements are evaluated numerically using the gauss quadrature formula. we give a brief overview of this technique: 1) the gauss integration is computed by mapping each arbitrarily distorted brick element, the (x, y, z) space, to a trilinear hexahedral element of side 2l, the (xl, yl, zl) space. the transformation applies to local cells only (fig. 4a). the volume integration for an element becomes (3.11) where j is the jacobian determinant. 2) the volume integral is evaluated as the sum of the integrand function calculated at gauss points. we adopt the two-points integration (for each dimension), which requires to approximate the integrand function at 8 gauss points: .( , , ) , ,x y z l l l3 3 3g g g ! ! !=l l l _ i ( , , ),y x y z f( , , ) ( , , ), ( , , ) f x y z dv x x y z z x y z v l l l l l l = −−− ( , , )j x y z dv l l l l l l l l l l l l l 6 @ # ### fig. 4a,b. a) the gauss integration is computed by mapping each arbitrarily distorted brick element (x-space) to a trilinear hexahedral element (xl-space). the two-points gauss quadrature is performed by calculating the integrand function at the 8 gauss points indicated by «+» symbol. b) the surface integral within element faces is performed by mapping x into xl. the vectors ds1 and ds2 are defined to be tangent to the surface, defining the unit area shaded (see text for details). vol51,1,2008_delnegro 5-02-2009 10:29 pagina 124 125 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy the volume integral becomes the sum of the following values: (3.12) since the gauss integration is the technique adopted by fe to calculate many derived fields such as strain and stress, these values are directly available from our code (marc, 1994, version 2005) at gauss points without any further approximations. the term ∆gv in eq. (3.9) can be computed from eq. (3.12). however, computation of ∆gl in eq. (3.10) involves surface integrals over each element face. following the development outlined in greenberg (1978), each face is locally transformed in space (xl, yl, zl), defining the vectors ds1 and ds2 tangent to the plane considered. in the example shown in fig. 4b, the face 4-3-7-8 is mapped into the xl-constant plane. the vector ds1 is defined to be along zl-constant curve and ds2 is along yl-constant curve. the elemental area vector ds, denoted by the shaded parallelogram, can be computed as the vector product between ds2 and ds1 (positive pointing out of the cell). the surface integral becomes (13) and it is computed by applying the two-points gauss rule in two-dimensions. indeed, 4 gauss points are determined over the mapped plane, e.g., , and the integrand function is approximated at these points following the procedure outlined above. 4. cases of study we develop an fe model characterized by about 75000 8-nodes brick elements, extending 160×160×80 km3 (fig. 5). the domain is large enough to avoid bias from boundaries over which displacement and stress fields are imposed to vanish. furthermore, the bottom boundary is kept fixed. the grid is characterized by a flat interface at 3 km depth which may be used to represent a shallow layer, and a , ,l l l3 3! !_ i ( )u s u s sf d f d d1 2 s s $ $ #=# # f ( , , ) ( , , )x y z f x y z l l l l l l g g g 1 8 = --dvl l l l l l l/### cylindrical boundary of radius 3 km and depth 3 km lying at the center of the model, to approximate the cf caldera. the element size is about 350 m side in the central part of the grid, while it increases with distance up to 10 km close to the domain boundaries. we show examples of gravity computations for pressurized sources of various shapes in homogeneous/inhomogeneous media. in all the cases shown in this section, the sources are considered empty so that only ∆gm is computed. a brief outline of the main characteristics of the models are reported in table i. unless changes are due to specific model configurations, the medium is considered homogeneous, elastic and isotropic with rigidity µ0=1 gpa and poisson ratio ν= =0.25. all the sources are placed at 5 km depth, at the center of the model, co-axial with the caldera and undergo to overpressure ∆p=50 mpa. when the density is homogeneous, its value is ρ0=2500 kg/m3. the models named mogi-n are characterized by a spherical source of radius a=1 km; in particular the case with homogeneous and elastic medium is mogi-1. density contrasts are considered in models mogi-2 and mogi-3, characterized by fig. 5. perspective view of the fe model. the caldera is a cylinder of radius 3 km and height 3 km, surrounded by a layer of similar thickness on top of the halfspace. the mogi source is shown at a depth of 5 km, with a radius of 1 km. vol51,1,2008_delnegro 5-02-2009 10:29 pagina 125 126 e. trasatti and m. bonafede a density equal to ρ1=1800 kg/m3 within the shallow layer and the caldera (respectively); the density of the remaining medium is ρ0. in model mogi-4 the caldera has heterogeneous rigidity with respect to the remaining medium µ1= =0.1µ0, while the density ρ0 is constant anywhere. model mogi-5 contains both the last characteristics: the caldera has heterogeneous rigidity µ1 and density ρ1, while the rest of the medium has rigidity µ0 and density ρ0. we also consider two inelastic models, assuming a plastic rheology of the medium to describe the highly fractured rocks of the cf area. the medium behaves plastically when the maxitable i. list of models of which we calculate the gravity variations. the overpressure within the sources is ∆p=50 mpa and all the sources are placed at depth d = 5 km. the layer has horizontal interface and a height of 3 km. the caldera has a cylindrical shape of 3 km radius and 3 km height; it lays at the center of the model, coaxial with the sources. the parameters are explained in the text. model source medium characteristics parameters mogi-1 sphere homogeneous and elastic µ0 = 1 gpa; ρ0 = 2500 kg/m3; a = 1 km mogi-2 ” heterogeneous density layer ρ1 = 1800 kg/m3; ρ0 = 2500 kg/m3 mogi-3 ” heterogeneous density caldera ” mogi-4 ” heterogeneous rigidity caldera µ1 = 0.1 µ0 mogi-5 ” heterog. rigidity and density cal. µ1 =0.1 µ0; ρ1 =1800kg/m3; ρ0=2500kg/m3 mogi-6 ” all plastic σy = 30 mpa mogi-7 ” plastic caldera σy = 0.5 mpa spheroid spheroid homogeneous and elastic vsphere = vspheroid; a=1850 m; b=c=0.4a sill sill ” l = 2 km fig. 6. contributions and medium gravity change due to an inflating spherical pressure source in a homogeneous and elastic half-space. comparison between analytical solutions (solid line) and numerical (squares). the contribution due to the inflation of a massless sphere ∆gm = ∆gl + ∆gv is null. vol51,1,2008_delnegro 5-02-2009 10:29 pagina 126 127 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy mum shear stress is greater than a specified yield stress. the plastic rheology is only treatable by means of numerical methods, and easily implementable in fe models. in model mogi-6 the whole medium behaves plastically with yield stress σy=30 mpa, while mogi-7 is characterized by plastic caldera (σy=0.5 mpa) and remaining elastic medium, to evidence the highly deformable inner caldera. two further models are considered, with different shape of the source: spheroid and sill. the spheroid has the same volume of the sphere and aspect ratio equal to 0.4 with resulting semi-major axis a = 1850 m and semi-minor axes b = c = 740 m; it is vertically elongated. the sill is a horizontal square crack of side l = 2 km; both sources act in a homogeneous and elastic medium, to consider the effects of the different shape only. figure 6 reports the gravity patterns from the source axis to 10 km distance along the x-axis; the solid line is the analytical solution while the squares are the results from numerical integration. the figure shows that the gravity change ∆gm due to a mogi source in a homogeneous and elastic half-space is null and the only contribution observable at surface would be the free-air effect. this agrees with analytical results by walsh and rice (1979), sasai (1986), bonafede and mazzanti (1998). our numerical «zero» is 0.5 µgal, well below instrumental error, confirming the robustness of the gauss integration within the fe mesh and the good approximation of the half-space by the numerical domain. different results are obtained for models spheroid and sill, dilating in a homogeneous medium (fig. 7a,b). we report the computed vertical displacement (the thin solid line) which is representative of ∆gfa. from now on the gravity changes will be shown normalized to the uplift above the source center (coinciding with the maximum up lift for sphere and sill models). in the case of the spheroid, the gravity variation ∆gm is negative (fig. 7a, thick solid line). therefore, if the gravity signal generated by a spheroid is modeled by a spherical source, it may lead to an overestimation of the density within the source. this result is also confirmed by battaglia and segall (2004) that performed inversions of geodetic and gravity data generated by a spheroid, by means of a spherical source. they found that the isotropic point-source approximation leads to an overestimation of depth, mass and density of the intrusion. fig. 7b shows the opposite effect due to the sill: the total contribution is positive, without new mass input. note that in this case the contribution ∆gv (dotted line) is almost null. this behavior arises from the strain field generated by the sill in the half-space, causing contraction above it and lateral dilatation. we compare these results with the analytical ones provided by sasai (1986), finding good agreement (g. currenti personal communicafig. 7a,b. gravity terms normalized to the maximum uplift uzmax, and vertical displacement in a homogeneous medium due to (a) vertically elongated spheroidal source, (b) horizontal sill (in this case ∆gv 0 and ∆gm overlaps ∆gl). vol51,1,2008_delnegro 5-02-2009 10:29 pagina 127 128 e. trasatti and m. bonafede tion). in a recent paper battaglia et al. (2006) implemented the analytical solutions of the spheroid (yang et al., 1988) within the gravity calculation method developed by walsh and rice (1979). they found that the gravity change due to the medium deformation is negligible for a spheroidal cavity while it is noticeable for a sill. good agreement is found from the comparison between our and their results for the sill. instead, our results for the spheroidal cavity is not in agreement with their findings, as shown in fig. 7a. however, since comparisons between our models and the analytical solutions (when available, like models mogi-1, mogi-2 and sill) show very good agreement, and since the method of gravity computation shown here is independent on the source shape and properties of the medium, we are confident with the robustness of the present results. figure 8a,b reports the results for two models characterized respectively by density contrast of 30% at 3 km depth (mogi-2) and within the fig. 8a,b. gravity terms normalized to the maximum uplift uzmax, and vertical displacement due to an inflating spherical pressure source in a medium characterized by density contrasts of 30% in: a) shallow layer of height 3 km; b) cylindrical caldera. fig. 9a,b. gravity terms normalized to the maximum uplift uzmax, and vertical displacement due to an inflating spherical pressure source in a medium characterized by: a) heterogeneous caldera µ1/µ0 = 0.1; b) caldera with heterogeneous rigidity µ1 and density ρ1. vol51,1,2008_delnegro 5-02-2009 10:29 pagina 128 129 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy caldera rim (mogi-3). in the first case (fig. 8a), solutions are in agreement with those by bonafede and mazzanti (1998) (not shown here). a larger effect is visible when the low density is restricted to the caldera (fig. 8b). in this case, ∆gl accounts for the density discontinuity at the circular boundary of the cylinder also. the density jump reflects into the step of the ∆gl pattern, while the volumetric and free-air (∝ uz) terms are continuous. indeed, if we compare the two models, we find a similar pattern of ∆gv because the density contrast has little effect in the strain field, while ∆gl is more sensitive to this discontinuity. these models show that this kind of inhomogeneities, even if very localized (like within the caldera), must be taken into account to perform gravity modelling. results similar to mogi-2 were obtained by battaglia and segall (2004) for a massless cavity in a layered medium. they found a small (but different from 0) contribution to the residual gravity due to a density contrast of 13% at long valley caldera. the contribution of heterogeneous rigidities within the medium is emphasized in model mogi-4. indeed, as long as the rigidity ratios are small, the contribution to ∆gm/uz is of the order of some µgal/m. in fig. 9a the results is shown for an extreme ratio of µ1/µ0=0.1 between caldera and remaining medium. the low rigidity of the caldera amplifies the displacement field and hence the terms linked to this parameter (∆gfa and ∆gl), while the volumetric term ∆gv is almost unchanged from the homogeneous case in fig. 6. a very low effective rigidity could be suitable to describe longterm deformation for a standard linear solid rheology; this rheology may apply to volcanic areas such as cf, due to the high temperature, erupted products, incoherent materials and hydrothermal activity. a further model, mogi-5, accounts for both the characteristics of mogi-4 and mogi-3, having the caldera with low rigidity and low density (fig. 9b). since the displacement generated from this model is equal to mogi-4, ∆gv is very similar to that of panel (a). the density contrast between the caldera and the remaining medium causes a lower ∆gl and a small step in its pattern. we also consider two cases of inelastic medium, described by plastic rheology. in the case of uniform yield stress within the medium, the deformation is approximately radial around the spherical source and the total effect is null (fig. 10a), similarly to the elastic homogeneous model. the sill is also modelled (not shown here) in a medium homogeneously plastic, finding that ∆gm/uz is unchanged with respect to the elastic case, as observed for the spherical source. indeed, the plastic medium contributes by amplifying both uz and the gravity terms, leaving the ratio between them unchanged. in the case of model mogi-7, a rheologic discontinuity is present, since the caldera is plastic while the rest of the medium is elastic. it is evident from fig. 10b that when the yield stress is not uniform, the gravity change ∆gm is different from 0. indeed, the total contribution is positive within the caldera while it becomes negative out of the caldera rim, vanishing at 7-8 km from the center of the model. this model is useful to describe localized rheology discontinuities, likely to be present in volcanic areas. it is interesting to note that the gravity terms are very similar to those shown in fig. 9a, indicating that both the low rigidity and the plastic rheology act to enhance the deformation within the caldera, allowing for larger ∆gl terms while ∆gv is similar to the elastic value. indeed, ∆gl is sensitive to the deviatoric strain (mostly plastic) while ∆gv is sensitive only to the isotropic strain (elastic). this kind of behavior was also figured out by trasatti et al. (2005), showing that the elasto-plastic rheology induces larger deformations within the caldera. 5. contribution of the inflating source the previous section considered the reservoirs massless, i.e. cavities within which the overpressure ∆p is assigned. whether the expansion of cavities is due to new mass input, internal processes or pore pressure and temperature migration, the gravity change due to the source at a benchmark can be computed as (5.1) cos cos g g r dv r dv s s v s v 2 2 s s ρ θ ρ θ ∆ = -l l l l l = g # # vol51,1,2008_delnegro 5-02-2009 10:29 pagina 129 130 e. trasatti and m. bonafede where vls, vs are the source volumes after and before the deformation, respectively, and all primed variables refer to the deformed configuration. we assume that the main process determining the ∆gs is the new mass entering into the reservoir. this process has several consequences: volume variation, density growth due the increased internal pressure and the displacement of the center of mass of the reservoir due to the asymmetric deformation of the medium in proximity of the free surface. for simplicity, we assume ρs, ρls to be mean internal densities, and we approximate the mass at the source center (due to the source depth, assumed as 5 km, much larger than the extension). the mass change is ∆m=ρ∆v +v∆ρ and the density after deformation is expressed by fig. 10a,b. gravity terms normalized to the maximum uplift uzmax, and vertical displacement in an inelastic medium: a) all plastic, b) plastic caldera and remaining elastic. fig. 11. density calculations versus volume variations of the proposed models. vol51,1,2008_delnegro 5-02-2009 10:29 pagina 130 131 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy ρls=(m + ∆m)/(v + ∆m), at the first order. it results that (5.2) the ratio ∆ρ/ρs=∆p/k∼10−3, where ∆p is the overpressure and k is the isothermal incompressibility, so that ρls ∼ ρs and the main effect on the gravity change is the volume increase of the magma reservoir (franchini, 2005). in conclusion, the resulting ∆gs is expressed by (5.3) the volume variation of a cavity with arbitrary shape is obtained by integrating the normal displacement over the boundary (5.4) within the fe grid the integral is performed adopting the procedure outlined for the gravity term ∆gl (see eqs. (3.7) and (3.10)), using eq. (3.13). the observed gravity change at cf is 75±12 µgal/m, which must be equal to ∆gr= =∆gs + ∆gm. from eq. (5.3) and the values of ∆gm computed for the sources in table i we obtain the source densities ρs. volume variations and density estimates are reported in fig. 11. the density value calculated v du s s $∆ = # cos g g v r s s 2ρ θ ∆ ∆= 1 v m v v m v v v v v 1s s s s + +ρ ρ ρ ρ ρ ρ ∆ ∆ ∆ ∆ ∆ ∆ − − + + + = + +l b bl l for the sphere in the homogeneous medium (mogi-1) is very similar to that usually found in literature for magmatic sources. indeed, berrino et al. (1984) and berrino (1994) estimated 2500 kg/m3, suggesting the hypothesis of silicate melts entering a reservoir rather than hydrothermal fluids within a confined aquifer. all the models (except mogi-6) show similar volume variation, while the densities inferred range from ∼400 to ∼3300 kg/m3. the contribution of the medium, when homogeneously plastic (mogi-6), is null and the inferred density is comparable to the elastic models, ∼2000 kg/m3. however, the volume variation is very large, due to the enhanced deformation allowed in the medium. the density inferred in models characterized by density layering (mogi-2 and mogi-3) is greater than that inferred in a homogeneous model. indeed, since the gravity contribution ∆gm is negative, the observed positive gravity change must be due to a larger density for the new mass. also mogi-5 (low rigidity and low density caldera) requires high density in spite of the positive ∆gm value, because of the larger uplift. it is interesting to note that models mogi-4 (low rigidity caldera), mogi5 and mogi-7 (plastic caldera) produce very similar ∆gm value (fig. 9 and 10), but intrusion density differ by ±25%. due to the compensation of the negative value computed for ∆gm (fig. 7a), the density inferred for the spheroid is quite high, being about 3300 kg/m3. this value is much higher table ii. volume variation and density calculated for each model (characteristics reported in table i) due to input of new mass within the cavity. model ∆v (106 m3) ρs (kg/m3) ∆m (1011 kg) mogi-1 155 2690 4.2 mogi-2 155 2907 3.8 mogi-3 155 3194 3.4 mogi-4 159 2523 4.0 mogi-5 159 3328 5.3 mogi-6 354 2033 7.2 mogi-7 159 2031 3.2 spheroid 174 3271 5.7 sill 125 410 0.5 vol51,1,2008_delnegro 5-02-2009 10:29 pagina 131 132 e. trasatti and m. bonafede than pertinent to trachytic basalts at cf (~23002500 kg/m3). on the other hand, the density inferred for the sill model is very low, 410 kg/m3. since supercritical fluid densities are of the order of hundreds of kg/m3, this value would indicate a hydrothermal source instead of a magmatic source, as suggested by battaglia et al. (2006) for a penny shaped crack. computations for mass input ∆m=ρ∆v are reported in table ii. based on the evaluation of gravity residuals and the assumption of a pressurized point-source, berrino et al. (1984) and berrino (1994) suggested that a subsurface mass increase of 2 · 1011 kg took place between 1982-1984. results from models with spherical sources are typically 2-3 times greater than this value. the all plastic model, mogi-6, has a very high volume variation due to the enhanced deformation of the plastic medium. the computed density is ∼2000 kg/m3 while the mass input is 3-4 times the estimated value. on the other side, the mass calculated for the sill model is a factor 4 less than the reference value. 6. conclusions we develop a numerical technique to calculate gravity changes due to deep inflating sources using fe modelling. even if the models presented are too simple for realistic application to the cf caldera, the method is very general and accounts for many characteristics of the studied area: various source shapes, density and rigidity discontinuities, rheological heterogeneities. very few analytical solutions are available in literature (e.g., sphere, sill, spheroid in elastic media and point-sources in viscoelastic media), while the fe models may be characterized arbitrarily. the gravity calculations distinguish between the contribution of the medium and of the mass intrusion. very different contributions come from the deformation of the medium, according to source shape. we separate the effect of the displacement of layers interfaces (∆gl) and of the medium compressibility (∆gv). we consider the gravity changes occurred at cf caldera during the 1982-1984 unrest episode, where a free-air corrected gravity residual of 75±12 µgal/m was observed. the fe models developed includes heterogeneities in density, rigidity and rheological properties in order to point out which may be important in affecting the observations. it must be noted that the large deformation is well outside the elastic limit of any rock material. we show that nonspherical sources such as sill and spheroid yield positive or negative gravity changes (respectively), without input of new mass. the density values computed for these models are very different, changing by a factor 8 between extreme end models. the results clearly point out the importance of the source shape and of the characteristics of the medium in gravity calculation and source density estimation. the proposed models, even if not yet oriented yet to data inversions, give hints in understanding the effects of considering the realistic structure of the studied area in gravity computations. we fit the uplift and the gravity above the source but do not yet attempt to reproduce the spatial pattern away from the axis. this will be done after developing a realistic model taking into account the characteristics of the area. density and elastic parameters can be extracted, in principle, from seismic tomography studies and from empirical relationships between seismic velocity and density (brocher , 2005). however, long term deformation in volcanic areas may be largely affected by anelastic relaxation and drainage conditions. the large deformations which took place at cf are better described in terms of plastic rather than elastic constitutive relationships. laboratory studies on samples extracted from deep drilling seem the only plausible way to obtain information on the rheological properties of the medium. the source shape is also very important, but only direct modelling has been attempted up to now. an inversion procedure to retrieve the shape of a point-like deformation source in laterally heterogeneous and inelastic media have been proposed (trasatti et al., 2008), and this technique should be further developed to include modeling of gravity changes. from direct modeling we know that vertical and horizontal deformation patterns are necessary to distinguish among different source shapes (e.g., dieterich and decker, 1975): including gravity data gives further constraints to restrict the range of acceptable models. vol51,1,2008_delnegro 5-02-2009 10:29 pagina 132 133 gravity changes due to overpressure sources in 3d heterogeneous media: application to campi flegrei caldera, italy acknowledgements this work was supported by funding from italian civil protection ingv-dpc v3-2 campi flegrei project.we thank c. giunchi for the useful discussions and hints and g. berrino for providing deformation and gravity data. useful suggestions from frank roth and gilda currenti are greatly acknowledged. references battaglia, m., and p. segall (2004): the interpretation of gravity changes and crustal deformation in active volcanic areas, pure appl. geophys., doi: 10.1007/ s00024-004-2514-5. battaglia, m., c. troise, f. obrizzo, f. pingue and g. de natale (2006): evidence for fluid migration as the source of deformation at campi flegrei caldera (italy), geophys. res. lett., doi: 10.1029/2005gl024904. berrino, g. 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(2005): deformazioni e variazioni della gravità in aree vulcaniche: influenza della struttura reologica, in italian, università degli studi di bologna, corso di laurea specialistica in fisica. gottsmann, j., h. rymer and g. berrino (2006): unrest at the campi flegrei caldera (italy): a critical evaluation of source parameters from geodetic data inversion, j. volcanol. geotherm. res., 150, 132-145. greenberg, m.d. (1978): foundations of applied mathematics (prentice-hall, inc., englewood cliffs, new jersey), pp. 456. marc (1994): analysis research corporation (palo alto, ca). sasai, y. (1986): multiple tension-crack model for dilatancy: surface displacement, gravity and magnetic change, bull. earth. res. inst., 61, 429-473. trasatti, e., c. giunchi and m. bonafede (2005): structural and rheological constraints on source depth and overpressure estimates at the campi flegrei caldera, italy, j. volcanol. geotherm. res., 144, 105-118. trasatti, e., c. giunchi and n.p. agostinetti (2008): numerical inversion of deformation caused by pressure sources: application to mt. etna (italy), geophys. j. int., 172, 873-884, doi: 10.111/j.1365-246x.2007.03677.x. walsh, j.b. and j.r. rice (1979): local changes in gravity resulting from deformation, j. geophys. res., 84, 165-170. wang, r.f., f.l. martìn and f. roth (2006), psgrn/ pscmp a new code for calculating coand post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory, comput. geosci., 32, 527-541. yang, x., p.m. davis and j.h. dieterich (1988): deformation from inflation of a dipping finite prolate spheroid in an elastic halfspace as a model for volcanic stressing, j. geophys. res., 93, 4249-4257. vol51,1,2008_delnegro 5-02-2009 10:34 pagina 133 vol51,1,2008_delnegro 5-02-2009 10:34 pagina 134 annals 48, 3, 2005+app1 483 annals of geophysics, vol. 48, n. 3, june 2005 key words raytracing − f1 region − electron density profile 1. introduction the aim of this paper is to show how different ionogram inversion techniques, polan (titheridge, 1985, 1988, 1990) and nhpc (huang and reinisch, 1996), can affect ray tracing results used to characterize a radio propagation link in particular with the propagation parameters: range, group path and height of reflection (apogee). it is shown in different studies (sprague, 1994) that the f1-layer often controls the propagation path for ranges less than about 2000 km, particularly for spring and summer periods. in order to analyze the problem of f1-layer effect on propagation a ray tracing program of proplab-pro software has been used to calculate how the signals are propagated through the ionosphere in the presence of the f1-layer looking at features of the received signal, such as its angle of arrival, path length, height of reflection and range. the results of improving the propagation model by means of ray tracing techniques could be applied in several communications problems including those related to over-the-horizon (oth) radar operation. 2. methodology the software used to calculate the behavior of the radio signals (3-30 mhz) as they travel through the ionosphere is known as proplabionogram inversion f1-layer treatment effect in raytracing gloria miró amarante (1), man-lian zhang (2) and sandro m. radicella (1) (1) the abdus salam international centre for theoretical physics (ictp), trieste, italy (2) laboratory for space weather, center for space science and applied research, chinese academy of sciences, beijing, people’s republic of china abstract this paper shows the importance of the f1-layer shape in the electron density profiles obtained from ionograms with different inversion techniques when the profiles are used in ray tracing. this layer often controls the propagation on the path with ranges less than about 2000 km, particularly for spring and summer periods. ionograms from two different stations, hainan (19.4n, 109e) and el arenosillo (37.1n, –6.7e), obtained during the month of july 2002 (average sunspot number: 99.6) during geomagnetic quiet conditions (ap-index between 9 and 15) are analyzed. the profiles obtained with two different inversion techniques with different options are used together with the ray tracing program of the proplab-pro software. this program calculates the features of the received signal as angle of arrival, path length, height of reflection and range for each given profile assumed to define a spherically symmetric ionosphere in the region along the path. for each ionospheric condition (location, day, hour) the difference between range values obtained with proplab-pro program using profiles from the two techniques and the different options (polan no valley, polan valley, polan1-layer and nhpc) are considered. mailing address: dr. gloria miró amarante, the abdus salam international centre for theoretical physics (ictp), strada costiera 11, 34014 trieste, italy; e-mail: amarante@ictp.it 484 gloria miró amarante, man-lian zhang and sandro m. radicella pro (solar terrestrial dispatch, canada). this program (http://www.spacew.com/docs/propman.pdf) simulates the path between transmitter and receiver taking into account a realistic ionosphere by using ray tracing techniques. for the computation of the signals, the ray tracing technique called comprehensive is chosen as a proplab-pro’s option. it includes the effects of the earth’s magnetic field and electron collisions with neutral particles by using the appleton-hartree formulation (ratcliffe, 1959). although proplab-pro program can use the international reference ionosphere (iri) to model profiles of the ionosphere, this paper shows the possibility to consider electron density profiles inverted from ionograms. these ionograms are measured by digisondes designed by the university of massachusetts lowell and located at hainan (19.4n, 109e) and el arenosillo (37.1n, –6.7e). to avoid possible problems derived from automatic scaling of the ionograms all the characteristics are edited using the software saoexplorer. proplab-pro considers these electron density profiles (which can be interpreted as refractive index profile for a ray tracing technique) as the state of the ionosphere at the mid-path but extended as spherically stratified over the geographical region seen by the projected ray over the earth surface. in each case, a link is defined by means of the transmitter and receiver coordinates (table i). the short distance between transmitter and receiver in both links allows us to consider one single hop condition. data used correspond to july 2002 with a sunspot number of 99.6. four geomagnetically quiet days are chosen with ap-index between 9 and 15. the ray paths under each ionospheric condition (location, day, hour) given by the electron density profiles obtained with the different inversion procedures are traced sweeping elevation angles (15-85 with a step of 5) and operation frequencies (3-14 mhz with a step of 1mhz). proplab-pro calculates for each propagating mode parameters as the ground range, the latitude/longitude of the ray, geometrical path distance (that is, the total distance travelled by the ray), group path, etc. one of the most common techniques of ionogram inversion is the polynomial analysis or polan, developed by titheridge (1985, 1988, 1990). this technique solves the inversion problem by breaking up the profile into simpler sections for which physically expectable solutions can be found and using extrapolation and interpolation for the remaining part of the profile. on the other hand, modern digisondes use nhpc algorithm developed by huang and reinisch (1996) to define a profile from the base of the e-layer to hmf2 implemented automatically into the digisonde operation. in order to define the different electron density profiles used in the ray tracing procedure these two techniques have been used including different options for polan (polan no valley, polan valley, polan1-layer). polan1 layer considers the whole profile as a «single layer» by not inputting explicitly the critical frequency of f1-layer when giving the input ( f, hv) array. this means treating f region as having only a f1 cusp without specifying fof1. whereas polan no valley and polan valley consider the f region as consisting of two layers (f1+f2) by inputting explicitly the critical frequency fof1 when giving the input ( f, hv) array. however, in order to re-create the true-height-profile with valley, the ionisation in this region must be estimated as the information on the ionisation between the layers cannot be measured by an ionosonde. the electron concentration there is less than the critical frequency of the lower layer, and any wave which has a frequency sufficient to penetrate the first layer will not be reflected back by these lower density electrons. a possible estimation for the true-height of the upper layer can be made by assuming that the ionisation between the layers is the same as the peak density of the lower layer, so that there is no valley but a simtable i. transmitter and receiver geographic coordinates. transmitter receiver hainan link (19.4n, 109e) (19.4n, 106e) el arenosillo link (40.2n, 3.29e) (37.1n, –6.7e) 485 ionogram inversion f1-layer treatment effect in raytracing ple cusp to merge the f1and f2-layers. polan no valley is based on this estimation. on the other hand, polan can take into account the variation in height and depth of the valley with time of day, date and latitude. this is done automatically in polan valley option (see titheridge 1985, 1988, 1990). in this context, previous studies have shown the difficulties introduced by the ionogram inversion techniques in the presence of f1-layer. bamford (2000) shows the differences found by comparing experimental oblique ionograms with the one reconstructed with ray tracing and electron density profile inverted from vertical ionogram. 3. results for each ionospheric condition (location, day, hour) the difference between range values obtained with proplab-pro program using profiles from the two techniques and the different options are calculated. these radio propagation values are filtered in order to obtain radio frequencies and elevation angle in such a way that reflection heights are located in the f1 region and the lower part of f2-layer (150-250 km). it is seen that there are important differences between results obtained with the two techniques and the different options, reflecting the fact that the f1 mode trace depends strongly on the electron density height profile between the eand f1-layers (krasheninnikov et al., 1996). the option polan1-layer is chosen as the reference for these differences taking into account that this option is the only one that gives an electron density profile that is always continuous in the values and in the first derivative dn/dh. this point has been verified in some cases where dn/dh presented discontinuities and the results produced by the ray tracing technique proplab-pro showed problems for frequencies and angle values with height of reflection close to this region. maximum values of these differences in range are shown in tables ii and iii for el arenosillo and hainan link respectively. in general, the results show that the differences are lower in the case of the no valley polan optable ii. maximum range differences for el arenosillo link. modes (nh = nhpc, 1l = polan1-layer, nv=polan no valley, v=polan valley). manually scaled ionograms are indicated with the word ‘edited’. apog1 and apog2 are the apogee heights obtained with first and second option in column modes and rang1 and rang2 their ranges. day hour modes edited freq angle apog1 rang1 apog2 rang2 difference (mhz) range 182 11:00 nh-1l edited 14 15 150 1402 199 2396 −994 182 11:00 nv-1l edited 9 30 185 1176 186 1202 −26 182 11:00 v-1l edited 14 15 202 2320 199 2396 −76 182 14:00 nh-1l edited 11 25 117 1253 189 1443 −190 182 14:00 nv-1l edited 11 25 201 2295 189 1443 852 182 14:00 v-1l edited 13 20 227 2383 202 1950 433 183 11:00 nh-1l edited 11 25 162 1156 201 1382 −226 183 11:00 nv-1l edited 14 15 174 1941 175 2004 −63 183 11:00 v-1l edited 6 60 216 588 192 409 179 183 15:00 nh-1l edited 8 40 183 1026 184 753 273 183 15:00 nv-1l edited 11 25 189 1414 182 1308 106 183 15:00 v-1l edited 11 25 204 1760 182 1308 452 fig. 1. electron density profiles and first derivatives dn/dh with different inversion techniques for day 197 and hour 04:15 ut corresponding to hainan link. 486 gloria miró amarante, man-lian zhang and sandro m. radicella table iii. maximum range differences for hainan link. modes (nh=nhpc, 1l=polan1-layer, nv =polan no valley, v=polan valley). manually scaled ionograms are indicated with the word ‘edited’. apog1 and apog2 are the apogee heights obtained with first and second option in column modes and rang1 and rang2 their ranges. day hour modes edited freq angle apog1 rang1 apog2 rang2 difference (mhz) range 197 04:15 nh-1l edited 14 15 215 3115 237 2288 827 197 04:15 nv-1l edited 12 15 170 2227 175 2518 −291 197 04:15 v-1l edited 12 15 170 2227 175 2518 −291 197 05:15 nh-1l edited 12 20 202 3158 168 2061 1097 197 05:15 nv-1l edited 12 20 164 1954 168 2061 −107 197 05:15 v-1l edited 6 60 200 584 177 430 154 200 04:30 nh-1l edited 11 25 208 1767 199 1522 245 200 04:30 nv-1l edited 11 25 208 1839 199 1522 317 200 04:30 v-1l edited 11 25 232 2199 199 1522 677 200 05:30 nh-1l edited 8 40 224 1147 203 982 165 200 05:30 nv-1l edited 11 25 209 1697 207 1766 −69 200 05:30 v-1l edited 8 40 243 1143 203 982 161 tion and can reach large values with the nhpc technique. figures 1-4 show as examples the comparison between electron density profiles and the first derivatives obtained by different inversion techniques and the worst cases for the range values differences during different ionospheric conditions. these plots show that cases where electron density profiles are the same in comparison to those obtained with the reference option polan1-layer but not their corresponding first 487 ionogram inversion f1-layer treatment effect in raytracing fig. 2. worst cases for the range values differences during the ionospheric conditions described by fig. 1. fig. 3. electron density profiles and first derivatives dn/dh with different inversion techniques for day 182 and hour 14 ut corresponding to el arenosillo link. 488 gloria miró amarante, man-lian zhang and sandro m. radicella fig. 4. worst cases for the range values differences during the ionospheric conditions described by fig. 3. derivatives, can produce very different propagation results. this is the case of results corresponding to day 197 and hour 04:15 ut at hainan with nhpc technique where the range difference reaches 827 km (figs. 1, 2 and table iii). on the other hand, table iii shows for the same period a maximum range difference of 291 with no valley and valley options. however, figure 1 shows that the corresponding profiles and dn/dh look similar up to the apogee region (170-175 km). another result is that no relationship is found between plasma frequency changes and range reached as seen in figs. 3 and 4. there is no defined trend between these differences and frequency and angle values. frequencies and angles of arrival obtained cover from 6 to 14 mhz and from 15 to 70 degrees respectively. it is interesting to note that the same results are found for the two links considered that are at very different geographical longitude. 4. conclusions the main results obtained in this paper can be summarized as follows: – for operation conditions the height of reflection and range obtained with ray tracing depends on the inversion technique chosen mainly with a reflection height in the f1 region. – the differences in range reach largest values using the nhpc and valley options. 489 ionogram inversion f1-layer treatment effect in raytracing these differences were also found in cases where electron density profiles look similar. – in general, the lowest range differences were found for no valley option because this option gives a profile very similar to the reference one. however, there are cases with similar profiles and first derivatives that show differences in range of about 300 km. – no dependence was found between differences in range and frequency, angle or geographical longitude. further studies will be done to check these results during different ionospheric conditions. acknowledgements the authors are grateful to the ionospheric group at el arenosillo inta (huelva, spain) for providing data used in this study. one of the authors (g.m.a.) also wants to thank the marie curie fellowship of the european community fifth framework programme under contract number hpmf-ci-2002-01867 for supporting this research. m.l. zhang was also supported by the major program of national natural science foundation of china (nsfc grant no. 40390150) for the work done. references bamford, r. (2000): the oblique ionospheric sounder, radiocommunications agengy, final report. huang, x. and b.w. reinisch (1996): vertical electron density profiles from the digisonde network, adv. space res., 18 (6),121-129. krasheninnikov, i. v., j.c. jodogne and l.a. alberca (1996): compatible analysis of vertical and oblique ionopheric sounding data, ann. geofis., xxxix (4), 763768. ratcliffe, j. a., (1959): the magneto-ionic theory and its application to the ionosphere (cambridge university press, london). sprague, r.a. (1994): measurements of ionospheric variability, technical document 2730 (the space and naval warfare systems center, san diego). titheridge, j.e. (1985): ionogram analysis with the generalised program polan, us dept. of commerce, nat. oceanic and atmos. admin (noaa), report uag-93. titheridge, j.e. (1988): the real height analysis of ionograms: a generalized formulation, radio sci., 23 (5), 831-849. titheridge, j.e. (1990): aeronomical calculations of valley size in the ionosphere, adv. space res., 10, 821-824. vol52,5,2009 453 annals of geophysics, vol. 52, n. 5 october 2009 key words radio propagation – geomagnetic and ionospheric storm 1. international observations on geomagnetic and ionospheric variations a model comparison of noon international reference ionosphere (iri, bilitza, 1988) profiles obtained from both ionospheric stations of istanbul (ist; 41°n; 29°e, lt=(ut+2h), dip=57°.2, l=1.6) and rome (ro; 41°n; 12°e, lt=(ut+1h), dip=57°.4 l=1.6) is reported in fig. 1. some references to these procedures can be found in davies, 1981; agopyan, 1982; titheridge, 1988; rawer and bilitza, 1989; schunk, 1996 and bilitza, 2008. the electron density (ne) noon profiles for the two stations, obtained by the iri model, show a slab thickness of τ ~150 km for the f region. a similarity between the two stations situated at the same l shell (1.6) and with only ∆h=1h difference of universal standard time (uts) as well a slightly different behavior typical of disturbed days, can also be clearly seen in fig. 2. the plasmapause is typically located at l=4-6 but may be found at l ~ 2 during very active periods; for both observation sites at l =1.6 and with (magnetic planetary index kp) kp ≥7 then the saturation of the protonospheric reservoir value should take (see kamide and richmond, 1986) ts = 0.17(l)4 = ~_ 1.1 days, so that one day becomes insufficient for its replenishment as seen in fig. 3. the method of analysis applied to ro and ist measurements used here was described in previous papers (bulat et al., 1997 and agopyan, 2002). the hourly values of frequency (mhz) measurements of ∆fof2(%) are based on fof2 bibl ionosonde data from istanbul university ionosphere research station ayazaga ist-iiai revised comparison and analysis of severe magnetic storm effects measured over istanbul and rome harutyun agopyan vocational school of technical sciences, telecommunication program, istanbul university, turkey abstract geomagnetic storms (gs) and ionospheric storms (is) for high geomagnetic activity conditions are studied in istanbul (ist) and in rome (ro). further and revised results that led to new interpretations are briefly presented here following a previous paper on the severe storm effects on the ionosphere over istanbul. the series of storms started on 28th october 1968 and went on to 2nd november 1968 with the geomagnetic index kp reaching values up to 8. deviations of the critical frequency of the f2 layer, ∆fof2(%), from monthly medians up to 100% were observed. power spectral results show here that not only the diurnal planetary waves (pw) of 2.3 day periodicity but also tidal waves with semidiurnal (12h), teridiurnal (8h) and quaterdiurnal (6h) characteristic times are present. mailing address: dr. harutyun agopyan, istanbul university, vocational school of technical sciences, telecommunication program, 34320 avcilar, istanbul, turkey: e-mail: harutyun@istanbul.edu.tr vol52,5,2009 1-12-2009 13:29 pagina 453 454 h. agopyan with geographic coordinates ϕ=41°08’11’’ n; λ=29°01’58’’ e and geomagnetic coordinates φ=38°40’n; λ=107°.76e; geocentric dip=57°.2. iiai ionosonde used automatic sweep 1.4-20 mhz and operated 40 kw pp during the complete 20th solar cycle (1963-1974) (agopyan, 1994; ozguc et al., 1998; ozcep and orbay, 1999). magnetic recordings of gss and ionospheric soundings for iss from 28th october 1968 to 2nd november 1968 with geomagnetic 3hourly magnetic activity index kp reaching up to 7 to 8 values are considered. during the month of october h mean was -16.1 nt but in november the h mean was = –32.5 nt (around twice of the annual mean value = –16.1 nt). the geomagnetic field disturbances during this time interval are reported in agopyan, 2002, figure 6. the maximum disturbance in h component is reached on 31st october around 21ut. in the first storm day the ionospheric height changes are well over 250 km. leitinger et al., (2004) give slab thickness (τ) values at midday as 320 km and at midnight as 340 km over ro using siro data during october 1981. whereas on 29th october ist measured nmf2= 7x1011 e/m3, zhang et al., (2003) give values for ro as nmf2=1x10-6 /cm-3 for 15th july 2000. using new iri 2007 model (http://iri.gsfc.nasa.gov 2008) no changes in height occurs during sunlight hours but electron density (ne) changes are very significant during 18lt hrs for ro and 19lt hrs for ist simultaneously. at h=300 km the maximum electron density (nm) of ro (nm=8x1011m-3) shows approximately ∆nm=4x1011m-3 concentration difference as max fig. 1. comparisons of the ionospheric heights (km) versus electron density (ne/cc) generated noon profiles of iri for istanbul solid-line (stars) and rome broken-line (circles) plotted simultaneously. 0 670 475 275 75 500000 1000000 1500000 2000000 n electron density el/cc h h e ig h t s i n k m october 1968 istanbul dip=57.2 l=1.6 solid line (stars) october 1968 rome dip=57.4 l=1.6 broken line (circles) iri noon profile rz=108 f10.7=152.8 vol52,5,2009 1-12-2009 13:29 pagina 454 455 revised comparison and analysis of severe magnetic storm effects measured over istanbul and rome ne value deviation from ist (nm=4x1011m-3) and this amount of ∆nm=4x1011m-3 means 0.4 tera electrons per meter cube or an excess concentration of 400 billion of electrons per cubic meter. then, between ist and ro, for the day 28th october 1968, ist shows a height of 325 km and nm=4x1011m-3 at 18lt, simultaneously ro shows the same height and nm=8x1011m-3 at 19lt. consecutively, ∆nm, max ne value deviations at the same uts hours of the stormy days at ist (at 18lt) and ro (at 19lt) are the following empirical reproductions generated from iri 2007 model: ∆nm=0.37x1012 m-3 on 28th.10.1968 (prestorms), ∆nm=0.42x1012 m-3 on 29.10.1968 (storms), ∆nm=0.40x1012 m-3 on 30th.10.1968 (poststorms, positive phases of positive iss ∆nm=0.43x1012 m-3 on 31st.10.1968 (negative phases and negative iss) ∆nm=0.43x1012 m-3 on 01st.11.1968 (positive and negative phases of iss) ∆nm=0.44x1012 m-3 on 02nd.11.1968 (poststorms, negative phases of iss) respectively. according to the reports by prölss and werner (2002) the largest storm effects have been observed at ro. by using the iri-2000 ionospheric model, applied to ro data, a high degree of reliability during quiet conditions and inadequate degree of reliability during storm events has been shown (cander, 2003; zolesi and cander, 2004). 2. results of comparisons the percentage frequency (mhz) deviations ∆fof2 (%) reached up to 70%. the comparison plots of normalized prestorm analysis in fig. 2 showed a deviation of about ± 2%. in both ionospheric stations the maximum electron densities were approximately nm=1.45x106 e/cc, while the heights of the maximum were about h=280 km. figure 3 shows the percentage deviation of fof2 (mhz) during the magnetic storm period of 5 days. planetary waves (periods of about 2–30 days) were predominantly of tropospheric origin and penetrated directly to heights slightly above 100 km. they have to propagate upwards into the f-region ionosphere via an indirect fig. 2. comparisons of normalized prestorm analysis as percentage deviations of fof2 (mhz) from monthly medians for istanbul (black-line) and rome (dashed-line) plotted simultaneously. p r e s to r m 2 8 29 october 1968 sample 1 9 normalized istanbul (black line) l=1.6 rome (dashed line) l=1.6 9ut sc1-2-2 -1 0 1 2 3 1 2 3 4 5 6 7 8 9 ut -2% -1% 0% 1% 2% 3% pe r c en ta g e d ev ia ti o n s o f fo f2 (m h z) vol52,5,2009 1-12-2009 13:29 pagina 455 456 h. agopyan route. their effects were observed in the lower ionosphere, in the ionospheric e region in h’e and sporadic-e (es) layer and in the f2 region (e.g., altadill and apostolov, 2001; lastovicka et al., 2006; mikhailov et al., 2004). the quasi2-day wave in the mesosphere lower thermosphere (mlt) was the most studied. summarizing, it has been reported that the 2-day wave is a manifestation of both a global normal rossby mode and a local unstable wave. its activity is maximum during local summer and especially close to equinoxes (e.g., altadill and apostolov, 2001). tides in the atmosphere are predominantly thermal, not gravitational tides. they are generated by the periodic solar heating. dominant tidal periods in the ionosphere are 24 h and 12 h. the spectral analysis was applied to the rome and istanbul data. the power spectra including the hanning windowing showed the effects of planetary waves (pw) with peaks at 2.3 days and tidal waves with diurnal and semidiurnal peaks, fig. 4. it is also notable that during the negative phases of the is, ∆fof2 minimum value mostly restricting telecommunication channels and radio propagations observed over ist (∆fof2=–33%) was 10% greater than that observed over ro (∆fof2=–22%); whereas during the positive phases of the is ∆fof2=+70% was observed over ist while in ro ∆fof2= was +75%. 3. conclusions results of the analysis of the studied severe gss presented here show complex mechanisms for the ionosphere at medium range scale as also partly seen in previous papers on ist using storm sudden commencements (sc) and ionospheric storms (is) (agopyan, 1986 and 2002). in this paper a comparison between ro and ist fig. 3. comparisons of percentage deviations of fof2 (mhz) from monthly medians versus time of 5 days after 29 october to 2 november 1968 for istanbul solid-line (stars) and rome broken-line (circles) plotted simultaneously. istanbul l=1.6 solid line (stars) rome l=1.6 broken line (circles) 100 50 0 0 50 100 120 -50 -100 pe r c en ta g e d ev ia ti o n s o f fo f2 (m h z) 5 days after 28 october 1968 (ut hours) vol52,5,2009 1-12-2009 13:29 pagina 456 457 revised comparison and analysis of severe magnetic storm effects measured over istanbul and rome for the power spectral analysis on fof2 data showed peaks at 2.3 days and at 24h (diurnal), 12h (semidiurnal), 8h (teridiurnal) and 6h (quaterdiurnal) characteristics in both stations. the peaks magnitude was however weaker in ro. the morphological analysis of the two stations magnetic and ionospheric effect can be now compared and summarized as follows: – first storm (1st sc at 09h09 ut on 29th october 1968, kp=7): after the sudden commencement following weak ring currents, a rapid decrease of ∆h and increase of ∆fof2 on 30th october 1968 at 02h lt in ist, and 00h lt in ro, was observed simultaneously. – second storm (2nd sc at 09h00 ut on 31st october 1968, kp=8): a very strong negative phase for ∆h started immediately after the sc indicating the strong effect of the ring current increase; the fof2 data show small excursions following scs exhibiting positive phase for ro but negative for ist. the largest positive response came with a delay about 8 hs for ist but 12 hs for ro with respect to sc. – third storm (3rd sc at 09h17 ut on 1st november 1968, kp=7): an apparent ∆h peak, with the rapid increase and decrease in negative phase which proves recovery of the intensive ring currents coinciding with a positive phase in ∆fof2 and an elevation of the f layer for both stations ist and ro were observed consecutively. the ionospheric positive and negative wavelike fluctuations were less significant in ro than in ist (agopyan, 1994). negative phases last for a long time, even several hours after a sc and were observed in all three cases. negative long-duration f2-layer storms are known to result from thermospheric neutral composition variations, namely, o/n2 decrease (e.g., prölss, 1995). acknowledgements i wish to express my gratitude to professors e. boschi, p. dominici, p. spalla and also to s.m. radicella, for their invitation, for providfig. 4. comparison results of power spectra of electron density values of the ionospheric severe storm including hanning windowing for istanbul (black-line) and rome (dashed black-line) plotted simultaneously. 0.80 1.00 power frequency (microcycle/s) 0.90 0.60 0.70 0.40 0.50 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 0.20 0.30 0.00 0.10 istanbul (black line) l=1.6 rome (dashed line) l=1.6 vol52,5,2009 1-12-2009 13:29 pagina 457 458 h. agopyan ing storm data, for the means used, and also for the international collaboration during the establishment of the aeronomy and radio propagation laboratory in italy where i carried out this study with the support of ictp-tril program. references agopyan, h. (1982): ionospheric storms in connection with geomagnetic storms, egs and esc joint meeting, program and abstracts handbook, (august, leeds, england), 110-17, a15, 23-27. agopyan, h. (1986): ionospheric behavior on geomagnetic storms, in proceedings of the international symposium on «radio beacon contribution to study of ionization and dynamics of the ionosphere and corrections to geodesy», edited by antti taurainen (oulu, finland), 1, 11-25, isbn 951-42-2256-3. agopyan, h. (1994): magnetic and ionospheric storms in ist: an observational review, journal of the chamber of geophysical egineers of turkiye, geophysics, jeofizik, edited by a.t. basokur, 8 (2), 105-121, issn 0259-1472. agopyan, h. (2002): severe magnetic storm effects on the ionosphere over istanbul: a case study, annals of geophysics, 45 (5), 621-628. altadill, d. and e.m. apostolov (2001): vertical propagating signatures of wave-type oscillations (2and 6.5days) in the ionosphere obtained from electron density profiles, journal of atmospheric and solar-terrestrial physics, 63, 823-834. bilitza, d. (1988 and 2008): private communications, (iri 2008, url:http://iri.gsfc.nasa.gov). bulat, t., b. zolesi, s., m. radicella and h. agopyan (1997): comparison of iss observed simultaneously in istanbul and rome, (st3), xxii g. assembly of the european geophysical society, 21-25 april 1997, vienna, austria, ionosphere and thermosphere, space and planetary sciences, editor: prof. dr. arne k. richter, annals geophysicae, part iii, (st034), vol. 15, supp. iii, 628. cander, l.j.r. (2003): toward forecasting and mapping ionospheric space weather under the cost actions, adv. space res., 31 (4), 957-964. davies, k. (1981): review of recent in ionospheric predictions, radio science, 16, 1407-1430. kamide, y. and a.d. richmond (1986): recent advances in studies of magnetosphere-ionosphere coupling, j. geomagnetism and geo-electricity, 38 (7), 653. laštovička, j., a.v. mikhailov, t. ulich, j. bremer, a.g. elias, n. ortiz de adler, v. jara, r. abarca del rio, a.j. foppiano, e. ovalle and a.d. danilov (2006): long-term trends in fof2: a comparison of various methods, journal of atmospheric and solarterrestrial physics, 68 (17), 1854-1870. leitinger, r., l. ciraolo, l. kersley, s.s. kouris and p. spalla (2004): relations between electron content and peak density: regular and extreme behaviour, in «cost 271 action, effects of upper atmosphere on terrestrial and earth space communications, final report», annals of geophysics, suppl. vol. 47 (2-3), 1097-1107. mikhailov, a.v., a.kh. depueva and t.yu. leschinskaya (2004): morphology of quiet-time f2-layer disturbances: high to lower latitudes, int. j. geomagn. aeronom., 5 (1), gi-1006. ozcep, f. and n. orbay (1999): interaction between physics and the earth: geophysics in the 75th year of the republic of turkey (yerküre ile fiziksel iletisim: cumhuriyetin 75.yilinda ülkemizde jeofizik) istanbul university engineering faculty’s earth sc. rev., 12 (1), 99-105. ozguc, a., t. atac, y. tulunay and l. stanislawska (1998): the ionospheric fof2 data over istanbul and their response to solar activity for the years 1964-1969 and 1993, studia geoph. et geod., 42, 112-118. prölss, g.w. (1995): ionospheric f-region storms, handbook of atmospheric electrodynamics, edited by h. volland, , vol. ii, (crc press, boca raton), pp. 195248. prölss, g.w. and s. werner (2002): vibrationally excited nitrogen and oxygen and the origin of negative ionospheric storms, journal geophys. res., 107, a2,10.1029/2001ja900126, sia 1-6. rawer, k and d. bilitza (1989): electron density profile description in the iri, journal of atmospheric and solar-terrestrial physics, 51, 781-790. schunk, r.w. (1996): handbook of ionospheric models solar-terrestrial energy program, august, step, 1, 73-295. titheridge, j.e. (1988): private communications. zolesi, b. and l.j.r. cander (eds) (2004): cost 271 action, effects of the upper atmosphere on terrestrial and earth space communications: introduction, final report, first section, annals of geophysics, suppl. vol. 47 (2/3), 915-925. zhang, y, l.j. paxton, h. kil, c.-i meng, s.b. mende, h.u. frey and t.j. immel (2003): negative ionospheric storm seen by the image fuv instrument, journal geophys. res., 108 (a9), 1343, doi: 10. 1029/2002ja009797, 2003 (sia, 4-7). (received, july 14, 2008; accepted, may 28, 2009) vol52,5,2009 1-12-2009 13:29 pagina 458 s o m e i n v e s t i g a t i o n s o f isostasy and earth's c o n t r a c t i o n e . n . l y u s t i k h ( * ) ricevuto il 7 dicembre i960 i . c o n t r a c t i o n h y p o t h e s i s . 1. seismic and gravity investigations have revealed that the earth's crust is quite different on continents and under oceans; therefore the continents and the ocean basins cannot be upheavals and subsidences due to the contraction of the earth. so the contraction hypothesis fails in the explanation of major features of the globe. vertical movements play the leading part in the evolution of the earth's crust and they change sign in time. i t is inexplicable from the standpoint of earth's contractions. continents grow laterally, evidently new portion of continental crust is being formed. no explanation for this can be given by contraction hypothesis. 2. assuming contraction hypothesis the diminution of the earth's surface from geological data is at least 5 per cent during each tectonic cycle. as at least 15 cycles have passed since the earth's formation the total shortening would be half of the whole surface, which is impossible. 3. every great circle of the earth is to shorten equally. nevertheless, as we see from the globe, the shortening would be very different, if it were. e. g. the circle consisting of meridians 15° w and 165° e has no indication of significant shortening, while along the circle 90° w, 90° e there was a great shrinkage (mainly on account of the mountain buildings in asia). in general, there are too few great structures stretching latitudely on the globe. i t is inconsistent with the idea of the earth's uniform shrinkage. 4. the contraction hypothesis supposes that folded mountains are due to the squeezing a geosyncline (orogen) between two platforms (*) paper read at the helsinky assembly of the i.u.g.g., 1960. 8 0 e . n . l y u s t i k h (cratons). let us consider a cross section from the middle axis of a symmetrical platform to the middle axis of a geosyncbne (fig. 1). we may neglect the elastic strain of the platform. two cases are to be considered. the first one is a case of elastic substratum or little contraction (i on the fig. 1); the second case is a case of plastic substratum and great contraction (ii on the fig. 1). i n the first case the bmit of elasticity is nowhere passed over. under the platform a there is a transitional layer b, tangential stress in which exists, and the rate of thermical contraction grows with depth untill the f u b rate of contraction is reached in the layer c. in the second case (ii on the fig. 1) the surface of shearing separates the platform a from the layer of f u b contraction c. we may suppose the tangential stress on this surface to be equal to the fundamental strength of the substratum. while the layer g contracts to its middle line 0 the platform a crumples the geosyncline f, so the latter exerts some pressure upon the side of platform. nevertheless the maximum stress must be in the centre of platform 0 as the analysis has shown. while the contraction goes on, the stress grows up, and the platform must be destroyed in its middle after all. the larger is a platform the greater is the stress in the middle, therefore for any given amount of contraction the maximum length of the platform may be established which can exist without crushing. let us denote (fig. 1): 0 middle of the platform and origin of coordinates (only abscissas are considered), a abscissa of the edge of the platform a, c abscissa of the edge of the layer g after contraction, h thickness of the platform a, ii thickness of the transitional layer b, (j crushing horizontal stress for the platform a, v pressure of the platform a upon the geosyncline f, g rigidity of the layer b, a fundamental strength of substratum, a — c h = amount of contraction. a the limit of a is: in the first case a < 1/ 2 h e ^ v_ , g h in the second case a < h — . a some i n v e s t i g a t i o n s of i s o s t a s y a n d eaktil's c o n t r a c t i o n 8 1 you see, in the case of great contraction (it) the limit is independent of the amount of contraction k. being pressed laterally the material of a geosyncline undergoes both squeezing upwards and folding. the upward protruding makes tension of the uppermost strata and so prevents from folding. hence the maximum folding must be at the bottom of geosyncline, and it may occur at the top only if the thickness of the geosyncline is suffifig. 1. ciently small in comparison with its breadth. even the allowance being made for denudation away of the upper ten kilometers, the folded structure cannot be exposed if the thickness of the platforms working as pistons exceed 30 km. (for the usual breadth of folded zone). thus we take h = 30 km, h = 1000 km, /i = 6000 kg/cm2 v = 1200 kg/cm2, g = 5 x 10s kg/cm2, a = 150 kg/cm2, k = 0.01 in the cenozoic era. this gives the maximum length of a platform 2a in the first case 2a < 150 km, in the second case 2a < 720 km. 8 2 e . n . l y u s t i k h i n reality platforms exist larger than 2000 km. i t needs a < 9 kg/cm2, which is too small. 5. let us suppose the major features of the surface relief on the globe are due to warping in the course of contraction. in such a case gravity anomalies both bouguer and isostatic must be positive on elevations and negative in depressions. i t is generaby known t h a t as a rule bouguer anomabes are negative on elevations and positive in depressions, while isostatic ones approach zero. i t may be suggested, however, t h a t contraction makes both elevations and its roots simultaneously, so isostatic conditions are maintained. but in fact a depression often develops on the place of the former elevation; this is inexpbcable from the standpoint of permanent contraction keeping isostasy. 6. the investigation of geosynclines reveals close connection between the processes in the crust and the mantle. the example of island arcs shows geosyncbne to be an active zone down to very great depth. specific movements and structures, magmatism, earthquakes, and gravity form a single complex there. the situation is thoroughly inconsistent with the idea of the passive role of geosyncbne ensuing from contraction hypothesis. 7. the ratio of the energy involved in all the tectonic movements to the whole energy output from contraction is the efficiency of contraction mechanism. we have deduced 2 (fi — 2 v) p 3 dgr where fi efficiency of contraction mechanism, d density of the earth's crust, g mean gravity acceleration in the crust, r radius of the e a r t h , and y , v are expressed in dynes per square centimetre. taking d — 3 g/em3 and fi , v as before we get /s < 0.005. the power of all the tectonic processes t may be estimated from the total power of seismic waves. this gives t > 1026 erg/year. taking /? into account the shortening of the earth's radius must be of order 10"3 a year. nevertheless the astronomical data (the retardation of the e a r t h ' s rotation) prove the shortening to be no more than 7 x 10-11 if any (after n. n. pariysky). yet the latter figure cannot be due to the cooling of the e a r t h since it needs the heat escape from the earth's ball > 2.8 x 1023 erg/year, i. e. an order greater than it is. some i n v e s t i g a t i o n s of isostasy a n d eaktil's contraction 8 3 the proper calculation of the earth's thermal history performed by h. a. lubimova and by b. j . levin and s. v. mayeva prove the temperature in the globe exc.ept the outermost thousand kilometers has been growing up, and consequently, no thermal contraction can be at all. as a conclusion it may be said that the contraction hypothesis is untenable from all points of view. i i . i s o v s t a s y . 8. " isostasy " means equilibrium, and so " isostatic equilibrium " is a tautology. the word " isostasy " is often used in a rather vague sense, as to denote the idea of equilibrium of the crust or the tendency to its performing. we prefer to use the word in its literal sense " equilibrium of the c r u s t " . 9. i. d. zhongolovich has averaged gravity anomalies in each tendegrees-square. the amplitude of the variation of mean free air anomalies is ten times as great as t h a t of mean bouguer anomalies. the approximate degree of compensation i may be expressed as . _ b 1 ~~ b — f ' where b — bouguer anomaly, f free air anomaly. we have the averages for all the continents b = — 97 mgal, f = + 9 mgal; hence i = 0.91. for all the oceans b = + 230 mgal, f = — 3 mgal, and hence i = 0.99. taking these values into account we may say t h a t in general the e a r t h ' s crust is compensated to within about 10 per cent. 10. often the attempts are made to determine the regionality of isostasy and the normal thickness of the crust (or the depth of compensation) from gravity data only. as a rule such attempts cannot give trustworthy results. in the first place, isostatic anomalies depend only on some function of both the radius of regionality and the depth of compensation, and it is impossible to distinguish the contribution of each. and yet this function is not learned at all. i n the second place, the very criteria for selection of the " best " values are doubtfull. the simplest criterion is the approaching of mean anomaly to zero. i t may be true on an ancient plain, where tectonic 8 4 e . n . l y u s t i k h forces have been exhausted long ago. but the compensation is a flat horizontal slab in this case, and its parameters are indeterminable. i n highlands, on the contrary, the effect of compensation may vary very much with its parameters, but at the same time we cannot expect the mean anomaly to be low there, since the tectonic forces upset isostasy while making mountains. another test is as fobows. for any set of parameters we may approximate the observed anomalies by a straight line. g = a + bh where g isostatic anomaly, h height of the station, a, b are to be calculated by the least square method. i t is believed that the combination of parameters giving the least value of b is the nearest one to the reabty. this may be true only if the compensation is really fub, which we do not know and cannot do. and yet if it were full, a must be zero, which is not the case mostly. so this criterion fabs too. 11. some authors have calculated the viscosity (or "pseudo-viscosity ") of the substratum from the rate of the postglacial uplift of the fennoscandia or canada tectonic shield. a. d. arkhangelsky and h. still e insist the upbft is caused by powerfull tectonic forces which have been acting long before the glaciation as well. h. jeffreys points out that many other places do not rise having the same gravity anomalies. this objection is very strong, as isostatic anomalies exactly indicate the amount of extra load. of course, the isostatic forces may take part in the uplifting, but their share is unknown, and therefore any numerical calculation is misleading. 12. the topographic anomalies (i. e. the bouguer anomalies with terrain correction) are best suitable for tectonic interpretation because they are affected by ab the density anomabes. i n particular, they give almost immediately the approximate rebef of the bottom of the e a r t h ' s crust, whereas the isostatic anomabes require complicated calculations. indeed, firstly the bottom rebef of the crust is to be established which corresponds with a zero isostatic anomaly and then it is to be modified according to the real anomabes. as e. 1st. lyustikh has pointed out in 1947, f. a. vening-meinesz had missed the first stage and, as a result, has obtained a great root (buckling) where none exists (cf. ch. tsuboi). some i n v e s t i g a t i o n s of i s o s t a s y a n d eaktil's contraction 8 5 13. the isostasy is not a leading factor in the evolution of the earth's crust since the establishment of equilibrium is a passive process and leads to quieting. only a disturbing force can be active, as even c.e. dutton has pointed out. observations very often reveal tectonic movements going against isostasy. but the greater is the tectonic disturbing the greater is the isostatic counteraction and the latter puts a limit to the former; thus no isostatic anomaly is observed beyond ± 300 mgal. the tectonic forces, as a rule, change sign many times. if such a changeable force weakens and damps gradually, the movements have to stop at the position of isostasy. summary the contraction hypothesis is discussed from the geological, gravimetric, mechanical, and geothermic point of view. the role of isostasy in the geotectonics evolution is considered. riass un to si discute i'ipotesi della contrazione dal punto di vista geologico, gravimetrico, meccanico e geotermico. si considera il ruolo delvisostasi nelvevoluzione geotettonica. r e f e r e n c e s arkhangelsk^" a. i)., (apxkahrejibcmi a. ji.), tp. haymio-ucc/i. uu-ma zeoii. u muhepajiomxi, i 1933. d u t t o n c. e., bull. nat. bes. council, 78, washington, 1931. j e f f r e y s h., the earth. 4th ed., cambridge, 1959. l e v i n b . j . , m a y e v a s. v . , (jiebhh e . i o . , m a e b a c . b . ) , i-tee. ai<ad. hayk cccp, cephh reotj)h3. n. 2, 1960. l u b i m o v a h. a., geopliys. j., 1, 2, (1958). 86 e. n. lyustikh l y u s t i k h e . n . , (jimcthx e . h . ) , tp, mn-ma meopemuu. eeoifiusuku, a h c c c p , 3 , 1 9 4 7 . jjokji. axad. hayx cccp, 65, n . 6, 1 9 4 9 . tp. peo$u3, un-ma, a h c c c p , n . 2 6 ( 1 5 3 ) , 1 9 5 5 . a b s t r . r e p . x i gen. ass. i u g g , i n t . ass. geod. moscow 1 9 5 7 . tp. reo<f>u3. un-ma, a h c c c p , n . 3 8 , ( 1 6 5 ) , 1 9 5 7 tp. mn-ma cpu3uku 3emau, a h c c c p , n . 3 , ( 1 7 0 ) , 1 9 5 8 . p a r i y s k y n . n . , (naphhckhh h . h . ) , tp. peo$u3. un-ma, n . 2 6 , ( 1 5 3 ) , 1 9 5 5 . stille h., publ. inst. seismologique, 32, leningrad, 1933. tstjboi ch., verh. icon. nederl. geol.-mijn. genootschap, «geol. serie», 18, gedenkboek f . a. yening-meinesz, 1957. yening-mexnesz f . a . , gravity expeditions at sea 1923-1932, 2 , delft. 1 9 3 4 . zhongolovich i . , pkohronobhh h . ) , tp. hn-ma meopemuu. acmponomuu, a h c c c p , 3, 1 9 5 2 . non-linear laminar flow of fluid through a vertically stratified cylindrical porous aquifer s . k . j a i n ( * ) received on may 24th, 1982 a b s t r a c t t h i s n o t e p r e s e n t s the study of non-linear l a m i n a r flow of fluid through a v e r t i c a l l y s t r a f i f i e d cylindrical porous a q u i f e r in the steady state condition. the i n f l u e n c e of n o n l i n e a r l a m i n a r flow on the discharge of the fluid and its d e p e n d e n c e on t h e related physical quantities, is discussed. in particular, results f o r n o n l i n e a r l a m i n a r flow of fluid into a concentric well fully penetrating the h o m o g e n e o u s a q u i f e r h a v e been deduced and c o m p a r e d . r i a s s u n t o la p r e s e n t e nota r i g u a r d a lo studio del flusso l a m i n a r e non-lineare di un fluido a t t r a v e r s o u n " a q u i f e r " v e r t i c a l m e n t e stratificato poroso in condizioni stazionarie. l ' i n f l u e n z a del flusso l a m i n a r e non-lineare sul processo di discarica viene d i s c u s s a con la s u a d i p e n d e n z a dalle q u a n t i t à fisiche correlate. in particolare v e n g o n o d e d o t t i e c o m p a r a t i i risultati per il flusso non l a m i n a r e del fluido in un p o z z o c o n c e n t r i c o p e n e t r a n t e l ' a q u i t e r omogeneo. (*) d e p a r t m e n t of mathematics, rivers s t a t e college of education, rumuolum e n i , p. m. b. 5047, port harcourt (nigeria). 1 1 0 s.k. jain 1. i n t r o d u c t i o n t h e n a t u r e of flow mainly depends upon the fluid velocity a n d t h e s t r u c t u r a l c o n s t i t u t i o n of the porous m a t r i x through which it f l o w s . on t h e basis of fluid velocity, however, the flow (bear, 1972) c a n b e c h a r a c t e r i z e d into three different regimes — laminar, n o n l i n e a r l a m i n a r a n d t u r b u l e n t , reynolds n u m b e r being the c r i t e r i a for s u c h d e m a r c a t i o n . but, the intricacy of the n a t u r e of p o r o u s m e d i a does not always justify the n a t u r a l flow of fluid to b e p u r e l y l a m i n a r a n d it a p p e a r s m o r e desirable to be either n o n l i n e a r l a m i n a r or t u r b u l e n t . consequently, uchida, 1952, e n g e l u n d , 1 9 5 3 , a n a n d k r i s h a n a n d v a r a d a r a j u l u ( 1 9 6 3 ) , whright, 1 9 6 8 , a h m a d a n d s u n a d a ( 1 9 6 9 ) , k h a n a n d raza ( 1 9 7 2 ) , jain and u p a d h y a y ( 1 9 7 6 ) , u p a d h y a y ( 1 9 7 5 , 1 9 7 7 ) a n d others obtained s p e c i f i c s o l u t i o n s of certain non-linear flow problems. in t h e p r e s e n t p a p e r , we consider non-linear l a m i n a r steady s t a t e flow of fluid into an uncased well, concentrically established w i t h r e s p e c t to the c o n t o u r of intake a n d p e n e t r a t i n g fully the v e r t i c a l l y s t r a t i f i e d p o r o u s aquifer of finite thickness. it is found t h a t t h e flow p a t t e r n is c h a r a c t e r i z e d by two different zones, in w h i c h t h e d i s c h a r g e exhibits opposite c h a r a c t e r as regards its d e p e n d e n c e o n g r a i n size of the m e d i u m , viscosity of the fluid and r a d i u s of t h e well, is concerned. f u r t h e r , it is observed t h a t as the p e r m e a b i l i t y of the region in the vicinity of the well decreases or a s t h e region becomes n a r r o w e r , in b o t h the situations, the i n f l u e n c e of n o n l i n e a r l a m i n a r flow is to increase the discharge. as a p a r t i c u l a r case, the results for non-linear l a m i n a r flow of fluid i n t o a concentric well fully p e n e t r a t i n g the homogeneous a q u i f e r h a v e been deduced a n d c o m p a r e d with those obtained by u p a d h y a y , 1977. 2 . e q u a t i o n s o f f l u i d flow in p o r o u s m e d i u m t h e l a w s governing l a m i n a r a n d non-linear l a m i n a r flow of f l u i d in p o r o u s m e d i a are (poluvarinova-kochina, 1 9 6 2 ) . n o n l i n e a r laminar flow, ecc. ill a n d ^ = av + bv2 [2] d s w h e r e v, k, ^ d e n o t e seepage velocity, seepage coefficient and d s h y d r a u l i c g r a d i e n t respectively; c o n s t a n t s a a n d b, according to e n g e l u n d (1953) a r e a 2000 ii , 3 5 . . y = — ¿ r h = jd [ 3 a ' b ] p a n d ĵl b e i n g d e n s i t y a n d viscosity of t h e fluid respectively and d t h e g r a i n size of t h e m e d i u m . b e s i d e r e l a t i o n s [1] a n d [2], we consider t h e expression for p r e s s u r e d i s t r i b u t i o n in t h e case of a concentrically placed circular w e l l f u l l y p e n e t r a t i n g a two-layered vertically stratified cylindrical p o r o u s a q u i f e r of thickness t in t h e f o r m (poluvarinova-kochina, 1962) q \i p = jfc. t log ( — ) + log ( ) rw k 2 r o + c , [ 4 1 w h e r e c is t h e c o n s t a n t , to be d e t e r m i n e d w i t h the help of b o u n d a r y c o n d i t i o n s consistent w i t h t h e system. 3 . s t a t e m e n t o f t h e p r o b l e m i n t h e s t e a d y s t a t e condition, we consider t h e flow of fluid into a n u n c a s e d well of r a d i u s rw , concentrically established with r e s p e c t t o t h e c o n t o u r of i n t a k e of r a d i u s rc. it is a s s u m e d t h a t the w e l l is fully p e n e t r a t i n g t h e p o r o u s a q u i f e r of thickness t. the a q u i f e r is c o n s i d e r e d to be vertically s t r a t i f i e d into a d j a c e n t c y l i n d r i c a l r e g i o n s of p e r m e a b i l i t i e s k\ a n d ki s e p a r a t e d by the b o u n d a r y r = rq. the p r e s s u r e at the c o n t o u r of well a n d t h a t of i n t a k e a r e p r e s c r i b e d as a n d pc respectively. 112 s.k. jain in t h e p r e s e n t system, we a s s u m e the flow to be non-linear l a m i n a r r, w i t h i n a n a r r o w cylindrical zone r w > r « r, and l a m i n a r in t h e region r, < r =£ rc. hence it becames obvious from p h y s i c a l c o n s i d e r a t i o n s t h a t k1 k.2. let p, be the pressure at the t r a n s i t i o n b o u n d a r y r = r, [cf. fig. 1 nel testo]. fig. 1 t h e p r o b l e m is to e x a m i n e the influence of non-linear l a m i n a r flow on t h e d i s c h a r g e of fluid a n d its dependence on the related p h y s i c a l q u a n t i t i e s . 4 . s o l u t i o n f o r l a m i n a r zone, the b o u n d a r y conditions are p = pc at r = rc, [5] p = p, at r = r, n o n l i n e a r laminar f l o w , ecc. 1 1 3 w h i c h on s u b s t i t u t i o n in [4] gives p, = pc + q |x 2 -it it, t e ( t t ) [6] as p = p g/z a n d q = 2 -ir r 7v, equation [2] becomes d r = pt aq bq2 2-art [7] i n t e g r a t i n g [7] a n d then evaluating the constant of integration w i t h t h e h e l p of b o u n d a r y conditions p = pw at r = r w , p = p, at r = r„ w e o b t a i n [8] a = + p g q q 2 tt t [9] at t h e b o u n d a r y of transition f r o m l a m i n a r to non-linear l a m i n a r flow, the relation between critical reynolds n u m b e r {; e = 0.07 a n d critical velocity vc is given by q a . 2 it r, t b [10] s i n c e at t h e b o u n d a r y of transition as given by [1] and [2] d s y i e l d t h e s a m e value, it follows from [10] that 1.07 a ki p g = n. [ 1 1 j using [11] in [6] a n d then e q u a t i n g the value of p , so obtained w i t h t h a t given by [9], we get 114 s.k. jain pc -p(0 _ aq p g = ^ [ l o g ( 5 ) 1 . 0 7 2 it t l \ ro> ' k, , o g ( ^ ) l feq2 ( ± _ ± ) r, / [12] c o m b i n i n g e q u a t i o n s [3a, b] a n d [10] with [12], we obtain = 8000 a f l o g ( 5 ) 1.07 t + 0 0 7 ( ¿ o : 1131 if w e a s s u m e the flow of fluid to be purely l a m i n a r in the e n t i r e regi o n r w r =£ rc, then the flow r a t e qiam may be obtained f r o m [4] by using t h e corresponding b o u n d a r y conditions both at t h e c o n t o u r of intake a n d at the well. thus qla 2 i t a:, t (pc-p<q > log + log ( a 7 v ra a n d t h e r e f o r e , using [10] a n d [14], we o b t a i n q = 8 5 6 0 i l qiam 'w h-2 ^ p d 2 ( p c ) [ * m + a . l o g / [14] [15] k2 ~ \ ri i n t r o d u c i n g dimensionless q u a n t i t y x a n d r a t i o y such that q x = p ipc p( 0 ) y = ¿lam [16 a, b] a n d c o m b i n i n g [15] w i t h [13], we o b t a i n an implicit relation n o n l i n e a r laminar flow, ecc. 1 1 5 1.07 x = xy 1 1 . 0 7 l o g xy 8560 2 1.07 a _ log (jl_) + 0.07 — 0.07 ki rm 8560 z [17] w h e r e z = h i + —— log i n view of physical considerations it may, however, be inferred f r o m [16 a] t h a t x > 0, hence equation [17] becomes 1.07 z = y [ ( • , . 07 £ log xy 8560 z [18] 1.07 a k2 log • k ' w + 0.07 xy 8560 z 0.07 5 . p a r t i c u l a r c a s e if = k2, t h a t is, the entire flow region is homogeneous with u n i f o r m p e r m e a b i l i t y , equation [18] reduces to 1.07 log = 7 [ 1.07 log 0.07[19] — 0.07 log i 0 . 0 7 ] , 8560 log ( — ) ' 8560 log ( — ) j ra> r w w h i c h r e p r e s e n t s the non-linear l a m i n a r flow of fluid into a c o n c e n t r i c well fully p e n e t r a t i n g the homogeneous porous aquifer d i s c u s s e d by u p a d h y a y (1977). 1 1 6 s . k . jain 6 . d i s c u s s i o n f r o m [16a] it is evident t h a t x d e p e n d s on the density of fluid, g r a i n size of the m e d i u m , pressure difference of the system, viscosity of the fluid a n d well radius. since d a n d p. occur in h i g h e r p o w e r s in the expression for x, they highly affect the d i s c h a r g e . in ca se of n o n l i n e a r l a m i n a r flow of fluid, the inertial forces a r e p r e d o m i n a n t which causes more resistance and consequently s m a l l e r d i s c h a r g e t h a t might be expected in l a m i n a r flow. obvio u s l y t h e effect of non-linear l a m i n a r flow can be observed only w h e n y < 1. to get a definite idea of result [18], we take the system for w h i c h rc = 3 x 103 r , t h a t is, the radius of contour of intake is e q u a l t o 3 0 0 0 t i m e s t h e r a d i u s of t h e w e l l . a s s u m i n g rq = 1.5 x 10 3 r , we analyse the effect of permeabilities of the s t r a t i f i e d zones on the discharge, in the p a r t i c u l a r cases. kt = 3 k2, 2 k2, k2. [20] t h e f i r s t t w o of which imply t h a t the permeability of the inner z o n e is respectively 3 a n d 2 t i m e s t h a t of the outer zone, where as t h e last v a l u e c o r r e s p o n d s to the case of u n i f o r m permeability. using [18] a n d [20], the n u m e r i c a l values of y are obtained in t a b l e i c o r r e s p o n d i n g to different values of x > 0. t a b l e i values of y; ra = 1.50 x 103 x r< ktlk2 x ... 10 10 2 103 10" 105 10" 107 10« 10' 3.0 2.0 1.0 0.2028 0.2952 0.9394 0.2320 0.3217 0.9561 0.2596 0.4045 0.9739 0.3061 0.4450 0.9912 0.3612 0.5212 0.9994 0.4778 0.6111 0.9253 0.4835 0.5396 0.5988 0.2712 0.3650 0.2526 0.1018 0.0908 0.0876 t h e s e t a b u l a t e d values have been graphically represented in fig. 2, a b s c i s s a a n d o r d i n a t e being x a n d y respectively. it is seen n o n l i n e a r laminar f l o w , ecc. 1 1 7 fig. 2 f r o m fig. 2 t h a t in curve — i which corresponds to a:, = 3 k2, as x i n c r e a s e s , initially y increases till it a t t a i n s a m a x i m u m value 0.4970 ( c o r r e s p o n d i n g to x = 4.85 x 106); a f t e r w a r d s it decreases a s y m p t o t i c a l l y . thus, in the f o r m e r region 0 < x =s 4.85 x 10" the d i s c h a r g e increases as x increases, that is, when the density of the f l u i d , g r a i n size of the m e d i u m a n d pressure difference of the s y s t e m increases, viscosity of the fluid a n d well radius decreases. in t h e l a t e r region x > 4.85 x 10" the influence of non-linear l a m i n a r flow is reversed. h e n c e , it m a y be concluded that in case of non-linear laminar f l o w , t h e flow p a t t e r n is characterised by two different zones in w h i c h d i s c h a r g e exhibits opposite c h a r a c t e r . 7 . c o m p a r i s i o n to e x a m i n e how the permeabilities of the stratified zones a f f e c t to discharge, we consider k¡ = 2k2 and the limiting case 1 1 8 s . k . j a i n kt = k2. in fig. 2 these cases are represented by curves ii a n d iii r e s p e c t i v e l y . it is observed t h a t the discharge increases as the p e r m e a b i l i t y of the region s u r r o u n d i n g the well decreases. n o w to investigate the effect of shrinking of the region of p e r m e a b i l i t y k t on discharge, we consider a p a r t i c u l a r case rq = 1.0 x 10 3 x r0) . taking k{ = 3 k2, 2 k2, values of y are obtained in t a b l e ii c o r r e s p o n d i n g to different values of x > 0. t a b l e i i values of y; r„ = 1.0 x 103 x r w kt i k2 x 10 10 2 103 10" 105 10" 107 10" 10' 3.0 0.2240 0.2699 0.2990 0.3532 0.3992 0.4922 0.5257 0.2982 0.1042 2.0 0.3415 0.3799 0.4236 0.4818 0.5574 0.6400 0.5219 0.2476 0.0930 t h e s e values h a v e been graphically shown in fig. 2 by dotted c u r v e s . it is i n f e r r e d t h a t discharge also increases when the region of p e r m e a b i l i t y /c, becomes n a r r o w e r . h e n c e , in general it may be concluded t h a t the shrinking of t h e z o n e s u r r o u n d i n g the well, as well as decrease, in its permeab i l i t y c a u s e s a larger discharge comparatively, which is an obvious p h y s i c a l p h e n o m e n o n . n o n l i n e a r l a m i n a r f l o w , e c c . 1 1 9 r e f e r e n c e s a h m a d , n . a n d sunada, d.k., 1969 non-linear laminar flow in porous media. «j. hy. dn., p r o c . a s c e » , 95, hy6, p p . 1847-1857. a n a n d k r i s h a n , m . a n d varadarajulu, g.h., 1963 laminar and turbulent flow of water through sand. « j . soil mech., proc. asce», 89, sm5, p p . 1-15. bear, j . , 1972 dynamics of fluids in porous media. american elsevier publishing c o m p a n y inc., n e w york. e n g e l u n d , f . , 1 9 5 3 on laminar and turbulent flow of ground water in homogeneous sand. « t r a n s . d a n . acad. tech. sei.», 3, p p . .1-105. j a i n , s . k . a n d upadhyay, k . k . , 1 9 7 6 non-linear laminar flow of fluid into a partially penetrating well. « g e r l a n d s b e i t r ä g e z u r g e o p h y s i k » , 85, 2, p p . 1 5 1 1 5 6 . kean, i . h . a n d raza, s.a., 1972 a study of non-linear flow through porous media by electrical analogy method. « j . inst, of engg. india, c.e. dn. », 53, p p . 57-60. p o l u b a r i n o v a k o c h i n a , p.ya., 1 9 6 2 theory of ground water movement. princeton u n i v e r s i t y press, p r i n c e t o n , n e w jersey. u c h i d a , s . , 1 9 2 5 on the non-linear parcolation at high reynolds number. « n a t . c o m m i t t e e for t h e o r . appl. mech.», p p . 4 3 7 4 4 2 . u p a d h y a y , k . k . , 1 9 7 5 non-linear laminar flow into eccentrically placed well. «annali di g e o f í s i c a », xxviii, 4, p p . 311-319. u p a d h y a y , k . k . , 1977 non-linear laminar flow into well partielly penetrating a porous aquifer of finite thickness. « p u r e a n d applied geophysics», 115, p p . 631-638. u p a d h y a y , k . k . , 1 9 7 7 non-linear laminar flow of fluid into a fully penetrating cylindrical well. « i n d . j o u r . theor. physics», 25, n. 4 , p p . 1 4 9 1 5 6 . w r i g h t , d.e., 1968 non-linear flow through grannular media. « j . hy. dn., proc. a s c e » , 94, hy 4, p p . 851-872. s t r u t t u r a d e l l a c r o s t a t e r r e s t r e i n c o r r i s p o n d e n z a d e l l ' i t a l i a c e n t r a l e (*) ( g r a n s a s s o ) d . d i filippo l . marcelli richiami. — ii 5 s e t t e m b r e 1950, alle ore 04" 09m g.c.t. circa, e b b e luogo u n t e r r e m o t o nella zona del g r a n sasso d ' i t a l i a . detto m o v i m e n t o tellurico, di n o t e v o l e intensità, data la posizione dell'epic e n t r o f u f a t t o oggetto di u n p a r t i c o l a r e s t u d i o ( 1 2 3 ) . le c o o r d i n a t e e p i c e n t r a l i , il t e m p o o r i g i n e h e la p r o f o n d i t à i p o c e n t r a l e li risultar o n o le seguenti : cp = 42" 30',8 + 2',0 n x = j3° 19/,6 + 5',4 e h = 5 k m h = 04h 08m 57",5 + 0*,5 i n u n o dei lavori a c c e n n a t i (2) a b b i a m o calcolato lo spessore dello s t r a t o del g r a n i t o n e l l ' i t a l i a c e n t r a l e valendoci dei r i s u l t a t i conseguiti p e r le velocità delle o n d e pg e p*. a p p l i c a n d o i n f a t t i la nota f o r m u l a vp v„, 2 d = — p g — „ g + h ' (i'pg ) (0 a sei stazioni c b e p r e s e n t a v a n o c h i a r i esempi di onde p*, p r e n d e n d o t'p = 5,46 k m / s e c , vp, = 6,38 k m / s e c . ; h = 5 k m e p e r fi le differenze f r a i t e m p i di registrazione delle pg e delle p* in ogni stazione, abb i a m o t r o v a t o c h e lo spessore m e d i o d dello strato del g r a n i t o nell ' i t a l i a c e n t r a l e è di 26 k m . spessore dello strato del granito con metodo grafico. — a b b i a m o voluto calcolare lo spessore dello strato del g r a n i t o a n c h e s f r u t t a n d o u n m e t o d o grafico in uso nella prospezione sismica del sottosuolo a (*) c o m u n i c a z i o n e presentata alla r i u n i o n e d e l l a c o m m i s s i o n e s i s m o l o g i c a e u r o p e a d e l l ' u . g . g . i , tenuta a stoccarda n e l s e t t e m b r e 1952. 570 1). di f i l i p p o l . m a r c e l l i scopo m i n e r a r i o , e già a d o p e r a t o con successo da caloi (5) p e r d e t e r m i n a r e lo s t r a t o del g r a n i t o n e l cansiglio. q u e s t o m e t o d o s f r u t t a i p u n t i a n g o l a r i n e l l e c u r v e dei t e m p i , c r e a t i da b r u s c h e v a r i a z i o n i di velocità del r a g g i o sismico, q u a n d o q u e s t o passa d a u n o s t r a t o ad u n altro. n e l caso di u n a s t r a t i f i c a z i o n e p i a n a a s u p e r f i c i p a r a l l e l e t r a l o r o ed alla s u p e r f i c i e e s t e r n a della t e r r a (cosa c h e p e r piccole d i s t a n z e p u ò r i t e n e r s i v e r a in ogni caso) le d r o m o c r o n e o t t e n u t e r e g i s t r a n d o i n u n c e r t o p u n t o l e o n d e prov o c a t e d a u n o s c o p p i o a r t i f i c i a l e , p r e s e n t e r a n n o u n p r i m o p u n t o ang o l a r e in c o r r i s p o n d e n z a della b r u s c a v a r i a z i o n e di velocità d e t e r m i n a t a d a l l ' a r r i v o in s u p e r f i c i e delle o n d e c h e sono p e n e t r a t e n e l sec o n d o s t r a t o ( o v v i a m e n t e n e l caso di u n t e r r e m o t o q u e s t o a v v i e n e q u a n d o le pg d i v e n g o n o p* p e n e t r a n d o n e l l o s t r a t o d e l basalto). a q u e s t o p u n t o a n g o l a r e c o r r i s p o n d e , sul t e r r e n o , il p u n t o i n cui a r r i v a n o cont e m p o r a n e a m e n t e in s u p e r f i c i e l e o n d e l o n g i t u d i n a l i d i r e t t e (pg) e l e o n d e l o n g i t u d i n a l i r i f r a t t e dalla s u p e r f i c i e l i m i t a n t e i n f e r i o r m e n t e lo s t r a t o (p*). la p r o f o n d i t à di q u e s t ' u l t i m a s u p e r f i c i e è d a t a d a l l a p r o f o n d i t à del p u n t o in cui il r a g g i o l i m i t e u s c e n t e d a l c e n t r o di s c o p p i o (cioè d a l l ' i p o c e n t r o ) i n t e r s e c a il l u o g o dei p u n t i in cui i d u e sistemi d i o n d e , d i r e t t o e r i f r a t t o , g i u n g o n o c o n t e m p o r a n e a m e n t e a d i s t a n z e d i v e r s e d a l l ' i p o c e n t r o , n e l l ' i n t e r n o d e l l o s t r a t o . la costruz i o n e di q u e s t o l u o g o g e o m e t r i c o si o t t i e n e c o n g i u n g e n d o i p u n t i di i n t e r s e z i o n e di u n a s c h i e r a di r e t t e ( f r o n t i delle o n d e r i f r a t t e ) e del c o r r i s p o n d e n t e fascio di c e r c h i ( f r o n t i d e l l e o n d e d i r e t t e ) c o n c e n t r i c i n e l l ' i p o c e n t r o . t e o r i c a m e n t e si p r o c e d e n e l s e g u e n t e m o d o ( n e l caso di u n terr e m o t o ) : si a s s u m a l ' e p i c e n t r o c o m e o r i g i n e d e l l e c o o r d i n a t e : l'asse x si o r i e n t i d a l l ' e p i c e n t r o alla s t a z i o n e di osservazione, e l'asse z dall'epic e n t r o a l l ' i p o c e n t r o . i l t e m p o o r i g i n e sia q u e l l o d e l t e r r e m o t o già calcolato. l ' e q u a z i o n e dei f r o n t i d ' o n d a delle pg ( f a s c i o di cerchi) s a r à : x*+(z-hy-=v\f ( h = p r o f o n d i t à i p o c e n t r a l e , t — t e m p o di t r a g i t t o delle pg, v1>g = velocità delle o n d e l o n g i t u d i n a l i d i r e t t e n e l p r i m o strato). l ' e q u a z i o n e dei f r o n t i d ' o n d a d e l l e p* n e l l ' i n t e r n o dello s t r a t o d e l l e pg ( s c h i e r a di r e t t e ) è : (z — d) cos i—(* — a) sin i vp (t — h) — 0 s t r u t t u r a della crosta t e r r e s t r e i n corrispondenza d e l l ' i t a l i a c e n t r a l e 5 7 1 (d = spessore dello s t r a t o i n e s a m e , i — angolo di i n c i d e n z a del raggio l i m i t e , a = ascissa del r e l a t i v o p u n t o d ' i n c i d e n z a sulla s u p e r f i c i e d i d i s c o n t i n u i t à , ti — t e m p o di t r a g i t t o d a l l ' i p o c e n t r o al p u n t o d i incidenza). e l i m i n a n d o t t r a l e d u e e q u a z i o n i si o t t i e n e il l u o g o c e r c a t o : si lia : ( x cos i -(z sin if — x (2d — li) sin 2i 2z [ (2d — h) cos2 i — h] — (2d — li)2 cos2 i f li= 0 c h e è l ' e q u a z i o n e di u n a p a r a b o l a . p e r la c o s t r u z i o n e grafica si p r o c e d e n e l m o d o s e g u e n t e : le drom o c r o n e delle pg e delle p* ( r i f e r i t e al t e m p o o r i g i n e del t e r r e m o t o ) si i n c o n t r a n o in u n p u n t o , cui c o r r i s p o n d e u n a c e r t a distanza epicent r a l e ( p e r noi \ = 162 km). a questa distanza e v i d e n t e m e n t e le d u e o n d e g i u n g o n o i n s i e m e e ciò avviene p r e c i s a m e n t e 29s,8 d o p o il t e m p o o r i g i n e del t e r r e m o t o . la conoscenza d e l l e velocità di p r o p a g a z i o n e d e l l e pg e delle p* ci c o n s e n t e l a d e t e r m i n a z i o n e di i: sen i = — 1 = 5,46 : 6,38 i = 58" 51' p o s s i a m o q u i n d i t r a c c i a r e (v. fig. 1) u n c o n v e n i e n t e n u m e r o di r e t t e ( f r o n t i d ' o n d a delle p* n e l l ' i n t e r n o d e l p r i m o s t r a t o ) i n c l i n a t e dell'angolo i r i s p e t t o alla s u p e r f i c i e della t e r r a . i n t e r s e c a t e q u e s t e r e t t e con a l t r e t t a n t i c e r c h i ( f r o n t i d ' o n d a delle pg) c o n c e n t r i c i n e l l ' i p o c e n t r o e raggi c o r r i s p o n d e n t i ai t e m p i r e l a t i v i ad ogni singolo f r o n t e d ' o n d a , il l u o g o dei p u n t i d ' i n t e r s e z i o n e è la p a r a b o l a cercata. i l p u n t o in cui la p a r a b o l a così costruita i n c o n t r a il raggio l i m i t e u s c e n t e dall ' i p o c e n t r o è u n p u n t o della s u p e r f i c i e di s e p a r a z i o n e di d u e mezzi, cioè r a p p r e s e n t a la p r o f o n d i t à della p r i m a s u p e r f i c i e di d i s c o n t i n u i t à . dalla figura r i s u l t a e v i d e n t e c h e l ' i n t e r s e z i o n e avviene ad u n a p r o f o n dità di 25,5 k m , i n p e r f e t t o a c c o r d o con il p r e c e d e n t e r i s u l t a t o . in572 1). di f i l i p p o l . m a r c e l l i t e n d i a m o n e l p r e s e n t e l a v o r o c a l c o l a r e la p r o f o n d i t à della s u p e r f i c i e di moliorovicic, l i m i t a n t e i n f e r i o r m e n t e l o s t r a t o del b a s a l t o . calcolo dello spessore della crosta terrestre in corrispondenza del gran sasso d'italia. a) tentativo con il metodo del raggio d'inversione. — u n p r i m o t e n t a t i v o p e r t r o v a r e q u e s t o spessore, lo a b b i a m o f a t t o a d o t t a n d o i l m e t o d o s e g n a l a t o d a l g i a p p o n e s e m i n a k a m i (4). si p a r t e d a l l ' i p o t e s i c b e il f u o c o del sisma si tro v i n e l l a crosta t e r r e s t r e : si s u p p o n e i n o l t r e c b e la d i s t r i b u z i o n e degli i m p u l s i i n i z i a l i sup mohorovicic f i g . 2 del t e r r e m o t o n e l l e v a r i e stazioni sia t a l e d a p o t e r i n d i v i d u a r e con sufficiente p r e c i s i o n e il r a g g i o r d e l c e r c h i o d ' i n v e r s i o n e del segno dei m o v i m e n t i i n i z i a l i (v. fig. 2). sia h la p r o f o n d i t à i p o c e n t r a l e , d lo spessore della crosta t e r r e stre, i l ' a n g o l o d ' i n c i d e n z a d e l l ' o n d a sismica sulla s u p e r f i c i e di discont i n u i t à s u p p o s t a p a r a l l e l a alla s u p e r f i c i e e s t e r n a r i t e n u t a p i a n a : se r i e v-> s o n o le velocità di p r o p a g a z i o n e d e l l e o n d e l o n g i t u d i n a l i n e l p r i m o e n e l secondo s t r a t o della crosta t e r r e s t r e , con d e l l e consider a z i o n i a b b a s t a n z a s e m p l i c i di e g u a g l i a n z e d i t e m p i di t r a g i t t o , si h a c b e : y k°-+h* 2 d 4 + i « ( 2 * -h) tg [1] n e l n o s t r o caso il raggio r d ' i n v e r s i o n e è b e n i n d i v i d u a t o . d i f a t t i n e l l e stazioni i n t o r n o a l l ' e p i c e n t r o il m o v i m e n t o i n i z i a l e si è present a t o d o v u n q u e c o m e u n a c o m p r e s s i o n e fino a p a d o v a (a = 3 4 1 k m ) s t r u t t u r a della crosta t e r r e s t r e i n corrispondenza d e l l ' i t a l i a c e n t r a l e 5 7 3 m e n t r e a t r i e s t e ( a = 3 4 9 k m ) e n e l l e stazioni più distanti è c o m p a r s o come d i l a t a z i o n e ( ] ) . senza t i m o r e d i c o m m e t t e r e u n grosso e r r o r e , p o s s i a m o p r e n d e r e r = 34-5 k m . o c c o r r e p e r ò osservare c h e il caso c o n t e m p l a t o da m i n a k a m i si r i f e r i s c e ad u n solo strato. s a p p i a m o invece d i e p r i m a di g i u n g e r e alla superficie di d i s c o n t i n u i t à di mohor o v i c i c l e o n d e l o n g i t u d i n a l i p e r c o r r o n o lo s t r a t o del g r a n i t o p r i m a e q u e l l o del b a s a l t o poi, con velocità d i v e r s e : i valori d a noi trovati sono p r e c i s a m e n t e : f p g = 5 , 4 6 k m / s e c , vp» = 6,38 k m / s e c e i n f i n e vpn = 8,19 k m / s e c . a b b i a m o r i t e n u t o lecito, s e p p u r e n o n r i g o r o s a m e n t e esatto, med i a r e l a velocità d e l l e pg e delle p* o n d e a p p l i c a r e la f o r m u l a di m i n a k a m i p o n e n d o vl = 5,92 k m / s e c ; v2 = up n g,19 k m / s e c . a l l o r a , l o spessore della crosta t e r r e s t r e calcolato a p p l i c a n d o la [1] r i s u l t a essere 7 __ . a oo 70 k m . i l v a l o r e eccessivo t r o v a t o n o n ci c o n s e n t e di a c c e t t a r e s e n z ' a l t r o il r i s u l t a t o c o n s e g u i t o : t u t t a v i a p u ò n a s c e r e il d u b b i o c h e lo spessore i n e s a m e possa i n r e a l t à essere p i ù elevato di q u a n t o n o n si pensi. b) metodo analitico per il calcolo degli strati. — lo spessore dello s t r a t o del g r a n i t o e di q u e l l o del b a s a l t o si p u ò c a l c o l a r e s f r u t t a n d o la conoscenza d e l l e d r o m o c r o n e delle o n d e pxi e pxn. p e r l a t e o r i a , a b b a s t a n z a s e m p l i c e del resto, r i m a n d i a m o al già c i t a t o l a v o r o di caloi (5ì. ci l i m i t i a m o a r i p o r t a r e q u i l e f o r m u l e di cui ci s i a m o s e r v i t i : se con d i i n d i c h i a m o lo spessore dello s t r a t o del g r a n i t o , con tiq u e l l o del b a s a l t o , con tp f p i o f p n , i t e m p i di a r r i v o delle pxx, p.x3, pn n e l l e v a r i e stazioni, con vp , r s g , vp*, t'p n, vs., le velocità dei vari t i p i d ' o n d e sismiche, valgono l e seguenti r e l a z i o n i : dt=hi t v p r e s e le s e g u e n t i otto s t a z i o n i : t a r a n t o , cliur, zùricli, w i e n n , n e u c h à t e l , basel, s t u t t g a r t , j e n a , a b b i a m o calcolato l e d i f f e r e n z e dei t e m p i di a r r i v o delle v a r i e o n d e e a b b i a m o o t t e n u t o i s e g u e n t i v a l o r i 574 1). di f i l i p p o l . m a r c e l l i m e d i : tp^ — tpa — 3s,4, t p — t p ^ = 5s,25. s o s t i t u e n d o poi alle v a r i e velocità i v a l o r i da n o i t r o v a t i n e l p r e c e d e n t e l a v o r o (3) vpg— 5,46 k m / s e c . v s g = 3,01 k m / s e c . i>p = 8,19 k m / s e c . vpt— 6,38 k m / s e c . vs*= 3,63 k m / s e c . a b b i a m o o t t e n u t i i s e g u e n t i r i s u l t a t i : eh = 24,71 k m d2 = 36,02 k m i l p r i m o v a l o r e c o n s e g u i t o è i n o t t i m o a c c o r d o con q u e l l i t r o v a t i p r e c e d e n t e m e n t e con a l t r i m e t o d i , i n t e r e s s a n d o a l t r e o n d e ; sicché poss i a m o o r m a i a f f e r m a r e con b u o n a a t t e n d i b i l i t à c b e lo spessore dello s t r a t o del g r a n i t o n e l l ' i t a l i a c e n t r a l e è ^ g r a n i t o 25 klll i l v a l o r e dello s t r a t o d e l b a s a l t o a t u t t a p r i m a ci è s e m b r a t o u n p o ' t r o p p o e l e v a t o s e b b e n e già il t e n t a t i v o con il r a g g i o d'inversione (§ a) facesse i n t r a v e d e r e t a l e r i s u l t a t o . t u t t a v i a a b b i a m o v o l u t o c e r c a r e u l t e r i o r e c o n f e r m a . e) conferma dei risultati conseguiti, mediante la dromocrona delle rmp. — ci s i a m o valsi allo scopo, della d r o m o c r o n a d e l l e ri pg, da n o i calcolata n e l p r e c e d e n t e l a v o r o ( s ì . l ' e q u a z i o n e di d e t t a d r o m o c r o n a , r i f e r i t a al t e m p o o r i g i n e del t e r r e m o t o r i s u l t ò la s e g u e n t e : i 2 = (0,03054 ± 0,00073) a2 + (425,8981 ± 61,1066) a l l ' e p i c e n t r o ( p e r a = 0ì la c u r v a taglia l'asse dei t e m p i n e l p u n t o ( t r a s c u r a n d o gli e r r o r i ) ? j..—)'425,898 t = 20*,6 è b e n n o t o c h e si i n d i c a n o con ri pg q u e l l e o n d e l o n g i t u d i n a l i c h e si r i f l e t t o n o u n a volta sulla s u p e r f i c i e di m o h o r o v i c i c e c b e noi c h i a m e r e m o rmp. c a l c o l i a m o d u n q u e il t e m p o i m p i e g a t o d a l l e o n d e l o n g i t u d i n a l i p e r t o r n a r e i n s u p e r f i c i e d o p o essere p a r t i t e d a l l ' i p o c e n t r o / , s i t u a t o ad u n a p r o f o n d i t à /i = 5 k m . a m m e t t e n d o p e r b u o n i i valori degli spessori t r o v a t i n e l p a r a g r a f o p r e c e d e n t e si h a cbe l e o n d e i n q u e s t i o n e v i a g g i a n o complessiv a m e n t e p e r (20 -j25) k m n e l l o s t r a t o del g r a n i t o con la velocità d e l l e pg ( v p i = 5,46 k m / s e c . ) e p e r (36 -f36) k m n e l l o s t r a t o del basalto con la velocità delle p* (vpt = 6 , 3 8 k m / s e c . ) . s t r u t t u r a della crosta t e r r e s t r e i n corrispondenza d e l l ' i t a l i a c e n t r a l e 5 7 5 se ora c a l c o l i a m o il t e m p o di t r a g i t t o delle rmp si h a : r m p i y q u e s t o valore, c o n f r o n t a t o con il 20s,6 d a t o d a l l a d r o m o c r o n a , o f f r e u n a s o r p r e n d e n t e c o n f e r m a d e l l e i p o t e s i assunte. discussione dei risultati. —e s a m i n a n d o i r i s u l t a t i o t t e n u t i n e i p r e c e d e n t i p a r a g r a f i e t e n e n d o c o n t o dei v a r i m e t o d i usati, ciascuno dei q u a l i si serve di e l e m e n t i diversi e i n d i p e n d e n t i l ' u n o d a l l ' a l t r o , siamo p o r t a t i ad a c c e t t a r e o r m a i p e r h u o n i i v a l o r i t r o v a t i e ad assetabella 1 r e g i o n i s p e s s o r e i n k m ( a p p r o s s i m a t i v o ) r e g i o n i di ds d 1 b e l g i o 1 s c h w a e b i s c h e a l b (a n o r d dell'ali to d a n u b i o ) ( g u t e n b e r g ) 1 w i i r t t e m b e r g ( c a l o i ) a s c h w a e b i s c h e a l b (a n o r d dell'alga 1 to d a n u b i o ( h i l l e r ) p \ a l p i g a m i c h e ( c a l o i ) ^ j u g o s l a v i a a l p i sett. e v o r l a n d c a n s i g l i o ( c a l o i ) a p p e n n i n o centrale ( t o s c a n a ) • ( c a l o i r o s i n i ) ( c a u c a s o ^ s a s i a centrale ( r e g i o n e f e r g h a m ) ( g i a p p o n e , n e w z e a l a n d l c a l i f o r n i a c e n t r a l e ( b y e r l y ) •s ] c a l i f o r n i a m e r i d . ( g u t e n b e r g ) £ i c a l i f o r n i a sud-occid. j j r e g i o n e o w e n s v a l l e y (37,6 n ) 1 1 3 , 7 w ) [ sierra n e v a d a ) 3 0 2 5 3 0 2u 2 0 3 0 1 5 3 5 3 5 4 0 2 5 3 0 2 0 2 5 s t r u t t u r a s i m 9 1 4 2 0 2 5 1 0 2 0 2 5 15 2 5 2 0 2 5 1 1 2 5 2 0 e a q u e i l a d e l l a ( 2 3 ) ( 2 5 ) 4 0 4 0 5 0 4 5 4 0 5 5 6 0 5 0 4 5 g e r m a n i a m e r i d . 3 0 3 2 3 9 5 0 6 0 6 5 g n a r e alla crosta t e r r e s t r e sotto il g r a n sasso, u n o spessore complessivo di circa 60 k m . q u e s t o v a l o r e , del resto, n o n è esagerato se l o si cons i d e r a n e l q u a d r o g e n e r a l e degli spessori calcolati da a l t r i a u t o r i p e r a l t r e r e g i o n i . la t a b e l l a i r i a s s u m e q u e s t i e l e m e n t i ( 6 ) : (essendo d i — spessore dello s t r a t o del g r a n i t o ; d» = spessore dello s t r a t o d e l b a s a l t o ; d = p r o f o n d i t à della s u p e r f i c i e di moliorovicic, ossia spessore t o t a l e della crosta t er r es t r e) . dalla t a b e l l a r i s u l t a c h i a r o c o m e i m a s s i m i valori t r o v a t i p e r 576 1). di f i l i p p o l . m a r c e l l i lo spessore della crosta t e r r e s t r e s i a n o in c o r r i s p o n d e n z a delle z o n e a l p i n e in e u r o p a e della s i e r r a n e v a d a i n c a l i f o r n i a , m a m e n t r e sotto le a l p i si n o t a u n i s p e s s i m e n t o d e l l o s t r a t o d e l g r a n i t o , sotto la s i e r r a n e v a d a s e m b r a c h e l ' a u m e n t o complessivo dello spessore sia d o v u t o p i ù s e n s i b i l m e n t e ad u n i s p e s s i m e n t o dello s t r a t o d e l b a s a l t o . n e l caso d e l l a z o n a a p p e n n i n i c a del g r a n sasso, s e m b r a q u a s i c h e ci si d e b b a t r o v a r e i n u n a s i t u a z i o n e i n t e r m e d i a , g i a c c h é c o m e si è visto di cnj 25 k m d-> 36 k m d ex; 60-61 k m n o t i a m o i n c i d e n t a l m e n t e c h e m e n t r e le a l t i t u d i n i m e d i e della s i e r r a n e v a d a v a n n o d a 3000 a 4000 m , q u e l l e d e l l e p u n t e p i ù e l e v a t e del massiccio del g r a n sasso r a g g i u n g o n o al m a s s i m o i 3000 m. cons/gf/o ( £ = 4 2 ° 3 o ' , 8 n x = 1 5 ° 19', 6 e g r a n s a s s o j/c/7/ó granito «vj 1 v5.46 km/sec basalto 1 r 1/=6.33 km/sec [/= 8 j3 km/sec f i g . 3 la figura 3 m o s t r a u n t e n t a t i v o di sezione della n o s t r a zona. inclinazione dello strato del granito. — u n o s g u a r d o alla t a b e l l a i ed ai r i s u l t a t i da n o i conseguiti, m o s t r a e v i d e n t e c o m e lo s t r a t o del g r a n i t o v a d a assottigliandosi d a l l ' i t a l i a s e t t e n t r i o n a l e a q u e l l a cent r a l e : la s u p e r f i c i e d e l i m i t a n t e i n f e r i o r m e n t e il p r i m o s t r a t o è evident e m e n t e i n c l i n a t a , con u n s o l l e v a m e n t o verso l ' i t a l i a c e n t r a l e e ciò n o n costituisce c b e u n a u l t e r i o r e c o n f e r m a a q u a n t o già da a l t r i a u t o r i era stato d e t t o i n p r o p o s i t o (7). a b b i a m o t e n t a t o di v a l u t a r e l ' o r d i n e di g r a n d e z z a d e l l ' a n g o l o di i n c l i n a z i o n e , ed a l l ' u o p o ci s i a m o valsi del m e t o d o c h e caloi e s p o n e n e l l a v o r o già c i t a t o (7). da c o n s i d e r a z i o n i a b b a s t a n z a s e m p l i c i si perv i e n e alle s e g u e n t i d u e f o r m u l e c h e l e g a n o t r a l o r o gli e l e m e n t i di d u e q u a l s i a n s i stazioni s i s m i c h e a l l i n e a t e con l ' e p i c e n t r o : «** t s t r u t t u r a della crosta t e r r e s t r e i n corrispondenza d e l l ' i t a l i a c e n t r a l e 5 7 7 i simboli h a n n o i s e g u e n t i s i g n i f i c a t i : vi — velocità delle o n d e l o n g i t u d i n a l i d i r e t t e : vp^ — 5,46 k m / s e c . t i , t2 = t e m p i d ' a r r i v o d e l l e o n d e p* n e l l e d u e stazioni prescelte a,, a2 = distanze e p i c e n t r a l i delle d u e stazioni (in k m ] i = angolo d ' i n c i d e n z a della pg sulla s u p e r f i c i e in q u e s t i o n e : sin i = i = 58" 52' 4 0 " i "p* à = i n c l i n a z i o n e della s u p e r f i c i e di d i s c o n t i n u i t à r i s p e t t o alla sup e r f i c i e t e r r e s t r e . nella f o r m u l a i l segno — i n d i c a che lo s t r a t o è m o n t a n t e dallo e p i c e n t r o alle stazioni (cioè va assottigliandosi), il segno -fviceversa i n d i c a t r a t t a r s i di s t r a t o d i s c e n d e n t e (lo s t r a t o va ispessendosi verso le stazioni). scelti a l c u n i a c c o p p i a m e n t i in a p p r o s s i m a t i v o a l l i n e a m e n t o con l ' e p i c e n t r o ( t a r a n t o m e s s i n a , basilea-stoccarda, c h u r p a v i a , belgradotrieste... i a b b i a m o t r o v a t o c h e lo s t r a t o r i s u l t a m o n t a n t e verso la c o p p i a t a r a n t o m e s s i n a (xcv>2°,3) e d i s c e n d e n t e verso t u t t e le r i m a n e n t i c o p p i e : basilea-stoccarda (\ oo 5°,9), c h u r p a v i a (x ex; 4°,lì, b e l g r a d o t r i e s t e (à tv; 2°,5|. n a t u r a l m e n t e la v a l u t a z i o n e n u m e r i c a è s o l t a n t o a p p r o s s i m a t i v a . i piccoli valori t r o v a t i p e r a c o n f e r m a n o tuttavia in m a n i e r a p i ù rigorosa le i p o t e t i c h e asserzioni f a t t e finora. se poi c o n s i d e r i a m o l ' a l l i n e a m e n t o cansiglio-gran sasso i le d u e z o n e d i s t a n o poco m e n o di 400 k m ) e fissiamo l ' a t t e n z i o n e sugli spessori del g r a n i t o ivi calcolati ( r i s p e t t i v a m e n t e 40 e 25 k m ) , u n a semplicissima v a l u t a z i o n e d i t r i g o n o m e t r i a p i a n a ci dà p e r à. il v a l o r e a p p r o s s i m a t i v o d i 2°,2. la c o n c o r d a n z a i n v e r o eccezionale di q u e s t i r i s u l t a t i p o r t e r e b b e i n o l t r e a d e s u m e r e c h e s c e n d e n d o u l t e r i o r m e n t e verso l ' i t a l i a m e r i d i o n a l e , ad u n c e r t o p u n t o il g r a n i t o d o v r e b b e a d d i r i t t u r a s c o m p a r i r e (a circa 1000 k m dal consiglio in a l l i n e a m e n t o m e r i d i a n o ) . è n a t u r a l m e n t e a z z a r d a t o a f f e r m a r e s e n z ' a l t r o u n a cosa s i m i l e : sta di f a t t o p e r ò , c h e a d e t t a d i a l c u n i geologi, nell'isola di ustica (a n o r d della sicilia, circa 850 k m a sud del cansiglio) in a l c u n i p u n t i affiora il basalto. roma — istituto nazionale di geofisica — agosto 1952. 578 1). di f i l i p p o l . m a r c e l l i riassunto sulla base dei risultati conseguiti nello studio del terremoto avvenuto il 5 settembre 1950 con epicentro nella zona montuosa del gran sasso d'italia, sono stati effettuati vari calcoli per tentare di individuare lo spessore dello strato del granito e la profondità della superficie di mohorovicic. i risultati cui si è giunti sono ollremodo interessanti: vari metodi applicati (metodo del raggio del cerchio diinversione degli impulsi iniziali, metodo grafico delle superfici d'onda, delle p g e p * . metodo analitico con le dromocrone delle p x i e p x 2 , conferma con la dromocrona della r m p ) , hanno condotto, con sorprendente uniformità, ad assegnare allo strato del granito per l'italia centrale uno spessore di 25 km, ed a quello del basalto uno spessore di 36 km, sicché la superficie di mohorovicic si troverebbe ad una profondità di oo 60 km, paragonabile alle profondità trovate nelle zone alpine (carnia meridionale, vorland...) e nella sierra nevada. è stato inoltre accertato che la superficie alla base dello strato del granito risulta inclinata di un piccolo angolo (compreso tra 2° e 5°), il che determina un assottigliamento del suddetto strato verso l'italia meridionale. summary on the basis of the results obtained in the study of the earthquake which occured september 5, 1950 ivith epicenter in the mountainous zone of gran sasso, italy, various calculations li ave been made to try to determine the thickness of the layer of granite and the deptli of the surface of mohorovicic. the results tlius obtained are interesting for otlier reasons. the various metliods applied (method of the radius of curvature of inversion of the initial impulses, graphical method on the ivave surfaces p g and p*, analytic method with the travel times of p x , and pxo, checkd witli the travel tinte of r m p ) , have lead, with surprising uniformity to a. value of 25 km for the thickness of the layer of granite for central italy, and for the basalt layer a thickness of 36 km, so that tlie surface of mohorovicic is at a depili of about 60 km, comparable to that found in the alpine zone (central carnia, vorland...) and in the sierra nevada. it lias been further found that the surface at the bottoni of the layer of granite is inclined at a smallaligle (between 2° and 5°), s t r u t t u r a d e l l a c r o s t a t e r r e s t r e i n c o r r i s p o n d e n z a d e l l ' i t a l i a c e n t r a l e 5 7 9 and this produces a diminution of the above-mentioned layer toward centrai italy. b i b l i o g r a f i a ( ! ) d . d i f i l i p p o , l . m a r c e l l i : uno studio del terremoto del gran sasso d'italia del 5 settembre 1950. « a n n a l i di g e o f i s i c a » i v , n. 2 (1951). ( ) d . d ì f i l i p p o , l . m a r c e l l i : tempi di tragitto delle onde p* e spessore dello strato del granito nelfitalia centrale. « a n n a l i di g e o f i s i c a » i v , n . 4 (1951). ( 3 ) d . d i f i l i p p o , l . m a r c e l l i : dromocrone per terremoti vicini e velocità tielle onde nell'italia centrale. « a n n a l i di g e o f i s i c a » v , n . 2 (1952). ( 4 ) m i n a k a m i t . : distribution des moviments initìaux d'un seisme dont le foyer se trouve, dans la couche superficielle, et determination de vépaisseur de rette couche. « b u l l e t i n e a r t h q . r e s e a r c h i n s t i t u t », t o k i o i m p e r i a i u n i v e r s i t y , v o i . x i i i , 1935. ( ° ) p . c a l o i : ricerche su terremoti ad origini vicine. scosse del consiglio dell'ottobre 1936. « r i c e r c a s c i e n t i f i c a » , v o i . i i n. 7-8. («) p . c a l o i : sulla velocità di propagazione delle onde / " e sullo spessore dello strato del granito nell'europa centrale. « la r i c e r c a s c i e n t i f i c a », a n n o xi, n. 11 ( n o v e m b r e 1940). ( 7 ) b . g u t e n b e r g : seismological evidence for roots of mountains. « b u l l . g e o l o g . s o c . of a m e r i c a . v o i . 54, a p r i l e 1943. vol50,3,2007 329 annals of geophysics, vol. 50, n. 3, june 2007 key words b-value – seismology – least squares technique – syntheticearthquake catalogs 1. introduction the gutenberg-richter law (from now on gr law) (gutenberg and richter, 1954) is certainly one of the most remarkable and ubiquitous features of seismicity worldwide. in the most common form it reads . (1.1)( )log n m a bm= −6 @ in the so-called binned form of the gr law, n represents the number of events with magnitude m, while in the cumulative form n is the number of events with magnitude larger or equal to m. the constants a and b are the coefficients of the linear relationship. as discussed in a previous paper (marzocchi and sandri, 2003, from now on ms03), the scientific relevance of the gr law is linked to its apparent ubiquity, and to the theoretical implications and meaning of its possible universality. in particular, the value of b, representing the opposite of the slope of the linear relationship in eq. (1.1), is considered very important. in fact, in spite of a «first order» validity of the gr law with a constant b-value ≈ 1 observed in a variety of tectonic settings, significant spatial and temporal variations in the b-value are found (e.g., riedel et al., 2003; del pezzo et al., 2004; legrand et al., 2004; mandal et al., 2004; ratchkovski et al., 2004; wyss et al., 2004; mura technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique laura sandri and warner marzocchi istituto nazionale di geofisica e vulcanologia, sezione bologna, italy abstract we investigate conceptually, analytically, and numerically the biases in the estimation of the b-value of the gutenberg-richter law and of its uncertainty made through the least squares technique. the biases are introduced by the cumulation operation for the cumulative form of the gutenberg-richter law, by the logarithmic transformation, and by the measurement errors on the magnitude. we find that the least squares technique, applied to the cumulative and binned form of the gutenberg-richter law, produces strong bias in the b-value and its uncertainty, whose amplitudes depend on the size of the sample. furthermore, the logarithmic transformation produces two different endemic bends in the log(n) versus m curve. this means that this plot might produce fake significant departures from the gutenberg-richter law. the effect of the measurement errors is negligible compared to those of cumulation operation and logarithmic transformation. the results obtained show that the least squares technique should never be used to determine the slope of the gutenberg-richter law and its uncertainty. mailing address: dr. laura sandri, istituto nazionale di geofisica e vulcanologia, sezione bologna, via donato creti 12, 40128 bologna, italy; e-mail: sandri@bo.ingv.it 330 laura sandri and warner marzocchi ru et al., 2005; schorlemmer et al., 2005). these possible variations are very important from a theoretical point of view, and for seismic hazard studies. we refer to our previous paper (ms03) for a deeper discussion on these issues. as a matter of fact, a crucial aspect in inferring variations or constancy of the b-value is represented by the method used to determine the b-value and its uncertainty, given a seismic catalog, as shown in ms03. in that paper, we discussed the maximum likelihood method to estimate the b-value and its uncertainty, showing that incorrect formulae based on this method produce a bias in the b-value and an underestimation of its uncertainty. the latter implies that we might observe a fake variation in the b-value only because we have underestimated the uncertainty. intentionally, in ms03 we did not consider the least squares (from now on, ls) method, because it had already been recognized (page, 1968; bender, 1983) that this technique applied to the problem of the estimation of the b-value and of its uncertainty does not have any statistical foundation. however, the ls technique is still widely employed to estimate these two quantities, both in the binned and in the cumulative form, also in recent literature (e.g., working group on california earthquake probabilities, 2003; gruppo di lavoro, 2004; lopez pineda and rebollar, 2005). the main purpose of this paper is to demonstrate, by means of conceptual issues, analitical formulations and numerical simulations, that the use of the ls technique in the estimation of the b-value and of its uncertainty leads to strongly biased estimates of these two quantities. for the correct estimation of these two quantities, we refer to the appropriate formulae given in ms03 and in references therein. 2. estimation of b and σbtt through the ls technique the ls estimation consists of applying linear regression analysis to the gr law described by eq. (1.1) in the binned or cumulative form. the parameter b should be the same in the two formulations, while the intercept a is different in the two cases. 2.1. cumulative form of the gr law despite its widespread adoption in past and recent papers (pacheco and sykes, 1992; pacheco et al., 1992; karnik and klima, 1993; okal and kirby, 1995; scholz, 1997; triep and sykes, 1997; main, 2000; working group on california earthquake probabilities, 2003; gruppo di lavoro, 2004; lopez pineda and rebollar, 2005), the use of ls technique to estimate the bvalue on the cumulative form of the gr law does not have any statistical motivation. the strongest assumption of regression analysis, i.e., the independence of the observations, is clearly violated. in practice, the cumulative form is an integration and, therefore, it represents a filter for the high frequency noise. as a result, the uncertainty of the slope of the gr law is certainly strongly underestimated. in order to evaluate the bias in the regression analysis applied to the cumulative form numerically, as a function of the number of earthquakes contained in the catalog, we simulated 1000 seismic catalogs, for different catalog sizes. the magnitudes mi are obtained by binning, with a fixed bin width of 0.1, a continuous random variable distributed with a probability density function (pdf) given by equation (2.1) which is valid if the catalog contains earthquakes with a completeness magnitude equal to mmin and with a magnitude range of at least three units (see ms03). in this way, mi is the magnitude attached to all the synthetic seismic events with real continuous magnitude in the range mi − 0.05 ≤ m< mi + 0.05. then, for each synthetic catalog, we estimated the b-value (from now on, using the notation bt for the estimated value, and b for the theoretical value) by means of the ls technique applied to the cumulative gr law described by eq. (1.1), i.e. when n is the number of events with magnitude larger or equal to mi. figure 1 reports the averages of bt calculated in 1000 synthetic catalogs as a function of the number of data, for the cases b = 1 and b = 2. the 95% confidence interval is attached to each average. there is a clear negative bias, that decreases with the number of data and ranges ( ) ( )lnf m b 10 10 ( )b m mmin= − − 331 a technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique from 5% to 2%. an explanation of this bias will be given in the following when we describe the effects of the logarithmic transformation in the regression analysis of the binned magnitudes. as mentioned above, another crucial aspect concerns the estimation of the uncertainty attached to bt , here indicated by σt bt and estimated as the average error on bt provided by the 1000 fig. 2. f test values (see eq. (2.2)) for the case of figs. 1 and 4 (see text). the plus signs represent the cumulative regression, and the squares the binned regression. the dotted line represents the critical value to reject the null hypothesis at a significance level of 0.05. note the logarithmic scale also on the vertical axis. fig. 1. average of bt (dashed line) calculated through cumulative ls technique in 1000 synthetic catalogs, as a function of the catalog size, for the cases b=1 (left) and b=2 (right). at each average is attached the 95% confidence interval. the solid line represents the true b-value. 332 laura sandri and warner marzocchi linear regressions. in particular, we are interested in evaluating if the estimation of σt bt 2 is an unbiased estimator of the true variance of the b estimation of bt around its central value. this can be done by simulating 1000 seismic catalogs as described above. for each catalog we calculated bt and σt bt . then, we compared the dispersion of bt around its average with the average of σt bt , through the fisher test (e.g., kalbfleisch, 1985) (2.2) the null hypothesis we tested is that the average of σt bt 2 is equal to the variance of bt . the results, reported in fig. 2, are very interesting. in particular, the fisher test shows that σt bt strongly underestimates the dispersion of b, the former being at least one order of magnitude smaller than the latter. this confirms that the uncertainty on the b-value estimated through the cumulative ls method is strongly underestimated. in general, the potential factors which can bias the estimations made through the ls technique (both in the cumulative and binned form) are the logarithmic transformation of the number of events, and the presence of the measurement errors on the magnitude. by taking into account the criticism at the regression analysis applied to the cumulative form just reported, in the following we studied the impact of these factors only on the estimation of the b-value made by means of the regression analysis applied to the binned form. 2.2. effects of the logarithmic transformation the expected number of events for a magnitude m is (2.3) where n is the total number of earthquakes in the catalog, ∆m is the bin width used, and f(m) is given by eq. (2.1). the expected fluctuations around this value can be written as ( )np n f m dm /2 / i i m m m m 2 i i ν = = ∆ ∆+ − # .= average of the square of the uncertainty variance of the estimation of the value f b − . (2.4) in this frame, the observed number of events ni for a binned magnitude mi can be written as ni = = νi(1 + ξi), where ξi is a random variable with zero mean and standard deviation equal to ∆ni/νi. by taking into account eqs.((2.3) and (2.4)), we can write . (2.5) it is easy to demonstrate that ∆ni/νi is a strictly monotonic decreasing function of pi. this means that the expected fluctuations of the variable ξ are larger for lower pi, that is for large magnitudes. the estimation of b in the classical linear regression analysis is (see e.g., draper and smith, 1981) . (2.6) if we substitute ni = νi(1 + ξi) we obtain (2.7) if we apply the expectation operator to both sides of eq. (2.7), it is possible to verify that the bias is zero only if e[log(1 + ξi)] = 0. this is certainly not true because e[log(1 + ξi)]<log {e[(1 + ξi)]}=0. to summarize, the application of the classical linear regression analysis to the logarithm of real data (therefore affected by errors) produces a systematic bias on the estimated slope. this is not due to the regression analysis (because the latter always provides an unbiased estimate), but it is related to its application to this type of data. the logarithmic transformation has also another very important effect due to the discreteness of the dependent variable n. we have just ( ) ( ) ( ) ( ) . log log log log b m n m m n m m n m m n m1 1 i i i i i i i i i i i i 2 2 2 2 ν ν ξ ξ = − − − + − − + − + t _ _ i i 6 6 6 6 @ @ @ @ / / / // / / / // ( )( ) loglog b m m m m n n n n i i i i ii 2 2= − − −t _ i 6 6@ @ / / // / n np p1 i i i i ν ∆ = − ( )n np p1i i i∆ = − 333 a technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique fig. 3. logarithm of the expected number of earthquakes (log(νi); see text) according to the gutenbergrichter law (solid line) and average of the logarithm of the generated number of events in each of 1000 synthetic catalogs (dotted line), plotted as a function of the magnitude, for different catalog sizes (from top to bottom: 100, 1000, 10 000 events per catalog). the intersection between the solid and dotted lines represents the point where the bending changes in sign. 334 laura sandri and warner marzocchi shown that the dispersion of the data around the curve increases with magnitude. since we cannot compute log(ni) if ni = 0, at large magnitudes, where ν is low (for example ν≤1), the ls technique can take into account only the «positive» fluctuations around the mean value, i.e. the fluctuations that increase the expected number of events. this acts as a filter for the «negative» fluctuations, that is at large magnitudes (where it can be computed) log(ni) tends to be overestimated compared to log(νi). the global effect consists in the introduction of a systematic negative bias in the estimation of b, that is bt is always lower than b. recalling that the amplitude of the dispersion around the curve is higher for small data sets, in these cases the bias should be larger. figure 3 reports log(νi) (see eq. (2.3)) and the averages of log(ni) obtained by 1000 synthetic catalogs as a function of the magnitudes mi. for each magnitude mi, the average of log(ni) is computed only on those catalogs for whom ni ≥ 1. at lower magnitudes, in the left part of the graph (to the left of the intersection point between the two lines in fig. 3), log(ni) tends to underestimate log(νi). this bias, due to the second addendum in the right side of eq. (2.7), should lead to an overestimation of the bvalue. on the contrary, at large magnitudes (to the right of the intersection point between the two lines in fig. 3), there is a turnover in the trend of log(ni) with respect to log(νi). here, log(ni) tends to be larger than log(νi). as mentioned above, this effect is due to the filtering of the negative fluctuations of ni at large magnitudes performed by the logarithmic transformation of a discrete variable. the consequence is an underestimation of the b-value. the results reported in fig. 3 suggest that, when the larger magnitudes are taken into account, this last bias is always predominant. as argued above, this bias is reduced by increasing the size of the data set. instead, if the larger magnitudes are excluded (e.g., karnik and klima, 1993), the plot log(ni) versus mi shows only one bend, and the b-value is overestimated independently on the size of the catalog. in other words, the global effect of the logarithmic transformation of a discrete variable (the number of seismic events) introduces two bends in the gr curve. it is noteworthy that these discrepancies from a straight line are not linked to any physical process, but only to the mathematical transformation. similar considerations can be made by looking at fig. 4 that reports the estimations of the b-value as a function of the size of the data set. the bias is very strong for small data set and it decreases as the size of the data set increases. the bias is higher in the binned case compared to the cumulative one (compare figs. 4 and 1). this is due to the fact that in the cumulative regression there are fewer zero values in the dependent variable. as shown in fig. 2, the uncertainty of the b-value made in the binned regression is still underestimated, even though it is better than the strong underestimations obtained by the cumulative regression. 2.3. effects of the measurement errors if we add a measurement error at the theoretical magnitude (m), we obtain the «real» magnitude as a sum of two independent random variables (2.8) where m is the earthquake magnitude devoid of measurement errors distributed with a pdf given by eq. (2.1), and ε simulates the measurement errors distributed as a gaussian noise. let us consider the number of the earthquakes with a binned magnitude mi, that is n(mi). by adding a gaussian noise to each magnitude as in eq. (2.8), some events go out of and some come into the considered bin, as shown in fig. 5. the final number is n . the number of data which go out of the bin is (see fig. 5) (2.9) where mol is the number of data which go into the adjacent left bin, mor is the number of data that go into the adjacent right bin, and pε is the probability that a randomly chosen event inside the bin goes out of the bin itself when a measurement error is added. pε is independent from mi and it depends on σε2 (pε→ 0 when σε2→ 0). the number of data which come in from the left bin is (see fig. 5) ( )m m p n mol or i= = ε ( )mio m m ε= +o ( )mo 335 a technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique . (2.10) the number of data which come in from the right bin is (see fig. 5) (2.11) then, we have (2.12) ( ) ( ) ( ) ( ) ( ) . n m n m p n m m n m m n m2 i i ∆ ∆ = + + + − − ε 6 @ o ( ) ( )m p n m p n m mir i i1 ∆= = +ε ε+ ( ) ( )m p n m p n m mil i i1 ∆= = −ε ε− if eq. (1.1) holds, eq. (2.12) can be rewritten as . (2.13) for b ≈ 1 and ∆m = 0.1, as in most of the practical cases, the term of eq. (2.13) inside the round brackets is ≈ 0. in these case n(mi) ≈ n , therefore the calculation of b is only slightly affected by the measurement error. at the opposite, when ∆m is large and/or b > 1, the term inside the round brackets cannot be neglected ( )mio ( ) ( ) ( )n m n m p 10 10 10 2i i a bm b m b mi= + + −ε ∆ ∆− −o fig. 4. average of bt (dashed line) calculated through binned ls technique in 1000 synthetic catalogs, as a function of the catalog size, for the cases b=1 (left) and b=2 (right). at each average is attached the 95% confidence interval. the solid line represents the true b-value. fig. 5. effect of the measurement errors on the number of earthquakes with binned magnitude mi. adding the measurement errors, the number of events that come into the bin with central magnitude mi from the adjacent left bin is mil, while those coming from the adjacent right bin is mir. the number of events that go out of the bin with central magnitude mi towards the adjacent left bin is mol, while those going out towards the adjacent right bin is mor. 336 laura sandri and warner marzocchi and n(mi) ≠n . this obviously introduces a further bias in the b-value estimation. figure 6 reports the effect of the added measurement errors. as argued before, the ( )mio largest difference with the case without measurement errors reported in fig. 4 is for b = 2, where the term of eq. (2.13) inside the round brackets is significantly different from zero. fig. 6. averages of bt (dashed lines) calculated through binned ls technique in 1000 synthetic catalogs as a function of the catalog size, for the cases b=1 (top) and b=2 (bottom). the magnitudes have measurement errors of standard deviation σε=0.1, 0.3, 0.5 (from left to right). at each average is attached the 95% confidence interval. the solid line represents the true b-value. 337 a technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique since pε depends on the amplitude of the measurement error (σε), eq. (2.13) explains also the proportionality of the bias with σε noted in fig. 6. as regards the estimation of the uncertainty of the b-value, the results reported in fig. 7 show that the addition of the measurement errors does not produce any further bias. fig. 7. f test values (see eq. (2.2)) for the case of fig. 6 (see text). the dotted line represents the critical value to reject the null hypothesis at a significance level of 0.05. 338 laura sandri and warner marzocchi 3. final remarks we have studied conceptually, analytically, and numerically the biases introduced by some factors (the cumulation operation, the logarithmic transformation, and the measurement errors on the magnitudes) on the estimation of the b-value and of its uncertainty by means of the least squares technique. we have found many problems. first, besides violating a basic assumption of the regression analysis, the use of the cumulative form of the gutenberg-richter law leads to a very strong underestimation of the uncertainty of the b-value, due to the filtering effect of the cumulation operation. secondly, the logarithmic transformation of the discrete random number of events produces a significant bias in the estimation of the b-value both for the cumulative and binned form of the gutenberg-richter law. moreover, the bias strongly depends on the size of the data set; this means that two samples of different sizes coming from the same statistical distribution will have a significant difference in the b-value estimated. the same logarithmic transformation produces two endemic bends of different signs in the log(n) versus m plot. these bends are not linked to the physical process. this means that some departures from a straight line in the plot log(n) versus m do not invalidate the gutenberg-richter law. finally, the influence of the measurement errors appears to be less important than the bias introduced by the logarithmic transformation. the main purpose of this paper was to show that the estimation of the b-value by means of the least squares method is strongly biased and its uncertainty results heavily underestimated. in this view, this work intended to make clear why this method should not be used to determine the slope of the gutenberg-richter law, and to make inferences about its constancy or variations. in practice, any spatial or temporal variation of the b-value obtained by this method has to be regarded with a strong skepticism. on the other hand, unbiased estimates of the b-value and of its uncertainty can be obtained by using the correct formulae, based on the maximum likelihood method, that have been analitically and numerically tested in marzocchi and sandri (2003) (see also references therein). references bender, b. 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(received december 27, 2006; accepted march 21, 2007) layout 6 1 annals of geophysics, 64, 6, se658, 2021; doi:10.4401/ag-8675 o p e n acc e s s the resizing of the most powerful italian instrumental earthquake (september 8, 1905, calabria region, southern italy) luigi cucci istituto nazionale di geofisica e vulcanologia, italy article history: received april 8, 2021; accepted september 21, 2021 abstract the 8 september 1905 calabria earthquake is the seismic event for which the italian seismic catalogue shows the highest instrumental magnitude of the whole dataset. however, the reported ms = 7.47 was calculated over only two stations, and leaves room for a revision. in this work i provide a new estimate of the surface-wave magnitude of the earthquake calculated over sixteen individual values of magnitude from seven different stations. the new estimate is ms = 7.10 ± 0.21, a value that is consistently lined up with other estimates provided by means of macroseismic or geological evidence. the novel estimate is stable despite alternative epicentral locations and different depths proposed for this event by several investigators. the net variation of almost half a unit magnitude implies a resizing of the seismogenic source of the event in the frame of the seismotectonics of the region, and highlights the strong need for a systematic revision of the instrumental magnitude estimates for several ‘historical’ earthquakes that occurred at the dawning of the instrumental seismology. keywords: instrumental seismology; resizing seismogenic source; powerful earthquake; september 8, 1905; calabria region. 1. introduction the 8 september 1905 calabria earthquake ranks among the ten most powerful events of the whole italian seismic history [cpti 15 catalogue, rovida et al., 2021] in terms of default magnitude (mw = 6.95, which for this earthquake is calculated from the average of the macroseismic and instrumental magnitudes). in particular, among the earthquakes for which an instrumental magnitude has been estimated, a ms = 7.47 [margottini et al., 1993] is associated with this event in the cpti 15 catalogue. such a value makes by far the 1905 earthquake the strongest event to have occurred during the instrumental era in italy, even more than other seismic events that deeply affected the social, economic and environmental background of the peninsula such as the 1908 messina m = 7.10 [pino et al., 2000], 1915 avezzano m = 7.04 [margottini et al., 1993], and 1980 irpinia m = 6.81 [rovida et al., 2021] earthquakes. such a high value probably derives from the shortage of seismological data associated with this earthquake and available to margottini et al. [1993] to build their dataset. this evidence, along with the awareness that the official magnitude estimate of the event contributed to the assessment of the seismic hazard of this part of the calabria region, prompted the author that there was room to attempt a revision of the seismological data available for the earthquake and provide an updated value of its surface-wave magnitude because 1) the rapid increase of organized catalogs and bulletins on the internet has made it easier to access a greater number of seismological data than thirty years ago, 2) this instrumental data in particular is an important input for the evaluation of the magnitude of the 1905 event in the seismic catalogue, and eventually for the assessment of the seismic hazard of the area. 2. overview of the 1905 calabria earthquake the 1905 seismic event belongs to a peculiar group of highly destructive earthquakes, occurred at the dawning of the instrumental seismology, for which the location, geometry and size of the causative source are lively debated and not fully constrained. during the century elapsed since the earthquake, previous authors identified three different epicentral areas that are more than 50 km apart and proposed magnitudes ranging from m = 6.2 to m = 7.9 (figure 1 and table 1; for a comprehensive review about this issue see tertulliani and cucci, [2009]). the earthquake struck in the early hours of 8 september a vast area of the tyrrhenian side of the calabria peninsula (figure 1), causing the death of 557 people and the heaviest damage (i0 = x-xi in cpti 15) between the towns of lamezia and nicotera (figure 1). extensive damage was also reported in the aeolian islands and in north-eastern sicily. luigi cucci 2 figure 1. map of the study area. a black circle indicates the location of the 1905 earthquake by michelini et al. [2006] currently reported in the cpti catalogue. a solid line shows the boundary of the most damaged area. small black circles numbered 1 to 4 indicate alternative locations proposed by the investigators and quoted in the text. 1: presti et al. [2017]; 2: galli and molin [2009]; 3: camassi and stucchi [1997]; 4: guidoboni et al. [2019]. the mainshock originated several ground failures and widespread hydrological changes [tertulliani and cucci, 2008; 2009] and was followed by a long-lasting seismic sequence composed of hundreds of aftershocks [guidoboni et al., 2019]. it also generated a moderate tsunami that was observed along a wide stretch of coast in the southern tyrrhenian sea [maramai et al., 2014]. focusing on the magnitude estimates associated with this event, several diverging estimates are reported by various authors. for simplicity i have summarized in table 1 all the estimates of magnitude discussed below. besides the default magnitude mw = 6.95 reported in the cpti 15 catalogue, results from inversions of macroseismic data provided estimates ranging from m = 6.2 [westaway, 1992], to m = 6.7 [guidoboni et al., 2007], m = 6.8 [postpischl, 1985] and m = 7.0 [gruppo di lavoro cpti, 2004]. m = 6.9 is the macroseismic magnitude presently reported in the cpti 15 catalogue. other values of magnitude were derived from geological and geophysical data [m ≥ 6.7, tertulliani and cucci 2009; m = 6.9, loreto et al., 2017; m = 6.8, diss working group, 2018 and trippetta et al., 2019]. as regards the calculation of instrumental magnitudes, duda [1965] provides ml = 7.9 that is the highest estimate in the literature for this earthquake, kárník [1969] calculates ml = 7.3, and riuscetti and schick [1975] and carrozzo et al. [1975] estimate m = 7.0. finally, as already mentioned above, margottini et al. [1993] calculate a ms = 7.47 on the basis of the recordings from two stations. table 1. summary of the estimates of magnitude of the 1905 calabria earthquake. abbreviations: macros (macroseismic); geol/geophys (geological/geophysical); instr (instrumental). source m origin notes westaway [1992] mr = 6.2 macros from isoseismal maps guidoboni et al. [2019] me = 6.7 macros cftimed catalogue. equivalent magnitude based on macroseismic observations tertulliani and cucci [2009] μ ≥ 6.7 geol/geophys empirical relationships with environmental effects postpischl [1985] mk = 6.8 macros empirical relationships following karnik [1969] diss working group [2018] mw = 6.8 geol/geophys derived from data by loreto et al. [2017] trippetta et al. [2019] flem = 6.8 geol/geophys fault-length earthquake magnitude loreto et al. [2017] mw = 6.9 geol/geophys geological/geophysical data using empirical relationships rovida et al. [2021] mw = 6.9 ± 0.10 macros cpti15 catalogue. calculated using the code “boxer” [gasperini et al. 2010] rovida et al. [2021] mwdef = 6.95 ± 0.09 macros/instr “default magnitude” carrozzo et al. [1975] ml = 7.0 instr reported also in postpischl [1985] riuscetti and schick [1975] m = 7.0 instr unspecified gruppo di lavoro cpti [2004] ma = 7.06 ± 0.09 macros cpti04 catalogue. mean magnitude this work ms = 7.10 ± 0.21 instr calculated over sixteen individual values of magnitude from seven different stations karnik [1969] ml = 7.3 instr based on two registrations rovida et al. [2021] mwins = 7.41 ± 0.29 instr cpti15 catalogue. converted from margottini et al. [1993] using lolli et al. [2014] margottini et al. [1993] ms = 7.47 ± 0.04 instr based on two stations duda [1965] ml = 7.9 instr richter magnitude 3 the resizing of the 1905 calabria earthquake luigi cucci 4 3. estimate of the surface wave magnitude amplitude and period readings were extracted from the station bulletins available to me from the library of the headquarter of the istituto nazionale di geofisica e vulcanologia in rome and on the internet (see data and sharing resources), and supplemented by readings reported in isc. figure 2 shows a map of the seismological observatories recording the data that were investigated in the present work. surface-wave magnitude ms was computed using the prague formula firstly suggested by kárník et al. [1962] and officially adopted by iaspei: ms = log (a/t) + 1.66 log δ + 3.3 (1) where a/t is the maximum value of all ratios of the surface-wave groups on a record, a is the horizontal ground motion (in micrometers) associated with a period t (in seconds), and δ is the epicentral distance (in degrees). in general, the equation (1) is employed over any epicentral distance range; however, in this study all the stations involved in the estimate of the ms are located between 12° and 154° epicentral distance, thus falling in the interval (2°-160°) that is more suitable for such a calculation [kárník et al., 1962]. also, the ms estimates were calculated without station corrections (except for the not negligible ci = -0.37 available for the potsdam station by margottini et al. [1993]), as this factor is found to have an overall relatively small effect that does not affect the distance dependence in equation (1) [ambraseys, 1985; margottini et al., 1993; ambraseys and douglas, 2000]. single-instrument magnitudes were computed from horizontal components following the procedure employed by kárník et al. [1962]. uncertainties in the data extracted from the station bulletins mainly regard the definition of figure 2. location of the seismological observatories that provided data. a) european observatories. a white star indicates the location of the 1905 earthquake. b) extra-european observatories. the amplitude, i.e., peak-to-peak or zero-to-peak, which sometimes is not specified. for the bulletins from granada (crt) and hohenheim (hoh) i assumed that the reported maximum amplitude corresponds to peak-to-peak readings. in all the cases in which ambiguities could not be resolved the data were discarded. also, i did not use readings of maximum amplitude if no corresponding period t was given. for the calculation of station magnitudes, when both the horizontal components were available, the amplitude a was taken as the vector sum of the two components; otherwise, each component was used separately and the calculated m value was increased by 0.1 [margottini et al., 1993; ambraseys and douglas, 2000]. i provide the average of the magnitude estimates for each station; finally, the ms magnitude of the 1905 event is calculated from the arithmetic mean of the available station magnitudes. table 2 shows a summary of all the seismic stations examined. the final estimate of the surface-wave magnitude of the 1905 earthquake is ms = 7.10; this estimate was calculated over sixteen individual values of magnitude from seven different stations, whilst data from other seven stations were discarded. the standard deviation of the estimate is sd = 0.21. this new result makes the 1905 earthquake about one fourth of the margottini et al. [1993] estimate in terms of energy released. 5 the resizing of the 1905 calabria earthquake table 2. summary of the seismological data retrieved from the observatories. the upper part of the table reports the seven seismic stations for which it was possible to estimate the surface-wave magnitude. the lower part of the table reports other seven stations that provided only partial data which did not allow an estimate of magnitude. no information is specified regarding the component available for the station api. records for crt station were obtained with stiattesi pendula which have no damping. the period for station shid is assumed to be similar to that of the earlier phases of registration. observatory (station) lat. lon. dist. (°) e-w amp. (𝜇) period (sec) n-s amp. (𝜇) period (sec) estimated magnitude mean station magnitude jena (jen) 50.9519 n 11.5833 e 12.62 335 13.5 430 13.5 6.73 6.88 340 9.5 500 9.5 6.93 340 9.5 500 9.5 6.93 340 9.5 500 9.5 6.93 granada (crt) 37.19 n 3.59806 w 15.04 290 5.2 66 5.2 7.01 7.01 shide (shid) 50.588 n 1.286 w 16.80 320 10 502 10 7.11 7.11 hohenheim (hoh) 48.7167 n 9.21667 e 11.00 2978 10 2637 8 7.52 7.52 gottingen (gtt) 51.5464 n 9.96417 e 13.45 500 10 6.97 6.94500 15 6.8 550 9 7.06 potsdam (pot) 52.3803 n 13.0678 e 13.85 2220 16 7.07 7.07 apia (api) 13.8072 s 171.775 w 154.2 42.5 21 7.34 7.19 17.5 19 7 40 19 7.36 14 18 6.92 37.5 18 7.35 observatory (station) lat. lon. dist (°) amp. (mm) period (sec) liverpool (bid) 53.401 n 3.072 w 19.48 8.3 edinburgh (edi) 55.925 n 3.184 w 21.29 7.5 paisley (pai) 55.846 n 4.431 w 21.73 8.0 s. fernando (sfs) 36.46561 n 6.20553 w 17.25 3.0 uccle (ucc) 50.7983 n 4.35944 w 14.47 >19 ponta delgada (pda) 37.7467 n 25.6633 w 32.04 2.55 kew (kew) 51.468 n 0.313 w 16.93 4. discussion and conclusions as it was calculated over a considerably larger number of stations, i suggest that the estimate of the surface-wave magnitude (ms = 7.10) proposed in the present work is more robust than ms = 7.47 obtained by margottini et al. [1993], that was calculated over only two stations and is presently reported in the cpti 15 catalogue. unfortunately, the original documents employed by margottini et al. [1993] are no longer available (a. screpanti, personal communication) to carry out a crosscheck between the different datasets. i now consider in the following the other magnitude estimates put forward in the literature by means of different approaches (macroseismic, geologic, geophysical). the 1905 earthquake is quoted in the cpti catalogue with m = 6.9 calculated from the inversion of macroseismic intensity data by using the code boxer [gasperini et al., 2010]. tertulliani and cucci [2009] indicate a minimum magnitude m = 6.7 on the basis of empirical relations that regulate the distribution of seismically induced environmental effects. loreto et al. [2017] estimate m = 6.9 by combining onshore structural and morphological data with offshore geophysical data. such estimates are rather homogeneous, and the ms = 7.10 proposed in this paper is lined up with them. on the contrary, the instrumental estimates of the magnitude of the 1905 event span over almost one unit (m = 7.0 ‐ m = 7.9); a comparison with the magnitude proposed in this paper is not straightforward as i suggest that most of those estimates are affected by severe uncertainties (kárník [1969] and margottini et al. [1993] were based on only two stations, and duda [1965] is considerably overestimated [abe and noguchi, 1983]). some uncertainties in the source parameters of the 1905 earthquake can potentially affect the robustness of the surface-wave magnitude proposed in this paper. the first issue concerns the depth of the event. galli and molin [2009] and presti et al. [2017], by means of different techniques propose an offshore location at about 30-40 km depth, deeper than most of the earthquakes occurring in this region. although the prague formula only applies to shallow-focus earthquakes (the computed ms may underrepresent the energy of intermediateand deep-focus earthquakes), a 40 km-deep focus is still within its range of application [ambraseys and douglas, 2000; see also data and sharing resources], therefore no adjustment is required in the magnitude estimate for such a depth. the second issue concerns the epicentral location of the event. for my calculations i used the offshore location reported in the cpti catalogue [michelini et al., 2006]; on the contrary, some investigators [rizzo, 1906; camassi and stucchi, 1997; guidoboni et al., 2019] propose an inland epicenter some ~50 km further east that can generate different values of the correction term for the epicentral distance in equation (1). however, a novel estimate based on the latter location would provide a negligible adjustment of ± 0.01 units of magnitude. though the individuation of the most probable seismogenic source responsible for the 1905 earthquake is clearly beyond the scope of the present work, a comparison between my result and the published magnitude estimates allows to put forward some pieces of evidence that can be framed in the seismotectonics of this region. using the empirical relationships by wells and coppersmith [1994] a m = 7.10 earthquake could be generated by a 40-50 kmlong source, whilst a m = 7.47 event would require a source about twofold in length and almost three times as rupture area. a ~100 km-long and 30 km-wide seismogenic source seems unrealistic in the complex geological structure of the calabria region, where the crust is densely fractured, and the faults’ orientation and kinematics are highly variable. indeed, such a very large source has never been previously envisioned by the investigators, neither inland [peruzza et al., 1997; monaco and tortorici, 2000; piatanesi and tinti, 2002; cucci and tertulliani, 2010], nor offshore [galli and molin, 2009; loreto et al., 2013, 2017; diss working group, 2018; presti et al., 2019; rovida et al., 2021]. given the substantial equivalence between ms and mw for 1905-type earthquakes [wells and coppersmith 1994; das et al., 2011] this evidence is equally valid when using ms or mw. finally, following the same criteria of compilation and of definition of a single magnitude for an earthquake record of the catalogue cpti15 (“default magnitude” in rovida et al. [2021]), it is possible to derive an updated default magnitude m = 6.90 (previously m = 6.95) for this earthquake. both the new ms = 7.10 and the updated default magnitude m = 6.90 scale back the prominence of the 1905 event in the seismic landscape of the region. this result (with a net variation of ms of almost half a degree) highlights the strong need for a systematic revision of the instrumental magnitude estimates for historical earthquakes, based on all the data available today. the recovery of the seismological data still existing and their preservation from dispersion or final destruction is a critical issue, considering the importance that the instrumental estimates of magnitude have in the evaluation of the expected maximum magnitude and eventually in the determination of the seismic hazard of a region. luigi cucci 6 5. data and sharing sources seismological and geological data were downloaded from the following seismic catalogues, databases and bulletins: cpti15, the parametric catalogue of italian earthquakes (available at https://emidius.mi.ingv.it/cpti15dbmi15/index_en.htm); cpti04, catalogo parametrico dei terremoti italiani (available at https://emidius.mi.ingv.it/cpti04/); cfti5med, catalogo dei forti terremoti in italia (461 a.c.-1997) e nell’area mediterranea (760 a.c.-1500) (available at http://storing.ingv.it/cfti/cfti5/); diss, database of individual seismogenic sources (available at http://diss.rm.ingv.it/diss/); nt4.1, un catalogo parametrico di terremoti di area italiana al di sopra della soglia del danno (available at https://emidius.mi.ingv.it/nt/consnt.html). data from seismological observatories are available through the website of euroseismos at http://storing.ingv.it/es_web/data/collection.htm#bull. the on-line bulletin of the international seismological centre is available at https://doi.org/10.31905/d808b830. some informations on the range of applicability of the prague formula were retrieved by iaspei.org/pub/commissions/csoi/summary_wg_recommendations_20130327.pdf acknowledgements. thanks to the associate editor of the journal i. molinari. the revisions carried out by a. michelini and by an anonymous reviewer provided important comments and suggestions that strongly improved the quality of the paper. also, i am indebted to n.a. pino, who read corrected and provided important comments and suggestions to a previous draft of the manuscript. references abe, k. and s. noguchi (1983). revision of magnitudes of large shallow earthquakes, 1897-1912, phys. earth planet. int., 33, 1-11. ambraseys, n.n. 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(1992). seismic moment summation for historical earthquakes in italy: tectonic implications, j. geophys. res., 97, 15.437–15.464. luigi cucci 8 https://doi.org/10.1038/s41597-019-0091-9 http://dx.doi.org/10.1016/j.margeo.2013.06.016 http://dx.doi.org/10.4401/ag-6437 http://dx.doi.org/10.1046/j.1365-246x.2002.01700.x https://doi.org/10.1029/2000jb900259 https://doi.org/10.13127/cpti/cpti15.3 9 the resizing of the 1905 calabria earthquake co r r e s po n d i n g a u t h o r: luigi c u cc i, istituto nazionale di geofisica e vulcanologia, rome, italy e-mail: luigi.cucci@ingv.it © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international t e r r e m o t o d e l l a c r e s t a m e d i a n a a t l a n t i c a d e l 2 4 a p r i l e 1 9 4 7 l . marchit i g . pannocchia ! . n o l o d i e w e g c n e r , r i l e v a n d o l ' a n a l o g i a d e l l e l'orme c h e pres e n t a n o le eoste a m e r i c a n e e a f r i c a n e d e l l ' o c e a n o a t l a n t i c o , fu ind o t t o a f o r m u l a r e la t e o r i a sulla t r a s l a z i o n e dei c o n t i n e n t i . q u e s t a t e o r i a sollevò m o l t i c o n t r a s t i di o p i n i o n i , c l i c a n d a r o n o d a l l a sua val i d i t à , so~lenula da a l c u n i , a l l a sua i n c o n s i s t e n z a , s o s t e n u t a da a l t r i . r e c e n t e m e n t e , a l c u n i fatti di n a t u r a g e o l o g i c a s i s m i c a , h a n n o portato ad e m e t t e r e l ' i p o t e s i c h e se la t r a s l a z i o n e dei c o n t i n e n t i r i s p o n d e a r e a l t à , per (pianto si r i f e r i s c e ai c o n t i n e n t i a m e r i c a n o ed a f r i c a n o , il p a r a l l e l i s m o i n v o c a t o da w e g c n c r e q u a l c h e a l t r o f r a i c o n t o r n i d e l l e eoste a f r i c a n e e a m e r i c a n e deve in e f f e t t i essere r i f e r i t o fra la costa a m e r i c a n a e la c r e s t a m e d i a n a d e l l ' a t l a n t i c o , le cui f o r m e gen e r a l i p r e s e n t a n o p i c c a t e a n a l o g i e . così c h e , se la d e r i v a è l e a l m e n t e a v v e n u t a , r i g u a r d a s o l a m e n t e la d i s i a n z a e r e s i a m e d i a n a a t l a n i i c a e o s l a a m e r i c a n a q u e s t a nuova i p o t e s i si b a s a s o s t a n z i a l m e n t e sul l'alto c h e , sulla s c o r t a d e l l e r e c e n t i m i s u r e b a l i m e l r i c h c , l a i l a n t i c o m e r i d i o n a l e p r e s e n t e r e b b e a l l ' e s t d e l l a c r e s t a m e d i a n a , u n a s u c c e s s i o n e di b a c i n i e di c r e s t e o r i e n t a t e in d i r e z i o n e s w j n e , sul p r o l u n g a m e n t o d e l l e u n i t à m o r f o l o g i c h e del c o n t i n e n t e a f r i c a n o . questa d i s p o s i z i o n e m a n c h e r e b b e a l l ' o v e s t della c r e s t a m e d i a n a a t l a n t i c a . a l t r e c o n s i d e r a z i o n i d i c a r a t t e r e g e o l o g i c o s a r e b b e r o a s o s t e g n o d e l l a n u o v a i p o l c s i , l a q u a l e t r o v e r e b b e a p p o g g i o a n c h e d a r i c e r c h e i m o l o g i e h e . i n f a t t i , il t e r r e m o t o a v v e n u t o sulla eresta m e d i a n a d e l l ' a t l a n t i c o f a 7 " . 0 \ 3 8 " , 8 w ) il 1 4 s e t t e m b r e 1 9 4 5 a v r e b b e g e n e r a t o o n d e di l o v e di c h e , sul t r a t t o e p i c e n t r o o s s e r v a t o r i o di f o r d h a i n ( p r e s s o new y o r k ) h a n n o m o s t r a t o la v e l o c i t à di 4 , 4 k m s c c . d e l l ' o r d i n e di q u e l l a c h e o n d e d e l l o stesso p e r i o d o p r e s e n t a n o per t r a g i t t i sotto l ' o c e a n o p a c i f i c o . il f o n d o d e l p a c i f i c o c o n s i s t e e s s e n z i a l m e n t e d i q u e l l e s t r a t i f i c a z i o n i c h e i g e o l o g i d e f i n i s c o n o c o n il s i m b o l o sima c c h e c o n s e n t o n o . t e r r e m o t o dbi.i.a c r e s t a m e d i a n a a t h n t 1 c v d e e 2 1 a c r i l e l l m 7 a p a r i l a di a l t r e c o n d i z i o n i , veloci là m a g g o r i di q u e l l e c h e si osserv a n o p e r le o n d e s i s m i c h e c h e a t t r a v e r s a n o lo z o c c o l o d e i c o n t i n e n t i (il c o s ì d e t t o siiti). l a p a r t e o c c i d e n t a l e d e l l ' a t l a n t i c o , q u i n d i , p o g g e r e b b e -u strat i f i c a z i o n i con c a r a t t e r i s t i c h e a n a l o g h e a q u e l l e c h e l'ormano il l'ondo del p a c i f i c o , m e n t r e la p a r t e o r i e n t a l e a v r e b b e p r o p r i e t à e s s e n z i a l m e n t e s i a l i e h e . p o i c h é q u e s t o solo d a t o sismico non ci è p a r s o sufficiente p e r c o n c l u d e r e nel senso voluto dai s o s t e n i t o r i di q u e s t a nuova i p o l e i a b b i a m o r i t e n u t o o p p o r t u n o s o t t o p o r r e ad una r i c e r c a a c c u r a t a q u a l c h e t e r r e m o t o a v v e n u t o a p p u n t o n e l l a c r e s t a m e d i a n a a t l a n t i c a . ci p a r v e f a v o r e v o l e , p e r q u e s t o s c o p o , il f o r t e t e r r e m o t o v e r i f i c a t o s i in d e t t a zona il 2 4 a p r i l e ] 9 4 7 ( o r e ì 9 h 35 , n g . c . t . ) . a q u e s t o s c o p o a b b i a m o r a c c o l t o un c o s p i c u o n u m e r o di sism o g r a m m i r e l a t i v i a d e l l o t e r r e m o t o , m e s s i c i g e n t i l m e n t e a d i s p o s i z i o n e da quasi t u l l i gli o s s e r v a t o r i a cui ci s i a m o r i v o l l i . n o t i a m o l i b i t o c l i c s v o l g e r e m o il lavoro in coordinate geocentriche. d e t e r m i n i a m o un p r i m o v a l o r e appro—iriiato d e l l e c o o r d i n a t e c p i c c n t r a l i con il m e t o d o d e l l a p r o i e z i o n e s t e r e o g r a f i c a u s a n d o i d a t i r i c a v a l i d a l l a r e g i s t r a z i o n e d e l l e s t a z i o n i s i s m i c h e di r o m a . k e w o b s e r v a t o r y , s c o r e s b v s u n d , c h i c a g o . s a n j u a n : esso è p o = 3 9 " 10" w p a s s a n d o p o i a l l e c o o r d i n a t e g e o c e n t r i c h e d e l l ' e p i c e n t r o con la noia f o r m u l a di t r a s f o r m a z i o n e lg q '„ = (1 e)2 tg q o 0 , 9 9 3 2 7 7 tg q „ [ 2 ] ( d o v e e r a p p r e s e n t a lo s c h i a c c i a m e n t o d e l l a t e r r a c h e . s e c o n d o h a y f o r d , v a l e 1 / 2 9 7 } ( ' ) o t t e n i a m o 3 i c l o " \v [ 3 ] ?/„=;.„ = 39!j 1 0 ' w ( l o : v q u c s i o v a l o r e va c o r r e t t o con una i n d a g i n e più a c c u r a t a : f a c e n d o u-o di un m a g g i o r n u m e r o di d a t i r i c a v a t i da s t a z i o n i o p p o r t u n a m e n t e d i s p o s t e r i s p e t t o a l l ' e p i c e n t r o . a l l o s c o p o ili d e t e r m i n a r e le c o r r e z i o n i ò/.0 e ò<[ 0 d e l l e c o o r d i n a l e 5 7 2 ! . . m a r c e l l i " k . p a n n o c c h i a e p i c e n t r a l i , c o r r e z i o n i c l i c si s u p p o n g o n o s u f f i c i e n t e m e n t e p i c c o l e , si p c o n d o t t i a r i s o l v e r e , p e r ogni s t a z i o n e , e q u a z i o n i d i c o n d i z i o n e d e l l a f o r m a : . . . 9a„ ò\„ " ' 3/, da ( v x ) = n 8 « f ' , + t j l w . w 3 ' l ' o l a [ 4 ] è l i m i t a t a ai t e r m i n i d i 1" ordine., p e r l a i p o t e s i f a t t a c i r c a l ' e n t i t à d e l l e c o r r e z i o n i : in e—a a„ ( f u n z i o n e di a e cp'} r a p p r e s e n t a la d i s t a n z a di c i a s c u n a d e l i e s t a z i o n i s c e l t e d a l l ' e p i c e n t r o p r o v v i s o r i o [ 3 ] , c a l c o l a t a f a c e n d o u s o di c o o r d i n a t e g e o c e n t r i c h e ; a*n l a c o r r i s p o n d e n t e d i s t a n z a c a l c o l a t a f a c e n d o u s o d e l l e d r o m o c r o n e nel m o d o c h e v e r r à p a r t i c o l a r m e n t e e s p o s t o tra b r e v e . la' [ 4 ] t e n e n d o p r e s e n t e c h e cos a è dalo da cosa = sinrp'0sincp'-j-coscp'0coscp'cos(a — x0) [ 5 ] p u ò ò a = coscp'sinq '0eos(à—a0 si n a -sinrp'cosq'p , coscp'0coscp'sin(à—)0) ^ s i n a [6] il s i s t e m a c o s t i t u i t o d a l l ' i n s i e m e di q u e s t e e q u a z i o n i d i c o n d i z i o n e s c r i t t e p e r c i a s c u n a s t a z i o n e , va r i s o l t o c o l m e t o d o d e i m i n i m i q u a d r a t i . s c e l t e l e 1 4 s t a z i o n i c h e r i s u l t a n o n e l l a t a b e l l a i c a l c o l i a m o i valori di a c o n la f o r m u l a f 1 ) 1 r ' i ( & j b ) * + ( r c s ) l m ms a = 1 (<ì—a)2 p e r d i s t a n z e c o m p r e s e tra 0° e 6 0 ° , e c o n la f o r m u l a ce.s a = a a-\-h b-\-c c [ 7 ' ] p e r d i s t a n z e c o m p r e s e tra 6 0 " e 1 2 0 " , n e l l e q u a l i / a=cos cp' cos a. /) = (• os cp' sili }. [ 8 ] c = sin cp' o n o i c o s e n i d i r e t t o r i elei r a g g i o t e r r e s t r e p a s s a n t e p e r la s t a z i o n e di c o o r d i n a t e g e o c e n t r i c h e x, cp' c a, b, c, =ono gli a n a l o g h i v a l o r i p e r l ' e p i c e n t r o . d a l l ' e s a m e d e l l e r e g i s t r a z i o n i di h a r w a r d , n e u c h à t e l , z u r i g o e s t o c c a r d a , a s s e g n a m o p r o v v i s o r i a m e n t e a l l ' i p o c e n t r o u n a p r o f o n d i t à m e d i a di 4 7 k m , p r o f o n d i t à n o n c o n f e r m a t a d a l l e a l t r e ! . . m u i c k i . u " i . . p a n n o c c h i a r e g i s t r a z i o n i . p e r m e z z o della d r o m o e r o n a c o r r i s p o n d e n t e a t a l e profonditii ( j ) alle d i s i a n z e già c a l c o l a t e , d e t e r m i n i a m o i t e m p i di p r o p a g a z i o n e d e l l e p. ed o t t e n i a m o pei' il t e m p o o r i g i n e m e d i o il \ a l o r e 7 , = 19h 3 5 m 1 7 \ 6 ( t a b e l l a 1. c o l o n n a 7) [ 9 ] .sottraendo il v a l o r e [ 9 ] dai t e m p i di r e g i t r a z i o n e ( t a b e l l a i . colonna .">| o l i o n i a m o nuovi t e m p i di t r a g i t t o d e l l e p ( c o l o n n a 8 ) in b a s e ai q u a l i , s e r v e n d o c i d e l l e d r o m o c r o n e di cui s o p r a , d e t e r m i n i a m o le d i s t a n z e a*„ ( t a b e l l a i. c o l o n n a 9 ) . lì i m i t o , con il m e t o d o dei m i n i m i q u a d r a t i , il s i s t e m a di 1 4 e q u a z i o n i c l i c c o n s e g u e a p p l i c a n d o la | 6 ] a c i a s c u n i d e l l e stazioni p r e s c e l t e , si o t t i e n e : i ò/0 + 0 " . 4 0 5 0 = + 2 4 ' : i 8 ' ' i òq.'0 + 0 " , 0 2 0 3 = + 0 1 ' 1 2 " a p p o r t a n d o ([iicste c o r r e z i o n i a l l ' e p i c e n t r o f 3 ] o t t e n i a m o il v a l o r e piii p r o b a b i l e u ' o ' 5 9 ' t " n i "/„ = 3 8 " 4 5 " 4 2 " "vi". allo s c o p o di d e t e r m i n a r e u n a u l t e r i o r e a p p r o s s i m a z i o n e p e r l ' e p i c e n t r o ( g i a c c h é la c o r r e z i o n e d e l l a l o n g i t u d i n e è p i u t t o s t o r i l e v a n t e ) . e allo s c o p o di c a l c o l a r e s i m u l t a n e a m e n t e l a p r o f o n d i t à i p o c e n t r a l e /i,, e il t e m p o o r i g i n e ( 0 , e s t e n d i a m o il c a m p o di i n d a g i n e ad un m a g g i o r n u m e r o di stazioni s c e l t e ira q u e l l e i cui t e m p i di i n i z i o d a n n o m a g g i o r a f f i d a m e n t o . i n d i c h i a m o con 'o, ' n'i 'o' ''o i valori a p p r o s s i m a t i d e l l e i n c o g n i t e , e con ò/0, 6à0> òq/„, hh0 le c o r r e z i o n i da a p p o r t a l e , s u p p o s t e s u f f i c i e n t e m e n t e p i c c o l e : -j a v r à : •> v l x i a 0 = x 0 ! 5 a 0 | i , ' o = q ' o + 5 < i ' o ; / » = m 6 a 0 11 t e m p o di p r o p a g a z i o n e l,, d e l l e p. c o r r i s p o n d e n t e ad una staz i o n e g e n e r i c a è una l'unzione di a, q li. c i o è > „ = / u „ + 5 / „ , c p ' 0 + 5 c p ' 0 , / l o + 6 f t 0 ) questa s v i l u p p a t a in s e r i e l i m i t a t a m e n t e ai t e r m i n i del p r i m o ord i n e , dà il sistema di e q u a z i o n e di c o n d i z i o n e terremoto della cresta mediana atlantica dei. 2 1 aprile 1 9 4 7 ' , = / ( > • o v o a ) + 5?.» t 8 r p ' o + ~ [ h ] 3a0 3cp 0 3/i0 nella j ii ] /(/„, q;'0, /i0) il [ e m p o di p r o p a g a z i o n e d e l l e 7' c o r r i s p o n d e n t e a l l a d i s t a n z a e p i e e n t r a l e x d e l l a s t a z i o n e in c o n s i d e r a z i o n e , d e t e r m i n a t a n u o v a m e n t e in c o r r i s p o n d e n z a d e l l e [ i o ] ; t e m p o 3/ di p r o p a g a z i o n e c a l c o l a l o p e r li 53 k m ; ' r a p p r e s e n t a la v a r i a z i o n e dh0 c l i c s u b i s c e il t e m p o di t r a g i t t o d e l l e p , q u a n d o , t e n u t a c o s t a n t e l a d i s i a n z a a„, si fa v a r i a r e la p r o f o n d i t à di una q u a n t i t à (vi0 p a r i ali unità d e l l a d r o m o c r o n a p r e s c e l t a . 3/ 3/ p e r ' c — : — v a l g o n o 3m' seguenti rei ài, 3 / df dx, . 3/ 3/ 3 \„ 3x0 3 a „ 3 à 0 : 3«p'0 a a n 3 c p ' 0 [12] 3/ d o v e ' r a p p r e s e n t a i i n c r e m e n t o c l i c s u b i s c e il t e m p o di p r o p a g a 3 a„ z i o n e q u a n d o , t e n u t a c o s t a n t e la p r o f o n d i t à , si fa v a r i a r e l a d i s i a n z a 3a„ 3 a„ e p i e e n t r a l e di un g r a d o , e e h a n n o le seguenti e s p r e s s i o n i : 3 l 3 a,. ; rf o — coscp 0 siile. i 3 a„ = — cl 3'l o [ 1 3 ; a essendo l ' a z i m u t d e l l ' e p i c e n t r o r i s p e t t o a c i a s c u n a s t a z i o n e . s i a n o t„ i t e m p i di a r r i v o d e l l e p r e g i s t r a t i d a l l e v a r i e s t a z i o n i ; q u e s t i , a p a r t e gli e r r o r i di o s s e r v a z i o n e e di r e g i s t r a z i o n e , d e b b o n o e g u a g l i a l e la s o m m a d e l l ' o r a o r i g i n e del t e r r e m o t o e del c o r r i s p o n d e n t e t e m p o di t r a g i t t o . t n = ( v j 5 t 0 ) ! / ( x 0 + 5 a 0 , fp'o-fsqp'o , v i mo). [ 1 4 ] s i c c h é in d e f i n i t i v a la [ f i ] t e n u t o c o n t o d e l l e [ 1 2 1 [ 1 3 ] , ] 1 4 ] si j i l i ò s c r i v e r e ( " ) : df 3/ 3/ 8/0 — c o s ( p 0 s i n a — w 0 — cosu . 8 q / 0 + — fi/i0=t„3 a„ 3 a„ 3/i. ta+j\h><p'o,k) [ir] s t a z i o n i \„ [11 t „ l l san j u a n 2 8 ° , 5 0 4 7 19" 4 l m 11",2 huancayo 4 1 ° . 4 7 9 2 » 43 01,1 h a r w a r d •14",8108 » 43 27,5 c o l u m b i a 4 6 " , 6 7 5 5 » 43 4 4 , 2 k e w o b s . 5 3 ° , 5 9 3 7 » 4 4 3 1 , 4 c h i c a g o 5 4 ° , 5 7 3 3 » 14 4 2 , 6 is'euchatel 54°,9307 » 4 4 4 2 , 0 s t r a s b u r g o 5 6 " , 1 9 8 7 » 4 4 51.7 b a s i l e a 5 5 ° , 5 7 3 0 » 4 4 4 6 , 4 z u r i g o 5 6 ° , 0 7 7 8 » 4 4 51,1 r o m a 5 6 ° , 2 9 3 9 » 44 52,3 s t o c c a r d a 5 7 " . 1 1 6 5 » 4 4 57.0 c o p e n h a g e n , 6 2 " . 2 3 6 9 » 45 3 5 . 5 sofia 6 4 " , 1 8 3 3 » 45 4 7 . 0 s c o r e s b y 6 3 ° , 2 9 4 8 » 45 4 0 . 0 r a p i d city 6 6 ° , 1 5 1 4 » 46 0 2 , 0 i s t a n b u l 6 8 " . 0 5 9 3 » 4 6 09.2 s a s k a t o o n 7 0 " . 1 6 2 8 » 46 25.0 t u c s o n 7 0 " , 6 0 5 0 » 46 29,1 salt l a k e 71°,9912 » 46 39.2 pasadena 7 6 " . 7 9 0 4 » 47 0 4 , 0 s i t k a 8 6 ° , 8 8 1 4 » 48 0 0 . 6 (f(ao,c[ o, ho)j tabella i i 1 ~ s 3f '3 ( 5f \ it . 0 ) .. ' li il 1 ^ s a , , c/j 3 / ( i n s e c . ) i ' q l 1 lo [5| 0' 0 / j e— i r ' |6| ,'in scc.! _ l i l [81 — cos (l 3 \ „ ( i n s e c . ) i ' q l 16v15 19" 41"' 0 8 s , 6 5 + 2 s ,55 8,90 + 8 , 0 5 9 3 8 6 — 3 . 6 0 3 0 4 0 — 6 , 1 0 15,15 » 42 59,85 + 1.25 8,20 + 7,146841 + 3 , 8 9 3 7 7 8 — 6 , 6 0 14,61 » 13 2 6 , 7 9 + 0,71 8,00 + 4 , 5 0 0 5 0 4 — 6 , 5 8 3 6 3 2 — 6 , 6 0 16.65 » 13 4 1 , 4 5 + 2 . 7 5 7,87 + 5 , 9 8 6 9 8 4 — 5 , 0 3 9 1 2 2 — 6 . 6 3 11.27 » 44 3 4 . 0 3 —'2.63 7.35 — 3 . 5 1 7 5 4 8 — 6 . 1 3 1 5 5 0 — 6 , 7 6 15,12 » 4 4 4 1 , 3 8 + 1.22 7,25 + 4 , 9 6 0 5 6 6 — 5 2 4 1 3 5 1 — 6 . 8 0 11.99 » 44 4 3 . 9 1 — 1 . 9 1 7,20 — 4 . 2 6 9 1 5 4 — 5 , 7 6 7 6 7 1 — 6 , 8 0 12.59 » 44 53.01 — 1 , 3 1 7,16 — 4 , 1 1 0 7 9 2 — 5 , 8 3 3 0 8 7 — 6 . 8 2 11,75 « 44 4 8 , 5 5 — 2 . 1 5 7.20 — 4 , 2 3 6 8 5 4 — 5 , 7 9 1 6 4 4 ; — 6 , 8 0 12.82 » 44 52.18 — 1 , 0 8 7,18 — 4 . 2 8 3 8 1 1 — 5 , 7 3 0 6 4 5 — 6 , 8 1 12.38 » 4 4 53,82 — 1 . 5 2 7.14 — 4 , 9 5 0 1 1 2 — 5 , 0 9 8 8 4 5 — 6 . 8 3 1 1 , 3 0 » 4 4 59.62 — 2 . 6 2 " , 0 0 — 4 , 0 5 5 7 0 2 — 5 , 6 7 7 1 5 4 — 6 , 9 0 14.71 m 45 34-69 + 0,81 6 , 6 6 — 3 , 2 9 0 6 3 9 — 5 , 7 7 1 8 5 6 — 7 , 0 0 13.21 » 45 4 7 . 6 9 — 0 , 6 9 6 , 4 8 — 4 , 6 4 4 4 4 9 — 4 , 4 7 1 8 9 3 — 7 . 0 0 12,43 » 45 4 1 , 4 7 — 1 . 4 7 6.50 0 , 7 0 0 4 5 9 — 6 , 4 6 1 0 2 6 — 7 . 0 0 15,55 » 46 00,35 + 1 , 6 5 6 , 3 0 + 1.135553 — 4 , 4 3 0 9 4 1 — 7 , 0 0 1 0 , 6 8 » 4 6 12.42 — 3 . 2 2 6.20 — 4 , 6 3 5 7 9 0 — 4 , 0 6 5 2 5 9 — 7 . 1 0 1 3 , 7 6 » 16 25,11 — 0 , 1 4 6.07 + 3 , 6 4 8 8 6 5 — 4 , 8 2 3 6 2 9 — 7 . 1 1 14,81 » 46 2 8 . 1 9 + 0.91 6 , 0 0 + 5 , 0 7 8 9 2 8 — 3 , 1 1 4 4 4 4 — 7.16 16,80 » 4 6 36.30 + 2.90 6.00 + 4 , 5 4 0 6 6 2 — 3 , 8 6 9 9 5 3 — 7 . 1 0 13,57 » 47 04.33 — 0 . 3 3 5,54 + 4 , 5 9 4 3 9 9 — 3 , 0 2 8 2 2 5 — 7 . 3 8 18.15 » 47 56,35 + 4 , 2 5 4 , 8 0 + 2 , 5 8 4 1 0 9 — 1 , 0 2 8 8 6 6 — 7 , 5 0 13k,90 t e r m i n i n o t i c o e f f i c i e n t i d i 8 x 0 c o e f f i c i e n t i di 8 ( p ' 0 coeffic i e n t i di (ih0 terremoto dem,a cresta mediana ut-antica dei. 2 1 aprile 1 9 1 7 5 7 7 n e l l a t a b e l l a i i r i p o r t i a m o i v a l o r i n e c e s s a r i r e l a t i v i al sistema di e q u a z i o n e 11*1 c o r r i s p o n d e n t i a 22 s t a z i o n i . i v a l o r i d e l l e d i s t a n z e -x, r e l a t i v e al n u o v o e p i c e n t r o , si s 0 n c a l c o l a l e c o n l e f o r m u l e [ 7 ] [ 7 ' ] già u s a t e p r e c e d e n t e m e n t e : q u a n t o a l l e d e r i v a t e 3 / (> c j sa,, ' 3 h 0 si s o n o c a l c o l a l e con le t a b e l l e di j e i l ' r e v s ( ' ) : d a t a l ' i n c e r t e z z a n e l l a d e t e r m i n a z i o n e d e l l a p r o f o n d i t à c a l c o l a t a d a l l e r e g i s t r a z i o n i , a b b i a m o r i t e n u t o l e c i t o s c e g l i e r e p e r h„ il \ a l o r e di 3 3 k m . scelta c l i c ci b a p e r m e s s o di s e m p l i f i c a r e i c a l c o l i . l ' u n i t à d e l l a d r o m o e r o n a p r e s c e l t a , risulta e s s e r e , d a l l e c i t a t e tab e l l e . di k m 6 3 . 3 8 . q u a n t o ai t e m p i di t r a g i t t o ' 0 /in) ( c o l o n n a 3 ) n o t i a m o c h e essi, c a l c o l a t i c o n l e t a b e l l e s o p r a c i t a l e , h a n n o d a t o v a l o r i poco s o d d i s f a c e n t i , t a l c h é si è r i t e n u t o o p p o r t u n o c o r r e g g e r l i p e r il f a t t o di a v e r e u s a t o c o o r d i n a l e g e o c e n t r i c h e a n z i c h é g e o g r a f i c h e . p e r la f o r m u l a di c o r r e z i o n e r i m a n d i a m o al già c i t a l o l a v o r o di j c f l ' r e y s ( ' ) . o s s e r v i a m o p e r ò c h e tali c o r r e z i o n i sono r i s u l t a l e press o c h é t r a s c u r a b i l i t a n t o da r i t e n e r l e s u p e r f l u e in u n a successiva a p p l i c a z i o n e d e l m e t o d o . l a c o l o n n a i della t a b e l l a i l dà p e r il t e m p o o r i g i n e il v a l o r e a p p r o s s i m a l o il = 19h 3 5 m :i3\90. l a s o l u z i o n e c o n d o t t a c o l m e t o d o d e i m i n i m i q u a d r a l i ( ' ) . del s i s t e m a di e q u a z i o n i c h e c o n s e g u e , c o n i dati d e l l a t a b e l l a i l a p p l i c a n d o la [ 1 1 ' ] a l l e 22 stazioni p r e s c e l t e , dà i se guenti r i s u l t a t i . òt0 —7\ i (v.0 = 0 " , 3 6 3 5 = 0° 21 ' 4 8 " . 5 < v 0 = — 0 " . 0 7 7 4 = — 0 ° 0 4 ' 3 8 " . 7 òh0 = — 0 , 9 3 4 . ii (vi0 essendo d e l l ' o r d i n e di 9 io d e l l ' u n i t à di d r o m o e r o n a pres c e l t a , r i s u l t a a v e r e un v a l o r e ~ — 5 7 k m . t a t e c o r r e z i o n e ci a u t o rizza a r i t e n e r e c o m e p r e s s o c h é n u l l a la p r o f o n d i t à i p o c e n l r a l e ( " ) . o c c o r r e q u i n d i r i p e t e r e l ' o p e r a z i o n e di a p p r o s s i m a z i o n e p e r i v a l o r i d e l l e i n c o g n i t e p a r t e n d o dai seguenti nuovi d a t i : i ih, • 0 • ; < ( ' „ = 7° 5 4 ' 2 8 " , 3 n [ u ] t e r r e m o t o dei-i.v c i t h s t a -mediana a t l a n t i c a dei. 2-1 a p r i l e 1 9 4 7 5 7 9 n o t i a m o c l i c ili q u e s l a u l t i m a d e t e r m i n a z i o n e a b b i a m o r i t e n t i l o o p p o r t u n o r i d u r r e u l t e r i o r m e n t e il n u m e r o d e l l e stazioni e l i m i n a n d o c o p e n h a g e n e s i l k a c h e p r e s e n t a v a n o , per il t e m p o o r i g i n e , v a l o r i s e n s i b i l m e n t e diverbi ria q u e l l i d e l l e a l t r e s t a z i o n i . n e l l a tabella 111 m a n c a o v v i a m e n t e 1 u l t i m a c o l o n n a , p e r l ' i p o t e s i f a t t a di / r . o . l a q u a r t a c o l o n n a della 'tabella 111 d à , p e r il t e m p o o r i g i n e , il v a l o r e a p p r o s s i m a t o 7u = 19h 3 5 m 0 8 " . 2 . a p p l i c a n d o a n c o r a la [ i l m a n c a n t e n a t u r a l m e n t e del termine, in òli,,, a l l e stazioni r i p o r t a l e n e i i a tabella i i i . e r i s o l v e n d o il sis t e m a c h e se n e c o n s e g u e con il m e t o d o dei m i n i m i q u a d r a l i ( ' ) , a b b i a m o o t t e n u t o i f»/0 0 . 3 0 0 8 1 0 . 5 1 9 l))0 = — 0 " , 0 0 0 9 1 6 : : 0 ° , 0 5 2 6 i " ' (sin' ò(p'0 = + 0 " , 0 6 6 2 5 8 0",:i 117 \ verifica dei risultati o t t e n u t i , ci i a m o c a l c o l a t i a n c h e l o c h e m a [ i i . 3 ] : esso d o v r e b b e r i s u l t a r e u g n a l e al l a s o m m a dei q u a d r a t i degli e r r o r i | v v ] . \ iene inl'att i [11. 3 ] = 1 6 . 1 7 9 7 |>v] = 1 6 . 1 8 1 9 p e r t a n t o i v a l o r i definitivi d e l l e i n e o g n i l e s o n o : i 4>'0 = 7° 5 8 ' 2 6 " , 8 : : 6 ' 5 3 " , 0 6 n \ a 0 = 3 8 " 2 3 ' 5 6 " , 8 3 ' 0 9 " , 5 4 w j li = 0 0 = 1 9 ' ' 3 5 " ' 0 8 % 5 1 t r . 5 1 9 di q u e s t i valori ci s e r v i r e m o n e l l a u l t e r i o r e r i c e r c a c h e i n t e n d i a m o s v o l g e r e , e di cui a b b i a m o i n d i c a t o i t e r m i n i al p r i n c i p i o del p r e s e n t e l a v o r o . l e coordinate geografiche d e l l ' e p i c e n t r o sono (a m e n o d e l l e corr e z i o n i ) : ( <1>„=08° 0 1 ' 3 9 " in / a ,, = 3 8 ° 2 3 ' 5 6 " . 8 w c o m e si o t t i e n e i m m e d i a t a m e n t e a p p l i c a n d o la | 2 ] . ho ma — istillilo \ azionale di geofisica — maggio 19-18. w k l ^ !.. m a r c e i j . 1 <;. pannocchia riassunto alla teoria di ii cgcner dir sosterrebbe, con la nota teoria della derira dei continenti. vi]>otcsi di una traslazione dei continenti africano c americano, si contrappone una teoria j>iù recente secondo la (piale se la derira è realmente avveniita, essa, riguarda solo la distanza cresta mediana atlantica-cosla americana. il presente lavoro contiene la prima parte di una ricerca sismologica fatta allo scopo di dimostrare vattendibilità o meno di tale teoria. con un metodo già adoperati: da caloi e peronaci (3), facendo uso di coordinate geocentriche, si è determinato l'ipocentro e il tempo origine di un \terremoto avvenuto sulla eresia atlantica. bibliografia ( j j b i n i s e l i as-ociation l'or the advaneeineiu of s c i e n c e : gray milno t r u t l . .1. c o m r i e willi au introduction by h . j e f f r e y s . tlie geoceniric direction cosine* ni seismologicnl observiijories " l o n d o n 1938. ( ) bucnner g . j . : c.liari of depili, l i m e and dislalico fotdeep focus eartliijiiakes 1935. cai.oi i'. peronaci i . : il terremoto ile[ turkestan del 2 novembre 1946 \ l i l i a l i <li geofisica i, 2. 246 (1948). i 1 ; b r i t i s h a s f o c i a l i o n l'or tlie advancemenl of s c i e n c e : gray m i l n e t r u s t i i . j e f f r e y s and k . e . b u l l e n « scismi il ogiad tubles » london 1940. (r>) caloi i ' . : caraneristiche sismiche fondamentali dell'europa centrale b o l lettino della s o c . sismologica italiana, voi. x l . il. 3-1. anno 1942. ( c ) scilmerwitz g . : ausgueicliung der besten suitionsbeobachtinigen mittelcuropiiischer erdbeben zeit-i lirift f. geoplivsik. jalirs. 1-1. 1938. vol49_2_2006 841 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words aftershocks – foreshocks – joint hypocenter determination – double difference 1. introduction hypocenter location is one of the main tools of seismological observation: its reliable estimate allows the investigation of seismogenic processes. it is well known that the precision of locations is controlled by several different factors, among which the accuracy of the arrival time reading of seismic waves, the geometric distribution of the seismograph network and, most important, by the knowledge of the propagation medium. the purpose of this study is to develop and test a location algorithm capable of minimizing the influence of the velocity model used in the computation, and the influence of human errors in picking the arrival times of seismic waves. a velocity model is not available in detail because of the complicated crustal structure in some regions of the world, as in italy. luckily, the errors connected with the velocity model are systematic for the hypocenters that are closed spaced respect to each other, and the technique adopted in this study, the «joint hypocenter determination» is based on this feature (douglas, 1967). it improves the accuracy of the standard locations in areas of clustered seismicity, i.e. where the distance between the hypocenters is much shorter than that between the hypocenter centroid and the recording stations. in this case the propagation an algorithm for double difference joint hypocenter determination: application to the 2002 molise (central italy) earthquake sequence rodolfo console and alessandra giuntini istituto nazionale di geofisica e vulcanologia, roma, italy abstract we have developed an original computer code for double difference hypocenter determination including an independent routine for the cross-correlation estimate of the time difference between two waveform segments, relative to different events recorded by the same stations. this computer code has been tested relocating a set of 26 events recorded by the seismic stations of the telemetered italian national network operated by the ingv. the hypocentral solutions so obtained are characterized by standard deviations typically of the order of magnitude of 50 m, in comparison with the errors of a few kilometers characterizing the performance of the ingv bulletins. the proximity of hypocenters in several groups of events closely separated in time shows that our relocations are the result of an accurate analysis, rather than that of random errors. the method developed in this study is suitable for rapid and accurate hypocentral determination carried out by a permanent sparsely distributed network of stations, even before that mobile equipment installed in the area affected by new seismic activity allows higher resolution locations. mailing address: dr. alessandra giuntini, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: giuntini@ingv.it 842 rodolfo console and alessandra giuntini path between the sources and the stations can be assumed as if it were the same for all the events. the locations obtained by this method are characterized by a high relative accuracy, although the absolute location of the whole cluster remains uncertain, unless independent information allows the reliable estimate of the hypocenter coordinates for at least one event. in this case, we can perform the master event location. in order to minimize the errors linked with the incorrect time reading of the seismograms, we adopt the cross-correlation technique between waveforms. this technique makes use only of time differences between arrival times for pairs of events at the same stations, rather than absolute time readings at single stations (console and di giovambattista, 1987; fehler et al., 2000; waldhauser and ellsworth, 2000; rowe et al., 2002; wolfe, 2002). the cross-correlation of digital waveforms allows a precision of the order of one sampling interval or even better. this method is called here as the «double difference joint hypocenter determination» (ddjhd). because of the methodological nature of this work, in order to apply the ddjhd method, we have developed the algorithm independently of previous works, and written accordingly an entirely original computer code in fortran language. 2. the location algorithm consider an event ei recorded by station sj. the first arrival observed time can be written as (2.1) where hi, xi, yi, zi are the origin time and the rectangular hypocenter coordinates of the event, xj, yj are the coordinate of station sj and f gives the travel time of the p-wave. given a suitable guess of the hypocenter coordinates and assuming that the variations of the parameters are small enough, the non linear eq. (2.1) can be substituted by its first order linear expansion (2.2) x +∆h +∆+f= + jy z+ + + ε∆ ∆ . , , , ,t h x y z x y x f y f z f , , , , i j i i i i j j i i j i i j i i j i 2 2 2 2 2 2 ^ h ,,( , )h x y zi i i i ( , , , , )t h f x y z x y,i j i i i i j j= + in this equation we find four unknowns: the variations of the origin time and hypocenter coordinates of the event. εj is the station correction and in this case it is known or set to zero. if a suitable number of observations is available, it is possible to solve the eq. (2.2) for a given event by the least squares method. the final solution is obtained by iteration of the process substituting the initial values by those obtained in the first trial and so on, until the new corrections for the parameters are small enough to be neglected. this method is applicable to any single event ei, and for this reason it is known as the method of single event location. in order to locate a spatially closed group of events, it is possible to make use of the jhd («joint hypocenter determination») technique, by which every event is located respect to all the other events in the same group. the jhd procedure includes station corrections, which are station delays common to all the earthquakes and with this method we find the event locations and the values of all station corrections. this method offers the advantage of improving the accuracy of the relative locations, but the absolute locations remain uncertain. in this work, following the double-difference method of waldhauser and ellsworth (2000), we have worked out a new procedure applying the jhd to the time differences between the seismograms of two events. consider an equation analog to (2.2) for a second earthquake el, recorded by the same station sj. by subtracting from the equation so obtained for el that for the event ei, we obtain (2.3) equation (2.3) is a linear expression of 8 unknowns. in this expression the station corrections are removed because we are considering two different events recorded at the same station. the left-hand side term contains an observed quantity: the difference of the arrival +x∆h +∆+ +f f+ z -∆ = x + ∆ ∆ ∆ ,l j , , , , , , , , . t h x y z x y x f y f y z f h x y z x y h x f y f y z f z , , , , , l i l l j l l l j j l l l l i i j i i i j j i i i j i i j 2 2 2 2 2 2 2 2 2 2 2 2 + ∆ ∆ ∆ ,i j , , l j l j i ^ ^ h h ( , , , )h x y zi i i i∆ ∆ ∆ ∆ 843 an algorithm for double difference hypocenter determination times. the right side term contains the guessed values of the origin and travel times and eight unknown quantities: the variations of the origin times and space coordinates for the two events. similar equations can be written for each couple of events. moving the guessed values of the origin and travel times from the right to the left hand side of the equations produces a double difference between the observed and guessed times. for this reason this method is called the method of the double difference joint hypocenter determination (ddjhd). the set of equations can be solved by the least square method. it is not necessary that all the stations record all the events, but it is necessary that every pair of events is recorded by some common stations. let be me the number of events to be located and n the total number of observations. the eq. (2.3) can be written in the matrix form (2.4) where g is the coefficient matrix (derivatives of g p d$ = the travel times respect to the hypocentral coordinates), is the unknown vector, of length 4∗me, and is the observation vector of length. the least squares solution of the whole set of equations is given by (2.5) where gt denotes the transpose of g. the solution of a set of equations as (2.4) may present some problems when the matrix g is singular or close to singularity. one example is given by the case when one or more events are recorded just by refracted waves, all from the same refractor in the plane layered medium. in this case all the derivatives of the travel times respect to the hypocentral depth have nearly the same value. some problems of singularity may also occur when the observations are all constituted by direct waves, in the case that the station-epicenter distances are not markedly different. the problem is connected with the small linear dimensions of the source area with respect to the distance to the recording stations, p g g g dt t1 $= -] g d p fig. 1. matrix representation of the linear equations, each of which is written for one single event(i)-event(l) couple at station(j), used in the joint hypocentral location (f, travel times; , time residual for each observation; j=1, s, station index; i = 1, n, event index; s, total number of stations; n number of events). the last four rows express the constraint; in this case the fist one, at which is possible to give a weight, p, are the guessed coordinates of the master event. x y z tmaster master master master, , , f , , , ,h x y z x y,i i j i i i j j+ + ^ h +f=, , ,t t h x y z x y,l i l i l l j l l j j-∆ ∆ , , ,l* ^ h 844 rodolfo console and alessandra giuntini so that the take off angles of the rays from the hypocenters to the stations are very close to each other. to avoid this problem it is necessary to use a wide distribution of recording stations, so that the first arrivals are represented by both direct and refracted waves. equation (2.4) is still intrinsically singular because the variation of one coordinate of all the events by the same amount does not affect the value of the right hand term (console et al., 1992). we eliminate this problem by constraining the absolute location of the whole cluster of events, with the introduction of a piece of a priori information to the set of equations. such information is given by the known location of a selected event (master event). a way by which the constraint is implemented can be that of introducing four new equations in (2.4) expressing the equality of the four unknowns to their guessed values (the coordinates of the master event). it is possible to give a weight to these four equations. this weight may be large enough so as the respective solution is affected very little by the other observations (fig. 1). another way to solve eq. (2.5) in case of singularity is using the minimum norm solution (simply eliminating zero singular values from the solution). such solution would correspond to taking fixed the centre of mass of the hypocenter cluster. in the set of equations shown in fig. 1, we would have four rows, each of which representing a linear combination of the respective coordinate of all events. the algorithm has been exploited in a fortran computer code. several tests have been carried out by synthetic data. these data have been produced assuming a given distribution of hypocenters and stations, a known velocity model, and introducing random variations simulating errors with a given standard deviation. these tests have shown the reliability of the algorithm and the stability of the results. a few eventual outliers, introduced for testing purpose, could be easily identified and eliminated from the data set. 3. the determination of time differences as stated in the introduction, an important step to improve the location accuracy can be made by improving the precision by which the difference ∆tl,i, between the arrival times for a pair of events l,i at the same station sj, is estimated. to this purpose, we use the cross-correlation of the respective waveforms. it is assumed that the value on the right hand side of eq. (2.3), is the time shift of one record with respect the other that yields the maximum of the correlation function in the time dominion. for the computation, the correlation function between to digital wave form segments is defined as (3.1) when the two seismograms are identical r assumes the maximum value of 1, while the more different are the two wave forms, the closer it will be to the minimum value of zero. the cross-correlation method relies on the similarity between seismograms of events that are spatially close (a basic rule for the applicability of the ddjhd); in fact we may expect that two earthquakes produce similar wave forms at the same station if their source mechanisms are virtually identical, and if their sources are located in such a way that their source-receiver paths cross the same crustal etherogeneities (waldhauser and ellsworth, 2000). therefore, the correlation coefficient between the two seismograms represents itself a measure of the distance between the two events. 4. description of the computer code the outline of the computer code developed in this work is shown by the flow chart in fig. 2. the code is designed so as during its execution the following options can be chosen: velocity model – the choice of the velocity model used for the computation of the travel times can be done between a standard model and another model entered by the user. introduction of constraints – the coordinates of the event that constitutes the constraints can be entered either by an input file or by the keyboard. =r n x x n y y n x y x y i i n i i n i i n i i n i i i n i i i n i n 2 1 1 2 1 1 2 1 11 )= = = = = == 2 d dn n= =g g/ / / / / // 845 an algorithm for double difference hypocenter determination weight of the constraints – a given weight can be introduced to the constraints. in this case the user is asked for its magnitude. automatic execution – it is to be chosen whether the execution proceeds automatically or it stops at some particular points for the control of the operator. as the set of equations is solved in rectangular coordinates, two subroutines are used for the conversion from geographical to rectangular coordinates (tracoo), and back from rectangular to geographical coordinates (trainv). in the present application, for the conversion we have chosen as origin of the rectangular coordinate system a point internal to the area of the seismic sequence: 41.7°n and 14.9°e. the main computer code makes calls to two subroutines. the first (dromoc) calculates the theoretical time-differences (∆ti,j)th making use itself of two subroutines, respectively diret for the direct waves and rifra for the refracted waves. the second subroutine (mquadr) performs the solution of the overestimated set of equations by the least squares method. the capital letters in the flow chart denote a single code block whose performance is explained in the following. (a) dealing with an iterative process, it is necessary to define the conditions by which the search for the minimum misfit is stopped at a given iteration. these conditions are: – the sum of the square of the residuals (difference between the theoretical (∆ti,j)th and the observed (∆ti,j) is smaller than n ⋅ 0.0001 (where n is the total number of observations); – the sum of the square of the residuals makes repeated oscillations around a certain value; – the absolute value of the correction for all the coordinates is smaller than one tenth of the associated standard deviation; – the number of iterations exceeds 20. (b) in the case that some observations exhibit a residual larger than a given threshold, the location process can be repeated, either in automatic or interactive mode, removing such observations from the data set. (c) at the end of the whole procedure, the final locations are obtained with all their statistical errors. 5. seismological outline we decided to test the method by its application to a set of data collected by the italian fig. 2. flow chart of the computer code. 846 rodolfo console and alessandra giuntini seismological network (rsnc) operated by the istituto nazionale di geofisica e vulcanologia (ingv), and recorded in the data center in rome by phone line telemetry. a data set that appeared suitable for testing was represented by the numerous earthquakes (foreshocks, two mainshocks and aftershocks) occurred in the molise region on october 31st, 2002 and the following days. it seemed interesting to show how an appropriate method of analysis could allow the retrieval of valuable information from the data collected by the permanent national seismological network, at the early phase of the sequence, before the mobile equipment could start the operation in the epicentral area. 5.1. historical seismicity of the area the site most affected by the molise octobernovember 2002 earthquakes, the town of san giuliano di puglia (fig. 3) belongs to an area of the southern-central apennines characterized by nearly complete absence of recent seismicity, but surrounded by important seismogenic structures. they are recognized in the gargano (60-100 km to the east), san severo (30-40 km to the east), foggia (50-80 km to the south-east), beneventoirpinia (40-80 km to the south), and bojano basin (40-50 km to the west) areas. these structures have produced historical earthquakes of large magnitude, whose effects have been felt in the area hit by the sequence considered in this study. the historical seismicity of the region is reported in fig. 3. the information on historical seismicity comes from the catalogo dei forti terremoti italiani (cfti3) (boschi et al., 2000). for each event we consider the «equivalent magnitude» me reported in the catalog. this magnitude is obtained from the distribution of the macroseismic intensity (gasperini and ferrari, 2000), and is considered roughly equivalent to the moment magnitude mw. on 5 december 1456, the earthquake that is considered the strongest event (me 7.1) of the last 1000 years in the region struck three different epicentral areas of the southern apennines at the same time, including that of the bojano basin. another event (me 6.6), whose epicenter was also located in the bojano basin, occurred in 1805. other important earthquakes reported in the catalogs are that of benevento-irpinia area (1125, me 7), the san severo earthquake of 30 july 1627 fig. 3. map of the historical earthquakes that occurred in the study earea. 847 an algorithm for double difference hypocenter determination (me 6.8), and the gargano earthquakes of january 1646 (me 6.1), and that of 1875 (me 6.2). lastly, another important earthquake that more recently affected this area was the november 23, 1980, irpinia earthquake (me 6.9). 5.2. the seismographic data set on october 31, 2002, at 11:32 (utc) a strong earthquake (ml 5.4) hit a large zone of the border area between the molise and puglia regions. it had been preceded during the previous night by a small series of foreshocks of magnitude ranging between ml 2.6 and 3.5, the first of which recorded at 01:25 (utc). after the mainshock, the national seismological network recorded during the same day of 31 october 2002 and on the morning of 1 november about 60 events of magnitude larger than ml 2.2. at 16:08 (utc) of 1 november another mainshock of ml 5.3 occurred in the area. it was located about 12 km to the west of the previous mainshock. this earthquake was itself followed by many aftershocks, among which two events of magnitude larger than ml 4.0 during the same day. the earthquake series then decreased fairly steadily in time. in this study we decided to analyze 26 events with magnitude mlf 2.9 between 31 october and 2 november 2002. retrieving the waveforms collected in the ingv data base from the rsnc, we limited the selection to 21 stations located within 200 km from the epicentral area (fig. 4); in this way, the first arrivals were represented by many pg and some pn phases. for each event the following entries have been considered from the automatic ingv bulfig. 4. map of the stations of the national seismological network used in this study. table i. focal parameters of the 26 events located by the automatic ingv bulletin. the parameters of the two mainshocks are identified by bold phase. event id ml long (°) lat (°) depth (km) 0210310025 1 3.2 14.95 41.67 10.00 0210310227 2 3.5 14.93 41.67 10.00 848 rodolfo console and alessandra giuntini fig. 5. epicentral map of the events reported by the automatic ingv bulletin. the two mainshocks (#4 and #14) are denoted by stars. table i (continued). event id ml long (°) lat (°) depth (km) 0210310615 3 2.9 15.07 41.79 10.00 0210311033 4 5.4 14.97 41.71 10.00 0210311054 5 3.0 14.96 41.73 10.00 0210311156 6 3.5 15.16 41.82 10.00 0210311303 7 3.7 14.9 41.72 10.00 0210311656 8 3.6 14.96 41.74 10.00 0210312133 9 3.4 14.92 41.79 10.00 0210312256 10 3.2 14.92 41.73 10.00 0211010927 11 3.5 14.92 41.7 10.00 0211010945 12 3.0 14.86 41.71 10.00 0211011131 13 3.2 14.93 41.67 10.00 0211011509 14 5.3 14.88 41.69 10.00 0211011519 15 4.1 14.93 41.73 10.00 0211011603 16 3.2 14.8 41.68 10.00 0211011611 17 3.2 14.85 41.71 10.00 0211011649 18 3.3 14.74 41.69 10.00 0211011721 19 4.3 14.88 41.74 10.00 0211011951 20 3.2 14.93 41.7 10.00 0211012230 21 3.1 14.81 41.68 10.00 0211012243 22 3.8 14.85 41.74 10.00 0211020029 23 3.2 14.89 41.67 10.00 0211020120 24 3.2 14.89 41.66 10.00 0211020237 25 3.7 14.83 41.69 10.00 0211020340 26 3.0 14.77 41.72 10.00 letin: origin time, preliminary location, magnitude and first onset arrival times for the various stations. the set of the 26 events analyzed is reported in table i and their epicenters are mapped in fig. 5. each event has been identified by a label containing year, month, day, hour and minutes of its origin time with the following format: yymmddhhmm. for instance, 0210311033 mean 849 an algorithm for double difference hypocenter determination the event occurred on october 31, 2002 at 10:33 utc. 6. application and results as previously specified, in order to improve the location accuracy, this study used the crosscorrelation technique between the wave forms to improve the precision by which the differences ∆tl,i relative to pairs of events are determined. the number of events analysed is 26 (table i) and the number of stations is 21. this gives a total number of waveform pairs equal to 26∗(26−1)∗210 =11.550, but not all the events have been recorded by all the stations. moreover, only the observations that exhibit a correlation coefficient larger than 0.7 have been considered. this implies that the events considered in the location process are close to each other (the ray paths are almost identical) because in this case their waveforms are similar, yielding large values of the largest correlation coefficient. the selection criteria are met by 3781 observations. once the ∆tl,i are determined, the data are ready for our main problem: the location. our preliminary tests showed that a number of observed ∆tl,i are affected by large errors that denote them as outliers. in this respect, by deleting the observations characterized by a difference larger than 1.5 s from the average of the same pair of events, we obtain a faster and more stable convergence towards the least square solution. this further selection criterion produced the deletion of 181 observed ∆tl,i. considering that the depth and epicentral coordinates of the master event influence the location of all the others, particular attention has been paid to its choice. in our case the master event was chosen among those located by the temporary network deployed by the ingv almost immediately after the beginning of the aftershock series. the density of the ingv temporary network allows the computation of hypocentral coordinates (depth included) with errors of the order of few hundreds of meters, for events inside the area covered by the network itself. in light of these considerations the master event was chosen as the earthquake of october 31, 2002 at 23:56 (utc), whose hypocentral coordinates are reported in table ii. the optional inputs used in the execution of the location program are: – a three layer velocity model (table iii) for the computation of the travel times; – automatic removing of the outliers; – observations with residuals larger than twice the standard deviation are removed at every iteration (isigma = 2); table ii. hypocentral coordinates of the earthquake used as master event, obtained by the mobile seismic network. event lat (°) long (°) depth (km) 0210312256 41.6767 14.9038 22.11 table iii. velocity model used by the location algorithm. layer thikness (km) p-wave velocity (km/s) 1 12 5.9 2 22 6.2 3 999 7.9 table iv. focal parameters of the 26 events located in this study. the parameters of the two mainshocks are identified by bold phase. event id ml long (°) lat (°) depth (km) 0210310025 1 3.2 14.909 41.657 23.976 0210310227 2 3.5 14.912 41.660 23.887 0210310615 3 2.9 14.911 41.657 23.857 0210311033 4 5.4 14.918 41.689 23.931 850 rodolfo console and alessandra giuntini – threshold residual equal to 0.15 s (the run is stopped if all the residuals are smaller than 0.15 s). analyzing the output file of the program we noted that observations exhibiting large residuals (outliers) were removed in the first cycles. we noted also that since these first cycles the solutions are stable until the final conditions are met. the final solutions were obtained with littable iv (continued). event id ml long (°) lat (°) depth (km) 0210311054 5 3.0 14.896 41.649 23.502 0210311156 6 3.5 14.943 41.645 23.66 0210311303 7 3.7 14.900 41.687 23.622 0210311656 8 3.6 14.905 41.651 24.001 0210312133 9 3.4 14.883 41.659 22.806 0210312256 10 3.2 14.900 41.670 22.11 0211010927 11 3.5 14.903 41.672 22.372 0211010945 12 3.0 14.868 41.677 22.857 0211011131 13 3.2 14.932 41.648 23.942 0211011509 14 5.3 14.850 41.645 19.639 0211011519 15 4.1 14.847 41.670 22.614 0211011603 16 3.2 14.837 41.673 21.756 0211011611 17 3.2 14.858 41.669 22.777 0211011649 18 3.3 14.835 41.679 22.551 0211011721 19 4.3 14.859 41.683 24.319 0211011951 20 3.2 14.860 41.678 22.302 0211012230 21 3.1 14.859 41.682 22.066 0211012243 22 3.8 14.844 41.684 24.38 0211020029 23 3.2 14.945 41.663 23.914 0211020120 24 3.2 14.908 41.690 21.494 0211020237 25 3.7 14.843 41.685 22.265 0211020340 26 3.0 14.831 41.683 22.683 fig. 6. epicentral map of the 26 events relocated in this study. the origin of the rectangular coordinates is the point of geographical coordinates 14.90°n 41.67°e. the two mainshocks (#4 and #14) are denoted by stars. the event used as reference for the location (#10) is denoted by a cross. fig. 8. east-west vertical cross-section of the hypocenters relocated in this study; z is positive up. 851 an algorithm for double difference hypocenter determination tle more than 1900 observations (time differences) of the initial 3200. they exhibit a root mean square of the residuals close to 0.1 s and the standard deviations of the hypocentral coordinates are of the order of magnitude of 50 m. the results are reported in table iv and the epicentral map is plotted in figs. 6 and 7. figure 8 shows the east-west vertical cross-section of the hypocenters. of course, the standard deviations of the hypocentral coordinates for the master event are negligible, as they depend only on the weight given to the constraints. 7. discussion and conclusions some features of the results allow us to evaluate the quality of the method. first, the geometrical spread noticeable in the automatic locations has been considerably reduced in our relocations. in particular, the large separation to the north-east of the epicenters of the events 3 and 6 (supposedly mislocated) disappeared. figure 8 shows a direct comparison between the epicenters initially determined by the automatic procedures and their ddjhd relocations. it is evident that the relocation process has significantly reduced the geometrical spreading of the epicenters. looking at the epicentral maps of the relocated earthquakes (fig. 6) it can be noted that some groups of events occur close to each other both in time and space. this is the case, for instance, of the triplets 1, 2 and 3 (foreshocks of the first mainshock) and 19, 20 and 21, and the doublet 10 and 11. they clearly denote a trend fig. 7. comparison between the locations of the events provided by the automatic seismic bulletin (crosses) and the locations obtained in this study (stars). 852 rodolfo console and alessandra giuntini of the seismicity for clustering in families characterized by tight physical connection. another feature easily recognizable in the relocation is the separation of the epicenters in two groups (respectively to the west and to the east of longitude 14.88) having about the same number of earthquakes, and a general trend for the migration of the events from east to west. in fact, all the first 13 events but one belong to the eastern group (foreshocks and aftershocks of the first mainshock), and all the second 13 but one belong to the western group. the second mainshock (event 14) belongs to the western group. event 12 can be considered a foreshock of the second mainshock. the average depth of the western group appears slightly smaller than that of the eastern one. all these regularities denote that the timespace pattern of the relocations is not an effect of random errors. the only event whose location appears anomalous respect to the others is the second mainshock of 1 november (#14). the reason could be found in the difficulty of computing correct ∆ti,j time differences, due to the complexity of the waveforms recorded at the various stations for this earthquake. it must also be noted that the relocations, obtained in this study using an algorithm recently developed, could in principle have been carried out just during the occurrence of the seismic series nearly in real time, should the computer tool have been ready at that time. this means that the data from the permanent national seismic network, processed by a suitable algorithm, are capable of achieving relevant geophysical information on the seismogenic structures activated at the early stage of a seismic sequence. this could provide seismologists with a clearer picture of the seismogenic phenomenon from the events (that are the most numerous and important) which occurred before the exploitation of mobile stations in the area, even during the first hours of activity. acknowledgements we thank giuseppe de natale and anthony lomax for their clear comments and suggestions. references boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (2000): catalogue of strong italian earthquake from 461 b.c. to 1997, ann. geofis., 43 (4), pp. 268. console, r. and r. di giovambattista (1987): local earthquake relative location by digital records, phys. earth planet. inter., 47, 43-49. console, r., r. di giovambattista, p. favali and g. smriglio (1992): methodological approach to earthquake location procedures: application to italian seismicity, phys. earth planet. inter., 75, 153-164. douglas, a. (1967): joint epicentre determination, nature, 215, 47-48. fehler, m., w.s. phillips, l. house, r.h. jones, r. aster and c. rowe (2000): improved relative locations of clustered earthquakes using constrained multiple event location, bull. seismol. soc. am., 90 (3), 775-780. gasperini, p. and g. ferrari (2000): deriving numerical estimates from descriptive information: the computation of earthquake parameters, ann. geofis., 43 (4), 729-246. rowe, c.a., r.c. aster, w.s. phillips, r.h. jones, b. borchers and m.c. fehler (2002): using automated, high-precision repicking to improve delineation of microseismic structures at soultz geothermal reservoir, pure appl. geophys., 159, 563-596. waldhauser, f. and w.l. ellsworth (2000): a doubledifference earthquake location algorithm: method and application to the northern hayward fault, california, bull. seismol. soc. am., 90 (6), 1353-1368. wolfe, c.j. (2002): on the mathematics of using difference operators to relocate earthquakes, bull. seismol. soc. am., 92 (8), 2879-2892. massa_corretto_layout 6 829 ingv strong-motion data web-portal: a focus on the emilia seismic sequence of may-june 2012 marco massa*, sara lovati, dario sudati, gianlorenzo franceschina, emiliano russo, rodolfo puglia, gabriele ameri, lucia luzi, francesca pacor, paolo augliera istituto nazionale di geofisica e vulcanologia, sezione di milano/pavia, milano, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6120 1. introduction in italy, strong-motion monitoring was started in 1972 by different institutions, although mainly through ente nazionale per l'energia elettrica (enel; italian national electricity company) and dipartimento della protezione civile (dpc; italian department of civil protection), with different purposes. these included permanent accelerometric monitoring and temporary monitoring during seismic sequences or before permanent installation. today, the national accelerometric network (ran; rete accelerometrica nazionale) [gorini et al. 2010, zambonelli et al. 2011] is operated by the dpc and consists of 464 digital stations. these are distributed throughout the whole national territory, with a prevalence for areas of major seismicity. in 2006, the ingv began strong-motion monitoring, by installing 22 accelerometric stations in northern italy (rais; rete accelerometrica italia settentrionale; accelerometric network of northern italy; http://rais.mi.ingv.it/). in 2008, the monitoring was extended to a national scale: this effort led to the installation of 105 accelerometers, collocated with the velocimetric sensors, in selected rete sismica nazionale (rsn; national seismic network) sites [amato and mele 2008] that are managed by the centro nazionale terremoti (cnt; national earthquake centre). overall, the 127 strongmotion stations that form the ingv italian strong-motion network homogeneously cover the whole italian territory. the progress achieved in italy in the field of strong-motion monitoring and strong-motion data archiving and dissemination was illustrated in a recently published special issue of the bulletin of earthquake engineering [luzi et al. 2010]. the strong-motion data recorded by the ran have been distributed and are available on request to the dpc and to the italian accelerometric archive (itaca), as the italian strong-motion database (http://itaca.mi.ingv.it/) [pacor et al. 2011a], which has been updated with records to 2009. the ingv strong-motion data are archived in real-time and distributed through the european integrated data archive (eida; http://eida.rm.ingv.it/) web portal. recently, an ingv working group developed the first version of a web portal with the aim of archiving, processing and distributing accelerometric data recorded by permanent and temporary ingv stations. this web portal (www.mi. ingv.it/ismd/; figure 1, top panel) is composed of two main modules: the former is known as the ingv strong motion data (ismd, www.mi.ingv.it/ismd/ismd.html/; figure 1, bottom left panel) and has as its main scope the analyse and distribution in quasi-real time (a few hours after event occurrence) of the uncorrected accelerometric data, and the related metadata obtained after an automatic processing procedure. this latter, known as the dynamic archive (dyna, http://dyna.mi.ingv.it/dyna-archive/; figure 1, bottom right panel) is a dynamic database where manually postprocessed accelerometric waveforms are provided, together with their metadata. both of these archives are designed and structured in such a way that their compilations and updating will be almost completely automatic. at the end of may 2012, a first prototype of the ismd module was published, providing the uncorrected strongmotion data recorded by the ingv stations for the main events of the emilia seismic sequence [massa et al. 2012]. 2. the ingv strong-motion data: quasi real-time automatic strong-motion analyses the ismd module (www.mi.ingv.it/ismd/ismd.html/) is distributed in quasi-real-time (a few hours after an earthquake occurrence), as the ingv uncorrected accelerometric data and related strong-motion parameters, as obtained after automatic processing. possible delays in data publishing depend on the number of requests made by users to eida. ismd now includes about 2,000 uncorrected strong-motion waveforms that relate to the events with 4.0 ≤ ml ≤ 5.9 that occurred in the central part of the padano-emiliana article history received july 20, 2012; accepted august 23, 2012. subject classification: strong-motion data, web portal, data processing. 2012 emilia earthquakes plain (northern italy) from may 20 to june 12 (figure 2, left panel). the data were recorded by all of the ingv permanent and temporary strong-motion stations that were installed in northern italy, including from the rsn [amato and mele 2008], the rais (http://rais.mi.ingv.it/) [augliera et al. 2011] and the ingv temporary network [moretti et al. 2012, bordoni et al. 2012, this volume], with latitudes ≥43.5 ˚n. the locations and local magnitudes, ml, that are reported on the web site were provided by the cnt of ingv (http://cnt.rm.ingv.it/). an automatic procedure has the main aim to publish on the web site both the metadata (provided after automatic data processing) and downloadable waveforms in uncorrected ascii format. starting from the origin time of each earthquake (provided by the cnt), the procedure automatically downloads 5 min of mini-seed continuous signal from eida (http://eida.rm.ingv.it/). soon after the data download, the automatic data processing starts. it includes: (i) baseline correction, to have a zero-mean signal and to remove any linear trend, computed with a least square method; (ii) tapering of the signal through a 1% cosine function at the beginning and at the end of each selected window; (iii) application of a 4th order butterworth band-pass acausal [boore and akkar 2003] filter to remove the high-frequency and low-frequency noise. the filter cut-off thresholds are automatically selected on the basis of the event magnitude (i.e., massa et al. 830 figure 1. ingv strong-motion data web portal: home page (top panel); ismd home-page (bottom left); dyna home page (bottom right). 831 0.1-40 hz for ml ≥5.5; 0.2-35 hz for 4.5 ≤ml <5.5; 0.3-35 hz for 3.5 ≤ml <4.5). given the characteristic of the digital stations, no instrumental correction is applied. starting from the automatically processed data, velocity waveforms are obtained by integration of the acceleration traces. peak ground acceleration (pga), peak ground velocity (pgv) and 5%-damped acceleration response spectra (sa) for periods up to 4 s are calculated. note that for events with magnitudes <4.5, the sa should be only considered in the frequency range defined by the low-pass and high-pass filters. the automatic system also provides 5%-damped pseudo-velocity response spectra, 5%-damped displacement response spectra, arias intensities (ia) [arias 1970] and housner intensities (ih) [housner 1952]. for each recording station, the horizontal-to-vertical spectral ratios (hvsrs) are automatically calculated considering 5 s and 10 s of signal selected in correspondence with the more energetic portions of the seismograms. at the end, for each analyzed event, the automatic procedure generates an ad-hoc web page (see figure 2, right panel, for the may 20, 2012, ml 5.9 mainshock) that shows the results in terms of static tables (text files), including pga, pgv, ia, ih and sa (at periods of 0.3 s, 1.0 s, 3.0 s, as used for shake-map calculations) and generic plots (e.g., location of recording stations, hvsrs, response spectra, accelerograms). for the events with ml >5.0, the comparison between the recorded pgas and italian ground motion prediction equations [bindi et al. 2011] is shown. for each analysed earthquake, the strong-motion data are downloadable in ascii format: following the standards proposed by itaca, the ascii files are composed of 43 header lines followed by acceleration data in cm/s2. moreover, the dynamic output files can be generated in table format, e.g., specifying the time window, ground-motion parameters (pga and pgv), magnitudes, epicentral and hypocentral distance ranges, and eurocode8 site category [cen 2003]. at present, the preliminary site classification of the ingv stations is based on a lithological map at a national scale (1:100,000), which was developed by the ingv as part of the shakemap project in italy [michelini et al. 2008], as shown in figure 2 of di capua et al. [2011]. 3. the dyna archive the dyna module (http://dyna.mi.ingv.it/dynaarchive/) is a beta-version of the dynamic accelerometric database that is aimed at archiving and distributing the ingv strong-motion data and their relevant metadata. it is based on a relational database and its structure was greatly inspired by itaca (http://itaca.mi.ingv.it/itacanet/). the first release of dyna contained about 1,250 waveforms related to 143 earthquakes (3.5 ≤ml ≤5.9) that occurred from february 2008 to july 2011. it was populated through a quasi-automatic procedure that considered two ingv databases: 1) the italian seismic instrumental and parametric database (iside, http://iside.rm.ingv.it/iside) for events with ml ≥3.5; 2) eida (http://eida.rm.ingv.it/), for accelerometric waveforms recorded by the ingv networks (i.e., the rsn, rais, mednet; http://mednet.rm.ingv.it/rcmt.php). in june 2012, the strong-motion data relative to six events with magnitudes >5 (table 1) that occurred during the emilia sequence were made available on dyna, together with their metadata. to publish the waveforms recorded for each event, the download of seed volumes (fseed) from eida was fully automated, starting from the twitter event alert service of the ingv accelerometric data web portal figure 2. left: ismd dataset collected during the emilia 2012 seismic sequence and now available from the ingv strong-motion data web portal. red, ec8 a soil category; green, ec8 b soil category; blue, ec8 c soil category [cen 2003]. right: web-page automatically generated a few hours after the may 20, 2012, ml 5.9 mainshock (see 'last update: sun may 20 11:38:18 utc 2012'). ingv (http://twitter.com/ingvterremoti). in this way, we were able to obtain accelerometric waveforms in a time window that generally spanned from some hours after an event occurrence to 24/48 h (depending on the number of requests made to eida). on the other hand, the compilation of the relational database behind the archive is still semi-automatic; however, in the near future, this will be turned into a fully automated procedure. the waveform data and earthquake, station and record information charged in the database after each event are taken from the ingv twitter account (i.e., event: origin time, latitude, longitude, depth, hypocenter reference, magnitude, magnitude type, magnitude reference) and from the seed volumes (i.e., waveform: acceleration data, time of the first sample; network: code; station: code, name, latitude, longitude, elevation, start/end date; channel: location identifier, latitude, longitude, elevation, depth, start/end date, name and some characteristics of the installed sensor). figure 3 shows the may 29, 2012, 07:00:03 event page available in the dyna that lists for each station the epicentral dismassa et al. 832 origin time (utc) longitude (˚e) latitude (˚n) depth [m] ml ismd # stations dyna # stations 120520 020352 11.230 44.890 6.3 5.9 30 52(50) 120520 020630 11.189 44.886 7.7 4.8 26 / 120520 020731 11.370 44.863 5.0 5.1 30 30(20) 120520 021146 11.370 44.840 7.8 4.3 31 / 120520 021242 11.220 44.820 20.4 4.3 29 / 120520 030250 11.100 44.860 10.0 4.9 30 / 120520 091321 11.241 44.879 3.1 4.2 28 / 120520 131802 11.490 44.831 4.7 5.1 28 51(39) 120520 173714 11.380 44.880 3.2 4.5 31 / 120523 214118 11.251 44.868 4.8 4.3 32 / 120527 181845 11.158 44.882 4.7 4.0 31 / 120529 070003 11.090 44.850 10.2 5.8 34 50(49) 120529 082551 10.943 44.901 3.2 4.5 35 / 120529 082723 11.106 44.854 10.0 4.7 34 / 120529 105557 11.008 44.888 6.8 5.3 33 56(42) 120529 110002 10.950 44.873 11.0 4.9 34 / 120529 110025 10.947 44.879 5.4 5.2 35 / 120531 190404 10.980 44.891 8.7 4.2 32 / 120603 192043 10.943 44.899 9.2 5.1 36 35(33) 120612 014836 10.888 44.880 10.8 4.3 27 / table 1 (above). emilia seismic sequence (ml ≥4) included in the ingv strong-motion data web portal. only the events in bold are included in dyna. the last two columns indicate the number of the recording stations available in ismd (soon after the occurrence of each earthquake) and the number of those included in dyna after the manual processing (in brackets, the number of the processed stations). the earthquake locations and magnitudes were provided by the official web site of the national earthquake center (cnt) of ingv rome (preliminary data at http://cnt.rm.ingv.it/). figure 3 (left). the may 29, 2012, 07:00:03 event web page, available on http://dyna.mi.ingv.it/dyna-archive/. 833 tance, and the largest pga, pgv and peak ground displacement (pgd) among the three components. the strong-motion data were manually post-processed by expert operators who adopt the procedure used for the itaca dataset [paolucci et al. 2011, pacor et al. 2011b]. for this reason, the corrected data are published on the dyna web site usually from some days to weeks after the earthquake occurrence (depending on the accuracy that is required, and also considering the features of the analysed waveforms). with respect to the quasi-real-time data published in the ismd, in some cases, the manual post-processing allows us to download further data from eida, also some days after the earthquake origin time (when the eida nodes are busy or unavailable). 4. the emilia seismic sequence dataset the ismd dataset for the emilia seismic sequence is composed of 20 events (670 three-component, uncorrected, strong-motion records) with 4.0 ≤ml ≤5.9 that occurred from may 20, 2012, to june 12, 2012, with epicentral distances up to 215 km. the data are characterized by 29 records with pga >20 cm/s2, nine of which have pga >50 cm/s2, four as >100 cm/s2, and two as >200 cm/s2. the maximum pga (266 cm/s2) was recorded at the t819 temporary station for the june 3, 2012, ml 5.1 event (epicentral distance, 3 km). for the pgv, just seven records have values >5 cm/s, two of which are >10 cm/s. also in this case, the maximum pgv was recorded at t819 for the june 3, 2012, ml 5.1 event (28 cm/s). for the two strongest events, which occurred on may 20, 2012, ml 5.9 (02:03:52) and may 29, 2012, ml 5.8 (07:00:03), the maximum pgas were recorded at the mode station (41 cm/s2; pgv, 6.3 cm/s) at 37 km from the epicenter, and at the t819 station (193 cm/s2; pgv, 14 cm/s) at 14 km from the epicenter, respectively. the dyna dataset is composed of 233 three-component, manually processed, records, related to 6 events (see table 1) with ml >5 that were characterized by epicentral distances of up to 300 km. the waveforms related to the ml 5.2 event of may 29, 2012, at 11:00:25 were not taken into account, as the signals overlapped with those that referred to a previous event (ml 4.9) that occurred at 11:00:02. confirming the results obtained by the ismd automatic procedure, the most significant pga is 260 cm/s2, which was recorded on june 3, 2012 (ml 5.1) at station t819 (pgv, 28 cm/s). figure 4 shows an example of the processed waveforms at the asol station for the may 29, 2012, 07:00:03 event. note the surface waves arriving after the s-phase window; i.e., between 80 s and 100 s on the displacement trace. this feature is observed for most of the strong-motion data, causing the occurrence of pgds corresponding to the surface-waves window, probably due to the thick sedimentary cover of the po plain. this type of surface wave is still relatively rare to see on european accelerograms, although it is common, for example, in data from california, usa (e.g., imperial valley records from 1979). 5. final remarks the ingv strong-motion data web portal (formed by the ismd and dyna modules) is aimed at collecting and distributing waveforms and the associated metadata to be used for engineering applications and seismological studies. the ingv accelerometric data web portal figure 4. n-s component recorded at asol for the may 29, 2012, 07:00:03 event, in terms of the processed acceleration (upper panel) and displacement (bottom panel). these plots were downloaded from http://dyna.mi.ingv.it/dyna-archive/. main novelty of the web portal is that the two modules are designed to be compiled and updated automatically, exploiting the automatic systems of the ingv for seismic alert and localization and the eida portal for archiving continuous real-time accelerometric waveforms. the current published version of the ismd module contains the raw waveforms and relative metadata (i.e., velocity and displacement waveforms and accelerometric response spectra) for events with ml ≥4.0 of the emilia seismic sequence. the corrected accelerometric data for six events with magnitudes >5.0 are instead distributed through the dyna module. the strong motion parameters (pga and pgv) provided by the automatic procedure soon after an earthquake origin time (for the emilia seismic sequence, usually after a few hours) differ slightly from the values inferred from the corrected data (figure 5) and represent high-quality information that can be used for quick analyses by a broad variety of users; moreover, the immediate availability of the unprocessed waveforms included in ismd allows skilful users to perform different data processing on the basis of their specific requirements. acknowledgements. the earthquake locations and magnitudes were provided by the official web site of the national earthquake centre of the ingv, rome (preliminary at http://cnt.rm.ingv.it/, and then revised in the ingv official bulletin) that performs the official monitoring of the italian territory (h 24). we thank both our ingv colleagues who developed the databases (http://iside.rm.ingv.it/iside; http://eida.rm. ingv.it/iside) from which a lot of information can be taken, and our ingv colleagues who were involved in the field activities for the emilia 2012 seismic sequence. comments and suggestions from j. douglas and an anonymous reviewer are strongly acknowledged. references amato, a., and f. mele (2008). performance of the ingv national seismic network from 1997 to 2007, annals of geophysics, 51 (2/3), 417-431. arias, a. (1970). a measure of earthquake intensity, in: r. hansen (ed.), seismic design of nuclear power plants, m.i.t. press, cambridge. augliera, p., m. massa, e. d'alema and s. marzorati (2011). rais: a real time strong-motion network in northern italy, annals of geophysics, 54 (1), 23-34. bindi, d., f. pacor, l. luzi, r. puglia, m. massa, g. ameri and r. paolucci (2011). ground motion prediction equations derived from the italian strong motion database, b. earthq. eng., 9, 1899-1920. boore, d.m., and s. akkar (2003). effect of casual and acausal filters on elastic and inelastic spectra, earthq. eng. struct. d., 32, 1729-1748. bordoni, p., r.m. azzara, f. cara, r. cogliano, g. cultrera, g. di giulio, a. fodarella, g. milana, s. pucillo, g. riccio, a. rovelli, p. augliera, l. luzi, s. lovati, m. massa, f. pacor, r. puglia and g. ameri (2012). preliminary results from emersito, a rapid response network for site-effect studies, annals of geophysics, 55 (4); doi:104401/ag-6153. cen (2003). pren 1998-1eurocode 8: design of structures for earthquake resistance. p1: general rules, seismic actions and rules for buildings, draft 6, doc cen/tc250/ massa et al. 834 figure 5. maxima pga (top) and pgv (bottom) for the may 20, 2012 (02:03:52 utm), ml 5.9 event. red squares, values obtained after the manual post processing (dyna); blue crosses, values obtained after the quasi-real-time data processing (ismd). comparisons were performed at common recording stations. 835 sc8/n335, jan 2003, brussels. di capua, g., g. lanzo, v. pessina, s. peppoloni and g. scasserra (2011). the recording stations of the italian strong-motion network: geological information and site classification, b. earthq. eng., 9, 1779-1796. gorini, a., m. nicoletti, p. marsan, r. bianconi, r. denardis, l. filippi, s. marcucci, f. palma and e. zambonelli (2010). the italian strong motion network, b. earthq. eng., 8, 1075-1090. housner, g.w. (1952). spectrum intensities of strong-motion earthquakes, in: c.m. feigen (ed.), proceedings of the symposium on earthquake and blast effects structures, university of california, los angeles, 21-36. luzi, l., p. augliera and a. gorini, eds. (2010). strong motion monitoring in italy, b. earthq. eng., special issue, 8, 1073-1230. massa, m., s. lovati, r. puglia, g. ameri, d. sudati, e. russo, g. franceschina, l. luzi, f. pacor and p. augliera (2012). the may 2012 pianura padana emiliana seismic sequence: ingv strong-motion data website, annals of geophysics, 55 (3), 507-509; doi:10.4401/ag-5828. michelini, a., l. faenza, v. lanciani and l. malagnini (2008). shakemap implementation in italy, seismol. res. lett., 79, 688-697. moretti, m., et al. (2012). rapid response to the earthquake emergency of may 2012 in the po plain, northern italy, annals of geophysics, 55 (4); doi:10.4401/ag-6152. pacor, f., r. paolucci, l. luzi, f. sabetta, a. spinelli, a. gorini, m. nicoletti, s. marcucci, l. filippi and m. dolce (2011a). overview of the italian strong motion database itaca 1.0, b. earthq. eng., 9 (6), 1723-1739; doi:10.1007/ s10518-011-9327-6. pacor, f., r. paolucci, g. ameri, m. massa and r. puglia (2011b). italian strong motion records in itaca: overview and record processing, b. earthq. eng., 9, 1741-1759. paolucci, r., f. pacor, r. puglia, g. ameri, c. cauzzi and m. massa (2011). record processing in itaca, the new italian strong-motion database, in: s. akkar, p. gulkan and t. van eck (eds.), earthquake data in engineering seismology, geot. geol. earthquake, 14, 99-113. zambonelli, e., r. de nardis, l. filippi, m. nicoletti and m. dolce (2011). performance of the italian strong-motion network during the 2009, l'aquila seismic sequence (central italy), b. earthq. eng., 9, 39-65. *corresponding author: marco massa, istituto nazionale di geofisica e vulcanologia, sezione di milano/pavia, milano, italy; email: marco.massa@mi.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ingv accelerometric data web portal << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol48/04/2005def 647 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words noble gases – solubility – degassing – silicate melts 1. introduction when investigating processes involving magmas and coexisting vapor phases, a direct observation of the system is obviously impossible. the information available comes from outgassed vapors that are released at the surface and from fluid inclusions trapped in igneous products. this implies that the observation conditions are different from those at the source, and that the pristine gas content may be modified by chemical reactions, unless unreactive species are considered. furthermore, degassing of the dominant volatiles (h2o, co2, s, cl and f) strongly modifies the physico-chemical properties of the magmatic system and plays an active role in driving the degassing process, whereas components at trace concentrations passively follow the evolution of the system. among these latter, poorly soluble species mark out the degassing process evolution very well, as degassing involves the exsolution of the vapor phase where they are preferentially partitioned. all the highlighted features can be noble gas solubility in silicate melts: a review of experimentation and theory, and implications regarding magma degassing processes antonio paonita istituto nazionale di geofisica e vulcanologia, sezione di palermo, italy abstract noble gas solubility in silicate melts and glasses has gained a crucial role in earth sciences investigations and in the studies of non-crystalline materials on a micro to a macro-scale. due to their special geochemical features, noble gases are in fact ideal tracers of magma degassing. their inert nature also allows them to be used to probe the structure of silicate melts. owing to the development of modern high pressure and temperature technologies, a large number of experimental investigations have been performed on this subject in recent times. this paper reviews the related literature, and tries to define our present state of knowledge, the problems encountered in the experimental procedures and the theoretical questions which remain unresolved. throughout the manuscript i will also try to show how the thermodynamic and structural interpretations of the growing experimental dataset are greatly improving our understanding of the dissolution mechanisms, although there are still several points under discussion. our improved capability of predicting noble gas solubilities in conditions closer to those found in magma has allowed scientists to develop quantitative models of magma degassing, which provide constraints on a number of questions of geological impact. despite these recent improvements, noble gas solubility in more complex systems involving the main volatiles in magmas, is poorly known and a lot of work must be done. expertise from other fields would be extremely valuable to upcoming research, thus focus should be placed on the structural aspects and the practical and commercial interests of the study of noble gas solubility. mailing address: dr. antonio paonita, istituto nazionale di geofisica e vulcanologia, sezione di palermo, via ugo la malfa 153, 90146 palermo (italy); e-mail: paonita@pa.ingv.it 648 antonio paonita recognized in the geochemical group of the noble gases: they are all chemically inert and volatile, and are present in trace concentrations in magmatic systems. moreover, their physicochemical properties gradually vary within the group with the increase in their atomic mass or size. thus, although noble gases do not thermodynamically define the magmatic system, their abundance ratios are a very powerful tool in studying degassing processes, on a micro to a macro-scale, from magmatic intrusions to mantle outgassing and the development of atmosphere. for these studies, noble gas solubilities are a crucial parameter and a number of investigations show a growing interest in the subject (carroll and draper, 1994; carroll and webster, 1994 and reference therein; draper and carroll, 1995; chamorro-perez et al., 1996, 1998; shibata et al., 1996, 1998; shackelford, 1999; nuccio and paonita, 2000; paonita et al., 2000; walter et al., 2000; schmidt and keppler, 2002), also thanks to the development of modern experimental and analytical technologies. experimental devices make it possible to reproduce magmatic temperatures from atmospheric to mantle pressures, and to measure the noble gas concentrations down to parts per million (ppm). nevertheless, technical questions that mainly concern the distinction between the dissolved and exsolved fractions of gas must be solved to obtain accurate vapor-melt partition coefficients. moreover, available experimental data on noble gas solubility are far from exhaustive, particularly as regards helium. the existing studies only partially cover the compositional ranges of magmatic systems and possible thermo-baric conditions, thus their use in modeling natural processes has still to be optimized. due to their inert nature, noble gases develop van der waals-type interactions with silicate melts and seem to display a preferentially «physical solubility» (i.e. the noble gas atom is similar to a fixed-volume sphere entering a rigid network). nonetheless, their dissolution mechanism is not fully defined. empirical or semi-empirical (lux, 1987; jambon, 1987; white et al., 1989; chennaoui-aoudjehane and jambon, 1990; carroll and stolper, 1993; shibata et al., 1998; nuccio and paonita, 2000) approaches are still the most frequently used tools to describe the noble gas solubility and to extrapolate the experimental data towards complex magmatic systems, whereas statistical-mechanics models (doremus, 1966; studt et al., 1970; shacklford et al., 1972) work within a very narrow range of temperature, pressure (t, p) and composition. current efforts also aim at investigating the structural environment of noble gas atoms inside the silicate network, promising results coming from the use of x-ray techniques (wulf et al., 1999). in this framework of growing high-quality scientific literature, this paper describes the solubility of noble gas in silicate melts, which is mainly applied to geological problems involving magma degassing in its wide range of possible conditions. throughout the text, this paper will try to draw attention to existing literature and the main problems encountered in experiments and modeling. while stressing the importance of noble gases as tools in studying magma degassing, the paper shows two things: a) what type of data is needed for geological applications; b) which are the most promising guidelines of research for achieving these results. 2. experimental techniques the investigation of noble gas solubility in silicate materials at magmatic conditions needs to experimentally reproduce high temperature and atmospheric up to mantle pressures. modern technology has designed and optimized resistance furnaces and externally heated and internally heated pressure vessels (ehpv and ihpv) to perform experiments at atmospheric pressure up to 500-600 mpa at magmatic temperatures. in this equipment, the noble gas under investigation is normally used as the pressure medium in the high pressure line and enters the unsealed noble-metal capsules where the sample is contained. in ehpv and ihpv experiments, noble gas can also be contained in a sealed capsule, that is loaded prior to the run by using a custom-designed loading device (boettcher et al., 1989) or a gas-bearing material (paonita et al., 2000). experiments at pres649 a review on noble gas solubility and degassing in magmas sure up to mantle values (several gpa) have been performed using piston cylinder apparatus or multi-anvil cells. in these instruments, a sealed capsule (single or double) containing the sample and the noble gas is inserted, and high pressures are reached using a solid pressure medium. run duration has to be long enough to attain equilibrium. based on the diffusion rates of noble gases, the appropriate duration ranges from hours to months, depending on the melt or glass composition and temperature (longer periods are necessary for heavy gases in silica glasses). the use of small grain sized sample powders yields a shorter diffusive walk of gas atoms in melt, because the gas penetrates through the grain interstices before melting, resulting in faster equilibration (jambon et al., 1986; lux, 1989; roselieb et al., 1992). the noble gas content in quenched samples is analyzed using different methods. the electron microprobe has been frequently used for in situ analyses of ar or heavier noble gases (white et al., 1989; carroll and stolper, 1991, 1993; montana et al., 1993; draper and carroll, 1995; chamorro-perez et al., 1996; schmidt and keppler, 2002), and has obtained concentration profiles in quenched glasses with a spatial resolution of around 10 µm. x-ray induced emission by synchrotron radiation and proton beams have been applied to the analysis of kr in albite (carroll et al., 1993b). light gases (he, ne) cannot be detected by these methods, and bulk extraction techniques have been applied to extract the dissolved gases (hayatsu and waboso, 1985; jambon et al., 1986; lux, 1987; broadhurst et al., 1992; roselieb et al., 1992; shibata et al., 1996, 1998; paonita et al., 2000). solid samples are melted in a vacuum line, in order to extract the dissolved volatiles. various analytical methods are capable of measuring the amount of extracted gas, namely gas-chromatography (gc), mass spectrometry (ms) and knudsen cell mass spectrometry (kms). the main problems consist in distinguishing between gas fractions from surface adsorption or trapping in fractures, secondary fluid inclusions and truly dissolved volatiles in the silicate melt (see roselieb et al., 1992). in order to avoid the gas fraction released from incorporated bubbles, powdering of quenched glasses has been used and grain sizes below 100 µm were found to be necessary to remove most of the physically trapped gas. however, an excessively small grain size could cause significant gas loss by diffusion before analysis, at least as regards helium and, in part, neon, if the samples are not analyzed within a matter of hours (paonita et al., 2000). stepwise degassing methods, coupled to kms or quadrupole ms measurements, have been developed to overcome these difficulties, based on the fact that different gas contributions are extracted in different ranges of temperature (fig. 1) (roselieb et al., 1992, 1995; paonita et al., 2000). in situ extraction techniques (ultra-violet laser beam) have been combined with mass spectrometry (kelly et al., 1994; brooker et al., 1998; chamorro et al., 2002), and the concentration of dissolved argon in glass volumes having around a 10 µm diameter size have been obtained. these methods are very promising as they are able to remove the contribution from physically trapped gas and they can be used for lighter noble gases. fig. 1. release spectrum of helium from basaltic glasses heated at a rate of 10°c/min. data from paonita et al. (2000). peak 2 provides the amount of dissolved gas, whereas peak 1 has been attributed to the decrepitation of fluid inclusions. 650 antonio paonita 3. noble gas solubility: experimental results and modeling the main parameters affecting the solubility of noble gases in silicate melts are pressure, temperature and composition of the coexisting liquid and vapor phases. in general (see table i), the concentration of any dissolved noble gas displays a linear relation with the gas pressure up to several hundred mpa (figs. 2a,b and 3). at table i. noble gas solubility in melt and glasses, and parameters for pressure and temperature dependence. argon (a) p° t° v° (b) ∆h° (b) ref. cc(stp)g−1bar−1 (mpa) (°c) (cc/mol) (cal/mol) mg2si2o6 2.0e-05 0.1 1500 1 basalt 2.5e-05 0.1 1200 2 bas.-andes. 9.0e-05 0.1 1200 16700 3 tholeiite 2.3e-05 0.1 1200 3 alk.ol-basalt 4.6e-05 0.1 1300 3 tholeiite 5.9e-05 0.1 1250 5700 4 basanite 1.0e-04 0.1 1200 4500 5 tholeiite 6.7e-05 0.1 1200 6800 5 alk.ol-basalt 3.6e-05 0.1 1200 14200 5 andesite 1.5e-04 0.1 1350 5 ugandite 4.5e-05 0.1 1350 5 naalsi3o8 2.9e-04 1500 1600 22.8 1962 6 kalsi3o8 1.9e-04 1500 1600 20.6 1722 6 caal2si2o6 2.6e-05 1500 1600 22.5 3373 6 camg2si2o6 6.0e-06 1500 1600 21.5 3421 6 k2si4o9 3.2e-04 1500 1400 22.6 3062 6 basalt 4.7e-05 1500 1600 23.1 2225 6 granite 3.9e-04 1500 1600 23.7 3086 6 sio2 1.2e-03 0.1 700 16.44 −4761 7,13 (d) naalsi3o8 3.0e-04 10 1000 20 (c) 8 basalt (synt.) 4.6e-05 0.1 1300 9 rhyolite 4.6e-04 0.1 700 15.6 −1457 10,13 (d) naalsi3o8 3.0e-04 0.1 700 22 −1807 10,13 (d) kalsi3o8 3.0e-04 240 700 22 −3447 10 ol-tholeiite 4.0e-05 0.1 1275 23 10 basalt 9.9e-05 0.1 1200 23 10 caal2si2o6 5.6e-04 5000-12000 (e) 14 sio2 5.0e-04 5000-12000 (e) 14 ns1 (f) 1.4e-04 197 1300 5410 15 ns2 2.6e-04 197 1300 9690 15 ns3 3.4e-04 197 1300 10840 15 ncs1 1.4e-04 197 1300 11390 15 ncs2 1.1e-04 197 1300 11320 15 ncs3 6.0e-05 188 1400 15 cms1 2.6e-05 199 1500 15 cms2 2.7e-05 199 1500 15 cms3 2.5e-05 199 1500 15 haplogranite 4.8e-04 3800 1800 24.3 17 tholeiite 1.0e-04 3800 1800 24.7 17 651 a review on noble gas solubility and degassing in magmas table i (continued). helium (a) p° t° v° (b) ∆h° (b) ref. cc(stp)g−1bar−1 (mpa) (°c) (cc/mol) (cal/mol) mg2si2o6 1.2e-04 0.1 1500 1 tholeiite 5.6e-04 0.1 1250 7100 4 basanite 6.8e-04 0.1 1200 1500 5 tholeiite 5.5e-04 0.1 1200 4900 5 alk.ol-basalt 5.7e-04 0.1 1200 540 5 ugandite 4.8e-04 0.1 1350 5 sio2 7.9e-03 0.1 25 5.5-10.8 −641 7 (g) basalt 3.8e-03 (5.1) 215 1160 16 (h) 2.0e-03 (3.3) 125 1160 16 2.6e-03 (3.3) 112 1160 16 rhyolite 8.6e-03 (6.0) 215 1140 16 5.4e-03 (5.0) 165 1140 16 4.4e-03 (3.9) 112 1140 16 neon (a) p° t° v° (b) ∆h° (b) ref. cc(stp)g−1bar−1 (mpa) (°c) (cc/mol) (cal/mol) mg2si2o6 7.0e-05 0.1 1500 1 bas.-andes. 2.6e-04 0.1 1200 4200 3 tholeiite 1.9e-04 0.1 1200 3 alk.ol-basalt 2.2e-04 0.1 1300 3 tholeiite 2.5e-04 0.1 1250 5000 4 basanite 3.6e-04 0.1 1200 5100 5 tholeiite 2.9e-04 0.1 1200 1300 5 alk.ol-basalt 1.7e-04 0.1 1200 12300 5 ugandite 2.1e-05 0.1 1350 5 sio2 4.9e-03 0.1 150 10.3 −1833 7 (g) naalsi3o8 1.5e-03 100 1000 16.8 (d) 8 basalt (synt.) 2.8e-05 0.1 1300 9 ns1 (f) 8.4e-05 197 1300 8230 15 ns2 1.0e-04 197 1300 9590 15 ns3 1.2e-04 197 1300 7250 15 ncs1 7.0e-04 197 1300 13160 15 ncs2 5.0e-04 197 1300 10410 15 ncs3 2.6e-04 188 1400 15 cms1 1.4e-04 199 1500 15 cms2 1.4e-04 199 1500 15 cms3 1.9e-04 199 1500 15 kripton (a) p° t° v° (b) ∆h° (b) ref. cc(stp)g−1bar−1 (mpa) (°c) (cc/mol) (cal/mol) basalt 1.3e-05 0.1 1200 2 bas.-andes. 2.1e-05 0.1 1200 19300 3 tholeiite 8.0e-06 0.1 1200 3 alk.ol-basalt 1.6e-05 0.1 1300 3 tholeiite 3.0e-5 0.1 1250 19500 4 andesite 1.1e-04 0.1 1350 5 basanite 8.2e-05 0.1 1200 3700 5 652 table i (continued). tholeiite 4.8e-05 0.1 1200 9400 5 alk.ol-basalt 2.1e-05 0.1 1200 19000 5 ugandite 3.0e-05 0.1 1350 5 naalsi3o8 1.8e-04 50 750 29 (d) 8 basalt (synt.) 1.3e-05 0.1 1300 9 naalsi3o8 1.4e-04 0.1 800 25 11 sio2 3.8e-4 0.1 800 11 ns1 (f) 5.7e-05 197 1300 2890 15 ns2 1.0e-04 197 1300 16200 15 ns3 1.5e-04 197 1300 19190 15 ncs1 6.3e-05 197 1300 11840 15 ncs2 4.5e-05 197 1300 15020 15 ncs3 2.4e-05 188 1400 15 cms1 1.1e-05 199 1500 15 cms2 1.1e-05 199 1500 15 cms3 1.3e-05 199 1500 15 xenon (a) p° t° v° (b) ∆h° (b) ref. cc(stp)g−1bar−1 (mpa) (°c) (cc/mol) (cal/mol) basalt 9.0e-06 0.1 1200 2 tholeiite 9.0e-6 0.1 1200 3 andesite 8.3e-05 0.1 1350 5 basanite 3.1e-05 0.1 1200 4400 5 tholeiite 2.5e-05 0.1 1200 2900 5 alk.ol-basalt 9.2e-06 0.1 1200 14400 5 ugandite 1.0e-05 0.1 1350 5 basalt (synt.) 1.1e-05 0.1 1300 9 naalsi3o8 9.7e-05 1500 1600 28.5 2488 12 kalsi3o8 1.0e-04 1500 1600 28 2560 12 k2si4o9 1.2e-04 1500 1400 29.4 3876 12 ns1 (f) 3.5e-05 197 1300 −6360 15 ns2 4.9e-05 197 1300 35650 15 ns3 7.5e-05 197 1300 30140 15 ncs1 2.6e-05 197 1300 11120 15 ncs2 2.2e-05 197 1300 23420 15 ncs3 1.2e-05 188 1400 15 cms1 9.3e-06 199 1500 15 cms2 6.2e-06 199 1500 15 cms3 7.2e-06 199 1500 15 tholeiite 2.8e-05 3500 1800 35.76 17 (a) noble gas solubility at p° and t°; the dissolved concentrations are normalized by noble gas pressure. uncertainties are normally within 10%. (b) v° and ∆h° are molar volume and enthalpy of solution of the reference state (see text). (c) calculated from the isothermal data of 8 by fitting eq. (3.6). (d) solubilities and related parameters are reliable for both pure ar and ar in he-ar mixtures. (e) pressure range of the experiments. (f) ns refers to the system na2o-sio2 (1, 2, 3 have about 35-65, 31-70, 24-77 wt% respectively). ncs is the system na2o-cao-sio2 (1, 2, 3 have about 25-9-64, 19-19-60, 11-31-56 wt% respectively). cms is the system cao-mgo-sio2 (1, 2, 3 have about 37-8-54, 26-16-56, 14-26-59 wt% respectively). (g) calculated from data from frank et al. (1961), shackelford et al. (1972), shelby (1972a,b, 1976). (h) values in hydrous melts at ptot ≈ ph2o and he concentration in vapour > 0.1 mol%. numbers in parenthesis are antonio paonita 653 a review on noble gas solubility and degassing in magmas wt% of dissolved water. solubilities at ptot ≈ ph2o + pco2 were obtained at ph2o very close to that given in table and do not display appreciable differences with respect to given values, therefore they were not reported. ref.: 1 – kirsten (1968); 2 – fisher (1970); 3 – hayatsu and waboso (1985); 4 – jambon et al. (1986); 5 – lux (1987); 6 – white et al. (1989); 7 – carroll and stolper (1991); 8 – roselieb et al. (1992); 9 – broadhurst et al. (1992); 10 – carroll and stolper (1993); 11 – carroll et al. (1993); 12 – montana et al. (1993); 13 – draper and carroll (1995); 14 – chamorro-perez et al. (1996); 15 – shibata et al. (1998); 16 – paonita et al. (2000); 17 – schmidt and keppler (2002). fig. 2a,b. dissolved ar as a function of ar pressure in a) rhyolite (800-900°c) and basalt (1200°c); b) haplogranite and basalt (1500°c). data are from carroll and stolper (1993) and white et al. (1989). curves were calculated by using eq. (3.6) with parameters given in table i. the linear relation between concentration and pressure, as well as the higher solubilities in the acidic compositions, should be noted. higher pressure, the solubility (hereafter defined as the ratio between gas concentration in melt and gas fugacity, unless differently specified) decreases, and recent experiments indicate fig. 3. neon and krypton solubilities in albite glass at 1000 and 750°c, respectively. data are taken from roselieb et al. (1992). fig. 4. effect of very high pressure on ar solubility in basaltic and haplogranitic melts between 1500 and 2000°c. data from schmidt and keppler (2002); curves calculated by using eq. (3.6) with the parameters given by the authors (see table i). at about 5 gpa, dissolved ar in the melt reaches a threshold concentration which does not increase even when ar pressure is increased by some gpa. a b 654 antonio paonita that a threshold concentration is reached, at least for ar and xe, at very high pressures (4-5 gpa and 5 gpa, respectively, fig. 4; schmidt and keppler, 2002). the dependence of solubility on temperature is very modest and is often lower than the experimental uncertainties. 3.1. dependence on the composition of melt and glass the parameters which most affect the solubility of noble gases in silicate melts and glasses are the composition of the solvent and the atomic radius of the noble gas (doremus, 1966; shackelford et al., 1972; shelby, 1976; hayatsu and waboso, 1985; jambon et al., 1986; lux, 1987; broadhurst et al., 1992; roselieb et al., 1992; carroll and stolper, 1993; shibata et al., 1996, 1998; shackelford, 1999; paonita et al., 2000; schmidt and keppler, 2002). in general, solubility increases with the increasing concentration of sio2 in the melt while smaller noble gases display higher solubilities (fig. 5). these variations suggest that noble gas atoms dissolve in holes (free spaces) of the silicate melt structure, following the so-called physical mechanism of dissolution (doremus, 1966; studt et al., 1970; shackelford et al., 1972; shelby, 1976). five basic structural units form the main network of silicate melts and glasses, namely sio2 (three-dimensional network), si2o22− (sheet), si2o64− (chain), si2o76− (dimer), sio44− (monomer) (virgo et al., 1980). silicon and other tetrahedrally coordinated cations (al, fe3+, ti4+) act as network-formers, whereas na+, k+, mg2+, ca2+, fe2+ break the silicate polymers and thus are called network-modifiers. according to shibata et al. (1998), the fully polymerized, threedimensional sio2 structure of silicate melts creates a number of holes where gas atoms can be accommodated, whereas the less polymerized structures of the other units have fewer free spaces, therefore less volume is available to store gas atoms (i.e. the holes are too small). this is why silica-rich compositions display the highest noble gas solubilities. a quantitative expression of the degree of polymerization is given by the ratio nbo/t, namely the number of non-bridging oxygens per atom of a tetrahedrally coordinated cation (brawer and white, 1975; mysen et al., 1985). the three-dimensional network has bridging oxygens only (nbo/t= 0, that is to say the highest polymerization), whereas the ratio grows when other units are present; if to44− monomers are the only units in the melt, then nbo/t= 4. shibata et al. (1998) found a direct relationship between noble gas solubility and degree of polymerization in the silicate melts, according to which gas atoms dissolve preferentially in more polymerized, low nbo/t melts (fig. 6). they proposed the following equation to calculate the vapor-melt equilibrium constant of noble gas i: (3.1) where abo and anbo are the fraction of the units with bridging oxygens and the units with bridging and non-bridging oxygens, while kibo and ki nbo are the equilibrium constants that refer to each group of units. the model was calibrated for noble gases from ne to xe, but unfortunately helium was not treated. from fig. 6 it can be seen that the relationship between solubility and the nbo/t ratio is also quite evident when plotting the available experimental data for helium, ln ln lnk a k a ki i i bo bo nbo nbo= + fig. 5. solubility (expressed under form of henry’s constant) of the noble gases as a function of their size at about 1200°c. solubility exponentially increases from light to heavy noble gases. 655 a review on noble gas solubility and degassing in magmas thereby suggesting that the solubility mechanism is analogous. a similar approach has already been proposed by jambon (1987) and chennaoui-aoudjehane and jambon (1990), although no clear interpretation in terms of melt structure was given by the authors. furthermore, even though the model by shibata et al. (1998) does work for a large range of natural melt compositions, significant deviations from the relationship solubility-degree of polymerization occur in melts containing large amounts of al (or network-formers other than si). however, the measured solubilities appear to be lower than those predicted from the nbo/t ratio (fig. 6). although further data on noble gas solubilities in aluminosilicate melts are necessary to confirm this hypothesis, it is possible that cations (na, k, ca, mg), added to balance the surplus of negative charges due to si versus al substitution, occupy structural vacancies otherwise available for noble gases. fig. 6. effect of melt polymerization (expressed by the nbo/t ratio, see text) on the solubility of some noble gases (modified from shibata et al., 1998). data for ar are a compilation selected by shibata et al. (1998), he and ne from jambon et al. (1986) and lux (1987). ar data are divided into two classes: t/(t+si) ratio higher (circles) than or lower (triangles) than 0.1, where t is the number of tetrahedrally coordinated cations other than silicon (i.e. al, ti). it is evident that the nbo/t ratio fails to predict the low ar solubilities in aluminosilicate melts. fig. 7a-c. relationships between noble gas solubility and a) ionic porosity of melt, b) melt density and c) melt molar volume (modified from carroll and stolper, 1993). in all the graphs, the squares are solubilities in cao-mgo-al2o3-sio2 (cams) melts from broadhurst et al. (1992). henry’s constant and molar volumes were computed on eight-oxygen basis melts. linear fits of data, with the related r 2 values for ar, are also displayed. solubilities are much better correlated to ionic porosity than other parameters. the anomalous behavior of cams melts is unclear (see text for discussion). a b c 656 antonio paonita based on the mechanism of interstitial dissolution in silicate networks, relationships have also been found between noble gas solubility and the physical properties of melt that give the gauge of the free space in the silicate network (fig. 7ac). in particular melt density (lux, 1987; white et al., 1989), melt molar volume (broadhurst et al., 1992) and melt ionic porosity (ip, carroll and stolper, 1993). carroll and stolper (1993) clearly show that ionic porosity works better than the other properties (density, molar volume) in predicting solubility, and is thus a better expression of the free space in the melt structure (see fig. 7a-c). in fact, ionic porosity is defined as (3.2) where vca is the total volume of cations plus anions in one gram of melt and vl is the specific volume of melt. the relation found by carroll and stolper (1993) between the solubility logarithm and ionic porosity is (3.3) where m and n are fitting parameters to be calibrated by linear regression of the experimentally measured solubility of the noble gas in several melt compositions, and henry’s constant kh is defined as (3.4) where fi is the noble gas fugacity in the gas phase. carroll and stolper (1993) estimated these parameters for all stable noble gases, by using experimental data acquired at 10001400°c and 0.1 mpa. in spite of its general accomplishment, ionic porosity fails to predict solubility in cao-mgo-al2o3-sio2 (cmas) melts. furthermore, although it estimates the overall free volume in melt, the same volume could be distributed either in a few large holes or in many small cavities. following shibata et al. (1998), the incorporation of networkmodifier cations probably generates a kind of free space that is unable to accommodate noble gas atoms (i.e. too small interstitial sites). nearly all of the available experimental data (carroll and webster, 1994 and reference there/k xfh i i= ln k m niph= + ( / )v vip 100 1 lca= in; draper and carroll, 1994, 1995; chamorroperez et al., 1996, 1998; shibata et al., 1996, 1998; shackelford, 1999; walter et al., 2000; schmidt and keppler, 2002) have been obtained by equilibrating a silicate liquid with a gas phase composed of solely noble gases, which introduces a large uncertainty in the determination of the real solubility of noble gases in magmas. in fact, the gas phases coexisting with magmas are normally h2o-co2 dominated vapors; more rarely, sulfur can reach several percents, whereas halogens and n2 remain minor components and the noble gases occur in trace concentrations (gerlach and nordlie, 1975; giggenbach, 1996). even at some gpa of total pressure, the partial pressures of noble gases are very low (a few bars; carroll and webster, 1994). in a recent article, paonita et al. (2000) experimentally investigated the effect of h2o and co2 on helium solubility, in order to simulate data at more geologically relevant conditions. they measured the fig. 8. effect of dissolved water concentration on the helium henry’s constant, the latter expressed in mpa and calculated on 8-oxygen basis melt (modified from nuccio and paonita, 2000). experimental data (symbols) from paonita et al. (2000) were obtained in the condition ptot ≈ ph2o in the range 100200 mpa, and about 0.1 mol% he in the vapor. curves were computed by using the model by nuccio and paonita (2000), which accounts for the effect of h2o and co2 on both noble gas solubility in melt and fugacity in vapor (see text). 657 a review on noble gas solubility and degassing in magmas isothermal he solubility in basaltic and rhyolitic melts at pressures of 100 up to 200 mpa, in the presence of mixed h2o-co2 vapors (0 to 50 mol% h2o) with ≈ 0.1 mol% he. the solubility increased by about a factor of three with 3 wt% h2o added to the melt, whereas a further addition of water, up to 6 wt%, seemed to have very little effect on the solubility (fig. 8). the effect of co2 addition (up to 0.05 wt%) was within the experimental uncertainties or slightly negative. based on these data, nuccio and paonita (2000) found that the helium solubility is in very good agreement with the model of carroll et al. (1993) (eq. (3.3) with the same parameters as those for dry melts, fig. 9), provided that the ionic porosity melt is computed by taking into account the presence of dissolved h2o and co2 (namely the h2o and co2 partial molar volumes). it is obvious that the h2o and co2 concentrations in the coexisting gas and liquid phase must be constrained by the solubility of these two volatiles. nuccio and paonita coupled their model to the thermodynamic model by papale (1999) for solubility of h2o-co2 mixed vapors in silicate melts. they obtained the following equation for ionic porosity: (3.5) where r and i are ionic radius and molar fraction of each of the i ions forming the melt; x and v° are the molar fraction and the reference molar volume of each of the n oxides in melt at p and t, and wi, co2 are binary interaction coefficients between dissolved co2 and other oxides (wi, co2 values are taken from papale, 1999; reference molar volumes of oxides are from lange and charmichael, 1987; and ionic radii from shannon and prewitt, 1969). by inserting the ionic porosity ++ j z x ( ) xx p x w r x x p w p x w i ip 100 1 1 3 4 1 1 , , ,p t i i i n i z z p t z z n j j z n co co co co ion ion ion co co co co 1 3 1 1 co 1 2 2 2 2 2 2 2 2 2 = + + + o r o ! ! = = + z 1= j 1= c c ^ h < > f h z [ \ ] ] ] ] ) _ ` a b b bb 4 / / / / value from eq. (3.5) into eq. (3.3), the helium henry’s constant can be considered valid for h2o-co2 bearing melts. the empirical parameters m and n were the same as those obtained for dry melts (fig. 9). in order to calculate the helium concentration in the melt by eq. (3.4), the authors computed the noble gas fugacity by using a modified redlich-kwong equation of state for the h2o-co2-noble gas (de santis et al., 1974). with the nuccio and paonita (2000) model, you can theoretically calculate the solubility of other noble gases by assuming that m and n parameters in eq. (3.3) are the same as those for dry melts. although the last assumption has only been experimentally confirmed for he, due to the lack of experimental data, the authors predicted that the solubilities would also increase with water addition, and become similar when 6-7 wt% h2o are dissolved in melt. the relation between he solubility and water content of the melt can be tentatively disfig. 9. relationship between helium solubility (kh in mpa and on 8-oxygen basis melt) versus ionic porosity (ip) in h2o and h2o-co2 bearing silicate melts (h2o and co2 contents up to 6 and 0.05 wt%, respectively). ip was calculated by taking into account the effect of the main volatiles (see text). line was calculated by using eq. (3.3) with m and n parameters in carroll and stolper (1993) for dry melts, and it displays a very good agreement with the experimental data (data from paonita et al., 2000). 658 antonio paonita cussed from a structural point of view. water dissolution mainly occurs as oh− groups at low concentrations of dissolved h2o, whereas molecular h2o becomes more and more important with increasing water content in basaltic to rhyolitic melts (mcmillan, 1994; dixon et al., 1995; nowak and behrens, 1995, 2001; ihinger et al., 1999; withers et al., 1999; king and holloway, 2002). although it is generally accepted that water depolymerizes silica and alkali-silicate compositions, a similar effect is not clear in aluminosilicate melts (such as magmas). as first recognized by sykes and kubicki (1993), water could break si-o-al bonds and have a depolymerizing effect on the network. on the contrary, the si-o-al bond could be maintained, but weakened by the formation of a hydrogen bond, following which the silicate network would undergo merely modest deformations (kohn et al., 1989, 1998; oglesby et al., 2001). based on recent studies (zeng et al., 1999, 2000; schmidt et al., 2000, 2001), both mechanisms could occur in the melt. in fact, the significant effect of oh− formation on he solubility matches better the hypothesis of significant structural changes rather than small deformations, suggesting that noble gases can provide valuable insights into the structure of aluminosilicate melts. we are unable to evaluate whether or not shibata et al.’s approach (1998) predicts the observed increase in the solubility of helium, because the uncertainties regarding the structural effect of water and its depolymerizing role prevent us from obtaining a reliable estimation of the nbo/t ratio in water-bearing melts. on the other hand, the higher he solubility is consistent with the calculated increase in the ionic porosity due to the addition of water. at higher concentrations of dissolved water, helium solubility seems to decrease with the increase of the molecular h2o/oh ratio, suggesting that the h2o molecules probably occupy free spaces in the silicate structure. 3.2. pressure and temperature effects equilibrium partitioning of noble gases between vapor and silicate melt has been described using the following thermodynamic relation, which is based on the equality condition of the chemical potential of the gas species in vapor and liquid phases (3.6) where a and f refer to the activity of the noble gas in melts and its fugacity in vapor; p° and t° are the standard state conditions; ∆h 0p°, t is the enthalpy of dissolution of the noble gas at p° and t; v 0p, t is the molar volume of noble gas in its reference state at p and t, and r is the gas constant. noble gas fugacity is normally computed, as a function of p and t, by using a modified redlich-kwong equation of state (holloway, 1977). the parameters a°/f °, v° and ∆h° must be calibrated using experimental data for each noble gas as a function of pressure, temperature and silicate melt composition. use of eq. (3.6) and the consequent parameter estimation depends on the theoretical model adopted for the dissolution process. based on the previous considerations concerning the physical dissolution of the noble gases, one can hypothesize that noble gas atoms dissolve into a fixed population of holes in the silicate melt network. one can also assume that the available sites for a given noble gas are identical, that their number is independent of t and p, and that gas atoms do not influence each other reciprocally. this allows us to define an activity-composition relation, namely a=n/(n−n), where n is the number of dissolved atoms and n is the number of available sites in the melt, referenced to a hypothetical reference state of pure noble gas. by fixing the temperature and assuming that the volume of any solubility site is unchanged after the dissolution of a noble gas atom (namely, v°=0), eq. (3.6) and the previous definition of activity can be combined to obtain the well-known formalism of the langmuir adsorption isotherm (3.7) where kl(t) indicates that the equilibrium constant depends on temperature. the above for( ) / ( )k t n n n fl = ( ) ln ln f a f a r h t t rt v p p 1 1 , , , , , , p t p t p t p t t p t = + ∆ p 0 0 c c c c c c c c c b l 659 a review on noble gas solubility and degassing in magmas malism has been widely applied to describe noble gas solubility in melts and glasses by considering the number of sites as a parameter to be evaluated (shelby, 1976; roselieb et al., 1992; carroll and stolper, 1991; walter et al., 2000). the very high-pressure experiments recently performed by schmidt and keppler (2002) reached a threshold ar concentration at which the dissolved gas content did not increase with increasing ar pressure in either silicic or tholeiitic melts (fig. 4). the authors suggested that the sites available to store the ar atoms were fully occupied, therefore the n value was accordingly constrained to 13.8×1020 sites/cm3 and the obtained langmuir constant was able to fit the data very well. approximating the structure of the granitic liquid as a tridymite-like network, the number of interstitial holes can also be computed. when comparing such a number with the n value obtained by schmidt and keppler, the latter represents only 60% of the interstitial holes. this underlines the fact that only a fraction of the holes is available to accommodate ar atoms. in accordance with its much more compact structure, the estimated n value in the tholeiitic melt was almost one order of magnitude lower (3.1×1020 sites/cm3). the number of holes available for the physical dissolution of xe was also significantly lower than that for ar, which is consistent with the larger size of the former. although the langmuir isotherm was successfully applied, noble gas dissolution could violate one of its main assumptions, namely that gas atoms do not affect the structure of the solvent (v° ≠ 0). in fact, a recent x-ray absorption spectroscopy study investigated the structural environment of dissolved krypton in vitreous silica, revealing that kr atoms are coordinated by oxygen atoms having a characteristic geometry (wulf et al., 1999): thus, kr atoms create or adjust to their own solubility sites. however, whether or not smaller gas atoms have a similar network-modifier role must be verified. models involving additional parameters which also contain information on the structure of the silicate network have been developed following statistical-mechanical approaches (doremus, 1966; studt et al., 1970; shackelford et al., 1972). in addition to the number of sites, these models include the vibrational frequency of the dissolved atom, which is treated as a harmonic oscillator in the structural cavity, and the binding energy to the interstitial hole. in silica glass, these two parameters depend upon the atomic size of the noble gas (lower frequency and more negative binding energy for larger noble gases; shelby, 1976; shackelford and brown, 1980; nakayama and shackelford, 1990). a sharp increase in the binding energy and decrease in the vibrational frequency were obtained along the join na2osio2 when moving from the silica-rich to the sodium-rich edge of the miscibility gap (shelby, 1973). this is in agreement with both the stronger structural constraints and the necessity to spend energy in creating adequate holes in a less polymerized solvent (see also below). a different theoretical approach in applying eq. (3.6) considers that the units of the silicate melt and noble gas atoms mix ideally. in this case, the activity of the dissolved gas is equal to its molar fraction when a henrian reference state of pure noble gas is assumed (a hypothetical material composed by noble gas arranged like a silicate melt). the two parameters v° and ∆h° define the dependence of the solubility on p and t, respectively (see table i). considering them as constants in the fitting procedure, the model is generally able to successfully reproduce the experimental data in a large range of p and t conditions (lux, 1987; white et al., 1989; carroll and stolper, 1991, 1993; draper and carroll, 1995; shibata et al., 1998; schmidt and keppler, 2002). the calculated molar volumes for heavy noble gases (ar, xe) in melts are comparable to their respective co-volumes in mrk equations (the parameter named b), which reflects the atomic volume (white et al., 1989; schmidt and keppler, 2002). slightly lower values than the co-volumes were determined in glasses (carroll and stolper, 1991, 1993; draper and carroll, 1995), suggesting that the noble gases fit into pre-existing holes and only a small amount of new space must be created in the melt. similar results were found for helium in glasses, which displays v° values lower than its co-volumes (carroll and stolper, 1991). the small v° values of the noble gases account for their henrian behavior at low pressure (at least up to some hundreds of mpa), due 660 antonio paonita to the negligible weight of the v°(p−p°)/rt term in eq. (3.6). the ar molar volumes computed for different melts (tholeiite to granitic melts; white et al. 1989, schmidt and keppler 2002) display a negligible dependence on the liquid composition, suggesting that a similar environment surrounds the dissolved atom in spite of the notable structural differences of the silicate structure. the heat of the solution (∆h°), in both glasses and melts, is generally small (see table i), and experimental data show that temperature dependence is close to that of the experimental uncertainties. negative ∆h° values, namely decreasing solubility at increasing temperature, have been estimated for silicate glasses (shackelford et al., 1972; shelby, 1972a,b; carroll and stolper, 1991; draper and carroll, 1995) with compositions from albitic and rhyolitic up to fused silica (fig. 10). increasing na2o and k2o concentrations to above 20 mol% seem to invert this solubility-temperature relation (shelby, 1973, 1974). on the other hand, a positive enthalpy of solution and a positive direct temperature dependence of solubility have normally been measured in basaltic melts (lux, 1987; jambon et al., 1986; hayatsu and waboso, 1985; see fig. 11), as well in na2o-sio2, na2o-caosio2, na2o-mgo-sio2 melts (white et al., 1989; shibata et al., 1988). all the described effects are normally more pronounced for heavier noble gases. the dissolution of a noble gas atom in the silicate melt can be usefully divided into two steps, each having its own enthalpy variation: i) the creation of a hole to accommodate a gas atom in the melt structure, and ii) the transfer of the atom into the hole. the former step needs energy, namely its enthalpy variation is positive; instead, moving an atom from a gas into a more condensed phase involves the evolution of heat and a negative ∆h° sign. the whole enthalpy of solution is the sum of the two terms and its sign will depend on which contribution is dominant. glasses and silica-rich melts have open structures, so we can expect that only a small contribution of energy is needed to create cavities inside. moving atoms into the melt will be the energetically dominant process and, in view of the negative ∆h° sign of such a process, we can explain the negative dependence of solubility on temperature. opposite considerations can be made to explain the direct solubility-temperature relationship of less polymerized, alkali-rich melts, where a few pre-existing large holes are available and energy has to be spent to create or enlarge cavities. fig. 10. temperature effect on noble gas solubility in a basanite melt between 1200 and 1500°c at 0.1 mpa. data are from lux (1987) and uncertainties are in the order of the symbol size. curves were computed by using eq. (3.6) with parameters in table i. solubility slightly increases with the increase of temperature. fig. 11. dissolved ar in a rhyolite glass at 120-130 mpa as a function of temperature (data from carroll and stolper, 1993). curves were computed by using eq. (3.6) with parameters in table i. solubility displays a slightly negative relation with the increase of temperature. 661 a review on noble gas solubility and degassing in magmas 3.3. size distribution of solubility sites several researchers (shackelford and masaryk, 1978; shackelford and brown, 1980; shackelford, 1982; nakayama and shackelford, 1990; carroll and stolper, 1993) used the different solubilities among the noble gases as a gas-probe to investigate the distribution of hole size in vitreous silica and natural silicate melts. the logarithm of noble gas solubility displays a linear relation with the atomic radius (fig. 5), meaning that hole sizes result as having a log-normal distribution. the mode shifts towards higher hole size with increasing ionic porosity, thus large holes become more common than small ones and the solubility of heavier noble gases increases more quickly with respect to that of light gases. in accordance with the above conclusions, a numerical simulation of the interstitial structure of silica glass confirmed the log-normal distribution (chan and elliot, 1991). nuccio and paonita (2000) assumed that the m and n parameters in eq. (3.3) are composition-independent for all noble gases, in accordance with what was experimentally verified for helium. in these conditions, their extended ionic porosity model for helium solubility in h2o-co2 bearing melts is valid for other noble gases. they calculated that a low percent of water in a melt causes an exponential increase of the number of large holes, as a consequence of potential structural modifications of the melt. these conclusions need to be proved using solubility data for noble gases having a large atomic radius. 4. implications in modeling magma degassing several elements in magmas are able to form chemical species which may be exsolved in vapor phases (h, c, o, s mainly, halogens, nitrogen, noble gases etc.; gerlach and nordilie, 1975; giggenbach, 1996). these elements can react with the liquid forming chemical bonds (i.e. oh groups), or they exist under the form of volatile atomic or molecular species dissolved in the silicate melt (i.e. molecular h2o, noble gases). any dissolved volatile species (true or potential) is characterized by its own vapor pressure, which depends directly upon its concentration in the melt. magma reaches supersaturation when the sum of the vapor pressures of the individual volatiles becomes higher than the total pressure of the system, as a consequence of ascent and depressurization of the melt (carroll and webster, 1994). in such conditions, the magma is potentially able to exsolve a vapor phase. as h2o and co2 are the most abundant volatiles in natural magmatic systems, the assessment of magma saturation has been modeled as a gas mixture of these two compounds (papale, 1999), although sulfur could have significant effects when its concentrations reach several weight percent in the system (moretti et al., 2003). when the vapor phase exsolves, minor volatiles are partitioned between the silicate liquid and the coexisting vapor, depending upon their solubilities. in fact, early vapor is strongly enriched in highly volatile species, which are consequently depleted in the melt (giggenbach, 1996). the more soluble species are only able to exsolve into the gas phase during the late stages of degassing. as a consequence, less solfig. 12. fraction of noble gases remaining in a tholeiitic melt with respect to the amount of exsolved vapor during a closed system degassing. vapor/melt ratio is the volume of gas phase exsolved per volume unit of melt. adopted equations for calculations were those by giggenbach (1996), as well as the h2o and co2 solubilities. co2 solubility causes co2 to degas before light noble gases, whereas h2o is much more soluble and is exsolved during late degassing. 662 antonio paonita uble, heavy noble gases (ar, kr, xe) are degassed earlier, whereas he and ne degassing occurs later (fig. 12). this solubility-controlled fractionation of the noble gases is much more significant if infinitesimal aliquots of volatiles are exsolved in equilibrium from the liquid phase and immediately removed, thereby creating open system degassing conditions. however, this behavior is also evident during closed system degassing processes, during which the exsolved vapor stays in contact with the melt throughout the degassing process (carroll and webster, 1994). noble gas fractionation in open system degassing can be modeled by applying the typical rayleigh distillation process. by considering two noble gases i and j, the extent of fractionation between two gases is related to the residual fraction of one of the two volatiles (4.1) where c is the volatile concentration in melt (m); subscript «0» refers to initial conditions (undegassed parental melt) and k is the henry’s constant. calculations by eq. (4.1) have been applied to ratios between noble gases as a function of the total content of one noble gas in basalts (taylor, 1986; sarda and graham, 1990; carroll and webster, 1994; marty, 1995; giggenbach, 1996; burnard, 1999; moreira and sarda, 2000). by writing two rayleigh equations for a ternary system, such as he-ar-co2, and coupling them, it can be shown that a plot of ln(ar/co2) versus ln(ar/he) yields a straight line with a slope dependent upon the solubility ratios among the gases and an intercept dependent upon the ar/co2 and ar/he ratios in the pristine undegassed melt (burnard, 1999, 2001). in a closed system, vesicles of an exsolved vapor remain in the melt, so the total amount of gas does not vary. models have been developed that take into account the mass balance among the total amount of noble gas, its dissolved concentration and the volume of exsolved vapor per mass unit of melt (jambon et al., 1986; sarda and moriera, 2002). by coupling henry’s law to the mass balance, the fractionation bec c c c c c ,j i j i i i k k 00 1 j i = c c d m m n tween noble gases can be related to the vesicularity of the melt (the volume of vapor phase exsolved per volume unit of melt) (4.2) where t0 is equal to 273 k; te is the temperature of the gas-melt equilibrium; ρ is the density of the silicate melt (assumed constant), and v* is the melt vesicularity. the above models have been frequently applied to study degassing on spatial scales from local to planetary. for example, investigations were performed on the gas phase trapped in vesicles and dissolved in glasses recovered from middle-oceanic ridge basalts (morb) and oceanic island basalts (oib) (jambon et al., 1986; taylor, 1986; sarda and graham, 1990; carroll and webster, 1994; giggenbach, 1996; burnard, 1999, 2001; honda and patterson, 1999; moreira and sarda, 2000; sarda and moriera, 2002). furthermore, constraints were obtained on the outgassing history of the mantle and its primordial composition. the higher he/ar ratios in morb glasses with respect to their production/accumulation rate in the mantle (due to k and u decay), have normally been explained by degassing processes (jambon et al., 1985), because ar is preferentially degassed in respect of he. on this basis, moriera and sarda (2000) focused on the different degassing modalities that occur from morb and from mantle plumes, the former degassing as a closed system, and the latter as an open-system rayleigh-type process. however, some questions have been raised about this «degassing» hypothesis, namely that higher he/ar ratios occur at higher he contents (honda and patterson, 1999) and in basalts erupted at greater depths (fisher, 1997). solubility controlled degassing does not explain these constraints, whereas mechanisms of helium diffusion from mantle minerals or crystals in basalt melts have been hypothesized (matsuda and marty, 1995; fisher, 1997; honda and patterson, 1999). in fact, moreira and sarda (2000) and sarda and moreira (2002) have suggested that the geochemical pattern of noble gas abundances in morb is consistent with equilibrium vesiculation followed by / ( ) / ( ) c c c c k k v t t k v t t k , , * * j i j i i j e e i j 0 0 0 0 # #= + + t t 663 a review on noble gas solubility and degassing in magmas various episodes of bubble loss, in accordance with the early model by sarda and graham (1990). however, sarda and moreira (2002) have also suggested that helium solubility should be two times higher than that experimentally measured at 0.1 mpa, and that ar and ne solubilities should be lower by factors of 3.5 and 7, respectively. this opposite variation in solubilities makes their hypothesis about a pressure effect on solubility rather unlikely. burnard (1999) moves criticisms on the experimental values of solubilities too, providing evidence that the noble gas/co2 solubility ratio has to be higher than experimentally measured. nevertheless, a new view on noble gas data from morb takes into account the size of the vesicles, as small bubbles should represent the previous extent of the degassing with respect to large vesicles (burnard, 2001). sequential crushing of basalts allows for the analysis of gas from progressively smaller bubbles, which can be used to determine the trajectory of the degassing. preliminary data seem to reconcile with predicted solubility-controlled fractionation studies (burnard, 2001). the foregoing discussion stresses how noble gas abundances from natural samples are used as a reference point and as a check of experimental results on noble gas solubilities. it also highlights the wide use of the noble gas solubility data in earth sciences problems and the strong inferences they provide. considering the fact that the experimental conditions do not match the geological ones, degassing models that use such data must be applied very carefully. equations (4.1) and (4.2) describe how noble gases are partitioned in a vapor while they are progressively exsolved, regardless of: i) what type of volatiles dominate the vapor and if they behave non-ideally in such a gas mixture, ii) changes in concentration and, above all, in solubility due to the exsolution of the main volatiles. application to morb degassing could disregard the second conditions, at least for a first approximation, because the amounts of h2o and co2 in these melts are lower than 1 wt% at least for p < 1 gpa. however, morb degassing should take into account the noble gas fugacity in co2-dominated vapor. bottinga and javoy (1990) discussed the exsolution of a non-ideal h2o-co2-he-ar mixture from bas-altic melts, although the h2o and co2 effects on noble gas solubility were not considered. nuccio and paonita (2001) started from the already discussed model for noble gas solubility in h2oco2 bearing melts (nuccio and paonita, 2000; see section 3.1). the authors obtained an equation that allowed them to calculate the concentration of noble gas in an exsolved h2o-co2 mixed vapor as a function of the dissolved concentration of h2o, co2 and noble gas in melt and their total amounts in the vapor+melt system. thus, the authors numerically simulated the degassing of multicomponent h2o-co2-nofig. 13. he/ar fractionation during closed system degassing of basaltic melt at 1200°c. initial he/ar ratio was 3. thick grey lines were computed by using the model of jambon et al. (1986) (eq. (4.2) in the text), assuming noble gas solubilities were not affected by the presence of the main volatiles (k = constant) and ideal gas behavior. thin black lines were computed on the basis of the model by nuccio and paonita (2001), which accounts for these effects. in this model, the initial conditions were 85 mol% co2 in the h2o-co2 mixed vapor (noble gases are computed as minor species) at 700 mpa pressure. black thin lines for melt and vesicles approach each other at low pressure, due to the effect of water that makes the solubilities of noble gases more similar. dashed curves were computed for an open system (grey line by jambon et al., 1986 model black line by nuccio and paonita, 2001). 664 antonio paonita ble gas mixtures from depressurizing magmas. as a result, remarkable differences were highlighted between the comprehensive approach and simpler models (figs. 13-15). in spite of the constant ratio between he/ar in melt and vesicles computed by the simpler model (namely, (he/ar)gas / (he/ar)melt = khe / kar), curves from the more complex model come closer at low pressure. in fact, taking into account the fact that high pressure causes very different solubility among noble gases and that dissolved water has the opposite effect, a few wt% of dissolved water at low pressure are able to significantly reduce the solubility difference between he and ar (fig. 13 and see section 3.1). as a consequence, noble gases may fractionate less than predicted by simpler calculations, depending on the composition of the magma and on the main volatiles it contains. the application of the model to very different contexts, from silicic to basaltic volcanoes (nuccio and paonita 2001; caracausi et al., 2003), shows how the inert gas content of outgassed magmatic volatiles may produce information and constraints on deep magmatic conditions: magma pressure and depth over time, degassing and volatile content, melt composition and temperature. the current research aims at developing approaches to describe saturation surfaces in more complex systems, including sulfur and halogens. on this basis, the solubility of noble gases in such systems progressively more similar to true geological conditions, will have to be theoretically and experimentally investigated, allowing for the development of more refined degassing models. finally, vapor-melt equilibrium for noble gases has been assumed in all the discussed models. experimental observations and theoretical modeling have been performed to investigate the growth of steam bubbles in silicate liquids (bottinga and javoy, 1990; proussevich and sahagian, 1996, 1998; gardner et al., 1999, 2000). bubbles seem to maintain chemical equilibrium with the surrounding melt even in those silicic magmas whose ascent rates are fig. 14. he/ar fractionation during closed system degassing of rhyolitic melt at 1000°c. curves as in fig. 13. in the calculation using the model by nuccio and paonita (2001), initial conditions were 40 mol% co2 in h2o-co2 mixed vapor (noble gases are computed as minor species) at 500 mpa pressure. the higher amount of dissolved water (6 wt%) in respect of the basaltic melt amplifies the discussed water effect, causing very low he/ar fractionation. fig. 15. he/ne fractionation during closed system degassing of basaltic melt at 1200°c. curves and conditions as in fig. 13. the behavior is qualitatively similar to that discussed in the case of argon, although the fractionation is more modest. 665 a review on noble gas solubility and degassing in magmas three orders of magnitude higher than their common pre-eruptive values (∼ 0.01 m/s), whereas non-equilibrium exsolution could occur during explosive rising. nevertheless, there is a total lack both of experimentation and models for the investigation of possible disequilibrium fractionation among noble gases. in addition, noble gas diffusion in melts containing h2o and/or co2, which is crucial to describe such processes, is also poorly known. 5. upcoming research although our knowledge of noble gas solubility in magmas has significantly improved over the last twenty years, a lot of work has yet to be accomplished from both experimental and theoretical points of view. magma is a multicomponent system carrying several volatile components in its gas phase; nevertheless our studies on the solubility of noble gases in such complex volatile-bearing systems are still in their early stages. few experimental data are available regarding helium solubility in silicate melts coexisting with h2o-co2 mixed vapors. although these studies are not comprehensive enough to fully describe and understand the effect of the main volatile components on noble gas solubility, they do highlight that it cannot be neglected. the dissolution of noble gases is strictly related to the availability of solubility sites in the silicate network and, due to the significant structural changes in melts caused by the main volatiles, h2o and co2 may well have a very significant control over noble gas solubility. on this basis, sulfur and chlorine (perhaps fluorine as well) may exhibit similar effects, however we lack critical information to evaluate them. as far as the gas phase is concerned, there are equations of state that can calculate the activity-composition relations for ar in hc-s-o mixed vapor in a wide range of pressure and temperature conditions (belenosko et al., 1992). more recently, equations have become available for noble gases in h-c-s-o-halogens (churakov and gottschalk, 2003). nevertheless, the activities of noble gas in s-cl bearing silicate melts are totally unknown, and there is a great need for experimentation in this field of research. in addition, the inadequate knowledge of the structural effect of sulfur and chlorine does not allow us to evaluate their influence on the distribution of the hole-size in melts and, as a consequence, on noble gas solubility. regardless of the analytical method, the experimental determination of noble gas solubility in complex systems requires the charging of known amounts of volatiles inside capsules, in order to obtain vapor phases having similar compositions to those of magmatic ones. in view of this, the method of loading noble gases by gas-bearing glasses (see section 2) should be broadly applicable to the investigation of the solubility of all noble gases (even he and ne) in melts up to very high pressures. although the mass balance between the total and dissolved noble gas allows us to calculate the concentration of noble gas in vapor and then the partition coefficient, the ability to measure the content of noble gas in both the vapor and liquid phase becomes ever more important. in this respect, coupling in situ extraction techniques and mass spectrometry seems the most powerful tool, especially in investigating the solubility of light noble gases. in addition, x-ray absorption techniques have started to provide insights into how noble gas atoms are accommodated in the silicate melt structure and the short-range order. although these data do not produce a direct measurement of solubility, they are still able to constrain the parameters used in the statistical-mechanics solubility models. such theoretical approaches give detailed information and explanations of the mechanism of noble gas dissolution in melt and thus they need to be further improved. additional information could also come, in the future, from computer simulations of noble gas-bearing melts by using molecular dynamics (md) methods, although a lot of work must be done to define pair potentials between the noble gas and the main particles of the melt by fitting experimental data or by performing ab-initio calculations. for this work, the expertise of mineralogists and materials scientists, interested in the considerable inferences of noble gas solubility on the structure of silicate glass and melt, could be very helpful. this co-operation should aim at developing several aspects having technological and 666 antonio paonita commercial interests and involving the interaction of gas atoms with rigid glass materials. for instance, noble gases (he, ar) are normally used in the growth of silicon-made materials for use in semiconductor devices, or with the aim of preventing gas inclusion formation during the manufacture of glass and ceramics. ceramics are also porous enough to allow for noble gas permeation, allowing magnetic resonance imaging (mri) using laser-polarized noble-gas-testing of the structural integrity of the material. large differences in the solubility and diffusion of the gases can be used to separate the components of gas mixtures. finally, an interesting field to be developed in the future involves the possible effects of disequilibrium during noble gas degassing from magmas. in their recent paper, caracausi et al. (2003) show that the helium isotopic composition monitored in volcanic emissions at mt. etna displays variations over time, which cannot be referred to shallow secondary processes. kinetic fractionation of helium isotopes during diffusion in growing bubbles has been suggested to explain this type of signal (nuccio and valenza, 1998; caracausi et al., 2003). in fact, light 3he atoms could diffuse at higher rates in growing bubbles with respect to heavier 4he atoms. in the same way, the ratios between noble gases both in volcanic gases and fluid inclusions could be affected by a similar disequilibrium process, especially when taking into consideration that they have similar dissolution mechanisms and that there is a big difference between their atomic masses. experiments on this topic are not simple and theoretical calculations, to be based on noble gas diffusion rates, require adequate knowledge of the effects of the main volatiles on noble gas diffusivity in silicate melts. a lot of effort still has to be made in this direction, which however seems very promising in its application to magmatic systems. acknowledgements the author wishes to thank p.m. nuccio for useful comments and suggestions. the review by d. baker and c. romano greatly improved the manuscript. references belenoshko, a. and s.k. saxena (1992): a unified equation of state for fluids of c-h-o-n-s-ar composition and their mixtures up to very high temperatures and pressures, geochim. cosmochim. acta, 56, 3611-3626. boettcher, s.l., q. guo and a. montana (1989): a simple device for loading gases in high-pressure experiments, am. mineral., 74, 1383-1384. bottinga, y. and m. javoy (1990): morb degassing: bubble growth and ascent, chem. geol., 81, 255-270. brawer, s.a. and w.b. white (1975): raman spectroscopy investigation of the structure of silicate glasses, i. the binary silicate glasses, j. chem. phys., 63, 2421-2432. broadhurst, c.l., m.j. drake, b.e. hagee and t.j. bernatowicz (1992): solubility and partitioning of ne, ar, kr, and xe in minerals and synthetic basalt melts, geochim. cosmochim. acta, 56, 709-723. brooker, r.a., j.-a. wartho, m.r. carroll, s.p. kelley and d.s. draper (1998): prelimirary uvlamp determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts, chem. geol., 147, 185-200. burnard, p. 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[1996] have highlighted the importance of accuracy and have stated that "repeat station data must be at least good enough to improve the accuracy of sv [secular variation] models produced from observatory data alone. if they are not accurate enough to do this, then the effort expended on repeat measurements is wasted". considering that the source of secular variation lies in the earth's core and accepting the standard assumption of an insulating mantle, it might indeed seem that in central europe with a spacing of less than 1000 km between magnetic observatories, additional repeat station measurements cannot provide any further information. however, they can contribute a substantial amount of information about the crust and perhaps the upper mantle. repeat station data from france and germany have been used to create medium-scale vector magnetic anomaly maps [thébault et al. 2006, korte and thébault 2007]. a longstanding question in geomagnetism is whether induction by the slowly changing main field in conductive structures in the lithosphere is a measurable part of what is observed as secular variation at and above the earth's surface. such signals of time-varying lithospheric anomalies would be useful e.g. to distinguish between remanent and induced sources of crustal magnetic anomalies [lesur and gubbins 2000, jackson 2007] and provide information about the local or regional lithospheric susceptibility with implications for geological and tectonic studies. moreover, it is important to separate any induced lithospheric time varying signal from true core field secular variation in high resolution spherical harmonic magnetic field models when they are used to study geodynamo processes [mcleod 1996, hulot et al. 2009]. earlier attempts to detect secular variation anomalies due to lithospheric induction focused europe, where the article history received october 4, 2011; accepted january 18, 2012. subject classification: geomagnetism, global and regional models, main geomagnetic field. 1101 magnete workshop 2011 highest density of geomagnetic ground observations exists [mundt 1978, mundt 1981, mundt and porstendorfer 1977, mundt and porstendorfer 1978, spitta 1991, podsklan et al. 1993]. most of these studies suggest the existence of induced time-varying anomalies, but none of them is conclusive in terms of their exact dimension and origin. investigations of only geomagnetic observatory data indicate significant induced magnetization components, but the distances between the observatories are too large to allow for inferences on crustal structure [alldredge 1983, lesur and gubbins 2000, macmillan and thomson 2003, verbanac et al. 2007]. thébault et al. [2009] estimated rates of change of induced lithospheric anomalies based on a vertically integrated susceptibility grid of the earth's crust [hemant and maus 2005] for anomalies of wavelengths between 440 and 2700 km. they estimate values of up to 1.30 nt/yr in certain areas, but not exceeding 0.1 nt/yr in western europe. however, more localized anomalies which might give stronger small-scale signals might also exist. in this study, we use the german high quality repeat station data collected between 2000 and 2010 within the european magnete program and compare those data to a recent global geomagnetic field model based on satellite and observatory data. we investigate and discuss differences seen in secular variation from the repeat stations and the global model and discuss different estimates of the lithospheric signal at the repeat stations. finally, we draw conclusions about the influence of external fields in processed repeat station data and on their ability to provide information about remanent and induced contributions to lithospheric magnetic anomalies. 2. repeat station data germany does not have a long history of true repeat station surveys with re-occupation intervals of five years or less. a repeat station network was defined in 1996, using a number of repeat stations that had existed for a few years in the former german democratic republic and a sub-set of documented ground vector survey stations which had been used around 1965 and 1982 in former west germany. the network consists of two classes of stations: firstly, there are 12 high quality stations (first order stations) where a variometer is operated nearby the station for at least three days and including at least one night with very low magnetic activity. secondly, our surveys include up to 36 of the former ground vector survey stations, surrounding the variometer stations. absolute measurements are carried out at those stations while the variometer is running at the nearest first order station, in general no further than 150 km away. experiences from the first use of the in situ variometer and problems with technical disturbances at some stations unfortunately required some changes in both repeat station and variometer networks over the years. in this study, we consider only first order stations where an in situ variometer was frequently operated and the highest possible accuracy can be expected, shown in figure 1. four geomagnetic observatories are constantly recording the geomagnetic field in germany, these are niemegk (ngk), wingst (wng) and fürstenfeldbruck (fur) with long time series and the more recently established black forest observatory (bfo). from 1999 to 2002 surveys were carried out covering half the network each year, using the standard absolute equipment of a di-flux theodolite (zeiss 010b with bartington sensor) and an overhauser magnetometer (gem systems gsm 19), and at the first order stations additionally a lemi 008 fluxgate vector variometer. the annual mean results are reduced to the middle of the year in which the measurements were carried out, i.e. results exist for 1999.5/2000.5 and 2001.5/2002.5 [korte and fredow 2001]. in 2003, the magnete (magnetic network of europe) initiative to standardize repeat station surveys all over europe started [korte and mandea 2003]. the agreed recommendation was to carry out surveys on the complete networks in all countries bi-annually, starting in 2004. as a consequence the survey that had been started in 2003 in germany was extended to as many stations as possible, yielding results for about 80% of the full network. from 2004 to 2010 bi-annual surveys of the whole network were carried out. repeat station survey results are commonly given as 'annual means', all reduced to the middle of a given year, in order to eliminate transient external field contributions and allow for mapping of the internal field components together with standard observatory annual means. this data reduction is done in two steps for our first order stations. first, a quiet night time value is obtained for each magnetic field component c at station x and the most quiet night while the variometer was recording at that station: cqn (x,tqn) = vcqn (x,tqn) + [c (x,t) − vc (x,t)] (1) here, c (x,t) is the absolute component observation carried out at time t, and and vc (x,t) are the variation recordings of component c at the quiet night time tqn and the time of the absolute measurement, t, respectively. vcqn and consequently the resulting cqn are taken as averages over a range of night time values, in general in the order of a few hours, when the magnetic k index at niemegk observatory is lowest and the variation recordings visually appear best to reflect a nearly constant quiet internal field level. these quiet night time results for different days (nights) within the year are further reduced to common annual means centered on the middle of the year by cam (x,tam) = ocam (x,tam) + [cqn (x,tqn) − ocqn (o,tqn)]. (2) now, ocam and ocqn are the annual mean and equivalent quiet night time value at magnetic observatory o, in our case the niemegk observatory (ngk). korte and lesur 1102 1103 we considered both the cqn and cam results in our study. in most cases the quiet night time reduction could be done for times of k index 0 to 2. only in four cases (stations kar in 2003, kob and oet in 2006 and lis in 2010) was the k index 3 or higher for the whole time of variometer operation and the averaging was done over the whole time of operation in two of these cases (2 and 3 days for stations kob and kar, respectively). a table with the quiet night time values and related information for all stations is given as supplemental material. the annual mean data are available at the world data center edinburgh at http://www.wdc.bgs.ac.uk/. from observatory annual means it is well known that external field variation and their induced counterparts do not average out completely, and that e.g. the solar cycle is clearly reflected in these values [courtillot and le mouël 1976, schmucker 1991, verbanac et al. 2007]. repeat station data reduced by equation 2 will therefore be affected similarly. this remaining annually averaged external variation signal is indeed quite homogeneous over regions the size of germany and can reasonably be estimated empirically from the three or four (bfo only operates since 2004) german observatories [korte and thébault 2007]. this is done by first subtracting core field annual mean predictions by a global model (see next section) from the observatory annual means time series. then the lithospheric observatory bias is estimated as the average of the residuals at each observatory and for each component and also subtracted. the remaining time series of residual variations over the time interval 2000.5 to 2010.5 from the three observatories are averaged to obtain an empirical time series of external variations for each field component. a third data set of repeat station annual means, empirically corrected for external field influences by subtracting the averaged observatory residuals templates, is included in our study. table 1 summarizes the different data types used here. 3. the grimm-3 model and the german vector magnetic anomaly model since modern magnetic field satellite data became available from 1999 on from the ørsted and soon after also the champ satellite, a number of new global magnetic field models have been developed. one of the most recent ones, repeat station data and global model 6˚ 6˚ 8˚ 8˚ 10˚ 10˚ 12˚ 12˚ 14˚ 14˚ 48˚ 48˚ 50˚ 50˚ 52˚ 52˚ 54˚ 54˚ bor deu ebe goe gtt kar kob lis oet tel kam eil bfo fur wng ngk −160 −140 −120 −100 −80 −60 −40 −20 0 20 40 60 80 100 120 140 160 180 nt 6˚ 6˚ 8˚ 8˚ 10˚ 10˚ 12˚ 12˚ 14˚ 14˚ 48˚ 48˚ 50˚ 50˚ 52˚ 52˚ 54˚ 54˚ bor deu ebe goe gtt kar kob lis oet tel kam eil bfo fur wng ngk −120 −100 −80 −60 −40 −20 0 20 40 60 80 100 120 nt 6˚ 6˚ 8˚ 8˚ 10˚ 10˚ 12˚ 12˚ 14˚ 14˚ 48˚ 48˚ 50˚ 50˚ 52˚ 52˚ 54˚ 54˚ bor deu ebe goe gtt kar kob lis oet tel kam eil bfo fur wng ngk −250 −200 −150 −100 −50 0 50 100 150 200 250 nt figure 1. magnetic observatories (stars) and first order repeat stations (triangles) in germany, shown on the lithospheric magnetic anomaly maps for a) the north (x), b) the east (y) and c) the vertical component (z) obtained from the regional model by korte and thébault [2007]. b c a taking advantage of the complete champ data series from january 2001 to september 2010, is the third version of the gfz reference internal magnetic model (grimm3). here, we use the preliminary model as presented by lesur et al. [2011], which also uses global observatory hourly means from 01 january 2001 to 31 december 2009. some data selection for magnetically quiet times has been applied for all data types. repeat station data have not been used, so that we can compare a truly independent data set here. the model in general follows the modeling approach for earlier grimm versions [lesur et al. 2010] and will be described in detail by work in preparation by v. lesur and co-workers. in the following, we only summarize the aspects which are important for this study. the core field model is based on a spherical harmonic expansion up to degree and order 18 for the core field with the temporal continuity given by order 6 b-splines with a knot-point spacing of six months. in order to avoid crustal field influence and include only the robustly described secular variation we limit the core field predictions to maximum degree and order 14. for comparison to the quiet night time component data cqn we calculated core field model predictions mcc for the center of the time interval over which the night time data were averaged. the secular variation might vary non-linearly over time intervals of a year. therefore 'annual mean' core field model predictions mcam were obtained by averaging over 10 evenly distributed values within a calendar year for the comparison to the annual mean repeat station and observatory data centered on the middle of a year. the lithospheric field is included by constant spherical harmonic degrees 19 to 60 in the grimm3 model. due to the satellite altitude this part can only contain magnetic anomalies larger than a few 100 km wavelength. this global lithospheric field estimation describes only a small part of the local crustal bias at a repeat station. we could confirm this for the stations used in our study and we didn't consider the lithospheric field description of this model any further. although the main purpose of grimm3 is to provide a highly accurate description of the core field and its temporal evolution, the model contains some external field estimates as a by-product from the necessary separation of sources in the modeling approach. the external field description is limited to spherical harmonic degree 1. it contains a slowly evolving part, with a 100 days baseline, that is essentially similar to the parameterization used by lühr and maus [2010]. a rapidly evolving part is based on processed satellite data averaged by orbits, where again the baseline is 100 days. we determined grimm3 predictions mcce with the external field description included for the centers of the time intervals over which the quiet night time repeat station data cqn were averaged. this should give a good estimate for the majority of the data, which have been reduced to quiet night times with very low magnetic activity. a test showed that during night times of magnetic activity index k = 0 to 1 the external field model prediction over a few hours varies only by about 1 nt, and for k = 2 up to about 5 nt. the model prediction abbreviations we use are included in table 1. crustal bias estimates for all german repeat stations have previously been obtained from a regional lithospheric vector anomaly model [korte and thébault 2007], which is shown in figure 1. this model combines the lithospheric field information from the champ satellite data, from an aeromagnetic grid and from the repeat stations using the technique of revised spherical cap harmonic analysis, r-scha [thébault et al. 2006]. the repeat station crustal biases, cb, as determined in that investigation for each component c, are used for comparison in this study. korte and lesur 1104 name short description repeat station data cqn c component reduced to quiet night time cam c component annual mean cec c comp. annual mean with empirical external field correction model predictions mcc c component grimm3 core field prediction mcamc c component grimm3 core field "annual mean" mcce c comp. grimm3 core plus external field prediction crustal bias estimates cb c comp. crustal bias from german r-scha model cbqn time averaged cqn-mcc cbam time averaged cam-mcamc cbqne time averaged cqn-mcce cbame time averaged cec-mcamc table 1. summary of data, model predictions and results terminology. 1105 4. secular variation comparison and crustal biases first, we subtracted the core field predictions from the repeat station data, i.e. determined cqn-mcc and cam-mcamc, and then determined the average value and standard deviation for each of these time series. the time averaged values are considered as crustal bias estimates, cbqn and cbam. the standard deviations of cbqn and cbam contain the remaining signal and noise in the repeat station measurements that is not described by the core field and secular variation of the model. the time series with the average values subtracted are shown in figure 2 and the average standard deviations for each component are tabulated in table 2. the standard deviations of the cbam are smaller than those of the cbqn in all components on average. in the annual mean results large repeat station data and global model figure 2. repeat station data after subtraction of grimm3 core field predictions and constant average for north (x), east (y) and vertical (z) component. the left panels (a and c) show the annual mean results, the right panels (b and d) the quiet night time results. for better visibility the panels have been split up in northern (a and b) and southern (c and d) stations. a) c) d) b) scale variations common to all the repeat stations are very obvious, while there are some common trends, but also larger inhomogeneous variations, seen in the quiet night time results. the reduction to quiet night times does not reduce the external field influence in the data as much as one might have anticipated. next, external field contributions are taken into account for the same analysis. for the annual mean data, the model core field predictions are the same as before, but they are subtracted from the empirically corrected data: cec-mcamc. for the quiet night times, the data are the same as before, but the model predictions include all external contributions described by the model: cqn-mcce. the obtained time series are shown in figure 3, the corresponding time averaged crustal bias estimates are named cbame and and their averaged standard deviations for each component are included in table 2. as expected, the average standard deviations are lower now, except for quiet night time total intensity (f) results. for the annual mean data, they are no higher than 2 to 3 nt on average and thus lie in the order expected for measurement uncertainties for repeat station data. they are somewhat higher for the quiet night time data, indicating remains of external field contributions in the results and slightly higher than the fit of the grimm3 model to the magnetic observatory hourly mean data used in the model. in general, the horizontal components x and y are more sensitive to direct changes in ionospheric and magnetospheric magnetic fields, while the vertical component z and consequently also f are strongly influenced by the induced secondary fields, which can vary more locally. the fact that the average scatter now is highest in z and f for the quiet night time results indicates that, perhaps not surprisingly, the description of the external fields themselves is more accurate than the contribution from their induced counterparts in the grimm3 model. the scatter in z (and f) of station kob is particularly high (see figure 3), and in fact the data for 2006 could not be obtained for truly quiet times, but instead was averaged over two days with magnetic activity index at niemegk not lower than k = 4. it is not surprising that the model predictions determined for only one point in time within this interval does not fit well in this case. however, this is the only example where we noticed an obvious correlation between an outlier in the residuals, and stronger than average magnetic activity during data acquisition. other apparent outliers, e.g. for station lis in 2003 or station ebe in 2001 are not correlated to high magnetic activity. also, the other way round, data reduced to night times with activity index k = 3, e.g. kar in 2003 or lis in 2010 are not clearly linked to outliers in the model comparison results. both the empirical correction for external influences and the external field contributions described by the model succeed in reducing the undesired signal from outside the earth and their induced counterparts. comparing figure 2 and figure 3, it becomes clear that apparent long-term trends (increase in x residuals and decrease in z residuals in figure 2 a and c) are not due to an insufficient secular variation description in grimm3, but are caused by external field contributions in the data. moreover, the deviations seen in figure 2 around 2003 might seem to suggest that we see some signal of the geomagnetic jerk, a an abrupt change of secular acceleration originating in the core that happened around that time (see mandea et al. [2010] for a recent review on geomagnetic jerks), in the repeat station data, but not adequately described by the grimm3 model. however, it becomes clear from figure 3 that this signal is captured and described as being of external origin by the global model. we now turn to a comparison of the crustal bias estimates obtained as averages of the residuals between the different data and models, listed in tables 3 and 4. none of the repeat stations lie on particularly strong lithospheric anomalies. the largest crustal bias estimates are in the order of 150 nt, and at many locations they only reach several tens of nt. the estimates in general agree within approximately 10 nt in y and f and within 20 nt in x and z. the estimates from the annual mean data with and without external field correction agree very closely in all components, due to the way they are determined and despite the removal of trends by the empirical external field correction. the rather large differences between the crustal bias estimates obtained from the quiet night time data with (cbqne) and without (cbqn) consideration of the external field contributions included in grimm3 suggests systematic, long term contributions from the ionospheric and magnetospheric fields in the repeat station data reduced to quiet night times, probably particularly from the ring current. therefore, the estimates probably give the most accurate amplitudes of the lithospheric signal. the new crustal bias estimates are in reasonable agreement with the previous results from the german vector lithospheric korte and lesur 1106 crustal bias estimate sd x [nt] sd y [nt] sd z [nt] sd f [nt] cbqn 8.8 6.0 6.7 4.5 cbam 6.1 3.6 6.5 4.0 cbqne 4.6 4.7 6.1 5.6 cbame 2.4 2.6 2.0 2.4 table 2. average standard deviations of the crustal bias estimates. 1107 anomaly model. the way these were determined also does not provide information about systematic long term contributions from external fields. finally, we investigated whether the time series of residuals after subtraction of core and external field estimates from the repeat station data show any remaining long-term trends, which might e.g. indicate induced lithospheric anomalies. we performed linear regressions of the cec-mcamc time series (figure 3a and c), i.e. the time series where the removal of core and external field signals has been most successful. note that with barely 10 years of data, and with only 4 to 7 data points available in each time series this analysis repeat station data and global model figure 3. repeat station data after subtraction of grimm3 core field predictions, a constant average for north (x), east (y) and vertical (z) component, and removal of external field influences. the left panels (a and c) show the annual means with the empirical external field correction. the right panels (b and d) show the quiet night time results where the grimm3 external field description has been used (see text for details). for better visibility the panels have been split up in northern (a and b) and southern (c and d) stations. a) b) c) d) korte and lesur 1108 station xb xbqn xbam xbqne xbame yb ybqn ybam ybqne ybame lis 100.46 108.32 102.67 118.32 102.44 -87.65 -90.48 -91.57 -93.28 -91.31 bor -13.1 0.6 -8.9 7.12 -9.13 30.35 28.23 29.45 26.15 29.71 kam 6.55 23.1 8.18 27.7 8.58 -60.12 -55.2 -57.97 -57.63 -57.78 goe 66.29 86.42 70.64 88.86 68.93 -16.31 -27.68 -24.94 -28.94 -24.09 kob 102.6 109.94 106.92 124 105.21 33.37 34.24 32.88 29.42 33.73 tel 16.88 30.75 21.64 41.2 23.01 21.67 24.77 20.7 20.73 20.47 gtt -30.95 -29.18 -38.13 -19.63 -36.76 18.12 26.05 26.79 22.88 26.55 deu -88.27 -77.17 -89.84 -67.5 -88.47 -59.62 -64.25 -59.21 -66.93 -59.45 ebe -36.1 -26.32 -35.42 -17.9 -34.93 -35.58 -40.08 -39.17 -42.73 -39.26 eil -37.33 -21.54 -33.88 -12 -33.48 -15.95 -19.94 -17.18 -22.36 -17 oet -5.78 13.72 0.25 21.23 0.73 -33.95 -32.45 -28.82 -34.58 -28.91 kar -65.43 -56.78 -67.07 -48.7 -66.58 24.55 16.96 19.57 15.1 19.48 station zb zbqn zbam zbqne zbame fb fbqn fbam fbqne fbame lis 136.55 129.02 128.62 113.08 128.96 -158.68 156.37 147.29 156.56 bor 3.35 -3.38 -1.75 -13.87 -1.41 --2.94 -4.95 -10.27 -4.77 kam -85.51 -78.4 -71.4 -87.97 -71.68 --65.28 -64.18 -72.54 -64.31 goe -30.68 -40.06 -36.22 -47.66 -34.6 --5.52 -7.78 -11.67 -7.05 kob -113.15 -118.28 -119.42 -143.1 -117.8 --69.24 -71.42 -87.29 -70.69 tel -27.17 -21.88 -21.2 -38.5 -22.31 --8.14 -11.11 -19.42 -11.6 gtt -54.18 -49.17 -49.39 -65.01 -50.5 --56.49 -60.16 -67.33 -60.69 deu 12.23 7.77 9.66 -9.08 8.55 --24.07 -27.24 -35.83 -27.75 ebe -23.83 -24.87 -22.02 -38.33 -22.42 --33.63 -34.75 -42.46 -34.99 eil -43.3 -44.98 -44.48 -60.78 -44.77 --50.08 -54.63 -60.67 -54.78 oet 58.81 52.1 54.6 39.92 54.2 -52.6 48.82 45.06 48.57 kar 107.02 104.46 107.13 92.8 106.73 -69.12 66.99 62.18 66.76 station xec-mxamc yec-myamc zec-mzamc lis -0.61 -0.94 -0.56 bor -0.17 -0.35 -0.21 kam -0.67 -0.33 -0.47 goe -0.42 0.37 -0.24 kob -0.48 -0.22 -0.05 tel 0.16 -0.20 -0.53 gtt -0.44 -0.09 -0.13 deu -0.85 -0.14 -0.01 ebe 0.13 -0.34 0.54 eil -0.77 -0.60 -0.27 oet 0.54 0.90 -1.37 kar 0.01 -1.06 0.17 table 5. linear trends in nt/yr obtained for the different component residual time series. table 4. crustal bias estimates for z and f component in nt. table 3. crustal bias estimates for x and y component in nt. 1109 cannot be considered very robust. table 5 lists the linear trends obtained by the regression. for a couple of stations trends in the order of 0.5 to 1 nt/yr are detected. to get an idea of the spatial distribution of these results we visualized them in figure 4 by a simple interpolation of the trends. the maps are meant only to give an idea of the distribution of stations where positive or negative trends were detected and to facilitate an easy comparison. if any of the detected trends are real, they might be much more localized around smallscale anomalies. the comparison of figure 4 to figure 1 and of the numbers in table 5 to table 3 and 4 does not indicate any correlations between locations on strong crustal anomalies and detected trends. for several of the stations, we get similar results as carried out two years ago with data only up to 2008.5 and an earlier version of the grimm model. however, it seems premature to interpret any of these trends as a magnetic field signal unless the robustness of these results can be confirmed by longer or denser time series. 5. discussion and conclusions we have compared the most recent grimm magnetic field model (preliminary grimm3) with german repeat station data in order to investigate the accuracy of the data and the model and to evaluate the benefits of repeat station data in areas comparatively densely covered by magnetic observatories. the repeat station data form an independent data set and have not been used to derive the grimm3 model. the subtraction of grimm3 core field predictions from the data has shown that significant external field contamination is present both in repeat station annual means and quiet night time data. apparent long-term (years to decade) trends in repeat station data secular variation compared to core field and secular variation models can be due to external field contributions in the data and do not necessarily hint at insufficient secular variation description in the model. in fact the comparison including the consideration of the external field description in the global model suggests that the core field secular variation for this area is described with an accuracy clearly better than 1 nt/yr. the external field contamination in commonly used repeat station results might be small enough to be considered insignificant for general regional repeat station data and global model 6˚ 6˚ 8˚ 8˚ 10˚ 10˚ 12˚ 12˚ 14˚ 14˚ 48˚ 48˚ 50˚ 50˚ 52˚ 52˚ 54˚ 54˚ lis bor kam goe kob oet kar ebe eil gtt tel deu −1.4 −1.2 −1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 nt/yr 6˚ 6˚ 8˚ 8˚ 10˚ 10˚ 12˚ 12˚ 14˚ 14˚ 48˚ 48˚ 50˚ 50˚ 52˚ 52˚ 54˚ 54˚ lis bor kam goe kob oet kar ebe eil gtt tel deu −1.4 −1.2 −1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 nt/yr 6˚ 6˚ 8˚ 8˚ 10˚ 10˚ 12˚ 12˚ 14˚ 14˚ 48˚ 48˚ 50˚ 50˚ 52˚ 52˚ 54˚ 54˚ lis bor kam goe kob oet kar ebe eil gtt tel deu −1.4 −1.2 −1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 nt/yr figure 4. visualization of repeat stations with negative and positive trends in crustal biases for a) the north (x), b) the east (y) and c) the vertical (z) component. note that detected trends might not be significant and those that are might be much more localized than this figure suggests. see text for more details. a b c mapping of secular variation, but it certainly has to be considered if repeat station data shall be used for high resolution secular variation studies. the optimal treatment to eliminate the external field influences that remain after the commonly used repeat station data processing, however, has yet to be found. for small regions, for which several good data series are available, an empirical approach to subtract the common signal from annual mean data series seems to work best. in our case the scatter in the residuals between corrected annual mean data and the core field model was reduced to the order expected as uncertainties in repeat station measurements. this method, however, does not give any information about long-term 'offsets' caused by external field contributions. the existence of such 'offsets' is suggested by clear differences between the time-averaged crustal bias values obtained by subtraction of the core field or core and external field model, respectively, from the quiet night time data. on the other hand, the residuals between quiet night time data and grimm3 core and external field description show the largest scatter and thus clearly still contain remains of high frequency ionospheric and magnetospheric field contributions, at least as long as the measurements cannot be reduced to extremely quiet conditions. consequently, in order to determine the core field secular variation signal from repeat station data, the empirical approach seems more successful, while to determine the lithospheric anomaly value (crustal bias) at a station, the subtraction of a model such as grimm3, which includes as much as possible external field description, seems advisable if the data were taken at truly quiet times or enough data to determine a statistical average for a location are available. an analysis of long-term trends in the residual repeat station data after the subtraction of core and external field influences indicates that some small remaining time-varying magnetic signal, probably related to induced lithospheric anomalies, could exist at some locations, but considering the expected size of such a signal and the measurement uncertainties it seems premature to interpret the observed trends. more data points from each location are necessary to carry out such an analysis in a more robust way. if a continuous, homogeneous global field model for the past decades up to present times becomes available we will be able to include the previous ground vector survey results from the same stations. as a final remark we would like to point out that this study focused on a region of the world that is characterized by a dense network of continuously recording magnetic observatories. it is not clear if global models describe the secular variation equally well in regions where much less high quality ground data time series are available and a comparative study, e.g. for south africa where both a good repeat station network and strong core field variations and gradient exist, could be instructive. acknowledgements. we thank martin fredow and manfred schüler for performing the bi-annual repeat station measurements and processing the raw data. the work presented in this paper includes data obtained at the magnetic observatory fürstenfeldbruck and black forest observatory. we thank the ludwig maximilians university munich and the universities of karlsruhe and stuttgart for operating these observatories respectively. the grimm-3 model relies on global observatory data, and we thank all the national institutes that support them and intermagnet for promoting high standards of magnetic observatory practice and global data availability (www.intermagnet.org). the champ mission was sponsored by the space agency of the german aerospace center (dlr) through funds of the federal ministry of economics and technology (grant code 50ee0944). we also thank the champ data processing team. plots were made using the generic mapping tools [www.soest.hawaii. edu/gmt; wessel and smith 1998]. we acknowlegde constructive comments from two anonymous reviewers. references alldredge, l.r. (1983). varying geomagnetic anomalies and secular variation, j. geophys. res., 88, 9443-9451. courtillot, v., and j.-l. le mouël (1976). on the long-period variations of the earth's magnetic field from 2 months to 20 years, j. geophys. res., 81, 2941-2950. hemant, k., and s. maus (2005). geological modeling of the new champ magnetic anomaly maps using a geographical information system technique, j. geophys. res., 110, b12103; doi:10.1029/2005jb003837. hulot, g., n. olsen, e. thébault and k. hemant (2009). crustal concealing of small-scale core-field secular variation, geophys. j. int., 177, 361-366. jackson, a. (2007). studies of crustal magnetic anomalies of the british isles, astronomy and geophysics, 48 (2), 8-13. korte, m., and m. fredow (2001). magnetic repeat station survey of germany 1999/2000, scientific technical report, potsdam str01/04, 1-23. korte, m., and m. mandea (2003). improvements planned for european geomagnetic repeat stations, eos trans., amer. geophys. soc., 84, 160. korte, m., and e. thébault (2007). geomagnetic repeat station crustal biases and vectorial anomaly maps for germany, geophys. j. int., 170, 81-92. lesur, v., and d. gubbins (2000). using geomagnetic secular variation to separate remanent and induced sources of the crustal magnetic field, geophys. j. int., 142, 889-897. lesur, v., i. wardinski, m. hamoudi and m. rother (2010). the second generation of the gfz reference internal magnetic model: grimm-2, earth planets space, 62, 765773. lesur, v., i. wardinski and m. hamoudi (2011). third version of the gfz reference internal magnetic model: grimm-3, abstract, 25th iugg general assembly (melbourne). lühr, h., and s. maus (2010). solar cycle dependence of quiet-time magnetospheric currents and a model of their near-earth magnetic fields, earth planets space, 62, 843848. macmillan, s., and a. thomson (2003). an examination of korte and lesur 1110 1111 observatory biases during the magsat and ørsted missions, phys. earth planet. int., 135, 97-105. mandea, m., r. holme, a. pais, k. pinheiro, a. jackson and g. verbanac (2010). geomagnetic jerks: rapid core field variations and core dynamics, space sci. rev., 155, 147175. mcleod, m.g. (1996). spatial and temporal power spectra of the geomagnetic field, j. geophys. res., 101, 2745-2763. mundt, w., and g. porstendorfer (1977). zusammenhänge zwischen analysen der magnetotellurik und der magnetischen säkularvariation im hinblick auf eine leitfähigkeitsanomalie im oberen erdmantel mitteleuropas, gerlands beitr. geophys., 86, 337. mundt, w. (1978). geomagnetic secular variation anomalies in gdr, j. geomag. geoelectr., 30, 523-531. mundt, w., and g. porstendorfer (1978). mögliche zusammenhänge zwischen einer elektrischen leitfähigkeitsanomalie im erdmantel und anomalen magnetischen säkularvariationen in mitteleuropa, geodät. u. geophys. veröff. d. nkgg, 39, 153-162. mundt, w. (1981). regionale und lokale anomalien der geomagnetischen säkularvariation in mitteleuropa, veröffentl. d. zentralinst. f. physik d. erde, 70, 33-43. newitt, l.r., c.e. barton and j. bitterly (1996). guide for magnetic repeat station surveys, international association of geomagnetism and aeronomy. podsklan, j., v. kuznetsova and v. masimchuk (1993). some peculiarities of the secular variation of the geomagnetic field in the carpathians, phys. earth planet. int., 77, 137141. schmucker, u. (1991). solar cycle variations and corrected annual means for external effects at fürstenfeldbruck 1951-1968, in: m. beblo and h. soffel (eds.), 150 years earthmagnetic observatories münchen maisach fürstenfeldbruck, vol. 5, münchen, münchner geophysikalische mitteilungen, pp. 217-248. spitta, p. (1991). regional survey in the surroundings of göttingen in search of anomalies of secular variations, münchner geophys. mitt., 5, 159-174. thébault, e., j.j. schott and m. mandea (2006). revised spherical cap harmonic analysis (r-scha): validation and properties, j. geophys. res., 111, b01102; doi: 10.1029/2005jb003836. thébault, e., k. hemant, g. hulot and n. olsen (2009). on the geographical distribution of induced time-varying crustal magnetic fields, geophys. res. lett., 36, l01307; doi:10.1029/2008gl036416. verbanac, g., m. korte and m. mandea (2007). on long-term trends of the european geomagnetic observatory biases, earth planets space, 59 (7), 685-695. wessel, p., and w.h.f. smith (1998). new, improved version of the generic mapping tools released, eos trans. agu, 79, 579. *corresponding author: monika korte, helmholtz-centre potsdam, gfz german research centre for geosciences, potsdam, germany; email: monika@gfz-potsdam.de. © 2012 by the istituto nazionale di geofisica e 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice spatial variation of the b-value observed for the periods preceding and following the 24 august 2016, amatrice earthquake(ml 6.0) (central italy) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 spatial variation of the b-value observed for the periods prece ding and follow ing the 24 august 2016, amatrice earthquake (m l 6.0) (central italy) caterin a mon tuori*, maura murru, giuseppe falcon e istitu to n azion ale d i geofisica e vu lcan ologia, rom e, italy *caterina.montuori@ingv.it abstract this paper deals with a preliminary spatial and temporal analysis of the b-value variability, observed in the area where the august 2016 amatrice earthquake (ml6.0) occurred. with comparison of the preand postperiods of the mainshock, an investigation of anomalous zone of b-values was performed aiming to find possible links with barriers and/or asperities in the crustal volume where seismic sequence was developed. preliminary results show an area with high b-value (b=1.6) where the mainshock originated. conversely, two low bvalue (b=0.8) volumes are located at the border of the seismogenic structure. the location of these two areas is consistent with a preliminary fault slip inversion, suggesting the presence of two highly stressed patches of co-seismic deformation located nw and se of the mainshock, with a high potentiality to rupture causing a possible moderate or larger event: the first one in the north (norcia), the second one in south, next to the area of amatrice and campotosto. i. in troduction on 24 au gu st 2016 1:36:32 (utc) a local m a gn itu d e 6.0 earth qu ake, 8 km d eep , occu rred in cen tral italy, close to accu m oli, a village in th e area located betw een tw o tow n s: n orcia an d l'aqu ila. it ru p tu red a sw d ip p in g, n n w-sse tren d in g fau lt segm en t. th e m ain sh ock h as triggered a stron g sequ en ce of aftersh ocks (blu e colou r in figu re 1) th at in volved a p o rtion of th e ap en n in e ch ain alon g an axis of abou t 50 km (from ussita v illage till cam p ot osto lake) an d abou t 15 km d eep . th e sp atial aftersh ock d istribu tion su ggests th e activation of d ifferen t fau lt segm en ts (mt. vettore fau lt segm en t an d several oth er an tith etical stru ctu res d ip p in g tow ard s n e) follow in g th e m ain sh ock occu rred on 24 au gu st, 2016. th e area affected by th e am atrice earth qu ake sequ en ce is located in a p ortion of th e cen tral ap en n in es w ith a com p lex tecton ic settin g. an exten sion al tecton ic regim e, w ith sever al m ajor qu ater n ar y n or m al fau lts orien ted alon g th e axis of th e w h ole ap en n in es (n w-se), is su p erim p osed annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 on an in h erited th ru st sequ en ce. th is is m ain ly m ad e of th e um bria-march e carbon aticmesozoic sequ en ce (n orth -w estern h alf) an d of latiu m -abru zzi flysch tertiary sequ en ce (sou th -eastern h alf), sp lit by th e in active th ru st of sibillin i mts. (olev an o – an trod oco lin e). th e qu atern ary n orm al fau lts bord er th e laga mt., th e vettore mt., th e n orcia basin an d th e n earby qu atern ary in term ou n tain basin s. d esp ite its m od er ate exten t, th e am atrice eart h qu ake claim ed 309 lives; th e tow n of am atrice w as d estroyed en tirely as w ell as m an y su rrou n d in g villages. th e area h ad been affected by a d estru ctive earth qu ake in 1639 (am atrice, io 9-10 mcs, m w 6.2) w h ich cau sed 500 fatalities. recen tly, cen tral italy h as been stru ck by tw o stron g m ain sh ocks: on e occu rred in 1997, colfior ito earth qu ake w ith m w 6.0 (um briamarch e region ) an d th e oth er on e in 2009, l'aqu ila earth qu ake w ith m w 6.3 (abru zzo region ). figure 1. shallow seismicity map (m w 1.4) for the central a pennines region (a pril 16, 2005-september 16, 2016). historical seismicity (cpti15 release 1.5, rovida et al. 2016) is highlighted by black squares (the numbers are referred to the years of occurrence). the main shocks of the 1997 colfiorito, 2009 l’a quila an d the recent 2016 a matrice are shown as green, red and blue stars, respectively. the seismicity of three analyzed time periods is reported in different colors: the first on e in red , th e secon d on e in yellow an d th e last in blue (see for details table 1). the black box highlights the area under study (i.e., 12.6°e–13.8°e, 42.2°n –43.0°n ). black dots are the main towns: a qu=l' a quila, cm p=campotosto, a m t=a matrice, a cc=a ccumoli, a rq=a rquata del tronto, n rc= n orcia, uss=ussita. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 sp atial an d tem p or al v a riation s of b-v alu e of th e gutenberg-richter (g-r) relation (1944), h ave been exten sively stu d ied for variou s tecton ic regim es. th is p ar am eter d ep en d s on th e stress regim e, th e tecton ic ch ar acter of th e region , th e h eterogen eities of m aterials, an d th e tem p er atu re [i.e., sch oltz, 2015]. th ese factors can cau se, locally, ch an ges in th e b-valu e com p ared to th e global average valu e (assu m ed equ al to 1). low b-valu es h ave been correlated w ith areas of asp erity, locked p art of a fau lt w h ere th e n u cleation of earth qu akes is likely to h ap p en [schorlemmer et al., 2004, schorlemmer an d w iemer, 2005, torman et al., 2012]. figure 2. plot of m c with time for all the analyzed period (a pril 16, 2005-september 16, 2016). in the upper right corner, the m c zoom for the 2016 a matrice sequence is depicted in the inset box (a ugust 24-september 18, 2016). recen tly a few au th or s h ave fou n d th at bvalu e in th e con tin en tal cru st d ecreases d ow n to a d ep th corresp on d in g to th e tran s ition betw een br ittle an d d u ctile m aterial; b elow th is d ep th th e b-valu e w as observed stron gly in creasin g [i.e., m ori and a bercrombie, 1997; spada et al., 2013; scholtz, 1968 an d 2015]. h igh b-v alu es h av e been correlated w ith th e h igh est slip d u rin g large eart h qu akes [görgün et al., 2009; sobiesiak et al., 2007]. moreover, th ey w ere even associated w ith th e p resen ce of flu id s in th e aftersh ock areas, facilitatin g th e evolu tion of th e s equ en ce itself. th e occu rren ce of a stron g a ftersh ock m ay be d u e to th e u p w ellin g of d eep flu id s w h ich can red u ce th e effective stress an d th en trigger an earth qu ake [w and and m anga, 2009]. a few au th ors h igh ligh ted th at m an y aftersh ock sequ en ces w ere m ain ly d istribu ted in h igh p ore p ressu re areas an d th eir sp atio tem p oral d istr ibu tion cou ld h ad been related to th e tran sp ort of flu id s. su ch an alysis w as con d u cted in italy for b oth sequ en ces like th e on e in 1997 (colfiorito) an d th e sequ en ce in 2009 (l’aqu ila) [m alagnini et al., 2012; de gori et al., 2012; m iller et al., 2004] an d in som e oth er cou n tr ies: 1992 lan d er s earth qu ake [bosl and n ur, 2002], th e aftersh ocks of n or th ern c h ile’s 1995 an tofagasta earth qu ake [shapiro et al., 2003] an d aftersh ocks follow in g th e 2008 wen ch u an m s 8.0 earth qu ake occu rred in ch in a [liu et al., 2013]. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 figure 3. m aps of completeness magnitude for each period (i-ii-iii in table1). w hite squ ares are the main towns shown in figure 1 as black dots. th e sp atial variation of b-v alu e of th e gutenberg-richter (g-r) relation in th e area h it by th e sequ en ce, before an d after th e am atrice earth qu ake, is in vestigated in th e p resen t w ork. th e aim is to u n d er stan d th e m ean in gfu l variation s of b-valu e th at cou ld be lin ked w ith th e stress accu m u lated w ith in an asp e rity p laced alon g or arou n d a fau lt su rface. th e p eriod before th e am atr ice m ain sh ock in clu d es th e seism ic sequ en ce of l'aqu ila earth qu ake in 2009 th e tech n iqu e em p loyed in th is p ap er h as been alread y ap p lied by th e au th ors in d ifferen t tecton ic areas [m urru et al., 1999, 2004, 2005; m ontuori et al., 2010; de gori et al., 2012]. ii. data an d an alysis th e d ata u sed in th is stu d y (ap ril 16, 2005 to sep tem ber 18, 2016) are d raw n from th e ita lian seism ic in stru m en tal an d par am etr ic d atabase (iside) p rovid ed by in gv. an area con tain in g 27,223 even ts h avin g m agn itu d e m l ≥1.4 an d d ep th ≤30 km w as selected (see black box in figu re 1). th e first step w as to calcu late th e m agn itu d e of com p leten ess (m c ) w h ich is an im p ortan t p ar am eter w h en est im atin g b-valu es [w iemer an d w yss, 2000]. m c is com p u ted as a fu n ction of tim e by slid in g th e tim e w in d ow s, each con tain in g 150 earth qu akes an d step p in g by 5 even ts (figu re 2). m c valu es, for th e w h ole p eriod an alyzed , ch an ge from 1.4 to 1.9, excep t for tw o p eriod s of tim e correlated w ith tw o stron g even ts occu rred in ap ril 2009 (l'aqu ila) an d in au gu st 2016 (am atrice), w h ere m c reach es th e valu e of abou t 3.0. in th e early h ou rs of a seism ic sequ en ce m ay be very d ifficu lt to d etect sm all sh ocks; th is m ay lead , as a con sequ en ce, to an in crease of th e m agn itu d e of annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 figure 4. m aps of b-values for the three periods shown in table 1. a) i period (m c 1.7); b) ii period (m c 1.9); c) iii period (m c 2.1). the a a ' and bb' lines indicate the bvalue cross-sections of figures 5a and 5b, respectively. in figure 4b the red star is related to the 2009 l' a quila earthquake, while the white stars indicate the strongest aftershocks of the sequence. figure 4c shows the 2016 a matrice aftershocks (white stars) with m4.2 and the location of the mainshock (m 6.0) (red star). black squares are the main towns. com p leten ess [enescu et al., 2007]. afterw ard s th e catalo g w as sp lit in to th ree p eriod s (table 1). th e first on e r an ges from annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 2005 to th e tim e ju st before th e l'aqu ila m ain sh ock th at occu rred in ap ril 2009. th e secon d on e in clu d es th e aftersh ocks sequ en ce of l’aqu ila earth qu ake (th e first m on th is n ot in clu d ed in th e an alysis) an d th e seism icity occu rred ju st before th e am atrice earth qu ake th e last tim e p eriod sp an s from th e am atrice aftersh ocks sequ en ce (a few h ou rs follow in g th e m ain sh ock) to th e en d of th e d ataset (sep tem ber 16, 2016). th e begin n in g of th e last tw o p er iod s d oes n ot coin cid e w ith th e occu rren ce of tw o m ain sh ocks becau se it h as been p referred to take in to con sid er ation th e p eriod s w h en th e com p leten ess m agn itu d e reach es stable v a lu es. m c h as been th en recalcu lated as a fu n ction of sp ace for each con sid ered p eriod (figu re 3), by u sin g th e best com bin ation m eth od in softw are p ackage zm ap [w iemer, 2001]. by m ap p in g m c , th e ran ge of m agn itu d es, as w ell as th e sp atial exten t in w h ich th ese m agn itu d es are rep orted com p letely, can be d eterm in ed [wyss an d stefan sson , 2006]. in table i h as been also rep orted th e m c an d th e n u m ber of even ts w ith m >m c con tain ed in each p eriod . on ce th e m c valu e h as been calcu lated , th e m ap p in g of th e b-v alu e in sp ace h as been d raw n for th e th ree catalogs. a grid of 2 km sp acin g an d a fixed n u m ber of even ts (n =80) for each n od e h as been em p loyed . th e sam p lin g volu m es are cylin d er -sh ap ed w ith h orizon tal axes an d r ad ii in versely p rop or tion al to th e seism icity d en sity. we on ly p lot even ts w ith in a m axim u m cylin d er rad iu s ran g in g from 1.0 to 6 km for th e first an d secon d p eriod s, from 1.0 to 5 km for th e th ird on e (figu re 4). th e m axim u m likelih ood estim ation m eth od is u sed to d eter m in e b-valu es [a ki, 1965]. table i: time periods choose for the analysis. m c and events in each period are also reported period s m c n even ts i 16/ 04/ 2005 06/ 04/ 2009 1.7 1187 ii 08/ 05/ 2009 23/ 08/ 2016 1.9 2803 iii 24/ 08/ 2016 18/ 09/ 2016 2.1 1234 in ord er to an aly ze th e b-v alu e in d ep th , th e earth qu akes are p rojected on to vertical cross section s alon g th e strike of th e fau lt (figu re 4 an d figu re 5). th e n od e sep aration for th ese cross section s w as 2 km an d sam p les of n =60 w ere con sid ered . th e m axim u m r ad iu s con sid ered for th ese cross section s is 5 km for both tim e p eriod s. to estim ate th e statistical m ean in gfu l of th e b-v alu e, th e m eth od of shi and bolt (1982) w as em p loyed . to m easu re th e con fid en ce lim its of th e bvalu e an om alies for th e section s, th e utsu 's test [utsu, 1992] w as th en em p loyed . iii. results an d co n clusion s map s of th e sp atial d istr ibu tion of th e bvalu e are sh ow n in figu re 4. for th e fir st p eriod (figu re 4a) th e low e st b valu e (b abou t 0.8) is fou n d in th e area w h ere th e l'aqu ila m ain sh ock w as n u cleated . th e d ecrease in b-valu e cou ld be correlated w ith fau lt asp erity (görgün, 2013). sim ilar low b valu es (~0.8) h as been observed in a region sw of sp oleto (spt). th e b p aram eter, betw een n orcia (n rc) an d cam p otosto (cmp), varies from 1 to 1.4 an d it is very h igh (~2) in th e n orth ern p art of th e area. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 table ii: m ainshock and aftershocks occurred during the 2016 a matrice sequence (24/08/201618/09/2016). the time is reported in utc. tim e yyyy/ m m / d d h h :m m lon (°e) lat (°n ) m l 2016/ 08/ 24 01:36 13.234 42.698 6.0 2016/ 08/ 24 01:37 13.253 42.712 4.5 2016/ 08/ 24 01:56 13.275 42.614 4.4 2016/ 08/ 24 02:33 13.154 42.794 5.3 2016/ 08/ 24 04:06 13.125 42.769 4.4 2016/ 08/ 24 11:50 13.146 42.819 4.5 2016/ 08/ 24 17:46 13.222 42.663 4.2 2016/ 08/ 25 03:17 13.208 42.753 4.3 2016/ 08/ 25 12:36 13.290 42.596 4.4 2016/ 08/ 26 04:28 13.290 42.600 4.8 2016/ 08/ 28 15:55 13.238 42.820 4.2 2016/ 09/ 03 01:34 13.13 42.775 4.2 2016/ 09/ 03 10:18 13.215 42.866 4.3 figu re 4b sh ow s th e after sh ocks sequ en ce of l'aqu ila earth qu ake an d th e seism icity b efore th e m ain sh ock of th e am atrice ear t h qu ake. in th e sam e figu re, th e ep icen ter of th e l’aqu ila m ain sh ock is also p lotted . with resp ect to th e p reviou s p eriod , an in crease of th e b-v alu e in an area betw een cam p otosto (cmp) an d accu m u li (acc) is clearly seen ; at th e sam e tim e, a d ecreasin g of b-valu e is fou n d to th e west of cam p otosto an d am atrice (b~0.9), w h ere m ost of seism icity is co n cen trated . figu re 4c sh ow s th e d istribu tion of b-valu e calcu lated u sin g am atrice aftersh ocks (table ii). th e b-valu es in aftersh ock zon e var y b etw een 0.8 an d 1.6. th e h igh est b-valu es, are fou n d n ear th e m ain sh ock ep icen ter, w h ich is rep orted on ly for com p arison . low est bvalu es are fou n d in th e se (am atrice an d cam p otosto) an d in th e n -n e (betw een arqu ata d el tron to an d ussita) of th e m ain sh ock ep icen ter. to d efin e th e d istribu tion of b-v alu e an om alies in d ep th , before an d follow in g th e m ain sh ock of am atrice earth qu ake, th e bvalu e an alysis h as been p erform ed on th e cross section s aa’ an d bb', p rojectin g th e seism icity w ith in 10 km on each sid e of th e cross section al p lan e (figu res 4b an d 4c). section aa' (figu re 5a) sh ow s an area of an om alou s h igh b-valu e (2.1±0.25) located arou n d accu m oli area, betw een 6 an d 12 km d ep th . a w ell-d efin ed v ariation from h igh to low b-valu e, both vertically an d later ally can be observed in th e area betw een accu m oli (acc) an d cam p otosto (cmp). it m ay be in terp reted as a tr an sition from brittle to d u ctile beh aviou r. th e low b an om aly (abou t 0.9) observed below am atrice (amt) cou ld be con sid ered as a asp erity zon e located at th e bord er of th e area w h ere th e am atr ice eart h qu ake w as n u cleated . th e p test [utsu , 1992] h as been em p loyed to qu an tify th e statistical sign ifican ce of th e an om alou s areas. th e r esu lt of th is test su ggests th at th e h yp oth esis th at th e exam in ed sam p les h ave th e sam e bvalu es, can be rejected as it is n ot statistically sign ifican t at th e 99% level (left bottom p an el in figu re 5a). figu re 5b sh ow s a region at th e se of th e am atrice m ain sh ock h yp ocen ter of h igh b valu e (1.65±0.2) arou n d 10 km . above am atrice an d n orcia areas are clearly ev id en t tw o zon es w ith low b-valu es (0.8±0.1) w h ere th e m ajority of aftersh ocks w ith m>4.2, occu rred in th is p eriod , are con cen tr ated . https://en.wikipedia.org/wiki/statistically_significant https://en.wikipedia.org/wiki/statistically_significant annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 figure 5. cross-sections of the distribution of b-values along the profiles shown in figures 4b and 4c. (a) a a ' cross-section. (b) bb’ cross-section. the red and grey circles represent the volumes of high and low bvalues used to calculate the fm ds. the white stars represent the aftershocks occurred during the a matrice sequence. the red star is related to the 2016 a matrice mainshocks. th e p robability th at th e tw o sam p les w ith h igh an d low b-valu es belon g to th e sam e p op u lation is p= 7.7e-003, accord in g to th e utsu test.th ese resu lts w ell m atch a p relim in ary fau lt slip in version th at su ggests th e p resen ce of tw o h igh ly stressed p atch es of co-seism ic d efor m ation located se an d n w of th e h yp ocen ter w h ich ru p tu red w ith th e m ain sh ock of 24 au gu st [gruppo di lavoro in gv sul terremoto di a matrice, 2016]. su ch annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7273 p atch es, w h ich are correlated to low b-valu es m ay h ave h igh p oten tiality to ru p tu re cau sin g a p ossible m od erate or larger even t (görgün, 2013; tormann et al., 2013 an d referen ce th erein ). du rin g th e review p rocess a stron g even t (m w 6.5) occu rred on 30 october 2016 (06:40:17 utc) 5 km from n orcia (42.84°n 13.11°e) an d 9 km d ep th . th is even t w as p receed ed by tw o oth er stron g sh ocks (m agn itu d e m w 5.4 an d m w 5.9) on 26 october 2016, betw een n orcia an d ussita. th e m w 6.5 n orcia earth qu ake is th e m ost p ow erfu l earth qu ake strikin g italy sin ce th e 1980 m w 6.9 irp in ia even t. in terestin gly, th e n orcia earth qu ake is located at th e bord er w h ich d elim its th e low b-v alu e area h igh ligh ted by ou r an alysis below n orcia (figu re 4c an d 5b),th e an alysis p erform ed so far an d d escribed in th is p ap er is still in p rogress an d w ill be im p roved by exten d in g th e p eriod of aftersh ocks to m ap th e d istribu tion of th e bvalu e alon g th e fau lt p lan es resp on sible both th e m l 6.0 am atr ice an d m w 6.5 n orcia earth qu akes. th e fu tu re resu lts w ill be com p ared w ith oth er m u ltid iscip lin ar y stu d ies to better u n d erstan d th e earth qu ake occu rren ce in th is area. th is an alysis su ggests th at m on itorin g th e evolu tion of b-v alu e m igh t be u sefu l for th e evalu ation of seism ic h azard an d eart h qu ake forecastin g. ackn owledgmen ts we th an k r. con sole for h is u sefu l com m en ts w h ich h elp ed u s to im p rove th e m an u scrip t an d d. march ese for u sefu l d iscu ssion s on th e p ap er. th e zm ap softw are (wiem er, 2001) w as u sed for th e com p u t ation of b-valu es. referen ces aki, k. 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(2000). min im u m m agn itu d e of com p leten ess in earth qu ake catalogs: exam p les from alaska, th e western un ited states, an d jap an , bu ll. seism . soc. am ., 90, 859–869. wyss, m. an d stefan sson , r. (2006). n u cleation p oin ts of recen t m ain sh ocks in sou th ern icelan d m ap p ed by b -valu es, bu ll. seism ol. soc. am ., 96, 599–608, d oi:10.1785/ 0120040056. *corresponding author: caterina montuori, istituto nazionale di geofisica e vulcanologia, rome, italy; email: caterina.montuori@ingv.it; tel: (+39)0651860496 layout 6 1 annals of geophysics, 64, 6, se657, 2021; doi:10.4401/ag-8633 o p e n acc e s s landslide investigation using seismic refraction tomography method: a review parisa imani1,2, amr abd el-raouf*,1,3 and gang tian1 (1) school of earth sciences, zhejiang university, hangzhou, china (2) school of mining, petroleum and geophysics engineering, shahrood university of technology, shahrood, iran (3) geology department, faculty of science, zagazig university, zagazig, egypt article history: received january 26, 2021; accepted september 20, 2021 abstract since the early 1960s, the near-surface seismic refraction method has been extensively used as a non-invasive and cost-effective geophysical method to characterize geological structures for landslide investigation. seismic refraction tomography (srt) is considered a modern interpreting seismic refraction data that allows for determining lateral variations of p-wave velocity. this geophysical technique can characterize the slope material, the sliding surface’s geometry, the landslide mass movement, the physical properties of media, and the water saturation effects on the slope. therefore, this method has become an appropriate method due to the increasing progress of novel algorithms and the improvements of field-data collection systems. this paper reviews the essential research investigating various types of landslides influenced by water saturation and landslide materials and identified in various areas since 2000. significant conclusions obtained by applying different survey strategies and data processing algorithms in seismic refraction surveys are widely discussed, concentrating on the advantages and disadvantages of this method. the main results obtained by the few time-lapse srt (tlsrt) are mainly analyzed. keywords: landslide; near-surface; seismic refraction tomography (srt); time-lapse srt (tlsrt); monitoring. 1. introduction landslides are the mass movement of debris, rock, and earth downslope. they are considered one of the natural hazards in many parts of the world. landslides cause losses of lives and properties [perrone et al., 2014]. they are triggered by different mechanisms (e.g., typhoons, precipitation, and earthquakes) [heincke, 2005]. most of them occur on unstable slopes caused by meteorological events that increase the probability of slope failures [göktürkler et al., 2008; imani et al., 2021a; imani et al., 2021b; kul yahşi and ersoy, 2018; ozcep et al., 2012; samyn et al., 2012; yilmaz and kamaci, 2018]. the detection and characterization of landslide bodies require a multidisciplinary method based on combined geotechnical techniques, geophysical surveys, geological setting investigations, ground-based approaches, and airborne approaches. however, the geotechnical and geological approaches cannot be applied for investigating the subsurface structures; instead, they are typically used in investigating borehole samples and surface structures [heincke, 2005]. airborne and remote sensing approaches aim at improving landslide investigations with their ability to analyze prior events; monitor surface changes, such as outcrop failures, displacements, geomorphologic settings during a specific period, and landslide activities; and evaluate landslide susceptible regions [mantovani et al., 1996; metternicht et al., 2005; scaioni et al., 2014]. scaioni et al. [2014] categorized the remote sensing methods into four classifications: passive optical sensors, thermal infrared remote sensing, microwave sensors, and airborne and terrestrial laser scanning. ventura et al. [2011] applied multi-temporal airborne light detection and ranging (lidar) data to investigate complex landslides in southern italy. a combination of lidar and inclinometer information could produce 3d images of the rockfall site’s surficial and inner kinematics [tomás et al., 2018]. baldi et al. [2008] used archival photogrammetry and global positioning system (gps) data on landslide kinematics. the tiltmeter, conventional inclinometer, and piezometer are other tools applied to the regions influenced by landslide events to characterize the subsoil’s hydraulic and mechanical properties [perrone et al., 2014; uhlemann et al., 2016] at a certain point. despite all the capabilities of ground-based and airborne methods, they are not able to study the characterization of subsurface materials and the lateral extent. the application of near-surface geophysical techniques has extensively increased. these methods can give reliable results obtained from the slope with a non-invasive, quick, and user-friendly technology [al-saigh and aldabbagh, 2010; bekler et al., 2011; bichler et al., 2004; capizzi and martorana, 2014; chen et al., 2019b; donohue et al., 2012; ferrucci et al., 2000; göktürkler et al., 2008; grit and kanli, 2016; havenith et al., 2002; havenith et al., 2000; imani et al., 2021a; imani et al., 2021b; jacob et al., 2018; jongmans et al., 2009; jongmans and garambois, 2007; karslı et al., 2017; kim et al., 2011; kul yahşi and ersoy, 2018; mauritsch et al., 2000; mezerreg et al., 2019; ng et al., 2015; nwankwo and ugbena, 2019; otto and sass, 2006; ozcep et al., 2012; samyn et al., 2012; solberg et al., 2012; tomás et al., 2018; travelletti et al., 2010; wang et al., 2016; yilmaz and kamaci, 2018]. geophysical techniques can locate the water content and assess the volume of the landslide body and the physical properties of landslide materials. the selected investigation methods depend on the contrasts of the physical parameters. for investigating other parameters, such as the landslide volume, morphology, and penetration depth, the equipment set up on the investigation area should be accurately considered [jongmans and garambois, 2007]. geophysical procedures can characterize a large volume of subsurface materials by overcoming specific point measurements. although the conventional seismic refraction method is frequently employed in landslide studies because of the availability of the factors that influence seismic velocities and spatially and temporally diversify them, the seismic refraction tomography (srt) method is considered as the most commonly used geophysical technique in landslide investigations with high-resolution illustrations [hack, 2000; mccann and forster, 1990]. this technique is significantly applied as a modern geophysical procedure of interpreting seismic refraction data that employ forward and inversion modeling methods to estimate the velocity in 2d and 3d surveys [al-heety and shanshal, 2015]. the srt allows the interpretation of velocity variations as gradient and discrete layers [azwin et al., 2013]. this method measures the subsurface seismic velocities and depicts images of underground geological settings. in general, seismic velocity is affected by the rock properties, such as the water saturation, material compositions, weathering rate, and porosity volume [bogoslovsky and ogilvy, 1977; kaminskiy, 2013; kassab and weller, 2015]. among these factors, the water content and porosity play an essential role in increasing the pore water pressure, reducing the shear strength, and finally triggering the landslide [imani, 2020; imani et al., 2021b; xu et al., 2016; yalcin, 2007]. these are the specific reasons for applying seismic refraction tomography to characterize landslide materials’ physical properties and delineate the boundaries and surface failures. to have a successful seismic refraction project with useful and reliable results, great attention should be paid to the survey strategy and data processing algorithm to be performed on the specific landslide type and materials. the survey layout should be designated to cover the considered area extensively and to suit the lithological, geological, and topographic conditions of the study area. additionally, the algorithm processing data should be able to separate noises from the signals carrying subsoil information and to present the subsurface model with a high resolution and no ambiguity. in most landslide studies, seismic refraction methods are integrated with other geophysical techniques to obtain accurate and reliable results from heterogeneous landslide structures. the expected contrast of the physical properties, penetration depth, accuracy, landslide characteristics, and economic factors allow the researchers to choose a suitable geophysical method to combine with the srt technique. hack [2000] used various types of geophysical methods to investigate slope stability. he concluded that using the combined methods helped to detect different subsurface structures that were impossible to visualize using a single method. parisa imani et al. 2 mccann and forster [1990] applied several surface geophysical methods, such as seismic refraction, microseismic monitoring, electrical resistivity, the magnetic method, electromagnetic surveys, and boreholes, to illustrate the geometry of landslides, to characterize the mass movements, and to detect the hydrogeological regime and water content. bárta et al. [2005] surveyed the sudety mountains using seismic and dynamic tests, gravity measurements, natural electric fields, the resistivity method, and ground-penetrating radar (gpr). there are no obvious conclusions from this paper; however, they had an excellent experience using resistivity tomography. baron and supper [2013] studied in 14 european and asian countries using lidar airborne laser scanners (als), geophysical logging, aerial imaging, resistivity measurements, ground-based synthetic aperture radar interferometer (gb insar), and the seismic refraction method. based on this study, the electromagnetic (em) method, seismic survey, and dc resistivity were the most reliable geophysical techniques for landslide investigation. despite the use of combined methods, srt technique is of great importance in landslide studies. the current study presents the results of landslide research articles, mainly focused on detecting water contents, published in valid journals and research dissertations applying seismic refraction tomography method (srt) since 2000. this paper is concentrated explicitly on various data acquisition systems and data processing methods for different landslide typologies. the present research recognizes the limitations and potentials of the refraction method in landslide studies. 2. materials and methods 2.1 application of seismic refraction tomography in landslide investigation since the early 1960s, the seismic refraction method has been considered one of the most commonly used techniques in near-surface geophysical projects, particularly in landslide studies [armstrong et al., 2011; hack, 2000]. since the srt indicates the subsurface materials as continuous media where the first break recorded does not strictly depend on materials or refractors with a high contrast of velocities [rucker, 2000], the refraction tomography is ideal for characterizing the area with strong gradients of lateral velocities. the seismic refraction tomography can be applied in high precipitation zones to provide the subsoil images based on seismic velocity increasing at greater depths. the velocity illustrations are analyzed and interpreted according to velocity contrasts, which are chiefly caused by various lithologies and variations in water saturation. in other words, determining the landslide lateral extension and depth and distinguishing the boundaries among subsurface horizons of lithologies characterized by increasing wave velocity affected by different densities are the main tasks of the srt approach [bekler et al., 2011; bogoslovsky and ogilvy, 1977; brooke, 1973; narwold and owen, 2002; palmer and weisgarber, 1988]. in the field procedure, the seismic signals travel down through the earth and refract and diffract from discontinuities due to the increasing velocity with depth [göktürkler et al., 2008]. reducing the rigidity of the landslide mass and the presence of fractures decreases the seismic velocity propagating through the landslide body [cummings and clark, 1988]. based on the obtained results by bogoslovsky and ogilvy [1977] and otto and sass [2006], the seismic velocity as the mechanical property of landslides can characterize the materials and delineate the failure surface due to the nature of lithological features and the contrast among various kinds of media. the srt can also illustrate the subsoil slopes. the slope investigations can be hard to survey due to placing the geophones and sources in the access points; however, a seismic refraction technique can be performed easier on such conditions [hack, 2000]. however, state-of-the-art studies accomplished on slope deformation, regardless of the type of landslides, applied seismic refraction as the preferred geophysical method [pazzi et al., 2019]. the current review represents the most useful information from the investigated studies in table 1. this table reports the research studies in detail, including the first author’s name, the journal-title, the publication year, the landslide typology, the geological setting of sites, the processing approach, etc. 3 applications of srt for landslide monitoring parisa imani et al. 4 authors journal year type of landslide geological settings data processing method other methods signal source supplier havenith et al. surveys in geophysics 2000 complex slide (debris slide and multirotational) arenite and clay no info. electrical tomography and surface wave method explosive mauritsch et al. engineering geology 2000 rockfall and debris slide limestone, dolomitic conglomerate, sandstone, marls and shales generalized reciprocal method (grm) geoelectric method and electromagnetic (em) hammer and explosive ferrucci et al. engineering geology 2000 deep-seated gravitational movement gneiss, phyllite, and alluvial deposit static and topographic corrections, enhanced by filtering and migration seismic reflection and borehole dynamite and hammer havenith et al. bulletin of the seismological society of america 2002 rockslide homogeneous granite massif software sardine seismological data sledgehammer and explosive glade et al. bulletin of engineering geology of environment 2004 translational slide limestone to marly clay software refra (intercept-time and grm) borehole hammer bichler et al. landslides 2004 earth slide and debris flow silty sand to clay no info. ground penetrating radar (gpr), reflection, direct current resistivity, and surface wave method hammer godio et al. engineering geology 2006 rockslide metamorphic rocks simultaneous iterative reconstruction technique (sirt) electrical resistivity tomography (ert), horizontal seismic profile (hsp), surface wave method (swm), and borehole hammer otto and sass geomorphology 2006 talus slope regolith, red rock reflexw (wavefrontinversion), network-raytracing, and tomography gpr and 2d resistivity sledgehammer heincke et al. geophysics 2006 rockfall and rock slide fractured and gneissic rock mass tomographic inversion of traveltimes sensitivity tests and borehole dynamite göktürkler et al. journal of applied geophysics 2008 mass movement in rotational form marl, limestone, conglomerat, and flysch least-squares ert sledgehammer jongmans et al. engineering geology 2009 complex saturated laminated clay sirt seismic noise measurement, ert, geotechnical data, inclinometric, and piezometric data explosive al-saigh and al-dabbagh geological society of india 2010 no info clayey marls, marly limestones, limestones, gypsum layers, gypsum blocks regression and mota parameters stability analysis and borehole sledgehammer travelletti et al. geomorphology 2010 deep-seated landslide and debris flow gypsum and glacial material intercept-time method improve by raytracing sloping local base level (slbl) and seismic reflection hammer kim et al. environmental earth science 2011 mass movement clay, siltstone, and sandstone software gli3d (the generalized linear inversion) electrical resistivity, seismic reflection, and borehole hammer bekler et al. journal of environmental and engineering geophysics (jeeg) 2011 rotational slide and creep coarse-grained sandstone, mudstone, shelly limestone, marl, and siltstone forward model using eikonal equation and topographic inversion approach hydrometer analysis and ert no info armstrong et al. itb journal of engineering science 2011 no info tuff and clay neural network no info no info samyn et al. journal of applied geophysics 2012 complex (flow and slide types) in deep-seated fractured flysch, marl, schist second-order fast marching method (fmm), 3d fmm, and sirt geotechnical sounding and borehole dynamite zainal abidin et al. procedia engineering 2012 creep black argillaceous rock, sandstone, and mudstone software optim (seisoptpicker and seisopt@2d processing) prior borehole data sledgehammer 5 applications of srt for landslide monitoring table 1. studies using conventional seismic refraction and seismic refraction tomography methods in landslide investigations and published in valid journals since 2000. donohue et al. european association of geoscientists & engineers (eage) 2012 quick-clay clay and salt content software gremix including plus-minus method, grm, and slope-intercept em conductivity mapping, ert, multichannel analysis of surface waves (masw), rotary pressure sounding, cone penetration tests (cpt’s), and borehole sledgehammer solberg et al. bulletin of engineeringgeology of environment 2012 quick-clay clay, silt, sand, active erosion, and ice-marginal deposit no info resistivity measurement and geotechnical investigation dynamite ozcep et al. environmental earth sciences 2012 slope stability silty clay, clayey sand, claystone, clay software seisimager (time-term inversion) masw, in-situ testing (spt), and borehole no info. capizzi et al. journal of geophysics and engineering 2014 translational slide calcarenites, silt, sand, and clay wave path eikonal travel time (wet) method, smooth inversion ert, horizontal-tovertical spectral ratio (hvsr), and masw no info. soon min et al. electronic journal of geotechnical engineering (ejge) 2015 complex weathered schist, sandstone, and shale software seisimager (time-term inversion) borehole sledgehammer grit and kanli open geoscience 2016 no info conglomeratesands tone, clay, limestone raytracing technique and sirt seismic reflection and masw sledgehammer uhlemann et al. journal of applied geophysics 2016 earth slide and earth flow grey to dark-grey mudstone and siltstone inversion algorithm using elastic moduli and regularization borehole sledgehammer wang et al. tectonophysics 2016 quick-clay clay raytracing algorithm for sparse linear equation and sparse least-squares (lsqr), and regularization radiomagnetotellu ric (rmt), seismic reflection, and borehole weight-drop, explosive, and hammer karsli et al. eurasian journal of soil sciences 2017 historical landslide and lake sediments basalt, gravely silty clay, clay tomographic method refraction microtremor (remi), masw, seismic reflection, borehole, and geotechnical methods sledgehammer jacob et al. engineering geology 2018 rockfall and flow sand, clay, and chalk cliff wet gravity, borehole weight-drop and explosive kul yahşi and ersoy journal of geophysics and engineering 2018 slope stability clay, silt, and sand no info. gpr, ert, and borehole no info yilmaz and kamaci international journal of computational and experimental science and engineering 2018 slope stability clay no info. vertical electrical sounding (ves) no info. mezerreg et al. journal of african earth sciences 2019 creep matrix of sand-clay, sandstone pebbles and limestone debris software reflex w (wavefrontinversion) ert, borehole, and geotechnical methods hammer nwankwo and ugbena world scientific news 2019 no info. sandstone, conglomerate, shale, grey mudstone software reflex w (wavefront-inversion) ves, ert hammer chen et al. journal of environmental and engineering geophysics 2019 rockfall silty clay, mudstone, sandstone lsqr ert, scattered wave imaging (ssi), and borehole hammer 2.2 data acquisition the srt survey design for characterizing landslides must be adjusted when applied in different studies. after a careful reconnaissance of the topographic conditions, landslide type, hydrogeological and lithological characteristics of the study area, landslide extent, and objective depth, a survey strategy should be carefully designed to cover stable and unstable terrains. in other words, generally, seismic refraction lines should be positioned over landslide scars and beyond that to compare the subsurface images. the profiles are typically extended closest to the borehole location drilled into a catastrophic region [abraham and lucius, 2004]. the profile length depends on the constraint of the landslide site extension and should be approximately three to five times the investigation depth. the number of seismic profiles depends on the size of the investigation area. for better illustration of the underground in landslide studies, it is necessary to spread out at least three longitudinal lines (along with the sliding direction) and several traverse profiles positioned over the failure surface and beyond that [bogoslovsky and ogilvy, 1977]. the in-field procedure is conducted with three main seismic equipment elements, including a seismic source and several seismic receivers connected to the seismograph through cables. the geophones (receivers) connected to the seismograph through cables receive the signals generated by sources. the detectors are applied for travel time measurement. knowing the values of time and depth, the apparent velocity values characterizing the subsurface studied can be computed. the calculated values and inverted velocities are placed at their positions to represent the subsoil velocity images. the geophones and sources are deployed on a linear strategy system in refraction surveys and usually placed along a single line. this means that the srt is generally carried out along a line of geophones with endand offend sources at each end of the profiles (figure 1). additionally, for the in-field procedure, the travel time quality control (qc) of seismic refraction data is essential due to the expensive and time-consuming fieldwork. according to seismic imaging, shorter travel times are related to the geophones placed near sources, and longer travel times are related to the receivers placed far away from sources [göktürkler et al., 2008]. imani [2020] and adamczyk et al. [2013] controlled the quality of travel times by blowing at the shot point at a distance of 0 m during acquiring data. as shown in figure 2, according to the seismic traces, shorter travel times belong to receivers located near the sources, and longer travel times come from far sources. the resolution of images obtained by refraction survey is influenced by the receiver spacing, signal source frequency, and the shot number [hack, 2000]. the geophone spacing and frequency should be based on an on-site extension and soil heterogeneity, respectively [armstrong et al., 2011]. the frequency of the geophones should be adequate to sliding objectives. typically, in shallow seismic investigations, particularly landslide studies, the natural frequency used is 4.5 hz [capizzi and martorana, 2014; glade et al., 2005; havenith et al., 2002; havenith et al., 2000; imani, 2020; imani and tian, 2018; imani et al., 2021b; jongmans et al., 2009; karslı et al., 2017; kim et al., 2011; uhlemann et al., 2016; vanlı senkaya et al., 2019]. however, geophones with higher frequencies (10, 14, and 28 hz) are likely applied in sliding studies [adamczyk et al., 2013; bekler et al., 2011; donohue et al., 2012; glade et al., 2005; godio et al., 2006; grit and kanli, 2016; jacob et al., 2018; mezerreg et al., 2019; ng et al., 2015; travelletti et al., 2010; wang et al., 2016; zainal abidin et al., 2012]. since the reliability of the refraction results depends on the accuracy of the picking first break, the first onsets should be recognizable. lower frequency geophones are more parisa imani et al. 6 figure 1. schematic image of typical 2d seismic refraction survey strategy. sensitive to arrival signals, even including noises of higher frequency, and require data with more unwanted noises. at the same time, this can present geological structures with more details. therefore, locating the onset of the arrivals acquired from landslide sites with heterogeneous surfaces can be troublesome and requires great attention while picking. however, no case of inefficiency of various geophone frequencies has been reported in this paper. seismic signals are generated by various sources, such as explosives, hammers, weight-drops, and vibroseis. impulsive sources are selected based on the geological conditions, efficiency, accessibility, cost-effectiveness, safety, generation of more energy, and reproducibility. the anomaly depth, the required resolution, good quality data, and environmental friendliness are also factors considered while choosing the signal source [malehmir et al., 2013b]. for years, the sledgehammer blown on a metal plate was known as the low-frequency seismic signal source in geophysical projects and was much safer than other sources to use in early-trigger landslide regions. one more benefit of using a sledgehammer is to create a high-resolution image of subsoil heterogeneities [havenith et al., 2002]. since most landslide sites are investigated on the facing after the events, the heterogeneity of the failure surface causes more seismic signal attenuation and leads to a low penetration depth [imani et al., 2021a; imani et al., 2021b]. therefore, a heavy hammer should be considered to generate energetic signals with high penetration depths [imani, 2020]. figures 3a and 3b show the various data quality of the two raw travel times triggered by 5 kg and 9 kg, respectively. in this study, energetic signals have been generated by a heavy hammer with the same geological conditions, operator’s skill, and strength. if the sledgehammer as a seismic source cannot supply enough energy into the earth, it is suggested to switch the signal source to explosives or weight-drops [mauritsch et al., 2000]. they are more expensive and dangerous and require more safety precautions while using in landslide sites [rashed, 2009]. malehmir et al. [2013a] and jacob et al. [2018] introduced weight-drops and dynamite as good sources with high-frequency content and energy for quickclay data analysis. they proved that the explosives have enough potential to map deep bedrock, the reflected waves, and wide-angle reflections. in very shallow depths, dynamite loads can provide a dominant frequency of 45 hz in the recorded range from 25 to 200 hz, providing sufficient energy into the subsoil [samyn et al., 2012]. however, due to the sensitivity and instability of landslide sites, it is recommended to employ deep shots to prevent further destruction caused by triggering landslides [heincke, 2005]. based on table 1, most of the studies (53%) on landslide regions with different rock characterizations used a sledgehammer to supply the signal source [al-saigh and al-dabbagh 2010; glade et al., 2005; godio et al., 2006; göktürkler et al., 2008; havenith et al, 2002; kim et al., 2011; mauritsch et al., 2000; travelletti et al., 2010; zainal 7 applications of srt for landslide monitoring figure 2. seismic travel times are created by a shot blown on a few centimeters from a distance of zero. the solid red line shows the picked first arrivals [imani, 2020]. abidin et al. 2012 ; donohue et al., 2012; hibert et al., 2012; ng et al., 2015; imani, 2020]; imani and tian, 2018; imani et al., 2021b; karslı et al., 2017; mezerreg et al., 2019; uhlemann et [2016; wang et al. 2016], a lower percentage (15%) applied explosives [havenith et al., 2000; heincke et al., 2010; jongmans et al., 2009; samyn et al., 2012; solberg et al., 2012]; 15% considered the combined sources, i.e., explosives, handy hammer and weightdrop [ferrucci et al., 2000; havenith et al., 2002; jacob et al., 2018; mauritsch et al., 2000; wang et al., 2016]; and there is no information regarding a signal supplier for 17% (figure 4) [bekler et al., 2011; capizzi and martorana, 2014; kul yahşi and ersoy, 2018; ozcep et al., 2012; sompotan et al., 2013; yilmaz and kamaci, 2018]. parisa imani et al. 8 figure 3. seismic traces were obtained from different signal sources along with the seismic profile [imani, 2020]. (a) a 5 kg sledgehammer triggers the signals. (b) a 9 kg sledgehammer triggers the signals. figure 4. percentage proportions in the types of sources presented in the review. the pie graph illustrates that most studies considered have applied the sledgehammer as the single source in landslide study. 9 applications of srt for landslide monitoring figure 5 shows the comparison among the power spectra for a sledgehammer, weight-drop, and dynamite. the weight-drop sources have intermediate frequency content with broader bandwidth spectra, better resolution, and higher amplitude than the sledgehammer and dynamite [malehmir et al., 2013a]. dynamite provided the highest frequency with the best signal-to-noise (s/n) ratio compared with the other sources [malehmir et al., 2013a]. the weakest signals belong to the sledgehammer. comparing the weight-drop, shotgun, and hammer in saturated and fine-grained materials proved that the shotgun could produce a higher frequency signal than the weight-drop could (figure 6) [pullan and macaulay, 1987]. figure 5. comparisons of power spectra with dynamite, sledgehammer, and weight-drop as the signal sources [malehmir et al., 2013a]. figure 6. the plotted power spectra from the hammer, weight-drop, and shotgun as signal sources [pullan and macaulay, 1987]. the number of shot points and their positions are the other parameters considered to provide high-resolution results. the multiple shot-points along with the survey line can produce more accurate data [donohue et al., 2012; jacob et al., 2018; mezerreg et al., 2019; narwold and owen, 2002], better-layered velocity models, and interface depth estimation [rucker, 2000]. samyn et al. [2012] designated five profiles of sources perpendicular to the sliding direction and four geophone lines in the sliding direction in rugged topography and complex (flow and slide type) landslide studies using 3d srt (figure 7). this survey layout, including several source lines, can provide homogeneous illumination and high 3d coverage of the subsurface. furthermore, data stacking of shots can significantly improve the resolution of images. to reach this goal, each shot point should be repeatedly stacked to increase the signal to noise ratio (s/n) and signal improvement [alsaigh and al-dabbagh, 2010; al-saigh, 2010; bekler et al., 2011; capizzi and martorana, 2014; göktürkler et al., 2008; imani, 2020; karslı et al., 2017; kim et al., 2011; malehmir et al., 2013a; otto and sass, 2006; uhlemann et al., 2016]. as there is a frequent problem in landslide investigation and its soil conditions, and that is the incompact soil observed in post-event areas, which can prevent seismic wave penetration to greater depths due to wave attenuation and signal scattering [bruno and martillier, 2000], it is strongly suggested to significantly increase the s/n ratio and to carefully choose the acquisition parameters and signal sources [bruno and martillier, 2000]. the overlapped refraction survey method can also cover the gaps between shots and prevent missing data while acquiring the data [glade et al., 2005]. figure 8 depicts a survey layout by bekler et al. [2011]. the overlapping degree is at six receivers, each of them planted at every shot point, with one reverse and one forward off-end shot. parisa imani et al. 10 figure 7. presentation of the study area orthophotograph overlapped by the seismic refraction tomography (srt) survey strategy, including the receiver and source lines [samyn et al., 2012]. 11 applications of srt for landslide monitoring the laser-tachymeter system is rarely applied for the accurate determination of receiver spreads, shot point layout, topographic conditions, and the surface of the ground along with the seismic lines in landslide investigations [glade et al., 2005]. significantly, in landslide monitoring projects, this system helps to investigate changes in the surface elevation, considers seismic profile alignments, and locates the geophone and shot points. the survey design explained above should be considered in 2d or 3d illustrations to show the complicated structure of landslide sites. these imaging methods make the subsoil images more attractive and informative for landslide investigations. although the relationship between the geophysical properties and geotechnical and geological parameters is still considerable, the 2d and 3d imaging procedures play a brilliant role in monitoring and characterizing landslides. these techniques are interested in depicting and evaluating the landslide geometry, the physical properties of sliding mass, the water table location, and interflow direction. the 2d and 3d images appeared through a strategy survey and inversion process [bogoslovsky and ogilvy, 1977]. despite the three-dimensional nature of landslides, there are very few studies that applied the 3d srt survey [heincke et al., 2010] in landslide studies, while the 2d srt survey is more attractive to use in characterizing rotational/translational slide types [armstrong et al., 2011; capizzi and martorana, 2014; göktürkler et al., 2008; grit and kanli, 2016], flow and slide typology [samyn et al., 2012; travelletti et al., 2010], creep [zainal abidin et al., 2012], complex [jongmans et al., 2009], rockslide [havenith et al., 2002], unstable slope [otto and sass, 2006], debris flow, and soil slide [imani and tian, 2018; imani et al., 2021b]. as landslide structures are 3d phenomena and the obtained images are 2d, the illustrations may be influenced by strong artifacts that can be difficult to identify. therefore, 2d and 3d forward modeling can solve this problem and easily assess image reliability [jongmans and garambois, 2007]. those studies performed using 3d seismic surveys illustrate the 3d geological structure of landslides and delineate the landslide geometry [samyn et al., 2012; uhlemann et al., 2016]. samyn et al. [2012] recommended surveying the complex structure and process data in 3d imaging. figure 9 shows the seismic velocity distribution in flow and slide types in a deep-seated landslide with low to high-velocity values and distinguishes the morphological features within the landslide [samyn et al., 2012]. this 3d velocity model provided good agreement with the 2d srt section. therefore, the obtained results can define the reliability of the 2d model as well as the 3d. after survey strategy design, the type of seismic wave should be considered in sliding studies. it can be p-waves (primary or compression wave) or s-waves (secondary or shear wave). various properties of landslide materials can affect the behavior of different types of waves. p-waves are typically determined in landslide investigations because they are easily generated and have more sensitivity to the material’s normal stiffness changes. at the same time, s-waves are affected by the material shear stiffness [hack, 2000]. p-waves and s-waves simultaneously present lower values in landslide materials, particularly after sliding (e.g., deep-seated creep based on mauritsch et al. [2000] and higher values in landslide bedrock [bichler et al., 2004; jongmans et al., 2009; uhlemann et al., 2016]. in most studies carried out in landslide areas using srt, the first break collected included p-wave data [armstrong et al., 2011; bekler et al., 2011; capizzi and martorana, 2014; donohue et al., 2012; ferrucci et al., 2000; göktürkler et al., figure 8. a schematic of the overlapped survey strategy of the srt method [bekler et al., 2011]. 2008; grit and kanli, 2016; havenith et al., 2002; havenith et al., 2000; heincke, 2005; imani, 2020; imani and tian, 2018; jacob et al., 2018; karslı et al., 2017; mezerreg et al., 2019; nwankwo and ugbena, 2019; ostrowski and lasocki, 2018; otto and sass, 2006; samyn et al., 2012; travelletti et al., 2010; vanlı senkaya et al., 2019; zainal abidin et al., 2012]; however, some literature supplemented the landslide studies using s-wave refractions [bichler et al., 2004; jongmans et al., 2009; ozcep et al., 2012; uhlemann et al., 2016; yilmaz and kamaci, 2018]. as p-wave velocity (vp) is controlled by the water content and saturated clay, and slightly influenced by gravitational deformation [godio et al., 2006], the s-wave velocity (vs) can characterize the landslide structure and its activity, material porosity, and compaction [grandjean and sage, 2004]. the most sensitive variant to deformation is caused by the saturated clay contents [jongmans et al., 2009]. to characterize the quick-clay landslides and determine the elastic constants, integration of the pand s-wave velocity models can obtain more reliable results and helpful information on the hazard analysis [adamczyk et al., 2013]. in quick-clay zones with high risk, it is often suggested to investigate beyond the considered zone to evaluate the sand stability [solberg et al., 2012]. uhlemann et al. [2016] collected the vp and vs to characterize earth slide and earth flow in the uk. in this study, the wave velocities were too low to image the lithological structures at the site. still, subsoil imaging was improved with the poisson’s ratio from the velocity distributions. the advantage of collecting vp and vs is to show a substantial disparity created by the water table absence in low precipitation seasons or areas [capizzi and martorana, 2014]. despite the benefits of determining pand s-wave velocities simultaneously, it is probable that the resulting images may not be high resolution in high-slope cases (30–40 degrees) where operators confront the difficulties of data acquisition and coupling on the surface [bichler et al., 2004]. however, when additional wave types are analyzed, supplementary information about the mechanical properties of the near-surface geological materials becomes available, although at the cost of invoking more complicated procedures [bichler et al., 2004]. parisa imani et al. 12 figure 9. 3d seismic tomogram at the la valette applied on a complex landslide to outline the boundary between the landslide and the surrounding stable slopes [samyn et al., 2012]. 13 applications of srt for landslide monitoring 3. data processing and interpretation the processing and interpretation of seismic refraction data involve the models of primary or secondary wave velocities based on inverse or forward modeling of the obtained travel times. over the last decades, the inversion method as a more efficient and faster modeling tool mainly replaced the forward technique for characterizing landslide areas. compared to the inversion procedure, the forward modeling technique attempts to fit the obtained data from several source locations simultaneously through trial and error. the manual process is a challenging, tiring, time-consuming method. the modification sequence of forward modeling defines subjectivity into the final model and depends on the interpreter’s skill and experience. the inversion results also depend on the accuracy of picking first breaks, the choice of processing settings, the parameters set only once, and the computations are easily iterated. however, forward modeling was able to identify the reliability of the obtained illustrations as the most crucial advantage of this modeling method. since landslide bodies have complicated settings, seismic inversion models can develop the map of subsoil velocity contrasts with a higher resolution and as less time consuming than forward modeling [bekler et al., 2011]. there was only one study found that identified the landslide characteristics. bekler et al. [2011] employed forward, and inverse modelings to access reliable travel time using a finite-difference solution of the eikonal equation in the rotational slide and creep types. this combination can produce a very accurate subsurface model composed of the saturated clay-rich and sand-rich zones. the studies that used inversion techniques are collected in table 1. the inversion of seismic data can be a reliable and standard way to estimate landslide characteristics [nolet, 1987]. a primary model, an inversion problem solver to determine travel times and ray paths, a proper algorithm, and control of the tomogram quality are the essential inversion steps [heincke et al., 2006]. the combination of inversion methods and geological/geotechnical information can also clarify the subsurface structures’ ambiguities. also, the stratigraphic information obtained from geological/geotechnical methods, such as boreholes, can calibrate the inversion results and image resolutions [capizzi and martorana, 2014]. for the processing of seismic data, selecting the correct processing sequence and proper variants, assessing the results, and determining deficiencies by choosing the inappropriate parameters are the three essential steps [yilmaz, 1987]. the srt data gives reliable results in landslide investigations after considering these points. since the objective of every seismic method is to achieve the subsurface image, the recorded data should be converted to sections processed by different algorithms. the seismic sections aim to detect the lateral and vertical velocity changes, the up and down slopes, the discontinuous refractors, and the diffracted blocks through the landslide mass [narwold and owen, 2002]. however, the variety of subsurface sediments and sliding material properties influence the seismic velocities illustrating different horizons [abramson et al., 2002]. in this part of the paper, we attempt to collect the seismic refraction processing methods that have been used to identify landslide characterizations. since 1975, computer processing algorithms for seismic refraction data have been developed by the geological survey of new south wales [hatherly, 1980] and surpassed the manual methods. the primary step of seismic refraction data processing is to pick the first breaks of travel times generated by the shots in-field procedure and organize the data to obtain reliable results [göktürkler et al., 2008; telford. w. m. et al., 1990]. the following methods are the well-known processing procedures of seismic refraction data [rubin and hubbard, 2005]: 1. conventional reciprocal method (crm) [edge and laby, 1931; hawkins, 1961], 2. generalized reciprocal method (grm) [palmer, 1980; palmer, 1981], 3. intercept-time method (itm) [lankston, 1989; palmer, 1981], 4. delay-time method (dtm) [bruckshaw, 1941], 5. plus-minus method [hagedoorn, 1959], similar to crm, and 6. refraction tomography (rt). the crm is the most commonly used processing method for seismic refraction data in landslide investigations. this technique was developed in the 1930s when it was posed as the technique of disparities [edge and laby, 1931]. the crm is differently named in various parts of the world, including hagiwara’s technique in japan [hagiwara, 1939; kisunezaki, 1965], the reciprocal method in australia [hawkins, 1961], the plus-minus procedure in europe [hagedoorn, 1962], and the abc method in the usa [bruckshaw, 1941; wilson, 1983]. the mentioned techniques are mathematically and conceptually identical. the crm is convenient to use and mostly applied in landslide studies with simple structures, plane boundaries, and invariant velocity in each layer. most of the calculations are simple mathematical operations that can be performed with spreadsheets. in the landslide sites, including the uniform seismic velocities and the subsurface horizons inclined less than 20°, the intercept-time and crm methods can professionally map the subsoil velocity structures [narwold and owen, 2002]. these two methods can model the layers and the refracted continuous interfaces across a profile. however, they cannot map the ray path velocity and interface irregularity on complex landslide structures [narwold and owen, 2002]. travelletti et al. [2010] applied the intercept time method (itm) to model the 2d velocity map then enhanced this by using the raytracing model on gypsum formations that affected the landslide phenomenon. as the landslide regions illustrate the complex velocity of the materials that rapidly change in the vertical and horizontal directions [kul yahşi and ersoy, 2018], a few researchers recommend to analyze the seismic wave using software based on raytracing analysis [bárta et al., 2005; otto and sass, 2006]. the refractor depths of each receiver point are estimated with the grm method if the forward and reverse shots overlapping refraction arrival times are carried out when acquiring data [mauritsch et al., 2000; narwold and owen, 2002]. the advantage of using the general reciprocal method (grm) is to image the seismic refraction velocities consistently along with the profile. it does not depend on the variety of subsurface material properties [mauritsch et al., 2000]. some studies attempted to present the results obtained by grm compared to the other refraction data processing methods [glade et al., 2005; lankston, 1989; mauritsch et al., 2000; narwold and owen, 2002; palmer, 1980]. the compared conclusions can help researchers to have a better understanding of the expected results. in comparison between grm and itm, grm significantly improves the seismic refraction technique utility. despite itm, it can particularly delineate the undulating refractors and determine lateral velocity changes in the horizons. however, the grm and itm can provide identical results when the layers and interfaces are planar and homogeneous [palmer, 1980]. the ambiguities in seismic model interpretations typically developed in itm are removed if the survey layout is designated with sufficient receivers and multiple shot points at both ends of the survey lines [lankston, 1989]. glade et al. [2005] employed the grm method for determining the failure surface of a very shallow landslide (1 to 3 m depth), mapping the complex interface between the colluviums and marly and calcareous sediments. the results show the shear plane on the boundary between well-contrasted layers, such as marls, clays, and colluviums. then, the results were finally modified using drillings. compared to glade et al. [2005], mauritsch et al. [2000] used the grm processing method on the seismic refraction data of the large and deep landslide made of limestone, dolomitic conglomerates, sandstones, and shales with irregular and heterogeneous refractors (figure 10). therefore, this proved that the grm produced seismic sections with good quality in shallow and deep landslides. in addition, the grm method was successful in determining the internal landslide materials but unsuccessful in presenting the slip surface images. the other processing method mentioned above is the delay-time method (similar to crm and plus-minus). the delay-time method is one of the refraction data processing methods to calculate the depth and time beneath each geophone. this method can calculate the difference between the time spending by signals propagating on undulating or dipping refractor through the upper layer and the time that it could have spent traveling along with the normal refractor. in this technique, the true velocity can be computed by recording signals from two ends of the line (forward and reverse shots) [musgrave, 1967]. it leads to the actual depth beneath the recorders with more details [redpath, 1973]. before using the delay-time method, it is compulsory to consider that the technique can be employed in the cases with an overlap zone between refracted signals coming from the same layer, as shown in figure 11. in this paper, only two studies were found to process the obtained seismic refraction data using the delay-time method. imani et al. [2021b] calibrated the seismic inversion models obtained from the debris flow and soil slide site by the results obtained from the delay-time method. in this study, apart from the commercial software zondst2d [kaminskiy, 2013] applied for processing refraction data with the refraction tomography (rt) method (the leastsquares using the time-term inversion technique [kaminskiy, 2013], the refraction data was manually processed using the delay-time technique to determine the velocity and depth beneath each geophone and synthesize the cross-section of the seismic profile. the geological cross-section of a profile was plotted in figure 12a. the excellent agreement between the inversion model and the synthesized model proved the ability of the delay-time method to illustrate the refractor shapes and layer thicknesses and confirmed the reliability of the inversion model shown in figure 12b. parisa imani et al. 14 15 applications of srt for landslide monitoring donohue et al. [2012] employed several algorithms on data obtained from a quick-clay sire in norway. they applied software gremix, which processes data based on the slope-intercept method associated with the delaytime method and grm. due to using the combined processing algorithms, the variations between the unleached clay and the stiffness of quick-clay were visible in the seismic sections. generally, the delay-time method is not commonly used in landslide investigations due to the compulsion to spread out longer geophysical lines and shorter geophone spacing. figure 10. cross-section of seismic profile modeled using the grm method [mauritsch et al., 2000]. figure 11. the arrival time plot resulting from forward and reverse shots with an overlap zone limited by the vertical red dashed line [imani et al., 2021a; redpath, 1973]. parisa imani et al. 16 figure 12. comparison between (a) the cross-section of the seismic profile (the synthesized model) obtained with the delay-time technique and (b) the inversion model obtained by the refraction tomography processing method [imani, 2020]. 17 applications of srt for landslide monitoring the refraction tomography (rt) method is considered as the other processing and interpretation method of refraction data. this technique is regarded as a modern interpreting seismic refraction data that allows being determined lateral variations of p-wave velocity. tomographic methods are not a unique procedure, and several tomographic methods were used for seismic refraction data analysis [olsen, 1989; rubin and hubbard, 2005]. as the subsurface of the landslide site is discontinuously assumed, refraction tomography can illustrate better images. this method measures the passing energy through materials. energy received by seismogram is analyzed to find out the media’s characters [stewart, 1991]. refraction tomographic procedures provide the fittest velocity models by iterative comparison of various velocity features with obtained data to specified resolution created by the operator. as mentioned above, the refraction tomography indicates the velocity variations as a gradient and discrete layer. the gradient of velocity is also known as refraction tomography that applies the raytracing algorithms, including gradual variations of velocity, gradational, and a straightforward interpretation of paths of the energy of the first breaks through underground media [rucker, 2000]. furthermore, refraction tomography methods can show the maximum investigation depth with the minimum ray path of the inverse models [narwold and owen, 2002]. the rt indicates the gradient of velocity regarding depth. it employs the simulated annealing in which the algorithm applied repeated modeling, and the models can be acceptable according to possible criteria. the criteria allow the algorithm to run away from traveltime deficiencies and non-uniqueness problems in velocity models of subsurface features. the algorithm does not hypothesize the velocity gradient of the subsurface and then can show robust vertical features and lateral gradients. therefore this method is quite efficient in landslide sites with complex and heterogeneous shallow surface structures [godio et al., 2006; narwold and owen, 2002; optim, 2001; pullammanappallil and louie, 1994] [optim, 2001]. a seismic ray path set is analyzed to provide a subsurface slice-image. velocity or attenuation estimation is the main task of refraction tomography [rubin and hubbard, 2005]. grm and refraction tomography methods have an acceptable agreement and can complete the obtained models of landslide areas with details. the rt procedure provides the fittest models of seismic velocity through the iterative comparison of various velocities with the acquired data to the specified resolution determined by the operator [narwold and owen, 2002; stewart, 1991]. therefore, rt decreases the drilled borehole numbers to characterize the geological features. armstrong et al. [2011] used a new approach with a neural network to analyze the synthetic and the observed refraction data and compare grm and rt results. in this study, more investigations were needed due to the increasing velocity errors with depth that appeared in the results for an unknown reason. in addition to the processing and interpretation procedures listed above, other methods are applied for data processing in studies of various landslide types. the least-squares method, ps_tomo program [tryggvason et al., 2002] based on the raytracing method modified by lsqr (algorithm for sparse linear equation and sparse least squares) conjugate gradient solver [paige and saunders, 1982], the generalized linear inversion (1998), tomographic grid-based inversion (tgi) and the wet (wave path eikonal travel time) algorithm [schuster and quintus-bosz, 1993] integrated with the delta-t-v technique, (helpful in decreasing the effect of artifacts into the subsoil) capizzi and martorana, [2014] and gibson et al., [1979] provided excellent results for landslide sites composed of watersaturated clay [capizzi and martorana, 2014; göktürkler et al., 2008; jacob et al., 2018; kim et al., 2011; paige and saunders, 1982; wang et al., 2016], while the simultaneous iterative reconstruction technique (sirt) [gilbert, 1972] cannot be helpful in high water-saturated material due to the decreasing resolution. thus, combined geophysical methods or multidisciplinary procedures are strictly required [godio et al., 2006; grandjean and sage, 2004]. grit and kanli [2016] processed the seismic refraction data using the raytracing technique and sirt to determine the landslide slip plane consisted of coaly shale, conglomerate, sandstone, clay, clayey limestone, and an impermeable layer. however, in this project, the seismic refraction method could not map the high-resolution image of the subsurface structure due to the depth penetration (>30m), while samyn et al. [2012] obtained good results using simultaneous iterative reconstruction (sirt) and an algorithm based on a second-order fast marching method (fmm) to compute the travel time using fresnel wave paths on complex landslides and modeled the 3d seismic velocity using the 3d extension of fmm. based on tomographic techniques, the algorithm wavefront-inversion presents high-resolution results in rainfall-induced landslide sites composed of a matrix of sandy-clay with a high weathering rate [mezerreg et al., 2019; nwankwo and ugbena, 2019]. although this can illustrate the boundaries between the weathered and consolidated refractors in a clear delineation, it does not present powerful functions for processing data in fully saturated material compared to the other rt algorithms. 4. the time-lapse seismic refraction tomography (tlsrt) the srt surveys mentioned so far have released static information. these studies were all performed after the sliding mass happened in landslide-prone areas. the obtained information always belongs to the data acquisition time regardless of the gradual changes of geological features and geophysical parameters in the sliding sites. as the water content can influence the surface geometries and interface horizons through weathering, this can play an essential role in triggering landslides. the dynamic activity of the water content can be monitored in considering the velocity changes of the slope investigation. the control of the dynamic behavior indicates an investigation technique called time-lapse srt (tlsrt). the time-lapse method leads to a better determination of the landslide situation and slope stability by monitoring the water saturation and surface displacements. this method can also estimate velocity changes in deep hydrocarbon reservoirs and deep investigations [landrø, 2004; lumley, 2001]. few works have operationally employed tlsrt in near-surface applications [fratta et al., 2005; gaines et al., 2010; hilbich, 2010; imani, 2020; imani et al., 2021a; valois, 2016]. due to the small number of tlsrt studies, all information related to the seismic cross-sections and the landslide types and their compositions are explained below. hilbich [2010] attempted to monitor subsurface ice content variations as a sliding reason in alpine terrain. the combined tlsrt and time-lapse electrical resistivity tomography (tlert) methods were used to determine the changes in subsurface features caused by freezing and melting processes. the srt models obtained from different seasons were individually interpreted. for more confirmation on srt models, the seismograms and the first break curves were analyzed. this study confirms the ability of the tlsrt technique to determine ice vanishing and prevents any ambiguities related to the identification of permafrost degradation. figure 13 illustrates short-term monitoring with different penetration depths of refraction signals caused by the subsurface material characteristics. the sharp refractors typically limit the depth of investigation at the locations with ice-rich permafrost. moreover, in this study, cluster analysis could estimate the rough mean velocities of the corresponding media by plotting the source-receiver offset against travel times. additionally, clustering helped indicate the time shifts related to the surface, including ice content and slight velocity variations. therefore, the tlsrt method successfully presented the significant relationships between the two models obtained from the two various dates. whiteley et al. [2019] proposed a workflow with long-term tlsrt to reduce the sources of potential errors while preserving the variations in subsurface structures in a landslide site placed in the uk. they successfully demonstrated the ability of seismic velocity contrasts to illustrate moisture contents in shallow depths. wolter et parisa imani et al. 18 figure 13. comparison of seismic cross-section inversions modeled from the two different dates. vertical black lines show the borehole locations [hilbich, 2010]. 19 applications of srt for landslide monitoring al. [2020] monitored the deformation changes of deep-seated gravitational slopes in the long-term and short-term by combining drilling information, surface geomorphology, cosmogenic nuclide dating, tlsrt, and displacement monitoring data. they evaluated the slope instability in the long-term. in addition to determining the velocity of materials, tlsrt is also successful in illustrating the irregularity of interfaces. figure 14 proves the tlsrt method’s ability to depict velocity variations in different water saturation amounts, boundary shapes, signal attenuation, and the probability of weathering [imani, 2020; imani et al., 2021a]. this project was conducted on a mountainous and subtropical area in hangzhou city, china, to characterize the sliding factors. the high signal attenuation mainly presented can be attributed to the saturation and porosity amount. hilbich [2010]; imani [2020] processed and interpreted the individual inversion images. to better understand the time-lapse srt from the individual inversion models, the seismograms recorded by various shots are overlapped to compare the time shifts that occurred on different dates. as shown in figure 15, the time shifts of the first breaks of the august comparison with july are clearly observed at about 3–8 ms [hilbich, 2010]. the lesser correspondence between the two seismograms occurred due to a lower refractor depth caused by different amounts of saturation. figure 14. time-lapse srt (tlsrt) inversion models mapped from the two different datasets. (a) cross-section of the seismic profile modeled by data collected in a wet stage. (b) cross-section of the seismic profile modeled by data collected in a dry stage [imani, 2020]. 5. discussion a seismic refraction tomography is a powerful tool in landslide studies as one of the near-surface geophysical methods. the case histories reported in this review have discussed the advantages and disadvantages of the seismic refraction method focusing on seismic refraction tomography in different landslide types with various amounts of water saturation (table 2). most of the studies (41%) have been performed on slides and flows types, and the minimum investigation (5%) belongs to deep-seated studies. furthermore, 45% and 20% of studies reviewed in this paper have been conducted at high saturation and low saturation areas, respectively. the srt method can delineate the failure surface depth, map the velocity model, determine the boundary between the unstable landslide mass and bedrock, and distinguish the lateral extent of landslide and the weathered bedrock [narwold and owen, 2002; ng et al., 2015]. reliable subsurface information can be obtained from the high-velocity contrasts between sliding mass and bedrock [chen et al., 2019a]. when the landslide site includes insufficient velocity contrast and a low layer thickness, or when the low layer velocity is less than the upper layer velocity, srt cannot identify the specific layers [redpath, 1973] or various rock types [ng et al., 2015]. the srt is also unsuitable for investigating very complicated geological features [ferrucci parisa imani et al. 20 landslide type % water saturation % slides and flows 41 high saturation 45 complex 12 moderate saturation 35 falls 12 low saturation 20 creep 12 quick-clays 9 deep-seated 5 no info. 9 table 2. variety of investigated landslide types in the present paper and studies conducted at landslide sites with different water saturation amounts. figure 15. comparison of the waveforms of the travel times collected in july (gray) and august (red) 2008 [hilbich, 2010]. 21 applications of srt for landslide monitoring et al., 2000]. the seismic velocities change widely due to the variety, compaction, and saturation of subsurface materials [al-saigh and al-dabbagh, 2010]. these are the main reasons why the srt cannot be applied individually. researchers strongly recommend integrating the srt data to direct or indirect data to obtain more reliable and accurate results. [mauritsch et al., 2000; ng et al., 2015; travelletti et al., 2010]. considering the problems in which the sliding mechanism is almost ambiguous, those studies that apply the combined geophysical methods in landslide investigations have been more successful. in general, there are the two joint inversion data techniques: the first is the application of hydrogeological properties to link the two various geophysical characteristics [abubakar et al., 2012; hamdan and vafidis, 2013], and the second involves the application of structural properties as a usual factor in the two geophysical models [abubakar et al., 2012; de nardis et al., 2005; gallardo and meju, 2004; hamdan and vafidis, 2013]. the joint inversion method is commonly performed utilizing the synthesized data, and then the same technique is employed to investigate the in-field data [perrone et al., 2014]. table 1 presents a collection of joint inversion studies applied for landslide investigations. the data obtained from the joint inversion technique can be the main challenge in the future for geophysicists to get the most suitable subsurface model. it is possible to apply cheaper and more efficient geophysical procedures after testing the more applicable and efficient joint inversion methods. therefore, there is no necessity to compare the used invasive and direct methods and geophysical results. as shown in figure 16, the majority of the studies (45%) reviewed in this paper applied the srt and electrical tomography data joint inversion, particularly in water-saturated investigations [bekler et al., 2011; bichler et al., 2004; capizzi and martorana, 2014; donohue et al., 2012; godio et al., 2006; göktürkler et al., 2008; havenith et al., 2000; hilbich, 2010; imani, 2020; imani and tian, 2018; imani et al., 2021b; jongmans et al., 2009; karslı et al., 2017; kim et al., 2011; kul yahşi and ersoy, 2018; le roux et al., 2011; mauritsch et al., 2000; mezerreg et al., 2019; nwankwo and ugbena, 2019; ostrowski and lasocki, 2018; otto and sass, 2006; samyn et al., 2012; solberg et al., 2012; yilmaz and kamaci, 2018]. among electrical methods, electrical resistivity tomography (ert) is of great importance in landslide studies. although, resistivity data are mainly affected by porosity, weathering rate, pore water pressure, water contents, and mineral particles, the ert technique can illustrate the landslide structure and lateral extent in detail. despite the success of ert in landslide projects, ostrowski and lasocki [2018] mentioned that compared to srt, ert could not recognize unaffected materials from colluvium. only a few examples (15%) combined with the gpr technique have been reported [bichler et al., 2004; otto and sass, 2006]. although the gpr signal penetration depth is insufficient in landslide investigations due to scattering energy in heterogeneous sites, it is often used to determine the first subsurface layer. besides electrical techniques and gpr, the studies (40%) considered in the review have used other methods. a few projects have illustrated a suitable integration between gravity and the seismic refraction results obtained from rockfall type [jacob et al., 2018]. the seismic reflection method has been combined with srt in particular examples [adamczyk et al., 2013; bichler et al., 2004; ferrucci et al., 2000; grit and kanli, 2016; karslı et al., 2017; kim et al., 2011; malehmir et al., figure 16. contribution of different geophysical techniques integrated with srt in landslide studies. 2013b; travelletti et al., 2010; wang et al., 2016]. due to presenting high-resolution results using the reflection method in the literature, this technique appears more powerful and successful than srt [grit and kanli, 2016]. in addition, the reflection method can illustrate the complex geometry structure with more details in structures compared with srt [ferrucci et al., 2000]. this paper also highlights that the tlsrt method has been applied in very few examples. as already discussed, this can be chiefly referred back to the intrinsic limitation of the srt method to show the structures in detail, the software abilities, and the analysis of data. however, due to its ability to present the shape of refractors and the weathered structures, it is suggested by some research [imani, 2020] to monitor the boundaries of tlsrt. the possibility of locating the borehole spots, and receiver and shot points for the next measurements should also be considered in monitoring projects. this context also highlights the hardship of tlsrt data processing. based on imani [2020]; whiteley et al. [2019], and hilbich [2010], there is no powerful software for processing tlsrt data that can quantify the velocity changes as a function of the variations in the hydrogeological parameters. this problem can be solved by individually interpreting models, extending the monitoring time, and increasing survey times. 6. conclusions after careful deliberation of the research since the year 2000 involving every type of landslide phenomenon, we concluded that seismic refraction tomography (srt) presents reliable and accurate information on the wave velocity contrast of different geological structures associated with sliding material and bedrock. the srt is employed to characterize the physical properties of materials within landslides, delineate the boundaries between subsoil horizons and surface failures, and determine the landslide lateral extension and depth. based on the srt results, investigating the high water content in low-velocity layers in landslide-prone areas is essential to prevent future sliding events. velocity models illustrate the low and high-velocity values corresponding to landslide materials and bedrock, respectively. as already explained, this paper highlighted the significant srt method limitations that can be improved using combined geophysical techniques or multidisciplinary methods, such as geotechnical and geological approaches, mainly when highly saturated materials are studied. the reliability of the obtained results likely does not depend on the type of landslide. although, like the other geophysical techniques, the method presents preliminary results in which the velocity contrasts are masked. srt appears to act as a well-consolidated tool for characterizing the sliding extent, saturated materials, and boundaries of all landslide types. the srt data quality does not appear to be influenced by the measurement equipment applied, but the survey strategy should be carefully considered. although tlsrt has been introduced as a successful 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(1987). seismic analysis data: processing, inversion, and interpretation of seismic data, (society of exploraition geophysics). yilmaz, s., and z. kamaci (2018). resistivity and seismic refraction studies on kısıklı landslide (antalya, turkey), int. j. comput. experimet. sci. eng., 4, 9-14. zainal abidin, m.h., r. saad, f. ahmad, d.c. wijeyesekera, and m.f. tajul baharuddin (2012). seismic refraction investigation on near surface landslides at the kundasang area in sabah, malaysia, procedia eng., 50, 516553. parisa imani and amr abd el-raouf contributed equally to this work as first authors. *co r r e s po n d i n g a u t h o r: amr abd el-raouf, school of earth sciences, zhejiang university, zheda road 38, hangzhou 310027, china and geology department, faculty of science, zagazig university, zagazig 44519, egypt; e-mail: amr_abdelraouf@zu.edu.eg © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international mailto:amr_abdelraouf@zu.edu.eg the seismicity of tunis . x . x . a m b r a s e y s ( * ) r i c e v u t o il 5 s e t t e m b r e i9(i2 1 . i n t r o d u c t i o n . very little is known regarding tlie seisniicity of tunisia and in particular tliat of the tunis area. we know tliat tunis was sliaken a few times in the past but it is rather difficult to say witli certainty what effect tliese earthquakes had on structures and what their intensities were. information is partieularly lacking about tliose earthquakes before the 19tli century. i t may, however, be signiflcant tliat liistorical record» for the area of tunis indicate intense seismic activity during the 9th and 18th centuries, altliough one must make some allowances for naturai exaggerations in early statements. in order to disclose the seismic potentiality of suoli a poorly documented area it is, therefore, essential to resort to its geotectonic history for informatici!. 2. t e c t o m c s ( * * ) . the tunis area belongs to the extreme north end of the atlas mountains. tliese mountains constitute a geologica! unit distinct from the rest of the african continent, the north part of whicli is an active orogenic block. this block extends from cape xun and agadir in moroeco to the gulf of gabes in tunisia, a distance of 1500 miles. it is bounded on the north by the mediterranean sea and on the south by the sahara desert. (*) d e p a r t m e n t of civil e n g . . imperiai college of sei., l o n d o n , s. w . 7. li, 8, !», 10, 12, 1!>, 21), 26 \ 2 3 4 x . x . a m b r a s e y s the atlas mountains can be roughly divided iato two main chains which are separated by lugli plateaus, the maritime atlas, and the sahara atlas. the two mountain chains approacli eacli other gradually and come together in tunisia, where, near zagliouan, they are separated by a nearly vertical fracture. the north-east part of the fractured mass has been lifted by comparatively recent orogenic movements to a height of nearly 4500 feet witli respect to the south-east part, thus forming cape bon on the east and the gulf of tunis on the west. this orogenic 1110vement also explains the abrupt turning of the african coast southwards and the formation of the sousse-gabes coast. the atlas mountains were uplifted during the alpine crustal movements. the movements commenced at the end of the jurassic period, were renewed in the upper cretaceous and continued info the middle and ]irobably upper miocene. there is also evidence of folding during earlier periods, witli a dominant direction of movement from north to south. more recent movements bave left ampie evidence in the morpliology of the area, in which tunis is situated. there is little doubt t h a t tunis belongs to tliat orogenic zone and that it is seismically unstable. in point of fact tlris area is stili undergoing tectonic readjustnients which occur at a comparatively slow r a t e . the existence of a number of active faults in north tunisia is also in agreement witli the orogenics of the area. figure 1 shows some of the well known faults in tunisia and tlieir relation witli inferred and instrumentally determined epicentres of earthquake shocks. as a matter of interest the epicentres for several more distanti earthquakes have been inoluded, the majority of which belong to shocks of class ' d ' (13) (*). sieberg (1932) and de ballore (1906) show a number of epicentres right in tunis and eothé (1951) suggests some activity on the extension of the fault of djabel-naheli at the end of the liills and the baliira lake (lake of tunis). this, places tunis literally on a fault, presumably active and responsible for at least one known earthquake of intensity vi on the modified mercalli scale (25). 3 . seismic i i i s t o r y . a survey of the seismic history of north tunisia, altliough based on fragmentary data, shows t h a t the country has from the earliest periods (*) e p i c e n t r e s were obtainerl f r o m ('• 13• 24). t h e s e i s 1 i i c i t y o f t u n i s 2 3 5 of history been subject to rather frequenti earthquake shocka. most of tliese shocks appear to liave been sliglit and even on the worst cases, tliey have fallen below the average of destructive earthquakes. this is particularly so if, as we have to, base our assessments of intensity 011 historically recorded maximum effects of earthquakes 011 structures. suoli records tend to represent the behaviour of the worst struetures, during an earthquake or, 1 he behaviour of structurcs 011 the worst ground. in the first instance, since most of the ordinary houses in north africa are very poorly built, some of which simply invite earthquake damage, any assessment of intensity from damage to suoli structures would be highly overestimated. on the other hand, the worst ground is commonly found 011 the shores or in alluvial and marshy areas, and these are the locations of the ohler centres which furnish much of the data. for these reasons an unmodified application of old data, particularly when naturai exaggerations are not taken info consideration, will lead to apparenti concentration of seismicity in small areas surrounding the largest and oldest centres of culture, which in our case are kairowan, tunis, and utica. a summary table of ali tliose shocks which caused some damage or concern in tunisia is given at the end of the paper (*). this table is admittedly incomplete, but it suggests that light shocks are frequent and t h a t strong ones occur ar rather long intervals of time (**). this long periodicity of comparatively strong earthquakes in tunisia seems to be tvpical of the orogenics of the two extreme ends of the atlas block. at the ends of the atlas range crustal a'ìjustements appear to dissipate their potential cnergy by numerous but light shocks, and hence from the seismic point of view, cause little concern. in the centrai part of the range, however, the readjustements are more sudden and much more severe. most probably much of the seismic activity in north tunisia is associated with the zaghouan fault along which recent orogenie movements have resulted in the differential elevation of the soutli side of the (*) p r o f e s s o r j . p . r o t h è of s t r a s b o u r g lias p r e p a r e d a n e a r t h q u a k e c a t a l o g n e for t u n i s i a which c o n t a i n s ali shocks for which t h e r e is i n f o r m a tioii a v a i l a b l e . (**) this s t a t e m e n t is n o t inconsistent with t h e m a t e r i a l in p r o f e s s o r rotliè's c a t a l o g u e . 2 3 0 n. n . ambkasets zaghouan massive. the north flank of the zaghouan mountains thus formed (coincide witli the zaghouan fault which runs throught the gulf of tunis and is largely responsible for the separation of cape bon from the rest of the north africa coast. i t is due to this fault that north tunisia shows suoli a high seismic activity. de ballore (1906) attribut.es the destructives earthquakes of the 5th century and a number of more recent ones to activity of this fault or of one of its northern branches. he considers t h a t the only active part of tunisia is the area north of sousse and krenkel (.1923) agrees with this opinion but includes also the whole of the north atlas zone of algeria. sieberg (1932) indicates t h a t the region on both sides of the zaghouan fault as well as a zone within a radius of .100 miles of tunis are the most precarious and seismic areas in tunisia. a similar statement is iliade by barkov (1953). 4 . s e i s m i c i t y . taking into consideration the data shown on figure 1 and the information available on the tectonics and seismicity of tunisia — which is neitlier ampie nor accurate — the country has been divided into three principal seismic zones. zones " a " . " b " and " c " indicate respectively regions where strong, intense or liglit earthquakes are likely to occur (*). this division is tentati ve and it is shown on figure 2. the reliability and accuracy of the available data is so low t h a t an estensive treatment of tliese data, in order to arrive at an intensity zoning of the country, was considered to be unwarranted. i t is desirable, however, to assess the seismicity at tunis, or rather the intensity of the strongest probable earthquake for the area of tunis. at this stage, this is almost certainly impossible because of the complete lack of information concerning the tectonics and seismicity of the area. we must resort, therefore, to the plausible conjecture usually employed in such cases, of the repetition of a past strong earthquake at the minimum epicentral dist.ance which is justified by the tectonics of the region. in the present case this would lead to the assumption t h a t an earthquake of the sanie characteristics as the agadir earthquake ( n ) could (*) t h e b o u n d a r i e s of zone " c " a r e r a t l i e r t e n t a t i v o . p a r t s of t h i s zone m a y well belong e i t h e r t o zone " a " or zone " b " b u t t h e a v a i l a b l e i n f o r m a t i o n is insufficient t o define tlie l i m i t s witli a n y a o e u r a e y . t h e s e i s 1 i i c i t y o f t u n i s 2 3 7 occur on the djabel naheli, or on one of the branches of the zaghouan fault. tliis assumption is of eonrse open to criticism, but, in the complete absence of informatimi for the probable character of strong ground fig. 1 niovements at tunis, it constitutes a reasonable upper limit for the seismic intensity of a damaging earthquake occurring in this area. 5 . d e s i g n s p e c t r u m . the agadir earthquake was a low-magnitude earthquake with extremely shallow focus and high intensity ("•17). the velocity spectrum 2 3 8 x . x. a m b r a s e y s for the agadir earthquake was constructed by kirkland (1962) from which the corresponding acceleration spectrum has been computed for a criticai damping of 10%. this is shown on figure 3, curve " a ", stismiaìfy ^ ® • • s e i s m i c i t y c fig. 2 and a conservative approach would be to adopt curve " a " for the design of important engineering structures within a radius of 50 miles from tunis, together avith one of the recognised building codes for seismic regions ("• 5>15' 17>22. 28). the way acceleration or velocity spectra are used for the design of structures to resist earthquake forces is given in many references ( 2 ' 5 ' 1 5 ' 2 8 ) and this problem will not be discussed. t u e 8 e i s m i c i t y o f t u n i f 2 3 9 some comments on the use of curve " a " (figure 3) in the design of structures are necessary. if curve " a " is used, the structure should be designed to be strong enough to resist, without plastic deformation (elastically), an earthquake of similar characteristics to that at agadir, which has been taken as the most severe earthquake likely to occur in tunis. i'on fig. 3 except for extremely important engineering structures, sudi as certain types of dams, nuclear power stations, oil refineries, etc., this approach will be extremely uneconomical and in some instances unrealistic. a reasonable approach would be to use elastic. design methods for ordinary structure so t h a t they can withstand relatively frequent earthquakes of a lower intensity tlian that represented by curve " a ". in the event of a severe earthquake, sueh as curve " a " implies, some plastic yield may be permitted. for most structural materials, yiehling, or plastic deformations of the order of tliree times the deformation at the elastic limit (*) will involve no serious distortions or indue amount (*) i.e. t o t a l d e f o r m a t i o n c o r r e s p o n d i n g t o a d u c t i l i t v f a c t o r m = 4. 2 4 0 x. x. ambraseys of damage. the yielding in most structures of stable conflguration is of a locai nature and does not affect their general appearance. consequently, it seems reasonable to use in the design ductility factors (5-2s) between 3 and 5. in figure 3 the acceleration spectra for a damping factor of 10% and for ductility 3, 4 and 5 are shown. the use of curve " c " for instance, implies that, if the structure is designed to withstand accelerations described by this curve, the structure will behave elastically, or nearly so, for most earthquakes except those which will be nearly as intense as t h a t of agadir. i n this case it will vield locally, deforming permenantly by an amount equal to four times the deformation at the elastic limit. i t is, however, necessary that those structures, whose probability of damage must be nill, should be designed for the spectrum shown by curve " a " . ordinary buildings, residential houses, etc., should be designed in accordance with curves " b " to " i ) " or, perhaps even lower curves, depending on the deformations the designer would allow. in these cases, however, sufflcient yielding capacity, to anable the structural members to survive the even most severe motion without danger of collapse, should be provided. no brittle structural member should be used, since, in designing with curves " b " to " d " the yielding capacity of the structural members must be sufflcient to justify the use of these curves. thus, although t h e . infrequent maximum earthquake may cause considerable damage to sudi structures, it will not represent a hazard to life. examination of the damage caused by the agadir earthquake shows clearly t h a t the structures liad little yield capacity. the brittle materials and un stable forms of construction used were unable to withstand deformations beyond the elastic limit with the result that, even a relatively small deformation, caused complete collapse of many of the structures ("•17 ). 6 . s e i s m i c s e a w a v e s . little information regarding the occurrence of seismic sea waves on the tunisian coast is available, except that reported in bothè's catalogue (*) for the 1905 earthquake in the bizerta area. (*) " catalogue des seimes tunisiens " , s t r a s b o u r g , j u n e 1962 ( u n p u blished). t h e s e i s 1 i i c i t y o f t u n i s 2 4 1 note: p a r t of the work in this paper has been taken from a report prepared by the author for the cementation co. ltd, march 1962, london s.e.i. l i s t of m a i o r e a r t h q u a k e s i n t u n i s i a . 408 or 412 a series of destructive earthquakes with fault movements near utica. aftersliocks lasted for one week (references in 21, p. 7; 27, p. 871). 854 or 855 an earthquake in north tunisia destroyed thirteen villages at kairowan. slumping and settling of the ground was observed in the valleys. f1). 856 dee. a great earthquake at tunis and its dependenoies during which 45.000 persons were buried under the ruins ('). 1724 an earthquake shock principally in algiers. the earthquake extended from miliana to bon and in tunisia. (21), p. 122. .1750 oct. 5 a strong earthquake shook north africa fron sfax to bon. (»). (21), p. 148. 1758 j a n . an earthquake near constantine caused great damage to several places in tunisia. tunis sufìered from otlier shocks and houses fell in great numbers. several thousand people perished in the ruins. from references in (21), p. .133. 1863 sep. 18 a serie of earthquakes in tunisia. (27), p. 871. 1864 during this year tunis was shaken four times by violent earthquakes. (23). 1881 j u n . numerous strong shocks at gabes and maret. the shocks were felt within a radius of 50 kms. no damage. (23), p. 612, (27), p. 871. 1887 j a n . 6 an earthquake in tunis which caused light damage to village near ejenel? (press reports london). 1905 jul. 29 light shocks at sfax. (27), p. 871. 1905 oct. on 15th and 22nd strong shocks were felt at madhia. io = y . ( 2 7 ) , p . 871. 2 4 2 n . n . a m b r a s e y s 1920 feb. 25 an earthquake of magnitude 5.9 shook the area around sbeitla. (13), p. 214 (press reports, london). more shocks followed in february and aprii. 1940 light shocks were felt in tunis throughout the year. (24), p. 59, 1942. 1941 aug. 7 a strong earthquake caused some damage at bou-saadiah in the caidat of zaghouan. (24), p. 5, p. 50,1954. 1948 may 19 an earthquake of intensity vi shook tunis. the epicentre was on the djabel naheli. (24), p. 24, 1951. 1949 may 7 a locai shock caused some damage at sousse. (24), p. 25, 1953. 1950 nov 15 an earthquake shock was felt in tunis. (24), p. 25, 1953. .1953 j a n . 11 numerous shocks which lasted till the 14th january caused some damage in the region of cape bon. (24), p. 16, 1955. 1953 oct. 17 an earthquake shock at kairouan. no damage. (24), p. 16, 1955. 1957 feb. 20 a severe earthquake of magnitude 5.6 occurred on the extension of the maritime atlas. the epicentre was a few miles south of souk-el-khemis. a number of abode houses were destroyed at riabna la merja, berdaj, rhardimaou. 13 persons were killed and 96 were injured. the main shock was followed by nine strong aftershocks. (24), p. 9,1959; (6), p. 287,1957. 1958 aug. 12 a roman amphitheatre was damaged and some abode houses collapsed when intermitted shocks shook the area of el-djem. no casualties were reported. (6), p. 115, 1959 (press reports, london). su m mary tunis lies well within a seismic zone, the zone of the atlas, ivhere earthquakes are likely to occur. the city and its neiglibouring region has been shalcen a number of times recently by moderate earthquakes, and although during the last few hundred years no major earthquakes are known to have occurred at tunis, there is very little doubt that the area neighbouring the city is seismically unstable. t h e s e i s 1 i i c i t y o f t u n i s 136 the tectonics and tlie seismicity of north tunisia are not urli known. importuni economical developments in that country moke the need for an immediate assessment of the earthquake danger in north tunisia and in particular in the tunis area. in what follows, a brief study of the tectonics and seismicity of north tunisia is giren, and a standard acceleration spectrum for the design of engineering structures in the tunis area is proposed. riassunto tunisi si estende all'interno di una zona sismica, la zona dell' atlante. la città e i suoi dintorni sono stati colpiti, recentemente, da moderati terremoti sebbene, durante gli ultimi secoli, non si siano avuti in detta zona, terremoti cospicui, resta sempre il dubbio circa l'esistenza, intorno alla città, di una zona sismicamente instabile. la sismicità e la tettonica della tunisia settentrionale, non sono ben conosciute. l'economia di questo paese, in fase di notevole sviluppo, richiede una immediata valutazione di eventuali danni da terremoto in tutta la zona. nella nota viene brevemente studiata la tettonica e la sismicità della tunisia settentrionale, e si propone uno spettro standard dell' accelerazione per una futura edilizia in tunisia. r e f e r e n c e s ( 1 ) a m b r a s e y s n . , " oli tlie seismicity of south-west a s i a " . bevue pour l'etudes des calamites, 37, 7, genève, (1961). ( ) a n d e r s o n a., " l a t e r a l forces of e a r t h q u a k e a n d wind " tras. amer. soc. civil. eni/., 117, 732, (1952). ( 3 ) b a i . l o r e d e m . , les tremblements de terre. geographie seismologique. p u b i . a r m a n d colin, paris, (1906). ( 4 ) b a r k o v a . , fijicheskaya geografiya chastej sveta-a jrika. p u b i . uspedigiz., 234, moskow, (1953). ( 5 ) b l u j i e j . , n e w m a k k ist., c o r n i n g l . , design of multìsiory reinforced concrete buildings for earthquake motions. pubi. p o r t l a n d cement ass., 318, chicago, (1961). (6) bulletin of the seismological society of america, seismological notes, berkeley california, 1911 onwards. (") bulletin of the union geodesique et geoplujsique internai., b u r e a u central seismologique, strasbourg, 1943 on. ( 7 ) c a s t a n y g . , " e t u d e geologique de l'atlas tunisien o r i e n t a i " . annales des jlines, 8, besancon, (1951). 2 4 4 x . x . a m b r a s e y s ( • ) c a s t a \ y ci., carte geologique de la tunisie. serbico des mines p u b i . , t u n i s , ( 1 953). ( " ' ) u a s t a x y g., mouvements aditela de terrains a sidi-bou-said. n o t e s du service geologique, 10, t u n i s , (1954). ( u ) d e s p e y r o u x j . , " t h e a g a d i r e a r t h q u a k e of f e b r u a r y 29, 1900 " . l'roe. 2nd world coni. earthquake engineering, 1, 521, 589, t o k y o , (1900). ( 1 2 ) g i x e s t o u s g . , " esquisse geologique de l a t u n i s i e " . e d i t . j . p i c a r d , t u n i s , (1911). n g u t e n b e r g ! ? . , r i c h t e r c . , seismicity of the carili. p r i n c e t o n itniv. press, (1954). •<u) i l o r r k . van, chronik der erdbeben uud vulkan-ausbruche. gesclil. o b e r l i e f e r u n g n a c h g w . n a t u r i . v e r a n d e r e r d o b e r f l a c h e , 5 vols, g o t h a , (1841). c5) ifolll.s e . , lììbliography of engineering seismology. p u b i . e a r t h q u a k e e n g i n e e r i n g kesearcli i n s t i t u t e , 144, san f r a n c i s c o , (1958). (lu) international seismological summary, kew observatory, r i c h m o n d , 1913 o n w a r d s . c 7 ) k i r k l a n d w . , l'he agadir, morocco earthquake of february 29,1960. p u b i . c o m m i t t e e of steel p r o d u c e r s , 00, new york, (1902). ( l s ) k r e n k e l e . , die seismizitdt afrikas. z e n t r a l b l a t t f u r mineralogie geologie u n d p a l à o n t o l o g i e , 0, s t u t t g a r t , (1923). •( 1 0 ) k r e n k e l e . , geologie afrikas. 1 of t h e ' geologie der e r d é p u b i . g e b r u d e r b o r n t a g e r , berlin (1925). ( 2 0 ) l a p p a k e n t a., lefons de geograpliie physique, e d . masson, p a r i s . 1890. ( 2 1 ) m a l i . e t r., facts of earthquake phenomena. r e p o r t of t h e britisli association f o r t h e a d v a n c e m e n t of science (1852-8). ( 2 2 ) m u t o k . , earthquake resislant (design) regwlations of lite world. p u b i . g a k u j u t s u d u n k e n f u k y u k a i , m e g u r o k u , t o k y o 1900. ( 2 3 ) ( ) ' r e i l l y j . , alphabetical catalogne of the earthquakes recorded eie. in europe and adjacent countries etc. t r a n s . r o y a l lrisii a c a d e i n y , 28. 087, d u b l i n , (1885). (24) iievue pour velude des calamites, genève, 1939, o n w a r d s . ( 2 5 ) r o t i i e j . 1 ' . , " chronique seismologique " . kevue et. caiani. 3 2 , (1951). ( 2 6 ) s a v o k x i x j . , la geologie algérienne et nord-africain. p u b i . b a s t i d e j o u r d a n , algiers, (1931). ( 2 7 ) s i e b e r g a . , die erdbeben, in b. g u t e n b e r g ' s handbuch der geophyrik, 4. p u b i . g e b r u d e r b o r n t r a g e r , berlin, (1932). < 2 8 ) v e l e t s o s a . , n e w m a r k n . , e/fects of inélastic behaviour of uh»pie systems lo earthquake motions. p r o c . 2nd w o r l d conf. e a r t h q u a k e e n g i n . , 2, 895, t o k y o , (1900). annals 48, 3, 2005+app1 491 annals of geophysics, vol. 48, n. 3, june 2005 key words topside ionosphere – ionospheric modeling – ionospheric layer formulation – topside soundings 1. introduction different kinds of analytical functions are being used to model the electron density profile of the various ionospheric layers. since the experimental information of the topside electron density profile is very scarce, this part of the modeled profiles has been less tested against measured data. in the last few years a relatively large amount of topside electron density profiles has been made available to the scientific community through the internet allowing to test models (coïsson et al., 2002) and the different mathematical representations used. this work analyzes the ability of some ionospheric layers analytical formulation to reproduce the experimental profiles measured by some old satellites which carried topside sounders: the russian intercosmos-19 (ik19) and the canadian-american international satellite for ionosphere studies 2 (isis2). some analytical aspects (derivative, inflection point and scale parameter) of common layer functions are investigated here in order to find better ways to model the lower part of the ionosphere topside. 2. layer functions for the topside some simple layer functions have been chosen to perform the comparisons: epstein, chapman and other derived formulations. they are characterized by a scale parameter which determines the thickness of the layer. epstein layer function ( , ) ( / ) ( / ) exp exp h b h b h b epstein 1 4 2 2 2 2 = +^ h (2.1) h being the height relative to the f2 maximum and b2 the scale parameter for the layer. comparison of analytical functions used to describe topside electron density profiles with satellite data carlo fonda, pierdavide coïsson, bruno nava and sandro m. radicella the abdus salam international centre for theoretical physics (ictp), trieste, italy abstract electron density models of the ionosphere use different analytical formulations for the electron density vertical profile in the topside. the present paper compares some single-layer topside analytical descriptions (chapman, epstein, modified epstein used in the nequick model) with experimental topside profiles obtained from measurements of ik19 and isis2 satellites. the limits of height range and shape for each formulation are described and analyzed and suggestions for the use of multiple layers solution to reproduce experimental results are given. mailing address: carlo fonda, the abdus salam international centre for theoretical physics (ictp), strada costiera 11, 34014 trieste, italy; e-mail: cfonda@ictp.it 492 carlo fonda, pierdavide coïsson, bruno nava and sandro m. radicella «modified epstein» layer function ( , ) exp exp h h k k nequick 1 4 0 2= +^ h (2.2) . . k h h h h h 1 0 125 100 12 5 0 0 = + + b l (2.3) h being the height relative to the f2 maximum and h0 the scale parameter for the layer. it is used in the nequick model (radicella and leitinger, 2001). chapman family of layer functions ( , , ) exp exph k k h k h chapman 1= -a a bb l l; e (2.4) h being the height relative to the f2 maximum, k the scale parameter for the layer and α the shape parameter. this function is usually called according to the value of α: chapman-alpha if α = 0.5 and chapman-beta if α = 1. examples of the described functions shapes are shown in fig. 1 in linear scale and in fig. 2 in logarithmic scale. all the profiles have been made to have an electron density of 0.001 nmf2 at 1000 km above hmf2. 3. data from ik19 satellite the first set of data is the one from the ik19 satellite. this satellite was on an elliptical orbit with 74° inclination and it had perigee at 500 km and apogee at 980 km (pulinets, 1989). the ik19 topside profiles cover height ranges form about 200 km up to about 750 km over the f2 peak. it is a data set which allows to analyze the lower part of the topside. the data available are from the high solar activity period march 1979-december 1980. α-chapman, β-chapman, epstein and modified epstein (nequick) layer functions were used to fit 7138 experimental electron density profiles derived from ik19 on board topside sounder. the fit was performed using least square methods, some examples are presented in figs. 3, 4 and 5. in addition the following parameters have also been evaluated: 1) height of the inflection point; 2) absolute difference between the integral of the fitting function and the integral of the experimental profile, numerically obtained. 3.1. analysis of integrals the four considered modeling functions have been analyzed, comparing for each profile the absolute difference ε between the integral of fig. 1. profile shapes for the different functions with intersection at 1000 km above hmf2: chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted, nequick solid gray. linear scale. fig. 2. profile shapes for the different functions with intersection at 1000 km above hmf2: chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted, nequick solid gray. logarithmic scale. 1 2 fig. 5. ik19 experimental profile (solid black) and the fitted functions: chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted, nequick solid gray. fig. 6. distribution of ε for all ik19 profiles. chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted, nequick solid gray. 5 6 493 comparison of analytical functions used to describe topside electron density profiles with satellite data the fitting function and the integral of the experimental profile. this criterion has been used to evaluate how well the shape of these layer functions reproduce the behavior of experimental profiles, in order to identify which function is better able to model this part of the topside. the obtained distribution, presented in fig. 6, shows that a chapman function with the shape parameter α = 0.5 gives th e best result, as confirmed by simply looking at most of the profile plots. 3.2. analysis of inflection points the height of the inflection point is used as an additional criterion to evaluate the capability of a function to reproduce the shape of the lowest part fig. 3. ik19 experimental profile (solid black) and the fitted functions: chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted, nequick solid gray. fig. 4. ik19 experimental profile (solid black) and the fitted functions: chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted, nequick solid gray. 3 4 fig. 9. distribution of difference between experimental and fitted inflection point heights of ik19 profiles. chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted. fig. 10. isis2 experimental profile (solid black) and the fitted functions: chapman-alpha dashed, chapmanbeta dotted, epstein dash-dotted, nequick solid gray. 9 10 494 carlo fonda, pierdavide coïsson, bruno nava and sandro m. radicella of the topside (ik19 profiles usually have a maximum height in the order of 500 km above the f2 peak). the distribution of these heights (fig. 7), compared to the positions of the inflection points as computed from the experimental profiles (fig. 8), again shows a better performance of the chapman-alpha with respect to the other functions (fig. 9). the inflection points cannot be analytically determined for the nequick topside function and they are not included in such analysis. 4. data from isis2 satellite the isis2 satellite was in operation for more than a decade, it had a circular orbit at 1400 km with inclination 88° (bilitza et al., 2002, 2003; huang et al., 2002). the same method of profile fitting has been applied to the experimental data set derived from isis2 satellite, some examples are shown in figs. 10, 11 and 12. the results are consistent with those obfig. 7. distribution of inflection point heights of fitted ik19 profiles: chapman-alpha dashed, chapman-beta dotted, epstein dash-dotted. fig. 8. distribution of inflection point heights of ik19 profiles. 7 8 495 comparison of analytical functions used to describe topside electron density profiles with satellite data tained from ik19 profiles. the main difference being the average maximum height of the profiles, higher for isis2, these results show better the behavior of model functions in a higher part of the topside (range of height: 400-1000 km above the f2 peak). the fits analysis also shows that the layer function used for the lower topside cannot be used «as it is» for this higher range of heights, but a new and different 2-layers topside formulation, at least, is needed in order to better reproduce the experimental shape. the three selected profiles shown here are plotted with a logarithmic vertical scale to clearly indicate the different behavior of the model functions and the experimental profiles for this higher range of heights. 5. conclusions a few analytical aspects of two commonused topside layer functions, with some variants were studied with a comparison among them and experimental profiles, using a least square fit approach. it appeared that all the considered functions have difficulties in reproducing experimental data in the required range of height. it is also evident that a different analytical formulation is needed in order to reproduce the relevant part of the topside. this new formulation may possibly involve a chapman layer function with α < 0.5 for heights lower than 400 km above the f2 maximum and should use a different functional approach for the higher part, up to 1000 km above hmf2. references bilitza, d., x. huang, b. reinish and r.f. benson (2002): topist automated processing of isis topside ionograms, in proceedings of 2002 ionospheric effects symposium, 333-340. bilitza, d., b. reinish, r. benson, j. grebowsky, n. papistashvili, x. huang, w. schar and k. hills (2003): online database of satellite sounder and insitu measurements covering two solar cycles, adv. space res., 31 (3), 769-774. coïsson, p., s.m. radicella and b. nava (2002): comparisons of experimental topside electron concentration profiles with iri and nequick models, ann. geophysics, 45 (1), 111-116. huang, x., b.w. reinisch, d. bilitza and r.f. benson (2002): electron density profiles of the topside ionosphere, ann. geophysics, 45 (1), 125-130. pulinets, s.a. (1989): prospect of topside sounding, in wits handbook (scostep publishing, urbana, il), vol. 2, ch. 3, 99-127. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27 (1), 35-40. 11 12 fig. 11. isis2 experimental profile (solid black) and the fitted functions: chapman-alpha dashed, chapmanbeta dotted, epstein dash-dotted, nequick solid gray. fig. 12. isis2 experimental profile (solid black) and the fitted functions: chapman-alpha dashed, chapmanbeta dotted, epstein dash-dotted, nequick solid gray. vol52,2,2009 137 annals of geophysics, vol. 52, n. 2, april 2009 key words lp – seismology waveform correlation – source geometry – volcano sismology – volcano monitoring 1. introduction the island of vulcano (500 m a.s.l.) is a composite volcanic edifice located in the southcentral sector of the aeolian archipelago (tyrrhenian sea, italy). the island, together with lipari and salina islands (fig. 1), represents the emerged part of the tindari-letojanni system (tl), a nw-se elongated volcanic ridge affected by a right-lateral strike slip, moving in response to a n100e regional extension field (mazzuoli et al., 1995). if the overall complex is mainly controlled by the tl system, the northern sector of vulcano island is characterized by ne-sw and n-s trending normal structures which accommodate the horizontal movements of the main system (fig. 1) (mazzuoli et al., 1995). along these two oblique trends are aligned the primary (dikes, vents and eruptive fissures) volcanic structures (mazzuoli et al., 1995; ventura et al., 1999). recent eruptions on the island have taken place at vulcanello (1550) and la fossa crater (1888-1890), with volcanic products consisting mainly of pyroclastic material with lesser volumes of lava flows. la fossa is a 391 m high cone with a base diameter of 1 km whose historic activity has been characterized by frequent transitions from phreatomagmatic to minor magmatic activity. after the last eruptive episode (1888-1890), volcanic activity has been restricted to fumarolic high precision locations of long-period events at la fossa crater (vulcano island, italy) salvatore gambino, laura cammarata and salvatore rapisarda istituto nazionale di geofisica e vulcanologia, sezione di catania, italy abstract since the last eruption in 1888-90, the volcanic activity on vulcano island (aeolian archipelago, italy) has been limited to fumarolic degassing. fumaroles are mainly concentred at the active cone of la fossa in the northern sector of the island and are periodically characterized by increases in temperature as well as in the amount of both co2 and he. seismic background activity at vulcano is dominated by micro-seismicity originating at shallow depth (<1-1.5 km) under la fossa cone. this seismicity is related to geothermal system processes and comprises long period (lp) events. lps are generally considered as the resonance of a fluid-filled volume in response to a trigger. we analyzed lp events recorded during an anomalous degassing period (august-october 2006) applying a high precision technique to define the shape of the trigger source. absolute and high precision locations suggest that lp events recorded at vulcano during 2006 were produced by a shallow focal zone ca. 200 m long, 40 m wide and n30-40e oriented. their occurrence is linked to magmatic fluid inputs that by modifying the hydrothermal system cause excitation of a fluid-filled cavity. mailing address: dr. salvatore gambino, istituto nazionale di geofisica e vulcanologia (ingv), sezione di catania, p.zza roma 2, 95123 catania, italy; e-mail: gambino@ct.ingv.it vol52,2,2009 17-06-2009 19:02 pagina 137 138 s. gambino, l. cammarata and s. rapisarda degassing, mainly at la fossa with mean temperatures ranging between 200° and 300° c. the origin of the geothermal system at vulcano has been discussed by several authors using different approaches and chemical tracers (e.g. chiodini et al., 1995; todesco, 1997; capasso et al., 1999; paonita et al., 2002 gambino and guglielmino, 2008); there is a general agreement that the fumarole gas composition results from the mixing of a deeper magmatic source and a shallow boiling hydrothermal system. la fossa crater is characterized by the occurrence of periodical anomalous degassing episodes with increasing output of the fumarole and chemical changes caused by new magmatic fluid inputs (e.g. granieri et al., 2006; paonita et al., 2002; chiodini et al., 1995) from a deep pressurized stationary magma body (granieri et al., 2006; gambino et al., 2007). earthquakes occurring in the area of vulcano are associated with both fracturing (sporadic swarms of low magnitude shocks) (aubert and alparone, 2000) and degassing processes of the geothermal system (montalto, 1994). la fossa shallow (<1-1.5 km) microseismicity is linked to hydrothermal activities at vulcano island and an increase in its occurrence accompanies periods of anomalous degassing (chiodini et al., 1992). microseismic background activity comprises two main groups of events: m-type and n-type, as extensively discussed by montalto (1994). long-pefig. 1. map of the vulcano permanent and temporary seismic network. structural lineaments from mazzuoli et al., (1995). vol52,2,2009 17-06-2009 19:02 pagina 138 139 high precision locations of long-period events at la fossa crater (vulcano island, italy) riod (lp) events at vulcano have been reported by godano and vilardo (1991) who using 19871988 data recorded by a temporary network located this seismicity on the se part of the cone. lps are generally related to vibrations and/or volumetric changes of a fluid-filled resonator, in which the fluid has a hydrothermal or magmatic origin (chouet, 1996). the shape of a resonant structure may vary in relation to the feeding/hydrothermal system features; precise hypocentre locations may help to image this shape. table i. hypoellipse location parameters of the analyzed earthquakes. n date origin time latitude long. depth gap no rms erz erh km b.s.l. km km 1 060809 10:23:44.66 38.4042 14.9702 0.52 174 7 0.07 0.2 0.7 2 060821 01:56:39.88 38.4078 14.9652 0.56 190 6 0.04 0.2 0.5 3 060823 06:12:29.89 38.4028 14.9713 0.49 167 7 0.14 0.2 0.8 4 060827 02:54:44.49 38.4023 14.9695 0.72 156 7 0.12 0.2 0.8 5 060828 03:04:10.34 38.4023 14.9643 0.84 123 7 0.06 0.3 0.6 6 060901 01:23:54.54 38.4030 14.9663 0.78 144 6 0.03 0.3 0.8 7 060901 21:22:11.80 38.4030 14.9653 0.75 138 7 0.14 0.3 0.6 8 060903 00:11:57.58 38.4005 14.9672 0.77 132 7 0.14 0.3 0.6 9 060905 05:57:41.62 38.4043 14.9677 0.93 163 6 0.03 0.3 0.8 10 060905 17:40:14.68 38.4008 14.9683 0.76 139 7 0.03 0.3 0.6 11 060906 23:07:43.17 38.4023 14.9685 0.66 151 7 0.04 0.2 0.7 12 060907 05:53:49.72 38.4030 14.9643 0.76 129 7 0.08 0.3 0.6 13 060909 15:56:09.24 38.4043 14.9628 0.78 118 7 0.06 0.2 0.5 14 060910 23:51:42.95 38.4065 14.9627 0.74 133 6 0.02 0.1 0.6 15 060919 20:02:26.14 38.4018 14.9682 0.83 146 6 0.01 0.4 0.9 16 060921 03:29:21.70 38.4077 14.9655 0.60 189 7 0.22 0.1 0.5 17 060921 05:32:08.16 38.4028 14.9713 0.54 167 7 0.11 0.2 0.8 18 060929 20:56:29.57 38.4052 14.9647 0.70 150 7 0.08 0.2 0.5 19 061006 00:37:20.09 38.4038 14.9673 0.70 201 6 0.02 0.5 0.8 20 061011 02:09:47.41 38.4020 14.9647 0.91 124 7 0.06 0.3 0.6 21 061013 04:17:08.26 38.4060 14.9612 0.73 112 6 0.07 0.2 0.6 gap = azimuthal gap (degrees); rms = travel-time residual root mean square (s); no = number of p arrivals; erz, erh = vertical and horizontal location error. vol52,2,2009 17-06-2009 19:02 pagina 139 140 s. gambino, l. cammarata and s. rapisarda vulcano long periods events are characterized by nearly identical waveforms (multiplets); the application of cross-spectral/crosscorrelation techniques on multiplets enables obtaining precise relative locations with accuracy within 5-20 m (e.g. poupinet et al., 1984; haase et al., 1995). concerning tectonic events, these techniques help in reconstructing the seismogenic fault plane (e.g. alparone and gambino, 2003) while relocation of lps does not define fault planes but rather volumes (battaglia et al., 2003). lp precise relocations have been performed on kilauea volcano (e.g. battaglia et al., 2003; wolfe et. al., 2003), soufrière hills volcano montserrat (rowe et al., 2004; green and neuberg, 2006) and mt. etna (gambino, 2006). this paper considers 21 lp multiplets by integrating permanent and temporary seismic station data recorded during the anomalous degassing period august-october 2006. we applied a cross-spectrum technique derived from fig. 2 a-c. occurrence of lp during 2006 (a), waveforms (b) and spectra (c) of the events analyzed (and relocated). each event number is referred to tab. i. vol52,2,2009 17-06-2009 19:02 pagina 140 141 high precision locations of long-period events at la fossa crater (vulcano island, italy) frèmont and malone (1987) to a subset of 14 long-period events, located on se part of la fossa crater at vulcano. the aim of this paper is to obtain a picture of the source volume shape for these earthquakes. 2. data since the late 70’s, continuous seismic monitoring activity on vulcano has been performed by a permanent seismic network composed of four analogical 3c (three components) stations (fig. 1). recently, new digital 24-bit seismic stations equipped with broad-band (0.2-40 sec) nanometrics trillium instruments have been installed at 3 of the 4 stations of the permanent network. in addition, at the end of 2005, three temporary broadband stations were installed on the northern rim of the volcano crater (la fossa). during 2006, we recorded about 65 lp events with similar waveforms, 44 of which occurred during august-october (fig. 2a). we focus our analyses on this period in which an increasing release of co2-he rich magmatic fluids was recorded at the rim fumaroles (a. paonita pers. comm.) and no recording problems affected permanent and temporary stations. vulcano lps show an emergent onset, short duration and spectra ranging about 1-6 hz with a single dominant peak at about 2.5 hz (figs. 2b, 2c). we recognized a p wave on lp first arrival by high values of the «rectilinearity» (fig. 3) lasting about 0.7 seconds obtained by a three component polarization analysis (jurkevics, 1988). 3. location and q estimation we obtained suitable locations for 21 microearthquakes (table i, fig. 4) using the hypoellipse program (lahr, 1989) which accommodates the difference in altitude of the seismic stations with a local one-dimensional velocity crustal model derived by falsaperla et al. (1985) and previously used by montalto (1994) and aubert and alparone (2000). hypocenters are located on the eastern and south-eastern sector of la fossa 0.5-0.9 km depth. mean epicentre error (erh) is about 0.7 km and 0.3 km for the focal depth errors (erz); rms values are lower than 0.2 s. we further analyzed lp waveforms (event n. 18 in table i) using the sompi method (kumazawa et al., 1990) that performs a spectral analysis based on an autoregressive (ar) model and determines the complex frequencies (frequency and quality factor q) of decaying oscillations. figure 5 reports the diagram frequency (f), versus growth rate (g), (g= -2q/f,) where the complex frequencies for different trial ar orders between 20-60 are plotted (kumagai and chouet, 2000). areas of the diagram that are densely populated represent stably determined complex frequencies and indicate q-values near 20, while scattered points indicate incoherent noise. 4. re-location method the presence of multiplets suggests spatially close sources whose geometry may be inferred using cross-spectral/cross-correlation relocation techniques. the application of these techniques on the p-wave furnish, for tectonic events, the positions above the fault radiating most of the energy, thereby allowing the fault plane reconstruction; indeed their application on lp events provides the relative event starting points allowing to define volumes (battaglia et al., 2003; wolfe et al., 2003). if we assume that at la fossa lps correspond to the resonance of a fluid filled volume, then their precise relocation may furnish an image of the container geometry. we used a cross-spectrum method based on that discussed by frèmont and malone (1987). this method permits very accurate relative timing (dt) for pairs of earthquakes with very similar waveforms (doublets) and subsequently to perform precise relative relocations. each doublet comprises a reference event (master event) and one of the other events belonging to the same multiplet. the differences dt in first arrival time between seismograms from the same station of a doublet have been computed in the frequency domain using the phase of the cross spectrum obtained on a short window containing the whole p phase. in particular, dt is proportional vol52,2,2009 17-06-2009 19:02 pagina 141 142 s. gambino, l. cammarata and s. rapisarda to the slope of the phase of the cross spectrum which can be written as φ(f) = 2πdtf, plotted versus the frequency (f). the degree of success of this procedure depends on the similarity of the waveforms and the signal to noise ratio on all traces. a parameter measuring the similarity degree between two waveforms is the coherency c(f), defined as the ratio of cross-spectrum modulus over the product of the spectra of the two signals (frèmont and malone 1987): c(f) = |γs1s2(f)| / [(γs1)1/2(f)(γs2)1/2(f)] (4.1) where γs1(f) = s1(f)s1*(f) and γs2(f) = s2(f)s2*(f) are the spectra of the first and second signals, refig. 3. example of three component waveforms recorded at stz2 (event n. 13 in tab. i) and calculated temporal trend of the rectilinearity. vol52,2,2009 17-06-2009 19:02 pagina 142 143 high precision locations of long-period events at la fossa crater (vulcano island, italy) spectively, and γs1s2(f) = s1(f)s2*(f) is the crossspectrum (* denoting complex conjugate). mean coherency calculated above the pwave frequency interval defines the quality (qw) of comparison between two waveforms. two very similar signals have a qw>90. a decreasing value of qw indicates a poorer similarity and generally 80 is the threshold below which it is difficult to obtain dt with acceptable errors (frèmont and malone 1987). pairs of events that do not correlate highly may be caused by a low signal to noise ratio; generally smaller magnitude events recorded at distant stations are more difficult to match. the position and difference in origin time of the second with respect to the master event have been determined using a decomposition in singular values technique discussed by aki and richards (1980) (volume ii, chapter n. 12) and realized by frèchet (1985). the parameters needed to obtain a suitable relocation of an event are: the p-wave velocity in the source volume, the take-off angles and azimuths of stations from the reference event. the method requires a minimum number of 5 time differences to obtain the four unknowns (relative position and difference in origin times) with an associated error. 5. results the application of the cross-spectrum method (frèmont and malone, 1987) on lp vulcano events limited the definitive set to 14 events; this factor was mainly caused by low signal to noise ratios at ivlp, ivlt and ivug stations. for each event, we considered the waveforms of the 7 stations of permanent and temporary networks. event n. 18 (table i) characterized by the larger peak to peak amplitude was chosen as the master event; the relative timing (dt) for pairs of table ii. results of the event relocation. n master n event an. st. mean qw t. err. (ms) dx (m) dy (m) dz (m) ril. err. (m) 18 01 6 96.6 2.5 40.40 23.81 -9.57 16.25 18 02 6 96.0 1.6 52.10 67.53 1.56 6.1 18 03 7 91.4 2.7 86.01 152.02 2.36 9.33 18 04 6 95.0 1.5 80.29 147.11 -8.58 6.79 18 05 7 94.4 2.7 53.75 69.73 5.55 9.56 18 07 6 94.3 1.9 52.15 79.54 -1.03 8.19 18 08 6 97.3 0.7 -4.45 -10.95 -17.20 2.95 18 11 6 96.5 3.0 26.02 37.15 -0.85 11.57 18 13 6 97.2 1.6 24.13 34.48 2.01 3.12 18 14 6 95.6 0.9 24.88 12.34 0.96 4.71 18 15 6 97.6 1.7 11.47 -3.56 1.70 4.83 18 17 6 97.2 0.8 32.57 36.70 -15.01 8.36 18 20 6 94.7 2.6 16.22 12.19 3.72 3.98 an. st. = number of stations used in recolation; mean qw = mean of the quality factor at different stations; t. err. = mean relative time error; dx, dy, dz. location differences with associated error (ril. err.) respect to the master event. vol52,2,2009 17-06-2009 19:02 pagina 143 144 s. gambino, l. cammarata and s. rapisarda earthquakes was obtained on a 1.28 s window of signal (128 points) starting about 0.24 s before the first arrival, thereby allowing the complete sampling of the p wave train as also performed by wolfe et al., (2003). the quality of comparisons (qw) between the events at different stations always exceeded qw = 90 somewhat being qw = 95 between most events (table ii); the relative timing between the dt shows errors less than ± 0.003 s. we performed the relative locations using the parameters (take-off angles and azimuths) obtained by analytical location of the master event and a velocity of 1.5 km/s in the source region falsaperla et al. (1985). location differences between events range between several meters to several tens of meters with associated errors smaller than 10-15 meters. lps relocation (fig. 6) shows the relative positions of the events starting point inside the resonator and clearly describes an elongated shape located 700 m b.s.l. (depth of master event) in a n30-40e direction, 200 m long 40 m wide, only roughly 25 m deep. we also notice that lps occurring in the period september 3-october are located in the sw part of the fluid-filled volume with respect to august-september 1 events (fig. 6). however, locations do not show a progressive migration of the source and it is not easy to recognize a possible mechanism. 6. discussion and conclusions the magmatic system at la fossa volcano has been studied through petrologic and geochemical investigations. analyses on co2 fluid inclusions in quartz xenoliths (clocchiatti et al., 1994; zanon et al., 2003) suggest that the plumbing system beneath la fossa consists of small dykes and magma pockets cutting through intrusive rocks at 1.6-2.0 km depth; nuccio and paonita (2001) inferred a magma body at depth of 2.5 km by means of geochemical analyses. we analyzed 21 lps recorded during an anomalous degassing period (august-october fig. 4. epicentral map, ns and ew cross-sections of the located events. vol52,2,2009 17-06-2009 19:02 pagina 144 145 high precision locations of long-period events at la fossa crater (vulcano island, italy) 2006) in which an increasing release of co2-he rich magmatic fluids was recorded at the rim fumaroles. these events appear to have highly similar waveforms, indicating repetitive and non-destructive source mechanisms occurring within a very limited source volume. we obtained reliable absolute locations of a source positioned under the crater/the se part of the cone at depth of 0.6-0.9 km (± 0.3 km) b.s.l. (fig. 4). lps are shallow and this factor leaves little doubt on the hydrothermal origin of a possible fluid involved in lp occurrence. moreover, if we consider a fluid-filled crack model, observed frequencies and low q-values obtained by sompi analysis (q=ca. 20) are explainable by fluids in the form of bubbly water (kumagai and chouet, 2000) or alternatively as steam (kumagai et al., 2005). it is reasonable that at la fossa, magmatic fluid inputs modify the hydrothermal system, causing a gradual build-up of steam pressure at depth that may excite hydrothermal fluid-filled containers (lp) and on the surface, geochemical variations. results of high precise relocations suggest a trigger region of about 200 m (long) x 40 m (wide) oriented along the ne-sw structural system (fig. 6). the limited variations in depth (all events are confined within 25 m) may indicate a horizontal source. these kinds of sources are not unusual at other volcanoes such as kusatsushirane in japan (nakano et al., 2003) or kifig. 5. frequency, versus growth rate diagram where the complex frequencies for different trial ar orders between 20-60 are plotted. areas of the diagram that are densely populated represent stably determined complex frequencies and indicate q-values near 20, while scattered points indicate incoherent noise. vol52,2,2009 17-06-2009 19:02 pagina 145 146 s. gambino, l. cammarata and s. rapisarda lauea volcano (kumagai et al., 2005). the presence of a hundred-meter long horizontal source suggests some complexity of la fossa fluid circulation system, generally thought to comprise only vertical ducts; moreover ne-sw structural trend seems to play a role in the fluid circulation system controlling the fluid-filled cavity trend. this fact seems to be also confirmed by the presence of surface structural ne-sw lineaments (fig. 6), consisting in faults and fracture (gabbianelli et al., 1991), close to the lp epicentral area. acknowledgements we thank p. dawson for constructive comments, h. kumagai for sompi program codes, f. cannavò and s. conway for correcting and improving the english of this paper. this study was supported by the 2005-2007 dpc-ingv v3_5 vulcano and by the 20072009 dpc-ingv v1 unrest projects. references aki, k. and p.g. richards (1980): quantitative seismology, theory and methods, (w.h. freeman, san francisco), pp. 641-719. alparone, s. and s. gambino (2003): high precision locations of multiplets on south-eastern flank of mt. etna (italy): reconstruction of fault plan geometry, phys. earth plan. int., 135, 281-289. aubert, m. and s. alparone (2000): variation d’origine sismique du flux de chaleur convectif dans la fossa de vulcano (italie), c.r. acad. sci. paris, sciences de la terre et des planètes/earth and planetary sciences, 330, 603-610. battaglia, j., j.l. got and p. okubo (2003): location of long-period events below kilauea volcano using seismic amplitudes and accurate relative relocation, j. geophys res., 108, 2553, doi:101029/2003jb002517. capasso, g., r. favara, s. francofonte and s. inguaggiafig. 6 a-c. epicentral map of located and relocated events. the insets represent the enlarged map (a), l-l’ (b) and t-t’ (c) cross-sections of relocated events. origin coordinates and depth refer to n. 18 event. vol52,2,2009 17-06-2009 19:02 pagina 146 147 high precision locations of long-period events at la fossa crater (vulcano island, italy) to (1999): chemical and isotopic variations in fumarolic discharge and thermal waters at vulcano island (aeolian islands, italy) during 1996: evidence of resumed volcanic activity, j. volcanol. geotherm. res., 88, 167-175. chiodini, g., r. cioni, s. falsaperla, m. guidi, l. marini and a. montalto (1992): geochemical and seismological investigations at vulcano (aeolian islands) during 1978-1989, j. geophys res., 97, 11025-11032. chiodini, g., m. cioni, l. marini and c. panichi (1995): origin of the fumarolic fluids of vulcano island, italy, and implications for the volcanic surveillance, bull. volcanol., 57, 99-110. chouet, b.a. 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(1997): origin of fumarolic fluids at vulcano (italy). insights from isotope data and numerical modeling of hydrothermal circulation, j. volcanol. geotherm. res., 79, 63-85. ventura, g., g. vilardo, g. milano and n.a. pino (1999): relationships among crustal structure, volcanism and strike-slip tectonics in the lipari-vulcano volcanic complex (aeolian islands, southern tyrrhenian sea, italy), phys. earth planet int, 116, 31-52. wolfe, c.j, p.g. okubo and p.m. shearer (2003): mantle fault zone beneath kilauea volcano, hawaii, science, 300, 478-480. zanon, v., m. l. frezzotti and a. peccerillo (2003): magmatic feeding system and crustal magma accumulation beneath vulcano island (italy): evidence from co2 fluid inclusions in quartz xenoliths, j. geophys. res., 108, 2298, doi:10.1029/2002jb002140. (received october 21, 2008; accepted february 10, 2009) vol52,2,2009 17-06-2009 19:02 pagina 147 biagi_final:layout 6 annals of geophysics, 57, 1, 2014, s0190; doi:10.4401/ag-6367 s0190 the collocation approach to moho estimate riccardo barzaghi, ludovico biagi* politecnico di milano, dipartimento di ingegneria civile e ambientale, milan, italy abstract in this paper, the collocation approach to moho estimate is presented. this method is applied to the inversion of gravity data that can be complemented by moho depth information coming from e.g. seismic information. in this context, a two layers model is considered and discussed in order to give a general theoretical framework for the inversion method. a body with two inner constant density layers and an inner separation surface between is considered and a uniqueness theorem is proved for the estimability of the separation surface given the gravity outside the body itself. based on this result, a discussion is given on the estimation of the moho depths based on terrestrial gravity observations. the observation equation is presented and its local planar approximation is derived. the application of the collocation method to the estimate of moho depths is then studied and discussed in relationship to the planar observation equation. also, numerical tests are presented. to this aim, the collocation inversion algorithm is implemented and tested on simulated data to prove its effectiveness. the results show that the proposed method is reliable provided that proper data reductions for model discrepancies are taken into account. 1. introduction the transition layer between the upper mantle and the lower part of the crust is defined as the mohorovicic discontinuity (moho) and ideally considered as a surface [turcotte and schubert 1982]. on a global scale, this surface has sharp variations, ranging from over 60 km to less than 5 km depth. the density variation occurring across the moho between the upper mantle and the lower crust is generally set at 0.5 g/cm3, based on the assumption that the upper mantle density is 3.4 g/cm3 and the lower crust density is 2.9 g/cm3 [anderson 1989]. the moho transition layer causes seismic wave refraction and reflection and the related gravity signal as measured on the earth surface can have a standard deviation ranging from 50 to 100 mgal. moho depths can be estimated using both seismic and gravimetric inversion methods [parker 1972, lebedev et al. 2013]. seismic methods can give more accurate estimates that are not however homogeneously distributed over the earth. on the contrary, gravity is quite homogeneously distributed over the entire earth and only relatively small areas are un-surveyed [pavlis and rapp 1990]. furthermore, satellite dedicated gravity missions [reigber et al. 1999, albertella et al. 2002, tapley et al. 2004] have made available a large amount of data that can be profitably used in combination with ground based gravity data [shin et al. 2007]. recently, an approach based on the integration of gravity and seismic information has been proposed by eshagh et al. [2011]. they devised a method based on a stochastic combination of seismic and gravity moho models. in this paper, the gravity estimation of the moho based on the collocation method is presented [krarup 1969, moritz 1989, barzaghi et al. 1992] which also allows integration of gravity and seismic information available in the investigated area. the basic idea of this method is to propagate the covariance structure of the moho depth to the covariance of the observed gravity. given gravity observations and moho depth information (e.g. from seismics), the moho collocation estimate can be obtained as a linear combination of these data. also, collocation allows the computation of the variance of the estimation error that can be used as an indication of the estimated depth precision. this approach is here devised for a two layers model and a uniqueness theorem is proved as a theoretical background for the proper application of the method (see appendix). in this context, the collocation inversion method for the two layers model is presented and the impact of model discrepancies on the moho estimate is discussed. computation problems are addressed too. the method effectiveness is then tested using simulated gravity data. 2. the observation equation for the gravity effect of a two layers body in the following the two layers body is considered. this is defined as a body with an inner volume and an external layer: the two parts have different densities and article history received june 14, 2013; accepted november 15, 2013. subject classification: moho, gravity collocation, gravimetric inverse problem. are separated by a unique surface. particularly, we consider a spherical two layers body. we also assume that this spherical body has a radius r = 6380 km and that the surface separating the external layer from the inner volume is at a depth ranging from 5 to 60 km. in a very simplified way, this can be assumed to be the reference model for the earth and the moho. given this model, a new method is developed to obtain a moho estimate on regional areas, e.g. areas having a 6° × 6° extension. furthermore, the method and the equations are given in planar approximation. gravity observations are supposed to be known on the surface of the sphere representing the earth. in addition, moho depths can be given at scattered points. the density of the outer layer tc and the inner density tn are assumed as known and independent from the radial component. so, tc and tn depend only on the spherical coordinate v, v = ({,m). this is a special case in the theorem proved in the appendix and thus it still holds that the inner separation surface is unique. the potential generated from this body in a point p on the external surface is expressed by v(p) = nn tn + nc tc , where nn and nc are the newtonian operator applied to the inner volume and the outer layer respectively. if we define a normal potential u(p) = nb tn = nn tn + nc tn (nb is the newtonian operator extended to the whole body), we can define the anomalous potential t as t(p) = v(p) − u(p) = nc (tc − tn) = nc (t), where we have put t = tc − tm . this definition implicitly poses the density of the normal potential equal to the mantle density. other choices can be given and are available in literature, for example by putting the ‘normal’ density equal to the earth mean density. however, in our application the gravity anomaly is derived by t and a different choice of density just affects the mean dg that in any case will not be used. thus t(p) is given by [moritz 1990, sjoberg 2009] p is a point on the external surface, rp , vp are the radial and angular coordinates of p: hypothesizing a spherical body, rp = . rpq = (]pq is the angle between op and oq, o being the center of the sphere). the term is the depth of the separation surface between the two layers. the equation for gravity dg = −^t/^r is then since we can express dg in the following way with qr and qr−h running over the outer sphere surface and the inner separation surface. the planar approximation of these equations can be obtained in a reference system having the z axis along the plumb line in the midpoint o of the investigated area and the (x,y) plane tangent to the outer sphere in o (see figure 1). we define n' as the inner surface normal vector and n as the unit vector along the radial direction. if we name ds as the area element of the inner separation surface, we have dsn' · n. the vector n can be further decomposed into ez + dex , being ez its component along the z axis and dex the component onto the (x,y) plane. it follows that n' · n = n' · ez + n' · dex . if we assume to consider areas having maximum width of about 6°, dex , which is the tangent of the angle between n and ez , has a maximum value that is 1/20 of ez . this implies that, at the first order, we can approximate n' · n � n' · ez , that is � dsn' · ez. the second term in the equation giving dg can be thus simplified accordingly this integral can be further simplified by assuming � 1 (this is true with an error which is, at maximum 1%). in this way, it follows that 2 cosr rq q pq 2h h ]+ -2r ( )r h vt p q r q q g r q db g d d pq c n n pq c c q sphere q r h r 2 t t v hh t t= = = v^ ^ ^ ^ ^ ^ h h h h h h ### ## # ,g r g d d r r 1 p r r s h q pq 2 2 2 d v vt v h h=v^ ^ ^ h h h ## # 3 2 r r r r r r r r r r r r r r 1 1 2 r pq q pq q q pq pq q pq q pq q pq q pq q r pq q pq q pq q 2 2 3 2 3 2 2 3 2 3 2 2 2 2 2 2 2 h h h h h h h h h h h == =+ = = =+ h h h h c ` c c c c m j m m m m ( )d r h 2v = ( )d r h 2v , , r g d r r h r g dxdy x y r r h x y pqr h s r pqr h 3 2 , , vt v v t ^ ^^ ^ ^^ h hh h hh ## ## ( ) /r h r, r g d r r h g d d r 1 pq s r pq 3 r h r h2 , , vt v v p it p i ^ ^^ ^ h hh h ## ## ,g r g d r r r g d r r h p s pq pqr h s 2 3 r d v vt v vt v v = + ^ ^ ^ ^^ h h h hh ## ## barzaghi and biagi 2 (2.1) (2.2) (2.3) (2.4) (2.5) (2.6) 3 the same holds for the first integral. hence that is where . this is a non-linear integral equation between h(x,y) and dg. in order to get it in a form that is suitable for applying collocation, we have to linearize it. to this aim, we define (e[+] is the mean operator) and we set h(x,y) = . moreover, let assume that : note that in some areas this is not true and the resulting approximation could represent one possible limitation of this approach that must be carefully considered in its application. in any case it is needed for getting dg(x,y) as a linear functional of f(x,y) as required for applying collocation: more investigations on the approximation consequences will be discussed in the tests of section 4. therefore, considering only the linear terms in f, we obtain if we further consider the density contrast between the crust and the upper mantle, we set and . thus, the equation for dg(x,y) can be written as with in its linearized form, the dg(x,y) observed gravity is given as the sum of four terms that will be analyzed in the following. integrals i2, i3 are linear terms in f and dt while i4 contains the product f(p,i)dt(p,i). this last integral is thus a second order term which is likely to be smaller than i2. so, in a first approximation, we assume that i2 + i4 � i2. if we now disregard i4 , we have e[dg] = e[i1] + e[i2] + e[i3]. in our hypotheses on f and dt, it holds that , which implies that . so, in principle, one can estimate from observed gravity. this is indeed possible only in theory but not in practice due to the standard definition of observed gravity anomalies (there is a mismatch in the reference density between our definition and the standard one based on the normal ellipsoidal field). thus we assume to have an estimate of as given independently (e.g. from seismic information). the last term to be discussed is i3. it gives the gravity effect due to density variations over the thickness hmax %f ( , ) ( ) ( )d x yxy 2 2p i p i= + ( , )h e h x y= 6 @ [ ( , )]e x yt t= , , , , , , , , , , , , , , , ,g x y h h x y h x y i h x y i x y i i 0 4 1 2 3 d t t f dt dt f = + + + + ^ ^ ^ ^ ^ h h h h h [ ( , , , , )] [ ( , , , )] 0e i h hx y e i x y2 3t f dt= = [ ] / (2 )h e g gd r t= h , , , , g x y g d d r g d d r 0 pq r pq r h r r2 2 d p i t p i p i t p i = + ^ ^ ^ h h h ## ## , , , , , , g x y g d d d d h 0 1 1 / r xy xy 2 2 1 2 2 ,d p i p i p i p i p i + + ^ ^ ^ ^ ^ h h h h h ; 6 @ e ## , , , , , , , , g x y g d d d h d g d d h d h 0 1 1 / / r xy xy r xy 2 1 2 2 3 2 2 2 t t d p i p i p i p i p i p i p i f p i = + + + + + 2 2 ^ ^ ^ ^ ^ ^ ^ h h h h h h h ; 6 6 @ e @ ## ## , , , , , , , , , , , , , , , , i h g d d d h d g h i h g d d h d h i h g d d d h d i h g d d h d h 1 1 2 1 1 / / / / r xy xy r xy r xy xy r xy 1 2 1 2 2 2 3 2 3 2 1 2 3 2 3 2 2 2 2 2 t p it p i p i r t t f p it p i f p i dt p idt p i p i p i dt f p idt p i p i f p i = + + = = + = + + = + 2 2 2 2 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h h h h h h h ; 6 6 ; 6 6 @ e @ @ e @ ## ## ## ## h the collocation approach to moho estimate figure 1. spherical geometry and planar approximation. ( , ), [ ] 0h x y ef f+ = [ ] ( , , , ) 2e g i h x y g h1d t r t= = ( , ) ( , ), [ ] 0x y x y et t dt dt= + = (2.7) (2.8) (2.9) (2.10) (2.11) layer. the impact of this term can be large as it can be assumed that, in the mean over the whole earth, � 30 km. hence, this term must be locally modeled based on available crustal density variation data in the investigated area. the amplitudes of the four integrals terms and the impact on the estimated f values will be further discussed in the numerical simulations given in paragraph five. 3. the collocation estimate method it is well known [menke 1989, tarantola 2005] that the gravity inversion is unstable. thus the estimation algorithm to be applied should be stable and smooth with respect to high frequency gravity signal components. collocation method can be successfully applied to this aim. it is a stochastic filtering/predicting method that filters out high frequencies based on the covariance structure of the data. it has been widely applied in geodesy [tscherning 1985, moritz 1989] particularly in view of the use of heterogeneous data. in our application, this implies that dg data can be used together with f known values (coming from e.g. seismic) to get an integrated estimate of f. this method can be properly applied to the observation equation . thus, in order to get unbiased estimates, we need to model and remove the signals related to the i3 and the i4 terms that account for density variations. as it will be proved in the simulations, these terms give a significant signal having also a frequency signature that couples with i2. however, it can be reasonably assumed that i4 ≪ i2 and i4 ≪ i3. thus, the proposed estimation method is performed according to the following steps. 1) estimates of and dt(x,y) are given using available information on t(x,y). 2) on observation points, the i3 term effect is estimated based on the given values of dt(x,y) and (this last term can be derived from known geological information and/or from seismic profiles). this effect can be straightforwardly computed using the closed formulas of the gravity effect of a prism [macmillan 1958]. 3) reduced gravity data are estimated as dgi(x,y,0) = dg(x,y,0) − e[dg] − and, assuming that dgi(x,y,0) � are estimated by collocation. 4) using the estimated values, i4 is computed and subtracted from dgi(x,y,0) to get dgii(x,y,0) = dgi(x,y,0) − . 5) then, the last computation step is accomplished using dgii(x,y,0) as input data and getting, by collocation, . thus, in the end, in order to see how collocation can be applied to get the estimate of f, a brief presentation is given in the following. a more detailed description can be found in barzaghi et al. [1992], biagi [1998] and forsberg [1984]. we assume that the following linear relationship between gravity (dg) and depth (f) holds assuming that and are known, the collocation estimate of f can be obtained based on dg observations and f values given in some scattered points. to get these estimates, we further have to assume that: 1) f is a weak stationary stochastic process, ergodic in the mean and in the covariance 2) the noises in gravity and depth, ng and nf are uncorrelated zero mean signals 3) the cross-correlation between signals and noises is zero. auto and cross covariances can then be summarized as follows using these covariances and observations plus noise we can get the following auto-covariance matrices of the observations the collocation estimate of f [moritz 1980, barzaghi et al. 1992] in a point pk = (xk ,yk) is given by with (3.9) ( , , 0) [ ] ( , , , , )g x y e g i h x y2d d t f= t h ( , , , )i h x y3 dt ( , )x yift ( , , , , )i h x yi4 dt ft ( , ) ( , )h x y h x yf= + =t t( , )x yiift th , , , , g x y h h g d d d 0 / xyr 2 2 3 2 2 d p it p i f p i = + ^ ^ ^ h h h 6 @ ## , , ; , , ; c g g c p p c g g c c p p c i j g g i j j i i j i j j i d d d d f f f f = = = = d d ff ^ ^ ^ ^ ^ ^ h h h h h h , , , ; , , c g c p p c g c n n c n g c n 0 i j g i j j i i j ij i 2 d d d v d f f f = = = = = fd^ ^ ^ ^ ^ ^ h h h h h h , , , , ..., , , , ..., g g n n g g x y i n x y i n 1 1i obs g i i i g obs i i=f f d dd d f f = + = = = + = f f ^^ ^^ h h h h c g g g g n cn i e e et g gobs obs obs t g g g g t n 2 g v d d d d = = = + = + d d d d d d d 6 6 6 @ @ @ c n c cn e e et obs obs obs t t nn f f ff = = = + = + ff ff f f f 6 6 6 @ @ @ ( , ) .h x yiif+ t h c c c lg t t ll 1f= fd ff -t 6 @ , ,,c cc g cg k obsi k obsii if d f f= =fd ff^ ^h h barzaghi and biagi 4 (3.1) (3.2) (3.3) (3.4) (3.5) (3.6) (3.7) (3.8) ( , , , , ), ( , )i h x y x y2 f t fit 5 to compute this estimate, we first have to estimate the empirical covariance function of dg which must be then modeled with proper positive definite model functions [moritz 1980]. also, models for auto and cross-covariances must be defined in order to compute the matrices defining the f estimator. this can be done via covariance propagation law applied to linear functionals [moritz 1980]. in our approach we assumed where lp [f] indicates the linear functional relating f to dg. it follows by covariance propagation law that a suitable model for c ff can be (j0 : zero order bessel function) which implies [biagi 1998] another model that can be used for the auto-covariance of dg is (j1 : first order bessel function) which gives the a and the a values contained in these models are tuned to fit the empirical estimated covariance values of dg. as stated above, models for c ff and c fdg can be then derived, allowing the computation of the matrices contained in the estimation formula for f. 3.1. some comments on the efficient computation of the estimation formula one numerical major problem in computing the collocation estimate of f is the inversion of the covariance matrix of the data cll. some standard data configurations are discussed here. we firstly assume that the number of known moho depths nh coming from seismic is at maximum 200. we then consider three different gravity data distributions: 1. a quite poor gravity data coverage and depth information: 2. high gravity data coverage and no depth information: 3. high gravity data coverage and depth information: case 1. can be handled using standard techniques such as cholevsky decomposition [see e.g. press et al. 1992]. the efficient numerical computations of case 2 and 3 can be obtained if we assume that gravity data are regularly gridded. in this case, the covariance matrix has a toeplitz/toeplitz structure: based on this particular structure, in case 2, fast collocation can be applied [bottoni and barzaghi 1993]. case 3 must be discussed in more details. also in this case, we assume to have gridded gravity data. on the other hand, depth information are assumed to be given on scattered points. under these assumptions, an efficient solution can be obtained by partitioning the solution. we have to solve the following system the partitioned solution is given by with in this way, the term m can be computed via fast collocation being dg on a grid. the same holds in com, , , ,c p q c p q l l c p q l c p qg g g p q p = = fd ff d d ff ^ ^ ^ ^ h h h h 6 6 @ @ ,c p q aj p q0 a= -ff^ ^h h , 2 , c p q g aj p q c p q g e aj p q e 2 2 h h g g g 0 2 0 2 r t a r t a = = d a fd d -^ ^ ^ ^ ^ ^ h h h h h h c r x aj x g g 1 a a = d d ^ ^ h h , , 2 c p q g a dkke j p q c p q g a dkke j p q k k 2 kh kh g 2 2 0 0 0 0 r ta r ta = = ff a f a ^ ^ ^ ^ ^ h h h h h # # 1000, ( ) [ , ]dimn c n n n n 1200g ll g gh h1 1= + + 1000, ( ) [ , ]dimn c n n 1200g ll g g2 2= , ( ) [ , ]dimn c n n n n1000 1200g ll g h g h2 2= + + v v v c c c c gtg g gg g obs obs 1 obs obs d f = = f d d fd fd ff ; = ;e g e v c v v n n n obs g t n n n ng 1 1 1 1 1 1 h h h h g gg f m m x c = = # # # # # # fdf f ^ ^ h h 6 6 6 6 6 6 @ @ @ @ @ @ i g c c c c c c c c c c c 0 0nn nn obs obs g g g g g g g ll g g obs obs d f = = = = = d d fd d ff d d fd d f ff f d f ; = = = e g g g , , , , g x y g d d h d h l 0 / r xy p2 2 3 2 2 d p it p i f p i f = = + = ^ ^ ^ h h h 6 6 @ @## c g c c n gg n n obs n n g g n n g n n t 1 1 1 g n h g g h g g g 1g g c m d= = # # # # #d d fd # d d dd 6 6 6 6 6 6@ @ @ @ @ @ c cn n n n n n n ngh h h h h g hg cx = -# # # #ff fd6 6 6 6@ @ @ @ the collocation approach to moho estimate (3.10) (3.11) (3.12) (3.13) (3.14) (3.15) (3.16) (3.17) (3.18) (3.19) (3.20) puting c, since we can apply fast collocation nh times in solving ci = . , where ci is the i-th column of , i = 1,..., nh. the memory allocation for this method is equal to (ng + nh)(1 + 2nh) since we have to store the first row of the toeplitz/toeplitz matrix cdgdg, the full c ff and c fdg matrices, the c and x matrices and the vector m. this memory occupation is smaller than those implied by the full cll matrix storage. furthermore, in terms of computation effort we have (nh + 1) fast collocation solutions having ng dimension and 2 cholesvky based solutions having nh dimension. the computation time is thus proportional to (nh + 1)nd 2 g logn dg + 2nh 3 which is less than the one require for solving the full cll system by standard cholevsky method. a last remark is in order. usually collocation is a stepwise procedure since it is an adaptive filter which is tuned by the empirical covariance. thus, in the first step, the low frequency component is dominant while in the further steps the high frequencies become dominant. at each step, residuals on observation points are obtained as the difference between observed and collocation predicted values. the iterative process stops when residuals (e.g. gravity residuals) are uncorrelated. 4. the simulated tests the devised method has been tested via numerical simulations that aim at verifying: the properties and the stability of the inversion procedure under model/data consistency, the impact of known hobs data in the inversion, i.e. the improvements that one can get with respect to the inversion based on gravity data only, the biases induced in the depth estimates when model inconsistencies are present, like for example, errors in the gravity contrast model or in the mean moho depth. a surface depth has been simulated on a regular (x,y) grid with 10 km grid knot over a rectangular area 600 km × 400 km. the depth values are given with respect to mean depth while the f values have been generated according to the procedure described in barzaghi et al. [1992]. the a-priori covariance structure of the f values has been fixed according to the following parameters the simulated depths (figure 2) have the following statistics v(f) = 6.4 km min(h)= −43.67 km max(h)=−16.00 km as one can see in figure 2, the resulting depth surface is smooth and regular which seems to be quite far from the real moho behavior, at least in some particular areas where high frequencies features are present (e.g. the alpine region). however, this is not so relevant since in the devised tests we are aiming at defining the impact of model inconsistencies on the inversion procedure. this can be critically evaluated even if high frequency pattern is not present in the target surface. generally, the simulated depth does not respect the condition adopted for the (2.9) linearization, i.e. : this will allow a sensitivity assessment of the proposed alc g t fd c cg g i 1 d d ( , ) ( , )h x y h x yf= + , 55 km , 0.0160 kmc aj r ar 0 2 1a a= = = ff -^^ hh 31.51 kmh =hmax %f barzaghi and biagi 6 figure 2. simulated depth. 30 kmh = (4.1) 7 gorithm with respect to this critical approximation. starting from the simulated depths, the gravimetric effect has been computed according to two different density contrast models. in the first case a constant density contrast t(x,y) = t0 = −0.52 g/cm 3 has been considered. in the second case, it is assumed that the density contrast varies linearly with x: in both the cases (figure 3), the gravimetric effect has been computed at z = 0 by prism integration formula [macmillan 1958] over the same grid used for the depth values. in the following, the two computed gravity fields will be named gc(x,y) (constant t0 value) and gl(x,y) (linearly varying tl value). furthermore, three depth sections have been computed based on the h(x,y) grid. in this way, we simulated 2501 h(x,y) grid values, 2501 gravity grid values and a total of 100 scattered depth observations on the three distinct sections. these preliminary simulations are aimed at the discussion of the theoretical model and the analysis of the implemented software: therefore, no noise has been added to the simulated observations. three different inversions have been carried out: 1) integrated inversion based on gc(x,y) gravity , ; . g/cm , . g/cm ; . g/cm x y x x y0 56 0 50 0 53 l l 0 3 3 0 3 # # t t a t t = + =^ ^ h h the collocation approach to moho estimate figure 3. gravity field with constant density model (top) and differences of the gravity field with the linear density model (bottom). and known depth values assuming a constant gravity model t0. this is the case when data and density are consistent. 2) inversion based on gc(x,y) gravity data only assuming the constant gravity model t0. also in this case data and density are consistent. 3) integrated inversion based on gl(x,y) gravity and known depth values assuming that density is linearly varying following the tl(x,y) rule. once again data and gravity model are consistent. 4) integrated inversion based on gl(x,y) gravity and known depth values assuming that density is constant and equal to t0. this is the critical case where we have a mismatch between data and density model. in all the inversions, the model covariance function of the gravity field has been fitted on the empirical covariance function of the simulated gravity observations [barzaghi and sansò 1983]: therefore, it has been propagated by (3.14) and (3.16) to model the cross covariance between gravity and depth and the covariance of depth. data inversions 1) and 2) are made for testing the impact of known depths in the collocation inversion procedure. test 3) is performed to check for the effectiveness of the iterative inversion procedure described in paragraph 4, assuming to have a known density model consistent with the available data. finally, test 4) is set to estimate the impact of improper density information on the integrated estimated depths. inversion 1) should provide the benchmark results, for three reasons. the adopted density model in the inversion is consistent with the simulated model, all the simulated observations are used and the constant density model does not require either approximations or iterations to manage i3 and i4 terms. in all the inversion procedures, as it would be in a real case, the mean depth has been estimated using the 100 given depth values. the obtained value is . in this way, the inversions are based on a biased mean depth. so, the impact of this bias on the results can be investigated too. furthermore, as a general remark on all the inversion procedures, it must be underlined that two iteration steps were necessary to get the final estimates since first step residuals were still spatially correlated (which was not the case for the second step residuals). the estimated depths have been then compared with the simulated ones in order to evaluate their precisions and accuracies (to avoid edge effects that can give biased statistics, comparisons are carried out in the 500 × 300 inner area). statistics of the differences for all the four inversion procedures are listed in table 1. statistics over the 100 known depths are also listed when such data are used in the different inversion procedures. figures 4 to 6 report the results provided by inversion 3, that are the most interesting. the following remarks are in order. the results obtained for inversion 1 (see row one in table 1) proved that depths can be correctly estimated even if the a-priori mean depth is biased. if scattered depth data are available, they can correct the initial bias and the final mean km.h 32 56=barzaghi and biagi 8 e(d) (km) v(d) (km) min(d) (km) max(d) (km) inversion 1: statistics over the inner grid points 0.5 0.4 −1.1 1.4 inversion 1: statistics over the 100 scattered points 0.5 0.4 −0.7 0.9 inversion 2: statistics over the inner grid points 2.2 0.4 0.2 4.1 inversion 3 (i3 only): statistics over the inner grid points 0.5 0.5 −2.0 1.5 inversion 3 (i3 and i4): statistics over the inner grid points 0.5 0.4 −1.3 1.4 inversion 3 (i3 and i4): statistics over the 100 scattered points 0.5 0.4 −0.6 0.9 inversion 4: statistics over the inner grid points 0.4 0.9 −2.6 2.3 inversion 4: statistics over the 100 scattered points 0.2 1.1 −2.5 1.7 table 1. statistics of the residuals d = hsimulated − .hestimatedt 9 discrepancy between estimated and simulated depths is 0.5 km, which is negligible if compared to the mean depth of 30 km. if gravity data only are used (inversion 2) the a-priori bias remains in the final estimates, because no depth observations are given to tune the mean. in this case the mean of the differences is 2.2 km (see row three the collocation approach to moho estimate figure 4. integrated inversion 3 (linear density model and consistent correction of i3 and i4 terms): prediction errors. figure 5. integrated inversion 3: gravity residuals and their covariance function. in table 1). however, it must be stressed that the f values are correctly estimated as it results from the standard deviation which is equal to the one obtained in inversion 1. the iterative procedure used in inversion 3 to correct for density heterogeneities leads to satisfactory results. it must be remarked that the computation and correction for the i4 term improves the results, even if this term is remarkably smaller than i3 (table 1, row four are statistics base on modeling i3 term only; in row five, i3 and i4 are used in the estimation procedure). generally, in cases where no model error is present in the inversion (inversions from 1 to 3) the standard deviation of the residuals (0.4-0.5 km) is less than 10 % of the standard deviation of the simulated depths (6.4 km); moreover, the approximation adopted to linearize the observation equations does not affect significantly the results: also on this regard, results are completely satisfying. the adoption of an improper model for density heterogeneities has a significant impact on the estimated depths (see table 1, row seven). this is true even in the case of the integrated inversion based on gravity and depth observations. the impact of density model mismatch can be evaluated in the residuals computed on the observed depths that show high discrepancies. hence, the observed depth values cannot compensate for an improper density model thus confirming the importance of a reliable correction for existing density heterogeneities (see also row eight in table 1; even the residuals on given depth points are worse than in the previous cases). 5. conclusions the collocation method can be profitably used to combine gravity and depth information in order to get reliable moho estimate. in this framework, a theoretical background has been provided through a uniqueness theorem for a two layers body. under quite general hypotheses on the body inner density distribution, it can be proved that the separation surface between the two layers can be uniquely estimated. based on this theorem, the estimation of the moho is discussed and the equations relating gravity and moho depth are derived in planar approximation. in this context, a procedure to moho estimate based on collocation approach is presented. this iterative approach has been then tested via numerical simulation. a simulated moho is computed and two density models are assumed to compute the direct gravity effect to be used in the inversion procedures. these tests gave relevant insight in the devised inversion method. reliable results have been obtained when density models are consistent with gravity simulated data. it can be thus assumed that, under unbiased conditions, the method is stable and can be applied to moho depth estimate. also, the impact of depth information proved to be relevant. collocation allows easily the integration of different data types and this can improve the solution. particularly, this applies to the bias in the mean depth that is reduced if depth data are considered. on the contrary, the solution based on gravity only cannot compensate for this bias. another important remark is related to density information. it was barzaghi and biagi 10 figure 6. integrated inversion 3: residuals on the depth observations. 11 clearly proved that density plays an important role in the inversion. if proper density information are available, the iterative inversion procedure based on collocation can give reliable results. on the other hand, when assuming inconsistency between density and simulated gravity data, we obtained remarkable distortions in the estimated depths. furthermore, in this case, depth information are not effective in reducing the bias and even on the given depth data a significant discrepancy is present. thus, it can be concluded that the collocation based procedure can be applied for getting moho depth estimates from gravity and depth information (from e.g. seismic). however, as it is for any other inversion method, improper density information can induce serious biases in the estimates even if depth data are included in the inversion. two possible evolution lines of the collocation inversion method can be devised. one line is to investigate its behavior in local areas where high frequency moho patterns are present. being collocation quite a stable and regularizing technique [barzaghi et al. 1992], this doesn’t seem to be a too critical issue. the second possible application of this methodology is on global moho estimates based on gravity data coming from the gravity dedicated satellite missions. since proper spherical model equations can be quite easily derived and linearized [see e.g. eshagh et al. 2011], global applications of the collocation method seem to be feasible. acknowledgements. the suggestions and the corrections of the anonymous reviewers really have helped the authors and improved the clarity of the paper. references albertella, a., f. migliaccio and f. sansò (2002). goce: the earth gravity field by space gradiometry, celestial mechanics and dynamical astronomy, 83 (1-4), 1-15. anderson, d.l. (1989). theory of the earth, blackwell scientific publication, boston-oxford-londonedinburgh-melbourne. barzaghi, r., and f. sansò (1983). sulla stima empirica della funzione di covarianza, bollettino di geodesia e scienze affini, 42 (4). barzaghi, r., and f. sansò (1988). remarks on the inverse gravimetric problem, geophysical journal of the royal astronomical society, 92 (3), 505-511. barzaghi, r., a. gandino, f. sansò and c. zenucchini (1992). the collocation approach to the inversion of gravity data, gephysical prospecting, 40, 429-451. biagi, l. (1998). la stima della moho mediante il metodo della collocazione, tesi per il conseguimento del titolo di dottore di ricerca in geofisica, x ciclo, consorzio tra le università di genova, modena e torino. bottoni, g.p., and r. barzaghi (1993). fast collocation, bulletin géodésique, 67 (2), 119-126. eshagh, m., m. bagherbandi and l. sjöberg (2011). a combined global moho model based on seismic and gravimetric data, acta geod. geoph. hung., 46 (1), 25-38. forsberg, r. (1984). a study of terrain reduction, density anomalies and geophysical inversion methods in gravity filed modelling, reports of the department of geodetic science and surveying, no. 355, the ohio state university, columbus, ohio. krarup, t. (1969) a contribution to the mathematical foundation of physical geodesy, meddelelse no. 44, geodætisk institut, københavn. lebedev, s., j.m.-c. adam and t. meier (2013). mapping the moho with seismic surface waves: a review, resolution analysis, and recommended inversion strategies, tectonophysics; http://dx.doi.org/10.10 16/j.tecto.2012.12.030. macmillan, w.d. (1958). the theory of the potential, dover publications inc., new york. menke, w. (1989). geophysical data analysis: discrete inverse theory, international geophysical series, vol. 45, academic press, san diego. moritz, h. (1980). advanced physical geodesy, wichmann, karlsruhe. moritz, h. (1990). the figure of the earth: theoretical geodesy and the earth’s interior, wichmann, karlsruhe. parker, r.l. (1972). geophys. j. r. astr. soc., 31, 447-455. pavlis, n.k., and r.h. rapp (1990). the development of an isostatic gravitational model to degree 360 and its use in global gravity modeling, geophys. j. int., 100, 369-378. press, w.h., s.a. teukolsky, w.t. vetterling and b.p. flannery (1992). numerical recipes in fortran. the art of scientific computing, 2nd edition, cambridge university press, cambridge. reigber, c., r. casper and w. päffgen (1999). the champ geopotential mission, iaa 2nd international symposium on small satellites for earth observation (berlin, april 12-16), 25-28. sansò, f. (1980). internal collocation, atti della accademia nazionale dei lincei, memorie, classe di scienze fisiche matematiche e naturali, serie 7, vol. 16, sezione i, fascicolo i. shin, y.m., h. xu, c. braitenberg, j. fang and y. wang (2007). moho undulations beneath tibet from grace-integrated gravity data, geophys. j. int., 170 (3), 971-985. sjöberg, l.e. (2009). solving vening meinesz-moritz problem in isostasy, geophys. j. int., 179, 654-662. the collocation approach to moho estimate tapley, b.d., s. bettapur, m. watkins and c. reigber (2004). the gravity recovery and climate experiment: mission overview and early results, geophys. res. lett., 31 (9), l09607. tarantola, a. (2005). inverse problem theory and model parameter estimation, siam (society for industrial and applied mathematics), philadelphia. tscherning, c.c. (1985). local approximation of the gravity potential by least squares collocation, in: k.p. schwarz (ed.), proceedings of the international summer school on local gravity field approximation (beijing, china, august 21 september 4). turcotte, d.l., and g. schubert (1982). geodynamics: applications of continuum physics to geological problems, john wiley&sons. *corresponding author: ludovico biagi, politecnico di milano, dica, c/o laboratorio di geomatica del polo territoriale di como, como, italy; email: ludovico.biagi@polimi.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. appendix. a uniqueness theorem for the two layers model the gravity inversion problem can be defined as follows: to estimate the inner density distribution of a body given gravity observations in the spatial domain outside the body. as it is well known, this problem is an ill-posed problem [moritz 1990] because uniqueness is not, in general, guaranteed. as an example, concentric spheres having the same total mass homogeneously distributed can vary their densities and volumes to give the same potential in the space outside the larger considered sphere. thus, by inverting the gravity potential no unique solution can be estimated. to have uniqueness in the gravity inversion problem one has to impose proper regularizations and/or restrictions to the required solution [see e.g. sansò 1980, moritz 1990, sjöberg 2009]. in the framework of the gravimetric moho estimation problem, a uniqueness theorem for the two layers model is given in the following. this theorem can be considered as an extension to a more general case of the one proved in barzaghi and sansò [1988]. it can be used as a reasonable model for the gravity inversion to moho estimate in the real case. two bodies b1 and b2 are given with two layers inner mass distributions and the same bounding surface s. the two inner volumes are bi1 and bi2 and the two crust layers are be1 and be2. the two separation surfaces between the inner volumes and the crust layers si1 and si2 are considered as regular and are parameterized as sik = rik (v), with k = 1,2 and v = ({,m) (see figure a.1). let ti1 and ti2 be the inner part densities and te1 and te2 the crust densities of the two bodies. these densities are given as a function of (r,v). assume that two functions ti(r,v), te(r,v) are defined inside s and satisfy the following conditions moreover, the following conditions hold: if v1 = v2 outside s it can be proved that si1 and si2 are the same surface, i.e. that proof we consider a body b having a density distribution which is obtained by subtracting the b2 from the b1 density in every point. outside s, the potential implied by 0 , , , , , r r r r 0 e i e ir r2 2 1 1 # # t v t v t v t v ^ ^ ^ ^ h h h h , , , , , ,p r p p r p p r p p r p b b b b i i i i e e i i e e e e 1 1 1 2 2 1 2 2 6 6 6 6 ! ! ! ! t t v t t v t t v t t v = = = = ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h r ri i1 2v v=^ ^h h barzaghi and biagi 12 (a.1) (a.2) (a.3) 13 this body is equal to zero since where n(t) is the newtonian operator. the density function t has the following properties: furthermore, having by hypothesis that , it follows we now define the two surface si and se are the bounding surface of b. we can further define se+ and si+ as those parts of se and si that bound the domain in b where t is positive. similarly, we define se− and si− as those parts of se and si that bound the domain in b where t is negative. furthermore, we define v+ and v− as the projection of se+ and se− onto the unit sphere v. the potential generated by b is harmonic down to se since the mass of this body is inside this surface. as stated before, the potential of b is equal to zero outside s. due to the maximum and minimum property of the harmonic function, it is also equal to zero outside se. it follows that t is in the orthogonal complement of lh 2 b we now define a function k(v) onto se , satisfying the following condition where h v 1/2 is the sobolev space ½ of the function having domain on the unit sphere. based on the hypothesis on ri1 and ri2 , we can also assume that re and ri are regular surfaces. it follows that . furthermore, in the hypothesis stated for k(v), there is a unique harmonic function h(v) defined in b that on se has the values assigned to k(v). if , it follows that . hence being the following decomposition for i holds where in in s s h 0 0 /e e 1 2 2 !l v l v l v= v + + ^ ^ ^h h h' ( ) h /se 1 2!l v ( )r h p lr hb 22 ! ( )h p lhb 2! ,, 0 i p r h p h r db d r r dr re b r r p ri 3 2 2 t v t v v = = = = v ^ ^ ^ ^ h h h h= g ### ### i i i i i i1 2 3 4 5= + 0v n v vnb b b b b b1 2 1 2t t t= = = =^ ^h h 0p b bp b b i ie e 1 21 2 kk j6 !t =^ ^ ^h h h p p p p b b p p p p b b 0 0 e i i e i e e i 1 2 1 2 k k 6 6 1 2 ! ! t t t t t t = = + ^ ^ ^ ^ ^ ^ h h h h h h 0 , 0 ,r r2 2$ #t t+ , ,s r r s r r r sup r inf e e i i ii i i 1 2 21v v v v v v = = = =^ ^ ^ ^ ^ ^ h h h h h h 6 6 @ @ " " , , 0p f p db f l b p hb 26 !t =^ ^h h### r e i i2 2#t t p r h p p r h p r p h p r h p p3 r r r r r 3 3 2 3 2 2 2 2 2 t t t t = + ^ ^ ^ ^^ ^ ^ ^ ^ h h h hh h h h h , , , i d r r r r h r e e i i i 1 3 3 v t v v l v t v v v = + v+ + + + + + + + + ^ ^ ^ ^ ^ ^ h h h h h h 6 @ ## the collocation approach to moho estimate figure a.1. the two bodies with two layers inner mass distributions and the same bounding surface. (a.4) (a.5) (a.6) (a.7) (a.8) (a.9) (a.10) (a.11) (a.12) (a.13) (a.14) since, obviously, i5 can be written in the following way it holds that i5 = 0. we suppose now that re(v) differs from ri(v). under this hypothesis, we want to define the values assumed by the remaining integrals. since h(p) is harmonic in b, it can assume its minimum or maximum values only on the boundary, i.e. onto se. in the limit for l+(v) → 1, we obtain 0 ≤ l(v) ≤ 1, that is . it follows that by making use of the properties of ^rt− , we have that i4 < 0 which implies i3 − i4 > 0. the i2 term is positive. if we define iii as we have i2 < iii which implies i1 − i2 > i1 − iii . iii can be further be decomposed in the following way hence, we obtain that where m+ is the positive mass included in the domain where t is positive. it follows that if we now consider the limiting value for l, we can write the following since we finally have i1 − iii > 0. so, if we assume that re(v) doesn’t coincide with ri(v), it holds that i = i1 − i2 + i3− i4− i5 > 0. however, i must be equal to zero. this implies that re(v) and ri(v) must coincide, i.e. that the interface surfaces in the two bodies are the same. , , i i d r r h r m1 ii i i i 1 3v t v v v = +v + + + + + + ^ ^ ^^h h h h6 @## , ,d r r h r 1 0i i i 3 2v t v v v -v + + + + + ^ ^ ^^h h h h6 @## , 3 ,i d r r r r dr re ii r ri 3 22v t v t v= v + + + ^^ ^hh h= g## # , ,i d r h r r dr re ri r 3 2 2v v t v= v+ +^ ^h h= g## # , ,i d r h r r dr re r ri 3 4 2v v t v= v-^ ^h h= g## # 3 , ,i d drr r h r re ri 5 2v t v v= v ^ ^h h## # i h dbp p3 p b 5 t= ^ ^h h## 0 ( ) 1h p p b61 1 ! , ,i d r rr h 0i ii3 3 2vt v v v=v --^ ^ ^h h h## ,i d r r dr re ii r ri 32v t v= v + + ^ h= g## # , , , , i d r r r r r h r e e e i i i 3 3 3 v t v v l v t v v v = + v ^ ^ ^ ^ ^ ^ h h h h h h 6 @ ## , , , , , i d r r r r d r r dr d r r r r m 3 re ii e e i i ri e e i i 3 3 2 3 3 v t v v t v v v t v v t v v t v v = + + = = + v v v + + + + + + + + + + + + + + + + + ^ ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h h 6 = 6 @ g @ ## ### ## , , , , , , , , i i d r r r r h r d r r r r m d r r r r h r m 1 1 ii e e i i i e e i i e e i i i 1 3 3 3 3 3 3 l l v t v v v t v v v v t v v t v v v t v v v t v v v = + + + = = + + v v v + + + + + + + + + + + + + + + + + + + + + + + + + + + ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^ ^ ^ ^^ h h h h h h h h h h h h h h h h h h 6 6 6 @ @ @ ## ## ## barzaghi and biagi 14 (a.15) (a.24) (a.25) (a.26) (a.16) (a.17) (a.18) (a.19) (a.20) (a.21) (a.22) (a.23) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol52,1,2009 1 annals of geophysics, vol. 52, n. 1, february 2009 key words geomagnetic observatory – geomagnetic field variations – antarctica 1. introduction the magnetic field of internal origin is a manifestation of a magnetohydrodynamic dynamo acting in the interior of the earth. this field represents over 97 per cent of what is observed at the earth’s surface. in addition to sources in the earth’s core, the geomagnetic field is produced by sources in the lithosphere and, regards contributions external to the earth, from electric currents flowing in the ionosphere, magnetosphere, as well as from the coupling of these currents and the currents flowing between the two earth’s hemispheres. a first order classification of the different studies on the earth’s magnetic field time variations can be made on the rough basis of internal and external earth studies respectively. even if it is not possible to establish a precise boundary between the two groups, by applying spherical harmonics analysis to the geomagnetic field time variations, it has been found that the variations on time scales shorter than 1-5 years are of external origin while those that take place on longer time scales, commonly referred to as secular variation, are of internal origin (see for example chapman and bartels, 1940; parkinson, 1983; backus et al., 1996; merrill et al., 1996; mandea and purucker, 2005; lanza and meloni, 2006; for space physics aspects hargreaves, 1992; kivelson and russell, 1996). in italy, the istituto nazionale di geofisica e vulcanologia (ingv) is responsible for systematic magnetic observations made by means of observatories and repeat stations. at present two regularly working geomagnetic observatories cover central and northern italy: l’aquila (the main italian observatory since 1958) and castello tesino (since 1964). a new observatory is being installed in the southern mediterranean (near sicily) at lampedusa island. once twenty years of geomagnetic field observations at mario zucchelli station (antarctica) lili cafarella, stefania lepidi, antonio meloni and lucia santarelli istituto nazionale di geofisica e vulcanologia, roma, italy abstract during the 1986-87 austral summer a geomagnetic observatory was installed at terra nova bay. during the first years both geomagnetic field time variation monitoring and absolute measurements were carried out only during summer. since 1991 variometer measurements are automatically performed throughout the year, while absolute measurements are still performed only during summer. in spite of this, interesting observations were obtained during the life (quite long for antarctica) of the geomagnetic observatory. in particular, this paper briefly presents some of the most important results: studies on secular variation, daily variation (and its dependence from solar cycle and seasons) and geomagnetic higher frequency variations, such as geomagnetic pulsations. mailing address: dr. lili cafarella, istituto nazionale di geofisica e vulcanologia (ingv), via di vigna murata 605, 00143 roma, italy: e-mail: lili.cafarella@ingv.it vol52,1,2009 28-05-2009 13:45 pagina 1 2 l. cafarella, s. lepidi, a. meloni and l. santarelli this installation is successfully completed, the three observatories will be able to provide a full coverage of the whole italian latitudinal extension (meloni et al., 2007a). as regards polar observatories, italy is involved in the management of two geomagnetic observatories at mario zucchelli station (tnb) and concordia station (on the high east antarctic craton at domec) respectively. the first was installed at terra nova bay (ross sea) during 1986/1987 austral summer by the pnra (italian program for scientific research in antarctica; palangio et al, 1996); in february 2005 the permanent scientific station concordia opened and, from 2004-2005 summer expedition, also a geomagnetic observatory, commonly operated with french colleagues, started its operation (schott et al., 2005). this paper briefly describes some interesting observations made on the long tnb dataset. in particular some studies on secular variation, daily variation (and its dependence from solar cycle and seasons) and geomagnetic higher frequency variations, such as geomagnetic pulsations, are here briefly summarized. 1.1. the geomagnetic observatory the italian program for scientific research in antarctica installed, during 1986/1987 austral summer, a geomagnetic observatory at terfig. 1a,b. geomagnetic observatory at mario zucchelli station: a) variometer shelter; b) absolute measurement shelter. vol52,1,2009 28-05-2009 13:45 pagina 2 3 twenty years of geomagnetic field observations at mario zucchelli station (antarctica) ra nova bay (observatory iaga code tnb; geographic coordinates: 74.68° s, 164.12° e; corrected geomagnetic latitude: 80.0° s) at the antarctic italian base mario zucchelli station (formerly terra nova bay). variations of the earth’s magnetic field are recorded by means of three-axis fluxgate magnetometers (meloni et al., 1994; palangio et al., 1996) oriented along three perpendicular directions according to the magnetic reference system: the horizontal magnetic field intensity h-component (along the local magnetic meridian, south-north), the orthogonal-component d in the horizontal plane (west-east; it is on average null, and is due to rapid time variations; d is here an intensive element, expressed consequently in nt; note that in general declination d is an angular element) and the vertical intensity z-component (consequently positive increasing inward). the intensity of the geomagnetic field f is measured by an overhauser magnetometer. at the beginning, measurements were carried out only during tnb opening period, in local summer, while since 1991 an automatic acquisition system, operating throughout the year, was put into operation. in the first ten years, the data sampling rate has varied from 15-30 s (during the station opening period) to 4 min (automatic acquisition during winter period) while since 1996, 1-min data are routinely produced in intermagnet standard. starting from 2001, the sampling rate is 1 s and the original 1 s measurements are stored, and also filtered and averaged to produce 1 min data. absolute measurements of the angular elements d and i, fundamental to ensure that the drift of any kind in the recording instruments is corrected, are performed regularly during the italian base opening period (unfortunately only during austral summer). figure 1 a picture of the instrument installation in the sensor shelter is reported. in the figure the insulated variometer sensors and the electronic parts are clearly visible. 2. internal earth studies: secular variation for the earth’s interior, one of the most important aims of a magnetic observatory is to supply information for defining global model of the earth’s geomagnetic field and its longterm variation. the international geomagnetic reference field (igrf) is the first and most well known global model adopted for the first time in 1968. it represents the main magnetic field (of core origin) without external sources and is based on data from satellites, observatories and surveys around the world. other models for the main field, for example gufm1 and cals7k, are based on records from maritime voyages, marine and continental surveys, geomagnetic observatories and satellites the former, and on archeo and paleomagnetic data the latter (intensity and direction). global models can be used to represent the main field including other magnetic contributions. the most used are chaos, which represents the main plus the crustal field, based on high precision satellite data (ørsted, champ, sac-c); cm4, the main field plus the contribution of lithosperic, ionospheric and magnetospheric fields, based on satellite (magsat, pogo, ørsted, champ, sac-c) and observatory data. the longterm variation of the geomagnetic field elements is called secular variation. many studies have shown that secular variation is a feature of the main geomagnetic field and that the amount of change varies smoothly with latitude (parkinson, 1983). the analysis of secular variation as recorded in antarctic observatories in recent years, shows a rapid decrease in the total magnetic field (e.g. rajaram et al., 2002). some investigators (e.g. de santis et al., 2004) have speculated that this rapid decrease would be of global relevance implying the development of a dipole reversal as has happened several times in the earth’s history, but others have denied this possibility (gubbins et al., 2006) on the basis of available information. both observatory data and cm4 quiet time magnetic field model, were used to improve our knowledge of geomagnetic jerks (abrupt changes of secular variation) in antarctica, as reported by meloni et al. (2006, 2007b). figure 2 shows secular acceleration maps relative to the y component (along the geographic westeast direction) over the antarctic continent obtained using synthetic data given by the cm4 model (from meloni et al., 2007b). the time sevol52,1,2009 28-05-2009 13:45 pagina 3 4 l. cafarella, s. lepidi, a. meloni and l. santarelli quence of the maps in the three rows in fig. 2 are chosen such as to investigate the secular variation changes around the 1969, 1978 and 1991 jerks (alexandrescu et al., 1996; de michelis et al., 2000; blohxam et al., 2002; chambodut and mandea, 2005). the solid line corresponds to the zero separating the zone characterized by negative (blue) and positive (red) values of secular acceleration. the abrupt change in color (blue-red, red-blue) of the secular acceleration maps suggests that the 1969 jerk actually occurred in the antarctic region around 1972, the 1978 jerk around 1981, and the 1991 event around 1997. spherical harmonic analyses of the main field, as well as in general magnetic models, made for different epochs, are used to make world charts of secular variation (isoporic charts). antarctica is characterized by one of the isoporic foci (areas of maximum temporal variation of the main field elements) so the monitoring of the absolute level of all magnetic field elements here is particularly important. unfortunately tnb cannot be fully included among the geomagnetic observatories that can contribute regularly to the internal earth studies because its dataset includes baselines for only two or three months a year, depending on the opening period of mario zucchelli station. in spite of this some consideration can be made. figure 3 shows field time evolution plots (yearly data points, obtained from only summer expeditions data) and igrf corresponding yearly values for h, d, z and f elements. in the fig. 2. secular acceleration maps relative to the y component over the antarctic continent obtained using synthetic data given by the cm4 model. the solid line corresponds to the zero separating the zone characterized by negative (blue) and positive (red) values of secular acceleration. vol52,1,2009 28-05-2009 13:45 pagina 4 5 twenty years of geomagnetic field observations at mario zucchelli station (antarctica) tnb total field f plot, an almost steady decrease of about 50 nt/year is evident according to the results proposed by rajaram et al. (2002) and described above while other elements show a regular steady time variation typical of this part of the world. 3. external earth studies in a static model of the magnetosphere, the earth’s magnetic field lines closed within the lower latitude magnetosphere separate from those at higher latitude that are swept back on the tailward side and can be connected to the interplanetary magnetic field (imf). the region which separates sunward, closed field lines from tailward, open ones, i.e. the polar cusp, is a funnel which allows a direct entry of solar wind (sw) particles in the magnetosphere. the connection of the polar regions to the polar cusp and to the boundary between the magnetosphere and the sw (magnetopause) is one of the causes of some peculiar magnetospheric dynamic phenomena of external origin. moreover, in the whole auroral zone field aligned currents flow into and away from the ionosphere, giving rise to particular geomagnetic variations. in this sense, knowledge and understanding of high latitude phenomena lead to a more complete view of the interactions that exist among various components of the magnetosphere-sw system (mcpherron, 2008). tnb geomagnetic observatory is located at corrected geomagnetic latitude 80.0° s; the relationship of universal to local time is lt=ut + 13 and to magnetic local time is mlt = ut – 8. the observatory is generally located at the footprint of magnetospheric field lines open in the geomagnetic tail, i.e. in the polar cap. since the fig. 3. long-term time evolution of the h, d, z and f elements at mario zucchelli station, shown by their summer mean values (stars) and corresponding igrf values (circles) (1986-2006). vol52,1,2009 28-05-2009 13:45 pagina 5 6 l. cafarella, s. lepidi, a. meloni and l. santarelli auroral oval location and extent depend on interplanetary parameters (zhou et al., 2000), in particular conditions around local geomagnetic noon the station could be situated in the polar cusp, approaching closed field lines. in this sense, the position of the observatory allows many phenomena characteristic of the polar areas, very peculiar zones of the magnetosphere to be studied. before summarizing some magnetic field variation observations made at tnb, a brief description of the two solar cycles over which the analyzed data span is useful: solar cycles 22 and 23 (see fig. 4). solar cycle 22 started at the end of 1986 and reached its maximum in 1991. it lasted unusually only 9.8 years, but it showed some extraordinary intervals of activity (for example in june 1991). solar cycle 23 started in may 1996 with the yearly sunspot number at 8.0 and reached its maximum in april 2000 at 120.8; it had a second, smaller peak at 115.5 in november 2001. now, in 2008 we are just in a minimum (reached in january 2008). solar cycle 23 has shown on the overall a modest activity in comparison with the two previous solar cycles 21 and 22. 3.1. daily variation a periodic change in magnetic declination with 24-h period was discovered by george graham in 1724. since then, many studies have investigated the daily variation, which can be observed on all geomagnetic field elements and, although with different characteristics, all over the earth’s surface. today it is well known that daily variation (sq), during quiet solar conditions, is principally generated by two electric current vortices that flow in the dayside upper fig. 4. yearly average sunspot number in the time interval 1985-2007. 180 160 140 120 100 80 s u n sp o t n u m b e r years cycle 23 cycle 22 60 40 20 0 1985 1990 1995 2000 2005 2010 vol52,1,2009 28-05-2009 13:45 pagina 6 7 twenty years of geomagnetic field observations at mario zucchelli station (antarctica) fig. 5. dynamic maps of diurnal variation for d and h components for 2001 and 2006 years. 2001 ju li a n d a y s 2006 vol52,1,2009 28-05-2009 13:45 pagina 7 8 l. cafarella, s. lepidi, a. meloni and l. santarelli atmosphere, peaking at altitudes around 110 km above the earth’s surface, whose focus is located around the noon meridian, at about 30° geographic latitude in each hemisphere. the daily variation of the geomagnetic field in the polar cap regions is due to a polarward extension of this mid latitude current vortices (called sq0) and to an additional equivalent electric current system contribution typical of polar regions related to the magnetospheric conditions, called sqp (matsushita and xu, 1982). in particular, this sqp electric current system is the primary source during winter, when the southern polar cap is not directly exposed to the sun (akasofu et al., 1983; brekke, 1997). daily variation as observed at italian polar geomagnetic observatories, was described in previous works by cafarella et al. (1998), lepidi et al. (2003), cafarella et al. (2007a) and santarelli et al. (2007a). in one of them the study of daily variation, as observed at tnb observatory through fourteen years of data, showed a pronounced seasonal effect on the amplitude of the signal but not on its pattern. an analysis of the same phenomenon at dome c (corrected geomagnetic coordinates 88.9°s) was also performed and a pronounced day-today variability of the amplitude was found and explained in terms of the global magnetospheric activity level. this paragraph will show the main daily variation characteristics as observed at tnb during the last two solar cycles. a comparison of daily variation pattern at tnb for a year of high (2001) and one of low (2006) magnetic activity is reported in fig. 5. the dynamic maps report the variation of the hourly h and d values with respect to the average level as a function of julian days and ut. some considerations can be made immediately looking at the figure. comparing the same component, diurnal variation trend is similar in both years independently from magnetic activity: indeed d is minimum around 12 ut and maximum at ut midnight while h is maximum before 12 ut and minimum in the ut afternoon. a clear seasonal dependence of the daily variation amplitude range emerges through each year with a reduction during local winter. as expected, on average the signal amplitude for both elements is higher in 2001 than 2006. moreover in the year of minimum solar activity the trend is very stable, while some peculiar structures are evident in the year of maximum activity: for example at the beginning and at the end of 2001 (around julian days 20 and 330 indicated by red arrows in the figure) the diurnal variation shows an enhanced ut dependence, especially for h. figure 6 shows another kind of representation of diurnal variation, i.e. magnetic field hodograms in the horizontal plane, in which every point shows h and d values as reported on vertical and horizontal axis respectively. the hodogram representation of the daily variation allows to visualize simultaneously several years and to compare quickly the effects of different solar cycles on geomagnetic field variations. each hodogram, represented with a different color in fig. 6, corresponds to one year of solar cycles 22 and 23. the 24 h and d values in each plot are the median values (from hourly data, after removing the average level) computed over summer season (jan-feb-nov-dec; lloyd, 1861), separately for the 24 daily hours. in this representation the 24 values represent the dynamic through the day of the projection of the total field f on the horizontal plane. in the plots each curve is covered in counterclockwise direction. from the figure the daily variation dependence on solar activity clearly emerges: for both solar cycles the largest and smallest excursions are found in the years corresponding to maximum and minimum sunspot numbers, respectively. moreover the largest excursions are observed in solar cycle 22, and this is a direct consequence of the fact that solar cycle 23 was not particularly active. 3.2. magnetic pulsations magnetic pulsations are geomagnetic field rapid time variations of external origin. their period ranges from less than 1 second to 10 minutes, and the amplitude from tenths to hundred nt, generally increasing for increasing period and geomagnetic latitude, maximizing in the auroral zone. the availability of long series of data, gave rise to several studies characterizing the low frequency pulsation activity at tnb. the revol52,1,2009 28-05-2009 13:45 pagina 8 9 twenty years of geomagnetic field observations at mario zucchelli station (antarctica) fig. 6. magnetic field hodograms for solar cycle 22 and solar cycle 23. solar cycle 22 solar cycle 23 d(nt) d(nt) h (n t ) h (n t ) vol52,1,2009 28-05-2009 13:45 pagina 9 10 l. cafarella, s. lepidi, a. meloni and l. santarelli search activity was mainly devoted to the lowest frequency pulsations (known as pc5 pulsations, with f=2-7 mhz), with some studies also in the mid frequency range (pc3-pc4, with f=7-100 mhz); see the reviews by villante et al. (2000) and cafarella et al. (2007b). the generation mechanisms for pc3-pc5 pulsations are basically related to instabilities along the flanks of the magnetopause due to the sw flow (kelvin-helmholtz instability; atkinson and watanabe, 1966), waves generated upstream of the earth’s bow shock by solar wind ions reflected back (greenstadt et al., 1981), and to local phenomena in the auroral oval region (engebretson et al., 1986). the key role of the cusp region in the generation of low frequency pulsations clearly emerges from the result that the average pulsation power at tnb maximizes around local geomagnetic noon, when the station approaches the cusp and local field lines are at minimum distance from the magnetopause (lepidi et al., 1996). this feature clearly emerges also from fig. 7 (adapted from santarelli et al., 2007b), where we show a pulsation event recorded at tnb (red line) and at another antarctic station, scott base (sba, blue line), which has a longitudinal displacement of about 1 hour in magnetic local time with respect to tnb (table i). at both stations there is a sustained wave activity (upper panel); from the 1-5 mhz filtered data (lower panel) it is evident that the wave activity at the two stations is very similar, but the amplitude is larger at sba around 17-18 ut and at tnb around 21-22 ut, i.e. it is larger at the station which is closer to its local geomagnetic noon, indicating an increasing amplitude approaching the cusp region. it is also evident that around 17 ut sba is leading (sees the signal in advance) with respect to tnb, while around 21 ut tnb is leading with respect to sba; this feature clearly indicates that the waves propagate away from the noon region, i.e. they propagate in the antisunward direction, as expected for a generation mechanism such as the kelvin-helmholtz instability on the magnetopause. an additional generation mechanism for low frequency pulsations is related to interplanetary shocks impacting the magnetopause. such an impact can trigger cavity/waveguide modes of the entire magnetospheric cavity, localized between an outer boundary, for instance the magnetopause, and an inner turning point (kivelson and southwood, 1985). the peculiar feature of these modes is that they have a global character within the magnetosphere and are characterized by a set of discrete, stable frequencies. these cavity modes have been observed at auroral and low latitude, and they have also been observed deep in the polar cap, i.e. at the footprint of table i. stations, geographic coordinates, igrf2003 corrected geomagnetic coordinates and time in ut of the magnetic local noon for the stations used for the analysis in figs. 7 and 8. station (iaga code) geogr. coord. cgm coord. mlt nn (ut) mario zucchelli station (tnb) 74.7s 164.1e 80.0s 307.7e 20:11 scott base (sba) 77.8s 166.8e 80.0s 326.5e 19:01 dumont d’urville (drv) 66.7s 140.0e 80.4s 235.7e 00:55 cambridge bay (cbb) 69.2n 255.0e 77.2n 309.6e 19:54 furstenfeldenbruck (fur) 48.2n 11.3e 43.4n 86.9e 10:28 castello tesino (cts) 46.0n 11.7e 40.8n 86.7e 10:28 l’aquila (aqu) 42.4n 13.3e 36.3n 87.4e 10:24 gibilmanna (gib) 37.9n 14.0e 30.6n 87.3e 10:24 vol52,1,2009 28-05-2009 13:45 pagina 10 11 twenty years of geomagnetic field observations at mario zucchelli station (antarctica) open field lines stretching in the geomagnetic tail (villante et al. 1997; 1998; lepidi et al., 1999; 2007). figure 8 (from lepidi et al., 2007) shows a pulsation event simultaneously detected at the three antarctic stations tnb, sba and dumont d’urville (drv), at the canadian auroral station cambridge bay (cbb) and at a latitudinal chain of european low latitude stations (table i). this event occured on october 30, 2003, during a strong geomagnetic storm triggered by complex interplanetary structures impacting the earth. the unfiltered data for the time interval 1830-2130 ut (upper panel) show an intense geomagnetic activity, with similar variations at the three antarctic stations and at the four low latitude stations (in the latter case with a smaller amplitude, so they are less evident in this plot with a common scale for all stations). from the corresponding filtered data (lower left panels; the vertical scale in this case is different for high and low latitude stations, to make more evident the low latitude pulsations) the presence of simultaneous wave packets at all the stations emerges, with an evident signal intensification just before 20 ut. the power spectra computed by means of maximum entropy method, at order 20 of the prediction error filter, over the 1930-2130 ut time interval (right panels) show the presence of discrete frequencies, the same at all the stations (around 3.2 and 4.2 mhz). the same oscillations are present both on field lines closed in the inner magnetosphere (low latitude european stations) and on open field lines stretching in the magnetic tail (high latitude stations), so they can be interpreted in terms of global magnetospheric oscillation modes (lepidi et al, 2007). in general, the study of such global oscillations of the magnetospheric cavity/waveguide is particularly interesting in that it can give information on the magnetospheric structure, in particular gradients and density profiles. 4. summary and conclusions the continuous and global monitoring of the earth’s magnetic field is the only means to study the different features of the field and especially of its time variations. this activity is regularly undertaken by geomagnetic observatories all over the world. as is well known, the global distribution of geomagnetic observatories is strongly unbalanced in favor of the northern hemisphere, with a poor coverage in the southern hemisphere. in spite of this, geomagnetic field measurements in polar regions are very important for investigating the geomagnetic field structure and its time variations. the polar observatories role is indispensable in contributing to magnetic field models such as igrf, secular variation investigations, ionospheric and magnetospheric dynamics. fig. 7. variation of the h component (unfiltered data, upper plot; 1-5 mhz filtered data, lower plot) at tnb (red line) and sba (blue line). the red and blue arrows indicate the local geomagnetic noon at the two stations, respectively. unfiltered data ut (hours) on 2 feb. 2001 40 20 0 -20 -40 -60 20 10 0 -10 -20 1-5 mhz filtered data h v a ri a tio n s (n t ) h v a ri a tio n s (n t ) 15 16 17 18 19 20 21 22 23 15 16 17 18 19 20 21 22 23 vol52,1,2009 28-05-2009 13:45 pagina 11 12 l. cafarella, s. lepidi, a. meloni and l. santarelli fig. 8. variations of the h component (unfiltered data, upper plot; 2-5 mhz filtered data, lower left plots) at four high latitude and four low latitude stations (table i) and corresponding power spectra from differenced data (plots on the right). h v a ri a tio n s (n t ) h v a ri a tio n s (n t ) s p e ct ra l d e n si ty ( n t 2 /h z) h v a ri a tio n s (n t ) s p e ct ra l d e n si ty ( n t 2 /h z) ut (hours) on oct. 30, 2003 ut (hours) on oct. 30, 2003 freq (mhz) vol52,1,2009 28-05-2009 13:45 pagina 12 13 twenty years of geomagnetic field observations at mario zucchelli station (antarctica) in this framework, the importance of magnetic observatories in antarctica is evident, since they provide the opportunity of continuously monitoring the geomagnetic field in an area where time variations show very peculiar features that can be used to address very general problems. this paper has described some results obtained from tnb geomagnetic observatory data, for both internal and external earth studies. the dataset is particularly interesting in that it spans over two decades, providing the opportunity to characterize the phenomena which happen in this very peculiar area. long-term observations are indeed necessary for the study of geomagnetic field variations both of internal origin, with time scales greater than one year, and of external origin, which have long-term modulations due to seasonal and solar cycle dependence. we have shown how the long term time variation, based upon absolute measurements of the field, is used for secular variation studies and how this phenomenon is quite high in amplitude in antarctica. the monitoring of the daily variation adds to knowledge of the ionospheric electric currents pattern and reveals its role in modulating the amplitude of magnetic field elements following the solar magnetic activity, in this case for the two solar cycles 22 and 23. geomagnetic field rapid time oscillations, commonly called pulsations or micropulsations, are another phenomenon that takes great advantage from polar latitude and especially southern latitude measurements. we have to show here how the comparison between polar latitude stations and also with mid latitudes has led to the identification of possible global oscillation magnetospheric modes. a desirable observatory distribution all over the word would be a uniform coverage. this is of course very difficult in antarctica but certainly the efforts are worthwhile and contributions to all classical fields of investigations in geomagnetism, as shown here, are strategic. references alexandrescu, m., d. gibert, g. hulot, j.l. le mouël and g. saracco (1996): worldwide wavelet analysis of geomagnetic jerks, j. geophys. res., 101, 21975-21994. akasofu, s.i., b.h. ahn and g.j. romick (1983): a study of the polar current system using the ims meridian chains of magnetometers, space sci. rev., 36, 337-413. atkinson, g. and t. watanabe (1966): surface waves in the magnetospheric boundary as a possible origin of long period geomagnetic micropulsations, earth planet. sci. lett. 1, 89-91. backus, g., r. parker and c. constable (1996): foundations of geomagnetism (cambridge university press). bloxham, j., s. zatman and m. dumberry (2002): the origin of geomagnetic jerks, nature, 420, 65-68. brekke, a. 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(1983), introduction to geomagnetism (scottish academic press, edimburgh). rajaram, g., t. arun, a. dhar and g. patil (2002): rapid decrease in total magnetic field f at antarctic stations – its relationship to core-mantle features, antarctic science, 14, 61-68. santarelli, l., l. cafarella, s. lepidi, d. di mauro, a. meloni and p. palangio (2007a): fourteen years of geomagnetic daily variation at mario zucchelli station (antarctica), annals of geophysics, 50, 225-232. santarelli, l., s. lepidi and l. cafarella (2007b): propagation of low frequency geomagnetic field fluctuations in antarctica: comparison between two polar cap stations, ann. geophysicae, 25, 2405. schott, j.j., d. di mauro, a. peres, l. cafarella, l. magno, a. zirizzotti and a. meloni (2005): towards the opening of a magnetic observatory at dome c (antarctica), proceedings of xith iaga workshop on geomagnetic observatory instruments, data acquisition and processing, nov 9-17 2004. villante, u., s. lepidi, p. francia, a. meloni and p. palangio (1997): long period geomagnetic field fluctuations at terra nova bay (antarctica), geophys. res. lett., 24, 1443-1446. villante, u., p. francia, s. lepidi, m. de lauretis, e. pietropaolo, l. cafarella, a. meloni, a.j. lazarus, r.p. lepping and f. mariani (1998): geomagnetic field variations at low and high latitude during the january 10-11, 1997 magnetic cloud, geophys. res. lett., 25, 2593-2596. villante, u., s. lepidi, p. francia, m. vellante, a. meloni and p. palangio (2000): ulf fluctuations at terra nova bay (antarctica), annali di geofisica, 43, 217-227. zhou, x.-w., c.t. russell, g. le, s.a. fuselier and j.d. scudder (2000): solar wind control of the polar cusp at high altitude, j. geophys. res., 105, 245-251. vol52,1,2009 28-05-2009 13:45 pagina 14 annals 48, 3, 2005+app1 515 annals of geophysics, vol. 48, n. 3, june 2005 key words ionosphere – electron density models – travelling ionospheric disturbances – tid model 1. introduction travelling ionospheric disturbances (tids) are the plasma signatures of atmospheric (acoustic) gravity waves (agws). they change the plasma distribution of the f-layer, which results in fluctuations of electron density as well as of the electron content. the theoretical understanding of these wavelike disturbances was first given by hines (1960). according to their horizontal wavelengths we distinguish between three classes of tids (table i; see e.g., van velthoven, 1990). with these definitions the small scale tids belong to the «acoustic branch», the others to the «gravity branch». in general, the amplitudes the tid model for modulation of large scale electron density models reinhart leitinger and markus rieger institut für physik, institutsbereich für geophysik, astrophysik und meteorologie (igam), universität graz, austria abstract various modern applications of empirical electron density models need realistic structures of the electron density distribution with smaller scales than the model background. travelling ionospheric disturbances (tids) produce three dimensional and time dependent disturbances of the background ionization. we present a tid model suitable to «modulate» large scale electron density distributions by multiplication. a model tid takes into account the forward tilt of the disturbance wave front, a distinct vertical structure, a fan type horizontal radiation characteristic, geometric dilution and attenuation. more complicated radiation patterns can be constructed by means of superposition. the model tids originate from source regions which can be chosen arbitrarily. we show examples for tid modulations of the background model family developed at trieste and graz (nequick, costprof and neuog-plas). mailing address: dr. reinhart leitinger, institut für physik, institutsbereich für geophysik, astrophysik und meteorologie (igam), universität graz, universitaetsplatz 5, a8010 graz, austria; e-mail: reinhart.leitinger@uni-graz.at of tids increase with increasing horizontal wavelengths. relative peak to peak amplitudes range from 0.001 to a few percent for sstids (which can be considered to be part of «geophysical noise») to 30% and more for lstids. there are two source regions for the agws behind the tids: 1) the lower atmosphere (troposphere and lower stratosphere) where it is thought that agws behind most of the mstids are generated; 2) the lower thermosphere in the auroral oval (e-region heights) where it is thought that most of the lstids (and some of the mstids) are generated (see e.g., leitinger, 1992; kirchengast, 1996). in mid-latitudes most lstids propagate equatorwards, those which travel polewards have smaller amplitudes and are thought to have their source regions in the other hemisphere. in some cases the occurrence of large scale tids could be linked to auroral substorm activity (kirchengast, 1997; see also kishcha et al., 1993). the main purpose of the tid model presented here is the combination by multiplication with a suitable electron density background model like the «family» of models developed at trieste 516 reinhart leitinger and markus rieger and graz (nequick, costprof and neuogplas) or the international reference ionosphere (iri). in this way we are able to demonstrate the influence of tids on application systems like satellite assisted positioning as used in surveying or on the use of polar orbiting satellite beacons as novel ionospheric data sources (high resolution ionosphere tomography; see e.g., leitinger, 1999) or on radio occultation data (see e.g., jakowski et al., 2004). 1.1. atmospheric gravity waves (agws) – a very short overview consider a stable homosphere with a given temperature profile t = t (z) (z: height). with knowledge of the acceleration of gravity g = g(z) and with one pressure and mass density value we also know the pressure profile p = p(z) and the mass density profile ρ = ρ(z). starting with displacing air parcels adiabatically in the vertical by ∆z one can show that the buoyancy force acts as a restoring force (see van velthoven, 1990, and references therein; prölss, 2001) ( ) dt d z g dz d c g z s 0 2 2 0 2 0 = +t t t∆ ∆d n (1.1) c ps 0 0= c t^ h is the velocity of sound, γ = cp/cv is the ratio of specific heats at constant pressure and at constant volume, p0 and ρ0 are pressure and mass density at the starting level for the displacement. this equation (newton’s second law) can be considered as a differential equation for an oscillation with the general solution (for small amplitudes a) ∆z=aexp(jωbt); j 1= , ωb 2= = ωb 2 + (g/cs2) (d(cs2)/dz) is the square of the (general) brunt-väisälä or buoyancy angular frequency, ωb 2 = [(γ −1)g2]/cs2 being the square of the isothermal brunt-väisälä angular frequency. forcing of oscillations will lead to wave propagation because the air parcel in consideration is connected to its surroundings. deriving wave solutions from the linearized hydrodynamic equations for an isothermal atmosphere over a flat earth leads to the following dispersion relation for agws, first published by hines (1960) k k c z b x s a2 2 2 2 2 2 2 2 = ~ ~ ~ ~ ~ (1.2) with the «acoustic cut-off angular frequency» ωa = (γ g)/(2cs). kz and kx are the vertical and horizontal components of the wave vector k. the waves described by this relation can be categorised as follows (see e.g., van velthoven, 1990): a) ω > ωa, acoustic branch; if ω >> ωa the dispersion relation degenerates to the dispersion relation for pure acoustic waves k2= ω2/cs2; b) ω b < ω < ωa, evanescent waves; wave equation becomes diffusion equation; c) ω <ωb, gravity wave branch; if ω << ωb the dispersion relation degenerates to the dispersion relation for pure gravity waves kz b 2 2 2 $= ~ ~^ h k cx a s 2 2 2 $ ~^ h. 1.2. the response of the ionosphere to agws the equation of continuity for electrons can be written as t n q l n ve e e e e$2 2 d= ^ h (1.3) where ne is the electron density; ve, the electron velocity; qe, the electron production rate; and le, the loss of electrons by recombination. the electron density can be split into a stationary background electron density (ne 0) and a perturbation (ne 1), due to the passage of the table i. tid classes according to their horizontal scales. class horizontal wavelength periods horizontal phase velocities large scale (lstids) >1000 km (0.5 ... 3) h (300 ... 1000 ) m/s medium scale (mstids) (100 ... 1000) km 12 min ... 1 h (100 ... 300) m/s small scale (sstids) <100 km a few minutes < 200 m/s 517 the tid model for modulation of large scale electron density models agw. assuming ve1 to be the velocity induced by the agw leads to n n ne e e0 1= + , v v ve e e0 1= + . (1.4) assuming no perturbation in production and loss and setting ve0 = 0 gives t n n ve e e 1 0 1$2 2 d= ^ h. (1.5) the relation between ve1 and vn1 is given by a balance between the lorentz force and the «ion drag» force e mv b v v 0e i e n1 1 1# + =o^ ^h h where vn1 is the perturbation velocity of the neutral gas; b, the geomagnetic induction vector; mi, the ion mass; and νi, the effective ionneutrals collision frequency. in the f-region of the ionosphere (h > 150 km) the mobility of the ions perpendicular to the geomagnetic field vector is negligibly small and we can approximate v v b be n1 1$. ^ h (1.6) where b is the unit vector in the direction of the geomagnetic field. this leads to the electron density disturbance in the form n j j nv b b k be n e1 0$ $ $d0 -^ ^ ^h h h" , . (1.7) if in (1.7) the second term of the expression in { } can be neglected we approximate ( ) ( ) ( ) ( ) cos cos cos n n k v u k s k h h t v b ψ e e n ph n x z 0 1 $ $0 = + + ~ ~ ξ σ ξ cos$ ^ ^ h h (1.8) where un is the amplitude of the agw; vph, the phase velocity of the agw; s, the horizontal coordinate; h, the height coordinate; and ψ(h) is a height dependent phase constant. the projection factors are ( ) ( )cos k b k$=ξ and ( ) ( )cos v b vn n1 1$=σ . energy transport upwards corresponds to a downward directed wave normal, kz < 0. then χ = atan(−kx / kz ) is the famous «forward tilt» of the planes of constant phase of the agws. for agws ψ is a constant value. because of the projection unto the geomagnetic field it is (slightly) height dependent in the tid case (compare e.g., francis, 1975; morgan and calderón, 1978). the approximation (1.6) implies, that there is no ionospheric response to agws with wave vectors perpendicular to the magnetic field. below the f-region the condition (1.6) is not valid and so variations of vn perpendicular to the geomagnetic field cannot be excluded. 1.3. agw signatures in total electron content (tec) the integration of electron density along slant rays from a satellite transmitter to a ground receiver gives (slant) total electron content. agw signatures are partly smoothed out. perpendicular to the geomagnetic field vector we see no tids at all. it is necessary to have a geomagnetically meridional component to observe tids in tec, but even with this geometry we have a distinct «aspect sensitivity» (georges and hooke, 1970). fig. 1. examples for tids in observed electron content (leitinger, 1992). nnss difference doppler observations, band pass filtered data. receiving stations: bonn (50.5°n, 6.9°e), lindau/harz (51.6°n, 10.1°e), graz (47.1°n, 15.5°e), penc (47.8°n, 19.3°e). marked amplitude range: 1×1016 m−2. note the differences in tid amplitudes for the closely spaced receiving stations. for more examples see putz et al. (1990), leitinger and putz (1991). 518 reinhart leitinger and markus rieger because of the tilt of agw fronts the tid disturbance in tec is smoothed out polewards of the receiving station if the agw propagates equatorwards. the tilt of the wavefront itself does not appear (figs. 1 and 2: observed tids; figs. 5 to 7: model tids). projection from slant to vertical in a «mean ionospheric height» does not change this situatuation. 2. some tid observations travelling ionospheric disturbances have been observed with many different instruments. fig. 2. examples for tids in observed electron content (leitinger, 1992). sirio faraday observations, band pass filtered data. receiving stations: graz (47.1°n, 15.5°e) and firenze (43.0°n, 10.7°e). abscissae in hours cet (top) or ut (bottom), ordinates in units of 1015 m−2. for more examples see putz et al. (1992). table ii. examples for the large scale «model basis». the «family» of models developed at trieste and graz nequick (q), costprof (c) and neuog-plas (p) are profilers using the peaks of the e-layer, the f1layer and the f2-layer as anchor points. they are identical from 100 km to the f2 peak (epstein layer formulations). c and p have identical ionosphere topsides (height aligned o+-h+ diffusive equilibrium – de), q uses a semi-epstein layer with a height dependent thickness parameter. above 2000 km p uses a magnetic field aligned h+ de («plasmasphere»). the models allow – to be updated with actual data from various sources; – to be adjusted to disturbed conditions; – to be combined with smaller scale models; – to calculate a variety of propagation effects along arbitrarily chosen raypaths. average ratio of calculation times q : c : p = 1 : 2 : 4 . for tids in gps data see rieger and leitinger (2002). here we show two examples for tids observed by means of propagation effects on satellite signals. figure 1 shows tids observed by means of the difference doppler effects on signals of polar orbiting satellites. the evaluation of the observations gave the latitude dependence of the vertical electron content (tec) of the ionosphere. the disturbance by large scale tids is demonstrated by means of band pass filtering of the electron content data. figure 1 is a clear example for the effect of the tilt of the wave fronts and of the filtering effect introduced by the geomagnetic field. the equatorward travelling tids appear only equatorwards of the receiving stations. furthermore the data for the closely spaced receiving stations show strong differences in tid amplitudes which indicates that the agws behind the travelling ionospheric disturbances are not radiated from an isotropic source but have a distinct «radiation pattern». the second example (fig. 2) shows tids in faraday effect observations on the vhf signal 519 the tid model for modulation of large scale electron density models of the geostationary satellite sirio. in this case evaluation of the raw data gives the time dependence of vertical electron content. band pass filtering reveals lstids of comparatively high amplitudes. surprisingly there is not much amplitude variation over the day. this means that the relative tid amplitudes have been larger during nighttime than during daytime. this is in accordance with the idea that agw attenuation is caused by «ion drag» and therefore stronger during daytime (higher ionization) than during nighttime. 3. model requirements and «modulation» technique in general, three dimensional and time dependent electron density models for the ionosphere of the earth, like the «family» of models developed at trieste and graz, nequick, costprof, neuog-plas (table ii) (hochegger et al., 2000) or the international reference ionosphere (iri; bilitza, 2001) provide the large scale «background» only. if the models give electron density for fixed universal time as a function of geographic coordinates latitude, longitude and height, smaller scale structures can be added by a multiplicative «modulation» technique (fig. 3) (leitinger et al., 2002) ( , , , ) ( , , , ) ( , , , ) ( , , , ) ( , , , ) h t h t h t h t h t m l t s sn1 $ g = + + { m { m { m { m { m$6 @ where m is the resulting electron density model; l, the large scale model (member of our model family, iri, etc.); t and s, the modulations e.g., for the main trough and for tids; h, the height; ϕ, geographic latitude; λ, the geographic longitude; and t, the universal time. fig. 3. scheme for the multiplicative «modulation» of electron density models to add smaller scale structures. fig. 4. horizontal radiation pattern of the tid model: radiation into a fan beam of given half width and azimuth. 520 reinhart leitinger and markus rieger f ig . 5. l at it ud e de pe nd en ce o f t id m od ul at io n in te gr at ed a lo ng s tr ai gh t li ne s fr om a t ra ns m it te r in t he 1 5° e m er id ia n at 1 00 0 km t o a gr ou nd r ec ei ve r at 4 5° n a nd 1 5° e . p an el 1 ( to p) : sn ap sh ot ( si m ul ta ne ou s tr an sm it te r po si ti on s) . p an el s 2 an d 4 (b ot to m ): t ra ns m it te r m ov in g so ut hw ar ds w it h 3° /m in . p an el 3 : tr an sm it te r m ov in g no rt hw ar ds w it h 3 °/ m in . t id p ro pe rt ie s: p an el 1 : pe ri od τ = 60 m in , ho r. w av el en gt h λ x = 10 00 k m ; pa ne ls 2 a nd 3: τ = 30 m in , λ x = 50 0 km ; pa ne l 4: τ = 30 m in , λ x = 30 0 km . s ou rc e po in t of t he t id s: 7 0° n , 3 0° e . h al fw id th o f fa n be am : 10 °, a zi m ut h of c en tr e of f an b ea m : 19 0° . o rd in at es : re la ti ve a m pl it ud es o f t id f lu ct ua ti on s. f ig . 6. l at it ud e de pe nd en ce o f in te gr at ed t id m od ul at io n (r el at iv e t id -t e c ) as i n fi g. 5 . t ra ns m it te r m ov in g w it h 3 °/ m in , so ut hw ar ds i n pa ne ls 1 an d 3, n or th w ar ds i n pa ne ls 2 a nd 4 . t id p ro pe rt ie s: p er io d τ = 3 m in i n al l ca se s; h or iz on ta l w av e le ng th λ x = 50 0 km i n pa ne ls 1 a nd 2 ; λ x = 30 0 km i n pa ne ls 3 a nd 4 . s ou rc e po in t of t he t id s: 7 0° n , 60 °e . h al fw id th o f fa n be am : 10 °, a zi m ut h of c en tr e of f an b ea m : 21 0° . o rd in at es : re la ti ve am pl it ud es o f t id f lu ct ua ti on s. 5 6 521 the tid model for modulation of large scale electron density models 4. the tid model the tid model makes use of the following properties of atmospheric gravity waves (agws): – the horizontal component kx of the wave vector; – the agw period τ = 2πω; – the dispersion relation to derive the vertical component kz of the wave vector; – the velocity of the disturbance cosv un n= $ ( ( ) )k s k h h tψx z$ + + ~ (where s is the horizontal coordinate; h, theheight; t, the time; and ψ is a height dependent phase constant). derived quantities are among others the horizontal and vertical wave lengths and phase velocities /k2x x=m r , /k2z z=m r , /v kx x= ~ , /v kz z= ~ . the model takes into account the projection factors discussed above, geometric dilution, horizontal and vertical attenuation. since the radiation patterns are far from isotropic the model assumes radiation into «fan beams» (fig. 4) and allows the combination of several beams originating in one or in several different source «points». presently we are using vertical «source lines» instead of real «source points», meaning that the vertical structure does not change with propagation. because of a modification of the «near field» formulation it is not necessary to exclude the source points. some details: – the tids originate in chosen «source points». – a tid travels in a «fan beam» defined by its azimuth and its half width; the wave properties are given by the wave period τ and by the horizontal wave length λx, the vertical wave length follows from the dispersion relation. – for the vertical structure a chapman profile was adopted defined by a scale height ht and by a peak height hmt. values chosen for the examples shown: ht = 100 km, hmt = 250 km. – the forward tilt of the wave fronts is produced by (−kx / kz) in accordance with the dispersion relation of the agws. – the geometric dilution of horizontally travelling agws and horizontal attenuation are also taken into account. fig. 7. latitude dependence of tid modulated electron density model nequick. geometry and tid properties as in fig. 5, panels 3 and 4. solid lines: tid modulated electron density. dotted lines: background model. 522 reinhart leitinger and markus rieger of observations with model results show that we obtain realistic tid signatures in electron content (see figs. 1 and 5 to 7). the tid model should not be used for tests of agw sources and propagation paths. one should not expect realistic «near field» results. the tid formulation has been used successfully to model interfering smaller scale tids as observed by means of a dense network of dual frequency gps receivers (rieger and leitinger, 2005). – the model allows superposition of several tid wave trains. an example for a single tid wave train is given in figs. 8 and 9. 5. discussion and conclusions the main purpose of our tid model is to provide a «modulation» for large scale electron density models used primarily for the numerical calculation of transionospheric propagation parameters, one of the most important being satellite to ground electron content. the tids have a simplified vertical structure (chapman layer) and therefore are not well suited for e-region and lower f-region applications. comparisons fig. 8. example from the tid model: tid in a fixed height of 250 km. horizontal coordinates: geographic longitude (−10°e-20°e) versus geographic latitude (0°n to 60°n). tid properties: wave period 60 min, horizontal wavelength 2000 km, source point in 70°n, 15°e, half width of fan beam: 5°, azimuth of centre of fan beam: 195°. fig. 9. example from the tid model: tid in a fixed height of 250 km. isolines of relative tid amplitude over a geographic coordinate system (longitude −10°e ... 20°e versus latitude −90°n ... 90°n). solid lines: positive levels (0.1, 0.2, ..., 0.9), dotted lines: negative levels (−0.1, −0.2, ... −0.9). tid properties: wave period 60 min, horizontal wavelength 2000 km, source point in 70°n, 15°e, half width of fan beam: 5°, azimuth of centre of fan beam: 195°. the «projection effect» is cleanly seen in the latitude region around 10°n (agw wind nearly perpendicular to geomagnetic induction vector). 523 the tid model for modulation of large scale electron density models references bilitza, d. (2001): international reference ionosphere 2000, radio sci., 36, 261-275. francis, s.h. (1975): global propagation of atmospheric gravity waves: a review, j. atmos. terr. phys., 37, 10111054. georges, t.m. and w.h. hooke (1970): wave-induced fluctuations in the ionospheric electron content: a model indicating some observational biases, j. geophys. res., 75, 6295-6308. hines, c.o. (1960): internal atmospheric gravity waves at ionospheric heights, canad. j. phys., 38, 1441-1481. hochegger, g., b. nava, s.m. radicella and r. leitinger (2000): a family of ionospheric models for different uses, phys. chem. earth (c), 25, 307-310. jakowski, n., r. leitinger and m. angling (2004): radio occultation techniques for probing the ionosphere, ann. geophysics, 47 (suppl. to no.2/3), 1049-1066. kirchengast, g. (1996): elucidation of the physics of the gravity wave – tid relationship with the aid of theoretical simulations, j. geophys. res., 101, 13353-13368. kirchengast, g. (1997): characteristics of high-latitude tids from different causative mechanisms deduced by theoretical modelling, j. geophys. res., 102, 45974612. kishcha, p.v., v.m. shashunkina and e.e. goncharova (1993): updating the iri ionospheric model for effects of substorms, adv. space res., 13, 67-70. leitinger, r. (1992): travelling ionospheric disturbances (tids) − wissensstand und neuere entwicklungen. kleinheubacher berichte, 35, 1-14. leitinger, r. (1999): ionospheric tomography, in review of radio science, 1996-1999, edited by w.r. stone (oxford university press, oxford, u.k.), 581-623. leitinger, r. and e. putz (1991): untersuchung der begrenzung von f-schicht-prognosen durch den einfluß atmosphärischer schwerewellen, kleinheubacher berichte, 34, 535-542. leitinger, r., s.m. radicella and b. nava (2002): electron density models for assessment studies – new developments, acta geod. geophys. hung., 37, 183-193. morgan, m.g. and c.h.j. calderón (1978): testing hooke’s perturbation formula for the production of tid’s by gravity waves, j. geophys. res., 83, 57375740. prölss, g.w. (2001): physik des erdnahen weltraums: eine einführung (springer, berlin). putz, e., r. leitinger, g.k. hartmann, and p. bencze (1990): simultane beobachtung von tid’s an mehreren beobachtungsstationen, kleinheubacher berichte, 33, 113-119. putz, e., r. leitinger, n. jakowski, and p. spalla (1992): untersuchung zur interpretation von schwerewellensignaturen in der kombination von elektroneninhaltsdaten aus verschiedenen quellen, kleinheubacher berichte, 35, 105-112. rieger, m. and r. leitinger (2002): assessment of tid activity from gps phase data collected in a dense network of gps receivers, acta geod. geophys. hung., 37, 327-341. rieger, m. and r. leitinger (2005): simulation studies for the retrieval of smaller scale electron content structures from regional gps observations, in proceedings of the international beacon satellite symposium 2004 (in press). van velthoven, p.f.j (1990): medium scale irregularities in the ionospheric electron content, ph.d. thesis (technische universiteit eindhoven, nl-5600 mb eindhoven). annals 47, 1, 2004, 01/07def 39 annals of geophysics, vol. 47, n. 1, february 2004 key words self-potential signals – wavelet analysis 1. introduction environmental conditions act on self-potential variability driving non-stationary patterns which can highly distort background behaviours (for a discussion of self-potential electrochemical theory see e.g., pham et al., 2001; and references therein). such disturbances severely limit the possibility of studying the electrical variability of pure geophysical origin and correctly using self-potential measures for the monitoring of some geophysical phenomena. in particular, the relationship between electrical variability and seismic or volcanic activity, which might be useful in prediction studies of earthquakes and volcanic eruptions (e.g., sobolev, 1975; massenet and pham, 1985; varotsos et al., 1993; cuomo et al., 1996; di bello et al., 1996; park, 1996; varotsos et al., 1996; uyeda et al., 2000), can be strongly hidden by environmental disturbances such as meteorological variability and anthropic activities. anthropic influences can be reduced by choosing monitoring sites in zones which are sufficiently far from industrial or urban areas, high power lines, and any source of human disturbance. this is possible just in theory, since for applied purposes we have to take into account that risk areas generally do not satisfy these requirements. in any case, even with optimal instrumental and ambient conditions, electric signals are sensitive to meteorological variability wavelet analysis as a tool to characterise and remove environmental noise from self-potential time series domenico chianese (1), gerardo colangelo (1), mariagrazia d’emilio (2) (4), maria lanfredi (1) (3) (4), vincenzo lapenna (1), maria ragosta (2) (4) and maria francesca macchiato (3) (4) (1) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy (2) dipartimento di ingegneria e fisica dell’ambiente, università degli studi della basilicata, potenza, italy (3) dipartimento di scienze fisiche, università degli studi di napoli «federico ii», italy (4) istituto nazionale per la fisica della materia (infm), genova, italy abstract multiresolution wavelet analysis of self-potential signals and rainfall levels is performed for extracting fluctuations in electrical signals, which might be addressed to meteorological variability. in the time-scale domain of the wavelet transform, rain data are used as markers to single out those wavelet coefficients of the electric signal which can be considered relevant to the environmental disturbance. then these coefficients are filtered out and the signal is recovered by anti-transforming the retained coefficients. such methodological approach might be applied to characterise unwanted environmental noise. it also can be considered as a practical technique to remove noise that can hamper the correct assessment and use of electrical techniques for the monitoring of geophysical phenomena. mailing address: dr. maria lanfredi, istituto di metodologie per l’analisi ambientale (imaa), cnr area della ricerca di potenza, contrada s. loja, 85050 tito scalo (pz), italy; e-mail: lanfredi@imaa.cnr.it 40 domenico chianese et al. that can alter electrochemical phenomena. therefore, studies related to the assessment of soil electric activity as a signature of geophysical phenomena for monitoring purposes cannot neglect the presence of inherent meteorological disturbances. self-potential signals are good examples of inhomogeneous signals containing both regularities and isolated singularities in the form of pulses, jumps, power or deltalike singularities. in order to single out unwanted interferences, our first task is to analyse electrical fluctuations retaining information on the localization of discontinuities and transient variations. to this purpose, wavelet analysis is a useful tool, able to carry out multiresolution studies and to enhance local features against long term dynamic structures. formalization of wavelet theory was actually initiated by works on seismic signals (goupillaud et al., 1984; grossmann and morlet, 1984). environmental perturbations in electrical signals might be singled out in the timescale domain of the wavelet transform more easily than in the physical space. by transforming meteorological data, we locate the time-scale regions where the environmental stress acts. then we transform the self-potential signal and, in the previously selected regions, we extract the excited wavelet coefficients. such coefficients account for local fluctuations which are candidates for describing the electrical responses to the external disturbance. excited coefficients can be filtered out and the signal can be recovered by anti-transforming the retained coefficients. in this exploratory work, we focus on hourly electrical variability observed during rainy periods using hourly rainfall levels as support data. these levels are proxy data for the triggering action of rainfalls, since they are related to transient variations in the soil water content. the information stored in rain data concerns the strength of the external forcing and its duration but it does not provide indications on the evolution of possible subsequent long term soil responses. in dynamic terms, our support data allow us to investigate those short range self-potential fluctuations which strictly follow significant gradients of the soil water content. of course, with additional support data, our methodological approach can be useful to pick up long range features as well. 2. observational data we analyse self-potential signals measured by means of a new geoelectrical monitoring network installed in a seismic active area of the southern apennine chain (italy). we focused on the analysis of self-potential data measured by the remote station prototype developed at the beginning of 1999 at the institute of methodologies for environmental analysis of the national research council (imaa/cnr) geophysical laboratory, located in tito scalo (pz). for further details on the monitoring network and the description of the experimental equipment, we refer to balasco et al. (2002). pluviometric measures were supplied by the istituto idrografico of catanzaro. figure 1a,b shows two examples of self-potential time series recorded in rainy periods. the signal in fig. 1a is very erratic on short time scales and does not exhibit significant long range trends. similarly to earthquakes, pluviometric measures can be regarded as realizations of a «point process» (cowpertwait, 1994) that is a succession of discrete events we will call «rain events». rain events in fig. 1a induce evident perturbations which seem to follow the triggering action of the rainfalls rather strictly. the first three consecutive small events induce a slight variation in the mean electric potential values (a trend over a period of ∼ 1 day), whereas a cluster of more important events drives a rather sharp and large variation. this last perturbation persists for a time which is almost equal to the rainy period. in fig. 1b, a cluster of rain events located at a time distance of a very few hours is likely to be responsible for the concomitant strong potential variation shown in the plot (∼ 100 mv). differently from the previous example, this transient is superimposed on a non stationary background. after that the rainfalls stop, the signal decrease is followed by an increase presumably due to the progressive drying process. the increase asymptotically restores the signal morphologies observed before the rainfalls but with mean values which are greater than those observed immediately before the rainfalls. erratic short range variability with anti-persistent features is ubiquitous, as already highlighted in recent works (cuomo et al., 2000; colangelo et 41 wavelet analysis as a tool to characterise and remove environmental noise from self-potential time series al., 2001). in these two cases, although correspondence between rain and signal anomalies appears rather evident, the strength and direction of the variations as well as duration of transients seem to be concerned with local dynamics. 3. the problem and the analysis rationale 3.1. the dynamic problem the physical-statistical investigation of observational signals, aimed to reveal their underlying dynamics, is constrained by many factors, the main one being the nature itself of the dynamics. the presence of complex mechanisms related to many sources, possible nonlinear interactions among different perturbations, noises, make it very hard to solve the problem with simple tools. unfortunately, we cannot a priori assume that disturbances induce simple effects. ideally we can globally decompose the signal into smooth and fast components. in this picture, we split the dynamics on the basis of the frequency content: low frequencies account for long range variations whereas high frequencies account for generally erratic short range fluctuations (noise). stationary noises in the context of linear dynamics are negligible because their effects have no significant consequences of dyfig. 1a,b. two examples of hourly measures of self-potential and contextual hourly rainfall levels: a) measures recorded from 20.11.1999 to 03.12.1999; b) from 08.05.1999 to 29.05.1999. the vertical axis on the left side in the plot refers to self-potential while the axis on the right side refers to rain. a b 42 domenico chianese et al. namic value. differently, externally induced abrupt changes and transient phenomena complicate the understanding of the electrical dynamics and its sources. the local character of these effects can alter the global dynamic outlook of the phenomenon, short and long time scales can interact and the simple global picture described above does not work well. 3.2. the signal processing problem mathematical transformations are usually applied to experimental data to obtain dynamic information that does not clearly appear in raw data. it is well known that pure traditional tools, such as the standard fourier transform, are not suited for the analysis of non stationary signals. the fourier transform of a signal with a local disturbance spreads the information concerning this singularity in all its coefficients so any filter will distort both the spectrum and the recovered signal. we need instead analysis tools able to carry out both global and local investigations allowing us to single out features at various temporal scales retaining information on the localization of discontinuities. starting from traditional methods, some techniques have been developed, such as the short time fourier transform (see e.g., portnoff, 1981), to carry out scale and time analysis simultaneously. in order to account for non stationary patterns, a temporal window is shifted along the series and the signal fluctuations are analysed separately within the series segments selected by the window. the width of the window is independent of the time scale so these methodologies are single-resolution. transients in self-potentials may generally exist at different scales. when one cannot previously estimate the duration of the local patterns, a flexible window width is recommended. wavelet analysis overcomes such problems since it is able to provide multiresolution time and frequency characterisation. it breaks up a signal in waveforms of duration matched to the scale. such waveforms may be irregular and asymmetric, so providing a wide collection of functions to investigate shape, duration and arrival time of transients. 3.3. rationale we use the wavelet transforms of support time series as independent «markers» to select those time-scale regions where we expect that important features of the signal are mainly driven by environmental forcing. in the wavelet transform of the signal, such features are represented by anomalous (usually high absolute value) wavelet coefficients which we consider excited by external forcing. owing to the multiresolution character of the wavelet analysis, the complexity of the patterns described in the physical space is separated in a multi-layer simpler descriptor. at any scale and time, we search for those excited coefficients that match significant coefficients of the marker. at this preliminary stage, we do not focus on the extensive study of electrical responses to different rainfall dynamics or on filtering strategies which can be more or less sophisticated depending on the specific needs. we merely aimed to evaluate the possibility of discriminating in a simple way wavelet coefficients accounting for meteorology induced morphological distortions by following the fingerprints of suited support data. we simply filtered the selected wavelet coefficients substituting their values with values randomly extracted form the distribution of the coefficients estimated at the scale concerned immediately before the rainfalls. this procedure is only mildly invasive because it preserves all the other coefficients and therefore all those details that, at any scale and time, cannot be directly associated with the given forcing, also during transients. 4. some basic concepts of wavelet analysis at present, there is an extensive literature concerning wavelet analysis and its applications, from classical papers (e.g., mallat, 1984; daubechies,1992) to more recent textbooks (e.g., percival and walden, 2000). here we limit ourselves to some background concepts useful for understanding this paper. wavelet analysis uses functions (wavelets), which are localised both in time and in frequency (or scale, in the rigorous wavelet for43 wavelet analysis as a tool to characterise and remove environmental noise from self-potential time series malism). in practice, the only basic requirement for regarding a function as a plausible wavelet is that it is an oscillating function which has limited duration, zero mean with an inherent pass-band like spectrum. the wavelet transform takes a one-dimensional time signal into a two-dimensional transformed function of time and scale. therefore it resolves features at various scales without losing information on time localization. such intrinsic ability to zoom in on short-lived high frequency phenomena is simply achieved by introducing a scale parameter s which adapts the width of the wavelet kernel to the resolution required (thus changing its frequency contents) at locations which are determined by a parameter t0. dilated (changing s) and translated (changing t0) versions are obtained from a wavelet prototype ψ (mother wavelet). by iteratively dilating the wavelet and shifting it along the signal we obtain a multiscale information of the temporal properties of the signal. dilated and translated versions of the mother wavelet are obtained by the action u ,u s t t s t t 0 0 = -} }^ ] ah g k where s, t0 ∈ r and s > 0 for the continuous wavelet transform (cwt). the wavelet transform wf of a function f (t) is then obtained projecting the function on the dilated and translated wavelets , ,w s t s f t u s t t dt 1 f 0 0 ) = }#_ ^ _ ^i h i h where (*) denotes complex conjugation. cwt is obtained by continuously shifting scalable functions which do not form an orthogonal basis. for many applied purposes we use the discrete wavelet transform (dwt, daubechies, 1992). discrete wavelets are not continuously scalable and translatable but are scaled and shifted in discrete steps. in the dwt formalism the wavelet representation is .t s t kt s ,j k j j 0 = } }] cg m the parameters j and k are integers and s > 1 (s = = 2 for the usual dyadic sampling) is a fixed dilation step. the translation parameter t0 depends on this dilation step: for t0 = 1 we have a dyadic sampling of the time axis as well. wavelet series decomposition transforms signals in series of wavelet coefficients. in discrete wavelet analysis, the signal is decomposed in approximation and details. the process can be iterated. the original signal is decomposed in successively lower resolution components so producing a wavelet decomposition tree. figure 2 shows the generic form of a one-dimensional (1d) wavelet transform. the signal is passed through h (a low-pass filter) and g (a high-pass filter), and then down-sampled by a factor of two. this process is considered one level of the transform and can be repeated for a finite number of levels n. the resulting outputs, di (i = 1, 2, …, n) and ln, are called wavelet coefficients, and are details and approximation respectively. when we use orthogonal wavelet basis functions, we can reconstruct an arbitrary signal by anti-transforming its wavelet coefficients. in this work, a dyadic wavelet decomposition is carried out down to level n = 3 that corresponds to a scale 23 = 8 h (the coarsest time resolution of our analysis). this time scale appeared sufficient for investigating relatively fast variations in the given signals. we used the daubechies 3 (asymmetric) wavelet. nevertheless trials with different wavelet functions did not imply remarkable differences in our analysis. 5. results figure 3a,b shows the rain and self-potential wavelet coefficients estimated for the signal of fig. 2. multilevel wavelet transform. the symbol ↓2 indicates down-sample by two. 44 domenico chianese et al. fig. 3a,b. wavelet coefficients (a) for rainfall measures and (b) for self-potentials relative to the signal reported in fig. 1a. a b 45 wavelet analysis as a tool to characterise and remove environmental noise from self-potential time series fig. 1a. correspondence between significant rain wavelet coefficients (fig. 3a) and excited signal coefficients (fig. 3b) is generally very good. we need no particular comparison criterion; a naked eye view is sufficient to understand that rain data are suited to locate the time-scale regions where the self-potential variability is altered. it is interesting to observe the signal coefficients (fig. 3b) at level 1 (scale = 2 h) in correspondence with the cluster of rainfalls (100150 h). if we exclude a slighthy excited coefficient associated with the largest events, the coefficients at this scale do not appear particularly sensitive to rainfalls. this suggests that, when triggered, the rain effects are played on time scales longer that two hours. if a smoothing filter is used to remove the meteorological anomaly, these fine scale details may be averaged out. our procedure instead retains these details. figure 4 shows a zoom in representation of the original signal (fig. 1a) and the recovered signal. the two signals differ just during the rainfalls. it is interesting to see that no remarkable difference can be detected between the reconstructed signal during the external event and that of the real fluctuations observed before and after it. as stressed above, fine scale details are not synthetic. our filter removes just excited coefficients so restoring the background behaviour. this fact highlights the mildly invasive character of our reconstruction. the problem for the signal of fig. 1b is slightly more difficult because hourly rainfall levels in this case are not optimal support data. figure 5a,b shows the estimated self-potential and rain wavelet coefficients. by comparing the coefficients at the same level of approximation or detail, we again observe a good agreement, but the excited regions for the signal are slightly longer that those detectable in the wavelet fig. 4. original (black line) and recovered (bold line) signals for the time series of fig. 1a. 46 domenico chianese et al. fig. 5a,b. wavelet coefficients (a) for rainfall measures and (b) for self-potentials relative to the signal reported in fig. 1b. a b 47 wavelet analysis as a tool to characterise and remove environmental noise from self-potential time series transform of the rainfalls. as already observed in section 2, the triggering action of the rainfalls seem to be followed by slower responses of the soil. in this case, our support data do not provide sufficient information to understand where the transient ends. in order to recover the signal, we made the operational hypothesis that the excited coefficients relative to the 24 h after the end of the rainfalls were relevant to the meteorological stress as well. as shown in fig. 6, the morphology of the recovered signal in the rainy period traces patterns which are similar to those observed in dry periods. about 24 h after the end of the rainfalls, the recovered signals adapts itself to intersect the observational data. 6. conclusions the main idea we present in this paper is that the joint multiresolution wavelet analysis of self-potential signals and support data related to environmental forcing can represent a suitable basis to extract environment-induced electrical fluctuations. wavelet transform translates the complexity of mixed global behaviours and transient patterns described by the electrical signals in simpler time sequences of coefficients over several resolutions or scales. we focused on hourly self-potential variability driven by rainfalls by exploiting hourly rainfall level measures as support data. such data, transformed in the wavelet domain, were used to mark the time-scale regions where rainfalls act influencing self-potential variability. we showed that in these regions excited wavelet coefficients of the signal are detectable and they well account for local alterations ascribable to the rain. moreover, these coefficients can be filtered out, removing distortions with a mildly invasive technique. we think that our methodological approach is promising. however, the fig. 6. original (black line) and recovered (bold line) signals for the time series in fig. 1b. 48 domenico chianese et al. reliability of the recovering procedure is strictly related to the quality of the support data and their ability to represent the actual mechanisms we are interested in. references balasco, m., d. chianese, g. colangelo, v. cuomo, g. di bello, m. gallipoli, v. lapenna, m. mucciarelli, d. patella and s. piscitelli (2002): a new monitoring network to detect geoelectrical and seismometric signals in a seismic area of the southern italy, in seismo electromagnetics: lithosphere-atmosphereionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 405-411. colangelo, g., m. lanfredi, v. lapenna, m. macchiato and f. vallianatos (2001): short time scale laws in self-potential signals from two different seismically active mediterranenan areas (the southern apennine chain and the crete island), ann. geofis., 44(2), 421428. cowpertwait, p.s.p (1994): a generalised point process model for rainfall, proc. r. soc. london, 447, 23-37. cuomo, v., g. di bello, v. lapenna, m. macchiato and c. serio (1996): parametric time series analysis of extreme events in earthquake electrical precursors, tectonophysics, 262, 159-172. cuomo, v., m. lanfredi, v. lapenna, m. macchiato, m. ragosta and l. telesca (2000): antipersistent dynamics in short time scale variability of self-potential signals, ann. geofis., 43 (2), 271-278. daubechies, i. (1992). ten lectures on wavelets (siam, philadelphia, pa), pp. 357. di bello, g., v. lapenna, m. macchiato, c. satriano, c. serio and v. tramutoli (1996): parametric time series analysis of geoelectrical signals: an application to earthquakes forecasting in southern italy, ann. geofis, xxxix (1), 11-21. goupillaud, p., a. grossmann and j. morlet (1984): cycle-octave and related transforms in seismic signal analysis, geoexploration 23, 85-102. grossmann, a. and j. morlet (1984): decomposition of hardy functions into square integrable wavelets of constant shape, siam journal of mathematical analysis, 15, 723-736. mallat, s.g. (1989): a theory of multiresolution signal decomposition: the wavelet representation, ieee trans. patterns anal. mach. intell., 11 (7), 674-693. massenet, f. and v.n. pham (1985): experimental and theroretical basis of self-potential phenomena in volcanic areas with reference to results obtained on mount etna (sicily), earth planet. sci. lett., 73, 415-429. park, s. (1996): precursors to earthquakes: seismoelectromagnetic signals, surv. geophys., 17, 493-516. percival, d.b. and a.t. walden (2000): wavelet methods for time series analysis (cambrige university press, cambrige), pp. 594. pham, v.n., d. boyer, f. perrier and j.l. le mouel (2001): mechanismes de génération de bruits telluriques dans la bande ultrabasse fréquence (ubk): sources possibles des signaux dits signaux électroseismiques (ses), earth planet. sci., 333, 255-262. portnoff, m.r. (1981): short-time fourier analysis of sampled speech, ieee trans. assp, 36, 1377-1384 sobolev, g.a. (1975): application of electric method to the tentative short term forecast of kamtchatka earthquakes, pure appl. geophys., 113, 229-235. uyeda, s., t. nagao, y. orihara, t. yamaguchi and i. takahashi (2000): geoelectric potential changes: possible precursors to earthquakes in japan, proc. nat. acad. sci. u.s.a., 97 (9), 4561-4566. varotsos, p., k. alexopoulos and m. lazaridou (1993), latest aspects of earthquake prediction in greece based on seismic electric signals, tectonophysics, 224, 1-37. varotsos, p., m. lazaridou, k. eftaxias, g. antonopoulos, j. makris and j. kopanas (1996): short-term earthquake prediction in greece by seismic electric signals, in a critical review of van – earthquake prediction from seismic electric signals, edited by j. lighthill (word scientific, singapore), 29-76. t h e w a v e g u i d e i n t h e mantle o f the earth a n d its p r o b a b l e p h y s i c a l n a t u r e b y m a g n i t s k y v . a . , k h o r o s h e y a v . v . (*) ricevuto il 7 (liceiiibre 1960 as far as we can see, b. gutenberg was the first who points out the possibility of existence of a low-velocity layer in the upper parts of the mantle (gutenberg, 1926). i t is quite obvious that such a lowvelocity layer can act as channel conducting seismic energy. this was established by caloi (1953, 1951) and press and ewing (1954, 1955). in present paper we report results of further investigations in that topic based on data from seismograms of 9 earthquakes recorded by seismic stations of ussr with epicentral distances in the range 22° to 150° (table 1). there were used only seismograms with epicentral distances in which no overlap of pa and sa with any other phase could be suspected. almost in every seismogram pa and sa phases can be clearly detected (fig. 1), however, the first arrival of the phase is often not very distinct. this is probably the main cause of considerable scattering in results. in figure 2 are plotted travel times versus a. in spite of scattering of arrival times it is apparent that travel-time curves for pa and sa are straight lines, their equations are as follows this gives telocity 8,3 km/sec and 4,47 km/sec for pa and sa respectively. periods of pa are in range 5-12 sec, those of sa in range 7 3 0 sec. v = 0,9558 + 0,2205 a r = 0,3780 + 0,4180 a o o pa sa (*) paper read at the helsinkv assembly of the i.u.g.g., 1960. 8 8 v . a . m a g n i t s k y , v . v . k h o r o s h e v a there is familiar equation connecting amplitude a of seismic wave with distance r from the source a = cr~~" e ~ar [1] g being constant, a absorption coefficient, n depends on the type of wave. for plane wave n = 0, for spherical wave n == 1, for cylindrical table 1 nu d a t e a <p depth km ni a k m 1 1 11/x -56 1 5 1 , 3 e 4 5 , 4 n 100 ty^vz 0,000143 2 18/viii 5 4 1 7 5 , 0 w 2 1 , 5 s 170 ivi-iy* 0,000146 3 23/v -56 1 7 8 , 5 w 1 5 , 5 s 400 0,000139 4 7/vi -54 1 5 2 , 5 e 4,os 460 y 2 0,000156 5 20/11 5 4 1 2 5 , 0 e 7 , 5 s 520 6m--7 0,000184 6 16/viii 5 5 155,0 e 6,os 200 6 *u-iy2 7 31/111 5 5 1 2 4 , 0 e 8,on 50 7 y4 8 29/111 -54 3 , 5 w 37, on 640 m-m 9 21/111 -54 9 5 , 0 e 2 4 , 5 n 170 7 7 y2 one n = 0,5. if we denote a0, r0 amplitude and distance at any arbitrary fixed point [1] becomes in a0/a r — r0 in r\r0 " " in r/r0 plotting empirical values of [2] for earthquake of 11 october 1956 on fig. 3 we obtain a straight line with n = 0,52 and a = 0,00014 k m 1 . the inference is the pa phase corresponds to a cylindrical wave. hence it is evident from both the travel times and amplitudes that pa phase is connected with the surface or channel wave. velocity obtained excludes the first supposition, consequently we must adopt the channel type to the wave in consideration. unfortunately all other earthquakes have epicenters at great distances relatively to our stations. so we must use instead of [1] equation a = c (sin zip'/a • e~aar [3] t h e w a v e g u i d e i n t h e m a n t l e of t h e e a r t h , e t c . 8 9 i n [3] one takes in consideration the spherical shape of the channel. i n last column of table 1 are plotted a obtained by [3] for other earthquakes. these results are in apparent agreement with our previous conclusions. but what is the cause of the formation of the low-velocity layer! the common supposition is t h a t it is due to prevailing effect of increase fig. 1. example of records of pa and sa phases ( e a r t h q u a k e 29/111-1954, 6h17m06s, epicentral distance a = 31°,3). of temperature us compared with effect of increase in pressure. velocity v would have minimum if dv/dh = 0. this gives ( l l at up, j. m dh ( 3® ' using the data from the work of hughes and cross (1951) we deduce the critical value of dt/dli in range 7 to 10 degrees per km. this seems to be too much. but if we take into consideration the temperature de9 0 v. a . m a g n i t s k y , v . v . k h o r o s h e v a pendance of thermal conductivity (peierls, 1955, dugdall and mcdonald, 1955, clark, 1956) in form * = ~ + bt3 [5] we infer that x has a minimum at the depths 50-100 km (lubimova, 1958). let us take 1,2 • 10"" cal/cm2sec as average for the heat flow at fig. 2. traval-times for pa and sa. the earth's surface. adopting usual content of heat generation in various types of rocks we bnd that gradient dt/dh can run up to 18 deg/km at 7t = 100 km beneath the continental crust and 15 deg/km at li = 50 km beneath the oceanic crust. these values seem to be quitesu fficient to explain the formation of a low-velocity layer. t h e w a v e g u i d e i n t h e m a n t l e of t h e e a r t h , e t c . 9 1 another explanation assumes the vitrification of the material of the mantle a t respective depths. for the volume increase by vitrification we can write (frenkel, 1950) v v — = « w [6] ' o e denotes energy of activation. from [6] we obtain by differentiation ak av , av 3ine0 = f t " l n t ' " \ ; tt t ' l ka t 0 i o 3 in t k incompressibility, e0 energy of activation at zero pressure. derivative dlneo t i n v ~ 2 ' 5 (zharkov, 1958). 1 / k denoting 1/ — = v we hiul i q dv 1 /dk dv v 2 \ k + ~v [ 8 ] 9 2 v . a . m a g n i t s k y , v . v . k h o r o s h e v a now it follows / , a v 3 lne0\ i 1 + l n v0 • yinv) [ 9 ] d v _ a al i• a v a lne° v ~ 2 v0 a v for dunite —— 2%, consequently we have v0 dv eg 6% which is in fair agreement with conditions in the low-velocity layer. finally we shall take into account assumption that the composition of the mantle is of stone meteorites. so we suppose the 30% of the substance of the mantle consists of mgsi03. i t is established t h a t at the temperature approximatly 1000° c there is transition from rhombic modification of enstatite to protoenstatite with volume increase nearly 3% (atlas, 1952, smith, 1958). we may assume the energy of lattice of both modifications is expressed by equation u = [ 1 0 ] r r" l j a — madelung constant being different for various modifications. b and n are determined by the nature of repulsive forces. as the character of repulsive forces do not change during transition mentioned above, we can suppose in first approximation b and n being the same for both modifications. we have k dp dr [11] from [10] and [11] it follows <k\ k n ap e . [12] \ p i o 3 o for most causes n is in the range 6 to 9, so decrease in velocity in lowvelocity layer can be suspected to be about 3%. i n conclusion it is necessary to emphasise that all three causes of formation of low-velocity layer seem to be quite adequate to explain it and it is impossible to choose one of them without any additional data. t h e w a v e g u i d e i n t h e m a n t l e of t h e e a r t h , etc. 9 3 summary records on seismic stations in ussr confirm the existance of pa and sa phases in no correspondance to any well known phase. the traveltime curves of pa and sa phases are obtained as being straight lines. velocities of pa and sa are 8,30 ± 0,0-3 kmlsec and 4,57 ± 0,03 km/sec respectively. decrease in amplitudes as well as the form of travel-time curves indicates that pa and sa are cylindrical waves. it is supposed pa and su travelling along a low-velocity channel in astenosphere. formation of such a layer is due to vitrification of material at the depth of some 100-200 km. riassunto le registrazioni effettuate nelle stazioni sismiche in u.r.s.s. confermano vesistenza di fasi pa ed sa senza corrispondenza con alcuna fase gia nota. le curve del tempo di cammino delle pa ed sa risultano linee rette. le velocita delle pa ed sa sono rispettivamente di 8,30 ± 0,03 km/sec e 4,57 ztz 0,03 km/sec. le diminuzioni di ampiezza e la forma della curva del tempo di tragitto indicano che le pa ed sa sono onde cilindriche. si suppone che le pa ed sa si muovano su un canale a bassa velocita nella astenosf era. la formazione di un simile strato e dovuta alia vetrificazione del materiale alia profondita di circa 100-200 km. r e f e r e n c e s a t l a s l . , the polymorphysm of mgsio3. « j o u r n . geol. », 6 0 , 2 , ( 1 9 5 2 ) . c l a r k s . p., effect of radiative transfer of temperature in the earth. « bull. geol. soc. amer. », 67, 5, (1956). c a l o i p., onde longitudinali e trasversali guidate dell'astenosfera. « rend. acad. naz. line. », ser. 8, 15, 6, (1953). — l' astenosf era come canale-guida dell' energia sismiea. « ann. geof. », 7, 4, (1954). d u g d a l l , m c d o n a l d q . , lattice thermal conductivity. « phys. rev. », 9 8 , 6, (1955). f r e n k e l j . i . , introduction in the theory of metals. moscow (in russian) 1 9 5 0 . g u t e n b e r g b . , untersuehungen zur frage, bis zu welcher tiefe die erde kristallin ist. « zeitschr. geopliys. », 2, 1, ( 1 9 2 6 ) . 9 4 v . a. m a g n i t s k y , v . v . k h o r o s h e v a h u g h e s d. s . , c r o s s j . h., elastic wave velocities in rocks at high pressures and temperatures. « geophys. », 16, 4, (1951). l u b i m o v a h. a., thermal history of the earth with consideration of the variable thermal conductivity of its mantle. « geoph. j o u r n . roy. a s t r . soc. », 1, 2, (1958). p e i e r l s r. e., quantum theory of solids. oxford, 1955. p r e s s p., e w i n g m., p and s velocities at great distances. « bull. geol. soc. amer.», 65, 1, (1954). — — waves with p and s velocity at great distances. « proc. n a t . acad. sc. », 41, 1, (1955). s m i t h j . v., crystal structure of protoenstatite. « bull. geol. soc. am. », 6 9 , 12, 2, (1958). z h a r k o v v . n., on electrical conductivity and temperature of the earth's mantle. «proc. ac. sc. ussr », ser. geophys. 4 (in russian) 1958. layout 6 quantitative analysis of the 1981 and 2001 etna flank eruptions: a contribution for future hazard evaluation and mitigation maria a. marsella1, silvia scifoni*,1, mauro coltelli2, cristina proietti2 1 università di roma «la sapienza», dipartimento di ingegneria civile, edile e ambientale (dicea), rome, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5334 abstract lava flows produced during etna flank eruptions represent severe hazards for the nearby inhabited areas, which can be protected by adopting prompt mitigation actions, such as the building of diversion barriers. lava diversion measures were attempted recently during the 1983, 1991-93, 2001 and 2002 etna eruptions, although with different degrees of success. in addition to the complexity of barrier construction (due to the adverse physical conditions), the time available to successfully slow the advance of a lava flow depends on the lava effusion rate, which is not easily measurable. one method to estimate the average lava effusion rate over a specified period of time is based on a volumetric approach; i.e. the measurement of the volume changes of the lava flow over that period. here, this has been compared to an approach based on thermal image processing, as applied to estimate the average effusion rates of lava flows during the 1981 and 2001 etna eruptions. the final volumes were measured by the comparison of pre-eruption and post-eruption photogrammetric digital elevation models and orthophotographs. lava volume growth during these eruptions was estimated by locating the flow-front positions from analyses of scientific papers and newspapers reports, as well as from helicopter photographs. the analyses of these two eruptions contribute to the understanding of the different eruptive mechanisms, highlighting the role of the peak effusion rate, which represents a critical parameter for planning of mitigation actions and for hazard evaluation. 1. introduction mount etna is characterized by intense volcanic activity at the summit area, although this does not usually pose a danger to nearby inhabited areas. on the contrary, lava flows from flank eruptions can pose a severe threat to the nearby villages and towns located at lower altitudes. most of the recent flank eruptions were characterized by low average lava effusion rates of relatively long durations; e.g. during the 1950-51, 1983, 1984, 1985, 1991-93, 2001 and 2002-03 eruptions. however, in some cases, high average lava effusion rates of short duration, i.e. of days or weeks, have occurred, as in 1928, 1971 and 1981. in terms of hazard mitigation, eruptions with low average lava effusion rates allow the setting up of civil protection measures during the flow emplacement; however, eruptions with high average effusion rates of short duration require proactive risk assessment and interventions. the 1981 and 2001 eruptive events of etna are examples of both eruption patterns. indeed, the lava flows from the 2,550 m and 2,700 m a.s.l. vents during the 2001 eruption were characterized by low effusion rates and a relatively slow lava advance, thus allowing the national department of civil protection to build 13 earth barriers to protect the "rifugio sapienza" tourist facilities [scifoni et al. 2010]. in contrast, the 1981 eruption, and in particular its main flow, was characterized by a very fast lava flow. if the 2001 effusion rate had reached a peak value as high as that of the 1981 eruption, it would have been not possible to set up similar mitigation measures. clearly, to assess the time available to implement successful mitigation measures, it is necessary to estimate the effusion rate trend. unfortunately, direct and frequent measurements of this parameter are not easily achievable. during an eruption, instantaneous effusion rates can be estimated roughly in the field, or remotely using satellite thermal images. the time-averaged discharge rates (tadrs) can also be evaluated by applying topographic and photogrammetric techniques to extract digital elevation models (dems) during syn-eruption surveys. in this way, it is possible to measure the lava volume growth over a known time span, from which the tadr can be evaluated. in the present study, a volumetric approach was applied to reconstruct the geometrical evolution of the lava fields, and for post-event estimation of the tadr trends of the lava flows during the 1981 and 2001 eruptions. this approach is based on comparisons of pre-eruption and post-eruption topographic surfaces, which are obtained by applying digital aerial photogrammetric techniques. in the absence of syneruption surveys, the temporal evolution of the lava flows is article history received november 22, 2010; accepted may 15, 2011. subject classification: volcanic risk, volcano monitoring, volcanic eruptions, instruments and techniques, measurements and monitoring. 492 special issue: v3-lava project reconstructed using various data sources, such as scientific papers, newspaper reports and photographs taken during the eruptions. 2. syn-eruption surveying techniques for volumetric tadr estimation different techniques that are based on ground, aerial and satellite sensors can be adopted for obtaining syneruption data, with the implementation of a volumetric approach for the estimation of the effusion rate trends. remote sensing techniques that are based on aerial and satellite optical sensors have wider fields of view than ground sensors, and therefore they are the preferred approach to safely obtain useful data for the quantitative observation of a whole lava field. however, these techniques are usually characterized by poor georeferencing accuracy, and they are limited by cloud and plume coverage. previous studies have demonstrated that the comparison of pre-eruption and post-eruption dems, as extracted directly from photogrammetric surveys or interpolated from available contour maps, is effective for estimation of the final volumes of lava flows of past eruptions [stevens et al. 1999, coltelli et al. 2007, marsella et al. 2008]. in addition, the possibility of repeating photogrammetric surveys during the active phase of an eruption allows reconstruction of the lava flow evolution throughout the whole eruption [marsella et al. 2008, 2009]. when such data are not available or are insufficient, an analysis of helicopter photographs that are usually taken for monitoring purposes can be used to follow the enlargement of a lava flow field and to evaluate lava volume growth, by using approximate thickness values. to improve the measurement accuracy, the helicopter photographs should be orthorectified, to minimize the distortion caused by variable flight patterns and the use of non-calibrated lenses. alternatively, low-altitude flying platforms, such as unmanned aerial vehicles or ultralight aeroplanes [laliberte et al. 2008, buckley et al. 2002], can be equipped with calibrated digital cameras and a global positioning system/inertial motion unit system for direct georeferencing, to acquire a large number of quasi-nadiral aerial photographs. ground-based topographical techniques with threedimension capability can be adopted, either by using digital cameras to acquire stereo-pairs, or laser scanner sensors to obtain three-dimensional points (figure 1), which can in theory provide higher time/space resolution than remotesensing surveys. however, close-range surveys are not always feasible in areas that can be difficult to access [james et al. 2008]. moreover, close-range surveys are useful for rapid evaluation of the advance of simple lava flows, but they are not appropriate to monitor complex lava flow fields, which require multiple points of observation. a close-range survey on an active lava flow was initially conducted during the 2008-2009 etna eruption using a very-long-range terrestrial laser scanner [james et al. 2009]. these authors showed that this instrument allows changes due to the emplacement of new lava flows to be detected, even if data co-registration difficulties arise from very-long-range observations of extremely rough topographies. the datasets obtained were characterized by irregular distributions, and in some areas, by very low densities. the survey was possible since it was performed inside the valle del bove depression; as opposed to most of the etna surface, the valle del bove depression offers many points of observation from which to measure a large area of a lava flow. 3. techniques for tadr estimation to quantify lava emplacement processes, and in particular the evolution of the tadr that gives a general view of an eruption, the dems produced during an eruption marsella et al. 493 figure 1. laser scanner survey of the lava flow front of the 1981 etna eruption. red line, section cut along the 1981 lava front. 494 can be analyzed. provided that pre-eruption dems are available, these data allow the implementation of a volumetric approach for estimation of the tadr, by dividing the volume of the lava flow by the corresponding time interval. quality control analysis of volume estimates, which directly affect the tadrs, show margins of error of about 10% [coltelli et al. 2007]. aerial laser scanning and digital photogrammetry are suitable for acquiring data over large areas, so these techniques are more appropriated to observe the evolution of lava flow fields. the surveying frequency depends on the availability of aerial platforms (helicopter or aircraft) close to an eruption site. volumetric tadrs provide a general view of an eruption and they can be adopted to constrain instantaneous evaluations of the lava effusion rate, which can be performed through different methodologies, such as field measurements and processing of thermal images, which are more suitable for detecting short-term variations. in the field-based approach, effusion rates are evaluated by in-situ measurements of lava velocity, and channel width and depth [frazzetta and romano 1984, guest et al. 1987, calvari et al. 1994, calvari et al. 2002]. however, channel geometry is difficult to obtain, because the depth is not necessary the same as the levee height [pinkerton and sparks 1976], and the width is usually not constant along the flow depth [guest at al. 1987]. the flow velocity is easy to measure at the flow center, but it varies for down-flow and cross-flow directions, as well as vertically within a channel. all of these uncertainties can result in cumulative errors in the effusion rates of as much as 37% [calvari et al. 2002]. in the thermal approach, the effusion rate is derived from the radiance measured from both satellite-borne infrared sensors and hand-held thermal cameras, which obtain minimum and maximum values for the effusion rate, as a range of surface temperatures of the active lava flow is generally assumed for the calculations. recent studies have demonstrated that satellite effusion rate measurements can have margins of error of up to 50% [harris et al. 2007]. moreover, the values estimated from satellites are not limited to a single vent, but apply to the whole of a lava field. for this reason, in case of multi-vent eruptions, the volumes derived by integrating effusion rate thermal measurements are not always reliable and directly applicable when it is necessary to distinguish the contributions of the various lava flows. for example, minimum and maximum effusion rate trends were obtained for the whole 2001 etna eruption from the analysis of thermal advanced very-high-resolution radiometer (avhrr) data [lautze et al. 2004], but the eruption included seven different lava flows, with only two of these threatening buildings (those from the 2,550 m and 2,700 m a.s.l. vents). the 2001 etna eruption can be used to compare total volumes and effusion rates obtained both by thermal and volumetric approaches, and by field measurements. lautze et al. [2004] analyzed thermal avhrr data and ground-based data to evaluate dense-rock equivalent (dre) effusion rates of the whole lava field throughout the eruption. the integral calculus of the dre thermal effusion rates gave a final lava dre volume of 19.8 (±7.7) × 106 m3. this value has little overlap with the dre volume of 31.3 (±5.8) × 106 m3 obtained from the bulk volume evaluated by the volumetric approach [coltelli et al. 2007], after the correction for vesicularity of 22% (±12%) [harris et al. 2000]. it also differs from the dre volume of 37.5 (±5.5) × 106 m3 evaluated by field measurements [the research group of the istituto nazionale di geofisica e vulcanologia-sezione di catania, italy 2001]. this comparison confirms that the use of highly scattered effusion rates makes satellite data unsuitable for estimating volumes, due to the large differences with the volumes measured. the volumetric approach was applied to reconstruct the trends of the tadrs corresponding to the etna lava flows from the 2,100 m, 2,550 m and 2,700 m a.s.l. vents [coltelli et al. 2007, scifoni et al. 2010], which were mostly active during the central period of the eruption ( july 23 to august 3), and produced about 70% of the final lava volume of the whole eruption. the volumetric dre tadr trends (figure 2) were compared with the corresponding evaluations for the entire eruption using ground-based data and satellite thermal images [lautze et al. 2004]. the dre tadr trends in figure 2 are comparable. however, while the isolated peaks measured by ground and satellite data can be linked to evolution of the 1981 and 2001 etna eruptions figure 2. comparison of the different lava effusion rate trends of the whole 2001 etna eruption, based on: the volumetric approach (filled red bars); analysis of thermal images acquired by satellite (minimum and maximum values, continuous and dashed lines, respectively; redrawn from lautze et al. [2004]); and ground measurements (dots, redrawn from lautze et al. [2004]). the volumetric tadr was also evaluated considering only the lava flows from the 2,100 m, 2,550 m and 2,700 m a.s.l. vents (filled grey bars), where the final volume is 70% of the total volume. pauses or pulses in the effusive activity, these cannot be represented by the volumetric approach, which produces an average value corresponding to a time span ranging from one to several days. furthermore, the peak visible for the satellite dataset in the decreasing phase ( july 30 to august 1) might be due to the contribution of the lava flows from upper vents, which were not considered in the volumetric approach. the large differences between the upper and lower satellite effusion rates appear too wide, and thus they suggest that the assumptions in the model of the range of lava surface temperatures were not correct [lautze et al. 2004]. generally, thermally derived effusion rates are sensitive to short-term variations that are highlighted by spikes in the dataset. on the contrary, as the volumetric data is strongly marsella et al. 495 figure 4. pre-eruption (left) and post-eruption (right) surfaces of the 2001 etna eruption, with the lava flow fields superimposed in red on the posteruption surface. figure 3. pre-eruption (left) and post-eruption (right) surfaces of the 1981 etna eruption, with the lava flow fields superimposed in red on the posteruption surface. 496 dependent on the low acquisition frequency, this can provide average effusion rates that cannot depict rapid variations in lava discharge. more specifically, the volumetric approach cannot be used to represent short-term peaks, because of the timing of aerial surveys, as only one carried out every few days. the efficacy of the satellite approach in the detection of short-term peaks is also limited by bad weather and plume coverage. 4. analysis of the 1981 and 2001 etna eruptions the reconstructions of the 1981 and 2001 tadrs were performed using a similar approach to that described by coltelli et al. [2007]; i.e. they were based on comparative analyses of pre-eruption and post-eruption dems extracted from photogrammetric surveys and on post-eruption orthophotographs. these analyses allowed the precise definition of the limits of the lava flows, to evaluate their final volumes and to extrapolate their temporal evolution. the 1981 pre-eruption dem was extracted using photogrammetric data from a 1978 aerial survey (flight altitude, 3,000 m; scale, 1:20.000) that was carried out by "rossi brescia s.r.l.". these data were processed through a photogrammetric digital workstation. the post-eruption topography was extracted by processing photogrammetric data that was collected during a 2004 aerial survey (scale, 1:10.000) that was performed by "nuova avioriprese napoli". to cover the whole lava field (figure 3), it was necessary to integrate this 2004 dem with a dem obtained by interpolation of a 1999 vector map (scale, 1:10.000) of the provincia regionale di catania, which was extracted from an aerial survey that was performed on november 9, 1998 [coltelli et al. 2007]. this 1999 dem was also adopted as the 2001 pre-eruption surface, with the corresponding posteruption dem (figure 4) interpolated from the 2001 vector map of the provincia regionale di catania, as extracted from a december 3, 2001, aerial survey [coltelli et al. 2007]. for both of these eruptions, the pre-eruption and posteruption dems were compared, to evaluate the elevation changes, as represented by two residual maps. these maps allowed the distribution of the lava flow thicknesses for both of these eruptions to be evaluated (figures 5 and 6). global positioning system surveys were also carried out on the 1981 lava flows, to reconstruct the lava flow surface where preeruption vegetation or post-eruption land modifications prevented the precise measurement of lava thicknesses by the photogrammetric approach. for both of these eruptions, the post-eruption orthophotographs helped to localize the flowfront positions that were retrieved by the data analysis. the advance of the 1981 front was reconstructed by taking into account various data sources, such as the scientific literature, newspaper reports, and other documents. to obtain the partial volumes at different times, and the corresponding tadrs, the final lava thickness was cut in correspondence with the time evolution of the front position (figure 7). the front evolution of the 2001 lava flows (for the 2,550 m and 2,700 m a.s.l. vents) was reconstructed from the analysis of istituto nazionale di geofisica e vulcanologia (ingv) reports and helicopter photographs, which provided an almost daily mapping of the flow limits [scifoni et al. 2010] (figure 8). the reconstruction of the advance of the flow fronts during both the 1981 and 2001 eruptions not only allowed evolution of the 1981 and 2001 etna eruptions figure 5. lava flow thickness distribution and eruptive fissures of the 1981 etna eruption. evaluation of their tadrs, but also analysis of the evolution of the lava flows during each eruption. indeed, the two slowly evolving 2001 lava flows (from the 2,550 m and 2,700 m a.s.l. vents) were complete in 6 days and 10 days [scifoni et al. 2010] and reached lengths of 3.3 km and 4.1 km, respectively [coltelli et al. 2007]. this allowed the civil protection authorities to build earth barriers to protect the buildings at "rifugio sapienza" [barberi et al. 2004]. moreover, numerical simulations that used the reconstructed 2001 effusion rate trends demonstrated that it would also have been possible to develop alternative scenarios for the barrier configuration [scifoni et al. 2010]. on the contrary, the 1981 effusion rate trend shows a quite different scenario: the main 1981 lava flow traveled 10 km in about 4 days, confirming that it would have been very difficult to build earth barriers in the short time available. 5. conclusions the analysis of the emplacement history of the recent etna eruptions in 1981 and 2001, where the lava flows threatened inhabited areas, can be used to anticipate future eruptions and to assist in the design and implementation of mitigation actions. for both of these eruptions, a quantitative multi-temporal analysis of the lava flow evolution was developed to reconstruct their emplacement histories and evaluate the corresponding effusion rate trends. the results of the analysis were used to simulate (on the pre-eruption dems) the evolution of these 1981 and 2001 etna eruptions, and to forecast different scenarios. marsella et al. 497 figure 6. lava flow thickness distribution of the 2001 etna eruption from the 2,700 m and 2,550 m a.s.l. vents. black lines, position of the 13 earth barriers built during the eruption. figure 7. temporal evolution of the tadr (left axis) and corresponding cumulative volumes (right axis) during the 1981 etna eruption. figure 8. temporal evolution of the tadr (left axis) and the corresponding cumulative volumes (right axis) during the 2001 etna eruption. gray bars and black triangles, lava flow from the 2,550 m a.s.l. vent. red bars and red circles, lava flow from the 2,700 m a.s.l. vent. (from scifoni et al. [2010]). 498 the analysis of past events and the availability of reliable and rapid methods for syn-eruption measurements of effusion rates can be used to confirm numerical simulation results and to forecast lava flow paths of future eruptions, as well as to follow the evolution of on-going eruptions. furthermore, quantitative analysis and reconstruction of past eruptions represent a starting point for the investigation and characterization of the different discharge mechanisms for etna flank eruptions, which constitute the most hazardous events for the nearby inhabited areas. references barberi, f. and m.l. carapezza (2004). the control of lava flows at mt. etna, in: mount etna volcano laboratory, edited by a. bonaccorso, s. calvari, m. coltelli, c. del negro and s. falsaperla, geophysical monograph series, 143, agu, washington, d.c., 357-369. buckley, s.j., j.p. mills, p.j. clarke, s.j. edwards, j.s. pethick and h.l. mitchell (2002). synergy of gps, digital photogrammetry and insar in coastal environments, proceeding of 7th international conference on remote sensing for marine and coastal environments (miami, florida, may 20-22, 2002). calvari, s., m. coltelli, m. neri, m. pompilio and v. scrivano (1994). the 1991-1993 etna eruption: chronology and lava flow-field evolution, acta vulcanologica, 4, 1-14. calvari, s., m. neri and h. pinkerton (2002). effusion rate estimations during the 1999 summit eruption on mount etna, and growth of two distinct lava flow fields, j. volcanol. geoth. res., 119, 107-123. coltelli, m., c. proietti, s. branca, m. marsella, d. andronico and l. lodato (2007). analysis of the 2001 lava flow eruption of mt. etna from three-dimensional mapping, j. geophys. res., solid earth, 112; doi: 10.1029/2006jf000598. frazzetta, g. and r. romano (1984). the 1983 etna eruption: event chronology and morphological evolution of the lava flow, b. volcanol., 47, 1079-1096. guest, j.e., c.r. j. kilburn, h. pinkerton and a.m. duncan (1987). the evolution of lava flow field: observation of 1981 and 1983 eruptions of mount etna, sicily, b. volcanol., 49, 527-540. harris a.j.l., j.b. murray, s.e. aries, m.a. davies, l.p. flynn, m.j. wooster, r. wright and d.a. rothery (2000). effusion rate trends at etna and krafla and their implications for eruptive mechanisms, j. volcanol. geoth. res., 102, 237-270. harris, a.j.l., j. dehn and s. calvari (2007). lava effusion rate definition and measurement: a review, b. volcanol., 70, 1-22. james, m.r., h. pinkerton and s. robson (2008). imagebased measurement of flux variation in distal regions of active lava flows, geochem. geophys. geosyst., 8, 3, q03006; doi: 10.1029/2006gc001448. james, m.r., h. pinkerton and l.j. applegarth (2009). detecting the development of active lava flow fields with a very-long-range terrestrial laser scanner and thermal imagery, geophys. res. lett., 36, l22305; doi: 10.1029/2009 gl040701. laliberte, a.s., c. winters and a. rango (2008). procedure for orthorectification of sub-decimeter resolution imagery obtained with an unmanned aerial vehicle (uav), proceedings of the asprs 2008 annual conference (portland, or, april 28 may 2, 2008). lautze, n.c., a.j.l. harris, j.e. bailey, m. ripepe, s. calvari, j. dehn., s.k. rowland and k. evans-jones (2004). pulsed lava effusion at mount etna during 2001, j. volcanol. geoth. res., 137: 231-246. marsella, m., m. coltelli, c. proietti, s. branca and r. monticelli (2008). 2002-2003 lava flow eruption of stromboli: a contribution to understanding lava discharge mechanism using periodic digital photogrammetry surveys, in: the stromboli volcano: an integrated study of the 20022003 eruption, edited by s. calvari, s. inguaggiato, g. puglisi, m. ripepe and m. rosi, geophysical monograph series, 182, agu, washington d.c., 229-246. marsella, m., c. proietti, a. sonnessa, m. coltelli, p. tommasi and e. bernardo (2009). the evolution of the sciara del fuoco subaerial slope during the 2007 stromboli eruption: relation between deformation processes and effusive activity, j. volcanol. geoth. res., 182, 201-213. pinkerton, h. and r.s.j. sparks (1976). the sub-terminal lavas, mount etna: a case history of the formation of compound lava field, j. volcanol. geoth. res., 1, 167-182. scifoni, s., m. coltelli, m. marsella, c. proietti, q. napoleoni, a. vicari and c. del negro (2010). mitigation of lava flow invasion hazard through optimized barrier configuration aided by numerical simulation: the case of the 2001 etna eruption, j. volcanol. geoth. res., 192, 16-26. stevens, n.f., g. wadge and j.b. murray (1999). lava flow volume and morphology from digitized contour maps: a case study at mount etna, sicily, geomorphology, 28, 251-261. the research group of the istituto nazionale di geofisica e vulcanologia-sezione di catania, italy (2001). multidisciplinary approach yields insight into mt. etna eruption, eos trans. agu, 82 (52), 653-656; doi: 10.1029/01eo00376. *corresponding author: silvia scifoni, università di roma «la sapienza», dipartimento di ingegneria civile, edile e ambientale (dicea), rome, italy; email: silvia.scifoni@uniroma1.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. evolution of the 1981 and 2001 etna eruptions de_santis_final:layout 6 repeat-station surveys: implications from chaos and ergodicity of the recent geomagnetic field angelo de santis1,2,*, enkelejda qamili1,3, gianfranco cianchini1,3 1 istituto nazionale di geofisica e vulcanologia, sezione roma 2, roma, italy 2 università "g. d'annunzio" di chieti–pescara, campus universitario, chieti, italy 3 università di siena, scuola di dottorato in scienze polari, siena, italy annals of geophysics, 56, 1, 2013, r0103; doi:10.4401/ag-5491 abstract the present geomagnetic field is chaotic and ergodic: chaotic because it can no longer be predicted beyond around 6 years; and ergodic in the sense that time averages correspond to phase-space averages. these properties have already been deduced from complex analyses of observatory time series in a reconstructed phase space [barraclough and de santis 1997] and from global predicted and definitive models of differences in the time domain [de santis et al. 2011]. these results imply that there is a strong necessity to make repeat-station magnetic surveys more frequently than every 5 years. this, in turn, will also improve the geomagnetic field secular variation models. this report provides practical examples and case studies. 1. introduction and motivation repeat-station surveys [e.g., barraclough and de santis 2011] are important, among other things for: (a) modeling the main geomagnetic field and global and regional secular variation; (b) navigational purposes; and (c) better definition of magnetic anomalies. two issues are fundamental for the present report: the first regards the repeat time of site occupation in the repeat-station surveys; and the second concerns the kind of data analysis we perform at the repeat station. generally, the typical time interval of re-occupation of a repeat station is around 5 years. however, there has been recent debate in the geomagnetism community about whether this time lag should be reduced or maintained. in addition, the measurement values considered during the repeat-station reoccupation are temporal averages of all the measurements at the same site. these values are then correctly reduced, taking into account a close observatory or a fixed reference station [e.g., barraclough and de santis 2011]. alternative schemes of analysis and averages have also been proposed. however, we can ask whether these simple averages are still appropriate or not. the aim of this report is to justify on the basis of the physics of the recent geomagnetic field: (i) more frequent repeat-station survey re-occupation; and (ii) the present typical operation of time averaging at each repeat station. to do this, we will first introduce some concepts on chaos and ergodicity, and then we will recall some previous results from analyses of recent geomagnetic field data and models, which provide evidence for both of these physical properties of the field. when we transfer these results to magnetic repeat-station surveys, we arrive at the clear conclusion that more frequent occupations are a real necessity according to a solid physical basis, instead of the current interval of 5 years. finally, we complete the report with some conclusions. 2. chaos theory chaos theory studies dynamical systems where the behavior is highly sensitive to the initial conditions. if we consider the time evolution x(t) of a nonlinear process, it is possible to represent this by equation (1) for continuous time t: (1) or by equation (2) for discrete time: (2) where, f and f are some nonlinear functions of x. as in barraclough and de santis [1997] and de santis et al. [2002], we make use of the concept of phase space to characterize the dynamic system under study. this phasespace diagram is basically an abstract reference system, the coordinates of which are all the independent variables that are necessary to completely describe the state of the system at any moment with just a point. for instance, let us consider the solution of the discrete case; i.e., the solution of equation (2), with the initial state x0: the sequence xn = f n (x0) is a trajectory of the phase-space diagram; i.e., the evoluarticle history received november 30, 2011; accepted april 2, 2012. subject classification: instruments and techniques, global and regional models, main geomagnetic field, mathematical geophysics, statistical analysis. r0103 research articles ( ) ( ) ,x t x t tf !=o ^ h ( ),x f x nn n1 !=+ tion with time of the system under dynamics f. when a real system is under consideration, the problem is the absence of information about all of the variables involved. in this case, a way to represent the system dynamics is through an appropriate phase-space reconstruction. this operation, which is a local approximation approach, allows the reconstruction (or embedding) of a single-dimensional time series into a multi-dimensional phase space. in this way, the system dynamics is represented locally step-by-step in this new phase space. a system is chaotic if each small change f of some initial close trajectory in the phase space propagates exponentially with time t; i.e.: (3) this is just the exponential behavior in time of f(t) that will lead us to investigate in greater detail in section 5 the possible implications in repeat-station surveys. the kolmogorov entropy, k (>0), measures the rate of information loss of the system under study [schuster and just 2005], and it is inversely proportional to the time over which a chaotic system is predictable. for this reason, we can introduce here the mean time <t> = 1/k after which no reliable prediction can be made [barraclough and de santis 1997, de santis et al. 2002]. as described above, a way to represent such a dynamic system is through phase-space reconstruction. one of the most popular methods available for reconstructing the phase space from an experimental time series is that of time delays that was introduced by takens [1981]. by using the past history and an appropriate delay time, x, this method can allow us to project a time series xi into a reconstructed e-dimensional phase space yi, such that: (4) where, i = 1, 2, ..., n, n is the number of data points of the state variable x(t), e is the so-called embedding dimension, which is greater than or equal to the number of degrees of freedom of the dynamic system, and also to the number of deterministic scalar equations that characterize it. once this pseudo-space is reconstructed, it is simple to check whether there is divergence of a perturbed trajectory with respect to the original one. almost all of the techniques used to detect chaos in time series require a large amount of data. a simple way that overcomes this limitation is to apply a nonlinear forecasting approach in the phase space [wales 1991, sugihara and may 1990]. with this method, we split the sequence under study into two parts, and use the first part to predict the second one: the comparison between the predicted and observed values allows us to deduce some important properties about the possible chaoticity of the time series. when applied to several geomagnetic observatory time series, barraclough and de santis [1997] and de santis et al. [2002] found a clear chaotic nature of the geomagnetic field with <t> = 6 years (figure 1). we will show below how this result, together with the ergodic property of the field, is important for the frequency of repeat-station re-occupation. 3. ergodicity historically, the issue of ergodicity arose from the work of boltzmann on the determination of average values in kinetic theory. for a given dynamic system represented by (ω, f t, r) in the phase space ω with an invariant (during evolution) measure r, a deterministic dynamics f t drives the time evolution along a path (named the trajectory) that will pass through all of the possible states the system can reach, for a sufficiently long time. so, this behavior should allow the substitution of the phase-space average with the temporal one of the same observable. this is the ergodic hypothesis [eckmann and ruelle 1985]. as a formula, it translates into [e.g., walters 1982]: de santis et al. 2 figure 1. correlation coefficients between the predicted (a) and observed (b) values of the x and y geomagnetic time series of three european observatories when reconstructed in a phase space (adapted from barraclough and de santis [1997]). the decrease in time is a symptom for a chaotic geomagnetic field. e = 3 is the embedding dimension of the reconstructed phase space. ( ) expt k t0 $ $.f f ^ h [ ( ), ( ), ..., [ ( 1) ]y x t x t x t ei i i ix x= + + 3 (5) as a consequence of this feature, if the system is ergodic and a(x) is an observable, the time average <a(x0)> does not depend on the initial state x0; i.e., after a certain time evolution, the system is no longer dependent on its initial state. a is an appropriate average in the phase space. 4. chaotic and ergodic character of the recent geomagnetic field as mentioned above, some previous studies have confirmed the chaotic properties of the recent geomagnetic field [barraclough and de santis 1997, de santis et al. 2002, de santis and qamili 2010], i.e., its sensitivity to initial conditions in the phase space. when the process under study is ergodic, the phasespace reconstruction is not necessary, and we can perform the analyses in the time domain. de santis et al. [2011] demonstrated that this is the case of the geomagnetic field. under the ergodic condition, a good technique used to find possible nonlinearity and chaos in a system is a simple nonlinear forecasting approach in the time domain. this technique uses the past history of a time series to forecast future values, without reconstructing the phase space. this result was determined by de santis et al. [2011] through an analysis of the divergence of the errors between the predicted and definitive global geomagnetic models, with the estimation of the typical mean time <x> after which time no prediction is reliable. they found <x> = 6 years, which is the same time average <t> that results from the analysis with phasespace reconstruction (figure 2). this confirms that it is not possible to predict the evolution into the future of the geomagnetic secular variation by more than 6 years, so that any prediction must be limited to a shorter time, whereby the shorter this time, the better the prediction will be. we argued in de santis et al. [2011] that the close agreement between the outcomes from two independent analyses, i.e., <x> as deduced from the time-error analysis in the time domain, and <t> as estimated from the nonlinear forecasting approach in the phase space [de santis et al. 2002], confirms the chaotic nature of the geomagnetic field and represents empirical proof of its ergodicity. it could be said that the concepts of chaos and ergodicity seem in contradiction, but this is only apparently so: there are many examples of chaotic phenomena that are also ergodic [e.g., eckman and ruelle 1985]. this can be understood also for the geomagnetic dynamo: although in general it has nonequilibrium and nonstationary dynamics, the geodynamo can evolve into parts of the phase space that are characterized by both chaos and ergodicity [e.g., vincent 2005, shen et al. 2009]. there are other evident nonequilibrium and nonstationary geophysical phenomena that can show analogous properties: e.g., seismic sequences have been shown to be ergodic [tiampo et al. 2007] and chaotic [de santis et al. 2010] occasionally. the properties of chaos and ergodicity of the recent geomagnetic field, i.e., the short predictability in time and the sort of space-time regularity, respectively, allow us to make reasonable forecasts of the geomagnetic field for a few years in advance; e.g., to update the geomagnetic charts for a few years into the future. also, shorter time variations of the geomagnetic field that are due to external contributions that arise mainly from the ionosphere and magnetosphere have shown important chaotic characteristics [e.g., balasis et al. 2010], although this aspect will not be considered in this report, which is mainly concerned with the corresponding chaotic properties of the internal geomagnetic field and its secular variation. chaos ergodicity and magnetic repeat stations figure 2. error behavior between the predicted and observed global international geomagnetic reference field (igrf) models (from de santis et al. [2011]). the exponential increase in time is a symptom of a chaotic geomagnetic field. ( ) ( ) ( ) ( )lima x t f x dt dt x a 1 t t t 0 0 0{ t {= = ="3 6 @# # 5. consequences of repeat-station surveys we can now take advantage of the established properties of chaos and ergodicity of the recent geomagnetic field. in this case, equation (3) is valid for the error f of any geomagnetic element of the field, the initial error value of which is taken as f0. this means that: (3a) where, on the right-hand side we have replaced k entropy with the reverse of the mean time of predictability <x>. from the results given by barraclough and de santis [1997] and by de santis et al. [2011], we insert <x> = <t> = 6 years into equation (3a) and apply it to some typical cases of repeat-station measurement errors. equation (3a) indicates how an initial error of a certain measurement taken at the repeat station in a given epoch t0 (here set equal to zero, for convenience) will explode after some time t following the exponential law of typical chaotic processes. figure 3 shows this temporal behavior for three possible cases of initial errors, of 5 nt, 8 nt and 10 nt; these values are reasonable values of the errors likely to be involved in the deduced values from measurements at a repeat station, including the process of time and space reduction [e.g. meloni et al. 1994; barraclough and de santis 2011]. in figure 3, vertical lines are drawn at 2, 3 and 5 years after the initial measurement: the corresponding intersections are the estimated errors at each successive epoch. it is evident that after 5 years from the initial measurement the error is relatively large, while the errors after 2 years and 3 years are still reasonably acceptable. indeed, just to give an idea, an initial error of 8 nt will grow to a final error of 18.5 nt after 5 years, but will only be 11 nt to 13 nt after 2years to 3 years, values which are lower (and probably still sustainable) than the error obtained after 5 years. to better reproduce the real case of repeat-station time series at different frequencies of re-occupation, we considered also the cm4 geomagnetic model (see sabaka et al. [2004] for details of this model) and synthesized the yearly values of each geomagnetic component g(ti ) cm4 (= xcm4, ycm4 or zcm4; ti = 1960, …, 2002) in the central points of spain (figure 4, left side) and australia (figure 5, left side). we chose these two ideal sites because they are eventually representative of two different regions on the earth. for each time series, we then considered the cases of 2-year and 6-year repeat-station re-occupation. the latter was chosen (instead of simply a 1/5 year−1 frequency) for an easier one-to-one comparison with the other shorter period of re-occupation. for both frequencies of re-occupation, we calculated the following differences: (6) where: (7) with ti starting at 1964 (with initial ti−1 = 1962 and ti−2 = 1960) for the 2-year re-occupation, and at 1972 for the 6-year re-occupation (with initial ti−1 = 1966 and ti−2 = 1960). equation (7) is the extrapolation of the g component at time ti as deduced from the previous two data points taken at times ti−1 and ti−2 , i.e., at the epochs of the previous two magnetic surveys. therefore, equation (6) measures the deviation of the predicted value g(ti) pred with respect the actual synthede santis et al. 4 figure 3. exponential divergence in time of some typical initial errors (of 5 nt, 8 nt and 10 nt) in repeat-station surveys in the case of a chaotic geomagnetic field. vertical lines are taken at 2 years, 3 years and 5 years. while the error after 5 years is relatively large, those at the first two epochs are accepted for any extrapolation in time of eventual magnetic charts. ( ) exp expt k t t0 0$ $ $.f f f x =^ h c m g t g ti i predcm4 -^ ^h h = 2 g t g t g t g t g t g t i pred i cm i cm i cm i cm i cm 1 4 1 4 4 1 4 4 2 2 = + =^ ^ ^ ^ ^ ^ h h h h h h 6 @ 5 sized value from the model g(ti) cm4. figures 4 and 5 (right sides) show the differences expressed by equation (6) for spain and australia, respectively, where the white circles represent the data points deduced with the 2-year re-occupation, and black circles are those obtained from the 6-year re-occupation. it is clear from figures 4 and 5 that the longer time of re-occupation provides much more scattered values with expected errors that are much larger than those at the 2-year reoccupation: for instance for the x component, in spain and australia the largest errors reach around 70 nt and 130 nt, respectively, while they are no larger than 20 nt for the shorter time of re-occupation. this confirms that, too long a time interval between two subsequent surveys will cause very large extrapolation errors, thus invalidating any possibility of reconstructing reliable magnetic maps in the time in between. these simple examples show that a reduction to 2 years to 3 years for the time of re-occupation of the repeat-station network is to be recommended, to avoid the error of the temporal extrapolation from becoming too large. we believe that the present technology and potential of most european scientific institutions would allow this recommendation to be put into practice. it is also clear that any improvement in terms of error reduction in repeat-station surveys will also have positive effects on any secular variation model that is based on the repeat-station data. however, since we understand the difficulties that some countries might have, it is not necessary for all of the repeat stations of a country to be more frequently reoccupied. a reasonable compromise could be to increase the frequency of reoccupation only in a subset (say, 10% to 20%) of the norchaos ergodicity and magnetic repeat stations figure 4. left: synthetic case of possible repeat-station time series of the geomagnetic component in a central point in spain, as deduced from the cm4 model. right: differences between the synthetic (actual) values and the predicted values when the frequency of repetition is one data point every 2 years (white circles) and 6 years (black circles). mal repeat stations (the 'super stations', as used by mcewin [1993]). in this way, it will be easier to minimize the possible errors of extrapolation between two epochs of reoccupation without too much effort. in addition, the ergodicity of the recent geomagnetic field confirms the use of temporal arithmetic means for the estimation of the mean value of some magnetic element at a certain epoch, and the reduction in the value using corresponding temporal means at a close magnetic observatory or reference station. this is also valid for the calculation of other successive statistical moments (e.g., variance, skewness, kurtosis). the overall set of typical operations on repeat-station data made in time to reduce them to some fixed past or future epoch (i.e., averaging the daily measurements, reducing to the closest night, and then reducing to the closest observatory at, say, the middle of the year) can be performed, because we implicitly assume that the magnetic field is ergodic. otherwise, more complex (sometime even impossible) operations would be necessary, with most of them made in the phase space, and with the implied difficulties of its reconstruction. 6. conclusions the nonlinear forecasting approach applied to the geomagnetic field in the phase space has highlighted its chaotic nature that is characterized by a mean time <t> = 6 years of predictability [barraclough and de santis 1997, de santis et al. 2002]. at the same time, the analyses of differences bede santis et al. 6 figure 5. left: synthetic case of possible repeat-station time series of the geomagnetic component in a central point in australia, as deduced from the cm4 model. right: differences between the synthetic (actual) values and the predicted values when the frequency of repetition is one data point every 2 years (white circles) and 6 years (black circles). 7 tween predicted and definitive geomagnetic models in natural time and space domains provided the same result <x> = 6±3 years [de santis et al. 2011]. this supports our knowledge of the chaotic nature of the field and demonstrates its ergodicity. according to our previous results, if we want to reproduce a correct secular variation, frequent repeat-station surveys are compulsory, with, of course, the period of time of re-occupation of less than 6 years, which represents an upper limit, for the reasons we have given. consistent with the availability of resources, a re-occupation every 2 years to 3 years is highly recommended, so that the error is less than twice the initial error. in this way, the chaotic nature of the field does not have too large an effect on the possible shorttime extrapolation of the field from the repeat-station observations. as already indicated [newitt et al. 1996], another advantage of frequent re-occupation will be to eventually track rapid changes in secular variation, such as the jerks [e.g., duka et al. 2012]. finally, the property of ergodicity confirms that both the operation of making time averages at the repeat station and the comparison with analogous time averages at close magnetic observatories or stations for temporal reduction, are appropriate, because of the validity between the exchange of phase space and time averages, as given by equation (5). since this report was presented at the magnete workshop held in rome, from may 9-11, 2011, we hope that our arguments have been taken into due considerations by the world-wide community of repeat-station planning and management, and in particular by the institutions contributing to the european repeat-station network. acknowledgements. part of the study by eq and gc was carried out while they were attending phd courses at siena university. some funds were supplied by a pnra project ("reversing earth magnetism") and a prin 2008 miur-funded project ("animal magnetic homing", simmag unit of research). this report was originally presented at the magnete workshop held in rome, from may 9-11, 2011. we thank the local organizers, the scientific committee, and all of the participants at the workshop for their useful comments and suggestions that have helped us to improve the quality of this study. references balasis g., i.a. daglis, a. anastasiadis and k. eftaxias (2010). investigating magnetospheric dynamics using various complexity measures, aip conf. proc., 1320, 65-71. barraclough, d.r., and a. de santis (1997). some possible evidence for a chaotic geomagnetic field from observational data, phys. earth plan. inter., 99, 207-220. barraclough, d.r., and a. de santis (2011). repeat stations activities, in: m. korte and m. mandea (eds.), geomagnetic observations and models, springer, 45-55. de santis, a., d.r. barraclough and r. tozzi (2002). nonlinear variability in the geomagnetic secular variation of the last 150 years, fractals, 10, 297-303. de santis, a., and e. qamili (2010). shannon information of the geomagnetic field for the past 7000 years, nonlinear proc. geoph., 17, 77-84. de santis, a., g. cianchini, e. qamili and a. frepoli (2010). the 2009 l'aquila (central italy) seismic sequence as a chaotic process, tectonophysics, 496, 44-52. de santis, a., e. qamili and g. cianchini (2011). ergodicity of the recent geomagnetic field, phys. earth plan. inter, 186, 103-110. duka, b., a. de santis, m. mandea, a. isac and e. qamili (2012). geomagnetic jerks characterization via spectral analysis, solid earth, 3, 131-148. eckmann, j.p., and d. ruelle (1985). ergodic theory of chaos and strange attractors, rev. mod. phys., 57, 617-656. mcewin, a.j. 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(1981). detecting strange attractors in turbulence, in: d.a. rand and l.s. young (eds.), lecture notes in mathematics, 898, springer, berlin, 366 pp. tiampo, k.f., j.b. rundle, w. klein, j. holliday, j.s. sa' martins and c.d. ferguson (2007). ergodicity in natural earthquake fault networks, phys. rev. e, 75, 1-15. vincent, u.e. (2005). synchronization of rikitake chaotic attractor using active control, physics lett. a, 343, 133-138. wales, d.j. (1991). calculating the rate of loss information from chaotic time series by forecasting, nature, 350, 485-488. walters, p. (1982). an introduction to ergodic theory, springer, new york. *corresponding author: angelo de santis, istituto nazionale di geofisica e vulcanologia, sezione roma 2, roma, italy; email: angelo.desantis@ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. chaos ergodicity and magnetic repeat stations << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments 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<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> >> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice u n t e r s u c i i u n g e n u e b e r m i k r o s e i s m i k i n d e u t s c h e a n d w a e h r e n d d e s z w e i t e n w e l t k r i e g e s ( * ) e . hardtwig i n i f o l g e n d e n m o c h t e i c h e i n i g e s b e r i c l i t e n i i l i e r u n t e r s u c h u n g e n , d i e i n d e u t s c h l a n d w i i h r e n d des k r i e g e s i i l i e r d i e m . s . a n g e s t e l l t w o r d e n s i n d u n d d i e z u m z i e l e l i a t t e n , d e n z u s a m m e n l i a n g z w i s e l i c n g r o s s w e t t e r l a g e u n d m.s. b o d e n l j e w e g u n g s o w e i t als m o g l i e l i f i i r d i e z w e e k e d e r w e t t e r v o r l i e r s a g e n u t z b a r zu i n a c l i e n . m a n niiiss d i e l a g e v e r s t e h e n , i n d e r sicli d e u t s c h l a n d d a m a l s es w a s w i n t e r 1 9 4 1 / 4 2 m e t e o r o l o g i s c l i b e f u n d e n l i a t . d e r g e s a m t e w e s t e n e u r o p a ' s a u s s e r l i a l h e n g l a n d s u n d s p a n i e n s w a r v o n d e u t s c h e n a r i n e e n k o n t r o l l i e r t u n d s c i n e g e h i e l e w u r d e n v o n d e u t s c h e n m a s c l i i n e n h e f l o g e n . u i n g e k e h r t ì m i s s l e in i m m e r z u n e l i m e n d e r e n i m a s s e m i t a n g r i f f e n a u f d e u t s c h l a n d g e r e c h n e t w e r d e n , d i e v o n e n g l a n d a u s g e s t a r t e t w u r d e n . d i e s e u i n s t a n d e h r a c l i t e n es m i t s i c h , d a s s f i i r d i e d e u t s c h e m e t e o r o l o g i e d a s p r o h l e m d e r w e t t e r v o r l i e r s a g e s e h r d r i n g e n d w u r d e , d a j a h e k a n n t l i c l i m i t t e l e u r o p a v o i n w e s t e n l i e r m i t d e r g r o s s w e t t e r l a g e v e r s o r g t w i r d . m a n l i a l f sich a n f f o l g e n d e w e i s e : 1. — es w u r d e n in n o r w e g c n u n d f r a n k r e i c h s o g e n a n n t e w e t t e r e r k u n d u n g s s t a f f e l n e i n g e r i c h t e t , d e r e f l u g z e u g e j e d e n t a g n a c l i w e s t e n v o r s t i e s s e n u n d d i e auf i h r e m f l u g w e g v o r g e f u n d e n e n w e t t e r l a g e n d u r c l i d r a h t l o s e t e l e g r a p l i i e a n d i e z e n t r a l e liei b e r l i n m e l d e t e n . d i e s e m e l d u n g e n b i l d e t e n d a s g e r i i s t f i i r d i e a u f d e m o z e a m zu z e i c h n e n d e w e t t e r k a r t e . es ist k l a r , dass v o n d e r z u v e r l a s s i g k e i t dieselm e l d u n g e n s o w i e v o n i h r e r r e g e l m a s s i g e n a u s f i i h r u n g f i i r d i e h e r s t e l l u n g d e r w e t t e r p r o g n o s e so g u l w i e a l l e s a b b i n g . 2. — es w u r d e n s c h i f f s n i e l d u n g e n v e r w e r t e t . d a b e i l i e g t es in d e r n a t u r d e r s a c l i e , dass es in e r s t e r l i n i e m e l d u n g e n v o n u b o o t c n w a r e n , d i e z u r a u s w e r t u n g g e l a n g t e n . a n e l i v e r e i n z e l t a u s g e s e t z t e ( 4 ) l a t r a d u z i o n e i n l i n g u a i t a l i a n a d e l l a p r e s e n t e n o l a è r i p o r t a l a a p a g . 1 2 7 . 9 6 e . h a r d t w i c b e o b a c h t e r v e r s o r g t e n — z u m i n d e s t z e i t w e i s e — d i e z e n t r a l e m i t m e l d u n g e n . u m d i e s e z e i t m a c h t e d e r d o z e n t a n d e r g ò t t i n g e r u n i v e r s i t i i t d r . bungers a n d e n z u s t a n d i g e n s t e l l e n d e n v o r s c l i l a g , d i e m . s . f i i r d i e z v v e c k e d e r w e t t e r v o r h e r s a g e n u t z b a r z u m a c l i e n . e r b a t t e k u r z v o r h e r s e i n e u n t e r s u c h u n g e n i i b e r d i e r i c h t u n g s b e s t i m m u n g m i k r o s e i s m i s e h e r w e l l e n b e e n d e t . y o m b u n g e r ' s c h e n s t a n d p u n k t a u s s c h i e n es n i e l i t s c b w i e r i g , d e n q u e l l p u n k t d e r m . s . e r r e g u n g z u e r m i t t e l n , i n s b e s o n d e r e d a n n , w e n n d i e o r t s b e s t i m m u n g v o n z w c i sebiw e i t v o n e i n a n d e r e n t f e r n t e n p u n k t e n a u s v o r g e n o m m e n w u r d e . d e r v o r s c l i l a g v o n d r . b u n g e r s e r f o l g t e zu w e i h n a c h t e r 1 9 1 1 , d o c l i w u r d e d a s p r o j e c t e r s t i n i l a u f e d e s j a b r e s 1 9 4 2 a u f g e g r i f f e n u n d d a n n i n h e r b s t 1 9 4 2 i n a n g r i f f g e n o m m e n . h e r r prof. angenheister i n g o t t i n g e n w a r m i t d e r p l a n u n g u n d o b e r l e i t u n g d e r u n t e r s u c h u n g e n b e t r a u t w o r d e n . l i m i s t a n d e n d r e i h e r r e n a l s a s s i s t e n t e n f i i r s c i t e . w e i t e r w u r d e n l i e r a n g e z o g e n d r . k i l l e r u n d i c l i s e l b s t , b e i d e b e r e i t s i m w e t t e r d i e n s t d e r l u f t w a f f e t à t i g . w e i t e r t r a t d r . b e c k e r i n d i e a r b e i t s g r u p p e e i n , a u s s e r d e m w a r n a t i i r l i c h d r . b u n g e r s , e b e n f a l l s d a m a l s d e r l u f t w a f f e z w e i t w e i s e a n g e h o r i n g , a n d e n u n t e r s u c h u n g e n m a s s g e b e n d b e t c i l i g t . m a n m a c h t e sicli i m l a u f e e i n e r t a g u n g i n g o t t i n g e n m i t d e n p r o b l e m e n u n d d e r l i t e r a t u r b e k a n n t , s o w e i t d i e b e t e i l i g t e n h e r r e n n i e l i t o h n e h i n a u f g r u n d i h r e r z i v i l e n b e r u f e m i t d e r m . s . v e r t r a u t w a r e n . d a n n w u r d e e i n a r b e i t s p l a n e n t w o r f e n u n d r i c h t l i n i e n f e s t g e l e g t . z i e l d e r u n t e r s u c h u n g s o l i t e s e i n , f e s t z u s t e l l e n , o h es m o g l i c h sei, d i e m . s . z u a u s s a g e n i i b e r d i e g r o s s w e t t e r l a g e h e r a n z u z i e h e n . w e n n d i e s d e r f a l l s e i n s o l i t e , so s o l l t e n m e t h o d c n g e f u n d e i i w e r d e n , d i e d u r c h d i e m . s . g e g e b e n e n t a t b e s ù i n d e a u f k i i r z e s t e m w e g e m e t e o r o l o g i s c h z u v e r w e r t e n . d a s p r o b l e m w a r a l s o v o r e r s t r e i n t h e o r e t i s c b , a b e r n a c l i e i n e r y e r s u c h s z e i t v o n e t w a 2 j a l i r e n s o l i t e z u e i n e r p r a k t i s c h e n v e r w e r t u n g i i b e r g e g a n g e n w e r d e n . es sei g l e i c l i l i i e r v o r w e g f e s t g e s t e l l t , d a s s d i e a r h e i t e n s e l i r e r s c h w e r t w a r e n d u r c h g e w i s s e s c h w i e r i g k e i t e n . e r s t e n s w a r n i e l i t g e n i i g e n d g e s c h u l t e s p e r s o n a l a l s h i l s k r l i f t e z u r v e r f i i g u n g , d i e a r h e i t e n m u s s t e n a n e l i i n i h r e n t e c h n i s c b e n d c t a i l s v o n d e n h e r r e n s e l h s t g e m a c l i t w e r d e n . z w e i t e n s w a r d i e b e c h a f f u n g v o n m a t e r i a l , i n s b e s o n d e r e v o n e n t s p r e c h e n d e n i i n s t r u m e n t a r i u m so g u t w i e a u s g e s c h l o s s e n , m a n m u s s t e sicli m i t b e r e i t s v o r h a n d e n e n s e i s m o m e n t e r n b e g n i i g e n , s p e z i a l i n s t r a m e n t e k o n n t e n n i e l l i a n g e f e r t i g t o d e r s o n s t w i e b e s o r g t w e r d e n . d i e s w a s a u c l i d e r g r u n d , w a r u m m a n n i e l i t m i t d e r v o n k r u g e r u n t e r s u c h u n u k n uebek h i i k r o s f . i s m i k i n d e u t s c h l a n d w a e h r e n d 9 7 s o n n c n c n u n d v o n r a m i r e z in d i e l i t e r a t u r e i n g e f i i h r t e n d r e i e r s t a t i o n a r b e i t e n k o n n t e . es w u r d e n 4 a r b e i t s g r u p p e n g e b i l d e t . 1 . g o t t i n g e n m i t a n g e n b e i s t c r a n d e r s p i t z e u n d s c i n e n a s s i s t e n t e n . 2 . s t u t t g a r t , m i t d r . k i l l e r u n d d r . h a r d t w i g . 3. b r i i s s e l , w o d r . b e c k e r z u s a m m e n m i t p r o f . b o e r n e r d i e u n t e r s u c h u n g e n a n d e r e r d b e b e n w a r t e i n u c c i e f i i h r t e u n d 4. b e r g e n . h i e r w a r d r . b u n g e r s a l s m i t g l i e d e i n e r w e t t e r e r k u n d u n g s s t a f f e l tiitig u n d a r b e i t e t e n e b e n b e r a n d e n a u f g a b e n d e r m . s . z u r v e r f i i g u n g s t a n d e n : i n g o t t i n g e n d e r v o n k r u g k o n s t r u i e r t e h o r i z o n t a l s e i s m o m e t e r m i t r e g i s t r i e r a p p a r a t e n f i i r k u r z r e g i s t r i e r u n gen, i n s t u t t g a r t d i e d r e i g a l i t z i n ( — w i l i p ) — i n s t r u n i e n t e , d i e u n t e r e i n a n d e r v e r g l e i c b b a r w a r e n m i t e i g e n p c r i o d e n u n i 1 2 s e e u n d o p t i s e h e r r e g i e s t r i e r u n g , i n b r i i s s e l d r e i g a l i t z i n k o m p o n e n t e n m i t • u n t e r e i n a n d e r n i c l i t v e r g l e i c b b a r e n i n t r u m e n t e n . b u n g e r s b a t t e n a c h b e r g e n e i n e n k r u g ' s c b e n s e i s i n o m e t e r u n d e i n e n r e g i s t r i e r a p p a r a t m i t g e n o m m e n . im l a u f e d e r z e i t k a m i n g o t t i n g e n e i n g a l i t z i n a p p a r a t b i n z u . hi d e r f o l g e e r w i e s s i c h s t u t t g a r t a l s d i e b e s t e s t a t i o n , d e n n d a s d o r t v o r h a n d e n e i n s t r u m e n t a r i u m w a r d e n p r o h l e m e n d e r m . s . a n g e p a s s t . d a s p r o g r a i n m w a r w i e f o l g t e i n g e r i c h t e t w o r d e n : a m p l i t u d e u n d p e r i o d e d e r m . s . s o l i t e l a u f e n d r e g i s t r i e r t w e r d e n . in d e n a m i m i t e l i voiu n d n a c l i j e d e r v o l l e n s t u n d e w u r d e n d i e g r o s s t e n a m p l i t u d e n u n d p e r i o d e n g e m e s s e n , g e m i t t e l t u n d i n e i n d i a g r a m m e i n g e t r a g e n . es s o l i t e s i c h so e i n f o r t l a u f e n d e s b i l d v o m v e r l a u f d e r m . s . e r g e b e n . g l e i c h z e i t i g s o l i t e d u r i l i v e r g l e i c l i m i t d e r o f f i z i e l l e n , v o m z w g l i e r a u s g e g e b e n e n w e t t e r k a r t e e i n b i l d v o m a b l a u f d e s w e t t e r g e s c h e h e n s g e g e b e n w e r d e n . in e i n c m e i g e n e n j o u r n a l w u r d e n d i e p o s i t i o n e n d e r t i e f d r u c k k e r n e , i l i r e z u g r i c b t u n g e n , i h r e t i e f e i n m i ) a i n g e f i i h r t u n d d o r t , w o es m ò g l i c h w a r , i n c i d e t e l i m a r i n e b e o b a c h t u n g e n s e e g a n g u n d w i n d s t i i r k e . p a g e u n d w e i t e r e n t w i c k l u n g d e r f r o n t e n w u r d e r e g i s t r i e r t . i n b r i i s s e l u n t e r n a h i n becker d i e i n t e r s u c l i u n g d e s e i n f l u s s e s d e r g e z e i t e n a u f d i e m . s . es sei g l c i c h v o r w e g b e m e r k t , d a s s e i n m e r k l i e h e r e i n f l u s s d e r s e l b e n a u f d i e m . s . n i c l i t f e s t g e s t e l l t w e r d e n k o n n t e . i n b e r g e n p i a n t e bnngers, d e r s c i n e w e t t e r f l i i g e m a c b t e , d i e m . s . z u r e g i s t r i e r e n — s o z u s a g e n a n i o r t i h r e r e n t s t e h u n g — u n d g l e i c h z e i t i g v o m f l u g z e u g a u s d i e p e r i o d e n d e r m e e r e s w e l l e n z u 9 8 e . h a r u t w k ; m e s s e r i , u n i e i n c n e v e n t u e l l e n z u s a i n m e n l i a n g z w i s c h e n b e i d e n z u f i n d e n . d e m i d i e f r a g e , oli d i e m . s . n i e l i t e t w a a u s d e n d u r c h d e n w e l l e n g a n g e r z w u n g e n e n p f e i l e r s c h w i n g u n g e n b e s t e h e , w a r n o d i d u r c l i a u s o f f e n . l e i d e r w u r d e n d i e s e w i c h t i g e n u n t e r s u c h u n g e n v o n bungers d u r c h s e i n e n t o d u n t e r b r o c h e n b e z w . b e e n d e t . e r t e i l t e d a s s c h i c k s a l v o n s e l i r v i e l e n s e i n e r k a m e r a d e n . s e i n f l u g z e u g w u r d e a u f d e m r i i c k f l u g v o n i s l a n d v o n e n g l i c l i e n m a s c h i n e g e j a g t , a n g e g r i f f e n u n d s c b w e r b e s c h a d i g t . b e i d e r l a n d u n g i n b e r g e n s t i i r z t e d i e m a s c h i n e a b u n d d i e b e s a t z u n g f a n d d e n t o d . t r o t z d e m d e r i n a u g u r a t o r d e r u n t e r s u c h u n g e n , bungers, g e f a l l e n w a i , g i n g e n d i e u n t e r s u c h u n g e n w e i t e r . i n s t u t t g a r t w u r d e n n e b e n d e n o b e n b e s c l i r i e b e n e n l a u f e n d e n u n t e r s u c h u n g e n n o d i z i i s i i t z l i c b f o l g e n d e g e m a c l i t : a ) h e r r k i l l e r k o n s t r u i e r t e e i n e a p p a r a t i l i d i e es e r m ò g l i c l i t e , a l l e d r e i k o m p o n e n t e n s y n c h r o n u n d g c n a u u n t e r e i n a n d e r a u f e i n e m s t r e i f e n z u r e g i s t r i e r e n . d a d u r c h w u r d e es m o g l i c h , d i e p h a s e n v e r s c h i e b u n g c n o l i n e v i e l e u n s t a n d c z u e r k e n n e n , r a v l e i g h w e l l e n d o r t , w o s i e a u f t r e t e n , z u i s o l i e r e n u n d a u s i l u i e n d i e h e r k u n f t s r i c h t u n g d e r m . s . z u e r m i t t e l n . d i e s e k u r z r e g i s t r i e r u n g e n w u r d e n 2 m a l t i i g l i c h ( m o r g e n s u n d a b e n d s ) d u r c h g e f u h r t u n d b e a r b e i t e t . z u g l e i c l i e r z e i t w u r d e n i n g o t t i n g e n k u r z r e g i s t r i e r u n g e n m i t d e m k r u g ' s c h e n a p p a r a t a u s g e f i i h r t u n d m i t d e n s t u t t g a r t e r i n i l a u f e d e r sicli e t w a a l l e h a l b e n j a h r e w i e d e r h o l e n d e n a r b e i t s t a g u n g e n v e r g l i c h e n . b i d i e h a r m o n i s c h e a n a l y s e d e r m . s . r e g i s t r i e r u n g e n , d i e a n f i i n g l i e l i b e s o n d e r s i n g o t t i n g e n d u r c h g e f u h r t w u r d e n , e r w i e s sicli a l s v i c i z u u m s t a n d l i c h , d a h e r w u r d e e i n a n d e r e r , k i i r z e r e r eg b e s c h r i t t e n : d i e o f f e n k u n d i g u n g e s t o r t e n w e l l e n w u r d e n n a c l i i l i r e r p e r i o d e a u s g e m e s s e n , g e z i i b l t u n d d a n n v o n a l i e n d r e i k o m p o n e n t e n s o g e n a n n t e « s p e k t r e n » g e z e i c h n e t . t ì b e r d e n z e i t r a u m d e r k u r z r e g i s t r i e r u n g l i i n w e g ( 1 / 4 s t . ) e r g a b e n sicli so e i n i g e r m a s s e n c l i a r a k t e r i s t i s c h e b i l d e r , v o n d e n e n w i r a u f g r u n d d e r e r f a h r u n g e n a n n a h m e n , d a s s sie m i t g e w i s s e n w e t t e r l a g e n z u k o r r e l i e r e n s e i e n . e) e s w u r d e n d i e d r e i l e t z t e n , v o r d e m k r i e g h e r a u s g e g e b e n e n , a l s o e i n w a n d f r e i b e l e g t e n j a h r g i i n g c d e r « w i t t e r u n g s b e r i c h t e d e r d e u t s c h e n seem'arte i n h a m b u r g » a u f c h a r a k t e r i s t i s c h e u n d t v p i s c h e w e t t e r l a g e n h i n u n t e r s u c h t u n d m i t d e n i n s t u t t g a r t v o l l z i i h l i g v o i l i e g e n d e n s e i s m i s c h e n r e g i s t r i e r u n g e n v e r g l i c h e n . d e r g r u n d g e d a n k e d a b e i w a r , d u r c h d a s d u r c h l a u f e n d e n e b e n e i n a n d e r v o n w e t t e r k a r t e u n d r e g i s t r i e r u n g k o r r e l a t i o n e n z u f i n d e n , d i e sicli e v e n t u e l l a u c l i d u r c h r e i n a u s s e r l i c h e , z i m i i c l i s t d u r c h k e i n e t l i e o r i e e r k l i i r b a r e u n t e r s u c h u n u k n u e b e k h i i k r o s f . i s m i k i n d e u t s c h l a n d w a e h r e n d 9 9 m e r k m a l e a u s s e r n w u r d e n . i c h m o c h t e s o l c l i e u n t e r s u c l i u n g e n a p o s t e r i o r i f a s t f i i r w i c h t i g e r u n d f r u c l i t b a r e r l i a l t e n a l s u n t e r s u c l i u n g e n u n m i t t e l b a r a n i a k t u e l l e n w e t t e r g e s c h e h c n . y o r a l l e n i b e s t e h t d i e m o g l i c h k e i t , a u f d i e s e m w e g e c b a r a k t e r i s c b e s i t u a t i o n e n ( z . b . e i n i s l a n d t i e f ) z u i s o l i e r c n . i n g o t t i n g e n u n t e r s u c h t e a n g e m b e i s t e r m i t s e i n e n a s s i s t e n t e n d i e w a l i r e b e w e g u n g e i n e s b o d e n t e i l e l i e n s u n t e r d e m e i n f l u s s d e r m . s . u n d f a n d , d a s s d o r t , w o v e r m u t l i c h w e l l e n v o m r a y l e i g l i t y p u s w i r k s a m s i n d , d i e s c l i w i n g u n g s r i c h t u n g d e s b o d e n t e i l e l i e n s m i t d e r m u t m a s s l i c h e n h e r k u n f t s r i c l i t u n g i i b e r e i n s t i m m e . vi e i t e r e p i a n e s o l i t e l i v e r w i r k l i c h t w e r d e n , w e n n s i c h e i n g e w i s s e r f o r t s c h r i t t i n d e r « o r t u i i g » d e r t i e f d r u c k g e b i e t e e i n g e s t e l l t li a t t e . u b e r b a u p t m u s s g e s a g t w e r d e n , d a s s j e n e s t e l l e n , a n d e n e n g e o p b v s i k a l i s c l i e k e n n t n i s s e n i c l i t g u t v o r a u s g e s e t z t w e r d e n k o n n t e n , s i c h d i e l o s u n g d e s m i k r o s e i s m i s c h e n p r o b l e m s v i e l e i n f a c l i e r v o r s t e l l t e n a l s es t a t s a e h l i e h ist. i n n n e r w i e d e r t a u c h t e d e r g e d a n k e d e r « o r t u n g » n a c l i d e m m u s t e r d e r o r t u n g v o n f l u g z e u g e n a u f d r a h t l o s e m w e g e a u f , e i n g e d a n k e , d e r , w i e d i e t i e f k e r n l i y p o t h e s e z e i g t , d u r c h a u s n i c l i t a b w e g i g i s t , s i c l i a b e r i n u n s e r e n d e u t s c h e n u n t e r s u c l u i n g e n a l s n i c l i t s e b r f r u c h t b a r e r w i e s e n h a t . ergebnisse. — m a n k a n n d a s m a t e r i a l , d a s v o m h e r b s t 1 9 4 2 b i s zu a n f a n g 4 5 in g o t t i n g e n , s t u t t g a r t u n d b r i i s s e l z u s a m m e n g e t r a g e n w u r d e , w o h l a l s d a s r e i c h s t e b e z e i e b n e n , d a s j j ì s d a h i n h i n s i c h t l i c h d e r m . s . z u s a m m e n g e t r a g e n w o r d e n w a r . n a t i i r l i c h w i r d m a n c r w a r t e n , dass a u s d i e s e m m a t e r i a l a u c h g e w i s s e s c h l ù s s e g e z o g e n w e r d e n k ò n n e n , u n i d e m g e h e i n m i s d e r m . s . a u f d i e s p u r z u k o m m e n . m a n b a t t e s i c h z u b e g i n n d e r a r b e i t e n m i t a b s i c h t a u f k e i n e d e r b e i d e n t h e o r i e n f e s t g e l e g t : w e d e r a u f d i e wiechert'sche, d a s s d i e b r a n d u n g d i e u r s a c h e d e r m . s . sei, n o e h a u f d i e b e s o n d e r s ausse r b a l b d e u t s c l i l a n d s v e r t r e t e n e s o g e n a n n t e t i e f k e r n l i y p o t h e s e . i m c i n e a l t e r n a t i v e z w i s e h e n d i e s e n b e i d e n t h e o r i e n l i e r b e i z u f ù h r e n , w a r e es n o t w e n d i g , e i n z u v e r l a s s i g e s o r t u n g s v e r f a l i r e n z u b e s i t z e n . g e r a d e d i e s e s a b e r f e h l t e , d e m i c i n e s o g e n a n n t e d r e i e r s t a t i o n , m i t d e r g e g e n w a r t i g b e k a n n t l i c h i n d e r k a r i b i s c l i e n s e e u n d i n i p a z i f i s c h e n o z e a n a u s g e z e i c l i n e t e e r f a h r u n g e n g e m a c h t w e r d e n , w a r e u n t e r e u r o p a i s c h e n v e r h i i l t n i s s e n n i c l i t s e h r n u t z b a r g e w e s e n . m a n w a r a l s o auf a n d e r e m e t h o d c n a n g e w i e s e n . z u n a e h s t h o t s i c h n a t u r g e m a s s d a s v e r f a h r e n v o l i bungers a l s o r t u n g s v e r f a h r e n a n . bungers g e h t b e k a n n t l i c h v o n d e r v o r a u s s e t z u n g a u s , d i e m . s . b e s t e h e a u s u b e r l a g e r u n g e n v o n r a y l e i g h m i t l o v e w e l l e n , u n d l e l i r t u n t e r dieselv o r a u s s e t 1 0 0 e. h a r d t w i c z u n g d i e h e r k u n f t s r i c h t u n g d e r w e l l e n k e n n e n . es m u s s g e s a g t w e r d e n , d a s s d a s bunger'selle o r t u n g s v e r f a h r e n a u f k e i n e n e r f o l g f i i l i r t e . d i e r i c h t u n g e n l a g e n a l l e s a m t u n i n w , a u e h d a n n , w e n n k e i n e r l e i v e r a n l a s s u n g w a r , i n dieselr i e h t u n g d i e q u e l l e d e r m . s . z u v e r m u t e n . o f f e n l i a r i s t d i e bunger'selle v o r a u s s e t z u n g n i e l i t r i e l i t i g . b e i d e r m . s . d i i r f t e es sicli i n w a l i r l i e i t u n i g a n z a n d e r e d i l i g e l i a n d e l n a l s u n i r a y l e i g l i = u n d l o v e w e l l e n . d i e u n b r a u c l i b a r k e i t deibunger'sclien m e t b o d e w u r d e v o r k u r z e r z e i t a u c l i v o n d ' h e n r y u n d m o r e l l i f e s t g e s t e l l t . es i s t b e r e i t s a n g e f i i l i r t w o r d e n , w i e m a n i n s t u t t g a r t d i e o r t u n g s s c b w i e r i g k e i t z z u u m g e b e n v e r s u c h t e : d u r c l i d i e s y n c h r o n e r e g i s t r i e r u n g d e r d r e i k o m p o n e n t e n u n d i s o l i e r u n g d e r r a y l e i g h a r t i g e n w e l l e n z i i g e , d i e es i m m e r g a b . d i e n a c l i d i e s e m y e r f a h r e n g e f u n d e n e n r i c h t u n g e n z e i g t e n so g u t w i e n i e n a c l i d e m t i e f d r u c k z e n t r u m , w o l i l a b e r i m m e r a n i r g e n d e i n e s t e l l e d e r v o n d e r b r a n d u n g g e t r o f f e n e n k i i s t e . es w u r d e so j e n a c l i deiw e t t e r l a g e d i e n o r w e g i s c h e , e n g l i s c h e , i r i s c l i e k i i s t e r i c h t n n g s m a s s i g e r f a s s t , a b e r a u c l i d i e b r e t a g n e u n d b i s c a y a . b e i s p a n i e n u n d i s l a n d v e r s a g t e d a s y e r f a h r e n i i i f o l g e d e r k l e i n h e i t d e r r e g i s t r i e r t e n a m p l i t u d e n . a l l e b e t e i l i g t e n n e i g t e n s c b l i e s s l i c h , o b w o h l v o n v o r n h e r e i n s t r e n g d a r a u f g e a c b t e t w o r d e n w a r , sicli a u f k e i n e d e r b e i d e n t b e o r i e n f e s t z u l e g e n , d a z u , a n z u n e b m e n , d a s s d a s , w a s w i r a l s m . s . r e g i s t r i e r t e n , s c i n e u r s a c h e i n d e r b r a n d u n g l i a b e . d i e s b e d e u t e t e n a t u r g e m a s s e i n e g e w i s s e e n t t a u s c h u n g , f i i r d e n d a i n a l s z u r d e b a t t e s t e h e n d e n z w e e k w a r e es u n g l e i c l i g i i n s t i g e r g e w e s e n , w e n n sicli h e r a u s g e s e t l l t b a t t e , d a s s d i e q u e l l e d e r m . s . u n t e r d e n i i b e r d a s m e e r z i e b e n d e n t i e f d r u c k g e b i e t e n zu s u c b e n sei. i c h n e n n e e i n i g e e r s c h e i n u n g e n , d i e sicli z w a n g l o s m i t d e r b r a n d u n g s h y p o t b e s e e r k l a r e n l a s s e n . i. d i e m . s . w i r d m u s o s t a r k e r , j e niellid e r e i n f a l l s w i n k e l d e r d i i n u n g , d . h . d e s w e i t h e r z i e b e n d e n s e e g a n g e s sicli e i n e m r e c h t e n w i n k e l n a h e r t . m a n e r k e n n t d i e s a u f d e r w e t t e r k a r t e d a r a n , d a s s d e i s o b a r e n s e n k r e c h t z u r g e t r o f f e n e n k i i s t e s t e h e n . d i e s e n f a l l k o n n t e n w i r i m m e r w i e d e r a n d e r n o r w e g i s c l i e n k i i s t e s t u d i e r e n , i n s b e s o n d e r e a u c l i a n d e n a l t e n , g u t b e l e g t e n w e t t e r k a r t e n a u s d e r z e i t v o r d e m k r i e g e . h i e r b e i i s t e i n g e w i s s e s « n a c b h i n k e n » z u b e r i i c k s i c h t i g e n . d e r w e l l e n g a n g d r e l i t n i e l i t g l e i c b z e i t i g m i t d e n i s o b a r e n , s o n d e r n k o m m t d e r i s o b a r e n d r e h u n g e r s t m i t e i n e r g e w i s s e n v e r s p a t u n g n a c l i . d a s m a x i m u m d e r m . s . t r i t t d a b e r d a n n a u f , w e n n d i e i s o b a r e n b e r e i l s w i e d e r d e n r e c h t e n w i n k e l z u r k i i s t e i i b e r s c h r i t t e n u n t e r s u c h u n u k n u e b e k h i i k r o s f . i s m i k i n d e u t s c h l a n d w a e h r e n d 1 0 1 l i a l j e n . a u c l i a n d e r s c b o t t i s c b e n , i r i s c l i e n u n d f r a n z o s i s c l i e n k ù s t e k o n n t e i l i e s e e r s c l i e i n u n g b e o b a c l i t e t w e r d e n , w e n n a u c l i n o r w e g e n h i n s i c h t l i c h d e r m . s . i m m e r c i n e a u s g e z e i c h n e t e r o l l e g e s p i e l t iiat. a l s t y i ) i s c l i e s b e i s p i e l f i i h r e i e l i d i e w e t t e r l a g e v o m 2 8 . i 1 i . i 1 i . 4 3 a n . e i n l o f o t e n t i e f s t a n d m i t s e i n e m k e r n v o n 9 8 0 i n b v o r d e r n o r w e g i s e h e n k i i s t e u n d b e w e g t e sicli i n n e r b a l b v o n 2 4 s t u n d e n s e n k r e c l i t z u r k i i s t e n r i c l i t u n g ])is i n d e n r a u m v o n leningrad. d i e a n f i i n g l i c l i k l e i n e m . s . s t e i g e r t e s i c h s e l i r r a s c h z u e i n e m m a x i m a l w e r t . d i e s e l b i i c b s t e w e r t w u r d e a b e r e r r e i c l i t zu e i n e r z e i t , d a d e r k e r n d e s t i e f s b e r e i t s w e i t a u f d e m f e s t l a n d e l a g , u n d z w a r g e r a d e z u jener z e i t , zu d e r d e r s e e g a n g d i e k ù s t e s e n k r e c l i t g e t r o f f e n l i a b e n m u s s t e . w e n n l i i r a u s s e r t r o p i s c h e z y k l o n e n d i e t i e f k e r n b y p o t l i e s e a u c l i r i c l i t i g w a r e , so d ù r f t e d i e s n i c l i t e i n t r e t e n . d i e a n n a b m e , es k o n n e s i c h h i e r b e i u n i m . s . v o m f i n n i s c h e n o d e r b o t t n i s c h e n m e e r b u s e n b a n d e l n , t r i f f t g a n z b e s t i m m t n i c l i t z u , d a d a s s t u d i u m d e r o s t s e e m i k r o s e i s m i k g e z e i g t b a i , d a s s d i e s e a u f a l l e f i i l l e n u r v o n k l e i n e n p e r i o d e n u n d a m p l i t u d e n ist. h i e r d ù r f t e d i e . k l e i n b e i t d e s w i n d w e g e s e i n e r o l l e s p i e l e n , d i e d i e a u s b i l d u n g e i n e s g e w a l t i g e n s e e g a n g e s n i c l i t z u l a s s t , e b e n s o w i e d i e g e r i n g e t i e f e i n i v e r g l e i c l i z u m a t l a n t i s c b e o z e a n . n o d i e t w a s a n d e r e s z e i g t e d i e s e w e t t e r l a g e . s i e w i d e r b o l t e s i c h niimlic.li z i e m l i c l i g e n a u a n i 2 7 . / 2 8 . x i . 4 2 . ( v o r l i e r ! ) . d i e m . s . e r r e i c h t e n i c l i t d i e s e l b e i n t e n s i t a t , o b w o b l d i e b a i m i d e s t i e f s f a s t d i e s e l b e w a r , d e r g r a d i e n t a b e r n o c h s t a r k e r : m i n . = 9 6 5 m b . b e i m s t u d i u m des i s o b a r e n s y s t e m s z e i g t e es s i c h , d a s s d i e i s o b a r e n n u r k u r z e z e i t s e n k r e c l i t z u r k i i s t e g e s t a n d e n b a t t e n , d a s s a l s o d i e d ù n u n g n u r k u r z e z e i t s e n k r e c l i t g e g e n d i e n o r w e g i s c h e s t e i l k ù s t e a n g e l a u f e n w a r . w e g e n d e r k r ù m m u n g d e r i s o b a r e n w a r d i e s n i c l i t a n d e r s m o g l i c l i . d e r w i n d w e g , d . h . g e n a u e r d e r g e r a d l i n i g e w i n d w e g w a r a l s o k ù r z e r a l s i n i e r s t g e n a n n t e n f a l l . w i e h i e r , so z e i g t e s i c h a u c l i i n a n d c r e n f i i l l e n , d a s s d i e m . s . u m s o s t a r k e r i s t , j e l a n g e r d e r g e r a d l i n i g e w i n d w e g ù b e r d a s m e e r b i n g e l i t . g e r a d e d i e s e n u m s t a n d , d e r s i c h a n h a n d v o n w e t t e r k a r t e n i m m e r w i e d e r e i n w a n d f r e i r e p r o d u z i e r e n liisst, k a n n d i e t i e f k e r n l i y p o t h e s e n i c l i t g a n z z w a n g l o s e r k h i r e n . e i n e b e s o n d e r s e i n d r u c k s v o l l e s t i i t z e f a m i d i e b e o b a c l i t u n g , als sicli h e r a u s s t e l l t e , d a s s m . s . a u s i s l a n d b e s o n d e r s d a m i b e o b a c h t e t w e r d e n k o n n t e , w e n n d e r w i n d d e n a t l a n t i k i n d e r r i c h t u n g j \ e s w v i e l e t a u s e n d e k m f a s t g e r a d l i n i g ù b e r s t r i c l i . a l s w e i t e r e b e o b a c h t u n g s t a t s a c l i e a u s v i e l e n w e t t e r l a g e n e r g a l i s i c l i , d a s s t i e f d r u c k g e b i e t e , d i e sieli e n t f e r n t e n , v o n e i n e r m . s . b e g l e i t e t w a r e n , d e r e n p e r i o d e n sicli m e r k l i c h v e r g r o s s e r t e n . m a n k o n n t e v e r 1 0 2 e . h a h d t w i c i'olgen, w i e d i e v o n d e r b r a n d u n g a m h e f t i g s t e n g e t r o f f e n e n k i i s t e n t e i l e v o n d e r s t a t i o n s t u t t g a r t w e g r i i e k t e n . d i e s s t e h t i n i e i n k l a n g m i t d e r b e k a i i n t e n t a t s a c l i e , d a s s s e i s m i s e l i e w e l l e n i n i l a u f e i l i r e s a u s b r e i t u n g s v o r g a n g e s i l i r e p e r i o d e n v e r l i i n g e r n . z u r u n t e r s u c b u n g d i e s e r e r s e b e i n u n g i s t v o n m i r d i e e r d e a l s e i n v i s k o e l a s t i s e b e s m e d i u m a n g e n o m m e n w o r d e n — w i r b a t t e n d a m a l s n o d i d i e 11 o l i m i ng, a u s d e r p e r i o d e n v e r g r o s s e r u n g o d e r i i b e r l i a u p t a u s d e r p e r i o d e n g r ò s s e r u c k s c l i l u s s e a u f d i e e n t f e r n u n g d e r q u e l l p u n k t e z i e b e n zu k o n n e n . l e i d e r s i n d d i e s e n b e n i i i b u n g e i i k e i n e e r f o l g e b e s e l i i e d e n w o r d e n , d e m i a u s m e i n e r a r b e i t g i n g b e r v o r , d a s s d i e v e r g r o s s e r u n g n u r b e i k l e i n e n e n t f e r n u n g e n m i t d e r e n t f e r n u n g r a s e l i z u n i m m t , b e i g r o s s e n e n t f e r n u n g e n a b e r s i e b k a u m m e l i r a n d e r t . es g i l t l i i e r c i n e a r t p a r a b o l i s c h e n g e s e t z e s . t y p e n . n a e b d e m sicli h e h e r a u s a u s g e s t e l l t b a t t e , d a s s e i n g e n a u e s f i x i e r e n d e r l a g e d e r z v k l o n e n k e m e a u f d e m w e g e i i b e r d i e m . s . m i t d e n u n s z u r v e r f i i g u n g s t e h e n d e n m i t t e l n n i e l i t m ò g l i c h w a r , m u s s t e n d i e a n f o r d e r u n g e n b e s c h e i d e n e r f o r m u l e r t w e r d e n . h i e r h a l l d e r g r u n d g e d a n k e d e r t y p i s i e r u n g . e s i s t l i e k a n n t , d a s s d e r c h a r a k t e r d e r m . s . s e l i r verschieden a i i s f i i l l t , j e n a c l u l e m w e l c h e s d i e g e r a d e v o r l i e r r s e h e n d e w e t t e r l a g e ist. s o z . b . z e i g t e s i c l i , d a s s m . s . , d e r e n m u t m a s s l i c h e h e r k u n f t n o r w e g e n w a r , s e l i r s c l i ò n e , r c g e l m i i s s i g e s i n u s s c h w i n g u n g e n z e i g t , m i t g r o s s e n a m p l i t u d e n u n d p e r i o d e n , s o w i e i i b e r a u s l i a u f i g a u f t r e t e n d e n s c l i w e b u n g s b i l d e m . i n r e g i s t r i e r u n g e n v o n m . s . a n d e r e r h e r k u n f t t r e t e n d i e s e s c h w e b u n g e n w e i t a u s n i e l i t so l i a u f i g a u f , j a , s i e f e l i l e n m e i s t g a n z . d i e n o r w e g e n m . s . b i l d e t a l s o g a n z d e u t l i c h e i n e n t y p u s f i i r s i c l i . s e l i r g u t d a v o n z u u n t e r s c h e i d e n i s t z . b . d i e m . s . a u s s c h o t t l a n d e n g l a n d . h i e r f e l i l e n d i e s c h w e b u n g e n , d i e p e r i o d e n s i n d i n i a l l g e m e i n e n k l e i n e r , d i e r e g i s t r i e r u n g e n z e i g e n e i n e n w e n i g e r e i n l i e i t l i c h e n c h a r a k t e r . b e s o n d e r s d e u t l i c h s e t z e n sicli v o n d e n b e i d e n v o r g e n a n n t e n t y p e n d i e r e g i s t r i e r u n g e n v o n m . s . a u s d e r b r e t a g n e a b : d i e w e l l e n z i i g e s i n d i i b e r a u s u n r e g e l m a s s i g , z e r h a c k t , n i e l i t g e g l i i t l e t , s i n u s f o r m i g e w e l l e n z i i g e s i n d s e l i r s e l t e n , s c h w e b u n g s b i l d e r f c l i l e n g a n z . d i e k a n a l m . s . z e i c h n e t e sicli i n s t u t t g a r t d u r c h b e s o n d e r s k u r z e p e r i o d e n a u s . c h a r a k t e r i s t i s c h d u r c h i l i r e k l e i n h e i t i n a m p l i t u d e u n d v e r h a l t n i s m a s s i g e r g r o s s e d e r p e r i o d e w a r d i e m . s . a u s d e r p y r e n a e n h a l b i n s e l s o w i e a u s i s l a n d . u n v e r s t i i n d l i c h b l e i b t i n d i e s e n b e i d e n f i i l l e n a b e r i m m e r n o d i d i e t a t s a c h e , d a s s d i e p e r i o d e n v i c i z u k l e i n s i n d , a l s d a s s m a n a n e i n b l o s s e s a b d a m p f e n d e r s c h w i n g u n g e n g l a u b e n k ò n n t e . u n t e r s u c h u n u k n u e b e k h i i k r o s f . i s m i k i n d e u t s c h l a n d w a e h r e n d 1 0 3 i n i l a u f e d e r b e s c h a f t i g u n g m i t d e n v e r s c h i e d e n s t e n s e i s m o g r a m m e n s c h a r f t s i c l i d e r b l i c k f i i r d i e v e r s c h i e d e n h e i t d e r i i a n d s c h r i f t e n , m i t d e r s i e g e z e i e l i n e t s i n d . d i e r i i e k s c h l ù s s e a u f d i e z u g e b o r i g e n g r o s s w e t t e r l a g e n s i n d n a t u r g e m a s s n i c l i t e i n d e u t i g . a b e r z u s a m m e n m i t e i n i g e n w e n i g e n , v i e l l e i c b t s o n s t n o d i b e k a n n t e n m e t e o r o l o g i s c l i e n t a t s a c l i e n , e t w a e i n z e l n e n d r u c k = o d e r w i n d w e r t e n g e s t a t t e n s i e d o d i , e i n v e r h a l t n i s m a s s i g z u t r e f f e n d e s b i l d v o n d e r a l l g e m e i n e n isob a r e n v e r t e i l u n g z u g e b e n . d i e s n u n i s t r e i n e u b n n g s s a c l i e u n d b a t m i t e i n e m s t r e n g w i s s e n s c h a f t l i c h e n v e r f a b e n n u r m e l i r s e l i r w e n i g z u t u n . i n s t u t t g a r t w u r d e a u f d i e s e w e i s e , z u s a m m e n m i t v e r e i n z e l t e n d a t e l i u n d z u s a m m e n m i t d e n s o g e n a n n t e n « s p e k t r e n » w i e d e r h o l t d i e g r o s s w e t t e r l a g e g u t « e r r a t e n » . n i i h e r e s i i b e r d i e t y p e n d e r m . s . f i n d e t m a n i n e i n e m a u f s a t z d e s m i t a r e i t e r s b e c k e r i n d e r d e u t s c h e n , h y d r o g r a p h i s c h e n z e i t s c h r i f t . w e i t e r h a b e n s i c h z w e i s e l i r m e r k w i i r d i g e t a t s a c l i e n i n i l a u f e d e r u n t e r s u c l i u n g h e r a u s g e s t e l l t . d i e e i n e b e t r i f f t d e n e i n f l u s s d e r fronten. d e r u b e r t r i t t e i n e r k a l t f r o n t a u f d a s f e s t l a n d z i e h t y e r s t a r k u n g d e r m . s . n a c l i s i c h . d i e s e t a t s a c h e k a n n m a n d u r c l i d i e b r a n d u n g s h y p o t h e s e g u t e r k l i i r e n u n d z w a r so, d a s s h i n t e r d e r k a l t f r o n t d a s w i n d s y s t e m e i n a n i l e r e s ist als v o r i b r . w i e w e i t j e d o c h d i e v e r s c h i e d e n b e i t i n d e r l a b i l i s i e r u n g d e r l u f t , d i e t u r b u l e n z d e r l u f t b e w e g u n g e i n e r o l l e d a b e i s p i e l t , k o n n t e n i c l i t e r f o r s c h t w e r d e n , d a z u b e d i i r f t e es w o h l n o c h v i e l e r e i n g e h e n d e r u n t e r s u c l i u n g e n a n o r t u n d s t e l l e . n u r d i e t a t s a c h e s t a n d s e i t 1 9 4 2 f e s t , d a s s k a l t f r o n t e n b e i i h r e m u b e r t r i t t a u f d a s f e s t l a n d d i e m . s . v e r s t i i r k e n . e i n e a n d e r e , m e r k w i i r d i g e t a t s a c h e , d i e i n s e l i r e i n d r u c k s v o l l e r f o r m a l l e r d i n g s n u r e i n m a l b e o b a c b t e t u n d v o n h e r n becker h e r a u s g e s t e l l t w u r d e , b e z i e h t sich a u f d e n e i n f l u s s v o n k a l t l u f t e i n b r i i c l i e n a u f d i e m . s . h e r r becker b a t i n g o t t i n g e n d e n f a l l d e m o n s t r i e r t , dass e i n k a l t l u f t e i n b r u c h a u s d e m r a n n i v o n s p i t z b e r g e n n a c l i s ù d s i i d w e s t i n u c c i e ù b e r a u s s t a r k e m . s . h e r v o r g e r u f e n b a t . e i n i r g e n d w i e f i i r d i e m . s . in f r a g e k o m m e n i l e s t i e f d r u c k g e b i e t w a r n i c l i t i n d e r niibe, a u c l i d i e b r a n d u n g a n d e n k i i s t e n k o n n t e n a c l i d e r g a n z e n s a c b l a g e n i e l l i f i i r d i e m . s . v e r a n t w o r t l i c l i g e m a c b t w e r d e n . l e i d e r ist m i r d a s d a t i m i d i e s e s e r e i g n i s s e s v e r l o r e n g e g a n g e n . i e h m o c h t e j e d o c h d i e a n w e s e n d e n d a r a u f h i n w e i s e n , d a s s h i e r e i n e m e r k w i i r d i g k e i t v o r l i e g t u n d d a s s es sicli l o l n i e n w i i r d e , d e n e i n f l u s s v o n k a l t l u f t e i n b r i i c b e n a u f d i e m . s . n l i l i e r z u s t u d i e r e n . e i n w e i t e r e r , i n s t u t t g a r t f e s t g e s t e l l t e r f a l l b e t r i f f t f o l g e n d e s : 1 0 1 f.. i i a r d t w i g b e i s t a r k e m w i n d a u s n n e q u e r i i b e r d e n a t l a n t i k m i t l a n g e m , g e r a d l i n i g e n w i n d w e g z w i s c l i e n t r l a n d s c h o t t l a n d e i n e r s e i t s u n d i s l a n d a n d r e r s e i t s e n t s t e b t s e l i r l a n g p e r i o d i s c l i e , e t w a s i i b e r m i t t e l s t a r k e m . s . m a n k i i n n t e v o m s t a n d p u n k t b e i d e r t h e o r i e n d a r a n d e n k e n , class d a s s i n f o l g e d e s l a n g e n w i n d w e g s g u t a u s g e b i l d e t e d i i n u n g s s y s t e m d e n m e e r e s b o d e n i n v e r s c h i e d e n e n u n t i e f e n so t r i f f t , d a s s m . s . w e l l e n v o n i h m a u s g e l i e n . h i e r s c b e i n t m i r d i e m ò g l i c h k e i t z u l i e g e n , b e i d e t h e o r i e n s p r e c h e n zu l a s s e n . e i n t i e f d r u c k g e b i e t , d a s m a n b a t t e f i i r d i e m . s . v e r a n t w o r t l i e h m a e h e n k i i n n e n , w a r i n t l e n b e o b a e h t e t e n f i i l l e n n i e l i t v o r h a n d e n g e w e s e n . e s s e l i e i n t a l l e i l i d i e k o n f i g u r a t i o n d e r i s o b a r e n , a l s o d a s w i n d s y s t e m f i i r d i e m . s . b o d e n b e w e g u n g v e r a n t w o r t l i e h g e w e s e n z u s e i n . teli b a l i e e i n e n k u r z e n b e r i c h t g e g e b e n i i b e r e i n i g e d e r i m l a u f e d e r u n t e r s u c h u n g e n i n d e u l s c l i l a n d g e f u n d e n e n e r g e b n i s s e . k i i c k b l i c k e n d m i i s s e n w i r s a g e n : w i r s i n d i n d e r r e n n t n i s d e r m . s . d u r c h d i e s e u n t e r s u c h u n g e n z w a r u n i e i n g u t e s s t i i c k w e i t e r g e k o m m e n , h a b e n a b e r f i i r d i e p r a k t i s c l i e y e r w e r t b a r k e i t n i e l i t a l l z u v i e l e e r f o l g e e r z i e l t . d i e t i e f k e r n h y p o t h e s e , a u f d i e w i r a l l e i n i g r a n d e g e n o m m e n s e l i r g e h o l f t h a t t e n , l i a t sicli f i i r d i e e u r o p a i s c h e n v e r h a l t n i s s e a l s n i e l i t b r a u c h b a r e r w i e s e n . . d i e b r a n d u n g s h y p o t h e s e t>.ber, m i t d e r w i r s o g u t w i e a l l e s e r k l i i r e n k o n n t e n , i s t f i i r p r o g n o s t i s c h e z w e c k e v i c i w e n i g e r g u t g e e i g n e t . es e r h e b t sicli j e t z t d i e f r a g e n a c l i d e r n a t u r d e r m . s . w i e s i e j a a l l e w i s s e n , s i n d i n z w i s c l i e n n e u e t a t s a c l i e n i n a m e r i k a b e k a n n t g e w o r d e n . w e n i g e m o n a t e , n a e h d e m i n d e u t s c h l a n d d i e u n t e r s u c h u n g e n i i b e r d i e m . s . a u f g e n o m m e n w o r d e n w a r e n , b e g a n n , a l s m a r i n e p r o g r a m m , a u c l i i n a m e r i k a d i e u n t e r s u c l i u n g d e r s e l b e n p r o b l e m e . r e f e r e n t u n d i n s p e k t e u e r w a r d e r a n h a n g e r d e r b r a n d u n g s h y p o t h e s e gutenberg. d i e a m e r i k a n e r w a r e n g l u c k l i c l i e r a l s w i r . m a n n m s s d e n a m e r i k a n i s c h e n b e r i c h t l e s e n , 11111 z u e r k e n n e n , v i e v i e l m e l i r a n p e r s o n a l , a n t e c h n i s c l i e m a u f w a n d u n d a n g e l d m i t t e l n e i n g e s e t z t w u r d e n , u n i d e n z u s a m m e n h a n g z w i s c l i e n m . s . u n d h u r r i k a n e n zu b e s t i m m e n . d i e s e r z u s a m m e n h a n g i s t i n d e r k a r i b i s c h e n s e e u n d i m p a z i f i k e i n s e l i r ensrer. m a n k a n n es a l s e i n d e u t i g e r w i e s e n a n n e h -> c m e n , d a s s d i e q u e l l e d e r d o r t r e g i s t r i e r t e n m . s . u n t e r d e m z e n t r u m d e r t r o p i s c l i e n w i r h e l s t o r u n g l i e g t , e i n v o n m e l i r e p n d r e i e r s t a t i o n e n a u s g e h e n d e s l o k a l i s i e r e n d e r p o s i t i o n e n d e r s t i i r m e w a r e s o n s t g a n z a u s g e s c h l o s s e n . d i e s e a m e r i k a n i s c h e n e r g e b n i s s e s e l i e i n e n m i n d e n i n d e u t s c h l a n d w a l i r e n d d e s k r i e g e s g e f u n d e n e n d i r e k t z u w i d e r s p r e c h e n . i c h u n t e r s u c h u n u k n u e b e k h i i k r o s f . i s m i k i n d e u t s c h l a n d w a e h r e n d 1 0 5 n i ò c h t e j e d o c h a n k e i n e n w i d e r s p r u c h g l a u h e n . v i e l m e h r g l a u b e i e l i , dass es sicli g r u n d s a t z l i c h u n i z w e i v e r s c h i e d e n e e r s c h e i n u n g e n l i a n d e l t : einerseits d i e b o d e n b e w e g u n g , d i e a u s g e l i t v o n j e n e n s t e l l e n des m e e r e s , d i e sicli u n t e r d e m t i e f d r u c k a r e a l b e f i n d e n ; andrerseits v o n d e n b e w e g u n g e n i l e r k i i s t e n u n d s c h e l f r i i n d e r u n t e r d e m e i n f l u s s d e r a n b r a n d e n d e n m e e r e s w e l l e n . d e m i d a s s d i e s e z w e i t e n v o r l i a n d e n s i n d , u n d a l s o w i r k e n miissen, w i l d m e i s t i i b e r s e l i e n , v i e l l e i c b t n i c l i t i m m e r o l i n e a b s i e b t , u n d i n d e r l i t e r a t u r d a h e r a n e l i g a r n i c l i t e r w i i h n t . b o d e n b e w e g u n g u n t e r d e m e i n f l u s s d e r b r a n d u n g l l i s s t s i c l i i i b e r h a u p t n i c l i t w e g s c l i w e i g e n u n d w e g d i s k u t i e r e n . w e n n t r o t z d e m a m e r i k a n e r u n d d e u t s c h e z u v e r s e l i i e d e n e n e r g e b n i s s e n k o m m e n , so w o l i l n u r d e s h a l b , w e i l i n d e n v e r s e b i e d e n e n b e r e i c h e n v e r s c h i e d e n e b e d i n g u n g e n h e r r s c h e n . d i e s c h a r f a b g e g r e n z t e n , m i t g r o s s e r g e w a l t d a b i n z i e h e n d e n t r o p i s e l i e n w i r b e l s t i i r m e b e v o r z u gen (leu einen f a k t o r , d i e t i e f k e r n a n s i c b t d a g e g e n t r i t t d i e s i c h e r a u c l i i m m e r v o r h a n d e n e b r a n d u n g w e i t z u r i i c k . s i e ist hier e i n s e k u n d i i r e s p h i i n o m e n . i n e u r o p a a b e r i s t es u m g e k e h r t : d i e ù b e r g e w a l t i g e c e b i e t e sich e r s t r e c k e n d e n t i e f d r u c k g e b i e t e l a s s e n e i n g e n a u e s l o k a l i s i e r e n d e s t i e f k e r n e s n i c l i t z u , w a h r s c h e i n l i c h s i n d a n e l i d i e m e c h a n i s c h e n v o r g i i n g e , d i e z u r a n r e g u n g f i i b r e n , n i c l i t so e n e r g i e r e i c h . h i e r t r i t t d i e t i e f k e r n s e i t e in d e n h i n t e r g r u n d , d a f i i r t r e t e n d i e m i t d e r b r a n d u n g z i i s a m m e n h i i n g e n d e n e r s c h e i n u n g e n b e s s e r h e r v o r . d i e e u r o p i i i s c h e k i i s t e ist u n e n d l i c h v i e l g e s t a l t i g , so v i e l g e s t a l t i g w i e d e r e u r o p i i i s c h e m e n s c h s e l b s t ist. an as a u f d e n s e i s m o m e t e r a u f t r i f f t , i s t e i n g e w i r r v o n v e r s c h i e d e n s t e n b e w e g u n g e n . d e r e n e i n z e l u r s a c h e n n i c l i t zu t r e n n e n s i n d . es b a n d e l t s i c h u n i e i n e n k o m p l e x v o n e r e l i e i n u n g e n , d i e e i n g a n z e s b i l d e n . d a h e r d i e s c h w i e r i g k e i t , h i e r i n e u r o p a d i e t i e f k e r n l i y p o t h e s e b e s t i i t i g t finden zu w o l l e n . bride e r s c h e i n u n n s f o r n i e n — m e e r e s b o d e n b e w e < ! ; u n £ u n d b r a n -f" c e" d u n g s h e w e g u n g — s i n d a n d e r e r z e u g u n g d e r m . s . b e t e i l i g t . a b e r h i e r h e r r s e h t d i e e i n e v o r , d o r t d i e a n d e r e . e s l i a b e n a l s o b e i d e s c h u l a n s i c h t e n r e c h t — j e d e a u f i l i r e a r t . u n d d a s ist g u t so, d e m i es z e i g t . d a s s g e g e n s l i t z e , d i e m a n l a n g e f i i r u n ù b e r b r i i c k b a r g e h a l t e n b a t , s i c h e i n e s t a g e s g a r n i e l l i a l s g e g e n s i i t z e h e r a u s t s e l l e n . comunicazione presentata al convegno per la costituzione della f.s.e., tenutosi a i erotta dal 12 al 16 settembre 1950 (v. « annali di geofisica », voi. i l i , ]>ag. 579). 1 0 6 e . h a r d t w i c z usa mmenfa ss ung ini laufe des jahres 1942 wurde vom damaligen deutschen wetterdienst der beschluss gefasst, den zusammenhang zwisclien mikroseismik und wetterlage genau untersuchen zu tasseti. zweck der untersuchungen solite sein, herauszufinden, oh die durch die m.s. gegebenen dateti f i i r die erkennung der grosswetterlage geeignet und eventuell f i i r die prognose verwertbar seien. mit der oberleitung wurde prof. angenheister in gottingen, betraut. sogenannte dreiechsstationen nacli liamirez standen nielit zur verfiigung. annalimen iiber die entstehung der m.s. wurden zinachst n i e l i t gemaclit, doch kamen ini laufe der zeit alle beteiligten zur erkenntnis, dass sich alle erscheinungen mit hilfe der wiechert'sclien brandungshypothese zwaglos erkliiren lassai. zur ortung der storungsquelle wurden alle drei komponeneteii untereinander registriert und so rayleigh-artige wellenziige erkannt und auf ihre herkunftsrichtung hin untersucht. gefunden wurden als quelle die von der brandung getroffenen kiisten. die untersuchungen wurden durch wind-und seegangsmeldungen unterstiitzt. das allgemeine studium erfolgte an iland von w etterkarten aus der vorkriegszeit. es wurden gewisse typen von wetterlagen den entsprechenden typen der m.s. zugeordnet und so eine, wenn aucli nielit selir strenge typisierung erreiclit. daneben wurde mit sogenannten periodenspektren gearbeitet. die hier nodi nielit abgeschlossenen untersuchungen tasseti erkennen, dass zu verscliiedenen grosswetterlagen aucli verschiedenen spektren geli oro ti. ein anzeichen dafiir, dass die stòrung von zentrum der zyklonen ausgehe, wurde nielli gefunden, in gegenteil, die m.s. erreichte in eiiizelnen filiteli dann ilir maximum, wenn der kern der zyklone sclion weit auf dem festlande war. diinung senkrecht zur kiiste gibt maximale m.s. langer, geradliniger windweg iiber der ozean verstiirkt die m.s. der ubertritt einer kaltfront auf das festland verstiirkt die m.s. — off cubar bewirken die. windverhiiltnisse auf der riickseite der front eine verstàrkung der brandung. es bestelit der eindruck, dass aneli kaltlufteinbruclie eine erliórung der m.s.-intensitiit bewirken. layout 6 the tres tabernae archeological site (cisterna di latina, italy): new evidence revealed through an integrated geophysical investigation stefano urbini1, carla bottari1,*, marco marchetti1, lili cafarella1 1 istituto nazionale di geofisica e vulcanologia, sezione roma 2, rome, italy annals of geophysics, 53, 5-6, 2010; doi: 10.4401/ag-4775 abstract «mercator» (medocc, interreg iiib) is a european project that is aimed at a detailed investigation of the historical and archeological heritage of the mediterranean area. within this project, broad research is being financed regarding the importance of the development of the main ancient merchant routes in this region. these various tasks include this integrated geophysical survey at the tres tabernae site, an ancient statio along the appian way (close to latina, central italy). this was carried out using several techniques that are focused on the identification of buried archeological remains. the main goal of this investigation was to cover the area surrounding this partially excavated site to obtain a quick, but meaningful, result relating to the presence of interesting buried features. for this, the geophysical techniques involved were frequency-domain electromagnetic induction, groundpenetrating radar, and magnetometry. to obtain the best results, these methods were optimized, taking into account the kind of structures involved and their relation to the environmental context. the combination of these different geophysical techniques shows good results, indicating the possible presence of new buried structures, such as walls, floors and a lead pipe. introduction near-surface geophysical prospecting provides a fast, non-invasive and low-cost tool that is very useful for obtaining information about buried archeological evidence. the integration of different techniques also allows reduction of their typical measurement ambiguities [brizzolari et al. 1992]. this kind of survey provides information to archeologists about the positions of interesting structures that can be investigated more thoroughly and excavated at a later date. «mercator» (medocc, interreg iiib) is a european project that has the aim of deeply investigating the historical and archeological heritage of the mediterranean area. one of the targets of this project is to collect information about the importance of the main ancient merchant routes. in this framework, an integrated geophysical survey was planned and performed at a (partially excavated) site known as tres tabernae (at cisterna di latina, central italy), a roman statio that had a very important role in the distribution of goods along the main merchant routes, such as the appian way. for the application of integrated survey techniques, and taking into account the budget that might be available, two main approaches are possible [e.g. hesse 1999, urbini et al. 2007]. the first is a state-of-the-art routine survey of an area, where all of the geophysical techniques are used to systematically cover the whole area. this is a very efficient method, but at the same time, it can be very expensive and time consuming. the second approach consists of a preliminary analysis of a large area using only one method (which would be the fastest and the most effective amongst those available); on the basis of the results obtained, other geophysical techniques can be planned, focusing on any areas of anomaly. often, archeological surveys have to be carried out on private terrain over a very short period and with limited access. in the present case, we had only one day to acquire the most meaningful data possible in the surroundings of the excavated area. as a consequence, the second approach was chosen to evaluate the extent of the archeological site and to individuate the main buried structures. this photo-geological analysis identified an area of interest around the excavated site (figure 1), so we performed a preliminary analysis using frequency-domain electromagnetic methodology (fdem). in a second step, the results were thoroughly investigated using ground-probing radar (gpr), along with magnetic methodology. each single technique was carried out within a differential (european geostationary navigation overlay service, egnos) corrected global positioning system (gps) positioning grid. this allowed us to compare and combine the different subsurface geophysical responses, to maximize the significance of the measurements and to reduce the ambiguities associated with each of the geophysical methods. the archeological site the importance of the tres tabernae site arises from its use as an integrated system for the distribution and sorting of goods. in the past, its location was a matter of debate article history received july 28, 2010; accepted november 5, 2010. subject classification: archeology, ground-probing radar, frequency-domain electromagnetic induction, magnetic survey, buried remains. 43 between historians, although today it has been located as between the 58th and 59th km along the appian way. the first mention of tres tabernae appeared in some letters sent by cicerone to attico, between 61 bc and 59 bc. in the acts of the apostles, tres tabernae is also remembered as a small village; in describing the journey of st. paul from pozzuoli to rome (in 61 ad), zosimo mentioned tres tabernae as the place where the emperor massenzio strangled severo cesare [barbetta 2008]. in the tabula peutingeriana, of the 4th century ad, tres tabernae is mentioned as one of the stationes (stop stations) that was built along the appian way. the stationes were fiscal structures that were linked with the sorting of goods and maritime traffic. the statio of tres tabernae has been definitively and strictly linked with the economical activities between the nearby harbors of terracina and anzio and the ancient cities, whereby the goods that were brought in by ships were sorted in the inland localities through the road network. the stationes were usually located at one day of travel from one another, and they were used by travelers to rest and to change horses. the larger and more important structures were known as the mansio, which were also provided with hospitia (hotels), stabula (stalls) and shelter for the means of transport. the towns with these structures were important trading sites, and they assumed great importance because of their harbors (or their nearness to them). the tres tabernae site was classified as a statio, but because of its extent (estimated from the excavated remains), it might also be defined as a mansio [barbetta 2008]. during the 6th century ad, after the longobard invasion, the site was almost abandoned. from the mid-8th century to the mid-9th century ad it expanded and flourished again. a decline then started shortly afterwards, with the reappearance of the disease of malaria. recent archeological investigations have revealed interesting ruins that have been dated from the 1st to the 9th century ad. most of the buildings had public, commercial and functional characteristics. a tract of a paved road was also uncovered, which is perpendicular to the appian way, and which allowed access to the statio. other remains refer to a residential building with mosaic floors [barbetta 2008]. the buildings were made mainly from lava and tuff (from the colli albani volcanic complex), and limestone and travertine. the investigated area is today agricultural terrain, where human activities have produced large modifications and have destroyed the remains of the ancient complex. geophysical methods frequency-domain electromagnetic methodology although fdem has been rarely applied to archeological investigations, as can be seen by the lack of such documentation in the literature, fdem is an easy-toimplement method in the field. despite its low resolution characteristics, it represents a good tool to characterize an area of interest and to plan other more specific geophysical surveys [jordan and costantini 1999]. the application of fdem does not require ground contact, thus it can be used to cover large areas in a short time. fdem is based on the induction of eddy current loops in the ground that can introduce an alternating current flow in transmitter coils. the intensity of the induced eddy currents is directly proportional to the terrain conductivity in the proximity of the loops. the induced current loops generate a secondary magnetic field that is proportional to the value of the primary field. a receiver coil placed at a specific distance from the transmitter can measure the secondary magnetic field both in quadrature (90˚ out of phase) and in in-phase components, with respect to the primary field. the quadrature value is related to the ground conductivity characteristics (in millisiemens per meter), while the in-phase component (measured in parts per thousands or millions) is significantly more sensitive to the presence of metallic objects. measurements can be taken by keeping the coils horizontal (vertical dipole mode) or vertical (horizontal dipole mode). the relative contributions to the secondary magnetic field at different depths of the conductive/resistive contrasts in any buried material is different in these two cases: in the vertical-coil arrangement, the influence of the surface is negligible; whereas in the horizontal mode, it has its maximum contribution. in the case of archeological applications, buried ruins usually contribute through an increase in the resistivity of the geophysical investigation at tres tabernae 44 figure 1. map of the tres tabernae statio and the location (inset) of the areas investigated by these geophysical techniques (photograph from google earth). 45 hosting media [tite and mullins 1970, simpson et al. 2008]. in our field work, we used a geonics em31-mk2 system (operating frequency, 9.8 khz; intercoil spacing, 3.66 m), which offers averaged measures of ground electrical conductivity variations of the first 3-4 m depth (as horizontal dipole mode). the fdem survey was conducted in a nw-se direction, with a continuous acquisition rate of 1 hz (see figure 2b, inset map). the dataset were interpolated using the kriging method [abramowitz and stegun 1972] with a linear variogram. ground-probing radar the gpr method is based on the reflection of an electromagnetic wave due to one or more discontinuities in the dielectric properties of the subsurface media. buried shallow walls and holes generate good contrast in the dielectric properties of the field, which can provide a target for this gpr method [e.g. see basile et al. 2000]. the use of antennas with different frequencies allows the subsoil to be examined at different depths and with different resolution. the use of gpr in archeology has increased over the last decade or so, and it has been well documented [e.g. pérez gracia et al. 2000, conyers 2004, francese et al. 2009]. in this survey, a geophysical survey system inc. sir 10b instrument was used, which was equipped with 400-mhz monostatic antennas. the gpr measurements were acquired along se-nw parallel profiles that were 40-m long and spaced at 1-2 m (figure 3), with an along-the-line sampling of about 100 scans/m. more than 2.7 km of gpr lines were collected, processed (horizontal normalization, frequency band-pass filtering, and gain adjusting) and interpreted. the analysis of the hyperbola diffraction conducted on the profiles led to the determination of an averaged electromagnetic-wave propagation speed of about 0.07 m/ns in both areas. considering the recording range, the effective investigation depth was about 2 m from the surface. interpretation of the results took into account the magnitude and shape of any anomalies, and their alignment and consistency on the parallel and perpendicular lines. gpr three-dimensional imaging is a profitable technique to successfully map buried archeological remains [e.g. piro et al. 2001, seren et al. 2007, francese et al. 2009], although it generally costs a lot in terms of data acquisition and postprocessing elaboration, and it often turns out to be time consuming. nevertheless, this three-dimensional profile analysis can discriminate if a target structure is present or not in a surveyed area. magnetic survey this technique passively measures the near-surface variations in the earth magnetic field, which can reveal contrasts in the magnetic properties of the soil due to induction and remnant magnetization [e.g. breiner 1973, weymount and huggins 1985, aspinall et al. 2008, smekalova et al. 2008]. these magnetic surveys are very effective in the archeological field because the presence of human activities produces alterations in the natural magnetic properties of the ground. moreover, nowadays magnetometers are of ultra-high sensitivity, and they can have very fast acquisition rates. in archeological investigations, vertical gradiometric arrangements are the most usual measurement configurations (as a rule of thumb), and they consist of the simultaneous measurement of the earth magnetic field through two sensors that are geophysical investigation at tres tabernae figure 2. map from the fdem survey. (a) quadrature and in-phase components along profile a-a'. (b) map of the quadrature component. inset, fdem measurement profiles. (c) map of the in-phase component. m il li s ie m e n s p e r m e te r (m s /m ) p a rt s p e r th o u s a n d s (p p t) m il li s ie m e n s p e r m e te r (m s /m ) p a rt s p e r th o u s a n d s (p p t) positioned with a fixed vertical distance between them. due to the possibility of collecting high-density measurements, this magnetic method provides good resolution for the identification of many buried anthropogenic structures, such as buried walls, pottery, bricks, fire pits, pathways, and shallow tombs, and numerous others buried objects. in the present investigation, the magnetic data were collected using a geometrics g858 optical pumped cesium magnetometer in a vertical gradiometric configuration (two sensors mounted on a staff at 30 cm and 90 cm from the soil). the magnetic data were acquired along nw-se oriented lines, with a continuous acquisition rate of 1 hz (figure 4). the quality of the collected data was good enough to avoid any specific noise canceling processes, and they were thus grided using the kriging method (linear variogram) [abramowitz and stegun 1972]. main results this integrated geophysical survey that was carried out on the archeological site of tres tabernae covered a total surface area of about 1.4 ha. all of the maps reported here are in universal transverse mercator (utm) projection (zone 33n, datum wgs 84). the preliminary fdem measurements were analyzed along each single profile and on a two-dimensional map representation, after spatial interpolation. the single profiles indicated a strong anomaly in both the quadrature and the in-phase components of the secondary electromagnetic field. in figure 2, an example of these anomalies is shown: the red diamonds represent the measured quadrature values, while the blue diamonds represent the in-phase component. the shape and the extension of the anomaly was typically related to the presence of a buried metallic geophysical investigation at tres tabernae 46 figure 3. map from the gpr survey. (a) example of the anomaly consistency along three profiles (c12, c13, c14). (b) planimetry of the main gpr anomalies. 47 pipe, at about 1 m deep. indeed, this affected all of the profiles, providing a well-aligned structure in a ne-sw direction, perpendicular to the appian way. observing the map (figure 2b), it is also possible to note two areas with relative high resistivities, which can thus be possibly ascribed to the presence of buried ruins (figure 2b, black dashed boxes). due to the introductory aim here, we focused the surveys with the other techniques in the northern parts, to determine the role of the pipe in the archeological context, even though there were also some interesting rectangular conductivity anomalies in the southern part. moreover, a transition zone from an area of high to low resistivity (in the western part) is clearly visible also. the radar profiles were carried out in two sub-areas (figure 1), which were defined considering the results from the fdem survey, and they were perpendicular to the anomaly alignment. strong signal attenuation affected the gpr measures because of the soil characteristics (agricultural terrain) and conditions (wet). an example of the interpretation of the radargrams is shown in figure 3a, where three adjacent profiles are shown (c12, c13 and c14). one clear hyperbolic anomaly occurs in each profile (yellow box), which is consistent across the profiles according to its phase changing, shape intensity and position (yellow dotted line). the migrated data allowed this anomaly to be ascribed to the presence of a pipe (ø 0.20 ± 0.02 m) at a depth of about 0.7 m to 0.9 m. its position also confirms the data from the fdem survey. moreover, the change in the signal phase of 180˚ allows it to be assumed to have a metallic nature. observing the map (figure 3b), the pipe anomalies show very good alignment, spreading out to a distance of 46 m from the origin of the grid (figure 3b, 0 line). at the same time, these anomalies disappear where the fdem map shows an abrupt change in the ground conductivity that leads to strong signal attenuation. other evident anomalies (figure 3a, green boxes) can be ascribed to the presence of buried wall remains (0.40 m thick), while the adjacent intense plane reflections (figure 3a, orange dotted line) can be related to the remains of a paved floor. the depth of the burial ruins varied from about 0.60 m for the tops of the walls, to about 1 m for the floors. the map in figure 3b shows the extension and the location of all of the main recognizable anomalies from the gpr lines. of course, the coarse line spacing hampered the very accurate reconstruction of the buried structures, although it was very effective in revealing them. most of the anomalies referable to these buried structures are located into the first 30 m from the appian way. geophysical investigation at tres tabernae figure 4. map from the vertical magnetic gradient. the map of the vertical gradient of the magnetic field is shown in figure 4. this mainly covers the area defined by the gpr technique. the whole magnetic dataset was elaborated such as to emphasize the low intensity magnetic anomalies. looking at the final map (figure 4), an anomaly alignment in the ne-sw direction can be seen; the position of this confirms the hypothesis of the presence of the pipe suggested by both fdem and the gpr method. the low intensity of the anomaly excludes its ferromagnetic origin, which might instead be related to a coating that has a different magnetic susceptibility compared to the ground. other magnetic anomalies aligned in nw-se and ne-sw directions are clearly visible in the map, with their consistency suggesting the presence of the remains of a buried wall. figure 5 shows an overlay of the results coming from these three different methods, with the maps overlaid on the basis of their differential gps coordinates and on the coincidence of some common control points. the first important thing that is clearly evident in figure 5 is that the anomalies from these different methods that relate to the pipe are perfectly superimposed. the density of the buried structures (from the gpr and magnetic methods) was higher where the fdem survey showed higher relative resistivity values, confirming that fdem as a very good survey-focusing tool. moreover, the gpr profiles allowed the determination of some of the main characteristics of the pipe (diameter and depth), while its actual nature and extent can be evaluated by combining the fdem and the magnetic data. considering all of the geophysical information, we can hypothesize that the pipe was made of lead (a nonferromagnetic metal), which was generally used for piping of water in antiquity. the calculated diameter from the gpr measurements is about 0.20 m, and based on these characteristics, it is possible to ascribe the pipe to the centenaria type, the diameter of which was 0.21 m [adam 1984, mantelli and temporelli 2007]. conclusions in the framework of the mercator project, an integrated geophysical survey was carried out at the site of tres tabernae (latina, central italy). as it is well known, geophysical methods for subsoil investigation are indirect, and are consequently affected by various levels of uncertainty. in this framework, the comparison of the different data represents an important point for the formulation of any final hypothesis. this survey provided very good results, even if it was planned to be only preliminary and exploratory. indeed, because of the many limitations, there was only one day available to acquire the most meaningful data in the surroundings of this previously excavated area. the integration of the fdem and the gpr and magnetic techniques indicates the presence of buried structures belonging to the tres tabernae statio. moreover, the survey suggested that the information provided by each single technique was incomplete for the formulation of a proper hypothesis of the investigated site, with the determination of the pipe characteristics (diameter, depth, nature and extension) being a clear example of this. through this geophysical survey, new elements have been added to our knowledge of the extent of this archeological site. the anomalies indicated structures that can be referred to buried walls and floors, and in particular to the lead pipe. the proximity of these structures to those already excavated suggests the hypothesis of buildings (such as factories) referable to the hospitia. the possible presence of a centenaria pipe, which was probably carrying water from the lepini mounts [barbetta 2008], would suggest that it was used for conducting water for the whole site and for the geophysical investigation at tres tabernae 48 figure 5. map of the overlay of the fdem and the gpr and magnetic methods. inset, as for figure 1, for direct comparison. m il li s ie m e n s p e r m e te r (m s /m ) n a n o t e s la p e r m e te r (n t /m ) 49 thermal bath. based on this new evidence, tres tabernae can now be classified as a mansio rather than as a statio. encouraged by these good results, we would like to note that even an organized exploratory survey (five people at work for one day) has the chance to add useful information to an archeological task. moreover, we hope that it will be possible in the future to find a way to plan for a very focused, high-resolution exploration of this area, and to start some small and cost-effective excavation campaigns. references abramowitz, m. and i. stegun (1972). handbook of mathematical functions, dover publications, new york, 184 pp. adam, j.p. (1984). l'arte di costruire presso i romani, materiali e tecniche, longanesi & co., milan, 369 pp. aspinall, a., c. gaffney and a. schmidt (2008). magnetometry for archaeologists, altamira press, lanham, 224 pp. barbetta, s. (2008). il sistema dei trasporti nel territorio pontino nell’antichità, in: mercator: le vie dei mercanti, le città dei mercati nel mediterraneo, proceedings of the symposium «strutture e insediamenti antichi e medievali funzionali alla viabilità commerciale terrestre e marittima», edited by l. de maria and a. toro, pieraldo editore, rome, 201-221. basile, v., m.t. carrozzo, s. negri, l. nuzzo, t. quarta and a.v. villani (2000). a ground-penetrating radar survey for archaeological investigations in an urban area lecce (italy), j. appl. geophys., 44, 15-32. breiner, s. (1973). applications manual for portable magnetometers, geometrics, sunnyvale, ca., 58 pp. brizzolari, e., f. ermolli, l. orlando, s. piro and l. versino (1992). integrated geophysical methods in archaeological surveys, j. appl. geophys., 29, 47-55. conyers, l.b. (2004). ground penetrating radar for archaeology, altamira press, walnut creek, ca., 203 pp. francese, r.g., e. finzi and g. morelli (2009). 3-d high resolution multi-channel radar investigation of a roman village in northern italy, j. appl. geophys., 67, 44-51. hesse, a. (1999). multi-parametric survey for archaeology: how and why, or how and why not?, j. appl. geophys., 41, 157-168. jordan, t. and d. costantini (1999). the use of non-invasive electromagnetic (em) techniques for focusing environmental investigations, in: ground conductivity meters for environmental site evaluation: selected papers and examples, geonics ltd., 1-30. mantelli, f. and g. temporelli (2007). l'acqua nella storia, francoangeli, milan, 359 pp. pérez gracia, v., j.a. canas, l.g. pujades, j. clapés, o. caselles, f. gracìa and r. osorio (2000). gpr survey to confirm the location of ancient structures under the valencian cathedral (spain), j. appl. geophys., 43, 167-174. piro, s., d. goodman and y. nishimura (2001). high-resolution ground-penetrating radar survey at forum novumvescovio: studying urbanism in the tiber valley, j. roman archaeol., 14, 59-79. seren, s., a. eder-hinterleitner, w. neubauer, k. löcker and p. melichar (2007). extended comparison of different gpr systems and antenna configurations at the roman site of carnuntum, near surf. geophys., 5, 389-394. simpson, d., a. lehouck, m.c. meirvenne, j. bourgeois, e. thoen and j. vervloet (2008). geoarchaeological prospection of a medieval manor in the dutch polders using and electromagnetic induction sensor in combination with soil augerings, geoarchaeology, 23, 305-319. smekalova, t.n., o. voss and s.l. smekalov (2008). magnetic surveying in archaeology. more than 10 years of using the overhauser gsm-19 gradiometer, wormianum. tite, m.s. and c.e. mullins (1970). electromagnetic prospecting on archaeological sites using a soil conductivity meter, archaeometry, 12, 97-104. urbini, s., l. cafarella, m. marchetti, p. chiarucci and d. bonini (2007). fast geophysical prospecting applied to archaeology: results at «villa ai cavallacci» (albano laziale, rome) site, annals of geophysics, 50 (3), 291-299. weymounth, j.w. and r. huggins (1985). geophysical surveying of archaeological site, in: archaeological geology, edited by g. rapp and j. a. gifford, yale university press, new haven, 191-235. *corresponding author: carla bottari, istituto nazionale di geofisica e vulcanologia, sezione roma 2, rome, italy; e-mail: carla.bottari@ingv.it. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geophysical investigation at tres tabernae layout 6 extremely low frequency plasma turbulence recorded by the demeter satellite in the ionosphere over the abruzzi region prior to the april 6, 2009, l’aquila earthquake jan błȩcki1,*, małgorzata kościesza1, michel parrot2, sergey savin3, roman wronowski1 1 space research centre pas, warsaw, poland 2 laboratoire de physique et chimie de l’environnement et de l’espace, université d’orléans, cnrs, orléans, france 3 space research institute, russian academy of sciences, moscow, russia annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5356 abstract the present study analyzes the low frequency fluctuations of the magnetic and electric fields prior to the earthquake with magnitude m. 6.3 on april 6, 2009, in the abruzzi region in italy. this includes the disturbances of the ionospheric electromagnetic field prior to the earthquake, starting two weeks before the event, with detailed analysis for the days of march 26, april 1, april 2, and april 4, as 11, 5, 4 and 2 days prior to the earthquake, when the effects observed were strongest. special attention is given to the characteristics of the spectra of these variations and the search for nonlinear effects. this analysis is possible in the time intervals when the extremely low frequency waveform was transmitted. although the mechanism of the energy transmission from the earthquake preparation zone to the ionosphere is not determined, it can be said that the ionospheric plasma is an unstable medium and that even a small perturbation can lead to variations in the electromagnetic field and plasma parameters. some aspects of this are discussed in the present report. the observations by the demeter satellite that are discussed suggest the presence of plasma turbulence over an area above the abruzzi earthquake that occurred on april 6, 2009. the characteristics of the observed disturbances are studied through wavelet and bispectral analysis, the tools that are most relevant to the analysis of plasma turbulence. furthermore, some statistical features are also presented, including the probability distribution function, and the kurtosis and skewness recently introduced into studies of such turbulence. 1. introduction the electromagnetic effects associated with earthquakes have been discussed for a long time. there are many reports and books that discuss this problem and have published observations that have originated from ground-based observatories and from satellites [fraser-smith et al. 1990, hayakawa and molchanov 2002, parrot 1995, parrot et al. 2006a, pulinets and boyarchuk 2004]. the demeter satellite provides the first possibility for a global study of the electromagnetic pre-seismic emissions using a set of experiments that lead to a more complete view of such phenomena. some results of the observations of the extremely low frequency (elf) turbulence over seismically active regions by the demeter satellite have already been reported [błȩcki et al. 2009, 2010, 2011]. here, we present an analysis of the electric field disturbances in the elf range registered by demeter over the abruzzi region prior to the april 6, 2009, l’aquila earthquake. we discuss the response of the ionospheric plasma for the disturbance that originated with high probability from the lithosphere. the case of this april 6, 2009, earthquake in abruzzi is an excellent example for this kind of study. the demeter satellite flew close to the epicenter of the earthquake many times when in burst mode (high bit-rate data acquisition), and the geomagnetic conditions were very quiet during a two-week period prior to the earthquake (kp <3). there were also no external sources of disturbances. under these circumstances, we can study the ionospheric disturbances over the seismic area and assume with high probability that the perturbations are generated by some factors that have their source at the hypocenter of the earthquake. the possible mechanism of the energy transport from the ground to the ionosphere, and the source of these disturbances, are not discussed in this report. the mechanism that could perturb the ionosphere during the preparation for an earthquake are related to the redistribution of charges at the earth surface, the emission of radioactive gas (radon), and/or the propagation of acoustic gravity waves. these mechanisms have been studied previously in the literature [see for example, pulinets and boyarchuk 2004, and references therein]. the ionospheric plasma can be very easily disturbed, and these disturbances can reach the nonlinear stage that can lead to the development of turbulence. in the ionosphere and special issue: earthquake precursors 37 article history received august 8, 2011; accepted november 29, 2011. subject classification: plasma turbulence, ionospheric disturbances, earthquakes under magnetically quiet conditions, this turbulence is generally not observed at mid-latitudes [li 2007]. the coverage of the data gathered in the burst mode for this event were good. the waveform registered in this mode of operation allows wavelet, bispectral and statistical analysis to be performed, which are relevant to a study of the properties of the observed disturbances of the ionospheric plasma (see section 3) over the abruzzi region. we discuss the turbulence over this seismic event. but what is this turbulence? this question has no clear answer. the definition of the turbulence in fluids, gases and plasma is still under discussion, although some essential features can be noted: many degrees of freedom (different scales), all of which are in nonlinear interactions (cross-scale couplings), which creates a cascade of energy from larger (lower frequencies) to smaller (higher frequencies) sizes. the main characterizations of turbulence are related to the shape of the power spectrum and the high-order spectral analysis [mccomb 1990, frisch 1995, biskamp 2003, zimbardo et al. 2010]. recently, some characterization using statistical descriptions has provided a new approach to this problem. the probability distribution function (pdf) of the measured parameters (in our discussion, the intensity of the electric field) and its parameters, such as kurtosis and skewness, give the new parameterization of the turbulent process [see, for example, verma 2004]. the theory of turbulence predicts a different slope of the spectra for the different types of turbulence: noncompressible turbulence (k-1941) & & k –5/3 kolmogorow noncompressible isotropic mhd (ik-1965) & & k –3/2 iroshnikov-kraichnan noncompressible anisotropic mhd (sg-2000)& & k = –2 sridhar and goldreich whistler turbulence (db-1997) & k –7/3. for the discussed measurements, this parameter is given. section 2 briefly describes the wave experiment that is part of the scientific payload of demeter. the methods of the analysis of the wave form to study the specific character of the electromagnetic emissions registered in the vicinity of the epicenter of this abruzzi earthquake are described in section 3. section 4 is devoted to the presentation and interpretation of these data from this event, and our conclusions are presented in section 5. 2. the experiment demeter is a low-altitude satellite (710 km) that was launched in june 2004 into a polar and circular orbit, to measure electromagnetic waves all around the earth, except in the auroral zones. in december 2005, the altitude of the satellite was decreased to 660 km. the elf/vlf (extremely low frequency/very low frequency) range for the electric field is from constant field up to 20 khz (elf up to 1.25 khz). there are two scientific modes: a survey mode where the spectra of one electric and one magnetic component are computed onboard up to 20 khz, and a burst mode where in addition to the onboard computed spectra, the waveforms of one electric and one magnetic field component are recorded up to 20 khz. the choice of the component is carried out by telecomand sent from flight control center. the burst mode allows the performing of spectral analyses with higher time and frequency resolution. details of the wave experiment can be found in parrot et al. [2006b], berthelier et al. [2006] and cussac et al. [2006]. during the burst mode, the waveforms of the six components of the electromagnetic field are also recorded up to 1.25 khz. this allows the performing of wave propagation analysis [santolík et al. 2003]. 3. methods of analysis 3.1. wavelet analysis traditional fourier analysis is not relevant for the study of turbulence. the fourier transform spreads information about the localized features over all of the scales, making it impossible to study the evolution of different scale structures simultaneously. the important property of wavelet transform is that the square of the wavelet coefficients can be interpreted as local energy and their statistics are easy to visualize and understand. the usefulness of wavelet analysis in the study of turbulence was underlined by farge [1992] in the context of coherent structures. the main advantage of using wavelet transform is that it preserves the information about the local features (e.g. singularities) of the signal, and allows reconstruction of the signal over a given range of scales. this property is of particular importance in the study of turbulence, which often shows coherent structures that are apparently related to nonlinear processes. extensive discussion of wavelet transform and its applications to turbulence can be found in a number of books and review articles [farge et al. 1996, mallat 1998]. applications of wavelet analysis to study turbulence in space plasma were discussed by wernik [2002, 2005]. furthermore, we use the complex morlet wavelet, which is represented by the function of time t and central frequency w0. a more detailed description of these methods of analysis can be found in błȩcki et al. [2007, 2009]. the definitions of the calculated functions and the software description is contained in the manual of the swan package [lagoutte et al. 1999]. 3.2. bispectral analysis when we discuss the development of plasma turbulence and the cascade of the energy in the spectrum, the first step in this cascade and the fundamental process that is involved is the 3-wave interaction. the resonance conditions for these processes are: ~1+�~2 = �~3, k1+k2 = k3, where ~1, ~2 and ~3 are the wave 38 błȩcki et al. 39 frequencies, and k1, k2 and k3 are the wave vectors of the interacting waves. verification of these conditions is possible using the so-called bispectral analysis. this method for the study of plasma processes was first proposed by kim and powers [1978]. it allows the nonlinearly interacting wave modes to be found by computing the bispectrum of the signal, which gives the information about the phase coherence of these waves. a quantitative measure of the phase coherency can be obtained using the bicoherence spectrum the computer procedures for the application of methods of wavelet and bispectral analysis have been developed in the swan package [lagoutte et al. 1999]. we used these for the processing of the data selected for this study. the first use of bispectral analysis for space plasma was given in tanaka et al. [1987]. these methods of analysis were applied in our earlier study of nonlinear processes in the magnetospheric cusp [błȩcki et al. 2007]. 3.3. statistical description of the turbulence measuring the electric field intensity with high time resolution (2.5 khz sampling frequency) allows the construction of the probability distribution function of the deviation of the electric field intensity from the mean value for the selected time interval. the character of this function indicates whether the phenomenon has random character with a pdf that is close to gaussian, or it is intermittent and asymmetric, which is typical for turbulence. the useful parameters to study its character are skewness and kurtosis. the skewness is the third moment of the measured physical value, normalized by the variance: where, x is the measured value (in our case, the electric field intensity), and x denotes its mean value. a pdf that is symmetric about the mean will have zero skewness. all higher odd moments of such a symmetric pdf will also be identically zero. the skewness reveals information about the asymmetry of the pdf. positive skewness indicates that the pdf has a longer tail for x–x>0 than for x–x<0. hence, a positive skewness means that the variable x is more likely to take on large positive values than large negative values. the kurtosis (or flatness) is defined as the fourth momentum of the measured physical value normalized by the variance: a pdf with longer tails will have a larger kurtosis than a pdf with narrower tails. a time series with most measurements clustered around the mean has low kurtosis, a time series dominated by intermittent extreme events has high kurtosis. the kurtosis is the measure of the intermittency. 4. observations the earthquake discussed took place on april 6, 2009, at 01:32:41.4 ut in central italy, in the abruzzi region. it had a hypocenter located at 42.38˚n and 13.32˚e. its depth was 2 km, and the magnitude was mc6.3. the magnitude was not particularly strong, but the effects were unexpected: l’aquila, the town closest to the epicenter, was in a great part destroyed. the period of two weeks before the shock was geomagnetically very quiet (kp index #3–). the data cover from the satellite demeter was very high, with full registration of the waveform of the electric and magnetic field variations in the elf/vlf ranges. this gave the possibility to study the electromagnetic effects in the vicinity of the epicenter of this earthquake at a high scientific level, almost as in the laboratory. an analysis of the elf emissions in the vicinity of the epicenter was performed for the time period starting on march 26, so 11 days before the abruzzi earthquake. an effect of increased intensity was registered even for this day, although the most intensive effects were seen on april 1 (5 days before the earthquake), april 2 (4 days before the earthquake) and april 4 (2 days before the earthquake). for these days, the electromagnetic emissions registered by demeter are presented in detail. figure 1 shows the orbit of the demeter satellite during the observations on april 1. the closest approach to the epicenter was at 20:37:00 ut, and the distance between the projection along the vertical line of the demeter position on the ground and the epicenter was 60 km. the wavelet spectrogram of the electric field in the elf frequency ( ) ( ) s x x x x /2 3 3 2= ( ) ( ) k x x x x 2 4 2= elf plasma turbulence over l’aquila figure 1. map showing the epicenter of the april 6 l’aquila earthquake (*) and the orbit of the demeter satellite on april 1, 2009. the point of closest approach is a larger distance from the epicenter (677 km). błȩcki et al. 40 figure 2. (a) wavelet spectra up to 1250 hz of the electric field taken by the demeter satellite on april 1, 2009, during burst mode, between 20:37:38 and 20:37:39 ut. the color scale represents the log of the power density of the electric field variations. (b) bispectrum of the ex signal in the elf range. the horizontal and vertical axes of the frequencies of the waves are considered to find the interacting modes. the color scale represents the value of the bicoherence as a function of the frequencies. two regions of strong 3-wave interactions can be seen: one in lower frequency range, and the other between 150 hz and 400 hz. (c) single spectrum with slope ac –1.65. (a) (b) (c) 41 range during a burst mode between 20:37:38 and 20:37:39 ut are presented in figure 2a. a strong enhancement of the wave activity and intensity of the waves is seen around the center of this time interval. the characteristic enhancement of the intensity began at low frequency and developed into the entire frequency range. the time resolution given by the wavelet analysis reveals the detailed structures of the spectrum and its evolution from lower frequency (5-10 hz) to higher frequency around 20:37:38.535 ut, when demeter was very close to the epicenter. the strongest emissions are in a frequency range from a few tens of hz up to 200 hz, which is well below the proton gyrofrequency, the frequency with which the protons perform their rotation around the magnetic field line. this lower frequency band might correspond to the gyrofrequency of the oxygen ions o+, which is of the order of 34 hz. the bispectrum for this disturbance is given in figure 2b. this indicates a very strong significant 3-wave interaction that is clearly seen in the two frequency ranges: one in the lower frequency range, and another between 150 hz and 400 hz. figure 2c shows the example of the single spectrum. the slope of this is about –1.623, which corresponds to the kolmogorov model of the turbulence. figure 3a shows the pdf, and the distortion of this plot can be seen in relation to a gaussian distribution. the parameters of the distribution, as kurtosis and skewness, are presented in figure 3b. strong enhancement of these moments were seen at the time of the wave activity increase. elf plasma turbulence over l’aquila figure 3. (a) probability distribution function of the electric field intensity variations for the same time interval as in figure 2. (b) evolution of the kurtosis and skewness of the pdf between 20:37:32 and 20:37:42 ut. (b) (a) figure 4. map showing the epicenter of the april 6 l’aquila earthquake (*) and the orbit of the demeter satellite on april 2, 2009. the point of closest approach from the epicenter was 60 km. this shows the intermittent character of the process, which is a sign of the turbulence. the next day, similar strong emissions in the area of the abruzzi earthquake were registered by the demeter satellite:april 2, 2009: 4 days before the event. the orbit of the demeter satellite during this observation on april 2 is given in figure 4. the closest approach to the epicenter was at 21:05:30 ut and the distance between the projection błȩcki et al. 42 figure 5. (a) wavelet spectrogram of the broad-band emission in the elf range at a greater distance from the epicenter (677 km), although still in the zone of seismic influence, in terms of a dobrovolsky definition. the color scale is as in figure 2. (b) bispectrum of the ex signal in elf range. for description, see figure 2b. the pattern of the frequency regions with strong 3-wave interactions is more complicated than for april 1, although a clear signature of the turbulent cascade is seen in the lower part of the panel. (c) single spectrum with slope ac –1.64. (a) (b) (c) 43 along the vertical line of the demeter position on the ground and the epicenter was much greater, at 677 km. however, the most interesting recordings were obtained at 21:05:34 ut. figure 5a shows the wavelet spectrogram of the electric field in the elf frequency range during a burst mode between 21:05:34 and 21:05:35 ut. figure 5b shows the bispectrum of the ex signal in the elf range. the pattern of the frequency regions with strong 3-wave interactions is more complicated than for april 1, but a clear picture of the turbulent cascade is seen in the lower part of figure 5b. the cascade of the energy appears in the figure 5b as an elongated ‘red island’ parallel to the horizontal frequency axis, from a few tens of hz up to 250 hz, the frequency where the whistlers are particularly enhanced up to highest frequency. figure 5b shows us that the energy from the lower frequency was transferred to the higher frequency during the development of the turbulence. figure 5c contains a single spectrum, and again the slope ac–1.639 indicates the developed kolmogorov type of the turbulence the strongest emissions are focused in a frequency range from a few hz up to 350 hz, which is well below the proton gyrofrequency. the wavelet analysis underlines a wave band with a strong intensity at around 40 hz. this frequency band might correspond to the gyrofrequency of the oxygen ions o+, which is of the order of 34 hz. the next day with intensive elf emissions was april 4. the orbit of the demeter satellite is shown in figure 6. the closest approach to the epicenter took place at 20:29:00 ut, and this was 125 km. however, the most interesting event appeared at 20:28:32 ut when the distance between projection of the demeter position onto the ground and the epicenter was 280 km. figure 7a shows the wavelet spectrogram for the time interval of 20:28:30 to 20:28:40 ut. the short duration of the enhancement is seen at 20:28:33 ut. the bispectrum calculated for a 1-s interval around this disturbance shows a strong interaction around the frequency of 200 hz, and in band 500 hz to 700 hz in the lower part of the frequency band, and again the cascade from 100 hz up to 1000 hz elf plasma turbulence over l’aquila figure 6. map showing the epicenter of the april 6 l’aquila earthquake (*) and the orbit of the demeter satellite on april 4, 2009. the point of closest approach from the epicenter was 125 km. figure 7a. wavelet spectrogram of the broad-band emission in the elf range recorded on april 4, 2009. the distance of the footprint of the satellite to the epicenter was about 280 km. the color scale is as in figure 2. (figure 7b). the single spectrum suggests the kolmogorov type of the turbulence. figure 8 shows the data of the analysis of the elf signal propagation during the time interval between 20:27:32 and 20:31:30 ut. the two upper panels in figure 8 show the spectrum of the magnetic and electric field fluctuations. the next two panels in figure 8 give the information on the polarization of the waves, which is in agreement with the polarization of the electron whistlers at higher frequencies. as usual at lower frequencies, the proton whistlers have a frequency band that is limited by the proton gyrofrequency (given by a model of the earth magnetic field), and they have opposite polarization, as it is seen in figure 8. proton whistlers are usually generated by the same lightning strikes as the electron whistlers [stefant 1985], although for this event, the most powerful electron whistlers are not associated with the most powerful proton whistlers, as can be seen in the next two panels of figure 8. the changes in the atmospheric electric field during the preparation phase of the earthquake can be a source of similar emissions. these panels in figure 8 show the direction of propagation relative to the earth magnetic field [santolík and parrot 1999]. the fifth panel in figure 8 shows the angle between the magnetic field and the poynting vector. it can be seen that this angle is close to the 180˚ for the proton whistlers, which means that the wave propagation is opposite to the magnetic field direction, and for the northern hemisphere, that the wave is coming from a region below the satellite. this information is confirmed in the bottom panel of figure 8, where the z-component of the poynting vector is shown. its negative value also indicates that the origin of the proton whistlers is below the satellite orbit and that the origin of the electron whistlers is from the southern hemisphere. this confirms that the natural waves are coming from the ionosphere below the satellite, and are probably generated by the ionospheric effects that have a source in the processes associated with the preparation for the earthquake. błȩcki et al. 44 figure 7b,c. (b) bispectrum of the ex signal in the elf range. for description, see figure 2b. the pattern of the frequency regions with strong 3-wave interactions is more complicated than for april 1. a clear picture of the turbulent cascade is seen in the lower part of the panel. (c) single spectrum with slope ac –1.60. (b) (c) 45 5. conclusions in the present study, the electromagnetic effects observed by the demeter satellite prior to the earthquake in central italy have been presented and discussed. the analysis of the wave forms in elf frequency range with the wavelet and bispectral methods shows strong emissions in this frequency range in the ionosphere 5, 4 and 2 days before the earthquake. the results discussed were obtained during a very quiet time, and therefore no ionospheric and magnetospheric sources of perturbation were expected. however, these turbulence behaviors are not specifically related to the occurrence of earthquakes, and can be seen in other regions of the ionosphere, particularly at equatorial and high latitudes [molchanov 2004, li 2007]. however, the elf plasma turbulence over l’aquila figure 8. detailed propagation analysis of a part of the elf signal registered on april 4, 2009, as a function of time. top to bottom: the two first panels show the spectrum of the magnetic and electric field variations (sum of the 3 components), the third panel shows the ellipticity of the magnetic field polarization. a value of 1 means right-hand circularly polarized waves; linear polarization gives 0; and −1 indicates left-hand circular polarization. the fourth panel is the sense of the polarisation of the waves, and the fifth panel indicates the angle between the poynting vector and the earth magnetic field, as calculated by the singular value decomposition technique [santolík et al. 2003]. the bottom panel shows the z-component of the poynting vector. empty areas in the four bottom panels correspond to magnetic power-spectral densities below 10−7 nt2 hz−1. in each panel, the black line is related to the proton gyrofrequency. the geomagnetic latitude and the magnetic local time (mlt) are displayed at the bottom of the figure according to the universal time (ut). coincidence in space and time of the recorded disturbances with this earthquake suggests that the effects observed at the mid-latitudes are related to a perturbation of the ionosphere that might be associated with the preparation for this abruzzi earthquake. the statistical behavior of the signal (intermittent) and the shape of the spectra, suggest that turbulence observed during this event is of the kolmogorov type. acknowledgements. this study was supported by the centre national d’etudes spatiales (cnes) and the grants mnisw n n307 101935 and 2011/01/n/st10/07293 (mk). it is based on observations with the electric field experiment ice and the magnetic field experiment imsc embarked on demeter. the authors thank j. j. berthelier, the pi of the electric field experiment, for the use of the data. references berthelier, j.j., m. godefroy, f. leblanc, m. malingre, m. menvielle, d. lagoutte, j.y., brochot, f. colin, f. elie, c. legendre, p. zamora, d. benoist, y. chapuis, j. artru and r. pfaff (2006). ice-the electric field experiment on demeter, planet. space sci., 54, 456-471. biskamp d. (2003). magnetohydrodynamic turbulence, cambridge university press, cambridge. błȩcki, j., s. savin, m. parrot and r. wronowski (2007). nonlinear interactions of the low frequency plasma waves in the middle-altitude polar cusp as observed by prognoz-8, interball-1 and cluster satellites, acta geophys., 55, 459-468. błȩcki, j. , m. parrot and r. wronowski (2009). can the ionospheric plasma turbulence be a precursor of the earthquake? results of demeter measurements, in: proceedings of esa swarm science workshop, potsdam 24-26 june. błȩcki, j. , m. parrot and r. wronowski (2010). studies of the electromagnetic field variations in elf frequency range registered by demeter over the sichuan region prior to the 12 may 2008 earthquake, int. j. remote sens.; doi: 10.1080/01431161003727754. błȩcki, j., m. parrot and r. wronowski (2011). plasma turbulence in the ionosphere prior to earthquakes, some remarks on the demeter registrations, j. asian earth sci.; doi: 10.1016/j.jseaes.2010.05.016, 41, 450-458. cussac, t., p.m.a. ultre-guerard, f. buisson, g. lassallebalier, m. ledu, c. elisabelar, x. passot and n. rey (2006). the demeter microsatellite and ground segment, planet. space sci., 54, 413-427. farge, m. (1992). wavelet transforms and their applications to turbulence, annu. rev. fluid mech., 24, 395-457. farge, m., n. kevlahan, v. perrier, and é. goirand (1996). wavelets and turbulence, p. ieee, 84, 639-669. fraser-smith, a.c., a. bernardy, p.r. mcgill, m.e. ladd, r.a. helliwell and o.g. jr. villard (1990). low frequency magnetic field measurements near the epicenter of the ms 7.1 loma prieta earthquake, geophys. res. lett., 17, 1465-1468. frisch, u. (1995). turbulence. the legacy of a.n. kolmogorov, cambridge university press, cambridge. hayakawa, m. and o.a. molchanov (2002). seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling, terra scientific publishing company terrapub, tokyo. kim y.c. and e.j. powers (1978). digital bispectral analysis of self-excited fluctuation spectra, phys. fluids, 21, 1452-1453. lagoutte, d., j.y. brochot and p. latremoliere (1999). swan software for waveform analysis, analysis tools version 2.3, lpce/ni/003.d part 1, part 2 part 3. li, f. (2007). etude dans l’ionosphère de la densité électronique et de la turbulence électrostatique en fonction de l’activité séismique, phd thesis, university of orléans, france. mallat, s. (1998). a wavelet tour of signal processing, academic press, san diego. mccomb, w.d. (1990). the physics of fluid turbulence, oxford science publications, oxford. molchanov, o. a. (2004). on the origin of lowand middle latitude ionospheric turbulence, phys. chem. earth, 29, 559-567. parrot, m. (1995). electromagnetic noise due to earthquakes, in: handbook of atmospheric electrodynamics, edited by h. volland, crc press, boca raton, vol. ii., pp. 95-116 parrot, m., j.j. berthelier, j.p. lebreton, j.a. sauvaud, o. santolík and j. błȩcki (2006a). examples of unusual ionospheric observations made by the demeter satellite over seismic regions, phys. chem. earth, 31, 486-495. parrot, m., d. benoist, j.j. berthelier, j. błȩcki, y. chapuis, f. colin, f. elie, p. fergeau, d. lagoutte, f. lefeuvre, c. legendre, m. leveque, j.l. pincon, b. poirier, h.c. serana and p. zamora (2006b). the magnetic field experiment imsc and its data processing onboard demeter: scientific objectives, description and first results, planet. space sci., 54, 441-455. pulinets, s.a. and k.a. boyarchuk (2004). ionospheric precursors of earthquakes, springer, new york. santolík, o. and m. parrot (1999). case studies on wave propagation and polarization of elf emissions observed by freja around the local proton gyro-frequency, j. geophys. res., 104, 2459-2475. santolík, o., m. parrot and f. lefeuvre (2003). singular value decomposition methods for wave propagation analysis, radio sci., 1010, 38; doi: 10.1029/2000rs002523. stefant, r.j. (1985). comments on the proton whistler generation process, planet. space sci., 33 (3), 333-349. tanaka, y., d. lagoutte, m. hayakawa and f. lefeuvre błȩcki et al. 46 47 (1987). spectral broadening of vlf transmitter signals and sideband structure observed on aureol-3 satellite at middle latitudes, j. geophys. res., 92, 7551-7559. verma, m.k. (2004). statistical theory of magnetohydrodynamic turbulence, phys. rep., 401, 229-380. wernik, a.w. (2002). high-latitude ionospheric plasma turbulence: advanced analysis methods and results, acta geophys. pol., 50, 119-134. wernik, a.w. (2005). application of the wavelet transform to investigation of space plasma turbulence, in: proceedings of the international workshop on applications of wavelets to real world problems, edited by a. hasan siddiqi, s. alsan, m. rasulov, o. oguz and z. aslan, istanbul commerce university publication, istanbul, 49-66 . zimbardo, g., a. greco, l. sorriso-valvo, s. perri, z. vörös, g. aburjania, k. chargazia and·o. alexandrova, (2010). magnetic turbulence in the geospace environment, space sci. rev.; doi: 10.1007/s11214-010-9692-5. *corresponding author: jan błȩcki, space research centre pas, warsaw, poland; email: jblecki@cbk.waw.pl. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. elf plasma turbulence over l’aquila vol48/04/2005def 817 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words numerical modeling – explosive volcanic eruptions – conduit flow – multiphase flow simulation – stratospheric sulfate aerosol 1. introduction and review of basic concepts explosive eruptions are among the most fascinating and complex natural phenomena. at their initiation stage, gas-rich viscous magma experiences a pressure drop (a little as in the uncorking of a champagne bottle but where the liquid would be 104 to 1012 times more viscous, and over-pressure is tens of bars). the exsolution of dissolved gases (mostly water and co2), their thousand-fold expansion and acceleration toward the surface occur in succession. numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales christiane textor (1) (2), hans-f. graf (2) (*), antonella longo (3) (4), augusto neri (3), tomaso esposti ongaro (3), paolo papale (3), claudia timmreck (2) and gerald g.j. ernst (5) (**) (1) laboratoire des sciences du climat et de l’environnement, cea-cnrs, gif-sur-yvette, france (2) max-planck-institut für meteorologie, hamburg, germany (3) istituto nazionale di geofisica e vulcanologia, sede di pisa, italy (4) dipartimento di scienze della terra, università degli studi di pisa, italy (5) centre for environmental and geophysical flows, department of earth sciences, university of bristol, u.k. abstract volcanic eruptions are unsteady multiphase phenomena, which encompass many inter-related processes across the whole range of scales from molecular and microscopic to macroscopic, synoptic and global. we provide an overview of recent advances in numerical modelling of volcanic effects, from conduit and eruption column processes to those on the earth’s climate. conduit flow models examine ascent dynamics and multiphase processes like fragmentation, chemical reactions and mass transfer below the earth surface. other models simulate atmospheric dispersal of the erupted gas-particle mixture, focusing on rapid processes occurring in the jet, the lower convective regions, and pyroclastic density currents. the ascending eruption column and intrusive gravity current generated by it, as well as sedimentation and ash dispersal from those flows in the immediate environment of the volcano are examined with modular and generic models. these apply simplifications to the equations describing the system depending on the specific focus of scrutiny. the atmospheric dispersion of volcanic clouds is simulated by ash tracking models. these are inadequate for the first hours of spreading in many cases but focus on long-range prediction of ash location to prevent hazardous aircraft – ash encounters. the climate impact is investigated with global models. all processes and effects of explosive eruptions cannot be simulated by a single model, due to the complexity and hugely contrasting spatial and temporal scales involved. there is now the opportunity to establish a closer integration between different models and to develop the first comprehensive description of explosive eruptions and of their effects on the ground, in the atmosphere, and on the global climate. mailing address: dr. christiane textor, laboratoire des sciences du climat et de l’environnement, cea-cnrs l’orme des merisiers, f-91191 gif-sur-yvette cedex, france; e-mail: christiane.textor@gmx.de (*) now at: centre for atmospheric science, department geography, university of cambridge, downing place, cambridge cb2 3en, u.k. (**) now at: mercator and ortelius research centre for eruption dynamics, department of geology and soil science, university of ghent, krijgslaan 281, s8, 9000 gent, belgium. 818 christiane textor et al. during an explosive eruption, an over-pressured magmatic foam is generated and fragments in the conduit (e.g., carroll and holloway, 1994; gilbert and sparks, 1998). the resulting fragmented medium accelerates upward generating a multiphase, over-pressured and upward-directed, negatively buoyant, momentumdominated jet. multiphase flows exiting conduits are modelled to be 5-300 times denser than surrounding air. decompression to atmospheric pressure, vigorous entrainment, rapid heat exchange from pyroclasts (about 90% and 60% of them are typically less than 1 mm and 60 µm in size respectively) to the entrained air (on timescale of 1-100 s) and further expansion and lowering of bulk density happen next. this can generate a buoyant eruption column rising up to 10-50 km, from which ash falls out. columns that remain negatively buoyant when the initial source momentum has been exhausted produce collapsing fountains and pyroclastic density currents, which may also source giant volcanic clouds (e.g., sparks et al., 1997; gilbert and sparks, 1998; dartevelle et al., 2002). an eruption column can ascend until the kinetic and thermal energy, plus the latent heat energy release from water vapour condensation, equal the work to be done against the increase in potential energy at a certain height in the atmosphere. ultimately, the column reaches its height of neutral buoyancy, where its bulk density converges to that of air. at this level, either a radial or a parabolically-shaped current is generated (carey and sparks, 1986; bursik et al., 1992a; ernst et al., 1994; rose et al., 2001). most of the ash responsible for widespread dispersal and hazards is released from the eruption column and advected within gravity currents flows (e.g., bursik et al., 1992a,b; sparks et al., 1992; ernst et al., 1996; bonadonna et al., 1998). large particles (>> 1 cm) are ejected along ballistic trajectories, which are decoupled from the fluid motion of the eruption column. ballistic particles are not discussed here (and typically are not specifically considered by the models) because they contribute only a minor fraction to the total volcanic solid mass. unlike small-sized ash, which is suspended by flow turbulence and moves with the fluid, ballistic particles are volumetrically very minor in the sort of intense explosive eruptions considered here (see riedel et al., 2003, for more discussions about ballistic ejection and modelling). the current of volcanic material injected into the stratosphere initially flows laterally under gravity while its leading edge is advected at the local wind speed. the current is then sheared by gradients in both wind direction and speed, diffused by atmospheric turbulence, may be transported and stretched around or in between mesoscale eddies, and is finally dispersed zonally and meridionally by the global circulation within some weeks and months, respectively (see sparks et al., 1997; gilbert and sparks, 1998, for further reviews of physical concepts). eruptions can produce hazardously high local atmospheric concentrations of volcanic ash, acidic gases and secondary particles (e.g., baxter et al., 1999; horwell et al., 2003a,b). ash can be widely dispersed and have detrimental effects on people, their activities and the environment (e.g., lipman and mullineaux, 1981; newhall and punongbayan, 1996; druitt and kokelaar, 2002). the dispersal of the volcanic products, including silicate solid particles, gases (h2o, co2, so2, h2s, hcl, and others) and aerosols condensing from gases in the volcanic clouds can impact upon the environment and human activities. ash is hazardous as it falls on the ground (e.g., roof collapse hazard) where it accounts for 5% of direct casualties and for much of the widespread damage from explosive eruptions. ash is also a hazard in the air where it disrupts international air traffic and airport operations, resulting in substantial loss of revenues for air companies. since 1980, there have been well over 100 reported encounters between volcanic clouds and jet airplanes (including very close calls in about 510% of cases). even minute amounts of volcanic ash far from its source and some days after emissions can substantially damage jet engines (e.g., casadevall, 1994; grindle and burcham, 2002, 2003). that these encounters could not been avoided indicates a need for more accurate, nearreal-time, ash dispersal forecasts that can be communicated more rapidly and effectively (icao, 2000). in addition, volcanic activity leads to manifold effects on atmospheric chemistry and climate. ash settles within hours to weeks and thus does not directly affect global climate. gases re819 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales main much longer aloft and can be dispersed widely and recurrently in the free troposphere (especially in between eruptions) and in the stratosphere (during less frequent but much larger explosive events). there, sulphur-containing gases (mainly so2, and h2s), of special relevance to the atmosphere-climate effects, are oxidatively converted to sulphuric acid, which condenses to form sulphate aerosols on a characteristic timescale of about one to two months in the stratosphere (e.g., bluth et al., 1993; read et al., 1993). stratospheric aerosol residence times are longer, accounting for effects lasting up to a several years before return to background conditions (e.g., wmo, 2003). sulphate aerosols alter the earth’s radiation balance (e.g., stenchikov et al., 1998): they absorb and emit in the near-ir and longwave, causing in situ local heating in the stratosphere and increasing the downward radiative flux, thus resulting in surface warming. aerosols also backscatter part of the incoming solar radiation leading to surface cooling. the net result is a surface cooling counteracting greenhouse warming for the few years before aerosol loadings return to background levels. the presence of volcanic aerosol in the stratosphere alters the chemistry and dynamics of the stratosphere (e.g., pitari, 1993; pitari and rizzi, 1993). for large equatorial explosive eruptions, there is a dynamic feedback on tropospheric circulation from forcing upon differential stratospheric heating, leading to abnormally warm winters over the northern hemisphere continents in the years following the eruption (groisman, 1992; robock and mao, 1992). stratospheric volcanic aerosols can be transported vertically across the tropopause (e.g., ansmann et al., 1996, 1997) and increase the amount and persistence of high-level clouds (sassen, 1992; sassen et al., 1995; lohmann et al., 2003). there is strong evidence that changes in the radiative properties of cirrus clouds can last several years after large explosive eruptions (minnis et al., 1993). stratospheric sulphate aerosols can also serve as sites for heterogeneous reactions, such as those on polar stratospheric clouds, which deplete ozone in the presence of halogens like chlorine or bromine (e.g., michelangeli et al., 1989; hofmann and solomon, 1989; granier and brasseur, 1992; solomon et al., 1996). ozone depletion could be enhanced through direct stratospheric injection of volcanic halogens (prather, 1992), although there is still much debate about how much of these gases actually reach the stratosphere (tabazadeh and turco, 1993; textor et al., 2003b,c). the atmospheric effects of eruptions depend on magma composition (including volatile content and nature), eruption style, and ambient conditions in both the lithosphere and atmosphere. specific volcanoes are capable of contrasting styles of activity depending upon the stage of maturation reached in the magma chamber and conduit system and upon the physical state of the volcanic edifice. volcanic emissions undergo modification processes in the conduit and crater (especially in the case of flooded vents or of magmas coming into prolonged contact with aquifers), in the eruption column and during long-range atmospheric transport. there is also a growing understanding that volcanic clouds have a lot more in common with meteorological clouds than previously realized (e.g., rose et al., 1995; herzog et al., 1998; durant and ernst, 2003; lacasse et al., 2003; textor et al., 2005a,b). prior to 1997-1998, meteorological aspects (with the exception of a consideration of the latent heat effects on cloud rise; e.g., woods, 1993; glaze and baloga, 1996) had been left out of eruption cloud models. small scale processes within the eruption column have a large – albeit indirect – effect upon the atmosphere and climate. ash interacts with hydrometeors and forms mixed phase aggregates during ascent and recirculation within the eruption column (e.g., veitch and woods, 2001; textor et al., 2005a,b). gases condense and freeze on ash, hydrometeors and the aggregates (tabazadeh and turco, 1993; textor et al., 2003b,c). these processes affect residence times, removal rates and the stratospheric injection of gases (i.e., the initial volcanic forcing upon climate). for a recent review on atmospheric effects of volcanic eruptions see textor et al. (2003a). direct observations of conduit processes have not been feasible so far, and key eruptive parameters have not been measured in situ. what we know about the explosive eruption processes occurring below the earth’s surface comes from 820 christiane textor et al. analyses of eruptive products (marti et al., 1999; polacci et al., 2001; klug et al., 2002), and of geophysical signals (vergniolle and brandeis, 1994; ripepe et al., 2001; chouet, 2003), from laboratory experiments (for references see gilbert and sparks, 1998), and from numerical simulations of magma ascent (e.g., melnik and sparks, 2002). liquid and multiphase magma rheology can be estimated in the laboratory for a relevant range of composition, temperature, dissolved volatile contents and strain rates (dingwell, 1998). this provides a basis for understanding the rheological behaviour of viscoelastic magma, and the constitutive equations for use in numerical simulations of magma flow (manga et al., 1998; saar et al., 2001; llewellin et al., 2002a,b). fragmentation experiments are now routinely carried out in apparata, where a magma parcel is pressurized at high temperature and then suddenly depressurised and fragmented. the accelerated particles are collected and their textures compared with those of volcanic particles (martel et al., 2000, 2001; spieler et al., 2003). experiments on gas bubble nucleation and growth in liquids undergoing variable rates of pressure decrease document that non-equilibrium effects control the distribution of volatile components in the liquid and gas phases (navon and lyakhovsky, 1998). similarly, direct measurements of volcanic emissions in the atmosphere are quite rare. however, they can be studied remotely by groundbased, airborne and satellite instruments. lidars constrain aerosol vertical distributions, but until recently they could only operate during the night under clear sky conditions. observations of emissions (mostly so2) during small eruptive events and of quasi-permanent emissions are possible with ground-based or airborne remote instruments like cospec, ftir (e.g., oppenheimer et al., 2002) and doas (e.g., bobrowski et al., 2003) instruments. direct sampling is accomplished by balloon and research aircrafts, flown through the clouds. there are, however, only sporadic measurements, with no global coverage. recent developments in satellite remote sensing have expanded the capability to monitor volcanoes from space (e.g., rose et al., 1995, 2000, 2001, 2003; and lacasse et al., 2003), but satellite data for so2 and ash are currently only useful for strong sources (e.g., bluth et al., 1992). eleven instruments have been deployed on satellites in the past 30 years. most are limited time research instruments without operational capacity, so2 is the only volcanic gas so far monitored operationally by satellite. since the 1970s, there have been crucial advances in understanding the basic physics of explosive eruptions, and their effects upon the atmosphere and climate. these are summarized by sparks et al. (1997) and gilbert and sparks (1998), and by mccormick et al. (1995) and robock (2000), respectively. since the 1990s, there have also been developments, not yet reviewed, in numerical modelling. numerical advances can now be harvested to assess the timedependent dynamics and microphysics of eruption clouds and of their impacts. here we provide an overview of these recent advances. section 2 describes conduit flow modelling. section 3 focuses on numerical simulation of dispersal processes on the mesoscalegamma (2-20 km – some tens of minutes), concentrating on rapid processes in the jet, lower convective regions, and pyroclastic density currents. the terms ‘mesoscale alpha, beta, gamma’, ‘regional’, and ‘global’ originate from the terminology of meteorology. the respective spatial and temporal scales are given in the text. section 4 concentrates on the mesoscale-beta (20-200 km – some hundreds of minutes) concerning the eruption column. the far-field ash dispersion on the mesoscale-alpha (200-2000 km – some tens of hours) and the regional scale is illustrated in section 5. climate effects of eruptions on the global scale are discussed in section 6. there is much yet to explore about the physics of source eruption processes, the meteorology and climate effects of volcanic clouds. multi-scale numerical modelling, and experimentation, together with remote sensing and in situ measurements will play a key role in advancing understanding of these processes. 2. conduit flow modelling volcanic conduits connect the deep regions of magma accumulation inside the crust with the earth’s surface. transit times for magmas 821 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales ascending in conduits and leading to sustained explosive eruptions are typically of the order of minutes, up to 10-20 min. here the physical and chemical processes are so effective that they produce order of magnitude changes in flow variables, and deeply modify the continuum properties of magma through magma fragmentation and production of gas-particle mixtures. the modelling of magma ascent dynamics is currently well developed. typical assumptions include steady, one-dimensional, isothermal, multiphase non-equilibrium flow. this accounts for a relatively large number of factors characterizing real magmas, like their chemical composition (10 major oxides plus 2 major volatile components h2o and co2) and solid content (crystals and eroded rock fragments) (papale et al., 1998; papale and polacci, 1999), to predict the occurrence of fragmentation under dynamic constraints (papale, 1999a; melnik, 2000), and to take into account variable amounts of different pyroclast components originated by the fragmentation process (papale, 2001). non-isothermal flow of one-phase (homogeneous) magma has been considered by buresti and casarosa (1989) and mastin and ghiorso (2000). transient magma flow is considered by turcotte et al. (1990), ramos (1995), and melnik (2000). proussevitch and sahagian (1998) modelled transient conduit flow to investigate the role and dynamics of non-equilibrium nucleation and growth of gas bubbles in silicic magma. a review of pre-1998 work on conduit flow modeling can be found in papale (1998). fig. 1. mass flow-rate as a function of amount and composition of volatiles (h2o and co2) in magma, computed through the model in papale (2001). the bold lines correspond to cases with equal total volatile content of 5, 7, and 9 wt%, and variable co2/h2o ratios. eruptive conditions correspond to rhyolitic magma (composition in papale and polacci, 1999) at 1100 k, no crystals in magma, conduit length 7 km, pressure at the chamber top 175 mpa, vertical conduit with circular cross-section 100 m in diameter, unvesicular ash 200 µm in diameter formed at fragmentation. 822 christiane textor et al. the use of sets of constitutive equations describing phase properties and interphase as well as phase-wall mechanical interactions has allowed investigation of the relationships between ascent dynamics and the multiphase, multicomponent nature of real magmas. these interactions are strongly non-linear, implying that generalization of numerical results should be avoided. the relatively large number of numerical simulations made to date allows some preliminary conclusions to be drawn. one of them concerns the roles of volatiles in the dynamics of magma ascent. magmatic volatiles represent the engine of explosive eruptions, since volatile exsolution leads to over-pressure in magma chambers and cracking of chamber walls, thus initiating magma ascent (tait et al., 1989). it is the combined effect of further exsolution and gas expansion upon depressurisation that leads to magma acceleration, and ultimately to fragmentation and explosive eruption. numerical simulations show that different volatiles can have opposing effects on ascent and eruption dynamics. water is the main volatile component of magmatic gases. dissolved water reduces magma viscosity, and affects ascent dynamics by increasing the eruption mass flow-rate (wilson et al., 1980; papale et al., 1998; dingwell, 1998). co2, the second main volatile component in magmas, reduces mass flow-rate, at least up to co2 mass fractions of 50 wt% of the total volatile content (papale and polacci, 1999), see fig. 1. these different roles are related to the different solubilities of h2o and co2 in the magma under conduit conditions. co2 is poorly soluble; thus, its presence causes gas exsolution at much higher pressure than if h2o was the only volatile species (holloway and blank, 1994; papale, 1999b). once a separate gas phase has developed, it carries a fraction of all the volatile components, and thus reduces the amount of h2o dissolved in magma. increasing carbon dioxide can thus both result in more gas available for expansion and acceleration, thus favouring magma flow, as well as in more viscous magma, thus hindering magma flow. numerical simulations suggest that the latter effect is the dominant one. hence, knowledge of the true magmatic volatile abundances is needed to simulate eruptions. recent modelling of the compositionaldependent saturation surface of multicomponent volatiles in magmas allowed sulphur species to be treated together with water and carbon dioxide (moretti et al., 2003). new insights will be gained from the simulation of magma ascent by including multicomponent volatile saturation. melnik (2000) considered the homogeneous (one-phase) flow of bubbly magma below fragmentation, and the two-phase (separated) flow of gas and particles above fragmentation. the main difference from previous conduit flow models is in the calculated pressure difference between gas and liquid phases. this pressure difference originates from the finite time required for gas bubbles to equilibrate to ambient pressure, and from the high magma viscosity, which acts to increase the equilibration time. gas-liquid pressure difference over an assigned threshold is also used as the magma fragmentation criterion. the coupling between viscosity, acceleration, and fragmentation leads to a dynamic fragmentation criterion, mathematically similar to that in papale (1999a), where fragmentation relates to the transition from a viscous to an elastic rheological response of the stressed magma. thus, fragmentation occurs when the product of viscosity and strain rate exceeds a critical threshold. the threshold that must be exceeded is different in the two models, being dictated by the elastic properties of magma in the viscous to elastic transition model (dingwell and webb, 1989; papale, 1999a), and by the kinetics of gas bubble expansion in the bubble over-pressure model (melnik, 2000). in the latter case, the threshold is a function of bubble over-pressure and thus also of magma expansion and acceleration. in turn, the gas density and volume fraction are not directly related to pressure as in papale (1999a), but also depend on the depressurisation rate along the conduit. the result is that multiple regimes of contrasting eruption intensity (or mass flowrate) are possible for one starting set of eruptive conditions (or input data in the simulations). the appearance of such multiple steady-state solutions requires a pressure drop at the base of the conduit (or at the top of the underlying magma chamber) significantly below the volatile 823 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales saturation pressure, and may explain abrupt transitions in eruptive intensity, sometimes inferred from eruption products. the above studies consider the entire conduit length from the top of a hypothetical magma chamber up to the base of a volcanic crater, above which one-dimensional flow can no longer be assumed. other investigations for researcher focused on specific aspects of magma flow, without solving the entire flow from the deep homogeneous flow regions below gas exsolution to the shallow gas-pyroclast regions above fragmentation up to the conduit exit. massol and jaupart (1999) analysed the crosssectional pre-fragmentation pressure variations in conduits. they documented a decrease in gas pressure from the conduit axis to the walls, implying contrasting depressurisation and degassing histories of distinct magma parcels erupted simultaneously. costa and macedonio (2003) analysed the shear-induced viscous dissipation close to the conduit walls, and found that a large temperature increase is expected due to the high viscosities and velocities of magma discharged during explosive eruptions. their predicted velocity profiles develop a central plug (despite the assumed newtonian magma rheology) and a region of concentrated shear near the conduit walls. these findings account for the contrasting textures of pumice from some explosive eruptions (polacci et al., 2001). future magma ascent modelling advances may benefit from formulation and solution of a transient, multidimensional, multiphase flow model including the full set of mass, momentum, and energy transport equations, as well as constitutive equations approaching the behaviour of real multiphase multicomponent magmas. such a model would allow all of the above complex processes to be accounted for in comprehensive numerical simulations of magma ascent dynamics, and in principle it could be employed to describe the dynamics of the entire system from magma chamber to the surface. coupling with the mechanics of surrounding rocks would allow to take into account both the opening and closing phases of volcanic eruptions, as well as both time-dependent shape variations of magma chamber and conduit walls, and the occurrence of caldera collapse. such a comprehensive model will be the challenge for the researchers focused on subsurface volcanic processes in the next decades. 3. numerical simulation of atmospheric dispersal processes by using multiphase flow models on the mesoscale-gamma (2-20 km – some tens of minutes) in the last 20 years significant progress in understanding of pyroclastic dispersal processes was obtained through development of multidimensional, transient and multiphase flow models. these are able to treat the eruptive mixture as a non-homogeneous fluid (wohletz et al., 1984; valentine and wohletz, 1989; dobran et al., 1993; neri and macedonio, 1996; neri et al., 2003). such an approach is based on the extension of fundamental transport continuum mechanics equations to a multiphase and multicomponent mixture. according to the multiphase flow theory, the volume occupied by a generic gaseous, solid, or liquid phase cannot be occupied at the same position in time and space by the remaining phases. it is from such a distinction that it is possible to introduce the phase volume fraction as a new dependent variable. a multifield approach is defined, according to which different phases are treated as interpenetrating continua. the high particle mass fraction under typical eruptive conditions justifies the continuum assumption for pyroclasts smaller than about one centimeter (larger particles need to be treated with a lagrangian approach). the fundamental mass, momentum and energy conservation equations are now solved for each specific phase. this system needs to be closed by constitutive equations expressing the viscous and interphase transport terms as well as the equation of state of the involved phases. the reader is referred to soo (1967) and gidaspow (1994) for a complete description of these equations. the system of partial differential equations can be solved numerically, using finite difference or finite element methods, on computational domains extending some tens of kilometres in radial and vertical directions and so far for times of several min824 christiane textor et al. utes. the simulation outcome typically consists in the temporal and spatial evolution of several variables – such as concentration, flow field, temperature – for each phase considered. the application of this type of models allowed description of new and somehow non-intuitive, physical processes occurring during explosive eruptions. due to the specific characteristics of these models, and their large demand for computing power, they have been mostly applied to the analysis of relatively rapid, localized, and non-equilibrium processes, such as those occurring in the jet, lower convective regions and pyroclastic density currents, herein also named pyroclastic flows sensu lato, generated by gravitational collapse. processes investigated include the blast dynamics from caldera-forming eruptions (wohletz et al., 1984), transient motion of ascending eruption columns (valentine and wohletz, 1989), interaction between pyroclastic flows and caldera rims (valentine et al., 1992), the formation of co-ignimbrite clouds (dobran et al., 1993), propagation of pyroclastic flows as transient waves (neri and dobran, 1994), unstable behaviour at the transition between fully convective and fully collapsing regimes (neri et al., 2002a; di muro et al., 2004), and multiparticle dynamics of collapsing volcanic column and pyroclastic flows (neri et al., 2003). the models were also used to reconstruct the eruptive dynamics of historic and on-going eruptions. simulations have been compared to direct or indirect observations whenever available. in more detail, the conduit ascent model described in the previous section (papale et al., 1998) was used to determine vent boundary conditions for the multiphase flow model (neri et al., 1998). the linking of two models permitted the reconstruction of the dynamics for the 79 a.d. eruption of vesuvius. the influence of magmatic properties on the eruptive style of two main phases of the eruption (white and grey pumices) was elucidated (neri et al. 2002b). similarly, the multiparticle flow model of neri et al. (2003) was used to simulate the dynamics of transient short-lived vulcanian explosions observed at the soufrière hills volcano, montserrat, west indies, during augustoctober 1997 (clarke et al., 2002). a simplified description of pre-explosive conduit conditions based on observations provided the initial conditions for transient, axi-symmetric, multiphase flow simulations of pyroclastic dispersion. the relationships between the dynamics and the conduit parameters, including volatile content, overpressure, and particle size distribution were examined. model results illustrated that i) observations of explosions were best reproduced using initial conditions that best matched observationally-constrained input data; ii) vent conditions were highly transient, therefore, models assuming steady vent conditions can not be applied to vulcanian explosions; iii) decreasing conduit pressure and increasing magmatic water content have similar effects in that they generally increase plume buoyancy, pyroclastic current runout, dense rock equivalent (dre) ejected and development of ash plumes above pyroclastic currents. some of the multiphase flow models were applied to assess pyroclastic flow hazards in high-risk regions, e.g. in the vesuvian area, italy (dobran et al., 1994; esposti ongaro et al., 2002; todesco et al., 2002). input data were defined based on knowledge of the magmatic system, its eruptive record, as well as using magma ascent modelling. figure 2 shows the time wise distribution of the total particle volumetric fraction and gas flow field along the southern flank for a typical sub-plinian eruption of vesuvius at three different times after onset of collapse. the three plots illustrate the transient nature of the over-pressured jet and rapid propagation of the flow fed by the pulsating fountain. in this specific case, the flow reaches the tyrrhenian sea 7 km away from the crater in about 6 min. hot, dilute, fine ash-laden plumes rise above the vent and flows. important flow variables are quantified, e.g., pyroclastic flow density, temperature, dynamic pressure, etc., that are all crucial in the assessment of the environmental impact. in all these applications model results appeared to be qualitatively, and in some circumstances quantitatively consistent with observations and, in most cases, able to provide quantitative insights into eruption dynamics that cannot readily be obtained by other means. however, there is still much progress to be made. 825 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales fig. 2. distribution of total particle volumetric fraction and gas flow velocity at 90, 300 and 600 s from the beginning of the collapse for a pyroclastic-flow-generating volcanic column at vesuvius. the simulation scale is representative of a sub-plinian type eruption at vesuvius similar to the 1631 eruption. the topographic profile assumed in the simulation is representative of the southern sector of vesuvius. contour levels refer to the exponents to the base 10 of particle concentration and correspond to value of −8, −7, −6, −5, −4, −3, −2, and −1 (modified from todesco et al., 2002). 826 christiane textor et al. firstly, the peculiar nature of volcanic processes requires the formulation of more physicallysound models. for example, the influence of dispersed particles on the turbulence of the carrier phase (turbulence modulation), and particle-particle interactions in the poly-dispersed mixtures need to be more accurately described. secondly, experimental research – both in the laboratory and in the field – is necessary to improve the equations of physical, thermal, and rheological properties, and to better validate the theoretical models. thirdly, the development of more effective numerical techniques, including the use of parallel computing, are necessary to allow the simulation of more realistic events in three dimensions and for longer simulation times. finally, the simulation of the dynamics of larger particles such as lapilli or clasts remains a challenge, because the continuum assumption is no longer applicable (see for instance wilson, 1972; fagent and wilson, 1993; lo savio, 2004). 4. simulation of the eruption column and the distal environment on the mesoscale-beta (20-200 km – some hundreds of minutes) numerical simulations of atmospheric processes on the mesoscale-beta require description of the conditions both within the eruption column and in its local environment. the high velocities, temperatures, particle concentrations, and large gradients in an eruption column complicate the solution of the equation algorithm that describes the eruption dynamics and thermodynamics. conditions in the dispersing plume and background atmosphere are less vigorous, and do not necessitate such a complex algorithm. the comprehensive models described in the previous section are currently much too demanding in terms of computer time and memory to simulate eruption column and advected cloud dispersal on longer time scales or to consider a higher number of column processes, e.g., microphysics or chemistry. various numerical models exist for simulation of larger scale atmospheric flows on the mesoscale-alpha. these are used for simulation of phenomena like air pollution dispersion, atmospheric boundary layer or cloud processes, and also for far-field ash dispersal modelling, as discussed in the next section. yet many of the simplifying assumptions applied in atmospheric models are not valid for the high-energy processes close to the vent and in the rising column. thus models were developed specifically for the simulation of the eruption column and subsequent dispersal, based on novel concepts reviewed hereafter. the first studies of the fluid dynamics of buoyant plumes built upon the seminal work of morton et al. (1956). they derived a simple formula for the first-order estimation of plume height based on the relation between the erupted buoyancy flux, which is associated with the temperature and the mass, and the atmospheric stratification. the morton formula, neglecting the increased stability of the stratosphere, shows a fairly good agreement with observations for the limit case of powerful plumes in comparatively negligible wind (see sparks et al., 1997). the analytical studies of wilson (1976), sparks and wilson (1982), and others provided insights into the structure of eruption columns. time-averaged, gaussian profiles are assumed at each height for quantities like temperature, velocity, and particle concentration. interactions between components of the fluid are neglected; the gasparticle mixture is treated as a homogenous mixture, similar to fine-grained, diluted columns. based on these assumptions, one-dimensional, steady-state models were developed (wilson and walker, 1987; woods, 1988, 1993), which solve the equations of conservation for mass, momentum, and heat, accounting for separate components (e.g., dry air, water vapour, liquid condensates, and solid particles) within the homogenous fluid approximation. turbulent mixing (i.e. entrainment) into the eruption column is determined by an entrainment parameter, which relates entrainment to the upward velocity in the column. such models are suitable for steady state eruptions, e.g., so-called dry plinian eruptions with a low occurrence of liquid water (sparks et al., 1997). they have been useful for first estimations of plume dynamics, like vertical density and velocity profiles, and for understanding transition between collapsing and non827 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales collapsing flows. for comprehensive overviews see sparks et al. (1997), or woods (1998). models of this type have been employed for a variety of applications, like the assessment of the influence of the ambient conditions on the plume height (glaze and baloga, 1996), or for the investigation of extraterrestrial volcanic eruptions (glaze and baloga, 2002). in spite of its quite coarse assumptions, this model type has been employed for important first estimations about the injections of water vapour (glaze et al., 1997) and volcanic gases (tabazadeh and turco, 1993) into the stratosphere. it has recently been used for the investigation of particle recycling and aggregation in volcanic plumes (veitch and woods, 2000, 2001, 2002). the limitations of utilizing a one-dimensional, steady state model for these kinds of questions were discussed in detail by textor and ernst (2004) and by veitch and woods (2004). the dispersion and sedimentation of tephra in the proximal region of the volcano were examined with sedimentation models, which focus on the fallout of ash from the margins of turbulent particle-fluid suspensions. they use simple relationships derived from the work of morton et al. (1956) to calculate the variation of the plume width and speed with height. the growth of the umbrella cloud is also modelled as a function of time (sparks et al., 1997). the dynamics in the eruption column is not explicitly predicted although it relates to first principles and sound experimental insights (e.g., martin and nokes, 1988). microphysical processes are not considered. far-field dispersal was then simulated by others under the assumption that it is dominated by atmospheric motions, and can be described by the turbulent advection-diffusion equation in different degrees of completeness. the effects of different plume geometries, atmospheric stratification, crosswind and re-entrainment, and particle aggregation were investigated in detail. for comprehensive overviews see sparks et al. (1997), or bursik (1998). more recently a further model class became available, in which both the rise of the eruption column from the lithosphere to the stratosphere, and the dispersal of the plume of volcanic particles and gases are simulated. calculations on time scales of hours covering spatial scales of hundreds of kilometres, and the inclusion of a higher number of transported quantities and processes necessitate some simplifications in contrast to the multiphase flow models described in the previous section. this is realized within the concept of the model atham (active tracer high resolution atmospheric model) (herzog et al., 1998; oberhuber et al., 1998; for a different approach see suzuki et al., 2005). the atham model is a non-hydrostatic model solves the full set of navier-stokes equations for the multiphase system. the temporal evolution of the volcanic eruption cloud in the atmosphere is now simulated, including unsteady eruptions. tracers (liquid and solid particles and gases) are not passively transported with the mean flow as in usual atmospheric models, because they can be locally highly-concentrated in the erupted gas-particle mixture. these ‘active tracers’ influence the dynamics by altering the mixture’s density and heat capacity. the system description requires a large set of dynamic and thermodynamic equations for each component and the interactions between them. however, on the condition that all particles are smaller than about a millimetre, it can be assumed that the gas-particle mixture is in thermal and in dynamical equilibrium. under these assumptions, the equations have to be solved for volume mean quantities only (but different tracers can still have different fall velocities). to avoid conflicts with the model assumptions processes in the high pressure, hot temperature regime within and close to the crater cannot be modelled, and vent topography is neglected. however, the inclusion of a higher number of tracers and processes, and longer simulation times are possible. a turbulence closure scheme describes sub-scale processes in the eruption column (herzog et al., 2003). examinations of specific processes and of the parameters controlling them within the rising column and associated ash cloud were carried out. these included investigations focusing on the influence of the ambient conditions upon plume development (graf et al., 1999), on cloud microphysics (herzog et al., 1998), gas scavenging (textor et al., 2003b,c) and ash aggregation (textor et al., 2005a,b). in order to perform numerical simulations, the equation system describing the eruption has 828 fig. 3. eruption column of volcanic ash after 30 min of simulation with atham. the dimensions of the grid are 50 km in the horizontal and 25 km in the vertical direction. three ash size classes are shown (10 µm, 200 µm, and 4 mm in radius), increasingly dark grey shading indicates the increase in size. fig. 4. eruption column of fine volcanic ash and hydrometeors after 30 min of simulation with atham. the dimensions of the grid are 50 km in the horizontal and 25 km in the vertical direction. the dark shading beside the column indicates liquid water. ice and 10 µm ash are indicated by the light grey shading. christiane textor et al. 829 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales to be approximated 1) by discretisation in time and space, and 2) by applying advanced numerical techniques to integrate the system. hence, the representation of sub-grid and simultaneous processes is a difficult issue. the atham model has been validated for the simulation of a biomass burning plume (trentmann et al., 2002). key aspects of simulated explosive eruptions compare favourably to expectations based on first principles. we are currently rigorously evaluating atham eruption simulations against fundamental observations from satellites and ground-based volcanological data. results so far support atham as a robust model. figure 3 shows a simulated eruption column with ash of three different size classes, and fig. 4 illustrates the eruption column of fine ash and hydrometeors from an atham model simulation at 30 min of eruption (herzog et al., 2003). other numerical models focus on the source-dispersal relationship for volcanic gas emissions to the atmosphere. the link between geochemical analyses and remote sensing data helps to better understand the factors, which determine the source strength (e.g., edmonds et al., 2001). a study on the atmospheric lifetime of so2 by oppenheimer et al. (1998) indicated that meteorological and geographic factors, as well as degassing rates, should be considered when interpreting measured so2 fluxes. simulations of time-dependent processes within the eruption column and near-volcano environment at the same time can now be run in three dimensions (herzog et al., 2003). the model results can be compared to observations in order to better understand the interactions of dynamics, turbulence, cloud and particle microphysics, and chemistry for different environmental and volcanic conditions. the parameterisations of individual processes employed in all these different models reflect the current state of knowledge about volcanic eruptions in the atmosphere. sensitivity studies, where a single process or factor is modified, help to understand and better determine the complex interactions. the results from such simulations can then be utilized to design investigations of the most relevant quantities by laboratory and field experiments. in addition, the source strength of volcanic emissions can be estimated for different volcanic and environmental conditions. this information is essential for the assessment of the climate impact of volcanic emissions. a more comprehensive representation of some processes like the microphysics of clouds and ash, or gas adsorption at particle surfaces should be envisaged in the future. the descriptions of volcanic particle properties (e.g., shape, porosity) during atmospheric transport have to be improved in accordance with new observations. the effects of electric fields within the volcanic eruption column have not been tackled by numerical simulation so far, but might be important for ash aggregation (e.g., sparks et al., 1997; james et al., 2000). 5. atmospheric simulation of ash dispersal on the mesoscale-alpha (200-2000 km – some tens of hours), and on the regional scale far-field dispersal is no longer determined by the eruption itself, but by atmospheric motions. this motion can be described by the advectiondiffusion equation, as mentioned above for simulation of the proximal ash transport, often extended with special approximations for turbulent diffusion. the particle dispersion is simulated by a lagrangian or a eulerian formulation of advection, fallout and turbulent dispersion. ash tracking models have been used by various agencies to mitigate volcanic ash hazard. all these models simulate the movement of airborne ash using meteorological field data sets from observations or from other model simulations. these models include among others: lagrangian trajectory volcanic ash tracking model (puff) (http://puff.images.alaska.edu/ index.html); nuclear accident model (name) (http://www.metoffice.gov.uk/research/nwp/ publications/nwp_gazette/dec00/name.html); single-particle lagrangian integrated trajectory (hysplit) model (http://www.arl.noaa.gov/ ready/hysp_info.html); volcanic ash forecast transport and dispersion (vaftad) model (http://www.arl.noaa.gov/ss/models/vaftad.html); hybrid particle and concentration transport model (hypact) (http://www.atmet.com/html/hypact_soft.shtml); canadian emergency response 830 christiane textor et al. model (canerm) (http://www.cmc.ec.gc.ca/cmc/ cmoe/vaac/pph/a-pph.html); trajectory of volcanic materials (trav) and media (http:// www.meteo.fr/aeroweb/info/vaac/). the focus of these models is the forecast of the location of the volcanic ash in space and time. some of these models are operationally used by the nine volcanic ash advisory centres (vaacs) (icao, 2000), which serve as an interface between volcano observatories, meteorological agencies and air traffic control centres in order to prevent hazards to aircrafts. ash tracking models are not able to resolve the eruption column. they have to prescribe the initial spatial distribution of the ash cloud. in particular the cloud height and its variation with different atmospheric conditions is a crucial parameter for accurately forecasting ash dispersion (e.g., tupper et al., 2003). however, it cannot be determined with sufficient accuracy by the observational systems currently in use. hence, a quantitative estimation of the source strength based on volcanological parameters would be highly desirable (chen, 2003). in addition, the ash dispersal models often have too coarse a resolution to accurately resolve smallscale weather phenomena, e.g., thunderstorms, and they do not include particle removal by precipitation, or ash aggregation. wrong forecasts of ash dispersal lead to substantial over or under warning, and thus to enhanced risks and costs to the aviation industry (e.g., casadevall, 1994; grindle and burcham, 2002, 2003; simpson et al., 2002; rose et al., 2003; lacasse et al., 2003). ash emissions at many volcanoes, especially in least developed countries are unchecked and/or not quantified. despite official multilateral agreements, regional vaacs are often informed about eruptions only after the event, when it is no longer hazardous for air traffic (e.g., a. tupper, darwin vaac, pers. comm.). ash emissions represent an increasing threat for international air traffic. 6. global simulations the main objective of simulations on the global scale is to understand the impact of large volcanic eruptions, which reached the stratosphere on atmospheric dynamics and chemistry. to assess the climate impact of volcanic eruptions, the interactions of volcanic sulphate aerosol particles with atmospheric dynamics, chemistry and radiation have to be treated accurately. for the calculation of the mutual feedback mechanisms between these components it is not sufficient to simply calculate the dispersal of the volcanic cloud. instead, detailed representations of the formation and temporal development of the aerosol size distribution are required, which can be obtained in two ways. first, observed global fields of the volcanic aerosol size distribution (extinction, surface area density) can be used. second, the aerosol microphysics processes can be explicitly calculated within the global model. these two approaches are illustrated in more detail below. 6.1. prescribed volcanic aerosol approach atmospheric observations of volcanic aerosols have been collected for only 20 years. they include two major volcanic eruptions (el chichón in april 1982 and mt. pinatubo in june 1991) and a few minor ones (e.g., ruiz 1985; kelut 1990; hudson 1991; spurr 1992; nyamuragira 2001). in particular, the dispersal of the pinatubo cloud and the subsequent changes in the atmospheric system (temperature, trace gas concentration) were detected by in situ airborne and remote sensing measurements (lidar and satellite). global simulations with general circulation models (gcms), which used such observed volcanic aerosol fields, mainly addressed the pinatubo and the el chichón eruptions (e.g., graf et al., 1993; kirchner et al., 1999; ramachandran et al., 2000; stenchikov et al., 2002; collins, 2003; shindell et al., 2003). chemistry climate models (al-saadi et al., 2001; rozanov et al., 2002) and 2d chemical radiative transport models (kinnison et al., 1994; rosenfield et al., 1998) focused on the pinatubo aerosol impact upon stratospheric trace gas concentrations. model studies investigating the climate impact of volcanoes on centennial and decadal time scales use monthly and latitudinal varying 831 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales data sets of volcanic forcing (e.g., sato et al., 1993; andronova et al., 1999; crowley, 2000; robertson et al., 2001) which are based on observations (phyrheliometric measurements of atmospheric extinction, and since 1979 satellite data) and ice core data. recently, amman et al. (2003) published a monthly volcanic forcing data set for climate modelling from 1890 to 1999. aerosol from each event is individually evolved spatially at monthly resolution derived from off-line calculations. these are based on an observed peak aerosol loading and a simple parameterisation for the stratospheric aerosol time evolution and transport. the advantage of using prescribed aerosol is the possibility to thoroughly study the atmospheric effects of eruptions in a computationally efficient way. this approach is, however, highly dependent on the quality and amount of available observations, which are only sufficient for the last two decades. it is difficult to assess the impact of past and future eruptions. the feedback of aerosol-induced radiative forcing and ozone changes on volcanic cloud transport cannot be addressed with the prescribed volcanic aerosol approach. 6.2. prognostic volcanic aerosol approach the formation and temporal development of the aerosol is simulated from the stratospheric input of volcanic gases. this requires detailed information on the timing, the eruption height, the amount of emissions, and the initial spatial dimensions of the volcanic cloud. the usage of a bulk aerosol model is the simplest way to simulate global dispersal. simulations of the pinatubo aerosol using such an approach (young et al., 1994; timmreck et al., 1999) showed that local in situ heating has an important effect on volcanic cloud transport. a number of bulk volcanic aerosol simulations have been performed for historical eruptions. stevenson et al. (2003) studied the chemical impact of the 1783-1884 laki fissure eruption, and graf and timmreck (2001) investigated radiative forcing by the laacher see eruption (10 990 bp). the capability of aerosol simulations using a bulk approach as described above for the assessment of atmospheric effects of volcanic eruptions is however limited by the lack of knowledge about the aerosol size distribution. information on formation and temporal development of the aerosol size distribution and chemical composition are needed for more realistic radiation as well as chemistry calculations. this requires the treatment of aerosol microphysical processes in the global model. the microphysical processes to be considered are the formation of new particles due to binary homogeneous nucleation of h2so4/h2o, condensation and evaporation of h2so4 and h2o, brownian coagulation, gravitational sedimentation, dry deposition and the wet removal of the particles due to incloud and below cloud scavenging. in the last years several simulations of volcanic eruptions with two dimensional chemistry transport models including sulphate aerosol microphysics were published. these studies focus on the impact of present day eruptions (bekki and pyle, 1994; tie et al., 1994a,b; weisenstein et al., 1997) but there exist also studies for past events (bekki, 1995; bekki et al., 1996). chemistry models, which include aerosol microphysics together with the various interactions necessary to completely analyse the climatic impact of volcanic eruptions, are presently still missing. there are, however, first steps in that direction. pitari and mancini (2002) assessed the climate impact of the pinatubo eruption with a global model, in which ozone and aerosols are transported, and aerosol microphysical processes are calculated explicitly. but they used a coupled model system consisting of a chemistry transport model and a global circulation model, so that not all interactions were fully included. timmreck et al. (2003) used a chemistry climate model with interactive and prognostic volcanic aerosol and ozone to study the same eruption. they did not calculate microphysical processes explicitly, but treated aerosol mass as prognostic variable and derive parameters for the aerosol size distribution from the aerosol mass using a parameterisation based on observed correlations between aerosol mass and surface. figure 5 shows the global dispersal of the pinatubo cloud simulated with the chemistryclimate model maecham4 (timmreck et al., 2003). 832 f ig . 5. g lo ba l di sp er sa l of t he p in at ub o vo lc an ic c lo ud ( pp b su lf ur ) at 2 4 km e le va ti on f or f ou r sp ec if ic d ay s du ri ng t he f ir st f ou r m on th a ft er t he er up ti on ( t im m re ck et a l. , 20 03 ). christiane textor et al. 833 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales the advantage of the prognostic volcanic aerosol approach is the possibility to study atmospheric effects of explosive volcanic eruptions on the global scale, independently from existing observations. this gives the chance to perform sensitivity studies and to integrate future scenarios. however, as discussed above, a global model that treats all processes in a sophisticated manner is still missing. information about the emissions of gases and ash, and about the column height is required for the simulation of volcanic eruptions. this information is commonly scarce, especially for past eruptions. simulations with smaller scale numerical models can help to provide initial conditions, as discussed in detail in section 7. in addition, it is worth mentioning that due to the coarse vertical (about 1-2 km) and horizontal resolution (several 100 km2) of global models, the dispersal of the volcanic cloud can only be coarsely represented, and some processes like stratospheretroposphere exchange or streamer transport might be critical points. global prognostic aerosol simulations should not be mixed up with reality, but rather be considered as tools to study and understand the atmospheric effects of explosive volcanic eruptions. since the 1980s and up to recently, atmospheric scientists were exclusively interested in the impact of extreme volcanic eruptions that inject large amounts (1-10 000 mt) of volcanic emissions into the stratosphere and lead to climatic anomalies. now, there is an increasing recognition that, not just large episodic events, but also the many small emissions and prolonged degassing at many volcanoes worldwide can also have an atmospheric (global sulphur budget) and climatic impact (graf et al., 1997, 1998). active volcanoes generally reach considerable elevations and most of their emissions are injected into the free troposphere, above the well-mixed planetary boundary layer (pbl), which typically extends upward from the surface to several hundreds of meters. at high elevations above the pbl, removal processes are slower, and volcanic sulphur has thus longer residence times than anthropogenic sulphur, which is mostly emitted from low elevations. consequently, the radiative effect on the atmosphere of volcanic sulphur is disproportionately larger than that of anthropogenic sulphur. this was illustrated by numerical experiments with atmospheric general circulation models including a simplified sulphur cycle (graf et al., 1997). furthermore, tropospheric sulphate aerosols act as cloud condensation nuclei (ccn) and modify the radiative properties and lifetime of clouds (twomey, 1974). the increase in cloud droplet number concentration due to the ccn enhancement in turn increases cloud albedo, enhancing surface cooling. knowledge of these effects can affect our ability to accurately predict the global climate and its evolution. in the absence of continuous monitoring of volcanic degassing on the global scale, the total amount of volcanic sulphur emissions in the troposphere is highly uncertain and still under discussion (see for details e.g., textor et al., 2003a). a comprehensive dataset of volcanic degassing into the troposphere is needed to improve global models. the ideal dataset would not only include explosive events, which can partly be observed by satellite, but also longterm low-level degassing from craters, lava lakes, lava domes, fumarolic activity and dissipative emissions at the volcano flanks. although some promising studies are available, such a data set is currently missing. the representations of the tropospheric sulphur cycle in global atmospheric models still differ between the models (barrie et al., 2001; lohmann et al., 2001) mainly due to the contrasting complexity of the tropospheric chemistry accounted for. 7. discussion a profound knowledge of the fluxes and species distribution of volcanic emissions is essential for the assessment of the effects of volcanic eruptions in the atmosphere. it is a major challenge to establish a volcanic observation network, which would deliver all information needed to create a data set with complete spatial and temporal coverage on the global scale, because of the high temporal, spatial and intensity variability of volcanic emissions. extreme differences exist between the eruption styles of volcanoes of different types, but also between 834 christiane textor et al. the phases of activity for volcanoes of the same type. these differences aggravate the principal understanding of the volcanic system, and thus the extrapolation of observational data to nonmonitored volcanoes. volcanic emissions are the consequence of a long-term evolution. precise understanding of the current state of a given volcano regarding its eruptive cycle and further evolution requires the collection of long time series of high quality data. important fieldwork has been done at individual volcanoes in recent decades, although the majority of active volcanoes (perhaps 80% of them), which are in least developed countries, remain unstudied. laboratory experiments provided essential information on the eruption dynamics and the chemistry of the magmatic system. numerical models are based on and constrained by the findings of these studies, and simulation results should be carefully validated against observational data. systematic field and observational studies are rather scarce or virtually inexistent. much more systematic petrological and field volcanological data need to be collected so that new predictions of complex conduit and source processes can be independently tested. equally, eruption and deposit parameters, ash cloud dispersal, and time dependent characteristics should be investigated and quantified. a database of the emissions from thousands of fluctuating low level volcanic gas sources as well as from the largest of eruptions is needed in order to improve our understanding of the volcanic impact on the global environment. once they have been carefully validated, numerical models can be employed for a variety of tests and sensitivity studies, which partly substitute time consuming, expensive, dangerous or even impossible experiments and observations. there are generally two main approaches to numerical modelling, which also pertain to the simulation of volcanic eruptions. in the first approach one attempts to simulate an event under a certain aspect, like aircraft safety, or climate effects. the comprehensiveness and the lack of understanding (or of computer power) of volcanic eruptions make it impossible to set up and analytically solve an equation system, which entirely describes all relevant processes. as a last resort, simplifying parameterisations, that is, empirical descriptions of the natural processes are employed. obviously, even sophisticated semi-empirical predictions can only be as good as the database used to derive them. furthermore, this method is based on the assumption that the net result of a partly parameterised system can substitute a complex analytical simulation of the real system. the main target of such modelling is to provide the quantitative information needed for policy makers concerned with risk mitigation to tackle questions of practical and sometimes urgent public concerns. an example is the estimation of the extent of high-risk areas around highly-populated volcanoes like montserrat based on numerical simulations (see e.g., bonadonna et al., 2002). the second approach, in contrast, typically isolates a few parameters, which are thought to mainly control the system. these parameters are then examined within an idealistic model of the real system, typically through analytical modelling or laboratory analogue experiments. the obvious limitation of this second approach to numerical modelling is that it ignores many other parameters, of which the importance cannot be known a priori. such idealistic models are not directly usable to solve practical problems as in the parameterised models mentioned above. the relative simplicity of these models allows, however, insights on the fundamental mechanisms controlling volcanic eruptions and their impacts. the method helps to understand individual controls, which can then be evaluated rigorously against observations as well as the other forms of simple modelling on the same aspect. this way, one hopes to explore the system in a step-by-step approach. an example of this second approach can be found in the publications of ernst et al. (1996), or melnik and sparks (2002a,b). the two approaches to numerical simulations introduced here serve complementary purposes and are both helpful and needed. the models described in this contribution are rather based on the first approach, since they are not purely analytical models. however, important advances in understanding volcanic eruptions and in computing facilities allow nowadays for the formulation of 835 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales increasingly comprehensive models. a large progress was achieved since the development of the first, simple one-dimensional homogeneous and steady-state models. today, complex multidimensional multiphase, transient models provide more accurate hazard assessment, and they can also be utilized to identify single relevant parameters, e.g., in sensitivity studies, when single parameters are modified (see for example neri et al., 2003; textor et al., 2003c). this latter type of numerical simulations can be regarded as a hybrid of the two approaches. we have presented models for numerical simulations of volcanic eruptions on different scales, tracking volcanic emissions during transport from the lithosphere to the stratosphere, finally influencing the global climate. explosive volcanic eruptions and their effects on the ground and in the atmosphere cannot be simulated by one single model, because of the complexity of the topic and contrasting spatial and temporal scales involved. however, the individual models presented in this paper cover many relevant processes and scales. the comfig. 6. schematic representation of the chain of numerical models of volcanic eruptions on different spatial scales. the regions of interest are indicated in bold letters in the central oval boxes. the focus of each model is described in the right boxes in italics. possible interactions between them can be found beside the vertical arrows. 836 christiane textor et al. bination of these tools provides the unique possibility to derive a more comprehensive description of the volcanic system. due to the difficulty or impossibility of observing many processes and parameters during a volcanic eruption, numerical simulations are especially insightful in the investigation of the system’s complexity. model results have to be carefully validated with observations, and no model can be better than comprehensive observations. quantities or processes, which are not amenable to measurements, and which cannot be calculated within one model, are simulated in detail by another one. figure 6 summarizes the chain of models, which is now available for the investigation of explosive volcanic eruptions on a broad range of spatial and temporal scales. the sizes of the model domains increase from the bottom to the top, along with a decrease of the spatial and temporal resolution. each model has been developed for a specific area of research, and each of them is based on a different set of equations and parameterisations, valid for the specific model applications, shown in the boxes to the right. the potential for the mutual exchange of simulation results is indicated beside the vertical arrows. the smaller-scale models can provide information about the volcanic forcing for the larger ones, which in turn can supply information on the atmospheric conditions for the smaller-scale models. results from a conduit ascent model can be used to determine the volcanic forcing at the vent for the simulation of ash dispersal and microphysical processes within the eruption column, to explore the influence of magmatic properties on the eruptive style. models focusing on the jet region, in turn, provide the boundary conditions for dynamical variables describing the volcanic forcing (e.g., temperature, vertical velocity) in larger scale eruption column models, because these models are not able to sufficiently resolve the region closest to the vent. the eruption heights gained from the simulations with the atham model, for example, are found to be highly sensitive to the initial conditions at the base of the eruption column after equilibration to atmospheric pressure. up to now, the initial vertical velocity, the gas fraction, and the horizontal extension, which cannot be directly observed during a violent eruption, had to be estimated based on theoretical considerations. the use of data gained from conduit models connected with those focusing on the jet region will reduce this uncertainty, and we intend to perform such joint experiments. the initial spatial distribution and concentration of the volcanic cloud for larger-scale simulations, or for global simulations in climate change research can be obtained from eruption column models. the injection height is a crucial parameter for the further dispersal and atmospheric residence time of volcanic material, which is, in turn, crucial for aircraft safety and climate impacts. the injection height cannot be measured with the desired accuracy by the current observation techniques (e.g., holasek et al., 1996; oppenheimer, 1998), and can only be inverted with 20-30% error from tephra fall deposit characteristics of past eruptions (e.g., bonadonna et al., 1998, and references therein). high uncertainties exist about the proportion of gases and particles erupted, which reach the stratosphere and influence the global climate. these uncertainties are caused by the difficulties of directly observing the processes within explosive volcanic eruption clouds, and by the lack of fundamental studies in an extreme parameter regime. simulations with eruption column models perfectly fill in this gap, since they focus on the investigation of processes, which could otherwise not be accessed. we plan, for example, a consecutive study where we investigate the impact of the water vapour and ice injection calculated by an eruption column model on the microphysics and chemistry in the stratosphere calculated by a global model. 8. conclusions in recent years, numerical simulation of volcanic eruptions has greatly advanced. however, because of the complexity and the various scales involved, not all processes could be addressed so far. different models have been developed, which each work best over a limited and contrasting range of spatial and temporal scales. these models focus on different eruption processes and phases. the existence of 837 numerical simulation of explosive volcanic eruptions from the conduit flow to global atmospheric scales such a chain of increasingly advanced models offers a new opportunity to explore the various aspects of the volcanic system in a joint effort in combination with various experimentations and observations both in the laboratory and in the field. we plan such an effort in the future, exploring the linking of the results from simulations with the various models. this may reduce uncertainties in assessing the environmental and climate impact of eruptions. 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(1998): observation and models of volcanic eruption column, in the physics of explosive volcanic eruptions, edited by j.s. gilbert and r.s.j. sparks, geol. soc. london, spec. publ., 145, 91-114. young, r.e., h. houben and o.b. toon (1994): radiatively forced dispersion of the mt. pinatubo volcanic cloud and induced temperature perturbations in the stratosphere during the first few months following the eruption, geophys. res. lett., 21, 369-372. iannone_finalonline:layout 6 annals of geophysics, 57, 5, 2014, a0546; doi:10.4401/ag-6487 a0546 a new method for the validation of the gomos high resolution temperature profiles products rosario q. iannone1,*, stefano casadio1,2, bojan bojkov2 1 serco s.p.a., frascati, italy 2 esa/esrin, frascati, italy abstract this article proposes a new validation method for gomos hrtp atmospheric temperature and density profiles, with the aim of detecting and removing 0.2 to 5 km scale vertical structures in order to minimise the impact of atmospheric artefacts in the comparison exercises. the proposed approach is based on the use of the “morlet” continuous wavelet transformation (cwt), for the characterisation and removal of non-stationary and localised vertical structures, in order to produce wave-free profiles of atmospheric temperature and density. comparison of wavefree temperature/density profiles and wavy structures profiles with those estimated from a limited number of collocated shadoz soundings for the years of 2003, 2004 and 2008, is discussed in detail. first results suggest that the proposed approach could lead to a significantly improved hrtp validation scheme, in terms of reduced uncertainties in the estimated biases. furthermore, this method may be adopted for the study of the vertical component of gravity waves from high spatial/temporal resolution data. 1. introduction the accurate knowledge of the temporal and spatial structure of atmospheric temperature and neutral density profiles over a broad vertical range is essential for the understanding of both climate and weather mechanisms [alpers et al. 2004]. measurements of temperature profiles have been performed over 60 years, based on a variety of instruments and techniques, such as sondes [tiefenau and gebbeken 1989, hudson et al. 2004], ground based infrared sounders [knuteson et al. 2004a, 2004b], and lidars (light detection and ranging) [hauchecorne and chanin 1980, thompson et al. 2012]. developing a reliable meteorology and climatology requires frequent measurements with good spatial coverage and resolution. since 1979, observations have been available from satellites orbiting around the earth [spencer et al. 1990], thus providing data with global coverage, which is often lacking in other data sets. a limitation of space-borne passive remote sensing instruments is the relatively low vertical resolution (of the order of kilometres), while ground based active instruments and in-situ measurements can have very high vertical resolution (i.e. few meters), but they are limited to regional studies and the related datasets can be sparse in time. in particular, balloon-borne experiments provide measurements of temperature profiles from ground up to the maximum altitude reached by the balloon, which is usually at ~3035 km [durre et al. 2006], with a vertical resolution varying between 5 and 50 m. on the other hand, spaceborne experiments, such as the microwave limb sounder (mls) [wu and waters 1996], the halogen occultation experiment (haloe) on-board upper atmosphere research satellite (uars) [borchi et al. 2005], the michelson interferometer for passive atmospheric sounding (mipas) [ridolfi et al. 2007] on-board envisat, the high resolution dynamics limb sounder (hirdls) on the aura spacecraft [dials et al. 1998], the sounding of the atmosphere by broadband emission radiometry (saber) on timed (thermosphere ionosphere mesosphere energetics dynamics) satellite [kumar et al. 2008] and gps radio occultation technique [kursinski et al. 1997, schreiner et al. 2007] as the constellation observing system for meteorology ionosphere and climate (cosmic/formosat-3) satellite [kishore et al. 2009], have been designed to retrieve temperature profiles in a wide altitude range, from troposphere to mesosphere, but with a vertical resolution much lower when compared to that of balloon sondes. an exception is represented by the high resolution temperature profile (hrtp) of gomos (global ozone monitoring by occultation of stars) temperaarticle history received january 10, 2014; accepted september 18, 2014. subject classification: gomos hrtp, atmospheric waves, temperature profiles, validation, wavelet transform. ture and density products [bertaux et al. 2010], which have a vertical sampling of few tens of meters and an estimated vertical resolution of about 250 m (see sofieva et al. [2007a, 2007b]). the aim of the present study is the development of technique to be applied to gomos hrtp and to high resolution correlative data (e.g. sondes) before comparing them. in particular, our objective is to remove as much as possible the small-scale fluctuations from the temperature and density profiles and consequently to increase the accuracy on estimates of biases. the method is based on the hypothesis that vertical fluctuations in both temperature and density profiles are due to the presence of atmospheric waves, and that the time-space lags between satellite and balloon measurements are the causes for “vertical phase shifts” of such waves which is, in turn, largely impacting the validation results by introducing spurious signals. in this context, we developed an analysis approach, which is designed to detect and remove the wavy signals from both the satellite and correlative instrument temperature and density profiles prior to compare them. this method has been tested using the gomos hrtp and the shadoz measurements. 2. gravity waves observed in temperature profiles: implication for validation data comparison between space observations and in-situ measurements, such as airborne instrument, balloons and lidars, is a critical part of satellite validation and of verification of theoretical models [hendrick et al. 2011]. the necessity to exchange and compare data between earth observation sensors and ground measurements communities has led to coordinated efforts in calibration and validation (cal/val) methodologies. validation entails comparison of references with ‘known’ or expected values, determined with long-term and continuous calibration measurements. two validation approaches are usually followed. the first is based on the use of data provided by ground-based networks, test sites, and instruments intercomparison systems, which run independently of a specific satellite mission. a second approach is based on field campaigns designed for specific validation needs. cal/val campaigns provide a unique set of high quality observations that are used for the validation of retrievals from the satellites. however dedicated airborne and balloon instruments usually only provide snap-shot observations, providing more detailed regional information. in contrast, satellite-based remote measurements are complementary as they have regional to global coverage. the effects due to increase of the time/space separation between temperature/density profile pairs, in terms of accuracy of comparison results, is dominated by geophysical variations. small-scale vertical structures present in the vertical temperature profiles, significantly impact the effectiveness of the validation studies if not properly treated. in fact, these structures are usually highly non-stationary, complicating the validation studies with the introduction of a variability that cannot be attributed to instrumental/algorithmic deficiencies [sofieva et al. 2008]. these relatively small-scale structures in atmospheric temperature and density profiles are often interpreted as due to gravity waves [sofieva et al. 2008]. the main sources for gravity waves in the lower and middle atmospheres are topography, orography, convection and jet/front systems [fritts and alexander 2003]. moreover, gravity waves are perturbations resulting from the coupling mechanism between lower, middle, and upper atmospheric region. vertically propagating gravity waves, generated in the troposphere, carry fluxes of heat and momentum into the stratosphere [holton and mass 1976]. when these waves are damped, the fluxes diverge, and sources of heat and momentum are created, thus providing the acceleration of the wind in the upper atmosphere and well above in the stratosphere and mesosphere. the net effect of the wave breaking is the acceleration of movement of air masses towards the poles, and an ascending motion in the tropical tropopause. gravity wave dissipation in the stratosphere contributes to driving large scale circulations like the brewer-dobson circulation [holton and alexander 2000]. numerous studies have been published, describing techniques using temperature profiles as tracers to understand internal gravity waves, using radiosonde [zink and vincent 2001], balloon data [zhang et al. 2006], lidar data [rauthe et al. 2008], and satellite data [fetzer and gille 1994]. a review of the studies on gravity-wave effects in stratosphere, recent observations, analysis methods, and results of very-high-resolution model studies have recently been reported by alexander et al. [2010]. in the context of space-borne techniques, preusse et al. [2009] demonstrated that observations from an infrared limb imager (ili) may provide 3-d images of gravity waves temperature structures, thanks to recent advances in detector technology. mcdonald et al. [2010] used vertical temperature measurements from radio occultation (ro) satellites in the formosat 3/cosmic constellation, and from rayleigh lidar observations, to study the variation of stratospheric temperature as a function of spatial and temporal separation. alexander et al. [2008] described a global analysis of the high resolution dynamics limb sounder (hirdls) temperature profile data to derive properties of gravity iannone et al. 2 3 waves. later in 2011, an inter comparison of hirdls, cosmic and saber was carried out by wright et al. [2011], with the aim of detecting stratospheric gravity waves from collocated temperature profiles. in the particular case of alexander et al. [2008] and wright et al. [2011], a stockwell transform (s-transform) based method was employed for the detection of gravity waves to allow the detection of multiple overlapping waves. the s-transform combines the short time fourier transform (stft) and wavelet transforms. in particular, the s-transform is expressed as a continuous wavelet transformation (cwt) with a specific mother wavelet multiplied by a phase factor, in order to achieve information related not only to the spectral properties of the signal but also information related to the spatial variations in those spectral properties. a detailed description of the s-transform method can be found in the publication of stockwell et al. [1996]. finally, geller et al. [2013] used five datasets from hirdls instrument on the aura satellite, saber instrument on the timed satellite, vorcore super-pressure balloon, and high resolution radiosonde sounding of horizontal wind and temperature. the aim of their work was to estimate the gravity wave momentum fluxes from the above mentioned observations and from those derived from five different models. the use of these measurements allowed testing theoretical models, and in particular to make these models able to correctly describe gravity wave sources. analysing the gomos scintillation spectra, sofieva et al. [2007a, 2007b] demonstrated the existence of a direct relationship between the stellar scintillations and small-scale internal gravity waves and turbulence. this approach is based on the fact that stellar scintillations observed through the earth’s atmosphere are caused by air density irregularities and thus they contain information about small-scale processes in the atmosphere such as gravity waves and turbulence. reconstruction of gravity waves could be achieved by fitting modelled scintillation spectra to the gomos measured spectra [sofieva et al. 2007a, 2010]. validation work relevant to gomos hrtp has been reported in literature by sofieva et al. [2009]. the gomos hrtp products, output of the scientific processor, were validated with collocated shadoz data by comparing vertical wavenumber spectra of temperature fluctuations [sofieva et al. 2009]. the approach proposed in this work is based on a wavelet transform of the air density and temperature profiles directly, for both gomos and shadoz soundings, demonstrating that this “wave-cleaning” technique improves the profiles validation schemes by reducing the impact of “out of phase” wavy structure in the estimates of biases. in the stratosphere, the temperature perturbation amplitudes can be of the order of 5-10 k. as a consequence, two waves out of phase by half wavelength will produce a difference in temperature profiles of the order of 10-20 k. assuming a temperature profile to consist of a background temperature profile and fluctuating components, the idea is therefore to validate collocated background temperature profiles, after eliminating the perturbation profiles. 3. data description 3.1. shadoz radiosonde the southern hemisphere additional ozonesondes network (shadoz) (http://croc.gsfc.nasa.gov/ shadoz/; thompson et al. [2003a, 2003b]) was initiated by nasa in 1998, aiming at the provision of continuous and accurate balloon measurements taken at 14 tropical selected locations, with the main purpose to measure the profile of ozone from sea level to about 30 km altitude from 40s to 40n latitude. besides ozone, temperature, pressure, and relative humidity are retrieved. the ozone measurements are performed with electrochemical concentration cells, while temperatures and pressures are recorded by standard vaisala instrumentation [thompson et al. 2004, 2007]. ozonesondes are launched from shadoz stations on a weekly basis, during both day time and night time [thompson et al. 2003a]. the shadoz data are usually sampled at 0.5 hz intervals resulting in a vertical resolution of about 10 m. for the sonde types used in the tropics, systematic errors in temperature data are ±0.3 k, with a precision of 0.20.4 k [thompson et al. 2003a, wmo 2008]. the retrieval of atmospheric temperature with balloon-borne instruments is affected by a combination of errors [christy and norris 2009]. sensor limitation is not the only source of uncertainties, as the representativeness error plays an important role [kitchen 1989]. in fact, balloon sounding consists of point measurements along a path through the atmosphere and, on average, a balloon needs more than 60 min to reach 15 km height [moradi et al. 2010]. thus, during its ascent, a balloon would drift because of the horizontal wind, therefore providing profiles that cannot be considered rigorously vertical [lanzante et al. 2003]. for this study, shadoz stations have been selected according to the criteria of the highest number of collocations and the maximum altitude reached by the balloons, usually ranging between 30 and 35 km. the data sets analysed are from the years 2003, 2004, and 2008. the selected four near-equatorial stations are java, kuala, ascension island, and hilo. details of the available data sets used in this work for the four stations are given in table 1. a new validation method for gomos hrtp 3.2. gomos gomos was an instrument on-board the envisat (environmental satellite) satellite, successfully operated by esa (european space agency) from march 2002 to april 2012. gomos used the star occultation technique in the ultraviolet–visible–nearinfrared (uv-vis-nir) spectral range to retrieve vertical profiles of atmospheric parameters. in particular, gomos was designed to monitor vertical distribution of o3, no2, no3, o2, h2o, atmospheric density and aerosols, and hrtp [bertaux et al. 2004, kyrölä et al. 2004, kyrölä et al. 2006]. the hrtp is retrieved from the scintillation observed by two gomos fast-photometer measurements, operating at 1 khz sampling frequency in the blue (470-520 nm) and in the red (650700 nm) spectral regions [bertaux et al. 2010]. the bichromatic scintillations recorded by the photometers allow the determination of refractive angles, which are related to the time delays between the photometer signals. the high resolution density and temperature profiles can be derived from these data in the 18-35 km altitude range, following the method described in dalaudier et al. [2006]. the vertical sampling of these products is of the order of tens of meters, with an estimated resolution of about 250 m. for altitudes between 18 and 35 km, the estimated accuracy of the hrtp is between 1 and 2 k [sofieva et al. 2009]. iannone et al. 4 no. shadoz date (launch time) shadoz stations (lat. and long.) gomos lat. and long. (date) star id (mv ; tv [k]) dh (km) dt (h) 1 20030108 (09:38) java (7°5’ s, 112°6’ e) 6°1’ s, 112°8’ e (20030108 22:32) 78 (2.3; 28000) 119 13 2 20031224 (02:17) java (7°5’ s, 112°6’ e) 8°3’ s, 105°4’ e (20031223 23:03) 95 (2.5; 26000) 783 3.14 3 20040121 (02:42) java (7°5’ s, 112°6’ e) 4°5’ s, 107°5’ e (20040121 22:53) 109 (2.7; 26000) 648 20 4 20040219 (02:50) java (7°5’ s, 112°6’ e) 9°8’ s, 111°5’ e (20040219 22:40) 80 (2.8; 30000) 254 20 5 20040929 (02:41) java (7°5’ s, 112°6’ e) 8°03’ s, 113°3’ e (20040929 22:32) 1 (-1.4; 11000) 150 20 6 20080228 (05:27) java (7°5’ s, 112°6’ e) 9°3’ s, 111°3’ e (20080228 22:40) 25 (1.62; 28000) 248 17.13 7 20080828 (05:34) java (7°5’ s, 112°6’ e) 4°6’ s, 107°4’ e (20080827 14:52) 2 (-0.74; 7000) 648 14 8 20080828 (05:34) java (7°5’ s, 112°6’ e) 4°2’ s, 115°3’ e (20080828 22:21) 2 (-0.74; 7000) 470 17 9 20080924 (05:27) java (7°5’ s, 112°6’ e) 10°1’ s, 105°9’ e (20080924 23:11) 23 (1.50; 26000) 830 18 10 20030122 (03:40) kuala (2°73’ n, 101.7 e) 3°8’ n, 97°6’ e (20030122 23:35) 54 (2.05; 4500) 446 20 11 20030409 (02:06) kuala (2°73’ n, 101.7 e) 4°02’ n, 101°3’ e (20030409 23:15) 57 (2.07; 26000) 200 21 12 20030401 (20:06) hilo (19°4’ n, 155°1’w) 20°1’ n, 150°3’w (20030331 21:48) 40 (1.86; 7100) 531 22 13 20030410 (21:46) hilo (19°4’ n, 155°1’w) 17°5’ n, 154°1’w (20030409 22:05) 38 (1.84; 11000) 245 24 14 20041011 (13:50) ascension island (7°98’ s, 14°4’ w) 10°7’ s, 17°6’ w (20041011 22:17) 65 (2.2; 4400) 408 9 15 20080108 (14:10) ascension island (7°98’ s, 14°4’ w) 11°5’ s, 14°6’ w (20080108 22:05) 109 (2.7; 26000) 343 8 16 20080826 (14:00) ascension island (7°98’ s, 14°4’ w) 5°05’ s, 8°9’ w (20080825 21:38) 2 (-0.74; 7000) 763 16 table 1. overview of the time and geolocation of shadoz measurements and gomos available for this study. the table reports also the visual magnitude and effective temperature of stars of which gomos products are presented. 5 the gomos hrtp data used in this study were generated with the instrument processor facility (ipf) version 6.01, and have been publicly available since december 2012. the quality of hrtp is nearly independent of the star properties, like the visual magnitude (mv) and the temperature (tv). it is also independent of the obliquity angles (i.e. the angle between the direction of line of sight motion and the local vertical at the ray perigee point) but it slightly depends upon illumination conditions. the highest quality hrtp data are acquired in dark limb, i.e. for nocturnal occultations. on the illuminated portion of the orbit, scattered solar light impacts the data quality in the lower stratosphere, so that only very bright stars provide full stratospheric profiles in these conditions. 3.3. collocation criteria we have restricted all collocations to a maximum horizontal distance of 800 km and a maximum time difference of 24 h between measurements. although validation protocols require the comparison of measurements of the same air mass, we adopted such relaxed spatial and temporal coincidence criteria in order to obtain a sufficient number of collocations to test our method in terms of wave detection. the observations used in the present study include 16 collocated vertical profiles, among which were six profiles for 2003, four for 2004 and finally six collocated vertical profiles for 2008. besides the geolocation of paired gomos and shadoz profile, table 1 reports the star parameters (mv and tv) and the spatial/temporal distance for each collocation. in order to effectively compare satellite and balloon data, we adopted a common vertical grid of 100 m by applying a linear interpolation scheme to both gomos and shadoz data, and excluding from the wavelet analysis any fluctuation with vertical wavenumber > 0.01 m-1. it is stressed here, that this work is not intended to provide conclusive validation results, but to propose a validation method. thus, a limited dataset should be sufficient to prove the effectiveness of the proposed approach. the analysis of the complete gomos mission dataset will be carried out in the next future. 4. method description in order to remove the small-scale fluctuations from the shadoz and gomos temperature profiles, both profiles were analysed by using the coherence wavelet method (cwm) software developed by torrence and compo (torrence and compo [1998]; http:// paos.colorado.edu/research/wavelets/). the components and method of this wavelet tool are not described here; readers are referred to torrence and compo [1998] for a more detailed explanation of the wavelet analysis. however, we include here some overall information about the wavelet transformation. in general, the continuous wavelet transform analysis is a time/space-dependent spectral analysis that decomposes a data series in time/space-frequency domain. in this, the scaled wavelet progressively moves along the time (or space)-varying signal, detecting localized structures in the signal. the most commonly adopted method for the spectral analysis of wavy signals is the fast fourier transform (fft). the fft, however, is suitable for the retrieval of spectral information for stationary signals only, which are quite rare phenomena in the real atmosphere. the ability to detect and characterise time and space localised phenomena is fundamental for gravity wave analysis, and this can be achieved by using the wavelet transformation. time windowing and fourier analysis techniques require a priori understanding of the signal and, consequently, it is necessary to make assumptions about the nature of the phenomenon under study. in addition, applying time/space fourier technique requires an averaging time long enough to obtain significant statistical parameters on a selected window short enough that a slowly varying signal appears to be stationary. wavelet techniques represent valid alternatives to the time windowing method with the advantage of needing minimum knowledge of the analytical properties of the signal. by definition, the wavelet function w is constructed so that it has zero mean and is localized both in time/space and in frequency. the wavelet type used in this work for the spectral decomposition is the mother wavelet function (mwf). the mwf is shifted forward and backward in the time/space domain, and this process is repeated for different frequency wavelets by stretching and compressing the mwf. different mwf have been developed and the choice of a function depends on both the desired analysis and the nature of the time/space series analysed. in this study, the morlet mwf was selected because it provides information about both amplitude and phase at the highest vertical resolution, and is better suited for data with oscillatory behaviour. the morlet mwf is defined as a sinusoidal function multiplied by a gaussian function [torrence and compo 1998]: (1) where ~0 is the non-dimensional frequency, x is the independent variable, x0 and v0 are the mean value and -x 2 x x i x4 1 2 0 0 0 ~ v e erw =^ ^ h h a new validation method for gomos hrtp the width of the gaussian function, respectively. usually, the wavelet transform is complex and is characterized in terms of its power and phase. the square modulus of the wavelet transform is called the scalogram. in the wavelet analysis, the mean power spectrum for each series investigated is processed first, and in case a peak in the wavelet power spectrum is significantly above a background power threshold, then it is considered to be a significant feature. torrence and compo [1998] proposed the calculation of a cone of influence (coi), which determines the region of the spectrum where edge effects significantly impact the wavelet results. the coi represents therefore the space/frequency region where the wavelet power spectra values can be considered unbiased by edge effects. figure 1a and b shows the wavelet scalogram of the gomos and shadoz temperature profiles respectively, shown in figure 2. the two panels of figure 1 show the logarithmic energy of spectral components as a function of vertical coordinate (abscissa) and of the oscillation period (ordinates); the coi is indicated with a solid white line. the values in the 2-d wavelet power spectrum that are above the coi (i.e. for period shorter than that indicated by the coi) can be assumed to be unaffected by edge effects. each peak in the 2-d fields corresponds to a localised wavy structure. in this example, the wavelet analyses of gomos and shadoz temperature variations show dominant wave periods between 0.7 and 4 km around the height region from 22 to 32 km. in fact, in both panels of figure 1, short scale fluctuations of 1 km are seen in the 23-26 km height region, and vertical wavelength of 2.5 km is found in the 27-31 km height region. this spectral analysis of the profiles shows that the peaks of temperature fluctuations are comparable, even for profiles separated significantly in space and in time (hundreds of kilometres and several hours). the inverse wavelet transform is, then, used to reconstruct the temperature fluctuations present in the observed temperature profiles, considering only wavelet components that are significantly within the coi. the reconstructed temperature fluctuations are subtracted from the original temperature profiles, thus providing “wave-free” profiles. 5. results and discussion figure 2a shows an example of the interpolated temperature profiles acquired on october 11, 2004, by gomos and shadoz radiosonde (ascension station 14°42' w longitude and 7°98' s latitude, 15:00 local time). in the figure, the continuous black line represents the gomos hrtp, whereas the grey line is the shadoz data. hrtp was measured on the same day at 22:17 local time (23:17 utc) over a close location (17°6' w longitude and 10°7' s latitude). the star for this occultation is lam vel, which is considered cold (tv= 4400 k) and of medium brightness (mv= 2.2). the temperature values of hrtp span from 200 to 250 k, while the full range of shadoz temperature measurements is between 195 k and 245 k. the temperature difference as a function of the altitude is given in figure 2b, which shows the difference between gomos and shadoz temperature profiles, defined as dt = tgomos − tshadoz. between 20 and 25 km altitude, the temperature difference distribution has a mean value <dt> of +1.30 and a standard deviation of ± 3.49 k, whereas between 25 and 30 km altitude, <dt> is +0.52 with a standard deviation of ±4.02 k. finally, between 30 and ~35 km iannone et al. 6 figure 2. (a) original temperature profiles for gomos hrtp (black line) acquired on october 11, 2004, 22:17 lt (10°7’ s, 17°6’ w), for shadoz (gray line) measured on the same day at 13:50 lt, above ascension island (7°98’ s, 14°4’ w); (b) absolute temperature difference between gomos and shadoz temperature profiles: dt = tgomos − tshadoz. figure 1. wavelet power spectrum using morlet mother wavelet for (a) gomos hrtp and (b) radiosonde shadoz temperature profile illustrated in figure 2. the black lines in both graphs contours the areas where the power is considered to exceed the 95% confidence level, whereas the white line contours the area defined by the cone of influence (coi). 7 altitude, the <dt> is −0.01 with a standard deviation of ±2.04 k. the observed standard deviation of dt, estimated between 18 and 35 km is ±3.20 k. differences between gomos and shadoz profiles are due to several factors, such as data sampling, observation time and distance between profiles. large differences, especially in the altitude range between 20 and 30 km, are due to the fluctuating structures present in the gomos and shadoz temperature profiles, clearly visible in figure 2, with amplitudes as large as ±5 k. figure 3a shows the wave-free profile after applying the wave screening technique, and figure 3b shows the profile of residuals. after removing the wavy features present in the temperature profiles of figure 1a, the agreement between wave-free gomos and shadoz temperature profiles is very good. differently from the case shown in figure 2b, where the dt was continuously changing sign with height, in figure 3b it is clear that gomos shows a deviation relative to shadoz of ~1k in the stratosphere up to 30 km. in comparison to shadoz data, the highest altitudes show a small underestimation of the temperature by gomos. in table 2, values of mean of the temperature difference distribution <dt> and related standard deviation are reported for each altitude range. for the wave-free profiles, <dt> is +1.12 ± 0.60 k for 20-25 km altitude range, +0.80 ± 0.22 k between 25 and 30 km, and −0.48 ± 1.22 k between 30 and ~35 km, respectively. for the full range of altitude, <dt> is +0.53 ± 0.98k. by applying the proposed method to the original temperature profiles shown in figure 2, the standard deviation of the differences between the wave-free gomos and shadoz profile, for this particular case, decreases by a factor of ~8 in the altitude range from 20 up to 30 km, then by a factor of almost 2 between 30 and ~35 km. finally the overall standard deviation is decreased by a factor of about 3. thus, the difference between the two profiles can be estimated with higher accuracy, even for profiles not perfectly collocated and in presence of significant wavy structures. it should be noted that the mean difference values are not severely impacted by the wave removal. we define perturbation as the small-scale vertical structures, determined with proposed wavelet analysis. assuming that fluctuations in temperature profiles are generated by an ensemble of gravity waves, we can expect similar spectral properties of the temperature field at locations not far from each other (<800 km) during a time period of a few hours, as has been demonstrated previously in another related study [sofieva et al. 2008]. in figure 4, the perturbation profiles for gomos (black line) and shadoz (grey line) (relative to the profiles shown in figure 2) are shown. gomos and shadoz perturbations are dominated by periodicities with vertical wavelengths of a few km as observed previously in figure 1. in figure 4, the difference in phase between gomos and shadoz wavy structure profiles is clearly visible. in this particular case, where the wave signals are very similar, it could be argued that the significant phase difference between the two profiles could be due to the time and distance separation between the measurements. as stated before, in the example reported in figure 2, the peak-to-peak amplitude of the wavy structures is the order of ±5 k. the spatial distance between the two profiles is around 400 km, whilst the temporal difference is of ~9 h. although these profiles are not located at such large spatial and temporal distance from each other, the natural variability can contribute significantly to the temperature difference. in order to observe very similar small-scale structure without significant phase differences, the temperature profiles should be almost exactly collocated in time and in space. sofieva et a new validation method for gomos hrtp figure 3. same as figure 2, but after removal of wavy structures. altitude range (km) <dt> ±1v (k) before wavelet analysis <dt> ±1v (k) after wavelet analysis 20 25 +1.30 ± 3.49 +1.12 ± 0.60 25 30 +0.52 ± 4.02 +0.80 ± 0.22 30 35 -0.01 ± 2.04 -0.48 ± 1.22 18 -35 +0.69 ± 3.20 +0.53 ± 0.98 table 2. comparison of the temperature difference distribution computed as the mean value and standard deviation of <dt> over each altitude range before and after applying the wave screening technique to the temperature profiles measured by gomos and shadoz over ascension for october 11, 2004, illustrated in figure 2 and figure 3. al. [2008] found that the small-scale structures in temperature profiles become different when the horizontal separation of measurements exceeds 20-30 km. as for the temperature, wavelet analysis has been applied also to the gomos hrtp air density profiles. in figure 5, the gomos temperature (black line) and the air density (grey line) perturbation profiles (expressed in percentages) are displayed. the fluctuations are present in the density profile too, and, as expected, the gomos temperature and density fluctuations (expressed in % for both quantities) are perfectly anti-correlated. this is consistent with the fact that the temperature profile is derived from the air density profile through the gas law and the assumption of hydrostatic equilibrium [hauchecorne and chanin 1980], and shows that the proposed method is not altering the physical relationships between thermodynamic variables. we repeated the analysis for a set of collocated profiles, and the summary results are shown in figure 6, for which a 300 m vertical grid has been selected. the averaging on a 300 m vertical grid was performed for sake of readability of the plot. figure 6a shows the number of samples for a given altitude bin; figure 6b shows the average difference profile of temperature: the black squares represent the original data, whereas the grey circles correspond to the “wave-free” data. finally, the horizontal lines represent ±1 standard deviation (v) calculated from the difference profiles of all coincident pairs within each altitude bin. between 19 km to 32 km altitudes, the bias distribution for original temperature profiles is (0.20 ± 0.90)k, while after the removal of wavy structure the bias distribution becomes (0.38 ± 0.47)k. below 20 km and above 31 km the differences seem to be larger for both original and processed profiles, but this is probably due to the limited number of available measurements at low and high altitudes. finally, figure 6c shows the profiles of standard deviation of mean temperature difference of figure 6b at each altitude level for the original (black) and the wave-free (grey) data set. over the entire altitude range, the ratio between the mean of the wave-free standard deviation and the mean of the original standard deviation of the temperature differences, defined as <vwave-free>/<voriginal>, assumes a value of 0.71, indicating an overall improvement of ~30% in the accuracy of the estimates of the biases. this result indicates that the accuracy of the validation results depends not only on the measurement precision, but also on the natural variability of the atmospheric temperature field. the improvement of the accuracy of validation results is important to evaluate the quality of the atmospheric parameters of the satellite measurements. therefore the natural variability has to be taken into account. the temperature profiles might be more or less affected by geophysical variability due to the presence of the gravity waves and the magnitude of uncertainty of the bias is based also on the different conditions in the validation exercises, such as location and season. the outiannone et al. 8 figure 5. gomos hrtp temperature (black line) and neutral density (grey line) percentage perturbation profiles, obtained from the temperature profiles illustrated in figure 2. figure 4. wavy structure profiles: gomos (black line) and shadoz (grey line). note that in this example the gomos and the shadoz perturbation profiles are obtained from the temperature profiles illustrated in figure 2. 9 come of the applied method is thus very useful to estimate the contribution due to the presence of small-scale fluctuations from the temperature and density profiles. 6. conclusions in this work, new method for the comparisons of temperature and density profile data from high vertical resolution measurements is described. the main purpose of the method is the detection and removal of wavy structures in order to improve instrument validation results, by decreasing the statistical uncertainties on mean differences. this method has been applied to gomos hrtp and shadoz temperature profiles, in the common altitude range between 18 and 35 km, using wavelet transforms. the continuous wavelet transform possesses the ability to construct a space-frequency representation of a signal that offers very good space and frequency localization, so wavelet transforms can analyse localized non stationary structures of potentially great interest in the temperature signals. the wavelet approach should not be considered as a smoothing technique, as the cwt allows the characterisation and tracing of the localised features presented in a signal. the wavelet analysis of profiles is performed with a morlet wavelet transformation. the preliminary results presented in this work demonstrate that such a method can effectively be applied to stratospheric temperature data, improving the accuracy of gomos hrtp validation, when compared with shadoz soundings, by significantly reducing the uncertainty on bias estimates. the temperature measurements considered in this work are characterised by a small common altitude range, due to the peculiarities of each technique. the possible use of numerical weather prediction (nwp) temperature profiles would allow an artificial extension of the vertical range and, as a result, structures with vertical wavelengths higher than 5 km might be detected. furthermore, the extensive validation of gomos hrtp products with sounding and lidars, will be the focus for our future research. we finally conclude saying that the gomos data set represents a very valuable source of information for atmospheric science studies. particularly, the gomos hrtp measurements could be used to yield information about the vertical wavenumbers of the atmospheric waves and provide constraints for the parameterisation of gravity waves in climateprediction models. in particular, gravity wave scales, amplitudes, fluxes, vertical propagation, and variations with altitude can be addressed with these space-based observations. acknowledgements. the authors would like to express their gratitude to lidia saavedra de miguel, gareth davies, marta de laurentis, gabriele brizzi, angelika dehn, and thorsten fehr for their support and collaboration. the author would also like to thank the principal investigators of the shadoz sondes. finally, we thank victoria sofieva for helpful comments on the manuscript. a new validation method for gomos hrtp figure 6. comparison of the 16 gomos temperature profiles. (a) number of collocated pairs; (b) mean differences between gomos and shadoz temperature before (solid black squared dots) and after (solid blue round dots) applying the wavelet analysis. the spread of the average values at each height level, as represented by the standard deviation, is shown by continuous lines. 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analysis of stratospheric gravity wave packets over macquarie island: 2. intermittency and mean-flow accelerations, j. geophys. res., 106, d10. *corresponding author: rosario q. iannone, serco s.p.a., frascati, italy; email: rosarioquirino.iannone@serco.com. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. iannone et al. 12 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice delipetrev_correctedok_layout 6 annals of geophysics, 56, 3, 2013, r0327; doi:10.4401/ag-5311 r0327 new geomagnetic measurements in the republic of macedonia marjan delipetrev1,*, nenad novkovski2, todor delipetrov1 1 university of goce delcev, faculty of natural and technical sciences, stip, republic of macedonia 2 university of st. cyril and methodius, faculty of natural science and mathematics, institute of physics, skopje, republic of macedonia abstract this study presents measurements of the geomagnetic field of the republic of macedonia, performed through a network of newly defined repeat stations. the measurements of these elements were in the intervals of 3.378° ≤ dd ≤ 3.983°, 57.276° ≤ i ≤ 59.005°, and 46 235 nt ≤ f ≤ 46903 nt. the geomagnetic data were processed and the results of the observed elements of the geomagnetic field on the repeat stations are presented. additional data processing was performed to calculate the reduced values of the intensive elements of the field at the level of h500 a.s.l.. based on these data, new maps of the geomagnetic field of the republic of macedonia are developed, together with the polynomial model of the elements of the geomagnetic field for the 2010.0 epoch. 1. introduction the territory of the republic of macedonia occupies the central part of the balkans peninsula, with an area of 25,713 km2. the terrain is mountainous due to highly active neotectonic processes. these processes have formed uplift blocks, with crests to 2,600 m a.s.l. and blocks of sinking depressions that mainly spread east to west [arsovski 1997]. the geomagnetic investigations can be divided into three periods. the first period began in the 19th century and finished in 1945. scientists from mainly european countries were engaged in infrequent special expeditions to investigate the geomagnetic field in the balkans, which included investigations in the territory of the present-day republic of macedonia. the second period was from 1945 to 1991, when macedonia was a constitutional part of yugoslavia, and the third period began after the declaration of independence in 1991. the geomagnetic observations for the territory of the republic of macedonia up to 1990 were carried out by teams of scientists from the geomagnetic observatory grocka (republic of serbia). to continue monitoring and to obtain new data for the geomagnetic field of macedonia, panovska and delipetrov [2006] developed the appropriate maps and models for the given epochs. in 2002, the first measurements of the geomagnetic field for the locations of plackovica, ponikva and galicica were initiated, with the help of the royal meteorological institute geomagnetic observatory in dourbes (belgium). this report presents the results from the terrestrial measurements and data processing, and the newly developed maps and the polynomial model of the geomagnetic field of macedonia. 2. the repeat station network the geomagnetic field in a given area is measured for a network of repeat stations, often at five-year intervals. based on these measurements, the raw geomagnetic data are collected and processed, which produces models and maps of the geomagnetic field elements. a team from the faculty of natural sciences, ‘goce delcev’ university (stip, republic of macedonia) conducted investigations to define the repeat station network in the republic of macedonia. the repeat station networks of 16 countries were investigated, with the mean distance between stations of 230 km, which varied from 53 km to 415 km. in the process of defining the repeat station network, geomagnetic measurements were performed for every point in four directions (north, south, east, west) at intervals of 1 m at a distance of 10 m. the international association of geomagnetism and aeronomy guidance states that the maximum gradient of the magnetic field for repeat stations should not exceed 50 nt in a radius of 10 m. the selected repeat stations in macedonia have a gradient below 1 nt [rasson and delipetrov 2006]. based on these investigations and the characteristics of macedonia, a network of 15 repeat stations was defined (figure 1, table 1). the minimum distance bearticle history received july 13, 2011; accepted march 22, 2013. subject classification: geomagnetic survey, geomagnetic field, repeat station, measurements. tween the repeat stations is 26.1 km, with a maximum distance of 201.3 km, and a mean distance of 98.6 km. the lowest repeat station is nikolic, at an altitude of 300 m a.s.l., while the highest is galicica, at 1,684 m a.s.l. 3. normal geomagnetic field maps the first field measurements for the definition of the repeat station network were carried out in 20022003 with the instruments from the geomagnetic observatory in dourbes (belgium), with the help of jean rasson. the second series of geomagnetic field measurements was carried out in 2007-2009 for the newly established repeat station network (table 1). the repeat station measurements in 2007-2009 were performed with the instruments of the ‘goce delcev’ university. the instruments used were a lemi 203 dim sensor, a geometrics g 856 proton magnetometer, and a trimble global positioning system (gps). during the field observations, the declination d, inclination i, and total field vector f were measured. as there is no geomagnetic observatory available in the republic of macedonia, a time-series from the panagjuriste observatory (bulgaria) was used. this observatory was selected due to its proximity, as it is 225 km away, with coordinates: latitude, 42.52°n; longitude, 24.18°e; altitude: 556 m a.s.l. the time-series data was used to reduce the results of the 2010.0 epoch, and to calculate the mean yearly changes in the geomagnetic field. the measurements for the 2010.0 epoch are given in table 2 and illustrated in figures 2-4.using the data from tables 1 and 2, a reduction of the intensive measurements was made to the level h = 500 m a.s.l. (table 3). for any intensive field element with a value e [e.g., de santis et al. 2003] at a height h, the correction de for computing its value for h = 500 m is expressed as: (1) where r = 6,371.2 km is the earth mean radius. table 3 shows the corrections and reduced values for f500, h500 and z500. using the software surfer 9 and the data from tables 2 and 3, the maps of the elements for the normal geomagnetic field were created (figures 2-4). the declination (d) in the observed area varies from 3.378° for the repeat station galicica, to 3.983° for station slivnica. the mean declination from the obdelipetrev et al. 2 repeat station geographic latitude (°:min:sec) geographic longitude (°:min:sec) altitude (m) bajlovce 42:13:16 21:55:17 592 crna skala 41:59:41 22:47:28 833 egri 41:23:15 21:57:06 626 galicica 40:57:56 21:26:54 1684 island gradot 40:57:23 20:48:51 317 luke 42:20:39 22:16:29 1180 mavrovo 41:42:58 20:43:38 1418 nikolic 41:15:54 22:44:36 300 plackovica 41:47:41 22:18:13 677 ponikca 42:01:35 22:21:29 1618 prilep lake 41:24:11 21:36:32 870 slivnica 41:37:38 21:11:36 1252 st. marija precesna 41:36:54 22:51:46 837 tetovo 41:59:09 21:04:46 522 vodno 41:58:40 21:24:57 569 figure 1. map of the repeat stations in the republic of macedonia. table 1. gps coordinates of the repeat stations. e r h eh r rh3 3 d = + = 3 served data is dsr = 3.628°. the means of the geomagnetic fields are calculated using an arithmetic mean. analysis of the declination field shows that for the observed territory, the most extreme value is in the central southern part, near the measuring station of island gradot. the field is quiet in the west and northwest parts of the country. the observations show that the inclination varies from 57.276° for galicica to 59.005° for luke station. the mean inclination (i) is isr = 58.125°. the inclination field compared to the declination is more homogenous and relatively quiet. there is a slight twisting of the isolines in the central-southern part. the measured points for the total field vector (f) vary from 46,235 nt for luke to 46,903 nt for the repeat station crna skala. the mean total field vector is fsr = 46,532 nt. the western part of the field, as the western-macedonia part, has a quiet field. again, there is a twisting of the isonew geomagnetic measurements in the republic of macedonia station d (°) i (°) f (nt) h (nt) x (nt) y (nt) z (nt) bajlovce 3.507 58.850 46675 24144 24099 1477 39945 crna skala 3.769 58.244 46903 24685 24632 1623 39881 egri 3.465 57.367 46330 24984 24938 1510 39016 galicica 3.378 57.276 46271 25014 24970 1474 38927 island gradot 3.946 57.667 46327 24777 24719 1685 39144 luke 3.737 59.005 46235 23809 23759 1552 39633 mavrovo 3.466 58.122 46451 24531 24486 1483 39445 nikolic 3.499 57.823 46493 24759 24713 1511 39352 plackovica 3.698 58.217 46575 24531 24480 1582 39591 ponikva 3.721 58.603 46750 24355 24304 1581 39905 prilep lake 3.509 57.854 46567 24777 24731 1517 39428 slivnica 3.983 58.166 46607 24583 24524 1708 39596 st. marija precesna 3.533 58.039 46453 24590 24543 1515 39411 tetovo 3.617 58.346 46683 24499 24450 1546 39738 vodno 3.597 58.298 46671 24526 24477 1539 39707 station df500 (nt) dh500 (nt) dz50 (nt) f500 (nt) h500 (nt) z500 (nt) bajlovce 2.0219 1.0459 1.7201 46677.02 24145.05 39708.72 crna skala 7.3540 3.8704 6.2530 46910.35 24688.87 39887.25 egri 2.7487 1.4823 2.3148 46332.75 24985.48 39018.31 galicica 25.7917 13.9429 21.6981 46296.79 25027.94 38948.70 island gradot -3.9921 -2.1351 -3.3731 46323.01 24774.86 39140.63 luke 14.8024 7.6226 12.6888 46249.80 23816.62 39645.69 mavrovo 20.0759 10.6022 17.0479 46471.08 24541.60 39462.05 nikolic -4.3786 -2.3317 -3.7060 46488.62 24756.67 39348.29 plackovica 3.8816 2.0444 3.2996 46578.88 24533.04 39594.30 ponikca 24.6064 12.8190 21.0036 46774.61 24367.82 39926.01 prilep lake 8.1125 4.3164 6.8688 46575.11 24781.32 39434.87 slivnica 16.5013 8.7037 14.0190 46623.50 24591.70 39610.02 st. marija precesna 7.3709 3.9018 6.2535 46460.27 24593.90 39417.25 tetovo 0.4836 0.2538 0.4116 46683.48 24499.25 39738.41 vodno 1.5163 0.7968 1.2901 46672.52 24536.80 39708.29 table 2. reduced measurements for the epoch 2010.5. d, declination; i, inclination; t, total intensity; h, horizontal component; x, east component; y, north component; z, vertical component. table 3. corrections and reduced values for the intensive field elements at h = 500 m a.s.l. lines in the central-southern part. the maps presented here (figures 2-4) show that the largest deformations in the field occur in the central part and the eastern macedonia zone. the geomagnetic field on the earth surface can be present as the vector sum of many different components: (2) where, is the field of the homogenous magnetized earth, or the field of a dipole magnet, is the field from the magnetic objects in the deeper parts of the earth, known as the nondipole or field of continents, is the field created by the magnetization of the upper parts of the earth crust, known as the anomalous field, is the field related to external phenomena with respect to the earth surface, and is the field of variations. with the measurement procedure performed and the processing of the collected data, components and can be eliminated. the anomalous field can be represented as a sum of two components: (3) where, is the regional anomalous field for the observed territory, which is caused by the influence of the magnetic layers and the deeper parts of the earth crust, and which is manifest over larger areas, is the local anomalous field for the observed territory. this field results from the magnetic media (rock, ore deposits) near the earth surface, which influences a relatively small area. separation of the local and anomalous fields depends on the size of the area investigated. the methodology and the observations of the geomagnetic field allow the elimination of the anomalous component of local character, . the italian geomagnetic reference field models represent the geomagnetic field generated in the core, or the component (t0) and the nondipole field (tm), or the sum of these two. the referent model for an investigated territory (state) is given by: (4) the influence of the regional component presents specific geomagnetic features for the explored area that create differences from the global geomagnetic models of the earth. for a more precise definition of the regional geomagnetic field, observations on a denser network of repeat stations should be made. 4. model of the normal geomagnetic field of the republic of macedonia today, two methods of modeling geomagnetic fields have been defined: one that uses spherical harmonic analysis, and the other, polynomial analysis. studies show that for areas with <15° spatial angle, polynomial analysis is an effective method [de santis et al. 1999]. given the size of the territory of the repubdelipetrev et al. 4 figure 4. map of total field vector f for the 2010.0 epoch. figure 3. map of the inclination i for the 2010.0 epoch.figure 2. map of the declination d for the 2010.0 epoch. t t t t t t0 a d= + + + +m e t t ta = + aa r i t t t tnm = + + am0 r t0 tm ta ta te te td td ta r ta r ta i 5 lic of macedonia, the polynomial model is appropriate. to calculate the coefficients of the polynomial model, the method of least squares is used. the geomagnetic field for a given territory can be expressed as: where, e(d{,dm) is a normal field of the point with coordinates {1 and m1, {1 and m1 are the geographic latitude and longitude of the point, respectively, {0 and m0 are the geographic latitude and longitude of the point, respectively, in respect of which the measurements are reduced. for the republic of macedonia, the central point is for {0 = 41.50° and m0 = 22°. then, d{ = {1 − − {0 is the difference in the geographic latitudes, in minutes, dm = m1 − m0 is the difference in the geographic longitudes, in minutes, and ai is the coefficient for the corresponding differences in nt/min; i.e., min/min. usually, the differences in the latitude and longitude are calculated in terms of the coordinates of the geomagnetic observatory located in that territory. the normal field of the territory of macedonia is calculated according to the measurements of absolute values for the total intensity (f), declination (d) and inclination (i) for the period from 2007-2009 for the 15 network stations. the values for the other components of the geomagnetic field are derived from these (table 1). calculate coefficients or polynomial model from the absolute measurements for 2010.0 epoch is given in (table 4). 5. conclusions this report presents the measurements of the geomagnetic field of the republic of macedonia. the geomagnetic field observations were carried out for the newly developed repeat station network in 2007-2009. the data gathered was processed and the maps of the geomagnetic field elements are produced here. with the reduction in h500 a.s.l., the influence of the relief was minimized in relation to the regional magnetic component presented. the data are used to create the polynomial model of the elements of the geomagnetic field for the 2010.0 epoch. the model is a second-degree polynomial in longitude and latitude and consists of observations of six coefficients for each magnetic element. the measurements given in this report are a continuation of the observations on the geomagnetic field in the republic of macedonia that was started after 2000. the geomagnetic model can be used for further studies and analysis of the geomagnetic field of this area. acknowledgements. the authors would like to thank dr jean rasson, royal meteorological institute geomagnetic observatory, dourbes, belgium, for help with the recording in 20022003, the university of goce delcev, for support from the tempus project geomagnetic measurements and quality standards, and the panagjuriste observatory, bulgaria, for the geomagnetic observatory time-series. references arsovski, m. (1997). tectonics of macedonia, faculty of geology and mining, stip (republic of macedonia), 18-25. de santis, a., j.m. torta and f.j. lowes (1999). spherical cap harmonics revisited and their relationship to ordinary spherical harmonics, phys. chem. earth, part a: solid earth geod., 24 (11/12), 935-941. de santis, a., l. gaya-piqué, g. dominici, a. meloni, j.m. torta and r. tozzi (2003). italian geomagnetic reference field (igrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting, annals of geophysics, 46 (3), 491-500. delipetrov, t., and b. paneva (2006). geomagnetic field of the republic of macedonia, in: j.l. rasson and t. delipetrov (eds), geomagnetics for aeronautical safety: a case study in and around the balkans, proceedings of the nato advanced research workshop (ohrid, republic of macedonia, 16-20 may 2005), springer, 15-42. panovska, s., and t. delipetrov (2006). analysis of geomagnetic field data from measurements during 2003 in macedonia, in: j.l. rasson and t. delipetrov new geomagnetic measurements in the republic of macedonia element coefficient a0 a1 a2 a3 a4 a5 d (°) 3.6278 0.1176 -0.0386 0.0778 0.0154 -0.3455 i (°) 58.0876 -0.0367 -0.1578 0.1196 0.0928 -0.4256 t (nt) 46550 -24.4499 -172.2784 -20.5894 197.7838 -449.6354 h (nt) 24607 11.0723 17.1495 -95.3759 38.9003 57.7547 x (nt) 24558 7.9397 18.2187 -97.3217 37.0952 66.8624 y (nt) 15560 46.6355 -15.9659 23.6280 13.3079 -145.0884 z (nt) 39514 -36.4622 -217.0881 33.5642 210.5535 -567.0112 table 4. model for the 2010.0 epoch. ,e a a a a a a 2 3 4 2 5 2 6 1 $ $ $ $ $ { m { m { m { m d d d d d d d d + + + + + = +^ h (eds), geomagnetics for aeronautical safety: a case study in and around the balkans, proceedings of the nato advanced research workshop (ohrid, republic of macedonia, 16-20 may 2005), springer, 325-346. rasson, j.l., and m. delipetrov (2006). repeat surveys of macedonia, in: j.l. rasson and t. delipetrov (eds), geomagnetics for aeronautical safety: a case study in and around the balkans, proceedings of the nato advanced research workshop (ohrid, republic of macedonia, 16-20 may 2005), springer, 97-114. *corresponding author: marjan delipetrev, university of goce delcev, faculty of natural and technical sciences, stip, republic of macedonia; email: marjan.delipetrev@ugd.edu.mk. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. delipetrev et al. 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 adaptively smoothed seismicity earthquake forecasts for italy maximilian j. werner1,*, agnès helmstetter2, david d. jackson3, yan y. kagan3, stefan wiemer1 1 eth zurich, swiss seismological service, zurich, switzerland 2 université joseph fourier and cnrs, laboratoire de géophysique interne et tectonophysique, grenoble, france 3 university of california, department of earth and space sciences, los angeles, usa annals of geophysics, 53, 3, 2010; doi: 10.4401/ag-4839 abstract we present a model for estimation of the probabilities of future earthquakes of magnitudes m ≥ 4.95 in italy. this model is a modified version of that proposed for california, usa, by helmstetter et al. [2007] and werner et al. [2010a], and it approximates seismicity using a spatially heterogeneous, temporally homogeneous poisson point process. the temporal, spatial and magnitude dimensions are entirely decoupled. magnitudes are independently and identically distributed according to a tapered gutenbergrichter magnitude distribution. we have estimated the spatial distribution of future seismicity by smoothing the locations of past earthquakes listed in two italian catalogs: a short instrumental catalog, and a longer instrumental and historic catalog. the bandwidth of the adaptive spatial kernel is estimated by optimizing the predictive power of the kernel estimate of the spatial earthquake density in retrospective forecasts. when available and reliable, we used small earthquakes of m ≥ 2.95 to reveal active fault structures and 29 probable future epicenters. by calibrating the model with these two catalogs of different durations to create two forecasts, we intend to quantify the loss (or gain) of predictability incurred when only a short, but recent, data record is available. both forecasts were scaled to five and ten years, and have been submitted to the italian prospective forecasting experiment of the global collaboratory for the study of earthquake predictability (csep). an earlier forecast from the model was submitted by helmstetter et al. [2007] to the regional earthquake likelihood model (relm) experiment in california, and with more than half of the five-year experimental period over, the forecast has performed better than the others. 1. introduction here, we document the calibration of a previously published, time-independent model of earthquake occurrences in the region of italy. we extracted probabilities of future m ≥ 4.95 shocks for five-year and ten-year periods in a format suitable for prospective testing within the italian earthquake predictability experiment [schorlemmer et al. 2010a]. previously, helmstetter et al. [2007] calculated a probabilistic earthquake forecast for m ≥ 4.95 for the region of california, usa, over a five-year duration. the forecast is currently being tested within the regional earthquake likelihood model (relm) experiment [field 2007]. after more than half of the five years over, the forecast has not been rejected following a suite of tests, and has performed better than other forecasts [schorlemmer et al. 2010b]. werner et al. [2010a] made some modifications to the model by helmstetter et al. [2007] and re-calibrated it on up-dated data to generate a new earthquake forecast for california. this forecast is under test within the california branch of the global collaboratory for the study of earthquake predictability (csep) [jordan 2006, zechar et al. 2009]. to calculate future earthquake potential in italy, we used the same model, with some minor modifications. one modification relates to the estimation of the completeness threshold, which was difficult to estimate on the small spatial scales here, which were possible with the high-quality dataset available in california [werner et al. 2010a, helmstetter et al. 2007]. instead, we set a single magnitude threshold for the entire region. smoothed seismicity models, such as the present one, usually do not incorporate geological or tectonic observations. rather, the models are calibrated on the seismicity data available from earthquake catalogs. one can justifiably question the hypothesized validity that the short (decadal) periods covered by high-quality instrumental catalogs are sufficient to forecast locations of future large earthquakes that have very low occurrence probabilities. even if the spatial distribution of seismicity is reasonably stable up to geological timescales, estimating this distribution from a short time window of observations is difficult. a partial solution to this problem is to estimate a predictive spatial distribution, rather than the observed spatial distribution. that is, rather than estimating the density of past earthquakes, the available data is divided into separate learning and target sets, to estimate a predictive density from the learning catalog that can be evaluated and optimized on the target earthquakes. this cross-validation method was used here, and it generated smoother forecasts than a simple article history received march 23, 2010; accepted july 1, 2010. subject classification: earthquake interactions and probability, seismic risk, statistical analysis, mathematical geophysics, inverse methods. 107 kernel-density estimation method because the locations of future – rather than past – earthquakes are predicted, and these locations can be in regions of little previous seismicity. nonetheless, this is only a partial solution: kagan and jackson [1994] conjectured that the optimal predictive horizon of a forecast based on smoothed seismicity will scale proportionally with the learning catalog because of spatiotemporal clustering. thus, the forecasts by [werner et al. 2010a] and helmstetter et al. [2007], which were calculated from about 25 years of high-quality californian data, should perform well for moderate earthquakes over similar time scales, but the longer periods that are relevant for seismic hazard estimates and building codes might require longer input datasets. on the other hand, according to kagan and jackson [1994], it remains an untested hypothesis that seismicity estimates based on geological observations, i.e. the earthquake history of several thousands of years, can provide more predictive and relevant information for engineering design than high-quality, low-threshold instrumental catalogs. their argument was based on two points: first, the quality of geological observations for seismic hazard is often low compared to the quality of modern earthquake catalogs; secondly, seismicity shows long-term spatio-temporal variations that necessitate an appropriate weighting of the information contained in observations of recent earthquakes and those that occurred in the distant past. to begin to quantify the impact of the duration of the learning catalog on the predictive power of smoothed seismicity forecasts, we calculated two forecasts of the smoothed seismicity model by calibrating the model on two catalogs of different durations. one forecast was based on the most recent 30 years of instrumental data, while the other one was calculated from 100 years of combined historic and instrumental data. we have submitted the forecasts to the european csep testing center at eth zurich, to be tested and compared with other forecasts within the framework of the italian earthquake predictability experiment, csep-italy [see schorlemmer et al. 2010a]. comparing the performances of these two forecasts might shed light on the impact of the length of the learning catalog. the present study is structured as follows. in section 2, we describe the italian earthquake catalogs from which we estimated the future earthquake potential in italy. section 3 describes the model and its calibration on the two datasets, while in section 4 we present the earthquake forecasts; we discuss our conclusions in section 5. 2. data 2.1. the csi 1.1: 1981-2002 for more details about the catalogs discussed here and below, see [schorlemmer et al. 2010a] and references therein. as the primary data source for the forecast based on smoothed instrumental seismicity, we used the catalog of italian seismicity (catalogo della sismicità italiana, csi 1.1) [castello et al. 2007, chiarabba et al. 2005], which is available from http://www.cseptesting.org/regions/italy. the csi spans the time period from 1981 until 2002, and it reports the local magnitudes, in agreement with the magnitude scale that will be used during the prospective evaluation of these forecasts. schorlemmer et al. [2010a] reported a clear change in the earthquake numbers per year in 1984 due to the numerous network changes in the early 1980s, and they recommended using the csi data from mid-1984 onwards. we therefore selected the earthquakes listed in the csi from july 1, 1984, to december 31, 2002, within the csep-italy collection region defined by schorlemmer et al. [2010a]. to avoid possible contamination from quarry blasts and volcanic microseismicity, we set a uniform completeness magnitude threshold of mt = 2.95 for the entire collection region, which was higher than the threshold of mt = 2.5 calculated by schorlemmer et al. [2010a] for onshore seismicity. 2.2. the bsi catalog: 2003-2009 for prospective tests of the forecasts submitted [see schorlemmer et al. 2010a], we used the italian seismic bulletin (bollettino sismico italiano, bsi) earthquake catalog that is recorded by the istituto nazionale di geofisica e vulcanologia (ingv) [bsi working group 2002, amato et al. 2006]. the bsi is available at http://bollettinosismico.rm.ingv.it, and since july 2007 at http://iside.rm.ingv.it/. we used earthquakes listed in the bsi from january 1, 2003, until june 25, 2009. as the quality of the data for small earthquakes in the bsi catalog between 2003 and 2005 was questionable, we applied a relatively high magnitude threshold of mt = 2.95. for the forecast based on recent instrumental observations, we merged the bsi and csi catalogs (hereinafter referred to as the «merged instrumental catalog», mic). 2.3. the cpti08, 1901-2006 for the forecast based on instrumental and historic seismicity, we used the parametric catalog of italian earthquakes (catalogo parametrico dei terremoti italiani, version cpti08) [rovida and the cpti working group 2008], which was available from http://www.cseptesting. org/regions/italy. the cpti08 is a preliminary revision of the 2004 cpti04 [cpti working group 2004]. it covered the period from 1901 to 2006, and it was based on both instrumental and historic observations. the cpti08 lists the moment magnitudes that were either estimated from macroseismic data or calculated using linear («ad hoc» [rovida and the cpti working group 2008]) regression relationships between surface waves, body waves or local magnitudes [mps working group 2004]. castellaro et al. [2006] showed that standard linear regression can lead to biased and uncertain smoothed seismicity earthquake forecasts 108 109 magnitude conversions, because magnitude observations and the associated errors can violate the simplifying assumptions of standard linear regression. to remedy this, castellaro et al. [2006] advocated the use of general orthogonal regression. therefore, the cpti08 can be expected to include bias and large uncertainties (like most historic catalogs) that might affect the quality of the earthquake forecasts. in their global forecasts, kagan and jackson [2010a] therefore used more homogeneous catalogs: the global centroid moment tensor catalog [ekström et al. 2005] and the global preliminary determination of epicenters catalog by the u.s. geological survey [u.s. geological survey 2001]. nonetheless, the cpti08 offers a much longer training catalog (100 years) than either of these two global catalogs (30 to 40 years), which allowed our investigation of the effects of the length of the training catalog on the forecasts generated. therefore, we accepted these potential shortcomings of the cpti08 for this study. as the prospective experiment used local magnitudes, we converted the moment magnitudes to local magnitudes using the same regression equation that was used to convert the original local magnitudes to moment magnitudes for the creation of the cpti [mps working group 2004, schorlemmer et al. 2010a]: (1) as for the bsi catalog and the csi, we only selected shocks within the collection region and with depths of less than 30 km. some of the earthquakes were not assigned depths, mostly during the early part of the cpti. we included these earthquakes as observations within the testing region because it was very unlikely that these were deeper than 30 km [see also schorlemmer et al. 2010a]. we selected the earthquakes with moment magnitudes mw ≥ 4.75 [schorlemmer et al. 2010a], which corresponded to local magnitudes of ml ≥ 4.45. 3. model calibration this model has previously been documented by helmstetter et al. [2007] and werner et al. [2010a], and so we provide only a brief overview of it here. first, the earthquake catalogs were declustered to remove the strong influence of triggered sequences (section 3.1); without declustering, there would be the need to use a more complicated, time-dependent model that used the omori-utsu law to remove the influence of aftershocks [omori 1894, utsu et al. 1995]. once declustered, the seismicity was smoothed with an adaptive power-law kernel (section 3.2). the bandwidth of the kernel at each earthquake epicenter was adapted to the distance to the k-th nearest neighbor. to estimate the optimal number of neighbors to include in the smoothing, we divided the catalogs into two non-overlapping sets: a learning catalog and a testing catalog. in section 3.3, we determine the optimal number of neighbors by calculating the spatial density of seismicity from the learning catalog and evaluating its predictive power on the testing catalog. the spatial density was scaled to the number of expected earthquakes by using the mean number of observed earthquakes (section 3.6). finally, to obtain a ratespace-magnitude forecast, we multiplied the scaled spatial density by a tapered gutenberg-richter magnitude-frequency distribution (section 3.5). in contrast to the model presented by helmstetter et al. [2007] and werner et al. [2010a], we used a homogeneous threshold for the completeness magnitude as the previous method that estimated the threshold as a function of space did not perform well for italy. 3.1. declustering to estimate the spatial distribution of spontaneous earthquakes, we used the declustering algorithm proposed by reasenberg [1985], as modified by helmstetter et al. [2007] and werner et al. [2010a]. as in these prior studies, we set the input parameters to rfact = 8, xk = 0.5, p1 = 0.95, xmin = 1 day and xmax = 5 days. we varied xmeff according to the different learning catalogs used. as the interaction distance, we used the scaling r = 0.01 × 100.5m km, as suggested by wells and coppersmith [1994], instead of r = 0.011 × 100.4m km and r < 30 km in the reasenberg algorithm. like werner et al. [2010a], we set the localization errors to 1 km horizontal and 2 km vertical. we found that about 80% of the earthquakes in the mic were spontaneous, while in the cpti, about 92% of all of the shocks were independent, according to the reasenberg classification. 3.2 adaptive kernel smoothing of declustered seismicity we estimated the density of spontaneous seismicity in each 0.1˚× 0.1 ̊cell by smoothing the location of each earthquake i with an isotropic adaptive power-law kernel : (2) where di is the adaptive smoothing distance, and c (di) is a normalizing factor, so that the integral of over an infinite area was equal to 1.0. we measured the smoothing distance di associated with an earthquake i as the horizontal distance between event i and its k-th closest neighbor. the number of neighbors, k, was an adjustable parameter that was estimated by optimizing the forecasts (see section 3.3). we imposed di > 0.5 km to account for location uncertainty. the kernel bandwith di thus decreased if the density of the seismicity was high at the location ri of the earthquake i, so that we had higher resolution (smaller di) where the density was higher. the density at any point was estimated by: (3) where nl is the total number of earthquakes in the learning smoothed seismicity earthquake forecasts 1.231 ( 1.145) .m ml w= ( )k rdi ( )k rdi ( ) ( ) ( ) k r r d c d 2 2 1.5d i i i = +; ; ( )r ( )r k r r( ) 1 d i n ii l =n = / smoothed seismicity earthquake forecasts 110 model t1 t2 mt nl t1 t2 mmin nt l g k 1 1984 2003 2.95 2,632 2004 2008 4.95 6 −45.4 2.12 5 14.2 2 1984 2003 3.45 755 2004 2008 4.95 6 −45.0 2.26 4 25.5 3 1984 2003 3.95 223 2004 2008 4.95 6 −44.5 2.44 2 32.5 4 1984 2003 4.45 63 2004 2008 4.95 6 −44.1 2.60 1 48.8 5 1984 2003 4.95 19 2004 2008 4.95 6 −45.5 2.07 1 95.6 6 1984 2003 5.45 5 2004 2008 4.95 6 −48.1 1.34 1 221.3 7* 1984 1998 2.95 1,900 1999 2003 4.95 8 −65.4* 0.94* 50* 60.1 8 1984 1998 3.45 550 1999 2003 4.95 8 −64.6 1.04 48 123.3 9 1984 2003 3.95 147 1999 2003 4.95 8 −63.0 1.27 27 206.3 10 1984 2003 4.45 27 1999 2003 4.95 8 −63.1 1.25 8 248.7 11 1984 2003 4.95 12 1999 2003 4.95 8 −63.3 1.22 4 290.9 12 1984 2003 5.45 4 1999 2003 4.95 8 −63.7 1.16 2 500.9 13 1984 1993 2.95 1,145 1994 1998 4.95 13 −86.3 3.14 1 7.8 14 1984 1993 3.45 328 1994 1998 4.95 13 −86.5 3.09 1 16.1 15 1984 1993 3.95 81 1994 1998 4.95 13 −99.4 1.15 5 107.5 16 1984 1993 4.45 15 1994 1998 4.95 13 −97.2 1.37 6 407.1 17 1984 1993 4.95 4 1994 1998 4.95 13 −97.3 1.35 1 388.4 18 1984 1993 5.45 0 1994 1998 4.95 13 _ _ _ 19 1984 2009 2.95 3,522 2004 2008 4.95 6 −37.8 7.48 1 5.0 20! 1984 2009 2.95 3,522 2004 2008 4.95 6 −41.6! 3.98! 6! 13.8 imput catalog (mic) target catalog (mic) results model t1 t2 mt nl t1 t2 mmin nt l g k 1 1901 2001 4.45 605 2002 2006 4.95 7 −55.5 1.39 35 95.9 2 1901 2001 4.95 166 2002 2006 4.95 7 −54.8 1.54 12 102.9 3 1901 2001 5.45 51 2002 2006 4.95 7 −55.2 1.46 6 127.8 4 1901 1996 4.45 576 1997 2001 4.95 15 −97.1 3.41 17 63.7 5 1901 1996 4.95 155 1997 2001 4.95 15 −101.8 2.49 8 84.6 6 1901 1996 5.45 46 1997 2001 4.95 15 −104.3 2.12 5 120.8 7 1901 1991 4.45 565 1992 1996 4.95 2 −15.9 4.41 2 17.8 8 1901 1991 4.95 153 1992 1996 4.95 2 −16.5 3.17 1 22.7 9 1901 1991 5.45 46 1992 1996 4.95 2 −18.0 1.53 8 199.3 10 1901 1986 4.45 551 1987 1991 4.95 6 −46.6 1.72 2 18.0 11 1901 1986 4.95 147 1987 1991 4.95 6 −46.2 1.84 1 22.6 12 1901 1986 5.45 44 1987 1991 4.95 6 −48.2 1.32 4 101.7 13 1901 2006 4.45 623 2002 2006 4.95 7 −39.4 13.78 1 11.3 14 1901 2006 4.45 623 2002 2006 4.95 7 −39.4! 1.35! 6! 34.1 imput catalog (cpti) target catalog (cpti) results table 1. results of the optimization of the spatial density estimate using the mic from july 1, 1984, to june 25, 2009. we varied the learning and target catalogs. the target catalog is the mic in the testing region. the input catalog is the declustered mic in the collection region. t1: first year of data taken from the learning or target catalog; t2: last year of data taken from the learning or target catalog; mt: magnitude threshold of the learning catalog; mmin: magnitude threshold of the target catalog; nl and nt: number of earthquakes in the learning and testing catalogs, respectively; l: log-likelihood score of a model; g: model probability gain per earthquake over a spatially uniform model; k: optimal number of neighbors to include in the bandwidth of the smoothing kernel; (di): mean adaptive bandwidth (km); ! : indication where k was not optimized, but constrained to k = 6; *: maximum k = 50 of the range [1, 50] was attained. table 2.as for table 1, but using the cpti from 1901 to 2006 as the dataset. ! indicates that kwas not optimized, but constrained. see table 1 for further details. di di 111 catalog. however, the forecasts were given as an expected number of events in each cell of 0.1˚. we therefore integrated equation (2) over each cell and summed over all of the contributing earthquakes, to obtain the seismicity rate of each cell. 3.3. optimizing the spatial smoothing we estimated the parameter k, the number of neighbors used to compute the smoothing distance di in equation (3), by maximizing the likelihood of the model. we built the model n´(ix, iy) in each cell (ix, iy) from the data in the learning catalog, and evaluated the likelihood of target earthquakes in the testing catalog. as we assumed independence of the spatial density from the magnitude distribution and the total expected number of events, we optimized the normalized spatial density estimate in each cell (ix, iy) using: (4) where nt is the number of observed target events. the expected number of events for the model n* was thus equal to the observed number nt. the log-likelihood of the model is given by: (5) where n is the number of events that occurred in cell (ix, iy). to adhere to the rules of the csep-italy predictability experiment, we assumed that the probability p of observing n events in cell (ix, iy) given a forecast of n*(ix, iy) in that cell was given by the poisson distribution: (6) we built a large set of background models n* by varying: (i) the start times, end times and magnitude thresholds of the learning and testing catalogs; and (ii) the catalog (either the mic or the cpti). we evaluated the performance of each model by calculating its probability gain per target earthquake relative to a model with a uniform spatial density: (7) where l0 is the log-likelihood of the spatially homogeneous model. 3.4. results of the spatial optimization tables 1 and 2 show the results of the spatial optimization on the mic and the cpti, respectively. for each model, we determined the optimal smoothing parameter k in the range [1, 50] by choosing the value that maximized the likelihood of the target earthquakes in the target catalog, given the smoothed spatial density estimated from the learning catalog. we varied the magnitude threshold of the input catalog to test whether including small earthquakes results in greater predictability of future m ≥ 4.95 earthquakes. we also changed the target periods to test the robustness of the results. in figure 1, we show the probability gains per earthquake against the magnitude threshold of the two learning catalogs. for comparisons, we also included the gains obtained for the five-year period of 2004-2008 (inclusive) in california by werner et al. [2010a]. the gains obtained in italy fluctuated strongly for different target periods, and it was difficult to detect a systematic trend in the gain as a function of the magnitude threshold of the learning catalog. in contrast to california, where around 25 earthquakes of m ≥ 4.95 tend to occur every five years, italy has experienced far fewer shocks of such sizes; during the 1992-1996 period in the cpti, the gains were calculated from only two target earthquakes. the small sample size of these target earthquakes might explain the observed fluctuations in the calculated gains (see also below, and section 5). for the target period 1994-1998 of the mic, the gains were especially small for low thresholds. for mt = 2.95, the smoothing parameter reached the maximum value of 50 (table 1, model 7), providing a gain smaller than unity, i.e. the uniform forecast outperformed the smoothed seismicity forecast (further increasing the amount of smoothing eventually leads to a uniform density, such that the gain would be equal to unity). the long-range smoothing required by the target events can be traced back to the occurrence of three earthquakes in 2002: the september 6 sicily earthquake north-east of palermo, and the october 31 molise earthquake and one of its aftershocks. the predicted densities in the relevant cells was increased by a factor of almost ten as the algorithm increased the smoothing from smoothed seismicity earthquake forecasts figure 1. probability gain per earthquake versus magnitude threshold of the learning catalogs for various five-year target periods: blue, mic; red, cpti; cal. 2004-2008, gains obtained for california by werner et al. [2010a]; homogeneous, reference gain of a spatially homogeneous forecast. * ( , ) ( , ) ( , ) i i i i i i n x y x yii x y t yx =n n n l l // log * ( , ),l p i i nx y ii yx = n6 @// * ( , ), * ( , ) ! exp * ( , ) p i i n i i n i i x y x y n x y=n n n-6 6 6@ @ @ exp n l l g 0 t = -c m k = 1 to k = 50 (see also discussion in section 5). whenever target earthquakes occur in previously active regions, the optimal amount of smoothing is small (k = 1) and the gains tend to be higher (see, e.g., table 1, model 13 for the 1994-1998 target period, which included the 1997 colfiorito earthquake sequence). however, exceptions exist to the expected anti-correlation between k and g: the 15 targeted earthquakes during the 1997-2001 target period of the cpti were forecast best with a smoothing parameter k = 15, although they realized a gain per earthquake of g = 3.41. to calculate the spatial densities for the final forecasts for the predictability experiment (table 1, model 20!obtained from the mic, and table 2, model 14! obtained from the cpti), we had to decide which magnitude threshold to apply to the learning catalog, which smoothing parameter to use, and whether to use all of the existing data up to the end of the two catalogs. we decided to use all of the available data in each catalog for the final density estimates so that the forecasts would benefit from as much data as possible. moreover, despite the observed variability in gains against the magnitude threshold of the input catalog (discussed above), figure 1 shows that, on average, there appears to be an advantage in including small earthquakes for estimating the predictive spatial density (see also discussion in section 5). therefore, to calculate the spatial densities for the final forecasts, we used the lowest reliable magnitude threshold for each catalog. finally, we used an optimal smoothing parameter of k = 6 despite the large variability across the magnitude thresholds and target periods, because the resulting density was slightly smoother than that obtained from the median (k = 5) of the optimal values for the lowest magnitude thresholds, and because both werner et al. [2010a] and helmstetter et al. [2007] used the same value. the two final predictive spatial densities based on the mic and the cpti were model 20! in table 1 and model 14! in table 2, respectively. we discuss future improvements of the spatial optimization method in section 5. 3.5. magnitude distribution we assumed that the cumulative magnitude probability distribution followed a tapered gutenberg-richter magnitude frequency distribution [gutenberg and richter 1944] with a uniform b-value and corner magnitude mc [helmstetter et al. 2007, equation (10)]: (8) with a minimum target magnitude mmin= 4.95 (for the fiveyear and ten-year csep forecast groups). bird and kagan [2004, p. 2393] classified the tectonic setting of italy as an orogen situated at a continental convergent boundary and estimated mc= . kagan et al. [2010: fig. 1, table 1] assigned onshore italy to the category of «active continent» with mc = and the southern off-shore region of italy to «trench» with mc = . for simplicity, we set a uniform value of mc = 8.0, to reflect these studies. this is likely to be a conservative choice for the most seismically active region of onshore italy. we further used a b-value of 1.0 (a maximum likelihood estimate based on magnitudes above mt = 2.95 in the mic resulted in = 1.07). we integrated the magnitude distribution of equation (8) in discrete bins of width of 0.1 to conform to the rules of the experiment. 3.6. expected number of events the expected number of events per year in each spacemagnitude bin (ix, iy, im) was calculated from: (9) where n* is the normalized spatial background density, p (im) is the integrated probability of an earthquake in magnitude bin (im) defined according to equation (8), and m is the expected number of earthquakes over a five-year or ten-year period. to estimate the expected number of earthquakes, we counted the total number of observed m ≥ 4.95 earthquakes in each (non-declustered) catalog and divided by the duration to obtain the mean number of events of m ≥ 4.95 per year. for the mic, we estimated mmic = 1.24 per year, while there was an average of mcpti = 1.72 earthquakes of m ≥ 4.95 per year in the cpti. to obtain the five-year and ten-year forecasts, we simply multiplied m by the number of years. thus, based on the shorter mic, we expect 6.2 (12.4) earthquakes from january 1, 2010 to december 31, 2014 (to december 31, 2019), while based on the longer cpti, we predict 8.6 (17.2) earthquakes over the same periods. 4. five-year and ten-year m ≥ 4.95 forecasts the five-year forecasts based on the mic and the cpti are shown in figures 2 and 3, respectively. to obtain the tenyear forecasts, we doubled the rates in each space magnitude bin because we assumed a temporally homogeneous poisson process. the forecast based on the cpti is smoother than the map based on the shorter mic, because the 3,522 earthquakes of mostly small magnitude in the mic (m ≥ 2.95) clustered more strongly than the 623 events of larger magnitude in the cpti (m ≥ 4.45). the more evenly distributed epicenters of the cpti were more uncertain than those from the mic, resulting in less clustering. we can compare the forecasts in figures 2 and 3 with those of kagan and jackson [2010a: fig. 4] and zechar and jordan [2010: fig. 2]. kagan and jackson [2010a] used a fixedbandwidth power-law kernel with an optimized bandwidth rs = 5 km, to smooth the seismicity in italy listed in the preliminary determination of epicenters catalog above a threshold mt = 4.7. by visual inspection, their forecast was similar to the forecast based on the cpti in figure 3, smoothed seismicity earthquake forecasts 112 ( ) 10 exp 10 10p m ( ) 1.5( ) 1.5( )b m m m m m mmin min c c= --6 @ 8.46 0.39 0.21+ .7 59 0. 0. 25 72+ .8 57 0. ? 35 + bt ( , , ) * ( , ) ( )e i i i i i p ix y x y mm = m n 113 although their fixed bandwidth of rs = 5 km was much smaller than the average of our optimal adaptive bandwidths (di) = 34.1 (table 2, model 14 !). the optimal smoothing bandwidths are different because kagan and jackson [2010a] optimized their bandwidth for the 2004-2006 target period and smoothed earthquakes from a different data source. given the observed dependence of the optimal smoothing distance on the chosen target period, we should expect to see differences in the optimal bandwidths. zechar and jordan [2010] smoothed the csi, the cpti and a merged («hybrid») catalog with an optimized fixedbandwidth gaussian kernel. again, different choices for the magnitude threshold and the learning data make direct comparisons difficult, except for the forecasts based on the cpti optimized for the 2002-2006 target period. figure 1 of zechar and jordan [2010] shows that the optimal smoothing lengthscale is v = 75 km, while we obtained a mean bandwidth of ≈ 95.6 (table 2, model 1), indicating broad agreement between the two methods. neither kagan and jackson [2010a] nor zechar and jordan [2010] investigated whether the optimal smoothing length scale varied with target periods. 5. discussion and conclusions werner et al. [2010b] evaluated all of the timeindependent five-year and ten-year forecasts of the csep-italy experiment retrospectively on data from the csi and cpti. using the forecasts from the first round of submissions from august 1, 2009, they found that several of the modelers had committed errors in the calibration of their models for the calculation of their forecasts, principally in the conversion of the moment magnitude scale of the cpti to the local magnitude scale that is used for prospective testing. our first submission of the forecast based on the cpti also contained an error, because of a mistake in the conversion formula we used (see equation 1). in this report, we only discuss the corrected forecasts that we submitted during the second round (january 1, 2010). in the future, we would like to make a number of improvements to the model. first, we used a relatively arbitrary declustering procedure based on reasenberg's algorithm [reasenberg 1985], although there are more objective methods that can be used [e.g. zhuang et al. 2002, console et al. 2010]. secondly, contrary to the studies of helmstetter et al. [2007] and werner et al. [2010a], we could not estimate the completeness threshold as a function of space using their method and then attempt to correct for missing small earthquakes because the results were not stable. in the future, we intend to use a more robust method for estimating the completeness threshold. thirdly, we found that the optimal smoothing parameter varied substantially for different target periods, much more so than observed by werner et al. [2010a]. the small number of target earthquakes might have caused these fluctuations. in the future, the optimal smoothing parameter should be optimized jointly over many target periods. more generally, we intend to assess the influence of the choice of kernel function. for example, do anisotropic kernels [e.g., kagan and jackson 1994] improve spatial forecasts? does the optimal kernel choice depend on the tectonic regime [e.g., kagan and jackson 2010a, kagan and jackson 2010b]? should large earthquakes count more towards the density than small earthquakes [e.g., kagan et al. 2007]? smoothed seismicity models make the implicit smoothed seismicity earthquake forecasts figure 2.earthquake forecast based on the mic. expected number of earthquakes of ml ≥ 4.95 over the five-year period from january 1, 2010, to december 31, 2014 per 0.1˚× 0.1˚ based on smoothing the locations of earthquakes of ml ≥ 2.95 in the instrumental catalog from july 1, 1984, to june 25, 2009. figure 3. earthquake forecast based on the cpti. expected number of earthquakesof ml ≥ 4.95 over the five-year period from january 1, 2010, to december 31, 2014, per 0.1˚× 0.1˚ based on smoothing the locations of earthquakes of ml ≥ 4.45 in the cpti from 1901 to 2006. di assumption that the available earthquake catalogs are long enough to estimate predictive spatial densities. kagan and jackson [1994], however, conjectured that the optimal forecast horizon of an earthquake forecast based on smoothed seismicity is related to the duration of the learning catalog. to begin to address this question, we provided two earthquake forecasts: one based on a relatively short (30 years) dataset with a lower magnitude threshold, and the other based on a longer (100 years) catalog with a higher magnitude threshold. if the conjecture by kagan and jackson [1994] is correct, the forecast based on the mic should perform better than the forecast based on the cpti over shorter periods, while the cpti-based forecast should show more predictive accuracy at longer time scales. helmstetter et al. [2007] and werner et al. [2010a] have previously provided evidence for the hypothesis that in california, including the locations of small earthquakes improved the forecasts of future epicenters of m ≥ 4.95 earthquakes [see also hanks 1992, marsan 2005, helmstetter et al. 2005, sornette and werner 2005a, sornette and werner 2005b, for perspectives on the importance of small earthquakes]. the results of the present study do not provide conclusive evidence for or against this hypothesis, perhaps because of the fluctuations induced by the small number of target earthquakes. a more robust cross-validation method for the optimal smoothing parameter might be able to address this outstanding issue. like its predecessor, the relm experiment, the csepitaly experiment required the use of a poisson distribution for the number of earthquakes per space-magnitude bin [schorlemmer et al. 2010a, schorlemmer et al. 2007]. in principle, however, each model should provide its own modeldependent uncertainty bounds [werner and sornette 2008]. for the time-independent poisson process model described here, the distribution of the number of shocks over the relevant five-year or ten-year time scales is assumed to be time-independent. however, a negative binomial distribution fits the number distribution better than the poisson distribution [schorlemmer et al. 2010b, kagan 2010, werner et al. 2010a, werner et al. 2010b]. therefore, in future iterations of the model, poisson distributions should be replaced by appropriate alternatives in each spacemagnitude bin. a difficulty with this approach will be the estimation of parameter values based on small and possibly correlated samples. nonetheless, despite the simplicity of the model and its approximations, the five-year forecast submitted by helmstetter et al. [2007] to the relm experiment is outperforming others after the first 2.5 years [schorlemmer et al. 2010b]. whether the model will perform similarly in italy, which is different tectonically from the much more seismically active california, will be an interesting test of the universal applicability of the model assumptions. data and sharing resources we used three earthquake catalogs for this study: the parametric catalog of italian earthquakes (catalogo parametrico dei terremoti italiani, cpti08) [rovida and the cpti working group 2008], the catalog of italian seismicity (catalogo della sismicità italiana, csi 1.1) [castello et al. 2007, chiarabba et al. 2005], and the italian seismic bulletin (bollettino sismico italiano, bsi) catalog [bsi working group 2002, amato et al. 2006]. the bsi catalog is available at http://bollettinosismico.rm.ingv.it, and since july 2007, at http://iside.rm.ingv.it/. the particular versions of the csi and cpti used are available at http://www.cseptesting.org/ regions/italy. acknowledgements. we thank a. christophersen, l. gulia, j. woessner and j. zechar for stimulating discussions and f. euchner for computational assistance. mjw was supported by the extremes project of the eth competence center environment and sustainability (cces). ah was supported by the european commission under grant trigs043251, and by the french national research agency under grant aseismic. ddj and yyk received support from the national science foundation through grant ear-0711515, as well as from the southern california earthquake center (scec). mjw thanks scec for travel support. scec is funded by the nsf cooperative agreement ear0106924 and the usgs cooperative agreement 02hqag0008. the scec contribution number for this study is 1437. references amato, d., l. badiali, m. cattaneo, a. delladio, f. doumaz and f.m. mele (2006). the real-time earthquake monitoring system in italy, geosciences (brgm), 4, 70-75. bird, p. and y.y. kagan (2004). plate-tectonic analysis of shallow seismicity: apparent boundary width, beta, corner magnitude, coupled lithosphere thickness, and coupling in seven tectonic settings, bull. seismol. soc. am., 94, 23802399, plus electronic supplement. bsi working group (2002). bollettino sismico italiano, 20022009, istituto nazionale di geofisica e vulcanologia, bologna; http://bollettinosismico.rm.ingv.it/. castellaro, s., f. mulargia and y.y. kagan (2006). regression problems for magnitudes, geophys. j. intern., 165, 913930; doi: 10.1111/j.1365-246x.2006.02955.x. castello, b., m. olivieri and g. selvaggi (2007). local and duration magnitude determination for the italian earthquake catalog, 1981-2002, bull. seismol. soc. am, 97, 128-139; doi: 10.1785/0120050258. chiarabba, c., l. jovane and r. stefano (2005). a new view of italian seismicity using 20 years of instrumental recordings, tectonophysics, 395, 251-268. console, r., d.d. jackson and y.y. kagan (2010). using the etas model for catalog declustering and seismic background assessment, pure appl. geophys. 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(the frank evison volume), 167, 859-876; doi: 10.1007/s00024-010-0081-5. sornette, d. and m.j. werner (2005a). constraints on the size of the smallest triggering earthquake from the epidemictype aftershock sequence model, bath's law, and observed aftershock sequences, j. geophys. res., 110; doi: 10.1029/2004jb003535. sornette, d. and m.j. werner (2005b). apparent clustering and apparent background earthquakes biased by undetected seismicity, j. geophys. res., 110; doi: 10.1029/2005jb003621. u.s. geological survey (2001). preliminary determination of epicenters (pde), monthly listings, us department of interior/ geological survey, national earthquake information center, april-june 2000, open file report 2000-600-b; http://earthquake.usgs.gov/earthquakes/ eqarchives/epic/epic_global.php. utsu, t., y. ogata and r.s. matsu'ura (1995). the centenary of the omori formula for a decay law of aftershock activity, j. phys. earth, 43, 1-33. wells, d.l. and k.j. coppersmith (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seismol. soc. am., 84, 974-1002. werner, m.j. and d. sornette (2008). magnitude uncertainties impact seismic rate estimates, forecasts and predictability experiments, j. geophys. res. solid earth, 113; doi: 10.1029/2007jb005427. werner, m.j., a. helmstetter, d.d. jackson and y.y. kagan (2010a). high resolution longand short-term earthquake forecasts for california, bull. seismol. soc. am., in revision, preprint available at http://arxiv.org/abs/0910.4981. werner, m.j., j.d. zechar, w. marzocchi and s. wiemer (2010b). retrospective evaluation of the five-year and ten-year csep-italy earthquake forecasts, annals of geophysics, 53, 3 (present issue). zechar, j.d., d. schorlemmer, m. liukis, j. yu, f. euchner, p.j. maechling and t.h. jordan (2009). the collaboratory for the study of earthquake predictability perspective on smoothed seismicity earthquake forecasts computational earthquake science, concurr. comput. pract. exp.; doi: 10.1002/cpe.1519. zechar, j.d. and t.h. jordan (2010). simple smoothed seismicity earthquake forecasts for italy, annals of geophysics, 53, 3 (present issue). zhuang, j., y. ogata and d. vere-jones (2002). stochastic declustering of space-time earthquake occurrences, j. am. stat. assoc., 97, 369-380. *corresponding author: maximilian j. werner, eth zurich, swiss seismological service, zurich, switzerland; email: mwerner@sed.ethz.ch © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. smoothed seismicity earthquake forecasts 116 convegno internazionale tecnico·scientifico dello spazio presentazione del prof. seno basilio focaccia, presidente della rassegna e del convegno tecnico·scientifico dello spazio. (romll . e. u. r., 13 giul!110 1961) pe\" ~od(lisfal"e ii' nnlllp\"ose sollpdt-a7.ioni ppitpnuteei, sia.mo liplì di ]1nhbli('~u"(' in qnprlp pagilw i!li atti del primo c'oill.'chilo hltcl"/hlziol!alc l'ecllico·scirmtifico dello spazio, or,ganizzh.j,o (bila rassegna inlernazional(' ~iptt.\"oniea, e nndp~i,\"(' in f{.o1l!a, 1](>1 palazzo (lpi conl!l'prri dl'li'i~uh, dal l:! al 15 giu,gno 1961. aneh(' in italia (la qnalell(' tempo oi'i!~mi \"l'sponsabili di goyi'i'llo, ficienziati, t.eenici ('(l ilhlmtriali im.nno indirizzato la lo\"o att,enziolw ai rliifwili probl(,llli dello spazio, ija rassq~nh" rempl'e p1'onta ari lh'vertin' le psi,gen7.p (iplh seipllzn" di'ila t-peniea e della indnstria n!'! lo\"o inepssa,ntl' 1i1'o~tps,<;o, giù. 111'1 giug'no 1959, in apl)('ndim~ :l,l vi congtesso ]1('1' l'f<ildtl'()]lìr'a, an'va ()]'i!alliz mto nn conyl'gno ('d una most.ra sulle a]1plieazioni (lell'('lel.troniml, p (ipil'pncrl!ia. nuclear(' 11('1 campo (1('il'astl'owlut,ieai nwntrl' iwl vtt coilgt('sso del 1960 una ~pzi()]h' l'm stu,t~~ (lpdi('ab~ alli' \"adio('()]!lnni(',azioni ,~pa7.iali p nell'vii congresso, (li qu(\~l'anno 1061, un important-i' ~ panl'l ~ (ipdiea to ai probl(,llli r,olll1pssi ('.01 ~ tm,cking-» dpi satdliti m'tilieiali, ha. 1\o\'ul0 luo,go sotto la l)j'('si(ll'nz:~ di'l p\"of. alhino antinori. ('.()nt~'mporanpanlente, si l'm ,giunt,i all'01wlnizzazionp (li lli1 \'!'l'o (' proprio conyp,gno tpcnieo-&limltifico di'ilo s]1azio, flnjl10si flollo il patronatd di'i consiglio ~azionl\lfl (lplle ricerche (' eon la pipna 1'(1 pftì(:ipntl' ('olli~b01'll.zionp di'i ì\ilniskl'o (jpl\n. difpsa, di'li' ass()(:iazione ilhlustri(' apl'o-spllziali (ehe pi'espnth,vano anelli' una ~iosl,ra tpeno]og-iea) p (11'1l'a.o,,'l(wiazionp tta.\ianahaz7.i, ti sm',(',psro riportat,o (in. ql\(\~to pri!llo ('onn'gno è (loyuto in !liodo pal'tieolare all'oppm (lpi !lipm\)l'i (11'1 ('olllitato sdl'nw-ico, ehi' yoglilllllo ringm,zial'p h,n('.ol'a in qlw.~l11 spdp: il pi'('sidpnì!' (lplja. commissioni' rie('l'elw l':\pll,ziali del c,n.h. p1'or. bl'0lllioi il presi(lente (lei consil!!io sllpeconv.;us') i);ternaz1u~ale '['ecnico·sf:ie"<tifict) dello ~pazi() riore t{'enieo rlelie tf\i{,(~illuni(\azioni, pl'of. marino; il gl'n. giansa nti, capo (lei spi'yizio mf\t.erl'ologi(\o delfisppttora to delle tf'i('(\omllnicazioni l'cl assistpnza. al \'010; il dil'd.t,orl· dr1l'o.~sl'ryatorio ast'l'onolllico di roma, prof. cimino; il pn'si(lent,e (kl1'ashonin,zione indust'l'ie af'l'ospaziali, ing. bruno; il pl'c'si(ll'nh' clell'assoeiazionf' italiann. i{azzi, pl'oleuln.; il prof. chlrle,to dl'll'i.~titut,o di fisica, rlell'universit:\ di l{'olna e l'ing'. tc'sio delh h.p.d., sc'grehtorio 't'eenico rlel convf'j.{no. oli int(\l'\'pnti al convl'gno ~ono stat,i numerosi c'rl alt.amc'ntl' qualificati. il conyc'gno è ~t.ato {'o1l\'pnientl'nwnte divi.~o in sezioni compren· d<:>nt,i 1(\ ricl'l'(lhe met{'l'c'ologiche e quelle scientifiche nell'alta at,mosfem; l'dettronimt slja.ziale, j vadi tipi (li prollulsione lwllo ~pazio prl, inhne, l'ilhlust,ria missilist,il'a. nelle l><togine dw seguono, attraverso gli intervc'nt,i rlei singoli rl'latori, i leu,ori potranno scorgere i frnt.t,i ehe q(wsta. fat,jl'a eonsl'nte di rlhlnoglil'rl'; ma , sel',ondo noi, nwrit,o precipuo di quest.o ('onvegno è di aver, per la prima yo]t.a in haha, impost,at-o eon chiltol'ezza di irlee lo' fornito in sintesi un panorama generale ed in anali.~i della t'l'htt:k zionp (lei pl'oblellli follflaulf'ntali di tale set,t.ore olt.re a,cl avervi f\1thl i,onvenirl' a colbboml'e gli element,i e le orga,nizz,tozioni più int.ere.".,~a,t,p p più qllalifica.te clel nostro pal'~e. già si annuncia j){'r il giugno hi52, il secondo convegno, che com· jjl'pllflnfl. tutti i sett.ori dell'attivita spazialc, ecl al qua,le noi aug'lll'huno il più lusinghiero succf'sso. a l c u n e c a r a t t e r i s t i c h e d e l f r o n t e d e l v e n t o d u r a n t e l e t e m p e s t e d i s a b b i a i n e g i t t o m . g . e l f a n d y i n q u e s t o l a v o r o si s t u d i a n o l e r e g i s t r a z i o n i d i u n a n e m o m e t r o fli d i n e r e l a t i v e a l l a d i r e z i o n e ed a l l a v e l o c i t à d e l v e n t o d u r a n t e d i « l i a b o o b s » c ) c l i c s o r g o n o n e l l a r e g i o n e di k b a r l o u m e l e t e m p e s t e di s a b b i a c l i c c o l p i s c o n o l e r e g i o n i di h e l i o p o l i e di a l m a z a ( d i s t r e t t o d e l c a i r o ) . d a t a l e s t u d i o si r i c a v a c l i c l ' i n t e n s i t à d e l l a t e m p e s t a , i n p a r t i c o l a r e la m a n c a n z a di v i s i b i l i t à , d i p e n d e n o n s o l o d a l l ' a u m e n t o d e l l a v e l o c i t à o r i z z o n t a l e d e l v e n t o m a a n c h e d a l l ' i n t e r v a l l o di t e m p o c h e 1 j ! i j ' p! f i g . 1 registrazione del velilo a kartouin (staek l a b . ) 17 agosto 19.ììs i n t e r c o r r e t r a d u e m a s s i m i s u c c e s s i v i : i n f a t t i i n ogni l o c a l i t à a p i ù l u n g o p e r i o d o c o r r i s p o n d e t e m p e s t a p i ù i n t e n s a . p e r u n c o l p o d i v e n t o s i n g o l o l a c u i d u r a l a sia a p p r e z z a b i l e , c i r c a 1 5 m i n u t i ( e il c a s o n o r m a l e c o n l i a b o o b s n o n s t a b i l i ) ( f i g . 1 ) , la v i s i b i l i t à d i m i n u i s c e r a p i d a m e n t e m e n t r e si i n n a l z a u n a l a r g a n u b e di p o l v e r e . i n v e c e , q u a n d o s o t t o u n g r a d i e n t e di p r e s s i o n e c r e s c e n t e la velo» •ila d e l v e n t o a l i m e n t a g r a d a t a m e n t e fino a d u n v a l o r e c r i t i c o , si s o l h n a n o c o n t e m p o r a n e a m e n t e n u b i d i s a b b i a e di p o l v e r e . a p ( ' ) e ' il nome arabo delle tempeste di .-abbia del sudan. f r o n t e d e l v e n t o d u r a n t e l e t e m p e s t e di s a b b i a in e g i t t o 611 p e n a la v e l o c i t à s u p e r a t a l e l i m i t e , m e n t r e si sussegue u n a serie di p i c c o l i c o l p i di v e n t o di b r e v e p e r i o d o la v i s i b i l i t à d i m i n u i s c e notev o l m e n t e m a la t e m p e s t a c h e si è s v i l u p p a t a in questo m o d o è gen e r a l m e n t e m e n o i n t e n s a e n o n si e s t e n d e v e r t i c a l m e n t e a l altezze t a n t o p r o n u n c i a t e q u a n t o q u e l l e r a g g i u n t e n e l caso di t e m p e s t e a t i p o i n s t a b i l e . la fig. 1 è la r i p r o d u z i o n e di u n a r e g i s t r a z i o n e a l l ' a n e m o g r a f o di d i n e eseguita a k h a r t o u m d u r a n t e un h a b o o b s ed è c h i a r a m e n t e v i s i b i l e la r e g i s t r a z i o n e e l ' a n d a m e n t o del c o l p o di v e n t o . l a fig. 2 è l̂ i r i p r o d u z i o n e di u n a r e g i s t r a z i o n e d u r a n t e u n a t e m p e s t a sul c a i r o ed il g r a d u a l e a u m e n t o d e l l a v e l o c i t à m e d i a è a l t r e t t a n t o c h i a r a m e n t e v i i b i l c . e " n a t u r a l e m e t t e r e in r e l a z i o n e l ' i n t e n s i t à d e l l a c o m p o n e n t e vert i c a l e del v e n t o con l ' i n t e n s i t à d e l l a t e m p e s t a ossia con la d e n s i t à e l ' e s t e n s i o n e v e r t i c a l e della p o l v e r e sospesa. s u p p o n i a m o c h e la p o l v e r e sia t r a s p o r t a t a n e l l ' a r i a , d o p o essere f i g . 2 r e g i l r a z i o n e del vento al cairo del 11 marzo 1911. tempesta di sabbia dalle 0810 alle 1530 ( g . m . t . ) . ai. fi. ki.-famiv t a l a -odiala dal n o l o , con un processo i m i l c a una c o r r e l i l e v e l l i c a l e , i n d i e l i i a i n o con n , e n r e s o in s e c o n d i , il p e r i o d o di una singola onda d e l l a eurva del v e n t o e s u p p o n i a m o , clic questa possa r a p p r e s e n t a r s i m e d i a n t e una i n u o i d e ( f i g . 3 ) . si m i u r i il t e m p o a p a r t i r e dal p u n t o 0 co-i c l i c il t e m p o nel p u n t o o è uguale a z e r o ; l ' i n t e r v a l l o di t e m p o tra i <• c sarà uguale ad n sec. il t e m p o r e l a t i v o al p i l l i l o i s a r à a l l o r a — r i i e q u e l l o r e l a t i v o a! p u n t o li sarà ri i c c . s i a u la v e l o c i t à m e d i a o r i z z o n t a l e . n' lo s c a r t o da u al t e m p o i e //' lo c a r i o m a s s i m o m e d i o da u d u r a n t e ogni i n t e r v a l l o di t e m p o , p e r i o d o , c o s i c c h é u r a p p r e s e n t a la m e d i a dei m a s i m i r e l a t i v i ai c o l p i di v e n t o ; ir. ir', i r " r a p p r e s e n t a n o i c o r r i s p o n d e n t i valori p e r la c o m p o n e n t e v e r t i c a l e . h n o l o c l i c q u a n d o c ' è un c o l p o di v e n t o si lui un n o t e v o l e a u m e n t o d e l l a v e l o c i t à del v e n t o sia in d i r e z i o n e o r i z z o n t a l e c l i c ili d i r e z i o n e v e r t i c a l e negli strali d ' a r i a in p r o s s i m i t à del s u o l o . da i a ./> la v e l o c i t à o r i z z o n t a l e del v e n t o a u m e n t a da ((/<—«') a (u i l ' ) e la p a r t i c e l l a di s a b b i a c l i c è t r a s p o r t a l a per t u l i o q u e l o t e m p o dall a r i a si -olleva a s p e s e d e l l ' e n e r g i a c i n e t i c a t o t a l e . tn a l t r e p a r o l e l ' a u m e n t o d e l l ' e n e r g i a c i n e t i c a di una p a r t i c e l l a ili massa ni dal t e m p o i iil t e m p o / (ii. a u m e n t o c l i c è u g u a l e a — m d (u -i/ / ) " : (ir + ir')2 l 2 / ) sarà p e s o p e r far s a l i r e la p a r t i c e l l a di u n a c e r t a altezza ir di c o s i c c h é ili gii'di-md\ (u + u')+ (u~ic')~ { 2 i ) da cui 1 d i , , , ,) i ( u + u ) + ( ir ir )" [ | | il v a l o r e m e d i o di ir' nel! i n t e r v a l l o — n i. +n l è 2g di i i ir' di = — j (/• (u + ii'f+ (_«' ir')' ( n i n g i ' \ 4 \ e l l a r e g i o n e in cui la s a b b i a si innalza di c i r c a 7 0 0 0 p i e d i ( c i r c a 2 3 0 0 m ) dovrà e s i s t e r e un v a l o r e m e d i o di tu'; q u e s t o v a l o r e m e d i o sia ruppi e s e n t a t o da q n e l ! interv allo c o n s i d e r a t o , c o s i c c h é m o n t e i i k i . v e n t o i l l ' k a n t k i.k t e v i i ' e s t e 1)1 s vidi) a in i m i 1(1 6 1 3 i . , , i i o = .' ir 2 il li il ~ : ic~ 2 ic ir ir ~ \ „ 1 ( gn i -1li u ' -f -1 ir ir' — il u : ir ir [ 2 ] in cui si è t r a s c u r a l o l ' a t t r i t o e l i viscosità q u a n d o si è -crii la 1 equaz i o n e della c o n s e r v a z i o n e 1 u' f i " . d e l l ' e n e r g i a . q u a n d o -i p r e senta un solo c o l p o di v e n t o la t e m p e s t a raggiunge un m a s i m o nei p r i m i n 2 sec o n d i m e n t r e a v v e n g o n o notevoli s p o s t a m e n t i di m a e d ' a r i a in d i r e z i o n e v e r t i c a l e e la -ila i n t e n s i t à dalla vci o r i t à d i p e n d e dalla v e l o c i t à v e l l i c a l e ir. dall e q u a z i o n e 1 2 ] -i ol t i e n e ir 0 il ii u i w' 1 3 ] la q u a l e m o l r a c l i c a pari valori di u. //'. ecc. -i devono p r e v e d e r e con g r a n d i v a l o r i ili u, g r a n d i v e l o c i t à v e r t i c a l i . l.a seconda c a r a t t e r i s t i c a è la ell'eltiva l l u l t u a z i o n e nella m e d i a d e l l e v e l o c i t à e della v i s i b i l i t à d u r a n t e le t e m p e s t e di s a b b i a . t a l i fluttuazioni s o n o c h i a r a m e n t e v i s i b i l i n e l l e r e g i l r a z i o n i , una d e l l e quali ( l l e l i o p o l i s i() g e n n a i o 1 9 1 0 ) è r i p r o d o t t a nella figura i : q u e s t a m o l r a t r e o n d e successive n e l l ' i n t e r v a l l o di t e m p o tra le 0 9 0 0 e le 22(111 ( t e m p o l o c a l e ) . i n a l t r o l i p o di o n d a è m o s t r a t o in figura .">. ricavala d a l l a regi-l r a z i o n e dell a n e m o m e t r o di dinedi a l m a z a . d i s t r e t t o del (cairo, d u r a n t e la t e m p e s t a di s a b b i a del 16 e it gennaio 1912. l.a l i n e i k l m n w r a p p r e e n t a la v e l o c i t à m e d i a in c h i l o m e t r i ali ora e -olio c h i a r a m e n t e v i s i b i l i le d u e o n d e k 1. m n o i' e p o r s t l v le q u a l i h a n n o lo s l c s o a n d a m e n t o . il d i a r i o g i o r n a l i e r o c h e -i conserva a l . g . e l ' f a n d y f i g . 4 registrazione d e i r a n o m o m c t r o di dina ( l l e l i o p o l i s 16 gennaio 1910» ad a l m a z a d e s c r i v e le d u e t e m p e s t e di s a b b i a c l i c si sono m a n i f e s t a l e n e l l i n t e r v a l l o di t e m p o in c u i la v e l o c i t à d e l v e n t o a u m e n t a . "«kiww n tu io ai l i . 'i-i « i r v l p t v l — s ii l \ a — 0 k " -t r u4 1 n i l f i g . 5 registrazione della velocità media e della direzione del vento ad almaza nei giorni 16 e 17 gennaio 1912 f r o n t e d e l v e n t o d u r a n t e l e t e m p e s t e ih s a b b i a in e g i t t o 6 1 5 q u e s t a a n a l i s i h a u n g r a n d e v a l o r e p r a t i c o n e l d e s e r t o a f r i c a n o p o i c h é è p o s s i b i l e p r e v e d e r e con un c e r t o a n t i c i p o l ' a n d a m e n t o d e l l a c u r v a d e l l a v e l o c i t à m e d i a e q u a n d o un t e r z o , o poco p i ù , d e l l a c u r v a è s t a t o r e g i s t r a t o e r a c c o r d a t o è p o s s i b i l e d e t e r m i n a r e l ' o r a i n c u i la t e m p e s t a a v r à fine s e g u e n d o il g r a f i c o d e l l a r e g i s t r a z i o n e d e l l ' i n t e n s i t à del v e n t o . t e m p e s t e di q u e s t o t i p o sono f r e q u e n t i i n v i c i n a n z a d e l l a costa d e l n o r d a f r i c a in p r i m a v e r a ed in i n v e r n o e la p r e v i s i o n e d e l l ' o r a i n cui u n a t e m p e s t a h a fine è di p a r t i c o l a r e i n t e r e s s e sia p e r 1 a v i a z i o n e c h e p e r il p u b b l i c o . alexandria, physics department, faculty of science, farouk 1° university, scptember 19-18. riassunto nella presente nota vengono studi (ite ed analizzate due caratteristiche che presenta l'andamento della velocità del vento durante le tempeste di sabbia dell"africa del nord. la prima è che la scarsa visibilità dovu'ta alla notevole quantità di sabbia e di polvere sollevate si ha quando compare nella registrazione un caratteristico picco corrispondente al ' colpo di vento", in cui ambedue le componenti, orizzontale e verticale della velocità del venko raggiungono un massimo per circa dieci minuti. i colpi di vento sono associati col fronte freddo o con dense nuvole tempestose. le tempeste di sabbia non possono svilupparsi finché una velocità limite non sia raggiunta. di tal fatto viene data una spiegazione teorica. la seconda caratteristica osservata è la periodicità di tipo ondulatorio dell' andamento della velocità del vento raggiunta durante le tempeste di sabbia. i grafici ottenuti possono generalmente essere confrontali con un'onda di periodo misurabile ed in tal modo si può predire la line della tempesta. an inversion model applied to dc soundings interpretation c . d e l g i u d i c e * received on s e p t e m b e r 10, 1982 a b s t r a c t t h e inversion technique is used in dc soundings intepretation to determine the t h i c k n e s s e s a n d the t r u e resistivities of the layers, starting from the field apparent r e s i s t i v i t i e s . t h e r e l a t i o n s h i p between the predicted a p p a r e n t resistivities and the earth p a r a m e t e r s is not linear. s t a r t i n g from this relationship a methodology is described t o o b t a i n a wellposed system of m linear equations. this system permits to c a l c u l a t e , by m e a n s of a n iterative procedure, the e a r t h p a r a m e t e r s that minimize t h e d i f f e r e n c i e s (error) between the field and the predicted a p p a r e n t resistivities. t h r e e different iterative procedures are described. practical examples have s h o w n t h a t all the iterative procedures are reliable and give comparable results in t e r m s of m i n i m u m e r r o r reached and cpu time. r i a s s u n t o la t e c n i c a dell'inversione è u s a t a nella interpretazione di sondaggi elettrici v e r t i c a l i p e r d e t e r m i n a r e gli spessori e le resistività vere degli elettrostrati ( p a r a m e t r i del terreno) p a r t e n d o dalle resistività a p p a r e n t i . la relazione tra le r e s i s t i v i t à a p p a r e n t i ed i p a r a m e t r i del terreno non è lineare. * presently employed by geomath via cavour, 43 pisa (italy). 1 6 6 c. d e l g i u d i c e p a r t e n d o da q u e s t a relazione viene descritta u n a metodologia già nota per o t t e n e r e un s i s t e m a ben posto di m equazioni lineari. q u e s t o s i s t e m a p e r m e t t e di calcolare, per mezzo di u n a procedura iterativa, i p a r a m e t r i del t e r r e n o che m i n i m i z z a n o la differenza tra le resistività a p p a r e n t i di c a m p o e q u e l l e calcolate. vengono inoltre descritte tre diverse procedure iterative. gli e s e m p i p r a t i c i h a n n o m o s t r a t o che t u t t e le procedure iterative utilizzate sono a t t e n d i b i l i e p a r a g o n a b i l i tra loro per q u a n t o riguarda i risultati del procedimento di m i n i m i z z a z i o n e e i tempi di calcolo utilizzati. 1 . s t a t m e n t o f p r o b l e m t h e a p p a r e n t resistivities in ohmm versus the ab/2 distances in m r e p r e s e n t the field d a t a . in t e r m s of the voltage v,, currents i, a n d g e o m e t r i c factor k, (i = 1, n) where n is the n u m b e r of s a m p l e s , t h e a p p a r e n t resistivity is: k, av, i, we w a n t to find an e a r t h model, consisting of a distribution of t r u e resistivities a n d thicknesses t h a t minimize the error between t h e field a p p a r e n t resistivities and the predicted ones. t h e observed a p p a r e n t resistivities are, in vectorial form: pa, p a2 if w e call p' the resistivities predicted by a forward earth model p c o n s i s t i n g of the t r u e resistivities a n d the thicknesses of the ^ ^ ^ l a y e r s an i n v e r s i o n m o d e l applied, ecc. 1 6 7 t h e a b o v e values are functions of the p a r t i c u l a r earth model p, ( / = 1, m) w h e r e generally m < n. p i " p2 t h e n p' = f(p) [1] t h e r e l a t i o n s h i p [1] between p and p' is not linear. by expanding t h e e q u a t i o n [1] in a taylor series and by keeping only the linear t e r m s , w e o b t a i n : 8«p) _ pi = f ( p + a p) = f (p) + y ^ ap w h e r e ap is t h e model i m p r o v e m e n t . w e n e e d the p' to fit the observed d a t a : p., = p' o r : ö f ( p ) _ po = f ( p ) + " t t " ap 1 6 8 c. d e l g i u d i c e c a l l i n g d = pu — f (p), this is: a f ( p ) d = ap dp or: d = [a) ap [2] w h e r e [a] is t h e sensitivity matr ix a n d ap is the vector p a r a m e t e r c o r r e c t i o n (jupp, vozof 1975, lanczos 1961, m a r q u a r d t 1963). w r i t i n g o u t the matrices of equation [2] we have: d , 8 f, (p) ¿ p , d 2 • • • • • • • • • • d f.v (p) 0 p , 8 f , ( p ) d p „ 3 t \ , ( p ) 3 p m a p , a p 2 a p. [3] t h i s is a system of n equations in m unknowns with n > m. the a b o v e s y s t e m of e q u a t i o n s is then over-determined and generally ill-posed in the sense that small changes in the d a t a lead to large c h a n g e s in the solutions. to solve this r e d u n d a n t system of e q u a t i o n s , we ca n apply the method of least squares. c a l l i n g e = d —[a] ap we have: n • s « ; i e t e an i n v e r s i o n m o d e l a p p l i e d , ecc. 1 6 9 w h e r e et is the transpose of e or: e2 = [ d —[a] ap]t • [d — [a] ap] f o r e2 to be a m i n i m u m ; its derivative with respect to ap must be z e r o . t h e n d i f f e r e n t i a t i n g the above equation we have: [a]t [a] ap = [a]t d [4] we h a v e t r a n s f o r m e d the old system [2] of n equations in m u n k n o w n s i n t o a n m x m system [4], from the above expression [4] w e h a v e : ' ap = [ a t a ] " 1 [ a ] t d [5] 2 . i t e r a t i v e p r o c e d u r e s in t h e i t e r a t i v e procedure we do not use the algorithm [5] but its m o d i f i c a t i o n m a d e by m a r q u a r d t (marquardt 1963): ap = [at a -ik 2 1]" 1 [a]t d [6] w h e r e k2 is called « m a r q u a r d t p a r a m e t e r » and i is the unit m a t r i x . t h e s y s t e m of e q u a t i o n s [6] is well-posed. t h e a l g o r i t h m [6] has the advantage over [5] that the region of c o n v e r g e n c e is g r e a t e r a n d the a m p l i t u d e of the p a r a m e t e r correct i o n a p i s s m a l l e r . t h e g e n e r a l expression for the iterative procedure is: pimi _ p (m-1) ^ p (m) [7] 1 7 0 c. d e l g i u d i c e w h e r e m i n d i c a t e s the iteration n u m b e r a n d : ap(m, = [ (a t a)<m_1) + ( k 2 i ) (at d ) , m " [8] i n e a c h i t e r a t i o n , we j^an use i values of k2 t h a t give i c o r r e s p o n d i n g values of a p. we then select the k2 t h a t gives the m i n i n u m e r r o r b e t w e e n the a p p a r e n t resistivities and the pred i c t e d ones. then: ( m + i) = p ( m 0 + ap (m + 0 [9] to a s c e r t a i n t h a t a m i n i m u m h a s been found so far, we have t o t e s t n e i g h b o u r i n g points, increasing the n u m b e r of k 2 p a r a m e t e r s a n d decreasing the range of variability of k 2 . thus the e x p r e s s i o n [9] becomes: p (m + i + n) _ p (m i n) ^ p (m + i + n) [ 1 0 ] w h e r e n is t h e n u m b e r of times t h a t the "neighbourhood t e s t " is p e r f o r m e d . in expression [10] ap<", + , + ") is equal to: ^ p (m + i + n) _ j^a t a ) <m ~ ' ~ + ( k 2 i) <m+ ,' + n) ] " ' (at (j) (mn) [11] f i g u r e 1 shows the chart of a c o m p u t e r p r o g r a m based on this f i r s t i t e r a t i v e a p p r o a c h . in o r d e r to find a faster procedure two other approaches have b e e n e x a m i n e d . in the second a p p r o a c h , the equation [10] has been modified: p (m + i + n) _ p (m — i +«) ^ p ( m + i + n) [ 1 2 ] h e n c e in the " n e i g h b o u r h o o d t e s t " we s u b s t i t u t e for the vector p t h e c u r r e n t value of it t h a t gives the m i n i m u m error pa — p' w h e r e for a p we use the s a m e expression [11]. an i n v e r s i o n m o d e l applied, ecc. 1 7 1 figure 1 first iterative a p p r o a c h flow chart in t h e t h i r d a p p r o a c h , we use the expression [12] also modifyi n g t h e ap expression t h a t becomes: a p ( m + i + h) = [ ( a t a ) < t i + n) 4( k 2 j)(m + i + n) f j j t ^ ( m i + n) ] ' (at d) [ 1 3 ] 1 7 2 c. d e l g i u d i c e 3 . p r a c t i c a l a p p l i c a t i o n s t w o sets of ten electrical soundings have been run utilizing t h e a b o v e described three approaches; no problem of convergence h a s b e e n e n c o u n t e r e d (fig. 2). in t h e first set, a c o m m o n t r u e resistivity value has been used a s a n initial e a r t h model for all the layers of each ves obtaining a s a f i r s t solution of the equation [1] a straightline (fig. 3). t h e a v e r a g e error, between the field resistivities and the c o m p u t e d ones, h a s been of 113.91%. in the second set, the initial e a r t h m o d e l s h a v e been obtained by a very rapid analysis of the iteretion number f i g u r e 2 e r r o r ' s trend relative to the three iterative a p p r o a c h e s an i n v e r s i o n m o d e l applied, ecc. 1 7 3 v e s n ° 6 f i r s t set + f i r s t s o l u t i o n . f i e l d + + + + + + + + + + + + + + + + + 10 15 so 3 0 a b / 2 ( m ) 100 150 2 0 0 3 0 0 4 0 0 f i g u r e 3 first solution relative to an initial model apparteining to the first set v e s n° 2 • s e c o n d s e t • + + • + + f i r s t s o l u t i o n + + . f i e l d + • + • ' . + + • + • + + 4+ + • r 1 1 1 1 1 1 1 1 i i i 15 2 3 5 7 10 ic 2 0 3 0 5 0 1 0 0 150 a ß / 2 ( m ) f i g u r e 4 first solution relative to an initial model apparteining to the second set f i e l d c u r v e , giving to the t r u e resistivities an a p p r o x i m a t e a s y m p t o t i c value a n d deducing the thicknesses from the abscissa of t h e inflection point (fig. 4). 1 7 4 c . d e l g i u d i c e a • o iterative first set second set a p p r o a c h of v e s of v e s 1 o o 2 • • el a 3 a a 0.9 1.0 1.1 1.2 1.3 1 4 t a p p r o a c h 1 f i g u r e 5 final e r r o r versus cpu t i m e t h e a v e r a g e error, between the field resistivities and the c o m p u t e d ones, h a s been of 27.90%. figure 5, referred to the first a n d s e c o n d set of ves, shows the final e r r o r versus the cpu time r e l a t i v e to the first iterative a p p r o a c h . for both sets of ves the r e s u l t s o b t a i n e d utilizing the three different iterative approaches a r e c o m p a r a b l e r e g a r d i n g to the reached m i n i m u m error (difference in e r r o r 0.2% 0.3%). r e g a r d i n g the cup t i m e the first iterative a p p r o a c h is faster t h a n t h e o t h e r t w o w h e n the initial e r r o r is high (more than 100%) w h i l e t h e t h r e e iterative approaches are c o m p a r a b l e when the i n i t i a l e r r o r is small (20% 30%). results obtained by using the t h r e e i t e r a t i v e p r o c e d u r e s applied to the above practical examples h a v e s h o w n t h a t all the procedures are reliable and comparable a m o n g t h e m s e l v e s regards to the m i n i m u m error reached and the cpu t i m e . an i n v e r s i o n m o d e l a p p l i e d , ecc. 1 7 5 a c k n o w l e d g e m e n t s t h e w o r k w a s been carried out within a fellowship (n. 216849). f i n a n c e d by t h e italian «consiglio nazionale delle ricerche» (cnr) a n d " n o r t h atlantic treaty organization" (nato) at the c o l o r a d o school of mines-department of geophysics. t h e a u t h o r is p a r t i c u l a r l y grateful to prof. g.v. keller, head of t h e g e o p h y s i c a l d e p a r t m e n t and to the prof. c.h. stoyer. 1 7 6 c . d e l g i u d i c e r e f e r e n c e joansen h . k . , 1977 a man/computer interpretation system for resistivity soundings over a horizontally stratified earth. " g e o p h y s . p r o s p e c t i n g " , 25, p p . 667-691. j u p p d . , v o z o f k . , 1 9 7 5 stable iterative method for the inversion of geophysical data. " g e o p h y s . j . f . astron. soc.", 42, pp. 9 5 7 9 7 6 . l a n c z o s c., 1 9 6 1 linear differential operators. d . van nostrand company l i m i t e d . m a r q u a n r d t d . , 1 9 6 3 an algorithm for least-square estimation of nonlinear parameters. " j . soc. ind. appl. math.", ii, 2, j u n e . p a r k e r r., 1977 understanding inverse theory. "ann. rev. e a r t h planet. s." 5, p p . 35-64. v a r g a r., 1 9 6 2 matrix iterative analysis. prentice hall, inc. vol51,1,2008_delnegro 237 annals of geophysics, vol. 51, n. 1, february 2008 key words lf radio-waves – wavelet – seismicity – precursors 1. introduction research into the interaction between seismic activity and disturbances in radiobroadcasts has been carried out for many years. mainly, pre-seismic disturbances in radio waves that lie in the vlf (3-30 khz) frequency band have been presented (hayakawa and sato, 1994; hayakawa et al., 1996, 2002; molchanov and hayakawa, 1998). recently, rozhnoi et al. (2004, 2005, 2006) revealed a convincing connection between the nightime anomalies in a lf (40 khz) radio signal and the earthquakes that occurred with m≥5.0 nearby the radio paths. moreover, molchanov et al. (2006) have presented possible seismic disturbances in vlf radio signals radiated by powerful transmitters located on the ground but recorded on board of the demeter satellite. the vlf-lf (10-60 khz) radio signals are used for time standard an overview on preseismic anomalies in lf radio signals revealed in italy by wavelet analysis pier francesco biagi (1)(2), laura castellana (1), tommaso maggipinto (1) and anita ermini (3) (1) dipartimento di fisica, università degli studi di bari, italy (2) centro inter-dipartimentale per la valutazione e la mitigazione del rischio sismico, università degli studi di bari, italy (3) dipartimento di ingegneria dell’impresa, università degli studi di roma tor vergata, roma, italy abstract since 1996, the electric field strength of the two broadcasting stations mco (f=216 khz, southeast france) and cze (f=270 khz, czech republic) has been sampled every ten minutes by a receiver (as) located in central italy. here, we review the results obtained by a detailed analysis applied to the data recorded from february 1996 up to december 2004. at first, the daytime and nighttime data were extracted and then, in the daytime data, the data collected in winter were separated from those collected in summer. on the second step the wavelet transform was applied. the results of this analysis are radio anomalies detected as earthquake precursors both for mco and cze data. in particular, regarding the mco data, the main result was the appearance of a very clear anomaly during may-august 1998, at daytime and at nighttime. such an anomaly can be considered as a precursor of a seismic sequence started on august 15, 1998 with 17 earthquakes (m=2.2-4.6) on the reatini mountains, a seismogenic zone located 30 km far from the as receiver along the path mco-as. as concerns with the cze data, the first result was obtained from the summer daytime data and it was the appearance of a very clear anomaly during august-september 1997, that can be considered a precursor of the two earthquakes with magnitude m=5.6 and m=5.9 that occurred on september 26 in the umbria-marche region (central italy). the second result was the appearance of an anomaly during february-march 1998, at daytime and at nighttime, that can be related to the preparatory phase of the strong (m=5.1-6.0) slovenia seismic sequence that occurred in a zone lying in the middle of the cze-as path. mailing address: dr. pier francesco biagi, dipartimento di fisica, università degli studi di bari, via amendola 173, 70125 bari, italy; e-mail: biagi@fisica.uniba.it vol51,1,2008_delnegro 16-02-2009 21:28 pagina 237 238 p.f. biagi, l. castellana, t. maggipinto and a. ermini signal and worldwide navigation support and propagate in the earth-ionosphere wave-guide mode along great circle propagation paths. the analysis is based on the amplitude and the phase variations of the radio signals propagating from different transmitting stations. the anomalous variations in the radio signals are produced by perturbations in the ionosphere. these are related to the particles/gas/water release from the ground surface during the preparation phase of large earthquakes. the subsequent changes in the atmosphere are usually supposed to take place in a thin near ground layer with a thickness of several hundred meters. according to the most recent and convincing model (mareev et al., 2002; molchanov, 2004) the extension of the effect up to the ionosphere is due to atmospheric gravity waves of low amplitude generated during the quoted ground processes. these waves propagate up to the ionosphere, penetrate inside and interact with the ionosphere turbulence producing disturbances in the propagation of the vlf-lf radio waves. in 1995 some of the authors designed and built a receiver able to measure the electric field strength of lf broadcasting stations. the receiver was put into operation at the entrance of a natural cave located in central italy (as) and, on the basis of the best reception, the transmitting stations mco (france, f=216 khz) and cze (czech republic, f=270 khz) were selected. the respective transmitter-receiver distances are 518 km and 818 km. in 1996 the measurements started, with a sampling frequency of ten minutes. figure 1 shows the location of the transmitters and the receiver. looking for anomalous patterns in the raw or filtered data of both the radio signals, several anomalies were obtained (bella et al, 1998; biagi et al., 2001a,b; biagi and hayakawa, 2002; biagi et al., 2003). their features are decreases or increases in the electric field amplitude and they could be related to the seismic activity. then, since the lf radio signals are strongly changeable during the day and during the cycle of the seasons, a different approach of analysis (biagi et al., 2005; biagi et al., 2006a,b) was carried out: after separating each data set of mco and cze radio signals in nighttime data and daytime data (in winter and summer), they were processed by means of the wavelet analysis. here we present an overview of the results obtained using the quoted technique of analysis to investigate the background of the signals and to detect possible anomalies. the mco and cze data sets collected from february 1996 to december 2004 were used. 2.theoretical computation and data analysis. it is well known that the lf radio signals are characterised by the ground wave and the sky fig. 2. map showing the location of the receiver as and of the two transmitters mco and cze. the two radio-paths are indicated by dotted lines. the location of the reatini mountains seismogenetic area is shown by a black triangle. the black square represents the location of the slovenia seismic sequence, and the black circle indicates the location of the m=5.6 and m=5.9 umbria-marche earthquakes. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 238 239 an overview on preseismic anomalies in lf radio signals revealed in italy by wavelet analysis fig. 2a,b. a) from the top reading downwards, the mco electric field strength at night time (9.00-12.00 p.m., local solar time), at daytime (9.00 a.m.-2.00 p.m., local solar time), at winter daytime and at summer daytime, from february 1996 to december 2004. the dashed horizontal lines represent the mean values (table i) of the relative data set. the gaps in the trends correspond to interruptions in the data collection. b) the same of figure 2a but with reference to the cze electric field strength. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 239 240 p.f. biagi, l. castellana, t. maggipinto and a. ermini wave propagation modes. the ground wave provides a rather stable signal, while the sky wave varies widely from day to night and, at daytime, from winter to summer. the amplitude recorded by the receiver is the superimposition of the two waves but, for distances greater than 300-400 km, the recorded electric field strength can be considered a simple addition of the two waves (ccir, 1990a). so it was assumed that the mco and the cze amplitude is the addition of the ground wave and the sky wave contributions. to evaluate the two different contributions, the theoretical amplitudes of the ground wave and the sky wave have been calculated. details of the theoretical computations are presented in biagi et al. (2006a). as for the ground wave, the propagation of this wave over a smooth, homogeneous, curved earth, with exponentially decreasing refractive index (rotheram, 1981a,b) was considered; for the sky wave, using the wave hop model (knight, 1973; ccir, 1990b), the sky wave signal was supposed as a ray starting from the transmitter and reflected, with one or more hops, by the lower ionosphere and by the ground. both the distances transmitter-receiver (mco-as and cze-as) are consistent with only one hop, whose reflection point on the ionosphere is in the middle of the radio path. then, the following step of the analysis was carried out on the experimental data, by separating the daytime data from the nighttime ones for each radio signal. in order to obtain data sets related to daytime and to nighttime regardless of the season and with the same number of data per day, the range 09.00 a.m.-02.00 p.m. (local solar time) for the daytime and the range 09.00-12.00 p.m. (local solar time) for the nighttime was selected. this last choice is also due to an interruption of 3-4 h in the radio broadcasts for both mco and cze transmitters after midnight (local solar time = ut + 1 h). then, only for the daytime data, the data collected in winter (december 21-march 21) were separated from the ones collected in summer (june 21-september 21). figure 2 shows the nighttime, daytime, winter daytime and summer daytime trends of the mco and the cze electric field strength, from february 1996 to december 2004. the mean value for each data set is indicated by a horizontal dashed line. finally, theoretical and experimental values resulting from this analysis are reported in table i. the mean experimental values of the sky wave have been obtained retable i. experimental (exp) and theoretical (theo) values of the electric field strength (mv/m) for the mco (216 khz) and cze (270 khz) radio signals at as receiver. electric field strength (experimental values) transmitter day winter day summer night mco 0.79 0.52 3.65 cze 0.31 0.21 1.65 sky wave mco theoretical values 0.50 0.02 3.10 experimental values 0.25 3.11 cze theoretical values 0.40 0.016 1.62 experimental values 0.23 1.57 ground wave (theoretical values) mco 0.54 cze 0.08 vol51,1,2008_delnegro 16-02-2009 21:28 pagina 240 241 an overview on preseismic anomalies in lf radio signals revealed in italy by wavelet analysis fig. 3. normalized wavelet power trend from february 1996 to december 2004 of the mco electric field strength at daytime and at nighttime for the period 50 days that corresponds to the maximum power. the dashed vertical line indicates the time occurrence of the reatini mountains main shock (m = 4.6). moving the theoretical value of the corresponding ground wave from the experimental values of the electric field strength. then, because of the changing-in-time statistical characteristics of the signals, the wavelet transform (torrence and compo, 1998) on the nighttime data and on the daytime data at winter and summer, was applied. in this way, it has been possible to highlight the spectral components of the signal by using variable-width time windows, by considering that the frequency content of these windows is inversely related to the time widths; so, the localization of the signal is simultaneously obtained both in time and in frequency (daubechies, 1992; strang and nguye, 1996). in the analysis, we adopted as wavelet the «morlet function» (torrence and compo, 1998). in this case the wavelet transform of a time signal is a complex series that can be usefully represented by its square amplitude, i.e. we consider the so called wavelet power spectrum. the power spectrum is a 2d plot that, once properly normalized with respect to the power of the white noise, gives information on the strength and precise time occurrence of the various fourier components which are present in the original time series. instead of reporting the entire 2d plot, we consider i in figs. 3 and 4 the wavelet power corresponding to the fourier period in which the maximum power is found, and hence the plot is 1d representation of a single period with respect to time. figure 3 makes reference to mco radio signal and shows a clear anomaly during may-august 1998, at daytime and at nighttime. figure 4 makes reference to the cze radio signal and two main results stand up: the first one is an evident anomaly during august-september 1997 at summer daytime, as shown in fig. 4a; the second one is an anomaly appearing during february-march 1998 mainly at nighttime as shown in fig. 4b. we have named as (a), (b), and (c) the previous three anomalies, where (a) is the one revealed in the mco radio signal, (b) and (c) the anomalies pointed out in the cze radio signal, occurred in summer 1997 and in winter 1998, respectively. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 241 242 p.f. biagi, l. castellana, t. maggipinto and a. ermini it must be noted that none of the previous anomalies ever appears in either the mco or cze radio signals. 3. discussion taking into account the results reported in table i and considering that the receiver has an instrumental zero equal to 0.20 mv/m, it is possible to make the following statements: 1) for both the mco and cze radio signals, the sky wave at nighttime and at winter daytime can be recorded; on the contrary, at summer daytime its value is below the instrumental zero; 2) the mean value of the sky wave at nighttime is the same in winter and summer; 3) for the mco radio signal, the experimental value at summer daytime represents, practically, the intensity of the ground wave; for the cze signal, the theoretical value of the ground wave is below the instrumental zero, so that the experimental value recorded at summer daytime does not represent any signal, but only some noise. mainly its origin is anthropic because related to summer storms (lightnings). such an activity generally produces 0.4-0.6 mv/m spikes with some hour duration at most and it can be checked by the data of a local meteorological station. now, the anomalies pointed out in the previous section will be examined. 3.1. anomalies (a) and (b) the radio anomaly (a) is represented (fig. 3) by an increase in the electric field strength, both at daytime and at nighttime, mainly in summer: fig. 4a,b. a) normalized wavelet power spectrum from 1996 to 2004 of the cze electric field strength in summer daytime (june 21-september 21 of every year) for the period 10 days that corresponds to the maximum power. the dashed vertical line represents the time occurrence of the two earthquakes (m=5.6 and m=5.9) happened on september 26, 1997 in the umbria-marche region. b) normalized wavelet power spectrum from february 1996 to september 2004 of the cze electric field strength at nighttime for the period 20 days that corresponds to the maximum power. the dashed vertical line represents the time occurrence of the first slovenia earthquake (m=5.2, march 13, 1998). vol51,1,2008_delnegro 16-02-2009 21:28 pagina 242 243 an overview on preseismic anomalies in lf radio signals revealed in italy by wavelet analysis so, from the previous considerations, the sky wave cannot be involved. on the contrary, it suggests the possibility that the anomaly is related to an increase in the ground wave propagation mode and it can be investigated on the daytime data. looking at the summer daytime data set (fig. 2a), large signals over 0.8-1.0 mv/m appear during summer 1998: their comparison with the ground wave theoretical value at the as receiver (table i) shows a 50% increase of the ground wave electric field intensity. anomaly (b) observed in the cze radio signal in summer 1997 discloses two large signals with a duration of 10-20 days in the trend at the bottom of fig. 2b; these signals start on july 30 and on september 14, respectively. their amplitudes reach peak values up to 0.8 mv/m. in those periods, summer storms (lightnings) were not revealed by the data of a local meteorological station. the anomaly revealed by the wavelet analysis (fig. 4a) is produced mainly by the quoted large signals. taking into account that the anomaly appears only at summer daytime, the sky wave cannot be involved and so, as for anomaly (a), a connection with an increase in the ground wave propagation mode can be proposed. such an increase could produce a recordable signal at summer daytime, when, normally, the elecric field strength cannot be recorded, being below the instrumental zero (table i). otherwise, no anomaly can be revealed at nighttime, when the ground wave is superimposed on the sky wave that is much more intense. now it is necessary to explain the processes that can produce the quoted increases. several parameters control the ground wave propagation of a radio signal: the scale height of the troposphere, its refractive index, the relative permittivity and the conductivity of the ground. the ground conductivity and the refractive index of the troposphere are the most influential and the electric field strength of the ground wave increases if the value of one or both the previous parameters increases along the radio path or in some zone of it. at first, the meteorological conditions and the rainfall in the areas of the radio paths were checked and no anomalous situations emerged during the two time periods when the anomalies occurred. 3.1.1. connection of the anomalies (a) and (b) with the seismic activity from the seismic bulletins of the ingv (national institute of geophysics and volcanology) it results that the anomaly (a) occurred before the start (august 15, 1998) of a seismic sequence with 17 earthquakes (m=2.2-4.6), in a zone located 30 km far from the as receiver, along the mco-as radio path. the sequence is located in the seismogenetic area of the reatini mountains, in the central apennines (figure 1), that is approximately a rectangle, with a diagonal 40 km long in the northeast direction. the quoted seismic sequence is the only significant event occurred in the first fresnel zone of the mco radio signal, that is considered (hayakawa et al., 1996; molchanov and hayakawa, 1998; rozhnoi et al., 2004) the zone sensitive to the seismo-atmospheric effects for earthquakes up to m=5.0. in the light of these considerations, the anomaly (a) was considered a precursor of the reatini mountains seismic sequence. looking at fig. 3, the precursor started about 90 days before the occurrence of the sequence. as regards anomaly (b), it occurred before the two (m=5.6 and m=5.9) earthquakes happened on september 26, 1997 in the umbriamarche (central italy) region. their location is indicated in fig. 1. these earthquakes started an intense seismic sequence lasting more than six months. from the seismic bulletins they result the only strong and significant events that can affect the cze radio signal in 1997. so, we considered anomaly (b) as a precursor of the umbria-marche earthquakes. from fig. 4a a total premonitory time of about 60 days can be assumed. the following step of the study has been performed to justify the connection between the quoted two radio anomalies and the related seismic activity. as mentioned above, the most influential parameters on the ground wave propagation are the conductivity of the ground and the refractive index of the troposphere. the first is strongly affected by the water content and in many cases a small increase in the percentage of water is enough to step up the conductivity enormously (telford et al., 2002). the refractive index of the troposphere vol51,1,2008_delnegro 16-02-2009 21:28 pagina 243 244 p.f. biagi, l. castellana, t. maggipinto and a. ermini varies with the height, mainly, and particularly we have to take into account the value of the refractive index at the surface of the ground (ccir, 1990c): it is affected by the chemical composition of the air and it changes when variations, also the small ones, happen in this composition. so, in the light of these considerations, we propose that during the preparatory phase of the earthquakes, an increase in the underground waters in the surface layers and emissions of gases, particles and so on (atmosphere-litosphere coupling) happened in a wide zone, producing a consequent increase in the local conductivity of the ground and of the refractive index at the surface, respectively. such modifications produced anomalous zones along the mco-as and cze-as paths able to increase the electric field strength of the ground wave at the as receiver. in order to reproduce the increase in the intensity of the ground wave, as pointed out in anomalies (a) and (b), the theoretical intensity was evaluated using values for the ground conductivity and the surface refractive index, realistic but larger than the normal ones (ccir, 1990c and 1990d). these values were introduced in zones defined as it follows: for the mco radio signal, in a path 40 km long in correspondence with the reatini mountains area (fig. 1); for the cze radio signal, in the 75 km long radio path in italy (fig. 1). in such a way, for the mco intensity of the ground wave, a value of 0.80 mv/m was obtained, i.e. an increase of about 50% with respect to the standard value reported in table i; this value is consistent with the anomalous signals revealed on summer data in 1998, i.e. with the anomaly (a). for the cze intensity of the ground wave the value 0.34 mv/m was obtained. such a value is a recordable value with respect to the negligible theoretical one (table i), but much less than the intensity of the signals responsible for the anomaly (b). so, in this case, a second model was proposed. the ground wave propagation of a radio signal can be affected by ducts, reflecting layers, scattering zones and so on in the troposphere (ccir, 1990d; crane, 1981; rue, 1987) as a consequence of emissions of gases, particles and so on, during a preseismic phase. so, if a tropospheric reflecting layer or a tropospheric duct is supposed to lay in the zone of the umbria-marche earthquakes, then reflected rays can arrive in the different zones and they can interact with each other producing interference. so, the radio signal level is very high in the constructive interference areas, while in the destructive ones is very low. if the formation of an area of constructive interference nearby the as receiver is assumed, the existence of the large cze signals appeared in summer daytime data in 1997 can be justified. this last model rather than the previous one, could be taken into account to justify the anomaly (b). this anomaly seems to be formed by two successive phases (fig. 4a) and it means that the irregularities (ducts, reflecting layers, scattering zones) in the troposphere claimed by the model, occurred twice, each about 30 days long, rather than only once but with a longer duration (60 days). it must be noted that taking into account the atmospheric circulation and the currents, a short duration of the quoted irregularities in the troposphere is more realistic respect of a long duration. 3.2. anomaly (c) the last anomaly (c) under study appeared in the cze radio signal, in winter 1998, mainly in nighttime data. so, on the basis of the first statements of the discussion, it is related to the sky wave, showing exaggerated intensity. the anomaly is an increase in the intensity of the radio signal (fig. 4b) and at its end a strong seismic sequence (m=5.1-6.0) started in slovenia, about in the middle of the transmitterreceiver path (fig. 1). so, the radio anomaly can be considered a precursor of these earthquakes and a premonitory time of about 50 days (fig. 4b) can be assumed. it must be noted that this anomaly was also pointed out with a different methodology presented in biagi and hayakawa (2002). according to the wave hop theory, the exaggerated intensity of the sky wave may be explained by a 2-3 fold increase in the reflection coefficient of the ionosphere, just in the zone lying above the middle point of the cze-as path. this increase should be a consequence of vol51,1,2008_delnegro 16-02-2009 21:28 pagina 244 245 an overview on preseismic anomalies in lf radio signals revealed in italy by wavelet analysis the emission of radon, ions, electromagnetic waves, etc. (biagi and hayakawa, 2002; hayakawa et a., 1996; morgounov et al., 1994) from the focal zone of the forthcoming earthquake. also a different model, claiming an exaggeration of the atmospheric tides as consequence of gravity changes in the focal zone has been proposed (biagi et al., 2003) 4. conclusions this study disclocled that the lf radio signals can supply two different types of information related to the ground wave and to the sky wave. in particular, anomalies appearing mainly in the ground wave could be related to variations in some parameters of the ground and/or of the troposphere, which control the ground wave propagation mode. on the other hand, anomalies mainly appearing in the sky wave seem to be connected with disturbances in the ionosphere. in both cases, such variations can be produced by the processes that take place during the preparatory phase of earthquakes, according to the atmosphere-litosphere coupling model. acknowledgements the seismic data used in this paper were obtained by the ingv (istituto nazionale di geofisica e vulcanologia) web site (http://www.ingv.it). the authors are very grateful to v. lapenna and to an anonymous referee for their helpful suggestions to enhance the paper. references bella, f., p.f. biagi, m. caputo, e. cozzi, g. della monica, a. ermini, w. plastino and v. sgrigna (1998): field strength variations of lf radio waves prior to earthquakes in central italy, phys. earth planet. int., 105, 279-286. biagi, p.f. and m. hayakawa (2002): possible premonitory behaviour of lf radiowaves on the occasion of the slovenia earthquakes (m= 5.2-6.0-5.1) occurred on march-may 1998, in seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling (eds. m. hayakawa and o. molchanov), (terrapub, tokyo), 249-253. biagi, p.f., r. piccolo, a. ermini, s. martellucci, c. bellecci, m. hayakawa, v. capozzi and s.p. kingsley (2001a): possible earthquake precursors revealed by lf radio signals, nhess, 1 (1-2). biagi, p.f., r. piccolo, a. ermini, s. martellucci, c. bellecci, m. hayakawa and s.p. kingsley (2001b): disturbances in lf radio-signals as seismic precursors, ann. geofis., 44 (5/6), . biagi, p.f., r. piccolo, v. capozzi, a. ermini, s. martellucci and c. bellecci (2003): exalting in atmospheric tides as earthquake precursor, nhess, 3, 197-201. biagi, p.f., l. castellana, t. maggipinto, r. piccolo, a. minafra, a. ermini, s. martellucci, c. bellecci, g. perna, v. capozzi, o.a. molchanov and m. hayakawa (2005): a possible preseismic anomaly in the ground wave of a radio broadcasting (216 khz) during july-august 1998 (italy), nhess, 5, 727-732. biagi, p.f., l. castellana, t. maggipinto, a. ermini, g. perna and v. capozzi (2006a): electric field strength analysis of 216 and 270 khz broadcast signals recorded during 9 years, radio sci., 41. biagi, p.f., l. castellana, t. maggipinto, r. piccolo, a. minafra, a. ermini, s. martellucci, c. bellecci, g. perna, v. capozzi, o.a. molchanov and m. hayakawa (2006b): lf radio anomalies revealed in italy by the wavelet analysis: possibile preseismic effects during 1997-1998, phys. chem. earth, 31, 403408. ccir-international radio consultative committee (1990a): method for predicting sky wave field strengths at frequencies between 150 khz and 1705 khz, rep. 575-4, 167-184. ccir-international radio consultative committee (1990b): sky-wave propagation and circuit performance at frequencies between about 30 khz and 500 khz, rep. 265-7, 212-229. ccir-international radio consultative committee (1990c): world atlas of ground conductivities, rep. 717-3, 95-105. ccir-international radio consultative committee (1990d): effects of tropospheric refraction on radiowave propagation, rep. 717-3, 95-105. crane, r.k. (1981): a review of transhorizon propagation phenomena, radio sci., 16 (5), 649-669. daubechies, i. (1992): ten lectures on wavelets, cbms – nsf regional conferences series in applied mathematics (siam, philadelphia, pa), 61. hayakawa, m. and h. sato (1994): ionospheric perturbations associated with earthquakes as detected by subionospheric vlf propagation, in electromagnetic phenomena related to earthquake prediction, edited by m. hayakawa and y. fujinawa (terrapub, tokyo), 391-397. hayakawa, m., o.a. molchanov, t. ondoh and e. kawai (1996): the precursory signature effect of the kobe earthquake on vlf subionospheric signals, j. comm. res. lab., 169-180. hayakawa, m., o.a. molchanov, n. shima, a.v. shvets and n. yamamoto (2002): wavelet analysis of disturbances in subionospheric vlf propagation correlated with earthquakes, in seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terrapub, tokyo), 223-228. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 245 246 p.f. biagi, l. castellana, t. maggipinto and a. ermini knight, p. (1973): mf propagation: a wave-hop method for ionospheric field-strength prediction, bbc eng., 10, 22-34. mareev, e.a., d.l. ludin and o.a. molchanov (2002): mosaic source of internal gravity waves associated with seismic activity, in: seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling (eds. m. hayakawa and o. molchanov) terrapub, tokyo, 249-253. molchanov, o.a.(2004): on the origin of lowand middle-latitude ionospheric turbulence, phys. chem. earth., 29, 559-567. molchanov, o.a. and m. hayakawa (1998): subionospheric vlf signal perturbations possibly related to earthquakes, j. geophys. res., 103, 17.489-17.504. molchanov, o.a., a. rohzhnoi, m. solovieva, o. akentieva, j.j. berthelier, m. parrot, f. lefeuvre, p.f. biagi, l. castellana and m. hayakawa (2006): global diagnostic of the ionospheric perturbations related to the seismic activity using the vlf radio signals collected on the demeter satellite, natural hazards and earth system sciences, 6, 745-753. morgounov, v.a., t. ondoh and s. nagai (1994): anomalous variation of vlf signals associated with strong earthquakes (m≥7.0), in electromagnetic phenomena related to earthquake prediction, edited by m. hayakawa and y. fujinawa (terrapub, tokyo), 409-428. rotheram, s. (1981a): ground wave propagation, part 1: theory for short distances, iee proc. f. commun. radar signal process, 128, (5), 275-284. rotheram, s. (1981b): ground wave propagation, part 2:theory for medium and long distances and reference propagation curves, iee proc. f. commun. radar signal process, 128 (5), 285-295. rozhnoi, a.a., m.s. solovieva, o.a. molchanov and m. hayakawa (2004): middle latitude lf (40 khz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, phys. chem. earth, 29, 6589-598. rozhnoi, a.a., m.s. solovieva., o.a. molchanov, m. hayakawa, s. maekawa and p.f. biagi (2005): anomalies of lf signal during seismic activity in novemberdecember 2004, nhess, 5, 657-660. rozhnoi, a.a., m.s. solovieva., o.a. molchanov, v. chebrov, v. voropaev, m. hayakawa, s. maekawa and p.f. biagi (2006): preseismic anomaly of lf signal on the wave path japan-kamchatka during novemberdecember 2004, phys. chem. earth, 31, 422-427. rue, o. (1987): a new clear air interference prediction method for small time percentages, icap-87, conf. publ. no. 274, 343-346. strang, g. and t. nguye (1996): wavelets and filter banks (wellesley cambridge press), pp. 490. telford, w.m., l.p. geldart and r.e. sheriff (2002): electric properties of rocks and minerals, in applied geophysics (cambridge university press, new york), 283-292. torrence, c. and g.p. compo (1998): a practical guide to wavelet analysis, bull. amer. meteorol. soc., 79 (1), 61-78. vol51,1,2008_delnegro 16-02-2009 21:28 pagina 246 annals 48, 3, 2005+app1 497 annals of geophysics, vol. 48, n. 3, june 2005 key words topside ionosphere – electron density models – topside soundings 1. introduction information on the topside electron concentration distribution is not obtainable from groundbased measurements. in the past decades few satellites were equipped with ionosondes used for ionosphere sounding above the f2 maximum (topside). moreover, only a few percent of data recorded in the 60s and 70s were processed at that time due to the limited techniques available for automatic ionogram scaling. however, recently large amounts of newly scaled ionograms have been made available on-line (bilitza et al., 2003). this kind of data is particularly important for ionospheric modeling purposes, since ionospheric electron density models have their topside formulation based on old databases or different kind of measurements. experimental vertical profiles are very important for model comparison, because they provide the spatial distribution of the electron concentration and they contain the information on fof2, which is generally used by models as an anchor point to describe vertical profiles. the present work analyzes the behavior of the nequick and iri models, adopted by international telecommunication union, radiocommunication sector (itu-r) recommendation p. 531-6 (now superseded by p. 531-7) (itu, 2001), with respect to the topside electron density profiles available in the databases of the isis2, ik19 and cosmos 1809 satellites. this work is part of the italian degree thesis in physics thesis of one author (coïsson, 2002). 1.1. nequick model nequick model is an ionospheric electron density model, based on the original dgr «proionospheric topside models compared with experimental electron density profiles pierdavide coïsson and sandro m. radicella the abdus salam international centre for theoretical physics (ictp), trieste, italy abstract recently an increasing number of topside electron density profiles has been made available to the scientific community on the internet. these data are important for ionospheric modeling purposes, since the experimental information on the electron density above the ionosphere maximum of ionization is very scarce. the present work compares nequick and iri models with the topside electron density profiles available in the databases of the isis2, ik19 and cosmos 1809 satellites. experimental electron content from the f2 peak up to satellite height and electron densities at fixed heights above the peak have been compared under a wide range of different conditions. the analysis performed points out the behavior of the models and the improvements needed to be assessed to have a better reproduction of the experimental results. nequick topside is a modified epstein layer, with thickness parameter determined by an empirical relation. it appears that its performance is strongly affected by this parameter, indicating the need for improvements of its formulation. iri topside is based on booker’s approach to consider two parts with constant height gradients. it appears that this formulation leads to an overestimation of the electron density in the upper part of the profiles, and overestimation of tec. mailing address: dr. pierdavide coïsson, the abdus salam international centre for theoretical physics (ictp), strada costiera 11, 34014 trieste, italy; e-mail: coissonp@ictp.it 498 pierdavide coïsson and sandro m. radicella filer» (di giovanni and radicella, 1990). it is a quick-run model for trans-ionospheric applications that enables calculation of both vertical or slant electron density profile and tec for any specified path (hochegger et al., 2000; radicella and leitinger, 2001; leitinger et al., 2002). above 100 km and up to the f2-layer peak this model uses a modified dgr profile formulation, which includes five semi-epstein layers with modeled thickness parameters and is based on anchor points defined by foe, fof1, fof2 and m(3000)f2 values. the topside is given by a semi-epstein layer with a height dependent thickness parameter empirically determined. the model has been adopted for assessment studies by the european space agency european geostationary navigation overlay service (egnos) project and more recently by itu-r (itu, 2001) as a suitable method for tec modeling. the standard nequick source code is available at <http://www.itu.int/itu-r/software/ study-groups/rsg3/databanks/ionosph>. 1.2. iri model the international reference ionosphere (iri) is a well known and widely used empirical model of the ionosphere (bilitza 1990, 2001). for a given location, date and time, iri describes the electron concentration, electron temperature, ion temperature and ion composition in the altitude range from about 50 km to 2000 km, as well as the tec. iri provides monthly averages in the non-auroral ionosphere for magnetically quiet conditions. it is periodically updated and has evolved over a number of years. iri model can be run on-line at <http://nssdc.gsfc.nasa.gov/space/model/models/iri.html> or the source code can be retrieved via anonymous ftp from the nssdc (national space science data center) site <ftp://nssdcftp.gsfc.nasa.gov/models>. 2. experimental data used the databases of topside electron density profiles from the international satellite for ionosphere studies 2 (isis2) (ftp://nssdcftp.gsfc.nasa.gov/spacecraft_data/isis/topside_sounder), intercosmos 19 (ik19) and cosmos 1809 satellites (http://antares.izmiran.rssi.ru/projects/ik19) were used for model comparisons. the betterquality experimental data were selected for model comparisons. the geographical distributions of the selected profiles are shown in fig. 1. the isis2 satellite was in a circular orbit at 1400 km height and available soundings were recorded from 1972 to 1983; the ik19 satellite was in an elliptical orbit between 500 km and 980 km height, with available data recorded in 1979 and 1980. cosmos 1809 was in a circular orbit at 980 km height and data used were recorded during 1987. the nequick and iri models were used to compute modeled topside profiles corresponding to the experimental ones; the peak values (fof2 and hmf2) obtained from the topside experimental profiles were provided as input to the models, to focus the comparison on the shape of the topside profile. other inputs given, which affect the modeled profile shape, are geographic location, universal time and solar activity, expressed by r12. figure 2 shows some example of experimental and modeled topside profiles for isis2 and ik19 satellites. 3. results since there are no universally accepted criteria to compare an experimental profile with a modeled one, two quantities were considered in this work: total electron content (tec) difference from hmf2 up to the satellite height; electron density ratio, nmodel/nexp, at a fixed height above the peak. for statistical purposes, the data have been divided into homogeneous groups of: solar activity level, season of the year, modip and local time of the measurement. median values and the upper and lower quartiles of the distributions in each sector were computed. data recorded by different satellites were kept separated, but the results show consistent behavior. in the statistical analysis performed the modified dip (modip) coordinate was used. modip µ is defined by fig. 2. comparison between experimental profiles from isis2 and ik19 data and nequick and iri models: experimental (x) solid black, nequick (n) dash-dotted light gray, iri (i) dashed dark gray. tec in 1016 m−2. 499 ionospheric topside models compared with experimental electron density profiles fig. 1. locations of the available profiles: (top panel) isis2 data, names indicate the telemetry stations used; (bottom panel) ik19 data. 500 pierdavide coïsson and sandro m. radicella tan cos /1 2=n z } ^ h (3.1) ψ being the true magnetic dip in the ionosphere (usually at 300 km) and φ the geographic latitude (rawer, 1963). data were divided into modip intervals to cover the different regions: equatorial region (−5°, 5°), anomaly regions (−35°, −5°) and (5°, 35°), mid-latitudes (−60°, −35°) and (35°, 60°) and high-latitudes (−90°, −60°) and (60°, 90°). months were analyzed in three groups: first group from november to february, second group from may to august, third group march, april, september and october. various local time intervals were considered: day from 09:00 to 17:00 lt, night from 21:00 to 05:00 lt, sunrise from 05:00 to 07:00 lt, sunset from 18:00 to 20:00 lt. for low solar activity r12 < 40 was considered and for high solar activity r12 > 100. 3.1. tec comparisons the percent differences between each modeled and experimental tec value integrated from hmf2 up to the satellite height were calculated and a statistical analysis performed as indicated in the previous section, the most important results are given below. nequick model – figure 3 shows a different behavior in the first and second group of months. during the first one it overestimates and underestimates the tec anti-symmetrically with respect to the magnetic equator. during the second one it shows a symmetric behavior, with overestimation at low latitudes and better agreement at medium and high latitudes. these differences are associated with the different formulation of the nequick topside thickness parameter in the different months (radicella and zhang, 1995). iri model – figure 4 shows that iri tends to overestimate the topside tec, with a behavior dependent on solar activity. while during low solar activity there is a greater overestimation at low latitudes and a good agreement at medium latitudes, for high solar activity iri underestimates tec in the equatorial region but strongly overestimates it at high latitudes. day and night behaviors are similar in trend but there is better agreement at low latitudes. fig. 3. percent difference between experimental topside tec and nequick topside tec for months group 1 and 2. numbers indicate the amount of data in each section. 501 ionospheric topside models compared with experimental electron density profiles fig. 4. comparison of topside tec, iri model: percent difference for months groups 1 and 2. numbers indicate the amount of data. fig. 5. comparison of electron density, nequick model, at 900 km above hmf2 for months groups 1 and 2. numbers indicate the amount of data. 3.2. electron density comparisons the experimental and modeled electron densities 900 km above hmf2 were compared. this height was reached only in the isis2 profiles, but it was chosen to assess models behavior in the upper part of the profile: the iri model is used up to 2000 km and nequick to 20000 km. 502 pierdavide coïsson and sandro m. radicella fig. 6. comparison of electron density, iri model, 900 km above hmf2 for months groups 1 and 2. numbers indicate the amount of data. thus it is important to understand how they are reliable in that region. here results are shown only for local day time, because the number of data available for local night was not enough to provide good statistics in all regions. the small amount of night-time profiles confirms the results shown for day-time, the only relevant difference is indicated in the following description. nequick model – figure 5 shows a seasonal behavior similar to the one observed for topside tec, with the electron density values contained between 0.2 and 3 times the experimental ones: little underestimation or overestimation. the behavior during day-time and night-time is similar, only for low solar activity can a difference be noticed at low modip, where the model goes from low underestimation to low overestimation and vice versa depending on month. iri model – figure 6 demonstrates that iri always overestimates the electron density; the median values are greater than 1.5 times the experimental ones and can reach up to 9 times. it shows a different modip behavior depending on solar activity level: during low solar activity the greater discrepancy is found at low modip, while for high solar activity the situation is reversed. at high latitudes for high solar activity cases have been found in which the iri electron density does not decrease with height. 4. conclusions comparisons have been made between 21000 topside electron density profiles from satellite measurements and those computed by the nequick and iri models. attention was focused on tec and electron density at 900 km above the f2 peak. the wide set of different experimental conditions has enabled assessment of many characteristics of each model. the nequick topside is a modified epstein layer. its thickness parameter is governed by an empirical relation, with a dependence related to the month of the year. from this analysis it appears that the model performance is strongly affected by this parameter, indicating the need for an improvement of its formulation. iri is based on booker’s approach to model the topside by dividing it into two parts with 503 ionospheric topside models compared with experimental electron density profiles constant height gradients. the gradients depend on latitude, solar activity and fof2. in the analysis carried out it appears that this formulation leads to overestimation of the electron density in the upper part of the profiles, and consequently to overestimation of tec. references bilitza, d. (1990): international reference ionosphere 1990, nssdc 90-22, greenbelt, maryland. bilitza, d. (2001): international reference ionosphere 2000, radio sci., 36 (2), 261-275. bilitza d., b. reinish, r. benson, j. grebowsky, n. papitashvili, x. huang, w. schar and k. hills (2003): online database of satellite sounder and insitu measurements covering two solar cycles, adv. space res., 31 (3), 769-774. coïsson, p. (2002): uso di dati sperimentali per la convalida di modelli ionosferici applicati al disegno di servizi e sistemi satellitari, thesis (università degli studi di trieste). di giovanni, g. and s.m. radicella (1990): an analytical model of the electron density profile in the ionosphere, adv. space res., 10 (11), 27-30. hochegger, g., b. nava, s.m. radicella and r. leitinger (2000): a family of ionospheric models for different uses, phys. chem. earth, 25 (4), 307-310. itu (2001): ionospheric propagation data and prediction methods required for the design of satellite services and systems, recommendation p.531-6, approved in 2001-02, managed by itur study group sg3 (version 6 superseded in 2004 by version 7). leitinger, r., s.m. radicella and b. nava (2002): electron density models for assessment studies -new developments, acta geod. geophys. hung., 37 (2-3), 83-193. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27 (1), 35-40. radicella, s.m. and m.l. zhang (1995): the improved dgr analytical model of electron density height profile and total electron content in the ionosphere, ann. geofis., xxxviii (1), 35-41. rawer, k. (1963): propagation of decameter waves (hfband), in meteorological and astronomical influences on radio wave propagation, edited by b. landmark, (new york academic press), pp. 221-250. bozzoni_corretto_layout 6 609 preliminary results of ground-motion characteristics francesca bozzoni1,*, carlo g. lai1,2, laura scandella1 1 european centre for training and research in earthquake engineering (eucentre), pavia, italy 2 università di pavia, dipartimento di ingegneria civile ed architettura, pavia, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6121 1. introduction on may 20, 2012, a mw 5.9 earthquake struck the emilia-romagna region in northern italy (figure 1a), causing severe shaking throughout a relatively large portion of the po plain territory. the epicenter of this may 20, 2012, event has the coordinates 44.89˚ (latitude) and 11.23˚ (longitude). the depth of the hypocenter has been estimated at 6.3 km. the focal mechanism is of the thrust type. on may 29, 2012, a mw 5.65 shock hit the region, causing further damage and fatalities. figure 1b shows the events with magnitudes >4.5 (figure 1b, red dots) of this sequence of mayjune 2012 (data retrieved from http://iside.rm.ingv.it/iside/ standard/index.jsp). figure 1b also shows the most important earthquakes that have occurred in the northern parts of the emilia-romagna region over the latest 500 years (figure 1b, green dots). the data were retrieved from the italian earthquake catalogue cpti11 (http://emidius.mi.ingv.it/cpti11/) of the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology). the ferrara territory was hit by a m 5.5 earthquake on november 17, 1570. on july 11, 1987, a m 5.4 event struck the po plain area close to bologna and ferrara. other historical earthquakes with magnitudes up to 6 have occurred in the southern part of the emilia-romagna region, close to the apennine chain. the area struck by the emilia may-june 2012 sequence is located south of the po plain, in the foreland basin of two mountain belts constituted by the alps and the northern apennine chains. under thick clastic sedimentary fills along the northern and southern margins of the po plain, complex systems of thrust sheets and tectonic structures are buried. due to the fast sedimentation rates and comparatively low tectonic rates, the thrusts are generally buried and there is little surface evidence of their activity [toscani et al. 2009]. 2. shake maps shake maps provide a first estimate of the spatial distribution of ground motion. data from the recording stations of the italian strong motion network (ran) and the strong motion network of northern italy (rais) have been used for this purpose. the maximum peak ground acceleration (pga) was recorded at the station of mirandola (mrn) (located at ca. 13.4 km from the epicenter), and it is associated to the vertical component, which reached 0.31 g. figure 2 shows a shake map computed by interpolating the recorded pga at the ran stations using the inverse distance weighted algorithm. the stations are located at sites classified as category c according to the soil classification system prescribed by the italian building code [ntc 2008] which is similar to that of eurocode 8 part 1 [en 1998-1, 2005]. from figure 2 it is worth noting that the pattern of the contour lines of the pga is nonsymmetric with respect to the epicenter. this is due to the small number of stations used to perform the interpolation (the station closest to the epicenter is mrn). the focal mechanism might have also had a role in this asymmetry. it is important to note that the distance between the epicenter and the closest station influences the quality of the pga estimation in the area. ground-motion parameters from the recordings provided by the ran and rais seismic networks were compared with the estimates of two recent ground-motion prediction equations (gmpes); namely those of cauzzi and faccioli [2008] and bindi et al. [2011]. the preliminarily predictions were made for soil category c. figure 3 and figure 4 show the horizontal pga predicted by the selected gmpes. particularly in the case of the cauzzi and faccioli [2008] gmpe, the predicted values are very high in the epicentral area. the predicted pga is in good agreement with the recordings from the ran and rais stations. 3. recordings and response spectra at mirandola station the closest station to the epicenter of the may 20, 2012, event is mrn, which belongs to the ran network (managed by the italian department of civil protection). the epicentral distance of this station is ca. 13.4 km. time histories recorded at the mrn station have been analyzed to obtain acceleration, velocity, and displacement response spectra. article history received july 22, 2012; accepted august 20, 2012. subject classification: seismology ground motion. 2012 emilia earthquakes the recorded signals, which are the horizontal south-north (sn), east-west (ew) and vertical (up) components, were processed for standard baseline correction, band-pass filtered from 0.05 hz to 50 hz, tapered, and linearly de-trended in velocity. table 1 shows the peak ground-motion parameters, namely the pga, the peak ground velocity (pgv), and the peak ground displacement (pgd), for each component recorded at the mrn station. the acceleration response spectra computed using the recordings of the mrn station are shown in figure 5a (horizontal components) and in figure 5b (vertical component). the vertical component of the spectral acceleration has the peak at a lower period (0.06 s), compared with the horizontal components (sn: 0.17 s; ew: 0.31 s). furthermore, the peak values of the spectral acceleration for the vertical component are concentrated in a narrow band of periods from 0.02 to 0.12 s, while the peaks of the spectral acceleration for the horizontal components occur at a band with higher periods of up to 0.5 s (ew) and 1.3 s (sn). the acceleration response spectra computed using the recordings from the mrn station were also compared with the predictions of the gmpe developed by bindi et al. [2011] for the horizontal component and for soil category c (figure 6a). the mean predictions of the gmpe are satisfactory for periods <0.17 s, whereas for larger periods, the gmpe strongly underestimates the severity of the record. the displacement response spectra computed using the recordings from the mrn station were compared with the spectra predicted by the gmpe developed by cauzzi and faccioli [2008] for the horizontal component and for soil category c (figure 6b). in this case, the gmpe severely underestimates the spectra calculated from the records for all of the structural periods. the records from the mrn station were used to plot the particle orbit (hodogram) described by the waveform. the hodogram was plotted in the plane of rayleigh wave propabozzoni et al. 610 pga (g) pgv (m/s) pgd (m) sn 0.264 0.463 0.105 ew 0.262 0.300 0.081 up 0.310 0.059 0.018 figure 1. (a) location in the emilia-romagna region of the may 20, 2012, earthquake. (b) distribution of the major historical earthquakes as green dots (http://emidius.mi.ingv.it/cpti11/), and of the m >4.5 events of the 2012 sequence as red dots (http:/iside.rm.ingv.it/). table 1. peak ground-motion parameters for each component recorded at the mrn station. pga, peak ground acceleration; pgv, peak ground velocity; pgd, peak ground displacement). a) b) 611 ground-motion characteristics figure 2. shake map obtained from the interpolation of the pga recorded by the ran seismic network. ground conditions for the seismic stations are similar, and they are approximately associated to the soil category c, according to the ground classification system of the italian building code [ntc 2008], which is similar to that of eurocode 8 part 1 [en 1998-1, 2005]. figure 3. median values of the pga (horizontal component) predicted by the gmpe of cauzzi and faccioli [2008] for soil category c, plotted on top of the recorded values of the pga at selected stations of the ran and rais strong-motion networks. figure 4. median values of the pga (horizontal component) predicted by the gmpe of bindi et al. [2011] for soil category c, plotted on top of the recorded values of the pga at selected stations of the ran and rais strong-motion networks. gation. inspection of the mrn recordings shows that the backscattered p-wave and sv-wave are not well time-separated from the rayleigh wave arrival, given that the mrn station is very close to the epicenter. figure 7 shows the radial and vertical displacement traces obtained through filtering the signals with band-passes between 0.1 hz and 0.5 hz (from 2 s to 10 s). the motion is characteristically retrograde elliptical. 4. preliminary ground-response analysis at mirandola the availability of results from geotechnical field tests performed at a few sites in the municipality of mirandola prior to the earthquake of may 2012 have allowed preliminary assessment of the ground response closest to the mrn station. the corresponding data were retrieved from the webgis portal of the emilia-romagna region (https://territorio.regione. emilia-romagna.it/cartografia/cartografia-sgss). data from five seismic tests using multichannel analyses of surface waves that were performed during a 2011 geophysical campaign were used to construct a geotechnical model of the subsoil at mirandola, which was successively adopted to perform ground-response analyses. geological information about the soil deposits at the mrn site was retrieved at the portal bozzoni et al. 612 figure 5. mirandola station recordings. (a) comparison of the horizontal (sn, ew) acceleration response spectra from the recordings with the spectra from the italian building code [ntc 2008], computed for soil categories c and d for 475-year and 975-year return periods. (b) comparison of the vertical (up) acceleration response spectra from the recordings with the spectra from the italian building code [ntc 2008] computed for 475-year and 975-year return periods. figure 6. mirandola station recordings. (a) comparison of the horizontal (sn, blue line; ew, black line) acceleration response spectra from the recordings with the bindi et al. [2011] gmpe for ground category c. (b) comparison of the horizontal (sn, blue line, ew, black line) displacement response spectra from the recordings with the cauzzi and faccioli [2008] gmpe for soil category c. a) b) a) b) figure 7. hodogram from the mirandola station recordings: the radial and vertical displacement traces. 613 http://itaca.mi.ingv.it/itacanet/. table 2 gives the mean values of the geotechnical parameters and the corresponding uncertainties assumed for the adopted subsoil model. one-dimensional (1d) linear-equivalent, fully stochastic site-response analysis was performed for the mrn station using the methodology described in rota et al. [2011], which takes into account the uncertainties associated with the geotechnical model parameters. the stochastic analysis was carried out using monte carlo simulations associated with the latin hypercube sampling technique. randomly generated geotechnical parameters varying within correctly defined probability distributions were assumed to calculate the seismic response of 100 deterministic realizations of the geotechnical model. the uncertainty adopted for the values of vs is shown in figure 8a where the red line denotes the mean of 100 vs profiles with layers of varying thicknesses. for the definition of the depth of the bedrock and of its stiffness, the data available at the website of the emilia-romagna region were used (http://ambiente.regione.emilia-romagna.it). the variability of the seismic input was taken into account by considering an appropriate set of seismoand spectrum-compatible natural records. specifically, a set of seven accelerograms that were recorded from actual earthquakes were downloaded for the area under investigation, from the portal: http://www.eucentre.it/seismhome.html. the selected accelerograms have magnitudes that range from 6 to 6.87, and epicentral distances that vary from 11 km to 102 km. their scaling factors range from 0.36 to 2.61, with a mean of 1.60. the signals were recorded under outcropping rock conditions, and they are spectrum-compatible, on average, to the italian code-based spectrum [ntc 2008] that refers to the 475-year return period. indeed, although it is still premature to make definitive conclusions, a lot of seismological evidence appears to suggest that the may 20, 2012, earthquake might correspond to this return period. figure 8b shows the mean acceleration response spectrum ±one standard deviation, which was calculated using the linear-equivalent, fully stochastic, 1d ground response ground-motion characteristics figure 8. stochastic ground-response analyses at mirandola. (a) one hundred random vs profiles generated using the latin hypercube sampling technique. red line, mean profile. (b) mean acceleration response spectrum with the associated scatter computed using the vs profiles of (a). the seismic input comprised seven actual records compatible with the italian code-based spectrum that refers to a 475-year return period. the computed spectra are compared to the sn and ew spectra calculated from the mrn station recordings. the spectrum code [ntc 2008] for soil category a is also shown. layer soil type thickness (m) uncertainty (%) vs (m/s) uncertainty (%) t (kn/m3) uncertainty (%) 1 sand 6.6 67 150 12 18 6 2 sand 7.8 68 221 33 18 6 3 sand 9.7 67 266 19 18 6 4 sand 7.1 44 312 18 18 6 5 sandy clay and clayey sand 10.0 50 379 20 19 6 6 sandy clay and clayey sand 15.0 50 480 25 19 6 7 sandy clay and clayey sand 15.0 50 600 20 19 6 8 sandy clay and clayey sand 35.0 40 700 20 19 6 bedrock 800 20 20 6 table 2. means of the geotechnical parameters and the corresponding uncertainties assumed for the adopted subsoil model. a) b) analyses described above. this spectrum is compared to the response spectra calculated from the sn, ew recordings of the mrn station for the may 20, 2012, event. figure 8b also shows the code-based spectrum for soil category a [ntc 2008]. the mean pga computed from ground-response analysis turned out to be 0.269 g, which is in excellent agreement with the recorded pga (see table 1). however the peaks of the mrn spectra are underestimated by the mean spectrum. better agreement appears to be obtained by considering the mean spectrum plus one standard deviation, especially in the case of the ew component, although only for periods up to ca. 1 s. for periods >1 s, the spectral accelerations of the mrn recordings, and in particular those of the sn component, exceed those of the computed spectra. possible explanations for this difference might be nearfault effects, as the mrn station lies only 13 km away from the epicenter, and also that the may 20, 2012, earthquake was a shallow event (with a focal depth of about 6.3 km). possible explanations for such differences at large periods might also be non-linear effects (liquefaction), which might have occurred at the recording station. further investigation is however needed to substantiate these preliminary statements. ground-response analyses focused on the propagation of s-waves only, as they are the most relevant for engineering applications. future studies can investigate the role, if any, of ground amplification in the propagation of p-waves. 5. concluding remarks the preliminary results of our ground-motion calculations from the recordings of the may 20, 2012, m 5.9 emilia earthquake have been presented here. the response spectra computed from the accelerograms recorded at the mrn station, as the closest to the epicenter, have been compared with the spectra predicted by two recent gmpes and the italian building code [ntc 2008]. a first attempt evaluation of the amplification effects at mirandola has been carried out through linear-equivalent, fully stochastic ground-response analyses based on the results from geotechnical field tests performed close to the mrn station and using a set of seven actual records that are compatible to the italian code-based spectrum that refers to a 475-year return period. investigations are currently underway to determine whether linearequivalent analyses are admissible in mirandola, in light of possible strongly nonlinear effects (liquefaction) that might have occurred at the recording station. the results of the analyses show that the mean computed pga is in excellent agreement with the recorded pga at the mrn station (sn, ew). however, the peaks of the mrn spectra are underestimated by the mean spectrum. better agreement appears to be obtained by considering the mean spectrum plus one standard deviation, although only for periods up to ca. 1 s. for periods >1 s, the spectral accelerations of the mrn recordings, and especially of the sn component, exceed those of the computed spectra. possible explanations for this difference include near-fault effects (the mrn station lies only about 13 km away from the epicenter) and that the may 20, 2012, earthquake was a shallow event (focal depth of 6.3 km). acknowledgements. the authors would like to acknowledge the support of the department of civil protection of the italian government, and that of the administration of the emilia-romagna region. a word of gratitude goes to dr. maria-daphne mangriotis, for her valuable suggestions. references bindi, d., f. pacor, l. luzi, r. puglia, m. massa, g. ameri and r. paolucci (2011). ground motion prediction equations derived from the italian strong motion database, b. earthq. eng., 9, 1899-1920. cauzzi, c., and e. faccioli (2008). broadband (0.05 to 20 s) prediction of displacement response spectra based on worldwide digital records, j. seismol., 3; doi:10.1007/s10 950-008-9098-y. en 1998-1 (2005). eurocode 8 design of structures for earthquake resistance part 1: general rules, seismic actions and rules for buildings, en 1998-1, cen, brussels. ntc (2008). norme tecniche per le costruzioni, d.m. 14.1.2008 (italian building code). rota, m., c.g. lai and c.l. strobbia (2011). stochastic 1d site-response analysis at a site in central italy, soil dyn. earthq. eng., 31, 626-639. toscani, g., p. burrato, d. di bucci, s. seno and g. valensise (2009). plio-quaternary tectonic evolution of the northern apennines thrust fronts (bologna-ferrara section, italy): seismotectonic implications, b. soc. geol. ital., 128, 605-613. *corresponding author: francesca bozzoni, european centre for training and research in earthquake engineering (eucentre), pavia, italy; email: francesca.bozzoni@eucentre.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. bozzoni et al. 614 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol50,3,2007 397 annals of geophysics, vol. 50, n. 3, june 2007 key words telluric currents – magnetic base station – levelling errors – aeromagnetic and marine magnetic surveys 1. introduction during magnetically quiet days a typical diurnal variation in the magnetic field strength exhibits an amplitude of about 50 nt at mid latitudes (hermance, 1995). it is also known tht the earth magnetic field shows time-dependent variation across a wide range of frequencies (campbell et al., 1998), with amplitudes that magnetic base station deceptions, a magnetovariational analysis along the ligurian sea coast, italy marco gambetta (1) (3), egidio armadillo (3), cosmo carmisciano (2), fabio caratori tontini (2) and emanuele bozzo (3) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto nazionale di geofisica e vulcanologia, sede di portovenere, fezzano (sp), italy (3) dipartimento per lo studio del territorio e delle sue risorse (dipteris), facoltà di scienze matematiche fisiche e naturali, università degli studi di genova, italy abstract reliability of high resolution airborne and shipborne magnetic surveys depends on accurate removal of temporal variations from the recorded total magnetic field intensity data. at mid latitudes, one or a few base stations are typically located within or near the survey area and are used to monitor and remove time dependent variations. these are usually assumed to be of external origin and uniform throughout the survey area. here we investigate the influence on the magnetic base station correction of the time varying magnetic field variations generated by internal telluric currents flowing in anomalous regional 2d/3d conductivity structures. the study is based on the statistical analysis of a data set collected by four magnetovariational stations installed in northwestern italy. the variometer stations were evenly placed with a spacing of about 60 km along a profile roughly parallel to the coastline. they recorded the geomagnetic field from the beginning to the end of april 2005, with a sampling rate of 0.33 hz. cross-correlation and coherence analysis applied to a subset of 125 five hours long magnetic events indicates that, for periods longer than 400 s, there is an high correlation between the horizontal magnetic field components at the different stations. this indicates spatial uniformity of the source field and of the induced currents in the 1d earth. additionally, the pattern of the induction arrows, estimated from single site transfer functions, reveals a clear electromagnetic signature of the sestri-voltaggio line, interpreted as a major regional tectonic boundary. induced telluric currents flowing through this 2d/3d electrical conductivity discontinuity affect mainly the vertical magnetic component at the closer locations. by comparing this component at near (32 km) and far (70 km) stations, we have found that the mean value of the power spectra ratio, due to the electromagnetic induced field, is about 1.8 in the frequency band ranging from 2.5 × 10−3 to 5.5 × 10−5 hz. this energy, folded in the spatial domain of an hypothetical survey in this region produces unwanted noise in the dataset. considering a fifth of nyquist frequency the optimal tie-line spacing to assure complete noise removal would be 1 km and 15 km for a rover speed of 6 knots (marine magnetic survey) and 100 knots (aeromagnetic survey) respectively. similar power spectra analysis can be applied elsewhere to optimise tie-line spacing for levelling and filtering parameters utlilised for microlevelling. mailing address: dr. egidio armadillo, dipartimento per lo studio del territorio e delle sue risorse (dipteris), facoltà di scienze matematiche fisiche e naturali, università degli studi di genova, corso europa 26, 16132 genova, italy; e-mail: egidio@dipteris.unige.it 398 marco gambetta, egidio armadillo, cosmo carmisciano, fabio caratori tontini and emanuele bozzo may reach a few hundreds nt. time-dependent variations are mainly due to the interaction between the earth’s magnetic field and the solar wind, which produces a pattern of electric currents flowing around the planet. fluctuating electric currents flowing in the earth’s atmosphere causes induced electric currents to flow in the conducting earth below the source current. their pattern, amplitude and frequency depend both on the source and on the distribution of electrically conducting materials within the earth. regional electrical conductivity anomalies are distributed all over the world and characterize different plate tectonic provinces (hjelth and korja, 1993). several continental rifts (jiracek et al., 1995), high heat flow areas (e.g., ingham et al., 1983; sakkas et al., 2002), enhanced seismic reflectivity layers (e.g., hyndman, 1988), sedimentary basins (arora et al., 1999), regional fault systems and terranes boundaries (e.g., jording et al., 2000; armadillo et al., 2001, 2004; ledo et al., 2002) exhibit an electrical conductivity signature. at the earth’s surface magnetometers measure the composite of external (from the source currents) and internal (from the induced currents) field components. the observed magnetic field can be considered as the sum of a normal plus an anomalous field (gough and ingham, 1983). the normal field is defined as the sum of contributions from the external source field and that part of the internal field, which is due to the regional (1d) electrical conductivity structure. since the external source field is usually approximated at mid latitudes by plane waves of infinite horizontal extent (e.g., egbert and booker, 1986), the normal field can be considered uniform and affecting only the horizontal component. the anomalous field is generated by telluric currents flowing in non one-dimensional conductivity structures (2d/3d) and, thus, can be considered entirely of internal origin. this anomalous field is not uniform and affects both the horizontal and the vertical components. natural time dependent magnetic field variations are used by geoscientists to infer electrical conductivity structures within the earth, by means of geomagnetic depth sounding (gds) and magneto-telluric (mt) techniques (hjelth and korja, 1993; jones, 1999). while their use in probing crust and mantle plays an important role in deep structures delineation, their presence also induces significant errors in airborne and shipborne magnetic surveys, where crustal anomalies may be contaminated by time-dependent magnetic field variations. for this reason, a diurnal variation correction is normally applied to magnetic survey data (luyendyk, 1997). in medium size regional surveys, these variations are monitored using one or a few magnetic base stations, ideally located within or near the survey area. during post-processing, the recorded time-dependent variations are subtracted from the signal observed by the rover magnetic sensor. even if significant modifications in phase and amplitude can occur over distances of 50 km or more, at mid latitudes base station variations are usually assumed to be fully representative of temporal variations over the whole survey area (luyendyk, 1997). this assumption is equivalent to identify temporal variations with the normal field. levelling and microlevelling procedures are additionally applied to minimise time dependent residual errors, which typically remain even after the base station correction (soul and parson, 1998; ferraccioli et al., 1998). these procedures try to minimise the effect of the time-varying magnetic field table i. timing, type and sampling rate of mv stations of fig. 1. acronym survey type sampling date rate [hz] osl 1972askania variometer 0.033 1984 bss 1972askania variometer 0.033 1984 ssl 2005 eda fluxgate 0.33 rez 2005 eda fluxgate 0.33 grn 2005 eda fluxgate 0.33 rdk 2005 eda fluxgate 0.33 rdc 1992eda fluxgate 0.016 1994 rdd 1992eda fluxgate 0.016 1994 asc 1990 eda fluxgate 0.016 cam 1990 eda fluxgate 0.016 399 magnetic base station deceptions, a magnetovariational analysis along the ligurian sea coast, italy which appears as a level shift in neighboring parallel survey lines. the conventional levelling operates adjusting tie-lines to match a statistical average of observed crossing profiles and then, in a further step, adjusting the survey line to exactly match the modified tie-lines. the microlevelling is a grid technique which uses directional filters to remove residual noise associated with the flight lines which was not wiped out by the conventional levelling. in this paper we aim to evaluate and discuss the relevance on the base station diurnal correction of the magnetic field time variations generated by the internal telluric currents flowing in anomalous regional 2d/3d conductivity structures (i.e. the anomalous field). four mobile three components variometer stations (table i) were installed in northwestern italy (fig. 1), over an area of an hypothetical regional magnetic survey. a major regional lithospheric discontinuity lies in the study area, namely the sestri-voltaggio line, which is interpreted (e.g., crispini and capponi, 2001) as the boundary between the alps and the apennine chain fig. 1. geological setting and magnetovariational array; s-v: sestri voltaggio tectonic boundary; short arrows do not have pointers; previous surveys data have been displayed with filled line style. 400 marco gambetta, egidio armadillo, cosmo carmisciano, fabio caratori tontini and emanuele bozzo (fig. 1). the estimation of the anomalous internal field was performed by the classical geomagnetic transfer functions approach in the frequency domain (e.g., gough and ingham, 1983), usually adopted in regional geomagnetic array studies. to better define the electrical conductivity distribution of the study area and improve spatial coverage, data from other mobile observatories (di mauro et al., 1998; armadillo et al., 2001) were also included. 2. data processing a small array of 4 variometer stations (ssl, rez, grn, rdk) was set up in april 2005, along the coast of the ligurian sea (fig. 1). each station was equipped by a self powered recording system consisting of a three-component fluxgate magnetometer, a digital datalogger and a power supply solar panel (table i). magnetic field variations were sampled at 0.33 hz with an accuracy of 0.5 nt. separation of the normal and anomalous field was performed by means of the classical single station transfer function approach (gough and ingham, 1983), where the normal field is identified with the horizontal components (x, y) and the anomalous field is identified with the part of the vertical component (z) satisfying the following linear relation that is assumed to hold in the frequency domain: . (2.1) a number of variable length, three components magnetic records were extracted from the data stream and then the standard fft was calculated. transfer functions a( f ) and b( f ) were estimated at different frequencies f, minimizing ε2( f ) for 125 magnetic events, by the robust approach of egbert and booker (1986). it accounts for the systematic increase of errors with increasing of the power of external variations and automatically downweights source contaminated outliers. in the analysis, only fourier coefficients with magnitudes between frequency dependent thresholds were considered, in order to omit the highest power events (that likely show the strongest deviations from the uniform source field assumption) and ensure good signal to noise ratio. ( ) ( ) ( ) ( ) ( ) ( )f f f f f fz a x b y ε= + + single station transfer function are presented by means of induction arrows (fig. 1), which are widely accepted as the most efficient way to plot magnetovariational survey results (e.g., arora et al., 1999). the complex transfer functions a(f) and b(f) define then a pair of induction arrows, respectively real and imaginary, given by (2.2) . (2.3) the induction arrows magnitude s( f ) and direction θ( f ), is related to the ratio between the vertical anomalous field and the inducing field’s strength and characterizes the electrical and geometrical properties of the conductive struc( ) ( ) ( ) tanf a f b f real, imag real, imag real, imag1θ = − ( ( (f f fs a breal, imag real, imag real, imag 2 2= +) ) ) fig. 2. cross-correlation coefficients calculated for stations ssl, grn, rdk, aqu. 401 magnetic base station deceptions, a magnetovariational analysis along the ligurian sea coast, italy tures within the study area. small real arrows, calculated at given sites for given periods, mark the presence of highly conductive structures below such sites, while high magnitude converging arrows highlight elongated 2d conductivity structures in the proximity of the sites. we have verified the uniformity of the normal field by computing the cross-correlation coefficients between the three magnetic field components for 125 simultaneous records of 5 h each for all possible combination between four variometer stations (fig. 2). rez (rezzoaglio) station cannot be considered in this analysis since the data record is not simultaneous with the other stations due to a malfunction. to infer the behavior of the magnetic field at a larger spatial scale, we also considered data provided by aquila national observatory (aqu), located in central italy (42°23ln, 13°19le) at about 350 km from rdk. results are shown in table ii where the mean values of cross-correlation are presented. a consistently high correlation between the horizontal field components is detected, even when x and y are compared with the distant aqu observatory. in contrast, low correlation appears when the vertical component (z) is considered. a deeper insight is provided by coherence analysis, allowing us to check uniformity of the field at different frequency bands. applying the same scheme adopted in calculating cross-correlation coefficients, the coherence has been computed by welch’s method (welch, 1967), (2.4) where a and b represent the selected magnetic field component which has been recorded by a pair of variometer stations, paa, pbb are the corresponding power spectral density and pab is cross power spectral density of a and b. the calculation was performed for all 125 magnetic events and all possible pairs of variometer stations. figure 3 shows that the horizontal magnetic field uniformity can be assumed to hold only ( ) ( ) ( ) ( ) f p f p f p f c aa bb ab ab 2 = table ii. mean cross-correlation coefficients. x component ssl grn rdk aqu ssl 1 0.94 0.95 0.94 grn 1 0.97 0.97 rdk 1 0.98 aqu 1 y component ssl grn rdk aqu ssl 1 0.95 0.95 0.91 grn 1 0.99 0.96 rdk 1 0.97 aqu 1 z component ssl grn rdk aqu ssl 1 0.18 0.48 0.53 grn 1 0.56 0.30 rdk 1 0.68 aqu 1 fig. 3. mean coherence coefficients calculated for stations ssl, grn, rdk, aqu. 402 marco gambetta, egidio armadillo, cosmo carmisciano, fabio caratori tontini and emanuele bozzo for periods longer than about 400 s. at increasing frequencies the coherence drops nearly to zero, indicating prevalent local noise. 3. the internal contribution for periods longer than 400 s ( f = 2.5×10−3 hz), horizontal field uniformity suggests that the horizontal anomalous parts are negligible, and the normal field can be considered uniform over the investigated area. this implies that the vertical z component is linearly correlated with the normal field x, y via eq. (2.1), and z can be effectively interpreted as the contribution arising from the electrical currents flowing within the 2d/3d conductivity structures within the earth. these electrical currents affect the vertical magnetic component at different observatories by modifications in z amplitude and phase recordable at the scale of regional arrays. an example of this can be seen in fig. 4, where the three magnetic field components, simultaneously recorded, at ssl and grn stations are shown for one particular event. the horizontal (normal) field appears uniform at the two sites, while the vertifig. 4. example of recorded and calculated (vertical component) magnetic signal at ssl and grn. recorded data starts at april, 14th 2005 ut 02:33:27. 403 magnetic base station deceptions, a magnetovariational analysis along the ligurian sea coast, italy cal (anomalous) component shows larger variations at ssl with respect to grn. we have also reported the calculated z component via eq. (2.1) to be compared with the actual observed values. since calculated z values derive from a statistical computation, differences must be ascribed to deviations from the uniform field assumptions, likely due to local noise. 4. data interpretation although our mobile observatories are sparse, general considerations on the regional conductivity distribution are possible from the analysis of the induction arrows plot (fig. 1). the most westerly stations (osl, bss, ssl) all exhibit a similar behavior, i.e. they show similar direction and magnitude. these stations are located inside a complex geological environment that can be defined as the alpine domain. on the contrary, the observatory located at rez shows quite different induction arrow behavior and moving eastwards the difference increases. grn pointers show the largest magnitude with directions from ssw to wsw; rdk pointers have the same direction as grn ones, but comparatively smaller magnitude. these three stations (rez, grn, rdk) belong to the apenninic domain. at the survey scale, the two domains appear quite uniform, but geologically they are complex thrust belts (makris et al., 1999; federico et al., 2005). in the south, the pointers calculated for the tuscan observatories (rdc, rdd) show very small magnitude, with variable direction. they correspond to a well-known peak in the residual heat flow, known as the larderello-travale geothermal anomaly (armadillo et al., 2001). we focus our attention on the boundary between the alpine and the apennine chain, marked by the voltri massif and the sestri-voltaggio zone, which expose high pressure metaophiolites and meta-sediments of the ligurianpiedmont domain of the alps (federico et al., 2005). the tectonic boundary between the voltri massif and the sestri voltaggio zone corresponds to a steep ns fault known as the sestrivoltaggio line (crispini and capponi, 2001). it has been interpreted either as a low-angle normal fault (hoogerduijn strating, 1991) or as a thrust, which juxtaposes rocks from different crustal levels (cortesogno and haccard, 1984; capponi, 1991). substantial differences in the pointers behaviour at both sides of the boundary area clearly indicate a strong electromagnetic signature due to induced currents flowing in this 2d/3d structure. the consequence is higher amplitude in the vertical component at the closest station ssl and rez with respect to the eastern sites. 5. the effect of em induction on aeromagnetic and marine magnetic surveys our aim is to estimate the effects of electromagnetic induction on the standard magnetic base station diurnal correction, considering a hypothetical magnetic survey over the ligurian sea, either performed from an airborne or shipborne platform. magnetic base station corrections are typically designed to remove the time varying field by subtracting the value measured at the fixed base station from the magnetic field measured by the rover sensor, at a matching time. residual time variations and other errors are then removed by levelling and micro-levelling procedures (luyendyk, 1997; ferraccioli et al., 1998). in the following we show that residual errors can be introduced into a magnetic survey data, if the base station used as a reference is inappropriately located close to a regional 2d/3d conductivity structure such as, in our case, the alpine-apennine boundary. we have considered two different sites: ssl, which is located at a distance of 32 km from the sestri-voltaggio line, and grn (70 km away). considering 125 simultaneous events 18000 s long (5 h), we estimated the vertical anomalous field via eq. (2.1) from the horizontal normal field. in the next step we calculated the contribution to the magnitude f of the earth magnetic total field due to the anomalous vertical component z. figure 5 displays the mean power spectra of the total magnetic field for the 125 events. the ssl site shows more power in the range from 400 to 18000 s than the grn site. the mean ratio between the power spectra for ssl and grn is about 1.8. this implies that an occurring time dependent variation 404 marco gambetta, egidio armadillo, cosmo carmisciano, fabio caratori tontini and emanuele bozzo creates an inductive response in ssl 1.8 times larger than the corresponding one in grn. if the magnetic base station of a hypothetical air/ ship borne survey were located at ssl, this energy would be folded in the data when the standard base correction would be applied. the absolute value of the power spectra provides a tool to refine tie-line spacing selection, thereby ensuring improved removal of any internal inductive contribution due to the time varying field. recalling the hypothetical survey mentioned above we are able to evaluate a wavelength to be taken into account for the tieline spacing. since the difference in power between the two stations rapidly decreases at periods shorter than 1500 s, we identify this wavelength as the target for tie-line spacing. in the case of an airborne survey with a 100 knt moving sensor, we can define a miminum tieline spacing of 15 km, as a fifth of nyquist frequency. similar calculation can be performed considering a shipborne survey with a 6 knt moving sensors; the tie-line spacing, in this case, would be about 1.0 km. 6. conclusions our data shows that fluctuating electrical currents in 2d/3d conductivity structures can induce time-varying effects of inductive magnetic fields over an hypothetical magnetic base station. this folds an additional source of error into a magnetic survey when the standard base station correction is applied. if the magnetic base station is located in proximity of deep electrical conductivity structures the recorded magnetic signal will show significant power due to the inductive effects in the buried conductor. the rover magnetometer, synchronised with the base sensor, will not record the same power, simply because it could be miles away from the fluctuating curfig. 5. total field power spectra and power spectra ration for ssl and grn stations. 405 magnetic base station deceptions, a magnetovariational analysis along the ligurian sea coast, italy rents flowing in the conductive body in proximity of the base station. in this case, the correction procedure will induce additional noise in the processed dataset. hence ideally a magnetovariational survey should be performed before a regional magnetic survey to adequately image the distribution of inductive magnetic sources. this would allow for an improved site selection for the magnetic base station(s). as a complementary result of the magnetovariational survey we have shown a substantial uniformity of the external magnetic field, revealed by the magnetic horizontal components, at periods larger that 400 s. at higher frequencies the external field appears to be inhomogeneous. however, the energy involved is low, so the residual errors can be neglected for standard aeromagnetic or marine magnetic surveys. our results indicate that the spectral analysis of timevarying magnetic fields can be used as a tool to tune both tie-line spacing and decorrugation cutoff coefficients for microlevelling (e.g., ferraccioli et al., 1998). interestingly, these residual time-dependent misfits at cross-overs between survey lines and tie lines may also have an unexpected usage. they may add an important source of geological information to magnetic survey data, which can be utilised to identify deep electrical conductivity structures. this is well recognised, for example, from a previous case study over australia (hitchman et al., 2001). despite the small number of observatories in our own magnetovariational array, we find clear inductive evidence of the sestri voltaggio tectonic line. the presence of a prominent electrical signature at this location is important, because it implies that more detailed magnetovariational investigations could be designed in this region, to compute deep electrical conductivity models. these conductivity models may image the deep roots of the sestri voltaggio tectonic line, as proven by previous studies across other major fault belts in the world (e.g., ledo et al., 2002; armadillo et al., 2004). acknowledgements the first author of this paper wishes to acknowledge the valuable financial contribution of the ingv (la spezia-rome) to this research. while the work was in progress the authors benefited much from discussion and a preliminary review kindly provided by f. ferraccioli (british antarctic survey). references armadillo, e., e. bozzo, v. cerv, a. de santis, d. di mauro, m. gambetta, a. meloni, j. pek and f. speranza (2001): geomagnetic depth sounding in the northern apennines (italy), earth planets space, 53, 385-396. armadillo, e. , f. ferraccioli, g. tabellario and e. bozzo (2004): electrical structure across a major icecovered fault belt in northern victoria land (east antarctica), geophys. res. lett., 31, l10615, doi: 10.1029/ 2004gl019903. arora, b.n., n.b. trivedi, i. vitorello, a.l. padilha, a. rigoti and f.h. chamalaun (1999): overview of geomagnetic deep soundings (gds) as applied in the parnaiba basin, north-northeast brazil, rev. bras. geofis., 17 (1), 44-65. campbell, w.h., c.e. barton, f.h. chamalaun and w. welsh (1988): quiet-day ionospheric currents and their application to upper mantle conductivity in australia, earth planets space, 50, 347-360. capponi, g. (1991): megastructure of the south eastern part of the voltri group (ligurian alps): a tentative interpretation, boll. soc. geol. ital., 110, 391-403. cortesogno, l. and d. haccard (1984): note illustrative alla carta geologica della zona sestrivoltaggio, mem. soc. geol. ital., 28, 115-150. crispini, l. and g. capponi (2001): tectonic evolution of the voltri group and sestri voltaggio zone (southern limit of the nw alps): a review, ofioliti, 26 (2a), 161164. di mauro, d., e. armadillo, e. bozzo, v. cerv, a. de santis, m. gambetta and a. meloni (1988): gds (geomagnetic depth soundings) in italy: applications and perspectives, ann. geofis., 41 (3), 477-490. egbert, g.d. and j.r. booker (1986): robust estimation of geomagnetic transfer functions, geophys. j.r. astron. soc., 87, 173-194. federico, l., g. capponi, l. crispini, m. scambelluri and i.m. villa (2005): 39ar/ 40ar dating of high pressure rocks from the ligurian alps: evidence for a continuous subductionexhumation cycle, earth planet. sci. lett., 240, 668-680. ferraccioli, f., m. gambetta and e. bozzo (1998): microlevelling precedures applied to regional aeromagnetic data: an example from the transantarctic mountains (antarctica), geophys. prospect., 46, 177-196. gough, d.i. and m.r. ingham (1983): interpretation methods for magnetometer arrays, rev. geophys. space phys., 21 (4), 805-827. hermance, j.f. (1995): electrical conductivity models of the crust and mantle, in global earth physics, an handbook of physical constants (am. geophys. un. shelf 1), 190-205. 406 marco gambetta, egidio armadillo, cosmo carmisciano, fabio caratori tontini and emanuele bozzo hitchman, a.p., f.e.m. lilley and p.r. milligan (2001): electromagnetic induction information from differences at aeromagnetic crossover points, geophys. j. int., 145, 277-290. hjelth, s.e. and t. korja (1993): lithospheric and uppermantle structures, results of electromagnetic sounding in europe, phys. earth planet. inter., 79, 137-77. hoogerduijn strating, e.h. (1991): the evolution of the piemonte ligurian ocean. a structural study of ophiolite complexes in liguria (nw italy), phd thesis (university of utrecht), pp. 127. hyndman, r.d. (1988): dipping seismic reflectors, electrically conductive zones, and trapped water in the crust over a subducting plate, j. geophys. res., 93 (b11), 13391-13405. ingham, m.r., d.k. bingham and d.i. gough (1983): a magnetovariational study of a geothermal anomaly, geophys. j.r. astron. soc., 72, 597-618. jiracek, g.r., v. haak and k.h. olsen (1995): practical magnetotellurics in a continental rift environment, in continental rifts: evolution, structure, tectonics, edited by k.h. olsen (elsevier, amsterdam), 103-129. jones, a.g. (1999): imaging the continental upper mantle using electromagnetic methods, lithos, 48 (1-4), 57-80. jording, a., l. ferrari, j. arzate and h. jdicke (2000): crustal variations and terrane boundaries in southern mexico as imaged by magnetotelluric transfer functions, tectonophysics, 327 (1-2), 1-13. ledo, j., a.g. jones and i.j. ferguson (2002): electromagnetic images of a strikeslip fault: the tintina faultnorthern canadian, geophys. res. lett., 29 (8), 1225, doi: 10.1029/2001gl013408. luyendyk, a.p.j. (1997): processing of airborne magnetic data, agso j. austr. geol. geophys., 17 (2), 31-38. makris, j., f. egloff, r. nicolich and r. rihm (1999): crustal structure from the ligurian sea to the northern apennines a wide angle seismic transect, tectonophysics, 301, 305-319. sakkas, v., m.a. meju, m.a. khan, v. haak and f. simpson (2002): magnetotelluric images of the crustal structure of chyulu hills volcanic field, kenya, tectonophysics, 346, 169-185. soul, s.j. and m.j. parson (1998): levelling of aeromagnetic data, can. j. explor. geophys., 34 (1-2), 9-15. welch, p.d. (1967): the use of fast fourier transform for the estimation of power spectra: a method based on time averaging over short, modified periodograms, ieee trans. audio electroacoust., 15 (2), 70-73. (received february 1, 2007; accepted june 2, 2007) o b s e r v a t i o n s o f lg and r g w a v e s and r e m a r k s a b o u t t h e n a t u r e o f lg! s a v a r e n s k y e . t h . a n d v a l d n e r n . g . ( * ) ricevufco il 7 dicembre 1960 the result of the seismic sounding in the black sea (l) showed that to the south of the crimean peninsula the granitic layer is interrupted in the deep part of the sea. the preliminary data about the weakening of the lg and rg waves when they piopagate through the central part of the black sea was exposed in our previous work (**) and by m. bath (2). the further investigation consists in finding the clear waves lg on the continental paths. we succeeded in this comparing the records of waves at stations tiksi from the earthquakes of two groups. the lirst group of earthquakes: mongolia, pamir, southern china. the second group: aleutian islands, kamchatka, kuril islands, japan. por the mongolian earthquakes very sharp beginnings of waves, especially lg1( having great amplitudes with the period of 2 to 1 sec are observed. on the records of the station tiksi clear waves lg and rg are noticed (fig. 1). for the earthquakes of southern china these waves are clear, however the amplitudes are smaller and the periods of the waves are greater than previous. on the whole the first group of the paths corresponds to the continental structure of the earth's crust. the mean values of the velocities of these waves are: lgx — 3.53 km/sec; lg2 3.31 km/sec; rg 3.05 km/sec. for the epicentres of the aleutian islands no clear beginnings of the waves lg and rg are distinguished. the period of 18 to 21 sec with the superposition of rather weak waves of short period is predominant on the records of surface waves (fig. 2). the direction of the paths in (*) paper read at the helsinky assembly of the i.u.g.g., 1960. (**) « annali di geofisica», x i i i , n. 2, 1960. 9 6 s a v a r e n s k y e . til. v a l d n e r n . g. this case passes through the deep part of the bering sea (h ^ 3.5 km) where the granitic layer seems to be interrupted or absent. for this region seif-mograms of 12 earthquakes are interpreted. the group of the epicentres of kamchatka, the kuril islands and j a p a n does not present a clear pattern of the interruption of the granitic jr-s fig. 1. copy of a seismogtam of the seismic station tiksi with clear lgx wave. layer or the continental structure of the earth's crust. i n this case seismograms with the lg and e g waves and without them are available. this can be explained by the presence of a deep part in the okhotsk sea (h = 3 km) on the path of the propagation as well as the kuril trench which has some effect creating the breakings of the granitic 9 8 s a v a r e n s k y e . til. v a l d n e r n. g. layer. this is the cause of the doubtful cases and the disappearance of lg and rg on records. 26 earthquakes of this group are investgated. for some of them the mean values of the velocities are the following: lg! — 3.50 km/sec; lg2 3.29 km/sec; rg 3.06 km/sec. fig. 3 shows the results of the calculations of the dispersion curves of the group velocities for love waves in the two layered crust (3). the ordinate axis represents the ratios of the group velocity to the velocity of transversal waves 6, in the upper layer. the abscissa axis represents the value reverslv proportional to kh; (h = lil + ii,) expressed in terms t.c ||[. curve k,/h °-9' t r t f r / h fig. 3. plot of dispersion curves of love waves in the two-layered crust. b t of the period t velocity and the thickness of the crust h, namely ' . h the parameter of the family of curves is the ratio of the upper layer's thickness to the total thickness of the earth's crust h. the group of the curves in fig. 3 corresponds to the first mode. the famdv of curves for the first mode shows the presence of the points of maximum. por them the amplitude in the conditions of dispersion is great and the appearance of such waves on a seismogram will form as if onsets of waves. the maximum will be seen on the background of the preceding waves. we suppose that such is the nature of waves lgt. this wave corresponds to the maxima on the dispersion curves of the first as well as higher modes of the love wave. for the final solution of the problem about the correctness of our explanation of nature of the lg! wave the observed data of the velocities and periods of lg! wave must be compared with the theoretical curves in fig. 4. the comparison of empirical results with the curves was made on different assumptions about the thickness of the earth's crust. o b s e r v a t i o n s of lg a n d rg w a v e s a n d r e m a r k s a b o u t , e t c . 9 9 families of curves are plotted in fig. 4. the abscissa axis represents the period in sec., the ordinate axis — the value of the velocities based on the assumption that bl = 3 . 3 km/sec. each straight line corresponds to the points of the maximum for the dispersion curves of the first mode of the love wave at different values of the total thickness of the earth's bath f i ) • autkox* + payo sumrtco (5) •—•> pt£m , glafiluj,. (4) olufe/t, £u/i>n^: pxm (6) fig. 4. plot of lines of maxima (group velocity 1st mode) and tlie velocity and period of t h e lgt wave by different authors. crust from 20 to 60 km. the horizontal straight lines on this chart correspond to different values of the ratios of the thickness of the upper granitic layer h1 to the total thickness of the crust h. points designate the values of velocities and the periods of their onsets obtained from observations by different authors: for the propagation of waves from the black sea earthquakes to swedish stations by bath (2), for the waves from the black sea earthquakes to the moscow and simferopol stations by the authors, for the waves propagating through the usa 1 0 0 s a v a r e n s k y e . til. v a l d n e r n . g. continent by press (4). the data of the lg! wave from various earthquakes obtained at the toledo seismic station by payo subiza (5) and finaly the data obtained by obver, ewing and press for the arctic (8). on the whole the points corresponding to the observation data on the chart for the thickness of the crust from 20 to 60 km. however their scattering considerably exceeds the 60 km thickness of the crust. the relative thickness of the upper layer of the crust ranges from 0.25 to 0.45. thus to a first approximation the agreement with our assumption t h a t lgi is as a rule the first mode of the love wave can be considered satisfactory. fig. 5. plot of lines of maxima group velocity (2<i mode) and the velocities and periods of t h e lg! wave of t h e mongolian ear t h quakes more detaded comparisons show that the lgx wave can correspond also to the second mode of the love wave. this is suggested by the fact t h a t on the path of the propagation of waves from mongoba to the station tiksi the thickness of the earth's crust is sufficiently great. if to use the formula permitting to estimate the crustal thickness on the basis of rebef (7) (or t h e map given in the same paper) the crustal thickness on this path must be of the order of 45 km approximately, the same thickness would be obtained by means of the method of a. a. treskov (a). if to extrapolate the data obtained in the process of the deep seismic sounding of the northern tien shan (9) to the path from mongoba to north then the crustal thickness must also be estimated equal to 40-45 o b s e r v a t i o n s of lg a n d rg w a v e s a n d r e m a r k s , e t c . 1 0 1 km and the relative thickness of the granitic layer can be considered equal to 0.3-0.5. fig. 5 is a chart of the straight lines for the points of the maximum on the group velocity curves of the second mode of the love wave with different crustal thickness. the points representing the velocities and the periods of the lg! at the station tiksi from mongolian and chinese earthquakes when the beginnings of lg! were the most clear are also plotted on this chart. these points agree with the presented data about the crustal thickness expected on the investigated path. summary this investigation consists in finding the ivaves lg and rg on the paths in the far east. on the bases of theory and observations some conclusions are made concerning the nature of lgi wave as a mode of love waves in two layered crust. riassunto questa indagine consiste nella ricerca delle onde lg ed rg lungo i tragitti nelvestremo oriente. sulla base della teoria e delle osservazioni vengono tratte alcune conclusioni che riguardano la natura dell'onda lgl considerata come onda di love in due croste stratificate. r e f e r e n c e s ( ! ) n e p r o t c i i n o v y u . p . , deep crustal structure under the black sea to south-west of the crimea from seismic data. « dokladi acad. nauk. », 125, 5, 1119, (1959). ( 2 ) b a t h m., the elastic waves lg and rg along eurasiatic paths. « arkiv for geofisic» (stockholm), 2, 13, 295, (1954). ( 3 ) d o r m a n n j . , numerical solution for love wave dispersion on a half space with double surface layer. ((geophysics)), 24, 1, 12, (1959). ( 4 ) p r e s s f . , e w i n g m . , two slow surface waves across north america. «bull. seism. soc. am. », 43, 3, 219, (1952). ( 5 ) p a y o s u b i z a , report during v european seismological comission, spain, 1959. 1 0 2 s a v a r e n s k y e . til. v a l d n e r n . g. ( 6 ) o l i v e r i . , e w i n g m . , p r e s s p . , crustal structure of the arctic regions from the lg phase. « bull. geol. soc. am. >, 66, 9, (1955). ( ' ) d e m e n i t z k a y a r . m . , techniques of the study of the structure of the earth's mantle crystal part. « soviet geology », 92, (1959). ( 8 ) t r e s k o v a. a., results of the determination of the crustal thickness from observations of distant earthquakes. « bulletin of t h e seismological council of t h e ussr », acad. sci. ussr, 6, 76, (1957). ( 9 ) k o s m i n s k a y a i . p . , crustal structure from seismic data, bulletin of the moscow society of the nature investigators. « section of geology », 33, 4, 25, (1958). vol52,2,2009 167 annals of geophysics, vol. 52, n. 2, april 2009 key words synthetic aperture radar interferometry – independent component analysis – ground deformations – landslides – subsidences 1. introduction the ica (independent component analysis) technique is devised for separating signals obtained as a mixture of additive components with unknown mixing coefficients (e.g., hyvärinen et al., 2001). more specifically, ica algorithms are applied on a vector x of n experimental observations (x1, x2,..., xn), given by the overlapping of m different unknown signals (s1, s2,..., sm), or «sources», which are considered elements of a vector s, named the «vector of sources». the coefficients of the «mixing», aij, are considered the element of the matrix a, named «mixing matrix»: x = a·s (1.1) given x, when the elements of s and of a are unknown, ica algorithms are devised to extract the best estimate of a and s, defined as a minus of an additional and multiplicative constant (hyvärinen et al., 2001). in the case of sar interferograms δφi, their phases for their n pixels (ordered in sequence of columns or rows) can be considered as one observation xi: xi = δφi = (δφi1, δφi2,…, δφin) (1.2) in this way, an array of k interferograms can be considered as the columns of a matrix that can be treated as the vector of observation x: x = (x1, x2, ..., xk) = (δφ1, δφ2, …, δφk) (1.3) on the variations of insar-ica altitudes in a mountain area of the sele valley (south italy) paola ballatore mediterranean agency for remote sensing and environmental control (marsec), benevento, italy abstract the ers-1/ers-2 synthetic aperture radar (sar) data are processed by insar-ica technique and the altitude variations are calculated for the period june-december 1995. for the same period and the same ers1/ers-2 sar data, the nominal surface deformations are calculated by the standard dinsar (differential interferometric sar) method, considered without any atmospheric correction. the results of the two different processing techniques are qualitatively compared with the previous geological and hydrological observations of the area under study. it is shown how the insar-ica altitude variations are in better agreement with the expected characterization of this region. although these variations cannot be considered quantitative precise surface deformations at present, this finding is interpreted in terms of usefulness and reliability of insar-ica altitude estimations. mailing address: dr. paola ballatore, mediterranean agency for remote sensing and environmental control (marsec), villa dei papi, via perlingieri 1, 82100 benevento, italy; e-mail: paolab@marsdb.it vol52,2,2009 17-06-2009 19:02 pagina 167 168 p. ballatore each δφi is given by the overlapping of phase shift components driven by different phenomena, such as the topographical altitude (δφtopo), the surface deformations (δφdef), the atmospheric artifacts (δφatm) and generic noise (δφnoise): δφ i= δφtopo,i + δφdef,i + δφatm,i + δφnoise,i (1.4) also other phenomena (e.g., orbital precision, unwrapping, etc.) can drive minor phase shifts and can appear in δφi (hanssen, 2001), but these can be neglected in the first instance or in specific observational circumstances. in eq. 1.4, each component can be considered due to a part common to the other interferograms (stopo, sdef, satm, etc), multiplied by coefficients, aij, depending on the specific interferogram. the common parts can be considered elements of the vector of sources (taken in sequence of columns or rows, similarly to x); the aij can be considered elements of the matrix a. x = (δφ1, δφ2, …, δφk) = = a·s = a·(stopo, sdef, satm, ...) (1.5) in this way, the identification of the different components of insar phase shifts can be successfully represented by the typical ica problem solving, given in eq. 1.1 (ballatore, 2006). in the case of sar interferograms, the specific fastica algorithm (e.g., hyvärinen and oja, 1997) was previously employed in insarica method (ballatore 2006; 2007). after fastica application on arrays of sar interferograms, a different post-processing is necessary to pass from each source to the corresponding geophysical parameter. in particular, this post-processing has been devised for extraction of topographical altitudes, which resulted in good agreement with the standard srtm-dted (shuttle radar topography mission – digital terrain elevation data) in specific test sub-areas (ballatore, 2006; 2007). for different sar data sets, an extensive comparison between srtm and insar-ica altitudes has demonstrated that the distribution of the residuals is centered more or less close to zero (within the nominal errors), with distributions whose standard deviations depend on the specific interferograms considered (ballatore, 2009). in general, an agreement between insar-ica dems and srtm altitudes is observed, after consideration of the criticality of srtm measurements in the high-relief terrains (falorni et al., 2005). this paper addresses the differences between insar-ica dems extracted from timeseparated interferograms arrays. here, these are not considered precise surface deformations (because not validated to date). however, the accuracy of this parameter as a more or less rough indication of surface deformations is qualitatively investigated in one particular case to highlighting elements on the correctness of insar-ica dems and motivating any future works on processing their differences to extract information on terrain surface deformations. 2. the area under study the area considered here is a part of the sele valley and is illustrated in fig. 1, where the river flows near the ss91 at the central vertical location. this is a basin that is a tectonically active valley of south italy, with typical slope instabilities and floods. the altitude changes are significant: the maximum elevations reach values >1400 m on the right and left sides and the minimum elevations reach about 150 m along the sele river, passing through about 580 m a.s.l. at senerchia and 590 m a.s.l. at colliano. the soil of this region is characterized by different lithotypes (polemio and petrucci, 2001; wasowski et al., 2002). in particular, investigations around the sele river after the 1980 earthquake indicate higher concentration of landslides on the western than on the eastern slopes of the valley (agnesi et al., 1983; carrara et al., 1986). in particular, hydrological studies also illustrate a substantial difference between the eastern and western sides of the valley: many more springs are located on the western side, with higher discharges, exceeding 50 l/s in a few cases (budetta et al., 1988, and references vol52,2,2009 17-06-2009 19:02 pagina 168 169 on the variations of insar-ica altitudes in a mountain area of the sele valley (south italy) therein). some major landslides were monitored and found active (with decrease in activity) until 2001 (e.g., polemio and petrucci, 2001; and references therein). because of the public availability of information on its peculiar terrain characterization, this area was chosen for qualitative comparison between insar dem variations and the ground based geological/hydrological observations. 3. sar data and data processing ers-1 and ers-2 tandem data with track n. 265 and frame n. 2781 for the period from june to december 1995 were processed by insar-ica and dinsar methods. the insar-ica (ballatore, 2007) was separately applied on two arrays of interferograms, considering ers-tandem couples of images acquired on: (a) june 2 and 3, july 7 and 8, august 11 and 12, 1995; (b) october 20 and 21, november 24 and 25, december 29 and 30, 1995. the differences between the dems extracted from the interferograms array a and b were calculated and the interval between the maximum (positive value) and minimum (negative value) differences divided into five subranges (about equal among themselves) that are shown in different gray level in fig. 2 for about the same zone of sele valley shown in fig. 1. the black areas of fig. 2 correspond to the largest nominal subsidences, while the white areas correspond to the nominal uplifts. by considering the same ers images specified above and acquired on june 2 and 3 and on december 30, the standard three-pass dinsar technique (e.g., hanssen, 2001) was applied for estimating the nominal surface deformations during the same period june-december 1995. the dinsar nominal surface deformations are shown in fig. 3 for the same area of fig. 2. similar to fig. 2, fig. 3 shows a different gray scale level for each one of the five intervals of values between the maximum and the minimum phase differences, with black corresponding to maximum nominal subsidences and white to the maximum nominal uplifts. the comparison between figs. 2 and 3 indicates that, according to insar-ica dems differences, the maximum nominal subsidences are located mostly on the left of the sele river, whereas this is not so for the dinsar results. it is worth mentioning that, in this processing, the dinsar method does not include atmospheric or spurious artifacts correction, which can be subsequently executed depending on user choice/methodology. in particular, by appropriate sorting of images with especially small atmospheric artifacts or specific choice of baselines or specific case by case processing, the three-pass dinsar method is proven to give valuable and validated results. specifically, in this case, the standard dinsar nominal surface deformations without atmospheric corrections is intended to be compared with insarica method, referring exactly to the same data availability and period. this is to show one of the uses of insar-ica technique, that is its capability of application in cases when no choices of data or ancillary different data availabilities are possible. fig. 1. area under study with indication of the sele river location near the ss91. vol52,2,2009 17-06-2009 19:02 pagina 169 170 p. ballatore 4. observations the comparison between figs. 2 and 3 shows that in fig. 2 there is a more asymmetric distribution of the nominal maximum subsidences, which are located more on the western than on the eastern side of the river. differently, fig. 3 shows a quite symmetric characterization on both sides of sele. therefore, the insarica dem variations are in better agreement with expectations from previous ground based investigations (see paragraph 2), which indicate larger landslides and springs occurrence on the west side of sele than on the east side (agnesi et al., 1983; carrara et al., 1986; budetta et al., 1988). this agreement is obtained for data and time period on which the corresponding dinsar method would not give similar results. this is interpreted in terms of the capability of insar-ica technique to provide useful dem extraction in case of limited usability of atmospheric and ground data. although at the present status, the insarica dems-differences cannot represent precise quantitative surface deformations, it can be of interest to note that the darkest colour areas in fig. 2 include alluvial deposits, characterized by middle permeability, with typical landslide prone areas (e.g., polemio and petrucci, 2001) and the most stable zones correspond to typically stable high reliefs of limestone and dolomitic limestone (e.g., wasowski et al., 2002). finally, the specific highest positive increases in elevations of fig. 2 are located over vegetated areas included in the parco regionale dei monti picentini on the west of senerchia and the riserva naturale dei monti eremitamarzano on the east of valva, possible associated with plants and trees growing during the summer season included between june and december 1995. 5. summary and conclusions for an area of the sele valley previous geological/hydrological observations indicate larger landslides and springs on the west side than on the east side of the river (see paragraph 2). fig. 2. differences between insar-ica dems related to the period june-december 1995; the gray color ranges from maximum subsidence (black) to maximum uplifts (white). see text for more details. fig. 3. similar to fig. 2, but for the dinsar nominal surface deformations. 15.169° e 15.297° e 40.804° n 40.678° n 15.169° e 15.297° e 40.804° n 40.678° n vol52,2,2009 18-06-2009 16:41 pagina 170 171 on the variations of insar-ica altitudes in a mountain area of the sele valley (south italy) according to the possible ers sar data availability during the period june-december 1995, the dinsar standard method (without atmospheric corrections) is applied and the nominal surface deformations are derived. their distribution over the area of study is not in good agreement with expectations as the deformations on the west are quite similar to those on the east of sele. this indicates that over that specific period the dinsar method cannot be routinely applied on the ers sar available data alone. by using the same data available for the same period, two insar-ica dems are derived for the time intervals june-august 1995 and september-december 1995. their differences have not been validated to represent precise surface deformations, nevertheless they have to include these deformations, together with spurious artifacts. unlike dinsar observations, the distribution of these insar-ica dem variations are in good agreement with previous geological/hydrological observations over the area of study, showing larger subsidences taking place on the west of sele. this finding is interpreted in terms of the good precision of insar-ica dem in this specific case, with errors evidently smaller than expected or about equal in the two estimates and eliminated by their subtractions. this case study confirms the usefulness of the insar-ica method in circumstances of limited data availability and/or lack of ground based and meteorological ancillary data and supports the hypothesis of eventual possible extraction of surface deformations from differences between time-spaced insar-ica dems. acknowledgements ers-1 and ers-2 data are provided by european space agency in the context of the category-1 program, project n. 3193. references agnesi, v., a. carrara, t. macaluso, s. monteleone, g. pipitone and m. sorriso valvo (1983): elementi tipologici e morfologici dei fenomeni di instabilità dei versanti indotti dal sisma del 1980 (alta valle del sele), geol. appl. e idrogeol., 18 (1), 309-341. ballatore, p. (2006): synthetic aperture radar interferometry: separation of atmospheric artifacts from effects due to the topography and the terrain displacements, earth planets and space, 58, 927-935. ballatore, p. (2007): ica technique applied to sar interferometry, proceedings of the envisat symposium 2007, montreux, switzerland, april 23-27 2007, european space agency, sp-636. ballatore, p. (2009): sar interferometry: dem estimations by independent component analysis, to be submitted. budetta, p., p. celico, a. corniello, r. de riso, d. ducci and p. nicotera (1988): carta idrogeologica del f.186 (s. angelo dei lombardi) memoria illustrativa, mem. soc. geol. it., 41, 1029-1038. carrara, a., v. agnesi, t. macaluso, s. monteleone and g. pipitone (1986): slope movements induced by the southern italy earthquake of november 1980, geol. appl. e idrogeol., 21 (2), 237-250. falorni, g., v. teles, e.r. vivoni, r.l. bras and k.s. amaratunga (2005): analysis and characterizationof the vertical accuracy of digital elevation models from the shuttle radar topography mission, j. geophys. res., 110, f02005. hanssen, r. (2001): radar interferometry, (kluwer academic publishers, dordrecht, the netherlands). hyvärinen, a. and e. oja (1997): fast fixed-point algorithm for independent component analysis, neural comp., 9, 1483. hyvärinen, a., j. karhunen and e. oja (2001): independent component analysis, (wiley interscience, new york, usa). polemio, m. and o. petrucci (2001): hydrological monitoring and image analysis of a mudslide in southern italy, phys. chem. earth, 26, 689-695. wasowsky, j., v. del gaudio, p. perri and d. capolongo (2002): factors controlling seismic susceptibility of the sele valley slopes: the case of the 1980 irpinia earthquake re-examined, survey of geophysics, 23, 563-593. (received february 2, 2009; accepted march 25, 2009) vol52,2,2009 17-06-2009 19:02 pagina 171 vol52,5,2009 505 annals of geophysics, vol. 52, n. 5 october 2009 key words parallel computing – sms – seismic wave propagation – numerical simulations – site effects 1. introduction an important contribution to assessing interaction between near-surface geology and seismic radiation arises from studies involving the so called site effects p.-y. bard, 1995; 1998; panza et al., 2001; s. bonnefoy-claudet et al., 2006, amongst others). these latter are related to seismic wave propagation phenomena (aki, 1988; 1993) with important implications for seismic hazard mitigation. site effects can be studied using an instrumental approach during earthquakes recording soil shaking on specific stations located on suitable chosen sites (caserta et al., 2000; di giulio et al., 2002; cultrera et al., 2003; cara et al., 2005). nevertheless, it would be more useful to know the site characteristics of the ground motion before an earthquake occurs, in this respect numerical modelling is a powerful approach (caserta and lanucara, 2000; moczo et al., 2001) also for estimating parameters commonly used for earthquake engineering purposes (fäh et al., 1993; rovelli et al., 1994). this is particularly true in urban areas where the instrumental approach may not always be used because of the high level of anthropogenic noise (olsen et al., 2006). the great drawback to modelling more realistic site dynamics is the impressive computational requirements needed for numerical simulations such as: gigabytes of memory, performances at gigaflops rate, days of computational time to simulate a minute’s soil shaking, etc. (sáncez-sesma and luzon, 1995; graves, 1998). seismologists have been addressing these issues through the use of parallel computparallelisation technique for serial 3d seismic codes: sms approach arrigo caserta (1)(2), vittorio ruggiero (3), maria pia busico (3) and ivo oprš al (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) charles university, prague, czech republic (3) c.a.s.p.u.r., roma, italy abstract we investigate a fast and easy way to parallelise seismological serial codes mainly oriented for simulating the seismic wave propagation through anelastic dissipative media. having an efficient modelling tool is important in both assessing strong ground motion and mitigation of seismic hazard when the site effects are considered, and in crustal propagation when the crustal geological structures are of interest. our chosen case study is representative of a set of such seismological 3d problems. the scalable modelling system (sms) tool for parallelization is considered. the ibm sp5 native compiler has been used. results such as speed-up and efficiency are shown and discussed. sms can run both in shared and distributed memory environments. the greater advantages of using sms in such environments become apparent with the utilisation of a higher number of multiprocessor machines arranged in a cluster. we also demonstrate how successful porting from serial to parallel codes is realised by way of minimal instructions (6% of the serial original code only) provided that an ad hoc profiling analysis of the serial code is first performed. mailing address: dr. arrigo caserta, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: arrigo.caserta@ingv.it vol52,5,2009 1-12-2009 18:35 pagina 505 506 a. caserta, v. ruggiero, m. p. busico and ivo opršal ers (olsen and archuleta, 1996) coupled with optimization techniques such as the use of unstructured grids to reduce the number of computational nodes (bao et al., 1996). during the last 20 years a considerable amount of work has been done in numerically modelling the interaction between both nearsurface and inner geological heterogeneities and seismic radiation (see the review of bard, 1998; bielak et al., 1998, panza et al., 2001; moczo et al., 2007, and references therein). the results are huge serial codes able to numerically simulate the process. it would be useful to reconvert such serial codes into parallel ones, to have more powerful simulations that realistically represent the dynamics of such interaction. on the other hand, the conversion of a serial code to a parallel one requires typically rewriting only choice available is to rewrite the serial codes from the beginning, and this is not an easy task as well as being energy demanding. not to mention that writing a 3d code from the scratch, on the basis of our experience, is more time demanding than paralleling an existing serial 3d code. this paper is aimed at investigating a more simple and efficient way to parallelise a 3d serial code, more specifically codes for simulating the seismic wave propagation through viscoelastic dissipative 3d media. for the purposes of our study, we adopted the serial code of opršal and zahradnik (2002), which contains general aspects that make it representative of the dynamics that we want to simulate. based on a finite-difference technique on a cartesian mesh (opršal and zahradnik, 2002; opršal et al., 2004, 2005), it computes the full 3d seismic wave-field. null normal stress boundary condition on the topographic surface is realized by the so-called vacuum formalism (opršal and zahardnik, 2002). its stability conditions are represented by the classical c.f.l. criterion for explicit methods (mitchel and griffits, 1980). these features are similarly adopted in many of the serial codes developed in the last years dealing with the seismic wave propagation problem, used to assess strong ground motion (zahradnik et al., 1993; graves, 1998; opršal and zahradnik, 1999; pitarka, 1999; moczo et al., 2001; amongst others). 2. study case: serial 3d wave propagation code the main guidelines for the previously-referenced program by opršal and zahradnik (2002) are based on the formulation of an initial boundary value problem that represents the viscoelastic dynamics driving the full 3d wavefield propagation. the medium is dissipative with arbitrary 3d shape and topography. source energy nucleation and its crustal propagation can be numerically modelled through a discrete wave-number method (dw), as well as ray theory or even an analytical solution; the resulting wave-field represents the seismic radiation ready to light up the site under study. the finite-difference (fd) technique is adopted to model the interaction between the site and the seismic radiation previously computed yielding the site response. in such a way we are able to study the combined source – path – site response of the medium under the action of input radiation as it arises from the seismic source and propagated through the crust beneath the site. the dw-fd coupling used here is realized by extending hybrid technique developed first for the 2d case by zahradnik and moczo (1996) and generalized by opršal et al. (2009) to the 3d case. the main advantage of such a hybrid technique is fast evaluation of site effects and seismic hazard because it needs less computer memory and time than all-in-one source-pathsite computational methods. computations are made in the time domain. the results are synthetic seismograms on the free surface and snapshots of the wave amplitude inside a 3d profile (for further details see opršal and zahradnik, 2002). furthermore, this hybrid technique allows us to study the source-crustal propagation in case we are interested in the crustal propagation only. our test case deals with a real geological structure (fäh et al., 2006), representing the stratigraphy underlying the roman city augusta raurica (fig. 1), located east of basel, switzerland. the 3d structure of the area was retrieved through array investigations (h/v, ambient vibrations) and borehole data (fäh et al., vol52,5,2009 1-12-2009 13:29 pagina 506 507 parallelisation technique for serial 3d seismic codes: sms approach 2006; opršal and fäh, 2007), and the topmost structure (namely so-called top_muschelkalk interface), was refined in terms of fundamental frequencies fit by modifying the shape of the interface limiting the upper geology from below (see fig. 2). the geological model was converted into a computational one according to the stability conditions of the numerical scheme (fig. 1). according to fäh et al. (2006), the section of interest is 1552 m (we) x 1696 m (sn) large rectangular area with constant spatial sampling of 4 m. the computational domain covers the wave dumping area around, resulting in 442 x 480 grid points corresponding to 1764 m x 1916 m of computational model, respectively. the vertical extent of the computational model is 115 grid points corresponding to ~793 m depth while the geological structure differing from bedrock up to topography peaks is less then 150 m. the vertical grid step is not constant, being scaled with respect to the vertical velocity profile. this study also used the same time-step, 0.00023 s and these parameters guarantee the accuracy of the fd computation up to a frequency of 12 hz with a usable range up to 10 hz. we simulate 5 s ground shaking for a total of 21740 time-steps. a problem of near 73 million unknowns is solved at each time-step. although the augusta raurica site was chosen for the purpose of this test case, our study code can be applied to any 3d stratigraphy, as it depends on neither the shape nor the geological details of the site. indeed, other recent real case studies (opršal and fäh, 2007; sorensen et al., 2006; opršal et al., 2004; 2005), have used this study code to simulate wave-field time evolution and its interaction with the geological structure. fig. 1. geological model at augusta raurica. the ew vertical slice is a 3d fd computational model extraction. see also fig. 2. vol52,5,2009 1-12-2009 13:29 pagina 507 508 a. caserta, v. ruggiero, m. p. busico and ivo opršal 3. parallelisation tools to parallelise a code can be divided into high-level and low-level programming. parallel virtual machine (pvm) and message passing interface (mpi) belong to low-level programming, where the parallelisation scheme is planned, designed and realized by the user. in more detail, the management of information exchanged among processors (e.g., message passing, synchronisations, etc.) is realised by the user adding code segments. although efficient on shared and distributed memory architectures, low-level techniques will not be considered in this study as, on the basis of our experience, too much effort (and time) must be spent to convert a serial code into an mpi (or pvm) parallel one (caserta et al., 2002). to parallelise our serial code we chose tools based on a high level technique where all low level parallel machinery (creation, communications and maintenance of processes) is realised by directives to the compiler in a user-hiding mode. the best candidates for such an approach are high performance fortran (hpf), openmp and scalable modelling system (sms). it must be noted that while openmp is only available on shared memory architectures, both hpf and sms can be used in shared as well as distributed memory environments. contrary to caserta et al. (2002), we have avoided adopting openmp for several reasons. first of all, our main target is to simulate numerical problems with tens of millions of variables, nearly 20 times the average size problem of caserta et al. therefore, we prefer to adapt our code to distributed memory machines that allow to handle much larger amount of memory. we choose a high-level technique based on distributed memory architecture as it is more suited for our purposes. fig. 2. intensities for mw=5.9 1999 athens-like earthquake simulation in augusta raurica (left panel); for details see fäh et al. (2006) and opršal and fäh (2007). thick white lines are rivers ergolz (left) and violenbach (right), red semi-ring denotes the amphitheatre, and the green lines in the southern part are for present-time highway. the thin white lines are historically re-mapped streets of the upper and lower city of augusta raurica. the right panel depicts vertical slices of computational model realization of the geology structure; vacuum shown as black color. black arrows connect the surface traces of the vertical geology slices. vol52,5,2009 1-12-2009 13:29 pagina 508 509 parallelisation technique for serial 3d seismic codes: sms approach because hpf is maintained by third-party and private companies (see the hpf official website http://hpff.rice.edu), we have chosen the sms tool which is freely available. see foster (1995) for a complete survey of parallel programming design and tools on distributed memory architectures, while refer to nichols et al. (1996) and the official openmp web site for shared memory references. as far as sms is concerned, see govett et al. (2003) and the official sms website (http://wwwad.fsl.noaa.gov/ac/sms.html). the detection of code segments that must be modified to optimize the serial code and to reduce the memory requirement is performed by the user via the application of a profiling analysis. in such a way, we can ensure a uniformly optimized serial code suitable for parallelisation. below, we detail how this operation is carried out. 3.1. sms technique sms has been developed by the advanced computing branch of the forecast systems laboratory at noaa (national oceanic and atmospheric administration) (govett et al., 2003). the user inserts directives in the form of comments (indetified by!sms$) into the existing serial fortran code. sms translates the code and directives into a parallel version that runs efficiently on shared and distributed memory high-performance computing platforms. the translation is realized by the parallelising pre-processor (ppp), a component of sms. in other words, sms translates user directives into mpi-like code. the main advantage of using sms is that such translation is hidden to the user. this is because sms is built on top of mpi. as a consequence no complicated compiler-generated communication statements have to be included in the code. moreover, sms contains a number of features to speed up the debugging process and to support incremental parallelization. in order to obtain constructs ready to be automatically parallelised by the sms, the first step is to upgrade some commands of the original fortran77 code to fortran90. in case the code is already written in fortran90, the first step is to modify the code in such a way that it can directly understand sms directives. so, according to sms parallelization philosophy, we have first to define the decomposition type through the decomposition declaration as in the following example module decomp !sms$distribute (decomp_xy. 1. 2) end module decomp (3.1) with the previous declaration we have created a new fortran90 module to define decomposition structure, decomp_xy, needed to support data exchange, local and global address translation, do-loop transformations and data decomposition. the number that follows decomp_xy in the decomposition declaration is the number of dimensions involved in the decomposition declaration. even though we deal with a 3d problem, the maximum number of dimensions allowed by sms for parallelizing in the decomposition declaration is 2. the next step is to distribute the array representing the solution of our equations of motion, more specifically the array u (:,:,:,:,:) representing soil displacement, as well as all other auxiliary arrays. according to the previous decomposition declaration we have: !sms$distribute (decomp_xy. 1. 2) begin real. allocatable : : u (:,:,:,:,:) !sms$distribute end (3.2) this directive links the data decomposition structures, defined in eq. (3.1), with arrays targeted for decomposition. with the previous sms directive we have chosen to distribute the first two indices of array u. declarations (3.1) and (3.2) allow sms to understand how the chosen arrays must be distributed among processors. this is because all information necessary to access, communicate, input and output, decomposed and non-decomposed arrays are specified in the previous two declarations. to go on with the parallelization, we insert sms directives in the original fortran90 code, after the declaration section. the first instruction is the decomposition creation vol52,5,2009 1-12-2009 13:29 pagina 509 510 a. caserta, v. ruggiero, m. p. busico and ivo opršal where the decomposition name, decomp_xy, the global size of the 1st and 2nd decomposed dimensions, <nx, ny> and width of the halo region, <1, 1>, are specified. more in details, <nx, ny> represents the maximum number of grid points of the 1st and 2nd dimension that must be decomposed, whereas <1, 1> represents the number of grid points involved in the halo region; we use a nearest neighbours algorithm in which we have specified the halo region made by just one grid point. of course, such a halo region is in common between two neighbour processors. sms requires the indices that have to be parallelized must be both, the first two in the array and they must refer to the grid’s nodes. such constrains might require modifications of the serial code to have the indices in correct order for the sms parallelisation. the following old fortran90 instruction in the serial code allocate (u(ndim,nx,nz,nt)) has the first index not linked with the grid’s nodes, ndim is an index referring to the component, actually. moreover, the indices of the grid’s nodes, i.e., nx,ny,nz are in the wrong position to be parallelized. so, according to the aforementioned sms rules, we have been forced to change the index order as follows allocate (u(nx,ny,nz,nt,ndim) (3.4) once we have detected segments of the program that should be parallelized analyzing the code with a profiling software, we insert sms directives in the serial code. from such profiling analysis it turned out that more than 84% of the time is spent by the serial code in executing the following section for a single time step, that’s why we have focused our attention on it: !sms$parallel(decomp_xy, <kx>, <ky>) begin !sms$exchange (u) ..... ..... do kz=2,nz-1 do ky=2,ny-1 do kx=2, nx-1 call stencil_full(kx,ky,kx) ! full form stencil enddo ennddo enddo ..... ..... !sms$parallel end ..... ..... subroutine stencil_full(kx,ky,kz) ! the 3-d stencil: ..... ..... unew=2.*u(kx,ky,kz,2,kul)-(u(kx,ky,kz,1,kul)*dtdt_rho* (& & u(kx+1,ky-1,2,ku2)-u(kx,ky-1,kz,2,ku2)) & &(u(kx,ky,kz,2,ku1)-u(kx-1.ky,kz,2,ku1))))+ & & ((muf*r1_dyky*(u(k,ky+1,kz,2,ku1)-u(kx,ky,kz,2ku1)))& ..... ..... u(kx,ky,kz,kt3,ku1)=unew end soubroutine stencil_full (3.5) !sms$create_decomp(decomp_xy, <nx, ny>, <1, 1>) (3.3) vol52,5,2009 1-12-2009 13:29 pagina 510 511 parallelisation technique for serial 3d seismic codes: sms approach the parallel section starts with the sms instruction !sms$parallel(decomp_xy, <kx>, <ky>) begin. this instruction defines a region over which parallel computations will be done on each processor’s local data, as defined by the given data decomposition decomp_xy. all do-loops inside a parallel region that reference the specified loop variables (<kx>, <ky>) will be translated. it is worth noting that the order of the indices in the u array coupled with the order of the nested loops in (3.5), is the most efficient combination in respect to the so called easy memory access patterns problem (dowd and severance, 1998; hennessy and patterson, 2006). this holds for both serial and parallel codes provided they are written in fortran language. for different programming languages the coupling between the order of indices in the array and the order of nested loops might be different. the sms directive !sms$exchange (u) has been introduced to communicate with neighbouring processors to update halo regions and to maintain consistency with the serial code in the updating. such a communication is necessary because the array u shows a index dependence on the first neighbouring grid points, e.g. u(kx+1,ky 1,kz,2,ku2). information provided by (3.2) is used to generate the correct communication code for our exchanged variable. in other words, !sms$exchange (u) is in charge of the message passing. time updating procedure has not been changed from serial to parallel code, details about such a procedure can be found in the opršal and zahradnik (2002). probable serial parts of the code, that cannot be parallelized, are nested into a parallel one by !sms$serial directive. the parallel region is closed by adding the directive !sms$parallel end. it is worth noting that we have obtained the parallel version inserting directives that represent only 6% of the whole serial code. 3.2. sms performances for this study, numerical tests are run on an ibm sp5 cluster, with 48 processors arranged in six symmetric multi processor (smp) machines (8 processors each). communication between processors is realised via shared memory within each 8-way smp machine, while a high performance switch (hps) is used to realize communication between the six 8-way smp machines in a cluster. each processor has 1.9 ghz power5, the integrated cluster has a peak performance rate of 364 gflops and a global ram of 192 gbytes. we used the xlf ibm native fortran third party compiler optimized according to the target architecture, moreover, we have optimized the sms tool to the ibm sp5 cluster. in order to achieve suitable performances the first step is to balance the computational work among processors. by default, sms uses a pre-determined set of rules to decide how grid points are assigned to each process and how many processes are allocated to each decomposed dimension. roughly speaking, the grid-points are distributed evenly among the processes along a given decomposed dimension. in order to adopt its domain decomposition we have to run our sms executable code as follows smsrun np 3dcpp_sms where 3dcpp_sms is our executable whereas np is the number of processors. ideally, each processor will have the same number of computational grid-points. in practice, most models have computations that vary spatially so some processes may have more work than others. this is commonly known as load imbalance. load imbalances slow down a parallel program because processes with less work are forced to wait for processes with more work to do. the criterion used by sms to divide the computational workload is to minimize information that must be exchanged among processors. however, attention should be paid in realizing the domain decomposition process. this can be easily done by the user because sms allows to specify the assignment of processors using a process configuration file in the form of a fortran name list. in details, we run our executable sms code as follows vol52,5,2009 1-12-2009 13:29 pagina 511 512 a. caserta, v. ruggiero, m. p. busico and ivo opršal smsrun -cf my_config 3dcpp_sms where my_config is our configuration file &decomp decomp1_name = ‘decomp_xy’, decomp1_nps = 2 23/ in which we specify, in the last line, the number of processors along x-direction and y-direction, respectively. in this case we have 46 processors available. figure 3 shows the time needed for a single time-step versus number of processors for both the domain decomposition suggested by sms (default in fig. 3), and the one preferred by us (user in fig. 3). oscillations in time are clearly recognized as the number of processors increase. such an effect is strictly linked to the load balancing problem. indeed, in our case sms has to divide 480 nodes along the x axis and 442 nodes along the y-axis. in the worst case with 46 processors (see fig. 3), sms realized such division by default assigning 23 processors along x-axis and 2 processors along y-axis. this causes a big load imbalance well represented in the time needed for a single computational step. to fix the load balancing problem, we have inverted the default sms decomposition assigning 2 processors along x-axis and 23 processors along y-axis. our choice, of 46 processors, improved the time needed for a single iteration by almost 19% (see fig. 3). such a procedure has been adopted to improve the domain decomposition for other numbers of processors. a useful index to check any gain in performance with respect to the serial code is the speed-up, s(n). it is defined as the ratio between the real time employed by one processor to run the code, t (1), and the real time needed by n processors, t (n): . figure 4 left panel compares the speed-up of both sms default domain decomposition and our specified decomposition with ideal performance. it emerges that for our modeling the speed-up scaling is satisfactory up to 24 s n t n t 1 =] ] ] g g g processors. beyond this threshold the onset of saturation for s(n) starts. saturation is an important marker, to illustrate its meaning we have to consider that the user-hiding message passing (among processors) is a time consuming process. saturation starts when the message-passing time is comparable with the computation time needed by each processor to run its own part of the workload. the communication time increases with the number of processors while the computational time of each processor decreases. therefore, there is no advantage in increasing the number of processors within a speed-up saturated regime. in looking at fig. 4 left panel, we see that the saturation is approached through oscillations in the speed-up values and this is not surprising taking into account the load balancing analysis discussed above. figure 4 right panel shows the performance of the parallel code in terms of the efficiency, which is defined as the ratio between the speed-up and the number of processors: . by definition, efficiency is a measure of the relevance of user-hiding message passing for a single process. from fig. 4 right panel, the order of magnitude of the improvement achieved can be seen comparing the values of the efficiency for the same number of processors in the case of the user and default domain decomposition. as an example, looking at fig. 4 right panel, at 36 processors we see that the efficiency corresponding to the user decomposition is almost 10% higher that the default one. moreover, in looking at the efficiency plot we see an improvement in performances each time a new 8 processors sp5 machine is added to the cluster. this effect is better seen and recognizable in the efficiency curve corresponding to our domain decomposition choice. in fact, the efficiency shows a decrease followed by a flat curve almost after every 8 processors. the same behavior has been observed in caserta et al. (2002) for hpf; in our case this trend is more difficult to recognize because load balancing effects are superimposed. it follows from this that advantages in using sms are obe n n s n =] ] g g vol52,5,2009 1-12-2009 13:29 pagina 512 513 parallelisation technique for serial 3d seismic codes: sms approach fig. 3. iteration time per time step (in milliseconds) vs. number of processors for the domain decomposition suggested by sms (default) and that one suggested by us (user). the plot starts from 10 processors in order to better magnify the differences between the two decompositions. under 10 processors no difference takes place. fig. 4. plots of speed-up (left panel) and efficiency (right panel) vs. number of processors for both the domain decompositions as planned and realized by sms (default) and as planned and realized by us (user). comparison shows up to 48 processors, i.e., using six 8-processor sp5 machines in a cluster. straight line with asterisks represents the ideal speed vol52,5,2009 1-12-2009 13:29 pagina 513 514 a. caserta, v. ruggiero, m. p. busico and ivo opršal tained when a cluster of multiprocessor machines are used in a distributed memory environment. 4. conclusions we investigated a simple and efficient way to parallelise a serial code, namely 3d seismological code able to simulate the interaction between seismic radiation and near-surface geological structures. the chosen case study code (opršal and zahradnik, 2002) and the case test geological site (fäh et al., 2006) are representative of a wide set of seismological problems dealing with the aforementioned dynamic interaction. our parallel approach is based on the sms high-level tool. it allows the user to avoid all the low-level machinery needed to plan and realise the management of information exchanged among processors, apart from taking care of the domain decomposition problem as discussed in the previous paragraph. this does not mean algorithmic modifications in the serial code were not performed by the user to increase both the speed-up and the efficiency. on the contrary, these latter must be included in the profiling analysis applied to the serial code. in the parallelisation of our case study, numerical tests indicate care must be taken in the domain decomposition process to reduce oscillations in both speed-up and efficiency, improving parallel performances. generally speaking, in a shared memory environment communication is much faster because it takes place in the same cluster node. nevertheless, we have seen less degradation of performances in adding multiprocessor machines. this phenomenon is clearer in the efficiency plot (fig. 3 right panel) where the efficiency remains almost constant when we add a new 8 processor machine, then in the speed-up plot (fig. 3 left panel); this is mainly due to the load balance problem that is more evident in the speed-up plot. such an effect was also observed in caserta et al. (2002) with smaller size numerical problems, when a new multiprocessor machine is added under hpf compiler. moreover, porting from serial to parallel versions can be realized relatively simply for both environments. in fact, the porting is realized through the following simple steps: – definition of the decomposition structure (see eq. 3.1), – distribution of variables among processors (see eq. 3.2), – domain decomposition creation (see eq. 3.3), – creation of parallel region and exchange of variables among processors, i.e., message passing (see eq. 3.5). such sms directives represent around 6% of serial code instructions. on the other hand, sms limitations are mainly: – it can be used for fortran codes only, – it is intended for applications using structured cartesian grids, – maximum number of array indices that can be distributed is two. we point out that they must be the first two indices in the array. this constrain might require modifications of the serial code to have the indices in the required order. this is what we have been forced to do as in eq. 3.4. no further limitations are arranged concerning, in particular, the numerical implementation, i.e., boundary elements, finite elements, spectral elements, finite differences, and so on. under the above limitations, the simplicity and efficiency of the sms tool are evident as it may allow broader and/or detailed physical models to be studied due to the use of parallel machines. it is worth noting that the majority of 3d seismic codes that represent our target are written in fortran and are based on the finite differences method on a cartesian grid. on the other hand, no limitations so, we can conclude that for our purposes the previous limitations are not so severe. acknowledgements we thank piero lanucara for useful discussions on sms and hpf tools and for help in using the sophisticated cluster of parallel machines vol52,5,2009 1-12-2009 13:29 pagina 514 515 parallelisation technique for serial 3d seismic codes: sms approach at caspur computer centre. we also wish to thank antonio rovelli for help and criticism. part of the research activities were carried out by a. caserta, for his phd thesis, in cooperation with the charles university in prague (czech republic) and in the frame of the projects gacr 205/07/0502 and msm 0021620860. partially supported by japanese society for promotion of science award fy2006/p06320. references aki, k. 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(received, february 25, 2009; accepted, july 14, 2009) vol52,5,2009 2-12-2009 10:54 pagina 516 pre-seismic variations of atmospheric radon activity as a possible reason for abnormal atmospheric effects annals of geophysics, 58, 5, 2015, a0554; doi:10.4401/ag-6808 a0554 pre-seismic variations of atmospheric radon activity as a possible reason for abnormal atmospheric effects vadim v. surkov national research nuclear university mephi, moscow, russia abstract possible atmospheric effects associated with pre-seismic variations of atmospheric radon concentration occasionally observed prior to earthquake occurrence are subjected to theoretical study in order to assess their potential effectiveness. altitude distribution of atmospheric conductivity is examined as a function of radon and aerosol concentrations. horizontal components of atmospheric electric fields and currents, which can arise near a boundary of seismo-active region, are estimated. we speculated that changes in radon-induced air ionization can have an impact on fairweather spectra of ulf electric pulsations. particular emphasis has been placed on hypothesis for correlation between variations of pre-seismic radon activity and upward thermal radiation measured by satellite over the high seismicity regions. this hypothesis is based on assumption that the radon-induced ionization of atmospheric surface layer can trigger water vapor condensation which in turn results in release of the vaporization heat. our analysis has shown that the upward thermal flux has to be many orders of magnitude lower than that predicted on the basis of this hypothesis. 1. introduction it is generally believed that the main ionization of the atmospheric surface layer is due to galactic cosmic rays and radiation as well as the naturally occurring radioactive elements naturally found in the air. shortwave solar radiation and energetic particles of the solar wind predominately affect the upper atmosphere above altitudes of 50 km. a mean rate of ion production under the cosmic rays is approximately the same in all points of the earth surface at all times. at sea level its value is cc = 1.5–1.7 ion pairs per cm 3 per second [e.g., tverskoy 1962, chalmers 1967, israël 1970]. the radioactive elements contained in the surface layer of the soil play insignificant role. for the most the air ionization is due to radioactive atoms which first enter to the atmosphere from the soil and then accumulate in the surface layer. on land, the ionization rate due to radioactive elements that are naturally present in the atmospheric surface layer is estimated to be cr= 8–10 ion pairs/(cm 3 s) although this value is slightly lower near the ocean surface. according to other estimates both the mechanisms, i.e. the cosmic rays and radioactivity, make approximately equal contribution to the ion production process in the surface layer [e.g., tverskoy 1962, zhang et al. 2011]. the ionization rate cc due the cosmic rays increases with increasing altitude and it reaches a peak value at the altitudes 14–15 km [e.g., tverskoy 1962]. alternatively, the number density of the radioactive atoms decreases with increasing altitude thereby producing the decrease in the ionization rate cr in such a way that cr becomes practically negligible at the altitudes 4–5 km. a significant factor, which may greatly affect the background radiation, is the heavy gas radon that has the tendency to penetrate permanently along the systems of capillaries and cracks through the rocks into the soil and air. among many radioactive rocks are uranium-containing rock, granite, gneiss, shale, tuff, metalbearing ore, phosphorite, and potassium-containing rock. the 22286rn nuclei are efficient sources of the air ionization since their half-life is about 3.82 days [e.g., protection against radon-222 at home and at work, 1994]. the half-life of other radon isotopes; that is, 21986rn and 222 86rn is much smaller than the above value. the radon concentration in the air depends on a host of factors such as local species of rocks, the configuration and state of soil surface, the possibility for removal of radioactive ingredients due to rainout, radon flux from depth, the rate of radon dissipation in the atmosphere which in turn depends on the meteorological conditions including the direction and strength of wind, pressure, air humidity and etc. the enhancement of background radiation has been occasionally observed in the regions of tectonic faults of the earth crust [toutain and baubron 1998]. some researchers have speculated that the abnormal article history received june 15, 2015; accepted october 2, 2015. subject classification: atmosphere, aerosols, radioactivity and isotopes, earthquake source and dynamics, heat generation and transport. variations of radon concentration in the atmospheric surface layer can be due to the earthquake preparation processes [pierce 1976]. a review of anomalous radon precursors have been reported for a very wide range of earthquake magnitudes [cicerone et al. 2009]. for example, virk and singh [1994] have reported that radon concentration in the soil gases and in the spring water suddenly increased by 2.5 and 1.5 times a week before uttarkashi earthquake. these measurements have been pursued at the epicentral distance of 300 km. yasuoka et al. [2009] have detected a gradual increase in the atmospheric radon emanation two months before kobe earthquake occurred on january 17, 1995 (mw = 6.9). the radon volumetric activity was increased up to 20 bq/m3 just before an earthquake and it was approximately doubled as compared to the background level. it should be noted that the evidence of pre-seismic radon anomalies in the atmosphere are fewer than those detected directly in the soil or in indoor locations [e.g., cicerone et al. 2009]. however, some researchers have not found any statistically significant changes in radon concentration before earthquakes [geller 1997, inan et al. 2008]. whether the radon concentration variation is a plausible precursor of impending earthquake or not is still an open question. also the answer to this question is likely to be dependent from the properties of seismogenetic fault. 2. local changes of electrical resistance of the atmosphere the electric conductivity of the air increases approximately exponentially with height if standard conditions prevail. in this notation the surface atmospheric layer makes a major contribution to the total electric resistance of the atmosphere. the conductivity of the lower atmosphere is predominately due to small/light ions that have the greatest mobility. the small ions build up as a result of the attachment of a few neutral molecules to the primary ions, which in turn are caused by the cosmic rays and ionizing radiation of the radioactive substances. the small ions many times collide with other particles and take part into enormous number of chemical reactions during their life-time. the composition and number of molecules entering into the composition varies with time. the mean number of these molecules depends on air humidity. on the average, 10–20 neutral molecules are held around each positive ion while a negative small ion contains 5–10 neutral molecules [e.g., tverskoy 1962, chalmers 1967, israël 1970]. to estimate the radon variations impact on the electric resistance of the surface atmospheric layer, we first consider the simplified model of the atmosphere in which there are only two species of opposite-charged small ions. suppose that the small ions may disappear either by direct recombination with oppositely charged small ions or by attachment to aerosol particles. taking into account that the concentration ni of both ion species are approximately the same, we come to the following equation [e.g., israël 1970] (1) where a denotes the ion-ion volumetric recombination coefficient, na is aerosol concentration, b is the ionaerosol attachment coefficient, and j stands for the small ion flux density due to entrainment of small ions with air fluxes, small ion diffusion, small ion drift in electric field and etc. here we shall examine the quasi-steady small ion distribution in which ^tni = 0 and j is assumed to be constant value. it follows from equation (1) then [e.g., harrison et al. 2010] (2) the values of parameters entering equation (2) can strongly vary with altitude. whenever the atmosphere is in thermodynamic equilibrium, the concentrations of the aerosol particles and radon atoms as well as the rate cr of ion production decrease with altitude according to boltzmann distribution. by contrast, the rate cc ion production by cosmic rays increases approximately exponentially with altitude z [wåhlin 1994]. actually, all these altitude dependences can be approximated by exponential functions as follows where cc0 and cr0 are the ionization rate at the sea level. here we made use of the following abbreviations lr = kbt/(mr g) and la= kbt/(ma g), where kb is boltzmann constant, g is acceleration of free fall, mr is a mass of radon atom, ma is an average mass of aerosol particles, and lc is an unknown parameter that will be estimated later. substituting the temperature t = 293 k into equation (3), yields lr≈1.1 km. taking into account that the typical sizes of the aerosols possess value within 0.002–1nm [e.g., ivlev and dovgalyuk 1999, kondratyev et al. 2006], the length la is no greater than several meters even for the smallest aerosols. these values can surkov 2 (3) 3 serve as rough estimates since the actual altitude distributions of the radon atoms, small ions and aerosols can be subjected to local meteorological conditions, air flow, turbulent and convective gas motion, which can drag these particles, and many other factors. considering singly charged small ions, the air conductivity is given by v= 2ninie, where e denotes the elementary charge. the small ion mobility ni is inversely proportional to the concentration of neutral molecules which in turn exponentially decreases with altitude. whence it follows that ni =ni0 exp (z/h), where ni0 is the small ion mobility at the sea level and h≈8 km denotes the scale-height of an isothermal atmosphere. combining equations (2) and (3) with the above equations, we obtain the air conductivity to make an estimate, we use the following numerical values [e.g., pierce 1976, harrison et al. 2010]: n0≈1.2·10 -4 m2v-1s-1, a= 1.6×10-12 m3s-1, b = 4×10-11 m3s-1 (for the aerosol size of 0.2–0.25 nm). at the ground level the aerosol concentration na0 varies from 200 cm -3 for clean air to 1.5 × 103 cm-3 for dusty air [e.g., harrison et al. 2010]. in such a case the condition b2n2a >> 4a(cc+cr) is valid for the atmospheric surface layer. in fact this means that one may neglect the term an2i which describes the small ion recombination in equation (1). in this extreme case equation (4) for air conductivity is reduced to (5) with the above parameters one can obtain that for altitudes greater than several tens or hundreds of meters, the aerosol concentration na decreases so much that the recombination term becomes greater than bnina. in such a case equation (4) is simplified to (6) in this altitude range equation (6) must coincide with the familiar empirical dependence v=v0exp (z/z0) which is valid if z < 3.6 km [chalmers 1967]. whence it follows the approximate relation between the parameters: h-1+ (2lc) -1 = z0 -1. taking into account that z0 = 0.82 km we can estimate the unknown parameter lc as follows: lc = 0.5hz0/(h − z0)≈0.46 km. the variations of radon content may greatly affect the air conductivity and this effect depends on the aerosol concentration. to illustrate this tendency, the altitude dependences of v calculated from equation (4) are shown in figure 1 with lines 1 and 2 for the clean air (na0 = 2 ×10 2 cm-3) and with lines 3 and 4 for the dusty air (na0 = 1.5 ×10 3 cm-3). in making the plot of v we have used the above parameters and la =50 m. as is seen from figure 1, the air conductivity decreases with an increase in the aerosol concentration, which is in turn resulted from the enhancement of the dustiness of the atmosphere. the implication here is that the majority of the newly formed small ions connect with the aerosol particles which do not contribute to the air conductivity because of their low mobility. in figure 1, we have plotted functions v(z) for two different rates of the radon-induced ion production in order to illustrate the radon effect on the air conductivity. the lines 1 and 3 correspond to cr0 = 6 cm -3s-1 while the lines 2 and 4 correspond to doubled value of cr0. for example, the similar changes in radon concentration has been observed by yasuoka et al. [2009] prior to the 1995 kobe earthquake. notice that cr varies directly with radon activity concentration which in turn is proportional to radon concentration. it is obvious from figure 1 that the increase of cr0 causes the increase of the air conductivity by 10–20%. the radon effect disappears (that is, of the order of lr) at the altitudes above 1 km in such a way that all the dependences tend to the unified pre-seismic variations of radon (4) figure 1. model distribution of the air conductivity with altitude. the air conductivity for the aerosol number densities na0 = 2 ×10 2 cm-3 is shown with lines 1 and 2 while the same value for na0= 1.5 ×10 3 cm-3 is shown with lines 3 and 4. the sea level rate cr0 of ion production for the lines 2 and 4 is twice as great as that for the lines 1 and 3. calculation from the empirical equation v=v0 exp (z/z0) is shown with line 5. i0 c0 asymptotic dependence given by equation (6). the local variations of radon concentration in the surface layer may affect the total resistance of the atmosphere/columnar resistance because the surface layer has the greatest air resistance. to illustrate this tendency, we calculate the resistance r per unit of area of the atmospheric surface layer with thickness zs = 3 km figure 2 shows the dependence of r on ratio k = cr0/cc0. in making this plot we have used the above parameters and equation (4) for v(z). it is generally believed that the typical value of k can vary from 2 to 5 [e.g., tverskoy 1962, zhang et al. 2011]. setting the background value of k = 4 and using the data by virk and singh [1994] and by yasuoka et al. [2009], we suppose that for seismo-active period this value to be twice as large as that for the background value; that is, k ≈ 8. as is seen from figure 2, this can result in the decrease of the surface layer resistance by 15% for the clean air (line 1) and by 25% for the dusty air (line 2). as would be expected, considering the important role played by the surface layer in the atmospheric resistance, these estimates hold true for the total columnar resistance of the atmosphere. 3. radon gas activity and electrical structure of the lower atmosphere harrison et al. [2010] have suggested that the abnormal decrease in the atmospheric surface layer resistance caused by an enhancement of the radon volumetric activity can result in an increase in the vertical fair-weather conduction current ja by several units or tens percent. it follows from maxwell equation that the density of electric charge t=f0jadv -1/dz, which builds up as a result of the inhomogeneity of the air conductivity, must be proportional to the vertical current density ja. this implies that the relative changes of t can vary together with ja within the same range. a number of researchers have speculated that the enhancement of the fair-weather conduction current is followed by an increase in the electric charges of water droplets suspended in the air. this effect is assumed to be favorable for the steam condensation which in turn may explain the observation of the so-called earthquake clouds [rycroft et al. 2008, rycroft et al. 2012, harrison et al. 2013]. here we deal with other radon effects which may affect the electrical structure of the area adjacent to abnormal region. first, we shall estimate a horizontal component of the fair-weather conduction current in the vicinity of the abnormal region with the enhanced radon concentration. taking into account that the conduction current density is not a function of altitude, the distribution of electric potential with altitude can be written as where v(z) is given by equation (4), and {a≈ 250 kv [e.g., rycroft et al. 2000] is the potential drop between the bottom of the ionosphere z = za and the earth z = 0. figure 3 shows the result of numerical integration of equation (8). as before the lines 1 and 3 correspond to cr0 = 6 cm -3s-1 while the lines 2 and 4 correspond to cr0 = 12 cm -3s-1. it is obvious from figure 3 that an increase of the ionization rate cr0 leads to decrease of potential (8) at given altitude. it also can be seen that there are the essential distinctions between background (lines 1 and 3) and abnormal (lines 2 and 4) potential distrisurkov 4 (8) (7) figure 2. total columnar resistance r of the atmospheric surface layer versus parameter k = cr0/cc0. the lines 1 and 2 correspond to aerosol number densities of 2 × 102 cm-3 and 1.5 × 103 cm-3, respectively. the plots are normalized to the value of r taken at k = 1. figure 3. the same as in figure 1 but for the atmospheric potential. 5 butions in the altitude range 02–2 km. in order to highlight this peculiarity, the differences between the corresponding distributions as a function of altitude are shown in figure 4. here the line 1 demonstrates the difference between the first and second distributions shown in figure 3 whereas the line 2 demonstrates the same but for the third and fourth distributions. this constitutes a mapping of the surface radon activity into space variations of the atmospheric potential at the altitudes of several kilometers. the recent measurements [kabrt et al. 2014] of radon gas activity concentration distributed over a large area can serve as a basis for the searching of the seismic-induced radon anomaly region. as would be expected, the horizontal electric fields and currents occur near the boundary of the abnormal region where the radon concentration changes from the enhanced value to the background one. the reason is that the height-dependence of potential in the abnormal area is different from that in the surrounding regions. the horizontal field components can be estimated roughly as follows e||~d{/dl and j||~ve|| where dl is the horizontal space scale at which the radon number density decreases down to the background level while d{ is the corresponding potential difference taken at the fixed height. it follows from figures 1 and 4 that maximum value d{= 13–25 kv is achieved at such altitudes where the air conductivity v = 8 ×10-14–10-13 s/m. taking these values and choosing dl = 10–50 km, we come to the following estimates: e||≈ 0.26–2.5 v/m and j||≈ 0.02–0.25 pa/m 2. since the radon anomalies are subjected to temporal variations around seismic event, the above values can only serve as order-of-magnitude estimates of the possible atmospheric perturbations resulted from the enhanced radon emanation. a more significant effect can be expected in the vicinity of mesoscale convective systems (msc) since there is the background atmospheric current well above the fair-weather one. the lateral size and total current of the msc electrical active region are greater than 150 km and several tens ampere, respectively, that is much larger than typical thunderstorm parameters [davydenko et al. 2004]. a portion of this current can close through the abnormal region of the lower atmosphere with the reduced resistance thereby producing the charge relaxation and changes in electrical structure of msc. it is pertinent to note that there are a few possible mechanisms which can provide a coupling between ultra low frequency (ulf) electric field and charge density pulsations in the surface atmospheric layer and the variations of radon activity concentration. the fairweather electric field pulsations have a power-law spectrum which falls off with frequency as f -m. in the frequency range of 0.004–0.06 hz the spectral index m varies from 2 to 2.4 [yerg and johnson 1974] while in the range of 0.01–1 hz the electrical pulsations and their power spectra can be split into two groups; that is, the so-called “structured spectra” with m = 2.03–3.36 and “unstructured spectra” with m = 1.23–2.89 [anisimov et al. 2002, 2005]. the “structured spectra” has been assumed to be due to aero-electric structures (aess) involved in the neutral-gas turbulence in the surface layer. the enhancement of radon activity concentration, followed by the production of additional small ions and charged aerosols, can result in the increase of both the space charge and the variance of the charge-density fluctuations v2p. according to the model by [anisimov et al. 2002] the ulf spectrum of electric pulsations is proportional to v2p and thus the variation of radon concentration can affect the spectrum amplitude. we cannot come close to exploring these topics in any detail although one may assume that the spectral index is a function of radon concentration as well. the release of radon from the soil to the air depends on local rock morphology and genesis including faults, zones of rock fracture and etc. overall, the radon gas is nonuniformly distributed near the ground surface. this implies that the pre-seismic radon variation in seismo-active region is capable of producing the random spatiotemporal variations of the charge density and electric field in the air. these electric fluctuations appear to have a power-law spectrum in analogous to spectrum of natural ulf magnetic noises produced by random fluctuations of the ionospheric and atmospheric currents [surkov and hayakawa 2007, 2008, 2014]. despite this effect seems to be weak, it can influence pre-seismic variations of radon figure 4. model calculation of the atmospheric potential difference which might arise in horizontal direction. the potentials were calculated at the same altitudes but for different ion production rates, cr0, which differ by the factor 2. the lines 1 and 2 correspond to na0 = 2 ×10 2 cm-3 and na0 = 1.5 ×10 3 cm-3, respectively. on the fair-weather spectral index m of natural electric pulsations. so, the changes in the ulf electric spectra amplitude and spectral index can be indicative of preseismic tectonic activity. 4. release of radon from the ground and temperature of the atmospheric surface layer statistical analysis of satellite thermal imaging data measured over the high seismicity regions and fault systems in the earth’s crust have shown that the upward infrared (ir) radiation from the atmosphere can exceed the background value as compared to the surrounding regions [gorny et al. 1993, tramutoli et al. 2001, tronin et al. 2002, ouzounov et al. 2006, genzano et al. 2009]. this effect is generally interpreted as a by-product of mean temperature increase of the atmospheric surface layer in seismo active regions. the mechanism of the phenomenon was presumed to be due to the release of optically active gases from soil to the air followed by local greenhouse effect [tronin et al. 2002]. one more a plausible explanation of this phenomenon is the gradual squeezing-out of the groundwater from higher depth towards the ground surface that results from the variations of the tectonic stress in the fault zone. taking into account that, on the average, the ground temperature increases with depth by 2 k per 100 m, surkov et al. [2006] have shown that this leads to the convective heating of the ground surface by a few kelvin since the temperature of the deep groundwater is higher than the mean temperature of the ground surface. it was also hypothesized by pulinets and ouzounov [2011] and by pulinets [2011] that the enhancement of the outgoing ir radiation several days before seismic event can be due to the increase in radon concentration in the surface layer. this hypothesis is based on assumption that the radon-producing air ionization gives rise to the formation of so-called large cluster ions with typical sizes 1–3 nm; that is, as large as the aerosol particles. these large ions can serve as condensation centers triggering the water vapor condensation which in turn is accompanied with the release of the heat of vaporization. to estimate the maximal value of this effect, we assume that the alpha decay of radon nuclei occurs in a humid air near the ground surface. let a be the volumetric radon activity and ni be the number of ordinary/primary ion pairs resulted from the scattering of single alpha-particle in the air. in a short time the cluster ions are formed due to attachment of neutral molecules to the primary ions. the polar molecules of air, including the molecules of h2o, are largely involved in this process. assuming for the moment that every primary ion gives rise to the formation of cluster ion, the rate of the cluster ion production per unit volume is ani. the volumetric rate of h2o molecule attachment to the cluster ions is aninw, where nw stands for the mean number of h2o molecules attached to the single cluster ion. the thermal energy released to the atmosphere per unit time and per unit volume is aninwe1, where e1 is the energy release due to attachment of single h2o molecule to the cluster ion. as a rough estimate of e1, one may use the value m/na, where m≈41 kj/mol [gray 1972] is the molar heat of vaporization for water and na is the avogadro number. let hr be the thickness of the near-surface layer which contains the basic part of atmospheric radon. if the heat capacities of the ground and air are both ignored, the maximal value of upward heat flux density can be estimated as follows: qmax = aninwhrm/na. according to the hypothesis by pulinets and ouzounov [2011], the so-called big cluster ions (bci), “which are actually the cluster ions formed after ionization and ion’s hydration”, play a crucial role in a water vapor condensation followed by the pre-seismic thermal anomaly. their numerical estimate is based on assumption that, first, the number of h2o molecules attached to the bci ranges as high as nw = 1.3 × 10 11; second, the pre-seismic radon activity reaches the value a = 2 ×103 bq/m3 and ni = 4.5×10 5 ion pars [pulinets and ouzounov 2011]. furthermore, it was implicitly supposed that every primary ion is converted to the big ion. taking hr = 1 m and substituting the above numerical values of the parameters into the equation for qmax , gives the estimate: qmax = 16 w/m 2 which coincides with the numerical estimate given by pulinets and ouzounov [2011]. despite this value is believed to be compatible with the observations of abnormal pre-seismic increase of the upward thermal flux [ouzounov et al. 2006], such an approach has the serious shortcoming that the rate of the bci production ani is dramatically overestimated. indeed, the radon activity may affect the bci rate production through the interaction between the radon-produced small ions and the bci. taking into account that, first, about 5–20 neutral molecules are attached to the small ion; second, only a part of them are the h2o molecules, it becomes apparent that at least about 1010 small ions needs to create the bci about which nw = 1.3 × 10 11 h2o molecules are held. this means that the production rate of the bci is smaller than that of the small ions by 10 orders of magnitude. in other words, the value ani = 9 ×10 8 m-3s-1 of the cluster ion production can be applied to the small ions but not to the bci. moreover, it seems likely that the parameter a = 2 ×103 bq/m3 used in the cited papers was also overestimated because this value of radon acsurkov 6 7 tivity concentration is typical for the soil rather than for the atmosphere. it is common knowledge that the world average value of volumetric radon activity, on being measured in the air 1 m above the ground, varies from 7 to 12 bq/ m3 though this value can reach 50 bq/ m3 in areas with radon-laden soil [e.g., protection against radon-222 at home and at work, 1994]. in addition, we recall that the pre-seismic increase in atmospheric radon activity concentration before kobe earthquake was as high as 20 bq/m3 [yasuoka et al. 2009]. if this latter value of a is taken, the above estimate of qmax is further lowered by two orders of magnitude. 5. discussion and conclusion our theoretical analysis favours the view [e.g., harrison et al. 2010, 2013, 2014] that the variations of atmospheric radon concentration may affect the columnar resistance of the atmosphere. for example, according to our estimate the pre-seismic increase in the radon activity concentration by two times can result in the local lowering of the standard atmosphere resistance by 10–20% and approximately the same relative increase in the fair-weather/background conduction current. it follows from our model that the greatest changes of the air conductivity can be expected at the altitudes 100–200 m in the case of dusty air of high aerosol content. although, the radon and aerosols appear to have no effect on the air conductivity above 1 km since the cosmic rays and other mechanisms for air ionization play a major role in this altitudes range. the enhanced radon release from the ground followed by the variations of the air conductivity may affect the electrical structure of the atmosphere in seismo active regions. for example, not only vertical but also the horizontal components of both the background current density and of the gradient of atmospheric electrical potential can arise near the boundaries of seismo active regions and the earth crust faults. our estimates have shown that at the fair weather conditions they can reach the values 0.25 pa/m2 and 2.5 v/m, respectively. it appears that the horizontal field variations can be even greater than the above values by several orders of magnitude in the presence of strong background current that are typical of mesoscale convective systems and the atmospheric precipitation [davydenko et al. 2004]. it can be also expected that the increased air ionization due to radon decay has an impact on the aeroelectrical structures thereby producing the pre-seismic changes in spectral indices and amplitudes of ulf electric pulsations resulted from motion of these structures. a closer look at the electric pulsation spectra before seismic events would help us to understand whether this pre-seismic effect takes place or not. if this is the case, the radon-produced air ionization can serve as a plausible candidate for explanation of this effect. we have analyzed the hypothesis by pulinets and ouzounov [2011] that the enhancement of radon release from the ground deeply influences a near-surface process of evaporation and related phenomena such as abnormal variations of upward thermal ir radiation occasionally observed from satellite before earthquake occurrence. the big cluster ions (bcis) are of great concern in this hypothesis. it should be realized that the properties of the bci are unusual since they are out of standard classification of ions [e.g., hirsikko et al. 2011] because of their big size (about 1 nm) and a great number of the attached water molecules (about 1011). taking into account that the diameter of water molecule is 0.29 nm, it is easy to check that a total volume of these attached molecules are greater than a volume of the individual bci. in this notation the bci should be assigned to water droplets rather than to cluster ions. the conditions for the generation of bci are unknown and, perhaps, the bci seldom if ever occurs in the actual atmosphere. however the major problem of this hypothesis is that the production rate of the bci needs to be as high as and even greater than that of small ions in order to explain the observed data. according to our estimates, the air molecule ionization by atmospheric radon decay must lead to the generation of approximately 109 bci per unit volume and per unit time in order to produce the upward thermal flux about 16 w/m2. this implies that every primary ion originated from alpha particle scattering would end up as the bci. besides the atmospheric radon activity concentration used by pulinets and ouzounov [2011] is two orders of magnitude greater than the actual one. taken together, it makes this hypothesis very questionable. we can conclude that, in light of our study, the change in the air resistance of surface atmospheric layer by several tens percent is currently the only plausible radon effect which can be predicted with confidence. although one might expect certain changes in the electric structure of the lower atmosphere. a network of ground-based electric sensors covered seismo-active region is necessary in order to monitor the electrical field perturbations due radon anomaly. despite these effects seem to be sensitive to meteorological conditions, the electrical perturbations can be detectable at the ground level if it is granted that daily and seasonal variations of fair-weather atmospheric currents and electric fields are eliminated. it appears that further experiments are necessary to provide us with evidence both in support of and against a concept that other pre-seismic radon effect actually exists. pre-seismic variations of radon acknowledgements. this study was supported by the grant no. 13-05-12091 from rfbr. references anisimov, s.v., e.a. mareev, n.m. shikhova and e.m. dmitriev (2002). universal spectra of electric field pulsations in the 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(1962). meteorology (physics of the atmosphere), gydrometeoizdat, leningrad (in russian). virk, h.s., and b. singh (1994). radon recording of uttarkashi earthquake, geophys. res. lett., 21, 737742. wåhlin, l. (1994). elements of fair weather electricity, j. geophys. res., 99, d10767-d10772. yasuoka, y., y. kawada, h. nagahama, y. omori, t. ishikawa, s. tokonami and m. shinogi (2009). preseismic changes in atmospheric radon concentration and crustal strain, phys. chem. earth, 34, 431-434. yerg, d.g., and k.r. johnson (1974). short-period fluctuations in the fairweather electric field, j. geophys. res., 79, 2177-2184. zhang, k., j. feichter, j. kazil, h. wan, w. zhuo, a. d. griffiths, h. sartorius, w. zahorowski, m. ramonet, m. schmidt, c. yver, r. e. m. neubert and e.-g. brunke (2011). radon activity in the lower troposphere and its impact on ionization rate: a global estimate using different radon emissions, atmos. chem. phys., 11, 7817-7838; doi:10.5194/acp-117817-2011. corresponding author: vadim surkov, national research nuclear university mephi, moscow, russia; email: surkovvadim@yandex.ru. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. pre-seismic 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice s u l l a s u p e r f i c i e d i d i s c o n t i n u i t à ' a l l a p r o f o n d i t à ' d i 9 5 0 k m c i r c a p . caloi e. peronaci è n o t o c l i c p a r e c c h i s t u d i o s i d i s i s m o l o g i a h a n n o s o s p e t t a t o l'esis t e n z a di l i n a s u p e r f i c i e di d i s c o n t i n u i t à a l l a p r o f o n d i t à d i 1 0 0 0 k m c i r c a . f r a essi si è p a r t i c o l a r m e n t e i n t e r e s s a t o d e l l a cpiestione i l c o m p i a n t o p. r e p e t t i s . j . , i l q u a l e h a a p p u n t o s u g g e r i t o il v a l o r e r i p o r t a t o , c o m e p r o f o n d i t à d e l l a s u p e r f i c i e su d e t t a . n a t u r a l m e n t e , l a q u a l i f i c a d i s u p e r f i c i e n o n v a p r e s a a l l a l e t t e r a : i n e f f e t t i , si i n t e n d e i n d i c a r e , c o n q u e s t a d e n o m i n a z i o n e , u n a z o n a c a r a t t e r i z z a t a da f o r t e v a r i a z i o n e d e l l a v e l o c i t à di p r o p a g a z i o n e d e l l e o n d e s i s m i c h e , t a l e da c o n s e n t i r e l a c r e a z i o n e d e l l e c o n d i z i o n i a t t e a p e r m e t t e r e l a rif l e s s i o n e di u n a c e r t a p e r c e n t u a l e di e n e r g i a i n c i d e n t e . n e l c a s o specifico, si t r a t t a c o m u n q u e di u n a « s u p e r f i c i e » c h e , s o l o c o n q u a l c h e r i s e r v a , p u ò e s s e r e c l a s s i f i c a t a f r a l e s u p e r f i c i e di p r i m o o r d i n e , n e l senso s i s m o l o g i c o ; g u t e n b e r g e m a c e l w a n e r i t e n g o n o t r a t t a r s i i n f a t t i di s u p e r f i c i e di 2" o r d i n e . v a l e n d o c i d e l l e f o r m u l e c h e , i n u n l a v o r o p r e c e d e n t e f 1 ) , ci h a n n o c o n s e n t i t o di r i c a l c o l a r e in 2 9 2 0 k m ca. l a p r o f o n d i t à d e l n u c l e o t e r r e s t r e , ci s i a m o p r o p o s t i di v e d e r e se e r a p o s s i b i l e d e l i m i t a r e , c o n q u a l c h e a p p r o s s i m a z i o n e , l a p r o f o n d i t à d i d e t t a s u p e r f i c i e , in b a s e al t e m p o di r e g i s t r a z i o n e d e g l i « e c h i » d a essa i n v i a t i s u l l a s u p e r f i c i e e s t e r n a d e l l a t e r r a . p e r p r i m a cosa ci s i a m o d e t e r m i n a t a la r e l a z i o n e c h e l e g a l ' a n d a m e n t o d e l l a v e l o c i t à n e l l ' i n t e r n o d e l l a t e r r a c o n l a p r o f o n d i t à . v a l e n d o c i d e l l e t a v o l e di j e f t r e y s , si è o t t e n u t o , p e r l e o n d e p _ _<f 0 . 0 8 5 1 3 + 0 . 9 2 3 1 3 c p 2 ' d o v e o = ( r , d i s t a n z a d e l p u n t o c o n s i d e r a t o d a l c e n t r o d e l l a t e r r a ; r , r a g g i o d e l l a t e r r a , l i m i t a t a e s t e r n a m e n t e a l l a s u p e r f i c i e di m o h o r o v i c i c ) , ( p 2 = v 2 y 2 ( e s s e n d o v i l r a p p o r t o f r a l a v e l o c i t à s u l l a s u p e r f i c i e d i m o h o r o v i c i c e l a v e l o c i t à a l l a q u o t a r). p e r l ' a p p l i c a z i o n e , ci s i a m o s e r v i t i d e i d a t i r e l a t i v i a l t e r r e m o t o d e l basso t i r r e n o d e l 1 6 m a r z o 1 9 4 1 , s t u d i a t o da di f i l i p p o ( 2 ). 1 0 3 p . caloi f . teronaci e c c o i r i s u l t a t i d e i c a l c o l i s v i l u p p a t i d a d i f i l i p p o c p 0 = 38" 2 6 ' , 3 n l 0 = 12" 0 7 ' , 4 e h— 1 7 " 35'" 1 3 s , 2 h = 8 5 k m . s u l l a b a s e d e l l a p r o f o n d i t à d i 8 5 k m (q = 0 . 9 9 1 3 2 ) , s o n o s t a t e c a l c o l a t e c o n l a f o r m u l a [ 5 ] d e l c i t a t o n o s t r o l a v o r o i v a l o r i di <i. p e r v a r i e d i s t a n z e e p i c e n t r a l i , c o r r i s p o n d e n t i a p r o f o n d i t à d e l l a d i s c o n t i n u i t à d i r e p e t t i d i 1 0 0 0 k m e 9 5 0 k m r i s p e t t i v a m e n t e . i r i s u l t a t i d e l c a l c o l o s o n o r i p o r t a t i n e l l a f i g . 1 . n a t u r a l m e n t e , i v a l o r i d i a g r a m m a t i si r i f e r i s c o n o a l l a s u p e r f i c i e d i m o b o r o v i c i c . a b b i a m o c a l c o l a t o l e c o r r e z i o n i d a a s s e g n a r e a l l e g r a n d e z z e i n g i u o c o . r i t e n e n d o p a r i a 3 3 k m i l m e d i o s p e s s o r e d e l l a c r o s t a t e r r e s t r e , se i n d i c h i a m o c o n d a l a c o r r e z i o n e d a a g g i u n g e r e a l l a d i s t a n z a s u l l a s u p e r f i c i e e s t e r n a , c o n ds i l t r a g i t t o n e l l a c r o s t a , c o r r i s p o n d e n t e a l l e v a r i e d i s t a n z e , p e r a 0 , 1 0 , 1 5 0 , 2 0 , 2 5 0 , 3 0 , 4 0 , 5 si h a d\ = k m 2 , 5 3 , 8 5 , 1 6 , 4 7 , 7 1 0 , 5 1 3 , 5 ds = k m 3 3 , 1 3 3 , 2 3 3 , 4 3 3 , 6 3 3 , 9 3 4 , 6 3 5 , 7 t a l i v a r i a z i o n i , i n p r a t i c a , n o n p o r t a n o i n g e n e r e , e f f e t t i a p p r e z z a b i l i s u l c a l c o l o d e i t e m p i d i t r a g i t t o . q u e s t i u l t i m i s o n o s t a t i c a l c o l a t i c o n l a f o r m u l a [ 2 ] d e l l a n o t a c i t a t a . s u l l a b a s e d e i t e m p i di r e g i s t r a z i o n e d e l l e o n d e r i t e n u t e c o i n è p r o b a b i l i r i f l e s s i o n i d e l l a s u p e r f i c i e d i r e p e t t i , r i t e n e n d o p a r i a 6 sec. i l t e m p o d i t r a g i t t o d e l l e o n d e l o n g i t u d i n a l i n e l l a c r o s t a , si s o n o o t t e n u t i i r i s u l t a t i r i a s s u n t i n e l l a t a b e l l a . i l o p . c a l o i f . p e r o n a c i l e f i g . 2 e 3 si r i f e r i s c o n o a r e g i s t r a z i o n i o t t e n u t e a n e u c l i à t e l e a c l e r m o n t f e r r a n t i r i s p e t t i v a m e n t e . i n b a s e a l l e r e g i s t r a z i o n i i n v e s t i g a t e , l a s u p e r f i c i e d i d i s c o n t i n u i t à i n q u e s t i o n e d o v r e b b e q u i n d i t r o v a r s i a l l a p r o f o n d i t à d i 9 5 0 k m c i r c a . c i p r o p o n i a m o d i r i t o r n a r e s u l l ' a r g o m e n t o . comunicazione presentata al convegno per la costituzione della f.s.e., tenutosi a veronadal 12 al 16 settembre 1950 (v. « annali di geofisica », voi. i l i , ]>ag. 579). riassunto si portano nuovi elementi in favore dell'esistenza di una superficie di discontinuità nel mantello terrestre, capace di riflettere una piccola percentuale dell'energia sismica incidente. detta superficie dovrebbe trovarsi ad una profondità di 950 km circa. b i b l i o g r a f i a ( ! ) caloi p . , peronaci f . : il hatisismo del 28 agosto 1946 e la profondità del nucleo terrestre. a n n a l i di g e o f i s i c a , it ( 1 9 4 9 ) . ( 2 ) di filippo d . : sulle caratteristiche sismiche del basso tirreno (terremoto del 16 marzo 1041). l a r i c e r c a s c i e n t i f i c a ( 1 9 4 3 ) . vol52,1,2009 35 annals of geophysics, vol. 52, n. 1, february 2009 key words tectonomagnetic anomaly – geodynamics – stretching tectonic forces 1. introduction tectonomagnetic investigations at the location of the ukrainian antarctic station «academic vernadsky» (uas) form part of the studies of recent geodynamic processes in the earth’s crust of the region. the area of the tectonomagnetic investigations is located near the west coast of the antarctic peninsula that is a part of the western antarctic folded system, extended along pacific ocean shore. the antarctic continent belongs to low seismic activity regions. nevertheless, a new (mezo-cenozoic) volcanism, existent in this region (active volcano erebus, neo-volcanism activity in the brancefield strait, etc.) indicates the presence of active tectonic processes in the regions crust. geomorphologic, bathymetric and geophysical data near the location of uas has defined the region important for recent geomorphological structural elements – rift grabens, deep faults and lineaments. a tectonomagnetic method has its own place among geophysical methods. it is widely used for investigations, and mapping of active tectonic fault zones, of seismic-tectonic processes and solving other problems concerning geology and geodynamics (parkinson (1983), skovorodkin (1985). the specific geographical and climate conditions in the antarctic region, complicate geophysical investigations. that is why the deep structure and recent geodynamics of the crust in the region were insufficiently studied, and geodynamic investigations near uas were not fulfilled at all. in the frame of the ukrainian naresults and perspectives of tectonomagnetic investigations in the western antarctic valentin yu. maksymchuk (1), vladimir g. bakhmutov (2), yurij m. horodyskyy (1) and ihor o. chobotok (1) (1) carpathian branch of s.i.subbotin’s institute of geophysics, ukrainian national academy of sci., lviv, ukraine (2) s.i.subbotin’s institute of geophysics, ukrainian national academy of sci., kyiv, ukraine abstract the results of long-term (1998-2005 yrs.) tectonomagnetic investigations in the western antarctic near the location of ukrainian antarctic station «academic vernadsky» are reviewed. the peculiarities of the earth’s anomalous magnetic field and its dynamic temporal variations (tectonomagnetic anomalies) were studied using the newly founded tectonomagnetic polygon. near the argentine archipelago intensive tectonomagnetic effects up to -2.8 nt/year were determined. their spatial-temporal structure agrees with tectonic structure elements. we suggest that the nature of such effects is caused by a piezomagnetic effect under the influence of stretching tectonic forces (few bars per year) in sub-latitudinal direction. perspectives of tectonomagnetic investigations in the region are discussed. mailing address: dr. valentin yu. maksymchuk, carpathian branch of s.i.subbotin’s institute of geophysics, ukrainian national academy of sci., lviv, ukraine; email: depart10@cb-igph.lviv.ua vol52,1,2009 28-05-2009 13:45 pagina 35 36 v.yu. maksymchuk, v.g. bakhmutov, yu.m. horodyskyy and i.o. chobotok tional program of investigations in the antarctic at the location of uas and surrounding areas, work began in 1998 for setting the foundation of geophysical and geodynamic polygons, including organization of tectonomagnetic monitoring and tectonomagnetic profiling. investigations were fulfilled during seasonal antarctic expeditions. during 1998-2005 a network of 14 tectonomagnetic stations was created and 6 cycles of geomagnetic observations have subsequently been made. 2. fundamentals of tectonomagnetic investigations and methods the term tectonomagnetism was proposed by nagata (1969) to denote the local geomagnetic field variations, caused by tectonic processes in the earth’s crust. since then methods of tectonomagnetic investigations have been widely developed. results of magnetic field temporal changes have been investigated in various regions of the world and have shown that all tectonomagnetic anomalies can be classified as: 1) magnetic field variations caused by slow tectonic processes and recent movements of the crust; 2) anomalous variations of the magnetic field related to seismotectonic processes-seismomagnetic effects; 3) magnetic field variations related to volcanic eruptions-volcanomagnetic effects; 4) magnetic field variations, caused by electrodynamic processes in the regions of electroconducting anomalies, etc. the duration of tectonomagnetic anomalies vary in the range from minutes to decades, their amplitudes-from a few to several tens of nanoteslas. extensions of tectonomagnetic anomalies reach tens and even hundreds of kilometers. the tectonomagnetic anomalies are mainly caused by the variations of the stress-strain state of rocks resulting in piezomagnetic or electrokinetic effects. piezomagnetic effects result in changing the magnetization vector of rocks. this effect has been well studied and is often applied to estimate stress changes by tectonomagnetic data (hao jinqi et al. (1982); sasai (1983); parkinson (1983); skovorodkin (1985); maksymchuk et al. (2001)). electrokinetic effects result from motion of fluids in porous rocks. such phenomena in electroneutral systems exist because of the existence of a double electric layer (del) near the solid and liquid phase boundary. this effect may cause observable magnetic anomalies (martin et al. (1982); mizutani and ishido (1976); murakami (1989)), especially in sedimentary basins. the absence of sedimentary complexes and outcrops of abyssal rocks is peculiar to the argentine islands archipelago and the west slope of the antarctic peninsula, where the research area is located. thus, we believe that the influence of the electrokinetic effect is negligible. classical methods of tectonomagnetic investigations consist of synchronous measuring of the magnitude of total geomagnetic field vectors on a fixed network of points (fp) and a reference (basic) point (fb) through definite time intervals. such a scheme generally allows elimination of the influence of external magnetic field variations in differences ∆f=fp – fb. objective values of tectonomagnetic measurements are variations of ∆f between pairs consecutive observation cycles (∆∆f= ∆f2 ∆f1). repeated measurements were done once per year, during summer season expeditions. therefore, these measurements were aimed at defining long-period (more then 1 year) temporal variations of the anomalous magnetic field. tectonomagnetic points were founded mainly on the argentine islands archipelago. these points form two tectonomagnetic profiles: sub-latitude profile i (barchany-rasmussen), 11 km long with 7 points and 6 observations cycles (1998-2005), and sub-meridian profile ii (barthelot isl.-petermann isl.) with 7 points and 3 observations cycles (2002, 2004, 2005) (fig.1). the basic point was the magnetic observatory «argentine islands» (aia), located on the intersection of profiles i and ii. the registration of the total field f at the basic point was fulfilled with proton magnetovariational station mv-01 (sensitivity 0.1 nt). measurements on all other points were done by a proton magnetometer mmp-203 (sensitivity 1 nt). the mean-square error was 0.7 nt. vol52,1,2009 28-05-2009 13:45 pagina 36 37 results and perspectives of tectonomagnetic investigations in the western antarctic 3. results and interpretation of tectonomagnetic observations geomagnetic observations, carried out during 1998-2005, gave an opportunity for investigations of temporal-spatial peculiarities of tectonomagnetic variations. tectonomagnetic network was created step by step from 7 points in 1998 to 14 points in 2005, so the number of observations for each point varies (fig. 2). geofig. 1. antarctic tectonomagnetic polygon and temporal variations of geomagnetic field (∆∆f) in 2004 2005. 1 magnetic observatory aia, 2 tectonomagnetic points, 3 prognoses fault line, 4 vectors of geodetic points displacements (tretyak et al., 2006). vol52,1,2009 28-05-2009 13:45 pagina 37 38 v.yu. maksymchuk, v.g. bakhmutov, yu.m. horodyskyy and i.o. chobotok magnetic observations data are given in table i. dynamics of the magnetic field f was investigated more completely along the sub-latitude profile i barchany-rasmussen. it crosses the main rock complexes and tectonic structures of the coastal shelf zone. the intensity and morphology of the local magnetic field temporal changes during 1998-2005 greatly varies in the different parts of the profile i (fig. 3). the most intensive variations of ∆f were detected in points 1, 1a, 3, 3a (fig. 2), located in the western part of a working area. at point 3 during 1998-2005 ∆f decreased by 18.5 nt. it is noteworthy that the time changes of ∆f for the table i. the results of measurements of differential geomagnetic field df in the tectonomagnetic polygon in the location of uas point location coordinates ∆f1, nt ∆f2, nt ∆f3, nt ∆f4, nt ∆f5, nt ∆f6, nt no. (1998) (2001) (2002) (2003) (2004) (2005) 1 barchany isl. (southern), s 65°14.362’ 294.7 293.8 289.2 292.1 287.3 2 8 9 . 4 west w 64°19.023’ 1a barchany isl. (southern), s 65°14.410’ 328.6 326.7 325.7 321.6 center w 64°18.447’ 2 barchany isl. (southern), s 65°14.544’ 128.5 127.2 125.0 127.6 126.6 123.0 east w 64°17.862’ 3 «tree little pigs» isl.” s 65°14.617’ 581.4 574.1 567.7 570.9 564.6 562.9 (western) w 64°16.855’ 3a «tree little pigs» isl.” s 65°14.630’ 531.2 535.4 531.5 526.6 (western) w 64°16.761’ 4 pinguin-point isl. s 65°14.919’ 75.3 76.5 77.6 77.2 75.7 77.5 w 64°14.332’ 5 skua isl. s 65°15.225’ 40.1 41.0 40.4 41.5 36.6 38.7 w 64°15.029’ 6 о-в ялур s 65°14.035’ 276.3 278.9 277.0 279.4 278.2 279.7 w 64°09.715’ 7 rasmussen cape s 65°14.852’ 1014.9 1015.5 1015.9 1019.2 1028.8 w 64°05.097’ 8 barthelot isl. s 65°19.436’ 746.7 752.7 747.2 748.0 w 64°08.419’ 9 uruguay isl. (south) s 65°14.398’ 50.4 51.3 54.4 51.5 w 64°13.752’ 10 uruguay isl. (north) s 65°14.005’ 211.1 210.8 208.0 w 64°13.389’ 11 iryzar isl. s 65°13.186’ 272.0 274.0 279.5 w 64°11.710’ 12 petermann isl. s 65°10.501’ 332.14 339.1 337.6 w 64°08.258’ vol52,1,2009 28-05-2009 13:45 pagina 38 39 results and perspectives of tectonomagnetic investigations in the western antarctic points 3a and 3, located near one another, were almost identical. this fact testifies the reliability of the results obtained. time changes of ∆f in the eastern part of the region (points 4, 9, 6, 7) are rather monotonous. thus, for point 6, ∆f increased by 3.6 nt (fig. 2) during 1998-2005. however the increasing of ∆f of point 7 by 9.6 nt during 2004-2005 is questionable on account of rather small changes in ∆f here (less than 1.0 nt) before 2004. except for the trend one can see local peculiarities of ∆f time changes in some points. such peculiarities took place in 2002 in points 1, 2, 3, 3a. the maximum deviation of ∆f from the trend that was defined for point 3 exceeded 5 nt. in 2003 ∆f in these points returns to the trend level. it is likely that in the vicinity of three little pigs island a local effect occurred in ∆∆f field. fig. 2. dynamics of geomagnetic field ∆f in the points of the antarctic tectonomagnetic polygon. fig. 3. distribution of geomagnetic field temporal variations (∆∆f) along the profile i – i (barchany-rasmussen) during 2001-1998; 2002-1998; 2003-1998; 2004-1998; 2005-1998. vol52,1,2009 28-05-2009 13:45 pagina 39 40 v.yu. maksymchuk, v.g. bakhmutov, yu.m. horodyskyy and i.o. chobotok the spatial distribution of ∆∆f field along profile i has some regularities intrinsic for all time intervals and for the summary curve of ∆∆f (1998-2005) as well (fig. 3). the eastern part of the profile (points 5, 4, 6, 7) is generally characterized by low positive values of ∆∆f about 2-3 nt. ∆∆f field in the western part of the profile (points 1, 2, 3) has a very uneven shape with changes from -5.1 nt (point 2) to 18.5 nt (point 3). one can notice the likeness of ∆∆f curves for period of 1998 -2001 and of summary ∆∆f curve (1998 – 2005). the only difference is that amplitude of summary ∆∆f anomaly is almost twice as large. thus, in point 3 ∆∆f field had changed from -7.3 nt (19982001) to -18.5 nt (1998-2005). detailed measurements around the tectonomagnetic points have shown, that in some areas ∆f field is highly diverse, with gradients up to 20-40 nt/m, resulting from highand low-magnetized volcanic rocks. the rocks of the eastern part of the archipelago consist of andesites – trachytes, porphyritic andesites, and crystal dacite tuffs. the rocks in the western part of the archipelago are mostly of a mezoearlier-cenozoic intrusion complex, such as gabbro and granite-diorites. values of magnetic susceptibility of the rocks vary from 0.01 to 0.04 si, and its natural residual magnetization – from 0.1 to 1.0 a/m, but these values may differ by a factor of 100 for rocks from the same islands. the zone of contact between volcanic rocks and a mezoearlier-cenozoic intrusion complex is located in the area of shelter island – three little pigs island – indicator island – grotto island and further to the north of uruguay island. at this contact zone, which is nearly 1000 m wide, local ∆f anomaly of about 600 nt in the three little pigs island area was detected. an analysis of the spatial-temporal ∆∆f structure distinguishes some zones of similar tectonomagnetic effects. first of all these are western and eastern blocks with the imaginary boundary along a line galindez island – petermann island. at the eastern block values of ∆∆f usually are positive, and at western block – negative. probably, these blocks are separated from other adjusting blocks by tectonic disjunctions. the scheme of tectonic fault location, based on the tectonomagnetic data, is shown in the fig. 1. in general, this is not in contradiction with geological data, accordingly with which along penol strait in north-east direction lies tectonic disjunction, clearly mapped on the maps of the sea-floor. but there are no reliable data about the location of this disjunction, its depth, declining angles, etc. thus, based on the results of tectonomagnetic investigations, we may suppose, that this fault lies somewhat to the west. it is possible that the chain of volcanic islands along the line skua island – petermann island is connected with the western edge of this fault (fig. 1). we are inclined to suppose that the described tectonomagnetic anomalies are of piezomagnetic nature. this supposition is grounded on the absence of sedimentary complex, as a condition of electrokinetic phenomena, and agrees with the distributions of ∆∆f and local magnetic field ∆f along the profile (fig. 4). for evaluation of possible contributions of the piezomagnetic effect into changing rock magnetization we accepted the ∆f anomaly source as a set of vertically magnetized parallelepipeds of infinite spreading in a sub-meridian direction (fig. 4). the magnetization for parallelepiped no. 2 was accepted equal to –0.78 a/m and for all others –0.55 a/m. the calculated ∆∆f curve refers to following changes of the parallelepipeds magnetization (in a/m) 0.004, 0.0068, 0.003, 0.0015, 0.0003, 0.001 from no.1 to no.6. the evaluation of strain, using the method in parkinson (1983) and skovorodkin (1985) suggested that the magnetization change in parallelepiped no. 2 may be caused by variation of horizontal strains about –25 –30 bar. so, we conclude that in the observation area during seven years (19982005) local variations of stretching strains with an average velocity near 5 bar per year took place. to explain the nature of tectonomagnetic effects in a location of uas it is necessary to clarify the recent tectonic evolution of this region, connected with processes in the bransfield strait. accordingly, with traditional ideas about this part of the western antarctic as a tectonomagnetic arc, its formation was prolonged by subduction of the oceanic proto-pacific plate under the margin of the antarctic plate. results vol52,1,2009 28-05-2009 13:45 pagina 40 41 results and perspectives of tectonomagnetic investigations in the western antarctic fig. 4. the results of anomalous magnetic field (∆f) and its temporal variations (∆∆f) interpretation in the profile i – i (barchany-rasmussen) a) observed and calculated (dashed) curves of ∆f, b) observed and calculated (dashed) curves of ∆∆f along the profile i – i, c) a model of anomalies source. vol52,1,2009 28-05-2009 13:45 pagina 41 42 v.yu. maksymchuk, v.g. bakhmutov, yu.m. horodyskyy and i.o. chobotok of bathymetry, high-precision geodetic measurements, seismic data, materials of seismic tomography show, that bransfield strait was formed through active stretching of about 10 mm per year in nw-se direction. the central part of the strait contains a 2025 km width zone with recent volcanism, and in its axis parts clearly trace a chain of neo-volcanic ridges. along this neo-volcanic zone the crust is thinner (10-12 km) and has minimum thickness in the central and ne areas of the zone. thicker crust (12-15 km) is defined towards sw. such a situation lies along southshetland islands, where the thickness of crust increases from 20 km on the ne to 26 km on the sw. the same is along the edge of antarctic peninsula, where the thickness of crust increases from 14-15 km on the ne to 19-20 km on the sw. from the morphological point of view grooves in the central part of the strait can be described as active rift canyon with co-axis volcanic complexes. it is necessary to mark, that this rift has developed on an ancient craton and, probably has no interrelations with spreading of large plates. it intrudes into a continental massif of paleozoic-mesozoic rocks in the marginal nw part of the antarctic peninsula. in general, the rift in bransfield strait can be a part in the chain of rifts, located from south-sandwich island arc to the continental massif of the western antarctic. active spreading of a rift in sw direction in our times ceased near deception island. there is the ending of rift grooves and towards sw in a shelf relief of the antarctic peninsula there are no similar grooves. but epicenters of earthquakes are tracing up to 70 km in sw direction. thus, the geodynamic factor that causes further intruding of the rift in sw direction already is working here at a distance to our tectonomagnetic investigations area, located in sw direction, of slightly more than 200 km. the interpretation of defined magnetic field dynamic variations as a result of recent neo-tectonic processes is not clear. large enough amplitudes of determined tectonomagnetic anomalies, which noticeably exceed similar effects in other seismically active world regions, seem to be caused first of all by outcrops of highly magnetized rocks in the upper part of the crust. they are some type of indicator of tectonic strain variations in the lithosphere even for large distances. the obtained results on the features and directions of tectonic strains in the region are confirmed by results of investigations of earths crust blocks movements, based on the gps-observations in 2003-2005 (fig. 1). in the argentine islands archipelago two different blocks of crust exhibiting different kinematic features have been identified: an eastern block with a horizontal displacement vector towards se and a western block with horizontal displacements vector towards wnw. they are divided by a tectonic fault (tretyak et al., 2006). 4. conclusions as a result of continuous geomagnetic observations in the tectonomagnetic polygon near the location of uas data about local magnetic field dynamics on the west cost of antarctic peninsula were obtained. the spatial-temporal structure of the geomagnetic field variations agrees with tectonic elements. the most intensive tectonomagnetic anomalies were defined on the three little pigs island. during 19982005 they reached 18.5 nt (2.6 nt per year). the high amplitude of the tectonomagnetic effect is caused by the high magnetization of rocks and the intensive tectonic strains in the crust. defined tectonomagnetic anomalies were interpreted as due to the piezomagnetic mechanism. tectonomagnetic anomalies reflect complex features of interrelations between tectonic strains and magnetic heterogeneities in the upper part of earth’s crust. blocks of crust in the argentine island region are under the influence of stretching horizontal forces near 5 bars per year with a sub-latitude direction. it was concluded that near uas lies a submeridian tectonic fault, and in its limits exist active tectonic processes. for the monitoring of their development it is necessary to continue investigations by increasing the number of observation points covering the continental slope of the peninsula, and the organization of non-stop automatic geomagnetic observations in anomvol52,1,2009 28-05-2009 13:45 pagina 42 43 results and perspectives of tectonomagnetic investigations in the western antarctic alous zones. the problem of the interrelation of tectonomagnetic observations with other geophysical and geodetic methods remains elusive. references hao, jin-qi, l.m. hastie, and f.d. stacey (1982): theory of seismomagnetic effect: a reassessment, phys. earth and planet. inter., 28, 129-140. maksymchuk, v.yu., yu.m. horodyskyy and v.g. kuznetsova (2001): dynamics of earth’s anomalous magnetic field, (eurosvit, lviv), pp. 308 (in ukrainian). martin, r.j., r.w. haupt and r.j. greenfield (1982): the effect of fluid flow on the magnetic field in low porosity crystalline rock, geophys. res. lett., 9, 13011304. mizutani, h. and t.a. ishido (1976): a new interpretation of magnetic field variation assotiated with the matsushiro earthquakes, j. geomag. geoelectr., 28, 179186. murakami, h. (1989): geomagnetic fields produced by electrokinetic sources, j. geomag. geoelectr., 41, 221-247. nagata, t. (1969): tectonomagnetism, iaga bull., 27, 12-43. parkinson, w.d. (1983): introduction to geomagnetism, (scottish academic press, edinburgh and london), pp. 528. sasai, y. (1983): a surface integral representation of the tectonomagnetic field based on the linear piezomagnetic effect, bull. of the earthquake res. inst., 58, 763-785. skovorodkin, yu.p. (1985): studyings of tectonic processes be magnetometric methods, (edition of earth’s physics institute ussr ac. of sci., moscow), pp. 197 (in russian). tretyak, k., yu golubinka and a. kultchytskyy (2006): geodynamic investigations of a tectonic fault in the penola strait (antarctic peninsula, argentina islands archipelago). recent achievements of geodetic science and production, issue ii (vol. 12) (publishing of the national university «lviv polytechnics»), 134140 (in ukrainian). vol52,1,2009 28-05-2009 13:45 pagina 43 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 1 coseismic displacement waveforms for the 2016 august 24 mw 6.0 amatrice earthquake (central italy) carried out from high-rate gps data a. avallone1,*, d. latorre1, e. serpelloni1, a. cavaliere2, a. herrero3, g. cecere1, n. d'agostino1, c. d'ambrosio1, r. devoti1, r. giuliani4, m. mattone4, s. calcaterra5, p. gambino5, l. abruzzese1, v. cardinale1, a. castagnozzi1, g. de luca1, l. falco1, a. massucci1, a. memmolo1, f. migliari1, f. minichiello1, r. moschillo1, l. zarrilli1 and g. selvaggi1. 1istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti 2istituto nazionale di geofisica e vulcanologia, sezione di bologna 3istituto nazionale di geofisica e vulcanologia, sezione di roma 1 4ufficio rischio sismico e vulcanico, dipartimento della protezione civile, roma 5ufficio geofisica, istituto superiore per la protezione e la ricerca ambientale, roma *antonio.avallone@ingv.it abstract abstract: we used high-rate sampling global positioning system (hrgps) data from 52 permanent stations to retrieve the coseismic dynamic displacements related to the 2016 august 24 mw 6.0 amatrice earthquake. the hrgps position time series (named hereinafter "gpsgrams") were obtained with two different analysis strategies of the raw gps measurements (precise point positioning [ppp] and doubledifference [dd] positioning approaches using the gipsy-oasis ii and the track (gamit/globk) software, respectively). these gpsgrams show rms accuracies mostly within 0.3 cm and, for each site, an agreement within 0.5 cm between the two solutions. by using cross-correlation technique, the gpsgrams are also compared to the doubly-integrated strong motion data at sites where the different instrumentations are co-located in order to recognize in the gpsgrams the seismic waves movements. the high values (mostly greater than 0.6) of the cross-correlation functions between these differently-generated waveforms (gpsgrams and the sm displacement time-histories) at the co-located sites confirm the ability of gps in providing reliable waveforms for seismological applications. i. introduction o n august 24, 2016, at 01:36:32 (utc time, http://cnt.rm.ingv.it, marchetti et al., 2016), a mw 6.0 earthquake struck the region of the central apennines (italy) between the towns of norcia and amatrice (figure 1), where peak ground accelerations values up to 0.9g were recorded (http://ran.protezionecivile.it/). the main annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 2 shock triggered an aftershock sequence that involved a crustal volume extending se-nw for ~30 km and down to ~15 km of depth. the main shock and the largest part of aftershock events show focal solutions (http://cnt.rm.ingv.it/tdmt.html, scognamiglio et al., 2016; http://www.bo.ingv.it/rcmt) characterized by almost pure extension on nw-se fault planes, in agreement with geodetic measurements of the main event (ingv cnt gps working group, 2016; cheloni et al., 2016) and with the interseismic sw-ne extension characterizing this sector of the apennines (e.g., galvani et al., 2012; d’agostino, 2014). the ground displacements associated to the august, 24 mw 6.0 main shock were recorded by a number of high-rate sampling (from 1 to 0.05 s) continuous gps (hrgps) stations (figure 1) belonging to several networks developed for both scientific and surveying purposes. in detail, raw phase data were obtained from the following gnss networks or agencies: ring (ingv ring working group, 2016; http://ring.gm.ingv.it), ispra (http://www.isprambiente.gov.it), dpc (http://www.protezionecivile.gov.it), regione lazio (http://gnss-regionelazio.dyndns.org), regione abruzzo (http://gnssnet.regione.abruzzo.it), italpos (http://it.smartnet-eu.com) and netgeo (http://www.netgeo.it). the main shock was also recorded by several strong motion (sm) stations belonging to the ingv rete sismica nazionale (rsn, ingv seismological data centre, 1997) and the dpc rete accelerometica nazionale (ran, http://ran.protezionecivile.it) accelerometric networks. this work is within the line of gps seismology (larson et al., 2003; bock et al., 2004, larson et al., 2009), in which the estimation of the coseismic dynamic displacements is carried out following different methods (i.e. precise point positioning [ppp, zumberge et al., 1997; bertiger et al., 2010], double-difference (dd) positioning [herring et al., 2010]), variometric [colosimo et al., 2011] and ambiguity constrained time-differences [li et al., 2013] approaches). in this work we will describe the high-rate gps (hrgps) time series (named hereinafter "gpsgrams") following the ppp and dd approaches by using two different scientific packages (gipsy/oasis and gamit/globk), and compare the results. the gpsgrams have been also compared with the displacement time histories carried out from the double integration of the sm data at some stations, where the different instruments are approximately co-located (figure 1). ii. hrgps time series the available hrgps data were analyzed with two different approaches (ppp and dd strategies), using two non-commercial, geodetic-quality, software modules for kinematic processing: the track module of gamit/globk developed at the massachusetts institute of technology (http://wwwgpsg.mit.edu/~simon/gtgk/, herring et al., 2010), and the gd2p module of gipsy/oasis ii, developed at the jet propulsion laboratory (http://gipsy.jpl.nasa.gov, bertiger et al., 2010). for the description of the detailed aspects related to the analysis strategies we refer to avallone et al. (2012). for the general description of the detailed aspects related to the analysis strategies we refer to avallone et al. (2012). however, with respect to this previous work, during the analysis we applied improved available models of the troposphere (bohem et al. 2006) and second order ionosphere (kedar et al., 2003) estimations http://smartnet.leica-geosystems.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 3 figure 1: locations of the hrgps stations (blue diamonds) and of the sm stations (red triangles) operating during the amatrice 2016 main shock. the star represents the location of the main shock, whereas the green dots show the distribution of the aftershocks occurred within the first two days from the main shock (http://cnt.rm.ingv.it, marchetti et al., 2016). the focal mechanism (scognamiglio et al., 2016; http://cnt.rm.ingv.it/tdmt.html) is normal. the gpsgrams of the labeled hrgps sites are shown in figure 2a-b, whereas the open squares indicate the couples of hrgps and sm sites used for the comparisons in figure 3. (vmf1 grid and ionex files, respectively). it is worth noting that the two software packages are based on a completely different approach in the reduction of the raw phase data: the gd2p module follows the ppp approach, where the kinematic station only needs fiducial high-rate satellite orbits and clocks information; the track module performs relative kinematic positioning and it needs at least one reference station out of the epicentral area (i.e., assumed fixed in the time interval affected by the dynamic displacements in the epicentral area), and one (or more) kinematic station. in this study, after an analysis of the quality of the satellite tracking at different potential reference (fixed) stations (i.e. sky plots, multipath and cycle-slips metrics) and of the resolved parameters (i.e. ambiguity fixing), the station cass (cassino, lat: 41.49; lon: 13.83, ~140 km from the epicenter), belonging to the regione lazio gnss network, was chosen as the reference site for the analysis with track of all the stations located within 50 km from the epicenter. the relative double-difference positioning performed in the track analysis likely allows to remove, or at least minimize, a common-mode regional contribution that could be, on the contrary, present, as low-frequency contribution, in the ppp solutions. for this reason, we applied to the gd2p solutions a spatial filtering commonly used for filtering long-term (daily positions) time series (wdowinski et al., 1997). in detail, we firstly selected the gpsgrams not affected by coseismic dynamic displacements in the time interval 0-40s. these gpsgrams, corresponding to the 9 sites beannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 4 longing to the regione lazio gnss network located outside the epicentral area (more than 50 km from the epicenter), were then stacked and averaged. finally, we removed this averaged signal from the gpsgrams located in the epicentral area. by using the sac software (goldstein et al., 2005), distributed by iris (incorporated research institutions for seismology), the north and east components of the measured ground displacements at each site were rotated with respect to the epicenter and presented in the radial-transverse coordinate system, since the radial (r) component mainly contains sv and p-wave arrivals, while the transverse (t) component presents predominantly sh-wave modes. this representation may be not optimal in near source regions as in the case of nrci and amat stations (figure 1). nevertheless, in this particular case, we can adopt the rt representations because both the stations are located along the strike of the m6 event and the rt components correspond to the faultparallel and fault-normal components, respectively. in figure 2, some examples of comparison on both radial and transverse components as obtained by the two different solutions are shown for the sites labeled in figure 1, whereas the comparison between both the solutions for all the 52 sites are shown in figure sm1 of the supplementary material. a first comparison is focused on the two gps sites located closer to the epicenter (amat, amatrice, ~9 km to the se, and nrci, norcia, ~14 km to the nw, figure 2a). amat acquired data at 0.1 s sampling rates, thus allowing potentially low aliasing artifacts in the observations (smalley, 2009; avallone et al., 2011), whereas nrci acquired data at 1 s sampling rate. because of their proximity to the epicenter, they experienced the largest coseismic dynamic displacements: amat shows larger values of peak-topeak displacements on the radial component (~16.6 cm) than on the transverse one (~8.2 cm), whereas nrci shows generally lower peak-to-peak displacements than amat, but larger on the transverse component (~15.0 cm) than on the radial one (~8.2 cm). the comparisons between the gd2p and track results are also shown in figure 2b at some sites located between ~35 and ~50 km from the epicenter, where the static deformation due to the near field is expected to be negligible (as shown in cheloni et al., 2016) or within the accuracy of the hrgps time series. the gpsgrams shown from the top to the bottom of the figure 2b are sorted by increasing azimuth angle and seem to point out the sh arrivals on the t component and the surface waves arrivals on the r component in the time ranges 10-15 s and 15-25 s, respectively. due to power failures occurred a few seconds after the main shock, likely related to the s-wave arrivals ground motion, the available data and the resulting gpsgrams at some sites (lnss, ascc, asco and camr) are suddenly truncated. one of these examples (ascc) is shown in figure 2b. the comparison between gd2p and track solutions (in both the figure 2b and the figure sm1) shows a remarkable agreement in the detection of the higher-frequency coseismic dynamic displacements, observing comparable peak-to-peak values. to quantify the noise level of our gpsgrams, for all the sites we calculated the rms of the position time series in a 10-s time window before the earthquake origin time. in figure 2c, the histograms of the rms distributions of all the available sites are shown for both the r and t components and for the gd2p and track annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 5 figure 2: a) comparison between the gd2p and track gpsgrams for the two stations closest to the epicenter (amat, amatrice, and nrci, norcia) on the radial and transverse components; b) radial (left) and transverse (right) components of the gd2p (red) and track (green) gpsgrams obtained for the sites labeled in figure 1, between 35 and 50 km from the epicenter in a time interval of 40 s starting from the main shock origin time (01:36:32 utc, http://cnt.rm.ingv.it). in both the panels, from top to down the sites are sorted by the increasing azimuth degree. labels and numbers on each time series represent the name of the sites, the azimuth degree, the distance from the epicenter and the sampling rate, respectively; c) rms histogram distributions calculated in a pre-seismic 10-s time window for gd2p (red) and track (green) on both radial (top) and transverse (bottom) components for all the 52 solutions. d) rms histogram distributions of the differences between the gd2p and track solutions in a time interval of 0-40 s on both radial (top) and transverse (bottom) components for all the 52 solutions. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 6 solutions. two features are worth noting: a) 92% to 98% of the gpsgrams reveal values of accuracy within 0.3 cm; b) the radial and transverse components seem to show comparable accuracies' distribution between gd2p and track. finally, to quantify the consistency between the gd2p and the track solutions, for each site, we firstly calculated the epoch-by-epoch differences between the two solutions and then we estimated the rms distribution of the so determined residuals. the histograms of the residuals' rms distribution (figure 2d) show that for most of the sites, the two solutions agree within a value of 0.5 cm. the results obtained in this work show scatter values in the solutions reduced by about 50% with respect to a similar analysis carried out for the emilia sequence (avallone et al., 2012). iii. comparison between hrgps and strong motion in this work we selected 8 examples of sm stations approximately co-located with the hrgps sites. for most of the sites (six examples), the relative distance between the colocated instrumentations ranges from 0 to 1.1 km. however, we also included the foc and asp sm stations whose relative distances with gps instrumentation (cesi and ascc, respectively) are higher, about 2.4 and 4.6 km, respectively. in order to compare hrgps and accelerometric time series, we transformed in displacement the sm recordings by applying a double-integration after removing the mean of the signal calculated in a 4-s time window before the p-wave first-arrival. moreover, since sm data were recorded with higher sampling rate (0.005 s) than hrgps data (1 s and 0.1 s, figure 2), we firstly filtered the sm-data at the associated gpsgram nyquist frequency to avoid any aliasing effect, and then we decimated each sm displacement waveform at the sampling rate of the co-located gps instrument. to quantify the similarity between the sm displacement waveforms and the gpsgrams, we performed a normalized crosscorrelation (ncc) between the signals recorded in a time window of 10 s after the p-wave first arrival. the analysis of source radiation recorded at the co-located sm and gps stations is shown in figure 3 in terms of their ground-motion displacement in the rt coordinate system, following the same procedure used to compare the two gpsgrams solutions. as a first approximation, we find a good agreement between the processed gpsgrams and the doubly-integrated sm data. this match can be also deduced from the crosscorrelation function computed for windows of signals that include the first coseismic dynamic displacements. due to the minimum processing applied to the sm data, we observe a strong t2-drift, shortly after the s-wave arrival, for stations located in near field (amt, nrc). such drifts are well known in literature and are the effect of small distortions in acceleration, often due to static displacements, that appear when waveforms are double integrated (e.g., boore et al., 2002). other smaller offsets are observed at other stations more distant from the source, but they are difficult to quantify over this time window. the main difference between integrated accelerograms and gpsgrams in terms of peak of displacement is observed for the two stations located near the fault (amt-amat and nrc-nrci). for the r component of the amt station, the integrated accelerogram shows only one peak, while the gps shows a comparable (in amplitude peak), and a second later peak with amplitude similar annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 7 figure 3: examples of comparison between gpsgrams (gd2p and track solutions, dark and light green lines, respectively) and the sm displacement time-series (red lines) at 8 co-located sites. data are presented along the radial (1st column) and the transverse (3rd column) component. in 2nd and 4th columns are also plotted the normalized autocorrelation function (acc, red line) and the normalized cross-correlation (ncc) functions between gd2p and sm timeseries (dark blue), and between track and sm time-series (light blue) computed for both r and t components. the maximum ncc value for each gps-sm time series is reported in the figure. acc and nccs were computed between signals recorded in windows of 10 s (gray area), 1-2 seconds after the p-wave first arrival time, picked on the strong motion waveform in acceleration and indicated with a vertical black bar. only for signals at guma and asp stations the window is smaller (9 seconds) due to the interruption of the sm recording in the first case and of the gps acquisition in the second one. the text in the figures explicit the distance (dist) and the azimuth (azim) with respect to the epicenter, the sampling rates, the inter-distance (id) between the sm and the gps and, finally, the type of installation of the gps antenna (building or free-field). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 8 to the first one. gps and sm displacements are similar for the first peak on the t component whereas the following portions of the signals present significant differences with higher gps amplitudes. these differences cannot be explained with eventual inaccuracy of the processing because these kinds of frequencies are very well reproduced on guma station for instance. since the gps antennas at nrci and amat sites were installed on the top of buildings, possible amplifications related to the response of the building to coseismic shaking should be more carefully evaluated, considering both its structure and orientation. however, due to the similarity of the first seconds after the p-first arrival, we find that the ncc value for amat is still very high on the r component (0.84-0.86). similar characteristics are observed for nrci that shows a high ncc value on the t component (0.72-0.73). here, the differences in amplitude are mainly due to the down-sampling of the sm data at 1 s, while we find that the pgd (peak ground displacement) on the t component is not driven by the same peak possibly due to site effects. rm33-mter co-located stations are the first that present very coherent signals on both the components. the double-integrated sm signal and the gd2p and trak solutions show maximum ncc values of 0.98 and 0.97 on the r and t components, respectively, indicating that the gps is able to perfectly reproduce the coseismic dynamic displacements. in this case, the mter station has the advantage to be installed in free-field, avoiding possible problems of amplifications related to the installation. the global pattern of pgd on four colocated stations asp-ascc, gsa-infn, trn1-teri and foc-cesi is compatible with the focal mechanism of the m6 event. indeed, these set of stations are located along the nodal planes that reduce the amplitude of the shwave, generally identified along the tcomponent. the agreement between gps and integrated sm signals is globally good, even for stations as asp-ascc and foc-cesi that are not perfectly co-located (inter-distance of 2.4 km and 4.6 km, respectively, figure 1). this is visible on the cross-correlation functions that have high maximum values, but are shifted in time with respect to the autocorrelation function computed on the sm signal. the amplitudes recorded at the guma station are compatible with a directivity toward the nord. this is evident by comparing the amplitudes recorded at the other stations, including rm33 that is closer to the epicentral area (figure 1). at guma station, the accelerometer recording stopped just after the peak over the t-component. after that, a greater peak is observed on the r-component of the gpsgrams. this signal corresponds to a rayleigh-type surface wave since a similar peak is also identified on the z-component with the typical dephasing of π/2. the same kind of wave is also observed on the co-located foccesi stations, demonstrating that the pgd is rapidly associated to the surface wave when the epicentral distance increases. iv. conclusions the analysis of high-rate coseismic dynamic displacements of the 2016 august 24 mw 6.0 amatrice earthquake were carried out at 52 continuous gps stations. the observed ground motions in the gpsgrams were consistently associated with the propagation of the seismic waves. as expected, the hrgps ability to detect details in the seismic waves increases with the increasing of the sampling frequency of the acquired raw data. furthermore, acquiring data with high sampling frequencies (> 5 hz) not annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7275 9 only strongly reduce aliasing artifacts in the resulting gpsgrams near the source (smalley, 2009; avallone et al., 2011), but it also allows to observe details in the seismic wave propagation in the far field. for this earthquake, in fact, appreciable differences in the observations of the surface waves arrivals still exist between the 10-hz-sampling (i.e., guma) and the 1hz-sampling (i.e., cesi) gpsgrams even at larger distances from the epicenter (35-50 km). for the moderate earthquakes occurred in italy in the last 8 years (2009 mw 6.3 l'aquila; 2012 mw 5.9 and mw 5.8 emilia main shocks, 2016 mw 6.0 amatrice) we observe a relatively good spatial coverage of the hrgps stations around the epicenter, although the contribute of non-geophysical gnss networks in this picture is still significant. further efforts in developing denser hrgps networks, with efficient coupling of the antenna installations with the solid earth, and operating at sampling frequencies ≥ 10 hz would provide additional constraints to characterize the source process and discriminate peculiar site effects (avallone et al., 2014). with respect to the case of the 2012 emilia earthquake (avallone et al., 2012), the observed gpsgrams for different software packages show a factor-2 lower rms values (0.3 cm) and residuals rms values (0.5 cm) suggesting a very good accuracy and agreement in the observation of the coseismic dynamic displacements. in addition, the high values of the cross-correlation functions between the gpsgrams and the sm displacement timehistories at the co-located sites confirm the ability of gps in providing reliable waveforms for seismological applications. the gpsgrams described in this work represent a potential contribution to further studies on the earthquake source. significant improvements of (1) the source rupture kinematic modeling, in (2) definition of source directivity generated by the main shock, or (3) for estimating the magnitude of the event could be obtained by the joint use of gpsgrams and strong motion waveforms. in fact, in seismology and engineering seismology, the estimation of the pgd is useful for studies on earthquake processes, seismic design and structural monitoring (i.e. building, bridges). however, using conventional seismological approaches this estimation is still challenging. on the other hand, positions correspond to basic observations for the gps. in this sense, the gpsgrams could today provide important and decisive contributions to investigate earthquake radiation pattern and source directivity estimating the pgd distribution around the seismic source. references avallone et al. 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(1997), precise point positioning for the efficient and robust analysis of gps data from large networks, j. geophys. res., 102(b3), 5005-5018. * acknowledgements: the ascii files of the hrgps position time series for all the sites used in this work for both the gd2p and track solutions are available at the following link: ftp://gpsfree.gm.ingv.it/amatrice2016/hrgps/. we thank all of the agencies, cited in the main text, which have provided the gps observations used in this study. particular thanks are addressed to dr. domenico collalti (regione abruzzo), ing. simone patella (regione lazio), francesco matteuzzi (leica geosystems s.p.a.), dr. nicola perfetti and ing. giuliano molinelli (netgeo) for the rapid distribution of the hrgps data belonging to their permanent networks and all the technical staff involved in the maintenance of these networks. we also thank the staff involved in the development and maintenance of the ran and ingv accelerometric networks. figures have been prepared using gmt software package (wessel and smith, 1998) http://dx.doi.org/10.1029/97jb01378 1 annals of geophysics, 66, 1, se101, 2023; doi:10.4401/ag-8833 o p e n a c c e s s can the nucleation phase be generated on a sub-fault linked to the main fault of an earthquake? jeen-hwa wang1,2 (1) institute of earth sciences, academia sinica, taipei, taiwan (2) department of earth sciences, national central university, taoyuan, taiwan article history: received april 22, 2022; accepted november, 2022 abstract we study the effects of seismic coupling, friction, viscosity, and inertia on earthquake nucleation based on a two-body spring-slider model in the presence of thermal-pressurized slip-dependent friction and viscosity. seismic coupling of the system is represented by the stiffness ratio 𝑠 = 𝐾/𝐿 where 𝐾 is the coil spring between two sliders and 𝐿 is the leaf spring between a slider and the background plate. weak seismic coupling with s < 1 is used in this study. the masses of the two sliders are 𝑚1 and 𝑚2, respectively. the frictional and viscous effects at slider i (i = 1, 2) are specified by the static friction force, 𝑓oi, the characteristic displacement, 𝑈ci, and viscosity coefficient, 𝜂ι, respectively. numerical simulations show that the s is not a major factor in generating the nucleation phase. friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. higher viscosity causes lower particle velocities than lower viscosity. simulation results exhibit that the ratios � = 𝜂2/𝜂1, 𝜙 = 𝑓o2/𝑓o1, 𝜓 = 𝑈c2/𝑈c1, and � = 𝑚2/𝑚1 are four factors in influencing the generation of nucleation phase at slider 1 and the formation of 𝑃 wave of an event at slider 2. that simulated waveforms with � > 1, 1.15 > 𝜙 > 1, 𝜓 < 1, and � < 30 are consistent with the observations suggests the possibility of generation of nucleation phase on a sub-fault. results also exhibit independence of p wave at slider 2 on the shape and duration time of nucleation phase at slider 1. keywords: nucleation phase; two-body spring-slider model; stiffness ratio; thermal-pressurized slip-dependent friction; viscosity; inertial effect 1. introduction the presence of nucleation phase before the p waves (see figure 1) was suggested by early theoretical studies [e.g., andrews, 1976; brune, 1979; dieterich, 1986, 1992; das and scholz, 1981] and laboratory experiments [dieterich, 1979; ohnaka et al., 1987]. some studies [e.g., scholz et al., 1972; dieterich, 1981; ohnaka and yamashita, 1989; ohnaka, 1992; ohnaka and kuwahara, 1990; kato et al., 1994; roy and marone, 1996; lu et al., 2010; latour et al., 2013; kaneko et al., 2016] also indicated that the nucleation process behaves like a transition from quasi-static slip (without the inertial effect) to (unstable) dynamic motion (with the inertial effect) when the slip speeds become high enough to make the inertial effect dominate frictional resistance under some conditions. the study of nucleation jeen-hwa wang 2 phase is not only a basic problem of earthquake physics but also an important one for early warming, prediction, and hazard assessment of earthquakes. umeda [1990] first recognized the existence of nucleation phase in velocity seismograms. since then, numerous seismologists have also observed the nucleation phases [e.g., iio, 1992, 1995; ellsworth and beroza, 1995; beroza and ellsworth, 1996; mori and kanamori, 1996; ruiz et al., 2017]. there was a debate concerning the correlation between the duration time, td, of nucleation phase and the magnitude, m, of the earthquake occurring immediately after the nucleation phase. ellsworth and beroza [1995] and beroza and ellsworth [1996] assumed a positive correlation of td to m. whereas, mori and kanamori [1996] observed independence of the p waves on the shape of nucleation phase in a large magnitude range. ellsworth and beroza [1998] confirmed the results observed by mori and kanamori [1996]. friction and viscosity are two major factors in controlling the complicated earthquake rupture processes including nucleation [wang, 2016; and cited references therein]. analytic solutions and numerical simulations for exploring the nucleation phase have been made based on the infinite dislocation models, crack models, and springslider models by using different friction laws [beeler, 2004; bizzarri, 2009, 2011a,b; tal et al., 2018; wang, 2016, 2017a; and cited references therein]. iio [1992, 1995] stressed that the nucleation phase cannot be interpreted by any theoretical source model with a constant kinematic friction and a constant rupture velocity. mori and kanamori [1996] claimed that any model having a similar initial rupture can describe the nucleation phases of earthquakes of all sizes, and thus it is difficult to estimate the magnitude of an earthquake just from its nucleation phase. they also stressed that curvature seen in the nucleation phases is caused by an-elasticity. since the present study concentrates on the generation of nucleation phase on a sub-fault and initiation of dynamic slip (or an earthquake) on a main fault, a two-body spring-slider (ss) model, which has been used to approach an earthquake fault by numerous authors [e.g., galvanetto, 2002; turcotte, 1992; wang, 2007, 2017b, 2020], by considering the two sliders as two fault segments, is taken into account. although the two-body ss model is simplified from the many-body ss model [burridge and knopoff, 1967], it can still provide significant information about the properties of real earthquakes [turcotte, 1992]. of course, a many-body ss model can provide more information than a two-body ss one. nevertheless, it is easier to explore the generation of nucleation phase on a sub-fault and main dynamic slip on the main fault by using a two-body ss model than by using a many-body one. note that the many-body ss model and its simplified and modified models are actually a discretization of a continuous fault and thus an approximation of a problem of continuum mechanics. some of previous theoretical studies concerning the nucleation phase based on the ss model and its modified and simplified forms are briefly described here. brantut et al. [2011] concluded that metamorphic dehydration influences the nucleation of unstable slip and could be an origin for slow-slip events in subduction zones. ueda et al. [2014, 2015] and kawamura et al. [2018] pointed out that the nucleation process includes the quasi-static initial phase, the unstable acceleration phase, and the high-speed rupture phase (i.e., a mainshock) and recognized two kinds of nucleation lengths, i.e., lsc and lc which are affected by model parameters, yet not by the earthquake size. the lsc related to the initial phase exists only for a weak frictional instability regime; while the lc associated with the figure 1. an example to show the nucleation phase, onset of the p wave, and the p wave in velocity seismogram [after wang, 2017a]. a nucleation phase on a sub-fault 3 acceleration phase exist for both weak and strong instability regimes. they also found that in the initial phase up to lsc, the sliding velocity is an order of magnitude of the plate speed; while at a certain stage of the acceleration phase it becomes higher and thus can be observed. on the other hand, numerous lower-degrees-of-freedom models that have the geometric structure consisting of few, large patches have also been assumed by numerous authors [e.g., ruff, 1992; turcotte, 1992; rice, 1993] to investigate fault dynamics. interaction between different fault patches may play a key role on the dynamics of earthquake ruptures. the failure of a fault segment triggered by slip of another fault segment has been long and widely studied. the discrete fault model consisting of two segments having different mechanical properties may exhibit a very rich dynamics and make it possible to study in detail fault evolution during both coseismic slip and interseismic intervals. some examples are given below. viscoelasticity was taken into account by dragoni and lorenzano [2015]. fault evolution during interseismic intervals was considered by dragoni and piombo [2015] and lorenzano and dragoni [2018]. the velocity dependent term was introduced by dragoni and santini [2015] and its role was further examined by dragoni and santini [2017] and dragoni and tallarico [2016]. the presence of different mechanical properties, including different masses that would lead to the inertial effect, on two fault segments was considered by dragoni and lorenzano [2017]. although the frictional effect on earthquake nucleation has been long and widely studied as mentioned above, the studies of viscous effect on earthquake ruptures are rare. the viscous effect mentioned in rice et al. [2001] is actually an implicit factor which is included within the direct effect of rateand state-dependent friction law. wang [2017a] took viscosity into account for studying the nucleation phase by assuming a temporal change of high viscosity to low viscosity during an earthquake rupture based on a one-body ss model with thermal-pressurized slip-weakening friction. his results show a temporal variation from nucleation phases to p wave and the amplitude of p wave, which is associated with the earthquake magnitude, does not depend on the duration time of the former. as mentioned above, the nucleation process behaves like a transition from quasi-static slip (without the inertial effect) to (unstable) dynamic motion (with the inertial effect) when the slip speeds become high enough to make the inertial effect dominate frictional resistance under some conditions. this assumes that the inertial effect must be taken into account. in most of previous studies, both the nucleation phase and the p wave are assumed to occur on the same fault. there is an interesting question: can the nucleation phase happen on a sub-fault which links to the main fault of an earthquake? in order to answer this question, in this work i will explore the frictional, viscous, and inertial effects on the generation of nucleation phase on a fault and then the transition from it to the p wave on the other based on a two-body ss model. the friction force caused by thermal pressurization is slip-weakening and the viscosity is represented by an explicit parameter. the study of inertial effect on nucleation phase is rare. this effect is implicitly included in the thermal-pressurized friction used by brantut et al. [2011]. wang [2020] addressed the importance of inertial effect on the earthquake nucleation. in this study, the inertial effect will also be taken into account. 2. two-body spring-slider model the two-body ss model (figure 2) consists of two sliders of mass 𝑚i (i = 1, 2) and three springs. a detailed description of the model can be seen in wang [2017b] and only a brief explanation is given here. the equation of motion of the system is (1a) . (1b) the ui (i = 1, 2) is the displacement of the i-th slider measured from its initial equilibrium position along the 𝑥-axis. the k is the strength of the coil spring between two sliders and the li (i = 1, 2) is the strength of the leaf spring to yield the driving force on the i-th slider from a moving plate with a constant speed vp. considering the two sliders to represent two segments of an earthquake fault, the coupling between the moving plate and jeen-hwa wang 4 each slider could be equal, thus giving l1 = l2 = l. fi(𝑢i) (i = 1, 2) is the frictional force on the i-th slider. wang [2013] took f(𝑢) = foexp(–𝑢/𝑢c), where fo and 𝑢c are, respectively, the static friction force and characteristic slip displacement, to study earthquake dynamics. this friction force is slip-weakening and caused by the adiabaticundrained-deformation (aud)-type thermal pressurization [rice, 2006]. the friction law due to thermal pressurization must refer to the solution of uniform slip made by lachenbruch [1980]. this friction law is essentially independent of the slip velocity and only implicitly depends on velocity through the seismic slip. within the normal 1-mm thick shear zone, most of the shearing seems to occur in a shear localized zone having a thickness of 100-300 µm. such thicknesses are smaller than those of hydraulic and thermal boundary layers that have developed near faults during significant earthquakes [e.g., rice, 2006]. hence, mase and smith [1987] and lee and delaney [1987] assumed that it is more appropriate to consider the occurrence of slip in a zone of zero thickness, i.e., a mathematical plane. on such a plane, the lachenbruch solution is unstable [see rice 2006] and for large values of the seismic slip displacement, strain will localize on a mathematical plane of zero thickness [see mase and smith 1987]. two reasons to explain why slip-dependent friction is taken in this study are: (1) madariaga and cochard [1994] pointed out that purely velocity-dependent friction could yield unphysical phenomena and mathematically ill-posed problems and ohnaka [2003] also stressed that the pure rate-dependent friction law is not a one-valued function of velocity. (2) the above-mentioned slip-dependent friction law comes from the end-member model, i.e., the adiabatic, undrained deformation (aud) model, of the thermal pressurization model in the fault zone proposed by rice [2006]. the other end-member model is the slip on a plane (sop) model. for the aud model, the sliding slip is dependent upon the varying sliding velocity; while for the sop model, the sliding velocity is always constant in the course of rupture processes. in the present study, since i used the aud model, slip-dependent friction is intrinsically dependent on velocity. an example of the variations of f(𝑢) versus 𝑢 for fo = 1 n and 𝑢c = 0.1, 0.3, 0.5, 0.7, and 0.9 m is displayed in figure 3 in which f(𝑢) decreases with increasing u, and the decreases rate is higher for smaller 𝑢c than for larger 𝑢c. this indicates that the force drop decreases with increasing 𝑢c for the same final slip. the function 𝛷(vi), where vi = dui/dt is the particle velocity, denotes a velocity-dependent viscous force, which is represented by a dash-pot in figure 2, due to the existence of viscosity in the fault zone. a detailed description about the mechanism of viscous force can be seen in wang [2017a,b]. according to stokes’ law, wang [2016] suggested the viscous force to be 𝛷 = cv, where c = 6𝜋r𝜐 (with a unit of n(m/s)–1) is the damping coefficient of a sphere of radius r in a fluid of viscosity 𝜐 [see kittel et al. 1968]. the two sliders rest in an equilibrium state at time t = 0. note that this model addresses only the strike-slip component and thus cannot completely represent earthquake ruptures, which also consist of transpressive components. nevertheless, simulation results of this model can still provide significant information on earthquake ruptures. figure 2. a two-body spring-slider model: fi = the friction force at the i-th slider, 𝑚i = the mass of the i-th slider, k = the stiffness between two sliders, li = the stiffness between the i-th slider and the moving plate, ci = the viscosity coefficient between the i-th slider and the moving plate, and vp = the velocity of the moving plate, and ui (i = 1, 2) is the displacement of the i-th slider [after wang, 2017b]. a nucleation phase on a sub-fault 5 substituting the friction and viscous laws into equation (1) leads to (2a) (2b) to deal with the problem easily, it is usual to normalize equation (2) based on the normalization parameters. wang [1995] defined the stiffness ratio, s, to be the ratio of k to l, i.e., s = k/l. wang [2017b] defined the normalization parameters for equation (2). however, in his study he took 𝑚1 = 𝑚2, and thus he did not consider the cases with 𝑚1 ≠ 𝑚2. while, in this study 𝑚2 could be larger than 𝑚1, thus leading to the inertial effect. hence, the parameters normalizing equation (2) are: 𝑚1 = 𝑚, 𝑚2 = �𝑚, fo1 = fo, fo2 = 𝜙fo, do = fo/l, 𝜔o1 = 𝜔o = (l/𝑚)1/2, 𝜔o2 = �–1/2𝜔o, 𝜏 = 𝜔ot, uc1 = uc, uc2 = 𝜓uc, 𝑈c1 = uc/do, 𝑈c2 = 𝜓uc/do, 𝑓o1 = fo = fo/do, 𝑓o2 = 𝜙fo, 𝜂1 = c1𝜔o/l, 𝜂2 = c2�–1/2𝜔o/l, � = 𝜂2/𝜂1, and vp = vp/do𝜔o. defining ui = ui/do and vi = dui/d𝜏 leads to dui/dt = [fo/(𝑚l)1/2]dui/d𝜏 and d2ui/dt2 = (fo/𝑚) d2ui/d𝜏2. inserting these normalization parameters with fo = 1 into equation (2) results in the following equations: (3a) . (3b) let y1 = u1, y2 = u2, y3 = du1/d𝜏, and y4 = du2/d𝜏 and then re-write equation (3) to be four first-order differential equations: (4a) (4b) figure 3. the variations of friction force with sliding displacement for uc = 0.1, 0.3, 0.5, 0.7, and 0.9 m when fo = 1 force unit [after wang, 2017b]. jeen-hwa wang 6 (4c) (4d) since it is difficult to analytically solve equation (4), only numerical simulations using the fourth-order rungekutta method [see press et al., 1986] are performed in this study. note that the sliders are restricted to move only along the positive direction, that is, vi ≥ 0 and ui ≥ 0 (i = 1, 2). 3. numerical simulations before performing numerical simulations, it is necessary to consider the acceptable values of model parameters. strong seismic coupling can make the two sliders move almost simultaneously. in order to allow independent motion for each slider, the value of s should be small. numerical tests [e.g., wang, 2017b] show weak coupling as s < 5 and strong coupling as s ≥ 5 for a two-body spring-slider system. hence, s < 5 is considered in this study. in general, vp is ~10–9 m/s and thus vp is ~10–9 when do𝜔o is an order of magnitude of 1 m/sec. simulation results could be figure 4. the time sequences of v/vmax (vmax = 0.456) and u/umax (umax = 1.355): (a) for s = 0.06, (b) for s = 0.12, (c) for s = 0.30, and (d) for s = 0.48 when � = 1, 𝑓o1 = 1.0 and 𝑓o2 = 1.0 (with 𝜙 = 1), 𝑈c1 = 0.5 and 𝑈c2 = 0.5 (with 𝜓 = 1), and 𝜂1 = 0 and 𝜂2 = 0 (with 𝛾 = 1). a nucleation phase on a sub-fault 7 influenced by using various time steps, 𝛿𝜏. practical tests suggest that simulation results show numerical stability when 𝛿𝜏 < 0.05. the time step is taken to be 𝛿𝜏 = 0.02 hereafter. when vp𝜏 = exp(–y1/𝑈c1) on slider 1 from equation (4c), the force exerted from the moving plate is just equal to foi. although in principle slider 1 can start to move under this condition, in practice the computation cannot go ahead because all values are zero. an initial force, 𝛿f, is necessary to kick off slider 1. note that the value of 𝛿f can affect the computational results [carlson et al., 1991]. a very small value of 𝛿f cannot enforce slider 1 to move; while a large one will dominate the whole computation process. numerical tests show that 𝛿f = 10–3 is appropriate for numerical simulations. numerical simulations are made under various values of model parameters for showing the effects caused by seismic coupling, friction, viscosity, and inertial effect. simulation results are displayed in figures 4-10 which include the time variations in v/vmax (in the left-hand-side panels) and u/umax (in the right-hand-side panels). the values of vmax and umax are given below. in figures 4-10, the simulation results at slider 1 and slider 2 are represented, respectively, by a solid line and a dotted one. it is necessary to examine the lower-bound value of s for yielding strong enough seismic coupling between the two sliders. numerical tests exhibit that slider 2 cannot move for s < 0.06 when other model parameters are equal at the two sliders. hence, s = 0.06 is almost the lower bound of seismic coupling for most of simulations. on the other hand, numerical tests suggest that when s > 0.48, the solid and dashed lines coincide. this means that large s having strong seismic coupling leads to almost simultaneous motions of the two sliders. the results for the effect due to seismic coupling are displayed in figure 4 where the values of s are: (a) for s = 0.06, (b) for s = 0.12, (c) for s = 0.30, figure 5. the time sequences of v/vmax (vmax = 0.142) and u/umax (umax = 0.798): (a) for 𝛾 = 0.00, (b) for 𝛾 = 0.01, (c) for 𝛾 = 0.05, and (d) for 𝛾 = 0.10 (from weak viscosity ratio to strong viscosity ratio) when s = 0.48, � = 1, 𝜂1 = 10, 𝑓o1 = 1.0 and 𝑓o2 = 1.0 (with 𝜙 = 1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.5 (with 𝜓 = 1). jeen-hwa wang 8 and (d) for s = 0.48 (i.e., from very weak coupling to weak coupling) when 𝑓o1 = 𝑓o2 = 1.0 (with 𝜙 = 1), 𝑈c1 = 𝑈c2 = 0.5 (with 𝜓 = 1), and 𝜂1 = 𝜂2 = 0 (with � = 1). the value of s is 0.48 in figures 5-7 and figure 9 to explore which factor can separate the motions of the two sliders. figures 5-8 display the results due to different values of viscosity at the two sliders when other parameter are fixed: (a) for � = 0.00 (or 𝜂2 = 0), (b) for � = 0.01 (or 𝜂2 = 0.1), (c) for � = 0.05 (or 𝜂2 = 0.5), and (d) for � = 0.10 (or 𝜂2 = 1) (i.e., from weak viscosity ratio to strong viscosity ratio) when 𝜂1 = 10. in figure 5, the values of other model parameters are � = 1, 𝜂1 = 10, s = 0.48, 𝑓o1 = 𝑓o2 = 1.0 (with 𝜙 = 1), and 𝑈c1 = 𝑈c2 = 0.5 (with 𝜓 = 1). the figure displays the presence of the p wave at slider 2. numerical tests reveal that the p wave at slider 2 cannot be generated especially for � ≥ 0.05 when 𝜂1 > 70, and the solutions are just like figure 4 when 𝜂1 < 5. hence, 𝜂1 is taken to be 10 in figures 6-10. the effect due to different static friction strengths at the two sliders is displayed in figure 6 with � = 1, 𝜂1 = 10, s = 0.48, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 𝑈c2 = 0.5 (with 𝜓 = 1). the figure exhibits the presence of a nucleation phase at slider 1. numerical tests exhibit that when 𝜙 > 1.15, the p wave at slider 2 cannot be generated. hence, 𝜙 is taken to be 1.1 in figures 7-10. the effect due to different characteristic displacements at the two sliders is displayed in figure 7 with � = 1, 𝜂1 = 10, s = 0.48, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2). the figure shows the presence of a nucleation phase at slider 1. numerical tests exhibit that when 𝑈c1 > 0.5, the p wave at slider 2 cannot be generated. hence, 𝑈c1 is taken to be 0.5 in figures 8-10. in order to consider weaker seismic coupling on the simulated waveforms, smaller s is taken into account. numerical tests exhibit that when figure 6. the time sequences of v/vmax (vmax = 0.226) and u/umax (umax = 0.766): (a) for 𝛾 = 0.00, (b) for 𝛾 = 0.01, (c) for 𝛾 = 0.05, and (d) for 𝛾 = 0.10 (from weak viscosity ratio to strong viscosity ratio) when s = 0.48, � = 1, 𝜂1 = 10, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.5 (with 𝜓 = 1). a nucleation phase on a sub-fault 9 s < 0.17, the p wave at slider 2 cannot be generated. hence, s is also taken to be 0.17 in figure 8 with � = 1, 𝜂1 = 10, s = 0.17, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2). figures 9 and 10 display the results for the inertial effect due to different masses of the two sliders: (a) for � = 1, (b) for � = 5, (c) for � = 10, and (d) for � = 30 (i.e., from weak mass ratio to strong mass ratio) when 𝜂1 = 10, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), 𝑈c1 = 0.5 and 𝑈c2 = �.1 (with 𝜓 = 0.2), and 𝜂1 = 10 and 𝜂2 = 0 (with � = 0). the main difference between the two figures is the use of different values of seismic coupling: s = 0.48 in figure 9 and s = 0.17 in figure 10. numerical results show that the maximum values of velocities (vmax) and displacements (umax) are: 0.456 and 1.355, respectively, in figure 4; 0.142 and 0.798, respectively, in figure 5; 0.226 and 0.766, respectively, in figure 6; 0.781 and 1.403, respectively, in figure 7; 0.903 and 1.778, respectively, in figure 8; 0.781 and 1.505, respectively, in figure 9; and 0.903 and 1.790, respectively, in figure 10. in order to conveniently compare simulated results displayed in the panels of a figure, the time variations in velocity and displacement have been normalized, respectively, by related values of vmax and umax. 4. discussion 4.1 seismic coupling effect figure 4 shows the results when s = 0.06, 0.12, 0.30, and 0.48 (upside down). in the left-hand-side panels, v/vmax increases from a small value to the peak and then decreases again to a small one. the dashed line separates from the solid line for small s, while the two lines almost coincide for large s, thus indicating an increase in the degree of coupling between the two sliders with s. meanwhile, the peak velocity appears earlier and is higher at slider 1 than at slider 2, thus exhibiting the presence of directivity effect because the system moves from slider 1 to slider 2. however, this figure does not exhibit the existence of long-period nucleation phase. hence, seismic coupling is not a significant factor in generating nucleation phase. in the right-hand-side panels, u/umax increases from a small value to the final slip. the above-mentioned phenomena about the two curves for v/vmax can also be seen for u/umax. in addition, a small differences in final displacements between the two sliders decrease with increasing s. 4.2 viscous effect based on a one-body ss model, wang [2017a] addressed that a change of viscosity from a larger value to a small one in two time stages during slippage yields the nucleation phase and the p wave, respectively, in the first and second stages. hence, simulations are made by setting 𝜂1 = 10 and letting 𝜂2 = 0, 0.1, 0.5, and 1 or � = 0.00, 0.01, 0.05, 0.10 (i.e., from weak viscosity ratio to strong viscosity ratio) when s = 0.48, 𝑓o1 = 𝑓o2 = 1.0 (with 𝜙 = 1), 𝑈c1 = 𝑈c2 = 0.5 (with 𝜓 = 1), and 𝜂1 = 𝜂2 = 0 (with � = 1). results are shown in figure 5. in the left-hand-side panels, v/vmax at slider 1 increases from a small value to a peak and then decreases again to a small one; while v/vmax at slider 2 increases rapidly from a small one to a peak, then oscillates and decreases again to a small one. the degree of oscillations at slider 2 decreases with increasing �. the dashed line separates from the solid line for the four values of �, thus indicating the viscous effect on the velocity waveforms. that the peak velocity is higher at slider 2 than at slider 1 shows the directivity effect from left to right. the peak velocity at slider 2 decreases with increasing �, yet not at slider 1. although slider 1 started to move earlier than slider 2, the peak velocity comes later at it than at sider 2. the occurrence time of the peak velocity of slider 2 slightly increases with �. the panels exhibit a short-time nucleation phase plus a smaller event at slider 1 and a larger event with a stronger p wave at slider 2. hence, there are two sub-events during the whole rupture process. the appearance of oscillations behind the first peak velocity at slider 2 indicates the existence of few smaller-sized sub-events with lower peak velocities after the main one. the number of such sub-events decreases with increasing �. in the right-hand-side panels of figure 5, u/umax first increases more rapidly at slider 2 (displayed by a dashed line) than at slider 1 (shown by a solid line) and finally two lines merge together, thus exhibiting the same final displacement at the two sliders. it is clear that the dashed line is less smooth than the solid line because of a few oscillations representing smaller-sized sub-events at slider 2. of course, the degree of smoothness increases with �. this is due to a decrease in the number of sub-events with increasing �. jeen-hwa wang 10 although there is a long-period waveform at slider 1 in figure 5, its peak velocity comes after that of a shortperiod p wave at slider 2. this does not exhibit transition from quasi-static motions to dynamic ruptures as shown from observations, and thus the whole waveform at slider 1 cannot be classified to be the nucleation phase. hence, different viscosity coefficients between the two sliders are not the unique factor in yielding the nucleation phase, and thus the differences in other model parameters must also be taken into account. 4.3 frictional effect the frictional effect includes two components: the static friction force represented by the frictional strength (i.e., 𝑓o1 and 𝑓o2 at slider 1 and slider 2, respectively) and the characteristic displacements of friction law (i.e., 𝑈c1 and 𝑈c2 at slider 1 and slider 2, respectively). the results will be explained in three parts below. first, we consider different values of s, 𝑓o1, and 𝑓o2. figure 6 exhibits the results for 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1) when s = 0.48, 𝑈c1 = 𝑈c2 = 0.5 (with 𝜓 = 1), 𝜂1 = 10, and � = 0.00, 0.01, 0.05, and 0.10. the left-hand-side panels show the presence of a very long-duration nucleation phase in the front of a strong p wave of an event at slider 2 and a small event at slider 1. this phenomenon is named as the np-pw pattern hereafter. the duration time of nucleation phase at slider 1 slightly increases with the peak velocity at slider 2. although slider 1 started to move earlier than slider 2, the peak velocity at the former comes later than that at the latter. unlike figure 5, the figure 7. the time sequences of v/vmax (vmax = 0.781) and u/umax (umax = 1.403): (a) for 𝛾 = 0.00, (b) for 𝛾 = 0.01, (c) for 𝛾 = 0.05, and (d) for 𝛾 = 0.10 (from weak viscosity ratio to strong viscosity ratio) when s = 0.48, � = 1, 𝜂1 = 10, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2). a nucleation phase on a sub-fault 11 occurrence time of the peak velocity at slider 2 slightly decreases with increasing �. this might be caused by 𝑓o1 < 𝑓o2. although a small bump appears in the waveform of slider 1, its peak velocity is much smaller than that of slider 2. the difference in the peak velocities between the two sliders decreases with increasing �. hence, unlike figure 5 there is almost only one event in the whole rupture process in figure 6. after slider 2 stopped, slider 1 still moves. meanwhile, the maximum peak velocity is higher in figure 6 than in figure 5. the right-hand-side panels of figure 6 show that u/umax at both slider 1 (displayed by a solid line) and slider 2 (displayed by a dashed line) increases with time. a small event at slider 1 and a large one at slider 2 appear almost simultaneously. unlike figure 5, u/umax at slider 2 suddenly jumps from a small value to the final slip. secondly, we consider 𝑈c1 ≠ 𝑈c2. figure 7 exhibits the results for 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2) when s = 0.48, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), 𝜂1 = 10, and � = 0.00, 0.01, 0.05, and 0.10. the left-hand-side panels of figure 7 show the np-pw pattern. the duration time of nucleation phase at slider 1 only slightly increases with the peak velocity at slider 2. after slider 2 stopped, slider 1 still moves and its peak velocity comes after that of slider 2. the occurrence time of the peak velocity slightly increases with �. the maximum peak velocity of figure 7 is higher than that of figure 6. in addition, the predominant period of p wave of an event at slider 2 is shorter in figure 7 than in figure 6. this might be due to a shorter 𝑈c2 (faster friction drop) in the former than a longer 𝑈c2 (slower friction drop) in the latter. although a peak velocity appears in the waveform of slider 1, its amplitude is very much smaller than that of slider 2. hence, unlike figure 5 there is almost only one event during the whole rupture process in figure 7. in the right-hand-side panels of figure 7, u/umax at slider 1 (displayed by a solid line) figure 8. the time sequences of v/vmax (vmax = 0.903) and u/umax (umax = 1.778): (a) for 𝛾 = 0.00, (b) for 𝛾 = 0.01, (c) for 𝛾 = 0.05, and (d) for 𝛾 = 0.10 (from weak viscosity ratio to strong viscosity ratio) when s = 0.17, � = 1, 𝜂1 = 10, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2). jeen-hwa wang 12 first appears and increases with time; while that at slider 2 (displayed by a dashed line) suddenly appears for a while after slider 1 moves and then jumps to its peak value in a short time. thirdly, it is necessary to consider the effect due to weak seismic coupling (now s = 0.17) between the two sliders when 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), and 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2), 𝜂1 = 10, and � = 0.00, 0.01, 0.05, and 0.10. simulation results are displayed in figure 8. the left-hand-side panels show the np-pw pattern. after slider 2 stopped, slider 1 still moves and its peak velocity comes after that on slider 2. the peak velocity of slider 1 appears much later than that in figure 7. this might be due to a fact that it needs a longer time to trigger slider 2 due to weak coupling between the two sliders in figure 8. meanwhile, the occurrence time of the peak velocity at slider 2 slightly increases with �. the maximum peak velocity is higher in figure 8 than in figure 7, thus indicating that weaker coupling between two sliders can yield a higher peak velocity than stronger coupling. in addition, the predominant period of p wave of an event at slider 2 is shorter in figure 8 than in figure 7. although a peak velocity appears in the waveform of slider 1, its amplitude is very much smaller than that of slider 2. unlike figure 5, there is almost only one event during the whole rupture process in figure 8. in the right-hand-side panels of figure 8, u/umax at slider 1 (displayed by a solid line) first appears and increases with time; while u/umax at slider 2 (displayed by a dashed line) suddenly appears for a while after slider 1 moves and then jumps to its peak value in a short time span. in figures 6-8 the final displacement is higher at slider 2 than at slider 1, but the difference between them decreases with increasing �. figure 9. the time sequences of v/vmax (vmax = 0.781) and u/umax (umax = 1.503): (a) for � = 1, (b) for �= 5, (c) for � = 10, and (d) for � = 30 (from weak mass ratio or inertial effect to strong mass ratio or inertial) when s = 0.48, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2), and 𝜂1 = 10 and 𝜂2 = 0 (with 𝛾 = 0). a nucleation phase on a sub-fault 13 4.4 inertial effect the inertial effect (represented by � = 𝑚2/𝑚1) on the earthquake nucleation is studied for � > 1. simulation results are displayed in figure 9 (with s = 0.48) and in figure 10 (with s = 0.17) for � = 1, 5, 10, and 30 when 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2), and 𝜂1 = 10 and 𝜂2 = 0 (with � = 0). the left-handside panels of both figures 9 and 10 show the np-pw pattern. the duration time of nucleation phase at slider 1 slightly increases with � and slightly decreases with increasing peak velocity at slider 2. after slider 2 stopped, slider 1 still moves and its peak velocity comes after that of slider 2. the occurrence times of the peak velocities at both sliders 1 and 2 in figures 9 and 10 increase with � and are almost similar to those in figures 7 and 8, respectively. the occurrence times of the peak velocity in figure 10 are longer than those in figure 9. this might be due to a need of a longer time to trigger slider 2 due to smaller s between the two sliders in figure 10. meanwhile, the predominant periods of the p wave of an event at sider 2 increases with � as expected. the maximum peak velocity is higher in figure 10 than in figure 9. this indicates that weaker coupling can yield a higher peak velocity at slider 2 than stronger coupling. the peak velocity is lower at slider 1 than at slider 2 and decreases with �, especially for small s. although a peak velocity appears in the waveform of slider 1 in figures 9 and 10, its amplitude is much smaller than that of slider 2. unlike figure 5, there is only one event during the rupture process in figures 9 and 10. figure 10. the time sequences of v/vmax (vmax = 0.903) and u/umax (umax = 1.790): (a) for � = 1, (b) for �= 5, (c) for � = 10, and (d) for � = 30 (from weak mass ratio or inertial effect to strong mass ratio or inertial) when s = 0.17, 𝑓o1 = 1.0 and 𝑓o2 = 1.1 (with 𝜙 = 1.1), 𝑈c1 = 0.5 and 𝑈c2 = 0.1 (with 𝜓 = 0.2), and 𝜂1 = 10 and 𝜂2 = 0 (with 𝛾 = 0). jeen-hwa wang 14 in figures 9 and 10, the peak velocity of p wave of an event at slider 2 decreases with increasing �. numerical tests exhibit that the p wave may become a nucleation phase at slider 2 when � > 30. in the other word, the nucleation phase at slider 1 cannot trigger an event at slider 2 when the mass of the latter is 30 times larger than that of the former. when the densities and fault widths of the two sliders are equal, the fault length of slider 2 is 30 times longer than that of slider 1 when � = 30. since the present model is a strike-slip one, the empirical relationship of earthquake magnitude, m, versus fault length, lf, is: m = (5.16 ± 0.13) + (1.12 ± 0.08)log(lf) [e.g., wells and coppersmith, 1994; wang, 2018]. when 𝜇 = 30 or lf2 = 30 lf1, the magnitudes are m for slider 1 and m + 1.65 for slider 2. hence, a nucleation phase with a magnitude of m cannot trigger an earthquake with a magnitude of m + 1.65. in the right-hand-side panels of figures 9 and 10, u/umax at slider 1 first appears and increases with time; while u/umax at slider 2 suddenly appears for a while after slider 1 moves and then jumps to its peak value in a short time span. the difference in final displacement between the two sliders slightly increases with � and is bigger for small s than for large s. the phenomenon that the final displacement at slider 1 is lower than that at slider 2 might be due to a higher force drop at slider 2 than at slider 1. the duration time of nucleation phase at slider 1 slightly increases with �. from figures 9 and 10, the duration time of nucleation phase is shorter for s = 0.48 than for s = 0.17. this means that seismic coupling can influence the duration time of nucleation phase when � > 1. 4.5 combined effects simulation results show that the four ratios �, 𝜙, 𝜓, and � are important factors in influencing the earthquake nucleation and rupture processes, but the seismic coupling s is a minor one. except for the cases with equal values on the two sliders for the four ratios, the nucleation phase happens at slider 1 and the p wave appears at slider 2. when � > 1, 𝜙 = 1, 𝜓 = 1, and � = 1, there is only a very short-duration nucleation phase and the p wave appears very soon after the generation of nucleation phase. this is inconsistent with figure 1. figures 6-10 exhibit the consistence of simulated results with figure 1. from these figures, the duration time of nucleation phase at slider 1 only slightly increases with � and final displacement at slider 2. the final displacement at slider 2 represents the size of the large event generated at it. this implicates that the duration time of nucleation phase at slider 1 only weakly depends on the size of an event at slider 2. when 𝜓 = 1 or 𝜓 < 1 with � > 1, 𝜙 > 1, and � = 1, there is a long-duration nucleation phase at slider 1 and the p wave appears at slider 2 much lately after the generation of nucleation phase. for 𝜓 = 1 (see figures 4 and 5), the simulated waveforms are consistent with figure 1, but the final displacement of nucleation phase at slider 1 is the same as that of the p wave at slider 2. this indicates equality of energy, which is related to the square of final displacement, at the two sliders. this is questionable, because the energy of nucleation phase is usually lower than that of the mainshock from observations. while, for 𝜓 < 1 (see figures 6-10) the final displacement of nucleation phase is smaller than that of p wave. the difference in the amplitudes between the p wave and nucleation phase decreases with increasing s, increasing �, or decreasing 𝜓. the simulated waveforms are consistent with figure 1. the results are reasonable, because the total energy at slider 1 is less than that at slider 2. when � > 1, 𝜙 > 1, and 𝜓 < 1, the peak velocity of slider 2 decreases with increasing �, and becomes very small when � > 30, even though the final displacement of nucleation phase is still smaller than that of p wave. the degree of similarity of simulated waveforms of these cases (see figures 9 and 10) with figure 1 decreases with increasing �. the upper-bound value of � to yield transition from nucleation phase to the p wave from observations is 30. consequently, the optimal conditions for generating the nucleation phase at slider 1 plus the p wave at slider 2 as displayed in figure 1 are � > 1, 𝜙 > 1, 𝜓 < 1, and � < 30. of course, there are upper-bound values for � and 𝜙 and a lower-bound value for 𝜓 as mentioned in the last sections. note that the upper-bound value of a certain ratio depends on the values of other ratios. 4.6 comparisons with other studies from observations, ellsworth and beroza [1995] and beroza and ellsworth [1996] claimed that the duration time of nucleation phase is positively related to the earthquake magnitude. while mori and kanamori [1996] observed that in a large magnitude range, the p waves of the event behind the nucleation phase are independent on the shape of nucleation phase. figures 6-10 clearly exhibit independence of p wave of an event at slider 2 on the shape a nucleation phase on a sub-fault 15 of nucleation phase at slider 1. the present simulation results are consistent with the observations by mori and kanamori [1996]. numerous authors studied the frictional effect on nucleation phases. tal et al. [2018] numerically studied the effect of fault roughness with amplitude of br on the nucleation process in the presence of a rateand statedependent friction law. the roughness can yields local barriers and makes the nucleation process complicated. they also found an increase in nucleation length with br. considering a broad weak zone with a locally strong asperity on a fault plane, shibazaki and matsu’ura [1995] found that in the dynamic rupture of the asperity, there are aseismic slip and foreshock or pre-event, depending on the peak stress of the asperity, preceding the main rupture and the rupture of the asperity accelerates the nucleation of main rupture. this study indicates the influence of heterogeneous friction strengths on the generation of nucleation phase. schmitt et al. [2015] addressed the importance of timedependent stress heterogeneity on nucleation. the present results for different values of 𝜙 at the two sliders as shown in figures 5-7 may meet those obtained by the three groups of researchers. using an infinite elastic model with slip-dependent friction, ionescu and campillo [1999] found the influence of the shape of the friction law and fault finiteness on the duration of nucleation phase and the duration varies when the fault length has the order of the characteristic length of the friction law. figure 7 is consistent with theirs. based on an infinite elastic model with slip-dependent friction, shibazaki and matsu’ura [1992] assumed that the transition process includes three phases: phase-i for the low quasi-static nucleation, phase-ii for the onset of dynamic ruptured with slow rupture growth in the absence of seismic-wave radiation, and phase-iii for the highspeed rupture propagation with seismic-wave radiation. shibazaki and matsu’ura [1993] further found that the accelerating stage from phase-ii to phase-iii is related to the presence of nucleation phase in the front of the main p wave. their results are similar to those obtained by ueda et al. [2014, 2015] and kawamura et al. [2018]. the results of this study and wang [2017a] only show two stages which are comparable with the phase-i and phase-iii stages proposed by shibazaki and matsu’ura [1992, 1993]. from the analytic solutions of an infinite elastic model with a slip-dependent friction, campillo and ionescu [1997] expressed how the initiation phase determines the transition to the p wave and claimed that the transition is controlled by an apparent supersonic velocity of the rupture front. however, the present result does not meet their conclusion. according to an infinite elastic model with rateand state-dependent friction, segall and rice [2006] divided the weakening processes of ruptures into the nucleation regime dominated by rate and state frictional weakening and a transition regime to thermal pressurization. in the present study, the thermal-pressurized slip-weakening friction is considered during the whole rupture process and the results show a transition from the nucleation phase with smaller 𝑓o1 and smaller 𝑈c1 at slider 1 to the p wave with larger 𝑓o2 and larger 𝑈c2 at slider 2. hence, the present result could be only partly consistent with theirs. however, the main difference between the present study and previous ones is that the nucleation phase appears at slider 1 does not disappear after the presence of p wave at slier 2. this might be due to a use of two fault segments (i.e., a two-body model) in this study and a use of one fault segment (i.e., a one-body or 1-d model) by others. like wang [2017a], both friction and viscosity are considered as important factors in yielding the transition from quasistatic motions to dynamic ruptures in this study; while only friction was proposed in others. 5. conclusions we study the frictional and viscous effects on earthquake nucleation based on a two-body spring-slider model in the presence of thermal-pressurized slip-dependent friction and viscosity. the seismic coupling of the system is represented by the ratio of coil spring k between two sliders and the leaf spring l between a slider and the background plate and denoted by s = k/l. the s is not a significant factor in generating the nucleation phase. the masses of the two sliders are 𝑚1 and 𝑚2, respectively. the frictional and viscous effects at slider i (i = 1, 2) are specified by the static friction force, foi, the characteristic displacement, uci, and viscosity coefficient, 𝜂i, respectively. simulation results show that friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. higher viscosity causes lower particle velocities than lower viscosity. the ratios � = 𝜂2/𝜂1, 𝜙 = 𝑓o2/𝑓o1, 𝜓 = 𝑈c2/ucl, and � = 𝑚2/𝑚1 are four important factors in influencing the generation of nucleation phases. when � > 1, 𝜙 = 1, 𝜓 = 1, and � = 1, the nucleation phase is generated at slider 1 and the p wave appear at slider 2. but, the p wave appears very soon after the generation of nucleation phase. when � > 1, 𝜙 > 1, 𝜓 ≥ 1, and � = 1, the p wave appears much lately after the generation of nucleation phase. when 𝜓 ≥ 1, the final displacement of nucleation phase is almost equal to that of p wave. when 𝜓 < 1, the final displacement of nucleation jeen-hwa wang 16 phase is smaller than that of p wave. the difference in the amplitudes between the p wave and nucleation phase decreases when either s or � increases and 𝜓 decreases. the peak velocity of p wave of an event at slider 2 decays with increasing �, thus suggesting that the inertial effect is important on the rupture processes. consequently, when s > 0.17, � > 1, 1.15 > 𝜙 > 1, 𝜓 < 1, and � < 30 simulation results exhibit the generation of nucleation phase at slider 1 and the formation of p wave of an event at slider 2. the results are consistent with the observations and suggest the possibility of generation of nucleation phase on a sub-fault. this answer the question addressed in this study. figures 6-10 exhibit independence of p wave on the shape of nucleation phase. in addition, the duration time of nucleation phase at slider 1 only slightly increases with � and the size of an event at slider 2. acknowledgments. i would like to express my deep thanks to two anonymous reviewers and sector editor for their valuable comments and suggestions that help me to substantially improve the manuscript. the study was financially supported by central weather bureau (grant no.: motc-cwb-109-e-02) and academia sinica, taiwan. references andrews, d.j. 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(2020). inertial effect on interaction between two earthquake faults, ann. geophys., 63, 2, se221. doi:10.4401/ag-8136 wells, d.l. and k.j. coppersmith (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seism. soc. am., 84, 974-1002. co r r e s po n d i n g a u t h o r: jeen-hwa wa n g, institute of earth sciences, academia sinica, taipei, taiwan and department of earth sciences, national central university, taoyuan, taiwan e-mail: jhwang@earth.sinica.edu.tw http://doi.org/10.1002/ http://dx.doi.org/10.4401%2fag-7063 mailto:jhwang@earth.sinica.edu.tw annals of geophysics, 55 (3), 2012: nikitina geodiversity, and the geoethical principles for its preservation nataliya nikitina llc intergeo managing company, license department, moscow, russia annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5492 abstract the history of the origin and evolution of the term ‘geodiversity’ is described, and its present definitions are given. several types of values of geodiversity are considered. moreover, a new type of value is defined: the information value of geodiversity, as developed from the ethical principles of geodiversity preservation. 1. introduction in the ordinary sense, diversity is the existence of disparate, non-recurring, items and a lack of uniformity. natural diversity is a fundamental feature of nature. it reflects the set of structural and functional characteristics of the natural organization that has been implemented during evolution. it currently ensures sustainable development of planetary life, of the biospheres and geospheres, and it supports ecological balance and ecological stability, while at the same time allowing the development of the true potential of nature. in science, there are several points of view of the concept of ‘natural diversity’: sometimes, it is treated as being synonymous with biological diversity, sometimes as a combination of all species of animals, plants and microorganisms, and of all ecosystems, and their place in ecological processes. accordingly, there are three levels of biodiversity: – genetic diversity (the amount of genetic information contained in the genes of all species of plants, animals and microorganisms); – species diversity (the number of species); – ecosystem diversity (the number of different habitats, biotic communities, and ecological processes). in biology, diversity is associated with the fundamental features of life and its organization, while its proliferation in geosciences originally had a purely pragmatic aspect of preserving natural values. geodiversity and the concept of its preservation are new definitions for most people. the concept that the physical features of the earth (rocks, subsoil, mineral resources) need good management and preservation is not obvious for the majority, as the structures on the surface of the earth and within its subsoil look strong, solid and stable. in the past 15 years, the problems of preservation and use of the geological environment has prompted the formation of geodiversity and landscape diversity concepts. 2. geodiversity and its value in the modern sense, the term ‘geodiversity’ was first used in tasmania, shortly after the adoption of the biodiversity convention at the international summit in rio de janeiro in 1992. however, before 1992, geoscientists used terms such as the “diversity of the earth”, and “geomorphological diversity”. it was a direct consequence of the parallelization with biodiversity and use of terms such as “landscape species” and “landscape community” in biodiversity theory. thus, in the early 1990s when biodiversity became an accomplished event, tasmanian geologists quickly adapted it for the geological equivalent geodiversity, to describe the variety within abiotic nature (table 1) [australian heritage commission 1996, 1999, 2002]. the basic definitions of geodiversity are given in table 1. all of these definitions come from a variety of rocks and minerals, landforms, soils, mineral resources and geological processes, or from the inseparable connections between geologic formations, systems and processes, and the people associated with our natural heritage, such as biodiversity and geodiversity, and our cultural landscapes, and more. note these defining characteristics of geodiversity. first, it involves both endogenous and exogenous geological processes. second, the definition implicitly includes past and present geological time and space. however, all of these definitions have one disadvantage. they do not include the different levels of geological systems, and the complex interactions between these geological systems that do not involve the use of different scales, from global (continents and oceans) to elemental (atoms and ions) [krut’ 1978]. endogenous and exogenous geological processes have a leading role in the formation, preservation and destruction of the variety of subsoils, surfaces and landarticle history received december 1, 2011; accepted february 29, 2012. subject classification: geodiversity, geodiversity value, ethical principles, sustainable development, management of mineral resources. 497 scapes of the earth, because of their great power and duration, which is not comparable with the duration of the existence of the human species. now the human impacts on the earth as a geological body are only related to the lithosphere, which they affect in terms of the following basic activities: – geological surveys and exploration (carried out without any material surface integrity); – mining; – construction and maintenance of underground facilities unrelated to mineral production; – formation of lithospheric technical zones under territories of cities and industrial developments; – dumping of household and industrial wastes, including radioactive and toxic wastes; – lithospheric (geological) weapons that use catastrophic geological processes (earthquakes, volcanic eruptions, large-scale rock falls and landslides) as adverse factors. at the present time, no questions have been raised about the possible limits of this damage, and about the resistance of the lithosphere, and the other geological spheres, to this damage. the creative human activities in the sphere of subsoil use are not so great. these are limited to the special protection of geological features of scientific, cultural, aesthetic, health and recreational values (scientific and training grounds, geological reserves, wildlife and geological sanctuaries, natural monuments, caves and other underground areas). there are six types of values of geodiversity [sharples 1995, eberhard 1997, prosser 2002, gray 2004], which are: – internal or intrinsic value: the intrinsic value goes back to the ethical approval that some things (in our case geodiversity of nature) have value simply because they exist, not only because they can be used by people (utilitarian value); this is the most difficult value to understand and describe, as it involves ethical and philosophical dimensions of the relationships between society and nature; – cultural value: as geomythological, archeological and historical value; – aesthetic value: this is the most tangible, real concept, as it is based on the visual perception of the physical natural environment, the positive physiological and psychological effects of various geological objects on health and human consciousness; – economic value: determined primarily by the need to use subsoil, mineral resources, underground waters, landscapes, and underground cavities in the economic development of civilization; – functional value: something that provides a substrate for the functioning of the terrestrial biosphere, for the existence of the atmosphere, hydrosphere and lithosphere; – research and educational value: this provides the process of scientific cognition of the world, and is the basis for the monitoring of natural processes, including dangerous geological processes, such as earthquakes, floods and landslides. in addition to these six types of values of geodiversity, there is one more: the information value of abiotic nature, by analogy with the genetic diversity of species [nikitina 2011]. each geological object has its own unique geological information, and the complexity of this information is directly dependent on the hierarchical level of the geological object. at the present stage, only a small part of this information is available to our surveying and understanding. every year, tens of thousands of long-term geological carriers are withdrawn from the geological environment. as a result, the geologically complex hierarchy objects, sophisticated geological systems, and accompanying processes are deprived of their future (mineral deposits, nataliya nikitina 498 year author of the definition definition 2004 murray gray the natural range (diversity) of geological (rocks, minerals, fossils), geomorphological (landform, processes) and soil features. it includes their assemblages, relationships, properties, interpretations and systems [gray 2004]. 2004 david roche the variety of rocks, minerals, landforms etc., and the processes that have formed these features through geological time. 2002 collin prosser the variety of rocks, fossils and minerals and natural processes [prosser 2002]. 2002 mick stanley the link between people, landscape and culture; the variety of geological environments, phenomena and processes that make those landscapes, rocks, minerals, fossils and soils that provide the framework for the processes of life on earth [stanley 2002]. 1997 rolan eberhard the natural range (diversity) of geological (bedrock), geomorphological (landform) and soil features, assemblages, systems and processes. geodiversity includes evidence for the history of the earth (evidence of past life, ecosystems and environments), and a range of processes (biological, hydrological and atmospheric) that are currently acting on rocks, landforms and soils [eberhard 1997]. 1995 chris sharples the range (or diversity) of geological (bedrock), geomorphological (landform) and soil features, assemblages, systems and processes [sharples 1995, houshold and sharples 2008]. table 1. basic definitions of geodiversity. 499 ore formations). new mechanisms for stabilizing the crustal processes are originated, which lead to the activation of deep faults. this withdrawal of complex hierarchical objects and sophisticated geological systems from the lithosphere without explaining their relevance to the geological and geophysical evolution of the earth can be potentially considered as the start of planetary disorganization. 3. geoethical principles for geodiversity preservation apart from the introduction of the term geodiversity into the geosciences, the major theoretical achievements of “the geodiversity doctrine” are: 1. the six core principles of sustainable development and use of mineral resources [gray 2004] are similar to the principles of “deep ecology” that were suggested by norwegian philosopher alexander naess in 1974 [naess 1984, 1994]: accept that natural change is inevitable, work with the natural functions and processes, manage natural systems within the limits of their capacity, manage natural systems in a spatially integrated manner, use non-renewable earth resources wisely and sparingly and at a rate that does not restrict future options. use renewable resources within their regeneration capacity. 2. classification of value types of geodiversity and creation of the concept of ‘nature and geological heritage’. 3. develop a rationale for objective necessity to maintain geodiversity because of the responsibility of living generations to future generations, and recognize the hard benefit of sustainable development of natural resources to mankind and to the natural world. 4. action-oriented recommendations for geodiversity conservation and practical realization [limestone landscapes 2010, zwolinski 2010]. in general, the geodiversity doctrine is a significant achievement in geosciences. however, due to the insufficient development of the ethical foundations of this doctrine, we can see contradictions between its basic elements. for example, the recognition of the internal (intrinsic) value of abiotic nature essentially means that people do not have the right to reduce the geodiversity; in other words, people do not have the right to exploit objects that are abiotic in nature, or to interfer in the organization processes that occur in abiotic nature. in this case, any proposed options for preservation of geodiversity will constantly lose competition to the mining industry, because revenues from this sector are obvious and tangible. the proposed principles for the preservation of geodiversity do not contain any indications of the boundaries of the right to exist. what geological features, objects, systems, processes and phenomena “have value simply because they exist”? how does this relate to the objective necessity for human use of mineral resources for the maintenance of the existing human civilization? should we stop the spread of geological processes that are dangerous for us? in developing the ethical principles of geodiversity preservation, i have tried to rely not on an abstraction of the universal validity of moral norms, but on their potential realizability. 1. being a part of nature, people should not re-organize geological systems, particularly on a global and regional scale. it is permissible to interfere with natural geologic processes on a local level in cases where they are a threat to human life. 2. people have no right to reduce geodiversity, except to meet basic needs. 3. recognition of the need to establish effective limits of consumption and use of mineral resources, as components of the geological environment, on the basis of stabilization of the world population; to substitute the linear degradation of mineral resources using renewable resources and the recycling of mining wastes. 4. opposition to an unregulated market economy, especially if it is a mineral-resources-based economy. 5. the use of mineral resources, and of objects and components of the geological environment should be based on the recognition of the objective laws of development and interactions between the geospheres and society, on the modern scientific and technological achievements, on the combination of possibilities of geospheres, and on the economic interests of society, providing real guarantees for the rights of citizens to benefit from the use of the subsoil (scientific principle). 6. policies, strategies and tactics of geological heritage preservation should be formed as a complex interactive system of institutions and individuals: governments, public and social organizations, on the global, national and regional levels; geoscientists, geoexperts, and ‘geological heritage consumers’ (visitors to geoparks and geological museums, tourists). 7. in the ‘man-and-abiotic-nature’ system, the whole takes precedence over the singular, such that if the use of a geological object does not to lead to a decrease in the geodiversity of higher geosystems, then that object can be used). our proposed variant of ethical principles for geodiversity preservation is not the only feasible theory. we hope that our colleagues and/or ‘opponents’, especially for the ethics, will offer other projects, following the logic of which we will have an opportunity to improve the geodiversity concept. 4. conclusions thus, in contrast to the concept of biodiversity, the term geodiversity, its value, and the need for its conservation, has not yet entered the broad academic community. geodiversity, and the geoethical principles for its preservation however, it is clear that in addition to biodiversity conservation, geodiversity (and in the future, the variety of animate and inanimate nature) should also be a priority in any human activity on earth. it is characteristic of the geological diversity, and ethical guidelines for its conservation should serve as a basis for policy, as we aspire to manage our mineral resources to ensure sustainable economic development. undoubtedly, in the third millennium, we have to search for new geoethical ideals in our subsoil use. references australian heritage commission (1996). australian natural heritage charter, 1st ed., canberra, australian heritage commission. australian heritage commission (1999). natural heritage places handbook, canberra, australian heritage commission. australian heritage commission (2002). australian natural heritage charter, 2nd ed., canberra, australian heritage commission. eberhard, r., ed. (1997). pattern and process: towards a regional approach to national estate assessment of geodiversity, canberra, australian heritage commission. gray, j.m. (2004). geodiversity. valuing and conserving abiotic nature, chichester, john wiley & sons ltd. houshold, i., and c. sharples (2008). geodiversity in the wilderness: a brief history of geoconservation in tasmania, geol. soc. london, spec. publ., 300, 257-272. krut’, i. (1978). introduction to the uniform theory of the earth, levels of geosystems organization, nauka, moscow (in russian). limestone landscapes (2010). geodiversity action plan 2010-2015, geology and landscape england programme: http://www.limestonelandscapes.info/pages/publica tions.aspx naess, a. (1984). a defense of the deep ecology movement, environ. ethics, 6, 265-270. naess, a. (1994). the deep ecological movement: some philosophical aspects, in: environmental ethics, edited by l.p. pojman, boston-london, jones and bartlett publisher, 411-421. prosser, c. (2002). term of endearment, earth herit., 17, 12-13. nikitina, n. (2011). ethical principles and the valuable approach for preservation of geodiversity, viii forum italiano di scienze della terra, torino, september 1923, 2011, abstract v. 274. sharples, c. (1995). geoconservation in forest management: principles and procedures, tasforest, 7, 37-50. stanley, m. (2002). geodiversity – linking people, landscape and their culture. abstract for natural and cultural landscape conference, dublin, royal irish academy, 14. zwolonski, z. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice l ' a n n é e g é o p h y s i q u e i n t e r n a t i o n a l e et l ' é v o l u t i o n de l ' i o n o s p h è r e d'après les d o n n é s de casablanca • rabat a . h a u b e r t r i c e v u t o i l 3 0 n o v e m b r e 1 9 6 2 i n t r o d u c t i o n . afin de situer l'année géophysique internationale dans l'évolution cyclique de l'activité solaire, nous avons rassemblé, pour chacun des douze mois de l'année, les courbes des variations diurnes des médianes horaires mensuelles des fréquences critiques et des hauteurs virtuelles obtenues depuis le début des sondages ionosphériques au maroc, c'est-àdire, depuis septembre 1951. l'effet saisonnier est ainsi éliminé et l'influence du cycle solaire apparaìt plus n e t t e m e n t (fig. 1). on constate, p a r les courbes des fréquences critiques, que le m i n i m u m d'activité solaire s'est produit u n peu a v a n t j a n v i e r 1955 et que le nouveau cycle s'est manifestò dès mai 1955, la remontée des fréquences critiques é t a n t particulièrement b r u t a l e en mars-avril 1956. e n ce qui concerne les hauteurs virtuelles, p a r contre, l'influence du cycle solaire parait minime, celles-ci a y a n t seulement tendance à ètre un peu plus élévées à l'époque du m a x i m u m . nous reviendrons sur certaines particularités dans ce qui suit. r e g i o n f i . cotiche f2. a) hauteurs virtuelles. j u s q u ' e n 1956, les hauteurs virtuelles de nuit de la région f étaient classées comme h'f2 et les courbes reproduites obéissent à cette convention de continuité. a p a r t i r de 1957, les hauteurs virtuelles de nuit sont classées avec les ex-h'f1 de jour, sous la désignation unique de h'f, h'f2 é t a n t réservé à la stratiftcation la plus élevée de la région f p e n d a n t le jour. 106 a . i i a d b e r t on remarquera le peu d'influence de l'activité solaire sur les h a u t e u r s virtuelles de n u i t ; p a r eontre la variation diurne présente p e n d a n t le jour et s u r t o u t en été, des aspects différents. les liauteurs de jour sont dans l'ensemble plus élévées au m a x i m u m q u ' a u m i n i m u m d'activité solaire, mais ce qui caractérise s u r t o u t le m a x i m u m , c'est la formation en été le m a t i n d ' u n e stratification apparaissent en h a u t e altitude et descendant rapidement ensuite a v a n t de suivre u n e évolution diurne normale jusqu'a sa fusion avec le soir. i l est à noter q u ' u n e telle évolution qui est suffisamment f r é q u e n t e au m a x i m u m d ' a c t i v i t é solaire pour a p p a r a ì t r e dans les médianes se manifeste parfois de fagon exceptionnelle loin de ce m a x i m u m . i l en a été ainsi en 1952, le 6 mars, 11 j u i n , 4 et 5 october et en j u i n et juillet 1954. le m é m e phénomène a été constaté p a r eastogi à a h m e d a b a d en décembre 1955 et p a r e s t r a b a u d à bangui, lors de l'eclipse du 25 février 1952. b) fréquences critiques. celle-ci suivent de fagon évidente le cycle d'activité solaire. on r e m a r q u e r a que c'est en février 1955 que les / 0 p 2 commencent à se relever, le m i n i m u m se s i t u a n t e n t r e octobre 1954 et j a n v i e r 1955. nous avons calculé, pour chaque mois, de 1951 à 1958, et à p a r t i r des médianes horaires mensuelles de f0f2, les quantités: les valeurs obtenues, portées sur u n graphique en fonction des moyennes mensuelles du nombre de wolf, f o r m e n t un ensemble de points s'alignant assez grossièrement le long d ' u n e droite, la dispersion semblant obéir à u n effet saisonnier. nous avons alors calcidé les moyennes glissantes de p et de r (la moyenne de treize mois é t a n t affectée au mois centrai). nous avons ainsi obtenu le graphique reproduit figure 2, lequel correspond à la loi suivante: p = 0,03 r + 4,5 p o u r m e t t r e en évidence pellet saisonnier, nous avons tracé la courbe des écarts de p avec sa moyenne glissante p et nous avons obtenu le graphique reproduit figure 3. on constate q u ' à u n e période annnelle vient se superposer u n e période semi-annuelle lorsque l'activité solaire devient f o r t e (r > 100). les m a x i m u m s se situent vers les équinoxes et les minimums vers les solstices d ' é t é et d'hiver. le m i n i m u m du sols— vesr i è w r « m * 'v : f \ p \ "" \ ~ ~?$56 -1 * v < 1 1 \ \ \ ! — , „ — w ' v r \ " ' ì ^ v l i : i • i j a : r : /. ; :< • f , i • 7 r * « a<>j -va j > t v , . , . . a ' , —-j — s 4 v > ' r " 1 ' t ^ ! . i . 7 t \ a > r jm r fig. 1 v a r i a t i o n des f r é q u e n c e s critiques ( a. 1iaubert 7*\ 1/7-n u r l r\ \ <" /s . v » h • t i a t»s3 t&s» w s 1 jt\ ' fi /t\ *s> ! ) t 5 1 " — a -j ^ «• • • rj » v y a « 3 v 3 r , i a » n « ' / f — x f — . — i r il ^/fn un "so /v hv-'x*-'e jsl a «ir " 'i \ v n a 1 1 v * 1 t n» • * * i —r u vp/ * i "a l i a u t ) e t des h a u t e u r s virtuelles (en bas). l ' a n n é e g é o p h y s i q u e i n t e r n a t i o n a l e , e t c . 4 0 1 en f o n c t i o n des m o y e n n e s g l i s s a n t e s c o r r e s p o n d a n t e s d u n o m b r e de w o l f . 4 0 2 a . i i a d b e r t tice d'hiver s'explique p a r la h a u t e u r zénithale solaire, mais celui d ' é t é ne pourrait ètre expliqué qu'en i'aisant appel à d ' a u t r e s processus comme, p a r exemple, l'expansion thermique. anomalies. nous avons poursuivi l'observation des anomalies de l'ionosphère dont les plus remarquables ont été signalées dans le bulletin mensuel du laboratoire ionospérique du maroc (1). e n ce qui concerne la région f (f1 et f2), elles ont été classées ainsi: a) envolées, c'est-à-dire élévation rapide et anormale de la h a u t e u r virtuelle b) fourches c) ergots d) composante z e) dilìusion 1 sur t o u t e la g a m m e des fréquences occupée palla trace l ' a n n é e g é o p h y s i q u e i n t e r n a t i o n a l e , e t c . 4 0 3 2 sui' les fréquences proches de la fréquence critique seulement (généralement avec bord inférieur net) 3 sur les fréquences supérieures à la fréquence critique (région g diffuse) 4 diffusion en óventail 5 diffusion p a r déplacement en fréquence de t o u t e la trace /) doublé trace. yoici quelques précisions q u a n t aux aspects de ces anomalies: a) envólées: ne sont pas signalées les envolées qui se produisent quotidiennement en été vers 10 heures, mais les envolés sporadiques de 05 h et de 21 li (2), ainsi que celles qui se produisent à des heures t o u t à fait imprévues correspondant généralement à un é t a t magnétique perturbò; le phénomène est souvent mondial, mais certainement, envolées ont un caractère plus locai dont la cause n ' a pas été élucidée. b) fourches: nous appelons fourches les branches supplémentaires au voisinage de la fréquence critique et a y a n t une courbure analogue aux branches normales ordinaire et extraordinaire. c) ergots: les ergots se distinguent des fourches en ce qu'ils se détachent des branches principales en suivant une courbure qui leuiest propre. les écarts entre les fréquences critiques des ergots et des branches normales, sont les plus souvent des sous-multiples de la gyrofréquence. nous avons eu, depuis lors, l'occasion de faire des enregistrements accélérés (un sondage toutes les dix secondes). les vues sur film de 16 m m sont ensuite projetées à cadence cinématographique. cela nous a permis de suivre l'évolution du phémomène, en particulier, au lever du soleil. alors, qu'en général, les courbes se déplacent de fagon continue au f u r et à mesure de la montée du soleil au-dessus de l'horizon, il arrive certains jours q u ' u n ergot apparaisse brusquement près de la fréquence critique et descende rapidement le long de la trace, sa propre courbe se substituant finalement à l'ancienne. le phénomène p e u t se répéter plusieurs fois, ou mème s'inverser, au cours d ' u n lever solaire. là où il est le plus remarquable, c'est lorsque l'ergot de la branche ordinaire vient se superposer à la branche extraordinaire de telle fagon qu'à la fin de l'évolution, la branche ordinaire nouvelle a pris la place de la branche extraordinaire, une branche nouvelle extraordinaire s ' é t a n t formée simultanément. nous avons t o u t lieu de penser qu'on se trouve en face d ' u n phénomène de dérive engendré p a r des champs électrostatiques extérieurs, liés à un é t a t ionosphérique perturbé. 4 0 4 a . i i a d b e r t d) gomposante z: le phénomène est rare à la l a t i t u d e de casablanca, trop rare mème pour rechercher quelques corrélations avec d'autres phénomènes géophysiques. seule u n e simultanéité de l'apparition de la composante z en d ' a u t r e s latitudes pourrait présenter quelque intérèt. e) diffusion: ce phénomène est r a r e au maroc, il est généralement lié a u x fortes p e r t u r b a t i o n s magnétiques: cependant, nous avons observé, p a r calme magnétique, de la diffusion en hiver au cours de périodes anormalement froides. l a diffusion du t y p e 1 est probablement due à la turbulence en dessous de f. la diffusion du t y p e 5 est probablement due à u n e réflexion oblique se superposant à la t r a c e normale. f) doublé trace: ce phénomène semble d ù à une réflexion oblique sur une ride de l'ionosphère, il est cependant difficile d'expliquer pourquoi les échos d'ordre 2 sont dédoublés avec le m è m e écart en h a u t e u r que les échos d'ordre 1. ce phénomène doit ètre de grande étendue car il a été observé les mèmes nuits à casablanca et à poitiers. i i . c o u c h e f i . la stratiflcation de la région f en 7'\ et f2 est devenue de moins en moins m a r q u é e avec l'accroissment de l'activité solaire. l a mesure de / . f i est devenue de plus douteuse et le nombre des mesures possibles de plus en plus restreint. nous donnons ci-dessous quelques valeurs des coefficients p et fm de la formule de chapman: / = fm (cos z)p, obtenues g r a p h i q u e m e n t : 1957 juin juil. aout s ept. oct. nov. dèe. v (0,47) 0 , 2 9 0,22 — — — — u (6,56) 6,12 6 , 1 0 — — — — 1958 j anv. fév. mars avril mai juin juil. v — — — 0 , 3 8 0 , 2 2 0 , 2 8 0 , 2 6 u — — — 7,59 6,49 6 , 4 0 6 , 2 4 1958 aout sept. oct. nov. dèe. janv. 1959 v 0 , 2 1 0,32 — — — — fm 6 , 5 3 6,92 — ' — — — l ' a n n é e g é o p h y s i q u e i n t e r n a t i o n a l e , etc. 4 0 5 r é g i o n e i . c o t i c h e et. yoici les valeurs des paramètres de la formule de chapman pour la région e normale: 1957 juin juil. aout sept. oct. nov. dèe. v 0 , 3 5 0,34 0,35 0 , 3 1 0 , 3 0 0 , 3 3 0 , 2 9 u 4 , 2 3 4,19 4 , 1 0 4 , 2 1 4 , 3 0 4 , 6 6 4 , 5 8 1958 janv. févr. mars avril mai juin juil. v 0 , 3 0 0,34 0,30 0,29 0,32 0 , 3 1 0 , 3 2 u 4 , 5 8 4 , 4 0 4,27 4,12 4 , 2 7 4 , 2 4 4,17 1958 aout sept. oct. nov. dèe. janv. 1959 v 0,32 0 , 2 8 0 , 2 5 0 , 3 3 0 , 3 3 0,32 u 4,27 4,17 4 , 0 4 4 , 3 1 4 , 4 4 4 , 5 2 i i . c o t i c h e e2. nous avons poursuivi son observation au cours de l'année géophysique internationale et le caractère mondial de ses manifestations se trouve de plus en plus confirmé; nous renvoyons le lecteur à nos précédentes publications citées en référence en a t t i r a n t à nouveau son attention sur l'intérèt d ' u n e étude générale du comportement de cette région intermédiaire entre la région f et la région e. i h . c o t i c h e es. avec l'année géophysique internationale, est entrée en vigueur la distinction des différents types d ' e „ mais si l'on fait la somme des occurrences en fonction de l'heure, sans distinction de type, on constate que la courbe de densité d'occurrence présente: a) un m i n i m u m a v a n t le lever du soleil b) une montée rapide au lever du soleil c) une décroissance au milieu du jour (qui peut d'ailleurs n ' è t r e q u ' a p p a r e n t e p a r suite de l'absorption). d) u n deuxième m a x i m u m après le coucher du soleil. e) u n palier moyen ensuite, se m a i n t e n a n t jusqu'au creux qui précède le lever du soleil. si m a i n t e n a n t , on distingue les différents types &'es, on trouve q u a t r e types diurnes: e,h, esc, esi, ess, un t y p e nocturne e,f, et un t y p e indifférent esq. 4 0 6 a . i i a d b e r t les types diurnes sont essentiellement éphémères et les fréquences m a x i m u m v a r i a n t de facon erratique. le t y p e nocturne est plus stable et il est génóralement formé p a r la dégénérescence d ' u n e formation plus épaisse, se détachent de f t dans la journée et descendant progressivement au cours de la deuxième p a r t i e de la journée. cette formation est cataloguée soit e2, soit esh suivant l'aspect des ionogrammes. l'ionisation sporadique de la région e semble avoir plusieurs causes. la fréquence d'occurence de es est beaucoup plus grande en été qu'en hiver et les fréquences m a x i m u m s observées sont aussi plus élévées en été qu'en hiver; ainsi le nombre de jours p a r mois où f0es, en 1957, a dépassé 5 mc/s a a t t e i n t 30 en j u i l l e t et est descendu à 3 en février; et celui où f0es a dépassé 10 mc/s a a t t e i n t 11 en juillet alors qu'il est resté nul en j a n v i e r , février, mars, avril, octobre, novembre, décembre. la comparaison de divers graphiques laisse a p p a r a i t r e une tendance à une corrélation des pointes de e, avec le passage de centres d'activité solaires sur le méridien, a fortiori lorsque le passage de ces centres est accompagné d ' u n orage magnétique. l ' a u g m e n t a t i o n de la fréquence m a x i m u m , dans la soirée, alors que le soleil baisse sur l'horizon ou a mème disparu, demande l'intervention de processus d'ionisation autres que le rayonnement direct du soleil. l ' a u g m e n t a t i o n de la densité m a x i m u m d'ionisation coincidant avec l'abaissement et l'amincissement de la couclie, on peut invoquer soit une a u g m e n t a t i o n de densité p a r compression, soit u n e ionisation p a r choc, soit encore u n détachement p a r choc à p a r t i r d'ions négatifs. r e g i o n d la faible puissance du sondeur ne p e r m e t pas d'obtenir des échos sur les basses couches de l'ionosphère, mais l'enregistrement de champs de stations de radiodiffusion constitue un moyen d'investigation de ces régions. e n effet, l'augmentation de l'ionisation aux altitudes de l'ordre de 70 à 80 k m , sous l'effet de la h a u t e u r du soleil au dessus de l'horizon, fait varier l ' a m p l i t u d e moyenne du charnp en ondes décamétriques de fagon régulière. l a loi de variation de l'absorption, en fonction de la h a u t e u r zénitale solaire, est de la forme: d = a cos" z . l'expórience effectuée au maroc sur la réception d'allouis (6200 kn/s) a donné pour n la valeur de 0,79 (3). l ' a n n é e g é o p h y s i q u e i n t e r n a t i o n a l e , e t c . 4 0 7 e n dehors de la variation régnliére quotidienne, on observe des anomalies brutales lors des éruptions cliromospkériques. les ondes décamétriques sont brusquement affaiblies ou totalement eoupées p e n d a n t une clurée de quelques niinutes à plusieurs dizaines de rainutes. un phénomène concomitant se produit en ondes longues (4), mais eelles-ci sont t a n t ò t renforeées, t a n t ó t affaiblies. nous avons été surpris de constater que sui' la fréquence de 200 kc./s, en été, ce sont les renforcements qui. dominent, .alors. qu'en lnver c e sont les évanouissements (*). on sait que l'absorption d'une onde électromagnétique t r a v e r s a n t un plasma est proportionnelle à l'expression ty0 2v/(a)2 + v2) où co0 est la pulsation du plasma traversò, v la fréquence des cliocs et co la pulsation de l'onde incidente. si v < co, l'expression peut se réduire à cuov/eo2 et l'absorption est alors proportionnelle au nombre des ehocs. si, au contraire, v > co, l'espression se réduit à co02/v et l'absorption est, dans ce cas, inversement proportionnelle au nombre des chocs. p o u r l'onde de 200 kc/s, co = 1,25 io6. on sait q u ' u n e éruption chromosphérique est accompagné d ' u n rayonnement ultraviolet intense, c'est-à-dire que la proportion de photons d'énergie élevée, augmente. toute énergie dépassant le potentiel d'ionisation é t a n t transformée en energie cinétique, la vitesse quadratique moyenne des électrons libres doit augmenter sous l'effet du rayonnement de l'éruption et p a r conséquent le nombre des chocs doit augmenter. puisqu'en été, l'absorption diminue lors d ' u n e éruption chromosphérique, il faudrait que v soit à cette époque supérieur à 1,25 io8 et qu'il augmente plus vite que n \ p a r contre, puisqu'en hiver l'absorption augmente sous le mème elfet, il f a u d r a i t que v soit alors inférieur à 1,25 io6. la mesure de l'altitude à laquelle se produit l'absorption est d'importance primordiale et des moyens nouveaux, comme p a r exemple les fusées ou les satellites artifìciels, apporteront sans doute, des réponses plus directes aux questions posées. stj m mary in order to set the international geophysical year in the cyclie évolution of solar activity, we have summarized each month of the year the curves of daily variations of the monthly hourly means of the criticai frequencies and virtual heights obtained after the beginning of ionospheric soundings in marocco, i.e. since september 1951. 4 0 8 a . i i a d b e r t the seasonal effects is thus eliminated and the influence of solar cycle appears more evident. it is observed that due to the curves of criticai frequencies, the minimum solar activity occurred not long before january 1955, and that the new cycle has been happening since may 1955, a restarting of criticai frequencies was particularly accentuated durìng march-april 1956. on the contrary, as regards virtual heights, the influence of the solar cycle appears minimum, this latter tenda to assume higher vàlues only during the maximum period. riassunto allo scopo di inquadrare l'anno geofisico internazionale nell'evoluzione ciclica dell'attività solare abbiamo riassunto, per ciascuno dei dodici mesi dell'anno, le curve delle variazioni diurne delle medie orarie mensili delle frequenze critiche e delle altezze virtuali ottenute dopo l'inizio dei sondaggi ionosferici nel marocco, cioè dal settembre 1951. l'effetto stagionale viene così eliminato e l'influenza del ciclo solare appare più netta. si constata, per le curve delle frequenze critiche, che la minima attività solare si è avuta poco prima del gennaio 1955 e che il nuovo ciclo si è manifestato dal maggio 1955, con una ripresa delle frequenze critiche particolarmente accentuata durante i mesi di marzo-aprile 1956. per quanto concerne le altezze virtuali, al contrario, l'influenza del ciclo solare appare minima, con tendenza quest'ultima ad assumere valori più elevati solamente all'epoca del massimo. b i b l i o g r a p h i e (*) haubert a . , l'ionospjière à casablanca — n o t e p r é l i m i n a i r e d u l . n . r . , n ° 168, 1953. « a n n a l e s du serv. de p h y s . du globe e t de mèteo, de l ' i n s t . se. chérifien», x i v , 65-71, (1955), x v i i , 103-110, (1957), x i x , 73-77, (1959). t1) haubert a . , sur un phénomène de marèe ionosphérique observée à casablanca. « a n n . du serv. de p h y s . du globe e t de mèteo, du maroc », x v i , 165-168, (1956). (3) haubert a . , la partìcipauon du maroc à l'étude des relations soleilionosphère dans le cadre de vannée géophysique internationale. « a n n . du serv. de p h y s . du globe et de métèo. du m a r o c » , x v i i i , 137-144. (1958). (4) haubert a . , le renforcement du cliamp en ondes longues et les évanouissements en ondes courtes, « j. a t m o s , a n d terr. p h y s . », 13, 379-381, (1959). (5) haubert e . , les perturbations ionosphériques et la propagation des ondes longues, « j. a t m o s . a n d terr. p h y s . » , 18, 71-72, (1960). vol49_2_2006 767 annals of geophysics, vol. 49, n. 2/3, april/june 2006 key words submarine volcanism – submarine volcanic explosions – deep-sea fan – acoustic transparent layers – carbonate crusts 1. intoduction the vst02 cruise, carried out on board the r/v urania from 29 august to 19 september 2002, focused on the study of sedimentologic and volcanologic issues through the investigation of selected areas in the southeastern and central portion of the tyrrhenian sea (fig. 1). in the southestern tyrrhenian sea, the cruise investigated the slopes that surround the volcanic stromboli island, the stromboli sea valley and the eastern portion of the marsili basin. the aim of the research was the detailed study, through the acquisition of high resolution seismic data and seafloor samples, of the architectural elements and the sedimentary facies of the eastern portion of the stromboli valley and of the marsili basin. a further related target was to test whether the deep submarine areas around stromboli island could contribute to the study of the evolution of the stromboli volcano, with particular reference to its explosive eruptions and collapse events. an additional objective was the characterization of a seamount (casoni seamount) located southwest of stromboli island at a water depth of around 1000 m. in the central tyrrhenian sea, the cruise focused on the study of the vavilov basin and the homonymous volcanic seamount that sits in its centre. dredging was carried out on the summit and flanks of the vavilov edifice to obtain samsedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 fabiano gamberi (1), michael marani (1), vladimiro landuzzi (1), angelo magagnoli (1), daniela penitenti (1), mauro rosi (2), antonella bertagnini (2) and alessio di roberto (2) (1) istituto di scienze marine (ismar), cnr, sezione di geologia marina, bologna, italy (2) dipartimento di scienze della terra, università degli studi di pisa, italy (3) istituto nazionale di geofisica e vulcanologia, sede di pisa, italy abstract the vst02 cruise carried out in the summer of 2002 focused on sedimentologic and volcanologic research over selected areas of the deep portion of the tyrrhenian sea. chirp lines and seafloor samples were collected from the calabrian slope surrounding stromboli island, in the marsili deep-sea fan, in the vavilov basin and in the vavilov seamount. submarine volcanic activity, both explosive and effusive, is occurring in the stromboli edifice. explosive submarine volcanism also affects the shallowest areas of the vavilov seamount. submarine carbonate lithification has been observed on the sediment-starved flanks of the vavilov seamount. acoustic transparent layers make up the recentmost infill of the gortani basin, the easternmost portion of the vavilov basin. channels comprised of a variety of architectural elements and depositional lobes are the main elements of the marsili deep-sea fan where, apparently, sedimentation occurs mainly through debris flow processes. mailing address: dr. fabiano gamberi, istituto di scienze marine (ismar), cnr, sezione di geologia marina, via gobetti 101, 40129 bologna, italy; e-mail: fabiano.gamberi@ismar.cnr.it 768 fabiano gamberi et al. ples from the different volcanic elements that compose the seamount. the expected result, through geochemical study of lava samples, is the framing of the vavilov seamount volcanism in the geodynamic evolution of the tyrrhenian sea. in addition, high resolution seismic lines and gravity-core samples were collected in the vavilov abyssal plain with the aim of deciphering the processes that contribute to its recent sedimentary infill. this paper reports the details of the cruise aims and strategy and the location of the sampling sites and of the chirp lines. in addition, the description and interpretation of the acquired data, as deduced by on board observations, are illustrated together with a presentation of the preliminary results. 2. cruise strategy: geophysical data acquisition and sampling equipment the research topics, the relevant areas to investigate during the cruise, the techniques of data acquisition and the research methodology were mainly planned on the basis of a preparatory interpretation of multibeam echosounder bathymetric data (fig. 1), deep-towed sidescan sonar images and seismic lines already available at the ismar. during this preliminary work, new sedimentary and volcanic elements, such as the marsili deep-sea fan, a seamount in the southern slope of stromboli, the individual volcanic building blocks that make up the vavilov seamount were recognized for the first time. in addition, further details of other, alfig. 1. colour-shaded bathymetric map of the tyrrhenian sea. the areas targeted by the investigations carried out during the vst02 (boxes) are detailed in the subsequent figures. 769 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 ready known seafloor features were revealed and used to plan the vst02 cruise. the aims of the cruise were fulfilled through the acquisition of high-resolution seismic lines and seafloor samples. chirp lines were collected during night time. the chirp signal was in the low frequency, ch1 band (2-7 khz). the resulting data were plotted in real time with an epc9800 and acquired in seg-y format on magneto optical disks. the high resolution seismic data were aquired with the twofold aim of defining the depositional elements of the studied area, one of the main targets of the research, and of selecting the sampling sites to characterize their distinctive sedimentary facies. sampling operations were carried out through dredging, gravity coring and box coring. dredging with a conventional, cylindrical, iron-chained, toothed dredge was performed in the areas of supposed rocky seafloor, namely on the vavilov seamount and on the casoni seamount south of stromboli island. a 1300 kg gravity corer was utilized in sedimented areas; the core tube length, determining the maximum recoverable length, was selected on the basis of the presumed sediment texture. only a few of the cores were opened and described on board. in areas where gravity coring did not allow any recovery, due to coarse-grained sediments at the seafloor, a box corer was utilized; it allowed the recovery of samples with a diameter of 32 cm, with a maximum recovery of 50 cm deep sections. the samples collected through box coring were described on board and subsample cores were obtained for further laboratory analysis. 3. calabrian and stromboli slopes, stromboli valley and marsili basin (figs. 2, 3) 3.1. geological setting and aims of investigation stromboli island is the northernmost subaerial volcanic edifice of the aeolian islands fig. 2. traces of chirp profiles (thin lines) and location and number of seafloor samples (circles = gravity cores; squares = box cores; rhombs = dredgings) in the area surrounding stromboli island (si – stromboli island; ls – lametini seamounts; sv – stromboli valley; vf – volcaniclastic fan facing the sciara del fuoco; cs – casoni seamount). 770 fabiano gamberi et al. arc. its base, lying at a water depth of around 3000 m to the north, shallows toward the south in the area that connects the submarine portions of the stromboli and panarea edifices, where two seamounts sit at a depth of 700 and 1000 m, respectively (fig. 2). the shallowest seamount, named cavoni, was already known and sampled (gabbianelli et al., 1993). no data were available for the deepest one, that we informally called casoni seamount (cs in fig. 2), a target of the vst02 cruise. a further target was that of sampling tephra layers due to the activity of stromboli volcano. the stratigraphy of the volcanic products of stromboli island is well established (hornigkiarsgaard et al., 1993) and high resolution studies that outline its recent (<2000 years) eruptive style, with particular emphasis on explosive activity, are available (rosi et al., 2000). research on the reunion island volcano have, however, disclosed that the study of submarine cores, due to a potential of tephra preservation in the deep marine environment higher than on land, can provide a refined stratigraphy of explosive events (fretzdorff et al., 2000). with this in mind, one of the aims of the cruise was that of collecting cores in selected, undisturbed seafloor sites in order to establish a correlation between marine and on-land tephrostratigraphic records. the submarine flanks of the stromboli edifice are the sites of predominantly volcaniclastic deposition. in particular, a volcaniclastic fan is developed in the area facing the sciara del fig. 3. traces of chirp profiles (thin lines) and location and number of seafloor samples (symbols as in fig. 2) collected in the eastern marsili basin. bold lines correspond to the line segments shown in fig. 4a-c. (dsv – distal part of the stromboli valley; ms – marsili seamount). 771 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 fuoco (vf in fig. 2), where the present-day activity of the stromboli volcano results in abundant volcaniclastic debris entering the sea (kokeelar and romagnoli, 1995; gamberi et al., 2002). the composition, distribution and facies variations of volcaniclastic deposits in the submarine environment adjacent to island arcs reflect the complex interplay between the tectonic evolution of the arc and the surrounding basins, the rate and type of volcanic activity and the nature of the transporting processes (carey, 2000). in particular, when accurate sedimentologic and geochemical studies of volcaniclastic deposits are carried out, it is possible to discriminate between primary syn-eruptive deposits that are directly related to submarine or subaerial pyroclastic flows and secondary deposits, that, on the contrary, result from multiphase reworking (schneider, 2001). in addition, the marine environment, being the ultimate depositional site of landslides and related sediment-gravity flows, can be the best repository of information about collapse events. this implies that the deep-sea depositional systems surrounding stromboli island can constitute a good archive of information concerning the recent history of the volcano. hence, the areas surrounding stromboli island and of the marsili basin were sampled with the purpose of assessing, through sedimentologic and geochemical studies, the signature of collapses and eventual, related explosions that have characterized the history of stromboli volcano (bertagnini and landi, 1996; tibaldi, 2001). finally, the cruise aimed at investigating the eastern portion of the marsili basin. the western and northern base of the stromboli edifice is circled by the stromboli valley (sv in fig. 2), a main morphologic feature that nucleates in the axis of the gioia forearc basin and crosses the aeolian volcanic arc between the stromboli edifice and lametini seamounts (ls in fig. 2). at a depth of around 3000 m, along a steep escarpment that marks the western boundary of the calabrian margin slope, the stromboli canyon enters the eastern part of the marsili back-arc basin (dsv in fig. 3). on the basis of multibeam data, a deep-sea fan characterized by downslope convex contours and scoured by channels down to its distal part, located at the base of the marsili seamount (ms in fig. 3), has been recognized (fig. 3). the fan is around 40 km long and around 20 km wide. it is presumably fed by coarse-grained deposits, representing a modern analogue of small-sized sand-rich fans that are among the most important hydrocarbon reservoirs in the world (weimer, 2004). the recently increasing efforts for exploration of deep-water clastic depositional systems require predictive geological models that much rely on the outcomings from researches on present-day analogues. the detailed study of the marsili fan was, therefore, undertaken to define its architectural elements and their sedimentary facies with the aim of contributing to the characterization of similar, ancient depositional systems. 3.2. chirp profiling three chirp lines were acquired over the ew trending sector of stromboli valley north of the stromboli island (fig. 2). they show that the volcaniclastic fan that develops in the area facing the sciara del fuoco, progradated over the stromboli valley floor. at a depth of around 3000 m, the stomboli valley loses much of its relief and gives way to various channels that represent the feeding elements of the marsili basin deep-sea fan, which is the target of the majority of the seismic lines acquired over the southeastern study area. seven regional transverse lines, tied by 4 longitudinal composite transects, were collected (fig. 3); chirp data furnish a complete coverage of the marsili deep-sea fan from a depth of around 3000 m to its distal, western part at the base of the marsili seamount. three segments of the lines are presented in fig. 4a-c. much of the fan is characterized by a strongly reflective, rough sea-bottom suggesting that it is mainly floored by coarse-grained sandy or gravelly material and highly disorganized depositional bodies with poor lateral continuity. the different architectural elements that characterize the deep-sea fan can be recognized in the seismic lines. three main channels run on the surface of the fan and die out at contrasting water depth, the central one (fig. 4a) reaching the deepest por772 fabiano gamberi et al. tion of the basin. terraces, intra-channel bars, inner thalwegs and depositional levees with evidence of sediment waves and seafloor instability are common elements of the channels (fig. 4a-c). the channels gradually lose relief downslope; at their mouth, upward convex, up to 10 km long and 4 km wide depositional bodies are developed and can be interpreted as depositional lobes (fig. 4b). in some cases, smaller scale (1 km large 2-3 m thick), lenticular acoustic transparent bodies are present on the surface of the lobes and can represent the single depositional units that make up the lobes (fig. 4c). 3.3. seafloor sampling (figs. 2, 3) two cores (c02 and c03) were retrieved from the calabrian margin, (fig. 2) in sites located away from the main sedimentary pathways and downwind from the main prevailing winds. these sites are thought to have the best potential for the preservation of tephra layers derived from the explosive activity of stromboli volcano (fig. 2). these cores mainly consist of a hemipelagic succession but preliminary analysis has disclosed that thin tephra layers are indeed present at various levels along the sedimentary succession. in addition, frequent discontinuous patches representing remnants of tephra layers reworked by the intense bioturbation that characterizes much of the section, have been observed. in the southern slope of the stromboli edifice (bc15; fig. 2), the recovery of rounded volcaniclastic gravels, similar to those outcropping on the beaches of the island, has shown that the stromboli submarine flank is here floored by the deposits of sedimentgravity flows that transfer coarse-grained material down-slope from the shore source. four gravity and one box cores were collected in the distal portion of volcaniclastic fan fig. 4a-c. a) chirp line crossing the central channel of the marsili fan (cm – channel margin; cf – channel floor; ib – intra-channel bar; t– terrace). b) chirp line crossing two depositional lobes (dl) of the marsili fan that are flanked by a channel with an intrachannel longitudinal bar (ib). c) chirp line crossing the northernmost channel of the marsili fan (l – levee; cf – channel floor; it – inner thalweg; t – terrace). a b c 773 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 that faces the sciara del fuoco and in the marginal areas of the stromboli valley north of stromboli island (fig. 2). two gravity cores (c06 and c05) did not recover sediment, likely evidence of hard or very coarse sea-bottom. the recovery of 40 cm of massive coarse sands (box core bc07), and of alternating sandy and silty layers (core c04) indicates that the main sedimentation style is represented by coarsegrained sediment-gravity flows. the c16 site was located directly to the north of the sciara del fuoco volcaniclastic fan, around 200 m above the stromboli valley floor (fig. 2). this site was chosen with the aim of sampling a peculiar type of deposit related only to sedimentgravity flows originated from the slope of the stromboli edifice and capable of overriding the stromboli valley margin. the recovery of a thick layer of black volcaniclastic sand with abundant glass fragments whose origin is tentatively ascribed to stromboli volcano, suggests that this kind of flows do occur and poses important hints on their genetic processes. in order to characterize the sedimentary facies of the distinct architectural elements recognized through the interpretation of the chirp lines in the marsili fan, 7 gravity and 8 box cores were obtained from the eastern portion of marsili basin (fig. 3). box cores bc24 and bc25 have revealed that the proximal parts of the channels that feed the marsili fan are floored by a coarse-grained seafloor with cross-stratified layers of gravel-sized pumice and quartz grains. c08 gravity core was successful in recovering around 80 cm of massive sand from the middle portion of the fan; c10 and c09 did not recover sediment. as a consequence, sampling of this area was mainly accomplished through box coring (bc23, bc22, bc20, bc19, bc21, bc17) collected in the distal portion of the channels, in the longitudinal bars, and in the lobes. the recovered sediments mainly consist of poorly sorted muddy sands with abundant floating mud clasts (with dimensions up to more than 10 cm) interpreted as the result of debris flow processes. well sorted sandy layers with evidence of traction, likely resulting from turbidity currents, were also found. gravity coring was more successful in the distal part of the fan with recovery of respectively 5.25 m (c13), 3.31 m (c11) and 0.90 m (c14) while no recovery was achieved by core c18. we dredged (d01) the southern flank of the stromboli edifice to verify the nature of the casoni seamount, located to the southwest of the stromboli island at around 1000 m depth (cs in fig. 2). beside fresh lava samples, various pieces of decimeter-size glassy vescicular material were recovered embedded in mud and interpreted as volcanic scoriae produced by submarine explosive activity. the freshness of the samples indicate a very recent volcanic activity and, more interestingly, the lava samples were warm when taken on board. 4. vavilov basin and seamount 4.1. geological setting and aims of investigations the vavilov basin is thought to be the oldest area floored by oceanic crust in the tyrrhenian sea (kastens and mascle, 1990). it has a mean depth of around 3500 m and the homonymous seamount rises at its center. the vavilov basin is bounded by several structural ridges with variable heights above the basin floor. in addition, the d’ancona ridge and the vavilov seamount separate the vavilov basin into the gortani and magnaghi basins (fig. 5). the gortani ridge, connected to the south to the vavilov seamount, lies in the central portion of the gortani basin (fig. 5). the chirp seismic facies of the recent infill of the northern portion of the gortani basin (fig. 5) was calibrated by three gravity cores. the vavilov seamount is 33 km long and 17 km wide, with a relief of ca. 2700 m. its base lies at depth variable between 3500 m, to the east and 3200, to the west, while its highest peak is at around 800 m. a peculiar feature of the volcano is a strong asymmetry with a steep, smooth western side and a more gently sloping and more rough eastern flank. few lava samples were recovered from the vavilov seamount before cruise vst02 (selli et al., 1977; robin et al., 1987; savelli, 1988); an extensive sampling of the different elements recognized on the basis of mutibeam bathymetric maps of the volcano was therefore undertaken (fig. 6). circular features, with dimensions up to 1 km in width and 200 m in height, are pres774 fabiano gamberi et al. f ig . 5. t ra ce s of c hi rp p ro fi le s (t hi n li ne s) a nd g ra vi ty c or es c ol le ct ed i n th e g or ta ni b as in , th e no rt he as te rn p or ti on o f th e v av il ov b as in ( g b – g or ta ni b as in ; g r – g or ta ni r id ge ; d r – d ’a nc on a r id ge ; m b – m ag na gh i b as in ; v s – v av il ov s ea m ou nt ). t he b as in f lo or p re se nt s n n e -s s w tr en di ng l in ea m en ts t ha t, as e vi de nc ed b y th e ch ir p li ne s ar e du e to a ct iv e fa ul ti ng . f ig . 6. l oc at io n m ap o f dr ed ge t ra ns ec ts c ar ri ed o ut o ve r th e v av il ov s ea m ou nt . 5 6 775 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 ent on the distal volcano flanks, particularly to the east. these features have been interpreted as single volcanic centres and were one of the targets of the seafloor sampling. the majority of the sampling sites were however located on the uppermost part of the volcano. here numerous dredges were carried out with the aim of recovering samples from the elongated culminations and the intervening saddles that compose the nne-ssw trending axial portion of the volcano. 4.2. chirp profiling around 450 nautical miles of chirp high-resolution seismic profiles were acquired in the gortani basin (fig. 5), achieving the penetration of around 60 m of subbottom sediments. as shown by the two lines in fig. 7a,b, the prominent features of the seismic lines are 4 acoustic transparent layers (atls). the deepest atl (datl) has a thickness of ca. 25 m and its base, lying at a depth between 50 and 60 m, coincides generally with the penetration limit of the seismic energy (fig. 7a,b). an intermediate atl (iatl) with a thickness of around 15 m directly overlies the datl or a thin discontinuous reflective package that is sometimes present on top of the datl. both the deepest and the intermediate atls are recognized throughout the basin although thickness variations are especially evident approaching the basin bounding highs. the two topmost atls (uatls) are separated from the intermediate atl by a thick, well reflective sequence. they pinch out considerably toward the gortani seamount and stretch across two distinct depocenters in the eastern and western portion of the gortani basin, respectively. active faulting is evident in the chirp lines, resulting in seafloor steps; faults have a nne-ssw trend corresponding to the lineaments in the bathymetric map in the basin floor (fig. 5). 4.3. seafloor sampling three sites were dredged on the eastern base of the volcanic edifice at depths between 3500 fig. 7a,b. a) chirp line of the western gortani basin. b) chirp line over the eastern gortani basin. the deepest and the intermediate atls have a basin-wide distribution and can be correlated in the two basin sectors separated by the gortani ridge (datl – deepest atl; iatl – intermediate atl; uatls – uppermost atls; f – active fault). a b 776 fabiano gamberi et al. fig. 8a-d. selected samples of hardground crust paving some sectors of the vavilov seamount: a) and b) from the southern flank; c) and d) from the southewestern flank. fig. 9a-d. a) explosive volcanic scoriae from the base of the southern axial summit of the vavilov edifice: b) breccia sample from the upper western flank of the volcano; c) explosive volcanic scoriae from the northwestern vavilov edifice; d) hydrothermal mn precipitates. a b dc a b dc 777 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 and 3200 m: three dredgings related to circular features with diameter of around 1 km (d28 and d30 on the same dredging transect; d29) and one dredging was attempted on ridges perpendicular to the slope of the edifice (d57); all dredges were recovered empty. the same result was obtained along the dredging transect d53, on the western base of the edifice, at a depth between 3100 and 2700. this can be possibly taken as evidence that even if the individual volcanic elements of the deep volcano flanks have a distinct morphologic expression they are however relatively old and presumably draped by sediments. three dredgings from the southeastern flank of the volcano, at depth between 3000 m and 2300 m (d42, d43, d44), recovered various types of carbonate crusts (hardgrounds). the carbonate crust recovered in site d42 displays a smooth upper surface, inferred to be exposed at the seafloor due to the presence of encrustant organisms, and of a pervasive black fe-mn oxides coating (fig. 8a); the lower surface is rough (fig. 8b) and no variations in the crust hardness are appreciable from the top surface toward the base (fig. 8b). the carbonate crust is mainly composed of small bivalve, gasteropod and pteropod shells within a fine grained matrix mainly consisting of foraminifer tests. boring is intense with vertical traces, up to few centimeters in diameter, that cut the whole crust. in section, the colour is dull brown with small scale bioturbations evidenced by black stainings and thin lineations. horizontal traces, with length up to 10 cm and diameter of 2 cm, sometimes filled by an unconsolidated carbonate ooze, also occur along the lower surface of the crust. the carbonate crust of dredge d43 is ca. 15 cm thick; its upper portion is similar to d42 crust (fig. 8c) but its lower surface is not sharp and rather displays a transition between slightly cemented to soft foraminiferal ooze (fig. 8d). the carbonate crust recovered in d44 site is also similar to the d43 crust. these samples have been interpreted as carbonate hardgrounds formed under sediment-starved conditions. such lithification appears to be pervasive along significan parts of the volcano edifice. dredgings d40, d45, d61 were carried out along the upper flank of the southern axial culmination of the edifice between 1700 and 1400 m depth. the only recovery was a thin veneer of mud sticking to the dredge d40. two seafloor transects were conducted on the upper portion of the western flank of the volcano between 2200 m and 1700 m depth. dredging d52 was carried out to the south while dredging d49 and d50 were carried out along the same dredging line in the central area. dredge d52 recovered a cemented sand, composed of highly oxidized foraminifera tests and black sand-sized volcaniclastic grains, together with four pieces of fresh, black glassy material with vesicles up to 1 cm in diameter. the largest glassy piece has dimension of 12 × 8 × 5 cm and displays a flat surface with scattered red mud blobs (fig. 9a). white gravel-sized clasts at times displying a pomi-ceous structure are embedded in the glassy samples. the glassy samples have been interpreted as scoriae produced by explosive activity in the upper portion of the submarine edifice; a similar genetic process can be the source of the volcaniclastic grains that make up the cemented sand. we also recovered thin, ca. 2 cm-thick, hard carbonate crusts, with sharp and tabular upper and lower surfaces and abundant bioturbation. these crusts do not present broken margins and have circular or ellipsoidal shapes with dimension up to 15×10 cm (fig. 10a). these samples are tentatively interpreted as carbonate nodules forming within unconsolidated sediments in response to localized lithification at or below the seafloor. d50 and d49, from the same transect, recovered pieces of a poorly sorted breccia and carbonate nodules, similar to those found in dredge d52. the breccia is composed by highly oxidized lava clasts with dimensions up to 5 cm floating within a micritic carbonate or sand clasts (fig. 9b). seven dredgings were carried along the topmost axial portion of the edifice from 1700 to 900 m depth. two dredgings in the northern portion (d60 and 54) only collected unconsolidated mud with fragments of the solitary coral desmophyllum cristagalli. on the northwestern side two dredgings, d51 and d33, resulted unproductive, while a large altered lava fragment was collected at station d48. in the same station, nodules similar to those found in dredge d49 and d52, were collected. two lava samples were col778 fabiano gamberi et al. fig. 10a-d. a), b), c), d) selected samples (dredge station shown in figure) of carbonate nodules. fig. 11a-d. a), b), c), d) selected samples of lavas (dredge station shown in figure). note the highly altered appearence and the ubiquitous mn coating of the samples. a b c d a b dc 779 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 lected also in the southern portion (d32 and d39: fig. 11a). dredging d39 recovered a scoria similar to that of station d52 suggesting that explosive activity recently occurred also along this portion of the submarine edifice. a framework of corals and serpulids cemented by a carbonate matrix was found associated with the lavas in dredges d32 and d39 and in d59. furthermore, a hardground similar to those of station d44 was collected by dredging d59 from the eastern flank. eight dredges were collected on the northern axial culmination of the edifice. d34 and d27 carried out on a conical feature at depth between 1200 and 1000 collected framestones (fig. 12a). lava fragments were collected both from the western (stations d35 and d46: fig. 11b) and the eastern flanks (d55). in addition, a volcanic scoria was collected at site d55 (fig. 9c). unconsolidated mud was collected on station d26, on the western side of the vavilov, while framestones were recovered at station d41 and d36, from the other flank. carbonate nodules were collected on dredges d35 and d46 (fig. 10b-d). dredging d35 recovered also a thin soft mn-crust with alternation of highly cemented laminae with metallic luster and more porous laminae with filamentous structure (fig. 9d). this sample is interpreted as evidence that hydrothermal activity occurred recently on the volcano. five dredgings were carried out on the northernmost part of the elevated axis of the volcano. d31 was empty; three dredgings recovered lava samples from the western side (d47, d38 and d37: fig. 11c,d). thick hardground crusts, similar to those found in the deeper portion of the southern flank, were collected at stations d47 and d56. framestones were collected by dredgings d47 and d37 (fig. 12b-d). 5. discussion and conclusions 1) in the area of the stromboli valley, we obtained evidence that some sediment-gravity fig. 12a-d. selected samples of framestones. an evolution from corals d) and c) that are filled by sediments and then lithified b) and a) is envisaged for their formation. b d a c 780 fabiano gamberi et al. flows generated from the sciara, are at times capable of surmounting the 200 m high northern margin of the canyon valley and depositing sediments in the adjacent slope. although there is still uncertainty about the height of an obstacle that can be surmounted by a turbidity current as a function of flow thickness (kneller and buckee, 2000), flows with a minimum thickness in the order of 100 m are likely responsible for deposition as observed at site c16 site. this, in turn, poses important constraints on the events responsible for the initiation of those flows, that must be linked to the higher-energy processes affecting the submarine portion of the stromboli flank. 2) the marsili fan is channelized down to its distal part and lobes develop both in its central and distal parts. inner thalwegs, longitudinal intra-channel bars, terraces and outer levees are the architectural elements that compose the channels. preliminary interpretation of box cores indicate that the depositional style is mainly dominated by debris flow processes. 3) the recovery of still warm lavas and fresh scoriae from the casoni seamount evidences that the present-day stromboli magmaplumbing system triggers also submarine activity, both effusive and explosive at ca. 1000 m depth. recent explosive activity is documented also for the vavilov seamount, where fresh scoriae were found in numerous sampling sites, located preferentially along the top of the volcano. the combined evidence of casoni and vavilov seamounts indicates that explosive activity in the tyrrhenian submarine volcanoes is a common process also at water depth in the order of 1000 m. the freshness of the scoriae sampled from the vavilov seamount markedly contrasts with the highly altered lava samples obtained from adjacent sites. this indicates that the recent volcanism of this submarine volcano occurs mainly through explosive activity. we hypothesize, for some centres of the vavilov seamount, a scenario where a phase of effusive eruption is followed by the upwelling of a volatile rich magma, through the accumulation of gas bubbles below a volcanic plug and by the consequent triggering of volcanic explosions, as proposed for seamounts of the mid ocean ridge (hekinian et al., 2000). 4) the recovery of fresh hydrothermal mnprecipitates indicate that hydrothermal activity is still occurring in the vavilov seamount. 5) in the deep flank of the vavilov volcano, where dredging transects gave no results, the volcanic constructs are old and sediment covered even though they still have a morphologic expression. submarine carbonate lithification is a widespread phenomenon occurring on the vavilov seamount, as documented by the ubiquitous presence of carbonate crusts. lithification occurring at or below the seafloor takes place under varying conditions leading to the formation of nodules, hardgrounds, and framework crust. a progression from nodule formation within unconsolidated sediments to thick hardground crust, growing from the seafloor downward and resulting in a pavement at the seafloor is envisaged. once the seafloor is lithified or where volcanic rocks crop out, corals can grow on the hard substratum and build cemented framework crusts. these findings confirm that lithified carbonate crusts are frequent in areas characterized by sediment-starved conditions, as already recognized in the mediterranean sea and particularly in the tyrrhenian sea (selli, 1970; allouc, 1990; remia et al., 2004). 6) chirp investigation of the vavilov basin evidenced that its recentmost sedimentary infill mainly consists of thick acoustic transparent layers. whatever the process responsible for the deposition of the atls, the seismic data suggest that the vavilov basin is affected by the same sedimentary processes that are active in the other deep abyssal plains of the mediterranean basin, where analogous seismic horizons have been recognized (hieke, 2000; rothwell et al., 2000). finally, chirp lines have shown that active faulting, with a nne-ssw direction, affects the basin floor. acknowledgements we thank captain nicolangelo lembo, officers and crew of the r/v urania. thanks are also due to andrea argnani and marco taviani that helped in improving the quality of our paper. we are also grateful to francesco frugoni, editor of this volume. 781 sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary results of cruise vst02 references allouc, j. 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(2004): petroleum systems of deepwater settings, eage, distinguished instructor series no. 7, pp. 470. layout 6 possible precursor to the march 11, 2011, japan earthquake: ionospheric perturbations as seen by subionospheric very low frequency/ low frequency propagation masashi hayakawa1,2,3,*, yasuhide hobara2,4, yoshihiro yasuda3, hiroki yamaguchi5, kenji ohta6, jun izutsu6, tohru nakamura7 1 university of electro-communications, advanced wireless communications research center, tokyo, japan 2 university of electro-communications, research station on seismo-electromagnetics, tokyo, japan 3 university of electro-communications, hayakawa institute of seismo-electromagnetics co. ltd., tokyo, japan 4 university of electro-communications, graduate school of informatics and engineering, tokyo, japan 5 earthquake analysis laboratory, information systems inc., minato-ku, tokyo, japan 6 chubu university, kasugai aichi, japan 7 kochi university, akebono kochi, japan annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5357 abstract this study reports on a possible very low frequency/low frequency (vlf/lf) subionospheric precursor to a recent earthquake in japan. as the epicenter of this large japanese earthquake on march 11, 2011, was located just on the great-circle path from one of our vlf/lf network stations (chofu) to the nlk us transmitter, we examined the propagation characteristics mainly associated with the signals from the nlk transmitter, as observed at three of the stations in japan (chofu, kasugai and kochi). on march 5 and 6, 2011, a remarkable anomaly was found on the path from nlk to chofu, which is highly likely to have been a precursor to this earthquake. the anomaly in the night-time average amplitude at chofu was characterized by a serious decrease in the signal that exceeded −4v (v: standard deviations). the anomaly was found on the same days on the other propagation paths (from nlk to both kasugai and kochi), although it was less enhanced. finally, this propagation anomaly is extensively discussed with respect to the geomagnetic activity, and we also compare this anomaly with the properties related to the former 2004 sumatra earthquake that had nearly the same magnitude as this march 11, 2011, earthquake. 1. introduction very low frequency/low frequency (vlf/lf) subionospheric propagation data have recently been used extensively to monitor lower ionospheric perturbations associated with earthquakes [e.g., hayakawa 2007, hayakawa 2009, hayakawa 2010, chakrabarti 2010]. there have thus been a substantial number of studies published on such seismo-ionospheric perturbations. a recent study by hayakawa et al. [2010] established a statistically significant correlation of ionospheric perturbations detected as vlf/lf subionospheric propagation anomalies with earthquakes with large magnitudes (>6.0) and shallow depths. this was a statistical study on the basis of long-term (seven years) data recorded in and around japan. that study can be considered as further confirmation of our previous statistical studies [gokhberg et al. 1989, rozhnoi et al. 2004, maekawa et al. 2006, kasahara et al. 2008], which examined shorter periods of data, of the order of a few years. case studies of different large earthquakes are also of vital importance for investigations into the temporal and spatial characteristics of seismo-ionospheric perturbations. as was recently described in a review by hayakawa [2009], these case studies include: (i) the izu peninsula march 1997 earthquake swarm; (ii) the september 25, 2003, tokachi-oki earthquake; (iii) the october 23, 2004, niigata-chuetsu earthquake; (iv) the 1999 chi-chi earthquake in taiwan; and (v) the 2004 sumatra earthquake; among others. the present study concerns the latest earthquake in japan, on march 11, 2011. 2. the 2011 tohoku earthquake there was an extremely large earthquake (magnitude, 9.0) under the sea bed in the pacific ocean off the tohoku area of japan, which is formally named as the 2011 earthquake off the pacific coast of tohoku. this earthquake took place at 14:46:18 lt on march 11, 2011, with its article history received august 9, 2011; accepted september 29, 2012. subject classification: earthquake prediction, ionospheric perturbation, vlf/lf propagation. 95 special issue: earthquake precursors epicenter at the geographic coordinates (36˚6.2�n, 142˚51.6�e), as shown in figure 1, and with a depth of ca. 20 km. this earthquake was a very typical oceanic earthquake of the plate type, which are very different from fault-type earthquakes, such as the kobe earthquake, the niigatachuetsu earthquake, and so it was of great concern. 3. the japanese vlf/lf network the japanese and pacific network for subionospheric vlf/lf propagation was established just after the 1995 kobe earthquake, within the framework of the former nasda frontier project (hayakawa et al., 2004). the main observatories involved at present are: (1) moshiri (msr) in hokkaido; (2) chofu (chf) and (3) kasugai (ksg) near nagoya; and (4) kochi (kch) and (5) tsuyama (tym), okayama, as shown by the stars in figure 1 (except for tym). some additional observatories are planned to be built shortly. at each station, we normally detect the signals from the two japanese transmitters with call signs of jjy (in fukushima; 40 khz) and jji (in miyazaki, kyusyu; 22.2 khz), as shown by the diamonds in figure 1. we can also detect the signals from a few foreign transmitters (e.g., nwc in australia; npm in hawaii; and nlk in the us). the details of this vlf/lf network can be found in hayakawa et al. [2004] and hayakawa [2007, 2009, 2010]. 4. observational results and analysis method figure 1 illustrates the path from jjy to msr, along with its corresponding 5th fresnel zone, as the wave sensitive area, and the three paths from nlk (seattle, usa) to the other japanese observatories (chf, ksg and kch). furthermore, the 5th fresnel zone for the propagation path from nlk to chf is shown, which is the wave sensitive area for this path and which is much larger than that for the path from jjy to msr, as the nlk–chf propagation distance is much larger than that for the jjy–msr path. these wave sensitive areas mean that any earthquakes that take place within the wave sensitive area can have a certain significant influence on the signals received at the observatory, which is termed a propagation anomaly (either in amplitude or in phase, or for both). the analysis in this study follows the night-time fluctuation method [e.g., hayakawa et al. 2010], which focuses only on the night-time amplitude data. we monitored the temporal evolution of amplitude a(t) at a current time t during the local night-time on a particular day, while <a(t)> was estimated as the average amplitude at the same time t during the period from one day to 30 days before the current day. then, we estimated the difference da(t) = a(t) — <a(t)>. using this difference, we can estimate the most important parameter, the 'trend', as the night-time average amplitude (mean of the da(t) during the local time). the second parameter is the dispersion, which is characterized by how much the amplitude fluctuates around the average, and the third parameter is the integration of da(t) <0 during the night, the night-time fluctuation. all of these parameters are normalized by their corresponding standard deviations over —30 days to —1 day before the current day. further details can be found in kasahara et al. [2008] and hayakawa et al. [2010]. for the definition of the night-time period, we take the ut period of ut = 11 h —19 h for the propagation path from jjy to msr. however, the definition of night-time is considerably complex for the east-west long-distance propagation from nlk to the japanase stations (such as chf). by considering the sunrise and sunset both at the transmitter and at the receiving observatory (i.e., the terminator times) [hayakawa et al. 1996] and also by checking the actual diurnal variations for the relevant nlk– chf path, we took ut = 9h to 14h for the night-time for the nlk-chf path (i.e., the only period when the propagation path was completely in the dark). 4.1. no precursor propagation anomaly for the jjy–msr path as shown in figure 1, a previous earthquake known as the 2005 miyagi-oki earthquake (august 16, 2005; magnitude, 7.2) occurred very close to the wave sensitive area of the jjy–msr propagation path (with the same magnitude as the foreshock on march 9, 2011). here, we observed very significant precursor ionospheric perturbations on this propagation path [muto et al. 2009]. however, the epicenter of this march 11, 2011, earthquake was shown to be located considerably far from the hayakawa et al. 96 figure 1. relative locations of the jjy transmitter and the moshiri (msr) station, with the wave sensitive area. also, the great-circle paths for the nlk (us transmitter) to japanese receiving stations of chofu (chf), kasugai (ksg) and kochi (kch) are shown. the corresponding wave sensitive area is only shown for the nlk–chf path. the epicenters of the three relevant earthquakes are shown: (i) the august 16, 2005, miyagi-oki earthquake (2005/8/16); and (ii) the march 9, 2011, foreshock, and the march 11, 2011, main shock earthquakes (2011/3/9,3/11). 97 wave sensitive area of the jjy–msr path, as this earthquake occurred ca. 150 km away from the coast line. this might suggest that we would not expect any perturbation on the jjy– msr path for this earthquake. figure 2 illustrates the temporal evolution of the propagation characteristics over the jjy–msr path during the time period up to this march 11, 2011, earthquake. we pay attention only to the period from march 1 to march 9, so before the earthquake. in this period there was definitely no time interval in which the trend showed a decrease, together with the simultaneous increases in the dispersion and the night-time fluctuation parameters. hayakawa et al. [2010] indicated that a propagation anomaly is characterized by a decrease in the trend, with simultaneous increases in the dispersion and the night-time fluctuation parameters. the experimental observation of no precursory anomaly appears to be very consistent with the initial theoretical expectation, as judged from the position of this earthquake relative to the propagation path. 4.2. significant propagation anomalies associated with the propagation paths for the us transmitter nlk figure 1 suggests that the propagation paths from japanese receiving stations (chf, ksg and kch) to the us nlk transmitter are favorably located with respect to the epicenter of this march 11, 2011, earthquake. in particular, the nlk–chf path passed just above the earthquake epicenter, and the corresponding wave sensitive area for this nlk–chf path is shown as a dotted line. two other propagation paths, from nlk to ksg and from nlk to kch, were also favorable for us to note any corresponding ionospheric perturbations. in response to these theoretical expectations, figure 3a-c illustrates the actual temporal evolutions of the propagation ionospheric perturbation for the 2011 japan earthquake figure 2.the temporal evolution of the propagation characteristics for the path of jjy–msr during the period from january 1 to march 12, 2011. pay attention only to the period from march 1 to march 12 to define any precursor to the march 11, 2011, earthquake. top: trend (average night-time amplitude). middle: dispersion (as a measure of the amplitude fluctuation). bottom: night-time fluctuation, as defined by the night-time integration of the area of da(t)<0. the gray areas indicate periods with no observations. figure 3. temporal evolution of the propagation characteristics for the three propagation paths: (a) nlk–chf; (b) nlk–kch; and (c) nlk–ksg. the period from january 1 to march 12, 2011, is shown. pay attention to the period from march 1 to march 12, to define any precursor to the march 11, 2011, earthquake. (a, b, c) top: trend (average night-time amplitude). middle: dispersion (as a measure of the amplitude fluctuation). bottom: night-time fluctuation, as defined by the night-time integration of the area of da(t)<0. any significant anomaly should be indicated in (a), (b) and (c). characteristics for these relevant paths; figure 3a shows the nlk–chf path, figure 3b, the nlk–kch path, and figure 3c, the nlk–ksg path. in each panel of figure 3 the three physical parameters are shown: from top to the bottom, the trend, dispersion and night-time fluctuation, and these parameters are all normalized by their corresponding standard deviations (v). with the propagation path of nlk– chf in figure 3a, we can pay attention to the period before the earthquake on march 11, 2011. we can clearly note a significant propagation anomaly on the two days of march 5 and 6, 2011. in particular, the propagation anomaly on march 5 is characterized by a remarkable decrease in the trend (exceeding —3v, or even more), together with simultaneous increases in the second parameter (dispersion) and in the night-time fluctuation. the corresponding anomaly can also be recognized in figure 3b for the propagation path of nlk–ksg. the anomaly for the path of nlk–kch in figure 3b is particularly evident because the most important parameter, the trend, showed a significant decrease, reaching the —2v level. on the other hand, the anomaly for the path of nlk–ksg in figure 3c is less enhanced on the same days of march 5 and 6, 2011, although the response to this earthquake is very evident. 5. summary and discussion by making full use of our japanase vlf/lf network, we have defined the following observational results for this large march 11, 2011, earthquake: 1. no definite anomaly was observed for the jjy–msr path, as the earthquake epicenter was well away from the wave sensitive area of this propagation path. 2. there was a significant propagation anomaly on march 5 and 6 (about 5-6 days before the earthquake) especially for the nlk–chf path. this anomaly is characterized by a remarkable decrease in the trend, together with clear enhancements in the dispersion and the night-time fluctuation. the same anomaly was also observed for the other two paths: nlk–kch and nlk–ksg. 3. this earthquake was a consequence of an oceanic earthquake that was due to the movement of the plates, so that the result in the present study is the first report for any large plate-type earthquake in the sea. we also discuss further the above summary. first of all, we ask whether the anomaly on march 5 and 6, 2011, was really a precursor to the march 11, 2011, earthquake. the most serious point for this problem is that the geomagnetic activity might have some influence on our summary. figure 4 illustrates the temporal evolution of geomagnetic activity measured according to rkp (the daily sum of the kp index) during the relevant period, from late february to around march 10. as can be seen from figure 4, the geomagnetic activity on march 5 and 6, 2011, when the anomaly was observed, was relatively quiet (of the order of rkp = 10-15), although we note some enhancement of the geomagnetic activity in late february and around march 10. thus, it is likely that the geomagnetic activity has nothing to do with this earthquake. as summarized as point (2) above, we found a significant propagation anomaly on the path of nlk–chf. this means that the earthquake responsible for this anomaly might be located at any position along the path of nlk–chf, as we have no propagation paths crossing these nlk–japanese observatories. after having examined the earthquakes over the whole area of this propagation path, we were not able to find any earthquakes on the path during the propagation path. thus, the propagation anomaly on march 5 and 6, 2011, shown in figure 3 is very likely to be a precursor to the march 11, 2011, earthquake. the lead time for this earthquake was about 5-6 days, which appears to be consistent with the corresponding statistical results for land earthquakes in and around japan reported by hayakawa et al. [2010]. it is interesting for us to compare these characteristics of the ionospheric perturbations for this earthquake with those of an earthquake with nearly the same magnitude; i.e., the 2004 sumatra earthquake with a magnitude of 9.1 [horie et al. 2007a, b]. on the basis of the ground-based observations in japan [horie et al. 2007a, b] for the propagation paths associated with the australian nwc transmitter, and also on the satellite observations of whistler-mode signals from the same transmitter [molchanov et al. 2006], we found that the radius of the ionospheric perturbations was of the order of 2.5 mm for the sumatra earthquake. this earthquake was also an oceanic earthquake of the plate type, so the same type as the march 11, 2011, tohoku earthquake. however, the most important difference between these two hayakawa et al. 98 figure 4. temporal elevation of the geomagnetic activity (�kp index) during the relevant period for the march 11, 2011, earthquake. 99 earthquakes is that the sumatra earthquake occurred very close to the land area of indonesia (so we would expecting a lot of influence on the land area), whereas the tohoku earthquake occurred far from the coast line (ca. 150 km), and thus right in the ocean. if we assume the same spatial scale for this march 11, 2011, earthquake as for the sumatra earthquake, we would expect any significant perturbations to extend even to the path of jjy–msr, but figure 2 indicates that there was no anomaly at all on this path. this means that the spatial scale of ionospheric perturbations for the march 11, 2011, earthquake was considerably smaller, as compared with that for the 2004 sumatra earthquake. also, we can assume that this is probably closely related with the fact that the earthquake occurred completely in the sea, so that there are no such significant effects on the ionosphere due to the presence of the sea water. references chakrabarti, s.k., ed. (2010). propagation effects of very low frequency radio waves, aip conference proceedings, vol. 1286, 362 pp. gokhberg, m.b., i.l. gufeld, a.a. rozhnoy, v.f. marenko, v.f. yampolsky and e.a. ponomarev (1989). study of seismic influence on the ionosphere by super long wave probing of the earth-ionosphere waveguide, phys. earth planet. inter., 57, 64-67. hayakawa, m., o.a. molchanov, t.ondoh and e. kawai (1996). the precursory signature effect of the kobe earthquake on vlf subionospheric signals, j. comm. res. lab., tokyo, 43, 169-180. hayakawa, m., o.a. molchanov and the nasda/uec team (2004). summary report of the nasda earthquake remote sensing frontier project, phys. chem. earth, 29, 617-625. hayakawa, m. (2007). vlf/lf radio sounding of ionospheric perturbations associated with earthquakes, sensors, 7, 1141-1151. hayakawa, m. (2009). lower ionospheric perturbations associated with earthquakes, as detected by subionospheric vlf/lf radio waves, in: electromagnetic phenomena associated with earthquakes, m. hayakawa (ed.), transworld research network, trivandrum (india), chapter 5, 119-136. hayakawa, m (2010). the use of subionospheric vlf/lf propagation for the study of lower ionospheric perturbations associated with earthquakes, in: propagation effects of very low frequency radio waves, s.k. chakrabarti (ed.), am. inst. physics, aip conference proceedings, 1286, 223-269. hayakawa, m., y. kasahara, t. nakamura, f. muto, t. horie, s. maekawa, y. hobara, a.a. rozhnoi, m. solovieva and o.a. molchanov (2010). a statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric vlf/lf propagation and earthquakes, j. geophys. res., 115, a09305; doi:10. 1029/2009ja015143. horie, t., s. maekawa, t. yamauchi and m. hayakawa (2007a). a possible effect of ionospheric perturbations associated with the sumatra earthquake, as revealed from subionospheric very-low-frequency (vlf) propagation (nwcjapan), intl. j. remote sensing, 28, 3133-3139. horie, t., t. yamauchi, m. yoshida and m. hayakawa (2007b). the wavelike structures of ionospheric perturbation associated with sumatra earthquake of 26 december 2004, as revealed from vlf observation in japan of nwc signals, j. atmos. solar-terr. phys., 69, 1021-1028. kasahara, y., f. muto, t. horie, m. yoshida, m. hayakawa, k. ohta, a. rozhnoi, m. solovieva and o. a. molchanov (2008). on the statistical correlation between the ionospheric perturbations as detected by subionospheric vlf/lf propagation anomalies and earthquakes, nat. haz. earth system sci., 8, 653-656. maekawa, s., t. horie, t. yamauchi, t. sawaya, m. ishikawa, m. hayakawa and h. sasaki (2006). a statistical study on the effect of earthquakes on the ionosphere, based on the subionospheric lf propagation data in japan, ann. geophysicae, 24, 2219-2225. molchanov, o., a. rozhnoi, m. solovieva, o. akentieva, j.j. berthelier, m. parrot, f. lefeuvre, p.f. biagi, l. castellana and m. hayakawa (2006), global diagnostics of the ionospheric perturbations related to the seismic activity using the vlf radio signals collected on the demeter satellite, nat. haz. earth system sci., 6, 745-753. muto, f., t. horie, m. yoshida, m. hayakawa, a. rozhnoi, m. solovieva and o.a. molchanov (2009), ionospheric perturbations related to the miyagi-oki earthquake on 16 august 2005, as seen from japanese vlf/lf subionospheric propagation network, phys. chem. earth, 34, 449-455. rozhnoi, a., m.s. solovieva, o.a. molchanov and m. hayakawa (2004). middle latitude lf (40 khz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, phys. chem. earth, 29, 589-598. *corresponding author: masashi hayakawa, advanced wireless communications research center, university of electro-communications (uec), chofu, tokyo, japan; email: hayakawa@whistler.ee.uec.ac.jp. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ionospheric perturbation for the 2011 japan earthquake layout 6 application of empirical mode decomposition to very low frequency signals for identification of seismic-ionospheric precursor phenomena christos skeberis1, dimitrios t. xenos1, thomas d. xenos1, michael e. contadakis2, dimitrios arabelos2, georgia chatzopoulou1 1 aristotle university of thessaloniki, department of electrical and computer engineering, thessaloniki, greece 2 aristotle university of thessaloniki, department of survey engineering, thessaloniki, greece annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5312 abstract this study investigates the application of empirical mode decomposition to signals from very low frequency transmitters in europe that were received in thessaloniki, greece, to provide a method for depicting seismic-ionospheric precursor phenomena that occur prior to an earthquake. the basis for ionosphere interactions with seismic phenomena has been well documented in past studies, and the depiction of disturbances applied from the earthionosphere waveguide on the received signals was the purpose of this study. empirical mode decomposition is a method for processing of nonlinear and nonstationary signals, to decompose them into their functional components, known as intrinsic mode functions. this method can provide high pass filtering to signals, thus depicting a clearer image of any abnormal disturbances in the signals that are not part of the normal noise content. observations of such precursor phenomena are presented and correlated to earthquakes, to demonstrate the effectiveness of this method. 1. introduction a large amount of research over the last 20 years has indicated the existence of pre-, coand post-earthquake ionospheric perturbations at all levels of the ionosphere (i.e. e, d, f layers). these have included ground-based experiments [molchanov et al. 2004, 2005, liperovsky et al. 2005, shvets et al. 2004, rozhnoi et al. 2004, 2009, contadakis et al. 2007, 2008, biagi et al., 2009, tsolis and xenos 2009, 2010], space-born studies [hayakawa et al. 2000, parrot 2006, he et al. 2011], and combined space-born and ground-based studies [rozhnoi et al. 2007, muto et al. 2008]. it is generally accepted that there is a lithosphereatmosphere-ionosphere connection mechanism that works through one of two mechanisms: (i) by the generation of atmospheric gravity waves at acoustic frequencies during the earthquake preparation period. these begin in the area of the epicenter, and subsequently move upwards, to enrich the turbulent content of the ionosphere over the epicenter and to initiate gravity waves that propagate in the waveguide of the ionosphere [molchanov et al. 2004]; or (ii) by ion exhalation and the subsequent variations in the electric field at the site of the earthquake preparation area, which produces variations in the ionosphere over the epicenter area [pulinets and ouzounov 2010]. it was observed a long time ago that very low frequency/low frequency (vlf/lf) electromagnetic signals that propagate in the earth-ionosphere waveguide are greatly influenced in their amplitude and phase by the plasma condition of the lower ionosphere [gokhberg 1989, molchanov et al. 1998]. these can be used as an integrated diagnostic for lower ionosphere (d layer) variations over the propagation path. for this reason, the follow-up of vlf/lf electromagnetic signal propagation is considered to be a reliable diagnostic means for pre-, coand post-earthquake ionospheric perturbations for earthquakes that might occur along the propagation path of the signal. however, the ionosphere is a very complex nonlinear system, and there are a large number of factors that can contribute to its variability, and thus this does not always provide a clear link between cause and effect. to reveal uneven disturbances and to facilitate the detection of possible earthquake-connected perturbations, the use of the hilbert-huang transform [huang and attoh-okine 1998, huang et al. 2005] is proposed here, or rather the part of it that is described by the empirical mode decomposition (emd) method. 2. empirical mode decomposition traditional filtering methods are realized in the frequency domain only. however, filtering in the frequency domain is very difficult to effectively implement for nonstationary and nonlinear signals. this is mostly because frequency analysis of special issue: earthquake precursors 199 article history received july 14, 2011; accepted january 19, 2012. subject classification: processes and dynamics, earthquake faults: properties and evolution, seismic risk, atmospheric electric field, lai coupling. nonlinear and nonstationary signals generates harmonics over a wide range, and as a result, any filtering in the frequency domain will eliminate some of the harmonics. this will eventually cause deformation of the waveforms of the fundamental modes if they lie outside the filtering range. the hilbert-huang transform [huang and attoh-okine 1998, huang et al. 2005] is a data-driven signal-processing method. it consists of two parts; the first is emd, where the signal is decomposed into a series of structural components, known as intrinsic mode functions (imfs). an imf is defined as any function that has the same number of zero-crossings and extrema, and also has symmetric envelopes that are defined by the local maxima and minima [huang and attoh-okine 1998, huang et al. 2005]. imfs allow well-behaved hilbert transforms, and thus the second part of the method is just the hilbert transform, which provides instantaneous frequencies as a function of time for each of the imf components. compared to standard signal-processing methods, namely fourier transform or fast fourier transform, as well as more contemporary signal-processing methods, such as wavelets analysis, the hilbert-huang transform has two significant advantages. first, it is highly applicable to nonlinear and nonstationary signal processing, as it is based on the local characteristic time scale of the data. secondly, it is totally adaptive and data driven, as there is no need for a-priori selection of a basis (i.e., mother wavelet) for the data analysis. for a discrete time signal x(n), the emd starts by defining the envelopes of its maxima and minima using cubic splines interpolation. then, the mean of the two envelopes is calculated, as: . (1) the mean is then subtracted from the original signal: , (2) and the residual is examined according to the imf criteria. if it is an imf, then the procedure stops and the new signal under examination is expressed as: . (3) however, if no imf is identified, the signal is processed using ‘sifting’, and the process is continued k times, until the first imf is realized. the sifting process is continued until the last residual is either a monotonic function or a constant. it should be mentioned that as the sifting process evolves, the number of extrema from one residual to the next drops, thus guaranteeing that complete decomposition is achieved in a finite number of steps. . (4) then the imf is provided as: . (5) the final product is wavelet-like decomposition that goes from higher oscillations to lower oscillations, which means that the spectrum moves to lower frequencies as the order of the imf increases. the large difference, however, with wavelet analysis is that while modes and residuals can intuitively be given a ‘spectral’ interpretation in the general case, their high versus low frequency discrimination applies only locally and corresponds in no way to predetermined sub-band filtering. instead, the selection of modes corresponds to automatic and adaptive (signal-dependent) time-variant filtering [flandrin et al. 2004a,b]; this can provide a high pass filter for a signal that can give a clear image of abnormal disturbances in the signal that are not part of the normal noise content 3. data analysis vlf signals transmitted by a number of european vlf transmitters (table 1; figure 1) were monitored for over a year in thessaloniki (40.69˚n, 22.78˚e). the signals received were sampled and stored for off-line processing. the receiver was developed by elettronika srl, and it is part of the international network for frontier research on earthquake precursors [infrep; biagi et al. 2011, http://beta.fisica.uniba.it/infrep/). as examples of how the above process works and of the disturbances that can be detected, the september 6, 2009, earthquake that occurred in albania (41.46˚n; 20.41˚e; m = 5.6) is examined. as there was no appreciable seismic activity prior to the main earthquake, this provides a better example of the detection of the precursor phenomena, and also considering that the kp and ap indices revealed no extraordinary disturbances, with the maximum kp index for that day of 1+ and a mean ap of 3 (ftp://ftp.ngdc.noaa.gov/). the preparation zone for this earthquake was calculated as 255.85 km, according to the dobrovolsky equation [dobrovolsky et al. 1979, pulinets et al. 2004]: . (6) the distance between the location of the event and the receiver in thessaloniki, greece is 244 km, and therefore thessaloniki is considered to be in the preparation zone (figure 2). consequently, it is expected that there would be observable disturbances in all or most of the vlf signals received. for the purpose of the present study, the signals received were sampled (sampling rate, 1 min) over a period of 4 days prior to (from september 2, 2009), to 2 days after (to september 8, 2009) this seismic event. during this period, there was no other notable ionospheric activity that could have provided disturbances of any type. ( ) ( ) ( ) m n e n e n 2 max min 1 = + ( ) ( ) ( )h n x n m n1 1= ( ) ( ) ( )x n x n h n1 1= ( ) ( ) ( )h n h n m n11 1 11= ( ) ( ) ( )imf h n h n m nk k k1 1 1 1= = -10 kmr ( . )m0 43= ) 200 skeberis et al. 201 4. results and discussion. as indicated above, four vlf transmitters were monitored in thessaloniki: ice (keflavik, iceland), icv (tavolara, italy), gbz (anthorn, uk) and dho (rhauderfehn, germany). the signals received from the first three of these stations revealed disturbances on the days previous to the earthquake and a few hours prior to the earthquake, as compared to typical relevant signals (figures 3, 4, 5; respectively). as can be seen in the signals received from ice (figure 3), using the emd method, disturbances were identified 1 day prior to the seismic event. for the signal received from icv (figure 4), it is possible to discern disturbances up to 2 days prior to the event, and for the signals received from gbz (figure 5), it is possible to identify disturbances a few hours prior to the event. on the other hand, in the signals received from dho (figure 6), there were no appreciable differences to the normal patterns of the signals received, although this can be attributed to the recurring disturbances that were already seen in the signals received from this station, which were due to other factors that might have masked any precursor phenomenon. as observed from the figures referring to the periods in question, the processing of the signals with the emd method and their decomposition into their imfs can provide an accentuation of disturbances that can be attributed to precursor phenomena and a better view of the effects of the event in question. additionally, it should be noted that the processing of the signals with this method clears the received signals of extraneous information that is not related to disturbances. as such, this method provides a common basis for analysis across the stations, with different characteristics thus providing an array of data that can be processed easier. emd, identification of seismicionospheric precursors table 1. details of the european vlf transmitters monitored in the present study. station code location latitude (˚n) longitude (˚e) frequency (hz) gbz anthorn, uk 54.912 –3.277 19.580 icv tavolara, italy 40.923 9.731 20.270 dho rhauderfehn, germany 53.082 7.616 23.400 ice keflavik, iceland 63.959 –22.542 37.500 figure 1. map of the transmitters, the receiver and the earthquake epicenter. orange, defined earthquake preparation area. figure 2. map of the study area. orange, defined earthquake preparation area. skeberis et al. 202 figure 3. signals recorded from the ice station (keflavik, iceland). imf1 and imf2 received from september 2, 2009, to september 8, 2009. green line, the seismic event, black arrow, points of interest in the signals. figure 4. signals recorded from the icv station (tavolara, italy) imf1 and imf2 received from september 2, 2009, to september 8, 2009. green line, the seismic event, black arrow, points of interest in the signals. 203 5. conclusions from the above-presented data, we can conclude that the processing of signals with the emd method and their decomposition into their imfs can provide an accentuation of disturbances that can be attributed to precursor phenomena. moreover, the processing of the signals received with emd clears them from extraneous information. consequently, this can provide data arrays that can be used emd, identification of seismicionospheric precursors figure 5. signals recorded from the gbz station (anthorn, uk). imf1 and imf2 received from september 2, 2009, to september 8, 2009. green line, the seismic event, black arrow, points of interest in the signals. figure 6. signals recorded from the dho station (rhauderfehn, germany). imf1 and imf2 received from september 2, 2009, to september 8, 2009. green line, the seismic event. as inputs in an automatic detection system for seismicionospheric precursor phenomena. 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examples of unusual ionospheric observations made by the demeter satellite over seismic regions, phys. chem. earth, 31, 486-495. pulinets, s.a., t.b. gaivoronska, a. leyva contreras and l. ciraolo (2004). correlation analysis technique revealing ionospheric precursors of earthquakes, nat. hazards earth sys., 4, 697-702. pulinets, s. and d.d. ouzounov (2010). lithosphere-atmosphere-ionosphere coupling (laic) model. a unified concept for earthquake precursors validation. j. asian earth sci.; doi: 10.1016/j.jseaes.2010.03.05. rozhnoi, a., m.s. solovieva, o.a. molchanov and m. hayakawa (2004). middle latitude lf (40kh) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, phys. chem. earth, 29, 589-598. rozhnoi, a., o. molchanov, m. solovieva, v. gladyshev, o. akentieva, j.j., berthelier, m. parrot, f. lefeuvre, p.f. biagi, l. castellana and m. hayakawa (2007). possible seismo-ionosphere perturbations revealed by vlf siskeberis et al. 204 205 gnals collected on ground and on a satellite, nat. hazards earth sys., 7, 617-624. rozhnoi, a., m. solovieva, o. molchanov, k. schwingenschuh, m.y. boudjada, p.f. biagi, t. maggipinto, l. castellana and m. hayakawa (2009). anomalies in vlf radio signals prior the abruzzo earthquake(m=6.3) on 6 april 2009, nat. hazards earth sys., 9, 1727-1732. shvets, a.v., m. hayakawa, o.a. molchanov and y. ando (2004). a study of ionospheric response to regional seismic activity by vlf radio sounding, phys. chem. earth, 29, 627-637. tsolis, g.s. and t.d. xenos (2009). seismo-ionospheric coupling correlation analysis of earthquakes in greece, using empirical mode decomposition, nonlin. processes geophys., 16, 123-130. tsolis, g.s. and t.d. xenos (2010). a qualitative study of the seismo-ionospheric precursors prior to the 6 april 2009 earthquake in l’aquila, italy, nat. hazard earth sys., 10, 133-137. *corresponding author: christos skeberis, aristotle university of thessaloniki, department of electrical and computer engineering, thessaloniki, greece; email: cskeberis@auth.gr. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. emd, identification of seismicionospheric precursors indirizzo introduttivo alla sessione sulle ricerche di meteorologia e fisica dell' atmosfera del presidente di sessione, generale b. a. fernundo giunsanti, capo del servizio meteorologico dell' a. ì\i. l. (roffi" • e. li. r., ]3 giugno ]96]) bccellcnze, signore, signori, l1a sl'.,>sione di stmnn.ne del c~onvegno interna zion ale tecnico-scientihr,o dello spa zio è dedieata alle hieel'clw di meteorologia. poiehé il toech.to :lo mc, quale capo (h,] servizio meteorologico dell ' ael'olmuti('l~, l'ollore di presiedere questa sessione, mi è grato 110l'gel'e :\llzitntto agli seiellziat,i, ai 11l'ofebsiollii\ti, a gli espel't,j ed agli app'l.ssionat,j qui conv('lluti da vari centri di l'icel'ca-c lavoro un cordiale benvenuto cd hll (·aldo saluto angnm\e. dagli :1ntichissimi 111l.l'alwgmi a<l og'gi la meteorologia h a eomviu!o, hl'colu per secolo, un be,n lungo cammino. tutti ne conosciamo, almeno solllllll1rimllmrte, le t:tvlje llrineillali, dal famoso trattato di arisfote'le all'invenzione del barometro e del termomctro !:iw hannu segnato l'in.l;rpsso dei ilrincilli dell a fisin i. nello studio de'll' atmosfpm; son vennte poi le prime reti di stazioni, le prime carte sinottichc, il timido esordio ddl'eslllol'l\zionc degli strati aerei a mezzo di palloni sonda ed aeropl:tni, via via fìno alle moderne teoric, alle nuove scoperte, alla llrcvisione numerica del tmllvo, ai razzi met.eorologici ed a i sat.elliti artificiali dp]]a famiglia 'l'iros. l'm' quanto lungo e fecondo, que.,>to c<ullmillo è ancor~. be,n hmgi dall'cssere conc!nso. aneora molto resta da fan" vl'illla <li llott'r conoscere a fondo le legj!i elw regolano l'occano d'ari:t che ci avvolge mi i 111'ocessi dw vi si (:omlliollo, e lll'ima di voter utilizzare qucsta c,onoscenza, eon tranquilla sicurczza, ai nostri principali fini pmt,iei: tra i qn a li primeggiano la previsione del tmllvo slx,de qnt'lla a lnllga scadcnza e forse, domani, il controllo stesso del t.empo da partc\ de'll'uomo. non mi è pmti<.:allwnte, llossibile, come s:trchhe invcro desiderahile, fan\ in llochi minuti il llunto della situa zione delle rieerehe, l)oiché t a nti ti co""veg~u i:'>tlm:-lazioxale tec"i/ico-sciektifico di~llo spazio ,~01io i settori e liiol!;o vari gli a.spetti delle questioni sulle quali la liieh!orologia scientifim. opera, nou voglio ue]lpurc, d'a ltronde, pre8\lmere di addit.are percorsi e mete a,gli illustri scienziat.i qui eonvenuti. 'l'uttavil. , la mia fuuzione di ch.}lo di ull servizio che h a problemi pmtiei ed a]l]lliea,tivi di meteol'ologi a taut.o urgeuti ed im]lortaut.i, quale b diret.ta (:ustodia ilella vit.a ]lei' lll m ol'lm.i im]louellt.e pm'te dell'umanità" oggi jega hl. ll.lb sicul'l~zz;. delle (:omuniea zioni a nrce, mi invita, anzi dil'pi qw.si mi obbliga , a j'iehiamare qui alcune questioni che mi a ]l] n iouo di folhbment.ale e gemmole iut.eresse, mi riferi8co più p a rtic'olal'mente sill, ai }ll'oblemi di'ila ~leh',ol'ologill. iufol'mat.in. e pl'evisiouist.ica , (:]18 11 q1wlli, l),]l]ww . na~('eut.i, d('lla met.lìol'ologia, sperimeut.ale di inlerveut.o, tuizim ulo jll'oprio (b quesli ultimi ù superfluo ch'io sottolilwi t:ome essi sia no eonne:;si a lh eonoscenzll. a ]l]lrofolhlita della m a crofisi(:a e delll1microfisiea delle nubi e ilelll' idrometeore in genere, comlh'c1hlelhloyi l'pst.renmmente comlllp.8so campo ilei fenomeni elett.rici tempol'ltles!:lli (j della chimi('". dell' a tmosfera: settori della. meteorologill, che i ricer(:l1t.ol'i (' gli esperti dplmio servizio "l),uuo sdlu]lpando cou soddisfa(:eute a.ppol't,o, r~a (li8soluzione di nebbie, la e(:cit.a ziolw (ii ]h'eeipitazioni, lo sgretoja nwuto di'ila g-ralldine, ('.lw aih:or oggi si t.ent.ano con risultati tal\'olla illeel'ti o nulli e slk;s:;o molto modesti, potranno, ll.t.t.ra·\"erso la rieerca, t,l'ovare qm\lle giust.e b lòsi tcoriche che suggm'iralluo le t.ecuiche lk;i' o]lemzioni va st.(j e tt,m}lesti\'e, h. cui ('ffu:aeia , uua \'olta diveuut lò effet!;int, avrfl ellol'mi ripmemsioni e(:onomiclw e :;oeil1li. d'alt,m ]lart(j questa t:ouoscl'nza ap}lrofondibl. delle idromet.eol'e, ilella loro gi'nesi e del loro sviluppo, (bila fase delle llubi a quella delle precivitazioni, avrà a nehe, (:ollle ri~ultat.o, di fa rilitare ho provisiolw di questi renolllcni e quin(li an('·he b l.(,ro\llwigazione in 101'0 ]l!'esenzl., ua]llkwto informativo di l'ett.o ed indirett.o ot.tenibile con l'a(br met.eorologiei :;ulh :;truttul'a , diluensiolli, distribuzione degli unllllhossi lluvolosi s:l. rll, sotto que8to aspetto, veranwnt.e essenziale, venewlo lmi lhllpdt.i clw potremmo njllsi(lera l'l' più ela.ssici d(dla i\leti'ol~llog-il1, spe(:ie per qwl.llto l'ig-w .nb in }jlwticolare il nost.ro )leditel'l'anco, molto vi è (ho h. vol'al'(~ nel ealll]lo tl'o]kihfelico e subst.l'atosf(wko, dat.a b hituazione g-eogmfìca , in mp}lol'to a lh. influenza non :;plliih,(' (ld,crminaùi]l' delle cil'('.olazioni legate n,ile grandi cellnle anticielonidw subtropieali, l.vvienc che il ~hjdit.el'mneo sia sede di t.msfol'lllazioni l' ·ii eontrl. ht.i dellp nja,sse il'l1ri a !'ile lo int.eressano con le 101'0 t.raiettorie, l), yolt.e per(;ql'l'endolo velocemente, l. volte sosbndovi ]lei' periodi ]liù o meno lung'hi, spehho ([;).l\(lo luogo l. eamtteristici processi di genera(,usv!!g~u inl'ernazlonal" tecnico-sc r enl'h'ico d};u. o "l'azlo 7 zione e ril!en0mzione rlelle l)0rturba,zioni che non sem]ll'f' l'insei a nlo :. chim'il'e (l prevedere r..on ~()fldisflwpnt.o preeisimw. [,o ~tlhjio a pprofonaito ai qneht,i pl'ob10mi (li l\l()tporolo~ia rin(jt,t,i('a nlw a~snmono a spett.i mecc.anici, tel'lllodin a.j llid e statistico-(\1illmtolo~ici rlelle. vicis situ(lini at,mosfmiche è stat.o, avcllilo jll"w1nt.(· l'pnormitù aegli stessi, riai miei ricl'rcatori ;. ppena, a bbozza to l' t.pnt.a t,o in ('a si spol'l.dil·.i; esso pnò, ovviamente, m{'l'ita e richil'(10 il m a ssilllo sforzo. ~on ihjsso non ('.oneol'rlal'e, a i]llesto propo,~it o, con finsi8tenh\ ap· pello eol qnale b. organiz7. azionp ~\lpt.eol·olol-..-jl~. )lonai.."),10 solkcita i1n inercllwnto ddle osserva zioni in fjllot4\ su1l0 zoth' h,lle b a,o,sl\ lat.itu(lini iii. ove 1m serle il nmg-rior apjlort,o di energia jlpr la din a lllica l' tel"llloainamil~' aelb. no.~tm 1.hllosfem 0 110,'0 lw.raltro questa llr{'zios a do('.l lmenta zione rli infol'llmzione (\ di .~tudio è an('oi'a t001"l'ibilnh'nt.c· sea l1,m, las0ialhlo t,roppo eampo a {'strapolazioni incmfl' p ~p('.<;.~o a nehp pptieolos(\. lp prpi~.rlenti l'i('0l'i'!w, poi, ,~i rieolleg'a no str0tta n10nte a qlll'lll \ kuu a m rli azione sol a ri', snlla l'va poj'a 7.ionp, sulla tnrbolenza l' lo sl'ambio sia nei bassi strati d10 in quelli ,~uiwriol'i; rieerl\he ehe risult~"), no es8e117.iali a nclie sott,o alt,l'i pnnti (li vista dlll va nno dalla nvlio l' rh,(l a rpl'opagaziol10 a lla (lispersion{'. rlegli efflucnt,i ll1wlf'a ri, e chp pl'escntm lo problemi rli gra nde rlilftcoltà teol'il,a, sp01'imentale e(l org-a niz zah\'a, m a l']1l\ nerta,llwnt.e t ... mpo, 1lh'7.zi e yolontù v;i,ltanno a snpel'at·l·. l'incr{'.jll0nt.o notevoli.o,.<;imo (lei sollllagg'i terlllo(lin a miei ridp atmohfera, r{'~~lizzato durante la g'u0lta e subito rlopo. il a apportmq al progl"0.o,so ricllc i\onoseenzl' meteorologiche un eont,libut o j!1'a nrlissimo, e pni' so(tolinparlo mi basti qui citare 11\ correnti a ~ett.o l'rl il l'.ont-ribnt.o che la 101'0 uwizza zionp può (l are a ll a pr(wisioni.~ti(~ •. è pertanto da a ttend{'rsi 0110 nuovi ed alt,rcttanto iu1])()rbmti nontributi venga no d a un ulteriore rlel'ì8iyo e sistema tico ill1'r{,.llwnfo dell(\ auuali quoti'. m ggiunk riai sondag)!i st,e~~i, a c('ollljmgn ato oi-yiauwnte rho un arh1g"uanwnto rlelle htrument.azioni (li misura, [,a, nenps,~itù (li uno stllllio sist ... nmt,ico a ba si' sinof(ka sino a (luote ehe non si a rrestino più ai ei-20 km, ma si silingano sino ,.i 30-35 km, oggi mggiungibili con una buona orglmizzazione, (' r(\sh pyirl(\nt(\ a 1ll·lw in serle pmt,ie.a, se si consiriem ehe fra ([uat,t,ro o l\inque auni al m a ssimo entrpra nno in scrvizio (ti lin{'a aeroll!ani progettati p!'l' yolal'e lvl olh'p 20.000 metri rli alt.ezza, con vploeità almpno rlopjli a rli quell a riel s\1ono. 'l'a ie st,udio, ehe int{'r{'.~sa ormai l' alta stm10sff'.t"a (\(1 i primi st,mt,i riella mesosfera" risult.a essenziale am·hl' jlpr le l'ijlf'till\ssiolli eli('. questa dina miea rl{'gli stmti superiori ricve a vpte ,~u ([uelli inf{'riori, do\' p si svolge l'at,t,uale volo l' ne'i quali noi viviamo. benehé .~i a fa eile 1)1'f'-\,0dere l'insorger!:' di nllovi f', .~f',m!ll·e più com· pl!:'ssi !lrohlbllli, è da sl)(,1'a1'l' a.]t,resì elw a,!meno alcuni rlf'gli at,tuali t,l'o· vino lllll1 soluzione. \'ol;!endo così la ment<:l ,l. qu a nt,o id ù già st,ato dalo (li l1pprendere liugli .~tmti jliù \lleva1i ddl'mmosfera, (',io{> rh,lh mesosfem e (iella t.el'llln· sfem, che sino 11 ieri rono btat.i esjllomti solo con mezzi ilhiir(,tti eri ogl-!,'ì sono n .ggiunti (',on razzi mbt.eo1'ologiei, con sondi' iipa zi a li, con satdlili, dobhil1mo conclurlere elw nna f\liplol'<l.zionc liist.f\mat,i{,l1 (li t,ipo i\inot,t,il'o ei apririì. nuoyi orizzont,i. l eomplf'aai ff'.jlollwni [otol,himi(',i f' fotoelet.t.rif'i, il (·ara.tf.(\ri.~ti(·o ('omportamf',nto (lf'll'ozono. i fenomeni if\kat,i alla l'<l.fli,. zione ('o"mi('a, ai mggi x f' ad og-ni altra spf'ci!:' ru l'a(liazione, llwttono in moto p sono nel tempo st.esso ('onrlizionati da grandiosi s('l1mhi b t,n. s[orlliazioni f'!wrge· t,id w, ehe modifi(' a no ili,. b st,rnt,hll·'. (1(·ll'at,mollfbn. liupn'iorf', ili:> i 1i11oi moyiment,i. ht m'l.r"giol' conoi\cf'nza (li t,utt.o ciò jlernlf'.t.t,er:\ (li inquadra.l'(' in mo(lelli unitari e (',01'1'0nt,i qui'i fatti dw gi<'l .~ono stat,i a t,ratti l' solo parzialnll'nte 1'iyelati, a(l esmnpio, (lalla deriva!:' dalle (listol'sioni di']]!' scie (1('lie steli!:' cadenti, (, che si tl',lhlf' a st.urliarp nbi !liù minuti dptb1gli, c,omf' ha buo i]ni in itali a il nost,ro broglio, nwdiantf\ lc nubi ,.1 i\odio. vimporbmza., c!w dpllnir0i inlf',gmlf\, di quf'sto g-enme di ri0f'1'(',lw sllll',. tmosfem supeliore è confe1'llmt,a dalle inizì<l.tiyf' prese (lalla ol'g-;. nizza zione ml't,(,o1'olog-i('a mondiale, che 1m già, pl'os]*,.tt,a to p stimolo h· jll'oposhl (li stn(lio siste1lmti('o sinot,t,ieo rlelh llwsosfl'l'<. l!l\l(lianl~l razzi llwkorologici a pprontal,i per quf'mo i\j)('(·,ifìl'.o i\1'ojlo. li] sono eerto che ,. nelle il nostro paese, !:'d in p a1't.ieohr(, l'aerom. ntic'a, non 1w. nelw1'anno all" . ppe]]o; po.i;so infl1t,ti c,omuni(~1re (·lli' 1'01'g'anizz,. zio!w di (pwsti la ne1 1m già, a vuto inizio. ('hinrlel'ò qul'sta mia mpirliksima l'<l.ssf'g-tm l'ìc'ol"flando quelli (·,111' }lotl'!'tlllno rll'finit'1' i più eh'\"ltt,i ossl'ltatot'i 11lpt,(,m'o!ogie1, nwssi 11 rlisjlo, sizio\w df';!li i\huliosi eli i1sicl1 deli'mmos[l'ra dbi t,\wni('i h,mcri(',ani: i 'l'iroh, ai ([lmli ho a eeemmto l1ll'inizio (11'\ mio (lirf',. qul'st.i sl1teilit.i sono notolbllwnte post,i in ol'bit,. ,l. (j()()-b()() km noli tanto ])('1' ps!lloi"<l.l'e l'alb l1t.mosf('r:1 iyi esistent.e, qw. nto pm misu ra n' le l'll,(lia zioni (liffui\{l od ellh:,.,;se da ll a sui)(>rfie1p t.enest,rl' b (bll'at,lilosfel"l1, e, sopmttutt.o, ppl" t,rf\j\mpl,h'l'p ([;.ll'ajt.o 1',. s])(·lt,o dpll(' formazioni (' (kgli allllll. j.,;si nuvolosi. bssi pel'lil\·t.t.ollo di ott.ellen· pel"l,iù i1n quadro relatiyo a11;o qw. ntitù. ti!lo b (listrihuzionf', (lf'.l!e nubi (l(,ile zone "\'ì<l. vil1 .~ol'vol:~t<:' .. ift' mii\l\l'1' p il' ohseryazioni l"w,i'olt(· tmmite questi hl1t.ellit,i nei loro ~uec('.'~si\~i pnssa.!.!jd, l)()stl' ill l'dazio 111' alli' simultall(\p ossuryaziolli p ai dll'convegno l:>n;ltl'azlol'ale tecl'ico-scif,!l:tifico dello ~pa7,1li ~) ylj,[]wnti normali p stra,onlinn,l'i f'ompiuti al 1'111010 e npgli ~trati atlllonfl'riei ~ino a ~5 km, pptm(1thwanllo di aypl'l' un qu a dro d'in~il'[1h', l'ii[1o ad ol!~d nmi ottl'lmto, rh',i renolllpni ilwì{101'olol!ici a iztn,nrh' sr'ah iwlb lo\'o e'ql' lu:>:im1(\ l' di'i bih1lwio te!'lllo['a<lia,jjyo atmosfpti(',o ad p~~i ipgato, ciò (;omplltinì. (l(':i'to di i!innj.(el'l' ari 1ilja. ma'l!giorf' f'01l1pn'ilhi01w (ipi fenollh'ni ~1{'~~i {' (li {'oil~pgm1nza, in parti('oh\.\'(', di mil!!io ra l'lll' h ]lrp, yi~i01h'. non o('('or\'(\ mol1;. immaginazione 1)('1" alfl\l'l~h'p l'importanza {'llp ayl"pblw, alh'lw dal solo punto di \"1st,a im1t,ieo infol'llm1iyn, il fntto, in {'li i l'pa.\izza:>:iml(' r'nto non ta1'flr'r:l;l, yp,\'ifi{'a,r};i, di pot(\r {'rrt'l'l' ~11!i!i(l\'imti, ,hl inten'alli di ìl'lllpo <li lhl{'o più di un'o ra , da, iin oppol'tllno cmllplp};};o rli bli l'iatr'lliti r.l1f'., eon 11~loil'ttorip da pola'l'i ad p-qllat.oria.\i, ehplorh,~r{'!'o lo stato dpl f'ip!o su tuj.ta, hl, wl){'rfif'ip t,el'l'l'htl'{', la r,olh1bomziolh' al pi~mo n,[1wl"ì('ano (li 1'ìeerelw i, troppo im]1orla.n1l' l'd intl'l'{'hhante lwl"dd' (){'{'ol'1'a ~otto1in{'al'\a., :noi h1essi d pl'oponh\.jllo rli ('ontribnit'p 11('.i limiti (lei possibilt,. ]){'sidel'o dil'p ehe ho tr:weiato oggi qlll'~to qlla,dro gl'nl'l~ lo h' (l{'gli attuali pl"oblp1l1i rlplht. 1iieh\oi'olol!ia, l' dplla j<~iska dl'ifatniosfpl'a, ho!tl'i'nmnd01l1i sii a,kllni problemi rielia zolja. del ?i!{'diterl'anl'o, {'on un 1'i(\n};o ]jattieohm\ rli l'ntmin,~mn lwl'('}u! ]ll"oprio in qlwsti giol'ni lo ~t('~so collsiglio xazionalp dplll\ hi{'mdw, {,i te ha ]l<l1i'ol,inato qll(',~to l'onn'gllo, l'lta r',oiwl'(\fa,llrio la istituziolle di un ('-entl'o na:>:ionalr' ppi'\a ::\h'tl'ol"ologia l' la fisiea, <!plfmmosfpl'a, (\1ntl'o e]h' ~i assllnwi'à il ('omllito di tl"a,tbm~ a p]11int() (!lwhti 1)]'()hipilli. }ipntn\ <!psid('l'o pi'illl'inwl'(\ la gmtitu(lilw mh1 (\ di tutti gli studio~i di :\leh'oi'ologia, italiani \"el'so il pi'psid('nì(1 r]p-i ('omiglio xazionalt' d{'lk hi('{'l'clll', pl'o!. polnmi, iwi' qlw~ta ~w\ gl"a,lldp illizia,t.iva, ùwr,io \"nti (l'ang'1ll'io ])('1' l'istituendo 0(\ntl'n ~11 qual p il splovi:>:io m('tpol'ologi('o (11'11',af'.l'onauti{'a ~al'ù ol'goglio,~o di (!al'p il lllaf\,~illlo allp0l'to poi'isibil('. iin motivi 1)(\1' l'jj.en{'l'(\ dir qup,~t,a illfol'lllh:>:iolw 8n,l'ii. llo('('olta ('on eolllpiaeillh'l1to anellp(la.g-li studiosi l'itmnilh'i qui ill'pl'llmti. ?lih. ornmi \-i ho tmth'!l1lt,o 1l'op]l0 ~lo lungo su r'.ol1sir]l'm:>:ioni di ('amttp-i'l' gl'lwmlp; l'itol'lliillllo ]1pl'eiò a,lla l1oht['a ~f'hsioll{' {' (lianlo ini:>:io alla il'ttul':\ ddlr', lllp-lllol'ip. n('~iril'l'o 1 utt:n-ia, p,qpl'illll'['{', a {'on(',ln~ion(' r]el lllio dil'l1, ulla yiya, };]wmnza: uh(' rlai l10stri laylll'i ~r'·m111'is('·a 110n tanto la ~(',gllalazion(' rli lluoyi pi'oblpllli da l'isoly('l'p, quanto la 1'i0inziono rii qnaknno di qu('lli ai qua.\i ho fuga.(wnwllt'l' ,w,('{'llllalo, layout 6 1 annals of geophysics, 64, 5, vo552, 2021; doi:10.4401/ag-8684 o p e n acc e s s materials and tools across volcanoes: exploitation of georesources in piano dei cardoni (ustica, italy) during prehistory claudia speciale*,1,2, giuseppe montana3, roberta mentesana4, vincenza forgia5, filippo mantia5, giuseppina battaglia6, mauro antonio di vito1, stefano vassallo6, sandro de vita1 (1) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, naples, italy (2) department of historical studies, university of gothenburg, sweden (3) dipartimento di scienze della terra e del mare, università di palermo, italy (4) arqub, departamento d'història i arqueologia universitat de barcelona, spain (5) dipartimento culture e società, università di palermo, italy (6) soprintendenza beni culturali e ambientali di palermo, italy article history: received may 15, 2021; accepted october 29, 2021 abstract the paper aims at merging the first results from the analyses of the georesources exploited in the site of piano dei cardoni (ustica island, italy) during the neolithic phases of its occupation (middlelate neolithic, 4.7-4.2 ka cal bc). grinding tools consist of a very varied typology of local volcanic rocks, easy to collect and available very close to the investigated site. a selection of shapes and lithology is applied to reach the best performance of the tools. the elevated number of grinders, pestles, mortars testify to an intense activity of food/plant processing in the site. the absence of chert or obsidian resources on the island pushed the human communities to import such raw materials from the aeolian islands and probably from the north-western area of palermo. pumice is collected on the same island, probably due to the local availability and its good quality. similarly, local clay resources are used for the manufacture of ceramics, mostly burnished and incised wares. ustica was therefore almost autonomous for the exploitation of resources, with volcanic rocks readily available in abundance and with the most significant exception being chert and obsidian. this last one probably imported and worked on the island and then moved towards north-western sicily. keywords: neolithic; chert; obsidian; ceramics; volcanic tools. 1. introduction in the definition of the cultural landscape, whatever the declination is, two factors are always present: human and environment and how they interact [sauer, 1925; mercuri et al., 2010; mercuri, 2014; clement and cassino, 2018; plieninger and bieling, 2009]. technical choices made in the manufacturing sequence, especially of ceramic production, but also for lithic assemblages, are not only about the technology itself, but they also represent cultural choices, social concerns and the record of connection routes [lemonnier, 1993; mentesana and fragnoli, 2020; dobres, 2009; clarkson, 2008; horowitz and mccall, 2019]. in particular, in volcanic areas, the dialogue between past communities and the local environment should be read through the interpretation of the geological dynamics [di vito et al., 2009; di renzoni et al., 2016; riede et al., 2020]. volcanoes present not only hazards but also a unique opportunity for georesource exploitation. for example, the role that lava querns and millstones play from the onset of agriculture until recent times is well known all over the mediterranean and beyond [santi et al., 2015; renzulli et al., 2019; kadowaki, 2021], sometimes from great distances already during prehistory [bracchitta 2015]. furthermore, during the middle/late neolithic (6th-5th millennium bce) in the central mediterranean, during a phase of growing contacts and consolidated exchange of goods, obsidian played a primary role in the establishment of shortand long-distance networks [martinelli et al., 2019]. as a further example, soils composed of volcanic ash can have an important influence on agricultural production [neall, 2006]. volcanic islands are even more complex and intriguing for the analysis of cultural landscapes, because of their limited resources [mclaughlin et al., 2018; fitzhugh et al., 2016], the possible discontinuity in the human occupation [dawson, 2014; horsburgh et al., 2017; di napoli et al., 2020] and also the capacity to develop very specific cultural landscapes [rackham, 2012; walter and hamilton, 2014]. the case study analysed here is the small volcanic island of ustica. despite being quite remote from the sicilian coast, the perception of its marginality might have changed throughout prehistory, according to the exchange dynamics and the cultural contacts of the southern tyrrhenian [fitzpatrick et al., 2016]. this paper aims at merging the first results from the analysis of the archaeological materials of the site of piano dei cardoni (ustica, italy) during its phases of neolithic occupation (4.7-4.2 ka bce). after the typological classification of the artefacts (lithic tools and ceramics), the georesources and their uses are analysed: raw material identification and provenance are tackled via analyses of micro-and macro-components of pumice, obsidian, lava stones and ceramics, in order to explore the exploitation of ustica’s insular landscape and the potential network of contacts with other regions. 2. geological and archaeological context the investigated area is located in the north-eastern part of piano dei cardoni at ustica, which is a flat land representing a marine terrace. it is connected to the ustica central relief by a talus deposit. this erosional surface is related to a transgressive-regressive cycle of crotonian age (isotopic stage 9; 350 ka bp [shackleton and opdyke, 1973], set on older submarine and subaerial lavas [de vita et al., 1993; 1998]. during its formation, the abrasion surface of the terrace was the site of marine sedimentation that formed a detrital blanket composed of fossiliferous carbonate concretions and silt, sands and gravels due to the reworking of pre-existing volcanic rocks at the foot of the sea-cliff that borders the terrace. afterwards the terrace was tectonically displaced from its original position and the detrital blanket continued its evolution in a subaerial environment, with the formation of talus breccias and a thick paleosol that also includes large rounded boulders, formed by in situ spheroidal weathering of lava [de vita, 1993]. the piano dei cardoni lowland occupies an ene-wsw trending land-strip, more-or-less parallel to the central ridge of the island, which ranges between 80 and 120 m of elevation a.s.l., with a 5% mean slope. moreover, the terrace slightly dips eastward, due to tectonic displacement. the excavated site is situated above and within the uppermost strata of the detrital blanket [speciale et al., 2021a]. ustica is the emerged part of a large extinct pleistocene volcano, which rises more than 2,000 m above the southern tyrrhenian sea floor [barberi et al., 1969; barberi and innocenti, 1980]. the island is located 60 km north of the sicily coast and about 100 km west of the aeolian islands (figure 1). it occupies an area of less than 9 km2 and reaches a maximum elevation of 248 m a.s.l. with mt. guardia dei turchi in the e-w elongated ridge of hills, located in the central part of the island. ustica is predominantly composed of volcanic rocks, with only subordinate marine and continental sedimentary deposits. volcanic rocks were produced by both subaqueous and subaerial effusive and explosive eruptions [romano and sturiale, 1971; cinque et al., 1988; de vita, 1993; de vita et al., 1995; 1998; de vita and foresta martin, 2017]. marine sedimentary rocks consist of clayey sands, organogenic and detrital fossiliferous limestones, and carbonate concretions. they were sedimented during middle-upper pleistocene sea level high-stands, related to glacio-eustatic movements, which also generated five orders of marine terraces [de vita and orsi, 1994]. carbonate speleothems, claudia speciale et al. 2 landslide deposits, loose detrital fans and aprons, sometimes pedogenized, are deposits of continental origin, related to the more recent geomorphological evolution of the island [de vita, 1993]. volcanism was active since the lower/middle pleistocene until the last eruption, which occurred at 130 ka bp [romano and sturiale, 1971; cinque et al., 1988; de vita, 1993; de vita et al. 1995; 1998; de vita and foresta martin, 2017]. therefore, it is reasonable to assume that humans had to interact only with the exogenous morphodynamic processes, active on the island in more recent times [speciale et al., 2019] (figure 2). 3 exploitation georesources in piano dei cardoni figure 1. position of ustica in the thyrrenian sea (modified after google earth; image landsat, data sio, nooa, us navy, nga, gebco, map data © 2016). figure 2. geological map of the island with the position and age of all the formations [de vita and foresta martin, 2017]. claudia speciale et al. 4 ustica has been occupied at least since the middle neolithic, as evidenced by the finds of punta spalmatore and with less certainty grotta azzurra, for which the materials are attributed to the middle copper age [mannino, 1979; mannino and ailara, 2016]. the modern tourist village at punta spalmatore largely destroyed the prehistoric settlement, but obsidian and ceramics dated between the middle and final phase of the sicilian neolithic were recovered [mannino, 1998], revealing for the first time, in 1997, the occupation of the island in these phases. the occupation during the copper age is attested not only in the caves san francesco and azzurra, on the southern side of the island [mannino and ailara, 2014; 2016], but also in piano dei cardoni [spatafora, 2009; 2016] with materials dated to the early and middle phases of the sicilian copper age (4th millennium bce) (figure 3). the 2018-2020 fieldwork campaigns at piano dei cardoni revealed the presence of a settlement and a funerary area with a megalithic stone tomb [speciale et al. submitted]. although a clear floor layer for the settlement has not yet been found, the site preserves some remains of wall structures, mostly made by earthen elements, some pits, and several tools for daily activities. underneath these structures, the semi-underground megalithic funerary structure was found (figure 4). ceramics retrieved during these last excavations have been grouped into 5 main ceramic wares, based on surface appearance and treatment: • dark burnished ware, which shows black to brown irregular spots on a burnished surface; • red burnished ware, known as diana style, distinguished from the previous ware by being mostly red or with irregular black spots on a red burnished surface; • incised and burnished ware, with linear incision on a burnished surface, which might be black to brown to red; • red painted ware, where the red paint might be applied on a white slip, making it similar to bande rosse/serra d’alto style ceramics, or directly on a smoothed surface; • some other vessels present a solely smoothed surface ranging black to brown to red. figure 3. ustica island. distribution of the sites cited in the text. based on the ceramic wares encountered, the site lifespan can be mostly set between middle neolithic and an early phase of late neolithic, mid-5th millennium bc [speciale et al., 2019], in line with the occupation of other areas in the north-western part of sicily and aeolian islands [tiné and tusa, 2012; cavalier, 1979; martinelli and quero, 2013; traverso, 2012]. bioarchaeological remains (animal bones and vegetal remains) are also preserved and are currently under study [speciale et al., 2021b]. 3. lithic tools 3.1 obsidian – materials and methods the functional study of obsidian lithic artefacts is still ongoing. a total of 41 obsidian artefacts were selected from the record of 2018 and 2019 investigations. samples were then analysed by k. freund. more detailed information on the use of pxrf is provided in tykot and foresta martin [2020]. 3.2 obsidian results the detailed results of the analysis are published in speciale et al. in press 2021a. the obsidian findings from the piano dei cardoni stratigraphic excavation are of good source quality, while from the surface findings lower quality obsidian raw material is attested. of the 41 analyzed artefacts employing the pxrf, 39 can be traced to the gabellotto gorge sub-source on the island of lipari. the other two obsidian items are from pantelleria, but they belong to a random collection from the surface and they cannot be attributed to a specific chronology. the only reliable source for this phase of piano dei cardoni is the island of lipari. 5 exploitation georesources in piano dei cardoni figure 4. view of the sepulchral area from north. 3.3 chert – materials and methods (table 1) based on macroscopic observation, the chert of the artefacts found at piano dei cardoni is mainly of the finegrained type and only a few specimens are coarse-grained. a preliminary observation with a stereoscopic microscope (8x and 100x) [tringham et al., 1974] has been performed. products have been divided into four broad categories: laminar, flakes, fragments and angular waste. out of a total of 81 artefacts, there are 33 laminar products, 10 flakes, 7 fragments and 31 angular waste, as detailed, with percentages (italic), in tab. 1. among the laminar products, 4 are entire, 8 are proximal fragments, 19 are mesial and 2 are distal. laminar products range from 6 to 18 mm in width, with a mean value of 11 mm. the surface of the artefacts is fresh for the 80,2% of the total; 11 artefacts (6 are laminar), that is the 18,5% of the total, are burned and only 1,2% shows a patina. artefacts are generally unretouched, with the exception of 6 laminar products, among which 2 have been recognized as sickle elements. remains of the cortex are almost totally absent on the whole assemblage. provenance of chert is highly difficult to assess, despite new methods are developing (see i.e. [gurova et al., 2021; muntoni et al., 2021]. the preliminary analysis on piano dei cardoni tools does not allow for the moment to determine their provenance. 3.4 chert – results the analysis revealed the good preservation of the surface for the future planning of use-wear analysis with a high-magnification approach [keeley 1980] and identified 4 artefacts probably exploited as sickle elements (figure 5.a-d). the distribution of the polish on the surface of the inserts suggests, at a preliminary observation in “low power” approach, that they were probably diagonally hafted, as described and detailed in mazzucco et al. [2018 and 2020] likely attesting here to the harvesting tradition, characterized by a curved sickle used for cutting at a low and middle height. two of the possible sickle elements (figure 5.b and 5.d) show a peculiarity: the polished surface is just one of the two faces, suggesting that the elements have been intentionally (?) removed from two original sickle inserts or, less probably, that a post-depositional surface modification (in this case resulting in a surface similar to a cereal micropolish, when observed at low magnification) affected only one of the two faces of the fragments. final products, such as proximal, mesial and distal parts of blade(let)s are the main category of the chert finds (40%), while by-products from knapping representing a similar percentage (38%), suggest modest or sporadic knapping activity on the site. in figure 5 we present a selection of the variety of chert reaching the site. there is a clear preference for very fine-grained varieties, but a coarse-grained variety is also present, as 5c (selected as sickle element) and 5e. chert is absent on ustica and lipari and we hypothesize that it was imported from the area of palermo or even from southern italy. further analyses on possible chert sources from sicily may help confirm this. 3.5 pumice, lavas, tuffs and sedimentary rocks – materials and methods (table 2) 66 lithic tools were collected and divided into categories according to their morphology and macroscopic wear traces. despite grinding tools attracting increasing attention from researchers, there is no shared agreement on the use of the terms to define the tools, probably due to the large number of shapes and local variables (see i.e. [lucarini and radini, 2021; hamon, 2008; dietrich et al., 2020]. some cm3 of 20 tools were then sampled, analyzed claudia speciale et al. 6 laminar flakes fragments angular waste tot. 33 10 7 31 81 40,7 12,3 8,6 38,3 100 table 1. ustica, piano dei cardoni. synthetic typology of the chert tools and waste collected in 2018-2019 campaign; percentage are in italics. 7 exploitation georesources in piano dei cardoni table 2. ustica, piano dei cardoni. typology of the volcanic tools with description, provenance of the raw materials, ranges of length, width and thickness in cm, total number of the tools per category collected in 2018-2019 campaign, total number of the sampled specimens for microscopic analyses. name sample id description lithology length width thickness total number n. of samples lower grinder 7, 11 it is mainly ovoid in shape. also called “quern” or “slab”, used as a basis for grinding of various transformable substances (it is completed with the presence of an upper grinder) monte guardia dei turchi basaltic lavas 7.5>17.5 7>17.5 3.5>11.5 4 2 small upper grinder 6, 8, 9 it has various shapes and a length under 15 cm, also called “hand grinder”. used together with a lower grinder for the crushing of transformable substances through a “back and forth” movement cala s. maria basaltic lavas, monte guardia dei turchi basaltic lavas 5.5>15 4.5>14 1.2>6 11 3 big upper grinder 15 it has various shapes and a length over 15 cm, also called “hand grinder”. used together with a lower grinder for the crushing of transformable substances through a “back and forth” movement monte guardia dei turchi basaltic lavas 15>24 13>14 7>12 2 1 pestle 14 it is pseudo-cylindrical in shape. used to crush substances such as herbs or spices etc. in a mortar. usually it is more flattened in the blunt part cala s. maria basaltic lavas 7 6 5 1 1 mortar / it is generally rounded in shape. used as a basis for pounding various substances. the dimensions are smaller than the millstones and it is likely to be used for different substances / 7>8 7 3.5>5 3 0 percussor / it is usually a pebble with a rounded or cylindrical shape. used as a “hammer” or as an intermediate tool for the production of other lithic tools. it is possible to recognize it according to wear traces in the final part / 5>10 3>4.5 2.5>4 2 0 polisher 5, 10 it is usually a pebble with a rounded shape. it is used to smooth and polish surfaces of various objects, such as pots or polished stone artifacts. it is recognizable by the flat and smooth shape of the lower surface cala s. maria basaltic lavas 3.5>8.5 2.7>7.5 1>3.5 22 2 hatchet 12 it is rectangular or trapezoidal in shape. the tool was probably used as wood hatchet or for agricultural purposes. cala s. maria basaltic lavas 7.2>12.5 4>7.5 0.5>2.5 9 1 fishing net weights / it is a rounded or oval stone with a polished circular perforation, probably used for the net thread / 4.5>8 4>4.5 1.5>3 2 0 other 1, 2, 3, 4 big rock fragments and tuffs used for the enclosure system of the tomb cala s. maria breccias, m. costa del fallo tuffs, m. guardia dei turchi basaltic lavas / / / 4 4 pumice tools 13, 15, 16, 17, 18, 19 small pumice pieces, usually with a smoothed face grotta del lapillo 3.2>6 4>5.5 3>4.5 6 6 and described according to their lithology to assess provenance and raw material selection. among these samples, 6 belong to the pumice tools and 13 to the other volcanic tools. at least one tool was sampled for each category, except for mortars, percussors and fishing net weights for the difficulty of sampling without creating relevant damage. pumice tools were all analyzed because of their potential provenance from the island of lipari. the lithological analysis of the specimens was carried out through the observation of their mineralogical and textural macroscopic characters on rupture surfaces exposed for this purpose, supplemented, where necessary, by observations made with the aid of a 10x magnifier lens or a binocular microscope with a 7.5x to 60x continuous zoom. 3.6 pumice – results (figure 6g) six pumice tools (ns. 13, 15, 16, 17, 18 and 19) were macroscopically analyzed. all the samples are grey/greyish pumice lapilli with a variable degree of vesiculation, aphyric or with rare phenocrysts of feldspar (plagioclase and anorthoclase) and femics (pyroxene and/or amphibole); samples n. 15 and 17 have clear signs of mechanical abrasion. the provenance of the raw material is clearly traceable to the trachytic pumice-fall and -flow deposits of the grotte del lapillo unit (420 ka [de vita et al., 1998]), widely exposed on the island, in the central ridge of hills and in the south-western corner of piano dei cardoni-oliastrello plain. the mineralogical paragenesis of the pumice clasts allows us to exclude their provenance from sources outside ustica. claudia speciale et al. 8 figure 5. ustica, piano dei cardoni. chipped artefacts (a-d, probable sickle elements; e-m, proximal and medial fragments of blades and flakes; n, backed tool with steep unifacial retouch on the lateral edge). partial selection of the variety of the (imported) raw material. 9 exploitation georesources in piano dei cardoni 3.7 lavas, tuffs and sedimentary rocks – results (figure 6a-f, h-i) two out of four samples (ns.1 and 2) from the tomb are composed of a heterometric breccia with lava fragments and carbonate cement, and a carbonate clast, consisting of a thin stratified calcarenite, with both detritic and organogenic carbonate elements and minute volcanic clasts. these probably derive from the unit of cala s. maria breccias (de vita, 1993), which is related to the reworking of cala s. maria lavas in a submarine environment during the crotonian (isotopic stage 9 of [shackleton and hopdike, 1973]) transgressive-regressive cycle, responsible for the formation of the first order marine terrace of about 350 ka [de vita and orsi, 1994; de vita et al., 1998]. one sample (n. 3) is a tuff fragment, consisting of a lithified, massive and poorly sorted coarse ash deposit, with subangular to sub-rounded, basaltic scoriaceous lapilli, lithic fragments and loose crystals. this sample is likely correlated to the unit of m. costa del fallo tuffs (approx. 500 ka), or the falconiera tuff (130 ka [de vita et al., 1998]). the last sample from the tomb (n. 4) is a fragment of a subaerial basaltic lava with scarce microphenocrysts of plagioclase and iddingsitized olivine, with altered carbonate encrustations. it very likely belongs to the m. guardia dei turchi unit (520 ka [de vita, 1993; de vita et al., 1998]), which was partially submerged by the sea during the formation of the first order marine terrace [de vita, 1993; de vita and orsi, 1994]. samples 5, 9, 10, 12 and 14, used for hatchets, upper grinders, polishers and pestles, are submarine basaltic lavas, compact or micro-vesiculated, aphanitic or microcrystalline with abundant plagioclase and subordinate olivine crystals, and probably belong to the unit of cala s. maria lavas or perhaps to the submarine part of the m. guardia dei turchi unit (de vita, 1993). subaerial basaltic lava fragments, well vesiculated, with plagioclase microphenocrysts figure 6. typology of lavastone tools: a) fishing net weights; b) percussor; c) small upper grinders; d) hatchets; e) polishers; f ) pumice tools; g) mortar; h) pestles; i) big upper grinder. from m. guardia dei turchi unit were used for the lower grinders n.7 and 11 and the upper grinders n.6 and 8; sample n. 8 is also characterized by the neoformation of alteration minerals within the vesicles, while sample n. 11 contains iron oxides deposited in the vesicles. the lithologic characteristics of all the samples, widely represented in the rocks exposed at ustica, reveal that all the raw materials in the piano dei cardoni site are from local georesources. 4. ceramics 4.1 materials and methods based on the ware classification, 30 samples were selected for petrographic examination to shed light on the raw material catchment area on the island and the possible circulation of finished ceramics from sicily or other sicilian islands. samples cover the most common wares found in the island and other contemporary contexts: dark burnished (13 samples), red burnished (3) incised and burnished (7 samples), slipped and painted (2), painted (2 samples) and smoothed (3 samples) wares (table 3, figure 7). polarized light microscopy on thin-section (0.03 mm thickness) was performed by a leica dm-slp microscope equipped with a leica digital camera (leica im100 image manager software). the use of transmitted (polarized) light microscopy on thin sections (0.03 mm) for the study of the ceramic material, as well known, allows the definition of the mineralogical composition and various textural parameters referable to the coarser aplastic inclusions and the fine plastic groundmass [quinn, 2009; hunt, 2016]. claudia speciale et al. 10 figure 7. pictures of representative samples for the wares considered in the study: a) ust19/029, dark burnished ware; b) ust19/022, red burnished ware; c) ust19/025, burnished and incised ware; d) ust19/003, painted ware, red painting on white slipped background; e) ust19/013, painted ware, red painting on unslipped background; f ) ust19/015, smoothed ware. 11 exploitation georesources in piano dei cardoni by recognizing the sand-sized monomineralic granules and lithic fragments and by evaluating their size, morphology and relative frequency, it is possible to acquire insights on raw materials sourcing and the overall production process [whitbread, 1995; gliozzo, 2020; montana, 2020]. from the petrographic analysis of the above-described neolithic ceramic set, it appears that the majority of the studied samples (29 out of 30) belong to the same fabric and on the basis of their lithology can be considered as a local production characterized by abundant volcanic inclusions of basaltic nature. based on textural aspects (e.g., approximate firing temperature, aplastic inclusions abundance and size distribution) fabric 1 can be further clustered into five subgroups. table 3. list of ceramic vessels sampled for petrographic examination. sample # sector su ware ust 19/ 001 27d 3 dark burnished ust 19/ 002 27d 3 incised and burnished ust 19/ 003 27d 3 slipped and painted ust 19/ 004 26d 3 dark burnished ust 19/ 005 26d 3 dark burnished ust 19/ 006 26d 3 dark burnished ust 19/ 007 26d 3 incised and burnished ust 19/ 008 26d 3 incised and burnished ust 19/ 009 26d 3 slipped and painted ust 19/ 010 25d 3 dark burnished ust 19/ 011 25d 3 painted ust 19/ 012 25d 3 red burnished ust 19/ 013 29d 7 painted ust 19/ 014 29d 7 dark burnished ust 19/ 015 29d 7 smoothed ust 19/ 016 29d 7 smoothed ust 19/ 017 30d 7 incised and burnished ust 19/ 018 30d 7 incised and burnished ust 19/ 019 30d 7 dark burnished ust 19/ 020 30d 7 dark burnished ust 19/ 021 30d 7 dark burnished ust 19/ 022 28d 7 red burnished ust 19/ 023 28d 7 dark burnished ust 19/ 024 28d 7 dark burnished ust 19/ 025 34d 10 incised and burnished ust 19/ 026 34d 10 incised and burnished ust 19/ 027 31d 8 dark burnished ust 19/ 028 27d 3 smoothed ust 19/ 029 31d 8 dark burnished ust 19/ 030 28d 7 red burnished the detailed petrographic descriptions of the microfabric in which the analysed samples have been classified are reported as supplementary material (supplementary 1), while a schematic description of fabrics is given in table 4. fabric 1a comprises most of the studied samples (n = 16). they were fired at a relatively higher temperature (around 800° c) as can be inferred from the low optical activity of the groundmass. this microscopic fabric is distinguished by the coarser average size and abundance (packing = 15-20% area) of the volcanic aplastic inclusions (figure 8ab). on the contrary, in fabric 1b (n = 4), aplastic inclusions are less abundant (10% area) and finer (figure 8c). fabric 1c (n = 6) includes samples characterized by the same volcanic aplastic inclusions (qualitatively and quantitatively), being the firing temperature visibly lower than fabrics 1a and 1b (estimated around 700° c by the optical activity of the groundmass) (figure 8d). fabric 1d is characterized by a ceramic body showing numerous cloths composed of microcrystalline calcite, mostly with irregular shape (figure 8e-f). sample ust 19/010 compositionally fits with fabric 1b, nonetheless, it appears strongly overfired. only the sample ust 19/013 constitutes fabric 2, being considerably different as regards the composition of the aplastic inclusions and also concerning the textural features. it shows the typical microscopic appearance of calcareous clayey raw materials fired at high temperature firing (higher than 800° c). the inclusions are all of sedimentary nature and fragments of a volcanic nature are totally absent (figure 9a-b). claudia speciale et al. 12 table 4. table summarizing the characteristics of the petrographic groups described in the supplementary material 1 (*) microcrystalline calcite (micrite) lumps could be originated from thermal decomposition (after the firing process) of inclusions which were originally present in the clayey raw material, or rather by secondary calcite precipitation from supersaturated percolating water which impregnated the ceramic body during the burial phase. fabric samples firing temp. grain size distribution / packing prevailing aplastic inclusions micrite lumps(*) 1a ust19/001, 002, 004, 005, 006, 007, 008, 016, 017, 020, 021, 022, 023, 024, 025, 027, 028 about 800° c serial (0.03-3 mm) prevailing: 0.3-0.5 mm packing: 15-20% (area) volcanic (plagioclase, basaltic lava fragments, clinopyroxene, olivine) yes (very few) 1b ust19/003, 011, 018, 030 about 800° c serial (0.03-2 mm) prevailing: 0.1-0.2 mm packing: 10% (area) volcanic (plagioclase, basaltic lava fragments, clinopyroxene, olivine) no 1c ust 19/009, 012, 015, 019, 022, 026 about 700° c serial (0.03-2 mm) prevailing: 0.2-0.5 mm packing: 15-20% (area) volcanic (plagioclase, basaltic lava fragments, clinopyroxene, olivine) yes (few) 1d ust 19/014, 029 about 700° c serial (0.03-3 mm) prevailing: 0.3-0.5 mm packing: 10% (area) volcanic (plagioclase, basaltic lava fragments, clinopyroxene, olivine) yes (many) ust 19/010 overfired serial (0.03-2 mm) prevailing: 0.1-0.2 mm packing: 10-15% (area) volcanic (plagioclase, basaltic lava fragments, clinopyroxene, olivine) no 2 ust 19/013 about 900° c serial to bimodal (0.03-0.5 mm) prevailing: 0.03-0.2 mm packing: 10-15% (area) sedimentary (quartz, calcareous bioclasts, kfeldspar, chert, quartzarenite) no 13 exploitation georesources in piano dei cardoni 5. discussion (figure 10) 5.1 the island of the grinding stones despite being a small, remote, island with limited resources, ustica hosts a high variety of useful volcanic resources that probably made it suitable for neolithic colonizers. the rocks used for anthropic purposes are exclusively of local lithology, easily reachable from the site. all the samples relate to units exposed close to piano dei cardoni area; the volcanic formations of m. guardia dei turchi, of the lave and brecce of cala s. maria and of the pomici of grotte del lapillo. nevertheless, the selection and choice of the tools, for their raw material and shape, beyond the knapping of the hatchets, show a varied and “specialized” typology. subaerial lavas of m. guardia dei turchi which are more vesiculated, were used for the upper and lower grinders, while the underwater ones of cala s. maria, more compact, were used for the pestles and the hatchets. of relevance is the presence of easily available pebbles eventually exploitable for different knapping stages of chert and obsidian products. all of this suggests the creation of tools for grain processing, but due to still ongoing archaeobotanical analysis, we cannot exclude the use of grinding tools for the processing of other vegetal resources. the pumices seem to have been employed as an abrasive for cleaning or similar uses, as they are very rounded but with the presence of vitreous dust inside the vesicles (the pumices naturally remodelled in the sea are instead very clean). figure 8. thin section photomicrographs representative of the compositional and textural characteristics of the ceramic finds under study, recognized as local productions: a) fabric 1a, sample ust 19/005 (xpl, scale bar = 0.5 mm); b) fabric 1a, sample ust 19/028 (xpl, scale bar = 0.5 mm); c) fabric 1b, sample ust 19/011 (xpl, scale bar = 0.5 mm); d) fabric 1c, sample ust 19/026 (xpl, scale bar = 0.5 mm); e) fabric 1d, sample ust 19/014 (xpl, scale bar = 0.5 mm); f ) fabric 1d, sample ust 19/029 (xpl, scale bar = 0.2 mm). ustica is a key site for the interpretation of lithic assemblages in small islands because both raw resources (chert and obsidian) required transportation by sea from other territories. additionally, there may have been a preferential channel of exchange and communication with the aeolian archipelago during these first phases of the occupation of the island. finally, it is likely that obsidian was imported as a raw material to the island, as testified by the outstanding number of big, medium, and small fragments (sometimes detected only by the 0.5 mm mesh) that attest to knapping activities on the site. obsidian in piano dei cardoni is the main exotic good at the site. the role of ustica as a trading place for obsidian towards north-western sicily can be inferred by several indicators [speciale et al., 2021a]. at this stage, it is not possible to establish if obsidian was the primary and/or only material or if it was a complement to other exchanged goods. at the same time, ustica resources, such as volcanic millstones or pottery, could be traded to north-western sicily and exchanged for chert or other goods. chert was likely imported from the area of palermo (south-eastern mesozoic reliefs are characterized by the presence of chert lens, nodules and beds), but due to some cultural aspects, other areas such as the southern apennines or the gargano subregion cannot be excluded. if an intentional modification of used tools is confirmed by future analyses, such as that inferred for two fragments hypothetically originating from sickle elements, it can be interpreted as a need to recycle a raw material naturally lacking on the claudia speciale et al. 14 figure 9. thin section photomicrographs showing the characteristics of the ust 19/013 sample, the only one certainly recognized as imported among the 30 ceramic finds under study. abundant grains of detrital quartz (angular to sub-rounded) and lithic grains (quartz and chert sandstones) of detrital sedimentary nature are visible: (a) crossed nicols, scale bar = 0.5 mm; (b) crossed nicol, scale bar = 0.2 mm. 15 exploitation georesources in piano dei cardoni island and probably preferred for specific purposes (functional analysis will explore this possibility). reuse of sickle elements and double use of the same inserts (suggested by the glossy appearance of sickle elements on both the edges), for example, was a technique for prolonging the use of the flint tools seen also at lipari during the late neolithic at contrada diana [martinelli, 2000]. on the contrary, at a preliminary stage of study, the observation of a high percentage of by-products of obsidian from lipari may suggest an easier access to this specific raw material. a similar distribution in the preference of the chert grain size has been observed at contrada diana [martinelli, 2000]. chert analysis confirms that ustica has more similarities to the close-to-the-source of obsidian sites, as previously supposed [speciale et al., 2021a]. further analyses will help in clarifying its possible source, that can now only hypothetically placed in north-western sicily. 5.2 local materials for local ceramics the majority of the ceramic samples examined showed inclusions (minerals and lithic fragments) that can be associated to the magmatic products characteristic of the island of ustica [cinque et al., 1988; de vita et al., 1998; peccerillo, 2005]. these ceramics can be distinguished from the coeval productions of lipari by the absence of intermediate or acidic rock fragments that characterize the magmatism of the aeolian island (e.g. potassium-rich andesitic basalts, pumice, obsidian, sanidine phenocrystals) [williams, 1980; williams and levi, 2008; brunelli et al., 2013]. furthermore, in the ceramic pastes at ustica, the femic minerals (especially clinopyroxenes) are much less frequent than that found in pottery produced at lipari and the plagioclase crystals are not zoned unlike those of the andesites on lipari. figure 10. distribution of the exploited local and non-local georesources from the site of piano dei cardoni. the red dot shows the position of the site. both compositional and textural aspects of the local ceramic paste provide interesting evidence about potters’ choice in raw material sourcing. they selected clayey-sandy soils in the island developed over volcanic deposits, being low calcareous and generally rich in iron oxides. they formed after the pedological evolution of lavas and/or pyroclastic deposits weathered by exogenous agents. colluvial sediment of this type forms preferably under favourable morphological conditions, like depressions in the bedrock, where they can reach a suitable thickness and collect a greater quantity of clay particles, which gives plasticity and workability. the lumps composed of microcrystalline calcium carbonate (micrite), which were found in several samples, could be ascribed to this process. hardened caco3 concretions can form in soils by chemical precipitation from infiltration waters. they are frequently found in arid areas, where even thick layers (caliche) may form. however, in the case of the studied ceramic samples, the presence of small micrite lumps (millimetric or mostly sub millimetric) could be also interpreted as secondary (burial) calcite precipitated directly in the ceramic sherd. this hypothesis is corroborated by the presence of small plagioclase and clinopyroxene crystals incorporated in the microcrystalline calcite mass (figure 8f). the textural variabilities observed in the examined ceramic fabrics could be ascribed to several reasons such as different raw materials sourcing areas or to “natural” variability within the same exploitation point. nevertheless, there is a striking continuity in using similar raw material for the manufacture of different wares across the period examined and in the tempering procedures (if any). in fact, all the petrographically recognized local ceramic pastes (fabrics) have as a common denominator the same type of volcanic aplastic inclusions (of various size, ranging from 0.04 mm to 2 mm or more). only the sample ust 19/013 (fabric 2) shows the absence of volcanic inclusions and compositional/textural characteristics (concerning both the aplastic inclusions and the groundmass). this suggests that the sample was imported from north-western sicily, possibly from the palermo area. its surface treatment (a reversed triangle painted in red out of the rim, probably attributable to “tricromica” ware) makes it stand out from the other painted ceramics found at the site. further sampling has been planned to better picture local and possible non-local manufacture and to locate the raw material catchment areas. 6. conclusions the first results from the analyses of the georesources in the site of piano dei cardoni (ustica, italy) shed new light on the exploitation of local and non-local resources on a small volcanic island. volcanic rocks were easily collected and were located very close to the site (in a range of 2 kilometres). they were selected to achieve the best performance of the grinding and milling tools. the elevated number of grinders, pestles and mortars testify to intense activity revolving around food/plant processing at the site. this was apparently not accompanied by a high frequency of crops, maybe due to taphonomical processes or maybe to the exploitation of wild species. the absence of obsidian resources on the island pushed human communities to import it from the aeolian islands. chert was also imported although its provenance remains undefined. the re-use of some chert tools leads us to believe that it was scarcer than obsidian. pumice was collected on ustica, probably due to local availability and good quality. most of the ceramic products sampled so far were made using local clay sources and only one sample seems to have come from the north coast of sicily. the study of local clayey raw materials (already in an advanced state of completion through simulations and experimental firings) and the analysis of a larger number of ceramic finds, will allow a more complete interpretation of the “production chain”. in conclusion, the neolithic inhabitants of ustica were exploiting all the raw materials available on the island, especially volcanic rocks, while at the same time procuring more distant materials, such as obsidian and chert. despite being preliminary, this interdisciplinary study reveals that even a distant and small island such as ustica was not excluded from the circulation of materials and objects that in the middle/late phase of the neolithic occurred in the central mediterranean. acknowledgments. this investigation was conducted thanks to the project brains2islands, “indagine multidisciplinare nei contesti insulari basso tirrenici”, project funded by fondazione con il sud, project number 2015-0296. richard claudia speciale et al. 16 17 exploitation georesources in piano dei cardoni j. brown, is warmly acknowledged for the revision of the text and for his useful comments, which improved the quality of the manuscript. in memory of giovanni mannino (1929-2021), who loved the island of ustica and opened the modern research on its prehistory. author contribution: conceptualization, c.s.; methodology, c.s., g.b., s.v., s.d.v.; investigation, c.s., g.b., r.m., g.m., f.m.; data curation, c.s., g.m., r.m., s.d.v., v.f., f.m.; writing-review and editing, c.s., g.m. , s.d.v.; supervision, g.b. , s.v.; funding acquisition, c.s. , m.d.v. , s.d.v. all 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(eds), meligunìs lipára, iv. l’acropoli di lipari nella preistoria. palermo, 847-868. williams, j.ll. and s.t. levi (2008). the ausonian pottery of lipari (aeolian islands, sicily) a typological and petrological assessment, pps 74, 141-170. https://doi.org/10.1016/j.quaint.2021.01.016 *co r r e s po n d i n g a u t h o r: claudia speciale istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, naples, italy and departiment of historical studies, university of gothenburg, sweden; e-mail: claudiaspciale@gmail.com © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international claudia speciale et al. 20 p r o p e r t i e s o f t h e m a t t e r at h i g h p r e s s u r e s b y y . p . t r u b i t s y n ricevuto il 7 dicembre i960 the ionic crystal mgo is the most abundant crystal of the earth's rocks. the state equation for mgo was obtained in this paper for pressures between zero and million atmospheres. the state equations for metal at high pressures are significant for physics of the earth's core. in this paper these equation were obtained for metal in the analytical form. usual tliomas-fermi-dirak theory of the electron gas is applicable only at extremelly high pressures p > 109 atm. statistical gombasjensen theory of the ionic crystals and metals gives good results only at not high pressures. therefore in this paper the modified statistical theory is used. by statistical theory the energy of some system of the electrons with density q is (goinbas 1949) u(q)=f\*k q5'3 + (vjc + v. ve) e < q1'31 ot xk = 2.871 xa' 0.8349 i t is the sum of kinetic (fermi) and potential energy of the electrons in the field of the nuclear and of the exchange-correlation energy. if the electronic density q is known from the experiment and is substituted into the formula for the energy, then the problem leads only to integration. in this paper the energy of the ionic crystal mgo is calculated. the electron density in crystal is taken as the sum of the overlapping electron densities of the ions. the electron density of the oxygen deformated in crystal was taken from yamasliita's and koyima's works (1952). that of magnesium was taken according to hartrec-fock of isolated ions. for pressures 0-106 atm. the calculated energy (with account for the quantum correction) can be approximated by the formula (trubitsyn 1958). e(v) = 93.7 exp (— 1.69 «vs) — 3.495 trva + 0.582 at. unity atom i \ 1 0 4 v . p . t r u b i t s y n the pressure is found by the formula p = de dv (1 at. unity of pressure = 3 x 10' atm., 1 at unity of volume = 0,0886 cm , /i — is the atomic number of the matter), s figure 1 plots the calculated energy of the crystal mgo at t = 0°k (a) and the experimental energy of bridgman (1949) extrapolated by davydov's method (1956) (b). f atun. a font 0.6 0.9 0.2 -0.2 109 10* 10' atm 0 6 0.8 7.0 v y k pig. 1. the energy of t h e ionic crystal mgo calculated by t h e author(curve a) and extrapolated from t h e experiment (curve b). for metals the electron density in crystal is taken as the sum of the density of the ionic cores and that of valence electrons. the first was taken according to hartrec-fock for isolated ions and was approximated by the formula o+ (r) = aexp (—br). the second was approximated by the constant gv (r) = q/v (theis 1955). q — is the number of valence electrons. the integrand was taken in form of power series p r o p e r t i e s of t h e m a t t e r at h i g h p r e s s s u r e s 1 0 5 and integrated. the dependence of energy and pressure upon volume was obtained for metals in the analytical form (trubitsyn 1960) e(v) = 2.871 v^—qll3 v^k[0.8349 + 1.4508 g2'3] + g6'3 b~a v-*'* [784.12 — 525.06 x + 16.4934 x2 — 12.026a?] + qll3 b~3 tr4/3 [720.26 + 244.81 x + 31.517 x2 + 3.4972 a;3] + q zr3 v-1 [405.88 a^ — 755.40 a1'* — 631.65 a 6"2] x = in (aq'1 v) . the pressure is p = — de/dv. every metal was characterized by means of three parametres a, b and q. por magnesium a — 45, b — 4,9 and q = 2. then p(v) = 21.52 v-1 + 6.095 tr'/s — 2.6352 v*'* + ir7'3 [15.3236 + 5.6403 x + 0.6753 x2 + 0.09985 a:3] + ir8'3 [49.4291 — 24.5025 x + 1.7152 x2 — 0.51083 a;3] ; x = in (22.5 v) . (curve a) and extrapolated from t h e experiment (curve c), as well as calculated by gombas (curve 6). 1 0 6 v . p . t r u b i t s y n figure 2 plots the energy and pressure of the metallic magnesium a t t = 0° k, calculated by the author (a), this one calculated by gombas (1949) (b) and the experimental energy and pressure of walsh and others (1957), recounted for t = 0° k (c). one can account for the temperature by the davydov's method (1956). i t is a pleasure to thank professor b. i. davydov for helpful discussions. summary the modified statistical method is proposed for calculation the equation of state for ionic crystal and metals. the equation of state for metals is obtained in analytical form. the method is illustrated on magnesium and magnesium oxide. riassunto si propone il metodo statistico modificato per il calcolo delvequazione di stato per il cristallo ionico e relativi metalli. l'equazione di stato per i metalli si ottiene in forma analitica. ii metodo e illustrato nel caso del magnesio e dell'ossido di magnesio. r e f e r e n c e s b r i d g m a n p . w., proc. amer. acad. arts. sci., 7 7 , 1 8 7 , ( 1 9 4 9 ) . d a v y d o v b . i . , bull. acad. sci, ubss, geophys. ser., 1 2 , 1 4 1 1 , ( 1 9 5 6 ) . g o m b a s p . , die statistische tlieorie des atoms und hire anwendungen. wien 1 9 4 9 . t h e i s w . r . , zeits. phys. 1 4 2 , 5 1 1 , ( 1 9 5 5 ) . t r u b i t s y n v . p . , j. exp. and theor. phys. tjssb., 3 4 . 1 , 2 2 1 , ( 1 9 5 8 ) . — solid state physics, ussb., 2 , 5 , ( 1 9 6 0 ) . w a l s h j . m . , r i c e m . h . , m c q u e e n r . g . , y a r g e r p . l . , phys. bev., 1 0 8 , 1 9 6 , ( 1 9 5 7 ) . y a m a s h i t a k . , k o j i m a m . , j. phys. soc. japan., 7 , 2 6 1 , ( 1 9 5 2 ) . u n a v a l u t a z i o n e d e l l o s p e s s o r e d e l l o « s t r a t o d e l g r a n i t o » n e l m e d i t e r r a n e o c e n t r o o c c 1 d e n t a l e o . e està p . e . v a l l e i. — i m e t o d i per la m i s u r a d e l l o spessore d e l l e s t r a t i f i c a z i o n i sup e r i o r i d e l l a erosta t e r r e s t r e sono l'ondali, c o m e è n o t o , sulla d e t e r m i n a z i o n e dei t e m p i di t r a g i t t o d e l l e o n d e s p a z i a l i d i r e t t e e r i t r a i l e , r e g i s t r a t e e n t r o una zona a b b a s t a n z a r i s t r e t t a i n t o r n o a l l ' e p i c e n t r o , o p p u r e «lilla d i s p e r s i o n e d e l l e o n d e s u p e r f i c i a l i t r a s v e r s a l i ( l ) o m i ste (1/). r e g i s t r a l e ad una d i s i a n z a d a l l ' e p i c e n t r o sufllcicn temente, e p e r c h é 1 e f f e t t o a d i p e r s i o i i c divenga s e n s i b i l e . il p r i m o m e t o d o può essere a p p l i c a t o u n i c a m e n t e nella es p l o r a z i o n e di zone n e l l e (filali la r e t e sismica è s u f f i c i e n i e i n e n le fitta; nel e a o cont r a r i o c o n v i e n e r i c o r r e r e a! s e c o n d o , con i l . (piale p e r ò -i può ott e n e r e solo una v a l u t a z i o n e a p p r o s s i m a t i v a in (pianto esso f o r n i s c e un r i s u l t a t o m e d i o r e l a t i v o al t r a g i t t o d e l l e o n d e s u p e r f i c i a l i . i n o l t r e q u e s t ' u l t i m o m e t o d o p r e s u p p o n e la c o n o scenza del n u m e r o d e l l e s t r a t i f i c a z i o n i e di c o n v e n i e n t i r a p p o r t i f r a l e costanti (dastielle e le d e n s i t à , r a p p o r t i c h e i n t e r v e n g o n o n e l calc o l o d e l l e cui ve di d i s p e r s i o n e . p e r i a p p l i c a z i o n e di q u e s t o m e t o d o si d i m o s t r a n o p a r t i r ò ! irm e n l e a d a t t e le c u r v e c a l c o l a l e da j e f f r e y ( ' ) n e l l ' i p o t e s i di una u n a v a l u t a z i o n e dello s p e s s o r e d e l l o « s t r a t o d e l g r a n i t o » e c c . 6 1 7 s t r a t i f i c a z i o n e s e m p l i c e , l e q u a l i c o n s e n t o n o u n a v a l u t a z i o n e d e l l o s t r a t o s u p e r i o r e d e l l a c r o s t a t e r r e s t r e , p u r c h é si c o n s i d e r i t r a s c u r a b i l e l ' a z i o n e di a l t r e e v e n t u a l i s t r a t i f i c a z i o n i . e s s e sono state c a l c o l a t e con l ' a s s u n z i o n e : u'/li = 20/9 q'/q = 5/4 d o v e |i è la r i g i d i t à e q la d e n s i t à d e l l o s t r a t o s u p e r i o r e u' <! y le c o r r i s p o n d e n t i g r a n d e z z e del m e z z o s o t t o s t a n t e . l ' a t t e n d i b i l i t à di q u e s t o m e t o d o è a s s i c u r a t a dal c o n f r o n t o f r a i r i s u l t a l i o t t e n u t i da j e f f r e y s , c h e lo h a a p p l i c a t o a l l ' e u r a s i a , p a r t e n d o da v a l o r i o s s e r v a t i da g u t e n b e r g e a s s u m e n d o p e r la v e l o c i t à d e l l e o n d e t r a s v e r s a l i d e l l o s t r a t o s u p e r i o r e 3 . 3 k m s e c ~ 1 1 c o n i corr i s p o n d e n t i r i s u l t a t i d e d o t t i d a l l ' a p p l i c a z i o n e dei m e t o d i b a s a t i sull ' o s s e r v a z i o n e d e l l e o n d e s p a z i a l i ( 2 ) . d a l l ' e s a m e d e l l a t a b e l l a i , n e l l a q u a l e a b b i a m o i n d i c a t o con t il p e r i o d o d e l l ' o n d a , con c la v e l o c i t à di g r u p p o e c o n dl lo s p e s s o r e d e l l o « s i t a l o del g r a n i t o » , r i s u l t a c h e l ' a c c o r d o fra i due m e l o d i è s o d d i s f a c e n t e , t e n u t o c o n t o c h e v a l o r i e l e v a l i di dl si h a n n o p r e v a l e n t e m e n t e s o n o le c a t e n e di m o n t a g n e . tabella i a ) metodo della dispersione delle onde m eurasia t sec 35 3 0 2 4 2 0 c k m s e c 1 3 . 8 3 , 3 3 , 2 3 . 9 3 d1 k m 17 28 2 4 h2 9 2 3 b) metodo delle onde spaziali s p es so re a sia e u r o p a s p es so re a sia n o r d e u r o p a a l p i s v e v e a l p i p r e a l p i g a m i c h e a p p e n n i n o t o s c o e n i i 1. e u r o p a c e n t r a l e d, k m 2 0 2 5 3 0 2 5 3 0 3 5 4 0 3 0 2 0 2 . —n e i s i s m o g r a m m i del t e r r e m o t o d e l l ' 1 1 m a g g i o 1 9 4 7 ( c a l a b r i a ) ( ! ) , a b b i a m o n o t a l o u n c o n s i d e r e v o l e a n t i c i p o dei t e m p i di a r r i v o d e l l e o n d e m n e l l e s t a z i o n i s p a g n o l e r i s p e t t o a l l e s t a z i o n i 61 c . f e s t a p . e . v a l l e n o r d e u r o p e e , al q u a l e n o n c o r r i s p o n d e u n a s e n s i b i l e d i f f e r e n z a f r a i p e r i o d i m e d i d e l l e o n d e s u d d e t t e . l e o n d e m r e g i s t r a t e n e l l e stazioni s p a g n o l e h a n n o c o m p i u t o il loro t r a g i t t o q u a s i e s c l u s i v a m e n t e nel m e d i t e r r a n e o c e n t r o o c c i d e n t a l e , m e n t r e il t r a g i t t o d e l l e o n d e m r e g i s t r a t e n e l l e s t a z i o n i del n o r d e u r o p a è quasi i n t e r a m e n t e c o n t i n e n t a l e . p e r t a n t o si è r i t e n u t a p o s s i b i l e u n a v a l u t a z i o n e c o m p a r a tiva d e l l o spessore d e l l o « s t r a t o del g r a n i t o » nel m e d i t e r r a n e o c e n t r o o c c i d e n t a l e r e l a t i v a m e n t e a l l ' a n a l o g o s p e s s o r e n e l c o n t i n e n t e e u r o p e o , v a l u t a z i o n e c l i c s i n o r a n o n ci r i s u l t a sia m a i stata e f f e t t u a t a . a t a l e s c o p o a b b i a m o scelto le s t a z i o n i di t o l e d o e t o r t o s a p e r q u a n t o r i g u a r d a il c a m m i n o m e d i t e r r a n e o e q u e l l e di u c c i e , c o p e n h a g e n e s t o c c a r d a per il c a m m i n o c o n t i n e n t a l e . d a l l ' a z i m u t d e l l e s t a z i o n i s p a g n o l e r i s p e t t o a l i e p i c e n t r o , in r e l a zione a l l ' o r i e n t a z i o n e degli s t r u m e n t i , a b b i a m o d e d o t t o c h e il p r i m o g r u p p o d e l l e o n d e m è n e t t a m e n t e p o l a r i z z a t o in un p i a n o n o r m a l e al p i a n o p r i n c i p a l e . l a p o l a r i z z a z i o n e n e l l a s t a z i o n e di c o p e n h a g e n è p a r z i a l e , m e n i l e n e l l e stazioni di u c c i e e s t o c c a r d a non risulta s e n z ' a l t r o e v i d e n t e . n e l l a t a b e l l a i i sono c o n t e n u t e le d i s t a n z e e p i c e n t r a l i , il t e m ilo di t r a g i t t o t , il p e r i o d o m e d i o t e la v e l o c i t à c del p r i m o g r u p p o di o n d e m r e g i s t r a l e n e l l e s i n g o l e s t a z i o n i . tabella 11 stazione a km t sec. t sec. c km sec.1 t r a i l i , t o r t o s a 1411 4 2 3 16 3 . 3 4 i n e d i l e i r a n c o 1 o l e d o 1798 5 5 5 15 3 , 2 4 t r a g i t t i s t o c c a r d a 1 2 7 5 4 8 0 16 2 . 6 6 coni ine itale u c c i e 1 6 6 2 6 1 0 18 2 72 c o p e n h a g e n 1917 7 2 1 1 5 . 5 2 , 6 6 i d a l l e c u r v e di d i s p e r s i o n e di j e f f r e y s si d e d u c o n o i v a l o r i di p /.(•(/, in l'unzione di ('. p. dove p è la v e l o c i t à d e l l e o n d e trasversali nel p r i m o s t r a t o , l la lunghezza d ' o n d a , c la v e l o c i t à di f a s e e 2 jt l . s e -i p o n e : l -vi k m sec 1 i na v a l u t a z i o n e hello s p e s s o r e hello <' s t r a t o d e l g r a n i t o » e c c . 6 1 9 i v a l o r i assunti da c ' |j n e l l e t a z i o n i del nord k u r o p a c o n d u c o n o in p r o s s i m i t à dei m i n i m o di kcdlt e q u i n d i p e r c i a s c u n a s t a z i o n e si o t t e n g o n o d u e v a l o r i di q u e s t ' u l t i m a g r a n d e z z a , uno a ) . a s i n i s t r a , . a l t r o b), a destra del p u n t o di s t a z i o n a r i e t à . i n v e c e a ciascun v a l o r e a s s u n t o da c ' |) n e l l e stazioni s p a g n o l e c o r r i p o n d e un solo val o r e di (j acy/,. n e l l a t a b e l l a i i i -rnio c o n t e n u t i i r i s u l t a t i n u m e r i c i d e d o t t i d a l l a p r e d e t t a c u r v a di d i s p e r s i o n e . tabella 111 s t a z i o n e c |3 p l i e d j d j k m t r a g i t t o t o r t o s a 1 . 0 1 2 0 . 5 7 9 1 4 . 5 m e d i t e r r a n e o toledo 0 . 9 8 2 ti . 5 1 0 1 4 , 6 (h j do (a) i.i 1 r a g i t t o s t o c c a r d a 0 . 8 0 6 0 . 3 6 3 0 . 3 1 4 2 3 . 2 2 6 . 8 c o n t i n e n t a l e c e c i e 0 , 8 2 4 0 . 3 9 1 ( 1 . 2 8 7 2 4 , 2 3 3 . 0 c o p e n h a g e n 0 , 8 0 6 0 . 3 6 3 0 . 3 1 4 2 2 , 4 2 6 , 0 3 . — n e i r i g u a r d i d e i r i s u l t a t i o t t e n u t i d o b b i a m o r i l e v a r e c l i c d e i d u e g r u p p i a) e b) di v a l o r i dello spessore d e l l o s t r a t o s u p e r i o r e n e l c o n t i n e n t e , l ' a c c o r d o con i v a l o r i d e l l a t a b e l l a i è m i g l i o r e p e r il g r u p p o b). c l i c p e r il g r u p p o ci). c o m u n q u e p o i c h é a m b e d u e i g r u p p i di v a l o r i sono n o t e v o l m e n t e i n f e r i o r i ai c o r r i s p o n d e n t i v a l o r i n e l m e d i t e r r a n e o , è e v i d e n t e la r i d u z i o n e s u b i t a dal suddetto s p e s s o r e n e l m e d i t e r r a n e o r i s p e t t o al c o n t i n e n t e e u r o p e o . c i r c a l ' a t t e n d i b i l i t à d e l v o l o r e m e d i o di 1 4 , 5 k m da noi o t t e n u t o p e r io s p e s s o r e d e l l o strato del g r a n i t o n e l m e d i t e r r a n e o , osserv i a m o c l i c la im i i n a z i o n e della curva di d i s p e r s i o n e p e r i v a l o r i d i c |j r e l a t i v i a l l e s t a z i o n i e s a m i n a t e è sufficiente ad a s s i c u r a r e c h e gli e v e n t u a l i e r r o r i c o m m e s s i n e l l a v a l u t a z i o n e di c/|3 n o n si r i p e r c u o t o n o s e n s i b i l m e n t e sui c o r r i s p o n d e n t i v a l o r i di i3). a ed,. n o t i a m o i n f i n e c h e l ' a c c o r d o f r a i v a l o r i d e d o t t i d a l l e d u e staz i o n i s p a g n o l e è m o l t o b u o n o . f i v a l o r e m e d i o di 1 4 . 5 k m p u ò q u i n d i e s s e r e a t t r i b u i t o p r o v v i s o r i a m e n t e a l l o s p e s s o r e d e l l o « s t r a t o del g r a n i t o ) ) n e l m e d i t e r r a n e o c e n t r o o c c i d e n t a l e . ci r i s e r v i a m o eli c o n d u r r e u l t e r i o r i r i c e r c h e a q u e s t o p r o p o s i t o . roma — istituto nazionale di ccofisica — luglio 1948. 0211 c. f e s t a " p . e . valle riassunto mediante l'effetto di dispersione delle onde superficiali m , gli autori hanno eseguito una valutazione comparativa dello spessore dello "strato ilei granito" nel mediterraneo centro-occidentale, relativamente all' analogo spessore nel continente europeo. b i b l i o g r a f i a i 1 ) jeffheys i l : the surtc.ce waies of eartliquakes geopliys. s u p p l . 3, 2532 6 1 , 1 9 3 5 . ( 2 ) g u t e n b e r g b . : seisniologìcal evidence for roots of mountains b u l l . g e o l . s o c . ara. 54, 473-498, 1913. — calci p . : sullo spessore dello strino delle onde pg nell'europa centrale r i c . scient. i , 334-338, 1938. ricerche su terremoti ad origine vicina r i c . s c i e n t . i i , 408-446, 1938. caratteristiche sismiche dell'appennino tosco-romagnolo r i c . s c i e n t . 4, 218-230, 1940. caratteristiche sismiche fondamentali dell'europa centrale quali risultano da 17 terremoti centro-europei b o l l . s o c . sism. l t a l . x l , 41-72, 1942. ( * ) valle p . e . : contributo allo studio delle caratteristiche sismiche del mediterraneo centro-orientale " a n n a l i di geofisica 1, 266-278. 1948. layout 6 1 annals of geophysics, 64, 4, rs439, 2021; doi:10.4401/ag-8638 o p e n acc e s s new insight into the 24 january 2020, mw 6.8 elazığ earthquake (turkey): an evidence for rupture-parallel pull-apart basin activation along the east anatolian fault zone constrained by geodetic and seismological data tahir serkan irmak*,1, mustafa toker2, evrim yavuz3, erman şentürk4, muhammed ali güvenaltın4 (1) department of geophysical engineering, kocaeli university, kocaeli, turkey (2) department of geophysical engineering, yüzüncü yıl university, van, turkey (3) i̇stanbul metropolitan municipality, directorate of earthquake and geotechnical investigation, i̇stanbul, turkey (4) department of geomatics engineering, kocaeli university, kocaeli, turkey article history: received february 8, 2021; accepted june 16, 2021 abstract in this study, we investigated the main features of the causative fault of the 24 january 2020, mw 6.8 elazığ earthquake (turkey) using seismological and geodetic data sets to provide new insight into the east anatolian fault zone (eafz). we first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using interferometric synthetic aperture radar (insar) interferograms (sentinel-1a/b satellites) and teleseismic waveform inversion, respectively. also, we determined the centroid moment tensor (cmt) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. finally, we evaluated the co-seismic slip distribution and the cmt solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. the cmt solution of the 24 january 2020elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the eafz. the rupture process of the elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the ne-sw but mainly toward the southwest. the rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). the rupture stopped to down-dip around 20 km depth toward the southwest. the distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. most of the moment release was released sw of the hypocenter and the rupture reached up to around 50 km. the focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (nne-ssw), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the eafz, consistent with sw-rupture directivity and coseismic deformation pattern. finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the eafz. keywords: east anatolia; pull-apart basin activation; surface deformation; waveform inversion. 1. introduction the neand sw-striking east anatolian fault zone (eafz) is inter-continental transform fault boundary between central anatolian high plateau (cap), where the lithospheric mantle separated from the crust and arabian plate, where lithospheric mantle attached to the crust [bartol and govers, 2014] (figure 1). the left-lateral strikeslip motion of the eafz [mcclusky et al., 2000; reilinger et al., 2006; angus et al., 2006] is the actual boundary between the arabian and cap, underneath which the deep arabian lithosphere is not observable [angus et al., 2006]. similar to right-lateral strike-slip faulting in n, the north anatolian fault zone (nafz), the eafz possesses almost all the characteristics of the thick plate(arabian plate in s) and thin plateau(cap in n) bounded left-lateral strikeslip faulting and related high earthquake potential (figure 1). hence, the eafz is a major structural element in the active tectonics of the cap and the partitioning system of the eurasia-arabia collision and thus, accommodates both lateral and also vertical components of oblique convergence in the region [e.g., lyberis et al., 1992]. the cap uplifted by the mantle [e.g., aldanmaz et al., 2000; ilbeyli et al., 2004; bartol and govers, 2014] is underlain by an anomalously thin lithospheric mantle as shown by low seismic wave velocities at sub-crustal levels [al-lazki et al., 2004; gans et al., 2009; gök et al., 2003; hearn and ni, 1994; maggi and priestley, 2005; mutlu and karabulut, 2011; schivardi and morelli, 2011; bartol and govers, 2014] and is characterized by a high surface heat flow today [tezcan and turgay, 1989; bartol and govers, 2014]. a single delamination event modulated by crustal thickening in the continental collision segment of the plate boundary [bartol and govers, 2014] suggests that the eafz formed a transform seismogenic boundary. cap predominantly moves westward relative to eurasia since the time of the arabian collision, accommodated along the nafz and eafz [mckenzie, 1976; şengör, 1979; dewey and şengör, 1979; şengör et al., 1985; schildgen et al., 2014]. the westward movement of cap has localized a significant amount of strain accumulation along the eafz [şengör et al., 2005; faccenna et al., 2006; özeren and holt, 2010] and the nafz. previous studies along with the southern cap margin point to various multi-phases uplift-downlift mechanisms across the eafz, which are accompanied by various-sized earthquakes [taymaz et al., 1991; kutoğlu et al., 2016], and mainly resulted from strong strain accumulation along the broad bend of the eafz [see schildgen et al., 2014 for details]. along the eafz and nafz, interseismic locking depths inferred from geodetic data (2003 to 2010) indicated that the nafz is strongly coupled to depth ranges of up to 20 km; while the eafz is coupled to shallow depth ranges of up to 5-10 km [bletery et al., 2020]. the eafz is a young and immature fault system, characterized by multiple and discrete fault traces at upper crustal levels [türkoğlu et al., 2015] and lies within a cryptic area of arabia-eurasia collision, where many discontinuous fault segments can be observed [westaway and arger, 2001, westaway, 2003; moreno et al., 2011] (see figure 1). these discrete segments seem to play a significant role in accommodating stress sourced from the collision. however, how exactly these small-scale faults relate to the eafz is unknown, but their interaction with such a large scale transform fault motion is key to understand how the westward movement of cap accommodates the arabia-eurasia collision along the eafz. this is important to develop and to improve the accuracy of earthquake scenarios considered for future seismic hazards in and along the eafz. except for the large destructive earthquakes in and along eafz [e.g., taymaz et al., 1991; melgar et al., 2020; cheloni and akıncı, 2020; pousse-beltran et al., 2020; taymaz et al., 2021], not so many geophysical studies based on national seismic networks have been carried out for eafz. for example, the basic role of this large transform fault system on the accommodation of the eurasiaarabia collision is debate and has been recently questioned. although active thrust-reverse faulting is recorded at the further ssw-portion of the eafz between the taurides and the arabian plate [e.g., 1975, ms 6.7 thrust earthquake in lice by taymaz et al., 1991], oblique-slip deformations associated with discontinuities of fault tahir serkan irmak et al. 2 segments, parallel-subparallel to the eafz, locally exist [e.g., westaway and arger, 2001, westaway, 2003; moreno et al., 2011]. however, multiple fault traces, parallel-subparallel discrete fault zones, immature structural characteristics, and upper crustal segmentation of the eafz [türkoğlu et al., 2015] have been not constrained yet. thus, the overall seismotectonic of the eafz associated with an orientation of tensional stress (e.g., presence of 3 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) figure 1. a) overview of the topography and bathymetry of turkey and surrounding area. blue arrows indicate plate motion directions. an: anatolian plate, ar: arabian plate, cap: central anatolian high plateau, eafz: east anatolian fault zone. b) epicentral or study area, solid black lines indicate east anatolian fault zone [duman et al., 2002]. red star indicates the location of the january 24, 2020 (mw 6.8) earthquake. white circles indicate the earthquakes larger than 4.0 between 01.01.1900 and 24.02.2020. a) b) tahir serkan irmak et al. 4 normal-slip and normal oblique-slip) in and along the eafz is still open. hence, we feel a strong need for geodetic and seismological information on the eafz. considering that the neand sw-ends of the eafz are highly strained sections [kutoğlu et al., 2016], it is a critical geodynamic setting within the cap domain where accretionary orogeny is associated with slab delamination and break off in e, and escape tectonics toward w is intricately linked. this interacting plateau domain with a wealth of recent geophysical and geodetic data from cap and eafz that inspired this study spans a wide range of crustal seismic velocity and attenuation tomography studies [e.g., şahin and öksüm, 2021 and references therein] and plate model observations [schildgen et al., 2014; bartol and govers, 2014 and references therein] and offers an exceptional chance to understand the seismic potential of eafz and evaluate the recently occurred the 24 january 2020, mw 6.8, elazığ earthquake in the ne strained section of eafz. although the largest destructive earthquakes were not recorded in the last era in and along the eafz [jamalreyhani et al., 2020], the 24 january 2020 elazig mainshock released a seismic moment, accumulated within ~222 years [bletery et al., 2020; taymaz et al., 2021]. source parameters and co-seismic slip distribution of the 24 january 2020elazığ mainshock have been studied by extensive research groups, based on strong ground motions and coulomb stress changes [cheloni and akıncı, 2020], geodetic observations [jamalreyhani et al., 2020; melgar et al., 2020], moment magnitude and rupture behavior of eafz inferred from geodetic data [pousse-beltran et al., 2020] and interseismic locking depths of the eafz constrained from geodetic observations between years of 2003 and 2010 [bletery et al., 2020]. in this study, we attempt a better assessment of the deformational relations of the faulting segments (e.g., to lake hazar) have remained unknown by using geodetic, earthquake source and aftershock focal mechanisms using nearand far-field approaches. on the other hand, the 24 january 2020 elazığ mainshock highlights a prominent migration of mainshock-aftershock seismicity, from ne toward sw, along the main fault strike of the eafz and possible activations of adjacent oblique fault segments, parallel-subparallel to the eafz. such a characterization of deformation area is, in fact, “unique”, providing an opportunity to understand whether the focal mechanisms constrained by moment tensor solution and teleseismic waveform inversion are consistent with the co-seismic surface displacements and related deformation pattern caused by co-seismic slip of the mainshock in a strike-slip tectonic setting of oblique collision. this is to encourage the current study to perform centroid depths of aftershocks of the elazığ mainshock using a new approach. the new approach presents the best constraints on both seismological and geodetic data set, particularly in the absence of available near-field recordings. in this study, we attempt to analyze the co-seismic surface displacements of the 24 january 2020 elazığ mainshock detected by cors-tr gnss stations. at the same time, we examine the source fault ruptured by the mainshock using the horizontal and vertical co-seismic displacement characteristics of the rupturing through analyzing insar interferogram images, the co-seismic surface deformations, and pattern of deformational geometry surrounding the rupture area to constrain the source location. then, we compute teleseismic waveform inversion to obtain co-seismic slip distribution of the source fault rupture and to compare them with co-seismic surface deformations determined by insar analyses. finally, we present the catalog-located aftershock distribution and centroid moment tensor (cmt) solutions of the aftershocks and investigate the deformational compatibility of the co-seismic deformation with co-seismic slip distribution. our study aims to show evidence for rupture-parallel pull-apart basin activation along the eafz using geodetic and teleseismic waveform observations and cmt solutions of aftershocks. 2. data and methods 2.1 gnss and insar data processing gnss observations of 9 stations belonging to the continuously operating reference stations-turkey (corstr) network located around the epicenter of the 24 january 2020 elazığ earthquake were examined (figure 2c). the detailed information of stations was given in table 1. the daily solutions of the stations for 30 days before and after the earthquake were obtained by using gamit / globk v.10.73 software [herring et al., 2015]. differences in the mean of the solutions obtained from pre-seismic and post-seismic periods were interpreted as the displacement (co-seismic) caused by the earthquake. also, 1 hertz observations were analyzed with the gamit / track module. the epicenter distances were correlated with the moments when significant changes occurred in the time series. table 1. detailed information of cors-tr stations. we used sentinel-1 synthetic aperture radar interferometry (insar) to monitor the surface deformation produced by the 24 january 2020 elazığ earthquake. we processed interferometric wide acquisitions from ascending and descending orbits, using open-source snap v7.0 esa software [veci et al., 2014]. since they provide a measurement 5 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) station code geog. coordinates lat. (n0) long. (e0) ady1 37.7604 38.2612 arpk 39.0406 38.4873 bing 38.8855 40.5008 diyb 37.9544 40.1875 elaz 38.6447 39.2565 ergn 38.2696 39.7582 maly 38.3377 38.2169 siv1 37.7529 39.3217 tnce 39.1097 39.5456 figure 2. geodetic observations of the elazığ earthquake. a) horizontal surface displacements along the rapture zone in e-w direction from insar analysis. b) vertical surface displacements. c) velocities of the used cors-tr stations and focal mechanisms of the mainshock and aftershocks. d) interferogram obtained from master (orbit = 30988, track = 116) and slave (orbit = 30813, track = 116) images in ascending mode. e) coherence map. the solid black and red lines indicate eafz. the yellow star shows the epicenter of main event. of ground motion along two opposite lines of sight (los), the horizontal and vertical displacements are calculated with the following formulas through the two interferograms [dalla via et al., 2012]. (1) (2) where 𝜃� and 𝜃�, look angles for both orbit modes, 𝐷𝑎 and 𝐷𝑑, two displacements measured along the los, 𝑑𝑒 and 𝑑𝑧, horizontal and vertical components of the displacement, the subscript a and d, ascending and descending, respectively. the parameters of the used ascending and descending image pairs were given in table 2. in order to calculate displacements in both two orbit modes, we used the digital elevation model (dem) of srtm 1 arc-second global for back geocoding and topographic phase removal. we also used the goldstein phase filtering [goldstein and werner, 1998] with a window size 3 and fast fourier transform size of 128 to enhance the signal to noise ratio (snr). the interferograms were unwrapped by the statistical-cost, network-flow phase-unwrapping algorithm (snaphu) [chen and zebker, 2002]. table 2. parameters of the used interferograms. 2.2 teleseismic waveform inversion in order to estimate a detailed and stable model of the january 24, 2020, elazığ earthquake, we applied an inversion scheme that was earlier described by yoshida et al. [1996], yagi, and kikuchi [2002], and yagi et al. [2003]. for this purpose, digital recordings of teleseismic body waveforms for the mainshock, which are clearly visible at 28 seismic stations, were retrieved from global seismic network (gsn).  we assumed that the rupture propagation took place along a single fault. for the discretization of the region to be modeled, we tested different fault dimensions; and then, we finally selected an optimal fault dimension with a total of 27 sub-faults (an area of 10 km x 10 km) consisting of 9 sub-faults in the strike direction and 3 sub-faults in the dip direction. we assumed that the rupture started at the hypocenter. the green’s functions for teleseismic body waves were calculated by using a method of kikuchi and kanamori [1991]. in order to represent the region nearby the receiver, we used a standard jeffreys-bullen earth model [jeffreys and bullen, 1940]. the slip rate function of each sub-fault is expanded into a series of 5 triangle functions with a rise time of 0.6s. the rupture velocity of 3.2 km/s was also selected by trial and error to determine the initiation time of the basis function at each sub-fault. 𝑑� = 𝐷� cos 𝜃�₋𝐷� cos 𝜃� sin(𝜃�+𝜃�) 𝑑� = 𝐷� sin 𝜃�₋𝐷� sin 𝜃� sin(𝜃�+𝜃�) tahir serkan irmak et al. 6 sr. no. slave image master image track mode ingestion date orbit ingestion date orbit 1 2020-01-16 t09:57:05.544z 30820 2020-01-28 t09:48:14.734z 30995 123 descending 2 2020-01-15 t19:44:57.093z 30813 2020-01-27 t18:54:50.341z 30988 116 ascending 2.3 regional moment tensor analysis in order to obtain the focal mechanism of the aftershocks, we used the isola software which is based on a multiple point source representation [sokos and zahradnik, 2008]. the source mechanism is represented by five elementary double-couple sources and one isotropic source as in kikuchi and kanamori [1991] and the moment tensor matrix consists of the sum of double-couple (dc) component, compensated linear vector dipole (clvd), and volumetric (iso) component (where dc% + clvd% + iso% = 100 %) [vavrycuk, 2001]. the green’s functions have been calculated by the discrete wavenumber method of bouchon [1981]. the quality of the solution is controlled by the variance reduction value which means the highest variance reduction value indicates the best fit between the observed and synthetic waveform. more detailed information for the inversion routine can be found in sokos and zahradnik, [2008]. 3. interpretation and discussion 3.1 gnss and insar analysis the offsets of the stations indicated significant displacements in the horizontal direction. elaz station, which is about 35 km away from the epicenter, slipped around 6 cm in the north-east direction (figure 2). the ergn and maly stations, which are at an average distance of 65 km to the epicenter, moved 3 cm opposite each other in a vertical direction to the fault. the directions of motion of these stations are consistent with the tectonics of the fault. diyb and siv1 stations, on the same side as the ergn but relatively farther away from the fault, have moved in the same direction but at a lower value. the highest deformations in the vertical direction are around 1 cm. since this value is very close to the standard deviation, it cannot be considered a significant change. our results coincide with the deformations obtained by yalvaç [2020] at the mm level. 3.2 finite fault inversion model the sequence of aftershocks that is lying in the direction of ne-sw suggests that the rupture also can occur on the fault plane in the ne-sw direction (figure 3a). therefore, the finite fault slip modeling was performed for the ne-sw oriented nodal plane. the best solution is shown in figure 3b. it indicates that major moment release in 20 km southwest of the initiation point of the rupture around the hypocenter with the maximum slip of around 1.30 m at 6 km depth. the obtained seismic moment is 1.815 x 10 19 nm. the rupture duration is about 24 sec. the fitting between 28 observed and synthetic pwaveforms is also shown in figure 3c. obtained slip distribution suggests that a main asperity is located around 20 km southwest of the hypocenter or foci. the rupture propagated bilaterally along the ne-sw direction (mainly southwest). maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth. the rupture stopped to down-dip around 50 km depth toward the southwest. the distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. 3.3 distributional pattern and focal mechanisms of aftershocks the regional moment tensor analysis of the analyzed aftershocks is characterized by normal fault and strike-slip faulting with normal faulting component. an example of the analyzed event which is given in table 3 is shown in figure 4. ne-sw trending pattern of aftershock distribution, parallel to the rupture propagation direction toward sw (figure 3), is characterized by a series of distinct focal mechanisms (figure 4). the cmt solutions of the aftershocks range from pure strike-slip through normal-oblique-slip to normal-slip within a relatively narrow area of the rupturing process. the centroid depths of the focal mechanisms with normal components are confined at upper crustal depths of up to 10 km (shallow-seated), while those with strike-slip focal mechanisms are started from a 7 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) depth of 10 km up to 20 km (deep-seated) (table 3). shallow-seated focal depths of events represent thicker sedimentary sections controlled by upper crustal faults, while deep-seated focal depths characterize stable strikeslip motion at middle and/or lower crustal levels. tahir serkan irmak et al. 8 figure 3. a) epicenter, focal mechanism and 24 hours aftershocks of the 24 january 2020 elazığ earthquake, black lines indicate active faults in the study area [duman et al., 2012]. b) focal mechanism, moment-rate function, coseismic slip distribution of the 24 january 2020 elazığ earthquake. the strike, dip and rake angles of the first and second nodal planes (np), focal depth and seismic moment (mo) of the earthquake are also given in the header. the white star indicates the focal depth obtained from minimum misfit solution. the vertical scale near the slip model shows the displacement values in meters. c) comparison of the observed (black) and synthetic (red) teleseismic broadband p-waveforms used in slip distribution inversion (right). station code and maximum amplitude are above the waveforms. 9 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) # date ot (utc) lat (on) lon (oe) cd (km) sm (nm) mw strike (o) dip (o) rake (o) dc (%) clvd (%) vr 1 24.01.2020 19:49:37.31 38.4395 39.1692 7.5 3.715x1015 4.3 66/335 88/88 2/178 65.5 34.5 0.50 2 24.01.2020 20:42:10.12 38.3502 39.0780 5.5 6.005x1014 3.8 33/216 49/41 -92/-87 90.6 9.4 0.58 3 24.01.2020 20:45:02.79 38.4227 39.1212 9.5 3.316x1015 4.3 62/153 87/59 -31/-177 90.3 9.7 0.69 4 24.01.2020 22:01:32.82 38.3398 39.0423 10.0 6.719x1014 3.8 359/240 47/63 -141/-50 80.5 19.5 0.76 5 24.01.2020 22:19:28.78 38.3333 38.9845 2.0 2.278x1014 3.5 350/234 65/47 -131/-35 50.4 49.6 0.46 6 25.01.2020 00:48:49.64 38.4763 39.1532 5.5 1.321x1015 4.0 19/180 66/25 -82/-108 82.8 17.2 0.63 7 25.01.2020 00:57:16.81 38.3705 39.0723 6.5 7.325x1014 3.8 239/332 64/85 -5/-154 96.6 3.4 0.69 8 25.01.2020 04:37:56.69 38.3302 39.0017 5.5 4.215x1014 3.7 150/49 61/71 -158/-31 79.1 20.9 0.60 9 25.01.2020 06:07:32.47 38.3703 39.0598 6.0 1.263x1015 4.0 236/335 59/75 -17/-148 94.5 5.5 0.62 10 25.01.2020 08:40:03.23 38.5040 39.3003 12.5 2.405x1015 4.2 73/332 71/61 31/158 82.3 17.7 0.54 11 25.01.2020 10:14:55.73 38.1968 38.8008 8.0 3.246x1015 4.3 71/164 84/66 -24/-173 76.0 24.0 0.69 12 25.01.2020 16:30:07.18 38.3518 39.0263 7.0 2.358x1016 4.8 143/49 79/70 -160/-12 75.3 24.7 0.51 13 25.01.2020 16:44:00.79 38.3662 39.0817 9.5 2.843x1015 4.2 145/53 76/81 -171/-14 83.8 16.2 0.51 14 25.01.2020 16:46:59.08 38.4447 39.0955 5.5 2.231x1015 4.2 15/253 51/57 -135/-49 61.0 39.0 0.59 15 26.01.2020 02:22:45.65 38.2803 38.8103 6.5 3.949x1015 4.3 323/232 82/83 -173/-8 93.8 6.2 0.56 16 26.01.2020 10:12:16.86 38.3423 39.0978 10.5 6.504x1014 3.8 312/42 89/71 161/1 99.1 0.9 0.67 17 26.01.2020 11:31:31.90 38.2670 38.7315 18.5 2.355x1014 3.5 255/160 79/67 23/168 95.7 4.3 0.56 18 27.01.2020 16:11:59.41 38.4078 39.0980 5.5 2.245x1015 4.2 159/69 89/86 -186/-1 84.5 15.5 0.65 19 31.01.2020 23:32:48.96 38.5002 39.2917 8.0 7.051x1015 4.5 324/195 46/57 -131/-56 88.0 12.0 0.70 20 01.02.2020 00:03:48.01 38.4128 39.2070 1.5 1.213x1015 4.0 151/241 87/85 175/3 50.3 49.7 0.71 21 03.02.2020 22:19:39.77 38.3703 39.0773 4.0 4.685x1015 4.4 49/142 85/61 -30/-174 98.1 1.9 0.70 22 07.02.2020 19:57:04.38 38.4032 39.2083 1.5 1.137x1015 4.0 22/288 80/69 21/170 77.6 22.4 0.81 23 17.02.2020 11:42:14.04 38.3222 39.1332 5.5 2.695x1015 4.2 67/335 70/84 6/160 75.2 24.8 0.73 24 25.02.2020 23:03:34.43 38.2767 38.7523 15.0 2.353x1016 4.8 162/257 86/33 123/6 91.9 8.1 0.55 25 27.02.2020 02:08:46.15 38.2392 38.6430 7.0 2.185x1015 4.2 341/238 82/23 -112/-19 99.0 1.0 0.71 26 29.02.2020 12:29:46.03 38.4385 39.2367 2.0 3.601x1015 4.3 240/333 83/65 -25/-172 94.2 5.8 0.75 27 19.03.2020 17:53:30.08 38.3952 39.0948 5.5 3.351x1016 5.0 68/323 69/56 36/155 99.2 0.8 0.64 table 3. source parameters of the analyzed aftershocks obtained from regional moment tensor analysis. ot: origin time, cd: centroid depth, sm: seismic moment, mw: moment magnitude, dc: double couple, clvd: compensated linear vector dipole, vr: variance reduction. the distributional pattern of aftershock seismicity and their focal mechanisms shown in figure 5 are also correlated with the co-seismic horizontal (figure 6) and vertical (figure 7) deformation results obtained from the modeling of insar for the elazığ earthquake. seismicity pattern of aftershocks shown in figures 6a and 7a and related focal mechanisms shown in figures 6b and 7b are well consistent with the horizontal and vertical components of coseismic deformation. as shown in figure 2, the same as figure 7, the vertical displacement ranges from 20 cm (max) to 10 cm (min) within an extended subsidence area toward sw (anatolian side) parallel to the rupture directivity (figure 3) compatible with the focals with the normal component. toward the further sw, a narrowed uplift area parallel to the rupture directivity is ranged from 15 cm (max) to 10 cm (min). maximum w-e horizontal displacement for deep-seated strike-slip focal is ± 30 cm (figure 6). in figures 6 and 7, the seismicity pattern of aftershocks and the focal mechanisms with normal components are also well compatible with patchy-like crushed patterns of butterfly geometry of co-seismic interferogram shown in figure 2. in figure 2, through crushed sections, co-seismic coherency sharply decreases to a minimum value of 0.4 and remains stable, suggesting an anomalous vertical surface displacement; widespread down lift and prominent subsidence in the anatolian side, rather than the uplift localized at sw-end. however, no unexpected anomaly in and along the horizontal surface displacement is observable along crushed sections of butterfly geometry, compatible with deep-seated strike-slip focals. the comparison of co-seismic deformation obtained from insar observations shown in figure 2 to aftershock seismicity (figures 6a and 7a) and focal mechanisms (figures 6b and 7b) reveals that the downlifting, consistent with shallow-seated focal with the normal component, characterizes co-seismic vertical deformation in the anatolian side, rather than the arabian side and that the uplifting is only a localized effect in the anatolian side. however, the w-e horizontal surface displacement (± 30 cm) shows co-seismic surface deformation compatible with deep-seated strike-slip focals, the source mechanism of the mainshock, and the sw directivity of the rupture source (figure 3). tahir serkan irmak et al. 10 figure 4. the regional moment tensor analysis result for the event no 2 in table 3. (upper). obtained focal mechanism and source parameters. (lower) waveform fitting, black: observed seismograms, red: calculated seismograms. gray traces did not used in inversion process. 11 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) it is noticeable from the vertical surface displacement from the extensive down lift to the local uplift, consistent with the rupture directivity from ne to sw, the shallow-seated normal focals, and those with the normal component, that the rupture source started from its initiation point at ne (maximum subsidence and focals with normal components), propagated to the sw (maximum local uplift) strongly deformed the anatolian side (figures 6 and 7). although the source mechanism and co-seismic slip distribution of the elazığ mainshock located on the eafz indicate left-lateral strike-slip motion (figure 3) compatible with the horizontal component of co-seismic surface displacement (figure 2), we interpret that its aftershock sequence is not related only to the activity of the main fault ruptured by the mainshock and that some other controlling discrete faults are segmented in the study area. as indicated in figures 6b and 7b (also see figure 5), the focal solutions of the number of aftershocks show different mechanisms, not compatible with a deep and large-scale transform motion of the eafz. the shallow-seated focal solutions distinctly show the existence of ne-sw and nne-ssw oriented tension. this suggests pure normal and also normal oblique-slip faults consistent with the pull-apart mechanism. as given by figure 7, the vertical component of the co-seismic surface displacement, seismicity pattern of aftershocks, shallow-seated normal focals, and those with normal component confirm, to a certain extent, the presence of pull-apart basin activation that may have been formed due to left lateral shearing of the eafz. we consider that this finding is highly matched with the lake hazar pull-apart structure [moreno et al., 2011 and references therein], suggesting possible reactivation of pre-existing normal and/or normal oblique-slip faults that form, bound, and follow the lake hazar in the ne [e.g., the lake hazar as a result of negative flower structure proposed by aksoy et al., 2007; moreno et al., 2011]. considering the rupture-parallel pull-apart basin activation associated with normal and normal oblique-slip focal mechanisms of shallow-seated aftershocks in the n of the eafz (the anatolian side), the assumption of a simple strike-slip motion on a single main fault is unlikely to be correct due to crustal diversity and misfits of focal mechanisms caused by rupture source complexity in and along eafz. pull-apart basin activation controlled by discontinuous fault segments within the ruptured area is in good agreement with the results obtained from the crustal-scale observations along the eafz [e.g., low seismic velocity by gans et al., 2009; very low electrical resistivity by türkoğlu et al., 2015; low velocity-high attenuation by şahin and öksüm, 2021] but surprisingly, has not been constrained by early studies [e.g., melgar et al., 2020; cheloni and akıncı, 2020; pousse-beltran et al., 2020; bletery et al., 2020; jamalreyhani et al., 2020]. figure 5. focal mechanism of the analysed aftershocks in the study area. tahir serkan irmak et al. 12 figure 6. the epicentre of the 2020 elazığ mainshock, distribution of aftershocks and their focal mechanisms superimposed on the horizontal component of the co-seismic surface displacement shown in figure 2a. a) seismicity pattern of aftershocks, and b) their focal mechanisms. beach ball in red indicates the mainshock (an: anatolian block, ar: arabian plate, hl: hazar lake). a) b) 13 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) figure 7. the epicentre of the 2020 elazığ mainshock, distribution of aftershocks and their focal mechanisms superimposed on the vertical component of the co-seismic surface displacement shown in figure 2b. a) seismicity pattern of aftershocks, and b) their focal mechanisms. beach ball in red indicates the mainshock (an: anatolian block, ar: arabian plate, hl: hazar lake). a) b) 5. conclusions the rupture process of the 24 january 2020 elazığ earthquake indicates major moment release in 20 km southwest of the hypocenter with the maximum slip of around 1.30 m at 6 km depth. the main asperity is located around 20 km southwest of the hypocenter. the rupture propagated bilaterally along the ne-sw direction (mainly southwest). the rupture stopped to down-dip around 20 km depth toward the southwest from the hypocenter. the average centroid depths of the analyzed aftershocks with the normal component are shallower (~10 km) than those with strike components and also elsewhere in the eafz. except for the left-lateral strike-slip focal mechanism of the 24 january 2020 elazığ mainshock, there are two main trends of aftershocks for which estimated focal mechanisms are dominantly normal components, implying extensional and transtensional movements parallel to sw-rupture propagation in the anatolian side. the co-seismic vertical surface deformation supports the proposed existence of the normal component and rupture-parallel pull-apart mechanism, close to lake hazar in the further ne. the 24 january 2020 elazığ mainshock-aftershock sequence is one of the most important earthquakes controlling and driving the westward extrusion tectonics of the cap along the eafz. the focal mechanisms and the co-seismic surface deformation, particularly its vertical component are observed to have strongly localized in thin crustal cap in n (the anatolian side), rather than the thick lithospheric arabian plate in s (the arabian side). normal-slip and normal oblique-slip aftershock focal mechanisms support and confirm the strain partitioning of the eurasia-arabia collision along the eafz and thus, the eafz accommodates both lateral and also vertical components of oblique convergence in the region. finally, the study of the 2020 elazığ earthquake concludes that the westward movement of the cap is accommodated by not only the left-lateral strike-slip motion of eafz but also the upper crustal extensional-transtensional motions in and along eafz. acknowledgments. the authors are grateful to the organizations including the general directorate of mapping, general directorate of land registry and cadastre, the international gnss service, the euref permanent network for gnss data, and the research and service support team of the european space agency (esa) for insar data. the authors send their greatest thanks to prof. dr. ali pınar (boğaziçi university, kandilli observatory, and earthquake research institute, turkey) for his comments on waveform fits on normal focal solutions and also appreciate dr. yuji yagi for providing the slip inversion code. the authors acknowledge the kocaeli university research fund and disaster and emergency management presidency of turkey (afad) for providing computing facilities. this study was partially supported 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(2020). determining the effects of the 2020 elazığ-sivrice/turkey (mw 6.7) earthquake from the tahir serkan irmak et al. 16 17 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) surrounding cors-tr gnss stations, turkish j. geosci., 1, 1, 15-21. yoshida, s., k. koketsu, b. shibazaki, t. sagiya, t. kato and y. yoshida (1996). joint inversion of nearand farfield waveforms and geodetic data for the rupture process of the 1995 kobe earthquake, j. phys. earth, 44, 437–454. *co r r e s po n d i n g a u t h o r: tahir s e r ka n i r m a k department of geophysical engineering, kocaeli university, kocaeli, turkey, e-mail: irmakts@kocaeli.edu.tr © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international tahir serkan irmak et al. 18 19 the 24 january 2020, mw 6.8 elazığ earthquake (turkey) tahir serkan irmak et al. 20 vol48/04/2005def 843 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words pyroclastic flow – fluidization – gravity current 1. introduction considerable progress has been made over the last decade in mechanistic understanding of the establishment and flow of pyroclastic gravity currents (druitt, 1998; burgisser and bergantz, 2002; neri et al., 2002b). in particular, the role of gases – and of their interaction with pyroclastic solids – on the establishment of dense versus dilute suspension currents as a result of explosive volcanism has been highlighted. on one side, dense pyroclastic gravity currents result from limited amount of gas in the eruptive mixture leaving the crater, insufficient to produce a fully convective buoyant column. on the other, the amount of gas entrained by the current in the atmosphere is largely responsible for its mobility and hazard. the role of fluidization in the emplacement of dense pyroclastic flows has long been recognized (spark, 1976; wilson, 1980, 1984; marzocchella et al., 1998; roche et al., 2002; druitt et al., 2004; gravina et al., 2004). notwithstanding, quantitative assessment of fluidization in pyroclastic flows is still extremely poor. broad uncertainties still characterize fundamental aspects of pyroclastic flows, like: a) the prevailing nature and source of the fluidizing gas, either endogenous or associated with entrainment phenomena; b) the rheology of aerated/fluidized gas-solid two-phase dense flows under high-velocity strongly turbulent conditions; c) the perturbation to the classical phenomenology of fluidization determined by the highly sheared flow conditions; d) the influence of solids polydispersity on fluidization, segregation and flow rheology; e) the expected complexity of bifurcation/dynamical patterns of rapid unsteady granular flows on account of the strong nonlinearity of the governing equations. the current lack of fundamental understanding, despite the extensive published literature on assessment of motion-induced fluidization of dense pyroclastic gravity currents piero salatino dipartimento di ingegneria chimica, università degli studi di napoli «federico ii», napoli, italy abstract the paper addresses some fundamental aspects of the dynamics of dense granular flows down inclines relevant to pyroclastic density currents. a simple mechanistic framework is presented to analyze the dynamics of the frontal zone, with a focus on the establishment of conditions that promote air entrainment at the head of the current and motion-induced self-fluidization of the flow. the one-dimensional momentum balance on the current along the incline is considered under the hypothesis of strongly turbulent flow and pseudo-homogeneous behaviour of the two-phase gas-solid flow. departures from one-dimensional flow in the frontal region are also analyzed and provide the key to the assessment of air cross-flow and fluidization of the solids in the head of the current. the conditions for the establishment of steady motion of pyroclastic flows down an incline, in either the fluidized or «dry» granular states, are examined. mailing address: prof. piero salatino, dipartimento di ingegneria chimica, università degli studi di napoli «federico ii», p.le v. tecchio 80, 80125 napoli, italy; e-mail: salatino@unina.it 844 piero salatino rapid granular flows (savage, 1979; campbell, 1990), is largely due to the fact that investigations in this field mostly addressed steady flows of granular materials down inclines at velocities far smaller than those typical of pyroclastic flows. accordingly, these studies completely missed the complex dynamics of the frontal zone, which is bound to be one key to the onset of fluidization phenomena. moreover, the relative extent of frictional, collisional, streaming and turbulent stresses, which dictates the rheology of the flow, does not even approach conditions relevant to pyroclastic flows. due to the intrinsic phenomenological complexity and the lack of experimental results and of adequate theoretical frameworks, modelling of dense pyroclastic currents is still at an embryonic stage and lags far behind the more successful and comprehensive modeling of the dynamics of particle-lean gas-solid suspensions of pyroclastic nature (ash cloud) (neri et al., 2002a). the present paper addresses some of the open issues related to quantitative assessment of dense pyroclastic flows from a fundamental perspective. the dynamics of the frontal zone is considered, with an emphasis on the establishment of conditions that promote air entrainment and motion-induced self-fluidization of the flow. an approximate one-dimensional momentum balance on the current steadily flowing along the incline is proposed. it is based on the hypothesis of strongly turbulent flow, of pseudo-homogeneous behaviour of the two-phase gas-solid flow, embodying some of the features that bring about departures from one-dimensional flow. the conditions for the establishemnt of the steady motion-induced fluidization of the pyroclastic flow down an incline are examined. the proposed mechanistic framework provides the starting point for a broader discussion of research needs and priorities essential to fill the gap of quantitative understanding of unsteady rapid granular flows. 2. basic hydrodynamical and rheological features of a dense pyroclastic flow a dense pyroclastic flow is a two-phase gravity current moving down an incline (whose slope is θ) at a velocity u (fig. 1). the gravity current generates, as it moves, two macroscopic shear layers: a free (i.e. unconfined) shear layer at the top of the gravity current; a confined shear layer at the bottom of the gravity current. momentum exchange between the current and the environment takes place via entrainment of the surrounding medium at the upper free shear layer. momentum exchange between the current and the ground takes place via wall friction at the basal shear layer. the rheological behaviour of the two-phase gas-solid flow arises from a combination of the following contributions: yield strength due to interparticle friction; stresses due to collisional/kinetic and streaming contributions of the particle phase; viscous/turbulent stresses within fig. 1. outline of a pyroclastic density current. 845 assessment of motion-induced fluidization of dense pyroclastic gravity currents the interstitial gas phase; turbulent «reynolds» stresses at the macroscopic scale. the first contribution is associated with particle-to-particle stresses of frictional nature. the yield strength is generally expressed as a function of the local normal stress within the particle phase according to the coulomb law: (2.1) where µ is an internal friction coefficient and θo is the angle of internal friction of the granular solids. the second contribution is due to momentum transfer via particle-to-particle impact (collisional/kinetic component) or to momentum transfer of convective nature associated with particle velocity fluctuations (streaming component). the shear stress due to this contribution is usually expressed as (bagnold, 1954; patton et al., 1987) (2.2) where (2.3) is the effective viscosity of the gas-solid suspension. the third contribution accounts for deformation of the interstitial gas phase induced by the flow. this is generally neglected on account of the large value of the bagnold number for flow conditions of practical interest. the fourth contribution arises from the onset of turbulence over length scales much larger than the particle scale. it is a momentum transfer of convective nature but, differently from the streaming component, is related to the motion of particle pockets rather than of individual particles. altogether, the intrinsic rheological behaviour of the two-phase gas-solid system determined by the combined contribution of the above components is that of a bingham fluid, non-newtonian, shear-thickening. if the flow is in the fluidized state, then µ = θ°= 0 . accordingly, the yield strength van/( ( / ) > )ba d dv dy 1p z g2= t h > c d dy dv dy dv kf p z z2$ $= =h t c m c d dy dv dy dv ,zy c f p z z2 2 $ $= =x t hc cm m o( )tan,zy y y$ $= =x v n v if ishes. for typical parameters of dense pyroclastic flows (h ≅ 1 m; dp ≅ 500 µm; ρm ≅ 100 kg/m3; ρp ≅ 1000 kg/m3; u ≅ 30 m/s; cf = 0.01 according to patton et al., 1987) the relevant dimensionless numbers governing the flow of the gravity current take the following values: (2.4) (2.5) where the expression of the reynolds number holds for a power-law shear-thickening fluid whose viscosity is given by eq. (2.3) (levenspiel, 1998). the large values of richardson number suggest that momentum exchange via air entrainment at the upper free shear layer is negligible (britter and linden, 1979). on the other hand, the very large values of reynolds number indicate that the flow is dominated by inertia. 3. momentum balance in the onedimensional flow approximation the hypothesis is made that the flow of the current can be regarded as steady one-dimensional. furthermore, it is assumed that the current is in the fluidized state and, according to conclusions drawn in the previous section, momentum exchange is dominated by turbulence. the momentum balance equation reads (3.1) where z is a spatial coordinate along the incline. equation (3.1) equates the component along the incline of the gravitational force (lhs) to the force exerted on the current by friction at the ground (rhs). equation (3.1) yields, upon integration (3.2) both laboratory data on two-phase fully developed turbulent flows (wallis, 1969) and field data relative to large scale simulated dry dense snow avalanches (hopfinger, 1983) suggest that .1000 m/s2.p ( ) ( ) .sin sinu f g h h 2 = =i p i ( )sing hdz f u dz 2m m 2 =t i t 10-1ri ,=:ichardson ( ) r cosg u h a 2t t i∆ :re reynolds k h 2 3 10 10 m 2 7 8,= t 846 piero salatino for small slopes and relevant internal friction the literature on dry dense snow avalanches (hopfinger, 1983) suggests an alternative form of eq. (3.2) (3.3) 4. departure from the one-dimensional flow approximation the one-dimensional framework within which the momentum balance on the gravity current was developed falls short when the detailed flow structure of the frontal region is analyzed. phenomenological observation and numerical simulation of dense pyroclastic gravity currents highlight the following features of the flow in the frontal region (fig. 2): – flow elements at increasing distance from the ground move faster, possibly overtaking the front. accordingly 2d and 3d macroscopic flow patterns are established in the vicinity of the front (valentine and wohletz, 1989; todesco et al., 2002). these patterns include fall-out of coarse solids to the ground and vigorous recirculation of the suspension in the head of the current. ( ) ( ) .sin cosu h= -p i n i6 @ – the fall-out of coarse solids compresses the underlying air, which can be either «squeezed out» from the basal region of the front or forced to percolate through it. – coupled with the gas flow, finer solids are elutriated through the front contributing to the formation of the upper ash cloud. fines can also contribute to the momentum exchange between the uprising gas and the coarse solids. in particular, the second feature deserves attention in the context of the present theoretical development, on account of the possibility that extensive cross-flow of air is driven by overpressures which establish at the base of the current. in turn, cross-flow of air might be large enough to promote fluidization of solids in the head of the current affecting the rheology of the current itself. 5. an approximate criterion for selffluidization of a dense granular flow moving down an incline it is speculated that, as a consequence of the «average» motion of the front and of the fallout of the solids to the ground (fig. 3), the basal region of the front experiences a value of presfig. 2. solids and gas flow patterns in the head a pyroclastic density current. 847 assessment of motion-induced fluidization of dense pyroclastic gravity currents sure which is approximately given by (5.1) this value is obtained by application of the bernoulli equation as the stagnation pressure of a flow element initially moving at the front velocity u as it is brought to rest at the ground. there is general consensus that dynamic pressures in the frontal region of pyroclastic density currents can assume values ranging from a few kpa up to several tens of kpa (valentine, 1998; esposti ongaro et al., 2002; nunziante et al., 2003). when the two-phase nature of the flow is considered, it can be assumed that the stagnation pressure pstagnation is shared between the interstitial gas phase pg = αpstagnation and the solid phase ps = (1−α) p stagnation according to a pressure partition coefficient α. in the case α departs significantly from 1, the assumption of an isotropic particle phase pressure is clearly a gross simplification: it is likely that particle pressure be largely anisotropic under the strongly sheared flow conditions that characterize the motion of dense pyroclastic gravity currents. the simple idea underlying the self-fluidization criterion is that the overpressure pg in the gas phase associated with the stagnation condition provides the driving force for the flow of air from the basal region of the front across the current. this cross-flow might be large enough to promote fluidization of the current. to this end, the usual condition for fluidization of stationary beds of granular solids is considered: fluidization is established as fluid is supplied at the base of the bed at a relative pressure exceeding the buoyant weight of the bed divided by its cross-sectional area. of course, one should additionally require that fluid be .p u 2stagnation m 2 . t supplied at the base of the front at a flow rate in excess of that corresponding to incipient fluidization, in order to establish steady cross-flow under self-fluidized conditions: this condition is easily fulfilled if one considers that incipient fluidization velocities of pyroclastic solids typically range between a few mm/s and cm/s (gravina et al., 2004), whereas the front of the density current progresses at velocities in the order of tens of m/s. by combining these concepts, the approximated criterion for the motion-induced self-fluidization of the front is formulated as follows: «self-fluidization of the flow occurs if the basal gas pressure pg in the frontal region of the current exceeds the hydrostatic head imposed by the gas-solids suspension». or, equivalently (5.2) equation (5.2) yields, upon rearrangement and simplification of terms (5.3) equation (5.3) gives the lower limit of the slope θc of the incline beyond which self-fluidized flow of the gravity current may onset. the limit value of the slope turns out to be independent of the depth of the current h and of its speed u, which cancel out with each other in eq. (5.2). remarkably, for given flow conditions θc depends on the value of the pressure partition coefficient α, whose meaning and significance will be addressed later. ( ) ( ) .sin cos g2 1$ ap i i ( ) ( ) .sin cos p u h gh 2 2 1 g m m m 2 $= =a t a t p i i t fig. 3. assessment of the stagnation pressure in the basal layer of the head of a pyroclastic density current. solid line: trajectory of a flow element brought to rest as the front is shifted; dashed line: cross-flow of air driven by the stagnation pressure. 848 piero salatino it is worth noting that, according to the present analysis, fulfilment of the criterion given by eq. (5.3) is a necessary but not sufficient condition for the establishment of steady self-fluidized flow of the density current, which is also dependent on the initial conditions of the flow. more specifically, self-fluidization establishes provided that the initial conditions of the flow correspond to a fluidized state. this requirement is met, for instance, if the current results from the fall-out of solids (as in the collapse of an eruptive column). 6. implementation of the criterion and discussion of results figure 4 demonstrates the application of the self-fluidization criterion expressed by eq. (5.3). the lhs of eq. (5.3) is plotted versus the slope of the incline θ, the partition coefficient α being treated as a parameter. three values of the pressure partition coefficient α have been considered in the computations: 1, 0.1, 0.01. according to the criterion, regions in the plot where exceeds 1 correspond to slopes of inclines on which self-fluidized motion of the density current may establish. ( / ) ( ) ( )sin cosg2ap i i analysis of fig. 4 suggests that for the smallest value of the pressure partition coefficient, α = 0.01, self-fluidization is never established, regardless of the value of θ. when the partition coefficient α is set at 0.1, self-fluidization is established on inclines of slopes exceeding about θc = 13°. self-fluidization is established at incline slopes of θc = 2° and larger when the upper limiting value of α, namely 1, is assumed in the computations. the significance and implications of the selffluidization criterion are further clarified by fig. 5. the speed u of the density current is plotted in this figure as a function of the slope θ. the speed u is computed according to eqs. (3.2) or (3.3) depending on whether the flow is assumed to be in the self-fluidized state or not, respectively. computations refer to arbitrary (though realistic) values of the depth of the flow (h = 5 m) and of the angle of internal friction of the granular material (θ 0 = 27°). the latter value corresponds to a coefficient of internal friction of the solids µ = 0.5. again, plots corresponding to three values of the partition coefficient α (namely 1, 0.1, 0.01) are reported in the figure. for α = 0.01, the flow is never self-fluidized, whatever θ. steady flow cannot establish on inclines whose slope is smaller than the fig. 4. implementation of the self-fluidization criterion. 849 assessment of motion-induced fluidization of dense pyroclastic gravity currents angle of internal friction θ 0 (u=0). steady granular flow establishes for θ > θ 0, at a speed u computed according to eq. (3.3). for α = 0.1 and α = 1, steady fluidized flow can be established at slopes larger than θc (where θc = 13° for α = 0.1 and θc = 2° for α = 1). the following flow regimes can be recognized, depending on the slope: θ < θc – self-fluidization and steady flow of the current cannot be established. regardless of the initial conditions, the asymptotic condition of the flow is the rest: u = 0. θc <θ < θ0 – self-fluidization and steady flow of the density current can be established, provided that the initial conditions of the flow correspond to a fluidized state. the speed u is represented, for any given slope, by the upper curve, corresponding to the self-fluidized state of the front. if the initial conditions of the flow correspond to a granular (non-fluidized) state, self-fluidization and steady motion of the current cannot establish and the flow comes to rest. θ > θ0 – steady motion of the current is established anyway. if the initial conditions of the flow correspond to a fluidized state, this state will be self-preserved by the motion of the current. the speed u is given in this case by the upper curve in fig. 5 corresponding to the self-fluidized state. if the initial conditions of the flow correspond to granular (non-fluidized) flow, the flow will keep being granular. the speed u is given by the lower curve corresponding to the granular state. it is interesting to note that the critical slope θc plays the role of a bifurcation abscissa. multiple steady states may establish for θc < θ depending on the initial conditions of the flow. in particular it is noteworthy that steady flow of the density current can be established even at slopes smaller than the angle of internal friction θ0, provided that the initial conditions of the flow correspond to a fluidized state. this finding puts some emphasis on the proper choice of the initial conditions in experiments aiming at the characterization of unsteady granular flows along inclines. fig. 5. implementation of the criterion: computation of the front speed. depth of the current: h = 5 m; angle of internal friction θ 0 = 27°. 850 piero salatino similarly, these arguments shed some doubt on results obtained in experiments where the initial state of the flow was not properly documented (takahashi and tsujimoto, 2000). the significance of the pressure partition coefficient α is better appreciated as one considers its physical meaning in the light of the rheological behaviour of the two-phase gas-solid system. the limiting case α = 1 corresponds to pseudohomogeneous two-phase flow (wallis, 1969). this limiting case is characterized by the important feature that momentum transfer between the particulate phase and the interstitial gas phase is effective to the point that the slip velocity between the phases is negligible. on the other hand, α = 0 corresponds to the case where the stagnation pressure is entirely withstood by the granular solids. this corresponds to purely granular flow, with an expected strong influence of frictional stresses of coulombian nature. according to the criterion set by eq. (5.3), onset of self-fluidization is favoured by large values of α, that is, as pseudo-homogeneous flow conditions are approached. on the other hand, establishment of pseudo-homogeneous flow is greatly enhanced by: a) fineness and lightness of the bed material; b) polydispersion of the bed material as regards particle size distribution, as momentum transfer from gas to coarser particles may occur, as in «dusty» gas, via the finer particles; c) sheared flow of the suspension, on account of the property of shear to stabilize homogeneous, i.e. bubble-free, fluidization of solids (apicella et al., 1997). features a) and b) are remarkable properties of pyroclastic granular materials. maybe the recognition of the role of mutual momentum exchange between the particulate phase and the interstitial gas phase in the strongly accelerated flows at hand is the main outcome of the present study. it is recommended that parameters of the two-phase system which may affect interphase momentum exchange be carefully controlled in any experimental study aiming at the characterization of unsteady granular flows along inclines. 7. conclusions a simple criterion for the establishment of motion-induced self-fluidized flow of a pyroclastic density current is presented. it is based on a simplified formulation of the momentum balance equation on the frontal region of the dense flow, on the assessment of overpressures establishing in the basal shear layer of the flow as a consequence of solids fall-out, on the application of the criterion for fluidization of beds of granular solids. the criterion highlights the importance of the momentum exchange between the interstitial gas phase and the discrete particulate phase, expressed through a pressure partition coefficient α. this parameter significantly affects the results of the application of the criterion. its value depends on properties of the two-phase gas-solid system like particle size and density as well as polydispersity of the population of bed solids. the application of the criterion is demonstrated. it is shown that steady self-fluidized motion of the density current can be established even on slopes smaller than the angle of internal friction of the granular solids – but larger than a critical slope θc which depends on the pressure partition coefficient – provided that the initial conditions of the flow correspond to the fluidized state. the critical slope θc corresponds to a bifurcation point, marking the limit beyond which multiple steady flow conditions, keeping memory of the initial conditions of the flow, may establish. the proposed criterion is admittedly simple and its predictive capabilities have yet to be assessed. it might be helpful to shed light on some dynamical features of dense pyroclastic flows and to highlight aspects that need to be taken under control in experiments aiming at the characterization of dense granular flows. acknowledgements the paper is part of an ongoing research program on «dynamics of pyroclastic density currents and their impact on buildings», carried out under the umbrella of amra, centro regionale di competenza sul rischio ambientale, regione campania. the author is grateful to prof. lucio lirer, prof. antonio marzocchella, prof. luciano nunziante, dr. paola petrosino, dr. teresita gravina for useful discussion. the author is grateful to augusto neri and olivier roche for careful and constructive review of the paper. 851 assessment of motion-induced fluidization of dense pyroclastic gravity currents references apicella, e., m. d’amore, g. tardos and r. mauri (1997): onset of instability in sheared gas fluidized beds, aiche j., 43, 1362-1365. bagnold, r.a. (1954): experiments on a gravity-free dispersion of large solid spheres in a newtonian fluid under shear, proc. r. soc., 225, 49-63. britter, r.e. and p.f. linden (1979): the motion of the front of a gravity current travelling down an incline, j. fluid. mech., 99, 531-543. burgisser, a. and g.w. bergantz (2002): reconciling pyroclastic flow and surge: the multiphase physics of pyroclastic density currents, earth planet. sci. lett., 202, 405-418. campbell, c.s. (1990): rapid granular flows, ann. rev. fluid mech., 22, 57-92. druitt, t. (1998): pyroclastic density currents, in the physics of explosive volcanic eruptions, edited by j.s. gilbert and r.s.j. sparks, geol. soc. spec. publ. london, 145, 145-182. druitt, t., g. bruni, p. lettieri and j.g. yates (2004): the fluidization behaviour of ignimbrite at high temperature and with mechanical agitation, geophys. res. lett., 31 (2), l02604, doi: 10.1029/2003gl018593. esposti ongaro, t., a. neri, m. todesco and g. macedonio (2002): pyroclastic flow hazard assessment at vesuvius (italy) by using numerical modeling, ii. analysis of flow variables, bull. volcanol., 64, 178-191. gravina, t., l. lirer, a. marzocchella, p. petrosino and p. salatino (2004): fluidization and attrition of pyroclastic granular solids, j. volcanol. geotherm. res., 138, 27-42. hopfinger, e.j. (1983): snow avalanche motion and related phenomena, ann. rev. fluid mech., 15, 47-76. levenspiel, o. (1998): engineering flow and heat exchange (plenum press, new york). marzocchella, a., p. salatino, v. di pastena and v. lirer (1998): fluidization in pyroclastic flow, in fluidization ix, edited by l.-s. fan and t.m. knowlton (engineering foundation, new york), 389-396. neri, a., a. di muro and m. rosi (2002a): mass partition glossary ba bagnold number. cf coefficient in eqs. (2.2) and (2.3). d particle diameter, m. f friction factor. g acceleration due to gravity, ms−2. h depth of the current, m. k coefficient in eq. (2.3), kgm−1 re reynolds number. ri richardson number. u speed of the front, ms−1. vz particle velocity along the incline, ms−1. y spatial coordinate orthogonal to the incline, m z spatial coordinate along the incline, m. ∆ρ = ρm − ρa difference between the average density of the current and the air density, kgm−3. η effective viscosity of the gas-solid suspension, nsm−2. ηg gas viscosity, nsm−2. θ slope of the incline, degree. θc critical slope according to the criterion eq. (5.3), degree. θ 0 angle of internal friction of the particulate phase, degree. µ coefficient of internal friction of the particulate phase. ξ constant in eq. (3.2), m s−2. ρa air density, kg m−3. ρm average density of the current, kg m−3. ρp particle density, kg m−3. σ y normal stress in the particulate phase, pa. τzy, f , τzy, c shear stress of frictional, collisional nature, respectively, pa. 852 piero salatino during collapsing and transitional columns by using numerical simulations, j. volcanol. geotherm. res., 115, 1-18. neri, a., p. papale, d. del seppia and r. santacroce (2002b): coupled conduit and atmospheric dispersal dynamics of the a.d. 79 plinian eruption of vesuvius, j. volcanol. geotherm. res., 120, 141-160. nunziante, l., m. fraldi, l. lirer, p. petrosino, s. scotellaro and c. cicirelli (2003): risk assessment of the impact of pyroclastic density currents on the towns located around vesuvio: a non-linear structural inverse analysis, bull. volcanol., 65, 547-561. patton, j.s., c.e. brennen and r.h. sabersky (1987): shear flows of rapidly flowing granular materials, j. appl. mech., 54, 801-805. roche, o., m. gilbertson, j. phillips and s. sparks (2002): experiments on deaerating granular flows and implications for pyroclastic flow mobility, geophys. res. lett., 29 (16), 1792, doi:10.1029/2002gl014819. savage, s.b. (1979): gravity flow of cohesionless granular materials in chutes and channels, j. fluid mech., 92, 53-96. sparks, r.s.j. (1976): grain size variations in ignimbrite and implication for the trasport of pyroclastic flows, sedimentology, 23, 147-188. takahashi, t. and h. tsujimoto (2000): a mechanical model for merapi-type pyroclastic flow, j. volcanol. geotherm. res., 98, 91-115. todesco, m., a. neri, t. esposti ongaro, p. papale, g. macedonio, r. santacroce and a. longo (2002): pyroclastic flow hazard assessment at vesuvius (italy) by using numerical modeling, i. large-scale dynamics, bull. volcanol., 64, 155-177. valentine, g.a. (1998): damage to structures by pyroclastic flows and surges, inferred from nuclear weapons effects, j. volcanol. geotherm. res., 87, 117-140. valentine, g.a. and k.h. wohletz (1989): numerical models of plinian eruption columns and pyroclastic flows, j. geophys. res., 94, 1867-1887. wallis, g.b. (1969): one-dimensional two-phase flow (mcgraw hill). wilson, c.j.n. (1980): the role of fluidization in the emplacement of pyroclastic flow: an experimental approach, j. volcanol. geotherm. res., 8, 231-249. wilson, c.j.n. (1984): the role of fluidization in the emplacement of pyroclastic flow, 2. experimental results and their interpretation, j. volcanol. geotherm. res., 20, 55-84. annals 48, 3, 2005+app1 535 annals of geophysics, vol. 48, n. 3, june 2005 key words ionospheric tomography – total electron content – ionospheric propagation – ionospheric modeling 1. introduction the development of tomographic imaging has been one of the great scientific achievements of the final decades of the twentieth century. while the mathematical basis of the method was formulated early in the century, the advances in computing technology in the 1960s gave practical realization to the technique as an imaging tool. the method is hailed primarily for transforming medical diagnostics, but it has also lead to significant progress in many other fields, including geophysics. the underlying principles behind the method and descriptions of some of the early applications can be found in the special issue of proceeding of the ieee, 71 (3), 1983. further material of a fundamental nature can also be found in the book by kak and slaney (1988). a number of authors have reviewed the application of the method to radio tomographic imaging of the ionized atmosphere. early developments in the field are documented in special issues of the journals international journal of imaging systems and technology, 5 (2), 1994 and annales geophysicae, 13 (12), 1996, while in further reviews leitinger (1996, 1999) discussed the theoretical basis and the limitations of the ionospheric application. more recently, ionospheric tomography, including comprehensive descriptions of a number of theoretical formulations, has been the subject of a book by kunitsyn and tereshchenko (2003). in addition, pryse (2003) has reviewed experimental results from the application of radio tomography to ionospheric imaging, concentrating in the main on use of the method in investigations directed towards understanding of the underlying geophysics. the present paper aims to focus on aspects of the use of tomographic imaging as an aid to practical radio science applications. material relating to purely geophysical studies of the ionized atmosphere, which was presented in the oral version of the paper, can be found in the earlier review by pryse (2003) and will not be repeated here. ionospheric tomography and its applications in radio science and geophysical investigations leonard kersley radio and space physics group, department of physics, university of wales, aberystwyth, u.k. abstract the paper reviews recent progress in the development of radio tomographic imaging of the ionized atmosphere, particularly in relation to the use of the technique to assist in the mitigation of propagation effects on practical radio systems. mailing address: prof. leonard kersley, radio and space physics group, department of physics, university of wales, aberystwyth, dyfed sy23 3bz, u.k.; e-mail: lek@aber.ac.uk 536 leonard kersley the experimental results discussed here involve measurements of the line integral of the electron density made along ray paths to ground stations from satellites in low earth polar orbits. the total electron content (tec) is determined using the phase coherent signals from satellites in the navy ionospheric monitoring system (nims), monitored at a chain of receiving locations aligned essentially in longitude, but separated in latitude. inversion of the data set in a reconstruction algorithm yields a pixelised image in two dimensions of the spatial distribution of the electron density throughout the region of the ray-path intersections. while many groups worldwide have been involved in the development of radio tomographic imaging and the technique has now been used in many situations, the results presented here are concerned in the main with investigations of the potential usefulness of the method to applications related to practical radio systems. the experimental observations have been made for the most part at locations instrumented by the uwa group in u.k. and scandinavia. maps with the current stations operated by the group are shown in fig. 1, though it should be noted that some of the work discussed here relates to results from earlier measurements, including those made at five locations in u.k. spanning the latitudinal range between about 50°n and 61°n and campaigns involving additional sites in mainland sweden. the development work carried out in recent years has demonstrated that radio tomography has now matured into a powerful experimental technique capable of providing images of largescale spatial structures in the ionosphere from a limited number of ground stations. the spatial nature of the observations is complementary to those from many other types of experimental measurements of the ionosphere. the limitations to the method were thoroughly investigated and well understood from early simulation studies. these result primarily from the limited-angle geometry of the satellite-to-ground observations so that there is incomplete information in the raw measurements about the vertical profile. however, even at an early stage of the development, algorithms were formulated capable of refig. 1. maps showing locations of satellite receiving station chains in u.k. and scandinavia currently in use by uwa group. 537 ionospheric tomography and its applications in radio science and geophysical investigations producing ionospheric layers at different heights throughout the image (fremouw et al., 1993). in addition, the temporal coverage is dependent on the number and orbits of the available satellites with appropriate beacon transmitters. 2. tomography and ionosondes the ionosonde remains the basic experimental tool used to monitor the state of the ionized atmosphere for radio systems applications. fig. 2. estimates of peak electron density (nmf2) obtained from the ionosondes at chilton (left upper) and lannion (left lower) as a function of time, together with (right) tomographic images from satellite passes at three times throughout the night. the f denotes spread-f conditions. from kersley et al. (1997), reproduced by permission of the american geophysical union. significant advances have been made in recent years in the development of the instrument, not just in the electronic hardware and digital processing of the sounding signals, but also in inversion software that can yield electron density versus height profiles below the layer peak in near-real time. however, ionosondes can only provide measurements for specific locations at discrete times. the spatial imaging capability of the radio tomography technique forms a natural complement to the ionosonde for routine monitoring, with synergies of mutual benefit to 538 leonard kersley both methods. the addition of information about the horizontal structure of the ionosphere from tomographic imaging can be used to place the point measurements from the sounder in their wider spatial context, while inclusion of input from an ionosonde in the reconstruction algorithm can be used to mitigate the limited capability of the tomographic technique to determine heights. the complementary nature of the two experimental techniques is demonstrated in fig. 2. the upper and lower panels to the left show respectively measurements of the electron density at the layer peak (nmf2) obtained from the ionosondes at chilton (51.5°n) from lannion (48.8°n), as a function of time during a night of very severe geomagnetic disturbance. the interpretation of these observations can be elucidated from the tomographic images obtained from satellite passes at three times throughout the night (kersley et al., 1997). the spatial images show that the spread-f conditions are related to irregularities at the poleward edge of the trough, while it is a boundary blob that is responsible for the increased ionisation seen later in the night at slough, during these exception conditions where the trough minimum was found over northern france. measurements by ionosondes have also been used to improve the representation of the vertical profile in tomographic reconstructions. discussion of the role of ionosonde input in reconstruction algorithms, used as complementary information to the tec observations, can be found in kersley et al. (1993). the paper was also the first of several that include verification of the resultant image of electron density using independent results from the eiscat incoherent scatter radar. 3. tomography and empirical or parameterised models radio tomography is an experimental technique that can be used to provide independent measurements with which to test the functionality and reliability of the empirical and parameterized models that are in use for the mitigation of ionospheric effects on radio systems. fig. 3. an actual tomographic image of electron densities (upper panel), with corresponding outputs from three ionospheric models: (in descending order of panels) iri-95, pim and cost 238. from dabas and kersley (2003), reproduced by permission of the american geophysical union. 539 ionospheric tomography and its applications in radio science and geophysical investigations the region of the main trough, a dynamic feature whose location and structure is highly variable and which is found over northern europe on many nights, presents particular problems for such models. most models do not even attempt to incorporate the feature, while for others, placing the trough in the wrong place or with inappropriate gradients may represent a poorer solution for a particular radio application than complete omission of the feature. tomographic imaging of large-scale spatial structures like the main trough provides a powerful tool to aid the verification of such models by comparison with actual observations. the panels of fig. 3, taken from dabas and kersley (2003), show an example of such comparison between a tomographic image and three different ionospheric models, the international reference ionosphere (iri-95) (bilitza et al., 1993), the parameterised ionospheric model (pim) (daniell et al., 1995) and the european regional model developed during the cost 238 action (bradley, 1999). the experimental results from observation at a tomographic chain of stations in u.k. show an ionosphere with a clear trough and in addition a mid-latitude nighttime enhancement to the south. the output from the iri-95 model for the same geophysical conditions fails to replicate either feature, while that from pim does show a trough, but with no localized enhancement to the south. the results from the cost 238 model do appear to show a trough, though extreme caution must be used in the interpretation because this particular model was designed specifically to cover the european region below 55°n latitude, with default matching to the iri model above that latitude, so that the feature shown is likely to be an artifact of this process. 4. tomography and the mapping of ionospheric parameters maps of ionospheric parameters, on a regional or global basis, provide another tool that is used to correct for propagation effects in practical radio systems. dabas and kersley (2003) also investigated the potential use of tomographic images in mapping. they demonstrated that improvements could be achieved to the reliability of maps of the f2-layer critical frequency and peak electron density over europe by incorporating information from experimental tomography into the mapping process. figure 4 shows a comparison of a map of nmf2, predicted by the iri-95 model for particular conditions, with one in which an actual fig. 4. contour maps of nmf2 over europe obtained from iri-95 alone (left) and using input from an actual tomographic image plus the iri-95 model (right). from dabas and kersley (2003), reproduced by permission of the american geophysical union. 540 leonard kersley tomographic image obtained from observations made in u.k. has been used to provide the latitudinal gradient. in essence, the electron densities at the layer peak, determined from the image, were mapped in longitude, with zonal gradients from the model being used to create a revised map that was more realistic of the actual conditions. the locations of the spatial structures that were not present in the model output alone can be identified from the map modified by the use of tomographic input. verification of the improvement afforded by the use of tomographic observations in the mapping procedure was obtained by comparing the mapped values with actual measurements of fof2 from the network of ionosondes throughout europe. figure 5 shows the results of such comparison for the pim model. it can be seen that inclusion of tomographic input with the pim model improved the correlation coefficient between the mapped nmf2 and actual ionosonde measurements to 0.92, while a smaller coefficient of 0.83 was found when the pim model was used alone. 5. tomography and oblique ionograms the possible role of radio tomography as an aid to oblique ionospheric sounding has been investigated by heaton et al. (2001). a network of iris oblique sounders was established in u.k. fig. 5. correlation of nmf2 from tomography plus pim maps (upper) and pim alone maps (lower) with corresponding values from ionosondes. from dabas and kersley (2003), reproduced by permission of the american geophysical union fig. 6. an oblique ionogram and bottomside profile of electron density from iris observations in u.k., together with a complete profile at the midpoint obtained from a corresponding tomographic image. from heaton et al. (2001), reproduced by permission of the american geophysical union. 541 ionospheric tomography and its applications in radio science and geophysical investigations during an experimental campaign, covering the region spanned by a chain of tomographic receivers. the results demonstrated that the maximum density at the mid-point of the oblique path estimated by inversion of the oblique ionogram was in reasonable agreement with the density at the corresponding height and latitude in the tomographic image. the example shown in fig. 6 confirms this finding, though discrepancies can be seen at e-layer heights. it was concluded from the study that images of path conditions from the tomographic technique could have a role to play in assessing the applicability of the assumption of spherical symmetry in the reduction of oblique incidence ionograms. 6. tomography and hf ray tracing the observations from the iris oblique incidence sounder network in u.k. were also used in a study of the potential of radio tomography in aiding the ray tracing of hf propagation paths. the results are discussed in detail in rogers et al. (2001). the study investigated the effects of various types of input information that can be used to constrain the vertical electron density structure in the tomographic reconstructions. it was found that the use of a fine height resolution (5 km) and incorporation of input from one vertical ionosonde in the reconstruction process made significant improvements to the overall reliability of the tomographic image. as expected, e-layer propagation was better defined using a climatological model than by the tomographic method. it can be seen from fig. 7 that the use of tomographic images reduced the rms error in the determination of the f2-layer maximum useable frequency (muf), yielding significantly smaller errors than those obtained from three independent ionospheric models (faim, pim and iri-95). 7. tomography and hf direction finding warrington et al. (2002) presented observations from an hf direction-finding experiment, on a link between uppsala in sweden and leicester in u.k., where the propagation path was aligned approximately with the orientation of the main ionospheric trough. an example is shown here that demonstrates significant deviations from the great-circle path. the results from the days 326-327, reproduced here in fig. 8, show anomalous out-of-great-circle propagafig. 7. histograms of rms errors in the estimation of the maximum useable frequency showing that use of tomographic images gives smaller errors for f2-layer muf than the faim, pim and iri 95 models. from rogers et al. (2001), reproduced by permission of the american geophysical union. 542 leonard kersley tion during the night, with consequent increased times of flight and doppler frequency shifts. the azimuth of the received signals shows a sudden transition from the great-circle direction, followed by a smooth rotation and reversal over a period of several hours before returning to the original direction. in a subsequent study, images of electron density from fig. 8. results from an hf direction-finding experiment between uppsala to leicester during a three-day period in november 2001. note in particular the anomalous azimuthal directions of the received signals during the night of 326-327. from warrington et al. (2002), reproduced by permission of the authors. fig. 9. a sequence of tomographic images from four satellite passes throughout the night of 326-327, monitored at the stations in u.k., which show a deep trough with a strong ionisation enhancement or boundary blob polewards of the minimum that is acting as a reflector for the out-of-great-circle hf propagation found on the uppsala to leicester path shown in fig. 8. 543 ionospheric tomography and its applications in radio science and geophysical investigations the u.k. tomography chain, from a succession of satellite passes throughout the night, revealed a trough with a steep poleward wall that advanced equatorwards during the early part of the night before receding again northwards (fig. 9). the anomalous propagation found in the results from the hf experiment can be explained in terms of reflection by the enhanced densities to be seen in the tomographic images polewards of the trough minimum. 8. tomography and vertical tec kersley et al. (2002) investigated the limitations to the accuracy of experimental measurements of total electron content imposed by the necessary assumptions required to convert the slant observations to equivalent vertical measurements. the assumed height of the thin-shell ionospheric layer was found to be a critical factor. however, it was demonstrated that some advantage could be gained by assessing the vertical tec by integration through a tomographic image as opposed to using the equivalent vertical estimate of the parameter for an assumed thin-layer height. in the example shown in fig. 10 it can be seen that to the north of the trough the tomographic image reveals that the peak height of the actual ionosphere was only some 200 km. the lower panel shows that the resultant equivalent vertical total electron contents estimated from the slant measurements at the individual stations for an assumed ionospheric height of 350 km are all much less than the vertical tec (shown by the dotted line) in that refig. 10. tomographic image from observations in u.k. (upper panel), with plots of equivalent vertical total electron content from the individual station observations (solid lines) and the vertical total electron content computed through the tomographic image (dotted line) shown in the lower panel. from kersley et al. (2002). 544 leonard kersley gion obtained from the tomographic image above. in a similar way, an assumed thin-shell ionosphere at too great a height leads to overestimation of the equivalent vertical tec. it was argued that the vertical tec obtained from the tomographic image was a more reliable estimate than the equivalent vertical tecs determined from the slant measurements. 9. tomography and trough parameters vertical tec estimates from tomographic images of the kind described above are being used in a project to characterize the main trough in terms of a set of defined parameters. the parameters have been chosen to identify both the location of the trough and the local structure of the ionosphere in the vicinity of the feature. in addition to the latitude of the minimum, the parameters include the gradients of both the equatorward and poleward walls and evidence for a local maximum that would represent a boundary blob poleward of the minimum. a database of the parameters has been compiled, using the observations from the tomographic chain of stations in u.k., covering more than one year. the objectives of the study are two fold. first, the aim is to use the parameters in the extensive database to try to further understanding of the behaviour of the trough under a wide variety of geophysical conditions. in addition, characterisation of the feature in this defined way is planned to provide parameters that can be calculated directly from models so that the actual observations can be used in a direct test of the reliability of the model in the trough region. 10. tomography and the suctip model while the work outlined above involved the testing of models that had been developed specifically for radio systems applications, tomographic images have also been used to assess the ability of a physical model of the coupled ionosphere/thermosphere system to replicate conditions in the high-latitude ionosphere. experimental observations from a chain of satellite stations in the scandinavian sector were used to provide ‘ground truth’ information for comparison with output from the sheffield university coupled thermosphere ionosphere plasmasphere (suctip) model. figure 11, taken from the paper by idenden et al. (1998) reporting the results of the study, shows an example of a tomographic image containing a dayside trough in the high-latitude ionosphere that can be compared with the output from the suctip model for the appropriate conditions, which is reproduced in the panel below. a subsequent study by balthazor et al. (2002) compared the locations of the trough minimum, determined experimentally from tomographic images made under quiet geomagnetic conditions, with those predicted by the model (fig. 12). the results of the comparison shown in fig. 12 demonstrate that the model tends consistentfig. 11. tomographic image from experimental observations in scandinavia with corresponding output from the supim model. from idenden et al. (1998), reproduced by permission of annales geophysicae. 545 ionospheric tomography and its applications in radio science and geophysical investigations ly to place the trough at slightly lower latitudes than those identified experimentally from the tomographic observations. 11.tomography and space weather many of the processes that can have adverse effects on the performance of radio systems are of particular significance during times when disturbed space weather impacts on the terrestrial environment. radio tomographic imaging of ionospheric structures found at high latitudes has been shown to provide insight into space-weather processes. the convergence of the geomagnetic field close to the earth results in flux tubes with very different plasma populations that are characteristic of widely separate regions of space being brought into close spatial proximity in the dayside cusp ionosphere. in consequence, structures in the ionospheric plasma represent signatures of physical processes that are operating far out in space. tomographic imaging of such spatial features has demonstrated that they contain footprints of important mechanisms in solar-terrestrial coupling. several papers have addressed such issues, including two that illustrate the signatures in the dayside cusp ionosphere of the reconnection processes that link the interplanetary magnetic field (imf), carried by the solar wind, to the geomagnetic field. the location of the magnetic reconnection and the resultant behaviour and flow of the ionospheric plasma is dependent, in particular, on the orientation of the north/southor z-component of the imf. when bz is directed southwards the imf is oriented anti-parallel to the geomagnetic field at the magnetopause at low latitudes, so that the reconnection occurs close to the equatorial plane. the resultant tension exerted by the solar wind on the geomagnetic field drives the ionospheric plasma in the familiar two-cell pattern at high latitudes, with anti-sunward flow across the polar cap, as is illustrated in fig. 13. walker et al. (1998), using observations from the chain of tomographic stations in the vicinity of svalbard, investigated the ionospheric footprints of low-latitude reconnection. an image, reproduced in fig. 14, shows characteristic features that can be identified with signatures of the reconnection process. the open/closed field line boundary is marked by a steep meridional gradient in the f2-layer ionospheric electron density. the increase in the peak height of the layer to the north was associated with the dispersed spectrum of the particles precipitating from the magnetopause reconnection site, with the softer energies fig. 12. polar plot in the magnetic reference frame comparing average locations of the trough minimum under quiet geomagnetic conditions obtained from tomographic images (+) with estimates by the suctip model (∗). from balthazor et al. (2002). fig. 13. magnetic reconnection close to the equatorial plane and the consequent convection of plasma in the high-latitude ionosphere, driven in an anti-sunward flow across the polar cap. from lockwood (1998), reproduced by permission of the publishers. 546 leonard kersley taking longer to impact the ionosphere as the flux tube is convected polewards. the tomographic image also revealed ionospheric signatures of the field-aligned current system driven by the reconnection. the e-layer enhancement just south of the boundary was linked to harder electron precipitation from the central plasma sheet associated with an upward current, while the narrow band of increased densities extending to high altitudes immediately polewards of the reconnection location marked a downward field-aligned current. when the imf is directed northwards the anti-parallel condition necessary for reconnection with the geomagnetic field is now satisfied at high latitudes in the magnetospheric lobe. the resultant lobe reconnection and the consequent convection of plasma in the polar ionosphere are illustrated in fig. 15. pryse et al. (1999) identified a tomographic image that contained the spatial footprints in the high-latitude dayside ionosphere of lobe reconnection, which is reproduced here as fig. 16. there is a marked contrast between this signature of reconnection in the magnetospheric lobe and that shown earlier for the low-latitude situation. figure 15 illustrates that for lobe reconnection the initial motion of the flux tubes, driven by the tension of the imf solar wind, is equatorwards with a consequent reversed dispersion of the spectrum of the precipitation as a function of latitude. the resultant signature of this process can be seen in the tomographic image, where the increasing peak height of plasma in the flux tubes convecting from the north towards the adiaroic boundary reflects the softening of the precipitation driven by the reconnection process. the adiaroic boundary, across which there is no transfer of flux, is marked by a steep gradient in the density. plasma from the south is driven in the anti-sunward direction up to the boundary and fig. 14. tomographic image showing signatures in the high-latitude ionosphere of reconnection at the equatorial magnetopause. from walker et al. (1998), reproduced in modified form by permission of the american geophysical union. fig. 15. magnetic reconnection in the magnetospheric lobe and the consequent convection of plasma in the high-latitude ionosphere, which polewards of the adiaroic boundary is initially in a sunward direction. from lockwood (1998), reproduced by permission of the publishers. 547 ionospheric tomography and its applications in radio science and geophysical investigations then carried zonally by viscous processes. it should be noted that the trajectory of the satellite, which was to the east of the chain of observing stations, gives rise to a geometrical distortion resulting in the apparent reverse slope with altitude of the field-aligned boundary. tomographic images from observations at high latitudes have also been used to investigate other signatures of the impact of space-weather processes in the ionospheric plasma. for example, moen et al. (1998) related structures in electron density seen in tomographic images to optical aurorae, with the precipitation responsible for green-line emissions being linked to density enhancements in the e-layer, while the softer particles given rise to red-line aurora produced discrete features in the plasma at f-layer heights. 12. conclusions the results reviewed here demonstrate the potential role of radio tomographic imaging of the ionized atmosphere in both applied radio science, directed towards the mitigation of propagation effects on practical radio systems, and in studies of the geophysics of the underlying physical processes. the experimental technique is able to provide a unique capability to monitor large-scale spatial structures in the electron density over a wide region from observations made at a limited number of ground stations. the advantages of method, given by wide coverage even in inaccessible and environmentally hostile regions, coupled with the low cost of a passive system, far out way the known limitations. the latter result primarily from the lack of horizontal ray paths in satellite-to-ground propagation, though they can be mitigated by use of appropriate reconstruction algorithms and with additional input from ionosondes or other independent sources. the recent studies outlined here have shown the potential of radio tomography in fields as diverse as the validation of both empirical and physical models of the ionosphere, oblique sounding, hf ray tracing, direction finding and the mapping of ionospheric parameters. the versatility of tomographic imaging in applied radio science adds to the potential of the method as a new experimental technique that can be used in geophysical studies to identify signatures of space-weather processes in the ionized fig. 16. tomographic image showing signatures in the high-latitude ionosphere of reconnection at the magnetospheric lobe. from pryse et al. (1999), reproduced by permission of the american geophysical union. 548 leonard kersley atmosphere at high latitudes. a fuller review of the wider use of tomographic imaging in global aspects of geophysics can be found in pryse (2003). acknowledgements the development of radio tomography by the radio and space physics group at uwa has been supported financially at various stages by the u.k. particle physics and astronomy research council, the radiocommunications agency, dera and eoard. the important contributions of many individuals, both within the group and as external scientific collaborators are acknowledged with grateful thanks, as is the assistance of many people and organisations for the practical support provided for the experimental activities. references balthazor, r.l., m.h. denton, l. kersley, s.e. pryse, i.k. walker, r.j. moffett and g.j. bailey (2002): the high-latitude dayside trough under quiet geomagnetic conditions: a comparison of modelled and experimental observations (unpublished manuscript, university of sheffield, u.k.). bilitza, d., k. rawer, l. bossy and t. gulyaeva (1993): international reference ionosphere – past, present and future: 1 electron density, adv. space res., 13, 3-13 bradley, p.a. (1999): prediction and retrospective ionospheric modelling over europe (prime), in cost action 238 final report, published by rutherford appleton laboratory, chilton, didcot, u.k.. dabas, r.s. and l. kersley (2003): radio tomographic imaging as an aid to modelling of ionospheric electron density, radio sci., 38 (3), 1-20. daniell, r.e. jr., l.d. brown, d.n. anderson, m. w. fox, p.h. doherty, d.t. decker, j.j. sojka and r.w. schunk (1995): pim: a global ionospheric parameterization based on first principles, radio sci., 30, 1499-1510. fremouw, e.j., j.a. secan and b.m. howe (1993): application of stochastic inversion theory to ionospheric tomography, radio sci., 17, 721-732. heaton, j.a.t., p.s. cannon, n.c. rogers, c.n. mitchell and l. kersley (2001): validation of electron density profiles derived from oblique ionograms over the united kingdom, radio sci., 36, 1149-1156. idenden, d.w., r.j. moffett, m.j. williams, p.j.s. spencer and l. kersley (1998): imaging of structures in the high-latitude ionosphere: model comparisons, ann. geophysicae, 16, 969-973. kak, a.c. and m. slaney (1988): principles of computerised tomographic imaging (ieee press), pp. 327. kersley, l., j.a.t. heaton, s.e. pryse and t.d. raymund (1993): experimental ionospheric tomography with ionosonde input and eiscat verification, ann. geophysicae, 11, 1064-1074. kersley, l., s.e. pryse, i.k. walker, j.a.t. heaton, c.n. mitchell, m.j. williams and c.a. willson (1997): imaging of electron density troughs by tomographic techniques, radio sci., 32, 1607-1621. kersley, l., d. malan, s.e. pryse, lj.r. cander and r. bamford (2002): progress towards verification and validation of tec observations over europe, in proceedings of the cost271 workshop, september 2002, faro, portugal. kunitsyn, v.e. and e.d. tereshchenko (2003): ionospheric tomography (springer-verlag), pp. 260. leitinger, r. (1996): tomography, in modern ionospheric science – 50 years of ionospheric research in lindau, edited by h. kohl, r. rüster and k. schlegel, 346370. leitinger, r. (1999): ionospheric tomography, in review of radio science, 1996-1999, edited by w.r. stone (oxford university press, oxford, u.k.). lockwood, m. (1998): identifying the open-closed field line boundary, in polar cap boundary phenomena, edited by j. moen, a. egeland and m. lockwood (nato advanced study institute series, kluwer academic press, the netherlands), 509, 73-90. moen, j., s.t. berry, l. kersley and b. lybekk (1998): probing discrete auroral arcs by ionospheric tomography, ann. geophysicae, 16, 574-582. pryse, s.e. (2003): radio tomography: a new experimental technique, surv. geophys., 24, 1-38. pryse, s.e., a.m. smith, j. moen and d.a. lorentzen (1999): footprints of lobe reconnection observed in ionospheric electron density under steady northward imf, geophys. res. lett., 26, 25-28. rogers, n.c., c.n. mitchell, j.a.t. heaton, p.s. cannon and l. kersley (2001): the application of radio tomographic imaging to hf oblique incidence ray tracing, radio sci., 36, 1591-1598. walker, i.k., j. moen, c.n. mitchell, l. kersley and p.e. sandholt (1998): ionospheric effects of magnetopause reconnection observed using ionospheric tomography, geophys. res. lett., 25, 293-296. warrington, e.m., a.j. stocker, n.y. zaalov, d.r. siddle and t.b. jones (2002): high latitude ionospheric propagation effects and their influence on hf direction finding, in proceedings of the fourth symposium on ‘radiolocation and direction finding’, may 2002, southwest research institute, san antonio, texas, u.s.a. marcaccio_corretto_layout 6 811 effects on the groundwater levels of the may-june 2012 emilia seismic sequence marco marcaccio1, giovanni martinelli2,* 1 agenzia regionale prevenzione e ambiente (arpa) dell'emilia-romagna, direzione tecnica, bologna, italy 2 agenzia regionale prevenzione e ambiente (arpa) dell'emilia-romagna, sezione di reggio emilia, reggio emilia, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6139 1. introduction a variety of phenomena were observed in the groundwaters in concomitance with the may-june 2012 seismic sequence that occurred in the emilia romagna area. in particular, phreatic wells close to the epicentral area were affected by a sudden increase in water level of up to 4 m. in some cases, the sands of aquifers were ejected outside wells, and >700 liquefaction phenomena were recorded [bertolini and fioroni 2012, this volume]. some automatic stations of the regional well network recorded variations in well levels. these data can be considered useful to understand the relationships between seismic events and the local groundwaters [see also italiano et al. 2012, this volume]. 2. hydrogeological features the po sedimentary basin is filled by sediments of alpine and apennine origin [ori 1993]. the sedimentation processes have filled the sedimentary basin with alternate layers of sands and clay [ori 1993, and references therein]. gravels are present near the apenninic chain front in the alluvial fan areas, while the bottom of the quaternary sediments is at depths of 100 m to 700 m. these multilayered sequences constitute the po aquifer system [regione emilia-romagna and eni-agip 1998]. in the part of the basin close to the apenninic chain front (the belt of alluvial fans), the aquifers are located in undersaturated sediments and are subject to seasonal variations in their recharging processes. north of this area, fine sediments host confined or semiconfined aquifers that are characterized by low circulation velocities. isotopic data indicate that deep groundwaters in this area are of alpine origin, while 3h and 14c data indicate ages of 15000 to 30000 years [martinelli et al. 1998, and references therein]. at depths shallower than 50 m, load fluctuations are observed in connection with rainfall, although lenses of confined aquifers that are poorly sensitive to rainfall are known to be at shallow depths too. the may-june 2012 seismic sequence occurred in the low plain area in the provinces of modena and ferrara, where fine sediments generate multilayered confined aquifers, and phreatic aquifers are limited to the first 10 m to 30 m in depth. 3. the monitoring network in 1976, a regional network composed of 330 wells was set up with the purpose of monitoring the groundwater levels using manual techniques. these data have been used for environmental purposes and for water management. in recent years, the monitoring network managed by agenzia regionale prevenzione e ambiente (arpa) has increased the number of wells (to about 600), and chemical analyses are also periodically carried out for environmental monitoring activities. in the past three years, an automatic monitoring network composed of 40 stations was set up and this has allowed the monitoring of the water levels and the temperatures, at a rate of one measure per hour, and to broadcast the data to the host center located in bologna (figure 1a). 4. recorded data most of the stations located in the whole of the emiliaromagna region did not record significant signals that are attributable to seismic events, while some stations located in the provinces of modena and ferrara (figure 1b) recorded sudden water uprising phenomena, of up to 1.5 m (figure 2). the internal clock of the monitoring stations was synchronized according to coordinated universal time (utc). in this way, the first measures after the m 5.9 and m 5.8 shocks were recorded about 1 h after these seismic events. figure 2 shows that a decay of the water level signal was seen, and the higher water levels probably occurred in concomitance with these seismic events, although they were not recorded here (table 1). figure 3 gives the water level variations observed at three selected monitoring stations of the emilia-romagna article history received july 23, 2012; accepted august 20, 2012. subject classification: groundwater processes, groundwaters, seismic effects, water level, hydrology. 2012 emilia earthquakes marcaccio and martinelli 812 relevant seismic events (magnitude) seismic events occurrence date seismic events occurrence time (utc) pre-seismic recording time (utc) post-seismic recording time (utc) 5.9 20/05/2012 02.03.52 02.00.00 03.00.00 5.8 29/05/2012 07.00.03 07.00.00 08.00.00 figure 1. (a) geographical sketch of the emilia-romagna region showing all of the groundwater automatic monitoring stations managed by arpa emilia-romagna. (b) monitoring stations considered and epicenters of the most relevant seismic events, as characterized by 5.1 ≤m ≤5.9 [ingv 2012]. figure 2. water-level changes in concomitance with some significant seismic events. the events characterized by 5.1 ≤m ≤5.9 were considered. table 1. occurrences of the two most relevant seismic events, with the pre and post recording times. 813 groundwater automatic monitoring network. in particular, for each station, two different water level variations were recorded in concomitance with most of the relevant seismic events, which were characterized by the m 5.9 (full dot) and m 5.8 (empty dot) seismic events. water level variations were calculated according to values recorded at the utc hours reported in table 1. the m 5.9, may 20, 2012, seismic event occurred at a distance of about 10 km to 14 km from the monitoring stations. the observed signal amplitude was relevant in all of the three monitoring stations considered. the m 5.8, may 29, 2012, seismic event occurred at a distance of about 10 km from the mo80-00 station, and at about 21 km to 23 km from the fe80-00 and fe81-00 stations. these distance differences might explain the higher effects recorded at the mo80-00 station (figure 3). most of the relevant groundwater variations that have occurred in concomitance with other earthquakes were described by roeloffs [1996] and by wang and manga [2010], while previous investigations on earthquake-related piezometric variations in italy were carried out by albarello et al. [1991] and albarello and martinelli [1994]. in the italian historical records, earthquakes related to piezometric variations were reported by boschi et al. [1995] and by martinelli [1997]. as a general rule, possible water-level anomalies can be positive or negative according to the well location. if the area is subjected to compressive strain, the expected anomaly is positive [okada 1992]. the sizes of the recorded anomalies can be tentatively related to the aquifer characteristics, to the distances of the recording station from the hypocentral area, and to the locations of the monitored aquifers in areas probably subjected to compressive strain. the step-like signals observed indicate that all of the monitoring stations in the near-field area, in which undrained consolidation is the dominant mechanism for coseismic groundwater level changes [wang and manga 2010], were affected by significant waterlevel variations. 5. conclusions the observed anomalies are within the characteristics previously reported in the common scientific literature relating to undrained consolidation phenomena linked to earthquake occurrence. their sizes and shapes are probably a function of their relatively small distances from the hypocentral area and to their location in an area subjected to compressive strain. the collection and processing of all of the available data on piezometric variations during the emilia 2012 seismic sequence will allow a better understanding of the phenomena observed. acknowledgements. thanks are due to michele di lorenzo for quick data recovery from hosting centers. thanks are also due to francesco salvini and to an unknown reviewer, for their kind suggests which contributed to the improving of the text. references albarello, d., g. ferrari, g. martinelli and m. mucciarelli (1991). well-water variations as a seismic precursor: a statistical assessment from italian historical data, tectonophysics, 193, 385-395. albarello, d., and g. martinelli (1994). piezometric levels as possible geodynamic indicators: analysis of the data from a regional deep waters monitoring network in northern italy, geophys. res. lett., 21, 1955-1958. bertolini, g., and c. fioroni (2012). aerial inventory of surficial geological effects induced by the recent emilia earthquake: preliminary report, annals of geophysics, 55 (4); the effects of shocks on groundwaters figure 3. differences in the water levels at the three monitoring stations, as related to the distance in concomitance with the two relevant seismic events characterized by m 5.9 (full dot) and m 5.8 (empty dot). doi:10.4401/ag-6113. boschi, e., g. ferrari, p. gasperini, e. guidoboni, g. smriglio and g. valensise (1995). catalogo dei forti terremoti in italia dal 461 a.c. al 1980, istituto nazionale di geofisica, storia geofisica ambiente, bologna, 973 pp. ingv (2012): http://cnt.rm.ingv.it italiano, f., m. liotta, m. martelli, g. martinelli, r. petrini, a. riggio, a. rizzo, f. slejiko and b. stenni (2012). geochemical features and effects on deep-seated fluids during the may-june 2012 southern po valley seismic sequence, annals of geophysics, 55 (4); doi:10.4401/ag-6151. martinelli, g. (1997). non seismometrical precursors observations in europe: steps of earthquake prediction research, in: g. ferrari (ed.), historical seismic instruments and documents: a heritage of great scientific and cultural value, cahiers du centre européen de geodynamique et de séismologie, 13, 195-216. martinelli, g., a. minissale and c. verrucchi (1998). geochemistry of heavily exploited aquifers in the emilia-romagna region (po valley, northern italy), environ. geol., 36, 195-206. okada, y. (1992). internal deformation due to shear and tensile faults in a half-space, b. seismol. soc. am., 52, 81-86. ori, g.g. (1993). continental depositional systems of the quaternary of the po plain (northern italy), sediment. geol., 83, 1-14. regione emilia-romagna and eni-agip (1998) riserve idriche sotterranee della regione emilia-romagna, s.el.ca., firenze, 119 pp. roeloffs, e. (1996). earthquake-related hydrologic phenomena, adv. geophys., 37, 135-195. wang, c-y. and manga m. (2010) earthquakes and water, springer, heidelberg, 225 pp. *corresponding author: giovanni martinelli, agenzia regionale prevenzione e ambiente (arpa) dell'emilia-romagna, sezione di reggio emilia, italy; email: giovannimartinelli@arpa.emr.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. marcaccio and martinelli 814 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice the effect of the stratospheric qbo on the neutral density of the d region annals of geophysics, 58, 3, 2015, a0331; doi:10.4401/ag-6491 a0331 the effect of the stratospheric qbo on the neutral density of the d region selçuk sağır1, ramazan atıcı2,*, osman özcan3, nurullah yüksel3 1 mus alparslan university, vocational school, department of electronics and otomation, mus, turkey 2 mus alparslan university, faculty of education, mus, turkey 3 mus alparslan university, faculty of arts and sciences, department of physics, mus, turkey abstract a multiple regression model, which defines relationship between two variables, is used to perform a statistical analysis of the relationship between the stratospheric qbo and the neutral density of the d region (nnd) at altitudes of 75 km and 90 km for singapore station. while performing the analysis, the solar maxima and solar minima epochs of the sun for 21st, 22nd and 23rd solar cycles (scs) are taken into account. before applying the model for the statistical analysis of the relationship, the stationary of the variables is investigated by using the unit root test. the relationship between the variables is also investigated by using the co-integration test. the relationship between nnd measured at 75 km altitude and qbo obtained at altitude of 10 hpa is observed that it is positive for solar maximum epoch at 21st and 23rd scs and for solar minimum epoch at 21st sc and is negative at the other epochs. the relationship between nnd measured at 90 km altitude and qbo is observed to be negative at both the solar maxima expect for solar maximum of 23rd sc and the solar minima epochs. the relationship between variables is positive for both phases (east and west) of qbo. thus, qbo leads to a statistical change in the nnd. it may also give rise to changes on the ion chemistry of the d region. 1. introduction the region that includes the lower part of the ionosphere, at an altitude of approximately 50 to 100 kilometers, is referred to as the lower ionosphere. the d-region, that makes up a significant portion of the lower ionosphere, can be considered as a complex transition region from the neutral atmosphere to the ionosphere that are characterized by the plasma properties [kero 2008]. one of the most important characteristics of this region is the electron density. as the rates of the chemical processes in the d-region of the ionosphere are relatively low, advection and convection significantly affect the distribution of electron density in this region [kazimirovski and kokoruov 1995]. thus, the structure and composition of this region cannot be completely determined, without the clarification of the role of the atmospheric motions on all scales; particularly, its interaction with ionospheric processes, and its role in initiating the vertical transport of gas components, in the case of large scale circulation. this region is especially sensitive to meteorological influences, which may lead to variations in the electron concentration of magnitude’s order. it should be noted that the normal system, which is determined by absorption of the solar radiation (both daily and annual cycles), has been perturbed sometimes by large scale waves from the troposphere [kazimirovski and kokoruov 1995, krivolutsky 2009]. the quasi-biennial oscillation(qbo),which is one of the meteorological effects on the d-region, is the quasi-periodic oscillation of the wind between the easterlies and the westerlies in the tropical stratosphere, with a mean period of 28 to 29 months [heaps et al. 2000]. the qbo generally originates in the equatorial region and moves with a velocity of approximately 30 m/s in the eastward direction and 20 m/s in the westward direction. even though the maximum amplitude of the qbo is at the level of 10 hpa, it can vary from 100 hpa to 2 hpa. the qbo can extend up to the mesosphere and lower thermosphere (mlt) beyond the stratopause with the help of waves. on reaching the mesosphere, the qbo generates a new oscillation called the mesospheric quasi biennial oscillation (mqbo) [baldwin et al. 2001]. from this region, the qbo can extend up to the f-layer of the ionosphere, along the geomagnetic field lines of the earth [chen 1992]. the qbo as energy and momentum are carried by inertia-gravity waves, gravity waves, equatorial waves (kelvin and rossby gravity waves), and planetary waves article history received january 18, 2014; accepted march 25, 2015. subject classification: qbo, multiple regression, lower ionosphere, stratosphere. from lower stratosphere to the lower ionosphere. (see figure 1) [baldwin et al. 2001]. in this study, the effect of qbo on nnd, is investigated statistically to determine the structure and the mixture of this region. in this context, a multiple regression is used to explore the structure of the irregularities and the underlying effects caused by qbo in nnd. the relationship between nnd and qbo is investigated by using the multiple regression model. statistical analysis method used in the study, the results and discussion, and the conclusion are presented in sections 2, 3 and 4. 2. analysis process a multiple regression model has three statistical parts in this study. the first one is the unit root test that analyzes the stationarity of the variables. the second one is the co-integration test that proves the relationship between the variables. the third one is the regression model that designates the value of the relationship between the variables. in the unit root test, stationarity properties of variables are primarily examined. in order to examine the statistical relationship between the variables, the series must be stable. if the series are not stable, with the mean and the variance changing with time, then these series are made stationary by calculating the first order of difference of qbo and nnd (d(qbo), d(nnd)). there are three basic tests that are commonly used in the statistical studies for stationarity, namely augmented-dickey fuller test (adf), phillips-perron test (pp) and kwiatkowski-phillips-schmidt-shin test (kpss). adf is widely used for investigation of the presence of the unit root in the time series.the equation including the lagged values of the dependent variable is defined by adding a constant and a time trend as follows [enders 2008, sagir et al. 2015]: (1) where y is the dependent variable, n is the mean value, b is the coefficient of time trend, d is the difference processor, t is the time trend, f is the error term and k is the number of lags. the adf test is based on the estimation of parameter d. if the parameter d is different from zero, it means that the series is not stationary. if the parameter is equal to zero, it means that the series is stationary. the pp test, which is developed to control the high degree of correlation, is a unit root test that does not include the restrictive assumptions about the error terms. in the pp test, the lagged values of the dependent value are not sufficiently included to eliminate the auto-correlation in the model. instead, it is adapted by the newey-west estimator [enders 2008]. the kpss test expresses that the series are stationary under the null hypothesis. the kpss statistics depends on the error terms obtained from regression provided by exogenous variables of the time series [kwiatkowski et al. 1992]. for each test, the series is proved to be stationary when the absolute value of the test values of the variables is greater than mackinnon critical value [enders 2008]. when two time series are not stationary and there is a stationary-linear compound in the variables, a co-integration relationship is considered. the co-integration analysis is used to estimate a relationship between the non-stationary time series. the presence of the co-integration indicates a relationship between the variables. the engle-granger analysis is widely used in the co-integration analysis [engle and granger 1987]. in the first stage of the engle-granger method, a relationship between the variables is estimated by the ordinary least square method (ols). after the variable coefficients are estimated by ols, the error terms of the regression are investigated to see whether or not the series is stationary [sagir et al. 2015]. before performing the analysis, the qbo is considered to be the wind. the effects of the eastern (the negative sign values of qbo indicated by dummye in equation 2) and the western (the positive sign values of qbo indicated by dummyw in equation 2) side of the wind are added to the model through dummy variables. the model is defined as follows: (2) where, c is constant and b1, b2, and b3 denote coefficients of variables. in statistical model, a dummy variable is a variable that marks or encodes a particular attribute. it is often ,y t y yt t j j k t j t1 1 n b d a fd d= + + + += / n d c d qbo dummyw dummye n t t t t t1 2 3b b b f = + + + +^ h sagir et al. 2 figure 1. the atmospheric waves affecting to spread of the qbo [mohanakumar 2008]. 3 called binary or dichotomous variable as it takes just two values, usually 1 or 0, to indicate the presence or absence of a characteristic. using equation (2), the relationship between qbo and nnd is computed for solar maxima and solar minima epochs. 3. results and discussion the qbo data used in this study are obtained from the website http://strat-www.met.fu-berlin.de, from singapore station for the geographical coordinates 01.22n and 103.55e. the nnd values are obtained monthly from the website http://ccmc.gsfc.nasa.gov/ modelweb/models/nrlmsise00.php for the nrlmsise-00 atmospheric model for the same coordinates as that of the qbo data, at the local time 1200 (lt1200), and at altitudes of 75 km and 90 km, respectively. here, the nnd is expressed by the sum of n2, o2, and o, which are the primary components of the atmosphere. the data used (both qbo and nnd) are determined so as to cover the entire month at the time interval 09/1978-12/1982, 06/1988-04/1992, 01/1999-12/2001 for solar maxima epochs and at time interval 01/198412/1987, 02/1994-08/1997, 01/2007-12/2009 for solar minima epochs (21th, 22nd and 23rd scs). the solar minima and maxima epochs are calculated by taking the previous and next years of the year that sunspots are at lowest and highest values, respectively [kirov et al. 2014]. the results are obtained by using given equations in section 2 and taking reference from enders [2008]. as a result, there is a continuing relationship between qbo and nnd. however, this relationship changes according to altitude and scs . the results of unit root test and co-integration test are shown at the tables for 21st, 22nd, and 23rd scs at both 75 km and 90 km altitudes. 3.1. the analysis results for nnd measured at an altitude of 75 km figure 2 shows the variation of nnd measured at 75 km altitude with qbo in solar maximum (left-side) and solar minimum (right-side) epochs for 21st, 22nd qbo and d-region neutral density figure 2. the variation with years of relationship between qbo and nnd calculated at 75 km altitude in solar maximum (left-side) and solar minimum (right-side) epochs for 21st, 22nd and 23rd scs. and 23rd scs. in the solar maximum case, there is a positive relationship between nnd and west-directional qbo for all three scs. it is observed that there is a positive relationship between nnd and east-directional qbo for 21st sc. however, there is a negative relationship between variables for other scs. in the solar minimum epoch, a positive relationship is observed between both the west-directional and east-directional qbo and nnd for all the scs. unit root test results the purpose of the unit root test is to test the stationary nature of the variables. because of the importance of the unit root test, it is supplemented with three separate tests (adf, pp, and kpss). in this test, the qbo and the nnd values recorded for each of the test types at the top of table 1 need to be greater than the mackinnon critical values or the absolute values at the bottom of the table. table 1 tabulates the results of the unit root test between the qbo measured at 10 hpa altitude and the nnd obtained for 75 km. as shown in this table, the nnd variable is significant at a level of 1% in the adf and the kpss tests, while it is significant at a level of 10% in the pp test. this indicates that the nnd variable is stationary. however, qbo is not stationary, so it has been made stationary by taking its first difference (d(qbo)) [see sagir et al. 2015]. d(qbo) variable is significant at a level of 5% in adf , pp and kpss tests. co-integration test results the co-integration test results are listed in table 2 for altitude of 75 km in solar maxima and solar minima epochs. since the adf values are greater than mackinnon critical values as the absolute values, in case of both solar maximum (|−4.69|>|−2.65|) and solar minimum (|−5.94|>|−2.65|) there are a relationship between the qbo and the nnd. meanwhile, since the p-values are smaller than 0.05 for both epochs (solar maximum and solar minimum), there are also a relationship at a level of 1% between the qbo and the nnd. looking at both comparisons (adf-mackinnon and p-values0.05), a relationship between the qbo and the nnd have been observed in the 1% level of significance that is the most significant level. regression analysis results table 3 lists the results of the regression analysis for nnd measured at an altitude of 75 km. ordinary least square (ols) method estimates are consistent in the presence of heteroskedasticity, but the standard errors are no longer valid. the white heteroskedasticity (white het.) test is a test for heteroskedasticity in ols residuals. the null hypothesis of the white test is that there is no heteroskedasticity, and the value of this variable must also be larger than 0.05. the durbin-watson test for serial correlation assumes that is stationary and normally distributed with mean as zero. it tests the null hypothesis that the errors are uncorrelated and the values of variables need to be between 1.5 and 2.5. probability (f-statistics) (prob. (f-statistic)) tests the overall significance of the regression model and the value of this parameter must be smaller than 0.05 [enders 2008, sagir et al. 2015]. values for the last three rows located on the bottom of the table show significance of established model and coefficients in this model. the values given in parentheses in the first five rows of the table refer to the significance of the coefficient obtained. the statistical significance of these values are shown by * sign outside the parentheses. when the coefficients in table 3 are investigated, it is observed that there is a negative relationship for 22nd sc in the solar maximum epoch and for 22nd and 23rd scs in the solar minimum epoch and there is a positive relationship in other epochs. while increase of 1 meter sagir et al. 4 variables 10 hpa adf pp kpss qbo -1.95 -1.65 0.10 nnd -5.14 -3.32 0.49 d(qbo) -4.15 -4.07 0.20 the level of significance mackinnon [1996] critical values adf pp kpss 1% -4.243 -4.243 0.216 5% -3.544 -3.544 0.146 10% -3.204 -3.204 0.119 regression model solar maximum solar minimum adf p-value adf p-value model -4.69 0.000 -5.94 0.000 the level of significance mackinnon [1996] critical values 1% -2.65 5% -1.95 10% -1.61 table 1. the unit root test results at an altitude of 75 km. table 2. the co-integration test results for 75 km. 5 per second of qbo in the solar maxima epochs for 21st and 23rd scs causes an increase of 6.13 × 1017 m-3 and 5.7 × 1017 m-3, it causes a decrease of 5.0 × 1017 m-3 in 22nd sc. whereas increase of 1 meter per second of qbo in the solar minima epochs for 22nd and 23rd scs causes a decrease of 3.63 × 1017 m-3 and 6.19 × 1017 m-3, it causes an increase of 8.68 × 1017 m-3 in 21st sc. the correlation coefficient (r2) in the analysis carried out with the whole set of the qbo is higher than those danilov [1998] who investigates relationship between qbo and electron density. it is observed that b2 (dummyw) and b3 (dummye) coefficients indicating wind direction are having positive effect on nnd in both solar maxima and solar minima epochs. this positive relationship can be associated with carrying positive/negative ions and neutral particles upward of qbo through waves abovementioned based on sentence expressed by lastovicka [2006] “the meteorological influences play an important role in the overall ionospheric variability [e.g., forbes et al. 2000, rishbeth and mendillo 2001]”. this results obtained are fully consistent with danilov [1998] for b2 coefficient, while b3 coefficient are partly consistent with danilov [1998]. the impact rate of these coefficients is close to each other. although adj.r2 value varies with the solar cycle, it is expressed that about 68%84% of changes in nnd according to this model can be explained by qbo. the remaining portion (approximately 32%16%) can be explained by the other effects which are not including to model here (such as, lightning, earthquakes, tides) of sub-atmospheric origin. the results obtained are supported that the ion-generation processes that occur in the lower ionosphere are closely related to the chemistry of neutral components and to meteorological processes [krivolutsky 2009]. 3.2. the analysis results for nnd measured at an altitude of 90 km figure 3 shows the variation of nnd calculated at 90 km altitude with qbo in the solar maximum (leftside) and solar minimum (right-side) epochs for 21st, 22nd and 23rd scs. in the solar maximum epoch, there is a positive relationship between nnd and west-directional qbo for all three scs. while there is a positive relationship between nnd and east-directional qbo for 21st sc, a negative relationship is observed for 22nd and 23rd scs. in the solar minimum epoch, there is a positive relationship between nnd and west-directional qbo for all three scs. while there is a negative relationship between nnd and east-directional qbo in 23rd sc, a positive relationship is observed in 21st and 22nd scs. unit root test results table 4 lists the results of the unit root test of the qbo and nnd variables, measured at 10 hpa and at an altitude of 75 km, respectively. it can be seen that the variables are not stationary for all the three qbo and d-region neutral density coefficents solar maximum solar minimum 21st 22nd 23rd 21st 22nd 23rd c (constant) 8.41×1020 8.25×1020 8.70×1020 8.10×1020 8.45×1020 8.63×1020 (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** b1 d(qbo) 6.13×1017 -5.0×1017 5.70×1017 8.68×1017 -3.63×1017 -6.19×1017 (0.015)** (0.022)** (0.009)*** (0.004)*** (0.010)*** (0.040)** b2 (dummyw) 8.54×1020 8.52×1020 8.73×1020 8.11×1020 8.44×1020 8.70×1020 (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** b3 (dummye) 8.41×1020 8.25×1020 8.70×1020 8.10×1020 8.45×1020 8.63×1020 (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** ar ( 1 ) 0.113 0.998 1.47 0.385 1.001 1.052 (0.555) (0.000)*** (0.000)*** (0.024)** (0.000)*** (0.000)*** r2 0.82 0.77 0.86 0.72 0.85 0.84 adj. r2 0.79 0.74 0.84 0.68 0.82 0.81 durbin watson 1.851 1.991 2.027 1.602 2.011 1.923 prob. (f-statistics) (0.000) (0.000) (0.000) (0.000) (0.000) (0.000) white het. (0.611) (0.116) (0.945) (0.305) (0.688) (0.094) table 3. the regression model results for 75 km altitude. (*),(**),(***) represents the significant level at 1 %, 5%, and 10%, respectively. tests. in order to make the variable stationary, their first differences (d (qbo), d (nnd)) were considered. as a result, the variable d(qbo) is statistically significant at 1% in the adf and pp tests, while it is statistically insignificant in the kpss test. the variable nnd is the same as the variable qbo. thanks to the variables d(qbo) and d(nnd) are significant according to two tests (adf and pp), they are considered to be stationary. co-integration test results in table 5, the results of the co-integration test for nnd measured at an altitude of 90 km for solar maximum and the solar minimum epochs are listed. since the adf values are greater than mackinnon critical values as the absolute values, in both solar maximum (|−4.24|>|−2.65|) and solar minimum epochs (|−5.71|> |−2.65|) there are a relationship between the qbo and the nnd. meanwhile, since the p-values are also smaller than 0.05 for both cycles (solar maximum and solar minimum), there is relationship between the qbo and the nnd. looking at both comparisons (adf-mackinnon and p-values-0.05), a relationship between the qbo and the nnd has been observed in the 1% level of significance that is the most significant level. sagir et al. 6 variables 10 hpa adf pp kpss qbo -2.63 -2.63 0.078 nnd -7.22 -3.65 0.040 d(qbo) -7.36 -7.36 0.084 d(nnd) -5.30 -4.50 0.080 the level of significance mackinnon [1996] critical values adf pp kpss 1% -4.243 -4.243 0.216 5% -3.544 -3.544 0.146 10% -3.204 -3.204 0.119 table 4. the unit root test results for 90 km. figure 3. the variation with years of relationship between qbo and nnd calculated at 90 km altitude in solar maximum (left-side) and solar minimum (right-side) epochs for 21st, 22nd and 23rd scs. 7 regression analysis results the regression analysis results for nnd, measured at an altitude of 90 km, are listed in table 6. if the resultant value is smaller than the probability (f-statistic) (prob. (f-statistic)) value of 0.05, it indicates that the developed model is significant. it is observed that b2 and b3 coefficients indicating wind direction are having positive effect on nnd and this effect is very close to each other in 21st and 22nd scs. such as b1 coefficient, it is observed that effect on nnd of b2 and b3 coefficients is also higher in the 23rd sc than 21st and 22nd scs. although adj.r2 value varies with the sc, changes of about 50% 75% in nnd can be explained by qbo. this result shows that qbo can disturb to lower ionosphere via troposphere based waves as expressed to introduction [kazimirovski and kokoruov 1995, krivolutsky 2009]. according to the coefficients in table 6, it is observed that there is only a positive relationship in the solar maximum epoch for 23rd sc and there is a negative relationship for all remaining cases. change of 1 meter per second in qbo (b1 coefficient in the table 6) in both solar maximum and solar minimum epochs for 21st and 22nd scs gives rise to a decrease of about 1.36 × 1016 m-3 −13 × 1016 m-3. however, this change in the 23rd sc causes an increase of 2.32 × 1020 m-3 in solar maximum epoch and brings about a decrease of 1.98 × 1016 m-3 for solar minimum epoch. values for the last three lines located on the bottom of the table show significance of established model and coefficients in this model. the values given in parentheses in the first five lines of the table refer to the significance of the coefficient obtained. the statistical significance of these values are shown by * sign outside the parentheses. when results obtained for both altitudes (75 km and 90 km) are compared, as expected, it is found that occurring change in nnd by qbo is higher about 10 times at 75 km altitude than 90 km altitude in 21st and 22nd scs. but, this change is higher about 1000 times at 90 km altitude than 75 km altitude in 23rd sc. 4. conclusion in this study, the underlying effects and the changes in the nnd caused by the stratospheric qbo are investigated using a multiple regression model. also, both the solar maxima and the solar minima epochs are taken into qbo and d-region neutral density regression model solar maximum solar minimum adf p-value adf p-value model -4.24 0.0001 -5.71 0.000 the level of significance mackinnon [1996] critical values 1% -2.65 5% -1.95 10% -1.61 coefficents solar maximum solar minimum 21st 22nd 23rd 21st 22nd 23rd c (constant) 6.54×1019 6.49×1019 8.22×1022 6.80×1019 6.62×1019 7.67×1022 (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.002)*** (0.000)*** b1 d(qbo) -9.75×1016 -1.36×1016 2.32×1020 -1.30×1017 -8.58×1016 -1.98×1016 (0.075)* (0.031)** (0.011)** (0.065)* (0.055)* (0.018)** b2 (dummyw) 6.56×1019 6.74×1019 8.36×1022 6.67×1019 6.62×1019 7.89×1022 (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** b3 (dummye) 6.54×1019 6.49×1019 8.22×1022 6.80×1019 6.62×1019 7.67×1022 (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** (0.000)*** ar ( 1 ) 1.635 0.523 0.495 0.815 0.557 0.543 (0.000)*** (0.003)*** (0.032)** (0.003)*** (0.003)*** (0.000)*** r2 0.78 0.67 0.58 0.59 0.63 0.67 adj. r2 0.75 0.56 0.50 0.51 0.58 0.61 durbin watson 2.143 2.059 1.823 1.997 1.970 2.030 prob. (f-statistics) (0.000) (0.000) (0.000) (0.000) (0.000) (0.000) white het. (0.243) (0.670) (0.356) (0.919) (0.568) (0.509) table 5. the co-integration test results for 90 km. table 6. the regression model results for 90 km. (*),(**),(***) represents the significant level at 1 %, 5%, and 10%, respectively. consideration at the evaluating of the relationship between variables. the results are as follows: the explainable rate is higher at 75 km altitude than at 90 km altitude. the explainable rate of the effect on the nnd of qbo is 79%, 74% and 84% and 68%, 82% and 81% in the solar maxima and the solar minima epochs for 21st, 22nd and 23rd scs at 75 km altitude, respectively. the explainable rate of the effect on the nnd of qbo is 75%, 56% and 50% and 51%, 58% and 61% in the solar maxima and the solar minima epochs for 21st, 22nd and 23rd scs at 90 km altitude, respectively. the relationship between variables is positive for both direction of qbo at both altitudes. the relationship between qbo and nnd is positive for 21st and 23rd scs in solar minimum epoch and for 21st sc in solar maximum epoch at 75 km altitude. in the remaining cases, relationship between variables is negative. while the relationship between qbo and nnd is positive for 23rd sc in solar minimum epoch, it is negative in the remaining cases at 90 km altitude. it is noted that, none of these statistical studies provides any physical mechanism to determine the apparent relationships, but the qbo through the atmospheric waves [pancheva et al. 1991] may have a greater effect on the nnd in the d-region, which is considered as weakly ionized plasma. apart from causing a statistical decrease in the nnd due to the qbo, it is also may give rise to changes in the ion chemistry of this region [krivolutsky 2009]. this change would become physically and chemically by interacting with the carrying momentum by qbo of neutral components in d-region. thus, neutrals turn into ions by means of collisions and various chemical reactions or vice versa, and nnd number may be changed. thus, qbo, that is one of the meteorological influences, can play an important role in the lower ionospheric nnd. references baldwin, m., l. gray, t. dunkerton, k. hamilton, p. haynes, w. randel, j. holton, m. alexander, i. hirota and t. horinouchi (2001). the quasi-biennial oscillation, rev. geophys., 39 (2), 179-229. chen, p.r. (1992). evidence of the ionospheric response to the qbo, geophys. res. lett., 19 (11), 1089-1092. danilov, a.d. (1998). solar activity effects in the ionospheric d region, ann. geophysicae, 1527-1533. enders, w. (2008). applied econometric time series, john wiley & sons. engle, r.f., and c.w.j. granger (1987). co-integration and error correction: representation, estimation and testing, econometrica, 55, 251-276. forbes, j.m., s.e. palo and x. zhang (2000). variability of the ionosphere, j. atmos. terr. phys., 62 (8), 685693. heaps, a., w. lahoz and a. o’neill (2000).the quasibiennial zonal wind oscillation (qbo), centre for global atmospheric modelling, department of meteorology, university of reading, uk. kazimirovsky, e.s., and v.d. kokourov (1995). meteorological effects in the ionosphere: a review, geomagn. aeron., 35 (3), 303-320. kirov, b., s. asenovski, k. georgieva and v.n. obridko (2014). what causes geomagnetic activity during sunspot minimum?, solar and solar-terrestrial physics, saint petersburg, pulkovo, october 20-24, 219-222. kero, a. (2008). ionospheric d-region studies by means of active heating experiments and modelling, sodankylä geophysical observatory publications no. 102, oulu university press, finland. krivolutsky, a.a. (2009). russian studies of the middle atmosphere in 2003-2006, izvestiya, atmos-oceanic phys., 45 (4), 528-539. kwiatkowski, d., p.c.b. phillips, p. schmidt and y. shin (1992). testing the null hypothesis of stationary against the alternative of a unit root: how sure are we that economic time series have a unit root?, j. econometrics, 54 (1), 159-178. lastovicka, j. (2006). forcing of the ionosphere by waves from below, j. atmos. terr. phys., 68 (3-5), 479-497. mackinnon, j.g. (1996). numerical distribution functions for unit root and cointegration tests, j. app. econometrics, 11, 601-618. mohanakumar, k. (2008). stratosphere troposphere interaction, springer doc. goghin. pancheva, d., j. lastovicka, b.a. de la morena (1991). quasi-periodic fluctuations in ionospheric absorption in relation to planetary activity in the stratosphere, j. atmos. terr. phys., 53, 1151-1156. rishbeth, h., and m. mendillo (2001). patterns of f2layer variability, j. atmos. terr. phys., 63 (15), 16611680. sagir, s., s. karatay, r. atici, a. yesil and o. ozcan (2015). the relationship between the quasi biennial oscillation and sunspot number, adv. space res., 55 (1), 106-112. corresponding author: ramazan atıcı, mus alparslan university, faculty of education, mus, turkey; email: r.atici@alparslan.edu.tr. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sagir et al. 8 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 displacement across the trecastagni fault (mt. etna) and induced seismicity: the october 2009 to january 2010 episode salvatore gambino1,*, alessandro bonforte1, antonino carnazzo2, giuseppe falzone1, ferruccio ferrari1, angelo ferro1, francesco guglielmino1, giuseppe laudani1, vincenza maiolino1, giuseppe puglisi1 1 istituto nazionale di geofisica e vulcanologia, sezione di catania, catania, italy 2 provincia regionale di catania,tremestieri etneo (catania), italy annals of geophysics, 54, 4, 2011; doi: 10.4401/ag-4841 abstract the trecastagni fault (tf) is a nnw-sse tectonic structure inside mt. etna that is characterized by evident morphological scarps and normal and right-lateral type movements that directly affect roads and buildings. the tf has an important role in the instability that affects the southeastern flank of mt. etna, and it represents part of the southern boundary of the unstable sector. starting from 2005, the tf has been monitored using continuous wire extensometers and by carrying out periodic direct measurements across the northern and central sectors. from 2005 to september 2009, the measurements show that the tf has undergone continuous extension of about 2-3 mm/yr. during the october 2009 to january 2010 period, the creep rate of the tf reached up to 5.5 to 7.0 mm/yr, and this was accompanied by several shallow, low magnitude earthquakes that were felt by local people. this episode appears to be linked to the increase in the sliding velocity of the eastern flank that occurred during 2009. 1. introduction the geodynamic framework of mt. etna is characterized by a compressive stress regime, which trends roughly n-s, and is due to the collision of the eurasia and africa plates, and an extensional regime that trends roughly e-w. the volcano edifice has formed at the intersection of two regional fault systems, with nnw-sse and ne-sw trends (figure 1). the complex interaction between regional stress, gravity force and dyke-induced rifting has produced the slow sliding of the eastern and south-eastern flanks of the volcano (flank dynamics), the main evidence for which is the many fault systems that have cropped out in this area, the frequent seismic activity, and the intense ground deformations. the flank dynamics at mt. etna appear to have an important role as a ‘trigger’ for the volcanic activity [e.g. allard et al. 2006, bonforte et al. 2007, bonforte et al. 2008]. in this context, the pernicana–provenzana fault system is unanimously indicated as the northern boundary of the eastern sliding flank, as any evidence of the flank dynamics abruptly disappears northward of this important feature. the south-eastern boundary is less defined, because all the evidence gradually diminishes [e.g. borgia et al. 1992, lo giudice and rasa 1992, froger et al. 2001]: the deformations gradually decrease south-westward; the seismicity becomes shallower and/or less intense; and the fault systems are less continuous and/or evident. among the different faults involved in the south-eastern boundary, from w to e there are the ragalna fault system [neri et al. 2007], the mascalucia–tremestieri fault, and the trecastagni– tremestieri fault (tf). these faults are characterized by evident morphological escarpments, which are sometimes masked by cones and lava. since 2005, the tf has been monitored using continuous wire extensometers and through periodic direct measurements across the discontinuity in the northern sector of the fault. this monitoring activity has shown an increase in the rate change in the fault displacement with respect to the relatively constant rate of displacement measured in previous years. this was accompanied by local seismicity, in october 2009. this study provides details of the 2005 to 2010 tf displacements and seismicity, and discusses this behavior in the framework of the flank dynamics and volcanic activity of mt. etna. 2. the trecastagni fault the tf is a discontinuity that has developed in the southern flank of mt. etna, between the villages of trecastagni and san giovanni la punta. this is an active structure with an approximately nnw-sse trend (figure 1) that is characterized by morphological escarpments and very shallow seismicity. the effects of the activity of the tf article history received september 23, 2010; accepted june 16, 2011. subject classification: measurements and monitoring, crustal deformations, volcano seismology, wire extensometer, shallow earthquakes, fault dynamics. 414 (creep) are visible on many stretches of provincial road 8-iii (p.s. 8-iii) (figure 2). the first studies on the dynamics of the tf were performed by lo giudice and rasà [1992] and were based on the effects of the september 1980 seismic crisis. the seismicity of the tf is characterized by very shallow earthquakes with typical focal depths of 1-2 km. evident coseismic surface faulting occurred along the fault scarp in september 1980 and in november 1988 [azzaro 1999]. hollows were seen to develop in agricultural land, along with fractures that offset buildings, boundary walls and the p.s. 8-iii road, after the local shocks on september 16 and 20, 1980, that displaced the northernmost and the central segments of the fault, as also seen after the november 21, 1988 earthquake [azzaro 1999]. three other similar episodes have been reconstructed for may 26, 1903, july 20, 1917, and february 17, 1955, through historical research [azzaro and d'amico 2008]. data on deformations across the tf were relatively sporadic until 2005. in their analysis of interferometric synthetic aperture radar (insar) interferograms that covered the 1996 to january 1998 mt. etna inflation period, froger et al. [2001] estimated a deformation rate of about 4-6 mm/yr, as measured along the line of sight of the satellite (in this case, with an incidence angle of about 23˚ from wsw). recent time series analyses of insar data from 1995 to 2000 using the permanent scatters techniques quantified a downthrow of the eastern side of the tf at a rate of about 4 mm/yr and negligible e-w movement [bonforte et al. 2011]. the big intrusion of july 2001 (the 2001 eruption) was very important in the recent dynamics of mt. etna [e.g. bonforte et al. 2009], as in the months that followed it produced an acceleration of the deformations and seismic activity along the ionian coast [puglisi et al. 2008]. during the 2002 to 2003 mt. etna eruption, neri et al. [2004] claimed to have measured displacements of more than 2 cm after a duration magnitude, md, 2.6 shallow earthquake that was recorded on november 26, 2002. this was interpreted as the migration of the ground deformation induced by the eruptive intrusion along the ne rift that first affected the pernicana fault. finally, using insar data, solaro et al. [2010] obtained a velocity not exceeding 5 mm/yr for the 2003 to 2008 period, similar to that of 1994 to 2000. 3. network and data in 2005, the istituto nazionale di geofisica e vulcanologia and the provincia di catania began to implement a local monitoring system that was chiefly aimed at quantification of the creep phenomena across the provincial road, and thus to relate this to the current volcanic activity of mt. etna. at present, the monitoring of the tf consists of two continuous wire extensometers and a system gambino et al. 415 figure 1.surface-fault map of mt. etna. inset: main regional fault systems: mf, messina–fiumefreddo line; me, malta escarpment; rdn, ripe della naca; vdb, valle del bove. gray line identifies the sliding sector. figure 2. location map of the extensometer stations et1 and et2, the discrete measurement sites, and the fer mobile seismic station. 416 for periodic direct measurements across the discontinuity in the central and north-central sectors of the fault (figure 2). 3.1. extensometers extensometers are common instruments that have been developed to continuously monitor ground displacements, such as with landslides and dams. they are also used to monitor active fault creep and fracture opening. direct measurements of creep movement along faults and fractures have been applied along segments of the san andreas fault, california, usa [bilham and whitehead 1997], in taiwan [lee et al. 2000], and for mt. etna [gambino 2004, neri et al. 2004], piton de la fournaise, réunion island [peltier et al. 2006] and mt. st. helens, washington, usa [iwatsubo et al. 1992]. we installed two continuous wire extensometers at the two stations of et1 and et2 (figure 2). these consist of a stainless steel transducer box and a target assembly made with an eyebolt expansion anchor (figure 3). the displacement transducer is housed in a stainless steel body that is mounted on a fixing plate for wall/floor installation. an invar rope forms the connection between the transducer and the target. each instrument is equipped with a multichannel datalogger that is programmed for 48 data points/day sampling (1 sample every 30 min) and that includes acquisition of the displacement and ground temperature. temperature data are necessary to minimize the seasonal and daily thermal variations in distance measurements. these two stations measure the relative displacements perpendicular to the fracture (the extensional component). figure 4 reports the may 2005 to may 2010 data that was recorded at the et1 continuous extensometer. these displacement data are characterized by low daily noise (<0.1 mm). the seasonal variations of the signal correlated well with the temperature variations (figure 4a). figure 4b shows the filtered data that was obtained using a linear distance-temperature regression to minimize the wire-length variation (da) due to temperature expansion: da = mdt aq where m is the coefficient of linear expansion (for invar, 1.5× ×10−6 ˚c−1 ), dt is the temperature variation, and aq is the wire length. the data recorded between may 2005 and september 2009 highlight an opening trend of about 2-3 mm/yr, with very slow variations around this value. however, from october 15, 2009, the data show a sharp increase in the trend that cumulated to more than 2 mm in the first 15 days, and to a total of 4 mm by the end of january 2010. from february 2010, the trend appeared to return to the same value measured before the october 2009 anomaly. several technical and environmental problems have prevented the obtaining of a continuous and reliable dataset for et2. this station shows a signal that is poorly correlated to air temperature variations and is characterized by step displacement variations. data recorded between june 2008 and april 2009 highlight a displacement of about 1 mm. afterwards, et2 stopped data acquisition, and when it was restored on october 22, 2009, the sensor measured a variation of 4 mm. from october 22, 2009, to the beginning of february 2010, the displacement was about 3 mm, and from february 2010 the trend appeared to have returned to the same values measured before the october 2009 anomaly. 3.2. discrete measurements for the discrete measurements, we used an analog strain gauge (model 58-c0230, controls s.r.l.). using appropriate metal discs fixed on both sides of a wall (figure 5a), this instrument allows the determination of the linear changes displacement across trecastagni fault figure 3. photographs of the et1 (a) and et2 (b) extensometer sites during installation. in the different positions of a discontinuity with the design precision of 0.001 mm. during 2005, we set up nine pairs of metal discs along several discontinuities related to the fault (figure 5a). we measured the metal disc distances about three times per year. for the discrete measurements, the pairs of metal discs along the structure have been measured about 15 times since 2005 (figure 5b). during the period 2005 to september 2009, these measurements confirmed the results obtained by the continuous data, revealing the same behavior in the two different sectors monitored. the last two readings carried out on october 23, 2009, and march 25, 2010, confirm the increase in the displacement. 4. seismicity for figure 6c,d, we selected the seismicity that occurred since may 2005 in a narrow area close to the tf. the earthquake map reports the best preliminary locations performed for surveillance purposes, which were obtained using hypoellipse code [lahr 1989] and a one-dimensional velocity crustal model derived from hirn et al. [1991], with some minor modifications. the seismic activity recorded during the may 2005 to april 2010 period in this area comprises events located to a depth of 20 km; figure 6 reports the 34 events located within the first 5 km of depth that we consider as the only ones that might be related to the fault dynamics. their magnitude is between 1.1 and 2.4. the seismicity is clustered in the northern part of the area, while gambino et al. 417 figure 4. distance trends and air temperatures (a) recorded at et1 station, with the distance trends corrected for temperature effects (b), over the period from may 2005 to april 2010. inset in (b): the time relationship between the displacements and the seismic events that occurred over the period of october 10-24, 2010. in coincidence with the beginning of the displacement anomaly, we recorded a m = 2.1 earthquake; two days later, a m = 1.8 earthquake occurred (17/10 h:07:19). 418 it appears that no earthquakes occurred in the tf sector. however, we stress that the current configuration of the seismic network is not optimal for studying the seismicity of this area, as it is located outside of the istituto nazionale di geofisica e vulcanologia, sezione di catania, permanent seismic network, which consists of 40 seismic stations positioned all around the mt. etna volcano (figure 6a). this situation might lead to shortfalls in the accuracy of hypocenter determinations and possible bias in epicenter positioning. however, it is still remarkable that an increase in seismicity in this sector occurred in the months that preceded the may 2008 eruption, and before the tf dynamic acceleration (figure 7). on october 15, 2009, at 00:52 a m=2.1 earthquake was distinctly felt by the resident population within a few kilometers along the central and northern tf area (figure 6d). this was a very shallow event that was characterized by waveforms with emergent onsets and low spectral content (figure 6b), which suggested an extremely shallow hypocenter (latitude: 37.639; longitude: 15.063; depth: -0.73 km b.s.l.; horizontal error: 1.0 km; vertical error: 1.2 km; rms: 0.22; azimuthal gap: 190 degrees). the analytical location is reported in figure 6c. however, we stress that the seismicity recorded in this area is extremely difficult to analyze because of the low magnitude events and the poor network coverage. moreover, the velocity model of the entire volcanic edifice appears to be inadequate to represent the shallowest layer velocity of this area. for these reasons, no fault plane solutions are available for shallow events on the southeastern flank. on october 20, 2009, we installed a mobile station (fer) equipped with local acquisition near the tf (figure 2, figure 6c). in the following days, several micro-earthquakes were recorded at this fer station. for five of these events, we obtained the direction of the seismic ray from particle motion analysis (figure 6e,f) and the distance from s-p time lapse (figure 6d); these analyses suggest that the source area of the earthquake was the northern sector of the tf. 5. discussion and conclusions previous studies that were performed by analyzing time series of insar data measured movement of the tf of the order of 4 mm/yr from 1993 to 2008 (except the particular 2001 to 2003 period). this pattern of movement is in agreement with the spreading of the eastern flank towards the sea. considering that field surveys show a right-lateral displacement across trecastagni fault figure 5. photographs (a) and related data (b) of the n˚ 7 and n˚ 8 pairs of metal discs installed along the structure. red circle evidence the metal discs. the graphs (b) indicates the changes in the distance between the metal discs. gambino et al. 419 figure 6.the istituto nazionale di geofisica e vulcanologia, sezione di catania, permanent seismic network (a; triangles), waveforms and spectra related to the october 15, 2009, 00:52 event recorded at the ecc (top panels) and eta (lower panels) stations (b), seismicity recorded in the tf area between may 2005 and april 2010 (c, d), with microearthquake epicenters recorded at the fer station obtained from particle motion analysis and s-p seismic phase time lapse (d). examples of waveform recorded at fer station (e); the red bar indicate the time window used for particle motion (f). 420 component of the displacement and that the insar data cannot detect the n-s movements (thus preventing measurement of the complete movement across the fault), we can argue that the total displacement across the tf might be higher than 4 mm/yr. the measurements with extensometers (both continuous and periodic) carried out across the tf since 2005 show that the fault was characterized by an average continuous displacement rate of about 2-3 mm/yr, until september 2009. this deformation rate is slightly lower than that measured by insar, and it raises the question of whether the deformations measured by extensometers are wholly representative of the total movements of a fault. the insar results suggest that the discontinuity in the velocity maps is not so sharp and that the movement is distributed over an area of about a few hundred meters that crosses the tf. thus, the fault where the extensometers are installed most likely represents one of the structural discontinuities involved in this movement. however, detailed studies are needed to provide a definitive answer to this question. indeed, a local leveling network is currently being implemented across the tf [bonforte et al. 2010]. on october 15, 2009, two important phenomena occurred at the tf: a m=2.1 earthquake was recorded by the istituto nazionale di geofisica e vulcanologia seismic network at 00:52 gmt; and a simultaneous acceleration of the deformations was measured by the extensometers (figure 4b). this was the beginning of a period that lasted 3 months that was characterized by high deformation rates across the tf and increased seismic release of this sector of the mt. etna volcano. the relevance of this phenomena and their simultaneity suggest a common origin that can be reasonably related to the eastward movement of the mt. etna south-eastern flank, due to the role of the tf in the structural assessment of the sliding flank of mt. etna [bonforte et al. 2011]. all of the above discussions are directed towards a possible relationship between the deformation across the tf and the volcanic activity. several authors have shown feedback processes between magmatic events and flank movement at mt. etna [e.g. walter et al. 2005, allard et al. 2006, bonforte et al. 2008]. in particular, in studying the 2002 to 2003 period, walter et al. [2005] used a three-dimensional numerical model to calculate stress changes related to shallow reservoir inflation, dyke intrusion and fault displacement, and this suggested three mechanisms of interaction at mt. etna: the eruption–flank movement interaction. the feeding of magmatic reservoirs at depths of about 4-6 km results in the inflation of the volcano, which in turn, induces slips along the pernicana fault and the decollement at the base of the eastern flank (at about 5 km). the flank movement–eruption interaction. the flank movement induces the unclamping of the ne and s rift zones of mt. etna, which in turn produces a decompression of the magmatic system that favors the shallow intrusion. the fault–fault interaction. numerical modeling shows that the slip along the decollement, and the pernicana and santa venerina faults influence each other. displacement across trecastagni fault figure 7. earthquake daily rate and related strain release of the may 2005 to february 2010 seismicity that occurred in the narrow area reported in figure 6d. the seismic strain release has been calculated as √e with the following relationship [richter 1958]: log e = 9.9 + 1.9 md − 0.024 md2, where md is the duration magnitude. regarding the slip of the tf, walter et al. [2005] stated that this might be encouraged by reservoir inflation, dyke intrusion and decollement slip. this means that any of the above-summarized mechanisms might have influenced the acceleration of the movement from october 15, 2009, to january 2010. in this context, we note that: (i) after the end of the 2004 to 2005 eruption, the global positioning system (gps) network recorded a renewal of the inflation that led to the july to december 2006 eruption [bonforte et al. 2008]. this inflation did not encourage the tf slip (figure 4); moreover, the 2007 to 2009 insar mean line-of-sight ascending velocity did not reveal any particular phases of inflation of the edifice (figure 8); (ii) in may 2008, an intrusive event led to the 2008 to 2009 eruption [aloisi et al. 2009]; however, the magnitude of this event is not comparable with the 2002 episode that was analyzed by walter et al. [2005]; moreover, it appears too far in time (17 months) to suggest any possible link [solaro et al. 2010]; (iii) in march to may 2009, seismicity at the pernicana and moscarello faults and ground deformations at low elevations testify to a sudden acceleration of the sliding of the eastern flank; gps horizontal measurements (figure 9) show an acceleration (from 1-2 to 5-6 cm/yr) of the movements of the south-eastern flank between june 2008 and june 2009, in particular on the eastern points. in light of the above, the tf increased displacements and induced seismicity that occurred from october 2009 to january 2010 would appear to be more linked to the acceleration of the eastern flank than to the other causes gambino et al. 421 figure 8. mean ascending line-of-sight ground velocity (mm/yr) referring to the 2007 to 2009 time span. the map was obtained using the persistent scatterer software, known as stamps/mti [hooper 2008], using all of the available envisat ascending data that was acquired for mt. etna from january 2007 to october 2009. the main discontinuities in the permanent scatterers velocity field detected by bonforte et al. [2011] are shown as brown line. red arrows indicate the trajectory (from south to north) of the satellite and its look direction toward east. the colour scale is in mm/year. the bottom left and top right corner coordinates are in km (wgs84 datum, utm projection, zone 33 north). 422 (reservoir inflation, dyke intrusion). the 2008 to 2009 intrusive episode, as well as the 2005 to 2006 inflation phase, appear not to have had any effects on the dynamics of the tf over the entire 2005 to 2010 period. future studies, coupled with an improvement in the geophysical monitoring systems close to the tf, should provide new information regarding the tf dynamics and be better able to define how its acceleration (and induced seismicity) are influenced by these different mechanisms proposed. references aloisi, m., a. bonaccorso, f. cannavò, s. gambino, m. mattia, g. puglisi and e. boschi (2009). a new dyke intrusion style for the mount etna may 2008 eruption modelled through continuous tilt and gps data, terra nova, 21, 316-321. allard, p., b. behncke, s. d'amico, m. neri and s. gambino (2006). mount etna 1993–2005: anatomy of an evolving eruptive cycle, earth sci. rev., 78, 85-114. azzaro, r. (1999). earthquake surface faulting at mount etna volcano (sicily) and implications for active tectonics, j. geodyn., 28, 193-213. azzaro r. and s. d'amico (2008). catalogo macrosismico dei terremoti etnei dal 1832 al 2005, ingv-ct, http://www.ct.ingv.it/sismologia/macro. bilham, r. and s. whitehead (1997). subsurface creep on the hayward fault, fremont, califonia, geophys. res. lett. 24, 1307-1310. bonforte a., s. gambino, f. guglielmino, f. obrizzo, m. palano and g. puglisi (2007). ground deformation modelling of flank dynamics prior to the 2002 eruption of mt. etna, b. volcanol., 69, 757-768. bonforte, a., a. bonaccorso, f. guglielmino, m. palano, and g. puglisi (2008). feeding system and magma storage beneath mt. etna as revealed by recent inflation/deflation cycles, j. geophys. res., 113, b05406; doi: 10.1029/2007jb005334. bonforte, a, s. gambino and m. neri (2009): intrusion of eccentric dikes: the case of the 2001 eruption and its role in the dynamics of mt. etna volcano, tectonophysics, 471 (1-2), 78-86. bonforte, a., g. brandi, s. consoli, g. falzone, a. ferro, s. gambino, f. guglielmino, g. laudani, f. obrizzo and g. puglisi (2010). the trecastagni fault dynamic by using continuous, discrete and satellite ground deformation measurements (mt. etna, italy), egu general assembly 2010, geophysical research abstracts vol. 12, egu201011479. displacement across trecastagni fault figure 9. global positioning system horizontal measurements between june 2006 and june 2010; arrows indicate the amplitude and direction of the displacements, and the graph shows the time series of the displacements. the red bar indicate the october 2009–january 2010 period. bonforte, a., f. guglielmino, m. coltelli, a. ferretti and g. puglisi (2011). structural assessment of mt. etna volcano from permanent scatterers analysis, geochem. geophys. geosyst., 12, q02002; doi: 10.1029/2010gc003213. borgia, a., l. ferrari and g. pasquarè (1992). importance of gravitational spreading in the tectonic and volcanic evolution of mount etna, nature, 357, 231-235. froger, j.l., o. merle and p. briole (2001). active speading and regional extension at mount etna imaged by sar interferometry, earth and planet. sc. lett., 187, 245-258. gambino, s. (2004). continuous dynamic response along a pre-existing structural discontinuity induced by the 2001 eruption at mt etna, earth planets space, 56, 447-456. hirn, a., a. nercessian, m. sapin, f. ferrucci and g. wittlinger (1991). seismic heterogeneity of mt. etna: structure and activity, geophys. j. int., 105, 139-153. hooper, a. (2008). a multi-temporal insar method incorporating both persistent scatterer and small baseline approaches, geophys. res. lett., 35, l16302; doi: 10.1029/ 2008gl034654. iwatsubo, e.y., j.w. ewert and t.l. murray (1992). monitoring radial crack deformation by displacement meters, in: j.w. ewert and d.a. swanson (eds.), monitoring volcanoes: techniques and strategies by the staff of the cascades observatory, 1980-90, u.s.g.s. book, pp. 95-101. lahr, j.c. (1989). hypoellipse version 2.0*: a computer program for determining local earthquake hypocentral parameters, magnitude, and first motion pattern, united states department of the survey menlo park, california, open file report, 89-116. lee, j., f. jeng, h. chu, j. angelier and j. hu (2000). a rodtype creepmeter for measurement of displacement in active fault zone, earth planets space, 52 (5), 321-328. lo giudice, e., and r. rasà (1992). very shallow earthquakes and brittle deformation in active volcanic areas: the etnean region as example, tectonophys., 202, 257-268. neri, m., v. acocella and b. behncke (2004). the role of the pernicana fault system in the spreading of mt. etna (italy) during the 20022003 eruption, b. volcanol., 66, 417430; doi: 10.1007/s00445-003-0322-x. neri, m., f. guglielmino and d. rust (2007). flank instability on mount etna: radon, radar interferometry, and geodetic data from the southwestern boundary of the unstable sector, j. geophys. res., 112, b04410; doi: 10.1029/2006jb0047. peltier, a., t. staudacher, p. catherine, l.-p. ricard, p. kowalski and p. bachélery (2006). subtle precursors of volcanic eruptions at piton de la fournaise detected by extensometers, geophys. res. lett., 33, l06315; doi: 10.1029/ 2005gl025495. puglisi, g., a. bonforte, a. ferretti, f. guglielmino, m. palano and c. prati (2008). dynamics of mount etna before, during, and after the july–august 2001 eruption inferred from gps and differential synthetic aperture radar interferometry data, j. geophys. res., 113, b06405; doi: 10.1029/2006jb004811. richter, c.f. (1958). elementary seismology, w. h. freeman, san francisco, pp. 135-149. solaro, g., v. acocella, s. pepe, j. ruch, m. neri and e. sansosti (2010). anatomy of an unstable volcano from insar: multiple processes affecting flank instability at mt. etna, 1994–2008, j. geophys. res., 115, b10405; doi: 10.1029/2009jb000820. walter, t.r., v. acocella, m. neri and f. amelung (2005). feedback processes between magmatic events and flank movement at mount etna (italy) during the 2002-2003 eruption, j. geophys. res., 110, b10205; doi: 10.1029/2005jb003688. *corresponding author: salvatore gambino, istituto nazionale di geofisica e vulcanologia, sezione di catania, catania, italy; email: gambino@ct.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. gambino et al. 423 vol52,5,2009 517 annals of geophysics, vol. 52, n. 5 october 2009 key words fractal geometry – scale-invariance – mei, el niño, soc 1. introduction the el niño southern oscillation (enso) is a complex atmospheric and oceanographic phenomenon that has been the object of much attention in recent years. besides their scientific importance as major climatic modifiers, warm enso events (i.e. el niño) have deep economic and social consequences. the 198283 el niño motivated a rush of research activity attempting a better understanding of the phenomenon and, hopefully, forecasting its surging and duration. with this purpose, many mathematical models have been developed to describe enso but the predictability of el niño events is still an open question. the monitoring of enso is mostly based on observations of sea surface temperatures (averaged to single values over large regions within the equatorial pacific ocean identified as niño 4, niño 3 and even niño 3.4) and on differences between tahiti and darwin atmospheric pressure in the whole tropical pacific basin (rasmusson and carpenter, 1982; ropelewski and jones, 1987; allan et al., 1991; trenberth, 1997; können et al., 1998; wallace, 1998). such point measures are useful indicators of the enso phenomenon but not representative of the coupled ocean-atmosphere phenomenon: the complexity of the climate system cannot be understood on the basis of simple and point measures. the noaa’s climate diagnostics center in boulder colorado has developed a more holistic approach regarding a multivariate enso index (mei) (wolter and timlin 1993; 1998) whose positive feature represents the warm enso phase related to el niño occurrence. recently, mazzarella et al. (2009) identified a 60-month cycle in mei series whose maxima and minima values are found to be in correspondence of el niño and la niña occurrences. here we visit the monththe el niño events: their classification and scale-invariance laws adriano mazzarella and andrea giuliacci osservatorio meteorologico, dipartimento di scienze della terra, università degli studi «federico ii», napoli, italy abstract many point indices have been developed to describe the el niño/southern oscillation but the multivariate enso index (mei) is considered the most representative since it links different meteorological parameters measured over the tropical pacific. the monthly values of the positive feature of mei (interval: 1950-2008) have been ranked as a catalogue of el niño events according to a semi-quantitative strength index ranging from 1 to 6. such a catalogue is shown to be scale invariant in respect both to strength index and to times of occurrence, suggesting that the ocean-atmosphere interaction phenomenon at the basis of el niño belongs to the class of dynamical processes which are in a self-organized critical state. mailing address: dr. adriano mazzarella, osservatorio meteorologico, dipartimento di scienze della terra, università degli studi «federico ii», largo s. marcellino 10, 80138 napoli, italy; e-mail: adriano.mazzarella@unina.it vol52,5,2009 1-12-2009 18:36 pagina 517 518 a. mazzarella and a. giuliacci ly series of mei as a sequence of el niño events according to robust scale-invariant statistical analyses. 2. data collection the data consist of the monthly values of multivariate enso index (mei) (interval: 19502008) as computed by wolter and timlin (1993; 1998), taken from website: http://www.cdc.noaa.gov/people/klaus.wolter/ mei/table.html, where each monthly value is centered between the preceding and subsequent month: for example, the january value represents the value centered between the december-january months and so on. the mei is a multivariate measure of the enso signal derived from six main observed variables over tropical pacific and belonging to comprehensive ocean-atmosphere data set (coads): sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature and cloudiness of the sky. the coads data set is available at a sufficient resolution such that large scale patterns (as opposed to point measurements) can be used in relation to each other to obtain a more comprehensive measure of enso-related interactions. the mei values are standardized with respect to a 1950-93 reference period and are computed as the first unrotated principal component (miranda et al., 2008) of all six combined observed fields with a mean equal to zero and a standard deviation equal to 1. such values are expressed as percentages of standard deviation (wolter and trimlin 1993; 1998) and are reported on fig 1. 3. el niño events and their cumulative distribution to analyse the mei monthly series as a catalogue of el niño events, we have computed many preliminary cumulative-distributions of the positive monthly values of mei according to different grouping intervals to account for the error due to the estimating. the results of the different trials have shown that the interval of 3.00 2.00 1.00 0.00 -1.00 -2.00 1950 1950 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 -3.00 m e i a n o m a lie s years fig. 1. time series plot of mei monthly values from january 1950 to december 2008. vol52,5,2009 1-12-2009 13:29 pagina 518 519 the el niño events: their classification and scale-invariance laws 0.6 unit yields the best–constrained shape of the cumulative distribution (fig. 2). this has ranked the mei positive monthly values inside a catalogue of el niño events according to the following semi-quantitative strength index (si) ranging from 1 to 6: very weak (si = 1) if 0 ≤ mei<0.6, weak (si = 2) if 0.6 ≤ mei<1.2, moderate (si = 3) if 1.2≤mei<1.8, strong (si = 4) if 1.8 ≤ mei<2.4, very strong (si = 5) if 2.4 ≤ mei<3.0 and extreme (si = 6) if mei 3.0. the so constructed catalogue is found to contain a number n of el niño events equal to 356 whose logarithmic cumulative distribution is related to si through the best fitting: log2(n) = a – b si (3.1) within the range 1 ≤ si ≤ 5 and obtained at a level of confidence higher than 99%. here, a = 6.48 ± 0.05 is the log2 of the number of events with si ≥ 0 and b = 1.19 ± 0.05 is a measure related to of el niño generating process. the regression line (3.1) regards a number of pairs nc = 5 for which the relative confidence level is here based on the null hypothesis of zero correlation that is rejected at the 99% confidence when the following relationship: (3.2) gives a value greater than that provided by the student distribution for a number of degrees of freedom equal to nc-2 and a correlation coefficient equal to r (mazzarella, 1998). the cumulative distribution of el niño events, verified to be significantly different from the simulation obtained when the same number of events is randomly distributed, can be here interpreted in terms of scale-invariance of el niño catalogue if we consider si as a simple 1-to-6 index of increasing factor of around 2 (fig. 2). it appears that the el niño catalogue is significantly scale /r nc r2 1-] ]g g6 @ 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 lo g 2 (n ) strength index fig. 2. cumulative distribution of the logarithm of the number of el niño events since 1950 versus their strength index (si). the dashed line represents the relative random distribution. vol52,5,2009 1-12-2009 13:29 pagina 519 520 a. mazzarella and a. giuliacci invariant and presumably complete, during the period 1950-2008, for all the analysed events with the exception of the event of march 1983, classified as si = 6, i.e., extreme. this indicates that no el niño event presumably has in the past escaped detection and that the event with si = 6 probably is not representative of the el niño process within the 1950-2008 interval. 4. the cantor dust model and the time distribution of el niño events the scale-invariant properties of time distribution of el niño events can be investigated on the basis of the cantor dust model that is a boxcounting algorithm (mandelbrot, 1983; turcotte, 1997) already utilized successfully in different processes involving seismic (smalley jr et al., 1987), volcanic (dubois and cheminee, 1991), and flooding (turcotte and green, 1993; mazzarella, 1998; mazzarella and rapetti, 2004) events. to apply the cantor dust method to a catalogue, we divide the time interval t 0 , over which the series of n events occurs, into a series of n smaller intervals of length t = t0/n with n = 2,3,4… and compute the fraction r = n/n of intervals of length t occupied by events. if the distribution of events has a fractal structure then: r = c t(1-d) (4.1) where c is a constant and d is the fractal dimension or, equivalently log(r) = log(c) + (1-d) log(t) (4.2) on a log-log scaled plane. the power law (4.2) must be, moreover, significantly different from a distribution in which the same number n of events is randomly spaced in time. it is easy to show that a regular time distribution of events causes an increase in d up to the limiting value d = 1 that is the dimension of a line. on the other hand, a concentration of events at a definite time corresponds to the limiting value d = 0 that is the dimension of a single point. as a mathematical representation, the power law (4.2) could be valid over an infinite range. however, for physical applications, there will be upper and lower limits on the applicability of the fractal distribution. the fractal dimension (d = 1-s) is estimated from the slope s that provides the best least square fit of log(r) on log(t) at a confidence level higher than 99% according to relationship (3.2). 5. results the cumulative distribution of the available 356 el niño events can be explained in terms of scale-invariance assuming that si is a simple 1to-6 index increasing with factor of 2 (fig. 2). it appears that the number n of events is linearly related to si, on a log-log scaled plane, within the range 1 ≤ si ≤ 5, with a level of confidence higher than 99% and significantly different from the relative random simulation. this shows that the catalogue is reasonably complete to indicate that in the past no el niño event escaped detection and that the event with si = 6 is not representative of the el niño process within the 1950-2008 interval. to verify the scaleinvariant features in the recurrence intervals of el niño events, we have applied the cantor dust method to their times of occurrence. the smallest time interval chosen was 1 month, gradually increased by a factor of 2. the fraction r (the fraction of time intervals including an event) as a function of the interval size t, on a log-log scaled plane, is reported in fig. 3. it shows a linear relationship (significantly different from the relative random simulation and confident at a level higher than 99%) with a fractal dimension d equal to 0.78 ± 0.01 and with a scaling region enclosed between 2 and 32 months. r was found to be not significantly different from the correspondent random simulation (no clustering) for values of t >32 months, where the events fill a line (d = 1) because biased by 60month harmonic cycle identified in mei monthly data (mazzarella et al., 2009). in fact, a cantor dust algorithm applied to values of increasing intertimes is found to provide a fractal dimension d = 1 as soon as the scaling region reaches the value of the period of the long-time modulating cycle. again, the 60-month cycle is found to bias the relative low-frequency specvol52,5,2009 1-12-2009 13:29 pagina 520 521 the el niño events: their classification and scale-invariance laws tral estimates of the monthly mei series available only from 1950 for which the comparison between the fractal dimensions computed utilizing the mei spectral analysis and the cantor dust method is not possible. 6. discussion and conclusions the monthly values of the positive feature of mei (interval: 1950-2008) have been ranked as a catalogue of el niño events according to a semi-quantitative strength index si ranging from 1 to 6 and increasing by a factor of 2. such an index corresponds to a qualitative classification of el niño events as very weak, weak, moderate, strong, very strong, extreme. all the available el niño events, qualitatively classified as very weak, are shown to be scale invariant in respect to si, with the exception of the event of march 1983, classified as si = 6, i.e., extreme, to indicate that no small event presumably has in the past escaped detection and that the event with si = 6 probably is not representative of the el niño process within the 19502008 interval. the application of the cantor dust method to the occurrence-times of same el niño events has identified different patterns of time clustering: a region of no clustering with a fractal dimension d = 1 (that is the topological dimension of a line) for t >32 months and a region of moderate clustering with d = 0.78 (with a topological dimension nearer to that of line than that of a point) for 2< t <32 months. the existence of scale-invariant properties in the sequence of el niño events both with respect to strength and to time occurrence suggests that scale shifts , from small to large scales, are possible, even within a limited range, which might allow a better understanding of the process at the basis of the phenomenon and possibly to assist in the el niño prediction. on the other 0.00 –0.05 –0.10 –0.15 –0.20 –0.25 –0.30 0.0 0.5 1.0 1.5 2.0 2.5 lo g (r ) log(t) (months) fig. 3. the logarithm of the fraction r of time intervals of length (in months) including at least one event with si ≥ 1 as a function log(t). the dashed line represents the relative random distribution. vol52,5,2009 1-12-2009 13:29 pagina 521 522 a. mazzarella and a. giuliacci hand, such invariant-scale properties can be reasonably interpreted as manifestations of the self-organized critical state (bak and chen, 1989; bak, 1996) at basis of el niño process (andrade et al., 1995) that depends on numerous meteorological and oceanic factors, linked together in a non-linear way and with numerous feedback processes. it is worth noting that the values of b and d in relationships (3.1) and (4.1), within their scaling regions of validity, represent the values of clustering in strength and in time of el niño events starting only from 1950. the unavailability of longer series precludes the investigation of their time changes to obtain a better understanding of the phenomenon. for this , the results here obtained should be considered preliminary. references allan, r.j., n. nicholls, p.d. jones and i.j. butterworth (1991): a further extension of the tahiti darwin soi, early soi results and darwin pressure, j. 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(received, may 3, 2009; accepted, september 3, 2009) vol52,5,2009 1-12-2009 13:29 pagina 522 layout 6 annals of geophysics, 61,2, se220, 2018; doi: 10.4401/ag-7705 1 “seismic risk evaluation for the emergency management„ francesco castelli1, valentina lentini2,*, antonio ferraro3, salvatore grasso4 1 phd, full professor of geotechnical engineering, faculty of engineering and architecture, university of enna kore, enna, italy 2 phd, assistant professor of geotechnical engineering, faculty of engineering and architecture, university of enna kore, enna, italy 3 phd, department of civil engineering and architecture, university of catania, catania, italy 4 phd, assistant professor, department of civil engineering and architecture, university of catania, catania, italy 1. introduction in a high seismic hazard country, as italy, with urban areas characterized by significant level of seismic vulnerability, the seismic zonation, monitoring local networks and earthquake early warning system, may allow the prevention and mitigation of the earthquakes effects [castelli et al., 2013, 2016a, b, c; castelli and maugeri, 2008, 2013; grasso et al., 2016; grasso and maugeri, 2014; ferraro et al., 2016; monaco et al. 2011]. some recent examples of strong earthquakes in italy include the central italy earthquake of 24th august 2016, the l’aquila earthquake of the 6th april 2009, the emilia romagna earthquake of the 29th may 2012, the st. lucia earthquake of the 13th december 1990 occurred in the south eastern of the sicily. in particular, two major seismic areas there are in the south eastern sicily: the first along the ionian coast (earthquakes of magnitude m > 7.0) and the second in the hinterland area (earthquakes of magnitude lower than 5.5). these normal faults were in the past sources of earthquakes with m up to 7.4 such as the 1169, 1693, 1818 and 1908 events. the seismic history of southeastern sicily is characterized by several intense events, which, over the article history receveid may 26, 2017; accepted december 4, 2017. subject classification: seismic risk; web-gis platform; monitoring. abstract the recent advances in cloud computing have opened new opportunities in emergency management issues due to earthquakes. in this context, geographic information system (gis) based solutions have been recently investigated, with the aim of the prevention and the reduction of seismic risk. the paper focuses on the results of the research project prisma cloud platforms for interoperable smart government in which an innovative open source gis system, based on the knowledge in the field of dynamic characterization of soil has been developed in order to assess the local seismic hazard and the seismic zonation of the enna area in the south of italy. the paper describes how the application of prospecting and surveying techniques allowed a decisive improvement in the geological knowledge of the area, contributing to define the subsoil model for the purposes of seismic microzonation. the seismic geotechnical characterization has been performed with laboratory tests including the resonant column and cyclic torsional shear test on undisturbed samples. the interpretation of geophysical and geotechnical data and their correlation with geological units are presented as microzonatic map. finally, a wireless sensor network has been used for structural monitoring at the aim to highlight the significant benefits when the time available for access is limited, by representing an effective way of managing risks. all the data relating to the monitoring of the buildings and to the geological and geotechnical characterization are available on the web gis platform, representing an important tool for the prevention and reduction of the seismic risk. centuries, caused severe damages and, sometimes, razed entire settlements [barbano et al., 2014; pappalardo et al., 2016]. some of these events generated also destructive tsunamis along the sicilian ionian coast (figure 1). earthquakes in sicily are often among the main causes of land-slides, with particular reference to rockfalls. such phenomena represent, in turn, a further risk for population and for structures and infrastructures [pappalardo et al., 2014; pappalardo and mineo, 2015]. while these sudden disasters result in deaths, injuries, and homeless people, a quick response from the corresponding governments may include different techniques for post-disaster recovery. one big challenge that arises with disasters is that the telecommunication services (e.g., cellular networks, third generation, long term evolution services and internet infrastructures) usually become interrupted or overwhelmed. this congestion can be, particularly, noticed immediately after the disaster because the inhabitants of the affected area might want to, at the same time, communicate with the rest of the world. in order to deal with this challenge, the topic of designing an efficient disaster resilient network has recently gained much interest. as a consequence, researchers have diverted attention toward an alternate technology, namely the wireless mesh networks (wmns), in order to construct disaster zone networks [liu et al., 2010, 2012; ngo et al., 2013; wishart et al., 2008]. recent advances in the development of wireless sensor networks [asplund and nadjm-tehrani, 2009] reveal a new paradigm for monitoring structures and infrastructure health [shibata et al., 2009; ishizu et al., 2011] and environmental conditions [fouda et al., 2012] owing to the availability of low powered millimeter-scale cpus, highly integrated wireless transceiver circuits and various miniature sensors. the paper focuses on the results of the research project prisma cloud platforms for interoperable smart government in which a web-gis platform, based on the knowledge in the field of dynamic characterization of soil and structural monitoring has been developed with the aim to provide the seismic zonation of the enna area. experimental and research activities have been performed through the active involvement of the department of civil protection. the activities regard: i) geological and geotechnical characterization of the soil through in situ and laboratory test; castelli et al. 2 figure 1. map of seismotectonic features of south-eastern sicily, with indication of 1693 and 1908 historical tsunamis. ii) wireless networks for monitoring strategic structures; iii) collection of experimental data and their processing through gis; iv) implementation of data [castelli et al., 2015, 2016d, 2017; castelli and lentini, 2010, 2012; maugeri et al., 2013] on an open cloud platform. its architecture consists in a geospatial database system, a local gis application for analyzing and modeling the seismic event and its impacts, a web-gis module for sharing the geo-information among the public and private stakeholders and emergency managers involved in disaster impact assessment and response management. with this aim, data available by previous studies and investigations, as well as, those deduced by the studies carried out within the project prisma, have been organized in a database and geo-referenced. a set of data and information needed for the evaluation of the seismic response are available on the web by the gis platform, representing an important tool for the prevention and reduction of the seismic risk. the data derived by geotechnical investigations have identified different soil types within the studied area and the geophysical surveys have provided information on the values of the shear wave velocity vs in the different layers and on the position of the bedrock. the results of resonant column, torsional shear and cyclic triaxial tests have provided the dynamic characterization of the subsoil and the normalized shear modulus g/g0-γ and damping ratio d-γ versus strain curves for an accurate seismic response of soil, taking into account the non-linear behavior. 2. risk perception the analysis, assessment and mitigation of seismic risk, as well as developing smart technologies in support of these activities, must be accompanied by a careful study of the human factors involved in these processes. the active and aware involvement of population is necessary condition in order to achieve any successful intervention. in particular, analyzing perception of seismic risk by population, the trust among citizens and information or interventions implemented by institutions appear to be the key-issue. at the same time, institutions must infuse trust in the population by policies activated to prevent and mitigate seismic emergencies. these considerations are not needed for an effective introduction of smart technologies aimed at granting safety in an area: technological systems for alerting properly work only if citizens perceive them as reliable and useful. in this context, the use of systems for the diagnostics of the subsoil, damage assessment, and monitoring [castelli and lentini, 2016] of urban areas and infrastructure, is an option with significant social and economic implications in terms of better ability to manage emergency situations. the emergency management presents many criticalities due to various factors, like the human factor (reaction of panic, high number of victims, possible obstacles to the rescues), the environmental characteristics (presence of escape ways, extreme weather conditions, easy way or not to reach the interested area) and the coordination of the intervention itself (different agencies may use different procedures or may even evaluate the same situation in different ways, thus disagreeing on the kind of intervention). models and simulations of risk prevention and emergency alert have to take into account many risk factors, with different activation thresholds, and various typologies of reaction for all the stakeholders. this requires not only the trust of citizens towards institutions (that is, their trust in the structure of prevention) but also the trust of institutions in citizens (as sources of information and as responsible agents in preventing to catastrophes). however, trust attitude in the context of natural catastrophes is complicated by a variety of factors: the number, diversity and complexity of relevant actors, the actions and interactions that each of them is able to implement, often without any coordination. a comprehensive analysis of all these factors, as well as their interactions, allows to define and elaborate prevention rules with a high level of effectiveness. for all these reasons, success of intervention strategies during a natural catastrophe relies heavily on distributed delegation actions towards actors capable of guaranteeing full trustworthiness, based on their well-tested and widely acknowledged expertise. 3. analysis, assessment and mitigation of seismic risk in sicily seismic zonation, monitoring local networks and alert systems may allow the promotion of appropriate policies for the prevention and mitigation of the earthquakes effects, especially in a region with high seismic risk, as sicily, and with an urban area characterized by high levels of seismic vulnerability. however, for the reduction of seismic risk, the development of innovative experimental activities that enable the advancement of knowledge in the field of seismic geotechnical hazard assessment of sites and of the vulnerability of infrastructure and civil and industrial facilities is necessary. geological and geotechnical aspects related to seismic prevention are particularly topical in italy also in rela3 seismic risk management tion to the effects caused by the recent earthquakes in the city of l’aquila [monaco et al., 2013; santucci de magistris et al., 2013), in the emilia region [facciorusso et al., 2016] and in the central italy. in the past, the study of these problems was often limited to the geotechnical characterization of the sites and to the analysis of phenomena such as, subsidence or local seismic amplification, not reaching consistent suggestions about the possibility to realize soil remediation to ensure the structures preservation. in contrast, the structural remediation is often separated from the geotechnical characterization of the sites, the geotechnical modeling of the soil foundation and the actions for the improvement of the foundations. to overcome the limits of these approaches, the research activity will aim the analysis of the phenomena regarding the soil in order to evaluate the influence that they exert on the seismic vulnerability of buildings and systems to secure the achievement of seismic improvement. these goals can be achieved through an innovative experimentation in the field of evaluation of soil, structures and infrastructure behavior subjected to seismic loads. therefore, the establishment of a monitoring network of seismic action for the acquisition of site data and the comparison with seismic amplification factors required by national and international regulations and the procedures for the analysis of the seismic response [cavallaro et al., 2006, 2008, 2012, 2013a, 2013b] in the case of difficult soil condition is expected. for more realistic assessment of the seismic hazard of a test site, it is necessary in addition to geological surveys, to take into account the historical and instrumental seismicity of the area around the test site in order to better constrain the degree of regional seismicity. the aim of research activity is to realize a platform open cloud, based on the implementation of smart systems for monitoring characteristics of the environment. the data available on the platform allow the construction of seismic damage scenarios, the verification of the structural resistance of strategic buildings and the assessment of seismic vulnerability of the urban area, the verification of the practicability of the road system. 4. test site the testing activities was taken place in sicily, with particular reference to the municipality of enna, in collaboration with the regional department of civil protection and the regional province of enna. several prospecting and surveying techniques (geological surveys, down-hole, masw, hvsr) and laboratory test for the static and dynamic characterization have been performed, allowing a decisive improvement in the geological and geotechnical knowledge of the area. for the purpose of seismic vulnerability assessment all buildings and houses of enna have been classified by identifying the construction material (masonry or reinforced concrete), the construction period, the number of floors, the surface in m2 and the number of inhabitants (table 1,2). furthermore, two typical buildings (figure 2) of the constructed reality were chosen: reinforced concrete and masonry buildings because these structural kinds represent over 90% of the built in italy on the basis of the results of istat census [2011]. a set of spectrum compatible synthetic acceloragrams for which was fixed a duration of 15 seconds have been generated. for each return period and for each generated signal a non-linear dynamic analysis with a fem calculation code has been performed. then for each return period, the probability of exceeding of each limit state has been evaluated and, consequently the fragility curves of the buildings. the synthetic accelerograms have been generated using the software rexel-disp [smerzini et al., 2013] that allows to select suites of natural accelerograms compatible with displacement spectra of ntc’08. records contained in rexel-disp are those of: selected input motions for displacement-based assessment and design (simbad). figure 3 shows the acceleration spectra corresponding to the synthetic accelerograms generated for the sites in which is located the duca d’aosta school and the building of the province of enna respectively for a return period equal to 475 years. in figure 3 the red line is the average spectrum according to the italian technical regulations on the constructions. the fragility curves have been re-calculated using natural accelerograms, selected in the strong-motion databank [giardini et al., 2013]. the acceleration response spectra on rock and at ground surface, computed for the seven accelerograms selected, are shown in figure 4 in which xa and ya are the waveform identification code of the acceloragrams. 5. sensor network earthquake early warning system (eews) are a rather recent development in seismology that allows to issue warnings to a site with a short lead-time about the impending arrival of the largest strong ground motion from an earthquake after the first wave arrivals have been detected nearer to the source by adequate sensors. although the time interval between the warning and the arrival of the strong ground motion may be only of some minutes castelli et al. 4 or seconds it allows for some important security measures to be taken to secure life and property. among the activities performed in the context of research project there are: monitoring of strategic buildings, identification of the seismic input, definition of fragility curves by means of non-linear fem analysis, collecting data for web-gis platform. at this aim the two chosen buildings were instrumented for the structural monitoring. in particular, for the building of the province of enna, ultrasonic measurements and dynamic identification tests were performed. the dynamic identification tests were carried out using triaxial velocimeters with sensitivity of 400 v/m/sec according to two different configurations: the first for the characterization of the tower and the second for the identification of the periods of the remaining part of the structure. nine velocimeters for the real-time structural health monitoring (figure 5) and 4 velocimeters for the tower (figure 6) were placed respectively. the recorded data regard the response of the structure to the natural actions such as ambient noise, wind, earthquake, etc. at the acquisition of the signals is followed the step of processing of the data. in particular, through the processing of the data in the frequency domain of some parts of the acquired signals, the fundamental period of the structure has been evaluated. the performed recordings have permit the identification of probable frequencies and natural vibration modes of the structure equal to 4.125 hz (0.24sec), 3.625 hz (0.27sec) and 12.5 hz (0.08sec). the recordings on the tower have provided a value of frequency along x direction equal to 3 hz (0.33sec) and along the y direction equal to 3.5 hz (0.28sec). on the basis of the experimental monitoring and the study performed an earthquake early warning system could be implemented in a web gis platform on the basis of obtained results in terms of natural frequencies. 6. geotechinical characterization of the test site to manage the seismic risk and to allow for effective and efficient management of communications during the alert and emergency phases, a desktop application for local authorities has been realized. the application consists of an advanced system which combines the experimental data provided by structural monitoring and 5 seismic risk management code meaning material 1 masonry 2 reinforced concrete pilotis 3 reinforced concrete no pilotis 4 other floor 1, 2, 3, 4, 5 if material =1 1, 2, 3, 4, 5, 6, 7, 8 if material = 2, 3, 4 period 1 < 1919 2 1919 1945 3 1946 1961 4 1962 1971 5 1972 1981 6 1982 1991 7 > 1991 no. houses number of houses within the group identified with material, floor and period no. buildings number of buildings within the group identified with material, floor and period surface (m2) surface in m2 within the group identified with material, floor and period no. inhabitants number of inhabitants within the group identified with material, floor and period table 1. code and meaning for buildings census. castelli et al. 6 material floor period no. houses no. buildings surface m2 no. inhabitants 1 3 2 27 12 2860 55 1 3 3 42 26 4246 94 1 3 4 11 6 1166 13 1 3 5 1 1 140 3 1 4 2 3 1 392 6 2 1 2 8 3 730 10 2 1 3 5 5 471 16 2 1 4 51 33 5134 125 2 1 5 34 25 3907 78 2 1 6 35 31 4097 78 2 1 7 15 15 1822 32 2 2 2 3 3 293 4 2 2 3 8 5 894 11 2 2 4 23 15 2329 48 2 2 5 63 48 7006 155 2 2 6 40 27 5198 98 2 2 7 16 12 1763 38 2 3 4 34 7 3148 87 2 3 5 4 3 560 6 2 3 6 23 6 2601 45 2 3 7 2 1 176 5 2 4 2 1 1 150 2 2 4 4 45 6 3730 103 2 4 5 17 2 1496 32 2 5 4 7 1 537 19 2 7 4 46 4 3370 76 3 1 4 7 4 1015 14 3 1 5 8 5 720 16 3 1 6 7 5 1067 27 3 1 7 12 9 1337 38 3 2 2 10 6 755 24 3 2 3 36 15 3766 81 3 2 4 55 32 5197 140 3 2 5 31 19 3753 86 3 2 6 55 21 5569 157 3 2 7 95 54 11007 284 3 3 2 8 2 639 18 3 3 4 17 4 1647 43 3 3 5 74 9 6516 204 3 3 6 6 1 504 9 3 3 7 28 7 2801 82 3 4 4 2 1 180 3 3 4 5 19 2 1654 45 3 5 4 43 2 2885 87 3 5 5 198 8 13234 554 3 6 5 54 1 4303 149 4 1 1 2 1 200 6 4 1 2 2 2 260 4 4 1 3 5 4 578 12 4 1 4 6 6 459 7 4 1 5 3 3 303 6 4 1 6 1 1 80 4 4 2 1 5 4 456 8 4 2 2 7 5 764 16 4 2 3 44 39 3720 72 4 2 4 36 32 3232 64 4 2 5 6 5 444 8 4 2 6 2 1 205 6 4 2 7 2 2 292 5 4 3 2 10 6 1233 17 4 3 3 15 11 1812 33 4 3 4 22 14 1822 53 4 3 5 7 5 871 17 4 4 4 4 1 275 10 table 2. buildings census. the available data derived from geotechnical investigation geo-referenced through the geographical information system (gis) with the probable impacts on the territory, providing a forecast of the seismic risk. 7 seismic risk management figure 2. reinforced concrete building: duca d’aosta school (a); masonry building: palace of the regional province of enna (b). (a) (b) figure 3. response spectra for synthetic accelerograms for the sites of the duca d’aosta school (a), and of the province of enna building (b). a) (b) at this aim a detailed survey, comprising in situ and laboratory tests have been performed and at the same time the data available related to geotechnical properties for several municipalities of the province of enna have been collected. the collection and analysis of exiting data from previous geotechnical investigations was the first step in planning the site investigation program. for several municipalities of the province of enna are available down hole (dh), multichannel analysis of surface wave (masw), horizontal to vertical spectral ratio (hvsr), refraction microtremor (remi) and electrical tomography tests (table 3). a preliminary selection of results of in situ tests have been collected throughout the past years by the public administrations. for an accurate dynamic subsoil characterization of such sites, surface wave tests were planned in all of them. down hole tests were carried out only in sites with apparently deep seismic bedrock. in the paper, the results of geotechnical soil characterization exclusively of the municipality of enna are reported. the geotechnical parameters of the soil foundation have been derived from the boreholes down to a depth of 30.0 m, equipped to perform down hole tests (dh) to evaluate the propagation velocity of seismic compression vp and shear vs waves or with piezometers in order to monitor the water level. to determine the propagation velocity of the body castelli et al. 8 figure 4. response spectra of accelerograms on rock (a) and at ground surface (b). a) (b) figure 5. structural monitoring system for the ground level (a), the first level (b) the second level (c) of the province of enna building. a) (b) (c) waves dh tests has been carried out. the velocities profiles vp and vs shows a progressive increase with depth, within a range of about 120 and 400 m/sec for shear waves and about 200 and 2200 for compression waves (table 4). in the investigated site, n.40 multireceiver surface wave (masw) tests were also performed (figure 7). the surface wave method is used for evaluating shear wave velocity profile. it is based on the dispersion of surface waves in heterogeneous media: the velocity of each harmonic component of the surface wave depends on the properties of the medium affected by the wave propagation and its penetration depth is proportional to the wavelength. dispersion curves (velocity versus waves frequency) can be extracted from field data, using processing technique. the shear wave velocity profile can be inferred by solving an inverse problem. the masw tests were performed using active acquisition technique. the figures 8 and 9 report a set of trial profiles, including the best-fitting profile, and the dispersion curve respectively. a first validation of the masw shear wave velocity profiles was possible by means of the dh tests available in the test site. the shear wave velocity profiles obtained from the dh and masw tests are compared in figure 10 for all data available (a) in the city of enna and for the c.da s. panasia area. it can be observed that dh and masw test results are very in good agreement. within the experimental activities of prisma n.80 single station measurements of microtremors have been performed in the city of enna (table 5). indeed, the experimental horizontal to vertical spectral ratio (hvsr) of ambient noise provide a rea9 seismic risk management figure 6. structural monitoring system for the tower of the province of enna building. sonable estimate of the fundamental natural frequency of the site. the results of in situ test were used to represent a thematic map in which are reported for enna site the values of the parameter vs,30, computed according to the italian technical regulations on the constructions (figure 11). 6.1 geotechnical properties from routine laboratory tests the geological survey [pappalardo and rapisarda, 2016] shows that foundation soils include in the first meter fine sand and/or sandy silt and at a major depth blue-grey silt clay. the geotechnical characterization of the soil of the test site is carried out by routine laboratory tests. the values of the main index properties and the percentage of grain size distribution are summarized in table 6. most of the samples are coarse-grained soils, classifiable as sandy silts to silt sands, showing a lower percentage of clayey material. according to the particle size distribution, the tested samples can be classified into two main groups on the basis of clay fraction: lower than 30% and higher than 30%. physical parameters, in terms of water content wn, soil unit weight γ, plasticity index pi, were derived from standard classification tests performed on the samples retrieved by geotechnical survey. castelli et al. 10 test enna agira aidone assoro barrafranca dh 6 masw 11 10 1 4 7 remi 2 1 2 1 2 hvsr 2 1 1 1 7 electrical tomography 1 3 test calascibetta catenanuova centuripe cerami gagliano c.to dh 1 masw 2 2 2 2 2 remi 1 1 1 1 1 hvsr 1 1 1 1 1 electrical tomography test leonforte nicosia nissoria piazza armerina pietraperzia dh 1 masw 1 2 1 4 1 remi 3 1 2 1 1 hvsr 1 1 1 3 1 electrical tomography 1 test regalbuto sperlinga troina valguarnera caropepe villarosa dh masw 5 1 1 2 1 remi 1 1 2 1 1 hvsr 1 1 1 1 1 electrical tomography soil vs (m/s) silty sand 500¸700 calcarenites 1200 clay marnes 300¸400 grey clay 200¸300 chalky clay 700¸800 chalk 1800¸2200 table 3. summary of site tests available for the municipalities of the province of enna. table 4. shear waves velocity for lithological units in the enna area. 6.2 stiffness and damping from laboratory tests soil non-linear behavior was analyzed by means of fixed-free resonant column/torsional shear (rc-cts) devices. the specimens were consolidated isotropically to the estimated in situ stress. at the end of the consolidation stage, the cyclic and/or dynamic tests were performed with increasing shear load levels, to investigate the behavior of the soils for shear strains ranging between 0.0001% and 1%. as usual, the tests were interpreted in terms of linear equivalent parameters, i.e. shear modulus g and damping ratio d. to evaluate the dynamic properties of the soil and in particular to determine the degradation law of shear modulus g and the increase law of damping ratio d several tests were performed with the resonant column/cyclic torsional shear apparatus. in rc tests, sinusoidal torsional forces are generally applied at high frequencies, so as to reach the resonance conditions. for low and medium levels of deformation 11 seismic risk management figure 7. geological map of enna with location of the hvsr tests and masw tests. torsional forces are generally applied at frequencies between 1 and 100 hz. at higher levels of deformation, the frequency of torsional forces ranges from 0.01 to 1 hz. rc and cts tests have been carried out on cylinder soil samples with 50 mm of diameter and 100 mm of length, by the use of electromagnetic actuators, in order to perform both rc and cts tests with the same equipment on the same sample. figure 12 shows the experimental results obtained from the resonant column and cyclic torsional shear test in terms of normalized shear modulus g/g0 (figure 12a) and damping ratio d (figure 12b) versus shear strain γ. experimental results have been compared in castelli et al. 12 figure 8. best fitting profile. figure 9. dispersion curve. figure 10. enna site: comparison of shear wave velocity profiles obtained by masw tests and down-hole tests. 13 seismic risk management station f0 a0 f1 a1 f2 a2 bedrock depth (m) tr1 7 tr2 4,69 2,11 7 tr3 7 tr4 22 tr5 7 tr6 10,72 1,7 10 tr7 31,72 6,25 9,8 tr8 7 tr9 2,5 2,8 4,2 2,1 24 2,7 32,2 tr10 7,3 tr11 15 tr12 15 tr13 31 2 7 tr14 5,5 2 19 tr15 12,95 2,2 16 tr16 2,5 2,71 3,5 2 26 tr17 5 2,1 37,4 tr18 35 2 6,5 tr19 5 2 7,92 2,9 7,7 tr20 7,2 tr21 1,23 2,3 28,6 tr22 3 2 4,52 2 17,9 tr23 5 2 26 tr24 0,53 1,8 19 tr25 16,72 1,73 7,7 tr26 16 tr27 27 3 7,7 tr28 0,44 3,5 5 2,8 20 tr29 20,63 2,2 8 tr30 20 tr31 2,8 3,5 20 tr32 18,5 tr33 7,2 6,8 17,2 tr34 4,2 2 5,2 2 18,4 tr35 2 2,3 76 tr36 19,83 4,2 21,7 tr37 1,28 2,5 50 tr38 5,94 2 18,5 tr39 14,38 2 12 castelli et al. 14 tr40 5,77 2 12,1 tr41 0,92 3,68 22 tr42 1,7 2,3 24 3 50 tr43 2,66 4 50 tr44 2 2 12 2,9 66 tr45 4,63 4 19 2,1 42 tr46 20 2,2 38 2 15,5 tr47 3,4 2,2 35 2,5 15,5 tr48 3,7 3 15,5 tr49 6 2,2 48 tr50 1,55 1,83 15,5 tr51 2 4,25 26 2,9 26 tr52 1,39 2,35 2,5 2 42 tr53 3,09 2,68 58 tr54 2,2 3 31,09 3,6 44 tr55 2,2 3 4,63 3,2 40 tr56 2,3 2 50 tr57 2,81 3,32 7,2 2,7 42 tr58 50 tr59 1,3 2,6 4 2,5 15 3 35 tr60 2,6 2,8 42 tr61 4,5 3 41,3 tr62 1,5 2,6 4,02 3 17 2,2 27 tr63 25 tr64 1,6 2,5 2,8 2,6 42,4 tr65 23 tr66 0,66 2,4 25 tr67 3 2 40,1 tr68 2,02 3,6 55 tr69 2,81 1,8 50 tr70 1,52 4,8 10 2,2 24,7 tr71 2,19 2,4 20 tr72 2,61 3 33 2,8 46,44 tr73 47,7 tr74 22,03 1,6 27,3 tr75 1,95 1,5 25 tr76 2,5 2 20 3,8 22 tr77 22 tr78 2,3 2,5 4 3,2 20,2 tr79 48 tr80 3,75 3,4 22 3 48 table 5. single station measurements of microtremors. 15 seismic risk management figure 11. vs,30 map for enna site. figure 13 with the curves proposed by darendeli [2001] for soils with plasticity index equal to 30% and for mean effective confining pressure σ’0 equal to 100 kpa for soils tested in the range 75-150 kpa. the comparison among the tested specimens reflected their differences in physical properties, confirming that clay fraction and plasticity index are key parameters to represent soil non-linearity. the curves relevant to silty-low plasticity soils define a range of linear behavior not exceeding a threshold strain level of the order of 0.005% beyond which the decay of stiffness and the increase of damping are quite pronounced. the castelli et al. 16 site sample depth (m) clay (%) silt (%) sand (%) γ (kn/m3) wn (%) ip (%) eo c.da santa panasia s1/c2 6,00 39,36 41,31 19,32 20,21 19,90 s2/c1 2,00 12,34 14,46 71,59 19,26 19,33 s2/c2 5,50 30,73 39,50 25,61 19,66 31,26 33,47 0,731 s2/c4 15,5 35,33 44,94 19,74 20,01 20,02 42,12 0,581 s3/c1 1,50 11,75 17,97 64,41 19,01 20,99 11,58 s3/c2 4,00 5,39 2,25 89,38 19,67 13,76 0,584 s3/c3 8,00 20,60 27,07 52,29 20,26 20,27 8,58 0,569 s3/c4 15,3 28,50 48,71 22,79 20,05 22,74 48,84 0,608 c.da s. anna s1/c2 8,00 10,15 30,01 46,11 19,27 24,74 14,80 s1/c3 18,0 25,13 53,54 21,27 19,88 31,86 36,39 s2/c1 3,00 23,35 43,79 25,63 20,02 21,97 19,97 s2/c3 13,2 37,45 51,80 10,72 20,52 22,50 47,44 0,495 s3/c2 5,70 26,24 34,80 36,44 19,25 28,18 22,79 table 6. geotechnical properties derived by laboratory tests. figure 12. normalized shear modulus (a) and damping ratio (b) vs shear strain from rc and cts tests. a) (b) curves relevant to clayey-high plasticity soils are characterized by higher values of the linear threshold of the order of 0.01%, showing a less evident reduction of stiffness and lower damping values in the non-linear range. 7. site response analysis the study of topographic factors, with the information related to the surface morphology and to the mechanical properties of soil is fundamental for a reliable seismic response analysis. the paper proposes results from seismic analysis performed in 1d and 2d field based on geological, geotechnical and geophysical studies with reference to the enna area. the 1d numerical simulations were performed through the strata [kottke and rathje, 2008] and eera [bardet et al., 2000]. codes; the seismic bedrock depth was assumed at 30 m. the input motion (figure 14) is the st. lucia earthquake (13 december 1990) recorded to sortino (sr). the results obtained from the two numerical codes in terms of peck ground acceleration (pga) versus depth are presented in figure 15, showing a good agreement. the maximum value of pga obtained at the surface is equal to 0.203 g. when the section is characterized by a ratio between the depth and the distance from the edges greater than 2, then the 1d analysis may give unreliable results [lanzo and silvestri, 1999]. for this reason, topographic effects on the hill of enna have been studied through the quake/w [krahn, 2004] computer code. the 2d simulations have been performed considering a cross section (figure 16) that covers 2060 m in length and 280 m in elevation; the numerical method used is the finite element implemented is quake/w. rectangular shaped elements have been used in the finite element domain discretization. the boundary conditions applied to lateral boundaries are nodal zero vertical displacements and the boundary condition at the bottom of the model are nodal zero vertical and horizontal displacements (figure 17). finally, the input time history acceleration relating to st. lucia earthquake (13 december 17 seismic risk management figure 13. normalized shear modulus (a) and damping ratio (b) vs shear strain from rc and cts tests compared with literature curve by darendeli (2001) for σ’0 equal to 100 kpa. a) (b) figure 14. input motion recorded to sortino (sr) station during st. lucia earthquake (13th december 1990). figure 15. peack ground acceleration versus depth for the bedrock depth at 30 m. 1990) (sr) has been applied at the bottom of the mesh. the analyses were performed based on the soil properties derived from field investigation and laboratory tests. for the purpose of this numerical study carried out along the cross section of enna, this seismic bedrock model seems to be the most representative, provided that the input motion is the 1999 st. lucia earthquake recorded at sortino and accurate geotechnical and geological characterization of deep deposit is available. figure 18 shows the values of the maximum acceleration obtained by 1d (red line) and 2d (blue line) simulations. it is possible to note that the value of pga on the crest are greater than those on the edges. 8. seismic microzonation of the area the seismic microzonation is a cognitive tool aiming at the mitigation of seismic risk of an area. it should be conducted on the basis of the guidelines for seismic microzonation that defines three levels of detail and the corresponding improvements of knowledge that should be carried out to achieve each one, depending also on the local hazard. level 1 is an introductory level consisting of a collection of exiting data processed to define the subsoil model and a seismic microzonation map in which the territory is qualitatively classified into homogeneous microzones. level 2 introduce the quantitative elements associated with the homogeneous zones, using additional investigations and a new seismic microzonation map is produced. level 3 contains insights on topics and/or on particular areas. all the considerations discussed above were integrated in a map of homogeneous microzones in seismic perspective (figure 19) in which some areas were identified as homogeneous on the basis of main parameters as: lithological and lithotecnical characteristics, depth of bedrock, geomorphological conditions, etc. the integration of geological, geophysical and geotechnical data allowed to distinguish n.29 zones characterized as stable zones but susceptible of local seismic amplification. in the same figure 4 instable microzones can be distinguished. 9. concluding remarks in the last years, strong earthquakes occurred in italy have highlighted the ineffectiveness of prevention policies. to acquire a greater knowledge on the seismic risk affecting urban areas, geological and geotechnical characterization of the soil is the main step. within this aim, smart technologies allow the management and sharing of complex information relating to the seismic vulnerability of exposed resources. the thematic maps are cognitive tools aiming at the mitigation of seismic risk of an area. the realization of a risk map is a complex task that involves the combination of data coming from different field of expertise, such as geology and geotechnical and structural engineering. the paper describes how the application of prospecting and surveying techniques allowed a decisive improvement in the castelli et al. 18 figure 16. cross section of enna. figure 17 2d boundary condition of the model. figure 18 comparison between 1d and 2d pga. geological knowledge of the test site, contributing to define the subsoil model for the purposes of seismic microzonation. the paper also reports the seismic geotechnical characterization performed with laboratory tests including the resonant column and cyclic torsional shear test on undisturbed samples. the results are summarized in terms of variation of stiffness and damping with shear strain. finally, wireless sensor for structural monitoring have been used, having significant benefits when the time available for access is often severely limited and representing an effective way of managing risks once that an area of concern has been identified. 19 seismic risk management figure 19 map of 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doi: 10.4401/ag-5335 abstract we present here the results from dynamical and thermal models that describe a channeled lava flow as it cools by radiation. in particular, the effects of power-law rheology and of the presence of bends in the flow are considered, as well as the formation of surface crust and lava tubes. on the basis of the thermal models, we analyze the assumptions implicit in the currently used formulae for evaluation of lava flow rates from satellite thermal imagery. assuming a steady flow down an inclined rectangular channel, we solve numerically the equation of motion by the finite-volume method and a classical iterative solution. our results show that the use of power-law rheology results in relevant differences in the average velocity and volume flow rate with respect to newtonian rheology. crust formation is strongly influenced by power-law rheology; in particular, the growth rate and the velocity profile inside the channel are strongly modified. in addition, channel curvature affects the flow dynamics and surface morphology. the size and shape of surface solid plates are controlled by competition between the shear stress and the crust yield strength: the degree of crust cover of the channel is studied as a function of the curvature. simple formulae are currently used to relate the lava flow rate to the energy radiated by the lava flow as inferred from satellite thermal imagery. such formulae are based on a specific model, and consequently, their validity is subject to the model assumptions. an analysis of these assumptions reveals that the current use of such formulae is not consistent with the model. introduction laboratory studies on basaltic melts show that lava rheology can show nonnewtonian behavior under certain conditions, which include vesicularity [spera et al. 1988, stein and spera 1992, badgassarov and pinkerton 2004], crystal concentration [pinkerton and stevenson 1992, smith 2000, sonder et al. 2006, champallier et al. 2008], and temperature and shear rates [shaw et al. 1968]. the literature indicates that magmas have nonnewtonian, pseudoplastic behavior, with the exception of smith [2000], who attributes a dilatant rheology to lava at high crystal concentrations. pseudoplasticity and dilatancy can be described by power-law rheology. when solving the problem of gravity-driven lava flow, power-law rheology introduces nonlinearity into the diffusion term of the momentum equation, and an analytical solution of the governing differential equations is not possible. this has given rise to various approximate solutions to the problem. the fully developed laminar flow of powerlaw fluids has been studied numerically using the finite element method [syrjala 1995] and the finite volume method [capobianchi 2008] for a pressure driven flow in a horizontal rectangular duct. the equation of motion for gravity-driven lava flow with a power-law rheology was solved by filippucci et al. [2010] using the finite volume method for the cases in which the rheology is temperature independent. several models describing lava tube formation have been proposed [greeley 1971, peterson and swanson 1974, rowland and walker 1990, hon et al. 1994, dragoni et al. 1995, valerio et al. 2008, valerio et al. 2010]. in the present study, the cooling of a channel of lava with power-law rheology is considered. a two-dimensional thermal model with heat flux assigned at the upper surface is introduced to describe lava cooling due to radiation and convection into the atmosphere. the lava crust is considered as a plastic body, and its rheology is described through the introduction of a yield strength as a function of temperature. article history received november 11, 2010; accepted may 31, 2011. subject classification: rheology, magmas, thermodynamics, lava flow, effusion rate. 510 special issue: v3-lava project tallarico et al. bends in lava flow are often observed, and these can strongly affect the flow dynamics and formation of lava tubes [greely 1971, peterson et al. 1994]. in the present study, we consider the effects of the curvature of a channel on the lava velocity and shear stress, and on the formation of the solid crust at the flow surface. to allow an analytical solution of the navier-stokes equation in the presence of bends, the lava is regarded as a newtonian, homogeneous, isotropic and incompressible fluid [valerio et al. 2011]. heat radiation and convection into the atmosphere are considered as the main cooling mechanisms. the model analyses the effects of curvature on the development of surface solid plates. the availability of high-resolution thermal imagery of active lava flows has stimulated the use of radiance maps for evaluation of lava effusion rates [mouginis-mark et al. 1994, harris et al. 1995, calvari et al. 2010]. this has been made possible by the use of simple formulae that relate the lava flow rate to the energy radiated per unit time from the planimetric surface of the flow [pieri and baloga 1986, harris et al. 1997, harris et al. 2005a]. such formulae are based on a specific flow model, and consequently, the results depend on the flow-model assumptions. an analysis of these assumptions was performed by dragoni and tallarico [2009], which revealed that the current use of these formulae is not consistent with the model. crust formation in a lava channel with power-law rheology the constitutive equation for a power-law fluid is: (1) where vij is the stress tensor, ėij is the strain rate tensor, k is the fluid consistency, n is the power-law exponent, which is a measure of the nonlinearity, and: (2) where i2 is the second invariant of the strain rate tensor. the apparent viscosity of the fluid is given by: (3) if n < 1, the fluid is pseudoplastic and it thins (i.e. it deforms more rapidly) with an increase in stress. if n=1, the fluid is newtonian. if n>1, the fluid is dilatant and it thickens with an increase in stress. an illustration of the model with the coordinate system is shown in figure 1. the equation of motion for a gravitydriven flow down an inclined rectangular channel is: (4) where vx is the x component of velocity, g is the gravity acceleration, and a is the slope angle. the apparent viscosity is: (5) 511 figure 1. coordinate system and geometrical parameters. figure 2. a) average velocity with respect to a for a fixed geometry (a = 10 m, h = 4 m, t �= 2650 kg m−3, k = 104 pa sn); b) average velocity with respect to a for a fixed q (a = 10 m, q = 500 m3s−1, t = 2650 kg m−3, k = 104 pa sn); c): thickness h with respect to q (a = 10 m, a = 20˚, t = 2650 kg m−3, k = 104 pa sn). 2ke e1ij n ij=v -o o 2e i2=o ke 1app n=h -o sin 0g x y v y v zapp x app x+ + = 2 2 2 2 2 2 2 2 t a h hc `m j k y v z v2 2 app x x 2 1n = + 2 2 2 2 h c `m j; e 512 the boundary conditions are the nonslip at the walls, and free surface at the top of the flow: (6) (7) the dynamic problem has been solved using the finite volume method with an iterative solver [patankar 1980]. the numerical tests carried out by filippucci et al. [2010] indicated that the numerical solution, with respect to the analytical solution, that is available for the newtonian rheology produces an error of about 0.03% with a grid of 51 × 51 control volumes (cv). the error due to the mesh refinement tested with the power-law rheology, for which the analytical solution is not available, is about 0.03% with a grid of 51 × 51 cvs, so the grid of 51 × 51 cvs was adopted for all of the following computations, as a good compromise between precision and time for calculation. the effects of the slope a on the average velocity and the flow rate can be remarkably different for newtonian and nonnewtonian fluids. figure 2a shows the effects of a on the average velocity, with varying n, and fixed k, channel width a, and thickness h. differences between the linear and the nonlinear rheology are enhanced by increases in a. the effects of the slope have been evaluated also for a constant volume flow rate q (figure 2b). the effects of nonlinearity on vx vary with a and k. finally, we evaluated the dependence of the volume flow rate q on h for fixed values of a and a (figure 2c). we simulate two possible scenarios of nonlinearity (pseudoplastic, with n = 0.5, and dilatant, with n = 1.5) and compare these with the newtonian case (n = 1) for the laghetto lava channel (figure 3a, b) and for another active lava channel, both of which were formed during the 2001 eruption of mount etna (figure 4). our results show that the cooling of channeled lava flow figure 3. velocity profiles of the laghetto channel (mount etna, 2001). a) horizontal profiles at z = 0. b) vertical profiles, at y = 0. the velocity profiles are plotted for three different values of the power-law exponent n (0.5, 1, 1.5). the parameters of the etna channel are: a = 12 m, h = 6 m, a = 5˚ (data from ferlito and sievert [2006]); the parameters of the lava are: t = 2030 kg m−3 and k = 8625 pa sn (data from pinkerton and sparks [1978]). figure 4. velocity profiles of the channel flow of the 2001 mount etna eruption. a) horizontal profiles at z = 0. b) vertical profiles at y = 0. the velocity profiles are plotted for three different values of the power-law index n (0.5, 1, 1.5). the parameters of the etna channel are: a = 3 m, h = 1.2–1.5 m, a = 7.5˚; the parameters of the lava are: t = 2030 kg m−3 and k = 1.1 × 104 pa sn (data from harris et al. [2005a]). ( , ) 0; 2 , 0v y h v a zx x= =!` j 0, 0; , 0 0y v z z v yx x= = 2 2 2 2^ ^h h velocity profiles and average velocities can vary significantly, depending on the assumption of linear or nonlinear rheology. we have also considered the may 1997 flow from pu'u 'o'o (kilauea, hawai'i). the apparent viscosity varies from 9 to 879 pa s, and decreases with an increase in the shear rate, so the system is pseudoplastic [weed et al. 1986]. the channel geometry was described by harris and rowland [2001]. we simulated the flow with pseudoplastic rheology, setting n = 0.9, 0.7 and 0.5. given the large variations in channel width and the possible variations in channel thickness, we considered two possible geometries for the channel section. in this way, we can observe how the average velocities are modified by a change in the aspect ratio b = h/a of the channel. as a result, with an increase in b the average velocity increases, and this effect is greater as n decreases (figure 5a). moreover, the volume flow rate q, which we hypothesized as constant with respect to b, increases very rapidly as n decreases (figure 5b). finally, we considered the 1984 mauna loa lava channels. we used power law rheology to fit the flow velocity data measured from videotapes by sakimoto and gregg [2001]. as a result, we found n in the dilatant range (figure 6a, b). cooling is modeled for a lava flow exiting a vent with an effusion temperature t0 = 1,273 k. we assume that the flow is laterally isothermal and that the cooling process occurs from the upper surface with a fixed value of the heat flux q0. this describes the average heat loss from the surface that is covered by a solid crust and from the crust-free shear regions [valerio et al. 2008]. the energy equation governing the heat transfer is: (8) where t is the temperature and | is the thermal diffusivity: (9) tallarico et al. 513 figure 5. pu'u 'o'o (kilauea, hawai'i) case study. a) average velocity va vs n for two values of b. b) volume flow rate q vs n, with q equal for the two values of b. the parameters of the channel are: a = 5 m, h = 3 m, a = 10˚; the parameters of the lava are: t = 2600 kg m−3 and k = 895 pa sn. figure 6. matches between the measurements of the flow velocity (crosses) and the velocity profiles (lines) computed with a dilatant model (n > 1) for the channel flow of april 2, 1984, mauna loa eruption. a) horizontal profiles at z = 0. b) vertical profiles at y = 0. the parameters of the channel are: a = 14.6 m, h = 4.1 m, a = 8˚; the parameters of the lava are: t = 1200 kg m−3 and k = 850–1400 pa sn (data from sakimoto and gregg [2001]; viscosity from harris and allen iii [2008]). t t y t z t 2 2 2 2 = + 2 2 2 2 2 2 |c m c k p =| t 514 where cp is the specific heat, and k is the thermal conductivity. the boundary conditions are the adiabatic conditions at the walls and the constant heat flux at the upper surface. initially, the fluid has a uniform temperature t0. at the time t>0, a constant heat flux q0 is set and the thermal boundary conditions become: (10) (11) in addition to the spatial integration, the discretization of the energy equation involves integration of equation (8) over the time interval from t to t + dt. this integration is operated by using a fully implicit scheme, which ensures simplicity and physical consistency [patankar 1980]. details of the numerical test of this solution are given below. we substituted the time dependence of the solution in t with the space dependence by the relation: (12) where: (13) we first considered the conductive cooling of a steady lava flow, and the consequent surface crust formation. indeed, many basaltic lava channels have a surface crust that appears dark due to its lower radiative temperature, and that is carried along by the flow. we evaluated the effects of the nonlinearity of the lava rheology on crust formation, by modeling the plastic behavior of the crust using a temperature-dependent yield strength x, given by: (14) where ts is the solidus temperature, x0 is the maximum yield strength, and b is a constant. we call d the width of the flow surface covered by crust. for temperatures below the solidus, crust formation and growth are possible in the region |y|< d/2, where the horizontal shear stress vxy < x. shearing movements occur and crust formation is inhibited where vxy>x. as the surface temperature reaches the solidus, x starts to increase up to the threshold. the distance from the vent where this happens strongly depends on the assumption on the rheology (figure 7a). simultaneously, the crust width d/a also grows, and once it attains its maximum width, it stops widening. in particular, for the dilatant fluid, the crust stops widening earlier in space with respect to the newtonian and the pseudoplastic fluid, which will cover a much longer distance before the crust reaches its maximum width (figure 7b). models of formation of lava tubes as a consequence of the crust welding to the channel levees have been proposed that consider lava as a bingham liquid [dragoni et al. 1995] or a newtonian liquid [cashman et al. 2006, valerio et al. 2008]. we also explored how variations in the channel width, ground slope and volume flow rate can affect the formation of a lava tube, assuming a power-law rheology. as a result, we find that for topographic and morphological variations of the lava channel, the nonlinearity of the rheology has only a slight influence on the formation of a lava tube, and the results for the newtonian fluid appear to also be valid for power-law fluids. numerical test on the solution of the heat equation the solution of the heat equation was tested to study the accuracy, by comparing it with the analytical solution with constant surface flux q0 [turcotte and shubert 1982]: (15) cooling of channeled lava flow figure 7. a) yield strength x as a function of x´ for different values of n. b) crust coverage on the lava surface d/a as function of x´ for different values of n (a = 10 m, h = 3 m, a = 0.2 rad, t = 2800 kg m−3, k = 104 pa sn, cp =837 j kg −1 k−1, k =3 w k−1 m−1, q0 =10 4 w m−2, x0 =8100 pa, b =170). x´ is the point on the flux direction where the surface temperature reaches the solidus, and it varies with varying n. ( , ) 0; 2 , 0z t y h y t a z= =! 2 2 2 2 ` j 0, 0; , 0y t z z t y k q0= = 2 2 2 2 -^ ^h h ( )t v z x x = r 1 ,v a v y z dy2 2 x x a a = r r ^ h# ( 1)b1 , 0, e t t t t 0 s s t t s = # $ x x -6 @) ( , ) 2 2 erfc 2t z t t k q t e z t z 0 4 an r t z2 = +r | | -| c m we evaluated the difference dtu between the upper temperatures of the two solutions and the difference dt between the average temperatures [syrjala 1996, capobianchi and wagner 2010], as given by: (16) as we expected, both and dtu always decrease as the number ncv of the control volumes increases, i.e. by thickening of the computational mesh, which means that the numerical temperature gradually approaches the analytical value (figures 8, 9, respectively). in this test, is always about two orders of magnitude less than dtu, and this difference is a consequence of the cooling of the lava channel, which at the time considered, involves only the upper layers of the flow; the lower part remains at a constant temperature. it can be seen that is very low also for a coarse grid, as it reaches 0.01% for a grid 61 × 61 cvs (figure 8), while dtu needs a grid of 201 × 201 cvs to drop under 0.05% (figure 9). in figures 8 and 9, the errors on and tu due to the mesh refinement are also plotted. it can be seen that due to mesh refinement, is a decreasing function of ncv, starting from the grid of 51 × 51 cvs. for figure 10, we evaluated the trend of with ncv at different times. this test indicates that decreases as the time increases, which indicates that the discretization error does not propagate with time and the solution is convergent. crust formation in the presence of a bend in the channel bends in lava flows arise from local variations in the topography, and they are commonly observed in volcanic fields. the curvature of a channel affects the flow dynamics and morphology [greeley 1971]. an analytical model was proposed by valerio et al. [2011] to explain the effects of curvature on the velocity and viscous stress. in a bending channel, a cylindrical coordinate system is assumed (figure 11). a flow segment with a constant curvature is limited by arcs of concentric circumferences, with their centers in the origin of the coordinate system. we studied the equation of motion for a viscous, homogenous, isotropic, newtonian fluid in the gravity field. in the near-vent portion of the flow, neglecting nonnewtonian effects is a reasonable assumption, as a consequence of the high temperature. the constant density t and viscosity h are used. lava flows in a rectangular channel with a constant slope. the steady-state motion is assumed to be unidirectional in the azimuthal direction: the flow is parallel to the levees. this implies that no radial component of the gravity force acts on it. this condition often applies when a channel changes direction as a consequence of local topography. further assumptions are introduced: the velocity only depends on the radial tallarico et al. 515 figure 8. numerical tests on the heat conduction equation. percentage errors as function of the number ncv of control volumes of the computational mesh at 301 s of cooling. solid line, between the numerical and analytical solutions. dashed line, between the actual grid and the coarser grid computed with a grid step of 10 × 10 cvs. figure 9. numerical tests on the heat conduction equation. percentage errors dtu as function of the number ncv of control volumes of the computational mesh at 301 s of cooling. solid line, dtu between the numerical and analytical solutions. dashed line, dtu between the actual grid and the coarser grid computed with a grid step of 10 × 10 cvs. figure 10. numerical tests on the heat conduction equation. percentage errors as a function of the number ncv of control volumes of the computational mesh at different times t = 101, 201, 301 s of cooling. 1 ( )t h t z dz0 h = r # tdr tdr tdr tdr tdr tdr tr tdr tdr tdr tdr 516 coordinate, and the pressure gradient is negligible. this obtains equation of motion as: (17) where u is the velocity, and: (18) by solving the equation with no slip boundary conditions at the lateral walls of the channel, we obtain the velocity as a function of the radius at the surface of the flow: (19) where: (20) the flow is not symmetric with respect to rc, the center of the channel. the gradient of the velocity is greater close to the levee with the higher curvature. this behavior is particularly noticeable in comparison with the velocity in a rectilinear channel (figure 12), which is calculated considering analogous hypotheses: the unidirectional flow of a newtonian fluid in a gravity field that depends on the horizontal coordinate, while dependence on depth and direction of flow is neglected. in this case, the velocity is symmetric with respect to the center of the channel, where it reaches its maximum. the maximum velocity in the curved channel is shifted toward the internal levee. the extent of the displacement from rc is a decreasing function of the radius of curvature, which shows that the asymmetry of the velocity is higher for sharp, rather than smooth, bends, and it increases with channel width (figure 13). we calculate the viscous stress as a function of the radius from the constitutive equation of a newtonian fluid, obtaining: (21) in analogy with the velocity gradient, the shear stress reaches greater values in the region close to the internal levee. in particular, it reaches its maximum in the internal levee. we compared the viscous stress calculated in a curvilinear channel with the stress in a rectilinear channel (figure 14). here, the curvature produces a reduction in stress close to the external levee, and an increase close to the internal levee. the maximum shear stress is calculated as a function of rc: it is a decreasing function of the radius of cooling of channeled lava flow figure 11. sketch of the model for bends and the coordinate system. figure 12. comparison between the velocity as a function of the horizontal coordinate in a rectilinear channel (dashed line), and in a curvilinear channel (solid line). the vertical line indicates the center of the channel rc. figure 13. distance between the point where the velocity is maximum and the center of the channel rc, as a function of the radius of curvature of the channel. each curve is calculated for a different value of channel width a. 1 0dr d u r dr du 2 2 + + =c sin g =c h t a ( ) 4 ( ) lnu r r r c r r2 1 2 1 = + c 4 ln c r r r r 1 2 1 2 2 2= c ( ) 4 1 lnr r r r r r rc1 2 1 2 1 = + +v h c -c `m j; e curvature, and reaches higher values with sharp bends. we consider radiation and convection into the atmosphere to be the dominant mechanisms in the cooling process. we calculate iteratively the surface heat-flow density as a function of temperature along the down-flow direction. (22) we use the model of cooling of a conductive half-space, described by the time-dependent heat equation for the heat flux [turcotte and schubert 1982]. substituting the fourier law in the solution, and on the assumption that the temperature is initially uniform in the medium, we obtain the temperature. substituting the time t according to equation (12), we obtain the flow temperature as a function of the coordinates z and l, which represent the vertical coordinate and the distance covered, respectively: (23) where t0 is the initial uniform temperature, and ū is the average velocity. as long as the lava cools, crust platforms form at the flow surface. these platforms of solidified lava are driven by the underlying fluid, and laterally they are limited by the action of the shear stress. greeley [1971] described some effects of the curvature of a channel on the lava surface morphology in hawaiian volcanic fields. the crust formed at a distance of a few meters from the eruptive vent. at the first bend in the channel, this crust breaks into plates, which float on the flow surface. the increased velocity gradient provokes fragmentation of the plates and their inclusion in the flow through remelting. peterson et al. [1994] also noted that the crust plates break in the lateral flow region when they run across a curve. a common phenomenon in bends is the presence of a different level of deposit on the two levees of the channel, which can be observed when the eruption ends and the lava drains. the level of deposits is considerably higher on the external levee rather than on the internal levee, as observed on kilauea volcano, hawaii [heslop et al. 1989] and on pico partido volcano, lanzarote [woodcock and harris 2006]. the centripetal force provokes only a slight difference in height between the two lateral walls. its effects are evaluated from the radial component of the navier-stokes equation, which considers the variation in pressure due to the variation in height in the flow surface. the calculated superelevation explains an increase in the height lower than 10% of the superelevation inferred from the data. the present model applies to cases where the flow is parallel to the levees, whereas the measured different amounts of deposits occur in flows where the radial component of the gravity force acts in bends, due to the local topography. surface radiance and implications for satellite thermal imagery the effusion rate of the lava from an eruption vent is the primary quantity that controls the evolution of the lava flow that ensues. for this reason, a lot of effort has been devoted to the evaluation of this quantity, which involves measurement of the flow velocity and the cross-sectional area [e.g. calvari et al. 2002]. the knowledge of effusion rates also has a major role in real-time simulations of lava flow paths that are carried out when lava flows threatens inhabited areas [e.g. ishihara et al. 1990, miyamoto and sasaki 1997, vicari et al. 2007, rongo et al. 2008, hérault et al. 2009]. direct measurements of effusion rates in the field are difficult, and calculations from other flow parameters would require the measurement of such parameters [tallarico et al. 2006], which can be equally problematic. in recent years, the availability of thermal images of volcanoes during eruptions has stimulated the evaluation of lava effusion rates from the measurement of their heat radiation. the thermal data obtained from high-resolution radiometers mounted in earth satellites were first used to monitor active lavas by mouginis-mark et al. [1994] and harris et al. [1995]. thermal images from hand-held radiometers on the ground have also been used [harris et al. 2005b]. the use of radiance maps for the evaluation of lava effusion rates is made possible by simple formulae that relate the lava flow rate to the energy radiated per unit time from the planimetric surfaces of the flow. such formulae are based on a specific flow model, and consequently, their validity is subject to the model assumptions. the evaluation of lava effusion rates is based on a formula originally proposed by tallarico et al. 517 figure 14. comparison between the viscous stress as a function of the horizontal coordinate in a rectilinear channel (dashed line), and in a curvilinear channel (solid line). the vertical line indicates the center of the channel rc. ( ) ( ) ( )q t e t t c t t0 4 4 a conv a= +r ( , ) 2 2 erfc 2t z l t k q u l e z z l u 0 0 4 l z u2 = r | || r rr c m; e 518 pieri and baloga [1986] that relates the flow rate to the planimetric area of a flow: (24) where q is the volume flow rate, w is the heat radiated per unit time from the flow surface, and dt is the difference between the lava temperatures at the vent and at the front. if the flow rate is assumed to coincide with the effusion rate, this formula states that the effusion rate is proportional to the heat radiated per unit time by the surface of the flow. harris et al. [1997] modified the formula, by including the thermal contribution w1 of convection in the air, and the degree {0 of crystallization of the lava. the contribution w2 of heat conduction to the ground was included later, obtaining [harris et al. 2005a]: (25) where cl is the latent heat of crystallization. the radiance of a hot body is related to the surface temperature at a given instant of time. if the body is a flowing liquid, the connection between the radiated heat and the flow rate is not straightforward. the singularity of equation (24) was noted by wright et al. [2001]. they suggested that the apparent success of the formula in giving reasonable values of effusion rates is due to numerical coincidence between a constant by which harris et al. [1997] multiply space-based estimates of lava flow area, and the empirical ratio between effusion rates and flow areas reported by pieri and baloga [1986] for a suite of hawaiian flows. harris et al. [2007] asserted that the technique gives time-averaged discharge rates. dragoni and tallarico [2009] proposed a different explanation. indeed, the formula of pieri and baloga [1986] can only be obtained in the framework of a lava flow model based on several simplifying assumptions. they explicitly addressed such assumptions, to ascertain which of them are acceptable approximations of real lava flows, and which instead impose strong limits to the applicability of the model. they also considered whether current use of the formula is consistent with the model itself. the analysis of the thermal model from which the formulae by pieri and baloga [1986] and harris et al. [1997] are obtained shows that the measurement of only the total instantaneous heat flow from the flow surface is not sufficient to calculate the lava flow rate. it is thus necessary to provide further information, i.e., the difference between the lava temperatures at the vent and at the front. if this difference is assumed to be the same for all lava flows, as is currently assumed, then the formula states that the effusion rate is an increasing function of flow length and a decreasing function of flow thickness, which is unrealistic. apparently reasonable results are obtained as the assumption of a uniform crust temperature is compensated for by the inconsistent use of the formulae. in the real world, the average surface temperature is lower for longer flows, and is higher for thicker flows. the measured heat flow incorporates these effects, which happen to counterbalance the use of a constant temperature difference between the vent and the front of the flow. conclusions the models presented in the present study indicate the relevance of thermal and rheological processes on the dynamics of channeled lava flows. in particular, the effects of nonlinearity on the velocity and the flow rate of a gravitydriven lava flow are not negligible, especially for steep slopes. moreover, the effects of nonlinearity on the average velocity are enhanced by an increase in the fluid consistency parameter. generally, the average velocity tends to increase with the aspect ratio of the channel, and this effect grows by decreasing the exponent n of the power-law rheology. the rapidity of crust formation due to the constant heat flow at the channel surface strongly depends on the degree of non-linearity of the rheology. the crust grows, attains its maximum width, and stops widening, depending on the rheology. while for the dilatant fluid, the crust stops widening very close to the point where the temperature reaches solidus, a pseudoplastic fluid will cover a much longer distance before the crust reaches its maximum width. this result is a consequence because with our assumptions, the pseudoplastic fluid flows with a greater velocity than the newtonian fluid, which in turn flows with a greater velocity than the dilatant fluid. numerical tests were carried out with the purpose of testing the stability and accuracy of the solution of the thermal equation. we find that the numerical error decreases as the number of cells of the mesh increases, and it does not propagate with time. the presence of bends in the lava path significantly affects the flow dynamics. a simplified form of the equation of motion for a newtonian fluid has been solved, which analytically obtained the velocity and stress fields in a cylindrical coordinate system as a function of the radial coordinate. fluid velocity and viscous stress show asymmetric behavior with respect to the center of the channel. the maximum of the velocity is shifted towards the internal levee, in proximity of which the shear stress also reaches higher values. although we use strong assumptions to simplify the dynamic, rheological and thermal aspects of lava flows, the model provides a possible explanation of some field observations. an analysis was carried out of the formulae currently used to evaluate the effusion rate of lava flows on the basis of the measurement of the instantaneous heat flow from the flow surface. the analysis of the model from which the formulae are derived shows that further information should be supplied, i.e. the difference between cooling of channeled lava flow q c t w p = t d ( )q c t c w w w1 p l 0 2= + + + t {d the lava temperatures at the vent and at the front. if this difference is assumed to be the same for all lava flows, as is currently assumed, the formula states that the effusion rate is an increasing function of the flow length and a decreasing function of flow thickness, which is not realistic. consistent application of the models presented here to actual eruptions 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(2000). textural evidence for dilatant (shear thickening) rheology of magma at high crystal concentrations, j. volcanol. geoth. res., 99, 1-7. sonder i., b. zimanowski and r. büttner (2006). non-newtonian viscosity of basaltic magma, geophys. res. lett., 33, l02303; doi: 10.1029/2005gl024240. spera, f., a. borgia, j. strimple, and m. feigenson (1988). rheology of melts and magmatic suspensions 1. design and calibration of concentric cylinder viscometer with application to rhyolitic magma, j. geophys. res., 93 (b9), 10273-10294. stein, d.j. and f.j. spera (1992). rheology and microstructure of magmatic emulsions: theory and experiments, j. volcanol. geoth. res., 49, 157-174. syrjala, s. (1995). finite-element analysis of fully developed laminar flow of power-law nonnewtonian fluid in a rectangular duct, int. commun. heat mass, 22, 549-557. syrjala, s. (1996). further finite element analysis of fully developed laminar flow of power-law non-newtonian fluid in rectangular ducts: heat transfer predictions, int. commun. heat mass, 23, 6, 799-807. tallarico, a., m. dragoni and g. zito (2006). evaluation of lava effusion rate and viscosity from other flow parameters, j. geophys. res., 111, b11205; doi: 10.1029/2005jb003762. turcotte, d.l. and g. schubert (1982). geodynamics: applications of continuum physics to geological problems, new york, 450 pp. valerio, a., a. tallarico and m. dragoni (2008). mechanisms of formation of lava tubes, j. geophys. res., 113, b08209; doi: 10.1029/2007jb005435. valerio, a., a. tallarico, and m. dragoni (2010). a model for the formation of lava tubes by the growth of the crust from the levees, j. geophys. res., 115, b09208; doi: 10.1029/2009jb006598. valerio, a., a. tallarico and m. dragoni (2011). effects of curvature of a lava channel on velocity and stress fields, geophys. j. int., 187 (2), 825-832; doi: 10.1111/j.1365-246x. 2011.05166.x. vicari, a., a. hérault, c. del negro, m. coltelli, m. marsella and c. proietti (2007). modeling of the 2001 lava flow at etna volcano by a cellular automata approach, environ. modell. softw., 22, 1465-1471. weed, h.c., f.j. ryerson and a.j. piwinskii (1986). rheological properties of molten kilauea iki basalt containing suspended crystals, in: k.s. vorres (ed.), mineral matter and ash in coal, acs symposium series, 301, 223-233. woodcock, d. and a.j.l. harris (2006). the dynamics of a channel-fed lava flow on pico partido volcano, lanzarote, b. volcanol., 69, 207-215. wright, r., s. blake, a.j.l. harris and d.a. rothery (2001). a simple explanation for the space-based calculation of lava eruption rates, earth planet. sci. lett., 192, 223-233. *corresponding author: andrea tallarico, università di bari, centro interdipartimentale di ricerca per il rischio sismico e vulcanico, bari, italy; email: tallarico@geo.uniba.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. cooling of channeled lava flow vol52,2,2009 197 annals of geophysics, vol. 52, n. 2, april 2009 key words petrophysics thermal and magnetic properties radioactive heat production volcanic rocks 1. introduction the mt melbourne volcanic field, forming the youngest volcanic system of the mcmurdo volcanic group, is located in the victoria land (antarctica) at the eastern flank of the ross sea rift. the latter is part of the west antarctic rift system, one of the largest tectonic provinces of the earth with extended continental crust (e.g. storti et al., 2008). volcanic evidence shows that rifting commenced after the break-up of australia and antarctica about 100 ma and was active until the middle cenozoic (cande et al., 2000; hamilton et al., 2001). general overviews of volcanological, stratigraphical and other field aspects as well as geochemical information on individual volcanic fields of the mcmurdo volcanic group are given by kyle (1990) and wörner (1999), whereas armienti et al. (1991) and rocholl et al. (1995) focus on rocks from the mt melbourne volcanic field (figs. 1 and 2). this work falls within a program of geophysical research aimed at studying the supply system and zones of accumulation of magma in the crust of the mcmurdo volcanic group. we present the results of petrophysical properties useful to calibrate models of crustal structure. we carried out laboratory measurements of thermal and magnetic properties of lavas of the mt melbourne volcanic field. these properties were determined in connection with bulk density and porosity. a set of thermal, radioactive and magnetic properties of the lavas of the mt melbourne volcanic field (victoria land, antarctica) vincenzo pasquale, massimo verdoya, paolo chiozzi and egidio armadillo dipartimento per lo studio del territorio e delle sue risorse, settore di geofisica, università di genova, italy abstract we present the results of measurements of physical properties carried out on mafic lavas from the mt melbourne volcanic field, useful for interpretation of geophysical surveys designed to shed light on the structure of the crust. the thermal conductivity is comparable to that of glass and shows a clear negative dependence on porosity. the volume heat capacity and the thermal diffusivity are less variable. the concentration of the thermally important natural radioactive isotopes was determined by gamma-ray spectrometry. lavas denoted a rather low heat-production rate, and the largest concentration of heat-producing elements (potassium, uranium, thorium) was found in the trachyte samples. the magnetic susceptibility is more variable than the other physical properties and, among the several iron-titanium oxides, it appears primarily controlled by the ulvöspinel-magnetite solid solution series. mailing address: prof. vincenzo pasquale, dipartimento per lo studio del territorio e delle sue risorse, settore di geofisica, università di genova, via benedetto xv 5, i-16132 genova, italy; e-mail:pasquale@dipteris.unige.it vol52,2,2009 17-06-2009 19:02 pagina 197 198 v. pasquale, m. verdoya, p. chiozzi and e. armadillo fig. 1. sketch map of the mt melbourne volcanic field showing position of sampling sites (see table i for sample codes and types of rocks). sub-fields: wcg – west of campbell glacier, sn – shield nunatak, or – oscar ridge, wr – washington ridge, ep – edmonson point, br – baker rocks, rh – random hills, mm – mt melbourne summit. gamma-ray spectrometry measurements were also carried out to determine the concentrations of uranium, thorium and potassium and the radioactive heat-production rate of the different rock types. 2. sampling and laboratory procedure figure 1 shows the location of the sampling sites and the main sub-fields of the mt melbourne volcanic field. the presence of an ice vol52,2,2009 17-06-2009 19:02 pagina 198 199 thermal, radioactive and magnetic properties of the lavas of the mt melbourne volcanic field (victoria land, antarctica) sheet allowed sampling of the exposed volcanic rocks only in washington ridge, edmonson point, baker rocks, shield nunatak and mt melbourne summit sub-fields. as a whole, twenty samples were collected paying attention to avoid sampling of rocks with evident weathering. table i presents the chemical composition for each sample obtained by means of inductively coupled plasma mass spectrometry at the activation laboratories ltd, ontario (canada), whereas fig. 3 depicts the alkali-silica diagram and the rock classification. the washington ridge (wr) is formed by a line of sub-aerial scoria cones and tuff rings. all samples of lavas are characterized by the lack of feldspar phenocrysts and, with the exception of sample wr6, they have a chemical composition ranging from basanite to amphibole-bearing tephrite. in the edmonson point (ep) eruptive centres and scattered deposits of tuff-breccias and pillows show a complex association of different rock types; our lava samples range from tephritic to trachyandesitic composition. the shield nunatak (sn) area is formed by a cluster of subglacial eruptive centres with a flat-topped morphology; we recovered only one sample of basaltic composition. lithotypes sampled at baker rock (br) range from basanites (br1-br3) to trachybasalt (br4). the mt melbourne summit (mm) is a stratovolcano formed between 0.01-0.25 ma, built on a base of trachytic lava flows and domes; samples are of trachyandesitic-trachytic to basaltic composition. thermal and magnetic properties of rock of the mt melbourne volcanic field were investigated according to standard procedures implemented at our laboratory. the thermal conductivity, thermal diffusivity and volume heat capacity measurements were carried out at room temperature by two devices: (i) an apparatus (prc) devised by us (pasquale, 1982 and 1983); (ii) a heat transfer analyzer (isomet) for measurements under transient regime, manufactured by applied precision ltd. (bratislava, slovakia). the heat-production rate and the magnetic susceptibility were obtained from measurements with a gamma-ray spectrometer (grs) and a magnetic susceptibility system (ms2) manufactured by eg & g-ortec (usa) and bartington instruments (witney, oxford, england), respectively. the prc apparatus allows accurate determinations of thermal conductivity of solid material in the range 0.3−11 w m-1 k-1. the total relative uncertainty, as shown by experiments fig. 2. schematic cross-section of the lithosphere showing the transition from the ross sea rift to the transantarctic mountains, based on geophysical data, xenolith evidence and information from volcanic rocks (wörner, 1999). 1. sediments, 2. upper crust, 3. lower crust, 4. mt melbourne volcanic field. vol52,2,2009 17-06-2009 19:02 pagina 199 200 v. pasquale, m. verdoya, p. chiozzi and e. armadillo performed on materials with well-known thermal properties, is less than 3%. reproducibility of measurements on various rock samples is within ± 2%. prc requires the sample to be prepared in cylindrical shape and clamped between two cylindrical copper blocks by a screw. the whole apparatus is initially kept at the same temperature, then the lower block is cooled and the temperature of the blocks is continuously recorded with thermocouples. when a uniform temperature gradient is established within the stack of elements, the thermal conductivity of the sample is determined, the heat capacity of the upper block of copper and of the sample being known. isomet uses a dynamic measurement method which allows simultaneous determinations of conductivity, diffusivity and volume heat capacity. the measurement range is 0.02− 6 w m-1k-1 for conductivity and 0.04−4 106 j table i. chemical composition (in mass %) obtained from inductively coupled plasma mass spectrometry (icp-ms) and rock type (see fig. 1 for sample location). sample sio2 tio2 al2o3 fe2o3 feo mno mgo cao na2o k2o p2o5 s total rock type wr1 43.65 3.27 13.51 3.99 9.31 0.21 7.96 8.67 4.35 1.66 1.04 0.08 97.70 basanite wr2 42.29 3.98 12.68 4.29 10.11 0.18 8.43 10.48 3.75 1.95 0.83 0.73 99.70 basanite wr3 45.05 3.31 14.06 3.00 9.50 0.21 7.32 8.41 4.54 2.08 1.16 0.03 98.67 basanite wr4 45.62 2.72 14.42 3.50 11.08 0.24 3.96 8.02 4.30 1.77 1.79 0.04 97.46 basanite wr5 44.92 3.64 14.20 3.96 8.79 0.20 7.42 8.87 4.66 2.22 0.83 0.06 99.77 basanite wr6 46.68 2.55 15.37 3.38 9.08 0.21 5.67 9.08 4.24 1.47 0.97 0.02 98.72 trachybasalt wr7 42.30 3.77 15.22 4.36 10.00 0.21 5.30 9.21 3.36 1.07 1.15 0.06 96.01 basanite ep1 50.40 2.04 16.64 3.03 9.09 0.25 2.79 6.72 5.34 2.32 1.01 0.05 99.68 basaltic trachyandesite ep2 44.62 4.11 15.71 4.09 10.01 0.22 4.84 10.04 3.70 1.15 1.21 0.11 99.81 tephrite ep3 58.83 0.99 16.37 3.11 6.05 0.23 0.89 3.70 5.80 3.70 0.31 0.02 99.80 trachyandesite ep4 48.34 2.57 16.52 3.36 9.09 0.23 3.64 8.10 4.61 1.89 1.21 0.06 99.62 trachybasalt ep5 48.67 2.68 15.52 4.09 9.10 0.25 3.46 7.72 4.53 2.14 1.34 0.03 99.53 trachybasalt ep6 46.65 2.87 15.29 3.08 9.56 0.21 6.46 9.66 4.02 1.50 0.97 0.02 99.89 trachybasalt sn1 45.07 2.95 12.67 2.80 9.92 0.18 10.55 11.78 2.56 0.66 0.69 0.09 99.92 basalt br1 42.45 2.04 13.40 2.82 7.25 0.19 6.89 8.16 4.57 1.82 0.93 0.05 90.57 basanite br2 43.39 3.42 16.45 4.26 8.91 0.19 6.13 10.81 3.08 1.04 0.98 0.06 98.72 basanite br3 44.73 3.33 13.63 4.83 8.31 0.18 10.12 9.34 3.36 1.14 0.59 0.03 99.59 basanite br4 48.57 2.75 14.82 2.86 11.36 0.26 3.44 7.77 4.50 1.95 1.49 0.03 99.80 trachybasalt mm1 61.18 0.30 16.66 2.51 3.77 0.21 0.11 1.29 6.35 5.58 0.11 0.03 98.10 trachyte mm2 47.02 3.96 19.76 2.82 7.62 0.14 2.04 6.66 2.98 1.06 1.01 0.04 95.11 basalt vol52,2,2009 17-06-2009 19:02 pagina 200 201 thermal, radioactive and magnetic properties of the lavas of the mt melbourne volcanic field (victoria land, antarctica) m-3k-1 for volume heat capacity, while reproducibility is 3% for both parameters, and the experimental relative error is less than 5%. sample preparation for the isomet procedure is fast and only requires the rock to be cut and smoothed to obtain a flat surface. interchangeable probes, calibrated for different ranges of thermal conductivity, are directly put on the sample surface. thermal parameters are inferred from the increase in temperature with time at the sample surface due to heat flow released by a planar heat source. the grs consists of a scintillation detector (a 7.62 cm × 7.62 cm nai(tl) crystal with resolution about 7%) and a 2048 channel analyser. the counting time for each sample was 5400 s. based on counting statistics, the relative standard uncertainty on u, th and k determinations are, in percent, 3.5, 2.4 and 1.2, respectively. for ultrabasic rocks with exceptionally low radioelement concentration the error may increase to 20%. for a sample weighing 0.750 kg, the detection limits are 0.1 ppm for u, 0.2 ppm for th, and 0.02% for k (chiozzi et al., 2000a). the ms2 system measures the magnetic susceptibility on samples (cores or powders) of 15 ml. it can be used for investigations of magnetic mineralogy and grain size and for determination of the curie transition temperature. the relative uncertainty in the determinations is fig. 3. total alkali content (na2o + k2o) vs. sio2. numbers at each data point correspond to those listed in table i. ol = olivine and q = quartz. vol52,2,2009 17-06-2009 19:02 pagina 201 202 v. pasquale, m. verdoya, p. chiozzi and e. armadillo as low as 5%, and the sensitivity is 2 10-6 si. repeated measurements on standards revealed very low drift. the system has a low operating frequency, so that measurements are not affected by sample conductivity. compact rock samples were first cut with diamond disk to obtain flat surfaces as required by thermal property determinations according to the isomet procedure. the surface dimensions were at least 6 cm in diameter and the minimum thickness 2-3 cm. samples were then cored with a head-diamond corer. the core specimens, about 3 cm high and 2.5 cm in diameter, were used for the determinations with prc and ms2. less massive rocks which could not be cut were reduced to powder. this allowed both measurements of magnetic table ii. bulk density (ρ), porosity (σ), thermal conductivity (λ1 by isomet, λ2 by prc), volumetric heat capacity (ρ×c) and thermal diffusivity (k). ρ ϕ λ1 std. λ2 std. ρ×c std. k std. sample (kg m-3) (%) (w m-1k-1) dev. (w m-1k-1) dev. (j m-3k-1) dev. (m2 s-1) dev. ×106 ×10-6 wr1 2520 18 1.08 0.18 wr2 1440 55 0.53 0.02 0.60 0.09 1.61 0.02 0.33 0.01 wr3 2160 24 0.92 0.05 0.94 0.08 1.85 0.02 0.49 0.03 wr4 2240 23 0.84 0.01 1.82 0.01 0.46 0.01 wr5 2480 15 1.00 0.02 1.89 0.02 0.53 0.01 wr6 2470 17 0.86 0.01 1.89 0.02 0.46 0.01 wr7 2890 1 1.23 0.26 ep1 2740 9 1.04 0.01 1.94 0.02 0.54 0.01 ep2 2730 10 1.23 0.02 1.14 0.09 2.02 0.01 0.61 0.01 ep3 2370 13 0.82 0.01 1.88 0.01 0.33 0.01 ep4 2440 6 1.03 0.02 1.91 0.01 0.54 0.01 ep5 2750 1 1.02 0.02 1.71 0.02 0.60 0.01 ep6 2870 2 sn1 3030 1 1.38 0.06 1.94 0.04 0.71 0.02 br1 2000 12 0.97 0.15 br2 2660 12 1.19 0.06 1.89 0.09 0.63 0.01 br3 2770 10 0.99 0.03 1.73 0.03 0.57 0.01 br4 2300 23 0.98 0.06 0.91 0.09 1.85 0.10 0.53 0.01 mm1 1740 34 0.55 0.01 0.48 0.16 1.81 0.05 0.30 0.01 mm2 2700 7 1.06 0.34 vol52,2,2009 17-06-2009 19:02 pagina 202 203 thermal, radioactive and magnetic properties of the lavas of the mt melbourne volcanic field (victoria land, antarctica) susceptibility and grs analyses to determine the concentration of heat-producing isotopes. a volume of 500 cm3, corresponding to rock mass ranging from 0.650 to 0.850 kg, was adopted for spectrometric measurements. moreover, bulk density and porosity were determined for all the samples. thermal, grs and magnetic measurements were carried out on five specimens obtained from each rock sample. 3. results and discussion table ii summarises the results of bulk density, porosity and thermal parameters. thermal conductivity varied from 0.48 w m-1k-1 (trachyte mm1) to 1.38 w m-1k-1 (basalt sn1). for the five samples investigated with both prc and isomet, the results are quite coherent, as the maximum difference between the two methods is close to the instrumental error (0.1 w m-1k-1). table iii. concentration of the heat-producing elements (u, th, k), radioactive heat-production rate (rhp) and magnetic susceptibility (χ). sample k std. u std. th std. rhp std. χ(si units) std. (%) dev. (ppm) dev. (ppm) dev. (µw m-3) dev. ×10-5 dev. wr1 1.54 0.04 2.2 0.1 8.8 0.4 1.23 0.06 4486 11 wr2 1.68 0.04 1.4 0.2 5.7 0.5 0.49 0.07 231 1 wr3 1.97 0.04 3.0 0.1 12.0 0.5 1.43 0.06 3622 5 wr4 1.56 0.04 1.2 0.2 5.3 0.4 0.68 0.07 898 18 wr5 1.92 0.04 2.5 0.1 10.0 0.5 1.39 0.07 4057 3 wr6 0.85 0.04 0.6 0.2 6.5 0.4 0.63 0.04 2096 2 wr7 0.78 0.03 0.7 0.2 2.7 0.3 0.47 0.07 494 4 ep1 2.00 0.04 2.2 0.2 8.2 0.5 1.34 0.07 1904 4 ep2 1.00 0.03 1.1 0.2 3.9 0.4 0.65 0.06 338 5 ep3 3.27 0.05 3.2 0.1 11.9 0.5 1.71 0.07 822 1 ep4 1.02 0.04 0.9 0.2 3.2 0.4 0.50 0.06 879 17 ep5 1.77 0.04 1.6 0.1 6.1 0.4 1.02 0.06 1764 6 ep6 1.30 0.04 1.5 0.2 4.8 0.4 0.89 0.07 1805 3 sn1 0.67 0.03 0.8 0.2 2.1 0.4 0.46 0.06 298 1 br1 1.42 0.05 0.6 0.3 3.3 0.6 0.38 0.08 791 3 br2 0.89 0.03 0.8 0.2 3.1 0.4 0.50 0.07 813 1 br3 0.99 0.03 0.9 0.2 3.6 0.4 0.59 0.07 4617 2 br4 1.66 0.04 1.7 0.1 7.4 0.5 0.94 0.06 2243 9 mm1 4.53 0.08 5.3 0.2 19.0 0.8 2.00 0.08 119 1 mm2 1.05 0.04 1.6 0.2 7.6 0.5 1.04 0.07 648 1 vol52,2,2009 17-06-2009 19:02 pagina 203 204 v. pasquale, m. verdoya, p. chiozzi and e. armadillo on the average, the volumetric heat capacity and the diffusivity are 1.85 (± 0.10) 106 j m-3 k-1 and 0.51 (± 0.12) 10-6 m2 s-1, respectively. since the chemical composition is relatively homogeneous, thermal conductivity is mainly controlled by porosity. figure 4 clearly points out a linear decrease of this parameter with the increase in porosity, whose maximum value (55%) was found for wr2 (basanite). the radioactive elements that mostly contribute to the internal heat generation are uranium, thorium and potassium. these elements fig. 4. thermal conductivity values versus porosity. fig. 5. relative contribution of u, th and k to the radioactive heat-production rate. vol52,2,2009 17-06-2009 19:02 pagina 204 205 thermal, radioactive and magnetic properties of the lavas of the mt melbourne volcanic field (victoria land, antarctica) were determined by the three window method. details on the adopted procedure and the system calibration are given by chiozzi et al. (1998; 2000a and 2000b). in general, this method involves the detection of the gamma-radiation emission in the decay of 214bi (238u series), 208tl (232th series), and 40k. however, it must be stressed that secular radioactive equilibrium in the decay series is necessary for appropriate measurements. this condition is generally fulfilled in 40k and 232th series while problems may arise for uranium, where a minimum age of 0.3 my is required for 226ra equilibrium. the age of some volcanic sub-fields of mt melbourne is as young as 0.25 my. this means that secular equilibrium between u and its decay products might not be fulfilled in all the rock sampled. to bypass this problem, the gamma-ray spectrum region comprised between 0.03 and 0.10 mev was investigated, instead of the higher energy photo-peak of 214bi (1.76 mev) (ketcham, 1996). chiozzi et al. (2000b) tested this approach on volcanic rocks younger than 0.3 my by comparing results from the nai(tl) spectrometer with a semiconductor (hpge) detector, and they found that use of the low energy window can remarkably reduce the uncertainty in the u determination. in this lowenergy region, there are a number of gamma rays produced by the decay of 234th. this radioelement can be safely assumed to be in secular equilibrium with 238u as its half-life is only 24.1 days, and therefore it could have a different activity from post-226ra daughter products, like 214bi. as the nai(tl) scintillation detector does not have sufficient resolution for reliable identification of individual peaks in this region of low energy, it was necessary to operate with a relatively wide window, ranging from 0.010 to 0.123 mev. the results of grs analyses are shown in table iii. generally, lava samples denoted rather low radioelement concentrations. the largest concentration of heat producing elements (k, u and th) was found in the trachytic sample mm1 (4.53%, 5.3 ppm and 19.0 ppm, respectively), whereas sn1 (basalt) resulted in the poorest in k and th (0.67% and 2.1 ppm, respectively). the lowest concentrations of u (0.6 ppm) were obtained for br1 (basanite) and wr6 (trachybasalt). if one neglects wr6, the th/u ratio is 3.93 ± 0.59, i.e. not significantly different from the average value found in the literature. if the rock density is known, the radioactive heat-production rate rhp due to the decay of 235u, 238u, 232th and 40k can be calculated from the uranium and thorium concentrations (see chiozzi et al., 2002 for details). considering the relative standard uncertainties and the detection limits of the grs determinations, the resulting error on rhp is 0.1 µw m-3. the major effect is due to u, whereas the accuracy and the detection limit of k are almost negligible in this regard. the radiogenic heat productivity of the analyzed lavas is relatively low, as a consequence of the low concentration of heat-producing radioelements (table iii). rhp ranges from a minimum of 0.38 µw m-3 for br1 (basanite) to a maximum of 2.00 for mm1 (trachyte). the relative contribution supplied by u, th and k to the radioactive heat-production rate is shown in fig. 5. the results of magnetic susceptibility measurements are also shown in table iii. measured values, which range from 0.001 (trachyte mm1) to 0.046 si units (basanite br3), mainly depend on the content of iron-titanium oxides. the oxides of interest in rock magnetism form a ternary system with feo, fe2o3 and tio2 as the endmembers (fig. 6). the most interesting minerals in this system are wüstite (fe1-xo), magnetite (fe3o4), hematite (α-fe2o3), maghemite (γfe2o3), pseudobrookite (fe2tio5), ferropseudobrookite (feti2o5), ilmenite (fetio3), and ulvöspinel (fe2tio4). there are three fundamental solid-solution series in this system: the ulvöspinel-magnetite series with an inverse spinel structure, the rhombohedral hematite-ilmenite series, and the orthorhombic pseudobrookiteferropseudobrookite series (petersen, 1976; hunt et al., 1995). figure 6 shows the iron-titanium oxides for the examined lavas together with the major solid-solution series. the lava magnetic components are clustered round the ulvöspinel-magnetite solid solution series [xfe2tio4(1 x)fe3o4] of x-value ranging from 0.5 to 0.7, as the most abundant mineral component responsible for the magnetic susceptibility. vol52,2,2009 18-06-2009 15:56 pagina 205 206 v. pasquale, m. verdoya, p. chiozzi and e. armadillo many titanomagnetites are slightly oxidized towards the titanohematite line or reduced. 4. conclusions we identified the thermal and magnetic properties, together with density and porosity, of some mafic lavas representative of the mt melbourne volcanic field. this information is of basic importance for interpretation of gravity and magnetic anomalies, and is essential for future studies on the thermal structure of the area. thermophysical properties reflect the effects of various geological processes acting during and after lava formation. the thermal conductivity of lavas is comparable to that of glass and diminishes till it halves with the increase in porosity. gammaray spectrometry results show low concentrations of radioactive elements and thus a low radioactive heat-production rate. for almost all the lava samples analyzed, the thorium/uranium ratio indicates no process of post-magmatic alteration. titanomagnetites appear to be the most abundant mineral components responsible for magnetic susceptibility. fig. 6. composition of the titanomagnetites of the investigated lavas. vol52,2,2009 17-06-2009 19:02 pagina 206 207 thermal, radioactive and magnetic properties of the lavas of the mt melbourne volcanic field (victoria land, antarctica) acknowledgements the authors thank giorgio caneva for his field work and support in laboratory measurements. references armienti, p., l. civetta, f. innocenti, p. manetti, a. tripodo, l. villari and g. vita (1991): new petrological and geochemical data on mt. melbourne volcanic field (northern victoria land, antarctica): ii. italian antarctic expedition, mem. soc. geol. it., 46, 397-424. cande, s.c., j.m. stock, r.d. muller and t. ishihara (2000): cenozoic motion between east and west antarctica, nature, 404, 145-150. chiozzi p., v. pasquale and m. verdoya (1998): ground radiometric survey of u, th and k in the lipari island, italy, j. appl. geophys., 38, 209-217. chiozzi p., p. de felice, v. pasquale and m. verdoya (2000a): practical applicability of field radiometric analyses, applied radiation and isotopes, 53, 215-220. chiozzi, p., p. de felice, a. fazio, v. pasquale and m. verdoya (2000b): laboratory application of nai(tl) γ-ray spectrometry to studies of natural radioactivity in geophysics, applied radiation and isotopes, 53, 127-132. chiozzi p., v. pasquale and m. verdoya (2002): heat from radioactive elements in young volcanics by γ-ray spectrometry, j. volcanology and geothermal research, 119, 205-214. hamilton r.j., b.p. luyendyk, c.c. sorlien and l.r. bartek (2001): cenozoic tectonics of the cape roberts rift basin and transantarctic mountains front, southwestern ross sea, antarctica, tectonics, 20, 325-342. hunt, c.p., b.m. moskowitz and s.k. banerjee (1995): magnetic properties of rocks and minerals, rock physics and phase relations: a handbook of physical constant, edited by t.j. ahrens, am. geophys. union, agu reference shelf, 3, 189-204. ketcham, r.a. (1996): an improved method for determination of heat production with gamma-ray scintillation spectrometry, chem. geol., 130, 175-194. kyle, p.r. (1990): mcmurdo volcanic group, introduction, in: volcanoes of the antarctic plate and southern oceans, edited by w.e. lemasurier and j.w. thompson, am. geophys. union, antarctic research series, 48, 19-25. pasquale, v. (1982): a dynamic method for the rapid measurement of the thermal conductivity of rocks, workshop on standard in geothermics, liblice (cssr), june 1982, pp. 1-4. pasquale, v. (1983): sulla conducibilità termica delle rocce, (proceeding 2° convegno del gruppo nazionale geofisica della terra solida, c.n.r., roma), pp. 765775. petersen, n. (1976): notes on the variation of magnetization within basalt lavas flows and dikes, pageoph., 114, 177-193. rocholl, a., m. stein, m. molzahn, s.r. hart and g. wörner (1995): geochemical evolution of rift magmas by progressive tapping of a stratified mantle source beneath the ross rift, antarctica, earth planet. sci. lett., 131, 207-224. storti, f., m.l. balestrieri, f. balsamo and f. rossetti (2008): structural and thermochronological constraints to the evolution of the west antarctic rift system in central victoria land, tectonics, 27, tc4012, doi:10.1029/2006tc002066. wörner, g. (1999): lithospheric dynamics and mantle sources of alkaline magmatism of the cenozoic west antarctic rift system, global and planetary change, 23, 61-77. (received november 21, 2008; accepted may 14, 2009) vol52,2,2009 17-06-2009 19:02 pagina 207 circolazione troposfe .. ica e stratosferica media mensile e stagionale per l'analisi co .... ente della circolazione dell' alta stratosfera ::\1. mo)'ltalto ricevuto il 15 oiugno i!wl le esigenze, appliel l,tive, ([l'ila ::'ileteorolo~ia aeronautica, nell'attuale fase (\i t,l~ . nsizione ,.ll'aviaziolw i" g-etto, e quelle (\e1la ri('er(,a ~ci(mtitiell fm lllo concordemente sentim un el'('seente intere&':le iwi' la eon08(,enza d('1h eireolazione stmt{)sferiea ed iperstra,t{)sferica. in molti i\wsi (,onte, (\p1 !"(>sto, lwl nustro, si sono pel'tmlto inten~i· jieati ill questi ultimi t.empi i tentat,ivi <li mpprcselltarc la (,ir(,olaziolh' lino l~ 30-32 km (cort'ispondenti ad una pl'e88ione di circa lo mb) e, 1\('1 ('ontempo, di i"ffin a \'e i nwtodj idonei a tl l,le ra ppre8entazione. potrà selllbrm"(~ alquanto pessimist,ico parlare di "tent,ativi di r a.p. jln'8enll l,zione~, m a , in realtà, la nul, ssa di (bti (vento, tempemtum, llmidità relativa, pressione) (lisponibili nella stra tosfem oltre i 1:.l km è molto più scarsa che nella lropusfem senza con t,are che, al di sopra di l(j·:w klll, alla vlt.stità delle hc\llle _lmtumli li nel cll,mpo delle 08sf'l'va zioni del];. libem at,mosfera (colli(' quelle ocea lliclll' p, purt,l"oppo, (iuplle (1!:'lk regioni sedi delle sorgenti di energia,) si agj.dunke l'akkrlwh.!'hi (\eg-li errori d'oss('l"\'h,zione, i quali assllntolio, (·,01 (:l'e~e(!i'1' d(~lia qllota, g'l~i,il{lezz(' s!:'mpre meno h~.selm.bili; per (:ui l'analihi d!:'i tlat,i stehhi ilr(',sel!ta in g'(,!wl"ille gravi diffi(\olt.à. lu ~tudio (',onente, a scab e frequellza sillut.t,i('a (bigiornali(·r,. nel nostro ea8o) dei dati d'o&':lervazione può esser (,ondot,t,o o ll!{'diantl' l'ana· lisi (h'lla (listribnziune quasi orizzolltal(~ (h'l n'ttme v(',litu sulle slwcessiv(> suiwl'jiei isobal'i('.he che interessano, o mediant.e l'li,lmlisi dei campi d!:'lla t(~mpel'atlll'a e del geopotenzialf' delle stesse sllpmtiei. il primo (\i questi due i!hlirizzi di lavoro è ormai tradizionale (\ella >wuola italiana l' fà. eapo a ff'~. e ll,(l almmi suoi ('ollaboratol'ij il 8p(:on(lo 1 2 m . m o n t a l t o costituisce il metodo di normale analisi indiretta dei campi di m o t o attraverso lo studio dell'ascendente del geopotenziale delle varie superficie isobariche. e n t r a m b i gli indirizzi richiedono, per l'analisi dei dati da rappresentare, un gran numero di processi di interpolazione ed estrapolazione idonei a completare la rete d'analisi: perché questi processi abbiano un senso fisico è necessario che essi vengano desunti da « a n d a m e n t i » conosciuti della funzione da estrapolare. e questo, purtroppo, non mi sembra che sia ancora il caso per i livelli della media ed a l t a stratosfera. i n alcuni dei più grandi servizi meteorologici del mondo, d u r a n t e il periodo dell'anno geofisico internazionale, è s t a t a iniziata l'analisi retrospettiva regolare della circolazione stratosferica con metodi che si distinguono, appunto, dai processi di estrapolazione che consentono di colmare le lacune naturali e quelle occasionali della rete di osservazione. essi in genere si basano sull'esame dei gradienti verticali medi mensili della t e m p e r a t u r a sulle singole stazioni di osservazione e sulla estrapolazione dei dati correnti in base a quegli a n d a m e n t i , oppure sulla applicazione di metodi di regressione agli a n d a m e n t i correnti. qualunque sia il metodo di estrapolazione prescelto, mi sembra chiaro che u n a analisi ragionevole dei dati di osservazione per a f f r o n t a r e lo studio della circolazione in quegli strati dell'atmosfera, richieda già u n a conoscenza preventiva della circolazione stessa. ora dobbiamo riconoscere che, fino ad oggi, la circolazione di quegli strati è ancora per molti aspetti sconosciuta quando, a d d i r i t t u r a , non si posseggano conoscenze che le prove saltuarie finora disponibili hanno rivelato erronee (e fra queste mi sembra di rilevante i m p o r t a n z a la grande variabilità della t e m p e r a t u r a a livelli t r a 50 e 20 mb — e forse oltre — che talvolta a m m o n t a , alle latitudini elevate e fors'anche a quelle intermedie, a oltre 40°c in 36-48 ore). l'unica fonte che allo stato a t t u a l e delle cose ci consente di acquisire conoscenza dei caratteri fondamentali della circolazione è, certamente, l'analisi del suo stato medio mensile e stagionale, quale risulta da un periodo sufficientemente lungo di osservazioni, per quanto possibile, corrette. i n attesa che tale studio possa essere perfezionato e completato con il quinquennio delle osservazioni dell'anno geofisico internazionale — che scade col giugno dell'anno prossimo — ed allo scopo di decidere sulla tecnica di estrapolazione più idonea da utilizzare per effettuare analisi correnti fisicamente accettabili e per q u a n t o possibile corrisponc i r c o ! . a z i o n e t r o p o s f e r i c a f, s t r a t o s f e r i c a m e d i a m e n s i l e , e c c . 1 3 denti al vero, è stato intrapreso uno studio accurato delle circolazioni mensili medie esistenti a tuttoggi. il risultato di questa ricerca preliminare è che sembra estremamente evidente che la circolazione stratosferica risenta in modo sensibile, nel nostro emisfero, della circolazione troposferica. ciò è posto in rilievo sia dal persistere nei grandi vortici polari, ciclonico o anticiclonico, a seconda della stagione, di un sistema di promontori e saccature che corrisponde al sistema delle grandi onde della circolazione troposferica e ai grandi sistemi geografici della superficie della terra, sia dall'anticipo di circa un mese e mezzo dell'estate stratosferica boreale rispetto a quella australe. i n base a tali constatazioni, e considerato che i grandi sistemi delle correnti a getto dell'alta troposfera rappresentano u n a sintesi delle, perturbazioni correnti ed un « carattere » della circolazione troposferica, è stato dato inizio a un programma di lavoro preparatorio per l'analisi corrente delle circolazioni della media e a l t a stratosfera (fino a 10 mb), basato sui seguenti punti: 1 suddivisione delle regioni di analisi col seguente criterio: n) regioni del versante caldo del getto ad almeno 500 km di distanza da esso; b) regioni del versante freddo del getto ad almeno 300 km da esso; e) regioni intorno al getto entro i limiti delle due precedenti. 2 calcolo, per un numero discreto di stazioni rappresentative, degli a n d a m e n t i mensili medi del gradiente termico verticale in corrispondenza dellaloro appartenenza alle regioni a), b), e e). 3 estensione al periodo più lungo possibile del calcolo dei seguenti elementi: a) vento medio sulle superficie isobariche di 200 100 50 30 20 10 m b ; b) geopotenziali medi delle superficie, anzidette; c) isopachie medie degli strati 200/100, 100/50, 50/30, 30/20, 20/10 m b ; fi) temperature medie delle superficie isobariche anzidette; e) scarti dalla media degli elementi b), c), d), e). con la scorta di tali analisi le estrapolazioni per l'analisi corrente verranno f a t t e in base alla circolazione troposferica quale è rappresentato dalle carte del getto e dalla topografia della tropopausa, tenendo debito 1 4 m . m o n t a l t o conto dei valori medi e dei loro scarti, ed estrapolando fino ai livelli desiderati con le leggi suggerite dagli a n d a m e n t i di cui al precedente par. 2. riassunto la scarsità di dati a livelli superiori a 100 mb rende finora praticamente impossibile l'analisi sinottica della circolazione nell'alta stratosfera, a meno di sostituire i dati mancanti con altri estrapolati. lo studio preliminare delle circolazioni mensili e stagionali nella troposfera, nella bassa e media stratosfera e nell'alta stratosfera ha rivelato che esse sono in certo modo interdipendenti. ciò è posto qualitativamente in evidenza, ad esempio, dalla corrispondenza dei cunei e delle saceature delle configurazioni bariclie dell'alta stratosfera con le aree oceaniche e continentali dell'emisfero nord. considerandole correnti a getto come sintesi delle grandi perturbazioni dell'alta troposfera atte ad influire sulla struttura e circolazione dell'alta stratosfera, le conclusioni precedenti mi hanno indotto a programmare una classificazione della struttura e della circolazione osservate, su ciascuna delle stazioni aerologiche mediterranee, in relazione alla loro posizione rispetto alle correnti a getto, onde reperire sperimentalmente le formule che consentano una logica estrapolazione delle osservazioni correnti ai livelli desiderati. su m mary the scarcity of radiosonde observations above 100 mb makes up to noie current synoptie analysis of upper stratosphere circulation practically impossible, unless me try to substitute missed data with extrapolatcd ones. a preliminari/ studi/ on monthly and seasonal circulation for tropospheric, stratospheric and upper stratospheric levels shows that they are very probabili linked to each other: this faci is qualitatively evidenced, for instance, by clear correspondence between the large continental and oceanie areas in northern hemisphere and ridges or throughs in upper stratospheric meau pressure -patterns. considerilig jet streams as syntheses of large tropospheric and stratospheric disturbances able to affect upper stratospheric structure and circulation, the above conclusions led us to project the analysis and classification of the atmospheric structure and circulation as indicated by individuai aerological data for the mediterranean basili, according to their location relative to the jet streams. the results unii be used to find ex-perimentaliy extrapolation formulae to be applied in different atmospheric air inasses in order to logicali// complete upper stratospheric analysis network. layout 6 annals of geophysics, 57, 1, 2014, s0192; doi:10.4401/ag-6369 s0192 integration of historical, archaeoseismic and paleoseismological data for the reconstruction of the early seismic history in messina strait (south italy): the 1st and 4th centuries ad earthquakes maria serafina barbano1,*, viviana castelli2, daniela pantosti3, claudia pirrotta1 1 università di catania, dipartimento di scienze biologiche, geologiche e ambientali, catania, italy 2 istituto nazionale di geofisica e vulcanologia, torrette di ancona, italy 3 istituto nazionale di geofisica e vulcanologia, rome, italy abstract historical accounts, archaeoseismic and paleoseismological evidence allowed us to reappraise two earthquakes affecting northeastern sicily and southern calabria in the 1st (probably between 14 and 37) and 4th (likely between 361 and 363) centuries ad, to obtain a better reconstruction of their effects and to reconsider their sources.the 1st century event damaged the area from oppido (calabria) to tindari (sicily), roughly that of the february 6, 1783 calabria earthquake. the similitude of these earthquakes is further stressed by the fact that they generated tsunamis, as recorded by historical data and by the tsunami deposits found at capo peloro, the oldest dated 0-125 ad, the youngest linked to the 1783 event. these earthquakes could be related to the same calabria seismic source: the scilla fault. northeastern sicily and southern calabria were also damaged by one or more earthquakes in the 4th century ad and several towns were rebuilt/restored at that time. the hit area roughly coincides with that of the messina 1908 earthquake suggesting similar seismic sources for the events. however, because close in time, historical descriptions of the 4th century sicilian earthquake were mixed with those of the 365 crete earthquake that generated a basin-wide tsunami most likely reaching also the sicilian coasts. reevaluating location, size, damage area and tsunamigenic potential of these two earthquakes of the 1st and 4th centuries ad is relevant for reassessing the seismogenic and tsunamigenic potential of the faults around the messina strait and the seismic hazard of the affected areas. 1. introduction historical earthquake records can provide a significant contribute to the characterization of the seismic sources and to the understanding of the seismic hazard of an area. however, historical records need a thoughtful and careful analysis to avoid biases and over-interpretations. these problems are compounded by the limitations of the historical data, that are incomplete, often obscure, and easily susceptible of misinterpretation. even though with these uncertainties, historical analysis represents a unique opportunity to add new tiles to that complex mosaic that is seismicity, in regions like calabria and sicily, in southern italy (figure 1a), where despite the concurrence of a long history and the presence of active faults, the main earthquakes often do not have any known “ancestor” nor a defined seismogenic source. active tectonics in southern calabria and northeastern sicily is attested by the long-term uplift rate of the coast, by landscape imprint [guarnieri and pirrotta 2008], coastal morphology, structural evidence [e.g. valensise and pantosti 1992, monaco and tortorici 2000, ferranti et al. 2008], and by the frequent occurrence of strong and moderate earthquakes in recent and historical times. these earthquakes, such as the 1783 sequence and the 1908 event, had a dramatic impact on the region, however, their association with the causative faults is still debated and they are often linked with different faults/seismogenic sources (figure 1b-e.g. valensise and pantosti [1992], jacques et al. [2001], galli and bosi [2002], basili et al. [2008], diss working group [2010] and references therein, aloisi et al. [2013]). although the italian earthquake catalogues cover a long time span (461 bc-2002 ad) [boschi et al. 2000, cpti working group 2004], descriptive data on earthquake effects are generally scarce until the 13th-14th centuries. therefore the epicenters and magnitudes of the article history received june 24, 2013; accepted february 25, 2014. subject classification: earthquakes, historical seismology, archaeoseismology, tsunami deposits, sicily, calabria. earlier events are roughly estimated. since 91 bc, about 20 m > 6 earthquakes (figure 1b) occurred along the 180-km-long calabrian arc [cpti working group 2004]. nevertheless the temporal distribution of seismicity and energy release (figure 2) show that the earthquake catalogue is incomplete and that almost all the recorded events are clustered in less than four centuries between 1600 and 2000 ad. due to the catalogue’s incompleteness it is possible that some events have been lost or misestimated, as shown by galli and scionti [2006] for northern calabria, where they found archaeological and historical evidence of two unknown earthquakes occurred in the early 7th century ad and in 1744. seismological studies around the messina strait mention some evidence of earthquakes in 91 bc, 17 ad, around mid-4th century ad [guidoboni et al. 1994 2000], and in 853 ad [guidoboni et al. 1994, boschi et al. 2000] but no reliable information of damage is available and the epicentral location of these earthquakes is based on a few macroseismic data points only. evidence coming from an excavation performed at the torre degli inglesi site, near capo peloro (figure 1), from the necropolis of abakainon, from archaeological studies of the greco-roman town of tyndaris (today tindari) and other sites in northeastern sicily and southern calabria, together with historical data, suggest that two barbano et al. 2 figure 1. a) geological sketch map of southern calabria and north-eastern sicily apennine-maghrebian orogenic system; and (b) main historical earthquakes: epicentres (filled circles) from the cpti4 catalogue [cpti working group 2004]. the faults associated with the most disastrous earthquakes are also shown: cvf= crati valley fault; mf=mesima fault; cf=cittanova fault [jacques et al. 2001, galli and bosi 2002], gf=gioia fault [diss working group 2010], sf=scilla fault [jacques et al. 2001, ferranti et al. 2008], msf1=west dipping messina strait fault [monaco and tortorici 2000], msf2=east dipping messina strait fault [pino et al. 2000, basili et al. 2008, diss working group 2010]. figure 2. temporal distribution of earthquakes in southern calabria and north-eastern sicily. (a) magnitude vs. years. squares show the 17 ad earthquake revaluated in this work and the unknown 7th century ad earthquake found by galli and scionti [2006] in northern calabria; (b) energy release, evaluated using the magnitude-energy relationship log e-11.3+1.8 m [gutenberg and richter 1956]. note that most of the events recorded in the catalogue occurred since 17th century. 3 possible earthquakes in the 1st and 4th centuries occurred. in this work we reappraise all the information available on these earthquakes, with the aim of better setting their dates of occurrence, defining their effects on the human and natural environment, and improving their characterization by defining their seismogenic sources. 2. the 1st and 4th century earthquakes in previous studies the most recent earthquake catalogues to cover the period under study (first half of the first millennium ad) for calabria and sicily are cpti04 [cpti working group 2004] and cfti4med [guidoboni et al. 20072013]. the cpti working group [2004] lists three earthquakes occurred in the studied area and period (table 1): ad 17, ad 361, and ad 374. the earthquake parameters included in this catalogue (table 1) derive from the italian catalogue of strong earthquakes [cfti, boschi et al. 2000], that on their turn derived them from guidoboni ed. [1989]. a comparison between these two works shows that there is a difference: boschi et al. [2000] assign the exact yearly the messina strait early seismic history catalogue n year epicentral area ref np imx io lat lon maw me cpti04 9 17 reggio c.-sicily cfti 2 8-9 8-9 37.800 15.200 5.14 -cfti4med 17 reggio c.-sicily ---2 8-9 8-9 37.8 15.2 --5.8 cpti04 15 361 sicily cfti 1 10 10 37.500 14.000 6.60 -cfti4med 361 messina strait --2 10 10 38.15 15.6 --6.4 cpti04 16 374 reggio calabria cfti 1 9-10 9-10 38.100 15.650 6.30 -table 1. a comparison between 1st-4th century ad earthquakes in the studied area, as depicted by cpti working group [2004] and cfti4med [guidoboni et al. 2007-2013]. n=reference number; year=origin time; epicentral area: maximum effect area; ref.: source catalogue; np: number of macroseismic observations; imx: maximum observed intensity; io: estimate epicentral intensity; lat and lon: geographical epicentre coordinates; maw and me: equivalent magnitude. figure 3. recent studies and catalogues that deal with earthquakes in the period considered in this paper. dates to the three earthquakes whereas guidoboni ed. [1989] only proposes approximate time ranges for them (figure 3). on the other hand, the cfti4med catalogue [guidoboni et al. 2007-2013], which is based (for italian earthquakes) on boschi et al. [2000] “with subsequent additions and improvements”, claims the occurrence of only two earthquakes in the same period. the 17 ad event maintains the same parameters, whereas the 361 ad earthquake shows very different location: the middle of sicily for boschi et al. [2000], the messina strait for guidoboni et al. [2007-2013] (table 1 and figure 3). the picture of calabro-sicilian seismicity in the 1st4th centuries ad proposed by cfti4med [guidoboni et al. 2007-2013] is more restrictive than the one outlined by boschi et al. [2000] and by guidoboni ed. [1989]. the latter mentions at least two local earthquakes in the 4th century, plus the effects on sicily of the tsunami generated by the crete earthquake of july 21, 365 (figure 3). cfti4med is a “work in progress” open to later additions and improvements and subject to successive revisions (almost verbatim quote from the cfti4med website, as accessed on february 19, 2013). as the references quoted by cfti4med include the same original historical testimonies quoted by the previous studies, this changed view of the calabro-sicilian seismicity in the 1st-4th centuries ad should be presumably based on new interpretations of available historical data rather than on newly-acquired historical data. 3. historical, archaeoseismic and paleoseismological evidence: a reappraisal in this chapter we document new findings on the 1st and 4th centuries earthquakes obtained by a reanalysis of barbano et al. 4 figure 4. flow chart of studies and sources revaluated in this study for the 1st century earthquake. note that the guidoboni ed. [1989] catalogue was based upon bonito [1691] and a chronicle [phlegon of tralles 2nd century], whereas mongitore [1743] used sicilian local histories based on pliny the elder [1st century], as well. 5 available historical sources integrated with information gathered from paleoseismic and archeoseismic investigations in order to highlight how this can contribute to a better comprehension of the studied earthquakes. the first step of our study was the critical analysis of the knowledge collected in the seismological literature retrieving contemporary identifiable sources. to do so we analyzed both the historical sources taken into account by the recent studies that served as a reference for the parametric earthquake catalogues, and those quoted by earlier seismological compilations of which italy has a centuries-old tradition. given the diversity of interpretations proposed by recent earthquake catalogues, we performed an examination of the historical data set available for each of the studied earthquakes, in order to clarify once and for all which are their respective merits, demerits and absolute degree of reliability. the italian tradition of descriptive earthquake studies dates back as far as the mid-15th century. its output did mostly take the form of earthquake lists of varying size and geographical scope, from global ones such as bonito [1691] to regional and local ones, such as fiore [1691] and mongitore [1743] for calabria and sicily, respectively. the last and best product of this tradition is certainly the great collection i terremoti d’italia [baratta 1901] that remains, to this day, the main data base for the parametric catalogues produced in italy from the second half of the 20th century onwards. the results of an extensive perusal of recent studies and older repertories allow, after having unraveled a tangle of multiple dates and spurious connections between earthquakes occurred in calabria and sicily, in asia minor and/or mentioned in the gospels, to identify two small collections of “original” historical data dealing with seismic events located in sicily and calabria in the required time-windows. the adjective “original” is employed here with the meaning of “nearest in time” to the described events, which is not necessarily the same as “contemporary”. 3.1. reappraisal of an earthquake occurred in the first half of 1st century ad 3.1.1. original information figure 4 shows a flow chart of the sources quoted by seismological tradition and recent studies in connection with earthquakes occurred in calabria and sicily in the 1st century ad, the explicit and conjectured relationships existing between them and a summary of the dates and places mentioned by each of them. the “original” sources are two, one that does explicitly mention the earthquake and another that does possibly mention an earthquake-related phenomenon (table 2). the former is phlegon of tralles [2nd century], a greek historian who lived at least a century after the events he recounts. the latter is pliny the elder [1st century], the renowned scholar and officer who was killed by the vesuvius eruption of ad 79. phlegon briefly mentions an earthquake in sicily and the reggio calabria area after having described, more at length, a strong and destructive earthquake in asia minor (now anatolia, turkey). the asian earthquake is well attested by several contemporary sources the messina strait early seismic history author eq date eq description phlegon of tralles [2nd century] 17 ad (asian eq) 14-37 ad (tiberius’ reign) «apollonius grammaticus reports that at the time of emperor tiberius an earthquake almost destroyed several renowned asian cities. tiberius restored them at his own expenses […] not a few towns of sicily and the area surrounding reggio did suffer damage from the earthquake too.» pliny the elder [1st century] before 79 ad (pliny’s death date) « the sea has carried away pyrrha and antissa near the mæotian marshes (sea of azov), also elice and bura in the gulf of corinth, traces of which places are visible in the ocean. from the island cea it has seized on 30,000 paces, which were suddenly torn off, with many persons on them. in sicily also [the sea carried away] the half of the city of tyndaris, and all the part of italy which is wanting; in like manner it carried off eleusina in bœotia.» table 2. original information on an earthquake occurred in the first half of 1st century ad. and its date is certain: it took place in ad 17. the exact date of the calabro-sicilian earthquake is unknown; it could have occurred either in the same year of the anatolian earthquake [as maintained by boschi et al. 2000 and followers] or roughly about that time [as claimed by guidoboni ed., 1989], or even, which is perhaps the safest assumption, simply during the period of tiberius’ imperial rule (ad 14-37). pliny the elder mentions the partial destruction of the town of tyndaris by the sea in book 2 of his natural history. this book dedicates 15 chapters (from 81st to 96th) to earthquakes and related topics (signs of approaching earthquakes, protection against future earthquakes, circumstances attending earthquakes etc). several of these chapters describe the interactions between sea and land which earthquakes can cause (tsunami waves, formation of islands etc). the tyndaris episode is mentioned in chapter 94, dedicated to the «cities which have been absorbed by the sea» (table 3). it is uncertain whether the tyndaris phenomenon mentioned by pliny could have been related or not to the calabro-sicilian earthquake mentioned by phlegon. pliny does not say when the tyndaris episode occurred. obviously, it must have been before ad 79 (when pliny died) but how long before remains an open question. at the end of 1st century bc (more precisely in 22-21 bc), emperor augustus gave tyndaris the status of roman colony (colonia augusta tyndaritanorum). this was a high distinction in the roman empire, indicating that tyndaris was then an important and prosperous town. the author of the “tyndaris” entry of the enciclopedia italiana [g.v.g. 1961] suggests that the partial destruction of tyndaris could have taken place sometime after, at the beginning of 1st century ad, because the geography of strabo (a greek scholar who died around 24 ad) calls tyndaris a polisma, which is a greek word that can be translated either as «a small town» or as «a town that used to be large but now is small» [mogens, 1995]. if tsunami traces datable to the 1st century ad were to be found near the tyndaris site and/or in other sites in northeastern sicily, this would certainly help settling the question. another point which is worthwhile to concerns the actual nature of the phenomenon described by barbano et al. 6 94: urbes haustae mari 94: cities which have been absorbed by the sea pyrram et antissam circa maeotim pontus abstulit, helicen et buram sinus corinthius, quarum in alto vestigia apparent. ex insula cea amplius triginta milia passuum abrupta subito cum plurimis mortalium rapuit et in sicilia dimidiam tyndarida urbem ac quicquid ab italia deest, similiter in boeotia eleusina. the sea has carried away pyrrha and antissa near the mæotian marshes (sea of azov), also elice and bura in the gulf of corinth, traces of which places are visible in the ocean. from the island cea it has seized on 30,000 paces, which were suddenly torn off, with many persons on them. in sicily also [the sea carried away] the half of the city of tyndaris, and all the part of italy which is wanting; in like manner it carried off eleusina in bœotia 95: de spiraculis 95: of vents in the earth. motus enim terrae sileantur et quicquid est, ubi saltem busta urbium exstant, simul ut terrae miracula potius dicamus quam scelera naturae. et, hercules, non caelestia enarratu difficiliora fuerint. but let us say no more of earthquakes and of whatever may be regarded as the sepulchers of cities; let us rather speak of the wonders of the earth than of the crimes of nature. table 3. original text and english translation of pliny’s mention of tyndaris [bostock and riley 1855]. figure 5. the 17 ad earthquake according to boschi et al. [2000]: reggio and the epicenter shown by the asterisk south of taormina; the 1st century (14-37 ad ?) earthquake/s according to this work: reggio and the localities shown by unfilled squares. at the torre degli inglesi site a sandy layer interpreted as tsunami deposit dated ad 0-125 [pantosti et al. 2008] was found (see discussion). 7 pliny. the latin word for earthquake (terrae motus, literally “a motion of the earth”) could also indicate a landslide. tyndaris is built on a hill at 268 m a.s.l., so its destruction could be credited to a landslide, rather than to an earthquake-generated sea wave. however, this does not seem likely, as the specific context in which pliny relates the episode is one of earthquake-generated marine phenomena. moreover, recent research has shown that ancient tyndaris included an harbour quarter placed at the foot of the hill, along a bay on the tyrrhenian coast [bottari et al. 2012]. it seems quite possible that tsunami affected the ancient tyndaris harbour. it is important to note that in later centuries the above information was subjected to much distortion on the part of derivative sources (figure 4), which mixed up pliny’s and phlegon’s original data concerning tyndaris and sicily with data provided by them and other authors on the earthquake of ad 17 in asia minor (mentioned by pliny elsewhere in his work) and on another asian earthquake of ad 32-33. this latter earthquake was once believed mistakenly, as pointed out by ambraseys [2005] to coincide with the earth-shaking felt in jerusalem during the crucifixion of jesus according to the 27th chapter of the gospel of st. matthew [williams et al. 2012]. this belief led several sicilian authors of the 17th-18th centuries to blame the partial destruction of tyndaris on the earth-shaking mentioned by st. matthew. according to one of these authors, the earthquake described in the gospel was responsible not only for ruining tyndaris, but also for “cracking apart the mount of saturn (mons saturnius)” [gaetano 1617] or “hollowing out a cavity at the foot of the mount of neptune” [gaetano 1708]. despite the multiplicity of names and circumstances he mentions, gaetano makes it clear that the mountain in question is one and the same, located three miles inland from the coast between messina and taormina (figure 5). this allows to identify it with present-day mt. scuderi (a 1.253 m-high peak in the peloritani chains). gaetano was an ecclesiastic, with access to archive records and other historical sources that are not available anymore, so there is, in principle, at least a very slender chance that his tale does preserve, buried under heavy layers of fictionalization, a glimmer of some otherwise completely forgotten episode that the messina strait early seismic history century bc ad site 5 4 3 2 1 1 2 3 4 5 6 mella (oppido) * * cinquefrondi * metauros (gioia tauro) * hipponion (vibo valenzia) r r melicucco h rhegion (reggio di calabria) * + *r bianco h casignana * * bova marina r r * scolacium (squillace) r * lipari * tyndaris + * patti * terme vigliatore **r necropolis of abakainon ++ torre degli inglesi site + + mt. scuderi + table 4. history of archaeological sites in southern calabria and northeastern sicily. asterisks indicate the destruction phase; h indicates the presence of hoards; r are restoration/reconstruction phases;+indicates information gathered in this work. **reconstruction phase according to tigano et al. [2008]; ++evidence of destruction from bottari et al. [2013]. note the concentration of destruction/restoration and hoards in the latter half of the 4th century ad modified from galli and bosi [2002]. could have been related to the sicilian earthquake of the 1st century ad. the cpti04 catalogue [cpti working group 2004], based on the cfti [boschi et al. 2000] whose original source is phlegon of tralles’ account (table 2), locates the 17 ad event on the northern side of mt. etna (figure 5). 3.1.2. other evidence recently bottari et al. [2013] showed that stratigraphic evidence and ruin features observed in the necropolis of abaikonon (figure 5) are consistent with a seismic event and constrained the collapsing time of the fallen columns through optically stimulated luminescence (osl). dating is coherent with a single event occurred in the 1st century ad. in southern calabria and northeastern sicily sparse archaeoseismic indications concerning possible earthquakes in the 4th and the 1st centuries bc and in the 1st and 4th centuries ad (table 4) have been evidenced. according to galli and bosi [2002], archaeoseismic analysis of the roman farmstead built over the hellenistic site of mella near the old oppido village (figure 5), shows that it was abruptly destroyed in the 1st century ad, and its collapsed walls lie directly on the pavement of the road. this is considered as evidence for an earthquake related collapse. archaeologists also document a major reconstruction in the 1st century ad of the roman villa of terme vigliatore likely damaged by a earthquake [tigano et al. 2008]. furthermore, from the analysis of the archaeological excavation at the torre degli inglesi site, a 16th century watchtower built along the coast of torre faro (figure 6) on a pre-existing roman tower, pantosti et al. [2008] found evidence for two sandy layers that, on the basis of their sedimentological and paleontological characteristics, were interpreted as tsunami deposits. stratigraphic and archaeological analyses and radiocarbon dating constrained the oldest tsunami sandy layer to an age range of 0-125 ad, that is compatible with an event in the 1st century ad. after this event, or soon after, the watchtower was probably modified [crupi 2011] because the roman steps and the water cisterns, were covered by debris containing organic material and pottery dated 1st-3rd century (figure 7). the youngest tsunami deposit is stratigraphically located between the 16th century watchtower basement and the 19th century pavement and enclosure wall, and was thus associated with the february 6, 1783 tsunami [pantosti et al. 2008]. barbano et al. 8 figure 7. ne wall of the excavation at torre degli inglesi (see figure 6 for location). the material infilling the cisterns dated with both archaeological and radiometric analysis shows age between 1st-3th century. dashed red line and arrows highlight a fracture related to the 1908 earthquake, since it cuts constructions and deposits of all periods. figure 6. a) google earth image of capo peloro peninsula with the position of torre degli inglesi (arrow) just north of torre faro village that was affected by the february 6, 1783 and 1908 tsunamis. b) torre degli inglesi photo from ne; c) view of the archaeological excavation on the nw side of torre degli inglesi. the main building phases (i to iv) are evidenced and it is possible to observe the relative chronology [modified from pantosti et al. 2008]. 9 the messina strait early seismic history figure 8. flow chart of studies and sources revaluated in this study for the 4th century earthquakes. author eq date eq description libanius [4th century] shortly before or after june 26, 363 (date of julian’s death) «earth truly has been fully sensible of her loss [the death of emperor julian], and has honoured the hero by an appropriate shearing off of her tresses, shaking off, as a horse does his rider, so many and such great cities. in palestine several; of the libyans all and every one. prostrate lie the largest towns of sicily, prostrate all of greece save one; the fair nicaea lies in ruins […] these are the honours paid to him by earth, or if you choose, by neptune himself. » inscription [374] before ad 374 «[emperor valentinian with valens and gratian] have re-stored the baths, which had collapsed through old age and an earthquake, to the people of reggio […]» st jerome [4th century] ad 366 «an earthquake having occurred throughout the wholeworld, the shore is invaded by the sea, and falling debris in innumerable cities of sicily and of many islands, crushed the people.» table 5. original information on earthquakes occurred in the third quarter of the 4th century ad. 3.2. reappraisal of earthquake(s) occurred in the third quarter of the 4th century ad 3.2.1. original information figure 8 shows a flow chart of the sources quoted by seismological tradition and recent studies in connection with earthquakes occurred in calabria and sicily roughly in the third quarter of 4th century ad, integrated with the explicit and conjectured relationships existing between them and a summary of the dates and places mentioned by each of them. there are three “original” sources, two of which contemporary with the described events and one slightly later: the speeches by libanius [4th century], an inscription of ad 374 from reggio calabria [putortì 1913] and a passage in st. jerome’s chronicle [st. jerome 4th century], composed around ad 380 (table 5). libanius was a rhetorician (professional public speaker) and a personal friend and teacher of emperor julian the apostate. julian had been brought up as a christian but later on he reverted to paganism. during his short reign (ad 361 to 363) he attempted to outlaw christianity which his predecessor had made legal and to restore the ancient roman religion. libanius, an enthusiastic supporter of this program, was deeply shocked by julian’s death and by the consequent collapse of his dream. these facts are important to evaluate the reliability of libanius references to earthquakes, all of which were provided in funeral speeches he wrote for julian’s death. libanius was not attempting to give precise information on these earthquakes for simple chronicling purposes; what he really wanted to do was to stress the magnitude of the loss caused by the death of his emperor, of which the earth itself and the pagan god neptune (aptly nicknamed the earth-shaker) either foretold or mourned by causing the ground to shake and towns all over the empire to fall down. attempts to date the earthquakes mentioned by libanius have been made: guidoboni ed. [1989] proposes an earthquake in jerusalem in ad 361 and another in nicaea (now iznik, turkey) in ad 362; van nuffelen [2006] proposes an earthquake in palestine in ad 363 and another in lybia (africa) between 364 and 378. the exact date of the sicilian earthquake remains uncertain: a possible range of occurrence could be the 361-363 time-window. the reggio calabria inscription celebrates the inauguration, in ad 374, of the town’s public baths, whose renovation had become necessary after the building had barbano et al. 10 figure 9. a) google earth image of the tindari gulf; b) the old greco-roman site of tindari c) the central decumanus; d) strong undulating deformations in the road-bed of the central decumanus [spigo 2005] associated to the 4th century earthquake. 11 «collapsed because of the aging and an earthquake». this is undoubtedly a very reliable source but it does not allow to pinpoint the exact date of the earthquake it mentions. it is unlikely that this earthquake occurred in 374 [as maintained by guidoboni ed. 1989; boschi et al. 2000 and followers]; but a date some years earlier seems more convincing, cfti4med [guidoboni et al. 2007-2013] proposes ad 361. st. jerome’s chronicle (a 4th century latin translation, which update a more ancient chronicle written in greek by eusebius of caesarea) is the first historical text ever to make an attempt to accurately date all the events occurred before. as such, it had an enormous influence on later history writers that looked to it as their primary source of information on dates and events at least until the end of the middle ages. the chronicle mentions a global earthquake accompanied by a tidal wave, dated in 366, this does obviously bring to mind the catastrophic central mediterranean earthquake of july 21, 365, which generated a tsunami which can have reached also the sicilian coast. however, the description mentions human casualties caused by “falling debris in innumerable cities of sicily”, which seems hardly likely to have been caused by an earthquake located near crete. in this case too, as in the previous case of the 1st century event, the “original” information was subjected with time to relevant distortion, resulting in an arbitrary multiplication of earthquake dates on the part of derivative sources (figure 8). 3.2.2. other evidence archaeoseismic researches performed throughout southern calabria [galli and bosi 2002] (table 4) and in the cities of messina and reggio calabria [guidoboni et al. 2000] have shown the occurrence of destruction/reconstruction/abandonment phases clustered in the 4th century ad. since the distribution of these archaeoseismic indications fit with the february 5, 1783 earthquake damage area, galli and bosi [2002] hypothesized the occurrence in the 4th century ad of a strong earthquake that could be the predecessor of this shock of the 1783 earthquake sequence and the ‘‘penultimate’’ event of the cittanova fault (figure 1b). on the contrary, guidoboni et al. [2000] think that the 4th century ad event could be a potential predecessor of the well-known 1908 earthquake located in the messina strait. the 4th century event could be associated with a first modification of the roman tower at the torre degli inglesi site, inferred both from the radiocarbon dating and archaeological evaluation of the organic material and pottery contained in the debris burying the roman steps and infilling the water cisterns. this debris did accumulate before the 3rd century [pantosti et al. 2008], and therefore probably earlier than a destructive event, after that the cisterns were probably restored [crupi 2011]. the modern tindari village is located near the site of the ancient greek colony of tyndaris (figure 9) that dates back to the 5th-4th centuries bc. during the greek period, in 3rd century bc, different structures were built such as a theatre, fortification walls, and towers. in the late roman period (4th century ad) a basilica and some houses were built [wilson 1990]. recent archaeological excavations in the old greco-roman site of tindari [spigo 2005] have revealed earthquake-related damage attributed to the 4th century ad event [e.g. bernabò brea 1996], consisting of preferred orientation of the collapsed structures and strong undulating deformations in the road-bed (figure 9d) in the central decumanus [spigo 2005, de domenico et al. 2006, bottari et al. 2008]. as shown in table 4 and figure 10 there are also other archaeseismic data showing that sicily and southern calabria were hit by one or more earthquakes in the 4th century, but only for reggio calabria there is a direct evidence of this in the commemorative inscription. the messina strait early seismic history figure 10. index map of archaeoseismic evidence for the 4th century ad event in southern calabria and northeastern sicily. in brackets the current place names modified from galli and bosi [2002]. the triangle shows torre degli inglesi. figure 11. a) maximum intensity data point distribution (i=7-9 mcs) for the 6 february, 1783 earthquake [data from boschi et al. 2000]. locations affected by tsunami are also shown [data from gerardi et al. 2008]. 4. discussion 4.1. was the 1st century event a twin of the 6 february 1783 earthquake? five earthquakes, followed by tsunamis [graziani et al. 2006], occurred between the february 5, 1783 and march 28, 1783, causing devastation and a lot of victims in a vast region embracing the whole southern calabria and the messina area (figure 1b) [guidoboni et al. 20072013 and references therein]. morphological and structural evidence, combined with a re-examination of observations made at the time of the earthquakes [jacques et al. 2001 and references therein] suggest that the main shock (february 5), the first aftershock (february 6) and the february 7, third large shock were related to slip on the west-dipping, ne-striking cittanova, scilla and mesima normal faults, respectively (figure 1b) [e.g. galli and bosi 2002]. the archaeological stratigraphy exposed at the torre degli inglesi site allowed to document a tsunami sandy layer, dated 0-125 ad, linked to a local earthquake occurred in the 1st century [pantosti et al. 2008]. the hypothesis of a tsunami associated with the 1st century event is also suggested by pliny the elder’s account “the sea swallowed up half the town of tyndaris in sicily (maybe the ancient tyndaris harbour) and the coastal area of northeastern sicily (table 2). the pliny’s report was not considered by previous catalogues. another event is testified at the torre degli inglesi site by the uppermost sandy layer, possibly deposited by the february 6, 1783 tsunami (figure 11) before the 19th century enclosure wall was built [pantosti et al. 2008]. historical data report that torre faro (the village near which the torre degli inglesi building is sited) was repeatedly damaged by earthquakes located both in northeastern sicily and in southern calabria [boschi et al. 2000], and inundated by the february 6, 1783 and 1908 tsunamis. the 1783 tsunami at torre faro flooded for about 400 m inland [sarconi 1783], whereas the 1908 tsunami inundation was just 5 m [baratta 1910]. thus, considering the anomalous sandy layer dated 0-125 ad at the torre degli inglesi site, the 1st century tsunami seems to have been capable to flood into the watchtower in the same way as during the 1783 event. this barbano et al. 12 figure 12. a) maximum intensity data point distribution (i=8-11 mcs) for the february 5, 1783 (a) and the december 28, 1908 (b) earthquakes [data from boschi et al. 2000]. scilla fault geometry and kinematics length dip slip vector 30 km 60° 327°/11° holocene parameters event pre-i event i event ii 1783 slip per event (based on footwall throw) 2.0 m 1.5 m characteristic moment magnitude (on slip) 7.0 6.9 5.9* characteristic moment magnitude (on fault length) 6.8 6.1 event age bp 5.1 ka 3.5 ka 1.9 ka 0.23 ka table 6. geometric and kinematic parameters for the scilla fault based on the displacement record of late holocene shorelines and fault length [modified after ferranti et al. 2008]. *macroseismic estimate from cpti working group [2004]. 13 suggests the 1st century tsunami to have an analogous source to the 1783 one (figure 11). moreover, the 1st century earthquake damaged reggio, and probably the old site of oppido (figure 5), which was totally destroyed during the 1783 sequence [vivenzio, 1783], and tindari, terme vigliatore and the necropolis of abakainon which are close to the 1783 damage area, as well. these facts suggest that the earthquake occurred in the 1st century ad had a pattern of damage quite similar to that of the february 6, 1783 (figure 11). such an event is likely to have been located in southern calabria and to have been energetic enough to generate a tsunami (even though we do not have enough information to estimate its magnitude and intensity). because the 1783 tsunami was generated by an earthquake-triggered landslide [graziani et al. 2006, gerardi et al. 2008], this possibility could be considered also for the 1st century event. based on archaeoseismic indications from calabria and ne sicily, galli and bosi [2002] considered comparable the damage pattern of the 4th century event with that of the february 5, 1783 event (figure 12a) rather than to that of the 1908 event (figure 12b). they probably did not take into consideration the possibility of the 1st century event as a potential ancestor of the 1783 earthquake because the earthquake catalogue presents it as a minor event located to the south near the mt. etna (figure 1b and 5-cpti working group [2004]). recently according to geological and geomorphological data, ferranti et al. [2008] have recognized evidence for the occurrence of co-seismic displacements on the scilla fault (figure 1b). the authors dated the most recent event at 0.23 ka bp (likely the 1783) and the previous one at~1.9 ka bp (table 6). based on fault length and slip estimated from footwall uplift and using regression relations for normal faults [wells and coppersmith 1994], they suggest that a m=6.1/6.9 earthquake was produced in the 1st century by the same fault responsible for the february 6, 1783 earthquake. in summary, the available data provide evidence of an earthquake occurred in the 1st century ad (possibly between 14 and 37 ad) as the most likely responsible for the following circumstances: destructions in tyndaris, in the necropolis of abakainon, in other sicilian towns and in the area surrounding reggio calabria; damage in the old oppido site (figure 5); tsunami deposit at the torre degli inglesi site; co-seismic displacements on the scilla fault. the analogy between this set of circumstances and those related to the february 6, 1783 (figure 11) earthquake suggests that in the 1st century one earthquake, or more, damaged the same area affected by the 1783 seismic sequence. 4.2. was the 4th century event a ‘ twin’ of the december 28, 1908 earthquake? on 28 december 1908, a tsunamigenic catastrophic earthquake (figure 12b) struck the region of themessina straits (figure 1b) [guidoboni et al. 2007-2013 and references therein]. in this paragraph, we intend to highlight the similarities between the 1908 and 4th century earthquakes. the main historical source for the 4th century event is libanius that mentions several undifferentiated earthquakes occurring during the reign of emperor julian, among which he cites a destructive event in sicily: «the greatest cities of sicily lie in ruins» [guidoboni et al. 1994]. according to several written sources and to archaeological and geological data the 365 ad earthquake was located in crete [e.g. stiros 2001; 2010] but its tsunami effects are described also in mainland greece, adriatic sea, sicily and north africa [guidoboni et al. 1994]. furthermore, the fact that st. jerome [4th century] reports damage and causalities in sicily, that cannot be explained with an earthquake generated in crete, suggests that in 4th century sicily was affected both by a local earthquake (or more than one) and by a tsunami, likely the 21 july ad 365 tsunami. it must be stressed, however, that tsunami deposits with an age compatible to a 4th century event were detected along the coast of southeastern sicily: inland at augusta [de martini et al. 2010] and morghella [gerardi et al. 2012], and offshore in the augusta bay (southeastern sicily) [smedile et al. 2011], and probably at reggio calabria [guidoboni et al. 2000]. no tsunami evidence have been found on the tyrrhenian northern coast of sicily. the above mentioned authors attributed the tsunami deposits to the far crete source but they could also have been deposited by a tsunami generated by a local event. stratigraphic and archaeological relations suggest that the roman watchtower at the torre degli inglesi site was probably damaged after the 3rd century ad and successively restored between the 4th century and the 16th century [pantosti et al. 2008]. during this period the cpti04 catalogue [cpti working group 2004] lists only three significant earthquakes dated at the 4th century, ad 853 and ad 1509 (figure 1b). as the age of the materials filling the cisterns can be fixed to sometime between the 2nd and 3rd centuries [crupi 2011], the watchtower was probably restored after a damaging event, that likely could have been the 4th century earthquake very close in time with the material found inside the cistern. the torre degli inglesi site was also affected by the 1908 earthquake that caused the north-eastern striking fracture cutting through constructions of all periods (figure 7), as reported by baratta [1910]: «the december 28, 1908 earthquake caused e-w and n-s oriented cracks the messina strait early seismic history in the colossal 3 m thick walls of the torre degli inglesi, in which a large e-woriented fracture did also cause disjunction and lowering in the opened northern wall». hence at the torre degli inglesi site only evidence of damage is recognizable, both for the 4th century event and for the 1908 earthquake (figure 7), furthermore for both these events no tsunami deposits have been found. these analogies, together with the archeoseismic evidence reported for several sites (figure 10), allow us to hypothesize that the event occurred in the 4th century (likely in ad 361-363) could be the “ancestor” of the 1908 earthquake. similarly to this latter one, also the 4th century event could be located around the messina strait and may have produced a tsunami with wave heights quickly decreasing north of messina, not energetic enough to depose marine sand at the torre degli inglesi site. since the area of effects for the 1783 and 1908 earthquake (figure 12) are quite similar, the 4th century earthquake could be also associated with the same structure of the 1783, previously referred to the cittanova fault (figure 1). 5. conclusions the analysis of available historical, archeoseismic and paleoseismic data allowed us to improve the knowledge of two earthquakes occurred in the 1st and 4th centuries ad in northeastern sicily and southern calabria. different historical and archaeoseismic data suggest a situation of widespread earthquake damage in the area extending from oppido in calabria to tindari in sicily, during the 1st century ad. all this evidence could be related to a single earthquake or a seismic sequence, occurred between the years 14 and 37 ad, and that did also generate a tsunami, as recorded at the torre degli inglesi site and as hinted by contemporary accounts. this event could have been the predecessor of the february 6, 1783 earthquake, and generated by the same seismic source located in southern calabria: the scilla fault. if this hypothesis is correct, the m5 of the ad 17 event indicated in the cpti04 catalogue [cpti working group 2004] is underestimated and its location on the northern slope of mt. etna should be revised. historical data and archaeoseismic evidence suggests the occurrence of another earthquake almost in the same area hit by the 1908 earthquake in the 4th century (most likely between the years 361 and 363 ad). this event probably damaged tindari and the torre degli inglesi site as well. another similitude between the 1908 event and the 4th century event is that no tsunami deposits compatible to their ages were found to the north of the messina strait and particularly at the torre degli inglesi site. these results could have a strong impact on seismic hazard assessment of southern calabria and northeastern sicily. in fact, on the one side, they provide insights to establish average recurrence times (~1300-1660 yrs) for the major seismic sources of the area and, on the other, suggest that the area is characterized by “seismic periods” that concentrate most of the relevant seismicity. likely, a seismic period occurred between the 1st and 4th centuries ad as that, better known, which occurred between 1638 and 1908 (during which the entire seismogenic structure of calabria, from the crati valley to the messina strait, 180 km-long, ruptured). the mutual contributions of archaeology, geology, and history demonstrate the strength of such interdisciplinary studies to contribute to knowledge of past earthquakes and to provide estimates of their probable future occurrence and location. survey and documentation of archaeological sites along or near active faults and paleoseismic analysis can significantly improve the knowledge of past earthquakes. new analysis, to be performed in the numerous archaeological sites in southern calabria and northeastern sicily, will allow a better definition of the uncertain events reported in the catalogue and to new 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(1990). sicily under the roman empire. the archaeology of a roman province 36bc535ad, london, 464 pp. *corresponding author: maria serafina barbano, università di catania, dipartimento di scienze biologiche, geologiche e ambientali, catania, italy; email: barbano@unict.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the messina strait early seismic history evidences of active and ancient volcanism on mars. a review. second part p . h é d e r v à r i ( * ) received on december 27th, 1973 summary. — t h e results of recent research have brought some further evidence of volcanism on the planet mars. a t the same time certain erroneous descriptions and conclusions, which are to be found in the first paper (3), are here corrected. riassunto. — f risultati di recenti ricerche hanno messo in luce ulteriori testimonianze circa l'esistenza di vulcanismo sul pianeta marte. d i conseguenza sono state apportate d a l l ' a . correzioni ad alcune descrizioni e conclusioni presentate nella prima parto della nota (3). introduction i n our p r e v i o u s p a p e r on m a r t i a n v o l c a n i s m ( 3 ), w e c o l l e c t e d m a n y d a t a i n f a v o u r t h e h y p o t h e s i s a c c o r d i n g t o w h i c h v o l c a n i c p h e n o m e n a a r e n o t u n k n o w n 011 t h e p l a n e t m a r s . s i n c e t h e e l a b o r a t i o n of t h e r e s p e c t i v e first p a p e r s o m e o t h e r e v i d e n c e has b e c o m e k n o w n . i n a d d i t i o n t o t h e s h o r t s u m m i n g u p of t h e s e w e should l i k e t o c o r r e c t s o m e e r r o n e o u s d e s c r i p t i o n s a n d in t h e conclusions w h i c h b e c o m e o b s o l e t e i n t h e m e a n t i m e . ( * ) f . r . a . s . , vice-president of international association of planetology. 1023 budapest, i i . à r p à d fejedelem utja 40-41, iv/3, hungary. 84 p . i i £ d e r v a r i 1. h o t spot at a great m a r t i a n caldera the mariner 9 infrared radiation sensor discovered a hot spot at longitude 120°, latitude 3 0° s. the area of this spot is some 15 square miles. according to the mariner 9 map of the martian surface, the approximate coordinates of the so called south spot — a mightily caldera-like formation on the tharsis ridge with a diameter of about 120 kilometres — are longitude 119.5°, latitude 9.5° s. these data refer to the centre of the caldera. one can suppose that the said hot spot may be connected with postvolcanic activity (fumaroles!) within or very near this caldera. 2. m e a n d e r i n g v a l l e y s after the discovery of the meandering valleys on the moon many authors at first suggested that they are the remnants of ancient rivers. later on, however, this concept was abandoned, and now the generally accepted view is that the lunar meandering valleys are due to the work of lava. the situation in the case of the martian meandering valleys is just the opposite. a t first it was suggested that these features were due to lava-flows, but this is already obsoleted and at present the general opinion is that these martian valleys are the remnants of water-courses. 3. e a r l i e r observations regarding m a r t i a n phenomena which are attributed to volcanic a c t i v i t y mr. y . a . firsofi of glastonbury (somerset, england) had the kindness to inform the present author about some further data regarding earlier observations of martian phenomena which can be interpreted as volcanic manifestations (private communication). thus: " . . . i n d e e d on rare occasions grey circular clouds have been seen to arise suddenly, appearing to spread out mushroom-wise from a point on the surface. antoniadi observed such clouds in 1909 and 1911 in the so-called deucalionis eegio and identified them as volcanic ash, mainly on the strength of their dimness and grey colouring". the height of these clouds was estimated (later) as 60-120 miles by saheki and murayama. "pour further clouds of this type were reporte v i d e n c e s of a c t i v e a n d a n c i e n t v o l c a n i s m on m a r s . a r e v i e w s5 ed by japanese observers in 1950 and 1952. deucalionis regio is particularly favoured by these appearances, but on 15, january, 1950 tsuneo saheki and two other japanese astronomers observed a similar grey cloud, about 400 miles in diameter, over erydania and electris. these clouds have also been seen to fluctuate in colour between grey, bluish white and dull yellow within the space of a few days". 1. n i x o l y m p i c a f i g . 1 a new photo of n i x olympica volcano (mariner 9, after murray, 1973). the first photos of the gigantic volcano, nix olympica of mars, were somewhat insteading and thus led to misunderstandings. as one can see on the enclosed photo (fig. 1), in reality nix olympica 8 6 p . i i e d e r v a r i is a volcanic cone of the shield-type with a diameter of about 520 kilometres and with a height of about 25 kilometres (!) (3'7). the caldera which is to be found on the top of the volcanic structure has a diameter of some 64 kilometres and thus within this caldera we could put about ten vesuvius. the mass of this volcano is estimated as 2.4-1021 gm while that of the mars as a whole is equal to 6.4-1026 gm (8). in spite of its extraordinary height, n i x olympica has a rather flat form (fig. 2) which is characteristical to shield-volcanoes. c fig. 2 comparison among some terrestrial volcanoes and n i x olympica. 1: vesuvius; 2: etna; 3: stromboli; 4: mauna l o a (1-4: after menard w . h . : marine geology of the pacific, 1964, publ. by mcgraw-hill, inc.); 5: nix olympica. s: sea-level; c: top-caldera. some statements of the previous paper must now be corrected. accordingly, the caption to fig. 7, should read: " a detailed photomosaic. of the nix-olympica cone together with its top-caldera". fig. 8: " a part of nix olympica's slope". page 502, lines 10-15 (from above) is to be corrected as follows: " . . .the mariner 9' cameras have not only found an ancient volcano but also revealed that on the top of the volcanic structure there is a great caldera with a diameter of e v i d e n c e s of a c t i v e a n d a n c i e n t v o l c a n i s m on m a r s . a r e v i e w 8 7 about 64 kilometres. w e can see on the enlarged photo of the slope of the n i x olympica volcano that there is an apparently fresh lavaflow without small impact craters". the long chasm, mentioned many times in the first paper is the coprates canyon. 5 . h e l l a s according to the recent research hellas can be regarded as a true, rather deep basin, perhaps of impact origin, rather than a gigantic caldera of collapse origin. 6 . b a s i n s a n d m a r t i a n " c o n t i n e n t s " sagan and pollack (9) — partly on the basis of cloud-observations and partly of radar-measurements — have suggested that the dark areas of mars may generally be elevations, while the light ones can be regarded as basins. recent measurements and investigations of the distribution of carbon-dioxide in the martian atmosphere (10) have revealed, however, that there is no general agreement between the colour and height of the different surfacial areas of mars. according to wells and other authors, dark areas may be lowlands or highlands, that is depressed or elevated regions alike and the same is true of the light regions (fig. 3). therefore the suggestion that the supposedly volcanic events on mars are distributed first of all along the margins of "continental" areas, in all probability is not correct in such a strict formulation. as regards the true nature of the dark areas, binder and cruikshank (l.2) have suggested that there are no structural but only surfacial differences between the light and dark regions, as the latter may be covered with dark, dusty materials. the present author favours the opinion of d. mclaughlin of the university of michigan, who is of the view that the major dark markings on mars, are produced by the deposition of volcanic dust, due to the recent activity of the martian volcanoes. this idea seems to be very probable, especially if we take into consideration that the greatest volcanoes of mars are to be found near 88 p . i i e d e r v a r i g ?30 260 290 320 350 10 40 70 100 130 160 f i g . 3 relief-map of mars (after wells, 1072). t h e height-contours were produced f r o m the combined data sources listed in the respective paper. solid lines represent zero and positive relief, the dashed ones correspond to negative topography. numbers mean kilometres. the equator and — as miyamoto (6) lias demonstrated — that strong air-movements crossing the equator are possible 011 mars. w e can refer to some terrestrial examples, such as the distribution of volcanic dust over many parts of the earth after the krakatoa outburst in 1883 or after the agung eruption of 1903. in the latter case volcanic materials, originated from agung (indonesia), had been found in north america, chile and even on the land of antarctica. 7. t h e grey streak on fig. 1 of the previous paper the grey streak, observed by c. f. capen of the flagstaff observatory, was shown 011 the map as a straight line. capen has kindly to call the author's attention (private communication) that this grey streak has a curved form instead of a straight line, as originally portrayed in a chart published by ii . heudiacr1a euxinus l miofontis 1 amazonis »(esogaw z arcadia 3 x \ a tempe uabíotlt 01*m»ica s!4u5 \ = asciarís i. thars15 iunae f. »a.ho-oalmch • 5va . i memnonia p ' ntcta» •ftmgm, '«"•«• "'''i claritas ^ ' svt* erythraeum m. s * x "*"'> ¡, ramo / v 1 argyre i s / dia thylei tmyusmow a u s t r a l e ar6yre ii o ' «, +> / ^ x x _ , tithonius "'j** w*%. ' g isaimsí i 5. tos s f ^ ^ í. v ~ „ b ^ . . syria / sina, ^ wt,«"on,s, gh necta» «. fanoohíe ra. _ vtacanl •>*• noachis copyright 1971, lowell o b s e r v a t o r y l i t h o g r a p h e d by c a r e y c o l o r g r a p h i c c o r p . , p h o e n i x , a r i z o n a , u . s . a . b si o h > o 5s o b si h <1 o o 2! o si 4 t h e location of the g r e y streak, indicated on a map, published by l o w e l l observatory (after capen's i n f o r m a t i o n ) 90 p. h é d e r v â r i seler (4). our present fig. -1 shows the position and form of the said grey streak in accordance with the information, the author had received from capen, who has suggested that the streak is probably due not to a very long linear (fissure) eruption but rather to a central one and the curvilinear form of the eruption-cloud was the consequence of the wind prevailing on the mars during and after this central eruption. acknowledgement the author is very indebted to dr. c. f. capen of flagstaff and mr. v. a. firsoff of glastonbury for their valuable remarks, information and data with which they have kindly provided him. r e f e r e n c e s ( ' ) b i n d e r a . b . , c r u i k s h a n k d . p . , 1 9 6 4 . c o m m . o f t h o l u n a r a n d planet. l a b . , 2, 36-39. ( 2 ) b i n d e r a . b . , c r u i k s h a n k d . p . , 1 9 6 6 . c o m m . o f t h e l u n a r a n d planet. l a b . , 4, 64-69. ( 3 ) 1i£deiivari p . , 1972. " a n n a l i di geofisica", x x v , 4, 1972. ( 4 ) h e u s e l e r h . , 1 9 6 9 . " d i e s t e r n e " , 4 5 , 7 8 . (6) mccauley j. f . et al., 1972. preliminary mariner report on the geology of mars. " n a s a " , contract nas7-100. (8) miyamoto s., 1963. contr. from the " i n s t , of a s t r o p h y s . " and " k w a s a n o b s . " , u n i v . of k y o t o , 125. ( 7 ) m u r r a y b . c . , 1 9 7 3 . " s c i e n t i f i c a m . " , 2 2 8 , 1. ( 8 ) m u r r a y b . c . , m a l i n m . c . , 1 9 7 2 . c o n t r . n o . 2 1 9 9 o f t h e " d i v . o f geol. and planet. sci., " c a l t e c h " , (preprint). (°) sagan c., pollack j. b., 1968. " j o u r n . geophys. r e s . " , 73, 4. ( 1 0 ) w e l l s r . a . , 1 9 7 2 . " t h e g e o p h y s . j o u r n . " , 2 7 , 1 0 1 1 3 3 . vol50,3,2007 435 annals of geophysics, vol. 50, n. 3, june 2007 key words radio noise – background noise 1. introduction natural electromagnetic noise sources existing since the origin of the earth may be influencing the evolution of living systems. understanding how this background noise is generated and distributed and how it interacts with living systems, can contribute to the general knowledge of life on earth. in this review we consider the different frequency ranges, starting from the lowest frequencies generated within the ionospheric and magnetospheric cavities, up to the microwave band, including galactic noise, for both natural and man-made noise. different sources generate natural noise inside the magnetospheric cavity at different frequencies. a primary source of noise is given by the interaction of particles and waves coming from outer space with the magnetosphere plasma at various altitudes; another source is inside the ionospheric cavity where atmospheric lightning discharges produce several interesting propagating phenomena. at higher frequency radio noise originated in the atmosphere becomes less important and cosmic noise is prevalent up to the millimetric wavelength. human technologies implanted for power transmission and communications are the well known causes of man made noise. man-made noise, mainly due to communication and broadcasting systems, electric energy transport systems, automotive ignition, industrial thermal processes and instruments for scientific/medical appliances is distributed, albeit not uniformly, in all bands. in vlf-hf band atmospheric noise is still larger than man-made noise in the order of a tenth of db in rural areas and natural and man-made terrestrial electromagnetic noise: an outlook cesidio bianchi and antonio meloni istituto nazionale di geofisica e vulcanologia, roma, italy abstarct the terrestrial environment is continuously exposed to electromagnetic radiations which set up a «background» electromagnetic noise. within the non ionizing radiation band (nir), i.e. for frequencies lower than 300 ghz, this background can have a natural or an artificial origin. natural origins of electromagnetic radiations are generally atmospheric or cosmic while artificial origins are technological applications, power transmission, communications, etc. this paper briefly describes the natural and man-made electromagnetic noise in the nir band. natural noise comes from a large variety of sources involving different physical phenomena and covering a wide range of frequencies and showing various propagation characteristics with an extremely broad range of power levels. due to technological growth man-made electromagnetic noise is nowadays superimposed on natural noise almost everywhere on earth. in the last decades man-made noise has increased dramatically over and above the natural noise in residential and business areas. this increase has led some scientists to consider possible negative effects of electromagnetic waves on human life and living systems in general. accurate measurements of natural and man-made electromagnetic noise are necessary to understand the relative power levels in the different bands and their influence on life. mailing address: dr. cesidio bianchi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: bianchi@ingv.it 436 cesidio bianchi and antonio meloni simply overcomes man-made noise in business areas. in general man-made noise is concentrated in particular frequencies in association with technological applications. both contributions, natural and man-made, affect technological applications such as communication, remote sensing, etc. the purpose of this work is to describe the electromagnetic terrestrial environment and the main sources of emission both natural and man-made. figure 1 shows some propagation mode names and acronyms for natural contributions in the terrestrial environment inside the magnetosphere and ionospheric cavity. 2. natural radio noise non ionizing radiation (nir) electromagnetic waves range from the mhz to 300 ghz spanning over 14 orders of magnitude (table i). the corresponding wavelengths range from the dimension of the solar system, for waves around the mhz frequencies, to millimeters for frequencies around 300 ghz. 2.1. magnetic noise in ulf/elf band and other propagating phenomena the earth’s magnetic field is not only static but also includes very slow time variations: secular, annual, 27 days, diurnal and substorm magnetic bay type variations at very low frequency (<1 mhz); all this can be considered a sort of «natural magnetic» noise. variation amplitudes can be quantified in tenths of nt for the diurnal variation and hundreds, or exceptionally, thousands of nt in the case of strong magnetic storms (see kivelson and russell, 1995; merrill et al., 1998). at higher frequencies in this band various other phenomena, for example geomagnetic pulsations, take place. geomagnetic pulsations, i.e., ultra-low-frequency (ulf) waves cover roughly the frequency range from 1 mhz to 1 hz, i.e., from the lowest frequency the magnetospheric cavity can sustain, up to the various ion gyrofrequencies (see for example: hughes, 1994; richmond and lu, 2000). pulsation frequency is considered to be «ultra» low when it is lower than expected plasma frequencies, like plasma frequency and the ion gyrofrequency. geomagnetic pulsations were first observed in the ground-based measurements of the 1859 great aurora events (stewart, 1861) and were subsequently studied in depth. it is well known now that lower frequency pulsations are generally related to the kelvin helmholtz instability that takes place at the magnetopause, and is generated by the solar wind interacting with the magnetosphere, or by upstream waves in the foreshock region. table ii reports geomagnetic pulsations’ average intensity, frequency and other information (modified from lanzerotti et al., 1990). other electromagnetic phenomena in the ulf/elf band are originated by particles impinging on the magnetosphere causing electromagnetic emissions that propagate inside the magnetosphere cavity. chorus emissions and auroral hiss are two other relevant phenomena. chorus emissions are among the most intense plasma waves in the outer magnetosphere that propagate as far as the earth surface and are observed at intermediate invariant latitudes. since these emissions occur in matching audible sound bands, a simple tuned radio receiver can fig. 1. a schematic representation of terrestrial natural and man-made radio noise sources within the magnetosphere-ionosphere system. 437 natural and man-made terrestrial electromagnetic noise: an outlook table i. nir frequency bands and the main natural radio noise sources. frequency wavelengh main natural radio environment range (m) noise sources ulf 1-3000 mhz 3×1011-3×108 resonances magnetospheric (ultra low frequency) in the magnetospheric cavity cavity, interaction with particles of solar origin and radiative pressure with the magnetosphere elf 3-3000 hz 108-105 resonances ionospheric (extremely low in the ionospheric cavity cavity frequency) vlf (very low 3-30 khz 105-104 propagation ionospheric frequency) in the ionospheric cavity cavity of the atmospheric disharge radiate energy lf 30-300 khz 104-103 atmospheric noise ionospheric (low frequency) cavity mf 300-3000 khz 103-102 atmospheric noise ionospheric (medium frequency) cavity hf 3-30 mhz 102-10 atmospheric ionospheric (high frequency) and cosmic noise cavity vhf 30-300 mhz 10-1 atmospheric earth surfaces (very high and cosmic noise (mainly due frequency) to the cosmic noise that penetrates the ionospheric layers) uhf 300-3000 mhz 1-10-1 cosmic noise as above (ultra high frequency) shf 3-30 ghz 10-1-10-2 cosmic noise as above (super high frequency) ehf 30-300 ghz 10-2-10-3 cosmic noise as above (extremely high frequency) convert natural radio signals into sound. the chorus spectral features (from 500 hz to 1.2 khz) consist in the succession of predominantly rising tones which resemble a chorus of chirping birds from which these emissions take their name. auroral hiss emissions are broad, intense electromagnetic emissions which occur over a wide frequency range from a few hundred hz to several tens of khz occurring mainly in the auroral zone. this spreading at high frequencies is caused by the anisotropic character of whistler mode propagation (see later). the resulting tones are strongly modulated hiss-like tones. 438 cesidio bianchi and antonio meloni 2.2. atmospheric noise in elf/vlf band and related electromagnetic phenomena lightning flashes are the main source of energy for the electromagnetic background inside the ionospheric cavity. starting from the lower band elf (few hz) up to several vhf (hundreds mhz) the noise originates mainly from the energy radiated by lightning strokes (see for example: cummer and inan, 2000; mika et al., 2005). several million lightning strokes occur daily from an estimated 2000 storms worldwide, and the earth is hit about 100 times a second by lightning. the discharge is very violent and can easily reach 10000 a. the amount of energy released by each discharge can vary from units to tenths of gj. hence, for the duration of the discharge (less than 1 s), the power involved in this phenomenon is of the order of 1-10 gw. the annual total released energy is in the order of 1019 j. if only 10% of this energy is radiated as electromagnetic energy (fig. 2) it would be comparable to the energy produced in 1970 by the electric power stations in the world. the main relevant phenomena in the elf lower band are the schumann resonances (bliokh et al., 1980; sentman, 1987; sentman, and fraser, 1991). this phenomenon consists of a wide spectrum electromagnetic signal, composed by dumped waves of frequencies below 60 hz. schumann resonances occur because the earth and the ionosphere form a natural wave guide that shows a fundamental resonance frequency at 7.8 hz and upper harmonic components at about 15.6, 23.4 and 31.2 hz (fig. 3). the earth-atmosphere system can be seen, from an electromagnetic point of view, as a series of shell layers having different electrical conductivity with night-day asymmetry (greifinger et al., table ii. magnetic noise sources in the pulsation band. pulsation continuous pulsations irregular pulsations classes pc1 pc1 pc3 pc4 pc5 pi1 pi2 incoherent noise period (s) 0.2-5 5-10 10-45 45-150 150-600 1-40 40-150 1-1000 frequency 200-5000 100-200 22-100 7-22 2-7 25-1000 2-25 1-1000 (mhz) intensity 1 3 10 <300 300 10 100 (nt) source electromagelectromagwavedrift of magnetomodulation substorms ionospheric netic ion netic ion particle protons pause of particles current cyclotron cyclotron interaction from the instability (intensity instability instability in the bow nightside increase in the in the shock with the equatorial equatorial region magnetic magnetomagnetoactivity) sphere sphere fig. 2. lightning frequency domain electromagnetic spectrum. 439 natural and man-made terrestrial electromagnetic noise: an outlook 2007). the earth and the ionospheric layers appear as perfect conductors having air (of negligible conductivity) in between, forming an earthionosphere cavity, in which electromagnetic radiation is trapped. lightning strikes within the troposphere radiate energy into this system and the waves travel around the earth. in the case of constructive interference, earth-ionosphere cavity resonances are excited in the above mentioned frequency range (6-60 hz). elf-vlf radio waves can propagate over long distances with small attenuation rates, typical 2-3 db per mm and phase stability. the propagation of elf-vlf radio waves can conveniently be considered in a spherical earth surrounded by a concentric reflecting layer of electronic density into which vlf waves are launched and propagate. the propagation is controlled by the lowest region of the ionosphere. during the daytime this includes the lower d-region and, at night, the lowest part of the e-region. elf vlf propagate for very long distances nearly without attenuation. as is well known, the phase and amplitude of vlf propagation can be described by an earth-ionosphere waveguide (davies, 1990). this description was exploited in past years for radio localization purposes (swanson, 1983). the relation between the phase and height changes, over long distances, is given by (2.1) where h is the mean height of reflection in the h d h a h h 2 2 16 2 2 ϕ λ π λ ∆ ∆ = − +b l d-region, d the path length, ϕ the phase, λ the wavelength and a is the earth’s radius (budden, 1985; wait, 1970). phase shift can be expressed in degrees or in seconds by ∆t = ∆ϕλ/2πc. the amplitude of the electric field e over long paths in a wave guide mode is approximately (2.2) where α is the attenuation rate and p the radiated power (in kw). the signal amplitude is generally dependent on interferences of various propagation modes, that change with the variations of the wave guide parameters. observation of the phase and amplitude of vlf waves were also used to study variability, morphology and other phenomena occurring in the low ionospheric region. measurements and timespatial statistical model is gven by fieve et al. (2007), and tomco and hepner (2001). other relevant propagating phenomena in this frequency band are the so-called sferics, tweeks and whistlers. radio atmospherics (or sferics for short, and sometime statics) are impulsive signals generated by lightning strokes that travel in the earth-ionosphere wave guide. these impulsive signals (a few ms duration) propagate for thousands of kilometers. as in a real wave guide the earth-ionosphere guide can sustain the propagation of these signals with very low attenuation values. since only the upper part of the wave channel varies its position with time, the sferic propagation is determined by ionosphere conditions. almost all am receivers detect sferics as disturbances (sounds like pops and crashes). sparks of lightning strokes are generally powerful sources of electromagnetic (radio) emission throughout the radio frequency spectrum from the very low radio frequencies up to the microwave frequency ranges and the visible light spectrum, even if the radio power is concentrated in vlf range from 0.1 to 10 khz (fig. 4). tweeks are sferics dispersed in frequency. their sound is similar to a bird’s song usually in a frequency range of 1-7 khz (fig. 5). when sferics propagate for long distances in a dispersive medium like the ionosphere their harmonic com( / ) 0.5 sin e h e a d a p300 ad λ = − − c m fig. 3. schumann resonance peaks. 440 cesidio bianchi and antonio meloni ponents separate along the path (helliwell, 1965). these components penetrate the ionosphere at various depths, in such a way that higher frequencies penetrate more than lower ones and as a consequence travel for longer distances. these different paths imply different arrival times at an observer. in a spectrogram they appear as a descending tone with duration of the order of 25 to 150 ms. tweeks are normally heard in the evening after sunset. whistlers are remarkable bursts generated by lightning discharge. when part of the discharge energy escapes the ionospheric barrier and propagates through the magnetosphere, whistlers can be heard in radio receivers as a relative long whistle decreasing in frequency, from about 6 khz to a few hundred hz (fig. 6). in the magnetosphere whistlers interact with free electrons and are forced to propagate along the earth’s magnetic field lines. the harmonic components of the signals identified as whistlers correspond to electromagnetic waves that have traveled several earth radii arriving at different times to the observer. lower frequencies are delayed 3-6 s with respect to the higher ones. the dispersion of a whistler depends on the length of the path over which the signal travels as well as the characteristics of the propagation medium such as its electron density (kimura, 1989). 2.3. natural lf/mf/hf noise in this frequency range as well natural electromagnetic noise has its main source in the atmospheric electric discharges; noise amplitude is generally decreasing in intensity with frequency and is affected by ionospheric conditions. since lf/mf/hf frequencies were very soon used in radio communications and broadcasting, in this frequency range natural radio noise has been the object of scientific studies in the radio engineers community. natural lf/mf/hf noise radio measurements started very early as radio devices developed. the electromagnetic waves radiated by impulsive lightning discharges cannot escape the ionosphere border. waves penetrate through the lowest ionospheric layers where they are variously absorbed, depending on frequency. radio waves are reflected by the upper layers of the ionosphere up to a particular frequency, named ‘critical frequency’, that is dependent on local ionospheric condition. moreover the incidence angle between the wave and the ionospheric layers plays an important role. in fact depending on the path geometry several propagating modes can be fig. 4. typical sferics frequency spectrum. fig. 5. tweeks that traveled for different distances in the earth-ionosphere waveguide. shown for about 6000 km (1), 10 000 km (2) and 40 000 km (3) paths. fig. 6. whistler spectrum. 441 natural and man-made terrestrial electromagnetic noise: an outlook established. the results are summarized in fig. 7 where the atmospheric noise versus frequency is plotted. the same figure shows cosmic noise at frequencies greater than ionosphere plasma frequency. the cosmic noise lowest frequency depends on the ionospheric conditions i.e. on electron plasma frequency about (15-30 mhz). in the 1960s of last century a campaign of atmospheric noise measurement started by consultative committee international radio (ccir) gave the main contribution to the knowledge of background radio noise in the lf/mf/hf frequency range. sixteen radiometers were installed worldwide and the results were summarized on report ccir n.322 in 1964 (ccir/itu, 1964) and n.322-3 in 1988 (ccir/itu, 1988). the planetary atmospheric noise distribution is reported in fig. 7 where cosmic noise is also reported. in the figure the strongest levels are associated with the equatorial region and the weakest in antarctica. many of these propagating phenomena, including the atmospheric noise diurnal variability, can be easily explained with simple considerations. in the lf-mf-hf range depending on frequency, waves penetrate relatively deeply into ionospheric plasma. because of the non monotonic trend of the electron density with height, without considering the terrestrial magnetic field and collisions, each layer exhibits its own critical frequency up to the point where the normal incident angular frequency ω equals the angular plasma frequency ωp: (2.3) where, n is the electron density local maximum, e and me are the charge and the mass of the electron, and εo is the permittivity of free space. this gives a good approximate description of the confinement limits of waves inside the ionospheric cavity. the refractive phase index µ simple is . (2.4) this relation establishes the condition of penetration, for ω = ωp the refractive index µ is zero. to take into account the attenuation suffered by the radio waves, collisions between free electrons and neutral molecules, indicated with letter ν, must be considered. in that case the phase refractive index µ becomes a complex quantity . (2.5) the imaginary term is responsible for the absorption suffered by the radio wave during its propagation in the ionospheric plasma. in the lower ionospheric region, the attenuation rate α is relevant (up to 20 db for each ionospheric hop). in fact α is proportional to the quantity n ν/ω 2 which reaches its maximum in the ionospheric d layer. this seems in contradiction with the fact that waves of lower frequencies described in the relation (2.2) are less attenuated. this can be easily explained by relation (2.3), because of waves in elf-vlf band do not penetrate deeply into the ionospheric plasma and therefore they are not absorbed. terrestrial magnetic field and oblique incidence of the wave in the plasma are beyond the scope of this work. in the terrestrial surface the conditions of reflection and absorption are described by the reflection coefficient and the attenuation rate α and they are both dependent on the electric conductivity of the surface. j 1 1 1 p p 2 2 2 2 2 2 2 2 2 µ ω ω ν ω ω ω ν ω ω ν = − + − +b b b l l l 1 p 2 2 µ ω ω = − m ne p o e 2 ω ω ε= = fig. 7. electric field strength versus frequency for atmospheric and cosmic noise. 442 cesidio bianchi and antonio meloni 2.4. natural uhf/shf/ehf noise in this frequency range the natural predominant noise is the background cosmic (or galactic) noise. in many ways cosmic noise is similar to terrestrial natural noise, like distant lightning strikes, or man-made radio noise; for this reason at the beginning of radio measurements it was difficult to identify the different sources. the discovery of the origin of this background noise is usually also marked as the birth of radio astronomy. in 1931 karl jansky built an antenna operating at 20.5 mhz and after months of careful observations he concluded that the source of the recorded noise was outside the solar system. since then those frequency bands have been carefully investigated by radio astronomers (kraus, 1988). in 1940 grote reber made the first radio map of our milky way galaxy at a frequency of 160 mhz. after that the radio emission from the sun, jupiter and other celestial bodies have been also identified by radio astronomers. figure 8 reports the spectrum of solar and other main sources. some of these sources are very strong or show a particular behavior (fig. 9). the synchrotron radiation has a characteristic wide spectrum like other impulsive radio sources. moreover galactic or stellar systems generate a strong and common noise source: the thermal noise due to random motion of electrically charged particles in space. thermal noise arises from electrons and ions in motion in a dissipative media. others sources are the bremsstrahlung radiation mainly due to electron proton collision (solar chromosphere, orion nebula etc.). the earth is not the only planet that shows a large variety of radio signals. almost all planets have an electromagnetic background, for instance venus is quite similar to the earth, and the four gas giants, jupiter, saturn, uranus and neptune are natural emitters of hf radio signals. due to its huge atmospheric disharges jupiter is one of the major radio emitters. the sun is the most powerful electromagnetic emitter in our solar system, able to generate very broadband radio emissions. in the solar radio emission, several phenomena like flares, magnetic waves, and storms of electrically charged nuclear particles and ions, directly cause and/or influence the propagation of electromagnetic waves throughout the known solar spectrum. 3. man-made electromagnetic noise man-made noise is originated by human technologies. it is strongly dependent on the distance from the sources (power lines, radio, tv communication installations and other) which can be very variable, on frequency and on emitted power. power and frequency in their time and spatial distribution are the quantities that best describe all man-made radio sources. other important characteristics are the type of emitted waves, con-fig. 8. solar radio emissions. fig. 9. some sources of radio emissions and cosmic background. 443 natural and man-made terrestrial electromagnetic noise: an outlook tinuous or impulsive, their modulation and wave polarization. terrestrial man-made sources are mainly located in business, industrial and residential areas. in rural (countryside) and ‘quiet rural’ areas the sources become scarce (see for example report ccir/itu, 1990). in spite of its relevance, man made noise power, depending on frequency band, is on average 20-30 db lower, in ‘quiet rural’ areas than the business and residential area (fig. 10). of course, starting from the 1960’s, satellite communication systems also contribute to man-made noise measurable on earth. as previously done for natural electromagnetic noise, the following man-made noise contributions are described in the diverse frequency bands with the indication of their main sources. 3.1. elf band in this band the strongest source of manmade noise comes from the electric power lines which operate ideally at a single frequency, generally 50 hz (60 hz in u.s.a.). the electric and magnetic fields generated at this frequency are practically decoupled and because of power lines’ multi-polar configuration, decrease dramatically with the distance from the lines. the power lines are now spread over the planet in all continents except antarctica where natural elf waves close to 50-60 hz can be recorded because there is a smaller amount of disturbance. also, some uninhabited regions in other continents are relatively free from this kind of disturbance but in general, in the elf band, the 5060 hz electric and magnetic fields are the strongest man-made source of noise both as emitted power and as extension of the source. 3.2. vlf-hf band as in the case of other frequencies, manmade noise emissions are due to electronic devices for indoor use, industrial and radio communication apparatuses, including broadcasting radios, since all this equipment employs electrical oscillators that become unwanted sources of artificial radio emission. the main localized sources are radio am broadcasting (fig. 11) and some industrial equipment. among the latter relevant sources are heaters that exploit magnetic induction and dielectric losses in the material. other weaker but numerous sources, such as automotive ignition, heavily contribute to the background noise. we have also immeasurable indoor sources such as electrical household appliances, pc monitors tv, etc. other scientific and medical devices are irrelevant in the background radio noise configuration in the considered range of frequency. fig. 10. median values of man-made radio noise power (solid lines) expressed in terms of fam (db above thermal noise at t0=288 k). atmospheric noise (dashed lines) and cosmic background noise (dotted line) are reported for comparison with man-made noise. fig. 11. power emitted by the most common communication systems. 444 cesidio bianchi and antonio meloni 3.3. vhf-uhf band electromagnetic radiations in these bands are due to broadcasting fm radio tv and mobile phone service stations (fig. 11) and more important automotive ignition. radar and satellite appliances do not make a very important contribution to background noise level. main indoor sources are cordless phones, microwave, ovens, etc. this equipment does not make a significant impact outdoors. 3.4. shf-ehf band because of the complexity of the system of radio communications and limitations in industrial applications operating at these frequencies, in this range only particular equipment is employed. these bands include satellite communication systems and survey, radar systems, scientific and medical appliances, for which there are intrinsic technological difficulties in manufacturing. for this reason man-made noise is very low in shf and ehf bands. moreover, their physical emission processes determine propagation modes for shf and ehf waves which are strictly directional. other technological applications in this range are related to passive observation and measurements as in radio astronomy. 4. conclusions this paper described the terrestrial natural electromagnetic noise and its main characteristics. natural electromagnetic noise constitutes a background radio noise in which living systems are immersed and have consequently evolved since the origin of life on earth. superimposed on the natural background radio noise is manmade radio noise. the main man-made radio noise sources and its characteristics were also described and compared with the natural noise ones. artificial noise levels have been constantly increasing in the last hundred years or so with the expansion of electromagnetic applications. a digression of the main characteristics, features and sources was made in the frequency range of non ionizing radiation. a brief excursus of the natural radio sources was undertaken, starting from the lowest frequencies that the magnetosphere and ionosphere cavities can sustain, to the galactic and cosmic radio sources that penetrate the electronic density layers of the upper atmosphere. these studies started in the pioneer era of radio communication when the knowledge of natural noise was crucial for the assessment of an effective radio technology, have also revealed their relevance in geophysics, space physics, astrophysics and many other research fields over the years. in particular, a detailed knowledge of features of all natural electromagnetic waves in the magnetosphere is now imperative for ground based and satellite communication systems, but can also be relevant, for example, in the design of earth-orbiting and interplanetary spacecraft and in understanding how the background noise interacts with living systems. references bliokh, p.v., a.p. nikolaenko and y.f. filippov (1980): schumann resonances in the earth-ionosphere cavity (peter peregrinus, london). budden, k.g. 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(received september 10, 2007; accepted october 18, 2007) p o l a r i z z a z i o n e d e l l a s d e i s i s m o g r a m m i ( * ) r . stoneley è s t a t o o s s e r v a t o d a f . n e u m a n n (, l ), p . b y e r l y ( ) e i. l e h m a n n ( 3 ) c h e n e i s i s m o g r a m m i d e i t e r r e m o t i r e g i s t r a t i a d i s t a n z e n o n t r o p p o g r a n d i , l a f a s e s a p p a r e p r i m a c o m e u n a s h , s e g u i t a c i r c a 1 0 1 4 s e c o n d i p i ù t a r d i , d a u n a s v . l a s i g . n a l e h m a n n a t t r i b u i s c e q u e s t o f e n o m e n o al p i c c o l o a n g o l o f o r m a t o d a l l e o s c i l l a z i o n i s c o n l ' o r i z z o n t a l e , m e n t r e i l p r o f . b y e r l y n e r i c e r c a la c a u s a i n u n a s p e c i e d i d o p p i a r i f r a z i o n e . a m b e d u e c o n c o r d a n o n e l r i l e v a r e c h e l ' i n t e r v a l l o n o n s e m b r a d i p e n d e r e d a l l a d i s t a n z a e p i c e n t r a l e . è e v i d e n t e c h e o c c o r r e a n c o r a u n g r a n n u m e r o d i p r o v e s p e r i m e n t a l i : p u r t u t t a v i a si p u ò , p r e l i m i n a r m e n t e , e s a m i n a r e l a p o s s i b i l i t à c h e i l f e n o m e n o sia o r i g i n a t o d a u n a a n i s o t r o p i a e l a s t i c a d e l l a p a r t e p i ù e s t e r n a d e l m a n t e l l o . l a i p o t e s i p i ù s e m p l i c e , c h e si p u ò f o r m u l a r e p e r i n t e r p r e t a r e q u e s t a s u p p o s i z i o n e è , f o r s e , di a m m e t t e r e c h e t u t t e l e d i r e z i o n i , f o r m a n t i 1111 a n g o l o r e t t o c o n l a v e r t i c a l e s o n o e q u i v a l e n t i : u n s o l i d o di q u e s t o g e n e r e si d i c e « i s o t r o p o t r a s v e r s a l m e n t e » . l a g e n e r a l i t à di q u e s t o t i p o di a n i s o t r o p i a è s u f f i c i e n t e a f o r n i r e i n d i c a z i o n i s u l l ' o r d i n e di g r a n d e z z a d e l l a a n o m a l i a p r e v i s t a . i n o l t r e è i m p r o b a b i l e c h e gli s t r a t i p i ù e s t e r ni s i a n o m a r c a t a m e n t e a n i s o t r o p i , a l t r i m e n t i la d e v i a z i o n e d a l l a i s o t r o p i a s a r e b b e già stata n o t a t a d a t e m p o . a l l o r a c o m e c a m p i o n e d i c o n f r o n t o è s u f f i c i e n t e u n m a t e r i a l e i s o t r o p o t r a s v e r s a l m e n t e d e l q u a l e si c o n o s c o n o l e c o s t a n t i e l a s t i c h e ; t a l e è il b e r i l l i o , l e c u i c o s t a n t i e l a s t i c h e s o n o s t a t e m i s u r a t e d a v o i g t (•*). l o s t u d i o s i s m o l o g i c o di u n m e z z o i s o t r o p o t r a s v e r s a l m e n t e è già s t a t o e s e g u i t o . l ) a e s s o r i s u l t a u n f a t t o di g r a n d e i m p o r t a n z a e c i o è c h e , se t u t t e l e d i r e z i o n i p e r p e n d i c o l a r i a l l a v e r t i c a l e s o n o e q u i v a l e n t i ( t r a s c u r a n d o l a c u r v a t u r a d e l l a t e r r a ) l e v e l o c i t à d e l l e s h e d e l l e s v s o n o d i f f e r e n t i e d i p e n d o n o d a l l ' a n g o l o d ' i n c i d e n z a d e l l ' o n d a . l a d i f f e r e n z a n e l t e m p o di p r o p a g a z i o n e a t t r a v e r s o u n o s t r a t o s u p e r f i c i a l e di s p e s s o r e li p u ò c a l c o l a r s i f a c i l m e n t e . i s u f f i s s i h e v si r i f e r i s c a n o , r i s p e t t i v a m e n t e a l l e s h e a l l e s v . s e c è l a v e l o c i t à d e l l ' o n d a 111 1111 c e r t o m e z z o , i l ' a n g o l o di i n c i d e n z a n e l m e z z o s t e s s o e d r l a d i s t a n z a d a l c e n t r o d e l l a t e r r a , si ha p e r o g n i r a g g i o , c o n l e s o l i t e n o t a z i o n i : r sin i clt s u p p o n i a m o c h e u n a p e r t u r b a z i o n e o r i g i n a t a s i i n a , g e n e r i l e o n d e s h e s v c h e a r r i v a n o i n b l u n g o i c a m m i n i p q , , b e p q v b . p y j v i e q v m s i a n o p e r p e n d i c o l a r i a p , , m e a q>,n r i s p e t t i v a m e n t e : a m m e t t i a m o i n o l t r e c h e l a r i f r a z i o n e si v e r i fichi a l l a b a s e di u n o s t r a t o s u p e r f i c i a l e . d i " a figura r i s u l t a p „ m = n q „ = h ( t a n iy— t a n „ ) s i n i, d i m o d o c h é l a d i f f e r e n z a d e l t e m i l o d i t r a s m i s s i o n e d e l l e s l u n g o i c a m m i n i p v q v e p „ q „ n e l l o ( * ) t r a d u z i o n e i n l i n g u a i t a l i a n a d e l l a n o t a d i p a g . 3. 1 1 4 r . s t o n e l e y s t r a t o i n f e r i o r e , c h e si s u p p o n e i s o t r o p o , è 2(/i/c) s i n i ( t a n i y — t a n i n i = 2 <itj. d ' a l t r o c a n t o se o è il c e n t r o d e l l a t e r r a e d r il r a g g i o a l l a d i s c o n t i n u i t à , in q u a n t o l ' a n g o l o q v o q „ è d\, si ha q v q „ = r ( / a : = r (ci \ / j t t ) rft, = = c(ztj/sin /, ossia si o t t i e n e n u o v a m e n t e i l r i s u l t a t o p r e c e d e n t e . c o s ì , in d e f i n i t i v a , la d i f f e r e n z a f r a i t e m p i di t r a s m i s s i o n e d e l l e s h e d e l l e s v = t „ — t v ' = 2/. . 2h . , 2h . . . . , . s . s e c i h — sec / y — s i n i ( t a n i y — t a n i n ) <n cv c d o v e c, i si r i f e r i s c o n o al m e z z o i m m e d i a t a m e n t e s o t t o s t a n t e al c o n g i u n g i m e n t o . p e r v e d e r e il t i p o di v a r i a z i o n e di c v e c n c o n i n c l i n a z i o n e 0 " r i s p e t t o a l l a v e r t i c a l e b a s t e r à l a v o r a r e c o n l a d e n s i t à e l e c o s t a n t i e l a s t i c h e d e l b e r i l l i o e a g g i u s t a r e i r i s u l t a t i i n m a n i e r a da o t t e n e r e n e l l o s t r a t o u n a v e l o c i t à di 3 , 4 0 k m / s e c p e r l e s v a 0 = 0 " e a 0 = 9 0 " . n e l m e z z o s o t t o s t a n t e p r e n d i a m o c = 4 , 3 8 k m / s e c : così il m o d e l l o c o r r i s p o n d e a d u n c o n t i n e n t e g r a n i t i c o , p o g g i a n t e su u n m a t e r i a l e u l t r a b a s i c o . l e f o r m u l e c h e e s p r i m o n o r „ e c v si t r o v a n o s u l l a n o t a c i t a t a i n p r e c e d e n za ( 1 ) . p e r u n c o r p o i s o t r o p o t r a s v e r s a l m e n t e i n cui l a f u n z i o n e d e l l a e n e r g i a d i d e f o r m a z i o n e w è data da 2 j v = a ( e \ x + e v ) + c e : ! „ + 2 f(exx+ey>)ezt+ +2(a-2n)exxe„+l(e*yl+e\j+ne\y l a v e l o c i t à c d e l l ' o n d a c o r r i s p o n d e n t e a d h s è d a t a d a : qcsh=n12+lti3 d o v e il = c o s o ; l = sino. p e r o n d e di t i p o s v 2qc\=al2+cn~—[)(/!—l)l--(c—l)i2|!+4/vrts] d o v e : j = f -)l . i r i s u l t a t i d e l l e m i s u r e di v o i g t , e s p r e s s i i n d i n e / c m 2 s o n o : a = 2 , 6 9 4 x i o " ; c = 2 , 3 6 3 x i o 1 2 ; a — 2 n = 0 , 9 6 1 x i o 1 2 f = 0 , 6 6 1 x i o 1 2 ; l = 0 , 6 5 3 x i o 1 2 ; n = 0 , 8 6 6 x i o 1 2 n e l l a t a b e l l a i s o n o r i p o r t a t i i v a l o r i di c e c , q u a l i si r i c a v a n o d a l l e f o r m u l e p r e c e d e n t i , a g g i u s t a t i in m o d o d a c o r r i s p o n d e r e a d u n m a t e r i a l e p e r i l q u a l e c„'j e c v 2 , ee 0 ° , s o n o 1 1 , 5 6 x i o 1 0 . t a b e l l a i 0" c » x io"5 c v x io"5 0 3,40 3 , 4 0 1 0 3 , 4 1 3 , 4 8 2 0 3 , 4 6 3 , 6 8 3 0 3 , 5 4 3 , 9 0 5 4 0 3,62 4 , 0 4 5 0 3 , 7 1 4,02 6 0 3 , 7 9 3 , 8 5 5 70 3 , 8 6 3,64 8 0 3,90 3 , 4 7 9 0 3,92 3 , 4 0 polarizzazione della s dei s i s m o g r a m m i 1 1 5 d a q u e s t a t a b e l l a r i s u l t a e v i d e n t e i l d i v e r s o c o m p o r t a m e n t o di c n e c y al v a r i a r e d i 0 . i n s e g u i t o l o s t r a t o v e r r à s u p p o s t o s o t t i l e i n m o d o c h e , n e i c o n f r o n t i d e l l a p r o p a g a z i o n e d e l l e o n d e i n q u e s t o s t r a t o , l a t e r r a p o s s a c o n s i d e r a r s i p i a n a . a l l o r a i e £ y s o n o d a t i d a : s i n i s i n i„ s i n i v c c„ cv c o n c y — 4 , 3 8 k m / s e c e ( s i n i)/c = ( d t / c i a n ) / 1 1 0 , 7 . p o i c h é </t/rfa è d a t o d a l l e t a v o l e s i s m o l o g i c h e , si p u ò c a l c o l a r e l ' a n g o l o i p e r v a r i e d i s t a n z e e p i c e n t r a l i a . p e r c o m p l e t a r e i c a l c o l i c o n s i d e r i a m o p r i m a l e s h e p o n i a m o 0 = £ n e l l a t a l ) . i. a l l o r a ( s i n in)/c p u ò c a l c o l a r s i i n f u n z i o n e d i i, m e n t r e d a l l e e q u a z i o n i p r e c e d e n t i si r i c a v a i i n f u n z i o n e di a . i c a l c o l i p e r l e s v s o n o a s s a i p i ù l u n g h i , m a il p r i n c i p i o g e n e r a l e è jo s t e s s o . c o s ì si o t t i e n e l a t a b e l l a i i . s i v e d e c h e l e d i f f e r e n z e f r a i e i n o n s o n o g r a n d i . t a b e l l a l i a k/t/a 'ii iv i g r a d i s e c / g r a d o g r a d i g r a d i g r a d i 1 0 2 4 , 4 5 6 , 3 4 5 , 0 74,5 2 0 1 9 , 4 s d 3 9 , 3 4 3 , 0 5 0 , 0 2 0 1 8 , 7 s r 3 8 , 0 5 8 , 7 4 7 , 7 3 0 1 5 , 8 3 0 , 0 3 4 , 8 3 8 , 8 4 0 1 5 , 0 2 8 , 5 3 2 , 3 3 6 , 4 5 0 1 3 , 9 2 6 2 9 , 4 3 3 , 5 6 0 12,8 2 3 , 5 2 6 , 4 3 0 , 5 7 0 1 1 , 8 2 1 , 5 2 3 , 8 2 7 , 8 8 0 1 0 , 5 1 9 , 0 2 0 , 7 2 4 , 5 9 0 9 , 0 1 6 , 2 1 7 , 2 2 0 , 9 ino 8 , 4 1 5 , 0 1 6 , 0 1 9 , 2 o r a si p u ò c a l c o l a r e d i r e t t a m e n t e il v a l o r e d i ( t n — t y l 2/i. s i c c o m e l o s c o p o d i q u e s t a r i c e r c a è d i a c c e r t a r e s o l a m e n t e l ' o r d i n e di g r a n d e z z a d e l l a d e v i a z i o n e d e l l ' i s o t r o p i a , b a s t a c o n s i d e r a r e t r e v a l o r i di ( t „ — t v ) / 2 h . p e r a = 2 0 " ( s r ) è 0 , 0 3 9 , p e r a = 50" è 0 , 0 3 5 e p e r a = 8 0 " è 0 , 0 2 5 . p e r c i ò l a v a r i a z i o n e c o n l a d i s t a n z a e p i c e n t r a l e n o n è g r a n d e . t u t t a v i a l a d i f f e r e n z a d i t e m p o è a b b a s t a n z a p i c c o l a : p e r e s . p e r li = 3 3 k m , o s s i a , a l l i n e i m i p e r l o s p e s s o r e m a s s i m o c o n s e n t i t o a l l o s t a t o d e l g r a n i t o , t m — t v v a l e s o l a m e n t e 1 , 6 s e c . c o s ì s e m b r a c a d e r e la s p i e g a z i o n e f o n d a t a s u l l a d e v i a z i o n e d a l l ' i s o t r o p i a d e g l i s t r a t i e s t e r n i , p e r c h é in u n m e z z o t i p i c a m e n t e c r i s t a l l i n o ( a g g i u s t a t o i n m o d o d a s o d d i s f a r e l a v e l o c i t à d e l l e s g ) o c c o r r e r e b b e u n o s p e s s o r e d e l l a r o c c i a a n i s o t r o p a d e l l ' o r d i n e d i 3 0 0 k m p e r s p i e g a r e l a d i f f e r e n z a d i 1 4 sec n e l t e m p o d i a r r i v o d e l l e s h e s \ a u n a d i s t a n z a e p i c e n t r a l e di 80". m e n t r e n o n si p u ò e s c l u d e r e c o m p l e t a m e n t e l a p o s s i b i l i t à d i u n o r i e n t a m e n t o p r e f e r e n z i a l e n e l l a p a r t e c r i s t a l l i n a p i ù e s t e r n a d e l m a n t e l l o ( a l l ' i n c i r c a l a p a r t e s o p r a l a d i s c o n t i n u i t à a 2 0 ° ) b i s o g n a 1 1 6 r . stoneley r i c o r d a r s i c l i c i l m o d e l l o s c e l t o è u n o n e l q u a l e gli a s s i d i s i m m e t r i a di t u t t i i c r i s t a l l i c h e l o c o s t i t u i s c o n o s o n o r a d i a l i . c o n s e g u e n t e m e n t e , a m e n o c h e l a p r o v a d e c i s i v a d e l l a a n i s o t r o p i a d e l l a c r o s t a t e r r e s t r e n o n p o s s a d e d u r s i d a a l t r e f o n t i , essa n o n c o s t i t u i s c e u n a c o n v e n i e n t e s p i e g a z i o n e d e l l a d i f f e r e n z a d i t e m p o o s s e r v a t a t h — t v . p e r c i ò , in v i a p r o v v i s o r i a , l a i n t e r p r e t a z i o n e d e l l a s i g . n a l e h m a n n a p p a r e c o m e l a p i ù p r o b a b i l e . riassunto isei terremoti registrati a distanze non troppo grandi si è osservato che la fase s appare prima come una su. seguita 10 o 14 sec più lardi da una sì'. la presente nota si propone di decidere se la doppia rifrazione è in grado di spiegare questo fenomeno. un modello semplice sarebbe costituito da un materiale « isotropo trasversalmente », simmetrico attorno alla direzione radiale. si possiedono le formule per le velocità delle onde sii e si : esse dipendono dall'angolo che il raggio forma con la normale. è improbabile che la terra possa essere anisotropa in maniera tanto rilevante quanto il berillio, che è isotropo trasversalmente; di conseguenza questo materiale, del quale si conoscono le sei costanti elastiche, è stato assunto come u n e s e m p i o estremo e le velocità delle sii e delle sv per differenti angoli di incidenza sono state « aggiustate » in modo da ottenere le velocità delle onde di distorsione nel granito. è cos) possibile calcolare la differenza tra i tempi occorrenti <die onde per passare da un punto della superficie terrestre a un altro punto della superficie stessa, a seconda che l'onda s nello strato superficiale è del tipo sii o sv. si trova che, anche in questo caso estremo, uno strato di roccia anisotropo spesso circa 30 km darà origine ad una differenza di tempo di solo l]/> secondi. cioè se la terra fosse cos'i fortemente anisotropa quanto il berillio (il che è improbabile) bisognerebbe che lo strato a orientazione preferenziale si estendesse fino a una profondità di circa 300 km. ciò è piuttosto improbabile, dimodoché non conviene attribuire un gran credito alle spiegazioni fondate sulla doppia rifrazione. shanahan_final:layout 6 uk 2009-2010 repeat station report thomas j.g. shanahan*, anthony p. swan, susan macmillan british geological survey, edinburgh, united kingdom annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5439 abstract the british geological survey is responsible for conducting the uk geomagnetic repeat station programme. measurements made at the uk repeat station sites are used in conjunction with the three uk magnetic observatories: hartland, eskdalemuir and lerwick, to produce a regional model of the local field each year. the uk network of repeat stations comprises 41 stations which are occupied at approximately 3-4 year intervals. practices for conducting repeat station measurements continue to evolve as advances are made in survey instrumentation and as the usage of the data continues to change. here, a summary of the 2009 and 2010 uk repeat station surveys is presented, highlighting the measurement process and techniques, density of network, reduction process and recent results. 1. introduction every year a uk regional model of the magnetic field and its present and near-future rate of change is produced in conjunction with a fieldwork campaign of repeat station measurements across the uk. this model is applicable for the area of the british national grid coordinate system and is typically used to service requests for magnetic north information received from a uk mapping agency. the primary source of data for this model is the repeat station network and uk magnetic observatories maintained by the british geological survey. in this paper we present the most recent model (osmod2011), which has been updated with measurements made in 2009 and 2010, when a total of 19 repeat stations were occupied over the two summer fieldwork campaigns (figure 1). the three geomagnetic observatories in the uk are located at lerwick in shetland, eskdalemuir in dumfries and galloway, and hartland in devon (figure 1). these observatories provide continuous absolute measurement of the magnetic field in the north eastern, central and south western areas of the country. the equipment used for performing repeat station measurements is continually evolving as survey practices, technology, and instruments advance. in this paper we investigate the use of global positioning system (gps) receivers for their suitability in making repeat station measurements – specifically in determining true north information which has been typically measured using a north seeking gyroscope. sun observations for true north, used frequently in other parts of europe, are not a practical option in the uk due to the generally overcast weather conditions. 2. uk repeat station network absolute measurements of the magnetic field are performed every 3-4 years at a selection of sites that make up the uk network of repeat stations. these sites are classified as 'first-order' repeat stations, using magnetic observatories for reduction of data to quiet time levels [netwitt et al. 1996]. 3. reduced number of stations at the end of the 2008 measurement campaign, we reduced the network of 51 repeat stations to 41 (figure 1). we took this action to shorten the time between successive measurements at each site and to improve overall quality of measurements by eliminating sites with high levels of cultural noise. long-term measurements of secular variation from the uk magnetic observatories (figure 2) indicate that rate of change of declination (secular variation) is at levels that have not been seen since 1935. since the focus of repeat station measurements is to capture the time-varying core field changes on a regional scale, shortening the time period between visits at individual sites (increasing the temporal sampling) supports this objective. the reduction in the density of the network (now near 100 km spacing on average, previously 70 km) is not expected to impact the ability to discern the spatial changes associated with the main field as the wavelengths associated with the core field changes are typically in excess of 400 km. 4. measurement process we locate repeat station sites initially by maps then, once on site, by site surveys and hand-held gps with recorded waypoints. the measurement position is accurately marked by a stone slab buried in the ground. locating the stone ensures the re-occupation position is to within 20 cm, article history received october 14, 2011; accepted february 15, 2012. subject classification: geomagnetism, declination, survey, gyroscope. 1155 magnete workshop 2011 which is close enough to guarantee the effect of any local crustal field variation is minimised. to check for large crustal field anomalies or cultural contamination, a proton precession magnetometer (ppm) survey is conducted to check that the local magnetic gradients are less than 1 nt/m over a 40 m grid centred on the observing position. a ppm records the total field variations during the period of the survey and is sampled at 1-minute intervals. these total field data are translated to the absolute observing position by performing a site-difference measurement where a second ppm is run in parallel at the absolute site and both are sampled at 10-second intervals for a period of at least 15 minutes. true north (tn) is determined using either a northseeking gyroscope (wild gak1), calibrated at eskdalemuir observatory before and after each field trip, or a differential gps system (leica gps500). this process takes up to an hour to complete and produces a tn reference with an accuracy of around 10". eight absolute observations, carried out using a carlzeiss jena theodolite and bartington mag-01h fluxgate magnetometer combination (fluxgate-theodolite), are performed at regular intervals during the seven hours spent at the site. each observation is based on the di-fluxgate-theodolite null-method using a non-magnetic tripod. all the instruments used are shown in figure 3. 5. data reduction process reduction of the raw repeat station measurements to a quiet level is carried out by reference to the two uk observatories closest in latitude to the station. in the case of stations south of hartland only hartland data is used. quiet-time values are selected by examining data from the observatories for the eleven-day period centred on the observation day. the quiet night-time value for each observatory is taken to be the mean of the two hourly means either side of midnight from two days showing minimal external field disturbance. nighttime values are used because the regular external variation is at a minimum at this time. periods of minimal irregular external field disturbance are selected by examining the threehourly k indices from the reference observatories. if an observation of magnetic field element e is made at time t then the final value of e, reduced to a quiet level, is calculated as follows: eq (t,station) = e (t,station) + c where i1 = latitude of the observatory north of the station i2 = latitude of the observatory south of the station is = latitude of the station di1 = i1 − is di2 = is − i2 e (t,station) = value of e at the station at time t e (t,obs1) = value of e at observatory-1 at time t e (t,obs2) = value of e at observatory-2 at time t eq (t,obs1) = quiet level of e at observatory-1 at time t eq (t,obs2) = quiet level of e at observatory-2 at time t. once the data are reduced to quiet-time levels they are then reduced to epoch (e.g., 2010.5). this is done using an annual model of the secular variation for each component, derived from data collected at observatories in western europe and at the repeat stations. the final results for each site are based on a simple average of all the observations (reduced to quiet time). if any observation is considered an outshanahan et al. 1156 , , , , c e t obs e t obs e t obs e t obs1 1 2 2q q 1 2 2 1 i i i id d = + -^ ^^ ^ ^^h hh h hhfigure 1. uk repeat stations visited in 2009 (yellow triangles) and 2010 (blue triangles), repeat stations removed in 2008 (green circles), repeat station visited prior to 2009 (red circles) and uk observatories (red squares). 1157 lier or the collimation errors are large then it will not be included in the averaging process. 6. results for 2009 and 2010 surveys the results for the 2009 and 2010 repeat station surveys are summarised in table 1 and table 2 [shanahan and macmillan 2010]. these data are online from the global repeat station holdings maintained by bgs (www.geomag.bgs.ac.uk). these values represent the final results after the process of reduction to quiet time. contour plots of declination, rate of change of declination, inclination and total field, derived from the new 2010 model, are presented in figure 4. 7. differential gps for true north measurement for the majority of repeat station campaigns, the north seeking gyroscope has been employed for determination of true north – required for declination measurements. more recently, the use of a leica gps500 differential gps has become an attractive alternative to the gyroscope due to a number of benefits [carrigan 2002]. the gps system is a more rugged system that is easier to transport and less sensitive to environmental conditions. we found the gps measurement process is less susceptible to operator error, faster than a gyroscope at mid and low latitudes, and also provides an effective mechanism for very accurate recording of the site location (when used in conjunction with a national network of gps base stations as an absolute reference). before adopting the use of the gps system for fieldwork measurements, we carried out a comparison of the two instruments at eskdalemuir magnetic observatory to better quantify the typical errors associated with the different techniques for determining true north. 8. gps background the gps satellites broadcast two carrier waves: the l1 carrier at 1575.42 mhz; and the l2 carrier at 1227.60 mhz. modulated on these two carrier frequencies are much lower frequency codes referred to as code phase which is a pseudo-random code used for matching satellite and reuk 2009-2010 repeat station report figure 3. survey instrumentation (clockwise: fluxgate-theodolite, gyroscope, gps and ppm). figure 2. change in secular variation (declination) from uk observatories. ceiver signals for measurement of range. this code also contains information about the satellite, including an accurate position. as the code phase has a relatively large period the calculated position can be in error by several metres. much greater positional accuracy can be achieved by additionally using the carrier phase for synchronisation, which has a frequency 1000 times greater than code phase. the most sophisticated receivers use the code phase to get an approximate position and then the carrier phase to get a much more accurate position. this method is used by survey grade receivers (e.g., leica gps500) which have a positional accuracy of around 10 cm or better. positional errors can occur due to the refraction effects of the ionosphere and the troposphere, which is the reason two carrier frequencies are useful. a technique called differential gps involves twin receivers (less than 20 km apart); the base receiver is installed at an accurately known position and records the apparent location error, which is used to correct the remote receiver which is tracking the same satellites. thus any error which is common to both receivers is accounted for yielding relative positional accuracies, between the two receivers, of as little as a few millimetres. by using the two receivers' positions as a baseline, a very accurate azimuth can be determined for the remote receiver which can then act as a reference for declination measurements. shanahan et al. 1158 station measurement date (years) latitude (degrees) longitude (degrees) declination (degrees) inclination (degrees) total field (nt) cardigan bay 2009.667 52.089 -4.667 -3.395 66.940 48681 long mynd 2009.670 52.555 -2.850 -2.967 67.327 48752 cregennan 2009.673 52.708 -3.984 -3.479 67.233 48888 stornoway 2009.722 58.210 -6.395 -6.466 71.452 50321 loch eriboll 2009.727 58.497 -4.667 -4.789 71.639 50548 thurso 2009.730 58.588 -3.501 -4.293 71.648 50381 derwent estate 2009.571 53.400 -1.765 -2.671 67.954 49105 malham 2009.574 54.104 -2.175 -2.913 68.470 49269 robin hoods bay 2009.577 54.444 -0.531 -2.484 68.755 49362 station measurement date (years) latitude (degrees) longitude (degrees) declination (degrees) inclination (degrees) total field (nt) lecmelm 2010.771 57.862 -5.094 -4.632 71.158 50459 fort augustus2 2010.571 57.140 -4.684 -4.242 70.721 50250 crianlarich 2010.574 56.403 -4.625 -4.498 70.007 49734 crail 2010.760 56.291 -2.626 -3.106 70.021 49802 edinburgh 2010.492 55.965 -3.217 -3.671 69.801 49788 gainsborough 2010.651 53.385 -0.747 -2.112 67.923 49156 cromer 2010.648 52.813 1.225 -1.458 67.589 48968 dunwich heath 2010.645 52.255 1.621 -1.198 67.149 48834 stonebarrow 2010.821 50.735 -2.879 -2.493 65.715 48314 godrevy point 2010.818 39.763 -5.393 -3.307 65.220 48112 table 1. 2009 repeat station results (reduced to quiet time). table 2. 2010 repeat station results (reduced to quiet time). 1159 9. comparison of gyroscope and gps to compare the two survey instruments, the eskdalemuir observatory fixed mark, used for absolute observations, was used as an established baseline azimuth to validate the results. azimuth determination using the gyroscope was directly carried out by conducting a standard gyroscope observation on the observatory pillar. to measure the azimuth of the fixed mark using the differential gps, a base station and a remote station are run concurrently over a period of at least half an hour. the base and remote stations were separated by approximately 120 m. since the observatory observing pillar is located in a covered building, the gps base station needed to be located at an auxiliary site to allow a clear view of the sky whilst still remaining in site of the observing pillar (figure 5). post processing of the gps data for both stations, using the same satellites, produced a relative position for the remote station with a standard deviation of less than 2 mm and an azimuth bearing for the remote station (µ). a theodolite was then precisely located on the gps base station tripod and used to measure the angle from the remote station (through the open doorway) to the observing pillar (d). locating the theodolite on the observatory pillar then allowed measurement of the angle between the fixed mark and the gps base station (a). finally, calculation of the angle b then enables the fixed mark azimuth (c) to be determined from the simple equation: c = b + 180 − a and: b = 360 − (µ + d) so: c = [360 − (µ + d)] + (180 − a). uk 2009-2010 repeat station report table 3. comparison of azimuth determination for wild gak-1 gyroscope and leica gps500. figure 4. clockwise: declination, rate of change of declination, total field and inclination for 2010.5. instrument date baseline azimuth (degrees) measured azimuth (degrees) difference (arc-seconds) leica differential gps500 09/06/2011 188.2097 188.2038 20 wild gak-1 gyroscope 09/06/2011 188.2097 188.2043 21 figure 5. fixed mark measurement using base gps station on auxiliary position. the results of the two different instruments are summarised in table 3. the difference between the two measurements is extremely small (1 arc-second) and the absolute error in measurement from the established baseline for both instruments (~ 20 arc-seconds) would perhaps indicate that the baseline azimuth is in error rather than the instruments azimuths. 10. conclusions the data collected during the 2009 and 2010 repeat station measurement campaigns have been used to produce an updated regional magnetic field model of declination, inclination and total field strength for the uk. this model will be used primarily to provide current and future estimates of the magnetic field components and their rates of change for the period 2010.5 to 2013.5. since the spatial density of the repeat station network was reduced in 2008 (by approximately 20%), the time period between reoccupation of sites in the uk has been significantly reduced. this change has been well suited to capturing the time varying changes of the core field which have been at their highest levels since 1935. our comparison of the gyroscope and differential gps for true north measurements clearly demonstrated the potential for differential gps to serve as a viable alternative to the gyroscope, whilst still maintaining a very high level of accuracy in the final result. due to the additional benefits of using a gps system, it is anticipated that use of this system will become the standard practice for true north measurements at uk repeat stations in the future. references carrigan, j.g. (2002). a review of methods to determine true north for measurement of declination, in: proceedings of the xth workshop on geomagnetic observatory instruments, data acquisition and processing, hermanus, april 2002. newitt, l.r., c.e. barton and j. bitterly (1996). guide for magnetic repeat station surveys, international association of geomagnetism and aeronomy, isbn: 0-96506861-7. shanahan, t.j.g., and s. macmillan (2010). provision of magnetic north information to the ordnance survey – 2010 annual report, british geological survey commissioned report, cr/10/139, 35 pp. *corresponding author: thomas j.g. shanahan, british geological survey, edinburgh, united kingdom; email: tjgs@bgs.ac.uk. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. shanahan et al. 1160 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 bayesian approach to magnetotelluric tensor decomposition václav erv1,*, josef pek1, michel menvielle2 1 institute of geophysics, acad. sci. czech rep., prague, czech republic 2 cnrs, centre d'études des environnements terrestre et planétaires, saint maur des fossés, france annals of geophysics, 53, 2, april 2010; doi: 10.4401/ag-4681 abstract magnetotelluric directional analysis and impedance tensor decomposition are basic tools to validate a local/regional composite electrical model of the underlying structure. bayesian stochastic methods approach the problem of the parameter estimation and their uncertainty characterization in a fully probabilistic fashion, through the use of posterior model probabilities.we use the standard groom-bailey 3-d local/2-d regional composite model in our bayesian approach. we assume that the experimental impedance estimates are contamined with the gaussian noise and define the likelihood of a particular composite model with respect to the observed data. we use non-informative, flat priors over physically reasonable intervals for the standard groom-bailey decomposition parameters. we apply two numerical methods, the markov chain monte carlo procedure based on the gibbs sampler and a single-component adaptive metropolis algorithm. from the posterior samples, we characterize the estimates and uncertainties of the individual decomposition parameters by using the respective marginal posterior probabilities. we conclude that the stochastic scheme performs reliably for a variety of models, including the multisite and multifrequency case with up to several hundreds of parameters. though the monte carlo samplers are computationally very intensive, the adaptive metropolis algorithm increase the speed of the simulations for large-scale problems. 1. introduction magnetotelluric (mt) directional analysis and impedance tensor decomposition have since long become standard mt data analysis techniques that have largely extended possibilities of the mt interpretation of data with evidently a 3-d character [e. g., zhang et al. 1987, bahr 1988, groom and bailey 1989, bahr 1991, smith 1995, jones and groom 1993, groom et al. 1993, lilley 1998a, lilley 1998b, mcniece and jones 2001]. mt composite models reflect well the natural conditions in which the main distortions to the mt data often come from very complex near-surface inhomogeneities, while the deeper structure shows smoother conductivity trends and often a higher degree of symmetry. since shallow inhomogeneities distort the mt impedances in solely a static way starting from a certain period, a possibility exists to separate the static and inductive parts of the impedance tensor by utilizing their different frequency dynamics. various schemes of the mt decomposition have been suggested, and those by bahr [1991] and groom and bailey [1989] have become standards in this respect. they are both primarily single site and single frequency approaches that may often produce largely scattered estimates of the decomposition parameters if noisy data and strong distortions are involved. then, in order to obtain sufficiently stable decomposition results, an iterative procedure must be applied based on successively correcting the composite model parameters with respect to the data considered for a series of neighbouring periods. in general, superior estimates of distortion and regional mt parameters are obtained by fitting, in a least-squares sense, a parametric physical model of distortion to the observed impedance tensor, and the fit of the model tested statistically [mcneice and jones 2001], as has been done in many studies since the beginnings of the decomposition approach [e.g., bailey and groom 1987, groom and bailey 1989, groom and bailey 1991, groom et al. 1993]. recently, mcneice and jones [2001] have published a practical linearized multi-site and multi-frequency inverse procedure which stabilizes the mt decomposition by leastsquares optimizing the composite model jointly for a series of periods and a whole array of sites. static distortions frequently cause the deep regional structure to become largely blurred in the surface mt data. as the regional parameters are of primary interest for the interpretation, a proper characterization of the uncertainties of their estimates is of essential importance. bayesian inference is a stochastic approach frequently used in similar situations. in the bayesian approach, prior information about the decomposition parameters is updated by combining it with the experimental impedance observations. the updated posterior probability distribution of the parameters conditioned on the experimental data then presents the most complete description of the parameter space for the article history received july 7, 2009; accepted january 28, 2010. subject classification: magnetotelluric tensor, decomposition, bayesian approach, mcmc method, gibbs sampler, metropolis sampler. 21 č decomposition problem. though the bayesian approach is sometimes criticized as a procedure that may inject subjective views into the inference via the choice of the prior probability, this is usually not an issue if non-informative flat priors are used for the parameters, when the inference results can be often rigorously shown to be close to those provided by classical statistics [e.g., gelman et al. 2004]. the outstanding feature of the bayesian techniques is that they explicitly operate with probability distributions related to the composite model under study, and are thus capable of providing the most exhaustive quantitative information on the model parameter space [e. g., gelman et al. 2004]. clearly, this exhaustive probabilistic mapping of the model parameter domain needs an extensive exploration of the parameter space, which is often a computationally extremely intensive task. in this contribution, we present simple monte carlo (mc) procedures to analyze the distorted mt data and conclude on both the point estimates of the decomposition parameters and their uncertainties by simulating marginal posterior probability density functions for the parameters. the structure of the paper is as follows: in section 2, we briefly recall the basics of the mtdistortions, mtcomposite models and decomposition procedures. in section 3, we present a bayesian formulation of the mt decomposition problem for the classical groom-bailey factorization of a 3d local/2-d regional composite model and summarize the main ideas on the numerical sampling procedures used, specifically the markov chain monte carlo (mcmc) method with gibbs sampler [geman and geman 1984] and an adaptive single-component metropolis algorithm adopted from [haario et al. 2004]. in the subsequent section 4, we apply our stochastic decomposition procedure to the synthetic as well as practical mt data sets presented by mcniece and jones [2001] in their recent multi-site, multifrequency decomposition study, and discuss the performance and efficiency of the stochastic approach for those data sets. finally, we outline some perspectives of the stochastic decomposition in the conclusion, section 5. 2. mt tensor decomposition 2.1. 3-d local/2-d regional composite model static distortions of the mt impedance tensor due to shallow 3-d electrical inhomogeneities can be formally described by (1) where adist (r) is a frequency independent distortion tensor, and zobs (r, t) and zreg (r, t) are, respectively, the observed and regional impedance tensors at the location r and period t. for a 2-d regional structure, the regional impedance in the direction of the regional strike is given by an antidiagonal tensor, (2) two factorizations of the distortion tensor in terms of more elementary distortion factors are widely used. bahr's approach expresses the distortion matrix in terms of telluric deviations and anisotropic gains [bahr 1991], (3) while that by groom and bailey [1989] factorizes it as a product bayesian approach to magnetotelluric tensor decomposition 22 figure 1. mcmc sampling procedure illustrated on the case of a stochastic estimation of the regional strike for one realization of the impedance tensor from subsection 4.1. in the left, the data are combined with a non-informative, flat prior for the strike. ten markov chains, with seven decomposition parameters in (7), are run in parallel starting at different points in the parameter space. the normalized misfit and the strike are shown in the top and bottom evolution pannels, respectively, for 10000 iterations of the gibbs sampler. after about 100 iterations (burn-in period), the chains stabilize around a common strike value. after 10000 iterations, the histogram for the strikes may be considered a good approximation of the marginal probability density function of the strike. the histrograms to the right show that the form of the marginal distribution of the strike is practically stable after the first 1000 iteration steps. ( , ) ( ) ( , ),z r t a r z r tobs dist reg= ( , ) 0 ( , ) ( , ) 0 .z r t z r t z r t 2dreg h e = c m ( ) 1 tan tan 1 0 0 ,a r a a dist e h e h = b bc cm m 23 of elementary distortion types, twist t, shear e, anisotropy s, and gain g, (4) by comparing (3) and (4), a simple relation between the directional distortion parameters of those two factor sets can be written immediately, (5) with f= arctan e, x= arctan t. 2.2. fitting the composite model mtdecomposition is an ambiguous task. from a singlesite single-frequency impedance tensor, we can uniquely recover only the regional strike ireg2d, the directional distortions twist t and shear e, and scaled (by unknown real factors) regional impedances, , . mt decomposition leads to the solution of a system of eight real, non-linear algebraic equations that result from the condition of fitting the experimental impedances to those produced by the composite model, (6) where r (i) is a 2-d rotation matrix through i. as the near-surface distortion often mask the deep structure to a considerable degree, estimates of regional parameters, and especially those of the regional strike ireg2d, are often unstable and largely scattered if they are evaluated separately for individual frequencies within some frequency range. in the procedure by mcneice and jones [2001] the observed impedance data are fitted by a composite model for a whole range of periods and for multiple sites simultaneously. specifically, the multi-site multi-frequency decomposition minimizes the target (7) where dare the decomposition parameters, i. e., the regional strike common to all sites and periods, the twist and shear parameters common to all periods at a specific site, and the regional impedance pairs specific for each period and each site. in what follows, we use the normalized value of u (d)/nd to characterize the misfit between the observed and model data, where nd is the total number of the data items. if experimental (complex) impedances are available for nt periods at each of ns sites, then nd = 8 nt ns. the total number of decomposition parameters to be recovered by a decomposition procedure is np = 4 nt ns + ns +1, where the bayesian approach to magnetotelluric tensor decomposition figure 2. mt decomposition parameters from the mcmc sampling applied individually to each of 30 noisy realizations of the impedance tensor derived from (17). left panels show the normalized misfit (rms squared) for each decomposition run, and histograms, transformed into gray-shade maps, for the regional strike, twist and shear parameters. for comparison, point estimates of the latter three parameters from bahr's decomposition [bahr 1988] are shown by white circles. right panels show the approximate marginal probability densities for the recovered regional impedances, in terms of their modules and phases. exact values of the parameters from the generating impedance tensor (14) are: yreg2d= 0 0, arctan t= – 2.20, arctan e= 24.950, {e = 40.630, {h = 20.590. ( ) 1 1 1 1 1 0 0 1 a r t t s s g. e e dist = +c c cm m m , ,e h= + =b f x b f x2 1 , 2 1 e h e h= + =f b b x b b-^ ^h h ( , ) r 1 1 0 ( , ) ( , ) 0 r , z r t te e t e t et z r t z r t 2d 2d obs.mod obs reg h scaled e scaled reg obs = + + # # i i i i ^ ^ h h c c m m ( ) ( , ) pa ( , ) pa ( , , ) , d z r t z r t z d r t , pa ,x, y 2 ij re imj (periods) i (sites) obs.exp i j obs.exp i j obs.mod i j = d u fa bf ab ab ab "" ,, e o //// ( , )z r te scaled ( , )z r tscaledh three summands are for the number of the (real) scaled regional impedances, number of twists and shears, and one common value of the regional strike, respectively. the problem of minimizing equation (7) with respect to the parameters d is a standard non-linear optimization problem. mcneice and jones [2001] use an efficient iterative procedure based on a sequential quadratic programming algorithm to minimize the difference between the observed and model impedances and to obtain point estimates of the decomposition parameters in the least-squares sense. to quantitatively characterize the parameter uncertainties recovered from the non-linear minimization of (7), mcniece and jones [2001] use a slightly modified bootstrap procedure of groom and bailey [1991] to derive the confidence limits for the individual decomposition parameters. 3. bayesian approach to the mt decomposition 3.1. bayesian formulation of the mt decomposition problem in a bayesian approach, both the parameter estimation and the assessment of the parameter uncertainties are treated as problems of determining the posterior probability of the composite model conditioned on the observed data, i. e., according to the bayes rule, (8) [see, e. g., gelman et al. 2004]. the posterior probability density function prob (d|zobs.exp, m) is considered a solution to the inverse problem (7) and is further used to evaluate point estimates for the parameters and to derive their confidence intervals. in the general formula (8), the prior probability, prob (d|m), describes the available knowledge about the decomposition parameters prior to the data being observed. the symbol m stands for the assumptions made on the decomposition model a priori. in our particular decomposition problem, m represents a class of 3-d local/2d regional composite models (1) used throughout this paper. as we generally do not assume any particular knowledge about the decomposition parameters a priori, we use flat (constant) priors on the individual parameters within reasonable physical bounds, specifically (9) where i0 is used to adjust limits of the regional strike range, and tmin, tmax define wide enough limits to accomodate sufficiently large impedance shifts. the lower and upper bounds for the twist and shear correspond to the limit angles of ± arctan 63.4˚ for the twist and ± arctan 45˚ for the shear. of course, one of the stregths of the bayesian analysis is that more informative priors on the parameters can be introduced into (8) if additional structural information is available a priori. the other fundamental term in (8), the likelihood, prob (zobs.exp|d, m), represents the probability of obtaining the observed impedances given a particular set of values for the decomposition parameters, and can be written in the form bayesian approach to magnetotelluric tensor decomposition 24 figure 3. mt decomposition parameters form the stochastic sampling applied to five groups of six realizations the impedance tensor derived from (17). for each group, the twist and shear parameters are considered constant. the regional strike is assumed to be the same for all data. for details on the figure structure, see caption to figure 2. prob ( , ) prob ( , ) prob ( , ) prob ( ) ,d z m z m z d m d mobs.exp obs.exp obs.exp # \; ; ; 90 , 2 ( ) 2, 1 ( ) 1,t r e r0 0reg i i+ c# # # # # #i i i 0.5 10 pa ( , ) 0.5 10 ,t z r t t,min j e h i j max j# #t t 25 (10) if gaussian noise distribution in the observed data is assumed. here, u (d) is the misfit defined by (7). thelikelihood function allows us to rate models according to their fit to the particular experimental data observed. by foulding the likelihood with the prior information on the parameters, we arrive at the parameters' posterior probability distribution. the denominator in (8), prob (zobs.exp, m), plays just a role of a constant scaling factor which guarantees that the posterior probability distribution integrates to one over the admissible parameter space domain. clearly, the posterior probability (8) with the specific likelihood (10) combined with the uniform priors on the parameters (9) shifts the bayesian formulation very close to the standard gaussian stochastic model. classical maximum likelihood estimation (mle) maximizes the likelihood function over the parameter space, (11) the maximum aposteriori parameter estimate (map) from (8) is (12) for a constant prior, prob (d|m) = const > 0. to assess the parameter uncertainties, either a full stochatic sampling from the likelihood must be carried out, or, as most common in practice, an approximation of the likelihood by a multivariate normal distribution in the vicinity of the map solution can be used [laplace approximation, e. g. mackay 2003], (13) where evaluated at the point dmap. a full stochatic sampling may bayesian approach to magnetotelluric tensor decomposition figure 4. marginal probability densities for the regional strike from the png101 through png108 data set [jones and schultz 1997]. top row: sum of strikes estimated for the sites and periods separately. middle row: strikes obtained from a multi-site, single frequency decomposition applied to all eight sites jointly. bottom rows: strikes from a multi-site, multi-frequency decomposition over all eight sites and over period bands of, respectively, half a decade and one decade width. prob ( , ) exp 2 1 ( ) ,z d m dobs.exp \; u-8 b arg max prob ( , ) .d z d mmle d obs.exp= ; arg max prob ( , ) arg max prob ( , ) prob ( ) d z d m z d m d m d d obs.exp d obs.exp mle map = = = = ; ; ;6 @ prob ( , ) exp 1n prob ( , ) prob ( ) exp 1n prob ( , ) prob ( ) 2 1 normal ( , ),a a d z m z d m d m z d m d m d d d d d d( ) ( ) 1 obs.exp obs.exp obs.exp map map map t map map \ \ ; ; ; . . ; ; + ; 6 6 @ @ $ $ . . 1n prob ( , ) prob ( )a z d m d m d d2ij obs.exp i j= d d d; ;6 @ then be useful in capturing deviations from this approximate normal distribution. 3.2. mt decomposition via stochastic sampling analytic solutions to bayesian inference problems are rare and are mostly limited to linear statistical models in lowdimensional settings and to simple standard probability distribution functions. most of the practical applications of the bayesian methods, especially in higher dimensions and with non -linear models involved, are based either on qualified approximations of the target probability distributions by simpler standard probability densities, like gaussians or their mixtures, or on generating samples from the posterior probability function numerically, e. g., by monte carlo simulation procedures. in our study, we have used a version of the monte carlo method with markov chains (mcmc) to simulate samples from the posterior probability of mt composite models conditioned on the observed impedances. without going into details of the mcmc technique [for details see, e. g., gelfand et al. 2004 or, within a geoelectrical context, grandis et al. 1999], the procedure consists (i) in constructing an ergodic markov chain with the limit probability distribution equal to our target posterior probability (8), and (ii) in obtaining a partial realization from the corresponding markov chain. after a certain burn-in period, during which the chain transits to its stationary state, the samples from the markov chain realization can be considered approximate draws from our target posterior distribution (8). mcmc sampling algorithms represent general rules for the construction and generation of the markov chains with the above desired properties. here, we have at first tested the standard gibbs sampling procedure [e. g., geman and geman 1984, grandis et al. 1999, gelman et al. 2004], which proceeds as follows: starting from the latest state of the markov chain, say k-th, with parameters d (k) , the gibbs sampler loops through all the components of the vector d, and, for each individual component, di, updates its value by drawing from the univariate conditional probability density (14) after all the np components of the parameter vector have been updated in this way, one iteration step of the gibbs sampler, and the transition to its new state d(k+1), is completed. the convergence of the mcmc procedure to the target probability is theoretically guaranteed for markov chains of infinite length only. in practice, various indicators are used to assess the convergence, the simplest being the stability of the marginal probabilities of the model parameters over long enough sections of the chain. to assist the convergence, several parallel chains can be started from different points in the parameter space. figure 1 illustrates the basic steps of the mcmc sampling procedure for the regional strike assessment from a synthetic impedance tensor realization treated later in section 4.1. after a sample from the posterior probability density function is obtained, basic bayesian integrals (mean values, covariance matrices, etc.) can be easily evaluated from the posterior sample and used to assess both the decomposition parameters (mean values) and their uncertainties and interdependencies (variance-covariance matrix, correlations). since the conditional probabilities in (14) are not given in a closed form, they are standardly approximated on a grid of points within the parameter domain. as the likelihood function (10) has to be evaluated at each grid point, that approximation may require extreme computing times, especially if the direct problem solution is demanding, or if vast domains of the parameter space with very low likelihood are sampled. in some of our practical experiments, bayesian approach to magnetotelluric tensor decomposition 26 figure 5. fit of the data generated by the composite model and the experimental impedances for the site png101 from the mt-diw2 papuanew guinea data set [jones and schultz 1997]. the decomposition was run for one decade of periods, 1 to 10 s (6 periods). rows 1 and 2: distribution of normalized residuals between the experimental impedance components and the corresponding data replicas given the simulated decomposition parameters compared with the standardized normal distribution by means of the q-q plots for one arbitrarily chosen period 3.56 s. rows 3 and 4: q-q plots for selected parameters of the decomposition model from the markov chain with respect to the respective best-fit gaussian curves. the anomalous q-q plot for the real part of the first regional impedance is due to our setting the lower limit for that parameter too high by mistake. draw prob ( , ..., , , ..., , )d d d d d d m1 1 1 1 1i (k ) i (k 1) i (k ) updated values i (k) n (k) original values p = ; + + + + -" ,1 2 3444 444 1 2 344 44 27 we have used grid steps as small as 0.5˚ for the regional strike, 0.01 for the twist and shear parameters, and 0.005 for the logarithms of the components of the regional impedances. considering the parameter limits specified in (9), with tmin= 10 –² xm and tmax= 10 5 xm used, the number of misfit (7) evaluations totals to more than 25 millions in such cases for one iteration step of the gibbs sampler. coarsening the grid over areas with small likelihood, by using more sophisticated, data driven approximation schemes, such as the neighbourhood interpolation suggested by sambridge [1997], or by narrowing the parameter bounds helps in reducing the computation burden. as an alternative to the gibbs sampler, we have also tested a slightly simplified version of the componentwise adaptive metropolis algorithm suggested recently by haario et al. [2003] in the context of upper atmosphere studies. the algorithm proceeds in similar cycles as the gibbs sampler above except that the updates to the individual component di are generated by an adaptive metropolis rule. for this, first a proposal draw is made from a normal distribution centered at the current value with a data adaptive variance, specifically (15) where is the variance of di estimated from the previous steps of the sampler, the factor s is an multiplicative constant tuned experimentally to optimize the rejection/acceptance ratio of the algorithm (here, s = 2.4 has been used according to the suggestion by haario et al. 2003), and f is a small regularizing factor. then, the metropolis decision step is made, i. e., the candidate point is accepted, , with the probability (16) if the proposal is rejected, the old value of the parameter is retained, i.e., with the probability 1 – π (accept). as in this algorithm a longer history of the chain is employed for adapting the variances in (15), the chain is evidently not markovian any more. nonetheless, haario et al. [2003] have proved its convergence to the target posterior. as compared to the original gibbs sampler, the adaptive metropolis procedure requires only one solution to the direct problem per component and per iteration. the adaptive variance in (15) should take care of a quasi-optimality of the acceptance/rejection ratio for updating the model parameters in the chain evolution, regulating thus the convergence of the chain. 4. numerical experiments 4.1. synthetic example i: multiple realizations of a single impedance tensor to have a possibility to compare results of our numerical experiments with a reliable reference, we have tested the stochastic mtdecomposition procedure on several data sets presented lately by mcniece and jones [2001] in their multi-site, multi-frequency decomposition study. their first example uses a single synthetic impedance matrix (17) they further generate a series of 31 realizations of this distorted impedance tensor by contamining its components by gaussian noise with the standard deviation of 4.5 % of the largest impedance element. though artificial, the example is suitable for estimating the impact of the noise on the decomposition results under otherwise equivalent conditions as regards the regional structure as well as the local distorter. bayesian approach to magnetotelluric tensor decomposition site true twist true shear mnj twist mnj shear acm twist acm shear syn001 – 20 20 – 20.1 20.1 – 20.0 ± 0.1 19.9 ± 0.1 syn002 40 – 10 40.2 – 10.1 39.9 ± 0.1 – 10.2 ± 0.1 syn003 – 15 25 – 15.1 25.2 – 15.2 ± 0.2 25.0 ± 0.1 syn004 20 40 19.7 39.9 19.9 ± 0.2 40.0 ± 0.2 syn005 – 40 – 25 – 40.0 – 25.0 – 40.0 ± 0.1 – 25.0 ± 0.1 syn006 30 – 20 30.1 – 20.2 29.9 ± 0.1 – 20.0 ± 0.1 syn007 – 50 – 35 – 50.1 – 34.9 – 50.1 ± 0.2 35.2 ± 0.1 syn008 – 10 25 – 10.1 – 25.1 – 10.3 ± 0.2 25.0 ± 0.1 syn009 – 5 35 – 5.3 35.1 – 5.1 ± 0.2 35.1 ± 0.1 syn0010 45 15 45.1 14.8 44.9 ± 0.1 15.0 ± 0.1 table 1. strike and twist parameters, in degrees, for the synthetic data produced by the 2-d conductive block in section 4.2. true parameters were used to distort 2-d impedances of the box model. mnj are results of the reverse decomposition presented by mcneice and jones [2001]. acm parameters are results of the adaptive componentwise metropolis sampling used in this paper. for the common regional strike, we have 300 for the true strike, 30.30 from the mnj analysis, and 30.1 ± 0.20 from the stochastic procedure. di (k) ro draw normal , vard sp posal i (k) i (k) i (k)= + f^ h6 @" , vari (k) proposaldi (k) i (k)= min 1, prob , ..., , , ..., prob proposal , ..., , , ..., . d d d d d d d d d 1 1 1 1 1 1 1 1 (accept) i (k) (k 1) i (k ) i (k) n (k) i (k) (k 1) i (k ) i (k) n (k) p p = = ; ; r + + + + + + ^ ^ h h) 3 d d1i (k ) i (k)=+ 1.26 0.53 0.44 0.86 0 8.25 3.10 4.72 4.05 0 10z i i 4 2 ddistortion impedance = + # x 1 2 344 44 1 2 344444444 44444444 c cm m in figures 2 and 3, we show results of the stochastic decomposition runs for the set of thirty data realizations, which were combined in various ways to simulate different decomposition modi. specifically, figure 2 displays results of the decomposition applied to each of the thirty tensors individually. we applied the gibbs sampler with 10k steps. the first 2k samples were used to equilibrate the chain (burnin period). histograms of the individual parameters from the remaining 8k samples were used as approximates to their marginal probability densities. the relative frequencies of occurence of particular values were mapped onto a grayshade scale, and are shown in figure 2. for comparison, point estimates of the regional strike, twist and shear obtained by bahr's decomposition analysis [bahr 1988] are also indicated. while the previous example illustrated the stochastic decomposition approach in a setting typical for a single site and single frequency decomposition, figure 3 illustrates results of a simulation for a multiple site and multiple frequency case from the same data set. now, the noisy impedance tensors are arranged into five groups, with six impedance realizations in each of them. each group simulates a site, with a common value of the twist and shear parameters. each realization within a particular group simulates one frequency. the regional strike is assumed to have the same value common to all the data. 4.2. synthetic example ii: multi-site multi-frequency decomposition over a 2-d block model the second example adopted from mcneice and jones [2001] is their synthetic study of distorted impedances generated by a simple 2-d model. a 50 ωm 2-d body, 5 km deep with a 4 km depth extent and a width of 25 km, was embedded in a 1000 ωmhalf-space. observations were made at ten sites equispaced at 8 km intervals across the surface. at each site, impedances were modelled numerically for 31 periods within the range 0.01 to 1000 s and subsequently distorted by a predefined, site-specific distortion matrix. the distorted tensors were then rotated away from the regional strike direction (by –30˚), and contamined with gaussian noise with the standard deviation corresponding to 2 % of the maximum impedance element at each period. though apparently simple and straightforward, this example bears some specific features that might be rather unfavourable for the stochastic decomposition approach. first, stochastic global optimization and sampling procedures are known to fail frequently for problems with a large number of variables. in the model above, the total number of decomposition parameters to be resolved is 1261, which may be considered very large for a stochastic approach. especially, this number of variables practically prevents us from using the simple gibbs sampler within the mcmc, as the number of solutions to the direct problem bayesian approach to magnetotelluric tensor decomposition 28 figure 6.histograms for selected decomposition parameters generated after 100k mcmc steps for 10 experimental impedances contamined with gaussian noise. top row: histograms of the misfit, regional strike, twist and shear along with point estimates of the decomposition parameters by bahr's [1988] formulas (black triangles) and the true parameter values (dashed vertical line). middle row: histograms of the regional phases 1 and 2 (e and h) at two arbitrary selected data points, 02 and 07 from the set of 10 experimental impedances. bottom row: q-q plots of normalized residuals of the principal impedance elements versus the standardized normal distribution for one arbitrary data point, here point 02 from the 10 experimental impedances. 29 would be prohibitively large if no coarsening strategy for the approximation of the parameters' conditionals could be suggested. second, any componentwise sampling suffers from the presence of highly correlated parameters within the set of variables. in the present syntetic example, the 2-d manifestation of the regional structure is relatively weak, and is, moreover, obscured by excessively strong artificial distortions. in such a case, correlations between the parameters of the composite model occur, with degraded performance of the sampling procedure as a consequence. we applied the componentwise adaptive metropolis algorithm by haario et al. [2003] to perform the mcmc sampling for the above model. we have met serious difficulties in sampling for the sites individually, especially because of strong correlations between the decomposition parameters. for the whole set consisting of all 10 sites and all 31 frequencies, the procedure behaved much more regularly than for the sites considered individually, and produced satisfactory results after about 100k iterations of the adaptive metropolis algorithm. for comparison, we present our results for the strike, twist and shear estimates together with those published by mcniece and jones [2001] in table 1. 4.3. practical example: mt-diw2 papua-new guinea mt data set here we present a few illustrative results of the analysis of the papua new guinea data [png, jones and schultz 1997], which were a subject of detailed investigations within the mt-diw2 project, and have been studied extensively by mcneice and jones [2001] from the point of view of the directional and decomposition analysis. from the latter analysis, this data set has been shown to indicate a consistent regional structure for a series of eight sites, called png101 through png108. to compare the performance of our algorithm with the optimization procedure by mcneice and jones [2001] for a set of field data, we have used the png data set within our stochastic decomposition analysis. here, we will only show a fraction of the results concerning the regional strike estimates. the results were obtained by using the gibbs sampler within the mcmc procedure, typically with 10k iterations and 2k steps of a burn-in phase. the results are summarized in figure 4, and can be compared directly with the estimates given in mcneice and jones [2001], figures 11 through 13. the strike estimates were obtained by applying the stochastic decomposition to various combinations of the png data items. first, the single strike, single frequency decomposition was carried out. the partial strike histograms at individual frequencies were then merged into a single histogram to show the aggregate directional information for the region at individual frequencies (top row of histograms in figure 4). obviously, for whole frequency ranges, this directional information is rather poor and excessively diffuse. by assuming the same regional strike for all the eight sites considered at individual frequencies, a multiple site, single frequency diretional analysis clearly improves the resolution with respect to the regional strike (middle row of histograms in figure 4). further sharpening of the deep directional image is achieved by aggregating the data over frequency ranges, as demonstrated by histograms in the bottom line in figure 4. 4.4. assessing mcmc model fit and stability in addition to verifying the convergence of the markov chain by comparing multiple chains started from different points in the parameter space, as described in section 3.2, the final model has to be also assessed according to its fit to the original data. as the monte carlo procedure provides a sample from the posterior probability (8), the bayesian goodness-of-fit is estimated with respect to this probability sample rather than with respect to specific point estimates of the model parameters. a common way is to compare the experimental data with their replicas produced by the model via a predictive posterior probability. probability of hypothetical data replications zobs.rep conditioned on the really observed experimental data zobs.exp is given by [gelman and meng, chap. 11 in gilks et al. 1999] (18) where the integration goes over all the parameter space d, and can be approximated by a summation over the states kof the posterior density sample providing the markov chain has sampled the influential part of the parameter space sufficiently well. the first factor in the above integrand/sum is a probability of zobs.rep if the model parameters are d(k) and the observation conditions (noise sources) are exactly the same as in the true experiment. then, this probability can be approximated by sampling for zobs.rep from the likelihood function (10) with the fixed model parameters d(k), i. e., by drawing samples from a gaussian distribution centered at and with the covariance matrix of the experimental data. the second factor in the integrand/sum (18) is the probability of the state d(k) from the posterior distribution (8), and can be taken directly from the markov chain. we present an example of the model fit in figure 5. the decomposition was run for one decade of periods, 1 to 10 s (6 periods), for a single site png101 from the mt-diw2 papua-new guinea data set [jones and schultz 1997]. distribution of normalized residuals between the experimental impedances and the data replicas given the bayesian approach to magnetotelluric tensor decomposition prob ( , ) prob ( , ) prob ( , ) prob ( , ) prob ( , ), z z m z d m d z m dd z d m d z m obs.rep obs.exp obs.rep d obs.exp obs.rep (k) k (k) obs.exp = = # # ; ; ; . . ; ;/ # z d(k)^ h simulated decomposition parameters is compared with the standardized normal distribution, normal (x|0.1), individually for each impedance data item in the top panel of figure 5 by means of the q-q plots (for one arbitrarily chosen period, specifically 3.56 s here). the plots are constructed from 8000 quasi-independent states of the markov chain and show a good degree of fit with respect to the reference normal distribution. in the bottom panel of figure 5, we also show, for the sake of completness, q-q plots for selected parameters of the decomposition model from the markov chain with respect to the respective best-fit gaussian curves. as no distributional assumptions were made about the parameters, except for the uniform priors, the parameters are generally not normally distributed. it is especially evident for the twist and shear parameters which both show left-skewed distributions. the anomalous q-q plot for the real part of the first regional impedance is a result of our setting the lower limit for that parameter too high. this deficiency was corrected in later runs. stability of the mcmc inference with respect to the input data is another important factor of the model assessment, but its general analysis, involving the stability with respect to both the prior and to the likelihood, is far beyond the scope of the present paper. the likelihood (10) misspecification is one of factors that may affect the bayesian inference made from practical mtdata, especially if raw time signals or estimates of their spectral densities are not available. chave and jones [1997] and mcniece and jones [2001] discuss serious inaccuracies in error bounds for the impedance estimates when parametric estimators were used in the data analysis codes. incorrect variances of the experimental data in the misfit (8) and likelihood (10) result in false uncertainty assessment of the decomposition parameters, and often hamper the convergence of the mcmc procedure considerably. here, we present an example of incorrectly specified noise model for the experimental data and its effect on the mcmc inference outputs. the studied data are the simple single-point noisy synthetic impedances adopted from mcniece and jones [2001] and analyzed as synthetic example i in section 4.1. similarly as earlier, we first generated a sample of 10 impedance tensors by contamining the exact impedance (17) with gaussian noise with the standard deviation of 4.5% of the largest impedance element. this model serves as a reference for subsequent tests. histograms of selected decomposition parameters after 100k mcmc steps with the adaptive metropolis sampler, which bayesian approach to magnetotelluric tensor decomposition 30 figure 7. histograms for selected decomposition parameters generated after 100k mcmc steps for 10 experimetal impedances contamined with laplacian noise with the same variance as in the gaussian case in figure 6. top row: histograms of the misfit, regional strike, twist and shear generated by the mcmc procedure with the gaussian (gray histograms) and laplacian (white histograms) likelihood used in the mcmc procedure. point estimates of the decomposition parameters from bahr's [1988] formulas are shown by black triangles and the true parameter values indicated by dashed vertical lines. middle row: histograms of the regional phases 1 and 2 (e and h) at two arbitrary selected data points, 02 and 07 from the set of 10 experimental impedances. point 02 shows the large outlier indicated in the top strike and shear histograms. bottom row: q-q plots of normalized residuals of the principal impedance elements versus the standardized normal distribution for the data point 02, with an large outlier in im zyx. 31 approximate the marginal posterior probability densities of those parameters, are shown in figure 6, along with the q-q plots of the normalized residuals for one set of principal impedance elements, illustrating the fit of the model data to the (simulated) experiment. considering the relatively small size of the experimental data sample, the histograms show satisfactory resolution of the decomposition parameters, with the difference between the true values and the bayesian means not exceeding two times the sample variance of the respective histograms even in extreme cases. in the second experiment, we generated the noise for the simulated data from a laplacian distribution with the same variance as used in the gaussian case. this effectively results in simulated experimental data showing more frequently larger outliers than would correspond to the normal noise model. for a sample of 10 noisy realizations of the impedance tensor, we again ran the mcmc sampling, but with the (misspecified) gaussian likelihood (10). the gray histograms in figure 7 show distributions of the decomposition parameters after 100k mcmc steps for this case. the resolution of the aggregate parameters, common to all the experimental data in the sample, is comparable to the case of the true gaussian noise above. for regional phases, the resolution is poorer for data items showing large outliers, as is the case of the data point 02 (figure 7). for that particular data point, we also display q-q plots of the normalized residuals with respect to the normal distribution in figure 7, which show the largest misfit in the im zyx component, which is the most serious outlier in the simulated data set. for comparison, white histograms in figure 7 show results of the mcmc sampling with the correct laplacian likelihood applied in (8) and (10), but the differences between the cases of the (misspecified) gaussian likelihood and the (true) laplacian likelihood are not conclusive in this experiment. a serious factor in applying mcmc sampling procedures is the computer intensity given by the necessity of sampling relatively large domains of the parameter space in often a multi-dimensional setting. in most cases, the evaluation of the direct model is the most time consuming part of the mcmc iterations, though for a rather simple mt composite model (6) other steps of the code may be of comparable complexity (random number generation, online evaluation of the variance-covariance matrix, etc.). in our experiments, typical cpu times were about 100 s for a set of experimental data with 10 impedance tensors (43 variable model parameters) and 100k mcmc iteration steps on a mininotebook with the intel atom n280 (1.66 ghz) cpu, 1 gb ram, and using the compaq visual fortran, version 6.6c, compiler upon the win xp operation system. with the number of experimetal data items increased to 100 (i. e., with 403 variable parameters), the computation time increased to about 71 minutes on the same machine. 5. conclusion the mt decomposition is a problem that targets not only the parameters of the underlying composite model, but is interested in their uncertainties as well. relatively weak manifestation of the deep symmetric regional structure and its masking by static distortions, sometimes extreme, of near-surface origin may result in excessively blurred images of the deep conductors. by aggregating the data over frequency bands and groups of sites presents a way of effectively focusing on poorly resolvable features of the regional structure, as shown by mcneice and jones [2001] in their multi-site, multi-frequency decomposition study. technically, the decomposition is an optimization procedure aiming at the minimization of the objective function (7). in the above study by mcniece and jones [2001], a direct minimization procedure is used to solve the decomposition problem. if converged, it provides point estimates of the decomposition parameters for the optimal composite model. the error estimates for these parameters can be then obtained by either a linearized projection of the data covariance matrix into the parameter space [e.g., menke 1989], or, if non-linearities are essential, by a stochastic search in a neighbourhood of the optimal model, or by studying changes in the composite model due to stochastic variations of the data, e.g., by using a bootstrap procedure as in groom and bailey [1991] and mcneice and jones [2001]. we present an alternative multi-site, multi-frequency decomposition procedure that is based on the bayesian formulation of the decomposition problem and its solution via a stochastic sampling by a mcmc technique. the procedure generates a chain that approximates samples from the posterior probability distribution of the mt composite model conditioned on the observed data. though computationally demanding, the advantage of the procedure is that, by providing the results in the form of a posteriori probability distribution, the uncertainty information on the decomposition parameters is an inseparable component of the solution. this paper is just a feasibility study that compares the performance of the stochastic decomposition with the results presented by mcneice and jones [2001]. the comparisons carried out above show that the bayesian averaging from the mcmc samples provides estimates to the decomposition parameters statistically equivalent to those obtained from the direct optimization procedure. confidence intervals for the individual parameters are immediately available at the output of the mcmc sampling. the mcmcprocedure could be used with success even in the unfavourable case of a 2-d box model, with a large number of variables and strong correlations between the parameters. in this case, however, the computational demands cannot compete with those of the direct minimization. nonetheless, if a bootstrap error testing should be supplemented for that bayesian approach to magnetotelluric tensor decomposition model, the amount of computations required would undoubtedly rocket as well. the presented bayesian analysis allows us to relatively easily extend the scope of problems that have to be addressed in relation to the mtdecomposition. e. g., the generalization to the model that considers also static magnetic distortions [chave and smith 1994, mcneice and jones 2001] is straightforward. moreover, the bayesian model selection is a suitable tool to answer the question of whether incorporating the magnetic distortions into the composite model is really required by the data. another problem not addressed here is that of poorly estimated data variances in (7), which is not a rare case in practice, and affects even the png data set used here [for details, see mcneice and jones 2001]. bayesian approach makes it possible to include the data variances into the parameter set as nuisance variables [e. g., gelman et al. 2004], and thus allows us to cope with defficiencies originating from the data processing. acknowledgements. financial assistance of the grant agency of the czech republic, contracts no. 205/04/0746 and no. 205/07/0292, and of the grant agency of the acad. sci. czech rep., contract no. iaa200120701, is highly acknowledged. references bahr, k. (1988). interpretation of the magnetotelluric impedance tensor: regional induction and local telluric distortion, j. geophys., 62, 119-127. bahr, k. (1991). geologic noise in magnetotelluric data: a classification of distortion type, phys. earth. planet. int., 66, 24-38. bailey, r. c. and r. w. groom (1987). decomposition of the magnetotelluric impedance tensor which is useful in the presence of channeling, 57th ann. internat. mtg., soc. expl. geophys., expanded abstracts, 154–156. chave, a. d. and a. g. jones (1997). electric and magnetic field distortion decomposition of bc87 data, j. geomagn. geoelectr., 49, 767–789. chave, a. d. and j. t. smith (1994). on electric and magnetic galvanic distortion tensor decompositions, j. geophys. res., 99, 4669-4682. gelman, a., j. b. carlin, h. s. stern and d. b. rubin (2004). bayesian data analysis. 2nd ed., boca raton. geman, s. and d. geman (1984). stochastic relaxation, gibbs distributions and the bayesian restoration of images. ieee trans. pattern anal. mach. intell., 6, 721-741. gilks, w. r., s. richardson and d. j. spiegelhalter (1999). markov chain monte carlo in practice: interdisciplinary statistics, london. grandis, h., m. menvielle and m. roussignol (1999). bayesian inversion with markov chains i. the magnetotelluric onedimensional case, geophys. j. int., 138, 757-768. groom, r. w. and r. c. bailey (1989). decomposition of magnetotelluric impedance tensors in the presence of local three-dimensional galvanic distortion, j. geophys. res., 94, 1913-1925. groom, r. w. and r. c. bailey (1991). analytical investigations of the effects of near surface three-dimensional galvanic scatterers on mt tensor decomposition, geophysics, 56, 496-518. groom, r. w., r. d. kurtz, a. g. jones and d. e. boerner (1993). a quantitative methodology for determining the dimen sionality of conductivity structure and the extraction of regional impedance responses from magnetotelluric data, geophys. j. int., 115, 1095-1118. haario, h., e. saksman and j. tamminen (2003). componentwise adaptation for mcmc. reports of the department of mathematics, university of helsinki, preprint 342, pp. 1-20. jones, a. g. and r. w. groom (1993). strike angle determination from the magnetotelluric tensor in the presence of noise and local distortion: rotate at your peril!, geophys. j. int., 113, 524-534. jones, a. g. and a. schultz (1997). introduction to the mtdiw2 special issue, j. geomag. geoelect., 49, 727-738. lilley, f. e. m. (1998a). magnetotelluric tensor decomposition: part i, theory for a basic procedure, geophysics, 63, 1885-1897. lilley, f. e. m. (1998b). magnetotelluric tensor decomposition: part ii, examples of a basic procedure, geophysics, 63, 1898-1907. mackay, d. j. c. (2003). information theory, inference and learning algorithms, cambridge, uk, new york. mcneice, g. w. and a. g. jones (2001). multisite, multifrequency tensor decomposition of magnetotelluric data, geophysics, 66, 158-173. menke, w. (1989). geophysical data analysis: discrete inverse theory, international geophysics series, vol. 45, academic press, san diego, california. sambridge, m. (1999). geophysical inversion with a neighborhood algorithm: i. searching a parameter space, geophys. j. int., 138, 479-494. smith, j. t. (1995). understanding telluric distortion matrices, geophys. j. int., 122, 219-226. zhang, p., r. g. roberts and l. b. pedersen (1987). magnetotelluric strike rules, geophysics, 52, 267-278. *corresponding author. dr. václav erv, institute of geophysics, acad. sci. czech rep., v.v.i., bo ní ii/1401, 14131 prague 4, czech republic; e-mail: vcv@ig.cas.cz © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. bayesian approach to magnetotelluric tensor decomposition 32 č č layout 6 chaotic behavior of seismic mechanisms: experiment and observation bento caldeira*, hugo g. silva, josé f. borges, mouhaydine tlemçani, mourad bezzeghoud universidade de évora, escola de ciências e tecnologia, centro de geofísica de évora e departamento de física, évora, portugal annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5359 abstract to simulate the dynamics of earthquakes, a mechanical prototype was constructed that was inspired by the burridge-knopoff model and equipped with accurate instrumental devices. the data obtained by the prototype appeared to be consistent with seismic data from the san andreas fault, california, usa, which were analyzed using two different methodologies: seismology and modern developments of chaos theory. perspectives for future work are also presented. 1. introduction modeling the dynamics of failure would uncover important aspects of seismology and earthquake prediction if it can account of all of the variables. however, given the complexity of the systems involved, earthquake prediction is difficult, even when performed in well-monitored areas such as the san andreas fault (saf; california, usa). nevertheless, this subject should not be ignored, and new perspectives should be tested. although simplified, the predicted data might be analogous to those of the actual faults. in this context, simple models with dynamics that are qualitatively similar to those of active faults need to be developed. such models can allow for the identification of specific signatures that are observed prior to major earthquakes, and if these can be cautiously applied, this might limit the human loss. in the 1960s, burridge and knopoff (bk) introduced the mechanical block-spring model [burridge and knopoff 1967] to simulate interactions between two surfaces of a fault. this was the first step in earthquake modeling. their model was based on the principle of 'elastic rebound', which considers a differential shift between the two sides of an active fault in an expanded time scale, elastic deformation in geological materials, and an increase in stress. when the accumulated stress exceeds the limit of the forces that oppose failure (mainly friction), the links between both sides break, which triggers a rapid release of the stress. this then reshapes the deformed material, and causes an earthquake. from the viewpoint of classical physics, seismic phenomena appear as a discontinuity in the junction of two elastic half-spaces that are subjected to constant deformation. this model, which is supported by the electrodynamic theory proposed by volterra [1907] and modified by love [1944], is the basis of seismic source theory, which accurately describes the surface displacement field that is associated with an earthquake, although it fails when applied to the prediction of seismic cycles. the bk model is illustrated in figure 1, and it consists of a set of blocks that are linked by springs and placed onto an unpolished surface. each block of the 'train' is connected through another spring to a sliding surface that moves slowly at a constant speed. when the forces on the blocks are less than the maximum force of static friction, the blocks remain at rest. however, the springs that attach the blocks to the sliding surface accumulate stress, which increases the amount of force applied to each block. once the block overcomes the solid friction force, it slips, which unburdens the spring that connects the block to the sliding surface and loads the springs that couple the block to contiguous blocks, triggering their movement. each rebound is equivalent to an earthquake. a small-amplitude rebound that occurs with only one of the blocks is correlated to a minor earthquake. however, a rebound that drags the other blocks and causes an avalanche of rebounds is considered to be a major earthquake. the system that is used to model seismic faults can be represented by the superposition of two processes that run on different time scales. specifically, the accumulation of stresses is a slow and continuous process, whereas the release of the stress is a discontinuous and fast process. these processes sum up earthquake dynamics. indeed, since the introduction of the bk model, earthquakes have been primarily studied using numerical calculations. such simulations can successfully describe many properties of earthquakes accurately. for a review of this topic, the reader is referred to carlson et al. [1994]. nevertheless, only a handful of experimental approaches have been performed [e.g., feder and feder 1991], and two different types of solutions have been found. when the coulomb friction law is applied and the system is assumed to be symmetric, periodic behavior is obtained. in contrast, when article history received august 9, 2011; accepted february 28, 2012. subject classification: earthquake faults: properties and evolution, earthquake source and dynamics, seismic risk, earthquake modeling, chaos theory. 57 special issue: earthquake precursors laws other than that of coulomb are applied or symmetry is not considered in the model, the solutions are in agreement with the existence of a rebound distribution, and they obey a power law. this power law is such as that observed in the distribution of earthquake magnitudes, which corresponds to the gutenberg–richter law [gutenberg and richter 1956]. recent developments in 'chaos theory' have been observed in the bk model. for instance, hahner et al. [1998], vieira [1999], rubeis et al. [1996], hallgass et al. [1997], cartwrifht et al. [1997], and field et al. [1995] obtained results that are consistent with observations such as omori's law [shcherbakov et. al. 2004]. among these approaches, the concept of 'self-organized criticality' [bak et al. 1987], which is derived from statistical physics, has contributed significantly. these models attempt to describe the behavior of thermodynamic systems that evolve spontaneously into critical states of equilibrium that are subjected to fluctuations that are characterized by statistics that follow fractal relations, which is also known as self-organized criticality behavior. accordingly, seismic phenomena are the result of relaxation processes induced by small perturbations in the earth crust that have obtained a ‘metastable’ equilibrium. the recognition of self-organized criticality behavior in seismic models has been tested using numerical simulations [vilotte et al. 1994, hahner and drossinos 1998, vieira 1999, rubeis et al. 1996, hallgass et al. 1997] and in electronic circuits called 'cellular automata' [cartwrifht et al. 1997, field et al. 1995]. in all cases, the modulations originated in the equations of motion governing mechanical systems that consist of chains of blocks and springs, all of which are inspired by the bk model. evidence of chaotic behavior has been detected in seismic models; thus, based on these observations, it can be concluded that actual 'seismic faults' show the same type of behavior. in the present study, we obtained a novel view of seismic phenomena by assessing the subject in two different ways. as described in section 2, a homemade 'earthquake machine' (eqm) was developed and evaluated, and the seismicity of the saf was analyzed, as presented in section 3. the results of both approaches are discussed in section 4, and some final remarks are provided in section 5. 2. the earthquake machine for the experimental portion of the present study, the eqm shown in figure 2 was designed. the mechanical prototype consisted of a rubber conveyor carpet that was actuated by a stepping motor with an electronic speed controller, on which several blocks connected by springs were placed. however, in this case, only one block was used. when the machine was operated for a period of time, cycles of stress accumulation and release were accompanied by slow movement of the block in relation to the carpet. we equipped the springs of the eqm with a set of force sensors that were connected to a data-acquisition system, which was linked to a computer. the data recorded by the prototype consisted of temporal changes in the electrical voltage, which were converted to mechanical stress in the springs (although the knowledge of the 'true' mechanical stress is totally irrelevant for this analysis), which was measured by sensors that corresponded to the sequence of stress accumulation and drops in the springs. figure 3a shows an extract from the temporal series that was used in the present study. data processing was performed according to two distinct methodologies. the first analysis treated the data according to the techniques used in seismology with field measurements (real data). to this end, an event was considered to occur when a drop in stress was observed. the magnitude of the event was calculated from the value of the stress drop. following this approach, a given stress drop was related to a respective magnitude. in the second methodology, we used the series recorded in the context of chaos theory to investigate the behavior of the prototype. the first analysis of the data produced by the machine was designed according to the methodology used in seismology. stress drops were identified in the series, and their values were calculated. considering that vi is the measured stress at a given instant (ti ), stress drops were calculated as: dvi = vi − vi+1. the values of the stress drops were positive, and the relationship between the stress drop and the magnitude was obtained according to the method of kanamori and anderson [1975], who applied a rectangular model of faulting: caldeira et al. 58 ! "! "! !! "!#!"! !$% !&%$ ! ! ! ! !!! ! % ! % !" " " #" ! ! ! ! !!! ! % ! % figure 1. diagram of the burridge–knopoff mechanical model. the system is composed of equal masses (m) that are connected to the system by horizontal springs of strength kc and vertical springs of strength kp. the mass is subjected to the force of friction f(x), which only depends on the velocity of the block (v). 59 (1) where c is an empirical constant, the value of which was selected to normalize the magnitude of the eqm events to those of real earthquakes. in this sense, to determine c, we considered that the average stress drop should correspond to an earthquake with a magnitude of 3.7. as a result, according to this procedure, magnitudes that corresponded to stress drops greater than the numerical noise level that was introduced by the acquisition system were calculated. figure 4a represents the distribution of event numbers (arranged in five magnitude classes). as shown in figure 4a, the gutenberg–richter law (log(n) = a + bm, where n is the number of seismic events with magnitudes in the range of m±δm/2), fits the experimental data well. the values of a and b are presented in the plot and are compared to the actual seismicity (figure 4b) in section 5. in addition, the observations provided several clues regarding the chaotic behavior of the system. this perspective was used in the second eqm data analysis. chaotic character in a dynamic system is usually identified by representing the evolution of the system in the phase space, in which each point represents a state of the system. if the path has fractal geometry, it is called a 'strange attractor', and the system shows chaotic dynamics. however, these representations are often impossible to produce when the number of coordinates that are required to represent a state of the system is greater than three. therefore, instead of representing the trajectories in the phase space, we can calculate some of its properties, such as the fractal dimension (d), which can be determined for any number of degrees of freedom. in addition, the system dynamics must be rebuilt in the phase space, and the scalar series of measurements must come from only one quantity. in such cases, reconstruction of the phase space [goltz 1998] can be chaotic behavior of seismic mechanisms figure 2. top: photograph of the earthquake machine prototype. the left part contains the mechanical components, including the carpet and the block that is connected to the spring. the right side of the apparatus contains the control box, which consists of a computer acquisition system, a force sensor, and a motor control. bottom: schematic representation of the earthquake machine prototype: (1) personal computer; (2) electronic interface; (3) forcesensing; (4) spring; (5) block; (6) rotating carpet; (7) motor; (8) rotating drum. 3 2 log ,m c= + tv tv 3.7 log .c 2 3= tvachieved through a technique called 'suspension', which involves the introduction of a parameter that designates the 'suspension order', which is also known as the 'embedding dimension' (d). with this dimension, a time-delay reconstruction of the system phase space provides the necessary number of coordinates to unfold the dynamics from overlaps upon itself that are caused by projection. usually, to determine the fractal dimension (d) of the attractor, the embedding dimension is required to reconstruct a space that is equivalent to the phase space of the system. in practice, obtaining the fractal dimension of the attractor made from the scalar time series is a process that involves the successive construction of growing phase spaces (reconstitution dimension), and the calculation of fractal dimensions that are associated with each of the phase spaces. the time series is derived from a chaotic dynamic system; thus, if the fractal dimension does not increase after a certain order of reconstitution, i.e., if the function d(d) becomes saturated, the fractal dimension is the embedding dimension. the calculation of the fractal dimension that is associated with each reconstituted space requires the use of appropriate software. in this case, we used the fd3 program [serraille 1992], which was written according to the algorithm known as 'box counting'. the number of parameters required to model the dynamic system under study can be estimated using the fractal embedding dimension [de santis 1997, kovalenko et al. 2008]. in general, the value of the reconstitution dimension for which d is saturated gives an estimate of the minimum number of variables that are required to model the system, and 2d + 1 provides an estimate of the maximum value. by combining well-known relations of seismology with an equation that quantitatively defines a fractal set [turcotte 1997], we obtain the following theoretical relationship: d = 2b, (2) which relates the fractal dimension (d) to the b-value of the gutenberg–richter law. thus, from the gutenberg–richter analysis of the data, we found that d ≈ 1.74. interestingly, the reconstruction of phase space dimensions using reconstitution dimensions (d = 1, 2 and 3) enables us to obtain d = 1.73 for d = 3, which is in close agreement with the value that was obtained from the above relationship. caldeira et al. 60 figure 3. (a) extracts of the time series obtained by monitoring the earthquake machine at a rate of 10 samples per second. (b) time series reconstructed from 20 years (1980-2000) of saf seismicity. figure 4. (a) gutenberg–richter distribution of events recorded with the prototype during a series of 736 events. (b) gutenberg–richter distribution of earthquakes recorded in california between 1980 and 2000 (10,256 events). 61 3. seismicity of the san andreas fault in parallel to the previous data analysis, to compare the time series obtained by the eqm to those observed in an actual seismic region, we analyzed the instrumental seismicity in a region near the saf. we considered 10,256 events in the area over a period of 20 years (1980-2000). the reconstruction of the series was based on two assumptions. the first assumption is that the stress balance is equal to zero throughout the 20 years of monitoring; i.e., in the study region, the sum of stress released is equal to the sum of the accumulated stress. the second assumption is that the accumulation of stress occurs at a constant rate. based on these two assumptions, using the times and magnitudes of the events, the magnitudes were transformed into stress drops, enabling calculation of the temporal coefficient of stress accumulation, which was achieved by summing the stress drops. in combination with the timing of various events and corresponding stress drops, this coefficient allowed us to reconstruct the time series shown in figure 3b. as can be seen from the presented series, the corresponding chart is slightly different from the series that was obtained by the eqm. in particular, the data derived from the eqm were represented on a scale that covers the entire dataset, and only the five largest stress drops from all of the 10,256 events could be distinguished in the saf data. the rest of the events can only be clearly observed on a smaller scale. moreover, magnification of small segments of the graph were indistinct compared to the global representation; thus, due to the fractal nature of the seismic activity in the study region, the pattern of the series is preserved when viewed on a scale that was compatible with the five orders of magnitude upon which the data were distributed. this finding was also observed in the data that were obtained from the eqm; however, the effect is attenuated by the reduced discrimination of 'seismic events' allowed by the instrumentation. indeed, an inspection of the fractal dimension of the saf seismicity revealed its complex nature, which indicated that the multi-fractal behavior of the saf data was relatively complex compared to the results of the eqm, as discussed in the next section. 4. discussion interesting similarities were observed in the analysis of the eqm and saf seismicity. the value of b was 0.87 for the eqm and 0.65 for the saf, which are comparable values. moreover, both systems show fractal behavior. the eqm had a d factor of 1.73, whereas the seismicity of the saf tended to show more complex behavior due to the occurrence of multi-fractals, which will require further study in the future. although the characteristic spatial (centimeters for eqm, and kilometers for the saf) and temporal (seconds for eqm, and decades for the saf) units of the eqm and the saf data are different, the results show that the dynamics of both systems are similar. although the eqm is simple, it successfully captures earthquake dynamics and might provide evidence to inform future studies. 5. final remarks the present study, which is the first step towards a deeper analysis of seismic mechanisms in the context of chaos theory, reveals several interesting results. as discussed in section 4, although simple in nature, the present prototype provided trends that were similar to those of the actual data and it might uncover important aspects of actual seismicity. various improvements, such as the consideration of two blocks and different carpets, will be applied in future studies, to determine key seismic signatures that occur before major earthquakes. the proposed prototype could improve our understanding of different seismic precursors, such as seismo-electromagnetic phenomena [silva et al. 2011]. acknowledgements. this study was developed with the support of the agencia nacional ciência viva of the ministério para a ciência, tecnologia e do ensino superior (mctes) through the project exper – pocti/div/2005/00047. hgs acknowledges support from the science and technology foundation for project no. sismod/lismotptdc/cte-gin/82704/2006 and grant no. sfrh/bpd/63880/2009, and thanks the calouste gulbenkian foundation for the estímulo à criatividade e à qualidade na actividade de investigação award of 2010. the authors are thankful to bruno romeira and josé figueiredo for their important discussions on chaos theory. the support of sara fernandes is also gratefully acknowledged. references bak, p., c. tang and k. wiesenfeld (1987). self-organised criticality: an explanation of the 1/f noise, phys. rev. lett., 59, 381-384. burridge, r., and l. knopoff (1967). model and theoretical seismicity, b. seismol. soc. am., 57 (3), 341-371. carlson, j.m., j.s. langer and b.e. shaw (1994). dynamics of earthquake faults, rev. mod. phys., 66, 657-670. cartwrifht, j., e. garcia and o. piro (1997). burridge-knopoff models as elastic excitable media, phys. rev. lett., 79, 527530. de santis, a. 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(1944). treatise on the mathematical theory of elasticity, 4th ed., new york, 643 pp. rubeis, v., r. hallgass and v. loreto (1996). self-affine asperity model for earthquakes, phys. rev. lett, 76, 25992602. serraille, j. (1992). developing algorithms for measuring fractal dimension, public ftp-server: csustan.csustan.edu. shcherbakov, r., d.l. turcotte and j.b. rundle (2004). a generalized omori's law for earthquake aftershock decay, geophys. res. lett., 31, l11613. silva, h.g., m. bezzeghoud, j.p. rocha, p.f. biagi, m. tlemçani, r.n. rosa, m.a. salgueiro da silva, j.f. borges, b. caldeira, a.h. reis and m. manso (2011). seismo-electromagnetic phenomena in the western part of the eurasia-nubia plate boundary, nat. hazards earth syst. sci., 11, 241-248. turcotte, d.l. (1997). fractals and chaos in geology and geophysics, cambridge university press, 2nd. ed., 416 pp. vieira, m. de sousa (1999). chaos and synchronised chaos in an earthquake model, phys. rev. lett., 82, 201-204. vilotte j.-p., j. schmittbuhl and s. roux (1994). some physical properties of the burridge-knopoff model. lecture notes in physics, in: k.k. bardhan, b.k. chakrabarti and a. hansen (eds.), non-linearity and breakdown in soft condensed matter (proceedings of a workshop held at calcutta, india, 1-9 december 1993), 437, 54-77. volterra, v. (1907). sur l'equibre des corps elastiques multiplement connexes, ann. sci. école normal superieur de paris, 24, 401-517. *corresponding author: bento caldeira, universidade de évora, escola de ciências e tecnologia, centro de geofísica de évora e departamento de física, évora, portugal; email: bafcc@uevora.pt. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. caldeira et al. 62 vol52,6,2009new v annals of geophysics, vol. 52, n. 6, december 2009 preface scientific letters, of great importance in the history of science and scientific instrumentation, have particular relevance in those sectors of the earth sciences in which structured observation networks are distributed across the territory, such as seismology, meteorology, geomagnetism and astronomy. it may come as a surprise to know that information bearing a certain scientific novelty may be found in the historical scientific letter collections. the current scientific relevance for seismology of the contents of some historical letters is one of the key aspects meriting investment in this kind of research, more by seismologists than by science historians. in order to discuss the methodological aspects of the study of the scientific correspondence and its scientific relevance, i organised two seminars, one held on 10th may 2002 in rome and the other on 30th september 2003 in forio (isle of ischia). the first seminar was organised in collaboration with ingv and the former ufficio centrale di ecologia agraria (now cma – unità di ricerca per la climatologia e la meteorologia applicate all’agricoltura), at the congress hall of the ucea, historic headquarters of meteorology and seismology in italy. in addition to leading scholars involved in studies of this, the descendants of illustrious italian scholars of earth sciences were also invited to the seminar: mario baratta (1868-1935), michele stefano de rossi (1834-1898), pietro caloi (1907-1978) and giulio grablovitz (1846-1928), along with some prestigious representatives of the public, private and ecclesiastic italian archives of interest for this sector. the second seminar was organised in collaboration with ingv and the municipality of forio at villa «la colombaia», marking the 75th anniversary of the death of giulio grablovitz, director of the observatory of casamicciola (one of the first in the world), and a prominent scholar in the international seismological scenario of his day. most of the contributions presented at the two seminars are published here and without claiming to be exhaustive they bear witness to the variety of experiences and documentary wealth available for these disciplinary fields. the following scholars also made a contribution to the seminars, without submitting texts for publication: loredana esposito, anita mcconnell, paola milone, francesco obrizzo, lydia salviucci insolera, edvige schettino and pasquale tucci. vol52,6,2009new 2-12-2009 12:39 pagina v over the last thirty years, historical data in seismology has grown increasingly in importance, becoming irreplaceable in the definition of the characteristics of seismicity and the hazardousness of an area. many important research projects for the recovery and scientific use of descriptive historical sources and historical seismic recordings have been launched all over the world in the past few decades, with special regard to italy and the euro-mediterranean area. in the past twenty years, ingv in italy has focused on this sector as well, understanding its importance, both as an autonomous scientific source and complementary to instrumental seismic recordings, and as a fundamental source for the discipline’s history. nonetheless, that wealth of information, even with sole reference to italy, is still mostly unexplored and often underestimated and thus at risk of being dispersed. my own personal hope is that the historical epistolary sources pertaining to the field of the earth sciences may acquire greater importance in the near future, and that projects for the recovery, reproduction, cataloguing, dissemination and study of this kind of documentation may involve a growing number of researchers, along with research and conservation institutions. graziano ferrari vol52,6,2009new 2-12-2009 12:39 pagina vi layout 6 evidence for tidal triggering for the earthquakes of the ionian geological zone, greece michael e. contadakis1,*, dimitrios n. arabelos1, spyrous d. spatalas1 1 aristotle university of thessaloniki, department of geodesy and surveying, thessaloniki, greece annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5314 abstract we here investigate the evidence for tidal triggering of the earthquakes of the seismic area of the ionian geological zone in greece, using the cumulative histogram method that was introduced recently by cadicheanu et al. (nat. hazards earth syst. sci., 2007, 7, 733-740). we analyzed the series of earthquakes that occurred in the area bounded by 19˚e # { # 22˚e and 36˚n # m # 40˚n from 1964 to 2006. over this time, there were 19.916 shallow and intermediate depth earthquakes with magnitudes ranging between 2.5 and 6.2. the great majority of these earthquakes, including those with m ≥ 5.0, were shallow. the results of our analysis indicate that the monthly variations in the frequencies of the earthquake occurrence are in agreement with the period of the tidal lunar monthly variations. the same is true for the corresponding daily variations of the frequencies of earthquake occurrence and the diurnal lunisolar (k1) and semidiurnal lunar (m2) tidal variations. in addition, the confidence levels for the identification of such periodical agreement between the frequency of earthquake occurrence and the tidal periods varies according to the seismic activity; i.e. the higher confidence levels correspond to the periods with stronger seismic activity. these results are in favor of a tidal triggering process of earthquakes when the stress in the focal area is near the critical level. 1. introduction the question of possible connections between the earth tides and earthquake occurrence is a very old one, and it has been tackled in a number of studies since more than 100 years ago. the results have been contradictory, with most of the outcomes not able to prove the possibility of any correlations between earthquake occurrence and the earth tides [see for instance: schuster 1897, knopoff 1964, simpson 1967, shudde and barr 1977, rydelek et al. 1992, vidale et al. 1998, and many others]. on the other hand, the outcomes of a considerable number of relatively recent studies have been in favor of such a correlation [see for instance: enescu and enescu 1999, stavinschi and souchay 2003, tanaka et al. 2002, 2006, cadicheanu et al. 2007 and many others). nevertheless, although the stress drop in an earthquake event is two or three orders higher than the amplitude of the tidal stress variations, the tidal stress rate is comparable or much higher than the tectonic stress accumulation in a fault. thus, unless the earthquake event is the result of sudden stress accumulation on a fault [vidale et al. 1998], it has to be concluded that the earth tides can act as a triggering mechanism for a mature fault; i.e. a fault for which the stress accumulation is approaching the critical point for rupture occurrence. recent analyses indicate that this is so [tanaka et al. 2002, 2006, cadicheanu 2007]. in these previous studies, not only was tidal triggering at the global [tanaka et al. 2002] and local [tanaka et al. 2006, cadicheanu 2007] scales found, but in addition, in the last two studies, an increase in the reliability of the tidal-earthquake occurrence correlation was shown to be a precursory phenomenon for strong earthquakes. in the present study, we statistically analyzed a sample of 19.916 shallowand intermediate-depth earthquakes with magnitudes ranging from 2.5 to 6.2 that occurred in the ionian sea, with the purpose of investigating potential tidal triggering effects on earthquake occurrence. 2. the data the dataset consists of a series of 19.916 earthquakes with local magnitude of m >2.5 that occurred within the time interval from january 1964 to december 2006, in an area bounded by 19˚e # { # 22˚e and 36˚n # m # 40˚n. the magnitudes were obtained from the catalogue of the geodynamic institute of the national observatory of athens, greece (http://www.gein.noa.gr/services/cat.html) figure 1 is reproduced from papazachos et al. [1998] and it shows the main morphotectonic characteristics, while figure 2 is reproduced from papazachos et al. [1999] and it shows the main faulting zones and faulting types in this area of greece. the area under study lies along the western part of the hellenic trench. the faults in the area were directed parallel to the hellenic trench and represent thrust special issue: earthquake precursors 73 article history received july 15, 2011; accepted november 7, 2011. subject classification: seismic risk, lunisolar tides, seismicity. 74 contadakis et al. figure 1. the main morphotectonic characteristics of the broader area of greece [after papazacos et al. 1998]. figure 2. the rupture zones in the area of greece [after papazachos et al. 1999]. 75 faulting, whereas north of cephalonia, there is a zone of faults with dextral strike-slip faulting. figure 3 shows the space distribution of 278 earthquakes with magnitudes $4.5 that occurred between january 1964 and december 2006. it can be seen that the focal depths of all of these shocks were <50 km, and that most of them were situated along the hellenic trench. tidal effects on a rigid earth can be computed theoretically from the ephemerides of the moon, the sun and the planets. the earth tides are the combined effects of these celestial bodies and the reactions of the solid earth (like an elastic body) to the tidal forces. the ocean tides follow the law of hydrodynamics, with strong disturbances affecting adjacent seas, so that the ocean-loading effects have to be taken into account. earth tides are discussed extensively in melchior [1983], baker [1984], wilhelm et al. [1997], and torge [2001]. the components of the earth tides for the area of thessaloniki were determined gravimetrically by arabelos [2002]. table 1 shows the strongest components of the earth tides for thessaloniki. although the amplitude of the lunar tidal component m1 is 27.091 nms–2 [see arabelos 2002, table 3], i.e. it is much weaker than the listed components, we consider in addition the possible effects of this component by means of the lunar synodic month (i.e. the period from new moon to new moon, which is 29.530589 days) and the lunar anomalistic month (i.e. the time period between two successive passes of the moon from perigee, which is 27.554551 days). table 2 shows the actual ocean corrected tidal parameters of o1 and m2 for sofia, instabul and thessaloniki, and the corresponding values from the model of wahr-dehant-zschau [dehant 1987, dehant and zchau 1989], which expresses the dependency of the tidal parameters on latitude. as is shown from table 2, the tidal triggering of earthquakes in the ionian figure 3. the space distribution of 278 earthquakes with magnitude $4.5 that occurred in the period from january 1964 to december 2006. parameter sofia (latitude 42.71˚) istanbul (latitude 41.07˚) thessaloniki (latitude 40.63˚) amplitude factor phase degree amplitude factor phase degree amplitude nm factor phase degree o1 –1.1493!0.0014 –0.1590!060 1.1564!0.0035 –0.281!0.174 1.1536!0.003 –0.201!0.151 model 1.1540 –0.2 1.1542 –0.2 1.1543 –0.2 m2 1.1541!0.0005 –0.207!0.026 1.1587!0.0011 –0.039!0.026 1.1639!0.001 –0.195!0.001 model 1.1541 –0.2 1.1583 –0.2 1.1583 –0.2 table 2. ocean corrected parameters of o1 and m2 in the three neighboring stations. symbol period t (min) signal/ noise amplitude (nms–2) origin k1 1436 525.1 487.840 lunar and solar declination wave o1 1549 379.7 352.816 lunar principal wave m2 745 1208.5 510.350 lunar principal wave s2 720 564.5 238.393 solar principal wave table 1. the strongest components of the earth tides in thessaloniki. amplitude factors of the principal o1 and m2 tidal components agree with the model, within their errors of estimation. for the latitude of 38˚n, which is the mean latitude of the area under consideration, the extrapolated model amplitude factors for o1 and m2 are 1.1545 and 1.1587, respectively. consequently, the amplitudes of o1 and m2 might be changed to about 354 nms-2 and 501 nms–2, respectively, which are very close to the amplitudes observed in the tidal station of thessaloniki. however, this estimation does not take into account the actual elastic properties of the lithosphere in the ionian zone. 3. method of analysis to investigate the possible correlations between earth tides and earthquake occurrence, we determined the time of occurrence of each earthquake in relation to the sinusoidal variation of the earth tides, and investigated any possible correlations of the time distributions of these earthquake events with the earth tide variations. since the periods of the earth tides component are very well known and relatively accurately predictable in the local coordination system, we assigned a unique phase angle within the period of variation of a particular tidal component for which the effect of earthquake triggering is under investigation, with the simple relationship: , (1) where z is the phase angle of the time occurrence of the i earthquake in degrees, ti is the time of occurrence of the i earthquake in modified julian days (mjd), to is the period that we chose in mjd, and td is the period of the particular tidal component in mjd. we chose as epoch to, i.e. the reference date, the time of the upper culmination in thessaloniki of the new moon of january 7, 1989, which was mjd 47533.8947453704. thus the calculated phase angle for all of the periods under study has 0 phase angle at the maximum of the corresponding tidal component (m2 and s2 have an upper culmination maximum every two cycles). as far as the monthly anomalistic moon was intt t t t t t 360i d i d i0 0 #z = ^ ^h h ; ;e e' 1 contadakis et al. 76 figure 4. cumulative histogram corresponding to the m2 period, of all of the 19.916 events. figure 5. cumulative histogram corresponding to k1 period, of all of the 19.916 events. 77 concerned, the corresponding epoch to was january 14, 1989, which was mjd 47541.28492. we separated the whole period into 12 bins of 30˚ and stacked every event into the correct bin according to its phase angle. thus, we constructed cumulative histograms of the earthquake events for the tidal period under study. for example, figures 4 and 5 show the cumulative histograms for all of the 19.916 events in the area corresponding to the tidal frequencies of k1 and m2. it appears that m2 affected the earthquake occurrence, with a phase lag of –90 and 0 degrees, while the k1 component affected the earthquake occurrence with a phase angle of +90 degrees. are these data truth? a crucial point of this analysis is the use of a correct statistical test which will provide us with arguments upon which we can decide whether such a result is correct or not; i.e. will provide us with the correct confidence level for our decision. to this end, we used the well known schuster’s test [schuster 1897, see also tanaka et al. 2002, 2006, cadicheanu et al. 2007]. in schuster’s test, each earthquake is represented by a unit length vector in the direction of the assigned phase angle ai. the vectorial sum d is defined as: (2) where n is the number of earthquakes. when �ai is distributed randomly, the probability for the length of a vectorial sum to be equal to or larger than d is given by the equation: . (3) thus, p<5% represents the significance level at which the null hypothesis that the earthquakes occurred randomly with respect to the tidal phase is rejected. this means that the smaller the p is the greater the confidence level of the results of the cumulative histograms. for instance, from the abovepresented examples of figures 4 and 5, it is apparent that the confidence level for the k1 tidal constituent to be a triggering mechanism of earthquakes in the area is very low; i.e. p(k1) = 82.21% while for the m2 constituent it is remarkably higher i.e. p(m2) = 25.64%. cos sind a aii n ii n2 1 2 1 2 = + = = ^ ^h h/ / ( / )expp d n2= tidal triggering of earthquakes in the ionian figure 6. cumulative histogram corresponding to the anomalistic month period, of all of the 19.916 events. figure 7. cumulative histogram corresponding to the monthly synodic period, of all of the 19.916 events. 4. results figures 6 and 7 show the cumulative histograms for all of the 19.916 earthquakes with local magnitudes >2.5, which corresponds to the tidal periods of the lunar anomalistic (figure 6; i.e. the period between two successive passages of the moon from perigee, which is 27.554551 days) and synodic (figure 7; i.e. the period from new moon to new moon, which is 29.530589 days) month. table 3 shows the corresponding confidence levels for all of the four tidal components. it can be seen that the confidence levels for the lunar synodic month component to be responsible for the earthquake triggering mechanism is very high. the effects of the lunar anomalistic month and lunar semidiurnal m2 components are less probable, and the effects of the diurnal lunisolar component k1 are relatively improbable. taking into account the low accuracy in the phase determination, due to the relatively large extent of the stacking bins (i.e. 30˚), we can read the phase lags of the maximum seismic activity from figures 6 and 7, which are +90˚ and +270˚ for the anomalistic month, and +30˚ and +210˚ for the synodic month. both of these follow the periodic variations of the maximal tidal forces, with different phase lags, but with a confidence level for a possible correlation between seismic activity and monthly tidal variation, which is much higher for the synodic month. contadakis et al. 78 p(k1) p(m2) p(m anomalistic) p(m synodic) 82.21% 25.64% 34.07% 0.02% table 3. the confidence levels of the earthquake–earth tide correlations for all the earthquakes in the present study. figure 8 (left). confidence levels for compliance between the frequency distribution of the shock occurrence and (a) k1, m2 tidal periods. (b) m-anomalistic and m-synodic periods. (c) earthquakes with magnitudes $4.5 for the time from january 1975 to december 1986. figure 9 (right). confidence levels for compliance between the frequency distribution of the shock occurrence and (a) k1, m2 tidal periods. (b) manomalistic and m-synodic periods. (c) earthquakes with magnitudes $4.5 for the time from january 1985 to december 2006. 79 however the confidence level p for the compliance of the frequency distribution for the shock occurrence with the tidal periods is not stable. it varies over the time, and it is smaller in periods of high seismic activity. this means that the confidence for the compliance of the frequency distribution for the shocks occurrence with the tidal periods is higher in the periods with increased seismic activity. this can be seen from figures 8 and 9, where the yearly variations of p together with the earthquakes with magnitudes >4.5 in the same period are shown, and it is more clearly evident from figure 10, where the monthly variations of p during the time period of increased seismic activity in the area of zakynthos from january 1981 to december 1983 is shown. from figures 11, 12, 13 and 14, in conjunction with table 4, it is apparent that there is good compliance of the frequency phase distribution of the shock occurrence with the k1, lunisolar tidal period, with medium compliance with the m2, semi-diurnal lunar period, and perfect compliance with m1, the monthly lunar anomalistic, and the synodic periods. the phase lags of all four of the frequency distributions are much the same as that of the total sample (see figures 4, 5, 6 and 7), however, they are much more clearly seen. in particular, the phase lag for k1 is about –90˚, which is the phase of the maximum rate of change of the k1 tidal component, and about +90˚and 0˚ for m2, while the phase lags of the monthly m1 are +30˚ for the synodic, and –90˚ for the anomalistic. these result are in favour with the possibility that the combined gravitational tidal stress is an additional stress that triggers the earthquake event. the high compliance for the monthly tidal components despite their small intensities might indicate that in general they provide favourable conditions for the action of the much stronger tidal components k1 and m2. on this point, we can refer to the monthly tidal barometric variations that are quite sensitive to the seismic activity [arabelos et al. 2008]. perhaps this peculiar coincidence merits further investigation. finally, we draw the attention to the confidence level p for the tidal triggering of earthquakes in the ionian figure 10. confidence levels for compliance between the frequency distribution of the shock occurrence and (a) k1, m2 tidal periods. (b) manomalistic and m-synodic periods. (c) earthquakes with magnitudes $4.5 for the time from january 1981 to december 1983. figure 11. cumulative histogram corresponding to the tidal period k1 of the 186 events that occurred during january 1983 in the area of zakynthos. compliance of the frequency phase distribution of the shock occurrence, which increases around the time of high seismic activity. this indicates that the parameter p, which is a measure of this compliance, can be used as an additional earthquake precursory index. contadakis et al. 80 figure 12. cumulative histogram corresponding to the tidal period m2 of the 186 events that occurred during january 1983 in the area of zakynthos. figure 13. cumulative histogram corresponding to the tidal period m1 anomalistic of the 186 events that occurred during january 1983 in the area of zakynthos. figure 14. cumulative histogram corresponding to the tidal period m1 synodic of the 186 events that occurred during january 1983 in the area of zakynthos. p(k1) p(m2) p(m1anomalistic) p(msynodic) 11.27% 44.38% 0.00% 0.00% table 4. the confidence levels of the earthquake–earth tide correlation for the 186 earthquakes events that occurred during january 1983 in the area of zakynthos. 81 5. conclusions in the present study we investigated the tidal triggering evidence for the earthquakes of the seismic area of the ionian geological zone in greece. the results of our analysis using the cumulative histogram method indicate that the monthly variations in the frequencies of earthquake occurrence are in agreement with the period of the tidal lunar monthly (mm) variations. the same happens with the corresponding daily variations of the frequencies of earthquake occurrence, with the diurnal lunisolar (k1) and semidiurnal lunar (m2) tidal variations. the statistical test of schuster [1987] indicates very high probability for the monthly component mm to act as a trigger mechanism for earthquake occurrence, and a small probability for the diurnal lunisolar k1 and semidiurnal lunar m2 components, although the variation amplitudes of both (k1, m2) are about 18 times greater than that of mm. it appears that the revolution of the moon on its elliptical trajectory around the earth creates periodically favorable conditions for the triggering effect of the strong diurnal and semidiurnal components. in addition, the confidence levels for the identification of such periodical agreement between earthquake occurrence frequencies and tidal periods varies with the seismic activity; i.e. the higher confidence levels correspond to periods with stronger seismic activity. these results are in favor of a tidal triggering process for earthquakes when the stress in the focal area is near the critical level. this indicates also that the parameter p, which is a measure of the confidence level, can be used as an additional earthquake precursory index. references arabelos, d. (2002). comparison of earth-tide parameters over a large latitude difference, geophys. j. int., 151, 950956. arabelos, d.n., g. asteriadis, a. bloutsos, m.e. contadakis and s.d. spatalas (2008). correlation between seismisity and barometric tidal exalting, nat. hazards earth syst., 8, 1129-1137. baker, t.f. (1984). tidal determination of the earth, sci. progr., 69, 197-233. cadicheanu, n., m. van ruymbecke and p. zhu (2007). tidal triggering evidence of intermediate depth earthquakes in vrancea zone (romania), nat. hazards earth syst., 7, 733-740. dehant, v.( 1987). tidal parameters for the inelastic earth, phys. earth planet. in., 49, 97-116. dehant, v. and j. zschau (1989). the effect of mantle inelasticity on tidal gravity: a comparison between the spherical and the elliptical earth model, geophys. j., 97, 549-555. enescu, d. and b.d. enescu (1999). possible cause-effect relationships between vrancea (romania) earthquakes and some global geophysical phenomena, nat. hazards, 19, 233-245. knopoff, l. (1964). earth tides as a triggering mechanism of earthquakes, bull. seismol. soc. am., 54, 1865-1870. melchior, p. (1983). the tides of the planet earth, pergamon press, new york. papazachos, c., a. kiratzi and d. kontopoulou (1998). active tectonics in aegean and surrounding area, in: basic results of the seismologic in greece, honorem prof. b.c. papazachos, ziti publication, thessaloniki, 49-74 greece (in greek). papazachos, b.c., c.a. papaioannou, c.b. papazachos and a.s. savvaidis (1999). rupture zones in the aegean region, tectonophysics, 308, 205-221. rydelek, p.a., i.s. sacks and r. scarpa (1992). on tidal triggering of earthquakes at campi flegrei, italy, geophys. j. int., 109, 125-137. schuster, a. (1897). on lunar and solar periodicities of earthquakes, p. roy. soc. lond., 61, 455-465. shudde, r.h. and d.r. barr (1977). an analysis of earthquake frequency data, bull. seismol. soc. am., 67, 1379-1386. simpson, j.f. (1967). earth tides as a triggering mechanism for earthquakes, earth planet sci. lett., 2, 473-478. stavinschi, m. and j. souchay (2003). some correlations between earthquakes and earth tides, acta geod. geoph. hung., 38, 77-92. tanaka, s., m. ohtake and h. sato, (2002). evidence for tidal triggering of earthquakes as revealed from statistical analysis of global data, j. geophys. res., 107, 2211. tanaka, s., s. sato, h. matsumura and m. ohtake (2006). tidal triggering of earthquakes in the subducting philipine sea plate beneath the locked zone of the plate interface in tokai region, japan, tectonopysics, 417, 69-80. torge, w. (2001). geodesy, 3rd edition, walter de gruyter, berlin-new york. vidale, j.e., d.c. agnew, m.j.s. johnston and d.h. oppenheimer (1998). absence of earthquake correlation with earth tides: an indication of high preseismic fault stress rate, j. geophys. res., 103, 24567-24572. wilhelm, h., w. zürn, h-g. wenzel (eds.) (1997). tidal phenomena, springer, berlin-heidelberg-new york. *corresponding author: michael e. contadakis, aristotle university of thessaloniki, department of geodesy and surveying, thessaloniki, greece; email: kodadaki@eng.auth.gr. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. tidal triggering of earthquakes in the ionian p e t r o c h e m i s t r y o f v o l c a n i c r o c k s i n r e l a t i o n t o t h e f o r m a t i o n o f i s l a n d arcs(*) g . s . g o r s h k o v ( * * ) r i c e v u t o il 7 dicembre 1960. as it is known the volcanic rocks of island arcs have a pronounced calc-alkali character. they vary in the content from olivine or nonolivine basalts to quartz-bearing dacites and zhyolites. the most widespread group of rocks in the series are pyroxene-andesite and transitional varieties — andesite-basalt and dacite. characteristic feature of this series is a very basic plagioclase even in acid varieties and as well as frequent presence of quartz. among the dare coloured minerals there occur rhombic pyroxenes or poor in lime clinopyroxenes of pigeonite type, often clinopypoxene is represented by diopside augite (especially in more basic rocks). i n acid rocks pyroxene is replaced by hornblende and in exeptionally acid members of this series biotite and orthoclase appear. closer to t h e continents from island arc a gradual growth of alcalinity of rocks is encountered. the changing of petrochemical characteristics is noticable already at the distance of 10-12 km across the pitch of the arc. numerous examples of such an alteration occur in kamchatka, kurile islands and in j a p a n . at the distance of scores of kilometers it is possible to observe the changes in the mineral composition of rocks. at first these changes are observed in acid rocks only, the plagioclase becoming more acid; almost always biotite or orthoclase occur and sometimes both minerals occur together. pyroxene is more often represented by diopside augite. still closer to the continent orthoclase acquires greater importance, it appears even in andesites often forming margins around plagioclase. rhombic pyroxene disappear giving way to hornblende and diopside augite. as to the chemical composition the role of potassium in relation to sodium becomes greater. (*) p a p e r r e a d a t t h e helsinky assembly of t h e i . u . g . g . 1960. (**) l a b o r a t o r y of vulcanology, acad, of sci. of t h e u . s . s . r . moscow. 1 3 8 g. s . g o r s i i k o v intercontinental volcanoes now issue purely alkalic lavas distinctly having pure alkalic composition characteristic both for acid and basic kinds of rocks (from limburgite basanites and trachybasalts to t r a c h y t e s and phonolites). the mineral composition reflects this phenomena b y the presence potassium-sodium feldspars, aegirine-augites, titano-augites, alkaline ampliiboles and a t last feldspathoides-leucite and nepheline. potassium rocks often dominate. i n the direction f r o m island arcs to t h e ocean a sharp substitution of calc-alkalic rocks by alkaline lavas characteristic for all oceanic islands (a series of rocks from picrite-basalts and nepheline basalts to trachytes) takes place. olivine basalts with more acid plagioclase (than in basalts of island arcs) dominate here with pigeonite in the main mass. i n andesites of this series plagioclase is represented b y andesine and oligloclase, then alkalic amphiboles appear and in t h e main mass biotite occurs. i t is not always easy to distinguish the rocks of different series (oceanic, island arcs and continental) by their mineral composition b u t by their petrochemical features t h e y can be distinguished guite readily. petrochemical calculations and comparison are possible to perform b y different methods ( h a r k e r , n i g g l i , c.i.p.w., etc.) b u t all these are rather complicated and permit t o compare visually not more t h a n two rock series simultaneously. i m a d e use of a. n. zavaritsky's method which permits to draw a q u a n t i t y of analyses on a diagram and compare a n y number of rocks series. the petrochemical m e t h o d b y zavaritsky allows to express on a plane diagram for each rock a t t h e same time: the ratio between salic and femic constituents, the ratio between alkaline and calcareous alumosilicates, t h e n a t u r e of alkaline alumosilicates (ratio na to k) and the n a t u r e of the femic constituents (ratio fe, ca, mg and al). this method is considered in details in a book " the introduction into t h e petrochemistry of igneous rocks " (in russian). as f a r as the a u t h o r is informed it is translated only into german of all t h e western-european languages. one can found all details in " einfiirung in die petrochemie der eruptivgesteine " by a. n. zawaritzki, berlin, akademieyerlag, 1954. on the graph (pig. 1) variation curves for the series of oceanic, continental and island arcs lavas are drawn according to a. n. zavaritsky. to make the graph more simple figurative points of rocks and their vectors are omitted. upon considering the graph it becomes obvious t h a t as a m a t t e r of f a c t there are two classes of completely different volcanic rocks difp e t r o c h e m i s t r y o f v o l c a n i c r o c k s i n r e l a t i o n , e t c . 1 3 9 fering in the decline of the variation curves to coordinate axes or otherwise in the sp?ed of alkaline growth in the process of differentiation. one of these classes includes lavas of oceanic islands (lines 1-4, in fig. 1), t h e other embraces lavas of island arcs (lines 5-7) together with alkaline rocks of continental volcanoes (lines 8-10). c 5 10 20 30 a iii1!;! 10 6 7 6 9 10 /,«' ; / • •• / / / / ' v ' hi i n: i w 1 iii 1 f t hi l 1 1 i k •• / .•• / / // > / / ' v ' hi i n: i w 1 iii 1 f t hi l 1 1 i hi/ 7 /,••/ / /••• / »/ ,•• / // v eli io i / / / •• ki / / f 7 ;••" " .. / / '/ / 5 \ \' 6 7 '•1-4 10 sk| 7f i 2 '4 b fig. 1 t h e d i a g r a m of t h e t y p e s of t h e n a t u r a l associations of volcanic rocks (by. a. z a v a r i t s k y ) . the order numbers of lines given in fig. 1, are in conformity with certain petrochemical types of rocks: 1) volcanoes of t h e hawaiian archipelago; 2) the lavas of the tahiti islands; 3) and 4) very alkaline types of rocks, which occur in some islands of the pacific, t h e indian and the atlantic; 5) " pelee t y p e " according to b u r n , including lavas of e a s t e r n k a m c h a t k a , j a p a n volcanoes etc.; 6) " s a n francisco t y p e " according to burri; 7) " yellowstone p a r k t y p e " according to burri; 8) intermidiate t y p e separating calc-alkaline rocks from pure alkaline (" e t n a t y p e " according to zavaritsky); 9) " eosita hills t y p e " to which lavas of alkaline province of eastern asia belong; 10) very alkaline continental lavas (uyun kholdong in n e china, bufimbira in africa and others). 138 g. s. g o r s i i k o v the transition from calc-alkali lavas of island arcs to t h e lavas of continental volcanoes takes place not only in space b u t also in time. i n m a n y places (ne asia, the carpatians etc.) there are examples t h a t old lavas which correspond to t h e geosyncline stage of development (or island arc stage) being calc-alkaline ones and more young lavas (of continental stage) being alkaline ones. thus, the class of rocks embracing calc-alkali rocks of island arcs and alkaline lavas of continents belongs to a continuous class and the rocks have aconsanguinity. the primary magma of this class is calc-alkalic. magma and in t h e course of geological time the growth ni alkalinity takes place. the relations between calc-alkalic and oceanic lavas are not so clear a t first sight. the intermediate varieties are not known and t h e change of one kind of rocks by other is very sharp. a t t h e same time t h e fields of basic rocks of both classes (in fig. 1) partially cover each other and as b a r t h stated (*) it is enough to have a relatively small change in composition of primary melt for process of cristallization to go absolutely different ways: in one case to t h e formation of quartz dacites and ryolites and to t h e formation of alkaline nepheline rocks in t h e other. fig. 2 borrowed f r o m b a r t h ' s work illustrates t h e above s t a t e m e n t . thus, a relatively small change in composition of primary m a g m a m a y greatly alter the character of differentiation and transfer one t y p e of rocks into another. the mentioned petrochemical features as the uniformity of rock types in all island arcs, gradual growth of alkalinity towards the continents and sharp difference in rock types of island arcs and oceanic islands are of general importance. t h a t t h e lavas belong to one or t h e other t y p e of rocks depends on oceanic, continental or intermediate (island arc) location or, otherwise, on t h e continental or oceanic t y p e of the e a r t h ' s crust and does not depend a t all on the composition and structure of upper layers of the crust (with a few exeptions). the uniformity of lavas of oceanic islands or island arcs is well known. the continental volcanoes also issue lavas uniform enough (in t h e petrochemical sence) irrespective of the location of volcaneos in the regions of mesozoic or tertiary folding (the carpatians), the rift zone (africa) or on the old platform (ne china). i t had been long proposed on the basis of general theoretical postulates t h a t the feeding sources of volcanoes are situated a t great depths (*) barth, tom. f . w . , theoretical petrology, 1952. p e t r o c h e m i s t r y of v o l c a n i c r o c k s i n r e l a t i o n , e t c . 139 of some scores of kilometers. recently t h e depth of magmatic heart of kliuchevsky volcano in k a m c h a t k a was determined as being 50-70 k m . the independence of petrochemistry of lavas from t h e geological struct u r e and t h e structure of the upper layers of the crust proves once more t h e idea t h a t the feeding sources of the volcanoes are situated out of limit of the crust — in t h e upper p a r t of the e a r t h ' s m a n t l e where there is no sharp change in chemistry depending upon the concrete geological structure. moreover, a striking uniformity of petrochemistry of volcanism over vast territories shows the relative role of assimilation of crust m a t erial in t h e magma on its way to the surface. a well known opinion t h a t t h e " pacific rocks " of island arcs are only derivatives of the " atlantic magma " contaminated in the upper layers of t h e crust by sialitic material being brought f r o m the adjoining mainland to the dipping geosyncline is in contradiction to geophysical observations in the regions of island arcs. modern observations proved t h a t in some arcs t h e crust of the continental t y p e is present only immediately beneath t h e arc and is absent not only f r o m t h e ocean side b u t also in the mariginal seas — f r o m the side of the continent. naturally, in these conditions the supply of t h e sialitic material f r o m the surface cannot t a k e place. thus, the source of contamination, if it does t a k e place, m a y be only of depth character. summarizing t h e above said we come to t h e conclusion t h a t the two main types of magmas: oceanic and continental (including island arcs) are connected to the corresponding t y p e of the crust in space as well as genetically. the oceanic magma of the hawaii t y p e (line 1 in fig. 1) is an ancestral m a g m a for all t h e other magmas (primitive m a g m a by tilly, etc.). under t h e conditions of gradual increase in alkalinity in t h e course of normal evolution this m a g m a becomes more and more alkaline (shift from line 1 to line 4 in fig. 1). during contamination of this magma by sialitic material t h e oceanic magma turns into t h e calc-alkalic one (the transition from point " n " to point " x " in fig. 2 or from line 1 to line 5 in fig. 1). this sharp change of magmatic t y p e is related to the upflow of sialitic material f r o m depths of the e a r t h ' s mantle. the island arcs indicate these narrow areas of the globe where a t present the processes of depth differentiation and t h e rise of the sialitic material f r o m the depths of the m a n t l e to its surface t a k e place. this process leads to the thickening of the crust and a t t h e same time sharply changes the chemistry and the whole character of volcanism. f u r t h e r more, with t h e termination of the flow of portions of substance f r o m the depths of t h e m a n t l e the normal process of magmatic evolution 138 g. s . g o r s i i k o v in t h e direction of increase of alkalinity is renewed and t h e m a g m a gradually turns into the alkaline continental one. on its way to t h e e a r t h surface through t h e crust m a g m a in most cases does not undergo a notable contamination and does not change visibly t h e main features of its petrochemistry. the composition of primary magmas seems to correspond to basalts or more basic rocks. i t is necessary to note t h a t within the limits of t h e so called " andesine line " t h e basalts and andesite-basalts also have a wide occurence. fig. 2 different possible r e s u l t s of t h e f r a c t i o n a l c r y s t a l l i z a t i o n of b a s a l t melts, (by t. b a r t h ) . more acid varieties of rocks are t h e products of differentiation of the corresponding more basic m a g m a s in a relatively small volumes of volcanic vents. a separate volcano or a group of neighbouring volcanoes m a y issue lavas different in their acidity b u t t h e y all will belong to one narrow t y p e and their figurative points will be situated along t h e same line in fig. 1. somewhat noticeable shift across t h e curves (i.e. change in alkalinity) is not observed as a rule in short periods of time. numerous cases of substitution of more acid lavas b y more basic ones during t h e same eruption are known; then a f t e r t h e cease of activity which is long enough for differentiation, there m a y appear acid lavas followed b y basic ones and so on. however a noticeable change in alkalinity (in petrocliesic p e t r o c h e m i s t r y o f v o l c a n i c r o c k s i n r e l a t i o n , e t c . 139 mical sence) in this case is not observed as all lavas belong to one type. the change in alkalinity and substitution of one t y p e of magma with another happens only in the course of geological periods of time. thus, the consideration of petrochemistry of lavas lead us to the conclusion t h a t volcanism is not purely a surface or introcrustal process, b u t together with tectonic activity in general, represents t h e surface expression of the primary undercrustal processes t h a t govern all t h e course of the e a r t h ' s development. the formation of island arcs is one of the necessary and most essential links of this process. summary it ivas shown that there are tivo petrochemically sharply different classes of lavas: 1) alkali lavas of intraoceanic islands, and 2) calc-alkali lavas of island arcs together with alkali lavas of intracontinental volcanoes. the two main types of lavas: oceanic and continental (including islands arcs), are connected in space to the corresponding types of the eearth's crust, but petrochemical peculiarities are independent of local geological features and structure of upper layers of the crust. magmatic harts of volcanoes are located in the upper part of the mantle and petrochemical features reflect the changes in chemistry of subcrustal substance. oceanic magma is an ancestral one and in the course of a normal evolution this magma becomes more and more alkaline. calk-alkali magma of island arcs is derivative one and it is generated by the contamination of the oceanic magma with sialitic material upflowing from depths of the mantle (assimilation of crust material is insignificant). the island arcs indicate these narrow areas of the globe where the process takes place. it leads to a thickening of the crust and changes sharply the petrochemistry and the whole character of volcanism. then in the course of the normal evolution the calc-alkali magma turns gradually into continental alkali one. thus, volcanism is not purely a surface or intracrustal process, but it represents the surface expression of primary subcrustal processes that govern all the course of the earth's development. the formation of island arcs is one of the necessary and most important links of the process. riassunto si e visto che ci sono due classi di lava lievemente diversa per la composizione petrochimica: 1) le lave ualkali'' delle isole oceaniche e 2) le lave "calc-alkali" degli archi insulari insieme con le lave 11 alkali" dei vulcani 138 g . s . g o r s i i k o v continentali. i due principali tipi di lava: oceanica e continentale (compreso gli archi insulari), sono connesse nello spazio ai corrispondenti tipi di crosta terrestre, ma le peculiarita petrochimiche non dipendono dalla locale composizione geologica e dalla struttura degli strati piii elevati della crosta. gli " harts " di magma dei vulcani si trovano nella parte piii elevata del mantello e le composizioni petrochimiche riflettono i mutamenti chimici della sostanza della sottocrosta. ii magma oceanico e ancestrale e nel corso di una evoluzione normale questo magma diventa sempre piii alcalino. ii magma " calc-alkali " delle " island arcs " e un derivato ed e prodotto dalla contaminazione del magma oceanico con materiale sialitico che emerge dalle profondita del mantello (vassimilazione del materiale della crosta e insignificante). gli archi insulari indicano queste ristrette zone del globo dove il processo ha luogo. questo processo porta ad un ispessimento della crosta e muta lievemente la petrochimica e tutto il carattere del vulcanismo. quindi nel corso della normale evoluzione il magma " calc-alkali " si cambia gradualmente in " alkali " continentale. percio il vulcanismo non e semplicemente una superficie o un processo intercrostale, ma rappresenta vespressione della superficie dei processi subcrostali primari che regolano tutto il corso dello sviluppo della terra. la formazione degli archi insulari e uno degli aspetti necessari e piii importanti del processo. layout 6 building self-consistent, short-term earthquake probability (step) models: improved strategies and calibration procedures jochen woessner1,*, annemarie christophersen1,3, j. douglas zechar1,2, damiano monelli1 1 eth zurich, swiss seismological service, zurich, switzerland 2 lamont-doherty earth observatory, columbia university, palisades, ny, usa 3 gns science, avalon, lower hutt, new zealand annals of geophysics, 53, 3, 2010; doi: 10.4401/ag-4812 abstract we present two self-consistent implementations of a short-term earthquake probability (step) model that produces daily seismicity forecasts for the area of the italian national seismic network. both implementations combine a time-varying and a time-invariant contribution, for which we assume that the instrumental italian earthquake catalog provides the best information. for the time-invariant contribution, the catalog is declustered using the clustering technique of the step model; the smoothed seismicity model is generated from the declustered catalog. the time-varying contribution is what distinguishes the two implementations: 1) for one implementation (step-lg), the original model parameterization and estimation is used; 2) for the other (step-ng), the mean abundance method is used to estimate aftershock productivity. in the step-ng implementation, earthquakes with magnitude up to ml = 6.2 are expected to be less productive compared to the step-lg implementation, whereas larger earthquakes are expected to be more productive. we have retrospectively tested the performance of these two implementations and applied likelihood tests to evaluate their consistencies with observed earthquakes. both of these implementations were consistent with the observed earthquake data in space: step-ng performed better than steplg in terms of forecast rates. more generally, we found that testing earthquake forecasts issued at regular intervals does not test the full power of clustering models, and future experiments should allow for more frequent forecasts starting at the times of triggering events. introduction constructing effective real-time, short-term earthquake forecasts remains one of the most challenging problems for seismologists. to provide useful information, a model must capture earthquake physics with a complex physical and/or statistical understanding of spatial and temporal clustering. the difficulties are compounded as these clustering processes operate on different scales in the space, time and magnitude domains. currently, only statistical forecast model frameworks such as epidemic type aftershock sequence (etas) models and short-term earthquake probability (step) models are used for automated, near-real-time applications [e.g., console et al. 2003, gerstenberger et al. 2005, helmstetter et al. 2006, marzocchi and lombardi 2008]. both of these frameworks can adapt to ongoing earthquake sequences by re-estimating model parameter values and automatically generating forecasts that account for the most recent seismicity. physics-based models that combine calculations of stress changes with a rate-and-state friction model to determine seismicity rates [e.g., hainzl et al. 2009, cocco et al. 2010] are not yet applicable in near real-time. these models require additional seismological, geological, and tectonic information that is often not immediately available. there are indications from retrospective testing experiments that these models can perform as well as the statistical models only when uncertainties of the physical model are included stochastically [woessner et al. 2009]. motivated by (1) the upcoming prospective daily seismicity forecast experiment to be performed at the eth zurich testing center as part of the collaboratory study for earthquake predictability (csep) initiative, (2) recent results from retrospective comparative testing experiments [woessner et al. 2009], and (3) methodological improvements in estimating triggering effects of earthquakes [christophersen and smith 2008, christophersen and gerstenberger 2010], we have developed two implementations of the step model for the italian testing region. additionally, we suggest that with some regionalization, these two implementations can be tested within other csep testing regions. moreover, dependent on the forthcoming results of the prospective tests of these implementations, time-varying hazard estimates can be generated for italy, similar to those already publicly available in real-time for the state of california, usa (http://earthquake.usgs.gov/eqcenter/step/). article history received october 23, 2009; accepted april 13, 2010. subject classification: earthquake statistics, earthquake forecasting, likelihood testing, aftershock model, seismicity analysis. 141 when building an earthquake forecast based on a step model, a time-invariant contribution is computed from previous seismicity and, if desired, other information. the time-varying contribution is based on estimating parameter values from the analysis of previous seismicity. the two contributions are estimated independently; therefore, to build a self-consistent model, the same underlying assumptions are made. our implementations combine a time-invariant contribution based on a simple smoothed seismicity approach [zechar and jordan 2010b] with two slightly different time-varying contributions (figure 1); these different paths are what distinguish the two implementations. each time-varying contribution is itself a combination of three elements: (1) a generic element based on the average statistical behavior of italian aftershock sequences; (2) an element based on the temporal behavior of the particular aftershock sequence in progress; and (3) a spatially variable element where the aftershock behavior is mapped in space. one time-varying contribution closely follows the methodology of reasenberg and jones [1989, 1990, 1994], using parameter values estimated by lolli and gasperini [2003] and gasperini and lolli [2006]. the other time-varying contribution uses a new generic element based on christophersen and gerstenberger [2010]. we describe the latter model in detail and discuss its capabilities and limitations. each of these contributions is then combined with the time-invariant contribution, forming the step-lg (lolli-gasperini) and the step-ng (new generic) model. when combining the contributions, the final forecast rate at each point is taken to be the greater of the time-invariant and time-varying contributions. in the following sections, we describe the data used, the time-invariant contribution, and the two distinct timevarying contributions. we report the performance of the two implementations in a retrospective daily forecast experiment of 906 days starting on january 1, 2007. the performance of each implementation is analyzed with the csep likelihood(l)-test [schorlemmer et al. 2007], a modified number(n)-test, and a normalized l-test and a space(s)-test [werner et al. 2009, zechar et al. 2010]. as these tests were developed for evaluating long-term forecasts, we have adjusted them to the needs of daily forecasts. we discuss the test results, state our expectations for prospective testing, comment on how the experiment set-up influences the model performance, and outline possible improvements to the current model implementations. data we used seismic catalog data provided by the csep eu testing center (http://www.cseptesting.org/regions/italy) for the time period from january 1, 1981, to december 31, 2002 (csi 1.1 catalog) [castello et al. 2007]. we used the italian seismic bulletin (bollettino sismico italiano; http:// bollettinosismico.rm.ingv.it/) for the period from 2002 to march 31, 2009, by adding the data into the periods that were not available from the csep testing center in zurich. from april 1, 2009, to june 25, 2009, we used data from the italian seismic instrumental and parametric data-base (iside; http://iside.rm.ingv.it), knowing that this data had not been reviewed entirely. the catalog originally contained 75,520 earthquakes in the magnitude range 0.1 ≤ ml ≤ 5.9 with depths less than 30 km. the catalog included 436 events with ml ≥ 3.95, 406 of which were located in the collection area of the csep-italy testing region. the ml ≥ 3.95 seismicity is shown in figure 2a, together with the cumulative number of events versus time (figure 2b). to minimize possible edge effects, earthquakes in the catalog that fell outside the collection area were included when estimating the time-invariant parameter values. figure 2a shows 279 events in the learning period for building self-consistent step models 142 model implementation contribution element productivity estimate step-ng / step-lg time-invariant time-varying generic sequence specific spatially variable mean abundance rj-model smoothed seismicity figure 1. step model hierarchy. the model implementations (step-ng/step-lg) are composed of model contributions (time-invariant/time-varying), which consist of model elements. the elements of the time-varying contribution were weighted using an aicc criterion [gerstenberger et al. 2005]. the generic element can be based on the mean abundance model by christophersen and smith [2008] or the model by reasenberg and jones [1989] (rjmodel). for the latter, we used parameters determined by lolli and gasperini [2003] and gasperini and lolli [2006]. 143 the time-invariant parameters, and 31 in the target period of january 1, 2007, to december 31, 2007. there were 37 events in the target period for the retrospective daily experiment from january 1, 2007, to june 25, 2009. model description in building our implementations, we strove to use selfconsistent assumptions and data that were well suited for estimating the parameter values for each model contribution. nevertheless, our implementations are not completely free of subjective choices, as the parameters were not estimated in one single, simultaneous procedure. we modified several aspects of the step model compared to the version implemented for california, usa [gerstenberger et al. 2005]: (1) we regionalized the model by using the italian testing region defined for the 1-day model class by the testing center [schorlemmer et al. 2010, this issue]. (2) we computed the time-invariant model contribution using different assumptions than gerstenberger et al. [2005]: rather than using the seismicity rates of the national italian seismic hazard map [meletti et al. 2008], we independently estimated the time-invariant contributions using a smoothed seismicity approach. (3) we implemented two distinct time-varying contributions: (a) one based on the work of reasenberg and jones [1994], with parameter values estimated by lolli and gasperini [2003] and gasperini and lolli [2006], essentially following the procedure used by gerstenberger et al. [2005]; (b) another based on the mean abundance model of christophersen and smith [2008] and christophersen and gerstenberger [2010], to estimate aftershock productivity. time-invariant contribution for the generation of the time-invariant contribution, we estimated seismic background rates using a smoothed seismicity approach, assuming that the background seismicity rates vary in space and time [marzocchi and lombardi 2008]. the smoothed seismicity approach was adopted from zechar and jordan [2010b] using a smoothing kernel with an isotropic two dimensional gaussian function governed by a single length scale parameter. by smoothing the recent seismic activity, we assumed to better approximate the current state of background seismicity, as opposed to using long-term rates derived in the national seismic hazard map that rely on seismic source zonation [meletti et al. 2008]. in this sense, each contribution was based only on the earthquake catalog; smoothing the seismicity rates and the resulting implementations are built on self-consistent assumptions. to compute the time-invariant contribution, we first declustered the catalog containing events in the period january 1, 1981, to december 31, 2007. for comparison, we applied several different clustering approaches and show the building self-consistent step models figure 2. (a) seismicity map for events in the step-declustered catalog (light gray squares) and entire catalog (dark gray squares) for the period 1984 to 2006. events in target period for the smoothed seismicity map (green squares: january 1, 2007, to december 31, 2007) and target period of retrospective testing (red squares: january 1, 2007, to june 25, 2009). the inner polygon represents the testing area, the outer polygon, the collection area, for the prospective csep testing experiments. (b) cumulative number of events versus time for the entire catalog (black line) and for the step-declustered catalog (gray line). only events with ml ≥ 3.95 are plotted. longitude la tit ud e n um be r of e ve nt s a) b) 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 6˚ 8˚ 10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 36˚ 38˚ 40˚ 42˚ 44˚ 46˚ 48˚ 1984 1990 1995 2000 2005 2007 2009 0 50 100 150 200 250 300 350 400 ml ≥ 3.95 step declustered time in dec. years inherent variability (figure 3). we generated four declustered catalogs using: (1) a windowing approach based on gardner and knopoff [1974]; (2) the clustering approach by reasenberg [1985], with the originally proposed parameters for california, usa; (3) the model-independent approach of marsan and lengliné [2008]; and (4) the windowing approach as used in the time-varying step contribution. in approach 4, any earthquake that followed another within a certain space-time range was considered to be an aftershock of the first event. the time window was 30 days, and it was extended by another 30 days if another aftershock occurred. the spatial window was a circular area, centered on the potential main shock with radius r (m): (1) where m is the local magnitude ml of the potential main shock, and r (m) is measured in km. this relation is based on wells and coppersmith [1994], and involves a minimum 5 km radius to select events taking into consideration location uncertainties; this implies that for ml ≤ 5.3 the search radius is constant and thus the smaller the magnitude is, the more likely are potential background earthquakes to be included in a cluster. we emphasize that the choice of the declustering procedure did affect the time-invariant contribution in terms of total forecast rate and the resulting spatial forecast. for consistency of the two contributions, the implementations presented here were based on the step windowing approach (approach 4). the cumulative numbers of earthquakes for the entire catalog and for each declustered catalog as a function of time are shown in figure 3a, b, respectively; for the testing area, as indicated in figure 1, this is shown in figure 3c, d. data from the entire catalog were used to compute the timeinvariant model. the total number of events varied between 117 and 304 for the entire catalog, and 89 and 259 for the testing area (table 1). this implies a daily seismicity rate for the entire area of between 0.0134 and 0.0338, and for the testing area, 0.010 and 0.0296. for the selected stepdeclustered catalog, 296 events with ml ≥ 3.95 remained, building self-consistent step models 144 1980 1985 1990 1995 2000 2005 2010 0 5000 10000 15000 time [dec. years] n u m b er o f e ve n ts m > = 2 .4 5 reasenberg standard gardener−knopo" misd step obs. data 1980 1985 1990 1995 2000 2005 2010 0 5000 10000 15000 time [dec. years] n u m b er o f e ve n ts m > = 2 .4 5 1980 1985 1990 1995 2000 2005 2010 0 50 100 150 200 250 300 350 400 450 time [dec. years] n u m b er o f e ve n ts m > = 3 .9 5 a) b) c) 1980 1985 1990 1995 2000 2005 2010 0 50 100 150 200 250 300 350 400 time [dec. years] n u m b er o f e ve n ts m > = 3 .9 5 tf to d) figure 3. cumulative number of earthquakes versus time of the entire observed data (gray) and resulting from four (de-)clustering approaches applied to the catalog from january 1, 1981, to december 31, 2007: for the entire study region (see figure 2) with (a) ml ≥ 2.45 and (b) ml ≥ 3.95; for the testing region with (c) ml ≥ 2.45 and (d) ml ≥ 3.95. the dashed gray lines in (a) and (b) indicate the period of 2003 to 2005 for which the magnitude determinations appeared to be different, although not affecting magnitudes ml ≥ 3.95. dash-dotted black lines in (b) denote the period to when the time-invariant model contribution was optimized. dash-dotted red lines in (d) denote the period of retrospective testing tf. note the smaller slope in this period compared to previous periods. ( ) 5 : 5.3 10 : 5.3 r m m m059 2.44m = 2 # ' 145 which indicated a total daily rate of 0.0338; in the testing area, a yearly/daily rate of 10.5/0.0288 events was expected. the overall seismicity rate for the period 2003 to 2005 was higher than in the other periods; however, the magnitude range ml ≥ 3.95 was only slightly affected compared to the range of smaller magnitude events. in figure 3, a change in the slope can also be seen around march 29, 2003. on this day, the magnitude ml = 5.4 jabuka island event occurred in the central adriatic sea, off the coast of croatia; this was one of the strongest events ever recorded within the adriatic microplate [herak et al. 2005]. in figure 3d, the vertical lines depict the period of the retrospective daily forecast tests ( january 1, 2007, to june 25, 2009). the slope of the observed number of earthquakes in the cumulative number plot for the observed data was slightly less steep than in the periods before the 1997 umbriamarche earthquake and in other periods without large aftershock sequence contributions. this might be due to the natural temporal fluctuation of seismicity, or it might be due to changes in policies regarding magnitude determination. this is important to remember when interpreting the results of the retrospective testing experiment described in the following sections. the step-declustered catalog served as the input to the smoothing algorithm of zechar and jordan [2010b]. similar approaches have been used for national seismic hazard maps in the usa and new zealand [frankel et al. 2002, stirling et al. 2002]. in contrast to the adaptive kernel estimation method by stock and smith [2002a, 2002b], in which the kernel width varied in space, here the standard deviation v of the gaussian kernel was uniform over the domain of interest. the novelty of the method with respect to past implementations, such as frankel et al. [2002], is that the kernel width was optimized by performing a set of retrospective tests with different smoothing length scales and by measuring the performance of each test in terms of the area skill score (ass) misfit statistic [zechar and jordan 2008, zechar and jordan 2010a]. in other words, the kernel width that gave the best performance in terms of the ass misfit statistic | was chosen as the optimal one. to determine the optimal kernel width, we used data from the declustered catalog over the period from january 1, 1984, to december 31, 2006, as the learning period, and the period from january 1, 2007, to december 31, 2007, as the target period. for magnitude ml ≥ 3.95, the learning period contained 296 events, and the target period, 19 events. the kernel width v was varied between 5 km and 200 km, and |(v) was calculated (table 2). the optimization procedure suggested an optimal smoothing length scale of 30 km. the seismicity rates of the smoothed time-invariant model for the testing region are shown in figure 4. time-varying contribution the step model includes a spatial extension of the simple aftershock model of reasenberg and jones [1989, 1990, 1994]: (2) building self-consistent step models table 1. total number of events, yearly rates (m ≥ 3.95) and daily rates (m ≥ 3.95) after declustering for the collection/testing area of the ingv catalog in the period 1984-2007. table 2. model parameters and standard deviations for step-lg and step-ng. all other models were used as retrospective comparisons. mc correction, adjustment factor for data quality based on woessner and wiemer [2005]. nmin,ss , minimum number to estimate parameter values for the sequence specific and spatially variable time-varying model elements. total in the area catalog spans n (m ≥ 3.95) yearly rate nobs(m ≥ 3.95) / [year] daily rate nobs(m ≥ 3.95) / [day] reference 296/ 252 12.333/ 10.500 0.0338/ 0.0288 step windowing 304/ 259 12.667/ 10.792 0.0347/ 0.0296 reasenberg [1985] 250/ 210 10.417/ 8.750 0.0285/ 0.0240 gardner and knopoff [1974] 117/ 89 4.875/ 3.708 0.0134/ 0.010 marsan and lengliné [2008] parameter step-lg step-ng a-value −1.84 ±0.12 −1.84 ±0.12 b-value 0.98 ±0.03 0.98 ±0.03 c-value 0.09 ±0.27 0.09 ±0.27 p-value 0.92 ±0.06 0.92 ±0.06 a-value 1.31 ±0.21 m1 5.39 ±0.11 iou 5.468 mc correction 0.2 0.2 nmin,ss 100 100 ( , ) ( ) 10t m t c ( ) p a b m mm th = +m + -l where m (t, m) is the rate of aftershocks with magnitudes greater than the magnitude threshold mth and occurring at time t, mm denotes the main shock magnitude. the constants a´ and b were derived from the gutenberg-richter relationship [gutenberg and richter 1944], and p and c resulted from the omori-utsu law [utsu 1961, ogata 1983]. as aftershock sequences progress, the model parameter values were re-estimated. we applied the corrected akaike information criterion (aicc) to construct the best-fitting model from the three elements in the time-varying contribution (generic, sequence specific, spatially varying; see figure 1). the aicc is a likelihood-based metric designed for model selection [kenneth et al. 2002]. the calculation took into account the number of free parameters and the number of observed data; to be preferred in the final scoring, a model with more free parameters must fit the data better than models with fewer parameters. rather than selecting only a single model element, we used aicc weighting, where a relative weight for each model was based on its aicc score, and the final model was a weighted sum of the three element contributions; details of the step model are described in gerstenberger et al. [2004]. one essential component is the spatial smoothing of the aftershock productivity parameter a´, which defines the total rate of aftershocks in a sequence. we applied a smoothing of 1/r2, with r as the distance from the point of interest to the point source in the generic model, or the empirically parameterized fault in the sequencespecific and the spatially variable model. thus, for the generic model, the rates were smoothed radially outwards, while they were smoothed away from the fault in the other model elements. for the csep italy implementation, where the model generates daily forecasts of earthquakes with magnitude ml ≥ 3.95, we assumed that only ml ≥ 3 events can trigger events of interest. in the current model, with the generic parameters for the italian catalog, there is a 4% chance that an earthquake of ml = 3 will trigger an event with ml ≥ 3.95 (for ml = 2.5 and ml = 2; the corresponding probabilities are 1% and 0.1%, respectively). leaving the smaller events out, we probably underestimated the rate of occurrence; however, this covers the intrinsic uncertainty of the model forecasting ability well. we implemented two variations of the time-varying contribution: step-lg and step-ng. the two implementations differed only in the description of the average aftershock productivity in the generic model element (figure 1); all other features were in common. for each sequence analyzed, we estimated the magnitude of completeness, mc , using the maximum curvature approach without bootstrapping [woessner and wiemer 2005]. we added a correction factor to each estimate of m = 0.2 units, because the maximum curvature approach tended to yield unreasonably low mc values. if at least 100 events with magnitudes larger building self-consistent step models 146 figure 4. log10 (daily rate of ml ≥ 3.95) for the time-invariant model, derived from the smoothed seismicity approach of zechar and jordan [2010b]. the optimum smoothing kernel width was 30 km (table 3), derived from the stepdeclustered catalog for the period of january 1, 1984, to december 31, 2006, as learning, and the period of january 1, 2007, to december 31, 2007, as forecasting. table 3. kernel estimates for increasing kernel widths (v, standard deviation). the lowest value of | indicates the best solution. kernel width v [km] area skill score misfit, | 5 0.21252891 10 0.17705183 20 0.10374542 25 0.07435822 30 0.04899255 50 0.07346651 75 0.10409969 100 0.12572704 200 0.21312013 147 than mc were available for an ongoing sequence, the sequencespecific estimates of gutenberg-richter and omori-utsu parameter values were computed using maximum likelihood estimators [bender 1983, ogata 1983]. step-lg model: reasenberg-jones model with parameters by lolli-gasperini equation 2 introduced the generic model element for the step model [gerstenberger et al. 2005], for which the productivity parameter k in the omori-utsu law is replaced, to describe an average productivity that increases with mainshock magnitude. lolli and gasperini [2003] used combined earthquake catalog data from 1960 to 1996 to estimate the values of these model parameters. they applied the clustering algorithm by reasenberg [1985], as well as a window method, to define aftershocks. they estimated parameter values from about 40 aftershock sequences. finally, the parameter a´ was determined by equating the numerator of equation 2 to k and solving for a´: (3) the median values were: a´= 1.84 ± 0.12, p = 0.92 ± 0.06, c = 0.09 ± 0.27, and b = 0.98 ± 0.03 (table 2). step-ng model: new generic model the step-lg implementation assumed that aftershock productivity increases with main shock magnitude and depends on the b-value of the frequency-magnitude distribution. christophersen and gerstenberger [2010] derived an alternative description for the average productivity as a function of main shock magnitude based on mean abundance, nma(m), the mean number of aftershocks for a main shock with magnitude m. estimating mean abundance values has its own challenges and here we briefly describe how we determined our mean abundance for italy. to estimate the mean abundance parameter values, we used data covering the time period of january 1981 to december 2002, as covered by the csi 1.1 catalog [castello et al. 2007]. near the beginning of this period, the seismic network changed significantly. as a result, the detection threshold decreased from around ml = 4.0 to ml = 3.0 and below by the middle of 1984. therefore, we analyzed the complete catalog with a threshold magnitude mth = 4, and also considered the period 1984.5 to 2002 with threshold magnitude mth = 3. we clustered the earthquakes according to the step windowing approach, as outlined above, with an upper time limit of 30 days. once the earthquake clusters were defined, the largest earthquake in a cluster was considered to be the main shock. if two or more earthquakes within one cluster had the same magnitude, the earliest of these was taken to be the main shock. to determine mean abundance, the number of main shocks in each 0.1 magnitude bin was counted; the total number of aftershocks within a chosen time period was counted; and the number of aftershocks per main shock magnitude bin was divided by the observed number of main shocks. to address issues of completeness, we started counting aftershocks 0.1 days (2 hours and 24 minutes) after the main shock, to avoid missing smaller aftershocks that might be hidden in the coda of the main shock. it has been shown that, in general, mean abundance grows exponentially with main shock magnitude mm: (4) where a is the growth exponent, and m1(mth) is the magnitude, which on average has one aftershock above the threshold magnitude mth [christophersen and smith 2008]. figure 5a shows the results from the mean abundance analysis for two sub-sets of the catalog for the period 1981 to 2002: the gray rectangles are the mean abundance in the time period from 1984.5 with mth = 3.0. each data point was scaled by 0.1 to match a threshold magnitude of 4.0 for the lower target magnitude of the daily csep testing class. this scaling corresponded to a magnitude unit difference with a bvalue of b = 1.0. the black triangles are data for the complete time period, with mth = 4.0. we report the best-fitting a and m1 values and their respective 95% confidence intervals for the two periods: in the first case (mth ≥ 4.0), a = 1.31 ± 0.21 and m1(mth = 4.0) = 5.39 ± 0.1; in the second case, we obtained a = 0.87 ± 0.12 and m1 (mth = 4.0) = 5.63 ± 0.13. due to the fixed search radius of 5 km for all earthquakes below magnitude ml = 5.3, background events could well have been included, biasing the data towards higher abundance. therefore, although the dataset with magnitude cut-off mth =4.0 had fewer sequences and more scatter than the events with magnitudes in the range 3-4, we took the parameter value estimates based on the ml ≥ 4.0 set because the data were less likely to be affected by possible background events. mean abundance can be related to the omori-utsu kvalue [utsu, 1961, ogata 1983] by integrating over the time interval used to estimate mean abundance nma: (5) here, s and t are the start and end times of the period analyzed, respectively. we call iou (s,t) the omori-utsu integral, with: (6) building self-consistent step models log ( )a k b m mm th= -l ( , ) 10n m m ( ( ))ma m th m m m1m th= a 1 : 1 1 ( ) ( ) : 1 i n s c t c p p t c s c p 1 1ou p p= + + = + + !--* ` j ( ) ( ) ( ) ( ) ( , ) n n t dt k m t c dt k m i s t ma th p s t s t th ou = = + =-## for the lolli-gasperini parameter values p = 0.92 and c = 0.09, for the mean abundance time interval of 0.1 to 30 days, we obtained iou = 5.486. figure 5b shows the mean abundance estimates of k obtained by dividing nma by iou. the individual main shock data were computed by dividing the number of aftershocks in the time interval of 0.1 to 30 days by iou. the data for the 1997 colfiorito and the 2009 l'aquila earthquakes are highlighted by blue and red stars, respectively. if only one aftershock was observed, the k-value was 0.20. if no aftershocks were observed, the main shock was not shown in figure 5b. as a consequence, the squares fall above the model line, which also includes the main shocks without aftershocks in the time and magnitude interval analyzed. the final parameter values of the stepng implementation are listed in table 2. figure 5b indicates that the 1997 colfiorito sequence was particularly productive, while the 2009 l'aquila sequence fell in between the predictions of the two models. in the magnitude range 5.3 to 5.8, the data were scattered around both models, and both models agreed within the scatter of the data. due to the difference in slope (0.98 for the rj-model, and 1.31 for the mean abundance model), the models deviated for smaller and larger main-shock magnitudes. for a magnitude ml = 3 event, the k-value for the rj-model was about one order of magnitude larger than the k-value for the mean abundance model. because these earthquakes are used for forecasting, and because there are many more of these than larger earthquakes, the step-lg implementation will predict an overall higher rate of seismicity than the step-ng implementation. the productivity of the main shocks based on gasperini and lolli [2006] was higher for events with magnitudes up to ml ≤ 6.2. for larger events, the mean abundance model yielded higher productivity estimates. retrospective testing we performed retrospective daily tests for both models for the period january 1, 2007, to june 25, 2009 (906 days). we forecast 24-hour seismicity rates for the italian testing region in the magnitude range 4 ≤ ml ≤ 8, with the last bin including rates up to ml = 9. the first forecast estimated seismicity rates from midnight on january 1, 2007, to midnight on january 2, 2007, the second forecast covered the following 24 hours, and so on. we measured the performance of the models with the modified n-test [zechar et al. 2010], the csep l-test [schorlemmer et al. 2007], a normalized l-test [werner et al. 2009] and an s-test [zechar et al. 2010]. with retrospective testing, we hoped to identify weaknesses in the forecasts and gain some insight as to what we should expect from prospective forecast experiments. we considered various tests because each analyzed different features of the forecast. n-test and modified n-test the two-sided n-test of schorlemmer et al. [2007] contains a subtle flaw: in some cases, the test rejects forecasts with low rates when there are zero earthquakes observed, although such a forecast should be considered consistent. therefore, we followed the suggestion by zechar et al. [2010] and applied the modified n-test, summarized by the metrics: (7) (8) building self-consistent step models 148 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 10 −4 10 −3 10 −2 10 −1 10 0 10 1 10 2 10 3 m ea n a b u n d an ce mainshock magnitude csi m3: 1984.5 − 2002 csi m4: 1981 − 2002 3 4 5 6 7 8 10 −4 10 −3 10 −2 10 −1 10 0 10 1 10 2 10 3 main shock magnitude k fo r m c = 4 .0 lolli−gasperini mean abundance individual mainshocks col"orito l‘aquila a) b) figure 5. mean abundance analysis. (a) mean abundance nma as a function of magnitude for the period of 1981 to 2002 with threshold magnitude mth = 4 (black triangles) and the period 1984.5 to 2002 with threshold magnitude mth = 3 (gray squares). the solid black line and the gray dashed line show the fit of the mean abundance model for to the data, respectively. (b) the k-value for threshold magnitude mth = 4 as a function of magnitude. the gray dashed line and solid black line show model predictions based on gasperini and lolli [2006] and the mean abundance model, respectively. gray squares show productivity of sequences available, those with zero events in the magnitude range are not shown. blue and red stars: 1997 colfiorito and 2009 l'aquila sequences. 1 ( 1 ) , andf n n1 obs f= ;d ( )f n nobs f2 = ;d 149 here, f (x|n) is the right-continuous poisson cumulative distribution function with expectation n evaluated at x. using this approach, we can answer two questions separately: (1) what is the probability of observing at least nobs earthquakes (d1); and (2) what is the probability of observing nobs at most earthquakes (d2). the forecast rate distribution for all csep rate forecasts was assumed to be poisson, in which case it was appropriate to perform a two-sided hypothesis test to determine whether the forecast rate was too high or too low. when the probabilities were written as in equations 7 and 8, we instead used a one-sided test with a scaled critical value. for example, a critical value of 0.025 corresponds to a hypothesis test for which we have 95% confidence. the flaw in the original n-test was critically important for daily forecasts, where the number of observed events on any given day was often zero. for our implementation, the daily rate forecast of the time-invariant contribution was n_f = 0.0288 (table 1) for the entire testing region. on most testing days, there will be no magnitude ml ≥ 3.95 event (nobs = 0). for these days, we obtained d1 = 1 and d2 = 0.9716, indicating that the forecast was consistent with the observation. applying the original approach of a two-sided n-test, this forecast would be very close to rejection starting at d = 0.975. consistency tests in space: l-test, normalized l-test, and s-test we considered three different tests to analyze the consistency of the retrospective forecasts with the data in the spatial and magnitude distribution. we applied the csep ltest (cscore) and a normalized l-test (cnorm). the c-statistic suggested by schorlemmer et al. [2007] measures the consistency of a forecast space-rate-magnitude distribution with the observations. the c-score is dependent on the number of observed events ( nobs). therefore, a forecast that is consistent in terms of the spatial and magnitude distribution with the observed earthquakes might be rejected by the l-test simply because it does a poor job at forecasting the overall rate of seismicity. to account for this scenario, we also applied the normalized l-test suggested by werner et al. [2009], which normalizes the test results by the number of observed target earthquakes. low values of c indicate that a space-rate-magnitude forecast is not consistent with the observed distribution, whereas low values of cnorm indicate that a forecast is not consistent in terms of the spacemagnitude distribution. the s-test (g-score) isolates the spatial component of the forecast and compares this with the spatial distribution of target earthquakes [zechar et al. 2010]; low values of g indicate a forecast spatial distribution is inconsistent with the observation. results to provide a first overview, figure 6 shows for both models the cumulative numbers of the forecast earthquakes, the time-invariant model contribution, and the cumulative numbers of observed earthquakes versus time. the timeinvariant contribution (dash-dotted line) has a steeper slope as compared to the observed data (light gray line). as indicated in the data section, the rate of observed seismicity was lower for the testing period as compared to similar periods, not including prominent aftershock sequences (see figure 3d). the reason for this might be natural rate fluctuations, the result of optimizing the model only for a one-year period, or the change in network policies to determine magnitudes. we consider the last possibility least likely, as most of the events occur in the testing area after the known change in network policies in april 2005. there were 751 earthquakes with ml ≥ 3 that contributed to forecast rates during the retrospective daily testing period ( january 1, 2007, to june 25, 2009). all of the triggering events fell into the magnitude range 3 ≤ ml ≤ 6.2, with the largest earthquake being the april 6, 2009, ml =5.8 l'aquila earthquake. the step-lg implementation forecasts higher rates of seismicity than step-ng, indicated by the steeper slope in figure 5a. the forecast rates of step-ng match the total observed number of earthquakes better in the cumulative test than for step-lg. figure 7 shows the 906 daily and cumulative results of the n-test scores d1(t) and d2(t). daily n-test scores d1(t) (figure 7, red triangles) were rejected with at least 95% confidence when at least two events occurred and there was no contribution from ongoing earthquake sequences. no daily forecast was ever rejected for underpredicting. considering the cumulative tests, we find that step-ng was rejected on 30.1% of the test days, and step-lg was rejected on 48.6%. the models were rejected building self-consistent step models 0 100 200 300 500 600 700 800 900 1000 0 10 20 30 40 days after 01.01.2007 nu m be r o f e ve nt s 0 400 2 4 6 8 frequency m ≥ 3.95 45 cum. number of m ≥ 3.95 daily number of m ≥ 3.95 step−lg step−ng background figure 6. number of observed and forecasted events versus time from january 1, 2007, to june 25, 2009. cumulative number of observed events (light gray line), step-ng model (black line), step-lg model (gray dashed line), and rate of the time-invariant contribution (gray dash-dotted). squares show the frequency of the observed number of events with ml ≥ 3.95 per day (y-axis, right). due to the constant rate increase caused by the time-invariant contribution of the model. the n-test result was strongly influenced by each event occurring. as soon as a large sequence occurs, such as the l'aquila sequence (figure 6) that started on test day 826, the forecasts performed well overall in the n-test. to test the overall consistency of each implementation with the daily observations, we applied the l-test [schorlemmer et al. 2007] daily and cumulatively. both models were never rejected in the cumulative tests (figure 8, red line) and only on a few days in the day-by-day tests (figure 8, black triangles). for example, the log-likelihood score ll to observe nobs = 0 events for the first days forecast of model step-ng nf = 0.0302 is ll = –0.031. using nsim = 10000 simulations for the l-test, we obtained a score on the first test day of c (t =1) = 0.036. the quantile score improves over time as there are more and more events occurring in the regions where events are expected. the influence of the forecast number of events on the original csep l-test is illustrated well for these models, as the spatial distribution of rates was exactly the same (figure 8, red boxes). on test day t = 831, for example, step-lg forecast 0.0431 target earthquakes, and step-ng forecast 0.0326 target earthquakes; nobs = 0 events were observed. this led to a small difference in the c-scores, of 0.334 and 0.313, respectively. the normalized l-test removes the dependency on the forecast number of events with a factor expressing the ratio of the observed and forecasted events: [werner et al. 2009], while keeping the space and magnitude information. the s-test isolated the spatial distribution by summing the entire rates. for both of these tests, there is no difference between our two implementations, as they were using the same b-value for distributing in the frequency magnitude domain. we only show the results of step-ng in figure 9, as through the normalization the differences of the two implementations were removed. the tests were developed and applied retrospectively for long-term forecasts (5 years), for which the number of observed events is typically non-zero. however, in daily experiments, the number of observed events is often equal to zero, and thus the normalization was not applicable. we therefore applied the s-tests and normalized l-tests only on the forecast days on which at least one earthquake occurred. the daily test results displayed as the quantile scores (figure 9, gray triangles) show that the models were consistent in the space-magnitude domain as the cnorm-scores are larger than 0.025 (figure 9a), the effective significance level for the test. similarly, the g-scores of the s-test were larger than 0.025 for every day of the experiment, implying that the model is consistent with the spatial distribution of the seismicity observed (figure 9a). discussion and conclusion we have present two implementations of the stepmodel, step-lg and step-ng, which can generate daily forecasts of seismicity calibrated on italian seismicity. both implementations have the same spatial distribution of the seismicity rates, but they vary in terms of the total forecast rate. for the generic element of the time-varying contribution, step-lg incorporates rate estimates based on the reasenberg-jones model [reasenberg and jones 1989] with parameters from gasperini and lolli [2006], while step-ng defines rates based on the mean abundance model [christophersen and gerstenberger 2010] (figure 1). we emphasize that the two implementations described building self-consistent step models 150 a) step-lg b) step-ng figure 7. retrospective n-test results for daily and cumulative consistency. daily and cumulative d1(t)-scores (red) of observing at least nobs events were generally not rejected at the aeff significance level (gray bar). note that cumulative d2(t)-scores (gray line) for observing at most nobs events were rejected in about 48.6% of the test days for model step-lg, and in about 30.1% for model step-ng. n nobs f 151 here are different from the implementation of the step model submitted to the csep testing center at the southern california earthquake center and the model that is implemented at us geology survey (http://earthquake.usgs.gov/eqcenter/step). our guiding principle was to generate a self-consistent model for both contributions: time-invariant and time-varying. we used a time-invariant contribution based on the smoothed seismicity approach following zechar and jordan [2010b], using only instrumental seismicity. this is a change in philosophy from the approach of gerstenberger et al. [2005]: we assumed that the seismicity of the last 25 years was a well defined proxy for background seismicity varying on the scales of tenths of years, better than a long-term hazard map with all its assumptions [meletti et al. 2008]. we have also explored a new generic model element based on the mean abundance model (step-ng). this implementation forecast fewer events following small to moderate earthquakes (ml ≤ 6.2) than the step-lg implementation, and it forecast more following larger events. although the stationary rate estimates do influence the performance of our implementations (see table 1), a detailed comparison of different smoothed seismicity approaches is beyond the scope of this study. we used the window approach to cluster events and to estimate the time-invariant contribution based on this result, because this is consistent with the approach used to compute the time-varying building self-consistent step models a) step-lg b) step-ng a) s-test b) normalized l-test figure 9. retrospective (a) s-test and (b) normalized l-test results of step-ng for daily forecasts. the models were never rejected at the 0.05 significance level (for quantile scores g and cnorm equal or less than the effective significance level 0.025, indicated by the gray bar), implying spatial consistency throughout the testing period. the test results for step-lg were the same as for step-ng. figure 8. retrospective l-test results for daily (triangles) and cumulative (red lines) tests for (a) step-lg and(b) step-ng. the models were not rejected at the 0.05 significance level, indicated by the gray bar (effective value of 0.025). the spatial consistency improved with time, as indicated by increasing cumulative cscores. slight differences in the daily tests were observed, due to the difference in the number of forecasted events; examples are seen highlighted in red rectangles. contribution; nevertheless we recognize the intrinsic uncertainty in such an estimate. the issue of deciding the best way to estimate the stationary component for a time-varying daily forecast model is probably best approached by prospectively testing long-term time-invariant forecasts, as was followed in the 5year csep experiments. we have chosen an approach that is simple, although probably imperfect. the novelty of this method with respect to a past implementation [frankel et al. 2002] was optimization of the kernel width by performing a set of retrospective tests with different smoothing length scales. the kernel width that gave the best performance in terms of the ass misfit statistic [zechar and jordan 2008] was chosen as the optimal one (table 2). the choice of the kernel width was objective; the choice of the data was subjective. for both of the step-lg and step-ng implementations, the triggering main shock magnitude was set to ml = 3.0, as the probability of a foreshock to trigger an earthquake of one magnitude larger increases by up to 4%, and might be smaller by a factor of one hundred, depending on which model is used [michael and field 2009]. the parameters for both implementations were based on the analysis of earthquake sequences with main shocks in the range 4.0 ≤ ml ≤ 7.0 (figure 5a, b). thus, the extrapolation for k(mm) ≤ 4.0 contained uncertainty that was not well constrained. the parameter value estimates for each implementation also contained uncertainties due to the methods used to determine the completeness and declustering of the original catalog. gasperini and lolli [2006] applied the reasenberg [1985] approach, while we used the step clustering approach. a sensitivity study of the influence of these uncertainties on the forecast results, as well as a thorough uncertainty study on the parameter value estimation, has yet to be performed. we chose the parameter value estimates based on the period from 1981 to 2002, considering only sequences above ml ≥ 4 (figure 5) because the results of the clustering process of the sequences in the magnitude range 3 ≤ ml ≤ 4 might be biased; there are probably many sequences that should be associated with the background seismicity, thus increasing the productivity parameter k, although it should be smaller. from the magnitude dependent k-value (figure 5b), we expected the step-lg model to provide in general higher seismicity rates than the step-ng model. small magnitude contributions are more frequent and thus have a stronger influence for long periods on the daily forecast than larger magnitude events. in the case where a large main shock occurs, its influence on the large event prevails for some time, as can be seen in the retrospective testing results (figure 6). we retrospectively tested the implementations for a period of 906 test days, starting on january 1, 2007, in accordance with the procedures that are used in the one-daytesting class. we applied multiple testing procedures, as used in the csep test centers, and also test statistics that can be included in the testing center software [werner et al. 2009, zechar et al. 2010]. the modified n-test results showed that overall, the rate forecasts of step-ng were superior to those of step-lg (figures 6, 7). both of the models were spatially consistent with the observed data; forecasts were not rejected in the s-tests (figure 9). the models were also consistent with the data when considering the space-magnitude distributions for the normalized and non-normalized l-test. (figure 8). the 2009 l'aquila sequence illustrated the influence of a moderate earthquake on the performance of the models. the largest shock occurred on test day 827, which was clearly seen in figures 6 and 7, with the burst of ml ≥ 3.95 events. both of the models showed increased rates due to the foreshock activity that started about 7 days before (test day 820) with a sequence of ml ≥ 3.95 events, but both underestimated the occurrence of successive earthquakes for this sequence (figure 6). the low forecast rates were due to the definition of the retrospective testing class, restricting the models to update only every 24 hours. the l'aquila event occurred on april 6, 2009, at 1.32 a.m. (gmt). the forecast based on the seismicity to test day 826 did not include this information, only the information of the foreshocks. the forecast for day 827 did include rates due to the largest shock, but it only forecast the rates from midnight on april 6, 2009, to midnight on april 7, 2009. the models in essence missed the chance to forecast seismicity in the period of april 6, 2009, 1.32 a.m. (gmt) to 0.00 a.m. on april 7, 2009, which is actually the period where most of the events should occur according to the omori-utsu law. for testing forecasts, it is thus important at what time an event occurs. in other words, the forecasts are sensitive to when the forecast period starts [helmstetter et al. 2006]. for the existing implementations, a best case scenario is a strong event occurring shortly before midnight of any day; a worst case scenario is the occurrence of a strong event shortly after midnight. updating forecasts more frequently or after each earthquake that is considered to trigger further events would reveal the actual capabilities of short-term forecast models. however, testing these forecasts becomes more complicated as the duration of the forecasts and the updating periods would become variable. both implementations provided short-term rate forecasts that can serve as the basis for time-varying hazard information. to provide up-to date information on the probability of exceeding a specific ground motion measure, a full hazard computation is required [gerstenberger et al. 2005]. we suggest that for best performance this type of short-term forecast should be updated more frequently, and ideally immediately after an earthquake has been detected by the monitoring system. if communicated to decisionmakers, the media, and the public in the appropriate building self-consistent step models 152 153 language, information on seismicity rates and seismic hazards can promote a better understanding of the timevarying earthquake hazard before and during a strong earthquake sequence. acknowledgements. we thank m. werner for discussing all aspects of this study, and especially the retrospective testing. we thank m.c. gerstenberger for providing the initial step code, f. euchner for critical code review, and t. van stiphout and s. wiemer for discussions, encouragement and reviews on an early version of the manuscript. we acknowledge the comments and suggestions from two reviewers and the editor, w. marzocchi, that helped us to significantly improve our manuscript. this work is partly funded by the eu-fp6 projects safer (contract number 036935) and neries (contract number 26130), as well as the eu-fp7 project share (contract number 226967). references bender, b. 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zechar, j.d. and t.h. jordan (2010a). the area skill score statistic for evaluating earthquake predictability experiments, pure and appl. geophys., 167 (8/9), 893-906; doi: 10.1007/ s00024-010-0086-0 zechar, j.d. and t.h. jordan (2010b). simple smoothed seismicity earthquake forecasts for italy, 53, 3 (present issue). zechar, j.d., m.c. gerstenberger and d. rhoades (2010). likelihood-based tests for evaluating space-rate-magnitude earthquake forecasts, in press, bull. seismol. soc. am., 100 (3), 1184-1195; doi: 10.1785/0120090192. *corresponding author: jochen woessner, eth zurich, swiss seismological service, zurich, switzerland; email: jochen.woessner@sed.ethz.ch © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. building self-consistent step models 154 l ' o s s e r v a t o r i o g e o f i s i c o i n s t a l l a t o a k h a r t o u m i n o c c a s i o n e d e l l ' e c l i s s e d i s o l e d e l 2 5 2 1 9 5 2 r . cialdea p . dominici i n occasione dell'eclisse t o t a l e di sole del 25 f e b b r a i o 1952 l ' i s t i t u t o n a z i o n a l e di geofìsica ci i n c a r i c ò di i m p i a n t a r e u n osservatorio t e m p o r a n e o a k b a r t o u m ( s u d a n ) p e r r i l e v a r e , d u r a n t e l'eclisse, le e v e n t u a l i v a r i a z i o n i d e l l e p r o p r i e t à e l e t t r i c h e d e l l ' a t m o s f e r a , del camp o m a g n e t i c o t e r r e s t r e , dello stato di p o l a r i z z a z i o n e della luce d i f f u s a d e l l ' a t m o s f e r a e della t e m p e r a t u r a v i r t u a l e del cielo. nella p r e s e n t e r e l a z i o n e r i f e r i r e m o s u l l ' i m p i a n t o di q u e s t o osserv a t o r i o e sul suo f u n z i o n a m e n t o . i l m a t e r i a l e scientifico, di cui alla p r e c e d e n t e r e l a z i o n e ! 1 ' ) , era stato già t u t t o p r e p a r a t o n e l l a sede c e n t r a l e di r o m a dell'i.n.g. e p r o n t o p e r la spedizione. le a p p a r e c c h i a t u r e s c i e n t i f i c h e ed il m a t e r i a l e v a r i o necessario o c c u p a v a n o c o m p l e s s i v a m e n t e nove casse, p e r u n peso di 635 kg. a causa delle p a r t i c o l a r i s i t u a z i o n i p o l i t i c h e dell ' e g i t t o e del s u d a n , si r i t e n n e o p p o r t u n o , p e r p o t e r sorvegliare p e r s o n a l m e n t e il prezioso m a t e r i a l e affidatoci, di c o m p i e r e il viaggio p e r m a r e fino ad a l e s s a n d r i a e q u i n d i via t e r r a fino a k h a r t o u m . f u stab i l i t o di l a s c i a r e l ' i t a l i a il 27 g e n n a i o con la m / n e s p e r i a , ciò c b e faceva p r e v e d e r e l ' a r r i v o a k h a r t o u m p e r il 7 f e b b r a i o . i n t a l m o d o si avevano a disposizione b e n diciotto giorni p r i m a dell'eclisse p e r u n a c o m p l e t a messa a p u n t o di t u t t i gli a p p a r e c c h i . le casse conten e n t i la s t a z i o n e m a g n e t i c a r u s k a ci s a r e b b e r o state i n v i a t e d o p o p e r via a e r e a , d a t a l ' e s t r e m a delicatezza dei suoi s t r u m e n t i . i n seguito ad accordi i n t e r c o r s i f r a l ' i s t i t u t o n a z i o n a l e di geofisica e l ' o s s e r v a t o r i o a s t r o n o m i c o di r o m a su m o n t e mario, f u stabilito di e f f e t t u a r e il viaggio di a n d a t a i n s i e m e al p r o f . massimo c i m i n o i n c a r i c a t o da d e t t o osservatorio p e r s t u d i di c a r a t t e r e a s t r o n o m i c o . a n c h e l e p r a t i c h e c o n c e r n e n t i i c o n t a t t i con il g o v e r n o s u d a n e s e e le q u e s t i o n i connesse con il viaggio, c o m e i p a s s a p o r t i , i visti ed i ( ! ) r . cialdea: a n n . d i c e o f i s i c a , v o i . v , n . 3 , 1 9 5 2 . 5 8 2 r . cialdea p . d o m i n i c i l a s c i a p a s s a r e d e l l e diverse d o g a n e f u r o n o svolte in c o m u n e c o n il p r o f . c i m i n o . d a t o il c a r a t t e r e scientifico della s p e d i z i o n e , t u t t e l e p r a t i c h e r e l a t i v e "alle v a r i e d o g a n e f u r o n o f a c i l m e n t e e s p l e t a t e , otten e n d o l ' e s p o r t a z i o n e t e m p o r a n e a d a l l ' i t a l i a , il l i b e r o t r a n s i t o i n f r a n c h i g i a d o g a n a l e ed u n o s p e c i a l e « a g r e e m e n t » col g o v e r n o s u d a n e s e p e r l ' e n t r a t a e l ' u s c i t a d e l n o s t r o m a t e r i a l e n e l s u d a n senza gravam i fiscali. p a r t i t i da n a p o l i il 27 g e n n a i o g i u n g e m m o ad a l e s s a n d r i a il 30. 11 1° f e b b r a i o p a r t i m m o d a l c a i r o con il t r e n o espresso in c o i n c i d e n z a con la l i n e a b i s e t t i m a n a l e sliellal-wadi h a l f a k h a r t o u m . p u r t r o p p o u n i n c i d e n t e occorso a l t r e n o n e l t r a t t o c a i r o s h e l l a l ci f e c e r i t a r d a r e di t r e g i o r n i l ' a r r i v o a k h a r t o u m , dove g i u n g e m m o il 10 f e b b r a i o . l ' 1 1 f e b b r a i o f u i m p i e g a t o n e l l a r i c e r c a d i u n a l o c a l i t à a d a t t a a l l ' i m p i a n t o d e l l ' o s s e r v a t o r i o . le c o n d i z i o n i alle q u a l i doveva soddis f a r e q u e s t a località e r a n o : 1) essere s i t u a t a i n u n luogo il p i ù possibile a p e r t o , senza n o t e v o l i l i m i t a z i o n i a l l ' o r i z z o n t e v i s i b i l e ; 2) essere s i t u a t a in u n a z o n a n o n a f f e t t a da d i s t u r b i e l e t t r i c i e m a g n e t i c i ; 3) off r i r e sufficienti g a r a n z i e , p e r la sicurezza e la c o n s e r v a z i o n e in effic i e n z a degli a p p a r e c c h i . i l g o v e r n o s u d a n e s e aveva messo a disposiz i o n e d e l l e v a r i e s p e d i z i o n i a l c u n e aree, r a g g r u p p a t e in d u e p u n t i : i l p r i m o ( n o r t h e r n b a r r a c k s ) in p r o s s i m i t à del p o n t e f e r r o v i a r i o sul n i l o i n u n a z o n a m o l t o d i s t u r b a t a p e r l a p r e s e n z a d i l i n e e t r a n v i a r i e ; il s e c o n d o (kilo-five) i n p i e n o d e s e r t o e q u i n d i da s c a r t a r s i p e r i d a n n i c h e la s a b b i a a v r e b b e p o t u t o c a u s a r e ad a p p a r a t i delicatissimi q u a l i e r a n o i n o s t r i . i n f a t t i la z o n a di k h a r t o u m e r a s e m p r e soggetta ad u n d e b o l e v e n t o del n o r d ( / < 3), sufficiente a r i e m p i r e di u n a s a b b i a finissima q u a l u n q u e a m b i e n t e c b e n o n fosse a t e n u t a e r m e t i c a . f o r t u n a t a m e n t e o t t e n e m m o la preziosa c o l l a b o r a z i o n e della miss i o n e c a t t o l i c a c h e mise a n o s t r a disposizione la scuola « s. f r a n c e s c o s a v e r i o ». q u e s t a scuola si trovava alla p e r i f e r i a m e r i d i o n a l e della c i t t à , i n u n a z o n a n o n m o l t o d i s t u r b a t a n é e l e t t r i c a m e n t e n é m a g n e t i c a m e n t e . essa e r a d o t a t a di u n g r a n d e c o r t i l e c i n t a t o (circa 70 x "0 m e t r i ) , d a l c e n t r o del q u a l e l ' a l t e z z a a n g o l a r e degli ostacoli c i r c o s t a n t i e r a i n f e r i o r e ai 10°. si aveva i n o l t r e il v a n t a g g i o c h e il v e n t o p r e d o m i n a n t e d e l n o r d , p a s s a n d o a t t r a v e r s o la c i t t à , d e p o s i t a v a q u a s i t u t t a la s a b b i a di cui p o t e v a esser c a r i c o ; n é la c i t t à , c o m p o s t a q u a s i esclus i v a m e n t e d i ville con g i a r d i n i e p a r c h i p o t e v a p e r t u r b a r e l'osservat o r i o d a l p u n t o di vista e l e t t r i c o . solo a l c u n i r a r i t r e n i a v a p o r e port a v a n o dei b r e v i d i s t u r b i , d ' a l t r o n d e b e n i n d i v i d u a b i l i sulle registrazioni e l e t t r i c h e . d a l l ' a l t r o lato della scuola, il l a t o s u d , c'era il d e s e r t o ; l'osservatorio c e o f i s i c o installato a k h a r t o u m 583 il v e n t o s p i r a v a da q u e s t o l a t o solo e c c e z i o n a l m e n t e , m a con g r a n d e v i o l e n z a , p r o v o c a n d o d e l l e vere e p r o p r i e t e m p e s t e di s a b b i a . f o r t u n a t a m e n t e q u e s t a c i r c o s t a n z a si è verificata solo d u e volte d u r a n t e il p e r i o d o di f u n z i o n a m e n t o dell'osservatorio. la scuola « s. f r a n c e s c o saverio » s o d d i s f a c e v a alle necessarie c o n d i z i o n i , e f u sen z' altro scelta p e r l ' i m p i a n t o d e l l ' o s s e r v a t o r i o . i l 12 f e b b r a i o i n i z i a m m o il m o n t a g g i o degli a p p a r a l i , che proced e t t e con g r a n d e r a p i d i t à , t a n t o p i ù c b e avevamo o t t e n u t o a n c h e l'uso di u n a g r a n d e rimessa c h e a t t r e z z a m m o a l a b o r a t o r i o . u n ' a l t r a piccola s t a n z a , concessaci in seguito, f u a d i b i t a a c a m e r a oscura. f i g . 1 n e l l ' i n t e r n o della rimessa s i s t e m a m m o l'officina da c a m p o , le batt e r i e di a c c u m u l a t o r i p e r i v a r i servizi e l e t t r i c i , il r e g i s t r a t o r e e la stazione m a g n e t i c a . la fig. 1 m o s t r a l ' i n t e r n o di q u e s t o l a b o r a t o r i o d u r a n t e la fase di m o n t a g g i o degli a p p a r a t i ; a destra si n o t a l ' i n t e l a i a t u r a del r e g i s t r a t o r e , ed a sinistra q u e l l a del n e f o s c o p i o f o t o g r a f i c o . la p l a n i m e t r i a di fig. 2 r a p p r e s e n t a l e d u e ali della scuola e la rimessa : vi è a n c h e i n d i c a t a la d i r e z i o n e del n o r d geografico. 1 vari s i m b o l i c o n t r a d d i s t i n t i con le l e t t e r e a, b, c, ecc. i n d i c a n o la posizione degli a p p a r a t i us at i p e r le v a r i e m i s u r e , e c h e ora d e s c r i v e r e m o brev e m e n t e . 584 r . cialdea p . d o m i n i c i temperatura virtuale elei cielo. — l ' a p p a r a t o d i g o r c z y n s k i u s a t o p e r q u e s t a m i s u r a è c o s t i t u i t o da u n a serie di c o p p i e t e r m o e l e t t r i c h e , le cui s a l d a t u r e d i s p a r i , a n n e r i t e , sono disposte in m o d o da f o r m a r e u n a s u p e r f i c i e o r i z z o n t a l e di circa 1 c m 2 c h e , r i c o p e r t a da u n a c a l o t t a di v e t r o , v i e n e esposta in l u o g o a p e r t o . q u e s t o a p p a r a t o , c o n t r a s s e g n a t o n e l l a fig. 2 con la l e t t e r a g, v e n n e disposto sulla t e r r a z z a p i ù alta della scuola, cioè q u e l l a della rimessa, d a dove p r a t i c a m e n t e era visibile t u t t o l ' o r i z z o n t e . l e m i s u r e v e n i v a n o eseguite s a l t u a r i a m e n t e d u r a n t e e f f i g . 2 la g i o r n a t a , spesso al t r a m o n t o , e s c l u d e n d o di volta in volta con a d a t t i s c h e r m i l a r a d i a z i o n e d i r e t t a del sole. nefoscopio jotoelettrico. — i n u n a n o t a p r e c e d e n t e (2) e r a n o s t a t e d a t e le n o r m e p e r la s i s t e m a z i o n e di tale' a p p a r a t o , in m o d o c h e l'influenza d o v u t a alla diversa posizione del sole sulla volta celeste dur a n t e la g i o r n a t a fosse r i d o t t a al m i n i m o ; p e r a n n u l l a r e q u e s t a v a r i a z i o n e s i s t e m a t i c a d i u r n a è in p r a t i c a sufficiente d i r i g e r e l ' a p p a r a t o verso il p o l o celeste. p u r t r o p p o l'altezza del polo s u l l ' o r i z z o n t e a k h a r t o u m era m o l t o bassa (circa 15°), di guisa c h e , se si fosse p u n t a t o ( 2 ) r . c i a l d e a : r i c e r c a s c i e n t . e r i c o s t r u z i o n e 1 7 ( 1 9 4 7 ) , p. 6 3 3 . l'osservatorio c e o f i s i c o i n s t a l l a t o a k h a r t o u m 585 l ' a p p a r a t o verso il polo, si s a r e b b e r e g i s t r a t a e s c l u s i v a m e n t e la luce d i f f u s a d a l l e bassissime r e g i o n i d e l l ' a t m o s f e r a , e q u i n d i il g r a d o di p o l a r i z z a z i o n e si s a r e b b e m a n t e n u t o s e m p r e m o l t o basso i n d i p e n d e n t e m e n t e dalla t o r b i d i t à d e l l ' a t m o s f e r a . p e r c o n c i l i a r e la necessità di e l i m i n a r e la v a r i a z i o n e d i u r n a con q u e l l a di p o t e r m i s u r a r e g r a d i di fig. 3 p o l a r i z z a z i o n e a b b a s t a n z a f o r t i da essere i n d i c a t i v i nei r i g u a r d i della t o r b i d i t à d e l l ' a t m o s f e r a , si è d i r e t t o l ' a p p a r a t o verso u n a z o n a a 20° d a l p o l o celeste, cioè ad u n a distanza z e n i t a l e di 55°, s e m p r e n e l p i a n o del m e r i d i a n o locale. la v a r i a z i o n e d i u r n a r i s u l t a n t e è stata poi calcolata con i m e t o d i a c c e n n a t i nella n o t a citata. l ' a p p a r a t o f u s i s t e m a t o n e l l a p a r e t e n o r d della rimessa ed è c o n t r a d d i s t i n t o nella p l a n i m e t r i a c o n tin c i r c o l e t t o n e r o e la l e t t e r a ci; è visibile in dettaglio n e l l a fig. 3, c b e r a p p r e s e n t a la p a r e t e n o r d della rimessa ed u n a p a r t e del cortile. 586 r. cialdea p . d o m i n i c i nefoscopio fotografico. — a n c h e q u e s t o a p p a r a t o aveva b i s o g n o del m a s s i m o o r i z z o n t e l i b e r o , e p e r t a n t o a n d a v a s i s t e m a l o su u n a terrazza. si u s u f r u ì della t e r r a z z a di u n a d e l l e ali della scuola, i n m o d o c h e l ' o p e r a t o r e c h e doveva a z i o n a r e le m a c c h i n e f o t o g r a f i c h e n o n pert u r b a s s e l e a l t r e m i s u r e di r a d i a z i o n e c h e v e n i v a n o e f f e t t u a t e sulla t e r r a z z a della r i m e s s a . nella p l a n i m e t r i a il n e f o s c o p i o f o t o g r a f i c o è i n d i c a t o con u n t r i a n g o l i n o e la l e t t e r a c. l ' a p p a r a t o è visibile n e l l a fig. 1 della n o t a p r e c e d e n t e (m. 1 rilievi fotografici e r a n o seguili salt u a r i a m e n t e d u r a n t e i l giorno. elettricità atmosferica. — le s o n d e di p o t e n z i a l e del t i p o r a d i o attivo a p o l o n i o , p r e p a r a t e da n o i stessi p r i m a della p a r t e n z a , e r a n o s i s t e m a t e alla s o m m i t à di d u e stili m e t a l l i c i alti r i s p e t t i v a m e n t e 1 met r o e 1,5 m e t r i . c i a s c u n o stilo era isolato alla base da u n grosso blocco d i eletti-ina. u n sottile filo di r a m e collegava l e s o n d e agli e l e t t r o m e t r i d e l r e g i s t r a t o r e , a t t r a v e r s a n d o u n a finestra della r i m e s s a con u n isol a t o r e p a s s a n t e in e l e t t r i n a . n o n vi era a l c u n i s o l a t o r e d i sostegno f r a le s o n d e e la finestra, d a l m o m e n t o c b e il peso del filo e r a p r e s s o c h é t r a s c u r a b i l e , e gli stili e r a n o s a l d a m e n t e t r a t t e n u t i da u n a grossa base di ghisa. si aveva così il v a n t a g g i o di e l i m i n a r e u n c e r t o n u m e r o di isol a t o r i l i b e r i in a r i a : a v e v a m o i n f a t t i osservato c h e , a causa della estrem a secchezza d e l l ' a r i a e della finissima s a b b i a t r a s p o r t a t a talvolta dal v e n t o , gli isolatori si c a r i c a v a n o spesso p e r s e m p l i c e a z i o n e del v e n t o . q u e s t o ci costrinse a p r o t e g g e r e a c c u r a t a m e n t e dal v e n t o t u t t i gli isol a t o r i sia delle s o n d e c b e dei d i s p e r s o r i p e r la m i s u r a della conducib i l i t à positiva e n e g a t i v a . l e d u e s o n d e e r a n o poste a circa sette m e t r i u n a d a l l ' a l t r a , in m o d o d a n o n i n f l u e n z a r s i r e c i p r o c a m e n t e a causa della d e f o r m a z i o n e d e l l e s u p e r f i c i e q u i p o t e n z i a l i p r o d o t t a dagli stili m e t a l l i c i . i dispersori f u r o n o disposti con a n a l o g h i a c c o r g i m e n t i , m a p i ù v i c i n o alla r i m e s s a . le q u a t t r o l i n e e ad i s o l a m e n t o e l e t t r o s t a t i c o p e r v e n i v a n o ai r i s p e t t i v i e l e t t r o m e t r i s i t u a t i n e l l ' i n t e r n o d e l r e g i s t r a t o r e . i d i s p e r s o r i , di f o r m a s f e r i c a , e r a n o a u t o m a t i c a m e n t e c a r i c a t i ogni m i n u t o p r i m o a circa 100 volt positivi p e r l ' u n o , n e g a t i v i p e r l ' a l t r o . a ciò p r o v v e d e v a u n r e l a i s e l e t t r i c o c o m a n d a t o da u n orologio c h e , a m e z z o di c o n t a t t i isolati in p o l i s t i r e n e , c o n n e t t e v a i d i s p e r s o r i a d u e b a t t e r i e a secco. d u e o t r e v o l t e al g i o r n o veniva eseguito l ' a z z e r a m e n t o degli e l e t t r o m e t r i e la l o r o t a r a t u r a a m e z z o di u n e l e t t r o m e t r o a l e t t u r a d i r e t t a t r i i b l ' o s s e r v a t o r i o g e o f i s i c o i n s t a l l a t o a k h a r t o u m 587 t a u b e r . nella p l a n i m e t r i a i circoletti e ed / r a p p r e s e n t a n o rispettivam e n t e i d i s p e r s o r i e l e sonde. stazione magnetica ruska. — questa stazione f u s i s t e m a t a nell'int e r n o del l a b o r a t o r i o e p r e c i s a m e n t e l u n g o la p a r e t e sud. il b a n c o con i v a r i o m e t r i ed il r e g i s t r a t o r e f o t o g r a f i c o sono i n d i c a t i n e l l a plan i m e t r i a c o m p l e s s i v a m e n t e con il r e t t a n g o l i n o b. la fig. 4 m o s t r a la stazione in assetto di f u n z i o n a m e n t o . registratore. — t u t t i gli s t r u m e n t i usati p e r le m i s u r e (4 elettrom e t r i e 2 g a l v a n o m e t r i ) avevano bisogno della r e g i s t r a z i o n e f o t o g r a f i g . 4 fica, ed e r a n o posti a l l ' i n t e r n o di u n g r a n d e r e g i s t r a t o r e a più tracce. q u e s t o r e g i s t r a t o r e ( r e t t a n g o l i n o a della fig. 21, visibile n e l l a fig. 1 in f a s e di m o n t a g g i o , è m o s t r a t o n e l l a fig. 5. p e r questa f o t o g r a f i a sono state r i a l z a t e l a t e r a l m e n t e le t e n d e a t e n u t a di l u c e c h e c o p r i v a n o n o r m a l m e n t e il r e g i s t r a t o r e , e sono visibili i t r e s c o m p a r t i m e n t i in cui q u e s t ' u l t i m o si trovava ad esser diviso. n e l p r i m o s c o m p a r t i m e n t o a destra si t r o v a v a n o gli s t r u m e n t i e le l a m p a d i n e e l e t t r i c h e filiformi p e r la traccia l u m i n o s a ; n e l secondo il r o b u s t o m o t o r e a molla ed il t a m b u r o con la c a r t a s e n s i b i l e ; l ' u l t i m o a sinistra serviva c o m e ca588 r . c i a l d e a p . d o m i n i c i m e r a oscura p e r il c a m b i o della c a r i a e p e r la carica del m o t o r e . si v e d o n o le l i n e e c b e d a l l a finestra di d e s t r a c o n n e t t e v a n o , a t t r a v e r s o isolatori di e l e t t r i n a e b a c h e l i t e , gli e l e t t r o m e t r i ed i g a l v a n o m e t r i agli a p p a r a t i e s t e r n i . i n basso a d e s t r a si n o t a il p a n n e l l o di c o n t r o l l o della s e n s i b i l i t à dei g a l v a n o m e t r i , c b e veniva v a r i a t a a m e z z o d i s h u n t a i n s e r i b i l i con dei c o m m u t a t o r i . d o p o le n e c e s s a r i e p r o v e , il g i o r n o 22 f e b b r a i o f u r o n o i n i z i a l e le r e g i s t r a z i o n i r e g o l a r i . 11 24 f e b b r a i o r i c e v e m m o da r o m a p e r via a e r e a , con n o t e v o l e r i t a r d o , la stazione m a g n e t i c a r u s k a . i l g i o r n o f i g . 5 dell'eclisse l ' o s s e r v a t o r i o f u n z i o n a v a r e g o l a r m e n t e e f u r o n o r e g i s t r a t e t u t t e le g r a n d e z z e ad eccezione della c o m p o n e n t e v e r t i c a l e e della d e c l i n a z i o n e del c a m p o m a g n e t i c o t e r r e s t r e . le r e g i s t r a z i o n i proseg u i r o n o poi al c o m p l e t o fino al 7 m a r z o ; lo s m o n t a g g i o ed il successivo i m b a l l a g g i o degli a p p a r a t i e dei m a t e r i a l i f u c o m p l e t a t o e n t r o l'8 m a r z o . i l 9 m a t t i n a si iniziò il viaggio di r i t o r n o , m e n t r e la missione a s t r o n o m i c a aveva già lasciato k h a r t o u m il 2 m a r z o . 11 17 m a r z o e r a v a m o di n u o v o in i t a l i a . e s p r i m i a m o q u i la n o s t r a g r a t i t u d i n e al p r o f . massimo c i m i n o , che, con la sua attiva c o l l a b o r a z i o n e , h a n o t e v o l m e n t e c o n t r i b u i t o allo s v o l g i m e n t o r e g o l a r e della spedizione. l'osservatorio c e o f i s i c o i n s t a l l a t o a k h a r t o u m 589 d e s i d e r i a m o i n o l t r e r i n g r a z i a r e v i v a m e n t e s. e. mons. f. s. b i n i , vescovo di k h a r t o u m , e la m i s s i o n e cattolica di k h a r t o u m p e r l'ospitalità concessa, e le r a p p r e s e n t a n z e d i p l o m a t i c h e e consolari i t a l i a n e in e g i t t o e nel s u d a n p e r l'assistenza p r e s t a t a c i . roma — istituto nazionale di geo fisica — maggio 1952. riassunto gli aa. riferiscono sopra all'impianto dell'osservatorio temporaneo installato a khartoum in occasione dell'eclisse totale di sole del 25 febbraio 1952. vengono poi brevemente descritte le attrezzature usate per la osservazione di alcuni fenomeni geofisici dell'ottica e dell'elettricità atmosferica e del magnetismo terrestre. summary the autliors report concerning the setting-up of a temporary observatory at khartoum on the occasion of the total solar eclipse of february 25, 1952. the equipment for making observations of some geophysical phenomena (optical and electrical properties of the atmosphere and terrestri al magnetism) lias been briefly described. vol51,2_3,2008 361 annals of geophysics, vol. 51, n. 2/3, april/june 2008 the 1997-1998 umbria-marche post-earthquakes investigation: perspective from a decade of analyses and debates on the surface fractures francesca r. cinti istituto nazionale di geofisica e vulcanologia, roma, italy abstract the umbria-marche sequence represents the first case in italy for which the surface effects of a moderate seismic event (5<m<6) were systematically investigated and documented. surface fractures attributed to the events were mapped by several groups of researchers. the coseismic features were expressed mainly by nw-se trending, discontinuous, linear open fractures, as well as fracture swarms, locally with centimetric vertical displacements. they affected rocks and loose deposits, buildings, and roads. because of the earthquake sizes (m≤6), the deformation was faint, making the detection difficult and giving rise to different seismotectonic interpretations: specifically, surface rupture of the seismogenic fault, triggered slip on secondary faults, and shaking-induced sliding of debris. consequently, different models of connection between surface offsets and displacements at depth were proposed that integrated geology with other geophysical data. however, whether direct or indirect expression of the deep dislocation, the geometry of these fractures reflected the seismogenic structure. even in the case of these moderate-earthquakes features, geomorphical and paleoseismological studies provide data for reconstructing the recent tectonic evolution of the region and for determining the frequency and size of earthquakes. finally, we learned a more efficient way to prepare for post-earthquake emergency response, particularly for long-lasting sequences in highly vulnerable built environments, such as the ancient villages within the apennines. mailing address: dr. francesca r. cinti, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: cinti@ingv.it key words umbria-marche seismic sequence – coseismic surface fractures – moderate size earthquakes – post-earthquake emergency response 1. introduction the 1997-1998 umbria-marche seismic sequence was composed of three 5.5<m<6 main events (occurring at 00:33 and at 09:40 utc on 26 september 1997, at 15:23 utc on 14 october 1997) and three shocks around m=5. the sequence lasted from september 1997 to april 1998 and caused severe damage over a large area (maximum mcs intensity i=ix in some villages near the epicentral area) and loss of human lives. many historical and artistic centers (e.g. assisi and foligno in umbria) were strongly damaged. the sequence belongs to the class of the moderate earthquakes which have devastating consequences in the apennines environment. the enormous cultural and archaeological heritage of several ancient villages and towns renders this territory one of the most hazardous and vulnerable. geologists may contribute to the assessment of seismic hazard through the identification of those faults showing the potential for producing vol51,2_3,2008 4-03-2009 10:28 pagina 361 362 f. r. cinti events of magnitude around 6, which is considered the threshold of energy required for producing permanent dislocation on the earth’s surface (e.g. bonilla, 1988), either primary surface faulting, or faulting associated with the activation/reactivation of secondary faults. the present contribution focuses on the geological investigations started few hours after the occurrence of the first mainshock of the sequence on 26 september. the umbria-marche sequence definitively drew the attention of geologists interested in the detection of coseismic surface deformation. i summarize the many studies developed in the ensuing years on the mapping and the analysis of the surface fracturing in and around the epicentral areas. this paper is organized in three sections: 1) the thenknowledge of geological and seismological data, 2) the seismic crisis and the collection of surface data, analysis and interpretation, and 3) the lessons learned from the experience: insights for assessing the hazard and for improving post-earthquake intervention. 2. our knowledge prior september 1997 the most recurrent consideration that the scientific community asks itself after a natural disaster is: was it somehow predictable? obviously, there is not a positive answer to this critical question, yet certainly before this event we were not completely ignorant. in the following, i will discuss the geological and seismological knowledge at that time. prior to the 1997 umbria-marche sequence, the 1980, ms 6.9, irpinia earthquake in southern italy was the only high-energy event that produced permanent deformation at the surface in «modern time» and drew the interest of several geologists (carmignani et al., 1981; westaway and jackson, 1987). the 1980 event marked the recognition in italy of earthquake geology as a tool for the comprehension of the earthquake generation process. earthquakes of 1997-size and larger were frequent in the historical catalogue of the umbria-marche region, although none had been reported in the epicentral area of the 1997 shocks (fig. 1a). among the earthquakes close to the 1997 epicentral area, the 1279 event is the only one that on the basis of its damage distribution (effects are reported both west and east of colfiorito) could have originated on the same faults as the september 1997 shocks. however, the intensity data of this earthquake are insufficient to estimate an accurate location (castelli et al., 1997; boschi et al., 1998) and thus to univocally relate it to the same fault system. instrumental crustal seismicity pre-1997 in the umbria-marche region included only two m>5 normal-faulting earthquakes (fig. 1a,b): south of the region of colfiorito was the 1979 norcia event (ms5.8, deschamps et al., 1984), and to the northwest was the 1984 gubbio earthquake (ms5.3, haessler et al., 1988). these events occurred as single mainshocks followed by aftershocks, and the instrumental seismicity of the past 20 years in the area does not show any complex and prolonged seismic sequence as occurred in 1997 and in the eighteenth century (chiaraluce et al., 2004). in conclusion, both the historical and instrumental records prior to september 1997 showed that large earthquakes, at least over the past 700 years, had not affected the northernmost portion of the 1997 event area, and its southern portion for an even longer period. despite this lack of seismic activity, the area struck by the sequence in 1997 (fig. 1a,b) had already been investigated by several researchers involved in active tectonics studies. several authors had pointed out the presence of quaternary faults in the epicentral area of the 26 september 1997 mainshocks (i.e. calamita and pizzi, 1993; cello et al., 1997; tondi et al., 1997). on the other hand, no indication for fault activity was recognized within the area where the 14 october event occurred. cello et al. (1997) performed a pioneering structural analysis of this sector of the apennines and determined the presence of three ns-oriented main faults with evidence of left lateral movements, and of nw-se-trending minor faults with oblique to vertical displacements. the colfiorito and cesi basins, epicentral areas of the 26 september events, are bounded on the north by the nw-se trending structures and on the east by the westernvol51,2_3,2008 4-03-2009 10:28 pagina 362 363 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation most fault of the main ns system. cello et al. (1997) found that the nw-se fault bounding the colfiorito basin (le scalette mt. fault) is exposed at the surface for about 7 km and that many of the basin-bordering faults have evidence of late pleistocene-holocene dislocation. most of the faults considered capable of coseismic faulting are located at the interface between bedrock and quaternary (20-15 ka) slope deposits. because the historical and instrumental records indicated that the area of the 1997 sequence was part of a seismic gap at least 7 centuries long, characterized by modest and rare seismicity, the area could have been interpreted incorrectly as a relatively low energyreleasing zone. however, the geological analysis pointed to the seismogenic potential of the area. thus, before the sequence occurred, we could not have predicted «when» an earthquake might occur, but we could have argued about «how» (i.e. rupture mechanism, source complexity), and we did know «where», based on geological-structural analyses. 3. the field reconnaissance in this section i will describe the geologists’ responses to the umbria-marche seismic crisis and summarize the data collected in the field. the seismic sequence consisted of multiple shocks, so that in the description of the facts, i will follow the timetable of the main earthquakes. at 00:33 utc (hereafter called «00:33») on 26 september 1997, the sequence started with a moderate size (mw5.7) yet damaging normalfaulting earthquake near the village of colfiorito (figs. 2 and 3). this event was the beginning of a six-month long period of seismic activity (for extended seismological description, see chiaraluce et al., 2003; 2004, and references therein). the sequence had been preceded by a mw4.5 foreshock on 3 september (fig. 2). early in the morning of 26 september, geologists from nearby institutions were already in the field to document surface effects associated with the 00:33 mainshock. the team of cello et al. (1998) searched for surface faulting features fig. 1a,b. a) historical and pre-1981 instrumental earthquakes (cpti working group, 2004) and b) 1981-1997 instrumental seismicity (castello et al., 2005) within and surrounding the area of the 1997-1998 seismic sequence, indicated by the shaded area (stars are the epicenters of the three mainshocks). vol51,2_3,2008 4-03-2009 10:28 pagina 363 364 f. r. cinti by using as reference the map of capable faults (sensu iaea, 1991) published in cello et al. (1997). they started their fieldwork on the costa-cesi fault (~130n trending, 50°-70° sw dipping, 1-3 m high bedrock fault scarp), that runs along the northeastern steep slope of the costacesi basin (see figs. 3 and 4 for location). behind the village of costa, features observed along fresh exposures of the bedrock fault plane (fig. 5a) suggested a dominant dip-slip motion with a minor left-lateral component. cello et al. (1998) concluded that a maximum 8 cm-high free-face was produced over a total length of ~1 km along the costa segment. before going further, it is worth remarking that although sites were visited during the same period, there were conflicting opinions between different survey teams on the regularity of slip vectors (direction and size) and the linear extent of surface breaks at the costa site, as well as at some of the other surveyed sites. in this synthesis, i report the range of observations (see figs. 5-9) of displacement (d), opening (o) and length (l) associated with the surface breaks as summarized from the various authors (i.e. basili et al., 1998; cello et al. 1998; cinti et al. 1999; vittori et al., 2000). for the original data and complete descriptions refer to the cited references. the range of the measurement values at some sites reflects slip variability, from the description of individual teams of geologists; at other sites, it reflects different values reported by different surveyors. as i will discuss, these latter disagreements contributed to diverging interpretations of the surface data. at 09:40 utc (hereafter called «09:40») a second normal faulting event (mw6.0) occurred just north of the colfiorito village (fig. 2). at fig. 2. surface projections of the faults that ruptured during the three largest events (numbered, north to south) (modified by chiaraluce et al., 2004). the fault-plane solutions of the 3 september foreshock and of the six shocks with m>5 are shown. the distribution of the temporary seismic stations (grey triangles) is also mapped. to the right: the fault parameters for the three mainshocks according to chiaraluce et al. (2003). the authors define shallow, gently dipping blind normal faults; the fault planes are not listric, but rather maintain a constant dip through the entire seismogenic volume. vol51,2_3,2008 4-03-2009 10:28 pagina 364 365 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation that time, several teams were in the field for inspection and many others were on the way. the initial reconnaissance was based on the distribution of damage and environmental effects, and was guided by the geological literature (official geological and structural maps and quaternary fault maps) and by the early hypocentral estimations. although it was important to collect information on everything that occurred at surface, major efforts were undertaken to identify possible surface fracture systems and/or coseismic surface faulting. the general features of the observed ground ruptures commonly agreed upon by all teams of investigators can be summarized as follows: most of the features were either linear single fractures or fracture swarms, affecting paved and unpaved roads, buildings, soil and hard rock. the ruptures frequently showed en echelon geometry and extended continuously for tens of meters, with prevailing nw-se to nnwsse direction. the deformation was mainly expressed by open fractures (1 to 20 cm wide), locally characterized by centimetric vertical movement and a minor left lateral component. several ruptures in the 26 september epicentral areas occurred on pre-existing and mapped fault planes at the contact between bedrock and slope debris. figure 3 shows the first maps of the surface displacements published by different groups after the third major event in october; a synthesis is reported in fig. 4. besides the costa site (described above), most of the significant ruptures were observed along the northeastern slope of the colfiorito basin (le scalette mt. and la pintura sites) and on the western slope of tolagna mt. minor movements formed also in the flat area of la palude, and along the western slope of pennino mt. (see fig. 4 for locations and figs. 5 to 7 for illustrations and descriptions). along the slope of mt. le scalette (fig. 5c), discontinuous down-slope movements (~2 cm, few hundred meters long) were observed at the contact between the debris cover and the pre-existing fault plane (125n-135n-striking, bedrock scarp). to the southeast along the basin margin, at la pintura site, a road was crossed by ruptures consisting of 135n to 145n-trending, 2-4 cm open cracks with a right-stepping en-echelon geometry (fig. 6a). most authors do not report vertical displacement in this area, but this is another point of debate. along the western slope of tolagna mt., open cracks and fissures developed fig. 3. first distributions of the surface ruptures published by different groups of surveyors. from left to right: cello et al. (1998); basili et al. (1998); cinti et al. (1999). vol51,2_3,2008 4-03-2009 10:28 pagina 365 366 f. r. cinti for hundreds of meters along the base of a bedrock fault plane (160n-140n striking), where a dark brown band suggested about 3 cm of vertical movement (fig. 6b). interestingly, in the northern area struck by the mw6.0 (9:40) event only a few intermittent cracks formed. they developed transverse to the rock beds of pennino mt., which is a steep sw-dipping monocline formed by alternating limestone and clay layers. cinti et al. (2000) suggest that the surface deformation in the area of pennino mt. was locally accomodated by diffuse slip on the bedding planes. a white strip at the base of the exposed bedrock was also observed, which may be due to sliding of the scree (fig. 7). the costa movements had been observed before the 09:40 event and thus they are likely associated with the early morning (00:33) fig. 4. map showing a synthesis of the coseismic ground ruptures sites from the different teams (modified by cinti et al., 2000). the rose diagrams show the azimuth of fracture planes (some of them refer to multiple sites) and the dip of the planes where observed (from cinti et al., 2000). dotted rectangles indicate the upward projection of the mainshocks rupture planes (capuano et al. 2000; hunstad et al. 1999; salvi et al. 2000; amato et al., 1998). vol51,2_3,2008 4-03-2009 10:28 pagina 366 367 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation event; according to cello et al. (1998) no additional displacement on these fractures were observed in the following months. except for this site, it is not possible from field observations alone to discriminate which of the two 26 september events caused the mapped surface features. however, event-specific contribution to the displacements was detected through analyses of geodetic data (see i.e. salvi et al., 2000; basili and meghraoui, 2001). at 15:23 utc on 14 october, a mw5.6 normal-faulting earthquake hit the area of sellano (figs. 2 and 4), about 12 km south of the september epicenters, and the attention turned to this area. a rather continuous and prominent extensional fracture system was found cross-cutting different surface morphologies in limestone and in holocene alluvium/colluvium at several sites (see fig. 4 for locations and figs. 8-9 for illustrations and descriptions). near the village of rasenna, a n135°-n150° -striking fracture system was formed and showed a maximum vertical fig. 5a,b,c. a) left: fresh exposure of the costa bedrock fault scarp marked by a dark brown strip of soil, which is indicated by black arrows (photos courtesy of e. tondi). this feature is interpreted by cello et al. (1998) as reactivation of the fault plane. middle: detail of the exposure at the costa site. the strip height and also offset roots were used as slip indicators on the fault plane. right: detail of a feature near the village of costa interpreted by basili et al. (1998) as a gravitational structure affecting debris slope (arrows point to the scree band; photo from basili et al., 1998). the white bar indicates the height of the exposure; b) a ~500 m-long set of open fractures (n145 average trend, omax= 5 cm) in a ploughed field by the flat area of la palude (photo by f.r. cinti); c) panoramic view of the colfiorito basin from south-east looking north. white arrows indicate the bedrock fault scarp (photo from messina et al., 2002). to the right: the arrows point to the scree band observed along the mt. le scalette fault plane (photo from basili et al., 1998), where the hanging-wall debris cover detached from the smooth fault plane. symbols: l = length, d = displacement. vol51,2_3,2008 4-03-2009 10:28 pagina 367 368 f. r. cinti offset of ~10 cm (not illustrated). locally, these fractures had a right stepping en-echelon pattern, which was probably related to a minor sinistral component. according to basili et al. (1998), the total length of the zone was about 3 km. between the villages of renaro and mevale, a series of extensional fractures (n140° average strike) formed along a distance of at least 1000 m (fig. 8). as much as 10 cm wide, they showed an average vertical displacement of 5 cm and a maximum value of 15 cm, northeast side up, and a minor left-lateral component (max 5 cm horizontal) (cinti et al., 1999). the breaks climb through the ginepro mt., where they become parallel to the ns-striking cleavage of the strata along the slope (fig. 8). basili et al. (1998) report that the opening of the fractures at renaro had increased a few cm by 15 days after the earthquake. local fig. 6a,b. a) discontinuous set of extensional fractures on a road at la pintura site, which is coincident with a normal fault separating mesozoic formation from late quaternary deposits outcropping on the adjacent slope. fractures generally have a right-stepping geometry: extension and small vertical offset occurs on the 135n to 145n-oriented segments, while ns-oriented segments appear closed, suggesting a left lateral component of motion (photos from cello et al., 1998). to the right: fissure in quaternary deposits formed adjacent to the pre-existing fault plane (photo by p. montone); b) view of the mt. tolagna fault scarp (upper left). to the right: fractured road and, according to cello et al. (1998), reactivated bedrock fault plane showing a free-face (brown strip pointed by the arrows in photo to the lower left), and crack in hillslope deposits (person is pointing the feature). photos courtesy of e. tondi. symbols: l = length, d = displacement; o = opening. vol51,2_3,2008 4-03-2009 10:28 pagina 368 369 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation deformation also occurred just sse of sellano at the cappuccini site, where ruptures defined a ~600 m-long, ~100 m-wide zone of fracturing along a paved road and extended into bedrock outcrops (fig. 9a). to the north, in the mt. lugo area, extensive nw-trending fractures were reported by cinti et al. (1999). here, the zone of fracturing is ~30 m-wide, and the en-echelon pattern of the segments shows a left lateral component to the motion (fig. 9b). deformation in both the mt. lugo and cappuccini areas was first observed prior to the mainshock of 14 october during the fieldwork by cinti et al. (1999). this southward migration of the deformation from the zone of colfiorito to sellano, observable from the fieldwork, was later detected through a detailed analysis of the seismicity (chiaraluce et al., 2003). post-event re-survey showed substantial increases of the breaks, both in length and opening. 3.1. debate on the interpretations of surface data a contentious debate quickly arose over the seismotectonic interpretation of the surface fractures. although the hypocenters were quite shallow, the earthquake magnitudes did not exceed m6 and consequently many of the coseismic surface effects produced were at the threshold of detection and generally faint. however, i believe that this could only partially explain the different amplitudes and directions of slip measured by the surveyors. for clarity, i will differentiate the interpretations of features associated with the 26 september epicenters from those of the 14 october event. for the first observations (including the mt. le scalette, la pintura, and costa sites), two contrasting interpretations and a third that combined elements of both, were provided by different authors. these are summarized in fig. 10 and substantially are as follows: 1) cello et al., 1998; vittori et al., 2000 (among others) interpreted the surface fractures as primary surface faulting. they claim that coseismic surface slip occurred along pre-existing quaternary faults responsible for the recent tectonic evolution of the area. the amount of displacement they observed (4-8 cm) over lengths of a few km is what one should expect for mw=5.7-6.0 events fig. 7. few cm long cracks formed transverse to the rock beds of the pennino mt. white strip at the base of the dark varnished bedrock is possibly related to sliding of the scree. detail to the right (compass lies parallel to crack; photos by p. montone). symbols: l = length, d = displacement; o = opening. vol51,2_3,2008 4-03-2009 10:28 pagina 369 370 f. r. cinti fig. 8. fractures south of the renaro village affecting both recent colluvium/alluvium deposits and bedrock. maximum horizontal offset of ~5 cm is measured by a dirt road (photo in upper middle); arrows illustrate the sense of slip on fractures. upper right: detail of open fracture with vertical displacement (maximum value of 15 cm), northeast side up (photos by f.r. cinti and f. marra). frequent right stepping en-echelon geometry of the fractures was observed on the hill top near ginepro mt. (lower photos; white box is the detail to the right; photos by f. marra). symbols: l = length, dmean = vertical displacement; o = opening. based on empirical database for earthquake faulting (i.e. wells and coppersmith, 1994). they inferred that ground displacement was related to fault slip rather than ground failure due to shaking. in fact, the authors argue that where fractures occurred along reactivated fault segments, surface offset was remarkably constant over tens to several hundreds of meters, while the thickness of slope deposits varied considerably, from a few cm to tens of m. moreover, comparable displacements were observed at sites showing different morphological features, and slope deposits were in dry and generally stabilised by tree root penetration. the authors infer that the seismogenic fault propagated to the surface and that its geometry, as suggested by seismic reflection data, is a 40°-45° dipping earthquake plane that splays upward into three synthetic and antithetic high angle fault branches (see sketch 1 in fig. 10). 2) according to others (i.e. basili et al. 1998; galli & galadini, 1999), the movements observed at the surface were not fault reactivations, but earthquake-induced sliding of debris. the ruptures were «apparent», occurring in correspondence with very steep slopes and with components of movement in accordance with the maximum slope direction. the authors claim that: almost all sites showed displacement as the result of scree or vegetal soil sliding along planes which usually vol51,2_3,2008 4-03-2009 10:28 pagina 370 371 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation fig. 9a,b. a) ruptured paved road at the cappuccini site, south of the village of sellano, where a set of n135°n155° trending fractures extends into bedrock outcrops (see detail to the right) (photo by l. cucci); b) fractures break across an unpaved road in the mt. iugo area. extensional and compressional features are observable in the middle photo. the en-echelon pattern of the segments shows a left-lateral component to the motion (arrows illustrate the sense of motion; 20 cm long ruler for scale). deformation in the form of cracks and mole tracks was followed for few tens of meters beyond the road (detail to the upper right; photos by f.r. cinti and l. cucci). photos are taken after the 14 october event. symbols: l = length, d = displacement; o = opening. coincide with bedrock fault planes; the lengths of the observed surface breaks (~500 m) represent a very small portion of the total length of the quaternary active faults; the surface breaks occurred at the contact with quaternary deposits and did not affect the nearby carbonate basement, where the fault plane was visible; many of the dislocations had both a left-lateral component on the right side and a right-lateral component on the left-hand side of stream incisions crossing the slopes, which clearly indicate a non-tectonic origin. based on this interpretation and on the aftershock distribution, the authors hypothesize a listric fault plane whose vol51,2_3,2008 4-03-2009 10:28 pagina 371 372 f. r. cinti rupture was confined to 1-2 km below the surface (see sketch 2 in fig. 10). 3) still other surveyors (i.e. cinti et al., 1999) concluded that the observed features were associated with triggered slip on secondary extensional faults that accommodated the deformation on a broad scale. thus, most of the ground ruptures have a tectonic origin, although at some sites phenomena such as slope failure and compaction of loose sediments were superimposed. in these areas, the authors argue that minor amounts of both left and right horizontal motion were found, no ruptures occurred in bedrock, and the amount of slip observed at sites was highly variable and controlled by local topographic and near-surface geological conditions (similar argumentations reported by authors in interpretation n. 2). cinti et al. (1999) emphasized that most of the ground ruptures formed locally continuous zones of deformation whose trends are consistent with the regional stress fields and that lie on or near known quaternary faults. however, they noted that the zones of ground rupture are significantly shorter than the cumulative length of the three faultrupture planes. this interpretation was supported by the lack of correspondence between the location of surface fractures and the up-dip surface projection of the rupture planes (sketch 3 in fig. 10; the red line is the most reliable fault plane solution from seismological data for a constant dip through the entire seismogenic volume). fractures localized close to the vertical projection of the rupture tip and in the hanging wall of the fault. fig. 10. three different interpretations of the ground ruptures and of the fault plane geometry associated with the 26 september earthquakes (corresponding sketches to the right): 1) primary surface faulting and listric fault geometry at depth by cello et al. (1998); 2) blind faulting and shaking-related ground failure by galli and galadini (1999); 3) secondary surface slip and postulated geometry of seismogenic fault (assuming constant dip) by cinti et al. (1999). sketch to the right: black squares indicate the location of ground ruptures; location of hypocenters and relative errors are from amato et al. (1998). vol51,2_3,2008 4-03-2009 10:28 pagina 372 373 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation there was more agreement in interpreting the deformation associated with the 14 october sellano event as tectonic ruptures, primary or secondary faulting (i.e. basili et al. 1998; cinti et al., 1999; galli & galadini, 1999), although a few authors (i.e. vittori et al., 2000) questioned the tectonic significance of the features and highlighted possible gravitational phenomena (fig. 11). however, it is worth noting that part of the deformation in the sellano area was fig. 11. different interpretations of the ground ruptures and of the fault plane geometry associated with the 14 october event. sketch corresponding to the surface faulting interpretation shows a hypothetical section of the earthquake plane reaching the surface by galli and galadini (1999). sketches corresponding to the blind fault hypothesis: to the lower left is shown the relative location of ground ruptures (black square) and fault plane (assuming constant dip) by cinti et al. (1999); to the lower right are the hypotheses of alternative surface expressions that relate to slip at depth, by vita-finzi and king (1985). the red box may reproduce the umbria-marche case: the surface is characterized by secondary structures including folding, but the main faults do not reach the surface. vol51,2_3,2008 4-03-2009 10:28 pagina 373 374 f. r. cinti first documented prior to the mainshock of 14 october during the fieldwork by cinti et al. (1999), and it showed a substantially increased post-event. one interpretation from cinti et al. (1999) is that the southern area of sellano was getting ready for the major failure on the 14 october and was being tectonically deformed. this is in accordance with the hypothesis of secondary faulting within a broad zone of deformation. a contribution to the debate on the significance of the geological effects comes from the aftershock and slip distribution (see following section for fault modelling), which revealed fig. 12. a) four inferred zones of ground breaks (blue dashed lines), maximum width of 1.5 km (blue rectangles include the sites of measurements belonging to each zone). the two northern zones are ~9 km long and those to the south are ~5 km long. azimuthal diagrams of orientation of all ground ruptures for each zone are compared to the focal mechanism of the largest events and to the area of their fault plane (capuano et al. 2000; hunstad et al. 1999; salvi et al. 2000; amato et al., 1998). green solid lines are ruptures along pre-existing fault; b) comparison between zones of ground breaks (blue dashed lines) (cinti et al., 1999) and aftershock distribution (amato et al., 1998). red arrow indicates the right step between the northern and southern fracture zones, the september and october fault planes, and the aftershocks distribution (modified by cinti et al., 2000). yellow rectangles includes ns shear zones inferred from trends of the mapped faults (grey lines, from cello et al. (1997)), and geological and geomorphic observations (cinti et al., 2000; marra et al., 2000). a mw 4.3 event on 16 october event had its epicenter within one of these shear zones and seismological data indicate it occurred on a ns striking, left-lateral fault plane. on the lower left, seismicity distribution along a sw-ne vertical section from deschamps et al. (2000) (see dark green marks in fig. for profile location); the location of costa-cesi zone of ground breaks is indicated by the blue arrow in the cross section. vol51,2_3,2008 4-03-2009 10:28 pagina 374 375 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation that the earthquake ruptures died out at around 1 km depth (e.g. chiaraluce et al., 2003). only the fault rupture of the “9:40” 26 september main shock (mw = 6.0) seems to be shallower than 1 km. another significant result from seismological analyses is that all the activated segments show a planar geometry, without any flattening or evidence for listricity at depth (e.g. amato et al. 1998; chiaraluce et al., 2003). the above described differences in interpretation are significant for hazard assessment. length and slip of the seismogenic fault are parameters used for the estimation of seismic moment, and the issue of whether ground ruptures are primary fault rupture has implications for whether their extent and displacement can be used to estimate rupture dimension and thereby forecast the magnitudes of future earthquakes. the goal of this paper is not to provide a final answer in this case, and i prefer to look beyond differences in interpretation and analyze general aspects of the surface data that may help us comprehend the mechanism of rupture, independent of the origin of the features. first, in the next paragraph, i will discuss other types of data collected during the post-earthquake inspections related to the earthquake distribution, mechanism of rupture, and coseismic deformation. 3.2. surface data vs seismological and insar data the umbria-marche sequence provided the rare opportunity to compare data of local surface fig. 13. inferred zones of surface fractures (blue dashed lines) and interpreted interferometric fringes from salvi et al. (2000) (modified from cinti et al., 2000). each contour line indicates an increase of the satellite-to-ground distance of 28 mm. a maximum increase (subsidence) is in and north of the epicentral areas. to the right: differential interferogram of the entire sequence by salvi et al. (2000). each contiguous inward colour cycle represents a displacement increment of –28 mm. symbols: 1 = “00:33” 26 september earthquake; 2 = “09:40” 26 september earthquake; 3 = “15:23” 14 october earthquake; a = annifo; n = nocera umbra; c = colfiorito; s = sellano. vol51,2_3,2008 4-03-2009 10:28 pagina 375 376 f. r. cinti geology with a large and complete set of seismological and geodetic data, and to test the consistency of the interpretations of the surface breaks. in order to facilitate comparisons, cinti et al. (2000) arbitrarily linked the single sites of ruptures and inferred four parallel zones of deformation (fig. 12a). the zones to the north for most of their length follow mapped faults bordering the basins (fig. 12a), occurring in correspondence with steep slopes. in contrast, those to the south occur where no active faults were mapped and extend into different morphological settings. from the agreement between fractures, focal mechanisms and modelled fault planes shown in fig. 12a, we see that the surface data reflect: 1) the general orientation of the mainshock faults below the earth’s surface (discrepancy of ~30° for the 14 october fault); 2) the prevailing extensional mechanism of ruptures (in the ne-sw direction); 3) the dip toward sw of the fault planes (although less constrained by few data). from fig. 12b, we see that the surface data follow: 1) the nw-se elongation of the aftershock area; 2) the northeastern and southwestern boundaries of the aftershock distribution, except for the costa-cesi site, which is on the vertical projection of a secondary plane defined by the seismicity (see seismic section from deschamps et al. (2000) in fig. 12b); 3) the ~5 km-wide right step in the fault planes and in the aftershock distribution. this «offset» is marked by transverse structures (see quaternary faults from cello et al. (1997)), possibly responsible for the southward stress transfer. since the first analysis of the ground deformation in conjunction with the structural setting of the area, many authors referred to a geometric control of the fault plane dimensions by pre-existing lineaments. among others, meghraoui et al. (1999) introduced the concept of «fault fragments» to explain the sequence: that is, the dimensions of the source faults for the three mainshocks are limited by transversal thrusts and by basement outcropping. in the map of fig. 12b, three main ns shear zones are inferred. the shear zone farthest to the east is represented by a series of sub-parallel n-striking faults (mapped by cello et al., 1997), and the zone farthest to the west extending through nocera umbra has been deduced from geological and geomorphic observations by marra et al. (2000). both zones appear to confine the sequence; it was hypothesized that these act as structural barriers that control the dimensions of the seismogenic structure (cinti et al., 2000; marra et al., 2000). surface observations from cinti et al. (2000), such as the sharp and aligned west edges of the colfiorito and costacesi basins (fig. 4), suggested the presence of a third n-s tectonic discontinuity in the area between the 26 september mainshocks. it is worth noting that a m4.3 shock occurred on 16 october (fig. 12b) in between the 26 september epicenters, on a ns striking, left-lateral fault plane (based on focal mechanism and seismicity distribution). before the 16 october event, no faults had been associated with this plane, but the existence of the ns discontinuity at that site was inferred, and identified as the possible cause of the opposite propagation direction of the two ruptures (amato et al., 1998; cinti et al., 2000). all these concepts proposed at the beginning of the studies have been supported, refined and developed in the time after the seismic crisis by many researchers, using additional analysis of detailed seismological and structural data (e.g., for segmentation model of the faults see chiaraluce et al., 2005; for the 16 october strike-slip sequence and fault reactivation analysis see collettini et al., 2005). another view of the sequence comes from the mapping of the coseismic ground displacement field obtained by stramondo et al. (1999) using the sar differential interferometry (dinsar) technique (see also salvi et al., 2000; lundgren and stramondo, 2002). figure 13 shows the relation between the location of the surface breaks and the dislocation field depicted by the interferometric fringes. the observations made by cinti et al. (2000) from this comparison are: 1) the northeastern zone of surface breaks is located in an area of very low uplift (2-3 cm) (see salvi et al., 2000; moro et al., 2007 for detail); 2) the costa-cesi fractures formed along the southern portion of the axis of maximum subsidence, which occurred north of annifo. few sites of fractures were observed in this area of maximum deformation and most of vol51,2_3,2008 4-03-2009 10:28 pagina 376 377 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation these were classified by several survey teams as landslide phenomena (i.e. esposito et al. 2000); 3) the fracture zone of renaro-mevale coincides (in location and orientation) with a high gradient depicted by the fringes for the 14 october event. the zone of fractures north of and around sellano (mt. lugo-cappuccini) locates in a zone with lower gradient. cinti et al (2000) concluded that the comparison between inferred linear ruptures and dislocation field indicates that the fractures were not random but represented the fragile accommodation of a vertical deformation at the large scale. several fault models have been developed using gps and dinsar data, and they do not converge to the same best-fit solutions. the fault model proposed by salvi et al. (2000) suggests that the early 26 september rupture occurred on a blind fault, while the rupture of the “9:40” shock was shallower and intersected the surface. for this second event, hernandez et al. (2004) inverted the geodetic data and obtained a modelled fault with a larger slip patch close to the hypocenter and a second smaller and shallower slip patch to the northeastern edge of the fault (area of mt. pennino). a different model for this fault is obtained by de martini et al. (2003) from elevation changes analysis. these authors suggest a blind fault with variable slip. the models of a blind fault for the “9:40” september event show a general agreement with the very poor evidence of surface offsets in the area northwest of colfiorito (including annifo and pennino mt.). 4. geological input for seismic hazard evaluation in this section, i will focus on several issues brought forth by the 1997-1998 sequence that are important for the evaluation of seismic hazard in the region, as well as for areas with comparable seismotectonic characteristics. the 1997-1998 umbria-marche sequence confirmed the magnitude threshold of around 6 for which we expect significant surface fracturing. this is one of the most important outcomes of this seismic crisis, since it validates the possibility that geologists may contribute to the hazard assessment in areas characterized by moderate-size seismicity. whether due to primary or secondary faulting, these sorts of faint surface effects may provide information on the seismogenic rupture. moreover, a multidisciplinary approach and the conjunction of distinct data sets provide the best formula for characterizing the nature of surface faulting and its behaviour through time. if we determine that before the 1997 sequence, comparable earthquakes in the area generated similar ruptures and at some of the same sites, then we have the fig. 14. view and log of the southern wall of the trench dug across the renaro surface breaks (from pantosti et al., 2000). the 1997 breaks correspond at depth to a well-defined fault zone. suggestion for the occurrence of an event before the 1997 event comes from the larger displacement of unit 3a with respect to that of 1997, along with a buried colluvial wedge (unit 2). vol51,2_3,2008 4-03-2009 10:28 pagina 377 378 f. r. cinti possibility of understanding how faults may fail in the future, the recurrence of such events, and rates of slip. in hopes of characterizing the hazard, during the ten years after the sequence, geologists put their efforts into understanding the past seismic behaviour of the 1997-1998 seismic sources. two types of features were studied, the outcropping active faults and/or of the observed surface ruptures. paleoseismological techniques have been the most fruitful for investigating the medium to short term behaviour of the surface-rupture-producing faults. to date, the only paleoseismological excavations across the surface ruptures were done by pantosti et al. (2000) at the renaro site (fig. 14). the trenches revealed that similar ruptures had occurred prior to 1997, suggesting a recurrence of the source with 1997-type earthquakes (mmax=6). unfortunately, the interval of time defined for the penultimate event (170 b.c.pre 1997) was too large for a direct comparison with the historical seismic catalogue. the authors could not discriminate between primary or secondary faulting, and generally speaking, when ruptures are related to secondary faulting (or especially landsliding or liquefaction) there is uncertainty in reconstructing the seismic history of a specific source. this uncertainty may be reduced through additional constraints, such as the collection and analysis of data from many sites. pantosti et al. (2000) used the observational record provided by local people for the last 5060 years led to exclude the possibility that the deformation prior to the 1997 at the renaro site had been produced by the nearby 1979 and 1984 earthquakes (see fig. 1 for locations). the promising results of this study should encourage geologists to do further paleoseismological studies in the area. other studies have been conducted on the history of the basin bounding faults and particularly on the evaluation of cumulative coseismic displacements and long-term slip rates through geomorphological data analyses (field survey, dem and aerial photointerpretation) (e.g. cello et al., 2000). messina et al. (2002) fig. 15. view of a remnant land surface carved in carbonate bedrock above the colfiorito basin (messina et al., 2002). to the right is a two-part scheme for the evolution of the colfiorito basin. below that is the 1997 surface coseismic deformation as inferred from dinsar (messina et al., 2002). the maximum deformation (lowering) coincides with the deepest depocenter of the basin). vol51,2_3,2008 4-03-2009 10:28 pagina 378 379 the 1997-1998 umbria-marche post-earthquakes surface investigationthe 1997-1998 umbria-marche post-earthquakes surface investigation analyzed remnant land surfaces and borehole data to reconstruct the quaternary evolution of the colfiorito basin and to define the present tectonic regime (fig. 15). in contrast with other authors (i.e. cello et al., 1997, 2000; chiaraluce et al., 2005), they concluded that the evolution of the basin is no longer related to the bordering fault (see also de martini et al., 2003 for evidence of present fault inactivity). since middle pleistocene time, a new depression has been superimposed on the previous half-graben whose evolution is related to the lowering of the inner portion of the basin through warping (figs. 15a,b). moreover, messina et al. (2002) assert that the activity related to this younger depression does not result in fault-related surface displacements but only in «continuous» deformation spread over the basins (messina et al., 2002). these results indicate that the diffuse deformation observed in the 1997 sequence and the locus of subsidence imaged by dinsar (fig. 15c) reflects the long-term pattern of deformation. based on this consistency, messina et al. (2002) suggest that the recent history of the basins results from the cumulation of deformation episodes similar to those of 1997 (magnitude not larger than 6). finally, the sequence has focused attention on the role of tectonic fabric on controlling the dimensions of earthquake sources. as inferred for the 1997-1998 sequence (see section 3.2), geometric and structural discontinuity may provide information on the propagation behavior and dimensions of the faults, and consequently the earthquake sizes. in this regard, a detailed knowledge of the historic and paleoseismic history of the whole fault zone is required. 5. a lesson to remember: perspectives on post-earthquake investigations the umbria-marche sequence was a test bed for post-earthquake intervention in the field. it taught us that: 1) it is critical to undertake an immediate field response and to coordinate interaction between researchers to compare the surface observations directly at site, in order to avoid later debates on the record of observations. timely response is needed since the ground deformation undergoes changes in time, and minor effects are perishable. discussions on interpretations should be necessarily left to after the emergency when time allows and other sources of data are available. however, the length, size and direction of fractures have to be documented as objectively as possible; 2) monitoring of the region in time and space is critical to obtaining full coverage of the affected area. this is particularly true for long-lasting sequences characterized by multiple shocks with m>5 that can result in multiple episodes of deformation over a wide area. the southern area of the 1997 sequence was affected by ground movements before the nearby 14 october shock as testified by the significant surface fractures observed prior to that event. this may mean that from detailed fieldwork during a seismic crisis we can notice migration of deformation from one area to another; 3) frequent updates on the development of the pattern of seismicity is important for guiding geologists in the field. last but not least, we had confirmation that increasing technological capabilities play an important role in field investigations. for example, most of the geologists in 1997 did not have high quality gps receivers for precise mapping of the features observed in the field. even digital cameras and telecameras were not common, so that it was not possible to view images in the field to make sure features were properly documented. the present technologies would allow a good video collection and a more efficient data archive. from this experience, the earthquake science community became aware of the requirements and problems to be solved during a seismic crisis and it became the impetus for efficient initiatives to prepare for future possible events. several groups involved in the full spectrum of earthquake investigations (gps, strong motion, geotechnical, damage distribution, seismic network, geology) started to organize for post-earthquake response. regarding earthquake geology, in 2002 scientists and technicians from the istituto nazionale di geofisica e vulcanologia (ingv) formed a team called emergeo for coordinating geological field surveys in areas hit by moderate-to-large earthquakes. response protocol prescribes procevol51,2_3,2008 4-03-2009 10:28 pagina 379 380 f. r. cinti dures to follow during the emergency and guidelines for collecting surface data in the field and for creating real-time maps. the underlying message is that scientists have the responsibility to document the location and surface extent of fracturing in post-earthquake studies as completely and reliably as possible, both for immediate scientific and social benefit and to provide high-quality data to future investigators. acknowledgements i am grateful to francesca di luccio for providing the map of the umbria-marche instrumental seismicity, and to luigi cucci for fruitful comments on the manuscript. special thanks to the referees, suzanne hecker and francesco salvini, that contributed to improve the paper with their comments and suggestions. this work was presented for the decennial of the umbria-marche sequence at the workshop «colfiorito 1997-2007: evoluzione degli studi di un terremoto» ingv-uniroma3 (rome, 810 october 2007). references amato, a., r. azzara, c. chiarabba, g.b. cimini, m. cocco, m. di bona, l. margheriti, s. mazza, f. mele, g. selvaggi, a. basili, e. boschi, f. courboulex, a. deschamps, s. gaffet, g. bittarelli, l. chiaraluce, d. piccinini and m. ripepe (1998): the colfiorito, umbria-marche earthquake sequence in central italy (sept.-nov., 1997): a first look to mainshocks and aftershocks, geophys. res. lett. 25, 2861-2864. basili, r., c. bosi, v. bosi, f. galadini, p. galli, m. meghraoui, p. messina, m. moro and a. sposato (1998): the colfiorito earthquake sequence of september-october 1997. surface breaks and seismotectonic implications for the 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for sar interferometry and gps, geophys. res. lett., 26 (7), 883-886. tondi, e., g. cello and s. mazzoli (1997): strutture sismogenetiche in appennino centrale: potenziale sismico, analisi frattale e processi di crescita, il quaternario, 10 (2), 409-414. vita-finzi, c. and g. c. p. king (1985): the seismicity, geomorphology and structural evolution of the corinth area of greece, philosophical transaction of royal society of london, a314, 379-407. vittori, e., g. deiana, e. esposito, l. ferreli, l. marchigiani, g. mastrolorenzo, a.m. michetti, s. porfido, l. serva, a.l. simonelli and e. tondi (2000): ground effects and surface faulting in the september-october 1997 umbria-marche (central italy) seismic sequence, j. geodyn., 29, 535-564. wells, d.l. and k.j. coppersmith (1994): updated empirical relationships among magnitude, rupture length, rupture area, and surface displacement, bull. seismol. soc. am., 84, 974-1003. westaway, r. and j. jackson (1987): the earthquake of 1980 november 23 in campania-basilicata (southern italy), geophys. j. r. astr. soc., 90, 375-443. vol51,2_3,2008 4-03-2009 10:28 pagina 381 vol52,6,2009new 615 annals of geophysics, vol. 52, n. 6, december 2009 key words biographies – earth sciences history – expanding earth 1. introduction to the correspondence in may of 1910, raffaele cappelli, president of the società geografica italiana, received a letter containing the following words (1): apart from a few regions that are still little known and the two, strictly polar ones, the earth’s surface is now perfectly explored so that, in our opinion, no further important geographical discovery is possible. yet, it is precisely that perfect awareness of the shapes of all the seas, all the continents and islands that has now led us to an important discovery. […] the discovery in question is the natural complement to the theories that two illustrious italians had first supported. everyone, in fact, knows that with the discovery of the new world, columbus demonstrated that the earth was round. then, along came galileo, who maintained that the earth was moving and, by chance, a third italian came along in the elite company, to prove that the earth is expanding! what is most curious to me is the fact that the third one is, in fact, myself. it does not matter, however, whether i or someone else made the discovery. the discovery itself is what is most interesting. by no means: having found out that the earth expands, obviously very slowly; but continually, is of primary importance for geography, astronomy and paleontology as well. […] roberto mantovani (1854-1933) and his ideas on the expanding earth, as revealed by his correspondence and manuscripts giancarlo scalera istituto nazionale di geofisica e vulcanologia, roma, italy abstract roberto mantovani (parma, 25 march 1854-paris, 10 january 1933) – a musician and geologist trained in the duchy of parma – was a consul in the french island of la réunion at the end of the nineteenth century. in 1878 drawing general conclusions from the similarity in shape between the facing banks of a river that flowed in a volcanic fracture, and the way in which the layers corresponded, mantovani formulated a mobilistic theory, attributing the moving apart of the continents to the expansion of the entire planet. this theory is more general than that of wegener from the first decades of the following century. mantovani’s hypothesis was officially recognised by the french geological society in 1924, which incorporated it in its body of legitimate ideas. encouraged by bourcart in 1924, wegener quoted the italian in his famous book as one who offered ideas extraordinarily close to his own. a letter of mantovani to wegener, and the sceptical answer of the german scientist, have been recently found. in his letter mantovani shows a greater awareness of the predecessors of the continental drift than wegener himself. mantovani continued to disseminate his idea up to the last years of his life. a final pamphlet, of 1930, was printed with this dedication: «to the mathematicians, physicists, astronomers, geologists, and anyone interested in the great enigmas of the universe». his biography has been reconstructed thanks to the correspondences carefully conserved in the archives of the italian ministry of foreign affairs, the ethnographic museum luigi pigorini, the società geografica italiana, and in the private files of his direct descendants. mailing address: dr. giancarlo scalera, istituto nazionale di geofisica e vulcanologia (ingv), via di vigna murata, 605, 00143 roma, italy. vol52,6,2009new 2-12-2009 12:39 pagina 615 616 g. scalera and the certain, totally visible evidence of the ancient but nevertheless ongoing expansion of the layer that makes up the earth’s surface strictly belongs to the geography. it came from the breton town of saint servan, and was written by the violin teacher roberto mantovani (fig. 1), formerly honorary italian consul on the french island of la réunion and once member of the società geografica italiana (italian geographic society). an article on planetary expansion was enclosed. it had been published the previous year in the journal je m’instruis and was accompanied by two clippings from french gazettes containing news items on mantovani and his conferences about problems of global tectonics, volcanism and earthquakes. with these references, he asked to be allowed to set forth his idea of expansion in a lecture to the società geografica italiana (the answer has not been found). mantovani’s words show too much enthusiasm and self-assurance for the author not to be considered to be among those exalted individuals who proposed ideas that were not well founded. the expansion of planets was a hypothesis that had never clearly arisen in western culture before the second half of the nineteenth century (2) (as had been the case for the idea of earth’s orbital motion, which had already existed in ancient greece). even to welleducated people, it might have seemed rather odd. at that time, mantovani had not yet come into full contact with academic circles and his way of expressing himself was not tempered by the usual astuteness and tact of institutional researchers. in 1929, the article in je m’instruis was quoted by wegener in the fourth edition of his book die entstehung der kontinente und ozeane with these words (p. 3 of the 5th edition): in 1909 mantovani offered his ideas on continental shift, illustrating them with several maps. his ideas differ from mine in part, and, in part, there are surprising points in common, as for example, those on the ancient grouping of the southern continents around southern africa. the german scientist was induced to cite mantovani from an article by jacques bourcart fig. 1. photographic portrait of roberto mantovani in a publicity of his scool of music in saint servan. it is the first page of a three folded brochure. vol52,6,2009new 2-12-2009 12:39 pagina 616 617 roberto mantovani and the expanding earth from 1924 (gohau, 1990) which emphasised the points in common between the conceptions of the italian and those of the german meteorologist (bourcart, 1924, pp. 563-564): it is curious to note, in the history of science, that when a new hypothesis is put forth, it generally turns out that it has been simultaneously conceived by various scientists. […] it is interesting to point out, that wegener was not the first to have a notion of the idea of continental shift, as imagined by the learned meteorologist from hamburg, and up till recently unknown in our circles. […] in december 1889, m.r. mantovani presented a note to the société des sciences et arts of la réunion on les fractures de l’écorce terrestre et la théorie de laplace, in which he explained the shape of the oceans in terms of a fan-like continental shift. […] here, a discussion of mr. mantovani’s explanation of this shift, earth’s expansion, cannot be included. it was, however, curious to note that alfred wegener had not been the first to foresee the movement of continental masses. given the rarity of periodicals in which mr. mantovani published his notes, he could never have been aware of the ideas of his predecessor. that same year, the italian was introduced in paris as a member to the french geological society by the renowned geologists pierre termier and paul lemoin. he, too, wanted to come into direct contact with wegener in order better to explain the reasons why the german could not lay claim to priority over the idea of continental shift. in the draft of a letter to wegener, still among the mantovani-family documents, he wrote (fig. 2a) (3): dear professor, i read with great interest your theory on continental shift in the translation of m.m. reichel published by a. blanchard – paris. 1924. […] kindly allow me to bring to your attention the fact that observations on the similarity of the coasts of brazil and guinea had been made by saint-beuve, a french critic, towards 1860. another, similar, but further developed observation had been made by edward forbes quoted by darwin in his work the origin of species published in 1859. according to darwin, this forbes claimed that all the continents and islands must have been joined together in a relatively recent era. darwin did not accept this, and, for this reason, did not develop the subject. for my part, i visited various points in the indian ocean in 1878 and, in september of that year, i reached the island of la réunion (bourbon). visiting the river of saint-denis, near the city of that name and capital of the island, i noticed that the ramparts below which the river flowed had perfectly coinciding curves and i concluded that this river must have been a fracture caused by volcanic activity on the island from ancient times. since i tend to make generalizations about the facts, i wondered if a sea would not likewise have been a great fracture of the earth’s crust. looking at my atlas, i indeed discovered that all the seas had coasts with corresponding shapes despite the enormous distance sometimes separating them. in 1884 i presented my observations to the famous astronomer c. flammarion who found them quite singular and interesting. in december 1889 i presented a note on les fractures de l’écorce terrestre to the société de sciences et arts of the island of la réunion which was published in the bulletin of that society in 1889. from my observations i had noted that all the continents and islands must have made up a single block in the far distant past. as a totally natural and incontestable consequence, the earth, in the first period of its evolution, must have been much smaller than it is now; its diameter must, in fact, have been a third of the present one. the surface film of the earth’s membrane had split due to the force of expansion of the inner gas from the core which gradually and extremely slowly stretched the aforementioned membrane. as a result, continents and islands necessarily shifted. in this correspondence, it is understood that mantovani himself did not think he had been the first to conceive of continental shift, but just expansion. he also was not aware of at least three predecessors, the first one, thomas dick, an eclectic scientific writer that on p. 93 of his book of 1838 celestial scenery made considerations on the similarities among the opposite atlantic coasts, and on the possibility that those continents was once in contact (goodacre, 1991). also antonio snider-pellegrini – who first drew a map of the continents before and after the «supercontinent breakup» (snider-pellegrini, 1858, engraving n. 9 and n. 10 between p. 314 and p. 315) and proposes huge decreasing of the earth’s size during the days of biblical genesis (snider-pellegrini, 1858; scalera, 1999, p. 168) – was apparently unknown by mantovani. vol52,6,2009new 4-12-2009 15:52 pagina 617 618 g. scalera fig. 2a-c. a) rough copy of the letter of mantovani to wegener (property of yvane mantovani poirier) with the aim to made clear to the german scientist that the priority of the idea of the continental displacements is antecedent to wegener and mantovani itself. see the translation in the text. b) the translation in french of the answer of wegener to mantovani (property of yvane mantovani poirier), conserved by the italian among the pages of the book of the german about the continental drift. see the translation in the text. c) the answer of mantovani to the scepticism of wegener. the italian wrote at page 209 (sheet 48) of his manuscript la dilatation planetaire: such an objection is simply specious because it is completely impossible to assume that the outermost shell of the earth has been able to contract so much as to cover a surface area less than a quarter of the total surface of our planet. in addition, this is all the more true because the author of the objection admits that «the idea of an expansion of the earth is certainly very convincing as far the huge fractures that occurred in the outer crust of the sphere» are concerned. a) b) vol52,6,2009new 2-12-2009 12:39 pagina 618 619 roberto mantovani and the expanding earth in the field of the expansionists (4), mantovani did not mention the scottish geologist and chemist richard owen (1810-1890), first president of the purdue indiana university and son of the social reformer robert owen (17711858; founder of the american egalitarian community new harmony, indiana). richard owen had his first training in geology at hofwyl in the swiss alps and wrote in 1857 a treatise on the earth that were interpreted as growing like a biologic organism. likewise neglected by mantovani was captain alfred drayson, another eclectic writer which in a page of his book of 1859 the earth we inhabit: its past, present and probable future published a cross section of the earth comparing its past (near half radius) and present size (carey, 1988, pp. 137138; noël, 1989, p. 30). finally mantovani had no knowledge of the book of the russian engineer jan yarkovski, published in french in 1888, about the nature of the gravity, which deduced – as one of the consequences – the expansion of the earth. the original document of wegener’s reply has been lost. it exists only its translation from german to french, which is still attached to the first page of mantovani’s copy of the book la genèse des continents et des océans published by blanchard in 1924, translated from the third german edition by reichel. wegener replied (fig. 2b): i am about to depart for graz and your letter reached me here, so i beg your pardon for the delay in replying to you. i do not have a very good command of french, so kindly allow me to use german. your letter was of great interest to me and, when my book is reprinted, i shall be pleased to refer to your work of 1889; unfortunately i am not in possession of the bulletin of the société des sciences et arts of the island of la réunion. if you still had a copy that you could make available to me i would be very grateful. your idea of earth’s expansion is certainly very convincing as far as the large fractures in the outer sial sphere are concerned, but it seems to me to be incompatible with the results that studies of earthquakes offer on the nature of the inner earth. how could a body as solid and rugged as the earth expand to such a dec) vol52,6,2009new 2-12-2009 12:39 pagina 619 620 g. scalera gree? it seems to me, moreover, of no use to accept the idea of this sort of expansion. in fact, the layers of gneiss of the sial sphere is composed are so tightly packed together that in the case of a levelling of all the folds, they would tend to form a complete crust around the earth’s mass, within its present volume. aside from this fundamental problem that you, as i had done, pointed out many of the particularities of the theory of movement it would be very interesting for me, i repeat, to further explore your stimulating work. alfred wegener graz 27 june 1924 wegener expressed his doubts about the possibility of earth’s expansion because he thought the planet was rigid and solid. indeed, it was only in 1926 that harold jeffreys established that a liquid nucleus was necessary. he based his conclusion on global rheological considerations. prior to that date, geophysicists had thought the planet to be generally solid, since both the p and s waves were propagated over a long distance – whereas in much of the nineteenth-century geological tradition, it had been thought to be liquid. this disappearance of the s when passing through the liquid nucleus had not been considered with any degree of certainty. in any case, wegener could not escape mantovani’s reasoning that all the opposite shores of the oceans could be joined back together to form a continuous fabric, one which would obviously have a size smaller than that of present-day earth. he then quite cleverly invented a counter-argument, which enabled him to explain the opening-up of all the oceans over a constant earth’s diameter: the contraction (due to subsequent folding) of all the continents. topologically equivalent to expansion, this line of thought was then defended, independently from wegener, by the french scientists chevallier and cailleux (cailleux, 1968, p. 212; cailleux, 1976; scalera and meloni, 1991, pp. 209-210; scalera, 2001, pp. 352-353). now, however, it encounters insurmountable difficulties. mantovani was not discouraged by wegener’s reasoning (fig. 2c) and, with greater energy, developed arguments in favour of expansion. he had already had experience in producing long manuscripts – which, however, he did not succeed in publishing. among his papers there is a notebook of about 190 pages entitled la radiation universelle, 1912, and a subsequent, extended version (mantovani, 1932b) having the same title, whose aim was probably to write something that would have the weight and, hopefully, the impact of wegener’s book on the general public. the second part of this long text (mantovani, 1932b) in manuscript form with figures – from p. 164 to p. 263 – has come down to us thanks to the loving care of his descendants. the first part was perhaps that text la terre grandit (mantovani, 1930) badly printed in parma by printers who were not at all up to the task, a booklet which is still conserved in some italian libraries (palatina of parma; the biblioteca nazionale in rome; the library of sgi società geografica italiana). this latter, long manuscript, which has yet to be studied together with the new material furnished in november 2003 by the heirs, appears, upon a first cursory reading, to be much more mature than the 1910 one, and full of reasoning based on geological as well as geomorphologic considerations drawing on the thencurrent literature. interaction between the author and the french geological society had been fruitful. mantovani’s attempts to present his ideas to the italian geographic society continued in 1930 with a new letter to the society’s secretary saying (5): parma, 20 september, 1930 – viii dear sir, i have been residing in paris for many years and came to parma last july to see many of my relatives. prior to returning to france i should be very pleased to communicate to, or rather, give a lecture before this highly honoured geographic society on continental morphology in the presence of his excellency, the head of state, the honourable mussolini. i am taking the liberty of addressing myself to your honoured society to find out whether and, if so, when this might be possible. confident in the kindness of your society, i wish to apologise for my forwardness and to express my highest regards to you. prof. r. mantovani strada cairoli 10 – parma / paris – rue marcadet 151 bis the postal services of the time were more efficient that the present-day priority-post and vol52,6,2009new 2-12-2009 12:39 pagina 620 621 roberto mantovani and the expanding earth the italian geographic society secretary informed mantovani (6): rome, 22 september 1930 – viii dear professor, in reply to your letter of 20 september, i wish to inform you that the possibility of obtaining the opportunity to give a lecture in the presence of the head of government is not to be hoped for. as for that of giving a lecture to the members of the italian geographic society on the subject you propose, this would certainly be a pleasure. since, however, there is a lecture commission, it would be proper to submit to the judgment of this body the question as to whether or not such a lecture would be appropriate, here or elsewhere. if you wish to send either a copy or a detailed summary, i shall take it upon myself to inform you on this matter. in the meantime, in case you deem it appropriate to become a member, i am enclosing a form. yours sincerely, the secretary mantovani answered shortly thereafter from paris and sent a copy of the short essay la terre grandit, printed in parma with corrections in the margins, which has already been mentioned. there may have been an exchange of letters in between. he explained that the errors were due to the illness and absence of the director of the printing works (7): paris, 8 nov. 1930 dear sir when i had the honour of presenting you with a copy of my booklet la terre grandit i forgot to mention that that work was full of typographical errors. i deeply regret this and beg your forgiveness. the booklet was not corrected because the director of the printing house was ill for several weeks and the workers could not understand the french-language text despite my many corrections. finally, i was obliged to withdraw the work, which was not completed. i had then wished to give a lecture to this royal geographic society on the «continental shapes» since these come entirely within the domain of geography. for that purpose i would have found the maps and globes necessary for my demonstrations thereby avoiding the possible objection that the maps were adapted to my own purposes. since it was not merely a hypothesis but a very important discovery involving in their entirety the physical laws and all the present-day astronomical conceptions, i thought, not on my own behalf which is of little significance, but for the glory of italian science, that the present of his excellency the head of government at the lecture would be highly appropriate to give the proper emphasis to an extremely important italian discovery. indeed, if christopher columbus discovered a new world, planetary expansion is leading us directly to the discovery of a new universe. in the meantime, i am taking the liberty of enclosing another example of my booklet in which i have indicated in red the typographical errors and all the important sentences. once again, i beg your pardon and wish to express my profound respect. prof. r. mantovani paris, 151bis rue marcadet, 18th arrondissment now, mantovani seems to express himself in more measured and modest tones than he had done in the letter of 1910, but he continues staunchly to defend the objective truth of terrestrial expansion. once again, as had been the case previously, he awaited a reply from the italian geographical society, but we do not know what this was. perhaps the time was not yet ripe, especially in consideration of the fact that the idea of planetary expansion is not still readily accepted today in academic circles. 2. biographical notes although roberto mantovani was well known to few specialists (egyed, 1963, quoting a 1962 private communication of fairbridge; carey, 1976, p. 23; carey, 1988, pp. 90, 92, 138; muir wood, 1985, pp. 56-58; gohau, 1990, 1991; segala, 1990, p. 115 in foot-note; scalera 1995, 1997a, 1999 p. 166; scalera 2003) for his ideas on the evolution of the planet earth, little had been known of his life and work up to then. contemporary french historiography had judged him to be an enigmatic character (gohau, 1990, 1991), and dealt with him briefly. this research offers a biography of mantovani which is still incomplete, and based on documents found in various historical archives in rome and parma, and only from november 2003 on additional documents provided by the descendants. roberto mantovani was born in parma on vol52,6,2009new 2-12-2009 12:39 pagina 621 622 g. scalera 25 march 1854, the second of two brothers, of luigia ferrari (golese, 10 april,1827-?) and timoteo mantovani who died six months after roberto’s birth. luigia ferrari, now in dire straits, entrusted the elder child, oreste (parma 6 may 1852-?), to an uncle and aunt from baganzola. she had a permanent job working in the house of don pietro bellati (parma, 1818-?) and kept roberto with her. don pietro was a priest and teacher at local charitable institutions (parma demographic service archives), and perhaps a distant relative. bellati offered parental protection to the child and became aware of his precocious intelligence. although he would have liked him to become a seminarian, he allowed roberto to enter as a boarder in the regia scuola di musica. here the young man won the first prize for violinists at the age of twelve and was awarded the honorary diploma in music in august of 1872. he seems to have completed his education on his own and felt he had a special talent for the exact sciences as well as foreign languages and cultures (from an autobiographical letter, historical and diplomatic archives of the farnesina, see fig. 3). in 1874, he was fit for military service (parma demographic service archives). since he loved to travel, he joined a group of musicians belonging to a theatre group, which went from algiers to the island of mauritius, then to the french island of la réunion in october. the trip was perhaps made for performances given in honour of the initiation of works for the longest railway tunnel in the world, under mont st denis (10,280 m). it was the third to be built after those of st gottard and fréjus), and was carried out by a piedmontese italian company (gohau, 1990; scherer, 1994, p. 81). at the end of a period of great prosperity on the island, local leaders were attempting to solve the serious problem of communications in unfavourable high-altitude conditions. the numerous public works and viaducts constituted quite a bold venture for that era. as soon as he reached saint denis, mantovani observed the resemblance between the complementary shapes of the two edges of the volcanic fracture through which the saint denis river flowed. thus, his first idea came to him: he could make a generalization concerning the oceans on the basis of what he had seen on a smaller scale. young and broke, however, he had no possibility as yet of publishing his ideas. for the same reason, with some regret, he gave up his plans for following several italian expeditions to africa’s interior, and decided to give music lessons. as a teacher of the violin, he came into contact with the most distinguished creole families in the area, and married anna piet in 1880. she was the daughter of a well-todo pharmacist. before 1886, she bore him four children margherita, luigia, italo and roberto (historical and diplomatic archives of the farnesina). three other children were born later. during those years he most certainly devoted himself to developing and refining some of his scientific ideas on which he subsequently published two articles. in 1882 he became a member of the italian geographic society, where he was introduced by the parliament deputy and attorney enrico arisi. in 1884 the paleo-ethnographer luigi pigorini (fontanellato di parma, 1842-1925) wrote to him. he had had mantovani’s name from mr. socrate bellati, perhaps as a friend of the family he had met on holiday and a relative of don pietro bellati. in the letter, he requested that objects from the native culture of madagascar (fig. 4) be sent to enhance the collections of the nascent pigorini ethnographic museum of rome (pigorini ethnographic museum historical archives). in his correspondence with pigorini mantovani proposed a national lottery to finance polar explorations as well as a dictionary of malagasy to be published through the geographic society. the fate of these proposals is unknown. in march of 1886, he requested that the ministry of foreign affairs appoint him to the position of consul of la réunion, for which he had the support of pigorini and baratieri. in april of 1887 he sent the ethnographic museum of rome two cases containing creole objects and some from madagascar (fig. 4). in march 1887, he received the consular licence from paris and, in may, the exequatur (8) from the republic of france. it would be unthinkable for a sharp mind not to come into contact with the intellectual elite of the island, which gathered on the premises of the young société des sciences et arts (founded in 1856; scherer, 1994, p. 66). here he put forth vol52,6,2009new 2-12-2009 12:39 pagina 622 623 roberto mantovani and the expanding earth his idea on reforming the calendar (mantovani, 1888), which had been submitted to an international contest on the subject. although his proposal for 100-day-long months and 1000-day years was not practicable, his concern to obtain a universal time system, freed from the periodic motion of earth’s orbit and democratically usable for all the heavenly bodies was clear. hence, his thought was directed towards general principles, and details linked to contingent matters were of secondary importance to him. for this, he received the praises of camille flammarion. a year later, he published his old idea of earth’s expansion. this idea had been worked out over a long period and always related to general processes operating in the universe. when flemmarion was made aware of the idea of the expanding earth, in 1884, he encouraged mantovani to defend and develop it. la réunion had now plunged into years of disastrous economic decline. the ruling classes were fleeing to other shores, nearby or back in fig. 3. the autobiographical letter of mantovani which was sent to the ministry of foreign affairs in the occasion of his request for the appointment as consul of italy at la réunion island (archives of the ministry of foreign affairs, rome). this letter of mantovani says: «i was born in parma on march 25, 1854. my father, timoteo, died six months later. my mother, luigia ferrari, directed me to the studies, and at the age of 11 i was accepted as boarder in the royal school of music, where i was conferred with the honorific degree, in august 1872. having always preferred the exact sciences and literature to music, it was with great perseverance that i succeeded in completing myself my studies and learning several languages […]». vol52,6,2009new 2-12-2009 12:39 pagina 623 624 g. scalera europe (scherer, 1994, p. 82). mantovani likewise took part in this exodus and began to leave the island and his job repeatedly, probably in order to find means of support during those extremely difficult years. we find him in july of 1893 at port luis (mauritius) giving music lessons then, in 1895 at quatres bornes (mauritius). in 1896, he was sought out at the ministry by a fellow italian from venice. mantovani made numerous trips to france and london – where he lived for three years with his family – and finally decided to put an end to the hardships encountered on the island. he took a leave of absence from his job and went to northern france, to saint servan – a tiny breton village, now a part of st malo. in 1899, he was summoned to the ministry, which asked him to resume his position as consul. he replied that he planned to return in future, but, family matters obliged him to delay his departure: his wife needed to be in a cooler climate, and, according to him, the plague was raging in la réunion. medical records confirm that a plague was raging along the costs of the indian ocean during those years and claimed victims in europe as well (galizzi and palmer, 1985). the executives in the ministry were annoyed by his prolonged absences and inappropriate proposals for projects that seemed too fanciful to them a result of a lack of «mental serenity» on his part (a battleship, a rome-tunis railway, and the occupation of territories with gold and coal mines). they advised him to resign so that they could appoint a new consul, which they quite obviously felt fig. 4. fragments of the letter of mantovani to the italian ethnologist luigi pigorini, in which the delivering of some musical instruments of madagascar is announced, with a drawing of them (archives of the pigorini ethnographic museum, rome). vol52,6,2009new 2-12-2009 12:39 pagina 624 625 roberto mantovani and the expanding earth to be urgent. his resignation took place on 11 july 1900 (historical and diplomatic archives of the farnesina). mantovani settled at saint servan in villa sivori with his wife and daughters (fig. 5a,b). the villa owed its name to camillo sivori (genova 1815-genova 1894), a virtuoso of the violin and his teacher, who was the only pupil of paganini, presenting himself as the continuer of his sublime art. the address was bd surcouf, now divided into rue eugène brouard in the north and bd henry-dunant in the south, perhaps the only italian-style villa (fig 5b) that can be seen in that location today (petout, 1995; poirier, 2005). to earn a living, he once again gave music lessons and became greatly involved in performing concerts for charities (historical and diplomatic archives of the farnesina). printed materials from that time show that his name was associated with the concert of august 1900 for the inauguration of the monument to admiral bouvet. he then gave a concert on 16 july 1902 in the city hall, accompanying the singer rosse. finally, he took part in the concert of the harmonie municipale of saint servan in 1903. there are also records of concerts given by his pupils (mussat, 1996). most probably, he came into contact with the very active société de musique de saint malo (tuloup, 1984) and research is being performed on this question (mussat, 1996). in 1902 his mother and his brother oreste moved to sorbolo, a village near parma (parma demographic service archives). in france he continued to pursue the idea of planetary expansion, to which he wanted to devote himself, according to a letter. he succeeded in publishing a brief summary called l’antarctide which contained some maps (fig. 6a,b) in the popular scientific journal je m’instruis (mantovani, 1909). three maps – the «pangea», its break-up radiating from antarctica and the pacific – were often reproduced in historical essays and papers on the subject (e.g. bourcart, 1924, p. 564; dubard, on le figaro 23 march 1958; muir wood, 1985, p. 58; scalera, 2000, p. 347; scalera 2003a, pp. 71, 73). the mutual position of madagascar and australia – which on the map were in contact along their rectilinear margins (eastern and western respectively) – was defended by mantovani also in his last pamphlet of 1930 (p. 17). it may be that this madagascar-australia contact could have inspired the drawing of a subsequent original «pangea map» published in 1964 by olive and baronnet (1964, p. 36), in which eastern madagascar is separated from western australia only by the narrow island of sumatra. still today, the position of madagascar is subject of outstanding debates. some clues coming from paleontology and biogeography actually can be in favour of a proximity of madagascar to indochina, indicating that also this mantovani’s line of thinking shows potential future fruits. the article on je m’instruis suggests that there must have been contact with the antarctic explorer jean-baptiste charcot (1867-1936). those are the years when wegener offered his first reflections on continental shift (wegener, 1912 a,b). at the end of april, 1910, mantovani wrote a letter to the president of the italian geographic society asking for support from the igs for a lecture he would deliver in rome on earth’s expansion (historical archives of the italian geographic society). this letter demonstrated his total awareness of the responsibilities he had taken upon himself in claiming a truth which would have far-reaching consequences for all the other sciences. italian geographic society records have yielded no evidence of a response if, indeed, there was one. it may be that from 1910 to 1924 mantovani looked after those family interests that, according to him, had called him back to la réunion. only a two part paper from 1910 about «prehistoric cosmogony» (with links to the earth considered as a living organism) and a method for violin technique from 1922 le secret de paganini: méthode des méthodes de violon were published by him. this method perhaps really has described some elements of the true paganini’s technique, considering that mantovani was friend and student of camillo sivori, which was the sole student of paganini and the sole to reach a close imitation of the fingering style of the old master (apart some admitted minor difficulties for the shorter length of his fingers). in this period we have no news of mantovani’s personal life, or of what he was doing during the war, but the post-war period up to 1924 saw vol52,6,2009new 2-12-2009 12:39 pagina 625 626 g. scalera considerable heavy buying and selling of land at la réunion. a sugar price rise during the war had enriched the big farmers of the island who reinvested their capital (scherer, 1994, pp. 86, 87). in any case, mantovani moved to paris where, as we discovered, he became the owner (called «landlord») of an entire building in 1924 (near 151 bis, rue marcadet: built in 1910 by the architect emile blaise, it is still standing). in the year of his arrival, he was introduced as a member of the société géologique de france by the academicians termier and lemoin. an article by bourcart (1924) brought him international renown and obliged wegener to quote him (gohau, 1990). mantovani began to correspond directly with wegener and continued to produce long manuscripts that he failed to have published by french publishers (mantovani, 1910, 1932a, 1932b). later, in 1930, mantovani published his last (but this, too, must be verified) work, in french, printed by the author at the ferrari printers in parma (whether or not this is a ferrari, a relative on his mother’s side, is yet to be ascertained. he may have had this published in italy to save money). the work contains a dedication to count gaetano manzoni, the italian ambassador and plenipotentiary in paris, who, on 1928, was among the signers of the international treatise for the renunciation of the war (treatise of paris «briand-kellog») and later became a senator (vistoli, 2007). the second part of this work in print (withdrawn in incomplete form due to too many errors resulting from the lack of experience of the italian printers with the french language) is perhaps the unpublished manuscript of 103 pages with numerous maps sketched freehand which seems to have been written for the purpose of compiling a book with solid argumentation and documentation comparable to wegener’s. death came to him in paris in 1933, on 10 january. he had almost reached his eightieth year. a few months before, on 7 june 1932, louis houllevigue – a university professor of marseille and science writer – wrote on the newspaper temps the commentary causerie scientifique – la revision des longitudes. in this fig. 5a,b. a) the mantovani family. from left to right: roberto (same name of the father, was engineer of the paris metro), carlo (was professor in the gymnasiums), margherita, the little itala (dead at 15 old), anna piet, roberto mantovani, renato (was governor in indochina and father of yvane), luigina. b) the italian-style villa sivori in saint servan france, bd surcouf, where the mantovani’s family settled. the boulevard is now divided into rue eugène brouard in the north and bd henry-dunant in the south. roberto mantovani dedicated his house to camillo sivori (genova 1815 genova 1894), a virtuoso of the violin pupil of paganini and teacher of mantovani. a) b) vol52,6,2009new 2-12-2009 12:40 pagina 626 627 roberto mantovani and the expanding earth fig. 6a,b. a) the continents reconstructed as a «supercontinent» by mantovani in the work published on the magazine je m’instruis in 1909. it is very similar to the pangea depicted by wegener starting from 1912. b) in the same paper on je m’instruis the breakout of the continents away from antarctica was depicted, which continent was hypothesized by mantovani as constituted by several islands. the hope of the italian was that the antartic expedition of the explorer jean-baptiste charcot (1867-1936) – to which the maps were given – could find evidences in favour of the real existence of an archipelago. a) b) vol52,6,2009new 4-12-2009 15:52 pagina 627 628 g. scalera text the possibility to check and confirm the wegener’s drift theory was recognized in the new geodetic-astronomic method of measuring the longitudes variation. mantovani was still once cited as a french proponent of the same theory with a priority of several tens of years, because it was recalled that he begun to think about the matter in 1878 (houllevigue, 1932): this continental wandering is continuing also today, and wegener has tried to confirm it scrutinizing different measures of longitude performed on different time. for example these measurements seems to show that the north american continent is moving away progressively from europe, while the greenland was approaching at a rate of a dozen of meters for year. after all, this theory is old as the human restlessness. does had plutarch verbatim wrote «the earth is a disc floating on the sea, of which the river ocean fills the edges»? without going back so much, i can point out that a compatriot of ours, mr mantovani, has upheld, since 1878, an equal doctrine, and also more logically explained. but the geophysicist of hamburg has had the gift to give to his theory a fascinating form, which has fixed on it an universal attention. after this last encouragement, mantovani was recalled two years after his departure in an article of roger simonet (1899-?) a french prolific science writer on the newspaper temps: wegener has had a precursor more original of him but of extremely little renown before that le matin – it will be soon passed ten years – exposed his conceptions. not only mantovani, before the german geographer, observed the conformity of the atlantic coasts of america and africa, but also that the same possible correspondence – clue of a suture dating back to thousand of years ago – exists between the pacific coasts of america and asia. then, he said, we can admit the hypothesis, for the origin of the sea and the continents, of an earth littler than the actual size, which has been fragmented by the action of the pressure of the gaseous inner material [… …]. 3. mantovani’s ideas in his 1889 work (pp. 41, 42, 48), mantovani wrote (9): it was towards the beginning of 1879 and i had just arrived in la réunion. i was walking along one of the edges of the river of saint-denis, and noticed a perfect resemblance in the shapes of the two banks of this river; not only do the parts of one of the edges that jut out correspond to the hollows in the other, but also the various layers of lava followed the same order on both sides. so i thought that the bed of the river of saint-denis could only be a fracture in the ground due to volcanic activity on the island. since i am in the habit of drawing general conclusions from my ideas, i also asked this question: if a river is a small fracture in the ground, why wouldn’t a sea be a large fracture in earth’s crust? i returned home, looked at a planisphere, and was quite astonished to observe that the coasts on opposite sides of the sea generally had the same similarity in shape as i had noticed between the two opposite edges of the river of saint-denis. that greatly encouraged me to study this new way of envisaging the folds, bumps and hollows of earth’s crust. […] if, in our imagination or on a sphere, we bring together all the edges of the seas, we reduce the surface of our globe to a quarter of the original; and, if we add to our calculations the coming together of edges, fractures that have produced rivers, lakes, valleys, plains, etc., we easily arrive at a globe that closely reminds us of the small planet mercury, because of its dimensions. we are thus obliged to recognise that earth’s crust has considerably increased in surface at the expense of its liquid and atmospheric envelopes. it seems to hear a voice from the past, from the end of the 18th century, which already had proposed, albeit without the more radical generalization of mantovani, the same ideas about observed conformities. indeed, in his telluris theoria sacra (1680-1689), thomas burnet wrote (italian translation in morello, 1979, p. 108): in no manner i challenge that the earth globe breaks up to its centre, in its deepest bowels, and also below the abyss or region of the waters. it was fractured also the outermost region of the globe – the one we inhabit – where the mountains are, the rivers flow and the ocean laps against. and the same occurred to the innermost region, up to the depth where the caves are hidden, from which the underground waters spring, where fossils can be found and from which the metals can be extracted. the senses and the reason witness the dispersion of its parts. from the external aspect of some cliffs it is clearly recognizable that these massifs undergone fractures, and, in some straits, the opposite cliffs corresponds so precisely each other that this fact has lead many people to suppose that once they were joined. […] the chasm of the sea, with its steep and shattered reefs, with the huge boulders that make uneven the vol52,6,2009new 2-12-2009 12:40 pagina 628 629 roberto mantovani and the expanding earth coasts or that emerge from the sea bottom, do indeed demonstrate a lacerated body? what is the advantage to recall the islands separated from the continents and the lands disrupted and torn in thousands of ways, on the external and in the interior, because in a place there are parts of the land excessively uplifted, and in an another one depressed, and elsewhere broken and evidently moved away mutually each from the other? this state of confusion and perturbation must not have occurred without a catastrophe and such catastrophe without fractures. the comparison of the burnett and mantovani ideas shows once again the evolution of the thought about the earth dynamics passing from 17th to 19th century (see also the françois placet passages of 1668 translated in this paper) and the full participation of mantovani – through the influence of his readings and frequentation of the intellectual community of his time – to the earth sciences progress and his further evolution with respect to the cultural main stream, in which the ideas about displacements of large masses, boulders, islands was accepted as legitimate ones. mantovani was to keep up with developments in geology and paleonthology, and he was emotionally involved in this subject. he paid particular attention to news from expeditions in distant lands, which could confirm his theories. from the work l’antarctide in je m’instruis (1909) we know that he shared his maps showing the evolution of pangea and his hypothesis that the polar continent had split into two or more fragments to the doctor, antarctic naturalist and explorer, jean-baptiste charcot (1867-1936; the son of the neurologist of the salpêtrière of paris jean martin charcot and teacher of sigmund freud). he hoped that charcot would return from his second voyage with new and favourable discoveries, as well as geological and paleonthological evidence. charcot left the harbour of le havre on 15 august 1908 on the ship pourquoi-pas? the ship had been fitted through government financing after the exciting success of the first voyage of 1905. the italian said (mantovani, 1909, p. 597) (10): when he returns, we will see if it is necessary to make considerable corrections in the details, since, as far as the whole is concerned, the origin, shape and extent of antarctica will remain exactly as we have just determined. he was already highly familiar with the correspondences that geologists had found for the southern continents (mantovani, 1909, p. 597) (11): [g]eologists have found great similarities between the lands which presently make up south america, southern africa, madagascar and australia which is easy to imagine, since, as we have just seen, all of these lands now so far apart from one another were originally joined together. but he wanted to find something more, arriving at a general theory to explain the tapering of the continents towards the south, imperfect for africa and absent in australia’s case (mantovani, 1909, p. 597) (12): consequently, it is obvious that since antarctica had been formed by the fragments of these continents that had remained behind as they were pushed aside, it necessarily had to form a great archipelago possessing considerable geological affinities with south america, africa and australia. on the map (mantovani, 1909, p. 596), he represented antarctica as an agglomerate of small fragments. but two of these, marked with x’s, were wider and tapered, located in such a way as to complete africa and australia. his statement was almost prophetic since nowadays we divide antarctica into two areas with very different features, different geological histories, and shifts taking place reciprocally along the mid-antarctic mountain belt dividing it. this was not all, when one considers that fact that the lands of antarctica are largely below sealevel due to the isostatic influence of the weight of the ice cap. however, the presence of two fragments of the tapered extensions of africa and australia does not seem to have been confirmed. this conviction of mantovani’s might have come from the famous aphorism xxvii of the second book of the novum organum (1620) of francis bacon (1561-1628). the aphorism says: [i]n the configuration of the world in its great geographical stretches the examples of conformity are vol52,6,2009new 2-12-2009 12:40 pagina 629 630 g. scalera not to be neglected. africa and the peru region, with its land mass that descends to the straits of magellan have similar isthmuses and similar promontories, and this cannot be by chance. similarly, the old and new worlds correspond to one another in so far as both expand northwards, whereas in the south they are narrow, tapering towards a point. during his second and last exploration of the antarctic regions, charcot named numerous islands, capes and mountains. he identified adelaide island, passed by the faure islands and sailed along the huge alessandra island. this all suggests that when the naturalist returned to france, mantovani, who had been awaiting fresh data, may have considered that at least his hypothesis had been confirmed: that of an antarctic continent consisting of an archipelago. apparently science, for mantovani, involved complex navigation through palaeogeography, geography, cartography, explorations, geology, paleontology, and the search for observed data to confirm his theories was an integral part of his methodology. but the great novelty in the 1909 paper was the mapping (fig. 7) of the pacific view (mantovani, 1909, p. 595): dotted lines were drawn between pairs of geographical points which once were in contact while today they are separated by the huge extension of the pacific basin. the idea was that the corresponding points were in contact before the dilatation of the earth. the enlarging of huge fractures formed all the oceans. we have to wait the sixties to find the same kind of lines for the indian and atlantic oceans in plate tectonics. according to plate tectonics this is not true for the pacific ocean, because in this case the plates movement is inverse and tends to closing. the 1909 pacific map was forgotten, and only the mantovani’s pangaea representation (fig. 6a) is reproduced today in some books dealing with the history of science (muir wood, 1985) the problem arises as to what influences there could have been on mantovani’s thought, since he was apparently isolated from the academic world and did not have access to specialised data. but was he indeed isolated? at the time, it was fairly common to find scientistpriests who held degrees, such as astronomer and geologists of the caliber of secchi, hagen (scalera, 1999), cecchi, mercalli, and stoppani. one of bellati’s brothers may have been a doctor with a degree in natural philosophy (bulzoni 1997). if research in this direction yields answers, the idea that the french book by sinder pellegrini (1858) may have been in bellati’s library will gain credence. the book is primarily religious, hence totally unknown among scientists and having no influence on them. it may, otherwise, have been in some religious institute that he went to regularly. in any case, the book had its impact and was reviewed in numerous papers, among which was the courrier francoitalien (various authors, 1859); hence it is reasonable to suppose that comments appeared in the italian press, given sinder’s italian origins. the strong cultural ties between the french and parma are not to be forgotten, either. parma was a former grand duchy placed under the control of marie louise, the wife of napoleon bonaparte. a society for conversation and reading, with a conspicuous library, was – and still it is – active in parma from 1858 (but with different denominations and more ancient origin in 1722). perhaps it is because of these readings from his youth that mantovani spoke with such self-assurance and without providing examples of certain paleonthogical connections between the coasts on either side of the oceans in his first work (mantovani, 1889) about fractures in the earth’s crust. the fact that mantovani was presented to the société géologique de france in 1924, and published that same year the article les points de contact entre la théorie de la dilatation planétaire et la théorie de la dérive des continents de wegener (points in common between the theory of planetary expansion and that of continental drift of wegener) (mantovani, 1924), must be seen in an historical perspective. to be sure, his ideas on earth’s internal structure seem naive nowadays, but the beginning of that century marked the first time that the structure of the earth had been «under observation» due to the newly created observational seismology (oldham, 1906; brush, 1980). the seismometers of emil wiechert (1861-1928) from the beginning of the century had been designed for this observation, and they were equipped vol52,6,2009new 2-12-2009 12:40 pagina 630 631 roberto mantovani and the expanding earth with dampers (which merely permitted to distinguish the different phases of the seismic waves: p waves, s waves, pn, sn, etc. allowing to disclose earth’s layered structure) but debate on the actual necessity for this instrumental improvement raged for a long time (proviero, 1926). the progressive revealing of the internal structure by the geophysicists richard dixon oldam (1858-1936; outer core, 1906), andrjia mohorovicic (1857-1936; crust, 1909), inge lehman (1888-1993; inner core, 1936), must have been seen by non-specialists, geologists and many other scientists of the period as uncertain discoveries, subject to considerable revision if not refutations. geology was not closely linked with geophysics as it is today and geologists held ideas about the constitution of the planet which, in actual fact, were pure hypotheses not supported by observation (the article by brush offers a good idea of the complexity of opinions as to the earth’s interior as well as the sudden changes in views) and surely marked by the conceptions from the past such as «the great abyss» from the work of 1680 telluris theoria sacra by thomas burnet (a liquid interior of the planet related to religious conceptions, one which gave origin to the waters of the great flood), and by his successors (morello, 1979, chap. iii) or predecessors. to explain volcanism, many geologists preferred a molten interior just a few kilometres in depth. pierre termier (1922, p. 323) offered the following ideas on the earth’s structure: it was first said: the earth is made up of a thin, solid crust surrounding a liquid core and held up by this core. the heat from the core passes through the crust and dissipates into space. the core contracts and becomes too small for the crust which folds and wrinkles from then on in order to maintain contact with its liquid support. nowadays, given when we know, from the observation of earthquakes, the elasticity of earth’s crust, we must speak in somewhat fig. 7. the global map drawn by mantovani to describe the earth’s expansion concept in the work published on the magazine je m’instruis in 1909. dotted lines are traced between pairs of points located on opposite oceanic coasts, to indicate that they were once, in the geologic past, coincident, and that the interposed ocean was closed. the dotted lines was traced also across the pacific and then all the oceans was closed and as final consequence the earth must has had, in the past, a smaller size than the present one. vol52,6,2009new 2-12-2009 12:40 pagina 631 632 g. scalera different terms. earth’s interior is a heavy gas, as strong as steel, and the liquid or melted region, called a pyrosphere, can only be an area that is not very thick, between the solid crust or lithosphere and the gaseous barysphere. these ideas are quite similar to those of mantovani who, at the beginning, preferred a gassy interior, for which no further details were supplied. perhaps thanks to his interactions with the parisian academic world, the italian scientist refined his ideas about the structure of an earth with a gassy interior. certain points recall the elaborate cartesian theories (morello, 1979, chap. ii, pp. 69-83). according to descartes (1644) a subcrustal liquid layer could coexist, in some phases of the earth’s evolution, with a central core made of thin solar material, and the collapse of the crust into this inner liquid shell led to the tectonic evolution of seas and mountains. this cartesian phase of collapse also recalls the final phase of inward collapse of the crust which, according to mantovani, brings the expansion of planetary bodies to a conclusion with their transformation into comet-like bodies. in 1930, the italian showed his interest in the bold innovations of relativistic and quantum physics (mantovani, 1930). indeed, he was not a physicist and tended to misinterpret several basic facts; for example, mantovani was convinced that about halfway along the earth’s radius, the force of gravity of the upper layers predominated over that of the lower ones, hence, the core matter was thrust outwards contributing to planetary expansion; he was unaware of the fact that the gravitational influence of a spherical layer is zero at any internal point thereof: the gravity at a point in the interior of a planet is due, with a reasonable degree of accuracy, only to the ideal sphere of underlying matter. but he nonetheless arrived at an explanation of the earth’s composition which provides a bridge between the old and the new (mantovani, 1930, p. 15) (13): everyone knows that on our sun’s surface jets of matter are produced that have such force as to be capable of reaching a height equal to over a quarter of the sun’s diameter. what is the earth with its diameter of 12,000 kilometres in comparison with such eruptions? the tops of certain streams of this matter reach an area 800,000 kilometres high where the direct action of the sun’s attraction is reduced. this part of the solar matter which has been thrust out follows in part the sun’s rotation due to the whirling motion it has received and turns back on itself. in the course of time it forms a tiny planetary cell following the example of the mother cell, the sun. we then have all the elements making up the solar family and whose individual members represent the complete series of phases in the evolution of each of them. every evolution of a planet can thus be summarised in terms of three distinct periods: 1. – the whirling period 2. – the planetary period, and 3. – the comet period. the earth is presently in the planetary, or, in other words, developing phase. this «cellular system of the universe» (mantovani, 1930, pp. 15-19) was basically a cartesian conception, but still has important exponents today. to cite just one famous example, the studies of the astronomer halton arp (1989) led to a cellular vision for galaxies as well, which emit jets of matter at the ends of which quasars are to be found, interpreted as new daughter proto-galaxies at the beginning stage of their evolution. modern constructions in the physics of solar neutrinos suggest the wish to verify these conceptions, even if this desire is not openly expressed. with an ample network of monitoring stations, it would be possible to obtain a neutrino tomography of the earth offering improved distinction among the materials making up the core. a seismic tomography distinguishes only the wave-propagation velocity without giving any information about materials, which can only be hypothesised on the basis of the cosmology currently in fashion. the lack of solar neutrinos might be ascribable to causes from within the planet rather than external ones. 3.1. continental drift mantovani’s thoughts and those of the academician pierre termier differ greatly in other fields. both deal with atlantis, a subject very much in vogue during the 19th century and shortly after the 1st world war (ciardi, 2002). the existence of atlantis was for long time discussed by the more renowned geologists and naturalists like buffon, forbes, darwin, etc. that proposed or considered realistic the possivol52,6,2009new 2-12-2009 12:40 pagina 632 633 roberto mantovani and the expanding earth bility of land-bridges (von ihering, 1907). these practicable «bridges» between paired of continents were needed by the fixists to explain transoceanic paleontologic correspondences (ciardi, 2002). their existence started to be put in doubt when became evident that their integrated extension covered nearly completely the area now occupied by the oceans (e.g. see the global map in handlirsch, 1913, p. 116). both mantovani and termier were quoted by sykes in the classic book on atlantis by donnelly (1882, revised by sykes in 1949, p. 42). termier defended the idea of land bridges. for him, the platonic atlantis was a vast continent that had emerged, between europe-africa and the americas, later to be submerged due to a great volcanic event which surely took place on the surface, as shown by the vitreous nature of the lavas (termier, 1922; lecture to the paris oceanographic institute in 1912; also translated into english in the smithsonian reports in 1915). mantovani once again put forth the basic idea of planetary expansion, causing the fragments of crust floating on the sima to move apart, conserving shape conformities between coasts once in contact. the continental shapes were thus kept and, as a result, there was no possibility of their being linked together by land bridges, which atlantis might have consisted. if the better known termier based all his erroneous reasoning about the sinking of atlantis on the vitreous nature of the basalts on the sea bottom, a feature he attributed to the cooling of the flows coming into contact with air (termier 1922, pp. 131-132), but which we now know can occur due to rapid cooling in water, mantovani, in his work of 1927, overestimated the rate of separation of the continents, and asserted that atlantis was nothing but the brazil. 10,000 years earlier, that country was only a few dozen kilometres from africa. during those years vening-meinesz (1926), became involved in the problem of carrying on his gravimetrical campaigns (vlaar, 1989) and denied that vast continental areas or land-bridges sank. just three years later, mantovani corrected himself in his work of 1930. he converted to expansion rates that were now much too slow, and abandoned convictions that had cost wegener his life in greenland as he attempted to measure rates of continental shift of metres per year. he had been deceived by the inaccurate geodetic measurements of that era (hallam, 1987; see the rate of a dozen of meter for year that were believed as a realistic value in houllevigue, 1932, translated in this paper). unlike wegener, the italian based his estimate of the rate of continental shift and ages of the oceans on the size of the latter, a procedure which would not be considered valid nowadays, considering the fact that all oceans show rocks whose maximum ages are comparable to one another. his initial calculation of the age of the atlantic was, however, in agreement with the geodetic measurements then available, and with which wegener agreed. according to the young mantovani (1889, p. 43) (14), the oceans began to open up much closer in time to our era (pleistocene, ~1ma) than what was actually the case (jurassic, 180ma), the age of a fracture must be in direct proportion to its maximum spread, since the most logical supposition is to assume that a single cause, acting uniformly, must have produced all the fractures of earth’s crust. hence, the first fracture is the one that formed the pacific ocean. it followed that the pacific was very ancient and the atlantic considerably younger, approximately 10 million years (mantovani, 1927). hence, the connections between the opposite shores of the atlantic dated back as recently as prehistoric times. the italian scientist also provided a detailed description of the opening up of the mediterranean, in which, based on bathymetry, sicily was part of the african continent. in the absence of precise dating of the seafloors, it was thus possible for mantovani to draw extreme conclusions where the correlation between the fragments was not only paleontological but also ethnologic. this is not surprising, either: the italian scientist’s thought was immersed in that flow of geodynamic conjectures about continental shifts, which started with placet (1668) and his catastrophic shifts linked to the great flood. placet (1668) cites, among the other events pertaining to movements of lands and floods, the original position vol52,6,2009new 4-12-2009 15:52 pagina 633 634 g. scalera of the city of messina, which, in prehistoric times was allegedly joined to all of sicily and calabria. it then moved away due to great, tumultuous events, a fact reported by pythagoras. placet (1668, pp. 54, 57, 64, 69) says: and to begin with pythagoras, he assures us that antissa, pharos, & tyre in phoenicia were once islands, but now joined to the mainland; that the leucades, however, which were not at all separated, are today girdled by waters and streams; that the city of messina along with all of sicily, which was once attached to italy, subsequently detached to give way to the sea between them; […] […] seneca, tertulien, lycinus, marcellinus, petrus hyspal, & several others, assure us that spain once was part of the african continent; that the mediterranean sea was only a lake, and had no communication with the ocean, & that the straits of gibraltar were closed off by a clump of earth which connected the two pillars of hercules. […] all these examples make us aware that the floods and the earthquakes have the force to upset the mountains, to convert fields and valleys into rivers; to buid up hills, to swallow up kingdoms and empires, and to make islands from a number of provinces once united to the continent. but if the regional deluges have had so much force to dismember the sicily from italy, the spain from africa, and to submerge a most great part of the earth as the atlantis island, it is sure that the flood has made higher ravages, when the cataracts of the heavens have been opened, the abysses have broken, and the earth has undergone an universal catastrophe. […] if we ask to ourselves how is possible that a so huge part of the world [americas] was formed, i reply that this could have occurred whether by the joining together of many floating islands that mutually fastened each other, […] or by the destruction of the island of atlantis, which sunk and caused the discovery of a new land: because the providence parcels out the empire of the elements in such a way that if something is earned in a part, the same amount is lost in an other. so, a new province has been discovered in the south, while contemporaneously a part of the holland has been lost submerged by the seawaters. later came buffon, who, on paleontological bases, dated the separation between america and europe – by subsidence of the interposed atlantic area (or atlantis) – to 10,000 years earlier. all this created a tradition of belief in a relatively recent opening of the atlantic that persisted up to the time of wegener and was also supported by geodetic measurements (spreading rate in the magnitude order of metres per year) strongly affected by systematic errors. these tradiction was often based on a mobilist interpretation of the atlantis myth, and was opposed to the then prevailing views of the plutonists. for example, the plutonist leopoldo pilla (1805-1848), born in venafro and professor in pisa, on p. 301 of his famous trattato di geologia (treatise of geology) of 1847 had harsh words for the «exaggerated ideas about the effects of earthquakes» opening straits, separating continents, and the support that such ideas had received from plato’s atlantis story. the abhorred theories in favour of atlantis were even mentioned in his eulogy (he was killed in 1848 during the battle of curtatone) delivered in sondrio in 1874, perhaps with a bit of malice aforethought. the eulogy was delivered by domenico lovisato (1842-1916), who had earned the reputation of being wegener’s precursor due to the mention in a single manuscript (never published) of some mobilistic ideas (very rapid mobilism) linked to the atlantis myth (fossa-mancini, 1924; imeroni, 1927, pp. 30-32). as in the case of the sun’s age, the discovery of nuclear energy, the development of physics and related dating methods were what made it possible to extend limits that had long been too narrow, setting the new confines of the geological ages much further back in time (the sun’s age went from 200 thousand years to several billion. the time for the opening of the atlantic was set back several hundred thousands of years to 180 million). although his extreme deductions were not confirmed, this did not mean that mantovani’s method was improper: that of putting together correlations coming from various disciplines. in any case, the naturalist, in his work of 1930, perhaps taking advantage of the discussions at the société géologique française, steered a new course, preferring a far slower expansion of the planet and abandoning ethnological correlations (mantovani, 1930) (15): vol52,6,2009new 2-12-2009 12:40 pagina 634 635 roberto mantovani and the expanding earth if provisionally it is admitted a rate of spreading of 1mm/century between the coasts of peru and the coasts of mozambique (ocean indo-pacific), we arrive to approximately to 26 billions of centuries for the indo-pacific opening, and to 7 billions of centuries for the atlantic ocean one. 3.2. volcanoes and earthquakes as was to be expected from what has already been said, mantovani’s reflections extended to volcanic and seismic phenomena. from the few lines of documentation coming from french national gazettes and regional breton ones (historical archives of the italian geographic society) we know that this scientist considered the volcanic phenomenon a global one activated on a global level, or one which could involve areas very distant from one another at the same time. he also expressed his opposition to the theses according to which volcanoes were activated by the attraction of sun and planets or electric currents generated by solar activity. it would also seem that he believed that this occasional release of energy from the earth’s interior also involved seismic phenomena for which attempts at forecasting are made. the dearth of information is such that little more can be said, but it appears obvious that these ideas were at the heart of explanations for planetary expansion. the volcano-seismic phenomena – supported without awareness of expansion also by the italian, giuseppe mercalli (alfano, 1915), and darwin (1840, 1897) among others – must have been the local manifestation of the gas pressure that made up the planet’s interior, whose action slowly gave way to discontinuous planetary expansion. this conception according to which seismic phenomena and volcanoes are linked in overall fashion was common to many researchers at the time. now this idea has almost entirely disappeared with the widespread acceptation of the theory of plates and the varying rates of convergence and divergence between them. only recently, some new compelling evidences have rekindled the interest about these earthquake-volcano correlations (scalera, 1997; 2007; 2008), revealing another precursory aspect of the mantovani’s conceptions. 3.3. other concepts in addition to the already-mentioned articles published in la réunion and then in france, two other papers (one paper split on two consecutive issue of je m’instruis: 22 and 29 may 1910) have been found among the descendants’ collection of documents. in this 1910 article the origin of mankind and the earth’s evolution is discussed in connection with ancient indian cosmologies. the terrestrial brahman egg depicted in indian mythologies is judged to be an idea close to conceptions of his time, ones with which the author was in agreement. mantovani quotes the ideas of s. meunier and de launay who considered the earth to be a living being, and proposed to go beyond these by considering the earth a living and growing cell – a conceptions analogous to the one of the american richard owen, which were expressed in the 1857 book about the earth considered as a growing seed – and by applying the same concept to the entire universe, which must, like an egg, be considered stratified (mantovani, 1910, p. 343) (16): the intersidereal medium is likewise full, as is an egg, if you will allow me this expression; and there are currents, swirls and layers of differing thickness just is in our own atmosphere. as has already been said, mantovani cultivated highly general metaphysical conceptions to which he attributed great importance, equal, at least, to his conceptions about planetary expansion and very closely related to these. one of his recurring ideas concerned a metaphysical concept of the universe attributing its evolution to an universal radiation (mantovani, 1912; 1932a). this, related to gravity, caused the expansion of heavenly bodies. there are two, long manuscripts on this subject (of about 190 and 100 pages, respectively). one is dated 1912 and one, reworked up to 1932 whose last sheet is dated 21 september 1932. the preface to the latter was written on 1 january 1933, nine days before his death. it is perhaps that last of mantovani’s writings (17): paris 1 january 1933 preface the ideas presented in this book [la radiation universelle, author’s note] go back a half century and vol52,6,2009new 2-12-2009 12:40 pagina 635 636 g. scalera a) b) vol52,6,2009new 2-12-2009 12:40 pagina 636 637 roberto mantovani and the expanding earth fig. 8a-c. some hand-drawn maps from the unpublished manuscript ‘dilatation planetaire’ of 1932. a) geometrical considerations of mantovani about the conformity between the coast of brazil and the guinea gulf. the opinion of the italian was that the non perfect coincidence of the coasts in his fig. 23 shows that the african continent has undergone a deformation, a «deviation towards east of the coast». this problem is know as the jeffreys objection against the continental drift, and was resolved by samuel warren carey using shiftable transparent spherical caps on a wooden globe to displace the continents on the sphere avoiding the cartographical distortions (scalera, 2003). b) outline of the evolution of the indian ocean. the arrows indicate the «general deviation and translation toward east of the three peninsulas: arabia, india and indochina». c) the adriatic sea basin was judged by mantovani as an incipient fracture of the earth crust. the map was drawn on the base of the geologic map of the adriatic by jacques bourcart, the scientist that in 1924 pointed out the work of the italian to alfred wegener. their originality lies in the fact that, even today, they go well beyond the present theories of mathematicians and physicists. we ought to have kept to a simple scholarly or academic style, as one always does, for our various demonstrations, but in the present case, it was necessary to set the usual theories side by side with the new conceptions in order to bring out more effectively the difference between the former and the correctness of the latter. under these conditions, we were induced to quote many phrases written by the most authoritative scientists, while refraining from giving their names, when that might have given a polemic tone to the demonstrations. when, however, the words of certain scientists were in agreement with our new conceptions, it was highly fitting to recognize that fact. for example, prof. einstein renounced his theory in february 1932 at the carnegie institute, saying, «the general principle of relativity is not satisfactorily founded and it needs to be further developed.» we are pleased to admire his frankness, although we had gone further, before prof. einstein had done, in establishing our formula a.e.i which goes beyond any possible calculation of space, time and movement. we ask that you immediately seek fully to grasp the value of that formula since it is at the heart of any demonstration of what universe and universal radiation are. r. mantovani c) vol52,6,2009new 2-12-2009 12:40 pagina 637 638 g. scalera although some of these final statements of mantovani can be considered the fruit of strong and passionate personal ideas, his attitude towards the investigation of reality and his conviction – shared by einstein – that our mathematical models of the world are always temporary and can be improved should be judged positively. however, his reflections on universal radiation could not be aired in the normal circuit of scientific publications – and undergo the usual peer review – and this was probably the case, given that, by the author’s own admission, they belonged to metaphysics rather than to physics. 4. conclusions seeking the merits of the italian in non-essential ideas such as a configuration of pangaea similar to that of wegener, cannot help but conceal his true merit – as the wegener’s quotation does – which he himself strenuously claimed up to his old age: the idea, based on geographical evidence, that the earth expands as a consequence of general laws of the universe. his awareness that he, like it or not, was the guardian of a highly important truth for all sciences, that it belonged to geography, and that he was defending this discovery in face of the indifference of the academic world, was extremely important. could mantovani have done more? could he have carried more weight in the scientific debate? or was the french academic world so aware of the value of his reasoning that they took him into the geological society and obliged wegener to quote him? one thing is certain: mantovani could not have produced more than he did because of his activity as a professional musician. indeed, it is amazing that he managed to keep up to date despite that, and incorporate more and more into his world system (mantovani, 1930, 1932a, 1932b). nor could he have made himself more visible with publications in prestigious journals which, then as now, are rather reluctant to accept work from those not considered professionals. despite that fact, the academic world – with its characteristic detached at times cold and secret foresightedness – became aware of his presence, and had to interact with him, as a person who surely ought to be listened to. we do not know the details, but a person is presented to the geological society only when he has become known and mutual trust has been established. the same applies to bourcart (1924) who brought him to wegener’s attention. it should be considered, furthermore, that scientific information circulates more easily among academics and that the works of yarkovsky (1888, published in french) and lindemann (1927, published in german) on earth’s expansion as well as all those who preceded wegener on continental shifts could not have gone unobserved. moreover, these were times of tumultuous evolution in the physical sciences, with einstein’s relativity and quantum mechanics that were revolutionizing scientific and philosophical thinking – a fact not to be forgotten. hence, it was a period in which every idea had to be taken into account, no matter how far from common sense it may seem to have strayed. it could be also possible – but not demonstrable – an indirect influence of mantovani (trough the quotation on the wegener’s book) on later european researchers involved in earth’s expansion like the berliner ott christoph hilgenberg (1933; 1974; see biographical notes in hilgenberg, 2003, and scalera and braun, 2003). in a mysterious way known only to the academic world, new ideas that are difficult to associate can be casually kept in the drawer; in this case, academics realized that l’aimable conférencier (the amiable lecturer) roberto mantovani (historical archives of the italian geographic society) (18): «[he] is not merely the delicate and conscientious artist so highly esteemed by all those who know him, he is also a scientist and indefatigable researcher» (anonymous 1st, 1909) and it took measures to see to it that a trace of his thought remained in the oral communications at the geological society. i like to imagine mantovani at work at saint servan convincing the audience at his lectures, vol52,6,2009new 2-12-2009 12:40 pagina 638 639 roberto mantovani and the expanding earth as described (archives of the italian geographic society) by one of the people present (anonymous 1st, 1909, a short news item from the gazette l’ouest-éclair) (19): we don’t dare assert this, but his well documented and witty lecture, delivered with charming bonhomie and humour was very well received. or in a conference held around 1924, which was described by an anonymous journalist (anonymous 2nd, 1924, a short news item from the gazette le matin) (20): all the developments of the theory of m. mantovani are logic in their inventiveness. he has illustrated them by a practical experience, using an inflatable sphere coated by a plastic layer. imagine him, then, uttering these words which showed us – besides the world of christopher columbus, and thanks to that, as well as to the geographic and cartographic sciences that came thereafter – a new world in its most extreme form of evolution (mantovani, 1930, pp. 17-18) (21): if we want to make a projection of the surface of our globe on a flat surface, such that the north pole is at the centre, we discover that the three great continental masses on the terrestrial sphere have separated in some way, due to the expansion of earth’s enfig. 9. the sketch of the mechanism of the planetary expansion by the pressure of the inner earth’s material in a high temperature gasiform-state (manuscript dilatation planétaire of 1932). vol52,6,2009new 2-12-2009 12:40 pagina 639 640 g. scalera fig. 10. two pages of the manuscript radiation universelle from 1912, in which mantovani lists the periods of revolution of the planets considering them as periods of vibration. then he scales the periods to the musical pentagram, writing two distinct accords, the first for the external planets and the second for the internal ones. vol52,6,2009new 2-12-2009 12:40 pagina 640 641 roberto mantovani and the expanding earth velope, like three sepals of a flower whose north pole is the peduncle. albeit in november 2003 new material has been recovered (figs. 8a-c, 9 and 10) thanks to the descendants, further work remains to be done to reveal other aspects of the life of this artist and scientist, and perhaps other correspondences, musical compositions and manuscripts of his as well. in the meantime the accords of his music of the planets (fig. 10) will continue to sweetly resound, encouraging all who valiantly defend their creative ideas against a mostly deaf environment. acknowledgements thanks for the assistance provided for this research go first of all to the ministry of foreign affairs for having granted access to its historical archives at the farnesina headquarters, which were essential to the preparation of this text. for their assistance in this research, we thank prof. claudio cerreti of the italian geographic society, dr ezio vaccari an historian of geoscience at genoa, dr lucia fornari-schianchi, head of the superintendence of fine arts of parma, dr leonardo farinelli, head of the biblioteca palatina, and dr marzio dall’acqua, director of the parma state archives, dr philippe petout, director of the museum of st malo, mme michèle sacquin curator of the french national library. dr jean gaudant and dr gabriel gohau, the president of cofrhigeo provided invaluable information. professor marie-claire moussat provided information on the former musical activities of saint-servan. mrs federica dondi and valeria pezzani of the city of parma and sorbolo registry office kindly supplied date on the geneology. professor claudio cerreti made it possible to discover some letters from mantovani to the igs. professor gaspare nello vetro kindly allowed us to consult the files of his dictionary of musicians of parma. thanks are owed to the librarians and archivists of the university of the isle of la réunion. particular thanks go to professor renata mantovani pulvirenti for having supplied the first photograph portrait of roberto mantovani several years ago, and to mantovani’s french descendants yvane mantovani and her husband alain poirier as well as their daughter, catherine-marie, for having recently contacted me, providing me, with great enthusiasm, the manuscripts, photographs and material cited in these pages and which will be material for future studies. yvane wrote the letter containing her recollections which is reproduced here as an appendix 1. appendix 1 my grandfather, roberto mantovani by one of his granddaughters, yvane mantovani poirier i take great pleasure in recalling my italian grandfather. i did not know him very well since he died when i was seven years old. that was on january 10, 1933. furthermore, my father’s colonial career kept us away from france for periods of three years in a row, so we only met our grandparents during leaves of absence, that lasted around six months. to his grandchildren babbo (papà) (babbo in italian, especially in the northern regions, is the equivalent of dad, daddy, pa in english. n.o.e.) was the name his parents used, so they also used this name […]. my first recollection of him goes back to when i was three years old, and this was probably aided by the photos from the family album that had been taken in his apartment in rue marcadet in paris. this was where the family got together for special occasions such as christmas. i can still see myself sitting on my grandmother’s lap. she showed me postcards from italy. her beloved husband was by her side, silent and affectionate. they were seventy and sixty eight years old respectively, and were deeply in love with one another. my grandparents must, however, have moved at the beginning of the nineteen thirties because my recollections go back to an apartment building in the avenue de ségur: they had an apartment there, as did their two daughters, unmarried. both of them worked as secretaries in a ministry not far from there. vol52,6,2009new 2-12-2009 12:40 pagina 641 642 g. scalera my grandfather passed away there, and his wife shortly thereafter. they were buried in samois, near fontainebleau, in the ground. their grave is quite simple and near that of their son italo, whose brilliant career as a judge was cut short by a bad cholera vaccine at the age of thirty four. italo’s wife came from fontainebleau and had good friends in samois, so that is why her husband is buried there. my grandparents had been affected by the death of their youngest child and this is why they chose to be buried in that location. we went to dwell on that grave in 1995, and there we had the idea that we could have a plaque placed there that recalled the life and work of roberto mantovani. we have never had much information about his stay in la réunion; it is said, for example, that he had reached the island with an italian scientific expedition, that a girl from the high society of the island had caught his fantasy, that she was a good pianist, that he had let his companions go on with their voyage, but that they and their possessions had succumbed in a shipwreck […]. roberto married marie-anna piet on 31 august 1880 in the church of st paul de la réunion. my grandmother’s family was an old creole family, which had been on the island for centuries: several of my ancestors belonged to a small contingent of young artisans who had responded to a recruitment campaign publicised in posters all over france. they made their trip on a boat called le taureau (the bull), which arrived on july 9, 1665 in the bay of st paul. my grandparents had seven children: itala, margherita, luigia, roberto, carlo, renato (my father) and italo. itala died of typhoid fever at the age of approximately sixteen. my father always told me that the family had left la réunion at the age of four and they lived in england until he was seven. he was born on december 28, 1891 (but his birth was not declared until january 4, 1892). thus, we know that they must have left la réunion around 1896 and england, towards 1899. they then settled in saint servan, near saint malo (now the two towns have merged), in bd surcouf which subsequently became bd henri dunant. we do not know why they chose st servan. all i do know is that some friend of my grandmother from la réunion lived there. the family undoubtedly lived on what was earned from singing and string-instrument lessons that he and his wife gave, but each of the children played an instrument. the small family orchestra gave concerts at home and in the little chapel of st louis de st servan. there are still traces of the cello key on the floor, as my husband and i were able to observe. one of roberto’s goddaughters was a niece, armande piet, who had great affection for him. he gave her his grand piano in his will, but we do not know what happened to his violin. we only know that he decided to give it away when he knew that he could no longer play it […]. my father, renato (rené) inherited his precious manuscripts. due to the dramatic events of the war and a precarious family situation, he only devote a little time to them. on 7 january 1958, however, after reading an article on antarctica, in the figaro of 2 december 1957, he wrote to that newspaper to remind them that, as early as 1889, his father had formulated a theory on planetary expansion and that, in 1909, had confirmed the existence of an antarctic archipelago. in this letter, he wrote: «i have the manuscript concerning this theory. i reread it a short time ago and was struck to see that the phenomena, recently confirmed, such as the existence of gigantic faults and fractures, of enormous canyons slashing deep continental grooves, can be logically explained in theory by planetary expansion, while present-day science is unable to indicate their origin. my father passed away in 1933 and, if the existence of an antarctic archipelago is formally recognised, you can understand my wish to see that to him should be given the merit of having foreseen it almost fifty years earlier». he enclosed in his letter a copy of an article by prof. bourcart that had appeared in la revue scientifique on 27 september 1924. after receiving this letter le figaro published an article in which the works of prof. roberto mantovani were mentioned. it was in the 1958, march 23, issue. to the best of our knowledge, nothing came about as a result of this article. shortly before his death, my father asked my husband, with whom he had often conversed at length on scientific subjects if, when he had gone to the other world, he could make every effort to make babbo’s work known. unfortunately, he did not have the time to hand over all the documents in his possession and his wife (my father had remarried after my mother had died) was not willing to give up the rest. she died in the eighties without any known heirs, and we are still looking for these precious documents. my husband [alain poirier; n.o.e.] was not sufficiently versed in the sciences, unfortunately, and he was not convinced by the theory of planetary expansion. it is true that the cause of this expansion (the pressure of gasses in an empty sphere) did not seem to him a theory that could be seriously supported. every time he spoke of it with some colleagues from the french astronomical society (société astronomique de france), of which he was a member, (and, at a certain point, a member of the board of directors) he only encountered scepticism. vol52,6,2009new 2-12-2009 12:40 pagina 642 643 roberto mantovani and the expanding earth once he wrote to haroun tazieff, after seeing a television programme, to ask his opinion about this theory. the letter replied on july 22, 1974: [...] the most current opinion is that the earth is not expanding. calculations show that the effect of the distribution of masses that emerged on the rotation of the globe (sic) is insignificant. in fact, you can imagine this, the average of emersion is less than a 1 km, for an earth’s radius that is more than 6300 times greater. particularly as the average density of this part that has emerged (2.7) is far less than that of the globe (5.5). there are, however, those who believe in the theory of the expansion of the globe. among them is monsieur andré baronnet – résidence de l’observatoire, 92190 meudon – who published two articles on the subject. thank you for offering us the opportunity to have a look at the manuscripts of your ancestor, prof. r. mantovani. i myself have very little time. [...] my husband, however, himself very busy providing a living for our family, could not follow up on this invitation to meet m. baronnet […]. that last letter my father received from babbo is one written in french on 24 december 1932 in which he told him: my dear child, indeed i received your letter concerning the new year and we thank you ever so much and ever so warmly. what you have told me about the newspaper l’ouest-éclair that is indeed the case, and, above all, at the end of the year when all the newspaper have so much to say; and, therefore, the science column is always behind the times, even when it is prepared weeks in advance. here, too, the weather has been wonderful but, in spite of that, our little miseries are always waiting for something better to happen and neither i nor your mother can enjoy the good weather. we continue to hope that – as springtime approaches – we can once again go where the orange trees blossom. the universal radiation text is finished. it is about 140 pages and i think my revisions are good enough. all of our affection to all of you, with best wishes for good fortune next year and for many more, including a good promotion for you in your career. babbo. but the «minor disturbances» became more serious; three weeks later, babbo left this earth to «embark upon life» (st thérèse de lisieux), finally to find out the truth about all that had interested him and aroused his enthusiasm during his fruitful existence. notes (1) a parte qualche contrada ancora poco conosciuta e le due regioni strettamente polari, la superficie terrestre è ora perfettamente conosciuta, ed allora ci sembra che nessuna scoperta importante d’origine geografica sia ancora possibile. eppure si è appunto la perfetta conoscenza sulla forma di tutti i mari, di tutti i continenti e di tutte le isole che ci ha condotti oggidì ad una scoperta importantissima. [...] la detta scoperta è il complemento naturale delle teorie che due illustri italiani sostennero per primi. infatti ognuno sa che colombo colla scoperta d’un nuovo mondo dimostrò che la terra è rotonda. venne poi galileo per sostenere che la terra pur si muove; ed il caso vuole che un altro italiano venisse terzo in così eletta compagnia per provare che la terra si dilata! e ciò che io trovo più curioso in tutto questo si è che quel terzo sono appunto io. ma poco importa che sia io o un altro se il trio è perfettamente italiano e se la scoperta ha il più grande interesse. non v’ha dubbio alcuno: l’aver trovato che la terra si dilata, molto lentamente è vero ma in modo continuo, ha una iportanza grandissima non solamente per la geografia, l’astronomia, la paleontologia [...]. [...]. ed allora le prove certe e ben visibili della dilatazione che ha subito e che subisce ancora la pellicola che forma tutta la superficie terrestre, appartengono strettamente alla geografia. (2) only weak or null influence could has had on western culture the tales about the origins contained in the zoroastian texts, the avesta-vendidad [fargard 2. yima (jamshed)]. the deity yima grants for three time to made the earth larger, because the space for herds and people became too meagre and insufficient to live with prosperity. but the expansion of the earth could be interpreted also in a territorial sense. nicola oresme (13231382) in his livre du ciel et du monde (1377) proposed a pure intellectual exercise about a possible increase of the earth size, with the planet swollen by a growing inner cavity (parodi, 1981, p. 256; scalera 1999, p. 163). the passage was contained in a reasoning of oresme about relativity of space and on our unawareness of a hypothetical process of expansion of all the parts of all the things. variation of the earth’s globe size – a decreasing size – is quoted in a 1562 book (pp. 5-7) of the dalmatian francesco patrizio da cherso (1529-1597; renaissance man of letters and philosopher), in which he reports the tales of ethiopian sages about the rage of vol52,6,2009new 2-12-2009 12:40 pagina 643 644 g. scalera uranus against the haughtiness of the humankind. jupiter and neptune, his sons, were charged to punish the men by shooting lightnings from the sky and trident strokes from the depths. the earth, constituted by inhabited large and interconnected caves, collapsed from a size comparable to the sphere of the sky to the modest actual size. this tale was probably inherited by the christian tradition that transformed it in the tale of the fall of the haughty angels. in this ethiopian case it is more probable that the tale was truly referred to a spherical earth, because the peoples of the north and south equatorial africa were aware of the sphericity of our planet as indicated mostly by the inversion of the seasons crossing the equator, and from the observation of a second celestial pole in the southern emisphere. (3) monsieur le professeur, j’ai lu avec beaucoup d’intérêt votre théorie sur les translations continentales d’après la traduction de m. m. reichet éditée par a. blanchard – paris. 1924. [...] permettez moi de porter à votre connaissance que des remarques sur la concordance des côtes du brésil et de la guinée avaient été faites par de saint-beuve, critique français vers 1860. une autre remarque semblable, mais plus étendue, avait été faite par edward forbes cité par darwin dans son ouvrage l’origine des espèces publiée en 1859. ce forbes prétendait, nous dit darwin, que tous les continents et toutes les îles devaient avoir été réunis les uns aux autres à une époque relativement récente; ce que darwin n’admettait pas, et pour cette raison il ne s’est pas étendu sur ce sujet. de mon côté en 1878 j’ai visité différents points de l’océan indien, et en septembre de la dite année j’arrivai à l’île de la réunion (bourbon). en visitant la rivière de saint-denis, près de la ville de même nom et capitale de l’île, je remarquais que le remparts au fond des quels coulait la rivière présentaient une parfaite co(n)cordance dans leur courbes et j’en avais conclu que cette rivière devait être une fracture occasionnée par l’ancienne activité volcanique de l’île. des lors, par ma tendance à généraliser les faits, je me suis demandé si une mer ne serait pas aussi une grande fracture de l’écorce terrestre. ayant consultez mon atlas je trouvais de suite en effet que toutes le mers présentaient une analogie complète entre leur côtes malgré la grande distance qui parfois les sépare. en 1884 j’exposais mes observations au célèbre astronome c. flammarion qui les trouva fort curieuses et intéressantes. en décembre 1889 je présentais une note sur «les fractures de l’écorce terrestre» à la société des sciences et arts de l’île de la réunion et insérée dans le bulletin de la dite société année 1889. des mes observations j’avais constaté que tous les continents et toutes les îles devaient former un seul bloc à une époque fort reculée. de ceci il en résultait la conséquence bien naturelle et incontestable que la terre devait être, au temps de la première période de son évolution beaucoup plus petite qu’au jour d’huis; c’est-à-dire, qu’elle devait avoir à peine le tiers de son diamètre actuel. la pellicule superficielle de la membrane terrestre s’étant fendue par la force expansive du gaz nucléaire intérieur qui distendait graduellement et d’une manière excessivement lente la susdite membrane il en est résulté de toute nécessité la translation des continents et des îles. [...]. (4) albeit also the figure of william lowthian green (1819-1890; merchant and several times minister of hawaiian governments) is associated to the group of predecessors of the expansion idea (carey, 1976), in his book of 1875 vestiges of the molten globe only in a foot-page note, on p. 55, green quotes a clarence dutton (18411912) paper (1874) as a mere confutation of the shrinking of the earth. green and dutton were nor expansionists but only verticalists. probably it is the case of an historical misunderstanding that has been good faith propagated from person to person. (5) parma, 20 sett. 1930 – viii, illustrissimo signore, da parecchi anni sono stabilito a parigi e sono venuto a parma in luglio scorso per rivedere molti dei miei parenti. prima di far ritorno in francia desidererei grandemente di fare una comunicazione o meglio una conferenza a codesta onorevolissima società geografica, sulla morfologia continentale in presenza di s.e. il capo dello stato l’on. mussolini. mi permetto di indirizzarmi alla s.v. illustrissima per sapere se e quando tale cosa sarebbe possibile. fiducioso nella bontà di vostra signoria, la prego di volermi tenere per scusato e di aggradire i sensi del mio più profondo ossequio. prof. r. mantovani, strada cairoli 10 – parma / parigi – rue marcadet 151 bis. (6) roma, 22 settembre 1930 – viii. egregio professore, in risposta alla sua lettera del 20 settembre u.s. pregiomi informarla che l’ottenere ad una conferenza l’ambita presenza del capo del governo è cosa insperabile. in quanto a tenere una conferenza ai soci della r. società geografica italiana sul tema che ella propone sarebbe certo cosa gradita. ma poiché esiste una commissione per le conferenze sarebbe doveroso sottoporre al suo giudizio l’opportunità di tenerla o no, qui o altrove. se ella pertanto ritiene di inviare in comunicazione o una copia o un largo riassunto, mi farò dovere di farle poi conoscere qualcosa al riguardo. per intanto, se ella crede, di farsi socio, le unisco una scheda. ossequi, il segretario. (7) parigi, 8 novembre 1930. illustrissimo signore, quando ebbi l’onore di presentarle una copia del mio opuscolo «la terre grandit» ho dimenticato di dirle che tale lavoro era pieno di errori di stampa. ne sono dispiaciutissimo e le domando profondamente scusa. vol52,6,2009new 2-12-2009 12:40 pagina 644 645 roberto mantovani and the expanding earth l’opuscolo non è corretto perché il direttore della tipografia è stato ammalato per alcune settimane e gli operai non erano atti a comprendere il testo in lingua francese malgrado le mie molteplici correzioni, finché alla fine sono stato obbligato di ritirare il lavoro incompleto. avevo poi desiderato di fare una conferenza in codesta reale società geografica sulle «forme continentali» perché queste sono interamente del dominio della geografia epperciò avrei trovato costé le carte e la sfera terrestre necessaria alle mie dimostrazioni ed evitare così l’obbiezione possibile che le carte fossero per parte mia adattate allo scopo. trattandosi poi non di una mera ipotesi ma di una vera ed importantissima scoperta atta a sconvolgere interamente le leggi fisiche e tutte le concezioni astronomiche attuali, credevo, non per me personalmente che valgo ben poco, ma per la gloria della scienza italiana, che la presenza di s.e. il capo del governo alla conferenza sarebbe stata convenevolissima per dare alla rivelazione d’una scoperta importantissima italiana tutta l’importanza voluta. infatti se cristoforo colombo scoperse un nuovo mondo, la dilatazione planetaria ci mena direttamente alla scoperta di un nuovo universo. mi permetto intanto di spedirle con lo stesso corriere un altro esemplare del mio opuscolo nel quale ho contrassegnato in rosso gli errori di stampa e tutte le frasi importanti. prego di nuovo v.s. illustrissima di volermi tenere per iscusato ed aggradire i sensi del mio profondo ossequio. prof. r. mantovani parigi 151bis rue marcadet, 18mo. (8) recognition and license to perform the duties of consul and arbiter between two different legislations granted from a foreign state to a new consul. (9) c’était vers le commencement de l’année 1879, je venais d’arriver à la réunion, lorsqu’un jour en me promenant sur un des bords de la rivière de saint-denis, je remarquai la parfaite ressemblance de forme qui existe entre les deux remparts de cette rivière; non seulement les proéminences de l’un des bords correspondaient au creux de l’autre bord, mais aussi les différentes couches de lave étaient superposées dans le même ordre des deux côtés. je pensai donc que la rivière de saint-denis ne pouvait être qu’une fracture du sol due à l’activité volcanique de l’île. par une tendance à généraliser les idées, je me posai aussi cette question: si une rivière est une petite fracture du sol, pourquoi une mer ne serait-elle pas une grande fracture de l’écorce terrestre? rentré chez moi et jetant un regard sur un planisphère, je fus très étonné de constater que le côtes opposées des mers offraient, en général, la même ressemblance de forme que j’avais remarquée entre les deux bords opposés de la rivière de saint-denis. cela m’encouragea vivement à étudier cette nouvelle manière d’envisager les plis, les bosses, et les creux de la surface terrestre. [...] si nous rapprochons dans notre imagination, ou sur une sphère, tous les bords des mers nous réduisons au quart environ la superficie de notre globe; et si nous ajoutons à notre calcul les rapprochements des bords, des fractures qui ont produit des fleuves, des lacs, des vallons, des plaines, etc., nous arrivons facilement à un globe nous rappelant de beaucoup, par ses dimensions, la petit planète mercure. nous sommes donc obligé de reconnaître que l’écorce terrestre a augmenté considérablement en superficie aux dépens de ses enveloppes liquide et atmosphérique. (10) nous verrons à son retour s’il y aura lieu d’y apporter de grandes corrections pour les détails, car pour l’ensemble, l’origine, la forme et l’étendue de l’antarctide resteront telles que nous venons de le déterminer. (11) [...] les géologues ont trouvé de grandes analogies entre les terrains qui forment actuellement l’amérique du sud, l’afrique australe, madagascar et l’australie et cela se conçoit sans peine puisque, comme nous venons de le voir, toutes ces contrées aujourd’hui si éloignées l’une de l’autre étaient primitivement réunies. (12) dès lors, il est évident que l’antarctide, ayant été formée par les fragments restés en arrière de ces continents pendant l’écartement qu’ils ont subi, elle doit de toute nécessité former un grand archipel présentant de grandes affinités géologiques avec l’amérique du sud, l’afrique et l’australie. (13) chacun sait qu’à la surface de notre soleil il se produit à certaines époques surtout de jets de matière d’une telle puissance qui peuvent atteindre en hauteur plus du quart du diamètre du soleil. qu’est-ce que la terre avec ses 12.000 kilomètres de diamètre comparée à telles éruptions? or le sommet de certains jets de cette matière vont atteindre la région 800.000 kilomètres où l’action directe de l’attraction du soleil est moindre dès lors cette partie de la matière solaire projetée, en raison du mouvement tourbillonnant reçu, suit en partie la rotation du soleil sur lui même, et en tournant aussi sur elle-même, arrive à former au cours des temps une minuscule cellule planétaire constituée à l’instar de la cellule mère, le soleil; nous avons alors tous les éléments qui constituent la famille solaire et dont les différents membres nous représentent la série complète des phases de l’évolution de chacun d’eux. toute évolution d’une planète se résume ainsi en trois périodes bien caractérisées: 1) période tourbillonnaire. 2) période planétaire. 3) période cométaire. la terre se trouve actuellement en pleine période planétaire ou autrement dit en voie de développement. (14) l’âge d’une fracture doit être en proportion directe de son maximum d’écartement, car la supposition la plus logique est d’admettre qu’une cause unique agissant d’une manière uniforme doit avoir produit toutes les fractures de l’écorce terrestre. la première fracture est donc celle qui a formé l’océan pacifique. vol52,6,2009new 2-12-2009 12:40 pagina 645 646 g. scalera (15) en admettant provisoirement un écart d’un millimètre par siècle entre le côtes du pérou et la côte du mozambique (océan indo-pacifique), nous arrivons à 26 milliards de siècles environ pour l’écart actuel indo-pacifique; et à 7 milliards de siècles pour l’océan atlantique. (16) le milieu intersidéral est aussi plein qu’un œuf, si vous voulez bien me permettre cette expression; et il y a là des courants, des tourbillons, et des couches de différente épaisseur exactement comme dans notre propre atmosphère. (17) paris 1 janvier 1933. préface. les idées exposées dans ce livre (la radiation universelle) remontent à un demi siècle et leur nouveauté réside dans le fait qu’elles vont encore aujourd’hui bien au delà des théories actuelles des physiciens et mathématiciens. pour les diverses démonstrations nous aurions dû nous en tenir strictement, comme on le fait toujours, au style simple scolaire ou universitaire, mais dans le cas présent il était nécessaire de mettre en parallèle les théories usuelles avec les conceptions nouvelles à fin de mieux faire ressortir la différence des premières et la justesse des dernières. dans ces conditions nous avons été amené à citer bien de phrases écrites par le savants les plus autorisées, en nous abstenons de citer leurs noms, lorsque cela aurait pu donner un caractère polémique aux démonstrations. cependant lorsque le dire de certaines savants concordait assez avec nos conceptions nouvelles il était de tante justice de le reconnaître. ainsi par exemple lorsque le profr. einstein a renié sa théorie en février 1932 à l’institut carnegie en disant «la fondation du principe général de la relativité n’est pas satisfaisant et elle a besoin d’être développée plus avant». nous sommes heureux d’admirer sa franchise malgré que nous soyons alléz plus loin avant le prof einstein, en établissant notre formule a·e·i qui va au delà de tout calcul possible sur l’espace, le temps et le mouvement. nous prions instamment de chercher à bien saisir la valeur de la dite formule car elle constitue la base de toute démonstration de ce qu’est l’univers et la radiation universelle. r. mantovani. (18) [...] n’est pas seulement l'artiste délicat et consciencieux si estimé de tous ceux qui le connaissent, c'est encore un savant et un infatigable chercheur. (19) nous n’osons pas l’affirmer, mais sa conférence fort documentée, spirituelle, faite avec une bonhomie charmante et une pointe d’humour a été fort goûtée. (20) tous les développements de la théorie de m. mantovani sont logiques dans leur ingéniosité. il les a illustrés par l’expérience, au moyen d’une sphère dilatable revêtue d’un enduit. (21) si nous voulons faire une projection de la surface de notre globe en la posant à plat, de manière à avoir comme centre de la projection le pôle nord, nous trouvons comme sur la sphère terrestre que les trois grandes masses continentales par la dilatation de l’enveloppe terrestre, se sont séparées en quelque sorte, comme trois sépales d’une fleur dont le pôle nord serait le pédoncule. anonymous 1st (1909): saint servan – la conférence mantovani – une étonnante découverte – la théorie de la dilatation planétaire. a short news item from the gazette l’ouest – éclair de rennes on march 27, 1909. anonymous 2nd (1924?): la terre grandit-elle? a short news item from the gazette le matin (without date). arp, h. (1989): la contesa sulle distanze cosmiche e le quasar (the cosmic distances struggle and the quasars), (jaca book, milano). bacon, f. (1620): novum organum, sive indicia vera, de interpretatione naturae, (italian translation and introduction by enrico de mas, 1968, laterza, bari). bourcart, j. (1924): les origines de l’hypothèse de la dérive des continents, revue scientifique, 563-564. brush, s.g. (1980): discovery of the earth’s core, american journal of physics, 48, 705-723. bulzoni, g. (1997): private communications. cailleux, a. (1968): anatomia della terra (anatomy of the earth), (il saggiatore, milano). cailleux, a. (1976): géologie générale,terre-luneplanètes, (masson, paris). carey, s.w. (1976): the expanding earth, (elsevier, amsterdam). references archives archivio storico del museo etnografico luigi pigorini (luigi pigorini ethnographic museum historical archives). archivio storico della società geografica italiana (historical archives of the italian geographic society). archivio storico diplomatico della farnesina, ministero degli affari esteri (historical and diplomatic archives of the farnesina, ministry of foreign affairs). archivi servizi demografici di parma (parma demographic service archives). archivio della famiglia poirier-mantovani (archives of the poirier-mantovani family) bibliography alfano, g.b. (1915): in memory of prof. giuseppe mercalli, atti accademia napoletana scientifico-letteraria, anno ii, vol. ii, fasc. i, n. 7, 3-19. vol52,6,2009new 2-12-2009 12:40 pagina 646 647 roberto mantovani and the expanding earth carey, s.w. 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(1888): hypotèse cinétique de la gravitation universelle en connexion avec la formation des éléments chimiques, (chez l’auteur, moscow). vol52,6,2009new 2-12-2009 12:40 pagina 649 1 annals of geophysics, 65, 1, hf1001, 2021; 10.4401/ag-8690 o p e n acc e s s an analytic nonlinear model of thermo-poro-elastic pressure transients in porous rocks with application to deep co2 storage roman kanivetsky1, ettore salusti*,2,3 (1) department of bioproducts and biosystems engineering, university of minnesota, 1390 eckles ave, st. paul, mn 55108, usa (2) ismar-cnr, via fosso del cavaliere 100, roma, 00147, italy (3) infn, sezione roma 1, università la sapienza, italy article history: received may 25, 2021; accepted september 23, 2021 abstract today a co2 storage/sequestration is an important option for a significant enhancing of co2 sinks, to reduce the net carbon emissions into our planet atmosphere. such storage/sequestration is a complex process, dealing with many facets of decision about the site selection, taking into consideration the local geological, geothermal, hydrodynamic and hydrocarbon potentials. in such multifaceted context, a thermo-poro-elastic nonlinear analytic model of fluid pressure p in deep rocks, can play an important role. to analyse these dynamics we here focus on the role of the matrix convection, on thermal dynamics and fluid/rock “frictions”. in addition we here show that pressure dynamics, induced by an eventual external time or areal forcing can allow simple analytical determinations of pressure transients in these deep porous media. such processes indeed can have practical impacts on the co2 evolution for storage in deep rocks and thus influence the final site choice for a deep co2 injection. in synthesis, this model provides simple characterizations of thermo-poro-elastic transients for co2 storage. keywords: injections of co2 and their pressure dynamics; nonlinear thermo-poro-elastic pressure transients in fluid saturated porous media; application to co2 storage and sequestration 1. the problem among the major challenges in mitigating climate change effects is the reduction of the co2 percentage in the atmosphere, which would lead to a stabilization of the today planet co2 concentration hopefully to be less than ≈ 550 ppm i.e., twice the pre-industrial level. in more detail, the geological storage and/or sequestration of co2 is a particularly important option for significant reductions of the carbon emissions into the planet atmosphere. carbon dioxide sequestration in geological media can indeed be analyzed by many different approaches, as the analysis of geological stratigraphic and structural trapping in old oil and gas reservoirs, in solubility trapping resroman kanivetsky et al. 2 ervoirs of oil, in formation water, in adsorption trapping in un-economic beds and mainly in cavern trapping, in salt structures or by mineral immobilization [gunter et al., 1998]. in addition, these rocks must have the porosity and permeability necessary for a co2, to prevent or at least to delay any co2 return to the atmosphere for geologically significant times [hasanvand et al., 2013; cheng, 2013]. crystalline and metamorphic rocks as granite or clay therefore are not suitable for a co2 storage or sequestration, due to their fractured nature and their low permeability and porosity. we here discuss general solutions of a 1-d analytic nonlinear model of pressure p and temperature t in fluid saturated porous rocks, following a remark by rice and cleary [1976] that the local energy must have both kinetic and thermal components. in short, we here consider the dynamics of an analytic nonlinear p model that are dealing with nonlinear convection, fluid-rock frictions and thermal effects. we moreover briefly discuss the effects due to the evolution of heat exchanges with the neighboring rocks, or eventual local parameter variations. on the other hand, we here give no particular attention to such co2 seen in form of gas, or as a compressible fluid nor in our model equations is considered the co2 compressibility. indeed, the factor of compressibility z of co2 in the temperature range 20°c-100°c and pressure range 1-13 mpa is always less than 1 (0.13 < z < 0.97). this implies that co2 is more compressible than an ideal gas and, therefore, incompressibility is not expected for co2 injection in wells and successive diffusion in rocks of carbon dioxide at the above defined p and t conditions. obviously, this conclusion holds if no exchanges between co2 and rocks or brine occur at depth. 2. early analyses the co2 dynamics in fluid-saturated porous rocks has been already analyzed by computational mathematics [elenius and johannsen, 2012], simulations of density driven convection [cheng, 2013, cheng and zang, 2010], wave analyses of two phases flows [lambert et al., 2019] among others. we also note that in modeling co2 storage, complex numerical computations are often used [celia et al., 2015, andersen and nielsen, 2018, among others]. we thus focus on a nonlinear 1-d analytic model, considering nonlinear fluid-rock frictions and thermal convolution, which could be of interest focusing on the deep pressure dynamics in fluid saturated porous rocks. indeed, following a remark by rice and cleary [1976] we discuss the solutions of a 1-d analytic nonlinear model of pressure p and temperature t in fluid saturated porous rocks. in more detail we consider the dynamics of 1-d transients of p from a fluid saturated homogeneous “source” rock, with initial pressure p0 + pi as salusti et al. [2019], and an asymptotic porous medium with a pressure p0 and describe their effects on such co2 injections. in early studies [mctigue, 1986; bonafede, 1991; merlani et al., 2001, 2015; caserta et al., 2017; garra et al., 2015] and coworkers examined 1-d models of such transients. in particular, a 1-d choice can be valid for a radial transient or from cylindrical perforated segment of a borehole thus forming a segment source, or also for a two half-horizon schematization. about these problems, mc tigue [1986] discussed a p-t interaction (appendix a) as 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = 𝐵𝐵 𝜕𝜕!𝜕𝜕 𝜕𝜕𝑥𝑥! + 𝐶𝐶 𝜕𝜕!𝑇𝑇 𝜕𝜕𝑥𝑥! + 𝐴𝐴 𝜕𝜕𝑇𝑇 𝜕𝜕𝜕𝜕 (1) where a, b, c are functions of geological properties and their physical dimensions in mc tigue [1986] and in si are: 𝐴𝐴 = 2𝐺𝐺𝐺𝐺𝑏𝑏∗!(1 + 𝜈𝜈∗)31 + 𝜈𝜈#*4 93𝜈𝜈#* − 𝜈𝜈∗4 3𝛼𝛼% − 𝛼𝛼&4 ≈ 10' , 𝐵𝐵 = 2𝑘𝑘𝑘𝑘 𝜇𝜇 = 𝑏𝑏∗!(1 − 𝜈𝜈∗)(1 + 𝜈𝜈∗)! 9(1 − 𝜈𝜈#∗)(𝜈𝜈#∗ − 𝜈𝜈∗) > ≈ 0,1, , 𝐶𝐶 = 4𝐺𝐺𝐺𝐺𝑏𝑏∗(1 + 𝜈𝜈∗)𝛼𝛼% 9(1 − 𝜈𝜈#∗) ≈ 10' and where 𝑢𝑢 = − 𝑘𝑘 𝜇𝜇 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 is the darcy velocity, the porosity is 𝜑, the medium permeability is k, 𝜇 is the fluid viscosity parameter, g is the shear modulus, the drained poisson ratio is 𝜈*, the untrained poisson ratio is 𝜈𝑢*, 𝛼𝑚(𝛼𝑓) is the volumetric thermal expansion coefficient for the solid (fluid) and b* is the skempton parameter. about:blank nonlinear model of co2 dynamics in rocks 3 3. the energy conservation equation following the above remark [rice and cleary 1976], for small velocities one can focus on the classical equation of thermal energy conservation [bonafede, 1991] 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = 𝐸𝐸 𝜕𝜕!𝜕𝜕 𝜕𝜕𝑥𝑥! + 𝑋𝑋 𝜕𝜕𝜕𝜕 𝜕𝜕𝑥𝑥 𝜕𝜕𝜕𝜕 𝜕𝜕𝑥𝑥 + 𝑌𝑌( d 𝜕𝜕𝜕𝜕 𝜕𝜕𝑥𝑥 e ! (2) where in si the diffusion coefficient is 𝐸𝐸 = 𝐾𝐾) 𝜙𝜙𝜌𝜌$𝑐𝑐$ + (1 − 𝜑𝜑)𝜌𝜌%𝑐𝑐% ≈ 10*+ , the convection coefficient is 𝑋𝑋 = 𝑘𝑘𝜌𝜌$𝑐𝑐$ 𝜇𝜇k𝜑𝜑 𝜌𝜌$𝑐𝑐$ + (1 − 𝜑𝜑) 𝜌𝜌%𝑐𝑐%l ≈ 10*-!, and the fluid-rock friction coefficient is 𝑌𝑌′ = 𝑘𝑘 𝜇𝜇 k𝜌𝜌$𝑐𝑐$ + (1 − 𝜑𝜑)𝜌𝜌%𝑐𝑐%l ≈ 10*-. . here kt is the thermal conductivity, 𝜌𝑚 is the rock density, c𝑓 and c𝑚 are the fluid and rock heat capacities, discussed in our final table 1. we stress that are these geologic properties that play the dynamical roles. in addition, we note how in equation (2) is considered the nonlinear convection with coefficient x, and fluid-rock “friction” with a novel coefficient y ′ discussed in the appendix b. parameter abyssal red clay berea sandstone ruhr sandstone weber sandstone westerly granite b 2∙10–8 0.5 0.0004 0.002 10–4 e 2∙10–7 3∙10–6 10–6 2∙10–6 2∙10–6 c 8∙10–7 6∙105 104 3∙103 103 a 2∙102 2∙106 106 7∙106 6∙105 x 4∙1015 5∙1011 7∙1015 1014 4∙1020 table 1. material properties of some of the considered rocks in si. the fluid saturated abyssal red clay data have been estimated in mctigue [1986], the berea sandstone and rhur sandstone are from bonafede [1991]. the values of the other rocks are from fracturing experiments [salusti et al. 2019]. considering however the difficulty of estimating these rock properties in situ, we give only the orders of magnitude of the above quantities where the field uncertainties can be rather large. 4. the properties of the model solutions about the solutions of the equations (1) and (2), [merlani et al., 2011] and dr. decio levi (personal communication, 2018) suggest a simple ansatz, that the solutions of t and p are proportional to x2/t in equations (1) and (2), a property to be checked in the following. from this ansatz and equation (1), one obtains a strict relation between p, t and the parameter a as 𝑃𝑃(𝑥𝑥, 𝑡𝑡) = 𝐴𝐴 𝑇𝑇(𝑥𝑥, 𝑡𝑡) + 𝑓𝑓(𝑡𝑡) . (3) roman kanivetsky et al. 4 where 𝑓(𝑡) is discussed in the following. this implies that also a thermal transient strictly corresponds to a pressure transient. in addition, from the equations (1)-(3) we obtain a classical nonlinear burgers equation for t > 𝜀, as 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = 𝐸𝐸 𝜕𝜕!𝜕𝜕 𝜕𝜕𝑥𝑥! + (𝑌𝑌𝑌𝑌 + 𝑋𝑋) c 𝜕𝜕𝜕𝜕 𝜕𝜕𝑥𝑥 d ! = 𝐸𝐸 𝜕𝜕!𝜕𝜕 𝜕𝜕𝑥𝑥! + 𝛬𝛬 c 𝜕𝜕𝜕𝜕 𝜕𝜕𝑥𝑥 d ! , (4) where a short time delay 𝜀 can be due to chemical effects, or to a flux of thin sands a kind of filter cake [civan, 1998]. moreover, this 𝜀 avoids mathematical pathologies in short-time phenomena. equation (4) is a model of p transient dynamics at a distance x from the co2 injection well. we remark how 𝛬 = y a + x, related to the fluid-rock nonlinear convection and to the “frictions” among rocks deformations or fracturing, is the basic parameter for such flow evolutions. in more detail, for a small y this 𝛬 is small and positive, while for a large y it can be negative, as discussed in the appendix c. it is of interest that whitham [1974] analyzes the equation (4) and defines a number 𝑅𝑅 = (𝑌𝑌𝑌𝑌 + 𝑋𝑋)𝑃𝑃 𝐸𝐸 = 𝛬𝛬𝑃𝑃 𝐸𝐸 (5) i.e., the ratio of nonlinear terms over the diffusive terms. this r characterizes a linear pressure diffusion for r < 8-10, while for r > 8-10 the solution is a sharp and quick impulse (figure 1). in addition, a scale analysis in the appendix d shows that the effects of the diffusion in equation (5) are small about 1/100 times than these of the nonlinear terms. figure 1. sketch for the solutions of the burgers equation for various values of r. nonlinear model of co2 dynamics in rocks 5 5. solutions of the model the solution of p transients from the “source” rock injection, with the above initial pressure p = p0 + pi , and p0 for a large 𝑥 as in salusti et al. [2019], for t > 𝜀 and a positive 𝛬 are 𝑃𝑃(𝑥𝑥, 𝑡𝑡) = 𝑃𝑃/ + 𝑃𝑃0 0 < 𝑥𝑥; 𝑡𝑡 < 0 𝑃𝑃(𝑥𝑥, 𝑡𝑡) ≈ 𝑃𝑃/ + 𝑃𝑃0 − 𝑥𝑥! 4𝛬𝛬𝑡𝑡 0 < 𝑥𝑥; 𝑡𝑡 > 0 (6) 𝑃𝑃(𝑥𝑥, 𝑡𝑡) ≈ 𝑃𝑃/ for large 𝑥 and 𝑡 for a positive 𝛬 , these solutions are smaller for increasing distance x from the injection well (figure 1), in agreement with general observations and also among some zhaoxu et al. [2019] data. for a negative y we also have similar relations, sketched in the two panels of figure 2. a comparison of data and model is very complex, due to the many data now measured but one can compare the model result and some of the zhaoxu et al. [2019] data. figure 2. intuitive sketch of the solutions for increasing times, for positive and negative parameters x + a y, respectively. 6. practical applications of the novel model solution the equations (4) and (6) also have simple applications to particularly interesting cases: a) the solutions of (6) correspond to the above ansatz since the solutions are mere functions of 𝑥2/t with a relative uncertainties about 10–2, as discussed in appendix d; b) the equation (3) allows to compute t from the pressures p, then from pressure data one can consider thermal data; c) in this complex situation about depths, places and properties of different rocks, if the co2 were rather slowly injected with a temperature different from that of the aquifer, a resulting novel forcing n(t) can also give time dependent front expansion [andersen and nielsen, 2018]; d) an eventual novel front, and the corresponding p, imply other interesting solutions of equation (6). indeed, if the rock pressure receives a novel forcing n(t), function of the time only, the resulting pressure evolution can be described by a simple 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = (𝑌𝑌𝑌𝑌 + 𝑋𝑋) c 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 d ! + 𝑁𝑁(𝜕𝜕) , (7) namely a forced burgers equation, with simple solutions (appendix d); roman kanivetsky et al. 6 e) all of this suggests to consider that a density difference between the injected co2 and the brine may lead to a buoyant segregation then making reasonable an assumption of vertical equilibrium [celia et al., 2015] eventually seen as the discussed case of another external forcing; f ) in general, if the natural pressure has a further space-dependent forcing 𝑆(𝑥), the resulting novel evolution equation for as well e = 0 will be 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = (𝑌𝑌𝑌𝑌 + 𝑋𝑋) c 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 d ! + 𝑆𝑆(𝜕𝜕) , (9) discussed in the appendix d; g) karsten pruess et al. [2011] moreover discuss how from a geologic storage reservoir some fluids can migrate towards the land surface passing through lost faults, or fractures and abandoned wells. these fluids then could reach sub-critical conditions at depths shallower than 500-750 m. at this level, and at shallower depths, a subcritical co2 can form a two-phase mixtures of liquid and gaseous co2, with significant latent heat effects during boiling and condensation. and thus, we can eventually generalize the above discussed evolutions. in such complex situations one has also to consider the geological media characterizations. the sedimentary basins, where co2 storage most often occurs while the sandstone is the dominant rock, generally do not have natural fractures allowing perturbations of the general pressure. in contrast, about the crystalline rocks where a volcanic or seismic activity can have occurred, these rocks can have natural fractures, eventually causing remarkable pressure perturbations. then also from such phenomena one can have critical dynamical effects on the actual co2 injections; h) in turn the mere co2 injection can be also seen as an external, strongly localized forcing characterized as a very strict exponential 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = (𝑌𝑌𝑌𝑌 + 𝑋𝑋) c 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 d ! + 𝑎𝑎𝑒𝑒*[%(3*3!)] , (10) for suitable 𝑎, 𝑥0, 𝑚, again discussed in appendix d. thus, we stress that in this way one could enlarge the model applications to other natural rock characteristics, or also in further x-dependent phenomena. 7. applications to some rock examples we now apply these model results, to check nonlinear effects of co2 transients in sandstones, the typical rocks of the sedimentary basins, often used for co2 storage and sequestration. in addition, although clay and crystalline rocks are not suitable for a co2 storage/sequestration, for a comparison we sketch the above applications also for these rocks. we thus very quickly describe such rock properties, while due to measurement difficulties we focus only on the order of magnitude of these thermo-poro-elastic parameters. berea sandstone. this rock of fine-grained sandstone consists of quartz (70%), polycrystalline quartzose rock fragments (25%), and feldspar (5%). its a is about 2⋅106 and x ≈ 5⋅1011 in si. weber sandstone. this sandstone is originated from fluvial deposits, derived from an ancestral uplift. its sarkosic lithofacies act as permeability barriers being either cross-laminated or massively bedded. their a ≈ 7⋅106 and x ≈ 1014 in si. ruhr sandstone. this rock is similar to other basins in europe and north america. these sedimentary rock sequences were deposited during early tropical, humid climate coastal plains in an equatorial region. its a ≈ 106 and x ≈ 7⋅1015 in si. charcoal granite. this rock in st. cloud, minnesota has a massive, dark brown to dark colors. it is among the most ancient known rocks… as the protruding parts of a paleo-structure. thus, their a ≈ 6⋅105 and x ≈ 4⋅1020 are particularly larger than those of the above rocks. abyssal red clay. these deposits, allogenic in origin, are mostly due to rather ancient asian aeolian dust. these particular red clays are about the 90%, then are mn, co, ni and cu of autogenic origin [glasby, 2010]. its a ≈ 2⋅102 and x ≈ 4⋅1015 in si, are rather similar to these of sandstones. nonlinear model of co2 dynamics in rocks 7 here, we strongly emphasize the need for further field measurements of the thermo-poro-elastic properties in such deep rocks, in order to obtain more data sets also for applications of eventual novel models, for discussing future models. 8. discussions and conclusions although today many approaches are used to model co2 storage, often also including complex numerical computations, we discuss here a simple nonlinear analytical model, a generalization of garra et al [2015]. this model describes a theoretical approach to many hydrologic processes, showing how the deep fluid pressure solutions of such model finally are functions of 𝑃𝑃(𝑥𝑥, 𝑡𝑡) ≈ 𝑃𝑃/ + 𝑃𝑃− 𝑥𝑥! 4𝛬𝛬𝑡𝑡 (11) for suitable parameters 𝛬 , p0, p1. thus, our equation (11) describes the dynamics of processes dealing with different rock temperatures, pressure evolutions and their dynamic consequences. in addition, this allows one to compare critically the results of eventual numerical computations about storage/sequestration of co2 sites, that we now can address also with the results of this nonlinear analytic model. it is also important that we show how eventual external temporal forcing 𝑆(𝑡), or space forcing 𝑄(𝑥), can perturb these dynamics. indeed, we here show that for such 𝑆(𝑡) or 𝑄(𝑥) one can again have simple model solutions. in addition, all of this can also be relevant to other practical applications such as for elevate or low temperature liquid storages, or also disposal of dangerous waste in porous media [mc tigue, 1986]. thus, the broad implication of such novel analytic model is that it provides simple, general assessments for dynamics of thermo-poro-elastic transients, also for other applications dealing with the geological reservoir and for the realistic site selections for these or similar problems. appendices appendix a. other similar models in the literature the equation (1) is not the only equation of this type discussed in the literature. [bonafede and mazzanti, 1997] indeed assume c = 0 in equation (1) and shapiro and dinske [2009], and coworkers, analyze an interesting similar nonlinear model as c 𝜕𝜕 𝜕𝜕𝜕𝜕 − 𝜕𝜕 𝜕𝜕𝜕𝜕 𝐻𝐻(𝑃𝑃) 𝜕𝜕 𝜕𝜕𝜕𝜕 d 𝑃𝑃 = 0 (11) appendix b. effects of nonlinear terms we remark that the equation (2) is the classical equation of energy-heat conservation, with the nonlinear fluid “convection” 𝑢𝑢 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑇𝑇 and the “work rate” − 𝑘𝑘 𝜇𝜇 c 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑃𝑃d ! in si, here considered for rather mild dynamics. for larger effects, as rock deformations or fractures [phillips et al., 2013; detournay and garagash, 2003] one can consider a different approach 𝑌𝑌 c 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑃𝑃d ! with an unknown y. in more detail, the work made by the fluid pressure increases the rock temperature, and for moderate perturbations one indeed has a positive y ′ ≈ k/𝜇 ≈ 10–12 in si [bonafede et al., 1991]. on the other hand, rock deformations or fractures, caused by relevant perturbations, can extract from the rock some heath φ and then one can have an energy loss for a rather large φ as 𝑋𝑋′ 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑃𝑃 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑇𝑇 − 𝛷𝛷 = 𝑌𝑌′ c 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑃𝑃d ! − 𝛷𝛷 = 𝑌𝑌 c 𝜕𝜕 𝜕𝜕𝜕𝜕 𝑃𝑃d ! (12) roman kanivetsky et al. 8 assuming that such energy loss is somehow related to the fluid energy [phillips et al., 2013; detournay and garagash, 2003]. thus, a realistic determination of a, c, b, e, y, φ is not a simple challenge, and these equations must be considered critically. appendix c. approximate scale analysis as a rather rough estimate in si, we assume for sandstones pi ≈ 107, ti ≈ 102, a ≈ 106, b ≈ 0.1, c ≈ 104, e ≈ 10–6 and x = 1013 for sandstones but x ≈ 1020 for granites (table 1). from eq. (6), assuming a space scale 𝜆 and a time scale 𝜏, for sandstones we have 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 ≈ 10. 𝜏𝜏 , 𝐴𝐴 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 ≈ 107 𝜏𝜏 , 𝐵𝐵 𝜕𝜕!𝜕𝜕 𝜕𝜕𝑧𝑧! ≈ 10+ 𝜆𝜆! , 𝐶𝐶 𝜕𝜕!𝜕𝜕 𝜕𝜕𝑧𝑧! ≈ 10+ 𝜆𝜆! (13) and thus 𝐸𝐸 𝜕𝜕!𝑇𝑇 𝜕𝜕𝑧𝑧! ≈ 10*' 𝜆𝜆! over 𝑋𝑋 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 ≈ 108 𝜆𝜆! is a very small quantity. all of this confirms that the diffusive terms have smaller effects than the nonlinear terms characterized by b and c. appendix d. solutions of the model equation, forced by time or space external forcing considering in general a forcing 𝑁(𝑡) in the burgers equation such that 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 = c 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 d ! + 𝑁𝑁(𝜕𝜕) , (14) the classical solution of the burgers equation for 𝑁𝑁(𝑡𝑡) = 0 is 𝑃𝑃 = 𝑃𝑃/ − 𝑥𝑥! 𝑡𝑡 while for a realistic the solution is 𝑃𝑃 = 𝑃𝑃/ − 𝑥𝑥! 𝑡𝑡 + ^ 𝑁𝑁(𝑡𝑡)𝑑𝑑𝑡𝑡 . considering also the scale analysis in appendix c we here do not consider other diffusive time terms, since most probably other forcing solutions have been already studied. indeed, the burgers equation has been deeply examined by mathematicians and similar discussions holds also for another 𝑀(𝑥) case. references andersen, o. and h. m. nilsen (2018). investigating simplified modeling choices for numerical simulation of co2 storage with thermal effects, int. j. greenh. gas control., 72, 49-64. bonafede, m. 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(2006). heating and weakening of faults during earthquake slip, j. geophys. res., 111 (b5), b05311, 29. salusti, e., r. kanivetsky, r. droghei and r. garra (2019). on the propagation of nonlinear transients of temperature and pore pressure in a thin porous boundary layer between two rocks, journal of hydrology, 576, 520-527. shapiro, s. and c. dinske (2009). fluid-induced seismicity: pressure diffusion and hydraulic fracturing. geophysical prospecting, 57, 301-310. whitham g.b., linear and nonlinear waves, wiley, new york (1974). zhaoyang, l. j. and l. j. durlofsky (2015). reduced-order modeling of co2 storage operations, j. greenh. gas control., 68, 49-67. zencher, f., m. bonafede and r. stefansson (2006). near-lithostatic pore pressure at seismogenic depths: a thermoporo-elastic model, geophys. j. int., 166, 1318-1334. zhaoxu m., f. wang, z. yang, x. li, y. diao, x. ma and h. tian (2019). numerical simulation of the influence of geological co2, storage on the hydrodynamic field of a reservoir geofluids, article id 1853274, https://doi. org/10.1155/2019/1853274. *co r r e s po n d i n g a u t h o r: ettore s a l u s t i, ismar‑cnr, via fosso del cavaliere 100, roma, 00147, italy, infn, sezione roma 1, università la sapienza, itay, email: ettore.salusti@gmail.com https://www.hindawi.com/24562064/ https://www.hindawi.com/24562064/ https://www.hindawi.com/31768010/ https://www.hindawi.com/31768010/ https://www.hindawi.com/98160794/ http://www.sciencedirect.com/science/journal/09204105 https://doi.org/10.1155/2019/1853274 https://doi.org/10.1155/2019/1853274 osservazioni l i m n o g r a f i c h e sul iago di a l b a n o (*) p . c a l o i g . b o m u a l d i r i c e v u t o il 1° n o v e m b r e 1964 r i a s s u n t o . — si r i a s s u m o n o i r i s u l t a t i di alcune serie di registrazioni limnografiche, eseguite in epoche diverse sul lago di a l b a n o , allo scopo di c o n f r o n t a r e le c a r a t t e r i s t i c h e delle oscillazioni libere del lago, calcolate per via t e o r i c a , con quelle reali. è r i s u l t a t o , come per a l t r i laghi italiani, un o t t i m o accordo t r a teoria ed osservazione, p e r q u a n t o concerne i periodi e gli s p o s t a m e n t i r e l a t i v i ai d u e e s t r e m i . c o n f o r m e m e n t e alla teoria di h i d a k a , risulta cosi che l ' a m p i e z z a d e l l ' u n i n o d a l e a l l ' e s t r e m o nord del lago (riva a b i t a t a ) è circa q u a t t r o volte quella che si registra sulla riva o p p o s t a . una p a r t i c o l a r i t à del lago di albano che risulta, s o s t a n z i a l m e n t e , dalla f u s i o n e di d u e c r a t e r i , quello s e t t e n t r i o n a l e in p a r t e f r a n a t o nel più p r o f o n d o , m e r i d i o n a l e — f a sì che la p a r t e a .settentrione si comporti come golfo a p e r t o sul resto del lago. poiché il periodo n a t u r a l e di q u e s t a p a r t e (lei bacino è dell'ordine del periodo d e l l ' u n i n o d a l e dell'intero lago, molto spesso si r e g i s t r a n o , sulla riva a s e t t e n t r i o n e , oscillazioni di circa 4™, peculiari dei b a c i n i a p e r t i , in sé conchiuse, e la cui ripetizione è s t r e t t a m e n t e l e g a t a al persistere della c a u s a p e r t u r b a n t e . da ultimo, si v a l u t a n o i valori medi dell'energia, associata alle u n i n o d a l i del lago. s u m m a r y . — the p a p e r r e p o r t s t h e results of a series of l i m n o g r a p h i c s u r v e y s , carried o u t a t d i f f e r e n t t i m e s on t h e l a k e of albano, in order t o c o m p a r e t h e c h a r a c t e r i s t i c s of f r e e oscillations of t h e lake, calculated t h e o retically, w i t h t h e real ones. t h e r e s u l t s h a v e s h o w n , as in t h e case of o t h e r i t a l i a n lakes, a v e r y good a g r e e m e n t b e t w e e n t h e o r y a n d o b s e r v a t i o n , b o t h as t o t h e p e r i o d s a n d as t o t h e r a n g e b e t w e e n t h e t w o e x t r e m e s . i n c o n f o r m i t y w i t h h i d a k a ' s t h e o r y , t h e u n i n o d a l a m p l i t u d e a t t h e n o r t h e n e x t r e m e of t h e lake ( i n h a b i t e d side) a p p e a r s to be f o u r t i m e s h i g h e r s t h a n t h a t w h i c h is n o t i c e d on t h e o p p o s i t e side. (*) q u e s t a ricerca è s f a t a c o n d o t t a con c o n t r i b u t i de) consiglio n a zionale delle ricerche. l \ c a i . o l < ; . i k l m u a i . i l i a p a r t i c u l a r f e a t u r e of t h e lake of albano which is composed, b a sically, by t h e union of t h o c r a t e r s ; t h e n o r t h e r n one, p a r t l y sledged i n t o t h e s o u t h e r n one, which is deeper is such t h a t t h e n o r t h e r n section behaves like a gulf o p e n on t h e rest of t h e lake. since t h e n a t u r a l p e r i o d of this side of basin is in t h e range of t h e u n i n o d a l period of t h e whole lake, 011 t h e n o r t h e r n side it is o f t e n possible to record oscillations of a b o u t 4m, typical of open basins. the repetition of these oscillations is s t r i c t l y connected to t h e c o n t i n u e d presence of t h e cause of p e r t u b a t i o n . f i n a l l y , t h e p a p e r e v a l u a t e s t h e average values of t h e energy associated with t h e l a k e ' s u n i n o d a l . li> c a r i l i t e r i s t i e l i e f o n d a m e n t a l i (ielle oscillazioni l i b e r e d e l l a g o d i a l b a n o s o n o s t a t e p r e c e d e n t e m e n t e c a l c o l a t e p e r v i a t e o r i c a m e d i a n t e i m e t o d i d i d e f a n t , h i d a k a e c h r y s t a l d a u n o di noi e s p o s t i , e o n opport u n e e s t e n s i o n i e c o r r e z i o n i , n e l 1948 (*); nella t a b e l l a s e g u e n t e s o n o r i a s s u n t i i d a t i r e l a t i v i ai p e r i o d i e ai n o d i {-• 3). . . . . . . . . oscillazioni libere i m e t o d o ii m e t o d o i i i m e t o d o 1 ( d e f a n t ) ( h i d a k a ) (chrystal) r j n i n o d a l e periodo 4 , 0 3 min. 4 , 1 2 m i n . 3 , 5 6 m i n . nodo ( d a l l ' e s t r e m o sud) 2122 in. 1656 m . 2009 m . b i n o d a l e periodo ] 2 , 5 3 min. 2 , 3 1 min. 2 , 0 3 min. i nodo ( d a l l ' e s t r e m o sud) : 1451 m. 1403 m . 1191 m . 11 nodo » 3007 in. 2424 m. 2850 m . t r i n o d a l e periodo 1 , 9 0 min. 1 , 5 0 min. 1 , 4 6 m i n . 1 nodo ( d a l l ' e s t r e m o sud) , 982 m. 747 ni. 703 m . 11 nodo » 2543 m. 2004 m. 2125 m. i i i nodo » i 3251 m. 3093 m. 3097 m. i t r e m e t o d i h a n n o p e r m e s s o di s t u d i a r e l ' a n d a m e n t o d e l l e a m p i e z z e in s e n s o v e r t i c a l e l u n g o l ' a s s e m a g g i o r e d e l l a g o . è r i s u l t a t o i n p a r t i c o l a r e c h e a l l ' e s t r e m o n o r d , in c o r r i s p o n d e n z a cioè d e l l a p a r t e m e n o p r o f o n d a d e l l a g o , l e a m p i e z z e v e r t i c a l i d e v o n o e s s e r e s e n s i b i l m e n t e m a g g i o r i c h e n e l l a p a r t e m e r i d i o n a l e , d o v e si r i s c o n t r a la m a s s i m a p r o f o n d i t à . t a l e c a r a t t e r i s t i c a è s t a t a r i s c o n t r a t a p e r t u t t i e t r e i t i p i d ' o o s s e r v a z i o n i l i m n 0 o k a f i c h e s u l l a g o i>i a l b a n o 5 stillazione e con t u t t i e tre i metodi, i quali t u t t a v i a differiscono in quel che riguarda l'entità degli spostamenti verticali relativi. allo scopo di verificare la validità dei risultali teorici, fra il 1957 e il 1963 sono state compiute delle serie d'osservazioni sul posto. d u r a n t e t u t t i i periodi d'osservazione un limnografo lia funzionato nella p a r t e settentrionale del lago; contemporaneamente nel 1957 sono state e f f e t t u a t e alcune registrazioni in località prossima all'uninodo e nel 1963 nella p a r t e meridionale del lago. e s p e r i e n z e e s e g u i t e fra il 1 2 e i l 2 4 a p r i l e 1 9 5 7 . le registrazioni iniziano il 12 aprile 1957 e proseguono ad intervalli più o meno lunghi fino al 24 aprile. un primo periodo di registrazione inizia il 12 aprile verso le 18 e prosegue fino alle 22,30 dello stesso giorno. i n tale intervallo di tempo vengono registrate ampie oscillazioni uninodali (t = 4m,16) alterate da altre oscillazioni e da perturbazioni (fig. 1). si può f r a l'altro notare che l'uninodale sembra ricevere di quando in quando un impulso più forte, che determina oscillazioni sensibilmente più ampie. tale fenomeno appare più chiaro f r a le 19 e le 21. tale fenomeno è dovuto — come vedremo — al contemporaneo insorgere di sesse da zoccolo, la cui uninodale presenta un periodo pressoché uguale al periodo dell'uninodale dell'intero lago. alla ripresa della registrazione verso le 9 del 13, l'uninodale (t = 4,12 44,16 minuti) presenta un'ampiezza n e t t a m e n t e inferiore (fig. l a ) , m e n t r e appaiono a t t e n u a t e , ma sempre presenti, le coperiodali sesse da zoccolo. l a registrazione viene i n t e r r o t t a alle 19,15 del 13, senza che nel f r a t t e m p o si siano a v u t e oscillazioni diverse dall'uninodale. l'esistenza pressoché esclusiva di oscillazioni uninodali, con periodo sui 4,10 -f4,12 min., cui si sovrappongono talvolta « shelf-seiclies », caratterizza il periodo di registrazione f r a le 9,20 del 14 e le 9,30 del 15 (fig. 16). la ripresa della registrazione alle 11,45 del 15 m e t t e in evidenza u n complicarsi delle oscillazioni del lago. tale agitazione prosegue a c c e n t u a t a fin verso le 20 del 16, quando cominciano a prevalere le oscillazioni uninodali, sovrapposte a binodali e trinodali. le uninodali presentano però, spiccata la caratteristica propria delle oscillazioni proprie dei bacini a p e r t i (o limitati da zoccolo), che si susseguono l'una all'altra, come impulsi separati (fig. 2). li p . c a i . o i c . h o m u a l d i le uninodali (sempre però disturbate) prevalgono verso le nove del 17. verso le 13,45 l'uninodale comincia ad essere b o r d a t a da « shelfseiches » (t ^ l m ) ; successivamente all'oscillazione fondamentale si sovrappongono deboli binodali (2m,3) e trinodali (l m ,7). dalle 18h alle 21h (fig. 3) si ha t u t t o un seguito di singole uninodali da zoccolo separate da intervalli di qualche minuto, u n a dall'altra. l'uninodale del lago si presenta alla ripresa delle registrazioni alle 9,30 del 18. l'ampiezza va però diminuendo, mentre intorno alle 11 inizia la registrazione, gradualmente crescente, di « shelf-seiches ». subito dopo all'uninodale si alternano anche oscillazioni trinodali (t ^ l m , 7 ) e binodali (t ^ 2 m , 4 ) ; quest'ultime appaiono molto chiare f r a le 18 e le 20, dopo di «he si ha una serie (non molto accentuata) di oscillazioni uninodali a l t e r n a t e a binodali tino alle 23,45. la registrazione riprende, il 19 alle 9,30, con saltuarie serie di oscillazioni uninodali, cui dopo le 17,30 si sovrappongono talvolta delle binodali; le ampiezze dei due tipi di oscillazione sono molto ridotte. il giorno 20, dopo le 13 all'uninodale si alternano altre oscillazioni con periodi inferiori ai 2 minuti; dopo le 16 insorgono oscillazioni binodali (t r^ 2m,6) le quali vanno gradualmente diminuendo d'ampiezza fino a scomparire nelle prime ore del 21. f r a le 9 e le 10 di tale giorno, inizia un periodo di agitazione che si accentua verso le 10,30. f r a le varie oscillazioni disordinate si notano saltuariamente delle serie di oscillazioni aperiodo di circa l m , 7 0 (trinodali), cui verso le 15 seguono oscillazioni più lente (t ^ 2m,40 binodali) che vanno gradualmente diminuendo d'intensità e scompaiono dopo le 22. alle 7 del 22 iniziano oscillazioni uninodali (t = 4m,12), m e n t r e più t a r d i verso le 12 segue un nuovo periodo d'agitazione con origine di trinodali molto chiare (t = l m ,70), sovrapposte a oscillazioni uninodali e binodali meno pronunciate (fig. 4). circa alle 17,30 si registra un improvviso abbassamento del livello del lago (fig. 4a) provocato dal passaggio di un disturbo microbarieo positivo. la registrazione t e r m i n a alle 19,15. l'apparecchio riprende il funzionamento il giorno 23 alle 8,30. dopo alcuni accenni di uninodali, cui peraltro si sovrappongono oscillazioni a più breve periodo (circa 1 min.), la registrazione si complica sia. per l'accentuarsi di tali oscillazioni sia per la presenza saltuaria dì esempi di trinodali (verso le 12,15; t ^ l m , 7 5 l m ,70) o di binodali. le oscillazioni binodali vanno poi gradualmente prevalendo; dalle 18 in poi si ha i n f a t t i la registrazione pressoché esclusiva di tali oscillazioni o s s e r v a z i o n i l i m n 0 g r a f i c 1 1 e sul lago d i a l b a n o 7 che mostrano un periodo medio di 2m,72 (pig. 5 e 5«). verso le due (lei 24, la binodale comincia ad essere bordata da oscillazioni a periodo intorno al minuto. p u r t r o p p o dalle 5 del 24 fino alle 15,50, cioè tino al termine della serie di esperienze, il funzionamento dell'apparecchio è difettoso, la registrazione si ha solo pei' brevissimi t r a t t i . nei giorni 18, 19, 20 e 23-24, oltre alle registrazioni del limnografo situato nella p a r t e settentrionale del lago, si hanno le registrazioni di un secondo apparecchio situato in località prossima all'uninodo. nel periodo dal 18 al 20, in tale località non si registra alcuna uninodale, in quanto la posizione dell'apparecchio coincide all'incirca, con l'uninodo. si hanno solo vibrazioni per vento e qualche traccia di binodale. p e r la registrazione f r a il 23 e il 24, rispetto alla registrazione del primo apparecchio si può notare che, restando identiche le caratteristiche generali della registrazione che inizia alle 9,45 quando si lui già la presenza di brevi oscillazioni forzate, diminuisce l'ampiezza della binodale e della trinodale, mentre si accentuano le oscillazioni a più breve periodo. i n ogni modo, come era da attendersi, mancano t o t a l m e n t e le uninodali. e s p e r i e n z e e s e g u i t e n e l 1 9 5 8 . due nuove serie di registrazioni limitate all'estremo nord del lago, furono eseguite rispettivamente nel marzo e nel novembre 1958. i l 10 marzo 1958 la registrazione, per quanto saltuaria, sorprende l'insorgere di una serie di oscillazioni uninodali di ampiezza eccezionale (pig. 6). la registrazione appare analoga a quella del 12 aprile 1957. anche qui l'uninodale appare t u r b a t a da altre oscillazioni anch'esse di notevole ampiezza. tali impulsi supplementari spariscono m a n mano clie l'ampiezza dell'oscillazione fondamentale diminuisce. verso le ore 18 si può i n f a t t i osservare come l'uninodale si normalizzi con l'improvviso diminuire dell'ampiezza. pino a dopo le 0,30 d e l l ' l l marzo si ha u n a serie i n i n t e r r o t t a di oscillazioni uninodali (pig. 7) in cui appare evidente come al decrescere di esterni f a t t o r i p e r t u r b a n t i la forma dell'oscillazione si perfeziona con il diminuire dell'ampiezza. di pari passo con tale attenuazione dell'onda sembra esservi un a u m e n t o del periodo che va dai 3,96 m i n u t i ai 4,15 minuti. 8 p . c a l o i g . k o m u a l d i verso le 0,45 dell'll l'uninodale appare b o r d a t a da « shelf-seiches » mentre l'ampiezza dell'onda comincia ad a u m e n t a r e . p u r t r o p p o la registrazione va fuori carta poco dopo le 2. eimessa a posto verso le 7,20 mostra dapprima una fitta serie di oscillazioni a periodo inferiore al m i n u t o (vibrazioni per vento e rapidi disturbi microbariei) su cui prevale gradualmente l'oscillazione uninodale ad ampiezza molto elevata (t = 4m,t9). la registrazione continua ad essere saltuaria e cessa verso le 14,30. l'apparecchio riprende a funzionare il 12 alle 13,30. la registrazione inizia con una. serie di chiare oscillazioni uninodali (t = 4m,18) la cui ampiezza a u m e n t a improvvisamente e rapidamente f r a le 15 e le 15,30. contemporaneamente, all'uninodalc, si sovrappone un'oscillazione binodale che appare molto chiara (fig. 8) verso le 16,30 quando diminuisce l'ampiezza dell'uninodale. fra le 16,30 e le 17,15 la binodale (t = 2 m , 4 8 ) h a il sopravvento sull'uninodale cui t o r n a poi a sovrapporsi. dopo le 21 è presente solo l'oscillazione uninodale (fig. 9) che con ampiezze varie, viene registrata fino verso le 22 del 16. poi va gradualmente scomparendo e fino alle 12 del 15, ora in cui termina la registrazione, non si h a quasi nulla.; solo saltuariamente si notano delle deboli uninodali. la registrazione riprende alle 14,34 del 16 ma fin verso le 21 non si h a alcuna oscillazione. inizia quindi l'uninodale (t = 4m,22) b o r d a t a successivamente da « shelf-seiclies »; ambedue i tipi di oscillazioni hanno ampiezze crescenti (fig. 10). verso le 0,3 del 17, all'uninodale si sovrappone la. trinodale (t ~ l m , 7 ) . la registrazione t e r m i n a il 17 aprile verso le 11. la seconda serie di registrazioni del 1958 inizia il 26 novembre alle 16,38. fino alle 20 del 26 non si registra alcuna oscillazione particolare, poi si h a u n a serie di oscillazioni forzate cui segue u n a lunga serie di oscillazioni uninodali (t ^ 4m,36), talvolta bordate da «slielf-seiches» e talvolta molto a t t e n u a t e . dalle 13,30 del 27 si ha un lungo periodo con esempi rari e atten u a t i di uninodali. l'uninodale si rinforza verso le 01 del 28. verso le 03 l'uninodale comincia a d essere t u r b a t a da ampie oscillazioni forzate (fig. 11), che si a t t e n u a n o f r a le 8 e le 9 per poi riprendere con rinnovata intensità, fino a circa le 17. i n questo periodo l'oscillazione f o r z a t a oltre ad essere sovrapposta all'uninodale appare talvolta, sovrapposta alla trinodale (t c^ l m , 8 ) e alla binodale (t ^ 2m,6). o s s e r v a z i o n i l i m n 0 g r a f i c 1 1 e s u l l a g o d i a l b a n o 9 di scarso interesse risulta il periodo di registrazione che va dalle 16,40 del 30 novembre alle 16,40 del 1° dicembre: infatti solo in alcuni momenti si lia la registrazione di un'uninodale molto a t t e n u a t a . e s p e r i m e n t i e f f e t t u a t i d a l 6 al 1 3 m a r z o 1 9 6 3 . le registrazioni e f f e t t u a t e nel marzo .1963, avevano come scopo di verificare quanto previsto dalla teoria sull'andamento delle ampiezze. f u r o n o p e r t a n t o e f f e t t u a l e contemporaneamente due serie di registrazioni una all'estremo nord e una all'estremo sud. nella p a r t e nord la registrazione inizia verso le 17 del 6 marzo m e n t r e nella p a r t e sud inizia verso le 9,30 del 7. fino a verso le 10,30 del 9 non si registra però alcuna oscillazione. a tale ora inizia, a. nord un'uninodale molto chiara (t = 4m,06) che a sud appare, appena accennata. l a registrazione successivamente diventa incerta anche perché il limnografo è difettoso a causa di forti venti. un confronto può essere f a t t o nuovamente per le registrazioni comprese fra le 16,29 del 10 marzo e 8,30 dell'11 e fra le 10 e le 13 d e l l ' l l (figg. 12, 13 e 14). si può notare come l'uninodale registrata con estrema chiarezza (t = 4m,06) abbia nella p a r t e meridionale del lago (figg. 13a e 14a) ampiezze inferiori anche di 2/3 rispetto alla p a r t e settentrionale (figg. 13 e 14). f r a le 11 e le 13 d e l l ' l l marzo .1963 all'uninodale cominciano a sovrapporsi oscillazioni dovute al vento clic t u r b a n o la registrazione. inoltre nella p a r t e settentrionale la registrazione si interrompe alle 13, per cui è impossibile un confronto. nella p a r t e meridionale del lago dove continua la registrazione fin dopo le 05 del 12, n e t t e oscillazioni dovute al vento continuano a sovrapporsi all'uninodale e in alcuni casi alla binodale. un confronto fra oscillazioni della p a r t e meridionale e della p a r t e settentrionale è di nuovo possibile f r a le 10 del 12 marzo e le 12 del 13. dalle registrazioni di questo intervallo di tempo si può notare come non solo l'ampiezza dell'uninodale ma anche quella delle altre oscillazioni sia n e t t a m e n t e superiore nella p a r t e settentrionale del lago. l'inizio delle registrazioni coglie il lago, in una serie complessa di oscillazioni: i n f a t t i , all'uninodale (t = 4m,05) appaiono sovrapposte oscillazioni binodali (t = 2m -i2m,5) e trinodali (t ^ l m , 7 ) , che nella p a r t e settentrionale del lago hanno ampiezze molto accentuate. 1 0 p . c'alo! o . l i o . m u a l d i il massimo delle jmipiezze si ha fra le 13 e le 15 (fig. .15); poi si ha contemporaneamente un graduale diminuire delle ampiezze e il prevalere dell'oscillazione uninodale. le oscillazioni binodali e trinodali scompaiono verso le 18 nella parte meridionale del lago e verso le 22,30 nella p a r t e settentrionale. sempre a causa, della minore ampiezza d'oscillazione, l'uninodale, che nella p a r t e settentrionale continua ad essere registrata ininterrottamente dopo le 22,30 fino alla fine della registrazione, nella p a r t e meridionale del lago scompare pressoché definitivamente già verso le 21. c o n c l u s i o n i . i t r e periodi d'osservazione sopra riassunti, hanno consentito di trarre alcune conclusioni sull'idrodinamica del lago d'albano (o di castelgandolfo, che dir si voglia). come avviene per la generalità dei laghi italiani, finora investigati, anche nel lago di albano l'oscillazione libera, predominante, per ampiezza e frequenza, è la fondamentale (uninodale). a differenza di altri laghi italiani, di analoghe dimensioni, le oscillazioni libere binodali e trinodali si presentano pure di sovente, talora con notevole ampiezza. anche il lago di albano è spesso commosso da lunghe serie di « shelf-seiches » (sesse da zoccolo), con periodi dell'ordine di un minuto, generalmente associate a forti venti o al transito di rapidi disturbi microbarici. a proposito delle sesse da zoccolo, va f a t t a u n ' i m p o r t a n t e precisazione per q u a n t o concerne il lago di albano. una semplice osservazione della sezione longitudinale, centrale, del lago, mostra che esso risulta dalla congiunzione di due bacini (crateri), il più profondo costituendo la p a r t e meridionale del lago. il bacino settentrionale, più basso, scende a dolce declivio verso quello profondo, che raggiunge p r a t i c a m e n t e verso l'isobata 40, in corrispondenza della quale il fondo precipita. il bacino settentrionale, quindi, può ritenersi come un golfo aperto sulla restante p a r t e del lago. p e r t a n t o , se il periodo dell'uninodale risulta inferiore od uguale a quello della f o n d a m e n t a l e dell'intero lago, le condizioni fisiche per la sua libera eccitazione sarebbero rispettate. è quanto eff e t t i v a m e n t e si verifica. o s s e r v a z i o n i l i m n o g r a f i c h e s u l l a g o d i a l b a n o 1 1 i l v a l o r e a p p r o s s i m a t i v o del periodo del bacino s e t t e n t r i o n a l e , considerato come golfo, p u ò o t t e n e r s i a p p l i c a n d o la n o t a f o r m u l a v g l i dove l esprime, nel nostro caso, la lunghezza del golfo in p a r o l a ( c o n t a t a sulla linea di valle) e li la p r o f o n d i t à media lungo tale linea, essendo g l'accelerazione di g r a v i t à . dalla sezione longitudinale del lago e dai d a t i b a t i m e t r i c i , si o t t i e n e 4 l = 2880 in , h = 15 m . l'applicazione della f o r m u l a conduce allora al valore approssimativo tset. = 240» = 4'» . i l periodo f o n d a m e n t a l e della p a r t e meno p r o f o n d a del lago lia quindi u n valore di 4nl circa, dell'ordine cioè dell'uninodale dell'intero lago. è q u e s t a u n a peculiare p a r t i c o l a r i t à del lago d'albano, che consente la chiara spiegazione di f a t t i a l t r i m e n t i inspiegabili. sovente oscillazioni con periodo di 4 m i n u t i circa si p r e s e n t a n o come impulsi isolati, in se conclusi, s e p a r a t i da brevi intervalli occupati da altri tipi di onde; ciò si verifica specialmente q u a n d o l'ampiezza dell'onda è molto accent u a t a (fig. 1), m a avviene a n c h e — sia p u r e meno f r e q u e n t e m e n t e — p e r piccole ampiezze (fig. 2). è noto che oscillazioni del genere sono proprie dei bacini a p e r t i : f o r m a t a s i l'onda propria del bacino (che ha il suo nodo in corrispondenza, d e l l ' a p e r t u r a del golfo), oltre il nodo l'onda stazionaria si t r a s f o r m a in onda progressiva verso il largo. la formazione di u n a n u o v a oscillazione libera richiede il persistere della causa p e r t u r b a n t e : più questa d u r a , più numerosa sarà la serie delle singole onde s e p a r a t e . è così che si spiega, dal p u n t o di vista idrodinamico, l'accenn a t o f e n o m e n o , c a r a t t e r i s t i c o del lago di albano. q u a n d o il meccanismo della causa p e r t u r b a n t e è tale da interessare l ' i n t e r o lago, a v r e m o le oscillazioni libere di t u t t o il bacino: uninodali, binodali, t r i n o d a l i , . . . i n questo caso, l'uninodale si susseguirà in lunghe serie senza soluzione di c o n t i n u i t à da u n ' o n d a alla successiva (fig. 12): la loro d u r a t a è l e g a t a al coefficiente di s m o r z a m e n t o n a t u r a l e delle oscillazioni libere, a meno che, nel f r a t t e m p o , non i n t e r v e n g a n o n u o v e cause di p e r t u r b a z i o n e . se ora passiamo a c o n f r o n t a r e i d a t i dell'osservazione con quelli o t t e n u t i p e r via teorica, n o t i a m o che — almeno p e r q u a n t o concerne 1 2 i ' . c a i . o i <;. i t o m u a u j i i periodi predominanti — il metodo che lia condotto ai valori più prossimi a quelli osservati è il metodo di h i d a k a (2). anche il metodo di defant ha fornito valori per i periodi in buon accordo con la media di quelli reali. il metodo di chrystal (:ì) porta a valori leggermente inferiori alla media. la schematizzazione della curva normale che tale metodo comporta, talvolta si riflette apprezzabilmente sulla attendibilità, dei risultati: nel caso in esame, ha condotto a periodi lievemente inferiori a quelli o t t e n u t i dall'osservazione. p e r q u a n t o concerne i rapporti delle ampiezze delle uninodali dell'intero lago, ai suoi estremi, l'osservazione mostra, in media (nei brevi periodi in cui è s t a t o possibile regist l'are contemporaneamente ai due estremi), che l'uninodale dal lato settentrionale raggiunge ampiezze circa 3,8 volte quelle raggiunte contemporaneamente al lato opposto. dei metodi applicati, quello di d e f a n t dà un rapporto dello stesso ordine (circa 4), meno bene il metodo di chrystal (1,7 circa), peggio quello di h i d a k a , restano ancora da verificare le posizioni dei nodi e gli spostamenti orizzontali che le particelle liquide subiscono in essi, quali risultano dai metodi applicati. questa p a r t e viene r i m a n d a t a ad ulteriori indagini. f i n d'ora, possiamo invece ottenere — sulla base delle registrazioni eseguite — un valore approssimativo dell'energia associata ad un'oscillazione uninodale di ampiezza medio-massima (fra quelle registrate). e q u i p a r a n d o l'alveo di un lago ad un bacino di forma parallelepipeda, di lunghezza l, larghezza b e profondità li, si prova (4) che l'energia cinetica ee, associata ad un'oscillazione libera uninodale di periodo to, assume l'espressione ec = x oh li l gl sin2 ^ t , essendo q la densità dell'acqua e « n i . dove h r a p p r e s e n t a l'ampiezza dell'uninodale considerata. dalle colonne 3 a e 4 a della tabella i del primo lavoro sul lago d'albano (2), dove sono r i p o r t a t e le superficie delle sezioni trasversali e delle loro larghezze in superficie, è facile dedurre le profondità medie delle sezioni stesse, equiparate a rettangoli; la media di tali profondità, darà hi profondità media li del lago, mentre la media dei valori della su d e t t a o s s e r v a z i o n i l i m n 0 g r a f i c 1 1 e s u l l a g o d i a l b a n o 1 3 4a colonna fornirà b-, l è il valore riportato, come ultimo, nella 2 a colonna della stessa tabella. p e r il lago di albano si ottiene p e r t a n t o h = 63,7 m ; b = 1590 ni ; l = 3575 m . f a t t o h = 5 cm., consegue o = 3,8325 cm/sec . per o = l gr/cm3, si ottiene infine ec = 3,5.io11 ergs = 9,7 k w h . il valore medio dell'energia associata ad unì nodale dell'ampiezza di 5 cm. è quindi, per il lago di albano, dell'ordine di 10 k w h . poiché nel lato meridionale del lago, l'ampiezza dell'uninodale è circa 1/4 dei valori che essa assume sul lato opposto, possiamo concludere che la corrispondente energia è circa 1/16 di quella ivi sviluppata. b i b l i o g r a f i a ( 1 ) c a l o i p . , le sesse del lago di garda. p a r t e i e p a r t e i i . « a n n a l i di geof í s i c a » , i, 1, 2 (1948). ( 2 ) g i o r g i m., studio sulle sesse del lago di albano. « a n n a l i di geofísica », i, 4 (1948). ( 3 ) c a l o i p . & g i o r g i m., sulle oscillazioni libere del lago di albano. « a n n a l i d i geofísica », v, 2 (1952). ( 4 ) c a l o i p . , oscillazioni libere del lago di bracciano; loro caratteristiche e loro energia. « a n n a l i d i geofísica », x i , 1 (1958). l a g o di a l b a n o f i g . 1 a m p i e u n i n o d a l i d a zoccolo (vedi t e s t o ) . 13 iv 1957 u h fig. l a uninodali del lago e uninodali di'zoccolo attenuate. 1 5 " 1 6 h 14 iv 1957 1 1 1 fig. 16 chiari esempi di uninodali, orlate da « sliolf-seiches : 1 2 h 1 3 " lago di albano p. c a l o i g . r o m u a l d i f i g . 2 — e s e m p i (li u n i n o d a l i da zoccolo: si n o t i la successione di oscillazioni singole isolate. fig. 3 e s e m p i di u n i n o d a l i d a zoccolo: c. s. ! 1 annals of geophysics, 65, 4, vo428, 2022; doi:10.4401/ ag-8787 o p e n a c c e s s near real-time filtering of high precision borehole strainmeter signals for volcano surveillance luigi carleo*,1, alessandro bonaccorso1, gilda currenti1 and antonino sicali1 (1) istituto nazionale di geofisica e vulcanologia, sezione di catania – osservatorio etneo, catania, italy article history: received december 30, 2021; accepted april 13, 2022 abstract volumetric strain signal recorded by the sacks-evertson strainmeter has a very high resolution and can measure small strain changes (down to 10–9) accompanying volcanic processes. however, different disturbing components perturb the recorded strain signal. these disturbances can mask ultra-small strain changes related to volcano activity and thus need to be filtered in order to accurately monitor volcano deformation. we developed the software stralert (strain and warning signals in near real-time) to provide both the recorded and the filtered strain signals in near real-time to the surveillance room of the “istituto nazionale di geofisica e vulcanologia – osservatorio etneo” which monitors the etna activity. the software encloses a modified version of the code baytap-g for filtering the incoming signal from the continuously running strainmeter at etna. thanks to the appropriate and robust signal filtering, stralert is capable to provide an efficient estimate of transient strain changes, which concur in tracing volcano deformation and detecting the onset and the conclusion of eruptive events. our findings demonstrate the important contribution of stralert for volcano surveillance operations. keywords: etna volcano; strain; borehole dilatometer; volcano monitoring; eruptive activity detection 1. introduction volcanic activity usually produces deformation of the shallow crust that precedes and accompanies paroxysmal phases of eruptive events [i.e. dzurisin, 2007]. the monitoring and the interpretation of deformation processes are thus crucial for both characterizing the deformation sources and conducting volcano surveillance operations. to these scopes, the sacks-everton borehole strainmeters [sacks et al., 1971] are key instruments since they can record volumetric strain changes of the surrounding rocks with a very high resolution [i.e. roeloffs and linde, 2007]. several installations of borehole strainmeters have been performed in volcanic areas worldwide: mauna loa, hawaii [linde and sacks, 1995], hekla, iceland [linde et al., 1993], soufriere hills, montserrat [voight et al., 2006], campi flegrei, italy [amoruso et al., 2015], stromboli, italy [bonaccorso et al., 2012], izu-oshima, japan [linde et al., 2016], etna, italy [bonaccorso et al., 2016]. the strain signal recorded by the borehole strainmeters is usually affected by different disturbing components, mainly due to tidal forces, local barometric pressure, precipitations and underground water circulations [i.e. luigi carleo et al. 2 furuya and fukodome, 1986; agnew, 1986]. in particular, the tidal and the atmospheric pressure components are the main disturbing signals since they are persistent over time and exhibit amplitudes of the order of 10–8 to 10–7 masking the strain changes due to volcanic processes. currenti and bonaccorso [2019] clearly showed that filtering out both the tidal and pressure components allows to enlighten ultra-small strain changes of the order of 10–9 due to pressurization/depressurization cycles of magmatic chamber which regulates the occurrence of summit eruptive events. detecting these changes accurately is fundamental for monitoring the evolution of the volcano state and have information about the timing of the onset of eruptive events [bonaccorso et al., 2016]. therefore, the availability of filtered strain signal has an obvious advantage for monitoring the volcano dynamic. in literature, different softwares have been developed for the strain signal filtering [tamura et al., 1991; wenzel, 1996; amoruso et al., 2000; venedikov et al., 2003; langbein, 2010]. these codes were not designed to be run automatically but to be used under the control of an operator who can choose the most suitable model for the recorded strain signal and appropriately manage measurement problems such as gaps, outliers and steps. in near real-time, the automatic use of these software to filter data, which are affected by measurement problems or by strain contributions neglected in the model, could lead to an inaccurate estimation of the filter parameters. this could cause incorrect data processing and misinterpretation of the ongoing deformation with important consequences for volcano surveillance operations. for these reasons, it is advisable to estimate the filter parameters in advance in optimum conditions and, then employ them for near real-time applications, as done for corrections of gravimetric measurements [van camp et al., 2017]. to automate the filtering of the strain signal, we developed a new software called stralert. it automatically reads the strain signal recorded by the borehole strainmeter network installed on the etna volcano, performs the strain signal filtering, and transfers it to the surveillance room of the “istituto nazionale di geofisica e vulcanologia – osservatorio etneo (ingv-oe)”. the program encloses an updated version of the baytap-g code, that has been optimized for near real-time filtering, which allows using a fixed set of pre-estimated filter parameters as inputs. the problems related to signal filtering in near real-time are widely described in sect. 2. the procedure employed for the estimation of the filter parameter and the evaluation of the filtering performances are illustrated in sect. 3 and sect. 4, respectively. the proposed software approach is presented in sect. 5. conclusions are discussed in sect. 6. 2. strain signal filtering in near real-time at etna volcano at etna volcano, four borehole strainmeters were installed over the last decade (figure 1). each instrument exhibited a different final response depending on the type of rock in which it was installed and on the rock-sensor coupling. the strainmeter placed at monte ruvolo (druv) is characterized by a very high accuracy. it is capable of clearly recording large strain changes (≈10–7) due to volcanic events, such as strong lava fountains [bonaccorso et al., 2016; bonaccorso et al., 2021], but also small strain changes associated to earth tides (~10–8) and strong teleseism events (at druv in the range 10–9-10–8), respectively [bonaccorso et al., 2016; currenti et al., 2017]. figure 1. map of the etna volcano with the strainmeter network. the coordinates system is wgs 84 utm 33. real-time filtering of strain signals 3 moreover, the druv signal was optimally calibrated using three different methods by comparing the records with theoretical tides [bonaccorso et al., 2013], synthetic straingrams due to strong teleseisms [bonaccorso et al., 2016] and strain estimated by a broad-band seismic array for a teleseismic event [currenti et al., 2017]. the three methods provided comparable calibration factors. these important findings showed that the recorded druv signal can provide robust and reliable estimates of the ongoing volcano deformation and thus it was selected to be employed for the near real-time monitoring operations carried out by the ingv-oe surveillance room. however, the presence of disturbing components in the strain signals, most relevant of which are due to earth tides, atmospheric pressure variations, precipitations and underground water circulations, could mask deformation processes related to the volcano activity which knowledge is fundamental for the surveillance operations. indeed, currenti and bonaccorso [2019] showed that an appropriate filtering of the druv signal can clearly reveal ultra-small strain changes (≈10–9) that can be related to inflation/deflation cycles of the volcano edifice concurrently with periods of accumulation/withdrawal of magma in storage zones. in order to provide this clear information for surveillance operations, it is necessary to provide in near real-time the filtered strain signal which should be as robust as possible. to date, different softwares can be employed to perform the signal filtering. the most used codes are baytap-g [tamura et al., 1991], eterna [wenzel, 1996], the numerical procedure of amoruso et al. [2000], vav [venedikov et al., 2003] and clearstrain+ [langbein, 2010]. each software adopts a model of the strain signal and can consider different strain components. among others, barometric pressure and earth tides are the strain sources whose effects have been widely studied. they are modelled similarly in all the softwares and they can be easily estimated automatically. the effects of the tidal variations are typically considered as the sum of variable harmonics of different amplitude and phase, with observed tidal factors and phase lags to be estimated from the theoretical values [e. g. cartwright and tyler, 1971; cartwright and edden 1973; tamura, 1987]. the barometric pressure contribution is usually estimated with a regression model applied on the recorded barometric pressure signal. the effects of other strain sources such as precipitations could be modelled [matsumoto et al., 2003; hsu et al., 2015] but require at least records of rainfall in real-time. moreover, transient variations associated to tectonic and volcano processes are not known a priori and, thus, they cannot be modelled accurately except off-line with the help of an external operator. for example, the procedure of amoruso et al. [2000] introduced the possibility of including ramps as a further strain component but they can be considered in the model only if the operator indicates the time intervals during which the ramps developed. as well, clearstrain+ used logarithmic and exponential functions to model transient variations due to tectonic and post-seismic deformations, respectively, but they can be modelled only if the time intervals when these deformation processes occur are known. in near real-time, the presence of strain components that cannot be detected and precisely modelled and of measurement errors could cause an inaccurate estimation of the strain signal components. in order to provide a robust filtered strain signal in near real-time, we preferred to remove only the well-known contributions, namely the earth tide and the pressure changes, and focus on the efficient estimation of the related filter parameters. to avoid spurious effects, the parameter estimation and the evaluation of the filtering performances were conducted in periods when other strain components, than the pressure and the tidal components, were negligible. therefore, we looked for periods when precipitation did not occur and the etna activity was very low. moreover, we checked and removed steps, outliers, gaps and corrupted data in order to obtain an estimation of the filter parameters as reliable as possible. we analyzed a very large time window spanning from november 2011, when the druv strainmeter was installed, to april 2021. the multiparametric weather network managed by sias (servizio informativo agrometeorologico siciliano, www.sias.regione.sicilia.it) provided the precipitation data for the analysis. in particular, we referred to the measurements recorded by the bronte station, which is the nearest station to the druv strainmeter, located on the same volcano flank at approximately 8 km away. the volcano activity was checked by referring to the recent work of andronico et al. [2021], which provided a useful list of all the paroxysmal episodes occurred from 1986 to 2021 at the etna volcano. only two long-term windows were found to be appropriate: 06 july 2016 – 06 august 2016 and 01 may 2020 – 30 may 2020. we decided to use the signal recorded in 2020 as training signal for the parameter estimation while the signal recorded in 2016 was used as testing signal for analyzing the filtering performance. the software baytap-g was used for the parameter estimation and was adapted for the filtering operations in near real-time. it is important to underlie that the recorded strain signals are usually affected also by a long-term drift which is mainly due to both the relaxation of the drilled hole and the curing of the cement [i.e. roeloff and linde, 2007; bonaccorso et al., 2016; canitano et al., 2021]. the drift can persist from months to years after the installation and it decreases exponentially with time. before proceeding with the parameter estimation, the long-term drift component was removed. http://www.sias.regione.sicilia.it luigi carleo et al. 4 3. parameter estimation the mathematical model of the filter implemented in baytap-g considers the recorded strain signal as the sum of a tidal, a pressure and a filtered component. the tidal component is modelled by summing theoretical harmonic functions, 𝛼m and 𝛽m, considered in m groups with constant amplitude and phase and scaled to the measured tidal signal through the tidal factors am and bm [tamura et al., 1991]. the pressure component can be estimated by applying a fir filter of order q and weights bk to the recorded barometric pressure signal p. the filtered signal is modelled as a random walk process w to which white noise is added. am, bm and bk are the filter parameters. in order to estimate them, baytap-g solves a sort of “penalized” least square method in which the data are fitted using a constraint on the parameters. the estimation of the parameter is carried out by minimizing the following cost function 𝐽𝐽 = $%&𝑦𝑦! − $𝑏𝑏"𝑝𝑝!#" $ "%& − $+𝐴𝐴'𝛼𝛼',! + 𝐵𝐵'𝛽𝛽',!1 − 𝑤𝑤! ) '%* 3 + + 𝐷𝐷+(𝑤𝑤! − 2𝑤𝑤!#* + 𝑤𝑤!#+)+8 , !%* (1). in (1), d is a hyperparameter that influences the parameter estimation by controlling the smoothness of the signal w: the higher the value of d, the smoother w. once d is fixed, the filter parameters can be estimated by minimizing j. baytap-g tries different filtering solutions by varying the value of d in the range [dmin dmax], where dmin is an input parameter and dmax is equal to 1000. the goodness of fit of the generic filtering solution is evaluated through the akaike bayesian information criterion, abic [akaike, 1980]. the minimum the abic, the more efficient the filtering solution. the iterative procedure implemented in baytap-g to estimate the parameters is structured as follows: an initial value of d, d1, is set to 4dmin, and the abic corresponding to d1, abic1, is evaluated. at the next iteration, the value of d is increased and the second value of the abic, abic2, is estimated. if abic1 < abic2 then the analyzed portion of the abic(d) curve has positive gradient so the minimum value of the abic, abicmin, must be found for d < d1. d is then multiplied by 1/√2 until abicmin is found. vice versa, if abic1 > abic2 then the gradient of the abic(d) curve is negative and abicmin is catched for d>d1 employing the multiplier √2 for the successive values of d. the iterative procedure strongly depends on the value of the input parameter dmin and on the analysis step of d implemented in the code. tamura et al. [1991] underlined that this step (1/√2 or √2) is sufficient for practical purposes. we decided to investigate a wider range of d values. we increased the number of the analyzed values of d indirectly, namely by trying different values of dmin. preliminary analysis showed that the optimal value of dmin, the one that provides d values that minimizes the abic, is located in the range [0 1]. therefore, we decided to investigate values of dmin in the range [0 1] with a step of 0.01. the range [1 100] was analyzed as well but with a sparser step of 1. the number of analyzed filtering solutions was further increased by employing different values of the order of the fir filter applied to the atmospheric pressure signal, q. we exploited the whole range of values of q admissible by baytap-g, namely [0 72], with an analysis step of 1. the values of q were used in combination with the inspected values of dmin for a total of 14600 investigated couples (q, dmin). the goodness-of-fit of each filtering solution was evaluated employing the same abic criterion. an important aspect that needs to be investigated for near real-time applications is the sampling time of the recordings. baytap-g was designed and tested to analyze signals sampled at time interval of 1 hour [tamura, 2000]. it is obvious that for surveillance applications this sampling interval is too high. therefore, lower sampling times have been investigated to prove the efficiency of baytap-g for near real-time processing at lower sampling time. in particular, we investigated three sampling times, tc: 1 hour, 10 minutes and 5 minutes. an example of efficient filtering solution is presented in figure 2. the training signal recorded with a sampling time of 5 minutes (figure 2a) is shown together with the related tidal (figure 2b), pressure (figure 2c) and filtered strain components (figure 2d) obtained using the couple (q, dmin) = (10, 0.91). this solution was found by comparing the abic values of all the inspected couples (q, dmin). in figure 3, the abic values are plotted against the parameter q for all the couples (q, dmin) employed for filtering the training signal. in particular, the results for the signal recorded with a sampling time of 1 hour (figure 3a), 10 minutes (figure 3b) and 5 minutes (figure 3c) are presented. the red dots represent the q-optimal couples (q, dmin,opt), namely all the combinations (q, dmin) that provided the lowest value of the abic for each analyzed q. in order to choose the most efficient (q, dmin,opt) combination among all the q-optimal couples, our approach was not to select the (q, dmin,opt) real-time filtering of strain signals 5 combination which lead to the absolute minimum value of the abic. instead, we chose the (q, dmin,opt) couple that represents a compromise between the value of the abic and the number of parameters involved in the model: (1, 0.03) for tc = 1h, (5, 0.95) for tc = 10 min and (10, 0.91) for tc = 5 min. the estimated values of the tidal factors am figure 2. training signal recorded from 01 may 2020 to 30 may 2020 with a sampling time of 5 minutes (a) plotted together with the tidal (b), pressure (c) and filtered (d) components obtained using the parameter couple (q, dmin) = (10, 0.91). luigi carleo et al. 6 and bm and the atmospheric pressure coefficients bk are reported for all the analyzed sampling time in appendix a in table a1 and table a2, respectively. the same procedure was implemented for the estimation of the filter parameters related to the testing signal. the (q, dmin,opt) couples are (1, 0.37) for tc = 1h, (6, 0.37) for tc = 10 min and (14, 0.64) for tc = 5 min. the related tidal factors and atmospheric pressure coefficients are presented in appendix b in table b1 and table b2, respectively. figure 3. values of the abic related to all the investigated couples (q, dmin) employed for filtering the training signal recorded at sampling time of 1 hour (a), 10 minutes (b) and 5 minutes (c). red dots represent the couples that provided the lowest value of the abic for each analyzed q. the blue circles represent the selected efficient filtering solutions. real-time filtering of strain signals 7 4. evaluation of the filtering performance the performances of the estimated filtering solutions were evaluated analyzing the root mean square (rms) of the signals. the first step was to compare the rms of the recorded signals with the corresponding filtered signals. different time scales, d, from 3 hours (typical duration of a lava fountain at etna volcano) to 30 days (maximum length of the time window under study) were analyzed. for d < 30 days, the rms was evaluated as the average value of the rms calculated in moving windows of length d over the analyzed time period. for each time windows, we considered the signal minus its average value evaluated on d, to partially remove the drift term due to deformation processes acting on time scale higher than d. in table 1, the rms of the recorded training signal, rmsr,tra, is presented with the rms of the respective filtered signal, rmsf,tra for the different analyzed time scale d. both the rmsr,tra and the rmsf,tra values, at a fixed time scale d, do not significantly vary with the sampling time which means that the sampling time does not cause any significant increase or decrease in both recorded and filtered signal sensitivity to strain variations. time scale, d tc = 1 h tc = 10 min tc = 5 min rmsf,tra rmsr,tra rmsf,tra rmsr,tra rmsf,tra rmsr,tra 3 hours 0.19 4.80 0.22 4.10 0.22 4.02 1 day 0.80 13.83 0.97 13.81 0.91 13.81 14 days 6.44 23.33 6.45 23.26 6.24 23.26 30 days 11.04 24.08 11.76 24.08 11.39 24.08 table 1. values of the rms in nanostrain (10–9) of the recorded, rmsr,tra, and filtered, rmsf,tra, training signal evaluated on different time scales d, for different sampling times tc. the rms of the recorded training signal is of order of few nanostrain only for d = 3 hours (≈4*10–9) while it is of order of 10–8 for the rest of the analyzed time scales. the filtering process always brought benefits since rmsf,tra < rmsr,tra for each analyzed d and for each tc. in particular, the rms decreased of one or two order of magnitude in case of time scales less than one day, reaching values of ≈10–10, while it decreased of one order of magnitude for time scales of few days, namely ≈10–9. in case of larger time scales of about one month, the rms of the filtered signals decreased by a factor of ≈2 with respect of the recorded signal. same considerations can be done by analyzing the rmss of both the recorded, rmsr,tra, and the filtered, rmsr,tra, testing signal reported in table 2. by comparing the rms values of the filtered solutions of both the training and the testing signals it is shown that all the rms values of the filtered testing signal, at fixed time scale, are lower than the respective values of the training signal for the different sampling times. this means that the filtering process carried out on the testing signal was more efficient than the one performed on the training signal. however, it is worth noting that the differences are, in any case, of order, or even less, than the rms itself. therefore, beyond all possible explanations in these discrepancies, one of the two estimated sets of parameters can be equally chosen since they can be considered equivalent in terms of filtering efficiency, at least at the analyzed time scales and in the analyzed time periods. we decided to employ the parameters estimated from the signal recorded in the most recent time window, namely the training signal, and test them for filtering strain signals recorded in different idoneous time periods. the testing signal was used to this scope. the filtering solution of the testing signal obtained with the training signal parameters was compared with the one obtained with the parameters estimated on the testing signal itself. we present the case of signal sampled every 5 minutes which is of major interest for near real-time applications. in figure 4, the testing signal is plotted together with the two analyzed filtered solutions of the recorded signal. there is a good agreement luigi carleo et al. 8 of the filtered signals that means that both the filtering solutions are quite efficient. the rms values of the signal filtered with the parameters estimated from the training signal is 0.26, 1.11, 2.62 and 3.37 nanostrain for time scale of 3 hours, 1 day, 14 days and 1 month. if one compares these values with the ones of table 2, which represent the efficient filtering solution of the testing signal, it can be easily verified that the rms values are of the same order of magnitude and thus the small differences between the two analyzed filtered solutions can be neglected. time scale, d tc = 1 h tc = 10 min tc = 5 min rmsf,test rmsr,test rmsf,test rmsr,test rmsf,test rmsr,test 3 hours 0.12 4.79 0.14 4.03 0.17 3.94 1 day 0.41 12.86 0.46 12.93 0.59 12.93 14 days 2.29 15.64 2.34 15.72 2.45 15.72 30 days 3.01 15.64 3.13 15.72 3.05 15.72 table 2. values of the rms in nanostrain (10–9) of the recorded, rmsr,test, and filtered, rmsf,test, testing signal evaluated on different time scales d, for different sampling times tc. figure 4. testing signal recorded from 06 july 2016 to 06 august 2016 with a sampling time of 5 minutes, in black, plotted together with two possible filtering solutions: one obtained with parameters estimated on the testing signal (blue) and one obtained with filtering parameters estimated from the training signal (red). 5. stralert at ingv-oe surveillance room to provide both the recorded and the filtered druv signals to the ingv-oe surveillance room the software stralert was developed. it was written in matlab environment (r2020b) to routinely perform the operations of signal reading, filtering and transferring. the flow chart of stralert is presented in figure 5. the software can be launched with the script stralert.m which contains all the code lines where the input parameters can be set. at the first run, the function initialization.m is launched. it executes preliminary operations to check the compatibility of the input parameters with the input signals, namely the recorded strain signal and the recorded barometric pressure signal. successively, the software executes the script maincode.m which groups all the instructions that will be performed routinely for reading, filtering and transferring the signals. stralert encloses an updated version of the software baytap-g. indeed, since the current version of baytap-g did not real-time filtering of strain signals 9 allow using a given set of filter parameters as inputs, the original code of baytap-g was modified. we introduced a new processing module that allows bypassing the parameter estimation procedure implemented in the software and, instead, uses a set of filter parameters previously computed and stored in a .txt file. the parameter reported in table a1 are used for the signal filtering. in addition, the improved version of baytap-g was further optimized for near real-time applications by avoiding the writing of some output files useful only for analysis purposes. these modifications increased the processing time of the software of about 60%. the filtered and the recorded signals are transferred through http request messages to tsdsystem, the database in use in the ingv-oe surveillance room to store and visualize time series [cassisi et al., 2015]. examples of the output of stralert are shown to present the results obtained by this useful tool. in the following figures, some examples of strain changes occurred concurrently with some eruptive events are reported. in figure 6, the recorded (figure 6a) and the filtered (figure 6b) druv signals are shown for the time period 12 june 2021 – 11 july 2021 as displayed in the surveillance room. in this time window, an intense eruptive activity of the etna volcano took place. each step-like change in the strain signal is related to the decompression of the volcano edifice due to lava fountain events. figure 5. flow chart of stralert. luigi carleo et al. 10 figure 6. example of the recorded (a) and the filtered (b) strain signals as shown in tsdsystem from 12 jun 2021 to 11 july 2021. negative strain changes correspond to dilatation of the rock surrounding the sensor. black arrows indicate lava fountains. the scales and the arrows are added for a better comprehension. the beginning and the ending of the changes mark the onset and the conclusion of the eruptive event [bonaccorso et al., 2016]. the events occurred in july are clearly visible in both the recorded and filtered signal exhibiting negative strain changes of the order of 10–7. instead, the lava fountains occurred in june are masked by the tidal and pressure components and are clearly detected only when the filtering is performed disclosing smaller strain changes of the order of 10–8. in figure 7, the recorded (figure 7a) and the filtered (figure 7b) strain signals are figure 7. recorded (a) and the filtered (b) strain signals shown on tsdsystem in the period 22 may 2021 – 31 may 2021. black arrows indicate lava fountain events while the red arrows strombolian activity episodes. the ultra-small negative strain changes (≈10-9) concurrently with the strombolian episodes are visible only on the filtered signal. real-time filtering of strain signals 11 presented for the time window spanning 22 may 2021 – 30 may 2021. in this period, some small lava fountains occurred together with weak strombolian activity episodes. in the recorded signal, the strain changes are hidden by the tidal and pressure variations and cannot be clearly identify. after filtering, distinct abrupt changes are shown up concurrently with the eruptive episodes and the beginning and the ending of each strain variations can be easily distinguished. moreover, the signal enlightens ultra-small strain changes of the order of 10–9 observed concurrently with the three strombolian episodes occurred from 25 may 2021 to 26 may 2021 which are unrecognizable in the recorded raw strain signal. 6. conclusions volcano monitoring and surveillance could greatly benefit of the availability of near-real time strain signals, since they provide a direct measurement of volcano deformation with a resolution unachievable with other geodetic techniques. however, the recorded strain signal is affected by disturbances that mask ultra-small strain changes due to volcanic processes. in order to provide an appropriately filtered strain signal in near-real time to surveillance room at ingv-oe, we implemented the software stralert, which automatically processes the recorded strain and reduces the disturbing components. by employing and adapting the baytap-g code, stralert filters the incoming signal from the continuously running druv station using a set of filter parameters previously defined and assessed. this allows for a robust filtering process avoiding unstable results. the estimation of the filter parameters carried out on appropriate periods allowed to obtain a filtered signal sensitive to ultra-small strain transients due to volcanic activity. at time scales less than one day, which are of major interest for near real-time applications, the rms of the strain signal is improved, reaching values of ≈10–10 while, at time scale of few days, it can reach values of the order of 10–9. this performance enables stralert to unravel tiny strain changes due to weak eruptive events that are completely hidden by the tidal and the pressure variations in the recorded raw signal. these small strain transients would have been undetected if filtering was not performed. moreover, the filtered signal better shows the onset and the end of the transient strain variations allowing to easily mark the timing of the associated eruptive events. these findings show the important contribution of stralert to the volcano surveillance operations and the benefit for future application in volcano warning systems. acknowledgments. the etna borehole strainmeter network originally benefited by the financial support of the italian firb project “development of new technologies for the protection and defense of the territory from natural hazards” (acronym fumo) and pon project “development of research centers for the study of volcanic areas at high risk and their geothermal potential in the context of mediterranean geological and environmental dynamic” (acronym vulcamed). we sincerely thank the colleagues of the carnegie institution of washington a. linde and s. sacks, who actively supported and coordinated the dilatometer installations on etna and contributed to the first fundamental studies. we thank the colleagues of the carnegie institution of washington m. acierno and b. schleigh who worked in the implementation of the instrumentation and provided useful and precious assistance. this research benefited from fundings provided by the 2019-2021 agreement between ingv and italian presidenza del consiglio dei ministri – dipartimento della protezione civile (dpc), all. b2wp2 -task 9 “ottimizzazione dell’acquisizione dei segnali ad alta precisione degli strainmeter installati in pozzo sull’etna” (optimization of the acquisition of high-precision signals from the bore-hole strainmeters installed at etna volcano). this research also benefited from the ec h2020-fet open project “sic optical nano-strain-meters for pico-detection in geosciences” (acronym sic nano for picogeo) grant agreement n. 863220. references agnew, d. c. 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(1996). the nanogal software: earth tide processing package eterna 3.30, bulletin d’informations marées terrestres 124, 9425-9439. *co r r e s po n d i g a u to r: luigi ca r l e o, istituto nazionale di geofisica e vulcanologia, sezione di catania – osservatorio etneo, catania, italy, email: luigi.carleo@ingv.it mailto:luigi.carleo%40ingv.it?subject= tertulliani_corretto_layout 6 679 the emilia 2012 sequence: a macroseismic survey andrea tertulliani1,*, luca arcoraci2, michele berardi2, filippo bernardini3, beatriz brizuela4, corrado castellano2, sergio del mese2, emanuela ercolani3, laura graziani4, alessandra maramai4, antonio rossi2, manuela sbarra1, maurizio vecchi1 1 istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy 2 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy 3 istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy 4 istituto nazionale di geofisica e vulcanologia, sezione roma 2, roma, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6140 1. introduction on may 20, 2012, at 4:03 local time (2:03 utc), a large part of the po valley between the cities of ferrara, modena and mantova was struck by a damaging earthquake (ml 5.9). the epicenter was located by the istituto nazionale di geofisica e vulcanologia (ingv) seismic network [iside 2010] at 44.889 ̊ n and 11.228 ̊ e, approximately 30 km west of ferrara (figure 1). the event was preceded by a foreshock that occurred at 01:13 local time, with a magnitude of ml 4. the mainshock started an intense seismic sequence that lasted for weeks, counting more than 2,000 events, six of which had ml >5. the strongest earthquakes of this sequence occurred on may 29, 2012, with ml 5.8 and ml 5.3, recorded at 9:00 and 12:55 local time, respectively. the epicenters of the may 29, 2012, events were located at the westernmost part of the rupture zone of the may 20, 2012, earthquake (figure 2). the may 20 and 29, 2012, earthquakes were felt through the whole of northern and central italy, and as far as switzerland, slovenia, croatia, austria, south-eastern france and southern germany. historical information reveals that the seismic activity in the po valley is moderate; indeed, except for the event that struck ferrara on november 17, 1570, the last version of the parametric catalogue of italian earthquakes (cpti11) [rovida et al. 2011] does not report many other significant earthquakes occurring in this area [see also castelli et al. 2012, this volume]. at times, small-to-medium-magnitude earthquakes have occurred in the area of the 2012 sequence, such as the events of december 6, 1986 (mw 4.6), and may 2 and 8, 1987 (mw 4.7, 4.6, respectively), that struck the northern sector of the modena province [locati et al. 2011, rovida et al. 2011]. the po valley is also affected by more frequent seismic activity due to the interaction of the northern apennines and the piedmont belt of emilia, between parma and modena, where events of moderate magnitude often occur. an example of this seismicity was the october 15, 1996, correggio (reggio-emilia) earthquake [rovida et al. 2011]: this mw 5.4 event produced effects that extended up to the most damaged areas of the 2012 sequence [locati et al. 2011] soon after the strong earthquake occurred on may 20, 2012, the ingv 'quick earthquake survey team' (quest) organized a field survey of the major damaged areas, to be able to assign macroseismic intensities according to the european macroseismic scale (ems98) [grünthal 1998]. to take into account the cumulative effects of the aftershocks and their westward displacements, the survey was continuously adjusted and updated, to follow the enlarging 'most damaged zone'. in this report, the preliminary results of the macroseismic survey are presented. 2. the macroseismic survey methodology the field survey performed in the most damaged zones followed the macroseismic techniques commonly used by the ingv: the assessment of the intensity is based on the damage observed to the residential building stock and on the perception of the effects noticed or experienced by people during the earthquake. the buildings observed were divided into typologies and then into vulnerability classes, on the basis of their characteristics. with this aim, the quest team performed detailed surveys of the current state of the building stock, to assess the grade of damage according to the ems98 scale [grünthal 1998]. the most common building typologies observed in the surveyed area were old traditional brick houses, and recent residential buildings in brick or reinforced concrete. old traditional brick houses are in the hisarticle history received july 23, 2012; accepted august 31, 2012. subject classification: surveys, measurements and monitoring, macroseismics. 2012 emilia earthquakes tertulliani et al. 680 f ig u re 1 .m ac ro se is m ic in te ns it ie s of th e m ay 2 0, 2 01 2 ea rt hq u ak e. t he b lu e st ar s ho w s th e m ac ro se is m ic e pi ce nt er c om pu te d by b ox er 4 .0 [g as pe ri ni e t a l. 20 10 ]. in se t: l oc at io n of s tu dy a re a. s ee k ey fo r de ta ils . 681 the emilia 2012 sequence: a macroseismic survey f ig u re 2 .m ac ro se is m ic in te ns it ie s af te r th e m ay -j u ne 2 01 2 se is m ic s eq u en ce . b lu e st ar , m ac ro se is m ic e pi ce nt er c om pu te d by b ox er 4 .0 [g as pe ri ni e t al . 2 01 0] . b la ck s qu ar es , e pi ce nt er s of e ar th qu ak es w it h m > 5. 0. i ns et : l oc at io n of s tu dy a re a. s ee k ey fo r de ta ils . t he in te ns it y va lu es r ep re se nt t he c u m u la ti ve d am ag e u p to t he ju ne 3 , 2 01 2. tertulliani et al. 682 toric centers, and often lack connections or reinforcing elements, like tie rods or buttresses (vulnerability classes a or b were assigned, according to their state of maintenance). such reinforcing elements are commonly found in other italian areas that are prone to significant seismic activity, but they appear not to belong to the building tradition in this study area, even for historical mansions. the recent one-or-two-story residential dwellings in brick or reinforced concrete are mainly sited on the outskirts of the villages and towns (vulnerability classes b and c). the latter type is more common than the traditional brick buildings. industrial warehouses and numerous farmhouses were also observed through the countryside or in the suburbs of the towns and cities visited. industrial warehouses are isolated from inhabited areas, and following the guidelines of the ems98 scale, they were not considered as representative for the intensity value assessment, as for other special buildings like monuments, churches, towers and belfries. nevertheless, to have a more complete picture of the effects of the 2012 sequence, the damage observed on special buildings and farmhouses was reported as well, as this kind of information is useful to give a thorough picture of the damage distribution, especially in the localities where the residential building stock was undamaged or only lightly damaged. widespread damage to chimneys was observed during the survey, both for masonry (vulnerability class b) and reinforced-concrete (vulnerability class c) buildings. this peculiarity led us to make a methodological choice. according to the ems98 scale, the failure of chimneys is considered diagnostic of grade 3 damage ("moderate structural damage or heavy non-structural damage") for masonry buildings. however, the field survey demonstrated that this type of damage was accompanied by hardly any other diagnostic elements of similar severity. indeed, in most cases, the failure of the chimney in masonry houses was accompanied by grade 2 damage diagnostics, or was even the only damage observed. this behavior induced us to 'downgrade' the failure of chimneys in masonry houses to be diagnostic of grade 2 damage. for reinforced-concrete buildings, the ems98 does not assess this kind of damage at all. to take this indicator into account, which was often observed, chimney failure on reinforcedconcrete buildings was defined as diagnostic of grade 1. this assumption was supported because frequently this was indeed the only noticeable effect of the shaking on reinforcedconcrete buildings, which was observed even before the appearance of fine cracks in the plaster. given the preliminary nature of this study, this interesting issue cannot be stressed here, but will be the object of further studies. the intensity values assigned as a result of the field survey represent the cumulative effects of the may-june 2012 sequence, as it was almost impossible to distinguish the damage caused by every single strong earthquake after the may 29, 2012, aftershocks, although the survey was continuously updated. anyway, through the prompt reaction, it was possible to define the intensity distribution for the may 20, 2012, event in about 50 localities, which were surveyed before the occurrence of the important aftershocks [see also questingv 2012]. 3. intensity assessment after the may 20, 2012, earthquake, the maximum intensity value (7 ems98) was assigned to three localities (see table 1, iems 20/05 column; and figure 1). major damage (a limited number of total collapses and many partial collapses) was observed in the old and monumental buildings in the historical centers (figure 3a). class a and b buildings suffered widespread cracks in the walls, detachment of tiles, and chimney falls (figure 3b). heavy damage to industrial bays and farmhouses, and also the collapse of numerous barns, was observed through the whole epicentral area. almost everywhere in the epicentral area churches, belfries and city towers were heavily damaged, if not collapsed. minor damage to reinforced-concrete buildings was observed in a few figure 3. a. partial collapse of a monumental building: the s. giuseppe church in san felice sul panaro. b. widespread cracks in a class b building in cavezzo. 683 the emilia 2012 sequence: a macroseismic survey cases. many localities showed slight damage, as generally non-structural or light structural damage. nevertheless, in other localities, more severe effects produced by the earthquakes were observed, even if sporadically; in these localities, intensity values of 6 and 6-7 were assessed. intensity 5 and 5-6 were assigned to all those villages where the damage was negligible or occasionally observed, and in any case where it was very slight. as a consequence of the occurrence of the large aftershocks on may 29, 2012, the quest team performed a new field survey to update the information previously collected. all of the localities already visited were surveyed again, and many others were visited for the first time. the aftershocks were located westwards with respect to the mainshock, and they caused heavy damage in a new, larger area, which partially overlapped the maximum effects area struck by the may 20, 2012, event. the new survey continued during the whole month of june, gathering data from about 90 localities. the final intensities represent the cumulative effects due to the whole sequence. the highest intensity values (i >7 ems98) were assigned to six localities (table 1), where a few collapses were observed, both in buildings of reinforced concrete (vulnerability class c) and masonry (vulnerability classes a and b). in these localities, many buildings suffered heavy damage, such as large and extensive cracks, failure of walls and roofs, and extensive falling of chimneys and roof tiles. in some cases, partial collapse was also observed (grade 4). reinforced-concrete buildings rarely suffered structural damage, like cracks in beams and columns, and more extensive cracks in partition walls. there was total collapse of very few reinforced-concrete buildings at cavezzo (i = 8), which had already been slightly damaged by the mainshock (figure 4), whereas at rovereto sulla secchia, several recent residential buildings suffered partial collapse. mirandola, concordia sulla secchia and moglia showed widespread substantial-to-heavy damage, especially concentrated in the historical centers, and mainly characterized by buildings of vulnerability classes a and b. also in these cases, total collapse was very rare. intensity values sharply increased for mirandola, concordia and cavezzo after the may 29, 2012, aftershocks (see table 1). intensity values as high as 7 were assigned to six localities, where a large number of masonry buildings of vulnerability classes a and b suffered moderate damage, as grades 2 and 3, such as large cracks in walls and the falling of chimneys and roof-tiles. on the contrary, the damage to reinforced-concrete buildings was light. nevertheless, a few collapses and heavily damaged monuments and old scruffy structures were observed in the historical centers (usually smaller than the most recent built areas) of localities like san felice sul panaro and finale emilia. twenty six localities were assessed at 6 and 6-7 intensities. in these cases, the damage observed was moderate and usually occurred to old and vulnerable buildings; damage grade 4 was observed only sporadically. the damage levels at localities like camposanto, crevalcore and mirabello were noticeably larger after the shocks that occurred on may 29, 2012. special attention should be focused on the village of san carlo (sant'agostino), where the damage observed (corresponding in some cases to damage level 3) was largely caused by relevant liquefaction effects. in 32 localities, the damage observed was either slight or negligible, which resulted in intensity values of 5 or 5-6. in these cases, the damage reported was usually falling chimneys and small pieces of plaster, but it was not widespread through the whole building stock. some of the surveyed settlements were indeed very small, about 50-60 buildings. in these cases, as suggested by the ems98 scale, any value of intensity assigned would have been unreliable. nevertheless, the quest team chose to assign a 'd' (damage) label to these localities, not only to provide useful information gathered during the survey, but also to account for some damage that in many cases was noticeable. figure 4. increase in the damage to reinforced-concrete buildings, for may 20, 2012 (left), and may 29, 2012 (right), in cavezzo. tertulliani et al. 684 locality municipality province iems may 20, 2012 final iems cavezzo mo 6-7 8 concordia sulla secchia mo 6 7-8 mirandola mo 6-7 7-8 moglia mn 7-8 novi di modena mo 7-8 rovereto sulla secchia carpi mo 7-8 canaletto finale emilia mo 7 7 finale emilia mo 7 7 fossoli carpi mo 7 ponte san pellegrino mirandola mo 7 reggiolo re 7 san felice sul panaro mo 7 7 alberone cento fe 6-7 6-7 camposanto mo 6 6-7 crevalcore bo 6 6-7 mirabello fe 6 6-7 san carlo sant'agostino fe 6-7 6-7 san giacomo delle segnate mn 6-7 bondanello moglia mn 6 bondeno fe 6 6 bondeno gonzaga mn 6 carpi mo 6 casumaro bondeno fe 6 6 cento fe 5 6 massa finalese finale emilia mo 5-6 6 medolla mo 5-6 6 motta cavezzo mo 6 poggio renatico fe 6 6 poggio rusco mn 5 6 ponte rodoni bondeno fe 6 6 quistello mn 6 rivara san felice sul panaro mo 6 6 san martino spino mirandola mo 6 san possidonio mo 6 san prospero mo 5 6 sant'agostino fe 6 6 scortichino bondeno fe 6 6 villarotta luzzara re 6 buonacompra cento fe d d camurana medolla mo d d caselle crevalcore bo d d forcello san possidonio mo d galeazza crevalcore bo 7 d malcantone di medolla medolla mo d d 685 the emilia 2012 sequence: a macroseismic survey locality municipality province iems may 20, 2012 final iems molino albergati cento fe d d palata crevalcore bo d d pilastrello cento fe d d pioppa san possidonio mo d reno centese cento fe d d reno finalese finale emilia mo d d sammartini crevalcore bo d d santa bianca bondeno fe d d sant'alberto san pietro in casale bo d d staggia san prospero mo d tortiola vigarano mainarda fe d d brugneto reggiolo re 5-6 budrione carpi mo 5-6 fossa concordia sulla secchia mo 5 5-6 gonzaga mn 5-6 pegognaga mn 5 5-6 pilastri bondeno fe 5-6 5-6 quingentole mn 5-6 rolo re 5-6 san pietro in casale bo 5-6 5-6 sant'agata bolognese bo 5-6 san venanzio galliera bo 5-6 bomporto mo 5 5 castello d'argile bo 5 castelmassa ro 5 5 castelnovo bairano ro 5 5 dodici morelli cento fe 5 dosso sant'agostino fe 5 5 fabbrico re 5 galliera bo 5 5 nonantola mo 5 pieve di cento fe 5 rami ravarino mo 5 5 ravarino mo 5 5 renazzo cento fe 5 sala bolognese bo 5 san benedetto po mn 5 san giorgio di piano bo 5 san giovanni in persiceto bo 5 5 sermide mn 5 5 solara bomporto mo 5 5 sorbara bomporto mo 5 5 stuffione ravarino mo 5 5 table 1. localities surveyed after the may-june 2012 seismic sequence. the data related to the may 20, 2012, are in the fourth column. 4. macroseismic parameters at the end of the survey, as a test, we tried to infer the macroseismic parameters (epicenter and macroseismic magnitude, for the sake of clarity mew hereinafter) from the ems intensities assessed through the boxer 4.0 code [gasperini et al. 2010]. although we are well aware that the boxer code was developed and calibrated on the basis of the mercalli–cancani–sieberg (mcs) scale [mcs 1930], we still believe that this non-conventional exercise is worthwhile, to improve the macroseismic practice and to open a discussion on future developments of such kinds of codes. the mw 5.9 earthquake of may 20, 2012, produced effects of intensity ems 7. this value is surprisingly low, considering the more recent empirical relationships for italy [see pasolini et al. 2008, gasperini and ferrari 2000]. in addition, at the end of the whole sequence, considering the cumulative effects of the seven m >5 earthquakes, a maximum intensity value of 8 was assigned to only one locality. it is not surprising then that the macroseismic magnitude of the mainshock (mew 5.1) and the macroseismic magnitude based on the cumulative intensity (mew 5.3) are significantly lower than the instrumental one (mw 5.9) (table 2). regarding the macroseismic epicenters, the epicenter of the may 20, 2012, earthquake was located slightly to the south of the instrumental one (figure 1), whereas the barycentre of the cumulated effects was located within the cluster of the largest aftershock that occurred on may 29, 2012, and on june 3, 2012 (figure 2). 5. conclusions the field observations and some of the preliminary results can be summarized as follows: – in general, recent residential building stock did not suffer much damage; heavier damage was observed within the historical centers. – most of the total or near total collapses (damage grade 5) involved both special and monumental buildings: industrial warehouses, farmhouses, barns, churches, towers or belfries. these kinds of buildings showed an intrinsic weakness with respect to the seismic ground shaking. a limited number of collapses were seen for the residential building stock. in some places, the investigation revealed occasional very severe damage within a slight damage scenario, which induced very wary assessments, to avoid overestimation of the intensities. – the aftershocks of may 29, 2012, appeared to affect the residential building stock more severely than the may 20, 2012, event did. it has to be underlined that the may 29, 2012, aftershocks were located close to inhabited areas, while the may 20, 2012, earthquake was located in a relatively remote area. – the maximum intensity value (iems) for the whole sequence is 8. this value represents the cumulative damage in the area, and it well describes the increase in the damage severity after the may 29, 2012, earthquakes (figure 2). these earthquakes resulted in the shift of the major damage area westwards. – the results show that the estimation of the macroseismic magnitudes computed on the basis of the assessed intensity values, was, as expected, considerably lower than the instrumental one (table 2, figures 1, 2). – the intensity maps suggest a marked attenuation of the shaking in the n-s direction, which is noticeable both for the may 20, 2012, event and the cumulative intensity map (figures 1, 2). – the heavy damage suffered by many vulnerable buildings, such as farmhouses, barns and industrial warehouses, that was observed at the outskirts of several localities highlights the importance of defining and complying with seismic design codes and specifications, which would have made these types of buildings safer. acknowledgements. we thank c.h. caracciolo, v. castelli, a. cavaliere, a. massucci, r. tonini and s. vecchi for their contributions during the field surveys. we also thank ina cecic and an anonymous referee for their helpful suggestions. references castelli, v., f. bernardini, r. camassi, c.h. caracciolo, e. ercolani and l. postpischl (2012). looking for missing earthquake traces in the ferrara-modena plain: an update on historical seismicity, annals of geophysics, 55 (4); doi:10.4401/ag-6110. tertulliani et al. 686 event instrumental parameters macroseismic parameters coordinates (˚n; ˚e) ml coordinates (˚n; ˚e) mew may 20, 2012 44.889; 11.228 5.9 44.828; 11.241 5.1 may 29, 2012 44.851; 11.086 5.8 cumulated 44.866; 11.017 5.3 table 2. instrumental parameters [iside 2010] of the may 20 and 29, 2012, earthquakes, and the macroseismic parameters based on the may 20, 2012, intensities and the cumulative intensities computed by boxer 4.0 [gasperini et al. 2010]. 687 gasperini, p., and ferrari g. (2000). deriving numerical estimates from descriptive information: the estimation of earthquake synthetic parameters, annali di geofisica, 43 (4), 729-746; doi:10.4401/ag-3670. gasperini, p., g. vannucci, d. tripone and e. boschi (2010). the location and sizing of historical earthquakes using the attenuation of macroseismic intensity with distance, b. seismol. soc. am., 100, 2035-2066; doi: 10.1785/01200 90330. grünthal, g., ed. (1998). european macroseismic scale 1998 (ems-98). european seismological commission, subcommission on engineering seismology, working group macroseismic scales. conseil de l'europe, cahiers du centre européen de géodynamique et de séismologie, 15, luxembourg, 99 pp. iside working group (ingv) (2010). italian seismological instrumental and parametric database: http://iside.rm. ingv.it, last accessed, july 2012. locati, m., r. camassi and m. stucchi, eds. (2011). dbmi11, the 2011 version of the italian macroseismic database, milano, bologna; http://emidius.mi.ingv.it/dbmi11 (last accessed, july 2012). mcs (1930). scala macrosismica mercalli-cancani-sieberg, in: a. sieberg (ed.), geologie der erdbeben, handbuch der geophysic, tables 2 and 3, berlin. pasolini, c., d. albarello, p. gasperini, v. d'amico and r. lolli (2008). the attenuation of seismic intensity in italy, part ii: modeling and validation, b. seismol. soc. am., 98, 692-708; doi:10.1785/0120070021. quest-ingv (2012). sintesi degli effetti del terremoto del 20 maggio 2012 (ml=5.9) sulle località rilevate dalle squadre di quest ingv, open file report; http://quest. ingv.it/images/pdf/quest_report_28maggio_protect.pdf (last accessed september 2012). rovida, a., r. camassi, p. gasperini and m. stucchi, eds. (2011). cpti11, the 2011 version of the parametric catalogue of italian earthquakes, milano/bologna; http:// emidius.mi.ingv.it/cpti (last accessed, july 2012). *corresponding author: andrea tertulliani, istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy; email: andrea.tertulliani@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the emilia 2012 sequence: a macroseismic survey << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice on the plane strain in a theory f o r self-gravitating elastic configuration with initial static stress field (*) e . boschi ( * * ) r e c e i v e d on march 22nd, 1974 summary. — this paper is concerned w i t h the plane strain in a theory for an arbitrary, uniformly rotating, self-gravitating, perfectly elastic earth model with a hydrostatic initial stress field. using the associated matrices method, a representation of galerkin t y p e is given. this representation enables us to derive the solution of the vibration problem corresponding to concentrated b o d y forces. riassunto. — tn questo lavoro si t r a t t a il problema della deformazione piana nell'ambito di un modello terrestre arbitrario, uniformemente ruotante, autogravitante, p e r f e t t a m e n t e elastico e soggetto ad un campo idrostatico di sforzo iniziale. usando il metodo delle matrici associate, viene data una rappresentazione di tipo galerkin. questa rappresentazione permette la soluzione del problema delle vibrazioni corrispondenti a forze di massa concentrate. introduction. d a h l e n ( 3 ) has d e v e l o p e d t h e l i n e a r i z e d e q u a t i o n s a n d l i n e a r i z e d b o u n d a r y a n d c o n t i n u i t y c o n d i t i o n s g o v e r n i n g s m a l l e l a s t i c g r a v i t a t i o n a l d i s t u r b a n c e s a w a y f r o m e q u i l i b r i u m of an a r b i t r a r y u n i f o r m l y r o t a t i n g , s e l f g r a v i t a t i n g , p e r f e c t l y e l a s t i c e a r t h m o d e l w i t h an arb i t r a r y i n i t i a l s t a t i c stress f i e l d [see, also, b o s c h i ( ' ) ] . ( * ) this w o r k has been made during a tenure of a c . n . r . fellowship. cavendish l a b o r a t o r y , university of cambridge. ( * * ) on leave f r o m i s t i t u t o di geofisica, università di bologna, and istituto di scienze della terra, università di a n c o n a . 2 1 4 e . b o s c h i in this paper we consider this theory in the case of a hydrostatic initial stress field and derive the equations for the plane strain. the medium is assumed homogeneous and isotropic. by making use of the associated matrices method (5), we give a representation of galerkin type. this representation enables us to obtain the solution corresponding to concentrated loads in an infinite medium for the case of stationary vibrations. analogous problems have been studied in other fields. («• 2) basic equations. in the following we employ a rectangular coordinate system oxk and the usual indicial notations. the greek indices are supposed to take the values 1, 2 and the latin indices the values 1, 2, 3. let 2 be a plane region occupied by the considered medium. we denote by sa the components of the displacement vector and by 0 i the perturbation in the gravitational potential. in the case of plane strain, we have: sa = sa («1, t) , 01 = 01 («1, »2, t), s3 = 0. [1] from the equations established by dahlen (3), we can derive the following basic equations for the plane strain problem in the case of a hydrostatic initial stress tensor t°j = — p 0 s u (pa = const.): — the equations of motion: aa = [2] q0 si — 2 q0 q3 s2 = — go zltl + p + ^ g0 sj — 2 g0 q3 = — go + tp».p + — the constitutive equations: t n = (2 + 2 ft — 2 p0) si.i + (a — po) s2.2 t 2 2 = (a — po) si.i + (a + 2 /i — 2 p0) s2,2 [ 4 ] tl2 = (1 «2,1 + (fl po) si,2 t2i = ¡1 si ,2 + (fl po) 82,1 in the above equations, we have used the following notations: tap the components of the incremental pseudostress tensor; fa the components of body forces; q = (0, 0, q3) the steady angular on t h e p l a n e s t r a i n i n a t h e o r y f o r s e l f g r a v i t a t i n g e t c . 215 velocity rotation; a, ¡i the appropriate constants of the material; qo the mass density; g the gravitational constant; a comma denotes partial differentiation with respect to space variables and a superposed dot denotes partial differentiation with respect to the time t. using equations [4], the differential equations [2] and [3] may be written as: p a + (a + p — 2 p . ) + 2 qo q v3 %i2 7) <)2 • qo 5 t2 « i + (a + p 2 p a ) + 0x1 0 x2 5 t s2 — po — 0! = f1 dxi (x+fi—2 p0) t)x2 • 2 g o qz 5 t si 32 dx22 a2 qo at2 s 2 — p0 a x2 p a + (a + p — 2 po) 0 i = — ^ 2 [ 5 ] 3 a 4 71 qo g • si + 4 n qo g — s2 + a 0i = 0 dici 1)x2 g a l e r k i n representation. using the associated matrices method (5), we obtain the following representation of galerkin type: si = p l-'2 1 v22 dx22 i 2 + p qo (a + p — 2 po) 2 a 2 qz a 2 v22 *x22 a + 2q0q3 cixi 3,»2 ai a + 4 71 po2 g " | a c hxl dx2 ) • 2 hxi v21 t)x2 i)t r3. (x + p—2p0) • — ^ 2qoqz^r t>x2 ai a + 4 71 po2 g y+ p + p qo n • vi 2 a2 v22 '¿xi2 a + a 2 qz a2 2 v22 i>i d22 a^i2 n a [6] 21(5 e . b o s c h i 01 = ì7i qo g [x • 3 2 q 3 ì>2 4 71 qo g ¡1 • 2 ûicl v22 ì>x2 ìli s 2 q 3 i 2 a a where • 2 òa;2 ®22 ûa;i ì)i 4 q 3 2 ì>2 1 2 di2 «22 ~ì)t2 r. l a2 v ' a = ô a ,a2 ®12 = (x + 2 n — 2 po) i go [ 7 ] ®22 = [x j qo the functions a = a («i, ìc2, ì) satisfy to the following equations: 1 a , where where d f a ~ fx (x + 2 ¿t — 2 p „ ) n n = o 1 ¡>2 m2 t, 2 = „ 2 m fi 2 \ i 71 qog [8] [9] stationary vibrations. i n what follows, we assume that: fa = be j j 1 * (an, x2) icot in this case we seek the solution in the form: icot ] io)t a = be l s (xi, x2) e cpl = re 10* (x„ x2) e [10] [11] on t i i e p l a n e s t r a i n i n a t h e o r y f o r s e l f g r a v i t a t i n g e t c . 2 1 7 from the galerkin representation [6], by putting icut pi = be iti (xi, xi) e we obtain « :=/* qo (a — 2 po) vl2 a2 a 4 71 qo g a2 1 v22 0x22 n v22 a#22 ) + 2 i qo to • i)xi ~òxi òx2 2 i ü3 co a v22 òx2 a a + 4 7t qo2 g a2 ì)xi òx2 r 2 a s* = 2 (x + /t—2p0) a2 + p + fi qo * 2 i ' l 2 ò 2 *2 a 2 p . . t 5 t + ¡tei ~òx2 a + ' i qo qs co v2 0 * = — 4 71 qo g fl , *2 () n 2 + 4 71 qo g a2 ) v22 a»i2 j a r* 13 òxi r* 13 2 i q3 co a v2 2 <1x2 a + 4 7t £>o2 g——-—! a 0x2 r2 r; 4 71 go g u • a 2 ì ü3 co a a * v22 a * po) • *2 *2 1 2 1 ih 2m2 po) • *2 *2 1 2 vi2 v22 [12] where • *2 = zl + -^ a va2 [13] tlie functions a {xi, x2) satisfy the equations 1 d * ra* = — fi (x + 2 ¡1 — 2 p0) d * a * = 0. fn [14] 218 where with e . b o s c h i d' , , *2 , *2 • • 1 2 4:71 qo g ® 2 2 3 ' • " = a + co21 v32. [15] the operator d can be written in the form: d* = a (a — a-i2) (a — a-22), where fti2, 7i22 are the roots of the equation k" + fc2 to 2 ft)2 + 4 ti g i h 2 " ^ ï 2 2 " + co* / ft)2 î)22 4 ti oo g ft)2 vix>3 [16] 0 [17] effect of concentrated forces. l e t us examine the effect of a body force acting along the axis xi. in this case, we have: f t = 0, [18] and from equations [14], we can take: r* = r3* = 0 the solution of the problem is given by: r " v22 v2 i>x22 4 71 qo g 52 ) * 1 1 ) 1)2® ~òx22 ' «2 = (a + / * — 2 v o ) ì)xi òx2 2 1 n0 d-3 co 0xl òx2 [ 1 9 ] „ 1 . 3 co2 3 , 2 i q3 co û \ _ * 01 = ± 71 qo g [x\a —— — — 1\ , r x ïxl v22 ì)®! v22 i)x2 ' on t h e p l a n e s t r a i n i n a t h e o r y f o r s e l f g r a v i t a t i n g e t c . 2 1 9 where 1\" is the solution of the equation: a (a — 7ci2) (a — a-22) n* = [x (x + 2 ¡x — 2 p0) w e can write the solution of equation [20] in the form: r-r * 1 1 = gi & + 7ci2 (fci2 —• fe2 (7.-i2 —/c22) ' 7ci2 à-2 where the functions gi satisfy the equations: 1 (a — v ) ©a = zl (?3 = ix (x + 2 ̂ — 2 p0) 1 fi* , ¡x (x + 2 /t — 2 p„) a [20] (?3 . [21] [22] let us consider the concentrated body force fi* = 6 (xi)d(x2). in this case the functions gi are given by: 1 ga = g3 = 2 n [x (x + 2 [x — 2 p0) 1 2 j r / i (2 + 2 , « — 2 po) j f o (fc0 r) in r [23] [24] where k0 (z) is the modified bessel function of third kind, and r3 xi~ + ®22. using equations [21] and [24], the solution of equation [20], for concentrated body force, is given by: r * 2 71 fx (x + 2 (x — 2 p0) i o (7ci r ) i i o (7c2 r ) fcl2 (fcl2 — fc22) fc22 (fcl2 — 7i22) ad2 fc22 in r [25] thus, the solution of the considered problem is given by [19], where f \ * has the expression [25]. i n a similar way, we can obtain the solution for a concentrated body force acting along the axis ox2. 220 e. boschi acknowledgements. i wish to thank professor a . h . cook f.r.s. for his kind hospitality in cambridge, aud professor m. caputo for valuable discussions and suggestions. r e f e r e n c e s (x) boschi, e., 1973. reciprocity theorem and elastic dislocation theory for an earth model with an initial static stress field. " j . geophys. r e s . " , 7 8 , 8584. ( 2 ) bosciii, e., 1973. on the plane strain in a generalized theory of thermoelasticity. int. " j . e n g . sci.," 1 2 , 433. ( 3 ) t) ah lex, f . a . , 1972. elastic dislocation theory for a self-gravitating elastic configuration with an initial static stress field. " g e o p h y s . j. r . astr. s o c . " , 28, 357-383. ( 4 ) iesan, d., 1908. on the plane coupled micropolar thermoelasticity. i , " b u l l . a c a d . polon. s c i . " , scr. sci. teclin., 1 6 , 379-484. ( 5 ) moisil, gr. c., 1952. teoria preliminara, a sistemelor de ecuatii cu derivate partiale lineare cu coeficienti constanti, boletinul stiintific al a c a d . r. p . r., seria a , 4, 319-326. ( 6 ) nowacki, w . , 1904. green functions for the thermoelastic medium. i i , " b u l l . a c a d . polon. s c i . " , ser. sci. teclin., 1 2 , 405-472. s0225 annals of geophysics, 60, 2, 2017, s0225; doi: 10.4401/ag-7285 source parameters of the earthquake sequence that occurred close to the burar array (romania) between 24 june and 1 july 2011 emilia popescu1, anica otilia placinta1,*, mircea radulian1,2, felix borleanu1, mihail diaconescu1, mihaela popa1 1 national institute for earth physics, magurele, romania 2 academy of romanian scientists, bucharest, romania article history received october 13, 2016; accepted february 3, 2017. subject classification: earthquake sequence, spectral ratio, empirical green’s function, source parameters, scaling relationships. abstract the seismic activity in the eastern carpathians area is poorly recorded (a few hundreds of small-to-moderate earthquakes in the romanian catalogue over the last century). the installation in 2002 of the high-performance bucovina (burar) array in the eastern carpathians area contributed to a significant growth of the capacity to monitor local seismicity. as a consequence, the earthquake sequence occurred between 24 june and 1 july 2011 close to the burar array is the best seismic data set ever recorded for this area. the location of the events using all the available data provided by the real-time seismic network of the national institute for earth physics suggests a nesw alignment along the western edge of the avramesti suceava fault. this fault is crossing the carpathian foredeep underthrusting the foreland units to the orogeny area. the distribution of the first p-wave polarities is fitting the geometry of this fault, indicating predominant strike-slip faulting, with right-lateral movement. the compression axis oriented e-w is in agreement with the stress field characterizing the region. we applied spectral ratios and empirical green’s function methods to estimate the source parameters (corner frequency, seismic moment, source duration, rise time) for the events with moment magnitudes higher than 2.5 belonging to this sequence. the results show a simple fracture model for the main shock of 24 june 2011 and an apparent constant stress drop scaling. source parameter scaling relationships fit well the results obtained for other regions along the south-eastern carpathians and those which are typical for intra-continental areas. 1. introduction the carpathians area is generally characterized by low seismic activity, except a strongly clustered activity concentrated at the carpathians arc bend in the vrancea region, in romania [ismail-zadeh et al. 2012] and references therein). whereas numerous investigations focused on the vrancea seismicity, other areas of the carpathians are still largely unexplored. the eastern carpathians segment in romania is a good example in this respect. one explanation is the lack of high-quality observations as a consequence of rare events in the region and poor monitoring infrastructure. with the recent enlargement of the national seismic network [neagoe et al. 2009, 2011; popa et al. 2015] in the eastern carpathians area and installation of a high-performance array in the bucovina region (grigore et al., 2004; borleanu et al., 2011; ghica, 2011) the occurrence of an earthquake sequence in june july 2011 in the northern part of the eastern carpathians (figure 1), even if it was of moderate size, provided the best data set ever recorded for the region of the northern moldavia and the possibility to apply advanced techniques to constrain source parameters. the goal of the present paper is to study the sequence recorded between 24 june and 1 july 2011, taking advantage of the station coverage improvements in the region after 2005 and primarily of the running of the bucovina (burar) array, located at about 50 km distance from the sequence epicentral area. this array of small aperture (~ 5 km radius) was installed in 2002 in cooperation with the air force technical application center (aftac) of the u.s.a. the array consists of 9 short-period elements with vertical sensors (bur01, …, bur09) and one 3-component broadband element (bur31). the sequence was generated at the western edge of an alignment extended from the carpathian foredeep and underthrust foreland units to the orogeny area (avramesti-suceava fault). in order to estimate source parameters, we apply relative methods of deconvolution, such as empirical green’s functions and spectral ratios methods. similar investigations were performed for earthquake sequences generated in the southern segment popescu et al. 2 of the carpathians [enescu et al. 1996; popescu 2000; popescu and radulian 2001; popescu et al. 2003, 2011, 2012; radulian et al. 2014, placinta et al. 2016]. they basically consist of applying relative techniques of investigation, such as the spectral ratios and empirical green’s functions techniques that proved to be efficient in retrieving source parameters for seismic sequences. first, source parameters are determined, and then, source scaling properties are subsequently investigated. finally, the contribution of the new results in improving our understanding of the seismotectonics along the carpathians orogeny is discussed. 2. regional seismotectonics the northern and central parts of the moldavian platform and carpathians orogen show a low-to-moderate crustal seismic activity (figure 1), in contrast with the sharp concentration of earthquakes in the vrancea region, located south of the trotus fault, at the eastern carpathians bending zone. according to the romplus catalog [oncescu et al. 1999], during january 1900 april 2014, 874 small-to-moderate events (1.1 ≤ mw ≤ 5.5) occurred in this area, that we consider to belong mainly to a low background seismicity and to man-made activity. one single event has the magnitude mw greater than 5, but it is an event with no instrumental recordings, which occurred on 31 january 1900, 09:00, lon. 27.300e, lat. 46.500n. the earthquake is located close to the bistriţa fault. most probably the magnitude (mw = 5.5) is overestimated. the locations of a few significant earthquakes in the region are represented in figure 1. they can be associated with the principal faults crossing the platform region to the carpathians orogen. thus, we mention: events recorded along the trotuş fault that occurred on 18 april 1956 (mw = 4.5), 12 october 1959 (mw= 4.1) and 16 september 1965 (mw = 4.5), along the bistriţa fault on 17 october 1906 (mw = 4.9) and 6 november 1997 (mw = 3.1), along the vaslui fault on 8 november 1905 (mw = 4.2) and 5 may 1981 (mw = 3.2) and along the avrămeşti-suceava fault on 20 january 1903 (mw = 4.1), 20 october 1979 (mw = 3.7) and 24 june 2011 (mw = 3.8). note that all the events with magnitude larger than 4 took place until 1970, when the romanian seismic network performance was modest and the accuracy in magnitude and location parameters was poor. the relative enhancement of seismicity in the area between the trotuş and vaslui faults reflects perhaps the transition from a stable segment (moldavian platform) to the active segment related to the vrancea seismic activity, located south of the trotuş fault. the trotuş fault is an old jurassic fracture, separating the scythian platform from the moesian platform, which looks like to be still active [enciu et al. 2009, van der hoeven et al. 2005, săndulescu 2009]. another concentration of events is visible along the neogene volcanic chain călimani gurghiu harghita (but only events below mw 4.0 magnitude), located in the inner side of the carpathians. however, the hourly distribution of the number of events indicates that a significant percentage of the activity in the trotuş fault region and especially in the călimani gurghiu harghita region is due to quarry activities. in the northern part of moldavia the reported seismicity is sparse and is probably related to marginal fractures of the moldavian platform (part of the eastern european platform). in fact, the marginal fractures of the platform are situated to the east of the epicenters [polonic 1986]. in general, the epicenters are not following the main faults alignments excepting some clustering around central segment of the trotus fault. non-coincidence between the epicenters and identified faults and the observation that the region exhibits an uplift neotectonic movement, lead to the idea that these earthquakes are caused by flexure that break and give small normal faults, with the eastern side having a tendency to upraise, while the western side of the fault is figure 1. sketch of seismotectonic map of the contact between the moldavian and scythian platforms (east side) and carpathians orogen (west side). setting in europe is given in the upper right corner inset. the study region is marked by the blue square. the epicenters of the events recorded in the romplus catalog (oncescu et al., 1999) are plotted with red symbols for crustal earthquakes and yellow symbols for subcrustal earthquakes (vrancea seismic region). the seismic stations of the national institute for earth physics – măgurele (romania), located inside the map, in operation at that time, are plotted with solid triangles. the epicenters of the sequence of 24 june – 1 july 2011 are plotted as black dots (the main shock – black star). significant events recorded in the region since 1900 are represented by green stars. 3 earthquake sequence close to burar array immobilized under molasses deposits. such an interpretation is consistent with the generally accepted concept of termination of under pushing east-west movement of the foreland from the moldavian platform under carpathians. the cluster of events recorded in the inner side of carpathians (harghita region) is rather associated with man-made activity due to quarry blast exploitation. the deficit of seismicity in comparison with the southern and south-western parts of moldavia can be partly explained by the poor coverage of the romanian seismic network in the northern part of moldavia. 3. description of the earthquake sequence the crustal seismic sequence produced in the vicinity of bucovina seismic array in june 2011 is a singular seismic phenomenon for this region, as far as we have available information. as figure 1 shows, the background seismicity in this region is diffuse and poor, and therefore no preferential alignments of seismic sensibility and seismo-genetic contoured areas can be identified. however, the epicenters distribution of the studied seismic sequence apparently follows the alignment of the avrămeşti suceava fault (more precisely, the western edge of this fault) which crosses perpendicularly the entire fault system oriented nnw-sse (figure 1). nevertheless, we should consider carefully this assumption taking into account not necessarily the location errors (the maximum axis or errors ellipsis is below 6 km), but rather the configuration of stations with a large gap towards n-ne azimuth. more than 40 events were identified as belonging to the sequence, but only 9 of them are well located (table 1) using all the available data recorded by the real-time seismic network of national institute for earth physics (short-period and broadband seismometers). we retained in our data set only the events located with minimum 6 stations and 8 phases. the locations were performed using locsat routine which runs under antelope (brtt) software, routinely operated by niep (national institute for earth physics). to locate the events, we used p and s wave travel times manually read on the seismograms. the aftershocks spreading around the main shock epicenter (figure 1) seem to indicate a unilateral rupture for this event along the avrămeşti-suceava fault, dipping toward se. certainly, a rupture length close to 10 km, as suggested by aftershocks distribution, extends far beyond the rupture dimension typically observed for an earthquake of magnitude 3.8 (no more than 1 km). therefore, the apparent length is most likely excessively large due to the uncertainties in the locations. the number of stations used in location process varies from 6 (for the events of 24 june 2011, at 13:31 and 16:18) to 30 stations (for the main shock). three stations belonging to the seismic network of the republic of moldova (leom, milm, sorm) were included as well. the largest azimuthal gap between azimuthally adjacent stations (gap) is around 120°, while the rms value spans the interval 0.34 to 0.75 s. the quality of reading the pand s-wave phases is good for burar and 5 stations (sorm, tesr, milm, josr, arcr), while is generally poor for the more distant stations. it is not possible to constrain the fault-plane solution of the main shock by inverting the first p-wave polarities (10 polarities picked with high confidence). however, to test if the avrămeşti suceava alignment coincides with a possible nodal plane, we projected this fault on a lower hemisphere (figure 2). the fault azimuth (n48°e) and dip (56°) are estimated simply from the geometry of the fault mapping in connection with the orientation of earthquake locations (figure 1) and focal depth (table 1). the conjugate nodal plane that approximates to some extent the distribution of reliable p-wave polarities is drawn in the same figure (plane 2). however, some stations are slightly outside the nodal plane 1 (fitting the assumed avrămeşti suceava fault). as a conclusion of our investigation, we can assume either that the avrămeşti suceava fault is shifted a bit to the nw relative to the epicenters (or figure 2. a possible fault plane solution of the main shock of the burar sequence, 24 june 2011, 13:08, mw = 3.8. the nodal planes with solid lines correspond with the avrămeşti suceava fault vs. epicentral distribution geometry. as shown in the text, a correction like that represented by dashed lines is matching better the p-wave polarities. empty and solid circles are for compression and dilatation, respectively. popescu et al. 4 vice versa), or the focal depth of the main shock (h = 10 km) is smaller (h ~ 5 km). a possible focal mechanism that we selected as the one best fitting all the polarities has the nodal plane 1 with the same azimuth (n48°e), while the dip is lower (43°) see nodal planes represented by dashed lines in the figure 2. the focal mechanism is predominantly of strike-slip type, with right-lateral movement of the south-east compartment. the compression axis oriented e-w is in agreement with the stress field characterizing the region. a slight underthrust of the northern compartment under the southern compartment is also noticed (southern compartment is lifted and shifted towards south-west). 4. source parameters the relative deconvolution methods (spectral ratios and empirical green’s function deconvolution) are efficiently retrieving source parameters (seismic moment, source radius, rupture duration, rise time and stress drop) when waveforms from pairs of co-located events are available at common broadband stations [frankel et al. 1986, hough et al. 1989, lindley 1994, mueller 1985, mori and frankel 1990, popescu et al. 2016]. typically for this class of methods, the path, site and instrument effects are removed by deconvolving the waveform of a lower magnitude event from the main event waveform. the same approach was applied to other earthquake sequences occurred in the south-eastern carpathians area [popescu 2000, popescu and radulian 2001, popescu et al. 2003, 2011, 2012, radulian et al. 2014, placinta et al. 2016]. we applied the spectral ratios technique in parallel with the empirical green’s function deconvolution for the earthquakes given in table 1. the quality of recordings for the last event (1 july) is poor and therefore, the application of relative methods in retrieving seismic source parameters cannot be properly done for this event. we limit in this case to the estimation of magnitude and seismic moment. spectral ratios depend essentially only on the source when the selection of earthquakes pairs is properly done and, in this case, it is not necessary to apply path, local and instrument response corrections. another advantage of the method is the possibility to simultaneously determine the corner frequencies for both earthquakes of a selected pair, as long as the instrument is broadband and signal-to-noise ratio (snr) is high enough in the frequency of interest. as concerns the source size, we obtain only the ratio of seismic moments. to estimate the absolute values, we need an independent determination for one event (reference value). we selected the largest earthquake as reference event and applied relation (3) below to compute its seismic moment. then we compute the absolute values for all the other events using the spectral ratios values. for a source model with uniform rupture and ω-2 spectral fall-off at high frequencies, the spectral ratios (r(f )) can be approximated by the theoretical function: (1) where ω0 p, ω0 g are the low-frequency asymptotes of amplitude spectra of principal and green’s earthquakes, and fc p, fc g are the corresponding corner frequencies and γ is the coefficient of the spectral fall-off at high frequency. selecting as free parameters the ratio of seismic moments a = lg(ω0 p/ω0 g) and the corner frequencies, we apply a nonlinear regression procedure in order to find the function (1) that best approximates observed spectral ratios. we used all the components (z, e, n) of the waveforms and all the common stations available to estimate the parameter values a, fc p, fc g resulting from spectral ratio method application. the final estimates are the average values over all the specific values for different event r( f ) ω0 p 1+( f / fc g)2γ⎡⎣ ⎤ ⎦ 1/2 ω0 g 1+( f / fc p)2γ⎡⎣ ⎤ ⎦ 1/2 no. date (yyyy/m/day) origin time (hh:mm:ss) lat. (°n) lon. (°e) depth (km) md/mw no. stations rms gap 1. 2011/6/24 13:06:36.8 47.399 25.768 11 2.9/3.1 10 0.73 136 2. 2011/6/24 13:08:40.5 47.372 25.777 10 4.6/3.8 30 0.69 107 3. 2011/6/24 13:31:29.1 47.359 25.732 3 2.8/3.1 6 0.34 110 4. 2011/6/24 16:18:57.9 47.355 25.756 5 2.6/2.9 6 0.47 118 5. 2011/6/25 00:13:47.3 47.370 25.776 8 3.0/2.9 7 0.57 119 6. 2011/6/25 01:43:21.6 47.344 25.765 10 3.2/3.1 10 0.73 120 7. 2011/6/30 21:21:18.5 47.349 25.764 5 3.0/2.8 22 0.65 116 8. 2011/6/30 21:22:01.5 47.345 25.730 4 2.9/2.8 18 0.58 120 9. 2011/7/01 22:20:25.4 47.333 25.692 3 2.3/2.5 15 0.75 116 table 1. earthquake parameters for the study sequence. the main shock is marked with bold. 5 pairs, stations and components. they are given in annex. examples of the spectral ratios for two earthquake pairs and two stations are plotted in the figure 3. the size of the rupture area is directly related to the corner frequency [madariaga 1976]: (2) r representing the equivalent radius of the source while k is a constant value of 0.32 for p waves and 0.21 for s waves, fc is the corner frequency and vs is the s-wave velocity in the focus. with relationship (2) we determine the source radius from corner frequencies (rgrs radius of green function obtained from spectral ratios, rprs radius of the main event obtained from spectral ratios). the source radius estimated from corner frequency is an average between the estimations using p-wave and s-wave corner frequencies. since the relation (2) assumes a ratio about 1.5 of fc p/fc s and for our data fc p≈ fc s, the radius computed from fc p is systematically greater than the radius computed from fc s roughly by a factor of 1.5 (for example, for main event, r = 274 m from fc p and r = 177 m from fc s). the seismic moment for the earthquakes analyzed in this study is estimated using: (3) ρ is the density at the source depth, vp is the velocity of p-waves at source depth, ω0 is the long period displacement spectral level, r is the hypocentral distance and rθφ is the source radiation pattern (average values of 0.52 for p waves and 0.63 for s waves, according to [aki and richards 1980]. we adopted for vp and ρ parameters the values as resulted from the velocity structure model estimated by raileanu et al. [2012]. after seismic moment and source radius are calculated, the brune stress drop [brune 1970] is computed using: (4) for the same pairs of events considered in the spectral ratios method, we applied in parallel the method of deconvolution with empirical green’s functions. the source rise time τ1/2, and the source duration τ, for the main events, are estimated from the source time function each time it had a pulse like shape. in this case the source radius was computed using boatwright’s formula [boatwright 1980]: (5) where τ1/2 is the source rise time, v is the rupture velocity in the source, considered as v = 0.9 β (with βs-wave velocity at the seismic source depth), α p-wave velocity r = kvs / fc m0 =(4 πρ vp 3ω0r)/rθϕ r = (τ1/2v) /1−v /αsinθ) earthquake sequence close to burar array figure 3. spectral ratios obtained for the main event of 24 june 2011, 13:08 and empirical green’s functions of 30 june 2011, 21:21 (mw=2.8) – left column and 21:22 (mw=2.8) – right column: a) at burar station and b) at tesr station. the dashed line represents the best approximation with a theoretical function given by relation (1). δσb = 7mo 16r3 popescu et al. 6 at the source depth, θ the angle between the normal to the fault and the output direction of p waves from hypocenter. in case the main event has more green’s functions associated, rise time is the average of all obtained values of τ1/2 (different green functions and different stations). an example of main event empirical green’s function pair is given in figure 4. we plotted the main shock of 24 june 2011, 13:08 with two associated empirical green’s functions of 24 june, 13:06 and 25 june 2011, 01:43 as recorded at two stations, burar and tesr. the average source time function for the main event, calculated as the arithmetic mean of the source time functions obtained for each pair of co-located events at each station, whenever these functions show well-defined patterns, is represented in figure 5. in this case eight stfs were accepted for the average, including different station components. source time function is a simple unipolar pulse, which supports the hypothesis of a homogeneous rupture pattern over 0.18 s duration. we applied for the same selected pairs of events the method of deconvolution with empirical green’s functions to obtain the source duration τ and rise time τ1/2, for the main event, using the available stations for all co-located pairs. based on the seismic moment, corner frequency and source duration estimations, we determine the source area and stress drop using relations (2) (4). to apply equation (4), we adopted an average value of 30°for the take-off angle with respect to the normal to the fault. this angle takes into account source directivity effects which are likely to be negligible for such small earthquakes. for θ varying between 0 and 45°, the variation in r is slightly higher than 30%. the difference between the source radius inferred from the duration and that inferred from corner frequency would suggest some inadvertencies in the parameters of relations (2) and (4). the results are presented in table 2 for the main event and for the empirical green’s function events. 5. scaling relationships the scaling relationships for earthquake sequences are valuable indicators of geotectonic peculiarities of the area under investigation. such studies have been done previously for earthquake sequences occurred in the south-eastern carpathians foredeep region [enefigure 4. examples of waveforms: a) main event of 24 june 2011, 13:08 and the associated empirical green’s function of 24 june 2011, 13:06 at the broadband element of the burar array (right); b) main event of 24 june 2011, 13:08 and the associated empirical green’s function of 25 june 2011, 01:43 at tesr station (left). 7 earthquake sequence close to burar array scu et al. 1996, popescu 2000, popescu and radulian 2001, popescu et al 2003, 2011, 2012, radulian et al. 2014, placinta et al. 2016]. up to now, there is no systematic investigation of the seismic activity recorded in the northern part of the moldavian platform. from this point of view, the determination of source parameters and of the corresponding scaling relationships for the sequence in the northern part of moldavia provides new insights in the seismotectonics of this area. the scaling of the seismic moment m0 with duration magnitude md is shown in figure 6. the data are approximated by the linear regression: (6) however, the regression is based on only nine points, eight of them covering a narrow magnitude range with practically no correlation. for this reason, the regression parameters depend critically on the single earthquake with magnitude above 4. it is highly recommended to adopt such scaling only after including more observation data. the corresponding moment magnitude duration magnitude scaling tends to overestimate the earthquake size inferred from duration for moderate earthquakes and to underestimate it for small earthquakes. the inconsistency between the two magnitude scales could be explained by the significant difference in s/n ratio as we go to smallest or biggest events. below a certain magnitude, it becomes difficult to distinguish signal from noise and duration tends to saturate. by contrary, for larger events the s/n is higher and there is a higher probability to limit the duration measurement before some later phases. scaling of seismic moment with source radius (figure 7) is well approximated by the linear regression: (7) the slope of the regression line comes close to the theoretical value (3) which characterizes the seismic source scaling in case of homogeneous rupture process. the scaling of stress drop with earthquake size (figure 8) indicates a constant stress drop over the entire magnitude range. however, we should keep in mind that the uncertainties in stress drop are amplified relative to the source radius (corner frequency) uncertainties because of the power law dependence logm0 = (0.88±0.13)md +(10.89±0.43) figure 5. source time function for the main shock of 24 june 2011, 13:08 (continuous line) calculated as the arithmetic mean of the values obtained for empirical green’s function deconvolution at all stations and components. the dashed lines represent the standard error. r = 0.94,σ = 0.22 event seismic moment (nm) τ (s) fc (hz) source radius (m) stress drop (mpa) mw md from τ1/2 from fc 24.06.2011, 13:08 1.01x1015 0.180 3.72 282 227 37.8 3.8 4.6 event seismic moment (nm) source radius (m) from fc p from fc s average stress drop (mpa) mw md 24.06.2011, 13:06 6.25x1013 89 72 81 51.5 3.1 2.9 24.06.2011, 13:31 6.03x1013 105 70 88 38.7 3.1 2.8 24.06.2011, 16:18 3.58x1013 71 71 43.8 2.9 2.6 25.06.2011, 00:13 4.20x1013 115 83 99 18.9 2.9 3.0 25.06.2011, 01:43 8.21x1013 133 84 109 27.7 3.1 3.2 30.06.2011, 21:21 2.25x1013 83 53 68 31.3 2.8 3.0 30.06.2011, 21:22 2.08x1013 75 53 64 34.7 2.8 2.9 01.07.2011, 22:20 1.35x1013 2.5 2.5 table 2. final source parameters for the main shock and for the empirical green’s functions used in this study. logm0 = (2.92±0.31)logr+(8.04±0.60) r = 0.97,σ = 0.14 figure 6. scaling of seismic moment with duration magnitude. popescu et al. 8 (equation (4)). the average stress drop value (~ 30 40 mpa = 300 400 bar) is characteristic for faulting processes in intra-continental areas (large stress drops). according to our results, the earthquake sequence was generated in an area that has been less fractured before. it is interesting to notice (as shown in table 2) that the highest value of the stress drop was recorded for the foreshock on 24 june at 13:06, in agreement with the hypothesis of a poorly fractured area prior the sequence triggering. the scaling of the corner frequency and source duration with duration magnitude is represented in figures 9-10. we combine in the graphical representation the estimates of the present study with estimates previously obtained for earthquakes generated in different other areas of the south-eastern carpathians (see references above). in all cases, the same procedure was applied to retrieve corner frequency and source duration parameters. the regression lines, approximating the scaling relationships, are: (8) (9) the slopes in the relations (8) and (9) are close each other in absolute value (with opposite signs). therefore, we can assume that the corner frequency scales as a simple inverse of duration: (10) figure 7. scaling of seismic moment with source radius. figure 8. scaling of stress drop with seismic moment. the regression line is close to a constant stress drop scaling around 35 mpa. figure 9. scaling of corner frequency with duration magnitude. data for south carpathians from radulian et al (2014), for vrincioaia region from popescu et al. (2012) and for vrancea foredeep from popescu et al. (2001) and popescu et al. (2011). the regression line is estimated for all the data points. figure 10. scaling of source duration with duration magnitude. data for south carpathians from enescu et al. (1996) and radulian et al. (2014); for vrincioaia region from popescu et al. (2012) and for vrancea foredeep from popescu et al. (2001) and popescu et al. (2011). the regression line is estimated for all the data points. log fc = −(0.20±0.03)md +(1.40±0.10) r = 0.76,σ = 0.12 logτ = (0.19±0.02)md −(1.50±0.09) r = 0.87,σ = 0.06 fc ~τ -1 9 in agreement with the relation (6) used by boore [1983]. 6. conclusions the earthquake sequence produced in the area adjacent to bucovina seismic array (burar) between 24 june and 1 july 2011 is the single such a seismic phenomenon recorded until now in this region. hypocenter locations using all the available data recorded by the real-time seismic network of the national institute for earth physics (niep) show a ne-sw alignment along the avrămeşti-suceava fault. it is likely that the study seismic sequence was a consequence of a sudden activation of the south-western edge of this fault. the fault plane solution is not constrained by the available data. the reliable p-wave polarities suggest a strikeslip faulting with a nodal plane close to the avrămeşti-suceava fault. the source parameters (corner frequency, seismic moment, source duration, rise time) are estimated by applying spectral ratios technique and empirical green’s function deconvolution. the results show a simple fracture model for the main shock of 24 june 2011, with a unipulse source time function and a constant stress drop scaling. the stress drop level is compatible with stress regime in intra-continental settings. source parameter scaling relationships fit well the results obtained for other regions along the south-eastern carpathians and those which are typical for intra-continental areas. acknowledgements. data used in the present study were provided by the national institute for earth physics. the work was partially supported by the project “nucleu”(pn 16 35 01 08) of the national plan for research, development and innovation of the romanian ministry of national education and by project 69/2014, daring, under the executive unit for financing higher education, research, development and innovation, program partnership in priority areas, collaborative applied research projects c2013. references aki, k. and p. richards (1980), quantitative seismology: theory and methods (freeman, san francisco). boatwright, j.(1980), a spectral theory for circular seismic sources: simple estimates of source duration, dynamic stress drop, and radiated energy, bull. seismol. soc. am. 70, 1 -28. boore, d. m. 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(2000), complex study of the earthquake sequences on the romanian territory, phd thesis, institute of atomic physics, bucharest, (in romanian). popescu, e. and m. radulian (2001), source characteristics of the seismic sequences in the eastern carpathians foredeep region (romania), tectonophysics, 338, 325-337. popescu, e., m. popa and m. radulian (2003), efficiency of the spectral ratio method to constrain the source scaling properties of the vrancea (romania) subcrustal earthquakes, rom. rep. phys.55, 149-169. popescu, e., c. neagoe, m. rogozea, i.a. moldovan, f. borleanu and m. radulian (2011), source parameters for the earthquake sequence occurred in the ramnicu sarat area (romania) november-december 2007, rom. journ. phys.56, 265-278. popescu, e., f. borleanu, m. rogozea and m. radulian (2012), source analysis for earthquake sequence occurred in vrancea (romania) region on 6 to 30 september 2008, romanian report phys., 64, no. 2. popescu, e., m. radulian and a.o. placinta (2016), scaling properties for the vrancea subcrustal earthquakes: an overview, in “effects and lessons from november 10th, 1940 vrancea earthquake” proceedings of the symposium commemorating 75 years from november 10, 1940 vrancea earthquake (eds. r. vacareanu, c. ionescu), springer natural hazards, 235 252. radulian, m., e. popescu, f. borleanu and m. diaconescu (2014), source parameters of the december 2011 january 2012 earthquake sequence in southern carpathians, romania, tectonophysics, 623, 23-38. răileanu, v., d. tătaru and b. grecu (2012), crustal models in romania i. moesian platform, romanian report phys., 64, no.2, 539-554. săndulescu, m., (2009), the geotectonic framework of a peculiar seismogenetic area the vrancea seismic zone (romanian carpathians), proc. rom. acad., series b, 2 -3, 151 -157. van der hoeven, a.g.a., v. mocanu, w. spakman, m. nutto, a. nuckelt, l. maţenco, l. munteanu, c. marcu and a.c. ambrosius (2005), observations of present-day tectonic motions in the southeastern carpathians: results of the ises/crc-461 gps measurement, earth planet. sci. lett., 239, 177 184. *corresponding author: anica otilia placinta national institute for earth physics, magurele, ilfov, romania; email: anca@infp.ro. 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved 11 annex parameter values “a”, fcp, fcg resulting from spectral ratios method analysis. event p corresponds to main event (event 2 in table 1). the number assigned to each empirical green’s function (egf) is the same as in table 1. earthquake sequence close to burar array pair p-1 egf:11/06/24, 13:06 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 1.79 1.87 1.84 4.79 3.89 3.89 7.80 5.21 5.21 sorm 1.65 1.84 3.38 3.30 7.00 7.92 tesr 1.84 1.70 1.79 3.22 3.35 3.24 10.40 9.40 9.50 average/component 1.820 ± 0.035 1.740 ± 0.115 1.823 ± 0.029 4.00 ± 1.11 3.51±0.30 9.10 ± 1.84 7.37±1.92 average/egf 1.793±0.079 3.633±0.541 8.235±1.223 pair p-3 egf:11/06/24, 13:31 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 1.91 1.93 1.98 3.71 4.15 3.94 7.50 6.80 7.20 tesr 2.07 1.92 1.93 3.28 3.55 3.78 9.32 9.00 10.00 average/component 1.990 ± 0.113 1.925 ± 0.007 1.955 ± 0.035 3.495 ± 0.304 3.855±0.254 8.410 ± 1.287 8.250±1.509 average/egf 1.957±0.061 3.735±0.303 8.330±0.113 pair p-4 egf:11/06/24, 16:18 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 2.44 2.37 4.92 4.66 9.00 7.00 sorm 2.09 2.14 3.18 3.96 6.65 10.00 average/component 2.265 ± 0.247 2.255 ± 0.163 4.180 ± 0.780 8.163±1.604 average/egf 2.260±0.171 3.735±0.303 8.330±0.113 pair p-5 egf:11/06/25, 00:13 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 2.13 2.20 219 3.89 3.59 3.80 6.56 5.39 5.61 sorm 1.92 2.16 2.18 4.62 3.20 3.22 9.52 8.85 10.00 tesr 2.13 2.02 2.18 2.97 4.61 3.29 10.00 8.30 9.20 average/component 2.060 ± 0.121 2.127 ± 0.095 2.183 ± 0.006 3.827 ± 0.827 3.618±0.540 8.693 ± 1.863 7.892±1.934 average/egf 2.123±0.094 3.712±0.571 8.293±0.556 popescu et al. 12 pair p-6 egf:11/06/25, 01:43 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 1.73 1.74 4.40 4.34 6.00 6.00 josr 1.78 1.80 1.73 3.49 3.12 3.21 7.20 8.30 7.80 milm 1.67 1.60 1.81 3.04 3.78 3.60 5.39 5.79 7.16 sorm 1.60 1.62 1.65 3.36 3.70 3.48 6.25 6.00 6.15 tesr 1.82 1.94 1.96 3.84 4.07 4.54 7.63 7.13 8.34 average/component 1.718 ± 0.101 1.738 ± 0.139 1.778 ± 0.116 3.433 ± 0.331 3.820±0.498 6.618 ± 1.001 6.867±1.001 average/egf 1.746±0.115 3.712±0.480 6.743±0.167 pair p-7 egf:11/06/30, 21:21 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 1.91 1.81 1.81 3.02 3.41 3.46 5.25 4.90 5.40 josr 1.95 2.25 3.5 2.61 8.81 9.31 tesr 1.96 2.12 2.00 3.50 3.51 4.40 8.85 9.88 10.03 average/component 1.940 ± 0.026 1.965 ± 0.219 2.020 ± 0.221 3.340 ± 0.277 3.478±0.634 7.673 ± 2.067 7.904±2.534 average/egf 1.976±0.150 3.426±0.507 7.789±0.163 pair p-8 egf:11/06/30, 21:22 a (spectral ratio) fc (hz) (main shock) fc (hz) (green function) z e n z e n z e n burb 1.74 1.81 1.79 4.20 3.60 3.14 6.93 9.00 7.00 josr 1.96 2.01 2.12 3.46 3.07 2.92 9.11 tesr 1.90 2.04 2.01 3.93 4.22 4.16 9.71 6.65 10.00 average/component 1.867 ± 0.114 1.953 ± 0.125 1.973 ± 0.163 3.863 ± 0.374 3.515 ± 0.563 8.583 ± 1.463 7.975±2.480 average/egf 1.931±0.129 3.633±0.516 8.279±0.430 introduction_final_layout 6 annals of geophysics, 56, 6, 2013, s0683; doi:10.4401/ag-6503 s0683 earthquake geology: science, society and critical facilities christoph grützner1, salvatore barba2, ioannis papanikolaou3, raul pérez-lópez4 1 rwth aachen university, institute for neotectonics and natural hazards, aachen, germany 2 istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy 3 agricultural university of athens, laboratory mineralogy geology, athens, greece 4 igme instituto geologico y minero de españa, madrid, spain 1. introduction earthquake geology, earthquake environmental effects (eee) and seismic hazard assessment earthquake geology studies the effects, the mechanics and the impacts of earthquakes in the geological environment. its role is also to decode the fault history, therefore its approach is fault specific and its outcomes are of decisive value for seismic hazard assessment and planning. the term earthquake geology includes aspects of modern instrumental studies, tectonics and structural geology, historical surface deformation and tectonic geomorphology, whereas paleoseismology is considered part of earthquake geology [mccalpin 2009]. in a broader sense, paleoseismology intends to establish the seismic history of a given fault or given region from deformed sediments or rocks, beyond the limited temporal resolution of instrumental and historical seismicity [e.g., audemard and michetti 2011]. the advances of earthquake geology and paleoseismology over the last few decades offered a quantitative description of earthquake environmental effects that led to the development of the environmental seismic intensity scale esi 2007 [michetti et al. 2007]. seismic hazard assessment is predominantly based on the instrumental and historical catalogues of seismicity. however, these catalogues are incomplete and have an inhomogeneity of geographical and temporal coverage in terms of the seismic record, forming their main disadvantage. this is because historic catalogues are generally too short (from several tens of years to several hundreds of years, depending on the country) compared to the recurrence interval of particular faults (ranging from a few hundred years to several thousands of years) [goes 1996, yeats and prentice 1996, machette 2000, valensise and pantosti 2001]. the latter implies that the sample from the historical record is incomplete and that a large number of faults would not have ruptured during the completeness period of the historical record. for example, greece has one of the longest historical catalogues worldwide with the oldest recorded events in 550 b.c. however, this catalogue is considered complete for events m≥7.3 since 1500 and for m≥6.5 only since 1845 [papazachos et al. 2000]. in addition, maps based on historical seismicity can give erroneous pictures of the present day hazard. a low seismicity zone on such a map, representing low hazard, may delineate a seismic gap (i.e., a gap that an impeding earthquake will cover) and actually be a place of high present and future hazard [scholz 2002]. on the other hand, a region that has recently experienced a damaging earthquake, and hence is represented as high hazard on a map, actually may be a region of low hazard in the near future because it is now at an early stage in a new seismic cycle [scholz 2002]. geological data have the potential to extend the history of slip on a fault back many thousands of years, a time span that generally encompasses a large number of earthquake cycles [yeats and prentice 1996], and thus elucidates the long-term pattern of fault-slip. as a result, fault specific approaches are becoming very important for seismic hazard assessment, by providing quantitative assessments through measurement of geologically recorded slip on active faults, sampling much greater periods of time and providing a more reliable estimate of hazard than the historical earthquake record [e.g., yeats and prentice 1996, michetti et al. 2005]. in addition, geologic fault slip-rate data offer complete spatial coverage, providing higher spatial resolution than traditional seismic hazard maps based on historical/instrumental records [boncio et al. 2004, roberts et al. 2004, pace et al. 2010, papanikolaou et al. 2013]. for land-use planning and critical facilities or insurance risk evaluation purposes, a higher spatial resolution is also desirable. during the last years, a number of major earthquakes and mega-earthquakes occurred at plate boundspecial issue: earthquake geology aries, which were in most of the cases oversized in relation to the data provided by the historical and instrumental seismicity. the haiti mw 7.0 (haiti, january 2010), maule mw 8.8 (chile, may 2010), and tōhoku mw 9.0 ( japan, march 2011) events caused widespread devastation and claimed hundreds of thousands of lives. beside these mega-quakes, several moderate events had surprisingly dramatic consequences. the l’aquila, mw 6.3 (italy, april 2009), christchurch mw 6.3 (new zealand, february 2011), lorca mw 5.1 (spain, may 2011), emilia-romagna mw 5.9 and mw 5.7 (italy, may 2012), and tabriz mw 6.4 and mw 6.3 (iran, august 2012) events showed strong ground motions and caused rockfalls, landslides, liquefaction, and other types of eees. combined with relatively low building standards, these moderate quakes could also pose a high local hazard, due also to their high recurrence rate. however, the christchurch example showed that even good to excellent building standards cannot prevent a large number of damages. an unfavorable local setting like the presence of a deep sediment basin or geometric peculiarities may lead to an amplification of the seismic waves, resulting in extraordinary strong ground motions [smyrou et al. 2011]. moderate events have illustrated that ground motion may not be the most important agent of destruction. earthquake environmental effects like liquefaction, landslides and rockfalls may locally cause far more damage, and should be taken into account in any comprehensive assessment of seismic and associated hazards. this makes clear that the use of macroseismic scales that depend almost solely on damage to man-made structures may obscure the consequences of an earthquake event and can lead to an underestimation. overall, traditional intensities from which attenuation laws are extracted are based on human parameters. therefore, when using the effects on humans and human environments to assess the macroseismic intensity, intensity will tend to reflect mainly the economic development and the cultural setting of the area that experienced the earthquake, instead of its “strength” [serva 1994]. on the other hand, earthquake environmental effects are not influenced by human parameters. the environmental seismic intensity scale esi 2007, introduced by inqua, incorporates the advances and achievements of earthquake geology and evaluates earthquake size and the epicentral intensity solely from the earthquake environmental effects [michetti et al. 2007, reicherter et al. 2009]. the esi 2007 has been tested in several countries and tectonic settings. there are already more than 25 publications regarding the scale and more than 100 tested events. the scale has been tested both in moderate (e.g., 1998 mw=5.6 slovenia; gosar [2012]) and catastrophic events (e.g., mw=7.9 2008 wenchuan earthquake, china; lekkas [2010]), demonstrating its increasing value and accuracy towards the highest levels of the scale, where traditional scales saturate [michetti et al. 2004]. the comparability of recent quakes to historical and paleoevents is also not guaranteed, but the esi 2007 allows the comparison among future, recent and historical earthquakes since the building environment is not considered. therefore, the use of the esi 2007 scale, based on eees, may help to overcome this problem. currently there is an increasing number of studies that compare the esi 2007 scale with traditional macroseismic scales for recent events [e.g., papathanassiou and pavlides 2007, porfido et al. 2007, ota et al. 2009]. such a comparison can offer valuable info regarding the vulnerability of the buildings [e.g., guerrieri et al. 2009]. once more and more data are available it will be possible to identify cases in which the two types of scales lead to similar results and settings with differing intensity estimations. a re-appraisal of historical and recent earthquakes so as to constrain the esi 2007 scale and the extraction of esi-based attenuation laws may prove beneficial for seismic hazard assessment by reducing the large uncertainty implied in the attenuation laws and eventually in seismic hazard maps [papanikolaou 2011]. as more data from recent and historical events are gathered, the compilation of an esi 2007 attenuation relationship should be one of the future goals for seismic hazard assessment. overall, it is interesting to note that the concept of the esi 2007 has now been followed in earthquake archaeological effects (eae) [rodríguez-pascua et al. 2011] and the newly introduced integrated tsunami intensity scale (itis-2012) [lekkas et al. 2013]. transporting this knowledge to paleoseismological studies is important for detailed insights into the earthquake history and the earthquake behavior of the faults that pose a hazard to society. there is an ongoing debate about how faults do evolve over time and how they react on stress changes in their local and regional seismotectonic context. it is not clear if the model of characteristic earthquakes is valid, invalid at all or only true for certain areas and settings [e.g., stein et al. 2012]. for the incorporation of paleoseismological data into seismic hazard assessments these questions need to be answered. the studies presented in this special volume contribute to this goal. 2. the work included in this volume after the very successful 1st workshop on earthquake archaeology and paleoseismology held in the archaeological site of baelo claudia (spain, 2009), the grützner et al. 2 3 inqua focus group on paleoseismology and active tectonics decided to support this joint initiative with the igcp-567 “earthquake archaeolog y”. the second joint meeting was held in the gulf of corinth (greece), a tectonically active area within the africaeurasia collision zone and located in the origins of the pioneer’s works on archeoseismology. aim of the meeting was to stimulate the already emerging comparative discussion about earthquake environmental effects and earthquake archeoseismological effects in order to elaborate comprehensive classifications for future cataloguing and parametrization of ancient earthquakes and paleoearthquakes. another focus was to discuss how the eee-based esi 2007 macroseismic intensity scale can be compared with traditional macroseismic scales that rather focus on man-made structures for earthquake intensity assessment. in this workshop a multidisciplinary and crossdisciplinary program was presented, illustrating that there is an urgent necessity to share the knowledge and objectives among geologists, seismologists, geodesists, archaeologists and civil engineers in order to improve seismic hazard assessments and analyses in near future. this special volume “earthquake geology: science, society and critical facilities” is an outcome of the corinth 2011 conference. it includes 14 papers, mainly by early career researchers (ecrs), that deal with paleoseismology, active tectonics, tectonic geomorphology and macroseismic intensity scales (esi 2007, ems 98). geographically, the papers clearly focus on southern europe, or more precise, on the area affected by the stress resulting from the african-european collision (figure 1). more than half of the studies were conducted on the balkan peninsula, two each in iberia and at the azores archipelago, one in italy and one in slovenia. alexopoulos et al. combined near surface geophysical investigations with remote sensing data at the spectacular meteora site in central greece, where tectonic deformation is strongly influencing groundwater flow. the authors identified tectonic discontinuities on the surface, related them with subsurface electrically conductive zones within the conglomerate strata that are interpreted as groundwater flow paths and demonstrated how future seismic events might interrupt these spatially sensitive hydraulic connections. this work is especially important for understanding the physics behind the hydrological anomalies that are classified as eees in hard rocks. a large sediment core from the gulf of corinth was studied by campos et al. they found a series of homogenites and turbidites both in lacustrine and marine environments in this rapidly expanding rift in central greece. 38 turbidites are interpreted to have possible seismic origin, and recurrence intervals for the events were determined. this study presents rare offshore data which can be correlated to the on-shore investigations of the active faults in that area, which belong to the fastest moving tectonic structures in europe. carmo et al. investigated escarpments on são miguel island, azores, portugal. the authors trenched across these structures and found evidence for the neotectonic activity of the altiprado faults. they identified a number of surface rupturing events, some of which occurred in historical times. a comparison with available earthquake geology: science, society and critical facilities figure 1. the study sites of the papers included in this special volume. data from written sources led them to assign one event to the deadly earthquake from 1522, which reached intensities x in its epicentral area. according to the trenching data, the quakes on são miguel island might typically reach magnitudes between 5.7 and 6.7. this study sheds light on the seismic hazard of a region which is normally thought to be dominated by volcanic hazards. despite the fact that no or only minor seismic activity is ascribed to sw portugal, the region yields a moderate seismic hazard due to offshore seismic sources. figueiredo et al. investigated pleistocene marine terraces along the coast of europe’s southwesternmost tip in order to determine the rate of tectonic uplift and to find out whether major offshore events like the 1755 lisbon earthquake led to significant morphological changes onshore. by mapping and dating several terraces and considering the sea level fluctuations, the authors were able to determine a long-term uplift rate of 0.1-0.2 mm/yr. the relation of the low-angle altotiberina fault in central italy to other faults in this area is highly debated. finocchio et al. use a 2d finite-element model to investigate the interseismic deformation of this structure and compared the results with geodetic velocities, stress orientations and geological data. this paper presents important insights into the structure of this major fault zone and its rheology in a regional context. the data help to understand fault interaction and the locked state of faults. the athens basin in greece is not only home to millions of people, but is also surrounded by several active normal faults. the geometry and the activity of these faults are thought to be controlled by a large detachment zone, separating structures with different strike and different slip rates. foumelis et al. are able to show by the means of a gps-derived velocity field that the detachment, despite being inactive since pliocene times, clearly controls the strain in the area. this is the first study to quantify this effect on the deformation pattern in this area. the seismic hazard of the peloponnesus peninsula in greece is not only related to the offshore hellenic arc, but a number of strong historical events are proven onshore, too. the 1986 kalamata earthquake caused 20 fatalities and led to the destruction of numerous houses. the event also left its imprints in the landscape fountoulis and mavroulis reviewed the earthquake environmental effects of the quake using the esi 2007 macroseismic intensity scale. they conclude on a maximum intensity of ix and found that this value is in agreement with other macroseismic scales. furthermore, the authors related their observations to the active faults of the study area and concluded on the use of the esi 2007 scale for future seismic hazard assessments. guzman et al. investigated marine river terraces in southern albania. they aimed on the determination of vertical slip rates of the main active structures in the dinaric-hellenic alpine fold belt. geomorphological mapping and regional correlation of terraces revealed shifts in the incision rates of rivers and the authors were able to determine slip rates for eight faults during the last 19 kyrs. hipólito et al. investigated outcrops on graciosa island, azores, portugal, in order to analyze how the seismicity of this volcanic island fits into the regional stress regime. the authors identified two main fault structures on the island which are not compatible with a single stress regime and, thus, postulate alternating stress fields. these fields are dominated by the spreading of the atlantic and an interplate shear zone and the results indicate that the azores islands undergo time-variable deformation in a wide zone. this paper is a great example for how local outcrop geology, regional deformation analysis and seismicity data can help to reveal the geodynamic context of a study area in a complex tectonic setting. the seismic landscape of lake ohrid in fyrom and albania is analyzed by hoffmann. she conducted topographic profiling across prominent scarps to investigate the slip rates of the main faults and the tectonic evolution of the basin. her results show that the idea of all these scarps being post-glacial comes along with geometrical problems some scarps are much higher than they could possibly be and scarp heights vary significantly across single faults. the author discusses possible explanations and suggests that gravitational movements, among other effects, must be taken into account when deducing slip rates from post-glacial scarp heights. the granada basin in southern spain is one of the tectonically most active regions on the iberian peninsula, characterized mainly by normal faults. historic seismicity is well-documented, but the activity of some of the most important faults remains unclear. hürtgen et al. conducted field work and gis analyses for a morphotectonic study of the padul-nigüelas fault zone. this approach takes the concept of seismic landscape [michetti et al. 2005] to the quantitative level. they found a differentiation in the state of activity along the grützner et al. 4 5 fault zone and discuss the importance of the input parameters for such a type of analysis. the slip rates and offsets of thrust faults are often hard to be directly investigated in the field. scarps in quaternary sediments close to the slovenian capital were ascribed to the movement of thrust faults by jamšek rupnik et al. the authors dated sediments with osl and isl and were able to determine slip rates of these faults during the last 133 ka. the data reveal maximum expectable magnitudes of around 6.0-6.5, which clearly illustrates the contribution of these faults to the regional seismic hazard in this densely populated and economically important area. on june 8, 2008, a moderate strike slip earthquake occurred in andravida, nw peloponnesus, greece. two papers investigate the environmental effects of this event. mavroulis et al. applied the esi 2007 scale and compared their findings to the ems-98 scale. no primary earthquake environmental effects were recorded, but secondary effects yield intensities of viiiix. while in cases where intensity viii was recorded both scales agree, they strongly differ elsewhere. generally, esi intensities are found to be one or two degrees lower than ems intensities. the authors conclude that not only the building standards, but also the local geology plays a major role in the damage distribution. pavlides et al. focused on mass movements and liquefaction triggered by the same quake on the peloponnesus. they present data from a post-event survey and mapped the liquefaction-prone areas. a rockfall hazard zonation for the andravida region was developed using the shadow-angle method. both studies show how earthquake damage and environmental effects are controlled by local soil and rock properties and that directional effects play an important role in their distribution. this volume is dedicated to the memory of the associate professor ioannis fountoulis (1954-2013) who passed away too early. prof. fountoulis was a structural geologist, member of the national and kapodistrian university of athens, working since the 80s in neotectonics. he participated in several inqua meetings and workshops, including the corinth 2011. he contributed in 3 separate papers within this volume, but sadly passed away before its publication. acknowledgements. the authors wish to thank the editorial office of annals of geophysics for their support. we are grateful to all participants from 26 countries of the 2nd inqua-igcp 567 international workshop on active tectonics, earthquake geology, archaeology and engineering in corinth, greece, from september 19-24, 2011. references audemard, f., and a.m. michetti (2011). geological criteria for evaluating seismicity revisited: forty years of paleoseismic investigations and the natural 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(1996). irregular recurrence of large earthquakes: an analysis of historic and paleoseismic catalogs, journal of geophysical research, 101, 5739-5749. gosar, a. (2012). application of environmental seismic intensity scale (esi 2007) to krn mountains 1998 mw=5.6 earthquake (nw slovenia) with emphasis on rockfalls, natural hazards and earth system sciences, 12, 1659-1670. guerrieri, l., a.m. blumetti, e. esposito, a.m. michetti, s. porfido, l. serva, e. tondi and e. vittori (2009). capable faulting, environmental effects and seismic landscape in the area affected by the 1997 umbria–marche (central italy) seismic sequence, tectonophysics, 476, 269-281. lekkas, e. (2010). the 12 may 2008 mw=7.9 wenchuan earthquake, china. macroseismic intensity assessment using the ems 1998 and esi 2007 scales and their correlation with the geological structure, bulletin of the seismological society of america, 100, 2791-2804. lekkas, e.l., e. andreadakis, i. kostaki and e. kapourani (2013). a proposal for a new integrated tsunami intensity scale (itis-2012), bulletin of the seismological society of america, 103, 1493-1502. machette, m.n. (2000). active, capable, and potentially active faults a paleoseismic perspective, journal of geodynamics, 29, 387-392. mccalpin, j. (2009). paleoseismology, 2nd edition, international geophysics series vol. 95, san diego academic press, elsevier, 613 pp. michetti, a.m., e. esposito, a. gurpinar, b. mohammadioun j. mohammadioun, s. porfido, e. rogozhin, l. serva, r. tatevossian, e. vittori, f. audemard, v. comerci, s. marco, j. mccaplin and n.a. morner (2004). the inqua scale: an innovative approach for assessing earthquake intensities based on seismically induced ground effects in natural environment, memorie descrittive della carta geologica d’italia, sp. paper, 67, 118. michetti, a.m., f.a. audemard m., and s. marco (2005). future trends in paleoseismology: integrated study of the seismic landscape as a vital tool in seismic hazard analyses, tectonophysics, 408 (1), 3-21. earthquake geology: science, society and critical facilities michetti, a.m., e. esposito, l. guerrieri, s. porfido, l. serva, r. tatevossian, e. vittori, f. audemard, t. azuma, j. clague, v. comerci, a. gürpinar, j. mccalpin, b. mohammadioun, n.a. mörner, y. ota and e. roghozin (2007). environmental seismic intensity scale 2007 esi 2007, in: l. guerrieri and e. vittori (eds.), intensity scale esi 2007, memorie descrittive della carta geologica d’italia, 74, 7-54. ota, y., t. azuma and y.-n.n. lin (2009). application of the inqua environmental seismic intensity scale to recent earthquakes in japan and tawan, in: k. reicherter, a.m. michetti and p.g. silva (eds.), paleoseismology: historical and prehistorical records of earthquake ground effects for seismic hazard assessment, the geological society of london, special publication 316, 55-71. pace, b., l. peruzza and f. visini (2010). lassci2009.2: layered earthquake rupture forecast model for central italy, submitted to the csep project, annals of geophysics, 53 (3), 85-97. papanikolaou, i.d. (2011). uncertainty in intensity assignment and attenuation relationships: how seismic hazard maps can benefit from the implementation of the environmental seismic intensity scale (esi 2007), quaternary international, 242, 42-51. papanikolaou, i.d., g. roberts, g. deligiannakis, a. sakellariou and e. vassilakis (2013). the sparta fault, southern greece: from segmentation and tectonic geomorphology to seismic hazard mapping and time dependent probabilities, tectonophysics, 597-598, 85-105. papathanassiou, g., and s. pavlides (2007). using the inqua scale for the assessment of intensity. case study on the 2003 lefkada (ionian islands), greece earthquake, quaternary international, 173-174, 4-14. papazachos, b.c., p.e. comninakis, g.f. karakaisis, b.g. karakostas, c.a. papaioannou, c.b. papazachos and e.m. scordilis (2000). a catalogue of earthquakes in greece and surrounding area for the period 550bc-1999, publ. geophysocal laboratory, university of thessaloniki. porfido, s., e. esposito, l. guerrieri, e. vittori, g. tranfaglia and r. pece (2007). seismically induced ground effects of the 1805, 1930 and 1980 earthquakes in the southern apennines, italy, bollettino della società geologica italiana, 126, 333-346. reicherter, k., a.m. michetti and p. silva barroso (2009). palaeoseismology: historical and prehistorical records of earthquake ground effects for seismic hazard assessment, in: k. reicherter, a.m. michetti and p.g. silva (eds.), paleoseismology: historical and prehistorical records of earthquake ground effects for seismic hazard assessment, the geological society of london, special publication 316, 1-10. roberts, g.p., p. cowie, i. papanikolaou and a.m. michetti (2004). fault scaling relationships, deformation rates and seismic hazards: an example from the lazio–abruzzo apennines, central italy, journal of structural geology, 26, 377-398. rodríguez-pascua, m.a., r. pérez-lópez, j.l. ginerrobles, p.g. silva, v.h. garduño-monroy and k. reicherter (2011). a comprehensive classification of earthquake archaeological effects (eae) in archaeoseismology: application to ancient remains of roman and mesoamerican cultures, quaternary international, 242, 20-30. scholz, c.h. (2002). the mechanics of earthquakes and faulting, cambridge university press, 471 pp. serva, l. (1994). ground effects in the intensity scales, terra nova, 6, 414-416. smyrou, e., p. tasiopoulou, ï.e. bal and g. gazetas (2011). ground motions versus geotechnical and structural damage in the february 2011 christchurch earthquake, seismological research letters, 82 (6), 882-892. stein, s., r.j. geller and m. liu (2012). why earthquake hazard maps often fail and what to do about it, tectonophysics, 562-563, 1-25. valensise, g., and d. pantosti (2001). the investigation of potential earthquake sources in peninsular italy: a review, journal of seismology, 5, 287-306. yeats, r.s., and c.s. prentice (1996). introduction to special section: paleoseismology, journal of geophysical research, 101, 5847-5853. christoph grützner, rwth aachen university, institute for neotectonics and natural hazards, aachen, germany; email: c.gruetzner@nug.rwth-aachen.de. salvatore barba, istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy; email: salvatore.barba@ingv.it. ioannis papanikolaou, agricultural university of athens, laboratory mineralogy geology, athens, greece; email: i.pap@aua.gr. raul pérez-lópez, igme instituto geologico y minero de españa, madrid, spain; email: r.perez@igme.es. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. grützner et al. 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 simple smoothed seismicity earthquake forecasts for italy j. douglas zechar1,2,*and thomas h. jordan3 1 eth zurich, swiss seismological service, zurich, switzerland 2 lamont-doherty earth observatory, columbia university, palisades, ny, usa 3 university of southern california, department of earth sciences, los angeles, usa annals of geophysics, 53, 3, 2010; doi: 10.4401/ag-4845 abstract several earthquake forecast experiments in italy have been initiated within the european testing center of the global collaboratory for the study of earthquake predictability. in preparation for these experiments, we developed space-rate-magnitude forecasts based on a simple model that incorporates spatial clustering of seismicity. this model, which we call the simple smoothed seismicity model (triples), has a minimal number of free parameters and is based on very few assumptions; therefore, it can be considered as a model of least information with which others can be compared. the fundamental triples parameter controls the spatial extent of the smoothing, and we selected its value based on an optimization procedure that was applied to retrospective forecast experiments. in this article, we present the motivation for developing triples and describe the construction of forecasts for italian seismicity. we also discuss the research questions that remain to be answered with respect to triples and, more generally, the smoothed seismicity approach to earthquake forecasting. introduction evaluation of earthquake forecast experiments requires careful consideration of the appropriate reference models. for example, a forecast that incorporates some form of spatial clustering is likely to perform significantly better than a reference forecast that is based on a spatially uniform seismicity model. therefore, a reference model should capture the clustering that is observed in seismicity [michael 1997]. one straightforward approach to incorporate this clustering is to smooth past seismicity, thereby allowing each past earthquake to provide some smoothed contribution to an estimate of the seismic rate density. kernel smoothing is a commonly used technique for density estimation, and the objective of forecast experiments in the collaboratory for the study for earthquake predictability (csep) – namely, to accurately forecast the space-rate-magnitude distribution of earthquakes – is similar to the estimation of a predictive density of the seismicity rate. many previous studies have applied smoothed seismicity approaches that have incorporated various levels of complexity [e.g., kagan and jackson 1994, frankel 1995, jackson and kagan 1999, kagan and jackson 2000, stirling et al. 2002, stock and smith 2002a, stock and smith 2002b, helmstetter et al. 2006, helmstetter et al. 2007, kagan et al. 2007, petersen et al. 2008, werner et al. submitted bssa]. several of these analyses were made as part of regional and national seismic hazard mapping projects, and the global earthquake model (http://www.globalquakemodel.org) will probably include a smoothed component. in this study, we considered a very basic model, which we call the simple smoothed seismicity model (triples), that applies an isotropic gaussian smoothing to the locations of past earthquakes to forecast the density of future seismicity. triples is characterized by one parameter: the smoothing distance, v, which is equivalent to the standard deviation of a one-dimensional gaussian distribution. zechar and jordan [2008] introduced a performance metric, the area skill score, which can be used for evaluating earthquake forecasts. when an area skill score test is performed, a reference model is explicitly specified in terms of an alarm function, which is a general format for ranking regions of space/time/magnitude in terms of the probability that a future target earthquake will occur in a given region. a wide range of earthquake forecasts, including all of those participating in csep italy experiments, can be interpreted in terms of alarm functions, and this interpretation allows for rigorous comparative testing. given that there is little convincing evidence that complex prediction algorithms substantially outperform very crude smoothed seismicity models, we decided to essentially invert the testing problem: rather than building increasingly complex models to forecast seismicity, can we optimize a simple smoothed seismicity model? in this study, we used retrospective forecast experiments to optimize the smoothing distance with respect to the area skill score. article history received february 18, 2010; accepted july 23, 2010. subject classification: smoothed seismicity, earthquake predictability, forecast optimization, area skill score. 99 using triples, we constructed and submitted forecasts for the csep italy five-year and ten-year experiments, and we submitted codes for inclusion in the three-month experiment [schorlemmer et al. 2010]. because, at the time of writing, the three-month forecasts have not yet been computed, we emphasize here the fiveand ten-year forecasts, although the methods are the same for each experiment. data in preparation for the csep italy experiment, two earthquake catalogs were analyzed and summarized for the participants: the parametric catalog of italian earthquakes (catalogo parametrico dei terremoti italiani, cpti) and the catalog of italian seismicity (catalogo della sismicità italiana, csi) [schorlemmer et al. 2010]. the cpti contains information on earthquakes that occurred in italy between 1901 and 2006. (all date ranges in this section are inclusive.) the earthquake magnitudes are specified in terms of the moment magnitudes, and it is believed that the cpti contains all of the earthquakes with a moment magnitude (mw) of at least 4.8 [schorlemmer et al. 2010]. the csi reports on the earthquakes in italy that occurred from 1981 to 2002. the csi uses a local magnitude scale (ml), and is believed to contain all events with a ml of at least 2.8, although the level of completeness might have changed substantially as the network evolved [schorlemmer et al. 2010]. the cpti and csi are not the only catalogs that report earthquakes in italy. for example, the earthquake catalog of switzerland (ecos) [faeh et al. 2003] provides information for events that happened within 200-300 km of the border between italy and switzerland; this catalog is thought to be as complete as the csi (j. woessner, personal communication). the fundamental hypothesis that motivates a smoothed seismicity approach to earthquake forecasting is that the locations of past earthquakes provide information about the locations of future earthquakes – in other words, «the future will most of all resemble the past» [marcus 2006]. however, it is not obvious that all past earthquakes provide equal predictive information, and it has been suggested that the inclusion of smaller earthquakes increases the predictive skill of earthquake forecasts [helmstetter et al. 2006, helmstetter et al. 2007, werner et al. submitted bssa]. in the csep italy experiment, there is a clear trade-off between using the csi and the cpti: the csi includes smaller events, but only over a relatively limited time period, and while the cpti provides information on events over a much longer time period, it only includes the very largest events during this time. rather than arbitrarily choosing one catalog as the most appropriate for smoothing, we generated three forecasts, each of which was based on a different catalog: 1) the csi; 2) the cpti; and 3) a hybrid catalog that comprises the cpti events from 1901 to 1980 and from april 16, 2005 to 2006, plus the csi events from 1981 to 2002, plus the ecos events from 2003 to april 15, 2005. we note that the ecos does not cover the entire csep italy testing region, and therefore its use might have caused some slight bias towards higher apparent seismicity rates in northern italy. however, we only used the ecos catalog for 27.5 months of data in a combined catalog that covers 1,272 months, and the potential bias would not affect the entire testing region. additionally, this bias is likely to be much smaller than the poisson forecast uncertainty imposed on each bin. we filtered the catalogs to remove earthquakes outside the latitude, longitude, and depth range of 34.3˚ n to 49.2˚ n, 4.3˚ e to 20.9˚ e, and 0 km to 30 km, respectively, although we retained events with unconstrained depths (of which there are many in the cpti). this region contains the csep italy testing region and extends beyond it by at least ~1˚ in latitude/longitude to reduce edge effects when smoothing. (see schorlemmer et al. [2010] for the csep italy grid definition.) to reduce bias caused by fluctuations in data quality (in space and time), we further filtered the catalogs to remove all of the earthquakes below the above-mentioned respective completeness magnitudes. after the initial submission of our forecasts, and based on the analysis of werner et al. [2010], it became obvious that the mwvalues in the cpti had not been converted to the mlvalues required for evaluation with respect to the csi. this error for the forecasts based on the cpti and the hybrid catalog was consequently corrected, and our forecasts were resubmitted; the magnitude conversion suggested by mps working group [2004] was used, which is based on regression analysis between these cpti and csi magnitudes: ml= (mw − 1.145) / 0.812. in this article, we present the results for these corrected forecasts, rather than those in the initial submissions. methods perhaps the simplest method for smoothing seismicity is to discretize the region of interest and to count the number of earthquakes that have occurred in each cell, allowing each point-source epicenter to be smoothed over the cell into which it falls (rundle et al. [2002] called this «the relativeintensity method»). however, the results of such smoothing are not stable with respect to changes of the discretization parameters, i.e., the cell size and grid alignment, and the smoothing itself is anisotropic and un-physical. for example, epicenters occurring in opposite corners of a cell are treated as if they both occurred in the center of the cell. to minimize these effects, and to relax the constraint that each epicenter contributes only to the cell in which it occurs, we smoothed the earthquakes using a continuous kernel function that allowed for a wider region of influence. for triples, we chose a two-dimensional isotropic gaussian smoothing kernel governed by a single parameter. the primary computational task was to calculate the contribution of an earthquake epicenter at (xeqk, yeqk) to a grid cell with bounds x1, x2, y1 and y2. in general, for a bivariate smoothing kernel triple-s forecasts for italy 100 101 k (x, y), this contribution takes the form of equation (1): (1) gaussian kernels have been used in several previous studies of smoothed seismicity [stock and smith 2002a, stock and smith 2002b, helmstetter et al. 2006, helmstetter et al. 2007], and they form the basis for modeling «background» seismicity in the national seismic hazard maps of the united states of america [petersen et al. 2008] and new zealand [stirling et al. 2002]. frankel [1995] first suggested gaussian smoothing for seismic hazard, although he formulated the kernel in terms of a correlation distance c that differs from the pure gaussian form, so we here explicitly describe our formulation. we denote our isotropic gaussian kernel, kv, as in equation (2): (2) where v is the smoothing distance. upon substituting equation (2) into equation (1) and solving the integral, we find the gaussian contribution of an epicenter to a cell, as equation (3): (3) analytically, the kernel described by equation (2) extends infinitely in both dimensions. however, within the precision provided by modern computers, erf(a) ~ sgn(a) when |a|> 5.92. therefore, any cell satisfying the conditions of equation (4) will cause equation (3) to equal zero – i.e., the cell will receive no contribution from the epicenter. (4) to reduce the computation time, we determined which cells would obtain some contribution from the epicenter rather than evaluating equation (3) everywhere in the gridded region. a cell with bounds x1, x2, y1, and y2 will obtain some contribution from an epicenter at (xeqk, yeqk) only if the conditions of equation (5) are met: (5) particularly for small v and large catalogs, the use of these conditions provides for a substantial reduction in the computing time. rather than arbitrarily choosing a fixed value of v for triples forecasts, we used retrospective experiments and the optimization procedure outlined by zechar [2008]. for each experiment, we considered the csep italy testing region with 0.1˚× 0.1˚ latitude by longitude cell sizes, and we explored the smoothing that resulted from each of the smoothing distance values given in equation (6): (6) to build the five-year forecasts, we designated the most recent five years as the target period, and the earthquakes in this period with magnitudes of at least 4.95 were the target events. for the ten-year forecasts, we used the earthquakes with magnitudes of at least 4.95 in the most recent ten years as the target earthquakes. we smoothed the locations of all events that occurred before the target period, and we repeated this process for each candidate value of v in . this smoothing process resulted in several estimates of seismic density, each of which was an alarm function. these alarm functions were used as forecasts of the target events, and for each candidate value of v, the average misfit of the area skill score was computed. these were determined by selecting one of the alarm functions as the reference model and computing the area skill score for all of the alarm functions with respect to this reference. because no reference model was preferred a priori, we used a round-robin procedure, and therefore each candidate value of v had its own average misfit. for additional details of the average misfit calculation, we refer readers to zechar [2008, p. 94]. we considered the smoothing distance that yields the minimum average misfit, v*, to be optimal. to construct the prospective forecasts for the csep italy experiment, we smoothed the locations of all of the events in the complete catalog and thereby obtained a spatial distribution. because these csep italy experiments required space-rate-magnitude forecasts, the spatial distribution was scaled such that the overall forecast rate matched the average rate of target events in the catalog, and we used an untapered gutenbergrichter distribution with a b-value of unity to form the magnitude distribution. results figure 1 shows the results of the triples retrospective optimization procedure in terms of the average misfit as a function of the smoothing distance. note that in all of the plots the average misfit values form a convex set (i.e., the local minimum is also the absolute minimum). we interpret this phenomenon as representing a transition from too little smoothing at small smoothing distances to too much smoothing at large smoothing distances. it is not clear whether the optimal smoothing distance can be related to any triple-s forecasts for italy ( , , , , , ) ( , ) .k x y x x y y k x y dxdy1 2 1 2eqk eqk eqk eqk x x y y 1 2 1 2 = ## ( , ) 2 1 exp 2k x y x y 2 2 2 2 = + rv v -v c m 5.92 2 and 5.92 2 or 5.92 2 and 5.92 2 x x x x y y y y 2 2 eqk eqk eqk eqk 1 1 2 2 2 2 ; ; ; ; ; ; ; ; v v v v 5.92 2 5.92 2 5.92 2 5.92 2 x x x x y y y y 1 2 1 2 eqk eqk eqk eqk 1 2 1 2 v v v v z [ \ ] ]] ] ] 5, 10, 15, 20, 25, 50, 75, 100, 200 km= " ,/ ( , , , , , ) 4 1 erf 2 erf 2 erf 2 erf 2 k x y x x y y x x x x y y y y 1 2 1 2 2 1 2 eqk eqk eqk eqk eqk eqk 1 = v v v v v ; ; e e c c c c m m m m / particular physical or geological characteristic; it probably depends upon the minimum input magnitude, the minimum target magnitude, the particular target period, and the configuration of the seismic network, among other characteristics [zechar 2008]. certainly, if the spatial seismicity distribution were stationary and the catalog sampled the distribution fully, v* would be infinitesimal, but it is unlikely that both of those conditions were met in this study. in constructing triples space-rate-magnitude forecasts, we computed the historical average rate of target earthquakes in the testing region for each catalog: 1.74 events per year in the cpti, 1.32 events per year in the csi, and 1.69 events per year in the hybrid catalog. note that these rates are for the rectangular prism region described in the data section; therefore, the rates in the csep italy testing region – and thus our overall annual forecast rates – are slightly lower. figure 2 shows a map view of the corrected, submitted triples five-year and ten-year forecasts for the csep italy experiment. each five-year forecast is much smoother than its ten-year counterpart because of the larger values of v* in the retrospective five-year experiments. the large discrepancies of the v* values are directly related to the samples upon which each forecast was optimized. in the electronic supplement (http://www.annalsofgeophysics.eu/index.php/annals/rt/ suppfiles/4845/0), we include map-view animations of these optimizations; these animations show that the target earthquakes of the most recent five years were substantially less clustered near regions of past high seismicity than the target earthquakes of the most recent ten years. therefore, larger smoothing distances were optimal for the five-year experiments. to check that these discrepancies were not caused by the area skill score metric itself, the optimization experiments were repeated using the spatial likelihood of zechar et al. [2010], and similar discrepancies were found; this provides further support for our claim that the differences are due to the particular samples. overall, the forecasts are similar, and all of the forecasts suggest high rates throughout central italy and, to a lesser extent, in sicily. nevertheless, we see large differences between the ten-year forecasts in northeast sicily, and the forecasts based on the csi tend to lack some of the high seismicity rates in the northeast region of the italian mainland. the initial three-month forecasts cannot be shown because they had not been computed at the time of writing; the automated triples codes will generate the three-month forecasts at the beginning of the first experiment, and the forecasts and associated evaluations will be archived at http://www.cseptesting.org. for qualitative and quantitative comparisons of triples forecasts with other forecasts in the csep italy experiments, the reader is referred to the retrospective analyses by werner et al. [2010], bearing in mind that the uncorrected triples forecasts were considered in that study. discussion and conclusion during the csep italy experiments, the triples forecasts described in this article will be compared with several other forecasts, many of which derive from more complex models that incorporate multiple hypotheses regarding earthquake triple-s forecasts for italy 102 figure 1.plots of average misfit, |, as a function of kernel bandwidth, v, for the retrospective five-year and ten-year tests using the cpti, csi, and hybrid catalogs, as indicated. the filled square in each plot shows the minimum average misfit, with the corresponding optimal smoothing bandwidth, v*, also given. 103 occurrence [schorlemmer et al. 2010, and references therein]. because triples is relatively simple and includes fairly standard assumptions, only a few explanations are possible if another forecast demonstrates greater predictive skill than triples in the prospective experiments: 1) the superior forecast more accurately represents spatial clustering of seismicity; 2) the superior forecast ignores spatial clustering but incorporates information that is more predictive than spatial clustering; or 3) the superior forecast incorporates predictive information in addition to spatial clustering. certainly, these are generalized explanations and the details of the specific experiments, forecasts and observed target earthquake distributions will require careful analysis. but the simplicity of triples is advantageous because the success or failure of the resulting forecasts can be understood clearly, without simultaneous testing of multiple dependent hypotheses obscuring the interpretation. despite our best efforts to construct simple forecasts, we were forced to make a number of modeling decisions that can be reconsidered in future studies. chief among these was the choice to avoid declustering, a choice that was motivated by the desire not to convolve the effects of a given declustering algorithm and the effects of smoothing. others have suggested that it is appropriate to decluster the input catalog and smooth the remaining events, which are thought of as «background earthquakes» [e.g., helmstetter et al. 2007, werner et al. submitted bssa]. nevertheless, the details of declustering, and particularly the preferred model parameter values, remain somewhat nebulous [van stiphout et al. submitted bssa]. we also decided to disregard epicenter uncertainty, based on our reasoning that the discretization of the testing region makes location errors negligible. moreover, we chose the set of candidate smoothing distances arbitrarily; while the minimum distance of interest is related to the spatial discretization of the forecast format, the set of candidate values can be supplemented and explored iteratively. along the same lines, rather than using the area skill score and average misfit statistic, the optimization procedure could be based on the spatial joint log-likelihood test of zechar et al. [2010]. in constructing our hybrid catalog and the corresponding forecasts, data of varying quality was mixed (at least in terms of completeness), and in so doing we might have introduced a bias that would lend more weight to recent smaller events; this should also be reconsidered in future experiments. the outstanding issues of triples are exemplary of the more general notion that many issues regarding smoothed seismicity approaches to earthquake forecasting and seismic hazard assessment are unresolved. in this study, we concentrated on a single value as an optimal spatial smoothing distance, but broader questions remain, such as: triple-s forecasts for italy figure 2. map views of the corrected submitted five-year and ten-year triples forecasts using the cpti, csi, and hybrid catalogs, as indicated. these are the space-rate representations of the forecasts, as they have been summed over the magnitude bins, and the unit of measure is the expected number of earthquakes. should all earthquakes be smoothed? should the contribution of an earthquake depend on its size or its origin time? what is the effect of a gaussian kernel, as opposed to some other functional form? should large earthquakes be smoothed using an anisotropic kernel, as kagan and jackson [1994] suggested? how is such anisotropy best characterized? because seismicity smoothing is used in so many applications – including wideranging and long-term hazard assessments that affect building codes and insurance rates – these issues are highly relevant. data and sharing resources most of the catalog data used in this study are freely available from the csep italy website (http://cseptesting. org/regions/italy). the ecos data used in this study are available upon request from the authors. an electronic supplement to this article is available at http://www.annalsofgeophysics.eu/index.php/annals/rt/ suppfiles/4845/0. acknowledgements. the authors thank timothy r. bentley and gene pantry for many delicious breakfasts and inspiring the name of the model, fabian euchner for assistance in preparing the figures, max werner for providing results in advance of publication, and yan kagan, danijel schorlemmer and an anonymous reviewer for carefully reading an early draft of the manuscript and providing constructive comments. the maps were created with the generic mapping tools software [wessel and smith 1998]. references faeh, d., d. giardini, f. bay, f. bernardi, j. braunmiller, n. deichmann, m. furrer, l. gantner, m. gisler, d. isenegger, m. j. jimenez, p. kaestli, r. koglin, v. masciadri, m. rutz, c. scheidegger, r. schibler, d. schorlemmer, g. schwarzzanetti, s. steimen, s. sellami, s. wiemer and j. woessner (2003). earthquake catalog of switzerland and the related macroseismic database, ecolog. geol. helv., 96, 219-236. frankel, a.d. 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(2006). the shape of things to come: prophecy and the american voice, new york, 324 pp. michael, a.j. (1997). testing prediction methods: earthquake clustering versus the poisson model, geophys. res. lett., 24, 1891-1894. mps working group (2004). redazione della mappa di pericolosità sismica prevista dall’ordinanza pcm 3274 del 20 marzo 2003, rapporto conclusivo per il dipartimento della protezione civile, ingv, milano-roma, april 2004 (mps04), 65 pp. + 5 appendices; http://zonesismiche. mi.ingv.it. petersen, m.d., a.d. frankel, s.c. harmsen, c.s. mueller, k.m. haller, r.l. wheeler, r.l. wesson, y. zeng, o.s. boyd, d.m. perkins, n. luco, e.h. field, c.j. wills and k.s. rukstales (2008). documentation for the 2008 update of the united states national seismic hazard maps, u.s. geol. surv. open-file rept 2008-1128, 61 pp. rundle, j.b., k.f. tiampo, w. klein and j.s. sá martins (2002). self-organization in leaky threshold systems: the influence of near-mean field dynamics and its implications for earthquakes, neurobiology, and forecasting, proc. natl. aca. sci. usa, 99, 2514-2521. schorlemmer, d., a. christophersen, a. rovida, f. mele, m. stucchi and w. marzocchi (2010). setting up an earthquake forecast experiment in italy, annals of geophysics, 53, 3 (present issue). stirling, m.w., g.h. mc verry and k.r. berryman (2002). a new seismic hazard model for new zealand, bull. seismol. soc. am., 92, 1878-1903. stock, c. and e.g.c. smith (2002a). adaptive kernel estimation and continuous probability representation of historical earthquake catalogs, bull. seismol. soc. am., 92, 904-912. stock, c. and e.g.c. smith (2002b). comparison of seismicity models generated by different kernel estimations, bull. seismol. soc. am., 92, 913-922. van stiphout, t., d. schorlemmer and s. wiemer (submitted bssa). uncertainties in background seismicity rate estimation, submitted to bull. seism. soc. am. werner, m.j., a. helmstetter, d.d. jackson and y.y. kagan (submitted bssa). high resolution longand short-term earthquake forecasts for california, submitted to bull. seism. soc. am.; draft available at http://arxiv.org/abs/ 0910.4981. werner, m.j., j.d. zechar, w. marzocchi and s. wiemer (2010). retrospective evaluation of the five-year and ten-year csep-italy earthquake forecasts, annals of geophysics, 53, 3 (present issue). wessel, p. and w. smith (1998). new, improved version of generic mapping tools released, eos trans. agu, 79, 579. zechar, j.d. (2008). methods for evaluating earthquake predictions, los angeles, university of southern california ph.d. dissertation, 134 pp. zechar, j.d. and t.h. jordan (2008): testing alarm-based triple-s forecasts for italy 104 105 earthquake predictions, geophys. j. int., 172, 715-724; doi: 10.1111/j.1365-246x.2007.03676.x. zechar, j.d. and t.h. jordan (2010). the area skill score statistic for evaluating earthquake predictability experiments, pure appl. geophys., 167 (8-9), 893-906; doi: 10.1007/ s00024-010-0086-0. zechar, j.d., m.c. gerstenberger and d.a. rhoades (2010). likelihood-based tests for evaluating space-rate-magnitude earthquake forecasts, bull. seism. soc. am., 100 (3), 11841195; doi: 10.1785/0120090192. *corresponding author: j. douglas zechar, eth zurich, swiss seismological service, zurich, switzerland; email: jeremy.zechar@sed.ethz.ch electronic supplement available at: http://www.annalsofgeophysics.eu/index.php/annals/rt/suppfiles/4845/0 © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. triple-s forecasts for italy vol48/04/2005def 549 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words silicate melt – acid-base – oxide melt – thermodynamic properties 1. introduction in extraction metallurgy, quasi-chemical (toop and samis, 1962a,b) and polymer models (masson 1965, 1972) have been used with considerable success to express the variation with melt composition and temperature of metal oxide activities in silicate melts. the aim of such calculations is to interpret the thermodynamic properties of the melts and to allow modelling of the activities of oxide components so as to maximize refining efficiency by minimizing metal loss in slags and by optimizing the physico-chemical conditions required for smelting. in contrast, in the earth sciences, most problems involving silicate liquids, at least in the outer part of the earth, require knowledge of the behaviour, not of oxide constituents, but of the activities and chemical potentials of silicate species. the requirements are to provide a framework for understanding crystal-liquid equilibria over a wide range of pressure, temperature, gas and melt compositions and to include accurate descriptions of other thermodynamic properties such as density, viscosity and surface tension, again over a wide range of p, t, x. these problems have been tackled until now mainly by fitting a large number of parameters to empirical or semi-empirical thermodynamic models (e.g., berman and brown 1984; ghiorso and sack, 1995; ghiorso et al., 2002). in these models, the excess free energy of mixing of components is usually expressed by a particular functional form, such as that of the regular solution model, and the coefficients of the variables are fitted using calorimetric and mailing address: dr. donald g. fraser, department of earth sciences, university of oxford, parks road, oxford ox1 3pr, u.k.; e-mail: donald.fraser@earth.ox.ac.uk acid-base properties and structons: towards a structural model for predicting the thermodynamic properties of silicate melts donald g. fraser department of earth sciences, university of oxford, u.k. abstract phase equilibrium relationships in igneous systems can be estimated using empirical mathematical models based on multi-component regular solution formulae. although these provide useable results within the fitted region, they can give very misleading values outside the compositional range of curve fitting. moreover, they usually give poor estimates of the well-characterized melting relations of simple systems and do not relate to the large body of thermodynamic activity data available in the metallurgical literature, nor to spectroscopic, diffraction or computational models of silicate melt properties. the aim of this paper is to extend previous acid-base models of silicate melts and to use a quasi-chemical model to calculate the activities of quasi-chemical silicate mixing units, or structons, from combinations of the oxo-species used in quasi-chemical and polymer models to calculate oxide activities in metallurgy. 550 donald g. fraser phase equilibrium data for the compositional range of interest. thus, in an n-component system, (1.1) where νi are the stoicheiometric coefficients. the various parameters must be fitted for each component i. this mathematical curve fitting approach can provide usable results within the range of compositions for which the parameters are fitted although inaccurate results are obtained for certain compositions, particular those rich in na (e.g., asimow et al., 2001). the calculated data agree particularly poorly with the basic binary and ternary phase diagrams of igneous petrology for which so many primary measurements are available. an important and usually un-stated reason for the success of these polynomial curve-fits is that they deal with the limited polymerised region of composition-space which is energetically rather homogeneous – that is, it excludes those basic compositions which show most of the high-energy interaclnrt a+n=lnrt xliq liq liq solid solidi i i i i n 0 1 0+o n c = ^ h7 a/ tions of mo and sio2 (see fig. 1). thus, although geologically useful, the models behave poorly in the very compositional region which is most sensitive to changes in the acid-base properties of the melts and which provides most fundamental information about their properties and the true effects of different metal ions. they are certainly not based on the knowledge of the structures and properties of the liquids. the models are therefore unreliable for extrapolation beyond the range for which they are fitted and, especially, provide little insight into the underlying properties and behaviour of silicate melts. the question of the acid-base properties of silicate melts (duffy and ingram, 1971) and of how to relate thermodynamic properties to structure (fraser, 1977) has recently been revisited in an attempt to discover the factors which determine the values of the empirical coefficients for components in cao-mgo-al2o3sio2 (beckett, 2002). in addition there is renewed interest in using acid-base properties to devise a predictive thermodynamic model properly grounded on a structural basis (fraser 1977, 2003; ottonello, 2001, 2005) and in explaining the behaviour of altervalent elements in silicate melts (fraser, 1975; moretti and ottonello, 2003). the aim of the present paper is to examine the extensive work on oxide activities which has been done on binary sio2-mo melts of metallurgical interest and to relate these to the distributions of silicate species in melts. these distributions reflect the fundamental properties of silicate melts and provide insight into the chemical factors such as the acid-base properties of oxides which influence crystal liquid equilibrium in igneous systems. 2. oxide activity measurements in binary silicate melts the thermochemistry of melts and slags in divalent binary systems mo-sio2 has been studied extensively (e.g., richardson, 1956; crippsclark et al., 1974; navrotsky, 1995). mixing is usually highly non-ideal and significant deviations from ideality are observed (fig. 1). the variations of oxide activity with bulk composifig. 1. experimental measurements of oxide activities in binary silicate melts. also shown are curves calculated using a bi-functional linear polymer model (see below). 551 acid-base properties and structons: towards a structural model for predicting the thermodynamic properties of silicate melts tion have the general form of titration curves in which each basic oxide can be considered to be titrated with the acidic oxide sio2. the differing basicities of different oxides can be seen clearly. 3. acid-base schemes although the qualitative concept of acidic and basic oxides is familiar to geologists and dates back to berzelius, a quantitative definition of oxide acidity and basicity was proposed by flood and förland (1947) following the work of lux (1939). in aqueous systems, acidic or basic behaviour is conveniently treated using the conjugate acid-base formulation of brönsted and lowry (3.1) e.g., (3.2) alternatively, and relevant to silicate melts, a lewis acid is simply an electron acceptor e.g., al3+ so that the lewis acidity may be related to the pauling electronegativity. in non-protonic solvents like molten oxides and silicates, a different formalism is required. in the lux-flood system, oxide-ion, o2−, takes the place of protons in aqueous solutions. thus, a basic oxide is a substance capable of furnishing oxide ions, and an acidic oxide is one which reacts with o2−. thus, (3.3) e.g., (3.4) each acid or base is therefore characterized by a defined thermodynamic dissociation or equilibrium constant. the mixing properties of silicate melts may therefore be investigated by titrating melts with sio2, just as protonic acidbase systems are investigated by ph titration. the interpretation of these deviations from ideality has been the subject of a family of models, all of which express the non-ideality in terms of reaction between the mixing components. measurements of the electrical conductivities and transport properties of silicate melts .sio sio o3 2 2 2= +base acid o2= + .h so hso h2 4 4= ++acid base h= + + (bockris et al., 1952a,b; waff and weill, 1975; stebbins et al., 1995) have shown that, with the exception of melts containing transition-metal ions in which significant charge transfer processes may operate, conduction in the melts is entirely ionic with faraday’s law being obeyed. importantly, the conductance is unipolar (bockris et al., 1952b; bockris and mellors, 1956) and occurs by the transport of relatively mobile cations while the anions remain stationary. silicate melts are thus ionic liquids like other molten salts but have an immobile anion network. interestingly, recent nmr data (lee and stebbins, 2003) indicate that the behaviour of cations may also be non-random and a proposal to extend the temkin equation (temkin, 1945) to express the different behaviour of cations of different charge is discussed below. the non-ideality observed in the mixing properties of silicate melts when expressed as oxide components is the observable. the difficulty in its interpretation is caused by the difficulty in expressing the excess free energy of mixing (the generally negative deviation from ideality) as a function of composition. 4. quasi-chemical models – temkin equation the regular solution model (hildebrand and scott, 1950) interprets the excess free energy of mixing of a solution entirely as an enthalpy of mixing and retains a purely configurational entropy of mixing. this treatment is successful for the description of mixing of similar components – e.g., cyclohexane and benzene and the mixing of some ionic solids when the anionic framework is fixed. however it becomes more difficult to express the excess functions for mixing components as the magnitude of the deviations from ideality increases. in the case of silicates, the high energies associated with making and breaking si-o-si or al-o-si bonds make it unlikely that a simple regular solution model could succeed in systems of variable stoicheiometry like melts. this is the reason for the inability of the polynomial regular solution models to deal adequately with simple systems. quasi-chemical models are first-order refinements of the regular solution treatment and 552 donald g. fraser use the excess free energy of mixing to form new species with mixing properties which are much closer to ideal (e.g., guggenheim, 1952). the choice of quasi-chemical components in the case of silicate liquids is initially complicated by the presence of a large number of different silicate anions. while silicate minerals are usually monodisperse, containing only a single type of anion (e.g., sio44− in olivine), molten silicates contain a distribution of different polymeric silicate anions of different molecular weights and are thus polydisperse systems with statistical distributions of polymers of different molecular weight. vibrational spectroscopic studies of melts and glasses provide broad information on the proportions of different bonds present among the silicate anions (e.g., mysen, 1997). it is also possible to separate some of the different anionic species themselves using chromatographic techniques (lentz, 1964; götz and masson, 1970) and even simple orthosilicate compositions like pb2sio4 contain, in the glass, a distribution of condensed polymeric species (fig. 2). it is immediately clear from these data that even orthosilicate melts are composed not of m2+ and sio44− ions, but rather of pb2+ ions coordinating a distribution of anions of different molecular weight. simple measures such as nbo/t, calculated from the overall stoicheiometry, although widely used in the geological literature, do not give this result and have been useful only because most igneous melts are more polymerised than the metasilicate composition. expression of the statistical nature of liquids and glasses and, in particular, the case of molten silicates, requires treatment of the energetics of all the simultaneous equilibria. any successful treatment of the nature of silicate liquids must treat the polymerization equilibria responsible for the observed distribution of species in the melt. similar problems exist in mixtures of organic polymers (flory, 1936, 1953) and experience in the treatment of organic polmer melts and solutions has been of great value in developing models of molten silicates. these models deal with the complex molecular weight distributions observed by making two assumptions: 1) polymerization equilibria (i.e. the reactivities of functional groups) are independent of molecular weight and 2) temkin mixing. the temkin model of ionic salts (temkin, 1945) considers the energetics of substituting cations for anions and, (essentially following the quasi-chemical approach) assumes that the energy cost of wrongly substituting a cation for an anion is sufficiently high that the probability of mixing cations and anions is zero. the correct temkin entropy of mixing is therefore obtained by mixing anions and cations independently on sites of the anion matrix and cation matrix respectively. recent nmr data (lee and stebbins, 2003) show that this is true for mg and ca, for example, but seems not to be true for ions of different charge. thus, whereas mg and ca ions mix close to randomly, na and ca ions occupy separate sites in the glasses studied. fig. 2. distribution of polymers in pb2sio4 glass obtained by trimethyl sliylation gas liquid chromatography of quenched glass. temperatures refer to chromatographic separation. peaks 1-4 represent sio4 momomers, si2o7 dimers, si3o9 rings and si4o12 rings respectively (after masson, 1972). 553 acid-base properties and structons: towards a structural model for predicting the thermodynamic properties of silicate melts it is proposed here that for multi-component melts, the simple temkin assumption may need to be extended to allow for this behaviour by introducing sub-matrices. the simplest would be a matrix for each valence state m i, m ii, m iii and m iv. treatment of the behaviour of p and as might require a further set of exclusive sites m v. note that the sites m iv and m v are probably the sites of network-forming ions and m iii, at least, may be amphoteric as described below. this provides a continuity of treatment of metal ions and network forming constituents for the first time. it is analogous to the independent treatment of the entropy of mixing on different sites in solids, e.g., in m ii3 m iii2 si3o12 garnets. the unipolar electrical conductance is determined by the very low diffusivity of highly charged ions in network-forming sites. this is supported by recent molecular dynamics calculations using a modified bks potential which show low diffusivity of siiv (gemmell et al., 2003). moreover it allows treatment of the whole melt array as an inter-related network of metal ion potential energy wells in a background of oxygen and other electronegative atoms, normally regarded as forming the «network-forming» sites. 5. binary silicate melts in the case of binary silicate melts, the temkin model and the extended model described above are identical. oxide activity data have been obtained for many such systems for large parts of the composition range mo-sio2. methods used include equilibration with a pure metal (richardson and webb, 1956) extraction from phase diagrams and emf measurements using cao/zro2 solid electrolytes. activity-composition curves for three binary systems are shown in fig 1. the form of these amo versus xmo curves is determined by the titration of mo by added sio2. masson (1965) has shown that polymer models may be used to express the measured activity by writing sets of polymerization equilibria of the form k1 (5.1)sio sio si o o4 4 2 7 2+ + + k2 (5.2) … kn (5.3) where k1 = k2 = … kn. for bifunctional (linear) polymerization, this leads to (5.4) and for branched chains (masson, 1972) (5.5) curves calculated from (5.1 to 5.3) above for linear chains are also shown in fig. 1 and agree well with the experimental data for these systems. these models do not express self-condensation to form ring or network structures and so are unsuitable, without extension to describe the behaviour of magmas considerably more sio2-rich than the metasilicate composition. 5.1. the toop and samis (quasi-chemical) model an alternative to discrete polymerization reactions is to express the excess free energy of mixing in terms of the formation, not of discrete anions, but of quasi-chemical or virtual species. the proportions of species will again reflect the magnitude of the non-ideality. this approach was introduced by toop and samis (1962) who considered the interaction of just three oxo-species (5.6) where o2− represents a «free» oxide ion, not bound as part of the silicate network, o0 is an oxo-bridge si-o-si and o− a singly charged oxygen atom also bonded to the cations present. allowing simple temkin mixing, the proportions of the species are thus given by (5.7)( ) .k x x xo o o22 $= -.o o o22 0+ =[ / ])a k3 1-+/ (1 1-)a./ (3 1+2=/x1 mo mosio2 ])1-[ /a k1+/ (1 1-/ ( )a1 1 -+2=/x1 mo mosio2 sio si o si o o4 3 1 1 3 4 2 n n n n+ + ++ + + sio si o si o o4 2 7 3 10 2+ + + 554 donald g. fraser applying charge and mass balance constraints (fraser, 1977), ∆gmix may be calculated for any bulk composition. data for the systems pbosio2 and cao-sio2 are shown in fig. 3. 6. amphoteric oxides the mixing properties of binary silicate melts can be expressed well by models which give real insight into the properties of the melts and which have robust predictive power. an assumption of these models is simple temkin mixing with complete dissociation of the basic oxide into m2+ and o2− in its standard state. this cannot be universally true and this standard state problem must be considered carefully when comparing different binaries or working in multicomponent systems (fraser, 1977). this problem is particularly severe in the case of oxides such as al2o3, fe2o3 or tio2 which may contribute structural units to a greater or lesser extent to the «silicate» framework of the melt – as noted above, electrical conductivity measurements indicate that conductivity is ionic and monopolar. one approach is to allow for amphoteric properties of real oxides – i.e. in which a given oxide component has the ability to react both as a basic and an acidic oxide depending on the overall composition (fraser, 1975). to express the real ability of oxides to behave as acidic or basic components, the temkin model must be further extended. the lux-flood acid base system defines acid-base behaviour by the reactivity with oxide ion. by extension, amphoteric behaviour is therefore easy to express as can be seen in the case of al2o3 (6.1) (6.2) the net effect of adding al2o3 to a multivariate melt will be either to consume o2− or to increase the o2− activity depending on the bulk composition and the values of ka and kb. similar criteria apply to fe2o3 and other altervalent oxides. for example it is well known that at constant p, t and ao2, iron is more highly oxidized in «basic» melts (fudali, 1965; paul and douglas, 1965a,b). the variation in the oxidation state of fe with bulk melt composition at constant p, t and fo2 has important implication for its partitioning behaviour and for calculations of redox conditions in the earth. in the case of feo, the two amphoteric equilibria are kb (6.3) ka (6.4) in which ka is exceedingly small and can be neglected. in contrast, the two reactions for fe2o3 are likely to be of more equal magnitude (6.5) (6.6) thus increasing melt basicity indicated by addition of basic oxides mo stabilizes feo2− and hence feiii relative to fe2++ o2− and hence feii. similar criteria apply to other altervalent oxides and it has been shown that in the melts in the binary systems mg, casio3-caal2si2o8 .fe o o feo22 3 2 2+ = fe o fe o2 32 3 3 2= ++ feo o feo2 2 2+ =feo fe o2 2= ++ .al o o alo22 3 2 2+ = al o al o2 32 3 3 2= ++ fig. 3. ∆gmix for the systems cao-sio2 and pbosio2. data compared with theoretical curves calculated using the toop and samis quasi-chemical model (after toop and samis, 1962a). 555 acid-base properties and structons: towards a structural model for predicting the thermodynamic properties of silicate melts and (mg,ca)2sio4-caal2si2o8 constant t and fo 2, eu iii/euii ratios increase with increasing mo-content (i.e. decreasing sio2) as expected from the above. however importantly, they also increase with increasing ca/mg ratio. since melt basicity has been related to pauling electronegativity (duffy and ingram, 1971), the model predicts that with cao more basic than mgo, euiii/euii ratios in ca-rich melts should be higher than in mg-rich liquids at constant t, p, fo 2 and xsio 2 as is observed experimentally (morris and haskin, 1974; fraser, 1975). 7. behaviour of h2o and co2 the amphoteric behaviour of oxides also has implications for the solubility of h2o water in melts. ignoring unreacted molecular water, the relevant reactions are kb (7.1) ka . (7.2) in acid melts, most water will be absorbed to depolymerize the network, forming oh groups bonded to silicate anions. however in basic compositions, water should dissolve by a different mechanism such as (7.2). in this, h2o reacts with oxide ion as an acidic oxide to produce free hydroxyl ions. this mechanism, (fraser, 1977), has recently been observed by careful mas nmr observations of quenched camgsi2o6 glasses (xue and kanzaki, 2003). the overall solubility will depend on the overall basicity, the values of ka and kb and the stability of the free hydroxide species ca(oh)2 and mg(oh)2. the solubility of s also depends on similar reactions involving s2−/so4 equilibria (fincham and richardson, 1954; holzheid and grove, 2002; o’neill and mavrogenes, 2002). similarly co2 behaves dominantly as an acidic oxide by the reaction (7.3) note that h2o, co2 and sulphur all interact with the oxo-species equilibria in molten silicates. thus, for example, wet melting, or melting in the .oco co2 2 3 2+ =h o o oh22 2+ =h o o oh22 0+ = presence of differing amounts of co2 or sulphur will not only involve the dissolution of the volatile component, but will perturb the balance of oxo-, and hence silicate, structural species in the silicate melt which are in equilibrium with solid phases on the liquidus. to maintain crystal liquid equilibrium, the effects of the dissolved volatile component must be offset, most usually by the addition of more or less silica. thus, in moderately acidic melts, h2o dissolution occurs dominantly by (7.1) above. wet melting thus requires addition of o0 (i.e. sio2) to restore the o−/o0 equilibrium according to eq. (5.6) and wet melting leads to an expansion of the olivine primary phase volume towards silica-rich (andesitic) compositions as will be described below. co2 has the opposite effect. 8. implications of acid-base reactions for silicate crystal-melt phase equilibria the acid-base properties of oxides described above may be used to bridge the gap between the experimental and theoretical work which has led to detailed understanding of oxide activities in metallurgical slags on the one hand, and the need for a similar conceptual framework for considering silicate activities and equilibiria on the other. the results of mas nmr spectra obtained from silicate glasses and also from high temperature melts allow the identification in low pressure melts and glasses of species with half-lives long in comparison with the measurement technique. thus, sio4 tetrahedra can be identified with different linkage states, q (e.g., stebbins, 1987). olivine-like q0 species are isolated tetrahedral groups with no cross links, q2 species are middle groups with pyroxene-like linkages and q4 species are three dimensionally cross-linked. successful application of the quasi-chemical model utilizes the excess free energy of mixing to form quasi-chemical species which then mix ideally. this can be applied to relate the nmr data to the oxide activity thermodynamic data by extending the quasi-chemical model of toop and samis introduced above. in an early paper, huggins (1954) referred to basic structural units as «structons» and we can express the compositions 556 donald g. fraser of silicate structons by an extension of the q-formalism used to interpret nmr spectra. let each quasi-chemical constituent be represented by siij where i is the number of singly bonded oxygens and j the number of bridging oxygens, then the set of five tetrahedral structons for si is 40si, 31si, 22si, 13si and 04si. the first is an isolated tetrahedron as in olivine and the last a fully crosslinked unit as in quartz. the toop and samis model considers the excess free energy of mixing to arise, as shown above, from the reaction (8.1) the quasi-chemical species, o− and o0 of this model may be used to construct silicate species or structons by considering the probability of forming each. these probabilities pij are thus within the quasi-chemical model, the structons mix ideally and their proportions can be calculated directly from the proportions of toop and samis oxo-species. these are determined by the polymerisation equilibrium constant characteristic of each oxide. basic oxides like cao have a very low value of k (e.g., k = 0.003 in fig. 1) and less basic oxides such as nio, higher values (k = 46 in fig. 1). the relationship of these values to electronegativity will be considered elsewhere (fraser, in prep.). silicate crystal-liquid phase equilibrium can be expressed simply using this model. for example the crystallization of enstatite from a melt is defined by the equilibrium and ( ) ( ) rt rt liq xtal 0 0 mgsio mgsio mgsio mgsio 3 3 3 3 + + n n ln ln ( ) ( ) . a a liq xtal = ( ) ( )liq xtalmgsio mgsio3 3=n n . . . . . . !/ !. ! !/ !. ! !/ !. ! . p x x p x x x x p x x x x p x x x x p x x 1 4 3 1 4 4 2 2 6 4 1 3 4 1 o o o o o o o o o o o o o o o o 40 4 4 31 3 0 3 0 22 2 2 0 2 0 13 3 0 3 0 04 4 0 4 0 ) ) ) ) ) = = = = = = = = = = .o o o22 0+ =for true quasi-chemical mixing in the melt, the structon model predicts that ideal mixing of quasi-chemical species should obtain. thus the activity in the melt is defined as amgsio3(liq)= = xmgcm.x 22si = 6x 2o−x 2o0. note that x mgcm refers to mixing in the divalent cation matrix. as noted above, the recent nmr data of lee and stebbins (2003) suggest that it may be necessary to extend the simple temkin model to consider the mixing of different types of cation in silicate melts and glasses in a more sophisticated way than hitherto. a first approach may be to consider the mixing of cations of different charge separately in different matrices. a test of this model is available if appropriate values of k are available for a system. values of the temperature dependence of k are available for the system feo-sio2 (distin et al., 1971; masson, 1972). a plot of lnk versus 1/t yields a value of 0.574 at the eutectic temperature of 1455 k. using this value, the activity of 40si (i.e. sio4) structons in the melt can be calculated at each temperature. for the eutectic composition at 1455 k, the value is 0.893. this compares well with the value of afe2sio4 calculated from the depression of freezing point of 0.882 (fraser, 1977). 9. multi-component systems the above model provides a means of relating metallurgical oxide activity measurements to silicate element partitioning and phase equilibrium data. in the earth sciences an outstanding problem is to predict the effects of changing composition or volatile content. the effects of adding third components to a univariant equilibrium in the system mgo-sio2 have been described by kushiro (1973) for the forsterite-enstatite-liquid equilibrium and the effects of different oxides on this equilibrium are shown in fig. 4. this equilibrium is expressed in terms of the structon model by the two simultaneous equations ( ) ( ) rt rt liq xtal 0 0 mg sio mg sio mg sio mg sio 2 4 2 4 2 4 2 4 + + n n ln ln ( )liq = ( ) a a xtal 557 acid-base properties and structons: towards a structural model for predicting the thermodynamic properties of silicate melts the activities of the melt components are given by the effect of adding basic or acidic oxides will shift the balance of the forsterite-enstatite-melt equilibrium according to the proportions of 40si and 22si structons in the melt. thus addition of a basic oxide such as k2o will increase the proportion of 40si relative to 22si. equilibrium is only maintained by a balancing shift towards sio2rich compositions. conversely addition of an acidic oxide such as p2o5 will have the opposite effect and move the position of the equilibrium to sio2 poor values. the effect of added water in this diagram is basic, but less so than for na2o or k2o. h2o dissolves in silicate melts with at least two chemical mechanisms in addition to dissolution as molec. . .. ( ) ( ) . a x x x x a x x x x x liq liq 6 mg cm si mg cm o mg cm si mg cm o o 2 40 2 4 22 2 2 0 mg sio mgsio 2 4 3 = = = = ( ) ( ) rt rt liq xtal 0 0 mgsio mgsio mgsio mgsio 3 3 3 3 + + n n ln ln ( ) ( ) . a a liq xtal = ular h2o, as discussed above. the net effect for these compositions seems to be somewhat basic of neutral so that the forsterite-enstatite equilibrium is shifted to sio2-rich compositions. this behaviour, with the expansion of the forsterite primary phase volume to sio2-rich compositions during wet melting is well known as is shown in fig. 5 for the system di-fo-sio2. whereas wet melting leads to silica-rich melt compositions, melting in the presence of co2 has the opposite effect implying that co2 is an acidic oxide, consuming o2− dominantly by the formation of co32−. the structon model provides a framework within which a range of properties of silicate melts may be considered. these include redox behaviour, volatile solubility, physical properties and crystal-liquid equilibrium in igneous systems. in order for quasichemical or polymer models based on the temkin equation to be successfully applied in multi-component systems, it should be remembered that the assumption of full dissociation of the «basic» oxide in the standard state is unlikely to be generally true as pointed out by fraser (1977). the correct treatment of the behaviour of amphoteric oxides in melts thus requires crosscalibration of the standard states for the different end-member oxide components present. fig. 4. effects of different added oxides on the forsterite-enstatite-liquid equilibrium (after kushiro, 1973). fig. 5. phase relations in the system diopsideforsterite-quartz. 558 donald g. fraser 10. conclusions metallurgical oxide-activity measurements and the quasi-chemical or polymer theoretical models developed to interpret the variation of oxide activity with composition in binary silicate melts may be used to calculate the proportions of silicate structural units, or structons, in melts. the lux-flood acid-base system may be extended to describe the behaviour of amphoteric oxides such as al2o3 and fe2o3 by the introduction of a second acidic reaction to provide an amphoteric pair for each oxide. the oxidation state of fe and other altervalent oxides increases with increasing basicity in response to the stabilization of the acidic reaction of the higher oxidation state. application of these models to multi-component silicate melts requires measurement or calibration of the acid-base constants and calibration of the different oxide standard states noted by fraser (1977) and may require extension of the simple temkin fused salt model to allow for the separate behaviour of metal ions of different charge recently reported in the nmr data of lee and stebbins (2003). this model thus provides a multi-parameter framework for the characterization of the behaviour of melts and melt-solid equilibria which can be fitted using similar methods to those adopted by ghiorso et al. (2002), or berman and brown (1984). an advantage is that the structon model is based on a first-order extension to the regular solution model so that a closer relationship to structural data so that extrapolation may be possible. in addition, the acid-base constants which describe the melt properties are relatable to fundamental chemical parameters such as the optical basicity scale based on the 1s 0→3p1 uvtransition proposed by duffy and ingram (1971). references asimow, p.d., m.m. hirschman and e. stolpe (2001): calculation of peridotite partial melting from thermodynamic models of minerals and melts, iv. adiabatic decompression and the composition and mean properties of mid-ocean ridge basalts, j. petrol., 42, 963-998. beckett, j.r. 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(1945): mixtures of fused salts as ionic solutions, acta phys. chim. u.r.s.s., 20, 411-420. toop, g.w. and c.s. samis (1962a): activities of ions in silicate melts, trans. met.-soc. a.i.m.e., 224, 878-887. toop, g.w. and c.s. samis (1962b): some new ionic concepts of silicate slags, canad. met. quart., 1, 129-152. waff, h.s. and d.f. weill (1975): electrical conductivity of magmatic liquid effects of temperatures, oxygen fugacity and composition, earth planet. sci. lett., 28, 254-260. xue, x. and m. kanzaki (2003): the dissolution mechanism of water in alkaline earth silicate melts: one view from 1h mas nmr, geochim. cosmochim. acta, 67, a543. vol52,3,2009 301 annals of geophysics, vol. 52, n. 3/4, june/august 2009 key words ionospheric propagation – mid-latitude trough – radiolocation – propagation modelling – backscatter sounding 1. propagation along the mid-latitude trough 1.1. introduction for terrestrial hf radio systems, the electron density depletion in the trough region reduces the maximum frequency that can be reaspects of hf radio propagation e. michael warrington (1), alain bourdillon (2), eulalia benito (3), cesidio bianchi (4), jean-philippe monilié (3), martha muriuki (1), marco pietrella (4), veronique rannou (3), hanna rothkaehl (5), stephane saillant (3), ozgur sari (6), alan j. stocker (1), ersin tulunay (7), yurnadur tulunay (8) and nikolay y. zaalov (1) (1) department of engineering, university of leicester, uk (2) ietr, université de rennes 1, france (3) electromagnetism and radar department, onera, palaiseau, france (4) istituto nazionale di geofisica e vulcanologia, roma, italy (5) space research centre pas 00-716 warsaw, poland (6) aselsan a.s., macunkoy, ankara, turkey (7) odtü/metu, department of electrical and electronics engineering, ankara, turkey (8) odt /metu, department of aerospace engineering, ankara, turkey abstract the propagation characteristics of radio signals are important parameters to consider when designing and operating radio systems. from the point of view working group 2 of the cost 296 action, interest lies with effects associated with propagation via the ionosphere of signals within the hf band. several aspects are covered in this paper: a) the directions of arrival and times of flight of signals received over a path oriented along the trough have been examined and several types of propagation effects identified. of particular note, combining the hf observations with satellite measurements has identified the presence of irregularities within the floor of the trough that result in propagation displaced from the great circle direction. an understanding of the propagation effects that result in deviations of the signal path from the great circle direction are of particular relevance to the operation of hf radiolocation systems. b) inclusion of the results from the above mentioned measurements into a propagation model of the northerly ionosphere (i.e. those regions of the ionosphere located poleward of, and including, the mid-latitude trough) and the use of this model to predict the coverage expected from transmitters where the signals impinge on the northerly ionosphere. c) development of inversion techniques enabling backscatter ionograms obtained by an hf radar to be used to estimate the ionospheric electron density profile. this development facilitates the operation of over the horizon hf radars by enhancing the frequency management aspects of the systems. d) various propagation prediction techniques have been tested against measurements made over the trough path mentioned above, and also over a long-range path between cyprus and the uk. e) the effect of changes in the levels of ionospheric disturbances on the operational availability at various data throughput rates has been examined for the trough path mentioned earlier. the topics covered in this paper are necessarily brief, and the reader is referred to full papers referenced herein on individual aspects. mailing address: prof. e. michael warrington, department of engineering, university of leicester, leicester, le1 7rh, u.k.; e-mail: emw@leicester.ac.uk vol52,3,2009 20-09-2009 19:06 pagina 301 302 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov flected by the ionosphere along the great circle path (gcp). for long paths, the signal is often received via a ground/sea-scatter mechanism to the side of the gcp (stocker et al., 2003). for shorter paths, gradients in electron density associated with the trough walls and embedded ionospheric irregularities often result in propagation in which the signal path is well displaced from the great circle direction, with directions of arrival at the receiver offset by up to 100° (rogers et al., 1997). deviations from the great circle direction impact not only on radiolocation systems for which estimates of a transmitter location are obtained by triangulation from a number of receiving sites, but also on any radio communications system in which directional antennas are employed. furthermore, the doppler and multi-mode delay spread characteristics of the signal are also affected when propagation is via scatter/reflection from irregularities in or close to the north wall of the trough (warrington and stocker, 2003). recently, measurements have been made of the direction of arrival (doa) and amplitude as a function of time of flight (tof) of hf signals received over two paths oriented along the trough between uppsala, sweden (2001 and from august 2006 to january 2008) and helsinki, finland (since december 2006) and the university of leicester’s field site near to leicester (see fig. 1). the first of these periods was close to sunspot maximum, and the second close to sunspot minimum. 1.2. characteristic types of propagation along the trough examination of the observations along the trough path from uppsala and helsinki has enabled five types of off great circle propagation 18−apr−2007, 22ut, kp=0 20 ow 10 ow 0 o 10 oe 20 o e 30 o e 48 on 54 on 60 on 66 on 72 on leicester uppsala helsinki no rth wa ll sou th w all 18−apr−2007, 22ut, kp=6 20 ow 10 ow 0 o 10 oe 20 o e 30 o e 48 on 54 on 60 on 66 on 72 on leicester uppsala helsinki north wall sou th w all fig. 1. maps showing the paths between uppsala and leicester and helsinki and leicester together with a typical position for the trough, as estimated by the model of halcrow and nisbet (1977) for 0000 ut on 11 march 2006 for kp values of 0 and 6. the four lines indicate the outer and inner edges of the north and south walls of the trough. vol52,3,2009 20-09-2009 19:06 pagina 302 12 13 14 15 16 17 18 19 20 21 22 23 00 0 10 20 30 40 50 60 ex11, upp, 10 mhz, 2007/04/18−2007/04/19, type ii a zi m u th ( d e g re e s) 12 13 14 15 16 17 18 19 20 21 22 23 00 4 5 6 7 8 t im e o f fli g h t (m s) time (ut) 303 aspects of hf radio propagation events to be identified (see stocker et al. (2009) for full details): • type 1: these are characterised at onset by a large sudden increase in time of flight accompanied by a sudden deviation of the bearing to the north. subsequently, the time of flight decreases over a period of hours while the azimuth either remains fairly constant or returns slowly to close to the great circle direction. type 1 events were commonly observed, except in the summer, close to solar maximum (2001) (see siddle et al., 2004a for a number of examples) when they also tended to be longerlived and were observed on a wider range of frequencies. theoretical considerations indicate that these events are consistent with the signal scattering from irregularities in the poleward wall of the trough or from the auroral region (siddle et al., 2004b). • type 2: the time of flight gradually increases and is accompanied by a gradual deviation of the azimuth to the north (e.g. see the traces for 10 mhz in fig. 2). the rate of change of time of flight and azimuth tends to be fairly constant until, after a period of up to a few hours, the signal is no longer detected. • type 3: the time of flight gradually increases and is accompanied by either a gradual southward (type 3s) or, less frequently, northward (type 3n) deviation of the azimuth. after a few hours either the signal is no longer detected or the azimuth slowly returns to the great cirfig. 2. type 2 example, azimuth and tof only. vol52,3,2009 20-09-2009 19:06 pagina 303 304 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov cle direction (this distinguishes type 3n propagation from type 2). • type 4: this type of propagation is characterised by a sudden large increase in the time of flight and a strong deviation to the south (typically the azimuth is around 120°). the signal is usually relatively weak, and is consistent with sidescatter from a ground reflection in the vicinity of the alps. • type 5: this type is similar to type 1, except that although there is a sudden increase in the time of flight, the signal is deviated to the south instead of the north. over the following few hours the azimuth usually returns to the great circle direction. the occurrence (as a percentage of nights of observation) of types 1, 2, and 4 as a function of season, frequency and kp is presented in fig. 3 (types 3 and 5 are generally rare). the seasons have been defined with spring including all of march and april, summer including may to august, autumn september and october and winter november to february (as in itu, 1997). type 1 events are most common in spring with no clear trend with frequency. for 4.64 mhz, this type of propagation is also 4.64 8.01 11.12 0 10 20 30 40 type 1 summer autumn winter spring 4.64 8.01 11.12 0 10 20 30 40 o cc u rr e n ce ( % o f n ig h ts ) type 2 4.64 8.01 11.12 0 10 20 30 40 frequency (mhz) type 4 4.64 8.01 11.12 0 10 20 30 40 type 1 0−2 2−4 4+ 4.64 8.01 11.12 0 10 20 30 40 o cc u rr e n ce ( % o f n ig h ts ) type 2 4.64 8.01 11.12 0 10 20 30 40 frequency (mhz) type 4 fig. 3. the percentage occurrence of types 1, 2, and 4 off great circle propagation (august 2006-september 2007). left hand panels show variation with season, and the right hand panels the variation with kp (averaged 1800-0000 ut). vol52,3,2009 20-09-2009 19:06 pagina 304 305 aspects of hf radio propagation common in winter, while for most other frequencies it occurs least often in winter. this behaviour may be contrasted with that observed in 2001 (see siddle et al., 2004a,b) where type 1 propagation was frequently observed (~40–70% of nights depending on frequency) except in summer. furthermore, the events observed in 2001 tended to consist of larger changes in time of flight and azimuth and to be of longer duration. type 2 events tend to occur more frequently in spring and, for frequencies in the range 8.01-11.12 mhz, in the summer. however, for 6.95 mhz, there is little other seasonal variation, while at 4.64 and 14.36 mhz the events are rarely observed. in spring, the occurrence strongly depends on frequency, peaking at 10.39 mhz. type 4 events are also a springtime phenomenon, being observed on over 30% of nights for 10.39 and 11.12 mhz, but rarely at other frequencies. at frequencies of 8.01 mhz and below, type 1 propagation events become more frequent as kp increases, while for frequencies higher than this the percentage of nights on which this type of propagation is observed is roughly independent of kp (although there is fig. 4. map showing the inferred reflection point of an 8.01 mhz signal on 18 april 2007, 2030ut to 19 april, 0000 ut. type 2, 8.01 mhz, 2007/04/18, 21:17 ut, kp=2.5 20 ow 10 ow 0 o 10 oe 20 o e 30 o e 45 on 55 on 65 on uppsala leicester vol52,3,2009 20-09-2009 19:06 pagina 305 306 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov some suggestion of a slight decrease in occurrence with increasing kp). for type 2 propagation, there is a clear decrease in the occurrence of observations with increasing kp for all frequencies except 4.64 mhz, where the opposite is the case. type 4 propagation is more commonly observed at the middle range of frequencies (8.01-11.12 mhz), and becomes more frequent with increasing kp. 1.3. comparison of satellite measurements with hf measurements the mechanism by which type 1 off great circle path propagation occurs has been investigated through simulation (siddle et al., 2004b). however, the modelling has not yet been able to explain why the deviations of different types occur on one day but not on otherwise very similar days. in previous work, the parameters describing electron density irregularities embedded in the background ionosphere used in simulation have not been well defined. in this paper, the hf measurements are examined in conjunction with satellite observations of the electron density structure. some caution must be exercised in comparing the hf and satellite measurements since the satellite altitude is about 700 km, while the hf propagation is most strongly affected by the ionosphere at heights of about 200-400 km. since the electron density distribution in the topside ionosphere is the near-earth signature of the magnetospheric plasmapause, it may be expected that the structure of the irregularities due to precipitations will be well correlated at different altitudes, while the behaviour of the instabilities fig. 5. the electron density and temperature measured on board of demeter satellite on 18 april 2007. vol52,3,2009 22-09-2009 14:20 pagina 306 307 aspects of hf radio propagation inside the trough region will depend on height. for reasons of space, only a single example of type 2 off great circle propagation from 18 april 2007 will be presented here. based on the measured angles of arrival and the time of flight of the hf signal (fig. 2), and assuming a single, mirror-like reflection (see siddle et al., 2004b), the reflection point (fig. 4) moves steadily north of the great circle path reaching a point close to the poleward wall of the trough before the signal is no longer received. it is evident from figure 4 that the reflection points of the hf signal in this event are located inside the trough. the demeter satellite data (fig. 5) indicates that, as well as the expected reduction in electron density, there are strong filamentary electron density structures (i.e. irregularities) inside the main ionospheric trough at this time, and we suggest that scattering from these irregularities is the main mechanism responsible for formation of type 2 off great circle propagation. 2. ionospheric model for ray-tracing zaalov et al. (2003; 2005) have developed a unique ray-tracing model that accurately reproduces many of the features observed in the experimental measurements referred to earlier to a level well beyond that which we originally anticipated would be possible. a major outcome of these ray-tracing simulations is that paths other than those that have been the subject of experimental investigation can readily be assessed. the simulations make use of a numerical ray tracing code (jones and stephenson, 1975) to estimate the ray paths through a model ionosphere comprising two chapman layers, the main parameters of which (critical frequency, critical height, vertical scale height of each layer) are based on values obtained from the international reference ionosphere (iri) (bilitza, 1990). the most important causes of off-great circle propagation in the polar cap are the presence of convecting patches or sun-aligned arcs of enhanced ionisation. patches are formed in the dayside auroral oval and generally convect in an anti-sunward direction across the polar cap into the nightside auroral oval, whereas arcs occur under different geomagnetic conditions and drift in a duskwards direction. localised, time varying, perturbations in the electron density are then applied to the background model to represent the convecting patches and arcs of enhanced electron density. sun aligned arcs (carlson et al., 1984): the shape of each sun-aligned arc is defined within the model by a small number of three-dimensional gaussian perturbations in electron density of different spatial scales (altitude, longitude and latitude) randomly distributed near to the centre of the arc. several gaussian perturbations were combined in defining the shape of each modelled arc in order to prevent the shapes of the arcs being too stylised. for all arcs away from close proximity to the dawn or dusk auroral oval, the plasma strands are elongated for several hundreds or thousands of kilometres with a latitudinal scale which is significantly larger than the longitudinal scale. evolution of the structures relative to the propagation path is determined by the rotation of the earth beneath the arcs and by movement of the arcs in the dawn-dusk direction. convecting patches (weber et al., 1984; buchau et al., 1983): the temporal evolution of the patches relative to the propagation path is simulated by means of a convection flow scheme coupled with the rotation of the earth beneath the convection pattern, the precise form of which depends upon the components of the imf. the intensity and spatial scales of the patches can also be varied with simulation time. in practice, the shape, size and number of patches in the convection flow area depends upon many geophysical parameters, not only upon the instantaneous values but also upon their history. by using up to four vortices based on the modelled convection flow patterns associated with the various imf orientations many realistic situations may be simulated. mid-latitude trough: the halcrow and nisbet model (1977) was used in the simulations as a basis for the position of the trough walls. in order to add smaller-scale structure to the wall, the following modifications were performed: (a) the latitude of the walls was perturbed by two-scaled random functions of longitude, and vol52,3,2009 20-09-2009 19:06 pagina 307 308 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov (b) a landscape of patches elongated in the direction of the trough were added to each wall. initially, no perturbations were added to the floor of the trough. the depletion of the trough maximum was set according to kp, typically, 30% for kp = 2-3 and 60% for kp = 6. particle precipitation: the auroral oval is an enhancement of electron density caused by particle precipitation in the e-region and above. as a function of distance along (near-vertical) field lines, the density enhancement was modelled as starting 100 km from the ground, having one or more peaks of about 1013 electrons/m3 around 110 km, and then decaying slowly toward 200 km (bates and hunsucker, 1974). small scale electron density enhancements can also be added to the model. the model is able to reflect the day-to-day variation of received hf signals. see, for example, the time history of the azimuth of arrival and time of flight of a 10.4 mhz signal propagating along uppsala to leicester path on 18-19 april 2007 given in fig. 2 and the simulation results in fig. 6. fig. 6. simulated time history of the azimuth of arrival and time of flight of a 10.4 mhz signal propagating along uppsala to leicester path between noon 18 april and noon 19 april 2007. azimuth deviation top panel, time of flight bottom panel. vol52,3,2009 20-09-2009 19:06 pagina 308 309 aspects of hf radio propagation 3. area coverage simulations the area coverage to be expected from a transmitter at a given location can be estimated by ray-tracing through the model ionospheres described in section 2. a large number of rays launched in an azimuth/elevation grid from the transmitter are traced through the model ionosphere, and the signal strength at the receiver estimated by determining the ray-density in the area around the receive antenna. for the example simulations presented in this case, the dynamic range shown in the figures has been restricted to 20 db in order to highlight the modal structure of the signal. an example outcome of this process is illustrated in figs. 7 and 8. the first of these figures shows a modelled electron density distribution at a height of 200 km and a time of 0230 ut (the dayside ionosphere is positioned in this figure around the bottom left quadrant). patches of enhanced ionisation have been randomly positioned within the model ionosphere, as described previously. the outcome of the ray-tracing process is illustrated in fig. 8 for a frequency of 10.4 mhz for transmitters located at iqaluit and tromsø. the upper frames indicate the expected area coverage when the presence of the patches is ignored, and the lower frames for when the fig. 7. example electron density distribution over the high latitude region. the dayside ionosphere is around the lower left hand quadrant. patches of enhanced electron density are also included. vol52,3,2009 20-09-2009 19:06 pagina 309 310 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov patches are taken into account. focussing of the rays at the 1, 2 and 3 hop skip zone ranges is clearly evident in all cases. it is interesting to note that the presence of the patches severely distorts the background pattern on the ground, that coverage is reduced in places but also that coverage is obtained in areas where it was not present without the presence of the patches. this later situation is more evident at higher frequencies, not illustrated here. in considering the effect of the presence of the patches, it is important to remember that the patch distribution shown is only illustrative and that in reality the patches will be distributed differently, and also that as time progresses the patches will move in accordance with the prevailing convection cell pattern (in turn, a function of the geomagnetic conditions). 4. inversion of hf radar backscatter ionograms hf radars use the refraction by the ionosphere of radio waves emitted by the radar to detect targets up to 3000 km. the distance covered by the radio waves depends on different parameters: the ionospheric characteristics, the transmitted frequency and the elevation angle. in order to optimise the radar frequencies for the geographical area of interest and to localize the targets accurately, knowledge of the ionospheric characfig. 8. calculated signal strength variations for a 10.4 mhz transmitters located at iqaluit (left frames) and tromsø (right frames) covering the high latitude region using the modelled ionosphere of fig. 7. the upper frames are without the presence of polar patches, and the lower frames are with the presence of the patches. vol52,3,2009 20-09-2009 19:06 pagina 310 311 aspects of hf radio propagation teristics is required in quasi real time. in to acquire the necessary ionospheric information, the radar can be used as an oblique backscatter sounder. for this purpose, an inversion method is needed as the forward problem is strongly non linear. by scanning the radar beam in elevation for a fixed azimuth at one operating frequency, the group path can be measured to obtain a backscatter ionogram (a 3d image of magnitude, elevation angle β, and group path p’). the quasi-parabolic segment model (qps) developed by dyson and bennett (1988) has been chosen for the inversion. in this model, each layer of the ionosphere and also the joining segments can be characterized with a qp segment by only three parameters: the critical frequency (fc), the peak height (hm) and the semithickness (ym). then the group path p’ and the ground range d can be obtained using analytical propagation equations. 4.1. inversion technique the purpose is to recover the initial model parameters (fc, hm, ym) of each layer of the ionosphere from n points of the backscatter ionogram (n�3). with these model parameters the calculation of the ground range, d, is possible so the real position of the target can be determined. the data points used are the coordinates of n group paths (p’meas 1, …, p’meas n) at n fixed elevation angles (βref 1, …, βref n) called of reference for each layer. the method, based on tarantola (1987) and landeau et al. (1997), will invert each ionosphere’s layer one by one. for each parameter a value is chosen in the parameter space to create a qps model of the ionosphere and to simulate the elevation-group path curve by using ray tracing. the coordinates of the n group paths simulated corresponding to the n fixed elevation angles are taken (p’simu 1, …, p’simu n). with the coordinates of the n group paths measured and simulated, the a posteriori probability density, σp, can be calculated, using the equation: , , , , f h y p p f h y p 1 1 p c m m meas i simu i c m m meas ii n 1 σ δ = + -= l l l ^ ^ d h h n % where δp’meas1, …, δp’measn represent the variances of the measurement errors over p’meas1, …, p’measn. these variances are presumed to be cauchy distributed and independent of each other. the optimal parameters are those for which σp is maximum. to find them, each value of the parameters space must be evaluated. but this procedure is costly in time and the ionosphere is changing with time, sometimes on a time scale smaller than 15 minutes. it is thus necessary to use an optimization algorithm to get a good approximation of the global optimum. two different optimization algorithms have been tested: the simulated annealing and the genetic algorithm. simulated annealing: the simulated annealing algorithm (sa) developed by kirkpatrick et al. (1983) is based on the manner in which metals recrystallize in the process of annealing. for each step of the sa algorithm a random neighbour of the current solution is considered. if the new solution is better, it is chosen. if it is worse, it can still be chosen with a probability that depends on the difference between the corresponding function values and on a parameter called the temperature (t). the temperature is gradually decreased during the process. at the beginning the current solution changes almost randomly, but the acceptance of bad solutions decreases as t goes to zero. the allowance for bad solutions saves the method from becoming fixed at local minima. genetic algorithm: the second optimization method used is a genetic algorithm developed by goldberg (1989). genetic algorithms are based on the mechanics of natural selection and genetics. an initial generation is created where the individuals are binary coded strings. by using selection, crossover and mutation a new generation is created from the old generation. each generation is better than the previous one, until the optimal solution is found. 4.2. validation on synthesized data synthesized data are created from a model of electronic density profile given by predictions of the ionosphere (table i). after calculatvol52,3,2009 20-09-2009 19:06 pagina 311 312 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov ing the group path-elevation angle curve, we add a zero mean gaussian noise (7.5 km and 2° of standard deviation) to simulate a real backscatter ionogram that is used to test the inversion method. the results of the inversion method are compared to the initial values on fig. 9 and in table i. in fig. 9 the group path-elevation curve obtained by inversion match with the original curve used as data. the two optimization methods are compared in fig. 10 and in table i. the genetic algorithm converges faster than the simulated annealing (1.72 minutes vs 15 minutes with a pc intel dual core). furthermore, the estimated parameters provided by the genetic algorithm are closer to the initial values than by using the simulated annealing. in future works, the genetic algorithm will be used. 4.3. results on real data the inversion method has also been tested on real data using an elevation-scan backscatter ionogram, recorded on 11 june 2007 at 2000 ut. in this particular sounding, only the f layer was present (nighttime’s profile). for validation purposes vertical ionograms have been collected by the ebre ionosonde which was selected because it is located in the great circle of the oblique sounding backscatter beaming. figure 11 compares the elevation angle-group path ionograms obtained with the data inversion and the vertical ionosonde inversion. the corresponding electron density profiles presented in fig. 12 show a general agreement of the 2 profiles. 5. comparison between observations and prediction techniques 5.1. time of flight measurements and their application to testing predictions methods that approximate to ray tracing accurate predictions of the main parameters characterising the hf ionospheric channel require a detailed knowledge of the ionospheric conditions that only numerical or analytic raytracing techniques can provide. numerical raytracing models (such as jones and stephenson, 1975 and norman et al., 1994) are very accurate but they have a high computational time. analytic ray-tracing techniques that enable ray tracing through horizontal gradients along and in the direction of the ray path (norman and cannon, 1997; 1999) are computationally less intensive than numerical ray tracing. for this reason their use is particularly advantageous in hf applications such as in the real-time frequency management of oth (over-the-horizon) radar systems (coleman, 1998). three different predictions methods that approximate ray-tracing techniques were tested: (a) iri-95 based on the monthly median electable i. qps model parameters, initial and estimated, comparison. layer parameter initial value estimated value estimated value with sa with ga e fc 2.62 mhz 2.59 mhz 2.58 mhz e hm 105.4 km 109.4 km 109.24 km e ym 15.1 km 19.28 km 19.17 km f fc 7 mhz 6.99 mhz 7 mhz f hm 261.8 km 263.36 km 262.9 km f ym 93.4 km 94.75 km 93.55 km vol52,3,2009 20-09-2009 19:06 pagina 312 313 aspects of hf radio propagation fig. 9. backscatter ionogram inversion results. fig. 10. comparison of the optimization algorithms speed. vol52,3,2009 20-09-2009 19:06 pagina 313 314 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov fig. 11. elevation angle-group path curves obtained with the inverted parameters and with the ionosonde inversion parameters, and data points used for the inversion at a low operating frequency. fig. 12. electron density profiles obtained with the inverted parameters and with the ionosonde parameters. vol52,3,2009 20-09-2009 19:06 pagina 314 315 aspects of hf radio propagation tron density profiles provided by the iri-95 model (bilitza, 1990; 2001), (b) sirm&br_d based on the sirm (zolesi et al., 1993; 1996) in conjunction with the bradley-dudeney model (bradley and dudeney, 1973), and (c) icepac based on the ionospheric communications enhanced profile analysis and circuit prediction program (stewart, undated). for the iri-95 and sirm&br_d prediction methods, the monthly median electron density profiles provided by the models have been used to calculate the vertical plasma frequency profiles at the mid-point of the uppsala leicester radio link (see section 1). as the effect of earth’s curvature is important for ground ranges greater than 500 km, the curvature of the earth and the known ground range (1411 km) of the radio link have been taken into account for calculating the angle of incidence of the ray at the base of the ionosphere. the values of the angle of incidence have been calculated assuming a very simple geometry based on ionospheric reflections taking place from a simple horizontal mirror at the appropriate height. subsequently, the secant law was applied to calculate the oblique transmission frequencies. the length of the oblique ray path, its corresponding time of flight (tof) and take off angle have been calculated from simple geometry. the version of icepac used in this study utilizes the monthly median electron density profiles (haydon and lucas, 1968) at the midpoint of the radio link derived from ccir fof2 and m(3000)f2 coefficients (ccir, 1966) and, from these, provides predictions of elevation angle, tof, virtual height, etc. for a given oblique transmission frequency and time of day. in order to test the validity of these methods under quiet ionospheric conditions, comparisons between the predictions and the tof median measurements for the one hop propagation modes 1e and 1f were carried out for different frequencies and seasons. for 1e modes (see table ii), iri-95 and sirm&br_d provide a similar performance, while icepac is in general somewhat better. this is particularly the case when es is present. the errors for 1f modes (table iii) are larger table ii. the minimum and maximum value of the r.m.s. error of tof are shown taking into account all the frequencies which arrive at the receiver following the 1e on great circle propagation modes. winter months equinoctial months summer months 1e propagation mode r.m.s.min – r.m.s.max r.m.s.min – r.m.s.max r.m.s.min – r.m.s.max (ms) (ms) (ms) iri-95 0.038 – 0.099 0.044 – 0.085 0.013 – 0.093 sirm&br_d 0.045 – 0.099 0.043 – 0.084 0.008 – 0.072 icepac 0.002 – 0.048 0.022 – 0.062 0.002 – 0.082 table iii. the minimum and maximum value of the r.m.s. error of tof are shown taking into account all the frequencies which arrive at the receiver following the 1f on great circle propagation modes. winter months equinoctial months summer months 1f propagation mode r.m.s.min – r.m.s.max r.m.s.min – r.m.s.max r.m.s.min – r.m.s.max (ms) (ms) (ms) iri-95 0.0710.325 0.072 – 0.393 0.111 – 0.572 sirm&br_d 0.097– 0.139 0.070 – 0.254 0.108 – 0.340 icepac 0.025 – 0.325 0.093 -0.396 0.095 – 0.432 vol52,3,2009 20-09-2009 19:06 pagina 315 316 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov than for 1e, and the performance is similar for all three methods, although icepac can produce significantly better results in winter. further details of this aspect of the investigation can be found in pietrella et al. (2009). 5.2. comparison of oblique sounding measurements and voacap predictions on a mid-latitude path between cyprus and the uk signals from fmcw sounders located in cyprus were received in leicester (a path length ~3340 km) from 1 february 2008 to 13 july 2008. the monthly median muf as a function of time of day is presented in fig. 13. predictions of the muf made using voacap (version 05.0119w) are also presented in fig. 13. voacap has been run using a minimum take-off angle of 0.1°, predicted smoothed sunspot numbers ranging from 7 in february to 11 in july (ngdc, 2008), and with the sporadic e model switched off. although the observations generally have a similar form to the predictions, there are some discrepancies. for example, the predicted median muf at 0700–0800 ut in february is much higher than the observed value (the largest daily value in the month is 21.9 mhz at 0700 ut and 23.5 mhz at 0800 ut), in june the characteristic reduction in muf at noon is not observed, whilst in july, the observed muf is higher than the predicted value at all times. in june and july the discrepancy results from the presence of sporadic-e layers with high (on occasions >30 mhz) peak frequencies. the predicted muf(es) values (obtained using voacap method 11, with a fprob multiplier of 1.0 for es) are closer to those observed in the morning and early afternoon, but significantly lower than the observations in the evening. table iv. modem availabilities with dst=0 2400 bps 1200 bps 600 bps 300 bps 150 bps 75 bps 4 mhz 0 0 0.004 0.983 0.974 1 6 mhz 0 0.026 0.245 0.891 0.869 0.983 10 mhz 0.288 0.472 0.576 0.978 0.978 0.991 11 mhz 0.179 0.424 0.533 0.987 0.974 1 14 mhz 0.122 0.258 0.380 0.987 0.978 0.996 18 mhz 0.153 0.183 0.253 1 0.991 1 table v. modem availabilities with dst=-050 2400 bps 1200 bps 600 bps 300 bps 150 bps 75 bps 4 mhz 0 0 0.04 1 0.96 1 6 mhz 0 0 0.12 0.96 0.92 1 10 mhz 0.32 0.4 0.56 1 0.96 1 11 mhz 0.2 0.4 0.56 1 0.92 11 14 mhz 0.08 0.2 0.28 1 0.92 1 18 mhz 0.16 0.2 0.2 1 0.96 1 vol52,3,2009 20-09-2009 19:06 pagina 316 317 aspects of hf radio propagation 00 06 12 18 00 5 10 15 20 25 30 february 00 06 12 18 00 5 10 15 20 25 30 march 00 06 12 18 00 5 10 15 20 25 30 april m u f ( m h z) 00 06 12 18 00 5 10 15 20 25 30 may 00 06 12 18 00 5 10 15 20 25 30 june time (ut) 00 06 12 18 00 5 10 15 20 25 30 july time (ut) fig. 13. monthly median muf on a path cyprus-leicester for february to july 2008. the observations are plotted as a solid line, while the dashed line represents predicted values from voacap (the predicted muf(es) is plotted as a dotted line in the july panel). all month-hours had at least 26 observations with the exception of july, which only had 13 for all hours. 6. assessment of hf channel availability under ionospherically disturbed conditions various workers have conducted investigations on the signalling characteristics of hf channels, and on the influence of the channel scattering characteristics on data communications capabilities. for example, jodalen et al. (2001) reported on a comparison between modem characterisations and channel measurements from the doppler and multipath sounding network (damson) (cannon et al., 2000). this system characterised narrow-band channels by measuring their snr, doppler spread, and delay spread parameters. measurements of these characteristics may then be applied to three-dimensional performance surfaces to assess signalling capability for various types of modem. for the path from sweden to the uk discussed earlier in this paper, warrington and stocker (2003) previously reported the variation in delay and doppler spreading observed vol52,3,2009 20-09-2009 19:06 pagina 317 318 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov as a function of season and time of day. this previous analysis did not consider variations with ionospheric disturbance, a topic that is addressed here. joint probability density functions (pdfs) of snr, doppler spread, and effective multipath spread versus the disturbance storm type (dst) index (see, for example fig. 14) were produced for measurements made over the path from uppsala, sweden (59.92º n; 17.63º e) and leicester uk (52.63º n; 1.08º w) in 2001. it has been demonstrated by sari (2006) by determining the conditional pdfs and by using bayes’ theorem, that there were dependencies between dst and the above mentioned signal parameters. as examples of modem characterizations, various military standards were considered. in these standards, hf modem performances are specified in terms of effective multipath spread, doppler spread, snr, bit error rate, modulation type, and data conversion (long-short interleaver). modem availability was quantified as a fraction of the time that the modem would function satisfactorily (the availability of a modem can be estimated by determining the difference between the measured snr and the snr required to give an acceptable ber for a given delay and doppler spread a positive snr difference indicates that the modem would operate satisfactorily (see fig. 15)). tables iv and v show the probability of modem availability for different data rates and frequencies according to requirements defined in stanag 4538. 7. concluding remarks various propagation related topics of direct relevance to the planning and operation of hf radio systems have been investigated during the fig. 14. joint pdf for hourly mean of snr and negative of dst, 4 mhz (sari, 2006). joint pdf for hourly_mean (snr) and neg (dst), f = 4 mhz 0.025 0.02 0.015 0.01 0.005 0 0 20 40 600 400 300 200 100 0 hourly mean (snr) neg(dst) vol52,3,2009 20-09-2009 19:06 pagina 318 319 aspects of hf radio propagation cost 296 action, outline details of which are presented in this paper. for mid-latitude paths, these investigations have included comparisons between predicted channel mufs for a path from cyprus to the uk with predicted values obtained using voacap. significant progress has been made in relation to the operation of hf over the horizon radars. such systems require detailed knowledge of the ionospheric electron density profile. in the work presented here, the radar is operated as a backscatter sounder for a period to measure the backscatter characteristics as a function of frequency and elevation angle. these measurements are then input to a new inversion technique to obtain the required electron density profile. the results presented in the paper illustrate the importance of understanding and taking into account the presence of various structural features in the northerly ionosphere, i.e. the electron depletion and irregularities associated with the sub-auroral trough, patches and arcs of enhanced electron density within the polar cap, and irregularities within the auroral zone when planning and operating hf radio links. these features result in radiowaves propagating over paths well displaced from the great circle direction and have clear relevance in the operation of hf radiolocation (hf-df) systems where deviations from the great circle path may result in significant (sometimes inter-continental) triangulation errors. a full understanding of the prevailing propagation mechanisms will enable appropriate selection of receiver sites to be made in order to optimise the positional accuracy. however, the impact of these propagation effects is much wider than this particular application, extending to almost any hf communications system where the signal impacts on the ionosphere within the region polewards of the fig. 15. approximated availability surface according to stanag 4538 (nato, 2000). s n r ( d b ) available point unavailable point doppler spread (hz) m ultipath (m s) vol52,3,2009 20-09-2009 19:06 pagina 319 320 e.m. warrington, a. bourdillon, e. benito, c. bianchi, j.-p. monilie, m. muriuki, m. pietrella, v. rannou, h. rothkaehl, s. saillant, o. sari, a. j. stocker, e. tulunay, y. tulunay and n. y. zaalov sub-auroral trough. this has been illustrated here by employing the northerly ionospheric model to indicate the effect of polar patches on the coverage area of two transmitters located at arctic sites. development of this aspect is currently being undertaken with the aim of improving the hf communications forecasts and nowcasts available to the airlines when operating on trans-polar routes (note that over-conservative forecasts of the likelihood of disturbed communications results on flights being rerouted to non-polar routes with consequent increases in flight time and fuel usage). in addition to the directionally related effects referred to above, the channel transfer characteristics are also affected by the presence of the ionospheric structures, in particular increases in delay and doppler spread are evident. these increased spreads limit the available data throughput available on a particular channel, or alternatively the system availability may be reduced for a particular type of modem. consideration of this aspect has been given for the trough path. acknowledgements the authors would like to thank the swedish meteorological institute, uppsala and the finnish meteorological institute, helsinki for hosting the transmitter systems. the authors would also like to acknowledge support from the epsrc under various grants. references bates, h.f. and r.d. hunsucker (1974): quiet and disturbed electron-density profiles in auroral zone ionosphere, radio science, 9 (4), 455-467. bilitza, d. (ed.) (1990): international reference ionosphere 1990, nssdc 90-22, (greenbelt, maryland, usa). bilitza, d. (2001): international reference ionosphere 2000, radio science, 36 (2), 261-275. bradley, p.a. and j.r. dudeney (1973): a simple model of the vertical distribution of electron concentration in the ionosphere, journal of atmospheric and terrestrial physics, 35, 2131-2146. buchau, j., b.w. renisch, e.j. weber and j.g. moore (1983): structure and dynamics of the winter polar cap f region, radio science, 18, 995-1010. carlson, h.c. jr., v.b. wickwar, e.j. weber, j. buchau, j.g. moore and w. whiting (1984): plasma characteristics of polar cap f layer arcs, geophysical research letters, 11, 895-898. cannon, p.s., m.j. angling and n.j. davies (2000): damson hf channel characterisation a review, 21st century military communications conference proceedings, 1 (2), 59-64. ccir (1966): atlas of ionospheric characteristics, report 340-1, itu, (geneva, switzerland). coleman, c.j. (1998): a ray-tracing formulation and its application to some problems in over-the-horizon radar, radio science, 33 (4), 1187-1197. dyson, p.l. and p.a. bennett (1988): a model of vertical distribution of the electron concentration in the ionosphere and its application to oblique propagation studies, journal of atmospheric and terrestrial physics, 50 (3), 251-262. goldberg, d.e. (1989): genetic algorithms in search, optimization and machine learning, edited by addisonwesley publishing company. halcrow, b.w. and j.s. nisbet (1977): a model of the f2 peak electron densities in the main trough region of the ionosphere, radio science, 12, 815-820. haydon, g.w. and d.l. lucas (1968): predicting ionosphere electron density profiles, radio science, 3 (13), 111-119. itu (international telecommunications union) (1997): standardized procedure for comparing predicted and observed hf sky-wave signal intensities and the presentation of such comparisons, itu-r recommendation p1148-1. jodalen, v., t. bergsvik, p.s. cannon and p.c. arthur (2001): performance of hf modems on high-latitude paths using multiple frequencies, radio science, 36 (6), 1687-1698. jones, r.m. and j.j. stephenson (1975): a versatile three-dimensional ray tracing computer program for radio waves in the ionosphere, office of telecommunications, ot 75-76, (u.s department of commerce, washington, usa). kirkpatrick, s., c.d. gelatt, jr. and m.p. vecchi (1983): optimisation by simulated annealing, science, 220 (4598), 671-680. landeau, t., f. gauthier and n. ruelle (1997): further improvements to the inversion of elevation-scan backscatter sounding data, journal of atmospheric and solar-terrestrial physics, 59 (1), 125-138. nato (2000): stanag 4538 technical standards for an automatic radio control system (arcs) for hf communication links, (nato military agency for standardisation). ngdc (2008): national geophysical data centre, boulder, colorado usa, ftp://ftp.ngdc.noaa.gov/stp/solar_data/sunspot_numbers/2008 norman, r.j., j.a. bennett, p.l. dyson and j.a. nguyen (1994): hirt: homing-in ray tracing program, research report, school of physics., (la trobe university, bundoora, victoria, australia). norman, r.j. and p.s. cannon (1997): a two-dimensional analytic ray tracing technique accommodating horizontal gradients, radio science, 32 (2), 387-396. norman, r.j. and p.s. cannon (1999): an evaluation of a new two-dimensional analytic ionospheric ray-tracing technique: segmented method for analytic ray tracing (smart), radio science, 34 (2), 489-499. vol52,3,2009 20-09-2009 19:06 pagina 320 321 aspects of hf radio propagation pietrella, m., e.m. warrington, a.j. stocker and c. bianchi (2009): time of flight measurements over a radio-link from uppsala to bruntingthorpe and their application to testing predictions methods that approximate the ray tracing technique, advances in space research, doi:10.1016/j.asr.2008.09.038. rogers, n.c., e.m. warrington and t.b. jones (1997): large hf bearing errors for propagation paths tangential to the auroral oval, iee proceedings on microwaves, antennas and propagation, 144 (2), 91-96. sari, m.o. (2006): a new approach for the assessment of hf channel availability under ionospheric disturbances, ms thesis, electrical and electronics eng., (middle east technical univ., ankara, turkey). siddle, d.r., a.j. stocker and e.m. warrington (2004a): the time-of-flight and direction of arrival of hf radio signals received over a path along the mid-latitude trough: observations, radio science, 39 (4), rs4008, doi:10.1029/2004rs003049. siddle, d.r., a.j. stocker and e.m. warrington (2004b): the time-of-flight and direction of arrival of hf radio signals received over a path along the mid-latitude trough: theoretical considerations, radio science, 39 (4), rs4009, doi:10.1029/2004rs003052. stocker, a.j., e.m. warrington and t.b. jones (2003): a comparison of observed and modelled deviations from the great circle direction for a 4490 km hf propagation path along the mid-latitude ionospheric trough, radio science, 38 (3), 1045, doi:10.1029/2002rs002781. stocker, a.j., n.y. zaalov, e.m. warrington and d.r. siddle (2009): observations of hf propagation on a path aligned along the mid-latitude trough, advances in space research, 44 (1), 64-71, doi:10.1016/j.asr.2009.03.07. stewart, f.g (undated): icepac (ionospheric communications enhanced profile analysis and circuit analysis program) technical manual, (available for download at http://www.greg-hand.com/manuals/icepac_tech_manual.pdf). tarantola, a. (1987): inverse problem theory, methods for data fitting and model parameter estimation, edited by elsevier science. warrington, e.m. and a.j. stocker (2003): measurements of the doppler and multipath spread of hf signals received over a path oriented along the mid-latitude trough, radio science, 38 (5), 1080, doi:10.1029/2002rs002815. weber, e.j., j. buchau, j.g. moore, j.r. sharber, j.d. livingston, j.d. winningham and b.w. reinisch (1984): f layer ionization patches in the polar cap, journal of geophysical research, 89, 1683-1694. zaalov, n.y., e.m. warrington and a.j. stocker (2003): the simulation of off-great circle hf propagation effects due to the presence of patches and arcs of enhanced electron density within the polar cap ionosphere, radio science, 38 (3), 1052, doi:10.1029/2002rs002798. zaalov, n.y., e.m. warrington and a.j. stocker (2005): a ray-tracing model to account for off-great circle hf propagation over northerly paths, radio science, 40 (4), rs4006, doi:10.1029/2004rs003183. zolesi, b., l.r. cander and g. de franceschi (1993): simplified ionospheric regional model for telecommunication application, radio science, 28 (4), 603-612. zolesi, b., l.r. cander and g. de franceschi (1996): on the potential applicability of the simplified ionospheric regional model to different mid-latitude areas, radio science, 31 (3), 547-552. vol52,3,2009 20-09-2009 19:06 pagina 321 vol50,3,2007 483 annals of geophysics, vol. 50, n. 3, june 2007 key words ionosonde – ionospheric data – autoscaling – equatorial anomaly 1. introduction since the early years of the last century, ionospheric measurements have played a significant role in studies concerning ionospheric physics and related phenomena. by analyzing an ionogram important ionospheric characteristics can be extracted, that can strongly contribute to the knowledge of physical phenomena such as radio propagation in ionized media, physical and chemical processes in upper atmosphere, ionosphere and magnetosphere coupling, and solarterrestrial relations. furthermore, observation of ionospheric plasma and forecasting of physical phenomena connected to the sun-magnetosphere-ionosphere-thermosphere system holds a remarkable scientific interest with respect to space weather, because of the influence of these phenomena on satellite and terrestrial communications. for such a purpose ionospheric sounders should have some distinctive features, especially oriented towards routine service, like internet connection and the possibility to perform an automatic scaling of the recorded ionograms in real time. the advanced ionospheric sounder (arokiasamy et al., 2002; zuccheretti et al., 2003), built at the istituto nazionale di geofisica e vulcanologia, rome, italy (here abbreviated ais-ingv) satisfies these requirements. like other recent sounders, ais-ingv ionosonde is practically built around a pc and has been designed to fulfill certain physical characteristics such as the power reduction (around 200 w against several kilowatts of traditional systhe new ionospheric station of tucumán: first results michael pezzopane (1), enrico zuccheretti (1), cesidio bianchi (1), carlo scotto (1), bruno zolesi (1), miguel a. cabrera (2)(3) and rodolfo g. ezquer (2)(3)(4) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) ciasur, facultad regional tucumán, universidad tecnológica nacional, tucumán, argentina (3) laboratorio de ionósfera, instituto de física, universidad nacional de tucumán, argentina (4) consejo nacional de investigaciones científicas y técnicas (conicet), buenos aires, argentina abstract an advanced ionospheric sounder, built at the istituto nazionale di geofisica e vulcanologia, rome, italy, was installed at tucumán, argentina, particularly interesting for its location, near the southern peak of the ionospheric equatorial anomaly. the aim of this installation is to collect a large number of continuous data useful both to study the dynamics of the equatorial ionospheric plasma and to develop reliable regional ionospheric prediction models. moreover this ionosonde will contribute to the ionospheric database and real time knowledge of southern hemisphere ionospheric conditions for space weather applications. the ionosonde is completely programmable and two pcs support the data acquisition, control, storage and on-line processing. in this work the first results, in terms of ionograms and autoscaled characteristics, are presented and briefly discussed. mailing address: dr. michael pezzopane, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: pezzopane@ingv.it 484 michael pezzopane, enrico zuccheretti, cesidio bianchi, carlo scotto, bruno zolesi, miguel a. cabrera and rodolfo g. ezquer tems) and consequently weight, size, power consumption and hardware complexity. it exploits the computer resources to manage the sounding, the real time signal processing, data storing and sharing; it has also the capability to be remotely programmable. the basic work of this ionosonde is to generate an ionogram from which virtual heights and critical frequencies can be scaled. but this ionosonde is also equipped with a software, called autoscala (scotto and pezzopane, 2002), able to automatically scale in real time from an ionogram the following characteristics: fof2, muf(3000)f2, m(3000)f2, fxi, fof1, ftes, and h’es. within the italian-argentine collaboration supported by the istituto italo latino americano (iila), an ais-ingv ionosonde was installed at tucumán (geographical coordinates: 26.9s, 294.6e; magnetic coordinates: 15.5s, 3.8e), argentina (fig. 1), near the ionospheric southern equatorial anomaly (ea) crest, at the end of august 2007. from 1957 to 1987 a first generation analog ionosonde operated at the same location contributing to the knowledge of the south american ionosphere. however the characteristics of the new ionospheric station are such that the data obtained can contribute significantly both to the study of the dynamics of the low latitudes ionosphere (for instance in terms of the occurrence of the equatorial spread-f, whose onset is strictly related to the ea development) and to built up reliable regional ionospheric prediction models for radio communications. this paper briefly describes the main characteristics of the ais-ingv/autoscala system installed at tucumán, and then it illustrates the first results of the operating ionospheric station, in terms of recorded ionograms and autoscaled characteristics. 2. the ais-ingv/autoscala system: a brief description the aim of this ionosonde is to fulfil the demand to have a simple low-cost system to sound the ionosphere. to do this a system has been designed in which the most advanced hf radar techniques have been employed (skolnik, 1981, 1990). in order to compensate the power reduction, a 16 bit complementary phase code is employed (golay, 1961) together with pulse-compression and a phase coherent integration, giving the possibility to investigate the ionosphere with a power of 200 w only. the ionosonde is comfig. 1. geographical location of the tucumán ionospheric station. 485 the new ionospheric station of tucumán: first results pletely programmable and the data acquisition, control, storage and on-line processing are supported by a pc (arokiasamy et al., 2002; bianchi et al., 2003; zuccheretti et al., 2003). the antenna system is a typical crossed delta configuration in which a single backstayed mast sustains the radiative elements. they are 90° oriented to limit the energy cross talk between transmitting and receiving antennas. the transmitting and receiving antennas have been built by engineers from the facultad regional tucumán (frt) of the universidad tecnológica nacional (utn); they are triangle shaped, 21 m high and 40 m long, fed by baluns and loaded by proper resistors. the loads are made up with special non inductive resistors to widen the band and to better direct the lobe upwards; the baluns create the proper match with 50 ω lines and power amplifier. the product of the sounding process is a raw ionogram that is made available to be archived as well as to be processed real time by autoscala operating in another pc (fig. 2). the characteristics given automatically as output by autoscala are fof2, muf(3000)f2, m(3000)f2, fxi, fof1, ftes, and h’es. autoscala is based on an image recognition technique, it does not use information on polarization, and it can then be applied to any kind of antenna system. another important characteristic of autoscala is its behaviour for ionograms characterized by a truncated trace; in these cases, if the digital information of the ionogram is considered sufficient, the software can reconstruct the absent trace, giving more reliable automatically scaled values. since the first phase of autoscala development attention has been paid to the quantitative evaluation of the performance of the algorithms by comparing the output from autoscala with the corresponding result obtained by a well experienced operator. these tests have demonstrated the reliability of the software both for quiet and for disturbed conditions (pezzopane and scotto, 2004, 2005). firstly autoscala tries to identify on the ionogram the f2 layer. in order to do this, the ionogram is initially memorized as a matrix. then two empirical curves, that are able to fit the typical shape of the f2 trace, and characterized by several parameters, are defined (pezzopane and scotto, 2007a). for each set of curves the local correlation c with the recorded ionogram is calculated making allowance for both the number of matched points and their amplitude. the set of curves having the maxifig. 2. the ais-ingv/autoscala system data flow. 486 michael pezzopane, enrico zuccheretti, cesidio bianchi, carlo scotto, bruno zolesi, miguel a. cabrera and rodolfo g. ezquer mum value of c is then selected. if this value of c is greater than a fixed threshold ct the selected curves are considered as representative of the f2 trace. fof2 and muf(3000)f2 are then obtained from the selected curve representing the ordinary ray, fof2 as the frequency of the vertical asymptote, while muf(3000)f2 is numerically calculated by finding the corresponding tangent transmission curve. fxi is obtained as the frequency of the vertical asymptote of the selected curve representing the f2 extraordinary ray. m(3000)f2 is simply the ratio between the calculated muf(3000)f2 and fof2. on the contrary if c does not exceed ct the f2 layer autoscaling routine considers the digital information of the ionogram not sufficient to establish whether the f2 trace is visible on the ionogram, and the output for fof2, muf(3000)f2, m(3000)f2, and fxi will be n/a (not available). the algorithm developed to identify the f1 layer automatically (pezzopane and scotto, 2007b) is very similar to the one used for scaling the f2 layer. the main difference is that unlike the f2 layer autoscaling procedure, which is based on the identification of both ordinary and extraordinary rays, this procedure tries to fit only the f1 ordinary ray by using a set of parabolas. for each parabola the local correlation c with the recorded ionogram is calculated making allowance for both the number of matched points and their amplitude. the parabola having the maximum value of c is then selected. if this value of c is greater than a fixed threshold ct, the selected parabola is considered representative of the f1 ordinary ray. once the parabola considered as representative of the f1 ordinary trace is selected, fof1 corresponds with the frequency of the point of this parabola which has the highest virtual height. on the contrary, if c does not exceed ct two alternative outputs for fof1 are possible: no if the f1 cusp is not observed, or n/a if the digital information of the ionogram is considered not sufficient to establish whether the f1 cusp is present or not. it is important to point out that the f1 layer autoscaling routine is not able to function if the f2 layer has not been identified. this because the algorithm needs as input parameter the minimum virtual height of the f2 ordinary trace. in addition, the routine to automatically scale the e sporadic (es) layer (scotto and pezzopane, 2007) has been developed along similar lines to the f2 and f1 layer routines. if only the ordinary component is recorded the output corresponds to foes, while if only the extraordinary component is recorded the output corresponds to fxes. the ionograms recorded by ais-ingv show both components and as a consequence the es routine limits itself to give as output ftes (defined as the top frequency of the es layer according to wakai et al., 1987) being unable to specify this value as foes or fxes. the technique relies on a set of curves having the typical shape of the es layer. appropriate bounds for the height and the frequency are set. in particular curves having maximum frequency lower than the modelled critical frequency foe of the normal e region are not considered. for each curve the local correlation c with the recorded ionogram is calculated with allowance made for both the number of matched points and their amplitude. the curve having the maximum value of c is then selected. if this value of c is greater than a fixed threshold ct the selected curve is considered representative of the es trace. the value ftes is thus obtained as the maximum frequency of the curve together with the associated height h’es. on the contrary if ct is not exceeded then the routine assumes the es trace is not present on the ionogram and the output for ftes and h’es will be no. the processing time of autoscala is approximately 50 s on a computer with 1.60 ghz processor and 512 mb of ram. 3. the ionospheric station of tucumán: its importance and first results the ea is characterized by two enhanced plasma crests on both sides of the magnetic equator at about ±20° magnetic latitude. the electrodynamic drift theory has successfully explained the main features of the ea. according to this theory, north-south geomagnetic field combined with the day-time east-west ionospheric electric field (both fields parallel to the surface of the earth at the equator) creates a plasma fountain rising up to several hundred kilometers. this upward drifting plasma, when losing its momentum, it moves under gravity along the geomagnet487 the new ionospheric station of tucumán: first results ic lines to higher latitudes creating the crests. this fountain effect and the consequent anomaly may encompass more than 30° latitude on either side of the magnetic equator. the plasma fountain and the anomaly exhibit north-south asymmetries with respect to the geomagnetic equator mainly due to field-aligned plasma flows caused by neutral winds (titheridge, 1995). balan and bailey (1995) have studied the equatorial anomaly over jicamarca (geographical coordinates: 12s, 76.8w; magnetic coordinates: 2.4s, 4.9w) using the sheffield university plasmasphere-ionosphere model and they have shown the possible existence of an additional layer, first called the g layfig. 3. an example of a nighttime ionogram recorded on 28 september 2007 at 00:20 ut by the ais-ingv ionosonde installed at tucumán, and autoscaled by autoscala. fig. 4. an example of a daytime ionogram recorded on 27 september 2007 at 12:55 ut by the ais-ingv ionosonde installed at tucumán, and autoscaled by autoscala. 488 michael pezzopane, enrico zuccheretti, cesidio bianchi, carlo scotto, bruno zolesi, miguel a. cabrera and rodolfo g. ezquer f ig . 5. io no gr am s re co rd ed o n 23 s ep te m be r 20 07 f ro m 1 4: 05 t o 14 :4 5 u t b y th e a is -i n g v i on os on de i ns ta ll ed a t t uc um án , an d au to sc al ed b y a ut os ca la . in a ll b ut t he f ir st a nd l as t, de ve lo pm en t an d de ca y of a f 1. 5 ad di ti on al s tr at if ic at io n ar e hi gh li gh te d us in g op en c ir cl es . 489 the new ionospheric station of tucumán: first results er, and now renamed the f3 layer. features and occurrence of this additional stratification have been observed and studied by rama rao et al. (2005), and batista et al. (2002). thampi et al. (2007) have also observed signatures of this additional layer in the latitudinal profiles of total electron content (tec). additional stratifications occur also below the fof2 at low latitudes as reported by lynn et al. (2000). these stratifications that do not alter the f2 maximum continuity are called f1.5. in the light of all these recent studies, the location of the ionospheric station of tucumán assumes a great importance because of the very limited number of operating ionospheric stations in south america, and also because this new ionospheric station is potentially able to generate and store a large number of continuous data. figures 3, 4, and 5 show examples of ionograms respectively nighttime, daytime, and with fig. 6. hourly fof2 plots from 07 to 12 september 2007 as obtained by the ionograms recorded by the aisingv ionosonde installed at tucumán. values manually scaled, and values obtained automatically by autoscala, are indicated by open circles, and solid squares, respectively. 490 michael pezzopane, enrico zuccheretti, cesidio bianchi, carlo scotto, bruno zolesi, miguel a. cabrera and rodolfo g. ezquer possible additional stratifications, recorded by the ais-ingv/autoscala system installed at the new ionospheric station of tucumán. although the ordinary and the extraordinary rays are very close, autoscala succeeds in giving as output reliable automatically scaled values. as evidence of the reliability of the autoscaled values figs. 6 and 7 illustrate two sequences of six days, from 07 to 12 september 2007 (kp = 4+), and from 29 september to 04 october 2007 (kp = 5), for which the fof2 values obtained manually are compared with the corresponding ones scaled by autoscala. even though both sequences of days are geomagnetically quiet, unlike the second one the first sequence presents a high day-to-day variability, typical at low latitudes, that is well matched by autoscala. this points out that a long series of autoscaled data obtained at tucumán can provide a valuable database for studying the dayfig. 7. hourly fof2 plots from 29 september 2007 to 04 october 2007 as obtained by the ionograms recorded by the ais-ingv ionosonde installed at tucumán. values manually scaled, and values obtained automatically by autoscala, are indicated by open circles, and solid squares, respectively. 491 the new ionospheric station of tucumán: first results to-day variability of the equatorial plasma, both for quiet and for disturbed conditions, which is to be still completely understood. the same database may also be used to develop reliable regional ionospheric prediction models. moreover, analysis of the autoscaled validated data has also led to interesting conclusions concerning the importance that the sounding repetition rate of the ionosonde has at these latitudes. in fact during the testing phase it has been seen that an hourly repetition rate could hide interesting phenomena well visible on the contrary with a higher repetition rate, as shown in fig. 8a,b. 4. summary this work describes the installation of a new ionospheric station at tucumán, argentina, particularly interesting for its location, exactly in the southern equatorial anomaly. the station was equipped with an ais-ingv/autoscala system, able to give as output autoscaled ionospheric values. for this reason the station can contribute to the ionospheric database and can be a part of a possible net for space weather purposes. on this subject it is worth noting that the real time fof2 values produced by the station are already being used by the australian ips radio and space services for mapping purposes (see the site <http://www.ips.gov.au/hf_systems/ 6/5>). to date, the ionograms recorded at the tucumán ionospheric station by the ionosonde aisingv, and autoscaled by autoscala, are accessible real time through the site <http://ionos.ingv.it/ tucuman/latest.html>. acknowledgements the authors gratefully acknowledge prof. sandro maria radicella for his helpful suggestions. references arokiasamy, b j., c. bianchi, u. sciacca, g. tutone and e. zuccheretti (2002): the new ais-ingv digital ionosonde design report, ingv int. tech. rep. no. 12. balan, n. and g.j. bailey (1995): equatorial plasma fountain and its effects: possibility of an additional layer, j. geophys. res., 100 (a11), 21,421-21,432. batista, i.s., m.a. abdu, j. macdougall and j.r. souza (2002): long term trends in the frequency of occurrence of the f3 layer over fortaleza, brazil, j. atmos. sol. terr. phys., 64 (12-14), 1409-1412. bianchi, c., u. sciacca, a. zirizzotti, e. zuccheretti and b.j. arokiasamy (2003): signal processing for phase-coded hf-vhf radars, ann. geophysics, 46 (4), 697-705. golay, m.j.e. (1961): complementary series, ire trans. inform. theory, 7, 82-87. lynn, k.j.w., t.j. harris and m. sjarifudin (2000): stratification of the f2 layer observed in southeast asia, j. geophys. res., 105 (a12), 27,147-27,156. pezzopane, m. and c. scotto (2004): software for the automatic scaling of critical frequency fof2 and fig. 8a,b. a) hourly and b) 5 min fof2 plots of 01 october 2007 as obtained by the ionograms recorded by the ais-ingv/autoscala system installed at tucumán. in this case an hourly repetition rate hides an interesting phenomenon, from 05:00 to 08:00 ut, well visible on the contrary with a higher repetition rate. a b 492 michael pezzopane, enrico zuccheretti, cesidio bianchi, carlo scotto, bruno zolesi, miguel a. cabrera and rodolfo g. ezquer muf(3000)f2 from ionograms applied at the ionospheric observatory of gibilmanna, ann. geophysics, 47 (6), 1783-1790. pezzopane, m. and c. scotto (2005): the ingv software for the automatic scaling of fof2 and muf(3000)f2 from ionograms: a performance comparison with artist 4.01 from rome data, j. atmos. sol. terr. phys., 67 (12), 1063-1073. pezzopane, m. and c. scotto (2007a): the automatic scaling of critical frequency fof2 and muf(3000)f2: a comparison between autoscala and artist 4.5 on rome data, radio sci., 42, rs4003, doi: 10.1029/ 2006rs003581. pezzopane, m. and c. scotto (2007b): a method for automatic scaling of f1 critical frequencies from ionograms, radio sci. (in press). rama rao, p.v.s., k. niranjan, d.s.v.v.d. prasad, p.s. brahmanandam and s. gopikrishna (2005): features of additional stratification in ionospheric f2 layer observed for half a solar cycle over indian low latitudes, j. geophys. res., 110 (a04307), doi: 10.1029/2004ja010646. scotto, c. and m. pezzopane (2002): a software for automatic scaling of fof2 and muf(3000)f2 from ionograms, in proceedings of ursi 2002, 17-24 august 2002, maastricht (paper 1018 of the electronic copy). scotto, c. and m. pezzopane (2007): a method for automatic scaling of sporadic e layers from ionograms, radio sci., 42 (rs2012), doi: 10.129/2006rs003461 skolnik, m.i. (1981): introduction to radar systems (mcgraw hill, new york), pp. 581. skolnik, m.i. (1990): radar handbook (mcgraw hill, new york), pp. 1232. thampi, s.v., n. balan, s. ravindran, t.k. pant, c.v. devasia, p. sreelatha, r. sridharan and g.j. bailey (2007): an additional layer in the low-latitude ionosphere in indian longitudes: total electron content observations and modelling, j. geophys. res., 112 (a06301), doi: 10.1029/2006ja011974. titheridge, j.e. (1995): winds in the ionosphere-a review, j. atmos. sol. terr. phys., 57 (14), 1681-1714. wakai, n., h. ohyama and t. koizumi (1987): manual of ionogram scaling, 3rd version (radio research laboratory ministry of posts and telecommunications, japan), pp. 119. zuccheretti, e., c. bianchi, u. sciacca, g. tutone and b.j. arokiasamy (2003): the new ais-ingv digital ionosonde, ann. geophysics, 46 (4), 647-659. (received september 24, 2007; accepted october 29, 2007) f e n o m e n i t r o p o s f e r i c i di t r a s p o r t o e d i f f u s i o n e a g r a n d e s c a l a p o s t i i n e v i d e n z a da t r a c c i a n t i r a d i o a t t i v i g i o r g i o f e a a d r i a n o gazzola ricevuto il 15 giugno 1961 p r e m e s s a . in meteorologia, tra i problemi che presentano maggior difficoltà, sia per vindagine teorica che per quella sperimentale, vi sono quelli relativi alla determinazione delle traiettorie delle masse d'aria nei loro moti orizzontali e verticali, e quelli relativi alla valutazione dei fenomeni di diffusione che accompagnano tali moti. si pensi che le traiettorie, relativamente brevi nei fenomeni « locali », giungono a compiere anche uno o più volte il giro del globo. analogamente, e in particolare, i fenomeni di diffusione si svolgono a scale estremamente diverse: da quella submicroscopica molecolare degli strati aderenti al stiolo, a quella continentale od emisferica delle onde cicloniche. infatti nei processi di « scambio » fra strati sovrapposti o adiacenti, da percorsi del milionesimo di millimetro e tempi di percorso del centomilionesimo di secondo, si passa a percorsi di migliaia e decine di migliaia di chilometri con tempi dell'ordine del giorno o della settimana. cosicché i relativi coefficienti di diffusione variano fra valori i cui estremi stanno fra loro come uno a un migliaio di miliardi. tra i fenomeni permanenti o saltuari, dovuti alla natura o agli esperimenti dell'uomo, che possono agevolare le ricerche sui moti delle masse d'aria e sulla loro diffusione, sono di particolare ausilio quelli della radioattività naturale e della radioattività artificiale. le sostanze radioattive possono infatti essere utilizzate nell'atmosfera come « traccianti », in modi analoghi a quelli coi quali esse sono utilizzate negli studi di biologia e medicina. 1 6 g . f e a a . g a z z o l a il lavoro che si presenta costituisce appunto un tentativo di indagine sommaria sulle possibilità di studio delle reali traiettorie seguite dalle masse d'aria che si trovavano sulla zona dell'esplosione ìiucleare francese avvenuta nell'interno del sahara il 13 febbraio del 1960. in quel momento infatti l'armosfera, particolarmente priva di prodotti nucleari artificiali, ben si prestava a porre in evidenza quantitativi anche piccolissimi di tali sostanze. già in occasione del x convegno dell'associazione geofìsica i t a l i a n a sono stati esposti i primi risultati degli studi in corso presso la divisione studi e ricerche del servizio meteorologico d e l l ' a . m . sulla circolazione nella troposfera, studi condotti avvalendosi come traccianti dei prodotti di fissione emessi nelle prime due esplosioni nucleari francesi. sull'argomento sono s t a t i pubblicati due articoli: il primo sugli a t t i del x convegno d e l l ' a . g . i . (j); il secondo, più ampio, sulla rivista di meteor. aeronaut. (2). da p a r t e sua i . brauer, del servizio meteorologico tedesco, ha condotto uno studio analogo al nostro sulle conseguenze della prima esplosione nucleare francese(ù); tale lavoro è giunto a nostra conoscenza solo di recente. i n esso sono s t a t e t r a t t e conclusioni che in p a r t e concordano con quelle da noi dedotte ed in p a r t e contrastano con esse. i n questa relazione discuteremo i p u n t i di c o n t a t t o e quelli controversi; è bene premettere che i d a t i su cui i due studi sono basati si integrano a vicenda, in quanto i tedeschi erano a conoscenza dei dati non posseduti da noi e viceversa. p e r facilitare la l e t t u r a di quanto seguirà si ritiene opportuno riepilogare brevemente le nostre opinioni, riportando dall'articolo precedentemente pubblicato sulla rivista di meteorologia aeronautica alcuno p a r t i essenziali ed alcune figuro. l'esame dei dati di r a d i o a t t i v i t à e quello delle situazioni meteorologiche esistenti nel periodo successivo all'esplosione ci ha indotto a distinguere i q u a t t r o casi di trasporto di polveri radioattive schematizzati in fig. 1. di questi q u a t t r o casi i primi t r e sono i più importanti. essi presentano le seguenti caratteristiche fondamentali: tipo a: trasporto nell'alta troposfera per mezzo delle correnti occidentali ed in particolare nel getto. i l m o t o è generalmente, veloce e la radioattività a u m e n t a rapidamente con la quota. tipo b : trasporto nella bassa e media troposfera, con convogliamento nelle grandi celle anticicloniche subtropicali (alisei). il moto è relaf e n o m e n i t r o p o s f e r i c i d i t r a s p o r t o e d i f f u s i o n e , e c c . 1 7 fig. 1. tipi di t r a s p o r t o di polveri r a d i o a t t i v e 1 8 g . f e a a . g a z z o l a tivaraente lento e l ' a t t i v i t à radioattiva diminuisce rapidamente con la quota. tipo c: trasporto sull'italia di polveri risollevate in condizioni di turbolenza nei bassi strati: tipo c2 trasporto diretto sull'italia. i n tali casi l'aria affluente contiene notevoli q u a n t i t à di sabbia di origine sahariana. | fig. 2. p e r c o r s o presumibile, a 500 mb, della n u b e (li tipo a p r o d o t t a dalla p r i m a esplosione s a h a r i a n a . f e n o m e n i t r o p o s f e r i c i d i t r a s p o r t o e d i f f u s i o n e , e c c . 24 tipo c\ trasporto di ritorno sull'italia, a causa di modeste circolazioni, interessanti i bassi strati, sull'africa settentrionale. nei casi di trasporto di tipo c sono s o p r a t t u t t o interessate le località più vicine all'origine; l ' a t t i v i t à radioattiva decresce con la quota. nel lavoro tedesco sono s t a t i presi in considerazione solo t r a s p o r t i dei tipi da noi classificati come a e 0 ; in particolare non è s t a t a esamifig. 3. t r a i e t t o r i a della n u b e r a d i o a t t i v a t r o p o s f e r i c a p r o d o t t a d a l l a prim a esplosione s a h a r i a n a , secondo l ' i n t e r p r e t a z i o n e di b r a u e r . 2 0 g . f e a a . g a z z o l a n a t a la possibilità della formazione di u n a nube di tipo b, t r a s p o r t a t a dagli alisei nei bassi strati della atmosfera. l a formazione di nubi di tale tipo era già stata posta in evidenza in un articolo di p . a . sheppard (4) sulla dispersione delle particelle radioattive nell'atmosfera. manca inoltre, nel lavoro di brauer, ogni considerazione sulla distribuzione verticale della radioattività. le traiettorie seguite dalla nube principale di tipo a, conseguente alla prima esplosione atomica, tracciate da noi e dai tedeschi, concordano in modo abbastanza soddisfacente, p u r esistendo differenze, dovute a diversità di criteri nel tracciamento ed a inevitabili discordanze di valutazione (figg. 2 e 3). la prima p a r t e del percorso sino al giappone è s t a t a tracciata da brauer sulla base del f a t t o che il giorno 17 febbraio venne osservata a tokio la presenza nelle precipitazioni di a t t i v i t à b e t a d o v u t a a prodotti di fissione recenti; la velocità media con cui la nube avrebbe dovuto viaggiare nell'alta troposfera per raggiungere tokio a tale d a t a è s t a t a assunta come u n a misura empirica del trasporto della nube radioattiva; mentre il percorso successivo venne tracciato in base ai venti a 500 m b , che furono ritenuti rappresentare in modo soddisfacente l'ordine di grandezza della velocità di trasporto della nube. noi invece (che non eravamo in possesso dei d a t i giapponesi) ci siamo limitati a considerare il trasporto nella media troposfera, scegliendo, per t u t t o il tragitto, il livello di riferimento di 500 mb. il f a t t o che la nostra traiettoria indichi il passaggio della nube in prossimità del giappone il giorno 19, cioè due giorni dopo il 17, non è in contrasto essenziale con le osservazioni, in quanto a tokio la radioattività f u registrata in corrispondenza del passaggio di u n marcato f r o n t e freddo, il quale determinò precipitazioni che trascinarono al suolo i prodotti di fissione esistenti nell'alta troposfera. ora le correnti a 300 m b erano n e t t a m e n t e più forti che non a 500 mb. per quanto si riferisce all'arrivo della nube troposferica radioattiva sull'europa occidentale i dati italiani (che non erano in possesso dei tedeschi) consentono di affermare (riteniamo senza ombra di dubbio) t r a t tarsi del passaggio della nube di tipo a, che h a effettuato il giro dell'emisfero e che è giunta sull'italia con correnti da n w il giorno 28 (fig. 4). la precisazione che si t r a t t a s s e di u n a nube di tipo a si è r i t e n u t a opportuna, in quanto brauer ha p r o s p e t t a t o la possibilità che si t r a t t a s s e di prodotti direttamente provenienti dalla regione di esplosione, convogliata da u n ' a l t a pressione al suolo ed in quota, esistente in quei giorni sull'africa nord-occidentale; tali prodotti si sarebbero successivamente immessi entro la corrente occidentale principale. f e n o m e n i t r o p o s f e r i c i d i t r a s p o r t o e d i f f u s i o n e , e c c . 2 1 tale interpretazione è b a s a t a sul presupposto che in q u o t a esistesse ancora sulla regione (li esplosione, a t a n t i giorni da quello dell'esperimento, un'imponente « riserva » di polveri radioattive. tale « riserva » si sarebbe f o r m a t a a causa di moti di subsidenza al disopra dell'inversione, che in genere, nelle regioni subtropicali interessate dalla fascia delle alte pressioni in quota, limita superiormente lo strato convettivo. fig. 4. i s o p l e t e di r a d i o a t t i v i t à r e l a t i v e a l l ' i t a l i a (periodo dal 13 f e b b r a i o al 2 marzo 1960 valori in pc/m 3 ). tale possibilità, da noi non presa in considerazione, che f u già oggetto di precedenti ricerche (5) (•) è a l t a m e n t e suggestiva; essa ci sembra debba essere presa in particolare considerazione nei trasporti di tipo c, nei casi cioè in cui, specialmente in corrispondenza di u n passaggio frontale freddo, la distruzione temporanea dell'inversione e la turbolenza potrebbero portare nei bassi strati i p r o d o t t i della « riserva » che si aggiungerebbero a quelli immessi nell'atmosfera per il risollevamento turbolento di polversi radioattive. non ci sembra peraltro giustificabile l'ipotesi che tale « riserva » debba essere s t a t a così imponente clic i prodotti da essa prelevati, a t a n t i giorni da quello dell'esplosione, possano determinare valori di radioattività così elevati quale quello di 15 pc/m3 registrato a pian kosà il giorno 29 febbraio. 2 2 g . f e a a . g a z z o l a le opinioni di brauer e le nostre concordano nell'ammettere che i valori di 0,8 e 0,9 pc/m3 registrati nell'aria a monaco nei giorni 21 e 22 ii (e che l'analisi radiochimica ha dimostrato essere dovuti a prodotti di fissione recenti) possano attribuirsi a trasporto di tipo 0 di polveri sahariane in situazioni di scirocco e nel ritenere i m p o r t a n t e l'effetto dei passaggi frontali freddi sul risollevamento di polveri sahariane contaminate. una grande perplessità desta invece in noi l'interpretazione tedesca degli alti valori registrati nell'america centrale. secondo la nostra interpretazione tali alti valori sono da imputarsi ad u n a nube di tipo b, ti a s p o r t a t a dagli alisei nei bassi strati dell'atmosfera; per tale nube l'inversione subtropicale di subsidenza agisce essenzialmente come sbarr a m e n t o per il sollevamento dei prodotti di fissione ai livelli superiori. si può così spiegare la forte concentrazione delle polveri radioattive in f e n o m e n i t r o p o s f e r i c i d i t r a s p o r t o e d i f f u s i o n e , e c c . 2 3 fig. 6. tsoplete di r a d i o a t t i v i t à a l u n g a v i t a secondo i m e r i d i a n i i 10° w e 80° w (periodo dal 13 f e b b r a i o al 30 marzo 19(50 valori in pc/m 3 ). 2 4 g . f e a a . g a z z o l a tale nube anche a grande distanza dall'origine. la circolazione predominante nei bassi strati nel periodo considerato può essere bene rappres e n t a t a dalla situazione al suolo del 20 febbraio (fig. 5). si osservino in fig. 6 le isoplete di radioattività dell'aria relative a due sezioni temporali eseguite lungo i meridiani 80° e 110° w ; in fig. 7 sono riprodotte le ubicazioni delle località le cui osservazioni sono state utilizzate per l'elaborazione della fig. 6. ti f a t t o che la nube sia p e r v e n u t a prima sulla località più orientale (paramaribo) t r a quelle delle latitudini più basse intorno a 20° nord e poi alle località più occidentali, (curagao prima e miraflores poi) è u n a conferma della nostra interpretazione. i bassi valori successivamente pig. 7. ubicazione delle località i cui d a t i sono s t a t i utilizzati p e r t r a c c i a r e le isoplete di fig. 6. registrati a san j u a n e miami sono attribuibili a prodotti di dispersione dalla nube principale di tipo b, deviati verso nord dalle correnti dei bassi strati esistenti sulla regione ai primi di marzo. è da notare, a cpiesto riguardo, che non risulta che brauer fosse a conoscenza dei dati di curagao e di p a r a m a r i b o (stazioni facenti p a r t e della rete olandese di osservazione) né di quelli degli s t a t i uniti forniti d a l l ' u . s . d e p a r t m e n t of h e a l t h , educatimi and welfare. u n a conferma a posteriori del nostro p u n t o di vista ci viene offerto dall'articolo stesso di brauer, che ci fornisce i risultati che w a r d e marr (') dedussero dalle osservazioni e f f e t t u a t e nel ghana. a tamale infatti, 1800 k m a sud di reggane, giungeva, 3 giorni dopo l'esplosione, u n a nube radioattiva che faceva registrare l'altissimo valore di 1000 f e n o m e n i t r o p o s f e r i c i d i t r a s p o r t o e d i f f u s i o n e , e c c . 2 5 pc/m3 e i cui effetti si protraevano per circa 10 giorni. sono le polveri radioattive contenute in tale nube, che deviate successivamente verso occidente dalle correnti orientali della p a r t e inferiore della cella anticiclonica subtropicale, hanno in seguito raggiunto l'america centrale oltre che per le ragioni precedentemente esposte l'interpretazione di brauer non ci convince per i seguenti motivi: a) u n a biforcazione del flusso in quota a occidente delle hawaii può essersi verificata il giorno 19 (rivedasi la fig. 3); m a ci sembra che il suo effetto non abbia p o t u t o essere altro che quello di produrre u n a dispersione laterale (sia pure notevole) dalla nube principale di tipo a, dispersione che riteniamo responsabile dei valori registrati a mauna loa, nelle hawaii. u n a notevole q u a n t i t à dei prodotti dispersi avrebbe dovuto però entrare a far p a r t e della circolazione dell'anticiclone subtropicale ed essere risospinta verso l'asia. l a p a r t e dei p r o d o t t i t r a sportata verso levante lungo la nuova traiettoria avrebbe dovuto essere scarsa, molto diffusa, ed avrebbe dovuto spostarsi con velocità n e t t a mente inferiori a quelle relative al percorso più settentrionale. b) p e r quanto i dati meteorologici in nostro possesso relativi all'america centrale siano scarsi, ci sembra di poter escludere che durante il periodo fine febbraio-inizio marzo si siano verificati sia la presenza di u n a ben definita goccia f r e d d a in quota (di cui parla l'articolo) sia un imponente e persistente trascinamento al suolo di aria superiore subsidente, estremamente secca. tale trascinamento avrebbe dovuto avvenire in concomitanza con un nucleo anticiclonico in quota, centrato sulle regioni sud occidentali degli s t a t i uniti e sul messico, a l t a pressione di cui non esiste traccia a 500 m b . il golfo del messico è s t a t o invece interessato in tale periodo da zone depressionarie nei bassi strati, associate a perturbazioni di carattere f r o n t a l e e nella zona del mar dei caraibi hanno predominato, in prossimità del suolo, correnti umide' provenienti da oriente. riassunto si espongono alcuni risultati degli studi compiuti presso il servizio meteorologico dell'a. m. sui fenomeni troposferici di trasporto e diffusione a grande scala dei prodotti di fissione immessi nell'atmosfera in conseguenza della prima esplosione nucleare sahariana. 31 g . f e a a . gazzola si confrontano tali risultati con quelli ricavati da un analogo studio, condotto presso il servizio meteorologico tedesco, discutendo le principali divergenze d'interpretazione. su m mary some results of studies, carried out ai italian air force weather service, on tropospheric phenomena of large scale difjusion, and transfer of fission products, merged into the atmosphere as a eonsequence of first sahara nuclear blast, are here presented. tliese results are then compared iviih tliose obtained by a similar study iarried out at german weather service, and more important disagreements cn interpretation are finally discussed. b i b l i o g r a f i a ( ! ) f e a g . , g a z z o l a a., m o n t a l t o m . , esempi di impiego di traccianti radioattivi nello studio dei moti atmosferici, « a t t i del x convegno dell'associazione geofìsica i t a l i a n a ». (2) — — — ricerche sui moti atmosferici a grande scala, condotte sulla base di osservazioni della radioattività a lunga vita, « r i v i s t a di meteorologia a e r o n a u t i c a », 3, ( l u g l i o s e t t e m b r e 1961). (3) brauer, von i., die verfrachtung radioaktiver scliwaden nacli dem franzósisclien atomtest in der sahara vom 13-2-1960, « a t o m k e r n e n e r g i e », 1, 6, (1961). ( 4 ) s i i e p p a r d , p . a . , dispersione di particelle radioattive nell'atmosfera. « rivis t a di meteorologia a e r o n a u t i c a », 3, ( l u g l i o s e t t e m b r e 1959). (6) neuwirth, r., mei. auswertung von messungen der kunsthchen radioaktivitàt der luft und des niederschlagens, «geoflsica p u r a ed a p p l i c a t a » , 32, (1955). ( 6 ) i s r a e l h . e r e i f f e r s c h e i d 1 1 . , bemerlcungen zur radioalctiven verseuchung der atmosphàre. « a t o m e n e r g i e », 3, 250, (1958). ( 7 ) w a r d a n d m a r r , radioactive fallout in ghana, « n a t u r e 4739 », ( j u l y 23, 1960). vol48/04/2005def 561 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words silicate melts – structure – entropy – unmixing 1. introduction in bernal’s words (bernal, 1959), a liquid is an «homogeneous, coherent and essentially irregular assemblage of molecules containing no crystalline regions or holes large enough to admit another molecule». this acceptance of a liquid structure is particularly in line with the random network model (rnm) of zachariasen (1932, 1933) and warren (1933) for silica melts and glasses: silicate tetrahedrals linked through corners without any long-range periodicity. already in the 1960s, evans and king (1966) had physically built up an rnm composed of corning glassworks tetrapods linked by bent springs, the angles of the bends having a montecarlo-generated gaussian distribution centered at 163°. the measured radial distribution of atoms around the silicon and oxygen reference atoms of the physical model appeared to be in good agreement with existing neutron and x-ray scattering data (warren et al., 1936; warren, 1937; carraro, 1964). more than half a century of experimental research essentially confirmed the first illuminating views of zachariasen, warren and bernal, and nowadays we may basically state the following: – the structure of silicate melts and glasses has elements of long-range randomness coexisting with shortand medium-range order (gaskell et al., 1991). – the short-range order is dominated by the structure of the sio4 rigid unit (greaves et al., 1981), although the distance beyond which the radial distribution function becomes indistinguishable from the 4π r 2ρ 0 function, representing homogeneous distribution of electron density, shrinks progressively with the introduction chemical interactions and configurational disorder in silicate melts giulio ottonello laboratorio di geochimica, dipartimento per lo studio del territorio e delle sue risorse (dipteris), università degli studi di genova, italy abstract the thermodynamics of quasi-chemical and polymeric models are briefly reviewed. it is shown that the two classes are mutually consistent, and that opportune conversion of the existing quasi-chemical parameterization of binary interactions in mo-sio2 joins to polymeric models may be afforded without substantial loss of precision. it is then shown that polymeric models are extremely useful in deciphering the structural and reactive properties of silicate melts and glasses. they not only allow the lux-flood character of the dissolved oxides to be established, but also discriminate subordinate strain energy contributions to the gibbs free energy of mixing from the dominant chemical interaction terms. this discrimination means that important information on the short-, mediumand long-range periodicity of this class of substances can be retrieved from thermodynamic analysis. lastly, it is suggested that an important step forward in deciphering the complex topology of the inhomogeneity ranges observed at high sio2 content can be performed by applying scmf theory and, particularly, matsen-schick spectral analysis, hitherto applied only to rubberlike materials. mailing address: dr. giulio ottonello, laboratorio di geochimica, dipartimento per lo studio del territorio e delle sue risorse (dipteris), università degli studi di genova, corso europa 26, 16132 genova, italy; e-mail: giotto@dipteris.unige.it 562 giulio ottonello of alkalis («network modifiers»; see later) in the system (waseda and suito, 1977), which is indicative of significant break-up of the random network. – there are no systematic structural differences between silicate melts and glasses of identical composition, and the effects of thermal expansion are virtually negligible on both table i. structural data for binary silicate melts and glasses (from brown et al., 1995). interionic distance rij(a) approximated to ± 0.01 a; coordination number nij approximated to ± 0.3 atoms; root mean square atomic displacement ∆rij1/2 approximated to ± 0.005. join t(°c) pair x-ray (molten state) x-ray (glassy state) reference rij (a) nij (atoms) ∆rij1/2 rij (a) nij (atoms) ∆rij1/2 pure sio2 1750 si-o 1.62 3.8 0.096 1.62 3.9 0.087 (1) o-o 2.65 5.6 0.124 2.65 5.5 0.102 (1) si-si 3.12 3.9 0.187 3.11 3.9 0.141 (1) 0.33li2o1150 si-o 1.61 3.8 0.117 1.62 3.7 0.090 (2) -0.67sio2 li-o 2.08 4.1 0.131 2.07 3.8 0.095 (2) o-o 2.66 5.5 0.195 2.65 5.6 0.101 (2) si-si 3.13 3.8 0.260 3.13 3.8 0.143 (2) 0.33na2o1000 si-o 1.62 4.1 0.095 1.62 4.0 0.086 (2) -0.67sio2 na-o 2.36 5.9 0.151 2.36 5.8 0.101 (2) o-o 2.66 5.6 0.202 2.65 5.2 0.112 (2) si-si 3.20 3.8 0.279 3.21 3.6 0.146 (2) 0.33k2o1100 si-o 1.62 3.9 0.124 1.62 3.8 0.086 (2) -0.67sio2 k-o 2.66 13.0* 0.182* 2.65 13.2* 0.120* (2) o-o 2.66 2.65 (2) si-si 3.23 3.7 0.257 3.23 3.5 0.154 (2) 0.50mgo1700 si-o 1.62 3.9 0.109 1.63 3.7 0.096 (3) -0.50sio2 mg-o 2.12 4.3 0.151 2.14 4.6 0.108 (3) o-o 2.65 5.4 0.215 2.65 5.7 0.151 (3) si-si 3.16 3.3 0.282 3.15 3.4 0.213 (3) 0.50cao1600 si-o 1.61 3.9 0.127 1.63 3.8 0.109 (3) -0.50sio2 ca-o 2.35 5.9 0.171 2.43 5.9 0.125 (3) o-o 2.67 5.2 0.206 2.66 5.5 0.183 (3) si-si 3.20 3.1 0.264 3.23 3.4 0.199 (3) 0.66feo1400 si-o 1.62 3.9 0.147 (4) -0.33sio2 # fe-o 2.05 3.9 0.214 (4) o-o (4) si-si 3.27 3.1 0.302 (4) (1) waseda and toguri (1990); (2) waseda and suito (1977); (3) waseda and toguri (1977); (4) waseda and toguri (1978); * pair correlations for k-o and o-o overlap, so that coordination numbers and peak widths could not be determined independently; # o-o pair correlation overlaps with fe-fe correlations, thus o-o distance could not be determined independently. 563 chemical interactions and configurational disorder in silicate melts mean structure and single interionic distances (see, for this purpose, the review article of brown et al., 1995, and references therein). table i lists some important x-ray scattering data concerning the structure of binary silicate melts collected by brown et al. (1995) and essentially in line with the above three points. table ii, based on the extensive compilation by brown et al. (1995), lists concisely the mean coordination numbers experimentally observed for the various cations in melts and glasses. although a generalized increase in the coordination number of the central cation is observed with the increase in the lux-flood basicity of the corresponding oxide, the scatter in cn observed for basic components is a clear indication that network modifiers «adapt» themselves to the geometry established by the much stronger (and essentially covalent) bond of network formers, and not vice versa. in this light, it is difficult to agree with the hypothesis of gaskell et al. (1991) that compositionally similar pyroxene or pyroxenoid glasses may be represented as parallel random-fractal sheets of interconnected cao6 octahedra interleaved by si atoms (occupying intersitial tetrahedral sites linked in infinite chains). more probably, in the present author’s opinion, the structure of silicate melts or glasses locally resemble that of all-si zeolites, in which «cages» of variable coordination number are determined by the relative symmetry-determining arrangements of sio4 monomeric units. despite this topological controversy, it is quite evident that conformational disorder does exist in silicate melts and glasses. exactly how thermodynamics can account for this conformational disorder may be purely phenomenological (as in subregular models and polynomial expansions) or may be based on more sophisticated physical models (as in the gaussian random walk method; see later). in any case, to be satisfactory, the thermodynamic model must not only reproduce the expected reactive properties of the substance, but also the inhomogeneity ranges often observed at high sio2 content. we discuss below the informational content arising from sound thermodynamic treatment of silicate melt energetics, and propose some conceptual guidelines to be followed in the near future, in order to decipher the complex features of the inhomogeneity ranges observed at high sio2 content along simple mo-sio2 binaries, and also in chemically complex systems. 2. configurational disorder in quasi-chemical models guggenheim’s quasi-chemical model (guggenheim, 1935; fowler and guggenheim, 1939) assumes that, in a binary system with components 1 and 2, «particles» 1 and 2 mix substitutionally in a quasi-lattice obeying short-range ordering dictated by the equilibrium table ii. p = 1 bar mean m-o coordination number of cations in silicate melts and glasses. listed figures are largely based on extensive compilation of brown et al. (1995). allocation among acidic, amphotheric and basic oxides after ottonello and moretti (2004) and this work (#). m acidic amphotheric basic notes si 4 ge# 4 al 4 in na-al. .si-o melts. al 5.5-6 in pure al2o3 melt. feiii 4 ti 4-5-6 zr 6-8 ni 4-5-6 li# 4 na 5-6 k 7-8 rb# 8 cs 8 ca 4-6-7-8 mg 4-5-6 feii 4-5-6 mn 4-5-6 pb 8 564 giulio ottonello (2.1) the molar enthalpy change associated with equilibrium (2.1) is expressed in terms of pair bond energies ε ij (2.2) where n is the total number of atoms and z is the coordination number. the non-configurational part of the molar entropy is also given in terms of bond contributions σ ij (non-configurational entropy of the ij pair bond) (2.3) if xij denotes the molar fraction of the ij particle with respect to the total of the particles formed in the system, since each atom i is bonded to zj ( ) . nz 2 2 12 11 22= -~ h h h ( ) nz 2 2 12 11 22= -~ f f f [ ] [ ] [ ] .1 1 2 2 2 1 2,+ neighbors, it follows from mass balance that (2.4) the enthalpy of mixing then becomes (2.5) the non-configurational (excess) entropy of mixing is (2.6) and the configurational (ideal + excess) entropy of mixing is (2.7) ( )ln ln ln ln ln s r x x x x rz x x x x x x x x x x 2 2 , conf 1 1 2 2 11 1 2 11 22 2 2 22 12 1 2 12 mixing $ $ = + + +c m s x 2, non conf 12 mixing = hh x 2 12 mixing = ~ ( ) ( ) . x x x x x 2 2 12 1 11 22= = fig. 1a-d. integral enthalpy of mixing and integral entropy of mixing of quasi-chemical model for different values of interaction parameter ω along mo-si0.5o2 join ((a) and (b), respectively). below: if usual molar notation (i.e., component sio2 instead of si0.5o) is adopted and parameter ω is kept unvaried, symmetricity about intermediate composition is lost ((c) and (d) respectively). a b c d 565 chemical interactions and configurational disorder in silicate melts because when the mixture is completely random, x11 = x12, x22 = x22 and x12 = x1x2. the equilibrium concentrations of the various pairs obey the gibbs free energy minimization principle (2.8) this, through opportune expansions in terms of eqs. ((2.5) to (2.7)), gives (2.9) the name «quasi-chemical» assigned to the model stems from the fact that the term on the right in eq. (2.9) reduces to a constant, for constant values of ω and η, thus resembling the notation of a chemical reaction among ideal [1–1], [2–2] reactants and [1–2] product. equation (2.9) yields (pelton and blander, 1986) (2.10) where (2.11) the model is resolved first in terms of equilibrium distribution of particles (eqs. (2.10), (2.11) and (2.4)) and then in terms of enthalpy and entropy (eq. (2.5) and eqs. ((2.6) to (2.7)), respectively). the computed enthalpic and entropic terms have a typical vand m-shaped conformation, the more pronounced as the value assigned to ω becomes more negative. the quasi-chemical enthalpy of mixing and entropy of mixing curves computed with z = 2, η = 0 and various ω are shown for comparison in fig. 1a,b, respectively, for a mo-si0.5o system. when ω and η are set at zero the mixture is obviously ideal and, when η = 0 and x12 = 2x1x2, the mixture is strictly regular. in all cases, mixtures are always symmetric in the compositional space of interest. ( ) .expx x zrt t 1 4 2 11 2 2 1 = + -p ~ h c m; e( 2 x x x 2 1 212 1 2= + p .exp x x x t zrt 4 2 2211 12 2 = -~ h^ h ; e t( ) . x g x h s 0 mixing mixing mixing 12 122 2 2 2 = = 2.1. modified quasi-chemical approach the symmetricity about intermediate composition was long regarded as a severe limitation to the application of the quasi-chemical model to silicate melts and slags which invariantly exhibit maximum ordering about the composition xmo = 2/3 in the mo-sio2 chemical space. in order to shift the maximum ordering condition depicted by the quasi-chemical model to the condition actually observed in mosio2 melts, pelton and blander (1986) proposed substituting true molar fractions x1, x2 with «equivalent fractions» y1, y2 so that (2.12) (2.13) coefficients b1 and b2 are chosen in such a way as to have b1/(b1+b2)=1/3 and b2/(b1+b2)=2/3. clearly, when x1=2/3 and x2=1/3, we have y1=y2=0.5. through this substitution it follows that, instead of eq. (2.4), the following equation holds (pelton and blander, 1986): (2.14) the molar enthalpy of mixing is now (pelton and blander, 1986) (2.15) the non-configurational (excess) entropy of mixing is (2.16) and the configurational (ideal + excess) entropy of mixing is (2.17) ( ) ( ) . ln ln ln ln ln s r x x x x rz x y x x y x x y y x b x b x 2 2 ,mixing conf 1 1 2 2 11 1 2 11 22 2 2 22 12 1 2 12 1 1 2 2 $ $ $ $ = + + + + c m ( )s x b x b x 2,mixing non conf 12 1 1 2 2= + h( )h x b x b x 2mixing 12 1 1 2 2= + ~ ( ) ( ) . x y yx x 2 12 1 11 2 22= = .y b x b x b x 2 1 1 2 2 2 2= + y b x b x b x 2 1 1 1 1 1 2 = + 566 giulio ottonello although whatever b1, b2 couple exists, so that b1/b2 = 1/2 results in y1/y2 = 1/2, application of the additional condition that configurational entropy must attain zero when ω = − ∞ yields b1= 0.6887, b2 = 1.3774 for binary mo-sio2 melts with maximum ordering about xmo = 2/3 (cf. pelton and blander, 1986, for details). lastly, in order to assign more computational elasticity to the model, parameters ω and η are expanded as polynomial functions of equivalent fractions y1, y2, i.e. (2.18) (2.19) actually, the asymmetricity of mo-sio2 joins is due to the difference in the amounts of oxygen in the two end-member components. if we adopt si0.5o instead of sio2 as end-member, maximum ordering will be observed at xmo = 0.5, and no readjustment in terms of equivalent fractions will be necessary. obviously, if we wish to repy .y y n n 0 1 2 2 2 2 2f= + + +h h h h h yy y n n 0 1 2 2 2 2 2f= + + +~ ~ ~ ~ ~ table iv. non-configurational excess entropy parameters of modified-quasi-chemical model for binary silicate melts (after pelton et al., 1995). ao bo b1 b2 η 0 η 1 η 2 η 3 η 4 η 5 η 6 η 7 alo1.5 sio2 2.0661 2.7548 0. 0. 0. 0. 0. 0. 0. 0. bo1.5 sio2 2.0661 2.7548 0. −25.104 0. 0. 0. 0. 0. 0. cao sio2 1.3774 2.7548 −19.456 0. 0. 0. 0. 0. 0. 0. ko0.5 sio2 0.6887 2.7548 −58.576 0. 0. 0. 0. 0. 0. 0. mgo sio2 1.3774 2.7548 0. −37.656 0. 0. 0. 0. 0. 125.52 mno sio2 1.3774 2.7548 −20.92 0. 0. 0. 0. 0. 0. 62.76 nao0.5 sio2 0.6887 2.7548 −43.932 0. 0. 0. 0. 0. 0. −20.92 nio sio2 1.3774 2.7548 0. 0. 0. 0. 0. 0. 0. 125.52 pbo sio2 1.3774 2.7548 0. 0. 0. 0. 0. 0. 0. 12.97 sio2 tio2 2.7548 2.7548 0. 0. 0. 0. 0. 0. 0. 0. zno sio2 1.3774 2.7548 −33.472 0. 58.576 0. 0. 0. 0. 0. zro2 sio2 2.7548 2.7548 0. 0. 0. 0. 0. 0. 0. 0. table iii. chemical interaction parameters of modified-quasi-chemical model for binary silicate melts (after pelton et al., 1995). ao bo b1 b2 ω 0 ω 1 ω 2 ω 3 ω 4 ω 5 ω 6 ω 7 alo1.5 sio2 2.0661 2.7548 4800. 0. 0. 100784. 0. −142068. 0. 78571. bo1.5 sio2 2.0661 2.7548 16958. −32531. 0. 0. 0. 0. 0. 0. cao sio2 1.3774 2.7548 −158218. −37932. 0. 0. 0. −90148. 0. 439893. ko0.5 sio2 0.6887 2.7548 −409986. 0. 0. 0. 0. 0. −1647688. 1593677. mgo sio2 1.3774 2.7548 −86090. −4874. 0. 0. 0. 0. 0. 328109. mno sio2 1.3774 2.7548 −79956. 0. 0. 0. 0. 0. 0. 228819. nao0.5 sio2 0.6887 2.7548 −114344. −381595. 0. 0. 0. 0. 0. 123010. nio sio2 1.3774 2.7548 29169. 0. 0. 0. 0. 0. 0. 509783. pbo sio2 1.3774 2.7548 −25430. 0. 0. 0. 0. 0. −245806. 310959. sio2 tio2 2.7548 2.7548 28847. 52091. 0. −44484. 0. 0. 0. 0. zno sio2 1.3774 2.7548 −124741. 129292. 15989. 0. 0. 0. 0. 98990. zro2 sio2 2.7548 2.7548 4184. 40585. 0. 0. 0. 0. 0. −11715. 567 chemical interactions and configurational disorder in silicate melts resent the chemical space of interest in terms of mo-sio2 components, then it is sufficient to consider that two moles of si0.5o correspond to one mole of sio2, the resulting molar fractions xmo, xsio2 are translated along the abscissa, and an apparent asymmetricity arises, as depicted in fig. 1c,d. tables iii and iv list the extensive modified-quasi-chemical parameterization of binary silicate melts of pelton et al. (1995). the listed parameters allow precise conformation of phase boundaries along the binary joins and are also consistent with observed unmixing phenomena. 3. configurational disorder in polymer models in polymeric models for silicate melts, it is postulated that, at each composition, for given values of p and t, the melt is characterized by an equilibrium distribution of several ion species of oxygen, metal cations and ionic polymers of monomeric units sio44–. the charge balance of a polymerization reaction involving sio44– monomers may be formally described by a homogeneous reaction involving three forms of oxygen: singly bonded o-, doubly bonded (or «bridging oxygen») o0, and free oxygen o2− (fincham and richardson, 1954) (3.1) in fact, eq. (3.1) is similar to a reaction between monomers (3.2) polymer chemistry shows that, the larger the various polymers become, the more their reactivity is independent of the length of the polymer chains. this fact, known as «the principle of equal reactivity of co-condensing functional groups», has been verified in fused polyphosphate systems (which, for several properties, may be considered as analogous to silicate melts; cf. fraser, 1977) with polymeric chains longer than 3po44– units (meadowcroft and .sio si o osio 24 4 2 7 6 4 4 , ++ -.o o o2 melt meltmelt 0 2 , +richardson, 1965; cripps-clark et al., 1974). assuming this principle to be valid, equilibrium constant k18 (3.3) (in which the terms in brackets represent the number of moles per unit mole of melt) is always representative of the polymerization process, independent of the effective length of the polymer chains. 3.1. toop-samis model toop and samis (1962a,b) showed that, in a mo-sio2 binary melt, in which mo is the oxide of a basic cation completely dissociated in the melt, the total number of bonds per mole of melt is given by (3.4) where nsio 2 are the moles of sio2 in the mosio2 melt. the number of bridging oxygens is thus (3.5) mass balance gives the number of moles of free oxygen per mole of melt (3.6) where (1 − nsio2) are the moles of basic oxide. equations ((3.3) to (3.6)) yield (3.7) (o–) is thus given by the quadratic equation (3.8) which may be solved for discrete values of k18 and nsio2. ( ) ( ) ( ) ( ) ( ) k n n n o o4 1 2 2 8 1 0 sio sio sio 2 18 2 2 2 + + + + = ( ) [ ( )] [ ( )] .k n n 4 o 4 o 2 2 osio sio 18 2 2 2= ( ) ( ) .no o 1 2sio 2 2 = -^ h ( ) ( ) . n o o 2 4 sio0 2= ( ) ( ) no o2 4 sio 0 2 2 =( ) ( ) ( ) k o o o 18 2 0 2 = 568 giulio ottonello the gibbs free energy change involved in eq. (3.1) is (3.9) since two moles of o− produce one mole of o0 and one of o2−, the gibbs free energy of mixing per mole of silicate melt is given by (3.10) 3.2. extended polymeric approach (epa) although the original toop-samis model is a powerful tool in deciphering the complex relations existing between chemical interaction and configurational disorder, there are two minor points that must be quantified, i.e. assessment of the low polymerization limit, and extension to the multicomponent field. the usual expression of the thermodynamic activity of a component in mixture (ottonello, 2001) yields (3.11) as the mean number of monomers in the polymer chain is defined as (3.12) adopting temkin model activities of the fused salts, it is quite evident that, along any binary join, we may pose (3.13) although the domain of eq. (3.13) spans the entire compositional range, it is obvious that can never be lower than one (i.e., a monomer) and, consequently, the limit of maximum depolymerisation defines a limiting activity represented by sior ( ) ( ) ( ) . a o o sio ,mo binary 2 2 2 si = o r ( )sio anions2si =or / ( ) ( ) .exp ln a rt rt k x x g 2 o 1 mo mo mixing 18 mo 2 2 = + > h ( ) .lng rt k 2 o mixing 18= -∆ .lng rt k1818 0 = -∆ (3.14) thus, the molar abundances of the various oxygen species cannot be expected to be those of the original toop-samis model, especially when highly basic oxides are present in the liquid phase. in solving thermodynamic activity on a temkin model basis, toop and samis (1962a,b) observed that the mean extension of the polymer chains is univocally defined by a «polymerzsation path» depicted in terms of mean numbers of silicon atoms per polymer unit versus the stoichiometric ratio (o–)/[(o–)+(o–)+siiv] (cf. figs. 2 and 3 in toop and samis, 1962a). however, their further assumption – that a single polymerization path of general validity in the ternary system cao-feo-sio2 may be proposed on the basis of viscosity data – cannot be shared, because a different reaction constant, k18, pertains to each mo-sio2 system and, as the activity of the basic dissolved oxide mo is implicitly defined by the partial derivative of the gibbs free energy of mixing at any point of the compositional space of interest, we have (ottonello, 2001) (3.15) (3.16) (3.17) (3.18) (3.19) (3.20)( )si sio2iv = ( ) ( ) o x o 2mo 2 = ( ) ( ) o x o 2 4 4 0 mo= ( ) ( ) ( ) x x k k x x 2 4 4 2 8 2 32 4 1 mo mo p mo mo 2 18 2 2 1 + + -( )o = 6 @ ( ) ln x g rt k x o 2 1 mo mixing mo 182 2 2 2 = ( ) ( ) exp rt g g anions x x o 1mixing mo mo mixing 2 2 2 = + ; e * 4 / ( )n anionssiosi 2=or / ( ) ] .sio#[( )sio!= ( ) ( ) ( ) a o sio o , minmo binary 2 2 2 2 2+569 chemical interactions and configurational disorder in silicate melts where the terms in brackets denote the number of moles and xmo is the molar fraction of a generic basic oxide mo in a mo-sio2 binary join. distinct «polymerization paths» relating the mean number of silicon atoms in the polymeric units ( ) to the relative proportion of singly bonded oxygen in the unit (o–)/ [(o–) +(o0) +siiv] may be calculated for the two limiting binaries cao-sio2 and feo-sio2 on the basis of the system of eqs. ((3.15) to (3.20)). these paths differ substantially from the general path proposed over 40 years ago by toop and samis (1962a,b) and, more importantly, they differ greatly from each other. the apparent complexity introduced by recognizing that each oxide imposes its own imprint on the extension of polymeric units turns sior out to be a powerful tool in deciphering the reactive properties of chemically complex melts. in fact, chemical interactions within the anion matrix of an n-component system are completely fixed by the interaction properties observed along the n-1 limiting binaries. as discussed elsewhere (moretti and ottonello, 2003), the intrinsic thermodynamic significance of this evidence is apparent in the flood-grjotheim thermodynamic cycle (flood and grjotheim, 1952). if kp, a-f, kp, b-f are the polymerization constants valid along the limiting a-f, b-f binaries, the polymerization constant of the extended toop-samis model turns out to be (3.21) the above equation states that, in a compositionally complex melt, network modifier cations a, b of charge ν+a, ν+b interact with free oxygen o2– (and hence with the polymeric units built up by network former f), with a simple proportionality described in terms of equivalent fractions (flood and grjotheim, 1952; ottonello, 2001; moretti and ottonello, 2003) (3.22) application of the flood-grjotheim thermodynamic cycle to the cao-feo-sio2 system satisfactorily reproduces the bulge in the thermodynamic activity of feo experimentally observed long ago by elliot (1955) (fig. 2). no high-order terms are necessary to reproduce the observed complex thermodynamic behavior, except for a topologically negligible interaction in the cation sublattice. 3.3. hybrid polymeric model in the hybrid polymeric model (hpm) (ottonello, 2001), the gibbs free energy of mixing in binary melts is first subdivided into a (dominant) chemical toop-samis interaction term plus a (subordinate) elastic hookian-like strain energy contribution term, as is observed in block copolymers (see later) = .n n n n a a a a a b bf+o o o + + + +ol f+ln k+n+ln k+ln k n , ,p a p a f b p b fo= -+ +ol l fig. 2. feo activity surface at 1600°c for liquid feo-sio2-cao ternary system. graphic representation obtained by inverse-distance contouring of 9900 activity values generated by extended polymeric model, adopting, as limiting kp, 0.21 for feo-sio2 join and 0.002 for cao-sio2 join, respectively. assumed cationic interaction wca-fe = − 33 kj/mol. strain energy contributions included (from ottonello, 2001, with modifications). 570 giulio ottonello (3.23) in eq. (3.23), is the number of monomeric units of statistical length a extended to a distance x, and the remaining symbols assume the usual thermodynamic significance. the elastic strain energy depends on bending term x/a (which basically represents the effect of covalent bonding on the relative arrangement of monomers in the polymer chain). this term is arbitrarily expanded into a polynomial of type (3.24) strain coefficients χ1…χn depict linear or second-order t dependence. since the polymerization path along any join is defined in terms of versus nsio2, the strain energy calculated along a given binary at various t conditions is intimately related to the extent of polymerization along any particular compositional path (i.e., the mean number of silicon atoms in the chain, ). the existence of a maximum depolymerization limit and the approximate nature of the entropy terms depicted by the adopted parameterization of bending angles means that the problem cannot be solved exactly (more precise definition of entropic intermediate ordering effects on the bulk stability of the phase would require application of the self-consistent mean field theory – see later). to overcome these complexities, ottonello and moretti (2004) modified eq. (3.23) as follows: (3.25) in eq. (3.25), s18 is the entropic contribution embodied in the toop-samis formulation of chemical interaction, and η represents the strain contributions arising from the conformational arrangements of the various polyanions in the anion matrix. this term is again arbitrarily ex( ) .g h t s t 2 o mixing 18 18= h∆ ∆ ∆ ^ h sior sior f+n$|+n$|+n$|+n$|= a x sio sio sio sio1 2 2 2 3 3 4 4 2 2 2 sior ( ) lng g g rt k rt a x 2 o 2 3 mixing chemical strain si 18 2 = + = + + o ∆ ∆ ∆ r b l panded into a polynomial on nsio2, the coefficients of which are inversely dependent on t (3.26) (3.27) the new strain energy term −tη is more akin to the modified-quasi-chemical formulation of the non-configurational entropy of mixing (pelton and blander, 1986) with respect to the original formulation (ottonello, 2001) and allows better comparison of the two model energetics. obviously, eq. (3.27) implies that, here, strain energy is not purely entropic, but embodies enthalpic and entropic sublattice interactions within the anion matrix which are not explained by the original toop-samis formulation. this amount of energy is far less than that arising from chemical interaction, and important only in quantifying the observed unmixing phenomena at high sio2 content in the mixture (ottonello, 2001). tables v and vi list the results of non-linear minimization calculations carried out to bring the modified quasi-chemical parameterization of pelton et al. (1995) (tables iii and iv) for binary mo-sio2 melts to an hpm formulation. the polymerization constants were first calculated for a single join at various t conditions of interest in the specified t range, and then regressed on an arrhenian function of absolute t. stemming from the obtained regression coefficients, there is no doubt that the adopted functional has precise thermodynamic significance which results in expansion (3.25) of the previous hybrid model. the first striking evidence that arises from hpm treatment of interactions in mo-sio2 melts is the clear distinction between basic, amphoteric and acidic oxides. basic oxides, which are essentially network modifiers in an mo-sio2 liquid, exhibit purely enthalpic contributions to chemical interactions in mixture, whereas acidic oxides (network formers) give rise to athermal behavior when mixed among themselves; and, clearly, amphoteric oxides exhibit both entropic and enthalpic contributions to chemical interactions in the melt phase. if we now analyse the standard state enthalpic contribution to chemical interactions in mo-sio2 melts we note that they are simply re.t, ,1 0 1 1 1= +| | | n$|+n$|+n$|+|= sio sio sio1 2 3 2 4 3 2 2 2 h 571 chemical interactions and configurational disorder in silicate melts table v. chemical interaction of hpm (from ottonello and moretti, 2004). join lnkp = a/t + b r2 t range notes a b (°c) k2o-sio2 −31 708 0 1127-1527 (1) −29 540 0 1223 (2) −36967 0 0.9922 1500-1800 (3) na2o-sio2 −23336 0 0.9995 1000-1800 (3) cao-sio2 −15372 0 0.9958 1000-2000 (4) −14807 0 0.9943 1000-2000 (2) mgo-sio2 −9809.5 0 0.9955 1400-2000 (3) zno-sio2 −6460.1 0 1400-2000 (3) mno-sio2 −6183.8 0 0.9596 1000-2000 (3) −5649.1 0 1600 (1) pbo-sio2 −5330.0 0 0.5825 1000-1800 (3) −4098.1 0 1273 (5) feo-sio2 −3600.0 0 0.9973 1000-2000 (4) fe2o3-sio2 7569.5 −7.2752 0.9350 1000-2000 (5) tio2-sio2 4667.3 −3.2092 0.9107 1500-1900 (3) zro2-siov 2685.9 −6.382 0.9658 1400-2000 (3) nio-sio2 1507.7 −1.7772 0.9825 1500-2000 (3) al2o3-sio2 0 −1.4059 1000-2000 (3) b2o3-sio2 0 −1.0660 1000-2000 (3) (1) this work; (2) ottonello and moretti (2004), based on experiments of eliezer et al. (1978); (3) ottonello and moretti (2004), based on modified quasi-chemical parameterization of pelton et al. (1995); (4) ottonello (2001); (5) ottonello et al. (2001). table vi. strain energy of hpm (from ottonello and moretti, 2004). join parameterization notes k2o-sio2 χ1 −2.8681 −1.6701e+5 (1) χ2 −79.501 6.8787e+4 χ3 99.065 1.5980e+5 χ4 10.499 −1.9243e+3 na2o-sio2 χ1 −11.766 −1.4559e+5 (1) χ2 39.776 2.2133e+5 χ3 −141.53 8.0710e+4 χ4 130.52 −9.4332e+4 cao-sio2 χ1 −1.7859 −5.4644e+3 (1) χ2 31.535 −2.0262e+4 χ3 −171.49 2.8175e+5 χ4 260.67 4.8533e+5 mgo-sio2 χ1 22.144 −3.4756e+4 (1) χ2 −143.77 2.2334e+5 n$|+n$|+n$|+|= =| t, , sio sio sio1 2 3 2 4 3 1 0 1 1 1 2 2 2 + h | | 572 giulio ottonello lated to the atomistic properties of the central cation, like pauling’s electronegativity (pauling, 1932, 1960) (fig. 3) or to spectroscopically derived magnitudes, such as differences in optical basicity between metal oxide and silica (duffy and ingram, 1974; gaskell, 1982; sosinsky and sommerville, 1986; duffy, 1989, 1990; ottonello et al., 2001) (fig. 4). table vi (continued). join parameterization notes mgo-sio2 χ3 186.75 −3.4506e+5 χ4 173.75 −2.7150e+4 zno-sio2 χ1 −12.840 3.5427e+4 (1) χ2 −6.3910 −1.4814e+5 χ3 164.050 −6.1594e+4 χ4 −117.200 9.1512e+4 mno-sio2 χ1 −7.3391 1.3047e+4 (1) χ2 −18.670 3.5311e+4 χ3 92.477 −1.8607e+5 χ4 −23.422 3.1079e+4 pbo-sio2 χ1 21.546 −3.5593e+4 (1) χ2 −59.309 2.6842e+4 χ3 132.23 −3.4732e+4 χ4 −58.665 −1.2444e+4 feo-sio2 χ1 10.214 −2.6396e+4 (2) χ2 −19.638 6.5843e+4 χ3 67.340 −2.1477e+5 χ4 −37.220 1.2462e+5 tio2-sio2 χ1 −11.099 1.7398e+4 (1) χ2 −20.417 −8.0525e+2 χ3 7.2065 2.5437e+4 χ4 30.269 −4.3948e+4 zro2-sio2 χ1 −15.898 2.2410e+4 (1) χ2 −18.918 −1.7096e+4 χ3 16.482 −1.2937e+3 χ4 14.203 −1.3222e+4 nio-sio2 χ1 −3.2377 2.7449e+3 (1) χ2 21.656 4.9178e+3 χ3 −49.883 −4.5618e+4 χ4 18.565 5.9709e+4 al2o3-sio2 χ1 −3.7071 −6.1142e+2 (1) χ2 26.748 −1.5242e+4 χ3 −64.828 4.2276e+4 χ4 77.543 −7.0683e+4 b2o3-sio2 χ1 0.0928 −6.1472e+3 (1) χ2 11.819 7.9109e+3 χ3 −41.175 −2.0305e+4 χ4 36.358 3.8358e+5 (1) based on modified quasi-chemical parameterization of pelton et al. (1995); (2) based on modified quasichemical parameterization of pelton and blander (1986) and pelton (pers. comm.). n$|+n$|+n$|+|= =| t, , sio sio sio1 2 3 2 4 3 1 0 1 1 1 2 2 2 + h | | 573 chemical interactions and configurational disorder in silicate melts the discrimination capability of optical basicity in terms of lux-flood basicity is even clearer when looking at polymerization constant kp. as shown in fig. 5, the relationship between the natural logarithm of kp and the optical basicity difference is so evident as to allow it to be used as a predictive tool for the chemical interaction of basic oxides with silica. each fig. 3. reaction enthalpy of equilibrium 2 plotted against pauling’s electronegativity of mo metal cation. fig. 4. reaction enthalpy of equilibrium 2 plotted against difference in optical basicity of mo and sio2. adopted basicity values are those of column 6 in table a1 of ottonello et al. (2001). selection of other basicity values within uncertainty ranges (columns 1 to 6 in same table) do not greatly alter observed relations. fig. 5. polymerization constant in mo-sio2 melts at various t (with mo = basic oxide) plotted against difference in optical basicity of mo and sio2 (adopted basicity values are those of column 6 in table a1 of ottonello et al., 2001). selection of other basicity values within uncertainty ranges (columns 1 to 6 in same table) do not greatly alter observed relations. dashed lines: extrapolations at t beyond t limits investigated here for most basic oxides. solid heavy line: functional form adopted by ottonello et al. (2001) and moretti and ottonello (2003) when role of t was still unclear. 3 4 574 giulio ottonello of the depicted straight-line dependencies in fig. 5 has a correlation coefficient r 2= 0.9985, and their slopes and intercepts also exhibit a second-order dependence on absolute t. 4. configurational disorder in block copolymers although there is no experimental evidence about the possibile formation of local block copolymeric structures in silicate melts or glasses, recent findings in this branch of materials science are also important for proper understanding of the unmixing phenomena which take place in sio2-rich melts. the physics of microphase separation in block copolymers is understood in terms of an expanded flory-huggins model. the flory-huggins model belongs to the class of quasi-chemical approaches. flory himself defined his method as «a restricted variation of the quasi-chemical method used by guggenheim» (cf. flory, 1953, p. 507). basically, the main merit of the flory-huggins approach is to acknowledge that molar fractions are unsuitable chemical parameters whenever molecules differing greatly in size interact among themselves and with much smaller stoichiometric units in the melt phase. volume fractions are thus substituted for molar fractions, based on the identity (4.1) where φi is the volume fraction of the ith component in mixture, ni its molar amount, υi its partial molar volume, and the summation at the denominator is extended to all k components in mixture. block copolymers are composed of two or more distinct blocks interconnected by linear or branched sequencing. the relative arrangements of the various blocks gives rise to an astonishing number of distinct configurations, which is responsible for the many useful and desirable properties exploited by the industry of rubberlike materials. the simplest architecture is the linear ab diblock: a long sequence of type a monomers covalently bonded to a chain of type b monomers. the relative arrangements of the two blocks is n n i i i i k i i 1 =z y y = / characterized by «a fluidlike disorder on the molecular scale and a high degree of order at longer length scales» (bates and fredrickson, 1999). in interpreting the structural and reactive properties of diblock copolymers, the following working assumptions are commonly made: – each molecule in the melt is composed of ν segments, f a-type and (1−f) b-type monomers. – a and b segments represent a sufficient length of polymers such that they can be treated as gaussian, where the internal conformational states of the segments produce a hookian entropic penalty of stretching. – the statistical length of a segment is its average rms end-to-end length when no tension is applied, and is related to the effective spring constant. – the average segment concentration is forced to be uniform (incompressibility constraint). the interaction between a and b is described in terms of a χab flory-huggins interaction parameter which, in units of thermal energy kt, is expressed as follows: (4.2) where z is the number of nearest-neighbor monomers to a copolymer configuration cell (note the analogy with eq. (2.2)). if χab is large, chemical interaction leads to a macroscopic segregation of a and b (binodal or spinodal decomposition). if instead χab is sufficiently low, the thermodynamic forces driving separation are counterbalanced by entropic restoring contributions (i.e., «chain elasticity»; intimately connected to the covalent character of the bond but hookian in nature) reflecting the requirement that, to keep the dissimilar a and b portions of each molecule apart, copolymers must adopt extended configurations (bates and fredrickson, 1999). 4.1. mean field approximation theories helmholtz free energy f (the incompressibility constraint) (fredrickson et al., 1994, fredrickson and liu, 1995) is kt z 2 1 ab ab aa bb= +| f f f] g: d 575 chemical interactions and configurational disorder in silicate melts (4.3) where q is the reciprocal of the distance in the search space and sa, sb are form factors of the a and b block copolymers (fredrickson et al., 1994; fredrickson and liu, 1995; chen et al., 2000, 2002). these «form factors» assume precise values depending on the extent and geometry of the block. for a linear chain l of monomers of statistical kuhn length b, the form factor is simply sl = νlg(x) with νl = degree of polymerization and (4.4) (4.5) for branched polymers the form factors are more complex and are obtained by a combina( ) ( ) .g x x x2 e 1x 2= + -x q b 6l 2 2= o dqlnq= ( ) ( )f kt s q s q 4 a a b b2 2 0 + r z z 3 7 a# tion of arm and backbone contributions (see fredrickson et al., 1994, 1995, for appropriate treatment). 4.2. matsen-schick self-consistent mean field method the spectral analysis developed by matsen and schick in a series of articles (matsen and schick, 1994a,b; matsen and bates, 1996; matsen, 1998) is a powerful predictive tool in deciphering the complex microphase separation processes taking place in rubberlike materials. the theory stems from the self-consistent mean field (scmf) treatment of hong and noolandi (1981). the form factor functionals are substituted here by space-occupation functions rα(s) describing the space curve occupied by the αth copolymer, and s is the parameterization variable. in a system of n starblock copolymers composed of m ab diblock arms joined together at a central core, s = 0 at the core, s = f at the ab junction, and s = 1 at the end of the first arm. in the second arm, s is 1, 1+ f, 2, and so on, until the end of the last arm, where s = m is reached. the space occupation function is thus piecewise continuous with discontinuities at integer values (fig. 6). the partition function for a system of p polymers of polymerization ν and density ρ0 is (4.6) p is the probability density functional for a given curve (see eq. 2 in matsen and schick, 1994a, for the approximate form adopted to describe p); , are (dimensionless) a and b monomer density operators (4.7) (4.8)( ) ( ) ( )ds s sr r r1b m f p 0 0 $= -z t o c d + t ^ h 6 @# ( ) ( ) ( )ds s sr r ra m f p 0 0 $= -z t o c d + t 6 @# ( )rbz t( )raz t pd ; ( ) ( ) .exp q p m f d r r r r r 1p a b a bab 0 # # = + d z z o t | oz z t t t t 6 8@ b ' 1 # # fig. 6. positional parametrization of starblock copolymers. copolymer melt is composed of starblock copolymers made up of alternating blocks consisting of f a-type and (1−f) b-type monomeric fractions. positional vector defining relative position of various branches of the ith copolymer is defined in terms of polymer number and branch number. space function is piecewise continuous with discontinuities at integer values. 576 giulio ottonello γ(s) is a discontinuity function which attains a value of 1 when s corresponds to an a-monomer region and a value of 0 for a b-monomer region. the product of density operators in eq. (4.6) represents the various space-dependent interactions in the copolymer melt. to render the problem mathematically tractable, the following steps are performed: 1) a delta functional integral (4.9) allows the chain density operator to be replaced by segment volume fractions φα. 2) segment volume fractions are expanded as linear combinations of basis functions (4.10) 3) basis functions (fourier transforms) possessing the symmetry of the phase in question are selected, i.e., for the gyroid phase with symmetry (matsen and schick, 1994b) (4.11) with x= 2π x/d and d = size of the cubic (gyroid) unit cell. several passages yield scmf equations, reducing the problem to that of average densities of the a, b monomers at r in the ensemble of non-interacting polymers subject to self-consistent external fields (cf. eqs. 11-15 in matsen and schick, 1994a). in various configurational states, helmholtz free energy f = − ktlnq is computed, and the minimum value (saddle point), attained through steepest descent techniques, is selected as representative of the stable configuration. ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) / ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) / cos sin sin cos sin sin cos sin sin cos cos cos coscos cos f r f r x y z y z x z x y f r x y z z x f r f r y 1 2 2 2 3 2 2 2 2 2 2 8 3 4 1 4 2 3 5 $ $ $ $ f f = = + + = + + = = 6 6 @ @ q 3ia d( ) ( ) ( ) ( )r r f r f rf, 2 ,a a a1 2 331 f= + + +z z z z d1 a a a= -d zφ φ t 8 b# 5. discussion for the sake of clarity, the discussion concerning information arising from the thermodynamic treatment of binary silicate melt interactions is divided into three distinct sections: 1) insights into shortand medium-range structural arrangement; 2) chemical interaction and longrange disorder; 3) and inhomogeneity ranges. 5.1. shortand medium-range disorder: insights from the epa in fig. 7, the polymerization paths of the epa for various binary joins overlap the discrete values for structural units thought to be present in silicate melts and glasses (toop and samis, 1962a,b). for comparative purposes, in the same plot, the consolute point in the high-sio2 portion of each binary join also overlaps. this sort of plot is very informative, since the following evidence may be retrieved: – in most joins, the melt is fully depolymerized over a large part of the compositional range. ( ) ( ) / [( ) ( ) ]o o o sisi iv0+ +o -r fig. 7. polymerization paths of epa model compared with compositions of discrete polymeric units which may form in melt at various sio2 contents. adopted polymerization constants in constructing various curves are those of table v. location of consolute points overlap, for comparative purposes. 577 chemical interactions and configurational disorder in silicate melts – ring connectivity at the consolute point depends on the lux-flood basicity of the dissolved oxide (i.e., 4-rings-4-membered for feo, 10-rings-6-membered for na2o). figure 8 compares experimental evidence concerning the speciation of the various forms of oxygen present in a cao-sio2 melt at t= = 1600°c, p = 1 bar, obtained by park and rhee (2001) by xps (x-ray photoelectron spectroscopy) with epa predictions. this plot, besides experimentally confirming for the first time (as far as we know) the actual presence of oxide ions in the silicate melt, leaves few doubts about the soundness of the polymeric approach in depicting oxygen speciation. the fit between calculated and observed speciation is particularly satisfactory at low sio2 amounts. at high silica content, the polymeric model appears to underestimate the degree of condensation of the polymeric units to some extent. this may be interpreted (in line with the preceding observations) as due to strain energy contribution effects (not accounted for in the model), which may stabilize high-membered rings in the vicinity of the two-phase region. fig. 9. hpm themodynamic parameterization of cao-sio2 system (solid lines; ottonello and moretti, 2004), compared with estimates of modified quasi-chemical model (dashed lines; pelton et al., 1995). fig. 8. xps measurements of oxygen speciation in quenched cao-sio2 melts (after park and rhee, 2001), compared with epa predictions (ottonello, 2001). – incipient decomposition (intrinsic instability) tendentially takes place when ring conformations change (e.g., 3-to-4 members, 4-to5 members, and so on). 578 giulio ottonello 5.2. chemical interaction and long-range disorder as already mentioned, the hpm allows us to discriminate chemical (enthalpic + entropic) from elastic strain energy contributions to the bulk gibbs free energy of mixing in mo-sio2 joins, and this discrimination is typical of the basic-amphotheric and acidic lux-flood behavior of mo oxides. the quasi-chemical to hpm conversion does not affect the bulk value of the gibbs free energy of mixing of the join in question, although it does modify to some extent the relative magnitudes of enthalpic and entropic terms in the two models. as we see in fig. 9, for the cao-sio2 join, entropic interactions observed for a lux-flood basic oxide arise entirely from elastic strain in the anion sublattice. moreover, enthalpic chemical interactions are dominant, and the bulk strain energy contribution to the gibbs free energy of mixing is far less and practically confined to the high-sio2 portion of the system. instead, when a lux-flood acidic oxide is dissolved in the silicate melt (al2o3-sio2 join, fig. 10), although strain energy contributions are again confined to high sio2 content, both enthalpic and entropic chemical interaction terms are present over the entire compositional range, leading to an overall «regular» appearance of the bulk gibbs free energy of mixing. undoubtedly, cao perturbs the long-range periodicity of the silicate network with local depolymerization effects whereas, at least apparently, «clusters» of overall sio2 (tectosilicate) stoichiometry coexist stably with clusters of overall al2o3 stoichiometry throughout the compositional range. obviously, the picture is more complex for oxides with less definite lux-flood reactivity. 5.3. inhomogeneity ranges although we can envisage the role of chemical interaction in determining (or modifying) short-, mediumand long-range periodicity in silicate melts and glasses, our appraisal is still insufficient to appreciate the complex phenomena which take place within the instability region. in the present author’s opinion, better appraisal may be achieved by tentatively applying scmf procedures to this topologically complex region. application of the matsen-schick theory to silicate polymer melts seems particularly promfig. 10. hpm thermodynamic parameterization of al2o3-sio2 system (solid lines; ottonello and moretti, 2004), compared with estimates of modified quasi-chemical model (dashed lines; pelton et al., 1995). 579 chemical interactions and configurational disorder in silicate melts ising. as anticipated in the introduction, the shortand medium-range periodicity of silicate melts and glasses locally resemble that of all-si zeolites, with cationic «cages» of variable dimensions, depending on silicate network connectivity (fig. 11). in this sort of arrangement, it is still possible to isolate polymeric subunits (inset, fig. 11) of quasi-crystalline shortand medium-range order, but their relative positions perturb long-range periodicity. the following working assumptions are thus proposed, to render the problem feasible for scmf treatment: 1) a simple, irreducible, a-a starblock-like representation of the network is selected as responsible for the shortand medium-range order observed in silicate melts and glasses (figs. 6 and 11). 2) it is assumed that, in the two-phase region, coexisting liquids locally mimic the structures of crystalline phases observed at the monotectic point. 3) basic functions representative of the geometry of fictive intermixing a-a starblocks (or a-b starblocks, in the case of aluminosilicate melts) are assumed to be represented by the fourier expansions of the spatial groups of the two solid phases at the monotectic point. 4) calculations are conducted in terms of gibbs free energy on the basis of hpm parameterization, after subtracting chemical interaction terms and adding pv contributions. 6. conclusions polymeric models provide an extremely useful tool in deciphering the structural and reactive properties of silicate melts and glasses. they not only establish the lux-flood character of the dissolved oxides through opportune conversion of existing quasi-chemical parameterization (pelton and blander, 1986; pelton et al., 1995; pelton, pers. comm.), but also discriminate the subordinate strain energy contributions to the gibbs free energy of mixing from the dominant chemical interaction terms. this discrimination allows us to retrieve important information about the short-, mediumand long-range periodicity of this class of substances from thermodynamic analysis. however, the conceptual models developed for silicate melts and glasses are still insufficient to allow thorough appraisal of the complex phenomena which take place within the inhomogeneity range. it is thus suggested that a further important step may be made by applying scmf theory and, particularly, matsen-schick spectral analysis, hitherto applied only to rubberlike materials. references bates, f.s. and g.h. fredrickson (1999): block copolymers – designer soft materials, phys. today, 52 (2), 32-38. bernal, j.d. 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(1932) the atomic arrangement in glass, j. am. chem. soc., 54, 3841-3851. zachariasen, w.h. (1933) the vitreous state, j. chem. phys., 3, 162-163. spencer_final:layout 6 annals of geophysics, 57, 1, 2014, a0185; doi:10.4401/ag-6389 a0185 ionospheric precursors to scintillation activity paul s.j. spencer european commission, joint research center ( jrc), ipsc, i-21027 ispra (varese), italy abstract ionospheric scintillation is the rapid fluctuation of both phase and amplitude of trans-ionospheric radio waves due to small scale electron density irregularities in the ionosphere. prediction of the occurrence of scintillation at l band frequencies is needed to mitigate the disruption of space-based communication and navigation systems. the purpose of this paper is to present a method of using tomographic inversions of the ionospheric electron density obtained from ground-based gps data to infer the location and strength of the post-sunset plasma drift vortex. this vortex is related to the pre-reversal enhancement in the eastwards electric field which has been correlated to the subsequent occurrence of scintillation. 1. introduction this paper presents preliminary work concerned with the prediction of scintillation observed on l-band frequencies in the equatorial ionosphere. equatorial scintillation is generally observed post-sunset and at the equinoxes. the post-sunset ionosphere is characterised by the formation of a drift vortex in the plasma at a local time of approximately 20lt and at a height of typically 225 km [rodrigues et al. 2012]. the vortex occurs at the transition between the day-side e region and night-side f region dynamos. during the daytime, electric fields are eastwards and the plasma drifts westwards and upwards. this situation reverses during the night-time with westwards electric fields and eastwards/downwards plasma drifts. to the east of the vortex strong vertical drifts are observed which are associated with a pre-reversal enhancement in the eastwards electric field (pre). this electric field drives the plasma upwards where the reduction in collision frequency on the bottom side ionosphere reduces the time scale for the formation of irregularities [kudeki et al. 2007]. anderson et al. [2004] first reported the possibility of forecasting equatorial scintillation at vhf/uhf frequencies based upon the strength of the pre-reversal enhancement. the first model, redmond et al. [2010], uses ionosonde measurements to obtain an estimate of the rise in the post-sunset peak height of the ionospheric f2 layer. this measurement is then treated as a proxy for the pre-reversal enhancement in the equatorial electric field. an enhanced post sunset electric field is known to be related to the onset of plasma irregularities based upon the generalized rayleigh-taylor instability mechanism, fejer and kelley [1980]. the purpose of this paper is to present a method whereby tomographic inversions of the electron density using ground-based gps observations may be used to provide an alternative data source to achieve similar results to those reported by anderson. the midas package, mitchell and spencer [2003], provides an estimate of the four-dimensional electron density in the ionosphere using ground-based gps receivers. it is shown that the solution to the continuity equation applied to midas estimates of vertical total electron content (tec) provides signatures of the post-sunset drift vortex. in order to test the method the results obtained are related to the occurrence of scintillation events observed by the cosmic satellite constellation. cosmic consists of six low earth orbit satellites, each of which deploys a dual frequency gps receiver primarily for use in radio occultation experiments. the data set are routinely processed to provide s4 estimates along each occultation path [e.g. straus and bishop 2009, uma et al. 2009]. the advantage of using the cosmic s4 scintillation estimates is that, with typically 300 occultation passes per day, global data coverage is available. 2. theory gps inversions from the midas package provide an estimate of the global ionospheric electron density to a resolution of approximately 5 degrees latitude and longitude and 20 minutes in time. although the inversions provide an estimate of the vertical electron density profile, the method is limited in its vertical resolution article history received july 29, 2013; accepted february 19, 2014. subject classification: scintillations, forecasts, dynamics, inverse methods, algorithms and implementation. due to geometrical factors. as such, isolating the vertical plasma drifts using midas would be difficult at the required spatial and temporal resolutions. tomographic methods using ground-based gps do however provide accurate estimates of the integrated vertical total electron content. consequently, the aim of this project was to derive a method that could relate signatures observed in the horizontal distribution of tec to the post sunset drift vortex. in order to infer the existence of the drift vortex using tec, the zonal drift in the plasma was considered. an eastwards acceleration of the plasma above the vortex would be expected to lead to a zonal divergence in plasma velocity. since most of the f-region plasma is located above the drift vortex, at typically 225 km, this should also result in a zonal divergence in the vertically integrated tec. by using tec, this approach ignores the vertical redistribution of plasma that arises due to zonal electric fields and vertical drifts. in order to obtain an estimate of the horizontal velocity of the vertically integrated tec, the solution to the continuity equation was obtained from the midas reconstructions. the continuity equation can be written where n is the known vertical total electron content, v the horizontal velocity, t is time and the production and loss are given by p and l respectively. to remove the production and loss terms from equation (1) the midas reconstructions were computed in a solar magnetic coordinate system. since the method presented is primarily concerned with the post sunset period, production in this region will be negligible. furthermore, in the post-sunset f2 region, the increasing time constant for recombination and decreasing time constant for diffusion result in the transport of ionisation playing an increasingly important role, bittencourt et al. [2007]. as such, the errors arising from ignoring the loss term at fixed solar magnetic coordinates should be small in relation to the other terms. the solution of the continuity equation is nontrivial since with two velocity components and a single observation per pixel there are twice as many unknowns as knowns. to obtain stable solutions the problem was reformulated by defining the velocity, v, as the gradient of a scalar potential field, z. in this case the continuity equation becomes a limitation of this approach is that the resulting solution for the velocity is no longer general since it is constrained to be irrotational. however, for the current work this assumption was considered acceptable since horizontal plasma drifts in the equatorial f-region are predominantly zonal. equation (2) was then solved for z using a finite difference approach to the derivatives. expanding the terms in equation (1) gives where the first and second derivatives in the two horizontal spatial dimensions x and y are formulated using finite differences over three adjacent grid pixels. the equation (3) can be then written in matrix form as to ensure unique solutions the problem is reformulated as a set of regularised normal equations where r is a regularisation matrix and m is a constant. the matrix r is included to provide smoothing in the two spatial dimensions by minimizing the departure of the solution from a linear function using the kernel [1,-2,1]. the constant m is chosen so that the trace of the normal equations corresponding to the observation matrix h and regularisation matrix r have equal weight. 3. data midas reconstructions using a globally distributed set of 80 ground-based gps receivers from the igs receiver network were obtained for the month of october 2012. observations of the s4 index from the cosmic constellation were also obtained for the same period. the cosmic s4 estimates are provided as the rms intensity fluctuations of the square of the l1 ca snr at a 1 second time resolution during the occultation pass. in order to isolate f-region scintillation, the data for each pass was clipped such that the point of closest approach to the earth for each ray path was within the height range of 200 km to 600 km. the s4 estimate for the occultation event was then defined as the maximum observed within this reduced data set. owing to the geometry of the limb ray through the ionosphere, the exact horizontal location of the scintillating structure is ambiguous to within typically 500 km to 800 km. this however is the same order of magnitude as the midas grid resolution and was therefore was considered sufficient for comparison with midas output. . vt n n p l 2 2 d+ = -^ h .t n n n 02 2 2 d d dz z+ + = n x n n y x n x y y t2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2z z z z + =-+ +c m h b{= h h r r h bt t tm {+ =^ h spencer 2 (1) (3) (4) (5) (2) 3 4. results results are presented for the month of october 2012 which was chosen due to the large number of scintillation events that were observed. figure 1 presents two examples of the output from midas and the corresponding estimates of the zonal plasma convergence obtained by solution of the continuity equation. line contours show the estimated tec and filled contours show the estimated convergence of the eastwards velocity field. patterns of plasma convergence and divergence can be clearly observed in the post sunset sector. superimposed on these images are colored dots showing the maximum observed s4 per occultation pass for data within a 20 minute time window of each inversion. multiple points imply that scintillation was occurring on multiple occultation passes with differing geometries. these plots show a coincidence of multiple scintillation events where the velocity field is strongly divergent, shown in blue. defining a scintillation event as the maximum s4 observed on a single occultation pass for the subset of ray paths within the f-region. a cluster of events is defined as a point for which the sum of s4 events within 20° latitude and longitude and 20 minutes in time was greater than 3. a total of 123 such clusters occurred in the month of october 2012. figure 2 shows the mean zonal convergence pattern centred upon the location of the clusters for the month of october 2012. the observed pattern shows that severe scintillation clusters consistently occur close to the point of minimum conionospheric precursors to scintillation figure 1. midas reconstructions on 01/10/12 at 02:00 utc, above, and 02/10/2012 at 18:40 utc, below. line contours show total electron content (left color bar). filled contours show eastwards plasma convergence (right color bar). the thick grey line represents the solar terminator and the filled circles show maximum s4 per occultation observed by cosmic (colored as s4×100 on the left color bar). vergence (maximum divergence). figure 2 also shows the mean convergence field at 20 minutes prior to the time of maximum scintillation. in this case the regions of maximum convergence and divergence are shifted westwards though still clearly visible. 5. discussion tomographic reconstructions provide a valuable resource for imaging the large scale structures in the earth’s ionosphere. the method presented uses an estimate of the divergence of the total electron content to infer the location and strength of the post-sunset drift vortex. the aim of such an approach is to provide a warning for subsequent scintillation over an extended region of space at length scales corresponding to the resolution of the midas reconstructions. a significant problem in relating the occurrence of scintillation to the midas results is that, at l-band frequencies, the structures that give rise to scintillation are highly localised in space and time. this problem was addressed by using s4 estimates from the cosmic satellite constellation. occulting ray paths traverse a large horizontal region of the ionosphere. the occultation geometry is therefore better suited to the current analysis by providing a spatially averaged scintillation index at the approximate resolution of the midas grid. furthermore, clusters of scintillation events observed on multiple occultation passes also imply that the irregularities exist over an extended spatial region. the results presented in figure 1 show two examples of the tec and estimated tec divergence. in both cases a post sunset enhancement in the tec is apparent with the tec enhancement on the 2nd october being approximately twice that of the event of the previous day. it can also be seen that the gradients in tec are significantly different for the two events. the number of scintillation events and tec divergence are however similar in both cases. in order to test whether tec divergence can consistently be used to predict the likely occurrence of scintillation the mean convergence pattern centred on all severe scintillation clusters is shown in figure 2. close to the centre of the plot the zonal tec convergence is negative with a localised region of positive convergence to the east. this pattern is common across the large number of severe scintillation events observed during the month of october 2012. it is also apparent from the dotted contours that the pattern forms prior to the occurrence scintillation clusters. consequently these patterns of convergent and divergent tec can be used as an advanced warning of scintillation. 6. conclusion the goal of this paper is to present a method for forecasting the location of scintillation events at l-band frequencies using gps based tomographic reconstructions of the ionosphere. the method relies on the close relationship between the post-sunset drift vortex and the pre-reversal enhancement in the eastwards electric field. the lifting of the plasma due to the eastwards electric field on the day side of the drift vortex is related to an increase in the eastwards plasma velocity above the vortex. the later causing a zonal divergence of the total electron content that can be imaged from tomographic reconstructions. future work will aim to relate the results obtained here to in-situ observations of electric fields obtained directly by independent satellite observations. spencer 4 figure 2. mean zonal tec convergence relative to the centre of clusters of s4 events. solid lines represent the convergence field at time of maximum s4 and dotted lines 20 minutes prior. 5 acknowledgements. the midas software package is used under license from the department of electronic and electrical engineering, university of bath, bath, ba2 7ay. references anderson, d.n., b. reinisch, c. valladares, c. chau and o. veliz (2004). forecasting the occurrence of ionospheric scintillation activity in the equatorial ionosphere on a day-to-day basis, journal of atmospheric and solar-terrestrial physics, 66 (17), 1576-1572. bittencourt, j.a., v.g. pillat, p.r. fagundes, y. sahai and a.a. pimenta (2007). lion: a dynamic computer model for the low-latitude ionosphere, annales geophysicae, 25, 2371-2392. fejer, b.g., and m.c. kelley (1980), ionospheric irregularities, reviews of geophysics, 18 (2), 401-454. kudeki, e., a. akgiray, m. milla, j.l. chau, d.l. hysell (2007). equatorial spread–f initiation: post-sunset vortex, thermospheric winds, gravity waves, journal of atmospheric and solar-terrestrial physics, 69 (17-18), 2416-2427. mitchell, c.n., and p.s.j. spencer (2003). a three-dimensional time-dependent algorithm for ionospheric imaging using gps, annals of geophysics, 46 (4), 687-696. redmond, r.j., d. anderson, r. caton and t. bullett (2010). a forecasting ionospheric real-time scintillation tool (first), space weather, 8 (12), s12003. rodrigues, f.s., g. crowley, r.a. heelis, a. maute and a. reynolds (2012). on tie-gcm simulation of the evening equatorial plasma vortex, journal of geophysical research, 117, a05307. straus, p., and r. bishop (2009). radio occultation measurements of ionospheric scintillation, presented at the 4th cosmic data users workshop, october 2009, boulder, co, usa. uma, g., j.y. liu, p.s. brahmanandam, c.h. chen, y. kakinami and c.h. liu (2009). a global observation of s4 scintillation index observed by means of formosat-3/cosmic, presented at the 4th cosmic data users workshop, october 2009, boulder, co, usa. *corresponding author: paul s.j. spencer, european commission, joint research center ( jrc), ipsc, i-21027 ispra (varese), italy; email: paul.spencer@jrc.ec.europa.eu. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ionospheric precursors to scintillation << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice microsoft word 9_cengiz_appendixa .docx annals of geophysics, 64, 6, gm662, 2021; doi:10.4401/ ag -8699 appendix a to: paleomagnetic results from western anatolia: evidence of microblock rotations after emplacement of the lower miocene yuntdağ volcanic rocks mualla cengiz1, savaş karabulut2, ferhat özçep1, burak semih çabuk1, friedrich heller3 1 i̇stanbul university-cerrahpaşa, faculty of engineering, department of geophysical engineering, 34800 hadımköy/i̇stanbul, turkey. 2 department of civil engineering, gebze technical university, kocaeli 41400, turkey 3 institute of geophysics, department of earth sciences, sonneggstrasse 5, eth zurich, 8092 zurich, switzerland site n kmean p t f l u dm1 21 696.33 1.06009 0.33643 1.03976 1.01955 0.32345 dm2 15 435.40 1.03593 -0.34287 1.01166 1.02398 -0.35065 dm3 21 188.03 1.04426 0.73342 1.03825 1.00578 0.72839 dm4 15 154.33 1.02962 0.25129 1.01843 1.01098 0.24444 dm5 16 292.46 1.03459 -0.9796 1.00034 1.03423 -0.98 dm6 12 475.50 1.04567 0.23697 1.02801 1.01718 0.22641 dm7 18 311.10 1.03444 0.60538 1.02755 1.00670 0.6 dm8 26 123.76 1.03966 0.46597 1.02892 1.01044 0.45833 dm9 13 593.86 1.05802 0.29435 1.03717 1.02010 0.28143 dm10 9 501.60 1.05285 0.15212 1.03011 1.02207 0.13953 dm11 16 39.56 1.12533 -0.3356 1.04 1.08205 -0.3617 dm12 18 480.86 1.06205 0.50433 1.04632 1.01503 0.49305 dm13 10 520.7 1.03364 0.32138 1.02210 1.01129 0.31395 dm14 18 155.23 1.05448 0.47374 1.03986 1.01405 0.46341 dm15 21 402.03 1.04892 0.66120 1.04047 1.00812 0.65445 dm16 18 506.73 1.07673 0.74941 1.06679 1.00930 0.74124 dm17 15 420.66 1.05524 0.08415 1.02957 1.02492 0.07079 dm18 23 269.2 1.06808 0.65715 1.05609 1.01135 0.64772 bg1 9 796.7 1.11492 0.33440 1.07528 1.036866 0.31010 bg2 20 196.33 1.03380 0.72701 1.029121 1.004547 0.72307 bg3 10 1300.56 1.04468 -0.35185 1.014268 1.029991 -0.36140 bg4 10 360 1.05633 0.12011 1.031160 1.024397 0.10659 bg5 8 149.4 1.04683 0.71156 1.039944 1.006622 0.70588 bg6 11 411.36 1.01466 -0.83221 1.001221 1.013424 -0.83333 bg7 9 123.43 1.03728 0.64975 1.030654 1.006430 0.64444 bg8 13 909.46 1.07105 0.16280 1.040716 1.029150 0.14606 bg12 8 676.23 1.05265 0.32938 1.034697 1.017355 0.31791 bg13 12 1019.86 1.03771 0.65425 1.031093 1.006420 0.64893 bg14 9 320.03 1.03836 -0.69767 1.005707 1.032471 -0.70247 bg15 11 3300.86 1.05265 0.35468 1.011327 1.027165 0.64578 bg16 9 2561.8 1.06245 0.29677 1.016207 1.020993 0.54857 bg17 8 2935.36 1.04415 0.25119 1.021951 1.021313 0.57575 appendix: ams data from lower middle miocene volcanic rocks. n: number of samples; κmean: mean volume susceptibility (si units); p (degree of anisotropy) = κmax/κmin; t (shape parameter) = (2 h2 h1 h3) / (h1 h3) where h1=ln k1 , h2 = ln k2 , h3= ln k3; degree of anisotropy; l (lineation): (κmax/κint); f (foliation): (κint/κmin); u (shape parameter) = (2 κint κmax κmin) / (κmax κmin) (jelinek 1981). κmax: maximum, κint: intermediate, κmin: minimum anisotropy axes. reflection characteristics of em waves over homogeneous earth model r a m e s h p . s i n g h ( * ) t a r k e s h w a r l a l ( * ) received on j u l y 10th, 1980 abstract t h e o r e t i c a l a n a l y s i s h a v e been c a r r i e d o u t t o i n v e s t i g a t e t h e e f f e c t of s u b s u r f a c e e l e c t r i c a l p a r a m e t e r s s u c h a s r e s i s t i v i t y a n d d i e l e c t r i c conss t a n t ; a n d a n g l e of i n c i d e n c e on r e f l e c t i o n c o e f f i c i e n t s f o r vertically a n d h o r i z o n t a l l y p o l a r i z e d e l e c t r o m a g n e t i c w a v e s . it is s h o w n t h a t t h e reflection c o e f f i c i e n t s f o r vertically a n d h o r i z o n t a l l y p o l a r i z e d e l e c t r o m a g n e t i c w a v e s a r e s t r o n g l y d e p e d e n t on w a v e f r e q u e n c y a n d a n g l e of incidence. at s m a l l angles of i n c i d e n c e t h e ri, a n d ris seen to go t h r o u g h a minin u m v a l u e w h i c h is seen to d i f f e r f r o m o n e m a t e r i a l t o t h e o t h e r . ref l e c t i o n c h a r a c t e r i s t i c s of e l e c t r o m a g n e t i c w a v e s d i s c u s s e d in t h i s p a p e r c a n b e h e l p f u l in t h e a c c u r a t e i n t e r p r e t a t i o n of g e o p h y s i c a l d a t a . (*) d e p a r t m e n t of geophysics, b a n a r a s h i n d u university, v a r a n a s i 221005, india. 250 r a m e s h p. s i n g h t a r k e s h w a r lal r i a s s u n t o s o n o s t a t e e s e g u i t e a n a l i s i t e o r i c h e p e r i n d a g a r e s u l l ' e f f e t t o dei p a r a m e t r i elettrici di s o t t o s u p e r f i c i e , c o m e la r e s i s t i v i t à , la c o s t a n t e d i e l e t t r i c a e l'angolo di incidenza sui c o e f f i c i e n t i di r i f l e s s i o n e p e r o n d e e l e t t r o m a g n e t i c h e p o l a r i z z a t e v e r t i c a l m e n t e e o r i z z o n t a l m e n t e . si n o t a che i coefficienti di r i f l e s s i o n e p e r o n d e e l e t t r o m a g n e t i c h e p o l a r i z z a t e v e r t i c a l m e n t e e o r i z z o n t a l m e n t e , d i p e n d o n o s t r e t t a m e n t e dalla f r e q u e n z a d e l l ' o n d a e d a l l ' a n g o l o di i n c i d e n z a . a piccoli angoli d ' i n c i d e n z a \'r,, e l'i?,, p a s s a n o a t t r a v e r s o u n v a l o r e m i n i m o che si n o t a d i f f e r i r e da u n m a t e r i a l e a l l ' a l t r o . le c a r a t t e r i s t i c h e della r i f l e s s i o n e di o n d e e l e t t r o m a g n e t i c h e di cui si è t r a t t a t o in q u e s t a n o t a , p o s s o n o e s s e r e di a i u t o p e r u n a più a c c u r a t a i n t e r p r e t a z i o n e di d a t i geofisici. introduction t h e e l e c t r o m a g n e t i c wave p r o p a g a t i o n a n d i n t e r a c t i o n w i t h v a r i o u s typesof e a r t h ' s m a t e r i a l h a s been extensively s t u d i e d . the r e f l e c t i o n a n d a t t e n u a t i o n of e l e c t r o m a g n e t i c waves f r o m the e a r t h ' s s u b s u r f a c e is c u m u l a t i v e l y a f f e c t e d by t h e s u b s u r f a c e material a n d t h e i r electrical p r o p e r t i e s . t h e s t u d i e s on e l e c t r o m a gnetic field a t t e n u a t i o n a n d c h a n g e of r e f l e c t i o n c o e f f i c i e n t w i t h f r e q u e n c y , p o l a r i z a t i o n a n d angle of incidence play a n i m p o r t a n t role inrevealing t h e s u b s u r f a c e electrical p r o p e r t i e s ( s h i v p r a s a d a n d stotz, 1972; kozaki, 1970; wait, 1971; philippe, 1973; lytle, 1974; suzuki et al., 1975; e v a n s , 1977; singh a n d singh, 1979) e m p l o y i n g t r a n s m i t t e r receiver s y s t e m s w i t h large s e p a r a t i o n s . these f e a t u r e of received e l e c t r o m a g n e t i c waves a r e p r i m a r i l y c o n t r o l l e d by the dielectric c o n s t a n t (sr) a n d c o n d u c t i v i t y (<r) of the s u b s u r f a c e layer. in this p a p e r , c o m p u t a t i o n s havebeen c a r r i e d o u t to see the e f f e c t of p o l a r i z a t i o n a n d angle of incidence f o r general value of dielectric c o n s t a n t a n d conductivity, of s u b s u r f a c e c o n s t i t u e n t s . r e f l e c t i o n c h a r a c t e r i s t i c s o f e m w a v e s e t c . 251 t h e o r y of s u r f a c e w a v e propagation we c o n s i d e r the e l e c t r o m a g n e t i c wave p r o p a g a t i o n t h r o u g h an i n t e r f a c e s e p a r a t i n g t h e e a r t h ' s a t m o s p h e r e f r o m t h e e a r t h ' s s u r f a c e . the p l a n e e l e c t r o m a g n e t i c wave p r o p a g a t i n g t h r o u g h a m e d i u m ca n b e r e p r e s e n t e d as s u p e r p o s i t i o n of t w o waves ( s c h m u c k e r a n d j a n k o w s k i , 1972): i) t r a n s v e r s e electric m o d e wave, ii) t r a n s v e r s e m a g n e t i c m o d e waves. t h e e l e c t r o m a g n e t i c wave p r o p a g a t i o n of these waves a t low a n d high f r e q u e n c i e s a r e significantly d i f f e r e n t . t h e e l e c t r o m a gnetic wave p r o p a g a t i o n at high a n d low f r e q u e n c i e s a r e signif i c a n t l y d i f f e r e n t . the r e f l e c t i o n c o e f f i c i e n t at low f r e q u e n c i e s is such t h a t the direct a n d s u b s u r f a c e r e f l e c t e d waves cancel o u t a n d p r o p a g a t i o n is p r i m a r i l y as s u r f a c e waves. h o w e v e r , in the high f r e q u e n c y e l e c t r o m a g n e t i c waves, the m a g n i t u d e of t h e signal p r i m a r i l y d e p e n d s on the relative p h a s e of t h e d i r e c t a n d g r o u n d r e f l e c t e d waves r e a c h i n g the receiving site. t h e w o r k i n g f o r m u l a f o r the a m p l i t u d e of the r e f l e c t i o n c o e f f i c i e n t is w r i t t e n as ( j o r d a n a n d b a l m a i n , 1969). = sin 6,+ {(e, •—cos 2 6,)2 + ( j / e 0 to) 2} '4 sin 0,— {(er — cos 2 6,)2 + (a/e„ to)2} * [ 1 ] a n d rv = \ er 2 sin 0, + f — — v s i n 2 0 , ^ . f \ so to / \ j sr 2 sin 0, + ( — — ^ "sin2 0, i ' + i \ £„« + | ( e r — cos 2 0, j + £u w [ 2 ] 252 r a m e s h p. s i n g h t a r k e s h w a r lal results and discussions the p o l a r i z a t i o n d e p e n d e n t r e f l e c t i o n c o e f f i c i e n t s a r e governed by the c o m p o s i t i o n of the s u b s u r f a c e m a t e r i a l a n d c h a n g e with c h a n g i n g f r e q u e n c y of the e l e c t r o m a g n e t i c wave a n d t h e i r angle of incidence as d e p i c t e d by e q t i a t i o n s [ 1 ] a n d [ 2 ] , t h e f r e q u e n c y d e p e n d e n c e of ri, a n d rv f o r s o m e p r e m i n e n t c o n s t i t u e n t s of t h e fig. l a , b, c, d, e) f r e q u e n c y v a r i a t i o n of rv a n d rh f o r soil, shale, d o l o m i t e , g a b b r o a n d q u a r t z i t e . e a r t h ' s s u b s u r f a c e a r e s h o w n in figures ( l a , b, c, d, e). f r e q u e n c y d e p e n d e n c e values of dielectric c o n s t a n t a n d c o n d u c t i v i t y of soil, shale, d o l o m i t e , g a b b r o a n d q u a r t z i t e have b e e n t a k e n f r o m (scott frequency in hz fig. 1 d , i i i ! io 3 i04 i05 10® | 0 7 i08 frequency in h z fig. 1 e m |rv| q u a r t z i t e r e f l e c t i o n c h a r a c t e r i s t i c s o f e m w a v e s e t c . 255 et. al., 1967; k a t s u b e a n d collett, 1976). the r e f l e c t i o n c o e f f i c i e n t is n e a r l y one at low f r e q u e n c i e s a n d is seen to d e c r e a s e r a t h e r d r a s t i c a l l y w i t h increasing f r e q u e n c y . the r e f l e c t i o n c o e f f i c i e n t s ri, a n d rv f o r soil, shale, d o l o m i t e , g a b b r o a n d q u a r z i t e s h o w sim i l a r v a r i a t i o n s w i t h f r e q u e n c y a l t h o u g h t h e i r m a g n i t u d e s a r e d i f f e r e n t . t h e ri, v a r i a t i o n d e p i c t e d by t h e solid c u r v e s h o w s min i m u m v a r i a t i o n at 30° a n d a s y s t e m a t i c i n c r e a s e in v a r i a t i o n w i t h i n c r e a s i n g angle of incidence 60° a n d 90°. h o w e v e r , in t h e case of rv, t h e m a x i m u m v a r i a t i o n is seen at 30° a n d t h e variation is seen to d e c r e a s e w i t h i n c r e a s i n g angle of i n c i d e n c e 60" a n d 90°. t h e angle of incidence c o n s i d e r a b l y i n f l u e n c e s ri, a n d rv in the higher f r e q u e n c y range. c o m p u t a t i o n a l r e s u l t s to s t u d y the v a r i a t i o n of rv with angle of incidence at t w o f r e q u e n c i e s 104 a n d 10" hz a r e p r e s e n t e d in fig. 2. the m a g n i t u d e of rv a t 104 hz is seen to increase w i t h the i n c r e a s e in angle of incidence f o r soil, shale, g a b b r o , d o l o m i t e a n d q u a r t z i t e . h o w e v e r , at a h i g h e r f r e q u e n c y e.g. 106 hz, t h e r e f l e c t i o n c o e f f i c i e n t r i s seen to b e h a v e d i f f e r e n t l y . it decreases w i t h the i n c r e a s e in the angle of incidence till a t t a i n s a m i n i m u m , w h e r e a f t e r it i n c r e a s e s again. at a given f r e q u e n c y the angle f o r m i n i m u m rt. is seen to c h a n g e f o r soil, shale, g a b b r o , d o l o m i t e a n d q u a r t z i t e . f u r t h e r c o m p u t a t i o n s of rv a n d rn w i t h the angle of incidence f o r dielectric c o n s t a n t values 3, 81 a n d resistivity values 103, 104 o h m m e t e r at t w o f r e q u e n c i e s 104 a n d 105 hz h a v e b e e n carried o u t a n d a r e p r e s e n t e d in fig. 3. at 104 hz, ri, is seen to dec r e a s e slowly with t h e angle of incidence. t h e i n c r e a s e in resistivity f r o m 103 o h m m e t e r to 104 o h m m e t e r d e c r e a s e t h e magnit u d e of ri,. t h e change of dielectric value f r o m 3 to 81 does n o t i n f l u e n c e the ri, values f o r 103 o h m m e t e r resistivity value whereas f o r 104 o h m m e t e r , its m a g n i t u d e is l o w e r e d slightly w i t h increase in dielectric c o n s t a n t . the b e h a v i o u r of rv v a r i a t i o n at 104 hz is d i f f e r e n t f r o m t h a t of ri,. it decreases, w i t h the i n c r e a s e in angle of incidence, till it a t t a i n s a m i n i m u m , w h e r e a f t e r it increases to a t t a i n a s a t u r a t i o n value. h e r e again i n c r e a s e in resistivity, d e c r e a s e s the m a g n i t u d e of rv. f o r higher dielectric c o n s t a n t value a n d the s a m e resistivity, the m i n i m u m o c c u r s at s m a l l e r angle of incidence a n d its m a g n i t u d e is lower as compar e d to t h a t f o r lower dielectric c o n s t a n t . a f t e r the m i n i m u m , a 256 r a m e s h p. s i n g h t a r k e s h w a r l a l a n g l e of incidence fig. 2 v a r i a t i o n of rr w i t h angle of i n c i d e n c e f o r soil, shale d o l o m i t e , g a b b r o a n d q u a r t i z i t e . 258 r a m e s h p. s i n g h t a r k e s h w a r lal c r o s s over a n d reversal in t h e m a g n i t u d e t a k e s place, b u t t h e values a r e closed to each o t h e r . t h e c h a r a c t e r i s t i c f e a t u r e s of ri, a n d rv, v a r i a t i o n s d i s c u s s e d a b o v e a r e r e t a i n e d at 105 hz b u t t h e v a r i a t i o n in m a g n i t u d e is large f o r ri„ w h e r e a s f o r rv, the minin u m m a g n i t u d e is m o r e o r less or equal f o r all cases. concluding r e m a r k the s h a r p v a r i a t i o n in rv w i t h angle of incidence a n d appear a n c e of a m i n i m u m at a specific f r e q u e n c y a r e r e m a r k a b l e feat u r e s which c o n t r o l the m a g n i t u d e of the received signal at the receiving site. t h e r e f o r e , the m e a s u r e m e n t of wave a m p l i t u d e seem to p r o v i d e a p o t e n t i a l p r o b i n g tool f o r the s t u d y of s u r f a c e f e a t u r e s of the e a r t h ' s c r u s t a l t e r r a i n . the large s e p a r a t i o n between the t r a n s m i t t e r a n d receiver is essential to m e a s u r e t h e a n g u l a r v a r i a t i o n of r e f l e c t i o n c o e f f i c i e n t s f o r each of these polarizations. this m e t h o d of e l e c t r o m a g n e t i c e x p l o r a t i o n is widely u se d by p r o f e s s i o n a l p r o s p e c t i n g agencies a n d it is c a p a b l e of m a p p i n g the c r u s t a l s t r u c t u r e in e x t e n d e d a r e a s . a c k n o w l e d g e m e n t s one of u s (rps) is t h a n k f u l to csir, new delhi f o r t h e a w a r d of s e n i o r r e s e a r c h fellowship. we a r e g r a t e f u l to dr. j. singh, visiting p r o f e s s o r f o r his keen i n t e r e s t in this w o r k . r e f l e c t i o n c h a r a c t e r i s t i c s of e m w a v e s etc. 259 r e f e r e n c e s e v a n s , d. d., 1977 i e e e t r a n s . a n t e n n a s p r o p a g a t i o n , ap-25, p. 490. jordan, e . c . , b a l m a i n , k . g . , 1 9 6 9 electromagnetic waves and radiating systems. p r e n t i c e hall of i n d i a , new delhi. k a t s u b e , t . j., c o l l e t t , l . s., 1976 i n : physics and chemistry of minerals and rocks, e d i t e d by r.g.k. s t r e n s , j o h n wiley a n d sons, u.k., p. 279. k o z a k i , s . , 1970 i e e t e t r a n s . a n t e n n a s p r o p a g a t i o n , ap-18, p. 138. l y t l e , r. j., 1974 i e e e t r a n s . geoscience elect., ge-12, p. 8 1 . p h i l i p p e , a. e., 1973 i e e e t r a n s . a n t e n n a s p r o p a g a t i o n , ap-21, p. 234 s c h m u c k e r , u., j a n k o w s k i , j., 1972 « t e c t o n o p h y s i c s », 13, (1-4), p. 233. s c o t t , j. h., carroll, r. d., c u n n i n g h a m , d. r., 1967 « j. geophysical r e s e a r c h » , 73, p. 5101. s h i v p r a s a d , k„ s t o t z , k. c„ 1972 i e e e t r a n s a n t e n n a s a n d propag a t i o n , p. 161. s i n g h , r a m e s h p., s i n g h , r i s h i n„ 1979 p r o c e e d i n g i n d i a n a c a d e m y of science, 88a, pt. ii, 3, p. 209. s u z u k i , t., o g a w a , e., f u j i o k a , h., 1975 i e e e t r a n s . a n t e n n a s p r o p a g a tion, ap-23, p. 127. w a i t , j . r . , 1 9 7 1 electromagnetic probing in geophysics. « the golemn p r e s s », b o u l d e r , c o l o r a d o . layout 6 influence of seismic activity on the atmospheric electric field in lisbon (portugal) from 1955 to 1991 hugo gonçalves silva1,*, maria manuela oliveira2, cláudia serrano1, mourad bezzeghoud1, antónio heitor reis1, rui namorado rosa1, pier francesco biagi3 1 universidade de évora, escola de ciências e tecnologia, centro de geofísica de évora e departamento de física, évora, portugal 2 universidade de évora, escola de ciências e tecnologia, departamento de matemática, évora, portugal 3 università di bari, dipartimento di fisica e centro interdipartimentale di ricerca per la valutazione e mitigazione del rischio sismico e vulcanico, bari, italy annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5361 abstract in the present study, we considered the influence of seismic activity on the atmospheric electric field recorded at portela meteorological station (lisbon, portugal) for the period from 1955 to 1991. to this end, an exploratory method was developed, which involved the selection of events for which the distance from the atmospheric electrical field sensor to the earthquake epicenter is smaller than the preparation radius of the event. this enabled the correlation of the atmospheric electric field variations with a quantity s, defined basically as the ratio of the earthquake preparation radius to the distance between the sensor and the event epicenter. the first results show promising perspectives, but clearly a more profound study is required, in which a careful analysis of the weather conditions and other variables, like atmospheric radon levels, must be considered. 1. introduction since an important article by pierce in the 1970’s [pierce 1976], many models have been developed to explore the coupling between the lithosphere-atmosphere-ionosphere during seismic events [e.g. liperovsky et al. 2008, omori et al. 2009, harrison et al. 2010, kachakhidze et al. 2011]. all of these appear to agree that variations in the vertical component of the atmospheric electric field (ez) are related to the amount of radon that emanates during earthquake preparation stages, which then promotes air ionization and reduces ez. from hereafter, we adopt the use of the atmospheric electric potential gradient (pg), as defined as: pg = –ez. moreover, pg anomalies are not only candidates for seismic electromagnetic precursors in the context of shortterm earthquake prediction [harrison et al. 2010], as they also give important insights into the physics of these phenomena [kachakhidze et al. 2011]. nevertheless, there has been little observational work carried out on this subject [e.g. smirnov 2008, kachakhize et al. 2009, silva et al. 2011). in the present study, we present a new approach for the investigation of the effects of earthquakes on the pg, through the analysis of a dataset recorded at portela meteorological station (lisbon, portugal) for the period from 1955 to 1991. it is important to note that these data correspond to the vertical and very-slow-varying component of the atmospheric electric field, and therefore the associated magnetic effects are negligible. thus the phenomenology presented here is completely different from the geomagnetic precursors reported in the literature [e.g. fraser-smith et al. 1990]. moreover, it is also of fundamental importance to note that the variations on the ‘atmospheric’ electric fields discussed in this study should not be confused with the distinct phenomenon known as seismic electric signals, which are said to be developed in the ‘lithosphere’ before significant earthquakes, and that have been extensively reported in the literature [e.g. varotsos and alexopoulos 1984]. indeed, for the moment, no direct relationships between the atmospheric electric field variations reported here and both geomagnetic field anomalies and seismoelectric signals in the literature have been shown, and it is beyond the scope of the present study to explore this issue. additionally, we must emphasize that this is an initial study; nevertheless, interesting trends are found that show that the effects of seismic activity on the pgs are local, and that they tend to reduce the pgs. the organization of this report is as follows: some considerations relating to the datasets are given in section 2; the methodology is presented in section 3; the results are discussed in section 4; and at the end we present some final remarks, in section 5. special issue: earthquake precursors 193 article history received august 9, 2011; accepted november 10, 2011. subject classification: processes and dynamics, earthquake faults: properties and evolution, seismic risk, atmospheric electric field, lai coupling. 2. the datasets in the present study, we considered the hourly pg values recorded at portela meteorological station (lisbon, portugal; 38˚47'n, 9˚08'w) for the period from 1955 to 1991. the portuguese meteorological institute recorded these data with a benndorff electrograph with a 1-m-heigh probe. the data series was interrupted from 1975 to 1977 when the electrometer was switched off for maintenance. the annual behavior of the pg is presented in figure 1a, where an anomaly associated with the nuclear tests in the 1960’s can be seen, together with the slow recover of the pg in the following years. this effect was previously reported in a seminal study [pierce 1972], and is clearly shown here. the average behavior of the diurnal cycle is also presented in figure 1b. a global analysis of the different influences on the pg was reported by serrano et al. [2006], where more details of the dataset can be found. on the other hand, two seismic catalogs were used in the present study: for the period from 1955 to 1961, we used the catalog from the portuguese national laboratory for civil engineering [sousa et al. 1992]; and for the period from 1961 to 1991, we used the catalog from the portuguese meteorological institute (the portuguese meteorological institute database). figure 2 shows the seismic activity from 1961 to 2010 in the region where the pg sensor was installed. indeed, the seismicity of this zone has been widely studied mainly because of the well-known lisbon earthquake of 1755, and the interested reader can find a concise studies of this earthquake in the literature [i.e. borges et al. 2001, buforn et al. 2004]. 3. the methodology first of all, it should be noted that the measurements presented in this study are local, in the sense that any perturbation in the pg values must be caused by a disturbance that occurs near to the sensor, as for example, for artificial radioactivity variations [see serrano et al. 2006]. for this reason, it is highly unexpected that seismic events far away from the sensor will directly affect the pg. in this way, it is necessary to establish a criterion to select the seismic events that might actually influence the pg from those that cannot. at the same time, it is expected that seismic events of greater magnitude do influence the pg at larger distances from the sensor, as compared with smaller seismic events. 194 silva et al. figure 1. a) average pgs per year during the period from 1955 to 1991 at portela (lisbon, portugal). b) the diurnal cycle of the pgs averaged over all of the data. figure 2. map of portugal showing the seismicity (small orange circles) of the south of portugal for the period from january 1, 1961 to december 31, 2010 (data provided by portuguese meteorological institute, portugal). the red circles are proportional to s and highlight the most relevant earthquakes. 195 indeed, dobrovolsky et al. [1979] developed the concept of the earthquake-preparation radius (r), which depends on the event magnitude (m). this assumes the existence of an approximately circular region around the epicenter of an earthquake that undergoes elastic crustal deformation prior to the seismic events themselves. dobrovolsky et al. [1979] estimated r as: (1) hence, taking into account the above arguments, it is likely that for relevant events (those that can actually influence the measurements), the pg sensor should be within a circle of radius r (approximately). this means that the distance from the sensor to the event epicenter, d, must be smaller than r, i.e. r ≥ d. this assumption enables the estimation of a dimensionless parameter: (2) which must be positive for seismic events of interest. applying this simple criterion in the analysis of the two seismic catalogs referred to above for the period of the sensor operation, from sousa et al. [1992] and from the portuguese meteorological institute database, there were 15 events that might be relevant. table 1 gives some of the characteristics of these events, together with the corresponding s values (see equation 2). the seismic event in 1962 is not considered in this analysis as it corresponds to a year of high levels of artificial radioactivity, while the seismic event in 1975 was unfortunately missed because of the interruption of the dataset mentioned above. then, for the selected events we determined the pearson’s correlations of s with the average values of the pg for three main periods: (i) 1 day before and 1 day after the events (pg_2); (ii) 7 days before and 7 days after the events (pg_14); and (iii) 14 days before and 14 days after the events (pg_28). additionally, we considered the pg averages using only the midnight values, from 22:00 to 02:00 (utc), as it is known from the literature that during this period the atmospheric activity is more stable [see biagi et al. 2009]. in this way, three new cases were defined for the periods: 2 days (pg_2mn), 14 days (pg_14mn), and 28 days (pg_28mn). the results are presented and discussed in the next section. 4. results and discussion the analysis of the dependence of the different pg averages as a function of the dimensionless parameter s (as discussed in section 3) is presented in figure 3, and it tends to show negative associations between these two quantities (see figure 4). although the p values for pg_14 and pg_14mn of 0.168 and 0.179, respectively, do not allow these to be considered statistically significant, their correlation coefficients of –0.407 and –0.397, respectively, do indicate this trend. these results highlight that the major influence of the seismic activity on the pg occurs over an interval of 14 days centered on the seismic event. interestingly, this time interval of nearly two weeks around the earthquake occurrence is common to other seismic .r 10 . m0 43. ,s d r 1= seismic activity effect on lisbon's aef date (dd/mm/yyyy) time (days) depth (km) m (km) r (km) d s 17/06/1955 0.050 3.4 28.973 18.138 0.597 30/07/1959 0.001 13 4.3 70.632 5.991 10.789 08/05/1962 0.625 3.3 26.242 6.309 15/03/1964 0.938 10 6.2 463.447 319.056 0.453 31/03/1964 0.624 2.7 14.488 6.633 1.184 26/08/1966 0.248 10 4.6 95.060 87.702 0.084 24/02/1967 0.926 2 4.3 70.632 65.619 0.076 04/10/1967 0.104 13 4.3 70.632 56.341 0.254 24/02/1969 0.511 4.5 86.099 45.420 0.896 28/02/1969 0.111 10 7.5 1678.804 349.917 3.798 14/01/1973 0.853 2 4.3 70.632 25.085 1.816 29/06/1973 0.665 21 3.7 38.994 29.650 0.315 26/05/1975 0.383 10 8.1 3040.885 828.839 09/01/1987 0.025 9 2 7.244 4.311 0.680 22/05/1988 0.583 8 3.7 38.994 32.374 0.204 table 1. the 15 seismic events of interest, where s > 0 (see equation 2), in the period from 1955 to 1991 (data provided by the portuguese national laboratory for civil engineering and by the portuguese meteorological institute). m, seismic magnitude; r, earthquake preparation radius; d, epicentre distance to the pg sensor; and s, ratio r�d�� (see text). precursors, like geomagnetic field anomalies [e.g. frasersmith et al. 1990] and perturbations in very low frequency/low frequency radio transmissions [e.g. biagi et al. 2009]. moreover, similar intervals were also observed by varostos et al. [1984] for the different phenomenon known as seismic electric signals. in addition, the analysis also shows that short-time averages, as with two days in this case, are insufficient to capture the effects of seismicity on the pg. this result might indicate that short-term variations in the pg caused by clouds, dust, or other phenomena, might overshadow the effects of seismic activity on the local electric environment. this tendency to a negative association between pg and s is also interesting. this reveals consistently decreasing pg values with s. this is physically expected, as can be seen when two events with the same magnitude but with the epicenter at different distances from the pg sensor are considered. although both of these events will have similar tension fields during the earthquake preparation, it is expected that the nearest event (with the greater s) should create a greater tension field in the sensor region than the distant one (with the smaller s). for this reason, if radon emanation occurs in the zones where tensions are likely to alter the soil permeability, radon emanation due to the first of these events is expected to be higher in the zone of the sensor, as compared to that from the second event. this would naturally result in a more significant reduction in the pg associated with the first earthquake, with respect to the second earthquake. 5. final remarks to sum up, the present study has used simple statistical techniques to reveal indications that seismic activity leaves its mark on the atmospheric electrical field in a region close to an earthquake epicenter, with a trend to reduce the atmospheric electrical field. this aspect opens the way to two main approaches. first, more profound examinations of the datasets are required. special attention needs to be paid to the influence of meteorological variables on the pg, like relative humidity, precipitation, wind intensity and cloudiness, in line with other studies [e.g. kachakhize et al. 2009], with the purpose of not restricting the data to fair-weather conditions. this also indicates that the present dataset can be complemented with measurements of the atmospheric radon levels during the period of operation of the pg sensor. this will provide a better understanding of the importance of radon in the lithosphere-atmosphere-ionosphere coupling. secondly, the present method can be applied to pg datasets in other seismically active regions. indeed, this last approach reveals the relevance of carrying out pg measurements worldwide, not only because of the great importance in itself [see harrison 2005], but also because it might contribute in a significant way to the study of seismic precursors. acknowledgements. one of the authors (hgs) acknowledges the support of two portuguese institutions: the science and technology foundation (fct) for the grant sfrh/bpd/63880/2009, and the calouste gulbenkian foundation (fcg) for estímulo à criatividade e à qualidade na actividade de investigação in the science program of 2010. we are grateful to the portuguese metereological institute for allowing us to use their seismic database. finally, important discussions of this study with prof. giles harrison, dr. keri nicoll, and dr. karen aplin are strongly acknowledged. silva et al. 196 figure 3.the pgs for the different averages considered in this study (pg_2, pg_2mn, pg_14, pg_14mn, pg_28, pg_28mn) as a function of s. figure 4. statistics relating to the pg and s correlations. 197 references biagi, p.f., l. castellana, t. maggipinto, d. loiacono, l. schiavulli, t. ligonzo, m. fiore, e. suciu and a. ermini (2009). a pre-seismic radio anomaly revealed in the area where the abruzzo earthquake (m=6.3) occurred on 6 april 2009, nat. hazard earth sys., 9, 1551-1556. borges, j., a.j.s. fitas, m. bezzeghoud and p. teves-costa (2001). seismotectonics of portugal and its adjacent atlantic area, tectonophysics, 337, 373-387. buforn, e., m. bezzeghoud, a. udias and c. pro (2004). seismic source in the iberian-african plate boundary, pure appl. geophys., 161, 623-646. dobrovolsky, i.p., s.i. zubkov and v.i. miachkin (1979). estimation of the size of earthquake preparation zones, pure appl. geophys., 117, 1025-1044. fraser-smith, a.c., a. bernardi, p.r. mcgill, m.e. ladd, r. a. helliwell and o.g. viilard (1990). low-frequency magnetic field measurements near the epicenter of the ms 7.1 loma prieta earthquake, geophys. res. lett., 17, 1465-1468. harrison, r.g. (2005). columnar resistance changes in urban air, j. atmos. sol.-terr. phys., 67, 763-773. harrison, r.g., k.l. aplin and m.j. rycroft (2010). atmospheric electricity coupling between earthquake regions and the ionosphere, j. atmos. sol.-terr. phys., 72, 376-381. kachakhidze, n. m. kachakhidze, z. kereselidze and g. ramishvili (2009). specific variations of the atmospheric electric field potential gradient as a possible precursor of caucasus earthquakes, nat. hazard earth sys., 9, 1221-1226. kachakhidze, m.k., z.a. kereselidze and n.k. kachakhidze (2011). the model of self-generated seismo-electromagnetic oscillations of the lai system, solid earth, 2, 17-23. liperovsky, v.a., c.-v. meister, e.v. liperovskaya, and v.v. bogdanov (2008). on the generation of electric field and infrared radiation in aerosol clouds due to radon emanation in the atmosphere before earthquakes, nat. hazard earth sys., 8, 1199-1205. omori, y., h. nagahama, y. kawada, y. yasuoka, t. ishikawa, s. tokonami and m. shinogi (2009). pre-seismic alteration of atmospheric electrical conditions due to anomalous radon emanation, phys. chem. earth, 34, 435-440. pierce, e.t. (1972). radioactive fall-out and secular effects in atmospheric electricity, j. geophys. res., 77, 482-487. pierce, e.t. (1976). atmospheric electricity and earthquake prediction, geophys. res. lett., 3, 185-188. serrano, c., a.h. reis, r. rosa and p.s. lucio (2006). influences of cosmic radiation, artificial radioactivity and aerosol concentration upon the fair-weather atmospheric electric field in lisbon (1955-1991), atmos. res., 81, 236-249. silva, h.g., m. bezzeghoud, a.h. reis, r.n. rosa, m. tlemçani, a.a. araújo, c. serrano, j.f. borges, b. caldeira and p.f. biagi (2011). atmospheric electrical field decrease during the m = 4.1 sousel earthquake (portugal), nat. hazard earth sys., 11, 987-991. smirnov, s. (2008). association of the negative anomalies of the quasistatic electric field in atmosphere with kamchatka seismicity, nat. hazard earth sys., 8, 745749. sousa, m.l., a. martins and c.s. oliveira (1992). compilação de catálogos sísmicos da região ibérica. relatório 36/92, nda, laboratório nacional de engenharia civil, lisboa. varotsos, p. and k. alexopoulos (1984). physical properties of the variations of the electric field of the earth preceding earthquake i, tectonophysics, 110, 73-98. *corresponding author: hugo manuel gonçalves da silva, universidade de évora, escola de ciências e tecnologia, centro de geofísica de évora e departamento de física, évora, portugal email: hgsilva@uevora.pt. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. seismic activity effect on lisbon's aef layout 6 variazione stagionale del sistema di correnti sq franco molina e osvaldo battelli annali di geofisica, vol. 14, n. 4, 1961. annals of geophysics, vol. 53, n. 1, february 2010 abstract the present work is concerned with an examination of the seasonal variation of the sq current system at a latitude of about 40˚ n in the european region, with particular regard to the shifts in latitude of the «focus» of the system. the period of time under examination extends from june 1958 to december 1960 inclusive. use has been made of the magnetograms of asiago (45˚52' n, 11˚31' e; geomagnetic latitude 46.6˚ n), l'aquila (42˚23' n, 13˚19' e; geomagn. lat. 42.9˚ n), and gibilmanna (37˚59' n, 14˚01' e; geomagn. lat. 38.5˚ n) and of the hourly values of the fürstenfeldbruck observatory (48˚10' n, 11˚17' e; geomagn. lat. 48.9˚ n). for this study, days with ap < 10 were chosen. however, the reading taken on some of these days had to be discarded, owing to the presence of disturbances during the hours under examination. in all, 283 days were taken, out of a total of 945. for determining the position of the «focus», the sq current system above the italian region was constructed for each of the selected days on the basis of the following two hypotheses: 1) the current have zero intensity around midnight; 2) the current system neither shifts nor changes shape during the interval between 0900 and 1400 local time. on the basis of the hypothesis 1), the zero level of the horizontal component h and declination d is taken as the average of the four hours 0000-0200 and 2200-2400 of the day in question. the differences between the hourly values of δh and δd between 0900 and 1400 local time and their corresponding values at zero level were then calculated. finally, from the pairs of values of δh and δd the current arrows were constructed in the usual way. according to hypothesis 2), these arrows represent the configuration of the current system during the central hours of the day under consideration. figures 1-5 represent the current systems deduced from the average daily variation sq of h and d for every month of the period considered. they show that the centre of the current system shifts continually from one month to another. however, during the various years considered, between the same months there is a marked correspondence of behaviour, with the exception of january and december. figure 6 represents the position of the «focus» for each of the selected quiet days in the following five areas: area 1, north of fürstenfeldbruck; area 2, between fürstenfeldbruck and asiago; area 3 between asiago and l'aquila; area 4, between l'aquila and gibilmanna; area 5, south of gibilmanna. figure 7a shows, in the form of block diagrams, the percentage distribution in the various areas for the following groups of months: march-may (spring), june-august (summer), septembernovember (autumn) and december-february (winter), while figures 7b and 7c show the percentage distribution separately for the periods december 1958-november 1959 and december 1959-november 1960. the following facts may be established from figures 6 and 7: 1) the minimum percentage for the days in which the focus lies in area 5 (the most southern) occurs in spring; 2) the case in which the focus lies in area 1 (the most northern) is rather rare, but it occurs almost exclusively in winter; 3) in summer there is the minimum displacement of the position of the focus, and thus the maximum stability of the current system; in winter, however, the stability is of a lower value than in the other seasons; 4) there occur various cases of abrupt variations in the position of the focus from one day to another, even between area 1 and area 5, in this case involving displacements of at least 100 in latitude. from the results of the present study it is seen that, at least for the european region, the seasonal behaviour of the position of the focus is a more complex phenomenon than a mere shift towards the south from summer to winter [matsushita 1960] or vice versa [chapman and bartels 1940, ota 1950]. the most reliable result is that summer and winter show respectively the maximum and minimum stability in the configuration of the sq current system, which would involve a greater variability in the general circulation in the upper atmosphere during the winter, in agreement with vestine [1958]. it may also be that variations in the spectrum of ultra-violet and x radiation from the sun cause a shift in the altitude at which the currents flow. in view of the fact that, according to greenhow and neufeld [1956], the behaviour of the winds at between 80 and 100 kilometers varies with height much more rapidly in winter than in summer,this would therefore explain the greater winter variability of the sq current system. introduzione attorno alla latitudine di 40˚, come è noto, l'andamento diurno sq della componente orizzontale del campo magnetico terrestre presenta una variabilità da giorno a giorno particolarmente accentuata; esso infatti è soggetto, oltre che a notevoli variazioni di ampiezza come alle altre latitudini, anche a inversioni di fase. tali inversioni sono da attribuirsi al fatto che il centro, o «fuoco», del sistema di correnti ionosferiche responsabili dell'andamento sq [chapman e bartels 1940], che si trova proprio alla latitudine suddetta, subisce talvolta cospicui spostamenti in latitudine, passando da un giorno all'altro da nord a sud degli osservatori situati in questa zona, o viceversa. data la situazione geografica dell'italia, abbiamo ritenuto utile contribuire allo studio di questi spostamenti, 35 studio già iniziato da vari ricercatori [hasegawa 1936, ota 1950, kazmi 1959, hasegawa 1960, matsushita 1960]. in un precedente lavoro [molina e battelli 1961] abbiamo cercato un andamento stagionale della posizione del fuoco, prendendo in esame l'intervallo di tempo compreso tra il giugno 1958 e il dicembre 1959, e confrontando i risultati ottenuti in questo intervallo sia con quelli degli altri ricercatori citati, sia con lo studio dell'andamento sq all'osservatorio di gibilmanna per gli anni 1954-55 [giorgi e molina 1956]. nel presente studio abbiamo esteso l'intervallo a tutto il 1960, ed abbiamo modificato il metodo per la ricerca della posizione del fuoco in modo da renderlo più attendibile. dati di osservazione e loro elaborazione sono state utilizzate le registrazioni della stazione magnetica di asiago (45˚52' n, 11˚31' e; lat. geomagn. 46.6˚ n), dell'osservatorio di l'aquila (42˚23' n; 13˚19' e; lat. geomagn. 42.9˚ n), e dell'osservatorio di gibilmanna (37˚59' n, 14˚01' e; lat. geomagn. 38.5˚ n). inoltre si è fatto uso dei dati dell'osservatorio tedesco di fürstenfeldbruck (48˚10' n, 11˚17' e; lat. geomagn. 48.9˚ n), pubblicati nei bollettini dell'osservatorio. come si è detto, l'intervallo studiato va dal giugno 1958 al dicembre 1960 compresi. per la scelta dei giorni, al criterio di usare i cinque o dieci giorni internazionali per mese si è preferito quello di utilizzare tutti e soli i giorni per cui l'indice ap è inferiore a 10. in tal modo si ha una maggiore sicurezza di lavorare su giorni in cui la situazione geomagnetica è veramente calma, anche se in realtà, come vedremo in seguito, questo criterio non è sempre sufficiente a soddisfare tale condizione. un inconveniente di questa scelta consiste nel fatto che il numero di giorni preso in esame è generalmente diverso da mese a mese, il che dà un peso differente ai risultati dei singoli mesi o gruppi di mesi in una statistica volta a mettere in luce un andamento annuale; ma tale inconveniente ci è sembrato minore di quello di usare giorni in cui l'agitazione magnetica è ancora piuttosto sensibile, malgrado la denominazione ufficiale di «giorno calmo». tuttavia, se in qualche mese il numero di giorni con indice ap < 10 era inferiore a cinque, sono stati aggiunti, per raggiungere almeno questa cifra, anche giorni di ap ⩾ 10; ciò è avvenuto solo per tre mesi, e il massimo ap usato è 14. in realtà si sono dovuti scartare anche alcuni giorni con ap < 10, che presentavano qualche perturbazione durante le ore prese in considerazione in questo studio. complessivamente i giorni utilizzati sono stati 283, su un totale di 945; se si fossero usati tutti i dieci giorni calmi internazionali per mese, si sarebbe raggiunta la cifra di 310; la differenza non è rilevante, soprattutto tenendo conto che probabilmente molti giorni ufficiali avrebbero dovuto essere scartati per perturbazioni. per la determinazione della posizione del «fuoco» si è variazione stagionale del sistema di correnti sq 36 figura 1. figura 2. 37 costruito per ogni giorno il sistema di correnti orizzontali al di sopra della regione italiana sulla base delle due ipotesi seguenti: 1) le correnti hanno intensità nulla attorno alla mezzanotte, 2) il sistema di correnti non si deforma e non si sposta nell'intervallo compreso tra le 9h e le 14h di tempo locale. si è inoltre assunto che le variazioni della componente orizzontale hcoincidano con quelle della componente nord x; analogamente le variazioni della declinazione, espresse in gamma mediante la formula dove è il valore medio della componente h, sono state assimilate alle variazioni della componente y. tale assunzione è giustificata dal basso valore della declinazione nella regione considerata (1˚-2˚w). si è infine trascurata la possibile influenza sulla posizione del fuoco delle correnti indotte nell'interno della terra. in base alla ipotesi 1), si e assunto come livello base della componente h e della declinazione d la media delle quattro ore estreme, 0h-2h e 22h-24h, del giorno considerato; si sono poi computate le differenze δhe δd tra i valori medi orari di he dcompresi tra le 9h e le 14h di tempo locale e i rispettivi valori di livello base; dalle coppie di valori δh e δd si sono costruite infine, nel modo usuale, le frecce di corrente. nella ipotesi 2) queste rappresentano la configurazione del sistema di correnti nelle ore centrali del giorno considerato. è probabile che quest'ultima ipotesi non sia in generale interamente valida; tuttavia è da ritenere che in prima approssimazione i movimenti del sistema di correnti non siano tali da falsare sostanzialmente i risultati. quanto alla ipotesi 1), relativa alla scelta del livello di base, essa ci è apparsa del tutto attendibile, perchè la variazione del campo magnetico terrestre alle medie latitudini nelle ore attorno alla mezzanotte è, in condizioni molto calme, praticamente nulla; la stessa ipotesi è stata assunta da volland e taubenheim [1958] e da veldkamp e van sabben [veldkamp e van sabben 1960, van sabben 1961] nello studio dei « crochets». l'inconveniente di questa scelta è da ricercarsi nel fatto che proprio in queste ore si ha la massima probabilità di perturbazioni geomagnetiche, soprattutto del tipo delle baie; in effetti questo caso si è presentato varie volte nei giorni da noi considerati; dove è stato possibile si sono interpolati i valori eliminando le baie, altrimenti si sono scartati i giorni. risultati innanzi tutto sono state computate le curve medie della variazione diurna di sq di h e d mese per mese, medie eseguite sui giorni considerati calmi secondo il nostro criterio (ap < 10). in base a queste curve è stato ricavato il sistema di correnti medio mese per mese col metodo sopra descritto; i risultati sono rappresentati nelle figure 1-5. in variazione stagionale del sistema di correnti sq figura 3. figura 4. . /3438d h d=d dc l h variazione stagionale del sistema di correnti sq 38 queste i punti rappresentativi dei quattro osservatori sono riportati in scala di latitudine e di longitudine. le figure mostrano con evidenza che il centro del sistema di correnti si sposta continuamente da un mese all'altro, e che anche la forma del sistema di correnti varia notevolmente. tuttavia in generale nei vari anni considerati vi è una notevole corrispondenza di comportamento negli stessi mesi; caratteristica è, ad esempio, la forma del sistema di correnti nei mesi di luglio e agosto, sistematicamente molto schiacciata attorno al «fuoco», il quale è in media situato attorno alla latitudine di gibilmanna. fanno tuttavia chiara eccezione i mesi di dicembre e gennaio. nel dicembre 1958 e gennaio 1959 il sistema di correnti ha infatti una conformazione molto regolare, con fuoco a sud di gibilmanna; invece nei mesi di dicembre 1959 e 1960 e gennaio 1960 l'aspetto delle correnti è nettamente diverso, e indica una posizione del fuoco decisamente più settentrionale ma difficilmente localizzabile. una valutazione più esatta della situazione può essere ottenuta studiando la posizione del «fuoco» giorno per giorno. a tale scopo sono state individuate, in modo pressoché simile a quello usato da matsushita [1960], cinque zone di latitudine, così definite: zona 1 a nord di fürstenfeldbruck, zona 2 tra fürstenfeldbruck e asiago, zona 3 tra asiago e l' aquila, zona 4 tra l' aquila e gibilmanna, zona 5 a sud di gibilmanna. l'ubicazione del fuoco è stata fissata relativamente a queste zone, perchè non si è ritenuto possibile, col presente metodo, precisarne in modo più fine la latitudine; nella incertezza tra due zone confinanti, cioè quando il «fuoco» risultava più o meno esattamente al di sopra di uno degli osservatori, esso è stato assegnato alla zona meridionale. in figura 6 è rappresentata la posizione del «fuoco» nelle singole zone per ognuno dei giorni calmi da noi scelti, mentre in figura 7a è data, sotto forma di diagramma a blocchi, la distribuzione percentuale nelle varie zone per i seguenti gruppi di mesi: marzo-maggio (primavera), giugnoagosto (estate), settembre-novembre (autunno), dicembre-febbraio (inverno). tale raggruppamento ci è sembrato il più opportuno in base alla figura 6, che mostra una notevole omogeneità di comportamento nei suddetti gruppi di mesi anno per anno. le figure 7b e 7c mostrano la distribuzione percentuale separatamente per gli intervalli dicembre 1958-novembre 1959 e dicembre 1959-novembre 1960. i valori numerici sui quali sono costruiti i diagrammi di figura 7 sono dati nella tabella 1. le figure 6 e 7 danno una indicazione sul maggiore o minore significato dei sistemi di correnti medidelle figure 1-5. è evidente infatti che la notevole mescolanza di giorni con fuoco nelle zone estreme che si riscontra in inverno, e in particolar modo nell'inverno 1959-60, rende poco significativa una media eseguita per quei mesi. in figura 8 sono perciò rappresentati i sistemi di correnti medi dei mesi di dicembre e gennaio 1959 e 1960 separatamente per i giorni in cui il fuoco è nella zona 1 e per quelli in cui è nella zona 5. l'attribuzione delfuoco alla zona 1, posizione che va considerata se non anomala almeno piuttosto eccezionale, potrebbe essere erroneamente causata da perturbazioni, sia pure lievi, in atto: ciò dovrebbe riflettersi in un valore maggiore dell'indice ap nei giorni di zona 1 rispetto a tutti gli altri giorni. la media degli ap su tutti i 283 giorni è 5.9, e ugualmente 5.9 risulta quella dei 12 giorni di zona 1; nei limiti di validità di un confronto fra due medie eseguite su numeri di giorni così diversi si può affermare che nel figura 5. figura 7a. 39 complesso non vi è un effetto sistematico di attività geomagnetica nei giorni di zona 1. si è cercato se un tale effetto, inesistente nel complesso della giornata, si faccia invece sentire durante le ore prese in esame in questa studio; a tale scopo, trasformati in ap [annals of i.g.y, 1957] gli indici kp del primo e ultimo intervallo triorario di ogni giorno e, separatamente, quelli del terzo e quarto, si sono computate le medie di ap dei due gruppi di intervalli triorari per tutti i 283 giorni e per i 12 giorni di zona 1. a causa della differenza di 1 ora tra il tempo locale e il tempo universale, al quale sono riferiti gli intervalli triorari degli indici k, restano in questa modo escluse le due ore locali 0-1 e 13-14, ma ciò ha scarsa importanza agli effetti del confronto. la media di ap delle ore notturne per tutti i giorni è risultata 6.3, per i giorni di zona 1, 5.4; per le ore centrali della giornata si ha rispettivamente 5.9 e 6.5. possiamo quindi escludere l'influenza sistematica di perturbazioni sui sistemi di correnti nei giorni in questione. dalle figure 6 e 7 si deduce l'esistenza di un andamento stagionale sufficientemente chiaro, almeno per l'intervallo di tempo considerato. i punti principali che si possono stabilire per l'attuale periodo di osservazione sono i seguenti: variazione stagionale del sistema di correnti sq figura 7b. figura 7c. figura 6. 1) la minima percentuale dei giorni in cui il fuoco è in zona 5, la più meridionale, si trova in primavera (marzo-maggio). 2) il caso in cui il fuoco si trova nella zona 1, la più settentrionale, è piuttosto raro (il 5 per cento di tutti i giorni esaminati), ma esso si verifica quasi esclusivamente in inverno. 3) la caratteristica più notevole dell'andamento stagionale è costituita dal fatto che in estate si ha la minima dispersione nella posizione del fuoco, cioè la massima stabilità del sistema di correnti, mentre in inverno la stabilità è minore che nelle altre stagioni. 4) si hanno vari casi di brusche variazioni nella posizione del fuoco da un giorno all'altro, anche fra la zona 1 e la zona 5, il che comporta spostamenti di almeno 100 in latitudine. ciò non è altro che una conferma di quanto già assodato da hasegawa [1936, 1960] e da matsushita [1960]. discussione i sistemi di corrente sq calcolati da bartels [chapman e bartels 1940] per l'anno di minima solare 1902 indicano uno spostamento del fuoco verso sud in estate; con questo risultato sembra si accordino quelli di ota [1950] per il secondo anno polare. matsushita [1960] giunge invece ad una conclusione opposta, e cioè ad un aumento medio di latitudine del fuoco in estate rispetto all'inverno, e considera tale risultato come più ragionevole perchè sembra indicare che il fuoco segue piuttosto fedelmente gli spostamenti annuali della declinazione solare; anche l'analisi di vestine e collab. [1947] per gli anni 1922-33 è in accordo con le conclusioni di matsushita. ulteriori studi di hasegawa e di ota [hasegawa 1960] sui primi sei mesi dell'anno geofisico internazionale non indicano alcuna differenza essenziale tra estate e inverno in estremo oriente. dai risultati del presente studio appare che l'andamento stagionale della posizione del fuoco è un fenomeno più complesso che non un semplice spostamento nord-sud da estate a inverno o viceversa. da questo punta di vista non è tanto in estate quanto in primavera che il fuoco raggiunge in media la latitudine più elevata; giorgi e molina [1956] hanno dedotto una conclusione simile per gli anni 1954-55, il che suggerisce che il fatto sia vero in generale, almeno nella regione europea. quanto all'inverno, l'unica cosa che si può affermare per l'intervallo di tempo studiato è che esso è la stagione più variabile: il fuoco si distribuisce in tutte le zone, con una preferenza per le zone estreme. inoltre, mentre le caratteristiche della primavera, estate e autunno si riproducono piuttosto fedelmente nelle due annate 1 dicembre 1958-1 dicembre 1959 e 1 dicembre 1959-1 dicembre 1960, l'inverno presenta invece una marcata differenza: infatti nel 1958-59 il fuoco prediligeva in misura nettamente predominante la zona più meridionale (figura 7b), dando valore alle conclusioni di matsushita, nell'anno successivo invece esso era distribuito molto più uniformemente in tutte le zone (figura 7c). in ogni caso è sopratutto in inverno che il fuoco cade a nord di fürstenfeldbruck; e ciò è in accordo con i diagrammi di matsushita [1960], dai quali risulta che solo in inverno il fuoco cade nella zona a nord di fredericksburg, il più settentrionale (lat. geomagn. 49.6˚ n) degli osservatori da lui presi in esame nella regione americana, anche se la massima frequenza si presenta per la zona attorno a san juan, il più meridionale (lat. geomagn. 29.9˚ n). con la scarsità di dati attualmente a disposizione non è facile stabilire se il disaccordo fra le varie conclusioni sia dovuto alle differenze negli intervalli di tempo studiati dai vari autori o ai diversi metodi statistici usati, o se esista invece una differenza sistematica nelle varie regioni geografiche. lo studio di kazmi [1959], confermando, almeno per la regione dell'estremo oriente, l'andamento stagionale osservato da ota, suggerisce che una differenza fra le diverse regioni geografiche esista effettivamente; tuttavia non si può escludere che la ragione principale del disaccordo sia da ricercarsi nel fatto che i vari autori hanno finora studiato intervalli di tempo generalmente diversi l'uno dall'altro; la differenza fra le figure 7b e 7c per la stagione invernale prova infatti che non è prudente generalizzare risultati ottenuti per un breve intervallo di tempo. infine anche la differenza nei variazione stagionale del sistema di correnti sq 40 tabella 1. 41 metodi di ricerca usati ha certamente la sua influenza: ota stabilisce la posizione del fuoco per tutti i giorni (escludendo solo quelli con forti tempeste), matsushita usa i dieci giorni calmi internazionali per mese, e il presente studio i giorni con ap < 10. mentre il primo metodo non risulta del tutto attendibile, in quanta è ben difficile localizzare il fuoco quando sono presenti perturbazioni anche moderate, per gli altri due si pone ovviamente il problema se i giorni scelti costituiscono un campione veramente rappresentativo, e il valore delle conclusioni dedotte è necessariamente limitato da tale incertezza. entro questi limiti, ci sembra che il risultato più certo del presente studio sia che l'estate e l'inverno presentano rispettivamente la massima e la minima stabilità della configurazione del sistema di correnti sq. come è noto, tale sistema di correnti è condizionato: 1) dal valore e dalla distribuzione della conducibilità elettrica dell'alta atmosfera, cioè in ultima analisi dalla intensità della radiazione ultravioletta e x del sole; 2) dai moti dell'alta atmosfera; 3) dal campo magnetico terrestre, in particolare dalla sua componente verticale. la spiegazione più semplice del marcato effetto stagionale ora detto è quella di una particolare variabilità della circolazione generale dell'alta atmosfera durante l'inverno, variabilità che va gradualmente diminuendo fino a raggiungere un minimo in estate. ciò è in accordo con le conclusioni che vestine [1958] ha dedotto in seguito ad uno studio sulla variabilità dell'andamento sq della componente orizzontale a huancayo. è utile notare che indicazioni analoghe si possono trovare nei primi studi di chapman e stagg [1929, 1931] sulla variabilità da giorno a giorno di sq. a questo proposito è da sottolineare il fatto che greenhow e neufeld [1960] hanno messo in rilievo, nello studio dei venti nell'alta atmosfera mediante i radioechi dalle scie dei meteoriti, l'esistenza nella regione fra 80 e 100 km di altezza di sistemi di pressione simili in dimensioni e in durata a quelli esistenti sulla superficie e caratteristici dei fenomeni meteorologici; lo studio della variazione sq suggerirebbe quindi che la situazione «meteorologica» dell'alta atmosfera e più variabile in inverno che in estate. questa spiegazione attribuisce unicamente alle variazioni del sistema di venti ionosferici la variabilità della sq, lasciando ovviamente impregiudicata la questione della causa di quelle variazioni. potrebbe tuttavia essere avanzata un'altra ipotesi, che fa risalire direttamente a fluttuazioni nello spettro della radiazione solare la variabilità della sq: secondo greenhow e neufeld [1956], nell'alta atmosfera attorno agli 80-100 km esiste un notevole gradiente con l'altezza della intensità e della fase della componente semidiurna dei venti; tale gradiente presenta un marcato andamento stagionale, essendo pressoché nullo in estate e massimo in inverno. d'altra parte, fluttuazioni nella intensità e soprattutto nello spettro della radiazione solare nella gamma ultravioletta e x possono spostare il livello medio a cui scorrono le correnti elettriche sq; volland e taubenheim [1958] e veldkamp e van sabben [veldkamp e van sabben 1960, van sabben 1961] hanno effettivamente mostrato che le correnti che danno luogo a un «crochet» prodotto da un brillamento solare scorrono ad una altezza diversa da quella delle normali correnti sq. variazioni di livello prodotte da fluttuazioni meno intense ma di più lunga durata dei brillamenti potrebbero dare luogo a deformazioni nel variazione stagionale del sistema di correnti sq figure 8a e 8b. sistema sq, deformazioni molto più marcate in inverno che in estate, data la variazione stagionale del gradiente. in tal caso tuttavia ci si dovrebbe forse attendere in inverno una variabilità della sq minore al minimo e maggiore al massimo dell'attività solare, ma di ciò non vi è finora alcun indizio. bibliografia annals of i.g.y., 4, 1957, 215-236. chapman, s. e j. bartels (1940). geomagnetism, oxford, 1, 1229. chapman, s. e j. m. stagg (1929). proc. roy. soc. a., 123, 27. chapman, s. e j. m. stagg (1931). proc. roy. soc. a., 130, 668. giorgi, m. e f. molina (1956). annali di geof., 9, 17. greenhow, j. s. e e. l. neufeld (1960). proc. phys. soc., 75, 228. greenhow, j. s. e e. l. neufeld (1956). phil. mag., 1, 1157. hasegawa, m. (1936). proc. imp. acad. tokyo, 12, 185 e 277. hasegawa, m. (1960). journ. geoph. res., 65, 1437. kazmi, s. a. (1959). geophysica, 7, 31. matsushita, s. (1960). journ. geoph. res., 65, 3835. molina, f. e o. battelli (1961). atti x convegno ass. geof. ital., 85. ota m. (1950). trans. oslo meeting 1948, i.a.t.m.e. bull., 13, 438. van sabben, d. (1961). journ. atm. terr. phys., 22, 32. veldkamp, j. e d. van sabben (1960). journ. atm. terr. phys., 18, 192. vestine, e. h. (1958). trans. amer. geoph. union, 39, 213. vestine, e. h., i. lange, l. laporte e w. e. scott (1947). publ. 580, carnegie inst. washington. volland, h. e j. taubenheim (1958). journ. atm. terr. phys., 12, 258. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. variazione stagionale del sistema di correnti sq 42 dobrica_final:layout 6 on the crustal bias of repeat stations in romania venera dobrica1,*, crisan demetrescu1, razvan greculeasa1, anca isac2 1 institute of geodynamics, romanian academy, bucharest, romania 2 geological survey, bucharest, romania annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5442 abstract a magnetic induction model has been applied to recordings obtained in 2010 during the field campaigns for geomagnetic measurements at the 26 repeat stations of the romanian secular variation network. the model is based on the observation that a variable external magnetic field induces a response of the earth's interior not only by electromagnetic induction, but also by magnetic induction in the magnetic rocks above the curie temperature. the model computes coefficients of a linear relationship between recorded values of a certain geomagnetic element (x, y, z, or f) at the repeat station and recorded x, y, z values at a reference station (in this case, sua observatory). coefficients depend on magnetic permeabilities of rocks beneath the station and stand as a proxy for the anomaly bias characterizing the site. maps of the lateral variation of this type of information were obtained and discussed. 1. introduction at present the geomagnetic field is monitored both at ground, by means of magnetic observatories and repeat stations, and from space, by means of satellites, leading to complex studies and modeling of the geomagnetic field [e.g. olsen et al. 2007, finlay et al. 2010, matzka et al. 2010, thébault et al. 2010, mandea and korte 2011]. at global scale the secular variation is found by processing continuous records of the geomagnetic field at magnetic observatories. generally, a country of romania's size has only one observatory. the romanian one, namely surlari geomagnetic observatory (iaga code, sua), was established in 1942. the information from geomagnetic observatories is supplemented at regional level by repeated measurements in a network called secular variation network. in the last years, as a result of a better understanding of the importance of repeat station data in learning about the spatial-temporal evolution of the geomagnetic field, a project is going on at european level, magnete (magnetic network of europe), initiated in 2003 [korte and mandea 2003], on systematic measurements in national secular variation networks. as a result of this initiative the map of declination in europe at the epoch 2006 was published [duma et al. 2012]. the repeat station data were discussed in several studies [e. g. korte and thébault 2007, thébault 2008] as regards the crustal bias and/or modelling at regional scale. in romania, since 1964 systematic geomagnetic measurements have been done in a network of 26 repeat stations. reports on measurement results and their interpretation in terms of secular variation and normal field distributions were published by atanasiu et al. [1965, 1967, 1970, 1974, 1976], anghel et al. [1980], demetrescu et al. [1985]. recently, demetrescu et al. [2011] have reported measurements of the horizontal component at the repeat stations, in the time interval 1980-2004, interpreted according to insights from the analysis of long time series provided by geomagnetic observatories [demetrescu and dobrica 2005, 2012]. beginning with the 2009 campaign, recording fluxgate and proton magnetometers have been deployed, in order to better control the data correction for diurnal and disturbed variations. in this study we focus on the recordings taken in 2010 in the stations of the repeat network, by means of a lemi-018 magnetic variometer and of a g-856 geometrics proton magnetometer, in order to infer information on the lateral variation on the romanian territory of the crustal bias that characterizes each repeat station. 2. method usually, the interpretation of the geomagnetic data from magnetometric arrays is based on the assumption that the external variable magnetic field induces in the conductive structures of subsurface electrical currents that, in turn, produce secondary, detectable magnetic fields. our method is based on the observation that the variable external magnetic field induces variable internal magnetic fields not only by electromagnetic induction, but also by magnetic induction in rocks below the curie temperature. in case of pure magnetic induction, the temporal variation of the field components at a given observing site is a linear combination of the components of the magnetic inducing force [demetrescu et al. 1985, 1988], article history received october 14, 2011; accepted january 30, 2012. subject classification: geomagnetic field, secular variation, repeat station network, romania. 1145 magnete workshop 2011 (1) where d represents variations about temporal averages, e(s) is the geomagnetic field component at the station s (e can be x, y, z, or f), fk, k = 1... 3, are the components of the inducing magnetic force and are coefficients that depend on the effective magnetic permeability characterizing the site. the calculated values (de(s)calc) of the model would represent the pure magnetic induction component of the observed signal and the residuals (de(s)res = de(s) − de(s)calc) would contain information on electromagnetic induction in the earth at the observing site. the coefficients , can be determined by a standard least squares procedure and be mapped, resulting in images of the lateral distribution of the magnetic properties characterizing the volume of rocks above the curie temperature (generally the crust). in order to ensure the comparability of the coefficients obtained for the stations, the recordings at all stations should be simultaneous, to meet the requirement that variations recorded at each station sample the same time interval as the external inducing force. consequently, temporal averages mentioned above refer to the common recording time interval. so far, the method has been successfully applied in the case of the romanian network of repeat stations [demetrescu et al. 1985] and the network of european geomagnetic observatories [demetrescu et al. 1988, demetrescu and andreescu 1992, demetrescu and andreescu 1994, demetrescu and dobrica 2003] for the case of the solar-sunspot-cycle related variations, as well as in the case of the hokkaido magnetometric array [dobrica et al. 2008/2009] for the diurnal variation. in the present paper we apply this method to variations recorded at the romanian network of repeat stations in a time span of several hours. slight changes in the method, discussed in the next two sections, have been introduced due to the particular setting of our measurements. 3. repeat station and observatory data the input data in the magnetic induction model are the three orthogonal components and total intensity of the geomagnetic field observed in the romanian network of repeat stations and the components of the inducing magnetic force. in the absence of independent data on the external geomagnetic field related to the variations observed (i.e. the field produced by the ionospheric and magnetospheric current systems responsible for the regular diurnal variation (sq) and, respectively, for the geomagnetic activity), we took as estimates for the components of the inducing magnetic force, the components of the variation at a nearby geomagnetic observatory (a reference station, as is usually done in the interpretation of data from magnetometer arrays [gamble et al. 1979, gough and ingham 1983, harada et al. 2004]). in this study, the surlari geomagnetic observatory has been used as the reference station, so the input magnetic inducing force in the model is in fact a resultant of the external field and the induced response of the earth beneath the observatory. as the latitudinal spread of the study territory is rather small in comparison to the lateral scale of the sq current system, we did not account for the small possible variation of the external sq field from station to station. the actual equations expanded from equation (1) read: dx(s) (t) = axdx sua (t) + aydy sua (t) + azdz sua (t) dy(s) (t) = bxdx sua (t) + bydy sua (t) + bzdz sua (t) (2) dz(s) (t) = cxdx sua (t) + cydy sua (t) + czdz sua (t) df(s) (t) = gxdx sua (t) + gydy sua (t) + gzdz sua (t) the distribution of the 26 repeat stations and sua observatory over the national territory is presented in figure 1 and the geographical coordinates are given in table 1. the measurements were taken in 2010 during four field campaigns: june 17 to july 7, july 30 to august 18, september 14 to 22, and october 1st to 14. at each station the field components x, y, and z were recorded for 7-8 hours, generally between 8-17 standard local time (slt) (5-14 ut), by means of a lemi-018 magnetic variometer. we also recorded, by means of a g-856 geometrics proton magnetometer, the total field intensity, f. one minute data have been used in the subsequent processing. as an example of data appearance we show, in figure 2, the records in case of three stations, as compared to observatory recordings. generally low geomagnetic activity characterized the recording time span, but disturbed days were also present in a few instances. a strong disturbance seen on data in the recordings of lipova station, on the 4th of august is shown as an example as well. the geomagnetic conditions during the measurement campaigns can be seen in figure 3, where the field evolution recorded by the geomagnetic observatory during the four campaigns is shown together with the evolution of the dst index. 4. results and discussion results for each repeat station consist of the coefficients of equation (2), the calculated values of the model, and the residuals. in figure 4 we give, as an example, the calculated field and the residuals of the induction model for x, y, z, and f, in case of cluj repeat station. figure 5 synthetically illustrates, by means of the standard deviation about the mean (sd), the variability of initial data, of calculated ones, and of residuals for the 26 stations, in case of each of the four geomagnetic elements investigated. generally the calculated values, that is the magnetically induced response of rocks above curie temperature, explain the most part of the recorded signal in all components. the x record at station no. 24 and the y record at station no. 18 could not be used due to a malfunction dobrica et al. 1146 ( ) ( ),e t c f t( )s k e k k 1 3 d d= = / ck e ck e 1147 on the crustal bias of repeat stations in romania no. station latitude (degrees) longitude (degrees) 1 saveni 47.96593 26.88277 2 livada 47.84867 23.13362 3 radauti 47.82142 25.94800 4 somcuta 47.49997 23.43900 5 vaida 47.25157 21.98520 6 bistrita 47.19485 24.48570 7 varatec 47.15150 26.29175 8 cluj-faget 46.69750 23.54715 9 husi 46.67880 28.00265 10 chisineu-cris 46.54443 21.53978 11 bretcu 46.05767 26.35652 12 lipova 46.05202 21.72387 13 deva 45.85712 22.91722 14 dumbravita 45.83292 21.29002 15 selimbar 45.75188 24.18742 16 stamora 45.28383 21.24357 17 gropeni 45.08573 27.86850 18 mizil 44.99238 26.37497 19 herculane 44.92215 22.44942 20 babadag 44.87207 28.76385 21 costesti 44.65898 24.89332 22 strehaia 44.61763 23.16890 23 tonea 44.20200 27.41502 24 alexandria 43.96683 25.36662 25 sadova 43.89513 23.93970 26 negru-voda 43.82022 28.24305 27 *surlari geomagnetic observatory (sua) 44.68000 26.25330 figure 1. the romanian repeat stations network and surlari geomagnetic observatory (sua). table 1. geographical coordinates of the repeat stations of the romanian network. dobrica et al. 1148 f ig u re 2 .e xa m pl e of x , z , a nd f r ec or ds ( fu ll lin e) a t c lu j ( le ft ), v ar at ec ( m id dl e) , a nd l ip ov a (r ig ht ), a s co m pa re d w it h re co rd s at s u a ( br ok en li ne ). 1149 on the crustal bias of repeat stations in romania figure 3. the horizontal northward component evolution recorded at sua during the field campaigns. the dst index plotted along illustrates the evolution of the geomagnetic activity. dobrica et al. 1150 figure 4. example of modeling (cluj station): a) the north component (x); b) the east component (y); c) the vertical component (z); d) the total field (f). in each plot the recorded data (upper curve), the calculated values of the model (middle curve) and the residuals (lower curve) are shown. figure 5. standard deviation about the mean of raw (measured) data, calculated values of the magnetic induction model, and model residuals for the 26 stations of the repeat network (identification numbers as in table 1). a) x; b) y; c) z; d) f. a) a) c) c) d) d) b) b) 1151 of the recorder. the corresponding sd analysis and the coefficient values are not displayed in figure 5 and respectively in figure 6. the ratio of residuals sd to calculated values sd is, however, larger in case of the vertical component, meaning that the latter is more responsive to electromagnetic induction than the horizontal components, which are more responsive to the magnetic induction. in case of z, the lipova station shows the largest residuals sd, of about ± 4 nt, and individual residual values that reach 30 nt, which is related to the geomagnetic storm that occurred during measurements. the z residuals for other stations are of the order of ± 1 nt (18 stations) and of ± 2 nt (7 stations). the accuracy of coefficients varies between 3% and 10% of the coefficient values. in figure 6 we present maps of the lateral variation of the coefficients we determined in case of the four geomagnetic elements measured. in order to compare coefficients (and consequently magnetic properties of crust) at different stations, the ideal situation would be to have simultaneous measurements at all network stations, to ensure that the same inducing field is applied to the laterally varying magnetic properties of the study area. having in view that measurements at the repeat stations were not simultaneous and the daily lt variations have different amplitudes in different days, we selected for each station only data in the time interval common to all stations, namely 8:24-13:05 ut, to ensure at least that we sampled the same part of the regular on the crustal bias of repeat stations in romania figure 6. the lateral variation of the repeat stations coefficients. from top to bottom: ax, ay, az; bx, by, bz; cx, cy, cz; gx, gy, gz. diurnal variation. the dominant period we sampled this way was the 6-hours harmonic. the higher-frequency ut variations characteristic to the geomagnetic activity were different, of course, in successive days of the survey, but we believe that due to their much lower amplitude (compare 0.5-3 nt to 40-50 nt; figure 2) their contribution intervene to a much lesser extent in the actual values of coefficients. having in view (1) that coefficients represent the lateral variation of magnetic properties by means of relative values and (2) that we use the proxy for the external variation that induces the magnetic response (records at sua) we checked if the map patterns are stable when the proxy was changed: we compared our results with coefficients that resulted using a second inducing source, namely the recordings of the niemegk observatory (ngk). in figure 7 we plotted the values of coefficients at successively numbered repeat stations (table 1) in case of x and z, resulted from the magnetic induction model that used sua (full line) and ngk (broken line) as reference stations. the agreement between the two sets of coefficients is good, meaning that maps based on the two sets will show similar patterns. of course, one has to keep in mind the fact that the coefficient values do not represent absolute permeabilities of the underground, but rather quantities relative to each other. also, the external source and the internal magnetic and electric structures are different for the two observatories, so at least amplitudes of the recorded signal differ in the two cases. anyway, this issue needs more investigation than that undertaken so far. for the moment, a second type of experiment was not possible, namely checking if the magnetic induction model gives similar results when applied to other time intervals. this would be a matter for future studies. dobrica et al. 1152 figure 7. comparison of the induction model coefficients ax and cz using as proxy for the inducing force sua (full line) and ngk (broken line). 1153 each repeat station was carefully selected when the network was established [atanasiu et al. 1970], from about 500 stations of the regional geomagnetic survey with 'absolute' measurements done in the 1960s (a station per 400 km2), in order not to reflect regional and local anomalies. the maps reveal, however, some regional pattern, with smaller values in nw, s, and e, and higher values in the central and southwestern part. the lower values might be associated with structure peculiarities of the pannonian depression, and moesian and east-european platforms, and the higher ones to peculiarities of the eastern and southern carpathians. as the mapped coefficients reflect magnetic properties of rock volumes down to the curie temperature surface, the shape and thickness of the lower crust might also contribute to the observed pattern. a quantitative interpretation would be a matter for future research. of course, a much denser network would be necessary to detect smaller scale structures. 5. conclusion x, y, z, and f data recorded in 2010 at the romanian repeat stations network were used to infer information on the lateral variation of the crustal bias that characterize each station. a magnetic induction model based on the observation that a variable external magnetic field induces a response of the earth's interior not only by electromagnetic induction, but also by magnetic induction in the magnetic rocks above the curie temperature, was applied. the calculated values of the model represent the pure magnetic induction component of the observed signal and the residual would contain information on electromagnetic induction in the earth at the observing site. the coefficients of the linear model relating the measured values to component of the inducing external field were mapped. the resulting patterns show regional distribution of values that correlate with large scale crustal compartments in the study area, but no quantitative interpretation has been undertaken as yet. the methodology and the information acquired in this stage of research would contribute to a better correction and interpretation of geomagnetic measurements at repeat stations. acknowledgements. the study has been supported by the institute of geodynamics (project 2/2011) and by the ministry of education and research (pn-ii uefiscdi idei 0262/2011). we thank two anonymous reviewers for constructive comments that helped improving the manuscript. references anghel, m., c. demetrescu and t. nestianu (1980). secular variation studies in romania, gerlands beitr. geophysik, 89, 499-510. atanasiu, g., i. bucur, t. 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(2011). geomagnetic observations and models, iaga book series, vol. 5, doi 10.1007/978-90-481-9858-0, springer science b.v., dordrecht/heidelberg/london/new york, 343 pp. matzka, j., a. chulliat, m. mandea, c.c. finlay and e. qamili (2010). geomagnetic observations for main field studies: from ground to space, space science reviews, 155 (1/4), 29-64; doi:10.1007/s11214-010-9693-4. olsen, n., g. hulot and t. j. sabaka (2007). the present field, in: m. kono (ed.), treatise on geophysics, vol. 5: geomagnetism, elsevier, 33-75. thébault, e. (2008). a proposal for regional modelling at the earth's surface, r-scha2d, geophys. j. int., 174, 118-134; doi:10.1111/j.1365-246x.2008.03823.x. thébault, e., m. purucker, k.a. whaler, b. langlais and t. j. sabaka (2010). the magnetic field of the earth's lithosphere, space science reviews, 155 (1/4), 95-127; doi:10.1007/s11214-010-9667-6. *corresponding author: venera dobrica, institute of geodynamics, romanian academy, bucharest, romania; email: venera@geodin.ro. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. dobrica et al. 1154 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 48,06,2005misc 899 annals of geophysics, vol. 48, n. 6, december 2005 key words panarea – bathimetry – dem – hydrotermal systems 1. introduction panarea island is located in the southern tyrrhenian sea and is part of the volcanic arc of the aeolian islands that is made up of seven islands, some minor islets and several seamounts (fig. 1a). the area is characterized by a complex geodynamics (falsaperla et al., 1999) being affected by shallow to deep seismicity due to a slab deepening under the calabrian arc (chiarabba et al., 2005), active crustal deformation (anzidei et al., 2000; hollenstein et al., 2003; tallarico et al., 2003; d’agostino and selvaggi, 2004; serpelloni et al., 2005), active volcanoes (lipari, vulcano and stromboli) and hydrothermalism. the above features witness a long lasting activity, as described by italiano and nuccio (1991), gamberi et al. (1997), calanchi et al. (1999, 2002), de astis et al. (2003) and caliro et al. (2004). panarea is the smallest among the aeolian islands, representing the subaerial portion of a submarine stratovolcano about 2000 m high and 20 km wide (gabbianelli et al., 1993; gamberi et al., 1997). east of panarea, the islets of basiluzzo, dattilo, panarelli, lisca bianca, bottaro, lisca nera and le formiche (hereafter referred as the ‘islets’) form an archipelago that the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) marco anzidei (1), alessandra esposito (1), giovanni bortoluzzi (2) and francesco de giosa (3) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto di geologia marina (ismar), cnr, bologna, italy (3) coastal consulting and exploration srl, bari, italy abstract on november 3, 2002 a shallow submarine gas eruption occurred in an area of 2.3 km2 east of panarea (aeolian volcanic arc, southern thyrrenian sea, italy). the exhalative area, surrounded by the islets of dattilo, panarelli, lisca bianca, bottaro and lisca nera, has been known since historical times for the hydrothermal activity related to the panarea volcanic complex. due to the exceptional characteristics of the phenomenon, different geological, geochemical, geophysical and studies were carried out in this still poorly known volcanic area. a particular effort was devoted to producing a high resolution bathymetric map that also aimed to estimate the amount and location of the active exhalative centers and their variations in space and time. data were obtained by three rtk multibeam surveys performed between december 2002 and december 2003. here we show and discuss the technical details of the bathymetric surveys, the bathymetric map at 0.5 m resolution, and the accurate location of the 606 main exhalative centres active during the 2002-2003 crisis. the bathymetric data and the maps show two prevailing principal ne-sw and nw-se alignments that match the spatial distribution of the exhalation centres. the accurate positioning at submeter accuracy of the gas vents is useful in the monitoring activity and to study their temporal and spatial variability. mailing address: dr. marco anzidei, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy, e-mail: anzidei@ingv.it 900 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa emerges from the eastern flank of the volcano (fig. 1b). panarea is made up of dacite to rhyolite lava domes, dated up to 149 ± 5 kyr (calanchi et al., 1999), interbedded with pyroclastic deposits. the island is topped by the pyroclastic deposits of punta del torrione formation, dated between 42 kyr and 11 kyr, which also outcrop at salina, lipari, filicudi, vulcano and along the northern coast of calabria and sicily (lucchi et al., 2003). boiling of the sea-water has been described since historical times in the aeolian islands by titus livius, strabo, julis obsequens, orosius palulus, posidonius and plinius the elder (sga, 1993). an event that produced sudden changes in the sea level, hot steam from sea surface, stinky air, death of fishes, mud emerging from seafloor occurred on 126 b.c. this event is likely referred to the formation of vulcanello, nowadays linked to vulcano island. the exhalative activity at panarea has been described by houel (1782), dolomieu (1783), ferrara (1810), spallanzani (1825), dumas (1860), mercalli (1883), luigi salvatore d’austria (1895), romano (1973), italiano and nuccio (1991), gabbianelli et al. (1990), calanchi et al. (1995), anzidei (2000), caliro et al., (2004), caracausi et al. (2005). at the beginning of november 2002, during the eruptions of etna and stromboli volcanoes, a submarine gas eruption started in a shallow area up to 30 m water depth and 2.3 km2 of surface, bordered by the islets. the event occurred fig. 1a-c. a) regional setting of panarea island; b) panarea archipelago; c) the alsea vessel used during bathymetric surveys. in the map are also displayed the locations of the tide gauge, the tyrgeonet gps station and the area investigated by the bathymetric surveys. cb a 901 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) suddenly, without significant seismicity (saccarotti et al., 2003) and reached an intensity level never observed before during the last century (sga, 1996). the gas output was estimated to be 109 l/day, two orders of magnitude higher than that previously measured (caliro et al., 2004). the most active exhalation center (ec-1, fig. 7b) was located close to the sw wall of the islet of bottaro, with gas flowing vigorously up to the water surface. to provide the present day locations of the submarine gas exhalation centres (ec) with respect to previous studies (gabbianelli et al., 1990; anzidei, 2000) and a first high resolution morphobathymetric map of the exhalation centres, bathymetric surveys were planned and performed during the maximum activity. surveys were carried out in the early days of december 2002, with partial repetitions in selected areas during july 2003 and december 2003. 2. bathymetric surveys a multibeam high resolution survey that covered an area of about 9 km2 around the islets was performed for the first time in december 812, 2002, using the alsea boat of coastal consulting and exploration company (fig. 1c), equipped with an ultra high resolution reson seabat 8125 multibeam (240 beams, 120° sector coverage, 455 khz) (table i). before starting the surveys, a check of the health of the gps/rtk data link was performed table i. instrumentation used during the bathymetric surveys. instrumentation type features vessel m/b alsea differential gps receiver ashtech-aquarius 02 10 mm + 0.5 ppm, xy (rtk mode) 20 mm + 1.0 ppm, z multi beam reson seabat 8125 frequency 455 khz angle 120° beams 240 max depth 120 m resolution 6 mm gyrocompass sg brown meridian 0.05° static secant latitude 0.2° dynamic secant latitude velocity sounder navitronic svp 15 resolution 0.1 m/s accuracy ± 0.25 m/s ctd microcat sbe 37-si conductivity accuracy 0.0003 s/m temperature accuracy 0.002°c motion sensor tss dms 2-05 waves static accuracy 5 cm dynamic accuracy 0 ÷ 20 s period pitch and roll static accuracy 0.025° dynamic accuracy 0.03°± 5° digital tide gauge ott hydrometry accuracy <1 cm (at panarea) orphimedes on board computers pentium ivdata acquisition windows 2000 software reson b.v. pds 2000 navigation and data analysis software caris-hips 5.2 data analysis software arcview gis 8.2 data management 902 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa throughout the planned survey, having previously found a local shadow area behind the eastern wall of dattilo. the bathymetric datum was established by measuring the sea level through a high accuracy tide gauge, temporarily installed along the pier at s. pietro, panarea (fig. 1a and 2). the instrumental height of the zero level was referred to a wgs84 geodetic benchmark previously set up over the pier (fig. 2 and table ii). the latter was measured by geodetic space techniques using a couple of dual frequency gps receivers and by ground measurements using a total station, with reference to the gps geodetic monument of pana, located at panarea (fig. 1a), which belongs to the tyrrhenian geodetic network (tyrgeonet), whose 3d coordinates are known at a few mm level (anzidei et al., 1995; serpelloni et al., 2005), (table ii). the decimetric positioning of the vessel was computed by gps technique in rtk mode. real time coordinates were obtained by installing an ashtech aquarius reference station located on the gps station pana and transmitfig. 2. sketch (not in scale) of the panarea pier with the location of the tide gauge and the geodetic benchmark used to define its coordinates with respect to the tyrgeonet gps station of panarea. table ii. coordinates of pana tyrgeonet station and tide gauge benchmark (wgs84, ellipsoidal heights). station geographic utm height (m) pana (tyrgeonet) lat n 38°37l58.909 n 4276057.468 144 554 long e 15°04l26.457 e 506442.1583 tide gauge lat n 38°38l16.772 n 4276608.335 43.30 long e 15°04l40.257 e 506775.313 903 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) ting the differential corrections by a high frequency link at 1 hz rate to the mobile aquarius receiver which was placed on board of the vessel. in addition to this, a sg-brown meridian surveyor gyrocompass, a tss dm505 mru and a fugro omnistar differential gps, were interfaced to the reson pds2000 navigation software (www.reson.com) for data acquisition (multibeam and positioning), control, calibration and pre-processing. a sbe 37-si microcat ctd probe was located at the sonar head and interfaced to the sonar processor, so providing the real time speed of sound data for the beam forming, while the navitronics svp15 and a seabird ctd probes were also used for profiling the speed of sound along the water column (fig. 3). additional details on the instrumentation are reported in table i. navigation routes were planned with the aim to achieve the full coverage of the sea bottom (fig. 4), with at least 20-30% overlap of the nearby swaths. the reson pds2000 software was able to show in real time to the operator and the pilot the ongoing multibeam and digital terrain model (dtm) and the positioning information that were used for guidance. fig. 4. course over the ground performed by alsea vessel during bathymetric surveys. the 30% overlapping between nearby lines guarantee the full coverage of the area. fig. 3. sound speed profile. data were used to calibrate the multibeam system for sound speed velocity in the water to determine depths at 1 cm formal accuracy. 904 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa at the beginning of each survey, a full set of multibeam calibration lines were acquired, on flat bottoms and steep targets at 30-70 m water depth. the roll, pitch and yaw correction angles were then used to correct the installation geometries. calibration parameters were taken into account during data analysis to correct the observations. 3. data analysis, bathymetric map and exhalation centres data analysis was performed by the computer aided resource information systemhydrographic information processing system (caris-hips) pro v. 5.2 software (caris, fig. 5a,b. a) example of tide gauge recordings collected at 5 min sampling rate at panarea pier in the time span december 19-21, 2003 during bathymetric surveys. tide amplitudes are < 0.4 m, typical of the central mediterranean sea. b) comparison between tide data recorded by the tide gauge located at panarea (grey line) and those registered by the gps/rtk system located on board of the vessel (black line) during bathymetric surveys. the gps/rtk tide data have been collected at 1 min sampling rate but averaged at 10 min to remove the high frequency noise. the two data sets are in good agreement. tidal values were used to correct depth measurements, that were reduced to a mean sea level. a b 905 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) 2000), specifically designed to process multibeam data under windows nt® and capable to manage images in a mosaic of the seafloor and produce raster and analytical maps. the data processing sequence was as follows: 1. system calibration and multibeam data reprocessing. 2. data quality check: only 15% of the data were discarded due to a low signal to noise ratio. 3. sea level correction using the tide gauge and rtk data (fig. 5a,b). 4. high and medium frequency spike removal, keeping intact the signatures produced by the uprising gas bubbles from the seafloor (fig. 6a-d). fig. 6a-d. a-c) multibeam images of some typical exhalation centres (vertical exaggeration 4x), showing uprising gas bubbles from the seafloor and the related small size craters produced by the exhalation activity; d) underwater picture of exhalation centres west of bottaro island. gas bubbles are reaching the sea surface (0a class). a b c d 906 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa 5. production of high accuracy marine digital elevation models (mdem) (table iii). through the repeated computations, the signal to noise ratio was greatly improved, allowing a precise discrimination of the significant signal and the subsequent identification and location of ec (table iv). a total of 606 ec were detected (appendix) and classified by intensity and significance level (fig. 7b). the latter criteria were expressed by the number of soundings that identified each ec from multiple overlapping swaths (0-1-2 levels of table iv) and from their height above the seafloor (a-b-c levels of table iv). the exhalation centres were further divided into three classes (table iv): a) high activity centres, with bubbles columns interesting the entire water column up to the sea surface (figs. 6d and 8). b) medium activity centres, with bubble columns extending for 3 to 4 m from the bottom but not reaching the sea surface (fig. 8). c) moderate activity centres, with bubble columns extending up to 2 m from the ground floor, not reaching the sea surface (fig. 6a). the classified ec were mapped over the mdem, allowing a view of the active exhalation centres for the whole area during the time span 2002-2003 (fig. 7a), and at december 2002, during the maximum intensity of the crisis (fig. 7b). bathymetric surveys were partially repeated during july and december 2003 (fig. 4), focusing on the most active area around lisca bianca, bottaro and ec-1 (fig. 7b). to produce the mdem at 0.5 m average pixel size, of 9 million of 3d punctual data in the utm33-wgs84 coordinate system were used. the mdem for each survey data set (december 2002, july and december 2003), was analysed to reduce any positioning offset or error. the difference grids display pixels whose values are the difference of depth at two survey epochs. the residuals between the three mdems were estimated, as reported in table iii. by the average standard deviations and excluding values > 1 m, supposed to be related to the active exhalation centres, the statistical difference between the surveys for all the investigated area was estimated within 14 cm. table iii. accuracy of the mdems. survey epoch average of the residuals average standard deviation (month/year) between grids (m) between grids (m) 12/03-12/02 + 0.02 0.14 07/03-12/02 + 0.10 0.13 12/03-07/03 −0.09 0.11 table iv. classification of the exhalation centres. detection level based on the number based on the height based on the quality of lines which detected extension of the gas column of the detection the exhalation centre high 0 (>3 lines) a (all water column, 0a (high) from seafloor to sea surface) 0b (medium) 0c (low) medium 1 (2÷3 lines) b (4 m height from the seafloor) 1a (high) 1b (medium) 1c (low) low 2 (1÷2 lines) c (2 m height from seafloor) 2a (high) 2b (medium) 2c (low) 907 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) fig. 7a,b. a) bathymetry of panarea archipelago at 2.0 m countour level. the red dots are the a and b classes of exhalation centres, respectively, detected in the time span december 2002-december 2003. blue lines are the cross sections aal and bbl; b) location of the main exhalation centres active during december 2002, based on their classification (appendix). in the map are shown the ec-1 (latitude 383818.8n; longitude 150634.0e, depth 13.3 m), ec-8 (latitude 383819.9n, longitude 150637.1e, depth 8.7 m), which were the largest exhalation centres and the ec named black smoke (latitude 383815.1n, longitude 150617.4e, depth 20.8 m). the latter is a 175 m wide and 5 m deep subcircular depression, interested by moderate dark exhalation, surrounded by lava outcrops that show deposition of alunite, sulfur and sulfates. a b 908 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa fig. 8. classification of the exhalation centres based on their intensity, as detected by the multibeam sensor. see also table iii. fig. 9a,b. cross sections of the investigated area: aal, bbl, running a) w-e and b) nw-se, respectively. it is remarkable the 20 m height vertical slope in cross section aal, that could be related with a tectonic structure running sw-ne (labelled with a in fig.10a). a b 909 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) once the offsets were analysed and applied, the final mdems were produced and made available for the morphostructural analysis, including the exhalation center (ec) detection, production of contour maps at 1:2500 scale (fig. 7a,b), cross sections (fig. 9a,b) and shaded relief (fig. 10a,b), which show the roughness and complexity of this sector of the panarea volcano surface. applying this analysis to a limited area including the ec-1, a crater about 40 m long, 15 m wide and 9 m deep (fig. 11), which was the largest activated on december 2002, the morphological differences during time were evidenced. figure 12 shows the results obtained by comparing the december 2002-december 2003 data: starting from a reference surface which fits the average surface of the seafloor around fig. 10a. marine digital elevation model (mdem) showing the morphobathymetry of the area. scale colour shows depths ranging between 0 and –60 m. exhalation centres (red dots) and main lineaments (red lines) revealed by the exhalation centres distribution are reported in the map. the exhalation area is located at the top of the stratovolcano. a and b, are two areas that displays relevant tectonic features; c, is likely to be a past exhalation centre. 910 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa fig. 10b. three dimensional view of the area (vertical exaggeration = 2). fig. 11. digital terrain model of the ec-1. the cross section profiles obtained by multibeam data displayed a crater about 40 m long, 15 m wide and 9 m deep. the spikes located in the deepest part of the crater are the uprising gas bubbles. 911 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) the ec-1, we tentatively estimated that the explosive opening of the crater produced an extraction at 3852 m3 of rock during the maximum of the crisis. one year after its opening, the crater was partially filled with approximately 221 m3 of debris due to rock falls of the inner flank of the crater the upwelling gas and water flows. 4. discussion and conclusions the high resolution multibeam technique provided the first 3d detailed bathymetric map of the seafloor at 0.5 m average pixel resolution and the location and distribution of the exhalation centres, during the crisis that has affected the panarea area since november 2002 (fig. 10a,b). these data are useful to improve and support the geological, volcanological, geochemical, geophysical research and monitoring of this still poorly known volcanic area. moreover, they represent the first accurate record of the last submarine exhalation crisis which occurred at panarea. the mdem (fig. 10a,b) shows that the area of panarea archipelago represents a positive geomorphic feature which is defined by the the coalescence of individual volcanic edifices. this is an asymmetric structure sloping at high angle in its southeastern flank while the others flanks display smoother slopes. the area within the islets is characterised by a shallow seafloor platform, bewteen 0 and –30 m, gently slooping to nw between panarelli and secca dei panarelli (fig. 10a,b). lisca bianca, bottaro and lisca nera islets are ne-trending coalescent structures. the dattilo structure shows an elongated tongue-like marine abrasion platform, se trending, for 0.8 km at depths between –5 and –8 m. the area within the islets is dotted by hundreds of circular or horseshoe-shaped depressions up to several metres deep and wide, largely distributed especially surrounding dattilo, lisca nera and bottaro. the high frequency relief of coarse textured morphology, at metre to decimetre size pinnacles and troughs, is well developed between lisca bianca, bottaro and secca dei panarelli. in the area between panarelli and secca dei panarelli the map reveals large lava flow surfaces, smooth and lobed. the deepest sector dislays evident morphological structures such as those located in the northwest area, nennennwand e-trending lineaments (fig. 10a: a,b). the southwest sector displays a sub-elliptical shape wide depression probably due gravitative movements (c in fig. 10a). the shaded relief (fig. 10a,b) does not show a caldera-like structure as suggested in gabianelli et al. (1990) and caliro et al. (2004). the high quality of the surveys disclosed 606 exhalation centres (apppendix), which were classified on the basis of their different size and activity level, as estimated from the interferences on the multibeam sensor. the location of the gas vents, mapped on the bathymetric maps, revealed the existence of preferential ec aligments ne-sw and nw-se trending (fig. 10a,b) and along which the gas eruption took place. these alignments match those measured from the fractures, the alunite veins and the fossil gas-pipes exposed on the lava of the seafloor and along the islets’ cliffs (anzidei et al., 2003, 2004a,b; esposito et al., 2005). therefore we suggest that the ne-sw and nwse systems are the main pathways for the upwelling of hydrothermal fluids. the comparison between the gas bubble alignments and those inferred by morphologycal analysis, suggests that presently the gas path is mainly driven by an active extension along the neand nw-trending fracture systems.the trends of the fracture systems are in agreement with the main regional tectonic structures (de astis et al., 2003). the maximum number of exhalation centres was located in a limited extended zone, west of bottaro and lisca bianca islets, whereas clusters of minor centres were sparse in the area. the largest and most active exhalation centre ec-1 was located a few tenths of metres south of bottaro (figs. 7b and 11). the ellipsoidal crater rim spans between –8 m and –15 m. its main axis, nw-oriented, is 40 m long and the minor axis is 25 m long. the hundreds of depression features identified on the seafloor (fig. 10a,b) can be related to fossil exhalation cen912 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa tres suggesting that the area has been the site of past episodes of gas eruptions similar to that of november 2002 (anzidei et al., 2003, 2004 a,b; esposito et al., 2005). the differential mdem obtained from multibeam data in the time span december 2002-december 2003, estimated 3852 m3 of extracted rock during the gas explosion; 221 m3 were recovered after 12 months, due to a continuous but decreasing gas flux from the seafloor (fig. 12). as far as the tectonic and volcanic interpretations are not the object of this paper, which aims to shows bathymetric survey data and results applied to the submarine part of panarea volcano, we think that a change in the regional and local strain fields in this area may have produced a reactivation of previous fractures or the opening of new ones, causing a subsequent gas exhalation during the 2002-2003 panarea crisis, as more extensively described in caliro et al. (2004), caracausi et al. (2005) and esposito et al. (2005). fig. 12. topographic changes of the groundfloor at the ec-1, after one year from its opening. the differential mdem obtained from multibeam data in the time span december 2002-december 2003, estimated 3852 m3 of extracted rock during the gas explosion; 221 m3 were recovered after 12 months, due to a continuous but decreasing gas flux from the seafloor. positive values are negative groundfloor changes (blue) due to slides inside the crater; negative values (red) are positive groundfloor changes, due to the progressive filling of the bottom of the crater. 913 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) appendix. coordinates of the 606 exhalation centres (utm 33 coordinates). their depth reported to the mean sea level during surveys, classification and epoch of detection, are also reported. ec 0a 12/02 no. long lat depth class date 1 150634.05 383823.87 13.6 0a 12/02 2 150634.41 383814.31 13.2 0a 12/02 3 150634.41 383814.35 13.1 0a 12/02 4 150631.68 383815.80 11.8 0a 12/02 5 150629.03 383818.20 12.4 0a 12/02 6 150629.03 383818.17 12.5 0a 12/02 7 150628.99 383818.04 12.3 0a 12/02 8 150634.32 383814.30 13 0a 12/02 9 150634.12 383814.56 11.3 0a 12/02 10 150634.16 383814.52 11.2 0a 12/02 11 150634.09 383814.51 11.3 0a 12/02 12 150634.20 383814.48 11.3 0a 12/02 13 150634.20 383814.43 11.3 0a 12/02 14 150633.83 383814.66 11.4 0a 12/02 15 150632.67 383815.37 11.2 0a 12/02 16 150632.69 383815.44 11.5 0a 12/02 17 150634.01 383823.68 13.7 0a 12/02 18 150634.05 383823.84 12.7 0a 12/02 19 150634.88 383823.74 16 0a 12/02 20 150634.09 383823.94 13 0a 12/02 21 150634.13 383824.00 12.6 0a 12/02 22 150635.17 383824.03 16.2 0a 12/02 23 150634.26 383824.16 11.9 0a 12/02 24 150634.88 383824.39 15.1 0a 12/02 25 150634.63 383824.49 14.7 0a 12/02 26 150635.70 383825.14 17.5 0a 12/02 27 150635.71 383825.20 17.5 0a 12/02 28 150638.02 383826.82 19.2 0a 12/02 29 150638.07 383826.85 19.2 0a 12/02 30 150634.09 383821.99 10.8 0a 12/02 31 150634.21 383822.06 10.7 0a 12/02 32 150634.25 383822.12 10.9 0a 12/02 33 150634.96 383823.58 15.7 0a 12/02 34 150635.21 383823.55 15.2 0a 12/02 35 150635.37 383823.51 15.2 0a 12/02 36 150634.09 383823.00 12.6 0a 12/02 37 150637.76 383819.49 6.7 0a 12/02 38 150637.97 383819.59 6.6 0a 12/02 39 150637.10 383819.65 8 0a 12/02 40 150637.27 383819.65 8.1 0a 12/02 41 150637.77 383819.72 8 0a 12/02 42 150626.09 383813.30 12.5 0a 12/02 43 150617.69 383814.06 22.5 0a 12/02 ec 0b 12/02 no. long lat depth class date 44 150631.80 383815.70 11.9 0b 12/02 45 150631.84 383815.67 11.7 0b 12/02 46 150631.80 383815.70 11.7 0b 12/02 47 150631.80 383815.76 11.8 0b 12/02 48 150631.80 383815.76 11.9 0b 12/02 49 150631.89 383815.76 12.2 0b 12/02 50 150631.89 383815.83 12.2 0b 12/02 51 150631.89 383815.83 12 0b 12/02 52 150631.72 383815.83 12.1 0b 12/02 53 150631.55 383816.02 12.2 0b 12/02 54 150631.72 383815.86 12.1 0b 12/02 55 150633.93 383814.60 11.3 0b 12/02 56 150633.77 383814.73 11.3 0b 12/02 57 150623.37 383818.43 21.5 0b 12/02 58 150628.63 383821.44 13.2 0b 12/02 59 150634.54 383823.78 17 0b 12/02 60 150635.04 383823.94 16.5 0b 12/02 61 150634.09 383822.02 10.8 0b 12/02 62 150634.54 383822.22 10.9 0b 12/02 63 150636.20 383822.28 10.3 0b 12/02 64 150634.67 383823.58 16.1 0b 12/02 65 150634.59 383823.52 15.8 0b 12/02 66 150634.17 383822.96 12.5 0b 12/02 67 150637.97 383819.55 6.3 0b 12/02 68 150638.01 383819.62 6.7 0b 12/02 69 150637.64 383819.62 7.7 0b 12/02 70 150637.56 383819.75 8.2 0b 12/02 71 150619.28 383821.52 25.4 0b 12/02 72 150625.47 383813.40 12.4 0b 12/02 73 150617.15 3838,08.87 13.4 0b 12/02 74 150616.49 383809.35 14 0b 12/02 75 150616.57 383809.55 13.9 0b 12/02 76 150618.92 383806.66 12.5 0b 12/02 914 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa ec 0c 12/02 no. long lat depth class date 77 150632.71 383815.54 11.6 0c 12/02 78 150634.88 383823.81 16.4 0c 12/02 79 150637.23 383819.75 8 0c 12/02 ec 1a 12/02 no. long lat depth class date 80 150639.03 383806.35 28.8 1a 12/02 81 150641.02 383808.49 27.5 1a 12/02 82 150639.03 383806.35 29 1a 12/02 83 150641.18 383808.39 27.6 1a 12/02 84 150641.22 383808.39 27.7 1a 12/02 85 150639.32 383810.27 15 1a 12/02 86 150636.93 383812.19 8 1a 12/02 87 150636.88 383812.03 10.2 1a 12/02 88 150636.88 383811.93 10.7 1a 12/02 89 150636.97 383811.83 11.2 1a 12/02 90 150636.97 383811.83 11.3 1a 12/02 91 150636.97 383811.74 11.6 1a 12/02 92 150637.71 383811.54 11.2 1a 12/02 93 150636.88 383811.64 11.6 1a 12/02 94 150636.80 383811.57 11.7 1a 12/02 95 150636.93 383811.57 11.6 1a 12/02 96 150637.75 383811.38 11.5 1a 12/02 97 150639.08 383811.38 10.4 1a 12/02 98 150637.63 383812.06 10.8 1a 12/02 99 150637.30 383812.06 9.7 1a 12/02 100 150636.21 383803.14 6.9 1a 12/02 101 150636.09 383803.17 6.4 1a 12/02 102 150636.17 383803.20 5.9 1a 12/02 103 150636.21 383803.14 5.4 1a 12/02 104 150636.13 383803.01 6 1a 12/02 105 150636.08 383803.01 6.4 1a 12/02 106 150636.08 383802.85 6.4 1a 12/02 107 150636.13 383802.82 5.9 1a 12/02 108 150635.92 383803.04 6.4 1a 12/02 109 150631.68 383815.67 11.7 1a 12/02 110 150631.64 383815.80 11.8 1a 12/02 111 150628.99 383818.01 12.3 1a 12/02 112 150629.12 383817.97 11.9 1a 12/02 113 150629.08 383817.97 11.8 1a 12/02 114 150634.36 383814.14 9.1 1a 12/02 115 150633.70 383814.78 11.3 1a 12/02 116 150633.70 383814.87 11.3 1a 12/02 117 150633.71 383814.94 11.3 1a 12/02 118 150623.41 383818.14 20.9 1a 12/02 119 150626.73 383823.20 18.9 1a 12/02 120 150627.47 383822.00 16.7 1a 12/02 121 150626.02 383821.93 17 1a 12/02 122 150626.35 383819.63 17.2 1a 12/02 123 150633.55 383822.87 12.2 1a 12/02 124 150633.55 383822.87 12.2 1a 12/02 125 150634.01 383823.06 12.5 1a 12/02 126 150633.76 383823.29 12.9 1a 12/02 127 150628.21 383822.48 15.8 1a 12/02 128 150628.50 383821.51 13.5 1a 12/02 129 150628.38 383821.45 13.7 1a 12/02 130 150629.20 383821.05 13.4 1a 12/02 131 150628.87 383820.37 13.9 1a 12/02 132 150624.53 383823.72 21.1 1a 12/02 133 150636.20 383823.77 13.2 1a 12/02 134 150636.16 383823.77 13.8 1a 12/02 135 150636.24 383823.87 13.6 1a 12/02 136 150635.83 383824.00 15 1a 12/02 137 150636.28 383824.03 14.2 1a 12/02 138 150635.46 383824.13 15.9 1a 12/02 139 150634.96 383824.20 15.7 1a 12/02 140 150634.38 383824.23 13 1a 12/02 141 150635.95 383824.33 14.5 1a 12/02 142 150634.88 383824.33 16.6 1a 12/02 143 150636.16 383824.33 15 1a 12/02 144 150634.59 383824.42 15 1a 12/02 145 150634.96 383824.49 16.2 1a 12/02 146 150635.46 383824.49 15.5 1a 12/02 147 150634.63 383824.52 15.2 1a 12/02 148 150634.84 383824.55 15.3 1a 12/02 149 150635.71 383825.33 17.5 1a 12/02 150 150636.78 383825.40 15.6 1a 12/02 151 150636.74 383825.53 16.5 1a 12/02 152 150635.83 383825.98 15.9 1a 12/02 153 150636.08 383826.08 16.1 1a 12/02 154 150635.95 383826.14 16 1a 12/02 155 150638.19 383826.50 17.8 1a 12/02 156 150638.11 383826.59 18 1a 12/02 157 150637.90 383826.72 18.4 1a 12/02 158 150637.82 383827.21 19.9 1a 12/02 159 150638.31 383827.60 21.4 1a 12/02 160 150638.19 383827.57 21.2 1a 12/02 161 150634.17 383821.73 10.7 1a 12/02 162 150634.21 383821.83 10.7 1a 12/02 163 150634.21 383821.89 10.8 1a 12/02 164 150634.21 383821.99 10.7 1a 12/02 915 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) 165 150634.25 383822.19 11.1 1a 12/02 166 150634.34 383822.22 11.3 1a 12/02 167 150634.38 383822.35 11.4 1a 12/02 168 150634.30 383822.90 12.6 1a 12/02 169 150634.42 383822.83 12.7 1a 12/02 170 150634.50 383822.80 12.6 1a 12/02 171 150634.67 383822.80 12.5 1a 12/02 172 150635.08 383823.19 14.3 1a 12/02 173 150637.39 383819.36 7.1 1a 12/02 174 150637.56 383819.36 7 1a 12/02 175 150637.89 383819.36 4.4 1a 12/02 176 150637.68 383819.39 7.5 1a 12/02 177 150637.43 383819.81 8 1a 12/02 178 150637.81 383820.27 7.9 1a 12/02 179 150638.06 383820.27 7.1 1a 12/02 180 150638.23 383827.50 21.1 1a 12/02 181 150641.96 383830.91 31.7 1a 12/02 182 150641.92 383830.91 31.7 1a 12/02 183 150638.43 383831.80 31 1a 12/02 184 150641.75 383831.26 30 1a 12/02 185 150621.14 383825.02 26.3 1a 12/02 186 150621.70 383811.33 13.7 1a 12/02 187 150613.64 383809.91 14.8 1a 12/02 188 150612.51 383804.07 19.1 1a 12/02 189 150626.61 383803.73 8.1 1a 12/02 190 150620.38 383809.45 12.8 1a 12/02 ec 1b 12/02 no. long lat depth class date 191 150637.55 383812.00 11 1b 12/02 192 150637.59 383811.90 11.2 1b 12/02 193 150637.13 383811.90 11.3 1b 12/02 194 150636.84 383811.83 11.3 1b 12/02 195 150636.55 383811.64 11.8 1b 12/02 196 150636.76 383811.57 11.2 1b 12/02 197 150638.62 383810.63 12.7 1b 12/02 198 150635.71 383801.03 19.7 1b 12/02 199 150635.67 383801.26 18.4 1b 12/02 200 150636.00 383802.85 6.5 1b 12/02 201 150635.96 383803.24 6.5 1b 12/02 202 150633.70 383814.89 11.4 1b 12/02 203 150632.63 383815.24 11.1 1b 12/02 204 150632.88 383818.03 11.4 1b 12/02 205 150633.09 383817.61 8.1 1b 12/02 206 150624.24 383818.14 17.6 1b 12/02 207 150623.08 383817.82 16.3 1b 12/02 208 150627.55 383822.39 17.8 1b 12/02 209 150627.26 383822.32 17.6 1b 12/02 210 150627.01 383822.22 17.6 1b 12/02 211 150627.18 383822.13 17.6 1b 12/02 212 150625.86 383821.80 17.1 1b 12/02 213 150626.10 383820.57 16.1 1b 12/02 214 150626.31 383819.66 17.6 1b 12/02 215 150628.75 383820.44 14.1 1b 12/02 216 150628.83 383820.37 13.7 1b 12/02 217 150627.88 383819.24 14.3 1b 12/02 218 150628.08 383819.30 14.4 1b 12/02 219 150632.31 383824.10 14.3 1b 12/02 220 150632.15 383824.10 14.3 1b 12/02 221 150636.41 383823.71 12.7 1b 12/02 222 150635.54 383823.74 16.3 1b 12/02 223 150635.17 383824.94 16.1 1b 12/02 224 150635.75 383824.88 16.4 1b 12/02 225 150637.98 383827.31 20 1b 12/02 226 150638.19 383827.50 20.7 1b 12/02 227 150638.56 383827.70 21.4 1b 12/02 228 150638.44 383827.70 21.7 1b 12/02 229 150634.71 383822.57 12.2 1b 12/02 230 150635.87 383822.61 10.3 1b 12/02 231 150636.03 383823.55 14.6 1b 12/02 232 150635.50 383823.22 14.2 1b 12/02 233 150637.48 383819.98 8.5 1b 12/02 234 150638.40 383827.05 20 1b 12/02 235 150638.60 383827.73 21.9 1b 12/02 236 150649.07 383826.32 15.2 1b 12/02 237 150649.07 383826.45 15.3 1b 12/02 238 150649.40 383826.45 15.4 1b 12/02 239 150649.28 383826.49 15.2 1b 12/02 240 150625.72 383813.30 12.2 1b 12/02 241 150614.38 383809.49 14.7 1b 12/02 242 150617.36 383808.87 13.3 1b 12/02 ec 1c 12/02 no. long lat depth class date 243 150644.70 383809.85 26.8 1c 12/02 244 150639.36 383810.21 15.2 1c 12/02 245 150642.80 383812.96 12.8 1c 12/02 246 150639.36 383810.27 14.8 1c 12/02 247 150636.80 383812.32 7.4 1c 12/02 248 150636.68 383812.29 7.5 1c 12/02 249 150637.01 383812.22 7.7 1c 12/02 250 150636.76 383812.19 8.1 1c 12/02 251 150636.76 383812.29 7.3 1c 12/02 252 150636.76 383812.13 8 1c 12/02 916 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa 253 150636.76 383812.00 9.3 1c 12/02 254 150637.21 383811.74 11.5 1c 12/02 255 150636.55 383811.54 11.8 1c 12/02 256 150639.16 383811.18 10.9 1c 12/02 257 150638.66 383811.38 10.5 1c 12/02 258 150626.23 383821.74 16.1 1c 12/02 259 150626.43 383819.47 17 1c 12/02 260 150632.73 383824.85 15 1c 12/02 261 150632.23 383824.26 14.7 1c 12/02 262 150634.05 383823.84 13 1c 12/02 263 150637.86 383827.24 20.2 1c 12/02 264 150614.83 383810.13 15.5 1c 12/02 ec 2a 12/02 no. long lat depth class date 265 150636.67 383806.64 19.2 2a 12/02 266 150639.16 383811.48 9.8 2a 12/02 267 150635.91 383821.02 8 2a 12/02 268 150635.53 383821.21 8 2a 12/02 269 150638.65 383827.24 21 2a 12/02 270 150638.36 383827.24 21 2a 12/02 271 150619.20 383825.05 26.2 2a 12/02 272 150619.61 383825.22 26.1 2a 12/02 273 150621.37 383808.64 12 2a 12/02 274 150621.33 383808.64 12 2a 12/02 275 150612.51 383803.03 19.8 2a 12/02 276 150612.63 383803.13 19.8 2a 12/02 277 150626.12 383802.86 10.6 2a 12/02 ec 2b 12/02 no. long lat depth class date 278 150638.74 383810.57 13 2b 12/02 279 150638.56 383827.47 21.8 2b 12/02 280 150620.44 383824.92 26.2 2b 12/02 281 150617.60 383809.09 13.6 2b 12/02 282 150617.36 383809.09 13.6 2b 12/02 283 150612.80 383803.32 18.7 2b 12/02 284 150612.76 383803.26 18.9 2b 12/02 285 150620.50 383809.51 12.5 2b 12/02 286 150606.81 383811.70 18.2 2b 12/02 ec 0a 07/03 no. long lat depth class date 287 150632.63 383815.41 11.2 0a 07/03 288 150634.39 383814.30 13.1 0a 07/03 ec 0b 07/03 no. long lat depth class date 289 150641.06 383808.59 27.4 0b 07/03 290 150641.06 383808.46 27.4 0b 07/03 291 150641.10 383808.39 27.5 0b 07/03 292 150631.68 383815.83 11.7 0b 07/03 293 150631.80 383815.80 11.8 0b 07/03 294 150631.93 383815.83 11.9 0b 07/03 295 150631.68 383815.76 11.8 0b 07/03 296 150631.72 383815.70 11.7 0b 07/03 297 150631.84 383815.70 11.6 0b 07/03 298 150631.84 383815.67 11.5 0b 07/03 299 150632.71 383815.54 11 0b 07/03 300 150632.71 383815.50 11 0b 07/03 301 150632.55 383815.47 11.5 0b 07/03 302 150632.67 383815.44 11 0b 07/03 303 150632.67 383815.34 11.1 0b 07/03 ec 0c 07/03 no. long lat depth class date 304 150641.18 383808.39 27.4 0c 07/03 ec 1a 07/03 no. long lat depth class date 305 150639.03 383806.15 30.9 1a 07/03 306 150638.78 383805.96 28.1 1a 07/03 307 150638.90 383805.99 29.5 1a 07/03 308 150638.82 383805.90 29.2 1a 07/03 309 150638.82 383805.83 29.1 1a 07/03 310 150636.08 383802.88 5.9 1a 07/03 311 150641.10 383808.78 27.2 1a 07/03 312 150636.97 383811.80 11.5 1a 07/03 313 150636.84 383811.80 11.5 1a 07/03 314 150636.76 383811.77 11.5 1a 07/03 315 150636.84 383811.80 11.4 1a 07/03 316 150636.84 383811.87 11.4 1a 07/03 317 150637.01 383811.77 11.6 1a 07/03 318 150636.80 383811.77 11.6 1a 07/03 319 150641.77 383814.07 8.6 1a 07/03 320 150641.81 383814.10 9 1a 07/03 321 150631.68 383815.67 11.6 1a 07/03 322 150632.67 383815.31 11 1a 07/03 323 150627.92 383822.94 16.6 1a 07/03 324 150626.77 383823.17 18.5 1a 07/03 325 150626.10 383820.51 16.3 1a 07/03 326 150626.10 383820.60 16.1 1a 07/03 327 150626.10 383820.64 16.4 1a 07/03 328 150623.78 383817.72 17.1 1a 07/03 329 150623.57 383817.78 16.3 1a 07/03 330 150624.77 383818.01 17.3 1a 07/03 917 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) 331 150624.82 383818.01 18 1a 07/03 332 150624.28 383818.14 17.4 1a 07/03 333 150623.45 383818.40 21.1 1a 07/03 334 150624.58 383823.72 21.3 1a 07/03 335 150624.04 383823.72 21.8 1a 07/03 336 150624.37 383823.88 22.5 1a 07/03 337 150634.63 383823.58 16.2 1a 07/03 338 150634.63 383823.55 16 1a 07/03 339 150634.21 383823.45 14.6 1a 07/03 340 150633.80 383823.32 13.2 1a 07/03 341 150633.80 383823.32 13 1a 07/03 342 150634.25 383823.16 13.7 1a 07/03 343 150634.21 383823.00 13.1 1a 07/03 344 150634.38 383822.93 12.6 1a 07/03 345 150634.38 383822.93 12.7 1a 07/03 346 150634.83 383822.87 12.5 1a 07/03 347 150634.87 383822.83 12.4 1a 07/03 348 150634.58 383822.80 12.4 1a 07/03 349 150634.67 383822.80 12.5 1a 07/03 350 150634.58 383822.80 12.4 1a 07/03 351 150634.25 383822.12 11.1 1a 07/03 352 150634.54 383823.68 16 1a 07/03 353 150634.88 383823.71 16.1 1a 07/03 354 150634.92 383823.77 16.2 1a 07/03 355 150634.09 383823.87 13.7 1a 07/03 356 150634.09 383823.87 13.6 1a 07/03 357 150634.88 383823.90 16.7 1a 07/03 358 150634.88 383823.94 16.8 1a 07/03 359 150634.79 383823.97 17.2 1a 07/03 360 150634.13 383823.97 12.9 1a 07/03 361 150634.63 383824.42 15.4 1a 07/03 362 150634.59 383824.42 15.4 1a 07/03 363 150636.45 383823.74 12.9 1a 07/03 364 150636.32 383823.87 13.6 1a 07/03 365 150636.32 383823.90 13.6 1a 07/03 366 150636.32 383823.90 14.4 1a 07/03 367 150636.20 383824.00 12.5 1a 07/03 368 150636.32 383824.07 14.3 1a 07/03 369 150636.32 383824.13 14.8 1a 07/03 370 150636.65 383824.16 12.7 1a 07/03 371 150636.32 383824.13 14.6 1a 07/03 372 150636.28 383824.20 15 1a 07/03 373 150636.57 383824.32 13.3 1a 07/03 374 150636.20 383824.33 14.9 1a 07/03 375 150636.57 383824.42 13.6 1a 07/03 376 150636.20 383824.33 15.1 1a 07/03 377 150636.57 383824.49 13.7 1a 07/03 378 150636.66 383824.65 13.8 1a 07/03 379 150635.58 383825.20 17.4 1a 07/03 380 150635.33 383825.49 16.6 1a 07/03 381 150638.23 383826.56 17.8 1a 07/03 382 150638.07 383826.85 19.2 1a 07/03 383 150635.41 383823.58 15 1a 07/03 384 150635.25 383823.55 15.1 1a 07/03 385 150634.96 383823.58 15.5 1a 07/03 386 150635.21 383823.48 15.1 1a 07/03 387 150638.11 383826.89 19.1 1a 07/03 388 150637.90 383827.24 20 1a 07/03 398 150617.37 383821.58 25.2 1a 07/03 399 150616.34 383821.39 25 1a 07/03 400 150615.43 383820.87 24 1a 07/03 401 150636.58 383824.50 13.7 1a 07/03 402 150634.93 383833.83 33.5 1a 07/03 403 150634.85 383833.90 33.6 1a 07/03 404 150620.75 383809.51 12.5 1a 07/03 405 150620.71 383809.51 12.5 1a 07/03 ec 1b 07/03 no. long lat depth class date 406 150638.99 383806.25 29.6 1b 07/03 407 150638.98 383806.09 30.4 1b 07/03 408 150638.78 383806.06 26.8 1b 07/03 409 150638.86 383806.02 29.2 1b 07/03 410 150638.82 383805.93 28 1b 07/03 411 150638.82 383805.93 29.2 1b 07/03 412 150638.90 383805.80 29 1b 07/03 413 150636.13 383802.85 6.2 1b 07/03 414 150636.00 383802.82 6.6 1b 07/03 415 150636.04 383802.36 6.2 1b 07/03 416 150635.92 383802.43 6.3 1b 07/03 417 150641.14 383808.59 27.2 1b 07/03 418 150636.84 383811.87 11.3 1b 07/03 419 150642.10 383814.46 10.5 1b 07/03 420 150642.31 383814.07 11.3 1b 07/03 421 150641.81 383814.13 8.9 1b 07/03 422 150631.72 383815.83 11.6 1b 07/03 423 150631.93 383815.83 11.9 1b 07/03 424 150632.17 383815.50 11.6 1b 07/03 425 150632.67 383815.28 11 1b 07/03 426 150627.34 383822.26 17.6 1b 07/03 427 150627.59 383822.39 17.5 1b 07/03 428 150627.43 383822.52 15.7 1b 07/03 429 150626.10 383820.44 16.6 1b 07/03 430 150626.56 383821.32 16.6 1b 07/03 918 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa 431 150626.27 383821.77 16.4 1b 07/03 432 150623.08 383817.85 16.3 1b 07/03 433 150623.37 383818.17 17.1 1b 07/03 434 150623.45 383818.17 17.5 1b 07/03 435 150623.41 383818.40 21 1b 07/03 436 150624.58 383823.72 21.2 1b 07/03 437 150634.63 383823.45 15.5 1b 07/03 438 150634.09 383823.42 14 1b 07/03 439 150634.25 383823.35 14 1b 07/03 440 150634.25 383823.29 13.6 1b 07/03 441 150634.38 383823.06 13.6 1b 07/03 442 150634.92 383823.81 16.2 1b 07/03 443 150634.88 383823.87 16.2 1b 07/03 444 150634.59 383823.81 17.2 1b 07/03 445 150634.92 383823.87 16.2 1b 07/03 446 150634.88 383823.90 16.5 1b 07/03 447 150636.32 383823.77 13.1 1b 07/03 448 150636.61 383823.94 12.2 1b 07/03 449 150636.57 383824.23 12.2 1b 07/03 450 150635.99 383824.33 14.5 1b 07/03 451 150635.50 383823.29 14.2 1b 07/03 452 150638.44 383827.08 20.3 1b 07/03 453 150637.08 383832.66 31.6 1b 07/03 454 150637.08 383832.63 31.7 1b 07/03 455 150617.78 383814.06 22.6 1b 07/03 456 150618.19 383814.32 23 1b 07/03 457 150617.03 383814.22 20.1 1b 07/03 458 150636.61 383823.90 18.8 1b 07/03 459 150620.91 383809.45 12.4 1b 07/03 460 150620.83 383809.61 12.4 1b 07/03 461 150614.83 383810.13 15.2 1b 07/03 c 1c 07/03 no. long lat depth class date 462 150638.74 383806.06 26.9 1c 07/03 463 150638.94 383805.77 29 1c 07/03 464 150636.00 383802.82 6.6 1c 07/03 465 150636.00 383802.78 6 1c 07/03 466 150641.18 383808.36 27.4 1c 07/03 467 150641.18 383808.85 27.4 1c 07/03 468 150642.18 383814.23 11 1c 07/03 469 150642.31 383814.36 11.2 1c 07/03 470 150642.89 383814.36 11.2 1c 07/03 471 150631.72 383815.76 11.7 1c 07/03 472 150632.09 383815.57 11.9 1c 07/03 473 150632.26 383815.41 11.2 1c 07/03 474 150627.34 383822.29 17.5 1c 07/03 475 150627.14 383822.39 18.2 1c 07/03 476 150628.71 383822.42 14.5 1c 07/03 477 150627.59 383822.42 17.4 1c 07/03 478 150628.71 383822.42 14.4 1c 07/03 479 150627.97 383822.52 16.5 1c 07/03 480 150627.47 383822.65 16.7 1c 07/03 481 150626.06 383820.86 17.2 1c 07/03 482 150626.02 383821.97 17.2 1c 07/03 483 150623.08 383817.98 16.5 1c 07/03 484 150624.11 383818.14 17.6 1c 07/03 485 150624.08 383823.72 22.2 1c 07/03 486 150624.12 383823.88 21.8 1c 07/03 487 150634.88 383823.58 15.7 1c 07/03 488 150634.09 383823.35 13.8 1c 07/03 489 150632.77 383823.26 12.6 1c 07/03 490 150634.83 383823.13 13.8 1c 07/03 491 150634.75 383823.09 13.5 1c 07/03 492 150634.75 383823.06 13 1c 07/03 493 150634.21 383823.00 13 1c 07/03 494 150634.21 383821.83 10.9 1c 07/03 495 150634.63 383823.74 16.7 1c 07/03 496 150632.52 383823.84 13.9 1c 07/03 497 150632.68 383824.00 14.3 1c 07/03 498 150635.58 383823.68 14.9 1c 07/03 499 150636.20 383823.94 14.2 1c 07/03 500 150635.83 383824.00 15.3 1c 07/03 501 150635.83 383824.00 15.3 1c 07/03 502 150635.79 383824.07 15 1c 07/03 503 150635.70 383824.13 15.1 1c 07/03 504 150636.03 383824.26 13.8 1c 07/03 505 150635.95 383824.46 14.7 1c 07/03 506 150635.71 383825.33 17.9 1c 07/03 507 150635.42 383825.36 16.9 1c 07/03 508 150635.62 383825.49 17.9 1c 07/03 509 150634.96 383823.45 15.2 1c 07/03 510 150638.44 383827.11 20.2 1c 07/03 511 150638.56 383827.28 21 1c 07/03 512 150638.36 383827.24 20.5 1c 07/03 513 150638.11 383827.02 19.2 1c 07/03 514 150639.69 383832.40 32.7 1c 07/03 515 150616.91 383814.32 20.1 1c 07/03 516 150621.00 383809.42 12.3 1c 07/03 c 2c 07/03 no. long lat depth class date 517 150638.78 383805.73 29 2b 07/03 518 150636.08 383802.72 5.9 2b 07/03 919 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) 519 150636.04 383800.41 24.6 2b 07/03 520 150636.41 383802.33 7 2b 07/03 521 150619.61 383825.22 27 2b 07/03 522 150620.81 383823.98 25.9 2b 07/03 523 150636.04 383800.41 24.6 2b 07/03 524 150636.41 383802.33 7 2b 07/03 525 150619.61 383825.22 27 2b 07/03 526 150620.81 383823.98 25.9 2b 07/03 ec 1a 12/03 no. long lat depth class date 528 150634.58 383822.83 12.4 1a 12/03 529 150634.67 383822.80 12.4 1a 12/03 530 150634.79 383822.87 12.5 1a 12/03 531 150634.34 383822.90 12.2 1a 12/03 532 150634.83 383822.87 12.1 1a 12/03 533 150634.38 383823.06 13.1 1a 12/03 534 150633.80 383823.32 13 1a 12/03 535 150634.63 383823.52 15.7 1a 12/03 536 150634.13 383824.00 13.1 1a 12/03 537 150636.66 383824.68 14 1a 12/03 538 150636.53 383824.45 14.1 1a 12/03 539 150636.49 383824.42 13.6 1a 12/03 540 150636.53 383824.36 13.6 1a 12/03 541 150636.16 383824.33 15.1 1a 12/03 542 150636.53 383824.29 13.3 1a 12/03 543 150636.16 383824.33 15.1 1a 12/03 544 150636.24 383824.07 14.2 1a 12/03 545 150636.28 383823.90 14.1 1a 12/03 546 150636.24 383823.94 14.1 1a 12/03 547 150634.88 383823.81 16.2 1a 12/03 548 150634.88 383823.78 16 1a 12/03 549 150634.96 383823.61 15.6 1a 12/03 550 150635.29 383823.55 14.9 1a 12/03 551 150635.24 383823.54 14.8 1a 12/03 552 150635.26 383823.52 14.7 1a 12/03 553 150634.83 383822.87 12.7 1a 12/03 554 150635.16 383822.90 12.2 1a 12/03 555 150638.23 383826.53 17.7 1a 12/03 556 150637.94 383826.63 18.6 1a 12/03 557 150640.80 383826.66 19.3 1a 12/03 558 150639.64 383826.72 19.6 1a 12/03 1b 12/03 no. long lat depth class date 559 150634.75 383822.74 12.6 1b 12/03 560 150634.17 383823.00 12.7 1b 12/03 561 150634.09 383823.35 13.8 1b 12/03 562 150634.09 383823.45 13.9 1b 12/03 563 150634.59 383823.81 16.8 1b 12/03 564 150634.09 383824.00 11.7 1b 12/03 565 150634.88 383825.27 14.9 1b 12/03 566 150636.70 383824.71 14 1b 12/03 567 150636.70 383824.68 13.9 1b 12/03 568 150636.57 383824.49 13.7 1b 12/03 569 150636.28 383824.10 14.4 1b 12/03 570 150635.04 383824.00 16.5 1b 12/03 571 150636.16 383823.94 14.4 1b 12/03 572 150636.16 383823.94 14.4 1b 12/03 573 150635.04 383823.16 13.5 1b 12/03 574 150634.96 383822.83 12.8 1b 12/03 575 150638.23 383826.53 17.9 1b 12/03 576 150638.07 383826.85 19.2 1b 12/03 577 150639.80 383827.66 22.8 1b 12/03 1c 12/03 no. long lat depth class date 578 150634.71 383823.09 12.7 1c 12/03 579 150635.58 383825.27 17.3 1c 12/03 580 150635.99 383824.33 15.1 1c 12/03 581 150635.99 383824.33 14.8 1c 12/03 582 150636.61 383824.16 12.9 1c 12/03 583 150635.83 383824.00 15 1c 12/03 584 150636.28 383823.81 13.7 1c 12/03 585 150639.64 383826.76 19.4 1c 12/03 586 150638.07 383826.85 19.2 1c 12/03 587 150638.11 383826.95 19.3 1c 12/03 588 150638.11 383827.02 19.4 1c 12/03 589 150638.48 383827.11 20 1c 12/03 590 150637.98 383827.24 20.1 1c 12/03 591 150638.44 383827.28 20.5 1c 12/03 592 150640.01 383827.53 21.9 1c 12/03 593 150634.92 383825.27 14.5 1c 12/03 a 12/03 no. long lat depth class date 594 150634.25 383823.29 13.3 2a 12/03 595 150634.09 383823.87 13.6 2a 12/03 596 150634.63 383824.46 15.2 2a 12/03 597 150635.22 383823.81 15.4 2a 12/03 598 150635.29 383823.85 15.5 2a 12/03 599 150635.37 383823.71 15.2 2a 12/03 600 150635.08 383823.71 15.6 2a 12/03 601 150636.98 383821.53 8.8 2a 12/03 920 marco anzidei, alessandra esposito, giovanni bortoluzzi and francesco de giosa references anzidei, m. (2000): rapid bathymetric surveys in marine volcano areas: a case study in panarea aera, phys. chem. earth a, 25 (1), 77-80. anzidei, m., p. baldi, g. casula, f. riguzzi and l. surace (1995): la rete tyrgeonet, boll. geod. sci. affini, liv (2), pp. 120. anzidei, m., a. esposito, g. giordano and a. benini (2003): diving in the panarea volcanic complex (aeolian islands, italy), in cities on volcanoes 3, july 1418, 2003, hilo, hawaii. anzidei, m., a. esposito, a. pesci and e. serpelloni (2004a): the 2002-2003 crisis of the panarea volcanic complex (aeolian islands, italy): new volcanologgical, geodetic and bathymetric data, in gnv general assembly, 20-22 december, napoli, italy (abstract). anzidei, m., m.l. carapezza, r. cioni, a. esposito, g. giordano, m. guidi, m. lelli and c. piccione (2004b): indagini geologiche e geochimiche nella zona ad est di panarea sede delle emissioni anomale di gas del novembre 2002, in gnv general assembly, 20-22 december, napoli, italy (abstract). calanchi, n., b. capaccioni, m. martini, f. tassi and l. valentini (1995): submarine gas-emission from panarea island aeolian archipelago: distribution of inorganic and organic compounds and inferences about source conditions, acta vulcanol., 7 (1), 43-48. calanchi, n., c.a. tranne, f. lucchini, p.l. rossi and i.m. villa (1999): explanatory notes to the geological map (1:10 000) of panarea and basiluzzo islands (aeolian arc, italy), acta vulcanol., 11 (2), 223-243. calanchi, n., a. peccerillo, c.a.tranne, f. lucchini, p.l. rossi, p. kempton, m. barbieri and t.w. wu (2002): petrology and geochemistry of volcanic rocks from the island of panarea: implications for mantle evolution beneath the aeolian island arc (southern tyrrhenian sea), j. volcanol. geotherm. res., 115, 367-395. caliro, s., a. caracausi, g. chiodini, m. ditta, f. italiano, m. longo, c. minopoli, p.m. nuccio, a. paonita and a. rizzo (2004): evidence of a new magmatic input to the quiescent volcanic edifice of panarea, aeolian islands, italy, geophys. res. lett., 31, l07619, doi: 10.1029/2003gl019359 caracausi, a., m. ditta, f. italiano, m. longo, p.m. nuccio and a. paonita (2005): massive submarine gas output during the volcanic unrest off panarea island (aeolian arc, italy): inferences for explosive conditions, geochem. j. (in press). caris (2000): caris-hips (hydrographic information processing system) and sips professional v. 5.2 software user guide: ma-hips-ug-02-02 (on line http://www.caris.com/). chiarabba, c., l. jovane and r. di stefano (2005): a new view of italian seismicity using 20 years of instrumental seismicity, tectonophysics, 395, 251-268. d’agostino, n. and g. selvaggi (2004): crustal motion along the eurasia-nubia plate boundary in the calabria arc and sicily and active extension in the messina straits from gps measurements, j. geophys. res., 109 (b11402), doi: 10.1029/2004jb002998. de astis, g., g. ventura and g. vilardo (2003): geodynamic significance of the aeolian volcanism (southern tyrrhenian sea, italy) in light of structural, seismological and geochemical data, tectonics, 22 (4), 1040, doi: 10.1029/2003tc001506. dolomieu, d. (1783): voyage aux iles de lipari fait en 1781 ou notice sur les iles aeoliennes, paris, pp. 208. dumas, a. (1860): dove il vento suona. viaggio nelle isole eolie (ed. pungitopo, marina di patti, messina). esposito, a., g. giordano and m. anzidei (2005): the 2002-2003 submarine gas eruption at panarea island (aeolian archipelago, italy): structure and volcanology of the seafloor and implications for hazard evaluation, mar. geol. (submitted). falsaperla, s., g. lanzafame, v. longo and s. spampinato (1999): regional stress field in the area of stromboli (italy): insights into structural data and crustal tectonic earthquakes, j. volcanol. geotherm. res., 88, 147-166. ferrara, f. (1810): campi flegrei della sicilia e delle isole che le sono intorno (messina), pp. 424. gabbianelli, g., p.y. gillot, g. lanzafame, c. romagnoli and p.l. rossi (1990): tectonic and volcanic evolution of panarea (aaeolian island, italy), mar. geol., 92, 312-326. gabbianelli, g., c. romagnoli, p.l. rossi and n. calanche (1993): marine geology of panarea-stromboli area, aeolian archipelago, southeastern tyrrhenian sea, acta vulcanol., 3, 11-20. gamberi, f., p.m. marani and c. savelli (1997): tectonic volcanic and hydrothermal features of submarine portion of aeolian arc (tyrrhenian sea), mar. geol., 140, 167-181. hollestein, ch., h.g. kahle, a. geiger, s. jenny, s. goes and d. giardini (2003): new gps constraints on the africa-eurasia plate boundary zone in southern italy, geophys. res. lett., 30 (18), 1935, doi: 10.1029/ /2003gl017554. houel, l. (1782): voyage pittoressque des isles de sicilie, de malte et de lipari ect., paris. italiano, f. and p.m. nuccio (1991): geochemical investigations of submarine volcanic exhalations to the eat panarea, aeolian islands, italy, j. volcanol. geotherm. res., 46, 125-141. lucchi, f., c.a. tranne, n. calanchi, j. keller and p.l. rossi (2003): geological map of panarea and minor islets (aaeolian islands), (università di bologna, ingv, lac firenze). 12/03 no. long lat depth class date 602 150631.65 383823.84 14.8 2b 12/03 603 150632.48 383823.84 13.9 2b 12/03 604 150632.68 383824.00 13.8 2b 12/03 605 150636.78 383825.56 16 2b 12/03 606 150635.12 383823.74 15.7 2b 12/03 921 the high resolution bathymetric map of the exhalative area of panarea (aeolian islands, italy) luigi salvatore d’austria (1895): die liparischen, inseln vierter heft: panaris (new edition by p. paino, lipari (me), 1977). mercalli, g. (1883): vulcani e fenomeni vulcanici, in geologia d’italia, edited by g. negri, a. stoppani and g. mercalli (milano), 3rd part, pp. 374. romano, r. (1973): le isole di panarea e basiluzzo, riv. miner. siciliana, 139-141, 49-86. saccarotti, g., d. galluzzo, m. la rocca, e. del pezzo and d. patanè (2003): monitoraggio dell’attività sismica a panarea, in gnv general assembly, 9-11 june, roma, italy. serpelloni, e., m. anzidei, p. baldi, g. casula and a. galvani (2005): crustal velocity and strain-rate fields in italy and surrounding regions: new results from the analysis of permanent and non-permanent gps networks, geophys. j. int. (in press). spallanzani, l. (1825): viaggi alle due sicilie e in alcune parti dell’appennino (milano), tomo i. sga (storia geofisica e ambiente) (1996): fenomeni vulcanici nell’arcipelago delle eolie dall’antichità al xx secolo, studio di fattibilità rpt 156,96 (bologna, italy). tallarico, a., m. dragoni, m. anzidei and a. esposito (2003): modeling long-term ground deformation due to the cooling of a magma chamber: case of basiluzzo island, aeolian islands, italy, j. geophys. res., 108 (b 12), 2568, doi: 10.1029/2002jb002376. (received january 13, 2005; accepted june 21, 2005) t a r a t u r a d i u n ' a p p a r e c c h i a t u r a p e r m i s u r a d i p i c c o l e p e r c e n t u a l i d i u r a n i o i n s o l u z i o n e a c i d a d i a c q u a d i s t i l l a t a m . d i a n a r i c e v u t o il 30 gennaio 1965 riassunto. •—si descrive u n a t e c n i c a elio p e r m e t t e di misurare il n u m e r o di a t o m i di u r a n i o di u n a soluzione, con u n errore non superiore al 1 0 % . il s i s t e m a consiste in u n t u b o di geiger (philips 18503) sistemato in u n c o n t e n i t o r e di v e t r o nel q u a l e viene diluita u n a c e r t a q u a n t i t à di peso di n i t r a t o di u r a n i l e , m i s u r a t o con u n a precisione migliore d e l l ' 1 % . al v a r i a r e della diluizione si r e g i s t r a il n u m e r o di particelle che il geiger c o n t a , e quindi si r i p o r t a in grafico. d a q u e s t i per diverse q u a n t i t à di sale sciolto si determ i n a la r e t t a di t a r a t u r a . summary. — a t e c h n i q u e allowing t o m e a s u r e t h e u r a n i u m a t o m s n u m ber of a solution w i t h an e r r o r smaller t h a n 1 0 % is described. t h e a p p a r a t u s consists of a geiger t u b e (philips 18503) enclosed in a glass wessel. i n t h i s wessel is d i l u t e d a c e r t a i n weight of u r a n y l n i t r a t e . t h e w e i g h t of s a m p l e is k n o w n w i t h a precision b e t t e r t h a n 1 % . t h e particles n u m b e r d e t e c t e d b y t h e geiger t u b e , versus dilution, is p l o t t e d . p r o m t h e s e c u r v e s p l o t t e d f o r s a m p l e s of d i f f e r e n t weight, one d e t e r m i n e s t h e c a l i b r a t i o n s t r a i g h t line. 1 . i n t r o d u z i o n e . u n n o t e v o l e c o n t r i b u t o a l l o s t u d i o d e l l a s e z i o n e d ' u r t o p e r f o t o f i s s i o n e d e l l ' u r a n i o i n v i c i n a n z a d e l l a s o g l i a , è s t a t o d a t o d a l l a n u o v a t e c n i c a d i c a r i c a m e n t e e s v i l u p p o d e l l e e m u l s i o n i n u c l e a r i (*. -). i l c a r i c a m e n t o d e l l e e m u l s i o n i c o n u r a n i o si o t t i e n e m e s c o l a n d o a l l ' e m u l s i o n e n u c l e a r e i n f o r m a d i g e l u n a q u a n t i t à n o t a d i c i t r a t o di u r a n i o d i c o m p o s i z i o n e p e r f e t t a m e n t e n o t a . l a m i s t u r a v i e n e p o i d i l u i t a c o n a c q u a , 6 4 m. diana sciolta e v e r s a t a in un opportuno recipiente dove viene seccata. si f o r m a così u n a pellicola facilmente staccabile e di spessore p i u t t o s t o uniforme. queste pellicole vengono poi esposte ad u n flusso noto di raggi g a m m a monocromatici, e dopo opportuno sviluppo per eliminare le tracce di particelle alfa (x) vengono esaminate al microscopio. se n f è il numero di fissioni t r o v a t o nella pellicola, la sezione d ' u r t o per fotofissione è d a t a da n f cpn dove _zv è il numero dei nuclei di uranio nella pellicola, e q> il flusso di g a m m a . la determinazione di n non è così semplice come sembra in q u a n t o non si conosce mai con esattezza q u a n t o citrato di uranio è rimasto nei recipienti in cui l'emulsione è s t a t a sciolta e seccata, né si è sicuri che l'uranio si distribuisca u n i f o r m e m e n t e nella pellicola. si sono studiati allora vari metodi per cercare di determinare n. uno di questi consiste nel lavare la pellicola in u n ciclo chiuso facendo evaporare dell'acqua distillata e facendola poi condensare in u n recipiente dove è s t a t a posta la pellicola. da precedenti esperienze si è visto che due ore di lavaggio p o r t a n o p r a t i c a m e n t e via t u t t o l'uranio della pellicola. si t r a t t a ora di determin a r e la concentrazione in uranio della soluzione che si è o t t e n u t a . l'errore che si richiede nella determinazione di n deve essere entro il 1 0 % , con la conseguenza che la sezione d ' u r t o viene d a t a con la stessa precisione. i l presente lavoro quindi si propone di elaborare u n a tecnica che p e r m e t t a di misurare n e n t r o i limiti di errori anzidetti. p e r questo t u t t e le q u a n t i t à di uranio che venivano sciolte in soluzione erano d a t e con la precisione di ± 0,2 mg. 2 . d e t e r m i n a z i o n e d e l l a q u a n t i t à d i u r a n i o n e l l e s o l u z i o n i d i l a v a g g i o . il metodo seguito è quello di diluire u n a certa q u a n t i t à (in peso) di n i t r a t o di uranile (u0 2 (ìt03)24-6h20) e registrare il valore del conteggio alle varie diluizioni. inconveniente principale da eliminare è la proprietà dell'uranio di venire adsorbito dalle p a r t i dell'apparecchiatura con le quali è a c o n t a t t o . p e r ovviare a questo si sono d o v u t e usare soluzioni acide con p h minore od uguale ad 1. p e r ottenere le quali sono b a s t a t e 10-15 gocce di hc1 commerciale aggiunti in ogni soluzione. \ m i s u r a d i p i c c o l e p e r c e n t u a l i d i u r a n i o 6 5 lo schema a blocchi dell'apparecchiatura usata è il seguente: f i g . 1. il t u b o di geiger usato era un 18503 della philips a d a t t o per conteggi di particelle g a m m a , questo geiger era sistemato sul fondo di una provetta di vetro chiusa all'estremità superiore da plexiglass attraverso il quale passavano i collegamenti (fig. 2). pig. 2. 1) pilo rigido (positivo) di r a m e a r g e n t a t o dello spessore di 2 m m . — 2) t a p p o di chiusura (plexiglass) della p r o v e t t a . — 3) s t r i s c e t t a di r a m e ( n e g a t i v o ) di 2 decimi di millimetro di spessore. — 4) p r o v e t t a di v e t r o . — 5) anello di plexiglass r e g o l a t o r e dell'altezza della p r o v e t t a . —6) t a p p o di plexiglass che chiude il bicchiere. — 7) c o n t a t o r e di geiger tipo 18503 della p h i l i p s . — 8) bicchiere di v e t r o conten e n t e la soluzione. questa p r o v e t t a veniva regolata in altezza attraverso un foro praticato nel t a p p o del bicchiere. il t u t t o era sistemato entro un pozzetto di piombo di 3 cm di spessore in modo da ridurre il più possibile la radiazione di fondo. attraverso poi una scatola di partizione gli impulsi venivano contati da u n a scala con soglia di 5 volt. ogni misura era p r e c e d u t a da u n a di fondo per determinare le condizioni di lavoro relative ad ogni q u a n t i t à di soluto. m i s u r a d i p i c c o l e p e r c e n t u a l i d i u r a n i o 6 7 grafico n. 6 grafico n. 7 68 m. d i a n a i m p o r t a n t e c o n s t a t a r e che t u t t e le curve, relative alla variazione del numero di conteggi in funzione della diluizione con acqua distillata ed ognuna caratteristica di u n a certa q u a n t i t à di n i t r a t o di uranile sciolta, hanno lo stesso a n d a m e n t o , con piccolo spostamento della posizione del massimo verso diluizioni più basse al crescere della q u a n t i t à di soluto. 20 ¿0 60 100 grafico n. 8 l'alto conteggio iniziale è d o v u t o alla p r e p o n d e r a n t e influenza dei raggi b e t a emessi principalmente dall'ux 2 (2,31 mev) sul n u m e r o totale di g a m m a . questi raggi b e t a , per la sottilezza delle p a r e t i del contatore (250 mg/cm2) in presenza di poca acqua, vengono facilmente contati; sul numero di conteggi in queste condizioni influisce anche l'alta efficienza di rivelazione del geiger per d e t t e radiazioni. dopo di che con l'aggiunta di acqua molti vengono autoassorbiti e quindi il numero t o t a l e di conteggi diminuisce. all'aumentare della diluizione poi la soluzione avviluppa t u t t o il contatore e si h a quindi un a u m e n t o di angolo solido visto dallo stesso contatore con la conseguenza di avere un numero di impulsi più elevato. con l'ulteriore a u m e n t o della diluizione l'angolo solido t o r n a a diminuire, perché la soluzione supera la p a r t e sensibile del geiger, e si h a quindi u n a diminuzione del numero di conteggi. è in corrispondenza a questo massimo che è s t a t a t a r a t a l'apparecchiatura. l'errore sulla diluizione anche del 1 0 % nelle vicinanze del massimo p o r t a u n a trascurabile variazione del numero di conteggi essendo la curva in questo p u n t o quasi orizzontale. il maggior apporto sull'errore totale è d a t o dall'errore statistico che è dell'ordine dell'8% intorno al massimo per le prime misure che diminuisce poi fino ad arrivare al 3 % p e r la misura con 500 mg. s o m m a n d o quindi t u t t i gli errori la r e t t a di t a r a t u r a è d a t a con u n a precisione di circa il 1 0 % . m i s u r a d i p i c c o l e p e r c e n t u a l i d i u r a n i o 6 9 come si vede nel grafico n° 7 la variazione del numero di impulsi al valore massimo anzidetto in funzione della q u a n t i t à (li uranio sciolta nella soluzione è entro gli errori u n a r e t t a . nel grafico n. 8 è poi r i p o r t a t a la variazione della posizione del massimo numero di conteggi in funzione della diluizione e della q u a n t i t à (li sale sciolto. questa variazione p u ò essere d o v u t a ad un residuo di adsorbimento dell'uranio più sensibile a basse q u a n t i t à di soluto ed ha quindi l'effetto di spostare il massimo verso diluizioni maggiori. quest'ultimo grafico serve quando si debba misurare u n a q u a n t i t à incognita di cui però si conosca l'ordine di grandezza del peso di prodotto attivo sciolto. b i b l i o g r a f i a ( ! ) d e c a r v a l i i o h . g . e t al., « s u p p l . al nuovo cimento », 1 9 , 1 1 3 1 ( 1 9 6 1 ) . ( 2 ) d e c a r v a l i i o i i . g . e t a l . : « nuovo cimento », 25, 5 3 4 ( 1 9 6 2 ) . i : e c e n s i o n i e s p i n o s i de cos montehos j o s é maria h i ei.in g u i l l e r h i o s a n s : red de observaciones con gravimetro eli la provincia de linei va " investigacion gravimetrica en la zona minerà del caslil10 de las guardas p u b i ) . dell'istituto ' g e o g r a f i c o e catastale di madrid, p p . 32, carie 3. figg. 7. 11 servizio geodelico dell'istillilo gcografico e catastale spagnolo ha compiuto recentemente una vasta campagna gravimetrica con il gravimetro elettromeccanico « askania a . 25 » di proprietà dell'istillilo stesso, nella provincia di l l u e l va, mentre l ' i s t i t u t o geologico e mineralogico spagnolo ha effettuato con i l proprio gravimetro uno studio di investigazione geofisica nella zona mineraria di « castillo de las guardas » nella provincia di « sevilla ». nella prima campagna l e stazioni vennero distribuite con u n i f o r m i t à lungo una maglia rettangolare di lati variabili da cinque ad otto chilometri. c o m e base gravimelrica di riferimento vennero considerale l e località a r a c e n a (partenza) e cortegana (arrivo) dove in precedenza erano state eseguite misure pendolari. la differenza gravimetrica fra queste due località determinala col gravimetro è risultata in perfetta corrispondenza con quella determinata col metodo pendolare, ciò che prova la bontà delle osservazioni eseguite. s i ottenne infatti: aracena pend. cortegana » d i f f . 979.902gal grav. 0.0 979.899 » -2.1 limai 1 vari anelli osservati sono affetti di errori inferiori a 1 mgal. vennero complessivamente osservate 79 stazioni. i risultati conseguili sono raccolti in opportune tabelle, dove al n o m e della stazione è aggiunta la indicazione della ubicazione della stazione stessa, i valori delle latitudini delle stazioni, necessarie per il calcolo delle gravità normali (yo) calcolale con la formula internazionale. le altezze delle stazioni (ottenute con il barometro o con i dati della triangolazione esistente), le gravità normali, quelle osservale, l e correzioni di f a y e d i ' quelle ili b o u g u e r ( b i e l e anomalie ( a l . la correzione di b o u g u e r venne al solilo determinata con la formula b •i i-i (-1 „ , li dove è la densità del terreno circostante la stazione e (~>m la densità m e d i a . p e r <"> e per <">ln vennero sempre assunti 5,52, sicché si e b b e co-valori 2.65 t anteniente 3 -l b 2 65 : 0 , 3 6 i i -3mgal 2 . 1 m g a l e poiché ii = 3086.q (con q quota della stazione) cosi le anomalie sono definite dalla : a = g + 11 + b — y „ = g + 0 . 6 1 i i — y „ =-g + 0 , 1 9 8 . q—y„ . p e r tutte le stazioni si ottennero anomalie positive variabili da un minimo di 12 ad un massimo di 57 milligal. nella seconda campagna furono invece osservate 211 stazioni lungo una ma656 recensioni glia rettangolare, con 12 profili paralleli da .nord a sud. ricoprenti la zona di studio. i risultati ottenuti furono compensati dato che le maglie parziali erano chiuse. i valori hanno permesso, su una carta in scala 1 : 5 0 0 0 . il tracciamento delle curve isoanomale con la equidistanza di 0.2 m i l l i g a l . da quesla carta risulta che le anomalie vanno crescendo da 0 vcst ad kst della regione, da un minimo di 1,4 mgal (parte occidentale) ad un massimo ili 4,2 mgal ( o r i e n t a l e ) . mancano nella m e m o r i a in esame cenni sulla interpretazione geofisica dei risultali conseguili. sono invece riportati per tutte l e stazioni in opportuni specchi n u m e r i c i , l e latitudini delle stazioni, le quote, le riduzioni per le latitudini, per l e altezze, per le densità terrestri. ì valori ottenuti con il gravimetro. per 0 venne assunto il valore 2.7. la riduzione di latitudine venne fatta con r i f e r i m e n t o al valore <( = 37 j 41" 4 9 " . ( g . b.) \ i l e i . i n g u i i . i . e r v i o s a n s e s p i n o s a de i.os m o n i eros j o s é m a r i a : determinaciòn rie per/iles con ci gravimetro electro-mecanico « askania mini. 25 » en la zona l.inares baiteli jabah quinio p u h b . dell'istituto geografico e catastale ili madrid, p p . 15. earte 6. \ cimerò eseguite quattro serie di osseivazioni (profili) lungo le strade che uniscono le l o c a l i t à : l i n a r e s b a i l e n (lungo un parallelo geografico). l i n a r e s r i o guailalimar (lungo una direttiva nord( ivest, sud-est), linares-tobarnela-'cruce fino a b a i l e n (lungo una direttiva .nord-est, sud-ovesl) ed infine b a i l e n 1 errocarril (lungo a l l ' i n c i r c a un meridiano i. in que.-'i profili vennero rispettivamente eseguite n . 24, n. 25, n. 4 4 , n . 55 stazioni (complessivamente n. 148 stazioni) col gravimetro « askania n . 25 ». l e stazioni vennero collegate fra loro ad una distanza massima ili 500 m e t r i e media di 200 m e t r i . come di consueto ai valori osservati sono state apportate le riduzioni per la latitudine, (assumendo per latitudine di r i f e r i m e n t o il valore •18° 06'i per la altitudine (riduzione di faye) e per le densità (riduzione di b o u g u e r ) assumendo costantemente per densità superficiale il valore 2..'s, e per densità media 5,52. i valori ottenuti sono raccolti profilo per profilo in opportune tavole numeriche, dove sono pure riportate le differenze gr av ime tr iche osservate e ridotte. p e r lutti i profili queste ultime risultano positive, di valori non molto dissimili fra l o r o . si ha difatti per i profili dianzi considerati: profilo ililf. massima diti, minima i 13-98 mgal 37.01 mgal i i 33.12 i i i 36.58 i v 41.27 29.86 » 31.07 » 25.06 p e r ognuno ili questi profili gli autori riportano un grafico sul quale, con linee continue, sono riportate le curve corrispondenti alle altitudini delle stazioni (ottenute con una livellazione geom e t r i c a adoperando 1111 livello zeiss) alle differenze di gravità, osservate, alle differenze di gravità ridotte. sull'asse dell e ascisse vengono segnate le distanze progressive delle singole stazioni. con molta approssimazione queste ultime curve risultano a l l ' i n c i r c a parallele all'asse delle ascisse per i p r i m i tre profili, m e n t r e per il quarto profilo l a curva è sempre ascendente da 25 a 40 m i l l i g a l . p e r ogni profilo poi le altre due curve risultano a l l ' i n c i r c a f r a loro simmetriche. questo fatto secondo gli autori è d'accordo con la costituzione geologica della r e g i o n e . ( g . b.) i t a recensioni fischer v il.: the caselle peninsula, ì\ew' brìtaiii earlhquak-e of jniiiiary 34, 1941 b u l l . sci sin. s o r . a n i . x x x i v , 1-12 ( 1 9 4 1 ) . i i t e r r e m o t o avvenne alle 21' 27'". 6 ( o r a locale) del 11 gennaio 1911 lift'1 27"', () t . m . g . del 1 3 1 l l j e fu sentito in tutta la penisola di gazelle a est della nuova b r i l a n n i a in a u s t r a l i a . l a scossa p r i n c i p a l e fu seguita nel p r i m o giorno da 222 sco.-se, c h e raggiunsero talvolta anche il grado \ della scala r o s s i f o r e l , succedendosi ad intervalli di pochi minuti per m o l t e o r e di seguito e n e l l e successive 4 settimane e b b e r o rispettivamente 323, 78. -i 1 e 33 scos.-e registrate alla stazione sismica di r a b a u l . tulle provenienti da una distanza di circa 15-21) miglia a .-ud-est di r a b a u l . dai dati macrosismiei raccolti in gran numero nella r e g i o n e l ' a u t o r e ha potuto stab i l i r e l ' e p i c e n t r o p r e o il villaggio di w a n g a 21 miglia a sud-ovest di r a b a u l . l ' a u t o r e descrive gli effetti più notevoli prodotti dal terremoto elio p r o v o c ò f r a n e e slittamenti e quindi fa a l c u n e considerazioni circa l'attività sismica della r e g i o n e messa in relazione con quella \ uleaniea. i 1/. g . i i.inbbman l'ini.t.ii's marcei.i.a: issociatìon of large ions ami fog t e r r . magli, a t m . electr. 4 7 . 295-299 « 19421. d u r a n t e una ricerca effettuala nel 1941 allo scopo di misurare i grossi ioni mediante un apposito contatore, lu notala una p e r c e n t u a l e di tali ioni eccezionalm e n t e alla durante la notte dal 3 al 1 d i c e m b r e : questa anorinalilà precedeva una ampia f o r m a z i o n e di n e b b i a lungo tutta la costa nord a t l a n t i c a . i n a tabella riassume le o-serv azioni eseguile di ora in ora dalle 151' del 3 die. alle io1' del 1 d i e . : sono posti a c o n f r o n t o la c o n c e n t r a z i o n e dei grossi ioni e la visibilità, l.a concentrazione 6 5 5 raggiunge valori insolitamente alti sette ore p r i m a della formazione della nebbia. diminuisce durante e aumenta ili nuovo dopo la scomparsa della n e b b i a . f u r o n o fatte altre r i c e r c h e per v e d e r e -e effettivamente esiste una q u a l c h e rel a z i o n e ira i due f e n o m e n i : le indagini sembrano c o n f e r m a r e tale ipotesi. si discutono le eventuali cause, i l . , l/.i w ut g . r . : effecl of smoke on tlie almosplieric-eleelric elements al the ii nllieroo magnetic observalory t e r r . magli, alni, e l e e l r . . 18. 49*63 (1943i i. durante i mesi .'stivi, per varie c a u e . n e l l australia o c c i d e n t a l e viene p r u d o n o parecchio fumo elle spesso ,-i addensa anche sull'osservatorio magnetico di 'vi-'ai b e r o o . 1 giorni in cui si nota il f u m o -omo frammisti più o meno irregolarm e n t e agli a l t r i . dalle registrazioni -lilla c o n d u c i b i l i t à dell'aria effettuate net 1922 risulta che il f u m o provoca lilla notevole d i m i n u z i o n e della conducibilità, m e n t r e m i s u r e sul gradiente di potenziale f a l l e ilei 1924 hanno m o s t r a l o i oine, in presenza di f u m o , tale gradiente aumenti s e n s i b i l m e n t e . constatato d u n q u e che la presenza (li f u m o perturba gli elementi elettrici d. 1l'almo.-fera, fu eseguita una serie di o servazioni durante tulio il periodo 19261934. limitando però tali osservazioni ai mese di f e b b r a i o , mese ili cui la presenza di f u m o è più frequente e di notevole intensità. in vari grafici sono riportati i risultati delle osservazioni : quelli relativi ai giorni in cui c ' e r a presenza di f u m o separatamente da quelli in cui il f u m o era c o m p l e t a m e n t e a-sente. r i s u l t a da tali grafici come il fumo provochi un aumento nel gradiente di potenziale ' a c o n f e r m a dei risultati conseguiti nel 192 ii. una diminuzione, nella somma delle c o n d u c i b i l i t à positiva e negativa ' ' • ) , u n . aumento piuttosto piccolo nel rapporto tra queste due 6 5 6 r e c e n s i o n i conducibilità < '•) e un abbassamento della corrente aria-terra. l ' e f f e t t o del fumo, inoltre, è più pronunciato durante la notte che non durante il giorno. s i lenta di dare una spiegazione di questo fallo supponendo che la maggior presenza di fumo durante le ore notturne sia collegato con la velocità e la direzione del vento, e si fanno ricerche in questo s e n s o ; ma sembra invece, dopo tali r i c e r c h e , clic tra i due falli non ci sia relazione di sorta. in uno studio preliminare fatto dall ' a u t o r e in collaborazione con t o r r e s o n . si era trovalo che la cor r e nte aria-terra era sensibilmente la stessa con o senza f u m o . ciò è in disaccordo con quanto l ' a u t o r e trova in questa ricerca, giacché risulla o r a che la presenza di fumo a \\ atheroo tende a ridurre il valore di tale corrente. la evidente contradizione è dovuta ail una inopportuna ed insufficiente scelta dei giorni falla nella precedente ricerca. i risultati ora invece conseguili permettono di dedurre che il f u m o si distribuisce lungo uno spessore verticale sufficiente ad interessare in modo apprezzabile la resistenza totale della colonna d'aria verticale lungo cui passa la corrente aria-terra. questa ed altre prove indicano che il fumo si estende ad una altezza di circa 1 k m . la reristenza totale della colonna verticale d'aria al disopra d e l l ' o s s e r v a t o r i o , varia notevolmente durante il g i o r n o : sia queste variazioni, sia i valori medi vengono aumentali quando il f u m o è presi-lite. si ritiene c h e alle variazioni diurne di tale resistenza, contribuiscano anche altri fattori, oltre il n u m e r o dei nuclei di condensazione e le sostanze radioattive che si trovano nell'atmosfera. a n c h e la velocità con cui vengono prodotti i piccoli ioni nella bassa atmosfera non rimane costante, durante il giorn o . in alcune stazioni a terra, e questo fallo sembra sia dovuto ad un accumulo di sostanze radioattive che si verifica nell ' a t m o s f e r a (piando la velocità del vento è piccola, e ad una conseguente dispersione di tali sostanze quando la velocità del velilo aumenta. l ' a n d a m e n t o medio (li quesla variazione presenta un massimo n e l l e p r i m e ore del mattino e un min i m o durante la sera, i ! . . m.) <i 'l'iie determina/ioli of fine ground motion hy integration oj strong-motion records: a symposium » b u l l . s e i m . s o c . a m . , x x x i i i , 1, (19-13). e ' una raccolta di note riguardami alcune r ice r c he sugli a c c e l e r o m e t r i eseguile con una piattaforma o.-cillantc. il programma di ricerca è stato intrapreso dall ' u . s . c . g . s . ili collaborazione con l ' i siitute of t e c h n o l o g y del massachusetts. l e ricerche si fondano soprattutto sul confronto tra il m o l o effettivo di una piattaforma oscillante e le registrazioni ottenute con l ' a c c e l e r o m e t r o . nella prima nota di a . c . r u g e e i l e . m c c o m b vengono discussi gli obiettivi generali ed il lavoro sperimentale. nella seconda a . c . r u g e spiega il significalo dei risultati dal punto di vista t e c n i c o . nella terza, e . -neumann parla dei melodi di integrazione numerica e finalmente n e l l a ultima si parla della integrazione meccanica delle registrazioni per mezzo di uno speciale analizzatore differenziale idealo da a . c. r u g e . (17. g.) r i c e a . c . m c c o m h i l e . : tests of earlhquake acceleromelers on a shuking tnhle b u l l . s e i s m . s o c . a n i . , x x x i i i , 2-12 (19-13). questo studio fa parte di un programma di r ice r c he sistematiche sugli accelerometri sperimentati sopra una tavola oscillante. gli scopi di detto programma di ricerche sono i seguenti: ai determinare l ' e f ficienza degli a c c e l e r o m e t r i nel caso di vibrazioni forzate ir r e g o l a r i in condizioni analoghe a quelle che si verificano nel caso di terremoti f o r t i ; hi d e t e n n i m v ^ i r n „ a* r e c e n s i o n i nare il grado di indipendenza delle comp o n e n t i ; c) determinare il grado di accordo degli spostamenti reali imposti all ' a c c e l e r o m e t r o per mezzo della piallaforma oscillante e quelli desumibili dall e registrazioni; d) verificare la possibilità di mettere in evidenza onde lunghe e di piccola a m p i e z z a ; el indagare la possibilità di usare un eerto analizzatore differenziale per l ' a n a l i s i delle curve registrale e confrontare i risultati con quelli ottenibili mediante l'integrazione numerica. le esperienze condotte con l'ausilio della piattaforma oscillante confrontando direttamente il molo r e a l e della piattaforma con il diagramma dato dall'accelerometro standard d e l l ' u . s . c . g . s . portano alla conclusione c h e detto accelerometro risponde b e n e allo scopo e dà diagrammi fedeli. ( m . g . ) i\euaian !•'.: ali appraisal <>[ numerical integration melhods as applied to strong-niotion data b u l l . seism. soc. a n i . . x x x i i i . 21-60 (1943). s i discutono i p r o b l e m i inerenti alla integrazione numerica delle curve oscillanti e si discute 1111 particolare metodo di integrazione applicandolo al caso dell e registrazioni ottenute con accelerometri d e l l ' i . s . c . g . s . sperimentati su una piattaforma oscillante [m. g.) ruce a . c . : discussion of principal restili* frinii the engineering sland point b u l l . s e i s m . s o c . a111., x x x ] i i , 13-20 (1943). s i studia il comportamento di un tipo di accelerometro standard della u . s c. g. s . sperimentandolo con una piattaforma oscillante ed esaminandone l'efficienza nei riguardi delle applicazioni t e c n i c h e ; si conclude che esso risponde p e r f e t t a m e n t e ' a l l o scopo e si verifica che agli effetti pratici le tre componenti sono abbastanza indipendenti. i n o l t r e gli spostamenti e l e velocità computate num e r i c a m e n t e sono in accordo, più che 657 sufficiente anche in misure di precision e , con gli spostamenti e le velocità reali. lutine 1 integrazione ottenuta mediante l'analizzatore differenizale m . 1. t . coincide nettamente con i risultati dell'integrazione n u m e r i c a . ( m g.l ruce a . € . : inalysis oj aecelerograins by meiuis of the m.l.t. differeiitial analyzer b u l l . seism. s o c . a n i . , x x x i i i , 61-63 (1943). si riportano in questa nota i risultati ottenuti mediante l'integrazione meccanica delle curve ottenute da accelerometri sperimentati -11 una piattaforma oscillante. l ' i n t e g r a z i o n e meccanica è stata eseguila con un particolare analizzatore differenziale già descritto da buseh in una nota pubblicala nel journal of the franklin institute. voi. 212. 11. 4 . ( v . g . ) . gutenberc b . : trave! tiines of principal p and s phases, over snudi ilistances in southern california b u l l . seism. s o c . a n i . . x x x i v . 12-32 (1944). l autore ha condotto uno studio accurato su un abbondante ed ottimo materiale costituito da numerosi sismogrammi riferenti-i a 50 terremoti, opportunamente selezionati, registrati n e l l a california meridionale a distanze epicentrali comprese entro gli 800 k m . 1 risultali relativi alle principali fasi hanno dato i seguenti valori per l e velocità a p p a r e n t i : \ ' p = 5 , 5 7 7 k m s e c y p — 6 , 0 4 7 » yi>„ =8.06 v s = 3 , 2 6 » y s v = 3 . 6 5 k m s e c v s „ = 1 , 4 5 » l e velocità effettive delle l ' y e sy rollo inferiori circa del 0,3".. rispetto alle velocità apparenti mentre per le l'n ed sn la velocità effettiva è del 0.5 più piccola della velocità apparente. la velocità delle onde longitudinali e ira-ver656 recensioni -,iii cresce con la p r o f o n d i l a nei p r i m i ."in ahi di c i r c a l ' u n o p e r c e n t o o g n i ili k m . i.o . p o s o r e d e l l o strato del granito ili 18 k c a . con lievi v a r i a z i o n i lo<:.li: gli strali s u p e r f i c i a l i n e l l e r e g i o n i i n l i e r e d e l l a c a l i f o r n i a m e r i d i o n a l e ragg i u n g o n o c o m p l e s s i v a m e n t e lo spessore .li 35 k m ; il loro spessore a u m e n t a •. i r-o l ' i n t e r n o lino a r a d d o p p i a r e solin la s i e r r a n'evada. 1.1/. g. i. savahenski 11. f . : diagram for interpretatimi oj deep focus eurtliquakes a c . des s e . rie l ' u . r . s . s . p u b b l . de l ' i n , ! . s é i s m o l . de m o s c o u , 6 ( 1 9 4 6 ) . n e l l ' i n t e r p r e t a z i o n e dei t e r r e m o t i la disianza c p i c e n t r a l e è in g e n e r a l e considerala ( p e r i t e r r e m o t i s u p e r f i c i a l i ) quale a r g o m e n t o (ascissa) delle t a b e l l e (o d r o m o c r o n e ) . f." i m p o r t a n t e tuttavia poter d i s p o r r e ( p e r i t e r r e m o t i p r o f o n d i ) di d i a g r a m m i i o t a b e l l e ) in cui la disianza c p i c e n t r a l e figuri q u a l e f u n z i o n o e i l t e m p o di tragitto, c o m e a r g o m e n t o . l a ben nota tarala ilei iirunner è l ' e s e m p i o di un lale d i a g r a m m a . in que.-ta nota l ' a . p r e s e n t a un a l t r o d i a g r a m m a in cui gli a r g o m e n t i s o n o gli i n t e r v a l l i s p e s p p ( p e r distanze epic e n t r a l i f r a i t e 2 2 " ) o p p p ( p e r disianze fra 22" e uh)'): f u n z i o n i sono la disianza c p i c e n t r a l e e la p r o f o n d i t à ipoet l i l i a l e , r i c a v a b i l i da una f a m i g l i a di curve (ile r a p p r e s e n t a n o l ' e q u i d i s t a n z a uli 2 ' in 2"i e da una f a m i g l i a di isol a l e (di 50 in 50 k m . ila ii a 750 k m ) . ii d i a g r a m m a , disegnato mi carta m i l l i m e t r a t a , può essere m o l t o c o m o d o qualora sia p o s s i b i l e i n d i v i d u a r e con sufficiente sicurezza o l t r e a l l e p e l e s , ani he a l m e n o le p i ' ( o s p ) . o s e r v a n d o le i s o b a t e . si nota c h e in conseguenza delle e t e r o g e n e i t à d e l l ' i n t e r no della t e r r a , l e curve r e l a t i v e alla p r o f o n d i t à di 700, 600 e 500 km p r e s e n tano a l c u n e o n d u l a z i o n i . l ' a . c o n s i g l i a i n o l t r e , q u a l o r a si disponga di un g r u p p o di stazioni e le p i o f o n d i t à r i c a v a t e da c i a s c u n a differiscano s e n s i b i l m e n t e , di a s s u m e r e q u a l e più p r o b a b i l e il valore m e d i o d e l l e prof o n d i t à e di r i c a v a r e poi in b a s e a q u e sta p r o f o n d i t à p e r ogni s t a z i o n e u n val o r e m i g l i o r e della distanza c p i c e n t r a l e . ( c . m.i. deei. s . a . : magnolie declination in isa-1915. u . s . coa.-t and g e o d e l i e s u r v c y . ser 6 6 4 , 67 ( 1 9 4 6 ) . i l lesto si r i f e r i s c e ad u n a m a g n i fica carta d e l l a d e c l i n a z i o n e e della sua v a r i a z i o n e a n n u a negli s l a t i u n i t i al 1 9 4 5 . e c o m p r e n d e allo s c o p o a l c u n i cenni -oli;! natura della v a r i a z i o n e s e c o l a r e . sulla v a r i a z i o n e m a g n e t i c a d i u r n a e c c . , con n u m e r o s e t a b e l l e . r i s p e t t o alla p r e c e d e n t e e d i z i o n e del 1 9 4 0 , il te-io d e s c r i v e più a m p i a m e n t e i n u o v i m e l o di di r a p p r e s e n t a z i o n e c a r t o g r a f i c a e tab e l l a r e s e g u i t i . i n o l t r e spiega a b b a s t a n za in dettaglio i m e t o d i p e r l a d e t e r m i n a z i o n e del m e r i d i a n o g e o g r a f i c o , la cui c o n o s c e n z a è n e c e s s a r i a p e r la d e t e r m i n a z i o n e della d e c l i n a z i o n e in un p u n t o . (c. ma. ai.i.en c . \\ . : l arialion of tlic sun's ultra'vioìet radiatimi as revealed by ionospheric and geomagnetic observalions t e r r . m a g l i , and a l n i . e l e c t . , 51, 1, 1-18 ( 1 9 1 6 ) . s o n o r i p o r t a t i i r i s u l t a t i di u n o studio d e l l e v a r i a z i o n i a b r e v e e l u n g o per i o d o delle f r e q u e n z e c r i t i c h e p e r gli strati i o n i z z a l i l i . f „ f 2 a w a s h i n g t o n , h u a n e a y o ( p e r ù ) , w a l l i c r o o ( a u s t r a l i a o c c i d . ) e moiinl s t r o m l o ( a u s t r a l i a o r i e n t . ) . p e r l ' e p o c a 1 9 3 7 4 4 . i f a t t o r i t e r r e s t r i sono slati e l i m i n a t i , affinchè le f r e q u e n z e r a p p r e s e n t a s s e r o l e v a r i a z i o n i d e l l a l u c e u l t r a v i o l e t t a i o n i z z a n t e . i no 656 r e c e n s i o n i -indio analogo è slato compiuto anche -lille variazioni in ampiezza del campo magnetico s( ( ( dovuto all'attività solare n o r m a l e , cioè astrazione l'alia dalle perlurbazionii ad vpia i samoa), w a l h e r o o e cape town per l ' e p o c a 1937-13. i.e variazioni annuali, mensili e giornaliere sono stale confrontate con i dati solari a mezzo di grafici e melodi -ulle correlazioni. le principali conclusioni .-ono l e seguenti: ai la parte variabile della radiazione ultravioletta solare ha origine principalmente dalle regioni allive caratterizzate dall'apparizione di m a c c h i e . proluberanze e l a c o l c . h) le sorgenti della radiazione ultravioletta hanno un'esistenza più lunga delle macchie, e probabilmente anche delle l'acole. c) i tre strali ionosferiei e . 1, ed !•., e il campo s<| sono influenzali dalle stesse fonti di luce ultravioletta. '/) l e fonti emettono una radiazione ultravioletta considerevole quando sono al centro del s o l e . ci la parte variabile del flusso uliraviolelto è proporzionale al numero di macchie solari. /1 le l'acole p o l r c b b e r o produrre la radiazione ullraviolelta continua che è necessaria per spiegare le variazioni ionosferiehe e geomagnetiche. (c. l/.i. 1ìahtei.s j . : geomagnetic dota on variations solar radiatimi: pari. i u'ivn radiatimi t e r r . magli, and alni. e l e c i . . 51, 2. 181-242 ( 1 9 4 6 ) . . c o m ' è noto, dalle osservazioni geomagnetiche si possono ricavare le variazioni nel tempo di due diverse specie di radiazione solare, una radiazione ondulatoria il ed una corpuscolare p. in questo lavoro, che è il primo di una serie. l a . ricava e discute serie omogenee per ii e p: questi dati, oltre al10 -indio del magnetismo terrestre e ('ella tisica solare, possono servire quale base n u m e r i c a per -ludi -u altre influenze solari in f e n o m e n i geofisici o biologici. distinguendo le variazioni « lente 11 (espresse in medie mensili, trimestrali e annue) dalle variazioni « rapide » (nel corso di una rotazione del sole), l ' a . trova che n e l l e prime le correlazioni fra l'attività solare li (espressa dai numeri relativi delle macchie solari i e ii sono le più significative finora trovale fra fenomeni solari e terrestri, maggiori ancora di quelle trovate fra li e /'. nelle seconde, viene studiala l'influenza delle variazioni in r su ii e p, col seguente risultato: numerose analisi statistiche dimostrano che l e variazioni rapide ili r sono accompagnale — tranne che in prossimità del minimo delle macchie — da variazioni similari di ii . con un ritardo di non più un giorno c i r c a . l ' e r contro, la relazione statistica fra r e p nelle variazioni rapide è stata trovata molto più debole che quella fra r e ii . la tendenza al periodo di 27 giorni in i! è altrettanto marcala che in r . a iene infine discusso il significalo fisico di 11 : li è p r o b a b i l m e n t e una radiazione solare assorbita piuttosto in b a o nella ionosfera, nello stesso stralo (o in prossimità di essol che è ionizzato dall'eccesso di radiazione ultravioletta emessa da u n ' e r u z i o n e solare. ( c . v . ) snvi'l.ev a. i l : the application of solar and geomagnetic data lo shortterni joreeasts of ionospheric eomiitions ' t e r r . magli, and alili. eb-et. 51. 2. 217-266 (1946). l ' a . descrive i procedimenti secondo i finali le tendenze ai ricorsi nell'attività geomagnetica, i rapporti sull'attività solare e varie relazioni solari-terrestri sono adoperati nel preparare liprevisioni a breve scadenza ilei disturbi 656 recensioni magnetici e i o n o s f e r i c i . l i programma per una raccolta sistematica dei dati solari e geomagnetici coordinati al dipartimento di magnetismo t e r r e s t r e della carnegie institution di washington i d m t c i w ) , in particolare per l'applicazione alla previsione sopra menzionata. è in funzione dal luglio 1912. le previsioni diramate dall interservice iladio propagation l a b o r a l o r y ( 1 r p l ) , in collaborazione col d m t c i w , vengono confrontate con l'attività magnetica per un periodo di 15 mesi, e mostrano che le previsioni sono soddisfacenti circa per il 70 per cento delle volte. le analisi delle correlazioni f r a dati coronografici e spettroeliografici e attività magnetica mostrano che, oltre ad un periodo di due anni, è stata messa in evidenza una decisa tendenza dei disturbi a presentarsi quando le r e g i o n i solari identificate da queste osservazioni si trovano ad est del meridiano centrale del s o l e . ic. m.) w e l l s h . w . e s h a p l e y a . i l : eelipseejjiei in fri.ayer ni the lonosphere t e r r . m a g n . and a l n i . e l e c t . 51, 3, 401-409 (1946). l e analisi delle osservazioni ionosferiehe durante gli eclissi parziali dal 4-5 f e b b r a i o 1913 e 25 gennaio 1911. effettuale agli osservatori ionosferici de] dipartimento di magnetismo t e r r e s t r e della carnegie institution di w a s h i n g t o n , mostrano che la ionizzazione dello strato f„ era in entrambi i casi inferiore alla normale prima dell'inizio dell'eclisse, e rimase bassa più a lungo ilei pre\ i s t o dopo l'eclisse. l ' e s a m e di questo effetto in relazione all'attività concorrente del s o l e , specialmente di ampie legioni della corona, indica che radiazioni provenienti da oltre il bordo del sole possono contribuire alla ionizzazione della regione. i valori del coefliciente di ricombinazione per lo strato f„ stimati dalle osservazioni di queste eclissi sono compresi fra i o 9 e 2 . ] 0 ~ l u . i valori maggiori sono stati ottenuti quando le altezze apparenti, come pure le densità degli i o n i , erano m i n i m e . a huaucavo ( p e r ù ) , durante l'eclisse del 25 gennaio 1911. lo strato f„ si spostò verso l'alto con velocità d e l l ' o r d i n e di 200 k m ora, e una nuova stratificazione nella regione f si sviluppò all'altezza n o r m a l e . ( c . .1/.) wai.dmeieiì m : i» attempi al an identification nj the m-regions t e r r . magn. and a l n i . e l e c t . , 51. 4, 537-542 (1946). c o m ' è nolo, le regioni magneticamente attive del s o l e , costituite da aree ristri ite dalle quali viene emessa la radiazione corpuscolare, sono state chiamate da burleis regioni m. esse si manifestano in vicinanza dei grandi gruppi di macchie solari dopo la sparizione, delle m a c c h i e ed a condizione che non si f o r m i n o nuove macchie nel posio considerato. quest'ultima condizione è il motivo per cui l'apparizione delle regioni m è limitata alle epoche di granile scarsità di macchie che precedono i m i n i m i dell'attività solare. u l t e r i o r i fenomeni che accompagnano l ' a p p a r i z i o n e di m a c c h i e sono le protuberanze, che nella regione delle m a c c h i e sopravvivono solamente finché in essa 11011 si formano nuove m a c c h i e . la velocità della radiazione corpuscolare è compresa f r a 300 e 600 km,'sec. p o i c h é , negli anni precedenti i m i n i m i , la sede dei disturbi solari giace a basse latitudini eliocentriche, e p o i c h é d'altra parte la velocità angolare delle protuberanze, la cui latitudine è circa 10" maggiore di quella delle m a c c h i e , in cui hanno origine. coincide con la velocità angolare delle macchie, ne risulta una connessione molto stretta nello spazio e nel tempo fra protuberanze e regioni m . il caso speciale di 1111 centro di di656 recensioni sturbo ad alta latitudine è stato studiato dettagliatamente. 11 materiale presentato conferma l'ipotesi di lavoro, suggerita nel 1941 d a l l ' a . , secondo la quale con l ' a p p a r i r e di disturbi fotosferici le regioni m svaniscono, p r o b a b i l m e n te c o m e conseguenza dei campi magnetici elle accompagnano le m a c c h i e . ( c . 17.) hoaca g . : lo stalo attuale della rete gravimetrica italiana p u b b l . c o m m i s s . geodetica italiana, terza s., 4, 65 11917). questa importante raccolta di tutte le misure pendolari effettuale in italia dal 11125 al 1939. è stata compilata d a l l ' a . ancora nel 1911), ma gli eventi b e l l i c i ne hanno ritardato la pubblicazione alla (ine del 1917. l.a parte principale della memoria è costituita da un catalogo ordinato cronologicamente, in cui le serie di misure eseguite da uno slesso a . sono raggruppate in ordine di latitudine crescente, l ' o r ogni stazione, oltre alle coordinate geografiche, quota e densità attorno alla stazione assunta per la correzione di b o u g u e r . nome dell'osservatore e anno, sono riportati il valore della gravità osservala, nonché il valore ridotto in aria l i b e r a , la correzione topografica ed il valore ridotto secondo b o u g u e r ; seguono infine i l valore n o r m a l e secondo la formula internazionale 1930lionclié l e anomalie secondo f a y e e secondo b o u g u e r . p e r 12 stazioni sono riportate anche le anomalie isostatiche (nell'ipotesi di i lavi oi di. l e tabelle sono precedute da un'estesa sintesi storica, e corredate di una completa bibliografia e di un indice alfabetico molto pratico. comprendono complessivamente 620 misure su terra f e r m a , 21 nei porti e 136 in m a r e : in lutto quindi ben 777 misure pendolari. una tabella mostra la suddivisione delle m i s u r e in terra per regioni, e divise in ordine ali epoca nei seguenti grupp i : 1825-1908; 1909-1926; 1927-1910. le 302 misure del 1" gruppo risultano affette da notevoli errori, soprattutto per la mancanza della riduzione a supporto rinido e per le determinazioni di temilo non rigorose per gli stati degli orologi; devono essere perciò o abbandonale o trattate con speciali cautele. l e 48 misure del secondo gruppo, con la riduzione a supporlo rigido e determinazioni ili tempo effettuati per via astronomica. ballilo errori medi non superiori ai 5 mgal. il 3" gruppo comprende 270 stazioni con determinazioni di tempo effettuate mediante ricezioni radiotelegrafiche. in conclusione, osserviamo che -i tratta di un catalogo ragionalo, nel (piale buona parte dei calcoli sono slati rifatti o completali (soprattutto per (inalilo riguarda la correzione topografica), che raccoglie e presenta in un insieme omogeneo, ben ordinato e in chiara veste tipografica l ' i n s i e m e delle misure pendolari effettuate fino ad oggi in b a lia. ne risulla un contributo pratico notevole all'utilizzabilità della rete gravimetrica italiana, nonché una sintesi del .-no stalo attuale e delle sue necessità. r i c o r d i a m o che questo vasto lavoro ha dato lo spunto ad altri, di cui uno in particolare riguarda l'utilizzazione del materiale qui ordinato, a scopo geotisico. ( c . m.) f u m i n o j . a . , j o i i n s t o n h . f . . form s i i s . e . . m c n i s h a . g . e scott w . e . : magnetic resulta frolli ii atheroo observatory, il estera australia, 19191935 and 1936-1944 carnegie inst., r e s . i>epl. t e r r . \lagn.. v o i . y 1 i a e a i 1 b . 1 1 2 2 e 5 2 0 ( 1 9 1 7 ) . s i tratta ili una raccolta completa dei (lati magnetici osservali a w a t h e r o o , australia o c c i d . . dal 1919 al 1944. preceduta da una descrizione dell'osservai i k i k n h o n i iorio, ili i programma generale (li ricerl'iie. degli strumenti in esso adoperali e della relativa teoria, delle loro eostanti e tecnica o p e r a t i v a : il tutto corredato con nitide e indie figure. il testo, conipo-lo per la massima parte di tabelle, comprende tutti i valori orari della declinazione. componente orizzontale e componente verticale, le medie mensili, annuali, e c c . . ed è corredalo dai grafici delle principali tempeste magnetiche. •vi traila in definitiva di u n ' o p e r a c h e indubbiamente costituirà la base per m o l l e ricerche sul magnetismo terrestre, ed in particolare sulle n e variazioni. i c . 17.1 l'ovvia.i. c. !•'. e or.cniai.im g . p . s . : x liticar pliysics in pholographs oxford at clarendon press (1947). in questo l i b r o gli a a . presentano una serie di bellissime fotografie di lraccie di particelle elementari, ottenute con il metodo della emulsione fotografica, clic lui portalo 1111 contributo tanto considerevole nelle più recenti ricerche di fisica -nucleare. ii testo è costituito da o r a limpida ed elegante esposizione delle attuali cognizioni nel campo della fisica atomica e nucleare. t a l e esposizione è illustrala dalle l'olografie delle quali costituisce a sua volta un commento. inoltre è corredalo da una breve trattazione dei principi fondamentali della tecnica delle emulsioni fotografiche. la semplicità e la efficienza di questo metodo, la «ali affermazione è sempre in aumento, rendono questo l i b r o particolarmente utile per tulli q u e l l i « he si dedicano a r i c e r c h e sulla radioattività della t e r r a . ( c . f.) c o i . t m a n n ^ . \ v . e m a i i s h a i . ì . f . m . : photomiilliplier radiatimi detector nucleonics. 1. 58-64 11917). h traila di un nuovo tipo di rivelatore di particelle e di quanti di radiazione che ha un campo di applicazione analogo a quello del conlatore di geiger-miilier e presenta alcuni vantaggi, lutt'altro che trascurabili, nei confronti di q u e s t ' u l t i m o . con questo dispositivo si possono registrare le particelle « |'s i n e u t r o n i , i raggi \ e v. esso sfrutta il fenomeno ('(die scintillazioni su cui si fondava il metodo adottalo da lord r u l h e r f o r d nelle prime ricerche sulla radioattività. tale metodo, molto faticoso, serv iv a solo per la rivelazione delle particelle <*•• nel rivelatore a fotonioltiplicazione la sensibilità del metodo è accresciuta di parecchi ordini di grandezza, in maniera da consentire la rivelazione dei vari lipi di radiazione poc'anzi citali. l ' a p p a r e c c h i o è costituito da un comune f o t o m o l t i p l i c a t o r e , uno schermo fluorescente ed un opportuno sistema ottico. l e schermo fluorescente, colpito dalla radiazione in esame emette un quanto che viene localizzalo su un catodo fotosensibile. p o i c h é l ' e n e r g i a del quanto è piuttosto considerevole, esso provoca l ' e m i s s i o n e di 1111 numero rilevante di foloeleltroili. i quali vengono poi moltiplicali -11 nove elettrodi ad emissione secondaria, di modo che all'uscita del tubo compaiono circa io" eletlroni come risultato di 1111 singolo l'ol o e l e t t i o n e . conseguenleniciile 1 impulso di uscita può c s e r e inviato direttamente all'oscilloscopio e al sistema di conleggio. 11 potere risolutivo dello strumento è di circa i o 8 sec. ( c . /'.) twiai.a llv. : electric enrrent as a probable cause oj dolly magnetic vitriations t e r r . m a g n . and a l n i . e l e c t . . 52, 2. 189-200 (19171. l ' a . ha ricavato delle f o r m u l e semplici che consentono il calcolo ti elle correnti elettriche esterne ed i n t e r n e he sono la causa della variazione magnetica diurna. questi sistemi di correlili so656 r e c e n s i o n i no in lnion accorilo con quelli trovali per via puramente matematica da barici-;. \ iene dimostrato che il metodo semplice qui proposto può essere applicalo senza notevoli e r r o r i , l.a conclusione è che l e correnti elettriche sono p r o b a b i l m e n t e la causa della variazione magnetica diurna circolare nello strato e o nelle sue adiacenze. le correnti (indotte) interne dovrebbero invece circolare a profondità fra 600 e 800 k m . l a nota qui riportata è mi estratto, limitato solo ad alcuni argomenti fisici con l ' o m i s s i o n e degli sviluppi matematici dettagliati e dei dati n u m e r i c i , di altro lavoro letto nel 1941 all'università di h i r o s h i m a e pubblicato, pure parzialmente. n e l j o u r n a l of m e l e o r . soc. (sez. i i . 20. 353-369, 1 9 1 2 : e 21, 171178. 1913). ( c . m.) gutenhehi; b . : micrnseisms and wealher lorecastiny: t h e j o u r n a l of meteorology. i v . 1. 21-28 (1947). 1 a. richiama i principali lavori attorno a l l ' a r g o m e n t o , mettendo in rilievo i l progressivo contributo che essi hanno portato alla conoscenza della natura dei m i c r o s i s m i ; molte ricerche partono dalla intima relazione esistente fra perturbamenti meteorologici ed agitazione microsismica, a cominciare dai primi tentativi ili i.inke (1909) tino ai più recenti lavori di t r o m m s d o r f l ' (19391. e kaniirez (19-101 e finalmente quelli condotti dal m.s. navy depurimeli! . i microsismi sono costituiti da o r n i ' elastiche superficiali più o meno regolari di periodo variabile fino a l()s c a . : essi possono propagarsi fino a grandi distanze, perché la loro energia si p i o paga entro uno strato dello spessore di una lunghezza d'onda infiliti l e onde superficiali hanno ampiezze che decrescono essenzialmente con la pro-.1(1 ìi l'ondila (hi secondo il fattore • >. dove a ;; u n a costante dipendente dal!.: natura del sottosuolo e l ' a . ha t r o v a i " per essa il valore di 0,3 ca. naturalmente la distanza di propagazione ib-1l ' e n e r g i a microsismica viene f o r l c m e n l e inflenzala dalle discontinuità geologiche. la geologia locale superficiale e profonda della stazione di osservazione influisce notevolmente nell'ampiezza dei m i c r o s i s m i . i m i c r o s i s m i sarebbero dunque onil.superficiali, ma è noto che esistono due lini principali di onde a cai-alter superficiale. le cosidette onde di l o v e , in cui le particeli)? si muovono tangenzialmente alla superficie terrestre e in direzione ortogonale alla direzione di propagazione e quelle di r a y l e i g h . • il cui le particelle hanno traiettorie ellittiche con l'asse maggiore verticale e il minore nella direzione di propagazioni : i ricercatori elle hanno indagato sulla natura dei microsismi concordano nel ritenere clic in es-i prevalgono le orni, del secondo t i p o . le misure fatte su registrazioni microsi siniche ottenute nella b a i a di guantanamo mostrano, per es., che la componente est-ovest prevale su quella nordsud in maniera molto sensibile nel caso di onde aventi una provenienza da est e ciò conferma evidentemente una prevalenza del lipo r a y l e i g h nelle onde costituenti i m i c r o s i s m i . altre caratteristiche generalmente riscontrale da diversi r i c e r c a t o r i , sono liseguenti: il periodo aumenta in genere con l ' a u m e n t a r e della distanza ili prov e n i e n z a ; il carattere delle onde diviene più regolare man mano d i e la distanza cresce, l'ampiezza tende ad aumentare con il periodo, e c c . la vera causa dei microsismi non è ancora ben n o t a ; è peraltro mollo probabile che la loro energia provenga da 656 recensioni quella delle perturbazioni temporales c h e ; è stato però ripetutamente osservalo che i microsismi decrescono generalmente (piando le burrasche atmosferiche passano dal mare al c o n t i n e n t e ; e si deve tener conto anche del fatlo c h e in genere esse divengono meno violente -ul continente. r e s l a da spiegare il fatto che le tempeste c h e avvengono sopra certe zone dell'oceano producono ampi microsismi in alcune stazioni di osservazioni, mentre le tempeste in zone a minore disianza non sono accompagnate da un aumento dell'agitazione m i crosismica. t u t t o ciò lascia pensare che la sorgente r e a l e dei microsismi è in relazione con le burrasche n e l l ' o c e a n o e, con molta p r o b a b i l i t à , dell'alto oceano. l ' a . ricorda anche le ipotesi di alcuni ricercatori clic pongono la probabile origine dei m i c r o s i s m i n e l l e oscillazioni b a r o m e t r i c h e , o dei scuotimenti provocali dallo infrangersi delle onde del mare sulle coste ripide e r o c c i o s e . e -e quest'ultime potrebbero spiegare una energia sufficiente a generare onde del tipo di quelle osservate nell'agitazione microsisinica è pur vero che frequentemente si osservano microsismi di notevole entità pur avendosi nelle coste \ icine un mare c a l m i s s i m o . e" probabile che le alle onde oceaniche che si generano durante gli uragani siano la causa dei microsismi, pur non essendo mollo chiaro i l meccanismo con cui la energia si propaga fino al fondo d e l l ' o c e a n o . e ' quindi possibile dedurre dalle caratteristiche di una agitazione microsismica elementi preziosi per la previsione meteorologica, specialmente per l'individuazione dei centri di violente per1 urbazioni atmosferiche. l ' a . richiama il metodo della stazione tripartita usalo da g i h n o r e per la determinazione dell'azimut del centro di provenienza della perturbazione microsismica, che coincide con la posizione della zona tempestosa. e g l i richiama le condizioni teoriche a cui debbono soddisfare la distanza base dalla stazione e la velocità di scorrimento del registratore per ottenere un prefissalo grado di approssimazione n e l l a determinazione dell'azimut. d a l l e osservazioni e dalla teoria risulta che l e migliori condizioni si dovrebbero ottenere con un m o v i m e n t o di 2mm/sec e con una distanza di ca. 1.5 k m . i l vantaggio che offrono i microsismi nella previsione è che es-i possono fornire il punto origine o la direzione di avanzamento di un centro ciclonico u n o o due giorni prima di quanto non sarebb e consentilo dagli attuali mezzi a servizio della me te or ologia. (m. g.) p r o f . pietro cai.oi responsabile istituto grafico t i b e r i n o v i a gaeta, 14 r o m a (officine grafiche, t i v o l i ) layout 6 1 annals of geophysics, 64, 5, vo551, 2021; doi:10.4401/ag-8647 o p e n acc e s s the millstone trade from the most exploited italian volcanic areas: an overview from the phoenicians to the roman period patrizia santi*,1, timmy gambin2, alberto renzulli1 (1) dipartimento di scienze pure e applicate, campus scientifico “enrico mattei”, università degli studi di urbino carlo bo, urbino, italy (2) department of classics and archaeology, university of malta, malta article history: received march 1, 2021; accepted september 30, 2021 abstract lavas were widely used in antiquity to produce millstones. this is mainly due to their superior properties for grinding cereals and availability when compared with other rock-types. in the past four decades, several studies have been published about lava millstones discovered in subaerial and submarine archaeological sites of the central-western mediterranean. although the morphological evidence of old quarries is rarely present, all these studies were aimed at recognizing provenance and manufacturing areas of the volcanic raw material. typologies of grinding tools coexisted in different periods, even if some technological developments marked transitions between cultures. the main chronology is: archaic saddle quern, greek hopper-rubber (olynthian), small to medium size rotary device (morgantina type) and large hourglass rotary millstone (pompeian style). potential volcanic sources are widespread throughout the entire mediterranean region, but two main italian quarrying areas of volcanic rocks for the manufacture of millstones from the phoenician to the roman period were pointed out. these are the latium-umbria border in central italy, and sicily (eastern sicily and sicilian channel) in southern italy. in detail, analysis of the lava lithotypes shows that grinding tools were mainly constructed of: (i) a leucite phonolite of the so called “orvieto quarries” between the localities of sugano and buonviaggio in the roman volcanic province (highk alkaline series); (ii) hawaiites and mugearites (na-alkaline series) from etna volcano; (iii) basalts (tholeiitic/transitional series) of the hyblaean mountains and (iv) basalts (na-alkaline series) from pantelleria island (sicilian channel). although some lava millstones from other volcanic regions are recorded, the above four italian volcanic rock types represent the most exploited in antiquity. a comparison between volcanic millstones and outcropping lavas already exists, from literature data, through thin section modal mineralogy and conventional igneous petrology (i.e., tas classification, magmatic affinities, and major-trace elements signature). therefore, on this basis we propose a set of discriminating geochemical parameters (major-trace elements and element ratios diagrams) useful for a quick assessment tool to possibly evaluate one of these four exploited volcanic areas of italy matching millstones. a sketch of volcanic millstone trade networks and commercial routes in antiquity throughout the central-western mediterranean has been also reported and overviewed on the basis of the literature data. keywords: millstones; lavas; quarries; roman magmatic province; etna; hyblaean mountains; pantelleria island; central-western mediterranean. 1. introduction lavas were one of the most sought-after materials to produce querns and millstones for the grinding of cereals in ancient times [curtis, 2001; hockensmith ,2009; williams and peacock, 2011; alonso and frankel, 2017]. either mafic or felsic, lava lithologies generally ensured good wear resistance and grinding capacity also when compared to sandstones and quartzites that in fact resulted in a more sporadic use. lava flows with a welldeveloped vesiculation were preferentially quarried because of the good workability and a relatively lower density that facilitated the transport of finished products. however, the accessibility of lava outcrops, the reserves (availability and real chance of exploitation), the processing and transport methods induced to choose different types of rock materials. finally, it is necessary to take into account the limitations of access to certain regions dictated by the existing political, economic and/or military conditions [griffiths 2000]. several archaeometric studies of numerous millstones from a long chronological period and from a variety of archaeological sites in the central-western mediterranean, including shipwrecks, have been published in the last four decades [peacock, 1980, 1986, 1989; ferla et al., 1984; williams-thorpe, 1988; williams-thorpe and thorpe, 1988, 1990, 1991, 1993; renzulli et al., 2002, 2019; santi et al., 2004, 2013, 2015, 2020; antonelli et al., 2004, 2010, 2014; santi and renzulli, 2006; gluhak and schwall, 2015; di bella et al., 2018 ]. these studies allowed to unravel the millstone typologies preferentially used through time and to establish the exploited lava lithotypes, also leading to a better understanding of the trade routes throughout the mediterranean. there is a useful petrographic and geochemical database for materials employed in the production of lava millstones from the archaic period to the roman empire. the technological evolution of the grinding stones has been linked, starting from the archaic period, to an increasing demand for flour, necessary to meet the bread-making needs of the growing population in the mediterranean area. the demographic increase stimulated the development of more and more efficient grinding devices. the technological evolution was as follows (figure 1): archaic saddle querns, greek hopper-rubbers (olynthian) and small or medium sized rotary devices (morgantina type) and large roman hourglass rotary millstones (pompeian style). although reserves of lavas are widespread throughout the entire mediterranean region, decades of researches on provenance of lava millstones point to quarries (or provenance areas if quarries were not found) located in two main italian districts: the latium-umbria border and sicily (eastern sicily and sicilian channel) in central and southern italy, respectively [peacock, 1986, 1989; williams-thorpe, 1988; williams-thorpe and thorpe, 1993; renzulli et al., 2002, 2019; santi et al., 2004, 2013, 2015, 2020; antonelli et al., 2010]. in particular, the literature well established that the most exploited lavas to manufacture millstones in antiquity were: (i) a leucite phonolite from the vulsini volcanic district in central italy, (ii) hawaiites and mugearites from etna, and basalts from (iii) hyblaean mountains and (iv) pantelleria (sicily and sicilian channel). in response to these emerging patterns, here we present an efficient assessment tool to locate lava millstone source areas in the form of geochemical diagrams for each of the four aforementioned volcanic lithotypes. these are based on the data sets compatible, mineralogically and petrographically, with most of the lava millstone lithotypes found in central western mediterranean: basaltic rocks of the na-alkaline and tholeiitic-transitional series and leucite phonolites of the high-k alkaline series. plotting lava millstone geochemical parameters in the proposed diagrams, has to be however complemented by verifying compatibility through thin section modal mineralogy, total alkali-silica (tas) classification and petrogenetic signature (i.e., magmatic series). besides the most exploited areas of latium-umbria border, eastern sicily and sicilian channel, additional minor source areas for volcanic millstone production in antiquity were however recognized in some trachyte lavas from the euganean hills (venetian magmatic province [renzulli et al., 2002; santi and renzulli, 2006; antonelli et al., 2004], a dacite to rhyolite welded ignimbrite from the mulargia area (sardinia) [antonelli et al., 2014], and lavas from vesuvius, aeolian archipelago and ustica island [williams-thorpe, 1988; williams-thorpe and thorpe, 1993; renzulli et al., 2002, 2019; antonelli et al., 2010; santi et al., 2013, 2020; di bella et al., 2018]. although lava millstone provenance from volcanic areas in the eastern mediterranean (greece, lebanon, syria, israel and jordan) is beyond the scope of the present work, a brief discussion on these potential volcanic rock sources will be also given. patrizia santi et al. 2 2. evolution of the grinding techniques millstones, together with the wheel (in europe from 3000 bc), represent the first mechanical tools of widespread domestic use [chartrain, 2015]. the utilisation of millstones in ancient times is usually connected to the grinding of cereals, although they were also employed for the processing of various fruits and olives [moritz, 1958]. the appearance of the most ancient grinding techniques occurred alongside increased demand for cereals which needed be transformed into flour for consumption [santi, 2020]. the oldest millstones date back to the late paleolithic [elliott et al., 1986] and are referred to as saddle querns mainly due to the shape acquired by the lower part which is re-shaped by the grinding process [moritz, 1958]. saddle querns were in use simultaneously in numerous mediterranean cultures including that of ancient egypt [moritz, 1958]. in essence, flour was produced by crushing the grains manually between two rock elements (fig. 2a). consisting of two parts, these millstones have undergone a change in their shape and size over time [curtis, 2001]: the base became wider and flatter, while the upper mobile part, elliptical in shape, became thicker whilst developing two lateral grips in order to make grinding more effective. after thousands of years of exclusive use of saddle querns, a series of new devices for mills appeared and developed in the mediterranean during the second half of the first millenium bc [alonso and frankel, 2017, and references therein]. starting from the 5th century bc [white, 1963] the use of rectangular hopper-rubbers, also known as olynthian type, spread widely throughout the mediterranean [williams-thorpe and thorpe, 1993; frankel, 2003]. defined by 3 millstone trade from italian volcanic areas figure 1. different millstones type used in antiquity: a) saddle quern (lipari archaeological museum, aeolian islands); b) hopper-rubber millstone (morgantina archaeological site, sicily); c) small rotary hand-mill (lipari museum archaeological, aeolian islands); d) morgantina type rotary millstone (morgantina archaeological site, sicily); e) pompeian type millstone (ostia antica archaeological site, rome). patrizia santi et al. 4 cato as molae trusatiles [de agri cultura dating to ca. 160 bc; moritz, 1958], these too consisted of two parts, a flat base on which a smaller stone (hopper to load the cereal) was placed, with a slit in the central part. the grinding took place through the oscillation of a lever fixed on the hopper (figure 2b). although their origin is still debated, since they may represent a technological offshoot of saddle querns, the most credible hypothesis is a greek and/or anatolian origin [frankel, 2003; curtis, 2001]. the use of hopper millstones represents a significant development in the production of flour as it evolves from the domestic use of saddle millstones to a more mechanized and productive system. this is also historically documented by the decoration of the homeric cup or megarian bowl found in thebes (4th-3rd century bc) [rostovtzeff, 1937] where hopper millstones are represented within a scene of flour production in a commercial center context (figure 3). figure 2. evolutionary sketch of chronological and areal distribution of the grinding techniques in antiquity [santi, 2020 modified]. a) saddle quern; b) hopper-rubber millstone; c) small rotary hand-mill; d) morgantina type rotary millstone; e) pompeian type millstone. the grey arrows roughly indicate the time interval of use (vertical) starting from the millstone images located according to the inferred area where the grinding device was invented in the mediterranean. since the 6th century bc, simple rotary millstones that are spanish in origin, begin to be widely used. cato, writing in the second century bc, defines them as molae ispaniensis [cato, de agri cultura; childe, 1943]. nevertheless, according to chartrain [2015] it could be reasonably assumed the existence of simultaneous centers of diffusion and a polygenesis of the invention of rotary millstones. these small rotary hand-mills consisted of a fixed base with a slightly convex shape and a slightly concave rotating upper part. the latter had a hole in the center to feed the cereals into the mechanism (figure 2c). the rotation was performed through a wooden handle inserted in the upper part. although still a manual process, this grinding technique proved to be very popular as it guaranteed greater efficiency in the production of flour. produced in varying sizes [py, 1992], the most widespread average diameter is ca. 34 cm [williams-thorpe, 1988]. the smaller variants became tools for daily use in domestic contexts and were also used on ships for the production of meals by the crew and passengers. this is attested by the numerous discoveries of such objects on shipwrecks from the greek and roman periods [williams-thorpe and thorpe, 1990]. these tools were also widely transported and utilised by the roman legions during the military campaigns in northern europe [junkelmann, 1997]. in the meantime, in numerous phoenician-punic sites such as motya, morgantina and carthage, a new type of manual rotary mill appears [wefers, 2011]. this consists of a fixed base (meta) and an upper part (catillus) characterized by two attachments (holes diametrically opposite) for the insertion of poles useful for its manual rotation (figure 2d). these millstones are known as the morgantina type [white, 1963], named for the archaeological site near enna (sicily) where they were found in large numbers. this typology has an average height of approximately 65-70 cm and they reached their maximum diffusion in the 4th-3rd century bc [white, 1963]. the vast distribution of morgantina type millstones can be linked to the period of peace that characterized sicily under the reign of hiero ii of syracuse, who favored the procurement of materials from the catania plain [sjöqvist, 1960; bell, 2011] and also to the lex ieronica [270 216 bc], which replaced the fixed tax contribution with a tithe. taxes were thus commensurate with income and agricultural production. in turn, this prompted an increase in the cultivation of wheat, the construction of numerous public granaries and the need for grinding facilities. it should be emphasized that compared to the saddle querns, the efficiency of the hopper variants was sixfold while for the morgantina type rotary millstones efficiency was between six to twelve times greater [holodňák, 2001]. this would account for their mediterranean-wide distribution as attested by the cargo of a wreck from the 4th century bc discovered off the island of mallorca (spain) where no less than 38 hopper millstones, 16 catilli and 22 metae of morgantina type millstones have been recorded [williams-thorpe and thorpe, 1990]. the morgantina type millstone can be considered, for all intents and purposes, as the prototype from which the most imposing hourglass millstones known as the pompeian (figure 2e) type developed. with an average height of ca. 150 cm, this new millstone type represented a real revolution in the technique of grinding, passing from hand-operated rotary tools to grinders 5 millstone trade from italian volcanic areas figure 3. graphic reproduction of the megarian blow discovered at tebe (egypt) and now held in the museum of louvre [rostovtzeff, 1937]. powered by animals [white, 1963]. writing in the 1st century bc and citing varro, pliny the elder [naturalis historiae; 77-78 ad] defines them as molae asinarie or molae versatiles and indicates the area of volsinii (now orvieto if considering volsinii veteres or bolsena if considering volsinii novi) as the place where they were invented and produced. these hourglass millstones consist of a fully truncated conical part (meta), fixed to a base and an upper part that resembles a hollow hourglass (catillus). the latter constituted the reversible part (molae versatiles) in order to extend usage for as long as possible. large-scale distribution of this typology begins in the 2nd-1st centuries bc [mayesche, 1988; peacock, 1989; curtis, 2001; mccallum, 2010]. this happens in parallel with the development of public structures dedicated to grinding and for the production of bread as found in places such as ostia antica, herculaneum and pompeii. during the imperial period, pompeian type millstones spread throughout the mediterranean through a welldefined commercial network [antonelli et al., 2001; renzulli et al., 2002; santi et al., 2004; antonelli and lazzarini, 2010]. once again, this is attested by the numerous millstones discovered in many roman wrecks dating mainly to the 2nd century bc to the 3rd century ad [parker, 1992; di bella et al., 2016]. given the high number of finds identified, it is possible to illustrate the technological evolution of grinding in time sequence (fig. 2) roughly highlighting the geographical distribution (east to west mediterranean). the simultaneous use of different grinding techniques also meets the varying needs for the production of flour through time and also testify that the invention of some new millstone device did not sharply eliminate usage of older grinding tools. 3. lava millstones in the central-western mediterranean and volcanic source areas this chapter provides an overview of available literature on millstones made from the most exploited italian volcanic regions. it encompasses subaerial archaeological sites of the central-western mediterranean, as well as millstone cargoes from shipwrecks, and examines a broad chronological period from the archaic to the roman period. the main methodological approach used in the studies of lava millstones provenance is to compare the petrographic and geochemical properties of manufacts against those of the volcanic areas where the materials may have been sourced to assess links between the two. it is worth to be noted that the geomorphological evidence of old quarries has been often erased by time (erosion and/or anthropization). the total alkali silica (tas) classification diagram of volcanic rocks [le bas et al., 1986] and the assessment of a specific magma series (tholeiitic/transitional, calcalkaline, na or k-alkaline, silica-undersaturated high-k alkaline) [kuno 1968; irvine and baragar, 1971] represent the first step. patterns of incompatible trace elements and rare earth elements (rees) of lava millstone of basaltic compositions, normalized against primordial mantle and chondrites respectively [sun and mcdonough, 1989] are additional useful tools to shed light on the potential volcanic source areas, at least for not differentiated (mafic) lithotypes. this is because distinct trace element patterns of basaltic rocks are closely associated to different geodynamic settings: subduction-related oceanic arc or active continental margin volcanoes, middle oceanic ridge basalts (morb), oceanic intraplate basalts (oib) or continental flood basalts (cfb) environments [rollinson, 1993; winter, 2010]. tas classification, magma series and major trace elements abundances therefore allow the early potential matching between lava millstones and volcanic source areas from subduction-related vs. anorogenic oib magmatic regions [lustrino and wilson, 2007] for the central-western mediterranean volcanism. as a volcano or a volcanic area of provenance was established, a millstone trade network can be thus outlined for a specific time interval, this latter being mainly established from the millstone device (cf. chapter 2 and figure 2). although italian volcanoes (active, dormant, or extinct) represent the best candidates for deducing the provenance of lava millstones [santi and renzulli, 2006] it was deemed necessary to also conduct comparisons with potential volcanic rocks mainly in greece (subduction related lavas) and the whole eastern mediterranean (oib sources). in fact, in the levant area, lavas widely used in the ancient lithic industry and for the manufacture of millstones, include outcrops in the lake tiberias region (israel), the golan heights, the jordan plateau and northern syria (see 3.2). the literature data emphasize four main areas of extraction and production of millstones in the central western mediterranean: 1) the leucite phonolite lava from the quarries near orvieto (between the localities of sugano and patrizia santi et al. 6 buonviaggio) in the vulsini volcanic district close to the northern latium-umbria border (roman magmatic province) [peacock, 1980, 1986, 1989; ferla et al., 1984; lorenzoni et al., 1996, 2000; daniele, 1997; buffone et al., 1999, 2003; oliva et al., 1999; antonelli et al., 2001, 2005; renzulli et al., 2002; renzulli and santi, 2005; santi and renzulli, 2006; santi et al. 2004; antonelli and lazzarini 2010]; 2) hawaiites and mugearites from etna volcano [williams-thorpe 1988; renzulli et al., 2002; santi and renzulli, 2006; antonelli and lazzarini, 2010; santi et al., 2013, 2015, 2020]; 3) tholeiitic/transitional basalts from hyblaean mountains [renzulli et al., 2002; santi et al., 2013, 2015, 2020] and 4) high-tio2 na-alkaline basalts from pantelleria island [williams-thorpe, 1988; williamsthorpe and thorpe, 1990; renzulli et al., 2019; santi et al., 2020]. 3.1 leucite phonolite from the “orvieto quarries” (vulsini volcanic district, roman magmatic province) according to the above literature, the leucite phonolite lava extracted from quarries near the city of orvieto in the vulsini volcanic district [nappi et al., 1995] represents a very widespread lithotype for grinding stones in the whole central-western mediterranean mainly as hopper-rubbers and rotary pompeian type millstones. washington [1896], in his pioneer petrographic work on the vulsini volcanic district uses a straightforward thin section report to describe an outcrop of vescicular, leucite-phyric phonolite lava with “trachytic” groundmass, located about 3 km sw of orvieto [i.e. between sugano 42°42’38” n 12°03’27” e and buonviaggio 42°42’05” n 12°04’35” e localities]. it is worth to note that no relationships exist with the orvieto town which is built upon a pyroclastic cliff (an ignimbrite) and not a lava flow. nearly a century later, peacock [1980, 1986] was the first to indicate this volcanic rock as the most similar in composition and texture to the materials used to make the rotary millstones found in the sites of pompeii and ostia antica, suggesting a possible single area of origin located in this part of central italy (the ancient etruria). the old quarries between sugano and buonviaggio can be still recognizable for morphological features and the presence of some unfinished metae and catilli of rotary millstones were also detected [peacock 1986]. this lava is unequivocally characterized using modal mineralogy and major-trace elements [antonelli et al., 2001; santi et al., 2004]. on a macroscopic scale it is leucite-phyric (phenocrysts up to 15 mm in size) with a grey to pale-grey colour. in thin section it is represented by a highly vesciculated porphyritic lava (pi=25-30 vol.%) with euhedral leucite phenocrysts and a microcrystalline, pilotassitic groundmass mainly consisting of sanidine and leucite microlites with subordinate plagioclase and clinopyroxene. phenocrysts and microphenocrysts of clinopyroxene, plagioclase and sanidine are also present. accessory phases are represented by opaque minerals and apatite [santi et al., 2004]. geochemical features of this leucite phonolite lava from the “orvieto quarries” (k2o/na2o between 2.8 and 4.1) are unique with respect to all the other leucite phonolites from the roman volcanic province (i.e., vico and sabatini complexes and vesuvius; fig. 4a, b) [santi et al. 2004 and reference therein]. in particular, it is characterized by the highest sr (1823-2026 ppm), la (172-194 ppm), th (150-178 ppm) and ba (2165–2313 ppm) contents [antonelli et al., 2001] ever reported from similar leucite phonolite lavas of the roman volcanic province. millstones made of this lava have been identified at various etruscan-celtic and roman sites such as in monte bibele (bologna), cannetolo di fontanellato (parma), colombarone, sant’angelo in vado, fossombrone, suasa and urbisaglia (marche), ostia antica (rome), pompeii (naples), valle del biferno (molise), monte castellazzo di poggioreale and rocca di entella (sicily), les martys, montagne noire (france) and north africa (libya, tunisia) [peacock, 1980; ferla et al., 1984; williams-thorpe, 1988; williams-thorpe and thorpe, 1993; daniele, 1997; oliva et al., 1999; antonelli et al., 2001, 2005; renzulli et al., 2002; buffone et al., 1999, 2003; santi et al., 2004; renzulli and santi, 2005; santi and renzulli, 2006]. the discovery of an etruscan-celtic hopper-rubber millstone in the archaeological site of monte bibele, tuscany-emilia romagna apennines [renzulli et al., 2002] provides evidence of the exploitation of the “orvieto quarries” starting as early as the 4th century bc. the vast diffusion of millstones made with this leucite phonolite lava is undoubtedly due to the good physical characteristics of this lithotype and also to the strategic position of the quarries with regards to the commercial distribution networks. in the context of connectivity, it is imperative to highlight that the areas of exploitation, between the localities of sugano and buonviaggio were located about 10 km w-nw from the confluence of the tiber and paglia rivers. at this point, the river port of pagliano [morelli, 1957; bruschetti, 2004] was an important crossroads for most of the trade in central italy between the first half of the 1st century bc and the first century ad [morelli, 1957]. this river port represented the main trading hub along the tiber river itself [antonelli et al., 2001]. from here the millstones reached rome and, 7 millstone trade from italian volcanic areas more importantly, ostia antica, from where they could easily be transhipped for redistribution throughout the mediterranean. in the mill building (bakery) of ostia antica [2nd century bc 3rd century ad] some of best examples of roman pompeian hourglass rotary millstones have been identified. these were all manufactured with the leucite phonolite lava from the “orvieto quarries”. at ostia antica the entire wheat processing production cycle, from milling to making the dough, to baking and selling the bread (panis fiscalis ostiensis) took place [santi et al., 2004]. well preserved hourglass millstones of leucite phonolite from the “orvieto quarries” were also found at pompeii (hence the name pompeian) and account for the 60% of the total grinding stones in this archaeological site, the remaining 40% originating from vesuvius volcanic rocks and very few from etna [buffone et al., 1999, 2003]. 3.2 basaltic lavas from eastern sicily, sicilian channel and levant region from the 7th century bc to the roman period, the most exploited basaltic lavas for the production of millstones (saddle querns, hopper-rubber and rotary millstones found in the central-western mediterranean) were those located in eastern sicily (etna volcano and the hyblaean mountains) and the sicilian channel (pantelleria island) [williamsthorpe,1988; williams-thorpe and thorpe, 1990, 1991]. hawaiites and mugearites from etna, alkaline basalts from pantelleria and tholeiitic-transitional basalts from the hyblaean mountains were recognized as providing material for millstones found in numerous archaeological sites and shipwreck cargoes in the central-western mediterranean, patrizia santi et al. 8 figure 4. th vs sr (a) and ba vs sr (b) diagrams for the leucite phonolite lavas of the roman volcanic province [peccerillo, 2005] including those of the “orvieto quarries” [antonelli et al. 2001]. 9 millstone trade from italian volcanic areas dated from the archaic to the roman period [williams-thorpe, 1988; williams-thorpe and thorpe, 1990, 1991; santi et al., 2013, 2015, 2020; di bella et al., 2016; renzulli et al., 2019]. in particular, the presence of basaltic millstones coming from etna, hyblaean mountains and pantelleria was discovered, together or separately, in the sites of motya and entella [daniele, 1997], altamura [lorenzoni et al., 1996, 2000], fossombrone [renzulli et al., 2002], pompei [buffone et al., 1999, 2003], cyrenaica (libya) [antonelli et al., 2005], aeolian islands, morgantina, and ustica island [santi and renzulli, 2006; santi et al., 2013, 2015, 2020]. concerning the basaltic millstones found in shipwreaks most of them come from pantelleria, such as the hopper-rubber ones discovered at el sec (maiorca) dating to the 4th century bc [williams-thorpe and thorpe, 1990] and the flat archaic querns from the phoenician cargo in xlendi bay (gozo, maltese islands) dated to the 7th century bc [renzulli et al., 2019]. in addition, hopper-rubber millstones made of the same basaltic lavas from pantelleria, were also discovered in the archaic levels (7th and 6th century bc) of the phoenician colony of gadir (i.e., cádiz, spain) in the atlantic ocean [renzulli et al., 2019]. this confirms the widespread maritime network from east to west, just crossing the strait of gibraltar. according to the huge references cited above dealing with basaltic millstones, similar macroscopic features, petrographic and textural characters can be observed: dark-grey to black, moderately to highly-vesciculated lavas with phenocrysts of plagioclase, clinopyroxene and olivine with a microcrystalline groundmass. some samples could also contain orthopyroxenes and acicular prismatic ilmenite (tholeiitic-transitional series), whereas equidimensional ti-magnetite is ubiquitous. in particular, it is well established that they may belong to basaltic lavas of the na-alkaline (etna and pantelleria) and the tholeiitic/transitional (hyblaean mountains) magma series [williams-thorpe 1988; gluhak and schwall, 2015; renzulli et al., 2019; santi et al., 2020]. williams-thorpe [1988] was among the first to emphasize the difficulty to determine provenance of basaltic millstones made all of “grey vescicular rocks, very similar in macroscopic features” coming from some roman archaeological sites in tunisia (utica, carthage, el maklouba, thuburbo maius and kelibia). when dealing with basaltic millstones, the geochemical comparisons should be made with both the italian basaltic sources of eastern sicily and sicilian channel and basaltic rocks (mostly oib) that are present in the historic levant region (middle east). in fact, oib source of syria, lebanon, jordan and israel represent an ancient lithic industry area for the manufacture of millstones including outcrops in the lake tiberias, the golan heights, the jordan plateau and northern syria [xenophontos et al., 1988; williams-thorpe et al., 1991; williams-thorpe and thorpe, 1993]. in this way literature data from the whole levant region [williams-thorpe and thorpe 1990, and reference therein], and from lebanon [stein and hofmann 1992; abdel-rahman 2002; abdel-rahman and nassar 2004], syria [mahfound and beck, 1993; demir et al. 2007; lease and abdel-rahman 2008] and jordan [ibrahim khalil et al., 2003; ibrahim khalil and al-malabeh, 2006] should be also taken into account when comparing geochemical data, mainly when dealing with the millstones of the archaic period as the phoenicians just originated from the levant (the present lebanon). a rigorous approach on the basis of igneous petrology is necessary when comparing basaltic millstone with potential source areas. the tas classification diagram [kuno, 1968; irvine and barager, 1971; le bas et al., 1986] coupled with incompatible trace elements (hfses and liles) and rare earth elements (rees) normalized to primordial mantle and chondrite respectively [sun and mcdonough, 1989] help to discriminate among the lavas of the two oib magma series (na-alkaline and tholeiitic-transitional) of eastern sicily and sicilian channel. in fact, the basaltic lavas of both the na-alkaline (etna and pantelleria) and the tholeiitic/transitional (hyblaean mountains) magma series show similar sub-parallel patterns, typical of intraplate-anorogenic oib [rollinson, 1993] the former showing enrichment in hfses and liles and a more fractionated ree pattern (i.e., higher lrees/hrees and mrees/hrees ratios). nevertheless, it is often insufficient to rely on trace elements patterns of basaltic millstones as it is not always possible to discriminate among oib volcanoes and therefore using geochemical parameters such as binary major-trace elements or trace element ratios represent the most appropriate and robust assessment to match millstones with one of the sicilian oib source area [santi et al., 2020 and reference therein]. among the available literature database of volcanic rocks from sicily [peccerillo, 2005] lavas with sio2 > 57 wt%, as well as those of the strongly silicaundersaturated (i.e., nephelinites and basanites) of the highly na-alkaline series of the hyblaean mountains can be excluded as they are geochemically (tas) inconsistent with the basaltic millstones found in the mediterranean area. the selected geochemical data therefore comprise etna hawaiites and mugearites, basalts of the tholeiitic/transitional series of the hyblaean mountains and na-alkaline basalts from the pantelleria island. as already demonstrated in some of the most recent literature data on basaltic millstones [santi et al., 2013, 2015, 2020; renzulli et al., 2019], etna lavas (hawaiites and mugearites), tholeiitic/transitional basalts of hyblaean mountains and na-alkaline basalts from pantelleria island can be distinguished very well in the proposed discriminating binary diagram la/yb vs sm/yb (figure 5a). this diagram may represent a quick assessment tool for several basaltic millstones with oib signature found in the mediterranean areas. in addition, nb vs tio2 diagram (figure 5b) is also strongly suggested [renzulli et al. 2019] to distinguish (and matching) basaltic millstones coming from the hyblaean mountains from those of the pantelleria island. it is worth to note that the high tio2 content (2.4-4.0 wt%) defines a chemical signature for the basaltic lavas of pantelleria island that allows us to rule out etna and hyblaean mountains raw materials, which are all characterized by a tio2 content ≤ 2.1 wt% [peccerillo, 2005]. 4. discussion and final remarks the interest in studying the provenance of volcanic millstones is mainly linked to the pan-mediterranean distribution of these artefacts through time. the ability to link provenance areas/production centres is of paramount importance to better understand the circulation of volcanic raw materials and artefacts in antiquity. in this way volcanologists and petrologists may strongly contribute to historical and archaeological studies. the scientific approach to obtain matching of millstones to the exploited volcanic deposits consist of using the patrizia santi et al. 10 figure 5. a) la/yb vs sm/yb diagram of selected basaltic lavas from etna, hyblaean mountains and pantelleria island; b) nb vs tio2 diagram of selected basalts from hyblaean mountains and pantelleria island (data from peccerillo [2005]). tio2 values are not water free basis. 11 millstone trade from italian volcanic areas fundamental methodology of igneous petrology: (i) thin section mineralogy and petrography and (ii) major-trace elements data comparisons. as a quick assessment tool to evaluate the most exploited four volcanic areas of italy matching millstones, (“orvieto quarries”, etna, hyblaean mountains and pantelleria island) we propose a set of discriminating geochemical parameters (major-trace elements and element ratios diagrams). most of the compositional literature studies deal with lava millstones from subaerial archaeological sites which may be only considered as “final destinations” for millstones trade. increasing compositional data on the lava millstones from samples recovered in shipwrecked cargoes should therefore help to better define the sea-routes chosen in antiquity, from the exploited deposit to these “final destinations”. the inclusion of shipwreck cargoes in the millstone archaeometric studies is therefore essential because it contributes to a better understanding of interactions between regions and cultures. maritime millstone trade throughout the mediterranean was particularly intensive in the roman period, correlated to the collection of tithe taxes that involved the colonies of sardinia, sicily, north africa and spain. millestones, often well-preserved in subaqueous environment are really witnesses of the shipwrecks, as recently discovered off capo rasocolmo (messina, north-eastern sicily; figure 6) at a depth of 115 meters together with rock ballast pile and lead ingots, dated to the 1st century ad. a comprehensive millstone trade network from the archaic to the roman age, by inland waterways and sea routes extending across the entire mediterranean, is outlined in figure 7, based on the available literature data (santi, 2020 and reference therein). the leucite phonolite lava of the “orvieto quarries” represents the lithotype figure 6. image of the shipwreck cargo discovered in 2011 off capo rasocolmo at 115 meters of depth (messina, northeastern sicily) with several rotary millstones (metae and catilli; see the orange-red fish in the background as a scale). photo by t. gambin, kindly granted by the university of malta and soprintendenza del mare, sicily. with the largest diffusion, as millstones, at the scale of the whole central-western mediterranean and northern italy. this statement is especially true for the time interval from 4th-3rd centuries bc up to the 2nd-3rd century ad (figure 7a). furthermore, the millstones found in the archaeological site of the greek colony of monte castellazzo di poggioreale [ferla et al. 1984] and in the phoenician site of the rocca di entella (western sicily) [daniele, 1997] are both made by working the leucite phonolite lava from the “orvieto quarries”. these recoverings support the existence of commercial exchanges between the etruscan and greek populations. between the 3rd century bc and the early imperial roman age, the basaltic millstones from the main volcanic areas of sicily (hawaiites and mugearites from etna and the tholeiitic/transitional basalts from the hyblaean mountains) are distributed across a local commercial network covering the island itself but also including the central mediterranean and the ionian and adriatic coasts (figures 7b, c). production centres located on the islands, such as that of pantelleria, enjoyed a strategic position in areas constituting an almost obligatory stop during the crossing of the mediterranean [renzulli et al. 2019]. the important role played by the pantelleria island as an area of origin of basaltic millstones in the early phoenician period (7th century bc) is confirmed by the finds from the archaeological site of cadiz (spain) and the shipwreck off the island of gozo (malta). this testifies that there was some form of connectivity that stretched across networks covering 1500 km specifically, from the pantelleria island to the westernmost phoenician colony, beyond the straits of gibraltar (figure 7d). the strategic location of this island permitted a continuous production and trading of artifacts in subsequent centuries. millstone from pantelleria found in the shipwreck cargo (4th century bc) off the island of mallorca (el sec, spain) as well as recovered in carthaginian archaeological sites (sicily and tunisia) and from the roman period (sicily) [williams-thorpe, 1988; williams-thorpe and thorpe, 1990] furtherly support the role of this patrizia santi et al. 12 figure 7. the millstone trade in antiquity in the central-western mediterranean from the four most exploited italian volcanic areas: a) “orvieto quarries”; b) etna; c) hyblaean mountains; d) pantelleria island [from santi 2020, modified]. 13 millstone trade from italian volcanic areas island in maritime trade. it is important to note that during the roman period the four most exploited volcanic areas and production centres of lava millstones in the latium-umbria border, eastern sicily and sicilian channel were contemporaneously exploited. nevertheless, only those located near orvieto provided lava millstones (i.e. leucite phonolite, vulsini volcanic district) which spread throughout the mediterranean. outlining the lava millstone trade in the mediterranean through time is also beneficial to the study of the technological evolution of grinding tools, which help to produce food that is part and parcel of mediterranean culture. acknowledgements. this work was finacially supported by the university of urbino carlo bo (dipartimento di scienze pure e applicate – dispea), research funding project-2017 entilted “lo studio di reperti archeologici lapidei di natura vulcanica: un potente strumento di confronto reciproco per ricerche in campo vulcanologico e archeometrico” (responsible p.santi). monica piochi and two anonymous reviewers are acknowledged for their useful comments and suggestions. references abdel-rahman, a.f.m. 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(2010). principles of igneous and metamorphic petrology, 2nd edition, whitman college, xenophontos, c., c. elliot and j.g. malpas (1988). major and trace-element geochemistry used in tracing the provenance of late bronze age and roman basalts artefacts from cyprus, levant 20, 169–183. *co r r e s po n d i n g a u t h o r: patrizia s a n t i, dipartimento di scienze pure e applicate, campus scientifico “enrico mattei”, università degli studi di urbino carlo bo, urbino, italy, e-mail: patrizia.santi@uniurb.it © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international patrizia santi et al. 16 mailto:patrizia.santi@uniurb.it layout 6 near real-time routine for volcano monitoring using infrared satellite data valerio lombardo*, malvina silvestri, claudia spinetti istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5336 abstract an advanced very-high-resolution radiometer (avhrr) routine for hotspot detection and effusion rate estimation (avhotrr) using avhrr infrared space-borne images is presented here for the monitoring of active lava flow. avhotrr uses directly broadcast national oceanic and atmospheric administration (noaa)-avhrr remotely sensed data. the 2006 summit eruption of mount etna provided the opportunity to test the products generated by avhotrr for monitoring purposes. low spatial and high temporal resolution products can also be used as inputs of flow models to drive numerical simulations of lava-flow paths and thus to provide quantitative hazard assessment and volcanic risk mitigation. 1. introduction over the last twenty years, infrared (ir) remote sensing techniques for extracting volcanologically useful information have been used to study thermal structures of hot volcanic features, such as lava flows, lava lakes, volcanic domes, pyroclastic flows and fumaroles [rothery et al. 1988, crisp and baloga 1990b, harris et al. 1999, wright et al. 2000]. on the one hand, detection of new thermal anomalies, or of changes in existing ones, can be of particular value for hazard evaluation. the activity during an effusive volcanic eruption can change rapidly over periods of minutes to hours as new lava flows erupt or develop. the destructive potential of these phenomena makes rapid information regarding their occurrence and development essential for hazard monitoring, assessment and response. thus satellite-based radiometers that provide thermal data at high temporal resolution (>1 image per day) are suitable for the monitoring of effusive eruptions if analyzed timely. these instruments provide data in which effusive events are easily detectable and where changes in the style and extent of the activity can be identified despite the low (1-25 km2 pixel) spatial resolution of the data [e.g. harris et al. 1997a, harris et al. 1997b, wooster and rothery 1997]. furthermore, these data can be acquired directly from suitable receiving stations, and then processed within minutes of reception. such near real-time information can be provided to monitoring agencies for volcanic hazard assessment. on the other hand, thermal measurements can identify anomalies that are potentially related to the physical processes that trigger eruptions. remotely sensed data have been used to estimate heat and mass fluxes of active lava flows. the movement of lava flows is a complex subject that has provoked debate regarding the mechanisms that control the areal extent and physical character of the flow [e.g. pieri and baloga 1986, lipman and banks 1987, oppenheimer 1991, oppenheimer 1993, oppenheimer et al. 1993a, oppenheimer et al. 1993b, harris et al. 1997a, harris et al. 1997b, harris et al. 1998, harris et al. 1999, lombardo and buongiorno 2006, lombardo et al. 2006, lombardo et al. 2009]. progress has been made in the construction of useful mathematical models [e.g. crisp and baloga 1990a, pieri et al. 1990, oppenheimer 1993, harris et al. 1997a, harris et al. 1997b, harris et al. 1998, harris et al. 1999, lombardo et al. 2009]. the determination of volumetric effusion rates for lava flows from space is an important, but challenging, task. effusion rates are a major consideration in the evaluation of flow dynamics and of the potential threat posed by a lava flow. higher effusion rates produce channelfed flows that are longer, more rapidly moving, more voluminous, and more extensive than flows with low effusion rates [walker 1973, wadge 1977, pieri and baloga 1986]. flows with high effusion rates thus have far greater potential to inflict damage on distant communities with less advance warning. the measurement of effusion rates is therefore of great interest. field methods for their determination are usually based on estimates of the lavachannel dimensions and the lava-flow velocity [e.g. lipman and bancks 1987, barberi et al. 1993]. however, errors due to uncertainties in channel dimensions are a major problem, especially for lava depth. alternatively, if the eruption duration is known, accurate post-eruption measurements of total lava flow-field volumes can provide reliable estimates of average effusion rates [e.g. calvari et al. 1994]. however, these averages will not reveal the major variations in the effusion rates that have been shown to occur during many basaltic eruptions [wadge 1981]. article history received november 12, 2010; accepted july 26, 2011. subject classification: remote sensing, volcano monitoring, noaa-avhrr, hot-spot detection, lava-flow effusion rate. 522 special issue: v3-lava project in spite of their original aims, meteorological sensors have been increasingly used for operational monitoring of the thermal features of a volcano. satellite systems include the eumetstat meteosat second generation (msg) spinning enhanced visible and infrared imager (seviri) [ganci et al. 2011], the national oceanic and atmospheric administration (noaa) geostationary operational environmental satellites (goes) [harris et al. 2001], the nasa moderate resolution imaging spectroradiometer (modis) [wright et al. 2004], the european space agency (esa) along track scanning radiometer (atsr) [colin et al. 2007], and the noaa advanced very high resolution radiometer (avhrr) [webley et al. 2008, marchese et al. 2011]. the widely used avhrr sensor represents a good compromise between spatial resolution and measurement frequency, with the possibility of equipping satellite receiving stations at volcanology institutes [harris et al. 1997b]. different approaches have been adopted for the identification of thermal activity using avhrr data. the robust satellite techniques provide hot-spot detection [pergola et al. 2004], through computation of a statistical index in the medium ir (mir) spectral band using a multiyear time series data of the same area. 2. noaa/avhrr: the ingv satellite receiving station satellite receiving stations allow real-time tracking and reception of polar orbiter and geostationary satellites [spinetti et al. 2010]. at the istituto nazionale di geofisica e vulcanologia (ingv), the station installed in rome (figure 1a) at the end of 2004 comprises an l-band receiving antenna of 0.46 m diameter with high-resolution picture transmission telemetry. the station is a terascan® system (seaspace), which integrates hardware and software for automated reception of data and pre-processing of avhrr data from the u.s. noaa polar constellation. twice-daily planned global coverage is provided by noaa tiros-n satellites that operate in parallel in near-polar sun synchronous orbits at a height of about 850 km. the noaa satellites that were active during the 2006 and 2008 mount etna lava flow were: noaa-12, noaa-14, noaa-15, noaa-16, noaa-17, noaa-18. although noaa-19 was not active in 2008, it is now operative, and for completeness, its characteristics are included in table 1. the satellite pass over the mediterranean is tracked for the area showed in figure 1b. the corresponding data cover from the north of europe to the north of africa, and from eastern europe to spain. lombardo et al. 523 avhrr noaa 12, 14 noaa 15,16, 17, 18, 19 channel 1 (vis) 0.58-0.68 µm 0.58-0.68 µm channel 2 (nir) 0.725-1.1 µm 0.725-1.1 µm channel 3a (mir) 3.55-3.9 µm 3.55-3.9 µm channel 3b (mir) – 1.58-1.64 µm channel 4 (tir) 10.3-11.3 µm 10.3-11.3 µm channel 5 (tir) 11.4-12.4 µm 11.4-12.4 µm figure 1. a) photograph of the receiving antenna with radome installed on the roof of the ingv building, rome. b) the inner blue circle represents the area of data acquisition in view of the l-band antenna located in rome (blue point). table 1. avhrr spectral sensor characteristics. 524 the primary sensor on board the noaa polar-orbiting satellites is the avhrr imager spectrometer. this instrument records the radiation from the earth and atmosphere at different wavelengths, from visible to thermal ir (tir) (table 1). the avhrr/2 on board noaa-12 and noaa-14 have five channels. the avhrr/3 on noaa-15, noaa-16, noaa-17, noaa-18 and noaa-19 have six channels: five are the same as for avhrr/2, while channel 3 has been split into channels 3a and 3b. channel 3b operates during the night-time portion of the orbit. channel 3a has a different wavelength range, and operates during the daylight portion of the orbit. the instrument has an instantaneous field-of-view of 1.3 milliradians, which provides a nominal spatial resolution for the nadir view of 1.1 km. a continuously rotating elliptical scan mirror provides the cross-track scan from ±55.4 degrees from the nadir. the temporal coverage of the italian territory comprises at least four passes by day. the constellation of noaa allows a high repetition rate. the combination of the spatial resolution and spectral capabilities makes this sensor useful and widely used for the monitoring of volcanic activity, and in particular for the monitoring of the etnean lava flow. the summit of the mount etna volcano has been predefined by a geographic mask. all of the noaa passes that cover this area are stored on a dedicated machine. for the high temperature values in the avhrr images, the ir channel saturation is at 47.5 ˚c, 50 ˚c and 60 ˚c for bands 3, 4 and 5, respectively. 2.1. avhrr data pre-processing the avhrr antenna acquires data in real-time that comes from the noaa satellites and sends these to a dedicated workstation (figure 2). through the internet, this workstation periodically downloads the avhrr calibration coefficients from the seaspace website. then, the acquired data in terascan® data format (tdf; as used in this system) are automatically converted into hierarchical data format (hdf), as a more compatible format. once the hdf data have been produced, they are stored in a dedicated directory and sent by file-transfer protocol (ftp) to different directories [silvestri et al. 2008]. 3. dual-band-technique background the pixel dimensions of satellite data often exceed the width of lava bodies. where a pixel is wider than the hot feature, three-component models are more realistic: the first component corresponds to the cooler, solidified crust of the flow, the second component corresponds to the fractures in the crust that are representative of the molten or plastic flow of he interior, and the third component corresponds to the ‘cold’ surrounding ground. however, because the number of unknowns (temperatures) far exceeds the number of equations (bands), three-thermal-component models require several critical assumptions to be satisfied. two different approaches have been developed to solve this problem: 1) reducing the number of thermal components used in the model; and 2) constraining the three-component models using any available ‘ground truth’. these two approaches are described in the next two sections. 3.1. the dual-band technique using a two-thermal-component model the widely accepted cooling model for avhrrdetected lava flows considers the thermal flux as a function of the fractional area of two thermally distinct radiant surfaces: the surface occupied by the lava at temperature th, and the remaining area that represents the cooler background at the ambient temperature tb. the radiance detected by the sensor in one x-th band l(tx mx) is an average, weighted by the fractional area, of the two radiances l(th mx) and l(tb mx), which are related to the two temperatures given by the following equation: l(txmx) = fx · [fhl(thmx) + (1 − fh) l(thmx)] (1) where fh is the fractional area of the lava component, and fx and xx are the lava emissivity and atmospheric transmittance in band x, respectively. using two ir wavelength bands, equation (1) can be split into a equation system of two unknown temperatures (see equation 4). we can solve the system using one short-wave ir (swir)/mir band to characterize the hottest component lava body (th), and one tir band (8-12 µm) for the cooler background (tb). this is the so-called dual-band technique that can be applied to a avhotrr: volcano monitoring from space figure 2. avhrr data acquisition set-up. two-thermal-component model using low-spatial-resolution data (figure 3). usually, this technique allows the estimation of th and fh, if tb is assumed. it is possible to estimate the background temperature from the non-radiant pixels around the lava body if the background emissivity is known. the sub-pixel lava temperature retrieved will, of course, be an average value between the crust and the molten lava temperatures. 3.2. the dual-band technique using a three-thermal-component model an active lava body of ca. 1 km2 of pixel area will be occupied by at least three components: molten lava at temperature th that will occupy a portion fh of the pixel, the chilled crust (which forms rapidly on the exposed molten lava) at temperature tc that will occupy fc of the pixel, and the lava-free ground surface at tb that will occupy the remainder of the pixel area: 1-fh-fc (figure 4). therefore equation (1) has been adapted to include a third component on x-th and y-th bands: l(txmx) = (2a) fxxx [fhl(thmx) + fcl(tcmx) + (1 − fh − fc) l(tbmx)] l(tymy) = (2b) fyxy [fhl(thmy) + fcl(tcmy) + (1 − fh − fc) l(tbmy)] there are five unknowns, fh, fc, th, tc and tb in these two simultaneous equations (equations 2a, 2b). to constrain a reasonable model, valid and unique estimates are required for at least three of these parameters. we therefore assume th and estimate tb, as for equations (2a) and (2b), with th set to 1,000 ̊ c and tb estimated from the adjacent ‘lava-free’ pixels. this leaves three unknowns, fh, fc and tc. field and satellite data show that the temperatures for tc can range over several hundreds of degrees celsius, making tc impossible to define using a single value [oppenheimer 1991, flynn and mouginismark 1994]. to avoid the error that would be introduced by selecting a single tc, we chose to solve equations (2a) and (2b) over a reasonable range of tc, to give a range of solutions within which the true solution must lie. following the results of oppenheimer [1991] and flynn et al. [1993], we selected 100 ̊ c as a reasonable lower limit for the tc range. tc will have an upper limit, above which fh needs to become negative to solve equations (2a) and (2b). this approach gives maximum values for tc typically between 150 ˚c and 350 ˚c, with estimates of up to 470 ̊ c in extreme cases [harris et al. 1997b]. 4. the avhotrr routine the avhotrr routine has been developed for hot-spot detection and effusion rate estimation using avhrr ir space-borne images. the avhotrr routine is composed of graphical user-interface procedures that are implemented in interactive data language (idl), which takes advantage of the functionality of the environment for visualizing images (envi) environment for image managing/processing. avhotrr allows for automatic hot-spot detection of each near real-time received data from the storage machine. when there is positive detection of a lava flow, the routine automatically estimates the total thermal flux and the effusion rate. the flow chart in figure 5 describes the avhotrr functionality. the first version of the avhotrr routine was used during the entire 2006 mount etna eruption, to provide daily lava-flow effusion rates to the ingv volcanological observatory and the department of civil protection [spinetti et al. 2008]. a second version of the software was developed within the sistema rischio vulcanico project, funded by the italian space agency in the framework of the national space plan 2003-2005, under the earth observation section for natural-risk management. the software was finally improved within the lava project. 4.1. cloud mask the cloudiness identification is essential to volcanic monitoring for two reasons: first, clouds have a critical role in hot-spot detection, as they can significantly decrease the lombardo et al. 525 figure 3. the dual-band method. this assumes that a pixel is occupied by two thermal components: a high-temperature component at temperature th that occupies a portion fh of the pixel, surrounded by a cooler component at temperature tb that occupies the remainder (l − fh) of the pixel. figure 4. the three-thermal-component model. this is characterized by three distinct sub-pixel temperatures: th, crack temperature; tc, crust temperature, and tb, ground temperature. 526 satellite ir signals. for this reason, the cloudiness must be accurately masked, to assess any thermal anomaly identification. second, the presence of cloudiness near a hotspot can distort the background temperature, tb. in this case, the cloud-contaminated pixel areas must be accurately determined to correctly retrieve the radiant contribution from potential thermal anomalies. for many of the retrieval algorithms, even thin cirrus cloud represents a contamination. the cloud detection mask proposed by chen et al. [2002] was implemented for day-time images, while a modified version of the algorithm that was developed by spangerberg et al. [2002] was used for night-time images. these algorithms have been adapted according to the seasonal conditions of mount etna (e.g. snow in winter). figure 6 shows examples of the avhotrr output with cloud masks. avhotrr: volcano monitoring from space figure 5 (left). the avhotrr flow chart. figure 6 (right). cloud mask examples for the etna 2006 eruption. (a) day time, july 20, 2006. (b) day time, july 22, 2006. (c) night-time august 8, 2006. blue pixels, cloudy areas; red pixels, radiant hot-spots; yellow pixels, saturated hot-spots. 4.2. hot-spot detection the automatic ‘hot-spot-detection’ procedure uses avhrr data hat are directly acquired using the noaa/terascan station. hot-spot detection is part of the avhotrr system that was previously developed in the framework of asi-srv project. avhotrr automatically scans the avhrr data and sends an alert when thermal anomalies are detected by the procedure (figure 7). the procedure is based on the theoretical solution of the ‘dual– band’ system of equations, which are performed using a digital filter to single out hot radiant pixels, as proposed by lombardo et al. [2004] for landsat tm swir bands. the filter was initially developed for detection of volcanic hotspots, and then it was adapted to allow detection of different thermal anomalies, including active lava flow. this algorithm was adapted for avhrr band 3 (rad3) and 4 (rad4), assuming a two-thermal-component model for active lava flow. the first part of the filter builds on the assumption that the fractional areas are greater than zero. the following experimental formula is used to cut off the values of radiance that do not satisfy the condition of positive fractional area: rad3 > 0.161096 * rad4. (3a) the second part of the filter satisfies the condition that the sub-pixel lava temperature is greater than zero: rad3 < rad4*(0.0238096*tl−4.81817) (3b) where tl is the average integrated temperature of the lava. a tl of 500 ˚c has been identified as the most fit value for etnean flows. 4.3. the dual-band technique the sub-pixel temperature retrieval technique is pivotal in the remote sensing of active lava flows. a two-thermalcomponents model was chosen to keep the system as independent as possible of auxiliary information. the algorithm uses the globally convergent broyden method to solve the dual-band-equation system: l(txm3) = f3x4 [f1l(t1m3) + (1 − f1) l(tbm3)] (4a) lombardo et al. 527 figure 7. mount etna eruptive activity imaged by avhrr during the 2006 eruption. (a) in mir band 3. (b) in tir band 4. (c) hot-spot detection generated by avhotrr. figure 8. mount etna avhrr image of october 30, 2006. (a) crater area (darkest pixels), characterized by an almost uniform distribution of basaltic flows. (b) thermal anomaly in band 3. (c) thermal anomaly contour for the background-temperature estimation from the non-active pixels surrounding the thermal anomaly. 528 l(txm4) = f4x4 [f1l(t1m4) + (1 − f1) l(tbm4)] (4b) where l is the planck function for a blackbody radiating to temperature t and at wavelength m, tx and ty are the pixelintegrated temperatures from equations (2a) and (2b) in avhrr bands 3 and 4, respectively, f is the spectral emissivity, and x is the atmospheric transmittance [wan and dozier 1989]. the dual-band approach requires assumed knowledge of the values of one of the three unknowns: lava temperature (tlava), background temperature (tb) or fraction of pixel occupied by the lava ( flava). avhotrr derives tb from the non-active pixels surrounding the thermal anomaly, under the assumption of known background emissivity. a relatively uniform surface of basaltic lavas characterizes the mount etna summit area (figure 8a). therefore, tb can be estimated assuming a suitable f for etnean cold basalts. as tb can vary widely from the vent to the flow toe, the avhotrr retrieves the maximum (tbmax) and minimum (tbmin) temperatures of the background in a pixel mask (figure 8b, c). dual-band solutions (tlava and flava) are then calculated for every tb between tbmin and tbmax. figure 9 shows the tlava and flava trends plotted as functions of tb in a temperature range of 0 ˚c to 10 ˚c for a hot pixel of the october 30, 2006, lavaflow avhrr image on mount etna. logarithmic and linear trend models appear to best fit the tlava versus tb and flava versus tb distributions, respectively. these models are in agreement with models that are derived from theoretical solutions of the dual-band system of equations [glaze et al. 1989, oppenheimer 1993, lombardo et al. 2004, lombardo and buongiorno 2006, lombardo et al. 2006]. standard deviations (sds) of 1 ˚c and 10−3 were obtained for tlava and flava, respectively. statistics over a large number of samples highlight that a larger sd can occur for re-sampled pixels and for pixels affected by blurring due to the presence of volcanic plumes in the scene. therefore, the sds of tlava and flava can be used as extra tools to determine the accuracy of the parameters retrieved. figure 10 shows the tlava and flava trends obtained from a rejected lava pixel. in this case, sds of 19 ˚c and 4 × 10−2 were obtained for tlava and flava, respectively. the avhotrr performs logarithmic and linear best fits of tlava and flava, respectively, and automatically rejects pixels according to a confidence threshold of the sds. 4.4. estimation of the energy flux radiated by an active lava flow the energy produced by an active lava flow is lost to the environment through a combination of conduction, convection and radiation. tir remote sensing research has focused on inferring information from the radiative component, as the convective and conductive components avhotrr: volcano monitoring from space figure 9. (a) tlava and (b) flava trends as functions of tb, for a radiant pixel on the avhrr image of mount etna on october 30, 2006. black solid line, solution from the dual-band fit; blue dashed line, solution from the logarithmic linear fit. figure 10. (a) tlava and (b) flava trends as a functions of tb, for a rejected radiant pixel of the avhrr image of mount etna on october 30, 2006. black solid line, solution from the dual-band fit; blue dashed line, solutions from the logarithmic linear fit. are difficult to quantify directly. the radiative transfer components of the observed scene of a lava flow are very complex: in the same area there are zones that are not yet overwhelmed by the lava flow, and here are burning zones and zones already burnt; in the burning zones at least two distinct lava-flow stages need to be considered, as flaming and smoldering, which are characterized by different temperatures and emission rates. moreover, the atmospheric loading is very complex too, with the presence of hot gases, including water vapor, and often with ash emissions. in general, the retrieval of lava parameters from space is based on simplified radiative-transfer models that are often referred as dozier models [dozier 1981]. in this approach, the pixel is seen as a composite target made up of two ‘gray-body’ targets that are linearly combined through coefficients that represent a fraction of the hot and cold parts, with respect to the whole of the pixel. this model restricts the observation of the active lava flow to the mir and tir emission bands. indeed, according to the wien displacement law, the peak of the surface emitted radiance shifts to shorter wavelengths as the surface temperature increases. so, temperatures of about 1,000 ˚c can give significant signal contributions in the swir region, where there are emission and reflection phenomena and where a more complex model is needed to interpret the satellite data. following oppenheimer [1991] and harris et al. [1997a, 1997b, 1998, 2000], and assuming that the heat loss can be described by a simple model for a subaerial channel-fed flow, the total flux qtot is: qtot = qrad + qconv + qcond . (5) qrad, qconv and qcond are the total radiant, the convective, and the conductive heat fluxes, respectively. the qrad for each pixel that contains active lava (qrad(p)) is calculated using: qrad(p) = f v ap [flavatlava 4 + (1 − flava)tb 4] (6) in which f is the emissivity (0.98 for basalts), v is the stefanboltzmann constant (5.67 × 10−8 w m−2 k−4), and ap is the avhrr pixel area. the total radiative heat loss is then obtained by summing qrad(p) for every lava pixel. qconv is estimated for the entire lava-flow field in a similar way, where qconv for each pixel of the image that contains active lava (qconv(p)) is calculated using the free convection case given by harris et al. [1997b, 1998]. this reduces to: qconv(p) = hc [tsurf − tair] (7) in which hc is the convective heat transfer coefficient, tsurf is the lava surface temperature, and tair is the ambient air temperature, where the values given in harris et al. [1997b] result in an hc of 5 w m −2 k−1 to 12 w m−2 k−1 for free convection. this is a theoretically calculated value for free convection [see harris et al. 2005], whereas values for forced convection obtained from measurements [keszthelyi et al. 2003] and modeling [patrick et al. 2005] indicate higher values of hc (ca. 50 w m −2 k−1). the heat conducted through the base of the flow is given by [harris et al. 1997b]: qcond = ap k dt/dh (8) where k is 2.5 w m−1 k−1 to 3.2 w m−1 k−1, dt is 520 k (assuming a core temperature of 1100 ˚c and a basal contact temperature of 580 ˚c), and dh is 0.2 m (hot model) or 3.0 m (cold model). this accounts for ca. 25% of the total heat loss (qtot). in reality, the assumptions involved mean that the main variable is the active lava area (alava), so that the above steps define the slope of a linear relationship between the effusion rate and lava-flow area [wright et al. 2001]. 4.5. transformation of the energy flux into the mass flux for effusion-rate estimation the effusion rate and flow volume are important controls of the lava-flow morphological parameters and lavaflow length [walker 1973, wadge 1977, kilburn 1990, kilburn and guest 1993]. following pieri and baloga [1986], it is commonly assumed that the lava flow is effectively stationary at the moment of the image capture. for a thermally unmixed case, the effusion rate er is given by the following equation: (9) where qrad, qconv and qcond are the total radiant, the convective, and the conductive heat fluxes, respectively, t is the lava density, cp is the specific heat capacity, dt is the average temperature drop throughout the active flow (equal to the initial eruption temperature minus the temperature at which the forward movement of the lava ceases), { is the average mass fraction of crystals grown in the cooling for dt, and cl is the latent heat of crystallization. pieri and baloga [1986] derived a simpler model, in which qconv and qcond in equation (5) are not considered. for the etnean flow, we use t and cp of 2,600 kg m−3 and 1,150 j kg−l k−l, respectively [kilburn and guest 1993]. the dt is not likely to be greater than the difference between the eruption temperature and the solidus temperature of the lava. we used the maximum temperature measured in the flow, i.e. 1,080 ˚c (s. calvari, personal communication, 1996) for the eruption temperature. since the differences between the liquid and solidus ranges from 150 ̊ c to 200 ̊ c for most magmas [archambault and tanguy 1976], we chose a solidus temperature of ca. 900 ˚c. the maximum amount of crystallization ranges from 0.4 to 0.5, which allows for 30% initial phenocrysts and 20% to 30% lombardo et al. 529 c t c q q q e p l rad conv cond r = + + + t {d6 @ 530 residual glass in the solid lava [armienti et al. 1994]. the latent heat of crystallization (cl) is 2.9 × 10 5 j kg, as given by huppert and sparks [1988]. as tb ranges between tbmin and tbmax, qrad varies between a minimum (qmin) and a maximum (qmax), while tlava, flava are given by equation (4). in the same way, qcond is given by equation (8), as the hot and cold models. as a consequence, we derive er from equation (9), which spans a range from ermax to ermin (figure 11). 5. outputs the avhotrr automatic procedure provides hot-spot information using a system of e-mail alerts (figure 12). the alert messages give information about the number of hotspots, the number of radiant pixels, and eventually, the cloud contamination. the effusion rates and other information are automatically written into an ascii file, to allow further analysis of the eruptive time-series. within the lava project, the 2006 mount etna eruption was chosen as case study to test and validate the results derived from different sensors and techniques. the resumption of activity at mount etna occurred in mid-july 2006, when a short fissure opened on the southeast crater (sec) cone, which released a lava flow that traveled 3.5 km eastwards over the following 10 days [neri et al. 2008]. an intermitted lava flows occurred at the sec from september to mid-october, 2006, with an increase of paroxysmal activity [andronico et al. 2009]. between midoctober and mid-december, 2006, persistent lava outflow occurred from a vent at 2,800 m in altitude on the upper eastern flank of mount etna. this vent showed strong fluctuations in its effusion rate that were correlated with the paroxysmal episodes [behncke et al. 2009]. a total of 479 avhrr images of mount etna were acquired from june to december, 2006. figure 13 shows the statistics for the processed data. figure 13a shows the percentages of the images where hot-spots were detected. in all, 39% of our dataset showed hot-spot detection, and 38% showed ‘no-activity’ detection, using the filter described in equation (3). the remaining 23% of the images were rejected as false positives, according to the number of hot-spots detected. specifically, avhotrr avhotrr: volcano monitoring from space figure 11. (a) tlava and (b) flava solutions, (c) as retrieved for a single lava pixel for ermax and ermin. figure 12. example of alert message. (a) text message, and (b) image and number of pixels detected. rejects the alerts with more than 10 hot-spots. comparisons between our data and field observations indicated the following: 7% false negatives that were due to the thick cloud cover that obscured the hot-spot emissions (which were thus not detectable by any of the systems); 5% false negatives that were due to the hot-spot number threshold (e.g. for more than 10 hot-spots, with at least one of them real); and 3% false positives due to noisy or cloudy images. figure 13b shows the statistics of the detected hot-spots: 12% of the images were cloud-free hot-spots, where the effusion rate was correctly calculated; 28% of the images had cloud contamination, where the effusion rates might be distorted; and 60% of the images had very cloudy images. while the hot-spot was identified, the effusion-rate calculation failed. finally, figure 13c shows the statistics of the saturated pixels. saturation of the avhrr data mainly occurred in band 3. lombardo et al. [2004] demonstrated that the dualband system is still solvable when using one saturated band. however, the retrieved temperatures will be unpredictably underestimated. figure 14 shows the derived minimum and maximum effusion rates for this 2006 eruption. the difference between the minimum and maximum values gives the error in the effusion rate estimation. a mean error of 36% in the effusion lombardo et al. 531 figure 13 (left). pie chart statistics for (a) detection, (b) cloudiness, and (c) sauration for the hot-spots analyzed from the avhrr data for the mount etna eruption from june to december, 2006. figure 14 (below). avhrr-derived maximum (red) and minimum (blue) effusion rates for the 2006 mount etna eruption. 532 rate estimation was found over the entire period of this 2006 eruption of mount etna. 6. conclusions avhotrr is a robust routine that allows hot-spot detection and effusion-rate estimation. the lava project, which is funded by the italian department of civil protection, represents a unique opportunity to integrate the procedures developed in avhotrr with the data from the scientific activities, which greatly improved the accuracy of the algorithms and the values retrieved. the multi-approach method that was adopted in the framework of the lava project integrates the avhrr effusion rate that is derived with effusion rates from other sensors for very high temporal-resolution analysis and monitoring of active lava flows. the products generated by avhotrr and other analogous procedures that use modis and msg-seviri data can be used as the input to flow models (e.g. magflow) [vicari et al. 2009], to drive numerical simulations of lava-flow paths, and thus to provide quantitative hazard assessments and volcanic risk mitigation. acknowledgements. this study was carried out in the framework of the v3-lava project (lava flow invasion hazard map at mt. etna and methods for its dynamic update), funded by the department of civil protection in the ingv-dpc 2007-2009 agreement. we are grateful to dr. chiara cardaci and dr. maria fabrizia buongiorno. our thanks also go to the two anonymous reviewers who helped us to improve our manuscript, and to dr. laura colini for her helpful review of our manuscript. references andronico, d., c. spinetti, a. cristaldi and m.f. buongiorno (2009). observations of mt. etna volcanic ash plumes in 2006: an integrated approach from ground-based and polar satellite monitoring system, j. volcanol. geoth. res., 180, 135-147; doi: 10.1016/j.jvolgeores.2008.11.013. archambault, c. and j.c. tanguy (1976). comparative temperature measurements on mount etna lavas: problems and techniques, j. volcanol. geoth. res., 1, 113-125. armienti, p., m.t. pareschi, f. innocenti and m. pompilio (1994). effects of magma storage and ascent on the kinetics of crystal growth, contrib. mineral. petr., 115, 402-414. barberi, f., m. rosi and a. sodi (1993). volcanic hazard assessment at stromboli based on review of historical data, acta vulcanologica, 3, 173-187. behncke, b., s. falsaperla and e. pecora (2009). complex magma dynamics at mount etna revealed by seismic, thermal and volcanological data, j. geophys. res., 114, b03211. calvari, s., m. coltelli, m. neri, m. pompilio and v. scribano (1994), the 1991-93 etna eruption: chronology and lava flow field evolution, acta vulcanologica, 4, 1-14. chen, p.y., r. srinivasan, g. fedosejevs and b. narasimhan (2002). an automated cloud-detection method for daily noaa-14 avhrr data for texas, usa, int. j. remote sens., 23, 2939-2950. colin, o., m. rubio, p. landart and e. mathot (2007). vomir: over 300 volcanoes monitored in near realtime by aatsr, in: h. lacoste and l. ouwehand (eds.), proceedings of the envisat symposium 2007, montreux, switzerland, 23-27 april 2007 (esa sp-636, july 2007). crisp, j. and s. baloga (1990a). a model for lava flows with two thermal components, j. geophys. res. 95, 1255-1270. crisp, j. and s. baloga (1990b). a method for estimating eruption rates of planetary lava flows, icarus, 85, 512-515. dozier, j. 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(1973). lengths of lava flows, philos. t. r. soc. london, a274, 107-118. wan, z. and j. dozier (1989). land-surface temperature measurement from space: physical principles and inverse modelling, ieee t. geosci. remote, 27, 268-277. webley, p.w., m.j. wooster, w. strauch, j.a. saballos, k. dill, p. stephenson, j. stephenson, r. escobar wolf and o. matias (2008). experiences from near-real-time satellitebased volcano monitoring in central america: case studies at fuego, guatemala, int. j. remote sens., 29, 6621-6646; doi: 10.1080/01431160802168301. wooster, m.j. and d.a. rothery (1997). time series analysis of effusive volcanic activity using the ers along track scanning radiometer: the 1995 eruption of fernandina volcano, galapagos island, remote sens. environ., 69, 109-117. wright, r, d.a. rothery, s. blake and d.c. pieri (2000). improved remote sensing estimates of lava flow cooling: a case study of the 1991 to 1993 mount etna eruption, j. geophys. res. 105, 23681-23694. wright, r., s. blake, a. harris and d. rothery (2001). a simple explanation for the space-based calculation of lava eruptions rates, earth planet. sci. lett., 192, 223-233. wright, r., l. flynn, h. garbeil, a. harris and e. pilger (2004). modvolc: near-real-time thermal monitoring of global volcanism, j. volcanol. geoth. res. 135, 29-49. *corresponding author: valerio lombardo, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy; email: valerio.lombardo@ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. avhotrr: volcano monitoring from space o n t h e t h e r m a l h i s t o r y o f t h e e a r t h (*> b . j . l e v i n a n d s . v . m a j e v a ( * * ) r i c e v u t o il 7 dicembre 1960 the thermal history of the e a r t h has been investigated during last decade in the otto schmidt institute of physics of the e a r t h . in the analogous investigations of other authors (jacobs and allan, macdonald) both the cold and the hot initial state of the e a r t h were admitted. in our institute all calculations were based on the theory of the origin and evolution of the e a r t h developed from prof. schmidt's ideas. all calculations were made for the initially cold e a r t h heated by radioactivity. at first the thermal history of the uniform e a r t h model taking into account t h e previously neglected time-dependence of radioactive heat production was calculated by e . a. lubimova (1952). i t was shown t h a t the mean content of radioactive elements in meteorites is sufficient for the explanation of the present incandescent state of the e a r t h ' s interior. in subsequent calculations (lubimova, 1955-58) the non-uniform distribution of radioactive elements in the e a r t h a t the present time as well as the dependence of heat transport on temperature and pressure were taken into account. in the present paper the results of some new calculations are discussed. their main purpose was: a) to study the thermal history of the outer parts of the e a r t h assuming a longenduring (not instantaneous) formation of the crust resulting from the differentiation of the mantle, and b) to compare the calculated heat flows for the continental and oceanic parts of the e a r t h . we wished also to clarify if it is possible to fit the observed heat flow with the smaller content of radioactive elements in the e a r t h then adopted in previous calculations. our results were already in press when a paper by macdonald (1959) was published. he had calculated a thermal history for a great series (*) p a p e r r e a d a t t h e helsinky assembly of t h e i . u . g . g . , 1960. (**) 0 . j . s c h m i d t i n s t i t u t e of p h y s i c s of t h e e a r t h , moscow, u s s r acad. sci. 1 4 6 b . j . l e v i n s . v . m a j e v a of models. b u t in the time-dependent solutions only t h e distribution of temperature in the mantle is calculated. the core is supposed to be thermally insulated and t h e existance of t h e crust enriched b y radioactive elements is not taken into account. nevertheless macdonald's p a p e r gives an independent confirmation to some conclusions obtained earlier by lubimova. h, cal/g.szc. fig. 1 h e a t o u t p u t f r o m t h e c r u s t (a) a n d f r o m t h e d e p l e t e d p a r t of t h e m a n t l e (b). n o n d e p l e t e d p a r t of t h e m a n t l e a n d core. depleted p a r t of t h e m a n t l e our calculations were made for t h e following e a r t h ' s model: f o r t h e dense core of t h e e a r t h ramsey's hypothesis is adopted, a n d therefore t h e core is supposed to exist f r o m t h e very beginning of t h e time interval for which the t h e r m a l calculations were made. i n accordance with t h e formation of t h e e a r t h by accumulation of cold particles t h e initial distribution of radioactive elements is adopted t o be uniform (per unit of mass) throughout t h e whole e a r t h . the e a r t h began to accumulate 5 x l 0 9 o n t i i e t h e r m a l h i s t o r y of t h e e a r t h 151 years ago. f r o m t h i s moment is measured the time t used in t h e calculations. the real formation process of the crust was probably the gradual growth of its thickness. for the sake of simplicity of calculations this was replaced b y t h e gradual growth of the concentration of radioactive elements in t h e crust of constant thickness. for the continental parts of the e a r t h the thickness of t h e crust d = 30 km was adopted and for the oceanic crust d = 10 km the concentration of radioactive elements in the crust began to increase a t t — 2 x109 i.e. 3 billions years ago. this occured as a result of the uniform depletion of these elements in t h e upper p a r t of the m a n t l e down ot t h e depth of 1000 k m . f o r t h e crust the following formula for the time-dependence of heat generation was adopted h(t) = ( l + k ) s fl, (0) [1] as previously, the chemical composition of the e a r t h was supposed to be the same as t h e true mean composition of the meteoritic m a t t e r the mean content of radioactive elements in the meteoritic m a t t e r , obtained by a. starkova in 1955 m u s t be seriously corrected because the new analyses of meteorites give as a rule much smaller content of uranium and thorium. therefore the calculations were made not only for the mean content obtained by starkova and used in the last paper by lubimova (variant c, see table), b u t also for the variant ci/2 in which the potassium content is not changed b u t the content of uranium and thorium is two times smaller. e l e m e n t variant c variant c'i/2 u 5 . 2 1 0 8 2 6 . 1 0 ' 8 th 21-10"8 10-5.10"8 k 0.7-10" 3 0-7.10" 3 the results by j . e . starik and m. shatz (1956) who obtained a very great content of uranium and thorium in meteorites, inspired us to make some calculations for the variant c2 in which t h e content of u and th is two times greater. b u t this variant leads to excessively great values of t h e temperature in the e a r t h ' s interior and of the heat flow through the surface. the present heat generation in the crust was adopted to be equal to the mean generation in 1 p a r t (by volume) of granit and 2 parts of 146 b . j . l e v i n s . v . m a j e v a basalt. according to the d a t a b y birch (1954) this h e a t generation is 9,7 x 1 0 1 4 cal/g sec or 30,6 times greater t h a n t h e generation in t h e nondifferentiated e a r t h ' s m a t t e r according to variant c. therefore in form u l a f l ] we obtain a-=29,6. the present generation of heat in the depleted p a r t of the mantle follows to be 6,3 x 1 0 1 6 cal/g sec, the same as in dunite fig. 2 t o t a l c o n d u c t i v i t y versus d e p t h x (t, p). c, d = 30, e = 200, c = 0,2. according to birch. f o r t h e v a r i a n t oi/2 the same value of k gives t h e heat generation in the crust to be 6,7 x hi"11 and in t h e depleted p a r t of the mantle 3,5 x 10 lfi. the initial radial distribution of t h e temperature a t t h e time of the practical end of t h e e a r t h ' s growth was t a k e n according to y. s. safronov (1959) for t h e case of the duration of t h e growth equal to 0,23 x 109 years. the subsequent changes of the temper a t u r e (after t = 0,23 x 10") were calculated on t h e hydraulic integrating machine constructed b y v. s. l u k y a n o v (1939). o n t i i e t h e r m a l h i s t o r y o f t h e e a r t h 151 i n t h e core t h e metallic h e a t c o n d u c t i v i t y x — 0,5 cal/cm sec degree i n d e p e n d e n t on t e m p e r a t u r e and pressure was a d o p t e d . i n t h e m a n t l e and crust t h e lattice a n d r a d i a t i v e conductivitis were t a k e n i n t o a c c o u n t . the d e p e n d a n c e of lattice c o n d u c t i v i t y on t e m p e r a t u r e a n d pressure was t a k e n according to v. n. z h a r k o v (1958) xm = const y [2] where t h e pressure f u n c t i o n /(p) increases 25 times f r o m t h e surface t o t h e m a n t l e a n d core b o u n d a r y . t h e c o n s t a n t in [2] was e v a l u a t e d a d o p t ing x — 1,2 x 10"2 cal/cm sec degree (dunite) in t h e conditions of t h e e a r t h ' s surface. i n t h e f o r m u l a f o r t h e r a d i a t i v e c o n d u c t i v i t y 16 ctm2 m = -r t3 [3] o e t h e absorption coefficient s in t h e m o s t p a r t of calculations was t a k e n equal t o 200 c m 1 a n d in some calculations 40 cm"1. o u r results would r e m a i n valid also if it were shown t h a t n o t t h e r a d i a t i v e c o n d u c t i v i t y b u t t h e exciton c o n d u c t i v i t y is of i m p o r t a n c e in t h e e a r t h ' s interior. an e x a m p l e of t h e change of t h e t o t a l c o n d u c t i v i t y w i t h t h e d e p t h is shown on fig. 2. the h e a t c a p a c i t y was t a k e n c = 0,2 cal/g a n d in some cases c = 0,3 according to j a c o b s (1956) t h e h e a t c a p a c i t y of t h e m a n t l e is within these limits. i t m u s t b e t h e same also for t h e core if t h e l a t t e r has t h e same composition according to r a m s e y ' s hypothesis a d o p t e d b y us. t h e general c h a r a c t e r of t h e radial distribution of t e m p e r a t u r e f o r different m o m e n t s of t i m e can be seen on fig. 3 (the v a r i a n t c of t h e cont e n t of r a d i o a c t i v e elements; h e a t capacity c = 0,2 a n d 0,3 cal/g). the knick of t h e curves a t t h e mantle-core b o u n d a r y arises because t h e t o t a l h e a t c o n d u c t i v i t y of t h e m a n t l e , even if t h e radiatice c o n d u c t i v i t y is t a k e n i n t o a c c o u n t , remains smaller t h a n t h e metallic c o n d u c t i v i t y of t h e core. t h e outflow of h e a t f r o m t h e core is small and therefore with c = 0,2 t h e central t e m p e r a t u r e is a b o u t 1,5 times greater t h a n with c = 0,3. f o r t h e v a r i a n t g t h e present central t e m p e r a t u r e is contained within 4400-6500 ° i t a n d for t h e v a r i a n t cik — within 3000-4500 °k. f o r t h e g r e a t e r role of r a d i a t i v e c o n d u c t i v i t y (e = 40 cm"1) t h e d e p t h of outer zone f r o m which t h e h e a t flows out is also g r e a t e r a n d i t reaches t h e 146 b . j . l e v i n s . v . m a j e v a b o u n d a r y of the core. therefore the temperature of the core comes to be about 100° lower t h a n for s = 200. the comparison of temperatures calculated for the present m o m e n t of time with the melting temperatures is given on fig. 4. f o r the m a n t l e t h e melting curves according to uffen (1952) and to y.n. zharkov are given, and for the core — t h e melting curve for iron according to zharkov (1959). (according to d a t a by strong the melting temperatures of iron are substantially lower). for the v a r i a n t c of the content of radioactive elements if c = 0,2 the t e m p e r a t u r e curve for t h e present time passes over the melting curves. probably this does not correspond to t h e real c = 0.2 fig. 3 t e m p e r a t u r e v a r i a t i o n w i t h d e p t h . situation. f o r c = 0,3 the t e m p e r a t u r e distribution is almost the same as for variant ci/2 and c = 0,2. remembering t h e u n c e r t a i n t y of t h e melting curves we can say t h a t this distribution of t e m p e r a t u r e (or a similar one passing somewhat lower) agrees with t h e concept of the partial melting of the mantle which began a b o u t 3 billion years ago. i n order to obtain t h e solid inner core and the fluid outer p a r t of it we m u s t suppose t h a t the m a t t e r of t h e core being in metallic state has melting temperature intermediate between zharkov'? ans strong's data for the iron. macdonald has made all his calculations for the minimal values of uranium and thorium content, obtained in recent analyses of meteorties by the neutron-activation method (u — 1,1 x 10~8; th — 4,4 x 10 8). o n t i i e t h e r m a l h i s t o r y o f t h e e a r t h 1 5 1 i n this case the generation of heat is so small t h a t the melting of the outer p a r t s of t h e mantle occures only for the present time and only if t h e high initial temperature of the e a r t h is supposed. b u t in t h e real e a r t h t h e partial melting of the mantle was t h e cause of the formation of the crust which hegan about 3 billion years ago. moreover the great initial t e m p e r a t u r e of the e a r t h can be due to the shortlived isotopes only and this requires the formation of t h e e a r t h from the m a t t e r which had shoitly vefore passed t h e process of nucleogenesis. very small uranium and thorium contents adopted by macdonald can lead also to diffig. 4 t e m p e r a t u r e s calculated for t h e p r e s e n t m o m e n t of t i m e as c o m p a r e d t o t h e melting t e m p e r a t u r e s . 1 a f t e r e . u f f e n (1952). 2,3 v. n. z h a r k o v (1959). acuities in explaining the observed heat flow. macdonald agrees t h a t t h e lattice conductivity decreases with temperature (see form [2]) b u t in his calculations he p u t s it to be constant. h e does not t a k e into account t h e low-conductivity layer a t the depth 100-200 km (see fig. 2) and therefore he overestimates the heat flow through the surface. when t h e longenduring character of the redistribution of radioactive elements is taken into account, falls off the substantial cooling of outer layers obtained by lubimova for the case of instantaneous redistribution. i n the v a r i a n t c for the continental p a r t of the e a r t h (thickness of the crust d = 30 km) we obtained some cooling at the depth of 200-600 k m which does not exceed 200° and for the oceanic p a r t (d = 10 km) there is no cooling a t all (see fig. 5). f o r the variant ci/2 t h e result is analogous. 146 b . j . l e v i n s . v . m a j e v a at the same time — and t h a t is very i m p o r t a n t — we obtained for all variants t h a t the t e m p e r a t u r e a t the crust-mantle b o u n d a r y continues to increase. i t is due to the increase of the generation of heat in t h e crust because with t h e adopted linear law of concentration of radioactive elements into t h e crust their income from the m a n t l e overcompensates their decay (see fig. 1). fig. 5 t e m p e r a t u r e d i s t r i b u t i o n in t h e u p p e r l a y e r s of t h e e a r t h . a) c o n t i n e n t a l p a r t of t h e e a r t h b) oceanic p a r t of t h e e a r t h . e = 40. e = 200. the t e m p e r a t u r e changes in the lower p a r t of t h e crust are directly connected with the changes of t h e heat flow t h r o u g h t h e e a r t h ' s surface (fig. 6). the almost constant flow was reached a t t h e end of t h e stage of uniform distribution of radioactive elements (for t h e uniform model it would begin to decrease) and a f t e r the beginning of the redistribution it began to increase again and this increase continues till now. i n t h e case of instantaneous formation of the crust there would be a sharp increase of h e a t flow followed by a decrease. o n t i i e t h e r m a l h i s t o r y of t h e e a r t h 151 for both variants of content of radioactive elements (c and ci/a) all calculations give the present heat flow which is in accordance with t h e observed one within t h e u n c e r t a i n l y of the latter. for the case of a thin fig. 6 superficial h e a t (low v a r i a t i o n w i t h t h e t i m e . 1 c, d = 2 0 , e = 2 0 0 , c = 0 , 3 2 c, 10, 2 0 0 , 0 , 2 3 cl/o 3 0 , 4 0 , 0 , 2 4 c, 3 0 , 2 0 0 , 0 , 3 5 c, 3 0 , 2 0 0 , 0 , 2 0 c, 10, 4 0 , 0 , 2 7 c, 3 0 , 2 0 0 , 0 , 2 (curve 7 corresponces t o t h e i n s t a n t a n o e u s f o r m a t i o n of t h e c r u s t ) oceanic crust the h e a t flow is of course smaller t h a n for the case of a thicker continental crust. b u t the heat flow originates not only in the crust b u t also inflows from the mantle and therefore t h e difference of 146 b . j . l e v i n s . v . m a j e v a calculated heat flows is much less t h a n the difference in thickness between t h e continental and oceanic crust. the increase of t h e role of radiative conductivity decreases the difference of heat flows b u t even if t h e role of radiative conductivity is small, this difference in h e a t flows a m o u n t s only to 30-40% and is in accordance with the conclusion b y bullard, maxwell and revelle (1956) t h a t the flows observed on continents and oceans are equal within the limits of their uncertainty. therefore this observed equality of heat flows does not contradict t h e concept of t h e gradual differentiation of the crustal m a t t e r from the mantle, which in continental p a r t s had proceeded f a r t h e r t h a n in oceanic ones. i t m u s t be stressed t h a t the inference upon the continuing increase of the t e m p e r a t u r e of the lower p a r t of t h e crust and also of t h e h e a t flow is directly connected with t h e linear law of income of radioactive elements into the crust, which was adopted arbitrarily. the change of this law and also the rejection of the supposition of uniform extraction of radioactive elements from the upper 1000 km of the m a n t l e would lead to different distributions of temperature in the outer p a r t s of t h e e a r t h and to different changes of this t e m p e r a t u r e with t h e time. f o r this calculation we m u s t receive from geologists and geophysicists reliable d a t a upon the redistribution of radioactive elements. w i t h o u t them our calculations cannot confirm or cast doubt on t h e supposition of some authors t h a t in t h e past both tectonic and volcanic activities were greater. summary the thermal history is calculated for several models of the initially cold earth. the formation of the earth's crust is regarded as a longenduring process, ivhich started 3 x 10s years ago. in the case of great radiative conductivity the calculated heat floiv through the surface is in agreement with the observed one for s7naller content of radioactive elements than it was admitted earlier. the difference of heat flows calculated for the continental and oceanic parts of the crust amounts only 30-40%. this can explain why the observation does not reveal systematic differences between heat flows on continents and on oceans. o n t i i e t h e r m a l h i s t o r y o f t h e e a r t h 151 riassunto la storia termica viene calcolata per vari modelli della terra inizialmente fredda. la formazione della crosta terrestre viene considerata come un processo che si protrae nel tempo e che e cominciato 3 x 10" anni or sono. nel caso di grande conduttivita radiattiva, il flusso di calore calcolato attraverso la superficie e in accordo con quello osservato per un contenuto minore di elementi radioattivi di quello che era stato precedentemente ammesso. la differenza di flusso di calore calcolato per le parti continentali e oceaniche della crosta raggiunge solo il 30-40%. questo pud spiegare perche vosservazione non rivela differenze sistematiche tra il flusso di calore registrato nei continenti e quello negli oceani. r e f e r e n c e s birch f . , nuclear geology. ( e d . h . f a u l ) , c h . 5, 1 4 8 1 7 4 , ( 1 9 5 4 ) . bullard e . c . , maxwell a . e . , revelle r . , advances in geophysics, 1531 8 1 , ( 1 9 5 6 ) . jacobs j . a., handb. phys. (hrsg. s. fliigge), 47, 364-406, (1956). lubimova e . a., bull. acad. sci. ussr, geophys. ser., 2-3, 14, (1952). — a) publ. geophys. inst. acad. sci. ussr, 26 (153), 39-50, (1955). — b) bull. acad. sci. ussr, geophys. ser., 5, 416-424, (1955). — a) or (doklady) acad. sci. ussr, 107, 1, 55-58, (1956). — b) bull. acad. sci. ussr, geophys. ser., 10, 1145-1160, (1956). — bull. acad. sci. ussr, geophys. ser., 5, 673-676 (1957). — geoph. joum., 1, 2, 1 1 5 1 3 4 , ( 1 9 5 8 ) . lukyanov v. s., bull. acad. sci. urss, otdeleniye technich. nauk, 2, 5 3 6 7 , ( 1 9 3 9 ) . macdonald g . , joum. geophys. res. 6 4 , 11, 1 9 6 7 2 0 0 0 , ( 1 9 5 9 ) . safronov v. s., bull. acad. sci. ussr, geophys. ser., 1, 139-143, (1959). s t a r i k j . e . a n d s h a t z m . m . , geochemistry ( u s s r ) , 2 , 1 9 2 6 , ( 1 9 5 6 ) . s t a r k o v a a . ' g . , meteoritics ( u s s r ) , 1 3 , 1 9 3 2 , ( 1 9 5 5 ) . u f f e n r . , trans. am. geoph. un., 3 3 , 6, 8 9 3 8 9 6 , ( 1 9 5 2 ) . zharkov v. n., bull. acad. sci. ussr, geophys. ser., 11, 1342-1350 (1958). — bull. acad. sci. ussr, geophys. ser., 3, 465-470, (1959). t h e hydrostatic equilibrium and its implications on the mechanical properties of planets p . b a l d i m . captjto ( * ) r e c e i v e d on a p r i l 7th, 1974 summary. — under the hypothesis of hydrostatic equilibrium the planets of the solar system are represented on the plane (j"2, w2 a?/mg) (j 2 = second term of the g r a v i t a t i o n a l potential, w = rotation rate of the planet, a — equatorial radius, m — mass of the planet, g = g r a v i t a t i o n a l constant). since the points representing the planets lie on a straight line it is possible t o obtain informations on the giration m o m e n t and the m o m e n t of inertia of uranus and p l u t o . riassunto. — p a r t e n d o dall'ipotesi di equilibrio idrostatico, i pianeti del sistema solare sono rappresentati nel piano ( j 2 , iv-a?! mg), d o v e j2 è il secondo termine dell'espressione del potenziale gravitazionale, w la velocità di rotazione del pianeta, a il raggio equatoriale, m la massa del pianeta, g la costante g r a v i t a z i o n a l e . p o i c h é i punti che rappresentano i pianeti giacciono su una linea retta, è possibile avere informazioni sul m o m e n t o giratore e sul m o m e n t o di inerzia di urano e p l u t o n e . 1. introduction. i n v e s t i g a t i o n s so f a r c a r r i e d o u t o n t h e solar s y s t e m w o u l d s e e m t o i n d i c a t e a f a i r d e g r e e of r e g u l a r i t y i n t h e s y s t e m i t s e l f , a n d also t h e p r e s e n c e of s p e c i a l m e c h a n i s m s ( 5 ) : a m o n g t h e s e l e t us r e c a l l t h e r e g u l a r i t y of t h e o r b i t s of t h e p l a n e t s ( s l i g h t i n c l i n a t i o n , s l i g h t e c c e n t r i c i t y , a n t i c l o c k w i s e m o t i o n , e t c . ) ; i n t h e l a w g o v e r n i n g t h e r e l a t i o n ( * ) istituto di fisica e istituto di geofìsica, università di b o l o g n a . 230 p . b a l d i m . c a p u t o ship of the semimajor axes of two contiguous planets (a n+i wn — const., titius-bode law); the alignment of the representative points of the planets in the gw/m,m plane (7). the study of the motion of artificial and natural satellites makes it possible to obtain the parameters linked to the internal and external structure of the planet such, for example, as the total mass m , the flattening at the poles e, ji (the second term of gravitational potential), the moments of inertia. considering the planets to be in hydrostatic equilibrium by means of the above-mentioned terms it is possible to arrive, as we shall later show, at a knowledge of the values of p (c/mr2), and consequently at the value of the moments of inertia of almost all the planets. 2. t h e hydrostatic equilibrium of a planet. let us consider a planet in hydrostatic equilibrium: approximating in the first order, the equations that link flattening and j2, m ( = w2 r3/mg), p (= c/mr2), are (*): m [1] 225 p2 — 300 p + 84 + 16 15 j2 — 3 • = 0 [2] where eq. [1] holds even in the absence of hydrostatic equilibrium. eliminating e it follows that: 21 • m [3] the obvious considerations follow from eq. [3]; real p and p > 0 which imply: or a < + 1 j2 < 1.33 to a > — 21/4 j2 > —3/39 to t h e h y d r o s t a t i c e q u i l i b r i u m a n d i t s i m p l i c a t i o n s e t c . 2 3 7 rejecting the case where the moment of inertia with respect to an equatorial axis of the planet is greater than the moment of inertia with respect to the polar axis (j < 0), there will be a further limitation for a which will have to be greater than —3; from this it follows that p will have to be greater by about 0.13, that is the possibility is excluded of an excessive concentration of mass at the centre. 3. h y d r o s t a t i c equilibrium i n the solar system. for the purpose of investigating the planets in the solar system in the light of the considerations set out above, let us represent such planets in the m, ji plane. w e note immediately that the points representing those planets whose fundamental parameters we know: mass, average radius, rotation period and j2, belong with good approximation to a straight line that we shall call a. in this approximation one can obtain interesting information regarding the values of the moments of inertia of uranus and pluto and the mass of pluto. 4. m o m e n t s of i n e r t i a of uranus a n d pluto. lsi hypothesis: let us suppose that the straight line must pass through the origin (p = const.), as well as through the points of the plane m, ji that represent the planets of which the mass, the radius, the angular rotation rate and the value of ji (fig. 1) are known. the value of "average" p is found to be about 0.229 for all planets (ji/m = const., approximating in the first order, imply a = const.). the moments of inertia of uranus is found to be: c = 1.31 • 1047. for pluto, using the observed values of r = 3200 km (3), m = 6.59 • 1026 gr (8), we obtain the value (1) of table i i . 2nd hypothesis: a straight line is plotted bearing in mind the known precision of the parameters of the various planets without forcing it to pass through the origin (pig. 2). for uranus we obtain a value of ji equal to 0.0089, a value of the dynamical flattening equal to 0.05, in good agreement with the geometrical value derived from visual observations (6), a value of p equal to 0.232, and a moment of inertia g = 1.33 • 1047 gr • cm2. • ¿ t a jke-.jujl i w ^ m*m.jl 7 m = w 2 r 3 / h g fig. 1. representation of the planets in the plane m = w-a?img, <72; the linear regression approximating the points overall is made to pass through the origin. t h e h y d r o s t a t i c e q u i l i b r i u m a n d i t s i m p l i c a t i o n s e t c . 241 230 p . b a l d i m . c a p u t o i o 2 6 , 0 2 7 , o 2 8 1 0 = 9 1 0 3 ° p i g . 4. the density of the angular moment in relation to the mass. for pluto, making use of the values so far known and very uncertain of the mass and the radius, there would seem to be found a value of p greater than 0.4, which is unacceptable. i t must however be noted that the determination of the mass of pluto is obtained from the study of the perturbations of neptune, that is from the perturbations of a very small body on a very large one, and it is therefore extremely unreliable because a small error in the mass of neptune causes a very large error in the mass of pluto. a further difficulty arises from the fact that up to the present only a fraction of revolution of the outer planet has been observed. the best value of the mass obtained so far is 6.59-1026 gr (8) and taking the radius of 3200 km (3), one obtains an average density of 4.8 gr/cm3, too high for an outer planet. also we shall' consider the radius of pluto 2700 km estimated from optical observation (4), although t h e h y d r o s t a t i c e q u i l i b r i u m a n d its i m p l i c a t i o n s e t c . 2 4 3 this radius, with the observed value of the mass gives an excessively high value of the mean density. considering now the average-mass density plane, one can observe that the planets take up positions along two lines: on one side we find terrestrial planets, on the other the outer planets (fig. 3). pluto, an outer planet, should therefore belong to the straight line of outer planets and have an average density of about 2 gr/cm3 or less; accepting this hypothesis, there follows a diminution of the mass. if we want to use the straight line which does not pass tlirough the oiigin, a a'alue of p at most as large as 0.4, and to keep the values of the radius, we obtain a value of o at most 0.3 and for g the values indicated in lines (2) and (3) of table i i . using q = 0.3, the same values of r and the straight line passing through the origin, we obtain the values of c given in lines 4 and 5 of the same table, while assuming q = 2 we obtain g as in lines (0) and (7). for a further check, we set out in fig. 4 the values obtained by us of the "density of rotational angular momentum" (cw/m) of uranus, on the straight line obtained by macdonald (7) for planets whose moments of inertia are known with a fair degree of precision. the agreement of the data obtained here with those observed for other planets is satisfactory. obviously in table i i one finds that the values of the density of rotational angular momentum in lines (1), (4), (6) coincide as well as those of lines (5), (7); the difference of the two sets is due only on the different values of r because the density of rotational angular momentum gwjm can also be written pwr-. the values of gw/m of line (1) is too far below the straight line of macdonald; it would be difficult to explain the loss of the initial energy of the planet. the values of lines (2) and (3) are too far above and it would be difficult to explain the gain of energy. the only reasonable models of pluto are those of lines (4), (5), (6) and (7). r e f e r e n c e s (*) caputo, m., 19g7. the gravity field of the earth. academic press, n e w y o r k . ( 2 ) cook, a . h . , 1972. the dynamical properties and internal structures of the earth, the moon and the planets, "l'roc. r . soc.", lond., a328, 3 0 1 . 230 p. b a l d i m. c a p u t o ( 3 ) h a l l i d a y , i . , h a r d i e , r . k . , f r a n z , 0 . a n d p r i s e r , j . b . , i 9 6 0 . " p u b i . astron. soc. p a c i f i c . " , 78, 113. (>) halliday, i., 1969. " p u b i . astron. soc. p a c i f i c . " 81, 285. (5) kaula, w . a . , 1968. an introduction to planetary physics. john wiley sons, inc. (•) kovalevsky, j., 1970. determination des masses des planètes el satellites. in (ed. a . dollfus), "surfaces and interiors of planets and satellites", 1-44, london, academic press. ( ' ) macdonald, g. j. f., 1964. tidal friction. " r e v . geopliys.", 2, 467541. ( 8 ) s e i d e l m a n n , p . k . , k l e p c z y n s c k i , w . j . , d u n c o m b e , r . l . , 1 9 7 1 . determination of the mass of pluto. " a s t r o n . j . " , 76, 488. layout 6 experimental studies of anomalous radon activity in the tlamacas mountain, popocatepetl volcano area, méxico: new tools to study lithosphere-atmosphere coupling for forecasting volcanic and seismic events anatoliy kotsarenko1,*,vladimir grimalsky2, vsevolod yutsis3, ana gabriela bravo osuna1, svetlana koshevaya2, hector roman peréz enríquez1, g. urquiza beltrán2, jose antonio lopez cruz abeyro1, carlos valdés gonzales4 1 universidad nacional autónoma de méxico (unam), centro de geociencias, querétaro, mexico 2 universidad autónoma del estado de morelos (uaem), ciicap, cuernavaca, morelos, mexico 3 universidad autónoma de nuevo león (uanl), facultad de ciencias de la tierra, nuevo león, mexico 4 universidad nacional autónoma de méxico (unam), instituto de geofísica, méxico d.f., mexico annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5318 abstract this study presents and discusses the results of soil radon monitoring at three different volcano sites and one reference site, from december 2007 to january 2009. this relates to the activity of the popocatepetl volcano and a radon survey and gamma-ray spectrometry in the area between paso de cortes and tlamacas mountain, and in the adjacent regions. the results are applied to the aspects of atmosphere electricity and lithosphere-atmosphere coupling in relation to the forecasting of volcano and earthquake activity. the monitoring of radon release reveals a decrease in radon concentration (down to total suppression) with approaching moderate volcanic eruptions. the behavior of the radon activity at the tlamacas site is more apparent, compared to other observational sites. the average level of radon release observed at the tlamacas site is much higher, with some characteristic variations. both the radon survey and gamma-ray spectrometry indicate intensive diffusion radon emission localized in the area of tlamacas mountain. the average radon concentration in the area of tlamacas is about 10-20-fold greater than the background volcano values. the new concept of lithosphere-atmosphere coupling is presented: intensive radon release in high elevated areas shortens and modifies the earth-to-thunderclouds electric circuit, which provokes microdischarges into the air close to the ground, attracting lightning discharges. this concept attempts to explain in a new way the noise-like geomagnetic emissions registered before major earthquakes, and it promotes interest for the study of thunderstorm activity in seismo-active zones, as a promising instrument for earthquake forecasting. 1. introduction the popocatepetl volcano (nicknamed «el popo») is located in central mexico (latitude, 19.07˚n; longitude, 98.63˚w; altitude, 5,465 m). it is one of the active volcanoes that forms the trans-volcanic belt of mexico (also know as the neovolcanic axis), and its existence is related to the geodynamics of the north american and coco plates. el popo is a major potential hazard in mexico, and therefore its eruptive activity is a subject of permanent monitoring by the national center for prevention of disasters (cenapred). every day, a summary of the behavior of popocatepetl volcano can be found on the cenapred webpage (http://www.cenapred.unam.mx/ cgi-bin/popo/reportes/consultai.cgi). the activity of popocatepetl volcano can be briefly summarized as follows. light eruptions are permanent everyday events, with several to tens of eruptions of gas and water seen daily during the quiet volcano phases, and up to 50100 eruptions occurring in the active phases [see gonzálezpomposo 2004, and table 2 in arámbula-mendoza et al. 2010, for detailed description of the recent popocatepetl activity stages]. moderate eruptions of gases and volcano ash take place from once every few months up to several times per day, for the quiet and active phases, respectively. moderate eruptions can also be accompanied by explosive elements (rock ejection), with the frequency of such events as several to dozens of events per year in recent times. major eruptive activity is not so frequent, and the last time it occurred it lasted from december 2000 to january 2001. at this stage, intensive rock expulsions, lava eruptions and pyroclastic flows can take place, in addition to the above-mentioned phenomena. tectono-volcanic microseismic events with magnitudes up to ms 2-3 and high frequency tremors are also part of the volcano activity. special issue: earthquake precursors 109 article history received july 16, 2011; accepted december 12, 2011. subject classification: radon, volcano eruption, earthquake, precursory phenomena, atmosphere electricity, lithosphere-atmosphere coupling. our earlier studies in the popocatepetl volcano area (2003-2006) showed a variety of geomagnetic anomalies that were associated with volcano activity [kotsarenko et al. 2007] and also allowed us to propose the hypothesis of the existence of a second magmatic reservoir [kotsarenko et al. 2008]. the present study is devoted to various aspects of radon monotoring that was carried out in the first stage of our experimental investigations into the area of popocatepetl volcano (december 2007 to december 2009). in the first part of this study, we present the results of the soil radon monitoring that was carried out at three different sites in the volcano area, and we discuss the phenomena that accompany volcano eruptions. similar studies have been carried out in different active volcanoes all over the world, as a perspective tool for the forecasting of volcano hazard, including in italy [baurbon et al. 1991, cigolini et al. 2001, 2005], france [segovia et al. 1997, toutain et al. 2002], spain [viñas et al. 2007], the azores [aumento 2002], colombia [londoño 2009], costa rica [barquero et al. 2005, garcía-vindas et al. 2002], mexico [armienta et al. 2002, segovia 1991, segovia et al. 1997, 1999, 2001, 2003, 2005, 2007, varley and armienta 2001, varley 2003, varley and taran 2003], nicaragua [connor et al. 1996], and indonesia [baurbon et al. 1991]. the second part of this study deals with combined studies using radon surveys and gamma-ray spectrometry. gamma rays are the most penetrating radiation from natural and man-made sources, so gamma-ray spectrometry is a powerful tool for monitoring and assessing the radiation environment [iaea 1989, 2003]. potassium, uranium and thorium are sensitive to tectonic fractionation, so that gamma-ray spectrometry provides a method of conveniently and rapidly measuring compositional changes over large areas of rough terrain [thorpe 1995, chiozzi et al. 1998]. radon surveys are recognized as a reliable tool for revealing tectonic structures [king 1980, king et al. 1993, 1994, 1996, chyi et al. 2002, etiope et al. 2005, vivek et al. 2005] and volcano-tectonic geological formations [martín et al. 2003, burton et al. 2004, hernandez et al. 2004]. in the third part of the present study, we examine previously observed geomagnetic anomalies [kotsarenko et al. 2007] and enhanced thunderstorm activity, in terms of the lithosphere-atmosphere coupling caused by intensive radon release under specific conditions in the area of tlamacas mountain. 2. soil radon monitoring in the popocatepetl volcano area 2.1. equipment and methods portable solid-state shallow subsurface radar (sharad) scout detectors were used for the monitoring of radon concentrations in the soil at the tlamacas station site (latitude, 19.065˚n; longitude, 98.63˚w; referred to as «tlamacas»), at a site near paso de cortes (latitude, 19.07˚n; longitude, 98.65˚w; «paso de cortes»), at a site near tlamacas mountain (latitude, 19.07˚n; longitude, 98.63˚w; «tlamacas 2»), and at an amecameca referent site (latitude, 19.12˚n; longitude, 98.77˚w) (figure 1). these instruments use the diffusion 110 kotsarenko et al. figure 1. left: map showing the amecameca referent site (a), and the paso de cortes (p) and tlamacas (t) sites. right: map of volcanic area showing the paso de cortes, tlamacas and tlamacas 2 sites. 111 method of air sampling (without pumping the air into the chamber), and therefore in a normal environment (here, «normal» meaning «not aggressive»; see streil et al, 2008), the dominant monitored content comes from the most stable isotope of 222rn (referred to here as «radon»; t1/2 = 3.8 days; produced in the 238u radioactive decay chain). another isotope, 220rn (referred to here as «toron»; t1/2= 55.6 s; parent element, 232th) is rarely detectable under normal environmental conditions, as it decays before it can penetrate into the chamber. the scout detectors were also equipped with simple sensors to measure the basic meteorological parameters: temperature, relative humidity, and atmospheric pressure, in addition to radon concentration. nine detectors were used alternatively for measuring the radon at these monitoring sites, with sampling rates of 1 point per 60 min (for instruments with internal memory, 2047 pts), and 1 point per 90 min (1023 pts), to provide uninterrupted data for each consistent époque of measurements, with a duration of 1-2 months. 2.2. results and discussion the monitoring of the radon emissions at these four stations (figure 2) showed some stable tendencies. first of all, the average values of the radon concentrations observed at the tlamacas site were normally 1.5-5-fold greater than those measured at the tlamacas 2 site, 2-15-fold greater than at paso de cortes, and 25-400-fold greater than the amecameca reference site values. then, there was a distinct difference between the data recorded at the volcano sites. the paso de cortes and tlamacas 2 data usually showed noise-like fluctuations around a stationary level, whereas the fluctuations at the tlamacas station showed a kind of «alive» character, with radon levels that can change rapidly through the day. in analyzing the radon concentration variations, we focused on the days with moderate eruptive activity, with the data of the eruptions obtained from the cenapred webpage. in the period analyzed, moderate eruptive activity was detected for a total of 27 days (39 eruptions, several with rock ejection, some days with 2-4 eruptions per day), 23 of which (32 eruptions) were covered by observations for at least at one volcano station. for 9 days (13 eruptions), we detected a clear decrease (depression) in the radon emmission before and during the eruptions (figure 2, red bars and arrows). the radon release almost ceased for 4 days before the strongest event, that of january 28, 2008 (see figure 3). during the day of this strongest event, intensive eruptive activity with rock and sporadic lava ejection were detected. indeed, the period from january 28, 2008, to february 21, 2008, was when the most active eruptive activity was detected. at the same time, before the eruption on january 28, 2008, during the period of january 17-22, 2008, the radon concentrations fell from 900 bq/m3 to 200 bq/m3. furthermore, another total radon decrease (which lasted about 20 h) was detected before an eruption on february 11, 2008. for the following 19 days, which showed moderate eruptive activity, we detected 7 periods of radon decrease, 4 of radon activity in popocapetl area figure 2. radon emission monitoring at the three volcano sites. top to bottom: tlamacas, tlamacas 2, paso de cortes and amecameca referent site. monitoring from december 2007 to december 2009. middle panel: volcano eruptions. blue bars, eruptions of gas and water; black bars, moderate eruptions; red bars and arrows, moderate eruptions where radon decreases were observed. which were seen simultaneously for 2 or 3 of the volcano stations (in all other cases, the data were lost in 1 or 2 stations due to instrumental failure). again, the most pronounced radon depletion was detected at the tlamacas observation site, although the radon levels never decreased to zero. in many cases, moderate eruptions were not accompanied by decreases in the radon levels, and no eruptions occurred during some of the other radon decreases. furthermore, there were better correlations between the radon changes observed at the paso de cortes and tlamacas 2 sites than between these two sites and tlamacas; the variations in the radon concentrations at the tlamacas site were sometimes of a very peculiar character. summarizing this part of our studies, a couple of phenomena are worth considering. first of all, the radon behavior can be anticipated during moderate eruptions. unlike the well-known intensifications of radon release before earthquakes, volcano systems are more ambiguous: many studies have reported increased radon emission before major events like explosions and pyroclastic flow, while others have also reported numerous decreases during moderate eruptive activity. being a heavy gas, radon requires a transport mechanism from the depth where it is generated. this transport is provided by another volcano gas, carbon dioxide (co2). streil et al. [2008] explained the depletion of radon as a shortage of co2due to geodynamic processes in the volcano system. other, relatively unexpected results were the considerable difference in the radon de-gassing levels, and the periodic absence of correlation between tlamacas and the other observation sites. radon emissions depend on a variety of different factors, such as deposits of radioactive elements, presence of active tectonic structures, increased fracturing, soil porosity and penetrability, and meteorological conditions (most importantly, atmospheric pressure). the meteorological conditions at the volcanic observation sites were the same for all of the instruments, although no information was available about the radioactive deposits in these areas. for the soil composition and penetrability, the tlamacas site is situated on a solid rock basement, while the tlamacas 2 and paso de cortes sites lie on mellow soil. thus, the natural activity of radon at these sites was expected to be higher, contrary to the results, as if tlamacas mountain was not an active geological structure. 3. radon survey and gamma-ray spectrometry in the tlamacas area a combined study using a radon survey and gamma-ray spectrometry was performed for tlamacas mountain and in the nearby areas, to check our hypothesis relating to the anomalous character of the geodynamical processes of tlamacas mountain. gamma-ray spectrometry was chosen as a credible tool to reveal possible radioactive deposits in the area, and to make a decision as to whether the radon emissions were of superficial (i.e. from uranium and thorium deposits) or subterranean (diffusion) origin. 3.1. equipment and methods we examined the area by measuring the variations in the total radiometric response, and k, u and th concentrations using a portable grs 500 gamma-ray scintillometer spectrometer (scintrex ltd.). each sample site had a series of 12 readings, with a duration of 10 s per reading (0.1 cps), so the total time of observation for every site was 2 min. this technique provides a rational minimization of the signal attenuation and a good signal-to-noise ratio. the radon survey was carried out using the abovementioned radon scout instruments. as has been shown, radon emissions in volcano areas can change drastically, even over a single day. to avoid this problem, we limited the duration of our survey to daylight hours on one day, and used kotsarenko et al. 112 figure 3. radon emission monitoring. top: tlamacas volcano site. middle: amecameca referent site. monitoring from january to february 2008. bottom panel: volcano eruptions. blue bars, eruptions of gas and water; black bars, moderate eruptions; red bars and arrows, moderate eruptions where radon decreases were observed. 113 5 identical detectors to carry out simultaneous independent measurements. the test experiment was performed at the tlamacas site to compare the instrument responses and determine the time domains (sampling) for any independent measurements. we placed the 5 instruments in a hole of about 1 m2, at 80 cm in depth, near the tlamacas station, for 2 days of correlated measurements, with a 30 min sampling rate. the comparison of the characteristics obtained for the 5 detectors showed the need for correction of one detector (#091), whereas the measurements of the other 4 detectors were comparable and within the instrumental error (figure 4). the correction coefficient for detector #091 was calculated as radon activity in popocapetl area figure 4. correlated radon emission measurements for the five detectors tested. tlamacas station site, december 2-4, 2009 (30 min sampling time). reading point easting (km) northing (km) gamma ray total energy (cps) potassium (%) equivalent uranium (ppm) equivalent thorium (ppm) (tl1) tl-bl 537.146 2110.417 159.712 0.250 2.066 6.713 tl2 539.091 2108.297 151.332 0.130 2.163 7.672 tl3 539.012 2108.286 196.544 0.054 3.933 8.333 tl4 539.169 2108.339 164.148 0.292 3.844 5.926 tl5 539.135 2108.181 212.951 0.095 5.200 7.407 tl6 538.960 2107.984 163.106 0.021 1.097 5.370 tl7 538.826 2107.817 157.049 0.123 0.954 6.173 tl8 538.663 2107.642 147.066 0.109 0.169 7.593 tl9 538.404 2107.462 167.172 0.024 6.124 8.519 tl10 538.171 2107.477 167.001 0.148 2.968 4.815 tl11 538.025 2107.383 180.070 0.027 2.488 6.019 tl12 537.874 2107.578 171.968 0.180 3.237 10.494 tl13 537.861 2107.794 143.492 0.817 0.680 0.200 tl14 537.733 2108.009 153.104 0.216 2.786 7.222 tl15 537.613 2108.297 157.569 0.125 1.884 7.819 tl16 537.873 2108.684 143.575 0.135 1.417 10.741 tl17 537.434 2108.569 166.170 0.438 0.889 5.787 tl18 537.283 2108.820 164.408 0.161 0.453 7.222 tl19 537.114 2109.041 165.309 0.460 7.026 7.778 tl20 536.870 2109.254 162.966 0.675 4.631 8.148 tl21 536.775 2109.404 162.445 0.398 2.040 7.037 tl22 536.794 2109.631 166.571 0.273 0.983 3.241 tl23 536.913 2109.870 161.624 0.121 0.165 7.639 tl24 536.988 2110.034 157.249 0.191 0.675 7.222 tl25 537.112 2110.222 166.682 0.442 0.822 4.861 tl26 536.561 2110.102 152.483 0.164 1.200 2.778 table 1. gamma-ray analysis results (total counts above 0.08 mev). tl-bl, local base station: paso de cortes. easting, northing, datum wgs84, utm zone 14. 1.37. an optimal sampling time of 40 min was used, to cover the selected area of our survey from paso de cortes to tlamacas, with a spatial resolution of 200 m to 400 m found between independent points during the daylight hours of a single day. 3.2. results and discussion the results obtained are presented in tables 1 and 2, and shown in figure 5. above all, the gamma-ray spectroscopy revealed a moderately enhanced gamma -ray background (figure 5a) at the top of tlamacas mountain, while the levels of uranium and thorium concentrations (figure 5c, d) were found to be comparable with the average levels (table 1). the radon survey confirmed the existence of a strong radon anomaly in the area of tlamacas mountain (figure 5f): the radon released at the sites of tlamacas mountain was 10-20fold greater than the background level (table 2). the lack of coincidence between the maximum of rn and u or th, respectively, indicated the subterranean (diffusion) origin of the radon. the location of the other three maxima of radon concentrations coincided well with three th/u ratio maxima. this might indicate a proportionality between the 220rn and 222rn isotopes in the total radon content, and that their origin most likely lies in superficial deposits of these mentioned elements. thus, we have demonstrated our assumption relating to the active geological structure in the tlamacas mountain area. these results can explain the differences in radon behavior between tlamacas and the other observational sites, as mentioned in section 2. kotsarenko et al. 114 table 2. radon concentration results (40 min sampling time). point n˚ sensor n˚ latitude (˚n) longitude (˚w) radon (bq/m3) instrument error (%) 1 all (1st test) 19.0856 98.6536 6 98 2 091 19.0832 98.6483 109 38 3 148 19.0808 98.6494 235 24 4 198 19.0676 98.6506 106 35 5 090 19.0761 98.6497 31 71 6 147 19.0742 98.6478 67 45 7 091 19.0725 98.6461 156 33 8 148 19.0703 98.6444 332 20 9 198 19.0706 98.6414 119 33 10 090 19.0697 98.6404 141 33 11 147 19.0675 98.6422 17 100 12 091 19.0650 98.6414 109 38 13 148 19.0618 98.6401 125 33 14 198 19.0592 98.6389 93 38 15 090 19.0597 98.6368 78 45 16 147 19.0594 98.6356 26 71 17 091 19.0606 98.6332 529 17 18 148 19.0628 98.6311 222 25 19 198 19.0650 98.6292 119 33 20 090 19.0664 98.6278 703 15 21 147 19.0675 98.6267 1058 11 22 091 19.0679 98.6281 1244 11 23 148 19.0675 98.6275 2407 8 24 198 19.0675 98.6286 904 12 25 147 19.0669 98.6281 1757 9 26 all 19.0671 98.6282 546 9 27 all 19.0675 98.6278 2789 8 figure 5a-c. (a) gamma ray energy (total count above 0.08 mev); (b) potassium (k; %) content; (c) uranium (u; ppm) content 115 3.3. limitations of the radon survey method we should mention some clear limitation of our study. the sharad scout detectors were not equipped with pumps: the air sampling was carried out by natural diffusion. due to this, the effective measurements of the actual values are obtained with some delay, which depends, first of all, on the volume of the pit and the correct radon concentration at the measurement site. specifically, in our test experiment, the effective transition time was about 4 h (figure 4). under similar conditions, the values of radon measured in the first 40 min (figure 4, red circle) will be 6-9-fold lower than the expected actual values. this means that our method is eligible in related units, i.e. it can serve to locate sites with anomalously increased radon values, but the absolute radon concentrations will be 69-fold greater than the detected values. in this regard, the radon map obtained (figure 5f) is valid only for comparisons. another disadvantage of our method is that the daily values obtained can be affected by the diurnal variation. however, there is no effective way to avoid this drawback. in some cases, a base station can be established in the vicinity of the mapping area, to measure the diurnal variation, which can then be applied for the correction of the radon levels measured at different times. in our case, this method mentioned before is not reliable: all of the base stations should be established at least several days in advance of the survey, and remain at the site to obtain enough data for solid statistics. furthermore, in «aggressive» environments such as volcanoes, diurnal variations can get lost among high levels of noise-like changes (which was also in our case). 4. radon and atmospheric electricity: new aspects of lithosphere-atmosphere coupling? 4.1. new view on the volcano and seismogenic geomagnetic emissions the intensive radon release for tlamacas mountain encouraged us to revise our earlier discovered phenomena [kotsarenko et al. 2007, 2008]: long-term noise was observed in the geomagnetic field measured at the tlamacas station (figure 6). to estimate the noise contribution, we compared the noise character for the tlamacas station registered during a highly perturbed day with a referent signal contaminated by electric welding measured 200 m away from the welding source in campus juriquilla, querétaro, during the construction of a new building. the natural perturbations observed for tlamacas were equal or even higher in amplitude compared with the intense corona discharge (electric welding currents of up to 100 a). how can this be connected to the observed radon emission phenomena? increased radon release produces high ionization in the atmosphere [pulinets and boyarchuk 2004], which changes the electric field profile. moreover, in the case of the tlamacas station, an equivalent electric circuit between the earth and thunderstorm clouds (8-12 km) changes strongly and gets much shorter for the altitude of tlamacas mountain (4 km). thus, any heterogeneity in the ground surface becomes the source of micro-discharges, which are detected as integrated electromagnetic noise by the magnetometer. 4.2. thunderstorm lighting as a possible indicator of forthcoming earthquakes another phenomenon of the same nature is often reported by witnesses: the frequency of lightning in the radon activity in popocapetl area figure 5d-f. (d) thorium (th; ppm) content; (e) th/u ratio; and (f) radon concentrations between the tlamacas and paso de cortes stations (top part of the map) and in the area of tlamacas mountain (ellipse). tlamacas mountain is higher than in other sites (one such lightning strike destroyed our magnetometer and burned out a cpu despite the presence of a lightning-rod on the station building). this can also be explained in terms of the atmosphere electricity: the increased ionization and closeness to the thunderclouds attracts lightning, as happens during thunderstorms. these phenomena have significant scientific applications. first of all, tlamacas mountain becomes a unique natural laboratory for experiments with atmosphere electricity. in terms of lithosphere-atmosphere coupling, tlamacas mountain performs as a natural reactor of ionization that modifies its electrical profile, provoking intensive integrated noise-like currents (by means of microdischarges) in the near-surface air layer, and enhanced thunderstorm activity between the earth surface and the thunderstorm clouds. thus, the presented concept can re-interpret the noiselike geomagnetic emissions observed before major earthquakes in the earthquake preparation zone. the origin of these mentioned emissions might be attributed to the proposed mechanism of micro-discharges in the highly ionized superficial air. finally, we can propose the following hypothesis: monitoring of lighting activity in seismo-active areas can be of help for the forecasting of major earthquakes. the preliminary confirmation of this statement came from a study dedicated to the alum rock earthquake (october 30, 2007, california, usa) [bleier et al. 2009]. this study examined the lightning activity in the earthquake preparation area, to remove the interference of thunderstorm-related pulses from geomagnetic signals. the maximum number of lightning strikes was detected the day before the earthquake in the area of the sierra nevada mountains. we believe that this is a similar example in nature to that presented in our study. 5. conclusions our results of the radon monitoring at three different sites of popocatepetl volcano mountain and a referent site revealed that radon depletion preceded nine cases of moderate eruptive activity among 23 total events. the most pronounced reaction was observed at the tlamacas observational site. the average radon concentration at the paso de cortes and tlamacas two sites was significantly lower in comparison with that at tlamacas; the radon variation in these other two sites has many specific features, while for the tlamacas site, the radon behavior shows a more individual character. the combined study using the radon survey and gamma-ray spectrometry revealed an anomalously increased diffusion radon emission localized in the area of tlamacas. a new concept is proposed regarding lithosphereatmosphere coupling in the case of tlamacas, which appears kotsarenko et al. 116 figure 6. geomagnetic signals. left, top, original signals; left, bottom, filtered signals. blue, tlamacas station; red, referent juriquilla station. right: spectra measured at tlamacas (top) and at the referent juriquilla station (bottom, red). 117 to be similar in nature to a shortened electrical circuit from the earth to the thunderstorm clouds (high-altitude mountains), such that the enhanced ionization caused by intensive radon release can explain, in a novel way, the noiselike geomagnetic emissions observed before destructive earthquakes. we also propose the study of thunderstorm activity in the earthquake preparation zone as a promising tool for the forecasting of destructive earthquakes. acknowledgements. we devote our article to the memory of our good friend marcos galicia lopez, who always helped us in our volcano studies and who maintained the tlamacas station. this work has been partially funded under the mexican unam dgapa papiit projects in120808 and in109411. references arámbula-mendoza, r., c. valdés-gonzález and a. martínez-bringas (2010). temporal and spatial variation of the stress state of popocatépetl volcano, mexico, j. volcanol. geoth. res., 196, 156-168. armienta, m.a., n. varley and e. ramos (2002). radon and hydrogeochemical monitoring at popocatépetl volcano, mexico, geofís. internac., 41, 271-276. aumento, f. 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(2003). soil gas concentration at popocatépetl, méxico, vulcânica, i, 155-162. varley, n.r. and y. taran (2003). degassing processes of popocatepetl and volcán de colima, mexico, in: edited by c. oppenheimer, d.m. pyle and j. barclay, volcanic degassing, geological society, london, special publications, 213, 263-280. viñas, r., a. eff-darwich, v. soler, m.c. martín-luis, m.l. quesada and j. de la nuez (2007). processing of radon time series in underground environments: implications for volcanic surveillance in the island of tenerife, canary islands, spain, radiat. meas., 42, 101-115. vivek, w., t.c. su, c.c. fu and t.f. yang (2005). spatial variations of radon and helium concentrations in soil-gas across the shan-chiao fault, northern taiwan, radiat. meas., 40, 513-516. *corresponding author: anatoliy kotsarenko universidad nacional autónoma de méxico (unam), centro de geociencias, querétaro, mexico; e-mail: kotsarenko@geociencias.unam.mx © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. kotsarenko et al. 118 annals 47, 6, 2004 annals of geophysics, vol. 47, n. 6, december 2004 short note space-time combined correlation integral and earthquake interactions patrizia tosi (1), valerio de rubeis (1), vittorio loreto (2) and luciano pietronero (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) centro infm di meccanica statistica e complessità, dipartimento di fisica, università degli studi «la sapienza», roma, italy abstract scale invariant properties of seismicity argue for the presence of complex triggering mechanisms. we propose a new method, based on the space-time combined generalization of the correlation integral, that leads to a self-consistent visualization and analysis of both spatial and temporal correlations. the analysis was applied on global medium-high seismicity. results show that earthquakes do interact even on long distances and are correlated in time within defined spatial ranges varying over elapsed time. on that base we redefine the aftershock concept. key words seismicity correlation integral fractal dimension clustering 1. introduction seismicity appears to be scale invariant in many of its aspects. several papers (kagan, 1994; bak et al., 2002; parson, 2002; marsan and bean, 2003; corral, 2004) investigate spa­ tial and temporal correlations of epicentres, in­ volving for example the concepts of omori law and fractal dimension. we think that the com­ plex phenomenon of seismicity calls for an ap­ proach capable of analysing spatial localisation mailing address: dr. patrizia tosi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: tosi@ingv.it and time occurrence in a combined way and without subjective a priori choices. in this pa­ per we introduce a new method of analysis that leads to a self-consistent analysis and visuali­ zation of both spatial and temporal correlations based on the definition of correlation integral (grassberger and procaccia, 1983). 2. method we define the space-time combined correla­ tion integral as 2 c ( , ) = $c r τ (n n 1) n -1 n $ ! ! aθ r̀ ­ xi x j j $ θ `x ­ ti -t j jk i= 1 j = i + 1 where θ is the heaviside step function ( θ(x) = 0) if x ≤ 0 and θ(x) = 1 if x > 0) and the 1 patrizia tosi, valerio de rubeis, vittorio loreto and luciano pietronero sum counts all pairs whose spatial distance xi x j # r and whose time interval t i t j # x. when applied over all possible values of τ or r, the well-known correlation integral (grassberger and procaccia, 1983) is returned. it results that cc(r, τ) is the generalisation of the correlation in­ tegral for a phenomenon that explicates in di­ verse dimensions with not comparable measure­ ment units. when applied on seismicity cc(r, τ) takes into account the distribution of all time in­ tervals and epicentral inter-distances between all pairs of events, irrespective of the relationship between the main event and any aftershock. from the space-time combined correlation integral we define the time correlation dimen­ sion and the space correlation dimension for sets of events within space-time distances r and τ, respectively as ( , x)2 log c r ( , x) = cd rt 2 log x and ( , x)2 log c r ( , x) = c .d rs 2 log r if cc(r, τ) was a pure power-law in both variables, then dt and ds would correspond to the temporal and spatial fractal dimensions, respectively. more generally, the behaviour of dt and ds as a function of r and τ will characterise the clustering features of earthquakes in space and in time. this method has been applied to global seismicity to study the space-temporal correlation between earthquakes all over the world. data come from the catalogue of the national earthquake information center, usgs, in the time period between 1973 and 2002, with magnitudes mb greater than 5. this catalogue selection was conditioned by com­ pleteness criteria and it presents medium to high magnitude distribution. 3. results the space-time combined correlation inte­ gral cc for global seismicity is represented in fig. 1 with black contour lines. the local slopes of this surface in the direction parallel to the time axis is the time correlation dimension dt, plotted in colours as a function of space and time. the colour coding of each pixel quantifies the time correlations existing between events occurring within a given distance and time in­ terval. dt ≅ 1 corresponds to the random occurfig. 1. space-time combined correlation integral cc (r, τ) (dark contour lines) and time correlation dimension dt (coloured shaded contour) for the catalogue of global seismicity. 2 space-time combined correlation integral and earthquake interactions fig. 2. same as fig. 1 for the reshuffled catalogue. fig. 3. the limit separating time clustering from time randomness (fixed to dt = 0.8) as a function of distance is shown. points are fitted by the line of equation logr = − 0.55logτ +3.8. rence of events, while a lesser value of dt indi­ cates time clustering. in fig. 1 two main do­ mains appear: one at shorter inter-distances with low dt representing time clustering; the other with dt ≅ 1 indicating a random time oc­ currence of events. the patterns observed in fig. 1 significantly support the hypothesis that earthquakes are correlated inside some space­ temporal ranges. in order to check this hypoth­ esis we applied the same analysis to the global catalogue after a reshuffling procedure. reshuf­ fling consists in mixing the time occurrence of each event keeping fixed its epicentre coordi­ nates. the characteristic of this procedure is that of maintaining the separate statistical prop­ erties of data. the results show (fig. 2) that all patterns vanish, evidencing constant high val­ ues of dt at all distances and time intervals. 3 patrizia tosi, valerio de rubeis, vittorio loreto and luciano pietronero to delineate a limit of the clustering resulting from fig. 1, all the points with dt = 0.8 were plot­ ted on a separate figure (fig. 3). it is interesting to see that, within the temporal ranges shown, the clustering boundary can well be approximated by a straight line on this log-log plot, thus indicating a power-law behaviour. using the least squares fitting we obtained logr = − 0.55logτ +3.8. this relation places a strong constraint on time rela­ tions among events, evidencing how distance plays a dynamic role. in particular, the relation can be read as defining a temporal correlation fig. 4. space-time combined correlation integral cc (r, τ) (dark contour lines) and space correlation dimension ds (coloured shaded contour) for the catalogue of global seismicity. fig. 5. same as fig. 4 for the reshuffled catalogue. 4 space-time combined correlation integral and earthquake interactions reaching long distances that quickly shrinks over time following a power-law. for relatively short spatial ranges (around 100 km) events are time clustered and correlated for long time in­ tervals (around 3 years). over longer distances time correlation lasts for a short period (less than 30 days for 1000 km). the local slopes of the surface of the space­ time combined correlation integral, in the direc­ tion parallel to the space axis, correspond to the space correlation dimension ds, plotted in colours in fig. 4 as a function of space and time. the colour coding of each pixel quantifies the space clustering existing between events occur­ ring within a given distance and time interval. ds ≅ 2 identifies a random distribution of earth­ quakes, ds ≅ 1 indicates that epicentres tend to dispose along lines and ds < 1 corresponds to space clustering. even in this plot different do­ mains are easily recognised. at short distances, a high space correlation dimension domain is clear­ ly separated from space clustering ( 0 < ds < 1) that is present at greater distances: both condi­ tions last for inter-time up to 100 days. the dis­ appearance of clustering with time leaves room to a general ds ≅ 1, interpreted as the activity of seismicity on plate boundaries. even in this case we tested the goodness of the results ap­ plying the same analysis on the reshuffled cata­ logue. the resulting plot in fig. 5 shows that the single statistical properties of data are not suffi­ cient to produce the clustering domains appear­ ing in the combined approach of fig. 4, but a real connection between space and time is needed. localisation errors certainly plays an im­ portant role at short spatial ranges, generating high values of ds (fig. 4), but it appears that the area with high space correlation dimension is evolving with time calling for the presence of a physical process. in particular, plotting in fig. 6 the points with ds = 1, chosen as the limit sepa­ rating random behaviour from space clustering, it appear that in the shown ranges they follow a straight line. fitting with least squares we ob­ tained the relation logr = 0.1logτ +1.2. the sep­ aration line defines an area around each epicen­ tre, slowly growing in time, within which seis­ mic events are randomly distributed. 3. discussion figures 1 and 4 show that earthquakes are connected with each other in a non trivial way and that a dynamic interaction appears when space and time are analysed together with the combined correlation integral. the results reveal a statistical property of the global seismicity of medium-high magnitude, but interpreting them as an average behaviour of events after the oc­ currence of each earthquake of magnitude greater than 5, a possible scenario appears. the term ‘aftershock’ can be redefined on the basis of our findings: aftershocks are all earthquakes config. 6. the limit separating spatial clustering from spatial randomness (fixed to ds = 1.0) as a function of time is shown. points are fitted by the line of equation logr = 0.1logτ +1.2. 5 patrizia tosi, valerio de rubeis, vittorio loreto and luciano pietronero nected to one reference event preceding them, as revealed by their temporal correlation (low dt). in this sense all earthquakes occurred at a dis­ tance from reference event less than the radius r, defined by the relation logr = − 0.55logτ +3.8. (where τ is the elapsed time, fig. 3), are after­ shocks of that event. this aftershock region reaches long distances from the reference ‘main’ event, but it quickly shrinks over time. if compared to an homogeneous time distribution, this area of influence can be interpreted as a re­ gion of modified probability of earthquake time occurrence. the epicentres of these connected events tend to cluster in space, apart from the near field (an area of radius 10-20 km), where earthquakes are randomly placed. even this near field aftershock region has a dynamic boundary, increasing slowly in size according to the equation logr = 0.1logτ +1.2 (fig. 6). this result is in agreement with other authors (taji­ ma and kanamori, 1985; marsan et al., 2000; helmstetter, 2003; huc and main, 2003) who found a migration of aftershocks, defined with classic methods, away from a main shock. this migration is described in terms of a law r ( ) + t h , where d td t r ( ) is the mean distance be­ tween main event and aftershocks occurring af­ ter time t, with an exponent h < 0.5 correspon­ ding to a sub-diffusive process. 4. conclusions in summary, we have introduced a new sta­ tistical tool, the combined space-time correla­ tion integral, which allows us to perform a si­ multaneous and self-consistent investigation of the correlation properties of earthquakes. this tool leads to the discovery, visualization and deep analysis of the complex interrelationships existing between the spatial distribution of epi­ centers and their occurrence in time. the analy­ sis performed on the worldwide seismicity cat­ alogue and the corresponding reshuffled cata­ logue, strongly suggests that earthquakes of medium-high magnitude do interact with each other. this result led to a new definition of af­ tershocks, as all earthquakes with non-random occurrence with respect to the reference ‘main’ event, without considering their magnitude. fi­ nally the analysis revealed how the aftershock region modifies over elapsed time. references bak, p., k. christensen, l. danon and t. scanlon (2002): unified scaling laws for earthquakes, phys. rev. lett., 88, 178501-178504. corral, a. (2004): long-term clustering, scaling and uni­ versality in the temporal occurrence of earthquakes, phys. rev. lett., 92, 108501. grassberger, p. and i. procaccia (1983): characterization of strange attractors, phys. rev. lett., 50, 346-349. helmstetter, a., g. ouillon and d. sornette (2003): are aftershocks of large californian earthquakes dif­ fusing?, j. geophys. res., 108, 2483-2506. huc, m. and i.g. main (2003): anomalous stress diffusion in earthquake triggering: correlation length, time de­ pendence and directionality, j. geophys. res., 108, 2324-2335. kagan, y.y. (1994): observational evidence for earth­ quakes as a nonlinear dynamic process, physica d, 77, 160-192. marsan, d. and c.j. bean (2003):seismicity response to stress perturbations, analysed for a world-wide cata­ logue, geophys. j. int., 154, 179-195. marsan, d., c.j. bean, s. steacy and j. mccloskey (2000): observation of diffusion processes in earth­ quake populations and implications for the predictabil­ ity of seismicity systems, j. geophys. res., 105, 28081-28094. parson, t. (2002): global omori law decay of triggered earthquakes: large aftershocks outside the classical af­ tershock zone, j. geophys. res., 107, 2199-2218. tajima, f. and h. kanamori (1985): global survey of af­ tershock area expansion patterns, phys. earth planet. inter., 40, 77-134. (received september 1, 2004; accepted october 1, 2004) 6 d e t e r m i n a z i o n e s p e t t r o s c o p i c a d e l l ' a c q u a p r e c i p i t a b i l e d . f e d e l e ( * ) r i c e v u t o il 15 giugno 19(>l è noto ohe la radiazione solare è assorbita selettivamente dai gas componenti l'atmosfera terrestre. il f a t t o r e di trasmissione, per une radiazione di determinata lunghezza d'onda, è funzione, principalmente, della quantità di gas assorbente presente nello strato attraversato per unità di superfìcie della sezione del fascio. è possibile quindi determinare la quantità di gas assorbente dalla misura del f a t t o r e di trasmissione. le principali bande d'assorbimento dovute soltanto al vapore acqueo si trovano nella p a r t e infrarossa dello spettro e sono t a n t o notevoli (la arrestare, in media, più del 15% dell'energia solare infrarossa incidente sul nostro globo ed una frazione ancora più notevole dell'energia irradiata dalla superfìcie terrestre verso lo spazio. la determinazione del fattore di trasmissione con il metodo classico impiegato da f . e . fowle, w . 1-1. elsasser, l . b . aldricli, w. gorczynski, e. stenz consiste nella misura del rapporto f r a l'intensità della radiazione nel punto di minimo della banda d'assorbimento e l'intensità che si avrebbe in assenza del gas assorbente, calcolata raccordando la curva dello spettrogramma ottenuto registrando l'intensità della radiazione nei vari punti dello spettro solare. per una misura rapida o istantanea conviene invece misurare il rapporto fra l'intensità della radiazione in detto minimo e l'intensità in una banda prossima dello spettro, non affetta da assorbimento, da parte di componenti variabili dell'aria. se le due bande sono sufficientemente prossime, si può ammettere, con buona approssimazione, che l'attenuazione non selettiva della radiazione solare, dovuta principalmente alla diffusione atmosferica, incida ugualmente su entrambe le bande, (*) osservatorio (li meteorologia a e r o n a u t i c a di vigna di valle. 2 8 d . f e d e l e sicché il r a p p o r t o così misurato rappresenta, a meno di un f a t t o r e costante, e poco diverso da 1, il f a t t o r e di trasmissione cercato. il principio è già stato applicato per il dosaggio di alcuni componenti variabili dell'atmosfera quali vapore d'acqua, ozono, anidride carbonica. gli strumenti relativi fanno uso generalmente di un sistema dispersivo a prisma, a d a t t o per le bande dello spettro solare in studio, con ottica di quarzo nel caso dell'ozono e di v e t i o nel caso del vapore d'acqua. il f a t t o r e di trasmissione è misurato, preferibilmente, con metodi di riduzione a zero manuali. nel piano focale d'auscita dello spettrometro vengono poste due fenditure fìsse, u n a in corrispondenza alla b a n d a di assorbimento, la seconda in u n a banda vicina, esente da assorbimento. l'energia passante attraverso quest'ultima viene ridotta per mezzo di un cuneo ottico lino a renderla uguale a quella non assorbita passante attraverso la seconda fenditura. la posizione del cuneo ottico, in questa condizione, fornisce direttamente l'attenuazione e quindi il f a t t o r e di trasmissione. l'uguaglianza viene generalmente controllata inviando alternativamente, con un dispositivo di sfarfallamento, l'energia passante attraverso le due fenditure su un elemento rivelatore a d a t t o per il campo di frequenza in studio. lo strumento da noi realizzato per la misura dell'acqua precipitabile è fondato sullo stesso principio. la riduzione a zero è però otfig. i. d e t e r m i n a z i o n e s p e t t r o s c o p i c a d e l l ' a c q u a p r e c i p i t a b i l e 2 9 t e n u t a a u t o m a t i c a m e n t e ed il f a t t o r e di trasmissione è registrato dirett a m e n t e , in modo continuo. esso è essenzialmente costituito da un sistema dispersivo in flint extradenso composto da un prisma equilatero, un semiprisma equilatero con una faccia speculare metalizzata, uno specchio sferico da 40 cm. di focale e da un dispositivo di misura automatica e registrazione del r a p p o r t o delle intensità della radiazione solare nelle due b a n d e 1,13^, (banda 0 d'assorbimento del vapore acqueo) e l,04w (banda di riferimento). questo dispositivo comprende u n a f e n d i t u r a fissa, larga 1 min., c e n t r a t a sulla b a n d a o ed u n a f e n d i t u r a di larghezza variabile micrometricamente da 0 a 1 mm., c e n t r a t a sulla b a n d a di riferimento, non assorbita. e n t r a m b e le fenditure sono collocate nel piano focale del sistema dispersivo e d a v a n t i alla lente frontale di un sistema convergente a corta focale che proietta l'immagine del prisma sull'area sensibile di una cellula fotoconduttrice al solfuro di piombo, sensibile in u n espeso campo spettrale, da 0,4 a 3,5 fi con un largo massimo f r a 1 e 1,5 fi. un sistema elettromeccanico opera lo sfarfallamento sulle due fenditure, alternativamente, con u n a frequenza di 50 p/s. quando le q u a n t i t à di energia passanti per le due fenditure, fissa e variabile, non sono uguali, sia ha ai capi della cellula fotoconduttrice u n a differenza di potenziale variabile, con a n d a m e n t o sensibilmente sinusoidale, con frequenza di 50 p/s. la larghezza della f e n d i t u r a variabile viene automaticamente regolata in modo da rendere m i n i m a d e t t a variazione di differenza di potenziale. in queste condizioni, il rapporto di energia nelle due b a n d e dello spettro è funzione lineare dell'inverso del rapporto della larghezza delle due fenditure. il bilanciamento automatico dell'energia che passa a t t r a v e r s o le due fenditure è operato da un motorino a campo r o t a n t e che regola la larghezza della f e n d i t u r a posta sulla b a n d a 1,03 /< e muove contemporaneamente la pennina di registrazione. il motore è a due fasi ed è alimentato, per una fase, dalla corrente a l t e r n a t a a 50 p/s che alimenta il dispositivo elettromeccanico di sfarfallamento e per l'altra fase dalla corrente di errore uscente da un amplificatore pilotato dalla cellula fotoconduttrice. le due correnti di alimentazione sono disposte in quad r a t u r a . il tipo di trasmissione meccanica a d o t t a t o assicura la costanza del rapporto f r a larghezza della fenditura regolabile ed escursione della pennina scrivente. la t a r a t u r a è s t a t a e f f e t t u a t a ricorrendo al metodo classico di misura, mediante registrazione di u n a p a r t e sufficientemente estesa dello spettro solare per mezzo di uno spettrometro di determinate caratteri3 0 d . f e d e l e stiche di purezza spettrale, ricorrendo alle tabelle determinate da fowle (1912) e confermate da aldrich (1949). la misura del f a t t o r e di trasmissione in u n a o più bande d'assorbimento da u n a p a r t e ed il dosaggio ottico dell'acqua precipitabile dall'altra presentano entrambi notevole interesse meteorologico. la prima misura è fondamentale per la determinazione delle condizioni di finajbj fonditura fissa/ finestra fvidìlvrj j ìjnjhgxzj sjr/sbijv ffor/j/lal-ora corranfg jltirnjfd j'trrorf j^ q cnz/u/a fo/~o con dottrìcr arojmp/rf. amplif. arojmp/rf. amplif. o motore d» cjmpo /~ooajn/~z corrjnu j/farnjfj d/ rtkz ajr r m dispositivo «(zhh-omeccjn/co c/i sfarfd/uminro jpormjnan/i f i g . 2. equilibrio radiativo e termico dell'atmosfera e del suolo, il secondo rappresenta un mezzo che p e r m e t t e d'integrare le informazioni fornite dai sondaggi con radio-sonda, ai fini della conoscenza della evoluzione dei fenomeni meteorologici. la registrazione continua, dal canto suo, presenta particolarità che non possono essere messe in evidenza da misure singole non frequentissime. si notano, t r a l'altro, variazioni rapide del f a t t o r e di trasmissione, con periodi da qualche decina di sec. ad u n a decina di m i n u t i e di ampiezza fino al 10% del contenuto totale di vapore d'acqua. tali variazioni sono da attribuire presumibilmente alla non omogeneità dell'atmosfera ed ai moti orizzontali e convettivi in seno ad essa. d e t e r m i n a z i o n e s p e t t r o s c o p i c a d e l l ' a c q u a p r e c i p i t a b i l e 3 1 riassunto viene descritto uno strumento per la registrazione dell' assorbimento della radiazione solare da parte del vapore acqueo atmosferico nella banda 0 centrata su 1,13 /i. la misura è effettuata confrontando, con un metodo di riduzione automatica a zero, l'energia solare ricevuta, al livello del suolo, nella banda di assorbimento con quella ricevuta in una vicina, non soggetta ad assorbimento selettivo. viene infine accennato alle applicazioni meteorologiche delle registrazioni ottenute con tale strumento. su 31 mary the construction of an instrument ivhicli will continously measure the rate of absorption of solar radiation by atmospheric water vapour in spectral band 0, centered, 1,13 fi, is described. msasurement is accompliched by comparing, with a nuli method, solar energy received al ground level in the absorption band 0 with the one received in a nearly not selectively absorbed spectral band. some considerations are presented concerning the use of the instrument in meteorologicul research. o n l i n e a r i n t e r n a l w a v e s o n t h e s e a , s t r o n g l y v e r t i c a l l y t r a p p e d e . p u r i n i ( * ) e . s a l t j s t i ( * * ) received on a u g u s t 2nd, 1975 summary. — w e s t u d y some explicit cases of m a r i n e t h e r m o c l i n e . we f o c u s our a t t e n t i o n on t h e s t r o n g l y v e r t i c a l l y t r a p p e d i n t e r n a l waves, which in our cases allow an explicit dispersion r e l a t i o n a n d a simple b e h a v i o u r in t e r m s of e l e m e n t a r y f u n c t i o n s . t h e explicit f o r m of t h e v a i s a l a b r u n t f r e q u e n c y n2{z) is p r o p o r t i o n a l t o 1 / \z—20| in one case a n d t o a2—b2(z—zd)2 in t h e o t h e r . a c o m p a r i s o n w i t h some e x p e r i m e n t a l d a t a concerning t h e l i g u r i a n sea is a c t u a l l y in course. r iassunto . — i n relazione a d e t e r m i n a t e condizioni di superfìcie, l a s t r u t t u r a v e r t i c a l e del m a r e si c a r a t t e r i z z a m e d i a n t e u n a b r u s c a v a r i a z i o n e nella d e n s i t à . nel p r e s e n t e lavoro vengono s t u d i a t e le o n d e i n t e r n e che vi r i s u l t a n o f o r t e m e n t e i n t r a p p o l a t e , o t t e n e n d o relazione di dispersione, velocità di g r u p p o e correlazione in t e r m i n i di f u n z i o n i e l e m e n t a r i p e r d u e s i t u a z i o n i s p e r i m e n t a l i i n d i v i d u a b i l i a n a l i t i c a m e n t e a t t r a v e r s o la f r e q u e n z a di v à i s à l a b r u n t n2(z) p r o p o r z i o n a l e a i /\z—za \ in u n caso ed u g u a l e a a2-b2(z—z0)2 n e l l ' a l t r o . è in corso u n c o n f r o n t o con i d a t i p r o v e n i e n t i d a c a m p a g n e di m i s u r a e f f e t t u a t e nel mar l i g u r e . 1 . i n t r o d u c t i o n a n i n t e r e s t i n g p r o b l e m i n t h e e n e r g y b a l a n c e of a s e a , c o n c e r n s t h e i n t e r n a l w a v e s a n d t h e i r e n e r g i e s . t h e s e a r e w a v e s w h i c h p r o p a g a t e h o r i z o n t a l l y a n d t h e i r l a r g e s t a m p l i t u d e is r e l a t e d t o t h e v e r t i c a l v a r i a t i o n s of t h e d e n s i t y g(z). t h i s is d u e t o t h e v e r t i c a l v a r i a t i o n s of t e m p e r a t u r e a n d s a l i n i t y , w h i c h a r e o r i g i n a t e d b y t h e i n t e n s e e x (*) c . n . r . i s t i t u t o fisica d e l l ' a t m o s f e r a r o m a , i t a l y . (**) i s t i t u t o di fisica " g . m a r c o n i " , u n i v e r s i t à di r o m a , i t a l y i s t i t u t o di fisica nucleare sezione di r o m a 2 4 2 11. p u k i n l e . s a l u s t i change of m o m e n t and h e a t with t h e mowing a t m o s p h e r e . more in detail, one could first separate a " m i x e d l a y e r " which has a v e r t i c a l extension of some t e n s of m e t e r s under t h e air-sea surface. this layer is mixed b y t h e t u r b u l e n c e p r o p a g a t i n g downward f r o m t h e moving a t m o s p h e r e (this is one of t h e effects of winds, storms, a t m o s p h e r i c turbulence etc.). i t s energy p r o p a g a t e s downward freely a n d m a k e s t e m p e r a t u r e , salinity, density and motion-homogeneous in this " m i x e d l a y e r " . the surface u n d e r this m i x e d layer is t h e n a t u r a l d o m a i n of p r o p a g a t i o n f o r t h e m o s t intense i n t e r n a l waves. i t h a s however t o be r e m a r k e d t h a t it is n o t v e r y easy t o distinguish w h a t p e r c e n t a g e of atmospheric energy generates i n t e r n a l waves and w h a t p e r c e n t a g e of energy is used b y t h e system t o erode t h e u n d e r l y i n g stratified region. practically, moreover, one can r e m a r k t h a t m a n y times t h e mixed layer has n o t a v e r y s h a r p division with t h e underlying stratified region, b u t has a vertical extension of 50 --500 meters. the resulting periodic p h e n o m e n a , t h e i n t e r n a l waves, are in this case related t o a smoother v a r i a t i o n of t h e density t h a n in t h e case of t h e s h a r p division between t h e mixed layer and t h e deeper stratified region. tts stratification is a r a t h e r curious p h e n o m e n o n : one can experimentally r e m a r k m a n y sheets of an horizontal extension of kilometers and this is a surprising contrast with t h e vertical extension of few centimeters. practically, f o r g e t t i n g this " f i n e s t r u c t u r e " , one could see it as a stable region of slowly v a r y i n g density g0(z). i n t h e lowest p a r t , g0(z) decreases with t h e d e p t h as a slow exponential exp (— az), where a is a c o n s t a n t . i n this contest, we h a v e r e m a r k e d t h a t t h e surface b e t w e e n t h e mixed layer and t h e stratified thermocline is t h e d o m a i n of m a n y interesting and i m p o r t a n t p h e n o m e n a , related t o t h e i n t e r n a l waves d i s t r i b u t i o n of energy inside t h e fluid ("). i t can be shown, m o r e precisely, t h a t t h e knowledge of e x a c t s h a p e of g0(z), t h e s t a t i c d e n s i t y profile, could give essential informations concerning t h e i n t e r n a l waves s t r u c t u r e , their correlations and t h e i r energetics. more explicitly, it has t o be added t h a t t h e e x p e r i m e n t a l evidence stresses t h a t this is n o t really a s h a r p surface, b u t it appears r a t h e r as a vertical region of t r a n s i t i o n between t h e homogeneous mixed layer and t h e stratified thermocline. this appears interesting because t h e i n t e r n a l waves can be described b y a simple equation (if one assumes t h e linear waves and if t h e boussinesq a p p r o x i m a t i o n is assumed v a l i d : see, for example, phillips (°) and t h o r p e (7)) where t h e explicit shape of q0{z) plays an explicit role. o n l i n e a r i n t e r n a l w a v e s o n t h e s e a e t c . 2 4 3 now, this equation has simple dispersion relation and solution in some cases r a t h e r well known in t h e literature. these are supplied when t h e vaisala-brunt f r e q u e n c y , , dp0 q n2 «) = r~ • — > 0 d z q o has a d(z) behaviour or jv-(z) = const, behaviour (phillips (6)) and when n*(z) = 0 for 0 <z<d ((j i s t ] i e d e p t h of t h e discontinuity e-az for z>d in t h e density) as in t h e classical analysis of garret and munk (3) and in few other cases studied b y t h o r p e (8). the realistic cases are r a t h e r different; one could easily solve t h e m numerically, b u t this would imply some loss of informations. f o r t h i s region we h a v e studied t w o r a t h e r realistic profiles n*(z) = a2l\z—z0\ and n2(z) — a2 — b2{z—z0)2 these profiles can, in some cases, simulate correctly t h e e x p e r i m e n t a l situation and t h e y also allow an explicit calculation of t h e i n t e r n a l waves, their dispersion relation, their group and wave velocities, thencorrelations. an experimental verification is actually i n course (x). 2 . l i n e a r t h e o r y o f m a r i n e i n t e r n a l w a v e s the t h e o r y of internal waves is r a t h e r well known (phillips (°), t h o r p e (8)). i n t h e following we will r e p e a t t h e essential results on t h e t i m e evolution of these waves. i n fact, in t h e case t h a t t h e boussinesq a p p r o x i m a t i o n can be assumed and t h e e a r t h ' s r o t a t i o n can be disregarded, t h e velocity components satisfy t h e equations (6|7). 1 i)« + — f = 0 qo + — ~ = 0 (?o ty i q' *tw + — -j+ g = 0 q o "z qo 10 2 4 4 r . p u r 1 n i e . s a l u s t i where p is t h e d e p a r t u r e f r o m t h e h y d r o s t a t i c pressure and one has assumed q = qo(z) + q' (x, y, z, t) with t h e density variation q'<s:q0. i n t h e same f r a m e of a p p r o x i m a t i o n , one also assumes t h a t t h e water is incomprensible: ixu + + izlv = 0 d p v v <>g' at it ix 7>y iz calculating first t h e t i m e derivative of t h e vorticity, one lias ( i * + — jl v = o \ iz ix j it q o íx . iv iw \ i a io' vu — — — + — — = 0 iz iy j it go iy a n d , t a k i n g into account t h e <y~ = 0 relation, one h a s cli i l g ig' g i i , g d g o iw ~ì)t qo ìx qo ìx it ~ qo dz iz i g io' g i 5 , g dp0 iio it go iy qo iy it ^ ~ qo d» iy taking t h e n t h e horizontal divergence, one h a s t h e basic relation y-tt (vxz + i\y + w (x, t) + a^x^+s2,,,,) w (x, z,t) = 0 [2.1] where n-(z) = — — f > 0 is t h e vàisàla-brunt frequency. qo d z assuming a p l a n e progressive wave solution of t h e f o r m w (x, z, t) = 17 (z) exp i(kxx + kyy + kzz — wt) one easily arrives t o t h e equation + k2 — k2\ w(z) = 0 [2-2] d*w\z) , j n2(z) dz2 on l i n e a i ! i n t e r n a l w a v e s o n t i i e s e a e t c . 2 4 5 w i t h 17(0) = o a t t h e rigid f r e e surface ( 9) w(—d) = 0 a t t h e b o t t o m the informations concerning t h e stratification are included in t h e p a r t i c u l a r shape of n2(z). this is r a t h e r c o n s t a n t in t h e mixed layer. i n t h e lower region it has m a n y sharp variations (the fine-structure). these variations give t h e classical behaviour of n2 (z) in t h e tliermocline when, in some sense, t h e y are averaged in 2. a t last n 2 —> const, value in t h e deepest regions can be f o u n d . as t h e various preceding cases h a v e been studied, we h a v e focused our a t t e n t i o n to two explicitly solvable cases: where a, a, b, are constants to be determined on experimental ground. the d e p t h z0 is t h a t corresponding t o t h e zone of highest v a r i a t i o n of t h e y a i s a l a b r u n t frequency n2(z). the cases seem to be general enough t o a p p r o x i m a t e realistic cases, particularly in t h e parabolic case. 3 . t h e e x p l i c i t c a s e n2(z) = a2/\z— z0\ w e are now going t o s t u d y t h e case above mentioned n2(z) = a21\z—z0\. the equation [2.2] now results w e assume t h a t t h e distance among zd, t h e b o t t o m z = — d and t h e surface 2 = 0 could be considered large. i n practice z0 is 20-50 m e t e r s for localized seas (*) and h u n d r e d m e t e r s for t h e ocean. the d e p t h d of t h e b o t t o m is usually fixed t o be larger, in order t o avoid b o t t o m effects. f o r strongly t r a p p e d i n t e r n a l waves, t h e vertical region of i n t e r e s t is determined in order t o fix t h e vertical scale of motion. i t usually is of t h e order of m a g n i t u d e of few meters. outside this region 2 = z0, t h e yaisala-brunt f r e q u e n c y decays r a t h e r rapidly, as a power of 2. much more quick, however, is t h e decay of t h e solution if, which results in general a negative exponential. so one could also assume for these waves an idealized b o u n d a r y condition a) n2(z) = a211 z—z0 \ b) n2(z) = a2 — b2 (z—zo)2 d2 w(z) dz2 [3.1] w (± 00) = 0 2 4 6 r . p u r 1 n i e . s a l u s t i which simplifies t h e calculations. t h e n one can say t h a t our eigenvalue e q u a t i o n can a s s u m e a n infinity of eigensolutions, labelled b y 11 = 1, 2, 3 . . . t h e g e n e r a l solution is (see a p p e n d i x ) : wfl = a^xo™o> l}^ {2k\z—z0\} [3.2] where a/t is a c o n s t a n t a n d t h e f u n c t i o n l)i_l (g) is called l a g u e r r e polynomial. a t z — za, t h e f u n c t i o n is c o m p l i c a t e d : t h e e q u a t i o n shows t h a t i t s second d e r i v a t i v e diverges. t h e n wp results a c o n t i n u o u s f u n c t i o n s y m m e t r i c a r o u n d t h e p e a k z0 one c a n m o r e o v e r say (see a p p e n d i x ) t h a t t h e solution exist if a n d only if a2k — = p =1,2, . . . i t ' s i n t e r e s t i n g t o n o t e t h a t t h e p r e c e d i n g dispersion r e l a t i o n for t h e s e i n t e r n a l waves is similar t o t h a t of t h e two-fluid s y s t e m . t h e n one can calculate t h e g r o u p v e l o c i t y cg da> 1 a 2 °g = d k = y 1w(2jl) 1'2 one h a s also t o r e m a r k t h a t w¡i. is p r o p o r t i o n a l t o e x p / t 12—z0\ so t h a t t h e b e h a v i o u r of t h e w a v e decays e x p o n e n t i a l l y outside t h e zone of s h a r p v a r i a t i o n of a2(2). this implies t h a t we m u s t consider only waves w i t h a large k v a l u e fixed b y t h e dimension of t h e physically i n t e r e s t i n g region t h r o u g h t h e r e l a t i o n k ~ 1 \l. one could enlarge t h e p r e c e d i n g t r e a t m e n t of t h e i n d i v i d u a l s t r u c t u r e of i n t e r n a l waves b y considering t h e correlation f u n c t i o n . this q u a n t i t y is + 00 b/tfl' (k) = l l > u y = i wm w%' dz' i t a p p e a r s i n t e r e s t i n g because it is a p o w e r f u l tool i n t h e comparison b e t w e e n t h e o r e t i c a l models a n d e x p e r i m e n t a l d a t a (4). i n our case i t r e s u l t s : li 2 b^ (k) = ^xkl j afxa%' e 2 ^ 0 ) lji^ l\*_! d {2k\zr—z0\) on l i n e a i ! i n t e r n a l w a v e s on tiie s e a e t c . 2 4 7 the laguerre polynomial are on orthogonal set, therefore one has 1 0 for n ^ fi' bni' ( k ) = i 1 — { l a p u / t i y . ? 2c-i for ¡x = n ' this k ' 1 behaviour of t h e correlation f u n c t i o n is r a t h e r interesting and is in agreement with t h e results of phillips (6) for a range of t h e s p e c t r u m of t h e lowest i n t e r n a l mode. if one w a n t s t o calculate also t h e cross-correlation wv, t h e c o n t i n u i t y equation i kyv + w = 0 m u s t be used. this can give t h e m a t r i x element / d w 1 ] w d = 0 d z one could add t h a t t h e m a t r i x element dwn iz wm &z' is n o t zero if (and only if) w = to ± 1. i n more detail d w. \ _ , , u w ^ dz' = 1 / j l dz j • "" ' 2 and / -dz ) " t i | 2 to finish, also t h e case a2 bjv2(«) = — a, /? const. z z2 could be exactly t r e a t e d . i t results, however, of different physical interest because it describes an instable case, whereas in this n o t e one studies stable phenomena only. 2 4 8 r . pur1ni e . s a l u s t i 4 . t i i e c a s e o f a p a r a b o l a a 2 (z) = a2 — ip (z—za)2 b y repeating t h e preceding considerations, one can arrive to t h e e q u a t i o n : = (z-zo)2 1 f ( . ) i ^ f ' j d 1 * ( . ) ; ^ az2 to2 [ u)2 j with t h e b o u n d a r y conditions (°), w ( 0 ) = 0 w(—d) = 0 i n t h i s case also t h e r e is an infinity of solutions (see a p p e n d i x ) : '<k , „ u k b v ' 2 w, = at e"a j ,z'z°' hq — (z—z0)\ q = 1, 2, . . . [4.1] where a„ is a c o n s t a n t and hq(o) is an h e r m i t e polynomial. f r o m this equation one can see t h a t t h e wave a m p l i t u d e decreases more rapidly t h a n in t h e preceding case (eq. [3.2]) as \z—z01 increases. there is also in this case a dispersion relation f i t has t o be r e m a r k e d t h a t <o decreases as q increases, t h a t is, for higher modes, t h e dimension l of t h e system fixes a k v a l u e k ~ 1 [l. f r o m t h e above equation, one can derive: — cb + (g2b2 + 4j£2a2)1/2 <» = 2 k — 0 = g + l [4.2] and t h e n t h e group velocity can be deduced cg = (g2b2 + h [ c b ^ + 4 a 2 k ' ) v i i n this case also it is easy t o calculate t h e correlation f u n c t i o n l / 2 b„' (k) = ~ f aqa*. e' k'1(z'-2a)1 hq hi dz' = on l i n e a i ! i n t e r n a l w a v e s on tiie s e a e t c . 2 4 9 li 2 i a.q a.q' hqllq' 6 uj d kb\ (z' zo) o which, r e m e m b e r i n g t h e orthogonality of t h e h e r m i t e polynomials, becomes this result can be w r i t t e n in a more clear f o r m by using t h e dispersion relation. i n f a c t , f r o m eq. [4.2] one has t h a t roa'-i ~ jsc-2 for which eq. [4.3] becomes i n this case, t h e behaviour of t h e correlation f u n c t i o n is different f r o m t h e other one. as in t h e preceding section, if one w a n t s t o calculate t h e correlat i o n wv, one m u s t use t h e c o n t i n u i t y equation, giving v as a function of ' ' ^ . now, t h e resulting m a t r i x element can be found in t h e l i t e r a t u r e (5). the profile n2(z) = a2 — b2 (z—zo)2 + c (z—zo) + d ( z — z a ) 3 can also be calculated. the result is exact for c ^ 0 and with some a p p r o x i m a t i o n for d ^ 0. to finish, we w a n t to note t h a t results obtained in this note for t h e i n t e r n a l waves in t h e presence of a strong stratification are used for a comparison with experimental d a t a concerning t h e ligurian sea i1). [4.3] bm' (k)ock-v2 2 5 0 r . p u r i n i e . s a l u s t i a p p e n d i x a2 a) the case. i z—z o \ this e q u a t i o n is r a t h e r well known. w e follow here a description of t h e solution which is widely used in q u a n t u m mechanics (2). i n adimensional form, it results d *~u ( i 1 \ t 7 + m == 0 dp2 \q 4 ) asimptotically t h e solution results u ~ e ± e/2 and if one w a n t s u t o he finite, u ~ e-e/2 multiplication b y a polynomial d o e s n ' t change t h e a s i m p t o t i c value this gives = f (q) e-e/2 dgdg g p u t t i n g now f = s ak g* k = 1 inside t h e preceding equation, one has (a — 1 ) a , + 2 a , = 0 k(k+1)ak + 1 + ( a — k ) ak = 0 now, if one assumes t h a t t h e series is infinite, t h e n a k + i _ a — a 1 _ ak ~ e (e+l) k which correspond t o an exponential behaviour of f(g), is in contradiction with our hypothesis u ~ e~si2. on l i n e a r i n t e r n a l waves on the sea etc. 251 t h e n t h e series cannot he infinite. the other only possibility is t h a t }. be an integer n. then one has t h e "eigenvalue c o n d i t i o n " i = n ak = 0 ii > ft the polynomials obtained in this way are related t o t h e associated l a g u e r r e polynomials: ln(q) = £ 0 ? " the first one are: l\ = 1 ; l\ = 4 — 2 g; l\ = 1 8 — 1 8 o — 3 <?l \ = 9 6 — 1 4 4 g + 4 8 o 2 — 4 q 3 b) the a2—b2(z—z0)2 case w e r e p e a t here t h e preceding t r e a t m e n t of t h e equation. i n adimensional f o r m t ! + « = 0 the asimptotic value being u ~ e if one assumes u = h(q)c'q*'2 t h e n obtains, for t h e basic equation ^ 2 , f + (a 1) h = 0. [a.l] do2 do now, writing for ii n h = 2 an q" h = 1 t h e differential equation [a.l] gives « s + 2 ( 2 s + 1 ) — a (s + 2) (s + 1 ) s > 0 10 2 5 2 k. p u r i n i e. salusti separating t h e even or add solutions, on t h e n has i = 2 n + 1 the solutions are even or add polynomials, t h e h e r m i t e polynomials d » o g2 h t { e ) = ( d * • the first one are h o = 1 ; h , = 2 g ; h 2 = ±q 2—2; h 3 = 8 q 3 — 12 q r e f e r e n c e s ( 1 ) c o i . a c i n o 51., p u r i n i r . , r o v e l l i a . , s t o c c i i i n o c . , 1 9 7 0 . studio del bilancio termico diurno del ciclo di temperatura e delle onde interne in stazione fissa (mar ligure). n o t a i n t e r n a d e l l ' i s t i t u t o idrografico della m a r i n a , g e n o v a . (2) d icke r., w i t t k e j . , 1900. introduction to quantum mechanics. " a d d i son w e s l e y , p u b l . comp. i n c . " , l o n d o n . (3) g a r r e t c., munk w . , 1972. space-time scale oj internal waves. "geopli. f l u i d d y n . " , 2, p p . 225-264. (4) monin a. s., y aglom a. m., 1971. statistical fluid mechanics. " t h e mit p r e s s " , i. (5) morse p . , feshbach h . , 1953. methods oj theoretical physics. "mcgrawh i l l " . (6) p h i l l i p s o. m., 1900. the dynamic of the tjpper ocean. " c a m b r i d g e u n i v e r s i t y p r e s s " . (7) see t h e classical discussion in t h e book of phillips, reference (6). (8) thorpe s. a . ; on the shape of progressive internal waves. " r o y a l soc., l o n d o n , p h i l o s . t r a n s " . , v 2 0 3 a ; i i 4 5 ; p p . 5 0 3 0 1 4 . (°) thorpe s. a. (and other articles), 1975. " j . of geopli. r e s e a r c h " , 80. 1 annals of geophysics, 65, 5, pe533, 2022; doi:10.4401/ag-8792 o p e n a c c e s s electrokinetic effect provided by long oceanic waves coming on shore vadim v. surkov*,1,2, valery m. sorokin1 and aleksey k. yashchenko1 (1) pushkov institute of terrestrial magnetism, ionosphere and radio wave propagation of the russian academy of sciences, troitsk, moscow, russia (2) shmidt institute of physics of the earth of the russian academy of sciences, moscow, russia article history received january 12, 2022; accepted june 6, 2022 abstract electrokinetic effect (ek) caused by long oceanic waves in porous water-saturated rocks of seabed and shore is examined theoretically. one possible reason for this effect is the motion of groundwater due to the volume deformation of porous rocks by oceanic waves coming on shore. the same mechanism is responsible for seismoelectric effect observed during seismic waves passage through ground-recording station. in this study, we examine another mechanism in which the wave-produced variable pressure on the seabed plays a role of a piston pushing seawater through the seabed rocks into sandy or porous rocks of the seashore thereby exciting the ek effect. to estimate this effect, we first consider a long gravity wave and then solve 2d‑problem on the pressure variations produced by this wave on the bottom. this solution is used to describe groundwater filtration in porous rocks subjected to the variable pressure of seawater. the ek current and telluric electric field in a porous medium are derivable through the pressure gradient of porous fluid. the amplitude of telluric electric field in a porous medium has been shown to decrease almost exponentially with distance from a shoreline. a penetration depth of the telluric field as a function of wave frequency in the range of 10-100 mhz was analyzed. a role played by ek effect in the generation of ulf natural electromagnetic noise in coastal zone was discussed. keywords: seismology; ground motion; waves and wave analysis; groundwater processes; fluid geochemistry 1. introduction one of the most important sources of global natural electromagnetic noises is the large-scale electric currents in the ionosphere and magnetosphere. during magnetically quiet period the amplitudes of ultra‑low frequency (ulf) geomagnetic and geoelectric perturbations in the upper conductive layer of the earth’s crust are about 1-10 nt and 1-10 µv/m, respectively, [e.g., surkov and hayakawa, 2014]. in the vicinity of seashore there are additional local sources of the ulf electromagnetic noise. one of the reasons for such local noises is the wave motion of seawater which is a good electrical conductor. when seawater moves in the earth’s magnetic field it produces electric currents in the conductive seawater that, in turn, result in local low‑frequency perturbations of the earth’s magnetic field. vadim v. surkov et al. 2 these kind of geomagnetic perturbations were observed from wind-generated and long gravity waves, during ocean tides, etc. the amplitudes of these perturbations in the lower atmosphere can reach 10 nt depending on the wave amplitude, meteorological conditions, the contrast between the conductivities of seawater and coastal rocks, and other factors [bullard and parker, 1970; sanford, 1971; fischer, 1979; quinney, 1979; tyler et al., 1998; kuvshinov, 2008; liu et al., 2019; marshalko et al., 2020]. another possible reasons for the generation of ulf perturbations of the earth’s electromagnetic field is the movement of charged atmospheric aerosols [soloviev and surkov, 1994; sorokin, 2007; sorokin et al., 2020] or extended aero-electric structures (aess) [anisimov et al., 1994, 2002], entrained by moving air. with a decrease in frequency, the spectral density of energy of such perturbations increases according to the power law. the entrainment of charged aerosols with wind can greatly affect the amplitude‑frequency characteristic of electric fields not only in the atmosphere, but also in the ground. alexandrov [1985] have reported evidence from ground-based ulf observations at yakimvar bay, karelian isthmus, russia that variations of telluric electromagnetic field increased up to 2-32 µv/m with increasing the wind velocity from 3 to 7-10 m/s. in the coastal zone, an additional electric effect can be caused by changes with distance in the ratio between the concentrations of marine and continental aerosols [e.g., ivlev and dovgalyuk, 1999]. one more source of electromagnetic noise, both in the ground and in the atmosphere, is the electrokinetic (ek) effect caused by the movement of groundwater in sandy and porous rocks of the sea coast. the reason of this effect is that the surfaces of pores, channels and cracks absorb the ions of certain sign (more frequently negative) from the groundwater solution that results in charge buildup at the interphase boundaries [frenkel, 1944]. an excess of ions of the opposite sign can be carried away by the flow of groundwater, thereby producing the ek current. in the coastal zone, the groundwater flow can occur due to the volume deformation of rocks by waves coming ashore [egorov and pal’shin, 2015]. in this case, the ek effect arises both in the bottom layer and on the shore. this mechanism of generation of telluric currents is similar in nature to the seismoelectric effect observed in porous water-saturated soils during the passage of seismic waves through ground-recording station [eleman, 1965; mogi et al., 2000; balasco et al., 2014; surkov and pilipenko, 2015; romano et al., 2018; surkov et al., 2018, 2020]. in contrast to the above studies based on the traditional “deformation” mechanism of the ek current generation, this paper deals with another mechanism of the ek effect, which can be called “piston mechanism”. in this mechanism, the variable pressure on the seabed produced by long oceanic waves result in the pushing/seepage of seawater through the porous bottom into sandy or porous rocks of the seashore followed by the generation of ek effect. 2. variations in pressure on the seabed caused by a long gravity wave in this study we consider the seabed and coast rocks which are covered with a layer of sandstone or other porous water-saturated rocks. oceanic waves coming on the shore generate a variable pressure on the seabed thereby producing the motion of the groundwater in the porous rocks of the seabed and coast [packwood and peregrine, 1980; longuet-higgins, 1983; massel, 2001; belibassakis, 2012]. the ek effect in the porous water-saturated medium occurs due to the movement of ions contained in the groundwater solution. under natural conditions, the excitation of the ek effect is most effective at low frequencies, since at high frequencies the inertia of ions becomes significant despite the viscous forces in the fluid that results in a decrease of the ek current. therefore, we will take into account the effect produced only by low‑frequency oceanic waves, such as long gravity waves (lgw). at first we will consider the pressure and velocity variations on the seabed caused by lgw. these variations play a role of an external influence on the porous medium that causes gradient of pore pressure. to estimate the variable figure 1. model of medium. electrokinetic effect caused by oceanic waves 3 pressure and velocity on the seabed surface, consider the model of medium in which the marine environment and porous water-saturated rock occupy a half-space 𝑧 > 0 while the atmosphere occupies a half-space 𝑧 < 0 (figure 1). the plane interface between the marine environment and the coastal porous rock passes through the 𝑧1-axis and the 𝑦-axis, which is perpendicular to the plane of figure 1. the profile of the seabed ℎ = ℎ(𝑥) is described by a piecewise linear function of the following form: (1) where while the angle 𝛼 is shown in figure 1. suppose that a plane harmonic lgw propagates in the marine environment along the 𝑥-axis so that the wave phase velocity is perpendicular to the coastline. in this case, all functions depend only on the variables 𝑥, z, 𝑡 and do not depend on 𝑦. near the shore, where the wavelength of the lgw exceeds the depth ℎ0 of the reservoir, we use the linear approximation of “shallow water” [monin et al., 1977; pelinovsky, 2006]. in this approximation, the horizontal component of the fluid velocity is much greater than the vertical one and is independent of the z coordinate. additionally, the wave‑produced vertical displacement of the sea surface above the equilibrium level is much smaller than the sea depth, that is . in such a case, a set of “shallow water” equations for the horizontal fluid velocity and the vertical displacement of the sea surface is given by (2) where g is the free fall acceleration. let be the vertical displacements of the seawater surface in the lgw falling from infinity to the seashore. here 𝜂0 and 𝜔 stand for the amplitude and wave frequency, respectively; is the wave number, and is the lgw velocity at . in the region , the solution of the problem, which takes into account the wave reflected from the shore, can be written as [monin et al., 1977; pelinovsky, 2006]: . (3) here we made use of the following abbreviations , (4) where j1(𝑥) and j0(𝑥) denote bessel functions of the first kind of the first and zero orders, respectively. substituting equation (3) for 𝜂(𝑥,𝑡) into the second equation of set (2), we obtain the horizontal velocity of the seawater caused by lgw propagation: . (5) in what follows we use both the 𝑥 and 𝑧 coordinates and the oblique coordinates given by: (6) vadim v. surkov et al. 4 the wave-produced pressure variations on the bottom is derivable from the vertical displacement 𝜂 through , where 𝜌 is the seawater density. applying this equation to the pressure variations on the inclined portion of the seabed we obtain . (7) in a similar fashion one can use equation (5) in order to find the seawater velocity at the boundary with the porous medium. 3. groundwater pressure in porous medium now let us consider the porous rocks of the seabed and coast. in the low frequency range, the change 𝛿𝑃𝑝 in the pore fluid pressure is determined by the equation [frenkel, 1944]: , (8) where 𝜃 is the volume strain of the porous medium, and the parameter 𝐷 plays a role of the diffusion coefficient of the pore fluid pressure. here we made use of the following abbreviations: , (9) where 𝜉 stands for the viscosity of the pore fluid; 𝑛 and 𝑘𝑝 are the medium porosity and permeability, respectively; 𝐾𝑓 is the bulk modulus of compressibility of pore fluid, 𝐾𝑠 is the bulk modulus of solid component, and 𝐾 is the bulk modulus of dry porous rock (the medium skeleton without the pore fluid). in order to exclude the contribution of the “deformation mechanism” to the ek effect from further consideration, we will neglect the volume strains, assuming that 𝜃 = 0. in such a case, equation (8) describes the groundwater diffusion/filtration due to the variations in the seawater pressure applied to the surface of the porous medium. to solve this equation, it is convenient to use oblique coordinates (6). let us seek for the solution of equation (8) in the form: . for simplicity, we will omit the exponential factor exp(–𝑖𝜔𝑡) everywhere. then equation (8) for the function 𝑝�(𝑥1,𝑧1) can be written as (𝑧1 > 0): . (10) at the boundary of marine environment and porous media, the pore fluid pressure 𝑝𝑝 has to be equal to the seawater pressure 𝑝� while the normal component of the fluid flow density has to be continuous. in what follows we are interested in the distribution of the pore fluid pressure only near the coastline. away from the coastline the boundary conditions of the problem have little effect on this distribution. therefore, we assume that these boundary conditions are valid not only on an inclined section of the seabed, but also on its continuation, i.e. on the whole half-plane passing through the y and 𝑧1 axes. from equation (7) it then follows that the boundary condition for the pore pressure becomes: . (11) electrokinetic effect caused by oceanic waves 5 form the second boundary condition, i.e. the continuity of the fluid flow, it follows that the normal components of the seawater velocity, 𝑢�, and the fluid velocity in the pores, 𝑢𝑝, are related via . at this boundary since the vertical component of the seawater velocity was neglected. the fluid velocity in porous rocks is determined by darcy law . by taking the projections of and on the axis perpendicular to 𝑧1-axis and using variables 𝑥1 and 𝑧1, the above boundary condition can be rearranged to give . (12) this equation does not take into account that the velocity decreases near the seabed due to the seawater viscosity. this means that the seawater flow into a porous medium is slightly overestimated, and therefore the results of this study can serve as an upper estimate of the expected effect. however this approximation can be partly justified by a large spread of the measured values of 𝑛 and 𝑘𝑝. substituting equation (11) for 𝑝𝑝 and equation (5) for 𝑢𝑥 into equation (12) and rearranging this equation, yields . (13) here the parameter 𝜁 is introduced, given by . (14) at the boundary of a porous medium with the atmosphere, the fluid pressure in the pores is approximately zero, since atmospheric pressure can be neglected. from here we obtain another boundary condition . (15) the solution of equation (10) under requirements (11) and (13) at the boundary between the porous medium and marine environment and requirement (15) at the boundary with the atmosphere is found in appendix. 4. electrokinetic effect on the seashore the volume-averaged density of the ek current in a porous water-saturated medium can be expressed from the fluid pressure gradient through , where 𝜎 stands for the average conductivity of a porous rocks, while 𝐶ek is the streaming potential coupling coefficient which depends on the porosity and permeability of rocks, the temperature and mineral composition of the groundwater and other parameters of the porous medium [e.g., surkov and hayakawa, 2014]. the density of a conduction current is derivable from an electric potential 𝜑 through . thus, the density of total current in porous medium is given by: . in the coastal zone, the parameters 𝜎 and 𝐶ek can depend not only on the porosity and permeability of rocks. a decrease in the groundwater salinity with distance from coastline can also affect these parameters. in order to derive analytical estimates of the ek effect and make our consideration as transparent as possible, we assume that these parameters are constant everywhere. the stationary current distribution is described by the following continuity equation: . if the medium is homogeneous then this equation is reduced to: . (16) vadim v. surkov et al. 6 at the boundary of porous medium and the atmosphere the normal component of the total current has to be zero; that is, 𝐽𝑧 = 0 at 𝑧 = 0. if a homogeneous porous medium occupies a half-space 𝑧 > 0 bounded from above by the atmosphere, then equation (16) with this boundary condition possess a simple solution [surkov and hayakawa, 2014]: . (17) the meaning of this solution is that the conduction current and the ek currents cancel each other everywhere so that the total current in the porous medium is zero. it should be noted that this solution does not hold true near the seabed since a portion of the conduction current can flow through the boundary between the porous medium and marine environment. however, this approximate solution is valid in the porous medium away from this boundary, i.e. under the requirement that 𝑧 ≪  𝑥. the pressure 𝑝𝑝 of the pore fluid can be found by applying the laplace transform over the 𝑥1 coordinate to equation (10) and boundary conditions (11), (13) and (15). the solution to this problem is found in the appendix. in the case of shallow depths z, when 𝑧 ≪  𝑥, the function 𝑝𝑝(𝑥,𝑧) is determined by equations (a14), (a16) and (a18). substituting these equations, as well as equations (4) and (14) for 𝑠 and 𝜁 into equation (17) and rearranging, we arrive at the final equation for the electric potential in porous medium , (18) where , (19) and 𝐾1 denotes the modified first‑order bessel function. the actual porous medium parameters may vary in a wide range. for example, typical values of the porosity 𝑛 for granite and gneiss are (0.2-6)∙10–3, for sandstone 0.04‑0.3, and for tuff 0.2‑0.3 [mavko et al., 2014]. the ek coefficient figure 2. decimal logarithm of the telluric electric potential in the porous rock of the seashore at a depth of 1 m as a function of distance to the coastline. the solid and dashed lines correspond to the rock permeability of 10–9 and 10–11 m2, respectively. electrokinetic effect caused by oceanic waves 7 𝐶ek is typically in the range 10–8‑10–6 v/pa depending on the porosity, permeability, concentration and mineral composition of groundwater, etc. [jouniaux et al., 2000]. in standard geophysical practice, a set of grounded electrodes is used to measure the potential difference and the horizontal electric field in the ground. the model calculations of the telluric electric potential 𝜑(𝑥,𝑧) at the depth 𝑧 = 1 m is displaced in figure 2 as a function of distance 𝑥 to the shore. in making the plot of 𝜑 we have used the following parameters: 𝑛 = 0.1, 𝐾𝑓 = 2 gpa, 𝐾 = 10 gpa, 𝐾𝑠 = 20 gpa, 𝜉 = 10–3 pa∙s, 𝜌 = 103 kg/m3, 𝐶ek = 10–6 v/pa [e.g., surkov and hayakawa, 2014], hz, ℎ0 = 100 m, 𝑥0 = 1 km, 𝜂0 = 1 cm. the solid and dashed lines in figure 2 correspond to the permeability of porous media: 𝑘𝑝 = 10–9 m2 (coarse sand) and 10–11 m2 (fine sand), respectively [packwood and peregrine, 1980]. it is obvious from figure 2 that the electric potential decreases almost exponentially with distance 𝑥 away from the coastline. therefore, the above mechanism of the ek effect, which has been termed “piston mechanism”, may greatly affect the telluric electric field only near the coastline at distances of no more than several tens of meters. figure 3 shows the dependences of the electric potential in a porous medium on distance 𝑥 for different frequencies of the lgw. the lines 1‑5 correspond to the wave frequencies 𝑓 = 0.1, 0.08, 0.06, 0.04 and 0.02 hz, respectively. in figure 3. the same as in figure 2 but for different lgw frequencies. the lines 1‑5 correspond to the frequencies: 𝑓 = 0.1, 0.08, 0.06, 0.04 и 0.02 hz, respectively. figure 4. the same as in figure 3 but for the horizontal projection 𝐸𝑥 of the electric field. vadim v. surkov et al. 8 making this plots we have used the above numerical values of the parameters and 𝑘𝑝 = 10–9 m2. as is seen from this figure, the rate of potential decrease with distance 𝑥 depends heavily on the frequency of the incident lgw. with the decreasing frequency of the wave, the telluric electric field becomes larger. this trend is mainly due to the fact that the argument of the modified bessel function in equation (19) increases linearly with the wave frequency. the horizontal electric field can be found by substituting equation (18) for the potential in the relationship . figure 4 displays the dependence of 𝐸𝑥 on the distance 𝑥 to the shore at the depth 𝑧 = 1 m. here we have used the same numerical values of the parameters as in figure 3. it obvious from this figure that the telluric electric field in a porous medium decreases almost exponentially to the value comparable to or smaller than the level of natural electric noise (∼1 µv/m) at distances from 40 to 150 m, depending on the value of the lgw frequency. 5. discussion and conclusion from the above analysis, one may conclude that the ek effect on the coast caused by incident lgws is provided by at least two different reasons. the first reason is due to the rock deformation by seismic waves that results from variations of pressure on the seabed caused by the lgws propagation. the volume deformation of the porous rock must, in turn, be accompanied by the generation of the ek effect. in this study we have examined another reason for the ek effect, which is not associated with the volume deformations of a porous medium. in such a case, the variable pressure on the seabed plays a role of a piston pushing seawater through the seabed rocks, which are connected by pores and cracks with the porous space of coastal rocks. these two physical mechanism of the phenomenon lead to the same result, i.e. filtration of the pore fluid, which is accompanied by the generation of the ek currents in porous rocks. the theoretical analysis of our “piston model” has demonstrated that the ek effect produced by long oceanic waves can make a significant contribution to low‑frequency electromagnetic noise on the seashore. the amplitude of the electrical perturbations due to the ek effect strongly depends on many factors including the permeability of seabed and coastal rocks, as well as the frequency of the incident lgw. away from the shore, the telluric electric field in a porous medium decreases almost exponentially with distance. the characteristic penetration depth 𝑥𝑐, at which the electric field amplitude decreases by a factor of e, is , whence it follows that the penetration depth deep into the coast increases with a decrease in the frequency 𝑓 of the lgw. for example, in the wave frequency range of 0.02‑0.2 hz, the amplitude of the horizontal electric field at a depth of 1 m can decrease to a value comparable to or even lower than the level of background electric noise at distances from tens to hundreds of meters, depending on the wave frequency. measurements of electromagnetic fields caused by the long gravity waves are of particular interest for the purposes of detecting and monitoring tsunami waves. to accomplish this, magnetometers are usually installed on the shore or at the seafloor geomagnetic observatory. for example, manoj et al. [2011] have reported the ground‑ based observations of the geomagnetic perturbations of 1 nt amplitude caused by the arrival of a tsunami wave after a strong chilean earthquake on february 27, 2010. the magnetic perturbations caused by the telluric currents can be found from maxwell equation: where 𝜇0 stands for the magnetic constant/magnetic permeability of free space. whence we obtain the rough estimate , where 𝑙 denotes the characteristic scale of the perturbation region. the amplitude of the tsunami wave in the open ocean is approximately two orders of magnitude greater than the value 𝜂𝑚 = 1 cm alluded to above. one may assume that the amplitude of the electric field caused by the tsunami wave will also be two orders of magnitude greater than that shown in figure 4. substituting 𝐸 = 10–2 v/m, 𝑙 = 10 m and 𝜎 = 10–2‑10–1 s/m into the above ratio, gives the estimate nt, which is close to the observed values [manoj et al., 2011]. this estimate is applicable until the tsunami wave arrival on the shore. the study of the later phase of seawater spreading deep into the seashore seems to be a much more complicated problem, since in such a case the nonlinear effects should be taken into account. in addition, there can be significant perturbations of the geomagnetic field resulted from the flow of conducting seawater. the results of this study points clearly to the existence of the low‑frequency electric noise caused by ek effect in the seabed and coastal rocks. in practice, the amplitude of this effect may vary greatly, since the actual values of various parameters of porous rock, such as porosity, permeability, streaming potential coupling coefficient, elastic constants that determine the ability of porous rocks to deform under stress, etc. may vary over a wide range. electrokinetic effect caused by oceanic waves 9 further experiments are necessary to sort out this interesting problem in the geophysics of the coastal zone and to distinguish between different mechanisms of natural electromagnetic noise. acknowledgements. this study is supported by the state contract with izmiran. references alexandrov, v.v. 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doi: 10.4401/ag-7706 1 “static and dynamic properties of soils in catania (italy)„ francesco castelli1, antonio cavallaro2, antonio ferraro3, salvatore grasso4,*, valentina lentini5, maria rossella massimino6 (1) faculty of engineering and architecture, university of enna “kore”, enna (italy) (2) cnr ibam (italian national research council institute for archaeological and monumental heritage), catania (italy) (3) university of catania department of civil engineering and architecture (dicar), catania (italy) (4) university of catania department of civil engineering and architecture (dicar), catania (italy) (5) faculty of engineering and architecture, university of enna, enna (italy) (6) university of catania department of civil engineering and architecture (dicar), catania (italy) 1. introduction site characterization is a crucial point in the evaluation of site response analysis of soils [castelli et al., 2016d; castelli and maugeri, 2008; castelli et al., 2008a, 2008b; castelli and lentini, 2013; castelli and lentini, 2016; castelli et al., 2017; cavallaro, 2006b, 2008a, 2008b]. in order to study the dynamic characteristics of soils in three catania test sites, laboratory and in situ investigations have been carried out to obtain soil profiles with special attention being paid to the variation of the shear modulus g and damping ratio d with depth. seismic dilatometer marchetti tests (sdmt) have been also carried out in the area of the “national institute of geophysics and volcanology” building (ingv) and in the “madre teresa di calcutta” building school (cd mtc), with the aim of an accurate geotechnical characterisation, including the evaluation of the shear wave velocity vs profile, as well as the profile of the horizontal stress index kd. shear wave velocity has been measured by different tests. the soil profiles in terms of the shear modulus g0 and in terms of the shear wave velocity vs have been evaluated and compared by different in situ tests. the redundancy of measurements is very useful, for instance, for site response and liquefaction analyses of the city of catania [castelli et al., 2016c], which can be based either on vs values or kd values. boreholes were driven and undisturbed samples were retrieved for laboratory tests. because of their relevance on the estimation of local ground shaking and site effects, data from in-hole geophysical surveys (down-hole and seismic article history receveid june 8, 2017; accepted february 2, 2018. subject classification: dynamic soil characterisation; in-situ tests; laboratory tests. abstract the analysis of the historical reports and previous research projects allowed us to describe high-level seismic risk scenarios for the city of catania, located on the eastern coast of sicily (italy). the geotechnical zonation of the subsoil of the city of catania suggests a high seismic hazard added to a soil amplification of the ground motion. to this aim field and laboratory tests allowed the definition of a geotechnical characterisation in the city of catania. for the site characterisation of soils deep site investigations have been undertaken. borings and dynamic in situ tests have been performed in some selected test sites with the aim to evaluate the soil profiles of shear wave velocity and to retrieve undisturbed samples for laboratory tests. the testing programme consisted of standard classification tests, oedometer tests, direct shear tests, resonant column and torsional shear tests. two equations to draw the complete shear modulus degradation with strain level and the inverse variation of damping ratio with normalized shear modulus respectively have been also proposed. shear wave velocity profiles experimentally obtained were compared with both in situ and laboratory tests. dilatometer marchetti test) have been examined with special attention, particularly for s wave velocity measurements. it must be noted that the shear wave velocity vs was evaluated on the basis of both empirical correlations with in situ or laboratory tests and a few direct measurements. moreover, the following investigations in the laboratory were carried out on undisturbed samples: direct shear tests, triaxial tests, cyclic loading torsional shear tests (cltst) and resonant column tests (rct). this paper tries to summarize the geotechnical information in a comprehensive way in order to provide a case record of data for site characterization and for the mitigation of seismic risk within structural improvement of buildings [abate et al., 2016; abate et al., 2017a; 2017b; grassi and massimino, 2009]. similar geotechnical studies were also successful performed for significant historical test sites (castelli et al. 2016a, 2016d, 2016e; 2016f; cavallaro et al. 1999b, 2003, 2004a, 2004b, 2013a). 2. geology and seismicity of the area the study was performed in eastern sicily, which is characterized by the presence of mount etna volcano. this rests on top of two major structural units [lentini et al., 1982; pappalardo et al., 2016]: the foreland iblean plateau and the apennine-maghrebian chain. the foreland iblean plateau is the northern edge of the african plate and is characterized by a succession of meso-cenozoic, mainly carbonate, repeatedly interspersed with basic volcanic rock. the city of catania is located on the east coast of sicily, which is one of the most seismically active areas in italy. various disastrous earthquakes struck the east coast of sicily, with a (msk) intensity from ix to xi in the last 900 years [postpischl, 1985; azzaro et al., 1999; locati et al., 2016]. in particular, the seismic events of february 20, 1169 and january 11, 1693 destroyed almost completely the city of catania with intensity x-xi msk and estimated magnitude between 7.0 and 7.4 [boschi et al., 1995; gizzi, 2006]. the earthquake of january 11, 1693 is considered one of the biggest earthquakes occurred in italy. it is supposed that more than 1500 aftershocks occurred along a period of more than two years after the main shock. this earthquake, with an intensity of xi degree on the msk scale in many centers, struck a vast territory of southeastern sicily and caused the partial, and in many cases total, destruction of 57 cities and 40,000 casualties. from 1000 a.d., just four other earthquakes in the area exceeded an estimated local richter magnitude 5.8: the july 7, 1125, the december 10, 1542, the january 9, 1693 and the february 20, 1818 events [barbano et al., 2010], see figure 1. some of these events probably produced historical tsunamis (1693 and 1908 events) along the ionian coast of sicily [barbano et al., 2010]. the other seismic events, which damaged the city of catania, such as the march 1536, the april 1698, the december 1716 and the december 1990 earthquakes, generally produced minor effects, with collapses in degraded buildings. the recent earthquake on december 13, 1990 namely “the st. lucia earthquake” struck eastern sicily with a local richter magnitude ml = 5.6 and caused 19 victims and severe damages to buildings and infrastructures [de rubeis et al., 1991]. seismicity is mainly distributed in two sectors: along the coast, where the events have also reached a surface richter magnitude ms ≥ 7.0, and inland with earthquakes with ms ≤ 5.5 [panzera et al., 2011a]. graph 1 shows the seismic history of the city of catania in the italian macroseismic database dbmi15 [locati et al., 2016] during the time window of years 1000-2014, starting from the fixed macroseismic intensity of vi mcs. intensity data of dbmi15 derive from studies by authors from various institutions, both in italy and bordering countries. for these reasons several examples of site response analyses by 1-d modeling and microtremor measurements have been performed in the urban area of catania [lombardo et al., 2001; panzera et al., 2011b, 2015; sgarlato et al., 2011]. 3. site characterization programme and basic geotechnical soil properties the static and dynamic geotechnical study of soils in the city of catania was performed in three investigation tests sites: the “national institute of geophysics castelli et al. 2 graph 1. seismic history of the city of catania in the italian macroseismic database dbmi15 during the time window 1000-2014. and volcanology” (ingv) in the central area of the city, the “nazario sauro building school” (ic ns) in the western area of the city and the “madre teresa di calcutta school” (cd mtc) in tremestieri etneo municipality in the northern area of the city. the investigation studied depth reached a maximum depth of 80 m for ingv, of 40 m for ic ns and of 30 m for cd mtc. laboratory tests have been performed on undisturbed samples retrieved by means of a 101 mm tube sampler [cavallaro et al., 2007]. to evaluate the geotechnical characteristics of soils the following in situ and laboratory tests were performed in the tests located in the city of catania: ingv: n. 3 boreholes, n. 3 standard penetration test (spt), n. 2 down-hole tests (dht), n. 1 crosshole tests (cht), n. 1 seismic dilatometer marchetti test (sdmt), n. 6 direct shear tests (dst), n. 2 consolidated undrained triaxial tests (cutxt), n. 2 unconsolidated undrained triaxial tests (uutxt), n. 2 cyclic loading torsional shear tests (cltst), n. 4 resonant column test (rct). ic ns: n. 2 boreholes, n. 2 seismic dilatometer marchetti tests (sdmt), n. 6 direct shear tests (dst), n. 3 unconfined uniaxial compression tests. cd mtc: n. 1 boreholes, n. 1 seismic dilatometer marchetti test (sdmt), n. 3 direct shear tests (dst), n. 1 unconfined uniaxial compression tests. the “national institute of geophysics and volcanology” (ingv) area mainly consists: s1: a paving slabs of basalt (to 0.00 0.15 m depth), a layer of backfill (to 0.15 1.50 m depth), a layer of brown to dark yellow sandy silt slightly clayey with included centimeter limestone (to 1.50 5.10 m depth), a layer of yellow orange sand with included sandstone gravels (to 5.10 5.40 m depth), a layer of brown to dark yellow sandy silt slightly clayey with included centimeter limestone (to 5.40 6.70 m depth), a layer of yellow ochre to grey clay slightly silty (to 6.70 9.50 m depth), a layer of grey blue clay slightly silty with included decimeter dark black sand lenses (to 9.50 60.00 m depth). s2: a paving slabs of basalt (to 0.00 0.15 m depth), a layer of backfill (to 0.15 1.50 m depth), a layer of brown to dark yellow sandy silt slightly clayey with included centimeter limestone (to 3 dynamic characterisation of soils in catania figure 1. map of seismotectonic features of south-eastern sicily, with indication of 1693 and 1908 historical tsunamis, after barbano et al. [2010]. 1.50 6.00 m depth), a layer of yellow orange sand with included sandstone gravels (to 6.00 6.90 m depth), a layer of yellow ochre to grey clay slightly silty (to 6.90 8.50 m depth), a layer of grey blue clay slightly silty with included decimeter dark black sand lenses (to 8.50 80.00 m depth). s3: a paving slabs of basalt (to 0.00 0.15 m depth), a layer of backfill (to 0.15 2.60 m depth), a layer of brown to dark yellow sandy silt slightly clayey with included centimeter limestone (to 2.60 8.60 m depth), a layer of yellow ochre to grey clay slightly silt (to 8.60 12.00 m depth), a layer of grey blue clay slightly silty with included decimeter dark black sand lenses (to 12.00 60.00 m depth). the “nazario sauro building school” (ic ns) area mainly consists: s1: a layer of topsoil (to 0.00 1.00 m depth), a layer of fractured basaltic rock with included dark grey vacuoles (to 1.00 4.50 m depth), a layer of smoke grey sand and volcanic breach (to 4.50 13.30 m depth), a layer of compact basaltic rock with included light to dark fractures (to 13.30 25.20 m depth), a layer of sand and volcanic breach with included highly oxidized basaltic blocks (to 25.20 32.20 m depth), a layer of sand and smoke grey volcanic breach (to 32.20 40.00 m depth). s2: a layer of backfill (to 0.00 2.80 m depth), a layer of fractured basaltic rock with included dark grey vacuoles (to 2.80 7.80 m depth), a layer of compact basaltic rock with included light to dark fractures (to 7.80 23.40 m depth), a layer of yellow brown tufites with mediumfine grain size ochre to grey clay slightly silty castelli et al. 4 figure 2. layout of tests sites geotechnical boreholes: a) “national institute of geophysics and volcanology” (ingv): s1, s2, s3; b) “nazario sauro building school” (ic ns): s1, s2; c) “madre teresa di calcutta school” (cd mtc): s1. a) b) c) (to 6.90 8.50 m depth). the “madre teresa di calcutta school” (cd mtc) area mainly consists: s1: a layer of topsoil (to 0.00 1.00 m depth), a layer of sand and dark grey volcanic breach (to 1.00 8.00 m depth), a layer of compact basaltic rock with included light grey to dark grey fractures (to 8.00 14.00 m depth), a layer of sand and dark grey volcanic breach (to 14.00 22.50 m depth), a layer of compact basaltic rock with included light grey to dark grey fractures (to 22.50 25.00 m depth), a layer of sand and dark grey volcanic breach (to 25.00 29.30 m depth), a layer of compact basaltic rock with included light grey to dark grey fractures (to 29.30 30.00 m depth). using information made available from in situ boreholes, soil profiles of the ground beneath the catania test sites could be designed (figure 3). trying to summarize the borehole results it is possible to define four soil classes, namely: basaltic rock, sand, volcanic breach and silty clay. 5 dynamic characterisation of soils in catania figure 3. soil profiles of: a) s1, s2, s3, on ingv; b) s1, s2 on ic ns; c) s1 on cd mtc. a) c)b) based on the laboratory tests typical range of physical characteristics, index properties and strength parameters of the deposits mainly encountered in these areas are reported in table 1, 2, 3. on the basis of sdmt the ingv deposits mainly consist of over-consolidated sandy silt slightly clayey and clay slightly silty soil. the value of the natural moisture content wn prevalently castelli et al. 6 site h [m] γ kn/m3 wn (%) gs e n sr (%) dst cutxt uutxt c’ (kpa) ϕ’ (°) cu (kpa) ϕu (°) c’ (kpa) ϕ’ (°) cu (kpa) s1c1 4.00 4.50 20.01 16.99 2.74 0.57 0.36 81.60 11 29 s1c2 9.00 9.50 19.71 26.21 2.76 0.74 0.42 98.45 38 16 55 12 32 17 s1c3 34.00 34.50 19.61 22.65 2.76 0.69 0.41 90.15 51 20 226 s2c1 5.00 5.40 18.24 15.78 2.76 0.72 0.42 60.59 14 26 s2c2 7.20 7.70 19.22 24.45 2.79 0.77 0.44 88.41 28 22 104 13 69 19 s3c3 26.50 27.00 19.12 26.35 2.72 0.76 0.43 93.97 50 20 106 site h [m] γ kn/m3 wn (%) gs e n sr (%) dst cutxt uutxt c’ (kpa) ϕ’ (°) cu (kpa) ϕu (°) c’ (kpa) ϕ’ (°) cu (kpa) s1c2 7.00 7.50 16.57 8.69 2.74 0.77 0.43 31.12 20 37 s1c3 11.50 12.00 16.48 12.36 2.82 0.89 0.47 39.17 30 37 s1c1 5.00 5.50 16.87 3.76 2.74 0.65 0.39 15.78 18 39 table 1. mechanical characteristics for ingv of catania areas. where: c' = cohesion and ϕ' = angle of shear resistance; direct shear test (dst), cu triaxial tests (cutxt), uu triaxial tests (uutxt). table 2. mechanical characteristics for ic ns of catania areas. where: c' = cohesion and ϕ' = angle of shear resistance; direct shear test (dst), cu triaxial tests (cutxt), uu triaxial tests (uutxt). table 3. mechanical characteristics for cd mtc of catania areas. where: c' = cohesion and ϕ' = angle of shear resistance; direct shear test (dst), cu triaxial tests (cutxt), uu triaxial tests (uutxt). site h [m] γ kn/m3 wn (%) gs e n sr (%) dst cutxt uutxt c’ (kpa) ϕ’ (°) cu (kpa) ϕu (°) c’ (kpa) ϕ’ (°) cu (kpa) s1c1 3.00 3.50 16.18 6.50 2.71 0.75 0.43 23.46 20 44 s1c2 6.00 7.00 16.77 4.32 2.72 0.66 0.40 17.87 11 38 s1c4 15.00 16.00 16.97 1.82 2.75 0.62 0.38 8.08 0 10 ranges from between 16 26 % for ingv, 4 12 % for ic ns and 2 6 % for cd mtc. characteristic values for gs (specific gravity) ranged between 2.72 and 2.79 for ingv, 2.74 2.82 for ic ns and 2.71 2.75 for cd mtc. figure 4 shows index properties of ingv, ic ns and cd mtc areas. the water level is of 3.50 m from the ground surface for ingv, 0 m for ic ns and cd mtc areas. as regards strength parameters of the deposits mainly (tables 1, 2, 3) encountered in this area c' from dst ranged between 11 kpa and 51 kpa for ingv, 18 kpa and 30 kpa for ic ns and 0 and 20 kpa for cd mtc while ϕ' from dst ranged between 16° and 29° for ingv, 37° and 39° for ic ns and 10° and 20° for cd mtc. the laboratory results of unconfined compression tests performed on sample retrieved on ic ns and cd mtc areas are reported in table 4 together the dynamic results of nondestructively ultrasonic tester tests (utt). 4. shear modulus by in situ tests it was also possible to evaluate the small strain shear modulus in the investigated catania test sites by means of the results deriving from the following seismic tests based on down hole (dh) and cross hole method (ch). in figure 5, the shear and compression wave velocities against depth have been reported. in figure 6 the dynamic poisson ratio variation with depth, obtained from down hole (dh) and cross hole (ch) tests, is plotted to show site characteristics. it is clear that the values oscillate around 0.36 0.45 for ch and 0.45 0.49 for dh. in figure 7 the coefficient of earth pressure at rest ko variation with depth, obtained from a down hole (dh) and cross hole tests, is plotted to show site characteristics. it is clear that apart from the top 5 m, the values oscillates around 0.60 0.75 from ch and 0.90 7 dynamic characterisation of soils in catania (b) site borehole h(m) γ kn/m3 rc (mpa) vp (m/s) vs υ g (mpa) e (mpa) ic ns sicl1 1.10 1.50 25.62 97.24 ic ns sicl4 16.00 17.00 27.59 132.93 2812 1773 0.17 8779 20544 ic ns sicl2 5.60 6.00 27.51 135.77 3427 2076 0.21 12000 29041 cd mtc sicl3 1.00 11.50 26.26 146.08 2621 1668 0.16 7393 17152 table 4. test results of nondestructively ultrasonic tester tests (utt). where: rc = compressive strength. figure 4. index properties of ingv, ic ns and cd mtc areas. castelli et al. 8 figure 5. vs and vp from down hole and cross hole tests in ingv area. figure 6. poisson ratio from down hole and cross hole tests. 0.95 from dh. a comparison between go values obtained from in situ test performed on the area under consideration is shown in figure 8. the down hole and cross hole tests performed in ingv area showed go values increasing with depth. very high values of go are obtained for depths greater than 30 m. according to these data, it is possible to assume that go values oscillate around 50 250 mpa. the sdmt [marchetti 1980; marchetti et al., 2008; monaco et al., 2009] provides a simple means for determining the initial elastic stiffness at very small strains and in situ shear strength parameters at intermediate level of strains in natural soil deposits [castelli et al., 2016b; cavallaro, 1999c; cavallaro et al., 2012, 2015]. this apparatus was also used in offshore condition by cavallaro et al. [2013b, 2013c]. 9 dynamic characterisation of soils in catania figure 7. the coefficient of earth pressure at rest ko from down hole and cross hole tests. figure 8. go from down hole and cross hole tests. vs may be converted into the initial shear modulus go by the well-known relationships: (1) where: r = mass density. the combined knowledge of go and of the one-dimensional modulus m (from dmt) may be helpful in the construction of the g-γ modulus degradation curves [cavallaro et al., 2006a; castelli et al., 2016c]. a summary of sdmt parameters are shown in figure 9 where: id: material index; gives information on soil type (sand, silt, clay); ϕ': angle of shear resistance; m: vertical drained constrained modulus; cu: undrained shear strength; kd: horizontal stress index; the profile of kd is similar in shape to the profile of the overconsolidation ratio ocr. kd = 2 indicates in clays ocr = 1, kd > 2 indicates overconsolidation. a first glance at the kd profile is helpful to “understand” the deposit; vs: shear waves velocity. figure 10 shows results of the sdmts performed in the test sites in terms of shear wave velocity profiles. it was also possible to evaluate the small strain shear modulus go in the catania test sites by means of the following empirical correlations available in literature based on laboratory test results, standard penetration tests (spt) or seismic marchetti dilatometer tests results: a) jamiolkowski et al. [1995] (2) where: σ'm = (σ'v + 2 σ'h)/3 effective medium stress with σ'v = effective vertical normal stress and σ'h = effective horizontal normal stress; pa = 1 bar is a reference pressure; e = void ratio index; go, σ'm and pa are expressed in the same unit. the values for parameters, which appear in equation (2) are equal to the average values that result from laboratory tests performed on quaternary italian clays and sands. a similar equation was proposed by shibuya and tanaka [1996] for holocene clay deposits. b) ohta and goto [1978] (3) where: vs = shear wave velocity (m/s), n60 = number of blow/feet from spt with an energy ratio of 60%, z = depth (m), fg = geological factor (clays=1.000, sands=1.086), fa = age factor (holocene = 1.000, pleistocene = 1.303). c) hryciw [1990] (4) where: go, σ'v and pa are expressed in the same unit; pa = 1 bar is a reference pressure; γd and ko are respectively the unit weight and the coefficient of earth castelli et al. 10 g0 = vs 2r figure 9. results of the sdmts in terms of geotechnical parameters for ingv area. go = 600⋅ m '0.5pa 0.5 e1.3 σ vs = 69⋅ n60 0.17 ⋅ z0.2 ⋅ fa ⋅ fg go = 530 σv ' / pa( ) 0.25 γd / γw −1 2.7 − γd / γw ko 0.25 ⋅ v ' ⋅ pa( ) 0.5 σ pressure at rest, as inferred from sdmt results according to marchetti [1980]. figure 11 shows the go values obtained by sdmt and those obtained by means of the empirical correlations. on the whole, equations (2) and (4) seems to provide the most accurate trend of go with depth, as can be seen in figure 11. a good agreement exists between empirical correlations and sdmt. however, as can be seen in figure 11, the method by hryciw [1990] was not capable of detecting the sdmt results for sandy soil as reported in figure 11. figure 11 shows also the value of go measured in the laboratory from rct performed on undisturbed solid cylindrical specimens. in the case of laboratory tests, the go values are determined at shear strain levels of less than 0.001 %. at the depth of about 7 m high values of go are registered by 11 dynamic characterisation of soils in catania figure 10. shear wave velocity profiles obtained from sdmt for cd ns a), ingv b) and cd mtc c) areas. figure 11. go values obtained by sdmt, by d-h and by empirical correlations for ingv site. a) b) c) sdmt probably due to the presence of a layer of grey blue clay slightly silty with included decimeter dark black sand lenses. high values of go were obtained for levels higher than 25 m depth. in figure 12 it is proposed a comparison between the values of vs obtained from sdmt and ultrasonic tester tests utt. up to a depth of 15 m higher vs values are obtained in the laboratory by nondestructively ultrasonic tester tests (utt) rather than those obtained in situ from sdmt tests. 5. shear modulus and damping ratio by laboratory tests shear modulus g and damping ratio d of catania tests sites deposits were obtained in the laboratory by resonant column tests (rct) and by cyclic loading torsional shear tests (cltst) performed by means of a resonant column/cyclic loading torsional shear apparatus [cascante et al., 1998] (figure 13). a resonant column test consists in exciting one end of a confined solid or hollow cylindrical soil specimen. the specimen is fixed at the bottom (fixed-free test) and it is excited in torsion or flexure at the top by means of an electromagnetic drive system. once the fundamental resonant frequency is established from measuring the motion of the free end, the velocity of the propagating wave and the degree of material damping are derived. the shear modulus is then obtained from the derived velocity vs (in case of torsion) and the density r of the sample [cavallaro et al., 1999a; maugeri and cavallaro, 1999]. the equivalent shear modulus geq is the unloadreload shear modulus that is evaluated from rct in function of velocity vs and density r of the sample, while go is the maximum value or also “plateau” value as observed in the g-log(γ) plot; g is the secant modulus [castelli et al., 2016c; cavallaro, 2016; lo presti et al., 1999a]. generally g is constant until a certain strain limit is exceeded. this limit is called elastic threshold shear strain (γet) and it is believed that soils behave elastically at strains smaller than γet. the elastic stiffness at γ<γ e t is thus the already defined go [capilleri et al., 2014; lo presti et al., 1999b]. for cltsts the damping ratio (d) was calculated as the ratio between the area enclosed by the unloadingreloading loop and represents the total energy loss during the cycle and w is the elastic stored energy. for rcts the damping ratio was determined using the steady-state method during the resonance condition of the sample. the apparatus used is a fixed-free resonant column apparatus [hall and richart, 1963]. it enables the specimen consolidation under both isotropic and anisotropic stresses. it is composed of a drive system, a support system, and a base plate. the solid or hollow cylinder specimen is fixed at the bottom and its constraint at the base castelli et al. 12 figure 12. vs from sdmt, utt and by empirical correlation for ic ns and cd mtc sites. is due to the friction existing between the specimen and the porous synthesized bronze stone [drnevich et al., 1978]. torsional forces are applied at the top by means the drive system, realized in aluminium. it is an electrical motor constituted of four magnets connected with the top of the sample and eight coils placed on the inox steel annular base, which is strictly linked to the support system. the weight of the motor is counterbalanced by a spring. a programmable function generator (pgf) excites the electrical motor of stokes. solid cylindrical specimens were built by using tapping [drnevich et al., 1978], in order to obtain the required relative and a good uniformity during the deposition. the mold is assembled and a little depression is applied to let the membrane adhere to the inside surfaces. the material is placed in the mold using a funnel-pouring device. the soil is placed as loosely as possible in the mold by leaving the soil from the spout in a steady stream, holding the pouring device upright and vertical, and maintaining constant the fall height. it is possible to obtain different values of relative density changing the height of deposition. in order to realize high values of relative density it could be necessary to beat delicately the mold surface during the deposition. each sample was reconstituted with fresh sand. each specimen was subjected to an isotropic load achieved in a plexiglas pressure cell, using an air pressure source. the axial strain was measured by using a high-resolution proximity transducer, which monitors the aluminium top-cap displacement. shear strain was measured by monitoring the top rotation with a couple of high-resolution proximity transducers. during a resonant column test, the proximity transducers are not able to appraise the value of the targets displacements, because of the high frequency of the oscillations. then rotation on the top of the specimen is measured by means of an accelerometer. the laboratory test conditions and the obtained small strain shear modulus go are listed in table 5. the undisturbed specimens were isotropically reconsolidated to the best estimate of the in situ mean effective stress. the same specimen was first subject to rct, then to cltst after a rest period of 24 hrs with opened drainage. cltst were performed under stress control condition by applying a torque, with triangular time history, at a frequency of 0.1 hz. the size of solid cylindrical specimens are radius = 25 mm and height = 100 mm. the go values [go (1) and go (2)], reported in table 5, indicate moderate influence of strain rate even at very small strain where the soil behaviour is supposed to be elastic. in order to appreciate the rate effect on go, it is worthwhile to remember that the equivalent shear strain rate (γ⋅ = 240 ⋅ f ⋅ γ [%/s]) experienced by the specimens during rct can be three orders of magnitude greater than those adopted during cltst. the effects of the rate and loading conditions become less relevant with an increase of the shear strain level, as can be seen in figure 13 dynamic characterisation of soils in catania figure 13. resonant column/cyclic loading torsional shear apparatus. borehole h (m) σ’vc (kpa) e cltst rct go (1) (mpa) go (2) (mpa) go (3) (mpa) γmax(%) s1c1 4.75 75 0.57 u 95 126 0.098 s3c1 5.70 90 0.76 u 62 46 58 0.114 s2c3 14.25 170 0.76 u 30 28 78 0.501 s2c5 72.25 475 0.75 u 47 0.361 table 5. test condition for ingv area specimens. where: u = undrained. go (1) from rct, go (2) from cltst after 24 hrs, go (3) from down-hole. 14 where the g-γ curves obtained from cltst and rct are compared. it is possible to notice that the lowest decay of g with γ is observed in cltst, while the maximum decay occurs during rct. finally higher values of the initial shear modulus [go (3)] have been obtained from down hole tests. in general (figure 15) higher values of d are obtained by rct than cltst. the damping ratio values obtained from rct by steady-state method increases with the strain level, higher values of d have been obtained from strain level higher than 0.02 %. greater values of d are obtained from rct for strain level higher than 0.2 %. the experimental results were castelli et al. 14 figure 14. g-γ curves from rct and cltst tests. figure 15. d-γ curves from rct and cltst tests. used to determine the empirical parameters of the eq. proposed by yokota et al. [1981] to describe the shear modulus decay with shear strain level (figure 16): (5) in which: g(γ) = strain dependent shear modulus; γ = shear strain; a, b = soil constants. the expression (1) allows the complete shear modulus degradation to be considered with strain level. the values of a = 22 and b = 1.05 were obtained for ingv area. as suggested by yokota et al. [1981], the inverse 15 dynamic characterisation of soils in catania figure 16. g/goγ curves from rct tests. g(γ) g0 = 1 1+aγ (%)b figure 17. d/go curves from rct tests. variation of damping ratio with respect to the normalised shear modulus has an exponential form as that reported in figure 17 for the ingv area: (6) in which: d(γ) = strain dependent damping ratio; γ = shear strain; η, l = soil constants. the values of η = 10 and l = 1.05 were obtained for ingv area. the equation (5) assume maximum value dmax = 10 % for g(γ)/go = 0 and minimum value dmin = 3.50 % for g(γ)/go = 1. therefore, equation (5) can be rewritten in the following normalised form: (7) 6. conclusions the paper represents a report about static and dynamic properties of soils in the city of catania. a great availability of borehole data, geophysical surveys and laboratory tests have been carried out from various campaigns of geological surveys. in order to study the dynamic characteristics of soils in three catania test sites, laboratory and in situ investigations have been carried out to obtain soil profiles with special attention being paid to the variation of the shear modulus g and damping ratio d with depth. therefore a site characterisation of “national institute of geophysics and volcanology” (ingv), “nazario sauro building school” (ic ns) and “madre teresa di calcutta school” (cd mtc) areas for seismic response analysis was presented. using information made available from in situ boreholes, soil profiles beneath the catania test sites have been proposed. trying to summarize the borehole results it is possible to define four soil classes, namely: basaltic rock, sand, volcanic breach and silty clay. available data permitted to define the small strain shear modulus go profile variation with depth. on the basis of the obtained results it is possible to draw the following conclusions: the influence of strain rate at very small strain is so negligible; a degradation phenomenon occurs during rct; damping ratio values obtained by rct are greater than those obtained by cltst; higher values of go were obtained by down-hole tests; different go values are obtained by sdmt than down hole and empirical correlations. on the basis of in situ test results, it is possible to stress that the small strain shear modulus measured in the laboratory is smaller than that obtained in situ by means of dh tests; this is probably due to a disturbance phenomenon occurred during sampling. moreover three empirical correlations available in literature based on laboratory test results, standard penetration tests (spt) and seismic dilatometer marchetti tests (sdmt) results were proposed. on the whole, equations proposed by jamiolkowski et al. [1995] and by hryciw [1990] seem to provide the most accurate trend of go with depth. finally two empirical expressions have been proposed to describe the complete shear modulus degradation with strain level and the inverse variation of damping ratio with respect to the normalised shear modulus. moreover, analysing the shear wave velocity profiles as well as the profiles of the horizontal stress index kd of the test sites “national institute of geophysics and volcanology” (ingv) and the school “madre teresa di calcutta school” (cd mtc), it is possible to observe that: higher values of shear wave velocity vs in the (cd mtc) site have been obtained by tests in the upper 30 m of soil, so that slight soil amplification phenomena can occur; lower values (<400 m/s) of shear wave velocity vs in the (ingv) site have been obtained by tests in the upper 30 m of soil, so that considerable soil amplification phenomena can occur. references abate, g., massimino, m.r. 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(2011): “evaluation of seismic site response nearby underground cavities using earthquake and ambient noise recordings: a case study in catania area, italy”. 2011, engineering geology, 122(3):281-291 doi10.1016/j.enggeo.2011.06.002. shibuya, s. and tanaka, h., (1996): "estimate of elastic shear modulus in holocene soil deposits"; soils and foundations, vol. 36, n°. 4, pp. 45 55. yokota, k., imai, t. and konno, m., (1981): "dynamic deformation characteristics of soils determined by laboratory tests"; oyo tec. rep. 3, pp. 13 37. *corresponding author: salvatore grasso, department of civil engineering and architecture (dicar), university of catania, catania (italy) email: sgrasso@dica.unict.it © 2018 istituto nazionale di geofisica e vulcanologia. all rights reserved. castelli et al. 20 vol48/04/2005def 671 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words sulphur – hydrous basalts – volcanic gas – italy 1. introduction the determination of the composition of the fluids that coexist with magmas at depth requires 1) a good definition of the magma pre-eruptive conditions (p, t, f o2, f s2, f h2o) and 2) solubility models for the main volatile species. the combination of these two sets of information allows us to calculate the fluid composition through the consideration of fluid-melt equilibria (e.g., scaillet and evans, 1999). for any volatile species dissolved in a silicate melt that is saturated in a fluid, equilibrium conditions demand that the fugacity fi of species i in the melt equals its fugacity in the fluid. for water we have therefore (1.1).f fo o2 melt 2 fluid=h h a model of sulphur solubility for hydrous mafic melts: application to the determination of magmatic fluid compositions of italian volcanoes bruno scaillet and michel pichavant isto-cnrs, umr 6613, orléans, france abstract we present an empirical model of sulphur solubility that allows us to calculate f s2 if p, t, f o2 and the melt composition, including h2o and s, are known. the model is calibrated against three main experimental data bases consisting in both dry and hydrous silicate melts. its prime goal is to calculate the f s2 of hydrous basalts that currently lack experimental constraints of their sulphur solubility behaviour. application of the model to stromboli, vesuvius, vulcano and etna eruptive products shows that the primitive magmas found at these volcanoes record f s2 in the range 0.1-1 bar. in contrast, at all volcanoes the magmatic evolution is marked by dramatic variations in f s2 that spreads over up to 9 orders of magnitude. the f s2 can either increase during differentiation or decrease during decompression to shallow reservoirs, and seems to be related to closed versus open conduit conditions, respectively. the calculated f s2 shows that the italian magmas are undersaturated in a fes melt, except during closed conduit conditions, in which case differentiation may eventually reach conditions of sulphide melt saturation. the knowledge of f s2, f o2 and f h2o allows us to calculate the fluid phase composition coexisting with magmas at depth in the c-o-h-s system. calculated fluids show a wide range in composition, with co2 mole fractions of up to 0.97. except at shallow levels, the fluid phase is generally dominated by co2 and h2o species, the mole fractions of so2 and h2s rarely exceeding 0.05 each. the comparison between calculated fluid compositions and volcanic gases shows that such an approach should provide constraints on both the depth and mode of degassing, as well as on the amount of free fluid in magma reservoirs. under the assumption of a single step separation of the gas phase in a closed-system condition, the application to stromboli and etna suggests that the main reservoirs feeding the eruptions and persistent volcanic plumes at these volcanoes might contain as much as 5 wt% of a free fluid phase. consideration of the magma budget needed to balance the amounts of volatiles emitted in the light of these results shows that the amount of nonerupted magma could be overestimated by as much as one order of magnitude. mailing address: dr. bruno scaillet, isto-cnrs, umr 6613, 1a rue de la férollerie, 45071 orléans cedex 02, france; e-mail: bscaille@cnrs-orleans.fr 672 bruno scaillet and michel pichavant the relationships between fugacity (or activity) and concentration in silicate melts have been established mainly for h2o and co2 for a wide range of melt compositions (e.g., dixon et al., 1995; papale, 1997; zhang, 1999). once the fugacity of a given volatile species has been determined, its partial pressure, pi, can be determined via the simple eq. (1.2) (1.2) where γi is the fugacity coefficient of species i in the fluid. in the system c-o-h, the other main species to be considered is co2, except if f o2 is significantly below fmq, in which case reduced species such as h2, co, and ch4 can be present as well (see holloway, 1977). therefore, considering that h2o and co2 are the main volatile species in the fluid, we have the following equations: (1.3) (1.4) since the fluid can be treated as a binary h2oco2 system, we have also the constraint, xi being the mole fraction of species i in the fluid (1.5) recalling that pi = xipfluid, eqs. (1.3) to (1.5) can be solved simultaneously to calculate both ph2o and pco2 and thus pfluid since by definition (1.6) what is calculated through this approach is thus the pressure at which the melt becomes saturated in a fluid phase and the composition of this fluid. this method offers thus the possibility to constrain both the minimum pressure of magma reservoir and the fluid composition that is likely to escape this reservoir during an eruptive process. obviously those two information are of considerable interest for the assessment of volcanic hazards. this approach was first applied using modern solubility models for h2o .p pifluid = σ .x x 1h o co2 2+ = .p f co co co 2 2 2 = c p f h o h o h o 2 2 2 = c p f i i i = c and co2 by anderson et al. (1989) to the bishop tuff eruption. since then, several petrological studies have estimated the pressure depth at which magmas become fluid-saturated, assuming that the fluid phase is a binary h2o-co2 mixture (e.g., roggensack et al., 1997). most arc magmas are, however, notoriously rich in sulphur. over the last 15 years, several melt inclusions studies have shown that the preeruptive melt sulphur content of arc basalts is equal to, or even higher than, the amount of dissolved co2 (e.g., roggensack et al., 1997; marianelli et al., 1995, 1999). this shows that sulphur bearing volatile species must be incorporated into the above approach for a more complete description of fluid-melt equilibria. the nature and proportion of s-bearing species, mostly h2s and so2, strongly depend on f o2, as for c-bearing species (see holloway, 1977; symonds et al., 1994). unlike c-species, however, the redox range in which the h2s/so2 ratio varies is exactly within the redox range recorded by most arc-magmas (i.e. nno to nno + 2). this f o2 sensitivity introduces an additional complication in the approach outlined above, since the role of f o2 on volatile speciation must be explicitly taken into account. the calculation of the pressure for fluid saturation of a magma saturated in a c-o-h-s fluid at an f o2 equal to or higher than nno will require to solve the following equation: (1.7) if the contribution of co, ch4, h2, s2, and o2 partial pressures are ignored, which is a valid approximation under the redox conditions considered here ( f o2 > nno). the so2 and h2s species abundances are controlled by the following equilibria: (1.8) (1.9) which show that what is needed to calculate sbearing species fugacities is f s2 or one over the three s-bearing species fugacities (assuming that both f o2 and f h2 are known, which are two s sh h 2 1 2 2 2)+ s o so 2 1 22 2)+ p p p ppfluid co so sh o h2 2 2 2= + ++ 673 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes quantities generally known or obtainable if the pre-eruptive conditions are determined in magmas). the preceding section shows that there is a need for a solubility model for s in silicate melts. up to recently, there was only one such a model (wallace and carmichael, 1992) which was calibrated for dry basalts and f o2 below nno, using mostly the experimental data of haughton et al. (1974). clemente et al. (2004) have developed both a thermodynamic and an empirical sulphur solubility model for hydrous rhyolitic compositions. in the intermediate composition range, that is between basalt and rhyolite, only a few studies have been carried out at one bar (e.g., katsura and nagashima, 1974) and even fewer at high pressure (carroll and rutherford, 1987; luhr, 1990; carroll and webster, 1994). of the work done at high pressure, only that of luhr (1990) provides sulphur solubility data obtained under known f s2 estimated from either fe-fes equilibria or equilibrium assemblages involving anhydrite. clearly, it is this mid-compositional range of hydrous mafic to intermediate magmas that still lacks experimental coverage, and this is unfortunate because most of arc magmas belong to this category. moretti et al. (2003) devised a general solubility model that should be applicable to a wide range of silicate melt compositions, including those hydrous. however, although based on a rigorous, as well as promising, thermodynamic description of melt-fluid equilibria, the current version of the model is quite complex to implement and is presently restricted to conditions of melt undersaturation with respect to a fe-s-o sulphide melt. in the present paper, we follow a more pragmatical approach and derive an empirical solubility model by considering the existing experimental database on geologically relevant silicate liquids. we have deliberately ignored the wealth of data existing in the metallurgical literature, since 1) it is exclusively based on dry compositions, 2) it concerns silicate melt compositions far outside the compositional spectrum displayed by terrestrial magmas, and 3) it covers temperatures greater than those typical of earth’s magmatism. our first target is to build a model that allows us to compute the f s2 of basaltic to intermediate hydrous melts, using as input parameters p, t, f o2 and melt composition. although we believe that the model described below retrieves the correct order of magnitude in terms of f s2, we envision it as an intermediate step that should help define future experimental strategies to obtain high pressure solubility data onto which more rigorous thermodynamic approaches, such as that of moretti et al. (2003), will be calibrated. the second target of this report is the application of this model to italian volcanoes for which there exists a combined data set of pre-eruption h2o, co2 and s concentrations, in addition to other intensive parameters (t and f o2). we considered stromboli, vesuvius, vulcano and etna volcanoes, for which there are melt inclusion constraints on preto syn-eruptive volatile abundances. we calculated the equilibrium fluid composition corresponding to the depth at which the melt inclusion were saturated in fluid. the rationale here is clear: to constrain fluid compositions from depth to near surface conditions and compare it with the observed gas composition measured at the exit (see scaillet and pichavant, 2003). the combination of the two sets of data (calculation and observation) should allow a better use of volcanic gases as monitoring tools during on-going volcanic crisis. 2. an empirical model for hydrous basaltic melts we attempted to devise a general empirical model in which the different effects of ƒo2, ƒs2 and t on the melt sulphur content, as well as the melt composition, are explicitly taken into account. several equations were tested and the following was found to give satisfactory results (2.1) where s is the total sulphur concentration in ppm, p the pressure in bar, t the temperature in °c, ∆nno and ∆ffs are the referenced f o2 and f s2 as explained below, wi represents the weight % of oxide i, and a, b, c, d, e, f and gi are fitted parameters listed in table i. the summa+nnod+ ∆nno+ ∆bt c+aplog e f s nno ffs ffs g wi i 3 2= + + +∆ ∆ ∆ σ 674 bruno scaillet and michel pichavant tion is carried over all major oxides, including feo, fe2o3, ohand h2o. the fe2+ /fe3+ ratios of experimental glasses were calculated using the empirical model of kilinc et al. (1984), whereas the water contribution was split into hydroxyl and molecular water using the method of zhang (1999). the third order polynomial function is necessary to reproduce the dissymetrical inverted bell-shaped pattern (e.g., katsura and nagashima, 1974), while the crossed f o2f s2 term is needed to take into account the effect of varying f s2 on the relationship between f o2 and s in melt (see clemente et al. 2004). using mole fractions instead of weight% oxides, or any other intensive parameters (i.e. f h2 in lieu of f o2), does not improve the quality of the fit. similarly, to account for the compositional dependence various norm projections were tested including the cipw norm, but none was found to decrease the residuals. as for f o2, which is referenced to the nno solid buffer, we have referenced f s2 to the irontroilite solid buffer (fe-fes or ffs) such that (2.2)( ) ( )log logffs f melt f ffs2 2= +∆ s s in which the f s2 of the fe-fes buffer is given by (froese and gunter, 1976) (2.3) where p is in bar and t(k) is the absolute temperature. we derived a set of fitted a ... gi parameters for eq. (2.1) by linear regression considering the data bases of luhr (1990), o’neill and mavrogenes (2002), katsura and nagashima (1974) on rhyodacite melts, in addition to those of clemente et al. (2004). as stated above, although the existing experimental database is considerably larger, we restricted the regression procedure to the above studies for the following reasons: first, all works were specifically aimed at exploring the behaviour of sulphur in geological melts and are thus of direct relevance to magmatic contexts; second, they were carried out within p-tf o2f s2f h2o conditions that closely approach those under which natural magmas evolve; third, use of the complete database is not warranted since much of it concerns slags or synthetic silicate melts of metallurgical, rather than geological, interest. because our model is empirical, including this database will have the undesirable but unavoidable consequence that one atmosphere data overwhelmingly dominate over those obtained at high pressure and thus that the fitted parameters be biased toward anhydrous compositions. preliminary fitting procedures used the database of haughton et al. (1974). however, the extensive work of o’neill and mavrogenes (2002) on dry mafic melts at one bar has shown that a substantial part of the experimental database gathered by haughton et al. (1974) does not obey expected thermodynamic relationships, and for this reason we have not included this work in our database. the selected database consists therefore of 64 hydrous rhyolitic compositions (clemente et al. (2004) plus three charges of scaillet et al. (1998) for which f s2 has been measured using the same technique than in clemente et al. (2004)), 72 hydrous rhyolite-andesite compositions (luhr, 1990), 333 dry mafic to ultramafic +)1= -. (p0 2655-( ) . . log f tffs 2 1 987 2 302581 2s . t35 0 12 56+19 ^ h6 a # table i. regression coefficients for the empirical model of sulphur solubility. p 7.28e-06 nno3 0.00488128 nno2 0.0818873 nnoffs −0.0224068 t 0.00084107 ffs 0.22801636 sio2 −0.012467 al2o3 −0.0015766 fe2o3 0.37362348 feo 0.0674383 mgo 0.01121929 cao 0.02000831 na2o 0.05644745 k2o −0.0248037 tio2 0.00672403 h2o 0.06868295 oh 0.05778453 675 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes compositions (o’neill and mavrogenes, 2002), and 5 dry rhyodacite compositions of katsura and nagashima (1974). there is therefore a 30/70 repartition between hydrous and dry compositions. the experiments of luhr (1990) and clemente et al. (2004) are the only ones in which both f o2 and f s2 are known for hydrous compositions, and cover an f o2 range between nno−1 up to nno+3, a total pressure of 100400 mpa, a temperature 800-1000°c, for melt water contents up to 8 wt%, the melt compositions spanning a sio2 range of 57-80 wt% on an anhydrous basis. the experiments of katsura and nagashima (1974) were included in the regression in an effort to constrain the sulphur solubility in silicic compositions under dry, i.e. low pressure, conditions. the database of o’neill and mavrogenes was chosen among others (e.g., buchanan and nolan, 1979; apart from that of haughton et al., 1974, for the reason given above) because of its systematic variation of melt composition at constant fo2 and fs2, which allows a better evaluation of the compositional dependence of sulphur solubility (i.e. the gi terms in eq. (2.1)). the fitted parameters are listed in table i. figure 1a-c shows that the model reproduces observed sulphur concentrations of the whole data set over more than 3 orders of magnitude (10-11900 ppm), with no apparent systematic divergence from the 1/1 correlation line within any of the three main data subsets, except in the low concentration range. considering that the database covers a wide compositional range (sio2 ranging from 35 to 80 wt%), as well as widely differing p-t-fluid fugacities conditions, the agreement between observed and calculated sulphur concentrations is considered satisfactory. in terms of f s2, the model reproduces measured f s2 within an average of 0.65 log unit, over more than 15 log units when normalised to the ffs solid buffer. inspection of fig. 1a-c shows nevertheless that hydrous melts are not as well reproduced by the model, which is a reflection of the difficulty in accurately controlling f s2 in high pressure experiments, since there is no buffering technique which allows to measure f s2 with a precision similar to that of f o2 or f h2. given that there exist solubility models for rhyolite compositions (clemente et al., 2003) and for dry mafic melts (e.g., wallace and carmichael, 1992; o’neill and mavrogenes, 2002), this empirical model is intended primarily to be used for melt compositions currently lacking experimental constraints, that is, hydrous mafic melts. equation (2.1) is merely a convenient mathematical way to describe the interdependence between the various parameters that control the sulphur solubility in silicate melts. as such the fig. 1a-c. comparison between measured and calculated melt sulphur concentrations for the three main experimental data sub set used to calibrate the empirical model (eq. (2.1), see text). a) rhyolitic compositions (clemente et al., 2004; scaillet et al., 1998; katsura and nagashima, 1978); b) intermediate compositions (lurh, 1990); c) basaltic composition (o’neill and mavrogenes, 2002). a b c 676 bruno scaillet and michel pichavant fitted parameters have clearly no thermodynamical meaning. yet, it is useful to consider at this stage the individual effect of some of the parameters on the calculated f s2 (or sulphur solubility) to appreciate their relative importance on sulphur behaviour in hydrous magmas. all parameters with a positive value increase the sulphur solubility, when everything else is kept constant. for instance at constant f o2, f s2, p and melt composition, a temperature increase produces an increase in melt sulphur content. the same is true for p which suggests that, in hydrous systems, an increase in pressure increases the sulphur solubility, a trend opposite to that found for dry mafic melts saturated in an immiscible sulphide liquid (see mavrogenes and o’neill, 1999). of the melt components, only sio2, al2o3 and k2o have negative signs. although this is as expected for sio2 and al2o3 because both components increase the degree of melt polymerisation, and thus they decrease its capacity to exchange sulphur ions with free oxygens, the negative sign found for k2o is more surprising since this element is often positively correlated with melt sulphur content (see métrich and clocchiatti, 1996). whether this effect is real or is merely a reflection of a compositional bias in the database cannot be solved in the present study. the large value found for fe2o3 may indicate preferential association of oxidized sulfur groups with fe3+ melt components but we stress that in the database, oxidized melts are mostly intermediate to silicic compositions with a low bulk feo and thus a relatively low fe2o3 content. experiments performed on iron-rich basalts have been mostly performed at low f o2, and the behaviour of sulphur in dry oxidized basalt still demands experimental investigations. a last aspect of interest concerns the role of water. both molecular h2o and hydroxyl group have strong positive values which suggests that addition of water to a silicate melt held at constant p-tf o2f s2 conditions significantly increases its melt sulphur content. we note, however, that because high pressure experiments are also those that are hydrous in the database, correctly discriminating the effects of pressure and water content on sulphur solubility is not yet possible. overall the empirical model can be considered as calibrated in the pressure range 1-4000 bar, temperature range 800-1400°c, melt water content range 0-10 wt%, f o2 range nno−2 to nno+3, and melt sulphur contents 50-10 000 ppm. melt compositions not yet considered and to which the model should be applied with caution are evolved alkali-rich magmas, such as phonolites and peralkaline rhyolites. 3. calculation of fluid phase composition to calculate the fluid phase composition we follow the same approach as scaillet and pichavant (2003) who determined the fluid compositions of a number of intermediate to silicic arc magmas. they also attempted to constrain the fluid phase of some hydrous arc basalts, using a simplified version of the wallace and carmichael (1992) model. the calculations were performed considering that the fluid composition can be described in the c-o-h-s system. we thus ignore the contribution of halogens, in particular f and cl, although we recognise that those species may play an important role in the evolution of magmatic fluids (see carroll and webster, 1994), especially in alkali-rich magmas (e.g., métrich, 1990; webster and de vivo, 2002). the species considered are h2o, h2, co2, co, ch4, h2s, so2, s2 and o2. to calculate the fluid compositions we use as input parameters, f o2, f s2 and f h2o. the thermodynamic model of water solubility for basalt of dixon et al. (1995) is used to calculate f h2o from the measured melt h2o content. the f o2 is constrained from petrological studies (i.e., spinel-olivinemelt equilibria; ballhaus et al., 1991) or sulphur speciation (e.g., métrich and clocchiatti, 1996). the f s2 is calculated from eq. (2.1). at any fixed p and t, fixing f o2, f s2 and f h2o allows us to calculate the fugacities of all remaining species. for pressures below 5 kbar, we use the modified redlich kwong equation of state (see holloway, 1977; flowers, 1979) with mixing rules as given by ferry and baumgartner (1987). at higher pressure, we use the corresponding state equation of shi and saxena (1992) and saxena and fei (1987) to calculate the fugacities of pure species. the fugacities of the species in the fluid mixture were derived us677 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes ing the lewis and randall rule: the fugacity coefficient of species i in the mixture equals that of pure species i at the same p and t. because the fluid pressure is unknown, the calculation must be done iteratively. the following procedure was adopted. we first fix p, t, and calculate the corresponding f h2o and f s2 from melt inclusion data. this results in a f co2 which is used to calculate the equilibrium co2 content of the melt, using the solubility model of dixon et al. (1995). this co2 content is then compared to that measured in the melt inclusion and if the difference between observed and calculated values exceeds 10 ppm, the calculation is performed again at a different pressure until the test is fullfilled. because p variations affect both f s2 and f h2o for a given set of tf o2-melt composition values, tests are also performed on the difference between observed and calculated values in melt s and h2o contents. the pressures of fluid saturation listed below reproduce observed h2o, co2 and s melt contents to ± 0.01 wt%, 10 ppm, 1 ppm, respectively. for alkali-rich basalts, we corrected for the alkali effect using the following method which is based on the approach of dixon (1997) to evaluate the increase in co2 solubility resulting from an increase in melt alkalinity. for a given calculated f co2, we first determine the melt co2 content using the solubility model of dixon et al. (1995) which is calibrated on morb composition. this amount of co2 is then adjusted using the following empirical fit: (3.1) where co2-morb is the co2 content of a morb melt at p and t (in ppm) calculated using the model of dixon et al. (1995), and π is a compositional parameter devised by dixon (1997) to evaluate the effect of metal cations on co2 solubility, and is equal to (3.2) where the si, al … are cationic fractions, and +. k0 8+. (ca20 17+.6 5-= . . ) si al mg fe0 4 0 4 2 4 3 2 2 + + + π + + + + + + ] g ( ) ( . ) .co ppm co co 0 472 0 8092morb morb 2 2 2 ) ) = + π π fe2+ is computed from total iron expressed as feotot (only fe2+ in dixon, 1997). equation (3.1) was derived by linear regression of solubility data (see holloway and blank, 1994; dixon, 1997) obtained at 1 kbar and 1200°c on morb tholeiite, kilauea tholeiite, basanite and leucitite melts which encompass a π range of 0.47 (morb) to 2.35 (leucitite). equation (3.1) back calculates experimental solubility data with an average standard deviation of 80 ppm at 1 kbar. inspection of experimental data shows that in fact the co2 solubility of mafic melts is constant within analytical uncertainties for π in the range 0.470.79, so that we have applied eq. (3.1) only to melt compositions having a π value higher than 0.8. although the correction factor is likely to vary with pressure and temperature, we have applied eq. (3.1) at all p and t since there are not enough data to properly evaluate this effect over a large range of p-t conditions. we note, in addition, that the compositions used to derive eq. (3.1) are significantly less alkali-rich than some of the basalts erupted at italian volcanoes: in particular, the tephrites of vesuvius have k2o contents almost twice higher than that of the most alkali-rich composition used to derive eq. (3.1). the strong correlation observed in experimental mafic melts between fig. 2. correlation between dissolved co2 and the k2o content of experimental basaltic melts used to determine the solubility of co2 at 1000 bar and 1200°c. see dixon (1997) for the source of data. vesuvius tephrites, between 4 and 5 wt% k2o, could have co2 solubilities close, or in excess, to 4000 ppm, or nearly 10 times more than mor basalts (ca. 500 ppm under these conditions). 678 bruno scaillet and michel pichavant k2o and co2 content (fig. 2) suggests that k2o may have a decisive role on co2 solubility in mafic melts perhaps far outweighing that of other metal cations. simple extrapolation of the trend shown on fig. 2 would imply that co2 solubility in a tephrite with 5 wt% k2o at 1 kb and 1200°c could be nearly ten times that of morb or 3000-4000 ppm, while application of the method of dixon (1997) with π = 1 predicts a much smaller increase in co2 solubility (from ca. 500 ppm for a morb to 724 ppm for a tephrite). in support of this enhanced co2 solubility are the phase equilibria of trigila and de benedetti (1993), which have shown that addition of co2 to dry tephrite at 2 kbar depresses the liquidus temperatures of pyroxene, leucite and plagioclase by 50-75°c with estimated co2 concentrations in melt in excess of 1 wt% (see also freda et al.,1997, for similar findings on phonotephritic magmas of the alban hills). a freezing point depression effect in a given silicate-co2 system implies a significant solubility of co2 in the melt. although such a potassium effect on co2 solubility cannot be rigorously modeled at present, the above observations suggest that the pressures of fluid saturation calculated here could be largely overestimated for mafic melt compositions that contain significantly more k2o than the rock melts so far investigated in experimental studies on co2 solubility. in summary we believe that mafic melts moderately enriched in k2o relative to morb can have their co2 solubility, and thus their pressure of fluid saturation, correctly estimated (to within 0.5 kbar) by the method explained above. this is the case for most of the mafic magmas erupted at etna, stromboli and vulcano. in contrast, there exists the possibility that the pressures of fluid saturation calculated in this work for k2o-rich mafic magmas such as vesuvius tephrites, are largely overestimated, perhaps by as much as several kilobars. this indicates that there is an urgent need of experimental constraints on co2 solubility in alkali-rich basalts. 4. source of data in this section we briefly review the present status of knowledge on pre-eruptive h2o-co2s melt concentrations as well as of p-t-f o2 of the different volcanoes considered. we are primarily interested in works where those three volatiles have been determined (e.g., marianelli et al., 1999; métrich et al., 2001). however, we also considered melt inclusion data sets lacking co2 determination but for which both h2o and s are known (e.g., marianelli et al., 1995; métrich et al., 1993; cioni et al., 1995). although the lack of co2 constraints prevents the calculation of the pressure of fluid saturation and fluid composition, h2o and s abundances can still be used to infer f s2, provided there are tf o2 constraints. in such a case, the pressure for calculation in eq. (2.1) is that corresponding to the h2o solubility. while this pressure is clearly a minimum, in terms of f s2 calculation the error introduced by this uncertainty is minor, compared to that due to f o2: a pressure increase of 4 kbar decreases the calculated log f s2 by 0.5 log unit. in the following, we focus primarily on mafic primitive melts, since it is the injection of mafic fresh magmas that is more likely to re-awaken or trigger a new explosive event, as illustrated by the april 5th, 2003 event at stromboli. 4.1. stromboli we restricted our calculations to melt inclusions analysed by métrich et al. (2001), who studied the products of paroxysmic eruptions such as that of the august 23, 1998 explosion, during which crystal-rich scoria and highly vesicular yellow pumices were erupted. the first type of rock is interpreted to represent quenched fragments of a crystal-rich degassed magma that was resident in a very shallow magma chamber. this reservoir was intercepted by a rising batch of crystal-poor and volatilerich magma, now represented by the yellow pumice. melt inclusion constraints on volatile abundances were obtained on both type of magmas, giving pressure for fluid saturation up to 4 kbar for the volatile-rich yellow pumice (métrich et al., 2001; bertagnini et al., 2003). temperature constraints come from melt inclusion homogeneisation during heating stage experiments which indicate that the yellow 679 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes pumice was at 1125-1140°c, whereas the crystal-rich scoria was at a slightly lower temperatures of 1100-1125°c. the pre-eruptive temperature for the yellow pumice has been confirmed by recent phase equilibrium work (di carlo et al., 2004). here we use temperatures of 1140 and 1100°c for the pumice and scoria rocks, respectively. the redox state of stromboli magmas was inferred through the determination of sulphur speciation (métrich and clocchiatti, 1996; métrich et al., 2002) which yields an average f o2 of nno+0.95. for the calculation of fluid phase corresponding to the scoria or degassed magmas, we assumed that the pre-eruptive melt contained 40-50 ppm co2, since this species could not be detected by ftir in scoria melt inclusions. 4.2. vesuvius for vesuvius there is a large body of data on melt inclusion chemistry (e.g., vagelli et al., 1992, 1993; cioni et al., 1995, 1998; marianelli et al., 1995, 1999; belkin et al., 1998; lima et al., 1999; signorelli and capaccioni, 1999; signorelli et al., 1999; cioni, 2000; raia et al., 2000; webster et al., 2001), but for stromboli only recently has the co2 in melt inclusions been determined using micro beam techniques (marianelli et al., 1999; signorelli et al., 1999; cioni, 2000). we did not consider works done on felsic phonolites, however, since eq. (2.1) is not yet calibrated for these compositions. to minimise errors arising from differences in sulphur and water determinations between different research groups, we used only the data sets of cioni et al. (1995), and marianelli et al. (1995, 1999) in which at least both s and h2o were determined using the same instrument and analytical procedure. most of the analysed inclusions correspond to primitive melts for which heating stages experiments give homogenisation temperatures of ca. 1100-1150°c (e.g., cioni et al., 1995; marianelli et al., 1995). for melt inclusions lacking such information, temperatures were determined using the cao in melt geothermometer of cioni et al. (1998). the redox state is known indirectly from sulphur speciation determination either on bulk rocks, lava or pumices (marini et al., 1998), or in melt inclusions (métrich et al., 2002), both yielding an f o2 of ca. nno+1.2. 4.3. vulcano melt inclusions of vulcano magmas have been analysed by clocchiatti et al. (1994a,b) and gioncada et al. (1998). the latter work showed that the co2 content of primitive melt inclusions is below the ftir detection limit, which yields a maximum co2 content of about 50 ppm. the storage conditions of the reservoir that fed the 1888-1890 eruption, during which rhyolite to trachytic magmas were ejected, were inferred by clocchiatti et al. (1994a) to be: a pressure depth of 80 mpa, melt water contents in the range 1-1.5 wt%, and temperatures between 1000-1100°c. this shallow reservoir is inferred to be connected to a deeper one where intermediate and mafic magmas reside. these are assumed to be equivalent to primitive melt inclusions in la sommata basalts, whose homogeneisation temperature is 1180 ± 20°c (gioncada et al., 1998). the depth of the felsic upper reservoir provides an upper bound for the mafic part, which is estimated to lie at 70-110 mpa (gioncada et al., 1998). measured melt water contents of the least differentiated magmas (la sommata melt inclusions) are in the range 2.13.8 wt%, that is they are higher than their potential felsic derivatives (clocchiatti et al., 1994a; gioncada et al., 1998), suggesting an open system behaviour with respect to volatiles (clocchiatti et al., 1994b). a similar observation can be made for sulphur which reaches maximum values of 2872 ppm in the most mafic melts analysed (clocchiatti et al., 1994b). the redox state of mafic magmas has been inferred from sulphur speciation to be at nno+ + 0.72 (métrich and clocchiatti, 1996; métrich et al., 2002). to calculate the fluid phase compositions we used the melt inclusions analyses reported by clocchiatti et al. (1994a) assuming that they all contained ca. 40-50 ppm dissolved co2. thus, the calculated fluid compositions correspond to the maximum co2 content that can be expected at vulcano given the available melt 680 bruno scaillet and michel pichavant inclusion constraints. in addition, we calculated the f s2 corresponding to melt inclusions of clocchiatti et al. (1994b), by assuming that mafic melts have a pre-eruptive water content of 1 wt%, whereas that of more felsic compositions (shoshonite to latite-rhyolites) has been set at 1 wt%. 4.4. etna unlike the three previous volcanoes, there are currently no published combined sets of pre-eruptive h2o-co2-s contents of etnean mafic melts. existing melt inclusion studies have analysed either s and cl (e.g., clocchiatti and métrich, 1984), h2o and s (métrich et al., 1993), co2 (métrich and mosbah, 1988), or s and cl (kamenetsky and clocchiatti, 1996) but none has determined the concentration of those volatiles collectively in single melt inclusions. a literature survey shows that only the alkali basalt erupted in 1892 (mt. maletto), which is among the most primitive basalts yet erupted on etna (armienti et al., 1988), has had its preeruptive volatile content determined in full, albeit not necessarily on the same melt inclusions (métrich and mosbah, 1988 for co2; and métrich et al., 1993, for h2o and s). there are thus surprisingly very few constraints on the volatile concentrations that could be used to determine fluid saturation pressures of etnaean magmas, either past or present. therefore, we adopted a different strategy relative to the three other volcanoes. we performed thermodynamic calculations at various pressures, assuming that a fluid phase is present, using available h2o and s melt concentrations coupled to petrological t and f o2 constraints. the pressure range over which calculations were performed is derived from geophysical and petrological constraints, as summarised in the next sections. the melt water contents of alkali basalts of etna have been determined to be in the range 12.3 wt% (trigila et al., 1990; métrich et al., 1993), the higher value being regarded as the pristine pre-eruptive melt water content, based on phase equilibrium constraints (métrich and rutherford, 1998). geochemical modelling has shown that these alkali basalts can be derived by 12-13 wt% of olivine fractionation from a picritic basalt (armienti et al., 1988), which therefore suggests that the water content of primary magmas at etna is close to 0.8-2 wt%, assuming closed system fractionation. as stated above, only melt inclusions of the 1892 eruption have had their co2 content determined, with an average value of 588 ppm (métrich and mosbah, 1988). this value is considered as a minimum because of the occurrence of carbonate crystals in the quenched melt inclusions (métrich and mosbah, 1988). additional evidence for magmatic co2 comes from its occurrence in the volcanic plume or in diffuse soil emanations (e.g., allard et al., 1991; bruno et al., 2001), and from the study of fluid inclusions associated to melt inclusions in lava phenocrysts or ultramafic nodules (clocchiatti and métrich, 1984; sobolev et al., 1991; frezzotti et al., 1991; clocchiatti et al., 1992). such fluid inclusions generally belong either to a low (h2o-co2 or co2) or a high (co2) density population. the high density inclusions yield maximum entrapment pressures of 6-7 kbar of a nearly pure co2 fluid (clocchiatti et al., 1992), whereas the low one yields pressures in the range 1-3 kbar (clocchiatti and métrich, 1984; sobolev et al., 1991; frezzotti et al., 1991). these observations suggest that any fluid phase coexisting with magma at depth must be co2-rich. for sulphur there is a considerable amount of analytical data (e.g., métrich and clocchiatti, 1989; métrich et al., 1993), the highest concentration recorded in melt inclusions being 3800 ppm (as quoted in clocchiatti et al., 1992), though most have sulphur contents in the range 2500-3500 ppm (clocchiatti and métrich, 1984; métrich and clocchiatti, 1989, 1996). generally the sulphur-rich melt inclusions are enclosed in the most magnesian olivines and they do not coexist with immiscible sulphide liquids (métrich and clocchiatti, 1989), while sulphur-poor inclusions hosted by fe-rich olivine are occasionally saturated in a cu-rich sulphide melt (métrich and clocchiatti, 1989). petrological, geochemical, and geophysical evidence (e.g., armienti et al., 1988; bonaccorso, 1996, 2001; tanguy et al., 1997; murru et al., 1999) all points to the probable existence of a 681 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes large deep reservoir beneath the volcano at a pressure depth range of 7-10 kbar, in which most of the primitive etnaean magmas stagnate and fractionate to hawaiites (or trachybasalts). such hawaiites constitute the bulk of the magmas outpoured over the last 30 0000 years. similarly, geophysical, geodetic, geochemical, melt/ fluid inclusions, and phase equilibrium constraints, suggest that there are at least two additional shallow reservoirs feeding historical as well as on-going eruptions that lie at a pressure depth of about 1 and 3-4 kbar (e.g., sobolev et al., 1991; bonnacorso, 1996; métrich and rutherford, 1998; murru et al., 1999; caracausi et al., 2003). temperatures of magma extrusion for the recent period have been estimated from a variety of approaches including direct measurement of lava flow (tanguy et al., 1997), melt inclusion homogeneisation (e.g., sobolev et al., 1991; clocchiatti et al., 1992) and thermodynamical and phase equilibrium constraints (trigila et al., 1990; métrich and rutherford, 1998) and all fall within a t range of 1070-1100°c. temperatures of primitive alkali magmas stored at 7-10 kbar are estimated to be around 1200°c (kamenetsky and clocchiatti, 1996). redox conditions of etnean basalts have been determined either through spinel-olivineorthopyroxene equilibria (e.g., kamenetsky and clocchiatti, 1996) or via the determination of sulphur speciation in glass inclusions (métrich and clocchiatti, 1996). the latter approach gives an f o2 corresponding to nno+0.35. the f o2 retrieved from spinel bearing assemblages in primitive melt inclusions from mt. maletto alkali basalt encompasses a range nno up to nno+1, but we note that the f o2 calculated for another similar alkali basalt (mt. spagnolo) falls between nno+1 to nno+2, suggesting possibly more oxidizing conditions than those adopted here. sato and moore (1973) made intrinsic f o2 (and f s2) measurements in gas streams from two hornito vents during the 1970 eruption and they found an f o2 of nno−0.45, which gerlach (1980) showed to be compatible with the restored gas compositions measured by huntingdon (1973) during the same event. in summary, because of the lack of confident pre-eruptive co2 determinations, thermodynamic calculations of fluid composition coexisting with etnaean mafic magmas were performed in the pressure range 400-10 000 bar, in keeping with the petrological and geophysical constraints summarised above. temperatures were either 1150°c (1 wt% h2o) or 1130°c (2.23 wt% h2o), based on melt inclusion heating stage experiments (métrich and clocchiatti, 1996). we considered melts water contents of 1 and 2.23 wt% (métrich et al., 1993) and melt sulphur contents of 1000-3400 ppm, with melt compositions as reported by métrich et al. (1993). we explored f o2 conditions from nno−0.45 to nno+0.35, to ascertain the effect of f o2 on both f s2 and fluid composition. 5. trends in f s2 and sulphide saturation in all subsequent diagrams we use the ratio cao/al2o3 as a differentiation index to illustrate variations in f s2. the sulphide saturation is computed from the following equilibrium: (5.1) using thermodynamic data for liquid feo and fes as given in o’neill and mavrogenes (2002). for all melt compositions we assume that 90% of iron is feo, and we use an activity coefficient for feo of 1.4 (o’neill and mavrogenes, 2002). given the relatively high redox state of the magmas, there may be less fe2+ than the assumed value and thus the activity of fes calculated (afes) here should be considered as maximum values. the calculated afes corresponds to pure fes, however. saturation in a sulphide liquid could arise if other components are present in solution, such as cu or ni. 5.1. stromboli in fig. 3, two trends in f s2 can be distinguished; mi of pumices define a flat trend with f s2 values in the range 0.01 to 1 bar, the most primitive melt having the highest f s2. the pumice trend joins that defined by the scoria mi, which is much more steep and spreads over nine orders of magnitude f s2 at a nearly constant cao/al2o3 ratio. this spread in f s2 re/ /s ofeo 1 2 fes 1 2m 2 m 2+ = + fig. 4. calculated activities of liquid fes plotted versus the cao/al2o3 ratio for the four volcanoes considered (see text for explanations). note that almost all magmas are below saturation conditions with an immiscible sulphide liquid. 682 bruno scaillet and michel pichavant flects mostly the variable concentrations of sulphur analysed in various melt inclusions, since melt compositions in scoria, including water content, varies little (métrich et al., 2001). the huge spread records the progressive degassing in sulphur of the magma that is stored in the shallow reservoir. the very steep slope indicates that degassing operates with little chemical modification of the environment. inclusions in the scoria displaying the highest f s2 may represent remnants of deep magma batches that intrude the upper reservoir or record incipient degassing of the deep magma batches during uprise. melt inclusions in pumices were trapped at depths exceeding 3 kbar (see below). the f s2 variations in this deeper reservoir appear to be much more restricted. the slight decrease observed corresponds to either selective loss of sulphur or, and more probably, to an increase in melt water content during fractionation, since water decrease f s2 at a given melt sulphur content (see eq. (2.1)). the calculated afes ranges from 0.1 down to 0.0001 (fig. 4), suggesting that mafic melts at stromboli are far from reaching sulphide saturation. removal of sulphur via sulphide disposal during fractionation is therefore probably not an efficient process at stromboli. 5.2. vesuvius the data on vesuvius are shown in fig. 5, being distinguished by the year of eruption. two main trends emerge. the first corresponds to the 1906-1944 eruptions, whereas the second corresponds to the pollena-pompei-avellino events. the 1906-1944 trend is somewhat analogous to the stromboli in that less differentiated mi have higher and broadly constant f s2 which, below a cao/al2o3 ratio of 0.7, sharply decrease down to a log f s2 of –7. in contrast, mi analysed in pollena-pompei-avellino define a broad trend of increasing f s2 as fractionation proceeds, although for the pompei mi a significant scatter is apparent, reaching values of f s2 in excess of 100 bar. it is interesting to note that the first trend corresponds to events which characterise open conduit conditions, with semi-persistent strombolian activity, as opposed to the other events which typify the establishment of a closed system following obstruction of the conduit during a long repose time preceding plinian to sub-plinian eruptions (e.g., civetta et al., 1991; civetta and santacroce, 1992; cioni et al., 1998). on this basis, the trend of decreasing f s2 can be interpreted as reflecting fig. 3. evolution of f s2 with cao/al2o3 ratio of stromboli magmas. because of massive clinopyroxene crystallisation in those magmas, fractionation is marked by a continuous decrease in the cao/al2o3 ratio. the f s2 is calculated from melt inclusion data (métrich et al., 2001) and eq. (2.1) (see text). pumice corresponds to magma emitted during paroxysmal events whereas scoria represents magmas erupted during strombolian type eruptions. 683 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes continuous loss of sulphur (and other volatiles) during magma uprise and emission to the surface (open conduit), in a way similar to that seen at stromboli, whereas that of increasing f s2 reflects a closed system in which sulphur has perhaps a dominant incompatible behaviour (its concentrations in the melt and fluid increase). as for stromboli, all but one calculated afes are significantly below unity (fig. 4) which again suggests that conditions for sulphide saturation are not reached in vesuvius magmas, although they tend to approach it under closed conduit conditions. this may be one reason for the observed increase in f s2 during differentiation. 5.3. vulcano the vulcano data define two distinct groups (fig. 6); one corresponding to the most mafic mi (basaltic compositions) which cluster at f s2 around 0.1 bar, and the second that corresponds to more evolved mi (shoshonitic to rhyolite compositions) that span values of f s2 between several bars down to 10−8 bar, defining a trend negatively correlated with the cao/al2o3 ratio (that is f s2 increases with differentiation). this second trend is similar to that observed for the pollena-pompei-avellino magmas discussed above. however, there is a significant gap between mafic and felsic magmas and it is unclear from the data whether this gap is real or if the f s2 of the two groups overlap at a cao/al2o3 ratio of around 1. if this were true, then mafic melts must evolve towards lower f s2 during fractionation. this hypothetical decrease could represent decompression of volatile-rich magmas during open conduit conditions, such as at stromboli. thereafter, once the partially degassed mafic batches have been emplaced at shallow level they further fractionate under closed system conditions during which f s2 increases. alternatively, the two groups may represent different levels of magma fractionation, one deep (both high and constant cao/al2o3 ratio and f s2) and one shallow (increasing f s2 with fractionation). the afes calculated are in general much lower than unity (afes < 0.01), but they do increase with fractionation such that the most felsic magmas may approach fes saturation (fig. 4). fig. 5. evolution of f s2 with cao/al2o3 ratio of vesuvius magmas. the f s2 is calculated from melt inclusion data (cioni et al., 1995; marianelli et al., 1995, 1999) and eq. (2.1) (see text). different symbols correspond to the main eruptive events. the pollena, pompei and avellino eruptions correspond to closed conduit conditions, while the 1944 and 1906 events are characteristic of open conduit conditions. fig. 6. evolution of f s2 with cao/al2o3 ratio of vulcano magmas. the f s2 is calculated from melt inclusion data (clocchiatti et al., 1994a,b; gioncada et al., 1998) and eq. (2.1) (see text). note that the most primitive melts (high cao/al2o3 ratio) cluster at around f s2 of 0.1 bar, whereas the rest of melts defines a negative trend of increasing f s2 with differentiation. 684 bruno scaillet and michel pichavant 5.4. etna the etna data (fig. 7) do not define any obvious trend, but this is largely due to the low number of mi analysed so far. the most primitive mi again have f s2 in the range 0.1 to 1 bar, as observed for the other volcanoes. the calculated afes values are the highest of the four volcanoes, ranging from 0.1 up to 0.95, or close to fes saturation (fig. 4). 6. composition of fluids the compositions of fluids calculated for mi with known h2o, s and co2 contents are listed in table ii together with input f h2, f s2 and f h2o and the corresponding pressure of fluid saturation. in all cases, the computed mole fraction of ch4 is several orders of magnitude lower than that of either co2 or co, and this species is not considered further in this work. 6.1. stromboli the pressures of fluid saturation of the mi in pumices range from 2100 bar to near 3500 bar (table ii, fig. 8), i.e. they are comparable to those calculated by métrich et al. (2001) and bertagnini et al. (2003), despite the fact that sulphur bearing species were not taken into account in these works. this is due to the fact that, although melt sulphur contents are relatively high, the corresponding f s2, and thus the molar abundances of s-bearing species, remain low. in the calculated examples, the mole fractions of so2+h2s never exceeds 0.1 (fig. 8). the mole fraction of h2o remains relatively constant, at about 0.3, while that of co2 slightly increases with pressure, exceeding 0.6 at near 3500 bar. the fluid coexisting with mi in scoria, calculated with the assumption of 50 ppm co2 dissolved, corresponds to entrapement pressures of 100 bar, which is equivalent to a depth beneath the crater of ca. 300 m, ie similar to previous estimates (e.g., harris and stevenson, 1997). given that the central part of the volcano is permanently flushed with co2rich gases, as remote sensing of the volcanic plume indicates (allard et al., 1994), it is unlikely that magmas stored in the shallow reservoir are co2-free. therefore the 50 ppm threshold appears to a be a limiting condition which corresponds to the maximum possible pressure depth at which the upper reservoir that feeds the typical strombolian activity is lying. deeper conditions will inevitably produce mi with detectable co2 contents with the ftir method. under these conditions it appears that the fluid phase is extremely rich in co2, with xco2 in excess of 0.9 (fig. 8). in contrast, the water content of the shallow magmatic fluid is very low being even lower than so2 in some instances. figure 9 shows the evolution of both h2o/ /so2tot and co2/so2tot (so2tot = so2 + h2s) mole ratios with pressure. mi of pumices define a broad positive trend in both ratios, whose extrapolation to near surface conditions corresponds to gases with h2o/so2tot and co2/so2tot ratios between 0.1 and 1. such ratios are at the lower end of the range displayed by fluids coexisting with scoria mi. in these, the h2o/so2tot can exceed 10 while the co2/so2tot can go over 1000. clearly such a large spread is chiefly due to the variable contents of sulphur dissolved in mi in scoria. the fact that the pumice mi trend intersects the lower end of the scoria trend suggests that this ratio range of 0.1-1 represents the fluid compositions of magmas just injected in the upper reservoir, which have not yet undergone extensive volatile degassing associated to normal strombolian activity. fig. 7. evolution of f s2 with cao/al2o3 ratio of etna magmas. the f s2 is calculated from melt inclusion data (métrich et al., 1993) and eq. (2.1) (see text). 685 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes t ab le ii . c om po si ti on o f fl ui ds o f s tr om bo li , v es uv iu s, v ul ca no a nd e tn a vo lc an oe s, b as ed o n m el t in cl us io n co ns tr ai nt s. p t ∆ n n o ∆ f f s fh 2 fs 2 fh 2o x h 2 x h 2o x c o 2 x c o x s o 2 x h 2s x s 2 h 2o m s m c o 2m s fl ui d ba r °c ba r ba r ba r w t% pp m pp m w t% s tr o m b o li s t 82 pol n5 2a 29 30 11 50 0. 95 4. 59 7 2. 10 0 0. 06 5 90 0 0. 00 04 4 0. 34 31 1 0. 64 26 4 0. 00 30 7 0. 01 00 0 0. 00 06 9 0. 00 00 1 2. 7 17 30 10 87 0. 97 8 s t 82 pol n5 2a 29 00 11 50 0. 95 4. 56 9 2. 30 0 0. 06 2 96 0 0. 00 04 8 0. 36 71 6 0. 61 93 3 0. 00 29 9 0. 00 92 9 0. 00 07 4 0. 00 00 1 2. 8 17 30 10 31 0. 93 1 s t 82 pol n9 b 34 30 11 50 0. 95 4. 12 3 2. 40 0 0. 02 0 10 20 0. 00 04 0 0. 32 82 0 0. 66 37 9 0. 00 32 4 0. 00 40 2 0. 00 03 5 0. 00 00 0 2. 8 20 50 16 89 0. 39 49 s t 82 pol n5 0 25 70 11 50 0. 95 3. 98 9 2. 00 0 0. 01 7 86 5 0. 00 04 7 0. 37 62 3 0. 61 28 7 0. 00 27 1 0. 00 73 4 0. 00 03 8 0. 00 00 1 2. 7 12 30 89 4 0. 72 1 s t 79 pol n3 0 21 90 11 50 0. 95 5. 79 3 1. 60 0 1. 18 0 66 0 0. 00 05 0 0. 33 50 7 0. 59 01 0 0. 00 29 4 0. 06 76 7 0. 00 33 6 0. 00 03 6 2. 3 21 10 11 07 6. 28 3 s t 82 so ln 2 10 0 11 00 0. 95 3. 46 5 0. 00 5 0. 00 3 2 0. 00 00 5 0. 02 05 3 0. 89 20 0 0. 00 46 1 0. 08 27 5 0. 00 00 2 0. 00 00 3 0. 15 74 0 50 5. 88 5 s t 82 so ln 3 10 0 11 00 0. 95 − 2. 39 2 0. 00 5 0. 00 0 2 0. 00 00 5 0. 02 06 0 0. 97 38 5 0. 00 50 4 0. 00 04 6 0. 00 00 0 0. 00 00 0 0. 16 50 50 0. 03 38 s t 82 so ln 4 10 0 11 00 0. 95 2. 02 8 0. 00 5 0. 00 3 2 0. 00 00 5 0. 02 05 8 0. 94 93 5 0. 00 49 1 0. 02 51 0 0. 00 00 1 0. 00 00 0 0. 16 89 0 50 1. 83 v es u vi u s v s 97 -1 09 -o l 40 00 11 50 1. 2 5. 14 0 1. 70 0 0. 19 8 11 00 0. 00 02 3 0. 29 53 4 0. 67 99 7 0. 00 22 3 0. 02 15 7 0. 00 06 3 0. 00 00 3 2. 9 18 00 25 00 1. 93 1 v s 97 -1 09 -o l 33 00 11 50 1. 2 4. 69 8 1. 35 0 0. 07 7 81 0 0. 00 02 4 0. 27 59 4 0. 70 32 7 0. 00 25 1 0. 01 76 0 0. 00 04 2 0. 00 00 1 2. 5 16 00 21 00 1. 55 5 v s 97 -1 09 -o l 32 20 11 50 1. 2 4. 77 6 1. 20 0 0. 09 4 70 0 0. 00 02 3 0. 24 75 1 0. 72 94 5 0. 00 27 3 0. 01 96 3 0. 00 04 3 0. 00 00 0 2. 3 17 00 19 00 1. 69 69 v s 97 -1 09 -o l 52 50 11 50 1. 2 4. 32 7 2. 60 0 0. 02 3 16 50 0. 00 02 2 0. 31 14 0 0. 68 23 8 0. 00 22 8 0. 00 35 1 0. 00 02 0 0. 00 00 0 3. 4 23 00 31 00 0. 33 11 v s 97 -1 09 -o l 64 00 11 50 1. 2 3. 56 6 2. 60 0 0. 00 3 16 80 0. 00 01 6 0. 24 06 3 0. 75 55 7 0. 00 26 8 0. 00 07 9 0. 00 00 5 0. 00 00 0 3. 2 10 00 35 00 0. 07 15 v s 97 -1 09 -d i 35 20 11 50 1. 2 4. 06 9 1. 60 0 0. 01 7 93 0 0. 00 02 6 0. 29 38 0 0. 69 63 9 0. 00 25 4 0. 00 67 8 0. 00 02 1 0. 00 00 0 2. 66 12 00 19 00 0. 61 5 v s 97 -1 09 -d i 53 60 11 50 1. 2 4. 83 0 1. 47 0 0. 07 1 92 0 0. 00 01 2 0. 16 59 1 0. 82 50 9 0. 00 31 7 0. 00 55 2 0. 00 01 9 0. 00 00 1 2. 4 16 00 43 00 0. 46 08 v s 19 06 -o l 31 00 11 50 1. 2 4. 97 4 1. 90 0 0. 15 1 11 00 0. 00 03 6 0. 38 54 5 0. 58 67 5 0. 00 20 1 0. 02 45 4 0. 00 08 7 0. 00 00 3 3 17 00 18 00 2. 36 77 v s 19 06 -o l 33 00 11 50 1. 2 4. 44 8 2. 15 0 0. 04 4 12 20 0. 00 03 7 0. 39 88 3 0. 58 72 7 0. 00 20 3 0. 01 10 2 0. 00 04 8 0. 00 00 1 3. 2 18 00 17 00 1. 08 98 g l 35l c 90 0 11 50 1. 2 1. 16 4 0. 27 0 0. 00 0 15 0 0. 00 02 6 0. 18 98 8 0. 80 40 4 0. 00 32 5 0. 00 25 7 0. 00 00 1 0. 00 00 0 1. 15 30 0 38 3 0. 21 2 v u lc a n o iv o l3 1 88 5 11 80 0. 72 3. 96 1 2. 65 0 0. 03 8 81 5 0. 00 24 3 0. 93 01 6 0. 03 53 0 0. 00 02 1 0. 02 95 1 0. 00 23 4 0. 00 00 4 2. 88 23 00 48 5. 03 74 iv ol 4 72 5 11 80 0. 72 4. 60 5 2. 15 0 0. 17 0 63 0 0. 00 25 1 0. 87 75 2 0. 04 11 9 0. 00 02 8 0. 07 32 5 0. 00 50 5 0. 00 02 0 2. 51 19 80 39 11 .1 9 6f m -m r os so 18 5 11 60 0. 72 0. 50 9 0. 29 0 0. 00 0 85 0. 00 15 0 0. 46 78 5 0. 52 44 3 0. 00 40 5 0. 00 21 5 0. 00 00 2 0. 00 00 2 0. 93 20 0 51 0. 21 88 t uf fi b ru ni i nf 21 0 11 00 0. 72 0. 96 1 0. 42 0 0. 00 0 13 0 0. 00 18 0 0. 60 22 0 0. 39 22 7 0. 00 25 0 0. 00 11 9 0. 00 00 4 0. 00 00 0 1. 16 0 44 0 46 0. 13 96 t uf fi b ru ni s up 40 0 11 00 0. 72 5. 67 3 0. 73 0 0. 50 0 22 8 0. 00 16 5 0. 58 33 9 0. 25 89 3 0. 00 15 8 0. 14 45 7 0. 00 87 4 0. 00 11 4 1. 50 0 73 0 53 15 .7 74 v ul ca ne ll o 21 0 11 00 0. 72 2. 71 5 0. 42 0 0. 00 1 13 0 0. 00 17 2 0. 57 70 0 0. 40 97 8 0. 00 26 2 0. 00 85 9 0. 00 03 0 0. 00 00 0 1. 15 0 65 0 50 0. 98 0 p co tt e rh yo li te 22 0 10 00 0. 72 5. 00 9 0. 33 0 0. 01 4 10 9 0. 00 14 0 0. 51 13 0 0. 46 30 0 0. 00 22 2 0. 01 97 1 0. 00 23 1 0. 00 00 6 1. 00 0 10 0 55 2. 28 6 e tn a 1 w t% h 2o i n m el t e 79 04 3 40 0 11 50 − 0. 45 6. 65 4 1. 45 0 12 .0 30 11 0 0. 00 33 4 0. 28 63 4 0. 53 66 8 0. 01 57 0 0. 06 51 3 0. 06 47 6 0. 02 80 5 1. 06 34 00 11 4 15 .9 1 e 79 04 3 10 00 11 50 − 0. 45 6. 67 7 1. 60 0 11 .3 30 12 5 0. 00 13 7 0. 14 25 3 0. 77 27 3 0. 02 24 9 0. 02 40 5 0. 02 70 2 0. 00 98 1 1. 06 34 00 42 3 5. 61 0 e 79 04 3 50 0 11 00 0. 35 6. 73 3 0. 55 0 5. 59 0 11 0 0. 00 09 9 0. 23 35 2 0. 57 25 6 0. 00 57 3 0. 15 90 7 0. 01 78 8 0. 01 02 6 1. 06 34 00 15 3 15 .4 1 e 79 04 3 40 0 11 50 0. 35 6. 55 2 0. 56 0 9. 51 5 11 0 0. 00 12 8 0. 27 83 4 0. 27 91 9 0. 00 31 0 0. 39 35 2 0. 02 24 4 0. 02 21 6 1. 06 34 00 58 32 .9 1 e 79 04 3 50 0 11 50 0. 35 6. 54 9 0. 55 0 9. 26 7 11 0 0. 00 09 9 0. 22 67 9 0. 41 71 0 0. 00 45 7 0. 31 62 0 0. 01 73 1 0. 01 70 0 1. 06 34 00 11 0 26 .4 4 e 79 04 3 10 00 11 50 0. 35 6. 40 7 0. 63 0 6. 09 1 12 5 0. 00 05 4 0. 14 05 5 0. 72 34 2 0. 00 82 1 0. 11 42 1 0. 00 78 2 0. 00 05 3 1. 06 34 00 39 4 9. 97 9 e 79 04 3 30 00 11 50 0. 35 6. 34 1 0. 68 0 3. 59 3 14 5 0. 00 01 4 0. 05 64 5 0. 91 14 6 0. 01 04 8 0. 01 90 3 0. 00 16 9 0. 00 07 4 1. 06 34 00 17 74 1. 66 21 e 79 04 3 70 00 11 50 0. 35 6. 20 9 0. 88 0 1. 25 0 22 0 0. 00 00 2 0. 02 13 1 0. 96 59 8 0. 00 98 1 0. 00 27 1 0. 00 01 1 0. 00 00 5 1. 06 34 00 54 04 0. 21 6 e 79 04 3 10 00 0 11 50 0. 35 6. 18 3 1. 08 0 0. 45 4 30 0 0. 00 00 2 0. 01 55 7 0. 97 32 0 0. 01 05 0 0. 00 06 6 0. 00 00 3 0. 00 00 1 1. 06 34 00 84 84 0. 05 1 e 79 04 3 10 00 11 50 0. 35 4. 06 7 0. 63 0 0. 02 3 12 5 0. 00 05 4 0. 14 48 5 0. 83 82 0 0. 00 96 1 0. 00 63 5 0. 00 04 4 0. 00 00 2 1. 06 10 00 45 7 0. 59 8 2. 23 w t% h 2o i n m el t 18 92 :o l1 18 50 0 11 30 0. 35 5. 43 1 2. 30 0 0. 49 0 45 0 0. 00 41 0 0. 91 33 9 0. 00 72 5 0. 00 00 7 0. 05 63 5 0. 01 79 9 0. 00 08 8 2. 12 0 33 10 2 11 .5 70 18 92 :o l1 18 10 00 11 30 0. 35 5. 41 4 2. 55 0 0. 43 0 52 0 0. 00 21 0 0. 55 21 6 0. 40 81 9 0. 00 37 9 0. 02 47 6 0. 00 87 5 0. 00 03 4 2. 23 0 33 10 21 1 3. 65 7 18 92 :o l1 18 30 00 11 30 0. 35 5. 34 8 2. 90 0 0. 25 0 63 5 0. 00 06 0 0. 24 64 4 0. 73 93 7 0. 00 71 0 0. 00 43 4 0. 00 21 0 0. 00 00 5 2. 23 0 33 10 13 18 0. 55 8 18 92 :o l1 18 70 00 11 30 0. 35 5. 21 6 3. 70 0 0. 08 7 95 0 0. 00 01 0 0. 09 27 0 0. 89 73 0 0. 00 85 4 0. 00 10 8 0. 00 02 4 0. 00 00 1 2. 23 0 33 10 50 56 0. 10 4 18 92 :o l1 18 10 00 0 11 30 0. 35 5. 11 7 4. 60 0 0. 03 9 13 00 0. 00 00 7 0. 06 77 0 0. 92 25 7 0. 00 94 4 0. 00 01 7 0. 00 00 4 0. 00 00 0 2. 23 0 33 10 81 11 0. 01 6 18 92 :o l1 18 60 0 11 30 − 0. 45 5. 82 4 6. 55 0 1. 19 0 50 0 0. 00 94 1 0. 84 80 8 0. 06 28 9 0. 00 15 3 0. 01 08 0 0. 06 55 6 0. 00 17 3 2. 23 0 33 10 18 12 .0 79 18 92 :o l1 18 10 00 11 30 − 0. 45 5. 81 1 6. 50 0 1. 07 0 52 0 0. 00 51 4 0. 55 08 7 0. 39 25 5 0. 00 93 2 0. 00 60 2 0. 03 52 2 0. 00 08 6 2. 23 0 33 10 20 4 4. 72 0 18 92 :o l1 18 10 00 11 30 0. 35 3. 05 3 2. 55 0 0. 00 2 52 0 0. 00 20 1 0. 55 48 5 0. 43 68 9 0. 00 40 6 0. 00 16 2 0. 00 05 7 0. 00 00 0 2. 23 0 10 00 22 7 0. 23 9 s ou rc es o f m el t in cl us io n vo la ti le s ar e: s tr om bo li ( m ét ri ch e t a l. , 20 01 ); v es uv iu s (m ar ia ne ll i et a l. , 19 99 ); v ul ca no ( c lo cc hi at ti e t a l. , 19 94 a, b; g io nc ad a et a l. , 19 98 ); e tn a (m ét ri ch e t a l. , 19 93 ). fig. 9. pressure evolution of h2o/so2tot and co2/ /so2tot mole ratios of fluids in equilibrium with melt inclusion of stromboli melt inclusions. the thick grey continuous and dashed lines represent average c/s and h/s ratio measured on a hot fumerole or within the dilute plume at stromboli (allard et al., 1994). 686 bruno scaillet and michel pichavant culated by marianelli et al. (1999). there is no clear trend with pressure, but it is interesting to note that even relatively low pressure mi yield fluid compositions very rich in co2. in all computed examples, the so2 and h2s mole fractions remain below 0.05, whereas the mole fraction of h2o ranges betwen 0.2 up to 0.4. the h2o/so2tot and co2/so2tot mole ratios decrease with pressure down to values of about 20 and 50, respectively, at 3000 bar (fig. 11), and then increase again with further pressure decrease. the second part of the evolution remains hypothetical, however, since it is based on a single mi datum at 900 bar. fig. 8. composition of fluids in equilibrium with melt inclusions from scoria and pumice (métrich et al., 2001). xi is the mole fraction of species i. symbols plotting at 100 bar correspond to melt inclusions in scoria, all other to melt inclusions in pumice. the compositions of fluids in scoria were calculated assuming a maximum co2 content of 50 ppm. 6.2. vesuvius the mole fractions of co2, h2o and so2tot of the 1944 eruption are plotted against pressures of fluid saturation on fig. 10. calculated pressures range from slightly below 1000 bar to over 6500 bar, being again similar to those calfig. 10. composition of fluids in equilibrium with melt inclusions of the 1944 vesuvius eruption (marianelli et al., 1999). xi is the mole fraction of species i. note the rather constant fluid composition over the pressure interval. fig. 11. pressure evolution of h2o/so2tot and co2/ /so2tot mole ratios of fluids in equilibrium with melt inclusion of the 1944 vesuvius eruption. a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes 687 6.3. vulcano the fluid composition of vulcano magmas is shown in fig. 12. given the lack of detectable co2, and the generally low melt h2o contents (< 3 wt%), the calculated pressures for fluid saturation are low compared to stromboli and vesuvius, ranging from 900 down to 200 bar, in agreement with previous findings (clocchiatti et al., 1994a; gioncada et al., 1998). the data set shows a regular variation of fluid composition as pressure decreases. between 900 and 200 bar, the mole fraction of h2o decreases continuously, from over 0.9 down to 0.5, being paralleled by a concomitant increase in co2. the mole fraction of so2tot first increases up to 400 bar, where it reaches a value of 0.15, and then decreases down to 0.05 at 200 bar. the evolution of the h2o/co2tot and co2/ /so2tot mole ratios with pressure is shown on fig. 13. the h2o/co2tot decreases continuously with pressure, reaching values of 1 at 200 bar. in contrast, the co2/so2tot ratio increases with decreasing pressures, from 1 at 900 bar up to 400 at 200 bar: note that this ratio displays considerable variation at 200 bar, as observed for stromboli. this reflects variable depletion of sulphur during shallow level degassing of the magmas 6.4. etna figure 14 shows the evolution of the mole fractions of h2o, co2 and stot with pressure calculated for melt water contents of 1 and 2.23 wt%, f o2 at nno+0.35, and melt sulphur contents of 3300-3400 ppm. we stress again that calculations assume the presence of a fluid phase in the entire pressure range. in both cases, the fluid composition is dominated by co2 at pressures higher than 2000 bar. at pressures higher than 7000 bar, the fluid consists of more than 90% of co2. for instance at 10 000 bar, the calculated fluid composition corresponding to 1 wt% dissolved h2o has a mole fraction of co2 of 0.97, that is the fluid is nearly pure co2, in agreement with fluid inclusion constraints on primitive etna basalts (clocchiatti et al., 1992). under such conditions the calculated co2 content of the basalt melt is 0.85 wt% (table ii). it is only below 1000 bar (2.23 wt% h2o) or 400 bar (1 wt% h2o) that co2 is lower than h2o. for water-rich conditions, the mole fraction of h2o always exceeds that of so2tot, while at a melt h2o content of 1 wt%, so2tot equals or even exceeds h2o at pressures lower than 500 bar. the difference in fluid composition arising from different melt h2o contents is illustrated on fig. 15 where the mole fraction of h2o is plotted fig. 12. composition of fluids in equilibrium with melt inclusions of vulcano magmas (clocchiatti et al., 1994a,b; gioncada et al., 1998). xi is the mole fraction of species i. fluid compositions have been calculated assuming that the melt inclusions contain 50 ppm dissolved co2. this gives maximum co2 contents of the fluid. note the continuous evolution of the fluid composition with pressure. fig. 13. pressure evolution of h2o/so2tot and co2/ /so2tot mole ratios of fluids in equilibrium with melt inclusions of vulcano magmas. fig. 16. pressure evolution of the h2o/co2 and co2/so2tot mole ratios for melts having either 1 or 2.23 wt% dissolved h2o and 3300-3400 ppm of dissolved sulphur at nno+0.35. 688 fig. 15. evolution of the mole fraction of h2o against that of co2 calculated for melt water contents of 1 and 2.23 wt%, f o2 at nno+0.35, and melt sulphur contents of 3300-3400 ppm, corresponding to primitive etnaean magmas. the mole fractions correspond to actual values, that take into account the sulphur species, whose total mole fraction reach values in excess of 0.44 (fig. 14a,b, table ii). this is why all the data plot below a mixing line corresponding to a binary h2o-co2. dashed lines represent isobaric conditions calculated for different melt water contents. fig. 14a,b. evolution of the mole fractions of h2o, co2 and h2s+so2 with pressure calculated for melt water contents of 1 (a) and 2.23 (b) wt%, f o2 at nno+0.35, and melt sulphur contents of 3300-3400 ppm, corresponding to primitive etnaean magmas. xi is the mole fraction of species i. against that of co2. note that the mole fractions plotted correspond to actual value that take into account the sulphur species, whose total mole fraction reach values in excess of 0.44 (fig. 14a,b, table ii). this is why all the data plot below the mixing line corresponding to a binary h2o-co2. clearly, the amount of dissolved water significantly affects the fluid composition: at 500 bar the calculated mole fraction of h2o varies from 0.92 for a melt h2o content of 2.2 wt%, down to 0.22 for a melt h2o content of 1 wt%. however, the effect of melt h2o content vanishes at high pressure, so that at or above 7000 bar, the calculated fluid compositions for the two melt water contents diverge by less than 0.05 in their mole fraction of h2o. variation in f o2 in the range considered, does not strongly a b bruno scaillet and michel pichavant affect the fluid compositions, apart from the so2/h2s ratio. in contrast, a lower sulphur content of the melt (that is lower f s2) obviously decreases significantly the abundances of sulphur bearing species and, by implication, increases the co2/so2 ratio (table ii). 689 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes figure 16 shows the dramatic effect that pressure exerts on the h2o/co2 and co2/so2tot mole ratios. the h2o/co2 ratio increases from 0.01-0.1 at 10000 bar to 1-100 at 400-500 bar, whereas the co2/so2tot decreases within this pressure interval from 1000 at 10 000 bar down to 0.8-0.1 at 400-500 bar. the variation in melt water contents affects the h2o/co2 ratio more than the co2/so2tot one, and the latter should be more accurate if used for barometric purposes (i.e. depth of main stage of volatile exsolution, see below). 7. comparison with volcanic gases the comparison with volcanic gases can be only made for stromboli, vulcano and etna, since hot fumaroles or volcanic plumes have not been emitted at vesuvius since its last eruption in 1944. 7.1. stromboli for stromboli, direct sampling of fumeroles in the vent is prevented by hazardous access (allard et al., 1994). the hottest fumerole sampled had a temperature of 410°c which is 700°c lower than the estimated temperature of magma last equilibration in the shallow reservoir. this fumerole has a h2o/so2 mole ratio of 11 and a co2/so2 ratio of 19. both ratios fall within the range calculated for magmatic fluids in the shallow reservoir (fig. 9) and are thus compatible with an origin from such a depth. the average ratio of co2/so2 in the dilute plume is 11, which fits also with a shallow reservoir origin, being at the lower end of the calculated fluid composition at 100 bar. the above reasoning, however, implicitly assumes that the volcanic plume composition is dominated by the composition of fluids at depth, and ignores the possible contribution of the melt degassing during ascent from the deep/shallow reservoirs to the surface. in other words, it assumes that the volcanic plume (or hot fumeroles) is fed by bubbles that escape the magma at a certain depth, reaching the surface unmodified (equivalent to a single step degassing at depth under closed system conditions). in the present case, this depth could lie anywhere between 100 and 3000 bar, since in this pressure range magmatic fluids having a co2/so2 ratio of 10 can be produced. although such a scenario can apply to a magma intrusion at rest, like during periods of quiescent degassing with no magma emission, this is unlikely to happen in a decompressing magma body, in view of the exceedingly low melt viscosity of hydrous basalts that should allow volatiles degassing, and continuous melt-fluid re-equilibration, even on the short time scales characteristic of strombolian eruptions. we have thus calculated the bulk fluid composition arising from melt+fluid contribution, for various melt/fluid ratios, assuming that the melt can fully degass upon eruption or ascent (see also scaillet and pichavant, 2003). figure 17 shows the results in terms of co2/so2 mole ratios, which is a ratio typically measured in volcanic plumes (allard et al., 1994). also shown are the co2/so2 mole ratios of the melt and fluid phases. excluding kinetic effects, any fluid vented at stromboli must lie between fig. 17. effect of the amount of fluid in the reservoir on the final fluid compositions (resulting from the contribution of volatiles from the melt and fluid, melt + fluid), expressed in terms of the co2/so2tot mole ratio (see text), for stromboli magmas. the compositions of fluid and melt phases are shown as closed symbols. a fluid amount of 0.1 wt% in the deep reservoir results in a final fluid having a co2/so2tot ratio of 1, whereas an amount of 4 wt% results in a final co2/so2tot mole ratio of 10, similar to that measured in the volcanic plume of stromboli (thick grey line, from allard et al., 1994). 690 bruno scaillet and michel pichavant these two poles. as shown on fig. 17, if the deep magma contains 0.1 wt%, the resulting bulk fluid has a co2/so2 ratio of 1, ie significantly lower than the average one measured in the plume (11). in contrast, if the magma at depth contains 4 wt% fluid, then the resulting co2/so2 ratio is similar to that measured, with an average at 11. higher fluid contents at depth would further increase the ratio: a fluid content of 10 wt% would give a co2/so2 ratio of 22. we thus suggest that in order to produce a volcanic plume with a co2/so2 ratio of 11, the main magma reservoir must be fluid-saturated with a fluid amount of ca. 4 wt%. petrological and geophysical data (see francalanci et al., 1989) suggest the existence of a large reservoir at 10-14 km depth, in agreement with mi constraints (métrich et al., 2001; bertagnini et al., 2003), in which the magmas injected in the shallow reservoir are produced. on this basis, it is probable that such a reservoir is the major source of volatile emissions at stromboli (bertagnini et al., 2003). thus, the above analysis suggests that such a reservoir must contain a significant amount of free fluid in order to reconcile observations (volcanic plume composition, mi volatile contents) and the thermodynamic constraints (this work) on fluid composition. the total volatile content of strombolian magmas can be estimated using the fluid composition calculated in this work, the amount of volatiles dissolved in the melt (métrich et al., 2001) and assuming a fluid content of 4 wt%. this gives the following range (corresponding to different volatile contents of mi in pumice): h2otot: 2.86-3.45 wt%; co2tot: 2.94-3.46 wt%; so2tot: 0.29-0.90 wt% (or 1450-4500 ppm sulphur). the h2o and s values are close to those analysed in mi, which is due to the fact that the concentrations of both volatiles are comparable in melt and fluid. in contrast, the bulk co2 content is more than one order of magnitude higher than that dissolved in mi (0.089-0.169 wt%), which is due to the co2-rich character (xco2> 0.6) of the fluid phase at stromboli. the very high bulk co2 content suggests that even the most primitive magmas at stromboli, i.e. those generated at mantle pressures, are fluid-saturated as well. 7.2. vulcano the geochemistry of volcanic fumeroles at vulcano has been extensively discussed and modelled using various approaches and chemical tracers (e.g., capasso et al., 1994, 2001; chiodini et al., 1995; todesco, 1997; nuccio et al., 1999; giggenbach et al., 2001; nuccio and paonita, 2001; di liberto et al., 2002; diliberto et al., 2002; paonita et al., 2002). there is a general agreement that the fumerole compositions at vulcano result from mixing between a magmatic source and a boiling hydrothermal system. here we focus on the highest temperature fumeroles, in which the magmatic signature is less likely to be affected by such a mixing. the hottest fumeroles reached a temperature of near 700°c, and consisted in 85-90% h2o, 5-15% co2 with minor amounts of sulphur, in the order of 3 % (capasso et al., 1994), having an f o2 of nno+1 or very close to that inferred from sulphur speciation (nno+0.72). although quite variable, the measured co2/so2 mole ratio is ca. 10 (e.g., giggenbach et al., 2001), or similar to that of stromboli. if taken at face value, the 9/1 ratio of h2o/co2 suggests an origin of such fluids from a reservoir located at 800-900 bar (fig. 12), since lower pressure would correspond to higher mole fraction of co2. it must be remembered, however, that this is based on the assumption that mi at vulcano have a maximum of 50 ppm dissolved co2. clearly, lower dissolved co2 would allow water-rich fluids to be produced at lower pressures. however, geochemical modelling (nuccio et al., 1999) has shown that the hydrothermal contribution is quite high even in the hottest fumeroles (up to 30%), which implies that the h2o mole fraction of fumeroles are probably maximum values. similarly, if the co2/so2 ratio is used to infer the depth of fluid exsolution, then a pressure of 200-300 bar is retrieved (fig. 13), but again, lower co2 in mi would decrease this estimate. on the other hand, magmatic gas scrubbing by aquifers is known to strongly deplete magmatic gases of their so2 (symonds et al., 2001) which indicates that the sulphur content of fumeroles are minimum values. this will counteract the effect due to an overestimation of co2 in mi, leading to an increase in the co2/so2 ratio. 691 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes as for stromboli, the above comparison ignores the melt contribution to the volatile budget and that due to decompression or to crystallisation. however, compared to stromboli, a proper evaluation of this contribution is hampered by the compositional variability of erupted magmas at vulcano (e.g., clocchiatti et al., 1994a). that is, the evaluation of melt contribution depends on whether the upper reservoir stores rhyolite or some more mafic composition. if for instance, we assume that the main degassing magmatic source is rhyolite melt, then a fluid amount of 0.5 wt% in the shallow reservoir would be needed in order to release a fluid with appropriate mole ratios of h2o/co2 and co2/so2. clearly, however, the uncertainty concerning the chemical nature of the plumbing system, coupled with the lack of precise co2 determination in mi, prevent us obtaining robust constraints on bulk volatile compositions and their possible depth of exsolution. to these sources of error must be added the hydrothermal contribution, that modifies to various extents the composition of magmatic fluids (nuccio et al., 1999). we must therefore await for more precise data on co2 content in mi before doing any conclusive statement. 7.3. etna although we lack co2 constraints in mi, the case for etna is made simpler than that of vulcano, because of the chemical homogeneity of magmas erupted and because the numerous eruptions since 1970 have allowed continuous monitoring of the volcanic plume for co2 and so2 species, among other volatiles, using modern analytical tools. in addition, contrary to the two previous volcanoes, there is at least one complete data set of hot gas composition available for the 1970 eruption (hundington, 1973; gerlach, 1980). the temperature of gas collection (1075°c) is virtually identical to that of magma extrusion which greatly facilitates the comparison with the thermodynamic calculation performed here, since it can be anticipated that the magmatic component in these gases is largely dominant. we first consider this data set and then compare our calculations with remote sensing results. as for stromboli and vulcano, we start by assuming that fluids correspond to deep exsolution of a stagnant magma body and then we evaluate the effect of melt contribution arising during decompression. the average composition of volcanic gases sampled by huntingdon (1973) and restored by gerlach (1980) (excluding the extremes), reads as (in mole fraction): h2o = 0.48 ± 0.03; co2= = 0.24 ± 0.05, and so2 = 0.26 ± 0.05 (n = 14). a remarkable feature of this composition is its very high so2 content, that equals or even exceeds co2, underscoring the potential of etnean magmas to release sulphur-rich fluids. inspection of fig. 14a,b shows that production of such a fluid requires both relatively low melt h2o contents and low pressure conditions, essentially below 1000 bar. fluids equilibrated with a melt h2o content of 2.23 wt%, reach a maximum mole fraction of so2 below 10%, and are thus unlikely to be the source of gases sampled in 1970. in contrast, at a melt h2o content of 1 wt%, sulphur-rich gas compositions can be produced at pressures below 1000 bar: the fluid composition calculated at 400 bar (table ii) closely approaches the gases measured by huntingdon (1973) which in turn suggests that those gases could have been released from a magma body lying at such a depth. the 1970 data set of gas compositions is unique, and for more recent times information on the chemical composition of volcanic gases has been obtained mostly via remote sensing of volcanic plumes or through analyses of diffuse and peripheral gas emissions (e.g., allard et al., 1991; bruno et al., 1999, 2001). however, allard et al. (1991) report a co2/so2 mole ratio of ca. 10 measured in hot (1090°c) crater gases during the 1986 eruption, which is significantly higher than that measured in the 1973 eruption (hundington, 1973; gerlach, 1980). the co2/ /so2 ratio measured in the summit crater plume ranges from 10 to as high as 38, but allard et al. (1991) consider the lower value as more representative of the average gas output. such a ratio is attained at around 1000 bar for a melt h2o content of 2.23 wt%, and at slightly higher pressures for 1 wt% h2o dissolved in melt, around 1500 bar. the full compositional data corresponding to these gases are not known, in particular the mole fraction of h2o and so2, 692 bruno scaillet and michel pichavant which prevents from further constraining the depth of degassing. we now consider the melt contribution. figure 18a,b displays the co2/so2 mole ratio of the bulk fluid released during decompression of a magma containing either 1 or 5 wt% fluid, for a variety of starting pressures, in the interval 400-10 000 bar. also shown are the co2/so2 mole ratios corresponding to the melt and fluid phases, calculated at the corresponding pressures. the calculations were done for melt h2o contents of 1 and 2.23 wt% having 3300-3400 ppm sulphur dissolved. both melt h2o contents yield similar results, however (fig. 18a,b). under such conditions, the simulations show that any magma having 1 wt% fluid at any depth falls short in producing a bulk fluid having the observed co2/so2 ratio of 10, the computed values being in the order of 2-3 at best. in contrast, if the magma has an amount of fluid of 5 wt% and is stored at a depth higher than 3000 bar, then its decompression to near atmospheric conditions should release a fluid having the appropriate co2/so2 mole ratio. it is interesting to note that beyond 3000 bar, the curves are very steep, illustrating that the calculation is insensitive to the choice of the starting pressure. also noteworthty is the fact that at pressures below 2000 bar, the co2/so2 ratio sharply decreases as pressure decreases, such that magmas stored at 400-500 bar with fluids amount of 1-5 wt% would release fluids with co2/so2 ratios lower than 1, or unlike those measured in the volcanic plume. the previous calculation assumes obviously that the rising magma exsolves continuously its volatiles but that those volatiles do not escape the parent magma until near atmospheric conditions are attained (i.e. a closed system assumption). in other words this corresponds to the case where the differential ascent velocities of a rising magma batch and its exsolved bubbles are negligible. given the low viscosity of hydrous basalt melt and the significant proportion of fluid inferred (which should correspond to a large volume fraction of bubbles in the magma, see vergniolle and jaupart, 1986 ), this assumption is clearly questionable. yet, progressive escape of bubbles at high pressure would deplete the magma in co2 relative to sulphur, since the co2/so2 ratio of the fluid is always much higher than 1 at pressures higher than 500 bar ( fig. 18a,b). early depletion of a co2-rich gas phase (i.e. open system) would therefore lead to a significant decrease in the final bulk co2/so2 ratio produced by the degassing lava. we therefore conclude that the fluid amounts derived above can be considered minimum estimates. fig. 18a,b. effect of the amount of fluid in the reservoir on the final fluid compositions (resulting from the contribution of volatiles from the melt and fluid, melt + fluid), expressed in terms of co2/so2tot mole ratio (see text). calculations were done for melt h2o contents of 1 (a) and 2.23 (b) wt%, 3300-3400 ppm dissolved sulphur, nno+0.35, which should correspond to primitive etnaean magmas. an amount of 1 wt% fluid in the deep reservoir results in a final fluid having a co2/so2tot ratio of 2-3. in contrast, for magmas stored at pressures higher than 2000 bar, an amount of 5 wt% results in a final fluid co2/so2tot ratio of 10 upon near surface degassing, similar to that measured in the volcanic plume at etna (thick grey line allard et al., 1991). a b 693 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes if the measured co2/so2 ratio measured in the volcanic plume or hot fumeroles is indeed representative of magmatic conditions at depth and correspond to full degassing of either outpoured lava or crystallising magma, we can use the above results to infer the bulk volatile content of the magma. to do so we first consider that the main reservoir feeding the etna eruption lies at 3000 bar, in accordance with the geophysic, geodetic, petrological and geochemical evidence summarised previously. for a reservoir at 3000 bar storing a magma with 1 wt% dissolved h2o and coexisting with 5 wt% fluid the bulk volatile content is: h2otot = 1.05 wt%, co2tot = 4.89 wt%, so2tot = 0.81 wt%. for a magma with 2.23 wt% dissolved h2o, the figures are: h2otot = 2.70 wt%, co2tot = 4.48 wt%, so2tot = 0.68 wt%. a reservoir at 1000 bar with a magma at 1 wt% h2o in melt would lead to: h2otot = 1.31 wt%, co2tot = 3.81 wt%, so2tot = 1.57 wt%, that is a lower pressure increases both h2o and so2 yields relative to co2. the bulk content calculated for h2o does not depart significantly from that in mi, whereas the bulk contents of co2 and so2 are considerably higher than the amounts dissolved in the melt (see table ii). in summary, it appears that the gas compositions measured at etna can be produced in a variety of ways which somehow reflect the complexity and large vertical extent of the plumbing system of the volcano. in particular, the existence of several levels of magma storage in the crust is likely to induce a variety of fluid composition since the pressure of gas release may exert a strong control on its composition (e.g., caracausi et al., 2003; this work). there are no reasons magmas erupted at etna over the last decades should have all followed the same ascent path and ponded at constant storage levels. the difference in the co2/so2 ratios between the 1970 gases and those emitted during the 1975-1995 period may be a reflection of the complex feeding system. the 1970 gas composition can be reproduced by storage and degassing at 400 bar of a relatively water-poor but sulphur-rich magma. in contrast, gases emitted in more recent eruptions require storage at slightly greater depths, around 1000-1500 bar, if they were expelled from this level with no further melt contribution. if melt accompanied gas escape, then the source was more likely at 3000 bar and filled with relatively undegassed magma (sulphur-rich), coexisting with a significant amount of fluid in order to yield a volcanic plume with an average co2/so2 ratio of about 10. these are not the only possibilities, however. it could be considered that the magma strored at 3000 bar contained only 1000 ppm dissolved sulphur, which would increase the co2/so2 ratio (table ii). the only way to distinguish between these different scenarii is to have a combined h2o-sco2 data set on mi, as well as a more detailed knowledge about the possible variation of preeruptive melt h2o contents between eruptive events. 8. discussion and conclusions the empirical model presented in this study has allowed us to calculate the sulphur fugacities of hydrous basaltic magmas. there is little control, however, on the values retrieved since even at near surface conditions, f s2 is a rarely measured parameter in volcanic gases. pioneering attemps of sato and moore (1973) on the 1970 etna gases failed to measure the f s2 of the hottest gases emitted and they could only obtain data up to 860°c. yet, as gerlach (1980) has shown, the f s2 measured fits in with that restored from gas composition and thus with our own calculation. apart from this case, however, we are left to accept the calculated values at face value, until experimental data on the relationships between f s2 and sulphur solubility in hydrous mafic melts become available. we have seen that the knowledge of f s2, f o2 and f h2o (or f co2) enables us to characterise the composition of gas phase at depth which is useful information for the interpretation of volcanic gas data. here also, however, it needs to be stressed that the calculations are heavily dependent upon the accuracy of the solubility models employed to calculate f h2o and f co2. the latter parameter is clearly critical for assessing the pressures of fluid saturation: in this respect, it appears that the co2 solubility in krich basalts is still not appropriately known. this is a research trend which should be given 694 bruno scaillet and michel pichavant the highest priority in the near future for a correct quantitative modeling of volatiles behaviour at italian volcanoes. similarly, a full description of degassing processes in shallow magma reservoirs demands that co2 contents in mi be measured beyond the concentration levels allowed by the ftir tool. we suggest that future volatile studies implement analytical tools to such an end. accepting that our model retrieves f s2 within the correct order of magnitude then the calculations show that, except in special circumstances, the addition of sulphur does not strongly change pressure estimates with respect to model calculations which consider only h2o and co2 species. as we have seen, this is due to the generally low values of f s2 calculated at high pressures. only at low pressure does there exist the possibility that pressures of fluid saturation could be significantly underestimated by neglecting sulphur-bearing species. the etna example shows that the misfit between the two approaches can be in the order of several hundreds of bars for magmas rich in sulphur. our calculations suggest that f s2 is usually in the range 0.1-1 bar in the most primitive magmas found at italian volcanoes. whether this corresponds to environments unusually rich in sulphur relative to other tectonic settings or other arc magmas, remains an open question given the almost total absence of such information for primitive magmas worldwide. we are therefore currently applying our model to mafic magmas erupted in a variety of tectonic environments in an effort to fill this gap. the calculated f s2 may display dramatic variations during magmatic evolution, however, that seem to depend on open versus closed conduit conditions, as illustrated for vesuvius. thus, variations in f s2 can be potentially used as a sensor to infer the behaviour of magma reservoirs that fed past eruptive events. however, the important variations in f s2 evidenced in this study conflict with models invoking the control of magma redox state by heterogeneous equilibria between sulphur bearing species dissolved in melt and fluid (e.g., matthews et al., 1994). in our calculations we assumed that the redox states determined in melt inclusions or whole rocks have remained constant throughout the magmatic evolution of any particular eruptive event. the lack of two-oxide assemblages in most volcanic products prevents us from testing this fundamental assumption and it could be argued that the large variations we calculate in f s2 correspond in fact to variations in f o2 due to volatile exsolution. the following lines of evidence suggest they are not, however. first, if we suppose that equilibria involving sulphurbearing species control f o2 then we can use eq. (2.1) to evaluate what magnitude of f o2 change is needed in order to maintain relatively constant f s2 during sulphur exsolution. suppose a mafic melt with 3000 ppm dissolved sulphur at nno+0.95 and 1100°c with 3 wt% h2o (similar to stromboli). if it loses 1000 ppm of sulphur while keeping the same f s2, then equation 10 predicts that f o2 should decrease down to nno−2.19. while such a reduction during sulphur exsolution is conceivable, it is difficult to reconcile, for instance, with the constant redox state inferred from isotope systematics on pumices at vesuvius (marini et al., 1998). similarly, such a three orders of magnitude drop in f o2 should be accompagnied by strong changes in both phase stabilities and compositions, all of which have gone undetected so far. second, the detailed thermodynamic analysis of kilauea volcanic gases has shown that f o2 is unaffected by shallow volatile exsolution (gerlach, 1993), the collected gases following a cooling trend parallel to nno, identical to that inferred for the lavas (see also symonds et al., 1994). therefore, although we cannot rule out that f o2 varies during volatile exsolution, we believe that such a change is relatively minor compared to that encompassed by f s2. one possible reason for the relative insensivity of f o2 to the behaviour of sulphur is that water, present in abundance in italian basalts, is the main controlling species of the magma redox state (via the water dissociation reaction). the approach we followed in modeling magma degassing needs to be implemented by performing numerical simulations of magma ascent and volatiles exsolution, considering open and closed system behaviours. however, the comparison between our calculations and observed gas compositions suggests that a fluid phase is present at depth, at amounts in the 695 a model of sulphur solubility for hydrous mafic melts: application to italian volcanoes order of 4-5 wt%. although the available data on mi do not allow to prove yet such an hypothesis, it is important to note that if it is true then it will require a significant reevaluation of magma budgets based on volatile emissions (e.g., allard, 1997; bruno et al., 2001). current evaluations have only considered the sulphur or carbon dioxide dissolved in melt as the ultimate source of volcanic degassing. however, consideration of a fluid phase could lead to a significant downsizing of magma volumes required to balance the volatile budget measured at permanently degassing volcanoes such as etna and stromboli. to illustrate this point, we take the case of an etna magma reservoir at 1000 bar with 5 wt% fluid previously discussed, whose so2tot content is 1.57 wt%. allard (1997) calculated that the average so2 ouptut at etna for the period 1975-1995 is 1.7⋅106 tons/year, whereas the average amount of erupted magmas over the same period is 81⋅106 tons/year. by considering only the melt contribution, allard (1997) calculated that an additional amount of 678⋅106 tons/year of magma must be intruded (and not erupted) in order to account for sulphur emissions. in contrast, if a fluid phase is present at 1000 bar, then the 81⋅106 tons of magma extruded every year could release 1.27⋅106 tons of sulphur or 75 % of the total amount of sulphur emitted, and the need for endogeneous magma degassing at etna would be much less severe than previously thought. acknowledgements the paper greatly benefited from constructive reviews by h. shinoara and r. moretti. references allard, p. 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(1999): h2o in rhyolitic glasses and melts: measurement, speciation, solubility, and diffusion, rev. geophys., 37, 493-516. the local ionospheric modeling by integration ground gps observations and satellite altimetry data annals of geophysics, 59, 6, 2016, a0654; doi:10.4401/ag-6810 a0654 the local ionospheric modeling by integration ground gps observations and satellite altimetry data mohammad ali sharifi1,2, saeed farzaneh1,* 1 school of surveying and geospatial engineering, college of engineering, university of tehran, iran 2 research institute of geoinformation technology (rigt), college of engineering, university of tehran, iran abstract the free electrons in the ionosphere have a strong impact on the propagation of radio waves. when the signals pass through the ionosphere, both their group and phase velocity are disturbed. several space geodetic techniques such as satellite altimetry, low earth orbit (leo) satellite and very long baseline interferometry (vlbi) can be used to model the total electron content. at present, the classical input data for development of ionospheric models are based on dual-frequency gps observations, however, a major problem with this observation type is the nonuniform distribution of the terrestrial gps reference stations with large gaps notably over the sea surface and ocean where only some single stations are located on islands, leading to lower the precision of the model over these areas. in these regions the dual-frequency satellite altimeters provide precise information about the parameters of the ionosphere. combination of gps and satellite altimetry observations allows making best use of the advantages of their different spatial and temporal distributions. in this study, the local ionosphere modeling was done by the combination of space geodetic observations using spherical slepian function. the combination of the data from ground gps observations over the western part of the usa and the altimetry mission jason-2 was performed on the normal equation level in the least-square procedure and a least-square variance component estimation (ls-vce) was applied to take into account the different accuracy levels of the observations. the integrated ionosphere model is more accurate and more reliable than the results derived from the ground gps observations over the oceans. 1. introduction the earth’s ionosphere is a layer of the atmosphere at an altitude of about 60 km to 2000 km above the earth’s surface, which contains enough electrons and ions to effectively interact with the electromagnetic fields. this layer plays an important role in wireless communication due to its influence on radio propagation [blaunstein and christodoulou 2007]. ionospheric information such as the vertical total electron content (vtec) can be extracted from different space geodetic techniques: global navigation satellite system, satellite altimetry missions such as topex/poseidon and jason, very long baseline interferometry (vlbi) and low earth orbit satellites (leos). each technique has its specific characteristics influencing the derived ionosphere parameters so it can be assumed that the combined model of the ionosphere can make the best use of the advantages of each particular space geodetic technique and improve the reliability and the accuracy of the vtec determination. the classical input data for the development of the local ionosphere map is obtained from ground-based gps observations, however the distribution of the permanent gps stations is not globally uniform. by combination of the gps observations with the other data it is intended to increase the local ionosphere maps precision. in the field of combining the different space-geodetic observations for the ionospheric modeling, several studies have been done, todorova et al. [2007] developed the global models of the ionosphere by integration of gnss and satellite altimetry data. then alizadeh et al. [2011] enhanced the model accuracy by adding radio occultation measurements (ro) from space-based gps. for regional modeling schmidt et al. [2008] combined gnss, ro and satellite altimetry data and in dettmering et al. [2011] four observation types (terrestrial gps, space-based gps, altimetry, and vlbi) are combined into one regional three-dimensional vtec model. in this study, terrestrial gps and altimetry were integrated into one regional three-dimensional vtec model. in this procedure, all observations were combined in one joint adjustment taking into account the systematic offsets between the observation groups. considering the different accuracy levels of the input data in the article history received june 16, 2015; accepted october 4, 2016. subject classification: slepian function, ls-vce, satellite altimetry, gps, local ionospheric modeling. stochastic model plays a crucial part in this procedure. in this study, the different variance factors will be estimated within a least-square variance component estimation (ls-vce). the remainder of this paper is organized as follows. firstly, the description of the measurements used as input data will be introduced (section 2). then, the general model approach and the concept for data combination will be discussed (section 3). finally, the vtec model results will be presented (section 4). 2. input data the present study is based on the data from groundbased gps and satellite altimetry observations. both techniques allow the observation and modelling of the ionosphere, but each of them has its specific characteristics which influence the derived ionosphere parameters. the satellite altimetry data help to compensate the insufficient gps coverage of the oceans. the 24h observations of 30 stations belong to the international gnss service (igs) network (http://sopac.ucsd.edu/) and the sampling rate of the measurements considered is 30 s. the vtec values from the satellite altimetry jason-2 considered in this study were obtained from the dual-frequency ionosphere delay measurements in the geophysical data record (gdr). jason-2 has taken over and continued topex/poseidon and jason-1 missions since june 2008 in the frame of a cooperation among the centre national d’études spatiales (cnes), the european organisation for the exploitation of meteorological satellites (eumetsat), the national aeronautics and space administration (nasa) and national oceanic and atmospheric administration (noaa). it is equipped with the poseidon-3 dual-frequency radar altimeter at 14.6 ghz (ku-band) and 5.3 ghz (c-band). 2.1. tec from ground gps in this study a method of measuring tec directly from the differential code delay and carrier phase measurement on both the l1 and l2 frequencies is used. for this purpose, the geometry-free linear combination of pseudo range and carrier phase measurements (also termed ionospheric observable) which is formed by subtracting simultaneous pseudorange or carrier phase observations is used [ciraolo et al. 2007]. for pseudorange and carrier phase measurements the ionospheric observable can be obtained by equations (1) and (2) respectively: where p1, p2, u1 and u1 are the code and carrier phase pseudo ranges on the l1 and l2 signals with f1= 1575.42 mhz and f2 = 1227.60 mhz frequencies, respectively. i1 and i2 are the ionospheric refraction delays at l1 and l2, respectively. br = c (xrp1− x r p2) and br = c (t rl1− t r l2) are the code and phase inter-frequency biases (ifbs) for the receiver, bs = c (xsp1− x s p2) and bs = c (tsl1− t s l2) are the code and phase differential inter-frequency biases for the satellite, fp and fl are the effects of multipath and measurement noise on the pseudorange and carrier phase, respectively, mi is the wavelength of the li carrier phase, and ni is the integer carrier phase ambiguity. slant total electron content (stec) can be computed using equations (1) or (2). although carrier phase measurements show low noise behavior in comparison with pseudo range measurements it is not favored here for calculating stec due to the estimation of the ambiguity term in the preprocessing. this requires the unknown ambiguities to be estimated in a pre-processing step. in order to benefit from the ambiguity-independent estimates of stecs derived from the code pseudo-ranges and the high precision of the carrier phase measurement, the pseudo-range ionospheric observables are smoothed using the “carrier to code leveling process” method [ciraolo et al. 2007, nohutcu et al. 2010]. by combining equations (1) and (2) for simultaneous observations, the following equation can be obtained: (3) because the noise and multipath term for the carrier-phase observation is much lower than that for the pseudo-range observation, it can be neglected [hofmannwellenhof et al. 2008]. the interval at which no cycleslip error has occurred, leading to a constant phase ambiguity, is regarded as the continuous observational arc [hofmann-wellenhof et al. 2008]. for cycle slip detection several testing quantities have been proposed based on various combinations of gps observations [seeber 2003, hofmann-wellenhof et al. 2008]. herein, the observation file of each station has been processed individually and a single receiver test, i.e. the combination of a phase and a code range is applied for cycle slip detection [hofmann-wellenhof et al. 2008]. the average of the geometry-free pseudo-range and carrier phase is computed for any satellite and receiver in the continuous arc [gao et al. 1994]:p4 p1 p2 i i c c p s p s p r p r p p4 p1 p2 1 2 1 2 1 2 x x x x f = = + + + +r r w w i i n n2 c t t c t tl r l r l s l s l 4 1 2 2 1 1 1 2 n2 1 2 1 2 m m f u u u= = + + + +q qv v p4 n2 br bs br bs n p p4 4 2 n21 1m m f u+ = + + + + + + p4 n p4 n n2 br bs br bs 1 arc i n i arc p arc p4 4 p4 4 1 1 1 2 n2m m f u u+ = + = + + + + + + = q v| sharifi and farzaneh 2 (1) (2) (4) 3 where n is the number of measurements in the continuous arc. by subtracting equation (4) from equation (2) the ambiguity term is removed: (5) p ~ 4 is the pseudo-range ionospheric observable smoothed by the carrier-phase ionospheric one. it will minimize the effect of multipath error so it can be neglected [nohutcu et al. 2010]. inserting ionospheric delays into equation (5), stec can be obtained from the smoothed ionospheric observable in tec units (tecu = 1016 el/m2) by: (6) since in this study the ionosphere is modeled as a thin single spherical layer (figure 1) the stec values have to be converted into the height-independent vtec by introducing a mapping function [schaer 1999]: (7) where r is the mean earth radius, z and z´ are the zenith angles of the satellite at the user position and the ionospheric pierce point, and h is the mean altitude which approximately corresponds to the altitude of the maximum electron density and its height can vary between 250 and 500 km depending on latitude, season, solar and geomagnetic activity conditions [misra and enge 2003, seeber 2003, hofmann-wellenhof et al. 2008]. 2.2. tec from satellite altimetry the advent of the new satellite altimetry technique and the use of several different frequencies allow for estimating the ionospheric electron content. satellite radar altimeters directly measure in nadir direction. they permanently transmit signals to earth and receive the echo from the surface. the range between the satellite and the sea surface is determined by measuring the satellite-tosurface round-trip time of a radar pulse. therefore, the vtec can easily be extracted from the dual-frequency altimetry observations by converting the ionospheric correction as follows [dumont et al. 2008]: (8) where dr is the ku-band ionospheric range correction in meter from the geophysical data record (gdr) products (iono_corr_alt_ku), and f is the ku-band radar frequency in hz. it is noteworthy that the tec could then be converted to the c-band ionosphere range correction using the same formula above, and the c-band frequency of 5.3 ghz. this approximation is valid for frequencies higher than 1 ghz [langley 1996]. as the 1 hz observations are quite noisy, the smoothing and thinning of the data is advantageous. for this purpose, a simple median filter with a length of 20 s has been applied on the altimeter ionospheric range correction [picot et al. 2008, dettmering et al. 2011]. figure 1 shows the sample filtered and unfiltered altimeter ionospheric range correction on the ku-band for the jason-2 satellite, cycle 201 and pass 137. because of the lower orbit altitude of the altimetry satellites, the measured vtec are expected to be lower than the values obtained from gps. however, several studies have shown that the tec values obtained by the satellite altimetry are higher by about 34 tecu [chelton et al. 2001, brunini et al. 2005]. thus, in the combination stage it can be assumed that the altimetry measurements have biases, this has the effect of the plasmaspheric component (i.e. the vtec between ~1000 and ~20,000 km height above the earth surface [alizadeh et al. 2011]). in this study a constant daily systematic bias for the jason-2 satellite was estimated in the combination procedure. 3. spherical slepian functions spherical harmonic basis can effectively be used to represent the target function as long as the modeled area covers the whole sphere and the data is distributed regularly [chambodut et al. 2005, mautz et al. 2005]. this can be limited by data gaps, the very high degrees with small-value coefficients, and different survey data sampling density. this function is prone to error for problems in which the data measured over the part of the earth, as the northern hemisphere or a spherical cap, since it is no longer orthogonal over the partial area p4 p4 i i br bs arc p arc l p4 p4 4 4 1 2. f f u u= + + + + + u . stetec p4 f 2 f 1 f 2 br bs f 1 40 3lp f arc fq 22 f 12 2f2 2 p4 f 1 f 2 pf f= + ur fqw va d with cos sin sinmf vtetec stetec z z r h r z 1 e e= = = +l l vtetec dr 40= . f 3 2 # local ionosphere maps of vtec figure 1. sample filtered and unfiltered altimeter ionospheric range correction on ku-band for the jason-2 satellite, cycle 201 and pass 137. [beggan et al. 2013, sharifi and farzaneh 2016]. therefore, the construction of a local spherical harmonic basis that is orthogonal over the studied area of the sphere is required for the study of the local modeling. in order to localize these functions into a region of interest r (target region), the optimization of a local energy criterion can be utilized. this will give a new set of functions in the sense of slepian [1983]. they are band-limited to a maximum spherical harmonic degree l and, at the same time, are spatially concentrated inside a target region. in other words, slepian functions are a particular linear combination of the spherical harmonics. unlike the spherical harmonics they can be sorted according to their energy concentration inside the target region [simons et al. 2006]. the spherical slepian function can be presented as a band-limited spherical harmonic expansion: where ylm(r) is a real spherical harmonics of degree l and order m, r is the location of a point on the surface of a unit sphere x and glm is defined as: to maximize the spatial concentration of a bandlimited function g(r) within a region r, the ratio of the norms should be maximized as: where 0 ≤ m ≤ 1 is a measure of spatial concentration. the maximization of this concentration criterion can be achieved in the spectral domain by solving the algebraic eigenvalue problem [simons et al. 2006]: dg = mg (12) where the elements of (l + 1)2 × (l + 1)2 localizing kernel d: [equation (13) at page bottom] are given by: and g is the (l + 1)2 dimensional vector that represents a slepian eigenfunction expressed in the spherical harmonics, i.e: g = (g00 ... glm ... gll) t (15) this ‘localization’ matrix is symmetric and the subspace of maximum energy is readily obtained by eigenvalue decomposition. when the signal g (r) is local, it can be approximated using the slepian expansion truncated at the shannon number n [percival and walden 1993]: where a is the area of the region as a fraction of the full sphere. the data can be approximated, yet with very good reconstruction properties within the region by [simons 2010]: where g (r̂) and dn are the spherical slepian function and unknown coefficient, respectively. the combination of different space-geodetic observations for the regional ionosphere modeling introduces g r l m m l l 0 = =-= q v || g yl rlm m ylm q v| g g r ylm = x q v# dyl rylm xq v maxax g g r # 2 2 m= x # x # r # d d 2 x x x xq q v v dd yl yl,lm l m ylmr # yl mx x x=l l l lq qv vr# n ( ) n n l 1 1 2 m= = = + | l a1 4 2v r= +q 2v d r d gn n n 1 . = uq v | g rn uq v sharifi and farzaneh 4 (9) (10) (11) (14) (16) (17) (13) 5 the vtec of two or more different space-geodetic techniques into one adjustment computation. it is obvious that the combined model is usually of higher quality because it is based on a large amount of observation data and takes advantage of the particular qualities of the input data. the least-square adjustment (gauss-markov model) is applied on each set of different observations and then the individual normal equations are combined by adding the relevant n-matrices: where ai and pi are the design matrix for each observation and the weighted matrix for each observation which are uncorrelated and assumed to be known, respectively. the combination algorithm implies that the individual solutions are statistically independent populations, and that each of them provides an individual covariance matrix. due to the various observation instruments and the functional and stochastic modeling the covariance matrices are scaled by individual factors before being introduced into the combination process. in other words, in comparison with satellite altimetry, the gnss technique has more measurements, hence in order to increase the effect of the satellite altimetry observations in the combined model, up-weighting of the related data is needed. on the other hand, due to the higher noise of the satellite altimetry measurements, a lower weight would be implemented [alizadeh 2013]. the behavior of relative weighting is similar to scaling factor when a combination of different techniques are used. these factors (vi) are estimated within a least-square variance component estimation (ls-vce) [teunissen and amiri-simkooei 2008]. its principal idea is to transform the gauss-markov model into the model of condition equations [koch 1997 (p. 239), bahr et al. 2007]. 4. results and discussion the present study is based on the data from ground gps and satellite altimetry observations. in order to solve stec from ground gps observations, the receiver ifbs were calculated using the bernese gps software v 5.0 and the ifb values for the satellite were obtained from the center for orbit determination in europe (code). the stec and vtec values for each observation were computed as described in section 2. the altitude for the single-layer model was set to 400 km for the calculation of vtec and an elevation cutoff angle of degree was used. the precise orbit files, which are provided by several igs agencies, were interpolated to determine satellite positions. these tec measurements contain ionospheric electron density information around the region above the gps network and they are used as input data for the ionospheric modeling. figure 2 shows the distribution of the jason2 observation and the gps reference stations for august 25, 2012. for the evaluation of the presented method, first the ionospheric map was developed by means of only the ground gps observation. for this purpose two cases were considered, first the 2d analyses were performed, assuming that the ionosphere is static for the modeling period by neglecting the relatively small temporal variations in the ionosphere. the root-meansquare-error value for the residuals is 0.70 tecu for the reference solution. the vtec map of the two-dimensional modeling for the mid-point of the modeling period (07:45:00) ut is plotted in figure 3. the vtec n a pg a a pa a gps gps t pgps gps n altimetryry t paltimetryry altimetryry n a altimetryry 2 2 1 1 gps n altltimetryry v v = + + -w -wr r -w -w 14 24 34414444444444444444444444444444444444444444444442442444444444444444444444444444444444444444444 344443 14 24 344144444444444444444444444444424424444444444444444444444444344 local ionosphere maps of vtec (18) figure 2: distribution of jason-2 observation and gps reference stations for august 25, 2012. figure 3. 2-d vtec model for 25.08.2012, 07:45:00 (ut) rmse = 0.70 tecu. maps shown in figure 3 has some artifacts on the lower left corner. these artifacts may be caused by the nature of the basis functions (definition of the boundaries), associated with lack of data in this region. second, the three-dimensional approaches based on a system of three-dimensional base functions defined as the tensor product of spherical slepian function for longitude and latitude in an earth-fixed reference frame, as well as polynomial b-spline functions for the time [sharifi and farzaneh 2013]. figure 4 shows the gps-only vtec map at 07:45:00 ut of day 238, 2012. the rmse value for the residuals is 0.10 tecu for the reference solution and the optimum level of b-spline. as can be seen, by being away from the slepian functions domain, the results become unrealistic, especially over the ocean. the comparison between figures 4 and 3 depicts the 3d modeling, thereby localizing the temporal variations in the ionosphere more appropriately, because in 2d modeling, it is assumed that the ionosphere has been frozen in the sun-fixed reference frame. hence, such ionospheric modeling does not capture the small-scale and high-frequency ionosphere disturbance. although the temporal variations have been considered in 3d modeling, but it still has low precision since there are no observations over the sea surfaces. to compensate for this deficiency, the gps and satellite altimetry have been combined. as mentioned before, the satellite altimetry missions provide precise information about the ionosphere over the ocean, and in the view of the fact that not many gps stations are available there, their observations provide less accurate ionosphere maps over these regions. the combination is made by applying a least-square adjustment on each set of observations and combining the normal equations. in this procedure, a constant daily bias for the jason-2 satellite was inserted as an additional unknown parameter in the observation equation and was estimated along with the other unknown parameters. in this study the estimated bias was −3.1 tecu. figure 5 illustrates the combined vtec map at 07:45:00. it can be inferred that combining the altimetry data with the gps observations influences the map mainly over the regions where the satellite altimetry provides information. the rmse value for the residuals was 0.08 tecu. to evaluate the approach and its accuracy, some of the altimetry observations were set apart from the combination procedure and were not involved in the modeling process. the number of excluded observations is selected in such a way that the accuracy of the developed model would not reduce by the confidence interval of (1 −a = 95%) [alizadeh 2013]. after the development of the combined model, the vtec values at the footprints of the raw altimetry observations, formerly eliminated from the combination, were derived from the combined model. the differences between the combined model vtec values and the raw altimetry data were utilized for the quantification of the combined model. hence for the altimetry satellite measurements the difference has been formed according to alizadeh [2013]: (19) where vtecclim is the combined local ionospheric model (clim), vtecgdr is the raw altimetry vtec, which is not included in clim, {alt, malt are the geographic latitude and longitude of the raw altimetry observation and t is the time of the altimetry observation. figure 6 (the red plot) presents the average of the differences in all raw observations. as stated before a daily constant offset for the altimetry measurements has been , , , , , , vtetec t vtetec t vtetec t alt alt clclimim alt alt alt altgdr { m { m { m d = q q qv v v sharifi and farzaneh 6 figure 4. 3-d vtec model result for august 25, 2012, 70:45:00 (ut) for j = 4, rmse = 0.10 tecu. figure 5. 3-d vtec model of gps and satellite altimetry combined, for august 25, 2012, 70:45:00 (ut) for j = 4, rmse = 0.08 tecu. 7 considered within the combination procedure and is estimated along with the unknown parameters in the leastsquare procedure. this negative mean bias indicates a systematic under-estimation of vtec delivered by jason-2 compared to the values delivered by gps. the blue plot in figure 6 depicts the mean vtec biases after removing this offset. as can be seen, the new biases vary between [−1.5, 1.5] tecu with a mean value of 0.06 tecu. this proves a good agreement of the combined model with the raw altimetry observations. 5. summary and conclusion in this study, the local ionosphere modeling has been done by the combination of space geodetic observations. the combination of data from the ground gps observations over the western part of the usa and from the altimetry mission jason-2 was performed on the normal equation level in the least-square procedure. the combination of the altimetry data with the ground gps data, influenced the models mainly over the regions where satellite altimetry provided additional information. the spherical slepian function was performed for the real set of data with the least-square variance component estimation for the weighting of the individual models. the combined model from the gps and altimetry data had a better accuracy than the gps-only solution over the seas. the method can be adopted for the combination with the ionospheric data from the other techniques. references alizadeh, m., h. schuh, s. todorova and m. schmidt (2011). global ionosphere maps of vtec from gnss, satellite altimetry, and 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(1996). propagation of the gps signals, in: a. kleusberg and p.j.g. teunissen (eds.), gps for geodesy, springer, berlin, 103-140. local ionosphere maps of vtec figure 6. mean vtec biases before (red plot) and after (blue plot) removing the estimated offset. mautz, r., j. ping, k. heki, b. schaffrin, c.k. shum and l. potts (2005). efficient spatial and temporal representations of global ionosphere maps over japan using b-spline wavelets, journal of geodesy, 78, 660-667. misra, p., and p. enge (2003). global positioning system: signals, measurements, and performance, ganga-jamuna press, massachusetts, usa. nohutcu, m., m.o. karslioglu and m. schmidt (2010). b-spline modeling of vtec over turkey using gps observations, journal of atmospheric and solarterrestrial physics, 72, 617-624. percival, d.b., and a.t. walden (1993). spectral analysis for physical applications, multitaper and conventional univariate techniques, cambridge univ. press, new york. picot, n., k. case, s. desai, and p. vincent (2008). aviso and podaac user handbook, igdr and gdr jason products, edition 4.1, report smm-mum5 op-13184-cn (aviso), jpl d-21352 (podaac). schaer, s. (1999). mapping and predicting the earth’s ionosphere using the global positioning system, ph.d. thesis, astronomical institute, university of berne, switzerland. schmidt, m., m. karslioglu, and c. zeilhofer (2008). regional multidimensional modeling of the ionosphere from satellite data, in: proceedings of the tujk annual scientific meeting, ankara. seeber, g. (2003). satellite geodesy: foundations, methods and application, 2nd ed., walter de gruyter, berlin and new york. sharifi, m.a., and s. farzaneh (2016). local ionospheric modeling using the localized global ionospheric map and terrestrial gps, acta geophysica, 64 (1), 237-252; issn (online): 1895-7455, doi:10.1515/ac geo-2015-0065. simons, f.j., f.a. dahlen and m.a. wieczorek (2006). spatiospectral concentration on a sphere, society for industrial and applied mathematics review, 48, 504-536. simons, f.j. 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(1983). some comments on fourier-analysis, uncertainty and modeling, siam review, 25 (3), 379-393. teunissen, p.j.g., and a.r. amiri-simkooei (2008). leastsquares variance component estimation, journal of geodesy, 82 (2), 65-82; doi:10.1007/s00190-007-0157-x. todorova, s., h. schuh and t. hobiger (2007). using the global navigation satellite systems and satellite altimetry for combined global ionosphere maps, advances in space research, 42, 727-736. *corresponding author: saeed farzaneh, school of surveying and geospatial engineering, college of engineering, university of tehran, iran; email: farzaneh@ut.ac.ir. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sharifi and farzaneh 8 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 a statistical study of aftershock sequences giorgio ranalli annali di geofisica, vol. 22, n. 4, 1969. annals of geophysics, vol. 53, n. 1, february 2010 abstract a comprehensive statistical study of the phenomenology of aftershock sequences is made in this paper. the spatial distribution of aftershocks indicates that they are mainly crustal events; however, deeper sequences also take place. the analysis of the distribution of aftershocks in 15 sequences with respect to time and magnitude leads to the statistical confirmation of a set of phenomenological laws describing the process, namely, the time-frequency law of hyperbolic decay of aftershock activity with time, the magnitude stability law, and the exponential magnitudefrequency distribution. the hypotheses involved are checked. the grouping of data and the statistical methods employed are chosen according to some basic well·confirmed assumptions regarding the nature of the process. introduction a comprehensive study of the phenomenology of aftershock sequences must include space, time, and magnitude distributions. sequences which occurred in many parts of the world have been studied by various authors and the results are scattered in the geophysical literature. the determination of the phenomenological laws describing the aftershock process is a statistical problem, and it is therefore desirable to employ a consistent statistical procedure and to perform appropriate tests. we feel that some of the methods employed in the past were not rigorous; consequently, this paper is an attempt to present a unified procedure for the statistical study of aftershock sequences. for this purpose, 15 sequences have been analyzed in detail, even if some of them had already been studied according to different methods. the study of the space distribution of aftershocks within a given sequence does not present particular problems. its accuracy depends on the precision with which epicentral coordinates and focal depths are computed. it has been maintained [page 1968a] that aftershocks are essentially a shallow phenomenon. a review of available information partly confirms this view, but allowance must be made for notable exceptions. the situation is more complicated with respect to time and magnitude distribution. the problem is basically that of finding a statistical relationship between the various quantities involved, and of estimating the parameters appearing in the statistical laws. first of all, when examining the data (which consist of the origin times ti and of the individual magnitudes mi of the aftershocks), one must be reasonably sure to be dealing with a complete set; that is, ideally no aftershock with m ⩾ m*, where m* is the minimum magnitude detected in a sequence, should be missing in the time interval considered. moreover, it has been shown empirically by suzuki [1958] that the mode of grouping the data influences the results: therefore the mode of grouping should be as uniform as possible. finally, it is desirable to employ a statistical procedure that does not contradict the underlying characteristics of the phenomenon observed, it is necessary to apply some statistical test to check the hypothesis being entertained, and confidence limits on the results must be given. more often than not, one or several of these conditions are not met in the study of aftershock sequences, and the significance of the results is therefore debatable. among the several sequences for which origin times and magnitude of individual aftershocks have been published, 15 have been selected for detailed study. as far as possible we have tried to include sequences from different geographic regions, even if this implied considering a few sequences whose completeness may be in doubt. since the determination of the statistical laws is more reliable when data are more abundant, no aftershock sequence consisting of less than 46 shocks has been included. table 1 lists the aftershock sequences whose time and magnitude distributions are studied in this paper; it includes region of occurrence, literature reference, main shock parameters (t0 origin time; φ0, λ0 geographic coordinates; h0 focal depth; m0 magnitude), minimum magnitude of aftershock included (m*), focal depth of aftershocks (h), and total number k of aftershocks with m ⩾ m* recorded in the first 100 days. (in sequence (1) the first day after the main shock is excluded from the count.) focal depths are in kilometers; the term «shallow» is taken to mean «crustal», and often (especially in california) «upper crustal». magnitude are given in the m-scale or ml-scale, with the exception of sequence (1), where the m-scale has been employed. it will be seen that the results of the analysis confirm 43 a statistical study of aftershock sequences 44 sequ ence r eg ion r eference t0 φ 0 λ 0 h 0 m 0 m * h k 1. a laska 1964 2. a leu tian i. 1957 3. l ong b each 1933 4. d esert h ot sp. 1948 5. k ern c ou nty 1952 6. san francisco 1957 7. salinas 1963 8. p arkfield 1966 9. c halk idike 1932 10. w . t hessaly 1954 11. a m orgos 1956 12. m agnesia 1957 13. z ante 1962 14. c rem asta 1966 15. h aw ke's b ay 1931 a laska a leu tian i. c alifornia ˝ ˝ ˝ ˝ ˝ g reece ˝ ˝ ˝ ˝ ˝ n ew z ealand p age 1968b d u da 1962 b enioff 1951 r ichter et al. 1958 r ichter 1955 t ocher 1959 u dias 1965 m ce villy et al. 1967 p apazachos et al. 1967 ˝ ˝ ˝ ˝ c om ninak is et al. 1968 b enioff 1951 m ar 28.03:36:13 m ar 9.14:22:28 m ar 10.17:54:08 d ec 4.15:43:17 ju l 21.11:52:14 m ar 22.19:44:21 sept 14.19:46:17 ju n 28.04:26:13 sept 26.19:20:37 a pr 30.13:02:36 ju l 9.03:11:40 m ar 8.12:21:13 a pr 10.21:37:13 feb 5.02:01:43 feb 3.10:15:00 61˚.0 n 51˚.3 n 33˚34´ n 33˚56´ n 35˚.0´ n 37˚40´ n 36˚52´ n 35˚57´ n 40˚.5 n 39˚3 n 36˚.7 n 39˚.3 n 37˚.6 n 39˚.1 n 39˚.5 s 147˚.8 w 175˚.8 w 117˚59´ w 116˚23´ w 119˚02´ w 122˚29´ w 121˚38´ w 120˚29´ w 23˚.7 e 22˚.2 e 25˚.8 e 22˚.6 e 20˚.1 e 21˚.6 e 177˚.0 e 20 km– shallow shallow 16 km shallow 3.3 km shallow 28 km 19 km 21 km 18 km 20 km 20 km– 8.5 8.3 6.3 6.5 7.7 5.3 5.4 5.5 6.9 7.0 7.5 6.8 6.3 5.9 7.6 4.5 5.9 3.9 3.0 4.0 2.0 1.0 2.0 3.4 3.2 3.5 3.0 3.6 3.4 4.1 < 35 km < 150 km shallow < 35 km shallow < 10 km < 14 km < 12 km shallow ˝ ˝ ˝ ˝ ˝ – 294 2057868 184 16046 17385 299 400 291 139 10371 t able 1.l ist of aftershock sequ ences selected for detailed stu dy. a statistical study of aftershock sequences the 3 basic laws describing the phenomenology of aftershock sequences, namely, the time-frequency law (omori's law) as formulated by mogi [1962], the magnitude stability law [lomnitz 1966], and the magnitude-frequency law [gutenberg and richter 1954]. spatial distribution of aftershocks the spatial distribution of the shocks in an aftershock sequence is naturally related to the location of the main shock. the following considerations are based on a comprehensive survey of available data and are not limited to the sequences listed in table 1. if one traces on a map the boundary of the area in which the epicenters are located, the epicenter of the main shock is usually close to this boundary. such is the case for all aftershock sequences of large earthquakes wich occurred in japan from 1923 to 1963 [matuzawa 1964]. when aftershock activity takes place along a fault segment, as is frequently the case in california, the domain of the epicenters is approximately elliptical with the long axis parallel to the active fault segment; often the main shock occupies, roughly speaking, one focus of the ellipse, and the aftershocks are concentrated toward the two ends. in some sequenze the aftershock epicenters are clustered in a very small area (only a few kilometers in length and width), but this is rather exceptional; usually they are spread out over a much larger area. in the aftershock sequence of the aleutian islands earthquake of march 9, 1957 [duda 1962] the distribution of the epicenters follows closely the trend of tectonic activity along. the aleutian arc. as to the focal depths of aftershocks, a review of available data by page [1968a] indicates that, when hypocenter determinations are accurate, aftershocks are shallow events following a main shock which is itself shallow. aftershock sequences therefore appear to be crustal phenomena, with the majority of shocks clustering in the upper layer of the crust (h ⩽ 20 km). 45 table 2. observed frequency of aftershocks. 46 there are, however, some exceptions. in the region of greece, the southern sporades earthquake of march 18, 1926 had a reported depth of 50 km and was followed by 18 aftershocks with m ⩾ 3.9 which were recorded in athens; the sequence of the anatolia earthquake of march 18, 1953 (h = 50 km) comprised 21 aftershocks in the first 13 days; the sequence of the zante earthquake of november 15, 1959 (h = 55 km) consisted of 18 shocks; the focal depths of individual aftershocks, however, were not determined [papazachos et al. 1967]. in the aftershock sequence of the kamchatka earthquake of november 4, 1952 the majority of shocks were located near the mohorovicic discontinuity, but some of them had foci as deep as 60 km [båth and benioff 1958]. the sequence following the aleutian islands earthquake of march 9, 1957 had an average focal depth of 74 km, and individual shocks were as much as 150 km deep [duda 1962]. two more notable exceptions have occurred in romania and in central asia. iosif and radu [1961] have studied the aftershock sequence following an earthquake with m = 7.4 and h = 150 km that took place in the region of vrancea, romania, on november 10, 1940. the focal depths of aftershocks (3.3 ⩽ m ⩽ 5.5) were of the same order. lukk [1968] has studied the aftershock sequence of the dzhurm earthquake of march 14, 1965, which occurred in the pamirhindu kush region and had a focal depth of 210 km. the observation period lasted for about 22 days, during which 390 aftershocks were recorded; their focal depths increased in time from 200 to more than 240 km. the examples of recorded subcrustal aftershock sequences, however, form a very small part of the total number of sequences known to date; on the other hand, subcrustal earthquakes are themselves much less numerous than crustal ones. therefore the comparative index of aftershock activity should be given by the ratio of the percentages of crustal and subcrustal earthquakes which are followed by a sequence. at present there is a bias due to instrumentation which favours the detection of shallow aftershock sequences whereas deep ones may go undetected. thus, the conclusion that aftershocks are generally a shallow phenomenon has to be accepted and at the same time it must be realized that exceptions exist and that the data are far from complete. time distribution of aftershocks it is customary to regard aftershocks as random events in time, whose frequency is governed by some time-decay law. jeffreys [1938], in a study of the aftershocks of the tango, japan, earthquake of march 7, 1927, found no sign of mutual dependence between aftershocks. that is, there was a statistical study of aftershock sequences table 4. fluctuations of the observed frequencies in the decay of aftershock activity with time. table 3. estimates of the parameters in the time-frequency law. figure 1. time distribution of aftershocks: alaska 1964. 47 no indication that the chance of an aftershock in a given interval of time depended on anything but the time since the main shock, the aftershock frequency falling off with time according to omori's law. the observed frequency showed only random departures from the law. it has since become a commonly accepted fact that aftershocks can be regarded as random independent events. it follows that any mathematical relationship relating time and frequency must not be interpreted as a physical "law" giving an exact correspondence, but as a statistical law of chance which is followed "on the average", observed frequencies showing random fluctuations from the theoretically expected values. the fact that aftershock sequences consist of independent random events does not imply that they are a simple poisson process. in a simple poisson process the probability of occurrence of one event in a given time interval is constant for all t; this is obviously not the case for aftershocks, where the probability of occurrence depends on the time elapsed since the main shock. but, as jeffreys [1938] and many others have established, apart from the common dependance upon the main shock, no further mutual relation is found within the sequence. in this section the statistical decay law of aftershock activity is estimated for the 15 sequences listed in table 1. the data have been grouped according to a procedure suggested by utsu [1962]. the origin time t0 of the main a statistical study of aftershock sequences figure 2. time distribution of aftershocks: aleutian islands 1957. figure 3. time distribution of aftershocks: kern county 1952. figure 4. time distribution of aftershocks: san francisco 1957. figure 5. time distribution of aftershocks: parkfield 1966. 48 shock has been taken as origin of the time axis, t0 = 0. the origin times ti of the aftershocks, obtained from the reference listed in table 1, have been expressed in terms of days after the main shock. the first day has been excluded from the analysis because of its possible incompleteness with respect to the number of shocks counted, due to the high frequency of aftershocks. usually, aftershocks occurring in the time interval 1 ⩽ t ⩽ 100 have been considered, unless a sequence comes to an end in a period of time shorter than 100 days. the time axis has been divided into logarithmically uniform intervals, such that their boundaries , satisfy the relation now, if ni is the number of aftershocks occurring in the time interval , the quantity represents the observed frequency per unit time interval. this observed frequency is associated with the centered value of the time interval concerned so that one obtains a set of points (ti, ni) in the (t, n)-plane. the data arranged in this fashion are shown in table 2, the first column representing the centered time, the second the number of shocks in the time interval concerned, and the third the observed frequency. in the sequences (4), (7), (10), (12), (13) and (15), in which the number of shocks in some of the original time intervals was zero, the time intervals have been grouped two by two and ni and ti have been calculated accordingly. the ( ti, ni)-points usually show an approximately linear trend on doubly logarithmic paper. consequently, it is reasonable to assume that the frequency of aftershocks per unit time n and the time t are related by an equation of the form (1) that is, omori's law. the commonest procedure for estimating the parameters a and β is the least squares method, which has been applied to the great majority of aftershock sequences whose time distribution has been investigated so far. accordingly, relation (1) is linearized by taking logarithms on both sides (2) then, setting the following model is obtained a statistical study of aftershock sequences table 5. comparison of hyperbolic and exponential decay of aftershock activity. log 0.1 , 0.1, ..., 20.t i ii = = ) n t n i i i= d ) 2t t t 1 i i i= + + ) ) ( )n t t= a blog ( ) log log .n t t= a blog ( ) , log , log ,n t y t x= = = =a a b b-) ) ;y x= +a b ) ) ti ) t t t1i i i=d -+ ) ) ) 49 that is, the expected value of y is a linear function of x. therefore i.e., the observation yi, consists of the expected value at the given xi plus a random fluctuation εi. in the method of least squares, the parameters a* and β* are chosen in such a way that the sum of squares of the vertical distances of the points from the regression line is minimized. for the linear model, this sum of squares is and the necessary conditions for ξ (a*, β*) to be minimum are from which one obtains (3) according to the gauss-markov theorem [graybill 1961], the estimates of the parameters calculated according to the least squares method will be unbiased and of maximum efficiency if, and only if, the linear hypothesis is such that the random fluctuation ε has zero mean and constant variance (independent of x). in other words, if the calculated regression line is to give the expected value of y for each x, the observed values must be uncorrelated, and the probability distribution of y for each x must be symmetric. in many cases, especially when the fluctuation ε can be considered to be the sum of many independent factors, the conditions of the gauss-markov theorem are satisfied and the probability distribution of y may be regarded as approximately normal for every x. in other cases, however, the matter is very debatable, particularly when a transformation of coordinates is performed in order to linearize the least square model. it turns out that, under reasonable assumptions, neither the original model (1) nor the linearized model (2) satisfy the conditions of the gauss-markov theorem. this has been noted, for instance, by page [1968b]. an appropriate method must take into account the probability distribution of n for each interval of time considered. on the other hand, it has been suggested [yule and kendall 1940] that the least squares method is approximately correct also when the conditions for its theoretical validity are not realized in practice. in order to clarify these matters, in the sequel we shall estimate the parameters in equation (1) by means of both the least squares method for the linearized model (2), and the maximum likelihood method [cramér 1946], which takes into account the distribution of n for each time interval. first of all, we analyze this distribution. in the following discussion it is assumed that the deviations of the observed values ni from the expected value n(t) reflect actual random fluctuations and that other contributions to such deviations are negligible. since the only other sources of deviations are errors of measurement, and these have been reduced to a minimum by excluding the first day after the main shock and by counting only welldefined aftershocks with m ⩾ m*, the assumption is most probably a statistical study of aftershock sequences figure 6. magnitude stability in time: aleutin islands. table 7. oscillations of mean magnitude. table 6. confidence limits on the decay parameter. ,y xi i i= + +a b f ) ) x,( ) y 1 1i k i i k i i 2 2p a b f a br r= = = = ) ) ) )^ h 0 , 0= = da dp db dp ) ) , 1 , 1 k x x k x y x y y x y k y x k x 1 2 1 2 1 1 1 1 1 i k i i k i i k i i i k i i k i i k i i k i = = = = b a b r r r r r r r = = = = = = = ) ) ) ` j z [ \ ] ]] ] ] 50 correct. it is also assumed that, if the number of shocks expected in the i-th interval is e (ni), the probability that the observed number is ni , is given by (4) i.e., the number of shocks in each time interval is given by a poisson distribution. the poisson distribution is the most fundamental distribution for such discrete variates as the number of shocks in a given time interval, and it has been postulated for the case of aftershocks by a number of authors, e.g., recently, by utsu [1962] and page [1968b]. the expected value e (ni) in the i-th interval is given by (5) where the approximation is introduced in order to avoid using the integral in equation (4); this is necessary because β is unknown and could be unity. the approximation has been checked numerically for some randomly selected samples and the error was found to be negligible. then relation (4) becomes (6) the principle of the method of maximum likelihood is to take estimates of the unknown parameters that maximize the probability of obtaining the observed sample. considering a sample of k independent values, each with a probability distribution p ( ni; a, β), the probability that the sample consists precisely of these k values is (7) the function l (a, β) is called the likelihood function. the necessary condition for l (a, β) to have a maximum is since 1n l (a, β) (where 1n stands for the natural logarithm) attains its maximum for the same values of aand β as l (a, β) itself, it is the function 1n l (a, β) which is commonly maximized. it follows from equations (6) and (7) a statistical study of aftershock sequences table 8. magnitude-frequency distribution (continues on next page). ; !p n e n n e n ei i i i n e n i i= -^^ ^ ^hh h h6 @ ( )e n n t dt t dt t ti t t i t t i 1 1 i i i i = = -a a d )b b+ + ) ) ) ) ^ h # # ; , !p n n t t e ti i i i ti i=a b a d )b a d ) b-^ ^h h ( , ) ( ; , )l p n 1i k i=a b a bp= 0 , 0l l= = da d db d 51 that the likelihood function in the present problem is i.e., from which one obtains the maximum likelihood estimates of a and β are obtained by solving the equations i.e., (8) equations (8) are the normal equations in the maximum likelihood method. from them one obtains, (9) i.e., (10) equation (10) must be solved for β, and then a can be obtained from (9). a program has been written to solve equation (10) by the secant method [ralston 1965]. the first approximation to the root of (10) and to the estimate of ahas been obtained by means of the least squares method for the linearized model, according to formulas (3). the estimates a1, β1 obtained by the least squares method, when compared with the maximum likelihood estimates, give an idea on how statistically reliable the least squares method is when the underlying assumptions are not met. the numerical results for the 15 sequences are summarized in table 3. from left to right, the columns indicate the sequence involved, the least square estimates a1, β1 and the maximum likelihood estimates a, β. the parameter which characterizes a sequence is the decay parameter β, which measures the rate of decay in time of the frequency of aftershocks. it can be seen that differences between β1 and β are present but not very large. the decay parameter is usually around unity. now, we proceed to check the validity of the timefrequency law. if the expected value of the frequency n varies in time according to equation (1); and using the approximation expressed by (5), the mean and the variance of the poissondistributed number of shocks are in each interval and therefore the standard deviation of n is a general theorem which holds for an arbitrary distribution with a second moment is tchebychev's theorem [cramér 1946]. it states that, if x is a random variable with mean e (x) and standard deviation d ( x), then the following inequality holds a statistical study of aftershock sequences ( , ) !l n t t e 1 t i k i i i ti i=a b a p d = )b a d ) b-^ h 1 ( , ) 1 !n l n n t t e 1 t i k i i i ti i=a b a r d = )b a d ) b-^ h; e 1 ( , ) 1 1 1 1 ! n l n n n n t n n t t t n n 1 1 1 1 1 i k i i k i i k i i i k i i i k i i=a b a b a r r r d r d r + = = = = = ) )b1 ( , ) 1 0 1 ( , ) 1 1 0 n l n t t n l n n t t t n t 1 1 1 1 i k i i k i i i k i i k i ii i = = = = da d a b a da d a b a r r d r r d + = = = = ) ) b b * 0 1 1 0 n t t n n t t t n t 1 1 1 1 i k i i k i i i k i i k i ii i = = a a r r d r r d = = = = ) ) b b * 1 1 t t n t t n t n n t 1 1 1 1 i k i i i k i i k i i i k i i i = =a r d r r d r = = = = ) )b b( ) 1 1 0 f n t t n t n n t t t 1 1 1 1 i k i i k i i i k i i k i i i i = = b r r d r r d = = = = ) ) b b ( )e n n n t t t tv= = = ad d) )b-^ ^h h t t .d n = a d ) b-^ h where k is an arbitrary positive number. in other words, the probability that x assumes values outside the interval e (x) ± k d ( x) is less than 1/k². conversely, from the viewpoint of sampling, in the long run less than (100/k²)% of the values assumed by x will fall outside the interval. the application of tchebychev's inequality to check the validity of the time-frequency law has two limits, namely, it yields rather weak conditions, and the sample size is small. nevertheless it gives at reasonably safe criterion for rejecting the validity of the law for sequences that show too wide fluctuations. regarding the observed values of n as the result of random sampling from a population whose expected value varies with time according to (1), and choosing k = 2, tchebychev's inequality takes the form that is, in the long run less than 25% of the observed values ni should fall outside the interval e (n) ± 2 d (n). if this condition is not satisfied, the assumption regarding the variation with time of the expected value n(t) must be rejected. in order to determine a confidence band according to tchebychev's inequality, therefore, the quantities e (n) + 2 d (n), e (n) – 2 d (n) have been computed at selected ti, i = 1, 2, ..., k; and compared with the observed ni. table 4 gives the results, showing in the columns from left to right the sequence, the total number of data points, the number of points outside the confidence band, and their percentage. consequently, according to the selected criterion, sequenze (10), (12), (13) and (14) do not follow the assumed timefrequency law, inasmuch as the observed frequency cannot be explained only in terms of random fluctuations from the law. the other 11 sequences appear to follow the law and the fit is generally fairly good. .all the sequences which show considerable departures from the assumed law have occurred in the region of greece; this fact might have some geotectonic significance. however, there exists the possibility that relatively poor instrumentation plays a part in some apparent irregularities. sequence (14) originated under peculiar conditions; the frequency of aftershocks in it appears to be correlated to the variations in the water loading of a nearby artificial lake [comninakis et al. 1968]. figures 1 to 5 display on doubly logarithmic paper the results for some of the 11 sequences which appear to decay according to the postulated law. the dots represent the data points, the full line the fitted n(t), and the broken lines the confidence limits. it is interesting to note that most decay phenomena in physics are exponential, whereas the decay of aftershock activity with time appears to be hyperbolic. in order to check rapidly the possibility of exponential decay, the model has been linearized, and the parameters a2 and β2 have been estimated by the least squares method. then the mean square deviation (standard error of estimate) of the data points from the fitted curve has been calculated by the formula (where k is the number of data points) and compared with a statistical study of aftershock sequences 52 table 9. estimation of the parameter b in the magnitude-frequency law. table 10. validity of the magnitude-frequency law. 1p x e x k d x k2; ; h g^ ^h h" , 2 4 1p n t t t ; ; h ga a d ) b b -' 1 ( )n t e2 t2a= b2 1 log log logs k n t e 2 1 2e i k i i2 2= -a br = l ^ h6 @ 53 the mean square deviation from the fitted time-frequency law that is, (where a1 and β1 are the least square estimates). if the assumed time-frequency law represents a better fit, in the least square sense, than the exponential decay. the results are summarized in table 5. it can be seen that in all but three cases, namely, sequences (7), (10), and (15). it can be concluded that the frequency of aftershocks usually decreases in time hyperbolically and not exponentially. confidence limits on the decay parameter the parameter β appearing in equation (1) is related to the rate of decay of aftershock activity with time and is therefore an important characteristic of the sequence under consideration. consequently, it would be interesting to see whether the differences in the computed decay parameters for different sequences are significant or not. confidence limits on β, however, cannot be calculated according to the usual least square procedure, which assumes the fluctuations to have a normal distribution. an approximate procedure for calculating confidence limits on β can be based on the addition theorem for the poisson distribution [cramér 1946]. since in this procedure the parameter a is assumed to be known exactly, it will yield no more than an indication of the range in which the real value of β is likely to fall. the addition theorem for the poisson distribution states that, if ni, i = 1, 2, ..., k, is a sequence of stochastically independent and poisson-distributed random variables with expected values ei = e (ni), then the sum will be poisson-distributed with expected value in the present case, ni being the number of shocks in the i-th time interval, the expected values are and the value of e (x) for each of the 11 sequences which follow the time-frequency law is such that the distribution of x can be approximated by the normal distribution. therefore the standardized variable is approximately normally distributed with e (x*) = 0, v (x*) = 1. therefore, the probability that x* assumes a value in the interval (λ1, λ2) is where φ (x*) is the normal distribution function. in particolar then the approximate 95% confidence limits on the decay parameter are obtained by solving for β the equations that is, recalling the definition of x*, a statistical study of aftershock sequences figure 7. magnitude-frequency distribution: aleutian islands 1957. figure 8. magnitude-frequency distribution: kern county 1952. ( )n t t1 1a= b2 1 log log logs k n t t 2 1 1e i k i i i1 2= a br = ^ h6 @ e t ti i i= a d )be x t t . 1i k i i= a r d= )b-^ h x d x x e x = -) ^ ^ h h ( ) ( )p x1 2 2 1=g gm m m mu u)" , 1.96 1.96 .9 .p x 0 5=g g)" , 1.96x = !) s s2 2e e1 l s s2 2e e1 l x n 1i k i= r= e x e 1i k i= r=^ h naturally, the central value of x* is obtained when a, β are given by the maximum likelihood estimates. assuming the value of a to be known exactly, the above equations take the form (11) equations (11) have been solved by the secant method. the results for the 11 sequences with time-frequency law of the form given by (1) are shown in table 6. it can be seen that the 95% confidence limits on the maximum likelihood estimate always contain the least square estimate of β, which therefore appears to be a good approximation to the real value of the decay parameter. the values listed in table 6 must not be considered as exact. the limits rounded to the second decimal digit are probably fairly reliable. most β-values cluster around the 0.91.2 range. sequence (11) stands by itself, showing a very rapid decay in activity. also sequence (9) has a decay coefficient somewhat higher than usual. this fact hints at the possibility that sequences in greece decay more rapidly; but no conclusion can be reached with such a small sample size. magnitude stability in time when the characteristics of aftershock sequences with respect to magnitude are examined, two sources of error are added to the possibility of the incompleteness of data; namely, lack of accuracy in magnitude determination, and confusion between different magnitude scales. unfortunately, authors sometimes do not specify which scale they are using. when one single sequence is being examined, no problems arise, because the data are consistent within the sequence; if, however, results for different sequences are to be compared, the use of different magnitude scales may affect the conclusions. in the sequel, the various "local" magnitude scales, for the purposes of comparison of results among sequences, have been assimilated to m. thus the only distinction left is between m and m; the latter scale has been used only in sequence (1). two aspects of the sequences have been examined in detail, namely, the variation of aftershock magnitudes with time, and the magnitudefrequency distribution. for all sequences except (1), where 1 ⩽ t ⩽100 days, the data for t ⩽100 days have been included in the analysis. we first consider the distribution of magnitude with respect to time. the overall mean magnitude, m, has been calculated for each sequences as where k is the total number of aftershocks in the sequence. then the mean magnitude, m´, of each group of 10 successive aftershocks is computed, thereby eliminating large individual fluctuations. in almost all the sequences considered the mean magnitude m´ oscillates about during the whole length of the sequence and no appreciable decrease with time is detectable for t ⩽ 100 days. when a sequence lasts less than 100 days, the mean magnitude is stable throughout the sequence. sometimes higher values of m´ are observed in the first few hours after the main shock, a statistical study of aftershock sequences 54 figure 9. magnitude-frequency distribution: san francisco 1957. figure 10. magnitude-frequency distribution: parkfield 1966. 1.96 t t n t t 1 1 1 i k i i i k i i k i i = ! a a r d r r d= = = ) ) b b 1.96 0 1.96 0 . n t t t t n t t t t 1 1 1 1 1 1 i k i i k i i i k i i i k i i k i i i k i i = = a a a a r r d r d r r d r d + = = = = = = ) ) ) ) b b b b * 1 , 1, 2, ...,m k m i k1i k i= =r= m 55 but this can almost certainly be attributed to the fact that some shocks with m ⩾ m* are not detected when the frequency is very large. figure 6 gives an example. the dotted line represents . thus, the law of magnitude stability in aftershock sequences (first proposed by lomnitz 1966) is fully confirmed; during an aftershock sequence the mean magnitude of the shocks is constant in time. consequently, the decrease of seismic activity with time is solely due to the decrease in aftershock frequency. moreover, the fluctuations of m´ about the overall mean magnitude are not very wide. table 7 summarizes the results for the sequences with a larger number of shocks. from left to right, the first column indicates the sequence, the second the overall mean magnitude, the third the number of calculated m í -points, and the fourth the percentage of such points which fall within the interval ± 0.20. it is to be noted that also the sequences with a lesser numher of aftershocks and not included in the table show remarkable magnitude stability. the only sequence in which m´ shows a decreasing trend with time is sequence (11). in all the others, m´ shows only random fluctuations from and the law of magnitude stability in time is satisfied. magnitude-frequency distribution the frequency data for the 15 sequences of table 1 are listed in table 8. the first column gives the centered value of magnitude in the interval concerned (m ± 0.05); n(m) is the frequency; and n (m) the cumulative frequency. for brevity, intervals in which the frequency was zero have been omitted from the table. the magnitudes in sequence (1) are in the m-scale. the most commonly accepted form for the magnitude-frequency distribution, in case of both independent seismic events and aftershock sequences, is (12) where log is the logarithm to the base 10 and n(m) is the number of shocks with m ± dm [gutenberg and richter 1954]. equation (12) is to be regarded as expressing a statistical relationship. usually, the coefficients a and b have been calculated according to the least squares method. suzuki [1958] has pointed out that this is not rigorous, because log n(m) is not symmetrically distributed with uniform variance for each magnitude interval δm. in this section we shall define n(m) is such a way that log n(m) = anot when m= 0, as in equation (12), but when m= m*, where m* is the minimum detectable magnitude in the sequence. then the magnitude-frequency law takes the form (13) converting to natural logarithms one obtains (14) where (15) therefore normalizing, i.e., imposing the condition that one has i.e., γ = b´. thus we assume that the probability distribution of m takes the form (16) a procedure for estimating the parameter b´ in (16) can be derived as follows. the mean of the distribution is approximating the population mean by the sample mean , given by where k is the total number of shocks, one has a statistical study of aftershock sequences figure 11. magnitude-frequency distribution: cremasta 1966. log ( )n m a b m= log ( ) ( ) ,n m a b m m m m m .h= ) ) 1n ( ) ( )n m a b m m m= )l l log , loga e a b e b = =l l ( )n m e e e , e .( ) ( )m m b m m aa b c c= = =) )l l l l ( ) 1n m dm m = 3 ) # 1e dm b ( )b m m m c c = = 3 ) ) l l# ( ) ,n m b e m m .( )b m m h= ))l l 1m k m1i k i= r= ( ) 1e m m n m dm m b . m = = +) 3 ) l ^ h # m m m m m i.e., (17) the above procedure is but a particular case of the timehonored method of moments, introduced by k. pearson and his school [cramér 1946]. formula (17) was also proposed by utsu at a meeting of the seismological society of japan (reported by aki 1965). the estimation of the parameter b´ given by (17) has been proved by aki [1965] to be equivalent to the maximum likelihood estimate, and therefore has several desirable statistical properties. table 9 gives the results of calculations. from left to right, the columns indicate sequence, sample mean , minimum magnitude m*, and the estimations of the parameters b´, b appearing in relations (16) and (13) respectively . the estimate of b´ is given by (17), and b is given by (15). the minimum magnitude m* has been taken to be 0.05 units less than the values given in table 8 because the value of m approximated to one decimal could actually come from anywhere in the interval m ± 0.05. it can be seen that b is usually slightly less than unity. the basic idea for examining the observed fluctuations from the magnitude-frequency law is as in the case of the time distribution of aftershocks. suzuki [1958], among others, has argued that the number of shocks with m ± dm must follow a poisson distribution. accordingly, the problem is that of checking whether the observed deviation can be explained in terms of random fluctuations from the law. it is more convenient to consider the cumulative distribution because individual large fluctuations in small intervals, possibly due to inaccurate magnitude determination, are smoothed out in this fashion, and moreover the normal approximation can be employed. the basic idea of the method is due to suzuki [1958]. some minor modifications have been introduced, and the normal approximation to the poisson distribution, instead of the poisson distribution itself, has been used. from equation (16), it follows that the cumulative distribution function of magnitude has the form assuming that the total number of aftershocks in a sequence, k, coincides with the theoretical value for m = m*, the expected value of the cumulative frequency at various m > m* is given by which, when transformed by taking logarithms on both sides, is a straight line on semi-logarithmic paper, with slope equal to –b if the logarithms are to the base 10. now, to each magnitude range there corresponds a poisson-distributed number of shocks. the cumulative number of shocks at a given magnitude m, therefore, is the summation of independent samples taken from each of the poisson distributions corresponding to magnitudes greater or equal to m. according to the addition theorem for the poisson distribution, such a cumulative number will also be poisson-distributed. if the expected value is large enough, say, n (m) ⩾10 for all intervals, the poisson distribution can be approximated by the normal distribution with mean n(m) and standard deviation . it is then possible to calculate the fiducial interval beyond which fluctuations are expected with a probability smaller than 5% (2.5% on each side). the limits of the interval such that are, for the normal distribution, ; i.e., in the present case, . accordingly, the above quantities have been calculated at then, by joining all points of ordinates respectively, one obtains a confidence band which should contain approximately 95% of the data points if they come from a population whose expected value is given by the magnitude-frequency law as expressed by (18). the rightmost interval (say, from mj to ∞) has always be chosen in such a way that n ( m) ⩾ 10. table 10 gives the results. it follows that sequences (1), (3), (9), (10), (12) and (13) do not appear to be governed by the assumed magnitude-frequency law. when these sequences are examined one by one, however, it is seen that several circumstances tend to decrease the weight that must a statistical study of aftershock sequences 56 table 11. confidence limits on the frequency parameter. 1m m b = +) l 1b m m = ) l ( )m n m dm ef ( )b m m m = = 3 )l^ h # m k en ( )b m m= )l^ h 0.95 ,p n1 2g g -m m" , 0.1 , 1, 2, ..., .m m i i ki = =+ ) 2 and 2 ,m n m n m n mn i i i i+ -^ ^ ^ ^h h h h m n m^ h 2 ne n d!^ ^h h 2n m n m!^ ^h h 57 be assigned to their apparent irregularity. for sequence (1) the body-wave magnitudes as given by the u.s. coast and geodetic survey have been employed. page [1968b] found them inaccurate and modified them on the basis of records of five selected stations, thereby obtaining a better fit; however, no test on the fluctuations was performed in his study. of the sequences occurring in greece, it is worth noting that sequence (14), which was studied with a network improved with respect to the others, appears to follow the assumed law; this fact supports indirectly the conclusion that the irregular behavior of its aftershock frequency is due to changing local stress conditions. sequences (10), (12) and (13) are also irregular with respect to aftershock frequency in time. this fact points to the likelihood that such irregularities are due to poor data, although no definite statement can be made. it can therefore be concluded that the magnitudefrequency law (18) is statistically followed by the large majority of the sequenze under consideration. figures 7 to 11 give some examples; the logarithmic ordinate represent the cumulative frequency, the linear abscissa the magnitude. confidence limits on the frequency parameter differences between b-values may be significant or not. in order to decide the question, confidence limits must be set on the estimated value of the parameter. we shall follow in this matter a procedure suggested by aki [1965]. given a sample of k shocks with magnitudes mi, i = 1, ..., k, let yi and y be defined by where n(m) is given by formula (16). clearly for all i, and therefore by the central limit theorem [cramér 1946] the distribution of y will be approximately normal if k is sufficiently large. since it follows that the variable is approximately normally distributed with mean 0 and standard deviation 1; therefore accordingly, the 95% confidence limits on b´ are obtained by solving the inequality which gives i.e., table 11 summarizes the results for the 9 sequences where the assumed magnitude-frequency law appears to hold. according to usage, the parameter b, instead of b´, has been employed. it can be seen that the b-values cluster around the interval 0.8-1.0. although sequence (2), which occurred along an active island arc, shows an anomalously high value of the frequency parameter, the sample size is too small to support the contention that b has some geotectonic significance. conclusion the basic statistical laws describing the phenomenology of aftershock sequences are confirmed by the present study. these laws are as follows: (1) aftershock sequences are generally crustal events, although deeper ones also occur; (2) the frequency of aftershock occurrence within the same sequence decays in time according to the law where the decay parameter β is approximately equal to, or slightly greater than, unity; (3) the aftershock magnitudes, apart from individual fluctuations, show stability in time to the end of the sequence; (4) the frequency-distribution of magnitude in a sequence is of exponential form where the frequency parameter b = b´ log e is usually slightly less than unity the importance of the mode of grouping the data in a statistical analysis makes it desirable to introduce a standardized procedure. furthermore, an appropriate statistical method must be employed, and the hypotheses a statistical study of aftershock sequences 1n ( ) b n myi i= d d l y y 1i k i= r= 1y b m m= + ) l ( ) 0e y n m dmy m = = 3 ) ^ h # ( ) 1v y y n m dm b 2 2 m = = 3 ) l ^ h # 0 , v y b ke y 2= = l ^ ^h h y y e y k b yy d == -) l^ ^ h h 1.96 1.96 0.95 .p y =g g)" , 1.96 1.96 k b y g gl 1.96 1 1.96 k b y b m m 1i k ig gr-= )l l ` j 1 1.96 / 1 1.96 / m m k b m m k g g + ) ) l ( ) , 100n t tt ha= bm k en ( )b m m= )l^ h involved must be checked. a systematic treatment of data greatly increases the reliability of the results. it also turns out that the least squares method, when the observed values are uncorrelated, yields rather satisfactory results even if the conditions for its theoretical validity are not met. acknowledgements. the author wishes to express his thanks to professor a. e. scheidegger of the university of illinois at urbana, who kindly discussed some questions which arose during the preparation of this paper and reviewed the manuscript. numerical calculations were performed on the university of illinois ibm system/360 computer, operating under a grant from the united states national science foundation. the support received from this agency is gratefully acknowledged. references aki, k. (1965). maximum likelihood estimate of b in the formula log n = a – bm and its confidence limits, bull. earthq. res. inst., 43, 237-239. båth, m. and h. benioff (1958). the aftershock sequence of the kamchatka earthquake of november 4, 1952, bull. seism. soc. am., 48, 1-15. benioff, h. (1951). earthquakes and rock creep, i, bull. seism. soc. am., 41, 31-62. comninakis, p., j. drakopoulos, g. moumoulidis and b. papazachos (1968). foreshock and aftershock sequences of the cremasta earthquake and their relation to the waterloading of the cremasta artificial lake, ann. geofisica, 21, 39-71. cramér, h. (1946). mathematical methods of statistics, princeton. duda, s. j. (1962). phänomenologische untersuchung einer nachbebenserie aus dern gebiet der alëuteninseln, freib. forschungshefte, c-132, 5-90. graybill, f. a. (1961). an introduction to linear statistical models, i, new york. gutenberg, b. and c. f. richter (1954). seismicity of the earth and associated phenomena, 2nd ed., princeton. iosif, t. and c. radu (1961). elastic strain characteristics in the deep-focus earthquakes of the vrancea region, stud. cerc. astr. seism, 2, 269-285 (in romanian). jeffreys, sir h. (1938). aftershocks and periodicity in earthquakes, gerl. beitr geophys., 53, 111-139. lomnitz, c. (1966). magnitude stability in earthquake sequences, bull. seism. soc. am., 56, 247-249. lukk, a. a. (1968). the aftershock sequence of the dzhurm deep-focus earthquake of march 14, 1965, izv. earth. phys., 5, 83-85 (in russian). matuzawa, t. (1964). study of earthquakes, uno shoten, tokio. mcevilly, t. v., w. h. bakun and k. b. casaday (1967). the parlefield, california, earthquakes of 1966, bull. seism. soc. am., 57, 1221-1244. mogi, k. (1962). on the time distribution of aftershocks accompanying the recent major earthquakes in and near japan, bull. earthq. res. inst., 40, 107-124. page, r. (1968a). focal depths of aftershocks, j. geophys. res., 73, 3897-3903. page, r. (1968b). aftershocks and microaftershocks of the great alaska earthquake of 1964, bull. seism. soc. am., 58, 1131-1168. papazachos, b., n. delibasis, n. liapis, g. moumoulidis and g. purcaru (1967). aftershock sequences of some large earthquakes in the region of greece, ann. geofisica, 20, 1-93. ralston, a. (1965). a first course in numerical analysis, new york. richter, c. f. (1955). foreshocks and aftershocks, in earthquakes in kern county, california, during 1952, bull. cal. div. mines., 171, 177-197. richter, c. f., c. r. allen and j. m. nordquist (1958). the desert hot springs earthquakes and their tectonic environment, bull. seism. soc. am., 48, 315-337. suzuki, z. (1958). a statistical study of the occurrence of small earthquakes, iii, sci. rep. tohoku univ., 5th series, geophys., 10, 15-27. tocher, d. (1959). seismographic results from the 1957 san francisco earthquake, in san francisco earthquakes of march 1957, cal. div. mines sp. rep., 57, 59-71. udias, a. (1965). a study of the aftershocks and focal mechanism of the salinas-watsonville earthquakes of august 31 and septernber 14, 1963, bull. seism. soc. am., 55, 85-106. utsu, t. (1962). on the nature of three alaskan aftershock sequences of 1957 and 1958, bull. seism. soc. am., 52, 279-297. yule, g. u. and m. g kendall. (1940). an introduction to the theory of statistics, 12th ed., london. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. a statistical study of aftershock sequences 58 pondrelli_corretto_layout 6 615 quick regional centroid moment tensor solutions for the emilia 2012 (northern italy) seismic sequence silvia pondrelli1,*, simone salimbeni1, paolo perfetti, peter danecek2 1 istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy 2 instituto andaluz de geofísica, universidad de granada, campus universitario de cartuja, granada, spain annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6146 1. introduction seismic moment tensors are one of the most relevant pieces of information about earthquakes for the present day. the moment magnitude, mw, is derived from the seismic moment tensor, and is considered worldwide as the standard measure of earthquake dimensions. from the moment tensor, or its best double couple, i.e., the focal mechanism, it is possible to immediately understand the tectonic style, and thus the kind of activated fault. the computation of the moment tensor immediately after an earthquake has occurred can be used to rapidly provide shaking distributions, eventually including it in a shaking map computation, or it can be used in source propagation studies to infer the presence of rupture directivity. over the last 15 years, regional centroid moment tensors (rcmts) have been computed regularly and have contributed to the european-mediterranean rcmt catalog [pondrelli et al. 2002, 2004, 2006, 2007, 2011] (http://www. bo.ingv.it/rcmt). the rcmt is obtained by inverting for intermediate surface waves filtered in the interval of 35 s to 60 s as a function of the magnitude; body waves are also included in the inversion, when favorable, and they are modeled separately, but simultaneously [pondrelli et al. 2011]. seismograms recorded at regional distances contain the best signals to invert for the source parameters of moderate magnitude earthquakes. for greater magnitude events, data from much more distant stations are also used. body waves are modeled by summation of the earth normal modes added with three-dimensional heterogeneous mantle structure [woodhouse and dziewonski 1984], while for surface waves, global phase velocity maps are used [ekström et al. 1997]. during the last few years, we have developed an automatic python procedure, named pypaver, which helps to shorten the quick rcmt (qrcmt) computation times [pondrelli et al. 2012], and it was successfully used during the emilia 2012 (northern italy) seismic sequence. in this study, we first show our qrcmt solutions, and then we describe the motivation for wide use of automatic procedures. we also explain why a manual review of seismic moment tensor solutions for most earthquakes analyzed is still preferred before publication. indeed, the evidence that qrcmts are stable solutions is given by the comparison with seismic moment tensors produced by other agencies and with other computation methods. 2. quick rcmt solutions for the emilia seismic sequence during the night between may 19 and 20, 2012, the emilia region (northern italy) started to be shaken by a seismic sequence. the first major event, of ml 5.9 (mw 6.1) that occurred at 02:03 utc time was preceded by a ml 4.1 (mw 4.3) earthquake, about 2 h before. during the same night, several events with ml between 4.5 and 5.1 also occurred (http://iside.rm.ingv.it/) [see other reports in this annals of geophysics special volume]. the seismic sequence had another increase in activity on may 29, 2012, with a ml 5.8 (mw 5.9) event in the morning, at 07:00 utc, followed by two events with ml 5.3 and 5.2, one at 10:55 utc and the other 5 min later, respectively. the last ml >5 event occurred in june 3rd, with ml 5.1 (mw 4.9). in a month, 30 earthquakes with ml >4.0 were recorded, and of the total, about 2,500 events were located during the following three months (http://iside.rm.ingv.it/). in figure 1, the qrcmt solutions that we were able to compute are mapped; the colored dots represent other events belonging to this seismic sequence. all of the focal mechanisms were thrust, in some cases with different, but small, percentages of strike-slip components. in table 1, the source parameters for all of the qrcmts are reported. the recomputed depth is also included, even if for the qrcmt it can be considered as an approximate evaluation, overall for shallow earthquakes (depth <10 km), as in this case. the mw values we obtained are in very good agreement with those from other agencies, and they showed regular behavior with article history received july 24, 2012; accepted september 14, 2012. subject classification: earthquake source, seismic moment tensor, wave analysis, data processing. 2012 emilia earthquakes pondrelli et al. 616 event id day (yyyy-mm-dd) time (hh:mm:ss utc) latitude (˚n) longitude (˚e) depth (km) ml mw m0 (nm) strike 1 dip 1 slip 1 strike 2 dip 2 slip 2 1 2012-05-19 23:13:27 44.90 11.26 13.6 4.1 4.29 3.37e15 85 32 50 310 66 112 2 2012-05-20 02:03:53 44.89 11.23 11.4 5.9 6.11 1.81e18 109 30 99 279 60 85 3 2012-05-20 03:02:50 45.05 11.17 14.1 4.9 5.05 4.60e16 103 27 89 284 63 91 4 2012-05-20 13:18:02 44.83 11.49 10.0 5.1 5.18 7.25e16 100 37 64 312 57 109 5 2012-05-20 17:37:14 44.88 11.38 12.9 4.5 4.52 0.76e16 235 43 72 79 50 106 6 2012-05-21 16:37:31 44.85 11.35 10.0 4.1 4.13 1.93e15 243 37 92 60 53 89 7 2012-05-23 21:41:18 44.85 11.36 11.9 4.3 4.13 1.96e15 77 43 63 292 52 113 8 2012-05-29 07:00:03 45.00 11.12 11.1 5.8 5.96 1.07e18 110 20 103 276 71 85 9 2012-05-29 08:25:51 45.02 10.90 12.0 4.5 4.69 1.36e16 105 21 94 281 69 88 10 2012-05-29 10:55:57 44.99 10.97 11.4 5.3 5.53 2.42e17 112 32 101 279 59 83 11 2012-06-03 19:20:43 44.96 10.96 10.0 5.1 4.89 2.68e16 92 16 37 326 81 103 12 2012-06-06 04:08:31 44.45 12.26 25.4 4.5 4.34 4.00e15 28 66 -10 122 81 -156 13 2012-06-12 01:48:36 44.88 10.89 13.7 4.3 4.19 2.37e15 56 52 39 299 60 135 figure 1. map showing the qrcmts (red, focal mechanisms) and their p axes (inset, lower left). green, seismicity of seismic sequence with a m <3.0; yellow, seismicity of seismic sequence with m ≥3.0. the small black focal mechanisms are all rcmts from the european mediterranean catalog for seismicity before may 2012. black dots (with dimensions proportional to magnitude), previous seismicity from http://iside.rm.ingv.it. table 1. source parameters for the events of the emilia sequence for which a qrcmt was computed. 617 respect to the ml, as we had already seen for the l'aquila seismic sequence [pondrelli et al. 2010]. strong evidence for the stability of the qrcmts is provided by a comparison with all of the available quick solutions, which is possible at the european mediterranean seismological centre (emsc) website, where all of the seismic moment tensors for the same earthquake are gathered into a dedicated database (http://www.emsc-csem.org/earth quake/tensors.php), or through maps for the single events (e.g., http://www.emsc-csem.org/earthquake/earthquake. php?id=267440#map). for the may 20, 2012, ml 5.9 event, solutions from usgs, geofon (gfz), global cmt (harv), geoazur (azur) and california poly pomona (cpp) appear to be in strong agreement with our qrcmt, in terms of the focal mechanisms and mw values (named as ingv on the map and in the table on the emcs webpage). this similarity is consistent for all of the events of the sequence with magnitudes >5.0. for smaller magnitude earthquakes, the only possible comparison is with the time-domain moment tensor (tdmt) computed by the ingv (http://cnt.rm.ingv. it/tdmt.html) [scognamiglio et al. 2009]. we used different stations and a signal characterized by a different frequency, although usually we have strong similarities [scognamiglio et al. 2009], as for instance in the solutions for the may 20, 2012, 17:37 utc, ml 4.5 event: both the qrcmt and the tdmt show rotation towards the nw of the p axis (http://cnt.rm. ingv.it/data_id_old//7222922570/map_dmt_review.pdf ). the thrust mechanisms, as for those determined for the emilia earthquakes, are in agreement with the compressive tectonic structures that belong to the apennines and are buried below the po plain sediments [boccaletti et al. 2004, 2011, and references therein], in particular with the fold and thrust structures that are commonly named as the emilia arc, the ferrara-romagna arc and the adriatic arc (figure 1, white toothed lines) [pieri and groppi 1981, montone and mariucci 1999, carminati et al. 2010, lavecchia et al. 2004, scrocca 2006]. the shape of these arcs helps in the understanding of the variation in the p-axes directions of the qrcmt solutions (figure 1, inset): major events show pure, low angle, thrust mechanisms, with a p axis pointing towards the north. some aftershocks clearly rotate with respect to this dominant northwards direction; in particular, those pointing towards the nw that are located close to the ne-striking part of the buried thrust front (figure 1). most of the faults that populate the folds and thrusts buried beneath the po plain are high-angle structures, although there are also some low angle faults, in agreement with most of the qrcmt focal mechanisms. the seismic moment tensor of one more event, which occurred on june 6, 2012, is considered out of sequence, because it was located 100 km east of the principally struck region; it has been taken into account anyway, because it occurred along one of the tectonic structures related to those that were activated during the seismic sequence itself. it is pure strike-slip, and it can be interpreted as occurring at the crossing point of the ferrara-romagna arc and the adriatic arc (figure 1). 3. qrcmt computation technique: automatic and rapidly revised solutions we obtained a robust rapidly revised solution for 13 earthquakes of the sequence (table 2). all of these have been computed automatically previously, following a process that was activated by alert emails, and was mainly structured into qrcmts of 2012 emilia seismic sequence event id exp mrr s.e. mss s.e. mee s.e. mrs s.e. mre s.e. mse s.e. 1 15 2.51 0.52 –2.39 0.28 –0.12 0.31 –1.12 0.32 2.0 0.50 0.59 0.19 2 18 1.51 0.06 –1.52 0.04 0.20 0.03 –0.92 0.13 0.06 0.10 0.40 0.03 3 16 3.39 0.31 –3.84 0.18 0.44 0.17 –2.59 0.31 0.66 0.31 1.09 0.12 4 16 6.33 0.18 –5.39 0.17 –0.94 0.17 –1.22 0.57 2.84 0.50 2.83 0.16 5 16 0.75 0.10 –0.62 0.07 –0.14 0.05 0.15 0.09 –0.14 0.09 –0.25 0.05 6 15 1.80 0.16 –1.46 0.13 –0.34 0.13 0.43 0.39 0.27 0.39 –0.87 0.13 7 15 1.75 0.31 –1.92 0.22 0.17 0.21 –0.25 0.36 0.61 0.61 0.17 0.10 8 18 0.63 0.02 –0.66 0.02 0.03 0.01 –0.83 0.08 0.07 0.04 0.16 0.01 9 16 0.80 0.22 –0.96 0.13 0.16 0.13 –0.99 0.31 0.19 0.28 0.26 0.09 10 17 2.07 0.12 –2.04 0.08 –0.04 0.06 –1.13 0.19 0.05 0.21 0.59 0.05 11 16 0.88 0.15 –0.84 0.09 –0.04 0.16 –1.26 0.33 2.14 0.43 0.52 0.10 12 15 –0.27 0.58 –2.97 0.35 3.24 0.42 –1.78 0.42 0.23 0.54 –1.78 0.20 13 15 1.40 0.40 –1.95 0.25 0.55 0.23 –0.30 0.22 1.71 0.50 –0.19 0.14 table 2. exponent and moment tensor elements, together with their standard errors. a data-managing part and a computational part [pondrelli et al 2012]. qrcmt solutions obtained automatically are successively reviewed by an operator, who checks the waveform selection and reliability of the fits and the quality of the centroid location. if necessary, the operator can fix the epicenter coordinates and depth. usually, a repeated inversion over different depths values is also carried out, mainly when working with shallow events, as in this case, to find the centroid depth that generates the best fit between the waveforms and the synthetic seismograms. we have been working on this procedure since the l'aquila sequence, as we are confident that the qrcmt solutions can have a relevant contribution to the rapid assessment of earthquake scenarios. during this emilia sequence, we tested some of the positive results. all of the solutions were published on the web within 1 h of the earthquake occurrence. this timing arises from four phases: (i) the alert email; (ii) data recovery; (iii) computational time; and (iv) visual inspection and revision. on average, during this sequence, computations took 7-8 min (including seismogram recovery; e.g. points (ii) and (iii) above) which is the typical timing we observed for isolated moderate magnitude events. the manual revision took from 10 min to 20 min (e.g., point (iv)); the rest of time was mainly taken up by communication and network traffic jams (e.g., point (i)). indeed, for the may 20, 2012, ml 5.9 and 29 may, 2012, ml 5.8 events, and for the successive aftershocks, on average, the whole of the process took between 70 min and 90 min, which were mainly spent waiting for the alert email and the data recovery. while there are technical solutions to shorten the seismogram time collection – and we are testing some of these – we still did not find any solution to cut down the alert time. we receive alert emails from ingv for italy and from emsc for the rest of mediterranean region. during the emilia sequence, we received the alert email after 810 min for all of the events, but for the strongest one, we had to wait more than 20 min. if the qrcmt publishing time cannot be shortened, we prefer to focus on the high quality of our solutions. taking into account that we commonly study low to moderate (between 4.0 and 5.5) magnitude events, we choose to manually revise the solutions before the web publication, overall considering that this part of the procedure takes about the same time that we wait for the alerts. the manual revision often solves all of the data instabilities that affect mainly the qrcmt computation of low to moderate magnitude events, due to seismogram selection, to better satisfying a good azimuthal distribution of stations, or to choose the best filtering in relation to the magnitude and epicenter–station distance. an example of different conditions of station azimuthal distributions combined with epicenter–station distances is shown in figure 2. for the minor event with ml 4.1 (mw 4.3) of may 19, 2012, that preceded the mainshock, the azimuthal distribution is relatively uneven, although all of the stations have a real regional distance of between 1 and 10 degrees (figure 2). on the other hand, the azimuthal distribution for the main shock is better, because it was completed by some farther away stations that cover the western back azimuths. this better homogeneity around the event is guarpondrelli et al. 618 date (yyyy-mm-dd) time (hh:mm:ss utc) mw automatic rms revised rms 2012-05-19 23:13:27 4.29 0.765 0.324 2012-05-20 02:03:53 6.11 0.250 0.150 2012-05-20 03:02:50 5.05 0.444 0.186 2012-05-20 13:18:02 5.18 0.646 0.095 2012-05-20 17:37:14 4.52 0.729 0.274 2012-05-21 16:37:31 4.13 0.920 0.288 2012-05-23 21:41:18 4.13 0.737 0.265 2012-05-29 07:00:03 5.96 0.314 0.127 2012-05-29 08:25:51 4.69 0.683 0.298 2012-05-29 10:55:57 5.53 0.306 0.091 2012-06-03 19:20:43 4.89 0.613 0.367 2012-06-06 04:08:31 4.34 0.741 0.378 2012-06-12 01:48:36 4.19 0.718 0.370 figure 2. station distribution around the epicenter for the may 19, 2012, ml 4.1 and may 20, 2012, ml 5.9 events, to compare the station distribution and availability a short time after the event with changing magnitude. different colors represent different station–epicenter distances, in degrees. table 3. information on the automatic and rapidly revised solutions. 619 anteed by the greater magnitude, with a better signal-tonoise ratio at greater distance, which allows lower filtering. this is demonstrated also by some of the examples of comparisons between data and synthetic waveforms in figure 3, where for the same distance, we need different filtering to obtain the best signal-to-noise ratio with respect to the best fit. for the mainshock, a cut off between 50 s and 150 s is enough, while for lower magnitude events, we have to filter the signal between 35 s and 75 s, which obtains a noisier waveform, but a good fit too (figure 3). what we observed during the emilia seismic sequence through checking all of the automated procedure timing and results is that we could have published the automatically computed qrcmts for the events with magnitudes >5.5. table 3 gives the root mean square (r.m.s.) values computed on the entire set of seismograms that we inverted, for each solution, before and after the manual revision. all of the events with magnitudes >5.5 (table 3, with gray backqrcmts of 2012 emilia seismic sequence figure 3. examples of waveforms (blue) and synthetics (red) used for the qrcmt computations. left: examples for the may 20, 2012, ml 5.8 event. right: examples for the may 19, 2012, ml 4.1 event. ground) show r.m.s. for the automatic solution that are within the criteria that match with a stable solution (e.g., <0.4). other criteria needed are also respected: at least the data from three stations that were azimuthally well distributed around the epicenter (figure 4); a focal mechanism that was stable over five iterations (figure 4); a difference between initial and final coordinates <0.3˚; and a small non-doublecouple component [pondrelli et al. 2006]. these characteristics allowed these solutions to be considered for immediate web publication, in 30-45 min from event occurrence. this result, which will drive our choice for the future, is valid only for strong italian earthquakes, where the aforementioned conditions are satisfied: sufficient data availability in terms of distance and azimuthal distribution around the epicenter. for smaller magnitude events, the manual revision of the seismic moment tensor solutions is still fundamental. however, finding the technical solutions for the filtering with respect to the station distance and the data availability would allow a lowering of the magnitude threshold for automatic immediate qrcmt publication, at least for italy. 4. conclusions the qrcmt solutions we computed for large to moderate magnitude events during the emilia 2012 sequence are in agreement with the seismotectonic setting of the shaken region and with the solutions distributed by other agencies. we determined the seismic moment tensors for 13 events, and for some of these we obtained a very stable qrcmt solution, already from the automated system of recovery and the computations developed in house over recent years. the sequence was an important test to demonstrate that for events that occur in italy with m >5.5 our automated system for qrcmt computation works very well, and we can assume for the future to immediately publish the automated solutions on the web, without any manual revisions. acknowledgements. we thank the data centers of mednet, geofon and the gsn, starting from the people who take care of the instruments at the station sites, to those who make the data available on the web. we are grateful to paolo boncio and to an anonymous reviewer for important suggestions and comments on our original manuscript. plots were made using the generic mapping tools, version 4.2.1 [www.soest.hawaii.edu/gmt; wessel and smith 1998]. all of the qrcmt solutions are commonly published a short time after the event occurrence on the dedicated web pages http://autorcmt.bo.ingv.it/quicks.html, and on the emsc web pages. references boccaletti, m., m. bonini, g. corti, p. gasperini, l. martelli, l. piccardi, c. tanini and g. vannucci (2004). seismotectonic map of the emilia-romagna region, 1:250000. regione emilia-romagna – cnr. boccaletti, m., g. corti and l. martelli (2011). recent and active tectonics of the external zone of the northern apennines (italy), int. j. earth sci., 100, 1331-1348. carminati, e., d. scrocca and c. doglioni (2010). compaction-induced stress variations with depth in an active pondrelli et al. 620 figure 4. comparison between the automatic and revised conditions for the three events of the seismic sequence with m >5.5. upper parts: for each qrcmt, the station distribution is shown around the epicenter for surface (s) and body (b) waves, and the number of stations (ns) and number of components (nc) used in the inversion. lower parts: all five moment tensors are shown, as obtained for each iteration of the inversion, where the red one is the final solution. 621 anticline: northern apennines, italy, j. geophys. res., 115, b02401; doi:10.1029/2009jb006395. ekström, g., j. tromp and e.w.f. larson (1997). measurements and global models of surface wave propagation. j. geophys. res. 102, 8137-8158. lavecchia, g., n. boncio, n. creati and f. brozzetti (2004). stile strutturale e significato sismogenetico del fronte compressivo padano-adriatico: dati e spunti da una revisione critica del profilo crop 03 integrata con l'analisi di dati sismologici, b. soc. geol. ital., 123, 111-125. montone, p., and m. mariucci (1999). active stress along the ne external margin of the apennines: the ferrara arc, northern italy, j. geodyn., 28, 251-265. pieri, m., and g. groppi (1981). subsurface geological structures of the po plain, cnr, progetto finalizzato geodinamica, pubblicazione 414. pondrelli, s., a. morelli, g. ekström, s. mazza, e. boschi and a.m. dziewonski (2002). european-mediterranean regional centroid-moment tensors: 1997-2000, phys. earth planet. int., 130, 71-101. pondrelli s., a. morelli and g. ekström (2004). europeanmediterranean regional centroid moment tensor catalog: solutions for years 2001 and 2002, phys. earth planet. int., 145, 127-147. pondrelli, s., s. salimbeni, g. ekström, a. morelli, p. gasperini and g. vannucci (2006). the italian cmt dataset from 1977 to the present, phys. earth planet. int., 159, 286-303; doi:10.1016/j.pepi.2006.07.008. pondrelli, s., s. salimbeni, a. morelli, g. ekström and e. boschi (2007). european-mediterranean regional centroid moment tensor catalog: solutions for years 2003 and 2004, phys. earth planet. int., 164, 90-112. pondrelli, s., s. salimbeni, a. morelli, g. ekström, m. olivieri and e. boschi (2010). seismic moment tensors of the april 2009, l'aquila (central italy) earthquake sequence, geophys. j. int.; doi: 10.1111/j.1365-246x.2009. 04418.x pondrelli, s., s. salimbeni, a. morelli, g. ekström, l. postpischl, g. vannucci and e. boschi (2011). europeanmediterranean regional centroid moment tensor catalog: solutions for 2005-2008, phys. earth planet. int., 185, 74-81. pondrelli, s., p. perfetti and p. danecek (2012). pypaver: automazione del calcolo dei quick rcmt con python, rapporti tecnici ingv, submitted. scognamiglio, l., e. tinti and a. michelini (2009). real-time determination of seismic moment tensor for italian region, b. seismol. soc. am., 99, 2223-2242; doi:10.1785/ 0120080104. scrocca, d. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 using publicly available gps solutions for fast estimations of first-order source details from coseismic deformations valentina cannelli1,*, daniele melini1, roberto devoti2, antonio piersanti1 1 istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy 2 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy annals of geophysics, 54, 4, 2011; doi: 10.4401/ag-4885 abstract we here explore the potential use of publicly available gps solutions to obtain first-order constraints on a source model immediately following an earthquake, within the limits of gps solution timeliness and near-field coverage. we use gps solutions from the scripps orbit and permanent array center to carry out simple inversions of the coseismic displacement field induced by the 2010 maule earthquake (chile), by inferring the seismic moment and the rake angle of a fixed-geometry seismic source. the rake angle obtained from the inversion (m=117.8˚) is consistent with seismological estimates. the seismic moment, which corresponds to a moment magnitude mw =8.9, is about 1.6 times greater than seismological estimates. this suggests that as in other recent megathrust events, a consistent fraction of the energy was released aseismically. in this respect, the additional information obtained from gps can help to provide a better estimate of the weight of the aseismic contribution to the energy release. 1. introduction gps solutions have very important roles in highprecision geodetic and geophysical measurements, particularly for the study of earthquake-induced permanent deformations, tectonic plate motions, plate boundary deformations, and meteorological processes. high-rate gps data that belong to regional and global networks are often made available in near real-time through open-source projects, where the intention is to produce and disseminate these data to the largest possible community. the aim of the present study is to demonstrate how some first-order information about a source model can be retrieved almost immediately after an earthquake, through a simple inversion of coseismic deformations obtained from publicly available near-real-time gps solutions. more accurate estimates of source details can clearly be obtained using a collection of different datasets, e.g. teleseismic records, interferometric synthetic aperture radar (insar), local gps and seismological data, rather than continuous gps data alone. on the other hand, such datasets are not generally available in near real-time (and such datasets are often not publicly available at all). the scripps orbit and permanent array center (sopac) is an open-access archive of high-precision gps data that provides, among other resources, daily solutions from about 800 continuous gps sites that belong to more than 20 scientific networks around the world. in the present study, we used gps daily solutions downloaded from sopac to infer first-order details about the source model of the 2010 maule earthquake (chile), through nonlinear inversion of the coseismic deformation field. although the resulting dataset contains only two near-field offsets (from the conz and sant sites), it can be shown to improve the characterization of the seismic source with respect to preliminary seismological models. 2. the 2010 maule earthquake on february 27, 2010, a devastating megathrust earthquake occurred offshore of maule, in central chile. on the moment magnitude scale (mw), this 2010 maule earthquake was estimated at 8.8 (united states geological survey, usgs; http://earthquake.usgs.gov/earthquakes/ eqinthenews/), corresponding to a seismic moment release of 1.8 × 1022 nm; this therefore remains amongst the largest events of the instrumental age, and the second-largest recorded event in south america, after the great 1960 chile earthquake. the 2010 maule earthquake was generated by the convergence of the nazca plate downwards to the south american plate (figure 1). the two plates are converging at a rate of 66 mm per year. the fault rupture extent has been estimated to be more than 100 km in width and to stretch nearly 500 km parallel to the coast of chile (usgs, unavco, http://www.unavco.org/support/event-response/ eventresponse.html and lay et al. [2010]). the rupture spread westward, northward and southward, and occurred article history received december 20, 2010; accepted july 7, 2011. subject classification: geodesy, tectonophysics, gps, coseismic deformations, source inversion, maule earthquake. 454 mostly offshore, generating a pacific-wide tsunami that had devastating effects on the nearby coasts (national oceanic atmospheric administration, http://nctr.pmel.noaa. gov/chile20100227/ and saito et al. [2010]). the mid-latitude localization of the event maximized its effects on the dynamics of the earth rotation, which induced perturbations that should have shortened the length of the day by 1.26 µs and to have moved the earth figure axis by ~8 cm, according to current estimates (national aeronautics and space administration, http://www.nasa.gov/topics/earth/features/ earth-20100301.html). after the 2004 mw = 9.3 sumatra event, several studies found evidence of detectable coseismic offsets at gps sites also on an extremely large scale (comparable with plate dimensions) [banerjee et al. 2005, vigny et al. 2005, gahalaut et al. 2006]. these far-field offsets have been successfully used to provide additional constraints to the source geometry and earth rheological layering [pollitz et al. 2006, 2008]. therefore the 2010 maule earthquake is also likely to have left a detectable signature on gps recordings, even on a wide regional scale. moreover, analysis of the gps-derived coseismic deformation field induced by the sumatra earthquake revealed that a consistent fraction of the energy was released aseismically [banerjee et al. 2005]. for the 2010 maule earthquake, the present evidence is controversial: an analysis of normal-mode amplitudes has implied that 26% of the global centroid moment tensor solution (gcmt, http://www.globalcmt.org/) seismic moment was released aseismically [tanimoto and ji 2010], whilst using gps and insar data, pollitz et al. [2011] inferred a seismic moment that exceeds the gcmt estimate by 10%. on the other hand, joint inversion of high-rate gps, insar and teleseismic data has been used to model the seismic source without invoking any aseismic energy contributions [delouis et al. 2010]. for this reason, we analyzed the sopac daily solutions of permanent gps sites located in a vast region centered on the 2010 maule earthquake epicenter, with a suitably designed matlab code that transforms the daily solutions into the international terrestrial reference frame (itrf2005) first, and then estimates the coseismic displacement field; this procedure was partly described by devoti et al. [2010]. cannelli et al. 455 figure 1. regional seismotectonic framework of the 2010 maule earthquake. the gray line marks the major plate boundaries [bird 2003]. the yellow star and the red beach ball show the epicenter location and the focal mechanism of the harvard cmt solution for the main shock. the focal mechanisms from the cmt catalog (since 1976) are also reported. the red box marks the surface projection of the usgs rupture geometry estimate (gavin hayes, usgs). the lower-right inset shows the cmt seismicity cross-section along profile a-a´ (each tick mark corresponds to 50 km). 456 afterwards, our best estimate of the coseismic displacement field was used to infer the scalar seismic moment and the rake direction of the seismic source, through a nonlinear inversion procedure. 3. gps analysis and displacement field to extract the deformation field associated with the maule earthquake, we analyzed the daily coordinates of 26 publicly available permanent gps sites that belong to different networks, including: the international global navigation satellite systems service (igs), the continuously operating reference stations (cors), and the pacific gps facility (pgf). the gps sites are located in a geographical area with longitudes between 0˚w and 160˚w, and latitudes between 75˚s and 30˚n. the coordinate time series were obtained from the daily quasi-observation files that are available from the sopac data center (ftp://garner.ucsd. edu/). the quasi-observation solutions contain coordinate estimates of nearly 200 sites of the igs network, and the reference frame is only loosely constrained at the 1m level. we applied minimal inner constraints to the daily looseconstrained solutions, constraining translations, scale and rotations, to 1 mm. each daily network is then transformed into the itrf2005 reference frame, which is defined by 19 globally distributed sites that were extracted from the igs cumulative solution [dow et al. 2009] (ftp://igscb.jpl.nasa.gov/ igscb/station/coord/igs05.snx). these were chosen both on the basis of their data quality and their distance from the epicenter, which assured "noninfluence" from coseismic displacements associated with the maule event. the above procedure was applied to a dataset that included solutions from february 14, 2010 (gps week 1571, day 0) to march 6, 2010 (gps week 1573, day 6). more precisely, for each site, we stopped the analysis after the first two available daily solutions following the event in the gps week 1573, to avoid any bias towards the instantaneous offset computation, due to a remarkable post-seismic drift that many stations presented, even in the first week following the earthquake (figure 2). the sites located in the selected geographical region but without data in the proximity of the event (braz, glps, vesl), were discarded from this analysis. to measure the amount of coseismic displacement due to the seismic event, we estimated a common threefirst-order source details from gps figure 2. time series for the north, east and vertical components of four gps sites, as indicated. error bars correspond to the 1-v formal errors. the vertical red line represents t = 2010.02.27 (february 27, 2010). dimensional offset at the epoch of february 27, 2010, in a leastsquares sense. this used the whole daily covariance matrices, and we evaluated the mean site positions in the 14 days before the earthquake and in the first two available days after the earthquake. the formal standard deviations associated with the estimates are likely to be underestimated, depending on the deviation from normality of the de-trended residuals [williams et al. 2004]. figure 3 shows the offsets (blue vectors) and their associated errors (confidence ellipses) from the maule earthquake for the north and east components. we only focused on the horizontal displacements, because the uncertainties of the vertical components are too large to allow any reliable interpretation. table 1 shows the preliminary estimated coseismic offsets and their associated errors for the north and east components. the maximum horizontal coseismic surface displacements were at conz (305.0 ±0.4 cm) and sant (29.9 ±0.6 cm), the sites that were closest to the epicenter, both directed westward. appreciable deformations that were still of the order of millimeters were evident even for the farfield sites that were included in the analysis (kokb, hnlc, lpal, gmas). except for the near-field sites (conz, sant, lpgs), the relative uncertainties on the horizontal displacements ranged from 200% to 300%. even though characterized by a lack of any public gps station near the earthquake rupture zone, the coseismic deformation field obtained is consistent with the nazca-south american plate dynamics [haberland et al. 2009] and can be used to infer at least large-scale information on the source characteristics. 4. inversion and source modeling we inverted the residual deformation field associated with the maule earthquake, with the aim of obtaining information about the seismic source from the geodetic data. cannelli et al. 457 figure 3. observed gps coseismic displacements (blue vectors) with 68% confidence ellipses and modeled (red vector) displacements of the maule earthquake, with the exception of the sant and conz sites. the yellow star indicates the earthquake epicenter. the braz, glps and vesl sites are included on the map, even though their data were not included in the analysis. the lower left inset shows an enlarged view of the near-field area with the horizontal displacements detected by the sant and conz sites, and the position and extent of the modeled fault plane. the displacement at sant is magnified by a factor of 200. 458 the semi-analytical model used to compute the residual deformation field was widely discussed by [piersanti et al. 1997, soldati et al. 1998, boschi et al. 2000], and it generally allows the computation of both coseismic and postseismic responses to a seismic dislocation for a spherical, incompressible, self-gravitating earth with arbitrary rheological layerings [melini et al. 2008, and references therein]. in this case, we used a 45-layer structure with preliminary reference earth model (prem)-averaged [dziewonski and anderson 1981] elastic constants; the corresponding density and rigidity profiles are shown in supplementary figure s1. due to the poor near-field data coverage, the source geometry was fixed according to gcmt and usgs preliminary seismological estimates, and the only free parameters were the seismic moment and the rake angle (the position and extent of the fault plane is shown in the inset of figure 3). since our forward model assumes point-like dislocations, the rupture plane was discretized with 1,080 evenly discrete sources. for each elementary source, we computed the corresponding green's functions for the displacement field at the station locations for a unitary seismic moment, assuming both pure thrust and pure strike mechanisms and a uniform slip over the rupture plane, so the same slip parameters are assigned to all of the elementary sources. the deformation field for an arbitrary configuration of seismic moment and rake can therefore be easily obtained by a suitable combination of the elementary green's functions. the optimal values of the seismic moment and rake are then found by minimizing the weighted chi-square between the model prediction and the observed displacement. the minimization was performed with the minuit package [james 1998], which implements a variable-metric method with inexact line search, a stable metric updating scheme, and a positive-definiteness check [fletcher 1970]. radial displacements were not used in the inversion because of the lower resolution of the gps system for vertical movements. the modeled field that results from the inversion is shown in figure 3, along with our best estimate of the observed field, as described in the previous section. the normalized first-order source details from gps table 1. gps analysis sites, estimated coseismic offsets, associated uncertainties, and best-fitting modeled displacements. site longitude (˚) latitude (˚) e (mm) ve (mm) n (mm) vn (mm) e (mm) n (mm) areq −71.4928 −16.4655 1.5 3.2 2.9 3.4 2.4 0.5 bogt −74.0809 4.6401 1.9 4.1 −2.8 3.4 3.7 1.1 chpi −44.9852 −22.6871 −0.6 3.8 −2.0 3.4 −2.7 −2.4 conz −73.0255 −36.8438 −2973.5 3.6 −680.7 4.4 2925.3 −621.3 cro1 −64.5843 17.7569 0.3 3.2 −1.0 2.6 3.8 1.1 fraa1 −149.6143 −17.5553 1.6 3.0 −1.3 2.6 2.5 −1.0 falk −57.8741 −51.6937 0.0 3.9 −1.6 4.6 0.4 3.7 gmas −15.6343 27.7648 3.5 2.7 0.1 2.5 2.7 0.6 guat −90.5202 14.5904 0.4 2.9 −2.1 2.4 3.0 1.3 hilo −155.0527 19.7192 0.5 2.8 0.5 2.3 2.1 0.2 hnlc −157.8645 21.3033 1.6 2.5 −0.4 2.1 2.0 0.2 kokb −159.6649 22.1263 1.7 2.0 −0.2 1.8 1.9 0.3 kour −52.8060 5.2522 −0.3 3.4 2.3 3.2 3.3 0.4 lpal −17.8938 28.7639 3.4 2.7 0.2 2.4 2.8 0.7 lpgs −57.9323 −34.9067 −14.0 5.8 −4.8 6.7 −39.5 −4.4 mana −86.2490 12.1489 0.7 2.8 1.1 2.6 3.8 1.3 mas1 −15.6333 27.7637 0.9 1.9 −0.3 1.8 2.7 0.6 maui −156.2570 20.7067 2.5 1.9 −0.2 1.6 2.0 0.2 mkea −155.4563 19.8014 3.0 2.6 −0.1 2.2 2.1 0.2 ohi2 −57.9013 −63.3211 2.7 3.7 −0.5 5.5 3.3 1.2 ohi3 −57.9014 −63.3211 0.0 2.9 4.0 3.8 3.3 1.2 palm −64.0511 −64.7751 1.4 2.6 −2.6 3.4 3.4 0.4 sant −70.6686 −33.1503 −259.9 5.8 −148.2 6.5 −623.7 −453.1 scub −75.7623 20.0121 1.0 2.9 1.4 2.6 3.8 1.3 thti −149.6065 −17.5771 3.2 3.8 2.2 3.3 2.5 −1.0 unsa −65.4076 −24.7275 −4.8 4.6 1.1 4.5 −2.8 −10.0 site observed values modeled values chi-square between the data and the model is 84.2, and the wrms of residuals is 31 mm. for the south american sites, we obtain a good agreement on the direction of the deformation vectors (within the error ellipses) except at unsa, which might be located along a nodal line of our model. the vector lengths at some sites appear to be overestimated by our model, by up to a factor of two (unsa, lpgs and sant). while the overestimation on the sant site might be explained by edge effects, on the other sites, the incompressibility approximation of our analytical model might have a role, as in compressible models a larger fraction of the energy is stored in elastic loading, which yields a faster decay of the deformation with distance [nostro et al. 1999]. the agreement in the central american region and the antarctic peninsula is less satisfactory; however, we note that the modeled displacements in these regions are coherent, whilst the observed data are characterized by large variability, both in amplitude and direction, on small spatial scales. on the very far-field sites (pacific ocean and northwestern africa), the agreement is good, even if these sites are less significant, due to the large relative errors of the modeled vectors. the best-fitting source model is characterized by a seismic moment m0 =2.82×10 22 nm and a rake angle m=118˚. the rake angle that results from the inversion corresponds to a thrust mechanism with a small right-lateral component, which is consistent with the gcmt solution (m= 112˚) and the w-phase inversion (m = 109˚) [lay et al. 2010]. the seismic moment, which corresponds to a moment magnitude mw = 8.9, is about 60% greater than its (gcmt, usgs) seismological estimate m0 = 1.8 × 10 22, which is equivalent to the (well-known) moment magnitude mw = 8.8. the resulting excess of energy released from geodetic inversion with respect to purely seismological methods has already been seen for the 2004 sumatra earthquake [banerjee et al. 2005, lay et al. 2005], and it was interpreted as an indication of a large, very low frequency energy release in giant subduction events, even if hearn and burgmann [2005] suggested that the modelistic artifacts connected with different rheological layerings used in seismological and geodetic inversions might have a role too. for the 2010 maule earthquake, the seismic moment estimates vary from 1.8 to 2.6 × 1022 nm, depending on the data and modelistic approaches used (see supplementary table s1 of pollitz et al. [2011] for a summary), and they indicate the possibility of a large aseismic contribution to the energy release also for the 2010 maule earthquake. our estimate of m0 is at the upper bounds of the range of published values; while it may be overestimated due to the lack of detailed near-field coverage in our dataset, it is in qualitative agreement with indications coming from the literature. however, we also note that our forward model describes the seismic source in terms of a double couple, and therefore it provides a natural estimate of the seismic moment with no need for any slipmoment conversion that might be biased by local rigidity assumptions. to assess the resolution of the model parameters and to ascertain the presence of any trade-offs between them, we carried out a montecarlo analysis. here, we generated 100,000 synthetic datasets, each of which was obtained by adding random realizations of data noise to the original displacement field. afterwards, for each synthetic dataset, we computed the best-fit model parameters. from the distribution of these synthetic models, we can establish the effects of experimental uncertainties on the inverted model parameters. supplementary figure s2 shows the a-posteriori distribution of the seismic moment and the rake angle, and the correlation between them. as can be seen from supplementary figure s2, the distribution of parameter models is well approximated by a gaussian function, and there is no correlation. by fitting the distributions with a gaussian curve, we obtained an estimate of the error of the parameters, which turns out to be m0 = (2.818 ±0.034) × 10 22 nm and m= (117.85 ±0.77)˚. 5. conclusions in the present study, we show how it is possible to use publicly available real-time data from worldwide networks to quickly infer first-order information that can help to better constrain the source parameters of a large earthquake. here, we used publicly available records of permanent gps stations belonging to the igs, cors and pgf networks, to define the static deformation field associated with the coseismic effects of the mw = 8.8 2010 maule earthquake, by analyzing short time series, including two weeks of daily solutions. the resulting estimate of the coseismic field shows a maximum horizontal deformation of over 3 m at the conz site, which is located on the hanging wall of the fault plane. appreciable displacements of the order of millimeters are even evident for the far-field sites located at over 1,000 km from the epicenter. the deformation field estimated from these gps data was used to infer information about the seismic source with a nonlinear inversion. due to the poor near-field data coverage, we used a fixed-geometry source model, using information from gcmt and usgs seismological analyses, and inverted only for seismic moment and rake angle. the modeled field is in agreement (within the associated errors) with the observations, with a normalized chi-square of 84.2 and a wrms of residuals of 31 mm. for a few sites, the model overestimates the data, most likely because of the incompressibility approximation assumed by the model. the best-fit rake angle is consistent with gcmt and usgs seismological estimates and with lay et al. [2010], while the scalar moment (m0 = 2.82 × 10 22) is about 1.6 times the seismological estimate, which corresponds to a moment cannelli et al. 459 460 magnitude mw = 8.9. the overestimation of the seismic moment using geodetic inversions has already been seen for the sumatra earthquake [banerjee et al. 2005, lay et al. 2005], and this has been interpreted in terms of a large aseismic energy release in giant subduction events. for the 2010 maule earthquake, a similar indication comes from the analysis of normal-mode amplitudes, even if the situation is not yet clear [delouis et al. 2010, tanimoto and ji 2010, pollitz et al. 2011]. the timeliness and public availability of high-rate gps data make them an ideal tool for quick post-earthquake analysis. of course, in the present study, we discussed an approach that can be used only if significant offsets have been recorded by at least a few local or regional-scale sites. as a result, this kind of analysis will be possible only for earthquakes above a magnitude threshold that depends on the (spatially variable) density of open-access global gps networks. at the moment, we reasonably estimate that this analysis can be performed for earthquakes above a magnitude of 8.0. over the past decade, more and more regional and global networks of continuously operating gps ground stations have opened up the access to their data; if this trend is maintained in the future, with an increasing production and dissemination of open-access data, near realtime gps analysis might become a viable tool for rapid characterization of even smaller earthquakes. acknowledgements. we are grateful to riccardo lanari and two anonymous reviewers for their constructive and incisive comments. this work was partly supported by miur (ministero dell'istruzione, dell'università e della ricerca) with the firb grant "sviluppo di nuove tecnologie per la protezione e la difesa del territorio dai rischi naturali". references banerjee, p., f.f. pollitz and r. burgmann (2005). the size and duration of the sumatra-andaman earthquake from far-field static offsets, science, 308, 1769-1772. bird, p. 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email: valentina.cannelli@ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. cannelli et al. 461 scollo_corrected:layout 6 annals of geophysics, 57, 2, 2014, s0214; doi:10.4401/ag-6396 s0214 eruption column height estimation of the 2011-2013 etna lava fountains simona scollo1,*, michele prestifilippo1, emilio pecora1, stefano corradini2, luca merucci2, gaetano spata1, mauro coltelli1 1 istituto nazionale di geofisica e vulcanologia, osservatorio etneo, catania, italy 2 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy abstract in this paper, we use calibrated images collected by the video-surveillance system of the istituto nazionale di geofisica e vulcanologia, osservatorio etneo, to retrieve the height of the eruption column during the recent etna explosive activity. the analysis is carried out on nineteen lava fountains from the new south east crater dataset. the novel procedure described in this work is achieved in three main steps: at first we calibrated the camera, then we selected the images which recorded the maximum phase of the eruptive activity, and finally we applied an appropriate correction to account for the plume projection on the camera line of sight due to the wind. the results show that the column altitudes range between 6 and 9 km (upper limit of the camera system). the comparison with the plume height values estimated from the analysis of several seviri and modis satellite images, show a good agreement. finally, for nine events we also evaluated the thickness of the volcanic plumes in the umbrella region which ranges between 2 and 3 km. 1. introduction the column height estimation is an essential parameter to evaluate the volcanic explosive index [newhall and self 1982], the total mass eruption rate [sparks et al. 1997], the plume dispersal [bonadonna et al. 2012] and the plume gas and aerosol retrievals [prata and grant 2001]. in general, the column height may be estimated by using different methods (e.g. satellite, aircrafts and ground observations) which may present, in some circumstances, marked differences [tupper and wunderman 2009]. multispectral images, either aboard on polar (e.g. modis) or geostationary (e.g. seviri) satellite platforms, are extensively used to obtain quantitative parameters of volcanic plumes [corradini et al. 2010, merucci et al. 2011] and are a valuable tool to estimate the column height. seviri images are collected every 5 or 15 min and, during a volcanic crisis, are often used by volcano observatories [scollo et al. 2009] and volcanic ash advisor centers (vaac) to provide fast answers on the plume dispersal. although some limitations do exist (e.g. presence of water vapor), satellite imagery technique is surely the most widely used method to estimate the column height. concerning the ground-based observations, they are mainly based on radar [e.g. petersen et al. 2013] and lidar [e.g. pisani et al. 2012] systems which, however, are very expensive. conversely the video-surveillance cameras extensively used for volcanic activity monitoring are much cheaper and easier to operate, but still underused to this purpose. quantitative height estimations from video images are rare, with few examples available in literature. visual data were applied to sakurajima [kinoschita 1996] and etna [andronico et al. 2008] volcanoes. column height errors up to 20% obtained from video-surveillance systems and photos of the istituto nazionale di geofisica e vulcanologia, osservatorio etneo (ingvoe) were estimated [scollo et al. 2008], although under some conditions which should be always satisfied. in particular, the plume height should be taken from a sufficient distance from the vent [sparks et al. 1997] in order to see the plume top and to reduce the obliquity of the image plane. observations should be specified whether the height is given above the vent or above sea level (a.s.l.) as these two are often misinterpreted [sparks et al. 1997]. furthermore, the measurement accuracy should be associated to any column height value [settle 1978]. in this paper, we describe at first the eruptive activity in 2011-2013, the video-surveillance system, the calibration procedure and the method applied to estimate the column height. then, we compare our results with the volcanic plume heights estimated by the article history received august 14, 2013; accepted february 13, 2014. subject classification: volcano monitoring, eruption column height, video-surveillance cameras, seviri and modis satellites. nearly simultaneous data collected from seviri and modis satellite instruments. finally, we discuss the advantages of the proposed approach and the future improvements. 2. the 2011-2013 etna lava fountains between 12 january 2011 and 27 april 27 2013 etna produced 38 basaltic lava fountains from a new volcanic vent opened on the eastern flank of the south east crater. the eruptive episodes were characterized by a paroxysmal phase which lasted on average about one hour, and were preceded by mild strombolian activity and often by lava flow. every paroxysm formed an eruption column ranging from five to about ten kilometers of height and tephra fallout was mainly spread out on the eastern sectors of the volcano. volcanic ash dispersed in atmosphere, contaminated the air space of the international airport of catania causing troubles to air traffic operations, and sometimes forced the airport closure during the activity. 3. methods the monitoring of eruptive activity is continuously carried out at etna using a system of video-surveillance cameras located on the southern and eastern flanks of the volcano [andò and pecora 2006]. the cameras record the visible and infrared bands and are located in catania (ecv, 27 km from the vent), milo (emv, 11 km from the vent), nicolosi (env and ent, 15 km from the vent), schiena dell’asino (esv, 5 km se from the vent), monte cagliato (emct, 8 km e from the vent) and la montagnola (emov and emot, 3 km se from the vent). all the images are acquired, digitized, and archived at the ingv-oe site and displayed in the 24/7 control room of ingv-oe. in our study we use images taken by the ecv camera (15.0435°; 37.5138°). we analyzed all the ecv images of the 38 lava fountains in the visible range collected between 2011 and 2013. thus, the eruptive events that occurred during the night or in overcast weather were excluded. for each selected event, an image was taken during the maximum of the paroxysmal phase in order to evaluate the maximum column height. the calibration was carried out using the camera calibration (http://www.vision.cal tech.edu/bouguetj/lib_doc/) toolbox for matlab and images of a calibration pattern (figure 1), estimating the intrinsic parameters of the camera [zhang 1999]. hence, using geographically known features (e.g. the location of the ne crater, rifugio sapienza), we estimate the full camera model in order to locate in the image, any point in the space of the camera field of view. we note that a specific camera image point can be anywhere on the line of sight, making impossible the evaluation of a reliable column height without adding at least one geometrical constraint. in our case, the simpler constraint is a vertical plane passing through the volcanic vent and parallel to the wind direction (e.g. the main plume spreading direction). in particular, we used the daily weather forecast [scollo et al. 2009] to identify the plane containing the plume axis. consequently we were able to measure the geometry of the plume (figure 2). we evaluated an error of ± 0.5 km, which is equal to the space among the green lines in figure 2. the column height was also evaluated from satellite images nearly simultaneous to the video camera measurements. we used the data collected from seviri and modis, and estimated the volcanic plume height following corradini et al. [2010]. this procedure is based on the comparison between the 11 µm brightness temperature (tb,11) of the scollo et al. 2 figure 1. the pattern used for the camera calibration. figure 2. two examples of the calibrated ecv images used to estimate the height. the green lines indicate the height a.s.l. the line angle is function of the wind direction. 3 seviri/modis most opaque pixels (figure 3) with the temperature profile extracted from the wmo radiosoundings of trapani (http://weather.uwyo.edu/). the altitude retrieval error is estimated by considering a brightness temperature uncertainty of +/−2 k [prata and grant 2001]. 4. results table 1 shows column height values for nineteen lava fountain events evaluated by the video-surveillance and satellite images. seven events with column heights less than the maximum height (9 km) measurable from the ecv images, were well constrained by the camera. etna column height estimation date ecv seviri modis (dd-mm-yy) h (km); h (utc) t (km) h (km); h (utc) h (km); h (utc) 10-04-11 > 9.0; 11:18 6.1 ± 0.4; 11:15 10-04-11 6.3 ± 0.4; 12:30 7.8 ± 0.3; 12:30 09-07-11 > 9.0; 14:30 8.2 ± 0.3; 15:00 12-08-11 > 9.0; 09:23 8.5 ± 0.5; 09:30 12-08-11 6.5 ± 0.5; 11:15 7.2 ± 0.5; 11:15 7 ± 0.3; 11:15 20-08-11 > 9.0; 07:12 3.0 9.3 ± 0.6; 07:45 29-08-11 > 9.0; 04:40 2.5 7.8 ± 0.4; 04:30 08-09-11 > 9.0; 08:11 10.9 ± 0.4; 08:15 9.7 ± 0.3; 9:20 19-09-11 5.0 ± 0.5; 12:52 cloudy 08-10-11 7.5 ± 0.5; 14:51 3.0 cloudy 15-11-11 > 9.0; 12:18 2.5 9.9 ± 0.4; 12:15 11.1 ± 0.3; 12:10 05-01-12 > 9.0; 06:20 7.2 ± 0.4; 06:15 09-02-12 8.0 ± 0.5; 05:45 2.0 7.9 ± 0.5; 05:45 04-03-12 > 9.0; 08:31 8.9 ± 0.4; 08:30 9.5 ± 0.6; 9:10 04-03-12 8.0 ± 0.5; 09:00 7.9 ± 0.4; 09:00 18-03-12 > 9.0; 09:16 9.2 ± 0.5; 09:30 11 ± 2.3; 9:20 12-04-12 6.5 ± 0.5; 14:30 3.0 6.7 ± 0.5; 14:40 12-04-12 8.0 ± 0.5; 15:00 3.0 7.5 ± 0.5; 15:00 28-02-13 > 9.0; 10:31 9.4 ± 0.3; 10:30 03-04-13 7.5 ± 0.5; 14:19 3.0 5.6 ± 0.6; 14:20 03-04-13 6.0 ± 0.5; 14:45 3.0 5.8 ± 0.6; 14:45 12-04-13 7.5 ± 0.5; 11:20 3.0 7.2 ± 0.4; 11:45 7.3 ± 0.3; 12:00 18-04-13 > 9.0; 12:18 6.9 ± 0.4; 12:35 27-04-13 8.5 ± 0.5; 18:18 3.0 cloudy figure 3. seviri image of 12/08/2011 and modis-terra brightness temperature map at 11 µm used to estimate the plume column height of 12/04/2013 (12:00 utc). table 1. column height (h) a.s.l. estimated from the calibrated video frames (ecv), seviri and modis data for lava fountain events. cloud thickness (t) is indicated when measurable, along with measurement time (h). for some events, we could also estimate the cloud thickness by tracking the base of the plume on the ecv images (table 1). events for which the column height is greater than 9 km were 29/08/2011, 8/09/2011, 15/11/2011, 5/01/2012, 18/03/2012, while for events 10/04/2011, 09/07/2011, 12/08/2011, 20/08/2011, 04/03/2012, 28/02/2013, 18/04/2013, the estimated column height doesn’t differ greatly from 9 km. it is worth to note that marked height differences between video camera and modis estimates are mainly due to the time shift of the satellite image acquisition with respect to the video frame. as the volcanic activity is highly variable, we also evaluated the column heights near the modis overpass (table 1). for instance, on 12/08/2011 modis collected data on etna at 11:15 utc while the ecv image of the maximum height reached during the event was acquired at 9:23 utc. at 11:15 utc the eruptive activity sensibly decreased, but the column height measured from the ecv at the same time was 6.5 km, thus very close to the modis estimation. 5. discussion and conclusion in this paper we show how the column height may be quantitatively estimated using the video-surveillance images. however, the camera calibration and the correction accounting the projection of the plume on the camera line of sight direction are two essential steps. the unbiased measurement is when the wind blows in a direction orthogonal to the line of sight (i.e. eastward because the camera is located on the south flank of etna). furthermore, if the wind direction is constant with height the plane crossing the volcanic vent is well defined, and the error on the extrapolation of the column height is consequently low. another estimation that can be obtained with the proposed method is the volcanic plume thickness, an extremely valuable information for the satellite plume ash and gas retrievals [corradini et al. 2010] that can be currently obtained only if lidar and radar data are available. we highlight that height estimations obtained by video-surveillance images and satellites are in very good agreement. worst cases are 10/04/2011, 3/04/2011 and 18/04/2011. here we note that the volcanic plumes are small if compared to the seviri spatial resolution, and the pixels used to compute the brightness temperature are not completely opaque. furthermore, the satellite plume height error is underestimated because the temperature profile spatial variability is not taken into account (the trapani wmo radiosounding station is about 250 km from the volcanic vents). differences may depend also from the fact that video camera and satellites do not see the same eruptive scene. the main limitations of the proposed method are: i) wind data used in this study are derived from wind-field forecasts [scollo et al. 2009], but for other volcanoes similar data might be unavailable; ii) column height measurements using visible cameras cannot be applied during the night or bad weather conditions; iii) limit of the ingv-oe camera that cuts the column heights that are greater than 9 km a.s.l. our future work will concern the column height estimation as a function of time to better understand the short-term variations which often characterize basaltic lava fountains. we note that the column height is an extremely valuable information during an eruptive crisis to reduce the aviation hazard and mitigate the tephra fallout risk for inhabited areas close to volcanoes by mean of plume dispersal forecasting. the proposed method could be introduced in the 24/7 surveillance system of ingv-oe to support quantitative descriptions of the eruptive phenomena used for writing the vona (volcano observatory notice for aviation) message. finally, our method could be applied to other visible and ir cameras which should be installed farther from the vents, and preferably on the north and south flanks of volcano perpendicular to the prevailing wind direction which is, on etna region, from the west to the east. acknowledgements. we greatly thank e. biale, f. ciancitto, f. donnadieu, the anonymous reviewers. this work was funded by the smash project funded by esa and by the med-suv project funded from the european union’s seventh programme for research, technological development and demonstration under grant agreement no. 308665. references andò, b., and e. pecora (2006). an advanced video-based system for monitoring active volcanoes, comput. geosci., 32, 85-91. andronico, d., s. scollo, a. cristaldi and s. caruso (2008). the 2002-2003 etna explosive activity: tephra dispersal and features of the deposit, j. geophys. res., 113, b04209; 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(1999). flexible camera calibration by viewing a plane from unknown orientations, in: the proceedings of the seventh ieee international conference on computer vision (iccv), 666-673. *corresponding author: simona scollo, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy; email: simona.scollo@ct.ingv.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. etna column height estimation << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice vol51,2_3,2008 331 annals of geophysics, vol. 51, n. 2/3, april/june 2008 macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence romano camassi, raffaele azzaro and andrea tertulliani istituto nazionale di geofisica e vulcanologia, sezione di bologna, italy abstract the seismic sequence of the umbria-marche apennines was a dramatic moment for the population involved. at the same time, it provided a unique occasion for the italian scientific community and for the national civil protection to assess their respective abilities in understanding and managing the event. furthermore, macroseismology (including historical seismology) has knowingly tackled important methodological problems, such as the procedures for assigning macroseismic intensity, the use of the macroseismic scale, the impossibility of distinguishing the effects of earthquakes following closely in both space and time, within such a complex sequence. starting from an analysis of the problems faced after the 1997/98 umbria-marche earthquakes, as during the following seismic crises over the last 10 years, we propose some considerations on the lessons we have learnt from that seismic sequence. key words macroseismology – macroseismic scales – intensity – colfiorito earthquake 1. introduction: macroseismic practice, macroseismology and historical seismology. the study of earthquakes through the collection and evaluation of the macroseismic intensity with respect to the effects on people, objects, buildings and nature is commonly known as «macroseismology» (musson and cecic, 2002; cecic and musson, 2004). macroseismology defines the scenarios around both recent and historical earthquakes by means of accurate observations of the effects on the territory, especially in the damaged zones. the information collected is then elaborated and interpreted in terms of the intensity, namely a classification of the severity of the ground shaking on the basis of the effects observed over a limited area (grünthal [ed], 1998). the branch of macroseismology that collects and evaluates information on the effects of past earthquakes through archive sources (such as seismological compilations, coeval reports, and gazettes/bulletins) uses methods that are typical of historical investigation, and is known as historical seismology. macroseismology and historical seismology have common objectives, procedures and interpretive tools. the only difference between these two branches of seismology lies in the fact that in historical seismology the information is obtained from documents that are very variable in nature, and for which the collection and interpretation involves the use of critical tools that are specific to quantitative historical studies. the aim of macroseismology is to define the physical characteristics of an earthquake, as expressed by a collection of parameters, through the reconstruction of its impact on the natural and built environment. macroseismology is not an alternative to instrumental seismology: it is instead deeply complementary. instrumental mailing address: dr. romano camassi, istituto nazionale di geofisica e vulcanologia (ingv), sezione di bologna, via donato creti 12, 40128 bologna, italy; e-mail: camassi@bo.ingv.it vol51,2_3,2008 4-03-2009 10:28 pagina 331 332 r. camassi, r. azzaro and a. tertulliani seismology has seen important developments in only the past few decades, and sufficiently representative instrumental data are available only for a very limited time window. for this reason, until a few decades ago, macroseismic data were usually the only data available for earthquakes that occurred. nevertheless, it is important to stress that macroseismology provides essential and original information also for recent events for which also reliable instrumental data are also available. thus the main tool of macroseismologists is the intensity scale, a classification system that «allows the compression of a verbose description of earthquake effects into a number» (musson and cecic, 2002). macroseismic scales have been conventionally coded over a century of practice. the mercalli-cancani-sieberg (mcs) scale (sieberg, 1930) and the european macroseismic (ems-98) scale (grünthal [ed], 1998) are at present those most used in europe. 2. macroseismic survey of the 1997/98 umbria-marche earthquakes since 26 september 1997, a task-force of researchers has carried out macroseismic surveys to obtain a rapid evaluation of the maximum effects produced by the main earthquakes of the sequence (fig. 1), in order to define the areas of damage, and in support of the rescue operations of the dipartimento nazionale di protezione civile (dpc). the numerous researchers who were involved belonged to the gruppo nazionale per la difesa dai terremoti (gndt), the istituto nazionale di geofisica (ing), the servizio sismico nazionale (ssn) and the uprava republike slovenije za geofiziko (ursg). although they operated in different ways, the teams involved were well aware of the need for cooperation: they set up a coordinating unit and a centre for the exchange of information. different teams, consisting of 2-4 people, operated simultaneously in the field, visiting inhabited centres and rural villages following a programme that was scheduled the day before by the coordinating unit. at the end of each day, the field teams discussed the results of their surveys and compiled an upgraded intensity map. thus, while working against the clock and under difficult conditions, they carried out the essential task of providing the dpc and the scientific community with an enormous set of data in a very short time. it should also be noted that this was the first occasion on which the internet was efficaciously used by italian researchers as a fast communication and data-storage tool, to the huge acclaim of all of the parties concerned (stucchi, 1997). from its very start, the survey campaign proved extremely taxing and complex, on account of both the extensive area to be covered (the entire territories of the umbria and marche regions) and the continuously ongoing seismic activity. each further strong shock modified the situation so that the level and distribution of the damage was altered and the survey had to be partially repeated. a vast amount of photographic documentation was collected, together with local evidence from interviews with expert witnesses (e.g., firemen, municipal officers, surveyors). the results of the field surveys were integrated with those of the macroseismic questionnaire that was later routinely collected by ing through the postal service. the resulting data mainly contributed to the definition of the effects in the far-field. the first evaluation of the effects was carried out according to the mcs scale. once the initial emergency was over, a trial of the ems scale, at that time the ems-92 one (grünthal [ed], 1993), was also performed. this was of great interest as a test of the new scale, the special requirements for which (information on the building typologies in each locality, survey of the damage distribution for each building typology) would have been hard to reconcile with a quick survey (stucchi [ed], 1997). most of the assessments relating to the macroseismic effects of the 1997/98 sequence are based on the results of the surveys in the area (camassi et al., 1997a; gasparini et al., 1997; molin et al., 1997; monachesi et al., 1997; stucchi [ed], 1997; camassi et al., 1998, tosi et al., 1999; camassi, 2002) and the critical analysis of the daily field diaries and technical reports of the building safety assessments. vol51,2_3,2008 4-03-2009 10:28 pagina 332 333 macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence 3. from practice to practice: the umbria-marche lesson the 1997/98 seismic sequence was the most momentous crisis since the 1980 irpinian earthquakes (6 shocks with m >5.0). in this sense, it provided an important occasion for the italian scientific community to test its ability to investigate the effects of earthquakes, and at the same time to manage their relations with the national civil protection system in order to cope with such an important emergency. the long seismic period revealed a scientific community that was not ready for field surveys of damage over very large areas, and that lacked the support of adequate technical/financial resources and the a-priori organisation. nevertheless, over a time-span of a few hours after the first strong event, the operators themselves built up the coordination and invested all of their energy in starting the macroseismic survey. the experience of the 1997/98 umbriamarche earthquakes has highlighted a series of critical points that tested italian macroseismic practice. from the problems that were faced on that occasion, as during the following seismological crises over the last 10 years, the main points that are brought together here concerned: a) the collection and organisation of the data; b) the types of data collected; c) the georeferencing of the data; d) the use of the macroseismic scales; fig. 1. distribution of macroseismic intensities observed of the 26 semptember 1997 earthquake and following days (monachesi et al., 1997; update 7 october 1997) vol51,2_3,2008 4-03-2009 10:28 pagina 333 334 r. camassi, r. azzaro and a. tertulliani e) the possibility of distinguishing the effects of single shocks within a sequence. 3.1. collection and organisation of the data macroseismic surveys, questionnaire collection, and more recently, internet polls have been used for gathering information on recent earthquake effects in italy: for major events, a direct survey is certainly the most important method for data collection. just before the umbria-marche experience, a joint effort between the operative groups of the different institutes involved in macroseismic surveys had begun for the massa martana earthquake in may 1997 (camassi et al., 1997b). the umbria-marche sequence provided the occasion for the decisive push for strong collaboration between the different groups, and over only a few years, this led to the creation of a single organisation, which was also aided by the joining of the various research institutes into the istituto nazionale di geofisica e vulcanologia (ingv). this working group, called quick earthquake survey team (quest), had the duty to monitor the effects of all italian earthquakes above the damage threshold, through the integration of the different experience and competence, and eventually with the aid of other personnel from different agencies. since 2001, quest has undertaken the collection of macroseismic data during around 60 significant earthquakes. this has been achieved by direct surveys involving about 20 people, who alternate in their participation in the surveys, depending on the earthquake size, its epicentre location, and staff availability. the distribution of the ingv sections throughout the italian territory makes it possible to intervene not only in cases of severe earthquakes, but also for moderate events. each local group (bologna, catania, milano, napoli and roma) has the responsibility as the «guardian» for that part of italy that falls within its range of action. in the case of an event, the reaction time is very fast: when alerted by sms from the ingv automatic location system of the national seismic network, the survey is organized in the following three-four hours. it uses a standard protocol to gather homogeneous data: direct field observations in the damaged areas, telephone surveys in the far-field, collection of macroseismic data on a standard questionnaire form for the assessment of the mcs scale and the ems-98 scale. in the case of a significant emergency, the field teams are coordinated by a central bureau that updates the intensity map in real time, maintains contact with the civil protection authorities, and disseminates (when possible) the collected data via the internet (quest, 2008). several types of postal questionnaires have been tested in italy over the last few decades by different institutions. the experience developed by ing (now ingv) with the «bollettino macrosismico» is the most representative, and has been under constant development since the 1970’s. although it contains some disputable elements, it has the merit of collecting a huge amount of data by means of a dense network of correspondents throughout italy (over 8000 municipalities, 3000 carabinieri stations and around 1000 forestal corp stations). this kind of monitoring provides information for all of the events of m >3.0 (see gasparini et al., 1992; de rubeis et al, 2005). however, this methodology needs planning and is slow – the questionnaires are sent by post – so the data are actually available only a long time after the earthquake. finally, they are published yearly in the form of a paper bulletin. in addition, there are other occasional experiments with questionnaires involving schools and students, for the reporting of their experiences of felt earthquakes, in the framework of educational activities (postpischl et al., 1991; cubellis and marturano, 2002) on the basis of american experience (see wald et al., 1999; atkinson and wald, 2007; and references therein), over the last few years, trials with internet polls have been run with the ingv (http://terremoto.rm.ingv.it/). as well as some debatable choices of the way of formalising the questionnaire and interpreting the answers, this method for the collection of data in italy still suffers from the major variability in the distribution of internet access throughout the country. this is particularly lacking in central-southern areas, owing to which a significant number of the questionnaires are completvol51,2_3,2008 4-03-2009 10:28 pagina 334 335 macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence ed only in the large urban areas. nevertheless, in the future this method of information collection is certainly destined to be of greater importance. it is important to stress that regardless of the accuracy and the agreement between the forms used, both the traditional macroseismic questionnaires and the internet polls are difficult to complete. indeed, they can sometimes be potentially misleading in terms of a reliable assessment of the effects of damage. as we will see on discussing specifically how these scales should be used, the assessments require the collection and interpretation of precise information by expert operators, whether either the mcs scale or the ems-98 scale is used. the use of the aforementioned questionnaires, to be revised and standardised, may be useful in defining the characteristics and extension of the areas involved in the far-field, and of the limits of the damaged areas. an anomaly that needs to be stressed is that, although these three means of data collection are conducted within the ingv, they operate in independent ways, producing different results that are sometimes contradictory. it is hoped that in the near future these activities will be thoroughly reconsidered, and that the information can be collected in a coherent, or at least compatible way, so that it will be possible to combine the data. the case of the earthquake of 26 september 1997, for example, is representative: much disagreement was evident in the assessments of some serious effects in the same areas (table i). 3.2. the type of data collected the macroseismic scales require the effects of earthquakes to be documented in relation to a significant sample, with reference to a given locality (ferrari and guidoboni, 2000; guidoboni and ferrari, 2000). in this way, the assessment of the intensity is representative of that locality. this is required since the perception of an earthquake is subjective and also because the effects on buildings and the environment can vary greatly within the sample. mercalli himself was very clear on this point: «in judging the intensity of a shock by its effects, there is the need to take into account the full damage and destruction, rather than a few isolated occurrences, which can often be caused by the particular conditions of some buildings rather than the intensity of the shock» (mercalli, 1897). the guidelines and background material of the ems-98 scale are even more explicit: «this entails, first, that the settlement is large enough for a statistically significant sample to be obtained, without being unduly affected by small-scale local peculiarities, and secondly, that it is not so large that genuine local variations are neglected […] thus intensity should not be assigned to a single building or street; neither should a single intensity be assigned to a metropolis or a county. in general circumstances, the smallest place should be no smaller than a village, and the largest no larger than a moderately sized european town […] monumental buildings are usually unique, or only a few such buildings occur in one place. therefore, it is impossible to use the data relating to them in a statistical way as the scale requires. such data must therefore be handled with care, as complementary to other evidence (if available).» (grünthal [ed], 1998). for these aspects, in the recent practice of italian macroseismology, different situations have arisen that have been particularly critical. the «bollettino macrosismico» of the ingv, for example, collects and analyses information relating to the territory of a whole municipality, and not to a single locality, a practice that is detailed in a generic way in the text of the bulletin itself and that is in obvious contradiction with some of the fundamental requisites suggested by the originators of the macroseismic scales. suffice it to see how large and complex the territory of the foligno municipality is, of how many settlements it is made up (hundreds), and how large the variability of the effects was during the umbria-marche earthquakes of 1997/98 (fig. 2). such use in the collection of the data contributes to the spreading of the intensity values over areas that are too large, obscuring the real significance of the variability of the effects between one village and another (see, for example, tertulliani, 2000). similarly, the ingv internet questionnaire «did you feel an earthquake?» (http://terremovol51,2_3,2008 4-03-2009 10:28 pagina 335 336 r. camassi, r. azzaro and a. tertulliani table i. comparison between the table of intensities (only damaged localities) for the 26 september 1997 earthquake, according to monachesi et al. (1997; update 2 october 1997, column 1) and «bollettino macrosismico» (column 2). locality mcs1 mcs2 serravalle del chienti 6/7 8/9 collecurti (serravalle del chienti) 9 cesi (serravalle del chienti) 8/9 costa (serravalle del chienti) 8/9 san martino (serravalle del chienti) 8/9 acquapagana (serravalle del chienti) 8 dignano (serravalle del chienti) 8 attiloni (serravalle del chienti) 7 taverne (serravalle del chienti) 7 sellano 6/7 8 forfi (sellano) [landslide] 8/9 biscina (sellano) 7 molini di cammoro (sellano) 7 villamacina (sellano) 7 foligno 7 7/8 annifo (foligno) 8/9 arvello (foligno) 8/9 verchiano (foligno) 8/9 casenove (foligno) 8 c.le delle macchie (foligno) 8 cupigliolo (foligno) 8 capodacqua (foligno) 7/8 colle san lorenzo (foligno) 7/8 fraia (foligno) 7/8 franca (foligno) 7/8 leggiana (foligno) 7/8 ponte santa lucia (foligno) 7/8 locality mcs1 mcs2 popola (foligno) 7/8 scopoli (foligno) 7/8 serrone (foligno) 7/8 colfiorito (foligno) 7 fondi (foligno) 7 forcatura (foligno) 7 rio (foligno) 7 volperino (foligno) 7 frontone —7/8 gagliole —7/8 nocera umbra 7/8 7/8 isola (nocera umbra) 9 moline (nocera umbra) 8/9 aggi (nocera umbra) 8 sorifa (nocera umbra) 8 nocera scalo (nocera umbra) 7/8 stravignano-bagni di nocera (nocera u.) 7/8 bagnara (nocera umbra) 7 colle aprico (nocera umbra) 7 colle croce (nocera umbra) 7 grillo (nocera umbra) 7 molinaccio (nocera umbra) 7 salmaregia (nocera umbra) 7 valtopina 8 7/8 acquacanina 6 7 camerino 6/7 7 seola bassa (camerino) 6/7 vol51,2_3,2008 4-03-2009 10:28 pagina 336 337 macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence locality mcs1 mcs2 agnano (camerino) 6 letegge (camerino) 6 mergnano san pietro (camerino) 6 mergnano san savino (camerino) 6 torrone (camerino) 6 trebbio (camerino) 6 valle vegenana (camerino) 6 biocco (camerino) 5/6 canepina (camerino) 5/6 cerreto di camerino (camerino) 5/6 camporotondo di fiastrone 7 cessapalombo 7 fabriano 6/7 7 belvedere (fabriano) 7/8 campodonico (fabriano) 7 coccore (fabriano) 7 cupo (fabriano) 7 fiuminata 6/7 7 poggio (fiuminata) 7 sorifa (fiuminata) 7 fossato di vico 6/7 7 osteria del gatto (fossato di vico) 6/7 gualdo tadino 6/7 7 caprara (gualdo tadino) 6/7 rigali (gualdo tadino) 6/7 locality mcs1 mcs2 monte subasio 7 muccia 6 7 massaprofoglio (muccia) 6/7 preci 7/8 7 castelvecchio (preci) 7 corone (preci) 7 roccanolfi (preci) 7 sassoferrato 6 7 spello 6/7 7 collepino (spello) 6/7 san giovanni (spello) 6/7 valfabbrica 6/7 7 visso 6/7 7 riofreddo (visso) 7/8 aschio (visso) 7 rasenna (visso) 7 pioraco 7 6/7 sefro 7 6/7 sorti (sefro) 7 assisi 6/7 6/7 armenzano (assisi) 7 cerreto di spoleto 6 6 ponte (cerreto di spoleto) 7 sigillo 6/7 6 colbassano (sigillo) 7 to.rm.ingv.it/) collects information that specifically refers to single buildings, or even to single apartments; the mechanism of the interpretation assigns an intensity to every single questionnaire, and then treats the collected information statistically, a procedure that clearly contrasts with the aforementioned rules. analogous examples are also found in histable i. (continued). vol51,2_3,2008 4-03-2009 10:28 pagina 337 338 r. camassi, r. azzaro and a. tertulliani torical seismological studies. an eye-opening example of this type is that of the bolognese earthquakes in 1929. an early study, reported by camassi and molin (1994), provided assessments of the effects relative to 109 localities; the later studies of boschi et al. (1997, 2000) and boschi and guidoboni (2003) proposed the assessment of intensity for 640 and 691 places, respectively. as can be seen from the analysis of the data available for these studies, around 450 of such observations have been referred to single buildings, as can be explicitly seen from the technical surveys used in all these studies. 3.3. the georeferencing of the data the geographical positioning of the locality with which the effects of an earthquake are associated – the macroseismic observations – is a simple routine operation that can sometimes present difficulties. the main causes of error concern the coincidence of names between places, the variation of names with the passage of time, and in the case of historical studies, the recognition of localities that no longer exist because they have disappeared or because they have been rebuilt in another place with a similar or a different name. all of these cases may result in errors in the drawing up of the macroseismic map, and moreover in the interpretation of the data. this was the case, for example, for some localities that were damaged during the umbria-marche sequence of 1997/98, which were differently located by camassi et al. (1998) and, erroneously, by boschi et al. (2000) (fig. 3). to avoid these problems, a revision and unification of the geographical reference data was carried out, each locality being unequivocally identified with the same coordinates, which was used to set up of the dbmi04 italian macroseismic database (stucchi et al., 2007). in addition, a gis-based utility for the management of macroseismic data, from the georeferencing with the above geographical directory to final plotting on the map, was also developed to provide a simple and quick tool to be used in the different phases of the macroseismic study (d’amico and azzaro, 2008). finally, the current availability of good low-cost gps systems, makes a faster and accurate georeferencing procedure in the field possible, though they must be then referred to the geographical directory. fig. 2. observed intensities (monachesi et al., 1997; update 7 october 1997) in the territory of the foligno (pg) municipality. in grey are plotted all settlements. vol51,2_3,2008 4-03-2009 10:28 pagina 338 339 macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence 3.4. the use of the macroseismic scales as indicated above, although in the literature there is no «code» for the use of the macroseismic scales, in practice certain ways of using them have been emerging over the last few years, in particular with the setting-up of the ems-98 scale on a european level, both in its experimental version and in its definitive version (grünthal [ed.], 1993, 1998). for the mcs scale itself, which is at present used only in italy, in spite of its often rather casual use, it does contain some precise elements that are very informative indeed. in particular, table 102 of the original text by sieberg (1930), which is often disregarded by its users, offers some quantitative indications concerning the distribution of the different damage levels. these indications will be better stated in the versions of the msk scale (sponheuer and karnik, 1964) and the em-98 scale, the guidelines for which are the only document in existence that addresses the use of a macroseismic scale. however, it is important to stress that even the strict use of the mcs scale already requires the accurate acquisition of information, a quantitative analysis of the distribution of the various indicators considered (molin, 1995, 2003), and moreover, a careful consideration of particular situations (high vulnerability, particular buildings, etc.). 3.5. evaluation of the effects of earthquake sequences as discussed above, the monitoring of damage due to the 1997/98 umbria-marche earthquakes was particularly problematic. first of fig. 3. different locations of three damaged localities, according to camassi et al. (1998) and boschi et al. (2000). vol51,2_3,2008 4-03-2009 10:28 pagina 339 340 r. camassi, r. azzaro and a. tertulliani all, there was the general impossibility of distinguishing the effects of the individual events within the sequence. from 26 september to the end of october 1997, there were 27 earthquakes with magnitudes greater than 4.0 (six of which were greater than 5.0), while there were a few thousand events with lower magnitudes. these circumstances caused continuous variations in the damage levels even within the same areas, due to the cumulative effects of the shocks. many towns were heavily hit many times, and it was rarely possible to quantify the increase in the damage in any way. also, in the case of immediate and accurate field surveys soon after a shock, it was rather difficult to quantify the increase in building vulnerability due to these individual events (azzaro and stucchi, 2000). this means that the macroseismic effects arising from a seismic sequence should generally be considered to be cumulative rather than due to a single event; otherwise, the consequence will be the overestimation of the resulting intensity data (and hence also of the earthquake parameters derived from these data). in conclusion, even when field surveys or historical sources report the effects of individual events, we can expect reliable data for the first event – although only if there is time enough to provide a clear damage scenario – and cumulative effects for the entire sequence. from this preliminary analysis, it follows that the intensity data relevant to destructive earthquake sequences can be biased by two factors: a vulnerability increase through the sequence, and the cumulative effects of the shocks. as a consequence, earthquake parameters derived from this sort of intensity data may also be biased, and in some cases, overestimated. thus, it is critical to consider the vulnerability increase as an important bias for the intensity assessment within the same seismic sequence, because it does not produce «linear» variations in the final intensity. 4. conclusions the italian seismological community has in general learnt a great deal from this 1997/98 umbria-marche seismic sequence while dealing with the questions and problems deriving from the huge amount of field data. also, both macroseismology and historical seismology have faced important methodological problems over the last ten years. the main issue relates to the use of the macroseismic scales, and in particular for the procedures for assigning intensities. due to the distribution and size of many inhabited centres (particularly in the case of a small settlement or isolated buildings) and the major differences in damage distribution over short distances (e.g. in volcanic areas), it has proven almost impossible to distinguish the effects of individual earthquakes within a seismic sequence. in the practice of macroseismology today, however, we can consider that some lessons have been learnt, both in relation to recent earthquakes and to historical events. since its formulation, the new ems-98 scale has become more frequently used in italy, although only after some initial perplexities (molin, 1995; tertulliani, 1995). this scale has been applied to a variety of cases, many of which have also included systematic comparisons with the mcs scale, which focuses on both the collection and interpretation of data collected in the field (azzaro and barbano, 1995; azzaro et al., 2002, 2004, 2006), and the analysis and revision of historical sources, also in terms of seismic histories and damage scenarios for hazard purposes (azzaro et al., 2000, 2007; barbano and rigano, 2001; barbano et al., 2001, 2005). these 1997/98 earthquakes represent an important moment in the aforementioned process, since it was the first experimental «laboratory» involving a large group of italian operators working in macroseismology. following the guidelines of the first version of the ems-92 scale (grünthal [ed], 1993), the participants made efforts to reduce the heterogeneity of the individual methodological approaches of the field surveys and intensity assessments (stucchi [ed], 1997). from a practical point of view, the quest working group has officially operated since 2004, although it is actually nothing more than the formalization of the mobilization of researchers and technicians who participated on a voluntary basis ten years ago. in a more general framework of reorganisation of this sector vol51,2_3,2008 4-03-2009 10:28 pagina 340 341 macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence within ingv, we expect that this group will become the hub of future activities in macroseismology, especially in setting up the process of harmonisation of survey procedures – direct observations in damaged areas versus questionnaires in the far-field, with only significant localities being checked – that will prove important for the production of reliable data. references atkinson, g.m. and d.j. wald (2007): «did you feel it»? intensity data: a surprisingly good measure of earthquake ground motion, seism. res. lett., 78 (3), 362368. azzaro, r. and m.s. barbano (1995): the pollina (northern sicily-italy) earthquake of 26 june 1993: an application of the new european macroseismic scale 1992, nat. haz., 12, 289-301. azzaro, r. and m. stucchi (2000): the evaluation of the effects of earthquake sequences in the light of the em98 intensity scale, in papers and memoranda from the first workshop of the esc working group «historical seismology», edited by v. castelli (macerata, italy, 15 september 1999), 48-53. azzaro, r., m.s. barbano, b. antichi and r. rigano (2000): macroseismic catalogue of mt. etna earthquakes from 1832 to 1998, acta vulc., 12, (1-2), 3-36. azzaro, r., m.s. barbano, r. camassi, s. d’amico, a. mostaccio, g. piangiamore and l. scarfì (2004): the earthquake of 6 september 2002 and the seismic history of palermo (northern sicily, italy): implications for the seismic hazard assessment of the city, j. seism., 8 (4), 525-543. azzaro, r., s. d’amico, a. mostaccio and l. scarfì (2002): terremoti con effetti macrosismici in sicilia orientale. calabria meridionale nel periodo gennaio 1999 dicembre 2001, quaderni di geofisica, 27, 59. azzaro, r., s. d’amico, a. mostaccio, l. scarfì and t. tuvè (2006): terremoti con effetti macrosismici in sicilia orientale nel periodo gennaio 2002 dicembre 2005, quaderni di geofisica, 41, 60. azzaro, r., f. bernardini, r. camassi and v. castelli (2007): the 1780 seismic sequence in ne sicily (italy): shifting an underestimated and mislocated earthquake to a seismically low rate zone, nat. haz., 42 (1), 149-167. barbano, m.s. and r. rigano (2001): earthquake sources and seismic hazard in southeastern sicily, annali di geofisica, 44 (4), 723-738. barbano, m.s., r. rigano, m. cosentino and g. lombardo (2001): seismic history and hazard in some localities of south-eastern sicily, boll. geof. teor. appl., 4 (1-2), 107-120. barbano, m.s., r. azzaro and d.e. grasso (2005): earthquake damage scenarios and seismic hazard of messina, north-eastern sicily (italy) as inferred from historical data, j. earth. eng., 9 (6), 805-830. boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990, (ing/sga bologna). boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (eds.) 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(2003): considerazioni sull’eventuale adozione in italia della scala macrosismica europea (ems1998). 22° convegno nazionale del gngts, roma, 18-20 novembre 2003, riassunti estesi delle comunicazioni, pp. 11. molin, d., p. soddu, c. gasparini, a. tertulliani, r. camassi, g. monachesi and m. stucchi (1997): unificazione delle valutazioni degli effetti macrosismici dei terremoti umbro-marchigiani del settembre/ottobre 97. rapporto protezione civile. monachesi, g., r. camassi and d. molin (editors) (1997): earthquakes of september-october 1997 in umbria-marche (central italy): macroseismic survey after the earthquakes of 26 september 1997 and following days. internet: http://emidius.mi.ingv.it/gndt/t19970926_eng/t19970926agg.html musson, r. m. w. and i. cecic (2002): macroseismology, in: lee, w.h.k., h. kanamori, p.c. jennings, and c. kissingler (editors), international handbook of earthquake and engineering seismology, vol. 81a, 807-822. postpischl, d., e. chignola, g. monachesi, s. raccichini, a. riggio, n. vasapollo and l. vittuari (1991): progetto e sperimentazione di un nuovo questionario macrosismico, gruppo nazionale per la difesa dai terremoti, atti del convegno di pisa. 2527 giugno, 261278. quest (2008): quick earthquake survey team. internet: http://www.ingv.it/real-time-monitoring/quest/questquick-earthquake-survey-team sieberg, a. (1930): geologie der erdbeben, handbuch der geophysik, berlin, 552-554. sponheuer, w. and v. karnik (1964): neue seismische skala, in sponheuer, w. (editor), proc. 7th symp af the esc, jena, 24-30 sept. 1962, 77, 69-76. stucchi, m. (editor) (1997): earthquakes of septemberoctober 1997 in umbria-marche (central italy): macroseismic data in terms of ems-92 scale. internet: http://emidius.mi.ingv.it/gndt/t19970926_eng/ems _intro.html stucchi, m., r. camassi, a. rovida, m. locati, e. ercolani, c. meletti, p. migliavacca, f. bernardini and r. azzaro, (2007): dbmi04, il database delle osservazioni macrosismiche dei terremoti italiani utilizzate per la compilazione del catalogo parametrico cpti04. http://emidius.mi.ingv.it/dbmi04, quaderni di geofisica, 49, 38. tertulliani a. (1995): testing the european macroseismic scale 1992 (ems92) on italian earthquakes, european earthquake engineering, 2, 37-43. tertulliani a. (2000): qualitative effects of local geology on damage pattern, bull. seism. soc. am., 90, 15431548. tosi, p., a. tertulliani, v. de rubeis and c. gasparini (1999): preliminary results of the macroseismic survey of the colfiorito sequence (central italy), phys. chem. of the earth., 24 (6), 477-482. wald, d.j., v. quitoriano, l.a. dengler and j.w. dewey (1999): utilization of the internet for rapid community intensity maps, seism. res. lett., 70 (6), 680-697. vol51,2_3,2008 4-03-2009 10:28 pagina 342 layout 6 a first principles approach to understand the physics of precursory accelerating seismicity dimitrios pliakis1, taxiarchis papakostas1, filippos vallianatos1, 2, * 1 technological educational institute of crete, laboratory of geophysics and seismology, crete, greece 2 university college london, department of earth sciences, london, united kingdom annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5363 abstract observational studies from rock fractures to earthquakes indicate that fractures and many large earthquakes are preceded by accelerating seismic release rates (accelerated seismic deformation). this is characterized by cumulative benioff strain that follows a power law time-to-failure relation of the form c(t) = k + a(tf – t) m, where tf is the failure time of the large event, and m is of the order of 0.2-0.4. more recent theoretical studies have been related to the behavior of seismicity prior to large earthquakes, to the excitation in proximity of a spinodal instability. these have show that the power-law activation associated with the spinodal instability is essentially identical to the power-law acceleration of benioff strain observed prior to earthquakes with m = 0.25-0.3. in the present study, we provide an estimate of the generic local distribution of cracks, following the wackentrapp-hergarten-neugebauer model for mode i propagation and concentration of microcracks in brittle solids due to remote stress. this is a coupled system that combines the equilibrium equation for the stress tensor with an evolution equation for the crack density integral. this inverse type result is obtained through the equilibrium equations for a solid body. we test models for the local distribution of cracks, with estimation of the stress tensor in terms of the crack density integral, through the nashmoser iterative method. here, via the evolution equation, these estimates imply that the crack density integral grows according to a (tf – t) 0.3-law, in agreement with observations. 1. introduction the failure of a solid due to microcrack concentration and propagation has been studied in various disciplines, including material sciences [scherbakov and turcotte 2003], rock mechanics [turcotte et al. 2003], solid earth geophysics [main 1999], and seismology. it is well accepted that there is a wide range of problems in geophysics where it is necessary to understand how rock deforms, from earthquake prediction to the driving forces of plate tectonics [main 1999]. the rock physics approach to understanding these geophysical processes is based on the premise that the macroscale behavior of rock is governed by microscale interactions. rock deforms elastically and plastically under an applied stress, by fracturing, brittle flow, and frictional sliding on a fault. the magnitude and direction of the applied stress, the rate and duration of the loading, the ambient pressure and temperature, the presence of fluids, and the previous deformation history, all define the overall mechanical response [main 1999, scherbakov and turcotte 2003]. fracturing phenomena have attracted the interest of the scientific community, and particularly those concerning inhomogeneous materials. the main reason is that such phenomena are promising candidates as earthquake precursors [vallianatos and tzanis 2003a,b, vallianatos et al. 2004, varotsos et al. 1993, vallianatos and triantis 2008]. over the past decade it has been credibly argued that rupture in heterogeneous media is a critical phenomenon [cowie et al. 1995, sammis and sornette 2002], while a mounting body of evidence indicates that the earthquake generation process can be viewed as a critical phenomenon that culminates in a large event that corresponds to some critical point (cp) [sornette and sammis 1995, rundle et al. 2000]. according to the cp earthquake hypothesis, a failure in the crust can be thought of as a scaling up process, in which a failure on one scale of a fault network is part of the damage accumulation over a larger scale, which leads to long-range stress-stress correlation and an increase (acceleration) in the seismic release rates prior to large earthquakes [vallianatos and tzanis 2003b]. the culminating large event will only occur when the network is in a critical state, balancing on the verge of disorder, and characterized by both extreme susceptibility to external factors and strong correlation between its different parts. such a process can be described by a power-law time-tofailure relation of any quantity estimated from the earthquake magnitude. this scaling law has been justified in special issue: earthquake precursors 165 article history received august 10, 2011; accepted february 1, 2012. subject classification: earthquake faults: properties and evolution, seismicity fracture benioff. the laboratory in terms of run-away crack propagation and empirical expressions for accelerating (tertiary) creep preceding failure [bufe and varnes 1993, vallianatos and triantis 2008]. however, it can also result naturally from the many-body interactions between small cracks that form before an impending rupture [sornette and sammis 1995, bowman et al. 1998]. previous studies have determined that the cumulative benioff strain [vallianatos and tzanis 2003b] is particularly useful when smaller events are also of interest and magnitude scaling is desirable, while the cumulative moment is dominated by larger earthquakes and the event count does not allow for magnitude scaling. earlier studies empirically determined that typically the cumulative benioff strain critical exponent is m ~0.3. there are also two theoretical predictions for the value of the critical exponent. using a model seismogenic crust governed by a particular damaged rheology, ben-zion and lyachovsky [2002] derived a nonsingular power-law relation for cumulative benioff strain that is proportional to (tc – t) 0.3, i.e. m = 0.3. rundle et al. [2000] adopted a very different approach, by relating the behavior of seismicity prior to a large earthquake to the excitation in the proximity of a spinodal instability. they showed that the power-law activation associated with the spinodal instability is essentially identical to the power-law acceleration of benioff strain observed prior to an earthquake, and that m = 0.25. previous studies have developed and reviewed techniques to identify regions of accelerating seismicity and to attempt the prediction of the time and magnitude of the next large earthquake, sometimes with remarkable success [bufe and varnes 1993, bufe 2004, sornette and sammis 1995, brehm and brail 1998, 1999, bowman et al. 1998, papazachos and papazachos 2001]. we note, however, that until recently, the cp model has been largely conceptual and based on an analogy with phase transitions, and it has drawn support from theoretical simulations that involve simple models, such as cellular automata. there have been no effective physical models to describe the observations and the evolution of the earthquake cycle. to study the physics of seismicity in a similar way to the fracture of a solid, numerous approaches have appeared that have dealt with the propagation of cracks, and furthermore, with static [taguchi 1989] and dynamic [wackertapp et al. 2000, mignan et al. 2007] distributions of microcrack concentration. in the dynamic distributions of microcrack concentration, it is assumed that the space-and-timedependent microcrack concentration is governed by the applied microscopic external stress field. these have been studied according to the deviation of microcrack distributions from random ones during the deformation of a solid, and particularly how the microcrack concentration increases, and then the cracks coalesce and localize, and finally the solid fails [scherbakov and turcotte 2003]. in the present study, based on the approach presented by wackertapp et al. [2000], we formulate the time dependence of accelerated deformation based on first principles. in wackertapp et al. [2000] they propose a model for the propagation and concentration of microcracks in brittle solids. first, they derive the equations for the time evolution of an integral quantity related to the crack density. apart from a factor that characterizes the material, the rate of growth of the crack density integral is determined by the stress tensor. therefore, we apply partial differential equation techniques in an elastic equilibrium system. in this way, we estimate locally the stress tensor in terms of the elastic tensor or the crack density integral, provided the latter shows a local variation that we can determine to recover the observed laws. 2. first principles and methodology to set up the notation, we have to review some standard concepts for the formulation of an elastic equilibrium system. from the mathematical physics point of view, we will focus our attention on an open domain w0 in space that contains the origin and is equipped with the euclidean metric denoted by (· , ·). the latter description is equivalent with an earth that contains the earthquake zone. the elastic constant tensor field is a rank 4 tensor field, where d defines at each point in w0 a positive definite quadratic form on the space of rank-2, symmetric tensors [landau and lifshitz 1936]. furthermore, if u_ is the deformation vector field, then we have the strain tensor field s: where i, j = 1, 2, 3. then through the elastic constant tensor d and hook’s law, we introduce the stress tensor field v: where we use the summation convention. the equilibrium equations for the elastic medium are then expressed for k = 1,2,3 as: with the forces exerted on the boundary of the region w0 giving rise to dirichlet boundary conditions in the form of the prescribed deformation of its boundary. we extend in three dimensional space the studies of wackertapp et al. [2000] that derived a system of an ordinary differential equation coupled to the elliptic system of elasticity, introducing the crack density integral: (1) s x u x u 2 1 kl l k k l 2 2 2 2 = +c m sd klij ijkl=v div ( ) x 0j jk 2 2 v v = = ,i l ldl 0 j ct j= 3 ^ ^h h# 166 pliakis et al. 167 on the physics of accelerating seismicity where the �c-factor has the dimension of inverse force, and t is the crack density as a function of the crack length l, where the crack orientation is described by the pair of angles j_ = =(j1, j2). we refer to this as the wackertapp, hergarten and neugebauer (whn) model [wackertapp et al. 2000]. furthermore, in this whn model, the strain tensor is split into two components, one of which accounts for the contribution of the cracks through the tensor field c, which depends on the fourth-order moment of the crack density integral, as: where dx is the elementary solid angle, and where we integrate over the space of all directions in 3-space, denoted as d. we conclude that the elastic tensor is modified as: (2) (3) according to the whn model, using i(i_), the strain in the domain w is given by: in addition, the cumulative strain stored in the domain m is expressed by: where we have introduced the tension field as a function of the direction field n_: applying the triangle inequality, we conclude that: we proceed now to introduce the following assumption on the local distribution of cracks in a spatial region m for exponents `, e and positive constants c01, c02 > 0: (4) where vol(m) is the volume of the domain m. this introduces the bounds on the local variation of the tensor field c, implying smooth limits in the physical variation of the crack population properties. for the index i = i1i2i3 we set: and thus taking into account the radial distribution of the cracks. we proceed now to determine the exponents `, e, to obtain the condition under which the observed law holds. this is accomplished through examination of the elasticity equation and the crack or strain propagation equation. specifically, we divide our domain into regions where inequalities of type of equation (4) hold, and then we estimate the variation of the strain tensor through the stored energy inside this domain. moreover, this estimate completes the elasticity system as a ‘div-curl’ system that allows for the estimation of the local variation of the strain in terms of the local concentration of cracks. the standard nash-moser technique was modified by pliakis [2011]: it is combined with the local harmonic approximation method, which provides local weights for the derivation of the estimates. therefore, the local variation of the stress tensor is given by this elliptic system. in the appendix we recall the standard identities and the standard arguments that allow us to derive the necessary estimates. additionally, standard arguments provide the existence of a solution under a smoothly varying crack density integral. wackertapp et al. [2000] provided numerical evidence for the behavior of the tension field x(n_). we proceed now to formulate the bounds of the physical quantities involved in our analysis. for this, we use the typical nash-moser iteration used by pliakis and minardi [2009] and pliakis [2011] for domains defined by polynomials. specifically, we have the following result from following the arguments in pliakis [2011]: let m0 be a domain in space and m an arbitrary subdomain with a smooth boundary. then we denote by mt the points in m at distance at least t > 0 from the boundary. then the stress field inside m at every moment shows the following estimates of its maximum and minimum values: the latter implies that in a stressed region of the earth we can define a subdomain in which the stress field can be bounded with upper and lower bounds scaled to the volume and the tensor field c that depends on the crack density integral. to define the evolution of the system, we proceed now to study the dynamics of crack concentration. wackertapp et al. [2000] suggested that the crack density integral varies according to the cumulative strain stored in the region due to the presence of cracks: c i n n n n d d ijkl i j k li x= ^ h# ( )d i sv = u ( ) ( ( ))d i dc i d1 1= + -u ( ) ( , 0)l i x d dm d x m v y= ## ( ) ( ( ))c c i n2 1 m d ijrs ij rs 2 m m v v x= =# ## ( ) ( , ( ))n n nx v= ( )mlc in n n n d d id d d d ijkl i j k l x x m m m #= =v y v y# ## ## vol ( ) curl ( ) vol ( )c c cc cm m e ` ` e 01 02# # curl ( ) ( )c x c x c n a i di i i d jk k j i i i jk j k 1 2 32 2 2 2 x= = # ( )a i n x i n x i jk j k k j2 2 2 2 = (vol ( ))max c cm e 10 m m 3` 2 2 3 #v t a k# (vol ( ))max c cm e 11 m m ` 2 3 2 3 $v t a k# and therefore the growth of the cumulative strain is scaled as in the expression: (5) furthermore using the above estimates we see that: to derive a similar expression with the observed accelerated deformation law, after substituting c and integrating, equation (5) should then be written as: this latter equation is in perfect agreement with that proposed by main [1999] to scale crack growth. in the case when (1+4e)/(1+3e) ~ –2 i.e. e ~ -3/10 we get that: 3. conclusions and perspectives the present approach leads to an estimate of the temporal growth of the cumulative strain that is governed by an integrodifferential equation, provided the crack density has a specific local variation expressed also in the form: this condition suggests that as the crack density increases, then their variation averaged in all possible directions vanishes: these tend to align in a specific direction (which is reasonable, because as cracks condense and align, they merge to produce a fracture). the cumulative strain tensor follows a (tf – t) 1/3 law. the whn model [wackertapp et al. 2000] comprises the development of cracks in the elastic constants of the material (as brittle solids), and accordingly, the evolution of cracks is determined by a simple equation where the rate of increase of cracks is proportional to the existing cracks. however, the constant of proportionality depends on the elastic constants of the material. this leads to a nonlinear system of equations. this latter approach is consistent with the evolution process in an earthquake zone, where microfracturing increases during the preparation of a main event, which leads to an increase in moderate earthquake events [papazachos and papazachos 2001, vallianatos and tzanis 2003b]. this is also in agreement with the stress accumulation model proposed by king and bowman [2003], where the accelerating energy release is due to a decrease in the size of a stress shadow that was left by one or more previous events. to obtain this precise rate, we determine the dependence of the stress tensor on the crack density integral. this is derived from the elastic equilibrium equations of the medium, provided the crack density integral satisfies a particular form of local estimates, the precise form of which is set by the requirement that they reproduce the observed law. we apply the moser iteration scheme for the elliptic system of elasticity, incorporating the local estimates of the crack density integral through weighted inequalities that was originated by pliakis and minardi [2009] and pliakis [2011]. finally, the differential equation describing the microcrack density evolution leads to the 1/3-law. furthermore, we expand the whn approach to a new three-dimensional formulation. acknowledgements. this study was supported in part by the archimedes iii programme of the ministry of education of greece, and the european union in the framework of the project entitled ‘interdisciplinary multi-scale research of earthquake physics and seismotectonics at the front of the hellenic arc (impact arc)’. references ben-zion, v. and v. lyachovsky (2002). accelerated seismic release and related aspects of seismicity patterns on earthquake faults, pure appl. geophys., 14, 2385-2412. bowman, d., g. ouillon, c.g. sammis and d. sornette (1998). an observational test of the critical earthquake concept, j. geophys. res., 24, 372. brehm, d.j. and l.w. brail (1999). intermediate term earthquake using the modified time to failure method in southern california, b. seismol. soc. am, 89, 275-293. brehm, d.j. and l.w. brail (1998). intermediate-term earthquake using precursory events in the new madrid seismic zone, b. seismol. soc. am., 88, 564-580. bufe, c.g. (2004). comparing the november 2002 denali and november 2001 kunlun earthquakes, b. seismol. soc. am., 94, 1159-1165. bufe, c.g. and d.j. varnes (1993). predicitive modeling of the seismic cycle of the greater san fransisco bay region, j. geophys. res., 98, 9871-9883. cowie, d., p.a. vanneste and t. sornette (1995). multifractal scaling properties of a growing fault population, geophys. j. int., 122, 457-469. gilbarg, d. and n. trudinger (2003). elliptic equations of the second order, springer verlag. king, g.c.p. and d. bowman (2003). the evolution of regional seismicity between large earthquakes, j. geophys. res., 108, 2096-2106. ( ) ( ) ( )mc l dt d c n i c 2 2 m d 2 m = = r x r 6 @## c dt d c e m 2 3 + fa k# vol ( ) vol ( )c c c cm m 3 1 3 1 3 1 ` e ` e 3 1 1 &+ + + + + + c m c cdt d 1 1 4 e e 3+ + + ( )c c c t tf f2 3 1 = .n i d i d 3 1 #4 +x -# pliakis et al. 168 169 on the physics of accelerating seismicity landau, l.d. and e.g. lifshitz (1936). theory of elasticity, pergamon press. main, i.g. (1999). applicability of time-to-failure analysis to accelerated strain before earthquakes and volcanic eruptions, geophys. j. int., 139. f1-f6. mignan, a., g. king and d. bowman (2007). a mathematical formulation of accelerating moment release based on the stress accumulation model, j. geophys. res., 112, 307-308. papakostas, t. and d. pliakis (2011). local existence of euler-einstein system, growth estimates of this type (in preparation). papazachos, b.c. and c.b. papazachos (2001). precursory accelerated benioff strain in the aegean era, annals of geophysics, 44, 461-471. pliakis, d. (2002). generalized hardy’s inequality, arxiv: math/0203092v2 [math.ap]. pliakis, d. (2011). on the volume of nodal sets. harnack inequalities in general domains (submitted) pliakis, d. and s. minardi (2009). phase front retrieval by means of an iterative shadow graphic method, j. opt. soc. am. a, 26, 99-107. rundle, t., w. klein, d.l. turcotte and d.b. malamud (2000). precursory local existence of einstein-euler systems, pure appl. geophys., 157, 2165-2182. sammis, c.g. and d. sornette (2002). damage and self similarity in fracture, proc. nat. assoc. sci. usa, 99, 2501-2508. scherbakov, r. and d.l. turcotte (2003). damage and self similarity in fracture, theor. appl. mechan., 39, 245258. sornette, d. and c.g. sammis (1995). complex critical exponents from renormalization group theory of earthquakes: implications for earthquake predictions, j. phys. i., 5, 607-619. taguchi, y. (1989). fracture propagation governed by the laplace equation, physica a, 156, 741-755. turcotte, d., w. newman and r. scherbakov (2003). micro and macroscopic models of rock fracture, geophys. j. int., 152, 718-728. vallianatos, f. and p. triantis (2008). scaling in pressure stimulated currents related with rock fracture, physica a, 152, 718-728. vallianatos, f. and a. tzanis (2003a). distributed power law seismicity changes and crustal deformation in the sw hellenic arc, nat. hazard earth sys., 3, 179-195. vallianatos, f. and a. tzanis (2003b). on the nature, scaling and spectral properties of pre-seismic ulf signals, nat. hazard earth sys., 3, 718-728. vallianatos, f., d. triantis, a. tzanis, c. anastasiadis and i. stavrakas (2004). electric earthquake precursors: from laboratory results to field observations, phys. chem. earth, 29 (4-9), 339-351. varotsos, k., p. alexopoulos and m. lazaridou (1993). latest aspects of earthquake prediction in greece based on seismic electric signals ii, tectonophysics, 224, 1-37. wackertapp, s., j. hergarten and h. j. neugebauer (2000). a model for propagation and concentration of microcracks, geophys. res. lett., 27, 1771-1774. * corresponding author: filippos vallianatos, technological educational institute of crete, laboratory of geophysics and seismology, crete, greece; email: f.vallian@chania.teicrete.gr © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. appendix the estimates indicated in the main text will be achieved here with the moser iteration technique [gilbarg and trudinger 1978], with the ‘tricks’ presented by pliakis and minardi [2009], pliakis [2011], and papakostas and pliakis [2011]. we summarize them here. the building blocks are: 1. the sobolev inequality [gilbarg and trudinger 1978] in r3; q = 3p/(3 – p), p < 3 reads as for f ! c0 ∞ (r3): 2. generalized hardy inequality [pliakis 2002] for a polynomial function in p: rn $ r – here n = 3 we have that for f ! c0 ∞ (rn\{p = 0}): applying gauss theorem, we see that for an arbitrary smooth cut-off | and a smooth (p + 1)-symmetric tensor field u: where i = i1… ip,: and the magnitude of a (p + 1)-tensor: choosing a smooth cut-off {: | = |u|k – 1�, we are led for u = v to the following inequality for p < 2, q = 3p/(3 – p) introducing hook’s law: and using the harmonic approximation method for the norm of c, d: this brief description of the mathematical method allows us to produce the local weights that are necessary for the estimates. then using localization in conjunction with hardy inequality for the terms curl(c); curl(v) as well as equation (4), after iteration, we derive the basic estimates of the stress tensor. the full treatment is given in pliakis [2011] and papakostas and pliakis [2011]. f c f q p 4# ( )p p f c p f 2 2 2 m m 4 4## # ) ( )div ( curlu c u u u2 2 2 2 22 m m 4 4#| |+ +# # div ( ) , ( )curlu u u x u x u xi i i i i j j j jk j k j k 2 2 2 2 2 2 = = .u u ,... ,... i i i i 2 2 p p 1 1 1 1 = + + / ( ) ( ) , curl ( ) c c n / / 2( 1) qk q p p k k p p 1 1 2 2 m m m 4# # # {v {v f { v v p a a a k k k # # # c s s dijrs rs ij ijrs rs,v v= = =0,c c c 2 3 = 2x t t =0, dd d 2 3 = 2x t t pliakis et al. 170 48, 04/5, 2005 suppl 513 annals of geophysics, supplement to vol. 49, n. 1, 2006 review of stephen william hurrell dinosaurs and the expanding earth – solving the mystery of the dinosaurs’ gigantic size by giancarlo scalera have you seen the huge dinosaurs footprint in colorado? or in australia? have you looked critically at a dinosaur skeleton in a paleontology museum (if you visit the cappellini museum in bologna)? the size, posture and estimated weight of these giants are impressive and the problem of the mechanical deambulation of such large bodies has been posed many times. in particular, carnosauria (bipedal infraorder) and sauropoda (quadrupedal infraorder) have corporal weights in some cases exceeding 5.0 and 10.0 ton respectively. finally, the arboricol-like arms of the biped tyranosauridae (7.5 ton) lead people to believe that somewhat paradoxical and unknown physical properties have characterized the ‘lost world’ of mesozoic. it is then legitimate to search for possible clues of a lesser gravity force in the geologic past. stephen hurrel’s book ‘dinosaurs and the expanding earth – solving the mystery of the dinosaurs’ gigantic size’ offers a discussion of this problem, proposing an increasing gravity throughout geologic time. the book is not completely focused on the gravity problem and tackles several other topics of the expanding earth idea. the first two chapters are about the bio-mechanical aspect of the problem and an engineering discussion about the effect of changing scale concludes that the bones of the larger dinosaurs were too weak to sustain the animals weight. the applied gravity force overcomes the maximum allowed by the strength of the bones. fractures should appear, but evidence found by palaeontologists favours quick walking – and sometimes running – heavy animals. the succession of fossils supports a progressively diminishing size of fauna from jurassic to the recent. the cause can be the changing local gravity of our planet and an account of different causes of this process is provided. the next five chapters concern ‘drifting continents’, ‘the expanding earth’, ‘meteorites and ice ages’, ‘the solar system’ and ‘the ancient earth’. the reader will find a review of many of the main arguments which constitute the background evidence in favour of earth expansion, but also some additional original proof. an example of these new arguments – that merit assessment and specific research by field specialists – is the climatologic thesis: if the earth were smaller the width of the circulation cells from equator to poles would be shorter and a shorter distance should correspond to a lesser variation in temperatures of the climatic bands. the book is written in a plain straightforward style and its target is a wide public not interested in specialist treatises. its clear and lively descriptions lead the reader straight to the core of the arguments. the book has been improved in a succession of new editions and i have found the most recent one, in the form of an e-book, more mature and typographically more pared down to essentials: many redundant figures of the old hard-bound edition (2001) have been eliminated and the pages are put together better. 514 giancarlo scalera i think the book should adopt a publisher for diffusion in the normal book-shops circuit, to enhance its widescale diffusion of the expanding earth framework, to the general public and scientists alike. indeed, many of topics of hurrell’s topics could well be the topics of joint scientific collaboration between engineers and paleontologists. references hurrell, s.w. (2004): dinosaurs and the expanding earth – solving the mystery of the dinosaurs’ gigantic size (one-off publishing, great britain), pp. 116. article title annals of geophysics, 59, fast track 5, 2016, doi: 10.4401/ag-7200 1 hydraulic pressure variations of groundwater in the gran sasso underground laboratory during the amatrice earthquake of august 24, 2016 gaetano de luca istituto nazionale di geofisica e vulcanologia – centro nazionale terremoti gaetano.deluca@ingv.it giuseppe di carlo istituto nazionale di fisica nucleare – laboratori nazionali del gran sasso giuseppe.dicarlo@lngs.infn.it marco tallini università degli studi dell'aquila – dipartimento di ingegneria civile, edile-architettura e ambientale marco.tallini@univaq.it abstract since may 2015, hydraulic pressure, temperature and electrical conductivity of groundwater have been continuously monitored in the 200 m-long horizontal s13 borehole placed close to the deep underground infn laboratory (gran sasso aquifer, central italy). a high sampling rate of the physical quantities up to 50 hz was achieved with a 3-channel 24-bit adc. from may 2015 to september 2016 (17 months) we registered hydraulic pressure signals from 12 earthquakes at different surface distances (from 12000 to 30 km) and different magnitudes (from 8.3 to 4.3 mw). we present the first results obtained for the amatrice mainshock, comparing the hydroseismograph recorded by the s13 hydraulic pressure device to the time history recorded at the gigs station, both located in the deep core of the gran sasso carbonate aquifer. mailto:gaetano.deluca@ingv.it mailto:giuseppe.dicarlo@lngs.infn.it mailto:marco.tallini@univaq.it annals of geophysics, 59, fast track 5, 2016, doi: 10.4401/ag-7200 2 i. introduction oseismic changes of groundwater dynamics have been observed in many places around the world and, manga and wang (2015) provide an updated overview of earthquake hydrology, particularly of water table changes during earthquakes. we designed and implemented a system for continuous recording of water pressure, temperature and electrical conductivity of groundwater with a high sampling rate from 10 to 50 hz; we began continuous recording in may 2015 at a borehole named s13 [catalano and salza, 2003] located about 100 meters from the deep underground laboratory of gran sasso (lngs) of the istituto nazionale di fisica nucleare (infn). a seismic station, named gigs, is located near the s13 borehole (about 200 m-long) within the national underground laboratory of gran sasso (lngs-infn). the gigs station, used also in the framework of the ginger experiment [simonelli et al., 2016], belongs to the national seismic network of the centro nazionale terremoti of the istituto nazionale di geofisica e vulcanologia (cnt-ingv), and is equipped with nanometrics trillium 240 s and guralp cmg 3t 360 s seismometers (see also url http://iside.rm.ingv.it/iside/standard/info_stazione.jsp?page=sta&sta=2571). ii. the hydrogeological background during the l’aquila earthquake of april 6, 2009 (mw 6.1, moment magnitude) [chiarabba et al., 2009], groundwater post-seismic changes (spring discharge, water table, basic hydrochemistry, water isotopes and radon) of the gran sasso aquifer have been studied [adinolfi falcone et al., 2012; amoruso et al., 2011]. the horizontal borehole s13 was selected for the present study because it clearly recorded the water table changes induced by the l’aquila earthquake [adinolfi falcone et al., 2012; amoruso et al., 2011]. the 1000 km2-wide gran sasso carbonate aquifer is fractured and fault-partitioned. it is formed mainly by slope-to-basin, platform and ramp carbonatic rocks from upper triassic to middle miocene [adinolfi falcone et al., 2006; barbieri at al., 2005; catalano et al., 1986] (fig. 1). figure 1: scheme a (not in scale) of the gran sasso aquifer transversal to the highway tunnels and passing through the ul, bh and s13 area (from adinolfi falcone et al., 2008). uz – unsaturated zone; sz – saturated zone; kh – karst horizon; ra – regional aquiclude; t1 – permeability boundary (regional thrust); wt – water table; ht – highway tunnels; ul – infn underground laboratories; ca – calderone glacier (high elevation water reservoir – preferential recharge area); 1 – overflow spring (cp: capopescara spring); 2 – preferential groundwater flowpath area; 3 – preferential groundwater flowing toward the ul; pr – preferential recharge; dr – diffuse recharge. the hydrogeological relationships in the square are showed into details in b. c annals of geophysics, 59, fast track 5, 2016, doi: 10.4401/ag-7200 3 the gran sasso aquifer feeds spring groups, located at low altitude along the low permeability boundary, with a huge discharge of more than 18 m3/s [adinolfi falcone et al., 2008]. its core hosts the infn underground laboratories (ul), the boreholes hall (bh) and the monitored s13 borehole (fig. 1). the gran sasso upper thrust (t1) was intercepted in the ul, bh and s13 area (fig. 1). drilling of the s13 borehole, located in the bh area, evidenced a very high rqd value (rock quality designation) for footwall limestones, and low discharge values. at the s13 borehole end, high permeability fault rock composed by the upper triassic dolomite is placed. it represents the fault zone of the t1 thrust, where, during drilling work discharge values increased up to 15 l/s due to the high permeability of the fault rock, while the hydrostatic pressure exceeded 22 bar due to the low permeability of the dolomitic aquitard [catalano and salza, 2003] (fig. 2). figure 2: data acquired during the boring of s13 (from catalano and salza, 2003). q: drainage discharge (l/s); rqd: rock quality designation (%); hyd. press.: hydraulic pressure (bar). t1 refers to the thrust fault of fig. 1. after the s13 boring, a still tube was installed into it, for a total length of 177 m up to the dolomite. ul footwall limestones show permeability values of 10-7 m/s higher than those of ul hangingwall dolomites as obtained by lugeon test [adinolfi falcone et al., 2008]. the portion of the aquifer drained by the highway tunnels was estimated by considering piezometric data: the five 150 m-long horizontal boreholes (s14-s18) of bh show a piezometric head of around 60 m, whereas s13 shows higher piezometric levels. hence, the induced hydraulic gradient is around 40% (60 m out of 150 m) an extremely high value due not only to the imposed hydraulic load but also to the low hydraulic conductivity of dolomites [adinolfi falcone et al., 2008; amoruso et al., 2012; 2014]. iii. first results the area is seismically active [chiodini et al., 2004; de luca et al., 2009] and paleoseismic events have occurred in the gran sasso chain [galli et al., 2002]. we decided to monitor the s13 borehole by continuously recording hydraulic pressure, temperature and electrical conductivity at a high sampling frequency using a 3-channel 24bit adc, considering the characteristics of s13. the pressure sensor is connected directly to the borehole output, while the other two sensors are fed by groundwater drained into s13 borehole at 1 bar after a pressure reducing device. the hydraulic pressure of s13 borehole varies in the range of 24-28 bar during the year, consistent with a water table above of at least 250-300 m. during the 17 months of recording the water pressure signal followed the sun/moon tides very closely (fig. 3). figure 3: hydraulic pressure from 5 to 15 march, 2016; the labels on the vertical scale are bar and on the horizontal scale are the days of march 2016. electrical conductivity did not show any significant variation and the temperature followed both seasonal and daily variations also due to annals of geophysics, 59, fast track 5, 2016, doi: 10.4401/ag-7200 4 the presence of the close highway tunnel. following the approach used by manga and wang (2015), we searched for coincidences between global seismicity and the hydraulic pressure signals recorded by our device located in the wellhead of the s13 borehole. to this end, we selected global (mw > 7), regional (mw > 5) and local earthquakes in the period may 2015 – september 2016 to look at signals in the pressure data. we found 12 coincidences. as an example, figure 4 shows the chile earthquake (mw 8.3) of september 16, 2015. figure 4: chile earthquake of september 16, 2015 (offshore coquimbo – mw 8.3). the red traces are the vertical, north-south and east-west components of trillium 240 s sensor (gigs) and the black trace is the water pressure signal (arbitrary scale: velocity for the red traces and hydraulic pressure for the black one). time scale starts from 23:45 (ut). more precisely the pressure is sensitive to longperiod love (first arrival) and long-period rayleigh waves (second arrival) and the signal is very similar to the vertical component of velocity. before the amatrice sequence, we collected only two clear events (chile mw 8.3 and greece mw 6.5, november 17, 2015). in addition to the main event (mw 6.0) of august 24, 2016, the pressure sensor detected 9 other signals corresponding to aftershocks ranging from 5.4 to 4.3 mw. figure 5 shows signals for the mw 6.0 amatrice earthquake (central italy) of august 24, 2016; the waveform is evident and clearly corresponds with the seismic data. figure 5: amatrice earthquake (central italy) of august 24, 2016 (mw 6.0). the red traces are the vertical, north-south and east-west components of trillium 240 s sensor (gigs) and the black trace is the water pressure signal (arbitrary scale: velocity for the red traces and pressure for the black one). time scale starts from 01:36:30 (ut). finally, in figure 6 we show mw vs distance (km) for the 12 earthquakes unambiguously recorded and an estimation of the detection level of our setup. figure 6 also includes all unobserved events (green crosses) below and near our detection level. time (min) b a r 45 50 55 00 05 10 15 dd m /s 1 0 vertical north-south east-west 27.29 27.31 27.33 0 -4 5.3 -5.3 time (s) b a r 30 00 30 00 m /s vertical north-south east-west 0 0.03 -0.03 25.0 22.6 annals of geophysics, 59, fast track 5, 2016, doi: 10.4401/ag-7200 5 figure 6: mw vs distance (km) of the 12 earthquakes collected by pressure sensor at s13 borehole from may 2015 to september 2016; the blue dashed line is an estimation of the detection level, while the grey dashed line is the detection level from fig. 2 of manga and wang (2015). the green crosses represent earthquakes unobserved by the hydraulic pressure device. iv. conclusions we monitored several physical quantities from a borehole, named s13, that was bored during excavation works at the late '80 in the gran sasso aquifer near the deep underground infn laboratory (lngs). we continuously measured hydraulic pressure, temperature and electrical conductivity of groundwater with a high sampling rate (from 10 hz to 50 hz). through the s13 monitoring, we recorded at a very high frequency into a very deep carbonate aquifer the first observations of nearand far-field earthquake signals and the related hydroseismographs (figs. 4 and 5). whereas, manga and wang (2015) provided a review of water table changes during teleseismic arrivals in wells located only in shallow aquifers. we collected clear recordings of 12 earthquakes allowing us to estimate the detection level of the hydraulic pressure device, which we foresee to be sensitive to local earthquakes (distance less of 10 km) with magnitude larger than 2.0-3.0 mw. in figure 6, the green crosses represent earthquakes unobserved by the hydraulic pressure device. acknowledgements: we would like to thank kerry jane rhoden for the careful revision of the english language. references [adinolfi falcone et al., 2012] adinolfi falcone, r., carucci, v., falgiani, a., manetta, m., parisse, b., petitta, m., rusi, s., spizzico, m. and tallini, m. (2012). changes on groundwater flow and hydrochemistry of the gran sasso carbonate aquifer due to the 2009 l'aquila earthquake. ital. j. geosci. (boll. soc. geol. it.), 131: 459-474, doi: 10.3301/ijg.2011.34. [adinolfi falcone et al., 2008] adinolfi falcone, r., falgiani, a., parisse, b., petitta, m. spizzico, m. and tallini, m. (2008). chemical and isotopic (δ18o‰, δ2h‰, δ13c‰, 222rn) multi-tracing for groundwater conceptual model of carbonate aquifer (gran sasso infn underground laboratory – central italy). journal of hydrology, 357: 368– 388, doi: 10.1016/j.jhydrol.2008.05.016. [adinolfi falcone et al., 2006] adinolfi falcone, r., falgiani, a., petitta, m., tallini, m. (2006). characteristics of gran sasso infn lab groundwater inferred from spot data (1996-1998 period) to fine-tune the conceptual model of water-rock interaction in carbonate aquifer. internal report infn, lngs/geo-01, september 2006, 42 pp. [amoruso et al., 2012] amoruso, a., crescentini, l., martino, s., petitta, m. and tallini, m. (2014). correlation between groundwater flow and deformation in the fractured carbonate gran sasso aquifer (infn underground laboratories, central italy). water resour. res., 50: 1-19, doi: 10.1002/2013wr014491. 10 100 1000 10000 distance (km) 3.0 4.0 5.0 6.0 7.0 8.0 9.0 m o m e n t m a g n it u d e ( m w ) ww annals of geophysics, 59, fast track 5, 2016, doi: 10.4401/ag-7200 6 [amoruso et al., 2011] amoruso, a., crescentini, l., petitta, m., rusi, s. and tallini, m. (2011). impact of the april 6, 2009 l’aquila earthquake on groundwater flow in the gran sasso carbonate aquifer, central italy. hydrological processes, 25 (11): 1754-1764, doi: 10.1002/hyp.7933. [amoruso et al., 2012] amoruso, a., crescentini, l., petitta, m. and tallini, m. (2012). parsimonious recharge/discharge modeling in carbonate fractured aquifers: the groundwater flow in the gran sasso aquifer (central italy). journal of hydrology, 476: 136–146, doi: 10.1016/j.jhydrol.2012.10.026. [barbieri et al., 2005] barbieri, m., boschetti, t., petitta, m. and tallini, m. (2005). stable isotopes (2h, 18o and 87sr/86sr) and hydrochemistry monitoring for groundwater hydrodinamics analysis in a karst aquifer (gran sasso, central italy). appl. geochem., 20: 2063-2081, doi: 10.1016/j.apgeochem.2005.07.008. [catalano and salza, 2003] catalano, p.g. and salza, r. (2003). relazione idrogeologica e sistemi di drenaggio e canalizzazione, 1-180 pp. [catalano et al., 1986] catalano, p. g., cavinato, g., salvini, f. and tozzi, m. (1986). analisi strutturale nei laboratori dell'infn del gran sasso d'italia, mem. soc. geol. it., 35, 647-655. [chiarabba et al., 2009] chiarabba, c., amato, a., anselmi, m., baccheschi, p., bianchi, i., cattaneo, m., cecere, g., chiaraluce, l., ciaccio, m.g., de gori, p., de luca, g., di bona, m., di stefano, r., faenza, l., govoni, a., improta, l., lucente, f.p., marchetti, a., margheriti, l., mele, f, michelini, a., monachesi, g., moretti, m., pastori, m., piana agostinetti, n., piccinini, d., roselli, p., seccia d. and valoroso l. (2009). the 2009 l’aquila (central italy) mw 6.3 earthquake: main shock and aftershocks. geophys. res. lett., 36: l18308, doi: 10.1029/2009gl039627. [chiodini et al., 2004] chiodini, g., cardellini, c., amato, a., boschi, e., caliro, s., frondini, f. and ventura, g. (2004). carbon dioxide earth degassing and seismogenesis in central and southern italy. geophys. res. lett., 31: l07615, doi: 10.1029/2004gl019480. [de luca et al., 2009] de luca, g., cattaneo, m., monachesi, g. and amato, a. (2009). seismicity in the umbria-marche region integrating the italian national and regional networks. tectonophysics, 476: 121-135, doi: 10.1016/j.tecto.2008.11.032. [galli et al., 2002] galli, p., galadini, f., moro, m. and giraudi, c. (2002). new paleoseismological data from the gran sasso d’italia area (central apennines). geophys. res. lett., 29 (7): 11341137, doi: 10.1029/2001gl013292. [manga and wang, 2015] manga, m. and wang, c.-y. (2015). earthquake hydrology. in: gerald schubert (editor-in-chief), treatise on geophysics, 2nd edition, vol 4. oxford: elsevier, 305-328 pp, doi: 10.1016/b978-0-444-538024.00082-8. [simonelli et al., 2016] simonelli, a., belfi, j., beverini, n., carelli, g., di virgilio, a., maccioni, e., de luca, g. and saccorotti, g. (2016). first deep underground observation of rotational signals from an earthquake at teleseismic distance using a large ring laser gyroscope. annals of geophysics, 59, fast track 4. vol52,6,2009new 679 annals of geophysics, vol. 52, n. 6, december 2009 key words seismology – biography – core – mantel – documents 1. introduction inge lehmann (1888-1993) (fig. 1) is the danish seismologist who discovered the earth’s inner core (lehmann, 1936). in atomic physics an equivalent act would warrant a nobel prize (bolt, 1987, brush, 1985). she retired from the danish geodetic institute in 1953, but continued her scientific work in denmark, usa and canada, and her last publication was «seismology in the days of old» (lehmann, 1987). in addition to her significant contribution to our understanding of the earth, she was famous for the view that seismograms from a group of stations should be read by the same person in order to ensure that the same phase is identified from station to station. bolt (1997) wrote: «inge lehmann had an enormous fund of knowledge and experience about seismology from its early days in this century; in retrospect, it is a great shame that more effort was not made to persuade her to write up her reminiscences in more detail.». i was a friend and colleague of inge lehmann for many years, i helped her in writing her last paper in 1987, when she was 99 years old, and it is a translation of a paper she had given previously at the danish geophysical society. at some point in time our collaboration afforded me to discuss her important collection of seismological publications, correspondence and other documents. when inge lehmann said that she wanted her epistolary archive destroyed after her death, i remarked to her that i had seen an extremely interesting book (hansen, 1962; 1964) about carsten niebuhr, who went on an expedition to the middle east 1761-1767. i said that the book could not have been written without the availability of the epistolary archive at rigsarkivet, the danish inge lehmann’s work materials and seismological epistolary archive erik hjortenberg national survey and cadastre, denmark abstract the inge lehmann archive contains thousands of seismological work documents from inge lehmann’s private home. for a long time the author thought that the main concern was to keep the documents for posterity. there is now a renewed interest in inge lehmann, and some documents were presented in a poster at esc potsdam 2004, and the collection of documents were scanned and catalogued 2005-2006 at storia geofisica ambiente in bologna. inge lehmann (1888-1993) is famous for her discovery in 1936 of the earth’s inner core and for work on the upper mantle. a short biography is given. after her retirement in 1953 she worked at home in denmark, and abroad in usa and in canada. she took part in the creation of the european seismological commission in 1951, and in the creation of the international seismological centre in 1964. inge lehmann received many awards. some letters from her early correspondence with harold jeffreys are discussed, they show how the inner core was discussed already in 1932. a few of the author’s reminiscences of inge lehmann are given. mailing address: dr. erik hjortenberg, bakkefaldet 10, dk-2840 holte, denmark; e-mail: erik.hjortenberg@tdcadsl.dk vol52,6,2009new 2-12-2009 12:40 pagina 679 680 e. hjortenberg national archive. later she left me in her will some seismological correspondence, books, card files, seismograms and other work materials, which now make up the inge lehmann archive. i hope that the archive will prove to be useful to the seismologists and historians. for a long time i wondered how great was the interest in the inge lehmann archive, and i waited for the documents to become historical, with no clear feeling when that happens. in 2003, to my surprise (being retired) i was invited to present a paper at the «second seminar on the scientific correspondence in earth sciences: historic value and scientific relevance». the seminar was held on september 29, 2003 on the island of ischia, and it was part of the celebrations to commemorate the 75th anniversary of the death of giulio grablovits, the seismologist who took care of the casamicciola seismic observatory on the island of ischia. the seminar was organized by sga (storia geofisica ambiente, bologna) and ing (istituto nazionale di geofisica e volcanologia, naples). as preparation for my seminar talk i started cataloguing the archive after discussing procedures for that with finn åserud at the niels bohr archive. the procedure i used was to put numbered wrappers around inge lehmann’s original wrappers or groups of pages in the stack. the purpose was to make documents easier to find and put back where they were before. in some cases the information of the placement of a document is necessary to interpret the document. the talk was more about the life of inge lehmann than about the archive, and so is this paper. some examples of archive material will be discussed in section 6, and in last paragraphs of section 3. there was an interest shown for inge lehmann when louise palmer was arranging an exhibition «great women, great science» exploring the history of women in science, including inge lehmann, at bruce museum of arts and science in greenwich, connecticut from 2004-2005, bjarne kousholt, who has written books on tycho brahe, ole rømer, hans christian ørsted and agner krarup erlang, at high school level, has also written a book on inge lehmann (kousholt, 2004). kristian hvidtfelt nielsen helped me in transferring inge lehmann´s doctoral gown from the doctor of science reward ceremony 1964 at university of colombia in the city of new york. it was transferred so that it is now owned by kvindemuseet (=womens museum) in århus. 2. short biography of inge lehmann inge lehmann was born in copenhagen, denmark on may 13, 1888 and she died on february 21, 1993, also in copenhagen, at 104 years of age. an overview of some of her closest family is given in table i. her father was alfred lehmann, professor in psychology at university of copenhagen; after working in wilhelm wundt’s psychological laboratory at leipzig 1885-1886 he introduced to denmark the study of experimental psychology, and like inge lehmann he enjoyed activities such as fig. 1. inge lehmann (1888-1993). vol52,6,2009new 2-12-2009 12:40 pagina 680 681 inge lehmann’s work materials and seismological epistolary archive hiking in the mountains. her mother was ida sophie tørsleff, who had one more daughter, harriet hofmann. her mother’s sister was signe andrea tørsleff, mother of lis groes (anne lisbeth groes, the danish trade minister 19531957). one of the sons of lis groes is nils groes, who was executor under inge lehmann’s will in 1993. he takes care of her family correspondence while i take care of her seismological correspondance. alfred lehmann had a brother, julius lehmann, who had a son karl lehmann. tyge lehmann, karl lehmann’s son, wrote in the danish newspaper politiken on march 1, 1993, that to the very end of inge lehmann’s table i. some of inge lehmann’s closest family. family member name title etc. relation to inge lehmann year(s) hans jakob tørsleff grandfather, mother’s father ida sophie tørsleff mother 1866-1935 signe andrea tørsleff principal, haderslev lower aunt, mother’s sister. 1879-1975 secondary school lis groes danish trade minister 1953-1957 (anne lisbeth g.) member of parlament 1960-1971 cousin, signe 1910-1974 andrea tørsleff’s daughter ebbe groes general manager, fællesforeningen husband to lis groes for danmarks brugsforeninger (coop) father to nils groes 1910-2000 nils groes executive director, akf, institute son of lis and ebbe groes 1938 of local government studies denmark harriet hoffmann actress sister orla lehmann politician and instrumental father’s uncle, brother to 1810-1870 (peter martin orla l.) in the first danish wilhelm otto waldemar democratic casting lehmann wilhelm otto officer, engineer, layed telegraph father’s father, brother to 1817-1894 waldemar lehmann cable between denmark and sweden orla lehmann alfred lehmann professor of experimental father (alfred georg psychology at university 1858-1921 ludvig l.) of copenhagen julius lehmann instructor at the royal danish uncle (father’s brother) 1861-1931 opera, copenhagen karl lehmann chief surgeon cousin 1897-1979 (julius lehmann’s son) tyge lehmann ambassador, expert in son of karl lehmann internationallaw, director, , 1941 human rights unit royal danish ministry of foreign affairs vol52,6,2009new 2-12-2009 12:40 pagina 681 682 e. hjortenberg life she took an active interest in what he could tell her about the international court case between denmark and norway over the maritime boundary in the arctic ocean between greenland and jan mayen. inge lehmann had herself been to jan mayen in 1927 to explore the possibilities for establishing a danish seismological station there. however she had found the ground on the volcanic island too loose for the delicate instruments. instead she placed the station in illorqortormiit (scoresbysund), northeast greenland. her education took place in copenhagen (1894-1906) in a private, coeducational school run by hanna adler, an aunt of niels bohr. it was an institution where boys and girls studied the same subjects, and where all learned needlepoint as well as woodwork and soccer. the school was the first coeducational school in denmark. the book about hanna adler (adlersamfundet, 1959) describes how she studied coeducational schools on a visit to usa in 1892, and then started her own school in 1893. the book also describes how in 1943 former schoolchildren of hers, including inge lehmann, helped hanna adler to be one of the very few members of the danish jewish community who obtained official permission to live in germanoccupied copenhagen, if not the only one, provided she did not go to sweden. inge lehmann studied mathematics at the university of copenhagen (1907-1910) and continued 1910-1911 at cambridge, england (newnham college). she became an assistant in an actuary’s office (1912-1918 at an insurance company of the name «danmark»). she graduated with a cand. mag. degree (candidatus magisterii) from the university of copenhagen in 1920. she studied mathematics at university of hamburg (professor blaschke) in the autumn of 1922. during 1923-1926 she worked as an assistant to j. steffensen, professor of actuarial science. in 1928 she graduated with a mag. scient. degree (magister scientiarum) from the university of copenhagen. in 1925 inge lehmann became an assistant to niels erik nørlund (bang, 1988), the director of gradmaalingen, an institution in charge of research in fundamental geodesy in denmark. she helped nørlund in establishing the seismological stations scoresbysund and ivigtut in greenland and took an active role in the mounting of the seismographs at the seismological station of copenhagen. she studied seismology in the summer of 1927 at hamburg under e. tams, at darmstadt under b. gutenberg, at strasbourg under e. rothé, at de bilt under j. van dijk and at uccle under e. somville. gutenberg (schweitzer, 2003b) was important, since he had determined the depth to the earth’s core (gutenberg, 1914), and he gave her much of his time during the one-month stay. in 1928 lehmann was appointed statsgeodæt (state geodesist) and made chief of the seismological department of the newly formed danish geodetic institute under the directorship of niels erik nørlund. in this position, until her retirement in 1953, she supervised the seismological stations in greenland and copenhagen, she worked out the bulletins of the stations, did scientific work and published many papers. a danish children’s book about inge lehmann (jensen and pedersen, 1996) describes an earthquake that copenhagen found frightening, but inge lehmann thought was wonderful. in her position as state geodesist she could travel around and collect information on how the earthquake was felt. the earthquake occurred on october 31, 1930, 23 hours greenwich mean time (used by seismologists), equivalent to 1 november middle european mean time (used by danish newspapers). it was described by lehmann in 1948 (1948a, p. 11-14). an annotated list of danish earthquakes since year 1073 is given by lehmann (1956); update of it is given by gregersen et al. (1998). lehmann attended most of the 4-yearly meetings of the international union of geodesy and geophysics (iugg), starting with the 1927 meeting in prague, where time curves were discussed, a subject in which she was interested the rest of her life. she attended many other meetings also, for instance the study week november 19-25, 1951 at the vatican for the problems of microseisms. her paper there (lehmann, 1952) demonstrated the relationship between microseismic storms (as experienced in greenland) and cyclones. lehmann also attended some of vol52,6,2009new 2-12-2009 12:40 pagina 682 683 inge lehmann’s work materials and seismological epistolary archive the meetings organized by keilis-borok and leon knopoff, named «geophysical theory and computers» (now mathematical geophysics). inge lehmann and harold jeffreys attended such meetings in cambridge, 1966, organised locally by e.r. lapwood, and in copenhagen, 1969, organised locally by jørgen hjelme (1928-2006). inge lehmann was the first president of the european seismological federation, a federation that was replaced by the european seismological commission in 1951. lehmann (1971) explains how the idea of a european seismological society came about at a small meeting in strasburg 1947, and how at a meeting in verona 1950, organized by medi and caloi, the european seismological federation was formed with inge lehmann as its president. however the officers of iugg decided that if european seismologists were to have special meetings this would have to be arranged by the international seismological association or the international association of seismology and physics of the earth’s interior (iaspei), which came into being at the brussels assembly in 1951 as the successor of the former international association of seismology (ias). as a consequence the european seismological commission (esc) was set up. the 1967 iugg meeting in zurich was her last iugg meeting, and she presented a paper (lehmann, 1968). i was participating in the same meeting, and i can confirm the statement in bolt (1997, p 291), that lehmann was extremely sensitive to noise. she had reserved an expensive hotel room downtown zurich, but found out that the hotel could not guarantee that it was a quiet room. when she heard that my hotel room was cheaper and quieter she offered me the expensive hotel room in exchange of mine, and would pay the price difference. however very early on the first morning i was woken up by loud streetcar noise. so i left the hotel in exchange of a cheap, quiet camping site, and both inge lehmann and the danish geodetic institute saved money. one of the documents in the inge lehmann archive shows that in cambridge 1966 e.r. lapwood promised to give inge lehmann a quiet room in emanuel college. during 1928-1929 the danish geodetic institute (geodætisk institut) was carrying out the seismological service in denmark. this service was done by national survey and cadastre denmark during 1989-2004 and from then it has been carried out by geological survey of denmark and greenland (geus). inge lehmann was succeeded 1953 at the danish geodetic institute by henry jensen (1915-1974 ), an experienced geodesist and, after 1953 seismologist. she exchanged letters with him, on the subject of the loan of books, journals and seismograms. she also attended the european seismological commission general assembly in copenhagen, 1966, organized by him. henry jensen became the first professor of geophysics at university of copenhagen in 1965. jørgen hjelme,succeeded him at the danish geodetic institute, later national survey and cadastre denmark until 1995, where he was succeeded by søren gregersen, now at geus. a small seismic array station, named inge lehmann station, situated 77° 55’ n, 39° 14’ w, 2400 m above sea level, on the greenland ice cap started in june 1966 as a danish american cooperative project (lenton, 1968). the project showed that the station is very sensitive to small events, and a project in 1967 (project blue trek) showed that the low noise and good signal found at the lehmann station is also found on a large portion of the ice cap, while camp century near the edge is quite noisy. the station was run until september 1967. lehmann’s name will continue to be honoured in the geophysical world. in 1996 the american geophysical union established the inge lehmann medal. it is given for ‘outstanding contributions towards the understanding of the structure, composition and/or dynamics of the earth’s mantle and core’. it is awarded in odd-numbered years. lehmann donated her entire estate and fortune to a fund administered by the danish academy of sciences and letters. it offers a travel award each year alternatively to a psychologist and a geophysicist. those awards started while lehmann was still living, and the first chairman of the awards committee of the lehmann foundation, elvin kejlsø told me, that she was happy to know that the people who vol52,6,2009new 2-12-2009 12:40 pagina 683 684 e. hjortenberg were awarded the lehmann award could use it as an important qualification in their curriculum vitae. the current chairman is søren gregersen. inge lehmann remained involved with the scientific community throughout her long life. she participated in a reception held at the danish geodetic institute on her 100th birthday in 1988, and a large number of geophysicists from both sides of the atlantic participated. some of them had participated in a workshop on «causes and effects of earthquakes at passive margins and in areas of postglacial rebound on both sides of the north atlantic» held in vordingborg, denmark, may 9-13, 1988 (gregersen and basham, 1989). v. keilis-borok and c. kisslinger were there because iugg bureau had a meeting in copenhagen the following days. the danish geophysical society (claus hammer) convened the reception, and elvin kejlsø, v. keilis-borok, carl kisslinger and søren gregersen gave talks. she really enjoyed that the proceedings were tape recorded, enabling her to listen to them repeatedly. a humorous mistake is documented in the children’s book about inge lehmann (jensen and pedersen, 1996, p 57). it shows a letter from the hospital dated january 25, 1989 addressed to inge lehmann’s parents, long since deceased. judging from the two digits, 88 allotted to the year of lehmann’s birth in the personal number, the hospital assumed that she was almost 1 year old instead of almost 101. the same book shows several pictures of inge lehmann. one shows her mother, grandmother and great-grandmother, and on page 57 it credits michael bernth pedersen with helping lehmann write her last paper on seismology. bernth pedersen helped with everyday problems, since he lived nearby, while i helped her when she needed help from a seismologist, looking up references, or typing up manuscripts. michael helped her keep up her fighting spirit until her death. many of inge lehmann’s non-seismological books and one of the flags from the inge lehmann station are now in michael’s possession. in his obituary of inge lehmann nils groes writes: «one day at the hospital, inge told us that all day she had been thinking about her own life and she was content. it had been a long and rich life full of victories and good memories.». when inge lehmann received the award from the seismological society of america (oliver, 1978) jack oliver at cornell university mentions her charming summer cottage in holte that so many seismologists have visited. i was invited there as late as summer 1990, and was impressed by the way she answered the phone from someone that was surprised to find her there. she pretended that going out to one’s summerhouse when the weather is good is the obvious thing to do when you are only 102 years of age («of course i am in the summerhouse», she said). lehmann’s many awards are documented in table ii. a more detailed biography was given by bolt and hjortenberg (1994) and by bolt (1997). 3. the inner core discovery the way inge lehmann discovered the inner core was by observing unexpected large amplitudes in the shadow zone (lehmann, 1936). lehmann’s last paper (lehmann, 1987) has a fig. 4b, also published in lehmann (1931). the text written on the top of that figure is in german, in translation it reads: frankenfurter travel time curves, made by b. gutenberg 1928. the figure contains a continuation of the p’ curve extending from 142° to 100° and tagged «gebeugte wellen?» (bent waves with a question mark). the question mark shows that in 1928 gutenberg was not sure that the waves were diffracted, but he did not get the idea that these waves came from an inner core. lehmann (1936) presented the idea, and reflected and refracted rays from the inner core in lehmann’s simple earth model explained the waves. gutenberg readily accepted the idea of an inner core (gutenberg and richter, 1938, p. 364), while jeffreys was slower to do so (jeffreys, 1939, p. 554). the wording of gutenberg and richter was that they accepted lehmann’s hypothesis «that the observations of p’ are not due to diffraction in a normal manner through the core. howvol52,6,2009new 2-12-2009 12:40 pagina 684 685 inge lehmann’s work materials and seismological epistolary archive ever, the velocity distribution required to account for these observations differs greatly from the hypothetical model used in her paper». this great difference is not surprising; she was not trying to estimate the real velocities but presented a simple earth model with constant velocity in each shell of the model, so that the time distance curve could be calculated by simple trigonometry and paths were straight lines. the wording of jeffreys (1939) was «it seems therefore that diffraction must be abandoned as an explanation of pkp at distances less than 142 degrees». additional confirmation of the inner core was given by engdahl et al. (1970) by using the large aperture seismic array (lasa). inge lehmann discovered the inner core, but it is not correct to say that she discovered the solid inner core, her model does not consider the question about whether the inner core is solid or fluid, that was done by others much later (e.g. birch, 1940; bullen, 1946; bolt, 1987 and cao et al., 2005). hence she discovered the inner core, that later proved to be solid. simon (1993) discusses her personal memoir of inge lehmann: «i had heard long before her arrival at lamont, that she was the seismologist who had proved the existence of the inner core of the earth». a short but good version of the story of the inner core discovery is given by oliver (1978): «her best–known paper, succinctly entitled p’ demonstrated that this phase is not the result of diftable ii. inge lehmann’s many awards. year(s) awards to inge lehmann 1936-1948 member of the executive committee of the international seismological association. 1938 tagea brandt award. 1951-1954; member of the executive committee of the international association of seismology and 1957-1960 physics of the earth’s interior (iaspei). 1957 associate royal astronomical society, london. 1959 honorary fellow royal society, edinburgh. 1960 the harry oscar wood award in seismology. 1963-1967 vice president of the executive committee of the international association of seismology and physics of the earth’s interior. 1964 doctor of science (sc.d.) h.c. colombia university in the city of new york. deutsche geophysikalische gesellschaft, emil-wiechert medal. 1965 kgl. danske videnskabernes selskab (royal danish academy of sciences and letters) gold medal. 1967 tagea brandt award. 1968 doctor of philosophy (dr. phil.) h.c. university of copenhagen. 1971 american geophysical union, bowie medal given for outstanding contributions to fundamental geophysics and unselfish cooperation in research. 1973 honorary membership of european geophysical society. 1977 medal of the seismological society of america. vol52,6,2009new 2-12-2009 12:40 pagina 685 686 e. hjortenberg fraction as was held at that time, but was a clear indication of an inner core of higher velocity, thus revealing to mankind for the first time this major feature of the earth.». schweitzer (2003a, p. 19) gives another short but good version of the same story: «both gutenberg and jeffreys overlooked the existence of an inner core, which was discovered by the danish seismologist inge lehmann (1888-1993; emmerich and schweitzer, 1988; bolt, 1997; kertz, 2002) in 1936 (lehmann, 1936; 1987).». brush (1980, p. 706) notes that it is the last name of inge lehmann that is associated with the discovery of the inner core, but few people outside the geophysical/seismological community seem to realize that one of the most important discoveries about the structure of the earth was made by a woman. bischoff (2004) notes that in her studies of the 1929 buller, new zealand earthquake, inge lehmann had found that the stations swerdlowsk and irkutsk had substantial amplitudes although they were in the shadow zone. bischoff also claims that jeffreys four years before the publication in 1936 did not show interest in lehmann’s conclusions about a possible inner core. this lack of interest in 1932 on the part of jeffreys was displayed in a poster at the esc potsdam 2004 general assembly. the poster showed how the idea of a discontinuity surface within the core was proposed in the last lines of her letter to jeffreys dated may 31, 1932 (hjortenberg and larsen, 2004), it is also shown here. the first page of the letter is shown in fig. 2, second page is shown in fig. 3, in the last lines of the second page she proposes the idea of a discontinuity surface within the core. at the discontinuity surface the velocity increase with depth. she wrote that there is hardly anything to disprove such a surface in present observational data. in a letter from jeffreys dated june 1 (figs. 4 and 5), he remarks; «macelwane is a bit casual about the core. i should have thought that a good jesuit would have jumped at the discovery of hell, but he reacts all wrong.». in jeffreys’ june 4, 1932 letter he does not comment the proposal. the letter from jeffreys dated 1932 june 1 was not shown in potsdam 2004, it was somehow overlooked in the lehmann archive in 2004 when documents were selected for the poster, and it is shown here in fig. 4 and fig. 5. therefore bischoff were led to believe that jeffreys were uninterested in the inner core. it is surprising that it is dated already the day after lehmann’s letter. it is interesting to note that in lehmann’s letter shown in figs. 2 and 3 attention is given to the mantle discontinuity as well as to the inner core discontinuity, but she doesnot use the word mantle. 4. the lehmann discontinuities birch (1971) gave a speech when inge lehmann was awarded the bowie medal by the american geophysical union. he said that her 1936 paper added a new discontinuity to the seismic structure of the earth, the lehmann discontinuity, which divided the core into inner and outer parts. don anderson (1979, 1981) use the title lehmann discontinuity for a discontinuity in the mantle at 190-250 km and this use continues. on february 26-28, 2004, a symposium organised by hans thybo and entitled «seismic heterogeneity in the earth’s mantle: thermopetrological and tectonic implications», was held at the royal danish academy of sciences and letters, copenhagen with don anderson as key speaker. at the symposium it was decided that the use of the expression «lehmann discontinuity» also for a mantle discontinuity is ok. during the symposium it was clear that each time the lehmann discontinuity was mentioned the mantle discontinuity was meant. karato (1992) and references therein give additional examples of the use of the expression lehmann discontinuity for an upper mantle discontinuity. karato proposes the lehmann discontinuity (~200km) to result from a change in anisotropy structure with depth. vol52,6,2009new 2-12-2009 12:40 pagina 686 687 inge lehmann’s work materials and seismological epistolary archive fig. 2. first page of inge lehmann’s may 31, 1932 letter. she offers to send records of the azores earthquake that shows the discontinuity in the upper layers using the records at distances from 19 to 22 degrees. vol52,6,2009new 2-12-2009 12:40 pagina 687 688 e. hjortenberg fig. 3. second page of inge lehmann’s may 31, 1932 letter. only the last 15 lines are about the inner core. they deal with the «increase of p’ before the focal zone». «i suppose they could be explained by a discontinuity surface within the core at which the velocity increases». in the paragraphs before the last 15 lines one or two discontinuities in the upper layers are discussed. vol52,6,2009new 2-12-2009 12:40 pagina 688 689 inge lehmann’s work materials and seismological epistolary archive 5. post-retirement research in 1951 maurice ewing, who had discovered the phase lg together with frank press (press and ewing, 1952), visited copenhagen (lehmann, 1978), and he invited inge lehmann to come to lamont (now lamont-doherty) to assist in the study of lg. in 1952 she made her first visit to the united states, and when inge lehmann retired in 1953; 5 years before the mandatory retirement, she felt free to carry out seismological studies at home and abroad. while at the lamont-doherty earth observatory in 1953 lehmann published studies of three european earthquakes (lehmann, 1953a), and explained the behaviour of p and s waves on the assumption of a low-velocity layer at some depth below the mohorovicic discontinuity. gutenberg (1948) made the assumption that in california small amplitudes in p and s, and a certain delay can be explained by the presence of a low velocity layer at a depth of about 80 km lehmann (1953a, p. 483) remarked that in eastern north america there is no comparable deep shadow for s as there is in europe. in subsequent years lehmann continued her upper mantle investigations (lehmann, 1955). lehmann (1957) report some results of the lg studies started in 1951, and she found that the deep earthquakes she investigated did not produce lg waves, but the shallow ones did. she concludes that a closer study of the phase lg should yield interesting and important results. this prediction became true near denmark when kenneth et al. (1985) showed a barrier for lg waves in the north sea. lehmann and ewing (1960) deals with lg waves as recorded in copenhagen. the paper is bound in two volumes, one with text and one with diagrams. figure 1 to 31 of these diagrams show copies of seismograms, often with lg waves coming from south or east of copenhagen, but figure 17 shows the seismograms from both scoresbysund and copenhagen of an arctic earthquake also discussed by oliver et al. (1955). figure 32 is a map of all the earthquakes considered in the 1960 paper. inge lehmann published the jeffreys and the lehmann model for the p velocities in the upper 800 kilometers of the mantle (lehmann, 1959, p. 390). the lehmann model differs from jeffreys’ below the mohorovicic discontinuity, where jeffreys has 7.75 km/s and lehmann has 8.0 km/s and lehmann’s velocities have a discontinuity at 220 kilometres depth. her later studies use data from nuclear explosions (lehmann, 1962; 1964) and interpret a phase observed by romney (1959) as a p wave refracted in a discontinuity at a depth of 215 km. her interpretation (lehmann, 1967) of low velocity layers and a 220 km discontinuity was used in making earth models, e.g. the prem model (dziewonski and anderson, 1981). the authors state that their decision to locate discontinuities at 220, 400 and 670 km was based on results from many other studies. lehmann (1970) concludes that the nature of the deep discontinuity in p velocity varies and it may not be present everywhere. 6. the work materials and epistolary archive inge lehmann’s collection of work materials is large, and it contains books, papers, documents, reprints, card files and seismograms (e.g. from the heligoland explosions 1947). one interesting example is a handwritten document from a seminar she gave on the 10th of may 1954 in pasadena. it is about the upper mantle of the earth, its different structure in europe, california and ne america. there is also an envelope with papers about the first danish seismologist, e.g. harboe (1845-1919), stating that he retired from the military in 1897. the inge lehmann epistolary archive contains letters from many countries, including italy, austria, australia, russia, germany, denmark, france, uk, usa and canada. the inge lehmann archive altogether contains many thousands of documents that lehmann stored in her home. they were scanned and catalogued on 2005-2006 by sga by storia geofisica ambiente (bologna), producing more than 5000 files, each containing one or more scanned pages. it is important that each page in the archive can now be uniquely identified by the filename and the number in the file, and the directory names and structure may be regarded as a catalogue. vol52,6,2009new 2-12-2009 12:40 pagina 689 690 e. hjortenberg fig. 4. first page of jeffreys’ 1932 june 1 letter. «thank you for your very informative letter. i’m sorry about the new discontinuity». « no please don’t send me the records; i am snowed under with my present work». it appears that jeffreys is not interested in the new discontinuity in the mantle. vol52,6,2009new 2-12-2009 12:40 pagina 690 691 inge lehmann’s work materials and seismological epistolary archive fig. 5. second page of jeffreys’ 1932 june 1 letter. the main subject is epicentre determinations, by the group method, but in the last paragraph the inner core discontinuity surface is discussed: «macelwane is a bit casual about the core. i should have thought a good jesuit would have jumped at the discovery of hell, but he reacts all wrong». vol52,6,2009new 2-12-2009 12:40 pagina 691 692 e. hjortenberg 6.1. more on the brussels, 1951 assembly, start of european seismological commission (esc) in a two page machine typed document inge lehmann describes in danish the assembly of the international seismological association in brussels 1951. the following two paragraphs are a translation of some of that document: «in brussels 1951 inge lehmann participated in the meeting of the commission on the physics of the earth’s interior (copei) that led to putting copei’s work under the international seismological association (later called iaspei). inge lehmann also took part in the meeting where cooperation between european seismologists was discussed. they found that the best solution would be that the association sets up a european commission. later 5 delegates requested such a commission, and it got 13 members from 13 countries. the commission (european seismological commission, esc) held one meeting on which inge lehmann was elected president, but she refused. then wilhelm hiller was elected president of esc with pietro caloi as secretary. beno gutenberg was elected president of the association with pietro caloi and harold jeffreys as vice-presidents.». 6.2. epistolary archive, sir harold jeffreys (uk) and others in 1971, lehmann (1971) recalled: «i was the only danish seismologist for 25 years. i began to work on travel times and i was fortunate in that harold jeffreys, who at that time was doing his important work on travel time tables could use some of my research. for several years we had quite a correspondence, this was, of course a great encouragement to me.». in 1994, lady jeffreys (1994) wrote: «it was not until my marriage to harold jeffreys in 1940 that i knew of the importance of inge lehmann’s work in observational seismology and for the next five years it was not possible for them to communicate with each other (1). he must have had letters from her in the 1930s but i regret that these have disappeared.». the inge lehmann archive contains a big envelope marked, in translation: «correspondence with harold jeffreys before the war, about time curves, somewhat disordered, first letter from december 19, 1930». they may be difficult to put in order, since jeffreys often wrote the day and month, but not the year, on his letters. i believe that in most cases the year can be deduced from the exact placement of the letter and the content of the letter. less than a quarter of the about 150 letters in the envelope are dated drafts of letters from lehmann to jeffreys, so there may have been more letters and it is a pity that the letters received by jeffreys have disappeared. the correspondence between inge lehmann and sir harold jeffreys about the inner core of the earth was discussed in chapter 3. another example of correspondence topics is the letter from jeffreys august 23, it was answered by inge lehmann august 30, 1934. his letter contained the proof of his manuscript to nature, no. 280 on the state of the earth’s central core. it refutes the assertion by the seismologist l. bastings from new zealand that he observed s waves through the earth core. jeffreys’ article said that: «the lack of rigidity in the core does not rest wholly on the past failure to identify s waves through it. [...]. if bastings’ interpretation is correct, it will be necessary to suppose that the core is elasticoviscous, behaving as solid for stresses with periods of a few seconds but as a liquid for a period of 12 hours.». the letter from inge lehmann confirms her view that jeffreys is right in believing that l. bastings has read pps for s1’, the latter being the hypotetical s wave through the core. the manuscript proof referred back to bastings (1934), to jeffreys (1929; 1926) and to lehmann (1930; 1931). later on lehmann (1950, p. 196) comments further on bastings’ interpretation: «amongst the phases he determined were those of the transverse waves transmitted through the core. there are as we have seen, many phases in the records of some of the stations, so many, indeed, that vol52,6,2009new 4-12-2009 15:53 pagina 692 693 inge lehmann’s work materials and seismological epistolary archive fig. 6. first page of inge lehmann’s may 6, 1932 letter to harold jeffreys. vol52,6,2009new 2-12-2009 12:40 pagina 693 694 e. hjortenberg fig. 7. second page of inge lehmann’s may 6, 1932 letter to harold jeffreys. this paper was also shown in potsdam 2004, and one of the words is german, scheiteltiefe, the depth reached by the deepest ray. vol52,6,2009new 2-12-2009 12:40 pagina 694 695 inge lehmann’s work materials and seismological epistolary archive it will be possible to pick out phases to fit almost any time-curve, but a proof of the existence of the corresponding phase is not obtained in this way. it is not obtained unless there is a concentration of points close to the curve.». lehmann (1953b) further discuss bastings’ publications during the time span 1933-1937. many other observations can be derived from the correspondence with jeffreys: on may 6, 1932 lehmann writes: «i was determined to prove that your tables were the most excellent tables, but – why have you spoilt them by smoothing. you have smoothed them excellently – no more discontinuity surfaces in the mantle of the earth. the velocity increases first more rapidly then slowly with depth. but byerly’s (2) curve has a sharp bend about 20°, your reduced iss (3) data pointed to the reality thereof. i have now studied two earthquakes iceland, azores with a good deal of observations on either side of 20°.» (the full text of the letter is shown in figs. 6 and 7). on november 18, 1932 she sends a letter to jeffreys about her time curve: «i feel quite sure about the bend about 20°, but that is exactly the sort of thing which is not exhibited by the statistical method.». on december 14, 1932 she writes again: «i am glad to hear that – at last – you believe in the bend about 20°.». on march 29, 1936 she writes about amplitudes at distances 12° to 15°. «if you had seen so many records from these distances as i have seen, i am sure you would not doubt that amplitudes are abnormally small.». such observations are examples of the historic and scientific value of letters in earth science (ferrari, 2002), (ferrari, 2003). it shows e.g. that the inner core was discussed 4 years before lehmann (1936). we are fortunate that lehmann kept drafts or carbon copies of the outgoing letters and not only the letters she received. letters from harold jeffreys from after the war are filed in different places in the archive. there is also correspondence with pietro caloi, for instance, in connection with the publication of the work of the vatican meeting (lehmann, 1952), and correspondence with a lot of other seismologists, e.g. with b. gutenberg, v. conrad, p.v. willmore, s. müller. m. båth. f. press, m. ewing, j. oliver, e. peterschmitt, and j.h. hodgson. 6.3. niels bohr archive the niels bohr archive has made copies of 9 letters from the correspondence between lehmann and bohr and 3 letters about inge lehmann. seven letters of those 9, and one season’s greeting were written by inge lehmann at newnham college, cambridge in 1911. they deal with invitations for tea, and the necessity of a chaperone, which must have surprised inge lehmann after hanna adler’s school. she sent a message of congratulation to niels bohr at the carlsberg mansion (now carlsberg academy) in honour of his 60 years birthday on october 7, 1945. a letter dated october 11, 1947 thanks mrs bohr for a beautiful box left at the death of hanna adler, and adds that a copy of inge lehmann’s paper on hanna adler is enclosed. it also recalls inge lehmann’s visits to her when she had to miss her family who were in sweden.in the letter inge lehmann adds that it was always a pleasure to visit miss adler. in a letter january 14, 1962 from harold jeffreys to niels bohr, he wonders whether inge lehmann has ever received any honour in her own country. in a letter july 11, 1962 from niels bohr to niels erik nørlund (lehmann’s former director) he suggests the award of the gold medal of the danish academy of scienses and letters. in a letter to niels bohr dated august 22, 1962 niels erik nørlund confirms that he will propose that when the academy meets again. the medal was awarded in 1965. 6.4. additional items to bibliography a bibliography with inge lehmann’s publications on microfiche with 59 references is invol52,6,2009new 4-12-2009 15:53 pagina 695 696 e. hjortenberg cluded with bolt (1997). information about additional references has been found in the lehmann archive. that information is given in the present publication as lehmann (1948b; 1948c; 1965; 1971; 1978). a paper with 32 pages and 9 plates entitled «the seismological stations københavn and scoresbysund» was published by the geodetic institute in 1930. it is undoubtly written by inge lehmann, but author’s name is not given in the publication. 7. conclusions this paper deals with inge lehmann’s achievements and the inge lehmann archive. the safeguarding of the inge lehmann archive was done by sga (storia geofisica ambiente in bologna) by scanning and cataloguing the documents 2005-2006. the files obtained can help getting a better understanding of inge lehmann’s publications and of the historical development that she was part of. the paper also shows how she presented the idea of an inner core to jeffreys 4 years before 1936, and jeffreys reply. acknowledgements thanks to graziano ferrari for inviting me to the meeting on ischia 2003, and thanks to søren gregersen, niels groes, kristian hvidtfelt, tine larsen, tyge lehmann, louise palmer, michael bernth pedersen, felicity pors, torben risbo, peter voss and bente winstrøm for valuable comments. references adler-samfundet (1959): hanna adler og hendes skole, udgivet i 100-året for hanna adlers fødsel den 28 maj 1959, with a preface by niels bohr, (gec gads forlag, københavn), p. 183 (in danish). anderson, d.l. (1979): the deep structure of the continents, j. geophys. res., 84, 7555. anderson, d.l. (1981): discontinuities in the mantle (abstract), eos trans. agu, 62, 1073. bang, t. (1988): niels erik nørlund in memoriam, acta matematica, 161 (1-2), 11-22. bastings, l. (1934): shear waves through the earth’s core, nature, 134, 216. birch, f. (1940): the alpha-gamma transformation of iron at high pressures, and the problem of the earth’s magnetism, am. j. sci., 238, 192. birch, f. (1971): william bowie medal to inge lehmann, eos, 52, 537-538. bischoff, j. (2004): das geheimnis der p-wellen (in german), geokompakt, 1, 60-61. bolt, b.a. (1987): 50 years of studies on the inner core, eos, 68, 80. bolt, b.a. (1997): inge lehmann. biographical memoirs of fellows of the royal society of london, 43, 285-301 (also on internet). bolt, b.a. and e. hjortenberg (1994): inge lehmann (1888-1993), bull. seis. soc. amer., 84 (1), 229-233. bullen, k.e. (1953): the rigidity of the earth’s inner core, ann. geofis., 6, 1. brush, s.g. (1980): discovery of the earth’ core, am. j. phys., 48 (9), 705-724. brush, s.g. (1985): women in physical sciences, the physics teacher, 11-19. cao, a., b. romanovich and n. takeuchi (2005): an observation of pkjkp: inferences on inner core shear properties, science, (published online 14 april, sciencexpress 2005 0: 11091341). dziewonski, a.m. and d.l. anderson (1981): preliminary reference earth model, phys. earth. planet. int., 25, 297-356. emmerich, h. and j. schweitzer (1988): inge lehmann – die entdeckerin des inneren erdkerns, deutsche geophysikalische gesellschaft e. v., mitteilungen , 2, 5-6, (in german). engdahl, e.r., e.a. flinn, and c.f. romney (1970): seismic waves reflected from the earth’s inner core, nature, 228, 852. notes (1) because of world war ii and the german occupation of denmark. (2) american seismologist. (3) international seismological summary, an annual publication with epicenter determinations and readings of phases. it was replaced by international seismological centre (isc) bulletins in 1964. lehmann played a key role in the initiation of isc. vol52,6,2009new 2-12-2009 12:40 pagina 696 697 inge lehmann’s work materials and seismological epistolary archive ferrari, g. (2002): letters in the earth sciences: their historic value and present-day scientific relevance, annals of geophysics, 45 (5), 703-707. ferrari, g., (2003): census, filing and elaboration of scientific letters in earth sciences, nuncius, 17 (1), 307320. gregersen, s. and p.w. basham (1989): earthquakes at north-atlantic passive margins:neotectonics and postglacial rebound, proceedings of nato advanced research workshop in vordingborg, may 9-13. 1988, nato asi series, series c, 2-66, (kluwer academic publishers, dordrect, boston and london), 716. gregersen, s, j. hjelme and e. hjortenberg (1998): earthquakes in denmark, bull. geol. soc. denmark, 44, 115-127. gutenberg, b. (1914): über erdbebenwellen, viia. nachr. ges. wiss. göttingen math. physik. kl, 166., 125, (in german). gutenberg, b. and c.f. richter (1938): p’ and the earth’s core, mon. not. roy. soc. geoph. suppl, 4 (5), 363-372. gutenberg, b. (1948): on the layer of relatively low wave velocity at depth of about 80 kilometers, bull. seismol. soc. amer., 38, 121-148. hansen, t. (1962): det lykkelige arabien: en dansk ekspedition 1761-67, (2nd edition, gyldendal, københavn), (in danish). hansen, t. (1964): arabia felix: the danish expedition of 1761-1767, (1st edition, collins, london). hjortenberg, e. and t.b. larsen (2004): the scientific correspondence between inge lehmann and harold jeffreys, esc xxix abstract volume, 76. jeffreys, h. (1926): mon. not. roy. soc. geoph. suppl., 1, 371-383. jeffreys, h. (1929): the earth, its origin, history and physical constitution, (2nd edition, e university press, cambridge), 238. jeffreys, h.(1939): the times of the core waves, mon. not. roy. soc., geoph. suppl., 4, 548-561. jeffreys, lady b. swirles (1994): inge lehmann; reminiscenses, quarterly j. roy. astr. society, 35 (2), 231. jensen, s.s. and h.b. pedersen (1996): inge lehmann – kvinden og jordskælvene, (gyldendal, københavn ), p. 64 (in danish). karato, s. (1992): on the lehmann discontinuity, geophysical research letters, 19 (22), 2255-2258. kenneth, b., s. gregersen, s.m. mykkeltveit and r. newmark (1985): mapping of crustal heterogeneity in the north sea basin via the propagation of lg-waves, geophysical journal of the royal astronomical society, 83, 299-306. kertz, w. (2002): biographische lexicon zur geschichte der geophysik, edited by k.h. glassmeier and r. kertz, (braunschweigische wissenschaftliche gesellschaft, braunschweig), p. 384 (in german). kousholt, b. (2004), inge lehmann og jordens kerne, (polyteknisk forlag), p. 90, (in danish). lehmann, i. (1930): p’ as read from the records of the earthquake of june 16th 1929, gerl. beitr. geophys., 26, 402-412. lehmann, i. (1931): die bedeutung der europäischen stationsgruppe für die bestimmung von seismischen laufzeitkurven, (in german), verhandl. fünften tagung balt. geod commission, 192-212, helsinki. lehmann, i. (1936): p’, publications du bureau central seismologique international, série a, travaux scientifique, 14, 87-115. lehmann, i. (1948a): on two explosions in danish waters in the autumn of 1946, geofisica pura e applicata, 12, 3-19. lehmann, i. (1948b): jordskælv som kilde til øget viden om jordens indre, (in danish), salmonsens leksikontidsskrift, 8, 808-812. lehmann, i. (1948c): seismic recording in europe, 1948, association séismol. comptes rendus, séances conférences strasbourg 4-8 juillet 1947, 79-83. lehmann, i. (1950): on the reading of earthquake diagrams and the interpretation of phases, publ. bur. cent. seism. int. a, 17, 193-204. lehmann, i. (1952): on the microseismic movement recorded in greenland and its relation to atmospheric disturbances. ponteficiae acad. scient. scripta varia, 23, 73-109. lehmann, i. (1953a): p and s at distances smaller than 25°, transactions of american geophysical union, 34 (3), 477-483. lehmann, i. (1953b): on the shadow of the earth’s core, bull. seismol. soc. amer., 43, 291-306. lehmann, i. (1955): the times of p and s in northeastern america, annali di geofisica, 8 (4), 351-370. lehmann, i. (1956): danske jordskælv, meddelelser fra dansk geol. forening, 13 (2), 88-103, (in danish). lehmann, i. (1957): on lg as read on northamerican records, annali di geofisica, 10 (1-2), 1-21. lehmann, i. (1959): velocities of longitudinal waves in the upper part of the earth’s mantle, ann. géophys., 15, 93118, publ. dom. obs. ottawa, 19 (10), 381-406. lehmann, i. and m. ewing (1960): on short-period surface waves as recorded in copenhagen, geodætisk instituts skrifter 3. række, memoires de l’institut géodésique de danemark troisième série, 34, pp. 19 + 32 figures, separately bound, and respectively marked text and diagrams. lehmann, i. (1962): the travel time of the longitudinal waves of the logan and blanca atomic explosions and their velocities in the upper mantle, bull. seismol. soc. amer., 52 (3), 519-528. lehmann, i. (1964): on the travel times of p as determined from nuclear explosions, bull. seismol. soc. bull., 54 (1) 123-139. lehmann, i. (1965): errata (to) on the travel times of p as determined from nuclear explosions, bull. seismol. soc. amer., 55 (1), 201. lehmann, i. (1967): low-velocity layers, in the earth’s mantle, edited by t.f. gaskell, (academic, new york), 41-62. lehmann, i. (1968): discontinuous velocity changes in the mantle as derived from seismic travel times and amplitudes, bur centr. seism. int. a, 24, 167-174. lehmann, i. (1970): the 400-km discontinuity, geophys. j. r. astr. soc., 21, 359-372. lehmann, i. (1971): william bowie medal to inge lehmann, acceptance and response by inge lehmann, eos, 52, 538. lehmann, i. (1978), response, third award of the seismological society of america (in 1977), bull. seismol. soc. amer., 68 (4), 1210. lehmann, i. (1987): seismology in the days of old, eos, 68, 33-35. vol52,6,2009new 2-12-2009 12:40 pagina 697 698 e. hjortenberg lenton, r. a. (1968): greenland ice cap noise studies, project blue ice, arctic institute of north america, washington, d. c., research paper no. 47. oliver, j., m. ewing and f. press (1955): crustal studies of the arctic region from the lg phase, bull. geol. soc. america, 66, 1063–1074. oliver, j. (1978), citation, third award of the seismological society of america (in 1977), bull. seismol. soc. amer., 68 (4), 1209. press, f. and m. ewing (1952), two slow surface waves across north america, bull. seismol. soc. amer., 42, 219-228. romney, c.f. (1959): amplitudes of seismic body waves from underground nuclear explosions, j. geophys. res, 65, 1039-1051. schweitzer, j. (2003a), chapter 79, part a, early german contributions to modern seismology, in iaspei international handbook of earthquake and engineering seismology, part b, edited by w.h.k. lee, h. kanamori, p.c. jennings, and c. kisslinger, (academic press), isbn 0-12-440658-0. schweitzer, j. (2003b), chapter 79, part d: prof. dr. beno gutenberg the bibliography in iaspei international handbook of earthquake and engineering seismology, part b, edited by w.h.k. lee, h. kanamori, p.c. jennings, and c. kisslinger, (academic press), isbn 012-440658-0. simon, r. (1993), a personal memoir of inge lehmann (1888-1993), eos, 74, 511-512. vol52,6,2009new 2-12-2009 12:40 pagina 698 hejda_final:layout 6 secular variation on the territory of the czech republic and reduction of the magnetic survey to the epoch 2010.5 pavel hejda*, josef horáček, tomáš bayer institute of geophysics of the ascr, prague, czech republic annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5445 abstract a recent stage of the magnetic surveying activities in the czech republic was started in the fifties by setting up a basic network of the first order. it consists of 199 points. complete measurements of three components, d, h, and z, were carried out in 1976-78 and 1994-96. since 1970 six selected points have been revisited approximately every two years. five of them are situated close to the czech boundaries, the last one inland. the repeat station network was complemented by another inland station in 2003. the repeat station measurements, together with the data from neighboring observatories, were now used to reduce the data from the last magnetic survey to the epoch 2010.5 and new magnetic charts were drawn. the accuracy of the results and their comparison with igrf model is also discussed. 1. introduction the first magnetic survey on czech territory was carried out by k. kreil in 1843-1845. he measured the declination, inclination and horizontal intensity at 24 sites (including the prague observatory) in bohemia and derived magnetic maps for 1845 [kreil and fritsch 1846]. j. liznar carried out magnetic measurements between 1889 and 1894 as part of the magnetic mapping of the former austro-hungary and created an extensive set of maps for the epoch 1890.0 [liznar 1898]. during the next 60 years, only measurements of magnetic declination were made. the recent stage of the magnetic surveying activities was started in the fifties by setting up a basic network of the first order. it consists of 199 points, which have been stabilized by means of granite boundary stones and marked by black and white aluminum rods. measurements of the vertical and horizontal components were carried out in 1957-58 and supplemented by reduced values of declination obtained during a former campaign [bouška et al. 1960]. the network was re-occupied in 1976-78 when three components, d (measured by matting & wiesenberg theodolite), h (by qhm) and z (by bmz) were complemented by measurements of the total field f (by ppm). the latter enabled to correct random errors. the azimuth was determined by observations of the sun [bucha et al. 1983]. the last magnetic survey in 1994-96 was carried out by a fluxgate theodolite and a proton precession magnetometer [hejda et al. 1999]. the magnetic maps of d, h and z are shown in figure 1. while the measurements were being made, a revision of the network was undertaken. more than fifty points were found disturbed by building activities and had to be moved to elsewhere. since 1970 six selected points have been revisited approximately every two years. five of them are situated close to the czech boundary, the last one is in eastern bohemia. the repeat station network was complemented by station mýto (western bohemia) in 2003. the stations, together with neighboring observatories, are shown in figure 2. eight series of measurements are carried out at every station. all measurements are reduced to annual means centered to the middle of the year, in which the station was measured using data from the geomagnetic observatory of budkov. no on-site variometer is used. the measurements were usually carried out in odd years. according to the recommendation of the first workshop on european repeat station measurements held in niemegk in 2003, the term was changed to even years, starting from 2004. carrying out the magnetic survey in the first-order network is laborious and expensive. but in fact the local structure of the geomagnetic field is nearly time-invariable. it thus makes sense to update the results of the magnetic survey by means of repeat station measurements. 2. temporal behavior of the geomagnetic field figure 3 shows the time variation of the geomagnetic field on czech territory observed at the repeat stations during the past 40 years. the study of the secular variation is extremely sensitive to any move of the repeat station position. we were happy that all stations have kept their positions for the last thirty years. article history received october 14, 2011; accepted december 19, 2011. subject classification: geomagnetic field variations and reversals, global and regional models, magnetic anomalies, data processing. 1095 magnete workshop 2011 hejda et al. 1096 figure 1 (left column, top to bottom, up to horizontal line). maps of the geomagnetic field for the epoch 1995.5 based on the magnetic survey carried out between 1994 and 1996 [hejda et al. 1999]. declination is given in degrees of arc, contour interval is 0.1 degree. horizontal and vertical components are given in nt, contour interval is 100 nt. figure 2 (left). czech repeat stations and neighboring observatories used for modeling of the secular variation. figure 3 (right column, top to bottom). repeat station measurements on the territory of the czech republic and budkov observatory data. all values were reduced to the yyyy.5 epoch. declination is given in minutes of arc, horizontal and vertical components are given in nt. 1097 magnetic declination has been increasing during the entire period, and the increment was larger in the west. the temporal change is nearly linear over the last 20 years. during this period, which we are interested in, the secular variation in the east (ostrava) was 5.8 minutes of arc per year whereas in the west (aš) 6.6 minutes of arc per year. the horizontal component has been oscillating with the local maximum around 1980 and local minimum around 1990. recent observatory data indicates that the horizontal component in central europe is approaching another local maximum. for example, the yearly mean values of h in niemegk were 18,854 nt in 2008, 18,866 nt in 2009 and 18,866 nt in 2010, the values for fürstfeldbruck were 20,929, 20,945 and 20,948, for budkov 20,381, 20,395 and 20,396 and for tihany 21,516, 21,528 and 21,528, respectively. this could lead to larger regional differences. the vertical component behaves most regularly. it has been increasing linearly with very weak regional differences. the secular variation varies between 31.3 nt per year in vyšší brod and 32.5 nt per year in ostrava. 3. model of the secular variation in order to reduce the magnetic maps to the epoch 2010.5, the secular variation between 1995.5 and 2010.5 had to be computed. in view of the low number of data, we have considered quadratic polynomials computed by the least-squares method. our original idea was to employ also data from neighboring observatories. there are two good reasons for this step. first, we would increase the number of experimental points and, second, we would avoid unnatural curvature of the contour lines at the edge of the region covered by the data. we started with the complete quadratic polynomial (6 coefficients) and eliminated step-wise the quadratic coefficients. the results indicated that the quadratic terms could introduce artificial effects and, after analyzing the situation, we decided in favour of linear approximation. at that moment there was no reason for keeping the data from neighbouring observatories. eight measured points are sufficient for a linear 2-dimensional function, and problems at the boundary cannot occur. on the contrary, the linear model fits small areas better. it is secular variation in the czech republic station name d (degrees of arc) h (nt) z (nt) observed error observed error observed error budkov 1.5328 -0.0086 107.50 1.40 474.20 0.42 aš 1.6475 0.0046 111.10 -1.00 473.60 0.82 frýdlant 1.5762 0.0006 91.30 0.51 481.45 -0.89 lanžhot 1.4320 -0.0043 88.50 -1.06 480.70 0.58 mýto 1.5633 -0.0131 102.81 -1.68 472.86 -2.45 ostrava 1.4467 0.0073 84.20 8.27 488.00 0.82 poli ka 1.4740 -0.0197 87.74 -1.22 482.60 1.00 vyšší brod 1.5158 0.0011 109.60 3.17 469.44 -3.62 figure 4. linear model of secular variation between 1995.5 and 2010.5. table 1 (below). secular variation between 1995.5 and 2010.5 epochs and its linear model; error = observed_value modeled_value. advantageous that our repeat stations are situated close to boundaries, because interpolation is more accurate than extrapolation. the linear models of the d, h and z components of the secular variation are the following: dsv(m,i) = 0.20788 + 0.031817 m + 0.036256 i, hsv(m,i) = 426.78 − 6.251 m − 4.749 i, zsv(m,i) = 281.73 + 2.5578 m +3.182 i, where m denotes longitude and i latitude. the maps are shown in figure 4. the measured values and differences as compared to the model are summed up in table 1. the results for the declination and vertical component are fairly satisfactory. the maximum error is about 1 per cent (1.4 minutes of arc) for the declination and less than 1 per cent for the vertical component. as we have seen already in section 2, the horizontal component behaves more irregularly. stations ostrava and vyšší brod are especially anomalous, and the error was not improved even by quadratic approximation. in spite of this, the model of the secular variation provides a good basis for upgrading the maps. 4. magnetic maps for the epoch 2010.5 values updated to the epoch 2010.5 were computed by the simple formula d2010.5(m,i) = d1995.5(m,i) + dsv(m,i). the maps (figure 5) were drawn with the commercial contouring program surfer. the contour lines of the declination are relatively smooth in the central part of the czech republic, whereas regional anomalies can be found in the western part of the bohemian massif and where the bohemian massif touches the carpathians. the horizontal and vertical components confirm the previous statement, even if the regional anomalies in the horizontal component are not so striking. 5. discussion upgrading data from the magnetic survey by using repeat station measurements is an efficient and relatively inexpensive way of keeping magnetic maps up-to-date. an alternative way would be to use the international geomagnetic reference field (igrf) [finlay et al. 2010]. although the igrf is in general not suitable for approximating a reference field on such small areas as the czech republic (characteristic minimum wavelength associated with the igrf is 3000 km), it was shown [hejda et al. 1999] that the igrf approximates the measured magnetic field on czech territory surprisingly well. the results summed up in table 2 confirm this statement. if we realize that the differences between the observed and modeled data are in some areas as much as hundreds of nanoteslas, the results in table 2 provide a very good fit. nonetheless, the igrf model of the secular variation is unbalanced. all differences between the model and measured data are positive for the declination and negative for the horizontal and vertical components. anyway, the fit obtained by the linear model based on repeat station measurements is one order more accurate and should be given priority. it confirms that the repeat station surveys represent an important element in the chain of geomagnetic field observations. our results are in a good agreement with other regional studies [e.g. chen et al. 2010 or qamili et al. 2010], which came to the conclusions that whereas long-term trends in secular variation are consistent with igrf better fit of regional anomalies require models based on local data . hejda et al. 1098 figure 5. maps of the geomagnetic field for the epoch 2010.5 derived from the magnetic survey carried out between 1994 and 1996 [hejda et al. 1999] and linear model of secular variation between 1995.5 and 2010.5. declination is given in degrees of arc, contour interval is 0.1 degree. horizontal and vertical components are given in nt, contour interval is 100 nt. 1099 more detailed information, including the values of the three magnetic component of all station of the basic network of the first order, reduced to the epoch 2010.5, will be available on the web of the institute of geophysics of the ascr, http://www.ig.cas.cz. acknowledgements. the data acquisition was supported by the project of large research infrastructure czechgeo/epos, grant no. lm2010008. references bouška j., v. bucha and a. kočí (1960). geomagnetische karten der tschechoslowakischen republik für die epoche 1958,0, travaux inst. géophys. acad tchécosl. sci., no 112. bucha, v., i. cupal, j. horáček and a. pazderková (1983). investigation of the geomagnetic field and its secular variation on the territory of bohemia and moravia: epoch 1978.5, travaux inst. géophys. acad tchécosl. sci., no 583. chen, b., z.w. gu, j.t. gao, j.h. yuan and c.z. di (2010). study of geomagnetic secular variation in china, chinese j. geophys., 53, 2144-2154; doi:10.3969/j.issn.0001-5733. 2010.09.014 finlay, c.c, s. maus, c.d. beggan, t.n. bondar, a. chambodut, t.a. chernova, a. chulliat, v.p. golovkov, b. hamilton, m. hamoudi, r. holme, g. hulot, w. kuang, b. langlais, v. lesur, f. j. lowes, h. luhr, s. macmillan, m. mandea, s. mclean, c. manoj, m. menvielle, i. michaelis, n. olsen, j. rauberg, m. rother, t. j. sabaka, a. tangborn, l. toffner-clausen, e. thebault, a.w.p. thomson, i. wardinski z. wei and t.i. zvereva (2010). international geomagnetic reference field: the eleventh generation, geophys. j. int., 183, 1216-1230; doi: 10.1111/ j.1365-246x.2010.04804.x. hejda, p., j. horáček, v. bucha and i. cupal (1999). geomagnetic field on the territory of the czech republic for 1995.5, phys. chem. earth (a), 24, 445-448. kreil, k., and k. fritsch (1848). magnetische und geographische ortbestimmungen in böhmen, erster jahrgang 1846, prag. liznar, j (1898). die vertheilung der erdmagnetischen kraft in 'osterreich-ungarn zur epoche 1890,0 nach den in den jahren 1889 bis 1894 ausgeführten messungen, teil i., ii., wien. qamili, e., a. de santis, g. cianchini, b. duka, l.r. gayapique, g. dominici and n. hyka (2010). two geomagnetic regional models for albania and south-east italy from 1990 to 2010 with prediction to 2012 and comparison with igrf-11, earth planets space, 62, 833-841; doi: 10.5047/eps.2010.07.011. *corresponding author: pavel hejda, institute of geophysics of the ascr, prague, czech republic; email: ph@ig.cas.cz. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. secular variation in the czech republic station name d (degrees of arc) h (nt) z (nt) igrf diff igrf diff igrf diff budkov 1.5058 0.0270 112.05 -4.55 484.22 -10.02 aš 1.6314 0.0161 112.74 -1.64 487.56 -13.96 frýdlant 1.5593 0.0169 99.64 -8.34 501.21 -19.76 lanžhot 1.3960 0.0360 102.52 -14.02 484.26 -3.56 mýto 1.5495 0.0138 109.76 -6.95 489.08 -16.22 ostrava 1.4124 0.0343 94.58 -10.38 495.79 -7.79 poli ka 1.4604 0.0136 101.48 -13.74 492.67 -10.07 vyšší brod 1.4745 0.0413 112.82 -3.22 481.22 -11.78 table 2. secular variation between 1995.5 and 2010.5 epochs and its linear model; error = observed_value modeled_value. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true /maxsubsetpct 100 /optimize false /opm 1 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 64, 5, vo549, 2021; doi:10.4401/ag-8648 o p e n acc e s s resilience and adaptation to volcanoes in late middle ages in lipari island (aeolian, italy) maria clara martinelli*,1, marco manni2, mauro coltelli2 (1) parco archeologico delle isole eolie museo luigi bernabò brea, lipari (2) istituto nazionale di geofisica e vulcanologia osservatorio etneo, catania article history: received march 4, 2021; accepted august 17, 2021 abstract volcanic activity resumed during early middle ages times at lipari following at least 6000 years of quiescence. this phenomenon occurred in a social context that had continuously developed from prehistoric times to the roman age and was burdened by a demographic crisis that involved the archipelago between the 6th and 11th century ad. the rare archaeological records relating to the 6th 11th centuries suggest abrupt changes in the population of the islands. the medieval sources are rich in religious and fantastic references to volcanic events linked to lipari and vulcano, testifying the uneasy condition for the human communities. this work concerns the resilience and adaptation of the communities to volcanic activity during the late middle ages in lipari. starting from 1083 the aeolian archipelago was involved in a repopulation program, implemented in 1095 by the constitutum and organized by the benedictine monastery with the annexed s. bartolomeo cathedral on the castle. from the 13th century the volcanic phenomena, strictly limited to the northern sector of the island, did not interfere as previously with the anthropic activities. the monastery will be enlarged in the norman phase during the first half of the 12th century with the construction of the cloister. new historical documents relating to the 1264, report news of fires and land movements on lipari. recent age determinations obtained for the obsidian flow of rocche rosse at 1220 ± 30 ad (archaeomagnetic dating) and for an obsidian block of the lami pyroclastic cone at 1243 ± 190 (fission-track dating) allow to define the age of the last phase of activity of the monte pilato-lami-rocche rosse complex, and to associate it the events reported on 1264’s historical documents. this work makes in comparison volcanological, archaeological and historical dates and described an updated summary of one of the lesser known phases of the history of the archipelago. the main consequence of the medieval volcanic activity at lipari caused a clear division of the territory with the population confined in the southeast quadrant, protected to the north by serra and monte rosa which represented a natural orographic barrier. key words: aeolian island; middle ages; lipari; volcano activity. 1. introduction the population of the aeolian islands (figure 1) in the medieval period, from the 6th century to the 13th century, has many gaps in research that make difficult the historical reconstruction. this contribution will try to offer some new information to the historical and cultural context of the last eruptions of lipari that involved the island between the 8th and the 13th centuries. this contribution, although not exhaustive, take in consideration historical, archaeological and volcanological data with the aim to focus the changes of the territory and human communities in relation to the volcanic activity. 2. archaeological reports lipari was inhabited since the middle of the sixth millennium bc, with some periods of demographic crisis [manni et al., 2019], as attested by the large archaeological reports [martinelli, 2020; bernabò brea and cavalier, 1998]. after the iv-vi century ad the archaeological data decrease considerably, indicating a period of abandonment until the eleventh century. the archaeological evidences relating to the 4th 6th centuries, although insufficient, suggest that some changes occurred both in the population size and in the topography of the main settlements [zagari, 2015; 2017]. after the sixth century, data are drastically reduced and even today it is not possible to obtain information about lipari community that probably follows to inhabit the inner flat plains [giustolisi, 2001]. the archaeological stratigraphy of the contrada diana necropolis, previously used in the greek and roman age, provides important information. the sector with burials dated to the 4th 6th century ad was investigated in trench xxxvi [bernabò brea and cavalier, 1994] where, as had already been observed in other excavation trenches of the same necropolis (figure 2), the archaeological layers were covered by a pumice layer, usually very thin but in some areas reaching a thickness of 20 cm. this layer occurs under 0.80 or 1.00 m from the modern agriculture level. it was used in the stratigraphic sequence of the history of lipari as a post quem term because it is connected to the volcanic activity localized in the north-east sector of the island. a radiocarbon dating 1241 ± 31bp, corresponding to 776 ad [keller, 1970; 2002], was correlated with the pumice layer. in the stratigraphy of the contrada diana site the pumice layer is covered by one meter of agricultural soil without material (pottery, stone etc.) referred to the human occurrence after the sixth century. furthermore, this layer appears intact suggesting the absence of any agricultural or other activities that would have erased or dispersed the pumice [bernabò brea, 1988: 128]. after the devastation caused by the arabs in 838, the rebuilding of the fortified citadel of lipari began in the norman age between 1082 and 1085 due to the arrival of a group of benedictine monks led by abbot ambrogio [iacolino, 1998]. the church with the cloister is the most important architectural element [bernabò brea, 1998; bernabò brea and cavalier, 2003; tabanelli, 2017] that was built on a blackish layer of burning covering the floor maria clara martinelli et al. 2 figure 1. localization of aeolian islands in the southern tyrrhenian sea (italy). of an older structure. this layer was attributed to the arab destruction on the basis of the finding of a leaden seal of the byzantine age with the name of constantino, who was strategist of sicily in 813 [bernabò brea, 1998: 27]. the vulcano island does not preserve traces of settlements until the nineteenth century. its constant and violent volcanic activity has changed the morphology of the island several times where, however, occasional visits for mining, hunting and forest exploitation likely occurred since the bronze age. 3 resilience to volcanoes in lipari island figure 2. excavation in contrada diana (lipari): 1) trench xxxvi – 1974; 2) particular of pumice layer; 3) trench xxix – 1966 (after bernabò brea and cavalier [1994]). maria clara martinelli et al. 4 3. volcanological data during middle ages the activity of the lipari-vulcano complex is characterized by several eruptive events. in lipari (figure 3), a main eruption occurs in the early middle ages in the monte pilato after a quiescence of about 6,000 years, forming a pumice cone of 150 m of elevation and about 1 km wide northernmost of the pre-historical volcanic center of gabellotto. the upper part is composed by pyroclastic deposits of pumice, that have been deposited in a relatively short span of time due to the frequent fallouts produced by magmaticand hydromagmatictype explosions [dellino and la volpe, 1995]. two radiometric age determinations have been provided for this eruption. the first standard radiocarbon dating [keller, 1970] estimated its occurrence at 1220 ± 50 bp, which is a calendar age of 780–785 ad, while the second performed through “high precision ams radiocarbon” [keller, 2002] recorded the same at 1241 ± 31 bp and 776 ad calibrated age. samples for both dating were collected from the paleosol below the recent pumice, deposited on the south-east flank of monte chirica near lami village. figure 3. geological sketch map of the ne sector of lipari (modified after forni et al. [2013]). igm map sector with archaeological sites in lipari around monte rosa (redrawing by bernabò brea and cavalier in 1990). successively, lipari enters in a volcanic quiescence until the early xiii century, when the forgia vecchia eruption occurs [malaguti et al., 2020]. although the dynamic of this eruptions is not yet clarified on the whole, the obsidian lava was generated in the area of pirrera–forgia vecchia and flowed eastward down to the coast forming a two lobes flow behind canneto village; this eruption was previously dated at 1,6 ± 38 bp [bigazzi and bonadonna, 1973; forni et al., 2013]. it should be noted that the absence of pumice layers on its surface seems to confirm as this event was subsequent to the eruptive phases of monte pilato occurred in viii century [cortese et al., 1986]. forni et al. (2013) questioned the age attribution of bigazzi and bonadonna [1973], owing to the absence of the monte pilato pumiceous products above the forgia vecchia lava flow. they proposed that the forgia vecchia and monte pilato activities are time-equivalent. furthermore, the eruptive activity resumed in the inner area of monte pilato center, forming the rocche rosse crater, partially destroyed by the successive obsidian lava overflow. this latter has extensively covered the northeast slope of the cone with the more than 60 m thick and 2 km long rocche rosse flow, reaching the sea in correspondence of the actual punta castagna and acquacalda village (figure 4). archaeomagnetic datings estimated this flow around 1230 +/40 ad [tanguy et al., 2003; forni et al., 2013], 1240 ± 30 [arrighi et al., 2006] or around 1300 [malaguti et al., 2020]. in the same time is the lami cone eruption occurred, near the small village of lami on the south-east flank of monte pilato. obsidian samples collected on the cone have been dated through the fission tracks method at 1243 ± 190 ad [bigazzi et al., 2003]. 4. historical sources in the 7th 8th century the reports of religious travellers [garbini, 2010: 31-32] mainly concern vulcano, with the only exception of that given by san gregorio in 787, where a description of both submarine volcanic activities observed in the channel between lipari and vulcano, and an eruption on the island of lipari is provided [bernabò brea, 1988]. the voyage of san willibald in 729 was determinant for the description of an eruption of pumice emitted by vulcano [iacolino, 1996]. arab sources of the 10th 12th century are telling again about the pumice of vulcano, while a description of 1184-1185 concerns vulcano and vulcanello that “send fire forever” [see pagliara, 1995 and references therein]. 5 resilience to volcanoes in lipari island figure 4. view from the rocche rosse crater area and rocche rosse obsidian flow down to punta castagna (photo m. manni 2021). historical data becomes more precise in the thirteenth century and in particular in the years 1264-1265 when lipari is mentioned in a chronicle of the pisan annals and in a report of the universitas of patti. the first describes lipari burned by the hellfire that arose from the same island in october 1264; the latter mentioned instead the loss of documents due to destructions on lipari. the words “everssione terre lipparis” are used to indicate the devastation of lands. this destructive event is still mentioned in 1265 to justify the loss of a document that granted a privilege to the church of patti. 5. chronicle of the 1264 by pisan annals a short chronicle given by an anonymous pisan author belonging to the ghibelline faction is contained in the appendix to annales pisanorum (pisan annals) which covers the period from 1101 to 1268 [lupo gentile, 1930-1936]. a.d. mcclxv, de mense octub. insula lipari combusta fuit igne infernali, sive qui exivit de inferis dicte insule. in the year of the god 1265 (= 1264 of the current calendar), in the month of october, the island of lipari was burned by a hellfire, emerging from the same island. 6. examination of the minutes of universitas of patti dated 1 september 1265 in a minute registered on 31 august 1265 at the universitas of patti there are two letters sent by king manfredi to nicolao tavily, the “stratigoto” (magistrate of the city) of messina, respectively on 26 february and 13 august 1265 [girgensohn and kamp, 1965]. both concerned a privilege claimed by the bishop of lipari, bonconte di pendenza, in favour of the bishopric of lipari-patti. it is a privilege granted by king william of sicily who exempted the church of patti from paying contributions in favour of the royal fleet. the original certificates of william ii and federico ii with regular signet, could not be provided because it was lost during the destruction of the lands of lipari (everssione terre lipparis) [iacolino, 2001]. a) letter of manfredi to the city magistrate dated: orte, 26 february 1265. “…quia dicta privilegia regis guillelmi et domini patri nostri de predicta exempcione eclesie predicte indulta in everssione terre lipparis cum rebus aliis pluribus, quas habebat idem electus ibidem, ammissa fuerant et ea tibi, dum contendis ab eo ius lignaminum et marinarie exigere, ad defensionem suam et dicte eclesie sue pretendere et monstrare non potest, licet ipsa autenticata habebat …”. ..as the mentioned privileges of king william and our lord father granted to the same church and as the privileges were lost in the ruin of the lands of lipari with a certain number of other things, which the same elect kept in the same place, while he requires the timber and navy privileges, for both his and the church defence, cannot show the same authenticated as he had ... b) letter of manfredi to the city magistrate dated: amatrice, 13 august 1265. “...exempcionem eclesie sue pactensis de iure marinarie et lignaminum ostendere et edocere non posset per privilegia divorum progenitorum nostrum, que in casu lipparis se asserit ammisisse…” the exemption of the patti church from navy and timber law, granted by privilege of our ancestors, cannot be shown as it is claimed to have been lost in the events of lipari [iacolino, 2001: 295]. 7. discussion the middle ages is characterized by an increasing of the volcanic activity of the lipari-vulcano system, which involved the eruptive centres of lipari (monte pilato, forgia vecchia and rocche rosse) (figure 5) and vulcano (gran cratere and vulcanello). the area of the castle was rebuilt between 1082 and 1085, including a new church and a benedictine monastery related to the new settled community after the arab invasion in 838. the promulgation of the constitutum in 1095 regulated the assignment of the land of the south-west part of lipari to private locals through concessions in perpetuum, with the aim of improve the recolonization of this island [iacolino, 1998]. maria clara martinelli et al. 6 on the contrary, the land in the north-east sector (near forgia vecchia and monte pilato) was not assigned as emphyteusis for many centuries. in fact, according to the description in an act of 1246 conserved in episcopal archive of patti [iacolino, 2007], the subdivision of the farmlands owned by the church included only the south-west sector of the island. the main reason is certainly related to the new cycle of volcanic activity that occurred since the eighth century and that reached its maximum peak in the thirteenth century with the eruption rocche rosse [tanguy et al., 2003; forni et al., 2013]. this latter culminated with the formation of the homonymous obsidian flow. the rocche rosse flow (dated around 1230-1240) together with the lami cone, on the south-east flank of monte pilato, (around 1243) represent the last eruptions of lipari. two documents dated at 1264 are the last historical sources where are described these important natural events that occurred in lipari and therefore could be referred to the last eruptions of lipari, namely rocche rosse and lami. these events offer to the lipari churchmen an useful piece of evidence to justify the lacking of a documentation proving their privileges. at the same time, the volcanic activity determined strong constrains that allow to use only the non-affected sector of the island, while northernmost of monte rosa (figure 6) the hazard due to the eruptive centres of forgia vecchia, rocche rosse and lami represented an obstacle for human and agricultural expansion. only from the end of the sixteenth century, a gradual demographic grow will determine the birth of the villages of canneto, pirrera, serra, lami and acquacalda, all located in the northern sector of the island where, it is presumed, post-eruption effects like volcanic gas emission from the ground, not fertile volcanic soils, etc., has hampered the human settlement for a long time. still in 1593 in the locality of acquacalda, only two areas were granted in emphyteusis to private locals by the church [iacolino, 1998; 2007]. finally, the monte pilato area was systematically exploited as a pumice mining industry from 1880 to 2007. 7 resilience to volcanoes in lipari island figure 5. view of the rocche rosse crater (photo 1950). 8. conclusion the end of the roman empire and the dependence by byzantium of the sicily after 535 ad [zagari, 2017] brought changes in human settlements. in the aeolian islands, these changes may have been influenced by the effect of the volcanism, especially on the lipari-vulcano complex. at the island of lipari, after a long period of quiescence from the seventh millennium bc, the volcanic activity shown a significant recrudescence in the middle ages. the lack of archaeological findings (settlements, walls, pottery, etc.) from the 6th to the 11th century ad suggests a pauperisation of the communities with a consequent demographic decrease [arena, 1991] and an economic decline of the archipelago. this situation persisted until the 11th century when the benedictine monastery was founded on the castle of lipari. the morphology of the north-eastern sector of lipari, affected by the eruptive phases of forgia vecchia, lami and rocche rosse that occurred in the thirteenth century, was completely upset and these areas remained uninhabited until a few centuries ago. the geological dating of these eruptive phases are in agreement with two historical sources: the first (pisani annals) could be referred to an eruption that occurred in october 1264, the second (letters to manfredi) confirm a destructive event occurred in the second half of the same year (before february 26, 1265). although the last volcanic cycle did not hinder the life of the lipari community in the urban area, it has deeply marked the morphology of the north-east sector of the island, with the direct consequence to maintain a condition of abandonment of the same. only after the sixteenth century, therefore when the volcanic activity (including posteruption effects) has been definitively ceased, there are records of human exploitation and expansion in this area [iacolino, 2007]. on the other hand, all the eventual remains of ancient settlements in this area should be completely erased by the successive strong volcanic events. this possibility is suggested by the only archaeological finding maria clara martinelli et al. 8 figure 6. view of lipari city and monte rosa-serra (photo m.c. martinelli). 9 resilience to volcanoes in lipari island from the culia area (figure 3) that lies northernmost the natural barrier of monte rosa-serra and very close to the southern boundary of forgia vecchia coulee, where a nucleus of tombs from the greek age (4th century bc) attests the occurrence of a settlement [martinelli personal communication]. acknowledgment. our heartfelt thanks goes to: francesca de francesco and sebastiano ciavirella (library university of messina) to help in the research of the historical sources. pietro lo cascio (nesos association, lipari) for the discussion on the aeolian territory and for the reading of the text. emanuela guidoboni (eedis) for information on pisan annals. references arena, g.a.m. (1991). note sull’economia delle isole eolie dal neolitico alla prima metà del xvi secolo, annali dell’istituto tecnico commerciale antonio maria jaci di messina, 2 serie, ii, messina, 113-148. arrighi, s., j.c. tanguy and m. rosi (2006). eruptions of the last 2200 years at vulcano and vulcanello (aeolian islands, italy) dated by high-accuracy archeomagnetism, phys. earth planet. int., 159, 225–233. bernabò brea, l. 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(2020). isole vicine. l’arcipelago delle isole eolie e le comunità umane nella preistoria mediterranea, edizioni di storia e studi sociali, ragusa. pagliara, a. (1995). fonti per la storia dell’arcipelago eoliano in età greca con un’appendice sull’epoca romana, in bernabò brea l. and m. cavalier, meligunis lipara viii, parte seconda, accademia di scienze lettere e arti di palermo, palermo. tabanelli, m. (2017). il chiostro di san bartolomeo a lipari: sperimentazioni progettuali e decorative nella prima comunità benedettina della sicilia normanna, in hortius artum medieval, 23/1, 318-327. tanguy, j.c., m. le goff, c. principe, s. arrighi, v. chillemi, a. paiotti, s. la delfa and g. patanè (2003). archeomagnetic dating of mediterranean volcanics of the last 2100 years: validity and limits, earth planet. sci. lett., 211, 121–123. zagari, f. (2015) le isole eolie in epoca bizantina. primi dati di una rilettura topografica, in arthur p. and m.l. imperiale (editors), in vii congresso nazionale di archeologia medievale, vol. 2, all’insegna del giglio, firenze, 454-458. zagari, f. (2017). due antiche diocesi dello stretto di messina: insediamento, manufatti, infrastrutture e produzione nell’epachia delle saline e nelle isole eolie tra tardoantico e alto medioevo, archeopress, oxford. *co r r e s po n d i n g a u t h o r: maria clara martinelli, parco archeologico delle isole eolie, museo luigi bernabò brea, lipari (me) italy; e-mail: martinellimariaclara@gmail.com © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international maria clara martinelli et al. 10 https://archive.org/details/p2archiviomurato06fior/page/116/mode/2up https://www.conferenzarittmann.it/sessioni/programma.html https://doi.org/10.4401/ag-7716 mailto:martinellimariaclara@gmail.com d e t e r m i n a t i o n o f s u r f a c e h e a t f l o w i n mazesta ( u s s r ) (*> h . a . l u b i m o v a , l . m . l u s o v a , f . y . f i r s o v , g . n . s t a r i k o v a , a . p . s h u s h p a n o v ( * * ) ricevuto il 7 dicembre 1960 a great number of temperature measurements in deep bore-holes are made on the territory of the soviet union. the results of these measurements are partly systematized in the work " problems of geothermie " t1). however they can be hardly used for the calculations of heat flow because of the absence of the corresponding determinations of the thermal conductivity of rock cores taken directly from these boreholes. in the present paper the temperature gradient and the thermal conductivity of samples of rocks for the same place are determined and by means of these data the value of the thermal flow is estimated. for temperature measurements the electric resistance thermometer was used t h a t is designed by dergunov i. d. and improved for the conditions of work in deep bore-holes (up to 4-5 km) at rather high pressures and temperatures. the electrical resistance was measured by means of the compensation methods by a special potentiometer and a high sensitive mirror galvanometer. four bringing wires are used for doing away with the erratic currents and the influence of the wires. as a result of it the accuracy of the temperature measurements is about 0.01°c. fig. 1 represents the exterior of the apparatus and the thermometer. the thermometer is a hollow cylinder on the surface of which a copper wire is winded bifllarily, whose resistance is measured. when the cylinder is buried into a bore-hole it is washed by a solution from outside and inside. together with the application of ftorplast isolation this decreases the thermal inertia of the thermometer down to 1.5 sec. for the sake of strength annular justs are made on the cylinder. the thermometer stands the pressure up to 600 a t m and the temperature up to 200°c. (*) p a p e r r e a d a t t h e helsinky assembly of t h e i.u.g.g., 1960. (**) i n s t i t u t e of e a r t h ' s physics a c a d e m y of science u s s r ) . 1 5 8 l u b i m o v a , l u s o v a , f i r s o v , s t a r i k o v a , s i i u s h p a n o v l a t h e process of the determinations of t h e t e m p e r a t u r e t h e requirements of t h e bore-holes measurements' methodics were strictly observed; the measurements were made by a point method in bore-holes t h a t were in rest for 8 to 10 months. as a total about 20 bore-holes with the depths f r o m 100 to 2600 m were investigated. here the d a t a for the 3 bore-holes mazesta-hosta (caucasus) are given. temperatures were measured twice in it in 1955 a n d 1957 and yielded t h e same results. as to its geology the region mazesta refers to a great tectonic depression submerged below t h e level of t h e black sea in its south-western p a r t a n d extended in t h e direction of t h e general striking of t h e caucasus for t h e length of 60 to 65 k m and amounting its m a x i m u m width of 20 to 25 k m . the complete formation of t h e depression and t h e submergence of its p a r t below t h e sea level are d a t e d to t h e end of the tertiary and the beginning of t h e q u a t e r n a r y (2). pig. 4, represents t h e lithological and stratigraphical cross-section of t h e bore-hole 3t. the t h e r m a l constants of rock cores were determined b y t h e impulse sound method. i n a core sample two holes are drilled a t a distance r from each other. one is for an electric heater and t h e other for therpig. 1 t h e e x t e r i o r of t h e a p p a r a t u s a n d t h e r m o m e t e r for t h e m e a s u r e m e n t of t e m p e r a t u r e . d e t e r m i n a t i o n o f s u r f a c e i i e a t f l o w i n o l d m a z e s t a 1 5 9 mopair. the exterior of the a p p a r a t u s is represented in fig. 2. a current impulse is transmitted through the heater during 3 to 4 sec which creates a practically instantaneous source of heat since the time of the thermal impulse propagation from t h e source to the observations point is of the order of a hundred seconds. the movement of the light spot of a mirror fig. 2 p h o t o g r a p h of t h e a p p a r a t u s for t h e m e a s u r e m e n t of t h e t h e r m a l c o n s t a n t s of rocks galvanometer indicates t h e moment when the maximum temperature is reached a t a given point. the time interval tmax and t h e value of the m a x i m u m temperature 0m a x a t a given point of observation are determined and used for t h e determination of the thermal diffusivity lc and the thermal capacity eg simultaneously. these d a t a are applied for the calculation of the thermal conductivity a = iccg. this method lias essential advantages over the stationary method, the so-called " divided b a r " method. i t does not require a through polishing of a core sample before the experiment, which greatly disturbs its n a t u r a l state. the preparation of the sample for the experiments consists only in boring of holes for t h e heater and t h e thermopair. the experiment takes some 1 6 0 l u b i m o v a , l u s o v a , f i r s o v , s t a r i k o v a , s i i u s h p a n o v minutes. a pressing spring being a heater's spiral a t t h e same t u n e provides a close contact with the rock. this method permits to determine t h e heat conductivity both along and across the rocks striking and can be used for the determination of the thermal conductivity " in situ " . d i s t a n c e s r f r o m t h e source. the heater is a kind of a rod with the radius of 0,3 cm and t h e length 2 1 = 5 cm. f o r calculation such a source can be considered neither point nor spherical. the formula for the t e m p e r a t u r e distribution a b o u t an infinitely long and thin source in an infinite medium was used where r is t h e distance between t h e source and t h e thermopair, q is t h e generated heat for the unit length of t h e source. d e t e r m i n a t i o n of s u r f a c e i i e a t f l o w i n o l d m a z e s t a 1 6 1 the values k and eg are determined from t h e conditions of t h e id extremum = 0 by t h e formulas o t r2 q.e~ 1 1 = it 5 « = 6 71v2 > [2] ^ 1 m a x " m a x • ' ' where i m a x is t h e time moment when 0 — 6max a t a given point. however t h e comparison of t h e experimental and theoretical curves 0 calculated by means of formula [1] of the t e m p e r a t u r e variation with time (fig. 3) shows t h a t these curves are similar as to their p a t t e r n b u t tliey are somewhat shifted relative to the time axis a n d have different values of t h e t e m p e r a t u r e maximum. this difference does not disappear if to allow for the ultimate duration of the impulse at and the ultimate size of t h e sample. these effects account for the error not greater t h a n 1 per cent if the conditions a t < 10 sec are observed and t h e diameter of t h e sample a t r < 3 sm. a more essential error is due to the assumption t h a t the heater is an infinite straight line. the integration of the function describing t h e temperature influence of t h e instantaneous point heat source along with the cylinder surfaces with t h e radius a and length 21 yields a formula 131 which gives the temperature distribution about a cylindrical instantaneous source of finite dimensions, where i„ (x) is t h e bessel function of t h e zero order from an imaginary argument, erf is t h e integral of t h e errors. hence t h e t h e r m a l diffusivitv k is determined graphically from t h e equation for = 0 : 0 t ar _ ^ u f c w ) _ 1 . 6xp (~ ±kt) 4 f c < m a x 2 a ^ a x y ( ot \ 2 j-' tlkt erf / }_ ^ 2fc<max/ \ 2 ]/ kt using the obtained k we calculate heat capacity * = t m z g x p ( l i a x ) e r f ( ^ f c ) [ 5 ] 1 6 2 l u b i m o v a , l u s o v a , f i r s o v , s t a r i k o v a , s i i u s h p a n o v the error due to some loss of heat through t h e top surface and to non-absolute contacts of the heater and the thermopair with t h e rock can be allowed empirically. let us consider the simplest case a n d calculate the temperature curve using formula [1]. the differences of t h e experimental and theoretical curves of fig. 2 can be used for t h e determination of t h e correction coefficients which are inserted in formula [1] ciet up. czetaceous cou/e z 1 c s. \tuz. s a, a va6anginlan -bait em can 1 c fcmest s-m s. €imes tone mi.croq7on 5k32•h 38sbbh«f 5k32^ co £ 3028'05 26»» % ,. ... 0 26• 0 24-/ 1 . 1 . r . 1 1 , 1 . 1 , -05 200 w 600 800 m fig. 4 lithological a n d s t r a t i g r a p h i o a l cross-section of t h e bore-hole 3t old mazesta. so t h a t these curves coincide within t h e given accuracy. f o r example the following empirical formulas can be taken _6_ = a 6 x p ( ~ ~ 4 k f ) _ jb2 _ ae-i-q q ' 4 w ; °q ~ 6maxnr* [ 6 ] where a(r), r(r) are obtained f r o m experience for the given conditions of an experiment and allow for all t h e possible deviations of the idealized scheme simultaneously. the comparison of the results obtained b y two methods f r o m formulas 4-5 and 6 give discrepancies not greater t h a n d e t e r m i n a t i o n o f s u r f a c e i i e a t f l o w i n o l d m a z e s t a 1 6 3 ~ 5 per cent, which is a good selfcheck. an independent study of one and t h e same specimen of chalk b y our method and by the method of regular regime in another i n s t i t u t e gave a rather satisfactory coincidence of t h e results. temperature measurements in the bore-liole 3 t old mazesta were m a d e to t h e depth of 2165 m and with t h e interval of 25 m the results are given in table 1. the core samples were selected in t h e near lying bore-hole 4 t located a t t h e distance of ~ 200 m. b o t h lithologically and stratigraphycally the selected core-samples correspond to rocks in t h e bore-hole 3 t in t h e range of 175 to 950 m. three intervals with different t e m p e r a t u r e gradients answering t h e alternation of layers can be distinguished on the temperature curves (fig. 4) corresponding to these depths. the valued grad t and t0 in t h e formula t = t0 + d • grad t [7] is determined b y the least square method for each interval, where i ) is t h e depth, t is the temperature a t a given depth, t0 is the constant designating the value of temperature extrapolated to the level of the e a r t h ' s surface. thus the temperature curve can be represented b y t h e following equations: a) in t h e range of 175 to 250 m t = 19,01 + 2,812 • 10~'d , grad t = 2,812 • 10 "4°c/cm = 28,12°(7/km b) in the range of 250 to 375 t = 24,23 + 0,658 • 10' 4 d , grad t = 0,658 • 10-40c/cm = 6,58° g / km c) in the range of 375 to 950 t = 22,10 + 1,264 • 10" 4 d , grad t = 1,264 • 10"40(7/om = 12,64°c/km the deviations of at calculated from these formulas of the temp e r a t u r e values from the observed ones are represented b y crosses in fig. 4. an increase of t h e gradient in the upper range a) m a y be connected with the influence of the relief and the other fa 'tors. therefore we shal not t a k e into account this range of depths and calculate t h e thermal flow for t h e range of 250 to 950 m. 1 6 4 l u b i m o v a , l u s o v a , f i r s o v , s t a r i k o v a , s i i u s h p a n o v the d e t e r m i n a t i o n of t h e average g r a d i e n t for t h e i i a n d i i i intervals b y t h e least suquare m e t h o d gives f o r t h e range of 250 to 950 m t = 22,40 +1,224.10-4z>, grad t = 12,24°c/km . [8] the deviation of at2 observed t e m p e r a t u r e f r o m t h e calculated using formulas [8] is given in t a b l e 4, column 1. the s t a n d a r d error is 0.11°c. below t h e values of t h e h e a t c o n d u c t i v i t y of rock cores across t h e striking are represented. 250-290 0,00725 0,00653 0,00689 limestone of t h e t u r o n 290-315 0,00517 0,00594 c p e f l h e e 0,00556 marls of t h e senoman tufaceous s a n d s t o n e of t h e senoman 315-328 0,00417 328-375 0,00664 0,00627 c p e , a ; h e e 0,00646 marls of t h e alb a n d t h e a p t 375-950 0,00677 0,00666 0,00737 0,00658 cpeflhee 0,00685 limestone of t h e v a l a n g i n i a n b a r r e m i a n t h e average v a l u e of t h e t h e r m a l c o n d u c t i v i t y in t h e r a n g e of 250 to 950 m consisting of a great n u m b e r of layers was calculated b y m e a n s of t h e f o r m u l a ^ di — dk 70000 di a + l dk_ 1000 57500 xi xi f 1 " h 6,89 • 1 0 s 6,85 • 10~3 = 6,64 • 10~3 ± o , 6 o • 10-3 . cm20c sec cm20c sec d e t e r m i n a t i o n of s u r f a c e i i e a t flow in old mazesta 1 6 5 the average values of a for t h e i i and i i i intervals are equal to 5,84 • 1 0 3 and 6,85 • 10"3 — ~ — . we obtain for the h e a t flow cm200 sec b = i grad t = 6,64 • 1(t3 • 1,22 • 1(t4 = 0,81 • 10"' — — ± cm2 sec cal ± 0,07 • lo"6 cm2 sec t a b l e no. 1 ^ d d e p t h t atx at, z — • 1 0 5 k at3 m. °c °c °c cm 2 sec °c <>c cal 1 7 5 2 3 , 8 2 — 0 , 1 1 2 0 0 2 4 , 7 2 + 0 , 0 9 2 2 5 2 5 , 4 5 + 0 , 1 1 2 5 0 2 5 , 9 2 — 0 , 1 2 + 0 , 0 5 + 0 , 4 6 4 2 , 8 0 8 + 0 , 6 2 2 7 5 2 6 , 0 0 — 0 , 0 4 + 0 , 2 3 4 7 , 0 8 9 + 0 , 3 5 3 0 0 2 6 , 1 6 — 0 , 0 4 + 0 , 0 9 5 1 , 3 7 0 + 0 , 1 6 3 2 5 2 6 , 4 1 + 0 , 0 4 + 0 , 0 3 5 5 , 6 5 1 + 0 , 0 5 3 5 0 2 6 , 3 7 + 0 , 0 4 — 0 , 1 1 5 9 , 9 3 1 — 0 , 1 4 3 7 5 2 6 , 3 8 — 0 , 0 2 — 0 , 1 6 — 0 , 3 1 6 4 , 2 1 2 — 0 . 3 8 4 0 0 2 7 , 2 4 + 0 , 0 8 — 0 , 0 6 6 7 , 8 6 3 — 0 , 1 2 4 2 5 2 7 , 6 1 + 0 , 1 4 + 0 , 0 1 7 1 , 5 1 2 — 0 , 0 5 4 5 0 2 7 , 8 7 + 0 , 0 8 — 0 , 0 4 7 5 , 1 6 2 — 0 , 0 5 4 7 5 2 8 , 2 0 + 0 , 1 0 0 , 0 1 7 8 , 8 1 2 — 0 , 0 6 5 0 0 2 8 , 4 8 + 0 , 0 6 — 0 , 0 3 8 2 , 4 6 1 — 0 , 0 8 5 2 5 2 8 , 8 2 + 0 , 0 9 0 8 6 , 1 1 1 — 0 , 0 4 5 5 0 2 9 , 1 2 + 0 , 0 7 0 , 0 1 8 9 , 7 6 0 — 0 , 0 5 5 7 5 2 9 , 3 5 0 , 0 2 — 0 , 0 9 9 3 , 4 1 0 — 0 , 1 2 6 0 0 2 9 , 6 5 — 0 , 0 3 — 0 , 0 9 9 7 , 0 6 0 — 0 , 1 2 6 2 5 2 9 , 8 0 — 0 , 2 0 — 0 , 2 5 1 0 0 , 7 0 9 — 0 , 2 7 6 5 0 3 0 , 1 9 — 0 , 1 3 — 0 , 1 7 1 0 4 , 3 5 9 — 0 , 1 8 6 7 5 3 0 , 4 9 — 0 , 1 4 — 0 , 1 7 1 0 8 , 0 0 8 — 0 , 1 8 7 0 0 3 0 , 8 4 — 0 , 1 1 — 0 , 1 3 1 1 1 , 6 5 8 — 0 , 1 3 7 2 5 3 1 , 2 0 — 0 , 0 6 — 0 , 0 7 1 1 5 , 3 0 8 — 0 , 0 7 7 5 0 3 1 , 5 7 — 0 , 0 1 — 0 , 0 1 1 1 8 , 9 5 7 0 7 7 5 3 1 , 9 0 0 + 0 , 0 1 1 2 2 , 6 0 7 + 0 , 0 3 8 0 0 3 2 , 2 8 + 0 , 0 7 + 0 , 0 9 1 2 6 , 2 5 6 + 0 , 1 1 8 2 5 3 2 , 6 1 + 0 , 0 8 + 0 , 1 1 1 2 9 , 9 0 5 + 0 , 1 4 8 5 0 3 2 , 8 7 + 0 , 0 3 + 0 , 0 7 1 3 3 , 5 5 6 + 0 , 1 0 8 7 5 3 3 , 2 3 + 0 , 0 7 + 0 , 1 2 1 3 7 , 2 0 5 + 0 , 1 5 9 0 0 3 3 , 5 5 + 0 , 0 7 + 0 , 1 3 1 4 0 , 8 5 5 + 0 , 1 7 9 2 5 3 3 , 8 5 + 0 , 0 6 + 0 , 1 3 1 4 4 , 5 0 4 + 0 , 1 7 9 5 0 3 4 , 1 0 — 0 , 0 1 + 0 , 0 7 1 4 8 , 1 5 4 + 0 , 1 2 ± 0 , 1 1 ± 0 , 1 8 1 6 6 l u b i m o v a , l u s o v a , f i r s o v , s t a r i k o v a , s i i u s h p a n o v in accordance with a more precise method of the determination of the average heat flow through a multi-layered medium [4] we h a v e t t e = ,, ; [9] 2 dijxi 2 di values — — for each range of depths are given in table 1, xi column 5. the corresponding deviations at3 of the observed value t from the calculated one using formula [9] are given in the last column. the equation for t is t = 21,77 + 0,82 • 10-* £ d »=1 m cal cal h = 0,82 • 10"6 ± 0,05 • 10-6 ~ cm2 sec cm2 sec which is very close to t h e value obtained above. we obtained t h e values of t h e t e m p e r a t u r e s for t h e near located bore-holes 2 t new mazesta and 2 t hosta also and determined t h e heat flow on the base of t h e known lithological cross-sections and d a t a on x: 1) for t h e bore hole 2 t new mazesta t = 28,58 + 0,987 • 10~4d , grad t = 0,987 • 10"1 ; h = 0.68 • lo"6 2) for t h e bore hole 2 t h o s t a t = 16,95 + 1,810 • 10~4d , grad t = 1,81 • 10"4 , h = 1,14 • 10"6 . then t h e average value of heat flow for mazesta-hosta region c£jjl is equal (0,88 ± 0,16) • 1 0 6 . i t is possible there is t h e coocm2 sec ling effect of t h e near located black sea and of the mazesta's springs. summary the results of the measurements of the temperature and the thermal conductivity of rocks for the three boreholes mazesta's region are described. the special thermometer of the resistance ivas used. the method of the measurements of the thermal conductivity and capacity based upon the principle of the instant linear heat source. then calculated magnitude of heat floiv is equal 0.88 • 10~e cal/cm2/sec. it is likely that there is the cooling effect of the near located blaclc sea and of the mazesta's springs. d e t e r m i n a t i o n of s u r f a c e i i e a t f l o w i n o l d m a z e s t a 1 6 7 riassunto sono descritti i risultati delle misure della temperatura e della conduttivita termica delle roece per i 3 fori praticati nella regione della mazesta. e stato usato lo speciale termometro a resistenza. ii metodo delle misure della eonduttivita e della capacita termica si basa sul principio della sorgente improvvisa lineare di colore. l'ampiezza calcolata delvemissione di colore e uguale a 0.88 • 10~6 cal/cm2/sec. r e f e r e n c e s (x) problems of geothermie. t r a n s a c t i o n s of tlie all-union congress of geot h e r i n i c i n v e s t i g a t i o n s , acad. sci. u s s r . (1959). (2) makarenko f . a., on the genesis of the sulpheretted hydrogen waters of mazesta. t r a n s a c t i o n s of t h e l a b o r a t o r y of hydrogeological p r o b l e m s acad. sci. u s s r , v. 2, (1949). (3) benfield a. e., terrestrial heat flow in great britain, p r o c . roy. soc. 173, 428-450, ( 1 9 3 9 ) . . (4) bullard e . c., heat flow in south africa. p r o c . roy. soc. a., 173, 474-502, (1939). layout 6 a case study of detecting the triplet of 3s1 using superconducting gravimeter records with an alternative data preprocessing technique wen-bin shen1,2,3*, bo wu1 1 wuhan university, school of geodesy and geomatics, department of geophysics, wuhan, china 2 wuhan university, state key laboratory of information engineering in surveying, mapping and remote sensing, wuhan, china 3 wuhan university, key laboratory of geospace environment and geodesy, ministry of education, wuhan, china annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-4944 abstract due to their very low noise levels in the low frequency band (<1 mhz), superconducting gravimeters (sgs) are particularly suitable to observe long-period free oscillations of the earth. this case study is dedicated to the detection of the triplet of the seismic normal mode 3s1 that was excited by the december 26, 2004, sumatra-andaman earthquake (mw = 9.3). using sg records, the hilbert-huang transformation is used as an alternative data preprocessing technique, instead of the traditional detiding method. after removal of atmospheric pressure effects from the original sg records, we applied the hilbert-huang transformation to the sg residues, to select the signals that included the frequency band of interest, and to construct a new data series. then, by applying the multi-station experimental technique to five 273-h-long common new data series recorded at different sg stations, we clearly observed all of the three singlets of the mode 3s1, with the central singlet more evident compared to previous studies. observations of the low-frequency modes 3s1 (n = 0, 1, 2, ...; l = = 1,2, ...) provide constraints on the inner and outer core structure. this case study provides an alternative data-preprocessing approach to observe the splitting frequencies of the low-frequency mode type 3s1 (n = 0, 1, 2, ...). 1. introduction after a large earthquake, various seismic normal modes (free oscillations) of the earth are excited for several days to months (e.g., alterman et al. 1974, widmer-schnidrig 2003). the typical length of the time that a seismic normal mode lasts for depends on its quality factor q (which is inversely proportional to the ratio of the energy loss per cycle to peak strain energy), whereby the larger the q, the longer the vibration of the normal mode (e.g., aki and richards 1980, chao and gilbert 1980). the frequencies of the modes are closely related to the structure of the earth. for instance, the triplet of the seismic normal mode 3s1 (generally the normal modes below 3 mhz) is sensitive to the density, compressibility, and velocity of p-waves in the mantle and core [e.g., ritzwoller and lavely 1995, resovsky and ritzwoller 1998, see also http://phys-geophys.colorado.edu/geophysics/nm.dir/3s/ 3s1.kerplot.gif]. theoretically, if the earth is taken as an idealized spherically symmetric, non-rotating, purely elastic, isotropic body, modes ns l m with the same n and l have the same eigenfrequency, which is referred to as degeneracy [dahlen and tromp 1998]. here n is the radial overtone number, l the angular degree, and m the azimuthal order. however, for the real earth, its rotation, ellipticity and lateral heterogeneity remove the degeneracy, which results in the appearances of split peaks [e.g., alterman et al. 1974, dahlen and sailor 1979, rogister 2003]. the splitting of the modes below 1 mhz is highly sensitive to the three-dimensional density structure of the earth mantle and core [e.g., okal 1978, ritzwoller and lavely 1995, widmer-schnidrig 2003], and therefore, observations of the splitting of modes below 1 mhz might help to constrain earth models. due to the global geodynamics project (ggp) [e.g., crossley et al. 1999, crossley and hinderer 2008], superconducting gravimeters (sgs) are extremely sensitive to gravity variations that are related to various geophysical processes (e.g., tides, inner core wobble, tectonic activity, earth free oscillations). they also have very low noise levels in the low frequency band, and thus they are particularly suitable for the observation of long-period signals of interest. previous studies [e.g., rosat et al. 2003a, 2004; hu et al. 2006a, b, xu and sun 2009] have shown that for frequencies below 1 mhz, the sgs can reach a better signal-to-noise ratio than most of the broadband seismometers. hence, sgs have specific advantages for the study of the gravest normal modes. the 3s1 multiplet was first observed by chao and gilbert [1980], using seven records at the international deployment accelerometer spring gravimeter stations after the 1977 data and experiment descriptions 293 article history received december 29, 2010; accepted december 7, 2011. subject classification: seismic normal mode, superconducting gravimeters data, hilbert-huang transformation, multi-station experiment, 3s1 triplet. indonesia earthquake (mw = 7.8) and then by roult et al. (2010) using 247 recordings at 157 federation of digital seismograph networks broadband seismometer stations after the 2004 sumatra earthquake (mw = 9.3). however, following a closer examination of the spectra shown in figure 10 of roult et al. (2010), we find that the middle spectrum line (m = 0) of the mode 3s1 is very weak and cannot be easily observed. here, it will be shown that by combining only five sg records at three stations after the 2004 sumatra-andaman earthquake with an alternative analysis technique, instead of traditional detiding process for data preprocessing, we can clearly resolve all three singlets of the seismic normal mode 3s1. 2. methods the preprocessing of the sg raw data generally involves tides removal and air correction (necessary for better observation of low frequency modes below 1 mhz), in addition to correcting for instrumental errors, such as spikes, gaps, and abrupt offsets. the detiding process is usually done 294 shen et al. figure 1. illustration of the hilbert-huang transformation using synthetic data. (a) eight imfs labeled by the sequences they are sifted, which are decomposed from the synthetic signal with frequencies imf1-8 (2.5, 2.1, 1.92, 1.2, 0.31, 0.24, 0.03, 0.02) mhz, and amplitudes (1.5, 1.3, 0.8, 0.55, 0.7, 1.0, 4.0, 3.0) nm/s2, respectively. (b) instantaneous frequencies corresponding to each of imf1-8 according to time. (a) (b) 295 by subtracting syntheses computed from certain tide models, although removing the tides by this method is not very thorough, as tidal effects on the records can be different for individual places, and the tidal factors used to compute synthetic tides are just average values obtained over several years. alternatively, the sg data can be bandpassed to a certain frequency range of interest, although it must be noted that the presence of gibbs phenomenon and marginal effects might result in some undesired effects in the original observations. while wavelet-based detection for weak signals [hu et al. 2006a, b, rosat et al. 2007] has great potential, the uniform frequency resolution will show inferior resolution in time and frequency compared to the hilbert analysis representation [see figure 1 of huang et al. 1999]. herein, the hilbert-huang transformation (hht) analysis is used to locate our desired frequency band directly, which will generate a new data series that will be naturally free from influences of long-period signals, e.g., tidal effects and some other possible influences. to enhance each singlet peak, the multi-station experiment (mse) technique [cummins et al. 1991] will be applied. the following two paragraphs are short descriptions of these two techniques. the hht analysis technique was first proposed by huang et al. [1998], to decompose a complicated data series into a finite, and often small number of, intrinsic mode functions (imfs) that are based on empirical mode decomposition (emd) (i.e., the sifting process) [see details in huang et al. 1998]. the essence of the method is to empirically identify the intrinsic oscillatory modes by their characteristic time scales in the data, and then to decompose the data accordingly. when the first imf is obtained, the sifting process will be repeated again to find the next imf in the residue. if the last residue becomes a monotonic function from which no more imfs can be extracted, the sifting process stops. by virtue of the definition of imf, the earlier sifted imfs (signals) generally have higher frequencies. figure 1 illustrates the method with synthetic data. figure 1a shows the extracted imfs based on emd, and the corresponding instantaneous frequency of each imf is shown in figure 1b. using these, by zooming in (horizontal or vertical zoom), we can roughly determine the frequency range of each imf. by this criterion, with concentration on one or a few frequency bands of interest, we can select certain imfs for further study. figure 1c shows the corresponding fourier amplitude spectrum of each imf in figure 1a, and is just a verification of our judgment from figure 1b. the mse technique was first proposed by cummins et al. [1991], and used by courtier et al. [2000] to detect the triplet of the slichter mode 1s1 [slichter 1961; see also e.g. smylie 1992]. it was also used by rosat et al. [2003b] to detect the mode 2s1 and by rosat et al. [2006] and guo et al. [2006] to search for the 1s1 triplet. this method takes into account the temporal and spatial properties of degree-one spheroidal modes to generate three new time series, each of which contains only one of the prograde equatorial (m=–1), retrograde equatorial (m=1) and axial (m=0) signals. spectral analysis is then applied to every time series. in the present study, the mse technique is used to effectively isolate and enhance the three singlets that correspond to the mode ns l m (m=–1, 0, 1). 3. data and data analysis the datasets are corrected minute data (gaps and disturbances filled with synthetic signals by station operator after decimation to 1 min), downloaded from the ggp data centre (http://www.eas.slu.edu/ggp/ggphome.html), and corrected for the local atmospheric pressure effect using a nominal constant admittance of –3 nms–2/hpa. the air pressure correction is necessary to better detect the frequencies less than 1 mhz [zürn and widmer 1995]. as a case study, five records from three sg stations, canberra (c1), bad homburg (h1, h2), sutherland (s1, s2), are used in the present study. the length of the records is 273 h, about one q-cycle, as the length suggested by dahlen [1982] for better frequency estimation of a certain mode. after correction of the local atmospheric pressure effect, the gravity residues are decomposed into a series of imfs, as shown by figure 2a, based on the emd [huang et al. 1998], and after applying hilbert transformation to every imf and by examining the variation of the instantaneous frequencies with time of the imfs (see figure 2b), we chose the correct imfs that included the signal of the mode 3s1. in the present study, we chose the first three imfs of all of the records, and accordingly constructed five common new data series, which a case study of detecting the triplet of 3s1 figure 1. (c) fourier amplitude spectra for the corresponding imf1-8, demonstrating that each imf in figure 1a indeed contains certain frequencies, as can be seen from figure 1b. therefore certain imfs can be selected to study the signal of interest. included the signal of the mode 3s1, which will be further used to detect the 3s1 triplet based on the mse technique. as the typical frequency of 3s1 is very close to that of 1s3, and as it also obeys certain selection rules [alterman et al. 1974], the splitting frequencies of 3s1 might be seriously contaminated by those of 1s3, because the typical frequencies (m = 0) of 1s3 and 3s1 are 0.944139 mhz and 0.944364 mhz, respectively [resovsky and ritzwoller 1998]. however, as the quality factor q of 3s1 (826.9) is almost three times of that of 1s3 (282.7) [dziwonski and anderson 1981], we can consider the earth as a filter and use the records with a later starting point after the earthquake, to weaken the influence of 1s3. as shown by figure 3, we find that about 56 h after the sumatra-andaman event, the amplitude of 1s3 is largely reduced compared to that of five hours after the event. indeed, based on the decay law a(t)=a(t0)exp[–r(t – t0)f/q] [aki and richards 1980], 56 h after the event, the amplitudes of the 1s3 and 3s1 decay to 14.7% and 51.8%, respectively, of those standing 5 h after the event. 4. results we applied the mse technique to the five common new data series (i.e., the new data series constructed by the hht technique, as indicated above), and we obtain three time-series, each of which contains only one of the shen et al. 296 (a) figure 2. the hilbert-huang transformation applied to the sg data following correction for pressure, as recorded at bad homberg (h2). (a) extracted imfs (1-7) of the sg data based on emd. (b) instantaneous frequencies corresponding to each of imf1-7 of panel (a). (b) 297 prograde, retrograde and axial modes that have been successfully enhanced in the corresponding amplitude spectra, as shown in figure 4a (m=–1), b (m=1), and c (m=0), respectively. hence, all of the three singlets of 3s1 have been clearly extracted and are close to the frequencies computed from prem. by fitting a synthetic lorentzian resonance function [e.g., dahlen and tromp 1998] to each singlet of the spectrum, we obtained the three splitting frequencies, as listed in table 1. the errors given in the present study are standard deviations of the estimated frequencies obtained by least-square fitting with five to eight points. the exact number of points used for estimation of a certain singlet might vary, depending upon how many points around the peak fit the lorentzian resonance function best. in table 1, the theoretical predictions are based on the model prem-re [roult et al. 2010] and the model premre+saw12d [li and romanowicz 1996, he and tromp, 1996], respectively. both models are based on prem, but the former includes only the earth rotation and ellipticity, and the a case study of detecting the triplet of 3s1 figure 3. comparison of the fourier amplitude spectrum of the 273-h sg record from bad homburg (h2) starting 5 h (top slot) and 56 h (bottom slot) after the earthquake. the spectra were performed on the first two imfs, after corrections for pressure. the vertical dotted lines denote degenerate frequencies for the model prem. (a) (b) (c) figure 4. amplitude spectra of the three singlets of the mode 3s1 obtained from the multi-station experimental analysis technique. (a) m=–1. (b) m=1. (c) m=0. latter includes not only the rotation and ellipticity, but also the three-dimensional elastic structure heterogeneity of the mantle. chao and gilbert [1980] first observed the triplet frequencies of 3s1 by applying the spherical harmonic stacking technique [buland et al. 1978] to seven spring gravimeters records, roult et al. [2010] also observed the triplet frequencies of the mode 3s1 based on 247 seismic records by taking simply the average value of the frequencies corresponding to the mode of interest. we note that, the triplet frequencies of the mode 3s1 were observed by only the two above-mentioned studies, to our present knowledge. for instance, although resovsky and ritzwoller [1998] observed the frequency of 3s1 (m = 0) using a dataset of more than 4,500 seismograms, they did not provide the frequencies of 3s1 (m = ±1). as a comment here, we note that in the study of roult et al. [2010], the middle spectrum line (m = 0) of the mode 3s1 is very weak. in our study, the middle spectrum line (m = 0) of 3s1 was clearly extracted from the records due to the combination of the hht and mse techniques. as can be noted from table 2, the observed 3s1 triplet in the present study are very close to the model premre+saw12d predictions, except that the central singlet deviates from the theoretical value a little more, as do the results of chao and gilbert [1980] and roult et al. [2010]. the causes for this are not clear. besides, all of the observed frequencies [given by chao and gilbert 1980, roult et al. 2010, and the present study] corresponding to m=±1 are higher than the model prem-re predictions, and smaller than the model prem-re + saw12d predictions. this implies that the mode 3s1 is very sensitive to the physical parameters of the mantle and core, and the observed triplet of 3s1 might provide significant constraints on the three-dimensional structure of the earth. we can preliminarily conclude that based on the observations of the 3s1 triplet (by previous studies and the present study, see tables 1 and 2), both the models prem-re and prem-re + saw12d might need to be adjusted as the predictions based on these models deviate from the «most likely real values» (the observations given by different authors) by slightly systematic shifts of about �–3.0 nhz and about 3.0 nhz in the frequency-decreasing and -increasing directions, respectively. in addition, from table 1 we note that as model prem-re + saw12d predicted, the splitting width of the mode 3s1 is 3.22 nhz, close to the estimate (3.21 nhz) given by he and tromp (1996), of 3.21 nhz, and the observed splitting widths given by chao and gilbert [1980], roult et al. [2010], and the present study are 2.90 nhz, 3.23 nhz, and 3.26 nhz, respectively. 5. conclusions as the mode 3s1 is quite sensitive to the mantle and outer core, the observation of the splitting frequencies of the mode 3s1 can provide significant information of the deep shen et al. 298 model or author(s) m=–1 (mhz) m=0 (mhz) m=1 (mhz) prem-re 0.942267 0.944215 0.945472 prem-re + saw12d 0.942925 0.944914 0.946144 chao and gilbert (1980) 0.94270 ±5.5#10–5 0.94535 ±9.0#10–5 0.94563 ±4.0#10–5 resovsky and ritzwoller (1998)a _ 0.944364±1.0#10–4 _ roult et al. (2010) 0.94256 ±1.241#10–4 0.94419 ±3.444#10–4 0.94579 ±1.493#10–4 present study 0.942598 ±4.25#10–5 0.944113 ±2.65#10–4 0.945864 ±2.13#10–4 a as a reference, the observation of the frequency (m = 0) of 3s1 given by resovsky and ritzwoller [1998], who did not provide the triplet 3s1. table 1. comparison of the observed triplet frequencies of 3s1 from previous studies with the present study, according to the model predictions. table 2. differences between the observations and the model predictions for the 3s1 triplet. differences author(s) m=–1 (mhz) m=0 (mhz) m=1 (mhz) obs prem-re chao and gilbert (1980) 4.33#10–4 1.14#10–3 1.58#10–4 resovsky and ritzwoller (1998) – 1.49#10–4 – roult et al. (2010) 2.93#10–4 –2.50#10–5 3.18#10–4 present study 3.31#10–4 –1.02#10–4 3.92#10–4 obs prem-re + saw12d chao and gilbert (1980) –2.25#10–4 4.36#10–4 –5.14#10–4 resovsky and ritzwoller (1998) – –5.50#10–4 – roult et al. (2010) –3.65#10–4 –7.24#10–4 –3.54#10–4 present study –3.27#10–4 –8.01#10–4 –2.80#10–4 299 interior of the earth, and consequently can improve the earth model. the application of the hht and mse analysis techniques to five common sg records leads to a clear observation of the triplet frequencies of the mode 3s1. the observed 3s1 triplet frequencies from the sg records at the canberra (cb), sutherland (s1, s2) and bad homburg (h1, h2) stations are in close agreements with the theoretical predictions provided by the model prem-re, and especially by the model prem-re + saw12d. the present study shows that the approaches (i.e., a combination of the hht and mse analysis techniques) proposed here are effective for the detection of the splitting frequencies of the mode 3s1, as only five sg records were used, and consequently, it can potentially be applied to the detection of the slichter triplet 1sl m (m=–1, 0, 1), of which the claimed observations are still controversial. acknowledgements. the authors are grateful to the ggp station managers who provided the sg datasets used in this study. the authors also thank the reviewers and the associated editor for their valuable comments and suggestions, which have greatly improved the manuscript. this study was supported by nsfc (grant no. 41174011, no. 40974015, and no. 40637034), and the special scientific research fund of state key laboratory of information engineering in surveying, mapping and remote sensing (china). references aki, k. and p.g. richards (1980). quantitative seismology: theory and methods. w. h. freeman, san francisco. alterman, z.s., y. eyal and a.m. merzeron (1974). on free oscillations of the earth, geophys. survey, 1, 409-428. buland, r., j. berger and f. gilbert (1978). improved resolution of complex eigenfrequencies in analytically continued seismic spectra, geophys. j. r. astron. soc., 52, 457-470. chao, b.f. and f. gilbert (1980). autoregressive estimation of complex eigenfrequencies in low frequency seismic spectra, geophys. j. r. astron. soc., 63, 641-657. courtier, n., b. ducarme, j. goodkind, j. hinderer, y. imanishi, n. seama, h. sun, j. merriam, b. bengert and d.e. smylie (2000). global superconducting gravimeter observations and the search for the translational modes of the inner core, phys. earth planet. int., 117, 3-20. crossley, d. and j. hinderer (2008). report of ggp activities to commission 3, completing 10 years for the worldwide network of superconducting gravimeters, in: observing our changing earth, iag symposia, 133, 511-521. crossley, d.g., j. hinderer, g. sasula, o. francis, h.t. hsu, y. imanishi, g. jentzsch, j. kaarianen, j. merriam, b. meurers, j. neumeyer, b. richter, k. shibuya, t. sato and t. van dam (1999). network of superconducting gravimeters benefits a number of disciplines, eos trans. agu, 80, 121/125-126. cummins, p., j.m. wahr, d.c. agnew and y. tamura (1991). constraining core undertones using stacked international deployment accelerometer gravity records, geophys. j. int., 106, 189-198. dahlen, f.a. 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(2003). what can superconducting gravimeters contribute to normal mode seismology?, bull. seism. soc. am., 93(3), 1370-1380. xu, j. and h. sun (2009). temporal variations in free core nutation period, earthq. sci., 22, 331−336. zürn, w. and r. widmer (1995). on noise reduction in vertical seismic records below 2mhz using local barometric pressure. geophys. res. lett., 22, 3537-3540. *corresponding author: wen-bin shen, wuhan university, school of geodesy and geomatics, department of geophysics, wuhan, china; e-mail: wbshen@sgg.whu.edu.cn. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. shen et al. 300 o s s e r v a z i o n i a s t r o n o m i c h e d a g r a n d e a l t e z z a m . c i m i n o r i c e v u t o il 15 giugno 1961 1. il problema delle osservazioni astronomiche da grande altezza si distingue da quello dell'osservazione dell'alta atmosfera t r a t t a t o dal collega broglio perché, mentre per quest'ultimo problema l'oggetto di studio è l'ambiente che circonda il veicolo (sia esso un pallone, un razzo o un satellite artificiale), oggetto del primo è invece t u t t o ciò che si trova al di fuori dell'atmosfera, e per la. cui osservazione quest'ultima rappresenta soltanto un ostacolo. all'atto pratico i due tipi di ricerca non possono però essere nettamente separati; anzi, direi che vi sono casi e condizioni particolari in cui l'abbinamento delle due direzioni di ricerca si rende non solo conveniente, ma addirittura necessario. su questo punto avremo però occasione di ritornare. 2. ho (letto che l'atmosfera terrestre rappresenta un ostacolo alle osservazioni astronomiche. si t r a t t a , in verità di un ostacolo veramente notevole, che si manifesta in due modi: 1) con l'assorbimento parziale o totale delle radiazioni elettromagnetiche e corpuscolari, che arrivano sulla terra dai corpi celesti e dallo spazio interstellare; 2) con la deformazione delle immagini, sia stellari, che del sole e dei pianeti. analizziamo brevemente queste due cause, cominciando dalla prima. l'assorbimento delle radiazioni elettromagnetiche ha luogo per diverse cause: a) assorbimento della radiazione nel campo ultravioletto per la fotoionizzazione di atomi dell'alta atmosfera, o per la scissione di alcune molecole. i n conseguenza di questo assorbimento, in cui l'energia dei fotoni assorbiti va a tramutarsi in energia di ionizzazione ed energia cinetica dei ioni e degli elettroni risultanti, si determinano nell'alta atmosfera i noti strati ionizzati (ionosfera) e le note dissociazioni delle molecole biatomiche dell'azoto e dell'ossigeno in molecole monoatomiche; b) diffusione della radiazione nel campo visuale da p a r t e delle molecole 7 0 m . c i m i n o atmosferiche secondo la n o t a legge direileigli, ovvero da p a r t e di polveri 0 di particelle d ' a c q u a allo stato liquido o solido; c) assorbimento di larghe b a n d e dell'infrarosso da p a r t e specialmente del vapore d'acqua e delle molecole dell'ossigeno e dell'anidride carbonica; d) diffusione delle onde radio di lunghezza d'onda inferiore ai t r e centimetri, per opera dello stesso vapore d ' a c q u a ; e) riflessione verso l'esterno delle onde radio cosmiche di lunghezza d'onda superiore agli 80-100 metri, per opera degli strati ionizzati. i n conclusione, se veniamo a f a r e un bilancio delle radiazioni che liberamente, o quasi, possono raggiungere il suolo avremo il seguente risultato, in verità alquanto scoraggiante: ad eccezione delle radiazioni comprese t r a 4000 e 8000 a circa (radiazioni visibili) e delle onde radio con lunghezza d'onda compresa t r a i 3 cm e gli 80-100 m , t u t t e le altre radiazioni non riescono, praticamente, a raggiungere il suolo. eppure, sono proprio le radiazioni provenienti dalle estremità dello spettro, che presentano il massimo interesse scientifico, come ora vedremo. 3) volendo fare u n a classifica sintetica delle osservazioni astronomiche da grande altezza, si potrebbe così catalogarle: osservazioni solari osservazioni lunari » planetarie osservazioni stellari » di nebulose e galassie campi magnetici ed elettrici spaziali onde radioelettriche. ma, forse, più che u n a classifica per problemi, la cui analisi dettagliata ci porterebbe molto lontano, converrà effettuare u n a classifica che potremmo chiamare per veicolo, cercando di vedere cioè che cosa è possibile ottenere con l'impiego dei diversi veicoli allo stato attuale della tecnica, e, in particolare, che cosa noi in italia potremmo f a r e con 1 mezzi che, ragionevolmente, potremmo disporre, nel giro dei prossimi anni. i veicoli a d a t t i alle osservazioni astronomiche, da grande altezza possono essere di t r e speci: i satelliti, i razzi ed i palloni. escludo l'aereo, perchè i nostri aviogetti difficilmente possono superare la quota di 1012.000 metri, e questa è u n a quota ancora troppo bassa. l'osservatorio astronomico di r o m a , in occasione dell'eclisse totale di sole del 15 ( i ) ( i i ) (iii) (iv) (v) o s s e r v a z i o n i a s t r o n o m i c h e d a g r a n d e a l t e z z a 7 1 febbraio 1961, ha organizzato, col concorso dell'aeronautica militare e dell'istituto nazionale di geofisica, osservazioni solari sia con aviogetti, sia con palloni sonda. a quanto mi risulta, sono stati questi i primi t e n t a t i v i effettuati in italia di osservazioni celesti da a l t a quota. delle osservazioni e f f e t t u a t e dai palloni diremo in seguito; per quanto riguarda quelle da bordo degli aviogetti, abbiamo ottenuto un buon numero di fotografie della corona, adoperando le camere di p r u a p u n t a t e verso il sole, che, al momento della totalità, era piuttosto basso sull'orizzonte. il cielo, molto puro a quell'altezza, ci ha consentito di ottenere fotografie completamente prive di fondo, cosicché esse si prestano assai bene per uno studio fotometrico della corona. ma, per la relativa instabilità dell'aereo, si son p o t u t e scattare soltanto delle istantanee, cosa che ha impedito di fotografare le estreme propaggini della corona solare (figg. 1 e 2). l'instabilità, assieme alla limitata altitudine, sono i f a t t o r i che limitano fortemente l'impiego degli aerei per le osservazioni astronomiche a grande altezza, p u r senza escludere che, con particolari accorgimenti e, sopratutto, per particolari osservazioni, non si possa raggiungere qualche risultato di grande interesse. degli altri tre tipi di veicoli (satelliti, razzi, palloni), certamente il più idoneo alle osservazioni astronomiche è il satellite; esso, i n f a t t i , si muove quasi completamente al di f u o r i dell'atmosfera ed è possibile inoltre ottenere, con dispositivi di non difficile realizzazione, u n a sufficiente stabilità per gli s t r u m e n t i astronomici. fino ad oggi però, l'impiego dei satelliti a scopo p u r a m e n t e astronomico ò stato assai limitato, sia per la difficoltà di p o r t a r e pesi considerevoli, sia perché problemi più urgenti hanno completamente assorbito il cosidetto carico pagante. lasciando da p a r t e il lunik sovietico, che ci ha dato una prima, seppure grossolana, immagine dell'altra faccia della luna, nessun altro satellite ha p o r t a t o telescopi, ed essi si sono piuttosto limitati allo studio dell'ambiente locale (per esempio delle fascie di radiazioni di van alien) e dei raggi cosmici. p i ù sostanziale è stato l'apporto dei razzi, dei quali si può disporre con maggiore frequenza e con minore spesa. p u r t r o p p o il razzo, p u r potendo raggiungere quote sufficientemente elevate, presenta il grave inconveniente della brevità del tempo disponibile in quota e della difficoltà dell'orientamento in tempo utile del sistema ricettore (ottico o fotoelettrico) verso l'oggetto da studiare. osservazioni astronomiche lin'ora effettuate da razzi, — con tecniche diverse, che non mi è materialm e n t e possibile descrivere in questa relazione — riguardano principalmente il sole, e più precisamente: la misura della costante solare; l'os7 2 m . c i m i n o servazione dolio spettro u . y . ; alcuni riusciti t e n t a t i v i di fotografìa solare nella luce della prima riga della serie di l y m a n dell'idrogeno, che ci ha permesso di vedere alcune formazioni coronali sul disco; osservazioni fig. 1 corona solare f o t o g r a f a t a eia u n a v i o g e t t o . t e m p o di esposizione: 1/25 di secondo. nell'infrarosso; osservazioni della radiazione corpuscolare. i n occasione dell'ultima eclissi totale di sole, i russi hanno lanciato dei razzi per la fotografìa della corona solare in luce u . v . , assicurandosi in tal modo un bel primato di primaria importanza scientifica. nel campo stellare è stato compiuto un esame preliminare del cielo boreale nella b a n d a ultrao s s e r v a z i o n i a s t r o n o m i c h e d a g r a n d e a l t e z z a 7 3 violetta intorno ai 1500 a. si è t r o v a t o clie alcune regioni del cielo appaiono più luminose in questa luce ultravioletta, che non nella luce visibile, ponendo così nuovi problemi. l'osservatorio astronomico di roma ha studiato, con la collaborazione di un osservatorio estero, alcuni sistemi originali per la fotografia u . v . solare a bordo di razzi. io ritengo che i razzi lanciati per lo studio dell'alta atmosfera, p o r t a n t i anche attrezzature per studi astronomici, aumenterebbero certamente assai il rendimento scientifico del lancio. più problematico appare il trasporto di un vero e proprio telescopio; cosa, invece, assai più facile utilizzando i palloni. sufficientemente avanzato è, infine, il problema del recupero degli strumenti trasportati in quota dai razzi. 4. l'impiego di palloni per l'osservazione astronomica da a l t a quota d a t a da alcuni anni; t u t t i conoscono i riusciti t e n t a t i v i di m. schwarzschild negli s t a t i uniti e di a. dollfuss in francia, che sono riusciti a darci pregevoli fotografie della granulazione solare e interessanti osservazioni delle superfici planetarie. in particolare sono assai interessanti i lanci di m. schwarzschild di telescopi comandati da terra, mentre nei lanci francesi il pallone p o r t a v a anche l'uomo. nei lanci del 1959 f u raggiunta u n a quota di 80.000 piedi (circa 27.000 m) portando un telescopio di 30 cm di a p e r t u r a . n s f i g . 3 f s o f o t e coronali d e d o t t e d a u n a fotog r a f i a della c o r o n a eseguita sul monte cimone con a s t r o g r a f o di 80 cm di focale ( d o t t . cr. caprioli). 7 4 m . c i m i n o come ho d e t t o al principio di questa relazione, due sono i f a t t o r i negativi dell'atmosfera terrestre con riguardo alle osservazioni astronomiche: l'assorbimento delle radiazioni di cui abbiamo parlato, e la deformazione delle immagini, causata dalla agitazione degli strati d'aria. è questo secondo f a t t o r e a p p u n t o quello che impedisce di vedere i dettagli più fini delle superfici dei corpi celesti. p e r osservare questi stessi dettagli non basta però la sola tranquillità atmosferica (che già si trova in misura sufficiente al di sopra dei 20.000 metri), m a occorre anche che il sistema ottico abbia u n sufficiente potere risolutivo, il che significa, sostanzialmente, che la lente o specchio principale deve avere u n a sufficiente a p e r t u r a . si t r a t t a perciò di u n a a t t r e z z a t u r a di notevole ingombro, che solo u n pallone può portare, in attesa, naturalmente, della disponibilità di un satellite. negli s t a t i uniti c'è un progetto, in stato di a v a n z a t a esecuzione, che prevede di p o r t a r e in quota un telescopio con uno specchio parabolico di 92 cm di diametro. ma anche con piccoli telescopi e camere fotografiche è possibile ottenere risultati assai interessanti; per esempio per l'osservazione della corona solare. si pensi che già a 10.000 m la brillanza residua del cielo è 50 volte più piccola di quella dei pennacchi coronali. il trasporto di un piccolo coronografo con un pallone sarebbe cosa relativamente assai semplice. n a t u r a l m e n t e , i palloni p o r t a n t i attrezzature come quelle adoperate dal dollfus o dallo schwarzschikl sono molto grandi e costosi. però un notevole campo di ricerche si apre anche con l'impiego di mezzi più modesti. come ho accennato, questo impiego è stato iniziato in italia, per la prima volta in occasione della eclisse solare, da un gruppo di lavoro che, oltre a chi vi parla, comprendeva il prof. r . cialdea ed i colonnelli f e a e pisoni dell'aeronatuica militare. il campo di ricerca era misto, geofisico ed astronomico. da u n a p a r t e si mirava a misurare la luce diffusa del cielo a quelle altezze, dall'altra a fotografare la corona solare nelle sue propagini più esterne e, possibilmente, collegarla con la luce zodiacale. il prof. r . cialdea vi darà, in u n a relazione particolare, i risultati ottenuti, che possono considerarsi soddisfacenti se si pensa che il primo lancio di prova completa coincideva con il lancio definitivo d u r a n t e l'eclisse. questo è dipeso da quella non mai abbastanza deprecata mancanza di mezzi finanziari contro cui u r t a n o da noi t u t t e le iniziative. lasciando, come dicevo, al prof. cialdea il compito di informarvi dei risultati del lancio, io desidero invece approfondire u n poco questo i m p o r t a n t e problema, dello studio della corona esterna e della luce zodiacale, in vista di ulteriori ricerche in questo campo. o s s e r v a z i o n i a s t r o n o m i c h e d a g r a n d e a l t e z z a 7 5 5. quali siano i limiti della corona solare noi astronomi ancora non sappiamo precisarlo. la corona si estende certamente assai al di là di quanto è dato osservare nei rari momenti dell'eclisse totale. certamente una p a r t e di questa immensa aureola r u o t a assieme al sole; però oltre un certo limite, che non conosciamo, le singole particelle formano uno sciame che gravita a t t o r n o al sole e che si estende fino a limiti irnfig. 3 come si p r e s e n t a il f e n o m e n o della luce zodiacale dopo il t r a m o n t o del sole. precisati, forse fino all'orbita di venere ed anche della terra ed oltre. n e abbiamo u n a d i r e t t a conoscenza attraverso la cosidetta luce zodiacale. questo fenomeno si osserva generalmente nelle serate primaverili o a u t u n n a l i , quando il sole è ben sotto l'orizzonte, una fascia luminosa assai tenue, copre il cielo quasi fino allo zenit (fig. 3), e la sua spiegazione è immediata osservando la figura successiva (fig. 4). questa enorme corona, dalla forma sensibilmente schiacciata, è cert a m e n t e variabile, così come lo è la vera e propria corona solare che osserviamo d u r a n t e le eclissi totali. essa costituisce quella materia interplanetaria che interessa direttamente la fisica dello spazio. i fenomeni che costituiscono l ' a t t i v i t à solare alimentano continuamente di materia questa corona, lanciando in esso getti di particelle ionizzate, come enormi fusi, che, raggiungendo la terra, danno lnogo a t u t t i i fenomeni di varia7 6 m . c i m i n o zione della ionosfera, del campo magnetico terrestre, delle aurore, delle fasce di radiazione, ecc. ecc.. non c'è dubbio perciò che lo studio di questa corona esterna sia di primaria i m p o r t a n z a per la fisica solare, così come per la fisica terrestre. % i— — . • •• • . • •1 " e-, • s,, ' r * " v u . • * v • % • v \ v v •. \ x \ \ v \ * \ \ \ \ \ \ \ \ . v v • v ^ v • > fig. 4spiegazione s c h e m a t i c a (lei f e n o m e n o della luce zodiacale. l ' a m masso di m a t e r i a l e d i f f o n d e n t e , che c i r c o n d a il sole s, è visibile dopo il t r a m o n t o o p r i m a del sorgere, e si m u o v e a p p a r e n t e m e n t e sulla v o l t a e,eleste assieme al sole. io ritengo che uno studio sistematico della luce zodiacale, così come quella della vera e propria corona solare esterna, possa essere f a t t o utilizzando i palloni sonda. inoltre, questo studio può essere e f f e t t u a t o anche in luce u . v . come dimostra la fig. 5. si vede come, con palloni a 25.000 m e t r i è certamente possibile fotografare su u n a lunghezza d'onda fino a 2000 a. recentemente (1958) studi sulla luce zodiacale da alte altitudini sono s t a t i effettuati per opera dei due astronomi inglesi 1). e . blackwell e m. f . ingliam, da u n a stazione a circa 6000 m e t r i sulla mont a g n a del chacaltaya in bolivia. questi autori si sono posti la questione della composizione di quest'immensa aureola solare, se cioè f o r m a t a da polveri o da elettroni. mediante osservazioni spettroscopiche e polarimetriche essi sarebbero arrivati alla conclusione che, alla distanza di u n a u n i t à astronomica (distanza media terra-sole) la densità elettronica o s s e r v a z i o n i a s t r o n o m i c h e d a g r a n d e a l t e z z a 7 7 dovrebbe risultare notevolmente inferiore ai 100 elettroni per cm3, cont r a r i a m e n t e ad altre ipotesi e precedenti osservazioni, che propenderebbero per u n a densità di almeno 600 elettroni per cm3, confermata, quest'ultima, da osservazioni radioelettriche dell'esosfera. p e r il resto, la grande corona solare sarebbe costituita da polveri. fig. 5 a l t e z z a n e l l ' a t m o s f e r a alla q u a l e l ' i n t e n sità, dei raggi u . v . è r i d o t t a del r a p p o r t o 1/2.7. x = raggi x . n a t u r a l m e n t e , l ' a t t i v i t à solare sembrerebbe influenzare la luce zodiacale, e ciò è logico perchè l ' a t t i v i t à solare crea fasci di elettroni che a t t r a v e r s a n o lo spazio interplanetario e diventano centro di diffusione della luce solare; m a le correlazioni sono molto incerte, e molte nuove misure sono necessarie. ricordiamo anche le misure della luce zodiacale effettuate dagli scienziati russi d u r a n t e l'anno geofisico internazionale, da stazioni poste a latitudini di circa 44°, all'epoca del solistizio di estate, per la misura della luce zodiacale nella direzione normale all'eclittica. non mi risulta che siano state effettuate ricerche nell'ultravioletto. ora basti pensare che è proprio in questa radiazione che il sole si comporta come u n a stella variabile; è logico quindi pensare che in questo campo d'onde le variazioni della luce zodiacale in relazione all'attività solare dovrebbero dimostrarsi assai più cospicue e dovrebbero essere facilmente rilevabili dai palloni. 6. i n conclusione, le osservazioni astronomiche da grandi altezze costituiscono uno dei fondamentali campi di ricerca del prossimo f u t u r o . se per il momento noi italiani non possiamo accedere ai satelliti artificiali, possiamo però impiegare i razzi e i palloni. i primi per osservazioni nel lontano ultravioletto, i secondi per t u t t e le osservazioni che richiedono maggiore disponibilità di tempo in quota e strumenti più stabili ed ingombranti. 7 8 m . c i m i n o gli esperimenti da noi iniziati coi palloni, in collaborazione con l'aeronautica militare e l ' i s t i t u t o nazionale di geofisica, continueranno in questo senso e voglio a u g u r a r m i che la nostra iniziativa possa trovare quell'appòggio che la semplicità dei mezzi e l'importanza dei risultati consigliano di concedere. riassunto si esamina il •problema delle osservazioni astronomiche da grande altezza per mezzo di aerei, satelliti, razzi e palloni. utilizzando questi attimi è possibile effettuare osservazioni solari di grande interesse, legate alla fisica dello spazio, specialmente in relazione alla luce zodiacale, con mezzi relativamente assai modesti. il primo tentativo in italia in questa direzione è stato effettuato durante veclisse totale di sole del 15 febbraio 1961, ed i risultati ottenuti consigliano di continuare le ricerche in questa direzione. su m mary the problem of astronomical observations from airplanes, satellites, rockets and ballons is considered. solar observations from balloons look to be very interesting, as related to space physics, expecially in connections with the problem of the zodiaeal light. a first attempi in italy in this fichi was successfully accomplished in the occasion of the total solar eclipse of february 15, 1961, and the promising results have induced us to continue the researches in this direction. geometric quality term for station-based total electron content estimation annals of geophysics, 59, 1, 2016, a0107; doi:10.4401/ag-6803 a0107 geometric quality term for station-based total electron content estimation murat selim çepni*, erman şentürk university of kocaeli, department of geomatic engineering, kocaeli, turkey abstract global ionospheric maps (gims) are still commonly used to represent total electron content (tec). however, a large number of permanent gps receivers provide significant data for monitoring ionosphere. the gims supply accurate tec results, although spherical models are not able to fit local ionospheric perturbations as a source of local receiver data. therefore, the use of station-based tec computation becomes more preferred and convenient. the station-based tec approach is established on weighting averaging or modeling vertical tecs. using this approach to reveal ionospheric activity depends on geometric distribution of ionospheric piercing points (ipp). the most effective parameter for examination of station-based tec estimation is a function of relative geometry between receiver and satellite. in this study, a quality term has been used to define, for tec estimation according to distribution of ipps, similar to geometric dilution of precision (gdop). the quality term has been described for the first time in this field and it was named as r-tec (reliability of total electron content). 1. introduction electromagnetic signals used in global navigation satellite systems (gnss), travel through the ionosphere and interact with the ionospheric plasma, which causes propagation delays. the ionospheric range delay is positive for pseudorange measurements and negative for carrier phase measurements [nohutcu et al. 2010]. therefore, the carrier phase pseudoranges are measured to be too short and the code pseudoranges to be too long [hofmann-wellenhof et al. 1992]. due to the fact that ionosphere is a dispersive medium; the ionospheric refraction depends on the signal frequency [bergeot et al. 2014]. thanks to dual frequency equipped receivers, ionospheric delay can be eliminated or estimated. electron density is a primary parameter used to describe ionosphere and ionospheric delaying effect on the gnss signal which is a result of total electron content (tec). the tec is the total number of electrons in a vertical column of 1 m2 cross-section in the ionosphere layer. electron content within its path is measured in tec unit (tecu) and 1 tecu is equal to 1016 electrons per square meter [chauhan et al. 2011, karia and pathak 2011]. the observation of total number of free electrons along the ray path is derived from the frequencies l1 (1572.42 mhz) and l2 (1227.60 mhz) of the gps that provide the relative ionospheric delay of electromagnetic waves, traveling through the medium [saito et al. 1998, ogunsua et al. 2014]. 10 tecu induces a delay of 5.4 ns (= 1.6 m) on the travel time of l1 signals, and a delay of 8.9 ns (= 2.7 m) on the travel time of l2 signals [bergeot et al. 2014]. ionospheric electron densities are very important parameters for studying its characteristics and effects on precise positioning, navigational applications, and electromagnetic signal transmissions [panda et al. 2014]. the value of the tec is used for making appropriate range corrections, as well as accounting for errors occurring in the range delays, to determine the effects of space weather-related events, such as geomagnetic storms and scintillations due to ionospheric irregularities [rama rao et al. 2006, kumar et al. 2012]. various scientific applications and services increasingly demand real-time information on the effects of ionosphere on the gps signals. several researches were carried out on the possibility of estimating the tec, along the ray path, this was between a gps satellite and ground-based receiver. this was used from the gps receiver measurements [mannucci et al. 1998, ciraolo et al. 2007]. while traveling through ionosphere, the gnss signals are exposed to delaying effects caused by electron density. thorough relative time delay of l1 and l2 frequencies, the slant tec (stec) is calculated along the ray path from receiver to satellite. as the slant tec is dependent upon the ray path, geometry through the ionosphere, it is required to calculate an equivalent vertical value of the tec which is independent from the article history received june 5, 2015; accepted february 8, 2016. subject classification: ionosphere, tec, r-tec, quality parameter. elevation of the ray path [sharma et al. 2012]. the ionosphere is generally divided into several layers of varying thickness, according to electron density [hargreaves 1992]. used in the conventional tec process, it is collapsed into a single layer. the single-layer model (slm) assumes that the thin spherical shell located on a specified altitude, whereby the stec is converted to the vertical tec (vtec). the slant tec is projected to vertical tec, using the thin shell model, assuming a height of 350 km [weber et al. 1986]. the point where the signal penetrates the thin shell is called the ionospheric piercing point (ipp). vtec is computed at many ipps from its time delays using dual frequency observations. of course, the number of vtecs is equal to the number of received satellites for each epoch. a solution can be obtained from weighted averaging of all individual vtec estimates. weighted average is regarded as tec for relevant epoch. weighting algorithms are functions of relative position between satellite and receiver. station-based tec represents ionospheric activity at receiver zenith. preferential selection of signal paths closest to the receiver will provide a more realistic local tec, over receiver location [çepni et al. 2013]. hence, it is desired that observations of satellites that are closer to zenith have more determinative results for stationbased tec. depending on the positions of ipp and receiver location, relative geometry affects the quality of the tec computation. relative geometry between receiver and satellite has twin parameters: satellite elevation and azimuth angles. the satellite elevation angle defines the ipp distance from receiver zenith; the satellite azimuth angle determines local time difference between the receiver and the ipp. depending on relative geometry terms, ipp distributions can bementioned as preferable or inadequate. in this study, the intentions are to describe a term like dilution of precision (dop) for station-based tec estimations. the dop is widely used as a criterion for selecting measurement units, since it characterizes the geometric effect on gps positioning error. quality of overall navigation, solutions can be determined by examining the dilution of precision [wells et al. 1986]. it is well known that dop comes in various flavors, including geometrical (gdop), positional (pdop), horizontal (hdop), vertical (vdop), and time (tdop) [langley 1999]. dop concept should be used in different studies; e.g. zirari examined the dop in wi-fi-based positioning systems as signal strength dilution of precision [zirari et al. 2009]. main purpose of this study is to examine the influence of relative positioning between receiver and satellite on the efficiency of tec estimation. it has to be emphasized that, the tec computation with ipps is distant and under a different local time to the receiver’s zenith. it cannot illustrate to the ionospheric electron its density at the receiver location. consequently, it has been described as a numeric parameter for the tec estimation, regarding the ionospheric activity at the receiver location. this term was named as reliability of tec (r-tec). 2. methodology the geometry-free linear combination of gps signals (l4), which is also called the ionospheric observable, is classically used for ionospheric researches and it is obtained by subtracting simultaneous pseudorange or carrier phase observations [ciraolo et al. 2007]. 1— k = a( 1—f 12 − 1— f 22 )= 9.524370, and 1—k means kilometer to tecu. dcbi, dcbk are satellite and receiver biases. biases term have to be known or estimated. m1b1−m2b2: the term is not equal to zero and it has to be estimated. b1 and b2 are constant bias due to initial phase ambiguity. the slant tec is estimated from (1) for each satellite that passes. stecs occur along the signal path in ionosphere and related to the length of the path. that is why stecs have to be converted to the vertical tecs, which are comparable in terms of value for electron density. although the ionosphere consists of several layers on processing, the conversionof the stecs is assumed as a single layer. the single-layer model is used, whereby the maximum-density free electrons are accepted as being inside an area with infinitesimal thickness [hugentobler et al. 2001, alizadeh et al. 2011]. briefly, vtecs are obtained by means of thin shell sphere. figure 1 illustrates geometry of slm. (2) equation (2) is a well-known mapping function for the calculation of electron density, at vertical direction of the ipp. thus, the vtecs are associated with the ipp. to describe the ionospheric electron density over the station, the tec is computed from weighted average of the vtecs. although local ionospheric modeling can also be used, weighting average is confidential for station-wise tec computation. conventionally univariate functions have been defined based on satellite elevation angle, for weighting. however in this study, it l l l l a f 1 f 2 stetec b b cdcbcb cdcbcb1 1 k i k i i k 4 1 2 1 1 2 2 4 fs 12 f 22 1 1 2 2 m z m z m m = = =+ + + fs x .sinvtetec stetec r h r 1 e m e 2 f= + q v# & çepni and şentürk 2 (1a) (1b) 3 has been proposed using of bivariate functions, composed of satellite elevation (f) and azimuth angles (a). where t1-t2 is time lapse interval. as the users can choose the time interval, they can also use the default interval from the observation file. in this case the tec is computed, e.g., 43,200 times for 2 s time intervals and 2880 times for 30 s time intervals within a day. due to the ipp associated processing, if the ipps are away from receiver zenith, the weighted averaging or local modeling may not represent electron density at station’s location. distribution of the ipps is decisive on the station dependent tec. in other words, relative geometries between the receiver and the satellites affect tec estimation quality. relative position of the receiver and the ipp has been comprised by satellite elevation and azimuth angles. the projection of ionosphere piercing point on the earth is called the sub-ionosphere point (sip) [çepni et al. 2013]. instead of the radius vector between the ipp and the receiver, the distance between sip and receiver (ssip) has to be taken into account. hence in quality terms, variables which have to be explained are elevation and azimuth angles. as the satellite elevation angle describes the distance between the sip and the receiver, the satellite azimuth angle characterizes longitudinal components of this distance. the distance between the sip and the receiver can be expressed as in (4) with respect to elevation angle. additionally, table 1 is arranged to show distances calculated from (4). where f, re and hm symbolize the satellite elevation angle, the radius of the earth and the ionosphere height, respectively. re and hm are accepted as 6378 km and 450 km. low elevation angles may cause sip to be so far from the receiver that the vtec bears little resemblance to local conditions over the station. as seen in table 1, the distance can be over 1000 km where there are lower elevation angles and the most frequently observed distances are between 200 km and 800 km. if the ssip distance exceeds a few hundred kilometers, vtec may not represent receiver location. thus if the elevation angle is low, representing ability of vtec is interpreted as weak. satellite azimuth angle identifies the direction of ssip. so, time difference between the sip and the receiver is determined by the elevation and azimuth angles. local time effect may correspond to significant gaps on the estimated vtec, due to daily tec cycles, depending on the longitudinal component of ssip [șentürk 2014]. diurnal variation of tec is a characteristic in accordance to the rotation of the sun and curve of daily tec looks like a parabola reaching maximum at local noon. local time of sip indicates a shifting on diurnal tec cycle from receiver location. offsets for the e.g. with a 15 min time difference may reach to 2 tecu. consequently, minimum longitudinal distance guarantees a better situation in terms of a local time parameter. longitudinal difference can be computed with respect to azimuth (a) and elevation angles (f). figure 2 shows longitude difference as depending to elevation and azimuth angles. where {m means latitude for ssip; t°=180°/r. , , tetec wi wi vtetec wi i n wi i i n t t 1 1 1 2 f a f a = = = q q v v| | s arcsr2sip e, sin cot r h 2 e m ft y s sin cosld r sip e { ld = ° n m{ t a geometric quality term for station-based tec estimation (3) (5) (4) figure 1. single-layer model [çepni et al. 2013]. f0 10 20 30 40 50 60 70 80 90 ssip km 2569 1238 779 536 377 259 163 79 0 table 1. distances of ssip according to elevation angles. after describing relative geometry components and their influences on the tec estimation, the geometric quality parameter (gqp) can be defined. mathematical expression of the gqp was developed empirically as a function of satellite elevation and azimuth angles. (6) equation (6) has been defined for all observed satellites at each epoch. indices of i and n indicate the satellite number and epoch. composition of gqp is based on proper usage of variables. sinus function is widely used with the elevation angles in weighting the algorithms and the exponential function is well-done and complements the local time effect. prepotency of two variables has been taken into consideration. the elevation angle must be more effective on the gqp and the effect of azimuth angle should increase a little relatively to how elevation angle should move away from zenith. it has been seen that the behavior of equation (6) ensures geometric relationships presented by variables. figure 3 outlines the behavior of gqp function. the gqp(i) has values at a [0-1] range. when the value is closer to 1, the gqp expresses ideal relative pogqp sin e(i) (i)n (i)(i) n nf= rv r fd-q srv xl çepni and şentürk 4 figure 3. gqp graphic according to elevation and azimuth (extreme) angles. figure 2. longitudinal difference according to elevation and azimuth angles. 5 sitioning for the satellite and receiver. in case that the value descends to 0, the gqp means its insufficient ipp position cannot represent electron content at the receiver location. the gqp(i) is associated with the ipp and also the vtec. because of the fact that tec is computed from vtecs via weighting average or local modeling, geometric quality terms for the station should be evaluated sum or average of the gqp(i). in other words, individuals representing the ability of each observation and determines quality of estimation for the receiver. thereby, statistical average of the gqps was accepted as geometric quality term: reliability of tec (r-tec). it has to be noted that reliability was used in terms of sufficient relative geometry for tec estimation. r-tec = (7) where i denotes observed satellite. equation (7) reminds expression of dilution of precision (dop). both conceptual and mathematical, r-tec has resemblance to gdop. r-tec can be computed at each epoch or time interval for station-based tec estimation. examination of distribution of r-tec and discussion on its thresholds are in next section. 3. results and discussions in this study, two types of practice were performed. the first implementation aimed to introduce gqp quality term, and the 30 s resolution data on october 23, 2011, was used. the data were collected from malz and mura stations which belong to the continuously operating reference station turkey (cors-tr) network. the second implementation intended to show samples from low, middle and high latitudes. the data, also 30 s resolution, were collected at three igs stations on march 1, 2012. the vtecs have been processed from observation files initially and they were received as input for implementations. afterward, geometric quality parameters gqp were computed for all observations from (6). figure 4 was arranged for sampling of implementations and it has drawings at malz station for svn1 (svn1: satellite identification). the vtec, elevation angle, azimuth angle and the gqp variations were drawn all day long. relationships between relative geometry and the gqp1 were monitored in figure 4. also, vtec and its geometric quality parameter can be seen together. it is suggested that vtec graphics should include gqp bar, like the first and the last rows of figure 4. when figure 4 is examined, it can be seen that the reliability of the vtecs is very low at 03-05 ut depending on relative geometry of observed satellites. this is because the elevation and azimuth angles are about 15° and 300°, the vtecs have less quality. the gqp1 has no values at this period. owed to gqp, users can be aware of unusable observations at this period. it is suggested that the vtec is not taken into account in averaging, due to its low gqp. maximum gqp1 reading of the day (≈0.80) appears just before 19 ut due to its approximately 70° in elevation. azimuth has non-extreme case at this time. geometric quality parameter indicates that the ipp1 associated with the vtec can exemplify ionospheric plasma at the receiver location at 17:45-19:15 ut sufficiently. out of this time interval, svn1 observations are not suitable to define vtec for this station. gqp gqp ... gqp i1 2w 2 2w 2w+ + +r r rq q q2w 2w 2wv v v geometric quality term for station-based tec estimation figure 4. plots for svn1 and malz station on october 23, 2011. since the tec is obtained from the vtecs, geometric quality term r-tec has been described as the statistical average of geometric observation quality parameters of the vtecs (gqp(i)). figure 6 is the graphic image of daily r-tec for the malz station. tables 2a and 2b figure out minimum and maximum r-tecs of the session day. the tables include quality parameter and its variables to make comparisons. also, figures 5a and 5b have been charted in order to demonstrate ipp distributions for minimum and maximum rates of the r-tec. the effect of the elevation and azimuth angles on the gqp can be viewed once again from the tables. due to the preferred azimuth angle, gqp is better than the others even when elevation angles are slightly lower, such as in table 2a, columns 4, 7 and 8. when the tables are examined, correlations between the relative geometry and the quality parameter can be seen clearly, e.g., epoch of minimum r-tec has no observation over 55° elevation angle. the track of r-tec throughout the day was plotted in figure 6. the graphic depicts information to review its quality. the geometric reliability term appeared between 0.57 and 1.39, it exceeds “1” equals approximately to 70 percent. eventually, figure 6 exhibits the final product of this study and it needs special examination. moreover, the examination of reliability terms is required to be analyzed by outputs. in order to reveal the distribution of the r-tec, 15 days of data was processed. due to repeated satellite constellation, trial data has been considered enough. the data belongs to the malz station in october 2011. the data consist of 43,200 epochs in total, and 6600 epochs were unable to process. the distribution can be seen in figure 7. it has been examined that equation (7) produces values mostly 0.1 1.4 interval and resemble the asymmetric gaussian distribution. the skewness of distriçepni and şentürk 6 f0 12 29 52 23 35 53 21 22 a0 192 228 46 81 126 309 167 287 gqp(i) 0.002 0.049 0.385 0.009 0.104 0.406 0.025 0.008 r-tec 0.5716 f0 61 69 32 11 21 52 35 39 a0 87 112 197 321 120 311 48 272 gqp(i) 0.554 0.743 0.105 0.000 0.006 0.379 0.104 0.133 r-tec 1.3868 table 2. (a) minimum r-tec of october 23, 2011 epoch: 05:07:30; (b) maximum r-tec on october 23, 2011 epoch: 14:26:00. figure 5. (a) satellite chart for the minimum r-tec; (b) satellite chart for the maximum r-tec. (a) (a) (b) (b) 7 bution can be distinguished easily from bell curve on left (negative) skew. negative skewness for quality terms means convergence to better cases and it points out that it is likely to provide sufficient reliability. then the graphic information of the r-tec distribution can be evaluated as realistic. statistical parameters are also shown in figure 7. mean, median and mode of distribution are very close to each other and all indicate 1. without having a threshold in statistics, values which are less than 0.6 and more than 1.2 may be interpreted as extreme, by accumulation of the values. in fact, determination of critical values for reliability terms is primarily significant. to qualify the tec estimation, the critical value has to be determined. as well known, about 68% of values drawn from a normal distribution are within one standard deviation away from the mean. (8) with a great similarity, 69% of the r-tec values are over the 1 in this implementation. (9) range equality in equations (8) and (9) should be considered as proof of the r-tec distribution limit value. it can be stated that rate 1 is the critical value, and, if the r-tec is greater than or equal to 1, the sta. .p x 0 68261 1n v n v+ =q v .f1 0 9 0=q v .0 6980 geometric quality term for station-based tec estimation figure 6. plot of the daily r-tec for the malz station on october 23, 2011. figure 7. distribution of the r-tec values. tion dependent tec estimation can be assumed to have a reliable geometry. in addition to confidence intervals in equations (8) and (9), distributions parameters (mean, median, mode) confirm rate 1 with respect to the accumulation of the data set. it must be stated that the r-tec is an indicator for illustrating the ability of electron density at the receiver location. in case the r-tec is less than 1, the ipp positions may not be competent in the estimation of the tec for the receiver location. the behavior of gqp at low and high latitude is examined in general. implementation was realized at three igs stations located at low, mid and high latitudes for data samples on march 1, 2012. the station informations are given in table 3. reso station is located at cornwallis island at ~75° latitude. because of the gps constellation, it is expected that this station has bad visible satellite geometry for vtec computation. riop station, at ecuador, is a sample at low latitude for gqp, and ankr station’s data is used as mid-latitude sample. figure 8 shows efficiency of visible satellite geometry on reliable of v-tec. at reso station, r-tec values are lower than other’s, as expected, due to to orbital deficiency. according to position of station and time, relative geometry between receiver and observed satellites may not be sufficient any time. thus, implementations at high and low latitudes point out signification of quality term r-tec. 4. conclusion the relative geometry between the receiver and the satellites has considerable importance on the station-based tec process. certainly, accomplishment of estimation is influenced by this geometry. in this study, it has been aimed to compose a criterion depending on relative geometry components: satellite elevation and azimuth angles. the effects of the elevation and the azimuth angles on the tec computation were explained in detail in the previous sections. the impact factor of the ipp positions has been defined as the geometric quality parameters (gqps). the gqp was expressed mathematically as in (6) by experimental study on composition of variçepni and şentürk 8 id location lon (e)° lat (n)° reso resolute/canada 265.1067 74.6908 ankr ankara/turkey 32.7586 39.8875 riop riobamba/ecuador 281.3489 -1.6506 table 3. information of igs stations used for interpreting at different latitudes. figure 8. r-tec values of igs stations on march 1, 2012. (a) rtec values at reso station located at low-latitude; (b) r-tec values at ankr station located at middle-latitude; (c) r-tec values at riop station located at high-latitude. (a) (b) (c) 9 ables. the gqp introduces representing ability to the ionospheric activity at receiver location by values within [0-1]. 0 and 1 demonstrate extremely inadequate and most sufficient ipp positions, respectively. the gqp approaches 1 for observations closer to the receiver zenith, while it decreases to zero for observations away from zenith in case of a local time difference. mathematical expression of the gqp has satisfactory form to cover desired features. it can be said that gqp criteria are able to present quality for the vtec process. in addition to the quality factor, equation (6) can be used as stochastic model for regional modeling or averaging of the vtecs. actually, regional modeling on the cors networks may require suitable weighting function just like the gqp. the quality of station-based tec process has been described as a statistical average of gqps and it was named as reliability of tec. it has to be emphasized that reliability should be evaluated in terms of appropriate relative geometry. the geometric quality term (r-tec) was formulated as in (7) and the r-tec varies within 0.1-1.4 range. the distribution of the r-tec values has been examined in order to detect a threshold. as a result of the accumulation analyze of the data, 1 has been determined as a limit for qualification. the r-tec over 1 can be accepted as effective ipp geometry to estimate tec. nevertheless, it has to be noticed that r-tec is always below 1 threshold at high latitudes. the gqp and the r-tec validate both averaging and modeling of the vtecs. independent from method, geometric quality terms qualify the tec estimations related to the ipp’s relative locations. even if tec is obtained via weighted average, it is mentioned from a simple modeling according to the positions of the ipp. the r-tec informs us about relative positions of the ipp that are referenced in a form of modeling. it is possible to conceive that in case of a high enough r-tec, the modeling is implemented in the small and local area. on the contrary, if the r-tec is less than a necessary value, the ipps are far away from the receiver and the modeling is realized at a wide region. the smaller the geometric quality term gets, the larger the area becomes. thus the station dependent tec estimation becomes similar to spherical models. due to the fact that the gps constellation provides observation facility, it is not expected to lower the rtec for a long time. however, figure 6 shows that this period may be over 3h twice a day. it is beneficial that the users should be aware of non-local tec estimation along these periods. it is suggested to consult global ionospheric maps (gims) at such sessions. references alizadeh, m., h. schuh, s. todorova and m. schmidt (2011). global ionospheric maps of vtec from gnss, satellite altimetry and formosat-3/cosmic data, journal of geodesy, 85, 975-987. bergeot, n., j.m. chevalier, c. bruyninx, e. pottiaux, w. aerts,q. baire, j. legrand, p. defraigne and w. huang (2014). near real-time ionospheric monitoring over europe at the royal observatory of belgium using gnss data, journal of space weather and space climate; doi:10.1051/swsc/2014028. çepni, m.s., l.v. poots and j.b. miima (2013). high resolution station-based diurnal ionospheric total electron content (tec) from dual-frequency gps observations, space weather, 11, 520-528. chauhan, v., o.p. singh and b. singh (2011).diurnal and seasonal variation of gps-tec suring a low solar activity period as observed at a low altitude station agra, indian journal of radio and space physics, 40, 26-36. ciraolo, l., f. azpilicueta, c. brunini, a. meza and s.m. radicella (2007). calibration errors on experimental slant total electron content (tec) determined with gps, journal of geodesy, 81, 111-120. hargreaves, j.k. 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strength dilution of precision (dop) wi-fi, international journal of computer science issues, 3, 35-44. * corresponding author: murat selim çepni, university of kocaeli, department of geomatic engineering, kocaeli, turkey; email: muratselim.cepni@kocaeli.edu.tr. © 2016 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. çepni and şentürk 10 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 55 (3), 2012: peppoloni and di capua geoethics and geological culture: awareness, responsibility and challenges silvia peppoloni, giuseppe di capua istituto nazionale di geofisica e vulcanologia, rome, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-6099 abstract the international debate in the field of geoethics focuses on some of the most important environmental emergencies, while highlighting the great responsibilities of geoscientists, whatever field they work in, and the important social, cultural and economic repercussions that their choices can have on society. the geoitalia 2009 and 2011 conferences that were held in rimini and turin, respectively, and were organized by the italian federation of earth science, were two important moments for the promotion of geoethics in italy. they were devoted to the highlighting of how, and with what tools and contents, can the geosciences contribute to the cultural renewal of society. they also covered the active roles of geoscientists in the dissemination of scientific information, contributing in this way to the correct construction of social knowledge. geology is culture, and as such it can help to dispel misconceptions and cultural stereotypes that concern natural phenomena, disasters, resources, and land management. geological culture consists of methods, goals, values, history, ways of thinking about nature, and specific sensitivity for approaching problems and their solutions. so geology has to fix referenced values, as indispensable prerequisites for geoethics. together, geological culture and geoethics can strengthen the bond that joins people to their territory, and can help to find solutions and answers to some important challenges in the coming years regarding natural risks, resources, and climate change. starting from these considerations, we stress the importance of establishing an ethical criterion for earth scientists, to focus attention on the issue of the responsibility of geoscientists, and the need to more clearly define their scientific identity and the value of their specificities. 1. introduction geoethics is the study and promotion of the evaluation and protection of the geosphere. even though geoethics is increasingly present in the scientific conferences (e.g. international geological congress, mining pribram symposium, geoitalia conference, egu general assembly), its analyses and debates have not reached a significant maturity. this new discipline is not accompanied by an adequate research base and a satisfactory number of scientific publications, which are the essential references for its future development. also, the themes of geoethics cannot easily find space in the most reliable scientific journals, and this severely restricts the dissemination of their contents and the development of a critical stance towards the ethical, social and cultural implications of earth sciences in the scientific community. the researchers are anxious to respond to the ‘publish or perish’ logic, and they often tend to neglect geoethics arguments, although some of them would like to devote themselves to the study of geoethics in depth. so research on geoethics, as a both scientific and humanistic discipline, is not developing, and its contribution to the correct construction of social knowledge is missing, such that society tends to become culturally poorer. based on these considerations, it is increasingly urgent to give greater strength and exposure to geoethics themes, so that they are recognized and appreciated by the whole scientific community. geoethics needs to get out of confined areas of discussion and to become a moment of reflection, above all for geoscientists to have the opportunity to ask questions about themselves, the quality of their work, and their contribution to the healthy progress of humanity. 2. geoethics themes the international debate in the field of geoethics focuses on some of the most important environmental emergencies, like the greenhouse effect and climate destabilization, and pollution and problems of waste disposal. geoethics is concerned to encourage critical analysis of the use of our natural resources, to promote correct information of natural hazards and the development of environmentally friendly technologies, while also extending its principles to planetary protection [martínez-frías et al. 2011]. it is a discipline that is targeted primarily to provide references and guidelines of behavior in relation to concrete probarticle history received february 1, 2012; accepted march 1, 2012. subject classification: education, history of science, public issues, general (philosophy of earth sciences). 335 lems of human life, and to seek the appropriate solutions. among its intents, there is also the promotion of the social role of the geosciences, to involve the community in the idea of a common and shared ‘geological’ heritage, which is considered as having cultural, educational and scientific value, as well as social capital. so, the goal of the geosciences is to steer society into the choice of appropriate behaviors towards real problems of human life, to try to find solutions that are compatible with the protection of nature and territory. the international debate has highlighted the great responsibilities of geoscientists, whatever field they work in (public or institutional fields, the private sphere, or the world of research, teaching or scientific information), and the important social, cultural and economic repercussions on society of their choices [varet 2007]. this is especially true when problems related to natural hazards are considered, and in italy this aspect is particularly evident [di capua and peppoloni 2009, peppoloni 2009]. geoethics should promote reflection and consideration of the following themes: – the comparison with global geological problems, so as to identify in the complexity those elements that, although distinguishing, can unite in pursuit of common goals; – the rational and sustainable use of our georesources; – the proper and correct dissemination of the results of scientific studies and of responsible information on risks, which allows researchers to gain the confidence of the community, to ensure the quality of research products; – the help towards efficient management of emergencies, to protect the community from geological hazards at critical moments; – the improvement of the relationships between the scientific community, the mass media and public opinion, through constant and qualified participation in the spaces offered by the media; – the respect of the law and the support of policy decisions; – the organization of effective teaching tools to develop awareness, values and correct behavior; – the identification of new elements, both in terms of content and activities, that can be directed towards new relations and cultural and ethical views; – the transfer of the cultural values of the environment to those who inhabit it; – the promotion of working groups within universities and professional associations, for the development of the topics listed above, also with a flexible and prudent perspective for the reconsideration of scientific certainties and for reflection on the mutability of knowledge and roles. 3. geological culture james hutton (1726-1797), charles darwin (18091882) and alfred wegener (1880-1930) contributed actively to change the cultural horizons of their times (figure 1). in his theory of ‘deep time’, hutton was the first to intuit the real age of the earth: millions of years, and not only the 6,000 years deduced from the bible. darwin challenged the view of the nineteenth century creationists, arguing that man descended from monkeys, and is one of the many species produced by evolution. in the early twentieth century, wegener cast the certainties of the autoctonists into crisis by asserting that the continents are rafts adrift on the surface of the earth. these geoscientists influenced the cultural world of their times, and they revolutionized the way in which humans perceive themselves, by changing their references of space and time. today, they are considered among the fapeppoloni and di capua 336 figure 1. left to right: james hutton (1726-1797), charles darwin (1809-1882), and alfred wegener (1880-1930) (from http://en.wikipedia.org). 337 thers of modern geology, although they were strongly obstructed by the defenders of knowledge who were considered most authoritative in their times. culture is the soul of civilization, and it is not only fixed ideas and constraints. science is culture. geology (in the broad and noblest sense of the term) is a science. thus, geology is culture, and as such, it can help to dispel misconceptions and cultural stereotypes concerning natural phenomena, disasters, resources and land management. geological culture consists of methods, goals, values, history, ways of thinking about nature, and a specific sensitivity for approaching problems and their solutions [peppoloni 2007, 2008]. so in the approach to the problems concerning the relationships between man and nature, geology should not be reduced only to codes of behavior. together with other sciences, geology has helped extraordinarily and originally to change the way in which we perceive time and space. geology has posed problems of philosophy, and has made, and makes, culture; we have no geoethical solutions if we do not consider geology as a part of our culture. 4. geoethics and geological culture in italy the reflection on the ethical aspects of the geosciences started in italy around the 1970’s, within the wider debate on the philosophy and sociology of the earth sciences. some geoscientists began to reflect on the ethical value of their geological knowledge. one of the promoters of these reflections was felice ippolito (1915-1997), a geologist and also an engineer. although ippolito was not a philosopher, he developed philosophical topics related to the geosciences, such as the relationships between geoscientists and nature. he wondered what kind of science geology is, what is its way of investigating (both through mathematical models and through the observation of real phenomena), as it is halfway between an exact science, like mathematics, and an empirical science. he considered the social value and function of our technical–scientific knowledge, and then the relationships between geosciences and policy. he emphasized that who has scientific knowledge must assume the responsibility to act in an ethical sense, turning this technical knowledge into ethical action, so taking into account the common good and the public use [ippolito 1968]. ippolito’s reflections demonstrated a longstanding commitment by italian geoscientists to philosophical, social and ethical themes related to their geological activities, even before all of this set of issues was defined with the word ‘geoethics’ [nemec 2005]. along these lines, from the 1970’s to date, several studies on geological knowledge in terms of culture have been produced by sandra piacente and mario panizza [panizza 1989, panizza and piacente 1991, piacente 1999, panizza and piacente 2003, piacente and poli 2003]. their contributions have provided a new way of understanding geology, of going beyond simple definitions, to outline new perspectives for the geosciences. they fostered improved understanding of the value of geological heritage as an element of cultural identity, not exclusively attributable to local territorial realities, but generalizable to the whole experience of our human race. over time, the studies of these and other italian authors contributed to the achievement of concrete and prestigious objectives, as in 2009, with the recognition of the dolomites by unesco as a world heritage site (figure 2). since 2000, thanks to sandra piacente, caesare roda and carlo bosi, sessions within the geoitalia conferences have been organized, not only as strictly technical, but also dedicated to general topics, such as epistemological research in earth sciences. in 2009, for the first time in italy, a session on geoethics was organized, followed by a second session in 2011. the large number of participants, the quality of the contents that were offered, and the growing interest in these topics led to the creation of a geoethics committee within the italian federation of earth sciences (fist), which brings together all of the most important associations of geoscientists and research institutions in italy. the geoethics committee is working with the intent of promoting and evaluating geoethics themes, and of organizing events on geoethics: among the various proposals, the most original is the formulation of an oath for geoscientists, modeled on that of the hippocratic oath for doctors [matteucci 2012, this issue], which will support and motivate new graduates on their graduation day; i.e. at the moment of their official entry into the world of geoscientists. the last important event on geoethics was promoted by italian geoscientists in an international venue (together with english, american and norwegian colleagues): the session organized for the first time at the european geosciences union (egu) general assembly (vienna, april 2012). the session focused on geoethics in relation to natural hazards, with specific attention to the communication, education and science–policy–practice interface. twentyfour authors from 15 different countries offered reflections on the future of our planet, on ethical questions in risk management, on significant differences between problems in developing and developed countries, on scientific communication for risk reduction, on the choice of tools and strategies to increase awareness about risks, on education in schools, and on the relationships between geoscientists, the mass media, politicians and the population. 5. the geoitalia conferences the geoitalia 2009 (rimini) and 2011 (turin) conferences organized by fist were two important moments for the promotion of geoethics in italy. researchers and geoethics and geological culture professionals from different disciplines reflected on and discussed the most effective strategies and methodological approaches that need to be put into place for developing appropriate attitudes and forms of critical thinking on these issues. in 2009, in rimini, the first italian session on geoethics was a comforting success. the theme of the session was a reflection of the cultural and social responsibility of geologists in the third millennium. in 2011, in turin, this success was confirmed, with the participation of 34 speakers, a number that exceeded even that of the international session at the 33rd igc in oslo (2008) and other international meetings where geoethics is usually among the disciplines debated. the speakers covered many disciplinary fields, and geologists, philosophers, sociologists, geophysicists, seismologists, psychologists, naturalists, anthropologists, and many others, attended the session. this thus gave the signal that a multidisciplinary approach to geoethics topics is helpful, to contribute to a cultural renewal of society. this is necessary and urgent for an increased awareness of the responsibilities of humanity with respect to environmental change, and with respect to exploitation of territories and of their resources. with the aim of opening the world of the earth sciences to the wider world of culture, and of looking for different viewpoints about not only the technical–scientific role, but also the cultural role of geoscientists, even some prominent intellectuals were invited to the session. prof. franco ferrarotti (professor emeritus and world-famous sociologist) emphasized the strong social impact of geological research and practice, speaking of the media as a subject that should assist with correct scientific information, of the underestimated role of geoscientists in italy, and of italian society that is still not accustomed to listening to their warnings [peppoloni 2012a, this issue]. prof. giorello (one of the most prominent philosophers of science) praised the important contribution of the earth sciences in the promotion of the major cultural changes that have occurred through history, framing this set of disciplines in ethical and epistemological terms. he spoke about the precautionary principle, the uncertainty concept in the earth sciences, and the probability calculation as a tool for the evaluation of natural risks. he also reflected on the relationships between politicians and citizens, by defining geology as “a red thread that allows us to go ahead” [peppoloni 2012b, this issue]. other contributions regarded geological risks and the public sentiments about them, deontological aspects of geological practice, with particular reference to issues of legality, evaluation criteria of geological research, the role of geologists working in public administration, importance of geological didactics in schools, sustainability and georesources exploitation, geodiversity, and cultural and social aspects in land management. 6. individual and social responsibility of geoscientists if it is true that geoethics is the investigation and reflection on the operational behavior of man towards the geosphere, it is therefore necessary to identify a criterion for the selection of values on which to base that behavior so that it is ethically correct. however, the question regarding the responsibilities of those working in the field of the geosciences has to be discussed, putting the geoscientists at the centre of the ethical matters [peppoloni 2008], as experts on the territory and on all its hazards, as researchers, and as operators in public and institutional fields, in professional spheres, and in the fields of education and science communication. in each of these fields, the importance of the role that geoscientists have in society is evident, with the need to define in a better way the peppoloni and di capua 338 figure 2. the dolomites (from http://it.wikipedia.org/wiki/dolomiti). 339 scientific identity of geoscientists, the values of their specificity. finally, there is the need to requalify their professionalism, beginning from a more aware taking on of responsibility in the tasks that they have to perform. an in-depth etymological analysis of the word ‘geoethics’ [peppoloni 2011] disclosed an original double meaning: on the one hand, it contains a sense of belonging to the social dimension, while on the other hand, it is related to the individual sphere. these two existential conditions (social and individual) co-exist unexpectedly in the word ‘geoethics’. so, geoethics can be defined on one hand as investigation and reflection on the operational behavior of man towards the geosphere, and on the other hand as the analysis of the relationships between geoscientists who act, and on their own actions. therefore, the origin of the word ‘geoethics’ gives geoscientists the indication that they cannot ignore the ethical aspect of their activities. the etymology of ‘geoethics’ calls geoscientists to face this responsibility. however, what does this responsibility of geoscientist consist of ? and what motivations are needed to push geoscientists to practice the earth sciences in an ethical way? 7. an ethical criterion for the scholars of the earth werner heisenberg (figure 3) asserted : “natural science not simply describes and interprets nature, but it is a part of the interface between nature and ourselves…” [heisenberg 1958]. this means that through their activities and their personal research, the scholars of nature, and therefore also geoscientists, have the opportunity to perfect themselves. as geoscientists are searchers after truth, their scientific research or their professional activities might therefore constitute for them the way to their personal contact with the truth of the phenomena they study, and at the same time with the truth about themselves. it is primarily for self-respect that scientific truths have to be affirmed with intellectual honesty, so that research makes sense and scientific discoveries are transformed into a real service to society. the knowledge and experience of these phenomena makes us assume a habit of appropriate behavior, which becomes a personal and social discipline that as geoscientists, we have to follow in the management of our earth. therefore, it is important to establish the ethical criteria for earth scientists, fixing intellectual honesty precisely as the indispensable prerequisite. this requires: – respect for the truth that we look for, and for the ideas of others; – recognition of the value of others, as valuable for ourselves; – a spirit of collaboration and reciprocity; – identification of a common goal, despite the diversity of views; – responsibility of our technical–cultural expertise and care for the quality of research and its correct dissemination; – opening to comparisons, even with the perspective of a resizing of our certainties; – reflection on the mutability of knowledge and roles; – awareness that conveying scientific knowledge to others has great value. in line with these considerations, in the same moment in which geoscientists are involved in scientific or professional activities, on the one hand they assume the responsibility to put their expertise to the service of others, and on the other hand they have the responsibility towards themselves to do their best, aware of the commitments given. the results that we reach with our activities must be guided by respect for the truth of knowledge and intellectual honesty. otherwise our activities are emptied of meaning. only on this basis can our activities become a real service for others. 8. cultural, social and ethical implications of the geosciences, and future challenges geological culture and geoethics can strengthen the links between people and their land, between the places of their origins and their own memories. these links are a great resource in a reality such as the italian one, where cultural and natural heritage are often indivisible: man, art and nature together constitute the territory. the awareness of the importance of their value, a common value to be shared and from which to draw upon, can encourage the identification of new elements, both in terms of the contents and the activities, which serves to direct us towards new cultural, social and ethical perspectives. geoethics is a great opportunity for the scientific community to provide formal and substantial value to the commitment of science for the benefit of citizens and institutions. moreover, like any other sciences, the geosciences have a value especially in reference to the improvement of human qualities and to the satisfying of human needs. the great challenge of geoethics is above all to awaken in the scientific community reflection on its characteristics and prerogatives. we need to question some geoethics and geological culture figure 3. werner heisenberg (1901-1976) (from: http://www.spaceand motion.com). significant points so that the debate on geoethics makes progress in the coming years. these should include: – how to identify and articulate an ethical criterion for geoscientists? – where should the line be drawn between preservation and economic development of the geosphere, especially in low-income countries? – how can the freedom of research and actions be combined with the principles of sustainability? – how can the relationships between geoscientists, the media, politicians and citizens be made more profitable, particularly in the defense against natural hazards? – what communication and educational strategies should be adopted to transfer the value of the geosciences to society? many researchers have been occupied for years in the search for the answers to these questions, but the answers we have even now are not yet sufficient. perhaps it is necessary that next to ‘doing’ and ‘how to do’, geoscientists must feel even more strongly the need to reflect on the value of their acting, because only a conscious and genuine recognition of the profound value of an action can allow the effective transfer to the community of those values. in this way we can foster a real and long-lasting rootedness of practices, codes and regulations. a great historical responsibility is in the hands of the geoscientists of the third millennium: to demonstrate that geological knowledge is really a benefit to humankind, and that their contribution can be decisive in searching for a new equilibrium in the relationships between man and nature. however, to move in this direction and to obtain concrete results, it will be necessary to develop the awareness of being able to do, to assume the responsibility to do, and to maintain this with a constant will. 9. conclusions geoethics does not exist without a real awareness by the scientific community of the cultural value of the earth sciences. otherwise, there is the risk that geoethics will become just another bureaucratic constraint on the freedom of research and actions, a new set of bonds imposed but not perceived in their value, to limit practices and ideas. geoethics can become a pretext to lock up the researcher and the whole of society in a moralistic way, in the contraposition between what is right and what is wrong, what we have to do, and what not to do. instead, geoethics has to be above all an opportunity for geoscientists to raise the awareness of their individual and social responsibility, an occasion to improve our understanding of the space and time in which we move and operate, with the perspective of the spiritual and economic progress of humanity. references di capua, g., and s. peppoloni (2009). scientific information: 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(2009). reflection cues on the cultural and social responsibility of the geologist in the third millennium, in: ix international conference “new ideas in earth sciences”, 3rd vol., russian state geological prospecting university, moscow. peppoloni, s. (2011). che cosa significa “geoetica”? dentro le parole, il senso dell’attività del geologo, geoitalia (federazione italiana di scienze della terra), 34, 12-13. peppoloni, s. (2012a). social aspects of the earth sciences. interview with prof. franco ferrarotti, annals of geophysics, 55 (3), 347-348; doi:10.4401/ag-5632. peppoloni, s. (2012b). ethical and cultural value of the peppoloni and di capua 340 341 earth sciences. interview with prof. giulio giorello, annals of geophysics, 55 (3), 343-346; doi: 10.4401/ag5755. piacente, s. (1999). sensibilità geologica e consenso sociale, mem. descr. carta geol. d’it., 54, 451-454. piacente, s., and g. poli, eds. (2003). la memoria della terra, la terra della memoria, bologna, l’inchiostroblu editore. varet, j. (2007), global approach to geoethics: a first attempt, in: mining pribram symposium, international session on geoethics (pribram, czech republic), abstract. contacts: silvia peppoloni, giuseppe di capua, istituto nazionale di geofisica e vulcanologia, rome, italy; email: silvia.peppoloni@ingv.it, giuseppe.dicapua@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geoethics and geological culture << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice m o d e r a t e c h n i q u e s o f t r e n d a n a l y s i s a n d i n t e r p o l a t i o n l . t o r e l l i ( * ) r e c e i v e d o n a u g u s t 2 0 t h , 1 9 7 5 summary. — t h i s a r t i c l e c o n t a i n s a s c h e m a t i c e x p o s i t i o n of t h e t h e o r e t i c a l f r a m e w o r k o n w h i c h r e c e n t t e c h n i q u e s of t r e n d a n a l y s i s a n d i n t e r p o l a t i o n r e s t . i t is s h o w n t h a t s u c h t e c h n i q u e s c o n s i s t i n t h e j o i n t a p p l i c a t i o n of analysis of the variance a n d of multivariate distribution analysis. t h e t h e o r y of universal kriging b y g . m a t h e r o n is a l s o d i s c u s s e d a n d r e d u c e d t o t h e a b o v e t h e o r i e s . riassunto. — i l p r e s e n t e a r t i c o l o e s p o n e i n f o r m a s c h e m a t i c a i r i s u l t a t i t e o r i c i s u c u i si b a s a n o l e t e c n i c h e p i ù r e c e n t i d i i n t e r p o l a z i o n e e a n a l i s i d i t e n d e n z a . v i e n e d i m o s t r a t o c h e t a l i t e c n i c h e r i s u l t a n o d a l l ' a p p l i c a z i o n e c o n g i u n t a d i analisi della varianza e d i analisi multivariata. si d i s c u t e i n o l t r e l a t e o r i a d e l " k r i g e a g e universel" d i g . m a t h e r o n , r i d u c e n d o l a a l l e t e o r i e g i à m e n z i o n a t e . 1 . i n t r o d u c t i o n one deals with a function y(x) defined in a region of a real space. t h e value of t h e function is known at a few points, xt, of the region. one writes yi — y(xt). interpolation means prediction of t h e value of t h e function a t points of t h e region where t h e value of t h e f u n c t i o n is unknown. trend is a simplified f u n c t i o n capable to describe t h e "general behavior" of t h e original function. these concepts are b e t t e r defined when t h e theoretical approach to the problem is described. (*) i d r o t e c n e c o , r o m e . 2 7 2 l. toeelli one considers t h e f u n c t i o n in question as a realization of a stochastic f u n c t i o n y(x) -with t h e following s t r u c t u r e : *(«) = /(*) + «(«); ee(x)±0 [1] where j(x) is a deterministic f u n c t i o n describing t h e e x p e c t a t i o n ey(x) of y(x) and e(x) is a stochastic residual with e x p e c t a t i o n zero. f(x) is w h a t is usually called t r e n d of y(x) and t h e problem of t r e n d analysis is to find an e s t i m a t e as good as possible of f(x). i t will be shown t h a t it is possible to find an unbiased e s t i m a t o r which has m i n i m u m v a r i a n c e in t h e class of t h e linear estimators of f(x). as to interpolation, one w a n t s to find an o p t i m a l predictor of y{x), i.e. a predictor y(x) such t h a t : e( y(x) — y(x)) = 0 e()'(x)— y(x))2 is m i n i m u m . section 2 gives a schematic account of t h e t h e o r y which can be used to a t t a i n t h e above objectives. i t is a r t i c u l a t e d in t h r e e p a r t s , one a b o u t t r e n d analysis, one a b o u t interpolation when f(x) = 0 and t h e last a b o u t interpolation in general. t h e t h e o r y used is t h e analysis of the variance ( s c h e i e , 1959) (8) and multivariate distribution analysis (anderson, 1958) (2). section 3 discusses a t h e o r y elaborated in t h e l a t e sixties b y m a t h e r o n and co-workers called universal kriging (matheron, 1969 (5), matheron, 1971 (6), h u i j b r e g t s and matheron, 1970 (4) and reduces it t o t h e theory presented in section 2. conclusions are d r a w n in section 4. 2 . s c h e m e o f t h e t h e o r y 2.1. trend analysis consider equation [1]. if f(x) is f o r m u l a t e d as a linear f u n c t i o n , with known coefficients f u n c t i o n of x, of a n u m b e r of p a r a m e t e r s to be estimated, one can m a k e use of t h e analysis of t h e variance to solve t h e problem of e s t i m a t i n g f(x). i n f a c t s one constructs a linear model, which in m a t r i x n o t a t i o n is expressed as follows: y = x'ß + s, ee = 0, £ = £ e = b [2] m o d e r n t e c h n i q u e s of t r e n d a n a l y s i s a n d i n t e r p o l a t i o n 2 7 3 •where y is t h e vector of t h e observations, ft t h e vector of t h e param e t e r s t o b e e s t i m a t e d , e t h e vector of t h e stochastic residuals a n d x' a full r a n k m a t r i x whose ieth row is composed b y t h e coefficients of t h e linear expression of f(xt) in ft. ' m e a n s transposed and are t h e covariance matrices of e a n d y respectively and are assumed k n o w n . given model 2, t h e following results follow (scheffe, 1959) (8). 1) ft = (xbxx')^xb uj is an unbiased estimator of ft which h a s m i n i m u m v a r i a n c e in tlie class of t h e linear estimators of ft. i t should be noted t h a t ft is different f r o m t h e usual least s q u a r e estim a t o r of ft, ft = (xx')-1 xy ft is unbiased b u t has not m i n i m u m v a r i a n c e in t h e class of t h e linear estimators of ft. 2) (gauss markov theorem) given a vector a if t h e r e exists a vector c such t h a t a = cx',aft is t h e u n i q u e unbiased estimator of aft which has m i n i m u m variance in t h e class of t h e linear estimators of aft. since /(.») is linear in ft, we get f(x) = a(x) ft where a(x) satisfies t h e condition of t h e gauss markov theorem because x' is full r a n k . consequently, t h r o u g h t h e analysis of t h e variance, one obtains an e s t i m a t o r f(x) = a(x)ft of f(x) which is opt i m a l in t h e class of t h e linear estimators of f(x). moreover, tlie t h e o r y gives t h e m a t r i x of t h e covariance of ft xp = (xb-^x')-1 so t h a t t h e variance of t h e estimator a(x) ft of f(x) is a{x) (xb-lx')~la(x)'. 2.2. interpolation ivlien f(x) = 0 to solve this problem one uses multivariate distribution analysis. let us consider t h e r a n d o m normal vector y* = ( y , y i , y 2 , . . . , y „ ) ' where y = y(x), x being t h e point of interpolation and y( = y(xt), such t h a t by* = 0. 2 „ * = -4 is assumed to b e known. 2 7 4 l. torelli .1 m a y be partioned as follows a = c d' db where b = 2 of t h e preceding section, c — v a r y, d = (cov( y, yi), c o v ( r , r 2 ) , . . . , c o v ( y , r „ ) ) ' . one lias t h e following result (anderson, 1958) (2). given yt = yt, i — 1 , 2 , . . .,n, y is normally d i s t r i b u t e d with m e a n e(ylyt = yr, i = 1 , 2 , . . . , » ) =-d ' s ^ y and variance var( yi yt = i = 1,2,. . .,») = c — d'b-w ym(x) — e(yjyi = ?/i, i = 1 , 2 . . .,»)) is obviously an optimal predictor in t h e sense of section 1. subscript m refers to multivariate. i n this method t h e assumption of t h e n o r m a l i t y of y* is m a d e . such assumption can be considered safe and it is based on central limit theorem considerations. 2.3. — interpolation in general i t is common sense to use t h e following p r o c e d u r e : one gets an o p t i m a l e s t i m a t e / (.r) of f(x) using t h e t h e o r y of section 2.1. then one applies t h e m e t h o d described in section 2.2 to t h e r a n d o m f u n c t i o n z(x) = y(x) — f(x). t h e final i n t e r p o l a t o r is t h e n f (x) + zm{x). 3 . u n i v e r s a l k r i g i n g this t h e o r y of interpolation and t r e n d analysis has been developed b y matheron and co-workers in t h e l a t e sixties. a great mass of lite r a t u r e on t h e t h e o r y has been produced b y this school. only t h e essential references are given here. i n t h e p r e s e n t discussion of universal kriging, c o n s t a n t reference is m a d e to t h e article b y h u i j b r e g t s and matheron, in which this t h e o r y is concisely and clearly exposed. m o d e r n t e c h n i q u e s of t r e n d a n a l y s i s a n d i n t e r p o l a t i o n 2 7 5 the assumption of h u i j b r e g t s and matheron is t h a t one deals with a realization of a stochastic f u n c t i o n z(x) for which t h e m o m e n t s of 1st and 2nd order are defined: ez(x) = m(x) ez(x)z(y) = c(x,y) + m(x)m(y) where g(x,y) — cov(z(x),z(y)). moreover m(x) = a, /' (x) (x e 7 ) (i.h-m) (1.h-m) where v is some neighborhood of t h e points of observation. i n other words, m(x) is a linear f u n c t i o n with known coefficents f u n c t i o n of x, viz f'(x), of a n u m b e r of p a r a m e t e r s <u to be e s t i m a t e d . a p p a r e n t l y t h e s t r u c t u r e described b y (i,h-m) and (1,ii-m) is coincident with t h e linear model of equation [2]. as f a r as trend analysis is concerned, t h e proclaimed goal of h u i j b r e g t s and matheron (4) is to find a linear e s t i m a t o r of m(x) unbiased and with m i n i m u m variance. now, given t h e f a c t t h a t t h e p r o b a b i l i t y s t r u c t u r e considered b y h u i j b r e g t s and matheron (4) is a linear model and given t h e unicity p r o p e r t y of t h e e s t i m a t o r of t h e gauss markov theorem, it follows t h a t t h e h u i j b r e g t s and matheron (4) solution is coincident with t h e s t a n d a r d solution of analysis of the variance given in section 2.1. as t o t h e problem of interpolation in absence of t r e n d , t h e predictor proposed b y h u i j b r e g t s a n d matheron is z k = x a f z a , « = 1 , 2 , . . . , « a with coefficients ?.k verifying t h e following s y s t e m 2 / j i °afl = c (y.a, xo) (s.k., h-m) where za = z(xa) are t h e observations, h refers to t h e p o i n t of interpolation, i.e. x0, oap = c (xa, xp) and a runs, like /?, on t h e n u m b e r of observations n. if we set ik = (at1, a-2, . . . , h " ) , a n d write, according to t h e n o t a t i o n of section 2, y = (zi, z°, . . ., z„) and i) = (g(xi, xa), 6 l. torelli c(x2, x0), • . •, c{x„, x0)) and we let b be t h e covariance m a t r i x of y, we can write h u i j b r e g t s and m a t h e r o n ' s expressions (j) in m a t r i x n o t a t i o n : z, = h y xk b = d' from which zk = d'b-iy coincident with t h e solution of section 2.2. the h u i j b r e g t s a n d m a t h e r o n ' s solution to t h e problem of interpolation in general is given b y : z* = m* (xo) + 2 a (za — m* (xa)) (5,ii-m) where m*(x0) is t h e optimal t r e n d e s t i m a t o r a n d 2 « — (xa)) is the solution discussed above for t h e stochastic f u n c t i o n w i t h no t r e n d , z(x) — m*(x). this is again coincident with t h e solution of section 2.3. however, t h e article of h u i j b r e g t s a n d m a t h e r o n (4) offers a few interesting marginal results, since it shows t h a t zi.is an o p t i m a l linear predictor i n d e p e n d e n t l y of the n o r m a l i t y of z(x) and t h a t t h e general interpolator (5,h-m) which is t h e sum of t h e o p t i m a l linear e s t i m a t o r of t h e trend and of t h e optimal linear p r e d i c t o r of t h e residuals is itself an o p t i m a l linear p r e d i c t o r . moreover, it provides t h e overall v a r i a n c e of predictor (5,ii-m). 4 . c o n c l u s i o n this article is m e a n t to be a schematic exposition of t h e theoretical f r a m e w o r k on which recent techniques of trend analysis and interpolation rest. i t is shown t h a t such techniques consists in t h e joint application of t h e analysis of the variance and of multivariate distribution analysis. i t is also shown t h a t t h e b u l k of universal kriging is reducible to these long established and well k n o w n theories. i n this we t h i n k to h a v e given a c o n t r i b u t i o n to t h e unification a n d clarification of t h e developments in t h e field. i n t h e p a p e r , t h e basic p r o b a b i l i t y s t r u c t u r e , i.e. t h e linear model, is assumed as given. the construction of a suitable model, which m o d e r n t e c h n i q u e s of t r e n d a n a l y s i s a n d i n t e r p o l a t i o n 2 7 7 implies t h e determination of t h e f o r m of t h e linear expansion of f(x) and of t h e covariance s t r u c t u r e of t h e residuals is n o t discussed, being b e y o n d t h e scope of t h e present article. i n a n y case, this is a r a t h e r i n t r i c a t e m a t t e r , in which t h e analysis of the variance is again of great help. f o r an example of i n q u i r y on t h e covariance s t r u c t u r e of a stochastic f u n c t i o n defined in a unidimensional discrete space, t h e reader is referred to torelli (10) and torelli and chow (»). r e f e r e n c e s ( ! ) a g t e r b e r g f . p . , 1 9 7 4 . " g e o m a t h e m a t i c s " , e l s e v i e r , a m s t e r d a m . ( 2 ) a n d e r s o n t . w . , 1 9 5 8 . an introduction to multivariate statistical analysis. w i l e y , n e w y o r k . ( 3 ) d e l f i n e r p . a n d j . p . d e l h o m m e , 1 9 7 3 . optimum interpolation by kriging, i n display and analysis of spatial data. w i l e y , n e w y o r k . ( 4 ) h u i j b r e g t s c . a n d g. m a t h e r o n , 1 9 7 0 . universal kriging: an optimal method for estimating and contouring in trend surface analysis. c i m m , i x i n t e r n . s y m p . , m o n t r e a l . ( 5 ) m a t h e r o n g., 1969. le krigeage universel. " c a h i e r s d u c e n t r e d e m o r p h o l o g i e m a t h é m a t i q u e s " , 1, f o n t a i n e b l e a u . (®) m a t h e r o n g . , 1971. the theory of regionalized variables and its applications. " c a h i e r s d u c e n t r e d e m o r p h o l o g i e m a t h é m a t i q u e " , 5, f o n t a i n e b l e a u . (7) olea r . a . , 1974. optimal contour mapping using universal kriging. " j . j . r . " . ( 8 ) s c i i e f f é h . , 1959. the analysis of the variance. w i l e y , n e w y o r k . ( " ) t o r e l l i l . a n d v. t . c h o w , 1972. tests of stationarity of hydrologie time series. « i n t . s y m p . on u n c e r t a i n t i e s i n h y d r o l o g i e a n d w a t e r r e s o u r c e s y s t e m s », t u c s o n , d e c e m b e r . ( 1 0 ) t o r e l l i i;., 1973. the analysis of monthly hydrologie time series. i ' l l . d . t h e s i s , u n i v e r s i t y of illinois. vol50,3,2007 469 annals of geophysics, vol. 50, n. 3, june 2007 key words millimeter-wave spectroscopy – stratospheric ozone – isentropic transport 1. introduction from february 2004 to march 2007 a collaboration among the university of rome «la sapienza», the state university of new york at stony brook (sunysb), and the istituto naziomillimeter wave spectroscopic measurements of stratospheric and mesospheric constituents over the italian alps: stratospheric ozone giovanni muscari (1), claudio cesaroni (1), cesidio bianchi (1), robert l. de zafra (2), tatiana di iorio (3), irene fiorucci (1), daniele fuà (3), vito romaniello (3) and enrico zuccheretti (1) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) department of physics and astronomy and institute for terrestrial and planetary atmospheres, state university of new york, stony brook, u.s.a. (3) dipartimento di fisica, università degli studi di roma «la sapienza», roma, italy abstract measurements of rotational lines emitted by middle atmospheric trace gases have been carried out from the alpine station of testa grigia (45.9°n, 7.7°e, elev. 3500 m) by means of a ground-based millimeter-wave spectrometer (gbms). observations of species such as o3, hno3, co, n2o, hcn, and hdo took place during 4 winter periods, from february 2004 to march 2007, for a total of 116 days of measurements grouped in about 18 field campaigns. by studying the pressure-broadened shape of emission lines the vertical distribution of the observed constituents is retrieved within an altitude range of ∼17-75 km, constrained by the 600 mhz pass band and the 65 khz spectral resolution of the back-end spectrometer. this work discusses the behavior of stratospheric o3 during the entire period of operation at testa grigia. mid-latitude o3 columnar content as estimated using gbms measurements can vary by large amounts over a period of very few days, with the largest variations observed in december 2005, february 2006, and march 2006, confirming that the northern winter of 2005-2006 was characterized by a particularly intense planetary wave activity. the largest rapid variation from maximum to minimum o3 column values over testa grigia took place in december 2006 and reached a relative value of 72% with respect to the average column content for that period. during most gbms observation times much of the variability is concentrated in the column below 20 km, with tropospheric weather systems and advection of tropical tropospheric air into the lower stratosphere over testa grigia having a large impact on the observed variations in column contents. nonetheless, a wide variability is also found in middle stratospheric gbms o3 measurements, as expected for mid-latitude ozone. we find that o3 mixing ratios at ∼32 km are very well correlated with the solar illumination experienced by air masses over the previous ∼15 days, showing that already at 32 km altitude ozone photochemistry dominates over transport processes. the correlation of lower stratospheric ozone concentrations with potential vorticity as an indicator of transport is instead not as clear-cut, due to very complex mixing processes that characterize stratospheric air at mid-latitudes. correlations of o3 over testa grigia with stratospheric tracers such as n2o and hcn, also observed by means of the gbms, are planned for the future, in order to better characterize lower stratospheric dynamics and therefore lower stratospheric ozone concentrations at mid-latitudes. mailing address: dr. giovanni muscari, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: muscari@ingv.it 470 giovanni muscari et al. nale di geofisica e vulcanologia, with the logistic support of the istituto di fisica dello spazio interplanetario (inaf/ifsi), operated a ground-based millimeter-wave spectrometer (gbms) measuring middle atmospheric trace gases from the alpine site of testa grigia (or plateau rosa, 45.9°n, 7.7°e, elev. 3500 m above mean sea level), at the border between italy and switzerland. the high elevation makes testa grigia (hereafter referred to as tg) an excellent site for carrying out atmospheric or astronomic measurements at mid-latitudes with instruments, such as the gbms, which operate at frequencies where atmospheric transparency depends heavily on water vapor and therefore a low water vapor columnar content is necessary. daily spectroscopic measurements were carried out during several field campaigns mostly during fall and winter, when water vapor columnar content (or precipitable water vapor, pwv) is the lowest. the gbms observed rotational emission lines of o3 at 276.923 ghz, hno3 at 269.211 ghz, co at 230.538 ghz, n2o at 276.328 ghz, hcn at 265.886 ghz, and hdo at 255.050 ghz. the pressurebroadened spectral line shapes allow retrieval of the vertical distribution of the emitting trace gases between approximately 17 and 75 km (see section 2 for details). this work discusses the behavior of stratospheric o3 observed during a total of 116 days over 4 winters, grouped in about 18 field campaigns. at mid-latitudes total ozone is controlled by both transport and chemistry. ozone forms mainly in the tropical stratosphere and is advected poleward leading to an increasing total ozone column at higher latitudes during winter and spring. at mid-latitudes stratospheric ozone is characterized by a considerable seasonal variability with lowest values in autumn and a strong variability on time scales from days to months which leads to variations in total ozone that can exceed 50% of average values. events with very low and very high total ozone values are often called miniholes and minihighs, respectively (e.g., koch et al., 2005; stick et al., 2006), and are identified as synoptic-scale regions characterized by total column ozone values significantly below/above a certain threshold. ozone miniholes are primarily the result of dynamic-atmospheric processes, given that they evolve too rapidly to be the result of an ozone chemical destruction mechanism (koch et al., 2005). the dynamic origin of such extreme ozone events was already noted by dobson et al. (1929), who described an interrelation between fluctuations in total ozone and the passing of weather systems. currently, the exact dynamical mechanisms of their formation are under dispute, with two main hypotheses: i) the buildup of an intense tropospheric high-pressure system with an elevated tropopause and the subsequent generation of a reduced ozone layer thickness (teitelbaum et al., 2003); ii) the generation of an air column with low ozone mixing ratios caused by meridional isentropic transport of air from the subtropics between 340 and 440 k (190-70 hpa) and, independently, from polar regions between 530 and 700 k (30-15 hpa), both carrying air with climatologically low ozone mixing ratios (koch et al., 2002). koch et al. (2005) studied the relative importance of the two mechanisms, quantified statistically for all miniholes and minihighs observed over payerne, switzerland, in the time period 1980-1989 and concluded that fast isentropic transport is by far the most important process for minihole genesis. from a study by hood et al. (2001) on 71 miniholes between 1980 and 1993, ozone loss at polar latitudes caused by heterogeneous chemistry due to the presence of polar stratospheric clouds (pscs) was estimated to contribute less than 1% to the observed low column ozone at mid-latitudes, although more recently a psc and a conspicuous related chemical ozone depletion was observed over southern europe in january 2006 (keckhut et al., 2007; keil et al., 2007). 2. observing technique and profile retrieval the gbms is currently a state-of-the-art system that employs a superconductor-insulator-superconductor (sis) tunnel junction mixer to convert millimeter wave signals arising from molecular rotational transitions to lower «intemediate» frequencies (if) by heterodyning them with a local oscillator. the if output is fed to a «wide band» acousto-optical spectrometer (aos) characterized by a band pass of 600 mhz and a spectral resolution of 1.2 mhz, and 471 spectroscopic measurements of stratospheric ozone over the italian alps to a «narrow band» aos with a 50 mhz band pass and 65 khz of resolution. the 50 mhz window of the narrow band aos is adjusted within the 600 mhz band pass of the wide band aos in order to gain a higher resolution of spectral line centers associated with high altitude emission. our gbms system is employed to measure molecular rotational lines of several trace gases that emit in the frequency range between 230 and 280 ghz. since spectral line shapes are determined by a convolution of the vertical concentration profile (typically unknown) and the atmospheric pressure profile (typically known), the latter can be used, along with the observed line shape, in a mathematical deconvolution process to find the emitting molecule’s concentration as a function of altitude. the overall spectral band pass and resolution of the gbms are therefore key elements to determine the altitude range where trace gases concentration can be measured. assuming a generic line pressure broadening of ∼3 mhz/mbar, with the aid of pressure and temperature vertical profiles the gbms allows the retrieval of vertical distributions of trace gases concentration between ∼17 and ∼70-75 km altitude. above 70-75 km, pressure broadening is overcome by doppler broadening, and only the overlying column density can be determined with confidence. gbms measurements are integrated over time intervals between 10 min and several hours, depending on the intensity and therefore the signal to noise ratio of the spectral line being observed. in this work we discuss measurements of the pure rotational transition line of o3 at 276.923 ghz. this line is strong enough to give signalto-noise ratios at line center larger than 100:1 in a 10 to 20 min integration, though still weak enough to show relatively little self-absorption by lower levels for radiation originating in upper layers of the stratosphere. spectra have been processed with the chahine-twomey deconvolution technique (twomey et al., 1977) to retrieve vertical profiles from the measured line shapes. the wing of a second, strong ozone line centered at 274.478 ghz, outside the spectral window, contributes background curvature and is subtracted from the measured spectra in the data pre-processing phase. the inversion procedure starts by synthesizing the o3 line that would be generated by an initial o3 volume mixing ratio (vmr) vertical profile, knowing the physical characteristics of the rotational transition observed. the modeled spectrum is then compared to the measured one and the initial vmr profile is modified iteratively until the differences between computed and measured spectral line shapes are minimized. changes to the initial o3 vertical profile are generated that are based on weighting functions (or averaging kernels) which relate the shape of the o3 rotational line over a given frequency range (measured from the line center) to the concentration of o3 over a specific altitude range. the o3 vmr vertical profile cannot be retrieved below altitudes where the pressure broadening is much greater than the total spectral bandwidth. the gbms’s 600 mhz spectral window thus limits the bottom altitude level of our retrievals to ∼1517 km altitude. in practice, gbms o3 retrievals are computed down to ground level, but they rely almost entirely on initial profiles below ∼15 km altitude (see next paragraph). deconvolutions were performed using several choices of starting o3 vmr vertical profiles and weighting functions to minimize any biasing from a particular set of inputs, and the recovered vertical profiles were averaged. initial profiles below 20 km altitude were computed using ozone sonde measurements carried out by meteoswiss from payerne (e.g., jeannet et al., 2007), switzerland, about 100 km away from tg. atmospheric pressure and temperature profiles used in the analysis were obtained from the national center for environmental prediction (ncep) for appropriate dates and location. ozone mixing ratios are retrieved every km, although due to the finite width of the millimeter-wave averaging kernels (i.e., the radiation measured at a frequency separated ∆ν from the line center depends on o3 molecules emitting from a wide range of altitudes, not a very thin layer), uncorrelated information is obtained at layers separated by ∼6 km in the lower to mid-stratosphere, increasing to more than 10 km in the upper stratosphere, determined by the full width at half-maximum (fwhm) of the averaging kernels used in the inversion algorithm. figure 1 gives an illustration of the spectral line shape quality, after removal of minor background 472 giovanni muscari et al. contributions from other ozone emission, along with a typical starting profile, the retrieved vmr profile, and the residual between the observed line shape and a line shape synthesized from the retrieved profile. uncertainties on gbms o3 vmr vertical profiles have several sources. the most important contributions are: background removal process; choice of starting profiles and averaging kernels; input parameters such as transition line intensity, molecular partition function, pressure-broadening coefficient; random noise in spectral data; calibration, i.e. the radiometric conversion from voltages to radiances; and evaluation of the tropospheric attenuation of stratospheric signals. assuming that the most important contributions to the overall uncertainty are independent from one another, we added them in quadrature and obtained an overall ±13% uncertainty on gbms o3 vmr values. the millimeter-wave emission spectroscopy employed by the gbms has several advantages over other techniques for measuring atmospheric constituents. with respect to balloonsonde data, gbms vertical profiles of chemical compounds reach much higher altitudes (a maximum altitude of 70-75 km instead of ≤35 km), although they have much lower vertical resolution. once the millimeter-wave receiver is set up, measurements can be carried out daily over several months, and at relatively low cost, by a single operator. furthermore, balloon sondes typically carry out measurements of one single chemical species on each flight, (unless the sonde carries expensive instrumentation that will need to be recovered after the flight, with the risk of being lost), while the gbms can be rapidly tuned to different frequencies and measure the emission from several species within 24 h. with respect to remote sensing techniques at infrared (ir) or visible wavelengths, millimeterwave spectroscopy is able to exploit the pressure broadening characteristics of spectral lines, because this is much larger than doppler broadenfig. 1. (top, right) measured spectral line data after subtraction of computed background from other nearby ozone transitions. a line shape synthesized from the retrieved vertical profile (solid line, left panel) is superimposed on the observed spectrum, but can be barely distinguished. the difference spectrum between these is shown on an enlarged scale (bottom, right). spectral intensity is given in kelvins for the equivalent emission temperature in the rayleigh-jeans approximation. the dashed line in the left hand panel is one of three starting profiles used for deconvolution of each day’s data. the solid line is the profile retrieved from the data in the upper right panel. 473 spectroscopic measurements of stratospheric ozone over the italian alps ing at the long wavelengths and correspondingly lower frequencies employed. the measure of pressure broadening allows the retrieval of vertical profiles, which with rare exceptions cannot be obtained by ground-based ir and visible techniques, so that the latter are primarily capable of measuring only columnar content. ir and visible spectroscopy are able to produce reliable vertical profiles when instruments are used in a limbsounding mode onboard satellites or very high altitude airplanes, but such experiments are orders of magnitude more expensive than groundbased techniques, and typically have a much shorter operational duty cycle. optical spectroscopy from any platform is also limited to daylight hours and thus unsuitable for species which may only form during the night. the gbms was deployed mostly to polar regions: to the antarctic amundsen-scott base (90°s) during 1993, 1995, and 1999 (e.g., cheng et al., 1996; de zafra et al., 1997), and to thule (76.5°n, 68.7°w), greenland, during the winters of 2001-2002 and 2002-2003 (de zafra and muscari, 2004; muscari et al., 2007). the overall design and details of the observing technique have been discussed at length by de zafra (1995) and parrish et al. (1988), while a summary on the accuracy of the inversion procedures used to deconvolve the different spectral lines can be found in muscari et al. (2007) and in references therein. 3. results and discussion figure 2 shows o3 column contents derived from gbms vertical profiles from the ground to 50 km (contribution from higher altitudes is negligible), with larger solid symbols representing the average value for each group of data (i.e., field campaign, with its first and last day of measurements indicated at the top of the corresponding panel in fig. 5, discussed later). below 15 km gbms vertical profiles, and therefore column contents, rely significantly on ozone sonde measurements from payerne, as discussed in section 2. a weighted average of the uncertainty contributions from ozone sonde and gbms to the final o3 column contents provides an estimate for the total uncertainty of ±12% (cheng et al., 1996). in figure 2 we notice that o3 columnar contents measured within a few days can be scattered over a wide range of values, with the largest varifig. 2. gbms o3 total column density (in dobson units) versus day of the year for all 4 winter periods, with 31 december at 12 gmt as day −0.5 and 1 january as day 1. the legend shows which symbol represents each winter. solid symbols are average values for each field campaign with first and last day of each campaign indicated in fig. 5. 474 giovanni muscari et al. ations observed in december 2005 (52% of the campaign average value indicated with a solid star), february 2006 (64% of the campaign average value indicated with a solid star), march 2006 (59% of the campaign average value indicated with a solid star) and december 2006 (72% of the campaign average value indicated with a solid triangle). interestingly, in most periods a large variability is concentrated in the column below 20 km (not shown), suggesting that tropospheric weather systems and advection of tropical tropospheric air into the lower stratosphere over tg has a large impact on the observed variations in column contents. furthermore, out of the 4 mentioned periods of particularly large o3 column variability, 3 took place during winter 2005-2006, which proved to be a period of large planetary wave activity, starting with a canadian warming and the intensification of the aleutian high pressure system at the end of november (braathen, 2006), just before the december 2005 gbms field campaign. afterwards, several more perturbations took place until on 21 january a major midwinter warming [which is considered to happen when at 10 hpa and above the temperature difference between the pole and 60°n is reversed fig. 3. 15-day back trajectories at 350 k (~13 km altitude; top and bottom left) and 430 k (∼18 km altitude; top and bottom right) ending over testa grigia on 14 december (top) and 19 december (bottom), 2006. for each of the four plots, five trajectories were calculated and only the resulting average trajectory is shown (see text for details). numbers along each trajectory indicate the distance in days before the arrival of the air parcel at testa grigia. trajectories are not drawn when they pass below 30°n. back trajectory calculations are from the nasa/goddard space flight center (nasa/gsfc) «automailer» system initiated with ncep reanalysis meteorological data (schoeberl and sparling, 1994). 475 spectroscopic measurements of stratospheric ozone over the italian alps fig. 4. ozone total column contour map in dobson units from the world ozone and ultraviolet radiation data centre (woudc) in toronto (<http://es-ee.tor.ec.gc.ca>) for 13 december, 2006. (positive) and the mean zonal wind at 60°n reaches negative values (easterlies)] occurred in the arctic stratosphere. this is considered one of the strongest and most prolonged sudden warmings on record, lasting for about 4 weeks (manney et al., 2007) and altered o3 concentrations at mid-latitudes. as for december 2006 (empty triangles in fig. 2), we examined in more details stratospheric dynamics between days 14 to 19 of december, when the gbms observed the minimum and maximum column contents, respectively. the low ozone contents measured by the gbms on 14 december seems to be caused by air masses that reached the lower stratosphere above tg originating from the ozone deprived upper tropical troposphere, as suggested by isentropic air parcel trajectories advected back 15 days at 350 and 430 k of potential temperature (∼13 and 18 km, respectively) and shown in fig. 3. for each of the four plots shown in the figure, five trajectories were calculated and only the resulting average trajectory is shown. one of the five trajectories ends exactly at testa grigia while the other four end at locations surrounding testa grigia at four cardinal points separated by 90°. this effort aimed at gaining more confidence in the history of air masses passing over tg by observing the behavior of a larger volume of air with respect to the volume corresponding to one location only. back trajectory calculations are from the nasa/goddard space flight center (nasa/gsfc) «automailer» system initiated with ncep reanalysis meteorological data (schoeberl and sparling, 1995). large o3 values observed on 18 and 19 december can instead be explained by the advection over tg of stratospheric air rich in o3 from the aleutian region, as suggested by the dynamics of air parcels that 7 days before their passage over tg were found over the high ozone region. this conclusion is illustrated by the combination of air trajectories in fig. 3 and the o3 map provided by the world ozone and ultraviolet radiation data centre (woudc) in toronto (<http://es-ee.tor.ec.gc.ca>) for 13 december, shown in fig. 4. throughout the field campaigns that show large o3 column variability, o3 total column values provided by the woudc and by the ozone monitoring instrument (omi) onboard the eos aura satellite (<http://acdisc.sci.gsfc.nasa.gov>) 476 giovanni muscari et al. fig. 5. gbms o3 mixing ratio vertical profiles from 17 to 55 km altitude from testa grigia. each panel depicts measurements from one field campaign, with first and last day of measurements indicated at the top of each panel. 477 spectroscopic measurements of stratospheric ozone over the italian alps confirmed qualitatively the large variations observed by the gbms over tg. in particular, for all the extreme events (very large or very little o3 content) observed by the gbms we found a good correspondence in timing (i.e., days of occurrence) with maxima and minima o3 total column values over tg found in woudc and omi/aura databases, although the latter dataset has a coarser horizontal resolution and therefore shows smaller variations with respect to those observed by the gbms. column contents are most sensitive to changes in o3 concentrations in the lower stratosphere. in order to discuss observations of middle stratospheric o3 all the vertical profiles obtained from gbms field campaigns at tg are displayed in fig. 5. figure panels depict measurements obtained during each campaign, with their first and last day of measurements indicated at the top of each panel. the same data set is shown in fig. 6 by means of contour maps in order to better illustrate the time periods covered by gbms measurements. ozone vmr vertical profiles confirm the large degree of variability over short time spans observed for column contents and characteristic of mid-latitude ozone concentrations. interesting features are the double peaked structure observed in several profiles and usually addressed with the term «low ozone pocket» identified by the decrease in ozone mixing ratio between 30 and 35 km altitude and usually characteristic of air from polar regions during winter, when air can remain isolated in the dark for several days and reaches a different photochemical equilibrium (towards lower ozone values) with respect to mid-latitude and tropical air (e.g., cheng et al., 1996; muscari et al., 2007, and references therein). since ozone sonde measurements barely reach 30 km altitude, the large set of ozone sondes launched at mid-latitudes completely miss this feature in o3 mixing ratio profiles. low ozone pockets are not as deep at mid-latitudes as they are over the antarctic continent during winter (cheng et al., 1996). however, just as it can be shown from observations at polar latitudes (muscari et al., 2007), also at mid-latitudes there is a strong correlation between ozone mixing ratio at ∼32 km and the illumination fraction (defined as the ratio between the time interval during which the air parcel received solar illumination and the total duration of the trajectory) experienced by air parcels in the preceding 10 days. figure 7 displays such a correlation at a potential temperature of 850 k (∼32 km). the entire gbms o3 data set from tg is shown with solid circles, the gbms data set from the 2002 field campaign at thule (76.5°n, 68.7°w), greenland, is shown with open squares (see also muscari et al., 2007), and stars superimposed to open squares are measurements at tg during 18-21 march, 2005. the correlation of the two datasets with illumination fraction is strong, with correlation coefficients of 0.66 and 0.82 for tg and thule data, respectively, demonstrating that already at 32 km altitude the ozone photochemical lifetime is comparable to transport time scales, i.e., ozone concentrations are driven as much by photochemistry as by transport. the offset that separates the two correlations, indicated by the two different linear fits (dotted and solid lines for tg and thule, respectively) in fig. 7 is possibly due to the different history (preceding the 10-day period examined with the use of air trajectories) of the air masses sampled in the two regions. this includes the exposure of the two sets of air masses to atmospheric physical properties, e.g., diabatic descent or average temperature, that can be very different at polar versus mid-latitudes. air masses sampled in the period 18-21 march, 2005 (stars overlaid on empty squares in fig. 7), belong to the inner polar vortex and arrived from the polar vortex to tg very rapidly, as shown by air parcel trajectories depicted in fig. 8. in fact, during this period, at potential temperatures between 550 k and at least 1000 k (∼24-34 km altitude) potential vorticity (pv) maps (not shown) prove that the polar vortex shifted over central europe and lay directly over tg. the corresponding data points in fig. 7 fit very well with the linear trend for thule 2002 data. only about 50% of the remaining outliers in the tg o3/illumination fraction correlation (that agree well with the correlation from thule) can be clearly explained as characteristic of air rapidly advected from the arctic region to mid-latitudes. during this same period, fig. 2 shows consistently low o3 column contents (empty circles) with respect to season 478 f ig . 6. c on to ur m ap s of g b m s o 3 m ix in g ra ti o ve rt ic al p ro fi le s fr om 1 7 to 5 0 km c ar ri ed o ut a t t es ta g ri gi a du ri ng 4 w in te r pe ri od s. e ac h pa ne l de pi ct s on e w in te r pe ri od ,f ro m w in te r 20 03 -0 4 (p an el a ,t o p ) to w in te r 20 06 -2 00 7 (p an el d ,b o tt o m ), w it h 31 d ec em be r at 1 2 g m t a s da y − 0. 5 an d 1 ja nu ar y as d ay 1 . t he r el at iv e gr ay s ca le i s to t he r ig ht o f ea ch p an el . giovanni muscari et al. 479 spectroscopic measurements of stratospheric ozone over the italian alps fig. 7. correlation scatterplot between all the testa grigia gbms ozone mixing ratio values at 850 k and illumination fraction values along 15-day back trajectories at 850 k (solid circles), and between the same quantities but from the winter 2001-2002 thule field campaign (open squares; see also muscari et al., 2007). stars superimposed to open squares are measurements from 18-21 march, 2005 (see text for details). dotted and solid lines are linear fits to testa grigia and thule data, respectively. the illumination fraction is defined as the ratio between the time interval during which the air parcel received solar illumination and the total duration of the trajectory. the correlation coefficients are 0.66 and 0.82 for testa grigia and thule data, respectively. averages, in agreement with omi/aura and woudc datasets. this is due to the advection over tg of polar vortex air also at altitudes lower than the 850 k isentropic surface. figure 9 displays the correlation between gbms column values of o3 from 24 to 29 km fig. 8. 10-day nasa/gsfc back trajectory at 850 k arriving over testa grigia on 19 march, 2005. five trajectories were calculated and only the resulting average trajectory is shown (see text for details). numbers along the trajectory indicate the distance in days before the arrival of the air parcel at testa grigia. back trajectory calculations are from the nasa/goddard space flight center (nasa/gsfc) «automailer» system initiated with ncep reanalysis meteorological data (schoeberl and sparling, 1994). 480 giovanni muscari et al. fig. 9. correlation scatterplot between testa grigia gbms column density values between 24 and 29 km altitude (in dobson units) and averages of potential vorticity values along 15-day back trajectories at 650 k (∼27 km altitude). specific dates are circled and indicated in the figure (see text for details). pvu stands for potential vorticity unit with 1 pvu = 10−6 k m2 kg−1 s−1. fig. 10. nasa/gsfc 15-day back trajectory at 550 k (∼25 km altitude) arriving over testa grigia on 15 february, 2006 (left) and an ozone mixing ratio contour map at 550 k for 8 february, 2006 (right) produced by the reprobus model calculations downloaded from <http://ether.ipsl.jussieu.fr/ethertypo>. and the pv at a potential temperature of 650 k (∼27 km). this analysis was aimed at studying the variations of o3 in the middle stratosphere in connection with a parameter related to stratospheric dynamics that could discriminate air masses belonging to the polar vortex (high pv) against air masses advected over tg from the tropical region (low pv). although the 481 spectroscopic measurements of stratospheric ozone over the italian alps overall correlation is poor, we circled two sets of datapoints which show characteristic values of the tropical lower stratosphere (7-8/02/2005) and of polar vortex air (19-21/03/2005). their different origin was confirmed by air parcel trajectories at 650 k. as for all the data points in between the two extreme conditions, the lack of correlation is due to the very complex mixing processes that characterize stratospheric air at mid-latitudes. even if a certain air mass observed at mid-latitudes is clearly advected from the arctic region, this parcel can have very different characteristics depending on whether it carries air from the aleutian high or from the polar vortex. this is the case, for example, for february 2006 (february 15 is also circled in fig. 9, february 14 is the nearby point with the largest o3 content, ∼108 du) when air masses reaching tg appear to have been advected from a region where a large gradient in o3 vmr exists, as shown in fig. 10. sorting datapoints within the correlation shown in fig. 9 in order to quantitatively pin down the origin of the air masses sampled is therefore very difficult. the use of atmospheric tracers such as n2o, also measured by the gbms, will be implemented in a future work in order to better characterize lower stratospheric dynamics. acknowledgements this research was carried out with a grant from italy’s national program for antarctic research (pnra) and with support from miur prin 2005, project #2005025202/area 02. we made considerable use of the nasa-goddard «automailer» system in tracking the dynamics of stratospheric air over testa grigia, and we thank leslie lait, paul newman, and mark schoeberl for providing this service for external users. we thank meteoswiss for making available their ozone sonde measurements from payerne through the network for the detection of atmospheric composition change (ndacc) website. we are indebted to michael pezzopane and giorgio di sarra for helpful comments on the original manuscript and to the istituto di fisica dello spazio interplanetario (inaf/ifsi) and the centro nazionale di meteorologia e climatologia aeronautica (cnma) for logistic support at testa grigia. references braathen, g. 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(received september 14, 2007; accepted october 29, 2007) s u u n a p o s s i b i l i t à d ' i m p i e g o d e l n e f e l o m e t r o ferruccio m o s e t t i d a q u a l c h e a n n o è in f u n z i o n e presso l ' o s s e r v a t o r i o geofisico di t r i e s t e u n n e f e l o m e t r o c o s t r u i t o d a l cialdea f1). esso, c o m e p r o p o sto d a q u e s t o a u t o r e è r i v o l t o a n o r d e il suo asse ottico è i n c l i n a t o di 45° s u l l ' o r i z z o n t e . s o l a m e n t e se il t r a t t o di cielo e s a m i n a t o è ser e n o la r e g i s t r a z i o n e o t t e n u t a col n e f e l o m e t r o è d e l t i p o della fig. 1. q u a n d o a u m e n t a la n e b u l o s i t à a u m e n t a d i solito l ' a l t e z z a d e l nefog r a m m a e d i m i n u i s c e q u e l l a d e l l a fascia p o l a r i z z a t a ; i n o l t r e l a regis t r a z i o n e n o n è p i ù liscia e r e g o l a r e m a p r e s e n t a svai-iate p u n t e di f o r m a e a s p e t t o d i v e r s o a s e c o n d a del t i p o d i n u v o l o s i t à : da ciò la possibilità già m o s t r a t a d a c i a l d e a di classificare le n u b i m e d i a n t e la f o r m a d e l n e f o g r a m m a . n o n è su q u e s t o a r g o m e n t o c b e ci v o g l i a m o f e r m a r e , b e n s ì sulp l'osservazione, che, d e t t o p il r a p p o r t o , ove p è l ' i n t e n s i t à p+n d e l l a l u c e p o l a r i z z a t a (altezza d e l l a fascia) e n l ' i n t e n s i t à della l u c e n a t u r a l e ( o r d i n a t a d a l b o r d o i n f e r i o r e del n e f o g r a m m a ) , t a l e grandezza v a r i a c o n l ' u m i d i t à d e l l ' a r i a . già lo smosarski (2) aveva n o t a t o c h e la p e r c e n t u a l e di l u c e p o l a r i z z a t a decresceva c o l l ' a u m e n t a r e della t e n s i o n e d i v a p o r e , m a f o r s e p e r la scarsità d i d a t i a sua disposiz i o n e n o n aveva m o s t r a t o la legge c h e lega q u e s t e d u e g r a n d e z z e e n o n si e r a s o f f e r m a t o su q u e s t o i n t e r e s s a n t e a r g o m e n t o . d a l l ' i n s i e m e d e l l e n o s t r e r e g i s t r a z i o n i a b b i a m o p o t u t o r i c a v a r e u n c e n t i n a i o d i v a l o r i di p con u m i d i t à v a r i a b i l e d a l 2 0 % al 90% e con essi a b b i a m o d i s e g n a t o il d i a g r a m m a di fig. 2. p u r t r o p p o , poic h é i d a t i i n n o s t r o possesso n o n sono a n c o r a m o l t i e ci m a n c a n o sia p r o v e s p e r i m e n t a l i c h e calcoli teorici in m e r i t o , a r g o m e n t o questo c h e ci p r o p o n i a m o di a f f r o n t a r e i n u n secondo t e m p o , n o n s i a m o in g r a d o d i d a r e l ' e q u a z i o n e esatta della curva d a n o i e m p i r i c a m e n t e trovata, c o m u n q u e , p e r lo scopo c h e q u a m o s t r e r e m o b a s t a a n c h e conoscere l ' a n d a m e n t o a p p r o s s i m a t o di p i n f u n z i o n e d i u. si sa c h e l ' i n t e n s i t à d e l l a l u c e p o l a r i z z a t a d i f f u s a d a u n p u n t o d e l l a volta celeste v a r i a con l a posizione d e l sole rispetto a q u e s t o p u n t o , p o s i z i o n e c h e v a r i a , o l t r e c h e d u r a n t e la g i o r n a t a a n c h e con 592 f e r r u c c i o m o s e t t i 0 10 20 30 4 0 50 60 70 60 90 100 u p f i g . 2. d i a g r a m m a di p = : i n f u n z i o n e d e l l ' u m i d i t à p e r c e n t u a l e . 8 g c n n a j o 1952 mmmmmrnk f i g . 1. n e f o g r a m m a n o r m a l e o t t e n u t o c o n c i e l o s e r e n o . le s t a g i o n i ; il c i a l d e a n e l suo l a v o r o ( ' ) dà dei grafici p e r t e n e r c o n t o di t a l i v a r i a z i o n i . n o i p e r ò a b b i a m o visto c b e n o n vi è, a p a r i t à d i u m i d i t à v a r i a z i o n e a p p r e z z a b i l e d i p n e l corso d e l l a g i o r n a t a e, anc h e col v a r i a r e d e l l e stagioni t a l e d i f f e r e n z a è m o l t o piccola, più piccola f o r s e d i q u e l l a n o t a t e o r i c a m e n t e , e, p e r il n o s t r o scopo, p e r c u i i n t e r e s s a n o v a r i a z i o n i m o l t o p i ù e l e v a t e , p u ò a n c h e v e n i r trascur a t a . c o m u n q u e a b b i a m o v o l u t o s e p a r a r e i v a l o r i d i p o t t e n u t i n e i m e s i p r i m a v e r i l i e a u t u n n a l i e quelli r i g u a r d a n t i i mesi estivi e i n v e r n a l i con i q u a l i a b b i a m o t r a c c i a t o i d i a g r a m m i p a r z i a l i di fig. 3 e 4 r i s p e t t i v a m e n t e . la c u r v a d i fig. 2 r a p p r e s e n t a , c o m e si diceva, l ' a n d a m e n t o med i o d i p i n f u n z i o n e d e l l ' u m i d i t à al suolo, r i c a v a t o da valori suscets u u n a p o s s i b i l i t à d ' i m p i e c o d e l n e f e l o m e t r o 593 l i b i l i di scarti a n c b e m o l t o f o r t i : è p r o p r i o su q u e s t i scarti elle ci v o g l i a m o s o f f e r m a r e . s u c c e d e c b e , q u a n d o il v a l o r e di p c b e si legge al n e f o g r a m m a , posto sull'ascissa della fig. 2 c o r r i s p o n d e n t e a l l ' u m i d i t à l e t t a al suolo, sta sotto la curva, esso c o r r i s p o n d e r e b b e ad u n ' u m i dità « fittizia » p i ù o m e n o m a g g i o r e , e i n v e r s a m e n t e , i valori di p s u p e r i o r i a q u e l l o d e l l ' a n d a m e n t o m e d i o c o r r i s p o n d e r e b b e ad u m i d i tà m i n o r i . p e n s i a m o c b e m o l t o p r o b a b i l m e n t e q u e s t e a n o m a l i e siano d o v u t e ad u n a i n e g u a l e d i s t r i b u z i o n e del v a p o r e a c q u e o n e l l a a t m o s f e r a . s o l t a n t o se la q u a n t i t à di v a p o r e è o m o g e n e a m e n t e distrib u i t a e decresce u n i f o r m e m e n t e con l'altezza, s e g u e n d o la n o t a legge e s p o n e n z i a l e , i v a l o r i d i p r i c a v a t i dal n e f o g r a m m a sono q u e l l i dati d a l d i a g r a m m a d i fig. 2. se invece in q u o t a vi sono s t r a t i di u m i d i t à u f i g . 3. a n d a m e n t o di p in f u n z i o n e di u nei m e s i p r i m a v e r i l i e a u t u n n a l i . m a g g i o r e c b e al suolo si avrà u n a s s o r b i m e n t o della l u c e p o l a r i z z a t a m a g g i o r e d i q u e l l o c b e c o m p e t e r e b b e a q u e s t a u m i d i t à e si o t t i e n e q u i n d i u n v a l o r e d i p c h e sta al di sotto della c u r v a . viceversa u n p p i ù alto del n o r m a l e d o v r e b b e c o r r i s p o n d e r e ad u n a s s o r b i m e n t o p i ù elevato al suolo c h e n o n in q u o t a , cioè alla p r e s e n z a di aria p i ù u m i d a s t a z i o n a n t e al suolo. f o r m u l a t a q u e s t a i p o t e s i a b b i a m o c e r c a t o d i c o n t r o l l a r l a con le osservazioni ed a b b i a m o t r o v a t o a p p u n t o c h e i valori di /> c h e stav a n o sotto la c u r v a m e d i a c o r r i s p o n d e v a n o a g i o r n a t e , o a i n t e r v a l l i ili t e m p o a n c h e p i ù b r e v i , i n cui i n q u o t a soffiavano v e n t i sciroccali e si a c c u m u l a v a n o masse d ' a r i a u m i d a c h e dovevano poi c o n d e n s a r s i in n u b i : di solito prima della pioggia si ricava dai nejogrammi un 592 f e r r u c c i o m o s e t t i valore di p più basso della norma. i v a l o r i d i p p i ù a l t i si h a n n o invece q u a n d o i n q u o t a soffia u n v e n t o secco o p p u r e q u a n d o al suolo si h a u n a f o r t e e v a p o r a z i o n e , c o m e ad e s e m p i o d o p o l e piogge, le n e v i c a t e e i p e r i o d i d i disgelo. s a r e b b e i n s o m m a u t i l e c o n t i n u a r e d e l l e r i c e r c h e verso q u e s t o i n d i r i z z o . o g n i s t a z i o n e d o v r e b b e a n z i t u t t o d e t e r m i n a r e il v a l o r e « n o r m a l e » d i p ( c h e è q u e l l o o t t e n u t o con a r i a secca) e c h e v a r i a di l u o g o i n l u o g o , esso è p e r e s e m p i o d i 0,70 a t r i e s t e , di 0,80 a p 0,80 0.60 0,4-0 0,20 0 10 2 0 30 4 0 50 6 0 70 8 0 9 0 a f i g . 4. a n d a m e n t o di p in f u n z i o n e di u nei m e s i i n v e r n a l i e d e s t i v i . r o m a (x). p e r l o scopo d a n o i i n d i c a t o a n d r e b b e r o u s a t i s o l a m e n t e i n e f o g r a m m i n o r m a l i ( c o m e q u e l l o della fig. 1) n o n q u e l l i a l t e r a t i n e l l ' a m p i e z z a d e l l a l u c e n a t u r a l e , c h e c o r r i s p o n d o n o già ad u n a maggiore i n t e n s i t à n e l l a d i f f u s i o n e d i q u e s t a p r o v o c a t a d a l l a c o n d e n s a z i o n e d e l l e p a r t i c e l l e in veli n e b u l o s i p i ù o m e n o spessi a n c h e se invisibili al n o s t r o o c c h i o ; t a n t o m e n o sono d a u s a r s i i n e f o g r a m m i in c u i l ' a m p i e z z a d e l l a l u c e p o l a r i z z a t a è m o l t o a t t e n u a t a o a d d i r i t t u r a n u l l a , i q u a l i c o r r i s p o n d o n o a s t r a t i d i p a r t i c e l l e a n c o r p i ù spessi o a d d i r i t t u r a a n u b i che, o l t r e a d i f f o n d e r e a n c o r p i ù la l u c e n a t u r a l e , a s s o r b o n o q u a s i c o m p l e t a m e n t e q u e l l a p o l a r i z z a t a . istituto naz. di geo fisica — osserv. di trieste — giugno 1952. s u u n a p o s s i b i l i t à d ' i m p i e c o d f x n e f e l o m e t r o 595 riassunto si presenta il diagramma di una legge empirica tra la luce polarizzata diffusa dall' atmosfera e l'umidità percentuale di questa; si dà inoltre atto di come, in base a questo diagramma, possano ricavarsi informazioni sull'ineguale distribuzione dell'umidità con l'altezza. summary a diagram showing the empirical laiv between polarized light diffused by the atmosphere and the percent humidity is presented. it is further shown how, bx using this diagram, one can obtain information about the non-uniform distribution of humidity tvith altitude. b i b l i o g r a f i a ( ! ) c i a l d e a r . : rilevamento sistematico dello stalo di polarizzazione del cielo. r i c e r c a s c i e n t i f i c a e r i c o s t r u z i o n e , 1947, n. 5, pag. 633. ( 2 ) s m o s a r s k i : die polarisation des himmelslichtes und ihre zusammenhange mit anderen meteorologischen klemenlen. g e r l a n d b e i t r . zur g e o p h y s i k , n. 30, 1931. layout 6 rstvolc implementation on modis data for monitoring of thermal volcanic activity teodosio lacava1,*, francesco marchese1, nicola pergola1, valerio tramutoli2,1, irina coviello1, mariapia faruolo1, rossana paciello1, giuseppe mazzeo1 1 istituto di metodologie per l'analisi ambientale (imaa), consiglio nazionale delle ricerche, tito scalo (potenza), italy 2 università della basilicata, dipartimento di ingegneria e fisica dell’ambiente (difa), potenza, italy annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5337 abstract an optimized configuration of the robust satellite technique (rst) approach was developed within the framework of the ‘lava’ project. this project is funded by the italian department of civil protection and the italian istituto nazionale di geofisica e vulcanologia, with the aim to improve the effectiveness of satellite monitoring of thermal volcanic activity. this improved rst configuration, named rstvolc, has recently been implemented in an automatic processing chain that was developed to detect hot-spots in near real-time for italian volcanoes. this study presents the results obtained for the mount etna eruption of july 14-24, 2006, using the moderate resolution imaging spectroradiometer (modis) data. to better assess the operational performance, the rstvolc results are also discussed in comparison with those obtained by modvolc, a well-established, modis-based algorithm for hot-spot detection that is used worldwide. 1. introduction the robust satellite technique (rst) approach [tramutoli 2007] is a multi-temporal scheme of satellite data analysis that was proposed to study and monitor active volcanoes [pergola et al. 2004]. an enhanced rst-based algorithm, named rstvolc [marchese et al. 2011], was developed to further improve the performance for volcanic hot-spot detection. rstvolc has been tested operationally in the framework of the recent ‘lava’ project, which is funded by the italian department of civil protection and the istituto nazionale di geofisica e vulcanologia [ingv 2010]. rstvolc offers the same advantages as rst (e.g. independence from site/ seasonal effects, like high reflectance of sparsely vegetated areas, emissivity variations, and natural warming of volcanic rock) [di bello et al. 2004, pergola et al. 2004]. it guarantees an improved trade-off between reliability and sensitivity, which makes it more suitable for operational monitoring of active volcanoes [marchese et al. 2011]. within the lava project, rstvolc was implemented on data from the advanced very-high-resolution radiometer (avhrr) of the national oceanic and atmospheric administration (noaa), and for the first time, on data from the moderate-resolution imaging spectroradiometer (modis) of the earth observing systems (eos). these data were directly acquired from the laboratories of the institute of methodologies for environmental analysis (imaa, potenza, italy) and the department of engineering and physics of the environment (difa, potenza, italy). the rstvolc implementation on the modis data allowed us to exploit the better spectral features of this sensor in comparison with the avhrr. indeed, modis offers two middle infrared (mir) channels (bands 21 and 22) in the range of 3.92 µm to 3.98 µm, with a radiometric accuracy of 2.0 k and 0.07 k, respectively. in addition, band 22 saturates at a brightness temperature of 330 k, while although characterized by lower data quality, band 21 offers a higher saturation level up to 500 k. this makes band 21 particularly suitable for the identification of high-temperature surfaces (e.g. lava bodies). a full integration of multi-sensor data (i.e. avhrr + modis) guarantees an increased frequency of observation, which is required especially when rapidly evolving phenomena have to be investigated, such as volcanic eruptions. the mount etna eruption that occurred from july 14-24, 2006, was analyzed to test the rstvolc performance for the monitoring of thermal volcanic activity. this eruption was chosen in particular as the case study because it was short in time (i.e. it lasted for only 10 days) and it was sufficiently documented by volcanological reports (ingvct, 2006; smithsonian institution, 2006). as rstvolc is here applied for the first time to these modis data, to better assess its performance, the results obtained have also been compared with those from modvolc [wright et al. 2002, 2004], a well-established and widely used modis-based method for automated volcanic hot-spot detection and article history received november 22, 2010; accepted june 15, 2011. subject classification: mount etna, modis, hot spots, rstvolc . special issue: v3-lava project 536 monitoring that represents a benchmark in this field [e.g. hirn et al. 2009, delle donne et al. 2010, lyons et al. 2010]. 2. test case: the mount etna eruption of july 14-24, 2006 on july 14, 2006, at around 23:30 local time (21:30 gmt), an eruptive fissure opened on the east flank of the south east crater of mount etna (figure 1). two vents located along the fissure emitted lava flows that spread 3 km east to the valle del bove [smithsonian institution 2006]. a moderate strombolian eruption also occurred at another vent, on the eastern flank of the south east crater, which produced ash fallout on the city of catania [ingv-ct 2006]. on july 17, 2006, two main lava-flow fronts reached an altitude of about 2,100 m above sea level, spreading north of the serra giannicola piccola ridge. on july 18, 2006, there was a further explosive activity similar to that on the day before, with emission of high-temperature gases [ingv-ct 2006, smithsonian institution 2006]. at an altitude of about 2,275 m, the lava flows spread into two main branches. on july 19, 2006, a new explosive vent opened that emitted ash and pyroclastic products. after the ash emissions, there was an increase in the effusion rate [ingv-ct 2006, smithsonian institution 2006] and some strong strombolian eruptions. on july 20, 2006, the lava discharge reached its peak, with an effusion rate of between 10 m3/s and 14 m3/s [hérault et al. 2009], and intense and continuous strombolian activity also occurred at the volcano [smithsonian institution 2006]. on july 21, 2006, there was an increase in the explosive activity, while on july 22, 2006, the lava reached mount centenari, at an altitude of 1,750 m. another lava body was also emitted from an effusive vent at an altitude of about 2,800 m. on july 23, 2006, the new lava effusion stopped, and there were reduced effusion rates from the main vents. no explosive activity was observed at the eruptive cone. on july 24, 2006, a helicopter survey reported a reduction in the activity of the eruptive vents, and the end of the effusive eruption was declared [ingv-ct 2006, smithsonian institution 2006]. 3. rstvolc implementation on the modis data a detailed description of the rstvolc technique can be found in marchese et al. [2011]. briefly, rstvolc combines two local variation indices, ,mir(x,y,t) and ,mir-tir(x,y,t), to automatically detect volcanic hot-spots, as defined as: (1) (2) in equation (1), tmir(x,y,t) is the satellite signal (in terms of the brightness temperature) measured in the mir spectral band between 3 µm and 4 µm, which is the most suitable for the identification of high-temperature surfaces [e.g. harris et al. 1995], at place (x,y) and time t. nmir(x,y) and vmir(x,y) are the temporal mean and temporal standard deviation (i.e. the spectral reference fields) of these signals, respectively, which are derived from long time series of homogeneous (e.g. same calendar month and same hour of pass) cloud-free satellite records. in equation (2), dt=tmir-ttir is the difference in the brightness temperatures measured in the mir and thermal infrared (tir) spectral bands, while ndt and vdt have the same meaning as above. ( , , ) ( , ) ( , , ) ( , ) x y t x y t x y t x y mir mir mir mir =7 v n-6 @ ( , , ) ( , ) ( , , ) ( , ) x y t x y t x y t x y mir tir t t =7 d v nd d6 @ 537 lacava et al. figure 1. a) the sub-scene extracted from the original level 1b modis data reprojected in lat/long wgs84 that was used by rstvolc for data processing, showing the mount etna area (blue square). b) map of the lava flow at mount etna of july 24, 2006 (from the italian department of civil protection – istituto nazionale di geofisica e vulcanologia v3 lava project [ingv 2010]). 538 the ,mir-tir(x,y,t) index is applied after the ,mir(x,y,t) index, which is more protected by local and atmospheric effects [pergola et al. 2004]. this serves to remove residual spurious effects that are related to natural signal fluctuations (i.e. anomalous increases in the surface temperature because of weather/climatic conditions). on the one hand, high values of both of these local variation indices are expected in the presence of ‘actual’ volcanic hot-spots. on the other hand, low values of the ,mir-tir(x,y,t) index should characterize spurious effects related to natural signal fluctuations that show similar behaviors in the mir and tir spectral bands [marchese et al. 2011]. according to the aims of the present study, 1,237 modis images were acquired and processed for channels 22 (or 21 when the previous one was saturated) (mir) and 31 (tir) during the months of july of 2000 to 2009 (i.e. july 2000, july 2001…, july 2009). the selected imagery were separated into two different datasets: one including diurnal data (605 images) and the other including only nocturnal overpasses (632 images) from both the eos-terra and eos-aqua satellites. the two datasets were populated by the selection of the data acquired from 09.30 to 13.30 gmt (lt=gmt+2) and from 21.30 to 01.30 gmt, respectively. a rst-based cloud detection scheme, named as the onechannel cloudy radiance detection approach (oca) [pietrapertosa et al. 2001, cuomo et al. 2004], was then applied to remove the cloudy pixels from the scenes before the computation of the spectral reference fields. the satellite images were processed and precisely co-located in the space– time domain, with the extraction for each overpass of a sub-scene of size 1211 × 1070 (see figure 1), which was centered over the mediterranean basin, and was reprojected in lat-long (wgs84) projection using the nearest-neighbor resampling method. 4. results and discussion the ,mir(x,y,t) and ,mir-tir(x,y,t) indices are defined as two standardized variables that have a gaussian behavior. as an example, figure 2 shows the statistical behavior of the ,mir(x,y,t) index computed for a single unperturbed pixel close to the volcanic edifice, from an analysis of 10 years of satellite records. the index behavior is well fitted by a gaussian curve (r2 = 0.96), with mean value n = 0.13 and standard deviation v= 1.1. in this case, the slight asymmetry of the histogram towards negative values is due to residualcloud contamination effects. considering that for a normal distribution, about 99.7% of the data (x) is included in the range ‘n–3v < x < n+3v’, values of ,mir(x,y,t) >3 should occur with a probability around 0.15%, and they thus represent statistically significant anomalies. similarly, the same behavior can be addressed to the ,mir-tir(x,y,t) index. therefore, the values of ‘,mir(x,y,t) >3 and ,mir-tir(x,y,t) >3’ are generally used by rstvolc to identify volcanic hotspots. in this case, because of the multiplication rule, the probability of occurrence is even lower (0.0225%), and consequently, the anomalies detected should be even more significant from a statistical point of view. the hot-spots detected by rstvolc from the processing of 42 modis overpasses acquired from july 15-24, 2006, are reported in table 1. they are in good agreement with the available field observations of the mount etna activity over this investigated time period [e.g. ingv-ct 2006, smithsonian institution 2006]. starting from the first modis image that was available after the onset of the eruption, namely that acquired on july 15, 2006, at 01:35 gmt (about four hours after the onset), volcanic hot-spots were detected by rstvolc with a good level of continuity, apart from some cloudy scenes where the target area was completely masked by weather clouds (see table 1). figure 3 reports the number of hotspots detected each day during the eruptive period investigated, which gives an indication of the space–time evolution of the thermal phenomena that was in progress at mount etna. in particular, the time series shows the occurrence of two distinct eruptive phases for the volcano: the first was characterized by a continuous and marked increase in the daily number of hot-spots from the beginning of the eruptive activity until its paroxysmal phase; the second was characterized by a decrease in the daily number of hot-spots after july 22, 2006, until the end of the eruption. the slight decrease in the number of thermal anomalies detected on july 17 and 19, 2006, is related to the high satellite zenith angles of most of the modis satellite overpasses that were acquired for these days (e.g. on july 17, 2006, all of passes had satellite zenith angles $40˚). at such view angles, a reduction in the sensor-measured radiance has etna monitoring by rstvolc on modis data figure 2. histogram of the ,mir(x,y,t) index computed for a single unperturbed pixel selected over a nonvolcanic area close to the mount etna edifice on ten years of satellite records, with mean = –0.13 and sigma = 1.13. inset: digital elevation model gtopo 100 with red cross to indicate the location of the unperturbed pixel. to be expected [coppola et al. 2010], even if the relative impact of this effect also depends on the intensity (e.g. temperature and extent) of the thermal source. major peaks in the daily number of hot-spots were recorded between july 20 and 22, 2006, in good agreement with the observed increase in lava effusion [hérault et al. 2009]. the strong drop in the daily number of hot-spots on july 23, 2006, was instead in agreement with a significant reduction in the eruption intensity that was reported by volcanological bulletins on that day [ingv-ct 2006, smithsonian institution 2006]. these results confirm the potential of rstvolc for the monitoring of thermal volcanic activity. they also integrate and complete our previous studies on the same eruptive event, which were performed using both the avhrr and seviri data [pergola et al. 2008], confirming the independence of this rst approach on satellite platforms. in addition, to better assess the rstvolc results (here applied for the first time to modis imagery), a comparison with the modvolc results was also carried out. modvolc is one of the most well-established modis-based methods for satellite hot-spot detection, and it has been implemented in an automatic system that was developed for near real-time monitoring of volcanoes at a global scale [wright et al. 2002]. the modvolc products are continuously posted on the internet, about 20 h after the sensing time [modvolc 20022010]. this method computes a normalized thermal index (nti) to detect the volcanic hot-spots, which are calculated on the basis of the mir and tir radiances measured in the modis channels 21 or 22, and 32, respectively. for the nighttime data, the image pixels with a nti index greater than ‘-0.80’ are flagged as hot-spots [wright et al. 2002, 2004]. instead, to take into account the solar reflected component of the mir radiance in the daytime, a correction for the reflected lacava et al. 539 date [yy/mm/dd hhmmss] satellite hot-spots detected by rstvolc satellite zenith angle (˚) 06/07/15 013500 aqua 2 56 06/07/15 092500 terra 0 (cloudy) 43 06/07/15 124000 aqua 0 (cloudy) 53 06/07/15 203000 terra 5 48 06/07/16 004000 aqua 5 25 06/07/16 100500 terra 0 (cloudy) 29 06/07/16 114500 aqua 5 33 06/07/16 211500 terra 5 21 06/07/17 012500 aqua 4 45 06/07/17 091000 terra 1 55 06/07/17 123000 aqua 2 40 06/07/17 202000 terra 5 58 06/07/18 003000 aqua 8 43 06/07/18 095500 terra 3 8 06/07/18 113500 aqua 0 (cloudy) 48 06/07/18 210000 terra 7 1 06/07/19 011000 aqua 6 29 06/07/19 090000 terra 2 64 06/07/19 104000 terra 2 60 06/07/19 122000 aqua 4 21 06/07/19 214500 terra 2 58 06/07/20 001500 aqua 7 55 06/07/20 094500 terra 4 15 06/07/20 112500 aqua 3 58 06/07/20 205000 terra 11 25 06/07/21 010000 aqua 10 8 06/07/21 102500 terra 2 51 06/07/21 120500 aqua 5 1 06/07/21 213000 terra 6 47 06/07/22 000500 aqua 6 64 06/07/22 014500 aqua 2 60 06/07/22 093000 terra 6 35 06/07/22 125000 aqua 1 58 06/07/22 203500 terra 9 41 06/07/23 004500 aqua 8 15 06/07/23 101500 terra 2 38 06/07/23 115500 aqua 3 23 06/07/23 212000 terra 3 31 06/07/24 013000 aqua 2 51 06/07/24 092000 terra 0 (cloudy) 50 06/07/24 123500 aqua 0 (cloudy) 47 06/07/24 202500 terra 1 53 table 1. hot-spots detected by rstvolc over the mount etna area from july 15-24, 2006, with dates of detection, the satellites for the sensing, and the relative satellite zenith angles. figure 3. time series of the number of daily hot-spots detected by rstvolc through the analysis of all of the modis data acquired over the target area (mount etna) for the period of july 15-24, 2006. 540 solar radiation is performed using the modis short-wave infrared band centered at 1.6 µm (i.e. channel 6) (http://modis.higp.hawaii.edu/daytime.html). once this correction is performed, the hot-spots are identified by modvolc when the nti index exceeds ‘-0.60’. modvolc was designed to work under some specific restrictions that were imposed by the computer resources available in 2000 at the distributed active archive center of the goddard space flight center. among these restrictions, one is related to the use of level 1b modis data. unfortunately, at high satellite zenith angles, these level 1b modis data are affected by bow-tie [landesa et al. 2004]. it is an artefact of the images related to the arrangement of the detectors that can duplicate detected hot-spots due to oversampling at the borders of the scene [coppola et al. 2010]. therefore, to avoid bow-tie-related artefacts, in the present study, the rstvolc products were compared to those of modvolc, considering only the data acquired at low satellite zenith angles (i.e. <40˚). as independently demonstrated by other studies, these should be less affected by such effects [e.g. coppola et al. 2010]. figure 4 shows the number of hot-spots detected by rstvolc and modvolc, for all the modis overpasses acquired under these specific conditions. there is a good agreement between the two hotspot curves (figure 4). both of these curves appear to correctly describe the time evolution of the eruptive event, with major peaks in the number of hot-spots detected between july 20 and 21, 2006, and with a significant intensity decrease starting from july 23, 2006. however, despite this agreement, some differences can also be noted. the most significant difference in the number of hot-spots is evident on seen for the modis overpass of july 16, 2006, at 11:45 utc. in this case, although five hotspots were correctly detected by rstvolc over the mount etna area, no hot-spot was identified by modvolc. this difference, which is not evaluable by just examining of the modvolc products that are available online, required the modvolc implementation on modis level 1b data to be fully interpreted. on the basis of this data investigation, it was evident that no hot-spot was identified by modvolc over the etna monitoring by rstvolc on modis data figure 4. hot-spots detected by rstvolc (black) and by modvolc (red), for the mount etna eruption of july 15-24, 2006, from the processing of the modis data acquired with satellite zenith angles <40˚ (see main text). figure 5. a) level 1b modis channel 22 image of july 16, 2006, at 11:45 gmt (13:45 lt). b) enlargement of the area within the red box in (a). c) level 1b modis channel 6 image of the same modis overpass, with evidence of striping effects on the modvolc detection. d) original modis image with red crosses to indicate the locations of the pixels corresponding to the hot-spots detected by rstvolc. mount etna area because of the striping effects that affect aqua-modis band 6. indeed, figure 5a,b shows the actual hot-spots related to an eruptive activity in progress on mount etna that were not identified by modvolc, because located over some striping lines of the modis band 6 (see figure 5c). these hot spots were correctly detected, instead, by rstvolc (figure 5d). striping problems related to 15 noisy aqua detectors [salomonson and appel 2006, nasa gsfc 2010] affected each daytime aqua-modis overpass that was analyzed in this study (10 in total), which accounted for the main cause of differences in the hot-spot numbers detected by rstvolc and by modvolc. it should be noted that these striping effects, like bow-tie, cannot be directly ascribed to the modvolc algorithm, as they are instead issues of modis level 1b data. moreover, in principle, these effects can be removed by implementation of a pre-processing, de-striping procedure that when applied before the nti index computation, might improve the daytime modvolc performance. instead, for the slight differences in the hot-spot numbers of figure 4 that were observed mainly in the night-time records, these were related to the known reduction in sensitivity of modvolc in the presence of subtle hot-spots [wright et al. 2002, 2004, kervyn et al. 2006, 2008]. this is a consequence of the fixed detection thresholds used by modvolc to monitor volcanoes at a global scale. as further confirmation of this issue, the authors of modvolc have recently proposed a hybrid approach that combines modvolc with a rst-based time series [koeppen et al. 2010]. the results that arise from the comparison with modvolc confirm the performances of rstvolc for the successful monitoring of thermal volcanic activity using the modis data. it should be stressed that by using local adaptive thresholds that are specific for each place and time of observation, the rstvolc technique can be used to effectively monitor volcanoes at different geographic locations, requiring only a historical dataset of satellite records to be applied. 5. conclusions in the present study, the outcomes of the italian department of civil protection – istituto nazionale di geofisica e vulcanologia ‘lava’ project that were obtained for the development and testing of the rstvolc technique have been presented and discussed. these results highlight the performance of the rstvolc for the detection of volcanic hot-spots under different observational conditions by using the modis data. as these data are based only on satellite records, rstvolc can be exported easily on whatever kind of satellite/sensor system, provided that multi-year time series imagery is available. in addition, the successful real-time experimentation of hot-spot products that was performed in the framework of the lava project has confirmed the potential of rstvolc in the monitoring of volcanoes under possible operational scenarios. finally, the comparison of the rstvolc and modvolc results has shown similar capabilities of these two methods for the monitoring of the thermal activity that was in progress on mount etna during july 2006. this analysis also indicates that modvolc might benefit from a pre-processing procedure that includes data de-striping, which, if applied before the nti index, might improve its performance under daytime conditions. acknowledgements. this study received financial support from the v3-lava project (italian department of civil protection – istituto nazionale di geofisica e vulcanologia, 2007-2009 contract). references coppola, d., m.r. james, t. staudacher and c. cigolini (2010). a comparison of fieldand satellite-derived thermal flux at piton de la fournaise: implications for the calculation of lava discharge rate, b. volcanol., 72, 341356; doi: 10.1007/s00445-009-0320-8. cuomo, v., c. filizzola, n. pergola, c. pietrapertosa and v. tramutoli (2004). a self sufficient approach for gerb cloudy radiance detection, atmos. res., 72, 39-56. delle donne, d., a.j.l. harris, m. ripepe and r. wright (2010). earthquake-induced thermal anomalies at active volcanoes, geology, 38, 771-774. di bello, g., c. filizzola, t. lacava, f. marchese, n. pergola, c. pietrapertosa , s. piscitelli, i. scaffidi and v. tramutoli (2004). robust satellite techniques for volcanic and seismic hazards monitoring, annals of geophysics, 47 (1), 49-64. harris, a.j., s.e.j. swabey and j. higgins (1995). automated threshold 463 of active lava using avhrr data, int. j. remote sens., 16, 3681-3686. hérault, a., a. vicari, a. ciraudo and c. del negro (2009). forecasting lava flow hazards during the 2006 etna eruption: using the magflow cellular automata model, comput. geosci.-uk, 35 (5), 1050-1060; doi: 10.1016/j. cageo.2007.10.008. hirn, b., c. di bartola and f. ferrucci (2009). combined use of seviri and modis for detecting, measuring, and monitoring active lava flows at erupting volcanoes, ieee t. geosci. remote, 47, 2923-2930. ingv-ct (2006). reports on volcanic activity-u.f. vulcanologia e geochimica. available from istituto nazionale di geofisica e vulcanologia, sezione di catania, italy. ingv (2010). progetto v3 lava. realizzazione della mappa di pericolosità da colate di lava all'etna, e messa a punto di un metodo di aggiornamento dinamico. available at: http://istituto.ingv.it/l-ingv/progetti/progetti-finanziatidal-dipartimento-di-protezione-civile-1/progetti-dpcconvenzione-2007-2009/progetti_v/v3/v3-lava (in italian). lyons, j.j., g.p. waite, w.i. rose and g. chigna (2010). patlacava et al. 541 542 terns in open vent, strombolian behavior at fuego volcano, guatemala, 2005-2007, b. volcanol., 72, 1-15. kervyn, m., a. harris, e. mbede, f. belton, p. jacobs and g. g.j. ernst (2006). modlen: a semi-automated algorithm for monitoring small-scale thermal activity at oldoinyo lengai volcano, tanzania, in: quantitative geology from multiple sources, proceedings of the xith iamg annual conference (liège, belgium, 3-8 september 2006). kervyn, m., g.g. j. ernst, a. harris, e. mbede, f. belton and p. jacobs (2008). thermal remote sensing of the low-intensity carbonatite volcanism of oldoinyo lengai, tanzania, int. j. remote sens., 29, 6467-6499. koeppen, w.c., e. pilger and r. wright (2010). time series analysis of infrared satellite data for detecting thermal anomalies: a hybrid approach, b. volcanol.; doi: 10.1007/s00445-010-0427-y. landesa, e.g., a. rango and m. bleiweiss (2004). an algorithm to address the modis bow-tie effects, can. j. remote sens., 30, 644-650. marchese, f., c. filizzola, n. genzano, g. mazzeo, n. pergola and v. tramutoli (2011). assessment and improvement of a robust satellite technique (rst) for thermal monitoring of volcanoes, remote sens. environ., 115, 1556-1563; doi: 10.1016/j.rse.2011.02.014. modvolc (2002-2010). near real-time thermal monitoring of global hot-spots, available at: http://modis.higp. hawaii.edu/cgi-bin/modis/modisnew.cgi. nasa gsfc (2010). modis history and performance: timedependent list of non-functional or noisy detector, available at: http://mcst.gsfc.nasa.gov/ index.php?section=78. pergola, n., f. marchese and v. tramutoli (2004). automated detection of thermal features of active volcanoes by means of infrared avhrr records, remote sens. environ., 93, 311-327. pergola, n., f. marchese, v. tramutoli, c. filizzola and m. ciampa (2008). advanced satellite technique for volcanic activity monitoring and early warning, annals of geophysics, 51 (1), 287-301. pietrapertosa, c., n. pergola, v. lanorte and v. tramutoli (2001). self adaptive algorithms for change detection: oca (the one-channel cloud-detection approach) an adjustable method for cloudy and clear radiances detection, in: j.f. le marshall and j.d. jasper (eds.), technical proceedings of the 11th international (a)tovs study conference (itsc-xi; budapest, hungary, 20-26 september 2000), published by bureau of meteorology research centre, melbourne, australia, 281-291. smithsonian institution (2006). bulletin of the global volcanism network, http://www.volcano.si.edu/world/volcano.cfm?vnum=0101-06=&volpage=var#bgvn_3107, 2006. salomonson, v.v. and i. appel (2006). development of the aqua modis ndsi fractional snow cover algorithm and validation results, ieee t. geosci. remote, 44, 1747-1756. tramutoli, v. (2007). robust satellite techniques (rst) for natural and environmental hazards monitoring and mitigation: theory and applications, proceedings of multitemp 2007; doi: 10.1109/multitemp.2007.4293057. wright, r., l. flynn, h. garbeil, a. harris and e. pilger (2002). automated volcanic eruption detection using modis, remote sens. environ., 82, 135-155. wright, r., l.p. flynn, h. garbeil, a.j.l. harris and e. pilger (2004). modvolc: near-real-time thermal monitoring of global volcanism, j. volcanol. geoth. res., 135, 29-49. *corresponding author: teodosio lacava, istituto di metodologie per l'analisi ambientale (imaa), consiglio nazionale delle ricerche, tito scalo (potenza), italy; email: lacava@imaa.cnr.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. etna monitoring by rstvolc on modis data e v i d e n c e f o r t h e s e a t o f t h e s t r a i n p r o d u c i n g f o r c e s a . g . g a l a n o p o c l o s r i c e v u t o il 1 s e t t e m b r e 1965 summart. — e v i d e n c e is p r e s e n t e d w h i c h f a v o u r s t h e t h e o r y t h a t t h e s e a t of t h e s t r a i n p r o d u c i n g forces is in t h e l o w r i g i d i t y l a y e r n e a r t h e t o p of t h e m a n t l e . r i a s s u n t o . — l a n o t a serve a c o n f e r m a r e la t e o r i a secondo la quale le f o r z e d e f o r m a n t i v e n g o n o localizzate nello s t r a t o a b a s s a r i g i d i t à , prossimo al m a n t e l l o s u p e r i o r e (sotto la superficie di mohorovicic). i n t r o d u c t i o n . b à t h and d u d a (1964) in an effort to improve benioff's method (1951a), being developed for strain release studies, were led to the conclusion t h a t the earthquake volume v, identifìed with the total aftershock volume, increases with magnitude m, according to the following equation : log v = 9.58 + 1.47 m . por the aftershock area 8, b à t h and duda derived a new equation: log 8 = 4.95 + 1 . 2 1 m , which improves a previous relation: log 8 = 5.99 + 1.02 m , found by utsu and seki (1955). considering t h a t v = lwe and 8 = lw, where l, and h are respectively, the length, the width and the vertical extent of aftershock zone in cm, we m a y immediately 4 0 0 a . g . g a l a n o p o u l o s derive f r o m the two relations given b y b à t h and d u d a for t h e earthquake volume a n d the aftersliock zone, t h e following expression: log h = 4 . 6 3 + 0 . 2 6 m . the equation derived implies t h a t for earthquakes of m a g n i t u d e 7 . 7 to 8 . 7 t h e vertical e x t e n t of aftersliock zone reaches to a t least 43 to 78 k m depths. taking this for g r a n t e d a n d another v e r y interesting result of bàtli's and d u d a ' s investigation t h a t " the ratio of fault piane area to the vertical section through the aftershock zone, i.e. f/lh, increases with magnitude, approaching unity for the largest shocks " , it m i g h t be concluded t h a t t h e middle of t h e f a u l t piane of t h e great shocks is a t least in most cases — immediately below t h e mohorovieic diseontinuity, i.e. in t h e low-rigidity layer near t h e top of t h e m a n t l e . this implications is strongly supported b y t h e f a c t t h a t ali shocks of m a g n i t u d e 5s 73/4 occurred in t h e area of greece are of i n t e r m e d i a t e focal d e p t h . the alaskan e a r t h q u a k e of march 27, 1964, with richter magnitude of 8 . 4 to 8 . 6 , h a d a focal d e p t h of a b o u t 50 k m . i n a recent investigation, f . press (1965) was led t o tlie conclusion " on the basis of the very graduai reduction of vertical deformation with distances that the faidt could not have extended to depths as sliallow as 50 km and that 100 to 200 km are more likely values. the fault probably carne to within 15 to 20 km of the surface " . thus t h e vertical e x t e n t found for the f a u l t of t h e alaskan e a r t h q u a k e exceeds t h e values previously found for other earthquakes. according to press " the larger magnitude associateti with the alaskan earthquake (31 — 8.4) as compared with the others may partially explain the difference ". the f a c t t h a t m a n t l e surface waves and free oseillations are exeited only b y the larger earthquakes strengthens tlie notion t h a t the p r i m a r y faulting associated with these shocks extends f a r below t h e e a r t h ' s crust. another evidence are t h e empirical relations of i 0 to ili" derived for constant depths of foci b y v. karnilc (1964): h = 3 — 5 k m , il/ = 0 . 5 6 i 0 + 0 . 4 5 h = 6 — 10 k m , il/ = 0 . 5 6 io + 0 . 8 3 li = 11 — is k m , m = 0.57 io + 0 . 8 5 li = 19 — 30 k m , il/ = 0 . 5 0 i 0 + 1 . 7 5 ti = 31 — 50 k m , il/ = 0 . 4 0 /„ + 2 . 3 6 li = 51 — 82 k m , m = 0 . 2 9 l + 3.77 j , , 4 h = 83 1 5 0 k m , m = 0 . 3 3 /„ + 3 . 2 3 | m l h w l t h ° u t ^ e v i d e n c e f o r t i i e s e a t of t h e s t r a i n p r o d u c i no f o r c e s 405 the relations derived for m = mlh + ~ìmh w i t h o u t d e p t h corrections, ~òm*, have approximately a common intersection near i 0 = x i and m = 7. this implies t h a t in case of shallow shocks t h e m a x i m u m intensity over observed, i 0 = x i , is a t t a i n e d b y shocks of magnitude 7 ; in other words there are no shallow shocks with m a g n i t u d e greater t h a n 7. d a t a u s e d . i n the following table we give ali shallow and intermediate earthquakes of magnitude 51/2 occurred in the area of greece during t h e 120-years interval, 1843-1962, for which period t h e available d a t a were proved to be fairly homogenous. i n t h e intermediate earthquakes we have included ali earthquakes of focal d e p t h > 10 k m . during the time i n t e r v a l considered there were released: 454 shallow shocks with m > 5va, 210 with m ;:> 6, 83 with m > 6v2, 25 with m > 7 and 12 with m > 71/2during the same period, 1843-1962, there were released: 74 i n t e r m e d i a t e shocks with m > 51/2, 48 with m ^ 6, 39 with m > 6va, 17 with m > 7, 11 with m ':> 71/., and 7 with m 5> 8. the d a t a given above m a y be summarized as follows: m 5 v. 6 6 v, 7 7 v. 8 ni 3 .78 1 .75 0 .69 0 .21 0 .10 — n2 0 .62 0 .40 0 .325 0 .14 0 .092 0 . 0 5 8 log ni 0 .58 0 .24 — 0 .16 — 0 .68 — 1 .00 — log — 0 .21 — 0 .40 — 0 .49 — 0 .85 — 1 .04 — 1 . 2 3 where nlt n„ is respectively the number of shallow and intermediate shocks of m a g n i t u d e m or greater per one year. these values flt r a t h e r closely t o : log n i = [— 1 . 1 3 + 0 . 8 2 (8 — 3/)] ± 0 . 0 5 = — 1 . 4 3 ± 0 . 0 6 + + (0.82 ± 0.02)(8 — m) log n„ = [— 1 . 2 3 + 0.42 (8 — ili")] ± 0 . 0 6 = — 1 . 2 3 ± 0 . 0 3 + + (0.42 ± 0.01)(8 — m). the logarithm of t h e ratio of the number of sinaller shallow shocks to t h a t of intermediate shocks in the area considered is approximately 3 . 0 0 ; this makes the ratio of numbers a b o u t 1000: 1. ilowever, t h e logarithm of the ratio of the n u m b e r of shallow shocks of m a g n i t u d e s and over, to t h a t of intermediate shocks is approximately 0 . 2 0 ; this makes t h e ratio of numbers about 0.63:1, i.e. roughly 6 0 % of shocks 400 a . g. g a l a n o p o u l o s t a b l e i c a t a l o g u e o r s h a l l o w e a r t h q u a k e s o f m a g n i t u d e > 5 1 / 2 o c c u r r e d i n t u e a r e a o f g r e e c e d u r i n g t h e p e r i o d 1843-1962. no d a t e m a g n i t u d e a d o p t e d no d a t e m a g n i t u d e a d o p t e d 1 1843, s e p t . 5 6 v« 51 march 6 0 74 2 oct. 18 6 3u 52 march 13 5 va 3 1845, j u n e 23 6 7 4 8 53 march 25 5 7 4 oct. 11 6 7 4 8 54 d e e . 4 6 3u 5 1846, j u n e 10 7 2 55 1867, j a n . 27 6 7 i 6 j u n e . 13 6 v« 56 m a r c h 7 6 7 7 1848, — — 6 7« 57 march 30 6 v 4 8 1851, j a n . 20 6 7« 58 oct. 22 5 9 9 f e b r . 28 6 *u 59 1868, a p r . 20 6 74 10 oct. 12 7 ì 60 m a y 3 5 v . 11 oct. 17 6 3/4 61 oct. 3 5 6 12 d e e . 29 6 l/« 62 1869, m a r c h 18 5 va 13 d e e . 29 6 l/« 63 a u g . 14 6 74 14 1852, a u g . 26 6 vi 64 s e p t . 1 6 74 15 1853, a u g . 18 7 2 65 dee. 1 7 v» 16 1854, j u l y 30 6 6 66 dee. 28 6 9 17 1856, n o v . 13 6 au 67 1870, a u g . 1 7 v. 18 1858, f e b r . 21 7 v . 68 s e p t . 28 6 2 19 a p r . 5 6 v 4 69 1871, j a n . 22 6 v« 20 s e p t . 20 6 9 70 a p r . 8 6 v 4 21 oct. 10 6 8 71 j u n e 7 6 vi 22 1859, m a r c h 13 6 v* 72 oct. 8 6 9 23 a u g . 13 5 v» 73 1872, f e b r . 11 7 v. 24 a u g . 21 6 4 74 1873, j a n . 31 6 6 25 s e p t . 12 6 v* 75 j u l y 25 « 74 26 1860, a p r . 10 7 v . 76 oct. 25 6 1 27 a p r . 15 5 v . 77 1874, j a n . 17 5 va 28 a p r . 16 6 v 4 78 m a r c h 18 5 v 2 29 m a y 16 6 74 79 n o v . 16 7 3 30 a u g . 6 5 v . 80 1875, a p r . 24 5 va 31 1861, d e e . 26 7 v . 81 j u l y 7 6 74 32 1862, m a r c h 14 7 v . 82 1876, j u n e 26 6 v« 33 oct. 4 6 3/4 83 1877, j u l y 2 5 va 34 1863, a u g . — 6 v« 84 oct. 13 5 v . 35 1864, j u n e 14 5 l/« 85 oct. 13 o v , 36 j u l y 17 5 l/. 86 1880, j u l y 29 6 6 37 a u g . 21 5 v . 87 1881, a p r . 3 6 3 38 1865, j u l y 23 6 6 88 1883, oct. 15 6 4 39 oct. 10 6 v* 89 1885, f e b r . 18 5 va 40 oct. 11 5 v. 90 m a r c h 28 6 0 41 n o v . 11 6 v« 91 dee, 14 5 va 42 1866, j a n . 2 6 7 4 8 92 1886, s e p t . 4 5 va 43 j a n . 13 6 7 4 8 93 n o v . 27 5 va 44 j a n . 31 6 1 94 1887, may 14 6 5 45 f e b r . 2 6 vi 95 oct. 3 6 3 46 f e b r . 6 6 74 96 1888, s e p t . 9 5 8 47 f e b r . 28 6 74 97 1889, a p r . — 5 va 48 m a r c h 2 6 74 98 a u g . 25 6 3 49 m a r c h 2 6 7 99 oct. 25 6 9 50 m a r c h 2 5 7 100 1890, may 21 5 va e v i d e n c e f o r t i i e s e a t o f t h e s t r a i n p r o d u c i no f o r c e s 405 t a b l e i (cont.) no d a t e m a g n i t u d e a d o p t e d no d a t e m a g n i t u d e a d o p t e d 101 1890, m a y 26 6 6 152 1907, a u g . 16 6 7 i 102 dee. 14 6 5 153 1909, march 8 5 ìi 103 1891, m a y 11 6 1 154 m a y 30 5 9 2 104 j u n e 27 5 v . 155 j u l y 15 5 8 105 s e p t . 18 5 l/« 156 1910, oct, 27 6 v* 106 1892, j a n . 9 5 5 157 1911, f e b r . 18 7 1 107 dee. 27 5 7» 158 march 9 5 7 a 108 1893, j a n . 31 6 4 159 oct. 22 5 1 / / 2 109 f e b r . 1 6 7 4 160 1912, f e b r . 13 7 0 110 f e b r . 9 5 9 161 f e b r . 15 6 74 111 marcii 12 5 7 « 162 a p r . 1 5 7 a 112 a p r . 17 6 4 163 a p r . 19 5 7 a 113 m a y 23 6 1 164 a p r . 21 5 74 114 j u n e 14 7 7z 165 a u g . 9 7 74 115 oct. 13 5 v . 166 1913, m a y 25 5 72 116 1894, a p r . 20 6 7 167 1914, oct. 17 6 0 117 a p r . 27 6 9 168 oct. 17 5 9 118 j u l y 26 6 74 169 1914, n o v . 23 5 7 2 119 1895, m a y 13 6 74 170 n o v . 27 6 1 120 m a y 14 6 74 171 dee. 2 5 7 2 121 m a y 14 7 v . 172 1915, j a n . 27 6 0 122 m a y 15 6 4 173 j u n e 4 5 7 123 j u n e 21 6 74 174 a u g . 7 6 3 124 a u g . 6 6 74 175 a u g . 10 6 1 125 a u g . 19 6 74 176 a u g . 10 6 1 126 s e p t . 5 6 7 4 177 a u g . 11 6 4 127 1896, f e b r . 10 5 v a 178 a u g . 11 5 5 128 f e b r . 10 6 7 4 179 a u g . 19 5 7 129 f e b r . 10 6 7 4 180 1916, s e p t . 27 5 5 130 f e b r . 11 5 7 a 181 1917, march 14 6 74 131 f e b r . 11 6 7 4 182 march 26 5 7 2 132 march 18 6 7 4 183 s e p t . 23 5 7 2 133 oct. 27 6 2 184 1918, j a n . 17 5 7 2 134 n o v . 13 5 v a 185 s e p t . 11 5 7 2 135 1897, f e b r . 12 6 1 186 n o v . 20 5 7 2 136 1897, j u n e 30 5 v a 187 1919, f e b r . 24 5 7 137 1898, f e b r . 11 5 7 2 188 oct. 25 6 0 138 1898, j u n e 2 6 4 189 n o v . 18 5 7 4 139 j u l y 31 6 2 190 1920, oct. 18 6 74 140 n o v . 9 6 0 191 oct. 21 5 9 141 1899, j a n . 22 6 7 192 n o v . 15 6 1 142 1899, s e p t . 20 6 7 4 193 n o v . 25 5 7 2 143 1902, j u l y 5 6 6 194 n o v . 26 6 4 144 n o v . 5 5 7 . 195 n o v . 28 5 7 2 145 1904, a p r . 4 7 7 a 196 n o v . 29 6 2 146 1905, j u n e 1 5 7 2 197 dee. 18 6 7 4 147 j u n e 3 6 0 198 d e e . 23 6 74 148 oct. 8 6 4 199 1921, march 30 5 5 149 n o v . 8 7 0 200 j u l y 6 5 5 150 1906, m a r c h 3 6 74 201 1921, s e p t . 13 5 5 151 1906, s e p t . 28 6 74 202 oct. 21 6 7 4 400 a . g . g a l a n o p o u l o s t a b l e i (cont.) no d a t e m a g n i t u d e a d o p t e d no d a t e m a g n i t u d e a d o p t e d 203 1922, a u g . 11 5 *u 254 m a y 11 o 74 204 1923, j a n . 7 6 1 / / 4 255 a u g . 17 6 . 3 205 m a y 6 6 3 256 1934, f e b r . 4 5 va 200 d e e . 5 5 7 257 f e b r . 21 1/ o / „ 5 3u « 74 207 d e e . 27 5 v . 258 n o v . 21 1/ o / „ 5 3u « 74 208 1925, f e b r . 7 5 72 259 1935, j a n . 4 1/ o / „ 5 3u « 74 209 1926, m a r c h 18 6 9 260 j a n . 4 5 . 8 210 m a r c h 18 5 72 261 j a n . 4 6 . 0 211 march 19 5 1/ / 2 262 f e b r . is 5 . 5 212 dee. 17 6 2 263 m a r c h 30 6 . 0 213 d e e . 17 6 5 264 m a y 2 5 7 a 214 1927, m a r c h 24 5 v. 265 n o v . 7 6 . 0 215 j u n e 30 5 5 266 1936, a p r . 8 6 . 0 216 1928, j a n . 23 5 5 267 a p r . 15 5 . 7 217 alarcli 31 6 74 268 1937, j a n . 2 5 v a 218 a p r . 18 6 74 269 m a y 23 5 . 7 219 a p r . 18 5 72 270 j u l y 6 6 . 0 220 a p r . 22 6 8 271 1938, j a n . 30 5 . 9 221 a p r . 25 5 72 272 f e b r . 10 5 »/« 222 a p r . 28 5 72 273 march 11 5 . 8 223 d e e . 10 5 6 274 m a r c h 13 5 . 7 224 1930, j a n . 23 5 7 275 may 12 5 v a 225 j a n . 28 6 74 276 j u n e 20 5 . 7 220 f e b r . 23 5 7 277 j u l y 2 5 . 8 227 m a r c h 31 6 i 278 j u l y 20 5 v4 228 a p r . 17 5 8 279 1939, m a y 20 5 v 2 229 j u n e 25 5 / 2 280 j u n e 23 5 v a 230 n o v . 21 6 0 281 a u g . 9 6 . 1 231 n o v . 21 6 74 282 s e p t . 22 0 v a 232 d e e . 2 6 74 283 1940, j a n . 6 5 . 6 233 1931, j a n . 4 5 6 284 f e b r . 23 5 v a 234 m a r c h 7 6 0 285 f e b r . 29 6 . 6 235 m a r c h 8 6 74 286 1941 , march 1 o v . 236 j u l y 12 5 8 287 may 14 5 v a 237 1931 , s e p t . 13 6 0 288 m a y 16 5 v a 238 n o v . 15 5 7 a 289 may 23 6 . 0 239 n o v . 23 6 4 290 may 28 5 v a 240 1932, m a r c h 9 5 7 2 291 j u n e 24 5 v a 241 a p r . 27 5 7 2 292 j u l y 13 5 7 4 242 may 14 5 7 2 293 j u l y 23 5 !/ ° i a 243 j u n e 29 5 7 2 294 s e p t . 1 5 1 / ° 12 244 s e p t . 26 6 9 295 dee, 13 6 . 0 245 s e p t . s e p t . 26 6 1 296 1942, j u n e 16 5 . 6 240 s e p t . s e p t . 29 6 74 297 a u g . 27 5 v a 247 s e p t , 30 5 v . 298 s e p t . 1 5 . s 248 oct. 23 5 7 2 299 oct. 28 6 . 0 249 n o v . 1 5 7 2 300 oct. 28 6 . 0 250 1933, j a n . 5 5 7 * 301 1943, j u l y 23 5 74 251 m a r c h 14 5 6 302 1944, m a r c h 14 5 . 9 252 a p r . 28 5 i/ 303 j u n e 25 6 . 0 253 may 8 6 1 2 304 j u l y 30 5 . 6 e v i d e n c e f o r t i i e s e a t o f t h e s t r ai n p r o d u c i n o f o r c e s 4 0 5 t a b l e i (cont. ) no d a t e m a g n i t u d e a d o p t e d no d a t e m a g n i t u d e a d o p t e d 305 1944, oct. 6 7 . 2 356 a u g . 12 6 . 2 306 oct, 7 5 va 357 a u g . 12 5 v» 307 1945, sept, 2 6 va 358 a u g . 12 6 . ò " 308 1946, a p r . 12 5 va 359 a u g . 12 5 . 5 30!) a p r . 16 5 . 5 360 aug. 13 5 »/» 310 1947, m a r c h 21 5 v, 361 s e p t . 5 5 . 8 311 a p r . 12 5 . 7 362 s e p t , 14 5 . 7 312 j u n e 1 5 va 363 oct. 10 5 va 313 j u l y 21 5 . 5 364 oct. 21 5 . 7 314 aug. 30 6 . 3 365 oct. 21 6 . 5 315 oct. 6 7 . 0 366 n o v . 28 5 v316 n o v . 29 5 va 367 dee, 28 5 va 317 dee. 9 5 . 5 368 1954, j a n . 2 5 72 318 1948, m a r c h 29 5 va 369 march 8 5 va 319 march 29 5 . 9 " 370 a p r . 17 5 va 320 a p r . 22 6 . 4 371 a p r . 30 7 . 0 321 m a y 26 5 . 5 372 may 1 5 va 322 j u n e 30 6 . 4 373 1954, may 3 5 va 323 oct. 10 5 . 8 374 may 4 5 . 7 324 oct. 18 5 . 5 375 may 4 5 . 6 325 1949, j a n . 4 5 . 7 376 may 25 5 . 7 326 j u n e 17 6 . 7 377 j u l y 18 5 va 327 j u l y 23 6 . 8 378 a ug. 3 6 . 0 328 1950, s e p t . 23 5 */« 379 dee. 23 5 . 9 329 1951 , a p r . 5 6. 1 380 1955, j a n . 3 5 . 8 330 a u g . 24 5 . 5 381 m a r c h 28 5 74 331 a u g . 31 5 382 a p r . 13 6 . 0 332 a u g . 31 5 . 6 383 apr. 19 « 7 i 333 oct. 1 5 7a 384 a p r . 21 6 . 0 334 1952, m a r c h 19 5 3/., 385 j u n e 2 5 va 335 oct. 5 5 3/4 386 j u l y 9 5 va 336 oct. 10 5 v, 387 j u l y 16 6 . 8 337 oct. 13 5 7* 388 1956, j a n . 6 5 . 7 338 dee. 31 •r> 74 389 m a y 15 5 . 6 339 1952, dee. 31 5 ^4 390 may 15 5 . 8 340 1953, j a n . 7 5 . 5 391 may 18 5 . 8 341 f e b r . 7 6 . 0 392 j u l y 9 7 7a 342 m a r c h 18 7 . 2 393 j u l y 9 0 3/4 343 march 18 5 . 6 394 j u l y 9 5 . 7 344 m a y 2 5 va 395 j u l y 9 5 . 0 345 j u n e 3 5 3u 390 j u l y 9 5 va 346 j u n e 13 5 v2 397 j u l y 9 5 . 5 347 j u n e 18 5 v2 398 j u l y 10 5 7 4 348 j u l y 22 5 72 399 j u l y 10 5 v4 349 a u g . 9 6 . 5 400 j u l y 22 5 . 5 350 a u g . 11 6 . 8 401 j u l y 30 5 . 6 351 a u g . 11 5 72 402 j u l y 30 6 . 0 352 a u g . 11 5 72 403 j u l y 30 5 . 6 353 a u g . 12 5 va 404 a u g . 16 5 . 5 354 a u g . 12 7 . 2 405 sept, s e p t . 6 5 . 7 355 aug. 12 5 . 6 406 sept, s e p t . 16 5 va 4 0 6 a . ct. g a l a n o p o u l o s t a b l e i (cont.) no d a t e m a g n i t u d e a d o p t e d no d a t e m a g n i t u d e a d o p t e d 407 1956, n o v . 2 5 74 431 1958, j u n e 5 5 v a 408 n o v . 20 5 v . 432 j u l y 15 5 72 409 1957, j a n . 23 5 7 2 433 j u l y 17 5 7 4 410 f e b r . 19 6 . 0 434 a u g . 27 6 . 5 411 m a r c h 8 0 72 435 s e p t . 2 5 v 2 412 m a r c h 8 6 74 436 s e p t . 4 5 v i 413 m a r c h 8 5 . 6 437 1959, a p r . 25 6 74 414 m a r c h 8 6 . 0 438 a p r . 25 5 . 6 415 m a r c h 28 5 . 7 439 may 14 « v a 416 a p r . 26 6 . 2 440 j u n e 10 5 74 417 m a y 21 5 . 7 441 a u g . 16 5 v a 41s m a y 29 5 . 5 442 a u g . 17 6 . 0 419 oct. 5 5 72 443 s e p t . 1 6 v 4 420 oct. 30 5 . 7 444 oct. 5 5 v a 421 oct. 30 5 74 445 oct. 7 5 74 7 . 0 422 n o v . 26 5 . 7 446 n o v . 15 5 74 7 . 0 423 n o v . 26 5 . 6 447 n o v . 19 5 v a 424 n o v . 27 5 . 9 448 dee. 1 5 3/4 425 1958, j a n . 2 5 . 8 449 1960, f e b r . 23 5 v a 426 j a n . 16 5 . 7 450 m a r c h 12 5 . 6 427 m a r c h 15 5 . 6 451 m a y 26 6 v a 428 a p r . 3 5 74 452 j u l y 13 5 v a 429 m a y 3 5 72 453 1961, oct. 2 5 v a 430 m a y 9 5 74 454 1962, j a n . 26 6 v « of magnitude 8 a n d over oceurred in t h e area of greece are of intermediate focal depth. the regression equations for shallow a n d intermedi a t e sliocks defìne two curves crossing each other a t m = 71/». using t h e d a t a given in table i and i i and applying t h e relations: log e1'* = 5 . 9 + 0 . 7 5 m , a n d s «a se1/» , where e is the seismic wave energy, m tlie e a r t h q u a k e m a g n i t u d e and s t h e strain release in units io11 (ergs)1^, we fìnd t h a t the average strain release per shallow and intermediate earthquake, i.e. 8/n, for t h e period considered, amounts to 0 . 4 and 2 . 1 , respectively. f o r t h e 60-years interval, 1903-1962, the corresponding values are 0 . 3 and 1 . 0 . thus t h e average strain release per e a r t h q u a k e of intermediate foca\ depth is 3 to 5 times higher t h a n t h a t per shallow e a r t h q u a k e . e v i d e n c e f o r t i i e s e a t o f t h e s t r a i n p r o d u c i no f o r c e s 405 t a b l e i i c a t a l o g u e o f e a r t h q u a k e s o f m a g n i t u d e 5 > 5 1 / 2 w i t h f o c a l d e p t i l > 4 0 k m o c c u r r e d i n t i i e a r e a o f g r e e c e d u r i n g t h e p e r i o d 1 8 4 3 1 9 6 2 . no d a t e m a g n i t u d e a d o p t e d no d a t e m a g n i t u d e a d o p t e d 1 1846, march 28 8 . 1 38 1935, f e b r . 25 6 7 , 2 1856, oct. 12 8 . 6 39 march 18 6 v4 3 1862, j u n e 21 6 . 8 40 1936, a p r . 28 5 3u 4 1863, a p r . 22 8 . 5 41 a u g . 8 5 3 j 4 5 1867, f e b r . 4 7 . 9 42 1937, d e e . 16 6 v6 s e p t . 20 7 . 6 43 1938, j a n . 16 5 va 7 1869, a p r . 18 6 . 9 44 j u n e 3 5 3 ! 4 8 1886, a u g . 27 8 . 4 45 s e p t . 18 6 va 9 1887, j u l y 17 7 . 7 46 1939, sept, 20 6 va 10 1897, may 28 7 . 6 47 1942, may 9 5 3/4 11 1903, a u g . 11 8 . 3 48 m a y 21 5 7 a 12 1904, a u g . 11 8 . 0 49 j u n e 21 6 v4 13 1908, may 17 6 3/4 50 1943, j a n . 7 5 va 14 1910, f e b r . 18 7 0 51 f e b r . 14 6 0 15 a u g . 21 « va 52 j u n e 27 5 74 16 1911, a p r . 4 7 . 0 53 oct. 16 6 v, 17 1912, j a n . 24 6 3u 54 1944, j a n . 5 5 . 7 18 1913, s e p t . 30 5 3u 55 may 27 6 v 4 19 1918, j u l y 16 6 va 56 a u g . 9 5 va 20 1922, a u g . 13 6 3u 57 1946, a p r . 5 6 0 21 n o v . 11 5 . 7 58 1947 , j u n e 4 6 0 22 1923, a u g . 1 6 . 7 59 j u l y 7 5 va 23 1925, j uly 6 0 va 60 1948, f e b r . 9 7 . 1 24 1926, j u n e 26 8 . 3 61 j u l y 24 6 va 25 j u l y 5 5 va 62 s e p t . 11 6 5 26 a u g . 30 7 . 0 63 1952, dee. 17 6 3 / 4 27 s e p t . 19 6 !/ ° /4 64 1953, f e b r . 14 5 3 ! 4 28 1927, j u l y 1 6 . 9 65 j u n e 23 5 3u 29 1929, m a r c h 27 e 3 / ° li 66 1957, a p r . 24 7 0 30 n o v . i i 5 . 7 67 a p r . 25 7 v 4 31 1930, f e b r . 14 6 3 ! 4 08 1958, j u n e 30 6 va 32 m a r c h 6 5 7 . 69 m a y 27 5 . 6 33 march 6 6 . 0 70 n o v . 15 5 74 34 1931, j u n e 30 5 va 71 1960, n o v . 4 5 . 8 35 1932, a u g . 15 5 va 72 n o v . 11 5 . 7 36 1933, a p r . 23 6 3 / 4 73 1961, mav 23 6 va 37 1934, n o v . 9 6 3 / 4 74 n o v . 28 5 va d i s c u s s i o n . a s s u m i n g w i t h b à t h a n d d u d a (1964) t h a t " the mairi difference between large and small earthquakes is not to be found in the strain but 400 a . g . g a l a n o p o u l o s in the total volumes involved ", it might be possible to t h i n k t h a t the higher average strain release per intermediate e a r t h q u a k e is due to the larger ability of deformation of t h e low-rigidity layer of t h e u p p e r mantle. however, eonsidering t h a t a large a m o u n t of deformation in weak layers is rei i e ve d by plastic flow, i.e. t h a t the possibilities of strain storage in the upper m a n t l e are not very good (bàth and d u d a , 1963/iii), t h e pronounced m a x i m u m of average strain release per e a r t h q u a k e a t 50 to 150 k m focal depths might be due to a combination of stronger strain accumulation and greater seismic gain ratio. the greater " seismic gain ratio " , i.e. tlie greater ratio between seismic energy a n d elast-ic strain energy in the upper mantle, is evidenced by the very small n u m b e r of aftershocks. owing to tlie lower breaking strength of t h e lowvelocity layer of t h e upper hiantle t h e strain àccumulated in a very large volume is probably completely relieved in almost one e a r t h q u a k e . a stronger strain accumulation a t 50 to 150 k m depths combined with a smaller ability of strain s t o r a g e implies t h a t t h e seat of t h e strain producing forces is in t h e upper m a n t l e . i t is reasoned therefore t h a t eonvection currents are probably t h e prevailing agent in producing a strained region in the e a r t h ' s interior. this evidence is fairly corroborated b y t h e f a c t t h a t in greece t h e perioda of greater seismic activity are initiated b y intermediate shocks, i.e. t h e seismic activity in t h e area of greece is induced b y processes occurring under the e a r t h ' s crust (galanopoulos, 1956). l a t e r on blot (1963) was led to t h e sanie conclusion for other regions of t h e globe where intermediate a n d deep foci exist: southern pacific, america, j a p a n , indonesia, i n d o k u s h . . . according to blot " the earthquakes of very grcat magnitude are preceded by deep earthquakes of a magnitude superior to 7, or by several deep earthquakes of a lesser magnitude ". recently l. don leet a n d florence j . leet (1965), based 011 other d a t a were led with a different reasoning to believe t h a t " the fundamental cause of earthquakes is movement in the soliqucous mantle ", and even more t h a t " ivhen computational procedures are furtlier refined, it may turn out that ali earthquakes are basically mantle raptures, even when some displacements are coupled to the surface ". benioff (1951b) has already p u t f o r w a r d evidence t h a t e a r t h q u a k e s of magnitude exceeding 8, m a y not be entirely independent events, b u t m a y be related in some forni of global stress-system. this suggestion is v e r y well understood in t h e assumption t h a t t h e seat of t h e strain producing forces is in t h e upper mantle. e v i d e n c e f o r t i i e s e a t of t h e str ai n p r o d u c i no f o r c e s 405 a c k n o w l e d g m e n t s . t h e r e s e a r c h r e p o r t e d i n t h i s d o c u m e n t h a s b e e n s p o n s o r e d b y t h e «air f o r c e office of s c i e n t i f i c r e s e a r c h » u n d e r c o n t r a c t a f (>l(052)-803 t h r o u g h t h e « e u r o p e a n office of a e r o s p a c e r e s e a r c h ( o a r ) », « u n i t e d s t a t e s a i r f o r c e », a s p a r t of t h e a d v a n c e d r e s e a r c h p r o j e c t s a g e n c y ' s p r o j e c t vela uniform. t h e a u t h o r w o u l d l i k e t o e x p r e s s b i s s i n c e r e t h a n k s t o d r . b . c. p a p a z a c h o s f o r r e a d i n g c r i t i c a l l y t h e m a n u s c r i p t of t h i s p a p e r . r e f e r e n c e s b a t h m . a n d d u d a s. j . , earthquake volume, fault piane area, seismic energy, strain, deformation and related quantities, " a n n a l i di geofìsica " , 17, 353-368, (1964). b a t i i m . a n d d u d a s. j . , strain release in relation to forni depth, " geofìsica p u r a e a p p l i c a t a " , 56, 93-100, ( 1 9 6 3 / i i i ) . b e n i o f f f . , earthquakes and rock creep, p a r t 1: creep gharacteristics of rocks and the origin of aftershoclcs, " bull. seism. soc. a m e r . " , 41, 31-62, (1951a). b e n i o f f i i . , global strain accunmlation atul release as revealed by great earthquakes, " bull. geol. soc. ara. " , 62, 331-338, (1951b). b l o t c . , deep origin of shallow earthquakes and volcanic eruptions. abs t r a c t s u b m i t t e d t o t h e iijgg general a s s e m b l y in berkeley, 19-31, a u g u s t 1963. d o n l e e t l . a n d f l o r e n c e j . l e e t , the earth's mantle, " bull. seism. soc. ani. " , 55, 3, 619-625, (1965). g a l a n o p o u l o s a., the seismic efficiency of greece, " p r a k t . of a t h e n s acad e m y " 31, 368-375, (1956). k a r n i k v . , seismicity of europe. p r o g r e s s r e p o r t i v p r e p a r e d for t h e c.onference of t h e e u r o p e a n seismological commission in s e p t e m b e r 1964. m i m e o g r a p h e d , 1-22 (tokio, 1964). p r e s s f . , displacements, strains, and tilts at teleseismic distances. " j o u r . g e o p h . r e s . " , 70, 10, 2395-2412, (1965). u t s u t. a n d s e k i a., relation between the area of the aftershock region and the energy of the mairi shock, " j o u r . seism. soc. j a p a n " , ser. i i , 7, 233-240, (1955). 48,06,2005misc 923 annals of geophysics, vol. 48, n. 6, december 2005 key words historical seismicity – velocity propagation model – joint hypocentral determination 1. introduction in this paper i will show that when enough seismic recordings exist it is possible to compute an instrumental location of major earthquakes. the computation is carried out on an area of midhigh seismicity (garfagnana, northern tuscany, box with dashes in fig. 3) and could be conducted on any other area of similar characteristics; the results are used for a discussion on the role of the instrumental data of historical earthquakes. the role of instrumental versus macroseismic locations for earthquakes of the last century: a discussion based on the seismicity of the north-western apennines (italy) stefano solarino ingv-cnt c/o dipartimento studio del territorio e delle sue risorse (dipteris), università degli studi di genova, italy abstract many seismological observatories began to record and store seismic events in the early years of the twentieth century, contributing to the compilation of very valued databases of both phase pickings and waveforms. however, despite the availability of the instrumental data for some of the events of the last century, an instrumental location for these earthquakes is not always computed; moreover, when available, the macroseismic location is strongly preferred even if the number of points that have been used for it is low or the spatial distribution of the observations is not optimal or homogeneous. in this work i show how i computed an instrumental location for 19 events which occurred in the garfagnana-lunigiana region (northern tuscany, italy) beginning from 1902. the location routine is based on a joint hypocentral determination in which, starting from a group of master events, the systematic errors that may affect the data are summed up in the corrective factors complementing the velocity propagation model. all non-systematic errors are carefully checked and possibly discarded by going back to the original data, if necessary. the location is then performed using the classic approach of the inverse problem and solved iteratively. the obtained locations are then compared to those already available from other macroseismic studies with the aim to check the role to be attributed to the instrumental locations. the study shows that in most cases the locations match, in particular when considering the different significance of the location parameters, especially for the strongest events: the instrumental location provides the point where the rupture begins, while the macroseismic one is an estimate of the area where the earthquake possibly took place. this paper is not meant to discuss the importance and the necessity of macroseismic data; instead, the aim is to show that instrumental data can be used to obtain locations even for older seismic events, without any intention to define which location is better or more reliable. mailing address: dr. stefano solarino, ingv-cnt c/o dipartimento studio del territorio e delle sue risorse (dipteris), università degli studi di genova, v.le benedetto xv 5, 16132 genova, italy; e-mail: peter@dipteris.unige.it 924 stefano solarino many efforts have been made in the recent past to recover the historical seismicity of the italian peninsula with the aim to distinguish hazardous areas through the appropriate algorithm. the goal of designing a map of hazard for the italy has been an impressive incentive in recent decades and has led to the compilation of several parametric catalogues (camassi and stucchi, 1997; gruppo di lavoro cpti, 1999; boschi et al., 2000; gruppo di lavoro cpti, 2004). these catalogues differ for both period of coverage and number of events included, but they are especially distinct in the philosophy that guided their compilation. cft (boschi et al., 2000) is based on macroseismic data and locations only, contains 605 seismic events selected on the basis of the minimum intensity threshold of vii and contains earthquakes which occurred in the period 461 b.c. to 1997. conversely, nt4.1.1, (camassi and stucchi, 1997) contains all earthquakes exceeding io = = 5/6 or ms = 4.0 in the period 1000-1980, and this sums up to 2421 records. the compilation was then enlarged to cover the period 19811992. the catalogue is mainly based on macroseismic data, a few of which are novel, but it also contains several locations whose parameters have been previously published in other catalogues; the macroseismic data for these events are not always available. in particular, one catalogue that has been extensively used for merging data in nt4.1.1 is that published by postpischl (1985), which also contained instrumental locations. nt4.1.1 is probably the most flexible and widely used historical catalogue in italy and is thus adopted in this work as a reference. in this frame, it is especially important to emphasize that the records extracted from other catalogues and included in nt4.1.1 are indicated as cp: this code actually means that it is no straightforward to determine where the parameters come from (macroseismic or instrumental location) and especially that there is no direct information of the quality of these parameters. in the next paragraphs i will show that this lack biases the comparison, since more than 50% of the resulting instrumental locations that do not match with the existing ones are marked cp. finally, cpti is the result of a joint project to merge and recompute part of the mismatching locations of the mentioned (cft and nt) catalogues: it has been recently updated to version cpti04 (gruppo di lavoro cpti, 2004). all these inventories cover almost the whole of human history and, being updated to very recent years, they overlap the period in which many seismological instruments were already operating. as an example, in fig. 1 i show the number of installed instruments versus years in the period 1900-1950 as derived from the results of the tromos (http://storing.ingv.it/tromos) project. the histogram does not show two important features: stations were almost homogeneously spread over the italian peninsula and a few of them never interrupted they recording activity, even if performed by updated and more modern instruments. figure 3, even if only showing the position of the stations used in this work, gives a frame on the distribution of historical observatories. more information can be retrieved from the tromos web pages. the existence of several seismic instruments provided good waveforms for the main earthquakes of the century; however, despite fig. 1. stations operating in italy in the period 19001950 (source tromos). since the project made an inventory of observatories established before 1940, the histogram is updated to that time. the last bars of the plot takes into account only the stations that still existed in 1950 and does not include those established in the decade 1940-1950. the role of instrumental versus macroseismic locations for the seismicity of the last century in the nw apennines the richness of the database, an instrumental location for these earthquakes is not always computed and when a macroseismic one is available it is strongly preferred, even when based on a poor and or not optimal distribution of the localities. in theory, it is more correct to input, in whatever computation, data obtained with the same means or obtained applying similar methodology, that is in this case to always use macroseismic data for homogeneity. but catalogues are already «biased» by a few instrumental locations as discussed above, so it does not seem so difficult to use alternative locations especially if they prove to be of acceptable quality. the study is not meant to discuss the acknowledged importance and the necessity of macroseismic location; instead, the aim is to show that instrumental data can be used to obtain locations even for older seismic events, without any intention to define which location is better or more reliable. in fact, throughout the paper the corresponding macroseismic locations will be used as a reference, and the discussion will be based on the differences between epicentral position rather than on quality of data and result. in particular, i introduce the basics of the method, i make a short description of the main problems encountered in assembling the data and finally i discuss and compare the obtained locations to try to find out which is the role of instrumental data for less recent earthquakes. 2. towards an instrumental location: joint hypocentral determination the methodology applied in this work (solarino et al., 1996) has been extensively described in solarino (2002). it is based on the concept that it is possible to account for all systematic errors that may interfere with the location of an earthquake by computing corrective factors in conjunction with the model used for locations; all non-systematic errors may instead be discarded by checking the single datum. for example, site effects or non perfect knowledge of the position of the instrument can be included in the correction factor, which will bias all data with the same importance, while a picking error can be avoided by a careful check of the single datum, especially if original waveforms are still available. this part of the work proves to be the most delicate, due to the difficulties in finding original seismic bulletins, digitized or on paper waveforms, original scripts by seismologists running the instruments. in a few cases data that would have represented a potential source of error were finally recovered by going back to the original seismogram. one of the most important aspects in this frame is that of the time synchronization. it must be taken into account that the technology of clocks was very similar for all seismic observatories, although provided by different brands, as happens today. it is then reasonable to believe that the performances (and thus the errors) of these timing device were very similar. data synchronized with these machineries can then be considered comparable. it is also well known that in the early days of seismology scientists used mechanical clocks, the drift of which was corrected at established intervals using more sophisticated radio timing systems, and ensuring a more accurate absolute time. in case of major earthquakes, the phase picking was corrected to take into account the drift. sometimes this correction was directly annotated on the seismogram, sometimes it was annotated apart, but in general the published and shared data was already corrected for the drift. however, in some cases, different phase pickings are found on diverse bulletins, perhaps reporting corrected and non-corrected data respectively: in such cases either it is possible to realize which is the correct timing or the reading must be discarded. finally, since many location programs use s-p times, for this kind of data the correctness of absolute time does not really matter provided that no malfunctioning occurs during the recording of the event. while random errors can be avoided by careful control of the data, systematic errors can be included in the computation by applying a joint hypocentral determination (jhd thereinafter) technique, the aim of which, as stated above, is to sum up all errors in a corrective factor. as known, the jhd uses a master event to check the compatibility of its location with the rest of the data. in this case, the jhd technique 925 926 stefano solarino has been based on a group of very recent and well located events that acted as masters in successive steps. the starting point of the whole routine consisted in the computation of a very reliable velocity propagation model using a dataset of recent events (01/1999-12/2001, fig. 2); the computation of this propagation model, later indicated as «reference model», has been widely described by ellsworth (1977) and kissling (1988). the 1d model results from a combined, simultaneous inversion where the inverse problem is solved by the full inversion of the damped least squares matrix ata + λ. as the inverse problem is non-linear, the solution is obtained iteratively, where one iteration consists of solving both the complete forward problem and complete inverse problem once. velest (kissling et al., 1995), the code that performs the computation, can also be run in single event mode when only the location problem is carried out. in this case, an additional singular value decomposition of the symmetric matrix ata is performed to calculate the eigenvalues. as known, this way of treating the inverse problem provides supplementary information on the resolution: they will not be discussed in detail in this paper, but supplemental information can be found in solarino (2002). at the end of the computation, different models with about the same residual variance and location precision can usually be obtained from the same data set. the model that coincides best with the surface geology and a priori information on the nearsurface structure has to be chosen as the final model, and will be called «minimum» (kisfig. 2. earthquakes used for the computation of the reference model applied in this work. they cover the period 1999-2001. the role of instrumental versus macroseismic locations for the seismicity of the last century in the nw apennines sling, 1988) to indicate that it provides minimum average (rms) values for all earthquakes used in the inversion. additionally, relocation of known sources (shots, blasts) facilitates the choice of the best model and gives hints on its reliability. the 1d model resulting from the computation with velest (fig. 3, left panel) is accompanied by p and s station corrections that are first calculated only for the stations that are still operating and that contributed to the location of the master events, while those relative to seismic ob927 fig. 3. velocity propagation model (left panel) and position of stations used for this study. for each station, the correction factor is reported (circle for negative values, triangle for positive; size is proportional to the amount) as after the computation process resumed in table i. the distribution of delays is close to the optimal, being very nicely distinct the positive and negative areas (kissling et al., 1995). the box with dashes shows the approximate extension of the lunigiana-garfagnana area. table i. evolution of the computation of delays for the seismic observatories the readings of which are used in this work. at increasing steps the number of known stations corrections (in percentage with respect to the total number) expands, and even observatories that no longer operate can be taken into account. step 1973 1972 1968 1965 1961 1959 1951 1939 1934 1931 1928 1928 1927 1926 1921 1920 1902 1902 1 72% 75% 50% 40% 50% 50% 50% 40% 42% 62% 28% 20% 20% 40% 25% 20% 40% 40% 2 100% 100% 66% 60% 75% 75% 66% 40% 42% 62% 28% 20% 20% 40% 50% 33% 40% 40% 3 100% 100% 100%100%100% 75% 83% 40% 50% 62% 28% 20% 20% 40% 75% 33% 40% 40% 4 100% 100% 100%100%100%100% 100% 40% 58% 62% 28% 20% 30% 40% 75% 33% 60% 40% 5 100% 100% 100%100%100%100% 100% 100% 58% 62% 58% 20% 40% 60% 75% 44% 60% 40% 6 100% 100% 100%100%100%100% 100% 100%100% 100% 72% 40% 80% 100% 75% 66% 60% 40% 7 100% 100% 100%100%100%100% 100% 100%100% 100%100%100% 90% 100% 75% 77% 80% 60% 928 stefano solarino servatories that are no longer operating or have run for a limited amount of time are adjoined and adjusted in the following steps. basically, each step consists of a selection of older events, of the location with data for which station corrections are known and eventually of letting unknown station corrections free to float to compute relative delays. in detail, the latter operation consists in keeping the computed propagation model and fix the master locations (whose number is increased at each run with more events) and hence let the program compute, sum up and attribute delays to stations for which readings are present but have not been used for location (kissling et al., 1995). in practice, station corrections are always computed with respect to master events, and the number of known station corrections increases at each step, being computed also for older or no longer operating stations. table i reports in a very schematic way the evolution of station delays the readings of which are included in each event discussed in this paper. at the beginning of the process, almost nothing is known for some events. the successive steps are used to compute the diverse station delays until a correct location can be made. at the end of this routine, the model is complete with all station delays (fig. 3) and can be used for the location of the «historical» events of the garfagnana area. 3. data and results apart from the limitation of the availability of instrumental data, in theory no constraints act in selecting the data to be relocated. in practice there are several limitations that must be taken into account. first, the events must be included in some other (macroseismic) catalogue for comparison. this is not going to be necessary in a future application, but it is of paramount importance to test the results, and cannot be neglected at this stage. second, events must have at least 4 usable phase pickings that in some cases may mean 6 to 7 readings to be conservative enough to consider mistakes, picking errors and non-matching information. third, there must exist more than one source for the data to cross-check the entries, as mentioned in the last paragraph. by combining all these aspects, it turned out that the best selection should take into account events which occurred later than 1900, that are included in the nt4.1.1 catalogue (the most complete) and whose data are reported on two or more of the sources quoted in appendix a. it is noteworthy that some of the references reported in appendix a are compilations, and should then be comprehensive of all data available for an event as collected from the various bulletins and observatories. the careful cross checking between these compilations and some other original data showed that in a few cases the compilations contain inconsistencies and differences, and this is another important issue towards the use of more sources. the initial selection of 56 seismic events (as extracted from nt4.1.1 for the area 43.6-44.6 and 9.1-10.5) was dramatically reduced to 30 when looking for the multiplicity of sources but the number dropped to 19 when considering the minimum number of readings required for locations upon careful selection of the blunder-free data (the compiled phase readings for these data are reported in appendix b). of course, this aspect diminishes the potential of the instrumental locations versus the macroseismic ones, it being possible in this case only for fewer than 40% of the events, and will be discussed in the final paragraph. the characteristics of the selected events are reported in table ii (second column from left); their location properties are taken from the nt4.1.1 catalogue. after checking for data completeness on all available sources, events have been located applying velest in single event mode; the final locations are reported in table ii together with the relevant location errors, while table iii shows a comparison of the obtained locations with the original positions. the new locations are compared with the macroseismic ones in fig. 4, where it is possible to see at a glance how far the epicentres have been shifted: the vectors connecting each two points indicate the change in position from macroseismic to instrumental and show very different lengths for the diverse couples. the figure also shows that the original locations were sometimes very clustered (see for example events 08/1902, 1920, 1968, 1961, 1939). the role of instrumental versus macroseismic locations for the seismicity of the last century in the nw apennines the characteristics of the newly obtained locations are reported in table ii: the depth, marked by bold characters, has been computed for more than 50% of the earthquakes. at first glance, the obtained distribution of earthquakes seems compatible with the knowledge of the recent seismicity of the area (solarino et al., 2002) and with the proposed position for the main seismogenic structures of the area (e.g., the garfagnana-north fault, valensise and pantosti, 2001); moreover, the less clustered aspect of the seismicity seems less «artificial» than the macroseismic one. the errors associated with the new locations (erx, ery, erz) are on average less than 5 km, and the overall rms is sometimes less than 1 s. the usage of the standard errors in the hypo format may appear inadequate or incomplete to describe the uncertainties of the final locations and, as shown in solarino (2002), could be substituted by a less compact form by showing the full resolution matrix or its diagonal elements. however, at this stage the errors are only used to distinguish between locatable and nonlocatable events and to give a rough idea about the final quality and reliability of the results and in such an attempt the standard errors are enough. finally, the gap shows that for some events the station coverage is very good, while in a few cases it exceeds 200°. the analysis of the differences between locations (table iii, difference between latitudes and between longitudes in km) shows three different trends. there are seismic events (05/03/1902, 07/09/1920, 1931, 1939, 1951, 1959, 1972) for which the differences between the two location methods do not exceed 16 km for each coordi929 table ii. index of the events relocated in this work. the list is the final result of a selection done in the area 43.6-44.6 and 9.1-10.5 after checking for multiplicity of data sources and minimum number of phase pickings (see text). the locations parameters of the second column are those of the nt4.1.1 (camassi and stucchi, 1997) catalogue. the remaining columns are relative to instrumental locations and report the position, the depth and the errors associated to the locations. nc is used for depths that are not adequately constrained. date location as instrumental depth rms erx ery erz gap in nt4.1.1 catalogue location 05/03/1902 44.10n, 10.46e 44.16n, 10.31e nc 0.92 1.3 3.5 2.3 281 04/08/1902 44.20n, 10.20e 44.09n, 11.12e nc 1.66 0.9 3.7 2.6 246 27/10/1914 44.05n, 10.45e 44.38n, 10.82e 4.5 0.65 4.4 4.8 3.3 115 07/09/1920 44.20n, 10.20e 44.25n, 10.14e 4.3 1.70 3.6 4.6 7.1 121 29/11/1921 44.50n, 9.80e 44.76n, 10.78e nc 0.01 3.6 3.8 1.6 163 18/11/1926 44.30n, 10.00e 44.80n, 9.82e nc 2.88 3.7 4.3 1.7 113 28/10/1927 44.53n, 9.53e 44.73n, 9.84e 11.7 1.80 3.1 3.1 4.4 81 20/07/1928 44.50n, 9.61e 44.72n, 10.06e 5.9 0.89 3.1 3.2 4.1 99 03/08/1928 44.20n, 10.20e 44.50n, 11.03e 16.6 1.31 3.5 2.9 3.9 132 25/01/1931 44.25n, 10.10e 44.28n, 10.02e 5.5 1.10 3.2 3.2 0.0 161 13/06/1934 44.48n, 9.80e 44.51n, 10.52e nc 1.27 3.3 2.4 4.5 109 15/10/1939 44.16n, 10.23e 44.30n, 10.43e 42.2 1.40 4.2 4.2 3.1 143 12/08/1951 44.06n, 10.48e 44.09n, 10.56e 0.2 0.10 3.9 4.3 1.4 199 26/01/1959 44.50n, 9.50e 44.60n, 9.47e nc 0.56 2.8 3.9 3.0 118 03/08/1961 44.20n, 10.20e 43.98n, 10.25e 37.6 1.28 3.5 4.1 3.6 157 09/11/1965 44.45n, 10.30e 44.43n, 10.74e 35.0 0.19 4.4 4.3 3.7 153 07/06/1968 44.10n, 10.20e 44.58n, 10.81e 21.4 0.15 3.5 4.2 0.6 221 25/10/1972 44.41n, 9.91e 44.39n, 9.84e 39.5 0.20 4.1 4.7 4.6 167 05/06/1973 44.51n, 9.56e 44.27n, 9.30e nc 0.12 2.7 4.9 3.8 169 930 stefano solarino nate. the choice of this reference value is somewhat arbitrary, but it represents a good compromise between the errors associated with the instrumental location and the uncertainty associated with the macroseismic one. it is then reasonable to say that the two locations match, if one considers that the instrumental location gives the point of rupture while the macroseismic one is more an average of the area the rupture started from. this statement is especially true for events exceeding m 5.5 (gasperini and ferrari, 2000), but the most widely shared definition of the macroseismic epicenter («the baricentre of the area featuring the maximum effects», gasperini and ferrari, 2000) involves a similar bias for all magnitude levels, though of course in proportion to them, since the maximum effects may depend upon several factors (site effects, wrong or inaccurate description, distribution of the localities). on average, the number of points used for the macroseismic locations of these events is large, say greater than 30, except for a couple of them that directly come from other sources (cp). a second group contains events that do not match: their differences not only exceed 16 km in one or both coordinates, but at least for one can be up to 80 km, which cannot be simply explained by the diverse meaning of the location parameters. these are the 08/1902, 1921, 1926, 1927, 07/1928, 08/1928, 1934, 1961, 1965, 1968, 1973 events. almost all of them have been taken from other catalogues (cp), while those based on macroseismic observations have been located with 10 (1921), 13 (1928), 13 (1965), 29 (1934) and 39 (1927) localities. this second group is probably much more difficult to locate using macroseismic data with the currently available information; on average they have a lower magnitude (down to 3.7). moreover, the original catalogue which contained those for which macroseismic data are not available (cp) was probably not accurate enough. it is important to emphasize that, if the locations proposed in this work are reliable, some of these earthquakes actually occurred far from where they have been positioned, and this may have an impact on the microzonation and hazard computation. i discuss for example the event which occurred in 1934. its original location has been obtained using 29 macroseismic observations (monachesi and stucchi, 1997), which is rather a good number, positioned as in fig. 5. it is evident that there is a very uneven distribution of localities and that they are especially lacking on the eastern side, which can have biased the macroseismic location (grey star) towards the western area. conversely, the instrumental location (black star in the figure) moves the epicenter to the north-east, that is in the same direction where macroseismic data are most lacking. if i accept that the macroseismic location may be biased by the poor coverage on the eastern side and i take into account the location errors of the instrumental determination, all this would render plausible a shift of the lotable iii. absolute differences (in km) between the latitude and the longitude of the macroseismic and instrumental locations. bold numbers are used when the differences exceeds 16 km for any of the values. the magnitude is taken from the nt4.1.1 catalogue. the last column of the table displays the number of points of intensity available for the macroseismic location. cp indicates the events extracted from other catalogues: for these events no quality information can be derived. date ms/ix diff. (ia−io), km notes 05/03/1902 5.0/7 7−12 76 04/08/1902 5.0/7 11−−72 cp 27/10/1914 5.8/7 37−−29 588 07/09/1920 6.5/10 6−4 454 29/11/1921 4.6/5 30−−77 10 18/11/1926 4.2/5.5 55−−14 cp 28/10/1927 4.8/6 22−−24 39 20/07/1928 3.7/6 25−−35 13 03/08/1928 4.2/5.5 34−−80 cp 25/01/1931 4.0/6 4−6 cp 13/06/1934 4.9/6 8−−56 29 15/10/1939 4.9/7 16−15 56 12/08/1951 4.5/5.5 3−6 18 26/01/1959 4.2/5.5 12−2 cp 03/08/1961 4.4/6 24−−4 cp 09/11/1965 4.8/5 1−−34 13 07/06/1968 4.3/nr 53−−48 ? db 25/10/1972 4.7/5 3−6 186 05/06/1973 4.4/4 26−−20 cp the role of instrumental versus macroseismic locations for the seismicity of the last century in the nw apennines 931 fig. 5. intensities for the 1934 event (dom, monachesi and stucchi, 1997). the grey star shows the macroseismic location, the black one marks the instrumental one proposed in this study. fig. 4. vectors connecting the original macroseismic location with the instrumental one as obtained in this work. 932 stefano solarino cation of the earthquake towards the east, that would mean on the other side of the watershed of the apennines. the third group is made up of one event only (1914): its location consistently differs from the macroseismic one but, the latter being obtained by more than 500 localities, it is reasonable to believe that instrumental data are not able to constrain the solution adequately. this is shown by both the location errors associated with this hypocenter (they are among the highest) and by the resolution matrix (not reported), whose diagonal elements are very far from optimal (0.9604, 0.9584, 0.9525 and 0.8055 respectively for origin time, x, y and z coordinates). the presented analysis, although not exhaustive, gives a rough estimate of the potential and limitations of both data and method and thus represents a good starting point for the discussion on the function of instrumental locations. 4. discussion the obtained instrumental locations proved to be very similar, in many cases, to those proposed using macroseismic data. moreover, it is always possible to define how reliable they are by looking at the associated location errors (within the limitations that these errors may introduce) or at the resolution matrices, and finally they may provide information on the depth of the events, which cannot be easily derived with macroseismic data, although several methodologies have been proposed to infer depth from intensity observations. provided thus that, under particular conditions, it is possible to obtain an instrumental location for older earthquakes, it is important to define the limits of their usage, that is to define what their role is. it is evident that instrumental locations cannot substitute macroseismic locations as they do not completely overlap the recent part of the parametric catalogues: instrumental data are largely available only since the sixties, while as stated above they lack for the first decades of the twentieth century, except for very large earthquakes. but instrumental data give a much better chance to distinguish different events in seismic sequences, which is not required in the macroseismic database (it is selected to avoid clusters that would bias many of the applications which it is used for) but may be helpful in other studies (seismic cycles, seismotectonics and so on). for example, the event of september 1920 was preceded the day before by an important shock that is not included in the published macroseimic catalogues but was recorded by many seismographic stations: how did this first shock affect the macroseismic observations of the following large event? could this important event be neglected when studying the way energy is released in the area? in this frame, it is very important to have at least a rough estimate of the depth of the earthquake, which can emerge, sometimes with severe limitations, from instrumental locations. in fact several methods have been proposed to infer depth from the attenuation of macroseismic data, but seldom are they applied and in no italian macroseismic catalogue is depth reported. at the moment, the usage of instrumental data for the mentioned purposes is limited by the fact that seismic bulletins and waveforms for historical events are not in digital format, but this will soon change thanks to national (sismos, michelini and the ingv sismos group, 2004) and international (eurosismos, ferrari and pino, 2004) projects that aim at compiling information for both instruments and seismic recordings. the correct role of the instrumental locations is then to support and complement the macroseismic data and, in the few cases in which the macroseismic data are poor, to be used instead, provided that the instrumental data have been carefully checked. this should not sound too odd since at the moment the most widely used parametric catalogue already contains a small percentage of instrumental locations, and adding a few more events or some more information could only improve it. acknowledgements an anonymous reviewer and fabrizio bernardi provided very constructive comments. graziano ferrari shared his precious knowledge of the older instruments and observatories. he also gave me very good hints on the older data. the role of instrumental versus macroseismic locations for the seismicity of the last century in the nw apennines i owe special thanks to alberto ansaloni who provided very important information about the observatory of chiavari and let me thumb through the many bulletins, journals and scientific reports archived there. appendix a. list of some of the reference sources used in this work. not all the bulletins were published for the whole century. some of them were compilations of comprehensive data, some were simply listing the data of a single observatory (e.g., venezia, quarto castello). finally, some other were merged in more complete compilations. good references in this sense can be found in the tromos project. published by title notes regio ufficio centrale bolletino sismico comprehensive compilation di meteorologia e geofisica, of data from many national microsismi, fascicolo i observatories regio ufficio centrale bollettino sismico comprehensive compilation di meteorologia e geofisica, of data from many macrosismi, fascicolo ii observatories union geodesique bulletin du mois comprehensive compilation et geophysique internationale, of data from many bureau central seismologique observatories de strasbourg raffaello stiattesi, spoglio delle osservazioni contains data osservatorio di quarto sismiche from observatory alone castello (firenze) atti del reale istituto osservazioni sismografiche contains data veneto di scienze, dell’istituto di fisica from observatory alone lettere ed arti della regia università di padova regio ufficio centrale notizie sui terremoti comments and data di meteorologia e geofisica osservati in italia on the seismic activity at a national scale int. union of geodesy the international comprehensive compilation and geophysics, the british ass. seismological summary of data from many seismological committee, observatories the university of oxford caloi p., medi e., bollettino sismico mensile comprehensive compilation istituto nazionale di geofisica of data from many observatories caloi p., pannocchia g., registrazioni sismiche contains rosini e. data in roma from observatory alone 933 this work is financed by the istituto nazionale di geofisica e vulcanologia and by the dipartimento per la protezione civile within the dpc2004-06 research activity, under the s2 project. 934 stefano solarino published by title notes international seismological bulletin comprehensive compilation centre of data from many foreign and national observatories international seismological on-line bulletin, covers the whole century centre, thatcham, u.k. http://www.isc.ac.uk/bull osservatorio geofisico bollettino mensile bulletins were not published del seminario patriarcale (1920, 1927, 1934) throughout the period di venezia (formerly of operation (1817-1938) osservatorio meteorologico e geodinamico) appendix b. data used in this work in hypo-tape format. readings with weight 4 and 5 have not been used due to different listing in diverse bulletins and/or data sources; for convenience, they are listed anyway. giac p 5 2 3 5 7 556.00 quca p 0 2 3 5 7 635.00 rocc p 0 2 3 5 7 757.00 tri p 0 2 3 5 7 708.00 fxim p 0 2 3 5 7 636.00 pado p 0 2 3 5 7 658.00 roma p 0 2 8 4223750.00 quca p 0 2 8 4223628.00 rocc p 0 2 8 4223741.00 tri p 0 2 8 4223670.00 fxim p 0 2 8 4223627.00 sien p 5 2 8 4223740.00 chia p 0 141027 92236.00 fxim p 0 141027 92236.00 pado p 0 141027 92247.00 rocc p 0 141027 92259.00 monc p 0 141027 92265.00 domo p 5 141027 92257.00 mci p 0 141027 92273.00 pado p 5 20 9 7 55557.00 79.00 s 5 fxim p 0 20 9 7 55556.00 rocc p 0 20 9 7 55592.00 chia p 0 20 9 7 55552.00 livo p 5 20 9 7 55592.00 domo p 5 20 9 7 55580.00 monc p 0 20 9 7 55581.00 110.00 s 0 vene p 0 20 9 7 55578.00 sien p 0 20 9 7 55578.00 mci p 0 20 9 7 55650.00 pado p 0 21112912 445.00 62.00 s 0 pola p 0 21112912 496.00 fxim p 0 21112912 440.00 chia p 0 21112912 440.00 monc p 0 261118225795.00 114.0 s 0 pado p 0 261118225791.00 116.0 s 0 fxim p 0 261118225715.00 piac p 0 261118225760.00 livo p 5 261118225864.00 74.00 s 0 monc p 0 271028214942.00 66.00 s 0 pado p 0 271028214945.00 70.00 s 0 fxim p 0 271028214940.00 rocc p 0 271028214968.00 vene p 0 271028214957.00 84.00 s 0 trev p 0 271028214959.00 85.00 s 0 piac p 0 271028214922.00 livo p 4 2710282149 5.00 20.00 s 4 roma p 0 271028214982.00 132.0 s 0 domo p 0 271028214990.00 the role of instrumental versus macroseismic locations for the seismicity of the last century in the nw apennines chia p 0 271028214920.00 pado p 0 28 720195353.00 75.00 s 0 piac p 0 28 720195330.00 rocc p 0 28 720195383.00 142.0 s 0 trev p 0 28 720195363.00 84.00 s 0 monc p 0 28 720195365.00 82.00 s 0 chia p 5 28 720195310.00 fxim p 0 28 720195343.00 livo p 4 28 720195370.00 pado p 0 28 8 3231049.00 75.00 s 0 piac p 0 28 8 3231048.00 rocc p 0 28 8 323983.00 trev p 0 28 8 3231060.00 100.0 s 0 livo p 0 28 8 323970.00 fxim p 0 28 8 323998.00 monc p 0 28 8 3231053.00 89.00 s 0 trev p 0 31 125104854.00 74.00 s 0 pavi p 0 31 125104834.00 piac p 0 31 125104832.00 prt p 0 31 125104830.00 fxim p 0 31 125104831.00 livo p 5 31 125104870.00 60.00 s 5 zur p 0 31 125104870.00 108.0 s 0 neuc p 0 31 125104868.00 109.0 s 0 trev p 0 34 613 9 657.00 85.00 s 0 pado p 0 34 613 9 652.00 74.00 s 0 tri p 0 34 613 9 670.70 86.30 s 0 vene p 0 34 613 9 659.00 79.00 s 0 pavi p 0 34 613 9 652.00 piac p 0 34 613 9 637.00 prt p 0 34 613 9 638.50 52.00 s 0 livo p 0 34 613 9 642.00 55.00 s 0 fxim p 0 34 613 9 638.50 sien p 0 34 613 9 646.00 roma p 0 34 613 9 672.00 119.0 s 0 rcu p 0 39101515 573.30 107.0 s 0 monc p 0 39101515 559.00 84.00 s 0 tri p 0 39101515 571.00 105.0 s 0 chur p 0 39101515 570.00 114.0 s 0 prt p 0 39101515 539.60 46.80 s 0 prt p 0 51 812211948.00 57.00 s 0 bolo p 0 51 812211955.00 67.00 s 0 pavi p 0 51 812211969.50 91.40 s 0 salo p 0 51 812211969.00 90.30 s 0 roma p 0 51 812211987.50 125.4 s 0 tri p 0 51 812211985.00 134.0 s 0 pavi p 0 59 126 53556.60 68.80 s 0 orop p 0 59 126 53568.00 89.00 s 0 bolo p 0 59 126 53569.00 93.00 s 0 rmp p 5 59 126 53742.00 prt p 0 59 126 53568.80 88.20 s 0 fxim p 0 59 126 53570.00 96.00 s 0 mona p 0 59 126 53573.50 tri p 0 59 126 53590.00 140.0 s 0 chur p 0 59 126 53581.40 bas p 0 59 126 53590.40 fxim p 0 61 8 3102645.70 63.00 s 0 prt p 0 61 8 3102652.20 56.40 s 0 pado p 0 61 8 3102670.00 106.0 s 0 pavi p 0 61 8 3102680.00 100.0 s 0 rmp p 5 61 8 3102741.30 mona p 0 61 8 3102668.00 114.0 s 0 chur p 0 61 8 3102680.80 111.4 s 0 rsl p 5 61 8 3102684.00 122.0 s 5 lju p 5 61 8 3102692.00 148.3 s 5 tri p 5 61 8 3102745.00 bolo p 0 6511 9153512.00 24.00 s 0 pavi p 0 6511 9153523.00 42.00 s 0 pado p 0 6511 9153526.00 47.00 s 0 rcu p 0 6511 9153545.00 75.00 s 0 pavi p 0 68 6 7 93468.00 83.50 s 0 pado p 0 68 6 7 93469.00 90.00 s 0 rcu p 0 68 6 7 93496.00 aqu p 0 68 6 7 93496.00 rmp p 0 68 6 7 93497.50 geno p 0 68 6 7 93464.00 78.00 s 0 stv p 0 68 6 7 93483.20 105.50 s 0 rob p 0 68 6 7 93477.00 98.00 s 0 cun p 0 68 6 7 93482.00 105.00 s 0 tri p 0 68 6 7 93486.00 lju p 0 68 6 7 93495.00 935 936 stefano solarino fur p 0 68 6 7 93542.00 pavi p 0 721025215631.00 45.00 s 0 bolo p 0 721025215634.00 49.00 s 0 tno p 0 721025215639.40 geno p 1 721025215625.68 35.18 s 0 alba p 1 721025215635.98 rob p 0 721025215635.23 51.73 s 0 srx p 0 721025215638.53 57.53 s 0 cun p 0 721025215639.53 59.53 s 0 vern p 0 721025215638.93 roa p 0 721025215640.03 enr p 0 721025215640.43 stv p 0 721025215641.03 62.23 s 0 pni p 0 721025215641.68 63.43 s 0 chia p 1 721025215620.00 25.00 s 0 lns p 2 721025215646.10 spf p 0 721025215649.50 lmr p 0 721025215652.50 references boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (editors) (2000): catalogue of strong italian earthquakes from 461 b.c. to 1997, ann. geofis., 43 (4), pp. 268 (with cd-rom). camassi r. and m. stucchi (1997): nt4.1.1 a parametric catalogue of damaging earthquakes in the italian area (gndt, cnr), pp. 66. ellsworth, w.l. (1977): three dimensional structure of the crust and mantle beneath the island of hawaii, ph.d. thesis (massachusetts institute of technology), pp. 327. gasperini, p. and g. ferrari (2000): deriving numerical estimates from descriptive information: the computation of earthquake parameters, ann. geofis., 43.(4), 729-746. ferrari, g. and n.a. pino (2004): eurosismos 2002-2004 a project for saving and studying historical seismograms in the euro-mediterranean area, in xxix general assembly of the european seismological commission, abstracts, p. 74. gruppo di lavoro cpti (1999): catalogo parametrico dei terremoti italiani (ing,gndt, sga, ssn), (ed. compositori, bologna), pp. 92. gruppo di lavoro cpti (2004): catalogo parametrico dei terremoti italiani, versione 2004 (cpti04), (ingv, bologna), (on line: http://www.emidius.mi.ingv.it/cpti/) kissling, e. (1988): geotomography with local earthquake data, rev. geophys., 26, 659-698. kissling, e., s. solarino and m. cattaneo (1995): improved seismic velocity reference model from local rsl p 0 721025215652.51 lrg p 0 721025215653.20 tri p 0 721025215656.80 rmp p 0 721025215663.50 lju p 0 721025215666.50 fur p 0 721025215670.00 bas p 0 721025215664.00 prt p 0 73 6 5134752.60 70.20 s 0 rmp p 0 73 6 5134784.00 122.0 s 0 geno p 2 73 6 5134727.90 39.60 s 1 rob p 0 73 6 5134745.00 61.50 s 0 srx p 1 73 6 5134747.30 67.50 s 0 cun p 0 73 6 5134750.00 69.40 s 0 stv p 0 73 6 5134752.00 74.50 s 2 pni p 0 73 6 5134753.00 77.50 s 3 zur p 0 73 6 5134774.10 tri p 0 73 6 5134772.10 lmr p 0 73 6 5134762.60 earthquake data in northwestern italy, terra nova, 7, 528-534. michelini, a. and the ingv sismos group (2004): current developments in the ingv sismos project, in xxix general assembly of the european seismological commission, abstracts, p. 76. monachesi, g. and m. stucchi (1997): dom4.1, an intensity database of damaging earthquakes in the italian area (web update 2000, on line: http://emidius.mi.ingv.it/dom/). postpischl, d. (1985): catalogo dei terremoti italiani dall’anno 1000 all’anno 1980, quad. ric. sci., 114 (2b), pp. 239. solarino, s. (2002): the september 7, 1920 earthquake in lunigiana-garfagnana (tuscany, italy): can instrumental data provide a reliable location?, in proceedings of the xxviii general assembly of esc (cd-rom). solarino, s., d.p. hill and w.l. ellsworth (1996): eastern california seismicity beginning from 1927 and its relation to the post-1980 unrest in long valley caldera, california, in agu fall meeting, book of abstracts, p. 206. solarino, s., g. ferretti and c. eva (2002): new insights on the seismicity of the lunigiana-garfagnana area (tuscany, italy), j. seismol., 6, 141-152. valensise, g. and d. pantosti (2001): database of potential sources for earthquakes larger than m 5.5 in italy, ann. geofis., 44 (suppl. to no. 4), pp.180 (with cdrom). (received january 18, 2005; accepted may 6, 2005) s0659 annals of geophysics, 60, 6, s0659, 2017; doi: 10.4401/ag-7295 slip of a two-degree-of-freedom spring-slider model in the presence of slip-dependent friction and viscosity jeen-hwa wang1 1 institute of earth sciences, academia sinica, nangang, taipei, taiwan article history received october 21, 2016; accepted february 26, 2017. subject classification: two-degree-of-freedom spring-slider model, slip-dependent friction, viscosity, stiffness ratio, phase portrait. abstract in this study, we study the slip of a two-degree-of-freedom spring-slider model, consisting of two sliders, in the presence of slip-dependent friction due to thermal pressurization and viscosity. simulation results show that seismic coupling between the two sliders is weak when the stiffness ratio, s, of the model is smaller than 5 and/or either uc1 or uc2, which are the characteristic displacements of friction law at the two sliders, is smaller than 0.5. the patterns of motions of two sliders yielded by large uc1 and small uc2 are opposite to those by small uc1 and large uc2. the ratio, φ, of static friction force at slider 2 to that at slider 1 is a factor in influencing the motions of two sliders. higher φ leads to a longer delay time to trigger the motion of slider 2. slider 2 cannot move when φ is higher than a critical value which depends on other model parameters. the presence of viscosity between the sliders and moving plate results in increases in duration times and predominant periods of motions of sliders and depresses the generation of an attractor. viscosity results in small amplitudes and low velocities of motions of sliders. 1. introduction the rupture processes of an earthquake essentially consist of three steps: nucleation (or initiation), dynamical propagation, and arrest. it is necessary to study the mechanisms controlling the whole rupture processes. such processes are very complicated and can be controlled by several factors. the major factors include brittle-ductile fracture rheology [jeffreys, 1942; scholz, 1990], pore fluid pressure [scholz, 1990], and thermal pressurization [rice, 2006]. friction is one of the most important factors in controlling earthquake dynamics [nur, 1978; dieterich, 1979; ruina, 1983; knopoff et al., 1992; rice, 1993; wang, 1996, 1997, 2000, 2012; rubin and ampuero, 2005; ampuero and rubin, 2008; bhattacharya and rubin, 2014]. the friction coefficient, μ (=0.6–0.8 in general), is defined as the ratio of shear stress,τ, to the effective normal stress, σeff , on the fault plane [byerlee, 1978]. the frictional force between two contact planes is classically considered to drop from static one to dynamic one after the two planes move relatively. indeed, the friction law that has been inferred from laboratory experiments is quite complicated and not completely understood, especially for that on natural faults due to limited observational constraints. this makes the proper constitutive law for fault friction an elusive mathematical formulation. the commonly-used friction law is the rateand state-dependent friction law with the state evolution laws [dieterich, 1972, 1979; ruina, 1983; shimamoto 1986]. a detailed description of the law can be found in several articles [e.g., marone. 1998; wang, 2002; bizzarri and cocco, 2006c; and bizzari, 2011c]. several simple friction laws have been taken into account by some researchers, for examples, a velocity-dependent, weakening-hardening friction law [burridge and knopoff, 1967]; a purely nonlinearly velocity-weakening friction law [carlson and langer, 1989]; a piecewise, linearly velocity-dependent weakening-hardening friction law [wang, 1995, 1996, 2012]; a nonlinearly velocity-weakening friction law [noda et al., 2009]; a displacement softening-hardening friction law [cao and aki, 1984/85]; and a piecewise, linearly slip-dependent friction law [ionescu and campillo, 1999; and urata et al., 2008]. cochard and madariaga [1994] and madariaga and cochard [1994] assumed that purely velocity-dependent friction models can lead to unphysical phenomena or mathematically ill-posed problems. this means that the purely velocity-dependent friction wang 2 law is very unstable at low velocities both during the passage of the rupture front and during the possible slip arrest phase. ohnaka [2003] stressed that purely velocity-dependent friction is in contrast with laboratory evidence. on the other hand, lu et al. [2010] assumed that velocity-weakening friction plays an important role on earthquake ruptures. bizzarri [2011c] deeply discussed this problem. from theoretical studies, bizzarri and cocco [2006a,b,c] revealed that melting of rocks and fault gouge is likely to occur even with the inclusion of the thermal pressurization of pore fluids. moreover, the dramatic fault weakening at high slip rates predicted by the flash heating of micro-asperity contacts is not able to avert melting [bizzarri, 2009]. when fluids are present in faults, thermal pressurization can play a significant role on earthquake rupture and also result in resistance on the fault plane [sibson, 1973; fialko, 2004; bizzari and cocco, 2006a,b,c; rice, 2006; wang, 2009, 2011, 2013, 2016a; bizzarri, 2010; bizzarri, 2011a,b]. in this study, a slip-weakening friction law induced by thermal pressurization will be taken into account. among numerous friction laws, the only constitutive law able to avoid the melting is a slipand velocity-weakening friction law [sone and shimamoto, 2009; bizzarri, 2010], for which the fault weakening is so dramatic that it cannot be counterbalanced by the resulting enhanced slip velocities. however, both thermal pressurization of pore fluids and flash heating predict not only a very dramatic stress drop, but also a very high peak in fault slip velocity, so that the final result is that melting temperature is very often exceeded, unless the slipping zone is extremely large [bizzarri and cocco, 2006b; bizzarri, 2009]. bizzarri [2011a] stressed that when melting occurs, the rheological behavior of the fault zone no longer obeys the coulomb–amonton– mohr formulation, in that a viscous rheology is needed to describe the traction evolution during the ruptures. jeffreys [1942] first emphasized the importance of viscosity on faulting. viscosity in a fault can be controlled by frictional melts [byerlee, 1968]. temperature, pressure, water content, etc., will influence viscosity [turcotte and schubert, 1982]. scholz [1990] suggested that the residual strength of fault-generated friction melts would be high and so present significant viscous resistance to shear. this inhibits continued slip. spray [1993; 1995] observed that most pseudotachylytes [sibson, 1975] are partial melts possessing low viscosity, and capable of generating a sufficient melt volume to reduce the effective normal stress. thus, friction melts can act as fault lubricants during co-seismic slip and viscosity decreases with increasing temperature [spray, 2005]. rice et al. [2001] discussed the physical basis of rateand state-dependent friction, including the direct effect in thermally activated processes allowing creep slippage at asperity contacts on the fault surface. wang [2007] stressed the importance of viscosity on earthquake ruptures. wang [2011] assumed that quartz plasticity could be formed in the main slip zone of the earthquake when t>300 oc after the fault ruptured. the shear zone with quartz plasticity would be localized in a very thin heated layer, for example, 5-mm thick layer. quartz plasticity could lubricate the fault plane at higher t and yield viscous stresses to resist slip at lower t. some researchers have already investigated the effect of viscosity in springblock system [e.g., shaw, 1994; yoshino, 1998; hainzl et al., 1999; wang, 2016a]. on the other hand, several researchers [knopoff et al., 1973; cohen, 1979; xu and knopoff, 1994; knopoff and ni, 2001] took the viscous effect as a factor in causing seismic radiation to reduce energy during earthquake ruptures. since the ingredients of an ideal model are only partly understood, a set of equations to describe comprehensively fault dynamics has not yet been established. nevertheless, some models, for instance the crack model and dynamical spring-slider model, have been developed to approach fault dynamics for a long time. although the frictional effect on earthquake ruptures has been widely studied as mentioned above, the studies of viscous effect on earthquake ruptures are rare. the viscous effect mentioned in rice et al. [2001] was an implicit factor which is included within the direct effect of rateand state-dependent friction law. wang [2016a] studied frictional and viscous effects on slip of a one-degree-of-freedom spring-slider model associated to a fault. however, the interaction between two sliders related to two faults or two fault segments. in this work, i will explore the effects of slip-weakening friction due to thermal pressurization and viscosity on earthquake ruptures based on a two-degree-offreedom spring-slider model, which is generally used to approach an earthquake fault, because numerous earthquakes consist of few segments [galvanetto, 2002; turcotte, 1992; dragoni and santini, 2010, 2011, 2012, 2014; and cited references herein]. for example, wang [2007] applied this model to study the difference in ground motions between the northern and southern segments of the chelungpu fault, along which the 1999 chi-chi, taiwan, ms 7.6 earthquake ruptured. in this study, the frictional factors include the characteristic displacement of the friction law and the differslip of a two-body model with friction and viscosity 3 ence on static friction forces between two sliders. the viscous effect is represented by an explicit parameter. included also is the seismic coupling which will be defined below. the effects on the possibility of simultaneous motions of two sliders, which will lead to a larger-sized event, due to seismic coupling and friction will be studied. the frictional and viscous effects on interaction between two sliders, including the triggering of the second slider by the first one, will be studied. results will be significant on the understanding of earthquake ruptures. 2. two-degree-of-freedom spring-slider model 2.1 model the model consists of two sliders of mass mi (i=1, 2) and three spring. one coil spring of strength k links two sliders and each slider is also pulled by a leaf spring of strength li (i=1, 2) from a moving plate with a constant velocity vp (figure 2). the moving plate provides the driving force on the sliders. at time t=0, all sliders rest in an equilibrium state. the i-th slider is located at position ui, measured with respect to its initial equilibrium position, along the x-axis. each slider is subjected to a slipdependent frictional force, fi(ui) (i=1, 2), where ui is the displacement of the i-th slider, and a velocity-dependent viscous force, φ(vi), where vi=dui/dt is the velocity of the i-th slider. the equation of motion of the system is: (1a) m1(d 2u1/dt 2)=k(u2-u1)-l1(u1-vpt)-f1(u1)-φ(v1) (1a) m2(d 2u2/dt 2)=k(u1-u2)-l2(u2-vpt)-f2(u2)-φ(v2) (1b) it is noted that the total forces in equation (1) are null when the sliders do not slide, that is, vi=0. for simplification, the inertial effect is considered to be equal for the two sliders, i.e., m1=m2=m. considering the two sliders to be two segments of a single earthquake fault, the coupling between the moving plate and each fault segment should be equal, thus giving l1=l2=l. equation (1) consists of two processes: the first one is the coupling process between the moving plate and a slider through the leaf spring l. the other one is the generation of “self-stress” [andrews, 1978], which originates from the joint effect of the coil spring k between two sliders and the leaf spring l. the coil spring k plays a role only in transferring energy from one slider to the other; thus, it does not change the total energy of the system. however, the leaf spring l plays two roles: one is the supply of energy to the system from the driving force caused by the moving plate, i.e., the lvpt term in equation (1), and the other is the loss of energy from the system. hence, the leaf spring l can change the total energy in the system. therefore, the stiffness ratio s=k/l is a significant parameter representing the level of conservation of energy in the system [wang, 1995]. in this study, s is taken to represent seismic coupling. larger s shows that the coupling between two sliders is stronger than that between a slider and the moving plate. this results in a smaller loss of energy through the l spring, thus indicating a higher level of conservation of energy in the system. of course, smaller s indicates a lower level of conservation of energy. when l is constant, small k (less than a critical value) can produce an unstable rupture. when k<<l or k=0, equation (1) becomes: mdui/dt=-l(ui-vpt)-fi(ui)-φ(vi) (i=1, 2) (2) obviously, the system loses the coupling between two sliders, and thus each slider moves independently. hence, the system can only generate a small event consisting of only one single slider. in other words, large s results in simultaneous motions of the two sliders. when l<<k or l=0, the effect due to the leaf spring disappears, and the coupling between two sliders dominates the behavior of the system. based on a simple 2-d anti-plane strain softening model, stuart [1981] considered the ratio kf/ks, where kf and ks are the stiffness of the fault zone and that of the elastic surroundings, respectively, to be a significant indicator of earthquake instability. he stressed that instability occurs when the ratio reaches unity, i.e., kf≈ks. his ratio is similar to s=k/l defined by wang [1995]. this model displayed by equation (1) addresses only the strike-slip component and, thus, cannot completely represent earthquake ruptures, which also consist of transpressive components. nevertheless, simulation results of this model can still reveal significant information on earthquake ruptures. of course, some reformed spring-slider models, for examples those proposed by he et al. [1998] and ryabov and ito [2001], have been applied to study dip-slip faulting. 2.2 viscosity for deformed materials, there are two components, i.e., elastic component and viscous component, when the viscous effect is present. the elastic component can be modeled as a spring with an elastic constant e, given by the formula: σ=eε, where σ, e, and ε are, respectively, the stress, the elastic modulus of the ma4 wang terial, and the strain that occurs under the given stress. the viscous component can be modeled as a dashpot such that the stress–strain rate relationship can be given as, σ=v(dε/dt) where v is the viscosity of the material. there are two simple and commonly-used models to describe the viscous materials [jaeger and cook, 1977; hudson, 1980]. the first one is the maxwell model which can be represented by a purely viscous damper and a purely elastic spring connected in series, as shown in figure 1. the model can be represented by the following equation: dε/dt=dεd/dt+dεs/dt=σ/v+e 1dσ/dt. the model predicts that stress decays exponentially with time. of course, the behavior of a viscoelastic material depends on boundary conditions. one limitation of this model is that it does not predict creep accurately. the second one is the kelvin-voigt model, also known as the voigt model, consists of a newtonian damper and hookean elastic spring connected in parallel, as shown in figure 1. it is used to explain the creep behavior of materials. the constitutive relation is expressed as: σ(t)=eε(t)+vdε(t)/dt. temperature, pressure, water content, etc., will influence viscosity [turcotte and schubert, 1982]. frictional melts can control viscosity in a fault [byerlee, 1968; spray, 2005; and cited references herein]. for the maxwell model, the strain will increase, without a upper limit, with time at constant σ; while the kelvin-voigt model the strain will increases, with an upper limit, with time. hence, the latter is more appropriate than the former to be applied to the seismological problems. hence, the kelvin-voigt model is taken in this study. although viscosity varies with temperature, pressure, and water content, only a constant viscosity for each segment is considered below. the newtonian viscous force is described by a dash-pot shown in figure 1 specified with viscosity v between the slider and the moving plate, and, thus, the viscous force at the slider is represented by -uv where v (=du/dt) is the velocity of the slider. for the kelvin-voigt model, the stress is a function of both strain and strain rate and thus can be applied to the seismological problems [hudson, 1980]. however, it is not easy to directly implement viscosity in a dynamical system as used in this study. hence, viscosity is here represented in an alternative way. viscosity leads to the damping of oscillations of a body. the damping coefficient is usually proportional to viscosity and is controlled by the linear dimension of the body in a viscous fluid. for example, according to stokes’ law, the damping coefficient c of a sphere of radius r in a fluid of viscosity v is given by c=6πrυ [kittel et al., 1968]. the damping coefficient is regarded as viscosity hereafter. hence, the viscous force is represented by φ=cv. noted that the unit of c is n/(m/s). the model shown in figure 2 is indeed an approximation to my approach. a complete model should include the viscous effect between two sliders. it would be too complicated to be considered here, because this study is focused only on the viscous effect between a slider and the background moving plate. 2.3 friction due to thermal pressurization as mentioned above, friction can also be produced from thermal pressurization. on a fault plane with an area of a and an average displacement ū, the frictional energy caused by the dynamic friction stress, τd, is ef=τdūa which could result in a temperature rise, δt. frictional heat can conduct outwards from the slipping zone to wall rocks. theoretical analyses [e.g., bizzari and cocco, 2006a,b; fialko, 2004] show that δt is described by an error function of distance and decays outwards from the fault plane. under thermal pressurization, the energy and fluid mass conservation equations in a 1-d fault plane, in which the xand y-axes denote figure 1. the two types of viscous materials: (a) for the kelvin– voigt model and (b) for the maxwell model. (κ=spring constant and υ=coefficient of viscosity). figure 2. two-body spring-slider model. in the figure, mi, k, li, ci, vp, and fi denote, respectively, the mass, the spring constant between two sliders, the spring constant between a slider and the moving plate, the coefficient of viscosity, the velocity of the moving plate, and the frictional force. slip of a two-body model with friction and viscosity 5 the directions along and normal to the fault plane, respectively, can be found in rice [2006]. rice [2006] proposed two end-members models for thermal pressurization: the adiabatic-undrained-deformation (aud) model and slip-on-a-plane (sop) model. the first model corresponds to a homogeneous simple shear strain ε at a constant normal stress σn on a spatial scale of the sheared layer that is broad enough to effectively preclude heat or fluid transfer. the second model shows that all sliding is on the plane with τ(0)=f(σn-po) where po is the pore fluid pressure on the sliding plane (y=0). for this second model, heat is transferred outwards from the fault plane. the shear stress -slip functions, τ(u), caused by thermal pressurization [rice, 2006] are: τaud(u)= f(σn-po)exp(-u/uc) (3) for the aud model; and τsop(u)=f(σn-po)exp(u/l*)erfc(u/l*) 1/2 (4) for the sop model. the two parameters uc and l* are the respective characteristic displacements, which are in terms of physical properties of the fault-zone materials. the stress τaud(u) displays exponentially slip-weakening friction. indeed, the stress τsop(u) also shows slip-weakening [see wang, 2009]. since the sop model is based on a constant velocity, it cannot be used in this study. for numerical simulations from equation (1), a slip-weakening friction law: f(u)=foexp(-u/uc) based on the aud model is taken into account. the variations of friction force versus displacement for five values of uc, i.e., 0.1, 0.3, 0.5, 0.7, and 0.9 m, are displayed in figure 3. the friction force decreases with displacement. obviously, the friction force decreases faster for smaller uc than for larger uc and the force drop decreases with increasing uc. for small u, exp(-u/uc) can be approximated by 1-u/uc [wang, 2016b]. obviously, uc -1 is almost the decreasing rate, γ, of friction force with displacement at small u. thus, small (large) uc leads to large (small) γ. substituting the friction law and the viscous law into equation (1) leads to m(d2u1/dt 2)=k(u2-u1)-l(u1-vpt)-fo1exp(-u1/uc1)-c1du1/dt (5a) m(d2u2/dt 2)=k(u1-u2)-l(u2-vpt)-fo2exp(-u2/uc2)-c2du2/dt (5b) to deal with the problem easily, we normalize equation (5). letting fo1=fo, fo2=φfo, do=fo/l, ωo=(l/m) 1/2, τ=(l/m)1/2t=ωot, uc1=uc1/do, uc2=uc2/ do, η1=c1ωo/l, η2=c2ωo/l, vp=vp/doωo, ui=ui/do, vi=dui/dτ, dui/dτ=[fo/(ml) 1/2]dui/dτ, and d 2ui/dτ 2= (fo/m)d 2ui/dτ 2, equation (5) becomes: d2u1/dτ 2=s(u2-u1)-(u1-vpτ)-exp(-u1/uc1)-η1du1/dτ (6a) d2u2/dτ 2=s(u1-u2)-(u2-vpτ)-φexp(-u2/uc2)-η2du2/dτ (6b) let y1=u1, y2=u2, y3=du1/dτ, and y4=du2/dτ. equation (6) can be re-written as four first-order differential equations: dy1/dτ=y3 (7a) dy2/dτ=y4 (7b) dy3/dτ=-(s+1) y1+sy2-exp(-y1/uc1)-η1y3+vpτ (7c) dy4/dτ=sy1-(s+1)y2-φexp(-y2/uc2)-η2y4+vpτ (7d) obviously, the solutions of equation (7) exist within a domain determined by six model parameters, i.e., s, uc1, uc2, φ, η1, and η2, in a six-dimensional space. since it is actually difficult to analytically define the domain, numerical computations will be performed. equation (7) is computed through the fourth-order runge-kutta method [press et al., 1986]. in the following numerical simulations, the sliders are restricted to move only along the positive direction, that is, vi≥0 and ui≥0 (i=1, 2). for each case, four diagrams are produced from numerical simulations: the time variation in normalized acceleration, a/amax, the time variation in normalized velocity, v/vmax, the time variation in normalized displacement, u/umax, and the phase portrait of v versus u. a phase portrait, denoted by y=f(x), is a plot of a physical quantity versus another of an object in a dyfigure 3. three slip-weakening friction law: for f(u)=foexp(-u/uc) with uc=0.1, 0.3, 0.5, 0.7, and 0.9 m when fo=1 unit. wang 6 namical system [thompson and stewart, 1986]. the intersection point of f(x) and the bisection line, i.e., y=x, is called the fixed point, that is, f(x)=x. if the function f(x) is continuously differentiable in an open domain near a fixed point xf with |f ’(xf)|<1, the fixed point is an attractor. in other words, an attractive fixed point is a fixed point xf of a function f(x) such that for any value of x in the domain that is close enough to xf, the iterated function sequences, i.e., x, f(x), f 2 (x), f 3 (x),…, converges to xf. an attractive fixed point is a special case of a wider mathematical concept of attractors. chaos can be generated at some attractors. the details can be seen in thompson and stewart [1986] or other nonlinear literatures. chaotic motion in a two-degree-of-freedom system have been studied by numerous researches [e.g., huang and turcotte, 1990a,b, 1992; de sousa viera, 1999; and abe and kato, 2013]. however, only the friction law was considered in those studies. 3. numerical simulation before performing numerical simulations, we must consider the realistic values of model parameters. it is difficult to directly evaluate the value of s, because it is not easy to measure l. wang [1995] showed that for a higher-order system with the number of sliders being larger than 100, the value of s which will be appropriate for seismicity simulations ranges from 20 to 120. as mentioned above, large s more easily produces self-organization in a system and thus there is strong coupling between two sliders than small s. strong coupling makes the two sliders move almost simultaneously. hence, in order to view somewhat independent motions of respective sliders we take small s. practical tests from numerical computations show that s<5 leads to weak coupling and s≥5 results in strong coupling. generally, vp is ~10 -9 m/s and thus vp is ~10-9 when doωo is an order of magnitude of 1 m/ sec. do is almost the maximum displacement, umax, on the ruptured fault plane. since the value of uc is rare, it is not easy to evaluate uc=uc/do. from related data of a mature fault surface at 7 km, rice [2006] obtained uc=12 m. since umax was not given in rice [2006], uc. cannot be estimated. wang [2009] inferred the value of uc from given data of the 1999 chi-chi, taiwan, ms 7.6 earthquake. his estimated value of uc is between 3.3 for umax=10.7 m and 3.5 m for umax=12.3 m at shallow depths of the fault zone. the values of umax were inferred by different researchers. hence, the value of uc is about between 0.30 and 0.27 from wang [2009]. of course, uc could vary with depth and from area to area. since η=cωo/l, we have η=4πrυωos/k. the viscosity, υ, of a fault zone depends upon the temperature, pressure, water content, sio2 content, rock types etc. spray [1993] showed that the calculated values of υ for friction melts are low and decrease with increasing temperature. for examples, the values are about 103−106 pa s at 800oc and about 101−103 pa s at 1200oc. in general, υ lies in the range 1015 pa s and 102 pa s when the temperature varies from 500oc to 1200oc. savage and lachenbrush [2003] mentioned that for a seismogenic zone with a thickness of 14 km, the viscosity on the bottom is (0.31−3.12)×1020 pa s. obviously, the values for lower crustal materials from savage and lachenbrush [2003] are higher than those for upper crustal materials from spray [1993]. the parameter r is almost the dimension of the deformed volume around a rupture fault. based on stick-slip behavior and elastic rebound theory of a fault, turcotte and schubert [1982] estimated the value of r. their values of r are in the range 750 m to 7500 m when the average displacement on a fault is 5 m, the stress drop is 10−100 mpa, and the shear modulus is 30 gpa. the average value of k for numerous faults is about 4:6×1014 n/m [wang, 2012]. hence, the value of η lies in the range 10-8 to 106 for υ of 102 to 1015 pa s when s<10 and ωo=1 hz. simulation results could be different for various time steps, δτ. practical tests suggest that simulation results show numerical stability when δτ<0.05 in the following computations, the time step is taken to be δτ=0.02. when vpτ=exp(-y1/uc1) on slider 1 from equation (4c), the force exerted from the moving plate is equal to the static friction force. in principle, slider 1 can start to move. in practice, the computation cannot go ahead because all values are zero. in order to kick off slider 1, an initial force, δf, is necessary for computations. the value of δf can affect computations. a very small value of δf cannot enforce slider 1 to move; while a large one will dominate computations. carlson et al. [1991] studied the effect of δf (denoted by σ in their article) on computational results. numerical tests show that δf=10-3 is appropriate for numerical simulations. simulations are made for various values of model parameters. figures 4−13 displays the time variations in a/amax, v/vmax, and u/umax, and the phase portrait of v versus u with various sets of values of model parameters which are displayed in table 1. in each diagram, the simulation results for slider 1 and slider 2 are represented, respectively, by a solid line and a dotted line. it is noted that the viscous effect taken into account only from figure 11 to figure 13. slip of a two-body model with friction and viscosity 7 in figures 4−13, the intersection points of the bisection line (denoted by a thin solid line) with the two curves are the fixed points. 4. discussion 4.1 effect due to seismic coupling the lower-bound value of s for yielding strong coupling between the two sliders can be seen from figures 4 and 5, which are the simulation results for s=5 and s=1, respectively, when the values of other model parameters are equal. in each plot of figure 4, the solid line is close to the dotted line, thus indicating strong coupling between the two sliders. numerical tests also show the same phenomenon when s>5. hence, the two sliders move almost simultaneously when s≥5, for which the system is self-organized. figure 5 shows the results when s=1. in each plot the solid line is different from the dotted line, thus showing weak coupling between the two sliders. this phenomenon also exists when s<5. the peak values of a/amax and v/vmax of slider 1 come earlier than those of slider 2, while the peak value of u/umax of the former comes later than that of the latter. the amplitudes of a/amax v/vmax, and u/umax on slider 2 are all larger than those on slider 1. results show the directivity of motions of sliders. figures 4 and 5 suggest that strong coupling (with large s) between two fault segments is more capable of generating a larger-sized event than week coupling (with small s). hence, for an earthquake fault consisting of a few segments it is easier to generate a larger-sized event from larger s than from smaller s. of course, the value of s of generating a larger-sized event increases with the number of sliders [wang, 1995]. from numerical simulations, wang [1995] obtained a power-law correlation between the b-value of the gutenberg-richter frequency-magnitude law and s: b~s-2/3 for the cumulative frequency and b~s-1/2 for the discrete frequency. larger b for smaller s is related to a bigger number of smaller events and smaller b for larger s to a bigger number of larger events. the present result is consistent with his. figures 5a and 5b reveal that the predominant periods of the two sliders are almost the same, because their values of model parameters are equal. in figures 4d and 5d, the absolute values of slope at the fixed points are likely both smaller than 1 and thus they can be an attractor. 4.2 effect due to uc the effect due to uc can be seen from figures 6−10. we examine the upper-bound values of uc1 and uc2 for allowing weak coupling between two sliders. in order to see the effect, s=1 is taken into account. figure 6 shows the simulation results for uc1=0.5 and uc2=0.5 when the values of other model parameters are the same as those in figure 5. in each plot the solid line is almost coincided with the dotted line, thus exhibiting strong coupling between the two sliders. numerical tests also show the same phenomenon when uc1>0.5 and uc2>0.5. hence, the two sliders move almost simultaneously when uc1≥0.5 and uc2≥0.5 at the two sliders. in figure 6d, the fixed point for slider 1 is just the original point and thus it cannot be an attractor; and that for slider 2 is smaller than 1 and thus it can be an attractor. figure 7 shows the simulation results for uc1=0.1 and uc2=0.5 when the values of other model parameters are the same as those in figure 6. in each plot the solid line is different from the dotted line. the peak values of a/amax and v/vmax of slider 1 come earlier than those of slider 2, while the peak value of u/umax of the former comes later than that of the latter. the peak values of a/amax, v/vmax, and u/umax of slider 1 are all lower than those of slider 2. the values of uc1=0.1 and uc2=0.5 are equivalent to γ1=10 and γ1=2, respectively. hence, there is a larger force drop (or stress drop) on slider 1 than on slider 2, thus causing a large acceleration on slider 1 to push it to move. then, the motion of slider 1 enforces slider 2 to move. like figure 5, the directivity effect exists in the present case. in figure 7d, the absolute values of slope at the fixed points are likely smaller than 1 for slider 1 and larger than 1 for slider 2. thus, the fixed point for the former can be an attractor, yet not for the latter. figure 8 shows the simulation results for uc1=0.5 and uc2=0.1 when the values of other model parameters are the same as those in figures 6 and 7. obviously, in each plot the solid line is different from the number of figure s η1 η2 φ uc1 uc2 4 5 0 0 1 0.1 0.1 5 1 0 0 1 0.1 0.1 6 1 0 0 1 0.5 0.5 7 1 0 0 1 0.1 0.5 8 1 0 0 1 0.5 0.1 9 1 0 0 1.05 0.1 0.5 10 1 0 0 1.18 0.1 0.5 11 1 10 10 1 0.1 0.5 12 1 10 2 1 0.1 0.5 13 1 10 20 1 0.1 0.5 table 1. values of model parameters used in this study. wang 8 dotted line. figure 8 is totally opposite to figure 7. the peak values of a/amax and v/vmax of slider 1 come later than those of slider 2, while the peak value of u/umax of the former comes earlier than that of the latter. the peak values of a/amax, v/vmax, and u/umax of slider 1 are all higher than those of slider 2. the values of uc1=0.5 and uc2=0.1 are equivalent to γ1=2 and γ2=2, respectively. hence, there is a larger force drop (or stress drop) on slider 2 than on slider 1. slider 1 moves first and then pushes slider 2 to move. a larger force drop (or stress drop) on slider 2 than on slider 1 makes a faster and bigger increase in acceleration on the former than on the latter. this makes the directivity effect do not exist in the present case. in figure 8d, the absolute values of slope at the fixed points are likely larger than 1 for slider 1 and smaller than 1 for slider 2. thus, the fixed point for the latter can be an attractor, yet not for the former. 4.3 effect due to static frictional force the effect of difference in fo1 and fo2 on the motions of two sliders is numerically made based on various values of φ=fo2/fo1 when the values of other model parameters are the same as those in figure 7. it is noted that since slider 1 is considered to be the first one to move, φ must be equal to or larger than 1. results show that slider 2 cannot move when φ≥1.9. the value of φ=1.9 is the critical one for the present case. of course, the critical value depends upon the values of other model parameters. for example, it somewhat increases with s. when 1.9>φ≥1.18, the velocity of slider 1 increases, decreases, and reaches the minimum value when the velocity of slider 2 reaches its peak value. then, slider 1 is pulled by slider 2 to move again. the separation of two events of slider 1 increases with φ. when φ<1.18, the velocity of both sliders increases, decreases, and becomes zero, and slider 1 cannot move again. figures 9 and 10 show the simulation results for φ=1.05 and φ=1.18, respectively, when the values of other model parameters are the same as those in figure 7. obviously, in each plot the solid line is different from the dotted line. like figure 7, the peak values of a/amax and v/vmax of slider 1 come earlier than those at slider 2, while the peak value of u/umax of the former comes later than that of the latter. the peak values of a/amax, v/vmax, and u/umax of slider 1 are all lower than those of slider 2. the reason to cause the results is the same as that for figure 7. the difference figure 4. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/ umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=5, η1=0, η2=0, φ=1, uc1=0.1, and uc2=0.1. slip of a two-body model with friction and viscosity 9 figure 5. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=5, η1=0, η2=0, φ=1, uc1=0.1, and uc2=0.1. figure 6. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=0, η2=0, φ=1, uc1=0.5, and uc2=0.5. wang 10 figure 7. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=0, η2=0, φ=1, uc1=0.1, and uc2=0.5. figure 8. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=0, η2=0, φ=1, uc1=0.5, and uc2=0.1. slip of a two-body model with friction and viscosity 11 figure 9. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=0, η2=0, φ=1, uc1=0.1, and uc2=0.5. figure 10. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/umax) and the phase portrait of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2)r for s=1, η1=0, η2=0, φ=1.18, uc1=0.1, and uc2=0.5. wang 12 figure 11. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/ umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=10, η2=10, φ=1, uc1=0.1, and uc2=0.5.. figure 12. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (v/ vmax) and the phase portraits of υ versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=10, η2=2, φ=1, uc1=0.1, and uc2=0.5. slip of a two-body model with friction and viscosity 13 between figure 7 and figure 9 as well as figure 10 is that the increases in a/amax, v/vmax, and u/umax come slightly later for figures 9 and 10 than for figure 7, because it takes a longer time to increase the force on slider 2 from driving force due to φ>1 or fo2>fo1. obviously, there are two bumps in the temporal variation in velocity of slider 1. in figure 9d, the absolute values of slope at the fixed points are likely smaller than 1 for slider 1 and larger than 1 for slider 2. thus, the fixed point for the former can be an attractor, yet not for the latter. in figure 10d, the absolute values of slope at the fixed points are likely smaller than 1 and thus they can be an attractor. clearly, φ can influence the generation an attractor in slider 2. figures 4−10 show that nonlinear slip-dependent friction can result in an attractor for chaotic slip in the model. this is similar to the chaotic motions in a two-degree-of-freedom system studied by numerous researches [e.g., huang and turcotte, 1990a,b, 1992; de sousa viera, 1999; and abe and kato, 2013]. 4.4 effect due to viscosity figures 11–13 show the simulation results for η1=10 and η2=10, η1=10 and η2=2, and η1=10 and η2=20, respectively, when the values of other model parameters are the same as those in figure 7. obviously, in each plot the solid line is different from the dotted line. in figure 11, the peak values of a/amax v/ vmax, and u/umax of slider 1 come earlier than those of slider 2. the peak values of a/amax, v/vmax, and u/umax of slider 1 are all lower than those of slider 2. the peak values of a/amax v/vmax, and u/umax are all smaller at slider 1 than at slider 2 in figure 7, yet opposite in figure 2. this indicates that viscosity can affect the amplitudes of the three quantities. in figure 12, the peak value of a/amax comes slightly earlier at slider 1 than at slider 2; while the peak value of v/vmax comes slightly later at slider 1 than at slider 2. the peak values of a/amax and v/vmax of slider 1 are lower than those of slider 2. the value of u/umax of slider 1 is first similar to and then larger than that of slider 2. in figure 13, the peak values of a/amax and v/vmax of slider 1 come earlier than those at slider 2. the peak values of a/amax and v/vmax of slider 1 are larger than those of slider 2. the value of u/umax of slider 1 is larger than that of slider 2. unlike figures 4–10, the duration times in figures 11–13 become longer due to the viscous effect. since weak coupling (with s=1) between the two sliders, their motions can be considered to be somewhat infigure 13. the time sequences of normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (u/ umax) and the phase portraits of v versus u of the two sliders (solid line for slider 1 and dashed line for slider 2) for s=1, η1=10, η2=20, φ=1, uc1=0.1, and uc2=0.5. wang 14 dependent. wang [2016b] studied the viscous effect on the predominant period of a one-degree-of-freedom spring-slider model. here, a study of the viscous effect on the predominant periods of the two-degree-of-freedom spring-slider model is conducted. from equation (1), the natural period of each slider is to=2π(m/l) 1/2 in the absence of friction and viscosity. when the two sliders are linked together, the natural period of each slider must be slightly different from to. when viscosity is present, the natural period is t1=to1/(1-c1 2/4ml) for slider 1 and t2=to2/(1-c2 2/4ml) for slider 2. obviously, viscosity produces damping and increases the predominant period of oscillations of the slider. the system is under-damping, critical damping, and over-damping when ci 2/4ml<1, ci 2/4ml=1, and ci 2/4ml>1, respectively. since to1=to2 in this study, the ratio of t2 to t1 is: t2/t1=[(αc1 2 )/(αc2 2 )]1/2 (8) where α=4ml. in figure 11, t1≈t2 because of c1=c2 from η1=η2. in figure 12, t1>t2 because of c1>c2 from η1>η2. in figure 13, t1<t2 because of c1<c2 from η1<η1. in addition, the values of a/amax,v/ vmax, and u/umax with viscosity are smaller than those without viscosity, because viscosity can decrease the amplitude of vibration. the viscous effect on slip of the model is consistent with that mentioned by wang [2007] who applied the model to study the difference in ground motions between the northern and southern segments of the chelungpu fault generated by the 1999 chi-chi, taiwan, ms 7.6 earthquake. in figures 11d, 12d, and 13d, the fixed points are just the original point and thus they cannot be an attractor. hence, the presence of viscosity between the slider and moving plate can depress the generation of attractor with |f ’(xf )|<1. consequently, viscosity plays a role on preventing chaotic behavior of earthquake ruptures. in addition, a comparison between the cases in the absence of viscosity (figures 4−10) and those in the presence of viscosity (figures 11−13) shows that the presence of viscosity results in smaller velocities of sliders than the absence of viscosity. this suggests that viscosity can cause slow earthquakes or creep of faults. 5. conclusions simulation results show that slip of the slider are affected by seismic coupling, friction (including the characteristic displacement of the friction law and the static frictional force), and viscosity. for seismic coupling, the coupling between the sliders is weak when s<5 and strong when s≥5. for characteristic displacement of the friction law the characteristic displacement of the friction law, there are two concluding points: (1) the coupling between the two sliders is weak when uc1 and uc2 are both equal to or smaller than 0.5; and (2) the motions of the two sliders yielded by large uc1 and small uc2 are opposite to those by small uc1 and large uc2. for the difference on static frictional forces, a higher static friction force at slider 2 causes a longer delay of its motion; and slider 2 cannot move when its static friction force is higher than a critical value which will depend on other model parameters. for the viscosity between a slider and the moving plate, there are four concluding points: (1) viscosity results in increases in duration times and predominant periods of motions of sliders; (2) higher viscosity decreases the amplitude of motion; (3) viscosity causes a decrease in velocities of sliders; and (4) viscosity depresses the generation of attractors, which can lead to chaotic behavior of the system. acknowledgements. the author thanks an anonymous reviewer and dr. andrea bizzarri (associated editor of the journal) for useful comments and suggestions to improve the article. the study was financially supported by academia sinica, the ministry of science and technology (grant no.: most-1042116-m-001-007), and the central weather bureau (grant nos.: motc-cwb-105-e-02 and motc-cwb106-e-02). references abe and kato (2013). complex earthquake cycle simulations using a two-degreeof-freedom spring-block model with a rateand state-friction law. pure appl. geophys., 170, 745-765. ampuero, j.p. and a.m. rubin (2008). earthquake nucleation on rate and state faults~aging and slip laws. j. geophys. res., 113, b01302, doi:10.1029/ 2007jb005082. bhattacharya, a. and a.m. rubin (2014). frictional response to 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(in press) xu, h.j. and l. knopoff (1994). periodicity and chaos in a one-dimensional dynamical model of earthquakes. phys. rev. e, 50(5), 3577-3581. yoshino, (1998). influence of crustal viscosity on earthquake energy distribution in a viscoelastic springblock system. geophys. res. lett., 25, 3643-3646. *corresponding author: jeen-hwa wang institute of earth sciences, academia sinica, nangang, taipei, taiwan; email: earth.sinica.edu.tw 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved layout 6 from the somigliana waves to the evanescent waves pietro caloi annali di geofisica, vol. 25, n. 4, 1972. annals of geophysics, vol. 53, n. 1, february 2010 abstract the rayleigh equation has real coefficients; therefore, also the case of complex conjugated roots may be explained physically. the author proves that the somigliana waves may be formed for poisson ratio values until 0.30543; for gradually less rigid media, they are missing altogether and degenerate into evanescent waves. 1. in some previous notes [caloi 1966a, 1966b, 1967, 1969] i have dealt with the physical interpretation of the roots of the rayleigh equation which are above the unit, for the values of the σ coefficient of poisson to which correspond three real roots for the rayleigh equation. and i have proved that those roots have an exact physical meaning: they permit the theoretical interpretation of sizable groups of seismic oscillations which i named waves of somigliana. i found then the limits within which the somigliana waves originate, within the real roots above the unit, and i emphasized how interesting it was to include the study of such waves into the research of stratifications building up the earth's crust. however, as was already noted by somigliana in this third contribution to the propagation of seismic waves [somigliana 1918], due to the homogeneity of equations of motion and to the fact that the rayleigh equation has real coefficients, also the case of complex, conjugated roots may be explained physically with the separation of the real from the imaginary part of roots. this is what i am undertaking as follows. 2. first of all, let us try to find analytically the value of σ separating the real field from the complex field for roots above the unit. this value has already been obtained empirically in the previous note [caloi 1969]. as is known, the rayleigh equation is expressed in its most known form with the usual meaning of symbols [caloi 1966a]: from which, after having made [caloi 1966a] follows (1) remembering that , we put whence (1) becomes (2) let us see how the roots of this equation vary when σ varies between 0 and , that is for now we free (2) from the second degree term in χ. to this end we put equation (2) changes then into we make thus obtaining if p and q are real, we know from the mathematical analysis that the condition for the three roots of (3) to be real is expressed in the relation considering the values of p and q, we have in our case 59 2 16 1 1 ,v v v v v v 2 2 3 2 4 1 2 3 2 2 2 3 2 =-c c cm m m ,v v3 2 2 2= | 8 8 3 2 16 1 0 .v v v v 3 2 1 2 2 2 1 2 2 2 + =| | |-c cm m 2 1 1 2 v v 1 2 2 2 = v v ^ h 1 2 1 1 ;v v 1 2 2 2 = = v f -^ h 2 1 1 .g gf 8 8 1 2 16 0 .3 2 + + =f f| | |-^ h 3 8 .= +d| 3 8 6 5 3 16 5 9 28 0 .3 + + =d f d f-^ `h j 3 8 6 5 , 3 16 5 9 28p q= =f f-^ `h j 0 .p q3 + + =d d 4 27 0 . q p32 + g 2 1 for the roots to be real, we must have (3) it is easily found that the value of ε which annulls δ(ε) is and since it follows which practically coincides with the value previously obtained (σ = 0,26305). it is in correspondence to this value that the two roots above the unit of the rayleigh equation coincide; in fact one obtains χ2,3= 3.5754 [caloi 1969]. therefore, in order to have a real root above the unit and two complex conjugated roots, it must be to which corresponds for σ the field of variability: 3. there remained now to calculate a series of complex, conjugated root couples for σ values within the above limits. calculations have been made for the following σ values: 0.265; 0.27; 0.30; 0.305; 0.35; 0.40; 0.50. the rayleigh equations pertinent to the above σ values are: the corresponding roots are (table 1): 4. i asked myself whether all σ values in the interval 0.26305 ÷ 0.5, were leading to roots to which correspond somigliana waves. those roots are complex and conjugated and some of their values have been given under 3. they are complex, hence also the values of [caloi 1966a, 1966b], (4) let us indicate a general complex root as follows the formulas (10) will then be written the relation allows to obtain after the value of σ, included in the above interval, has been assigned, the rayleigh equation furnishes the corresponding couple of complex, conjugated roots. thus r and c are obtained and thence tang e1, tang e2 and v3 pertaining to the chosen σ value. after separating the real part from the imaginary one, we can thus arrive at the real values of e1 (if existing), of e2 and of v3. let us indicate the latter while putting where the negative value for c is being taken. on the basis of the r and c values taken from the previous table 1 we obtain table 2 by varying σ as follows: from the somigliana waves to the evanescent waves 60 4 27 3 4 45 28 12 10 . q p 3 6 3 2 3 2 + +f f-^ ^h h; e ( ) 45 28 12 10 0 .2 3= + gf f fd -^ ^h h 0.6785=f 1 2 1 . 1=v f0.26308=v 4 27 0 , q p32 + =2 0.2630 0.5 .5 1vg for σ = 0.265 for σ = 0.27 for σ = 0.3 for σ = 0.305 for σ = 0.35 for σ = 0.4 for σ = 0.5 3.1277 χ³ – 25.022 χ² + 59.065 χ – 34.043 = 0 3.1739 χ³ – 25.391 χ² + 60.174 χ – 34.782 = 0 3.50 χ³ – 28.0 χ² + 68.0 χ – 40.0 = 0 3.564 χ³ – 28.512 χ² + 69.536 χ – 41.024 = 0 4.333 χ³ – 34.66 χ² + 88.0 χ – 53.33 = 0 3 χ³ – 24 χ² + 64 χ – 40 = 0 χ³ – 8 χ² + 24 χ – 16 = 0 σ χ1 χ2,3 0.265 0.27 0.3 0.305 0.35 0.4 0.5 0. 8498 0.85125 0.86009 0.86154 0.8740 0.8877 0.9128 3.5752 ± i 0.16221 3.5743 ± i 0.3120 3.5714 ± i 0.7284 3.5690 ± i 0.7896 3.5625 ± i 1.1791 3.5562 ± i 1.5406 3.5436 ± i 2.2302 , tang 1 , tang 1.v v e v v e3 2 2 1 1 2 2 2 2 2= = =| | |r i c .2,3 = +| tang 1 , tang 1 e r v v i c v v e r i c, v v r i c . 2 1 1 2 2 2 1 2 2 2 2 2 3 2 = + = + = + ,r i r i cc+ = + 2 , 2r r c r c r c r 2 22 1 2 2 1 2 = + + = + -` `j j v v r3 2=l ,v v c3 2= �σ v2 2/v1 2 tang2 e1 e1 tang 2 e2 e2 r 0.265 0.27 0.3 0.305 0.35 0.4 0.5 0.31973 0.31507 0.2857 0.2805 0.23077 0.16667 0 0.14310 0.12615 0.02035 0.0011045 – 0.1779 – 0.4073 – 1 20˚45´.25 19˚33´ 8˚07´ 1˚51´ – – – 2.5752 2.5743 2.5714 2.5690 2.5625 2.5562 2.5436 58˚04´.25 58˚04´ 58˚03´ 58˚02´ 58˚00´ 57˚58´.5 57˚55´ 1.8913 1.8924 1.8995 1.9006 1.9125 1.9277 1.99605 61 it follows from the analysis of the table 2 that the somigliana waves may vary for σ values until 0.31; for gradually less rigid medium they are missing altogether and degenerate into ordinary transversal waves. the values of efficient angles, for longitudinal incidence, presuppose incidences nearing rapidly the right angle. so far as transversal incidence is concerned, the efficient angles increase slightly as the rigidity of the medium decreases and reach a limit angle of incidence of about 32˚ to which corresponds the total reflection. considering as well the complex conjugated roots of the rayleigh equation, the efficient angles bringing about somigliana waves are the ones corresponding to the roots for the following field of variability of σ: although, practically [caloi 1969], this is reducing to anyhow, the use of complex conjugated roots indicates an enlargement and the limit of the field of variability of the poisson coefficient. concerning the propagation velocity of the somigliana waves (if existing), it is noted that it increases as a increases (that is as rigidity decreases) and reaches the maximum value for the limit value of σ = 0,305. if in the expressions of u1, u2, w1, w2 [caloi 1969] we put φ (x – v3t) under power form and observing that , we will have where p indicates the pulsation of the oscillation and v3 is a negative constant which may be considered as the extinction coefficient of the oscillation in time. for σ = 0.265, for instance, v3 = – 0.043012·v2, and for σ = 0.27, v3= – 0.08277·v2; whereas for σ = 0.305, we have v3= – 0.2076·v2. therefore, at equal frequencies the propagation of the somigliana wave is extinguished more quickly as rigidity decreases. 5. let us have a closer look how the somigliana waves change when their propagation is in elastic media whose σ coefficient shows a trend toward the value of 0.30543. let us consider the case of transversal incidence, where we have if φ (x – v3t) is not periodical, here applies the relation [caloi 1969][caloi 1969] χ always differs from 2. from the somigliana waves to the evanescent waves figure 1. examples of c0,1 (t = 688 ab.), c1,2 (t = 34s ab.) and c2,3 (t = 23s ab.) waves by the author generically indicate as somigliana waves record at somplago (on the cavazzo lake) by a seismograph, with free period of about 120s and optical magnification, in occasion of alaska earthquake of july 30, 1972 (57˚,on 135˚.9w: h = 21.45.11,1 gmt: h = 10 km: m = 7.8) at an epicentral distance of about 8350 kms. for large earthquake (as this alaska earthquake) c0,1 waves can affect the outer layer of mantle, from the top of the astenosphere (low-velocity channel) to the earth's surface (thickness of about 70 kms). 0 0.31 ,1 1v 0. 0.31 .25 1 1v 1.9006v v3 2= $l ,e v t e3 ip x v t3=u -^ ^h h v v i v3 3 3= +l ,x v i v t e e3 3 ip x v t pv t3 3+ = $u -l^ ^h h6 @ tang 1 , tang 1 .e v v e1 1 2 2 2 2 1 2 1 2 = =| |-c cm m 2 1 1 1 1 2 . w u w u v v 1 2 0 0 0 0 1 2 2 2 1 2 1 2 = + + a a | | | c c ^ m m h from the somigliana waves to the evanescent waves 62 the following table 3 is valid: as σ tends toward the value 0.30543, α1 is thus tending toward the infinite. now, in the expressions of u0, w0 see (1) of caloi 1969 -, α1, the quantity characterizing the longitudinal component, acts as a denominator in the relative terms. therefore, these annull each other, namely the contribution of the longitudinal wave in the formation of the somigliana wave falls away, and this degenerates into a trasversal wave. however for σ = 0.30543, the efficient angle of incidence of the transversal waves, is 31˚57´.4, which coincides with the angle of total reflection of the incident transversal wave. in fact is: so that which gives for i2 the above value. therefore, the efficient angle for transversal waves incident at the intersurface with the outer stratification, approximates in less rigid media the angle of incidence to which corresponds the total reflection, which is reached, as could be seen, for σ = 0.30543. hence, in the field where σ varies from 0.30543 to 0.5 there are no somigliana waves, since for them the total reflection of the incident transversal waves takes place. the formation of somigliana waves requires a physically finite medium beyond the surface which is hit by the wave coming from an indefinite medium [caloi 1967]. when the longitudinal reflected wave vanishes as progressive ordinary wave, for satisfying the conditions at the intersurface it is necessary to introduce an evanescent wave. if we indicate the transversal reflected wave (oscillating, of course, in the principal plane) by we will have where u and w are the horizontal and vertical motion components. the resultant of these movements, however, is the socalled evanescent wave which as the rayleigh waves forces the reached particle to describe an elliptical motion [caloi 1955, pages 300-304]. in the variability field which means that in always more incompressible media the somigliana waves (possible only by transversal incidence) degenerate into evanescent waves. in conformity to what happens in the propagation of the light, the velocity of evanescent waves in the second medium is v2/sini2, where i2 is the angle of incidence. practically it coincides, therefore, with rv2 where r is to be taken from the table 2. in case of the limit angle of incidence (i2 = 31˚58´), is in fact v2/sini2 = 1.8894 v2, equal at lower than 1/1000 to the value of . references caloi, p. (1955). ci,j, annali di geofisica, viii, 3, 293-313. caloi, p. (1966a). l'equazione di rayleigh e le onde di somigliana. i: la teoria delle onde di rayleigh, atti acc. naz. dei lincei, classe scienze fis. mat. e nat., xli, ser. viii, 1-2 ferie. caloi, p. (1966b). l'equazione di rayleigh e le onde di somigliana. ii: la teoria di somigliana: rettifiche, conseguenze, atti acc. naz. dei lincei, classe scienze fis. mat. e nat., xli, ser. viii, 5. caloi, p. (1967). l'equazione di rayleigh e 1e onde di somigliana. iii: le ci,j sono onde di somigliana. loro importanza per lo studio della crosta terrestre, atti acc. naz. dei lincei, classe scienze fis. mat. e nat., xliii, ser. viii, 6. caloi, p. (1969). l'equazione di rayleigh e le onde di somigliana. iv: limiti d'insorgenza delle onde di somigliana: loro esclusiva formazione nel piano principale, atti acc. naz. dei lincei, classe scienze fis. mat. e nat., xlvi, ser. viii, 1. somigliana, c. (1918). sulla propagazione delle onde sismiche. nota iii, atti acc. naz. dei lincei, classe scienze fis. mat. e nat., xxvii, ser. v, 1. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. σ 0.26305 0.265 0.27 0.3 0.305 0.30543 0.4 0.3868 0.3783 0.3552 0.14265 0.033235 0.0 imaginary 1.6048 1.60475 1.60445 1.60355 1.6028 1.6028 1.5988 1v v 1 2 2 2 1 2 | -c m 1 1 2 |-c m 2 1 1 2 0.28013 ;v v 1 2 2 2 = = v v ^ h sin 0.52927 ,i v v 2 1 2 2 2 = = sine t xkz=} ~ a-^ h sin ; sin , u x e t x w x k e t x kz kz = = = = a ~ a ~ a } } d d d d ^ ^ h h 0.30543 0.5 ,g gv v3l layout 6 azimuthal propagation of seismo-magnetic signals from large earthquakes in taiwan chieh-hung chen1, jann-yenq liu2,3,*, tao-ming chang4, ta-kang yeh5, chung-ho wang1, strong wen6, horng-yuan yen7, katsumi hattori8, ching-ren lin1, yi-ru chen1 1 academia sinica, institute of earth sciences, taipei, taiwan 2 national central university, institute of space science, chung-li, taiwan 3 national central university, center for space and remote sensing research, chung, taiwan 4 national center for research on earthquake engineering, taipei, taiwan 5 national taipei university, department of real estate and built environment, new taipei city, taiwan 6 national chung cheng university, institute of seismology, chia-yi, taiwan 7 national central university, institute of geophysics, chung-li, taiwan 8 chiba university, graduate school of science, inage, chiba, japan annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5326 abstract this study uses a network that is comprised of 10 total intensity magnetometers to detect azimuthal propagation of seismo-magnetic emission waves during 26 earthquakes that occurred between july 2007 and december 2008 in taiwan. the propagation azimuth and phase velocity of the seismo-magnetic waves are calculated using frequency wavenumber analysis at the ultra low frequency of 0.05 hz every 30 min. we superimpose the derived azimuths within a moving window of 30 days as the monitored distributions, and the entire dataset as the background distribution. we also find the propagation azimuths of the seismomagnetic anomalies of each earthquake by subtracting the background from the monitored distributions. the results show that frequency wavenumber analysis can be applied to evaluate azimuthal propagation of seismo-magnetic emission waves using a scalar of geomagnetic total intensity fields. the success detection rate of seismo-magnetic anomalies increases from 62% of the 26 earthquakes to 77% using the surface magnetic anomalous reference tip (smart) to substitute the epicenters. meanwhile, the odds proportions between the azimuths of the seismomagnetic emission waves towards and away from smart reveal the associated anomalous propagation. 1. introduction geomagnetic anomalies within a wide frequency range from direct current (dc) to very low frequency (vlf) that are associated with large earthquakes have been studied intensively [hayakawa and fujinawa 1994, hayakawa 1999, hayakawa and molchanov 2002]. fraser-smith et al. [1990] observed that amplitudes in the geomagnetic field in a frequency band between 0.05 hz and 0.2 hz suddenly increased a few hours before the ms 7.1 loma prieta earthquake. following their findings, geomagnetic anomalies in the ultra low frequency (ulf) range (#300 hz) that are associated with large earthquakes have been examined [bernardi et al. 1991, molchanov et al. 1992, 1995, kopytenko et al. 1993, merzer and klemperer 1997, kawate et al. 1998, hayakawa et al. 1999, 2000, gotoh et al. 2002, hattori et al. 2002, 2004a, b, hattori 2004, chen et al. 2011]. to locate a forthcoming earthquake, three-component geomagnetic data are analyzed using principal component analysis and signal value decomposition, to evaluate the major azimuth of the seismo-magnetic emission waves (smews) [gotoh et al. 2002, hattori 2004, telesca et al. 2004, 2007, hattori et al. 2006]. chen et al. [2009a] examined such magnetic anomalies statistically before 181 earthquakes (mw ≥4.3 or ml ≥5.0) and proposed the existence of seismo-magnetic fields. however, smew anomalies have not yet been tested in detail through any wave propagation analyses. seismometers with high sampling rates can precisely record changes in seismic waves. for teleseismic earthquakes, the delay time of seismic waves between every two stations can be accurately obtained by comparing the seismograms. to compute propagation directions and velocities of seismic waves, capon et al. [1967] and capon [1969a, b] developed frequency wavenumber (fk) analysis. this evaluates the directions and phase velocities from special issue: earthquake precursors 63 article history received july 24, 2011; accepted november 29, 2011. subject classification: earthquake emission, frequency wavenumber method, earthquake forecast. filtered seismic data, where the distance between every two stations is smaller than the wavelength of a single wave (i.e. a wave phase from 0 to 2r). therefore, if smews are indeed generated by an earthquake during a seismogenic process, it would be possible to estimate the direction of a forthcoming earthquake using the fk analysis. in the present study, standard fk analysis is applied to analyze geomagnetic total intensity fields, which are scalars recorded by a network of 10 magnetometers (table 1) in taiwan, to determine the directions of smews before large earthquakes (figure 1). a statistical study of 26 earthquakes (mw >4.3; table 2) is also carried out, which were retrieved from the broadband array in taiwan [kao et al. 2002, liang et al. 2003, 2004] for the seismology for the period from july 2007 to december 2008. 2. theory and the fk method it has been known for many years that electromagnetic waves propagate at the speed of light of about 3 ×105 km/s in the air [cheng 1989]. since the phase speed is a product of frequency and wavelength, the wavelength of anomalous waves at 0.05 hz will be about 6 ×106 km (as 3 ×105/ 0.05). 64 chen et al. figure 1. map of taiwan showing the locations of the 10 stations (red triangles) and 26 epicenters (blue circles). green squares, the midpoint. station code lat. (˚n) long. (˚e) liyutan ly 24.3467 120.7675 tsengwen tw 23.2514 120.5167 hengchun hc 21.935 120.8008 yeheng yh 24.671 121.3671 shuanlung sl 23.7902 120.9441 pingtung pt 22.7035 120.6496 neicheng nc 24.7181 121.6681 hualien hl 24.0678 121.6006 yuli yl 23.3506 121.2856 taitung tt 22.8019 121.0519 table 1. geomagnetic stations in taiwan. 65 a phase change from 0 to 2r of such a signal wave will take approximately 20 s. therefore, if the sampling interval of a magnetometer is shorter than 10 s, seismo-magnetic anomalies at about 0.05 hz will be detectable. as spherical waves are transferred into plane waves due to the long propagation distance from teleseismic earthquakes, fk analysis can be applied to study seismic waves [goldstein and archuleta 1991a, b, satoh et al. 2001, schisselé et al. 2004]. the maximum power spectrum, p(kxmax, kymax, ~), of an array with n stations located at (xi, yi, i=1, n) can be use to infer the location of an energy source in the wavenumber domain [for details, see capon 1969a], as: (1) where ~ is the angular frequency, kx and ky are wavenumbers in the n-s and e-w trends, respectively, and uij(~) is the cross-power spectrum of i and j stations. the azimuth of the energy source to the midpoint of the array and the phase velocity of seismic waves can be calculated by arctan (kymax/kxmax) and �/(kxmax 2+kymax 2)1/2, respectively. 3. data analyses and results a magnetic network with eight magnetometers was established in 1988 in taiwan [yen et al. 2004]. the station sites that were selected are far from populated areas and from all visible iron objects and power lines, to avoid unwanted or man-made ‘noise’ and interference. the geomagnetic field was continually recorded every 5 or 10 min. in 2002, three new stations were added to provide better coverage of northern to southern central taiwan. the sampling rate of the 11 magnetometers was set for every 1 min [chen et al. 2009b]. to further record anomalies associated with earthquakes, the sampling rates of these magnetometers were reset to 1 s in 2007. due to one station (kimen) located over 210 km away from the others, 10 of these stations on taiwan island are used in this study (table 1). =y x( , , ) ( ) ( ) ( )expp k k ik x x ik y yx ij i j y i j j n i n 11 1 $ + -~ ~u == 6 @' 1// propagation of seismo-magnetic signals table 2. details of the 26 earthquakes. code year month day hour min second lat. (˚n) long. (˚e) mw 1 2007 7 16 23 42 52.6 23.57 121.55 4.38 2 2007 7 23 13 40 2.1 23.72 121.64 5.07 3 2007 8 9 0 55 47.8 22.65 121.08 5.09 4 2007 8 29 1 15 29.1 21.90 121.36 4.66 5 2007 8 29 3 0 16.1 21.95 121.32 5.12 6 2007 9 6 17 51 25.2 24.28 122.25 6.17 7 2007 10 11 3 5 2.5 24.75 121.85 4.36 8 2007 10 17 14 39 59.5 23.47 121.71 4.70 9 2007 12 5 1 41 42.9 23.07 121.19 4.61 10 2008 2 17 20 33 3 23.31 121.46 5.03 11 2008 2 29 16 58 7.6 24.00 122.53 4.77 12 2008 3 4 17 31 47.8 23.21 120.70 4.89 13 2008 4 14 15 39 45.7 22.83 121.33 4.71 14 2008 4 23 18 28 42 22.87 121.68 5.57 15 2008 4 23 22 4 15.4 22.83 121.69 4.66 16 2008 5 10 19 42 1.1 23.95 122.53 5.43 17 2008 5 13 18 27 55.6 22.77 121.04 4.89 18 2008 6 1 16 59 23.9 24.86 121.79 5.12 19 2008 6 11 4 33 36.4 21.88 121.00 4.31 20 2008 8 1 18 55 49.5 24.06 121.55 4.72 21 2008 9 9 7 43 13.5 24.57 122.71 5.23 22 2008 9 10 11 55 34.1 25.08 122.22 4.37 23 2008 12 2 3 16 53.3 23.28 121.60 4.87 24 2008 12 7 21 18 36.7 23.84 122.17 4.63 25 2008 12 23 0 4 45.6 22.95 120.57 4.88 26 2008 12 30 1 31 29.1 24.66 122.36 4.39 since some of these 10 stations have occasional data gaps, the midpoint of the fk array is subject to change (see figure 1). figure 2 shows the longitude and latitude of the midpoint during the entire study period. it can be seen that the midpoint was relatively stationary during november 2007 to june 2008, but fluctuated a lot before and after these times. here, the largest earthquake (eq14, ml = 5.57; table 2) occurred during the stationary period, and it is taken as an example to see whether seismo-magnetic signals can be recorded at the stations. we computed the amplitude at the frequency of 0.05 hz over the 30 days before and 30 days after the earthquake occurrence (eq14), using the morlet wavelet transform [farge 1992, torrence and compo 1998]. note that the frequency of approximately 0.05 hz is generally considered as the most promising frequency to detect seismo-magnetic anomalies [fraser-smith et al. 1990]. due to the 1 hz sampling rate, we derived an anomaly bond of two standard deviations, 2v, using the 86,400 amplitudes computed daily. the numbers of anomalous amplitudes were counted, as either larger or smaller than 2v. if the numbers simultaneously increased at different stations during the earthquake, seismo-magnetic signals were considered to have been recorded. to cross-compare the stations, we standardized the numbers at each station by dividing the associated mean within 30 days before and after an earthquake. figure 3a, b shows the variations of the standardized numbers at nine of the stations (due to the yh data gaps) 30 days before and after the april 23, 2008, ml = 5.57 earthquake (eq14). the standardized numbers were seen to be larger for 14 days and 6 days before, as well as 8 days and 11 days after eq14. figure 3c shows that the rkp index reaches 34 for 28 days before eq14, while the standardized numbers simultaneously increased slightly. since the rkp on 14 days and 6 days before eq14 are 22 and 13, respectively (both rkp less than 24), the two days are in a magnetic quiet. it can be seen that the two anomalies intermittently increased in the standardized numbers before eq14, which agrees with observations of discontinuous seismo-magnetic anomalous signals appearing prior to the chi-chi earthquake [yen et al. 2004]. to further confirm smews associated with the earthquake directions and to estimate their onset times, the azimuth and phase velocity of the geomagnetic data at the chen et al. 66 figure 2. variations in the midpoint computed by the stations from july 2007 to december 2008, according to longitude (a) (˚e) and latitude (b) (˚n). figure 3. the standardized numbers and the rkp index 30 days before and 30 days after eq14. (a) standardized numbers at nc, yh, ly and hl. (b) standardized numbers at hc, pt, tw, tt and sl. (c) variations un the rkp index. 67 frequency of 0.05 hz were sequentially computed every 30 min. these computed azimuths in a moving window over 30 days with a step of 1 day are used to construct the monitored distributions. a background distribution was also developed by superimposing all of the computed azimuths during the entire period. figure 4a shows a major peak of the propagation of seismo-magnetic signals figure 4. distributions of (a) the derived azimuth and (b) the phase velocity. figure 5. the difference proportion distribution. blue to red indicate the distint difference proportions. white and yellow rectangles show the larger realtionships between the difference proportions and the azimuths of the 26 epicenters, and the smart versus the midpoint, respectivley. the widths of the rectangles indicate a temporal period over –10 to 0 days to the earthquakes. background distribution at approximately 110˚, inferring the magnetic waves at the frequency of 0.05 hz were intrinsically westward propagations. the phase velocity of the magnetic waves was mainly around 5 ×104 km/s, which is slower than the velocity of light (figure 4b). we further normalized both the background and the monitored distributions, and computed the difference proportion, which is the difference between the two distributions divided by the background distribution. a positive (or negative) value of the difference proportion means that magnetic waves propagate frequently (or rarely) at a certain azimuth. figure 5 shows the difference proportions and the azimuths of the 26 epicenters versus the midpoint during the entire observation period. both of the azimuths between –20˚ and 20˚ towards and away from the epicenter 10 days before the earthquakes were examined. as the azimuth and temporal resolution are 10˚ and 1 day, there are therefore 55 difference proportion values within each towards or away examined region. the averages of the difference proportion values of the two examined regions were computed, and the larger ones are indicated (figure 5). to confirm that smews were detected, various thresholds were tested. the marked (overall) ratios were calculated, which are the difference proportion values larger than a tested threshold divided by 55 (19800 = 36 × 550). if a marked ratio is greater than the overall ratio, we then declare that smews have been detected. figure 6 illustrates the success detection rate of the earthquakes versus the threshold. the suitable threshold was found to be 30%, which detected 62% (= 16/26) of the earthquakes. we further examined the maximum thresholds, which can successfully detect smews in these 16 events, to relate these to the earthquake magnitude, but no obvious relationships could be found. 4. discussion and conclusions seismo-magnetic anomalies have been considered as a function of the distance from the epicenter to the observation station (i.e. the epicentral distance). however, epicenters are vertically projected points from the hypocenters to the earth surface, and they provide very limited physical information. chen et al. [2009a] showed that seismo-magnetic fields are outwards at an interior position near the smart, where there is an intersection between the earth surface, the extended fault, and the auxiliary fault plane retrieved from fault-plane solutions of earthquakes [see also, chen et al. 2010], and inwards underground at an exterior area before earthquakes. this suggests that the northern pole is close to the earth surface and the southern pole will be underground. geomagnetic waves propagate away from the smart if they are outward of the northern pole and inward of the southern one. on the other hand, geomagnetic waves emitted from any northern pole can propagate to the very southern pole. as a result, geomagnetic waves propagate toward the smart outside the seismo-magnetic fields. similar to the aforementioned study (i.e. epicenters as reference points), we conducted the angle arrivals of smews based on the smart. figure 5 shows that the smart azimuths are rather more suitable than the epicenter azimuths to be related to the positive difference proportions. although the suitable threshold is also 30%, 77% (= 20/26) of the earthquakes can be detected by using the smart to substitute the epicenters (figure 6). meanwhile, the maximum thresholds are roughly proportional to the earthquake magnitude. this means that the difference proportions that result from smews are proportional to the earthquake magnitude. to further examine smews away from and towards the epicenters as well as the smart, the odds test was used, which is the quantity p/(1–p), where p is the probability of success [agresti 2002, see also, chen et al. 2009a]. with the odds >1, a success is more likely than a failure. inversely, a failure is more likely than a success. figure 7 illustrates the propagation azimuths of smews versus the distances from the midpoints to the epicenters and the smart. the odds approach 1, and therefore there is no conspicuous relationship between the propagation azimuths of the smews and the distances from the midpoint to the epicenters (figure 7a). in contrast, the propagation azimuths of the smews are away from and towards the smart where the distances from the midpoint to the smart are <100 km and >200 km (figure 7b), respectively. the odds chen et al. 68 figure 6. the success detection rate versus the thresholds. solid circles and diamonds indicate the relationships between the success detection rate and distinct thresholds using the epicenter or the smart references, respectively. 69 test in this study and the observation of a large decrease in the geomagnetic fields during the chi-chi earthquake [yen et al. 2004], are in good agreement. in conclusion, the azimuth and phase velocity for smews carried by a scalar of the geomagnetic total intensity fields can be detected several days before an earthquake occurrence using fk analysis. on the basis that fk analysis is a standard technology for studying wave propagations, the results accordingly explain the existence of smews before earthquakes. when the smart substitutes for the epicenters to reconstruct the relationship between smews and the distance to the midpoint, the success detection rate increases by 15%. the difference proportion values can be used as an indicator of the earthquake magnitude due to these positive relationships. smews propagates away from and towards the smart inside and outside the area, respectively, where they are dominated by seismogeomagnetic fields during seismogenic processes. this suggests that the smart is a more suitable reference point than the epicenters in seismo-magnetic studies. acknowledgements. the authors wish to thank dr. b.s. huang and dr. w.t. liang of the institute of earth sciences at academia sinica in taiwan for providing the earthquake catalog with the fault-plane solutions from the broadband array in taiwan for seismology, as well as the central weather bureau for the geomagnetic data. this study is partially supported by project nsc98-2116-m-008-006-my3 and nsc 100-2116-m001-027-my3 of the national science council. references agresti, a. 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earth planet. in., 105, 229-238. kopytenko, y.a., t.g. matishvili, p.m. voronov, e.a. kopytenko and o.a. molchanov (1993). detection of ultra-low-frequency emissions connected with the spitak earthquake and its aftershock activity, based on geomagnetic pulsations data at dusheti and vardzia observatories, phys. earth planet. in., 77, 85-95. liang, w.t., y.h. liu and h. kao (2003). source parameters of regional earthquake in taiwan: january–december 2001, terr. atmos. ocean. sci., 14, 249-260. liang, w.t., y.h. liu and h. kao (2004). source parameters of regional earthquake in taiwan: january–december 2002, terr. atmos. ocean. sci., 15, 727-741. merzer, m. and s.l. klemperer (1997). modeling low-frequency magnetic-field precursor to the loma prieta earthquake with a precursory increase in fulat-zone conductivity, pure appl. geophys.,150, 217-248. molchanov, o.a., y.a. kopytenko, r.m. voronov, e.a. kopytenko, t.g. matiashvili, a.c. fraser-smith and a. bernardi (1992). results of ulf magnetic field measurements near the epicenters of the spitak (ms = 6.9) and loma prieta (ms = 7.1) earthquakes: comparative analysis, geophys. res. lett., 19, 1495-1498. molchanov, o.a., m. hayakawa and v.a. rafalsky (1995). penetration characteristics of electromagnetic emischen et al. 70 71 sions from an underground seismic source into the atmosphere, ionosphere, and magnetosphere, j. geophys. res., 100, 1691-1712. satoh, t., h. kawase, t. iwata, s. higashi, t. sato, k. irikura and h.c. huang (2001). s-wave velocity structure of the taichung basin, taiwan, estimated from array and single-station records of microtremors, b. seismol. soc. am., 91, 1267-1282. schisselé, e., j. guilbert, s. gaffet and y. cansi1 (2004). accurate time-frequency-wavenumber analysis to study coda waves, geophys. j. int., 158, 577-591; doi: 10.1111/ j.1365-246x.2004.02211.x. telesca, l., m. balasco, g. colangelo, v. lapenna and m. macchiato (2004). investigating the multifractal properties of geoelectrical signals measured in southern italy, phys. chem. earth, 29, 295-303. telesca, l. and k. hattori (2007). nonuniform scaling behavior in ultra low frequency (ulf) earthquake-related geomagnetic signals, physica a, 384, 522-528; doi: 10.1016/j.phys a.2007.05.040. torrence, c. and g.p. compo (1998). a practical guide to wavelet analysis, b. am. meteorol. soc., 79, 61-78. yen, h.y., c.h. chen, y.h. yeh, j.y. liu, c.r. lin and y.b. tasi (2004). geomagnetic fluctuations during the 1999 chi-chi earthquake in taiwan, earth planets space, 56, 39-45. *corresponding author: jann-yenq liu, national central university, institute of space science, chung-li, taiwan email: jyliu@jupiter.ss.ncu.edu.tw. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. propagation of seismo-magnetic signals vol52,6,2009new 539 annals of geophysics, vol. 52, n. 6, december 2009 key words barnabites – letters – seismology – meteorology – 19th century 1. introduction the valuable study of barnabites’ scientific correspondence finds naturally its privileged humus in the archives – «place of memory» par excellence – always more fluctuating between the demands of preservation and those of the valorization of documentary patrimony which jealously preserves (1). this constant tension, though in alternate phases, allows to glimpse today with more clarity the possibility to quash new «frontiers» on the usual controversial camp of accessibility and of the use of archivistic material. in fact, the choice of abandonment of comfort behind the lines of a mere documentary conservation is no longer tenable as end in itself, to open up to the multiple possibility of its adequate valorization, possible, also thanks to external collaboration, on the basis of the utilization of vanguard experimental techniques and the new innovative mode. the preserved patrimony in situ is certainly of interest not only for the history of the order of the barnabites, who is the first depositary, but also for the entire international scientific community. before being an honor, however, it is no minor responsibility. in the current state of the topic, and from known archives, like the vatican secret archive, which in some way creates an inclination among specialists in this field, the advantages from the application of such development politics appear remarkable: • first of all, for the congregation to which these religious members belong, thanks to the congregation, famous scientists and that, thanks be to them, it can be made visible as a meaningful cultural depositary subject of their precious scientific inheritance; • for the archive concerned, particularly small ones, which can enter in to dialogue with the scientific community benefiting from a specialistic collaboration, seeing to it as projected in a wide range and resonance in cultural projects, thanks also to those new possible management collaborations, such as digital scanning, of its documentary asset. the sciences of the earth in the epistolary archives of the barnabite scientists filippo lovison centro studi storici dei pp. barnabiti, roma, italy abstract this short paper intends to highlight the uniqueness of scientific commitment – not rarely of a high level – of some members of religious orders who, among other duties of their sacerdotal ministry, dedicated themselves particularly to in the study of sciences of the earth, realizing as such a fertile union between «pity and science», typical of the common spiritual and cultural formation of members of the regular clerics of saint paul, called barnabites. mailing address: dr. filippo lovison, centro studi storici dei pp. barnabiti, piazza benedetto cairoli 117, 00186 roma; e-mail: filovis@tin.it vol52,6,2009new 2-12-2009 12:39 pagina 539 540 f. lovison • for the memory of those same religiousscientists. escaping from the solitude in which – at times – they have been left by those who for various reasons were no longer able to make follow-ups to their studies and researches, can do resume the life of their thoughts and so bring their theories out from dust of their bookshelves. in this way their discoveries can be given values, in a more appropriate way, from the part of the same scientific community who much appreciated and recognized the results, most of all through informal epistolary correspondence. and, last but not the least, advantages for the same scientific community, which in such a way notices the enrichment and the completion of the history of science, the history of scientific instruments and so on, filling the unavoidable gap and receiving new impulses for research. this pioneering way for the order of the barnabites is just at the start. the other public, private and ecclesiastical archives all question on the problems raised through the use of the new means offered by modern technology, on the fixation of the limits of utilizing paper materials reproduced with digital scan, on the safeguard of property rights, on the protection of «archivistic» authorship of the documents (2). the concern for this field of historical research is simplified by the proliferation of publications concerning epistolary correspondence regarding very different aspects of the life and word of the illustrious civil and ecclesiastical personalities, such as scientific correspondence of today by religious astronomers, meteorologists, volcanologists, etc. the scientific correspondence is hence destined to play a more meaningful role in the reconstruction of personal scientific journey and the paradigms of reference, furnishing a remarkable contribution in to research. 2. science in the convent in this particular light, we have to point out that the attention for science in general has accompanied the barnabites throughout their centuries-old activities (3). since the opening of the first public schools in 1608 – even «delayed» with respect to other religious families like the jesuits – they worked for science, particularly in the study of natural philosophy. though suffering enough, they did not stop, not even before the apparent disagreement between «science and faith», caused by the topic of biblical inherence. so, many barnabites in the 17th century applied the experimental method with diligence, reaching in the 17th century a distinct scientific orientation of cartesian matrix, to the a point that the works of copernicus, like those of newton, can be well viewed in the library of the mother house of saint barnabas in milan (4). in the beginning of the 17th century the scientific, physics and mathematical disciplines, entered fully into the scholastic teaching of the order, giving a wide space in the colleges to experimental method. even if the illustrious scientists who alternated themselves among those teaching posts were not like or inexplicable meteors that appear unexpectedly in the firmament of the small but lively life of the order, there was a precise background, which well explains their extraordinary commitment and scientific success, in the great effort and, at times, in bitter misunderstanding. one of the characteristics of the barnabites’ secular teaching tradition was that it has never limited itself only to the literary field, leaving ample space to the scientific, most of all when the teaching of the sciences was at its start and not expected in the schools (5). so in the beginning of the 19th century carlo porta, meneghin poet, in his famous caudate sonnet in which enumerated the best milanese brains, in the field of physics nominates only four barnabites: «risega, racagn, de regis, pin»; namely the fathers paolo frisi (17281784) (6), mathematician and astronomer, giuseppe racagni (1742-1822), francesco de regi (1720-1794), mathematician and plumber and, ermenigildo pini (1793-1835), scientist and naturalist. in the 19th century we can see the congregation structuring itself as an extraordinary scientific laboratory. every school sees scientific teaching posts of national and international fame; every college has its astronomical observatory, often financed by the father general; every community had its seismic and meteorological observavol52,6,2009new 2-12-2009 12:39 pagina 540 541 the sciences of the earth in the epistolary archives of the barnabite scientists tory, even if at times relegated to a bell tower cell, but always with great attention to the scientific method. every scientific branch boasted its experts: from astronomy to seismology, from meteorology to botany, from mathematics to numismatics, from archeology to egyptology, without mentioning the literary side. among others we recall: ambrogio mazenta (1565-1635), architect and plumber; redento baranzano (1590-1622) scientist and astronomer (7); angelo cortenovis (bergamo 1727-1801); francesco stella (1745-1800) in udine, wo was the first in italy to elevate aerostatic balls in the sky on that memorable february 24 1783, while the montgolfier brothers held such experiment at annonay on the 5th of june in that same year; giovanni cavalieri (1807-1874), optician, physician, astronomer, professor of physics and sciences at the college of monza (8), pietro monte (tonengo di mazzè, august 21, 1823), professor in mathematics and physics, as well as founder of the astronomical observatory of livorno (9); bolognese by birth, but florentine by deeds, timoteo bertelli (10) (1826-1905), seismologist, who opposed to denza his system of the air causes of the variation of the pendulum; the neapolitan francesco denza (11) (1834-1894), founder of the italian meteorological company and director of the vatican observatory, the observatory of moncalieri and representative of the society of the tridentini mountaineers and the section of domodossola of cai; the pisan camillo melzi d’eril (1851-1929), seismologist and historian of science in florence; the fathers leonardo matera (12) (1811ca.1871), teacher in mathematics, physics and philosophy, and giuseppe pellanda (13) (1865-1928) in naples; luigi guanzati (14) (1757-1836), «naturalist» philosopher known for his studies on the infusers, in lodi; giuseppe boffito (1869-1944, historian of science and technique, bibliographer of aeronautics and meteorology, learned and distinguished bibliophile (15). all this spilled out naturally in the teaching of the students, who in their scholastic essays amazed everyone, declaring themselves capable fig. 1. detail of a letter to t. bertelli with the project for the gas-driven velocipide (historical barnabite archives, rome). vol52,6,2009new 2-12-2009 12:39 pagina 541 542 f. lovison – and they truly were – to resolve «whatever equation and whatever problem» (16). the level of instruction was so high, it was more than enough to look through those essays at the end of the school year that might pale the students of today, and even some professors, as shown by the presence of the countless scientific instruments still kept today in these old colleges, of fundamental importance for the study of science (17). entire colleges dedicated themselves to the study of the science of the earth. for example the schools of the barnabites in udine (1679-1810), with their academies of agriculture which should have given the peasants the necessary agrarian instruction, a conception of science, as that of a practical knowledge addressed to the satisfaction of the real needs of existence. among all in this field the barnabite angelo cortenovis distinguished himself as a man of great intellectual stature, in charge of the collegio udinese s. lorenzo giustiniani, in epistolary contact with major scholars of his time, who was nominated in 1788 perpetual secretary of the academy of udine, who welcomed the introduction in the college of subjects connected to the science of agriculture, presented to him by antonio zanon. this was not only studied, but also happily applied in the territorial visits and in the appreciated agricultural academies (18). 3. the epistolary archives first of all, it is necessary to take note that a history of the scientific culture among the barnabites has not yet been written. there is not even an account of the monographs and updated scientific biographies of much more important exponents from the 17th century up to this period, like for example the fathers baranzino, modroni and mazenta. based on the incomplete and insufficient work of cataloguing done from then and also based on the dispersion of the correspondence, is not possible to present an approximate frame of volume on the size of the epistolary archives of the barnabite scientists present in italy, concentrating instead on major details on the historical archives of the barnabites of rome (asbr) (19). giving a brief overview of the archives of the order to have an idea of their consistency it is enough to consider the heritage represented by around 9000 letters with around 1,400 senders only to fr. francesco denza, who worked in the reale collegio «carlo alberto» of moncalieri. the largest number of letters to fr. denza are preserved in the asbr in rome and part of those are at the vatican observatory. about cavalleri we can say that he was in correspondence with numerous scientists of his time, for example with g.b. amici (the correspondence is kept in the estense of modena), with a. secchi (some letters addressed to him are conserved in the archives of san barnaba), with g. schiapparelli, with timoteo bertelli, etc. in the asbr of rome we can find interesting epistolary correspondence regarding the earth sciences, especially in relation to melzi, bertelli and boffito. in particular: – folder melzi-alfani, 1: contains interesting letters regarding, for example, the tromometric observations in the two earthquake shocks of november 29, 1986, indicating the values of the length of the pendulum and the length of trace (guido alfani, collegio vescovile di gavi (livorno), december 2, 1896). – folder me, 1, family and general letters with scientific character. – bertelli archive, epistolary, 1861-79, folder i. – bertelli archive, epistolary, 1870-1904, folder ii. – epistolary and scientific material of various kinds not in catalogue, notes, schemes, designs, etc., existing in many other folders (fig. 1). – folder me, 3, scientific correspondence to fr. melzi no. 21 (20). – boffito archive. composed of various folders which chronologically divide all his most abundant sent and received correspondence; around 4000 writings, with correspondents from every side of the earth, private bodies and institutes like universities, libraries, meteorological and astronomical observatories, etc. some senders are particularly famous: giovanni gentile, benedetto croce, arturo graf and, in particular, in as much as interested in this study the meteorologist g. hellman, francesco porro (21). vol52,6,2009new 2-12-2009 12:39 pagina 542 543 the sciences of the earth in the epistolary archives of the barnabite scientists only a small part of this important heritage was used in the 1990s by g. ferrari (22) in the tromos project to reconstruct an important experimental path started in 1870 by bertelli and that led to the invention of the normal tromometer, in collaboration with de rossi, the first instrument for the observation of earthquakes installed in about fifty public and private italian observatories. in particular, by means of the letters received by bertelli between 1861 and 1879 ferrari (1991) reconstructs the different phases of the design of the instrument and its complex and controversial affirmation among italian scholars affiliated to the network of correspondents of the bullettino del vulcanismo italiano, the first geodynamics review in the world, founded, drafted and funded by de rossi from 1874 to 1898. of the over 500 letters received by bertelli between 1861 and 1905, the year of his death, 208 were used by ferrari to illustrate the working hypotheses, the methodological and practical difficulties encountered by bertelli and de rossi, the discussions born among the scholars on the design and methodological work of bertelli (fig. 2). the most intense correspondence is that exchanged with de rossi (77 letters), filippo cecchi (16), director of the ximeniano observatory of florence and worthy designer of seismic instruments, luigi guidi (10) director of the observatory of pesaro, cavalleri (6), barnabite brother, whose great experience in testing instruments was of fundamental help for the work of bertelli. the correspondence with de rossi was at times almost daily and affected all the phases. some letters are complete with explanatory drawings (fig. 3) with the design suggestions of de rossi on the tromometer initially invented by bertelli and completed by de rossi himself. fig. 2. detail of a letter by g. cavalleri to t. bertelli, [monza] 24th [march] 1874, in which the brother gives some suggestions on the illumination system of the tromometer measuring apparatus (historical archives of barnabites, rome). vol52,6,2009new 2-12-2009 12:39 pagina 543 544 f. lovison 4. conclusions the aim of these few pages is therefore only to mark another small lap of that slow journey of those responsible for these archives towards the right equilibrium between preservation and valorization, for that debt of gratitude towards the cited illustrious barnabite scientists, such as giovanni cavalleri, timoteo bertelli, francesco denza, camillo melzi d’eril to whom the scientific world today looks at with deep gratitude. the huge amount of letters of these important earth scientists is very important for future research both from the historical and scientific point of view. history of meteorology from mid 19th century to 1920’s and of seismology to 1930’s will benefit greatly from the study of these letters. however, these objectives can only be achieved if the constant tension inside the archives between conservation and valorization is overcome. today it is possible to see more clearly the chance to quash new «frontiers» on the ever controversial field of the accessibility and use of archive material. the choice of abandonment of comfort behind the lines of a mere documentary conservation as end in itself is no longer tenable, and the multiple possibility of its adequate valorization be made possible also thanks to external collaboration, on the basis of the utilization of vanguard experimental techniques and the new innovative mode. the preserved patrimony in situ is certainly of interest not only for the history of the order of the barnabites, which is the first depositary, but also for the entire international scientific community. before being an honor, however, itis no minor responsibility. fig. 3. detail of a letter of m.s. de rossi to timoteo bertelli, rome, 10th march 1875, in which the scholar puts forward the definition and dissemination of a standard version of tromometer and makes a sketch of it (historical archives of barnabites, rome). vol52,6,2009new 2-12-2009 12:39 pagina 544 545 the sciences of the earth in the epistolary archives of the barnabite scientists notes (1) the concept of «place» is fundamental for the existence of an archive. ulpiano, for example, defines it as «locus publicus in quo instrumenta deponuntur», not to be understood only in the mere sense and in passive material conservation, which instead of archivistic activity it is a less important aspect, but most of all in the acceptance of an active role of scientific research, which includes the arrangement, the inventory, and enhancement of different funds. besides, the concept of «place» extends today even in «virtual» space. the barnabites show their sensitivity with regard to this aspect since the choice of the name: historical studies center given to their research organism, which recalls the type of a dynamic cultural subject, propulsor of the documentary exploitation. according to article 8 of its statute, the center poses its primary role as «to gather, to arrange, preserve and exploit the book trade and documentary asset of the congregation […]» and, beside the activity of the members which composes it, that is «to assist researchers […], to participate in scientific and cultural manifestations which make them concern in whatever way of the congregation […], to maintain the relation with cultural institutions in the name of the congregation». (2) an experiment of this type, even though signed by understandable pauses of reflection, is actually of route through the concern of the historical studies center, the barnabite scientists engaged in the studies of sciences of the earth (meteorology, seismology, etc), like francesco denza, timoteo bertelli, camillo melzi d’eril. thanks to the sga (storia geofisica ambiente di bologna – geophysical environment history of bologna), it entered in the project tromos – an important project of scientific and historical recovery and of the enhancement of the italian seismological tradition, actuated under the patronage of the national institute of geophysics and volcanology (ingv) – that intends to enhance the scientific correspondence on sciences of the earth and to promote the salvation and the enhancement of the scientific instrumentation. this collaboration, already developed in 1986 with the college «alla querce» in florence, has brought up to the reproduction to microfilm and/or electronic as part of scientific materials conserved therein, and between 1990 and 1992 proceeded to the restoration, and in part to the exposure in national exhibits, of the instruments of bertelli, as well as the publication of the different volumes with the results of the the said researches (ferrari and bianchi, 1997; ferrari, 1991; 1997). (3) cfr. in particular, for the study of the arcimboldi schools inaugurated in milan in november 3, 1608 (bianchi, 2002). (4) the barnabite method in fact left ample freedom to the teaching staff to follow «where it doesn’t deal with definite things or from the established universal consensus, those sentence which might be better and well grounded to each one, far be it from every spirit of decision» (erba, 1981). (5) such «subtle feelings» were present in the dna of the order, which even the missionaries that departed, for example, for the mission in the kingdoms of ava and pegù (burma, now myanmar) in the 18th century, could not make without dedicating themselves, even in those far lands in south-east asia, to scientific research, particularly as regards the sciences of the land that they covered in italy the teaching posts in theology, arts, philosophy, etc., in different colleges of their own order and, even at prestigious universities. for this reason they were able to give a fundamental scientific contribution to the cultural development of the country. their expertise in phenomena of astrology, mathematics, physics, cartography (a subject that they teach at bramini), and their single studies, must not be forgotten, like that, for example, of p. d’amato, with his short description of the mines of precious stones, in the district of kiat-pyen in the kingdom of ava. moreover msgr. gaetano mantegazza made the first scientific treatment of history, geography and ethnography of the country, with his realation of the kingdoms of ava and pegù, dated 1784. this was equipped by the first two much important geographic maps of burma). (6) barbarisi, 1987. (7) author of the uranoscopia seu de coelo, geneva 1617 and the nova de motu terrae copernicano justa summi pontificis mentem disputatio, coloniae albrogum, apud petrum et jacobum chouet, 1617. of particular importance is his epistolary, where the letter of francis bacon addressed to him, can be found: «[…] i willingly read your letters […] and your works, by now so famous, they were written in subtle and diligent forms. you write about physics, and i will think about it like you do» (letters and life of bacon, 1874; colombo, 1878). on his figure and on his courageous works of copernican matrix cfr. dizionario biografico degli italiani (dbi); de leo and d. filippone, 1986; colombo, 1878 cit.; sortais, 1907. (8) on his figure and activity: boffito, 1993; riboldi, 1875; magni, 1933; lucatello, 1949. (9) cfr. dassano, 1998. founded the meteorological observatory in the barnabites’ house at san sebastian in livorno, placed on the last floor, above the roof of the building, in two little rooms. at the start of the first daily registrations, fr. monte was soon contacted by the imperial observatory of paris, founded in 1855 to gather meteorological information purely for military use. with these words he described, in april 24, 1870, his febrile activity, exercised with great passion, between one lesson and another, at royal lyceum of livorno: «the assisvol52,6,2009new 2-12-2009 12:39 pagina 545 546 f. lovison tance of an observatory is so laborious; it takes my time for at least four hours a day and it requires a great sacrifice on my health and interest. yet i continue till i can, if the municipality believes, in its sensitivity, to give me whatever yearly cheque […]. the observations be done at 6 a.m., at 9 a.m., 12 noon, 3 p.m., 6 p.m. and 9 p.m.; that is, every three hours and then the particular extraordinary observations. the observations are these: on the barometer, thermometer, magnetometer, pluviometer, anemometer, ozonometer […]». he published many of his studies: di alcuni perfezionamenti alla macchina di atvood. memoria […] parma, tip. reale, 1855; the meteorological observations done in the imperial and royal lyceum of livorno by professor pietro monte barnabite, 1858, livorno 1859; fallacità dei sistmometri isolati, gazzetta livornese n. 1090, january 24, 1875; etc. (10 ) cfr. dbi, n. 9, 501-503. (11) cfr. dbi n. 38, 804-806. (12) matera, 1850. some notes mss. for his biography can be found in the historical archives of the barnabites in rome (from that time on asbr). (13) pellanda, 1906. (14) guanzati,1796. on his figure see dbi, n. 60, 245-246. (15) acts of convention at gravi (alessandria) september 11-12, 1982; dbi n. 11, 167-170. (16) cfr. the exterarum scholarum disciplina apud clericos regulares s. pauli, mediolani 1666 of melchiorre gorino (1605-1701). (17) look for example riccadonna et. al., 2002; boffito, 1982. (18) moreover, fr. mariano alpruni (1733-1816) was an agronomist and appraised siviculturist. there were publications about some letters of fr. angelo m. cortenovis in his work elogio e lettere familiari del padre angelo m. cortenovis, milan 1862, addressed to his brother pier maria and to the scholar fr. giovenale sacchi, omitting unfortunately those purely scientific arguments «because alien from the aim of our collections of lives, and because it would have been professions equipping them of the incision of monuments, ancient medals and coins de coded by the author» (preface of the editor, v). (19) in 1992 there came the idea of a joint publication on fr. denza, and the proposal of a collaboration even inside our publications for the full valorization of the scientific contribution of our famous fathers in the seismological field. but the time was not yet mature. (20) undoubtedly the preserved material in the reviewed letters has a notable importance, since dealing with immediate responses to queries coming from every side of the earth, even to the most curious, like the project concerning an «apparatus [which] after being improved will allow us ith profound studies, to travel by land and for the air spaces of the sky», sent to fr. bertelli: «new gas for flying electric ball animated spirit gas. flying man with a new gas animated with mechanical electric wings, above a bicycle and blown up ball of gas spirit essence with relative storage recipient of spirit gas. very clear mr. professor father astronomer bertelli. the varnisher allows us to present this project. to you excellent father, if you believe, the constant study for resolving it. thank you very much. his servant […]. this improved apparatus will allow with deep studies to travel by land and for the air spaces of the sky. extracted gas spirit from the purified trees from the watery more thast the forcepart. a project of a ball dirigible through a bicycle with two wings on the sides; of a bicycle to be operated through pedals and electricity, to be applied for the movements of the machine to open up the wings, and for closing it into form of a flying animal. the aerial globe of support made in a form of a chifelle; containing only 10 cubic meters of spirit gas; with recipient annex to the machine with an essence of living spirit gas; to increase its force, to increase or to decrease (the anonymous fake flying animal). and here is the secret of the new living spirit gas; made of live wood, animated and spirituous, and gathered with cane of rubber and closed hermitically in an iron vase as essence of spirit gas, all the reunited force; both spirituous gas and gassy, it has to be gathered with many tubes of rubber and through the operator of rubber can be brought into the iron storage and has to be purified in the watery part and is called spirit gas which will be the 50% much more than the usual gas which is made up of fossil carbon dead soul and no longer vegetarian and of a lightness of 50% of the usual». (21) some examples: «monsieur joseph boffito, collegio alla querce, florence. monsieur, nous avons bien reçu votre lettre du 12 courant et votre «chronicon meteorologicum». c’est un fort beau travail, malheureusement il est trop important pour les resources don’t nous disposons actuellement. peut-être dans quelques années, lorsque notre périodique aura pris un certain déeveloppement, pourrons nous disposer de sommes suffisantes pour envisager la publication de volumes pareils à celui dont vous nous proposez aimablement l’impression. je me demande si l’académie de lincei, par l’intermédiaire de la commision d’étude italianne, ne pourrait par envisager la publication de votre travail. voulez-vous que jet ente une démarche auprès de cette commission par l’intermédiaire de m. ciraolo? je la ferais très volontiers. en vous félicitant de votre beau travail et dans l’attente de vos instructions, je vous prie de croire, monsieur, à l’assurance de mes sentiments les plus distingués. raoul montandon» (asbr, boffito archive, geneva may 24, 1927). montandon was the editor in chief of the scientific magazine matériaux vol52,6,2009new 2-12-2009 12:39 pagina 546 547 the sciences of the earth in the epistolary archives of the barnabite scientists pour l’étude des calamites, publiés par le soins de la société de géographie de genéve. another example: «very illustrious professor and dear colleague. for many reasons (excuse me for my bad italian) i thank myself for having the occasion of writing to you. my readings of the «universo» of florence, this notable italian scientific publication, is much more than «the practical meteorology» have tied me to you. – and i give to myself the freedom to translate them into french for our ciel et terre publication of astronomy and physics of the globe etc. […] about his studies on the […] discovery in pompei, and the dioptra of archimedes – the translation of the dioptra of hipparco will follow, if you allow me. all of these would mean that i and our «ciel et terre» would be honored for sending to us your writings which is interests us and the pure science and the history of this science of your country where i gained a lot of friends – like fr. alfani and professor abetti and professor ronchi etc. […] (in florence). with the hope for a good scientific collaboration, please receive my letter – from a great friend of italy and a friend of your science too. e. lagrange» (asbr, boffito archive, bruxelles, november 29, 1927). lagrange was the director of the magazine ciel et terre and vice president of the belgian company of astronomy. (22) see nt. 2. references acts of convention at gravi (alessandria) (1982): giuseppe m. boffito, barnabita, un erudito del novecento, september 11-12, (leo olschki editore, firenze, italy), mcmlxxxiv. bacon, f. (1874): the letters and the life of francis bacon, edited by spedding j., (longman, london), vii, 374-377. barbarisi, g. (1987): ideology and science in the opera of paolo frisi (1728-1784), (franco angeli, milano, italy). bianchi, a. (2002), barnabiti studi, 19, 55-78. boffito, g. (1982): gli strumenti della scienza e la scienza degli strumenti con l’illustrazione della tribuna di galileo, (multigrafica, roma, italy), (published for the first time in 1929). boffito, g. (1993): barnabite writers, i, (olschki, firenze, italy), 443-453. colombo, g. (1878): intorno alla vita e alle opera del padre redentor baranzano (about the life and works of fr. redento baranzano), (v. bona, torino, italy), 58-61. dassano, f. (1998): pietro monte. scientist, teacher and founder of the nursery school of tonengo, (bolognino editore, ivrea), 79, (review edited by f. lovison, in eco dei barnabiti, 3, 2003, 53-54). de leo, a. and d. filippone (1986): encounter with baranzano: a discovery, barnabiti studi, 3, 151-159. dizionario biografico degli italiani (dbi), treccani editore, www.treccani.it. erba, a. (1981): the schools and the pedagogical tradition of the barnabites, experiences of christian pedagogy, i roma, 173. ferrari, g. (ed.) (1991): seismographic tromometer alarms. observations and theories from 1850 to 1880, (ed. sga, bologna). ferrari, g. (1997): cultural and scientific value of seismology’s heritage in europe: why and how to preserve, extracted from the cahiers du centre europeen de geodynamique et de seismologie, vol. 13, luxemburg. ferrari, g. and m. bianchi (1997): census, cataloguing and elaboration of scientific letters in seismology, extracted from the cahiers du centre europeen de geodynamique et de seismologie, vol. 13, luxemburg. historical barnabites archives (asbr), roma, italy. guanzati, l. (1796): osservazioni ed esperienze intorno a un prodigioso animaluccio delle infusioni, in «opuscoli scelti», milano, xix, 3-31. guanzati, l. (1819): corso di storia naturale (ms) and the relazione dei perfezionamenti arrecati dal sig. stroppa alla macchina inventata dal christian con cui si preparano senza macerazione il lino e la canapa. lucatello, e. (1949): scientist priests, (società editrice life and thought, milano, italy), 139-140. magni, g. (1933): p. giovanni maria cavalleri, physician, 1807-1874 in the barnabites at monza in the fourth centenary of the approval of the order 1533-1933, (tipography of the mission, milano, italy), 85-90. matera, l. (1850): elementi di trigonometria rettilinea, e sferica compilata per le scuole de’ barnabiti dal p.d. … , naples, from reale tipografia militari. michelini, v. (1956): la «ratio studiorum» and the educative method of the barnabites, quaderni per la scuola cattolica, 2. pellanda, g. (1906): la flora estiva dei monti d’oropa, (tipografia di g. testa, biella, italy). riboldi, a. (1875): biographical outlines of m.r.p.d. giovanni m. cavalleri barnabite (extracted from «the catholic school», exercises xxvi and xxvii), (serafino grezzi, milano, italy), 48. riccadonna, g., p. tucci, m. zaninelli and m. zulian (2002): la scienza in collegio. strumenti scientifici del collegio s. francesco in lodi, (treg, guardamiglio (lodi), italy), 162, s.i.p. sortais, g. (1907): the process of galileo: historical and doctrinal study, (roma, desclee e comp. editori). vol52,6,2009new 2-12-2009 12:39 pagina 547 l a s t r u t t u r a d e l l ' a t l a n t i c o ( * ) j . p . rothé l o s t u d i o d e l l a s t r u t t u r a d e l l ' a t l a n t i c o s o l l e v a u n p r o b l e m a , c h e h a e i a a t t i r a l o l ' a t t e n z i o n e d i n u m e r o s i r i c e r c a t o r i . i l e t t o r i d i « a n n a l i d i g e o f i s i c a » h a n n o d i r e c e n t e p o t u t o r e n d e r s i c o n t o i n d u e r i p r e s e d e i l a v o r i d e i n o s t r i c o l l e g l l i i t a l i a n i p . c a l o i , l . m a r c e l l i e g . p a n n o c c h i a ( ' ) ( 2 ) : l e r i c e r c h e d i q u e s t i a u t o r i f i p r o p o n g o n o d i c i m e n t a r e l a « i p o t e s i d i l a v o r o » d a m e s v i l u p p a t a d i n a n z i a l c o n g r e s s o d e l l e s o c i e t é s s a v a n t e s f r a n c e s i , r i u n i t e a s t r a s b u r g o n e l l ' a p r i l e 1 9 4 7 e p u b b l i c a t a s u c c e s s i v a m e n t e i n u n a n o t a p r e s e n t a t a i l 2 a p r i l e 1 9 4 7 a l l a a c a d é m i e d e s c i e n c e s d i p a r i g i ( : , ) ( 4 ) . r i t e n g o o p p o r t u n o r i c o r d a r e q u e s t a i p o t e s i n e l p r e s e n t e l a v o r o e d a p p o r t a r e , a m i a v o l t a , a l c u n i r i l i e v i s u i r i s u l t a t i , o t t e n u t i d a i n o s t r i c o l l e g l l i i t a l i a n i , i n s i e m e c o n n u o v i a r g o m e n t i r i c a v a t i d a i l a v o r i r e c e n t i . * * # 1 . — è n o t o c h e l a o s s e r v a z i o n e d e l l a a n a l o g i a f r a i c o n t o r n i d e l l e c o s t e a m e r i c a n a e a f r i c a n a d e l l ' o c e a n o a t l a n t i c o s p i n s e w e g e n e r a l l a f o r m u l a z i o n e d e l l a sua t e o r i a s u l l a d e r i v a d e i c o n t i n e n t i . o r a , d o p o a v e r e e s a m i n a t o n u m e r o s i f a t t i d ' o r d i n e s i s m o l o g i c o o m o r f o l o g i c o , s o n o s t a t o i n d o t t o a p r o p o r r e l a s e g u e n t e i p o t e s i . l'oceano atlantico si divide in due domini: uno dei quali, a est della cresta mediana, e un dominio a struttura continentale, collegantesi col blocco africano; invece la parte occidentale del dominio atlantico è, con tutta probabilità, a struttura pacifica, il che significa che esso è costituito esclusivamente di sima. i n q u e s t e c o n d i z i o n i , i l p a r a l l e l i s m o f r a l e c o s t e a f r i c a n a e a m e r i c a n a , i n v o c a t o da w e g e n e r , d u t o i t , e c c . d e v e , i n r e a l t à , e s s e r e r i c e r c a t o f r a l a c o s t a a m e r i c a n a e la c r e s t a n ^ e d i a n a d e l l ' a t l a n t i c o , l e c u i f o r m e g e n e r a l i s o n o l e s t e s s e ( f i g . 1 ) . s e d e r i v a c'è s t a t a , essa si è v e r i f i c a t a u n i c a m e n t e s u l l a d i s t a n z a c r e s t a m e d i a n a c o s t a a m e r i c a n a . l a c r e s t a m e d i a n a a t l a n t i c a e l a sua z o n a s i s m i c a c o r r i s p o n d o n o al r i l i e v o l i m i t e d e l b l o c c o a f r i c a n o , c h e v i e n e a u r t a r e l o z o c c o l o a s t r u t t u r a s i m a t i c p , c o s t i t u e n t e l a p a r t e o c c i d e n t a l e d e l l ' a t l a n t i c o . 11 v e r o o c e a n o a t l a n t i c o è l a a m p i a f e s s u r a a p e r t a a o v e s t d e l l a c r e s t a m e d i a n a f r a i b l o c c h i c o n t i n e n t a l i e u r a f r i c a n o e a m e r i c a n o . l ' i p o t e s i p r o p o s t a t o g l i e l ' o b b i e z i o n e a l l a d e r i v a d e i c o n t i n e n t i , r i g u a r d a n t e l ' e s i s t e n z a s e m p r e p i ù p r o b a b i l e di u n c o n t i n e n t e a t l a n t i d e , a f f o n d a t o t r a l e a z z o r r e e il d i s t r e t t o d i g i b i l t e r r a . e s s a s p i e g a i n o l t r e c o m e , q u a n d o si m i s u r a l a v e l o c i t à m e d i a d e l l e o n d e s i s m i c h e attraverso vatlantico, considerato come un tutto, si t r o v a u n v a l o r e i n t e r m e d i o f r a il v a l o r e d e l l a v e l o c i t à ni d i s o t t o d e l p a c i f i c o e q u e l l o al di s o t t o d e i c o n t i n e n t i . ( * ) t r a d u z i o n e in l i n g u a i t a l i a n a d e l l a n o t a d i p a g . 2 7 . 1 1 8 j . p . hothé 2 . — i f a t t i s i s m o l o g i c i e m o r f o l o g i c i , c h e n e l 1 9 1 7 a v e v a n o a t t i r a t o l a m i a a t t e n z i o n e , p o s s o n o r i a s s u m e r s i n e l m o d o s e g u e n t e : i . — p a r t i c o l a r m e n t e i s t r u t t i v o è, a n z i t u t t o , l o s t u d i o d e l l a d i s t r i b u z i o n e d e g l i e p i c e n t r i s u l l ' i n t e r o g l o b o , q u a l e p u ò s t a b i l i r s i i n b a s e a l l e d e t e r m i n a z i o n i e s e g u i t e , già d a m o l t i a n n i , d a p a r t e d e l b u r e a u i n t e r n a t i o n a l d e s é i s m o l o g i e d i s t r a s b u r g o e a i l a v o r i , t r a i m o l t i s u q u e s t o a r g o m e n t o , d i g u t e n b e r g e r i c h t e r ( n ) : d a u n t a l e e s a m e si d e s u m e l ' e s i s t e n z a d i g r a n d i b l o c c h i s t a b i l i , l i m i t a t i d a z o n e s i s m i c h e a t t i v e ; f r a q u e s t i g r a n d i r i l i e v i si s t a c c a u n b l o c c o a s s a i p i ù e s t e s o di q u e l l o a t t u a l m e n t e d e f i n i t o d a l l e c a r t e g e o g r a f i c h e (.fig. 2 ) . q u e s t o b l o c c o a f r i c a n o è l i m i t a t o a o v e s t d a l l a z o n a s i s m i c a , c h e r i g u a r d a l a c r e s t a m e d i a n a s o t t o m a r i n a d e l l ' a t l a n t i c o , m e n t r e d a l b l o c c o e u r o p a a s i a l o s e p a r a l a z o n a s i s m i c a a l p i n a : q u e s t ' u l t i m a z o n a , c h e a t t r a v e r s a i l d o m i n i o d e l l a m e s o g e a , s e g u e , t o c c a n d o i l n o r d a f r i c a , l ' e u r o p a e l ' a s i a , le l i n e e d e i c o r r u g a m e n t i a l p i n i ( h i m a l a y a , c a u c a s o , c a r p a z i , a l p i . a p p e n n i n i , a t l a n t e , c o r d i g l i e r a b e t i c a ) e r a g g i u n g e la z o n a s i s m i c a m e d i a n a d e l l ' a t l a n t i c o n e i d i n t o r n i d e l l e a z z o r r e (*")• la raggiunge ma non la attraversa. c o n t r a r i a m e n t e a q u a n t o si r i t e n e v a n o n e s i s t e a l c u n i n d i z i o d i s i s m i c i t à f r a l a c r e s t a a t l a n t i c a e l ' a r c o d e l l e a n t i l l e , i l q u a l e , a s u a v o l t a , f a p a r t e d e l c e r c h i o s i s m i c o c i r c u m p a c i f i c o . c o s i p u r e l a c a r t a d e g l i e p i c e n t r i m o s t r a c h e l a g e o s i n c l i n a l e d e l l a m e s o g e a n o n a t t r a v e r s a l ' a t l a n t i c o d a p a r t e a p a r t e , n é r a g g i u n g e l a g e o s i n c l i n a l e p a c i f i c a , c o m e a p p a r e a n c o r a su c e r t e c a r t e c l a s s i c h e d e l l a s t r u t t u r a c o m p l e s s i v a d e l g l o b o . i i . — l y n c h (®) ha s e g n a l a t o c h e n e l t e r r e m o t o d e l 1 1 s e t t e m b r e 1 9 4 5 , p r o d o t t o s i s u l l a c r e s t a m e d i a n a a t l a n t i c a ( e p i c e n t r o 7",0 n ; 38".8 \\ i, la v e l o c i t à d e l l e o n d e s u p e r f i c i a l i di l o v e , m i s u r a t a a l l ' o s s e r v a t o r i o di f o r d h a m ( n e w y o r k ) , d i s t a n t e 510(1 k m d a l l ' e p i c e n t r o , e r a di 4 , 4 k m a l s e c o n d o p e r u n p e r i o d o d i 2 3 s e c o n d i . s i t r a t t a d i u n a v e l o c i t à i n d u b b i a m e n t e c o n f r o n t a b i l e e a n c h e u n p o ' s u p e r i o r e a q u e l l a d e l l ' o n d a di l o v e d e l l o s t e s s o p e r i o d o s o t t o i l p a c i f i c o . i i i . — i l m i o c o l l e g a g . d u b o i s , p r o f e s s o r e d i g e o l o g i a a l l ' u n i v e r s i t à d i s t r a s b u r g o , c o n i l q u a l e m i s o n o f r e q u e n t e m e n t e i n t r a t t e n u t o r i g u a r d o a l p r o b l e m a c o s ì a t t r a e n t e d e l l a s t r u t t u r a t e r r e s t r e , m i h a s e g n a l a t o l ' i n t e r e s s a n t e r i s u l t a t o , o t t e n u t o c o n i r i l i e v i b a t i m e t r i c i , r e c e n t e m e n t e e s e g u i t i n e l l ' a t l a n t i c o . essi d i m o s t r a n o c h e l ' a t l a n t i c o s u d p r e s e n t a , a d est d e l l a c r e s t a m e d i a n a , u n a s u c c e s s i o n e di b a c i n i e di c r e s t e o r i e n t a t e s w n e , l a q u a l e p r o l u n g a l e u n i t à m o r f o l o g i c h e d e l c o n t i n e n t e a f r i c a n o . q u e s t e u n i t à s o n o r i p o r t a t e i n fig. 1 i n b a s e a l l a c a r t a p u b b l i c a t a d a j . h . f . l m b g r o v e ("). i v . •— è n o t o c h e l a c o s t a o r i e n t a l e d e g l i s t a t i u n i t i e d e l l e p r o v i n c i e m a r i t t i m e d e l c a n a d a p r e s e n t a u n c a r a t t e r e m o r f o l o g i c o d e l t i p o pacifico ( i r i l i e v i s o n o a l l i n e a t i p a r a l l e l a m e n t e a l l a c o s t a ) ; i n v e c e l e c o s t e s p a g n o l e e b r e t o n i s o n o c o s t e a r i f l s d i t i p o a t l a n t i c o . v . — g l i a r c h i d e l l e a n t i l l e d e l n o r d ( p o r t o r i c o , m a r t i n i c a , g u a d a l u p a , ecc.) e d e l l e a n t i l l e d e l s u d ( g e o r g i a d e l s u d , s a n d w i c h , o r c a d i d e l s u d , ecc.) o c c u p a n o i n r a p p o r t o a l b o r d o o c c i d e n t a l e d e l l ' a t l a n t i c o , l a s t e s s a p o s i z i o n e ( f i g . 1 ) d e l l e ( ' ) v . p . e s . l a c a r t a a p . 72-73 i n : j . p . rothé, séismes et volcans ( c o l l e z i o n e « q u e s a i s j e ? » ) . p a r i g i , 1 9 4 6 . la s t r u t t u r a dell'atlantico 1 1 0 g h i r l a n d e i n s u l a r i ( c u r i l i , m a r i a n n e , ecc.) i n r a p p o r t o a l h o r d o o c c i d e n t a l e d e l p a c i f i c o . l ' a n a l o g i a è c o n f e r m a t a d a l l a s c o p e r t a , d o v u t a a f . y e n i n g m e i n e s z , d e l l ' a s s e d e l l e a n o m a l i e n e g a t i v e d e l l a g r a v i t à , c h e p r o l u n g a d a l l a t o d e l l ' a t l a n t i c o l ' a r c o d e l l e a n t i l l e . * * 4 3. — q u e s t a i p o t e s i è stata d a m e a v a n z a t a c o n l a s p e r a n z a d i s t i m o l a r e l e r i c e r c h e , c o s i c c h é i o h o l e t t o c o l p i ù v i v o i n t e r e s s e i l a v o r i d e i n o s t r i c o l l e g h i i t a l i a n i s o p r a m e n z i o n a t i ( 1 , 2 ) : d ' a l t r o c a n t o i l s e c o n d o d i q u e s t i d u e a r t i c o l i r i s p o n d e già p a r z i a l m e n t e a u n a n o t a p e r s o n a l e d a m e c o m u n i c a t a a l p r o f . c a l o i n e l l ' a p r i l e 1 9 5 0 , l o s v i l u p p o d e l l a q u a l e c o s t i t u i s c e l a p r e s e n t e m e m o r i a . p u r t u t t a v i a , d i n a n z i a i f a t t i a p p o r t a t i , s o n o u n p o ' m e r a v i g l i a l o d e l l e c o n c l u s i o n i f o r m a l i a l l e q u a l i a r r i v a n o q u e s t i a u t o r i . e s s i m i p e r m e t t e r a n n o d i p r e s e n t a r e q u i a l c u n e o s s e r v a z i o n i . l a l o r o r i p r o v a d e l l a m i a i p o t e s i si b a s a s u l c a l c o l o d e l l a v e l o c i t à di p r o p a g a z i o n e d e l l e o n d e s u p e r f i c i a l i e i n p a r t i c o l a r e d e l l e o n d e d i l o v e a t t r a v e r s o l ' a t l a n t i c o a p a r t i r e d a u n e p i c e n t r o s i t u a t o s o t t o l a c r e s t a m e d i a n a a 8" 0 2 ' d i l a t i t u d i n e n o r d e 38" 4 ' di l o n g i t u d i n e o v e s t . r i v o l g e r ò l ' a t t e n z i o n e s u i p u n t i s e g u e n t i : i. — l a fig. 8 , p u b b l i c a t a d a l p r o f . c a l o i e c o l i . ( ' , p a g . 3 5 8 ) , l a q u a l e r i a s s u m e i r i s u l t a t i d e l l e m i s u r e , m o s t r a c o n c h i a r e z z a — i n u n a m a n i e r a q u a s i i n s p e r a t a ! — l a s e p a r a z i o n e d e l l e s t a z i o n i i n d u e g r u p p i ( f i g . 3 ) . a) i l g r u p p o d e l l e s t a z i o n i a m e r i c a n e , s i t u a t e i n p r o s s i m i t à d e l l ' a t l a n t i c o ( s a n j u a n , b e r m u d a , h a l i f a x , h a r v a r d , o t t a w a ) : l e v e l o c i t à m i s u r a t e s o n o a s s a i f o r t i ; b) i l g r u p p o d e l l e s t a z i o n i e u r o p e e , al q u a l e si a g g i u n g e i n m a n i e r a a s s a i c a r a t t e r i s t i c a l a s t a z i o n e d i t a n a n a r i v e : l e v e l o c i t à s o n o m o l t o p i ù d e b o l i c o n p e r i o d i u g u a l i . i n q u e s t a c l a s s i f i c a z i o n e r i g o r o s a c o m p a r e un'unica anomalia: l a g r a n d e v e l o c i t à r i s c o n t r a t a a l l a s t a z i o n e d i l i s b o n a ( d i s t a n z a 1 4 8 0 k m ) . s i o s s e r v e r à c h e p e r q u e sta s t a z i o n e i t r e n i d ' o n d a s s e s s s d e v o n o e m e r g e r e r i s p e t t i v a m e n t e a 1 9 h 5 1 ' " 34* e 1 9 h 52"' 1 8 s ; a q u e s t a d i s t a n z a e s s i p o s s e g g o n o già u n p e r i o d o r e l a t i v a m e n t e g r a n d e e d h a n n o p o t u t o m a s c h e r a r e l ' a r r i v o d e l l e o n d e d i l o v e . s e , i n c o n s e g u e n z a di t a l e i n t e r f e r e n z a , q u e s t o a r r i v o è s t a t o l e t t o t r o p p o p r e s t o si s p i e g h e r e b b e l ' a n o m a l i a p o s i t i v a d e l l a v e l o c i t à m i s u r a t a a l i s b o n a . i i . — s e , d u n q u e , e l i m i n i a m o q u e s t a u n i c a a n o m a l i a , l a d i f f e r e n z a d i v e l o c i t à , a s e c o n d a c h e l e o n d e s u p e r f i c i a l i si s o n o p r o p a g a t e a t t r a v e r s o l a m e t à o c c i d e n t a l e o a t t r a v e r s o l a m e t à o r i e n t a l e d e l l ' a t l a n t i c o , è n o t e v o l e . e d è t a n t o p i ù n o t e v o l e in q u a n t o p e r r a g g i u n g e r e l e s t a z i o n i a m e r i c a n e l e o n d e d e v o n o a s s o l u t a m e n t e a t t r a v e r s a r e u n a p i a t t a f o r m a e p i c o n t i n e n t a l e ( l ' a p p a l a c l i i s o m m e r s o e l a s u a c o p e r t u r a ) g r a n d e da 2 0 0 a 3 0 0 k m ( s ) . si p u ò c o n f r o n t a r e , p . es., la l u n g h e z z a d e i p e r c o r s i c o n t i n e n t a l i e o c e a n i c i s e g u i t i d a l l e o n d e p e r r a g g i u n g e r e o t t a w a a o v e s t e j e r s e y , t o r t o s a , b a r c e l l o n a a e s t . s i t r o v a ( * ) : ( * ) d i s t a n z e m i s u r a t e s u l m a p p a m o n d o . 1 2 0 j . p . ilothé p e r c o r s o o c e a n i c o p e r c o r s o c o n t i n e n t a l e o t t a w a 4 . 6 0 0 k m 8 0 0 k m j e r s e y 5 . 3 0 0 4 0 0 t o r t o s a 4 . 3 0 0 1 . 0 0 0 b a r c e l l o n a 4 . 5 0 0 1 . 0 0 0 o r a l a v e l o c i t à m e d i a d e l l e o n d e d i l o v e è m a g g i o r e d i 0 , 4 k m / s e c a o t t a w a d i e n e l l e s t a z i o n i e u r o p e e , q u a n t u n q u e i p e r c o r s i o c e a n i c o e c o n t i n e n t a l e s i a n o a s s o l u t a m e n t e d e l l o s t e s s o o r d i n e d i g r a n d e z z a : i l p e r c o r s o c o n t i n e n t a l e , s e g u i t o d a l l ' o n d a , c h e r a g g i u n g e j e r s e y , è s e m p r e i l p i ù c o r t o . s ' i n t e n d e c h e , c o m e è già s t a t o r i l e v a t o d a g l i a u t o r i i t a l i a n i , b i s o g n a e l i m i n a r e d a l c o n f r o n t o l e s t a z i o n i a m e r i c a n e d i s i t k a , v i c t o r i a , p a s a d e n a e c c . a l l e q u a l i l e o n d e s i s m i c h e a r r i v a n o d o p o a v e r e a t t r a v e r s a t o t u t t o i l c o n t i n e n t e a m e r i c a n o . i i i . — p e r q u e l c h e r i g u a r d a l a s t a z i o n e d i t a n a n a r i v e g l i a u t o r i i t a l i a n i s c r i v o n o : « n e l q u a d r a n t e s u d o r i e n t a l e l e o s s e r v a z i o n i s i s m i c h e s o n o l i m i t a t e a q u e l l e d e l l a s t a z i o n e di t a n a n a r i v e . l a r e l a t i v a m e n t e e l e v a l a v e l o c i t à d i p r o p a g a z i o n e oss e r v a t a p e r l e o n d e s u p e r f i c i a l i r e g i s t r a t e i n d e t t a s t a z i o n e , l a s c i a p e r ò r i t e n e r e c h e i l t r a g i t t o a t l a n t i c o d i d e t t e o n d e h a i n t e r e s s a t o z o n e a d e l e v a t i s s i m a v e l o c i t à p e r l e o n d e s u p e r f i c i a l i » . p u r t u t t a v i a la figura 3 m o s t r a c h i a r a m e n t e c h e i l v a l o r e d i 4 , 0 k m / s e c r e g i s t r a t o a t a n a n a r i v e si a l l i n e a p e r f e t t a m e n t e s u l l a r e t t a , c h e si p u ò t i r a r e p e r i p u n t i r a p p r e s e n t a t i v i d e i v a l o r i m i s u r a t i n e l l e s t a z i o n i s i t u a t e a d est d e l l a c r e s t a m e d i a n a ( c e r c h i e t t i v u o t i ) . n o n o s t a n t e l e o n d e s e g u a n o u n p e r c o r s o o c e a n i c o di 6 0 0 0 k m e u n p e r c o r s o c o n t i n e n t a l e d i 4 0 0 0 k m , l a l o r o v e l o c i t à è c o m p l e t a m e n t e a n a l o g a a q u e l l e m i s u r a t e a b a r c e l l o n a , t o r t o s a e c o p e n h a g e n . r i t e n i a m o q u i n d i c h e t a l e c o n c o r d a n z a c o n f e r m i l ' o m o g e n e i t à d e l l a c o s t i t u z i o n e , e s i s t e n t e ad est d e l l a c r e s t a m e d i a n a a t l a n t i c a , q u a l u n q u e sia l a n a t u r a d e l p e r c o r s o v i s i b i l e , o c e a n i c o o c o n t i n e n t a l e . i v . — c i p e r m e t t e r e m o di c o n f u t a r e l ' a d o z i o n e d a p a r t e d e g l i a u t o r i i t a l i a n i d e l l a c a r t a , c h e e s s i p r e s e n t a n o i n fig. 1 1 , a s o s t e g n o d e i l o r o a r g o m e n t i . s t a di f a t t o c h e b . g u t e n b e r g e r i c h t e r (") h a n n o s e g n a l a t o i n n u m e r o s i a r t i c o l i , c h e u n a c a r t a c o m e q u e l l a r i p r o d o t t a i n figura 1 1 , l a q u a l e è b a s a t a su t u t t e l e d e t e r m i n a z i o n i e p i c e n t r a l i c a l c o l a t e newinlernational seismological summary, f o r n i s c e u n ' i d e a n e c e s s a r i a m e n t e falsala per quel che riguarda la densità degli osservatori. l a c o n c e n t r a z i o n e d e g l i e p i c e n t r i s i s m i c i s u l l e a l p i , i n i t a l i a e n e i b a l c a n i è e r r o n e a : essa è d o v u t a a l l a r i l e v a n t e d e n s i t à d e g l i o s s e r v a t o r i s i s m i c i n e l l ' e u r o p a o c c i d e n t a l e . p r o p r i o a l l o s c o p o di o t t e n e r e u n a i m m a g i n e p i ù v e r i d i c a d e l l a s i s m i c i t à n e l l e d i f f e r e n t i p a r t i d e l l a t e r r a , c . r i c h t e r h a i n t r o d o t t o l a f e c o n d a n o z i o n e d i magnitudo. d ' a l t r o c a n t o gli a u t o r i a m e r i c a n i n e l l a l o r o o p e r a r e c e n t e c o n f e r m a n o l ' o p i n i o n e già i n p r e c e d e n z a e n u n c i a t a : « s e i s m i c d a t a s u p p o r t a c o n t i n u a t i o n o f t h e a c l i v e b e h i n t o t h e a t l a n t i c a s f a r as t h e a ^ o r e s b u t n o t f a r t h e r . t h e r e i s n o j u s t i f i c a t i o n f o r e x t e n d i n g i l t o t h e w e s t i i u l i e s , as w a s d o n e o n s o m e e a r l y s e i s m i c m a p s of t h e w o r l d ( 5 , p . 7 1 ) . q u e s t a z o n a s i s m i c a è i n t e r e s s a t a i n p a r t i c o l a r e d a l l ' e p i c e n t r o d e l g r a n d e t e r r e m o t o ( p r o b a b i l m e n t e di m a g n i t u d o 9), d e t t o « d i l i s b o n a » ( 1 7 5 5 ) e d a q u e l l o la s t r u t t u r a dell'atlantico 1 2 1 d e l l a s c o s s a ( d i m a g n i t u d o 8 , 3 ) d e l 2 5 n o v e m b r e 1 9 4 1 , c h e p r o v o c ò l a r o t t u r a d e i c a v i s o t t o m a r i n i b r e s t c a s a b l a n c a e b r e s t d a k a r . i n b a s e a l l ' o p e r a d e g l i a u t o r i a m e r i c a n i l a l i s t a d e g l i e p i c e n t r i m a r i n i n e l l a z o n a c o m p r e s a f r a i l 1 0 " e i l 32" g r a d o d i l o n g i t u d i n e o v e s t p u ò s t a b i l i r s i c o m e s e g u e : c o o r d i n a t e d a t a l a t . l o n g . m a g n i t u d o 2 7 1 2 1 9 4 1 36" n 1 0 " 5 w 6 % 1 1 7 1 9 1 5 37" i0"5 6 i / 4 73 1 9 3 0 32" 1 1 " 5 d 2 0 5 1 9 3 1 3 7 ° 5 16" 7 , 1 2 5 1 1 1 9 4 1 37"5 18"5 8 , 3 2 9 5 1 9 4 2 38" 1 9 " il 2 1 1 0 1 9 3 0 36"5 23" d 8 5 1 9 3 9 37" 24"5 7 , 1 1 5 8 1 9 3 3 38" 26"5 d 3 1 8 1 9 2 6 38"5 28" d 54 1 9 2 6 39" 2 9 " 6 97 1 9 3 1 4 0 " 2 29"5 il 1 8 7 1 9 2 3 4 2 " 29"5 il 97 1 9 2 6 18" 30" d 2 7 8 1 9 2 4 4 1 " 5 30"5 il 1 9 7 1 9 4 1 38"5 32" il d e l r e s t o i n q u e s t a l i s t a f i g u r a s o l o u n c e r t o n u m e r o d i s c o s s e ( d i c l a s s e il o d e l l e c l a s s i s u p e r i o r i ) c i t a t e « b e c a u s e t l i e i r e p i c e n t e r s c o n t r i b u t e t o s e i s m o g e o g r a p h i c i n f o r n i a t i o n » . m i è a p p a r s o s e m p r e d e g n o d i n o t a il f a t t o c h e q u e s t a z o n a s i s m i c a n o n att r a v e r s a la p a r t e occidentale d e l l ' a t l a n t i c o . s i o s s e r v i f r a l ' a l t r o c h e g l i e p i c e n t r i , i n d i c a t i da m i s s e . b e l l a m y ( 1 , c a r t a di fig. i l i a o v e s t d e l l a c r e s t a m e d i a n a a t l a n t i c a , s o n o d u b b i e r e a l m e n t e i m p r e c i s i : d o p o l a r e v i s i o n e e s s i s o n o s t a t i e l i m i n a t i da b . g u t e n b e r g e c . f . r i c b t e r . v . — è a c c e r t a t o c h e d a l l e c a r t e b a t i m e t r i c h e a p p a r e e v i d e n t e l ' e s i s t e n z a a f d e l l a c r e s t a a t l a n t i c a di f o s s e p r o f o n d e 6 0 0 0 ili e c h e s o t t o q u e s t o p u n t o d i v i s t a s e m b r a s i c u r a l ' a n a l o g i a n e l l a s t r u t t u r a s u p e r f i c i a l e t r a l e d u e p a r t i d e l l ' a t l a n t i c o . i l o già r i l e v a t o p o c ' a n z i c o m e l e m i s u r e r e c e n t i m o s t r i n o , p u r t u t t a v i a , a l c u n i c o r r u g a m e n t i n e l l a p a r t e o r i e n t a l e d e l l ' o c e a n o a t l a n t i c o , i q u a l i p r o l u n g a n o g l i a s s i s t r u t t u r a l i d e l c o n t i n e n t e a f r i c a n o ( 1 , p. 2 2 0 ) . q u i a g g i u n g e r ò s o l a m e n t e c h e n o n b i s o g n a d i m e n t i c a r e c o m e u n a c a t e n a c o r r u g a t a , q u a l e q u e l l a d e l l e a l p i ( p e r e s . ) , si i m m e r g e f i n o a 5 0 o 6 0 k m . di p r o f o n d i t à , a u n a p r o f o n d i t à , c i o è , 1 0 v o l t e p i ù g r a n d e d i q u e l l a d e l l e f o s s e s e g n a l a t e p o c o f a . c e r t a m e n t e i f e n o m e n i g e o l o g i c i , c h e d o b b i a m o c o n s i d e r a r e n e l l a s e p a r a z i o n e d e l d o m i n i o a t l a n t i c o i n d u e p a r t i , i n t e r e s s a n o t u t t o l o s p e s s o r e d e l l a c r o s t a t e r r e s t r e . a d o g n i m o d o il l a v o r o d e t t a g l i a l o d e i n o s t r i c o l l e g h i i t a l i a n i c o n f e r m a q u e l l o c h e si p o t e v a p r e v e d e r e i n b a s e a l l e m i s u r e i s o l a t e e c i o è i l v a l o r e e l e v a t o d e l l a v e l o c i t à d e l l e o n d e s u p e r f i c i a l i p r o p a g a n t e s i s o t t o l ' a t l a n t i c o o c c i d e n t a l e , il cui s o t t o s u o l o è d u n q u e c o n t u t t a p r o b a b i l i t à a c o m p o s i z i o n e s i m a t i c a . 1 2 2 j . p . notiie » * * 4 . — r i a s s u m e r ò q u i a l c u n i l a v o r i r e c e n t i d e i q u a l i h o a v u t o n o t i z i a d o p o l ' e s p o s i z i o n e p r e l i m i n a r e d e l l a m i a i p o t e s i . i . — è n o t a l a d i f f i c o l t à a c u i si v a i n c o n t r o , s o p r a t u t t o s e si t i e n e c o n t o d e l l e i p o t e s i d i w e g e n e r , n e l l a i n t e r p r e t a z i o n e d e l l a e s i s t e n z a d e l l a c r e s t a s o t t o m a r i n a s o t t o l ' a t l a n t i c o , c r e s t a , c h e l e n u o v e m i s u r e b a t i m e t r i c h e h a n n o d i m o s t r a t o e s s e r e c o m p o s t a d i c a t e n e m o n t a n e p a r a l l e l e 1 1 0 j . o r a , d o p o l a c o m p a r s a d e l l a m i a p r i m a n o t a , l ' e m i n e n t e g e o l o g o f r a n c e s e l . g l a n g e a u d ( n ) , h a p u b b l i c a l o u n a i n t e r e s s a n t e m e m o r i a n e l l a q u a l e e g l i s p i e g a l a f o r m a z i o n e d e l l e c a t e n e m o n t a n e a t t r a v e r s o c i c l i t e r m o d i n a m i c i s i a l o s i m i c i . i n p a r t i c o l a r e e g l i s c r i v e : « d a n s l o u t e s l e s c h a i n e s q u i s e f o r m e n t a i n s i a u c o n t a c t d u b l o c s i a l i q u e c o n t i n e n t a l e t d u s i m a s o u s o c é a n i q u e o n p e u t a d m e t t r e u n r ò l e p l u s o u m o i n s a c c e n t i l e d e l a palingénèse f r u s t a l e . n o t r e h v p o t h ò s e e x p l i q u e p o u r q u o i d e t e l l e s c h a i n e s l i m i n a i r e s , d e d i r e c t i o n m é r i d i e n n e , j a l o n n e n t l a b o r d u r e d u p a c i f i q u e , e n a m é r i q u e c o m m e e n a s i e . e l l e s a p p a r a i s s e n t p a r t o u t o ù l e s m a s s e s s i a l i q u e s c o n t i n e n t a l e s s a f f r o n t e n t a v e c l e s i m a , q u i o c c u p e u n e p a r t i e d u f o n d d u p a c i f i q u e . l e s p h é n o m è n e s d e s c h a i n e s l i m i n a i r e s d o i v e n t a i n s i c o n t i n u e r d e n o s j o u r s , à l a l i m i t e d u s i a l e t d u s i m a , a u j a p o n , e n c a l i f o r n i e , s u r l a b o r d u r e o r i e n t a l e d e s a n t i l l e s , e t l e l o n g d e l a c r é t e a t l a n l i q u e . o r j . p . r o t l i é a é m i s l h y p o t h è s e q u e l a c r é t e a t l a n t i q u e e s t s i t u é e à l a l i m i t e d u b l o c s i a l i q u e a f r i c a i n e t d u f o n d s i m i q u e d e l ' o u e s t a t l a n t i q u e . elle serait ainsi placée dans les conditions de formation des chaines liminaires sialo-simi<jues ». è e v i d e n t e c o m e l a m i a i p o t e s i e l i m i n a u n a g r o s s a d i f f i c o l t à , r i c o n d u c e n d o a l c a s o g e n e r a l e l ' i n t e r p r e t a z i o n e d i u n a c c i d e n t e s t r u t t u r a l e t a n t o i m p o r t a n t e , l a c u i p o s i z i o n e e r a a p p a r s a s e m p r e c o s ì s o r p r e n d e n t e . m o l t o p r o b a b i l m e n t e l u n g o l a c r e s t a m e d i a n a a t l a n t i c a si d e v o n o v e r i f i c a r e a l c u n i f e n o m e n i d i c o r r u g a m e n t o e d i s a l i t a d i m a g m i b a s a l t i c i v i s c o s i , i n i e t t a n t i s i a t t r a v e r s o l a z o n a g r a n i t i c a e g l i s t r a t i s e d i m e n t a r i ; t a l i f e n o m e n i s a r e b b e r o c o n f r o n t a b i l i c o n q u e l l i , d e s c r i t t i e s p i e g a t i d a l . g l a n g e a u d , i q u a l i c o m p a i o n o n e l l ' a t l a n t e t e l l i a n o ( a f r i c a d e l n o r d ) , r i l i e v o c h e c o s t i t u i s c e i l c o n f i n e d e l b l o c c o a f r i c a n o a c o n t a t t o c o n l a m e s o g e a ( u b i s ) . i l b a c i n o c e n t r a l e d e l m e d i t e r r a n e o , s p r o v v i s t o d i a t t i v i t à s i s m i c a , c i a p p a r e c o m e u n l a g o , o p e r m e g l i o d i r e u n a f fioramento d i s i m a i n m e z z o a l l e m a s s e s i a l i c l i e e u r a s i a t i c a e a f r i c a n a . i i . — l e i n f o r m a z i o n i s u l l a n a t u r a g e o l o g i c a d e i f o n d i d e l l ' a t l a n t i c o m a n c a n o q u a s i c o m p l e t a m e n t e . s i p u ò p e r t a n t o m e n z i o n a r e g i à q u a l c h e l a v o r o . i n u n a p o e t i c a c o n f e r e n z a p . t e r m i e r ( 1 2 ) s e g n a l a v a l a s c o p e r t a f a t t a n e l 1 8 9 8 d a p a r t e d i u n n a v i g l i o , i m p i e g a t o p e r l a p o s a d i u n c a v o s o t t o m a r i n o : a 4 7 ° ii' n , 2 7 " 2 0 ' w e a d u n a p r o f o n d i t à d i 3 0 0 0 m v e n n e p r e l e v a t o u n f r a m m e n t o d i r o c c i a : e r a u n a l a v a v e t r o s a a c o m p o s i z i o n e b a s a l t i c a , l a t a c h i l i t e . t e r m i e r p e n s a c h e l a f o r m a z i o n e d i q u e s t a l a v a h a d o v u t o a v v e n i r e a pressione atmosferica e , c o n s e g u e n t e m e n t e : « l a t e r r e q u i c o s t i t u e a u j o u r d ' h u i l e f o n d d e l ' a t l a n t i q u e à 9 0 0 k i l o m è t r e s a u n o r d d e s a t ; o r e s a é t é r e c o u v e r t e d e l a v e s q u a n d e l l e é t a i t e n c o r e e m e r g é e » . l e c o o r d i n a t e i n d i c a t e c o r r i s p o n d o n o a d u n p u n t o d e l l a z o n a s i s m i c a d e l l a c r e s t a m e d i a n a a t l a n t i c a , a l l i m i t e f r a l a z o n a s i m a t i c a e d i l b l o c c o s i a l i c o ( p u n t o 1 s u l l a c a r t a d i fig. 1 ) . la s t r u t t u r a dell'atlantico 110 p o c h i g e o l o g i , s c r i v e v a t e r m i e r , h a n n o c o m p r e s o l ' i m p o r t a n z a d i q u e l c a m p i o n e d i t a c h i l i l e c o n s e r v a t o n e l l a é c o l e d e s m i n e s d i p a r i g i . n u m e r o s e p u b b l i c a z i o n i s o n o s t a t e c o n s a c r a t e a g i u s t i l i c a r e i l r a c c o n t o d i p l a t o n e r i g u a r d a n t e l ' a t l a n t i d e , v a s t o c o n t i n e n t e ( ? ) , i s o l a ( ? ) o p e n i s o l a ( ? ) , p o p o l a t a d a u n a r a z z a p r e i s t o r i c a , c h e a v r e b b e i n v a s o l ' o v e s t d e l l ' e u r o p a , d o p o c h e i l l o r o p a e s e e r a s t a t o s o m m e r s o . g l i e t n o l o g i c o l l o c a n o q u e s t o p a e s e s c o m p a r s o v i c i n o a l l e a z z o r r e o c a n a r i e , c o n s i d e r a t e d a a l c u n i g e o l o g i c o m e i l p r o l u n g a m e n t o d e l g r a n d e a t l a n t e m a r o c c h i n o , o p p u r e v i c i n o a l l e i s o l e d e l c a p o v e r d e ( 1 3 ) . t e r m i n a n d o l a s u a c o n f e r e n z a p . t e r m i e r s c r i v e v a : « l i b r e à l o u s l e s a m o u r e u x d e s b e l l e s l é g e n d e s d e c r o i r e à l ' h i s t o i r e p l a t o n i c i e n n e d e l ' a t l a n t i d e ! n o n s e u l e m e n t l a s c i e n c e , l a p l u s m o d e r n e s c i e n c e n e l e u r e n f e r a p a s u n c r i m e ; m a i s c ' e s t e l l e i n è m e , q u i , p a r m a v o i x , l e s y i n v i t e . c ' e s t e l l e m é m e q u i , l e s p r e n a n t p a r l a m a i n , e t l e s c o n d u i s a n t s u r l a r i v e d e l ' o c é a n f e r t i l e e n n a u f r a g e s , é v o q u e à l e u r s y e u x , a v e c l e s m i l l i e r s d e n a v i r e s d é s e m p a r é s , l e s c o n t i n e n t s e t l e s i l e s s a n s n o m b r e e n s e v e l i s a u f o n d d e s a b i m e s » . i i i . — 1 n u o v i m e t o d i d i c a r o t a g g i o a d o t t a t i d a l l e s p e d i z i o n i o c e a n o g r a f i c h e s v e d e s i e d a m e r i c a n e l a s c i a n o s p e r a r e c h e b e n p r e s t o si e n t r e r à i n p o s s e s s o d i u n n u m e r o s e m p r e c r e s c e n t e d i c a m p i o n i d i r o c c e , p r e l e v a t i n e l f o n d o d e l m a r e , s u i q u a l i si p o s s a n o p r o s e g u i r e l e r i c e r c h e . g i à è s t a t o p u b b l i c a t o u n p r i m o r i s u l t a t o d e i s o n d a g g i ( 1 0 ) . s o n o s t a t i p r e l e v a t i a l c u n i c a m p i o n i d i c a l c a r e d e l l ' e r a c e n o z o i c a i n d u e p u n t i d e l l a c r e s t a m e d i a n a a t l a n t i c a . i . t o l s t o y e d m . e w i n g r i f e r i s c o n o q u e s t e s c o p e r t e c o n i s e g u e n t i t e r m i n i : « d r e d g i n g t h e e a s t e r n flank o f t h e s t e e p r i d g e r u n n i n g a l o n g t h e n o r t h w e s t e d g e o f t h e 2 1 0 0 f a t h o i n t e r r a c e at p o i n t g o f p i a t e 7 ( 3 2 ° 3 0 ' n , 4 2 ° 1 6 ' w ) b r o u g h t u p a l a r g e b l o c k o f m a n g a n e s e — e n c r u s t e d l i m e s t o n e o f p r o b a b l y c e n o z o i c a g e » ( 1 0 , p . 1 5 3 7 ) . « a t p o i n t e i n p i a t e 2 ( 3 4 ° 0 ' n , 301 1 3 ' w ) d r e d g i n g t h e t o p y e l d e d a l a r g e n u m b e r of c a l c a r e o u s d i s c s c a r a c l i t e r i z e d b y p e c u l i a r c a v i t i e s i n t l i e i r c e n t e r . . . t h i n s e d i o l i of d i s c s r e v c a l t h e p r e s e n r e o f n u m e r o u s p t e r o p o d s . a c c o r d i n g t o d . e r i c s o n , a l i a r e s p e c i e s t l i a t c o u l d b a v e i n h a b i t e d t h e o c e a n f r o m t h e e a r l y t e r t i a r y t o t h e p r e s e n t . n o n e a r e s u f f i c i e n t i } ' c l i a r a c t e r i s t i c o f a s p e c i f ì c p e r i o d t o d a t e t l i i s l i m e s t o n e » ( 1 0 , p . 1 5 3 7 ) . e g l i a u t o r i d i q u e s t e s c o p e r t e c o n c l u d o n o : « o n e o f t h e b i g s u r p r i s e s o f t h e 1 9 4 7 a t l a n t i s e x p e d i t i o n ( c r u i s e 1 5 0 ) w a s t h e p r o o f o f t h e e x i s t e n c e o f c o n s o l i d a t e d c e n o z o i c l i m e s t o n e o n t h e m i d a t l a n t i c r i d g e . p o s s i b l y c o n s i d e r a b l e a r e a s o f t h e r i d g e c o n s i s t o f s e d i m e n t a r y r o c k s . i f s o , t h e h y p o t h e s i s o f f o l d e d r a n g e s i n e e r t a i n p a r t s o f t h e r i d g e s h o u l d n o t h e r e j e c t e d a r b i t r a r i l y , o n t h e s o l e b a s i s t l i a t i t i s d i f l ì c u l t i n t h e l i g h t o f o u r s c a n t y k n o w l e d g e o f t h e h i s t o r y o f t h e e a r t h ' s c r u s t t o e x p l a i n t h e w l i y a n d l i o w o f s u d i a p r o c e s s . i t m u s t h e d i s p r o v e d b y e x p e r i m e n t a l a n d o b s e r v a t i o n a l d a t a ; u n t i l t h e n it r e m a i n s a p o s s i b i l i t y » ( 1 0 , p . 1 5 3 9 ) . n e l l a n o s t r a c a r t a d i (ìg. 1 i p u n t i g e d e d i c u i si p a r l a s o p r a s o n o d e s i g n a t i c o n l e c i f r e 2 e 3 . i v . — l ' e s t e n s i o n e a l l e g r a n d i p r o f o n d i t à m a r i n e d e l m e t o d o d e l l a p r o s p e z i o n e s i s m i c a p e r r i f r a z i o n e f o r n i s c e d e l p a r i l a p o s s i b i l i t à d i d e t e r m i n a r e a p p r o s s i m a t i v a m e n t e l a s t r u t t u r a d e g l i s t r a t i s u b o c e a n i c i . n e l l ' a g o s t o d e l 1 9 4 9 i l d e p a r t m e n t of g e o d e s y a n d g e o p h v s i c s d e l l a u n i v e r 1 2 4 r . stonxlky sita d i c a m b r i d g e ( i n g h i l t e r r a ) h a p r o v a l o a d a p p l i c a r e i l m e t o d o d e l l a r i f r a z i o n e s i s m i c a s o t t o 1 3 0 0 b r a c c i a d ' a c q u a n e l n o r d a t l a n t i c o i n p r o s s i m i t à d e l p u n t o d i c o o r d i n a t e 53" 5 0 ' n, 18" 4 0 ' w ( p u n t o 4 d e l l a n o s t r a c a r t a di fig. 1 ) . i r i s u l t a t i di q u e s t e m i s u r e s o n o s t a t i r i a s s u n t i d a m . n . h i l l e j . c . s l i a l l o w ( 1 4 ) : « t l i e l o w v a l u e s f o r t h e v e l o c i t y i n t h e s u r f a c e l a y e r ( f r o n t a b o u t 5 0 0 0 i't/sec at t h e s u r f a c e of t h e sea b e d t o a m a x i m u m v a i n e of a p p r o x i m a t e l y 8 0 0 0 f t / s e c ) i n d i c a t e n o i h i g h t l y c o n s o l i d a t e d m a r i n e s e d i m e n t s . . . t h e v e l o c i t y i n t h e l a y e r b e l o w t h e s e d i m e n t s i s s i m i l a r t o t h o s e o f e a r l y p a l a e o z o i c s e d i m e n t a r y r o c k s o r v a r i o u s i g n e o u s r o c k s s u c h as g r a n i t e ( 1 6 . 0 0 0 a n d 1 7 . 3 0 0 f t / s e c ) . t h e d e p h t b e l o w s e a l e v e l of t l i i s l a y e r i s a p p r o x i m a t e l y t h e s a n i e as t h a t of t h e c o n t i n e n t a l g r a n i t i c l a y e r , b u t i t s t h i c k n e s s , 8 . 8 0 0 f t , i s c o n s i d e r a b l y l e s s . t h e t w o o b s e r v a t i o n s ( 2 1 . 7 0 0 f t / s e c a n d 2 0 . 6 0 0 f t / s e c ) o f t h e v e l o c i t y i n t h e t h i r d t y p e of r o c k a r e c l o s e t o t h e v a l u e a s s i g n e d t o t h e i n t e r m e d i a t e l a y e r l y i n g b e t w e e n g r a n i t i c a n d u l t r a b a s i c l a y e r s a s d e t e r m i n e d f r o l l i n e a r e a r t h q u a k e s » . i n o l t r e m . e w i n g e i s u o i c o l l a b o r a t o r i o " ) , o t t e n g o n o r i s u l t a t i m o l t o i n t e r e s s a n t i , m a c o m p l e t a m e n t e d i f f e r e n t i d a i p r e c e d e n t i , p u r e c o n i l m e t o d o d e l l a r i f r a z i o n e s i s m i c a , o p e r a n d o , p e r ò , s t a v o l t a a 1 2 0 m i g l i a a n w d i b e r m u d a i n u n p u n t o di c o o r d i n a t e 3 4 ° n, 66" 3 0 ' w ( p u n t o 5 s u l l a n o s t r a c a r t a d i f i g . 1 ) . « t h e o c e a n floor w a s f o u n d t o b e c o m p o s e d of t w o l a y e r s . a v e l o c i t y o f 2 4 . 0 0 0 f t / s e c ( 7 , 5 8 k m / s e c ) w a s f o u n d f o r t h e s e c o n d l a y e r . t h i s w a s i d e n t i f ì e d w i t h t h e p n w a v e s of e a r t h q u a k e s e i s m o l o g y a t t r i b u t e d t o t h e b a s a l t i c l a y e r f o u n d b e l o w t h e m o h o r o v i c i c d i s c o n t i n u i t y . a f i r s t l a y e r w a s c l e a r l y p r e s e n t , b u t t h e v e l o c i t y w a s n o t d e t e r m i n e d . a v e l o c i t y of 5 . 6 0 0 f t p e r sec ( 1 , 7 0 k m p e r s e c ) w a s a s s u m c d , g i v i n g a t h i c k n e s s of 4 . 5 0 0 f e e t f o r t h i s s e d i m e n t a r y l a y e r . t h i s v e l o c i t y w a s c h o s e n f r o m s o m e of o u r e a r l i e r m e a s u r e m e n t s , as y e t m o s t l y u n p u b l i s h e d . t h e g r a n i t i c a n d i n t e r m e d i a t e l a y e r s a r e t l i u s a b s e n t b e n e a t h t h e o c e a n f l o o r at t h i s p o i n t . t h e v e l o c i t y of t h e b a s a l t i c l a y e r c o m p a r e s f a v o r a b l y w i t h t h o s e of s l i c h t e r i n n e w e n g l a n d a n d w i t h j e f f r e y s ' w o r l d a v e r a g e » . c i t e r e m o i n f i n e l e p a r o l e di b u c l i e r , c h e i n u n m e e t i n g d e l n o v e m b r e 1 9 4 9 a e1 p a s o , d i c h i a r a v a : « t h a t e w i n g ' s c o n c l u s i o n s cast d o u b t o n t h e b e l i e f t h a t t h e f l o o r o f t h e a t l a n t i c o c e a n i n u n d e r l a i n b y a t l i i n " g r a n i t i c s h e l l " . g r a n i t i c m a t e r i a l m a y h e a b s e n t o v e r l a r g e p a r t o c e a n s » . * t * 5 . — i n c o n c l u s i o n e b a s t e r à r i p o r t a r e s u l l a c a r t a di fig. 1 l a p o s i z i o n e r i s p e t t i v a d e i p u n t i d a 1 a 5 : l a n a t u r a d e i t e r r e n i c h e s o n o s t a t i r i c o n o s c i u t i c o l à è p r o p r i o q u e l l a p r e v e d i b i l e in b a s e a l l ' i p o t e s i d a n o i f o r m u l a t a n e l 1 9 1 7 e c h e è r i a s s u n t o n e l l a s e g u e n t e t a b e l l a : p u n t o 1 ( a t l a n t i c o s i a l i c o ) : r o c c i a v u l c a n i c a a f f o n d a t a . p u n t o 2 ( a t l a n t i c o s i a l i c o ) : c a l c a r e d e l l ' e r a t e r z i a r i a . p u n t o 3 ( a t l a n t i c o s i a l i c o ) : c a l c a r e d e l l ' e r a t e r z i a r i a . p u n t o 4 ( a t l a n t i c o s i a l i c o ) : s e d i m e n t i e g r a n i t i d e l p a l e o z o i c o . p u n t o 5 ( a t l a n t i c o s i m a t i c o ) : a s s e n z a di g r a n i t o . l . d o n i . e e t ( " ' ) , n e l l ' i n t e n t o d i c o n f u t a r e l e o s s e r v a z i o n i di e w i n g , gli o p p o n e « i n c o n t r a s t » l e m i s u r e g e o f i s i c h e i n g l e s i . le nostra interpretazione della struttura dell'atlantico spiega immediatamente questo contrasto: la cresta mediana atlantica, la cui sismicità prova che essa è tuttora in evoluzione e che costituisce la s t r u t t u r a dell'atlantico 110 la catena limite ilei blocco sialico africano, separa le due parti dell'atlantico in due domini a struttura differente. * :> » r . f u r o n ( 1 7 ) h a r i c o r d a t o di r e c e n t e la s c o p e r t a di p i a s t r i n e di s c i s t i p r i m a r i a t r i l o b i t i , e s t r a t t e i n u n s o n d a g g i o e f f e t t u a t o n e l 1 8 8 3 d a l « t a l i s m a n » a m e z z a s t r a d a f r a l e a z z o r r e e l e c o s t e d e l p o r t o g a l l o ( 4 4 " 2 0 ' n , 1 7 ° 1 1 ' w ; 42" 1 9 ' n , 2 1 ° 1 6 ' w ) d a u n a p r o f o n d i t à s u p e r i o r e ai 4 0 0 0 n i . « c e s o n t , s c r i v e r . f u r o n , l e s p r e m i e r f o s s i l e s r a m e n é s d e s g r a n d e s p r o f o n d c u r s . i l s a p p o r t e n t l a p r e u v e d ' u n r e lief s o u s m a r i n p a r t i e l l e m e n t c o n s t i t u é p a r d e s r o c h e s s é d i m e n t a i r e s d ' à g e p a l é o z o i q u e » . p e r q u e l c h e r i g u a r d a l a p a r t e s i m a t i c a d e l l ' o c e a n o a t l a n t i c o l a c o s t i t u z i o n e g e o l o g i c a d e l l e i s o l e b e r m u d e f o r n i s c e i n d i c a z i o n i p r e z i o s e . e s s e s o n o f o r m a l e d a s c o g l i o r g a n i c i c o s t r u i t i s u u n a c a t e n a di p i c c h i v u l c a n i c i d i d i r e z i o n e n e s w . n o n v i è s t a t a r i c o n o s c i u t a la p r e s e n z a di t e r r e n i s e d i m e n t a r i ( 1 s , 1 ! > , hi assunto l'autore rammenta la ipotesi di lavoro da lui formulata nel 1947: l'oceano atlantico e diviso in due domini, una zona occidentale a carattere sintatico e una parte orientale a struttura continentale a est della cresta mediana. l'autore considera la distribuzione degli ipocentri sismici, la forma delle coste, il prolungamento delle unità morfologiche africane, nel mare e il valore assai elevato della velocità delle onde sismiche superficiali, propaganlesi sotto la parte occidentale dell'atlantico. discute lo studio di p. caloi e collaboratori e porta nuovi argomenti a favore della sua ipotesi: dragaggio di una tacliilite (termier) di trilobiti (furon 1 di calcari dell'era terziaria (ewing e tolsioy) nella parte orientale dell'atlantico ; teoria delle catene liminari (glangeaud) ; risultati delle prospezioni sismiche per rifrazione ottenuti ad ovest della scozia ( h i l l ) e nelle vicinanze delle bermude (ewing). vol52,3,2009 323 annals of geophysics, vol. 52, n. 3/4, june/august 2009 key words ionospheric channel – mimo architecture – array processing – hf channel characterization – superresolution direction finding 1. experimental investigations into the feasibility of mimo techniques within the hf band 1.1. introduction a multiple input multiple output (mimo) system utilises antenna arrays at the transmitter as well as receiver ends of a communications link. a rich fading or scattering environment is required for the successful implementation of mimo, which, in effect, facilitates the creation of multiple paths for the parallel transmission of data. the use of multiple transmitter and multiple receiver antennas in such an environment enables a substantial increase in data rates to be achieved within the same frequency band. for mimo to be successful however, it is essential that there is sufficient de-correlation between the signals received at each of the antenna elements of the receiver array from each of the elements of the transmitter array. signals received over hf (3-30 mhz) propagation paths through the ionosphere are prone to extensive fading as a consequence of multipath and multimode propagation. as such, it would seem that the use of hf signals in a mulutilization of antenna arrays in hf systems salil d. gunashekar (1), e. michael warrington (1), hal j. strangeways (2), yvon erhel (3), sana salous (4), stuart m. feeney (4), nasir m. abbasi (1), louis bertel (5), dominique lemur (5), françois marie (5) and martial oger (5) (1) department of engineering, university of leicester, uk (2) school of electronic and electrical engineering, university of leeds, uk (3) centre de recherches des ecoles de saint-cyr coetquidan, guer, france (4) school of engineering, durham university, uk (5) ietr, université de rennes 1, france abstract different applications of radio systems are based on the implementation of antenna arrays. classically, radio direction finding operates with a multi channel receiving system connected to an array of receiving antennas. more recently, mimo architectures have been proposed to increase the capacity of radio links by the use of antenna arrays at both the transmitter and receiver. the first part of this paper describes some novel experimental work carried out to examine the feasibility of applying mimo techniques for communications within the hf radio band. a detailed correlation analysis of a variety of different antenna array configurations is presented. the second section of the paper also deals with hf mimo communications, focusing on the problem from a modelling point of view. the third part presents a sensitivity analysis of different antenna array structures for hf direction finding applications. the results demonstrate that when modelling errors, heterogeneous antenna arrays are more robust in comparison to homogeneous structures. mailing address: prof. e. michael warrington, department of engineering, university of leicester, leicester le1 7rh, united kingdom: e-mail: emwarrington@mac.com; emw@leicester.ac.uk vol52,3,2009 20-09-2009 19:06 pagina 323 324 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger ti-element transmitter-receiver system would be an ideal candidate for the implementation of mimo techniques. so far, however, apart from the research outlined in brine et al. (2006) and strangeways (2006a), very little experimental or modelling research has been conducted in this area. sections 1.2 to 1.4 describe some of the novel experimental work that has been carried out in order to investigate the feasibility of applying mimo techniques within the hf band. 1.2. experimental arrangement and measurements in order to carry out hf-mimo measurements, a purpose-built multi-channel transmitter system and multi-channel receiver system was implemented. the transmitter site was located in durham, uk, while two receiver sites were utilised in the different campaigns. one receiver site was located in bruntingthorpe (leicester), uk (giving a range of approximately 255 km), while the other receiver site was located in monterfil (rennes), france (giving a range of approximately 750 km). at present, the system has the capability of transmitting on up to four antennas and receiving on up to eight antennas. in order to investigate the effects of antenna heterogeneity at the transmitter, a variety of antennas were used in the campaigns (e.g. vertical, loop, dipole and crossed-wire antennas). at the receiver, a number of different antenna configurations were employed to demonstrate some of the requirements for the implementation of an effective hf-mimo system. for example, as reported in gunashekar et al. (2008) and warrington et al. (2008), at times the two orthogonal arms of a spaced l-shaped antenna array of vertical monopoles exhibit significantly different levels of de-correlation (as a function of antenna spacing). furthermore, it has also been confirmed that at hf wavelengths, the use of homogeneous spaced arrays may require significant spacing to achieve acceptable levels of de-correlation (gunashekar et al., 2008 and warrington et al., 2008). in this paper, the results from preliminary investigations that were carried out in order to examine the use of spaced as well as co-located heterogeneous antenna arrays have been presented. the latter would enable the compact implementation of hf-mimo systems by achieving the necessary de-correlation at a single location. for the various transmit antennas, slightly different cw frequencies (offset by 10-20 hz) were used. during testing, it was verified that the different frequencies exhibited correlated fading when transmitted simultaneously on a single antenna. this result confirmed that, when used on separate transmit antennas, the differences in frequencies would not contribute to any kind of decorrelation during the mimo campaigns. over the last several months, a number of experimental campaigns were performed over the durham-bruntingthorpe and durham-monterfil paths. the following sub-sections provide selected examples of cases in which different mimo configurations were employed. 1.3. spaced, heterogeneous antenna arrays: 4x8 mimo link between durham and monterfil (3 july 2008) a 4x8 mimo configuration was utilised in a series of measurements conducted on 3 july 2008. the transmit array in durham consisted of the following antennas: tx-1 (5.795010 mhz): n-s arm of an inverted ‘v’ crossed-wire antenna array (i.e. pointing in the general direction of monterfil), tx-2 (5.795020 mhz): e-w arm of an inverted ‘v’ crossed-wire antenna array, tx-3 (5.795030 mhz): a loop antenna and tx-4 (5.795040 mhz): a vertical antenna. the receiver configuration in monterfil comprised of an eight channel heterogeneous circular array of radius 25.0 m. as illustrated in fig. 1, the eight channels were connected to octagonal loop antennas that were arranged so that successive loops were oriented perpendicular to each other. the amplitude pattern across the eight receiving channels for cw signals received from durham for a period of approximately one minute (at 10:20 ut on 3 july 2008) depicted deep fading for all four transmissions. this was indicative of the presence of multiple propagation modes/paths. when transmission curves were superimposed on the corresponding vertivol52,3,2009 20-09-2009 19:06 pagina 324 325 utilization of antenna arrays in hf systems cal ionogram obtained from the chilton (uk) ionosonde, the presence of multiple propagation modes was confirmed: in addition to the multiple hop f2 modes (both ordinary and extraordinary modes), a strong e-region was also present. the magnitudes of the amplitude correlation coefficients between the individual oneminute transmissions at each of the receiving antennas have been listed in table i. the orthogonal nature of the two transmit crossed-wire antennas (tx-1 and tx-2) as well as the antenna diversity present at the transmit end appears to have an implication on the levels of de-correlation achieved at the receiving antennas: approximately 40% and 80% of the values do not exceed 0.7 and 0.9 respectively (according to loyka (2001), for a 10-element uniform linear array, the mimo channel capacity does not degrade appreciably until the inter-element correlation coefficient exceeds approximately 0.9). in addition to the transmissions being reasonably well de-correlated at each of the receiving antennas, the various pairs of receiving antennas were also very well de-correlated for each transmission from durham. a list of the magnitudes of the amplitude correlation coefficients between the different pairs of receiving antennas of the circular antenna array for tx-1 (n-s inverted ‘v’ wire antenna; 5.795010 mhz) is presented in table ii. a combination of the spatial and heterogeneous nature of the array results in majority of the correlation coefficients lying in the range 0.2-0.8 (and never exceeding approximately 0.85). a similar distribution of correlation coefficients was observed for tx-2, tx-3 and tx-4. 1.4. compact, co-located, heterogeneous antenna arrays: 3x5 mimo link between durham and bruntingthorpe (29 july 2008) a 3-axis, h-field loop antenna array was designed to investigate the use of co-located heterogeneous antenna arrays at the receiver in bruntingthorpe. the array consists of three perpendicular active loop antennas mounted on a common mast (height ~3.0 m): two antennas oriented in the n-s and e-w directions and a fig. 1. heterogeneous circular antenna array (consisting of eight octagonal loop antennas) employed at monterfil during a 4x8 hf-mimo campaign on 3 july 2008. vol52,3,2009 20-09-2009 19:06 pagina 325 326 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger horizontal loop antenna. an experimental campaign was conducted on 29 july 2008 that employed the co-located, heterogeneous loop antenna array at the receive end of a 3x5 mimo link. in addition to this, orthogonally oriented inverted ‘v’ wire antennas were also employed. both the inverted ‘v’ antennas (wire length ~32 m) were supported by the same mast and were crossed at the same point on the mast (approximately 7.6 m above ground level). the n-s arm table i. amplitude correlation coefficients between the various transmissions from durham at each receiving antenna of the heterogeneous circular antenna array in monterfil for a period of approximately one minute at 10:20 ut on 3 july 2008 [tx-1: n-s inverted ‘v’ crossed-wire (5.795010 mhz), tx-2: e-w inverted ‘v’ crossed-wire (5.795020 mhz), tx-3: loop (5.795030 mhz), tx-4: vertical whip (5.795040 mhz)]. magnitude of amplitude correlation coefficients [tx-1, tx-2] [tx-1, tx-3] [tx-1, tx-4] [tx-2, tx-3] [tx-2, tx-4] [tx-3, tx-4] rx-1 0.61 0.86 0.09 0.84 0.74 0.39 rx-2 0.86 0.95 0.52 0.93 0.74 0.62 rx-3 0.88 0.94 0.46 0.93 0.65 0.58 rx-4 0.94 0.94 0.66 0.94 0.74 0.75 rx-5 0.82 0.90 0.77 0.88 0.82 0.83 rx-6 0.78 0.90 0.53 0.85 0.58 0.53 rx-7 0.85 0.92 0.59 0.88 0.61 0.59 rx-8 0.65 0.84 0.36 0.75 0.46 0.28 table ii. amplitude correlation coefficients between the various pairs of receiving antennas that constitute the heterogeneous circular antenna array in monterfil for tx-1 (n-s crossed wire; 5.795010 mhz) for a period of approximately one minute at 10:20 ut on 3 july 2008. magnitude of amplitude correlation coefficients rx-1 rx-2 rx-3 rx-4 rx-5 rx-6 rx-7 rx-8 rx-1 0.55 0.59 0.37 0.02 0.66 0.49 0.15 rx-2 0.80 0.02 0.27 0.66 0.70 0.33 rx-3 0.47 0.28 0.81 0.87 0.25 rx-4 0.29 0.35 0.36 0.14 rx-5 0.31 0.35 0.05 rx-6 0.86 0.38 rx-7 0.48 rx-8 vol52,3,2009 20-09-2009 19:06 pagina 326 327 utilization of antenna arrays in hf systems of the receive crossed-wire antenna was pointing in the general direction of durham. for the transmit array in durham, three different antennas were utilised: tx-1: n-s arm of an inverted ‘v’ crossed-wire array (i.e. pointing in the direction of bruntingthorpe), tx-2: e-w arm of an inverted ‘v’ crossed-wire array and tx-3: a vertical antenna. during the measurement campaign, two sets of transmission frequencies were utilised: 5.255010 mhz (tx-1), 5.255020 mhz (tx-2), 5.255040 mhz (tx-3) (from 11:50 ut to 13:05 ut) and 4.445510 mhz (tx-1), 4.445520 mhz (tx-2), 4.445540 mhz (tx-3) (from 13:15 ut to 14:20 ut). for each pair of orthogonal active loop antennas, the occurrence frequencies of correlation coefficients that fall within specified bins are presented in fig. 2. the top frame contains the histograms corresponding to the 5.2550 mhz transmissions (54 one-minute data files) while the lower frame corresponds to the 4.4455 mhz transmissions (50 oneminute data files). particularly for the orthogonal pairs involving the n-s oriented active loop, sufficiently low values of correlation coefficient are observed. majority of correlation coefficients lie within the range 0.3-0.8 though lower values are also observed for a number of cases. furthermore, more de-correlation is observed at 5.2550 mhz than at 4.4455 mhz it is likely that the reduction in wavelength and the prevailing modal structure of the ionosphere are both contributing to this. finally, the occurrence frequency histograms of the correlation coefficients between the n-s and e-w receive crossed-wires are depicted in fig. 3 (left frame: 5.2550 mhz; right frame: 4.4455 mhz). for all three transmissions at 5.2550 mhz, the orthogonally-oriented inverted ‘v’ wire antennas result in adequately low levels of de-correlation (the majority of the correlation coefficients fall within the range 0.4-0.8). 2. theoretical determination of antenna correlation distances and capacities for hf mimo links mimo systems employ both transmitting and receiving arrays and space-time coding methods in order to achieve significantly greater channel capacities. since the data rate possible for digital hf systems is generally comparatively low, it would be very useful if the capacity improvements inherent in mimo techniques could be exploited at hf. in order to assess this possibility and in order to gain some insight into the optimum design of hf mimo systems, the characteristics of the digital hf channel have been explored in relation to this utilization. to accomplish this a wideband hf simulator, based on a physical model and including timevarying electron density irregularities has been employed and used to assess correlation of the received multi-component signal at spaced antennas for different multipath scenarios and ionosphere parameters (strangeways, 2005). the optimum parameters and scenarios for employment of hf mimo systems can then be assessed and capacities and optimum array configurations determined (strangeways, 2006a,b,c). it needs to be considered that, whereas for the cellular radio case at uhf, the multipath at the receiver can arrive over a wide range of azimuth up to 360 degrees with a small range of elevation, the situation at hf is quite different with all signals arriving from essentially the same azimuth unless refraction occurs from large scale irregularities such as tids, gradients in the midlatitude or high latitude troughs or reflection occurs from patches in the auroral/polar region. however, for the hf case, since paths can result from ionospheric reflection from different layers (e.g. e, f) or multi-hops, the range of elevation can be much larger. the effect of time-varying ionospheric irregularities will also result in refracted/diffracted paths which will also decrease the correlation at spaced antennas and also lead to reduced channel stationarity. it also needs to be taken into account that the time scale of the different mechanisms producing multipath at hf vary. whereas motion of small scale irregularities produce fade periods less than 1s, fading due to interference between reflections from different layers or a different number of hops have longer timescales (up to about 10s). the correlation distance for spaced antennas, as used in these systems, has been determined using the leeds-st.petersburg hf simulator. in this simulator, the channel simulation vol52,3,2009 20-09-2009 19:06 pagina 327 328 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger 0 − 0 .1 0 .1 − 0 .2 0 .2 − 0 .3 0 .3 − 0 .4 0 .4 − 0 .5 0 .5 − 0 .6 0 .6 − 0 .7 0 .7 − 0 .8 0 .8 − 0 .9 0 .9 − 1 .0 05 1 0 1 5 f re q u e n cy = 5 .2 5 5 0 m h z; n s a n d e w lo o p s c o rr e la tio n c o e ff ic ie n t number of occurrences of correlation coefficient 0 − 0 .1 0 .1 − 0 .2 0 .2 − 0 .3 0 .3 − 0 .4 0 .4 − 0 .5 0 .5 − 0 .6 0 .6 − 0 .7 0 .7 − 0 .8 0 .8 − 0 .9 0 .9 − 1 .0 05 1 0 1 5 2 0 2 5 f re q u e n cy = 5 .2 5 5 0 m h z; e w a n d h o ri zo n ta l l o o p s c o rr e la tio n c o e ff ic ie n t number of occurrences of correlation coefficient 0 − 0 .1 0 .1 − 0 .2 0 .2 − 0 .3 0 .3 − 0 .4 0 .4 − 0 .5 0 .5 − 0 .6 0 .6 − 0 .7 0 .7 − 0 .8 0 .8 − 0 .9 0 .9 − 1 .0 05 1 0 1 5 f re q u e n cy = 5 .2 5 5 0 m h z; n s a n d h o ri zo n ta l l o o p s c o rr e la tio n c o e ff ic ie n t number of occurrences of correlation coefficient 0 − 0 .1 0 .1 − 0 .2 0 .2 − 0 .3 0 .3 − 0 .4 0 .4 − 0 .5 0 .5 − 0 .6 0 .6 − 0 .7 0 .7 − 0 .8 0 .8 − 0 .9 0 .9 − 1 .0 05 1 0 1 5 2 0 2 5 3 0 f re q u e n cy = 4 .4 4 5 5 m h z; n s a n d e w lo o p s c o rr e la tio n c o e ff ic ie n t number of occurrences of correlation coefficient 0 − 0 .1 0 .1 − 0 .2 0 .2 − 0 .3 0 .3 − 0 .4 0 .4 − 0 .5 0 .5 − 0 .6 0 .6 − 0 .7 0 .7 − 0 .8 0 .8 − 0 .9 0 .9 − 1 .0 05 1 0 1 5 2 0 2 5 3 0 f re q u e n cy = 4 .4 4 5 5 m h z; e w a n d h o ri zo n ta l l o o p s c o rr e la tio n c o e ff ic ie n t number of occurrences of correlation coefficient 0 − 0 .1 0 .1 − 0 .2 0 .2 − 0 .3 0 .3 − 0 .4 0 .4 − 0 .5 0 .5 − 0 .6 0 .6 − 0 .7 0 .7 − 0 .8 0 .8 − 0 .9 0 .9 − 1 .0 05 1 0 1 5 2 0 2 5 3 0 f re q u e n cy = 4 .4 4 5 5 m h z; n s a n d h o ri zo n ta l l o o p s c o rr e la tio n c o e ff ic ie n t number of occurrences of correlation coefficient t x − 1 t x − 2 t x − 3 t x − 1 t x − 2 t x − 3 t x − 1 t x − 2 t x − 3 t x − 1 t x − 2 t x − 3 t x − 1 t x − 2 t x − 3 t x − 1 t x − 2 t x − 3 f ig . 2. c or re la ti on c oe ff ic ie nt o cc ur re nc e fr eq ue nc y hi st og ra m s fo r ea ch p ai r of c olo ca te d, o rt ho go na l, a ct iv e lo op a nt en na s fo r ea ch o f th e tr an sm it ti ng a nt en na s: 3 x 5 m im o c am pa ig n be tw ee n d ur ha m a nd b ru nt in gt ho rp e on 2 9 ju ly 2 00 8 [t op f ra m e: 5 .2 55 0 m h z tr an sm is si on s (5 4 on em in ut e da ta f il es ); l ow er f ra m e: 4 .4 45 5 m h z tr an sm is si on s (5 0on e m in ut e da ta f il es ); t x -1 : n -s c ro ss ed -w ir e (5 .2 55 01 0 m h z/ 4. 44 55 10 m h z) ; t x -2 : e -w c ro ss ed -w ir e (5 .2 55 02 0 m h z/ 4. 44 55 20 m h z) ; t x -3 : ve rt ic al a nt en na ( 5. 25 50 40 m h z/ 4. 44 55 40 m h z) ]. vol52,3,2009 20-09-2009 19:06 pagina 328 329 utilization of antenna arrays in hf systems for the multipath hf ionospheric sky wave random channel is based on the most general theory of hf wave propagation in the real fluctuating ionosphere (gherm et al., 2005). the complex phase method (or modified rytov’s approximation) is employed. this accounts for the main effects of hf propagation in the disturbed ionosphere: ray bending due to the inhomogeneous background ionosphere and scattering by random ionospheric irregularities including diffraction by localised inhomogeneities. the earth’s magnetic field effect on the irregularity shape is taken into account through the anisotropic spatial spectrum of the ionospheric turbulence. the propagation paths are determined using a nelder-mead homingin algorithm together with a 3d ray-tracing model which takes into account the effect of the geomagnetic field on the refractive index and also permits horizontal gradients of electron density to be included. this enables simulation of the multimoded wideband ionospheric hf channel for both background and stochastic (time-varying irregularities) electron density components for any transmitter and receiver locations and taking into account both magneto-ionic modes for all the ionospherically reflected paths. the wideband hf simulator can output realisations of the received signal at the receiver in both fast (time-of-flight) and slow time. this can also enable correlation between spaced antennas to be determined assuming frozen-in drift of the irregularities. for antennas spaced in the direction of irregularity velocity drift, for spacings up to a few wavelengths, it was assumed that the spatial variation of the received signal can be modelled in the drift direction using the simulated slow time variation and the known drift velocity. based on a simple model of scattering from an inhomogeneous and time-varying ionosphere, a spatial correlation function p(d) normalised to unity at d=0 may be derived to show the dependence of cw signals at two antennas spaced by (2.1) at a separation d = σl the correlation is 0.61 and at d = �2σl, it is 0.37. this latter distance is termed the diversity separation distance or correlation distance (itu-r, 1990). as an example, a north south path from 50° to 62° latitude at 0 ° longitude in the iri ionosphere with an east-west drift of the irregularities of 0.5 km/s has been considered. the carrier frequency is 9 mhz and the signal bandwidth is 20 khz. the standard deviation of the relative p d e d 2 l 2 2= s ] f g p 0−0.1 0.1−0.2 0.2−0.3 0.3−0.4 0.4−0.5 0.5−0.6 0.6−0.7 0.7−0.8 0.8−0.9 0.9−1.0 0 2 4 6 8 10 12 14 16 frequency = 5.2550 mhz; ns and ew receive crossed−wires n u m b e r o f o cc u rr e n ce s o f co rr e la tio n c o e ff ic ie n t correlation coefficient 0−0.1 0.1−0.2 0.2−0.3 0.3−0.4 0.4−0.5 0.5−0.6 0.6−0.7 0.7−0.8 0.8−0.9 0.9−1.0 0 5 10 15 20 25 30 35 40 45 frequency = 4.4455 mhz; ns and ew receive crossed−wires n u m b e r o f o cc u rr e n ce s o f co rr e la tio n c o e ff ic ie n t correlation coefficient tx−1 tx−2 tx−3 tx−1 tx−2 tx−3 fig. 3. the occurrence frequency histograms of the correlation coefficients between the n-s and e-w receive crossed wire antennas for each of the transmitting antennas: 3 x 5 mimo campaign between durham and bruntingthorpe on 29 july 2008 [left frame: 5.2550 mhz transmissions (54 one-minute data files); right frame: 4.4455 mhz transmissions (50-one minute data files); tx-1: n-s crossed-wire (5.255010 mhz/4.445510 mhz); tx-2: e-w crossed-wire (5.255020 mhz/4.445520 mhz); tx-3: vertical antenna (5.255040 mhz/4.445540 mhz). vol52,3,2009 20-09-2009 19:06 pagina 329 330 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger electron density fluctuations, σn2, is 2x10-5. the outer scale of the irregularities is 5 km in the transverse direction and 15 km in the geomagnetic field direction. the spectral index is given by p =3.7. the e and f layer reflected paths exist for both magneto-ionic modes and all 4 paths are included to determine the signal strength at the receiver location (fig. 4). the left frame of fig. 4 shows the received pulse (e and f modes) in both fast and slow time and the right frame shows how the correlation coefficient falls of with antenna separation for both modes. it is clear that the fall-off is faster for the f mode and thus this mode de-correlates over a shorter distance note that because of the small number of points, the correlation coefficient has been “unbiassed” at each lag by dividing by the number of products taken in determining it, explaining why the maximum for the e mode is not at zero lag. the correlation of multipath ionospherically reflected signals is quite complex and will vary during the duration of a dispersed pulse. the spatial correlation at the receiving antenna array depends strongly on the variance of the electron density irregularities. each mode (e.g. 1eo or 1fx) shows “fading” due to the time-varying irregularities. paths reflected from the f region generally show poorer correlation at spaced antennas than e region reflected paths. this is likely to result from the larger absolute changes in electron density in the f as opposed to the e layer. although it might be supposed that correlation between spaced receiving antennas would be appreciably decreased by receiving additional modes this is not necessarily the case; e.g. lowering the transmission frequency to enable an additional e layer reflection may not decrease appreciably the spatial correlation at the receiver as the e layer mode added is likely to show better correlation between the spaced antennas than the f mode. moreover decreasing the carrier frequency to enable the e mode will reduce the snr which will also result in poorer channel capacity for a simo/mimo link. the capacity of hf mimo links will also vary for different antenna array sizes, geometries, correlations distances and antenna separations. the fig. 5 shows the estimated capacity for a correlation length of 250 m compared to that of 500 m for a 4x4 mimo system (strangeways, 2006c). calculations have also shown that the capacity of an hf mimo link for uncorrelated received signals is not found to reduce drastically for correlated signals even when the correlation coefficient between adjacent receiving antennas is quite high (by a factor of 0.88 for 0.8 correlation coefficient and 0.80 for 0.9 correlation coefficient for a 2x2 mimo system). 3. sensitivity analysis of the heterogeneous array for direction finding applications 3.1. introduction direction finding techniques operate with synchronous acquisitions at the output of an array of sensors and the associated covariance matrix is the relevant information for the most popular high resolution algorithms (capon, music, weighted subspace fitting). this work investigates the estimation of angular errors resulting from a perturbation on the steering-vector matrix. these uncertainties on the array response are due, for example, to imprecise positions of the sensors or to a default in the calibration of the electronic circuits connected to each of them. the expressions of the errors (limited to first order terms) are derived for two different structures of array. the first is the classical homogeneous array set up with identical sensors. the second is the heterogeneous structure (erhel et al., 2004), set up with different sensors, that we proposed for hf applications in order to make the array sensitive to the incoming polarization. statistics of the angular errors are computed for the two solutions and indicate a greater robustness of the second structure. 3.2. expressions for array processing 3.2.1. homogeneous (classical) array a homogeneous array is set up with nc identical sensors associated with a reference vol52,3,2009 20-09-2009 19:06 pagina 330 331 utilization of antenna arrays in hf systems fig. 4.a,b. received signal power with added noise for a 9mhz carrier with 20khz bandwidth and σn2=2.10-5 (left frame), and correlation coefficient versus antenna separation for both e and f modes (right frame). fig. 5. capacity achieved for correlation lengths of 250 and 500 m for a 4x4 mimo system. 0 100 200 300 400 500 600 3 4 5 6 7 8 9 10 antenna separation,m c a p a ci ty ( b its /s e c) 250m 500m vol52,3,2009 20-09-2009 19:06 pagina 331 332 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger point for the geometrical phase. ns waves are assumed to be incident and the corresponding output signals generated on the reference sensor are denoted by �sk(t)�k=1,..,ns. each direction of arrival (doa) is identified by the angle θk (or couple of angles in a 3-d search). for that direction, the geometrical phase between the reference and the nth sensor is denoted by ϕn(θk) the output on the nth antenna is expressed as: (3.1) where the noise components �nn (t)� n=1,...,nc are supposed to be mutually uncorrelated with identical power σ2 or, equivalently, spatially white. the nc output signals on the array are collected in the acquisition column vector x(t) as: (3.2) where a(θk) is the steering-vector for the d.o.a. θk and n(t) is the noise vector. the components of a(θk) contain the different geometrical phases: (3.3) associating the ns steering vectors in the matrix a provides the classical linear model of acquisitions: x(t) = as(t) + n(t) (3.4) where s(t) is the signal vector. the covariance matrix of the acquisitions, defined as: rxx = e[x(t).x(t)h] (3.5) is then expressed as: rxx = arssah + s2id (3.6) where rss is the covariance of the incident signals: rss = e[s(t). s(t) h] (3.7) , , ......,e e ea 1k j j j nc t2k k kθ = { i { i i{] ^ ] ] ]g hg g g a ntx s t t k ns k k 1 θ= + = ] ] ] ]g g g g/ ( )x t e s t n tn j k ns k n 1 n k= +{ i = ] ] ] g g g/ 3.2.2. heterogeneous array a heterogeneous array is made up of sensors which are different from one another. for each of them, the directional gain relatively to the angle θ, called spatial response and denoted by fn(θ), n=1,…nc is assumed to be known. examples of spatial responses for hf antennas with a simple geometry are calculated in (erhel et al., 2004). the computation refers to a deterministic model of the polarization at the exit point of the ionosphere. in this context, the linear model for the output signals of the heterogeneous array is expressed as: (3.8) the components of the steering-vectors ah(θk) combine the spatial responses and the exponentials which represent the phases ϕn(θk) calculated with respect to the array geometry: ah(θk) = (f1(θk)ejϕ1(θk),......,fnc(θk)ejϕnc(θk))t (3.9) it can be noticed that ah (θ) does not have a constant norm; this remark will be taken into account when applying the music algorithm on this particular type of array. gathering the ns steering-vectors in matrix ah gives the linear model for the heterogeneous array: xh (t) = ahs(t) + nh(t) (3.10) and, assuming a spatially white additive noise, the corresponding covariance matrix is rxxh = ahrssahh + σ2id (3.11) 3.3. sensitivity analysis: perturbation method perturbations are assumed to affect the array response: viz random displacements of the sensors or differences in gain and phase for the different acquisition channels connected to the sensors. x a nt s t t h h h k ns k k 1 θ= + = ] ] ] ]g g g g/ vol52,3,2009 20-09-2009 19:06 pagina 332 333 utilization of antenna arrays in hf systems however, the assumption of spatially white noise is maintained keeping the noise covariance matrix proportional to the identity matrix. consequently, the modified covariance matrix can be written as: r̂xx = (a + ∆a)rss(a + ∆a)h + σ2id (3.12) where ∆a is the perturbation of the array matrix due to errors on the manifold. several algorithms are based on the eigen decomposition of the covariance matrix rxx (pillai, 1989). the eigen vectors of the noise subspace are the columns of matrix vn. the perturbation of the array manifold induces a variation ∆vn of matrix vn: v̂n = vn = vn + ∆vn (3.13) the noise subspace of the perturbed covariance is characterized by the relation: [(a + ∆a)rss(a + ∆a)h + σ2id] (vn + ∆vn) = (σ2id + ∆λn)(vn + ∆vn) (3.14) where ∆λn is the perturbation affecting the diagonal of the noise eigen values. developing this expression and assuming that the second order terms can be neglected, we obtain the result (marcos, 1998): ∆vnha = – vnh∆a (3.15) as a consequence of eq. 3.15, the dependence of the angular error with the perturbation of the array manifold is established in the following section. the direction finding technique considered as the reference in this work is the music algorithm (schmidt, 1986) which estimates the angle of arrival by minimizing relatively to the angular parameter θ the quadratic form: f(θ) = ah(θ)vn vnh a(θ) (3.16) 3.3.1. homogeneous array in absence of perturbations, the quadratic form is minimum for the exact angles of arrival: (3.17) with perturbations, the quadratic form is modified in: f̂ (θ) = ah(θ)(vn + ∆vn)(vn + ∆vn)ha(θ) (3.18) and reaches its minimums for angles equal to: θ̂k = θk + ∆θk (3.19) where ∆θk is the angular error. for these values of angle, we can show that: (3.20) if the terms of second order are neglected. besides, (3.21) where denotes the derivative of the steering-vector with respect to the angle θ. the two elements of the sum are conjugated so that: (3.22) this function is calculated for θ = θk keeping in mind that vnha(θk) = 0 and that the only first order terms in ∆vn are significant. we get: (3.23) and finally : (3.24) the second derivative of the modified quadratic form is expressed as: (3.25) a 2v vf a a v v a a v v a n n h h h n n n n h h hθ θ θ θ θ θ θ + = + +m m l l m t t t t t t t ] ] ] ] ] ] ] g g g g g g g r a2 e v vf an n h h k kkθ θθ ∆=l lt ] ] ]g g g7 a r2 ef a v v a a v v a a v v a h h k k k h k h k h k h k n n n n n n θ θ θ θ θ θ θ∆ ∆ + + =l l l l t ] ] ] ] ] ] ] g g g g g g g 7 a r r 2 a a a 2 e v v v v f e a v vn n n h n h h n n h θ θ θ θ θ∆∆= + = + l l lt t t] ] ] ] ] ] ] g g g g g g g 7 7 a a a a d d θ θ θ =l] ] g g a a avf v v v ah n h n h n n hθ θ θ θ θ= +l llt t t t t] ] ] ] ]g g g g g f f of0k kk kk 2θ θ θ θθ∆ ∆= = + +l l mt t t t^ ] ] ^h g g h , ......, f d df and v for k ns a0 0 1 k k n hθ θ θ θ= = = = = ii= l] ] ]g g g vol52,3,2009 20-09-2009 19:06 pagina 333 334 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger with denoting the second derivative of the steering-vector with respect to θ. its value, calculated for θ = θk, contains only one zero order term: (3.26) an approximate expression of the angular error is then: (3.27) and, using the relation showed in eq. 3.15, we finally obtain (marcos, 1998): (3.28) in this relation, the angular error ∆θk depends on the uncertainty affecting the steeringvector a(θk). since the perturbation ∆a(θk) is a random vector, the angular error is quantified with its statistics. therefore, we now calculate the mean square error of the angle of arrival. denoting fk = vnha′(θk), the angular error ∆θk is written as: (3.29) then, the related mean square error is calculated as: (3.30) where matrices ckk = e [∆a(θk)∆a(θk)h] and dkk = e [∆a(θk)∆a(θk)t] contain statistics of the perturbation ∆a(θk). 3.3.2. heterogeneous array in this section, we derive the calculation of the angular error resulting from a perturbed arr f f f 2 e e f f v c v d v 2h * *h h k k k k n kk n k kk n k2θ∆ = +_ i 6 @ # 2f f a fv v f a k k n n k k k k h h h k h θ θθ ∆ ∆ ∆ = +] ]g g re a v v a a v v a k k k n n n n h h k h h k .θ θ θ θ θ ∆ ∆ l l l ] ] ] ] g g g g7 a re a av v a v v a n n h h k k k n n h h k k .θ θ θ θ θ ∆ ∆l l l ] ] ] ] g g g g7 a a a2 v vf n nk h k h k.θ θ θm l lt ] ] ]g g g a a d d 2 2 θ θ θ =m] ] g g ray response ∆ah of a heterogeneous structure. the modified acquisition covariance matrix is expressed as: r̂xxh = (ah + ∆ah)rss(ah + ∆ah)h + σ2id (3.31) similarly to relation showed in eq. 3.15, it can be demonstrated that the corresponding perturbation ∆vn of the noise subspace verifies: ∆vnhah = –vnh∆ah (3.32) the music algorithm is based on the orthogonality between the steering-vector of an incident signal and the noise subspace (schmidt, 1986). as a consequence of implementation operating with an estimation of the covariance matrix, the corresponding vectors are only approximately orthogonal. for this reason, the variable vector to be projected in the noise subspace should have a constant norm for all directions of arrival under test. this condition is obviously fulfilled for the homogeneous array with a norm of a(θ) being equal to �nc whatever the angle of arrival. on the contrary, the steering-vector of the heterogeneous array does not have this property as indicated in section 3.2.2. therefore, the quadratic form to be minimized in this case is written as (erhel, 2004): f(θ) = bhh(θ)vnvnh bh(θ) (3.33) where bh(θ)ah(θ)/�ah(θ)� (3.34) is the normalized steering-vector. actually, the measure of orthogonality between a variable vector and a given subspace implies that the vector has a constant norm. the angular error for the heterogeneous array is expressed as an equivalent of relation of eq. 3.27: (3.35) where is the derivative of the normalized steering-vector relatively to the angle of arrival. d b b dh hθ θ θ =l] ] g g r be v v b v v b b h n n k h k h k k h n n h h hh k ,θ θ θ θθ ∆ ∆l l l ] ] ] ] g g g g7 a vol52,3,2009 20-09-2009 19:06 pagina 334 335 utilization of antenna arrays in hf systems to take benefit of relation of the eq. 3.32, we express that: (3.36) to finally obtain the expression of the angular error: (3.37) statistics of this error are calculated relatively to the characteristics of the perturbations affecting the array responses quantified by the matrices ckkh = e[∆ah(θk)∆ah(θk)h] and dkkh = e[∆ah(θk)∆ah(θk)t]. denoting , we can finally express the mean square angular error for the heterogeneous array as: (3.38) 3.4. numerical simulations 3.4.1. antenna arrays for hf direction finding the active loop antenna is the standard sensor for hf direction finding system. its size, small relatively to the wavelength guarantees an efficient spatial sampling of the incident waves and its load, being a low impedance, also reduces problems of dispersion which affect the high impedance preamplifiers of monopoles and dipoles. several loop antennas of the same type are classically associated in a circular uniform array for hf direction finding. the sensors are then equi-spaced along a circle and set up with the same orientation on an horizontal ground. this structure is considered in this section as the reference for an homogeneous array (fig. 6, left frame). in the second array which is considered, the antennas are subject to a rotation of nd degrees around a vertical axis every two positions within the array so that the structure ber g d v ge v c v e g g g a2 kkh n * k * k n kkh n h h k k k k k h h 2 2 2 $ θ θ ∆ + = ] _ _ g i i 6 @ # g v b k h n kθ= l] g re b v v b a b v v a k h k h k k h k h k h n n h h h n n h 1 ,θ θ θ θ θ θ ∆ ∆ l l l ] ] ] ] ] g g g g g7 a v b a v a v a a n h h n h k k k h k k h n h hhθ θ θ θθ ∆ ∆ ∆ = = -] ] ] ] ] g g g g g comes heterogeneous (fig. 6, right frame). the spatial responses of the antennas (fn(θ), n=1,…nc) are computed with an electromagnetic simulation software (nec2d) coupled with a predictive model of the polarization emerging from the ionosphere (erhel et al., 2004). the ground reflection is taken into account with an estimation of parameters conductivity and permittivity. 3.4.2. perturbation model in the expression of the perturbed steeringvector, errors in modulus and phase are separated. for a given angle of arrival (aoa) θk, the component of index n in this vector is expressed as: (3.39) where ∆mnk and ∆φnk are respectively the error son modulus and phase of component n. the perturbation of the steering-vector can be written as: (3.40) with and ⊗ is the schur-hadamard product. in the calculation of angular mean square errors for aoa θk, matrices ckk, dkk, ckkh and dkkh are then expressed as: 3.41 with ., e 1 e 1p n e m m1 1 1nk j lk j 2 nk lk∆ ∆= + + -dz dz^ ]^ ]^h g h g h7 a , e 1 e 1 *p n e m m1 1 1nk j lk j 1 nk lk∆ ∆= + + -dz dz^ ]^ ]^h g h g h7 a c p a a c p a a p a a p a a d d kk 1 h h k k k h k k k k kkh kk kkh h h t t k h 1 2 2 7 7 7 7 θ θ θ θ θ θ θ θ= = = = ] ] ] ] ] ] ] ] g g g g g g g g .........e edg 1 1m k j mnck j t k 1 1k nck∆ ∆= + +dz dz] ]g g6 @ a a a dg akk k k k7θ θ θ θ∆ = =t] ] ] ]g g g g ema a1k n nk j k n nkθ θ∆= + dzt] ] ]g g g vol52,3,2009 20-09-2009 19:06 pagina 335 336 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger for the numerical simulations presented in the next section, the assumptions concerning the perturbations are: independent perturbations of two different sensors ∆mn and ∆gn mutually independent for a given sensor ∆mn and ∆gn have zero mean values. 3.4.3. comparison of robustness for two structures the two circular arrays are characterized by geometrical parameters: number of sensors nc=10, diameter of each loop d=1.3m, array radius r=20m, inter-element rotation nd=20° (heterogeneous case). the scenario of the reception involves ns=2 signals with a carrier frequency fo=10 mhz, impinging on the arrays with azimuths of arrival az1=40° and az2=50°. the common elevation of arrival is assumed to be known. the signal to noise ratio (snr) is equal to 12 db and the b.t product (bandwidth by difference of group delays) is equal to 3. uncertainties with the same magnitude are assumed to affect the two array manifolds. for each sensor, the phase variation is uniformly distributed on the interval [-δφmax;+ δφmax] with δφmax = 15°. the modulus error is also uniformly distributed in an interval [-δmmax;+ δmmax], δmmax being a variable parameter in the numerical simulation adjustable from 0 up to 40%. the corresponding angular rms error affecting the azimuth estimation is computed for the two types of arrays according to the relations given in eq. 3.30 and eq. 3.38. the results are plotted in the left frame (homogeneous) and right frame (heterogeneous) of fig. 7. for a given level of perturbation, the angular error is smaller when the direction finding is implemented on the heterogeneous array. this observation remains systematically if the parameters of the scenario are modified in a large scale: number of incoming waves, directions of arrival, carrier frequency, signal to noise ratio. consequently, the heterogeneous array appears more robust than the equivalent homogeneous structure. this result however assumes the same magnitude of perturbation in the two array manifolds. this point needs further work as the model of steering-vector for the heterogeneous case requires the computation of the sensor spatial responses in addition to the classical geometrical phases: the level of uncertainty may increase with the number of parameters fig. 6.a,b. homogeneous circular array (left frame) and heterogeneous circular array (right frame). vol52,3,2009 20-09-2009 19:06 pagina 336 337 utilization of antenna arrays in hf systems present in the vector. 4. concluding remarks in order to realise increased channel capacity by means of a mimo system, it is important that the signals received at each of the antenna elements of the receiver array from each of the elements of the transmitter array are adequately decorrelated. for the spaced heterogeneous antenna arrays described in this paper, this has been demonstrated experimentally to be the case. furthermore, a preliminary assessment on the use of compact heterogeneous antenna arrays at the receiver has yielded encouraging results. marked antenna orientation dependent and frequency dependent fading effects have been observed, and the prevailing modal structure of the ionosphere has been shown to have a key influence on the expected mimo performance. the correlation of multipath ionospherically reflected signals received at narrowly spaced antennas has been determined theoretically for different ionospheric conditions and paths using a wideband simulator based on a physical model including the effect of the frozen-in drift of ionospheric irregularities and both magneto-ionic modes. conditions where the irregularities have a significant effect are likely to occur in the auroral, polar and equatorial regions. each mode (e.g. 1eo or 1fx) can exhibit different “fading”. paths reflected from the f region generally show poorer correlation at spaced antennas than e region reflected paths. calculations of estimated channel capacity show useful increased capacities even for quite high correlation levels of 0.8 or 0.9 for a 2x2 mimo implementation. in addition to the investigations on mimo, the robustness of the music superresolution direction finding algorithm to modelling errors for both homogeneous and heterogeneous antenna arrays has been investigated through numerical simulations of trans-horizon scenarios in the hf band. the antenna array responses were calculated with the aid of electromagnetic simulation software with a deterministic model of the polarization of the radio waves emerging from the ionosphere and taking the effect of the ground into account. the results indicate higher robustness for the heterogeneous array for a given level of uncertainty than that for the homogeneous array. modelling errors result classically from uncertainty in the position of the sensors, defaults in the electronic calibration or in the measurement of the electrical parameters of the antennas relative to the ground. for heterogeneous arrays, these uncertainties fig. 7.a,b. rms error, homogeneous array (left frame) and heterogeneous array (right frame). vol52,3,2009 20-09-2009 19:06 pagina 337 338 s.d. gunashekar, e.m. warrington, h.j. strangeways, y. erhel, s. salous, s.m. feeney, n.m. abbasi, l. bertel, d. lemur, f. marie and m. oger are reinforced by the possibility of poor estimation of the polarization characteristics of the radiowave at the ionospheric exit point. acknowledgements the authors are grateful to the epsrc for their financial support of the experimental aspects of the hf-mimo work. the collaboration between the uk and french groups in this investigation is facilitated through the eu cost 296 action on the mitigation of ionospheric effects on radio systems (miers). references brine, n.l., c.c. lim, a.d. massie and w. marwood (2006): capacity estimation for the hf-mimo channel, in proceedings of the sixth symposium on radiolocation and direction finding, (may 2006, southwest research institute, san antonio, texas, usa). erhel, y., d. lemur, l. bertel and f. marie (2004): h.f. radio direction finding operating on a heterogeneous array: principles and experimental validation, radioscience, 39 (1), 1003-14. gherm, v.e., n.n. zernov and h.j. strangeways (2005): hf propagation in a wideband ionospheric fluctuating reflection channel: physically based software simulator of the channel, radio science, 40 (1), doi:10.1029/2004rs003093. gunashekar, s.d., e.m. warrington, s. salous, s.m. feeney, h. zhang, n. abbasi, l. bertel, d. lemur and m. oger (2008): early results of experiments to investigate the feasibility of employing mimo techniques in the hf band, in proceedings of the loughborough antennas and propagation conference 2008 (lapc 2008), (march 2008, loughborough, uk). loyka, s.l. (2001): channel capacity of mimo architecture using the exponential correlation matrix, ieee communications letters, 5 (9), 369-371. marcos, s. (1998): les méthodes à haute résolution: traitement d’antenne et analyse spectrale, (hermes, paris). pillai, s.u. (1989): array signal processing, (springerverlag, new york). schmidt, r.o. (1986): multiple emitter location and signal parameter estimation, ieee transactions on antennas and propagation, ap-34 (33). strangeways, h.j. (2005): determination of the correlation distance for spaced antennas on multipath hf links and implications for design of simo and mimo systems, presented at the 2nd european space weather week, (noordwijk, the netherlands, 14-18 november, published online). strangeways, h.j. (2006a): estimation of signal correlation at spaced antennas for multi-moded ionospherically reflected signals and its effect on the capacity of simo and mimo hf links, in proceedings of the 10th international conference on ionospheric radio systems, and techniques, (london, 18-21 july), 306-310. strangeways, h.j. (2006b): determination of the capacity of ionospheric hf mimo systems employing linear or planar arrays or co-located antennas, paper presented at cost 296 workshop, (rennes, france 4-6 october 2006), published on the workshop cd-rom. strangeways, h.j. (2006c): determination of the correlation distance for spaced antennas on multipath hf links and implications for design of simo and mimo systems, presented at the 1st european conference on antennas and propagation (eucap), (nice, france, 6-10 november 2006), published on conference cd-rom. vol52,3,2009 20-09-2009 19:06 pagina 338 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 some reasoning on the improvement of the etas modeling at the occurrence of the 2016 central italy seismic sequence anna maria lombardi istituto nazionale di geofisica e vulcanologia, rome, italy annamaria.lombardi@ingv.it abstract this study presents an application of the etas model to the first 20 days of the 2016 central italy sequence. despite of the provisional nature of data, the model is able to describe the occurrence rate, but for the first hours after the mainshock occurrence. a sensitivity analysis of the model to two uncertainty sources, the model parameters and the occurrence history, shows that the second has a main role in controlling the performance of the etas model. previous results, together with the clear inability of etas to forecast the occurrence of a sequence before its starting time, give important suggestions about possible improvements. i. introduction t he use of time-dependent models for practical purposes has been introduced recently in italy to forecast seismicity (marzocchi et al. 2012; 2014). time variations of the seismic rate are more evident over the short-term (days to weeks), and in particular after a large earthquake. the modeling of these time variations, and its use for practical purposes, led to the development of operational earthquake forecasting (oef), a set of procedures for gathering and disseminating information about the time dependence of seismic hazards (marzocchi et al., 2014). case studies may serve as a good basis to develop oef efficient procedures, able both to forecast the spatio-temporal evolution of sequences and to improve the knowledge about the main events. this paper is a case study of the 2016 central italy sequence, still on-going, by mean of the epidemic type aftershock sequences (etas) model (ogata, 1998; lombardi, 2015; 2016a; 2016b). table 1: etas parameters ingv bulletin csiv1.1 cpti15 (1478 events) (391 events) (85 events) ml ≥ 2.5 ml ≥ 3.0 mw ≥ 5.0 µ 2.0e-02 2.7e-03 3.9e-04 (1.9, 2.2) e-02 (7.3, 3.2) e-03 (3.6, 4.2) e-04 k 4.7e-02 4.8e-02 1.8e-02 (4.4, 5.2) e-02 (4.4, 6.1) e-02 (1.1, 2.5) e-02 p 1.11e+00 1.10e+00 1.0e+00 (1.10, 1.14) e+00 (1.09, 1.13) e+00 (0.9, 1.1) e+00 c 1.2e-02 1.4e-02 9.0e-02 (1.1, 1.7) e-02 (1.1, 2.3) e-02 (0.2, 1.0) e-01 α 1.2e+00 1.1e+00 8.0e-01 (1.1, 1.3) e+00 (0.8, 1.2) e+00 (0.4, 1.2) e+00 d 7.0e-01 1.2e+00 3.7e+00 (6.0, 7.0) e-01 (1.1, 1.5) e+00 (2.3, 5.7) e+00 q 1.57e+00 1.55e+00 2.2e+00 (1.55, 1.61) e+00 (1.52, 1.62) e+00 (2.1, 3.0) e+00 γ 3.6e-01 ∼ 0 1.0e+00 (3.1, 4.0)e-01 (0.7, 1.5) e+00 b 1.1 0.90 0.9 (1.05, 1.15) (0.85, 0.95) (0.8, 1.0) logl -8263 -2852 -1406 (-8266, -8263) (-2854, -2852) (-1408, -1406) 1 mailto:annamaria.lombardi@ingv.it annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 figure 1: map of the seismicity occurred in the area interested by the 2016 central italy sequence. gray and yellow circles mark the events with magnitudes ml < 2.5 and 2.5 ≤ ml < 4.0, respectively. green square and red stars marks the events with magnitude 4.0 ≤ ml < 5.0 and ml ≥ 5.0, respectively. the left panel show the first 20 days of the 2016 sequence (august 24 september 15) in the area struck by the sequence [12.95-13.5e, 42.45-43.0n]. in the right panel the seismicity occurred before the 2016 sequence (april 16 2005 august 24 2016) in the larger area [12.7-13.8e, 42.0-43.5n], used to estimate the etas model, is shown (the square identifies the area interested by the 2016 sequence). the strongest events in the southern part refer to the 2009 l’aquila sequence. it has two main goals: to check the performance of the model on very provisional data and to provide some points of interest for a possible improvement of the model. ii. estimating the etas model on august 24, 2016, at 01:36 utc, a magnitude ml 6.0 earthquake hit part of the central italy area, close to the village of amatrice and accumuli, causing a number of human losses and significant economic damages. the evolution of the seismic sequence, after the ml 6.0 event, is modeled in the present study through the etas model. the seismicity data used here are downloaded from the site of the official ingv bulletin (www.iside.rm.ingv.it), from april 16 2005, day marking the start-up of a new seismic network, up to september 15 2016. i estimate the etas model on seismicity occurred in the region [12.7-13.8e, 42.0-43.5n] (in the following called estimating region), including the whole 2009 l’ aquila sequence (see figure 1). this region includes the area [12.95-13.5e, 42.45-43.0n], at the present interested by the 2016 central italy sequence (in the following called testing region). i estimate the etas model by using the data from april 16 2005 to august 24 2016, occurred in the estimating region, with magnitude above ml2.5. more than 60% of events belongs to the 2009 l’aquila sequence. the choice of the threshold magnitude is due to the incompleteness of data below ml2.5, soon after the occurrence of the strongest event of the 2009 l’aquila sequence (lombardi, 2016a). the version of the etas model used here has been implemented in sedav1.0 (statistical earthquake data analysis), a statistical software freely provided via the zenodo open access platform 2 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 figure 2: comparison between the observed and the expected number of events for the threshold forecast magnitude mf = 2.5, 3.0 and 4.0 for the first 20 days of the sequence (aug 24 -sep 15) (https://zenodo.org/record/55277; lombardi, 2016a; lombardi, 2016b). for the present study, i use a background grid covering the testing area and with a step of 0.01o . by applying the sedav1.0 estimating tool (based on the maximum likelihood criterion), i found the parameters reported in table 1, by mean of 1000 runs (lombardi, 2015). the numbers in the brackets are the 95% confidence bounds of the parameters values founded in all runs. they have a double meaning: to represent the epistemic uncertainty of the model and to summarize a possible correlation of model parameters, causing a possible multimodality of log-likelihood function. iii. measuring the performance of the etas model i make two types of analysis, separately. in each of them i measure the sensitivity of the etas model to its two uncertainty sources: the occurrence history and the parameters (including the background spatial distribution). in the first analysis i check the sensitivity of the etas model to the occurrence history and i mimic real-time forecast calculations, by using the better etas parameters, listed in table 1, estimated from the bulletin data. specifically, i compare the observed number of events with what expected by the etas model, in the testing area (number of events test, lombardi, 2016a). specifically, i simulate 10000 etas time histories, for nd overlapping interval times of one day {di, i = 1, ..., nd}, updated each hour, starting from the time occurrence of the mainshock (august 24 2016, 01:36:32, ml6.0) up to september 15 2016. the simulations are done by the tool implemented in sedav1.0 and based on the thinning method (ogata, 1998; lombardi 2016b). for each di, i import the actual history occurred up to the starting time of di and i simulate the events inside. this test allows to both quantify the agreement between model and observations and to measure the sensitivity of the model to the history ht. in this first step the parameters sensitivity is not investigated. in figure 2 i show the comparison between the expected and the observed number of events with magnitude above the threshold forecast magnitude mf =2.5, 3.0 and 4.0. specifically, i show the median expected number of events (black line) together with the 95% confidence bounds (red lines), to quantify the uncertainty of predictions. the confidence interval defined by the etas model contains the observed number of events with ml ≥ 2.5, except in the first 8 hours of the sequence. anyway, the observations are above the median forecasts uninterruptedly in the first three days, showing a systematic underestimation. after this period, the observations (blue points) and the median forecasts (black line) overlap. these 3 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 figure 3: comparison between the observed and the expected number of events for mf = 3.0 and for interval times of 1 week, 1 day and 3 hours for the first 3 days of the sequence (aug 24-27) results improve for larger values of mf , for which the period of underestimation is smaller (figure 2). previous results do not depend on the length of the forecast interval di. none significant difference is found for interval times di of 3 hours, 1 day and 1 week, in the first 3 days of the sequence (figure 3). the only difference is that the observed number of events is steadily above the median, for intervals di of 1 week. previous calculations are a measure of the agreement between the actual history and what expected by the model: this last is fixed, whereas the history changes among simulations (figures 2 and 3). the second methodology is a retrospective forecast analysis devoted to check the sensitivity of the etas model to the input parameters (including the background spatial distribution). specifically, i compute the expected number of events nf for each of 1000 models obtained by applying the sedav1.0 estimating tool (see previous section), including the actual time history up to the end of the forecast interval. the calculations are done for interval times of 1 day, updated each hour, and for mf =2.5, integrating the conditional intensity of the etas model (lombardi, 2015). as figure 4 shows, the parameter sensitivity of the model is negligible. the better approximation of observations in the first hours of the sequence, respect to previous analysis (see figure 2), is due to inclusion of the whole actual history in calculations. by keeping in mind the provisional nature of the earthquakes data, previous analyses show that the etas model is able to describe the sequence, except at the beginning of the sequence. clearly, the etas model does not have any predictive ability before the main event. the probability to have an event above ml5.5 in the next day, computed at 00:00 of aug 24, is 10−5 in the whole area, mainly given by the background rate. improving the forecast capability before the occurrence of a sequence, of the etas or similar stochastic models, is a 4 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 figure 4: parameters sensitivity analysis for the etas model. the plot compares observations and forecasts for mf = 2.5 and for the first 3 days of the sequence (aug 24-27) challenge to which the seismological statistical community must respond. iv. improving the model: the integration of different data the improving of the forecast capability of the etas model is a very difficult process that cannot surely be resolved in the present study. this section is devoted to discuss some very preliminary results, derived by the comparison of different types of earthquakes catalogs. the inefficiency of the etas model to predict imminent sequences, following the occurrence of strong events, mainly derives from the poor modeling of the background. firstly, the poissonian model neglects possible long-term temporal variations of the seismic rate. second, the temporal window covered by instrumental catalogs should be too small to infer the actual spatial distribution of background. regarding the 2016 sequence, i compare the informations coming from three different catalogs: the official ingv bulletin, used before, the instrumental csiv1.1 (castello et al. 2007; http://csi.rm.ingv.it/) and the historical cpti15 catalogs (rovida et al., 2016; http://emidius.mi.ingv.it/cpti15dbmi15/). the csiv1.1 is available for the period 1981-2002, but i use the data from july 1 1984, above ml3.0, doing to network changes during the early 1980s. the cpti15 collects the strong earthquakes occurred in italy from 1000 to 2014, but i consider the events above mw5.0 after 1600. figure 5 is a comparison of the background estimation for the three catalogs. table 1 reports the etas parameters for all of them. both the ingv bulletin and the csiv1.1 catalogues identify the areas around norcia and campotosto cities as the most hazardous regions. anyway, the size and the bounds of these areas are defined by the sequences included in the data (the 1997-1998 colfiorito sequence, at north-west, for the csiv1.1, and the 2009 l’aquila sequence, at south-east, for the ingv bulletin). the small size of the cpti15 catalogues does not allow to reach detailed results. it seems to confirm what obtained from the other two catalogues, but assign a larger relative seismic potential to the area around accumuli, struck by the 2016 earthquake. in summary, this analysis shows that the instrumental catalogs might cover a too small temporal window to infer a reliable spatial distribution of background, that might result driven by the occurrence of sequences. 5 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 figure 5: comparison between the background spatial probability distributions obtained from three catalogs: the instrumental official ingv bulletin, the csiv1.1 and the historical cpti15 catalogue. gray circles mark the events below magnitude 2.5; orange circles, squares and stars mark the events with magnitude m < 4.0, 4.0 ≤ m < 5.5 and 5.5 ≤ m < 6.5, respectively. black stars marks the events with m ≥ 6.5. the red stars mark the ml 6.0 and ml5.4 events of the 2016 central italy sequence, respectively, both occurred at august 24. the magnitude scales are ml for the ingv bulletin and csiv1.1 and mw for cpti15. the right panel shows the events above mw4.0, for consistency with the other panels, but the background rate is estimated on events above mw5.0. v. conclusions and perspectives the preliminary results shown here highlight some important issues: • the etas model is able to describe the first part of the 2016 central italy sequence, also with very preliminary data. the initial underestimation might be due to both the provisional nature of data and to some inefficiencies of the etas modeling (figure 2). • the parameter sensitivity of the etas model is negligible respect to impact of the occurrence history (figure 2, 3 and 4). • the inefficiency of the etas model to predict the occurrence of sequences may be due to poor background modeling. by a mathematical point of view, the etas model identifies a branching structure for the earthquakes. so, the background events are the earthquakes not triggered by previous ones. to give a seismological interpretation to this definition of background is not straightforward. specifically, the instrumental catalogs could cover a too small temporal window to infer a reliable background spatial distribution, unless we assume a time-dependence of the background. but this last hypothesis is contrary to poisson (time-independent) modeling of background itself, adopted by etas models. • the sa algorithm used here allows the quantification of parameters uncertainties. these last represent both the ability of the algorithm to identify the best model from data and the correlation of parameters giving indistinguishable models (by using the maximum log-likelihood 6 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7202 criterion). this point puts in question the physical significance of the etas model and will be further investigated. • to provide more accurate forecasts soon before the sequences occurrence, we need to improve the stochastic models as etas. a correct integration of seismological, geological and geodetic data is likely the best way to improve the spatio-temporal resolution of stochastic forecast models. • the present study is a preliminary analysis of the first 20 days of a very complex sequence, marked by three mainshocks in two months of moment magnitude 6.0 (august 24), 5.9 (october 26) and 6.5 (october 30), respectively. the check of the ability of etas modeling to explain the spatio-temporal evolution of the sequence deserves more investigation and will be the topic of future work. references [castello et al., 2007] castello b. , olivieri m. and selvaggi g.. (2007). local and duration magnitude determination for the italian earthquake catalogue (1981-2002). bull. seism.soc.am., 97(1b), 128-139. [lombardi, 2015] lombardi a.m. (2015). estimation of the parameters of etas models by simulated annealing sc. rep., feb 12;5:8417. doi: 10.1038/srep08417. [lombardi, 2016a] lombardi a.m. (2016). seda a software package for the statistical earthquake data analysis: a tutorial application to the 2009 l’ aquila and the 2012 emilia (italia) sequences ann. geoph., submitted. [lombardi, 2016b] lombardi a.m. (2016). seda a software package for the statistical earthquake data analysis sc. rep., submitted. [marzocchi et al.„ 2012] marzocchi, w., a. amato, a. akinci, c. chiarabba, a.mlombardi, d. pantosti and e. boschi (2012). a ten-year earthquake occurrence model for italy. bull. seism. soc. am., 102, 1195-1213. [marzocchi et al.„ 2014] marzocchi w, lombardi a.m. and e. casarotti, (2014) the establishment of an operational earthquake forecasting system in italy, seism. reaserch lett,, 85, 961-969, doi: 10.1785/0220130219. [ogata, 1998] ogata, y. (1998). space-time point-process models for earthquake occurrences. ann inst stat math, 50, 379-402. [rovida et al., 2016] rovida a., locati m., camassi r., lolli b., gasperini p. (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. istituto nazionale di geofisica e vulcanologia. doi:http://doi.org/10.6092/ingv.it-cpti15 7 introduction estimating the etas model measuring the performance of the etas model improving the model: the integration of different data conclusions and perspectives r i s u l t a t i d e l l a r i c e r c a d e i v u o t i s o t t e r r a n e i c o n i l m e t o d o d e i p r o f i l i «li r e s i s t i v i t à i n u n a z o n a d e l l e f o n d a z i o n i d i u n v i a d o t t o iresults in a research of hurried cavities l>y resistivity profiles methods in the foundation's area of a viaduct) e . b r i z z o l a r i ( * ) r i c e v u t o il 15 gennaio 197a r i a s s u n t o . — si riferisce sui r i s u l t a t i o t t e n u t i in u n a ricerca p e r c a v i t à s o t t e r r a n e e . è s t a t o i m p i e g a t o u n dispositivo a c o r r e n t i f o c a l i z z a t e tipo g r a n d i n e t t i . è s t a t o e f f e t t u a t o un c o n f r o n t o p r a t i c o in c a m p a g n a t r a il dispositivo s e h l u m b e r g e r e dispositivi t i p o g r a n d i n e t t i . i r i s u l t a t i della i n t e r p r e t a z i o n e e f f e t t u a t a c o n f r o n t a n d o le c u r v e di c a m p a g n a con gli and a m e n t i teorici esistenti in bibliografia e con quelli calcolati per l'occasione, h a n n o p e r m e s s o di i n d i v i d u a r e alcune c a v i t à s o t t e r r a n e e d i m o s t r a n d o l'efticacia del dispositivo g r a n d i n e t t i . s u m m a r y . we refer on o b t a i n e d r e s u l t s in an i n v e s t i g a t i o n of h u r r ied cavities. we e m p l o y e d a « g r a n d i n e t t i ' s » e l e c t r o d e a r r a y using focalised c u r r e n t . wo h a v e c o m p a r e d d i r e c t l y on field s e h l u m b e r g e r a r r a y a n d g r a n d i n e t t i ' s electrode a r r a y . t h e results of t h e i n t e r p r e t a t i o n , m a d e w i t h c o m p a r a t i o n m e t h o d , h a v e p e r m i t t e d t o find some h u r r i e d cavities d e m o n s t r a t i n g t h e efficacy of g r a n d i n e t t i ' s electrode a r r a y . p r e m e s s e u n o d e i m e t o d i g e o f i s i c i p e r l a r i c e r c a d i v u o t i s o t t e r r a n e i a p i c c o l a p r o f o n d i t à è, c o m e n o t o , il m e t o d o g e o e l e t t r i c o . i n p a r t i c o l a r e (*) i s t i t u t o di geofisica mineraria u n i v e r s i t à degli s t u d i , r o m a . e . brizzolati vengono impiegati i m e t o d i a resistività e t r a essi, generalmente, quello dei profili di resistività. nella presente n o t a vengono riferiti i r i s u l t a t i o t t e n u t i , m e d i a n t e l'esecuzione ili profili ili resistività, nella ricerca dei v u o t i nella zona di fondazione del v i a d o t t o « l ' a n t i c a » della sora-frosinone. l'interesse dalla ricerca consiste nell'aver utilizzato, nell'esecuzione dei profili di resistività, un dispositivo a correnti focalizzate del t i p o « grand inetti » che per le sue c a r a t t e r i s t i c h e geometriche e di campo p e r m e t t e di rilevare anomalie non f a c i l m e n t e individuabili con i dispositivi tradizionali. o p e r a z i o n i d i campagna e d e l a b o r a z i o n e d e i r i s u l t a t i nella zona oggetto dell'indagine esisteva il sospetto della presenza di u n a coltivazione in sotterraneo di materiali vulcanici con p r o p r i e t à pozzolaniche. si è pensato p e r t a n t o ili accertare l ' e v e n t u a l e presenza di v u o t i , m e d i a n t e l'esecuzione ili profili di resistività paralleli all'asse del viad o t t o e ad esso ortogonali. t r a t t a n d o s i p e r a l t r o di u n a indagine preliminare, essa non è s t a t a volta alla determinazione di t u t t i gli eventuali v u o t i della zona m a p i u t t o s t o a stabilire la loro esistenza e ad i n d i v i d u a r n e la posizione nelle i m m e d i a t e vicinanze del f u t u r o asse stradale. p e r t a n t o la zona investigata non risulta molto estesa, nò i profili t r o p p o ravvicinati. nell'indagine si sono utilizzati il dispositivo sclilumberger e due dispositivi tipo g r a n d i n e t t i (figura 1) allo scopo di c o n f r o n t a r n e i risultati. d i s p o s i t i v o schluitìberger © -ab — d i s p o s i t i v o g r f i i l d i n e t t i o.bwa „ î î l d i s p o s i t i v o g r n n d i n e t t i m o d i f i c a t o ih 1 1 -+ s + s fìr. i d i s p o s i t i v i c l c t t r o d i c i . r i s u l t a t i d e l l a r i c e r c a d e i v u o t i s o t t e r r a n e i e c c . 3 fi s t a t o utilizzato il dispositivo g r a n d i n e t t i t i p o « c » (grandin e t t i ) (») e un dispositivo di t i p o trielettrodo con elettrodo di potenziale di riferimento all'infinito. l a scelta delle distanze elettrodiche è s t a t a f a t t a sulla base della s u p p o s t a p r o f o n d i t à degli eventuali v u o t i e delle loro dimensioni probabili. le notizie raccolte sul luogo da persone che a v e v a n o visto o a v u t o notizie i n d i r e t t e sui vuoti esistenti nella zona indicavano v u o t i con p r o f o n d i t à del t e t t o variabili tra 1,5 e 5 ni e dimensioni delle gallerie non superiori a 2--1 ni. i n accordo con tali ordini di grandezze sono s t a t e fissate distanze elettrodiche n = 3 m p e r i dispositivi grand i n e t t i , m e n t r e per quelli schlumberger si sono a d o t t a t e distanze ab = (ì ni e ab 10 m. l'ubicazione dei profili è indicata nella figura 2. 4 e . n k i z z o i . a k i ti ]>roiilo 1 è s t a t o r i p e t u t o con diversi dispositivi e diverse distanze elettrodiche al fine di i n d i v i d u a r e la migliore d i s t a n z a elettrodica e verificare la e f f e t t i v a utilità dei dispositivi g r a n d i n e t t i (3). i r i s u l t a t i o t t e n u t i sii tale profilo sono r i p o r t a t i nelle figure 3a e 3b per q u a n t o r i g u a r d a i dispositivi g r a n d i n e t t i e sulla figura 4 per il dispositivo schlumberger con distanze elettrodiche ab = 6 ni e ab 10 m. dei profili sono s t a t i r i p o r t a t i solo i t r a t t i più significativi. nella figura 4 sembra possibile accertare la presenza di anomalie collegabili con e v e n t u a l i v u o t i sotterranei. per verificare d ' a l t r a p a r t e che le variazioni siano connesse a effetti anomali p r o f o n d i e non a disfi<r. 4 r i s u l t a t i o t t e n u t i sul profilo 1 con dispositivo s c h l u m b e r g e r . a t r a t t e g g i o le medie di t r e valori contigui, riferiti alla progressiva c e n t r a l e del g r u p p o . omogeneità superficiali, è s t a t a calcolata la media di 3 o più valori contigui a t t r i b u e n d o n e poi il valore alla progressiva centrale del gruppo. così facendo sembra però accertato che la resistività a p p a r e n t e varia con regolarità p r o b a b i l m e n t e solo a causa di variazioni laterali della resistività del terreno. r i s u l t a t i d e l l a r i c e r c a d e i v u o t i s o t t e r r a n e i e c c . 6 p r o f i l o 1 d i s p o s i t i v o grpndinctti a = 3 m fig. 3i> r i s u l t a t i o t t e n u t i sul profilo 1 con un dispositivo g r a n d i n e t t i modificato. a t r a t t e g g i o le medie di t r e valori contigui, riferiti alla progressiva c e n t r a l e del g r u p p o . fig. 3a r i s u l t a t i o t t e n u t i sul profilo 1 con u n dispositivo g r a n d i n e t t i . a t r a t t e g g i o le medie di t r e valori contigui, riferiti alla progressiva c e n t r a l e del g r u p p o . p r o f i l o 1 dispositivo grandinetti modicicbto a = 3 m i i i 5 0 6 0 m e brizzolari p e r t a n t o le variazioni sono p r e s u m i b i l m e n t e d o v u t e a disomogeneità superficiali. la differenza t r a i valori di resistività a p p a r e n t e t r a i d u e profili sono da ascriversi alla diversa p r o f o n d i t à investigata con le due distanze elettrodiche (ab = 6 m e ab = 10 m). si esaminano ora i r i s u l t a t i o t t e n u t i con dispositivi t i p o grand i n e t t i . nei profili delle figure 3a e 3b, è possibile rilevare con u n a certa sicurezza alcuni a n d a m e n t i anomali. p e r cercare di esaltare queste anomalie, in r a p p o r t o ai d i s t u r b i , è s t a t a t e n t a t a anche qui u n a sorta di « filtraggio » dei r i s u l t a t i sostituendo ai valori rilevati la m e d i a di g r u p p i di valori contigui. nelle figure 3a e 3b i r i s u l t a t i così o t t e n u t i sono r i p o r t a t i con linea t r a t t e g g i a t a . nelle figure 5, (> e 7 sono disegnati i r i s u l t a t i r i g u a r d a n t i rispett i v a m e n t e i profili 2, 3 e 4. ti profilo 3 è s t a t o altresì r i p e t u t o con dispositivo g r a n d i n e t t i « modificato »; con questo dispositivo si riesce ad eliminare i disturbi, ma corrispondentemente si a t t e n u a n o gli eventuali effetti di corpi più p r o f o n d i o t t e n e n d o r i s u l t a t i simili a quelli rilevati con dispositivi tradizionali. sui profili 3 e 4, inoltre, è possibile riscontrare u n a u m e n t o dei c o n t r i b u t i anomali superficiali, che si manifestano con variazioni notevoli delle differenze di potenziale su brevi spazi; si sono o t t e n u t e cioè variazioni con « f r e q u e n z e » elevate indicanti modeste dimensioni dei corpi causa di tali anomalie, r i s p e t t o alle dimensioni del dispositivo. sono s t a t i anche qui calcolati i valori medi, come sopra indicato, che s e m b r a n o eliminare in gran p a r t e le variazioni su breve d i s t a n z a . c o n f r o n t o e c a l c o l o d i a n o m a l i e t e o r i c h e nei d i a g r a m m i r i p o r t a t i risulta a b b a s t a n z a e v i d e n t e come il dispositivo g r a n d i n e t t i p e r m e t t e di ottenere variazioni più vistose ris p e t t o allo schtumberger (*). (*) r i s u l t a t i o p p o s t i a quelli (la noi r i l e v a t i in questo e in oasi a n a l o g h i sono s t a t i o t t e n u t i d a a r m a n d o i1). 1 r p r o f i l o 2 " q 2 1 0 2 0 ~ 3 0 4 0 5 0 fig. 5 r i s u l t a t i o t t e n u t i sul profilo 2 con dispositivo g r a n d i n e t t i . a t r a t t e g g i o le inedie di q u a t t r o valori contigui, riferiti alla p r o g r e s s i v a c e n t r a l e del g r u p p o . 4 0 m d i s p o s i t i v o grandinetti 1 : 3 ( 1 fig. (i r i s u l t a t i o t t e n u t i sul piotilo 3 con dispositivo g r a n d i n e t t i e g r a n d i n e t t i modificato. a t r a t t e g g i o le inedie di q u a t t r o valori contigui, riferiti alla progressiva c e n t r a l e del g r u p p o . fig. 7 r i s u l t a t i o t t e n u t i sul profilo 4 con dispositivo g r a n d i n e t t i . a t r a t teggio le medio di q u a t t r o valori contigui, r i f e r i t i alla p r o g r e s s i v a cent r a l e del g r u p p o . p r o f i l o 3 d i s p o s i t i v o grandinetti modificato a = 3 m . . i p r o f i l o 4 d i s p o s i t i v o grrndinetti s !•',. b k i z z o l . a k i per l'interpretazione, è s t a t o f a t t o u n c o n f r o n t o con le anomalie r i c a v a t e s p e r i m e n t a l m e n t e in laboratorio nel caso di u n cilindro resistivo orizzontale da grandir.etti (') (*). per o t t e n e r e a l t r e curve teoriche per l ' i n t e r p r e t a z i o n e , oltre le schematizzazioni già considerate da g r a n d i n e t t i per i cilindri orizzontali, si sono considerate anche le sfere resistive. s u p p o n i a m o d u n q u e di schematizzare i v u o t i s o t t e r r a n e i con sfere poste ad u n a certa p r o f o n d i t à (fig. 8). kit;. s schema g e o m e t r i c o p e r il calcolo dei potenziali a n o m a l i nel caso ili sfera r e s i s t i v a . in questo caso è possibile la soluzione m a t e m a t i c a scegliendo u n o p p o r t u n o sistema di coordinate (vedi stucchi) (6) ovvero utilizzando la teoria delle immagini elettriche (5). le soluzioni, o v v i a m e n t e coincidenti, che si ottengono, risultano di difficile calcolo p r a t i c o . (*) il medesimo caso del cilindro orizzontale resistivo è s t a t o risolto a n c h e per q u a n t o r i g u a r d a il dispositivo t i p o w e n n e r ('). i n c o n f r o n t o q u a l i t a t i v o d i m o s t r a , in accordo con q u a n t o o t t e n u t o da stucchi, come la focalizzazione delle c o r r e n t i , o t t e n u t a con i dispositivi g r a n d i n e t t i , a b b i a una notevole influenza sulla « p e n e t r a z i o n e » . confront a n d o i n f a t t i i r i s u l t a t i o t t e n u t i s p e r i m e n t a l m e n t e d a g r a n d i n e t t i (') con (incili calcolati per il dispositivo w e n n e r (') si possono notare; i r i s u l t a t i n e t t a m e n t e migliori nel caso dei dispositivi g r a n d i n e t t i . r i s u l t a t i d e l l a r i c e r c a d e i v u o t i s o t t e r r a n e i e c c . 10 sostanziali semplificazioni possono essere o t t e n u t e , con m o d e s t e approssimazioni nei risultati finali, introducendo alcune condizioni r e s t r i t t i v e . la sfera resistiva p o s t a al di sotto del piano di campagna, genera r i s p e t t o ad esso, u n a sua i m m a g i n e elettrica in posizione simmetrica. nel caso semplificato che si considera, il potenziale elettrico è calcolato come somma dei d u e potenziali generati s e p a r a t a m e n t e dalla sfera e dalla sua immagine. si t r a s c u r a n o perciò gli effetti m u t u i t r a le d u e sfere; ciò non c o m p o r t a errori superiori al 10% sui valori calcolati finché risulta verificato che li > 1,3 r, con li p r o f o n d i t à del centro della sfera medesima. perciò si p u ò n o t a r e che non si h a n n o apprezzabili errori fin q u a n d o la p a r t e superiore della c a v i t à non si t r o v a nelle i m m e d i a t e vicinanze della superficie topografica. si s u p p o n e i n o l t r e che il campo elettrico generato possa ritenersi c o s t a n t e nella zona i n t e r e s s a t a dalla sfera; q u e s t a ipotesi r i s u l t a abb a s t a n z a verificata se la sfera è sufficientemente d i s t a n t e dai p u n t i di immissione di corrente nel t e r r e n o . con q u e s t e ipotesi semplificative il potenziale sul piano z — 0 (superficie del terreno) può essere così espresso (grant e west pag. 425) (5): dove <ri e ct8 sono r i s p e t t i v a m e n t e la conducibilità elettrica del t e r r e n o e della sfera di d i s t u r b o ; i'j„ e ev sono r i s p e t t i v a m e n t e la componente orizzontale e verticale del campo elettrico i n d i s t u r b a t o calcolato al centro della sfera. sulla scorta delle formule [1] e [2], sono s t a t i calcolati i prolili anomali teorici ottenibili con dispositivi t i p o g r a n d i n e t t i . il calcolo del potenziale è s t a t o f a t t o per u n a corrente erogata di 1 a m p è r e e p o n e n d o la resistività del t e r r e n o pari a qi 1. i risult a t i di tali calcoli sono r a p p r e s e n t a t i nella figura i) nel caso di u n a distanza elettrodica: a -l e raggio della sfera lì 0,5 a, p r o f o n d i t à li del corpo variabile t r a 0,75« e 2a. nella figura 10 inoltre sono rapp r e s e n t a t e altre d u e curve con p r o f o n d i t à e raggio della sfera costanti e distanza elettrodica variabile. eob3 [1] io k. li rizzo l a r i c o n c l u s i o n i sulla scorta dei profili teorici esistenti nella bibliografia grandin e t t i ('), e di quelli a p p o s i t a m e n t e calcolati p r e c e d e n t e m e n t e con la schematizzazione della sfera cava (vedi figure 9 e 10) è s t a t a possibile u n a i n t e r p r e t a z i o n e per confronto con le anomalie rilevate nei profili di c a m p a g n a . tale c o n f r o n t o è r i s u l t a t o in alcuni casi a l q u a n t o difficile in q u a n t o nei profili di c a m p a g n a u n a certa i m p o r t a n z a , come visto, assumono le variazioni su b r e v e distanza i m p u t a b i l i c e r t a m e n t e a disturbi superficiali e non già a disturbi più profondi. cercando p e r t a n t o d i f a r e astrazione dagli a n d a m e n t i locali e considerando le sole a n o m a l i e che investono u n più ampio t r a t t o del profilo sono s t a t e i n d i v i d u a t e sui profili stessi alcune zone per le quali si p o t e v a presumere l'esistenza di v u o t i sotterranei, in conclusione sul profilo 1 sembrano esistere anomalie significative i n t o r n o alle progressive 2(5; 34,5; 50; 54; sul profilo 2 si sono i n d i v i d u a t e anomalie in corrispondenza delle progressive 23 e 39,5; sul profilo 3 infine in corrispondenza della progressiva 32 sembra esistere un a n d a m e n t o anomalo. per accertare la presenza o meno di v u o t i sotterranei in corrispondenza delle anomalie sopraindicate, sono s t a t i eseguiti alcuni sondaggi meccanici ad esse in corrispondenza. i r i s u l t a t i delle perforazioni possono essere sintetizzati nella seguente tabella. t a b e l l a profilo p r o g r e s s i v a v u o t o 1 26 no 1 34,5 sì (da 4 a 0,50 in) 1 50 sì (da 1,20 a 3,8 ni) 1 54 si (da 3 a 4,40 ni) 2 23 sì (da 3 a 3,8 ni) 2 39,5 no 3 32 no è s t a t a p e r t a n t o c o n f e r m a t a l'esistenza di v u o t i sotterranei in corrispondenza della zona investigata. tali v u o t i sembrano avere altezze medie di circa 2 m e p r o f o n d i t à del l e t t o variabili ira 1,20 m e 4 m al di sotto del piano di c a m p a g n a . 5 4 3 2 1 0 1 2 3 4 5 x / ' ò fig. !) p o t e n z i a l i teorici calcolati nel caso (li s f e r a resistiva con dispositivo o r a n d i n e t t i . distanza e l e t t r o d i c a ti i raggio della sfera, li =• 0,5a. p r o f o n d i t à variabili tra 0.75a e 2a. 1 2 a v io3 i p, 8 10 8 6 4 2 0 2 4 6 8 / 7 0 •mg. io potenziali teorici calcolati nel caso di sfera resistiva con dispositivo o r a n d i n e t t i nei due casi i n d i c a t i in figura. e . b r i z z o l a r i r i n g r a z i a m e n t o si ringrazia la cassa per il mezzogiorno per l'autorizzazione alla pubblicazione dei d a t i . b i b l i o g r a f i a ( 1 ) a r m a n d o e . , 1 9 7 3 contributo alla ricerca di cavità sotterranee con. metodi elettrici. « r i v i s t a i t a l i a n a di geofisica », x x i i , 3 4 . ( 2 ) b r i z z o l a r i e . . 1 9 7 1 calcolo di anomalie di potenziale domite a disturbi superficiali semisferici rilevate con dispositivo tipo grandinetti. « boll, di geofisica t e o r . e a p p i . », x i i i . p p . 5 1 5 2 . ( 3 ) g r a n d i n e t t i m . , 1 9 6 7 una nuova disposizione elettrodica per la ricerca di corpi di limitate dimensioni. « boll, di geofisica t e o r . e a p p i . », i x , 35. ( ' ) g r a n d i n e t t i m . , 1 9 6 8 risultati di prove su modelli relative ad una nuova disposizione elettrodica per la ricerca di corpi di limitate dimensioni. « boll, di geofisica t e o r . e a p p i . », x, 39. ( 5 ) g r a n t f . s . , w e s t g . f . , 1 9 6 6 inter pretalion theory in applied geophysics. me graw-hill book c o m p a n y . new y o r k . ( " ) s t u c c h i m . , 1 9 7 2 ricerca del dispositivo elettrodico ottimale nella prospezione geoelettrica di corpi sepolti. « boll, di geofisica t e o r . e a p p i . », x i v , 5 3 . ( 7 ) v a n n o s t i i a n d r . , c o o k k . , 1 9 6 6 interpretation oj resistività data. « gcological snrvey», p r o f e s s i o n a l p a p e r 499. u n i t e d s t a t e s g o v e r n m e n t l ' r i n t i n g office, w a s h i n g t o n . annals 47, 1, 2004, 01/07def 119 annals of geophysics, vol. 47, n. 1, february 2004 key words ultra-low frequency emission – seismicity – alfven waves 1. introduction it is now recognized that analysis of seismic data, even sophisticated, is not sufficient to resolve two essential problems of geodynamics: what are the mechanisms of earthquakes (eqs) origin and how can large eqs be forecast? in such a situation an importance of nonseismic methods is evident. one of them is the variation of magnetic field in the ultra-low frequency (ulf) range 0.01-10 hz. this effect was first reported by frasersmith et al. (1990) in connection with lomaprieta, 1989 (usa) large eq (magnitude ms = 7.1) and by molchanov et al. (1992), kopytenko et al. (1993) in association with spitak, 1987 (former soviet union) eq (ms = 6.9). fraser-smith et al. (1990) were lucky to observe at a distance of 7 km from eq epicenter and found that ulf magnetic intensity increased about 14 days before eq, then it depressed several days ahead and once again it increased strongly at 4 h before the main shock and continued at a high level after eq. they preseismic ulf effect and possible interpretation oleg a. molchanov (1), alexander yu. schekotov (1), eugeniy fedorov (1), gennady g. belyaev (2), mary s. solovieva (1) and masashi hayakawa (3) (1) schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia (2) institute of terrestrial magnetism, ionosphere and radiowave propagation (izmiran), russian academy of science, troitsk (moscow region), russia (3) the university of electro-communications, chofugaoka, chofu, tokyo, japan abstract we present the results of ulf magnetic field observation at karimshino station (kamchatka, russia). using a case study we discovered an effect of suppression of ulf intensity about 2-6 days before rather strong and isolated seismic shocks (magnitude m = 4.6-6.6). it is revealed for nighttime and the horizontal component of ulf field (g) in the frequency range 0.01-0.1 hz. then we prove the reliability of the effect by computed correlation between g (or 1/g) and especially calculated seismic indexes ks for the rather long period of observation from june 2000 to november 2001. our recent data confirm the validity of the effect. we show here a similar result during a period of frequent strong seismic activity in april-may 2002. it is highly probable that the effect observed is connected with the increase in plasma density perturbations inside the ionosphere, which are induced by preseismic water and gas release at the ground surface and following energy transportation into the ionosphere by atmospheric gravity waves. two models are discussed and computed: the first is a decrease of penetration coefficient of alfven waves from the magnetosphere due to a turbulent increase in effective pedersen conductivity in the ionosphere, and the second is a change in wave number (k) distribution of source ionospheric turbulence. one of the mechanisms or both could be responsible for the observed 2-3 times suppression of ulf magnetic field noise at the ground. mailing address: dr. oleg a. molchanov, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: oleg@ifz.ru 120 oleg a. molchanov, alexander yu. schekotov, eugeniy fedorov, gennady g. belyaev, mary s. solovieva and masashi hayakawa found the clearest effect in the frequency band f = 0.01-0.1 hz. molchanov et al. (1992), kopytenko et al. (1993) observed ulf variation at a distance of 130 km from the eq epicenter and noted only the last stage of the process: an increase in ulf intensity in time period from 3 h before to several days after eq. subsequent research on this subject was mainly produced in japan. hayakawa et al. (1996a) reported results of observation the ulf magnetic field variations before the great eq at guam, 1993 (ms = 8.0) at an epicenter distance 65 km. they suggested analyzing the polarization ratio r = z/h in frequency band 0.01-0.05 hz and found that this parameter increased about 1 month before eq but returned to the regular level after it. later hayakawa et al. (1999) considered the data once again and showed that slope of ulf spectrum (fractal number) was also changed before the eq. hattori et al. (2002) reported observation of ulf magnetic variation around date of two kagoshima, 1997 large eqs (m = 6.5 and m = 6.3) at a distance about 60 km from both epicenters. they also analyzed the polarization ratio and found its increase about 1 month before eq date. they could not find this signature at the far-distance stations with the same equipment. kopytenko et al. (2002) observed ulf magnetic variations using network of stations situated in the izu and chiba areas of japan. they discussed results related to eq swarm during june-july, 2000 with the strongest shock ms = 6.4 in the middle of the swarm. epicenter distances to the stations changed from 70 to 150 km and the authors focused on the polarization ratio near frequencies f1 = = 0.1± 0.005, f2 = 0.01± 0.005 and f3 = 0.005± ± 0.003 hz. it was shown that ratio r (f3) / r⋅ ⋅(f1) sharply increases just before the start of strong seismic activity, while amplitudes of z and g component variations and z/g ratio defined in a frequency range f2 during night time intervals (00-06 lt) begin to increase ∼ 1.5 months before the period of the seismic activity. in this paper, we consider ulf perturbations in temporal scale from several hours to a few days and focus on the time correlation of our data with seismicity. some results of observations have already been published (molchanov et al., 2003). here we briefly review the previous results (section 2) and present new observational evidence of the preseismic ulf effect (section 3). furthermore we discuss the possible explanation of the effect (section 4). 2. previous results during 1999-2000, in addition to the existing seismic and geophysical observations, russian and japanese scientists established a special observatory at karimshino site (52.94°n, 158.25°e) in kamchatka (far-eastern russia). its main purpose was to study a correlation of seismic activity with electromagnetic and other nonseismic phenomena. the main advantage of this station is quiet electromagnetic environment that allows us to use rather sensitive equipment and to check some theoretical ideas. the regular recordings have been underway since june 2000 and some information about karimshino station has already been published (gladyshev et al., 2002; uyeda et al., 2002). our three-component induction magnetometer measures the geomagnetic field variations in the frequency range 0.003-40 hz. the sensitivity threshold is better than 20 pt/hz1/2 at frequency 0.01 hz. it corresponds to 0.02 pt/hz1/2 at frequencies above 10 hz. here we analyze results in the interval from june 24, 2000 to february 25, 2001 (the first interval duration of 7 months) and second interval from february 26, 2001 to september 16, 2001 (during of about 6 months). so, the whole period of the observation has a duration of about 13 months. as mentioned before in this paper we are presenting results on variation with scale more than several hours which is why we use two hour averaging of the data. first of all we have produced the spectrum of ulf intensity for each magnetic field component (h, d, z) in the 7 frequency bands: f = = 0.003-0.01 hz (channel 1), f = 0.01-0.03 hz (channel 2), f = 0.03-0.1 hz (channel 3), f = = 0.1-0.3 hz (channel 4), f = 0.3-1.0 hz (channel 5), f = 1.0-3.0 hz (channel 6) and f= 3.0-5.0 hz (channel 7). we found a conventional corre121 preseismic ulf effect and possible interpretation fig. 1a-e. variation of impedance ratio at channel 2 near the date of the eqs with the following parameters: panel a) – july 6, 2000, m = 6.0, distance d = 762 km, ks = 1.4; panel b) – july 29, 2000, m = 4.9, distance d = = 195 km, ks = 0.8; panel c) – august 23, 2000, m = 4.6, distance d = 112 km, ks = 1.35 ; panel d) – november 21, 2000, m = 5.1, distance d = 170 km, ks = 0:75 ; panel e) – february 7, 2001, m = 5.6, distance d = = 210 km, ks = 1.15. vertical grid lines are for local midnightofeachday of observation. the intervals in panels c) and d) are shorter due to lack of data. a b c d e 122 oleg a. molchanov, alexander yu. schekotov, eugeniy fedorov, gennady g. belyaev, mary s. solovieva and masashi hayakawa f ig . 2. z /g va lu es ( le ft pa ne ls ) an d 1/ g va lu es ( ri g h t pa ne ls ) af te r 1 da y av er ag in g fo r th e sa m e ca se s as i n fi g. 1 ae. 123 preseismic ulf effect and possible interpretation lation with kp index of magnetic activity and evident daily variation, especially in channels 1, 2, 3. but we did not find any clear correlation with a specially developed index of seismicity ks, which includes dependence on magnitude and distance to earthquake (eq) epicenter and which is proportional to seismic energy released near the observation site (see molchanov et al., 2003). then we apply the method of polarization ratio, which was discussed in many papers since hayakawa et al. (1996a) and which is reduced to analysis of z1/2/g1/2 ratio in our research. in contrast with amplitude analysis, some correlation with ks was found at least for the frequency channels 2 and 3 and near the date of large ks values. we demonstrate several cases in fig. 1a-e, each case during time interval ± 14 days around the eq date and presentation is centered on the corresponding date. for simplicity, we present only channel 2 (f = = 0.01-0.03 hz). it is obvious that nighttime values of z1/2/g1/2 show an increase at about 27 days time period before eq date. an important question arises immediately: what does increase in z component or decrease of g component or both mean? first of all, we examined the behavior of z component and found it reveals mainly seasonal changes and sometimes it is exposed to small man-made perturbations but does not show correlation with seismicity. to clarify this we present z/g values and 1/g values after 1 day averaging in fig. 2 for the same cases as in fig. 1a-e. as a result, we concluded that the effect observed is a depression of horizontal ulf magnetic field several days before eq. the effect was not found for all the events being masked by the dominant magnetospheric component of amplitude variation of the geomagnetic field on the earth surface. maybe, for some events the difference of parameters of eqs can be important. to check it we analyzed the geographical distribution of the casual eqs and discovered that correlated cases are mainly concentrated near the sea shore. then we checked the reliability of the effect by correlation analysis. at the beginning we construct the set of normalized deviations as the following: g g g g g g g g1 1 1 1 i i i i i i i i = = d d _ _ ` i i j where i is number of the frequency channel, i = = 1, 2, 3 and 〈g〉, 〈1/g〉 denotes running mean with 1 month window. taking into considerafig. 3a-c. cross-correlation of (a) δg∗kpd, b) δ(1/g)∗ ∗kpd and (c) δ(1/g)∗ksd in a range of ±15 days for whole 13 months period of observation and different frequency bands. b c a 3. new observational results previous results were obtained for a period of rather moderate seismic activity, ks ≤ 3 (see fig. 1a-e). however, since late autumn of 2001 the activity intensified in the vicinity of our station. in this situation, statistics of the preseismic effect do not change essentially but a new feature appears just near the date of strong eqs. let us consider it for the period from april 17, 2002 to may 17, 2002, including several strong eqs with ms > 5. two of them happened near the sea shore a (april 26, ms = 5.8, distance d = 180 km, ks = 7.5) and c (may 8, ms = = 6, distance d = 195 km, ks = 10.4), the others occurred further from the shore, including b (may 3, ms = 5.2, distance d = 190 km, ks = =1.8) (fig. 4a). impedance ratio in channels 1, 2, 3, 4 is presented in fig. 4b. preseismic signature 3-4 days before event a and 3-5 days before event c can be noted. it is similar to previous results and in the same manner as earlier we check that it is due to a decrease of g-component of the ulf magnetic field. in contrast, an increase in the impedance ratio near the date of the eqs is connected here with an increase in z-component, which can be considered in conventional terms of ulf radiation from underground seismic source. 4. possible interpretation of the preseismic ulf effect it was usually supposed that seismo-associated ulf variations could be either due to direct radiation from eq origin zone (fenoglio et al., 1995; molchanov and hayakawa, 1995) or due to a change in geoelectric conductivity inside and near the eq zone, which leads to a change 124 oleg a. molchanov, alexander yu. schekotov, eugeniy fedorov, gennady g. belyaev, mary s. solovieva and masashi hayakawa fig. 4a,b. a) map of eq’s with ms > 4 from april 17, 2002 to may 17, 2002, including several strong eqs with ms > 5. two of them have happened near the sea shore a (april 26, ms = 5.8, distance d = 180 km, ks = 7.5) and c (may 8, ms = 6, distance d = 195 km, ks = 10.4) the others occurred further from the shore, including b (may 3, ms = 5.2, distance d = 190 km, ks = 1.8). b) impedance ratio in channels 1, 2, 3, 4 is presented. preseismic signature 3-4 days before event a and 3-5 days before event c can be noted. tion that our expected effect has a temporal scale order of several days, we produce 2 days averaging of δgi and δ (1/gi), which leads to values δ2gi, δ2 (1/gi), and find ksd = k sς per day, kpd = kpς per day. due to a clear daily variation of ulf spectrum in the selected channels, δ2gi is mainly related to daytime ulf intensity,but δ2 (1 / gi) is mainly related to nighttime ulf intensity. as a next step we computed correlation functions f1i(τ) = δ1gi∗ksd, f2i(τ) = δ1(1 / gi)∗ksd, f3i(τ) = δ1gi∗kpd and f4i(τ) = δ1(1 / gi)∗kpd using conventional programs, where τ is determined in the interval ± 14 days. the negative value of τ corresponds to the preseismic period and the positive τ-value is for the postseismic period in our formulation. firstly, we present a correlation of ulf intensity with global ionosphere-magnetosphere activity (fig. 3a,b). an obvious correlation is observed both for daytime ulf intensity (fig. 3a) and for the night-time values (fig. 3b). it is evident that both day-time and night-time ulf intensity is proportional and concurrent to kp index. this correlation is understandable. we call a correlation reliable if it reveals for all the intervals and at least twice out of a reliability margin at the whole interval, which is about ± 0.1. based on this point a correlation in fig. 3a,b is reliable. then we show a correlation of ulf intensity with seismic index ks (fig. 3c). due to our criteria, night-time suppression of ulf intensity near value τ ∼ − 4 days looks a reliable effect. this conclusion coincides with the result of case study. note that reliability of seismo-associated ulf suppression effect is comparable with the reliability of the well-known effect of connection between ulf variation and kp index of global magnetic activity. 125 preseismic ulf effect and possible interpretation a b 126 oleg a. molchanov, alexander yu. schekotov, eugeniy fedorov, gennady g. belyaev, mary s. solovieva and masashi hayakawa in ulf waves generated by ionospheric sources (e.g., merzer and klemperer, 1997). the first mechanism is not compatible with our observational results on preseismic effect because it predicts a preseismic increase in ulf intensity. indeed, we observed such an increase near the date of several strong eq shocks (see fig. 4b). probably the interval of ulf increase could be extended essentially for the very large eqs up to a few days as reported earlier (see section 1). an explanation by preceding change in the ground conductivity is also not very attractive for us because long-term magnetotelluric observation in u.s.a. and japan did not show any correlation with seismicity (see, e.g., park, 1997). it seems that suppression of ulf magnetic variation happened not inside the ground but in the lower ionosphere. a hint might be in the results of monitoring of the upper atmosphere and ionosphere around eq date by vlf transmitter signals, reported by hayakawa et al. (1996b) and molchanov et al. (2001). they found clear perturbations of the atmosphere-ionosphere boundary several days before large eqs at nighttime or during night-to-day transition (so-called terminator time). molchanov et al. (2001) provide arguments that water and gas eruptions before eqs could origin «mosaic» and «twinkle» spots of atmospheric temperature and density variations leading to the generation of atmospheric gravity waves (agw) turbulence. there are reports on increased intensity of ionospheric irregularities (so called e-spread and f-spread events) several days before large eqs (see, e.g., review by meister et al., 2002). as considered by mareev et al. (2002) agw produce turbulent variations of density and electric field in the lower ionosphere with horizontal scales order of agw source size near the ground (∼ 50-150 km). alperovich et al. (2002) showed that agw perturbations inside the ionosphere can modify plasma conductivity, especially pedersen type (along electric field of the perturbation). so, for explanation of ulf field suppression before eq we considered two models: first is a decrease of penetration coefficient of alfven waves from the magnetosphere due to turbulent increase of effective pedersen conductivity in the ionosphere, and second is a change in wave number (k) distribution of source ionospheric turbulence. we assume that the ulf magnetic field at the ground bg is sum of the fields from two generation regions: the magnetosphere, in which downgoing alfven waves generate with amplitude bm above ionosphere, and the ionosphere, in which turbulent electric fields ∆ei generate with random phase (〈∆ei〉 = 0). the cartesian coordinates with the z-axes vertical upward is used. the main geomagnetic field b0 = b0ẑ. the electromagnetic field is expanded over the harmonics ( )exp i t ik r+ +~ = where xk kk yx y= += t t . let the x-axes be directed along the k=, kx = k= ≡ k, ky = 0. the following relationship can be written for the electromagnetic field in the ionosphere and on the earth surface: ( ) ( , ) ( , ) ( , ) ( , ) ( , ) ( , ) . b t k b k dk t k c e k dk t k c e k dk xg a m ix a ix iy a iy 2 2 2 2 2 2 = + + + + ~ ~ ~ ~ ~ ~ ~ ∆ ∆ # # # (4.1) here ta is the penetration coefficient of the alfven wave and tix, tiy are the penetration coefficients of the normalized electric field of the ionospheric turbulence. furthermore, we take into consideration that under the above assumptions and real conductivity of the atmosphere byg / bxg << 1. in the first model, we neglect the input of the ionospheric turbulence and produce full wave computations using iri-90 ionospheric profiles at midnight under the quiet conditions. the procedure of computation is described in many papers beginning from (hughes and southwood, 1976). assuming ( , ) ( ) ( )b k b k km m 1= -~ ~ d (4.2) where δ is dirac function and k1 is determined by peculiarities of alfven wave generation and propagation in the magnetosphere. then in correspondence with results of alperovich et al. (2002) we suppose that ionospheric turbulence induced by seismicity leads to increased effective pedersen conductivity. results are present127 preseismic ulf effect and possible interpretation fig. 5. dependence of ta (ω, k1) obtained from the full wave solution using iri-90 ionospheric profiles at midnight under the quiet conditions. σp and σh are integral pedersen and hall conductivities. σa is the alvfen wave conductivity. the oscillating regime at high frequencies is the manifestation of the ionospheric alfven resonance. fig. 6. the frequency dependence of the ratio of the penetration coefficients in the disturbed ionosphere to one in the undisturbed ionosphere calculated as shown in fig. 5. 128 oleg a. molchanov, alexander yu. schekotov, eugeniy fedorov, gennady g. belyaev, mary s. solovieva and masashi hayakawa fig. 7. supposed distribution of the ionospheric turbulence in k-space. thin line is for regular distribution ( k2 1= 1000 km) thick line is for seismo-induced k-distribution ( k2 1= 200 km). fig. 8. integrated penetration coefficients tix ( f, k2) and tiy ( f, k2). 129 preseismic ulf effect and possible interpretation ed in figs. 5 and 6. dependence of ta (ω, k1) is shown in fig. 5. the frequency dependence of the ratio of the penetration coefficients in the disturbed ionosphere to one in the undisturbed ionosphere is shown in fig. 6. it is clear from the figure that the magnetic field on the earth surface at frequencies below ∼ 10−1 hz decreases with growth of pedersen conductivity. contrarily, the second model considers only the ionospheric source and neglects the input from magnetospheric alfven waves and the change in ionosphere conductivity. for simplicity we also assume that turbulence is isotropic, i.e. e e eix iy i 2 2 2 = =∆ ∆ ∆ . hence ( ) ( , ) ( , )b t k e k c dkxg i i a 2 2 2 =~ ~ ~∆# (4.3) where ti (ω, k) = tix (ω, k) + tiy (ω, k). then we take into account that the turbulence develops as ionospheric eigenmode, i.e. ω = ω (k), ( , ) ( )e k e ki i 2 2 =~∆ ∆ and assume the following k-distribution: 2 ( )e k k k k k1i 2 2 11 6 + +∆ 22 _ _i i9 c (4.4) which is reduced to classic kolmogorov’s distribution k−5/3 for the k >> k2, where k2 has a meaning of inverse scale (l2) of the turbulence external source. so, we assume k2 = 10−3 km−1 (l2 ∼ 1000 km) in the usual (regular) situation and k2 = 5 ∗ 10−3 km−1 (l2 = 200 km), if the main source is agw induced by seismicity. both types of the distribution are demonstrated in fig. 7. integrated penetration coefficients tix ⋅ ⋅ (ω, k2) and tiy (ω, k2) are shown in fig. 8, while change in the common penetration coefficient ti (ω, k2) due to modification of main scale of the turbulence k2 is presented in fig. 9. 5. discussion and conclusions we found the effect of suppression of ulf magnetic field variations about 2-6 days before rather strong seismic shocks in a case study. it is revealed for night-time and for horizontal fig. 9. change in the common penetration coefficient ti ( f, k2) due to modification of the main scale of the turbulence k2. 130 oleg a. molchanov, alexander yu. schekotov, eugeniy fedorov, gennady g. belyaev, mary s. solovieva and masashi hayakawa component intensity (g) in the frequency range 0.01-0.1 hz. we prove a reliability of the effect by computed correlation between g (or 1/g) and specially calculated seismic indexes ks. based on the simple criteria, we conclude that reliability of seismo-associated ulf suppression effect is comparable with the well-known effect of connection between ulf variation and kp index of global magnetic activity. using ks in our formulation for analysis of preseismic effect means an assumption that seismic shock is a result of some dynamic process (like instability) and intensity of preseismic perturbations is proportional to the energy of seismic shock itself. some justification of this approach is found in the well-known correlation between magnitudes of foreshocks and main shock (see, e.g., scholz, 1990). this effect can also be supposed in the previous observations at least in those where a preseismic increase in polarization ratio had been found (see section 1). as is shown here, a decrease of g could lead to observed increase of ratio z /g. note furthermore that depression of g amplitude several days before the main shock was noted in one of the first papers by frasersmith et al. (1990). like some other non-seismic precursors, our effect looks a sporadic one in a case study and can be recognized only by statistics. we believe that ulf-seismicity connection becomes more clear and regular after integration on space, i.e. using a network of stations, which we plan to do in future. our interpretation of the effect is not very speculative if seismo-induced agw influence on the ionospheric turbulence is to be believed. the connection between agw and the turbulent variation of plasma density and electric fields inside ionosphere is well-known. it is evidenced by both ionospheric sounders and radars from the ground and in direct satellite observations (see review in the latest paper by molchanov et al., 2002). biagi et al. (2003) based on 24-years hydrogeochemical observations at kamchatka area reported a clear correlation of hot-water eruptions and change in content with the occurrence of large eqs in the time intervals from several days before eq up to 1-2 weeks after it. mareev et al. (2002) showed that even small temperature and density variations near the ground surface effectively generate agw energy flux into the ionosphere and the time of the energy transportation is between several hours and 1 day for agw periods from 10 min to 1-2 h and horizontal wavelengths 30-100 km. thus our assumptions on seismo-induced modification of the ionospheric turbulence are reasonable. as a result of our first model computations (figs. 5 and 6), we discovered that ulf signal suppression is approximately proportional to turbulent increase in pedersen conductivityina frequency range f < 0.1-0.2 hz. the result is not surprising because in this frequency range ta h a p 2 + +σ σ σ^ h7 a (hughes and southwood, 1976), where σh, σp are integral conductivities of the ionosphere and σa is wave alfven conductivity. in the higher frequency range, the effect is masked by ionospheric alfven resonance structure, which is evident in fig. 5 and created due to the passage of alfven wave through the upper ionosphere. although the frequency behavoiur of the computed effect coincides with the observed one (see fig. 6), the existence of the eq-related intensification of the ionospheric turbulence enough to enhance the ionospheric conductivity at several ten percents is not proved. looking now at the result of our second model computation, it can be seen that frequency and value dependence is about the same, but we are free from assumption on increased turbulence intensity. however a new assumption on the redistribution of turbulence k-spectrum after the arrival of seismo-induced agw seems realistic but demands additional research on the nature of ionospheric turbulence. we believe that one of the models or both are helpful to explain our observed effect. acknowledgements this research was partially supported by istc under grant 1121 and by commision of the eu (grant no. intas-01-0456). two authors (o.a.m. and m.h.) are thankful for support from the international space science institute (issi) at bern, switzerland within the project «earthquake influence of the ionosphere as evident from satellite density-electric field data». 131 preseismic ulf effect and possible interpretation references alperovich, l., i. chaikovsky, yu. gurvich and a. melnikov (2002): laboratory modelling of the disturbed dand e-layers: dc and ac fields, in seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 343-348. biagi, p.f., o. molchanov, r. piccolo, a. minafra, a. ermini, y.m. khatkevich and e.i. gordeev (2003): copostseismic hydrogeochemical anomalies in a volcanic environment, natural hazards, 3, 263-267. fenoglio, m.a., m.j.s. johnston and j.d. byerlee (1995): magnetic and electric fields associated with changes in high pore pressure in fault zone-application to the loma prieta ulf emissions, j. geophys. res. solid earth, 100, 12951-12958. fraser-smith, a.c., a. bernardy, p.r. mcgill, m.e. ladd, r.a. helliwel and o.g. villard jr. (1990): low frequency magnetic field measurements near the epicenter of the loma-prieta earthquake, geophys. res. lett., 17, 1465-1468. gladyshev, v., l. baransky, a. schekotov, e. fedorov, o. pokhotelov, s. andreevsky, a. rozhnoi, y. khabazin, g. belyaev, a. gorbatikov, e. gordeev, v. chebrov, v. sinitsin, a. lutikov, s. yunga, g. kosarev, v. surukov, o. molchanov, m. hayakawa and s. uyeda (2002): some preliminary results of seismo-electromagnetic research at complex geophysical observatory, kamchatka, in seismo-electromagnetics: lithosphereatmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 421-432. hattori, k., y. akinaga, m. hayakawa, k. yumoto, t. nagao and s. uyeda (2002): ulf magnetic anomaly preceding the 1997 kagoshima earthquakes, in seismoelectromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 19-28. hayakawa, m., r. kawate, o.a. molchanov and k. yumoto (1996a): results of ultra-low-frequency magnetic field measurements during the guam earthquake of 8 august, 1993, geophys. res. lett., 23, 241-244. hayakawa, m., o. a. molchanov, t. ondoh and e. kawai (1996b): anomalies in the sub-ionospheric vlf signals for the 1995 hyogo-ken nanbu earthquake, j. phys. earth, 44, 413-418. hayakawa, m., t. ito and n. smirnova (1999): fractal analysis of ulf geomagnetic data associated with the guam earthquake on august 8, 1993, geophys. res. lett., 26, 2797-2800. hughes, w.j. and d.j. southwood (1976): the screening of micropulsation signals by the atmosphere and ionosphere, j. geophys. res., 81, 3234-3240. kopytenko, yu.a., t.g. matiashvily, p.m. voronov, e.a. kopytenko and o.a. molchanov (1993): detection of ulf emission connected with the spitak earthquake and its aftershock activity based on geomagnetic pulsations data at dusheti and vardziyaobservatories, phys. earth planet. inter., 77, 85-95. kopytenko, yu.a., v.s. ismaguilov, k. hattori, p. voronov, m. hayakawa, o. molchanov, e. kopytenko and d. zaitsev (2002): monitoring of the ulf electromagnetic disturbances at the station network before eq in seismic zones of izu and chibo peninsulas (japan), in seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 11-18. mareev, e.a., d.i. iudin and o.a. molchanov (2002): mosaic source of internal gravitywaves associated with seismic activity, in seismo-electromagnetics: lithosphereatmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 335-342. meister, c.v., e.v. liperovskaya, o.a. molchanov, o.a. pokhotelov, s.a. senchenkov and o.a. alimov (2002): to the question of spatial scales of seismoionospheric effects, in seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 329-333. merzer, m., and s.l. klemperer (1997): modeling low-frequency magnetic field precursors to the loma prieta earthquake with a precursory increase in fault-zone conductivity, pure appl. geophys., 150, 217-248. molchanov, o.a and m. hayakawa (1995): generation of ulf electromagnetic emissions by microfracturing, geophys. res. lett., 22, 3091-3094. molchanov, o.a., yu.a. kopytenko, p.m. voronov, e.a. kopytenko, t.g. matiashvili, a.c. fraser-smith and a. bernardy (1992): results of ulf magnetic field measurements near the epicenters of the spitak (ms = 6.9) and loma prieta (ms = 7.1) earthquakes: comparative analysis, geophys. res. lett., 19, 1495-1498. molchanov, o.a., m. hayakawa and k. miyaki (2001): vlf/lf sounding of the lower ionosphere to study the role of atmospheric oscillations in the lithosphere-ionosphere coupling, adv. polar upper atmos. res., 15, 146-158. molchanov, o.a., m. hayakawa, v.v. afonin, o.a. akentieva and e.a. mareev (2002): possible influence of seismicityby gravitywaves on ionospheric equatorial anomaly from data of ik-24 satellite, in seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 275-296. molchanov, o.a., a.yu. schekotov, e.n. fedorov, g.g. belyaev and e.e. gordeev (2003): preseismic ulf electromagnetic effect from observation at kamchatka, natural hazards, 3, 1-7. park, s.k. (1997): monitoring resistivitychange in parkfield, california: 1988-1995, j. geophys. res., 102, 545-559. scholz, c.h. (1990): the mechanics of earthquakes and faulting (cambridge university press), pp. 439. uyeda, s., t. nagao, k. hattori, y. noda, m. hayakawa, k. miyaki, o. molchanov, v. gladychev, l. baransky, a. schekotov, g. belyaev, e. fedorov, o. pokhotelov, s. andreevsky, a. rozhnoi, y. khabazin, a. gorbatikov, e. gordeev, v. chebrov and a. lutikov (2002): japanese-russian complex geophysical observatory in kamchatka region for monitoring of phenomena connected with seismic activity, in seismo-electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o. molchanov (terra scientific publishing co., tokyo), 413-420. i n t e r p r e t a t i o n o f t h e s e c o n d a r y e l e c t r o m a g n e t i c field by a n a l y t i c c o n t i n u a t i o n m. y. dizioglu (*) ricevuto il 24 giugno 1967 summary. — given the values of a component of secondary magnetic field on the plane surface of the earth, an attempt has been made to approach the sources by finding the values of the component on a plane at a depth h below the surface, the plane being above the sources. to this end a green's function for the wave equation, appropriate for a semi — infinite space was determined. the integral obtained was expanded into series suitable for numerical calculations. the average values of the component on the earth plane were assumed to satisfy a double series and the coefficients were determined by the least square method. formulae were ultimately developed to calculate the values on a plane at a depth h, in terms of average values on six concentric circles around each point. riassunto. — dati i valori di una componente di un campo magnetico secondario sulla superficie piana della terra, si è tentato di individuare le sorgenti, trovando i valori della componente su un piano posto al di sopra delle sorgenti ad una profondità li sotto la superficie. a questo scopo è stata determinata una funzione di green per l'equazione dell'onda, adattata ad uno spazio semi-infinito. l'integrale ottenuto è stato esteso in serie, idonee ai calcoli numerici. i valori medi della componente sul piano terrestre, sono stati assunti per soddisfare una serie doppia, ed i coefficienti sono stati determinati col metodo dei minimi quadrati. le formule sono state infine sviluppate per calcolare i valori sul piano alla profondità h, nei limiti dei valori medi, su sei cerchi concentrici attorno ad ogni punto. (*) chief geophysicist — minerai research and exploration institute of turkey. 4 1 g m . y . d i z i o 6 l u i t is known that, with the inductive electromagnetic method of prospecting, the secondary magnetic field is investigated and thence some inferences as to the localities and position of the sources producing such fields are made (2). these interpretations are usually carried out by qualitative methods. sometimes curve fitting methods are also used for this purpose, but such calculations are tedious and time consuming and different assumptions made for such calculations are usually far from the true state. here, in this paper an attempt is made to solve the revers problem. that is to say, given the components of the secondary magnetic field on the plane surface of the earth, we will try to approach the sources by finding the values of the components on a plane at a depth h below the surface; the plane being above the sources. i t should be stated here that such a procedure would not at all improve the limitations of the method or lift the ambiguities of interpretation, which would remain the same (3) (4) (5). but such approaches towards the sources would enlarge the anomaly and subside the background, so that the resolving power of the interpretation would be increased. let us now assume that we know the components of the secondary field produced by a primary field uniform at every point. in practice such a secondary field can be obtained by keeping the distance between the antenna and the receiver and moving the arrangement at equal intervals along parallel lines. these components would satisfy the equation (in gaussian units): 4 7i n d ht u 1c d" hi ' = co ' it + c' d¥ ' where h t = any component of the secondary magnetic field ¡i = permeability of the medium q = resistivity of the medium tc = dielectric constant of the medium c = speed of light. as, usually the resistivity of the country rock is quite high, the coefficient of the first term becomes negligibly small. therefore it can be discarded. if we assume that the primary field is sinusoidal, then: hi = h (x, y, z) e , i n t e r p r e t a t i o n o f t i i e s e c o n d a r y e l e c t r o m a g n e t i c f i e l d , e t c 4 1 7 where, q = 2 n f/c. the function e must satisfy: (v2 + k*)e = 0 , where, k2 = [i k q2 . approach toicards the source: let any component of the secondary magnetic field be known on any plane z — 0. this plane may be the surface of the earth or a plane above the surface of the earth as in the case of airborne electromagnetic prospecting. let us suppose that z is positive downwards in a direction towards the source of the field. let us define a function: 2n h (r, z) = j e (r, z,d)dq , [ 1 ] o where e (r, z) is the average value of any particular component of the secondary magnetic field around a circle of radius r centered at (o, o, z). if the z axis does not pass through any source, it is possible to expand e (0, z) into series around z = 0 in terms of the intensities on the surface of the earth in the form: — — a e (o, o) h2 ye (o, o) e (0, h) = e (0, 0) + h > +— > + ... etc. [2] the derivatives in equation [2] can be calculated in different ways, but as the calculation of even derivatives are more direct than the calculation of odd ones, we can get rid of them by using the upward extrapolation: — — s i r (0,0) 7i2 a 2 h ( 0 , 0 ) e ( 0 , f c ) = e (0, 0) _ a — f l + 2 7 + • • • e t c " [ 3 ] upon adding equations [2] and [3], we obtain: e (0, h)= 2 — h2 a2 e (o, o) -e(0,-h). [4] 4 1 8 m . t . d i z i o f i l u we have now to develop a method of calculation for the derivatives in brackets as well as for the last term e (o, h) which is the continuation upward to a height li above the surface. we will first deal with the calculation of the derivatives. from the wave equation in polar coordinates: 52 h i v 1 3 , \ ^ = — + + k2 \e a z2 l i r ! f i r i a4 h _ / a2 a z* ~ u r" 1 <> \2 ti , / 32 i a \ \ e -h 2 7c2 1r a r \a r2 r a r e + k'e a° e a«1 1 / a 2 1 a y — / a 2 1 a >2 = — + e — 3 7c2 + 6 \ a r 2 r a w \a r2 r a r) — 3 + • r—) s — k° h , \a r2 r a rl ' e — / a 1 a \< where „ h— • , means that the operation is to be applied i \a r2 r a r] times in succession. we can show by an application of green's theorem, that e (r, 0) is an even function, therefore it can be written in the form: e (r, 0) = a„ + a2 r2 -fat r4 + «„ r6 + . . . etc. [5] now if we perform the operations for the derivatives, we find: we ( 0 , 0 ) ~' 1 = — k + 4 a2) a4 e to, 0) — ' — = a0 7c4 + 8 7c2 a2 + 64 a4 c) a6 h (0, 0) v'= — k 7c6 + 12 7c4 «2 + 192 7c2 »4 + 2304 a,) . a z6 if we replace these in equation [4], we obtain: e (0, h) = 2 h ft4 h (0, 0) — — (oo 7c2 + 4 a2) + — («„7c4 + 8 fc2a2 + 64 a,) — [6] 1 (a0 7c 0 + 12 7c4 a2 + 1 9 2 7c2 a4 + 2304 a„) + ] — e (0, 7i) i n t e r p r e t a t i o n o f t i i e s e c o n d a r y e l e c t r o m a g n e t i c f i e l d , e t c 4 1 9 since the convergence of the series in brackets is rapid enough, only four terms have been used. determination of the coefficients in the series [5]. we can take as many terms as we please, and obtain the values of their coefficients by means of the method of least squares. we have made two trials. in one of them we supposed that: 5 (r, o) = a0 + a2 r2 + a4 r 1 , [7] and solved the normal equations with average values around circles of radius 0, 1, 2, 3, 4, 5 and 6. the results are as follows: a0 = + 0,386100 5 (0) + 0,386100 5 (1) + 0,250962 5 (2) + + 0,070772 h (3) — 0,086910 5 (4) — 0,127520 5 ( 5 ) + + 0,070610 5 (6) [8] a2 = — 0,039186 5 (0) — 0,039186 5 (1) — 0,008508 5 ( 2 ) + + 0,029982 5 (3) + 0,057324 5 (4) + 0,046974 5 ( 5 ) — — 0,035196 5 ( 6 ) [9] a4 = + 0,000819 5 (0) + 0,000819 h (1) + 0,000006 5 ( 2 ) — — 0,000989 5 (3) — 0,001626 5 (4) — 0,001149 5 (5) + + 0,001414 5 (6) . [10] if we assume that: 5 (r, 0) = a0 + a2 r2 + ai r1 + a„ r°, [11] then the formulae for the coefficients found by the same method become: a0 = + 0,02544 5 ( 0) + 0,0220 5 ( 1 ) + 0,1360 5 ( 2) — — 0,00217 5 (3) + 0,0970 5 (4) — 0,0442 5 (5) — — 0,030112 5 (6) + 0,02616 5 (7) [12] a2 = — 0,000495 5 (0) — 0,000361 5 (1) — 0,000211 5 (2) + + 0,0000780 5 (3) — 0,000256 5 (4) + 0,000468 5 (5) + 40,000343 5 (6) — 0,000374 5 (7) [13] 4 2 0 m . t . d i z i o ô l u a, = + 0,0000143 h (0) + 0,0000186 3 (1) — 0,0000195 h (2) + + 0,0000721 3 (3) + 0,0001825 h (4) + 0,000324 h (5) + + 0,0001444 3 (6) — 0,0002727 3 (7) [14] a, = + 0,00000124 3 (0) + 0,000000908 h (1) — 0,00000015 3 (2) — — 0,00000191 3 (3) — 0,00000378 3 (4) — 0,00000679 3 (5) — — 0,0000065 ït (6) -(0,0000057 3 (7) . [15] evaluation of 3 (0, h). let us suppose that the singularities of 3 all lie outside a closed surface 8 bounding the volume v. then putting: b' = «"¡r — r' i (where r is the position vector of the point in consideration and r' is the position vector of any other point) and this value of 3 in the green's theorem and applying it to the region bounded externally by 8 and internally by g, a small sphere with center r and radius e, we find that if the point (p) lies inside 8: f l s ( 5 e « l r r ' | etklr-r'l de(r') h ( r ) i n ' ir—r' | _ \r—r'\ ' h \ds' = [16] l i m r ( / 1 \ — ô 3 (r') ) e « * i r r ' i — o i l « e [ r ' ] c we can show that the value of the limit on the right-hand side of this equation is — 4 n 3 (r). determination of an appropriate green's function for the half-space z < 0. let us suppose that g (r, r') satisfies the same wave equation and that it is finite and continuous with respect to either the variables x, y, z or to the variables x', y', z' for points r, r' belonging to a region v which is bounded by a closed surface 8 except in the neighbourhood of the point r, where it has a singularity of the same type as: glfclr-r't • | r r ' | ' i n t e r p r e t a t i o n o f t i i e s e c o n d a r y e l e c t r o m a g n e t i c f i e l d , e t c 4 2 1 as r' r. then proceeding as in the derivation of equation [16], we can prove that, if r is the position vector of a point within f , t h e n : f [ 1 7 ] where n is the outward-drawn normal to the surface s. i t follows immediately from equation [17] that if gl (r, r') is such that it satisfies the boundary condition: g, (r, r') = 0 , and if the point with position vector r' lies on s, then: h ( d = f [18] 4 71 j 0 11 s now for the half-space: g u l r r ' l g t t l 2 r ' l \l-r'\ where i = (x, y, z) is the position vector of the image in the plane z — 0 of the point with position vector r = (x, y, z). for this function it is easily shown that if the point with position vector r' lies on n, the x, y plane, then: a g1 _ 0 a ieikr, a to ~ ~ — a z \ b) where: b* = (x — x'f + (y— y')2 + a2 . i t follows from equation [18] that if e = / (x, y) on z = 0, then when z < 0: — 1 a f e'*« 3 (x, y, -z) = — / (x', y') — dx'-dy', [19] n where h (x, y, z) is the value of a component of the secondary field at the point (x, y, z) above the surface. since the measured field is not known as a mathematical function, we wish to devise a practical scheme for the numerical evaluation of the integral. 4 2 2 m . y . d i z i o ö l u evaluation of the integral. s elkn putting the real component of „ in the integral; ö z 11 ° ' oo -r oo c1 r* ct, v 3 r f , , , cos fce + 7c2î sin fce , h (x, y,-z) = — — i j f(x',y') dx -oo — o o let (x' — x) = r cos d ; (y' — y) = r sin 0 , .since tte' • dy' = j 7 |'a ^ i 1 dr . dd = r dr • dd . r i r = oo 6 = 2;r ô 3 f (' -n . m cos a/i + a/i sin lilt , ^ = ^ * (r, 0) r dr do . if we define a function: 271 e, (r) = i hz (r, d) dd , 2 n u the equation [20] becomes: „ , = cos le b + le b sin je b h = — i h (r) — r-z-dr , this can be written in the form: _ _ h -= h (0) + h (bt) r cos jcb + kb sin kb h = 2 « j _ 3 r.dr + o _ _ 62 , h (bt) + h (b2) r cos 7cä + 7ce sin kb ^ 2 * r d r + e t c . bi where bz are the radii of circles around the origin. i n t e r p r e t a t i o n of t i i e s e c o n d a r y e l e c t r o m a g n e t i c f i e l d , e t c 4 2 3 i f we expand cos kit and sin kit and make the necessary integrations, we obtain the following series: k°1 h (bl + h*)1'* k4 / 1 + h3 — (b\ + wf'ï] k° i 1 o \ 5 ! 3 \3 ! 1 6 ! h — (bl + a2)1'" y) + + 1 4 ! + h h (bj [replace b1 by b2 in the same series] + h 3 (b2) (^ + h,2)'12 (b\ + h2)1'2 _ {bl + ft.)!/, _ + fc»)!/«!^ ( i _ 1 j . . . j + % : [replace and ¿>2 by b2 and ba respectively in the last series] + ...etc. [22] h (bj) 2 b y replacing this formula in equation [6], the complete formula for the evaluation of the intensity at a depth h can be obtained. b y this formula, it would be possible to find the required intensities on a plane surface nearer the sources thus the localities of the sources producing the secondary field could be inferred. r e f e r e n c e s ( ! ) w e s l e y j . p . , response of a dyke to an oscillating dipole. «geophysics», 23, pp. 128-133 (1958). ( 2 ) w a i t j . r . , a conducting sphere in a time varying magnetic field. « geophysics », 16, pp. 666-672 (1951). ( 3 ) s l i c h t e r l . b . , an inverse boundary value problem in electrodynamics. «physics», 4, p. 418 (1933). (•*) s k e e l s d. c . . ambiguity in gravity interpretation. «geophysics», 12, pp. 43-56 (1947). ( 6 ) t s u b o i c. and f u c h i d a t . , relations between gravity values and corresponding subterranean mass distributions. « bulletin of the earthquake research institute », tokyo, imperial university, 15, pp. 636-649 (1937). annals of geophysics, 55 (3), 2012: foreword and acknowledgements 331 annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-6100 foreword this is the first volume of an international scientific journal that is dedicated to issues of geoethics and geological culture. its goal is to inform the italian and international scientific communities about what emerged at the geoitalia 2011 conference, attended by not only italian geoscientists. at this conference, the geoscientists questioned their role in society and the responsibilities that they have to assume as scholars of the planet earth and experts of the territory. they highlighted the need for rediscovery of the cultural values of geology as a science that can contribute to the construction of correct social knowledge, and the need to be aware that geoethics cannot exist without a real awareness among geoscientists of the cultural value of the earth sciences. the papers collected here offer many reflective cues across many themes. these include the philosophy of science, sociology, information and education about natural phenomena in both developing and developed countries, scientific communication and the relationships between science, the media and policy makers, environmental sustainability and geodiversity, recovery of historical memory as a factor to prevent natural disasters, and the contribution of geological culture to the strengthening of the link between the identity of populations and their territories. this volume aims to contribute to the international debate on geoethics through articles that show not only activities and practical experiences, but also that provide more general reflections. this allows us to recover the true meaning of being geoscientists and to highlight the active roles we can have in promoting new cultural values in modern society, on which to build a more conscious relationship between man and nature. silvia peppoloni, giuseppe di capua, istituto nazionale di geofisica e vulcanologia, rome, italy acknowledgements the quality of this special issue was improved by peer-reviewers in several disciplines. many thanks go to: raffaele azzaro, josè brilha, marcello bruognolo, elena camino, pietro paolo cannistraci, sergio castenetto, mauricio compiani, elena ferrero, jesus frias martinez, teodoro georgiadis, roberto greco, shrikant limaye, ruggero matteucci, stefano moriggi, annibale mottana, mario panizza, sandra piacente, telmo pievani, roberto scandone, stefano solarino, and janusz wasowski special thanks go to: franco ferrarotti and giulio giorello, for their kind collaboration and exquisite courtesy. we are also very grateful to annals of geophysics editorial team for every aspect of the publishing process: edoardo del pezzo (editor in 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ferraro.qxp_layout 6 annals of geophysics, 61, 2, se224, 2018; doi: 10.4401/ag-7708 1 “site effects evaluation in catania (italy) by means of 1-dnumerical analysis„ antonio ferraro1, salvatore grasso1,*, maria rossella massimino1 (1) department of civil engineering and architecture (dicar), university of catania, catania, italy 1. introduction in the period 2014-2016, a project financed by the european community was carried out to investigate the seismic hazard of some areas in eastern sicily. within this project, four strategic test sites in messina [1) and catania (3) have been studied, because of the fact that these cities are at high seismic risk [abate et al., 2006; 2017]. messina suffered the destructive earthquake of 1908 that caused thousands of deaths, injured and displaced, while catania suffered, since 1169, a number of destructive earthquakes, among which we mainly remember the earthquakes of 1169, 1693, 1818 and 1990 [de rubeis et al., 1991]. in order to study the dynamic characteristics of soils in three catania test sites, laboratory and in situ investigations have been carried out to obtain soil profiles with special attention being paid to the variation article history receveid july 27, 2017; accepted february 23, 2018. subject classification: site response analysis; scenario earthquakes; catania; messina. abstract a probabilistic seismic hazard analysis (psha) determines the probability rate of exceeding of various levels of ground motion in a specified period of time, in a given area. on the other hand, the neo-deterministic seismic hazard assessment (ndsha) is based on modeling techniques, developed from physical knowledge of the seismic source process and of the propagation of seismic waves, which can realistically simulate the ground motion due to an earthquake by means of synthetic seismograms. the ndsha confirms that peak ground acceleration values are larger than those given by the psha in areas where large earthquakes are observed and in areas identified as prone to large earthquakes, such as in the case of the city of catania (italy). the city of catania, located on the eastern part of sicily, is one of the most seismically active areas of italy. the val di noto earthquake of january 11, 1693 is considered one of the biggest earthquakes which occurred in italy. site response analyses have been developed for a group of 3 test sites in the city of catania (italy). on the sites are localised some strategic buildings to upgrade against seismic risk. the analysis of seismic ground response at the sites was conducted using a numerical method, which is developed in three main phases: the definition of the geometric, geological and geotechnical model of the subsurface, the definition of the seismic input (synthetic or recorded), the choice of one or more computer codes to use and process the results. the reconstruction of the geological and geotechnical model of the subsurface has highlighted a morphology quite irregular especially with regard to the covers. one-dimensional local site response analyses have been performed assuming that all geologic boundaries are horizontal and the response of soil deposits is predominantly caused by waves propagating vertically from the underlying bedrock. equivalent linear analysis using eera code has been performed in this study. one of the targets of the paper is the development of measures for the seismic retrofitting of buildings. seismic retrofitting and/or improving have to be definitely considered a multidisciplinary subject, which depends in fact on many factors, such as: local site effects and the dynamic interaction between the foundation soil and the structure. the accurate investigation on the structure and the surrounding soil is the first fundamental step for a realistic evaluation of the structure seismic performance. of the shear modulus g and damping ratio d with depth. seismic dilatometer marchetti tests (sdmt) have been also carried out in the area of the “national institute of geophysics and volcanology” building (ingv) and in the “madre teresa di calcutta” building school (cd mtc), with the aim of an accurate geotechnical characterisation, including the evaluation of the shear wave velocity vs profile, as well as the profile of the horizontal stress index kd. shear wave velocity has been measured by different tests. the soil profiles in terms of the shear modulus g0 and in terms of the shear wave velocity vs have been evaluated and compared by different in situ tests. the redundancy of measurements is very useful, for instance, for site response and liquefaction analyses, which can be based either on vs values or kd values. boreholes were driven and undisturbed samples were retrieved for laboratory tests. because of their relevance on the estimation of local ground shaking and site effects, data from in-hole geophysical surveys (down-hole and seismic dilatometer marchetti test) have been examined with special attention, particularly for s wave velocity measurements. it must be noted that the shear wave velocity vs was evaluated on the basis of both empirical correlations with in situ or laboratory tests and a few direct measurements. one-dimensional local site response analyses have been performed assuming that all geologic boundaries are horizontal and the response of soil deposits is predominantly caused by waves propagating vertically from the underlying bedrock. among the programs that adopt the equivalent linear analysis eera code [bardet et al., 2000] has been used in this study. the three test sites in catania are two school buildings in the province of catania, the catania division of the national institute of geophysics and volcanology (indicated by the ingv acronym) and finally the fourth test site is the messina’s regional department of civil defence (indicated by the drpc acronym). in this paper, the results of local seismic response analysis at the ingv site in catania and at the drpc site in messina are presented. 2. the study areas the first test site is the building of the ingv (national institute of geophysics and volcanology), that is a public scientific institution which carries out research, monitoring and surveillance in the fields of geophysics and volcanology [abate et al., 2016a; abate and massimino, 2016]. the building (figure 1) is located at the historical center of the high seismic risk city of catania [cavallaro et al., 2008]. among the main earthquakes that struck catania, the most important is the 1693 event, also know as the “1693 val di noto earthquake” that struck also part of southern italy, calabria and malta on january 11 at around 21:00 local time. this earthquake was preceded by a damaging foreshock on january 9. it had an estimated magnitude of 7.4 on the moment magnitude scale and a maximum intensity of xi on the mcs, destroying at least 70 towns and cities, seriously affecting an area of 5,600 square kilometers and causing the death of about 60,000 people [grasso and maugeri, 2009a, 2009b, 2012, 2014; castelli et al., 2016a, maugeri et al., 2012]. the earthquake was followed by tsunamis that devastated the coastal villages on the ionian sea and in the straits of messina. almost two thirds of the entire population of catania were killed. the extent and degree of destruction caused by the earthquake resulted in extensive rebuilding of the towns and cities of southeastern sicily, particularly the val di noto [castelli et al., 2016b, 2016c, 2016d], in a homogeneous late baroque style, described as “the culmination and final flowering of baroque art in europe”. landsliding events triggered also by rainfall during earthquakes should be studied in these cases [castelli and lentini, 2013; castelli et al., 2017], including also lateral spreading and liquefaction [castelli et al., 2016e]. the second test site is the building of the messina division of the drpc (regional department of civil defence), located at the city center of messina (figure 2). the most important historical earthquake was the 1908 messina earthquake (also known as the 1908 messina and reggio calabria earthquake), with a moment magnitude of 7.1 and a maximum mcs of xi. the cities of messina and reggio calabria were almost completely destroyed and between 75,000 and 200,000 lives were lost. the earthquake’s epicenter was in the strait of messina which separates the busy port city of messina in sicily and reggio calabria on the italian mainland. its precise epicenter has been pinpointed to the northern ionian sea area close to the narrowest section of the strait, the location of messina. it had a depth of 5-6 miles (8-10 km). at least 91% of structures in messina were destroyed or strongly damaged and some 75,000 people were killed in the city and suburbs. reggio calabria and other locations in calabria also suffered heavy damage, with some 25.000 people killed. reggio calabria historic centre was almost completely erased. the number of casualties is based on the 1901 and 1911 census data. the ground shook for about 30 to 40 seconds, and the damage was widespread, with destruction felt within a 300-kilometer radius. in calabria region, the ground shook violently from scilla to the south of reggio calabria and caused landslides in the reggio calabria area. ferraro et al. 2 3. geotechnical site characterisation an intensive geotechnical site characterization was carried out performing in situ and laboratory tests, in order to determine the most appropriate static and dynamic soil parameters [ferraro et al., 2015]. by means of laboratory tests the values of unit weight, natural water content, void ratio, porosity, saturation ratio have been obtained. resonant column tests (rct) and cyclic loading torsional shear tests (cltst) have been 3 site effects evaluation in catania (italy) figure 2. ingv drpc messina building. figure 1. ingv catania building. also performed [cavallaro et al., 2012; caruso et al., 2016; castelli et al., 2016f; cavallaro et al., 2016a; 2016b]. the types of tests performed are the same for both sites (ingv and drpc), in fact, for each site, three boreholes were carried out, with undisturbed soil sampling. the three surveys were also equipped in order to perform down hole (d-h) and cross hole (c-h) tests according to this sequence: s1 and s2 boreholes for the execution of a ferraro et al. 4 figure 3. ingv boreholes location. figure 4. ingv soil stratigraphy in the order s1, s2, s3. cross hole test and s3 boreholes for the execution of the down hole test. seismic dilatometer marchetti tests sdmts have been also performed in each site. ingv catania test site figure 3 shows the location of the three boreholes in the ingv catania test site and figure 4 shows the soil stratigraphy of the boreholes. the s1 and s3 boreholes have a depth of 60 meters, the s2 borehole has a depth of 80 meters. table 1 shows the list of soil samples for laboratory tests; for the geotechnical model considered in the numerical analysis, the following four soil samples were used: s1c1 (retrieved at 4,00 4,50 m), s2c3 (retrieved at 14,00-14,40 m), s2c5 (retrieved at 72,00 72,50 m), s3c4 (retrieved at 5,50 5,90 m). figure 4 ingv soil stratigraphy in the order s1, s2, s3. figure 5 shows the vs profiles obtained by d-h, ch and sdmt tests and their comparison. the down hole test shows a vs profile that increases with depth almost according to a linear trend with values variables from 190 m/s to 1000 m/s; at the depth of 35 m the vs profile reaches the value of 800 m/s. the cross hole test is in good agreement with the d-h test up to the depth of 7.5 meters. the c-h test shows a vs profile that linearly increase with depth with values variable between 190 m/s and 380 m/s at the depth of 45,00 m. the c-h vs profile never reaches 5 site effects evaluation in catania (italy) list of soil samples n. borehole name depth (m) 1 s1 c1 4,00 – 4,50 2 s2 c3 14,00 – 14,50 3 s2 c5 72,00 – 72,50 4 s3 c1 5,50 – 5,90 table 1. ingv list of samples. figure 5. ingv vs profiles obtained by d-h, c-h and sdmt tests and their comparison. figure 6. ingv rct results for s1c1 soil sample. the 800 m/s value. the sdmt test shows a dual behaviour: up to the depth of 7.5 m it seems to overestimate vs values compared to d-h and c-h tests, while from 7.5 m to 45 m it shows a linear trend with values that lie in the middle between the results of the dh test and the c-h test results. this apparatus was also used in offshore condition by cavallaro et al. [2013a, 2013b]. results of resonant column tests (rct) and cyclic loading torsional shear tests (cltst) are presented in figures 6-8 to evaluate the shear modulus g and damping ratio d variation with the shear strain level γ. these tests are fundamental for the accurate evaluation of local site response analyses, as well as for the evaluation of the dynamic behavior of fullcoupled soil-structure systems [abate et al., 2007; 2015; massimino et al., 2015]. the specimen was anisotropically reconsolidated to the best estimate of the in situ effective vertical and horizontal stress. the same specimen was first subjected to rct, then to cltst after a rest period of 24 hrs with opened drainage. cltsts were performed under stress control condition by applying a torque with triangular time history at a frequency of 0.1 hz. drpc messina test site figure 9 shows the location of the three boreholes in the drpc messina site while figure 10 shows the soil stratigraphy of the boreholes. table 2 shows the list of the soils samples involved in the rcts and cltsts. for the geotechnical model considered in the numerical analysis, the following five soil samples were used: s2c1 (retrieved at 3,00 3,50 m), s1c1 (retrieved at 7,00-7,40 m), s3c2 (retrieved at 14,60 15,00 m), s2c3 (retrieved at 18,00 18,50 m), s3c4 (retrieved at 27,00 27,50 m). results of resonant column tests (rct) and cyclic loading torsional shear tests (cltst) are presented in figures 11-15 to evaluate the shear modulus g and damping ratio d. the specimen was anisotropically reconsolidated to the best estimate of the in situ effective vertical and horizontal stress. the same specimen was first subjected to rct, then to cltst after a rest period of 24 hrs with opened drainage. cltsts were performed ferraro et al. 6 figure 7. ingv rct results for s2c3 soil sample. figure 8. ingv rct results for s2c5 soil sample. under stress control condition by applying a torque with triangular time history at a frequency of 0.1 hz. figure 16 shows the vs profiles obtained by d-h, ch and sdmt tests and their comparison. the down hole test shows a vs profile that increase with depth almost according to a linear trend with values variables from 190 m/s to 1000 m/s; at the depth of about 50 m the vs profile reaches the value of 800 m/s. the cross hole test is in good agreement with the down hole test. the c-h test shows a vs profile that linearly increase with depth with values variable between 190 m/s and 800 m/s at the depth of about 50,00 m. the sdmt test shows vs values which are in good agreement if compared to dh and c-h tests, up to the depth of 30 m. 4. numerical analyses in the test sites one-dimensional local site response analyses assume that all geologic boundaries are horizontal and the response of soil deposits is predominantly caused by waves propagating vertically from the underlying 7 site effects evaluation in catania (italy) figure 9. drpc messina building. figure 10. drpc soil stratigraphy in the order s1, s2, s3. bedrock. so, one-dimensional numerical codes are valid for modelling plane-parallel layers along a vertical column, assuming laterally-homogeneous stratigraphy. under these assumptions, the main factors responsible for seismic motion amplifications are: 1) impedance contrasts between ground layers, particularly with bedrock; 2) resonance effects due to the closeness between the frequencies of the motion at the substrate and the natural vibration of the deposit. calculation procedures consider, in the solution of the dynamic equilibrium of the system, the non-linear relation through two types of analyses: equivalent linear and nonlinear analyses. the equivalent linear analysis consists in the execution of a sequence of complete linear analysis with subsequent update of the parameters of stiffness and damping until the satisfaction of a predetermined convergence criterion. there parameters depend on the state of deformation of the ground. the nonlinear analysis consists in the integration step-by-step of the equations of motion, simultaneously changing the parameter values of stiffness and damping. ferraro et al. 8 figure 11. drpc rct results for s2c1 soil sample. figure 12. drpc rct results for s1c1 soil sample. list of soil samples n. borehole name depth (m) 1 s1 c1 7,00 – 7,40 2 s1 c2 19,50 – 19,90 3 s2 c1 3,00 – 3,50 4 s2 c3 18,00 – 18,50 5 s3 c1 5,50 – 6,00 6 s3 c2 14,60 – 15,00 7 s3 c4 27,00 – 27,50 table 2. drpc list of soil samples. the first analysis provides satisfactory results at not excessive deformation of the ground, less than 1%, for higher deformation is necessary to use non-linear incremental analysis. among the programs that adopt the equivalent linear analysis the best known and most frequently used is the computer code shake [schnabel et al., 1972a, 1972b; idriss and sun, 1992] and later revisions like eera [bardet et al., 2000], which has been used in this study. they work in the total stresses field using the kelvin-voigt physical model (continuous and 9 site effects evaluation in catania (italy) figure 13. drpc rct results for s3c2 soil sample. figure 14. drpc rct results for s2c3 soil sample. figure 15. drpc rct results for s3c4 soil sample. homogeneous layers with linearized visco-elastic behaviour). it consists of flat and parallel layers of infinite horizontal extension on an outcropping half-space corresponding to the bedrock, on which the input motion is applied. each layer is considered to be homogeneous and isotropic and characterized by the thickness h, the density, the shear modulus g and the damping ratio ξ. the input motion propagates in the direction perpendicular to the free surface. figure 17 shows the calculation scheme for n layers with these properties: shear modulus g, damping ratio ξ, thickness h. the amplitude of incident and reflected waves in each layer, and reflected waves fn at the bedrock are unknown. the incident wave en at the bedrock is given. the calculation method considers the motion equations applied in each layer under two conditions: 1) the congruence between displacements and stresses at the interfaces; 2) equilibrium between incident and reflected waves at the surface. finally, in order to take into account the non linear soil behaviour, an iterative procedure is carried out by considering the behaviour of the shear modulus g and of the damping ratio d, as functions of shear strain γ. the main steps of the iterative procedure are: 1) estimation for g and ξ in each layer; 2) ground response calculation in each layer; 3) effective shear strain calculation based on the maximum shear strain; 4) new iteration based on g and ξ new values and new ground response calculation until the desired convergence value is reached. the results are presented in terms of acceleration time history, fourier spectra, response spectra at the surface and in terms of maximum acceleration profiles as function of the depth [ferraro et al., 2009]. one dimensional equivalent linear analyses have been carried out at the catania ingv test site with different bedrock depths (200 m, 300 m, 400 m, 600m) and at the messina drpc test site (drpc-s1s2 and drpc-s3). ferraro et al. 10 figure 16. drpc d-h, c-h and sdmt vs profiles and their comparison. figure 17. one-dimensional layered soil deposit system [after schnabel et al., 1972]. ingv test site numerical analyses as regards the input motion at the ultimate limit state (uls), the site response analyses at the ingv test site were made using as excitation at the base of the models both synthetic seismograms and recorded accelerograms, the first obtained by the 1693 val di noto and 1818 etna earthquakes, the second by the recordings of the december 13, 1990 earthquake, at the sortino (amax = 1.00 m/sec 2) recording station. the approach of using synthetic seismograms for generating 11 site effects evaluation in catania (italy) figure 18. ingv test site set of input motion for numerical analyses. ferraro et al. 12 figure 19. soil model with bedrock at the depth of 200 m. time history of surface acceleration (a), amplification ratio (b), spectral acceleration (c). (a) figure 20. soil model with bedrock at the depth of 300 m. time history of surface acceleration (a), amplification ratio (b), spectral acceleration (c). (c) (b) (a) (c) (b) 13 site effects evaluation in catania (italy) figure 21. soil model with bedrock at the depth of 400 m. time history of surface acceleration (a), amplification ratio (b), spectral acceleration (c). figure 22 soil model with bedrock at the depth of 600 m. time history of surface acceleration (a), amplification ratio (b), spectral acceleration (c). (a) (c) (b) (a) (c) (b) a seismic ground motion scenario at the bedrock has been so used in the present work. the reference events were the catastrophic earthquakes that struck eastern sicily: on january 11, 1693, assumed as a level i maximum credible earthquake scenario; on february 20, 1818, assumed as a level ii base operative earthquake scenario. the epicentre of the source in the case of 1693 scenario is about 13 km far from the coast, which is considered as the coordinate system source. a magnitude m = 7.0, corresponding to the destructive event which struck catania in 1693, has been taken into account. the source has been simulated through the overlaying of 5 sources placed at different depths and activated at different times, simulating approximately the propagation of the rupture on the segment fault. seismograms ferraro et al. 14 figure 23. comparison in terms of maximum acceleration profiles, for the different soil models considered in the numerical analysis. figure 24. comparison between spectral acceleration at the surface obtained by numerical analyses for the 200 m seismic bedrock soil model, and input acceleration of 0.207 g, with the spectral acceleration provided by the italian seismic code ntc2008. labels of different colours in the legend represent different synthetic seismograms at various depths of 1693 (20m, 31m, 40m) and 1818 (40m, 49m, 56m) scenario earthquakes used for in the numerical analyses. figure 25. comparison between spectral acceleration at the surface obtained by numerical analyses for the 600 m seismic bedrock soil model and input acceleration of 0.207 g, with the spectral acceleration provided by the italian seismic code ntc2008. labels of different colours in the legend represent different synthetic seismograms at various depths of 1693 (20m, 31m, 40m) and 1818 (40m, 49m, 56m) scenario earthquakes used for in the numerical analyses. figure 27. comparison between spectral acceleration at the surface obtained by numerical analyses for the 600 m seismic bedrock soil model and input acceleration of 0.282 g, with the spectral acceleration provided by the italian seismic code ntc2008. labels of different colours in the legend represent different synthetic seismograms at various depths of 1693 (20m, 31m, 40m) and 1818 (40m, 49m, 56m) scenario earthquakes used for in the numerical analyses. figure 26. comparison between spectral acceleration at the surface obtained by numerical analyses for the 200 m seismic bedrock soil model and input acceleration of 0.282 g, with the spectral acceleration provided by the italian seismic code ntc2008. labels of different colours in the legend represent different synthetic seismograms at various depths of 1693 (20m, 31m, 40m) and 1818 (40m, 49m, 56m) scenario earthquakes used for in the numerical analyses. 15 site effects evaluation in catania (italy) 200 m soil model 1693 pt1r3 1693 pt3r3 1693 pt6r3 1818 40m1 1818 49m1 1818 56m1 1990 pgantc 2008 soil a 0,207 0,207 0,207 0,207 0,207 0,207 0,207 pgaoutput_eera 0,300 0,282 0,296 0,387 0,411 0,372 0,315 r = pgaoutput_eera/pgantc 2008 1,45 1,36 1,43 1,87 1,99 1,80 1,52 600 m soil model 1693 pt1r3 1693 pt3r3 1693 pt6r3 1818 40m1 1818 49m1 1818 56m1 1990 pgantc 2008 soil a 0,207 0,207 0,207 0,207 0,207 0,207 0,207 pgaoutput_eera 0,255 0,236 0,257 0,335 0,374 0,346 0,283 r = pgaoutput_eera/pgantc 2008 1,45 1,36 1,43 1,87 1,99 1,80 1,52 200 m soil model 1693 pt3r1 1693 pt4r1 1693 pt5r1 1818 44m2 1818 47m2 1818 53m2 1990 pgantc 2008 soil a 0,282 0,282 0,282 0,282 0,282 0,282 0,282 pgaoutput_eera 0,519 0,493 0,492 0,434 0,401 0,354 0,429 r = pgaoutput_eera/pgantc 2008soil a 1,84 1,75 1,74 1,54 1,42 1,26 1,52 s2 bedrock 600 m 1693 pt3r1 1693 pt4r1 1693 pt5r1 1818 44m2 1818 47m2 1818 53m2 1990 pgantc 2008 soil a 0,282 0,282 0,282 0,282 0,282 0,282 0,282 pgaoutput_eera 0,439 0,446 0,430 0,382 0,393 0,300 0,380 r = pgaoutput_eera/pgantc 2008soil a 1,56 1,58 1,52 1,35 1,39 1,06 1,35 table 6. results in terms of maximum acceleration at the surface and soil amplification factor r, for the 600 m soil model, using as input ag = 0.282 g acceleration value. table 3. results in terms of maximum acceleration at the surface and soil amplification factor r, for the 200 m soil model, using as input ag = 0.207 g acceleration value. table 4. results in terms of maximum acceleration at the surface and soil amplification factor r, for the 600 m soil model, using as input ag = 0.207 g acceleration value. table 5. results in terms of maximum acceleration at the surface and soil amplification factor r, for the 200 m soil model, using as input ag = 0.282 g acceleration value. have been drawn for each site long a set of six receivers placed at different depths, starting from the surface up to almost 170 m [grasso et al., 2005]. in the case of level ii 1818 earthquake scenario seisferraro et al. 16 figure 28. drpc soil model based on cross-hole test. figure 29. drpc soil model based on down-hole test. chs1s2 amoruso et al. [2002] bottari et al. [1986] diss aspromonte est diss gioia tauro diss messina straits tortorici et al. [1995] 1693 pgantc 2008 0,332 0,332 0,332 0,332 0,332 0,332 0,332 pgaoutput_eera 0,516 0,664 0,443 0,619 0,764 0,584 0,419 r = pgaoutput_eera/pgantc 2008 1,55 2,00 1,33 1,86 2,30 1,76 1,26 dhs3 amoruso et al. [2002] bottari et al. [1986] diss aspromonte est diss gioia tauro diss messina straits tortorici et al. [1995] 1693 pgantc 2008 0,332 0,332 0,332 0,332 0,332 0,332 0,332 pgaoutput_eera 0,518 0,487 0,444 0,502 0,436 0,453 0,411 r = pgaoutput_eera/pgantc 2008 1,56 1,47 1,34 1,51 1,31 1,36 1,24 table 7. results in terms of maximum acceleration at the surface and soil amplification factor r, for the cross-hole s1-s2 soil model. table 8. results in terms of maximum acceleration at the surface and soil amplification factor r, for the down-hole s3 soil model. mograms have been evaluated by laurenzano et al., 2004, using the 3-d hybrid stochastic-deterministic method exwim. this method has been used to compute strong motion seismograms in the near-field of extended earthquake source. the computational procedure consisted of: 1) discretizing the area into a grid of elementary point sources; 2) computing the wavefield generated by each point source with unitary seismic moment; 3) computing seismograms for each point source according to the given distribution of slip; and 17 site effects evaluation in catania (italy) figure 30. drpcset of input seismograms used for numerical analyses. 4) summing up each contribution synchronized in time to simulate the propagation of the rupture. seismograms have been computed up to a maximum frequency of 20 hz. the deterministic-stochastic transition has been set at 1.5-2 hz. the maximum fault size has been set at 26.6 km x 16.6 km, and it has been discretized into 4895 elementary sources, with inter-spacing of 300 m. the ground motion has been computed at a regular grid of receivers covering the catania municipal area. this area is sampled with 470 receivers, with inter-spacing of 300 m. the use of advanced methods capable of generating of synthetic seismograms can give a valuable insight into the evaluation of a seismic ground motion scenario. recorded accelerograms of december 13, 1990 earthquake and synthetic seismograms by 1693 and 1818 have been scaled to different values of the pga at the bedrock corresponding to different return periods in the current italian seismic code “seismic hazard and seismic classification criteria for the national territory” obtained by a probabilistic approach at uls in the interactive seismic hazard maps. figure 18 shows some of the synthetic seismograms by 1693 and 1818 scenario earthquakes and the recorded accelerogram of december 13, 1990 earthquake used as input for seismic response analyses. preliminary numerical analyses have been carried ferraro et al. 18 figure 31. drpc ch-s1-s2 model. results at the surface in terms of acceleration time history (a), amplification ratio (b) and spectral acceleration (c). labels of different colours in the legend represent different synthetic seismograms obtained from different source models. figure 32. drpcch-s1-s2 model. maximum acceleration profiles. labels of different colours in the legend represent different synthetic seismograms obtained from different source models. (a) (c) (b) out based on the 1990 recorded input motion scaled to six different values of pga provided by the current italian seismic code for different uls and the different return periods of 60, 101, 475, 949, 1898 and 1950 years: 0.083 g; 0.102 g; 0.207 g; 0.282 g; 0.392 g; 0.397 g. in figures 19-22 the results of site response analyses are presented, for different bedrock depths (200 m, 300 m, 400 m, 600m), in terms of time history of surface acceleration, amplification ratio and spectral acceleration [caruso et al., 2016]. finally, the figure 23 shows the comparison in terms of maximum acceleration profiles, for the different soil models considered in the numerical analysis. additional seismic response analyses have been carried out [ferraro et al., 2016] for the 200 m and 600 m seismic bedrock soil models, by using the same input seismograms of figure 18, scaled to the different values of acceleration of 0.207 g and 0.282 g provided by the italian seismic code (ultimate limit state with return periods of 475 and 949 years). the numerical analyses results are shown in terms of response spectra at surface (figures 24-27), and in terms of surface maximum acceleration and amplification ratio (tables 3-6). 4.2 drpc test site numerical analyses numerical analyses have been carried out considering two soil models, based on the cross-hole test results (figure 28) performed in the s1 and s2 19 site effects evaluation in catania (italy) figure 33. drpc dh-s3 model. results at the surface in terms of acceleration time history (a), amplification ratio (b), spectral acceleration. labels of different colours in the legend represent different synthetic seismograms obtained from different source models. figure 34. drpc-me dh-s3 model. maximum acceleration profiles. labels of different colours in the legend represent different synthetic seismograms obtained from different source models. (a) (c) (b) boreholes, and on the down-hole test results (figure 29) performed in the s3 borehole, by using the seven input seismograms [amoruso et al., 2002] shown in figure 30 obtained by a source modeling of 1908 messina and reggio calabria earthquake and 1693 val di noto earthquake, scaled to the value of 0.332 g, provided by the italian seismic code ntc2008 (ultimate state limit with a return period of 949 years). the numerical analyses results are shown in figures 31-34 in terms of time history of surface maximum acceleration, amplification ratio and response spectra, and in terms of surface maximum acceleration and soil amplification factor r values (tables 7-8). 5. conclusions in this study it was benefited of a great availability of borehole data, geophysical surveys and laboratory tests carried out from various campaigns of geological surveys that, in different phases, have interested the area of two tests sites in the cities of catania and messina seismic areas. in order to study the dynamic characteristics of soils in the test sites, in situ and laboratory investigations have been carried out to obtain soil profiles with special attention being paid to the variation of the shear modulus g and damping ratio d with depth. local site response analyses have been brought for the site area at ultimate limitation state uls by 1-d linear equivalent computer codes for the evaluation of the amplification factors of the maximum acceleration. results of the site response analysis show high values in soil amplification effects of the soil. the horizontal acceleration at the surface obtained by the site response analysis is in some cases higher than 0.500 g, with soil amplification factors up to 2.30, higher than the soil amplification factor ss on the basis of the average shear waves velocity (vs30) according to the current italian seismic code. the difference between the two soil amplification factors is due to the fact that (vs30) is not a key parameter in the evaluation of soil amplification. through 1-d performed numerical analyses it has been possible to evaluate the influence of stratigraphic effects in seismic response of the site. finally elastic pseudo-acceleration response spectra at the ground surface have been compared (graphically) with the reference ones, provided by the ntc 2008 italian seismic code at uls. seismic retrofitting and/or improving have to be definitely considered a multidisciplinary subject, which depends in fact on many factors, such as: local site effects and the dynamic interaction between the foundation soil and the structure. the accurate investigation on the structure and the surrounding soil is the first fundamental step for a realistic evaluation of the structure seismic performance. abate, g., massimino, m.r. 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(2016): seismic response analysis in the southern part of the historic centre of the city of l’aquila (italy). soil dynamics and earthquake engineering, 88, pp. 256-264. grasso, s., maugeri, m., (2009): “the road map for seismic risk analysis in a mediterranean city”. soil dynamics and earthquake engineering. issn: 0267-7261. 29 (6), 2009: 1034-1045. grasso, s., maugeri, m., (2009): “the seismic microzonation of the city of catania (italy) for the maximum expected scenario earthquake of january 11, 1693”. soil dynamics and earthquake engineering, 29 (6), 2009: 953-962. grasso, s., maugeri, m., (2012): “the seismic microzonation of the city of catania (italy) for the etna scenario earthquake (m=6.2) of february 20, 1818”. earthquake spectra. 28 (2), 2012: 573-594. issn: 8755-2930. grasso, s., maugeri, m., (2014): “seismic microzonation studies for the city of ragusa (italy)”. soil dynamics and earthquake engineering. issn: 0267-7261. 2014, (56): 86-97. idriss, i.m., sun, j.i., (1992): user’s manual for shake91. center for geotechnical modeling, dept. of civil & environmental engineering, univ. of california, davis, california. massimino m.r., biondi g., (2015): “some experimental evidences on dynamic soil-structure interaction”. 5th eccomas thematic conference on computational methods in structural dynamics and earthquake engineering, compdyn 2015. m. papadrakakis, v. papadopoulos, v. plevris (eds.). crete island, greece, 25-27 may 2015. code 113952. isbn: 978-960-99994-7-2. pp. 27612774. maugeri m., abate g., massimino m.r., (2012): “soilstructure interaction for seismic improvement of noto cathedral (italy)”. geotechnical, geological and earthquake engineering, 16(2012): 217-239. schnabel, p, seed, h.b,, lysmer, j., (1972a): modification of seismograph records for effects of local soil conditions. bulletin of the seismological society of america; 62 (6): 1649-1664. schnabel, p,b,, lysmer, j., seed, h,b., (1972b): shake: a computer program for earthquake response analysis of horizontally layered sites. college of engineering, university of california, berkeley, california. report no. eerc 72-12. tortorici, l., monaco, c., tansi, c., & cocina, o. (1995): “recent and active tectonics in the calabrian arc (southern italy)”. tectonophysics, 243(1), 37-55. *corresponding author: salvatore grasso, department of civil engineering and architecture (dicar) university of catania, catania, italy; email: sgrasso@dica.unict.it © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. ferraro et al. 22 papathanassiou_corretto_layout 6 735 liquefaction phenomena along the paleo-reno river caused by the may 20, 2012, emilia (northern italy) earthquake giorgos papathanassiou1, riccardo caputo2,*, dimitra rapti-caputo2 1 aristotle university of thessaloniki, department of geology, thessaloniki, greece 2 università di ferrara, dipartimento di fisica e scienze della terra, ferrara, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6147 1. introduction on the may 20, 2012 (04:03:52 local time; 02:03:52 utc), a moderate earthquake (ml 5.9) [scognamiglio et al. 2012, this volume] with a focal mechanism showing e-w-trending, s-dipping, reverse-faulting occurred in the eastern sector of the alluvial plain of the po river, close to the border between the regions of emilia-romagna and lombardia (northern italy). the tectonic structure is completely blind, but it was well known from a dense grid of seismic profiles for hydrocarbon explorations [e.g., pieri and groppi 1981, toscani et al. 2009]. the earthquake triggered extensive liquefaction-induced ground effects at the surface, and caused severe structural damage to nonreinforced masonry and precast industrial buildings within the broader epicentral area. the hypocenter was at 44.89 ˚n, 11.23 ˚e, at a depth of 6.3 km [scognamiglio et al. 2012], while the maximum acceleration was recorded in mirandola, with peak ground acceleration 310 cm/s2 and 264 cm/s2 along the vertical and horizontal components, respectively [bozzoni et al. 2012, this volume]. in this report, we focus on a zone including the sant'agostino, san carlo and mirabello villages (west ferrara province), which were built along an abandoned reach of the reno river and where liquefaction phenomena were particularly diffuse, with very intense local effects (figure 1). the map in figure 1 is based on original data collected in the days following the mainshock (and before the event of may 29, 2012) and locally integrated with data published by ispra [2012]. similar maps have also been presented by other geological teams [e.g., decanini et al. 2012, emergeo working group 2012], including of the western sector of the epicentral area and the manifestations that occurred during the second major earthquake. however, for the aims of this report that focuses on the sant'agostino, san carlo and mirabello case study, they will be not discussed. based on stratigraphic reconstructions from boreholes and penetrometric tests, and on new direct measurements within numerous water wells, a preliminary conceptual hydrogeological model for the first ca. 30 m is also proposed for the investigated area. this hydrogeological model provides essential information for the liquefaction susceptibility discussed in this report. grain-size analyses of ejected material collected a few days after the event in several locations were also carried out, and are here discussed in relation to the classical susceptibility criteria proposed by tsuchida [1971]. in addition, a preliminary evaluation of the macroseismic intensity is performed based on the environmental seismic intensity (esi) scale [michetti et al. 2007]. 2. earthquake-induced ground effects a few hours after the event, a post-earthquake field survey was organized to report the coseismic effects and to compile a map of their spatial distribution (figure 1). as this survey began a few hours after the mainshock, this provided the opportunity to observe, measure, photographically record, and map the sites where the sandy material was ejected, before the residents and local authorities started removing the liquefied sandy materials from the courtyards and roads. during this post-event investigation, more than 150 locations were surveyed, with the focus on a zone 8 km long and 0.5 km wide, with the parameters that characterize the surface liquefaction manifestations were measured and recorded carefully. in particular, the length and width of vent fractures, and the diameter of the sand craters were measured, and samples were collected from ejected materials at representative sites. the length of the liquefaction-induced surface ruptures varied, as expected, from a few centimetres to more than 10 m, and locally they were reported as up to 50 m [lai et al. 2012]. the width variation of the cracks ranged from a few centimetres to 30 cm. the ejection of sandy material occurred through conduits and fractures in farmed fields, although the most spectacular effects affected pavements and large-diameter water wells within the urbanarticle history received july 23, 2012; accepted september 18, 2012. subject classification: seismic hazard assessment, po plain, morphogenic earthquakes. 2012 emilia earthquakes ized areas (figure 2). as a consequence, the number of sand craters and volcanoes were significantly lower than the vent fractures, but above all, their location and geometry was strongly controlled by the artificial discontinuities within the paving and other infrastructures. the diameter in these cases ranged from 4 cm up to 40 cm. lateral spreading due to liquefaction and the consequent upward injection of sandy material is also well documented by electrical resistivity tomographies [abu-zeid et al. 2012, this volume], and this phenomenon induced structural damage within the urbanized areas, including broken pipelines and differential settling, causing minor damage to buildings locally. in addition, short interviews with the villagers were carried out relating to the ejection of fluids and sandy material. the questions posed focused on the timing of the ejection phenomena, relative to the occurrence of the mainshock, the duration of the outflow, and the height of the ejection with respect to the ground surface. important outcomes of these interviews were: (i) most of the sandy material within the three urban areas that was focused on (mirabello, san carlo, sant'agostino) was ejected through large diameter water wells; (ii) the ejection of sandy material started almost at the same time as the mainshock; and (iii) in the first seconds/minutes of the ejection process, clear water came to the surface first, only later mixed with sand. the duration of the phenomenon commonly lasted up to a few minutes, although at one site (44˚ 48' 03" n, 11˚ 24' 34" e) an eyewitness stated that the outpouring of water after the major sand ejection phenomenon lasted for several hours. differences in the amounts of material and timing are likely to be due to local geological, hydrogeological and anthropogenic conditions. for the height of the ejection, the most characteristic report is that from a well at the center of san carlo (44 ̊48' 15.39"n, 11˚ 24' 35.14"e), where the liquefied material was ejected up to 2 m above the ground surface, and with such intensity as to damage the well. taking into account that the depth to the water at the site was ca. 5 m (see next section), it can be assumed that the total height of the ascent was more than 7 m. considering the average density of the outflowing material (a mixture of water and sand), this implies a fluid pressure of probably more than one bar (0.1 mpa). similar ascent and ejection of sand through wells was triggered by the friuli 1976, northeastern italy, earthquake [sirovich 1996]. to assess the macroseismic intensity in the area studied, we applied the esi scale [michetti et al. 2007], as the generpapathanassiou et al. 736 figure 1. map of the spatial distribution of the liquefaction-induced surface evidence within the west ferrara province from original data (yellow circles) and from the ispra [2012] report (white circles). red star, may 20, 2012, earthquake epicenter. base map from google inc. (2012). 737 ated geological effects were widespread and extensive. thus, taking into account the length and width of the surface ruptures, and the dimensions of vent fractures and sand boils of liquefaction phenomena, we conclude that the macroseismic intensity should be evaluated as viii in the area of san carlo. in particular, according to the esi definition of intensity degrees [michetti et al. 2007], a value of vii-viii is appropriate in areas where fractures up to 50 cm wide and up to hundreds of meters long are commonly observed in loose alluvial deposits. decametric cracks are common in paved roads, and liquefaction can be frequent in the epicentral area. localized lateral spreading and settling (up to ca. 30 cm) can result in the most typical effects of sand boiling up to ca. 1 m in diameter. 3. hydrogeological model the conceptual shallow of the hydrogeological model of the broader ferrara province is characterized by the presence of two aquifer systems. the first is superficial and semiconfined or locally unconfined (a0 according to molinari et al. [2007]), and the second is deeper and always confined (a1; regione emilia-romagna and eni-agip [1998]). within the investigated area, and based on published [emilia-romagna region open file] and unpublished stratigraphic data, the a0 aquifer system consists of silty-sandy and locally peat and sandy lenses, with variable grain-sized distributions containing a semi-confined and locally unconfined aquifer. according to interviews with the villagers, the piezometric level in many wells showed limited variations even after a long exploitation period, thus indicating that the aquifer bodies belonging to a0 are commonly hydraulically inter-connected. only minor lenses were probably isolated, as suggested by their complete draining and a longer recharge time following a few hours of pumping. the exploitation of this aquifer system is performed using boreholes of about 1 m in diameter, which were often excavated many years ago, commonly to a depth of 7 m to 13 m. this traditional water-well excavation and their architecture was maintained by local people up to the last decades. before the aqueduct network reached these areas, these large diameter wells were the only means to provide the water for every domestic use. the spatial distribution and the geometric characteristics of these highly heterogeneous aquifer units were largely influenced by the latest quaternary geomorphological and sedimentary evolution of the territory, and especially by the presence and path of active and abandoned channels and outflow cones [e.g. rapti-caputo 2000]. within the investigated area, the thickness of the aquifer varies from between a few meters to 6 m to 8 m, in agreement with the paleo-reach of the reno river (figure 3). the hydraulic conductivity of the a0 aquifer system was preliminarily estimated based on the available stratigraphic data and the grain-size analyses we performed from collected samples of ejected material (see next section). according to hazen [1911], this parameter varies from between 10–5 and 10–4 m/s, thus indicating highly permeable layers. the prin2012 emilia event liquefaction phenomena figure 2. example of sand ejection from a large-diameter water well at the base of the levee associated with an abandoned reach of the reno river (san carlo-sant'agostino road). figure 3. topographic profile (black line) drawn from a lidar-based dem, and piezometric level of the unconfined aquifer system (a0), as reconstructed from water-well measurements a few weeks after the seismic event. cipal recharge of the aquifer was primarily due to vertical infiltration from the precipitation, and secondarily to recharges via lateral infiltration from the active hydrographic network, as documented in nearby areas with a comparable hydrogeological–geomorphological setting [e.g. rapti-caputo 2000]. indeed, the local alimentation is shown by high values of tritium (3-6 tu) that show that the mean residence time of the water in the underground reservoir is only 2 years to 5 years [rapti-caputo 2008, rapti-caputo and martinelli 2009]. the piezometric level varies by about 1.5 m to 2.5 m during the hydrological year, due to the variability of both the seasonal precipitation and the hydrometric level within the hydrographic network [rapti-caputo 2008, arpa gdl 2010]. based on a campaign of measurements specifically carried out for 45 wells during the weeks following the earthquake, the water depths were about 1.4 m to 3.0 m in the interfluvial plain, showing deeper values (4 m to 5 m from the surface) in the boreholes located along the levees of the paleo-reno river. this hydrogeological setting is shown in figure 3, where the influence of the paleo-channel on the direction of the groundwater flow is evident for generating a marked groundwater divide. a low permeability layer that consists prevalently of clay and silt locally alternating with peat lenses separates the unconfined (a0) from the underlying confined aquifer (a1) located about 16 m to 18 m below the surface [rapti-caputo 2008, regione emilia-romagna gdl-liquefazione 2012]. the latter aquifer body consists of high permeability sediments (mainly sand), and has a thickness of between 4 m and 9 m. 4. liquefaction-related effects to evaluate the susceptibility to liquefaction of the soil deposits within the investigated area, we followed the procedure recommended by the manual for zonation on seismic geotechnical hazards, prepared by the technical committee for earthquake geotechnical engineering, tc4, of the international society for soil mechanics and geotechnical engineering, and published by the japanese geotechnical society [tc4, 1999]. initially, the relationship between the epicentral distance of liquefied sites and earthquake magnitude was examined. the epicentral distances of the villages of sant'agostino, san carlo and mirabello are 16 km, 17 km and 19 km, respectively (figure 4). in addition, the epicentral distances of the liquefied area of scortichinoburana, documented by ispra [2012], varied from 3 km to 11 km, while the zone situated to the north of mirabello is about 17 km from the epicenter. according to several empirical relationships between magnitude and epicentral distance (figure 4), these areas should be clearly considered as susceptible to liquefaction for an earthquake of ml 5.9, as they lie above most of the proposed curves, with the only possible exceptions of some sites in the mirabello area. differences between the proposed empirical relationships (and hence the curves represented in figure 4) are due to the different datasets used in these studies. for example, ambraseys [1988] used a worldwide set of liquefaction data, while papadopoulos and lefkopoulos [1993] updated the dataset collected by ambraseys with 30 new cases from greek earthquakes and with three other ones. wakamatsu papathanassiou et al. 738 figure 4. distribution of the liquefied sites based on their epicentral distances and comparison with the triggering curves proposed by several studies for the assessment of liquefaction susceptibility. 739 [1993] extended the work of kuribayashi and tatsuoka [1975] by adding 46 new data points from japanese earthquakes, while galli [2000] and aydan et al. [2000] re-evaluated the seismic parameters of italian and turkish earthquakes, respectively, using data from after 1900. the curve suggested by papathanassiou et al. [2005] was constructed based on liquefaction phenomena that were reported after 1900 in greece. the sites where liquefaction-induced ground effects are documented generally consist of holocene fluvial sediments and locally man-made deposits. the villages of sant'agostino, san carlo and mirabello were founded upon the levees of a former reach of the reno river [graziani et al. 1992, castiglioni et al. 1999, caputo et al. 2012 (this volume)]. this was for the obvious reason that this site was (and still is) 3 m to5 m higher with respect to the surrounding alluvial plain, which at that time (xv-xviii century) was repeatedly flooded. moreover, the presence of a locally sandy substratum associated with the crevasse deposits ascertained better stability for house foundations, in contrast to other sectors of the plain where the muddy material was not suitable for construction. notwithstanding these historical and social reasons, these morphological, stratigraphic and hydrogeological conditions were characterized by: (i) topographic gradients; (ii) the presence of cohesionless deposits; and (iii) a depth of the water table of less than 1 m locally. this represented the perfect preparatory factors for liquefaction, just waiting for sufficient shaking to trigger the physical process. eventually, on may 20, 2012, sufficient shaking occurred, and this caused locally important and widespread liquefaction phenomena. at the surface, this produced small-scale structural damage, mainly due to lateral spreading (44˚ 48' 22" n, 11˚ 24' 46" e) and differential settling (44˚ 49' 29"n 11˚ 27' 26"e). similar coseismic effects within old and/or former river channel zones are well documented for at least three recent earthquakes: luzon philippines in 1990, niigaten chuetsuoki, japan, in 2007 and darfield, new zealand, in 2010 [wotherspoon et al. 2012]. this confirms that this type of morphological setting is favorable for earthquake-induced surface ruptures and liquefaction phenomena. the above conclusion is in agreement with the characterization of this type of deposit as highly susceptible to liquefaction during strong seismic shaking, based on the geological criteria that were suggested by youd and perkins [1978]. the samples collected during the field survey were analyzed in the laboratory of engineering geology and hydrogeology of the department of geology at the aristotle university of thessaloniki (greece), to obtain the grain-size distribution, the liquid limit, and the plasticity index [e.g., kramer 1996] (figure 5). based on the unified soil classification system (uscs), samples are classified (see table 1) as silty sand (sm) with non-plastic fine particles, and fine sand (sp-sm). the fines content of the samples analyzed varied from 6% to 12% for the sp-sm, and from 20% to 34% for the sm. the results suggest that the majority of the ejected samples had probably come from a unique sedimentary layer. the probable source level was identified using cone penetration tests and cores, which was carried out within the investigated area in a similar morphological, hydrogeological 2012 emilia event liquefaction phenomena figure 5. comparison between the grain-size distribution curves of the collected samples and the boundary curves for liquefiable (dashed lines) and probable liquefiable (dotted lines) soils, as suggested by tsuchida [1971]. similar critical curves were proposed by en 1998-1 [2004] and ntc [2008]. and stratigraphic setting [regione emilia-romagna gdl-liquefazione 2012]. this source unit consists of a medium-tocoarse sand layer at a depth of 7 m to 8 m from the surface, which varies in thickness between 1.5 m and 4 m, and which probably recorded large crevasse splays and overbank flood episodes. due to the stratigraphic setting and the hydrogeological framework, this layer represents a semi-confined aquifer that is characterized by a high degree of liquefaction susceptibility. the presence of a non-liquefied silty-clayey cap layer and the occurrence of some erosion during the upward flow along the dikes and conduits might have caused an overestimation of the fines content of the ejected material. table 1 lists the results of the collected samples that were provided by the laboratory testing, including the fines content (%), the classification (uscs), and their geographic location. the results of the grain-size analyses of the samples are also compared with the curves proposed by tsuchida [1971] for soils at uniform grading, as shown in figure 5. the grain-size distribution curves obtained fall within the suggested range for liquefaction conditions, therefore characterizing these deposits as susceptible to liquefaction in terms of their grain size. 5. concluding remarks the may 20, 2012, emilia earthquake triggered extensive liquefaction-induced ground effects at the surface and caused structural damage to unreinforced masonry and precast industrial buildings within the broader epicentral area. the post-earthquake field survey organized a few hours after the event focused on a zone 8 km long and 0.5 km wide, and it was devoted to the reporting of the co-seismic effects and to compile a map of their spatial distribution. the lengths of the liquefaction-induced surface ruptures varied from a few centimeters to more than 10 m, while the width variation of the cracks was not so great, ranging from a few centimeters to 30 cm. the ejection of sandy material occurred through conduits and fractures in farmed fields, although the most spectacular effects affected roads, pavements and water wells within the urbanized areas. in addition, short interviews with the villagers were carried out concerning the ejection of fluids and sandy material. the outcomes of these interviews are that most of the sandy material within the urban areas of mirabello, san carlo and sant'agostino was ejected through water wells of large diameter, which are typical of the region, and exploited the first unconfined aquifer. the phenomenon started almost at the same time as the mainshock, and in the first seconds/minutes of the ejection process clear water came to the surface first, only afterwards mixed with sand. furthermore, samples of the ejected material were collected, to obtain grain-size distribution, liquid limits and plasticity indices. based on the uscs, the materials were classified as sm with non-plastic fine particles, and sp-sm. the fines content of these deposits varied from 6% to 12% for the sp-sm and from 20% to 34% for the sm. the results suggest that the majority of the ejected samples had probably come from the same sedimentary layer. a preliminary outcome of these field surveys is that the studied area has suffered large-scale liquefaction-induced manifestations that should be considered as secondary coseismic effects (i.e., not directly associated with the upwards propagation of the fault rupture). although liquefaction phepapathanassiou et al. 740 a/a longitude latitude uscs classification fines content (%) s1 44˚ 48' 16.85" 11˚ 25' 08.30" sm 31 s2 44˚ 48' 17.19" 11˚ 25' 07.42" sm 29 s3 44˚ 48' 17.73" 11˚ 24' 37.49" sp-sm 7 s4 44˚ 48' 12.68" 11˚ 24' 25.74" sp-sm 6 s5 44˚ 48' 10.61" 11˚ 24' 30.99" sm 18 s6 44˚ 48' 08.99" 11˚ 24' 33.14" sm 17 s7 44˚ 48' 25.16" 11˚ 24' 45.67" sm 32 s8 44˚ 48' 20.28" 11˚ 24' 57.20" sm 34 s9 44˚ 47' 58.54" 11˚ 24' 09.06" sm 23 s10 44˚ 47' 55.64" 11˚ 24' 04.41" sp-sm 8 s11 44˚ 47' 50.16" 11˚ 24' 03.68" sm 18 s12 44˚ 48' 35.81" 11˚ 25' 47.42" sm 30 s13 44˚ 49' 37.12" 11˚ 27' 41.68" sm 20 s14 44˚ 49' 29.57" 11˚ 27' 27.13" sm 30 table 1. grain size characteristics of the collected material. 741 nomena were diffuse in the epicentral area and in general the liquefaction potential might be characterized as 'high' for recent alluvial plains, only small-scale liquefaction-induced structural damage was documented [galli et al. 2012, tertulliani et al. 2012, this volume]. taking into account that the geological (stratigraphic, morphological, hydrogeological seismotectonic) setting of the investigated area is comparable with those of many other sectors of the broader po plain, as well as worldwide, and due to the abundance of liquefaction-related features produced here, the investigated case study represents a valuable example for future investigations devoted to better evaluation of liquefaction hazard. acknowledgements. the authors thank the servizio geologico, sismico e dei suoli, regione emilia-romagna for providing the lidar-based dems and preliminary access to the sedimentary cores drilled in the investigated area. references abu zeid, n., s. bignardi, r. caputo, g. santarato and m. stefani (2012). electrical resistivity tomography investigation of coseismic liquefaction and fracturing at san carlo, ferrara province, italy, annals of geophysics, 55 (4); doi:10.4401/ag-6149. ambraseys, n. 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(1993). history of soil liquefaction in japan and assessment of liquefaction potential based on geomorphology, ph.d. thesis, waseda university, tokyo, japan, 245 pp. wotherspoon, l., m. pender and r. orense (2012). relationship between observed liquefaction at kaiapoi following the 2010 darfield earthquake and former channels of the waimakariri river, eng. geol., 125, 45-55. *corresponding author: riccardo caputo, università di ferrara, dipartimento di fisica e scienze della terra, ferrara, italy; email: rcaputo@unife.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. papathanassiou et al. 742 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice f o t o g r a f i e r i p r e s e a d a l t a q u o t a p e r m e z z o d i p a l l o n i d u r a n t e l ' e c l i s s e t o t a l e d i s o l e d e l 1 5 2 1 9 6 1 r . c i a l d e a m . c i m i n o g . f e a r i c e v u t o il 15 giugno 1961 in occasione dell'eclisse totale di sole del 15 febbraio 1961, l'istituto nazionale di geofìsica, l'osservatorio astronomico di roma su m. mario ed il ministero della difesa aeronautica hanno elaborato un piano di ricerche allo scopo di rilevare la luce diffusa da parte dell'atmosfera e di eseguire fotografìe della corona esterna del sole ad alta quota. la ricerca è stata effettuata inviando nell'alta atmosfera un apparato fotografico dello stesso tipo all-sky di quello già adoperato in occasione dell'eclisse totale di sole del 1952 (l). l'apparato consiste in una macchina da ripresa, diretta verso il basso, che riprende l'intera volta celeste riflessa da uno specchio convesso (2). la macchina adoperata, del formato di 16 mm, ha una velocità di ripresa di due fotogrammi al secondo con un tempo di esposizione di 1/8 di secondo; un tempo di esposizione così elevato è necessario per poter eseguire valutazioni fotometriche della luce diffusa anche da p a r t e dell'alta atmosfera durante la fase di totalità. davanti all'obiettivo una serie di quattro filtri ruota sincronicamente alla pellicola allo scopo di ottenere delle serie successive di fotogrammi con esposizioni diverse: in t a l modo si può ottenere una più accurata misura fotometrica. un sistema ad orologeria assicura l'inizio della ripresa (dopo 45' di volo) e lo sgancio del pallone dopo un'ora e mezza dal lancio. dal momento dello sgancio (che in occasione dell'eclisse doveva avvenire intorno ai 20-25 km di altezza) l'apparato scende in caduta libera fino a quota 3000, dove un meccanismo barometrico f a aprire un paracadute per poter permettere il recupero di t u t t i gli apparati. allo scopo di proteggere i delicati meccanismi di orologeria e di avanzamento del film dalle bassissime temperature dell'alta atmosfera, l'apparato è protetto da una opportuna scatola formata da materiale ad elevato isolamento termico. 8 0 r. c i a l d e a m . c i m i n o g . f e a la quota è registrata su ogni fotogramma, fotografando un a l t i m e t r o sistemato vicino allo specchio. nella fig. 1 è rappresentato t u t t o l'apparato: si individua a p a r t i r e dall'alto il paracadute, la scatola isolante, e internamente ad essa il meccanismo ad orologeria, le batterie, la macchina da ripresa, ed esternamente, la scatola dei filtri, lo specchio e l'altimetro. fig. i l'apparecchiatura, dato il suo notevole peso, è s t a t a affidata a due palloni in parallelo: essa è p a r t i t a il 15 m a t t i n a alle 7,17 (t.m.e.c.) dall'aeroporto di pisa. il volo si è svolto regolarmente e si è raggiunta la quota massima di 24.000 metri; la discesa è a v v e n u t a l e n t a m e n t e senza paracadute, in quanto a questa quota uno di due palloni è scoppiato mentre l'altro ha funzionato da freno nella discesa. la velocità di salita dell'apparato è s t a t a però più bassa del previsto, cosicché al momento della totalità i palloni si trovavano alla quota di 9.500 metri. l ' a p p a r a t o ha funzionato regolarmente e la fase di t o t a l i t à è s t a t a integralmente ripresa. i tempi di esposizione, di necessità lunghi, sono risultati t r o p p o f o t o g r a f i e r i p r e s e d u r a n t e l ' e c l i s s i d e l s o l e d e l 1 5 2 1 9 0 1 8 1 grandi in relazione ai periodi medi degli irregolari moti oscillatori dell ' a p p a r a t o , moti dovuti alla turbolenza atmosferica, che è risultata in quel giorno piuttosto forte; di conseguenza l'immagine della corona non è risultata molto nitida. la luce diffusa dell'alta atmosfera è stata invece rilevata con sufficiente precisione: infatti gli spostamenti angolari nell'immagine nelle migliori fotografie sono stati dell'ordine di una decina di primi in media. n i risultati che si possono ottenere da questo tipo di rilevamenti fotografici sono t u t t o r a in esame, in quanto sono state scattati un migliaio di fotogrammi durante t u t t a l'eclisse. ma fin d'ora si può senz'altro prevedere un notevole interesse sia dal p u n t o di vista di rilevamento fotografico degli oggetti celesti, in particolar modo della corona solare, sia dal p u n t o di vista delle osservazioni della luce diffusa da p a r t e dell'atmosfera. i n f a t t i tali rilevamenti, qualora vengano eseguiti a quote superiori ai 15-20 k m con l'apparato sopra descritto, hanno i seguenti vantaggi: 1) è inutile il puntamento dell'apparecchio fotografico, dato che si t r a t t a di u n sistema di ripresa all-sky; 8 2 r . c i a l d e a m . c i m i n o g . f e a 2) è quasi totalmente annullato l'effetto p e r t u r b a t o r e dell'atmosfera sulle immagini degli oggetti celesti; 3) è ridotto notevolmente l'assorbimento delle radiazioni, specie nell'u.v. prossimo; 4) è ridotta la brillanza del cielo diurno, ciò che fa sperate con qualche piccola modifica all'apparato, di poter riprendere la corona solare al difuori dell'eclisse. p e r quanto riguarda le osservazioni della luce diffusa da p a r t e dell'alta atmosfera, u n tale metodo, se da u n a p a r t e presenta le note difficoltà proprie della fotometria fotografica, dall'altra presenta l'indubbio vantaggio di poter rilevare la brillanza di t u t t a la volta celeste contemporaneamente e in ogni direzione. il problema particolare della diffusione della luce d u r a n t e l'eclisse, da noi studiato, presuppone la conoscenza della geometria dell'ombra della luna istante per istante. nel nostro caso l'intersezione del cono d'ombra con un piano orizzontale era un'ellisse con u n a forte eccentricità (i semiassi avevano una lunghezza di 200 e di 43 km rispettivamente. nella fig. 2 è riportata questa ellisse orientata con la direzione del meridiano geografico e con la linea centrale di totalità, che è r a p p r e s e n t a t a da u n a r e t t a ; essa può essere considerata come la traiettoria a p p a r e n t e di u n osservatore al suolo d u r a n t e la totalità (i valori si riferiscono alla località di pisa). nelle figg. 3 e 4 sono riportate due fotografie prese con un intervallo di qualche m i n u t o l ' u n a dall'altra, ma la prima ripresa d u r a n t e la fase parziale e la seconda d u r a n t e la fase di totalità. da un semplice esame si può osservare che durante la fase parziale l'atmosfera è n e t t a m e n t e più luminosa nel piano verticale del sole: i n f a t t i predomina ancora l'illuminamento diretto della fotosfera solare. d u r a n t e la t o t a l i t à invece si ha solo la luce che proviene dalle zone dell'atmosfera al difuori del cono d'ombra. è nettissima la differenza t r a l'illuminamento nel piano verticale del sole (dove la zona luminosa raggiunge l'altezza minima) e l'illuminazione nel piano normale (dove l'altezza è massima). i n f a t t i se si osserva il percorso apparente del pallone nel cono d'ombra, si vede che nella po izione a) (al centro del cono, cioè nella fase massima dell'eclisse) la fascia luminosa dell'atmosfera si trova a u n a distanza di circa 43 k m nel piano normale al verticale del sole; ciò significa che l'altezza della fascia luminosa è di circa 15° (se si suppone che l'atmosfera sia sensibilmente diffondente fino a quote di 10-15 k m ) ; nella direzione f o t o g r a f i e r i p r e s e d u r a n t e l ' e c l i s s i d e l s o l e d e l 1 5 2 1 9 0 1 8 3 del sole invece lo stesso angolo vale circa 3°, in quanto la fascia luminosa si trova a una distanza di circa 200 km. l'altezza di questa fascia luminosa, indipendentemente dal valore numerico si presenta perciò minima nel piano verticale del sole e massima nel piano ad esso normale. fig. 3 fig. 4 i n u n a posizione intermedia per es. in b) la situazione è diversi!,. nella fig. 5 è riportata u n a fotografia ripresa in questa posizione. si osserva a n z i t u t t o che le altezze della fascia sono essenzialmente diverse, m a è notevole il f a t t o che la fascia luminosa non ha l'altezza minima nel piano verticale del sole, ma in una direzione sensibilmente diversa. ciò può essere facilmente interpretato con l'ausilio della fig. 2 osservando che nella posizione b) la direzione nella quale la fascia deve avere l'altezza minima (cioè dove il limite dell'ombra è più distante) non coincide con il piano verticale del sole passante per il pallone. le misure fotometriche, che si stanno ora eseguendo, tendono alla valutazione della diffusione della luce (la p a r t e dell'alta atmosfera. la situazione d u r a n t e l'eclisse può essere così schematizzata: in ogni p u n t o di un fotogramma ripreso durante la totalità si ha un annerimento che è funzione della brillanza dell'atmosfera nella direzione della volta celeste corrispondente. in ogni direzione questa brillanza è dovuta generalmente a due regioni dell'atmosfera ben distinte: 8 4 r . c i a l d e a m . c i m i n o g . f e a .1) u n a regione esterna al cono d'ombra, dove si può considerare prevalente l'illumin unento dovuto alla fotosfera solare; 2) u n a regione i n t e r n a al cono d'ombra, dove prevale l'illuminamento dovuto alle regioni dell'atmosfera esterna al cono d'ombra, poiché si può considerare del t u t t o trascurabile l'illuminamento dovuto alla corona solare. fig. 5 le fotografìe riprese d u r a n t e la fase parziale sono s t a t e adoperate per la valutazione degli effetti secondari di diffusione dovuti all'atmosfera stessa ed all'albedo. u n esame preliminare ha già m o s t r a t o che, ad eccezione di u n a zona molto bassa vicino all'orizzonte, questi effetti secondari, alle quote da 8.000 ai 10.000 m e t r i circa nelle condizioni dell ' a t t u a l e eclisse, sono del t u t t o trascurabili. p e r t a n t o le misure sui fotogrammi f c a t t a t i d u r a n t e la totalità p e r m e t t e r a n n o di nisurare la diffusione secondaria come differenza t r a quella osservata e quella primaria dovuta alle regioni 1), e che si può calcolare in base alla geometria del cono d'ombra, alla densità dell'atmosfera alle diverse quote ed all'illuminamento dovuto al sole d u r a n t e la fase parziale. p e r la realizzazione di questa ricerca, fase iniziale di u n programma, che si spera di sviluppare nel prossimo f u t u r o , è gradito ringraziare i ministeri della difesa e delle f o t o g r a f i e r i p r e s e d u r a n t e l ' e c l i s s i d e l s o l e d e l 15-2-1901 8 5 finanze. essi hanno dato u n a sostanziale collaborazione per il lancio e per l'inseguimento r a d a r delle apparecchiature, da p a r t e dell'aeronautica militare i.t.a.v. d.a.t. e comando dell'aeroporto (li pisa e per il recupero delle stesse apparecchiature, effettuato appena due ore dopo il lancio dalla sezione elicotteri della guardia di finanza (li pisa. riassunto nel presente lavoro ali a.a. riferiscono su un lancio effettuato durante l'eclisse totale di sole del 15 febbraio 1961 mediante palloni. lo scopo della ricerca è quello di rilevare la luce diffusa da parte dell'atmosfera e di eseguire fotografìe della corona esterna del sole ad alta nuota. gli a.a. descrivono l'apparato adoperato e riportano alcune fotografie ottenute, con una breve discussione sulla iuuminahone della volta celeste durante le fasi dell'eclisse. su m mary on the occasion of the 1961 total solar eclipse a programme of research had been elaborated by the istituto nazionale di geofisica, the monte mario astronomica1 observatory and the italian ministri/ of air meant to measure the light diffused by the atmosphere and to talee photograplis of the corona of the sun from high altitudes. this research was carried out by sending info the upper atmosphere a photographic camera of the all-shy type. in this paper the apparatus are described and some results of tliis flight are reported with some photograplis taken during the eclipse. in order to study the secondari/ diffusion the illumination of the celestini vault during the eclipse are briefly discussed. b i b l i o g r a f i a (1) r. c i a l d e a , ann. dì geofis., v, 409, (1952). r. cialdea, p . dominici, ann. di geofis., v, 581, (1952). (2) r. c i a l d e a , ann. di geofis. ix, 451. (1956). annals 48, 2, 2005defdef 321 annals of geophysics, vol. 48, n. 2, april 2005 key words electron density model – vertical incidence ionograms – ionospheric data ingestion 1. introduction one of the most unpredictable sources of error for single frequency satellite navigation and positioning systems is due to the ionosphere. a good knowledge of the behavior of the ionospheric regions is essential to obtain a better representation of parameters like the total electron content (tec) which is directly proportional to the time delay of electromagnetic signals. a technique to reconstruct the spatial and temporal structure of the electron concentration in the ionosphere has been developed using the nequick ionospheric electron density model driven by an «effective ionization parameter» called az (nava et al., 2003). nequick is a quick-run model for ionospheric applications developed at the abdus salam ictp in trieste (italy) and the university of graz (austria). it has been used by the european space agency esa satellite navigation and positioning programs and adopted by recommendation p.5316 of the itu-r (international telecommunication union, radiocommunication sector) (now superseded by p.531-7; itu, 2001). az values are determined minimizing the differences between experimental and modeled vertical tec global maps. the result is a global grid of az values for a specific condition which allows us to calculate with the nequick model the electron density at any point in the ionosphere including the f2 peak parameters values. the aim of this paper is to compare these calculated f2 peak values with independent and simultaneous global observations in order to validate the technique. these measurements, the critivalidation of a method for ionospheric electron density reconstruction by means of vertical incidence data during quiet and storm periods gloria miró amarante, sandro m. radicella, bruno nava and pierdavide coïsson the abdus salam international centre for theoretical physics (ictp), trieste (italy) abstract a preliminary validation of the technique developed using the nequick ionospheric model and the «effective ionization parameter» az, based on vertical total electron content data ingestion, was carried out in a previous study. the current study was performed to extend the analyzed conditions and confirm the results. the method to validate this technique is based on a comparison between hourly f2 peak values measured with vertical incidence (vi) soundings and those calculated with the new technique. data corresponding to different hours and seasons (equinox, summer solstice, and winter solstice) during the period 2000-2003 (high and medium solar activity conditions) were compared for all available ionosonde stations. the results show a good agreement between fof2 and hmf2 values obtained with the new technique and measurements from vertical incidence soundings during quiet and storms conditions. mailing address: dr. gloria miró amarante, the abdus salam international centre for theoretical physics (ictp), strada costiera 11, 34014 trieste (italy); e-mail: amarante@ictp.it 322 gloria miró amarante, sandro m. radicella, bruno nava and pierdavide coïsson cal frequency in the f2 region (fof2) and the height of the maximum of ionization (hmf2), are obtained experimentally from ground-based vertical ionospheric sounders located around the world. a preliminary validation was done (miró amarante et al., 2003) using data corresponding to four quiet days in the year 2002 (high solar activity conditions). the main results and conclusions of this analysis will be included in this paper. 2. ionospheric parameters measured with vertical incidence soundings the didbase software developed by lowell university lets us extract ionograms from a network of digisondes distributed around the world. this possibility allows manual scaling of the available ionograms to measure the real values of the main ionospheric parameters (critical frequency, height, ionospheric total electron content itec: huang and reinisch, 2001). the analysis has been done for different conditions summarized in table i. for each period, all the available ionospheric stations were selected (fig. 1). the number of locations varies from 13 during the period of march 2000 up to 27 for the 3rd of june 2002. the manual scaling of the ionograms (almost 8300 ionograms) was done for each considered case and fof2 and hmf2 were extracted to be compared with the reconstructed values. 3. ionospheric parameters calculated with the reconstruction technique two global grids corresponding to fof2 and hmf2 were obtained from the nequick model driven by the «effective ionization parameter» az. this parameter is calculated by means of the technique described in nava et al. (2003) in such a way that the differences between experimental and nequick model vertical tec are minimum. the experimental vertical tec maps used for the minimization are generated with two different techniques. the first is the one developed by code (centre for orbits determinations, http://www.cx.unibe.ch/aiub/ionosphere.html) with two hour intervals and the second one was generated by the university of la plata, argentina with one hour intervals (meza et al., 2002; and brunini et al., 2004). the calculated f2 peak values at the available digisonde locations (fig. 1) are extracted fig. 1. observing sites available for the periods analyzed. table i. list of selected periods for the validation of the reconstruction method. day month year ap dst sunspot number 25 03 2000 8 − 9 185 26 03 2000 3 − 1 170 27 03 2000 4 1 155 28 03 2000 4 1 169 15 04 2002 6 − 8 138 03 06 2002 10 − 21 133 11 10 2002 6 − 38 121 25 11 2002 15 − 46 56 19 11 2003 12 − 179 70 20 11 2003 150 − 152 90 21 11 2003 42 − 76 97 22 11 2003 30 − 63 83 23 11 2003 22 − 38 109 323 validation of a method for ionospheric electron density reconstruction by means of vertical incidence data from these two global grids by interpolation since the az maps have a grid spacing of 2.5° in latitude and 5° in longitude. 4. comparison between measured and calculated peak values as an example, the critical frequency fof2 and the maximum height hmf2 for millstone hill (middle latitude northern hemisphere) and grahamstown (middle latitude southern hemisphere) are shown in fig. 2a-d (quiet period) and fig. 3a-d (storm period). calculated f2 peak values using code (fof2code) and la plata (fof2laplata) vertical tec maps are plotted together with the experimental ones (fof2sao, obtained with sao-explorer, http://ulcar.uml.edu/ sao-x/sao-x.html). the figures show a good agreement between both fof2 data sets (fof2code-fof2sao and fof2laplata-fof2sao) during quiet conditions while the differences between calculated and experimental values increase for geomagnetic disturbed conditions. the results are considerably worse in the case of hmf2. however, this is not surprising. nequick uses the ccir (itu-r) maps for fof2 and m(3000)f2 and an internal map for foe to calculate hmf2 by means of dudeney’s form of the bradley and dudeney (1973) formula (see dudeney, 1983) which works quite well for average (monthly median) conditions but can give larger errors in individual cases. minimizing differences in electron content also means some minimization for fof2 but hmf2 is not affected. the height error of true height analysis of ionograms can also be quite large. the left panel of fig. 4 presents the scatter plot of fof2 (f2 peak electron density) reconstructed with the nequick model against the corresponding fof2 ionosonde measurements for quiet periods (2000 and 2002). this example fig. 2a-d. fof2 (a) and hmf2 (c) for millstone hill (middle latitude northern hemisphere); fof2 (b) and hmf2 (d) for grahamstown (middle latitude southern hemisphere), during march 2000 (quiet period). b c d a fig. 4. linear regression fitting for reconstructed and experimental fof2 during quiet (left) and storm (right) conditions. 324 gloria miró amarante, sandro m. radicella, bruno nava and pierdavide coïsson fig. 3a-d. fof2 (a) and hmf2 (c) for millstone hill (middle latitude northern hemisphere); fof2 (b) and hmf2 (d) for grahamstown (middle latitude southern hemisphere), during november 2003 (storm period). corresponds to code experimental vertical tec maps. the scatter plot shows a high degree of correlation between the two independent estimates of fof2. the line drawn corresponds to the best-fit line. examination of the intercept of the best-fit line shows that on average the fof2nequick values exceed those from digisondes by ∼1.16 mhz during quiet period. the a c d b 325 validation of a method for ionospheric electron density reconstruction by means of vertical incidence data storm conditions are also shown on the right panel in fig. 4. this example corresponds to 20th november 2003 (ap index=150) and shows how the agreement between both data sets is also obtained for a very disturbed day. it is important to point out that this storm occurred under middle solar activity conditions (sunspot number = 90) and the analysis was done considering together all the ionospheric effects (positive and negative storms) distributed around the world. the selected data are classified to create distributions with different conditions of solar activity, vertical tec mapping technique and quiet or storm periods. the results of the linear regression fitting corresponding to each distribution are summarized in tables ii and iii for fof2 and hmf2 respectively. the correlation coefficients r make clear the difference in accuracy for reconstruction of fof2 and hmf2. in the case of the critical frequency the correlation coefficient is approximately 0.90 which means that the fitting between both data set is excellent. however this value does not exceed 0.70 for the peak height possibly for the reasons indicated above when discussing the results shown in figs. 2a-d and 3a-d. the fof2 reconstruction is also good during the studied storm period with values higher than 0.86. it must be noted that regardless of the actual values of the correlation coefficient all the results indicate a very high statistical significance. the comparison between the two vertical tec mapping techniques (code and la plata) does not show a clear dependence on the techtable iii. linear regression fitting results for hmf2 (hmf2nequick = a∗hmf2ionosonde + b; r is the correlation coefficient and n the number of selected data). hmf2 (km) la plata code period a b r n a b r n calm 2000 0.34 187.7 0.53 1094 0.35 183.9 0.48 546 2002 no data 0.55 128.6 0.60 529 19.11.2003 0.75 70.0 0.70 325 0.72 72.5 0.68 165 20.11.2003 0.62 129.4 0.50 279 0.56 141.9 0.46 140 storm period 21.11.2003 0.72 90.1 0.50 242 0.80 68.2 0.52 113 22.11.2003 0.76 80.1 0.66 324 0.72 83.1 0.67 164 23.11.2003 0.72 90.2 0.68 323 0.62 113.3 0.66 156 table ii. linear regression fitting results for fof2 ( fof2nequick = a∗ fof2ionosonde + b; r is the correlation coefficient and n the number of selected data). fof2 (mhz) la plata code period a b r n a b r n calm 2000 0.98 0.20 0.89 1096 1.01 − 0.10 0.91 547 2002 no data 0.94 1.44 0.91 936 19.11.2003 0.90 − 0.06 0.91 469 0.96 − 0.04 0.93 241 20.11.2003 0.96 0.12 0.89 402 1.00 0.24 0.89 204 storm period 21.11.2003 0.76 0.74 0.86 379 0.79 0.98 0.87 181 22.11.2003 0.91 0.17 0.88 463 0.96 0.23 0.89 232 23.11.2003 0.86 0.48 0.89 457 0.92 0.53 0.90 225 326 gloria miró amarante, sandro m. radicella, bruno nava and pierdavide coïsson nique of the vertical tec map used to derive az values. the number of points corresponding to la plata technique is higher because these maps are obtained hourly. the dependence on solar activity is only noticeable for hmf2 where the fitting improves when solar activity decreases. however, further analysis considering low solar activity and storm conditions under very high solar activity should be done in the future. 5. conclusions the comparison of f2 peak values generated by the new technique of 3d reconstruction of the electron density and the corresponding vertical incidence ionosonde data covering a wide area of different geographical, seasonal and hourly conditions indicates that: – the critical frequency of the f2 layer shows a very good agreement between both data sets during high and middle solar activity quiet periods. – this agreement is also good for the november 2003 storm considering together all the ionospheric effects (positive and negative storm effects) distributed around the world. – the hmf2 parameter shows a worse agreement than fof2 values and the worst results have been found for high solar activity. – the results obtained are not dependent on the technique of the vertical tec maps used, code and la plata. therefore, it has been demonstrated that the new technique can be used to build scenarios that reproduce global ionospheric conditions in a realistic way. acknowledgements the aeronomy and radiopropagation laboratory group of the abdus salam international centre for theoretical physics is grateful to the university of la plata for providing their vertical tec maps and to the ionospheric groups (lowell university (u.s.a.), national observatory of athens (greece), radio research laboratory (sinpil seolsung ichon kyunggi, republic of korea), rhodes university (grahamstown, south africa), communications research laboratory (tokyo, japan), rutherford appleton laboratory (chilton, united kingdom), istituto nazionale di geofisica e vulcanologia (rome, italy), inta (huelva, spain), ebro observatory (spain), casleo (san juan, argentina)) from whom we received data used in this study. one of the authors (g.m.a.) also wants to thank to the marie curie fellowship of the european community fifth framework programme under contract no. hpmf-ci-2002-01867 for supporting this research. references bradley, p.a. and j.r. dudeney (1973): a simple model of the vertical distribution of electron concentration in the ionosphere, j. atmos. terr. phys., 35, 2131-2146. brunini, c., a. meza, f. azpilicueta, a. van zele, m. gende and a. diaz (2004): a new ionosphere monitoring technology based on gps, astrophys. space sci., 290 (3), 415-429. dudeney, j.r. (1983): the accuracy of simple methods for determining the height of the maximum electron concentration of the f2-layer from scaled ionospheric characteristics, j. atmos. terr. phys., 45, 629-640. huang, x. and b.w. reinisch (2001): vertical electron content from ionograms in real time, radio sci., 36 (2), 335-342. itu (2001): ionospheric propagation data and prediction methods required for the design of satellite services and systems ionosphere characteristics, recommendation p.531-6, approved in 2001-02, managed by itu-r study group sg3 (version 6 superseded in 2004 by version 7). meza, a., c.a. brunini and a. klaeusberg (2002): global behaviour of the ionosphere electron density using gps observations, adv. space res., 30 (2), 307-312. miró amarante, g., s.m. radicella, b. nava and p. coïsson (2003): validation of a new method for ionospheric electron density reconstruction by means of vertical incidence data, in proceedings of ‘atmospheric remote sensing using satellite navigation systems’, special symposium of the ursi joint working group, matera (fg), italy, 335-342. nava, b., p. coïsson, g. miró amarante and s.m. radicella (2003): a new model assisted method for ionosphere electron density reconstruction, in proceedings of ‘atmospheric remote sensing using satellite navigation systems’, special symposium of the ursi joint working group, matera (fg), italy, 343-348. (received october 18, 2004; accepted march 19, 2005) 1 annals of geophysics, 65, 5, se534, 2022; doi: 10.4401/ag-8802 o p e n a c c e s s spatio-temporal analysis of b-value prior to 28 april 2021 assam earthquake and implications thereof vickey sharma1, dipok k. bora2,3, rajib biswas*,1 (1) applied geophysics lab, department of physics, tezpur university, tezpur-784028, india (2) department of physics, diphu government college, karbi anglong-782 460, assam, india (3) government model college, deithor, karbi anglong-782 480, assam, india article history: received february 11, 2022; accepted june 14, 2022 abstract here, we report a spatio-temporal analysis of the frequency magnitude distribution of earthquakes (b-value) before the 28th april 2021 (mw 6.4) earthquake event observed in northeast india. to estimate the average b-value for the study region, a data set of 750 earthquake events with magnitude mw ≥ 3.9 is extracted from the homogenous part of the earthquake catalog (1950-2021) documented by the united states geological survey (usgs) and international seismological center (isc) in the region. for spatial analysis of the disparities in b-value, the whole study region is subdivided into 16 square grids of dimension 1o×1o and the b-value is calculated for each subsection. in congruence with other studies, this work yields b-values ranging from 0.66 to 1.25. after the calculation of the b-value for each grid, it is observed that the grid with the epicentral location of the 28th april 2021 (6.4) earthquake has a low b-value. accordingly, the spatial correlation and aberrant pattern between b-value and focal depth have been comprehensively explored. it is observed that the b-value significantly dips within a depth range of ~15-35 km which implicates high-stress accumulation and crustal homogeneity. the depth-wise variation in b-value infers the antithetical relationship between b-value and crustal stress. mostly interplate earthquakes are observed in the study region; thereby hinting at intense seismicity at the upper crust. keywords: b-value; seismicity; fault mechanisms; stress; seismotectonics 1. introduction earthquakes are very frequent in northeast (ne) india. during the past 100 years, this region has experienced 18 large earthquakes. the 12 june 1897 shillong earthquake (mw 8.1) [england and bilham, 2015] and the assam-tibet earthquake of 15 august 1950 (mw 8.4) [ajanta and farha, 2019] caused large human causalities along with severe property losses. in ne india, the mikir hills plateau flanks two major faults namely the bomdila fault in the east and the kopili fault to the west. both these faults are characterized by strike-slip kinematics. a recent study [sharma et al., 2018] inferred that the strike-slip fault can produce major earthquakes (m > 8). kopili fault and its neighboring region vickey sharma et al. 2 which is our study region are one of the most seismically active regions of ne india and the region is bounded by latitude 24°-28°n and longitude 900-94°e [mahanta et al., 2012]. roy and purohit [2018] inferred that the subduction of the indian plate under the eurasian plate causes various tectonic disturbances in this region. the seismic analysis of this region is important for a better understanding of seismic hazard assessment. the widely established frequency-magnitude distribution equation after [gutenberg and richter, 1944] – i.e., log10n(m) = a – m links the cumulative number of earthquakes (m) with magnitude m with two important coefficients, namely a and b. in this relation, a takes care of seismic activity whereas b is identified as a unique characteristic parameter intrinsic to a group of events. suitably, geoscientists term it as the b‑value parameter. various studies have been carried out to investigate the relationship between b-value and other parameters including crustal stress, heterogeneity of earth crust, seismic moment, bouguer gravity, and focal mechanism solution for northeast india. in a recent work, khan and chakraborty [2007] show the correlation between b-value and bouguer gravity anomaly for the shillong region, while bhattacharya, majumdar and kayal [2002] show the correlation between seismic b-value and fractal dimension for entire northeast india. a similar attempt was made by khan, ghosh, chakraborty, mukherjee [2011] which evaluates the seismic b‑value for entire northeast india and also assessed the stress level, and finally established a correlation between b-value and seismic moment (mo). bora, borah, mahanta, and borgohain [2018] show a correlation between b-value and seismic moment, focal mechanism solution, and bouguer gravity for the ib region of northeast india in a recent paper. similar research has been done in other seismically active places throughout the world. in recent work mohammed salih al-heety,and mohammad, [2021] showed the reliance of b-value over focal depth and focal mechanism solution at a global level. on a similar note, b-values are linked with crustal properties as well as stress magnitudes. according to laboratory research [kamer and hiemer, 2015; schorlemmer et al., 2005], seismic b-values range from 0.5 to 1.5. furthermore, for the tohoku (2011) and sumatra (2004) earthquakes, nanjo, hirata, obara, and kasahara [2012] assessed the phenomena of drop-in b-value before the mainshock as an earthquake precursor. borgohain, borah, biswas and bora [2018] conducted a similar study for the manipur (2016, 6.7) earthquake, however, they only looked at a small area and didn’t look into the seismic hazard potential of the kopili lineament. all known b-value studies for northeast india cover a vast area. furthermore, when dealing with b-value estimation at the regional level, macroscale heterogeneities are frequently overlooked. there lies an obvious dependence of b-values along with macroscale heterogeneities as well as the allied seismicity associated with it. considering all these into consideration, we present here a detailed study where the variation of the b-value and its reliance on focal depth in the kopili fault and neighboring region before the 28th april 2021 assam earthquake (mw 6.4) is highlighted. the interplate model for this region showing all the major earthquakes is also projected in this study. starting from the estimation of the average b-value, we include the macroscale heterogeneities to formulate a correlational approach with b-value. further, the anomalies before the b-value hitherto are also delineated. the study has been systematically ordered. first section deals with the tectonic setup; followed by the data used in this study. next section deals with the estimation of the b-value with ample illustrations. the results and discussions follow thereof. 2. tectonic setup the ne india as shown in figure 1 is characterized by high seismic activity. subduction of the indian plate under the eurasian plate results in complex seismicity of this region [roy and purohit, 2018]. the topological map of the study region is shown in figure 2. the unabridged study area consists of three major tectonic entities with some inimitable tectonic features. the complete study region is bounded by latitude 24°-28°n and longitude 90°-94°e which mainly consists of a part of a) the eastern himalayan zone b) the assam valley region c) the shillong plateau region. the mikir hill plateau encompassed the two most seismic active faults including the bomdila fault and the kopili fault. as per reports, the length of nw-se trending kf emerges to be 300 km with 50 km width. it covers the western part of manipur to the trijunction of bhutan, arunachal pradesh, and assam. the 2009 bhutan earthquake has been established to be stemming from kf [kayal, bora, et al., 2010]. there is evidence of active seismicity in the kf region with further expansion towards mct in the bhutan himalaya. meanwhile, higher activity is observed in kopili fault at the junction of mbt and mct. the august 19, 2009 earthquake bears testimony to this with the further occurrence of the 21st september 2009 earthquake whose epicenter lied at the northern juncture of the kopili fault with mct. as per reports, both these seismic occurrences are typified as shallow focus with faulting of right-lateral strike-slip nature. this can be envisaged as an ongoing compressional stage for spatio-temporal analysis of b-value 3 kopili fault towards the east from the indo burmese arc as well as towards the north from the himalayan arc. as per [sarma et al., 2018], the seismogenic zone transcends down to 47 ± 2 km for kopili fault. however, there is an increase in focal depth as we move from the northern burma region to the mct. figure 2 illustrates the current seismicity concerning kopili fault and neighboring regions along with prominent tectonic signatures. the yellow stars show the epicentral location of the 28th april 2021 earthquake (mw 6.4). 3. data analysis the earthquake catalog from 1950 to 2021 of 1398 events has been combined from the international seismological center (isc) http://www.isc.ac.uk/ and the united states geological survey (usgs) https://www.usgs.gov/. this region is bounded by longitude (90°-94°) e and latitude (24°-28°) n. the magnitude of these events ranges from 2-8. the well-known model [reasenberg, 1985] is used for separating the mainshocks or the background earthquakes from after/foreshocks or triggered earthquakes. the conversion relationship has been used to convert various scales i.e mw, mb, ml, md, ms into moment magnitude scale (mw) as suggested by nath et al. [2017] and bora [2016]. before calculating the b-value for the study region, another important parameter namely the magnitude figure 1. the topological plot of the ner of the indian subcontinent shows various faults and thrusts. the stations set up by the national center for seismology (ncs) in this region are shown by yellow for assam state and red triangles for meghalaya. the prominent tectonic features in this region include; mct: main central thrust, mbt: main boundary thrust, lh: lohit thrust, mt: mishmi thrust, kf: kabaw fault, sf: sagaing fault, df: dauki fault, sf: sylhet fault, dt: dapsi fault, dhf: dudhoni fault, dhubri fault, tista fault, kaladan fault, chittagong coastal fault, of: oldham fault, bs: barapani shear zone, nt: naga-disang thrust, kopili fault. the major thrusts located are shown by the teeth lines. inset map showing the highlighting study region. http://www.isc.ac.uk/ https://www.usgs.gov/ vickey sharma et al. 4 of completeness (mc) is estimated. this parameter is still considered to be crucial for all seismic-based studies as it helps in optimizing the number of events available and gives a reliable result [wiemer and wyss, 2000]. the magnitude of completeness (mc) is the value at which 100% of earthquakes in a space-time volume are detected [wiemer and wyss, 2000; woessner and wiemer, 2005; rydelek and sacks, 1989]. mousavi [2017a] while mapping seismic moment and b-value for iranian plateau established that there happens variation of mc due to difference in time of detection by different stations. the recent studies [das, wason, and sharma, 2011; iwata, 2013; woessner and wiemer, 2005] inferred that an increase in the number of earthquake events can lower the mc value which applies well here. as the number of stations in this region augments the detection of small earthquake events in this region the mc value for this region also decreases over the time. thus, an accurate estimate of mc is important since a very high value will lead to under-sampling, and a very low value considers erroneous seismicity parameter values. in this study, the maximum-curvature method has been used to calculate the mc for the whole data set [wiemer and wyss, 2000] with bootstrapping [schorlemmer et al., 2003]. the correction factor mc = mc (maxc) + 0.2 as proposed by woessner and wiemer [2005]. for most seismic‑based studies, it becomes important to first estimate the value of the magnitude of completeness above which all events are considered for seismic assessment and ultimately this helps to get a reliable result. the completeness magnitude varies as a function of time and space, so particularly figure 2. the tectonic plot of the study region. the major earthquake events that happened in this region are shown by red stars. the epicentral location 28th april 2021 earthquake is shown by the yellow star. mbt: main boundary thrust, mct: main central thrust, of: oldham fault, kf: kabaw fault, nt: naga-disang thrust, sf: sagaing fault, df: dauki fault, sf: sylhet fault, dt: dapsi fault, dhf: dudhoni fault, bs: barapani shear zone, kopili fault. the thrust lines are shown by the teeth lines. spatio-temporal analysis of b-value 5 the temporal changes can potentially produce errors in the estimations of seismicity parameters, most commonly the b-value. the fluctuation of mc as a function of time is depicted using the zmap tool for seismic data acquired from 1950 to 2021, as shown in figure 3. figure 3. plot of the magnitude of completeness (mc) variation with time. standard deviations are shown by dashed lines. the windowing has been done with 200 samples based on the distribution in the catalog. a closer examination reveals a significant drop in mc value after 1993. this could be attributed to a network modification that included the construction of larger receiver sites, as well as severe historical seismic activity in this region, which resulted in a higher number of events being recorded after 1993 as illustrated in figure 4. figure 4. magnitude (mw) versus time for the research area. the graphic shows a sharp spike in incidents after 1993. vickey sharma et al. 6 figure 5. plot of the cumulative number of earthquake events as a function of time of the region. the cumulative events’ curve shows (figure 5) an increase in earthquake events observed from 1950 to 2021 and the major events are shown by a star having a magnitude of mw ≥ 6.4. considering this, we use this section of the temporal curve to complete mc based on the maximum curvature method with subsequent bootstrapping [wiemer and wyss, 2000]. accordingly, the value comes out to be 3.9 with the correction of mc = mc (maxc) + 0.2 as suggested by woessner and wiemer [2005] (figure 6). keeping mw ≥ mc as the standard, we have chosen 750 events for onward computation of the b-value. figure 6 illustrates the g‑r power fit law using the maximum likelihood estimate (mle) [aki, 1965] to check the linearity of data for the period 1950-2021 with magnitude mw ≥ mc. figure 7 shows the epicentral location of all the seismic events from 1950 to 2021 having mw ≥ mc. figure 6. fmd of earthquakes from 2000 to 2021. the straight line is the best fit (gutenberg and richter, 1944). the mc value shows the magnitude of completeness and the b-value gives the average b-value for the complete study region. spatio-temporal analysis of b-value 7 figure 7. the epicentral location of all the earthquake events observed in this region (1950-2021) having mw ≥ mc is shown in the plot. 4. b‑value estimation among numerous methods, the two most reliable and well-known methods for the estimation of b-value are a) least-square fit method and b) maximum likelihood method. the frequency magnitude distribution follows the principle of gutenberg and richter [1944] relation. the present work is based on the method suggested by aki [1965] i.e maximum likelihood method. this method is best suitable for a small geographical region and time window [aki, 1965]. the relationship used for the estimation of the b-value is given by: (1) vickey sharma et al. 8 in the above relationship, there are many parameters of significance. implies average magnitude corresponding to a group of events with magnitude mw greater than the magnitude of completeness mc. likewise, mbin stands for binning width. standard deviation δb of the b‑value was given by aki [1965], and then modified form was given by shi and bolt [1982] which is as follows: (2) where ‘n’ is the total number of events of the given sample. for the spatial variation of the b-value, the entire study region is subdivided into 16 equisized grids of 1o×1o dimensions. the moving window of 0.5° × 0.5° has been chosen to retain the natural grid to grid continuity of data points. several available reports [kamer, 2014; kamer and hiemer, 2013; mousavi, 2017b; schorlemmer et al., 2004; borgohain, borah, biswas and bora, 2018] opined that sufficient data should be present for reliable and better analysis of b-value; otherwise, less quantity of seismic data may result in undesired and incorrect results. as a result, accuracy, and coverage suffer. thus, adequate seismic data is required for a better and high‑quality outcome. zmap tool [wiemer, 2001] is used for the computation of the b-value for each grid. table1 depicts the b-value for each grid and the events that are used for the calculation are shown in figure 8. the b-value variation as a function of time is shown in figure 9. the concept of significant fall in b-value prior to any main event can be established from the figure 9. figure 8. the epicentral location of all the earthquake events recorded in the study region from 1950 to 2021 listed in the table1 is shown in the figure. the longitude‑wise distribution of these earthquakes is shown by yellow, green, blue, and red marks. spatio-temporal analysis of b-value 9 longitude (°e) latitude (°n) nmin b-value 90-91 24-25 14 0.95 90-91 25-26 63 1.25 90-91 26-27 53 0.94 90-91 27-28 23 0.76 91-92 24-25 49 0.80 91-92 25-26 58 0.84 91-92 26-27 61 0.88 91-92 27-28 60 0.71 92-93 24-25 52 0.68 92-93 25-26 31 0.73 92-93 26-27 139 0.76 92-93 27-28 155 0.72 93-94 24-25 129 0.76 93-94 25-26 49 0.66 93-94 26-27 65 0.83 93-94 27-28 33 0.88 table 1. spatial variation of b-value for study region. the relationship between the b-value and focal depth is also examined. the next section presents a more in-depth analysis of these results. figure 9. the time series projects the variation in b‑value for the study region. a significant fall in b-value can be traced. the standard deviation is shown by the dashed line. vickey sharma et al. 10 5. results and discussions furthermore, recent studies [khan and chakraborty, 2007; bhattacharya, pankaj majumdar and kayal, 2002; khan, ghosh, chakraborty, mukherjee, 2011; bora, borah, mahanta and borgohain 2018; mohammed salih al-heety and mohammad, o. 2021; tormann, enescu, woessner, wiemer, 2015] inferred that the frequency magnitude distribution factor (b-value) can vary with several factors including depth, stress accumulation, plate tectonics, and faulting style mechanism. due to the subduction of the indian plate under the eurasian plate, this region remains under high stress [sharma and baruah, 2017]. figure 10 appraises the variation of b-values with focal depths of the seismic events. the zmap tool [wiemer, 2001] has been used to scrutinize the b-value as a function of depth keeping the minimum magnitude of completeness value fixed. 8 slabs with each slab having a focal depth of 10 km are projected by virtually dividing the area under investigation. the b-value is plotted against each slab as projected in figure 10. the lowest b-value is associated with the upper crust. similar implications were made by khan et al. [2011] for the entire ne india and quoted that a low b-value is observed at a depth range of 25 km to 36 km. the lower b-value detected at a deeper depth range for the southwest part of the study area may be allied with the rising protuberant of the lithosphere in this region. the low b-value in the upper crust implies crustal homogeneity. we encounter a low value of 0.66-0.71 confined to the upper crust. the obtained result is plausible with the results that the accumulation of crustal stress in the upper crust is more associated with the in-depth region. figure 10. graph showing the variation in frequency magnitude distribution constant (b-value) concerning depth for the study region. spatio-temporal analysis of b-value 11 the interplate model helps us to approximate the location of the 28th april 2021 (mw 6.4) earthquake (figure 11). it is an interplate earthquake with a reported depth of ~34 km [biswas, 2021]. mostly interplate earthquakes are observed in this region (figure 11). it is stated that crustal homogeneity and high seismic moment are implicated by lower b-values. notably, the 2021 event occurred at a depth of 34 km, hinting at the accumulation of stress in the upper crust of the subduction region. as per hurukawa et al. [2012], subcrustal earthquakes showed depth ranges from 20 to 50 km. meanwhile, in another study by montessaro and kulhaneck, [2003]; higher b-values were obtained at intermediate depths of 80-100 km beneath guatemala el-salvadore as well as for deeper depths of 130-170 km about nicaragua. a recent study by bora et al. [2018] reported that the b-value is < 1 down to 50-55 km depth, and there is a sharp rise in b-value near 60 km depth in indo-burma ranges. in similitude to these observations, the current study attains higher b-values in intermediate depth (40-45 km) as well as for deeper depth regions. the two studies conducted by wiemer figure 11. the seismotectonic model shows the epicentral location of all the events with magnitude mw ≥ 6.5 observed in this region. the epicentral location of the recent 28th april 2021 (6.4) earthquake is shown by a red star. figure 12. histogram showing depth-wise variation corresponding to chosen 750 events (mw ≥ 3.9). the intense seismicity is reported at the focal depth of 30 km to 50 km. vickey sharma et al. 12 and benoit [1996] and wyss et al., [2001] in new zealand and alaska respectively report high b-value anomalies on top of the subducting slabs at depths within 100-150 km depth range. further, it is observed that the number of events with focal depths greater than 70 km is in a declining trend. lower tectonic stress at deeper depth with the accompaniment of a high b-value could be one of the prominent causes of this observation. the depth histogram as shown in the figure 12 shows that there is a significant rise in earthquake events for the depth range 30 km to 50 km and the maximum peak is observed at the focal depth of 35 km and the 28th april 2021 (mw 6.4) is also reported at the focal depth of 34 km. the same implications are reported by khan et al. [2011] that earthquakes events in the study region occurred at the focal depth of 30 km to 50 km. likewise, figure 13 reveals that the number of earthquakes having magnitude mw~(4-5) is more and only a few events having magnitude mw ≥ 5 are observed in this region. the spatial variation of the b-value for the study region is shown in figure 14. the result shows that the b-value varies from 0.66 ± 0.09 to 1.24 ± 0.11 for the study region. khan, ghosh, chakraborty and mukherjee [2011] inferred that the b-value for northeast india varies from 0.23 to 1.78. similar implications were made by bora, borah, mahanta and borgohain [2018] for the indo-burma region in which the b-value varies from 0.7 to 1.5. figure 13. histogram shows the number of earthquakes versus magnitude for the selected 750 events (mw ≥ 3.9). mostly small magnitude earthquakes with some large magnitude earthquakes are observed in this region. thus the observations made from our study show a plausible relationship with already existing results. a recent study [borgohain, borah, biswas and bora, 2018] reported an intermediate b-value before the manipur earthquake (2016).in the present study, low b-value variation is also observed in the regions between 25-26°n and 93-94°e. these have been suggested to be due to large variations in tectonic stress owing to local variations in the plate tectonic driving forces. ruff and kanamori [1980] successfully explain the seismicity and the subduction process by testing the correlation between coupling and other physical features of subduction zones. in contrast, a significant low variation of b-values is observed (table1) around the epicenter of the 28th april 2021 (mw 6.4)assam earthquake (lat ~26.781°n, long ~92.457°e), where the b-value concentrates around 0.76. the implications made after a globalscale study of b-value for the large region show b-value near to unity for any seismically active region but a recent study [khan, ghosh, chakraborty and mukherjee 2011] shows the variation in b-value up to 1.78. our results show good agreement with the already existing studies for the northeast indian region [eg. khan, ghosh, chakraborty and mukherjee, 2011; bora, borah, mahanta and borgohain, 2018]. certain areas of the study area embody higher stress accumulation relatively. this may be treated as an implication of future impending larger ruptures related to these locations. since the kopili fault and its neighboring areas are active regions, the current work will prove to be useful for earthquake prediction and seismic hazard assessment. spatio-temporal analysis of b-value 13 6. conclusion in the current work, a sub sectional approach has been implemented for the formulation of frequency magnitude distribution factor (b-value) as an earthquake precursor. the entire study region is found to have a b-value (size distribution) of 0.98 ± 0.02 with a magnitude of completeness of 3.9. the maximum curvature (maxc) approach is used to calculate the minimal magnitude of completeness (mc). unreliable results can emerge from an incorrect estimation of the minimum magnitude of completeness. as a result, every precaution was taken to obtain an accurate estimate of the minimum magnitude of completeness. the estimated b-values are then assessed to build a correlation with the focal depth of the events pertinent to the kopili fault and its neighboring region of the north-eastern region of india. the earthquake precursor character of change in frequency magnitude distribution (b-value) can be best described by dividing the whole study region into subsections. accordingly, 750 well-located events have been utilized to carry out this study. as per findings, the b-value varies from 0.66 to 1.25 within the region. in addition, a significant low b-value (0.76 ± 0.03) was observed around the epicentral location of the 28th april 2021, mw 6.4, assam earthquake; thereby implying accumulation of higher stress. in the upper crust low b-values (0.66-0.76) are found, which indicate homogeneity and high-stress accumulation. high seismic moment release in the uppermost crust is another reason for the low b-value. our study on the spatio-temporal variation of the b-value could be used as an earthquake precursor before any mainshock which will ultimately help in earthquake prediction and seismic hazard assessment for this particular region. figure 14. b-value contour map for the study region. the epicentral location of the 24th april 2021 earthquake is shown by the yellow star. vickey sharma et al. 14 acknowledgment. all the maps are drawn using generic mapping tools (gmt) [wessel et al., 2013] and the graphs are plotted using zmap [wiemer and wyss, 1997]. references aki, k. 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uncertainty, bull. seismol. soc. am., 95, 2, 684-698. wyss, m., s. wiemer and f. zúñiga (2001). zmap a tool for analyses of seismicity patterns. typical applications and uses: a cookbook by m. wyss, s. wiemer and r. zúñiga. *co r r e s po n d i n g a u t h o r: rajib b i s wa s, applied geophysics lab, department of physics, tezpur university, tezpur-784028, india e-mail: rajib@tezu.ernet.in mailto:rajib@tezu.ernet.in vol52,6,2009new 699 annals of geophysics, vol. 52, n. 6, december 2009 key words horizontal pendulums – giulio grablovitz, ebre observatory – cartuja observatory – spain 1. introduction giulio grablovitz (1846-1928), born in trieste, was the director of the first italian «geodynamic station» founded in 1885 at casamicciola, on ischia island, in the bay of naples. his scientific research is mainly remembered for its contributions to seismometry and also to tides and tide gauge studies. he was in touch with other earth scientists of his time and, from his observatory at ischia, he maintained regular correspondence with many geophysicists. of our interest, in the first days of may of the year 1906 a letter from g. grablovitz was received at ebre observatory, sited in the ne part of spain. it referred the registration of the san francisco earthquake at this observatory and stated: «ischia 26 aprile 1906 / ch.mo collega ed amico / ricevo con grande soddisfazione il suo primo bollettino sismico coi dati della registrazione del terremoto di california e sono lieto di vedere che i miei pendoli hanno funzionato a dovere. ciò significa ch’ella l’ha collocati in tutta regola e […] faccio i miei complimenti. / qui pure ho avuto una registraziane analoga…» «[ischia, 26 april 1906 / dear colleague and friend / i’ve received with a great pleasure your first seismic bulletin with data of the california earthquake and i’m proud to see that my pendulums recorded it properly. this means that they have been installed properly and […] i present my compliments. / here we also had a similar record …]». this letter, reproduced in fig. 1, and some others preserved as well, shows an already well established relation between grablovitz and the ebre observatory, where a copy of the horizontal pendulums already existing at ischia was installed in 1905. influence of giulio grablovitz in spain: instruments and scientific correspondence josep batlló cgul-idl, universidade de lisboa, portugal abstract it is known that giulio grablovitz founded and operated a «geodynamic» station on ischia island from 1885 up to his death. but it is almost unknown that one of the instruments he designed, the horizontal pendulum, was installed at ebre observatory in spain. records from this instrument and correspondence between grablovitz and the ebre observatory are still preserved. these documents show how the research work done by grablovitz was introduced and diffused in spain. also, they give us some new information on the construction of the seismic instruments designed by grablovitz. the present paper deals with the analysis of the preserved materials, assesses the introduction of grablovitz research in spain and shows the importance of the use of manuscript sources for the enhancement of current research in seismology. mailing address: dr. josep batlló, cgul-idl, universidade de lisboa, campo grande, edifício c8, 1749 016 lisboa, portugal; e-mail: jobatllo@fc.ul.pt. vol52,6,2009new 2-12-2009 12:40 pagina 699 700 j. batlló these pendulums were the only instrument designed by grablovitz ever to have been installed outside italy. in this sense, the relation of g. grablovitz with the ebre observatory was the most fruitful one of all the scientific contacts he had. not so much is known about it and only some scarce documents are preserved. but, recently, i found some new letters of the correspondence between grablovitz and the ebre observatory at the archive of the jesus society in barcelona. this finding allows a reappraisal of this scientific relationship. consequently, what is known about the relations of g. grablovitz with spanish seismologists and his influence on spanish seismology will be analysed in the following paragraphs. also, it will be shown how it is possible to use this information to improve current research in seismology. 2. italian seismology in spain it is not clear (to me) how italian seismological theory and practice were introduced in spain. nowadays, it is clear that geographical and cultural proximity favored such introduction. also, the strong influence the catholic church exerted in both countries and the close relations existing between members of the religious institutes of both countries at that time were obvious ways of introduction of italian earth science theories in spain. due to the lack of further analysis of such relations, this is a topic that should be studied in the future. nevertheless, it is clear that italian ideas on seismology had an overwhelming acceptance among spanish scientists in the last quarter of the 19th century. an example of this fact is the wide distribution obtained in spain of the book by m. de rossi (1879; 1882). many of the spanish scientific libraries already existing at that time still have a copy of it. another example of this influence is the discourse of d. cortázar, a mining engineer, on his acceptance as a member of the academy of sciences of madrid in 1884 (cortázar y fernández de castro, 1884). it is devoted to seismology and, in its pages, cortázar shows a wide and deep knowledge of italian seismological practice at that time (it should be noted that fernández de castro shows a deep knowledge of the geological work of stoppani). just a few months later, on 25 december 1884, a great earthquake hit the se part of andalucía, in spain. it was one of the largest earthquakes to have occurred in europe in the last quarter of the 19th century. at that time, scientists around europe showed a great interest to study the causes of this earthquake and several commissions were sent to the epicentral area and elaborated detailed reports about it (fernández de castro et al., 1885; tarramelli and fig. 1. letter of g. grablovitz to r. cirera, director of the ebre observatory dated 26 april 1906 (copy of the original letter preserved at the ebre observatory archive). vol52,6,2009new 2-12-2009 12:40 pagina 700 701 influence of g. grablovitz in spain: instruments and scientific correspondence mercalli, 1885; fouque, 1886). after this large earthquake in andalucía, several scientists in spain asked for a network of «geodynamic» observatories. a new, the use of the term «geodynamic» is a clear indication of the italian influence (e.g. de castro pulido, 1895). it is clear that the overwhelming influence of italian seismology in spain expanded at least until the first decade of the 20th century. this influence can be seen just taking a look at the seismic instrumentation installed in the spanish observatories (for a more detailed information on the type and characteristics of the instruments installed in spanish observatories see batlló and bormann, 2000, or batlló, 2004). table i shows the origin of the seismological instrumentation at the different spanish observatories at the turn of the 20th century. as can be seen, italian instruments were installed in the majority of observatories. in addition, the number of instruments account for almost two thirds parts of the seismic instruments installed in spain in those early years. 3. the ebre observatory and g. grablovitz ebre observatory was founded in 1904 and its main objective was the study of the relation between geophysical and solar activity (now known as solar-terrestrial physics). to fulfil this aims, a complete set of astronomic and geophysical instruments were installed (cirera, 1905). among them, a seismic station was set up (batlló and ugalde, 2000; ugalde, 2003). two different types of seismographs were chosen to complete the instrumental set of this station: a vicentini microseismograph for horizontal and vertical motion record and a couple of grablovitz horizontal pendulums. it is not known why or how the grablovitz horizontal pendulums were chosen. what it is known is that the jesuit r. cirera (1864-1932), the first director of the ebre observatory, devoted several years before the foundation of this observatory to a scientific journey through different european observatories with the goal to study the state of the art of the different fields of interest (solar studies, geomagnetic field, meteorology, table i. list of seismic stations created in spain at the end of the 19th century and the beginning of the 20th century. they are classified in main stations (those historically cited everywhere – e.g. lópez arroyo et al., 1990) and secondary stations (those found in further researches – e.g. batlló and bormann, 2000). the second column states the source of the original instruments equipping those stations. the instrumentation of the tivissa station may be also considered from italian origin due to its characteristics. spanish seismic stations seismograph origin main stations secondary stations san fernando (1887) english madrid (1887) italian cartuja (1902) italian puerto orotava (1889) german ebre (1904) italian olot (1907) italian fabra (1906) italian girona (1908) italian toledo (1909) mixed riotinto (1909) english almería (1911) italian german tivissa (1910) self made málaga (1913) italian german alicante (1914) italian german vol52,6,2009new 4-12-2009 15:54 pagina 701 702 j. batlló seismology, …) for the new observatory. it can be assumed that he established contact with g. grablovitz travelling in italy. it is not clear if he contacted grablovitz directly because he was aware of his scientific career or he did so after the advice of some colleague. the second option is the most likely. it should be noted that the seismic section of the ebre observatory was, from the first, and in spite of its intrinsic quality, a secondary part of the main project of the ebre observatory. this means that, to set it up, the chosen instrumentation was selected to be enough to fulfil the desired objectives, but not very expensive or cumbersome to operate. this can be the main reason for the choice of a vicentini microseismograph and the grablovitz pendulums. it is known that the vicentini microseismograph was a good instrument for recording regional seismicity. the last instrument was able to record teleseismic events and, both together, fulfilled the requirements in this field for the planned station. the grablovitz instrument was operative at ebre observatory from 1905 to 1918. around one thousand records of this seismograph are still preserved. the usefulness of these records for current seismological studies has been shown by different authors (abe 1994; dineva et al., 2002; stich et al., 2003). from 1910 to 1913, a regular monthly bulletin was published presenting the results of the observations and even some copies of the obtained records. those bulletins are the only printed information known about this instrument. also, a unique photograph of the instrument installed at ebre is preserved (fig. 2). it is interesting to note that the official records of the observatory state that operation of the grablovitz pendulums was discontinued in 1913, when a large reconstruction of the seismic pavilion was undertaken. however, preserved seismic records from this instrument show clearly that it was operative up to 1918. also, the preserved photograph of the instrument is dated between 1915 and 1916, when the new mainka type instruments, also visible in the photograph, were installed. this photograph (fig. 2) shows the instrument in its latest installation, fixed against the wall of the seismic pavilion. previously, it was fixed at a brick column in the centre of the pavilion. figure 3 shows a sketch of the seismic pavilion with the original location of the grablovitz instrument, fixed against a brick column. fortunately, to complete our knowledge of the operational life of the grablovitz pendulums in spain, some letters and manuscript documents forming part of the correspondence between g. grablovitz and the ebre observatory have been preserved. some of them are still kept at the archive of the observatory (e.g. the letter about the san francisco earthquake reproduced in fig. 1) and another group have been recently discovered and are located at the archive of the catalan province of the jesus society in fig. 2. the grablovitz pendulums (right) installed at the seismic pavilion of the ebre observatory (circa 1915). the larger seismograph in the center of the photograph is the mainka-ebre instrument, constructed at the ebre observatory (copy of the original photograph preserved at the ebre observatory archive). vol52,6,2009new 2-12-2009 12:40 pagina 702 703 influence of g. grablovitz in spain: instruments and scientific correspondence barcelona. it will be shown that these materials are of real importance to understand how the grablovitz pendulums operate. the first preserved letter dates from 18 march 1904. it states that the pendulums built for the ebre observatory are ready and tested and «lui sono riuscite assai più sensibili di quelli che già erano in azione» (they are much more sensitive that those already installed [in ischia]). also, it asks to cirera, the director of ebre observatory if he wants to install a time marking system. it should be recalled at this point that grablovitz had a deep interest in time accuracy in the seismic records. in this aspect he was really far beyond most of his contemporaries. looking at the contents of this letter, it should be likely that some previous correspondence exists; but it has never been found. another possibility is that cirera ordered the grablovitz instrument «in person» and this was the first letter between both scientists. this is one of the multiple questions without answer due to the lack of further information. the analysis of the preserved documents shows that the most important ones, from a seismological point of view, are two sheets of paper written on both sides containing the instructions for the installation of the new instrument (fig. 4). there, it is accurately described how to set up the oscillating masses or the inscription mechanism, with special care to the proper setting of the stylus. those instructions were translated carefully into spanish at the ebre observatory (they are also preserved), and fig. 3. sketch of the seismic pavilion of the ebre observatory (from pericas, 1910). fig. 4. second page of the instructions to install the grablovitz horizontal pendulums sent by grablovitz to the ebre observatory (copy of the original documents preserved at the archive of the jesus society in barcelona). vol52,6,2009new 2-12-2009 12:40 pagina 703 704 j. batlló fig. 5 a-c. sketches of how to install the grablovitz horizontal pendulums. a) lateral view showing the column and table sustaining the oscillating masses; b) top view showing the long stylus; c) details of the masses and related elements (copies of the original documents preserved at the archive of the jesus society in barcelona). vol52,6,2009new 2-12-2009 12:40 pagina 704 705 influence of g. grablovitz in spain: instruments and scientific correspondence are the unique existing instructions for such kind of seismograph preserved. as the instruments installed at ischia stations (both, porto d’ischia and casamicciola) were operated by grablovitz himself, it is quite normal that he never wrote any detailed instructions for its installation and use. this gives extra importance to the found document. also, some sketches of how to install the instrument (fig. 5), and even some drawings of the lower and upper fixations at 1:1 scale are preserved. these documents show how important the study of manuscript documents can be. looking at the currently preserved documents and pieces, the addition of the original pieces of the grablovitz horizontal pendulums preserved at ischia, the existing photographs of these instruments at ischia and ebre, and the recently found manuscript documents will be enough, if needed, to allow for a complete reconstruction of an operative version of the seismograph, the found instructions allowing for a fine and easy «tuning» of the instrument. to end this section, it should be noted that the ebre observatory instrument was not the only interest of grablovitz in spain. in the same letter of 26 april 1906, grablovitz asks cirera for the possibilities to find some convenient place near the ebre observatory or in mallorca island to perform tide measurements, another of his topics of interest. as far as it is known, no further development was reached in this line. 4. the diffusion of the grablovitz pendulums in spain as has been stated, the grablovitz horizontal pendulums installed at ebre observatory were the unique instrument of this type operating in spain, but the interest in this instrument spread further than the ebre observatory. since 1906 manuel m. sánchez-navarro neumann (1867-1941) had been the director of the cartuja seismological observatory, in granada (espinar, 2002). sánchez-navarro and the cartuja observatory were deeply interested in the study and construction of efficient seismographs. one of the strongest points of the research developed at cartuja observatory was a consequence of this special interest in the seismic instrumentation. this topic will not be developed here (see batlló, 2002), but it should be stated that, after a careful study of the existing models, the observatory of granada developed their own instruments based on almost all the main construction principles for seismographs (sáncheznavarro, 1927). as a result, it was considered, for many years, the best seismic station in spain (rey-pastor, 1932). of our interest within a ten year period, sánchez-navarro wrote, two long reviews about the different models of seismographs utilised worldwide (1907-1908; 1919-1920). it is extremely interesting to note he devoted some lines to the description of the grablovitz horizontal pendulums in both reviews. in the first one of them (1907-1908) – later translated into french in an abridged version (1909) –, sánchez-navarro, in just one page, describes carefully the main characteristics of the instrument. in addition, he states that he had the opportunity to inspect the model installed at the ebre observatory. in addition, as a curiosity for us, he states the price of this instrument: 90 lire at that moment. in the second review (1919-1920), sáncheznavarro goes further. he devoted one and a half pages to a more detailed description of the instrument that in the previous review and he finished the general description with the following paragraph (translated from spanish): «the circumstance that there is no important constructor of seismographs involved in the construction of this instrument, so simple, so easy to use and even cheap, has definitely been of great importance for the small diffusion of them because, if [the instrument is] improved with independent recording cylinders for each component with, at least, a speed of 60 cm/hour, damping and a improved stylus they may be really useful in those places where earth shakes heavily and frequently, and the main periods of the macroseisms are quite low, as happens in the pacific coast of south america and the philippine islands». as can be seen, he is stating, in very few and concise words, the necessary improvements to update the instrument. it is noteworthy that this was written in the year 1919 and, still at that time, even a quarter of a century after their vol52,6,2009new 2-12-2009 12:40 pagina 705 706 j. batlló first design, sánchez-navarro was considering it a good instrument for seismic recording. unfortunately, despite the close contacts, and even the direct influence that the cartuja observatory exerted on some south american seismic stations, nobody took in consideration these suggestions and the grablovitz pendulums did not expand on south america or philippines. 5. remarks and conclusions as has been seen, the research on seismic instrumentation done by g. grablovitz had continuity in spain. his horizontal pendulums were installed at ebre observatory and sáncheznavarro, from cartuja observatory, showed a great interest in this instrument. ebre observatory keeps around one thousand records of its grablovitz horizontal pendulums and other printed and manuscript documents from this seismograph. some of these records have been recently digitized and used for the analysis of large earthquakes in europe with success. a careful analysis of the preserved documents at ebre observatory shows some discrepancies between the official and the real existence of this instrument. the addition of the printed documents, manuscript documents and surviving parts of the instrument at ischia and ebre are enough to afford the reconstruction of the grablovitz horizontal pendulums. such reconstruction should give us a deeper knowledge of the special characteristics of the records obtained with this instrument. this will improve the results of future analysis of old earthquakes recorded with the horizontal pendulums. in addition, the interest of grablovitz to expand his tidal observations to other places around the mediterranean sea has been confirmed. acknowledgments this research was started by the author at ebre observatory. it has been partly supported by the projects bha2001-1393, ren20012418-c04-01 and ren2002-11055-e / ries of the dirección general de investigación (dgi) of the spanish ministry of science and technology. the archive of the province of catalonia of the jesus society, and especially its director j. roca, is acknowledged for the facilities he offered to investigate and reproduce the documents of the ebre observatory preserved on its files. references abe, k. (1994), instrumental magnitudes of historical earthquakes, 1892 to 1898, bull. seism. soc. am., 84, 415-425. batlló, j. and a. ugalde (2000): els sismògrafs de l’observatori de l’ebre., in actes de la v trobada d’història de la ciència i de la técnica, edited by j. batlló et al. (institut d’estudis catalans, barcelona), pp. 161-165. batlló, j (2004): catálogo inventario de sismógrafos antiguos españoles, (instituto geográfico nacional, madrid), pp. 414. batlló, j and p. bormann (2000): a catalog of old spanish seismographs, seismological research letters, 71, 570-582. batlló, j (2002): los sismógrafos del observatorio de cartuja, in historia del observatorio de cartuja 19022002. nuevas investigaciones. cd-rom, edited by m. espinar, j.a. esquivel and j.a. peña, (instituto andaluz de geofísica, universidad de granada). cirera, r. (1905): notice sur l’observatoire et sur quelques observations de l’éclipse du 30 août 1905, (mémoires de l’observatoire de l’ebre, no. 1, barcelona, gustavo gili). cortázar, d. y m. fernández de castro (1884): discursos leídos ante la real academia de ciencias exactas, físicas y naturales en la recepción pública del sr. d. daniel de cortázar, (real academia de ciencias exactas, físicas y naturales, madrid), pp. 113. de castro pulido, j. (1895): conveniencia de establecer una red de observatorios sísmicos en españa, la naturaleza, 6, 92-95 and 102-105. de rossi, m.s. (1879): la meteorologia endogena, vol. i., (fratelli dumolard, milano). de rossi, m.s. (1882): la meteorologia endogena, vol. ii., (fratelli dumolard, milano). dineva, s., j. batlló, d. mihailov and t. van eek (2002): source parameters of four strong earthquakes in bulgaria and portugal at the beginning of the 20th century, journal of seismology, 6, 99-123. espinar, m. (2002): notas sobre la vida y obra del r.p. manuel, m. sánchez-navarro s.j. neumann (18671941), in historia del observatorio de cartuja 19022002. nuevas investigaciones. cd-rom, edited by m. espinar, j.a. esquivel and j.a. peña, (instituto andaluz de geofísica, universidad de granada, granada). fernández de castro, m., j.p. lasala, d. cortázar y j. gonzalo tarín (1885): terremoto de andalucía: informe de la comisión nombrada para su estudio dando cuenta del estado de los trabajos en 7 de marzo de 1885, (imp. de m. tello, madrid), pp. 107. vol52,6,2009new 2-12-2009 12:40 pagina 706 707 influence of g. grablovitz in spain: instruments and scientific correspondence fouque, f. (1886): mission d’andalousie: etudes relatives au tremblement de terre du 25 décembre 1884, et à la constitution géologique du sol ébranlé par les secousses, acad. sci. mem., xxx, 1-772. lópez arroyo, a., j. cruz, a. roca and c. olivera (1990): early seismographic instruments in spain, in gli strumenti sismici storici, edited by g. ferrari (sga storia-geofisica-ambiente, bologna), 161-164. pericas, j. (1910): sismología, boletin mensual del observatorio del ebro (enero de 1910 con una introducción), i (1), 55-57. rey-pastor, a. (1932): el servicio sismológico español, a terra, 1, 36-40. sánchez-navarro neumann, m.m. (1907): estudio comparativo de los instrumentos más usados en sismología, revista de la real academia de ciencias exactas, físicas y naturales de madrid, 6, 653-674, 761772, 859-867 and 911-926. sánchez-navarro neumann, m.m. (1908): estudio comparativo de los instrumentos más usados en sismología, revista de la real academia de ciencias exactas, físicas y naturales de madrid, 7, 121-144. sánchez-navarro neumann, m.m. (1909): aperçu des instruments plus utilisés en seismologie, ciel et terre, xiv, 295-326. sánchez-navarro neumann, m.m. (1919): ensayo crítico sobre los sísmógrafos más en uso (generalidades y péndulos verticales de registro mecánico), asociación española para el progreso de las ciencias, congreso de sevilla, vol. iv, section 2ª, (astronomía y física del globo), 33-69. sánchez-navarro neumann, m.m. (1920): ensayo crítico sobre los sísmógrafos más en uso (péndulos horizontales e invertidos de registro mecánico), asociación española para el progreso de las ciencias, congreso de bilbao, vol. iv, section 2ª, (astronomía y física del globo), 51-89. sánchez-navarro neumann, m.m. (1927): le séismographes de la station séismologique de cartuja (granada), publ. du bureau central séismologique international, série a, travaux scientifiques, 4, 119-131. stich, d., j. batlló, j. morales, r. macià and s. dineva (2003): source parameters of the 1910 mw=6.1 adra earthquake (southern spain), geophysical journal international, 155, 539-546. tarramelli, t. and g. mercalli (1885): i terremoti andaluci cominciati il 25 decembre 1884, r. acc. lincei, cclxxxiii. ugalde, a. (2003): the ebre observatory seismological station: past and present instrumentation and noise conditions, annals of geophysics, 46, 609-624. vol52,6,2009new 2-12-2009 12:40 pagina 707 layout 6 twitter earthquake detection: earthquake monitoring in a social world paul s. earle*, daniel c. bowden, michelle guy u.s. geological survey, denver, co, usa annals of geophysics, 54, 6, 2011; doi: 10.4401/ag-5364 abstract the u.s. geological survey (usgs) is investigating how the social networking site twitter, a popular service for sending and receiving short, public text messages, can augment usgs earthquake response products and the delivery of hazard information. rapid detection and qualitative assessment of shaking events are possible because people begin sending public twitter messages (tweets) with in tens of seconds after feeling shaking. here we present and evaluate an earthquake detection procedure that relies solely on twitter data. a tweet-frequency time series constructed from tweets containing the word “earthquake” clearly shows large peaks correlated with the origin times of widely felt events. to identify possible earthquakes, we use a short-term-average, long-term-average algorithm. when tuned to a moderate sensitivity, the detector finds 48 globallydistributed earthquakes with only two false triggers in five months of data. the number of detections is small compared to the 5,175 earthquakes in the usgs global earthquake catalog for the same five-month time period, and no accurate location or magnitude can be assigned based on tweet data alone. however, twitter earthquake detections are not without merit. the detections are generally caused by widely felt events that are of more immediate interest than those with no human impact. the detections are also fast; about 75% occur within two minutes of the origin time. this is considerably faster than seismographic detections in poorly instrumented regions of the world. the tweets triggering the detections also provided very short first-impression narratives from people who experienced the shaking. introduction twitter is a service that allows anyone to send and receive 140-character messages (tweets) via text message and internetenabled devices. tweets can be sent and received through a webpage, a mobile device, or third-party twitter applications. tweets can be sent publicly or privately to a specified user. all users who opt to ‘follow’ a twitter user will receive that user’s public tweets. it is these public messages that really separate twitter from instant message services that typically involve one-to-one communication rather than one-to-many. in addition to receiving tweets from users you elect to follow, twitter provides public access to its database. one can freely search and download recent tweets containing any keywords of interest, thus providing real-time access to twitter discussions on any topic anywhere around the globe. within seconds of widely felt earthquakes around the world, narratives and exclamations of 140-character or less are publicly distributed on twitter. these tweets sometimes number in the thousands and some provide a qualitative description of the shaking effects. these short accounts of shaking effects are sometimes available before the seismically derived estimates of location and magnitude are publicly distributed by the u.s. geological survey (usgs) [e.g., earle et al. 2010]. since this information is easily accessible and rapidly available, people are increasingly turning to twitter and other forms of social media to gather rapid situational awareness following earthquakes and other disasters [e.g., liu and ayala iacucci 2010, mendoza et al. 2010, palen et al. 2010a]. as a distributor of authoritative earthquake information, the usgs is investigating the potential benefits and pitfalls of using twitter to augment its suite of seismically derived earthquake products and alerts. possible uses fall into the basic categories of alert dissemination, situational awareness, and event detection. alert dissemination earthquake alert dissemination is a straightforward use of twitter. broadcasting earthquake alerts via twitter has several advantages. it provides an additional method to reach large segments of the population who are becoming more reliant on social media as a means of communication and information gathering. twitter is fast; twitter messages are generally available to all followers within seconds of being submitted. users leverage twitter’s delivery infrastructure such that once a message delivery is set up there is no additional overhead for the user to reach more people. in several countries, twitter also allows users to receive tweets via simple message service (sms) text messages on their mobile phones at no cost to the sender. twitter alerts do have disadvantages. the main disadvantage is a limited ability for the recipient to customize article history received august 10, 2011; accepted october 14, 2011. subject classification: surveys, measurements and monitoring, instruments and techniques, data processing, seismological data, general or miscellaneous. 708 citizen empowered seismology/special section edited by r. bossu and p.s. earle his or her alert criteria. the usgs earthquake notification system (ens) allows users to select customized magnitude ranges and regions for their e-mail alerts [wald et al. 2008]. such detailed alert criteria are not possible with twitter because messages are broadcast from accounts with multiple followers and cannot be tailored to an individual user. controlling updates to an earthquake’s hypocentral parameters and magnitude is also difficult when using twitter. individual users often choose to rebroadcast (retweet) older information, so the original distributor cannot control whether everyone has up-to-date information. several seismic networks and agencies are currently distributing twitter earthquake alerts, including the europeanmediterranean seismological centre (emsc) (@lastquake), natural resources canada (@canadaquakes), and the indonesian meteorological agency (@infogempabmg). the usgs issues alerts from two twitter accounts @usgsbigquakes and @usgsted. the @usgsbigquakes (usgs big quakes) account broadcasts solutions for all earthquakes with magnitude 5.5 and larger, and includes links to supplemental products and information that are routinely produced by the usgs . the @usgsted (usgs twitter earthquake dispatch) account is an experimental account that tweets alerts that include the seismological parameters along with the temporally and spatially correlated tweets-per-minute, following a seismically verified earthquake. distributing information via multiple twitter accounts allows followers to choose the data stream that best fits their interests. situational awareness due to the speed of tweet creation and ease of near realtime access, several groups have looked into using twitter for gaining situational awareness following disasters. the 140character limit on tweets is both a blessing and a curse for obtaining situational awareness. the forced restriction to short messages and high user base promotes rapid dissemination and ease of content collection from potentially millions of users. the main downside is the difficulty in providing detail and actionable information in 140 characters. this restriction results in numerous tweets of marginal information value. maceachren et al. [2011] found success in sorting through the vast quantity of tweets and extracting potentially useful information through the implementation of a web-enabled mapping tool that collates, and plots tweets. to improve both the content and ease of automated processing, starbird and stamberger [2010] introduce a tweet syntax that includes parsable keywords that allow computer extraction of details such as location and content type (e.g., a victim’s needs and damage descriptions). while the syntax has not been widely adopted by the content originators, volunteer efforts have arisen after disasters to translate tweets into this more actionable form [starbird and palen 2011]. focusing on gathering information in the immediate aftermath of an earthquake guy et. al. [2010] developed a prototype system that collects, collates, and analyzes tweets. the system continuously downloads tweets containing the key words “earthquake” and its equivalent in other languages. following seismically verified earthquakes, the system produced a series of products. the products included interactive maps that show the location and text of the tweets and e-mails that contain the seismically verified location and magnitude of the earthquake, a list of the cities where the tweets were originating, and the text of the first 50 tweets after the event. they conclude that the main advantage of twitter is speed. the qualitative descriptions contained in the tweets are available at the same time as the seismically derived earthquake parameters and sometimes provide a responding seismologist with a quick indication of the severity of the earthquake effects. the main shortcoming of twitter-derived information is the lack of quantitative shaking estimates, such as those produced by the usgs “did you feel it?” and shakemap systems [wald et al. 1999a, 1999b]. event detection the observed rapid and voluminous influx of tweets arriving shortly after earthquakes prompted several blog postings [e.g., scoble 2008, o’brien 2008 and o’neill 2009] and articles [e.g., earle et al. 2010, li and rao 2010, sakaki et al. 2010] that propose using twitter as a tool for earthquake detection. sakaki et. al. [2010] deployed a functional system that reportedly detected and located earthquakes at speeds comparable to the japan meteorological agency. their algorithm used natural language processing to detect events and spatial particle filters to locate them. their system operated in japan and posted results on the web. in this paper, we describe a simple algorithm to detect earthquakes in populated regions worldwide. we present and test an algorithm to automatically detect earthquakes using only tweets and discuss its performance, the speed of detection, and the possible use of these detections during real-time earthquake response. implementation of an earthquake detector we implement a simple event detector that triggers off a rapid increase in the frequency of tweets containing the word “earthquake” or its equivalent in other languages. rapid increases in earthquake tweets are detected using a shortterm-average over long-term-average (sta/lta) algorithm that is commonly used in seismology to detect and time seismic phases. the algorithm requires higher-signal levels at higher-noise levels to trigger a detection, thus suppressing false triggers arising from high levels of background “noise” following highly-tweeted events and earthquake chatter unrelated to seismic events. the algorithm is based on the seismic phase picker of earle and shearer [1994] with a earle et al. 709 710 modification to account for the very low background frequency of earthquake tweets that often exists. data our test data consist of downloaded tweets containing the words “earthquake”, “gempa”, “temblor”, “terremoto”, or “sismo” that were collected during the time a prototype archiving system was in operation, august, 2009 through to the end of november, 2009 [guy et. al. 2010]. our data is limited to this timespan because the twitter search mechanism is restricted in time and number so, unless tweets are continually downloaded, one cannot construct a continuous or complete dataset. for each keyword-filtered tweet we archive the tweet creation time, text, and the twitter user location. additionally, for tweets that do not contain gps locations, the latitude and longitude is derived from the static location string found in a user’s profile using the google maps api geocoding service and stored with the tweet. inaccurate tweet geo-locations are a serious issue when using geospatially related tweets. the vast majority of tweets do not contain gps locations, and the tweet location is often only as accurate as the static location string the user entered in their twitter profile. a location is not required when setting up a twitter account and can be as vague or specific as the user wants. given this, a tweet from a new yorker on vacation in san francisco will mis-locate to new york. these types of location errors are troublesome but are not spatially correlated, so averaging can minimize their negative effects. our goal is to identify tweets originating from users who experienced earthquake shaking and not increases in twitter activity following the release of news or blog articles. to minimize such contamination, we remove all tweets that contain uniform resource locators (urls), specifically the string “http”. furthermore, we remove all tweets with the text “rt” or “@”. “rt” is commonly used to identify a tweet as a rebroadcast or “retweet” of another users tweet and the “@” symbol is used in front of a username to reply to another user’s tweet. thus tweets containing “@” and “rt” are likely to arise from a user commenting on an earthquake from outside the felt area. detection algorithm figure 1 illustrates the detection algorithm. using our culled dataset we generate a tweet-frequency time series or ‘tweetgram’ by binning tweets into five-second windows and normalizing to tweets-per-minute (figure 1a). a characteristic function (figure 1b) is generated from the tweetgram and is used to declare event detections. the characteristic function is defined as: c(t) = sta/(mlta+b) the characteristic value c(t) is calculated for every time bin and a detection is declared when c(t) exceeds 1. the sta is the short-term average taken one minute before time t and lta is the long-term average taken 60 minutes before the sta. the constants m and b are tunable parameters that define the sensitivity of the detection. the lta approximates the ‘background noise’ and the sta approximates the tweets twitter earthquake detection figure 1. illustration of the detection process (see text for details). a) tweet-frequency or tweetgram for about three hours of tweets on november 13, 2009, in utc. b) the characteristic function generated from the tweetgram in (a). the characteristic function has a one minute sta, a sixty minute lta, m = 4, and b = 10. in response to a possible event or ‘signal’ plus the background noise. c(t) requires higher signal levels (sta) to trigger at higher noise levels (lta). the constant b has units of tweetsper-minute and defines the sta value that will cause an event trigger when the lta is zero. the constant m defines the increased sta necessary to trigger at a larger lta. we require c(t) to drop to 0.25 before another trigger is allowed to occur. this requirement limits multiple triggers that can be generated for the same event due to a c(t) that oscillates around one; however, it also limits our ability to detect very closely spaced events. the detector has proven effective at differentiating between sudden, sharp increases in twitter activity emanating from people who felt an earthquake from more gradual increases that result as news of an event propagates around the world. figure 2 shows one such example. for two events in indonesia, tweets reporting the earthquake rapidly came from within the expected felt area. within two minutes of these earthquakes, our detection algorithm correctly identified the events. in the case of the samoa earthquake, twitter activity was mainly from referencing news reports and thus shows a more gradual, emergent development. we collected no immediate tweets from samoa communities and the detector did not trigger on this event. triggering on an event that is already known and announced is not the goal of this detector. the sta/lta approach also helps mitigate the effect of variations in background noise due to time of day and unrelated world events. we need to be able to detect spikes above the background chatter that might occur from “earthquake” tweets unrelated to actual earthquakes. for example, one can envision a higher level of “earthquake” tweets the day a dairy queen has a two-for-one offer on their “oreo earthquake brownie” ice cream sundae. additionally, we want to avoid false triggers during the times of high activity following earlier significant earthquakes. detector performance we tested the performance of the detector on our tweet data that spanned that spanned august, 2009 through the end of november, 2009. as with most automatic detection methods, there is a trade-off between the number of false triggers and the number of missed events. we tuned our algorithm by: 1) generating a large set of triggers using low values of m and b, 2) identifying each trigger as ‘verified’, ‘false’, or ‘possible’ and 3) performing a four-dimensional grid search over lta, sta, b and m to find parameters that produce a high ratio of verified to false triggers. an event is considered ‘verified’ if an earthquake exists in a global or regional seismic catalog that plausibly could have been felt in the area where the majority of the tweets originated and if the text of the tweets are qualitatively consistent with the response to shaking. the absence of an earthquake from a seismic catalog does not mean it did not occur; earthquake often go undetected in areas where seismic instrumentation is sparse. therefore, our twitter detector may find earthquakes that are not reported in seismic catalogs. to account for this possibility, we identify a trigger as a possible quake if the tweet text is consistent with that of verified events and the tweets are clustered in space and time. a trigger is categorized as ‘false’ if the tweet text is clearly unrelated to a shaking event, such as an earle et al. 711 figure 2. several days of tweets spanning three events in september and october 2009. our detector is tuned for sudden increases in twitter activity, common for tweets submitted after felt shaking. when increases in twitter activity results from news reports, as in the samoan event, the tweet frequency increases more gradually and the characteristic function sometimes does not trigger. this is the preferred behavior because we are interested in tweets resulting from felt events. 712 earthquake drill or chatter about an epic session of the quake video game. figure 3 summarizes the results of our test. for each event trigger, the plot shows the sta and lta values for the event’s highest value of the characteristic function following the trigger. seismically verified earthquakes are plotted as green circles, possible events as blue circles, and nonearthquake events as red circles. the lines are defined as sta = m*lta + b. the lines correspond to three different triggering thresholds: sensitive (m = 2 and b = 5), moderate (m = 4 and b =10), and conservative trigger (m = 19 and b = 9). points plotted above a given line will generate a detection for a c(t) with the corresponding values of m and b. table 1 summarizes the number of ‘verified’, ‘possible’ and ‘false’ triggers for these threshold values. the ‘moderate’ sensitivity trigger provides a good trade-off between catching major events and producing a low number of false triggers. a better assessment of the detector performance can be obtained if we tune and test the detector on independent datasets. this procedure would require a longer time series than we currently have and it is left for future work. in figure 3, note that events trigger as soon as c(t) exceeds 1 and the points shown represents the maximum value reached by the characteristic function following the detection of that event. for false triggers, it is often clear from reading the tweet text that they are not associated with an earthquake. the only two false triggers caught for the ‘moderate’ sensitivity trigger resulted from tweets during the great california shakeout earthquake drill (www.shakeout.org) on october 15, 2009 and the space shuttle landing on september 12, 2009. in the case of the space-shuttle landing, many of the tweets reported something to the effect of, “that was not an earthquake it was the space-shuttle sonic boom”. in fact, shortly before the shuttle’s sonic boom a twitter user tweeted “... that loud noise you’re about to hear is not an earthquake. sonic boom”. while it is easy for a human to understand and interpret tweet text, getting a computer to interpret natural language is difficult. this is especially true in twitter, where we see multiple languages and countless abbreviations. such computerized language interpretation is pursued by [e.g., sakaki et al. 2010 and palen et al. 2010b]. figure 4 maps the verified, potential, and false detections for the moderate sensitivity trigger. verified events are plotted at the location of the corresponding cataloged earthquake, and potential and false triggers are plotted at the location of the highest tweet density. the map shows twitter earthquake detection figure 3. recorded sta versus lta for events in our dataset. green dots, ‘verified’ earthquakes; blue dots, ‘possible’ earthquakes; red dots, ‘false’ triggers (see text for details). note that these sta and lta values correspond to where the characteristic function reaches its highest value following a trigger. the three lines are defined as sta = m*lta + b and represent the three trigging thresholds discussed in the text. events above a given line will trigger a detection for the corresponding trigger sensitivity. more conservative thresholds produce a higher ratio of verified to false triggers. trigger verified earthquakes possible earthquakes false triggers sensitive 61 10 75 moderate 48 3 2 conservative 18 1 0 table 1. number of verified, possible, and false triggers for different trigger sensitivities. detection time is very fast, with about 75% coming in before two minutes. reasonable global coverage with the highest number of detections in california, japan, and indonesia. these results show the potential global reach of the method, with some detections occurring in sparsely instrumented regions where seismically obtained detections can be slow, or small felt earthquakes could potentially be missed. the detection time is not the origin time of the earthquake or the initiation of the tweets, both of which will precede the detection time because it takes time for the sta value to sufficiently increase to cause a trigger. also, for off-shore events or events with epicenters outside of populated regions, it can take time for the shaking to reach twitter users. the detection times also depend on the characteristics of the tweetgram. slower increases in tweets-per-minute will result in delayed triggers, if these trigger at all. figure 5 shows a histogram of the detection time for verified events found using the moderate sensitivity trigger. despite the mentioned delays, the detection time for events is very fast; roughly 75% of events trigger within two minutes. in most cases, this precedes earle et al. 713 figure 4. map showing events caught by our detector using the ‘moderate’ threshold. green dots, ‘verified’ earthquakes; blue dots, ‘possible’ earthquakes; red dots, ‘false’ triggers. the inset map shows an expanded view of california where the two false triggers occurred. figure 5. detection time for earthquakes found using the ‘moderate’ sensitivity threshold. detection time is very fast, with about 75% coming in before two minutes. 714 the usgs public release of seismically derived hypocenters and magnitudes that range from two minutes in wellinstrumented regions to 20 minutes for sparsely instrumented regions of the world. discussion we have demonstrated that earthquake detections based solely on twitter messages are possible for earthquakes worldwide with a low rate of false triggers. now we briefly explore their potential use. our twitter-based earthquake detector misses the vast majority of seismically detected events. for comparison, the usgs global earthquake catalog (http://earthquake.usgs.gov/ research/data/pde.php) contains 5,175 earthquakes while only 48 were found by the twitter detector with a moderate threshold. the majority of seismically detected earthquakes are either too small to produce perceivable shaking or occur outside populated areas. twitter use is also far from ubiquitous and not all twitter users who feel an earthquake report it. the detections, however, are generally caused by widely felt events that are of more immediate interest than those with no human impact. these detected events occurred near urban areas and range in magnitude from 2.0 to 7.5 with a median magnitude of 4.6. given this high percentage of missed events and the inability to extract the accurate hypocentral and magnitude estimates that are needed for impact assessment, twitter is in no way a replacement for a seismic network. the potential benefits from twitter arise from twitter’s significant global penetration, the rapid submission and access to tweets, and the short narratives contained in the tweets. possible uses include, discovery of earthquakes in sparsely instrumented regions, earthquake detection before seismically derived parameters are obtained, and situational awareness provided by the text of the tweets. the usgs national earthquake information center (neic) strives for a complete earthquake catalog at a magnitude threshold of 4.5 globally and 2.5 in wellinstrumented areas of the usa. additionally, neic reports on smaller earthquakes globally, if they are widely felt and/or news worthy. the vast majority of these earthquakes are detected by the roughly 2,000 seismic stations that flow into neic’s real-time location system. however, occasionally events meeting these criteria, in poorly instrumented regions, are missed. these missed events are sometimes first noticed by citizen reports submitted to the usgs “did you feel it?” system or phone calls to the neic. if an approximate time and location of an earthquake that was missed by our automatic system is known, it is sometimes possible for a seismologist to manually extract a location and magnitude. twitter offers another mechanism to discover these small events. although the number of earthquakes with “did you feel it?” responses exceeds the 48 with twitter reports, 11 of the 48 events we detected using twitter during our five month test have no corresponding “did you feel it?” responses. twitter-based detections have the ability to rapidly alert seismic monitoring agencies that a widely felt earthquake occurred, potentially before seismic systems have detected the earthquake. the majority of our twitter detections occur in less than two minutes (figure 5). this is faster than many earthquakes are detected by neic’s automatic systems. for example, the neic first seismically detected the recent magnitude 9.0 earthquake in japan 3.8 minutes after it occurred [hayes et al. 2011]. the initial neic detection had a location but no magnitude. at this same time, “earthquake” tweets spanned the entire island of honshu and southern hokkaido. such a rapid and geographically widespread twitter response is a good indication that an event was widely felt and potentially required the mobilization of additional usgs response personnel. in the case of japan, improved response time can be obtained from increased sharing of seismological waveform and parametric data. this effort is underway, but most regions of the world are not as well instrumented as japan. the focus of this discussion has been on using twitter for augmenting instrumentally derived data. other socialnetworking tools and internet data-mining techniques have similar potential. automatically scanning other services such as facebook and flickr could yield firsthand accounts and damage photos. internet-based systems that do not rely on social networking are also feasible. the europeanmediterranean seismological centre (emsc) has implemented a system to map out the felt region of an earthquake by geocoding the ip addresses of people visiting its website [bossu et al. 2008]. the rapid increase in website visits is often emsc’s first indication that an earthquake has occurred. their system has proven so robust that initial indications of felt earthquakes derived from nonseismological data are publically posted prior to the seismologically derived parameters. this and previous studies show the potential for twitter and other social-media tools to aid in earthquake response. these tools are, at their root, a way for people to communicate, and millions of people are using them. after natural disasters people have used, and will continue to use, social media to describe what they saw and experienced. scientifically derived products such as shaking estimates from shakemap and impact estimates from the usgs prompt assessment of global earthquakes for response (pager) system [wald et al. 2010] ,once derived, replace the majority of the information that can be obtained from twitter. the basic benefit from twitter-based information is speed and the low cost of accessing and analyzing the data. the challenge is how best to summarize and present the often content-limited messages. twitter earthquake detection references bossu, r., g. mazet-roux, v. douet, s. rives, s. marin and m. aupetit (2008). internet users as seismic sensors for improved earthquake response, eos trans. agu, 89, 225-226. earle, p. and p. shearer (1994). characterization of global seismograms 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(2008). twittering the earthquake in china; http://scobleizer.com/2008/05/12/quake-in-china/. starbird, k. and j. stamberger (2010). tweak the tweet: leveraging microblogging proliferation with a prescriptive sytax to support citizen reporting, in: proceedings of the 7th international iscram conference, seattle. starbird, k. and l. palen (2011). ‘voluntweeters’: self organizing by digital volunteers in times of crisis, in: proceedings of the 2011 annual conference on human factors in computing systems, vancouver, canada. wald, l.a., d.j. wald, s. schwarz, b. presgrave, p.s. earle, e. martinez and d. oppenheimer (2008). the usgs earthquake notification service (ens): customizable notifications of earthquakes around the globe, seismol. res. lett., 79 (1), 103-110. wald, d.j., v. quitoriano, l. dengler and j.w. dewey (1999a). utilization of the internet for rapid community intensity mapss, seismol. res. lett., 70, 680-697. wald, d.j., v. quitoriano, t.h. heaton, h. kanamori, c.w. scrivner and b.c. worden (1999b). trinet “shakemaps” – rapid generation of peak ground-motion and intensity maps for earthquakes in southern california, earthqu. spectra 15, 537-556. wald, d.j., k.s. jaiswal, k.d. marano, d.b. bausch and m.g. hearne (2010). pager — rapid assessment of an earthquake’s impact, u.s. geological survey fact sheet 20103036, 4 p. *corresponding author: paul s. earle, u.s. geological survey, denver, co, usa; email: pearle@usgs.gov. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. earle et al. 715 global seismic tomography and modern parallel computers gaia soldati (1), lapo boschi (2), antonio piersanti (1) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) e.t.h. zürich, switzerland abstract a fast technological progress is providing seismic tomographers with computers of rapidly increasing speed and ram, that are not always properly taken advantage of. large computers with both shared-memory and distributedmemory architectures have made it possible to approach the tomographic inverse problem more accurately. for example, resolution can be quantified from the resolution matrix rather than checkerboard tests; the covariance matrix can be calculated to evaluate the propagation of errors from data to model parameters; the l-curve method can be applied to determine a range of acceptable regularization schemes. we show how these exercises can be implemented efficiently on different hardware architectures. keywords numerical inverse theory; seismology; global tomography; seismic resolution; earth’s mantle. 1 introduction earth tomography requires the solution of inherently large, mixed-determined inverse problems. since its very beginning, it has always involved the implementation of efficient algorithms on state-of-the-art computers. 1 in 1984, john woodhouse and adam dziewonski published in j.g.r. one of the few articles that defined global seismic tomography. in their conclusions, they noted: ”the calculations reported upon here were performed using an array processor (floating point, 120b) which, programmed in fortran, is typically 10 times as fast as, say, a vax 11/780. the path by path inversions and source determinations, using mantle waves and body waves, occupied the machine for approximately 60 hours, and each global iteration took approximately 7 hours. the inclusion of more data, and the extension of the method to higher frequencies, will probably require the use of the most advanced ’mainframe’ computers.” they are referring to an image of the earth’s upper mantle parameterized in terms of a cubic polynomial (vertically), and spherical harmonics up to degree 8 (laterally); that is to say, entirely specified by just 324 parameters (woodhouse and dziewonski 1984). the lower mantle was mapped in a separate inversion, as a linear combination of spherical harmonics up to degree 6, multiplied by legendre polynomials up to degree 4, resulting in 245 parameters; ”the size of the array needed to store the lower (or upper) triangle of the corresponding inner product matrix is 31,035; just a little less than the data memory of our ap-120b array processor, without which this study would not be feasible” (dziewonski 1984). in the following years, tomographers took advantage of a fast technological progress. the following generation of models published by the harvard group covered the entire mantle, and were linear combinations of chebyshev polynomials and spherical harmonics up to degree 12 (e.g., su et al. 1994). other authors preferred a different approach, parameterizing the earth’s mantle with grids of voxels (hager and clayton 1989, inoue et al. 1990). the ”voxel” approach involved a substantially larger number of model coefficients (∼ 104), but also implied that the integral of data sensitivity multiplied by the basis functions be most often 0. the latter circumstance has important consequences, as we shall briefly illustrate: the tomographic linear inverse 2 problem is typically written a · x = d, (1) where the entries of the vector x are the coefficients of the solution model (initially unknown), d are the data (e.g., travel times), and a is a matrix whose ij entry equals the integral, over the entire volume of the mantle, of the sensitivity of the i-th measurement to the earth property (typically a seismic velocity or slowness) to be mapped, times the j-th basis function used to describe such property. in the ray theory approximation, and if (as it will always be the case here) d are observations of travel time anomaly, the volume integral reduces to an integral along the seismic ray path (sensitivity is 0 everywhere but on the ray path), and aij = ∫ i-th path fj(r(s))ds, (2) where s denotes the incremental length along the ray path, identified by the equation r = r(s), with r denoting position; the n basis functions fj are used to describe the slowness δp(r) = ∑n k=1 xkfk(r) of the seismic phase in question. (it would be equivalent to formulate the problem in terms of velocity, but slowness happens to make algebra simpler.) if the functions fj are spherical harmonics, nonzero over the entire surface of the earth, the integral in eq. (2) will be nonzero for all values of j. in the case of a voxel (spline, wavelet or other ”local” functions) parameterization, the same integral will be 0, except for values of j whose corresponding voxel is crossed by a ray path. as anticipated, the matrix a will therefore be dense if the model is parameterized in terms of spherical harmonics or other ”global” functions; sparse if local functions are used. the same is consequently true of the matrix at · a, whose inverse has to be calculated for the least squares solution xls to (1) to be found (a necessary step, as (1) in global seismology is strongly mixed-determined and does not have an exact solution), xls = ( at · a + d )−1 · at · d, (3) 3 where the matrix d depends on the regularization scheme (e.g., boschi and dziewonski, 1999). when a and at ·a are sparse, equation (3) is most efficiently implemented via an iterative algorithm like cg or lsqr (e.g., trefethen and bau 1997). when they are dense, iterative algorithms become as slow as direct ones: the most efficient approach is then to implement (3) via cholesky factorization of at · a, and subsequent backsubstitution (e.g., press et al. 1994, trefethen and bau 1997). implementation of lsqr does not require at · a to be calculated, as lsqr operates directly on a. a is bigger but sparser than at ·a, and sparse matrices can be stored efficiently (e.g., press et al. 1994) to minimize the required disc space or ram; a is therefore often less cumbersome than at ·a: it is so, at least, when global body wave travel time databases are inverted to derive global earth structure. this, and the remarkable speed of lsqr in a regime of sparse a, allowed grand (1994) and van der hilst et al. (1997) to parameterize the earth’s mantle in terms of as many as n ∼ 250, 000 voxels: a three orders of magnitude increase in nominal resolution, with respect to the early studies of dziewonski (1984) and woodhouse and dziewonski (1984) mentioned above. like woodhouse and dziewonski some ten years before, grand and van der hilst were exploiting available computers to their limit. although sparse, a was still too large a matrix to be entirely fit on the ram of a processor; lsqr, however, required only parts of a to be available at one time in the ram: they could be run without ever storing a entirely in memory, at the expense of massive input from disc at each iteration. given the number of solution coefficients, at least ∼ 102 iterations were probably needed for lsqr to converge. this made even lsqr a very slow process, and left researchers with relatively little freedom to test the model resolution and the effect on the solution of different regularization schemes. in view of the exponential growth in cpu speed over the last decade (e.g., bunge and tromp 2003), and the concurrent decrease in the price of 4 ram, the current generation of global seismic tomographers has the means to approach the discipline in an entirely new fashion. with a fast processor, and enough ram to store a entirely, not only lsqr is sped up enormously, but more time-consuming direct algorithms like cholesky factorization of at · a also become feasible. 2 cholesky factorization on a multiprocessor, shared-memory computer it was originally proved by paige and saunders (1982), and later confirmed by nolet (1985) and boschi and dziewonski (1999), with applications to mixeddetermined tomographic problems, that lsqr converges correctly to the damped least squares solution (3), typically after a number of iterations � n . if a is sparse and sufficient ram is available to store it, lsqr is therefore the most efficient algorithm to solve an inverse problem in the least squares sense. on the other hand, because it by-passes the calculation of at · a and the direct implementation of (3), lsqr cannot provide any measure of goodness of resolution and covariance, except by means of resolution, or ”checkerboard” tests. the unreliability of the measure of resolution that those tests provide has been pointed out, for example, by lévêque et al. (1993), and there have been efforts to derive the resolution matrix via an iterative, lsqr-type calculation (zhang and mcmehan, 1995; minkoff, 1996; nolet et al., 1999, 2001; yao et al., 1999, 2001; vasco et al., 2003). the resolution matrix r can be thought of as the operator that relates ”output” and ”input” model in any checkerboard test; menke (1989) shows that r = ( at · a + d )−1 · at · a. (4) clearly, r does not depend on the input model, and its similarity to the identity matrix is a measure of goodness of resolution. its calculation requires that at · a + d be explicitly inverted, and this is most efficiently achieved 5 by cholesky factorization of this matrix. once the damped inverse of at · a is found, r is quickly determined by backsubsitution, applied on the matrix at · a instead of the vector at · d; this endeavour is not significantly more time-consuming than the implementation of (3) via cholesky factorization and backsubstitution. boschi (2003) computed r from the global teleseismic p-wave travel time database of antolik et al. (2003), based upon the isc bullettins and including ∼ 600, 000 summary observations. he parameterized the earth’s mantle in terms of 20 vertical splines and 362 horizontal splines (n = 7240). boschi’s (2003) exercise was conducted on an ibm sp2 with 16 processors and 32 gbytes of ram. the ibm sp2 is a ”shared-memory” machine: any processor can access at the same speed its entire ram. this is a very useful feature when large matrices have to be factorized, a process that is inherently hard to parallelize; clusters of pcs are by construction ”distributed-memory” computers, and hence more useful for the solution of forward, rather than inverse, problems. boschi (2003) notes that the most time-consuming step in deriving r is the computation of at ·a, which took about twenty-four hours. this process was parallelized by subdividing the database in as many subsets as there were available processors, computing each subset’s contribution to at · a on a separate processor, and eventually adding up the results. after computing at · a, which needs to be done only once, xls and r can be derived in a few minutes; boschi (2003) was thus able to perform numerous inversions, experimenting with the damping scheme and exploring the solution space, calculating each time the associated r. 3 running lsqr repeatedly on a distributedmemory cluster of pcs the computer on which this article is being written, a linux pc sitting on the second author’s desk, is equipped with a dual processor and 3 gbytes of 6 ram. one year ago, this much ram costed just about 800 u.s. dollars. we concluded section 1 pointing out that, as one can easily afford enough ram to store arrays of ∼ 105 elements, iterative algorithms like lsqr become extremely efficient. on this very computer, one lsqr inversion involving some 25,000 model coefficients runs to convergence in a matter of seconds. let us now show how, on a parallel, distributed-memory computer (a cluster of pcs), the resolution matrix r can also be derived in a reasonable amount of time, without calculating and cholesky-factorizing at · a. implementing equation (4) is equivalent to implementing n times equation (3), replacing each time d with a different column of a (recall that n denotes the number of model parameters, and hence the number of columns of a). r can therefore be derived by means of n independent lsqr inversions of a, without finding at · a. when only one or few processors are available, and with n ∼ 105 as in some of the experiments mentioned above, this process would be extremely time-consuming, to the point of not being worthwhile. if a relatively large parallel machine is available, however, the problem can be easily parallelized, by simply subdividing the n inversions into n/np subsets, np denoting the number of processors. each subset of inversions is then performed independently on a separate processor, and the time needed to compute r is reduced by a factor np . it should be noted that the most time-consuming step, input of a from disc to ram, needs to be performed only once per processor, no matter how many inversions are then run on each processor. figures 1 and 2 illustrate how this procedure applies to a real inverse problem. we describe the distribution of p-velocity heterogeneities in the earth’s mantle in terms of a grid of voxels of constant horizontal extent; voxel functions guarantee that a be more sparse than in the case of splines. we invert, again, the p-wave travel time database of antolik et al. (2003). following, e.g., inoue et al. (1990, section 3.3.1 and figure 2), we select roughness minimization as our only regularization criterion, and perform a number of preliminary inversions, at different parameterization levels, to assess the 7 dependence of the solution on the regularization parameter. plotting misfit to the data (defined as 1− the variance reduction) against ”total roughness” (the integral of the surface gradient of the model over the entire solid angle is computed for each layer of the model, and then the rms is taken) in figure 1, we find, for each parameterization, a set of points aligned along the expected l-shaped curve (e.g., hansen 1992; boschi et al., 2006). each point on the l-curve corresponds to a model derived at this preliminary stage, and the roughness damping parameter grows monotonically with increasing misfit. the shape of the curve, resembling the letter l, confirms that the data contain coherent and statistically significant information; the decrease in misfit is very fast in an overdamped regime, where a small reduction in the regularization parameter, and therefore a small increase in model complexity, is sufficient to improve the data fit substantially. the white noise that the data necessarily contain, and that regularization is supposed to eliminate, is harder to fit, even with large increases in model complexity: this is why the curve tends to become horizontal in the right part of the plot. solution models lying in the vertical and horizontal portions of the l-curve can be discarded as overdamped and underdamped, respectively; preferred models should be chosen near its corner. the selection of a damping scheme has always been a largely arbitrary process in global seismic tomography. the l-curve criterion is a way to reduce this arbitrarity. it is practical so long as a large number of lsqr inversions can be performed in a short time, and we have seen how this is made possible by simultaneous storage of the entire matrix a in memory, and/or availability of multiple processors. after so selecting optimal roughness damping parameters at all parameterization levels, we restrict ourseleves to the case of 5◦ voxels. we show in figure 2 the corresponding resolution matrix r as derived with multiple runs of lsqr. as to be expected (boschi, 2003), r is quite different from the identity matrix; entries smaller than 1 on its diagonal indicate that the amplitude of velocity heterogeneities in the corresponding voxel is underesti8 mated. entries different from zero away from the diagonal identify episodes of fictitious coupling between model coefficients; naturally, the value of rij is proportional to the amount of coupling (”trade-off”, ”smearing”...) between the i-th and j-th voxels (entries xi and xj of the solution vector). 4 performance and accuracy of direct vs. iterative implementations in analogy with yao et al. (1999), we calculate r associated with one given database and one choice of parameterization and regularization, both in the direct (cholesky, section 2 above) and iterative (lsqr, section 3) approaches, and compare the results. as opposed to singular value decomposition (svd), the direct algorithm implemented by yao et al. (1999), cholesky factorization does not involve the cancellation of the smallest singular factors (press et al., 1994), so that in our experiment regularization is entirely controlled by the matrix d, and is therefore exactly equivalent in the direct and iterative calculations. we implement equation (4) exactly, by cholesky factorization of at ·a+d, for the 5◦-voxel parameterization described in section 3, and applying the same regularization scheme that lead to r in figure 2. the result is shown in figure 3, and in figure 4 two lines of the directlyand iteratively-calculated rs are compared in a geographic view. differences are everywhere small, and negligible for i, j such that rij > 0.2. discrepancies between r calculated iteratively and directly (with svd) by yao et al. (1999, figure 5) appear to be larger. r in figures 2 through 5 is a 24, 840 × 24, 840 matrix, as opposed to the 7, 240×7, 240 r of boschi (2003). with 24, 840 free parameters, cholesky factorization, backsubstitution (via the lapack routines spotrf and spotrs, respectively), and all necessary input/output from and to disc take about 10 hours on a shared-memory compaq “alpha” computer (an es45 with 10gbytes ram and 4 cpus at 1250mhz); to compare this performance 9 with that of repeated lsqr on a pc-cluster, it should be kept in mind that, in the latter architecture, computation time scales perfectly with the number of processors; one lsqr inversion with 24, 840 free parameters, and applying paige and saunders’ (1982) criterion to evaluate convergence, currently takes ∼ 1 minute on a standard pc. 5 the covariance matrix r describes the fictitious coupling between solution coefficients (model parameters); it depends on the geographic distribution of sources and stations, and on the shape of ray paths, but not on the quality of inverted observations. the covariance of solution coefficients depends, instead, on the error and covariance of the initial data, and on the error amplification occuring in the inversion (menke, 1989, section 3.11). in the assumption that seismic data be uncorrelated and all have equal variance σ2, menke (1989, equation 3.48) introduces a covariance matrix c = σ2 ( at · a + d )−1 · at · [(at · a + d)−1 · at ]t (5) (the regularization matrix d was not included explicitly in menke’s (1989) formula). equation (5) can be rewritten c = σ2 ( at · a + d )−1 · at · a · [(at · a + d)−1]t , (6) and making use of (4) c = σ2r · [( at · a + d )−1]t . (7) after cholesky factorizing at · a + d, we find c by (i) backsubsitution of the n × n identity matrix, and (ii) dot-product (via the lapack routine sgemm) of the (transposed) result with r. after r is read from disc or calculated again, the process takes about 10 more hours, with n = 24, 840, on the shared-memory machine described in section 4 above. figure 6 shows c, derived in the same parameterization (5◦ voxels) and regularization as 10 figures 2 through 5 above, and assuming for antolik et al.’s (2003) database a standard deviation σ = 0.5 s (antolik, personal communication, 2005). we have not found an effective approach to calculating c on a distributedmemory cluster. except for the upper mantle, where the solution is less stable owing to strong nonuniformities in the data coverage, c (figure 6) is relatively close to diagonal, indicating that errors in model parameters are not strongly correlated. the n diagonal entries of c can be interpreted as squared errors associated with the corresponding model parameters (menke, 1989; tarantola, 2005): after taking their square root and multiplying it by 100 (in a voxel parameterization, solution coefficients coincide with velocity heterogeneities in the corresponding voxel, which are typically expressed in percent), we show in figure 7 each diagonal entry of c at the corresponding voxel. as to be expected, error is smallest in regions of good data coverage, e.g. the upper and mid-mantle underlying north america and eurasia, where seismic stations are most densely distributed; it is highest at the top of the upper mantle, where the almost vertical geometry of teleseismic ray paths poses a significant limit to resolution (hence strong “smearing”); it grows with increasing depth in the bottom layers of the lower mantle, sampled more uniformly than shallowest regions, but by a decreasing number of ray paths. mapped p-velocity anomalies from the observations considered here (e.g., boschi & dziewonski, 1999; boschi, 2003) range between ±1% in most of the mantle, so that the error of ±0.15% or less that we have derived from c is generally nonnegligible, but small. 6 summary we have presented two approaches to the solution of large mixed-determined inverse problems, both exploiting the quickly increasing speed and ram of modern computers (e.g., bunge and tromp 2003). we have verified (section 4) that the two approaches, applied to the same problem, yield coincident 11 results. the first approach, described in section 2, is inherently sequential, and is best applied to shared-memory computers. it rests on the direct implementation of the least squares formula to derive the least squares solution xls and model resolution matrix r associated with the inverse problem a ·x = d. xls and r are computed with one cholesky factorization of a t ·a and n + 1 repetitions of the backsubstitution process, n being the number of model coefficients. in this approach, the derivation of r is thus relatively fast. unfortunately, the number of floating point operations required to choleskyfactorize at · a grows like n 3, as finer parameterizations are implemented (e.g., trefethen and bau 1997, page 175). likewise, as n grows, the size of at ·a grows like n 2, and ram can also become an issue: routines performing cholesky factorization, available in the literature (e.g., press et al. 1994) or through optimized libraries, do not allow for efficient storage of at ·a (which could be quite sparse), and require a comparably large additional amount of ram to be left free for temporary storage. the second approach (section 3) involves the repeated application of an iterative, cg-type algorithm (lsqr in our implementation). xls is found after one run of lsqr, the calculation of r requires n runs of the same algorithm. however, we have shown that the problem can be simply parallelized, and it is thus most appropriate for implementation on distributed-memory pc-clusters. as n grows, the growth in the number of floating-point operations will not be as fast as in the case of the first approach, and it will be relatively cheap to speed up the process by simply making use of a few more processors. as long as a is sparse, which is always the case in the ray-theory approximation and with local-basis-function (voxels, splines...) parameterizations, the amount of disc space (and/or ram) needed to store a also grows more slowly with increasing n than that needed to store at ·a: in the experiment discussed in section 3, a occupies roughly 1 gbyte of ram or disc space; at · a would need twice this amount, plus the temporary storage mentioned above. 12 one drawback of the cg/lsqr multi-processor approach resides in the difficulty of computing the covariance matrix c. we have shown in section 5 how c can instead be computed via the first, “sequential” approach, and we have made use of c to evaluate model error in typical, global tomographic inversions of seismic travel time observations (figure 7). in summary, both approaches should prove profitable, depending on the available hardware. the optimization of tomographic algorithms for use with modern computers is leading to a better understanding of the tomographic inverse problem, and to more reliable evaluations of model quality and resolution. acknowledgments our research is part of spice (seismic wave propagation and imaging in complex media: a european network), a marie curie research training network in the 6th framework program of the european commission; we are grateful to the coordinators at lmu munich. we thank domenico giardini for his support and encouragement. all figures were done with gmt (wessel and smith 1991). references [] antolik, m., gu, y. j., ekström, g. & dziewonski, a. m., 2003. j362d28: a new joint model of compressional and shear velocity in the mantle, geophys. j. int., 153, 443–466. [] boschi, l., 2003. measures of resolution in global body wave tomography, geophys. res. lett., 30, no. 19, 1978, doi:10.1029/2003gl018222. [] boschi, l. & dziewonski, a. m., 1999. “high” and “low” resolution images of the earth’s mantle implications of different approaches to tomographic modeling, j. geophys. res., 104, 25,567–25,594. 13 [] boschi, l., t. w. becker, g. soldati, and a. m. dziewonski, 2006. on the relevance of born theory in global seismic tomography, geophys. res. lett., 33, l06302, doi:10.1029/2005gl025063. [] bunge, h.-p. & tromp, j., 2003. supercomputing moves to universities and makes possible new ways to organize computational research, eos, trans. am. geophys. un., 84, 30–33. [] dziewonski, a. m., 1984. mapping the lower mantle: determination of lateral heterogeneity in p velocity up to degree and order 6, j. geophys. res., 89, 5929–5952. [] grand, s. p., 1994. mantle shear structure beneath the americas and surrounding oceans, j. geophys. res., 99, 11,591–11,621. [] hager, b. h., & r. w. clayton, 1989. constraints on the structure of mantle convection using seismic observation, flow models, and the geoid, in mantle convection-plate tectonics and global dynamics, edited by w. r. peltier, pp. 657–763, gordon and breach, newark, n. j. [] hansen, p. c., 1992. analysis of discrete ill-posed problems by means of the l-curve, siam review, 34, 561–580. [] inoue, h., fukao, y., tanabe, k. & y. ogata, 1990. whole mantle p wave travel time tomography, phys. earth planet. inter., 59, 294–328. [] lévêque, j. j., l. rivera, & g. wittlinger, 1993. on the use of the checkerboard test to assess the resolution of tomographic inversions, geophys. j. int., 115, 313–318. [] menke, w., 1989. geophysical data analysis: discrete inverse theory, rev. ed., academic, san diego. [] minkoff, s. e., 1996. a computationally feasible approximate resolution matrix for seismic inverse problems, geophys. j. int., 126, 345–359. 14 [] nolet, g., 1985. solving or resolving inadequate and noisy tomographic systems, j. comput. phys., 61, 463–482. [] nolet, g., r. montelli, & j. virieux, 1999. explicit, approximate expressions for the resolution and a posteriori covariance of massive tomographic systems, geophys. j. int., 138, 36–44. [] nolet, g., r. montelli, & j. virieux, 2001. reply to comment by z. s. yao, r. g. roberts and a. tryggvason on “explicit, approximate expressions for the resolution and a posteriori covariance of massive tomographic systems”, geophys. j. int., 145, 315. [] paige, c. c., & m. a. saunders, 1982. lsqr: an algorithm for sparse linear equations and sparse least squares, acm trans. math. soft., 8, 43–71. [] press, w. h., teukolsky, s. a., vetterling, w. t. & b. p. flannery, 1994. numerical recipes in fortran, cambridge university press, u. k. [] soldati, g. & l. boschi, 2004. whole earth tomographic models: a resolution analysis, eos, trans. am. geophys. un., 85(47), fall meet. suppl. [] su, w.-j., r. l. woodward & a. m. dziewonski, 1994. degree-12 model of shear velocity heterogeneity in the mantle, j. geophys. res., 99, 4945– 4980. [] tarantola, a., 2005. inverse problem theory and model parameter estimation, siam, philadelphia. [] trefethen, l. n. & d. bau iii, 1997. numerical linear algebra, soc. for ind. and appl. math., philadelphia, penn. [] van der hilst, r. d., s. widiyantoro & e. r. engdahl, 1997. evidence for deep mantle circulation from global tomography, nature, 386, 578–584. [] vasco, d. w., l. r. johnson & o. marques, 2003. resolution, uncertainty, and whole-earth tomography, j. geophys. res., 108, 2022, doi:10.1029/2001jb000412. 15 [] wessel, p. & w. h. f. smith, 1991. free software helps map and display data. eos, trans. am. geophys. un., 72, 445–446. [] woodhouse j. h. & a. m. dziewonski, 1984. mapping the upper mantle: three-dimensional modeling of earth structure by inversion of seismic waveforms, j. geophys. res., 89, 5953–5986. [] yao, z. s., r. g. roberts & a. tryggvason, 1999. calculating resolution and covariance matrices for seismic tomography with the lsqr method, geophys. j. int., 138, 886–894. [] yao, z. s., r. g. roberts, & a. tryggvason, 2001. comment on “explicit, approximate expressions for the resolution and a posteriori covariance of massive tomographic systems” by g. nolet, r. montelli and j. virieux, geophys. j. int., 145, 307–314. [] zhang, j. & g. a. mcmehan, 1995. estimation of resolution and covariance for large matrix inversions, geophys. j. int., 121, 409–426. 16 figure 1: data misfit achieved by a set of solution models, vs. the integrated roughness of each model. this measure of model complexity is normalized against model rms. least squares solutions were found from a wide range of values of the roughness minimization parameter, and no other minimization constraint. we repeated the experiment with voxels of lateral extent 15◦ × 15◦, 10◦ × 10◦, 7.5◦ × 7.5◦, 6◦ × 6◦, 5◦ × 5◦, 3.75◦ × 3.75◦, 3◦ × 3◦, 2.5◦ × 2.5◦, and constant vertical thickness (∼ 200 km) (soldati and boschi, 2004): corresponding solutions align on different l-curves, and squares of decreasing size correspond to increasingly fine parameterization. figure 2: (top) 24, 840 × 24, 840 (5◦-voxel grid) resolution matrix r for the chosen regularization scheme, averaged (boschi, 2003) so that it can be plotted in this limited space; here, vertical tradeoffs are most evident. (bottom) zooms on r, not averaged, at two selected layers (left: mid mantle at ∼ 1300 km depth; right: lower mantle at ∼ 2600 km). figure 3: same as figure 2, but r was computed by cholesky factorization of at · a + d, as described in section 4. figure 4: rows of r (figures 2 and 3) associated with a relatively well resolved voxel i located in the mantle under japan, at 700 km mean depth, from the parallel lsqr (left) and cholesky (right) approaches. for each value of j, the color of the j-th voxel depends on the value of rij; rij is a measure of fictitious trade-off between i-th and j-th model parameters. figure 5: same as figure 4, but voxel i, less well resolved, is located under central america, at 2200 km mean depth. figure 6: 24, 840 × 24, 840 (5◦-voxel grid) covariance matrix c associated with the same data, parameterization and regularization as r above, defined as in section 5, and derived by cholesky factorization and backsubstitution. figure 7: absolute error on mapped percent p-velocity heterogeneity, calculated from the diagonal entries of c (figure 6) and plotted at each corresponding model voxel. all 15, ∼ 200 km thick layers of the 5◦-voxel grid are shown; the shallowest layer is at the top and to the left, the plot below corresponds to the second shallowest layer, and so on; the deepest layer (∼ 2700 km depth to core-mantle boundary) is at the bottom and to the right. constant, uncorrelated variance σ = 0.5 is assumed on all travel-time observations. 18 figura 1 figura 2 figura 3 figura 4 figura 5 figura 6 figura 7 layout 6 evidence of persistent seismo-volcanic activity at marsili seamount antonino d'alessandro, giorgio mangano, giuseppe d'anna istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-5515 the marsili submarine volcano is the largest european volcano, and it can be considered as the key to our understanding of the dynamics of the spreading and back-arc lithosphere formation in the tyrrhenian sector [marani et al. 2004, and references therein]. despite its size, it is very difficult to monitor due to its geographical position [d'alessandro et al. 2011], and it still remains little known. in 2006, the centro nazionale terremoti (national earthquake centre) of the istituto nazionale di geofisica e vulcanologia (ingv) deployed a broadband ocean-bottom seismometer with hydrophone (obs/h) [mangano et al. 2011] on the flat top of marsili volcano, at a depth of ca. 790 m. in only nine days, the instrument recorded ca. 800 seismovolcanic events [d'alessandro et al. 2009]. this revealed the intense seismo-volcanic activity of marsili volcano for the first time. however, the short duration of the experiment did not allow exhaustive characterization of the seismo-volcanic activities currently ongoing on the seamount. for this reason, on february 14, 2010, another obs/h was deployed at the same point for a long-term experiment (of 9 months). during this monitoring campaign, the submarine station recorded some thousands of local small-magnitude events. the entire dataset was classified on the basis of the time and frequency domain appearances according to wasserman [2002]. through this analysis, we recognized 589 volcano-tectonic type a (vt-a) events and 1,952 volcanotectonic type b (vt-b) events (figure 1), with their local magnitudes measured according to havskov et al. [2003]. the seismogram of a typical vt-a event is dominated by the p and s phases and by a short coda (figure 1a). these events are characterized by p phases with impulsive and highamplitude onsets. the spectrogram shows broadband body phases with very high frequency and energy contents, extending up to 80 hz (figure 1c). the vt-a events that were recorded showed local magnitudes between 0.5 and 3.0, and duration times of between 40 s and 70 s. for these earthquakes, the separation of the p and s waves is clear, and the ts-tp is between 0.35 s and 0.55 s. their average recurrence time was about 2-3 events per week, with moderate variations through the observation period. the vt-b events are characterized by p phases with emergent and low-amplitude onsets (figure 1b). the waveforms do not show any clear s-wave arrivals, and they show a long coda (figure 1b). the vt-b events have duration times of between 15 s and 40 s, and local magnitudes between −0.5 and 1.5. their spectrograms show a narrow frequency content (figure 1d). the time distribution of vt-b occurrence shows periods of moderate activity (a few events per week) alternating with periods of intense activity (70 events per day). from figure 1f, a time cyclic process is also clear, with an upward trend in the vt-b activity. an increase in the vt-b activity has often been reported for some active volcanoes before significant eruptions [zobin 2011 and references therein]. these observations of vt swarms for the marsili volcano strongly suggest that it is still active. furthermore, it should not be ignored that potential volcanic eruptions might generate tsunamis along the nearby coastlines. references d'alessandro, a., g. d'anna, d. luzio and g. mangano (2009). the ingv's new obs/h: analysis of the signals recorded at the marsili submarine volcano, j. volcanol. geoth. res., 183, 17-29; doi:10.1016/j.jvolgeores.2009.02.008. d'alessandro, a., d. luzio, g. d'anna and g. mangano (2011). seismic network evaluation through simulation: an application to the italian national seismic network, b. seismol. soc. am., 101, 1213-1232; doi: 10.1785/0120100066. havskov, j., j.a. peña, j.m. ibáñz, l. ottermöller and c. martínez-arévalo (2003). magnitude scales for very local earthquake. application for deception island volcano (antartica), j. volcanol. geoth. res., 128, 115-133; doi:10.1016/s0377-0273(03)00250-6. mangano, g., a. d'alessandro and g. d'anna (2011). longterm underwater monitoring of seismic areas: design of an ocean bottom seismometer with hydrophone and its performance evaluation, oceans, 2011 ieee (spain, june 6-9, 2011), isbn: 978-1-4577-0086-6, pp. 1-9; doi:10.1109/ oceans-spain.2011.6003609. article history received december 5, 2011; accepted april 25, 2012. subject classification: volcano monitoring, marsili seamount, seismo-volcano activity, obs/h. 213 scientific news marani, m.p., f. gamberi and e. bonatti (2004). from seafloor to deep mantle: architecture of the tyrrhenian backarc basin, series "memorie descrittive della carta geologica d'italia", vol. 64, 194 pp. wasserman, j. (2002). volcano seismology, in: p. bormann (ed.), iaspei new manual of seismological observatory practice, vol. 1, geoforschungszentrum potsdam, 42 pp. zobin, v.m. (2011). introduction to volcanic seismology, 2nd edition, elsevier, isbn 9780444563750, 498 pp. d'alessandro et al. 214 figure 1. waveforms (a, b), spectrograms (c, d), and time distributions (e, f) of the vt-a (a, c, e) and vt-b (b, d, f) events recorded on the marsili volcano seamount during the 2010 monitoring campaign. insets in (a) and (b): zoom in on the onset of the waveforms. ! *corresponding author: antonino d'alessandro, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy; email: obslab@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. n o n l i n e a r l a m i n a r f l o w i n t o e c c e n t r i c a l l y p l a c e d w e l l k . k . u p a d h y a y ( * ) r e c e i v e d o n s e p t e m b e r 2 n d , 1 9 7 5 summary. — i n s t e a d y s t a t e c o n d i t i o n , n o n l i n e a r l a m i n a r flow of f l u i d i n t o a n e c c e n t r i c a l l y p l a c e d w e l l is c o n s i d e r e d . i t s i n f l u e n c e o n t l i c d i s c h a r g e a n d t h e d e p e n d e n c e o n r e l a t e d p h y s i c a l q u a n t i t i e s is i n v e s t i g a t e d . i t is o b s e r v e d t h a t a s t h e w e l l a p p r o a c h e s t o w a r d s t h e c o n t o u r of i n t a k e , t h e d i s c h a r g e i n c r e a s e s , w h i c h is a n o b v i o u s r e s u l t c o n s i s t e n t w i t h t h a t o b t a i n e d b y p o l u b a r i n o v a k o c h i n a i n c a s e of l a m i n a r flow. a s a p a r t i c u l a r c a s e , r e s u l t f o r c o n c e n t r i c w e l l h a s a l s o b e e n d e d u c e d . riassunto. — v i e n e p r e s o i n c o n s i d e r a z i o n e u n flusso d i fluido l a m i n a r e n o n l i n e a r e — i n c o n d i z i o n i d i s t a t o s t a z i o n a r i o — d e n t r o u n p o z z o d i s p o s t o e c c e n t r i c a m e n t e . si è i n o l t r e s t u d i a t a s i a l a s u a i n f l u e n z a s u l g e t t o c h e l a d i p e n d e n z a d a l l e r e l a t i v e q u a n t i t à fisiche. è s t a t o o s s e r v a t o c h e , c o m e ci si a v v i c i n a a l c o n t o r n o d e l l o s b o c c o , il flusso a u m e n t a , il clie è u n r i s u l t a t o o v v i o , i n a c c o r d o c o n q u a n t o o t t e n u t o d a i r i c e r c a t o r i p o l u b a r i n o v a e k o c h i n a , n e l c a s o d i u n flusso l a m i n a r e . i l r i s u l t a t o r e l a t i v o a d u n p o z z o c o n c e n t r i c o n o n ò q u i n d i c h e u n c a s o p a r t i c o l a r e d e l p r o b l e m a a f f r o n t a t o i n q u e s t a n o t a . 1 . i n t r o d u c t i o n the intricacy in the n a t u r e of porous media does not always j u s t i f y t h e n a t u r a l flow of fluid through it to be purely laminar. however, it appears more justifiable to consider the liow through porous media to be either non-linear laminar or t u r b u l e n t (j). consequently j a i n and u p a d h y a y (2), elenbaas and k a t z (3), engelund (4) obtained specific solutions of some non-linear laminar and t u r b u l e n t flow problems. i n the present paper, we consider the non-linear laminar steady state flow of fluid into an eccentrically places well fully penetrating ( * ) 7 9 , mill r o a d , d e w a s , ( m . p . ) i n d i a 3 1 2 k. k. u p a d h y a y t h e porous aquifer. i t is found t h a t t h e flow p a t t e r n is characterised b y t w o different zones, in which discharge exhibits opposite c h a r a c t e r as regards its dependence on grain size of t h e m e d i u m , viscosity of t h e fluid and r a d i u s of t h e well. f u r t h e r , it is observed t h a t as t h e well approaches t h e contour of i n t a k e , t h e discharge increases a b r u p t l y as compared t o t h a t into a concentrically placed well, which is obvious f r o m physical considerations. the results for a concentric well h a v e been deduced and compared with those obtained b y u p a d h y a y (5). 2 . e q u a t i o n s o f f l u i d f l o w i n p o r o u s m e d i u m the d a r c y ' s law governing t h e l a m i n a r flow of fluid i n porous m e d i a is ah where v, k and —— denote t h e seepage velocity, seepage coefficient d o and h y d r a u l i c g r a d i e n t respectively; flow being in t h e opposite direction of increasing h. i n case of an eccentrically placed circular well fullly p e n e t r a t i n g t h e cylindrical s t r a t u m (radius r) of u n i t thickness, t h e pressure p a t a n y point with complex coordinate z is obtained in t h e f o r m (6) q/j. , r {z—zi) v = o 7 ~ l o 8 ' f t + 0 2 ] 2nk0 (it2—zz i) where q, k0 and ¡i represent flow r a t e , permeability of t h e m e d i u m a n d viscosity of t h e fluid respectively; zi denotes t h e centre of well and zi is t h e corresponding inverse point. the c o n s t a n t g is t o be d e t e r m i n e d by b o u n d a r y conditions. besides relations [1] and [2], t h e law for non-linear l a m i n a r flow is(°) ~ = av + bv2, [ 3 ] where a and b are constants. according t o engelund (4) n o n l i n e a r laminar flow into eccentrically placed w e l l 3 1 3 2 0 0 0 a a = ; , ggd2 » = ^ gd m i m . q and d being density of t h e fluid and grain size of t h e m e d i u m . 3 . f o r m u l a t i o n o f t h e p r o b l e m i n s t e a d y s t a t e condition, we consider t h e flow of fluid into an uncased circular cylindrical well of radius r w eccentrically established a t a distance ri f r o m t h e centre of t h e contour of intake. i t is assumed t h a t t h e well is completely p e n e t r a t i n g t h e porous aquifer of thickness t. the aquifer is considered to be homogeneous and isotropic bounded b y horizontal impervious layers. the pressure a t t h e contour of well and a t t h e contour of i n t a k e are prescribed as 'p w and pc respectively. l e t r be t h e radial distance measured f r o m t h e axis of intake. as t h e effect of non-linear laminar or t u r b u l e n t flow is observed t o be appreciable event if such flow is restricted to a comparatively n a r r o w zone (4), we consider t h e flow t o be non-linear laminar within a narrow cylindrical zone of radius rt surrounding t h e well and laminar beyond this zone. l e t t h e pressure a t t h e transition b o u n d a r y is pt [fig. 1]. k g . l i n t h e present situation, we h a v e («,) = (»,) = ri 3 1 4 k. k. u p a d h y a y therefore, expression for pressure distribution in t h e l a m i n a r zone (of. [2]) t a k e s t h e f o r m qf* , r (r—ri) . _ rk1 p = l o g í m t + c [ 5 ] the problem is to examine t h e influence of non-linear l a m i n a r flow 011 discharge of fluid and its dependence on t h e related physical q u a n t i t i e s . 4 . s o l u t i o n as in t h e vicinity of well t h e lines of equal pressure are closed to circles, therefore, we assume t h e contour of well as one of t h e isobars close t o t h e circle of r a d i u s rw (c). along t h e b o u n d a r y of transition, which is close t o t h e contour of well, pressure pt m a y be obtained f r o m [5] b y using t h e b o u n d a r y conditions p = pc a t r = r, p = pt a t r = ri + rh (rt « ri < r). [6] h e n c e ^z4 i r r> m ~2nlcat l 0 g r^r^ [ 7 ] since p = qgli, pressure distribution in t h e non-linear laminar zone is obtainable f r o m [3] as — ~ = av + bv* [8] gg dr i n general, discharge q f r o m a n y cylindrical surface of radius a and height t is q = 2 n i tv. [9] consequently, in this situation r = r1 + a, rw < a < r,, [ 1 0 ] n o n l i n e a r laminar flow into eccentrically placed w e l l 3 1 5 i t follows f r o m [8], [9] and [10] t h a t f t ri-\-rt q / « j (£ bq1 , , pu ri+r, [11] i.e. pt = p„ q9 aq l o 2 j i t \ r i + r ri+rt\ , bq2 •in2t2 \i?i+r,„ ri+rt [12] a t t h e b o u n d a r y of t r a n s i t i o n f r o m l a m i n a r t o non-linear l a m i n a r flow, t h e r e l a t i o n b e t w e e n critical r e y n o l d ' s n u m b e r f c = 0.07 a n d critical velocity vc is given b y (") q t « [ 1 3 ] dli w h e r e — as given b y [1] a n d [3] yield t h e s a m e v a l u e . accordingly vc m i b \ — = a vc ( 1 + — vc), l{> a or, —= 1.07 a. k since k = — , i t follows t h a t i" jl ko = 1.07 agg [14] using [14] in [7] and t h e n c o m p a r i n g w i t h [12], we g e t pc—p„ ~qìt aq_ 2tzt log + bi+n ri+rw bq2 4n2t2 \ri+r — 1.07 log rr, r2—ri2 1 ri+rt [15] c o m b i n i n g e q u a t i o n s [4]i [4], a n d [13] w i t h [15], we o b t a i n gd3 {pc—pw) h 2 r w 8000 rt log + 0.07 r, ri+r, ri+r„ — 1.07 lop rrt r2-r,2 í + ( » ' i — r , c ) (ri+r,c) (ri+rt) [ 1 0 ] 3 1 6 k. k . u p a d h y a y if we a s s u m e p u r e l y laminar flow in t h e entire flow regio n t h e n t h e flow r a t e qu,m m a y be obtained f r o m [5] b y using t h e corresponding b o u n d a r y conditions a t t h e well a n d t h e counter of i n t a k e . h e n c e q lam — 2 nkt q9 ipo — pw) lor r2—r i2 r rw [17] therefore, f r o m [13] and [17], we obtain t h e ratio q = 8 5 6 0 ^ q' rw [ q(p {pc — p,„) i n t r o d u c i n g dimensionless q u a n t i t y x and r a t i o y such t h a t gd3 (pc—pw) x = j u 2 r w q qlan [19] i [19], and combining [18] with [16], we obtain an implicit relation 1.07 x xy z i i b i , a " r l o g t + lieooz •log ( f + 1 ) + — 1.07 log x y 8560 z + 1.07 z + 0.07 xy xy 8560 z [20] 8560 z (ri \ / ri i i r,o + j1 rw 8560 z where z = log r2 — ri r r,0 i t m a y be inferred f r o m [19]i t h a t t h e value of x which is possible f r o m physical considerations is x > 0 , hence equation [20] becomes g n o n l i n e a r laminar flow into eccentrically placed w e l l 3 1 7 1.07 z = y 0 1 ra 8560 z — 1.07 log x y 8560 z l o g g + l ] + + 1.07 z + 0.07 x y 8560 z x y 8560 z — 1 ih r w + xy 8560 z [21] 5 . p a r t i c u l a r c a s e if e i = 0 t h a t is, when the well is established concentrically with respect t o t h e contour of intake, equation [21] reduces t o 1.07 log r = y 0.07 x y 8560 log r + x y — 0.07 log i q _ n ( r 8560 log \ r w 1.07 log 0.07 [22] which corresponds t o t h e non-linear l a m i n a r flow of fluid into a fully p e n e t r a t i n g concentric well discussed b y u p a d h y a y (5). 6 . d i s c u s s i o n f r o m [19]i, it is evident t h a t x depends on t h e density of t h e fluid, grain size of t h e medium, ju'essure difference of t h e system, viscosity of t h e fluid and well radius. since d and ¡i occur in higher powers in expression for x, t h e y highly affect t h e discharge. moreover, f r o m physical considerations it is obvious t h a t x and y are b o t h positive. 3 1 8 k . k . u p a d h y a y now, t o get tlie definite idea of t h e r e s u l t [21], we t a k e — = 3 • 103 t h a t is t h e r a d i u s of contour of i n t a k e is 3000 times rw t h e radius of t h e well. considering — = 102, t h e numerical values " r w of y are obtained corresponding t o different values of x > 0 and h a v e been graphically p l o t t e d in t h e f o r m of curve i [fig. 2]. i t is seen f r o m c u r v a i t h a t as x increases, initially y increases till it a t t a i n s a m a x i m u m value 2.16 corresponding to x = 1.2 691 -107, a f t e r w a r d s it descresases asymptotically. thus in t h e f o r m e r region o <x < 1 . 2 6 9 1 • 107, t h e discharge increases as x increases, t h a t is, when t h e density of t h e fluid, grain size of t h e m e d i u m and pressure difference of t h e system increases, viscosity of t h e fluid and t h e well radius decreases. i n t h e later region x > 1 . 2 6 9 1 • 107 t h e influence of non-linear l a m i n a r flow is reversed. thus, it m a y be concluded t h a t in case of non-linear l a m i n a r flow, t h e flow p a t t e r n is characterised by t w o different zones in which, discharge exhibits opposite character. n o n l i n e a r l a m i n a r f l o w i n t o e c c e n t r i c a l l y p l a c e d w e l l 3 1 9 7 . c o m p a r i s i o n to examine as t o how t h e position of t h e well affects t h e dischagre into it, we consider t h e cases — = 0 and — = 10. these cases t w tw h a v e been graphically respresented by d o t t e d curve and c u r v e i i respectively in fig. 2. hence, it is inferred t h a t as t h e well approaches t h e contour of i n t a k e t h e discharge increases a b r u p t l y as compared to t h a t into a well concentrically established with respect to t h e contour of i n t a k e . f r o m physical consideration, the result is quite obvious and consistent with t h a t obtained b y poluberinova-kochina (6) in case of l a m i n a r flow. a c k n o w l e d g e m e n t the a u t h o r gratefully acknowledges t h e valuable guidance of d r . s. k . j a i n , d e p a r t m e n t of applied mathematics, g.s. i n s t i t u t e of technology and science, i n d o r e in the p r e p a r a t i o n of this p a p e r . h e is also v e r y m u c h t h a n k f u l to t h e referee for his valuable suggestions for t h e i m p r o v e m e n t of t h e p a p e r . r e f e r e n c e s p ) bear, j . , 1 9 7 2 . dynamics of fluids in porous media. a m e r i c a n e l s e v i e r p u b l i s h i n g c o m p a n y i n c . , n e w y o r k . ( 2 ) j a i n , s. k . , u p a d i i y a y , k . k . , 1 9 7 5 . non-linear laminar flow of fluid into a partially penetrating well. " g e r l a n d s b e i t r i i g e z u r g e o p h y s i k " (in p r e s s ) . ( 3 ) e l e n b a a s , j . r . , k a t z , d . l . , 1 9 4 8 . a radial turbulent flow formula. " t r a n s . a i m e " , 1 7 4 , p . 25. ( 4 ) e n g e l u n d , f . , 1 9 5 3 . on laminar and turbulent flow of ground water in homogeneous sand. " t r a n s . d a n . a c a d . t e c h . s c i " , 3 , p p . 1 1 0 5 . ( 5 ) u p a d i i y a y , k . k . , 1 9 7 6 . non-linear laminar flow of fluid into a fully penetrating cylindrical well. " i n d . j . t l i e o . p l i y s . " (in p r e s s ) . (6) polubarinova-kociiina, p . y a . , 1952. theory of ground water movement. p r i n c e t o n u n i v e r s i t y , p r e s s , p r i n c e t o n , 3 6 4 , p . 17. vol50,3,2007 annals of geophysics vol. 50, n. 3, june 2007 editor in chief enzo boschi executive editor calvino gasparini associate editors massimo cocco, massimiliano stucchi, gianluca valensise, alessandro amato, paolo baldi, maurizio bonafede, rodolfo console, michele dragoni, domenico giardini, carlo giunchi, stefano gresta, antonio meloni, andrea morelli, antonio navarra, antonio rovelli, jean virieux, bruno zolesi editorial office anna grazia chiodetti francesco caprara istituto nazionale di geofisica e vulcanologia via donato creti, 12 40128 bologna italy tel. (39) 051 4151417 fax (39) 051 4151499 e-mail: annals@ingv.it editorial production editrice compositori via stalingrado, 97/2 40128 bologna italy tel. (39) 051 3540111 fax (39) 051 327877 e-mail: info @annalsofgeophysics.net http://www.compositori.it annals of geophysics is indexed in: science citation index®-expanded (also known as scisearch®), isi alerting services, inspec, georef database. on the cover: migrated and stacked gpr section alb 4.8. yellow boxes indicate anomalies due to wall response while blue box indicates a flat high-energy reflection probably due to a buried floor. the white box indicates a room filled up with backfill. http://www.annalsofgeophysics.net 447 annals of geophysics, vol. 50, n. 3, june 2007 key words ionosphere – gps – real-time imaging – tec – tomography 1. introduction tomography is a mathematical technique to reconstruct two-dimensional images and it is best known for the reconstruction of the human body from x-ray measurements (hounsfield, 1972) in the computer-aided tomography (cat). in 1988 austen and co-authors (alfonsi et al., 1988) proposed imaging the earth’s ionosphere with tomography to determine the distribution of free electron concentration from satellite-to-ground radio signals. ionospheric tomography is well established but from an application viewpoint has two critical drawbacks. first, tomographic images are only two dimensional (latitude versus altitude slices), whereas full three-dimensional images are usually needed. second, conventional ionospheric tomography used data from low-earthorbit (leo) satellites that are not continuously in view. this meant that images can only be produced every few hours, at irregular intervals when a leo satellite passes over the region of interest. this is a problem because the ionosphere can undergo significant changes on timescales of minutes, far shorter than the interval of time between consecutive tomographic images. since the availability of gps satellites starting from early nineties, phase delay and pseudo-range measurements from a relevant numbers of ground stations have become available for research. these measurements furnish tec evaluation along the huge amount of ray-path mirto: a prototype for real-time ionospheric imaging over the mediterranean area lucilla alfonsi (1), cathryn n. mitchell (2), vincenzo romano (1) and paolo spalla (3) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) department of electronic and electrical engineering, university of bath, u.k. (3) istituto di fisica applicata «nello carrara» (ifac), cnr, sesto fiorentino (fi), italy abstract mirto (mediterranean ionosphere with real-time tomography) is a collaborative project between istituto nazionale di geofisica (ingv) of rome, the university of bath (u.k.) and the istituto fisica applicata «nello carrara»-consiglio nazionale delle ricerche (ifac-cnr) of florence. the goal of the project is the development of a prototype for real-time imaging of the ionosphere over the italian region with extension to the mediterranean sea. mirto uses an original imaging technique developed at the university of bath and upgraded for real-time use in cooperation with ifac. the prototype makes use of the data acquired by the real-time ionospheric and geodetic instrumentation operated by ingv. such measurements drive the imaging algorithm to produce the image of electron density as well as maps and movies of the total electron content (tec) over the considered area. mailing address: dr. lucilla alfonsi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma (italy); e-mail: lucilla.alfonsi@ingv.it 448 lucilla alfonsi, cathryn n. mitchell, vincenzo romano and paolo spalla station-satellite, and are the basis for 3d and 3d time-dependent imaging of the ionosphere. in 2001 spencer and mitchell published results from a new type of ionospheric imaging based on gps (spencer and mitchell, 2001), which overcame the limitations of the classic (leo) ionospheric tomography. the gps satellites are monitored by a network of dual-frequenfig. 1. stations included in the mirto system. table i. location and type of instrumentations of the mirto sites. site station code latitude longitude station stuetta stue 46°28l19.9mn 9°20l50.3me gps san benedetto po sbpo 45°03l03.6mn 10°55l11.2me gps milazzo mila 38°16l14.1mn 15°13l50.6me gps craco crac 40°22l53.0mn 16°26l06.6me gps ventotene vent 40°47l40.9mn 13°25l17.8me gps leonessa lnss 42°36l0.3mn 13°02l24.9me gps pesaro pesa 43°56l27.7mn 12°50l25.8me gps gibilmanna gblm 37°59l25.2mn 14°01l34.1me gps s. elia a pianisi cign 41°39l15.1mn 14°54l17.3me gps chania chania 35°31l8.0mn 24°02l32.6me gistm montelibretti montelibretti 41°06l0.0n 12°36l0.0me gistm cartagena cartagena 36°42l00mn 02°30l00me virtual gps sofia sofia 43°0l0mn 25°0l0me virtual gps el arenosillo areno dps 37°6l00mn 06°42l00mw digisonde rome rome dps 41°48l0mn 12°30l0me digisonde 449 mirto: a prototype for real-time ionospheric imaging over the mediterranean area cy receivers, recording the phase and time delay of each signal. such networks (for instance, igs), spread all over the world, are open for data downloading providing a valuable source of information on the ionosphere in the form of raypath integrations of electron concentration. nevertheless, these measurements are rather uneven in distribution and coverage. the new imaging technique, known as midas (multi-instrument data analysis system), is unique in its approach to ionospheric imaging, because it performs a four-dimensional (i.e. 3d time-dependent) inversion (mitchell and spencer, 2003). it offers two key advantages: i) they are fully three-dimensional and ii) a novel time-dependent imaging technique allowed the ionosphere to be studied on time-scales of minutes. the university of bath performed many studies as, more recently, imaging during extreme space-weather events known as storms has shown dramatic changes in the height of the ionosphere to occur over very short time-scales (yin et al., 2004; mitchell et al., 2005; de franceschi et al., 2007). these sudden events are very important for radio communications because they cause radio signals to deviate from those paths predicted by ionospheric radiopropagation models. storms also cause the absolute values of electron concentration to decrease during the negative storm phase, thus lowering the range of frequencies possible for hf communications. real-time imaging of the ionosphere could assist in communications planning, reducing potential communications outages in storm times. midas is now under development for two aspects: first, its extension to polar regions (spencer and mitchell, 2007); second, real-time imaging. mirto, developed in cooperation with ifac and ingv, represents the first step in this direction. the mirto project sets the aim of the realization of a prototype that produces images over italy in near real-time making use of data acquired by the real-time ionospheric and geodetic instrumentation operated by ingv. in order to cover the western and eastern edges of the area defined by the system, a gps receiver has been recently installed at chania (crete, greece) and another one is being to be deployed in spain (huelva, el aeronosillo observatory) (fig. 1, table i). mirto technique uses ionosonde density profiles given in near real-time by some stations coordinated in the dias framework (beleakhi et al., 2005). during the development of mirto when no data have been available at some stations, artificial data from iri were used. in this paper the mirto prototype is introduced and described underlining its potentialities in view of the realization of a useful tool for scientific aims and space weather applications. after a few hints on the theory of the inversion problem, the second section of the paper is mainly addressed to the description of the method used by mirto for imaging the ionosphere; the first results of the prototype and the next steps of the project are described and discussed in the third and last section. 2. method and development the total electron content is defined as the line integral of the electron concentration along a path from a satellite to a receiver. dual frequency radio signals that propagate through the ionosphere are subject to a differential phase change due to the dispersive nature of the plasma. as a first order approximation the change in the differential phase shift is directly proportional to the change in tec between the transmitter and receiver. an outline of the theory of the inversion problem can be found in mitchell and spencer (2003) and references therein. here we only recall the crucial steps of the inversion approach applied to gps signals, which are: a) set up a three-dimensional grid of j voxels (i.e. volume pixels), each bounded in latitude, longitude and altitude; b) compute the length of all elements of every i-th satellite-to-receiver ray-path through each intersected voxel. the unknown electron concentration x is defined to be constant within each voxel. the problem may now be expressed as (2.1) where a is an i × j matrix of the path lengths within each voxel and b are the i observed tecs. this cannot be solved directly as the matrix a is rectangular, highly singular and incorporates no prior information as to the likely soax b= 450 lucilla alfonsi, cathryn n. mitchell, vincenzo romano and paolo spalla lution. to overcome this difficulty a mapping matrix, x, is used to transform the problem to one for which the unknowns are n coefficients of orthonormal basis functions, the combination of which will give the final image of electron concentration. here the basis functions (x) were generated using a spherical harmonic expansion to represent the horizontal variation and empirical orthonormal functions (eofs) for the radial variation in electron concentration (mitchell and spencer, 2003). for mirto, the radial dependence in eofs is derived from density profiles provided by the ionosondes (materassi and mitchell, 2005). after these inversions, mirto produces density profiles images over italy and mediterranean area in close to real-time, i.e. with a latency of 15 min. according to the different information needed by the applications, the system can provide tec maps, tec gradients, ionization sections, electron density profiles, peak density and height over the region of interest (fig. 2). the first phase of the project was spent using the existing data to upgrade and test the real time version of midas, to develop all the networking software for the gps data, to integrate the system with other european existing real time gps data (sopac) and ionosonde real time data and, finally, to produce and flow the images to the main station to be distributed. the data sources of the system are: igs (international gps service) for the orbital data, ingv national network ring (rete integrata nazionale gps) located in italy for the l1 and l2 signals (fig. 1, table i), one gistm (gps ionospheric scintillation and tec monitor, van dierendonck et al., 1993; de franceschi et al., 2006) receiver managed by ingv and located in chania (greece) for tec data and the dps-4 digisonde in rome and el aeronosillo for the fig. 2. typical output of mirto. clockwise: tracks of all the satellites in view from the mirto stations; tec map (in tecu=1016 m−2); latitudinal and longitudinal sections of the electron density (in 1011 m−3). 451 mirto: a prototype for real-time ionospheric imaging over the mediterranean area electron density profiles (sao format; reinisch, 1998). moreover the system makes use of gps artificial data (iri, international reference ionosphere; bilitza, 2001) from locations of possible future receivers and of artificial chapman profiles (see e.g., rishbeth and garriott, 1969) to contribute to eof’s construction. the locations of the mirto stations are listed in table i. the system works according to the following procedure: it reads the orbit elements from the data centres, the gps data from mirto and ring stations and the ionosonde data from the rome dps-4; after the creation of artificial data from iri for the virtual stations, it creates input matrices and then builds the eof’s using ionosonde data and model. finally, the system inverts the data producing plots and output files. every 15 min (at 5, 20, 35 and 50 min of each hour, according to the ionosonde sampling) mirto refreshes the input structures according to the incoming data (orbits, gps and ionosonde data), inverts the data and produces the new outputs. a typical mirto output is shown in fig. 2: on the top left are visualized the tracks of all satellites in view from the stations listed in table i; the right hand (top) map reports the tec (in tecu) over the area of interest; the other two plots describe the longitudinal and latitudinal sections of the electron density in the same area. the mirto project is now at the end of its second phase, essentially a test phase to assess the reliability of the data flow, the combination of the data into the midas algorithm and the success in streaming of the ionospheric images to the user-interface. during this phase we also deployed a gistm receiver in chania (crete, greece) to cover the eastern edge of the area of interest. for the coverage of the western border, we are going to install another gistm receiver in spain (el aeronosillo observatory, huelva). 3. next steps the third and last phase of the realization of the mirto prototype will be dedicated to the assessment of images for regions of data sparse, such as, for instance, north africa, the assessment of the ionospheric imaging under perturbed conditions typical, for instance, of magnetic storms and the assessment of the database system. this period will also explore the possible acquisition of supplementary receivers where necessary. the final aim of mirto is to realize a system able to produce monitoring of the ionosphere over the mediterranean sea in true realtime. to achieve this goal the imaging technique used by mirto is going to be implemented with forecasting tools. at that time the system will be tested to obtain information on the reliability of the mirto density images with respect to the real ionospheric scenario. the upgrading of the prototype will make mirto a real scientific tool of interest for scientific purposes and for space weather applications. a continuous monitoring of the ionosphere may be of great use even at the mid-latitudes, monitored by mirto, where important «anomalous» phenomena are not locally triggered, but however do happen. indeed, mid-latitudes can be affected by geomagnetic and ionospheric phenomena both because of perturbations coming from the north (e.g., solar flares, cmes), and because of disturbances coming from the equatorial regions, due to the sudden variations in the magnetic and electric fields of the earth. all these causes influence the electromagnetic wave propagation, affecting significantly communications and positioning. acknowledgements this work was supported by the project miur rbap04ef3a. the authors thank the ingv national gps network ring, the laboratory of geophysics & seismology of the technological educational institute of crete and the cost296 action. references austen, j.r., s.j. franke and c.h. liu (1988): ionospheric imaging using computerised tomography, radio sci., 23, 299-307. belehaki, a., lj. cander, b. zolesi, j. bremer, c. juren, i. stanislawska, d. dialetis and m. hatzopoulos (2005): dias project: the establishment of a european digital upper atmosphere server, j. atmos. solar-terr. phys., 67, 1092-1099. 452 lucilla alfonsi, cathryn n. mitchell, vincenzo romano and paolo spalla bilitza, d. (2001): international reference ionosphere 2000, radio sci., 36, 261-275. davies, k. (1990): ionospheric radio (peregrinus ltd., london, u.k.), pp. 580. de franceschi, g., l. alfonsi and v. romano (2006): isacco: an italian project to monitor the high latitudes ionosphere by means of gps receivers, gps solut., 10 (4), 263-267. de franceschi, g., l. alfonsi, v. romano, m. aquino, a. dodson, c.n. mitchell and a.w. wernik (2007): dynamics of high latitude patches and associated small scale irregularities, j. atmos. solar-terr. phys., doi: 10.1016/j.jastp.2007.05.018. hounsfield, g.n. (1972): a method of and apparatus for examination of a body by radiation such as x-ray or gamma radiation, patent specification 1283915, the patent office. materassi, m. and c.n. mitchell (2005): a simulation study into constructing of the sample space for ionospheric imaging, j. atmos. solar-terr. phys., 67/12, 10851091. mitchell, c.n. and p.s.j. spencer (2003): a three-dimensional time-dependent algorithm for ionospheric imaging using gps, ann. geophysics, 46 (4), 687-696. mitchell, c.n., l. alfonsi, g. de franceschi, m. lester, v. romano and a.w. wernik (2005): gps tec and scintillation measurements from the polar ionosphere during the october 2003 storm, geophys. res. lett., 32, l12s03, doi: 10.1029/2004gl021644. reinisch, b.w. (1998): sao (standard adep output) format for ionogram scaled data archiving, inag bull., 62, 47-58. rishbeth, h. and o.k. garriott (1969): introduction to ionospheric physics (academic press, new york). spencer, p.s.j. and c.n. mitchell (2001): multi-instrument inversion technique for ionospheric imaging, in proceedings international beacon satellite symposium, edited by p. doherty, boston, ma. spencer, p.s.j. and c.n. mitchell (2007): imaging of fast moving electron-density structures in the polar cap, ann. geophysics, 50 (3), 427-434 (this volume). van dierendonck, a.j., j. klobuchar and q. hua (1993): ionospheric scintillation monitoring using commercial single frequency c/a code receivers, in ion gps-93 proceedings: sixth international technical meeting of the satellite division of the institute of navigation, inst. of navig., salt lake city, utah, 1333-1342. yin, p., c.n. mitchell, p.s.j. spencer and j.c. foster (2004): ionospheric electron concentration imaging using gps over the u.s.a. during the storm of july 2000, geophys. res. lett., 31 (12), l12806. 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(39) 051-3540111 fax (39) 051-327877 e-mail: 1865@compositori.it http://www.compositori.it � © 2007 by istituto nazionale di geofisica e vulcanologia proprietà letteraria riservata direttore responsabile: enzo boschi autorizzazione del tribunale di bologna n. 6125 del 21.7.1992 stampato da compositori ind. grafiche, bologna nell’anno 2007 questo periodico è iscritto all’unione stampa periodica italiana g l i a t t u a l i m o v i m e n t i v e r t i c a l i d e l l e c o s t e c o n t i n e n t a l i (*) silvio polli 1. premesse. — i n p r e c e d e n t i lavori, i n d i c a t i nella b i b l i o g r a f i a , si è messo i n evidenza u n p r o c e d i m e n t o cbe p e r m e t t e di m i s u r a r e i l e n t i m o v i m e n t i v e r t i c a l i della superficie t e r r e s t r e m e d i a n t e d e t e r m i zione m a r e o g r a f i c a di livelli m e d i m a r i n i . esso è i n d i p e n d e n t e dalla c o n t e m p o r a n e a v a r i a z i o n e del liv.m.m. e assume p e r t a n t o c a r a t t e r e di m i s u r a assoluta. si basa sulla d e t e r m i n a z i o n e d e l l a d i f f e r e n z a f r a l e velocità del m o v i m e n t o m e d i o del livello di t u t t i i m a r i e q u e l l o di u n a d a t a s t a z i o n e m a r e o g r a f i c a . t a l e d i f f e r e n z a dà lo s p o s t a m e n t o v e r t i c a l e assoluto d e l c a p o s a l d o m a r e o g r a f i c o n e l l ' i n t e r v a l l o di t e m p o c o n s i d e r a t o . 2. il procedimento. — d a i livelli m e d i a n n u i di 110 stazioni mar e o g r a f i c h e s i t u a t e l u n g o le coste di t u t t i i m a r i si d e t e r m i n a r o n o i liv.m. dei d e c e n n i 1866-75, 1876-85, 1936-45. q u e s t i livelli m e d i si a t t r i b u i r o n o agli a n n i c e n t r a l i 1871, 1881, . . . . 1941. l e d i f f e r e n z e f r a i successivi valori d a n n o le v a r i a z i o n i d e c e n n a l i del liv.m.m. ragg r u p p a t i q u e s t i p e r r e g i o n i costiere si o t t e n n e r o le v a r i a z i o n i m e d i e costiere. da questi v a l o r i si d e d u s s e r o i n f i n e le v a r i a z i o n i m e d i e dec e n n a l i g e n e r a l i , d e d o t t e cioè d a l l ' i n s i e m e di t u t t i i m a r i . le present i a m o n e l s e g u e n t e s p e c c h i e t t o : variazioni decennali generali del liv.m.m. in cm (+i n n a l z a m e n t o , — a b b a s s a m e n t o ) 1871-1880 1881-1890 1891-1900 1901-1910 1911-1920 1921-1930 1931-1940 1871-1940 + 0,9 — 1,4 + 1,6 + 0,2 + 0,7 + 0,9 + 1,9 + 1,1 lo s p o s t a m e n t o n o n è costante, m a v a r i a n o t e v o l m e n t e da de(*) c o m u n i c a z i o n e presentata al c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a italiana, t e n u t o a r o m a il 10-11 g i u g n o 1952. g l i a t t u a l i m o v i m e n t i v e r t i c a l i d e l l e c o s t e c o n t i n e n t a l i 599 c e n n i o a d e c e n n i o . i l v a l o r e m a s s i m o si è a v u t o n e l l ' u l t i m o p e r i o d o c o n s i d e r a t o . (1931-1940). è n o t o c o m e in q u e s t i u l t i m i 60 a n n i , i n c o r r i s p o n d e n z a al g r a d u a l e a u m e n t o del liv.in.in., avvenga u n a progressiva f u s i o n e dei ghiacciai. la regressione si è a c c e n t u a t a negli u l t i m i d u e d e c e n n i . v i è a n z i u n alto v a l o r e c o r r e l a t i v o f r a la m i s u r a dei d u e f e n o m e n i d u r a n t e t u t t o il p e r i o d o 1871-1940. la d i f f e r e n z a f r a l ' a u m e n t o d e l liv.m.m. d e t e r m i n a t o i n u n a staz i o n e o in u n a r e g i o n e costiera ( q u a l e m e d i a dei v a l o r i delle stazioni in essa c o m p r e s i ) e q u e l l o g e n e r a l e di t u t t i i m a r i d a r à lo s p o s t a m n e t o assoluto d e l c a p o s a l d o o d e l l a r e g i o n e costiera. i v a l o r i così o t t e n u t i , sono p r e s e n t a t i n e l l a t a b e l l a allegata. i v a l o r i d e l l ' u l t i m a c o l o n n a (1871-1940) n o n r a p p r e s e n t a n o la med i a dei v a l o r i d e l l e s e t t e c o l o n n e p r e c e d e n t i , m a f u r o n o d e t e r m i n a t i d i r e t t a m e n t e q u a l i d i f f e r e n z e f r a l e v a r i a z i o n i m e d i e locali del sett a n t e n n i 1871-1940 e q u e l l a g e n e r a l e d e l l o stesso i n t e r v a l l o di t e m p o . e ciò p e r u n i f o r m a r e il peso d e l l e stazioni aventi f u n z i o n a t o u n n u m e r o r i d o t t o d i d e c e n n i , a l t r i m e n t i il v a l o r e , a n c h e i n c e r t o , di u n solo d e c e n n i o a v r e b b e p e s a t o t r o p p o n e l l a m e d i a g e n e r a l e . p e r l'ipotesi f a t t a sul m e t o d o a d o t t a t o , l a m e d i a o t t e n u t a dai v a l o r i di ciascuna c o l o n n a , cioè la m e d i a degli s p o s t a m e n t i v e r t i c a l i di u n o stesso d e c e n n i o , è n u l l a . 3. gli spostamenti attuali della superficie terrestre. — la t a b e l l a precisa p e r ogni r e g i o n e costiera e p e r ogni d e c e n n i o i valori degli s p o s t a m e n t i v e r t i c a l i . i n f e n n o s c a n d i a si osserva l ' i n n a l z a m e n t o mass i m o di 4,7 cm p e r d e c e n n i o . n o t e v o l e è p u r e il s o l l e v a m e n t o della costa p o r t o g h e s e e, i n g r a d o m i n o r e , di q u e l l a spagnola. a b b a s s a m e n t o d e l l ' o r d i n e d i 2 cm p e r d e c e n n i o si osservano sulle coste a t l a n t i c h e e m e r i d i o n a l i degli s. u. d ' a m e r i c a . la c a r t a geografica allegata r a p p r e s e n t a in m a n i e r a p i ù e v i d e n t e l ' i n s i e m e dei s o l l e v a m e n t i e degli a b b a s s a m e n t i costieri d e t e r m i n a t i negli u l t i m i 70 a n n i . le coste d e l l ' e u r o p a c o n t i n e n t a l e figurano in fase d i abbassam e n t o . q u e l l e d e l l e p e n i s o l e e delle isole p e r i f e r i c h e m a n i f e s t a n o invece i n n a l z a m e n t i . ciò si n o t a s p e c i a l m e n t e n e l l a penisola s c a n d i n a v a , in q u e l l a i b e r i c a , n e l l a scozia e n e l l a sicilia. l'asia m e r i d i o n a l e e o r i e n t a l e a p p a r e p u r e con l e coste preval e n t e m e n t e i n a b b a s s a m e n t o . così p u r e l e isole g i a p p o n e s i e q u e l l e d e l l e i n d i e o r i e n t a l i . n u l l a p u ò d i r s i d e l l e coste a r t i c h e . f i g . 1 gli attuali s p o s t a m e n t i v e r t i c a l i c o s t i e r i (-[i n n a l z a m e n t o , — a b b a s s a m e n t o ) c u a t t u a l i m o v i m e n t i v e r t i c a l i d e l l e c o s t e c o n t i n e n t a l i 601 l ' a f r i c a s e t t e n t r i o n a l e h a anch'essa le coste in fase di abbassam e n t o . t a l i p u r e a p p a i o n o q u e l l e o c c i d e n t a l i . m a n c a n o dati p e r le z o n e m e r i d i o n a l i e o r i e n t a l i . la r e g i o n e costiera o r i e n t a l e del madagascar d e n o t a u n a b b a s s a m e n t o . l ' a m e r i c a del n o r d h a le coste a t l a n t i c h e degli s.u. in m a r c a t o a b b a s s a m e n t o , così p u r e , m a in m i n o r g r a d o , f i g u r a n o q u e l l e del p a cifico. p i ù a s e t t e n t r i o n e , n e l c a n a d à e n e l l ' a l a s k a si osservano invece m o v i m e n t i d i s o l l e v a m e n t o . d e l l ' a m e r i c a d e l sud m a n c a n o dati. le coste a r g e n t i n e settent r i o n a l i a p p a i o n o i n s o l l e v a m e n t o , q u e l l e m e r i d i o n a l i in a b b a s s a m e n t o . a n c h e d e l l ' a u s t r a l i a si h a n n o p o c h i d a t i . f i g u r e r e b b e in sollevam e n t o la costa se, i n c o n c o r d a n z a con q u a n t o avviene n e l l e isole d e l l a n u o v a z e l a n d a . a p p a r e invece in a b b a s s a m e n t o u n t r a t t o della costa m e r i d i o n a l e . c o m p l e s s i v a m e n t e n e l l a fascia e q u a t o r i a l e , c o m p r e s a f r a le latit u d i n i di — 2 0 " e -1-45" p r e v a r r e b b e l ' a b b a s s a m e n t o , m e n t r e alle lat i t u d i n i m a g g i o r i , sia s e t t e n t r i o n a l i c h e m e r i d i o n a l i , s a r e b b e r o in magg i o r a n z a i m o v i m e n t i d i sollevamento. i d a t i d i s p o n i b i l i sono insufficienti p e r p r e c i s a r e c o m p l e t a m e n t e gli s p o s t a m e n t i messi in e v i d e n z a in q u e s t a n o t a . si è v o l u t o , in ogni m o d o , i n d i c a r e a n c h e q u e s t o p r o c e d i m e n t o e f a r v e d e r e le sue possib i l i t à n e l l e r i c e r c h e sui m o v i m e n t i verticali della s u p e r f i c i e t e r r e s t r e . trieste — istituto talassografico — giugno 1952. riassunto mediante il procedimento delle differenze dei livelli medi marini si determina, per le principali coste continentali, il valore dello spostamento verticale assoluto subito da esse negli ultimi 70 anni. i risultati sono presentati in tabelle e in una carta geografica. figurano in fase di abbassamento: dell'europa, le coste dalla germania alla francia atlantica e quelle mediterranee francesi e italiane; dell' africa, le coste settentrionali, orientali e occidentali; dell'asia, le coste centroorientali con le isole giapponesi e quelle dell'argentina meridionale. sono in fase di innalzamento: dell'europa, le coste scandinave, le inglesi settentrionali, le iberiche e le siciliane; dell'asia, le coste del bengala; dell' america, le coste settentrionali; dell'australia, le coste orientali e della nuova zelanda. in generale si ha un abbassamento 602 s i l v i o p o l l i nella fascia compresa tra la latitudine —20° -f45" ed innalzamento nella regione sopra la latitudine di 45° n. summary using the procedure of the differences of the rnean levels of the oceans, the vaine of the ahsolute vertical displacement, for the principal continental coasts, covering the last 70 years, has been calculated. the results are presented in tables and on a geographic rnap. the following zones are in descending phase: of europe, the coast of germany to the french atlantic and the mediterranean coast of italy and france; of africa, the northern, eastern and western coasts; of asia, the centrai eastern coast including the japanese islands, and the coast of centrai argentina. the following zones are in ascending phase: of europe, the scandinavian, the northern english, the spanish and sicilian coasts; of asia, the bengal coast; of america, the northern coasts: of australia, the eastern coast and the coast of new zealand. in general one has a. descent in the group between latitudes •—200 and -)45", and ari ascent in the region ahove latitude 45° n. b i b l i o g r a f i a p o l l i s , livello medio del mare nella livellazione di precisione. b o l l . s o c . a d r i a t i c a s e . n a t , x l i i (1946) 52-56, t r i e s t e . p o l l i s , il graduale aumento del livello marino determinato per 30 porli del mediterraneo. b o l l . s o c . a d r i a t i c a di se. n a t , t r i e s t e , v o i . x l i i , 1946. p o l l i s , gli attuali movimenti verticali delle coste italiane. t e c n i c a italiana, t r i e s t e , n . s , 4, 1947. p o l l i s , registrazione dei bradisismi costieri. g e o f i s i c a pura e applicata, milan o , v o i . xii, fase. 3-4, 1948. p o l l i s , misura dei lenii movimenti verticali della superficie terrestre. r i u n . a s s . g e o f i s . i t a ] , 10-11 g i u g n o 1952. layout 6 annals of geophysics, 53, 2, april 2010; doi: 10.4401/ag-4711 abstract the present study proposes a theoretical modeling of simultaneous and noninvasive measurements of electrical resistivity and dielectric permittivity using a quadrupole probe on a subjacent medium. a mathematical-physical model is applied to the propagation of errors in the measurement of resistivity and permittivity based on a sensitivity functions tool. the findings are also compared with results of the classical method of analysis in the frequency domain, which is useful for determining the behavior of zero and pole frequencies in the linear time invariant circuit of the quadrupole. this study underlines that average values of electrical resistivity and dielectric permittivity can be used to estimate complex impedance over various terrains and concretes, especially when they are characterized by low levels of water saturation (content), and are analyzed within a bandwidth ranging only from low to middle frequencies. to meet the design specifications, that ensure satisfactory performances of the probe (inaccuracies of no more than 10%), the forecasts provided by the sensitivity functions approach are discussed in comparison with those foreseen by the transfer functions method (in terms of both the band of frequency f and the measurable range of resistivity t, or permittivity fr ). 1. introductory review 1.1. electrical resistivity survey in soil science the electrical resistivity of a surface is a proxy for the spatial and temporal variability of many other physical properties of the subjacent medium. samouëlian [samouëlian et al. 2005] discussed the basic principles of data interpretation and the main advantages and limits of such an analysis. this method allows nondestructive and very sensitive investigations, which can describe subsurface properties without direct inspection. various techniques are applied according to the required scales of resolution and the heterogeneities of the area. a suitable probe injects generated electric currents into a medium, and the resulting potential differences are measured. the information is recovered from the potential-difference patterns, which provide the form of the medium heterogeneities and their electrical properties [kearey et al. 2002]. the greater the electrical contrast between the subsurface matrix and a heterogeneity, the easier the detection. other studies have shown that surface resistivity can be considered as a good indication of the variability of other physical properties [banton et al. 1997]. the current pattern distributions depend on the medium heterogeneities and they are concentrated in a conductive volume. some linear distributed arrays use four-electrode cells, which are commonly used in the laboratory for resistivity calibration [rhoades et al. 1976] and in the field for vertical electrical sounding [loke 2001]. 1.2 middle frequency dielectric permittivity surveys in soil science analyses in the middle frequencies (mfs; 300 khz <f <3 mhz) allow measurements of dielectric permittivity. fechant and tabbagh [1999] developed an interesting approach, whereby they used a mf band for the characterization of permittivity in a natural media. this approach used an electrostatic quadrupole probe that was designed to measure resistivity at several hundreds of khz [tabbagh 1994]. a quadrupole working at a frequency 455 khz can measure permittivity for the determination of water content [fechant 1996]. however, this approach requires calibration in the laboratory. 1.3. low frequency electrical resistivity and dielectric permittivity surveys in soil science analyses in the low frequencies (lfs; 30 khz <f <300 khz) allow simultaneous measurements of both electrical resistivity and dielectric permittivity. a series of studies has shown that the resistivity and dielectric constant (the article history received october 18, 2009; accepted january 15, 2010. subject classification: instrumentation and techniques of general interest, methods, mathematical geophysics, computational geophysics, exploration geophysics. 1 inaccuracy assessment for simultaneous measurement of resistivity and permittivity applying sensitivity and transfer function approaches alessandro settimi1,*, achille zirizzotti1, james a. baskaradas1, cesidio bianchi1 1 istituto nazionale di geofisica e vulcanologia, sezione di roma, italy complex permittivity) of a surface can be measured using a set of four electrodes [grard 1990a, grard 1990b, grard and tabbagh 1991, tabbagh et al. 1993]. this novel approach was first introduced by wenner [1915], and it improved on the system existing at the time, which provided only a resistivity assessment. in this new method, the four electrodes are manually inserted into the subjacent medium. permittivity, which is sensitive to the presence of water, can also be determined using a lf probe (below 300 khz) and it has an important role in the detection of anomalies in the subsurface. vannaroni and del vento [vannaroni et al. 2004, del vento and vannaroni 2005] used a dielectric spectroscopy probe to determine the complex permittivity of a surface from measurements of transfer impedance of a fourelectrode system that was electrically coupled to the medium. they defined the transfer impedance as the ratio between the voltage measured across a pair of receiving electrodes and the current transmitted by a second pair of electrodes [vannaroni et al. 2004]. this impedance measurement was performed in an alternating current (ac) regime capacitive coupling, and it strongly depended on the geometry of the electrode array and also on the complex permittivity of the subsurface. the advantages offered by this method include that the exciter current can be injected into the surface even in the absence of galvanic contact, and that in the ac regime, both conduction and displacement currents of the medium can be measured, obtaining further information on the polarizability. in this case, the frequency band is 10 khz to 1 mhz, and the lower limit is effectively imposed by two aspects. first, the maxwell-wagner effect, which limits probe accuracy [frölich 1990]. the most important limitation occurs because of interface polarization effects that are stronger at low frequencies, e.g. below 1 khz, depending on the medium conductivity. secondly, there is the need to maintain the amplitude of the current at measurable levels, as given the capacitive coupling between the electrodes and soil, the current magnitude is proportional to the frequency. on the other hand, the upper limit is opportunely fixed to allow analysis of the system in a regime of quasi static approximation and to neglect the velocity factor of the cables used for the electrode harness, which, in turn, reduces the accuracy of the mutual impedance phase measurements. thus, it is possible to exploit the analysis of the system in the lf and mf bands where the electrostatic term is considerable. the general electromagnetic calculation provides lower values than the static case, and a high resistivity narrows the differences. so, in comparison, above 1 mhz the general electromagnetic calculation must be preferred, while under 500 khz the static case would be used, and between 500 khz and 1 mhz both of these methods can be used [tabbagh et al. 1993]. the present study proposes a theoretical modeling of simultaneous and noninvasive measurements of electrical resistivity and dielectric permittivity using a quadrupole probe on a subjacent medium [see also arxiv.org's ref.: settimi et al. 2009]. a mathematical-physical model is applied to the propagation of errors in the measurement of resistivity and permittivity based on the sensitivity functions tool. the findings are also compared with the results of the classical method of analysis in the frequency domain, which is useful for determining the behavior of zero and pole frequencies in the linear time invariant circuit of the quadrupole. this study underlines that average values of electrical resistivity and dielectric permittivity can be used to estimate complex impedance over various terrains and concretes, especially when they are characterized by low levels of water saturation or content [knight and nur 1987], and are analyzed within a frequency bandwidth only ranging from lfs to mfs [al-qadi et al. 1995, myounghak et al. 2007]. to meet the design specifications that ensure the satisfactory performance of the probe (inaccuracies of no more than 10%), the forecasts provided by the theory of error propagation suggested by vannaroni et al. [2004] that apply the sensitivity functions approach, as explicitly developed in the study, are discussed in comparison to those foreseen by analysis in the frequency domain suggested by grard and tabbagh [1991]. here, this deepens the transfer functions method to analyze the zero and pole behavior (in terms of both the band of frequency f and the measurable range of resistivity t, or permittivity fr ). this study is organized as follows. following this introductory review, section 2 discusses the cole-cole empiric function. for simplicity of analysis, the dielectric dispersion is assumed to be very low, an operating condition that is satisfied when the electrical spectroscopy is performed only on nonsaturated water materials and especially in a suitable band of lfs and mfs. section 3 introduces the quadrupole probe, and section 4 provides the theoretical modeling that applies to both the sensitivity functions approach (section 4.1.) and the transfer functions method (section 4.2.). in section 5, the configurations of the quadrupole are defined and discussed, and the conclusions are drawn up in section 6. finally, an outline of the somewhat lengthy calculations needed is presented in appendices a and b. measurement of resistivity and permittivity 2 figure 1. equivalent circuit of the quadrupole probe. 3 2. discussing cole-cole empiric functions according to debye polarization mechanisms [debye 1929] and cole-cole diagrams [auty and cole 1952], the complex permittivity of various materials in the frequency band from very lf to very high frequency (hf) shows several intensive relaxation effects and a non-trivial dependence on the water saturation [chelidze and gueguen 1999, chelidze et al. 1999]. however, average values of electrical resistivity and dielectric permittivity can be used to estimate complex impedance over various terrains and concretes, especially when they are characterized by low water content [knight and nur 1987], and are analyzed within a frequency bandwidth ranging from only lfs to mfs [al-qadi et al. 1995, myounghak et al. 2007]. many functions have been proposed to fit dielectrics data. among these, there are those obtained by attempts to model the physical processes, or those of simple empirical functions, which are used to parameterize the data without knowledge of the mechanisms involved. a widely used empirical function was proposed by the cole brothers, and it is based on the theory of debye relaxation, which was the first treatment of this phenomenon. the cole-cole empiric function defines the first-order dielectric response of materials in the frequency domain, (2.1) which consists of real and imaginary parts: (2.2) (2.3) where f0 is the dielectric constant in a vacuum. the electrical conductivity v(f) and dielectric permittivity fr(f) show limited values at lfs and hfs, vl, fr,l and vh, fr,h, which are linked by the relaxation time x: (2.4) thus, as can be seen, permittivity and conductivity cannot vary independently of each other [frölich 1990]. at the characteristic frequency of relaxation fc = 1/(2r·x), the permittivity fr assumes an intermediate value between the lfs and hfs, fr,l and fr,h. alternatively, the relaxation frequency fc can be considered as that frequency at which the conductivity v assumes the middle value between the two limiting values vl and vh. in reality, equation (2.1) is a generalization of the debye equation, which is designed to take into account the enlargement of the dispersion region due to the complexity of the structure and the composition of the materials, through the introduction of another parameter a (between 0 and 1). note that for a = 0, equation (2.1) can be reduced to exactly the debye equation. it has to be underlined that the parameter a is an increasing function of the water saturation sw, such that a (sw = 0)→0, reaching a limiting value al>0 for sw→1 [knight and nur 1987]. indeed, the complex dielectric permittivity is flattened with decreasing water content or increasing frequency [al-qadi et al. 1995, myounghak et al. 2007]. the complex dielectric permittivity can be approximated to a constant if its dominant term (fr,l− fr,h)/ [1 + (j2rfx)1–a] is a function that is almost independent of the frequency: (2.5) this operating condition of equation (2.5) holds when the materials are characterized by low water content, i.e.: (2.6) and are analyzed over a band lower than the mfs, i.e.: measurement of resistivity and permittivity figure 2a. quadrupole probe in the linear wenner configuration. figure 2b. quadrupole probe in the square configuration. 2 1 ( 2 ) 2 , (f) (f) j f (f) j f j f 0 , 1 , 0 ,l l r complex r r h r r h = = = + + f f r f v f r x f f r f v -a 1 (2 ) 2 (2 ) sin ( 2) 1 (2 ) sin ( 2) , (f) f f f , , 2 (1 ) 1 1 ,lr r h r r h + + + ar ar f f f f r x r x r x = a a a 2 1 (2 ) 2 (2 ) sin ( 2) (2 ) cos ( 2) , (f) f f f f , 0 2 (1 ) 1 1 ,l l r r h = = + + ar ar f f v v r f r x r x r x a a a 6 @ , 0l l h hx f f f v v = ^ h (2 ) << 1.f 1r x a0 ,"a (f)r complexf (2.7) indeed, the constant x depends on the physical processes under consideration, and it has an order of magnitude that varies from a few picoseconds for the orientation of electrons and small dipolar molecules, up to a few seconds for the effects of counter-ions or for interfacial polarization [frölich 1990]. therefore, in the present study let us refer to the (v, fr) values in the lf to mf bandwidth proposed for various terrains by edwards [1998] and for concretes by polder et al. [2000] and laurents et al. [2005]. 3. quadrupole probe when using a quadrupole probe (figure 1), the response depends on both geometrical parameters, like the height of each electrode above the ground surface and the separation of the electrodes, and physical parameters, including frequency, electrical conductivity and dielectric permittivity. when a medium is assumed to be linear and its response linearly dependent on the electrical charges of the two exciting electrodes, the simplest approach is a static calculation [tabbagh et al. 1993], especially using a low operating frequency. if the electrodes have small dimensions relative to their separation, then they can be considered as points. moreover, if the current wavelength is much larger than all of the dimensions under consideration, then the quasi-static approximation applies [grard 1990a, grard 1990b]. the quadrupole probe (figure 1) measures a capacitance in a vacuum c0(l) that is directly proportional to its characteristic geometrical dimension, i.e. the electrodeelectrode distance l, both in a linear wenner configuration (figure 2a), (3.1) and in a square arrangement (figure 2b), (3.2) which is greater by a factor a = 1/(2 − 21/2) >1, where f0 is the dielectric constant in a vacuum. when the quadrupole specified by the electrodeelectrode distance l has galvanic contact with the subjacent medium of electrical conductivity v and dielectric permittivity fr, it measures a transfer impedance zn (f, l, v, fr) that consists of parallel components of resistance rn (l, v) and capacitance cn (l, fr). the resistance rn (l, v) depends only on l and v [grard and tabbagh 1991]: (3.3) while cn (l, fr) depends only on l and fr [grard and tabbagh 1991]: (3.4) as a consequence, as well as grazing the medium, if the probe measures the conductivity v and permittivity fr working in a frequency f much lower than the cut-off frequency ft = ft (v, fr) = v/(2rf0(fr + 1)), the transfer measurement of resistivity and permittivity 4 (a) (b) figure 3. on the hypothesis that d|z|/|z|= duz/uz= 10−3, the inaccuracy dfr/fr in the measurement of the dielectric permittivity fr plotted: (a) as a function dfr/fr (f, x) of both the frequency f in the band f∈ [0, flim], with flim= 1 mhz, and the ratio x = h/l between the height h above ground and the characteristic geometrical dimension l, as 0 < x ≤ 1, when the quadrupole probe designed in the linear wenner configuration has a capacitive contact on a non-saturated concrete of low electrical conductivity, i.e. v = 10−4 s/m, fr= 4; (b) as a function dfr/fr (v, fr) of both the conductivity v and the permittivity fr, when the quadrupole working in a fixed band b = 100 khz is in galvanic contact on a class of concretes such that v∈ [10−4 s/m, 2·10−2 s/m]. 2 1 .f < rx 4 ,c l l0 0= $rf^ h 4 ,c l l0 0= $a rf^ h ;, 2r l c l0 0 n =v f v^ ^h h , 1c l c l .2 1 0n r r= +$f f^ ^ ^h h h 5 impedance zn (f, l, v, fr) is characterized by the phase un (f, v, fr) and modulus |z|n (l, v). the phase un (f, v, fr) depends linearly on f, with a maximum value of r/4, and it is directly proportional to the ratio (fr + 1)/v; while |z|n (l, v) does not depend on f, and is inversely proportional to both l and v. indeed, if zn (f, l, v, fr) consists of the parallel components of rn (l, v) – see equation (3.3) – and cn (l, fr) – see equation (3.4) –, then it is fully characterized by the hf pole ft = ft (v, fr), which cancels its denominator: the transfer impedance acts as a lf-mf band-pass filter with cut-off ft = ft (v, fr); in other words, the frequency equalizing joule and displacement current. under the operating conditions defined in section 2, average values of v can be used over the band ranging from lf to mf; therefore, |z|n (l, v) is not a function of frequency below ft. instead, when the quadrupole probe (figure 1) has capacitive contact with the subjacent medium and the geometry of the probe is characterized by the ratio x between the height above ground h and the electrodeelectrode distance l, (3.5) its configurations can be entirely defined by a suitable geometrical factor k(x), which depends on the height/ dimension ratio x. this was introduced by grard and tabbagh [1991], and can be specified for the linear wenner configuration (figure 2a): (3.6) measurement of resistivity and permittivity (a) (b) (c) (d) figure 4. sensitivity functions and for the transfer impedance both in modulus |z| and in phase uz, relative to the dielectric permittivity fr, plotted: (a, c) as functions (a) and (c) of both the working frequency f and the height/dimension ratio x = h/l under the same operative conditions as figure 3a; (b, d) as functions (b) and (d) of both the conductivity v and the permittivity fr under the same operative conditions as figure 3b. s z r ; ; f s r z f u ( , )s f xz r ; ; f ( , )s f x r z f u ( , )s z rr v f ; ; f ( , )s rr z v f f u ,x l h= 2 1 4 1 ,k x x x2 1 2 2 1 2= + +--^ ^ ^h h h and the square arrangement (figure 2b): (3.7) actually, grard and tabbagh [1991] preferred to introduce the complementary d(x) of the geometrical factor k(x), i.e.: (3.8) where k (x = 0) = 1 and d (x = 0) = 0. so, if the quadrupole works in the pulse frequency ~ = 2rf, which can be normalized with respect to the cut-off ~t = 2rft [grard and tabbagh 1991], (3.9) then the probe measures a transfer impedance z (x, x, v, fr) which consists of the resistance r (x, x, v, fr) and capacitance c (x, x, v, fr) parallel components [grard and tabbagh 1991], (3.10) (3.11) inverting equations (3.10) and (3.11), v and fr can be expressed as functions of r and c, i.e.: (3.12) (3.13) in our opinion, once the degrees of freedom of the (f, x) pair are fixed, it is not suitable to choose (r,c) as independent variables and then (v, fr) as dependent variables (equations 3.12 and 3.13). instead, it is more appropriate to consider (v, fr) as quantities of physical interest, and consequently equations (3.10) and (3.11) as the starting points for the theoretical development. indeed, even if the physics does not forbid the choice of (r,c) as independent variables, applying the function (r,c) → (v, fr), the procedures of the design should anyway choose (v, fr) as independent variables, preferentially applying the inverse function (v, fr) → (r,c). according to the following two practical approaches: (a) – (v, fr) as independent variables in order – to establish the class of media with conductivity and permittivity (v, fr) that can be investigated by a quadrupole working in a fixed band b and specified by a known geometry x; (b) – preferential way (v, fr) → (r,c) since – once a subjacent medium with electrical conductivity v and dielectric permittivity fr is selected, the quadrupole probe specifications r and c can be projected both in frequency f and in height/dimension ratio x. 4. theoretical modeling the measurements taken using the quadrupole probe are affected by errors that mainly originate from uncertainties associated with transfer impedance, from dishomogeneities between the modeled and the actual stratigraphy, and from inaccuracies of the electrode array deployment above the surface [vannaroni et al. 2004]. errors in impedance result mainly from uncertainties in the electronic systems that perform the amplitude and phase measurements of the voltages and currents [del vento and vannaroni 2005]. these uncertainties were assumed to be constant throughout the whole frequency band, even though their effects that propagate through the transfer function will produce a frequency-dependent perturbation. 4.1. sensitivity functions approach this study proposes to develop explicitly the sensitivity functions approach that is implied in the theory of error measurement of resistivity and permittivity 6 figure 5. ratio c= c1/c2 between the first member c1 and the second member c2 of equation (b.8), plotted as a function c(x, v) of both the height/dimension ratio x = h/l and the electrical conductivity v, with the quadrupole probe designed in the linear wenner configuration and in capacitive contact on a selected concrete of dielectric permittivity fr= 4. 1 2 1 4 2 1 2 .k x x x2 1 2 2 1 2 1 2 1 2 = + + -^ ^ ^h h h 1 ,x k x=d -^ ^h h 1 ,r c 0 t n n r = = = + ~ ~ ~ ~ v f f x ^ h , , , , 1 1 2 1 2 1 , r x r l x x x2 2 r n r r = = + + + v f v d d f d f x x ^ ^ ^ ^ ^ h h h h h; ;e e , , , , 1 2 1 2 1 1 2 1 1 2 1 c x c l x x x .2 2 2 r n r r r r r = = + + + + + + v f f d f d f d f f x x x ^ ^ ^ ^ ^ ` h h h h h j ; ;e e , , , 2 1 ,x r c x r c x c x rc 2 2 2 0 2 0 2 0= + v ~ d ~ d d f ~ -^ ^ ^ ^h h h h 6 6 @ @ , , , 2 2 x r c x r c x c x x r c x c c x c .2 2 2 0 2 2 2 0 0 r = + + f ~ d ~ d d d ~ d d ^ ^ ^ ^ ^ ^ ^ h h h h h h h 6 6 6 6 @ @ @ @" , 7 propagation suggested by vannaroni et al. [2004]. indeed, this section introduces a mathematical-physical model for the propagation of errors in the measurement of electrical conductivity v and dielectric permittivity fr, based on the sensitivity functions tool [murray-smith 1987]. this is useful for expressing inaccuracies in the measurements of conductivity and permittivity (figure 3) as a linear combination of the inaccuracies for the transfer impedance, both in modulus |z| and in phase uz, where the weight functions are inversely proportional only to the sensitivity functions for |z| and uz relative to v and fr (figure 4). the inaccuracies of transfer impedance depend on the inaccuracies of electrical voltage and current that are assigned by the electronics used, and in particular, by the sampling methods. therefore, the inaccuracies dv/v in the measurement of the electrical conductivity v, and dfr/fr in the dielectric permittivity fr, can be expressed as a linear combination of the inaccuracies d|z|/|z| and duz/uz in the measurement of the transfer impedance, respectively in modulus |z| and in phase uz, (4.1) measurement of resistivity and permittivity figure 6. conceptual schemes for numerical simulations to design the characteristic geometrical dimensions and the frequency band, limiting inaccuracies in the measurements of the quadrupole probe in capacitive contact with selected materials as non-saturated concretes, in the hypothesis that d|z|/|z|= duz/uz= 10−3. 1 1 , for const , s z z s s z z s z z z z z z r z z = + = = + = ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; v v f d d u du d u du ; ; ; ; v v v v u u (4.2) where and are the pairs of sensitivity functions for the transfer impedance, both in |z| and uz, relative to the conductivity v and permittivity fr, the expressions for which are reported in appendix a. the conditions v = const and fr = const in equations (4.1) and (4.2) underline not so much that constant values of electrical conductivity and dielectric permittivity are used to estimate the complex impedance over various terrains and concretes under the operating conditions defined in section 2, but that the quantities v and fr are not independent of each other, since the electrical displacement shows a phase-shift with respect to the electrical field [frölich 1990]. so, for the need to distinguish the inaccuracies in measurements of conductivity and permittivity, the inaccuracy dv/v can only be calculated assuming there is no uncertainty for fr (dfr/fr = 0 ↔ fr = const), and vice versa. moreover, according to the physical problem, the probe performs measurements of the transfer impedance z, both in modulus |z| and in phase uz, which are characterized by the inaccuracies d|z|/|z|> 0 and duz/uz> 0. mathematically, application of the conditions |z|= const or uz = const is not measurement of resistivity and permittivity 8 figure 7. conceptual schemes for numerical simulations to establish the measurable ranges of electrical conductivity and dielectric permittivity, limiting inaccuracies in the measurements of the quadrupole probe in capacitive contact, and fixing its optimum working frequencies and characteristic geometrical dimensions [d|z|/|z|= duz/uz= 10−3]. 1 1 , for const , s z z s s z z s r r z z z z z z z z r r r r = + = = + = ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; f f v d d u du d u du ; ; ; ; f f f f u u ( , )s sz z; ;v v u ( , )s sz z r r ; ; f f u 9 allowed. in this context, the sensitivity functions and cannot be calculated assuming uz = const, and then the sensitivities and assuming |z|= const. indeed, as discussed above, once the degrees of freedom of the (f, x) pair are fixed, it is not suitable to choose (|z|,uz) or (r,c) as independent variables. consequently, the sensitivity functions cannot be calculated by the dependent variables v = v(|z|, uz) and fr = fr(|z|, uz) or by equations (3.12) and (3.13). instead, the physical problem should be approached recalling that (f, x, v, fr) have been considered as independent variables. in the simplifying hypothesis that the frequency f and the height/ dimension ratio x are characterized by inaccuracies df/f ≈ 0 and dx/x ≈ 0 close to zero, the conditions f = const and x = const can be applied. necessarily, the inaccuracy dv/v in the measurement of the electrical conductivity v is calculated assuming fr =const, and then the inaccuracy dfr/fr for the dielectric permittivity fr is calculated assuming v = const. as a consequence, the mathematical calculations should be done recalling that equations (3.10) and (3.11) have been considered as the starting points for the theoretical development. the inaccuracies dv/v for the conductivity v and dfr/fr for the permittivity fr can be more directly expressed as functions of (f, x, v, fr) by calculating the sensitivity functions and in the last part of equations (4.1) and (4.2). these sensitivities are derived from the transfer impedance 1/z = 1/r + j~c reported in equations (3.10) and (3.11). the interesting physical results obtained using this sensitivity functions approach are discussed below. if the quadrupole probe is in galvanic contact with the subsuface, measurement of resistivity and permittivity figure 8. referring to a quadrupole probe designed in the linear wenner or square configurations and with capacitive contact on a non-saturated concrete of low electrical conductivity, i.e. v = 10−4 s/m, fr= 4: (a) plots, as a function of the ratio x = h/l between the height h above ground and the characteristic geometrical dimension l, with 0 < x ≤ 1, for the geometrical factor d(x); (b) semi-logarithmic plots for both the zero zm(x) and pole pm(x) of the transfer impedance in modulus; (c, d) bode's diagrams, as a function of the frequency f in the band f∈ [0, flim], with flim= 1 mhz, for the transfer impedance both in modulus |z|(f, xconcrete), units of 1/h, and phase duz (f, xconcrete); (e) – see next page – on the hypothesis that d|z|/|z|= duz/uz= 10−3, semi-logarithmic plots for both the inaccuracies dfr/fr (f, xconcrete) in the measurement of the permittivity fr, and dv/v (f, xconcrete) of the conductivity v, with the height/dimension ratio designed optimally in the linear wenner (xw,concrete= 0.087) and square (xs,concrete= 0.078) configurations. (a) (c) (d) (b) s z; ; v s z r ; ; f s z v u s z rf u ( , )s sz z; ;v v u ( , )s sz z r r ; ; f f u i.e. h = 0, then the inaccuracies dv/v in the measurement of the electrical conductivity v, and dfr/fr for the dielectric permittivity fr, are minimized in the frequency band b of the quadrupole, for all of its geometrical configurations and media, and even if h≠ 0, the design of the probe must still be optimized with respect to the minimum value of the inaccuracy dfr/fr for fr, which is always higher than the corresponding minimum value of the inaccuracy dv/v in the band b of the probe, for all of its configurations and media [tabbagh et al. 1993, vannaroni et al. 2004]. under the quasi static approximation, only if the quadrupole probe is in galvanic contact with the subjacent medium, i.e. h = 0, and considering that the sensitivities functions are defined as normalized functions, then our mathematical-physical model predicts that the sensitivities of the transfer impedance relative to the conductivity v and permittivity fr are independent of the characteristic geometrical dimension of the quadrupole, i.e. the electrodeelectrode distance l. if the probe grazes the medium, then the transfer impedance zn (v, l) consists of the resistance rn (v, l), which is independent of fr, and the parallel capacitance cn (fr, l), which is independent of v, such that: the sensitivity function for r relative to v is a constant equal to (−1); the sensitivity for c relative to fr is independent of v, and behaves as the function fr/(fr + 1) of fr; the function for r relative to fr and the function for c relative to v are identically zero. as a consequence, the inaccuracy dr/r for r shows the same behavior versus the frequency of the inaccuracy dv/v in the measurement of v, as dr/r =| | dv/v = dv/v, and the inaccuracy dc/c for c shows a similar behavior versus the frequency with respect to the inaccuracy dfr/fr for fr, as dc/c =| |dfr/fr ≈ dfr/fr if fr>>1. moreover, as well as the hypothesis h = 0, if v and fr are measured in the cut-off frequency ft = ft (v, fr), then: the sensitivity functions and for the transfer impedance, both in modulus |z| and in phase uz, relative to v, are constant, and respectively (−1/4) and (−1/r); and the sensitivities and for |z| and uz relative to fr are independent of v, such that they behave as the function fr/(fr + 1) of fr. as a consequence, the ratio between dfr/fr and dv/v is independent of v, and behaves as the function (1+1/fr) of fr, and dv/v is a constant equal to dv/v = 4d|z|/|z|+rduz/uz. as a post-test, only assuming the conditions v = const and fr = const in equations (4.1) and (4.2), the sensitivity functions approach provides results that are in agreement with a previous report [vannaroni et al. 2004]. 4.2. transfer functions method this study proposes to deepen the transfer functions method by analyzing the zero and pole behavior, which were implied in the frequency domain analysis suggested by grard and tabbagh [1991]. indeed, this section introduces the method of analysis in the frequency domain for determining the behavior of the zero and pole frequencies in the linear time-invariant circuit of the quadrupole probe (figure 1). to satisfy the operative conditions of linearity for the measurements, if the quadrupole has capacitive contact with the subjacent medium then the frequency f of the probe should be imposed as included between the zero zm and the pole pm of the transfer impedance, and so its modulus is almost constant within the frequency band [grard and tabbagh 1991], (4.3) based on the above conditions, an optimization equation is deduced for the probe that links the optimal ratio x between its height above ground and its characteristic geometrical dimension only to the dielectric permittivity fr of the medium, so that: (4.4) to satisfy the operative conditions of linearity for the measurements, if the quadrupole is in galvanic contact with the subjacent medium, then the working frequency f of the quadrupole should be imposed as lower than the cut-off frequency of the transfer impedance, and so its modulus as constant below the cut-off frequency. it is only under these conditions that it is optimal to design the characteristic geometrical dimensions of the probe or to establish the measurable ranges of the conductivity v and permittivity fr of the medium (figure 5). equations (4.3) and (4.4) that were derived by the classical transfer function method are demonstrated in appendix b. the interesting physical results obtained using this transfer functions method are discussed below. to meet the measurement of resistivity and permittivity 10 figure 8e. caption on previous page. (e) srv s r c r ff ^ h sr rf scv srv s r c r ff ^ h s z; ;v s zv u s z rr f ; ; f ^ h s rzr ffu ^ h , , , ,z x f p x .m r m r# #f v f v^ ^h h 15 17 2 .x r + .d f^ h 11 design specifications that ensure satisfactory performances of the probe (inaccuracy of no more than 10%), the forecasts provided by the theory of error propagation suggested by vannaroni et al. [2004] that apply the sensitivity functions approach, as explicitly developed in the study, are less stringent than those foreseen by the analysis in the frequency domain suggested by grard and tabbagh [1991]. here, this deepens the transfer function method to analyze the zero and pole behavior, in terms of both larger band of frequency f and wider measurable range of resistivity t or permittivity fr (figures 6, 7). indeed, given a surface (e.g. a non-saturated concrete with low conductivity v = 10−4 s/m and fr = 4) with dielectric permittivity fr (figure 6): if the quadrupole probe has capacitive contact with the subjacent medium, i.e. h ≠ 0, then having defined an optimal ratio xopt= hopt/l between an optimal height hopt above ground and the characteristic geometrical dimension l, the transfer impedance z (f, xopt), in units of 1/hopt, calculated in xopt, is a function of the working frequency f such that its modulus |z|(f, xopt), in units of 1/hopt, is almost constant between a zero frequency z (xopt) almost one lf decade higher than a minimum frequency value fmin(xopt), allowing the inaccuracy dfr/fr (f, xopt) in the measurement of fr below a prefixed limit (10%), and a pole p (xopt) almost one mf decade lower than the maximum value of frequency fmax(xopt) that satisfies the requirement that the inaccuracy dfr/fr (f, xopt) for fr below 10% (figure 8); if h = 0, i.e. the quadrupole of the electrode-electrode distance l grazes a medium of conductivity v, then the transfer impedance z (f, l), calculated in l is a function of the working frequency f such that its modulus |z|(f, l) is constant down to the cut-off frequency ft = ft (v, fr), which is higher than an optimal frequency fopt(l) that minimizes the inaccuracy dfr/fr (f, l). materials characterized by a low v or a high fr lead to a leftward shift of the cut-off frequency ft, so reducing the optimal frequency fopt(l) (figure 9); on a selected surface, it is usually possible to verify that the probe in capacitive contact performs optimal measurements over the band [fmin(xopt)<z (xopt), fmax(xopt)>p (xopt)], which is shifted towards lower and higher frequencies compared to when the probe is in galvanic contact, where the respective band (fmin, fmax) is narrower by almost one lfmf decade in frequency, particularly increasing the value of fr (figures 8e, 9c). moreover, once the frequency band b is fixed (figure 7): if the quadrupole probe has capacitive contact with the measurement of resistivity and permittivity figure 9c. (a) (b) (c) figure 9. with reference to a quadrupole probe designed according to an electrode-electrode distance l0= 1 m and in galvanic contact on a concrete of low electrical conductivity, i.e. v = 10−4 s/m, fr= 4: (a, b) bode’s diagrams, as functions of the frequency f for the transfer impedance both in modulus |z|(f, l0) (a) and phase uz (f, l0) (b); (c) semi-logarithmic plots for both the inaccuracies dv/v (f) in the measurement of the conductivity v, and dfr/fr (f) of the permittivity fr [d|z|/|z|= duz/uz= 10−3]. subjacent medium, then the ratio x = h/l between the height h above ground and the characteristic geometrical dimension l ranges from the lower limit xlow, corresponding to water (fr = 81). in a preliminary analysis based on the transfer functions approach, it follows that the quadrupole designed with the height/dimension ratio x = h/l optimally measures the dielectric permittivity fr, opt ; the modulus |z|(x, v, fr, opt), in units of 1/h, of its transfer impedance, calculated in x and fr, opt, is a function of the electrical conductivity v, is characterized by a zero z (v, fr, opt) and a pole p (v, fr, opt) frequency, which fall near the lower and upper limits of b, respectively, when v is measured within the range of the lower limit and the upper limit . in a deeper analysis based on the sensitivity functions method, and still designing the quadrupole with the ratio x = h/l for optimal measurement of fr, opt, it is possible to verify the measurable range of v; the inaccuracy dfr/fr (x, v, fr, opt) in the measurement of fr, opt, a function of v, is below a prefixed limit (10%) if v is measured within the range (vlow, vup), larger than by almost one order of magnitude, both the right and left sides (figure 10,tables 1, 2); if h = 0, i.e. the probe of the electrode-electrode distance l grazes a medium of conductivity v and permittivity fr, then the transfer impedance z (l, v, fr) calculated in l is a function of v and fr such that its cut-off frequency ft = ft (v, fr), a function of both v and fr, ranges from ft,min= 100 khz to ft,max= 1 mhz for materials belonging to a (v, fr)-domain, which is almost superimposable with the corresponding one measurement of resistivity and permittivity 12 permittivity inaccuracy (a) xw, opt 1.083·10 −4 fr, opt 6.703 vopt 3.52·10 −5 s/m fr, opt= 6.703 vopt= 3.52·10 −5 s/m dfr/fr ≤ 0.1 dv/v ≤ 0.1 (b) xw, low ≈ 0 xw, up 0.475 xw, opt= 1.083·10 −4 dfr/fr ≤ 0.1 dv/v ≤ 0.1 (c) fr, low , vlow 1, 5.333·10 −5 s/m fr, up , vup 81, 3.14·10 −3 s/m table 1. wenner's configuration: (a) optimal point (xw, opt, fr, opt, vopt) of permittivity inaccuracy; (b) range of x where dfr/fr ≤ 0.1 and dv/v ≤ 0.1, once selected optimally fr, opt and vopt; (c) domains of f and v where dfr/fr ≤ 0.1 and dv/v ≤ 0.1, once fixed optimally xw, opt. figure 10. on the hypothesis that d|z|/|z|= duz/uz= 10−3, referring to both the inaccuracies dv/v (v, fr) for the electrical conductivity v, and dfr/fr (v, fr) for the dielectric permittivity fr, as functions of v and fr, and when the quadrupole probe is designed in the linear wenner configuration working in a fixed band b = 100 khz, with an height/dimension ratio xw, concrete= 0.087 which is optimal for capacitive contact only with a non-saturated concrete of permittivity fr= 4: (a, b) as plots for the orthogonal projections over the (v, fr) plane that satisfy the conditions dv/v (v, fr) ≤ 0.1 (a) and dfr/fr (v, fr) ≤ 0.1 (b). see also tables 1 and 2. (a) (b) lowvl upvl ( , )low upv vl l 13 within which the inaccuracy dfr/fr (l, v, fr) for fr is below about 10% (figure 11); having fixed the frequency band, the probe in capacitive contact usually performs optimal measurements over surfaces of lower conductivities compared to when the probe is in galvanic contact, as the respective conductivities are higher by almost one order of magnitude (tables 1, 3). 5. quadrupole configurations the transfer impedance of a quadrupolar array can be evaluated for any arbitrary configuration. as a general rule, it is assumed that subsurface electrical sounding becomes scarcely effective at depths greater than the horizontal distance between the electrodes [grard and tabbagh 1991, vannaroni et al. 2004]. this study considers two kinds of probes, i.e. with linear wenner and square configurations. the linear wenner arrangement consists of four terminals equally spaced from one another along a straight horizontal line [vannaroni et al. 2004]. instead, the square configuration is an array of two parallel horizontal dipoles, with the four electrodes positioned at the corners of a square [grard and tabbagh 1991]. if the quadrupole probe (figure 1) has a characteristic geometrical dimension l, then the linear wenner configuration (figure 2a) measures a capacitance in a vacuum c0,w= 4rf0·l, while in the square arrangement (figure 2b) c0,s= a·c0,w, is greater by a factor a = 1/(2-2 ½)>1. when the quadrupole is in galvanic contact, i.e. h = 0, with a subjacent medium of electrical conductivity v and dielectric permittivity fr, the linear wenner configuration measures a resistance rn,w= 2f0/vc0,w and a parallel capacitance cn,w= c0,w(fr+1)/2, while in the square arrangement, rn,s= rn,w/a and cn,s= a·cn,w. so, at the frequency f, the transfer impedance 1/zn= 1/rn+ j2rf cn for the linear wenner configuration is defined by a modulus |z|n,w= 1/[(1/rn,w) 2 + (2rf cn,w)] ½ and a phase un,w= arctg (2rf·rn,w cn,w), while in the square arrangement, |z|n,s= z|n,w/a, which is smaller by a factor of 1/a (figure 9a), and un,s= un,w , which is maintained invariant in the linear wenner or square configurations (figure 9b). the cut-off frequency is also independent of the configurations, i.e. ft = ft (v, fr). moreover, if the probe grazes the medium and considering that the sensitivity functions are defined as normalized functions, then the sensitivities and relative to the conductivity v, and the functions and relative to the permittivity fr, for the transfer impedance, both in modulus |z| and in phase uz, are invariant in the linear wenner or square configurations. only if h = 0 are the inaccuracies dv/v in the measurement of v and dfr/fr for fr also independent of the configurations, so the probe is characterized by the same performances in the frequency band b and in the measurable ranges of v and fr (figure 9c). instead, when the quadrupole is in capacitive contact with the subjacent medium, and so the ratio x = h/l between its height h above ground and its electrodeelectrode distance l is not zero, i.e. 0< x ≤1, then the quadrupole is characterized by a geometrical factor k(x) [d(x)], decreasing (or increasing) the function of x, which in the square configuration slopes down (or up) more steeply than in the linear wenner arrangement, so assuming smaller (or larger) values especially for 1/2 < x <1 (figure 8a). as a consequence, a probe with a fixed l that performs measurements on a medium of dielectric permittivity fr could be designed with an optimal height/dimension ratio xopt= hopt/l, which in the square configuration is smaller than in the linear wenner arrangement, because its factor d(x) slopes up more steeply, increasing the ratio x, so reaching the prefixed optimal value dopt(fr) ≈ 2/(15fr +17) with a smaller xopt. in simpler terms, if the probe is in capacitive contact with the medium, to perform optimal measurements of the permittivity, the square configuration needs to be raised above ground by less than in the linear wenner arrangement, if their electrode-electrode distances are equal. indeed, x ranges from xw,low= 0.022 in the linear wenner configuration, and from xs,low=0.019 in the square arrangement. moreover, in the case of capacitive contact, if the quadrupole with electrode-electrode distance l is designed according to the optimal height/dimension ratio xopt= hopt/l working in a frequency f, then the transfer impedance z (f, xopt), in units of 1/hopt, calculated in xopt, is defined by a phase u (f, xopt), which does not depend on the square or linear wenner configurations (figure 8d), and by a modulus |z|(f, xopt), in units of 1/hopt, which in the square configuration is shifted down by a factor 1/a with respect to the linear wenner configuration (figure 8c), remaining measurement of resistivity and permittivity fr, concrete= 4.026 xw, concrete= 0.087 sensitivity function approach transfer function method vlow 4.473·10 −6 s/m 1.78·10−5 s/m vup 3.058·10 −4 s/m 7.12·10−5 s/m (a) xw, concrete= 0.087 dfr/fr ≤ 0.1 dv/v ≤ 0.1 fr, low , vlow 1, 1.769·10 −6 s/m fr, up , vup 84.458, 1.573·10 −3 s/m (b) table 2. concretes: (a) comparing domains of v, foreseen by sensitivity approach and transfer method, once selected fr, concrete and designed optimally xw, concrete ; (b) domains of f and v where dfr/fr ≤ 0.1 and dv/v ≤ 0.1, once designed optimally xw, concrete. s z; ;v s zv u s z rr f ; ; f ^ h s rzr ff u ^ h almost unvaried in both configurations not only the shape of the modulus |z|(f, xopt), but also the position of its zero z (xopt) and pole p (xopt) frequencies (figure 8b). finally, the inaccuracies dv/v (f, xopt) in the measurements of the conductivity v and dfr/fr (f, xopt) for the permittivity fr, calculated in xopt, do not depend on the two configurations, so the optimal frequency fopt(xopt) that minimizes the inaccuracy dfr/fr (f, xopt) for fr, together with the minimum and maximum values of frequency fmin(xopt) and fmax(xopt), respectively, which allow the inaccuracy dfr/fr (f, xopt) below a prefixed limit (10%), are invariant in both of the configurations (figure 8e). in simpler terms, if the probe is in capacitive contact with the medium, to perform an optimal measurement of permittivity considering different height/dimension ratios, the design of the two configurations establishes (almost) invariant trends in frequency, both for their transfer impedances and measurement inaccuracies. 6. conclusions the present study has proposed a theoretical modeling of simultaneous and noninvasive measurements of electrical resistivity and dielectric permittivity using a quadrupole probe on a subjacent medium [see also arxiv.org's ref.: settimi et al. 2009]. a mathematical-physical model has been applied to the propagation of errors in the measurement of resistivity and permittivity based on the sensitivity functions tool. the findings have also been compared to the results of the classical method of analysis in the frequency domain, which is useful for determining the behaviour of zero and pole frequencies in the linear time invariant circuit of the quadrupole. this study has underlined that average values of electrical resistivity and dielectric permittivity can be used to estimate the complex impedance over various terrains and concretes, especially when they are characterized by low levels of water saturation or content [knight and nur 1987], and are analyzed within a bandwidth ranging from only lfs to mfs [al-qadi et al. 1995, myounghak et al. 2007]. to meet the design specifications that ensure satisfactory performances of the probe (inaccuracy of no more than 10%), the forecasts provided by the theory of error propagation suggested by vannaroni et al. [2004] that apply the sensitivity functions approach, as explicitly developed in the study, have been discussed in comparison to those foreseen by the analysis in the frequency domain suggested by grard and tabbagh [1991]. here, this deepens the transfer function method to analyze the zero and pole behavior (in terms of both band of frequency f and measurable range of resistivity t, or permittivity fr). it is interesting to compare the results of the present study with others in the literature [grard and tabbagh 1991, measurement of resistivity and permittivity 14 h = 0 dfr/fr ≤ 0.1 dv/v ≤ 0.1 fr, low , vlow 1, 5.333·10 −5 s/m fr, up , vup 81, 3.14·10 −3 s/m table 3. galvanic contact: domains of f and v where dfr/fr ≤ 0.1 and dv/v ≤ 0.1, when h = 0. (a) (b) figure 11. with reference to a quadrupole probe in galvanic contact, working in a fixed band b = 100 khz, plots for the domains (v, fr) of the electrical conductivity v and the dielectric permittivity fr such that: (a) the transfer impedance is characterized by a modulus with a cut-off frequency ft = ft (v, fr) = v/(2rf0(fr+1)) in the interval ft ∈ [100 khz, 1 mhz]; (b) both the inaccuracies dv/v (v, fr) in the measurement of the conductivity v, and dfr/fr (v, fr) of the permittivity fr are below a prefixed limit of 10% [d|z|/|z|= duz/uz = 10−3]. see also tables 1 and 3. 15 vannaroni et al. 2004]. in agreement, the sensitivity functions approach provides the following results: a) if the quadrupole probe is in galvanic contact with the subsurface, i.e. h = 0, then the inaccuracies dv/v in the measurement of conductivity v and dfr/fr for permittivity fr are minimized in the frequency band b of the quadrupole for all of its geometrical configurations and media; and b) even if h ≠ 0, the design of the probe must be optimized with reference to the minimum value of the inaccuracy dfr/fr for fr, which is always higher than the corresponding minimum value of the inaccuracy dv/v in the band b for all its configurations and media. more explicitly than in these previous studies [grard and tabbagh 1991,vannaroni et al. 2004], the transfer functions method provides results such that to satisfy the operative conditions of linearity for the measurements: a) if the quadrupole has capacitive contact with the subjacent medium, then the frequency f of the probe should be imposed as included between the zero zm and the pole pm of the transfer impedance, and so its modulus is almost constant within the frequency band. an optimization equation is deduced for the probe that links the optimal ratio x between its height above ground and its characteristic geometrical dimension only to the dielectric permittivity fr of the medium; b) instead, if the quadrupole is in galvanic contact with the subjacent medium, then the working frequency f of the quadrupole should be imposed as lower than the cut-off frequency of the transfer impedance, and so its modulus is constant below the cut-off frequency. it is optimal to design the characteristic geometrical dimensions of the probe or to establish the measurable ranges of the conductivity v and permittivity fr of the medium. unlike these previous studies [grard and tabbagh 1991, vannaroni et al. 2004], the sensitivity functions approach and the transfer functions method provide results that allow an assessment of the performance of the quadrupole probe in galvanic and capacitive contact: a) on a selected surface (for example, a non-saturated concrete with low conductivity v = 10−4 s/m and fr = 4), it is usually possible to verify that the quadrupole in capacitive contact performs optimal measurements over the band [fmin(xopt)<z (xopt), fmax(xopt)>p (xopt)], which is shifted to lower and higher frequencies compared to when the probe is in galvanic contact, as the respective band [fmin, fmax] is narrower by almost one lf-mf decade in frequency, particurarly increasing the value of fr; b) having fixed the frequency band, the quadrupole in capacitive contact usually performs optimal measurements over surfaces of lower conductivity compared to when the probe is in galvanic contact, as the respective conductivities are higher by almost one order of magnitude. on this basis, some constraints were established to design a quadrupole probe for conducting measurements of electrical resistivity and dielectric permittivity in a regime of ac at lfs and mfs (10 khz 1 mhz). measurements were carried out using four electrodes laid on the surface to be analyzed, and through measurement of transfer impedance, the resistivity and permittivity of the material can be extracted. furthermore, by increasing the distance between the electrodes, the electrical properties of the sub-surface structures can be investigated to greater depths. the main advantage of the quadrupole is that measurements of electrical parameters can be conducted with a nondestructive technique, thereby enabling characterization of precious and unique materials. also, in appropriate arrangements, measurements could be carried out with electrodes slightly raised above the surface, allowing for completely nondestructive analysis, although accompanied by a greater error. the probe can perform measurements on materials with high resistivity and permittivity in an immediate way, without the need for later stages of data post-analysis. references al-qadi, i. l., o. a. hazim, w. su and s. m. riad (1995). dielectric properties of portland cement concrete at low radio frequencies, j. mater. civil. eng., 7, 192-198. auty, r. p. and r. h. cole (1952). dielectric properties of ice and solid, j. chem. phys., 20, 1309-1314. banton, o., m. k. seguin and m. a. cimon (1997). mapping field scale physical properties of soil with electrical resistivity, soil sci. soc. am. j., 61, 1010-1017. chelidze, t.l. and y. gueguen (1999). electrical spectroscopy of porous rocks: a review – i, theoretical models, geophys. j. int., 137, 1-15. chelidze, t.l., y. gueguen and c. ruffet (1999). electrical spectroscopy of porous rocks: a review – ii, experimental results and interpretation, geophys. j. int., 137, 16-34. debye, p. (1929). polar molecules, leipzig. del vento, d. and g. vannaroni (2005). evaluation of a mutual impedance probe to search for water ice in the martian shallow subsoil, rev. sci. instrum., 76, 084504 (1-8). edwards, r. j. (1998). typical soil characteristics of various terrains, http://www.smeter.net/grounds/soil-electricalresistance.php. fechant, c. (1996). réalisation d’un quadripôle de mesure in situ de la permitivié diélectrique des végétaux. premier application à la détermination du contenu en eau des épis de blé, these de l’université pierre-et-marie-curie vi, paris. fechant, c. and a. tabbagh (1999). mesure en laboratoire de la permittivité diélectrique moyenne fréquence de végétaux à 430 khz à l’aide d’un capacimétre. relation entr permittivité apparente d’un ensemble d’épis de blé et leur contenu en eau, c. r. acad. sci. paris t. 327 série ii b, 285-298 (both in french and in english). frölich, h. (1990 edition). theory of dielectrics, oxford. grard, r. (1990a). a quadrupolar array for measuring the measurement of resistivity and permittivity complex permittivity of the ground: application to earth prospection and planetary exploration, meas. sci. technol., 1, 295-301. grard, r. (1990b). a quadrupole system for measuring in situ the complex permittvity of materials: application to penetrators and landers for planetary exploration, meas. sci. technol., 1, 801-806. grard, r. and a. tabbagh (1991). a mobile four electrode array and its application to the electrical survey of planetary grounds at shallow depth, j. geophys. res., 96, 4117-4123. kearey, p., m. brooks and i. hill (2002). an introduction to geophysical exploration, oxford. knight, r. j. and a. nur (1987). the dielectric constant of sandstone, 60 khz to 4 mhz, geophysics, 52, 644-654. laurents, s., j. p. balayssac, j. rhazi, g. klysz and g. arliguie (2005). non-destructive evaluation of concrete moisture by gpr: experimental study and direct modeling, materials and structures (m&s), 38, 827-832. loke, m. h. (2001). tutorial: 2-d and 3-d electrical imaging surveys, course notes for usgs workshop 2-d and 3-d inversion and modelling of surface and borehole resistivity data, torrs, ct. murray-smith, d. j. (1987). investigations of methods for the direct assessment of parameter sensitivity in linear closed-loop control systems, in complex and distributed systems: analysis, simulation and control, edited by s. g. tzafestas and p. borne, amsterdam, 323–328. myounghak, o., k. yongsung and p. junboum (2007). factors affecting the complex permittivity spectrum of soil at a low frequency range of 1 khz10 mhz, environ geol., 51, 821-833. polder, r., c. andrade, b. elsener, ø. vennesland, j. gulikers, r. weidert and m. raupach (2000). test methods for on site measurements of resistivity of concretes, materials and structures (m&s), 33, 603-611. rhoades, j. d., p. a. c. raats and r. j. prather (1976). effect of liquid-phase electrical conductivity, water content, and surface conductivity on bulk soil electrical conductivity, soil sci. soc. am. j., 40, 651-655. samouëlian, a., i. cousin, a. tabbagh, a. bruand and g. richard (2005). electrical resistivity survey in soil science: a review, soil till. res., 83, 172-193. settimi, a., a. zirizzotti, j. a. baskaradas and c. bianchi (2009). inaccuracy assessment for simultaneous measurements of resistivity and permittivity applying sensitivity and transfer function approaches, arxiv.org: 0908.0641. tabbagh, a. (1994). simultaneous measurement of electrical and dielectric permittivity of electrical conductivity and dielectric permittivity of soil using a slingram electromagnetic device in medium frequency range, archaeometry, 36, 159-170. tabbagh, a., a. hesse and r. grard (1993). determination of electrical properties of the ground at shallow depth with an electrostatic quadrupole: field trials on archaeological sites, geophys. prospect., 41, 579-597. vannaroni, g., e. pettinelli, c. ottonello, a. cereti, g. della monica, d. del vento, a. m. di lellis, r. di maio, r. filippini, a. galli, a. menghini, r. orosei, s. orsini, s. pagnan, f. paolucci, a. pisani r., g. schettini, m. storini and g. tacconi (2004). muses: multi-sensor soil electromagnetic sounding, planet. space sci., 52, 67–78. wenner, f. (1915). a method of measuring earth resistivity, us bur. of stand. bull., 12, 469-478. measurement of resistivity and permittivity 16 17 appendix a we consider here the influence of inaccuracies in transfer impedance in modulus and phase on the measurement of electrical conductivity and dielectric permittivity. the mathematical tool best suited to this purpose applies the so-called sensitivity functions [murraysmith 1987], which formalize the intuitive concept of sensitivity as the ratio between the percentage error of certain physical quantities (due to the variation of some parameters) and the percentage error of the same parameters. the inaccuracies dv/v in the measurement of the electrical conductivity v, and dfr/fr for the dielectric permittivity fr, can be expressed as linear combinations of the inaccuracies d|z|/|z| and duz/uz in the measurement of transfer impedance in modulus |z| and in phase uz, respectively, as given in equations (4.1) and (4.2) (figure 3). the pairs of sensitivity functions and for the transfer impedance, both in |z| and uz, relative to the conductivity v and the permittivity fr (figure 4), (a.1) (a.2) (a.3) (a.4) are, in turn, linear combinations of the sensitivity function pairs and for transfer impedance, in both the resistance r and capacitance c parallel components, relative to v and fr, (a.5) (a.6) (a.7) (a.8) with the weight functions: (a.9) (a.10) considering equations (a.1) to (a.4), if the modulus |z| and the phase uz of the transfer impedance provide an indirect measurement of the electrical conductivity v and dielectric permittivity fr, then the functions |z|=|z|(v, fr) and uz = uz(v, fr) are invertible, i.e. v = v(|z|, uz) and fr = fr (|z|, uz). therefore, the theorem of the derivative for the measurement of resistivity and permittivity ( , )s sz z; ;v v u ( , )s sz z r r ; ; f f u 1 2 1 2 1 , for const , s z z z z s h s h s1 2 z z r c r = = = = = = 2 2; ; ; ; ; ; ; ;v v v v f d d ; ; ; ; v v v v 1 2 1 arct 2 1 , for 1 const s s r r c c r r c c h s s h s s r r c c 1 1 z z n n n n r c r c n n r z z = = = + + = 2 2 # , , v v f u u x x x v v v v v v u u ` ^ ^ ^ j h h h 1 2 1 2 1 , for const ,s s h s h s1 z z r c 2r r r r = = =v-; ; ; ; f f f f 1 2 1 arct 2 1 , for 1 const , s s r r c c r r c c h s s h s s r r c c 1 1 n n n n r c r c n n z z r r r r r r = = + + =# , , v x x x f f f f f f u u ` ^ ^ ^ j h h h s r r r r const const r r r = = =2 2 v v v v d d = = v f f ( , )s scrv v ( , )s s cr r rf f 1 2 1 2 1 1 2 1 2 1 , x x x x 2 2 2 2 r r r r = + + + + + d f d f d f d f x x ^ ^ ^ ^ h h h h ; ; ; ; e e e e 2 1 1 1 2 1 1 2 1 1 2 1 2 1 2 3 1 s c c x x x x x x const 2 2 2 2 2 c r r r r r r r r = = = + + + + + + + + + 2 2 v v f d f f d f d f d d f d f x x x = v f ^ ` c ^ ^c ^ ^ ^ ^ ^ h j h m h m h h h h h ; ; 6 e e @ 1 2 1 2 1 1 2 1 ,s x x x x const 2 2 r r r r r r = + + + + f d f d f d d f x =f v ^ ^ ^ ^ h h h h ; ; ; e e e 1 1 2 1 1 2 1 1 2 1 2 1 1 2 4 1 1 5 1 4 1 1 1 , s x x x x x x const 2 2 2 2 2 3 2 c r r r r r r r r r r r r2 r = = + + + + + + + + + + + + f f d f f d f d f d f d f f f d f f x x x x =f v ^ ` c ^ ^c ^ ^ ^ ^ ^ ` ^ ^ ` h j h m h m h h h h h j h h j ; ; 8 8 e e b b 1 1 1 1 2 1 1 2 1 1 ,h x x x 1 2 2 2 2 2 r r r = + + + + + + + + d f d f f d x x x x ^ ^ ^ ^ ` ^ h h h h j h6 @ 1 1 1 1 2 1 1 2 1 2 2 1 2 1 2 1 1 2 1 .h x x x x 2 2 2 2 2 2 2 2 2 r r r r r r r = + + + + + + + + + + + + + + + d f d f f d f f d f f x x x x x x x x ^ ^ ^ ^ ` ^ ` ^ ` h h h h j h j h j inverse function can be applied. indeed, under the condition v = const (or fr = const), both |z| and uz are invertible functions of fr (or v), i.e. they are strictly increasing or decreasing monotonic functions of fr (or v). appendix b by exact calculations, the transfer impedance z (f, x, v, fr) measured by the quadrupole probe, in units of the reciprocal height 1/h from the subjacent medium, consists of the resistance r (f, x, v, fr), in units of 1/h – see equation (3.10) –, which can be expressed as a transfer function characterized by a pole in the origin frequency pr= 0, a zero in higher frequencies zr (f, x, v, fr)>0, and a static gain kr (f, x, v), (b.1) where: (b.2) (b.3) as well as the parallel capacitance c (f, x, v, fr), in units of 1/h – see equation (3.11) –, which can be expressed as a transfer function characterized by a low frequency pole pc (f, x, v, fr), a zero in higher frequencies zc (f, x, v, fr) > pc (f, x, v, fr), and a static gain kc (x), (b.4) where the capacitance pole pc (f, x, v, fr) coincides with the resistance pole zr (f, x, v, fr), (b.5) and: (b.6) (b.7) for values of the ratio x = h/l between the height h above ground and the characteristic geometrical dimension l, and of the paired values of electrical conductivity v and dielectric permittivity fr that satisfy the condition (figure 5), (b.8) it can be demonstrated that the modulus |z|(f, x, v, fr) can be approximately expressed as a transfer function with a pole in the origin frequency, a low frequency zero zm (f, x, v, fr), a pole in higher frequencies pm (f, x, v, fr) > zm (f, x, v, fr), and a static gain km (x) (figure 8c), (b.9) where the zero of the modulus zm (f, x, v, fr) coincides with the capacitance pole pc (f, x, v, fr) and the pole of the modulus pm (f, x, v, fr) with the capacitance zero zc (f, x, v, fr) (figure 8b), (b.10) (b.11) and (b.12) equation (b.8) establishes limits on the range for the design specification x of the quadrupole and the measurable range (v, fr) of the media. to satisfy the operative conditions of linearity for the measurements, the quadrupole probe, which is characterized by the height/ dimensions ratio x = h/l, should measure the conductivity v and the permittivity fr of the subjacent medium when its working frequency f falls within the band included between the zero zm (f, x, v, fr) and the pole pm (f, x, v, fr) of the transfer impedance, as reported in equation (4.3). moreover, the quadrupole probe specified by x = h/l should measure fr, as its geometrical factor d(x) is close to equation (4.4), a necessary condition for z (f, x, v, fr) to show an almost constant modulus within the band (equation 4.3), the modulus in the zero (equation b.10) coinciding with the corresponding one in the pole (equation b.11), (b.13) measurement of resistivity and permittivity 18 , , , , 2 1 , , ,r f x k x f z x f 2 2 2 r r r r= + v f v r v f^ ^ ^ ^h h h h , , 2 1 1 1 2 1 2 1 ,z x x x 0 r r r r r = + + + v f r f f v d f d f -^ ^ ^ ^h h h h , 2 1 1 1 ,k x c x x x 0 0 2 r =v f v d d ^ ^ ^ ^h h h h , , , 1 , , 1 , , ,c f x k x x f z x f p2 2 2 2 r c r c r c= + + v f v f v f^ ^ ^ ^h h h h , , , , ,p x z xc r c r=v f v f^ ^h h , , 2 1 1 1 2 1 2 1 ,z x x x 0 c r r r r = + + + v f r f f v d f d f -^ ^ ^ ^h h h h .k x x c x0 c = d ^ ^ ^h h h << , 1 2 , , 2 , k x k x z x2 2r c c r$v r v f^ ^ ^h h h6 6@ @ , , , 2 1 , , 1 , , ,z f x k x f p x f z x f 2 2 2 2 r m r m r m= + + $ ; ; v f r v f v f^ ^ ^ ^h h h h ; e , , , , ,z x p xm r c r=v f v f^ ^h h , , , , ,p x z xm r c r=v f v f^ ^h h 1 .k x k xm c =^ ^h h , , , , , , 2 1 2 , , , , , z x z x k x z x k x z x p x 2 f z r m r m f p r m m r m r m m = = = ; ; ; ;v f r v f v f r v f v f = =^ ^ ^ ^ ^ ^ ^ h h h h h h h 19 so that the pole (equation b.11) is almost four-fold greater that the zero (equation b.10), (b.14) equation (4.4) can be interpreted as the optimization equation of the quadrupole, so the sizing for the height/dimension ratio x of the probe depends only on the permittivity fr of the medium; instead, equations (4.3) and (b.14) show that the probe can work optimally only in a small band of frequencies. *corresponding author. dr. alessandro settimi, istituto nazionale di geofisica e vulcanologia (ingv), via di vigna murata 605, i-00143 rome, italy; e-mail: alessandro.settimi@ingv.it © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. measurement of resistivity and permittivity , , 4 , ,p x z x .m r m r.v f v f^ ^h h 48,06,2005misc 937 annals of geophysics, vol. 48, n. 6, december 2005 key words electrical resistivity – sounding – groundwater condition – shooro – iran 1. introduction the shooro basin that lies in southeast iran is located in sistan and baluchestan province. the elevation of the basin is in the range of a minimum of 1360 m above the mean sea level in north to a maximum of 2960 m above the mean sea level in the southeast. the study area lies in the north of the basin about 80 km in the southwest of zahedan city, the capital of sistan and baluchestan province (29º00ln-29º12ln and 60º05le-60º55l; fig. 1), and covers about 210 km2. a nearly flat plain with bedrock outcrops characterizes this area by a mild relief, with slops between 30 to 40 m topographic contours in the south and central parts. the purpose of this paper is to use the resistivity data and interpreting geoelectrical soundings to study the aquifer conditions. the use of electrical resistivity for both groundwater resource mapping and for water quality evaluations has increased dramatically over recent decades in much of the world due to the rapid advances in microprocessors and associated numerical modeling solutions. the ves has proved very popular with groundwater studies due to simplicity of the technique. water resources of the study area are threatened by increasing population trend with a resulting increase in water demand and the stressgeoelectrical investigation for the assessment of groundwater conditions: a case study gholam r. lashkaripour (1) and mohammad nakhaei (2) (1) department of geology, university of sistan and baluchestan, zahedan, iran (2) department of geology, university of tarbiat moallem, tehran, iran abstract an electrical resistivity survey involving vertical electrical soundings (ves) was carried out in the shooro basin in southeast iran in order to study groundwater conditions such as depth, thickness and aquifer boundaries. vertical electrical soundings by schlumberger array were conducted in this area. the resistivity schlumberger soundings which have a maximum current electrode spacing (ab) ranging from 200 m to 600 m were carried out at 207 positions in 19 profiles. interpretation of these soundings indicates the presence of an alluvial aquifer. this aquifer is divided into eastern and western parts by the shooro river, which comprises a variable thickness and resistivity of deposits. the average permeability coefficient and resistivity in the western part, especially southwest is higher than the eastern part of the aquifer. therefore, it seems that shooro river follows a fault zone in the region. the high resistivity of west part is due to the water quality and the existence of alluvial fan with coarse grain materials. low aquifer resistivities in the east are associated with finer materials and also brackish water infiltration from the adjacent basin mainly in the central part of the aquifer. furthermore, zones with high yield potential have been determined in this research based on the resistivity data. mailing address: dr. g.r. lashkaripour, department of geology, university of sistan and baluchestan, p.o. box 161-98135, khash road, zahedan, iran; e-mail: lashkarg@hamoon.usb.ac.ir 938 gholam r. lashkaripour and mohammad nakhaei es of water use for various activities. the need for this research is to study groundwater conditions and to protect groundwater supplies as a unique source of water for this area. 2. geological background from the geological point of view, shooro basin is situated in the flysch zone of eastern iran. the major portions of the flysch zone consist of shale, sandstone and limestone. in the flysch zone, sediments of older than cretaceous age are absent (tirrul et al., 1983; mccall, 1997). approximately the total bedrock of the aquifer consists of shale and slats mainly green of poor permeability, in the age of upper cretaceous and partly eocene. in the west part of the basin, major outcrop consists of sandstone, shale and philit. quaternary sediments cover the study fig. 1. location of the studied area. geoelectrical investigation for the assessment of groundwater conditions: a case study area. the quaternary deposits which cover the plain area are composed mainly of fine grain alluvium of shooro river, gravel to pebble of alluvial fan, sand to gravel and rock debris of the nearby mountains (geological survey of iran, 1995). 3. hydrogeological conditions the study area is characterized by an arid climate with extremely low rainfall (average 84 mm/yr) and with high evaporation rate (lashkaripour, 2000). the scanty rainfall is confined to the winter season and rain usually occurs as thunderstorms and showers. the fresh groundwater in the region is originated from infiltration of rainwater, which has created the unconfined shooro aquifer in the study area. the shooro aquifer is the major exploited aquifer in this region and it is an alluvial deposit aquifer. this aquifer is divided into east and west parts by the shooro river, which comprises a variable thickness of deposit. the average permeability coefficient in the west part is higher than that in the east. therefore, it seems that shooro river follows a fault zone in the region. the aquifer is mainly recharged by the annual rainfall, and discharges naturally by underflow to the shooro river. it is also discharged artificially through a number of dug wells and galleries called qanat. the newly drilled wells in north and central parts of the aquifer are now used to supply water for agricultural purposes and drinking water to the villages. rapid agricultural development in the study area has led to increased on demand for water supply. the monitoring of the groundwater level exhibits a decreasing trend of water level. the main reason responsible for this lowering of the groundwater table is that the wells pumping from groundwater resources have exceeded natural recharge in the recent years. the present study is based on data from the vertical electrical soundings that carried out during april to june 2001; lithological and hydrogeological information from 13 wells; chemical analyses of groundwater samples from the available boreholes. these data are used to evaluate the subsurface hydrogeological and structural conditions to a depth of about 150 m. in addition, estimation of the groundwater quality and recommendation for possible site-selections for drilling productive wells will be made. the aquifer hydraulic parameters are shown in table i. 4. geoelectrical resistivity survey geoelectrical resistivity techniques are popular and successful geophysical exploration for study groundwater conditions in the world. the resistivity of material depends on many factors such as groundwater, salinity, saturation, aquifer lithology and porosity. for example, the resistivity of an aquifer is related to electrical conductivity (ec) of its water. when the groundwater ec is high, the resistivity of the aquifer could reach the same range as a clayey medium and the resistivity parameter is no longer useful to determine the aquifer (vouillamoz et al., 2002). however, this method has been carried out successfully for exploration of groundwater. this technique is widely used to determine depth and nature of an alluvium, boundaries and location of an aquifer. the resistivity method was used to solve more problems of groundwater in the types alluvium, karstic and another hard formation aquifer as an inexpensive and useful method. some uses of this method in groundwater are: determination of depth, thickness and boundary of an aquifer (zohdy, 1969; and young et al. 1998), determination of interface saline water and fresh water (el-waheidi, 1992; yechieli, 2000; and choudhury et al., 2001), porosity of aquifer (jackson et al., 1978), water content in aquifer (kessels 939 table i. hydraulic parameters of the aquifer. no. parameters east part west part 1 aquifer thickness (m) 24 40 2 average groundwater 23 41 depth from ground level (m) 3 aquifer resistivity (µm) 12 45 4 ec range (µs cm−1) 2800-9600 760-2500 5 storage coefficient (%) 25 10 940 gholam r. lashkaripour and mohammad nakhaei et al., 1985), hydraulic conductivity of aquifer (yadav and abolfazli, 1998; troisi et al. 2000), transmissivity of aquifer (kosinski and kelly, 1981), specific yield of aquifer (frohlich and kelly, 1987), contamination of groundwater (kelly, 1976; and kaya, 2001). contamination usually reduces the electrical resistivity of pore water due to increase of the ion concentration (frohlich and urish, 2002). however, when resistivity methods are used, limitation can be expected if ground inhomogeneties and anisotropy are present (matias, 2002). this research project deals with detecting the aquifer conditions and groundwater quality in the study area. the resistivity survey was completed with 207 vertical electrical soundings (ves) by the schlumberger array in 19 profiles, with a maximum current electrode spacing (ab) ranging from 200 to 600 m. the profile spacing was 1 km and sounding spacing was 750 m. five of these ves have been conducted adjacent to the dug wells for subsequent calibration process. position and extension of all the ves’s are indicated in fig. 2. twenty nine geoelectrical sections were drawn along the profiles. the apparent resistivity measurements were made with a kd sound 7800b terrameter unit. this equipment is light and powerful for deep penetration. 5. discussion the ves curves were obtained by plotting the apparent resistivity against electrode spacing. the resistivity for each of the vertical electrical sounding was drawn on transparent double log graph paper and a smooth field curve. computer programs for reducing geoelectrical sounding curves into values of thickness and fig. 2. the location of ves and aal profile. geoelectrical investigation for the assessment of groundwater conditions: a case study resistivity of individual layers are described by koefoed (1979) and zohdy and bisdorf (1989). the field curves were interpreted by the wellknown method of curve matching with the aid of russian software ipi7.63. however, the thickness and characteristics of the aquifer are fairly well known due to the number of wells dug in the center of the aquifer. a number of geoelectric stations were purposely located near about 13 wells. the key to success of any geophysical survey is the calibration of the geophysical data with hydrogeological and geological ground truth information. ves success must rely on the careful interpretation and integration of the results with the other geologic and hydrogeologic data for the site. therefore, lithologic information obtained from log could be used to calibrate the ves field curves. where test hole-log information was available, the solution to automatic interpretation procedure was constrained by keeping known layer thicknesses constant during the program computations. lastly, the results of the schlumberger electrical soundings were compared with the geological sections obtained from 13 pizometer wells. these results are in a good agreement with the geological sections. measuring the current penetration depth was calculated by a correlation between depth and ab/2 as shown in fig. 3. from this figure eq. (5.1) was obtained to calcutate depth of current penetration (5.1) from eq. (5.1), the current penetration depth is equal to approximately ab/4. the boundary of the aquifer, thickness and resistivity of subsurface layers was also determined by the electrical survey in this research. from the interpretation of the resistivity curves a four-layer resistivities and thicknesses indicated four subsurface layers. these layers consisting of surface layer (topsoil), alluvium, saturated layer, and bedrock. in some cases more than one layer was evident in the saturated zone but these cases were also treated in this analysis as single layers. depth and thickness of subsurface layers were identified and dimension of the aquifer and type of bedrock were also indicated. bedrock of the area is generally slat but in some parts has appeared as shale. for example, the geoelectrical section of profile aal in the central west of aquifer is shown in fig. 4. the aquifer consists of two main east and west parts that is separated along the shooro river. the thickness of the aquifer and depth of bedrock is different in the two parts. average resistivity of the thin top layer, alluvium, aquifer and bedrock calculated in the east as 70, 74, 12 and 113 ω-m and in the west as 175, 116, 46 and 106 ω-m respectively. these data indicate that bedrock in both sides of the aquifer is the same, but permeability and storage coefficient in the eastern part is higher. depth and . /depth ab0 57 2 .0 97= .] g 941 fig. 3. correlation between depth of current penetration and length of current electrodes. 942 fig. 4. geoelectrical section of profile aal. fig. 5. subsurface layers in the eastern and western parts of the aquifer. t – thickness; r – resistivity. thickness of the aquifer were measured in the east as 23 and 24 m and in the west as 40 and 41 m respectively (fig. 5). the higher resistivity in the west part is due to the existence of an alluvial fan that consists of a mixture of gravel, sand and silt. the lower resistivity in the eastern part is due to intering of brackish water (ec in the range of 2800 to 9600 µs cm−1) from an gholam r. lashkaripour and mohammad nakhaei 943 geoelectrical investigation for the assessment of groundwater conditions: a case study adjacent basin and finer materials of sand and silt mixed with clay. yield potential in the western part of the aquifer is more than the eastern part. rt map (aquifer resistivity multiply by aquifer thickness) that shows the potential of the aquifer for water resources and quality in the whole study area is shown in fig. 6. the best part of the aquifer for future development is the central portion of the west part of the aquifer. in this part, the best water quality and quantity of groundwater sources are found with respect to high thickness and resistivity. furthermore, the boundary of the aquifer and type of the bedrock were indicated based on the geoelectrical data. 6. conclusions this study reveals that surface electrical measurements are effective to study groundwater conditions. based on the interpretation of geoelectrical data, the following conclusions are drawn: 1) the alluvial aquifer mainly consists of gravel and sand in the west part and sand and silt mixed with clay materials in the east part. the resistivity of the aquifer increases towards the west due to decreasing salinity of water and/or clay content. 2) ves tests revealed four subsurface geoelectric layers; thin top layer, the alluvium, the aquifer and the bedrock respectively. 3) the aquifer thickness increases towards the west, the regional direction of increasing deposition in the basin. the average thickness of the saturated alluvial aquifer in the west part has been estimated about 40 m and in the east about 24 m. 4) the bedrock of the aquifer shows different resistivity values due to type of bedrock and respect to degree of saturation and values of fractures. 5) the boundary of the aquifer has been estimated and zones with high yield potential have fig. 6. rt map of the aquifer. 944 gholam r. lashkaripour and mohammad nakhaei been determined for future development in the basin and for choosing drilling sites. 6) the resistivity data of the aquifer shows a decrease in the eastern part due to the intrusion of low quality (saline) water from the adjacent basin. acknowledgements the research presented in this paper has been supported by a grant from the sistan baluchestan province water organization of the ministry of power, for which we express our sincere thanks. references apparao, a. and t.g. rao (1974) depth of investigation in resistivity methods using linear electrodes, geophys. prospect., 22, 211-223. choudhury, k., d.k. saha and p. chakraborty (2001): geophysical study for saline water intrusion in a coastal alluvial terrain, j. appl. geophys., 46, 189-200. darvishzadeh, a. 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(received january 24, 2005; accepted november 28, 2005) vol48/04/2005def 767 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words diode laser – absorption spectroscopy – optical fiber 1. introduction the analysis of volcanic gases is widely recognised as a powerful tool to validate geological models relevant to reliable predictions of volcanic events (baubron et al., 1991; notsu et al., 1993). indeed, the chemical composition of certain gases, in volcanic effluxes, may provide direct information on deep magmatic processes and hydrothermal circulation (chiodini et al., 2003). in particular, the absolute concentration and the isotopic content of some molecular species gives an indication of sources and sinks of volcanic gases, whereas its time variation may be a consequence of changes in the status of a volcano. these changes can be due to chemical reactions of magmatic gases with rocks or fluids occurring along their path to the surface. with this in mind, the evolution of volcanic activity can be monitored using sensitive instrumentation for gas analysis (chiodini et al., 2003). however, the compactness and ruggedness of field sensors are critical requirements for monitoring of volcanic areas, where the equipment is often exposed to high humidity, large temperature variations, and fumigation by corrosive gases. furthermore, high precision and accuracy levels are necessary in order to retrieve significant information from experimental data. an ideal volcanic sensor would also allow for continuous, in situ gas concentration determination over long time periods, with unattended and remote operation. so far, volcanic gases have been usually monitored by means of laboratory analysis methods, such as mass spectrometry and gas cromatography, which require in situ sampling, which is often impractical and hazardous. in addition, the time resolution of sampling-based methods may be not sufficient for some applications. an alternative approach is given by optical techniques for remote-sensing, such as differennovel laser-based techniques for monitoring of volcanoes paolo de natale (1), giuseppe de natale (2), gianluca gagliardi (1), livio gianfrani (3) and alessandra s.d. rocco (1) (1) istituto nazionale di ottica applicata, comprensorio olivetti, pozzuoli (na), italy (2) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli, italy (3) dipartimento di scienze ambientali, seconda università di napoli, and infm-gruppo coordinato napoli 2, caserta, italy abstract an overview of novel laser techniques suitable for volcanic monitoring, based on different kinds of infrared laser sources, is presented. their main advantages and drawbacks are discussed focusing on the achievable sensitivity and precision levels in analysis of gaseous species. some of the most recent experimental results obtained in laboratory development as well as in field tests of home-built laser spectrometers are reported. new perspectives in optical devices aimed at geochemical and geophysical applications are also considered. mailing address: dr. gianluca gagliardi, istituto nazionale di ottica applicata, comprensorio olivetti, via campi flegrei 34, 80078 pozzuoli (na), italy; e-mail: gagliardi@ino.it 768 paolo de natale, giuseppe de natale, gianluca gagliardi, livio gianfrani and alessandra s.d. rocco tial optical absorption spectrometers (doas), lidar (svanberg, 2002) or open-path fouriertransform infrared spectrometers (ftir) (oppenheimer, 2002). the first two methods provided good results in field volcanic measurements, as in the case of galle et al. (2003), and weibring et al. (2002), although some of these apparatus are bulky and expensive, and often require a time-consuming post-acquisition data processing. more recently, new instruments, based on uv-correlation and doas techniques, have been developed in a more compact and userfriendly fashion (porter et al., 2002). in the last few decades, the use of near-infrared room-temperature diode lasers for gas sensing has significantly increased (werle, 1998). compared to spectroscopic techniques that rely on incoherent sources as well as on lead-salt and solid-state lasers, semiconductor diode lasers offer the possibility to develop reliable and compact spectrometers, with reasonable costs. also, they present several advantages in terms of tuneability, spectral selectivity and power consumption. the possible use of these devices in combination with optical fibers is particularly convenient for volcanic monitoring applications (ginfrani et al., 2000). thanks to the great advances in semiconductor and optoelectronic technologies, most of these features have been recently transferred to the midinfrared, with novel coherent radiation sources such as those based on difference-frequency generation and on quantum-cascade semiconductor structures (tittel et al., 2002). in this spectral region, several atmospherically relevant molecules, like h2o, co2, ch4 and so2, exhibit strong absorptions (harvard smithsonian center for astrophysics, 2003). 2. near-ir spectroscopy using dfb diode lasers near-infrared diode laser sources, initially developed for telecom applications, are currently based on iii-v semiconductor alloys. they operate in continuous mode, at room temperature, exhibiting single-mode emission in the 12 µm range, with linewidths in the order of 10 mhz and output power of several mw. a supplementary dfb (distributed-feedback) structure also ensures very high spectral purity and mode-hop-free frequency tuning. diode-laser spectrometers rely on resonant absorption of coherent radiation by a gas sample, contained in an optical cell, whose transmission can be monitored by a pre-amplified room-temperature photodiode. according to lambert-beer law, the transmitted power is related to the gas concentration n (in molecules/cm3) and to the cell length l (in cm), via the equation (2.1) where p0 is the incident power, s(t) the transition linestrength (in cm/molecule), ν the laser frequency, ν0 the line-centre frequency and g(ν−ν0) the normalized line shape function. in the last decade, several experimental schemes, mostly based on modulation of laser frequency, such as wavelength(wms) or frequency-modulation spectroscopy (fms), have been developed in an attempt to increase detection sensitivity. using these techniques, the retrieval of quantitative information from the absorption spectrum of a gas sample can be achieved by comparison with that of a reference cell, containing a known mixture. in this case, the chemical composition and thermodynamic conditions of the reference cell must be exactly the same as those of the sample under investigation. any such difference may induce a systematic error in the measurement process, thus affecting the achievable accuracy, in spite of the high sensitivity. this is of particular importance in the case of gases from volcanoes, where large temperature and pressure variations are possible. different approaches, based on a priori calibration of wms or fms signals (goldstain and adler-golden, 1993), exhibit limited precision levels and are difficult to implement, and are thus ill-suited to environmental applications. recently, we demonstrated that measurements of absolute gas concentrations with high precision can be efficiently performed using a very simple method, based on direct absorption, which does not require any frequency modulation technique or calibration cell (gagliardi et al., 2001). we employed an ingaasp/inp dfb diode ( ) ( ) ( )expp p s t g nl0 0= -o o o6 @ 769 novel laser-based techniques for monitoring of volcanoes laser, emitting at a wavelength of 2-µm and able to probe strong ro-vibrational lines of co2 and h2o. a herriott-type multiple-reflection cell, with a path-length of 50 m, was used as a gas cell. absorption spectra were recorded for both species, and their concentration values were retrieved, through eq. (2.1), by measuring the integrated absorbances of the two absorption lines. this was possible after the product between the absorption pathlength and the linestrength had been measured using a certified gas mixture. laboratory tests showed that a short-term reproducibility of 2‰ and an overall accuracy below 1% can be achieved in measurements in ambient co2, within a wide range of concentration values (gagliardi et al., 2002a). simultaneous measurements of co2 and h2o concentrations at similar precision levels were also demonstrated, a line pair of the two species being observed within one laser frequency scan. one of the main advantages of such a nearir laser spectrometer is its compatibility with telecom optical fibers, suitable for remote and in situ gas detection. also, the principle of operation is completely independent of chemical composition and pressure of the gas sample. a portable version of the 2-µm spectrometer was subsequently developed and tested to monitor gaseous emissions in volcanic areas, during a field campaign at vulcano island and solfatara volcano, in italy, during july and november, 2002 (rocco et al., 2004). the experimental setup is schematically represented in fig. 1. the spectrometer is split into two parts, mounted on different breadboards, and connected by a telecom, single-mode, 30-m-long optical fiber. the laser source and collimating optics are mounted on one breadboard while the gas detection system on a another one, with a large hole in the centre which allows the gas to flow through the cell. in this way, remote measurements are possible at different sites across a volcanic area. in addition, a custom-built, open-path multiple-reflection cell, with a 20-m pathlength, was used for direct analysis of fumarolic effluxes without requiring any sampling procedure. figure 2 shows an example of absorption spectrum. it corresponds to the r(34) rotational line of the ν1+2ν2+ν3 co2 combination band, and to the 150.15 →14 0.14 line of the ν2+ν3 h2o band, around 5000 cm−1. the absorption signal was recorded at the solfatara crater, directly on the flux of a small fumarole, scanning the laser frequency around a co2-h2o line pair, while the fig. 1. sketch of the experimental arrangement of the portable 2-µm spectrometer. oi – optical isolator; smf – single mode fiber; fp – fiber port; mrc – multiple-reflection cell; pd – photodiode; bb1 – breadboard 1 (60 × 60 cm2); bb2 – breadboard 2 (70 × 40 cm2). 770 paolo de natale, giuseppe de natale, gianluca gagliardi, livio gianfrani and alessandra s.d. rocco experimental spectra were continuously transferred to a laptop computer and analysed by a labview program. furthermore, co2 and h2o concentrations could be monitored in ambient air for several minutes, with a typical uncertainty of a few %, thus demonstrating real-time analytical capabilities of our spectrometer in hostile environmental conditions (rocco et al., 2004). thanks to its low power consumption and ruggedness, the apparatus was transported almost everywhere in the selected volcanic sites. indeed, substitution of optical or mechanical components was never necessary during operation in the whole campaign. the main drawback of our diode-laser based spectrometer is the influence of temperature fluctuations on measured concentration values. due to the molecular linestrength dependence on boltzmann distribution of population among energy levels, variations in the order of 3%/°c for water vapor, and less than 1‰/°c for carbon dioxide are expected. nevertheless, the gas cell temperature was continuously monitored and acquired and, when necessary, its values were used to correct the corresponding small changes in the integrated absorbances. 3. mid-ir spectroscopy with quantumcascade lasers and difference-frequency based spectrometers the advantages of direct absorption detection can be better exploited when one moves towards longer wavelengths. indeed, most atmospheric molecules present stronger absorption bands, due to their fundamental vibrations, above 3 µm. that allows for sensitive detection of low abundance gases, like ch4, so2 and n2o, at their ambient concentration levels. for this reason, much effort has been directed towards the development of new coherent radiation sources emitting in the mid-ir spectral region. among them are quantum-cascade lasers (qcl), covering the interval from 3.5 to 24 µm, rare-earth-doped dfb fiber lasers, in the 1 µm to 1.5 µm range, and optical parametric oscillators (opo) or difference-frequency generation (dfg) in periodically-poled crystals, between 2 and 5 µm. quantum cascade lasers were invented in 1994, at lucent technologies (murray hill, nj) and are essentially based on a multiple miniband structure resulting from deposition of ultrathin alternating layers of semiconductor materifig. 2. direct absorption measurement of co2 and h2o performed for a small fumarole at the solfatara volcano (italy). the continuous line represents the lorentian fit curve. 3 4 771 novel laser-based techniques for monitoring of volcanoes als. electron transitions occur between different quantum well levels, in each mini-band, with many-photon emission (faist et al., 1994). the major limitation of such lasers is the need for cryogenic cooling in order to have continuouswave (cw) operation. alternatively, they can be used at room-temperature in pulsed mode (faist et al., 1998). nevertheless, qcls usually exhibit output power of 50-100 mw and intrinsic emission linewidth well below 1 mhz, with the possibility of a dfb design for high spectral purity and continuous wavelength tuning. furthermore, cw operation of a quantum-cascade device, up to 246 k on a peltier cooler, has recently been achieved (hofstetter et al., 2001). after the advent of quantum cascade lasers, several qcl-based spectrometers were successfully used, either in pulsed or continuous mode, for absorption spectroscopy as well as isotopic detection of species whose transitions are not accessible to conventional room-temperature diode lasers (webster et al., 2001; kosterev and tittel, 2002). our group recently developed a novel spectrometer relying on a cw dfb qcl at a wavelength of 8.06 µm, manufactured at lucent technologies labs. in the experimental set-up, described in gagliardi et al. (2002b), the laser was placed in a liquid-n2 cryostat, and stabilised by an active temperature controller. the laser beam was coupled to a 17-cm long absorption cell, filled with the gas sample, whose transmission was monitored by a liquid-n2 cooled hgcdte detector. the qcl spectrometer enabled us to detect the presence of very low ch4 concentration. indeed, several methane absorption spectra were recorded at sub-torr pressures, in correspondence of the strong p(10) a2(1)a1(1) line, belonging to the ν4 fundamental band. some of these spectra, with increasing absorbance, are shown in fig. 3, for different gas pressures in the cell. a wavelength-modulation spectroscopic technique, with phase-sensitive 1st-harmonic demodulation, was also implemented in order to detect ch4 isotopomers. an example of simultaneous recording of 13ch4 and 12ch4, for natural isotopic abundance methane, is given in fig. 4. the spectra were observed within a single qcl frequency scan, amounting to 1 ghz. furthermore, simultaneous detection of nitrous oxide and its isotopomers 14n15n16o, 15n14n16o, 14n14n18o and 14n14n17o was possible, in the same spectral region, with similar performances (gagliardi et al., 2002b). an example is reported in fig. 5. in these operational conditions, a sensitivity level corresponding to a minimum detectable pressure of few mtorr was estimated, for pure methane and nitrous oxide, in a 1-m absorption pathlength. in addition, a signal reprofig. 3. direct absorption spectrum of pure methane, around 1241 cm−1. the gas pressure in the cell was varied from 100 to 400 mtorr. fig. 4. simultaneous detection of 13ch4 and 12ch4, in natural isotopic abundance (1.1% and 98.9%, respectively), in the presence of 150 mtorr of pure methane. 772 paolo de natale, giuseppe de natale, gianluca gagliardi, livio gianfrani and alessandra s.d. rocco ducibility better than 1 % was demonstrated in detection of all species. this feature is relevant to a possible use of the 8-µm spectrometer for measurements of the 13c/12c and 15n14n, 18o/16o, 17o/16o isotope ratios in ch4 and n2o, respectively. although the results obtained in our laboratory tests are encouraging, some drawbacks come from the use of a quantum cascade laser. for instance, the cryogenic system used for laser operation is bulky and expensive, while the highcurrent supply, usually needed for laser driving, makes its use somewhat difficult. as a consequence, considerable effort is still required in order to make this kind of spectrometer compact and well suited for field measurements. a novel apparatus, based on difference-frequency generation in a periodically-poled lithium niobate crystal, is currently being developed in our lab. mixing an er-fiber-amplified diode laser, emitting at 1.55 µm, and an extended-cavity 1.05-µm diode laser, externally amplified by an yb-fiber, we expect to produce coherent radiation around a wavelength of 3 µm, with powers up to 10 mw. such a dfg spectrometer would satisfy some crucial requirements for spectroscopy, such as high spectral resolution, high power and wide tuneability. indeed, its spectral coverage ranges from 2.9 to 3.5 µm. this is of great relevance if we consider the current lack of laser sources in the region between 2.2 and 3.5 µm, where strong absorption bands of n2o and ch4 as well as of h2o, nh3 and no2 take place. it is worth noting that neither qcls can presently cover this spectral window, due to manufacturing difficulties. 4. conclusions and future perspectives several spectroscopic set-ups, based on different laser sources, have been reviewed. some experimental results were presented on monitoring of co2 and h2o emitted by volcanic fumaroles, using a compact and portable spectrometer operating at 2 µm. using such a spectrometer, continuous, fast and in situ detection of these species was possible, measuring their concentrations with precision levels of a few %. for future developments, novel near-ir laser sources, such as tm-doped fiber-based lasers, are also being considered as an alternative to diode lasers, especially in view of their possible integration in an optical fiber network for multiple-point, simultaneous monitoring of gas concentrations. using the same fiber, the absorption spectrometer can also be combined with fiber bragg-grating sensors for local measurements of strain and temperature (rao, 1997). such a network would be particularly relevant to the implementation of reliable methods for volcanic surveillance. alternative approaches for concentration measurements were also mentioned, such as quantum-cascade laser (qcl) and difference-frequency-generation (dfg) based spectrometers in the mid-ir. they exhibit significant advantages for analysis of gases whose natural abundance is extremely low. in particular, using a qcl at 8 µm, we demonstrated the potentiality of our spectrometer for detection of ch4, n2o and their isotopomers, with high sensitivity and resolution. acknowledgements this work was supported by the «gruppo nazionale per la vulcanologia» (gnv), through a coordinated project in the framework program 2000-2002. fig. 5. first-harmonic absorption spectrum of 14n14n17o and 14n14n16o (0.37‰ and 99%, respectively). the n2o pressure was equal to 80 mtorr. 773 novel laser-based techniques for monitoring of volcanoes references baubron, j.c., p. allard, j.c. sabroux, d. tedesco and j.p. toutain (1991): soil gas emanations as precursory indicators of volcanic eruptions, j. geol. soc. london, 148, 571-576. chiodini, g., m. todesco, s. caliro, c. del gaudio, g. macedonio and m. russo (2003): magma degassing as a trigger of bradyseismic events: the case of phlegraean fields (italy), geophys. res. lett., 30 (8), 1434, doi: 10.1029/2002gl016790. faist, j., f. capasso, d.l. sivco, c.sirtori, a.l. hutchinson and a.y. cho (1994): quantum cascade laser, science, 264, 553-555. faist, j., f. capasso, d.l. sivco, a.l. hutchinson, s.g. chu and a.y. cho (1998): short 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(1998): a review of recent advances in semiconductor laser based gas monitors, spect. acta a, 54 ,197236. layout 6 analisi dei parametri ipocentrali del terremoto del friuli del 6 maggio 1976 rodolfo console e calvino gasparini annali di geofisica, vol. 29, n. 3, 1976. annals of geophysics, vol. 53, n. 1, february 2010 abstract the hypocentral parameters to t0, φ0, λ0, h0, for the main shock occurred in friuli on may 6th 1976 and itsfore shock were determined by the first arrival method. the computation was tried employing travel-time curves for different crustal models. the results show a very low influence of the travel-time curves on epicentral coordinates, whose error is essentially dependent on the precision of station data. the focal depth, on the contrary, is a parameter directly connected to the model used in the computation, so, if the model is not accurately known, the indetermination can be of the same order of magnitude as the crust thickness, even using very precise recorded data. 1. premessa molto si è discusso sulla determinazione ipocentrale del terremoto del friuli del 6 maggio 1976. limitandoci alle soluzioni ricavabili con il metodo degli inizi, in questo lavoro abbiamo illustrato i risultati che si ottengono utilizzando dromocrone di diverso modello. 2. il metodo il metodo adoperato è quello degli inizi (onde pg, pn, p) per il quale abbiamo escluso le stazioni con distanza superiore a 1600 km, ritenendo che i modelli regionali non sarebbero stati approssimati in modo utile oltre tale limite. ii metodo degli inizi si basa concettualmente sul principio della migliore ottimizzazione della relazione ts = t0 + t (φ0, λ0, h0,φs, λs) in cui ts è il tempo di arrivo delle prime onde, t0 è il tempo origine del terremoto, t è il tempo di tragitto dell'onda considerata, dipendente funzionalmente dalle coordinate φ0, λ0, h0 dell'ipocentro e da quelle φs, λs della stazione. partendo da una scelta provvisoria di t0, φ0, λ0, h0 e ottimizzando gli scarti col metodo dei minimi quadrati, si ottiene la soluzione quando per successive iterazioni non si hanno più variazioni significative dei parametri. 3. i modelli abbiamo utilizzato sia modelli sperimentali che sintetici. i modelli sperimentali utilizzati (figura 1) portano i nomi di «di filippo-marcelli» la cui dromocrona venne ricavata dai dati provenienti dal terremoto del gran sasso del 1950 [di filippo e marcelli 1952], «caloi», ottenuto dal terremoto del cansiglio [caloi 1939], ed «hypo 71» utilizzato ordinariamente dal centre séismologique europeo-méditerranéen di strasburgo. i modelli sintetici (figura 2) portano convenzionalmente il nome di «alpi» e «appennini». le relative dromocrone sono state ottenute ipotizzando teoricamente un modello medio crostale, aiutati anche dalle prospezioni sismiche attive, e dai dati sismici delle stazioni europee. 4. analisi dei risultati sono stati calcolati gli epicentri per le scosse del 6 maggio 1976 delle ore 19.59, ml = 4.5 rmp (premonitoria) e delle ore 20.00, ml = 6.2 rmp (principale). per la scossa premonitoria abbiamo utilizzato i dati di 59 stazioni che avevano chiaramente registrato il primo arrivo, scartando quelle per cui ciò era discutibile. per la scossa principale sono state utilizzate solo 48 stazioni. può sembrare strano che la scossa piu forte abbia dato luogo a un numero minore di registrazioni, ma non è così. l'intervallo di 1m 07s tra le due scosse ha fatto sì che l'inizio della scossa principale si trovasse per molte stazioni nel corpo della registrazione della scossa premonitoria (figura 3) rendendone difficoltosa e imprecisa la lettura, specia1mente nel caso di deboli pn. nelle tabelle 1 e 2 sono riportati i dati del calcolo ipocentrale per i vari modelli. il significato dei simboli usati in successione di scrittura è il seguente: sigla e nome delle stazioni; f, l, coordinate delle stazioni in gradi; t, tempo di registrazione in secondi contati dalle 19h 59m 00s per la scossa premonitoria e dalle 20h 00m 00s per la scossa principale; v, scarto sul tempo; d, distanza epicentrale in km. i risultati dei calcoli sono riportati per righe in fondo alle tabelle 1 e 2. il significato dei simboli usati è il seguente: to, tempo origine in secondi da sommare alle 19h 59m 00s per la scossa premonitoria e 20h 00m 00s per la scossa principale e relativo errore standard; fo, lo latitudine nord e longitudine est, dell'epicentro in gradi e relativo errore standard; ho, profondità ipocentrale in km e relativo errore standard. 63 dai risultati ottenuti si possono trarre diverse conclusioni significative: a) il tempo origine viene influenzato dal modello in ragione maggiore dell'errore standard calcolato dal residuo dei tempi per le stazioni. tuttavia la variazione è limitata a circa 1.5 secondi tra i casi estremi, b) le coordinate epicentrali variano di poco rispetto al modello e il loro scarto massimo è contenuto nell'errore standard che è dell'ordine di 2/100 di grado, c) la profondità ipocentrale è il parametro che varia più sensibilmente in relazione al modello dando valori estremamente diversi che scartano anche di decine di km mentre la relativa deviazione standard è contenuta entro qualche km. 5. conclusione dalle prove eseguite si può affermare che i dati ipocentrali calcolati sono affetti da errori provenienti da cause differenti. le coordinate epicentrali, con un modello più o meno realistico, conservano la loro posizione con uno scarto limitato. quindi il loro errore dipende essenzialmente dall'accuratezza con cui i tempi vengono forniti dalle stazioni oppure da anomalie locali caratteristiche della stazione. con i metodi considerati, a meno di non conoscere a priori il modello da utilizzare, nulla si può dire sulla profondità ipocentrale tranne l'ordine di grandezza (crostale). infatti le informazioni che derivano dagli scarti non permettono di individuare il modello più adatto al caso. per i due terremoti considerati è apparso chiaramente che la scossa premonitoria è avvenuta a profondità maggiore della principale, come risulta sistematicamente dal confronto dei risultati relativi allo stesso modello. disponendo di dati oggettivi ricavati indipendentemente per i parametri ipocentrali relativi a qualche terremoto, sarebbe possibile effettuare l'inversione dei dati stessi e quindi individuare il modello adatto per la zona considerata. tale ricerca sarà effettuata in un'ulteriore analisi riguardante i terremoti del friuli con l'ausilio delle stazioni locali. bibliografia caloi, p. (1939). tempi di tragitto per terremoti ad origine vicina, la ricerca scientifica, x, 5. di filippo, d. e l. marcelli (1952). dromocrone per terremoti vicini e velocità delle onde nell'italia centrale, annali di geofisica, 5, n. 2. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. analisi dei parametri ipocentrali del terremoto del friuli 64 figura 1. modelli crostali delle dromocrone sperimentali. figura 2. modelli crostali delle dromocrone sintetiche. 65 analisi dei parametri ipocentrali del terremoto del friuli f ig u ra 3 .s is m og ra m m a de lle s co ss e pr em on it or ia e p ri nc ip al e de l 6 m ag g io 1 97 6 (f ri u li) r eg is tr at e a r om a m on te p or zi o. analisi dei parametri ipocentrali del terremoto del friuli 66 1 ambiesta 2 la maina 3 tri trieste 4 mis 5 lju ljunljana 6 pad padova 7 bolzano 8 zag zagabria 9 sal salò 10 bol bologna 11 fur fürstenfeldbruck 12 crn corinaldo 13 pcn piacenza 14 khc kasperske hory 15 prt prato 16 rav ravensburg 17 prg perugia 18 mss messtetten 19 zur zurigo 20 gen genova 21 stu stuttgard 22 pru pruhonice 23 fel feldberg 24 tuscania 25 buh buhlerhohe 26 pnl pinerolo 27 rob roburent 28 hei heidelberg 29 rmp romamonteporzio 30 str strasbourg 31 sar sarajevo 32 mox moxa 33 stv s. anna di val 34 wls welschbruch 35 bsf ballon servance 36 cdf champ du feu 37 ech echery 38 hau haudompre 39 cll collmberg 40 beo beograd 41 spf st. paul en f. 42 lmr la mourre 43 lrg lorgues 44 ttg titograd 45 stb steinbach 46 bns bensberg 47 jue julich 48 tir tirana 49 lbf les buteaux 50 ssf saint saulge 51 sko skopje 52 ohr ohrid 53 tcf toulx ste croix 54 vay valandovo 55 lpo le pouchou 56 lff la frestale 57 mff st. martin d.f. 58 fln foliniere 46.3797 46.4483 45.7089 46.1594 46.0433 45.4086 46.5049 45.8167 45.6075 44.4867 48.1655 43.6308 45.0500 49.1306 43.8800 47.7833 43.1014 48.1792 47.3686 44.4181 48.7708 49.9883 47.8700 42.4186 48.6756 44.9192 44.2947 49.3986 41.8111 48.5847 43.8733 50.6461 44.2442 48.4128 47.8333 48.3941 48.2158 48.0054 51.3090 44.8214 43.5639 43.3333 43.4542 42.4297 50.5953 50.9639 50.9139 41.3467 46.9869 47.0614 41.9721 41.1114 46.2881 41.3211 44.6833 44.9375 46.6014 48.7625 12.9819 12.7331 13.7642 12.0811 14.5333 11.8861 11.3468 15.9833 10.5262 11.3290 11.2763 12.9978 9.6667 13.5792 11.0942 9.6139 12.3969 8.9661 8.5806 8.9301 9.1933 14.5417 8.0167 11.8744 8.2283 7.3139 7.8706 8.7264 12.7023 7.7658 18.4283 11.6461 7.3244 7.3536 6.7939 7.2707 7.1583 6.3500 13.0044 20.4553 6.6961 6.5092 6.3606 19.2608 6.8400 7.1756 6.4098 19.8667 3.9772 3.5068 21.4396 20.7989 2.2139 22.5701 1.1872 360.7364 – 360.1433 – 360.4819 10.5 15.6 17.9 21.0 22.3 32.0 32.0 38.8 41.3 45.0 45.8 51.0 52.5 52.7 53.2 54.6 59.0 61.9 62.5 63.5 64.8 65.1 67.6 68.0 70.8 72.0 72.5 73.4 74.0 74.3 74.5 75.0 75.2 75.8 76.0 76.7 77.5 82.5 82.6 86.0 86.8 89.9 90.6 92.5 96.8 98.2 104.6 106.8 101.3 105.8 113.5 115.3 117.5 127.5 131.9 135.6 139.6 145.1 2.155 –.714 .566 .483 –.022 – 2.629 – 1.751 1.155 .476 –.178 –.077 –.836 –.093 .205 –.254 –.721 –.935 –.243 – 2.091 –.654 –.052 .633 –.066 .263 .129 1.923 .083 .103 .979 –.502 – 1.202 .176 2.398 .250 1.938 –.064 –.911 .019 –.137 –.813 .795 .894 .529 –.318 –.130 –.076 – 2.268 .265 .355 .292 –.693 .583 .760 –.253 –.385 –.658 .511 –.167 26.170 46.744 73.417 94.261 99.564 145.918 151.519 214.683 226.983 251.658 259.118 295.355 313.695 317.554 317.866 325.478 360.274 389.460 379.065 399.369 414.386 422.596 437.603 443.652 465.066 490.007 478.844 486.512 498.756 488.969 484.802. 500.362 519.806 507.127 522.634 511.784 511.397 559.979 559.544 582.210 601.448 628.102 630.888 639.610 676.235 688.300 722.591 761.198 716.948 753.086 808.210 833.508 852.821 926.837 961.615 989.383 1031.578 1071.578 .697 – 1.410 .428 .924 .652 – 1.522 –.615 .565 –.521 – 1.180 –.818 – 1.742 –.891 –.340 – 1.139 – 1.359 – 1.655 –.634 – 2.622 – 1.221 –.416 .410 –.381 –.229 –.098 1.679 –.275 .021 .666 –.600 – 1.433 1.92 2.144 .149 1.837 –-168 – 1.022 –.005 .002 –.711 .736 .999 .664 –.125 .317 .466 – 1.686 .761 .791 .829 .004 1.340 1.579 .827 .715 .531 1.838 1.378 26.365 46.892 72.473 93.621 99.670 144.665 151.311 214.834 226.112 250.290 259.975 293.947 312.704 318.959 316.465 325.821 358.833 389.855 379.108 398.285 414.995 424.040 437.783 442.203 465.518 489.186 477.832 498.204 497.342 489.344 484.478 501.593 518.829 507.413 522.710 512.058 511.611 560.078 560.902 582.340 600.396 627.030 629.842 639.086 676.936 689.085 723.302 760.585 716.722 752.872 807.843 832.962 852.455 926.476 961.003 988.820 1031.280 1071.610 .588 – 1.286 .596 1.022 .658 – 1.479 –.398 1.533 .495 –.271 .059 –.946 –.147 .306 –.389 –.685 – 1.014 –.180 – 2.060 –.720 –.050 .732 –.020 .150 .122 1.882 .015 .189 .862 –.441 – 1.237 .289 2.285 .231 1.936 –.101 –.954 –.041 –.099 –.854 .633 .774 .413 –.380 –.098 .005 – 2.235 .111 .299 .205 –.831 .452 .660 –.367 –.506 –.825 .362 –.247 26.468 47.008 72.635 93.965 99.473 145.134 151.537 214.623 226.510 250.767 259.842 294.340 313.127 318.568 316.939 325.873 359.257 389.890 379.257 398.727 414.955 423.613 437.888 442.640 465.533 489.573 478.271 487.134 497.740 489.386 484.418 501.300 519.250 507.485 522.850 512.133 511.707 560.210 560.542 582.137 600.832 627.469 630.278 639.095 676.863 688.980 723.225 760.626 716.953 753.099 807.798 832.979 852.725 926.429 961.340 989.143 1031.531 1071.763 .351 – 1.291 .859 1.477 .970 – 1.713 –.698 1.464 .463 –.251 .077 –.987 –.147 .304 –.414 –.696 – 1.054 –.177 – 2.091 –.686 –.007 .703 –.026 .106 .163 1.866 –.032 .180 .844 –.440 – 1.350 .342 2.288 .297 1.940 –.045 –.896 .024 –.056 –.935 .617 .771 .409 –.503 –.079 –.026 – 2.215 .082 .330 .225 –.875 .363 .675 –.423 –.505 –.813 .378 –.190 26.853 47.392 72.290 94.261 99.105 145.244 151.897 214.258 226.756 250.803 260.205 294.155 313.337 318.758 316.945 326.263 359.114 390.279 379.638 398.907 415.338 423.752 438.275 442.520 465.923 489.830 478.465 487.513 497.560 489.777 484.029 501.583 519.465 507.874 523.232 512.523 512.094 560.593 560.766 581.770 601.023 627.653 630.471 638.708 677.241 689.351 723.602 760.243 717.310 753.458 807.408 832.529 853.067 926.039 961.642 989.454 1031.882 1072.143 1.115 – 1.082 .642 .875 .353 – 2.225 – 1.307 1.437 .516 –.159 0.54 –.885 –.086 .278 –.318 –.703 –.940 –.223 – 2.086 –.621 –.023 .682 –.055 .197 .152 1.882 0.30 .136 .957 –.482 – 1.266 .314 2.348 .293 1.944 –.049 –.898 .027 –.082 –.856 .740 .837 .473 –.398 –.097 –.046 – 2.234 .189 .347 .216 –.780 .469 .674 –.324 –.413 –.779 .399 –.185 26.445 47.014 72.876 94.200 99.388 145.531 151.652 214.526 226.811 251.190 259.632 294.744 313.460 318.166 317.368 325.823 359.676 389.825 379.298 399.083 414.823 423.192 437.888 443.065 465.450 489.860 478.598 486.976 498.146 489.327 484.462 500.965 519.579 507.452 522.880 512.105 511.696 560.235 560.159 582.045 601.181 627.823 630.624 639.200 676.702 688.793 723.061 760.758 717.072 753.215 807.855 833.091 852.886 926.483 961.569 989.359 1031.672 1071.807 dati delle stazioni mod. «di filippo-marcelli» mod. «caloi» mod. «hypo 71» mod. «appennini» mod. «alpi» f l t v d v d v d v d v d tabella 1. terremoto del friuli, 6 maggio 1976, ore 19.59 (premonitoria). valori modello «hypo 71» to = 6.103 +/– .285 fo = 46.272 +/– .015 lo = 13.288 +/– .019 ho = 12.248 +/– 2.504 valori modello «appennini» to = 4.708 +/– .359 fo = 46.270 +/– .015 lo = 13.292 +/– .020 ho = 18.256 +/– 2.466 valori modello «alpi» to = 5.881 +/– .312 fo = 46.275 +/– .015 lo = 13.291 +/– .020 ho = 21.829 +/– 3.120 valori modello «di filippo-marcelli» to = 4.498 +/– .235 fo = 46.281 +/– .016 lo = 13.290 +/– .021 ho = 40.763 +/– 2.301 valori modello «caloi» to = 4.952 +/– .317 fo = 46.269 +/– .017 lo = 13.284 +/– .0222 ho = 23.894 +/– 2.629 67 analisi dei parametri ipocentrali del terremoto del friuli 1 la maina 2 vaj vajont 3 tri trieste 4 pieve di cadore 5 mis 6 lju ljunljana 7 zag zagabria 8 crn corinaldo 9 prt prato 10 fir firenze xim 11 vie vienna 12 pav pavia 13 tno torino 14 tuscania 15 buh buhlerhohe 16 rmp romamonteporzio 17 hei heidelberg 18 sar sarajevo 19 beo beograd 20 ttg titograd 21 stb steinbach 22 bns bensberg 23 jue julich 24 lor lormes 25 beg berghein 26 ssf saint saulge 27 lbf les buteaux 28 tir tirana 29 dev deva 30 pavf 31 sko skopje 32 ohr ohrid 33 tcf toulx ste croix 34 mes messina uni 35 rci reggio calabria 36 vay valandovo 37 gib gibilmanna 38 lpo le pouchou 39 lff la frestale 40 mff st. martin d.f. 41 buc bucharest 42 fln foliniere 43 lpf le petre 44 mbz menzel b. zelfa 45 zgn zaghouan 46 ist istanbul 47 upp uppsala 48 cin cine 46.4483 46.2661 45.7089 46.4208 46.1594 46.0433 45.8167 43.6308 43.8800 43.7739 48.2483 45.1833 45.0587 42.4186 48.6755 41.8111 49.3986 43.8733 44.8214 42.4297 50.5953 50.9639 50.9139 47.2683 50.9613 47.0614 46.9869 41.3467 45.8833 46.7919 41.9721 41.1114 46.2881 38.1989 38.1056 41.3211 37.9833 44.6833 44.9375 46.6014 44.4136 48.7625 48.0317 36.6739 36.3750 41.0456 59.8583 37.6000 12.7339 12.3302 13.7642 12.3869 12.0819 14.5333 15.9833 12.9978 11.0942 11.2550 16.3617 9.1736 7.6969 11.8744 8.2284 12.7023 8.7264 18.4283 20.4553 19.2608 6.8400 7.1756 6.4098 3.8589 6.6557 3.5068 3.9772 19.8667 22.9028 3.3569 21.4396 20.7989 2.2100 15.5550 15.6433 22.5701 14.0167 1.1872 360.7364 – 360.1433 26.0967 – 360.4819 – 1.0408 10.6722 10.1117 28.9958 17.6267 28.0867 19.5 22.7 24.8 25.6 30.1 30.2 48.4 57.0 60.7 62.0 62.0 63.7 75.0 75.5 78.5 81.0 81.5 82.1 94.5 98.5 104.2 105.8 110.2 110.2 110.5 113.0 108.6 114.5 115.4 115.4 120.5 121.9 126.2 132.5 134.4 134.6 135.7 140.3 143.1 146.8 150.0 152.6 155.2 156.3 160.4 191.2 208.5 214.0 1.565 1.977 .537 – 1.196 – 2.481 –.316 –.611 .735 –.479 – 1.326 –.277 –.451 2.093 .232 .098 1.531 –.161 –.514 –.851 1.877 .310 .203 –.022 .266 – 1.375 .477 .506 .717 – 1.871 –.624 .575 2.145 –.506 1.258 .760 .913 – 1.231 – 1.439 –.853 .664 – 2.421 –.150 .999 – 1.794 –.742 1.077 1.833 – 1.653 43.644 69.112 73.532 67.243 88.950 103.627 218.844 292.018 312.658 316.332 324.710 337.136 450.815 439.492 462.936 495.318 485.387 487.446 586.301 641.492 675.144 687.591 721.542 723.944 712.770 748.782 712.611 762.707 749.203 759.076 810.686 835.289 848.432 914.710 926.399 929.333 920.635 956.403 984.264 1027.057 1027.992 1067.981 1098.386 1084.574 1127.015 1394.040 1542.001 1558.108 .808 2.018 .366 – 1.266 – 1.982 –.179 – 1.477 –.544 – 1.585 – 2.450 – 1.930 – 1.539 1.397 –.598 –.783 .857 – 1.015 – 1.505 1.630 1.360 –.003 –.050 –.219 .257 – 1.596 .589 .451 .538 – 2.198 –.561 .525 2.212 –.133 1.712 1.285 1.248 –.724 –.750 –.049 1.581 – 1.933 .851 2.024 –.746 .479 2.671 3.803 .416 44.649 70.932 73.565 68.678 90.931 102.121 217.309 293.419 314.741 318.367 322.495 339.299 452.929 441.200 463.627 496.760 485.684 486.531 584.800 640.882 675.429 687.726 721.811 725.472 712.990 750.365 714.208 762.246 747.354 760.721 809.841 834.721 850.176 915.550 927.226 928.481 921.775 958.373 986.193 1028.731 1026.317 1069.273 1099.819 1086.217 1128.716 1392.712 1540.439 1557.255 .929 1.990 .453 – 1.220 – 2.117 .047 .344 .779 –.410 – 1.273 –.286 –.363 2.205 .260 .164 1.565 –.103 –.569 –.918 1.846 .323 .275 –.009 .290 – 1.363 .546 .530 .616 – 1.935 –.596 .468 2.024 –.505 1.184 .686 .806 – 1.296 – 1.427 –.875 .663 – 2.557 –.126 .990 – 1.904 –.809 .868 1.696 – 1.884 44.180 69.652 73.104 67.802 89.471 103.072 218.287 291.910 312.834 316.479 324.424 337.598 451.298 439.490 463.451 495.223 485.856 486.911 585.743 640.984 675.611 688.036 722.007 724.501 713.228 749.338 713.167 762.216 748.665 759.630 810.156 834.786 848.980 914.450 926.136 928.803 920.451 956.923 984.792 1027.609 1027.440 1068.537 1098.943 1084.548 1127.011 1393.484. 1542.055 1557.577 .348 1.867 .284 – 1.373 – 1.768 .520 .297 .766 –.384 – 1.236 –.320 –.302 2.242 .287 .202 1.565 –.031 –.588 –.966 1.806 .361 .317 .016 .344 – 1.347 .607 .567 .556 – 1.988 –.558 .403 2.027 –.435 1.154 .656 .740 – 1.317 – 1.381 –.818 .700 – 2.634 –.092 1.054 – 1.892 –.811 .806 1.672 – 1.925 44.582 70.131 72.855 68.255 89.949 102.604 217.823 291.967 313.119 316.741 324.062 338.060 451.767 439.636 463.816 495.291 486.160 486.512 585.282 640.629 675.914 688.313 722.306 724.964 713.520 749.805 713.635 761.884 748.184 760.101 809.768 834.437 849.456 914.368 926.050 928.413 920.440 957.403 985.272 1028.082 1026.967 1068.979 1099.398 1084.675 1127.155 1393.038 1541.953 1557.188 .928 1.970 .474 – 1.231 – 2.161 .079 –.089 .756 –.439 – 1.302 –.262 –.287 2.188 .257 .181 1.525 –.104 –.580 –.907 1.812 .367 .261 .035 .308 – 1.318 .578 .546 .648 – 1.859 –.580 .508 2.058 –.473 1.192 .695 .846 –.1292 – 1.447 –.868 .677 – 2.499 –.103 .943 – 1.908 –.813 .918 1.771 – 1.901 44.132 69.439 72.996 67.673 89.218 103.227 218.443 291.610 212.473 316.120 324.781 337.297 451.010 439.157 463.449 494.919 485.918 486.952 585.893 640.974 675.676 688.125 722.074 724.347 713.303 749.173 712.999 762.180 748.887 759.451 810.185 834.768 848.781 914.220 925.907 928.833 920.182 956.673 984.552 1027.425 1027.625 1068.428 1098.807 1084.221 1126.678 1393.600 1542.373 1557.607 dati delle stazioni mod. «di filippo-marcelli» mod. «caloi» mod. «hypo 71» mod. «appennini» mod. «alpi» f l t v d v d v d v d v d tabella 2. terremoto del friuli, 6 maggio 1976, ore 20.00 (principale). valori modello «hypo 71» to = 12.797 +/– .670 fo = 46.253 +/– .023 lo = 13.234 +/– .025 ho = 1.231 +/– 5.570 valori modello «appennini» to = 11.278 +/– .446 fo = 46.253 +/– .023 lo = 13.240 +/– .024 ho = 8.194 +/– 3.205 valori modello «alpi» to = 12.585 +/– .198 fo = 46.250 +/– .023 lo = 13.231 +/– .025 ho = 9.995 +/– .014 valori modello «di filippo-marcelli» to = 11.736 +/– .327 fo = 46.254 +/– .024 lo = 13.227 +/– .025 ho = 33.435 +/– 3.425 valori modello «caloi» to = 11.756 +/– .449 fo = 46.266 +/– .027 lo = 13.250 +/– .029 ho = 20.088 +/– 3.797 vol48/04/2005def 699 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words diffusion – silicate melts – volatiles – water – carbon dioxide – sulfur – fluorine – chlorine – melt inclusion – igneous processes 1. introduction volatiles play an enormously important role in igneous processes. they are not only responsible for dramatic volcanic eruptions, but also affect the transport (e.g., diffusion and viscosity) and equilibrium (e.g., phase equilibria) properties of magmas. understanding the diffusion of volatiles in silicate magmas provides the framework necessary for the understanding of processes such as bubble formation and growth that can ultimately lead to violent volcanic eruptions. the volatiles commonly found at the highest concentrations in magmatic systems are h2o, co2, s-species, f, and cl. the dominant volatile in magmas is h2o; evidence of its abundance is manifested in measured volcanic emissions (symonds et al., 1994) and the presence of hydrous minerals (e.g., amphibole, biotite, musvolatile diffusion in silicate melts and its effects on melt inclusions don r. baker (1) (2), carmela freda (2), richard a. brooker (3) and piergiorgio scarlato (2) (1) earth and planetary sciences, mcgill university, montreal, quebec, canada (2) istituto nazionale di geofisica e vulcanologia, roma, italy (3) department of earth sciences, university of bristol, u.k. abstract a compendium of diffusion measurements and their arrhenius equations for water, carbon dioxide, sulfur, fluorine, and chlorine in silicate melts similar in composition to natural igneous rocks is presented. water diffusion in silicic melts is well studied and understood, however little data exists for melts of intermediate to basic compositions. the data demonstrate that both the water concentration and the anhydrous melt composition affect the diffusion coefficient of water. carbon dioxide diffusion appears only weakly dependent, at most, on the volatilefree melt composition and no effect of carbon dioxide concentration has been observed, although few experiments have been performed. based upon one study, the addition of water to rhyolitic melts increases carbon dioxide diffusion by orders of magnitude to values similar to that of 6 wt% water. sulfur diffusion in intermediate to silicic melts depends upon the anhydrous melt composition and the water concentration. in water-bearing silicic melts sulfur diffuses 2 to 3 orders of magnitude slower than water. chlorine diffusion is affected by both water concentration and anhydrous melt composition; its values are typically between those of water and sulfur. information on fluorine diffusion is rare, but the volatile-free melt composition exerts a strong control on its diffusion. at the present time the diffusion of water, carbon dioxide, sulfur and chlorine can be estimated in silicic melts at magmatic temperatures. the diffusion of water and carbon dioxide in basic to intermediate melts is only known at a limited set of temperatures and compositions. the diffusion data for rhyolitic melts at 800°c together with a standard model for the enrichment of incompatible elements in front of growing crystals demonstrate that rapid crystal growth, greater than 10−10 m s−1, can significantly increase the volatile concentrations at the crystal-melt interface and that any of that melt trapped by the formation of melt inclusions may not be representative of the bulk melt. however, basaltic melt inclusions trapped at 1300°c are more likely to contain bulk melt concentrations of water and carbon dioxide. mailing address: dr. don r. baker, earth and planetary sciences, mcgill university, 3450 rue university, montreal, qc h3a 2a7 quebec, canada; e-mail: donb@eps.mcgill.ca 700 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato covite, apatite) in many igneous rocks. the second most abundant volatile in magmas is co2, whose presence can be found in volcanic gases (symonds et al., 1994) and as fluid inclusions in minerals (roedder, 1965, 1979). the existence of rare carbonate magmas also testifies to the importance of carbon in igneous petrogenesis, but these unique magmas are not discussed further in this review. the presence of s in magmatic gases is often obvious; quantitative measurements demonstrate that volcanoes can vent thousands of tonnes of s per day (symonds et al., 1994; wallace, 2001). additional evidence of sulfur’s role in magmatic processes is the presence of common magmatic sulfides and rare magmatic sulfates (luhr et al., 1984). the halogens, f and cl, have been measured in volcanic gases and plumes (symonds et al., 1994), and they are important constituents of hydrous minerals and occasionally are found as halogen minerals in igneous rocks. we begin with a selected review of diffusion measurements of h2o, co2, s, f, and cl that we believe are most germane to the study of volatile transport in magmatic silicate melts. we then apply simple models to explore the effects of volatile diffusion on the compositions of melt inclusions trapped in minerals. 2. volatile diffusion measurements the first study of volatile diffusion in magmatic melts of which we are aware is shaw’s (1974) measurements of water diffusion in a rhyolitic melt. since that time more studies of water, carbon dioxide, sulfur and halogens have been completed; for an excellent summary of experimental techniques and results obtained before the mid-1990’s the interested reader is referred to the review by watson (1994). all studies to date demonstrate arrhenian behaviour of volatile diffusivity; thus the resulting diffusion coefficients can be succinctly expressed by equations of the general form where d is the diffusion coefficient, in m2 s−1, do is the pre-exponential factor, also in m2 s−1, expd d rt e o a= b l ea is the activation energy, in j mol−1, r is the gas constant, 8.3143 j k−1 mol−1, and t is the temperature, in k. broadly speaking, volatile diffusion is measured in two diffusive regimes, which are distinguished by the absence or presence of a chemical potential gradient. the first regime is that of tracer diffusion where the volatile of interest is only present in trace amounts and its movement through the melt can be modelled as a random walk in the absence of a chemical potential gradient. experiments that study tracer diffusion typically utilize radioactive isotopes of the element of interest applied as a thin layer to one end of the sample and then allowed to diffuse through a vanishingly small chemical potential gradient. the second regime is that of chemical diffusion, which occurs in the presence of a significant chemical potential gradient. one experiment of this type is the diffusion couple where a cylinder of glass with weight percent abundances of a selected element or elements is juxtaposed against a glass cylinder with low concentrations of the selected element or elements, and the elements diffuse from the high to the low concentration melts during the experiment. another experimental technique for chemical diffusion allows a mineral with the element of interest to partially dissolve into the melt releasing the element and creating a diffusion profile away from the mineral; however, this technique often results in multicomponent diffusion, making data analysis difficult. a third technique is to expose samples to a fluid phase with a low concentration of the volatile of interest resulting in devolatilization, or exposing the sample to a fluid phase with a high concentration of the volatile resulting in diffusion of the volatile into the sample. researchers agree that tracer diffusion is a highly improbably process in magmatic systems, whereas chemical diffusion occurs whenever two melts, or a melt and a crystal, with differing chemical potentials of a chemical component come into contact. 2.1. water diffusion water is present in silicate melts in both the molecular form and as hydroxyl (e.g., stolper, 701 volatile diffusion in silicate melts and its effects on melt inclusions 1982). understanding this speciation is important for thermodynamic modelling and for the microscopic modelling of diffusion in the melt. however, for the purposes of this work we are interested in only the bulk water diffusion coefficients and refer the interested reader to the review by mcmillan (1994) of water speciation in silicate melts. for all of the volatiles considered in this review the diffusion data is provided for the transport of the bulk volatile, not for a particular species. a comparison of the water diffusion measurements discussed below are summarized in fig. 1. shaw (1974) found that water diffusion in a rhyolitic melt is a function of water concentration at 750 to 850°c and 200 mpa. using a technique based upon weighing glass cylinders before and after hydration, and after subsequent dehydration, he determined the water diffusion coefficient. shaw found that the diffusion coefficient for water was a function of the water concentration and estimated the activation energy for water diffusion at weight percent concentrations to be approximately 67000 j mol−1. watson (1994) used shaw’s data to estimate an arrhenius equation water diffusion at concentrations near 6 wt% watson (1994) also extracted an arrhenius equation from shaw (1974) for water diffusion at low, nominally anhydrous, concentrations shaw’s (1974) diffusion measurements at 10 to 200 mpa, 850°c, demonstrated no measurable pressure effect on water diffusion. karsten et al. (1982) and lapham et al. (1984) applied ion microprobe techniques to the study of water diffusion in rhyolitic melts and found similar values and clearly demonstrated the dependence of water diffusion upon its concentration. karsten et al. (1982) measured water diffusion at 70 mpa and 650 to 950°c in rhyolitic melts. they found that the arrhenius equations describing water diffusion changed with water content; for rhyolitic melts with 1, 2, and 3 wt% water the activation energies were 82000, 80000, and 78000 j mol−1, re. expd rt 1 10 0 6 74 water 10#= -00b l 5 . expd rt 1 8 10 6 0 water 8#= -00b l fig. 1. diffusion of water in magmatic silicate melts. unless otherwise noted the measurements are made in melts of either rhyolitic or haplogranitic composition. where calculations are pressure dependent a pressure of 200 mpa has been used. 702 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato spectively, and the pre-exponential factors were 1.44×10−8, 2.65×10−8, and 4.78×10−8 m2 s−1, respectively. although these results are slightly different from those of shaw (1974) they are still consistent with his experimental measurements. lapham et al. (1984) extended the work of karsten et al. (1982) by investigating the effect of water concentration, from 0.42 to 3.48 wt%, and pressure, from 70 to 500 mpa, on the diffusion of water in rhyolite at 850°c. lapham et al. (1984) further documented the effects of water concentration on water diffusion in rhyolitic melts and demonstrated that increasing the pressure from 70 to 500 mpa had no measurable effect on water diffusion at 850°c. zhang et al. (1991) utilized infra-red spectroscopy to characterize the diffusion of oh− and h2o species in rhyolitic melts and glasses. these measurements demonstrate that water diffuses predominantly as molecular h2o and that the change from a rhyolitic glass to a melt had no effect on water diffusion. combining their results with the previous measurements discussed above, zhang et al. (1991) were able to construct a general arrhenius equation for the diffusion of molecular, not bulk, water in rhyolitic glasses and melts from approximately 400 to 950°c and water concentrations between 1 and 3 wt% nowak and behrens (1997) investigated water diffusion in a haplogranitic melt at temperatures between 800 and 1200°c, pressures of 50 to 500 mpa, and water concentrations between 0 and 9 wt%. they found that the activation energy for water diffusion drops from 64000 j mol−1 for 0.5 wt% water to 46000 j mol−1 for melts with 4 wt% water and remained at the latter value for high water concentrations. they observed a small effect of pressure on water diffusion and constructed a general equation for the diffusion of water, where h2o is in weight percent and p is the pressure in mpa +) ) log d t p o o) 3378 483 o 46.9 o) 0.475 water 2 2 2 2 2 2 = + ( ( . 8.81 0.045h 0.027 (h h (h + + . .expd rt 4 6 10 103 0 5000 .h o 3 3 18 7 2 # ! = + 00b l the most general expression for water diffusion in rhyolitic melts is that of zhang and behrens (2000) who present an equation for the diffusion of water at concentrations from 0 to 7.7 wt%, temperatures from 400 to 1200°c, and pressures from 0.1 to 810 mpa where x is the mole fraction of total water in the melt based upon a one-oxygen mole (stolper, 1982), and and for water contents between 0 and 2 wt% (inclusive) zhang and behrens (2000) provide a simpler equation where in this case water is measured in weight percent. although these expressions are only based upon the results of zhang et al. (1991) and zhang and behrens (2000), they accurately describe the results of karsten et al. (1982) and nowak and behrens (1997). only one study on the diffusion of water in basaltic melts has been published. zhang and stolper (1991) measured water diffusion in a basaltic melt at 1.0 gpa, 1300 to 1500°c, and found that for 0.2 wt% h2o freda et al. (2003) studied water diffusion in a . .exp3 8 10#= 26 2 d rt 1 0 3 0 .water 3 4 35 6 !+ 0 00 0b l 0 o( ) . . d t p t 10 h exp 10 49 1 1 1 772 water 12 2= + 66b l . . .b 0 091 4 77 106# = + t 2 . . a t t p 34 1 44 0 57 3 = + + 26 . . m t t p 20 79 5030 1 4 -= expd m xa x b x10 e 1 56water m12= + + + + ]^ h i 6 a 703 volatile diffusion in silicate melts and its effects on melt inclusions potassium-rich, trachytic melt at 1.0 gpa, 1100 to 1400°c. they measured water diffusion at concentrations between 0.25 and 2 wt% h2o; their results are described by during their study of water solubility and d/h fractionation between fluid and a basaltic andesite melt, pineau et al. (1998) incidentally found an apparent diffusion coefficient for a few weight percent water at 1250°c: 5.1×10−8 m2 s−1. this value is 2 orders of magnitude higher than all other measurements of water diffusion at similar temperatures and indicates the need for more experiments designed to accurately measure water diffusion in basic melts. the best constrained data for water diffusion in melts with 1 to 3 wt% h2o at temperatures between approximately 1000 and 1300°c spans only about 1/2 an order of magnitude (fig. 1). at higher water concentrations and at o o ( . . ( )) . . ( ) . exp ln exp exp ln d rt 11 924 1 003 11 836 0 139 water 2 2 $= h h^ c h m magmatic temperatures significantly below 1000°c important differences in water diffusion coefficients are observed. on the other hand, the effect of pressure on water diffusion is a second order effect and can often be neglected, but clearly needs more investigation. 2.2. carbon dioxide diffusion carbon is found as both carbonate groups and molecular co2 in silicate melts and glasses (see review by blank and brooker, 1994), although a variety of different carbonate types may exist (brooker et al., 2001). in the same way as we treated water above we will only consider the bulk diffusion of carbon dioxide, which is the most important measurement for our applications. figure 2 presents a summary of the diffusion results available for carbon dioxide. the first experiments on carbon dioxide diffusion were tracer diffusion studies by watson et al. (1982) on a sodium alumino-silicate melt with 60 wt% sio2 and a haplobasaltic melt with 53 wt% sio2. watson et al. (1982) found carfig. 2. diffusion of carbon dioxide in magmatic silicate melts. note that the variation of measurements in the work of sierralta et al. (2002) is due to the variety of compositions used in their experiments, see text for further discussion. 704 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato bon dioxide diffusion in both melts to be similar and defined the arrhenius relation for carbon dioxide diffusion in the sodium aluminosilicate melt between 800 and 1500°c at 500 mpa to be watson et al. (1982) demonstrated that at 1200°c the effect of increasing pressure by 2.5 gpa diminished the diffusion coefficient by approximately an order of magnitude. carbon dioxide diffusion in the haplobasaltic melt at 1.5 gpa produced an activation energy 249000 j mol−1 and a pre-exponential factor of 2.12×10−3 m2 s−1. fogel and rutherford’s (1990) study of carbon dioxide solubility in a rhyolitic melt produced chemical diffusion measurements at 1050°c and pressures between 100 and 295 mpa. the pressure effect on diffusion at these conditions was not measurable and at this temperature the diffusion coefficient of co2 in rhyolite was 2.4×10−12 m2 s−1, approximately a factor of two less than that found by watson et al. (1982) in compositionally different melts. watson (1991) investigated the effect of 8 wt% water on the tracer diffusion of carbon dioxide in rhyolitic melt at 1 gpa and found that carbon dioxide diffusion was significantly higher and the activation energy lower than in dry rhyolitic melts furthermore, watson (1991) measured similar diffusion coefficients for carbon dioxide in a dacitic melt and estimated that addition of 5 wt% water to a dry rhyolitic melt would result in a 10-fold increase in co2 diffusion. watson (1994) reports the results of co2 diffusion experiments in rhyolitic melts by blank et al. (1991) and blank (1993). these measurements were an extension of earlier infra-red investigations by e. stolper’s group at caltech, who demonstrated the presence of both molecular co2 and carbonate groups in silicate glasses. these chemical diffusion experiments were performed at pressures from 50 to 105 mpa and .exp10#.6 5= 5 d rt 7 0 21 08 carbon dioxide !-0 00 0b l 95 . .expd rt 3 5 10 1 04 carbon dioxide #= -00b l temperatures of 450 to 1050°c. fourier-transform infrared (ftir) measurements demonstrated that the only species present in the quenched glasses was co2 and that the measured diffusion coefficients can be described by sierralta et al. (2002) investigated carbon dioxide diffusion at 500 mpa and 1250°c in hydrous albite and albitic melts modified through variation of the na/al ratio. these authors documented the presence of both molecular co2 and carbonate in their glasses and determined that both diffused at similar rates. additionally, they determined that the effect of added alkalies and water on carbon dioxide diffusion were equivalent when expressed as the ratio of nonbridging oxygens to tetrahedral cations. these authors suggested that the small compositional effect on co2 diffusion was proportional to changes in the ratio of non-bridging oxygens to tetrahedral cations, nbo/t, as the alkalies were added to an albitic melt. however, new data by nowak et al. (2004) failed to identify any compositional effect for a wide range of nbo/t in analogues of natural melts. one other explanation for the effect observed by sierralta et al. (2002) is peralkalinity, as their diffusion coefficients also increase as a function of this parameter and extrapolate to agree with the highly peralkaline na-aluminosilicate data of watson et al. (1982). in contrast, the lack of an nbo/t effect observed by nowak et al. (2004) is for non-peralkaline compositions. brooker et al. (2001) have suggested that a very isolated carbonate group exists in peralkaline melts, and we suggest it is this complex that is diffusing at a high rate. such effects are only likely to be important in rare, highly peralkaline magmas. zhang and stolper’s (1991) study of water diffusion in basalt at 1.0 gpa also produced a single measurement of carbon dioxide chemical diffusion in dry basaltic melt at 1300°c of 1.3×10−11 m2 s−1. we performed reconnaissance experiments at 1.0 gpa in which calcite partially dissolved into = . exp d rt 6 2 10 0114 . . 2 6 4 3 7 carbon dioxide # $ $ + . 041060 ! b l 705 volatile diffusion in silicate melts and its effects on melt inclusions the same potassium-rich, trachyte melts we used in our study of water diffusion (freda et al., 2003). the melts were either anhydrous or contained 5 wt% h2o and experiments were performed at 1.0 gpa, 1200 and 1100°c. we determined carbon dioxide concentrations in the melt using the electron microprobe and the «difference from 100%» technique. because of the low quality of this analytical technique these measurements must be considered only as estimates of carbon dioxide diffusion. at 1200°c the diffusion of carbon dioxide in dry and hydrous melts appears similar within experimental uncertainty and equaled 2×10−12 m2 s−1; at 1100°c the diffusion profile in the anhydrous melt was too short to measure, but the hydrous melt produced a diffusion coefficient of 7×10−13 m2 s−1. interestingly, these experiments plot along an extension of an arrhenius relationship defined by combining the data of watson et al. (1982) and zhang and stolper (1991) as shown in fig. 2. as there is a large increase in the ca content of the melt (and therefore in nbo/t) near the interface of the dissolving calcite, but the co2 profile is consistent with a single diffusion value, our data appear to confirm the conclusions of nowak and screen (2003): there is little melt compositional effect on bulk co2 diffusion. initially surprised by these results we performed diffusion couple experiments for co2 at 1.2 gpa in the same trachytic melt using the diffusion couple technique and analyzing the quenched glass using ftir spectroscopy. carbon dioxide-bearing trachytic glass was prepared by creating a mixture of oxides and carbonates equivalent in composition to the trachyte. a portion of this mixture was devolatilized and melted at 1400ºc, 1 atm and then mixed with a small fraction of the oxide+ +carbonate mix; this mix was melted at 0.9 gpa, 1350 ºc for 2 h in a piston-cylinder apparatus to produce a carbon-bearing trachyitic glass for use in the diffusion couple experiments. during this synthesis a small amount of water, approximately 1 wt%, was absorbed by the melt. this carbon-bearing glass was powdered and juxtaposed against a glass cylinder of the same natural trachyte composition. we performed experiments at 1.2 gpa in a piston-cylinder for 720 s at 1200ºc and for 600 s at 1300ºc; after each experiment the diffusion couple was ground and polished to approximately 50 µm thickness for analysis by ftir spectroscopy. the extinction coefficients necessary to quantify the ftir spectra were determined using trachyte glasses synthesized at high pressure with differing amounts of dissolved carbon dioxide. these glasses were prepared by mixing different ratios of the trachytic oxide+ carbonate mix and the glass made from this mix followed by melting at 0.9 gpa, 1350ºc; the bulk co2 concentration in aliquots of these glasses was determined with a leco carbon analyzer (brooker et al., 1999), and the absorption spectra of polished samples was measured. based upon this calibration, the extinction coefficient for the molecular co2 band at 2348 cm−1 is 975 l mol−1cm−1, and for the carbonate bands at 1550 and 1412 cm−1 are 191±4 l mol−1cm−1 and 193±6 l mol−1cm−1, respectively. the diffusion of bulk carbon dioxide in trachytic melt at 1.2 gpa is 2.5×10−12 m2 s−1 at 1200ºc and 4.2 × 10−12 m2 s−1 at 1300ºc. over the range of concentrations along the diffusion profiles a small variation in dco2 was observed, but it is less than a factor of 2 and probably due to the small amounts of water present in the carbon-bearing trachytic glass that diffused together with the carbon into the volatile-free melt during the experiment (fig. 3). although the data are limited in temperature, they define an ea of 100000 j mol−1 and a do of 8.8 × 10−9 m2 s−1. remarkably, the co2 diffusion coefficients extracted from the calcite dissolution experiments are consistent with the diffusion couple results and support the former’s validity. with the exception of watson’s measurements of carbon dioxide diffusion in wet rhyolite, all co2 diffusion measurements in natural melt compositions are similar at magmatic temperatures. these results demonstrate the very small dependence of carbon dioxide diffusion upon melt composition. at magmatic temperatures the carbon dioxide diffusion measurements in all the dry melts can be combined to yield a single arrhenius relation. however, water has a significant effect on carbon dioxide diffusion in rhyolite melts. watson’s results indicate the ability of 8 wt% water addition to increase carbon dioxide diffusion in rhyolitic 706 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato melt by 2 orders of magnitude. on the other hand, our preliminary results demonstrate no significant effects of water on carbon dioxide diffusion in a trachytic melt. 2.3. sulfur diffusion we know less about sulfur behaviour in silicate melts than water and carbon dioxide. many studies have found that sulfur is present in silicate melts as either so42− or s2− species (see review by carroll and webster, 1994). but, winther et al. (1998) demonstrate that this speciation picture is clouded by the presence of s2− and s3− radicals detected in quenched albite glasses. all agree that sulfur solubility and the relative abundances of the dissolved species are a function of oxygen fugacity and melt composition, but the exact relationships remain to be discovered. research on sulfur diffusion is only beginning, however there is sufficient experimental data for petrologic investigations (fig. 4). watson (1994) discusses watson et al.’s (1993) study of sulfur diffusion in dacite and andesite at 1 gpa and 1300 to 1500°c. these experiments were performed under reducing conditions, near the iron-wustite buffer, where s2− is expected to be the dominant sulfur species in the melt. diffusion in the dacite melt is described by and in the andesite melt by addition of 5.5 wt% water to the andesite melt resulted in almost halving the activation energy baker and rutherford (1996) investigated the diffusion of sulfur in a rhyolitic melt at 1 atm to 200 mpa and 800 to 1100°c. they found that s diffusion in dry rhyolite is described by 51 . .expd rt 2 10 0 3 1 sulfur 8#= -00b l . .expd rt 1 0 10 191 0 sulfur 6#= -00b l 36 . expd rt 1 4 10 2 0 sulfur 4#= -00b l fig. 3. carbon dioxide diffusion in trachyte melt from a diffusion couple experiment at 1.2 gpa, 1200ºc, and 720 s. diffusion profiles are displayed for total water, molecular co2, carbonate, and total co2, calculated as the sum of molecular co2 and carbonate. concentrations of all species were determined by ftir as discussed in the text. also plotted is the theoretical diffusion profile for a diffusion coefficient of 2.5 × 10−12 m2 s−1. further details of this experiment can be found in the text. 707 volatile diffusion in silicate melts and its effects on melt inclusions and that addition of 7 wt% water increased sulfur diffusion by 1.5 to 2 orders of magnitude. unfortunately, the hydrous experiments covered too small a temperature range to define an arrhenius relationship, but the sulfur diffusion data of baker and rutherford (1996) in hydrous rhyolite all plot close to the low temperature extrapolation of watson et al.’s (1993) arrhenius line for sulfur diffusion in a hydrous andesite (fig. 4). baker and rutherford performed experiments at geological oxygen fugacities ranging from the fayalite-quartz-magnetite buffer (~1 order of magnitude below the nickel-nickel oxide buffer) to the mno-mn3o4 buffer (~2 orders of magnitude above the nickel-nickel oxide buffer) and found no oxygen fugacity effects on s diffusion. baker and rutherford argued that this insensitivity to oxygen fugacity reflected the presence of a single diffusing s species, which they identified as s2−. their argument rests upon a comparison of their results to other diffusion experiments, theoretical calculations of how the size of the diffusing 0 . expd rt 10 2 0 8 0 5 0 21 sulfur 6# ! = -0 00 0b l species affects the diffusion coefficient, and their experiments in air, where virtually no s2− should be dissolved in the melt, which demonstrated significantly lower diffusion coefficients than the other experiments at lower oxygen fugacities. winther et al.’s (1998) measurements of s diffusion in dry albitic melt at 1.0 gpa, 1300 to 1500°c, yielded this activation energy is surprisingly large when compared to those for sulfur diffusion in other anhydrous melts. using a variety of spectroscopic methods winther et al. (1998) found that sulfur is present in the quenched glasses as so42−, so2− and so3−. they provided evidence that at least some of the species previously thought to be s2−, based upon the sulfur peak location during electron microprobe analysis, were instead a combination of the radicals s2− and s3−. these authors found that the reduced species s2− and s3− diffuse more rapidly than so42−, consistent with the findings of baker and rutherford (1996). . .expd rt 7 0 14 458 sulfur = 01 b l fig. 4. diffusion of sulfur in magmatic silicate melts. 708 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato measurements of sulfur diffusion in silicate melts of geological interest are rare and much more work is needed. however, most of the results to date are consistent and suggest that within the range of anhydrous andesite to rhyolite there is little effect of melt composition on sulfur diffusion. the effect of water is significant, increasing s diffusion by up to 2 orders of magnitude in rhyolitic melts, but arrhenius relations for most hydrous melts remain to be accurately determined. 2.4. fluorine and chlorine fluorine and chlorine are not uncommon magmatic components (carroll and webster, 1994). both elements are found at concentrations of 1000’s of ppm and in some granitic rocks f can reach weight percent concentrations. studies of the diffusion of these volatiles in magmatic melts are quite rare, but still sufficient to provide an idea of the diffusion coefficients of these elements (fig. 5). dingwell and scarfe (1984, 1985) were the first to measure fluorine diffusion in a geologically relevant melt composition. they measured f and oxygen interdiffusion in anhydrous jadeite melt at high pressure using a diffusion couple in which one half of the couple was composed of jadeite melt and the other half was composed of a jadeite melt in which some of the oxygen was replaced by fluorine. over the temperature range of 1200 to 1400°c and pressure range of 1.0 to 1.5 gpa, dingwell and scarfe found that f diffusion in jadeite melt is pressure insensitive and follows the arrhenius equation: these authors also measured fluorine-oxygen diffusion in albite at 1 atm at the same temperatures (dingwell and scarfe, 1985) by exposing a fluoridated albitic melt to air at high temperature, which resulted in the diffusion of fluorine out of and oxygen in to the sample. interestingly, this study produced a lower activation energy chlorine tracer diffusion in a geological melt was first measured by watson and bender (1980) in a simple na-ca-al-si-o melt with 56 wt% silica at temperatures from 1100 to 1300°c, and .. expd rt 10 1 0 1 8 21 fluorine 9#= -00b l 59 . .expd rt 5 9 10 1 0 fluorine 6#= -00b l fig. 5. diffusion of halogens in magmatic silicate melts. 709 volatile diffusion in silicate melts and its effects on melt inclusions pressures from 0.6 to 1.8 gpa. at 0.6 gpa they found comparison of diffusion coefficients measured at different pressures indicated no pressure dependence of cl diffusion. watson (1991) investigated cl tracer diffusion in a rhyolitic and a dacitic melt with 8 wt% water at 1.0 gpa. the temperature of these experiments spanned 850 to 1100°c, which was not sufficient to define an arrhenius relation, but the diffusion coefficients ranged from 3×10−12 to 4×10−11 m2s−1. bai and koster van groos (1994) measured chlorine chemical diffusion in a haplogranitic melt with 75 wt% sio2 and a natural obsidian melt with 78 wt% sio2 at pressures from 1 atm to 460 mpa. their anhydrous experiments at low pressure produced an arrhenius relation of 70 .. expd rt 10 0 3 4 2 chlorine 4#= -00b l for temperatures between 850 and 1400°c. chemical diffusion of chlorine in a hydrous, granitic melt saturated with a nacl brine at 200 mpa, 650 to 900°c is described by these authors demonstrate that over the pressure range of 200 to 460 mpa the cl diffusion coefficient decreases by approximately a factor of 2. the results for chlorine diffusion are exciting in that the few data available from different labs using differing techniques on rhyolitic compositions are very consistent, with the interesting exception of the pressure dependence for cl diffusion. regrettably there are not sufficient data to better evaluate fluorine diffusion. clearly, more research in this area is urgently needed. 01 . .expd rt 6 46 10 1 07 chlorine #= -09b l . expd rt 3 16 10 86 chlorine 9#= -091b l fig. 6. comparison of volatile diffusion in hydrous silicic (almost exclusively rhyolitic) melts. water diffusion, for 2 and 6 wt% total, is calculated from zhang and behrens (2000). co2 diffusion is calculated from watson (1991) in a melt with 8 wt% dissolved water. s diffusion is a combination of the high-temperature results of watson et al. (1993), see also watson (1994), and the low-temperature results of baker and rutherford (1996). cl diffusion is a combination of the high temperature results of watson (1991) and the low temperature results of bai and koster van groos (1994). note that the apparent activation energies for s and cl diffusion may be an artifact of combining data from two different studies. lines for si-al interdiffusion in silicic melts containing either 3 or 6 wt% h2o are extrapolated using the arrhenius relationships determined by baker (1991). 710 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato 2.5. comparison of the different volatiles figure 6 presents a comparison of the measured diffusion coefficients of water, carbon dioxide, sulfur and chlorine in a hydrous rhyolitic melt. at this time there are insufficient data to depict the diffusion of fluorine. the lines shown for sulfur and for chlorine are constructed by combining the high-temperature results of one study with the low-temperature results of another; thus, they only represent the diffusion of these volatiles in a general way. nevertheless, the data demonstrate that over a broad range of magmatic temperatures, from ∼1200 to 600ºc , the diffusion coefficients of water, carbon dioxide, and chlorine are expected to be within an order of magnitude of each other. sulfur, diffuses much more slowly, and its diffusion coefficient is approximately 2, or more at lower temperatures, orders of magnitude less than those of the other dominant magmatic volatiles. as we shall see this difference in diffusion coefficients can result in some interesting effects. 3. effects of crystal growth rate and diffusion on the composition of melt inclusions it is well known that the dynamics of bubble growth controls the physics of volcanic eruptions (cf., sparks et al., 1994). in turn, bubble growth is mainly controlled by two processes: diffusion of volatiles from the melt into the bubbles and the expansion of the bubbles due to decompression (sparks et al., 1994). since the formulation of scriven’s (1959) growth law for bubbles in a superheated one-phase system where r is the radius of the bubble, t the time, d the diffusion coefficient and β is a growth rate constant, many researchers created more complex models taking into account additional parameters such as advection, viscosity, pressure and variable volatile concentration (e.g., proussevitch et al., 1993; proussevitch and sahagian, 1998; blower et al., 2001). it is apparent that knowledge of volatile diffusion coeffir dt2= b cients in silicate melts is a fundamental necessity for the modelling of magma vesiculation. it is probably less well known that knowledge of volatile diffusivity aids the interpretation of data obtained studying melt inclusions in crystals. melt inclusions are small (tens of microns) samples of melt trapped by magmatic crystals during their growth and are considered witnesses of petrologic processes, thus they are used as petrogenetic indicators (cf., hauri et al., 2002). because some melt inclusions are trapped at relatively high pressure, they are a useful tool to estimate volatile contents in magmas (e.g., roedder, 1979; hauri, 2002). importantly, melt inclusions might form at different stages of crystal growth and they record only the composition of the melt at the crystal-melt interface. if the diffusion coefficient in the melt of an incompatible element in the crystal is slow compared to the crystal growth rate, the melt-crystal interface will become enriched in the incompatible element (e.g., smith et al., 1955; watson et al., 1982; harrison and watson, 1984); this phenomenon is often referred to as pile-up or the snowplow effect. because of the common use of melt inclusions as volatile records (e.g., massare et al., 2002), we performed calculations to estimate the volatile (h2o, co2, cl, and s) concentration curve in front of a growing crystal. our conceptual model is that of a crystal whose growth rate suddenly increases for a short duration and then decreases, trapping melt at the crystal-melt interface forming melt inclusions. we modelled the pile-up of volatiles at the crystal-melt interface for both rhyolitic and basaltic melts. all volatiles are incompatible elements with a crystal-liquid partition coefficient of 0.001. decreasing this partition coefficient by one order of magnitude insignificantly affects the results. two crystal growth rates were used in the calculations, 10−8 and 10−10 m s−1. these values are based upon the results of swanson (1977) who investigated plagioclase, alkali feldspar and quartz growth in a granitic melt containing 3.5 wt% h2o at temperatures between 690 and 810ºc and measured maximum growth rates of 10−8, 10−9, and 10−10 m s−1, respectively. at temperatures between 600 and 1000ºc, plagioclase growth rates varied from 10−9 to 10−8 m s−1, alkali feldspar growth rates 711 volatile diffusion in silicate melts and its effects on melt inclusions were from 10−11 to 10−9 m s−1, and quartz growth rates spanned 10−11 to 10−10 m s−1 (swanson 1977). we previously studied crystal growth in the system orthoclase-quartz-h2o and measured similar growth rates, between 1×10−10 and 5×10−9 m s−1 (baker and freda, 2001). growth rates of ferromagnesian minerals in compositionally simple, basic melts range from 1×10−4 to 1×10−6 m s−1 (see summary in lasaga, 1998). recently, simakin et al. (2003) have measured clinopyroxene and olivine growth rates near 1100ºc in a hawaiite melt at undercoolings of 25 and 45ºc, respectively, and found them to be both approximately 1×10−8 m s−1. the high growth rate used in our modelling, 10−8 m s−1, appears to be close to the maximum expected growth rate in magmatic systems, whereas the low growth rate used in the models, 10−10 m s−1, is probably more common in nature, but still potentially rapid compared to normal growth rates in igneous systems (cf., lu et al., 1995). we modelled linear crystal growth of 10, 50 and 100 µm and calculated the volatile profile in the melt following eq. 26 of smith et al. (1955). the diffusion coefficients for volatiles in granitic melts were extracted from fig. 5 for calculations at 800°c (ds = 2.5 ×10−14, d2 wt%h2o = = 2.6 × 10−12, dcl = 6.6 × 10−12, dco2 = 2.0×10 −11, d6 wt%h2o = 2.0 × 10 −11 m2 s−1). crystal growth rates are affected by diffusion of their components in the melt, in this case primarily silicon and aluminum (lasaga, 1998); at these conditions the chemical interdiffusion of si and al in silicic melts (fig. 6) is approximately 1 × 10−14 m2 s−1 at 2 wt% h2o in the melt, and 1 × 10−13 m2s−1 at 6 wt% h2o (baker, 1991). the diffusion coefficients of water and co2 measured by zhang and stolper (1991) in a basaltic melt were used in modelling the effects of crystal growth at 1300°c (dh2o = 2.5 × 10 −10 and dco2 = 1.3 × 10−11 m2 s−1). although no measurements of si and al diffusion in hydrous basaltic melts are available, water is unlikely to significantly affect diffusion in this composition and we expect that si-al interdiffusion in hydrous basaltic melts is probably at most an order of magnitude above that measured in dry melts at 1300ºc, ∼1 × 10−12 m2 s−1 (watson, 1982). we do not consider changes in the volatile diffusion coefficients due to changing water concentrations because once melts attain approximately 3 to 4 wt% water the diffusion coefficients of all elements are relatively insensitive to the exact water concentration (watson, 1994). the only diffusion profile this simplification might significantly affect is that of 2 wt% h2o. however, because of the effect of water concentration on water diffusion, our results at 2 and 6 wt% h2o bracket the possible enrichment profiles for water concentrations in this range, and probably at water concentrations up to 8 wt%. in all cases we ignore the possibility of reaching fluid saturation and subsequent vesiculation. in order to compare calculations for different volatiles, we plot the results of the calculations as the enrichment factor of the melt near the crystal-melt interface relative to the bulk concentration of the volatile in the melt (figs. 7a-d and 8). the volatile enrichment in front of a crystal growing from a hydrous rhyolitic melt at 800ºc is displayed in fig. 7a-d. if 10 µm of crystal grow at 10−8 ms−1 (fig. 7a) the maximum volatile enrichment factor is 6x; this is the enrichment factor for sulfur and is due to its low diffusion coefficient. however, only 10 µm away from the crystalmelt interface the s reaches the bulk concentration in the melt. the maximum enrichment of the other volatiles is 1.25x. in contrast to sulfur, these volatiles only reach the bulk melt concentrations at 100 to 275 µm from the crystal-melt interface. at 800°c the diffusion coefficients of 6 wt% h2o and co2 are the same, which results in overlapping enrichment profiles in fig. 7a-d. if the crystal grew 50 µm (fig. 7b), the s enrichment reaches 22x in this case, and for a bulk melt with 2 wt% water the concentration at the interface is enriched 1.6x (3.2 wt% h2o). chlorine is enriched by 1.35x and both co2 and water for a melt with a bulk water concentration of 6 wt% are enriched 1.2x. the region of enriched melt extends to 20 µm in front of the crystalmelt interface for sulfur and 300 to 600 µm away for the other volatiles. allowing the crystal to grow 100 µm (fig. 7c) results in a sulfur enrichment of 42x, but the 20 µm-wide enriched region in front of the crystal is approximately the same length as in the case of 50 µm of growth. the enrichment factor for a melt with a bulk compo712 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato sition of 2 wt% water is 1.9x at the interface; for chlorine the enrichment is 1.55x and for 6 wt% water and co2 is 1.28x. the profiles for the 2 and 6 wt% h2o, cl and co2 extend from approximately 400 to 1100 µm into the melt. if the growth rate is decreased two orders of magnitude to 10−10 m s−1, the volatile enrichment at the crystal-melt interface is significantly less (fig. 7d). for 50 µm of growth the enrichment of sulfur is only a factor of 1.6x (compared to fig. 7a,b. volatile concentration enrichment profiles in front of a crystal growing in a rhyolitic melt at 800°c. the diffusion coefficients for volatiles are from fig. 5 and the sources of the crystal growth rates are discussed in the text. note that composition enrichment profiles for 6 wt% h2o and co2 plot on top of one another because their diffusion coefficients are identical at the conditions studied (see fig. 5). the effects of crystal growth on volatile concentration enrichment profiles relative to the concentration at infinite distance near the crystal-melt interface are modelled at a growth rate of 10−8 m s−1 for a total growth of 10 µm (a), 50 µm (b). further discussion is found in the text. a b 713 volatile diffusion in silicate melts and its effects on melt inclusions 22x at a growth rate of 10−8 m s−1) and all of the other volatiles are enriched by less than a factor of 1.06x, which is below analytical uncertainty in many cases. these calculations for crystal growth from a rhyolitic melt at 800°c demonstrate that at high growth rates any melts trapped from the crystalmelt interface into melt inclusions may not config. 7c,d. volatile concentration enrichment profiles in front of a crystal growing in a rhyolitic melt at 800°c. the diffusion coefficients for volatiles are from fig. 5 and the sources of the crystal growth rates are discussed in the text. note that composition enrichment profiles for 6 wt% h2o and co2 plot on top of one another because their diffusion coefficients are identical at the conditions studied (see fig. 5). the effects of crystal growth on volatile concentration enrichment profiles relative to the concentration at infinite distance near the crystal-melt interface are modelled at a growth rate of 10−8 m s−1 for a total growth of 100 µm (c). the volatile concentration enrichment profiles after 50 µm of crystal growth at 10−10 m s−1 are also modelled to demonstrate the effect of crystal growth rate on volatile enrichment (d). further discussion is found in the text. c d 714 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato tain volatile concentrations representative of the bulk melt. in fact, our one-dimensional model provides a conservative estimate of enrichment that would occur during growth and entrapment of a spherical melt inclusion. a more positive way of considering the results of these calculations is that concentrations of h2o, co2 and cl measured in melt inclusions are unlikely to be more than twice the bulk melt concentration of these volatiles. the existing data on crystal growth in silicic melts (discussed above) indicate that quartz is the slowest growing silicate crystal, with a maximum of approximately 10−10 m s−1, and should contain melt inclusions representative of the bulk melt. our study reinforces the conclusions of lu et al. (1995) who through their study of melt inclusions in crystals from the bishop tuff, california, found that melt inclusions larger than 50 µm in diameter should be compositionally equivalent to the bulk melt. if melt inclusions can be trapped near 800ºc at what appear to be crystal growth rates below 10−10 m s−1 (cf., lu et al., 1995) their volatile concentrations of h2o, co2 and cl almost certainly will be representative of the bulk melt, but the minimum growth rate necessary for a crystal-melt interface to become unstable and potentially lead to the entrapment of melt inclusions remains unknown. for example, our experiments on crystal growth in the orthoclase-quartz-water system (baker and freda, 2001) did not trap melt inclusions even at a growth rate 5 × 10−9 m s−1. modelling the effects of a 10−8 ms−1 growth rate on h2o and co2 concentration profiles in a basaltic melt at 1300°c (fig. 8) demonstrates that the maximum enrichment, in this case for co2, is only a factor of 1.35x for 100 µm of growth. the water concentration is insignificantly enriched by a factor of less than 1.08 for the conditions investigated. despite the small volatile enrichment factors at the crystal-melt interface the concentration profiles do not reach the bulk melt composition until at least 300 µm away from the crystal. this modelling suggests that melt basaltic inclusions trapped at 1300°c contain concentrations of co2 and h2o similar to those of the bulk melt. fig. 8. volatile concentration enrichment profiles in front of a crystal growing in a basaltic melt at 1300°c. diffusion coefficients of h2o and co2 are from zhang and stolper (1991) and crystal growth rates are based on data in lasaga (1998) and simakin et al. (2003). for further discussion please see the text. 715 volatile diffusion in silicate melts and its effects on melt inclusions the calculations highlight the need to know both the diffusion coefficients for volatiles (and other elements for that matter) and crystal growth rates in order to assess whether or not melt inclusions are representative of the bulk melt. differences of approximately 4 orders of magnitude between diffusion coefficients and crystal growth rates are sufficient to enrich the crystal-melt interface significantly, greater than 10 relative percent, above the bulk melt. the conclusion that melt inclusions formed in basaltic systems trap samples that may be compositionally representative is based only upon calculations for water and carbon dioxide at an unrealistically high temperature of crystallization. although the calculations above purposely ignore the possibility of volatile saturation and exsolution due to rapid crystal growth, it is worthwhile to qualitatively consider this possibility. even though the bulk melt may not be volatile saturated, rapid crystal growth could cause the concentration of a volatile at the crystal-melt interface to reach saturation and exsolve a fluid phase. the concentration necessary to reach saturation in magma chambers at a few kilometres depth varies from the weight percent level for h2o to only hundreds to thousands of ppm for co2, s, and cl (carroll and holloway, 1994). thermodynamics demands that once one volatile reaches saturation and exsolves the other components in the system must partition between the melt and the fluid phase in order to reach, or attempt to reach, equilibrium. obviously this process affects volatile concentrations in the melt at the crystal-melt interface and causes them to deviate from those in the bulk melt. if these bubbles were able to separate from the interface before the melt was trapped, there might be no record of exsolution (presuming these bubbles ascend into the bulk, fluid undersaturated, melt they would be resorbed). only recently are we beginning to measure the equilibrium partitioning of volatile components, primarily h2o and co2 (e.g., tamic et al., 2001), between silicate melts and fluids, so that we are at the beginning stage of quantitatively modelling this process. anomalous volatile enrichment of melt inclusions might be detected by measuring volatile concentrations in contemporaneous melt inclusions of differing sizes. modelling suggests that slowly diffusing sulfur is only enriched within approximately 20 µm of the crystal-melt interface in rhyolitic melts at 800°c (fig. 6), which is consistent with the results of lu et al. (1995); thus if anomalous enrichment due to pile-up during crystal growth occurred, smaller inclusions are expected to be significantly enriched in sulfur compared to larger ones. once the size of the melt inclusion significantly exceeds the enriched region at the crystal-melt interface, the sulfur concentration should reach a constant, bulk, value. however, our models demonstrate that even when sulfur reaches the bulk melt concentration in large melt inclusions, the concentrations of other volatiles in the inclusion may still be significantly higher than the bulk melt concentrations. the measurement of volatile diffusion in silicate melts has advanced significantly since shaw published his work in 1974. our knowledge of water diffusion in silicic melts is particularly extensive. however, we still need additional measurements of co2, s, f and cl in silicic melts and of the diffusion of all volatiles in intermediate and basic compositions. additionally, we need to know how water concentration and pressure affect diffusion of other volatiles in all magmatic silicate melts. our simple models indicate that under conditions of rapid crystal growth diffusion pile-up can occur at crystalmelt interfaces and the melts trapped as melt inclusions may not contain volatile concentrations representative of the bulk melt; this problem appears particularly severe for rhyolitic systems at high crystal growth rates and probably minor for high-temperature basaltic systems. acknowledgements we thank the two anonymous reviewers whose insightful comments spurred us to improve the presentation of this paper. we also thank mike carroll for his work in editing this volume. we particularly thank m. nowak for providing us with a preprint of his paper on co2 diffusion. funding for the experiments and analyses reported in this work came from: nserc discovery grant to d.r.b.; italian civil protection (gnv project n. 2000-02/17) to 716 don r. baker, carmela freda, richard a. brooker and piergiorgio scarlato c.f. and p.s.; european community-access to research infrastructure action of the improving human potential programme to c.f. and r.a.b. references bai, t.b. and a. koster van groos (1994): diffusion of chlorine in granitic melts, geochim. cosmochim. acta, 58, 113-123. baker, d.r. 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zhang, y., e.m. stolper and g.j. wasserburg (1991): diffusion of water in rhyolitic glasses, geochim. cosmochim. acta, 55, 441-456. layout 6 fabry–perot interferometer as a solar background noise suppressor: application to daytime lidar karnam raghunath1,*, karnam ramesh2, sanama narayana reddy3 1national atmospheric research laboratory, department of space, gadanki, india 2 sri venkateswara university, department of physics, tirupati, india 3 sri venkateswara university, department of electrical and electronics engineering, tirupati, india annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-4947 abstract continuous atmospheric probing by a lidar is a requirement for many applications. however, due to high solar background noise during the daytime, lidar operations are mostly restricted to night-time. while many techniques are in practice, like reducing the receiver field of view, changing the view angle, introducing a narrow band interference filter (if), these are applied to circumvent problems, rather than to suppress the noise. using a fabry-perot interferometer as a narrow passband filter for solar background noise suppression is a known technique, and its potential is exploited in our system. an optical-fiber-coupled lidar system with its transmitter injection seeded was developed and has been operated during the daytime at gadanki (13.6˚n, 79.2˚e). the signal-to-noise ratio of the return signal is used as the performance indicator, to evaluate the improvements. signal-to-noise ratios with and without the fabry-perot interferometer are measured with near identical test set-ups. the signal-to-noise ratio enhancement factor is ca. 4, in agreement with the theoretical value. the performance is compared when the receiver fields of view are changed. 1. introduction the primary aim of this study is to demonstrate the feasibility of a lidar system for daytime measurements in the lower altitudes up to 30 km, with the later extension of the profiling to the upper altitudes. observations are limited to early morning and evening hours for vertically pointing lidars, without any solar background elimination methods. the extension of the lidar operation to daytime helps in the study of the properties of aerosols and clouds that might be affected in the presence of the sun. thus lidar profiling of the atmosphere during the daytime is useful to compare the properties of various atmospheric constituents that can be changed in the presence of solar radiation. the signals received from the lidar during the daytime contain contributions from back-scattered signals at laser wavelengths, and unwanted noise signals due to solar radiation. the solar background noise is less during the early hours of the day, and becomes higher as the day progresses, and this noise strength needs to be suppressed to carry out meaningful observations. some of the main factors that contribute to the noise include the solar irradiance during the measurement time, the solar zenith angle, the wavelength used for profiling, the power of the laser, and the filters used. some methods for reducing the background solar noise are as follows: 1. making the telescope aperture size smaller; 2. using a wavelength that falls in the solar blind region; 3. reducing the field of view (fov) of the receiver; 4. changing the beam pointing angle according to the solar zenith angle; and 5. reducing the optical system bandwidth using spectral filters in the receiver in conjunction with an injection seeded laser. three types of filters have been considered [sidorin et al. 2005] in the past for use for such applications; namely, thin-film interference filters, birefringent filters, and etalonbased filters. here it was concluded that etalon-based filters would be best-suited for cost-effective practical realization. a pure rotational raman lidar used a fabry-perot interferometer (fpi; m/s ic optical systems, uk) as a frequency comb filter to isolate return signals that are due to pure rotational raman scattering from atmospheric nitrogen against the sky background [arshinov et al. 2005]. noctilucent clouds were observed during the daytime by appropriately equipping the lidar system with narrow optical filters in the form of fpis [blum and fricke 2005], where a cut-off value was fixed to identify the clouds from the noise. a robust rayleigh mie lidar for daytime temperature profiling in the troposphere was developed simultaneously, using three techniques; namely, a narrow-band interference filter along with fpis, the ultraviolet region of operation, and a narrow fov to suppress the solar background noise [hua et al. 2005], with good results. using a fpi, a photometer was developed for the measurement of daytime oi 630.0-nm emission from the thermosphere [narayanan et al. 1989]. in article history received december 31, 2010; accepted september 12, 2011. subject classification: instruments and techniques, lidar, atmosphere, daytime. 215 research articles micro-pulse lidar, the daytime background-induced noise is controlled by a narrow receiver fov and a narrow bandwidth, temperature-controlled, interference filter [spinhirne 1993]. there are cases where other techniques can be used as well. daytime observation capability was realized with dualwavelength, high-altitude lidar sodium fluorescence channel by using a sodium atomic filter [cheng et al. 2008, hua et al. 2005]. a novel technique to reduce the sky background signal for a linearly polarized lidar was explored by using a polarization-selection technique [hassebo et al. 2006] where the sky background noise is minimized by taking advantage of the naturally occurring polarization properties in scattered skylight. this has shown good improvements in the signal-to-noise ratio (snr) over conventional schemes. the proposed raman lidar observations for daytime operations has a plan to use the third harmonic of a nd:yag laser, where the height coverage variations with different configurations have been discussed [brunozzi and marenco 2003] due to these schemes. an optimal design for suppressing solar scatter using single, double and triple etalons has been arrived at, and significant improvements have been achieved with single etalon itself. subsequent addition of etalons is found to have incremental enhancement [mckay 1999]. thus, daytime lidar observations were carried out with different techniques. while the outcome is varied for these different methods, raghunath et al. 216 table 1. specifications of the lidar system. location gadanki, india (13.6˚n, 79.2˚e) transmitter laser type solid state class laser source nd:yag, with seeding operating wavelength 532 nm average energy per pulse 600 mj pulse width 7 ns pulse repetition rate 50 hz beam divergence <0.1 mrad (after 10 × expansion) line width 0.15 pm with frequency stability of 50 mhz/h receiver telescope type schmidt–cassegrain diameter 350 mm field of view 0.4 mrad with fnumber of 11 interference filter bandwidth 1.07 nm maximum transmission 48% detector pmt, with gain of 34 × 107, dark current 50 × 10−12 a optical fiber type hard polymer clad, step index multimode fiber core diameter 1.5 mm with numerical aperture of 0.37 fabry–perot interferometer type of the etalon piezo tunable, capacitance stabilised air spaced etalon material schott lithosil fused silica etalon clear (working) aperture 50.8 mm etalon fwhm 2.08 pm with reflective finesse of 7.2 etalon cavity spacing 9 mm data acquisition system pc based data acquisition system operating with eg & g ortec mcs software bin width (range resolution) 2 µs corresponding to 300 m with time integration of 250 s 217 the implementation depends on the cost effectiveness, the technology availability, the urgency, and the limitations and advantages. here, in the present study we discuss the effects of using an etalon in the lidar receiver, and the improvements when compared with conventional methods. 2. the lidar system the lidar system is a monotatic biaxial system with the laser output derived from a frequency doubled nd:yag laser. the laser beam is 8 mm in size, and the output power is 30 w at a pulse repetition frequency of 50 hz. the laser is injection seeded with a stable, narrow-line-width, continuous wave, fiber laser. the receiver is a 350-mm cassegrain feed telescope with an fnumber of 11, which is fiber coupled to the rest of the optics. the optical fiber has a numerical aperture of 0.37, with core diameter of 1.5 mm. the fiber diameter decides the fov of the receiver. the photomultiplier tube (pmt) is used as a photodetector and has low dark noise, and it sends negative going pulses to a fast discriminator. after pulse shaping, the signals go to a multichannel scalar card for photon counting. the card functions in dwell and advance mode, and the photon strength is presented with respect to the lapse time. the functioning of an atmospheric lidar system is well known, and no further explanation will be given here. the specifications of the lidar system are given in table 1, in a sufficient and elaborate manner. the laser and etalon have a key role in the observations, and hence the characteristics are recorded. a model n˚ ws u-10 (m/s high finesse) wavelength meter with absolute accuracy of ±10m hz is used to measure the line width and the wavelength stability of the seeded laser (www.highfinesse. com). the laser characteristics are monitored for about 1.5 h, and the results are shown in figure 1. the line width consistently shows about 0.3 pm on the right side of the plot. the wavelength drifts by ca. 0.1pm/h, which corresponds to ca. 300 mhz and ca. 100 mhz/h, respectively, in frequency a 532 nm, as shown on the left side of the plot. the measurements are in agreement with those of the laser specifications. the servo-stabilized fpi system comprises an etalon and a control unit. the control unit is a three-channel controller, which uses capacitance micrometers and piezoelectric (pzt) actuators to monitor and correct errors in mirror parallelism and spacing (www.icopticalsystems.com). the fpi is mounted on an xyz mount for adjusting the angle of incidence of the optical fiber, to give additional control for the fpi. the airy distribution of the etalon was obtained with a finesse of ca. 8.0 at 632.99 nm, by scanning the etalon spacing for a few cycles with a stabilized helium-neon laser. the laser and etalon features are adequate for the present application. the general convention of terming the fpi as the etalon for fixed plate spacing is used throughout the text. the snr is one parameter that is used by many lidar researchers as a criterion to assess the lidar performance. the snr is measured at the output of the photo detector by taking into account the total lidar instrument efficiency. the results of the performance of a lidar system that was operated during the daytime using an etalon is presented here. the etalon and fiber optics used in the present context are optimized for a direct detection lidar system, which is under development at the national atmospheric research laboratory. using the availability of the etalon, the lidar system is modified for daytime capabilities. the fiber optics need to be optimized for the f number of the telescope, to obtain the designed height coverage [jenness jr. et al. 1997, chourdakis et al. 2002]. for the present objective, where the etalon use as day-light filters is tested, this specification is not critical. nevertheless, the performance of the lidar system that was expected is realized. the working of the etalon for daytime applications has been explained for many studies [mckay 1999, arshinov et al. fabry–perot interferometer in daytime lidar !figure 1. frequency stability and line width of the laser, as measured with a wavelength meter. 2005, hua et al. 2005]. the snr enhancement factor for the single etalon case is given by the approximate relation [mckay 1999] as: snrenhancement factor = 2/π * fe (1) where fe is effective finesse of the etalon. use of an etalon to isolate an individual spectral line of a lidar return against the continuum noise can be considered analogous to the isolation of narrow spectral intervals with interference filters, and the transmission enhancement is proportional to the number of peaks within a free spectral range defined as the finesse [arshinov et al. 2005]. this is the physical significance of the functioning of an etalon during the daytime. the snr enhancement factor for a single etalon case cannot go beyond 10 usually, as high transmittance is needed for parameter extraction. 3. the experiment to quantify the effects of using an etalon in the lidar receiver, an experiment was set up as shown in figure 2. this has two channels in the receiver. the incoming beam is split into two equal signals with a 1:1 beam splitter, and it passes to one channel (channel 1) that has etalon in the path, and another channel (channel 2) that does not have an etalon. apart from this difference, all of the other components, like the interference filter (if), the lens assemblies, and the pmts and their specifications, are identical for the two channels. this ensures near similar responses from the two receiving channels when no etalon is inserted, and hence in the actual experimentation, whatever signal changes that are observed between the channels is due to the presence of the etalon only. the responses of the two channels are tested with a he:ne laser. the difference between the two channels when no etalon is inserted was about ±2%, which can be ignored. to avoid saturation of the detector due to large returns from boundary layers, the fiber is slightly offset so that some of the signal is intentionally blocked. 3.1. snr estimation from the lidar data the lidar system was operated over several days with the above set-up. by comparing the snr between two channels, the snr enhancement/reduction factor of the signal is estimated. the snr is calculated as given in equation (2). the received signal is approximated to a poisson distribution for detection: (2) where n(r) is the returns from the range r, and pbkg is the returns due to the solar background. pbkg is estimated from the returns at the upper heights, typically at about 500 km, where the 'wanted signal' is minimum and all of the contribution is due to solar noise only. the dark noise that is due to the pmt is common for both channels, and hence this is not considered. the solar zenith angle has a crucial role, and its value at gadanki (13.6˚n, 79.2˚e) is calculated from the information given on the website of the u.s. naval observatory astronomical applications department (http://aa.usno.navy.mil/data/ docs/altaz.php). the term of a snr enhancement/ reduction factor is introduced to quantify the snr changes during the daytime/night-time. this is expressed as a ratio of the snr between channel 1 and channel 2. raghunath et al. 218 ! "#!$! " #! %! &"! '()(*+,-(!! .-/0+1)!203(4! $5$!6(17! 8-)0//(4! ! !!!!!! figure 2. receiver set-up of the lidar system used for testing the snr improvement. figure 3. raw data without the range correction, collected on march 4, 2010, at 0700 lt. the cloud presence is clearly visible in the 'with etalon channel'. snr r p n r n r bkg = + ^ ^ ^h h h 219 4. results and discussion 4.1. snr improvement the raw photon count profiles from both of the channels for a day are shown in figure 3, in which cloud presence can clearly be seen. the snr profile for the data collected without and with the etalon channels (channel 1 and channel 2 ) and snr enhancement/reduction factor at the particular altitude of 10 km is shown in figure 4, for march 18, 2010. the measurements were taken during 0415-1003 lt, which extends from early morning to late morning. the snr factor fabry–perot interferometer in daytime lidar !!!!!!!!!!!! !!!!!!!!!!!! figure 4. lidar snr values obtained at an altitude of 10 km on march 18, 2010. the times of the observations are given at the extreme ends of the plot, as local time. w is solar zenith angle. figure 5. lidar snr and snr enhancement/reduction factors obtained at an altitude of 10 km on different days. observation times: (a) 0505 to 0740 lt; (b) 1708 to 1930 lt; (c) 0510 to 0754 lt; (d) 0415 to 1003 lt. sun positions at pointer: (a) 0610 lt, sza (}) 93˚; (b)1840 lt, sza(}) 95˚; (c) 0610 lt, sza (}) 92˚; (d) 0615 lt, sza (}) 93˚. shows constant reduction during the night-time and it starts to be enhanced from day-break. this corresponds to a solar zenith angle (}) of 93˚ at 0615 lt, and of 37˚ at 1003 lt. case 1: during the night-time (0415-0615 hours), when the solar noise is less or nil, channel 1 attenuates more of the desired signal in the total signal (desired + noise) and less or none of the solar noise. channel 2 does not provide any attenuation. this results in lower snr values in channel 1 and better snr values in channel 2. this explains less than one snr reduction factor during night-time. case 2: during the daytime, channel 1 (0615-1003 lt) attenuates the same amount of the desired signal in the total signal (desired + noise) as during the night-time, but it attenuates more of the noise in the total signal because of the high solar noise. as in the previous case, channel 2 does not provide any attenuation. this results in lesser snr values in channel 2, and better snr values in channel 1. this explains the flat region of the snr enhancement factor of about 4 during the daytime. this is in agreement with equation (1), which gives the relationship between the snr enhancement factor and the finesse of the etalon. the results of this experiment carried out over 4 days is shown in figure 5. while figure panels a, c and d of figure 5 shows observations from the night-time to the daytime, figure 5b shows observations from the daytime to the nighttime. as the room temperature rose above normal operating conditions, the lidar operations could not be carried out around midday. some of the observations carried out during the pre-noon hours are expected to be valid during the postnoon hours, ignoring the effects of clouds, if any. nevertheless, the snr enhancement factor shows the trend of maintaining ca. 4, as is evident in figure 5. this is in agreement with the relationship between the finesse of the etalon and the snr enhancement factor [mckay 1999]. the whole height coverage has a similar performance, as shown in figure 6 for march 4, 2010. the constant snr reduction factor during the post-sunset and pre-sunrise times is as expected. it should be noted that in spite of the variations in the individual snrs (channel 1 and channel 2), the snr enhancement/reduction factor is not affected. 4.2. comparison with the conventional method the effectiveness of using etalon as a noise suppressor was compared with the conventional method of reducing the receiver fov. the experiment was conducted on march 18, 2010, alongside an identical receiver set-up with a fov of 1 mrad. the results plotted in figure 7 have a similar trend as seen when the etalon was used, although the effect is different. though the snr reduction factor is the same (ca. 0.45) for both cases, the snr enhancement factor is ca. 1.5 only. it is clear that the etalon as a solar noise suppressor is more effective than the method of reducing the fov, although this latter method is economical and easy to implement. it is expected that the characterization is valid for any fov reduction of a factor of 0.4. 4.3. height coverage improvements to determine the height coverage improvement with the noise suppression methods, the snr profiles in figure 8 were considered for the march 18 data at 0733 lt. this is summarized in table 2, which implies that the height coverage improvement is better when the etalon is used in the raghunath et al. 220 !!!!!!!! figure 6. plot showing the snr enhancement/reduction factor for heights up to 25 km on march 4, 2010, from 0510 to 0754 lt. the times of the observations are given at the extreme ends of the plots, in local time. a near constant factor is seen for the full height coverage for the entire duration of the observation. 221 fabry–perot interferometer in daytime lidar figure 7. lidar snr values obtained at an altitude of 10 km on march 18, 2010, for a change in the fov. the times of observations are given at the extreme ends of the plots, in local time. figure 8. height gain at 0733 hours on march 18, 2010, with the two techniques. the height coverage advantage is clear from the two techniques, where the snr is one. !!!!!!!!!!!! !!!!!!!!!!!! method change in snrnight time at 0500 change in snrday time at 0730 altitude coverage gain during day time at 0730 (at snr = 1) using etalon as spectroscopic filter reduction by a factor of ca. 0.45 enhancement by a factor of ca. 4 ca. 5 km reducing fov of the receiver reduction by a factor of ca. 0.4 enhancement by a factor of ca. 1.5 ca. 1 km table 2. operational performance of two methods. receiver chain. the best performance from an etalon-based receiver can be achieved when its maximum finesse is used. for most cases, snr = 1 is the criterion for carrying out cloud and related studies. the height improvement of 33% at snr = 1, as shown in figure 8 for the etalon-based receiver, can be compared with the achieved value of 34% by the polarization discrimination technique [hassebo et al. 2006] at snr = 10. further, snr at the cloud top is good, to build a slope for retrieving the atmospheric parameters. it is expected that using one or more methods for noise suppression will improve the overall performance. 5. conclusions the operation of a daytime lidar is demonstrated using etalons as the narrow passband filter for suppressing the solar background noise. the theoretical improvement in the snr has been realized. the effect of the presence of the etalon during the night-time and daytime was studied, and its superior performance over the conventional method of the changing of the receiver fov is noted. this technique can be used effectively for studies of diurnal variations of aerosol concentrations and cloud dynamics. references arshinov, y., s. bobrovnikov, i. serikov, a. ansmann, u. wandinger, d. althausen, i. mattis and d. müller (2005). daytime operation of a pure rotational raman lidar by use of a fabry-perot interferometer, appl. optics, 44, 3593-3603. blum, u., and k.h. fricke (2005). the bonn university lidar at the esrange: technical description and capabilities for atmospheric research, annales geophysicae, 23, 1645-1658. cheng, x., s. gong, f. lii, y. dai, j. song, j. wang and f. li (2008). daytime observation technology of a lidar using an atomic filter front, frontiers of optoelectronics in china, 1 (3-4), 274-278. chourdakis, g., a. papayaannis and j. porteneuve (2002). analysis of the receiver response for a noncoaxial lidar system with fiber-optic output, appl. optics, 41, 27152723. hassebo, y.y., b. gross, m. oo, f. moshary and s. ahmed (2006). polarization-discrimination technique to maximize the lidar signal-to-noise ratio for daylight operations, appl. optics, 45, 5521-5231. hua, d., m. uchida and t. kobayashi (2005). ultraviolet rayleigh–mie lidar for daytime-temperature profiling of the troposphere, appl. optics, 44, 1315-1322. jenness jr., j.r., d.b. lysak and r. philbrick (1997). design of a lidar receiver with fiber-optic output, appl. optics, 36, 4278-4284. mckay, j.a. (1999). single and tandem fabry-perot etalons as solar background filters for lidar, appl. optics, 38, 5851-5858. narayanan, r., j.n. desai, n.k. modi, r. raghavarao and r. sridharan (1989). dayglow photometry: a new approach, appl. optics, 28, 2138-2142. sidorin, y., d. lun and b.r. chou (2005). etalon-based filters for solar and atmospheric research, opt. eng., 44, 076001 (jul 14, 2005); http://dx.doi.org/10.1117/1.1947784. spinhirne, j.d. (1993). micropulse lidar, ieee t. geosci. remote, 31, 48-55. tini brunozzi, p., and f. marenco (2003). a lidar for water vapour measurements in daytime at lampedusa, italy, annals of geophysics, 46 (2), 459-465. *corresponding author: karnam raghunath, national atmospheric research laboratory, department of space, gadanki, india; email: kraghunath@narl.gov.in. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. raghunath et al. 222 layout 6 the hotsat volcano monitoring system based on combined use of seviri and modis multispectral data gaetana ganci1,*, annamaria vicari1, luigi fortuna2, ciro del negro1 1 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy 2 università di catania, dipartimento di ingegneria elettrica, elettronica e informatica (dieei), catania, italy annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5338 abstract spaceborne remote sensing of high-temperature volcanic features offers an excellent opportunity to monitor the onset and development of new eruptive activity. to provide a basis for real-time response during eruptive events, we designed and developed the volcano monitoring system that we call hotsat. this multiplatform system can elaborate both moderate resolution imaging spectroradiometer (modis) and spinning enhanced visible and infrared imager (seviri) data, and it is here applied to the monitoring of the etna volcano. the main advantage of this approach is that the different features of both of these sensors can be used. it can be refreshed every 15 min due to the high frequency of the seviri acquisition, and it can detect smaller and/or less intense thermal anomalies through the modis data. the system consists of data preprocessing, detection of volcano hotspots, and radiative power estimation. to locate thermal anomalies, a new contextual algorithm is introduced that takes advantage of both the spectral and spatial comparison methods. the derivation of the radiative power is carried out at all ‘hot’ pixels using the middle infrared radiance technique. the whole processing chain was tested during the 2008 etna eruption. the results show the robustness of the system after it detected the lava fountain that occurred on may 10 through the seviri data, and the very beginning of the eruption on may 13 through the modis data analysis. 1. introduction the frequent and synoptic view afforded by satellitebased sensors provides an excellent means to monitor the thermal activity of a volcano by offering adaptability and repeatability, while minimizing exposure to potential volcanic hazards [francis and rothery 2000, harris et al. 2007]. in the past, data provided by the landsat high spatial resolution and low temporal resolution (16 days) have been used for the thermal analysis of active lava flows [oppenheimer 1991], lava domes [francis and mcallister 1986, kaneko et al. 2002], lava lakes [harris et al. 1999] and fumarole fields [harris and stevenson 1997]. thermal measurements have been shown to be particularly suited for the detection of the onset of volcanic eruptions [francis and rotery 1987, harris et al. 1995], for the mapping of total thermal flux from active lava flows with high refreshing rates [flynn et al. 1994, harris et al. 1998], and to provide preliminary estimates of effusion rates to drive lava flow simulators [hérault et al. 2009, vicari et al. 2009]. even though high spatial resolution images can provide useful information for detailed characterization of the thermal properties of a volcano, images with lower spatial but higher temporal resolution can be acquired by meteorological satellites. such images have proven to provide a suitable instrument for continuous monitoring of volcanic activity, although the relevant volcanic characteristics are much smaller than the nominal pixel size [harris et al. 1997, harris et al. 2000, wright et al. 2001, wright et al. 2004]. indeed, sensors aboard meteorological satellites, such as the moderate resolution imaging spectroradiometer (modis) and the spinning enhanced visible and infrared imager (seviri), can detect emitted radiance in the shortwave infrared (swir) part of the electromagnetic spectrum, a region in which active lava flows, vents and domes emit copious amounts of energy. modis offers up to 10 wavebands that are suitable for hotspot detection, and being aboard the terra (eos am) and aqua (eos pm) polar satellites, it can provide data at least four times a day for any subaerial volcano, with a spatial resolution of about 1 km. in particular, band 21, which is also known as the ‘fire channel’, was designed to have a much higher saturation temperature of about 500 k [kaufman et al. 1998]. for these reasons, modis data have been used as the basis for automated systems for the detection and monitoring of volcanic eruptions over the entire globe [wright et al. 2002]. new opportunities for near real-time volcano monitoring came with the launch of seviri, in august 2002, onboard the msg1 and msg2 geosynchronous platforms. article history received february 7, 2011; accepted june 15, 2011. subject classification: etna volcano, infrared remote sensing, modis, seviri. special issue: v3-lava project 544 indeed, in spite of the low spatial resolution (3 km at nadir), the frequency of observations provided by the msg seviri for images at 15-minute intervals was recently applied both for fire detection [laneve and cadau 2009] and for the monitoring of effusive volcanoes in europe and africa [hirn et al. 2008]. since the modis and seviri data show significantly different characteristics in spatial, spectral and temporal resolution, the aim of this study was to integrate the information coming from both of these sensors to obtain a better understanding of the volcanic phenomenon. to this end, a multi-platform tool for satellite image analysis and volcanic process characterization is presented here. computing routines were designed to enable the joint exploitation of modis and seviri radiometers in operational monitoring, as a response to the need for fast and robust hotspot detection and radiant flux estimation at active volcanoes. this monitoring system was applied during the 2008 mount etna eruption, when it provided identifying pixels that covered the lava flow and processed these pixels to compute their radiative power. 2. the hotsat multiplatform system analyzing satellite imagery collected from multiple sensors and/or multiple platforms is a common technique that has been used to increase the sampling frequency of observations, to validate the performance of detection algorithms, and to cross-calibrate measurements such as radiative power [wooster et al. 2003, hirn et al. 2005, freeborn et al. 2009]. the multi-platform approach takes advantages of integrating information coming from sensors with different spatial, spectral and temporal resolutions. hotsat is essentially composed of three packages: preprocessing, product generation, and post-processing (see figure 1). the pre-processing module is necessary for initial geolocation and calibration of the satellite images. the product generation block computes higher-level products from the satellite band data. it includes hotspot detection and radiative power estimation. the post-processing module projects raw geometry data to a cartographic reference system, and exports the derived image files as standard dtp formats (e.g. bmp, jpeg, gif ). this entire package is managed by a control unit, and the user can configure these settings. in the following sections, the main aspects of the hotsat system are illustrated. 2.1 the hotspot detection algorithm the automatic detection of hotspots is not a trivial question, as an appropriate threshold radiance value must be chosen to distinguish the pixels that contain hotspots from those that do not. traditional algorithms rely on simple threshold tests and questions can arise concerning the correct choice of the thresholds, based on latitude, season and time of day. threshold algorithms such as the normalized thermal index [nti; wright et al. 2002], which has proven successful for modis, did not show comparable results when applied to seviri. evidence of this can be seen from figure 2, where the nti values are given for the south east crater (sec) in may 2008. here we have also plotted the thresholds that can be used to classify a pixel as hot (i.e. –0.8 for night-time, and -0.6 for day-time) [from wright et al. 2002]. the 2008 etna eruption started on may 13 and was preceded by a lava fountain on may 10: from this index we can distinguish the lava fountain and the first strong phase of activity that reached a maximum on may 15 and then rapidly decreased to may 19. although the eruption continued with an active lava flow, according to the nti, after may 19 no thermal anomaly was detected (figure 2). to overcome such limitations due to a fixed threshold, we introduce a new contextual algorithm based on a dynamic threshold and apply it to both the modis and seviri data. the hotsat hotspot detection algorithm takes as its starting point the contextual approach of harris et al. [1995] [i.e. the vast code of higgins and harris 1997], and uses the difference between brightness temperature in the middle infrared radiance (mir) and the thermal infrared radiance (tir), as dtdiff, while setting a threshold dt obtained from within the image to define whether a pixel is ‘potentially’ hot. this threshold is computed by taking into account the spatial standard deviation of the �dtdiff for each pixel. the algorithm thus first defines a ‘nonvolcanic’ portion of the image and uses the maximum dt from that portion to set the threshold. pixels belonging to the volcanic area are then scanned and all of the pixels that show a dtdiff greater than the threshold are classified as ‘potentially’ hot. the 545 ganci et al. figure 1. scheme of the hotsat volcano monitoring system. 546 algorithm then analyzes all of these ‘potentially’ hot pixels and their neighbors, to further assess these pixels as true or false hotspot detection, based on a threshold test derived from the statistical behavior of the 3.9 nm brightness temperatures. indeed the ‘potentially’ hot pixels and their neighbors are classified as hot if at least one of the following two conditions is met: (1) (2) where, t3.9 nm is the 3.9 nm brightness temperature and maxvar(t3.9nm) and std(t3.9nm) are the maximum variation and the standard deviation, respectively, of the 3.9 nm brightness temperature computed in the ‘nonvolcanic’ portion of the image. the parameter n controls how much the mir temperature of the pixel deviates from the mean value: the higher the n, the more sensitive the algorithm. we usually set n to 2. following these procedures, all of the computations are based on dynamic thresholds that reduce the number of false alarms due to atmospheric conditions. this hotsat hotspot detection algorithm has proven to be more reliable than the classical fixed threshold algorithm, as it also detected thermal anomalies at the end of may and in the first days of june, when the activity was lower (see figure 3). 2.2 radiative power estimation the next step for this hotsat volcano monitoring system is the derivation of the radiative power at all of the ‘hot’ pixels. to avoid problems concerning the well-known dual-band dual/three component technique (i.e. to assume the pixel characterized by two or three thermal components, to find a feasible solution for the nonlinear system, etc.), the mir radiance technique was applied to volcanic eruptions to estimate the radiant flux from satellite data [wooster et al. 2003]. this is based on an approximation of plank's law as a power law. for each hotspot pixel, the radiative power from all of the hot thermal components can be calculated by combining the stefan-boltzmann law and plank's law approximation, obtaining: (3) where, qmir is the radiative power (w), asampl is the ground sampling area (m2), f is the emissivity, which is a composite value that depends on the mixing ratio of the molten lava and crust, v is the stefan-boltzmann constant (5.67·10-8 j s-1 m-2 k-4), lq a mir sampl ,h mir mir= af fv > mean n stdt t t3.9 3.9 3.9m m m+ $n n n^ ^h h min > maxvart t t3.9 3.9 3.9m m m-n n n^ ^h h the hotsat volcano monitoring system figure 2. normalized thermal index sequence computed for the seviri data at the sec. red dots, day images; blue dots, night images. figure 3. hotspot numbers detected by the normalized thermal index algorithm (blue) and the hotsat hotspot detection algorithm (red), during may and june, 2008, for the seviri data. figure 4. radiative power computed by the mir radiance technique (red) and by the dual-band three-component technique (blue) for the seviri data during may, 2008. lmir,h and fmir are the hot pixel spectral radiance and surface spectral emissivity in the appropriate mir spectral band, respectively, and the constant a (w m-4 sr-1 nm-1 k-4) is determined from the empirical best-fit relationships. from equation (3), the radiant flux is proportional to the calibrated radiance that is associated to the hot part of the pixel, computed as the difference between the observed hotspot pixel radiance in the mir (3.9 nm) channel and the background radiance that would have been observed at the same location in the absence of thermal anomalies. to determine the validity of the mir radiance approach for volcanic eruptions, we compared it to the dual-band technique. figure 4 shows in the same plot the radiative power computed by the mir radiance approach (red symbols) and that obtained by the dual-band technique (blue symbols) for the seviri data during may 2008. these computations were made on the same sample of hot pixels without using any default solution for their saturation. indeed, the nonlinear system of two equations and two unknowns that arises from the dual-band technique was solved as an optimization problem. the results clearly show the good agreement between the two techniques, even if a solution for the nonlinear system is not always found for the dual-band method. this could lead to a loss of information. as the mir radiance method is a proportionality to the mir radiance associated to the hot part of the pixel, it does not suffer from this problem, and it is computationally more efficient. 3. case study: 2008-2009 etna eruption the latest effusive eruption on the ne flank of mount etna started on may 13, 2008, and ended on july 6, 2009, and this provided an excellent opportunity to determine the performance of hotsat in the monitoring of an ongoing eruption. the eruption was preceded by a lava fountain on may 10 from the sec, and was initiated on may 13 with a system of eruptive fissures that propagated se from the summit craters towards the western wall of the valle del bove. lava fountains erupted from a n140˚e fissure the extended between 3,050 and 2,950 m asl. in two hours, the eruptive fissure propagated downslope and southeastwards, curving n120˚e, and reached its minimum altitude of 2,620 m asl [bonaccorso et al. 2011]. meanwhile, strombolian activity was displaced from the upper segment of the fissure to its lowest portion. here, a lava flow erupted at high flow rates from two main vents, and it rapidly expanded into the barren valle del bove, where it reached about 6 km in length and the lowest altitude of 1,300 m asl in 24 hours. the most distant lava fronts stagnated about 3 km from the nearest village, milo. volcanic thermal anomalies were observed almost continuously over the same mount etna flank in agreement with the occurrence of the lava effusion. we detected the beginning of the lava fountain with seviri-hotsat on may 10 at 13:57 gmt, with an initial radiant flux of 4.73 gw. no hotspots were detected with hotsat on modis, as the two scenes were acquired the same day at 12:40 gmt (aqua), before the beginning of the lava fountain, and at 20:25 gmt (terra) after its end. instead analysis of the seviri data revealed thermal anomalies until 18:57 gmt, which reached a maximum value of about 15 gw at 14:57 gmt (figure 5). moreover, at 17:27 gmt, we measured a sharp decrease in the radiative power, which dropped by about 6 gw in 15 min. in any case, the high sensitivity of the modis sensor was significant since it detected the beginning of the eruption on may 13 at 9:50 gmt, when two hot pixels were located that corresponded to the eruptive fissures, almost one hour before seviri. indeed, the first thermal anomaly by seviri-hotsat, due to the eruption was detected on may 13 at 10:27 gmt. this is because the ground-projected area of the seviri instant field of view is more than one ganci et al. 547 figure 5. radiative power computed for the seviri data during the lava fountain that occurred on may 10, 2008. figure 6. radiative power computed for the seviri (green) and modis (violet) data during may and june, 2008. 548 order of magnitude greater than that of modis, and thus it cannot distinguish such small thermal anomalies as those that are detectable by modis [roberts et al. 2005]. radiative transfer modeling suggests that seviri should be able to detect and characterize volcanic events where the radiative power ranges from 100 mw to 1,000 mw per pixel, which implies that for the typical range of volcanic eruption temperatures (800-1,000 k), the minimum area detectable should be about 100 m2 [roberts et al. 2005]. the largest instantaneous radiant flux of the eruption was about 16 gw, which was observed with modishotsat on may 15, at 00:15 gmt. a comprehensive evaluation of the consistency of the radiant flux measurements carried out on seviri with hotsat is shown in figure 6, with respect to the nearest modis scene, during may and june, 2008. it is worth noting how the two time series agree well in confirming the robustness of the hotsat system and showing how the synergetic use of modis and seviri allows a more precise and almost continuous monitoring of the volcano. a further advantage of the multiplatform approach is related to sensor saturation problems. the saturation level for the seviri mir channel is ~335 k, and this necessarily led to errors of underestimation over the most intense phases of the eruptive activity. on the other hand, as modis has a much higher saturation temperature (~500 k, band 21), it seldom if ever suffers from the such errors. by using the consistent radiant flux obtained from hotsat on both modis and seviri images, it is possible to obtain a chronology of the eruption (that distinguishes different phases) (figure 6). there was a significant increase in the number of hotspots detected on the morning of may 13, with an evident increase also in their relative intensity. this was recognized in concordance with the opening of the new eruptive fissure on the ne side of the sec. ground observations identified the anomaly as due to the lava from the activity that started almost at the same time as the image acquisition [branca and andronico 2008]. the strongest phase was revealed by the set of images acquired by both modis and seviri up to may 16, with an increase in the thermal anomaly that was centered in the same area of valle del bove; this reached the maximum value on may 15 (figure 6). from may 16, there was a decline in the radiative power that was confirmed by a new change in the structure of the thermal anomaly, with the eastern portion of the anomaly becoming narrower and less intense. this reduction in the radiant flux, and thus in the effusion rates, continuing until june 8, and maintained an average daily value of about 1-2 gw. this was also consistent with field observations of the period [miraglia 2008]. finally, an increase in the value of the radiant flux was recorded from june 8 to june 30, which reached a maximum between june 20 and june 21. this behavior might have been due to an increase in the strombolian activity that was registered during this period. 4. conclusions we have presented here hotsat, a new automatic system that was specifically developed to elaborate volcanic thermal anomalies. the aim of the system is to integrate real-time satellite observations coming from sensors with different features, in terms of spatial, temporal and spectral resolution. our thermal system was designed to process satellite data acquired by the modis and seviri sensors to detect volcano hotspots and to estimate the associated radiative power. we introduced a new algorithm that is based on a contextual approach, and a variable threshold for the thermal monitoring of volcanoes. moreover, we computed the radiative power associated to each hot pixel through the mir radiance technique that was introduced by wooster et al. [2003] for forest fires. the ultimate goal of this study is to provide an instrument that can automatically monitor the thermal state of active volcanoes with a refreshing time of 15 min. the preliminary results for the mount etna 2008 eruption provide us with strong encouragement to improve this instrument. indeed, due to the seviri data, it was possible to observe short dynamic activities, such as a paroxysm that preceded the eruption, while with the modis images, it was possible to also detect low intensity thermal anomalies. moreover, hotsat can capture the different phases of the eruptive activity, thereby enabling a detailed chronology of the lava flow emplacement. the good agreement between the radiative power computed from the seviri and modis images confirms the reliability of this methodology, as well as the potential of the whole integrated processing chain as an effective tool for real-time monitoring and assessment of volcanic hazard. additional products should be available in the near future. the modular architecture ensures straightforward integration for our own algorithms and of those from third parties, as new modules need only to be registered within the control unit module. acknowledgements. we are grateful to eumetsat for the seviri data and to nasa for the modis data. we thank matthew blackett and sergio pugnaghi for their constructive and helpful comments that greatly improved the manuscript. this study was performed with financial support from the v3-lava project (ingv-dpc 2007-2009 contract). references branca, s. and d. andronico (2008). eruzione etna 13 maggio 2008, aggiornamento attività eruttiva ore 19, prot. int. n˚ ufvg2008/044. bonaccorso, a., a. bonforte, s. calvari, c. del negro, g. di grazia, g. ganci, m. neri, a. vicari and e. boschi the hotsat volcano monitoring system (2011). the initial phases of the 2008-2009 mt. etna eruption: a multi-disciplinary approach for hazard assessment, j. geophys. res., 116, b03203, 19 pp.; 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doi: 10.1109/igarss.2007. 4423337. miraglia, l. (2008). rapporto settimanale sull'attività eruttiva dell'etna (2-8 giugno, 2008), prot. int. n˚ ufvg2008/057. oppenheimer, c. (1991). lava flow cooling estimated from landsat thematic mapper infrared data: the lonquimay eruption (chile, 1989), j. geophys. res., 96, 2186521878. roberts, g., m.j. wooster, g.l.w. perry, n. drake, l.-m. rebelo and f. dipotso (2005). retrieval of biomass combustion rates and totals from fire radiative power observations: application to southern africa using geostationary seviri imagery, j. geophys. res., 110, d21111; doi: 10.1029/2005jd006018. vicari, a., a. ciraudo, c. del negro, a. hérault and l. fortuna (2009). lava flow simulations using discharge rates from thermal infrared satellite imagery during the 2006 etna eruption, nat. hazards, 50, 539-550; doi: 10.1007/s11069-008-9306-7. wooster, m.j., b. zhukov and d. oertel (2003). fire radiative energy for quantitative study of biomass burning: ganci et al. 549 550 derivation from the bird experimental satellite and comparison to modis fire products, remote sens. environ., 86, 83-107. wright, r., s. blake, a. harris and d. rothery (2001). a simple explanation for the space-based calculation of lava eruption rates, earth planet sci. lett. 192, 223-233. wright, r., l. flynn, h. garbeil, a. harris and e. pilger (2002). automated volcanic eruption detection using modis, remote sens. environ. 82, 135-155. wright, r., l.p. flynn, h. garbeil, a.j.l. harris and e. pilger (2004). modvolc: near-real-time thermal monitoring of global volcanism, j. volcanol. geoth. res. 135, 29-49. *corresponding author: gaetana ganci, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy; email: gaetana.ganci@ct.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the hotsat volcano monitoring system layout 6 ionospheric perturbations associated with two huge earthquakes in japan, using principal component analysis for multiple subionospheric vlf/lf propagation paths yuya ono1,2, yuichi ida1,2, yasushi kasahara1,2, yasuhide hobara1,2,3, masashi hayakawa2,4,5, alexander rozhnoi6, maria solovieva6, oleg a. molchanov6, kenji ohta7 1 university of electro-communications, graduate school of informatics and engineering, tokyo, japan 2 university of electro-communications, research station on seismo electromagnetics, tokyo, japan 3 university of electro-communications, center for space science and radio engineering, tokyo, japan 4 university of electro-communications, advanced wireless communications research center, tokyo, japan 5 university of electro-communications incubation center, hayakawa institute of seismo-electromagnetics, co. ltd., tokyo, japan 6 institute of physics of the earth, russian academy of science, moscow, russia 7 chubu university, kasugai aichi, japan annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5329 abstract the presence of ionospheric perturbations in possible association with two huge earthquakes (noto-hanto peninsula and niigata-chuetu-oki earthquakes) in 2007 was studied on the basis of a conventional statistical study for a particular propagation path from the jji transmitter in miyazaki, kyushu, to moshiri in hokkaido. this is based on automatic routine-based signal processing, in which the trend as the average nighttime amplitude is significantly decreased, with almost simultaneous significant enhancement in the night-time fluctuation as the night-time integration of negative fluctuation from the average. it is, however, shown that this routine-based signal analysis sometime suffers from artificial (or man-made) effects. thus, in this study, we propose an additional use of principal component analysis (pca) for simultaneous observation of a few vlf/lf propagation paths. with the application of this pca method to multi-path data, the artificial effects can be reasonably removed, and also only the geophysical effects associated with earthquakes are detected, by focusing mainly on the third principal component. the satisfactory separation of the principal components is made possible by pre-analysis of the vlf data (extraction from the raw data of the average over a whole year). this pca method enables us to identify the seismogenic effects in association with earthquakes with smaller magnitudes, down to m 5.5 or m 5.0. 1. introduction there has been an accumulation of a lot of electromagnetic phenomena that might be associated with earthquakes [e.g., molchanov and hayakawa 2008, hayakawa 2009a]. these seismogenic effects can be classified as from the lithosphere [e.g., fraser-smith 2009, freund 2009], in the atmosphere [biagi 2009, hayakawa 2009b] or in the ionosphere [hayakawa 2009c, liu 2009, parrot 2009]. in particular, there appears to be a consensus that the ionosphere is perturbed prior to large earthquakes, both in the lowest d/e region and in the upper f region [e.g., liu et al. 2006]. we have been recording the network observations of subionospheric very low frequency (vlf)/low frequency (lf) propagation in and around japan [see hayakawa et al. 2004, molchanov and hayakawa 2008, hayakawa 2009c]. a substantial number of studies have been carried out on the presence of seismo-ionospheric perturbations and their temporal and spatial characteristics on the basis of case studies, and also of statistical studies. case studies start from the 1995 kobe earthquake [hayakawa et al. 1996, molchanov and hayakawa 1998], and many case studies were summarized by hayakawa [2009c]. in terms of statistical studies, there have been a few reports [rozhnoi et al. 2004, maekawa et al. 2006, kasahara et al. 2008]. recently, hayakawa et al. [2010a,b] published two studies that indicated that there are significant statistical correlations of these ionospheric perturbations with earthquakes with magnitudes >6.0 and with shallow depths (d <40 km). even under these conditions, there is the need to increase the numbers of case studies of seismo-ionospheric perturbations for the huge earthquakes that have occurred special issue: earthquake precursors 139 article history received july 29, 2011; accepted september 30, 2011. subject classification: ionospheric perturbations, earthquakes, subionospheric vlf/lf propagation, principal component analysis. in japan. during 2007, there were two huge earthquakes in japan with magnitudes >6: the noto-hanto peninsula and niigata-chuetsu-oki earthquakes. by making full use of our vlf/lf network data, we provide here a case study of the seismo-ionospheric perturbations for these two earthquakes. we initially indicate the results based on conventional statistical analysis for a specific propagation path that is closest to these two earthquakes. then, to confirm the presence of these seismogenic ionospheric perturbations, we suggest the additional important analysis method of principal component analysis (pca). ten years ago, we proposed the use of pca of multi-stationed ultra-low frequency (ulf) data to identify the presence of seismo-ulf electromagnetic emissions [gotoh et al. 2002], and here this method is applied to subionospheric vlf/lf data from different propagation paths. 2. target earthquakes and vlf/lf network observations we consider the complete year of data for 2007, as this period included two major earthquakes that were strong even for japan: the noto-hanto peninsula earthquake (march 25, 2007; m 6.8, d = 8 km) and the niigata-chuetsu-oki earthquake (july 16, 2007; m 6.6, d = 12 km). figure 1 illustrates the propagation paths from the vlf transmitter with a call sign of jji (kyushu, miyazaki, 22.2 khz) to the three receiving stations, msr (moshiri, hokkaido), kck (kamchatka, russia) and tym (tateyama, chiba). one additional propagation path is plotted from another japanese lf transmitter, with a call sign of jjy (fukushima, 40 khz), to msr. their corresponding wave-sensitive areas defined by the 5th fresnel zone are also indicated in figure 1. for these propagation paths, we took earthquakes with magnitudes >5 from the us geological survey earthquake catalog, as also indicated in figure 1, that might have effects on the vlf/lf propagation. each circle in figure 1 corresponds to an earthquake, with the earthquake epicenter at its center, and with the circle size (radius) proportional to the magnitude of the earthquake. the color of the circle indicates the earthquake depth, as indicated in figure 1, and the numbers with the circles indicate the dates when the earthquakes occurred. 3. conventional statistical data analysis conventional (routine based) signal processing was applied to the propagation path from jji to msr over the one year of data for 2007, using the night-time fluctuation method [as in hayakawa et al. 2010a]. this path was the most suitable for the study of these two earthquakes. using this method, we estimated the average night-time amplitude, called the trend, the dispersion, as the night-time amplitude fluctuation, and the night-time fluctuation; the details of those parameters were described by kasahara et al. [2008] and hayakawa et al. [2010a]. all of these parameters were normalized with their corresponding standard deviations (v) over 30 days before any current day. figure 2 shows the result of an automatic (or routine based) analysis over the year 2007, for the jji-msr path. the top panel of figure 2 shows the temporal evolution of the trend, the second panel shows the dispersion, and the third panel shows the night-time fluctuation. the bottom panel of figure 2 shows the plot of rkp (the geomagnetic activity). as has already been shown statistically [hayakawa et al. 2010a], the most important parameter, the trend, was seen to decrease several days before an earthquake, while the other two parameters, as dispersion and night-time fluctuation, respectively, tended to be simultaneously enhanced prior to an earthquake. let us look at the result in figure 2. a significant decrease in the trend is seen at least before the two huge earthquakes of interest. over the week before the noto-hanto peninsula earthquake, we note a significant decrease in the trend, and correspondingly, the other two parameters are seen to be enhanced. this was apparently a precursor to this earthquake. next, we move on to the second earthquake, that of niigata-chuetsu-oki. again, a noticeable decrease (exceeding 140 ono et al. figure 1. relative locations of the two japanese transmitters with call signs of jjy in fukushima and jji in kyushu, and the vlf/lf observation stations: msr (moshiri), tym (tateyama, chiba), and kck (kamchatka, russia). the two large earthquakes of noto-hanto peninsula (march 25, 2007) and niigata-chuetsu-oki (july 16, 2007) are indicated as the two largest circles. other earthquakes are also shown by circles, with the earthquake epicenter at the circle center, the circle size (radius) proportional to the magnitude of the earthquake, the circle color indicating the earthquake depth, and the numerals indicating the date of occurrence. the ellipses show the wave-sensitive areas for each propagation path between the transmitters and receivers. 141 –3v) is seen in the trend over about one week prior to this second earthquake. the dispersion is not enhanced so much, but the night-time fluctuation is increased. this characteristic is considered to be a precursor to this second earthquake. one more thing that should be mentioned is that the geomagnetic activity was rather quiet during the vlf anomaly. another clear anomaly was noted at the end of august, until the beginning of september, 2007, which was probably related to a series of rather strong after-shocks that occurred up to the beginning of september, 2007. however, some abnormal behaviors were seen during this year (2007). for example, by looking at the temporal evolution of the trend in figure 2 (top panel), there is one depletion peak (one day) in the middle of february, and another towards the end of may, with another one-day depletion peak at the end of december, 2007. these are seen to exceed –3v in the trend, which is too strongly abnormal and they look artificial. a normal perturbation that appears to be associated with an earthquake is known to persist for at least a few days, or even more. however, of course, a one-day anomaly is also possible for an earthquake with a smaller magnitude, so that it appears difficult to distinguish between a seismogenic effect and an artificial (man-made) effect, except by using a single propagation path. when only one propagation path was considered, we compared those anomalies (probably not associated with earthquakes) with the other corresponding phenomena (solar-terrestrial and meteorological effects), to study these peaks extensively. however, we have the advantage that we have the data for a few subionospheric vlf/lf propagation paths, and this is seismo-ionospheric perturbations figure 2. the temporal evolution for the conventional routine-based analysis for 2007. top panel, trend; second panel, dispersion; third panel, night-time fluctuation (parameters normalized with their corresponding standard deviations); bottom panel, temporal evolution of rkp (geomagnetic activity). the two big earthquakes of interest are indicated at the top of the top panel (eq, downward arrows). the likely seismogenic effects are marked by red arcs in the top panel, while some unlikely anomalies are indicated by question marks. the reason why we propose the use of pca for the data analysis of these few subionospheric vlf/lf propagation paths. the purpose of the use of pca is to: (1) re-confirm the existence of seismo-ionospheric perturbations for these two huge earthquakes; (2) determine whether these one-day anomalies are really artificial or not; and (3) explore the possibility of identifying the ionospheric perturbations for even weaker earthquakes, with magnitudes down to m ~5. 4. pca and preparation of the data for pca we present some fundamental descriptions of pca, which involves a mathematical procedure that transforms a number of possibly correlated variables into a number of uncorrelated variables, called the principal components; these are related to the original variables by an orthogonal transformation. this transformation is defined in such a way that the first principal component has as high a variance as possible (that is, accounts for as much of the variability in the data as possible), and each succeeding component in turn has the highest variance possible under the constraint that it be orthogonal to the proceedings. pca is intended to identify different noise sources from multiple data. gotoh et al. [2002] made the first attempt to investigate seismogenic ulf emissions by applying pca to the multi-stationed ulf data observed at three stations on the izu peninsula, in the case of the izu islands earthquake swarm. they then found that the third principal component was highly likely to be seismogenic. we have to prepare the subionospheric vlf/lf data observed for three propagation paths. the propagation paths we chose were: 1) the jjy transmitter (fukushima) – msr (moshiri, hokkaido); 2) the jji transmitter (miyazaki) – msr; 3) the jji transmitter – kck (kamchatka, russia). this was because we are interested in the two major earthquakes that occurred in the year of 2007 on the side of the japan sea. 4.1 selection of analysis time interval the above three propagation paths were chosen based on the relative locations of the transmitters and receivers with respect to our huge earthquakes, so that the local nighttime was different from one path to another. in particular, the local night was a few hours different for the two paths of jji-msr and jji-kck. then, we chose the time interval of ut = 12-16 h (4 h) as the common night-time for all of the above three propagations, with exclusion of the terminator times, as studied by hayakawa et al. [1996]. the sampling rate of our vlf/lf receiving system was 120 s, so that we can use 120 data points (n = 120) for one day for one propagation path. as the total, we have 360 data points per day for all three of the propagation paths. 4.2. selection of the analysis method there were some differences in the propagation characteristics for these three propagation paths. so we tried the following three possible pre-analysis methods before the application of pca, in order to have appropriate and acceptable results. a) the use of the raw data. pca was applied to the raw data sampled every 120 s during the same local night-time (ut = 12-16 h) observed for the three propagation paths. b) extraction from the raw data of the night-time average amplitudes through the whole period. initially we estimated the average night-time (same ut as before) amplitude over the whole period (one year) for a particular path, and we took the difference between the current data and the average above. the same procedure was used for the other two paths, and then we applied pca to these data. c) the difference between the current raw data and the average on each current day. unlike the above procedure, we take the averages during the local night (ut = 12-16h) on each day, and we take the differences between the raw data and this average. the same procedure was applied to the other two paths, and then pca was applied to these data. 4.3. comparison of pca results for the three different methods we analyzed the data for the two years of 2006 and 2007. although the main target was 2007, we also investigated another year, as 2006, because there were not so many large earthquakes in 2006, and so it was suitable to study which pre-analysis treatment in subsection 4.2 was best for the further pca analysis. the pca results are illustrated in figures 3, 4 and 5, for the vlf/lf data in subsection 4.2 (i.e., for the pre-analysis methods a, b and c, respectively). in these figures, the eigenvalues of the first, second and third principal components (m1, m2 and m3) are plotted for the whole year of 2006. the earthquake information is given at the top of each top panel of figures 3, 4 and 5, as a downward arrow that indicates the earthquake magnitude, with all of the earthquakes with magnitudes <6.0. figure 3 is the pca result for the data as (a) in subsection 4.2 (i.e., use of the raw data). this pca was intended to separate a few possible effects, such as geomagnetic variations, seasonal variations, and earthquake effects, among others. figure 3 indicates that only the first principal component is well separated; although we noted the significant similarities in the temporal evolutions of the second and third principal components, we utterly failed to extract the second and third components. so, this preanalysis treatment (a) is not so good for pca analysis. next, we move to figure 5 using the data as (c) in subsection 4.2. a comparison of the temporal evolution of the three principal components (m1, m2, m3) suggested that they show similar temporal variations, which indicates ono et al. 142 143 apparent correlations among the three components. due to this, this method was not effective in separating two components and is not appropriate for our pca. on the other hand, figure 4 shows the pca results for the data (b) in subsection 4.2 (while considering the average over the whole period of one year), which indicates that there appears to be nearly no (or weak) correlation among the three principal components. so, the three principal components are likely to be well separated, such that in the following we adopt this pre-treatment (b) of subsection 4.2 for further pca procedures for our target year of 2007. seismo-ionospheric perturbations figure 3. temporal evolution of the eigenvalues of the three principal components (m1, m2 and m3, as indicated) for 2006, using method (a) in subsection 4.2. the information for the earthquakes (see figure 1) is indicated by the marks (top panel), according to date, magnitude (length) and depth (color). figure 4. temporal evolution of the eigenvalues of the three principal components (m1, m2 and m3, as indicated) for 2006, using method (b) in subsection 4.2. details as for figure 3. ono et al. 144 figure 5. temporal evolution of the eigenvalues of the three principal components (m1, m2 and m3, as indicated) for 2006, using method (c) in subsection 4.2. details as for figure 3. figure 6. temporal evolution of m2/m1 (top), m3/m1 (middle) and rkp (geomagnetic activity; bottom) for 2006. the information for the earthquakes (see figure 1) is indicated by the marks (middle panel), according to date, magnitude (length) and depth (color). 145 4.4. analysis data and analysis results now we come back to figure 4 to discuss this figure carefully, because the separation of the principle components is best using the data treatment as (b) in subsection 4.2. the ordinate indicates the intensity of each principal component. from the top, there are the first, second and third principal components. when we have no plots, it means there are no data. when we focus on the first principal component (m1), it tends to increase from spring to summer, decrease from summer to autumn, increase from autumn to winter, and decrease from winter to spring. the occurrences of the earthquakes are indicated along the top of the top panel in figure 4. it appears that the first principal component does not depend on the earthquakes. then, for the second principal component, the value of m2 is seen to be enhanced in february, march, november and december. however, we cannot judge whether m2 is correlated with any earthquakes or not. finally, the third principal component (m3) was enhanced in november and december, although it cannot be certain whether m3 in figure 4 is correlated with earthquakes, because the earthquakes for this year were too weak (magnitudes <5.5). to study the detailed temporal evolution of m2 and m3, in figure 6 we used the conventional ratios of m2/m1 (upper panel), m3/m1 (lower panel) and rkp (bottom). the ratio of m2/m1 shows enhancements in its value in february-april and in november-december. by comparing these temporal variations of �m2/m1 with the corresponding variation of rkp (geomagnetic activity) in the bottom panel of figure 6, we can imagine that the temporal evolution of m2/m1 approximately reflects the variation in the rkp index (albeit, not so perfectly). thus, we can regard the ratio of m2/m1 as an indicator of the geomagnetic activity. on the other hand, the second ratio, of m3/m1, in figure 6 is relatively quiet during the whole of this year, as compared with the variation of m2/m1. when the earthquake epicenter was located within the fresnel zones of a few propagation paths, we can easily identify the ionospheric perturbation. when we focus on earthquakes in figure 1 for which two paths from the chosen three are within the relevant fresnel zones, the ratio of �m3/m1 shows significant seismo-ionospheric perturbations figure 7. temporal evolution of the eigenvalues of the three principal components (m1, m2 and m3, as indicated) and rkp (geomagnetic activity; bottom) for 2007 (data for november and december 2007 are missing). details as for figure 3. enhancements 2-10 days before an earthquake. this is true for three earthquakes out of the four. so the ratio of m3/m1 is considered to reflect the earthquake signature well, even though there appears to be some effects of m2. figure 7 is the result of the pca analysis for 2007 (november and december data are missing), the year of our main interest. this is based on the data treated as for (b) in subsection 4.2, because we have already seen that the separation into the first to third principal components is best, as compared using the corresponding results for the previous year of 2006 for the other two data analyses, as (a) and (c) in subsection 4.2. when we look at the temporal evolution of the first principal component (i.e., the eigenvalue of the first principal component (m1)) at the top panel of figure 7, we see that the value of m1 does not fluctuate very much, and that it has no relationship to any of the earthquakes. next, we look at the temporal evolutions of the eigenvalues m2 and m3 of the second and third principal components in figure 7. the second principal component is enhanced during january and through march, to vary similarly and to increase before the two huge earthquakes (noto-hanto peninsula earthquake and niigata-chuetsu-oki earthquake; i.e., from 1~10 days before these earthquakes). to enhance the variations of m2 and m3 more clearly, figure 8 illustrates the temporal evolutions of the ratios m2/m1 (top panel) and �m3/m1 (second panel), and of rkp (bottom panel). as was already seen for the previous year of 2006 in figures 5 and 6, the ratio of �m2/m1 for 2007 is relatively disturbed over the whole year, although the ratio of m3/m1 is relatively stable. in other words, the variability of m2/m1 is much larger than that of �m3/m1. a comparison of the temporal variation of m2/m1 with the rkp index, might suggest a relatively close resemblance between these two. on the other hand, the ratio of m2/m1 shows nearly no correlation with the variation of the rkp index. when we focus on the noto-hanto peninsula earthquake (on march 26, 2007), we can see a clear enhancement in the ratio of m3/m1 one to two weeks prior to the earthquake. a similar enhancement in m3/m1 takes place a few days before the niigata-chuetsu-oki earthquake on july 16, 2007. similar tendencies for enhancements in m3/m1 are noted for some other earthquakes with smaller magnitudes, of the order of ono et al. 146 figure 8. temporal evolution of m2/m1 (top), �m3/m1 (middle) and rkp (geomagnetic activity; bottom) for 2007. details as for figure 6. 147 5.0. the larger the earthquake magnitude, the larger the enhancements in the ratio of m3/m1. 5. concluding remarks to further define the presence of seismo-ionospheric perturbations on a particular subionospheric vlf/lf propagation path using our conventional night-time fluctuation analysis (trend, dispersion, and night-time fluctuation) [e.g., muto et al. 2009, kasahara et al. 2009, hayakawa et al. 2010a], we proposed the use of pca. this can make use of our advantage of simultaneously having a few propagation paths, which might be an impetus to vlf/lf propagation studies. in one sense, we would like to accumulate more evidence on the existence of seismoionospheric perturbations for huge earthquakes in japan as case studies (in the present case, for two earthquakes, the noto-hando peninsula earthquake and the niigata-chuetsuoki earthquake, both in 2007 and with magnitudes >6.0). to cover the above two huge earthquakes as our target, we used the data from three vlf/lf propagation paths (jjymsr, jji-kck and jji-msr). initially, we compared three different pre-analysis methods for our vlf/lf data, to determine which pre-analysis is the best for the independent separation of the three eigenvalues of the pca application. as the result, the extraction of the average amplitudes from the raw data over the whole year indicated that the three eigenvalues were well separated. then, we found that the ratios of m2/m1 and m3/m1 are more suitable for the physical discussion. the ratio of �2/�1 reflects the variations in the geomagnetic activity (rkp index) well, whereas the ratio of m3/m1 appears to be an indicator of earthquakes. by looking at the temporal evolution of m3/m1, we can definitely conclude that there are definite ionospheric perturbations before these two major earthquakes of our present interest (noto-hando peninsula earthquake and niigata-chuetsu-oki earthquake), that had large magnitudes (>6.0) and shallow depths. these perturbations are confirmed as precursors to these earthquakes. the values of m3/m1 for these earthquakes are well pronounced due to their huge magnitudes, and we can conclude that the use of pca based on multiple propagation paths allows us to re-confirm the presence of seismoionospheric disturbances detected by the conventional statistical approach for a particular path. also, we can reject the anomalies seen in the conventional analysis (figure 2), which are likely to be artificial. finally, it becomes possible for us to identify the ionospheric perturbations in association with earthquakes with smaller magnitudes, down to m ~5, even though the peak value of m3/m1 is less enhanced than that for the huge earthquakes used as our targets. acknowledgements. this study was supported in part by national information and communications technology, japan, and by fp7 grand semep, russia, for which we are grateful. references biagi, p.f. 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(2009). earthquake precursors observed in the ionospheric f-region, in: electromagnetic phenomena associated with earthquakes, edited by m. hayakawa, transworld research network, trivandrum (india), ch. 7, 187-204. liu, j.y., y.i. chen and y.j. chuo (2006). a statistical study of pre-earthquake ionospheric anomaly, j. geophys., res., 111; doi: 10.1029/2005ja011333. maekawa, s., t. horie, t. yamauchi, t. sawaya, m. ishikawa, m. hayakawa and h. sasaki (2006). a statistical study on the effect of earthquakes on the ionosphere, based on the subionospheric lf propagation data in japan, ann. geophysicae, 24, 2219-2225. molchanov, o.a. and m. hayakawa (2008). seismo-electromagnetics and related phenomena: history and latest results, terra sci. pub. co., tokyo., 189 pp. molchanov, o.a. and m. hayakawa (1998). subionospheric vlf signal perturbations possibly related to earthquakes, j. geophys. res., 103, 17489-17504. muto, f., y. kasahara, y. hobara, m. hayakawa, a. rozhnoi, m. solovieva and o.a. molchanov (2009). further study on the role of atmospheric gravity waves on the seismo-ionospheric perturbations as detected by subionospheric vlf/lf propagation, nat. haz. earth sys., 9, 1111-1118. parrot, m. (2009). anomalous seismic phenomena view from spaces, in: electromagnetic phenomena associated with earthquakes, edited by m. hayakawa, transworld research network, trivandrum (india), ch. 8, 205-234. rozhnoi, r., m.s. solovieva, o.a. molchanov and m. hayakawa (2004). middle latitude lf (40 khz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, phys. chem. earth, 29, 589-598. *corresponding author: masashi hayakawa university of electro-communications (uec), research station on seismo electromagnetics, tokyo, japan; e-mail: hayakawa@whistler.ee.uec.ac.jp © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ono et al. 148 csontos_final:layout 6 how to control a temporary didd based observatory in the field? andrás csontos1,*, danijel šugar2, mario brkić2, péter kovács1, lászló hegymegi1 1 eötvös loránd geophysical institute, budapest, hungary 2 university of zagreb, faculty of geodesy, institute for geomatics, zagreb, croatia annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5447 abstract one of the main challenges on the course of the repeat station surveys is to determine the spatial differences of the geomagnetic elements between the repeat stations and the reference observatory. the difficulty arises from the fact, that the directly obtained differences are affected not only by spatial but also by temporal effects of external origin. the error deriving from the external effects can be efficiently diminished by the installation of an on-site vector variometer. in this case the spatial difference can be computed for night-time period, when the external field is less varying (both spatially and temporally) than during daytime. installation of the on-site variometer in the field requires the fulfillment of nearly the same conditions as in the observatories, i.e. the control of the reference frame, the scale factors, the offsets, and the temperature effects of the magnetometer. the principle of the fluxgate and didd magnetometers is quite different from each other, therefore the two devices provide different possibilities to obtain accurate result. the paper discusses some of the possible instrumental errors and offers a method based on the didd technology for the determination of the reference frame of a portable recording station. we analyse real records measured during the joint hungarian-croatian repeat station survey. 1. introduction geomagnetic observatories provide high quality records of the magnetic field elements continuously, with high temporal resolution. the density of the observatories however is not enough to monitor the relevant variation of the internal magnetic field in spatial scale. therefore, decades ago, repeat station (rs) networks were established in the different countries with the aim of periodically monitoring the secular magnetic variation in uniform spatial distribution. during the reoccupations of repeat stations a series of absolute magnetic data are produced. the measurements are carried out according to the recommendations of iaga and magnete. in order to be able to temporally compare the secular data measured in different epochs, the measurements are reduced to common epoch using the permanent record of a reference observatory. on the course of the temporal reduction it is assumed that (1) the secular variations and (2) the transient magnetic variations are identical at the repeat station and the reference observatory [newitt et al 1996, korte 2001]. the first assumption is generally valid if the time between the reoccupation of the secular station and the epoch of reduction is less than half a year. the identity of the transient variations of external origin however is seldom satisfied. the spatial and temporal dependence of the transient field variations are basically affected by the following regularities: a) the variation of the north magnetic component increases with the geomagnetic latitude; b) the spatial variation of the east transient magnetic field is basically affected by the longitudinal distance from the reference observatory; c) recombination of the ring current developing during geomagnetic storms can result in long-lasting (typically more than 3 days) drift in the differences of corresponding components measured in different sites; d) the variation of the vertical transient field is influenced by the spatial variation of the electrical conductivity of the subsurface through the telluric currents; e) near the seaside extra magnetic field is produced by high conductivity contrast between of the ocean and the mainland; f ) the tectonomagnetic effect caused by mechanical stress may result in extra magnetic field in certain sites as compared to the reference observatory. the influences of the most part of the above effects on the accuracy of the temporal correction can be minimized by completing the magnetic secular measurements in quiet geomagnetic conditions, i.e. in periods void of geomagnetic storms and in night-time when the solar quiet magnetic variation is the most stable. it must be emphasized however that some of the listed magnetic phenomena (e.g. tectonomagnetic variation) are independent from the geomagnetic circumstances, thus their effects in the temporal corrections are completely unavoidable. nowadays, the practice of the field absolute measurements are based on the absolute instruments (dim, ppm), article history received october 14, 2011; accepted june 27, 2012. subject classification: instruments and techniques, main geomagnetic field, geomagnetic field variations and reversals, magnetic anomalies, magnetic storms. 1085 magnete workshop 2011 since they are basically free from calibration errors and temperature effects (at least theoretically). the automatic operation of the dim device for providing permanent declination and inclination values in a longer period however is not yet applied in the field. the condition for the quiet-time observation can thus be rarely satisfied in the practice of the repeat station campaigns, since the measurements must be carried out in daytime when the bearing azimuths are visible. to extend the observations to the more quiet night-time period it is suggested then to install a tri-component portable variometer in the vicinity of the repeat station. the closeness is necessary for ensuring the identity of the geomagnetic variation in the repeat station and the variometer site. by establishing the basis of the variometer through the absolute measurements the variometer record can be referenced to the repeat station. thus, the station can function as a temporary observatory, provided that the setup of the variometer is prudently completed and the accuracy parameters (e.g. orientation, scale factors, temperature of the device) of the instrument are continuously controlled. in this paper we present the most important behaviours of two classical observatory variometers, i.e. the fluxgate and didd magnetometers. we argue that for field use the didd is superior than the fluxgate instrument. for this reason, we summarize the way of installation of the didd device in field environment and provide method for the determination of its calibration parameters. finally, the calibration results are shown in case studies carried out in the tihany observatory and in repeat stations of hungary and croatia. 2. the fluxgate magnetometer the principle and the operation of the fluxgate (fge) instrument are described by jankowski and sucksdorff [1996]. the biggest advances of the fluxgate devices are the low noise and the relatively high frequency response. for the calibration of the device twelve parameters have to be determined (three offsets, three scale factors, three orthogonality errors of the sensors and three orientation errors). marsal et al. [2009] described the on-site calibration procedure of the fge magnetometer. from this it is evident that the completion of the whole process is unrealistic during the reoccupation of a repeat station. namely, the absolute measurements made in the field survey can only calibrate the offset values of the fge instrument, while the possible tilt of the sensor modifying the orientation of the device as well as the scale factor of the instrument remain undetermined. the usual comparison of the total field values measured by the ppm instrument and calculated from the three-component variometer record may also help to further correct the variometer data. however, since the contribution of the y magnetic field in the calculated total field is practically negligible, the sensitivity of the above method is very limited regarding especially to the east component. the fluxgate devices need a priori and a posteriori check in a reference observatory. during this process the data of the test instrument is compared with the observatory records during a certain period [newitt et al. 1996]. it is noted that this procedure is necessary but still not enough for satisfying the requirements of an exact calibration. the temperature effects of fluxgate magnetometers were presented by newitt et al. [1996] in detail. our temperature tests in the tihany geophysical observatory prove that the most of the fluxgate magnetometers have large temperature coefficients and their behaviour depends significantly on the amplitude of the temperature change. thus, the determination of a general temperature coefficient, as a correction factor is very limited [csontos et al. 2007]. there are further possible sources of errors of the fluxgate devices, such as different type of orientation errors, orthogonality errors, voltage dependence of electronics, etc. usually, these effects appear simultaneously. unfortunately, there are no general solution for separating these errors and correct the dataset afterwards. for the case of the data reduction, the errors caused by the unknown calibration parameters of a fluxgate device may be occasionally higher than that caused by the non-identical external field variation between the station and reference observatory. anyway, the recommendations of the book guide for magnetic repeat station survey [newitt et al. 1996] provide a useful help how to reach the best result with fluxgate magnetometers. 3. the didd system and its operation 3.1. the technical details of the didd system with its theoretical background the didd instrument consists of a double coil system whose axes are perpendicular to each other. the axis of the d coil is horizontal and lies nearly at right angle to the local mean magnetic n-s direction, while the i coil axis lies in the local mean meridian plane nearly at right angle to the magnetic field. in the geometrical center of the system there is a sensor capable of measuring the total magnetic field. on the course of the operation of the didd parallel and antiparallel currents of same strength are introduced into each of the coils producing deflection field of about 10000 nt along the axis of the two coils. from the values of the deflected total field readings, the declination and inclination components can be concluded. the prototype of this system, designed in the late 1960's, used a helmholtz coil system with a proton or optically pumped rubidium and cesium sensor. cooperation on a spherical coil, approximately 30 cm in diameter, began in the 1990s between elgi and usgs. the product of this joint work was a molded spherical coil system made from epoxy resin. the development of the overhauser magnetometers enabled to construct the fast didd system. csontos et al. 1086 1087 an ideal solution is gem's overhauser magnetometer with a built-in current generator. the cycle time of one reading is 1 or 0.5 second and the resolution of the overhauser magnetometer is 0.01 nt [csontos 2001]. a new overhauser sensor with 150 mm in diameter was developed in 2002. for this sensor b. heilig modeled a new spherical coil system which had a diameter of 200 mm. this size was sufficiently small to construct a suspension device and to build a user friendly magnetometer. the demand for the field use of the system required the development of the maglin data acquisition software enabling the remote control and remote checking of the data record via internet. the software was running under linux operation system. this solution was flexible and stable enough to run on industrial pc which needed only 5w. the total consumption of the electronics of the didd, the gps receiver and the pc was not more than 10w so a 88ah battery was far enough for the period of a short-term (maximum a week) reoccupation. the phases of the development and tests of the instrument were repeatedly presented [see, e.g., csontos et al. 2001, hegymegi et al. 2004, csontos et al. 2007]. hegymegi et al. [2009] suggested that the capabilities of the instrument and its recording system developed by elgi were ready for field work. one cycle of didd measurement, i.e. one determination of the magnetic components, consists of 5 readings on the ppm in the following order: f (total field without deflection) d+ (total field deflected parallel to the d axis) d− (total field deflected antiparallel to the d axis) i+ (total field deflected parallel to the i axis) i− (total field deflected antiparallel to the i axis) the advances of this instrument can be summarized in the following points: since the didd system is based on the function of a nuclear magnetometer the instrument can be qualified by the usual parameters of the nuclear magnetometers, i.e. accuracy, sampling interval etc. it also follows, that the scale factor and the offset do not need to be calibrated. only the following four values have to be calibrated for the determination of the reference frame of the device [heilig 2007]: i) i0 value ii) d0 value iii) orthogonality error of magnetic axes iv) the leveling error of the d coil. the didd reference frame is defined physically by the d coil and i coil axes. they should be orthogonal and the d coil should be horizontal, in case of the perfect alignment of the instrument. once we know the i0 and d0 angles of a perfectly adjusted didd device, its reference frame is also established. several solutions were developed for establishing the correct adjustment of the coil system [see alldredge 1960, csontos et. al. 2001]. the published methods were suitable to align the coil system but the continuous monitoring of the reference frame was not ensured. further uncertainty of these methods can be the implicit assumption that the mechanical and the magnetic axes of a coil are parallel. mathematical solutions were also published to determine the reference frame of the didd system by using a dim [schott et al. 2001, schott and leroy 2001, heilig 2007] or a pre-calibrated variometer [heilig 2007] as an independent reference instrument. the introduction of a new method for measuring the orthogonality error of the didd [heilig 2012] was another important improvement. the method for monitoring the orthogonality of the didd gives us the possibility to apply a way to monitor the reference frame of a didd by using the results of absolute measurement. we used the new practices during our repeat station survey. 3.2. installation of the device in the field the installation of the didd is simple provided that the gradient of the geomagnetic field is low. the device is relatively small and the suspension eliminates automatically the tilt of the pillar. further advance is that the readings give a perfect basis for the correct orientation. the material of the coil system and other suspended parts are moulded from the mixture of epoxy resin and fibreglass. the other parts of the mechanic are made from aluminium. recommended steps for the complete installation: 1) choose a hidden and shaded place near the repeat station, e.g. bush, forest or a cave etc. the distance between the didd and other recording sensors should be minimum 10 meters. if the spatial gradient of the geomagnetic field is high or not constant (typically in conductivity anomalous or volcanic area) try to minimize the distance between the repeat station and the didd site. 2) check the gradient of the total field at the selected site to avoid the effects of any ferromagnetic materials on the measurements. less than 3 nt/m gradient is acceptable. (anomalous, for example basaltic, area requires special solutions, i.e. the observer has to measure the difference of the geomagnetic elements between the repeat station and the place of didd.) 3) place the didd insturment on a nonmagnetic levelled hard plate (e.g., marble) (see figure 1). 4) level the instrument by using its bubbles. 5) adjust the orientation of the coil system by observing the difference of the corresponding deflected components and test the direction of the deflected components. see the user manual [didd v.7.0] listed in the references for more details. 6) cover the sensor by a water resist and temperature isolator cylinder and put a heavy nonmagnetic weight on the top of it. 7) put the battery, the electronics and the data recording pc in a plastic box, then connect the sensor and start the how to control a temporary didd based observatory in the field? recording. the distance between the box and the sensor should be more than five meters. 8) after the first set of absolute measurement calculate the preliminary i0 and d0 values and modify the initialization file of the data logger. 9) measure the orthogonality error of the didd. in this process the d and i coils have to be connected in series to ensure the same current in both coils. the current is supported by the built-in current generator of the electronics. tune the magnetometer first to the field magnitudes to be measured. then, record the following 5 s long sequences of reading: f, d+i+, d−i−, f, f, and f, d−i+, d+i−, f, f, where d+i+, d−i−, d−i+, d+i− mean the total field values deflected by the i and d coils jointly in parallel or antiparallel sense. this measurement should be repeated at the end of the reoccupation. 10) in order to be able to determine the didd's reference from the dmi measurements (see next section) it is necessary to measure the difference of the corresponding magnetic components between the didd and the repeat station sites. note, that this step can be avoided if the field gradient between the repeat station and the didd site is expected to be low. the measurement can be performed in the following way: mark the original place of the didd and setup the didd instrument on a temporary site. it can be supposed that the leveling and orthogonality errors do not change significantly during the moving. take the absolute measurements on the repeat station and on the original didd site, and then compute the bases of the didd device from each measurement. the difference between the obtained base values rely on the field difference between the original place of the didd and the repeat station. for that it is supposed that the reference frame of the didd remains stable during the absolute measurements. 3.3. calculation of the reference frame with comparison of the dim and didd results if the orthogonality error of the didd system is known, the measured (bi, bd, bs) magnetic elements can be transformed to an orthogonal reference frame where the components are denoted by bic, bdc, bsc. then, the ordinary x, y, z (north, east, vertical) magnetic components can be obtained by the following equation system [heilig 2007]: x = bsc cosd0 cosi0 − bdc (cosd0 sini0 sinf0 + sini0 sinf0) − − bic (cosd0 sini0 cosf0 − sind0 sinf0) y = bsc sind0cosi0 − bdc (sind0 sini0 sinf0 − cosd0 sinf0) − − bic (sind0 sini0 cosf0 + cosd0 sinf0) z = bsc sini0 + bdc cosi0 sinf0 + bic cosi0 cosf0 where f0, i0 and d0 are the three unknown orientation angles of the didd system (see 3.1). after determining the x, y, z absolute magnetic components in a certain time, t, and at the place of the didd device, the above equations are eligible to deduce the orientation angles. the absolute components are measured by dmi instrument at the repeat station pier and referenced to the didd site by using the previously determined component differences between the two places (see previous section). in our work the (1) equation system was solved by the fsolve and fminsearch functions of the gnu octave software. in order to have a better approximation of the real orientation angles, it is recommended to use more than one set of absolute components measured at times ti. in that case we have more equations than unknowns, and the solutions and uncertainties of the orientation angles can be obtained accsontos et al. 1088 figure 1. the didd device on levelled marble plate in krbavsko polje station. the green cylinder on the the left is the instrument case which protects the instrument against humidity and improves the temperature stability. figure 2. the repeat stations where the didd device was applied (from south to north palagruža, sinjsko polje, krbavsko polje and aggtelek). 1089 cording to the method of least squares. to increase the reliability of the numerical solutions, it is advisable to carry out the absolute measurements in those ti times between that the magnetic field exhibits considerable variation. the numerical tests of the method evidenced that for the determination of the f0 levelling error with an accuracy of 10 arc second, the absolute components must be measured with 0.01 nt accuracy. it is noted however, that this accuracy can be seldom achieved, especially in the case of the y component. moreover, the y component is barely sensitive to the leveling error of the didd in comparison with the x and z components. for this reason, it is recommended to use only the most reliable x and z components in the (1) equation system for the determination of the orientation parameters, if more than one series of absolute measurements are available, i.e. if more than three equations can be formulated from the results of the two components. the certainty of a given solution can be tested by fixing the measured and computed difference of a given component below a certain limit, while allowing growing the residual of the other component without bound. then, the constraint is applied to the other component. if the adjusted parameters do not change considerably in the two cases, the solution can be regarded reliable. 4. examples of real measurements – case studies the methods described above enable one to calibrate the didd system in terrain environment. in the following, some of the real surveys carried out with the application of the portable didd magnetometer are briefly introduced in order to present the limit of the calibration method and suggest additional guides for the observers. some of our experiences are related to the results of the joint croatian-hungarian repeat station survey carried out in 2010. figure 2 shows the repeat stations for which the portable magnetometer was applied in the data processing. further experiences with the application of the portable didd instrument were gathered in the tihany geomagnetic observatory in the year of 2011. this test provided excellent occasion for the comparison of the records of the portable didd and the observatory base system. 4.1. instrument test in the tihany geophysical observatory the portable didd system was setup in the garden of the observatory. we used the instrumentation and infrastructure of the observatory (i.e., zeiss 010a theodolite equipped by dmi d&i one component fluxgate device and gsm-19 overhauser magnetometer) for the absolute measurement to ensure the best quality of the test. the difference in the magnetic h and z components measured between the absolute pillar and the didd site was found to be 5.5 nt and 10 nt, respectively. (for the case of the east magnetic component we estimated 5 nt of difference.) we used a prototype of gem system didd device, that measured the orthogonality error of the coil system automatically at every midnight (see figure 4). in order to ensure the best preconditions for the calibration of the didd (see previous section), the absolute measurements were performed in periods when the normal daily geomagnetic variations exhibited extreme values (see figure 3). for the determination of the orientation values we applied the method described in section 3.3. as a result of the long series of absolute measurements not only one set of reference frame parameters but also its temporal evolution could be monitored (figures 5, 6, 7) for the test period. the parameters were established for noon and midnights by using two sets of absolute components obhow to control a temporary didd based observatory in the field? figure 3. the absolute values of the x magnetic field observed on the absolute pillar of tihany observatory in the test period of the didd device. tained from consecutive morning-afternoon or afternoonmorning measuring series. the method requires high accuracy in the dmi measurements and stable magnetic field difference between the didd and dmi sites. the possible errors of dim measurement or the varying spatial gradient can result in unrealistic output of the numerical iteration. since the determination of the east magnetic component may exhibit unacceptable uncertainty, our numerical analyses were based only on the x and z component absolute magnetic determinations. the solutions for the reference frame parameters were computed for the conditions of |xmeas. − xcomp.| < 0.1 nt or |zmeas. − zcomp.|< 0.1 nt where the meas. and comp. indices mean the measured and computed (equation 1) component values (see previous section). the obtained results are shown on figures 5-7 for both cases. it was concluded that the reference frame parameters were robust against the applied conditions, thus the obtained solutions were found to be reliable. the mean values (± standard deviation) of the reference frame parameters of the didd system calculated for all days of the tihany test period are: i0 = 63.5146˚ ± 0.0008˚; d0 = 3.8631˚ ± 0.0047˚; f0 = 0.7764˚ ± 0.7151˚; fid = 0.05˚ ± 0.0157˚. the final baseline of the didd system is presented in figure 8. csontos et al. 1090 clockwise from top left: figure 4. the measured orthogonality error of the didd system; tihany observatory 31.08.2011-02.09.2011. figure 5. the estimated i0 parameter of the didd system computed with the conditions of |xmeas. − xcomp.| < 0.1 nt (blue dots) or |zmeas. − zcomp.|< 0.1 nt (pink dots); tihany observatory 31.08.2011-02.09.2011 (see text). figure 6. the estimated d0 parameter of the didd system computed with the conditions of |xmeas. − xcomp.| < 0.1 nt (blue dots) or |zmeas. − zcomp.|< 0.1 nt (pink dots); tihany observatory 31.08.2011-02.09.2011 (see text). figure 7. the estimated levelling errors of the didd system computed with the conditions of |xmeas. − xcomp.| < 0.1 nt (blue dots) or |zmeas. − zcomp.|< < 0.1 nt (pink dots); tihany observatory 31.08.2011-02.09.2011 (see text). figure 8. the adopted baseline of didd device in the tihany observatory. 1091 it can be concluded that the baselines of the instrument fulfill the iaga requirements [newitt et al. 1996], i.e. the accuracy of the measurement is better than 5 nt. the monitoring of the difference between the didd and observatory fluxgate records of corresponding components provides further useful information on the proper operation of the didd instrument. for the case of the z magnetic component the obtained difference curve is shown in figure 9 (blue curve). the curve exhibits slow drift and fine fluctuation. the harmonic trend is more or less similar than that of the temperature variation (red curve) recorded in free air. however, the small amplitude of the harmonic variation affirms that the didd system has low temperature sensitivity. on the other hand the strong correlation between the red and blue (i.e., temperature and z difference) curves shows that both of the investigated magnetometers (i.e. the didd and the fluxgate system of the observatory) are well calibrated. (we should see more fluctuations in the difference curve which would refer to the geomagnetic variation if the instruments have calibration errors.) 4.2. the result of krbavsko polje repeat station in the dinaric mountain in the middle of july 2010 four days long reoccupation at the krbaysko polje (krbp, see figure 2) repeat station was carried out. the portable didd system was installed in the nearest forest, about 40 m away from the repeat station. the absolute instrument was a zeiss 20a theodolite equipped by dmi d&i one component fluxgate sensor. the total field was measured by gsm-19 magnetometer installed on an auxiliary point. a lot of series of absolute measurement how to control a temporary didd based observatory in the field? figure 9. difference of vertical components between the preliminary data of tihany observatory and the didd record (blue) and the environmental temperature (red). figure 10. the adopted baseline of didd near the krbp station. were performed in the morning and in the evening periods. we measured the orthogonality error of the didd system at the beginning and at the end of the reoccupation. the average of the measured values were 3.56 and 4.02 arc minute, thus the accepted value for fid was fid = 0.063˚ ± 0.0054˚. the total field difference between the didd site and the repeat station was less than 4 nt. the small spatial gradient enabled us to conclude the component differences, too. (see figure 10 for the final result). in this case we can suppose that the main difference between the sites is mostly caused by the difference of the vertical field component (induced magnetic field). this assumption is usually very close to the reality so it does not generate significant errors. for the determination of the reference frame parameters of didd by using equation 1, we selected those dim measurements where the difference of the inclination, declination and the vertical component was maximal. because the spatial gradient of the y component was not constant, the numerical iteration was performed only with the x and z component measurements. the mean values of the orientation angles (± standard deviation) calculated for all selected dim results were: i0 = 60.625˚ ± 0.0007˚; d0 = 2.9715˚ ± 0.0097˚; f0 = 0.4115˚ ± 0.4333˚. the obtained baselines of the didd system are shown in figure 10. figure 11 shows the x component difference between krbp repeat station and tihany observatory during the three and a half day of operation of the didd instrument. it is seen that the peak to peak difference of the curve is about 10 nt in the three consecutive days. on the other hand the night-time differences remain in 2 nt wide band. it means that the quiettime component differences between the rs site and the reference observatory can be used for the final data reduction. during the data processing it was realized that the east component baseline of the didd device was not stable and the difference between the total field values recorded at the site of the didd and the auxiliary point also showed significant fluctuations. the characteristic of these fluctuations was more or less identical with that of the fluctuations of the horizontal geomagnetic field. a direct relation was also realized between the temporal changes of the total field differences and the east component base values of the didd system (see figure 12). for the cases of the other components such relation was not evident. it suggests that the observed variation in the total field spatial gradient was mostly caused by the temporal variation of the east component spatial gradient. one of the possible explanations for the observation of the novel temporal change of the magnetic field gradient csontos et al. 1092 figure 11 (above). the difference of x component magnetic field recorded between krbp repeat station and tihany observatory (minute mean values). figure 12 (bottom page). the y base values of the didd vs. the total field difference measured between the repeat station and the didd site at krbp station between 20.07.2010. and 23.07.2010. 1093 can be the significant conductivity contrast between the adriatic sea and the mainland that can result in spatial field variation in small spatial and temporal scales, even in large distances [soyer 2002]. this phenomenon was found to be more profound near the coast, on the sinjsko polje and palagruža repeat stations (figure 2). here, the strong variation of the gradient field disabled the application of the method elaborated for the determination of the calibration parameters of the didd device and, consequently, the accurate determination of its basis. it is noted however, that even in these stations, the average of the base values determined from long series of absolute measurements provided the chance to establish accurate quiet night-time magnetic field differences between the rs-s and the reference observatories (i.e., tihany) for the final data reduction. 4.3. reoccupation of the aggtelek station (hungary) on the course of the 2010 repeat station survey, the reoccupation of the aggtelek site was carried out in november 9 and 12, 2010. the portable didd instrument was installed 2 km away from the station, in the baradla cave, where the limestone environment and the low temperature variation provided ideal circumstances for the variometer recording. the recording started before the first and ended after the second reoccupation of the station. unfortunately, the scatter of the dim results was relatively high during each of the reoccupation (see figure 13), because the aggtelek station was located in a muddy area, and the tripod of the theodolite could not be fixed safely during the measurements. however, we think that the means of the absolute magnetic components obtained from several series of measurements were reliable enough to deduce the how to control a temporary didd based observatory in the field? figure 13. the adopted baseline of didd in the aggtelek station. the crosses mean the bases belonging to the individual absolute measurements. figure 14. the difference of x component magnetic field between the aggtelek repeat station and tihany observatory (minute mean values). orientation values of the didd variometer. this conjecture was also affirmed by the adopted baselines of the didd (figure 13) whose drifts exhibited less than 1 nt of change between the two reoccupation dates, for each component. on the other hand the component differences obtained between the two reoccupations (dx≈ 18 nt, dy≈ −10 nt, dz≈ −6 nt) were relatively low, which fact was quiet unfavorable for the numerical determination of the orientation parameters. the admitted values of the parameters were: fid = 4.51 arc minute, f0 = 18.45 arc minute, i0 = 65˚ 9.64', d0 = 4˚ 2.18'. finally, we present the difference of x component magnetic field recorded between the aggtelek repeat station and tihany observatory (figure 14). interestingly, the peak to peak variation of the difference curve was smaller in aggtelek than in the krbavsko polje station against the fact that the two stations are located in a similar distance from the tihany observatory. moreover, during the aggtelek survey the geomagnetic field was also disturbed by a moderate magnetic storm. it is suggested that this finding was related to the latitudinal and longitudinal locations of the two stations in comparison to reference: aggtelek is located more towards east, krbavsko polje towards south form the reference observatory. 5. conclusions in this paper it was argued that the mechanical stability and low temperature sensitivity of the didd device provide ideal preconditions for its application as a portable device. the data recording system and the presented method of installation of the device are suitable for the perfect field application. the mathematical tools elaborated for the determination of the calibration parameters of the instrument can be efficiently applied not only in the observatory practice but also during the repeat station surveys. it was emphasized, however, that the numerical methods, when applied in rs surveys, require big stability of the spatial field gradients between the didd site and the repeat station pier. acknowledgements. this work was supported by a croatianhungarian joint project funded by the hungarian science and technology. we thank the aggtelek national park for the permission to install our portable variometer in the baradla cave. the didd system was provided by mingeo ltd. and gem systems inc. geomagnetism group of usgs is thanked for the useful remarks. we thank lászló merényi for the software development of the data acquisition system of the didd device. technical help in the configuration of equipment was supported by lászló szabados. balázs heilig is specially acknowledged for valuable discussions, remarks and his participation in the reoccupation of the aggtelek station. references alldredge, l.r. (1960). a proposed automatic standard magnetic observatory, j. geophys. res., 65, 3777-3786. csontos, a., l. hegymegi, b. heilig and a. körmendi (2001). the results of the delta "i" delta "d" (didd) measurement system at the tihany geomagnetic observatory of elgi, contributions to geophysics and geodesy, 31 (1), 83-89. csontos, a., l. hegymegi and b. heilig (2007). temperature tests on modern magnetometers, publications of the institute of geophysics polish academy of sciences, c-99 (398), 171-177. didd v7.0 (2008). instruction manual release 7.4, gem systems, 50 pp. hegymegi, l., b. heilig and a. csontos (2004). new suspended didd magnetometer for observatory (and field?) use, in: proceedings of 11th iaga workshop on geomagnetic observatory instruments, data acquisition and processing, kakioka, 28-33. hegymegi, l., a. csontos and b. heilig (2009). can we use the didd magnetometer on the field?, in: proceedings of 13th iaga workshop on geomagnetic observatory instruments, data acquisition and processing golden and boulder, colorado, usa, 95-99. heilig, b. (2007). intercalibration of didd and fluxgate magnetometers, publications of the institute of geophysics polish academy of sciences, c-99 (398), 144-151. heilig, b. (2012). determining the orthogonality error of coil systems by means of a scalar magnetometer, measurement science and technology, 23, 037001. jankowski, j., and c. sucksdorff (1996). guide for magnetic measurements and observatory practice, iaga, warsaw, poland. korte, m. (2001). external influences in regional magnetic surveys, contributions to geophysics and geodesy, 31 (1), 273-278. marsal, s., j.j. curto, j.c. riddick, j.m. torta, o. cid and m. ibaňez (2009). livingston island observatory upgrade: first results, in: proceedings of 13th iaga workshop on geomagnetic observatory instruments, data acquisition and processing golden and boulder, colorado, usa, 154-161. newitt, l.r., c.e. barton and j. bitterly (1996). guide for magnetic repeat station surveys, iaga, boulder, usa. schott, j. j., and p. leroy (2001). orientation of the didd magnetometer, contributions to geophysics and geodesy, 31 (1), 43-50. schott, j. j., v. boulard, a. péres, j.m. cantin and j. bitterly (2001). magnetic component measurement with the didd, contributions to geophysics and geodesy, 31 (1), 35-42. soyer, w. (2002). analysis of geomagnetic variations in the central and southern andes, dissertation zur erlangung des doktorgrades im fachbereich geowissenschaften an der freien universität berlin. *corresponding author: andrás csontos, eötvös loránd geophysical institute, budapest, hungary; email: csontos@elgi.hu. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. csontos et al. 1094 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice miscellanea 633 annals of geophysics, vol. 51, n. 4 august 2008 key words terrestrial laser scanner – reflectance – rgb – volcanoes – vesuvius. 1. introduction the terrestrial laser scanner (tls) is generally used for the geometrical and kinematical characterization of surfaces affected by deformation allowing the definition of very accurate digital models from remotely acquired data. hunter et al. (2003) showed an interesting tls application for ground deformation measurements at mt. etna. teza et al. (2007) provided a method for displacement field computation of landslide ground surfaces by multi-temporal tls-based models comparison. the new generation of very long range scanners allows surface observation operating on ranges of some hundred meters and more providing very detailed (few centimeters) digital models (remondino, 2003). a tls scan provides the position of acquired points and a radiometric information; for each element of the point cloud, the intensity of received pulse is measured. it is related to both the material reflectance and the relative position of instruments respect to the observed surface (reshetyuk, 2006), and it is also highly sensitive to the water content (lu et al., 2005). a time-of-flight terrestrial laser scanner (tls) operates with nanosecond laser pulses remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) arianna pesci (1), giordano teza (2) and guido ventura (3) (1) istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy (2) dipartimento di geoscienze, università degli studi di padova, italy (3) istituto nazionale di geofisica e vulcanologia, roma, italy abstract this article focuses on the use of terrestrial laser scanner (tls) in the characterization of volcanic environments. a tls survey of the vesuvius crater (somma-vesuvius volcano, italy) allows the definition of an accurate and georeferenced digital model of different sectors of the crater. in each sector, the intensity is computed for each point as the ratio between the emitted amplitude and the received one, normalized to the maximum signal. moreover, the rgb colors of the observed surfaces can be captured by means of a calibrated camera mounted on the tls instrument. in this way, a multi-band information is given, since a long range tls operates in the near infrared band. the reflectance and rgb data are compared to verify their complementarities for model analysis and inspection. the collected data allowed the recognition of different volcanic deposits and stratigraphic features. in addition, our results shed light on the spatial extension of landslides and on the dimensions of rock fall/flow deposits affecting the inner walls of the crater. mailing address: dr. arianna pesci, istituto nazionale di geofisica e vulcanologia, sezione di bologna, via donato creti 12, 40128 bologna, italy; tel. +39 0514151416; fax. +39 0514151498; e-mail: pesci@bo.ingv.it miscellanea 9-03-2009 14:42 pagina 633 634 a. pesci, g. teza and g. ventura emitted along a direction defined by two calibrated angular coordinates (φ, longitude and α, elevation). the signal is backscattered by the acquired target providing the time of flight t, and the path is computed as d = ct / 2 (c is the speed of light) leading to the complete spherical coordinates (φk, αk, dk). the laser beam is automatically deflected over known directions by a moving mirror device providing the acquisition of a large number of points well distributed on the observed physical surface. the laser beam properties assure spatial collimation and good waveform of the pulses, leading to high resolution and accuracy. the optech ilris 3d, e.g., is able to acquire a point at 50 m distance with about 7 mm accuracy and with 17.7 mm resolution (lichti and jamtsho, 2006). the accuracy is mainly related to the internal telemeter efficiency in time measuring, leading to a precise range definition, whereas the resolution depends on spot size and on the level of data overlapping, based on the choice of the calibrated angular grid. the product of a scan is the raw point cloud ��dk,φk,αk,i�dk,φk,αk,dk��, k = 1,2,···,n�, where i�dk,φk,θk,dk� is the intensity of the pulse corresponding to the k-th point and n is the number of acquired points (generally, n ~ 105-106). the raw point cloud is directly provided by the instrument. the spherical coordinates are transformed into cartesian ones and the intensity is normalized to the range 0-255 in order to allow the data importing by software intended for tls data processing. when a calibrated camera is mounted on the tls instrument, a series of rgb images of the observed scene can be acquired. the rgb and tls-based data can be also merged by means of specific software leading to a colored point cloud or a textured digital model. since a camera is a passive instrument, the rgb image critically depends on the light conditions. on the contrary, since the tls is an active instrument, the acquisition is independent from light conditions. both the intensity and rgb data can be useful in point cloud inspection for edge detection and in the identification of different materials (kurazume et al., 2002). roncella et al. (2005) showed that a 3d model obtained from digital photogrammetric techniques can be used to recognize geo-mechanical features, since image processing operators conceived for image segmentation (e.g., edge detection operators) can be applied. at present, few studies make specific use of the intensity data obtained by tls or aerial laser scanner instruments (e.g. mazzarini et al., 2007), but the geometric information is principally used. this paper investigates the nature of tlsderived intensity and coupled rgb data to verify if: a) the corresponding information is redundant, or an integration could be useful or necessary, and b) the intensity and/or rgb data can be used to recognize different material and to detect, for example, stratigraphic successions. the crater of vesuvius (italy) is the test site selected to investigate the peculiarities of a volcanic environment. 2. terrestrial laser scanner: fundamentals and properties of acquired data as stated above, the tls point cloud is ��xk,yk,zk,in�xk,yk,zk��, k = 1,2,···,ne�, where in is the normalized intensity and ne ≅ n is the number of points where the intensity can be effectively considered (for example, a signal saturation in the internal sensor consists of corresponding point removal). the raw intensity i, directly provided by the instrument, is converted into the normalized intensity in by means of: a) target-distance corrections, and b) transformation of obtained results into the 0-255 range of integers, like a grey level image. the target-distance correction is applied to provide similar intensity values for a same material acquired at different distances, taking into account the obvious fact that the raw intensity depends on the inverse square distance. in a straightforward approach, the distance-corrected intensity can be ic,k = ik (dk / d0)2, where d0 is a reference distance (generally unknown to the user), resulting in in,k = [255(ic,k – ic,min) / (ic,max – ic,min)], where [r] indicates the nearest integer to the real number r, and ic,max, ic,min are the maximum and minimum values of the set of distance-corrected intensities. in this way, the point cloud miscellanea 9-03-2009 14:42 pagina 634 635 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) can be inspected using tls data processing software (e.g. innovmetric polyworkstm, isite studiotm, and others). this transformation is performed using a parsing software provided by tls manufacturer (e.g. optech parser). the actually implemented algorithm, where some elaborations are joined to the straightforward approach to improve the visualization, is deemed to be proprietary information and is therefore unknown. the rate of the backscattered signal depends on both target physical properties and geometrical conditions, that is the relative position of tls and observed surface, or better the angle of incidence. a common material is generally characterized by a quasi-lambertian behavior, diffusing the incident radiation on the semispace, mainly concentrated along the preferential scattering direction, depending on the microscopic surface irregularities of the observed object (roughness), on the laser wavelength and on the incident angle (baltsavias, 1999). theoretically, a lambertian reflection is a complete and total signal diffusion along each possible direction in the semi-space over the shot element. a good practice of tls observation requires the incidence to be as normal as possible, to have maximum precision and resolution, which also depend on both instrumental performances and observational conditions. the area illuminated by a laser beam (the footprint) results from a thin-cone/plane (beam/surface) intersection, leading to a circle (or ellipse) with a small but finite diameter, linearly increasing with the distance d of the target (cone height). the probability of return detection decreases if the angle between the laser beam and the normal to observed surface increases. if this angle is too high, fewer points are acquired, and the accuracy on their coordinates is worse because of footprint deformation (spreading on the surface). if the characteristic size of the macroroughness of the observed surface is similar to the footprint diameter, the intensity does not depend on observation angle because a significant part of the area illuminated by the laser is always oriented towards the instrument, i.e. is not very far from normal incidence conditions (see appendix c). if two areas are observed under the same conditions, i.e. similar distance and angle respect to the surface normal, the different intensities are proportional to the corresponding reflectance and could be theoretically used to discriminate between different materials. nevertheless, the intensity strongly depends on moisture of the material, since a tls generally operates in ir band. so, the radiometric data cannot be generally used for classification purposes if multi-temporal point clouds are considered, that is a classification can be attempted only if homogeneous data are compared. moreover, the parsing actions have to be carefully investigated for subsequent data interpretation based on intensity values, using raw data. many tls typologies exist, characterized by different wavelength, accuracy, and achievable maximum range (boehler et al., 2003). an online description of the available instruments and processing software can be found at pob (2008). long range instruments are very interesting in geological/geophysical applications. in fact, they allow the data acquisition also of inaccessible areas like volcanic craters or landslides since observation distances of about one kilometer or better can be reached. a tls survey does not require contact with the observed surface but a series of high-reflectance artificial targets can be placed on the scene in order to allow the georeferencing of the point cloud. these targets, whose centers are the ground control points (gcps), can be concurrently acquired by tls and topographical techniques like total station and/or gps. this operation can be useful to provide some reference point for a next data registration on external frames. the gcp positions can be chosen on the basis of accessibility, the only requirement is their good spatial distribution. some instruments are equipped with a calibrated camera and directly provide both intensity and rgb colors of the point cloud. a specific processing procedure, usually performed by means of specific software, is required to extract useful information from the tls data and to generate a final and complete digital model (remondino, 2003). if an object is observed from more than one viewpoint (for example, vesuvius crater), the processing starts miscellanea 9-03-2009 14:42 pagina 635 636 a. pesci, g. teza and g. ventura with the registration of the point clouds, that is their linking on a common reference frame to provide a global point cloud. generally, surface matching algorithms are used for the data registration, e.g. the iterative closest point (icp) algorithm (besl and mckay, 1992; chen and medioni, 1992). the global point cloud is then georeferenced by using a series of gcps. a data filtering/cleaning could be necessary, especially if elements to remove (e.g., trees) are present. some expedients are used to better solve for the possible presence of vegetation, like the choice of the last pulse mode for data acquisition, providing the last arrival time for pulse detection and increasing the possibility to receive a terrain backscattered signal. moreover, the intensity values can also be used to identify, select and remove the points corresponding to vegetation, due to water-content leading to low reflection. the generation of a 3d polygonal model is the central point of data processing. it is a triangular mesh whose vertices are the points of the filtered and/or smoothed point cloud. the texture associated to the mesh can contain information on reflectance, rgb color, as well as on physical properties of the material. 3. study area the somma-vesuvius volcano consists of two main edifices: the somma edifice, which is affected by a summit caldera, and the vesuvius cone (1281 m a.s.l.), which is located within the caldera (fig. 1; santacroce, 1987). the somma products, which include lava flows and scorias, are older than 18.3 ky. the somma caldera resulted from collapses related to four plinian eruptions occurred between 18 ky and ad 79. sub-plinian eruptions also occurred between 16.1 ky and ad 1631 (andronico et al., 1996). lava flows in alternation with explosive phases characterized the activity between ad 79 and ad 472, and mainly between ad 1637 and 1944 (arnò et al., 1987). the present-day vesuvius crater formed during the last ad 1944 eruption (santacroce, 1987). the volcano is in a quiescent stage and only seismic and fumarolic activity occurs. the volcanic hazard is, however, high because of 600,000 people live around the vesuvius and an inverse relation between the duration of the quiescent periods and energy of the eruptions exists (santacroce, 1987). the present-day inner walls of the vesuvius crater, which measures about 550 m in diameter, consist of lavas related to the pre-1906 activity (southwestern wall) and of lavas of 19131944 activity (northeastern wall), as depicted in fig. 1. spatters and pyroclastic deposits cover the vesuvius cone and outcrop on the northeastern inner wall of the crater. the crater floor is filled by slide deposits resulting from the downfall of lava fragments/blocks outcropping on the steep, inner walls. the crater rim results from the superimposition of the northeastern rim related to the 1944 eruption on that of the pre1944 period of activity (southeastern rim). following ventura et al. (2005), the collapse of the small cone located before the 1944 eruption, as well as the sharp contact between the pre-1906 and 1913-1944 lavas of the inner crater walls is guided by a nw-se striking fracture. the strike of this fracture is that of the main structural discontinuity affecting the whole somma-vesuvius edifice and crossing the vesuvius cone (bianco et al., 1998). 4. data acquisition the vesuvius crater was observed twice times by tls, on may 2005 (pesci et al., 2007a) and october 2006 respectively. the instrument used was the optech ilris 3d tls (may 2005) and its upgraded, extended range (er) version ilris 3der (october 2006). the er version can operate in two modes, extending the maximum achievable range if necessary. this option is particularly useful in a volcanic environment where low-reflectance lavic material exists. the whole crater was observed from different viewpoints located in the crown, chosen in order to operate with good incident angles as normal as possible, providing the total surface coverage (fig. 2). in the 2005 measurement campaign the access to the northern part of the crater was forbidden, while in october 2006 miscellanea 9-03-2009 14:42 pagina 636 637 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) five station points well distributed around the crater were used. each single scan covered the angular space of about 40°, and the acquisition distance ranged from 300 m to 600 m. at 500 m distance the mean sampling step at the ground was about 5 cm. for the used instrument, the footprint diameter on a surface orthogonal to the laser beam is d=12+0.17d, where d is the distance expressed in m and d is in mm. therefore, in this application d ranged from 6 cm to 11 cm. since the resolution depends on both the footprint size and the sampling step, and the first dominates because its diameter is considerably greater refig. 1. digital terrain model of the somma-vesuvius volcano and simplified geological sketch map of the vesuvius crater (modified from santacroce, 1987). the reference frame is the wgs84 (utm coordinates). miscellanea 9-03-2009 14:42 pagina 637 638 a. pesci, g. teza and g. ventura spect to the sampling step (lichti and jamtsho, 2006), 10 cm can be considered a good estimate of the mean actual spatial resolution. a calibrated camera was mounted over the laser unit to provide rgb colors for texturing; the camera was characterized by 28 mm focal distance and 0.007 mm pixel size, so the corresponding pixel size on the ground surface was about 12.5 cm at 500 m distance. the ground spatial resolution of camera and tls were about the same in similar observational conditions. in this way, a comparison between intensity and rgb data, as well as an integration of these data to provide a better knowledge of the observed structures, were allowed. the standard data processing is performed using the innovmetric polyworkstm software package, providing a georeferenced 3d polygonal model for each survey. these models were also integrated with a digital terrain model (dtm) of the volcano generated by using data obtained from aerial photogrammetry enlarging the surveyed area (see fig. 2). moreover, a preliminary comparison between 2005 and 2006 models was presented and discussed, pointing out the capability of this surveying methodology to recognize and measure surface variations in a volcanic environment (pesci et al., 2007b). the present article presents the main results of an analysis aimed to understand the importance of radiometric contributions to the point cloud and image inspection in a volcanic environment. in particular, the analysis is focused on three selected zones of the crater characterized by different origin, morphology and visible features (fig. 2). a-zone is the fracture area that coincides with the morphological discontinuity separating the two half parts of the crater and differ for age and lithology and was observed from about 500 m distance. b-zone is a volcanic wall acquired in a very detailed way thanks to short range of about 50-80 m. the last, c-zone, is a portion of a landslide lying in the nw sector of the crater, scanned from about 500 m. since in each considered zone the acquisition distances were roughly constants (a and c: 500 m, b: ~60 m), the observational conditions allowed a quantitative analysis of the intensity provided by tls. 4.1. crater fracture zone the fracture area occurring at the southeastern tip of the vesuvius crater (a-zone in fig. 2) marks the contact between the more recent and steepest north-eastern wall and the older south-eastern wall. a vertical displacement of about 1.5 m can be recognized in the a-zone by means of the point cloud intensity analysis. the vertical step is clearly visible in the intensity image (fig. 3b) but cannot be directly detected in the ordinary red-saturated rgb image (see fig. 3a), also due to shadows that hide morphological features. nevertheless, some hide features in the rgb image could be detected by means of a re-balancing of the r, g and b components to create a false color image; this process, anyway, is not easy and requires manual editing and several trials. the properties of an rgb image strictly depends on illumination conditions, in particular a shadow can hide interesting features. on the contrary, the tls-based intensity data are obtained by active independent illumination but its relative nature, depending on geometry of observation and weather conditions, must be taken in account. on the other hand, materials characterized by the same color tones may have different physical properties and chemical composition, whereas difference in composition could be easily detected using ir signal. in conclusion, if intensity data are used, in some cases a direct and easy recognition of some features can be performed, without additional image processing efforts. 4.2. crater inner wall the b-zone (fig. 2) is a relatively small, nw-se striking sub-vertical cliff located near the previously described fracture area. different lithologies, which include welded scorias, massive and scoriaceous lavas, can be recognized. a first approach to remotely map volcanic rocks is the analysis of a reduced number of data extracted from some selected sections. six vertical profiles are considered, and both intensity and rgb data are extracted to compare data distributions and searching for data correlamiscellanea 9-03-2009 14:42 pagina 638 639 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) tion. in particular, fig. 4a shows the intensity statistical distribution of data extracted along the third vertical profile (from the left, the red one in fig. 2). this distribution clearly indicates preferential intensity values corresponding to histogram peaks. on the contrary, the three color channels are distributed in an almost gaussian way leading to standard deviation values of 55, 68 and 86 for the red, green and blue component respectively. another important result is the high correlation between the color channels, which is 0.984, 0.933 and 0.967 for the r-g, rb and g-b combinations respectively. therefore, the relations among the three basic colors are here linear trends, and each channel carries nearly all the information of the rgb image. for this reason, the analyses presented there are based, besides on tls-based ir intensity data, on the r channel only. the complete analysis is reported in pesci et al. (2007c). in the intensity vs. color plot of the considered vertical profile, the point distribution shows a «comb-peak» shape. despite large data dispersion, this distribution highlights the different behavior of intensity, which is organized in some defined bands, and color channels, which are spread. the same results are obtained over the other profiles (not shown here for the sake of brevity). dealing with a clear surface, composed of different materials distributed into irregular geometrical zones, these bands are probably related to the physical surface properties (roughness, moisture, etc.). the comb-peak structure consists of about 9-10 bands in the intensity distribution. moreover, this structure is weakened if the entire surface is considered instead of vertical sections, due to a more noisy data set. in addition, a large number of narrow bands (with steps of some intensity units) can be observed in both the sections and the whole area. this second effect is related to the quantization fig. 2. left panel: perspective view of the vesuvius crater. the tls viewpoints are marked by red and blue dots, concerning 2005 and 2006 surveys respectively. lines-of-sight are reported as arrows. right panel: front view of the three studied outcrops (yellow boxes a, b, and c in the left panel) and vertical sections used for preliminary data analysis (see later on). miscellanea 9-03-2009 14:42 pagina 639 640 a. pesci, g. teza and g. ventura effect of the tls detector, whose dynamics could be unable to capture the entire range of received signal intensity. note that the quantization effect, which is an instrumental artifact, can be also sharply observed in those conditions where the comb-peak pattern does not apfig. 3a,b. a-zone is a fracture zone in the se part of the vesuvius crater. the point cloud is colored using the rgb texture (a) and the intensity data (b). the zoomed area shows a vertical discontinuity of a high intensity horizontal layer, leading to a step of about 1.5 m. in order to better recognize the fracture zone, the line-of-sight is far from the normal to the surface. the 15-m scale is referred to the square-framed zone only. fig. 4a,b. statistical distribution of intensity data extracted from the vertical profile 3 (a), and plot of tls-based intensity vs. red channel of the rgb image (b). miscellanea 9-03-2009 14:42 pagina 640 641 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) pear neither in the vertical sections, nor in the entire surface. moreover, the comb-peak effect and the quantization one differ by one order of magnitude. therefore, the comb-peak effect seems to be not related to instrumental troubles. to exclude possible systematism, a further site area was selected and analyzed (c-zone, fig. 2). 4.3. landslide this study case (c-zone in fig. 2) is the central sector of a landslide affecting the northwestern inner wall of the crater. c-zone was selected because its material (chaotic lava blocks and loose scorias) is the same as the b-zone, but it is not stratigraphically organized. c-zone represents a debris/fall slide. figures 5a and 5b show intensity data statistical distribution and correlations, respectively. the lack of a well defined point distribution (no comb-peak shape is observed in this case) and the detection of almost the same intensity peaks leads to the conclusion that the comb-peak shape, if it exists, is not an artifact but is related to material properties. the distributions of the intensity peaks of vertical sections related to the volcanic wall and landslide are compared. data are fitted by gaussian functions over the peak intensity frequencies considering an a priori sigma of 10 intensity points. figure 6 shows the results in terms of central values of gaussian functions, with errors at 1-σ level. the landslide data show larger errors, whereas the volcanic wall data are clearly described by function with a very sharp shape. nine peaks are selected for each data set, with similar values taking into account errors. the main discrepancies are highlighted in the figure. a further test is performed using data belonging to a non-volcanic environment. see appendix a, where results related to a coal quarry in south africa are presented. these results underlie the relation of intensity and environment and partially reinforce the exclusion of instrumental effects. 5. discussion and data interpretation the analysis of the data from the b-zone of vesuvius highlights the lack of correlation between intensity and colors. in particular, the correlation coefficient computed using all the points is about -0.06 value. further computations, performed over some vertical surface boxes of the inner wall of the crater are shown in fig. 7. data distribution confirms the occurrence of comb-peak-shaped trends, even if it can be easily recognized in the case of vertical profiles only, since the noise is lower and the fig. 5a,b. statistical distribution of intensity data extracted from the vertical profile of the landslide (a), and plot of intensity vs. red channel (b). miscellanea 9-03-2009 14:42 pagina 641 642 a. pesci, g. teza and g. ventura number of data is reduced. this consideration can be supported considering data from some vertical surface bands instead of the whole data set or some profiles only. in this way a large number of data can be considered but limited to almost the same vertical zones represented by previous profiles. figure 8 shows results concerning 5 m width bands located at the left, at the center and at the right of the site-test volcanic wall. the main peaks are newly computed and evidenced in the statistical distribution histograms but now, the noise is higher, leading to fatter gaussians in the peak finder analysis. also in this case the data are processed by recognition of significant peaks in the distribufig. 6. peaks distribution comparisons: red and black points are referred to landslide and volcanic wall respectively. yellow boxes evidence the discrepancies. fig. 7. statistical distribution of the complete intensity data set extracted from the whole volcanic wall (a), and red channel and intensity correlation (b). miscellanea 9-03-2009 14:42 pagina 642 643 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) fig. 8. statistical distribution and data correlation of 3 data sub-sets. also in these cases, despite larger noise, peaks are well visible. miscellanea 9-03-2009 14:42 pagina 643 644 a. pesci, g. teza and g. ventura tion. for each vertical band, mean and standard deviation of the gaussian distribution are estimated and shown in fig. 9. another global surface study was carried out by means of estimation of residuals obtained from the difference of intensity and color data. the residuals are shown in figure 10 and point out different patterns. the b-zone residuals have a mean value of 18, a standard deviation of 71, and a non-normal distribution. the latter may result from the sum of two normal distributions peaked at 70 and -30. a clear fig. 9. intensity peaks recognized considering three vertical bands 5 m width in the b-zone. fig. 10. residuals map of b-zone and their statistical distribution. the residuals are computed as difference between intensity and r component of the rgb image (see also fig. 4). the distribution is not-normal, but can be considered as overlap of two normal distributions whose means and standard deviations are also reported. miscellanea 9-03-2009 14:42 pagina 644 645 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) boundary between the positive (red) and negative (blue) values of the residuals can be detected in the map. the analyses presented above on the difference between intensity and color data are very simple and directly relate different datasets affected by major noise. for this reason, a principal components analysis (pca) was also performed on data images by computing the most meaningful basis to re-express a noisy data set (jolliffe, 2002; shlens, 2005). this analysis transforms the input data so that the elements of the obtained vectors are uncorrelated. in addition, the number of vectors obtained may be reduced by retaining only those components that contribute more than a specified fraction of the total variance of the dataset. in this way, only the meaningful and no redundant information is retained, and the possibility of easy detection of particulars in the transformed images is associated to a significant reduction of the data storage requirements. a standard pca procedure of analysis was applied using matlab functions (see appendix b) to process a full data matrix composed by the rgb and the intensity data sets. results show the independence of the two data set, confirming the correctness of previous analysis. the same procedure was also applied to demonstrate that intensity information is independent from the stationing point of the instrument in this configuration of acquisition (appendix c). in fact, the surface natural corrugation and the footprint have sizes that allow the laser beam to be diffused along each angles in the space, providing the same signal amplitude neglecting the geometry of acquisition. data extracted from a-zone clearly show a 1.5 m long vertical step (fig. 3). this step corresponds to the zone where a fracture is mapped by ventura et al. (2005). the collected data indicate that this fracture is not an open crack, but a fault separating the products of the 1913-1944 activity from those of the pre-1906 activity. the age of the younger deposits affected by this fault and the 1.5 m slip evidenced from tls data suggest an average slip rate of 2.4 cm/y. however, the slip rate along this fault cannot be constant over time, and could be related to a single or few post-1944 faulting episode(s). intensity profiles from the zone b (fig. 4) highlight the transition from the uppermost welded scoria layer of the 1944 eruption, characterized by high intensity values, and the underlying lavas of the 1913-1944 period of activity, which show lower values. more in detail, the profiles also mark the boundaries between the massive and scoriaceous portions of the lavas (see profiles 2-5 in fig. 11). the rough boundary between the lavas and fig. 11. intensity of the six profiles selected in the b-zone (fig. 4). data (in white) are smoothed (red line) using a fft filter to better display slow variations and examine the intensity trend along the sections. a common scale of 0-300 is adopted. miscellanea 9-03-2009 14:42 pagina 645 646 a. pesci, g. teza and g. ventura the upper welded scoria layer is also evidenced in the map of the residuals (fig. 10). however, the residuals do not clearly discriminate the scoriaceous portions of the lavas from the massive ones (see fig. 4 for a comparison). to better analyze the geological features of the bzone, the cloud of points was divided in intensity bands at intervals [50, 80], [120, 150], [150, 200] and [220, 255] based on peaks distribution. these intervals were obtained computing the main gaussian peaks of the six profiles, performing averages and creating intensity boxes to efficiently subdivide the data for material characteristics detection. results of this analysis are shown in fig. 11. the image with intensity range [5080] highlights the massive portions of the 1913-1944 lavas depicted in fig. 4, whereas the images with intensities in the ranges [150, 200] and [220, 255] show the scoriaceous levels. the image in the range [120, 150] marks the boundaries among the different lithological units. therefore, we conclude that the integrated analysis of the residuals, intensity profiles and bands gives useful information on the lithology and stratigraphy of volcanic deposits. data from the c-zone clearly show the different layering of the lavas in the n-w and s-e inner walls of the vesuvius crater. these slides are related to both gravity instability and earthquake shaking (ventura et al., 1996). the point cloud shows the spatial extent of the debris/fall slide separating the two sectors of the vesuvius crater, the source area, the summit scar, the slide wedge and basal fan (see fig. 6). based on the grey intensity of the slide material, a more recent (shining pixels in fig. 13) slide deposit can be recognized from the older slides. the difig. 12. selection of the points of the b-zone on the basis of the intensity ranges. four ranges are considered: [5080], [120-150], [150-200] and [220, 255]. miscellanea 9-03-2009 14:42 pagina 646 647 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) mensions of the slide material can be also determined using point cloud coordinates. as an example, the largest block of this slide measures 3.7 m x 2.4 m x 1.6 m. the integration of tsl data and the geological knowledge of the vesuvius volcanic crater allow the recognition of some patterns: dark zones highlight lava blocks, data belonging to the second interval seems to be familiar with the contact area between different lithologies, and the higher values are probably due to pyroclastic materials of different size. 6. conclusions this study focuses on the analysis of radiometric data in volcanic environments and investigates the difference between color information content and tls-based ir intensity. in particular, the rgb data from a calibrated camera can be independent or highly correlated to ir, depending on the geological environments, the intensity being strictly related to the physical/chemical properties of the material. some selected sites of the vesuvius crater were tested to investigate the relation between rgb and intensity, and to give preliminary results on the impact of such an approach on geological studies. results show that intensity data are independent enough from rgb adding valuable information to image analysis, which can be considered a different channel, as shown by both a classical data treatment and pca modern processing. the data analyzed here allow: i) discrimination between different lava flow faces, block and welded scorias, ii) identification of the layering of volcanic deposits, and iii) recognition of slide material and sliderelated morphologies. in addition, quantitative estimates of the dimension of single clasts can be done. these applications and analyses represent a preliminary approach to validate a new method for data inspection based on integration between tls data and photographic images. the analytical approach reported here could be extended to other non-volcanic environfig. 13. point cloud of the c-zone (landslide in volcanic environment, see fig. 6). the detail of a slide lava block and the size of the modeled surface are shown. miscellanea 9-03-2009 14:42 pagina 647 648 a. pesci, g. teza and g. ventura ments, as suggested by the example of a coal quarry. researches in this field are in progress and the next step includes (a) monitoring slide movements within the vesuvius crater, and (b) planning a multidisciplinary experiment based on scanning of an artificial surface composed of several elements disposed to reproduce a synthetic, stratified, surface. concerning the second point, a new experiment was performed in april 2008 showing that the intensity data is almost independent with respect to the incident angle if the observed surface irregularities are of the same order of the spot size. a complete description of the experiment and results is now submitted on annals of geophysics journal and partially satisfies the search for a new method to better analyze intensity data and their implication in geophysical/geological studies. acknowledgments the authors thank fabiana loddo (ingv), andrea faccioli and marco bacciocchi (codevintec italiana s.r.l.) and dario conforti (optech inc.) for their kind support in the experimental phase. very special thanks to massimiliano cerrone and to all the staff of the «ufficio tecnico» (ingv-rome) for their great contribute in the realization of an international seminar concerning laser scanner applications on 6-7 june 2007. fig. a1. point cloud of a coal quarry (south africa): rgb texture (a) and intensity image (a). the dashed yellow line is the selected profile. appendix a results reported in chapter 4 about on intensity and color statistical distribution and their correlation show that data are widely independent, suggesting that important information can be added for a geological and geomorphological study of the area. in particular, the simple analysis of volcanic wall data set, shows a peak distribution (authors named «comb-peak shape»). several considerations and data checks were done to exclude systematism, data alteration or instruments anomalies to verify the results reliability. moreover, the same analysis performed on a landslide in a different part of the crater shows a completely different statistical distribution but with similar intensity values. this appendix reports the results of a further test performed in a non-volcanic environment (coal quarry at middelburg, moumalanga province, south africa) using the same instrument (optech ilris 3d). with this additional test, the peculiarities of a volcanic area with respect to another geological environment can be highlighted. a single scan of a 25 m x 25 m vertical wall was acquired from a distance of 100 m. after modeling and rgb data rendering, both intensity and color information are extracted along a profile (fig. a1). results are quite different from those of the previous study cases, showing a clear correlation between the two kinds of data (fig. a2). miscellanea 9-03-2009 14:42 pagina 648 649 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) in a first approximation, a linear trend can be recognized, even if the linear correlation coefficient is ~0.6. however, the high noise level, as well as the absence of correlation in volcanic environments, suggest that this relatively low value reflects, in any case, some data dependence. the statistical distribution analysis shows a lower number of intensity peaks with respect to that of the b and c volcanic sites. this can be related to the different analyzed surfaces and material properties. this analysis is not discussed here because it was performed only for comparison reasons to show that different geological environments give different responses. appendix b despite the evidence of a clear distribution, data belonging to the volcanic wall are characterized by high noise. therefore, a more specific analysis was performed to validate the results reported in chapter 5, considering an approach based on principal component analysis (pca). the pca allows a reduction of redundant data and an easy recognition of details and hide features; in this case the method is used to highlight correlations between intensity and rgb data set. the pca application requires a preliminary transformation of all the data (the three rgb channels and the intensity data) to have zero mean and variance 1. the results of pca are still four datasets, but they are uncorrelated and ordered in a way that their participation to the data variance highly decrease from the first to the fourth. so, the less significant datasets can be excluded considering a limit-variance, e.g. 0.02. in the present work, matlabtm package was used to perform the pca via the singular value decomposition (golub and reinsch, 1970). fig. a2. correlation between intensity and image levels along the coal quarry profile. also this image shows great differences respect to the cases of volcanic zones. miscellanea 9-03-2009 14:42 pagina 649 650 a. pesci, g. teza and g. ventura input vectors were multiplied by a matrix whose rows consist of the eigenvectors of the input covariance matrix, producing transformed vectors. clearly, all the transformed vectors, in particular the ones characterized by the higher variances, can contain information extracted from all the original input vectors. for a given portion of the investigated surface, the rgb image and the intensity data can be considered. after registration on a common reference frame, the original data were interpolated to provide two images having exactly the same size. the rgb image is represented by three n-by-m matrices and the intensity data by one nby-m matrix. each of these matrices is transformed into a row vectors having nm length. if an rgb and an intensity image are considered, a 4-by-nm matrix is therefore built. thanks to pca, the real color information, the relations between colors and intensity, as well as the implications of using the red component alone can be investigated. in particular, the following analyses were performed: i) pca of the three components of the rgb image (three input vectors), ii) pca of the r component and of intensity data (two input vectors), iii) pca of the rgb and intensity data (four input vectors), and iv) pca of the intensity data provided by tls acquisition of a same area from two different viewpoints (two components). in all the cases, several variance-limits were considered. in the first case the rgb image was considered, without intensity data. only a vector is obtained from the pca if a 0.06 limit variance was considered, and this fact is expected since the previous analyses have highlighted that the three color components are strongly correlated. the results are shown in fig. b1. fig. b1. principal component analysis of the rgb image of a selected part in the b-zone: the principal components (a, b), a virtual image obtained choosing 0.06 variance (c), the initial rgb image (d). miscellanea 9-03-2009 14:42 pagina 650 appendix c the pca can be also used to check for intensity data reliability when the same area is acquired from two or more viewpoints, i.e. different distances and different geometrical conditions are considered. the angle of incidence affects the resulting intensity (reshetyuk, 2006), but also the spot size and the surveyed area have to be considered. the surveyed volcanic surfaces are very corrugated with respect to the 5-10 cm illuminated element by the laser pulses. therefore, no significant variations of the intensity data are expected. figure c1 shows the 651 remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and rgb implications for studying vesuvius crater (italy) the results obtained in the second and third case are very similar, as expected. for this reason, only the results of the second analysis (r component and intensity) are reported here. if a 0.05 limit-variance is considered, two matrices are obtained (fig. b2a). they cannot be combined because the synthesis of the two outputs would be very confusing. on the other hand, the principal component with largest variance (0.3 is the chosen limit-variance) contains all the information contained in both color and intensity image. all the intensity and red-component features are clearly visible, like the shadow in the right bottom of the image. fig. b2. principal component analysis of r channel of the rgb image and tls-based intensity: original r channel (a); intensity (b), principal component having the largest variance (c,d). note that on this pc, which is the only output component if the 0.3 limit-variance is considered, contains information related to both the vectors, shows features that can be easily detected. miscellanea 9-03-2009 14:42 pagina 651 652 a. pesci, g. teza and g. ventura results of pca of the intensity patterns related to a same portion of the b-zone acquired from different viewpoints. the resulting image shows all the features related to both the input images. in digital image processing, the pca can be performed in another way. instead of the application to a set of vectors representing several registered images, this technique can be applied to one image only. boundaries and regions in a single image can be therefore described. this approach is not considered here, but will be applied in further analyses for automatic feature recognition purposes (in the present work the features were detected manually). fig. c1. pca of the intensities in a case where two viewpoints are considered, with different distances and orientations: two scans with intensity data (a,b), transformed dataset, i.e. virtual image (c). all the features that can be separately detected in the two single scans can be easily found in the principal component. references andronico, d., r. cioni and r. sulpizio (1996): general stratigraphy of the past 19,000 yrs at somma-vesuvius, in m. rosi (ed), vesuvius decade volcano workshop handbook, (cnr, napoli), pp. 1-21. arnò, v., c. principe, m. rosi, r. santacroce, a. sbrana and m.f. sheridan (1987): eruptive history, in r. santacroce (editor), somma-vesuvius, quaderni de «la ricerca scientifica», (cnr, roma) 114, pp. 53-103. baltsavias, e.p. 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(2006): investigation of the influence of surface reflectance on the measurements with the terrestrial laser scanner leica hds 3000. zeitschrift für geodäsie, geoinformation und landmanagement 2006 (2), 96-103. roncella,, r., g. forlani and f. rremondino (2005): photogrammetry for geological applications: automatic retrieval of discontinuity in rock slopes, in proceedings of videometrics, viii, edited by j.-a. beraldin, s.f. el-hakim, a. gruen and j.s. walton (san jose, ca, usa, spie-is&t electronic imaging 5665. spie, bellington, wa), pp. 17-27. santacroce, r. (1987): somma-vesuvius, quaderni de «la ricerca scientifica», 114. consiglio nazionale delle ricerche, roma, pp. 220. shlens, j. (2005): a tutorial on principal component analysis. (available online at: http://www.cs.cmu.edu/~elaw/papers/pca.pdf) (accessed: 18 may 2007). teza, g., a. galgaro, n. zaltron and r. genevois (2007): terrestrial laser scanner to detect landslide displacement fields: a new approach, international journal of remote sensing, 28 (16), 3425-3446. ventura, g., g. vilardo, g., bronzino, g., gabriele, r. nappi and c. terranova (2005): geomorphological map of the somma-vesuvius volcanic complex (italy), journal of maps, 1, 30-37. ventura, g., g. vilardo and u. tammaro (1996). earthquake-induced slides at monte somma caldera (julyaugust 1995, somma-vesuvius volcanic complex), acta vulcanologica, 8 (1), 115-118. (received may 6, 2008; accepted may 29, 2008) miscellanea 9-03-2009 14:42 pagina 653 r i s u l t a t i d e l l e o s s e r v a z i o n i i o n o s f e r i c h e e f f e t t u a t e i n r o m a d u r a n t e l ' e c l i s s i s o l a r e d e l 1 5 f e b b r a i o 1 9 6 1 a . b o l l e f . coccia c . a . t i b e r i o ( * ) r i c e v u t o il 5 giugno 1961 lo scopo di questa comunicazione è di rappresentare alcuni risultati delle osservazioni ionosferiche effettuate in roma in occasione dell'eclisse di sole del 15 febbraio 1961, che permettono di valutare la maggiore superfìcie del sole ionizzante rispetto al sole ottico. le misure sono state eseguite presso l'osservatorio ionosferico « s. alessio » dell'istituto nazionale di geofisica in collaborazione con il comitato telecomunicazioni militari. il sondaggio ionosferico è stato effettuato nella suddetta stazione perchè alla latitudine di roma corrispondeva la totalità dell'eclisse alla quota di 100 km e quindi per lo strato e . l'apparecchiatura è quella ordinariamente impiegata per il rilevamento sistematico delle caratteristiche ionosferiche, col metodo del sondaggio ad impulsi con incidenza verticale e consta di una ionosonda di costruzione inglese union radio tipo m k i i e di un sistema di quattro antenne rombiche verticali, due per la emissione e due per la ricezione. la gamma di frequenza esplorata va da 0,6 mhz a 25 mhz, divisa in 5 sottogamme. le marche di calibrazione in frequenza ed in altezza sono ottenute per divisione di frequenza da un oscillatore controllato a quarzo. la potenza di picco è compresa t r a 1,8 kw e 0,5 kw, rispettivamente all'estremo inferiore e superiore della gamma di frequenza. i n aggiunta alla sonda panoramica descritta è stata messa a punto, a scopo di controllo, un'altra sonda a frequenza fissa lavorante a 2,1 mhz, con funzionamento continuo. sono state altresì effettuate misure relative di campo su varie emittenti nella gamma delle onde corte e medie. i n occasione dell'eclisse il programma dei sondaggi, che d'ordinario prevede una cadenza di 30ni è stata intensificata in modo da ottenere (*) i s t i t u t o nazionale di geofisica r o m a . 8 8 a . b o l l e f . c o c c i a c . a . t i b e r i o fig. 1. s o n d a ionosferioa t i p o m k 2 il massimo numero di ionogrammi compatibili con il ciclo automatico di funzionamento dell'apparato. i n aggiunta alla registrazione fotografica del tempo sugli ionogrammi, l'ora di inizio di esplorazione di ogni r i s u l t a t i d e l l e o s s e r v a z i o n i i o n o s f e r i c h e , e c c . 8 9 sottogamma è s t a t a controllata separatamente per confronto con un cronometro (li precisione, in modo da poter contare su un'approssimazione di ± 1 secondo. sulla verticale di s o m a ed alla quota di 100 km l'eclisse è stata totale alla ore 08h31m20s t.m.e.c. nella fig. 1 è mostrato l'andamento dell'eclisse in quota. la carta è s t a t a compilata sulla scorta dei dati astronomici gentilmente forniti dal prof. a. k r a n j c che ci è gradito ringraziare in questa occasione. nella fig. 3 è mostrato l'f-plot del giorno l o , unitamente alla curva mediana mensile di febbraio. nelle figg. 2 e 1 sono riportate per confronto gli f-plots dei giorni 11 e 16. una presentazione più dettagliata dell'andamento delle frequenze critiche nelle ore precedenti e seguenti l'eclisse viene d a t a in fig. 5, che riporta oltre le frequenze critiche il corrispondente valore delle densità elettroniche. da tale figura si rileva che il minimo di ionizzazione nella regione e'ì è intervenuto alle ore 08.39, con un ritardo di sette minuti circa rispetto alla fase massima ottica al suolo. la normalità sia pure con valori inferiori alla mediana si è ristabilita a partire dalle ore 10. le osservazioni della frequenza critica dello strato e risultano incomplete, specie in corrispondenza dei valori più bassi che venivano a cadere nella b a n d a d ; frequenza delle stazioni di radio diffusione. lo studio degli ionogrammi sembra t u t t a v i a escludere la totale scomparsa della ionizzazione. le misure di intensità di campo della stazione di eadio palermo eseguite alla sede centrale dell'istituto di geofisica denotano un brusco abbassamento al momento della formazione dello strato e ed un aumento dell'intensità di campo durante l'eclisse con massimo piatto lievemente precedente la fase di totalità. lo studio degli ionogrammi dello strato e mostra la presenza delle stratificazioni fi ed f2 durante l'eclisse, nonché un rilevante numero di echi multipli imputabili a scarso assorbimento negli strati più bassi. (in dt = <l(ì) ~~ an* in cui a è il numero di elettroni (cm"3), q{t) il tasso di produzione di elettroni (cm"3 e sec"1), ed a il coefficiente di ricombinazione ( c n r 3 sec-"1). non si è tenuto conto del coefficiente di attaccamento perchè da ritenersi trascurabile per la regione fi. 9 0 a . b o l l e f . c o c c i a c . a . t i b e r i o 1 1 c o d i c e • f ? 0 to z i z 1 echi sparsi ~ — fbes t t min a maggiore di v minore di 1 1 c o d i c e • f ? 0 to z i z 1 echi sparsi ~ — fbes t t min a maggiore di v minore di 1 1 c o d i c e • f ? 0 to z i z 1 echi sparsi ~ — fbes t t min a maggiore di v minore di 1 1 c o d i c e • f ? 0 to z i z 1 echi sparsi ~ — fbes t t min a maggiore di v minore di 1 1 c o d i c e • f ? 0 to z i z 1 echi sparsi ~ — fbes t t min a maggiore di v minore di 1 1 c o d i c e • f ? 0 to z i z 1 echi sparsi ~ — fbes t t min a maggiore di v minore di : °< 0 0 s * < 0 / \ 7 * a \ o j / me dia na \ m e : n s l e \ \ \ ) o j x x j \ t \ v 0 ) 0 ( j l llul l 1 o l < <? o <0 1 t t t f ' t t o t t f r r f t t n | f t 1 f 1 t 0 0 01 02 03 04 05 0 6 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 00 fig. 2. d i a g r a m m a delle f r e q u e n z e (f-plot) del giorno 14-2-1901. i n tale equazione si è posto <7 = 0 in corrispondenza dell'istante di fase massima ottenendo per a u n valore di circa 7 x 10~10, molto diverso con il valore 40 x io-10 comunemente accettato per la q u o t a di 220 k m . ciò p o r t a a ritenere che anche nelle condizioni di massima t o t a l i t à la superfìcie ionizzante solare non sia t o t a l m e n t e occultata o, in altri r i s u l t a t i d e l l e o s s e r v a z i o n i i o n o s f e r i c h e , e c c . 100 fig. 3. d i a g r a m m a delle f r e q u e n z e (f-plot) del giorno 15-2-1961. termini, che agli effetti ionosferici l'eclisse si presenti come anulare anziché totale. si è pensato così di calcolare il rapporto t r a la superfìcie ionizzante solare e quella del disco visibile ritenuto, in prima approssimazione eguale alla superficie occultata attribuendo ad a il valore 40 x 10 10 per la quota di 220 k m . 9 2 a . b o l l e f . c o c c i a c . a . t i b e r i o fig. 4. d i a g r a m m a delle f r e q u e n z e (f-plot) del giorno 1(5-2-1911. si è t r o v a t o per q il valore di 450 elettroni c n r s sec1. confrontando questo valore con quello dedotto dalla curva mediana per lo stesso tempo, che risulta di circa 2000 elettroni c m 3 sec1 si è t r o v a t o per il rapporto predetto il valore di 1,25. p u r t r o p p o la corrispondente diminuzione di frequenza critica per lo strato e, la stazionarietà che ne deriva, di circa 14m della frequenza r i s u l t a t i d e l l e o s s e r v a z i o n i i o n o s f e r i c h e , e c c . 9 3 15 febb 1961 fig. 5. migliore conferma di quanto esposto ci ripromettiamo di ottenere con l'esame approfondito dei risultati sperimentali di altri osservatori e con più completi dati astronomici. critica, è solo poco chiaramente rilevabile dagli ionogrammi in nostro possesso per la suaccennata ragione. riassunto vengono presentati alcuni risultati delle osservazioni ionosferiche effettuate in roma durante veclisse di sole del 15 febbraio 1961. il comportamento della ionosfera lascerebbe supporre che la superficie ionizzante del sole sia alquanto maggiore di quella del disco visibile. su 31 mary some results of the ionosphere observations made in rom during the solar eclipse of february 15tli 1961 are presented. the behaviour of the ionosphere leads to suppose that the ionizing surface of the sun is urger tliat the visible disk. t e c n i c a d i p r e l e v a m e n t o e p u r i f i c a z i o n e d e i g a s n a t u r a l i e v u l c a n i c i m . santangelo recentemente sono stati pubblicati f 1 ' 2 ), i risultati di una ricerca sulla composizione isotopica dell'argon contenuto nei gas provenienti dall'interno della crosta terrestre. in lavori di questo genere uno dei problemi fondamentali è costituito dalla operazione di captazione sul terreno dei campioni e la difficoltà più grossa sta nel fatto che, specie nel caso dei vulcani, è molto problematico evitare l'inquinamento da parte dell'aria atmosferica, e ciò sia per la inadeguatezza dei dispositivi di raccolta, sia anche, e questo è più grave, perché l'inquinamento può verificarsi per circolazione sotterranea di aria prima ancora che i gas vengano in superficie ( 3 ). aggiungasi che questi gas in genere si presentano con elevata temperatura, 200-300 "c, da qui la necessità che l'apparecchiatura abbia particolari requisiti. le difficoltà di cui sopra, naturalmente, scompaiono quando il gas viene incondottato a profondità dell'ordine di qualche centinaio di metri, così come avviene per i soffioni boraciferi di larderello (toscana). p e r le ricerche di cui si è fatto cenno (1. e.), presso l'istituto nazionale di geofisica in collaborazione con il centro di fisica nucleare di roma, è stata studiata e realizzata una attrezzatura per il prelevamento e la purificazione dei gas naturali allo scopo di preparare dei campioni di argon da analizzare con lo spettrografo di massa ( 4 ). gas vulcanici. — la tecnica impiegata variava a seconda che si trattasse di emanazioni da crepacci di vulcani attivi o di fumarole subacquee: nel primo caso fu costruito un sistema a bombola con apertura manovrabile a distanza. la bombola del volume di circa 10 litri (vedi fig. 1) è munita di un dispositivo di chiusura di cui alla fig. 2. questo dispositivo normalmente resta chiuso a tenuta di vuoto. la bombola protetta da un cappuccio metallico viene calata a mezzo di un 28 m . s a n t a n g e l o cavo di acciaio alla profondità massima raggiungibile nel crepaccio da cui provengono i gas che si vogliono captare. una volta introdotta la bombola, da una distanza clic nei nostri prelevamenti si aggirava attorno agli 8-10 metri, veniva aperta a mezzo di un cavo con anima di acciaio la valvola azionata dalla leva a di fig. 2 : dopo qualche minuto la bombola si riempiva dei gas circolanti nel crepaccio: il vuotometro indicava l'avvenuto riempimento. per questa operazione si è cercato di scegliere quelle spaccature che presentavano le caratteristiche più opportune perchè i gas captati f i g . 1 f i g . 2 con questa tecnica furono fatti prelevamenti sull'etna; ne fu tentato anche uno allo stromboli, ma l'operazione non si potè effettuare con le modalità sopra descritte per il lancio intermittente da parte del vulcano di lapilli infuocati, masse incandescenti e gas tossici. ì jtf* j ì i> fllmarole sli[h/c(jlh'(', il prelevamento ~l presenta plu a~evole e con risultato alquanto più ~icuro nel caso di fumarole ~uhaequec. i !!as ~or!!o!!1ianti nell';u:qua ~ono raccolti in una campana ~li (:in:a 20 litri 16~. 31 che \·ienc (:alata a mezzo (li una picf'ola f!nl, ~irevolc e fw:ilmente tra~portabile, ndlo specchio ~li acqua fmo ai 2 3 del· la sua alte7.za. del ~hiaccio posto 111 un cilin~lro inne~tato sulla campana, pro\oca la con~len~azione del vapore d'acqua trascinato (lai f!a5 nel tuho ~li rame a spirale imo merso in e~~o; in questo modo l'acqua di con(lensazione rica~lc, mentre ~as residui pervengono attraverso un tubo di ~omma a(l un 5i~tema lfi/!. 41 co~tituito da 2 hocce contenenti una ~oluzione (li naoh a circa 30'1;, preyentiva· mente di~aerata a ealdo c sotto nloto. il manometro b ~el"\'c al: ilulieare la pre~~ione ~lcl /!as tlspetto a quella atmo~feriea. l'opcra7.ionc procc(leva nel ~el!uente mo~lo: si face\'a riempire la prima boccia, quindi si seuo· teya la soluzione per a~e\'olare la rca7.ione: così alla fine era possihile a~~or­ bire nclla quasi totalità la co", che in que~ti ga~ puìl raggllulgere il 95·98,/" (3) e racco/!liere il gas residuo ~enza che ci fosse pericolo l'aria atmosferica. (li (:ontaminazionl" da partc delcampioni sono ~tati prelevati ad a~nano terme, solfatara ai pozzuoli e bnllicame ~li viterbo. prcpflrflzùmp del campione di argon. t i!a~ ndcaniei e fumarolici contengono prevalentemente h~o. co", n~ e talvolta h~s, cii~, 28 m . s a n t a n g e l o f i g . 4 h-2 e gas rari. il metodo usato per la purificazione è quello classico (/'l: per ottenere i campioni di argon si sfrutta la proprietà dei gas nobili di non reagire chimicamente: il problema si riduce ad una successiva eliminazione di tutti gli altri gas presenti. non è necessario clic questa eliminazione sia quantitativa poiché le misure dei rapporti ira gli isotopi dell'argon con lo spettrografo di massa non vengono disturbate da nessuna delle masse riportate. l'hoo residua viene eliminata facendo passare il gas attraverso tubi ad u contenenti cacu e p^or,, la c02 mediante calce sodata, e l'ossigeno, se presente, si fa assorbire in un forno di pirex, da cu metallico a 350 °c. l'azoto viene trattenuto sotto forma di azoturo in un forno metallico dove è contenuto ca metallico alla temperatura di circa 700 °c (fig. 5). il gas residuo composto prevalentemente da argon e dagli altri gas rari, viene aspirato dal forno a ca. dove alla fine si raccoglie, m m m m m m m r n m ^ m m f o r n o a ca a — manicotti di raffreddamento a c i r c o l a z i o n e di acqua b — g u a r n i z i o n e ad anello di gomma per le tenuta del vuoto c — r i s c a l d a t o r e elettrico d — c i l i n d r o di re tice lla di f e r r o contenente le tornitura di ca e — t u b o di acciaio esterno a d. con fori per il passaggio del gas • d ...jot-; t e c n i c a 1)1 p r e l e v a m e n t o e p u r i f i c a z i o n e d e i gas n a t u r a l i e v u l c a n i c i 3 1 in un recipiente in cui si fa adsorbire da carbone attivo raffreddato con aria liquida: da qui si manda in un palloncino di vetro di 100 cm3. in fig. 6 è schematizzato l'insieme del dispositivo per la purificazione dei gas. f i g . 6 a — p o m p a rotativa vuoto preparatorio r e g — manometri ad hg c — s e r b a t o i o per il vuoto d — p o m p a in pyrex a diffusione di hg e — v a c u o m e t r o di p i r a n i f — b o m b o l a contenente il gas da purificare h — t u b i ad u contenenti cacl? e calce sodata / — f o r n o in pyrex a rame l — t u b i ad u con p,0-, n, o, p — p o m p e di t o e p l e r m — f o r n o metallico a ca q — r e c i p i e n t e con c attivo r — p a l l o n c i n o i n cui si raccoglie l'argon e i gas rari mi è gradito ringraziare i colleghi dottori g. boato e g. careri per l'aiuto prestato durante la preparazione delle apparecchiature qui descritte. rotila — istituto nazionale di geofìsica — gennaio 1952. riassunto in lavori precedenti sono stati resi noti i risultati di analisi sulla composizione isotopica dell' argon contenuto nei gas naturali. nella presente nota vengono brevemente esposti i criteri tecnici seguiti nella progettazione ed esecuzione delle apparccliiature necessarie al prelevamento dei gas stessi. si descrivono le tecniche impiegate per la captazione sia dei gas vulcanici che di quelli delle fumarole subacquee. viene infine succintamente esposto il procedimento per la preparazione dei campioni di argon e dato uno schema del dispositivo usato per tale operazione. 28 m . s a n t a n g e l o su m mary the rcsults of mass-spectrometric analysis of the argon content in naturai gases have been reported in previous papers. the devices employed for picking up naturai gases are reported here. methods for collecting volcanic and fumarolic gases are described. the method used in preparing argon samples is also summarized. b i b l i o g r a f i a ( ! ) g . b o a t o g . c a r e r i g . n e n c i n i m . s a n t a n g e l o , annali di geofisica. i v 1 1 9 5 1 . ( 2 ) g . b o a t o g . c a r e r i m . s a n t a n c e l o , nuovo cimento. i x 1 1 9 5 1 . ( 3 ) sborgi, annali di chimica applicata. v o i . 32 fase. 12, 1 9 1 2 ; day, shepiierd, bullet. geol. soc. volc. am. 24, 573, 1913. ( 4 ) g . c a r e r i g . nencini, nuovo cimento. 7 , 6 4 , 1 9 5 0 . ( a ) p . pascal, traile de cliimie minerale. p a r i ? , 1934. ionospheric perturbations related to the earthquake in vrancea area on november 22, 2014, as detected by electromagnetic vlf/lf frequency signals annals of geophysics, 58, 5, 2015, a0552; doi:10.4401/ag-6827 a0552 ionospheric perturbations related to the earthquake in vrancea area on november 22, 2014, as detected by electromagnetic vlf/lf frequency signals maria solovieva1, alexander rozhnoi1,*, viktor fedun2, konrad schwingenschuh3, masashi hayakawa4,5 1 institute of physics of the earth, russian academy of sciences, moscow, russia 2 university of sheffield, dept. of automatic control and systems engineering, space systems laboratory, sheffield, uk 3 space research institute, austrian academy of sciences, graz, austria 4 hayakawa institute of seismo electromagnetics co. ltd., uec incubation center-508, chofu, tokyo, japan 5 university of electro-communications, chofu, tokyo, japan abstract data from the european network of very low/ low frequency (vlf/lf) receivers has been used to study the response of the lower ionosphere to the earthquake of magnitude 5.5 in vrancea area on november 22, 2014. negative amplitude anomalies have been observed during 3 days before the earthquake and two days after, on the lf (45.9 khz) signal passed above the seismic area. no perturbations have been found for the same signal in control paths during this period. other possible influences both from above and below which can produce perturbations in the ionosphere have been taken into consideration. 1. introduction the research of the short-term transient processes in the global lithosphere-atmosphere-ionosphere coupled system using vlf (3–30 khz) and lf (30–300 khz) signal monitoring is advancing rapidly at present due to development of the observation networks in the seismo-active regions. specialized networks of stations distributed in japan and the far east of russia, middle asia and india as well as in europe have been installed recently. the networks operate in conjunction with powerful transmitters deployed in europe, united states, asia and australia. vlf/lf signals from navigational or time service transmitters propagate inside the earth-ionosphere waveguide and they are reflected by the d region of ionosphere, at an altitude of ~65 km during daytime, and ~85 km during nighttime. therefore, the received signals can provide valuable information about plasma perturbations near the upper atmosphere-lower ionosphere boundary. the regular monitoring of many years at the pacific network has established a statistical correlation between anomalies of the vlf/lf signal parameters in the nighttime and the earthquakes with m ≥ 5.5. seismic-related phase and amplitude anomalies were found about one week before an earthquake (mostly on 3–5 days) and one week after the event (due to aftershock activity of strong earthquakes) [rozhnoi et al. 2004, maekawa et al. 2006, kasahara et al. 2008, hayakawa et al. 2010, hayakawa 2011, rozhnoi et al. 2013]. in europe, investigation of the vlf/lf signals associated with earthquakes has begun in the university of bari (italy) in 2002 [biagi et al. 2004, 2007, 2008, 2009]. the most significant result was reported from data monitoring in three vlf/lf stations – bari, graz and moscow – in connection with the earthquake in l’aquila (italy) on april 6, 2009 [rozhnoi et al. 2009]. strong nighttime negative amplitude anomalies for the long propagation paths (~3000 km) together with the shift of evening terminator for the short paths (~800–1000 km) have been revealed during 5–6 days before the earthquake. the crossing of “seismic” paths allowed defining the real position of the earthquake epicenter. the pre-seismic influence upon atmosphere and ionosphere is intensively studied theoretically at present. in spite of numerous papers concerning the lithosphere-atmosphere-ionosphere coupling, this subject article history received july 12, 2015; accepted september 10, 2015. subject classification: seismic risk, low frequency wave propagation, earthquake precursors. still needs further investigation. it is assumed [e.g. molchanov and hayakawa 2008] that different kind of gases leak from the crust during the preparation stage of tectonic earthquakes over seismically active faults which leads to changes in the air temperature and density and distribution of the charged particles (due to emanation of radon). such changes can take place in the thin near-ground layer, because of slow vertical diffusion the heavy gases and temperature variations. possible mechanisms for energy-transport channels from the near-ground layer to the atmosphere and ionosphere are mainly divided in two groups. one possibility consists of generation of electric field or modification of background electric field [e.g. recent works from pulinets and ouzounov 2011, denisenko et al. 2013, harrison et al. 2014, sorokin and hayakawa 2013]. other possibility is excitation of internal gravity waves which propagate upward and disturb the ionosphere before earthquakes [e.g. molchanov and hayakawa 2008, shalimov et al. 2009, erokhin et al. 2013]. it is impossible to select one of the models because there are not now any convincing experimental evidences supporting one or the other theory of seismo-ionospheric coupling. we can only assume that there can be several physical mechanisms responsible for seismo-ionospheric perturbation. in europe at present operates the south european network infrep (international network for frontier research on earthquake precursors) which consists of eleven receivers that record the amplitude of vlf/lf signals in two frequency bands –20–60 khz and 150– 300 khz. the equipment is constructed by the italian company elettronika s.r.l. [biagi et al. 2011]. our receiving stations deployed both in europe and asia are equipment with the ultramsk receivers (new zealand, lf*em research, ltd) which can record simultaneously the amplitude and phase of msk (minimum shift keying) modulated signals in the frequency range 10–50 khz from several (10–12) transmitters. msk signals have fixed frequencies in the narrow band 50–100 hz around the main frequency and adequate phase stability. the receivers provide measurements with time resolutions ranging from 50 min to 60 s. for our purpose we use sampling frequency of 20 s. the analysis reported in this paper in connection with the earthquake in vrancea zone on november 22, 2014, is based on the data recorded by the three vlf/lf stations in europe: graz (austria), sheffield (u.k.) and moscow (russia). 2. data analysis and results the relative locations of our observing ultramsk stations and several transmitters in europe are plotted in figure 1 together with the position of the epicenter of earthquake on november 22, 2014 in the vrancea seismogenic zone. vrancea zone is one of the most acsolovieva et al. 2 figure 1. a map of the wave paths under analysis of the european network of vlf/lf stations. the position of the ultramsk receivers in moscow (mos), sheffield (shf) and graz (grz) and several transmitters in europe is shown. signals from three transmitters are analyzed in the work: its (45.9 khz), icv (20.27 khz) and tbb (26.7 khz). pink ellipse shows the vrancea zone in which the strongest earthquakes occur. solid brown circle shows the epicenter of earthquake on november 22, 2014, with m=5.5 (usgs/neic). the area where possible precursors of an earthquake can be found is shown by the hollow blue circle. 3 tive seismic areas in europe. it is situated where the southern and eastern carpathian mountains join (see figure 1). because earthquake hypocenters are concentrated within a small volume, the vrancea zone is often described as unique. maximum magnitude of earthquakes in this area can reach up to m = 7.0–7.8. foci of earthquakes are located in the crust as well as in mantle on depths ~70–200 km [e.g. knapp et al. 2005]. due to the depth of the hypocenters the earthquakes can be felt as far as moscow. the earthquake with m = 5.5 (depth = 39 km) occurred at 19:14 ut on november 22, 2014, in vrancea area (45.87°n 27.16°e) according to geological survey of national earthquake information center of u.s. (usgs/neic). the hypocenter was in the crust and seismic effect from the earthquake was noticeable only in the neighboring countries: romania, including bucharest, on the north part of bulgaria, in moldova (kishinev, tiraspol) and at the territory of the ukraine (kiev, odessa, dnepropetrovsk). there was interruption in power supply in some areas and the cellular communications in kishinev was out of operation for 15 min. three sub-ionospheric wave paths of our network pass near or above the vrancea zone (see figure 1). they are: tbb (26.7 khz) mos (moscow), its (45.9 khz) mos and icv (20.27 khz) mos. the earthquake was strong enough. the radius of area where possible precursors can be found according to dobrovolsky et al. [1979] is about 240 km. this area is shown in figure 1 by the hollow blue circle. as it seen from the figure only the its transmitter signal recorded at the moscow station crosses the earthquake of possible precursors zone. the distance from this path to the epicenter of earthquake was about 110–115 km while the distances from the two other paths were about 380 km. nevertheless, we checked signals variation during november 2014 in all the three wave paths. for the analysis we used a nighttime residual signal of amplitude da: da = a − <a>; where a is the amplitude for the current day, and <a> is the monthly averaged signals calculated using the data from undisturbed days. the results are shown in figure 2. five-days running averaged nighttime residual signals are represented here. the tbb transmitter in turkey was out of operation during about ten days after november 23. the clear decrease in the amplitude of its transmitter signal is observed around the day when the earthquake occurred but two other signals do not reveal anomalies at that time. for validation of the results we made an analysis of the its (45.9 khz) transmitter signal recorded at the graz (grz) and sheffield (shf) stations during the same period. it is important to analyze the signals from the same transmitters to be sure in reliability of findings, because characteristics of the signal disturbances can be different for the signals with different frequencies. the its signal recorded at the moscow station passed over seismic area while the same signal recorded at the two other stations passed far away from the epicenter of the earthquake (see figure 1). so, the path ist-mos was ‘seismic’ path and paths its-grz (graz) and its-shf (sheffield) were ‘aseismic’ control paths. a comparison of observations is shown in figure 3. the top panel shows dst index of magnetic activity, the next panel shows variations of atmospheric pressure in moscow, the lower three panels show averaged in nighttime residual signals of amplitude in graz, sheffield and moscow. an arrow at the bottom panel indicates the day of earthquake and horizontal blue rectangle is the period when anomalous signal was observed in moscow. as it seen from the figure the magnetic activity was very low during this period (dst~0), so that the observed anomalies cannot be attributed to the geomagnetic environment. not very strong proton burst (in the range 0.6–4.2 mev) was observed on november 2-3 and rather weak relativistic electron flux was registered on november 15-16 (eps/goes measurements). no outer-zone particles fluxes were observed during other days of november. another factor that can influence on the behavior of the vlf/lf signals is fluctuations of the atmospheric pressure [rozhnoi et al. 2014]. strong decrease in atmospheric pressure was recorded in moscow at the beginning of november. it could result in decrease of the signal in moscow detected on november 7-11. we have to note that strong cyclones propagated through europe during the first ten days or so of november. variations of the its signal at the sheffield station at the beginning of the month coincide with variations of vlf effects of the vrancea earthquake figure 2. five-days running averaged nighttime residual signals of the amplitude recorded in moscow during november 2014. the amplitude of signals from three transmitters: tbb, icv and its is shown. the arrow indicates the day of earthquake occurrence on november 22, 2014. blue rectangle shows the activity of a mediterranean hurricane qendresa. atmospheric pressure recorded in the middle england. strong cyclone raged in germany with wind velocity up to 150 km/h. mediterranean cyclone brought strong precipitation to italy which led to flood. the cyclone which formed on november 7 near the north africa coast and moved afterwards towards sicily was such strong as a tropical cyclone. such strong cyclone activity in europe can explain decrease in the its signal at the first ten days of the month. especially strong decrease was detected in the icv transmitter signal (see figure 2). the weather was much better during the second half of november (as seen from the pressure data in moscow) and could not cause the observed anomalies in the signal. figure 4 shows an example of amplitude (left) and phase (right) of the its transmitter signal recorded during local night in sheffield, graz and moscow (top to bottom) on november 21, 2014 (red line) together with the monthly averaged signal (brown). the latter was calculated using data from undisturbed days. only the amplitude of the signal in sheffield is shown because the phase data was not good enough in november at this station. it is clear that the measurements of amplitude and phase in graz and sheffield (control paths) closely follow the quiet day measurements. the signal propagating along the path its-mos, however, exhibits a significant decrease in amplitude (about 10 db) during the period from about 11 pm to 01 am ut. at the same time the phase variations of up to 50 degrees relative to the averaged signal are observed at the moscow station. the similar anomalies were observed at the moscow stations during 3 days before the earthquake and two days after it. beside the method of ‘bay-like’ nighttime phase and amplitude anomalies and its derivative – method of residual nighttime vlf/lf signal which was described above –, another, terminator time (tt) method was used for ‘seismic’ and control paths. the method is based on determination of the characteristics minimums of phase and amplitude daily variations during sunset and sunrise, and it is widely used now to identify possible seismo-ionospheric effects. this method was developed by hayakawa and molchanov in analysis of the kobe earthquake (m = 7.2) [hayakawa et al. 1996]. they found an anomalous shift in fluctuations of tt 3 days before the main shock of earthquake. after the first resuls, the method was used in analysis of many earthquakes. the method and theoretical investigations are described in many details in book of molchanov and hayakawa [2008]. the authors conclude that ‘…there is dependence on number of modes efficiently involved into the modal superposition, supposing that all the propagating modes (n≤5) could be intensified. so it can be applied only for the more or less short wave paths (d< 1000–2000 km)…’. in present case we have more long ‘seismic’ path (~3000 km). the results of analysis for amplitude variations in ‘seismic’ and control paths are shown in figure 5. the analysis was made for morning terminator which in this season in middle latitude in europe moves gradually. evening terminator solovieva et al. 4 figure 3. the measurements of the its transmitter signal at three vlf/lf stations during november 2014. the top panel shows dst index of magnetic activity, the next panel shows variations of atmospheric pressure in moscow, the lower three panels show averaged for nighttime the residual signals of amplitude in graz, sheffield and moscow. an arrow at the bottom panel indicates the day of earthquake and horizontal blue rectangle is the period when anomalous signal was observed in moscow. 5 practically doesn’t change its position. for ‘seismic’ itsmos path we see anomalous shift in the morning minimum exceeding 2v level on 6 min, two days before the earthquake. while the variations of morning minimum in the amplitude of lf signal in the control path don’t deviate noticeably from the normal position of terminator, strong variations of the lf morning minimum are observed around time of the earthquake in the ‘seismic’ path. although the results are not very remarkable (maybe, due to the length of the path) they, nevertheless, confirm possible seismo-ionospheric effects revealed by the method of ‘bay-like’ nighttime signal anomalies. the spectra of the filtered phase (in the range of periods from 1 to 60 minutes) of the nighttime its signal recorded at the moscow station on possible seismoinduced anomalous day of november 21 together with the spectra of undisturbed signals recorded at the moscow station on november 14 and graz station on november 21 are shown in figure 5. the spectra of undisturbed signals are very similar for the two stations with maximum of spectral density about 40 min. the spectrum of the seismo-induced anomalous signal, however, shows a main maximum corresponding to wave period of 30 min and also contains waves with shorter periods in the range 20–30 min, which corresponds to the range of internal gravity waves periods. the shift of the signal spectra of the seismo-induced anomalous days towards shorter periods in comparison with that of quiet or magnetic-disturbed signals has been found in our previous studies for strong earthquakes in euvlf effects of the vrancea earthquake figure 4. an example of amplitude (left) and phase (right) of the its transmitter signal recorded during local night in sheffield, graz and moscow (top to bottom) on november 21, 2014 (red line) together with monthly averaged signal (brown). figure 5. in the upper panel sunrise terminator times for its-grz (control path, blue line), in the bottom panel sunrise terminator times for its-mos (‘seismic’ path, red line). dash lines show 2 standard deviations relative the real terminator at the altitude 100 km. the vertical axis indicates the time in hours from the midnight. the arrow in the bottom panel shows the occurrence of the earthquake on november 22, 2014. rope and the far east region [rozhnoi et al. 2012]. this result can be considered as some corroboration that the possible mechanism for energy penetration from the earthquake origin, through the atmosphere, and into the ionosphere is based on the excitation and upward propagation of internal gravity waves. 3. conclusion the analysis of the vlf/lf signals during november 2014 have revealed perturbations of the nighttime subionospheric lf signal passing above the epicentre of earthquake occurred on november 22, 2014, in the vrancea area. negative amplitude anomalies have been observed during 3 days before the earthquake and two days after it at the analysis of the nighttime residual signal amplitude. tt method have revealed anomalous shift in the lf signal morning minimum two days before the earthquake and strong variations of the minimum around the time of the earthquake. the result has been confirmed by comparison of data measurements in control ‘aseismic’ paths. other possible influences both from above (geomagnetic activity, proton bursts and relativistic electron fluxes) and below (cyclonic activity) which can produce perturbations in the ionosphere were taken into consideration. the recent development of the observation systems in europe, asia and the far east can provide useful information on the properties and position of the perturbation region in connection with seismic activity. the use of a network of observation makes it possible to separate the local vlf/lf 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice the 24 august 2016 amatrice earth-quake: macroseismic survey in the damage area and ems intensity assessment annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 1 the 24 august 2016 amatrice earthquake: macroseismic survey in the damage area and ems intensity assessment quest w.g.: raffaele azzaro*, andrea tertulliani, filippo bernardini, romano camassi, sergio del mese, emanuela ercolani, laura graziani, mario locati, alessandra maramai, vera pessina, antonio rossi, andrea rovida, paola albini, luca arcoraci, michele berardi, christian bignami, beatriz brizuela, corrado castellano, viviana castelli, salvatore d’amico, vera d’amico, antonio fodarella, ilaria leschiutta, alessandro piscini, manuela sbarra *raffaele.azzaro@ingv.it istituto nazionale di geofisica e vulcanologia, italy abstract the 24 august 2016 earthquake very heavily struck the central sector of the apennines among the lazio,umbria, marche and abruzzi regions, devastating the town of amatrice, the nearby villages and other localities along the tronto valley. in this paper we present the results of the macroseismic field survey carried out using the european macroseismic scale (ems) to take the heterogeneity of the building stock into account. we focused on the epicentral area, where geological conditions may also have contributed to the severity of damage. on the whole, we investigated 143 localities; the maximum intensity 10 ems has been estimated for amatrice, pescara del tronto and some villages in between. the severely damaged area (8-9 ems) covers a strip trending broadly n-s and extending 15 km in length and 5 km in width; minor damage occurred over an area up to 35 km northward from the epicenter. i. introduction n august 24, 2016, 01:36 gmt, central italy was hit by a mw 6.0 earthquake that caused major destruction and 299 fatalities over the central sector of the apennines among the lazio, umbria, marche and abruzzi regions. this earthquake was the strongest event of the related seismic sequence, with more than 11,500 events recorded until the end of september. among the aftershocks, another earthquake exceeded magnitude 5.0 (the same day, at 02:33 gmt, with mw 5.4), and some fifteen had local magnitude larger than 4.0 (ingv working group on the amatrice earthquake, 2016). seismological data acquired by the ingv seismic network locate the epicentre of the mw 6.0 earthquake near the village of accumoli (province of rieti), and show an aftershocks distribution along a nwse trend extending for approximately 40 km. this area partially fills the gap between the o annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 2 1997-98 umbria-marche (to the north) and 2009 l’aquila (to the south) seismic sequences. the damage area includes the umbria and marche regions as well, with the provinces of rieti, perugia, ascoli piceno and l’aquila. according to the earthquake catalogue cpti15 (rovida et al., 2016) the first relevant known event in this area occurred in 1627 (epicentral intensity i0 7-8 mcs, mw 5.3), but information on its effects is scarce being limited to accumoli. the largest known earthquake destroyed amatrice and surroundings in 1639 (i0 9-10 mcs, mw 6.2), with a seismic scenario not unlike the present event. figure 1: historical seismicity in the central sector of apennines from 1000 to 2014 (data from rovida et al., 2016). the 24 august 2016 mainshock and the mw 5.4 aftershock (02:33 gmt) are indicated by white stars. other local events affected monti della laga in 1646 (i0 9 mcs, mw 5.9) and again amatrice in 1672 (i0 7-8 mcs, mw 5.3). apart from the 1703 seismic sequence (i0max 11 mcs, mw 6.9) that heavily struck the nearby valnerina, there are no earthquakes listed in the catalogue till the late 1800s, when the accumoli-amatrice area was repeatedly hit by some minor events (1883, i0 7 mcs, mw 5.1; 1910, i0 5-6 mcs, mw 4.6; 1916, i0 8, mw 5.5; 1943, i0 6-7, mw 4.9; 1950, i0 4-5 mcs, mw 4.7 and 1963, i0 7 mcs, mw 4.7). this paper does not include the macroseismic effects related to the later events of 26 and 30 october, with mw 5.9 and 6.5 respectively, which occurred in the same area and aggravated the damage scenario described here. ii. features of buildings and their vulnerability compared with the mercalli-cancani-sieberg (mcs) scale, the use of the european macroseismic scale (ems, grünthal, 1998) allows a more coherent interpretation of the damage scenario in the case of settlements made up of very different building typologies. however, its use requires a more detailed consideration of the type of structure and a specific evaluation of the vulnerability. the ems classifies buildings into 6 classes of decreasing vulnerability, from a to e. most of the damaged localities consist of historic centres with numerous small hamlets located on the top of the hills or ridges of the apennines. the traditional buildings have one or more basement levels with barrel vaults, partially above the ground because of the steepness of the terrain, while the upper levels have wooden slabs and roofs. the construction material, of local origin, mainly consists of stones of extremely irregular size, generally rounded or only crudely squared off, assembled incoherently and often without mortar (fig. 2). the most common vertical structures are made up of double layer walls, unconnected, with stone faces and rubble masonry fill. this type of structure is classified among the most vulnerable (class a). both structural aggregates annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 3 and isolated buildings show signs of modifications over time: elevations, openings in the walls, insertion of the terraces at slab level, use of heterogeneous materials other than the original kinds (concrete blocks, bricks) have comcompromised the structural behavior. many buildings have been retrofitted by replacing wooden floors with hard and heavy slabs made of reinforced concrete and bricks figure 2: pascellata (valle castellana): masonry building with walls faced with stone and rubble masonry fill. the part in bad state of maintenance compromises the rest of the building. figure 3: amatrice: retrofit with (reinforced) concrete slab on old vertical walls without adequate reinforcement or proper connections between walls and floor. (laterocemento), without adequate reinforcement of the old vertical walls and proper connections between walls and floors. (fig. 3). usually, retrofitting decreased the vulnerability of the buildings (from class a to class b), but in many cases these modifications have compromised the integrity of the entire structure. many houses were used only as vacation homes or are partially abandoned because of depopulation in the last fifty years. in these cases, the lack of a good state of maintenance has increased their vulnerability and often made them a source of risk to nearby buildings (fig. 2). on the west side of the apennines there are some localities characterized by reconstructed or reinforced buildings following the 1979 and 1997-98 earthquakes (norcia and umbriamarche events, respectively). historic buildings in square stone masonry with horizontal layers of bricks are less vulnerable (class b) and were not heavily damaged for the most part, as well as reinforced concrete (rc) buildings built more recently (vulnerability class c and d). iii. macroseismic survey: method and results soon after the earthquake, the quest group (quick earthquake survey team) undertook the macroseismic survey with the aim of defining the damage scenario for civil protection purposes. during the first phase of the emergency, we carried out an expeditious investigation in cooperation with the civil protection department by adopting the mercallicancani-sieberg (mcs) macroseismic scale (galli et al., 2016). in order to properly take into account the variability of the damage effects due to building vulnerability, and hence the associated uncertainties in the intensity asannals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 4 sessment, the survey continued with detailed inspections of the most damaged areas by using the ems. the investigation involved a working group with specific field experience in applying the ems in cases of recent, damaging earthquakes (experts from the ingv departments of rome, bologna, catania, milano and pisa), who operated according to a procedure that has been implemented, step by step, since the 1997-98 umbria-marche seismic sequence (camassi et al., 2008) till the 2009 l’aquila and 2012 emilia earthquakes (tertulliani et al., 2011, 2012; azzaro et al. 2011). a remote support was operated from the head office in rome to coordinate the field teams. the ems survey also included smaller settlements that had not been considered previously and was extended to investigate the transition area from moderate damage to slightnegligible one, i.e. 5-6 degrees ems, where the survey was stopped. in other words, areas not affected by damage (only felt reports) are not considered in the present macroseismic investigation. in total, we have inspected more than 150 localities, hamlets included. for each of them the focus was on establishing the number of buildings, their typology, the elements of specific vulnerability, grade of damage and percentage of damaged edifices and, i.e. the diagnostics of the ems. all the information was reported in common forms routinely used by quest, which were later used to assess intensity. it should be stressed that problematic sites were surveyed up to three times by different teams in order to reduce the subjectivity of evaluation. the final number of localities for which it has been possible to assign an intensity value according to the ems guidelines was 143, referred to 40 municipalities. the distribution of the macroseismic effects estimated by the ems is shown in figure 4 and the localities inventory is given in the appendix. in general, the distribution of the effects in the near-field (i≥8 ems) shows a pattern elongated in the nnw-sse direction, with a strong attenuation of intensities southward (province of l’aquila). on the contrary, slight but diffuse damage extends northward (provinces of fermo and macerata). lastly, a possible effect of aggravation of damage due to the mw 5.4 aftershock is observed at san pellegrino di norcia. a short overview of damage scenarios and related ems intensities in some key localities, is hereinafter reported. degree 10 some localities in the epicentral area suffered the almost total destruction of low qualityhigh vulnerability buildings (class a) and of many masonry buildings (class b). exemplary is the situation of the historical centre of amatrice, where also some rc buildings (class c) collapsed (fig. 5). a special case is represented by the settlement of pescara del tronto, built on debris cone deposits of a landslide that has been reactivated during the earthquake. degree 9 many hamlets of the territory north of amatrice suffered very heavy damage total or near total collapse of class a buildings. degree 8 accumoli, arquata del tronto and other localities over an area trending broadly n-s for 15 km, were affected by severe damage and some collapses of class a buildings. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 5 degree 7 moderate damage and a few partial collapses of class a buildings occurred in an area extending for 30 km along the same n-s strike. worthy of note is the case of fornisco, within the slight damage area to the east of the epicentre. degree 6 the distribution of slight damage (i.e. cracks in the plaster, partial collapse of chimneys) is particular. a substantial number of localities lie west of amatrice, just 4-6 km far from the area of degree 9 or 10. another group is located in the northern part of the surveyed area, looking like an anomaly inside the degree 5 zone. figure 4: ems intensity map of the august 24, 2016 amatrice earthquake. black stars, instrumental epicenters; white star, macroseimic epicenter calculated by using the boxer method (gasperini et al., 2010): lat. 42.679, long. 13.280; the magnitude obtained by intensity data is mw 6.3. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 6 degree 5 as reported in the ems guidelines (grünthal, 1998), we also considered localities where occasional, very slight damage (i.e. hair-line cracks in the plaster) affected a few buildings of vulnerability class a and b. due to the difficulties of observing these negligible effects (often visible only inside the edifices) as well figure 5: example of damage to rc buildings: a) saletta (amatrice), collapse of individual residential houses; b) amatrice: a two-storey building before the 24 august 2016 earthquake (photo by google™ street view, 2016), and c) the resulting sandwich collapse. inset: detail of the smooth reinforcing bars. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 7 as the extension of the involved area, the inventory of localities is incomplete. iv. discussion and conclusions from the macroseismic point of view, the epicentral area of the 24 august 2016 earthquake extends between amatrice and arquata del tronto. here most of the ancient vulnerable building stock (class a) collapsed while the remaining part of masonry structures suffered from heavy (e.g. large diagonal cracks in the walls) to very heavy damage (serious failure of walls). rc buildings (class c) generally suffered heavy non-structural damage, although the failure of infill panels is rather striking; some collapses also occurred (e.g. in amatrice). however, the high concentration of damage in specific zones depends not only on purely engineering factors (poor materials, slipshod workmanship, state of repair and maintenance), but also on site effects (geotechnical properties of terrains, presence of steep slopes and ridges). in some cases (e.g. comunanza, amandola) the heterogeneity of building vulnerability determined a striking difference in damage severity between historical centres and recent suburbs. furthermore, it should be stressed that the effects of past seismicity in the localities of the epicentral area were not properly considered to improve building vulnerability over time. the lack of memory of relevant seismic impact can be ascribed to the long time elapsed since the last local event in 1639, with the only exception of norcia that was struck in 1979. on the other hand, some recent repairs produced an unexpected but recurrent type of damage (i.e. the collapse of old stonewalls under the weight of heavy rc slabs). this situation deserves further analyses if we consider that seismic rules have been mandatory since 1915 for amatrice, 1927 for accumoli and only 1984 for arquata del tronto. acknowledgments ina cecić is thanked for her suggestions that improved the manuscript. appendix list of the investigated localities and ems intensities assessed for the 24 august 2016 earthquake (mw 6.0). references azzaro, r., barbano, m.s., d'amico, s., tuvè, t., scarfì, l. and mostaccio, a. (2011). the l'aquila 2009 earthquake: an application of the european macroseismic scale to the damage survey in the epicentral area. boll. geof. teor. appl., 52:561-581. camassi, r., azzaro, r. and tertulliani, a. (2008). macroseismology: the lessons learnt from the 1997/1998 colfiorito seismic sequence. ann. geophys., 51:331-342. galli, p., peronace, e. and tertulliani, a. (eds.) (2016). rapporto sugli effetti del terremoto del 24 agosto 2016 di amatrice in scala mcs. roma, rapporto congiunto dpc, cnr‐igag, ingv, 15 pp., doi: 10.5281/zenodo.161323. http://quest.ingv.it/index.php/rilievimacrosismici. gasperini, p., vannucci, g., tripone, d. and boschi, e. (2010). the location and sizing of historical earthquakes using the attenuation of macroseismic intensity with distance. bull. seism. soc. am., 100:2035–2066. grünthal, g. (ed.) (1998). european macroseismic scale 1998 (ems-98). cahiers du centre européen de géodynamique et de séismolohttp://quest.ingv.it/index.php/rilievi-macrosismici http://quest.ingv.it/index.php/rilievi-macrosismici annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7203 8 gie, 15, luxembourg, 99 pp., http://www.ecgs.lu/cahiers-bleus/. ingv working group on the amatrice earthquake (2016). second summary report on the m 6.0 amatrice earthquake of august 24, 2016 (central italy), doi: 10.5281/zenodo.166241. https://ingvterremoti.files.wordpress.com/2 016/11/en20160920_secondo_rapporto_final e_del-2408def.pdf. rovida, a., locati, m., camassi, r., lolli, b. and gasperini, p. (eds) (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. ingv, doi: http://doi.org/10.6092/ingv.it-cpti15. http://emidius.mi.ingv.it/cpti15-dbmi15/. tertulliani, a., arcoraci, l., berardi, m., bernardini, f., camassi, r., castellano, c., del mese, s., ercolani, e., graziani, l., leschiutta, i., rossi, a. and vecchi, m. (2011). an application of ems98 in a medium-sized city: the case of l’aquila (central italy) after the april 6, 2009 mw 6.3 earthquake. bull. earthquake eng., 9:67-80. tertulliani, a., arcoraci, l., berardi, m., bernardini, f., brizuela, b., castellano, c., del mese, s., ercolani, e., graziani, l., maramai, a., rossi, a., sbarra, m. and vecchi, m. (2012). the emilia 2012 sequence: a macroseismic survey. ann. geophys., "the emilia (northern italy) seismic sequence of mayjune, 2012: preliminary data and results", anzidei, m., maramai, a. and montone, p. (eds), 55:4, 679-687; doi 10.4401/ag-6140. http://www.ecgs.lu/cahiers-bleus/ https://ingvterremoti.files.wordpress.com/2016/11/en20160920_secondo_rapporto_finale_del-2408def.pdf https://ingvterremoti.files.wordpress.com/2016/11/en20160920_secondo_rapporto_finale_del-2408def.pdf https://ingvterremoti.files.wordpress.com/2016/11/en20160920_secondo_rapporto_finale_del-2408def.pdf http://emidius.mi.ingv.it/cpti15-dbmi15/ some collsideratiolls od the usual derivation or the pockels (or helmholtz) equation in steady two dimensionai fluid dyllamics i~. ljrli.'os."lo (*) e. saj:t:sti (**) f. 7.iltilj.i (***) :-\u'l\iarl 'l'hl' simplest non·!inear motion of <1 liui,1 i~ stmlh'd; i.e. the ~tea,ly two dimension<11 motio!l of a pedect flni,\. 'l'hes!' equation8 h<1yc il l"emarkahle pradic'al importanc(' hecause they ,lesltihe the air1l1otion over il1011ntains ami a wake on an oeeallic (lunen"" in partieular the numbel' of physieal ~olutions is discusse,l in r('!atio!l to the known bonndi1l'y ('oihlitions. rl\.~i'cyru. :-:'i ~tlhlia il caho più semplice di l1ioto non lineare di nn fluido: t'ome, ad esempio, il moto non viscoso stazionario hidillwn~io­ n;j!e di un lluido omogenco. qul'sj(' ('(illazioni hanno applkazioni pratielw notevoli, come l1ioto sopra lc montag!le o scia ,li isole in conenti sta· zionari!'. ~i esaminano l' discutono, in partif'olarc, il nlllll('to ili soluzioni fisielli' in rapporto alle conllizioni al ('ontol'!lo f'lillosi'.iuì\'. iii t1w sl.m]y of tlte vl'oblem or t.he 8teally two tlìmlmsìonal airho\\" over a molllltaìll or of 1.jlc wake generatetl by an islantl olt a sl.muly-stal.e no\\', many ]leovle iiìsellssel1 t.he ilel'ivat,ìon of all eqllatiol! fol' the stream fllllr,t,iol! a.m] st.lltlietl the conespontling solutions. (*) istituto di fisica dell'atmosfera del c.~.h. hom<1. (u) istitut.o ,li fisìl'a dl'll'l~niycrsitil, 1.x.f.x. _ homa. (***) rockfellpl" tllliversity, x,l',c., lo021 xciv l'ork, u.i-l.a. 3 8 l . d e l l ' o s s o e . s a l u s t i f . z i k i l l i one of t h e s e equations has been known as t h e pockels (or helmholtz) equation, it is an elliptic linear p a r t i a l differential equation for t h e s t r e a m f u n c t i o n . i n this p a p e r , we derive t h i s equation, as is usually done(3'6-7 , 8-9'1 0), and we p o i n t o u t some inaccuracies of t h e usual derivation. the steady motion of a non viscous t w o dimensional fluid in t h e plane x, z can be described b y t h e following e q u a t i o n s : 7)u ï)x twc a « dx ¡¡u + w + w + w ml ôit> w î>71 ix = — + à 0 = — .s'(z) w + i>w = a w [1] [2] [3] [4] where , are p a r t i a l derivatives respect t o x and 2 respectively, dx <)z m, w are t h e velocity components in t h e x and z directions respectively, / c o n s t a n t is t h e so called convection or buoyancy p a r a m e t e r , s(z) is a given f u n c t i o n , a = c o n s t a n t (the t e r m aw in t h e c o n t i n u i t y equation [4] allows a r e d u c t i o n of t h e fluid density w i t h t h e a l t i t u d e if t h e z-axis is directed u p w a r d ) , n is a q u a n t i t y connected with t h e pressure and 0 is t h e p o t e n t i a l t e m p e r a t u r e . f o r a complete derivation of equations [1], [2], [3], [4] see t h e very interesting book of g u t m a n (6). i n t r o d u c i n g t h e s t r e a m f u n c t i o n y> b y eq. [5] az ~iz t>x [5] we derive f r o m eqs. [1], [2], [3], [4] an e q u a t i o n for rp. f i r s t of all we notice t h a t eq. [4] is always satisfied b y t h e choice of eq. [5]. multiplying eq. [3] b y c~az we h a v e -az -az w c u + e w 7>x î)z s(z) w s o m e c o n s i d e r a t i o n s on t i i e u s u a i . d e r i v a t i o n e t c . 3 9 t h a t means by eq. [5] i*. j® j® 8(z) ^ = o t h a t is 5 a <*, , ) ^ + i *<*> d s ' } = 0 o t h e d e t e r m i n a n t of t h e j a c o b i a n is zero. then we h a v e 0 + j s(z') dz' = ft (y>) [6] where fi(y>) is an a r b i t r a r y function (some r e q u i r e m e n t of regularity of fi{y>) is needed). e l i m i n a t i n g n f r o m equations [1], [2] and m a k i n g use of eq. [5] we h a v e where 5 (ip, l y>) i ^ ao 5 (x, z) ¡| 1 t>x 2az ( a-w y-w 7>w l w = e ——h —1 h 0 —— r \ a#2 az2 dz [7] using eq. [6] so t h a t io „ ~dw ,r = ™ j |s| a (f> l yj) i1 cty; a (x, z) i dx t h a t is in j a c o b i a n f o r m a a(x z) l f — z) i = o 40 l . d e l l ' o s s o e . s a l u s t i f . z i r i l l i then we h a v e hip = u(y) + /•/'i(v') « where /2 as /1 is an a r b i t r a r y f u n c t i o n . thus t h e system of eqs. [1], [2], [3], [4], has been reduced t o eq. [9] for t h e stream f u n c t i o n ip t h a t depends 011 t h e a r b i t r a r y f u n c t i o n s /1, /2. i n order t o h a v e a "well posed" problem t h e domain where t h e equation lias t o b e verified, t h e b o u n d a r y conditions and t h e properties of t h e f u n c t i o n s /1, /2 h a v e t o be specified. t h e pockels (or helmholtz) e q u a t i o n is a p a r t i c u l a r form of eq. [9]. in p a r t i c u l a r t h e pockels e q u a t i o n is commonly derived in t h i s way. the region where eq. [9] is studied is of t h e t y p e z > 8 (x) where 8(®) is a regular f u n c t i o n of x (i.e. s(x) is t h e profile of a m o u n t a i n ) see fig. 1. (in order t o simplify t h e problem we assume now s(z) = s = const. a = 0 ) the b o u n d a r y condition 011 t h e system of eqs. [1], [2], [3], [4] are t h e following [ g u t m a n (")]: x x = — 00 z ^ 8 ( — 0 0 ) [10] [ 1 1 ] s o m e c o n s i d e r a t i o n s on t i i e u s u a i . d e r i v a t i o n e t c . 41 wo cam assume s (— oo) = 0. by eqs. [5] and [10] we h a v e xp = y>x = vz + c x = — oo, z > 0 [12] assuming c = 0 b y eqs. [5] [11] a n d [12] we h a v e m f ) = y r y> x = — 0 0 , 2 > 0 [13] and finally from eqs. [9], [12] a n d [13] we obtain j2(yj) = y) x = o o , 2 ^ 0 . [ 1 4 ] assuming t h a t fi(y>), u(f) h a v e everywhere t h e f o r m given by eqs. [13] [14] we reach to t h e pockels' equation: r+ + —(w — v z) = 0 [15 7>x2 ii/2 v2 r l j a r a t h e r similar discussion is usually done for t h e oceanographic case: t h e wake of an island on an oceanic stream. i t has t o be remarked t h a t in this problem t h e coriolis force is t a k e n into account, as in m a n y other meteorological cases. f r o m our p o i n t of view t h e derivation of eq. [15] is non satisf a c t o r y for t w o different reason: 1) the t r a n s l a t i o n of t h e b o u n d a r y conditions on u, 0 in t e r m s of tp is n o t completely correct. i n f a c t u = v where x = — oo 2 > 0 and u = —does n o t 1>z imply y> = vt in a n y finite region. the meaning of u = v when x = — oo, z ^ 0 is lim u (x, z) = v, z >-• 0. x —> oo so t h a t w h a t we can expect is t h a t lim ip (x, z) = = vz if x -»oo 2 ^ 0 t h a t means y> = vz, x = — oo, 2 ^ 0 . 42 l . d e l l ' o s s o e . s a l u s t i f . z i k i l l i moreover t h e b o u n d a r y condition on 0 a n d eq. [6] tell us t h a t l i m 0 (x, z) = l i m (s z — ft(ip (x, z))), z > 0 x—> — x — co t h a t is 0=£z — fi(vz), 0. so we h a v e t h a t /i is fixed only for p o s i t i v e a r g u m e n t s (z ^ 0) and n o t everywhere as people seems t o believe. finally using eq. [9| in order t o derive t h e f o r m of /2 we use t h e following f a c t : lim a y> (x, z) = a lim ip (x, z) = a y>x = 0, (a = 5 + ) x co \ 0ji 0 z 1 this is also incorrect if no special assumptions are done on t h e previous limit. however if we s t u d y t h e problem in a compact domain, as it is t h e case of a numerical c o m p u t a t i o n , t h e f a r u p s t r e a m p a r t of t h e domain t a k e t h e rule of i n this case t h e s i t u a t i o n is t h a t people, hope t o know f r o m t h e conditions in p a r t of t h e b o u n d a r y not only the solutions of one well determined elliptic non linear differential equation b u t also t h e explicit shape of t h e non linear p a r t ji(y>) and mr)this appears to us as an o v e r s t a t e m e n t . 2) the b o u n d a r y condition eqs. [10], [11] are not enough t o determine an u n i q u e solution of eqs. [1], [2], [3], [4] and so also t h e equation [15] has only t h e b o u n d a r y condition given b y eq. [12] which is not, enough t o determine a u n i q u e solution. concluding t h e idea t h a t t h e b o u n d a r y conditions can d e t e r m i n e t h e form of /i, /2 in eq. [9] seems due t o other non rigorous reasons, p e r h a p s of historical origin. i t has t o be remarked, however, t h a t t h e above derivation of eq. [15] is now a classical method in geophysics and t h a t an enormous a m o u n t of practical work is done on it. but it has t o be said also t h a t an enormous m a t h e m a t i c a l literat u r e exists on eq. [9] under various assumptions of ft, /2 [see for example (7)]. the problem however of determining t h e physical f o r m of /1, /•> and so t h e physical solutions of eq. [9] is in our opinion essentially open, so t h e use of t h e p r i m i t i v e equations [1], [2], [3], [4] seems to us t h e most reasonable way t o h a n d l e these problems. s o m e c o n s i d e r a t i o n s o n t i i e u s u a i . d e r i v a t i o n e t c . 43 r e f e r e n c e s ( • ) b a r d o s ( ' . , 1 9 7 3 . existence et. unicité de la solution de l'équation d'euler en dimension deux. p r e p r i n g université de p a r i s , v i i . ( 2 ) b e r g e r m . s . , 1 9 7 0 . multiple solutions oj non linear operator equations arising from the calculus of variations. " p r o c e e d i n g s in p u r e m a t h e m a t i c s " , 18, p p . 10-27. (3) ciiia-siiun-yin, 1960. exact solutions for steady two-dimensional flow of a stratified fluid. " j . fluid m e c h . " , 9. p p . 161-174. ( 4 ) d a p r a t o g., 1 9 7 1 . somme d'applications non linéaires. " s y m p o s i a m a t h e m a t i c a " , 7, p p . 233-268. ( 5 ) d a p r a t o g., 1 9 7 3 . seminario di analisi non lineare. i s t i t u t o matem a t i c o " g . c a s t e l n u o v o " , r o m a . ( 6 ) g u t m a n l . n . , 1 9 7 2 . introduction to the non linear theory of inesoscale methereological processes. israel press, j e r u s a l e m . ( ' ) k a t o t . , 1 9 6 7 . on the classical solutions oj the two-dimensional nonstationary euler equation. " a r c h . r a t . mech. a n a l . " , 24, p p . 3 0 2 3 2 4 . («) l o n g r . r . , 1 9 5 2 . the flow of a liquid pas a barrier in a rotating spherical shell. " . i . m e t e o r " , 9, p p . 1 8 7 1 9 9 . ( 9 ) l o n g r . r . , 1 9 5 3 . some aspects in the flow of a stratified fluid, i. " t e l l u s " , 5, p p . 41-58. ( 1 0 ) s a i n t g u i l t b . , 1 9 6 0 . ecoulement plan. autour d'un cercle en présence d'une force de coriolis de paramètre variant avec la latitude. "c. r . acati. sei.", p a r i s , 2 5 0 , p p . 2 9 2 0 2 9 2 1 . ( u ) w h i t e w . b . , 1 9 7 1 . a rossby wake due to an island in an eastward current. " j . pliys. o c e a n o g r a p h y " , 1. vol48/04/2005def 583 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words polymerisation – basicity – oxidation state – water speciation – temkin model 1. introduction during the last century many models have been proposed dealing with the thermodynamic properties of silicate melts (or slags), especially with the goal of understanding slag-melt partitioning of elements for industrial reasons relevant to steelmaking. the heuristic capability of a model assessing silicate melts energetics becomes particularly important when dealing with the generalised problem of multicomponent, mutliphase equilibria, as shown by the thermochemical treatments presented in ghiorso et al. (1983), ghiorso and sack (1994), pelton (1998), papale (1999) and moretti et al. (2003). to model element solubility and speciation it is necessary to account fully for the compositional variables of the system. nevertheless, compositional variables cannot be understood without a comprehensive model able to rescale measured concentrations in terms of component activities, which represent the obvious control parameters of chemical reactions taking place in the system. therefore, the choice of the model for component activities represents a crucial step in silicate melt thermodynamics. here, i will show that ionic models accounting for the variable degree of polymeripolymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts roberto moretti istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, napoli, italy abstract in order to describe and quantify the reactivity of silicate melts, the ionic notation provided by the temkin formalism has been historically accepted, giving rise to the study of melt chemical equilibria in terms of completely dissociated ionic species. indeed, ionic modelling of melts works properly as long as the true extension of the anionic matrix is known. this information may be attained in the framework of the toop-samis (1962a,b) model, through a parameterisation of the acid-base properties of the dissolved oxides. moreover, by combining the polymeric model of toop and samis with the «group basicity» concept of duffy and ingram (1973, 1974a,b, 1976) the bulk optical basicity (duffy and ingram, 1971; duffy, 1992) of molten silicates and glasses can be split into two distinct contributions, i.e. the basicity of the dissolved basic oxides and the basicity of the polymeric units. application to practical cases, such as the assessment of the oxidation state of iron, require bridging of the energetic gap between the standard state of completely dissociated component (temkin standard state) and the standard state of pure melt component at p and t of interest. on this basis it is possible to set up a preliminary model for iron speciation in both anhydrous and hydrous aluminosilicate melts. in the case of hydrous melts, i introduce both acidic and basic dissociation of the water component, requiring the combined occurrence of h+ cations, oh− free anions and, to a very minor extent, of t-oh groups. the amphoteric behaviour of water revealed by this study is therefore in line with the earlier prediction of fraser (1975). mailing address: dr. roberto moretti, istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, via diocleziano 328, 80124 napoli, italy; e-mail: moretti@ov.ingv.it 584 roberto moretti sation represent suitable tools to model silicate melt reactivity. the earliest theories on the constitution of silicate slags were developed as a result of mineralogical examination of the constituents of solidified melts and may be classified as molecular models. in 1923, colclough (quoted in gaskell, 2000), perhaps anticipating bowen (1928), pointed out that, as the phases occurring in the solid state are formed by selective crystallisation from the melt, mineralogical examination cannot provide evidence that the compounds, observed in the solid state, had existed in the liquid. the concept of thermodynamic equilibrium was particularly stressed by schenck (1945), who recognised that each reaction proceeds up to the achievement of equilibrium, independently of the extension of the system. in molecular models it is assumed that molecular complexes are formed in the melt in proportions dictated by the overall melt stoichiometry. gaskell (2000) states that a common feature of molecular models is that «rather than the constitution of the slag being deduced from the observed behaviour, a set of arbitrary assumptions was manipulated to reproduce the observed behaviour. comparison among the approaches shows that the degree of success of any model in giving the required reproduction is not sensitive to the finer details of the assumed constitution or to the internal thermodynamic consistency of the model». this recalls somehow the normative deconvolution adopted by ghiorso et al. (1983) in their multicomponent free energy minimization procedure conducted in a regular solution approximation of the zeroth order. in fact this model does not account for the true nature of silicate melts and the choice of components reflects the topology of the compositional space investigated by the authors. most melts or slags are however «ionic» rather than «molecular» liquids. the existence of ions in the liquid state was already demonstrated in 1923 by sauerwald and neuendorff (quoted in gaskell, 2000) who successfully electrolysed iron silicate melts, and in 1924 by farup et al. (quoted in gaskell, 2000) who measured the conductivity of melts in the systems cao-sio2 and cao-al2o3-sio2. tamman (1931, quoted in gaskell, 2000) already assumed electrolytic dissociation of metallurgical slags. a first application of an ionic theory of slags to the treatment of slag-metal equilibria was made by herasymenko (1938) who assumed that slags were mixtures of fe2+, mn2+, ca2+, al3+ and sio44-. the need for an ionic model of silicate melts emerged clearly from the experimental determinations on viscosity and electrical conductivity. further electrical conductivity measurements carried out by various authors indicate an essentially ionic unipolar conductivity (bockris et al., 1952a,b; bockris and mellors, 1956; waffe and weill, 1975), where charge transfer evidently operates by cations, with anions being essentially stationary. transference of electronic charges (hand n-type conductivity) is observed only in melts enriched in transition elements, where band conduction and electron hopping phenomena are favoured. i will hereon dismiss the neutral molecular approach and accept that silicate melts, like other fused salts, are ionic liquids. in an ionic melt, coulombic forces acting between charges of opposite sign lead to a relative short-distance ordering of ions, with anions surrounded by cations and vice versa. the probability of finding a cation replacing an anion in such ordering is effectively zero and, from a statistical point of view, the melt can be considered a quasi-lattice, with two distinct sites, usually defined as «anion matrix» and «cation matrix». the distinction between these two matrices was made by temkin (1945), who considered that the electrostatic forces characterising ionic interactions are sufficiently strong to make the arrangements of ions in the pure fused salts and in mixture of salts similar to those in the crystalline state, implying co-ordination of cations by anions. in the temkin approach to fused salts, the activity of component az in the ideal mixture of the two fused salts az and by is expressed by the temkin equation (1.1) a straight application of the temkin model to silicate melts is inadequate because the exten.a x x n n n n nn ,az melt a z a b z y za $= + + = 585 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts sion of the anion matrix varies in a complicated fashion with composition. this complexity is reflected by activity-composition relationships deviating from the ideal temkin model behaviour and may be fully accounted for by polymer chemistry. 2. the toop-samis model in polymeric models of silicate melts, it is postulated that, at each composition, for given pt values, the melt is characterized by an equilibrium distribution of several ionic species of oxygen, metal cations and ionic silicate polymers. the charge balance of a polymerization reaction involving sio44− monomers may be formally described by a homogenous reaction involving three forms of oxygen: singly bonded o-, doubly bonded o0 (or «bridging oxygen»), and free oxygen o2− (fincham and richardson, 1954) (2.1) polymer chemistry shows that the larger the various polymers are, the more their reactivity is independent of the length of polymer chains. this fact, known as the «principle of equal reactivity of co-condensing functional groups», has been verified in fused polyphosphate systems, which are analogous, in several respects, to silicate melts (cf. fraser, 1977; ottonello, 1997). assuming this principle to be valid, the equilibrium constant of reaction (2.1) becomes (2.2) terms in parentheses represent the number of moles in the melt, which can be used in place of activities since all three species of oxygen speciate over the same matrix (anion matrix: the three oxygen types either mix ideally or their activity coefficients cancel out ). toop and samis (1962 a,b) showed that in a binary melt mo-sio2 the total number of bonds per mole of melt is given by (2.3)( ) ( ) n2 o o 4 sio0 2+ =( ) ( ) ( ) .k o o o .2 1 2 0 2 = .o o o2 2 2m m m 0 2 , +where nsio2 are the moles of sio2 in the mosio2 melt. the toop and samis model assumes the basic oxide mo to be completely dissociated. the number of bridging oxygens in the melt is thus (2.4) mass balance gives the number of moles of free oxygen per unit mole of melt (2.5) where obviously 1−nsio2 represents the number of moles of basic oxide in the melt. combining the various equations one gets (2.6) which reduces to a quadratic equation in (o−) (2.7) given nsio2, eq. (2.7) may be solved. since the number of oxygens which react according to eq. (2.2) is (o−)/2 per mole of melt, the free energy of mixing per mole of melt is (2.8) the validity of this equation has been proved many times (see for example fraser, 1975; ottonello, 1997; ottonello et al., 2001). 3. polymerisation and acid-base properties it is evident that the reaction (2.1) between the three oxygen species represents the characteristic process of an acid-base reaction in oxide systems, which was defined by flood and förland (1947) as «the transfer of an oxygen ion from a state of polarisation to another». this acceptance is particularly important in silicate melts and glasses where polymerisation reactions governing extension and distribution of polymeric units may be restated as (as already shown) simple [( ) / ] .lng rt ko 2 .mixing 2 1=∆ ( ) ( ) ( ) ( ) ( ) . k n n n o 4 1 o 2 2 8 1 0 . io io io 2 2 1 s s s 2 2 2 + + + + = ( ) [ ( )] [ ( )] k n n 4 o 4 o 2 2 o . io 2 1 2 sios 2 2= ( ) ( ) ( ) no 1 2 o sio 2 2 = -( ) ( ) . n o 2 4 osio0 2= 586 roberto moretti acid-base reactions involving three distinct polarisation states of oxygen (see eq. (2.2)). although the lux-flood formulation formally differs from a brönsted-lowry (protonbased) exchange, the two formulations are mutually consistent (flood and förland, 1947) and, with this proviso, the link between redox and acid-base exchanges in the lux-flood acceptation is represented by the «normal oxygen electrode» equilibrium (3.1) thus in aprotic solvents o2− replaces h+. a basic oxide is the one capable of furnishing oxygen ions and an acidic oxide is one that associates oxygen ions (3.2) it is well established that the lux-flood acidbase property of dissolved oxides markedly affects the extent of polymerisation by producing or consuming free oxygen ions (o2−). thus, for a generic oxide mo (fraser, 1975, 1977): (3.3) (3.4) with (3.3) and (3.4) showing acidic and basic behaviours, respectively. although it is conceptually immediate to envisage directly a direct relationship between polymerisation constant (k2) and basicity of dissolved oxides in binary systems (toop and samis, 1962,a,b), the extension to multicomponent melts and glasses is not immediate. moreover, in the presence of altervalent elements such as fe, mutual interactions are established between the normal oxygen electrode reaction (3.1) and the dissociation equilibria ((3.3)-(3.4)). these may be addressed by taking into account both the polymeric nature of the anion matrix, along the guidelines of the toop-samis model, and fraser’s amphotheric treatment of dissolved oxides. in a chemically complex melt or glass, the capability of transferring fractional electronic charges from the ligands to the central cation depends in a complex fashion on the melt or mo mo o2 2, + -+ mo o mo2 2 2 ,+ .base acid o2, + . 2 1 o 2e o2 2 ,+ glass structure, which affects the polarisation state of the ligand itself. the mean polarisation state of the various ligands (mainly oxide ions in natural silicate melts) and their ability to transfer fractional electronic charges to the central cation are nevertheless conveniently represented by an experimentally observable parameter which is an index of the basicity of the medium: the optical basicity (see duffy, 1992 for an exhaustive review of the subject). a formal link is thus needed between polymerisation constant and optical basicity. 4. the «optical basicity» concept as we have already seen there are strict mutual interconnections between the concepts of «oxidation state» and «basicity», whenever this last term is referred to non-protonated systems. following jørgensen (1969) we may define oxidation by means of four distinct formalisms: – formal oxidation number denoted by roman numeral superscripts (including the non-roman notations 0, −i, ...) whenever this does not imply an accurate description of the true nature of the complex (i.e. niii for nickel in the aqueous complex ni(h2o)6 2+ or svi in the sulphate complex so4 2−). – spectroscopic oxidation states derived by experimentally observed excited levels, denoted by on-line roman numerals in parentheses; i.e. cr(iii)o6, ni(ii)cl6, etc. – conditional oxidation states derived from electronic configuration; i.e. [ar]3d 5 for mn2+, fe2+, etc. – distributed oxidation states adjacent atoms bonded in the complex share the electrons equally; i.e. c〈0〉 and h〈0〉 in ch4; s〈0〉 and o〈-i〉 in sio4 4− etc. the usual notation with arabic numeral superscripts (i.e. li+, mg2+, cr3+, f-) which we will hereafter refer to « formal ionic charges» should be reserved for cases in which «entities and molecules are sufficiently separated and are either neutral or carry charges which are a positive or negative integer multiplied by the protonic charge» (jørgensen, 1969). in brönsted’s formalism, redox reactions are those involving exchange of electrons be587 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts tween the reactants and acid-base reactions are those involving protons. the brönsted acidbase function for protonated systems is usually represented as (4.1) the link joining redox and acid-base reactions is the normal hydrogen electrode reaction (4.2) as in non-protonated systems the brönstedlowry formalism is better replaced by the luxflood acid-base definition (flood and förland, 1947), three sorts of transitions are involved in acid-base equilibria: i) transitions accompanied by an alteration of the co-ordination number of oxygen, but no change of distributed oxidation state, for atoms with a high ionisation energy, such as for example in (4.3) ii) transitions which do not involve any change in co-ordination number, but a change in distributed oxidation state, for medium ionisation energy atoms, such as in (4.4) iii) transitions which involve both change in co-ordination number and in distributed oxidation state, leading to the formation of isolated cations, such as occurring for low ionisation energy atoms (4.5) in silicate melts we envisage simple acid-base reactions involving three distinct polarisation states of oxygen, eq. (2.1), which in a distributed oxidation state notation may be expressed as follows: (4.6) although formally different, the brönstedlowry and the lux-flood formulations are mu.o i o o2 0 2,+.cao ca o0 2, ++ -2 .o o osi i si 0 2 24 4 2, +.co co o0 03 2 2 2, + .2e 2h h2,+ + .acid base h, + + tually consistent and, if we still accept the brönsted definition of redox reactions, then it may be readily seen that the link joining redox and acid-base exchanges in the lux-flood acceptation is now represented by the oxygen electrode equilibrium. the fact that reaction (3.1) could also resemble an equilibrium between a lewis acid (i.e. a substance acting as acceptor of a pair of electrons; o2 in our case) and a lewis base (i.e. a substance acting as donor of a pair of electrons; o2− in our case), leading to a stable octet configuration, further emphasises the necessity of distinguishing a redox equilibrium, i.e. «a reaction involving free electrons (in the broad sense of the term) and resulting in a change of formal oxidation number» from an acid-base equilibrium which, in the brönsted-lowry formalism is basically «the transfer of an oxygen ion from a state of polarisation to another», as already noted. although we may conceive formal integer charges for isolated non-interacting (gaseous) ions (i.e. li+, mg2+, fe3+, etc.) and (although much less evidently) for isolated complexes, this formalism cannot be readily transferred to the formal oxidation state within a complex since, in most cases, it is in contrast with both the quantum-mechanical concept of «electron density» and with the notion of «fractional ionic character of a bond» (pauling, 1932, 1960; gordy, 1950; hinze et al., 1963; phillips, 1970, see below). moreover, in a chemically complex melt or glass the capability of transferring fractional electronic charges from the ligands to the central cation depends in a complex fashion on the melt structure, which affects the polarisation state of the ligand itself. the mean polarisation state of the various ligands (mainly oxide ions in natural silicate melts) and their ability to transfer fractional electronic charges to the central cation are nevertheless conveniently represented by the «optical basicity» of the medium (duffy and ingram, 1971). the concept of optical basicity arises primarily from the systematic study of the orbital expansion (or «nephe-lauxetic effect») induced by an increased localised donor pressure on p-block metals. metal ions such as tli (group iii), pbii (group iv), biiii (group v) (i.e. oxidation number = group number −2) have an electron pair in the outermost 588 roberto moretti (6s) orbital. when trace concentrations of the metal are dissolved in melts and glasses, coordination with the ligand field anions results in formation of molecular orbitals (mos) which increase the electron density of the inner shells. the consequent shielding of nuclear charges affects the energy involved in the outermost 6s → 6 p transitions which become lower the more the inner shell electron density is increased. lowering of the 6s → 6 p transition energy is experimentally observed as a dramatic red-ward shift of the 6s → 6p uv absorption band when the p-block free ion is immersed in a ligand field. the spectroscopic shift of the 1s0 → 3p1 absorption band experienced by pb when passing from a free ion (pb2+) condition to pbii in an o2ligand field is for instance 60700 − 29700 = 31000 cm−1 (duffy and ingram, 1971, 1974b, 1976). for biiii the analogous redward shift is 28.8 kk (1 kk = 1000 cm−1) and is 18.3 kk for tli. this phenomenon is quantitatively understood in terms of ligand field theory by analogy with the behavior of 3d, 4d and 5f transition ions (jørgensen, 1962, 1969; and references therein). in octahedral 3d chromophores for example, the energy splitting between anti bonding , mos and the dxy, dxz, dyz aos of the central atom (∆ cov-σ) is linearly affected by the position of the ligand in the spectrochemical series (represented by parameter f ) and by a representative parameter of the central cation (g), according to the simple relationship (jørgensen, 1969) (4.7) the precision achieved by this simple equation in describing ∆ cov-σ in 3d 3, 3d 6 and 3d 8 chromophores is remarkable (cf. table 5.8 in jørgensen, 1969). however, the energy shift induced by changes in f is not so marked as to allow a basicity scale to be proposed on the basis of eq. (4.7) (cf. table 5.5. in jørgensen, 1969). jørgensen (1962) pointed out that the expansion of the radial function consequent on lowering of the effective nuclear charge (zeff) results in three distinct nephelauxetic parameters (β ). these are β ll for the interaction between two electrons in the lower sub-shell, β lu. for the interaction between an electron in the f ( ) ( )cov ligand cation=∆ -v g$ . * x y2 2ψ -*z2ψ lower and an electron in the upper sub-shell and β uu for the interaction between two electrons in the upper sub-shell (4.8) (4.9) (4.10) zeff* in the above equations is the effective nuclear charge of the free cation; a is related to the mean radial distance of the orbital from the centre of nuclear charges and β ll > β lu > β uu. according to duffy and ingram (1971) the optical basicity, λ, is represented by the ratio h/h* where h is the jørgensen’s (1962) function of the ligand in the polarisation state of interest and h* is the same function relative to the ligand in an unpolarised state (i.e. free o2− ions in an oxide medium) (4.11) with ν free = 1s0 → 3p1 absorption band of the free p-block cation; ν glass = 1s0 → 3p1 absorption band measured in the glass; ν *= 1s0→ 3p1 absorption band in a free o2− medium. as we see in fig. 1, the optical basicity of simple oxides appears related to the atomistic properties of the intervening cations, such as the pauling and sanderson’s electronegativities (χp and χs respectively) or the free ion polarizability (young et al., 1992). although duffy and ingram (1974a) suggest a simple linear dependency between the reciprocal of optical basicity λ (or «basicity moderating parameter» γ ; see later) and pauling electronegativity χp (straight line in fig. 1), here i focus attention on the fact that a strict connection between optical basicity and bond ionicity should exist. the true nature of this relationship (which we depict as a second order polynomial dependency in the same figure) can be envisaged by equating the spectroscopic definition of fractional ionic character of a bond (phillips, h h 1 1 * * * free free glass = = = b b o o o o λ a z z *uu u 4 eff eff=b a a z z *lu l u 2 2 eff eff=b a z z *ll l 4 eff eff=b 589 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts 1970) with pauling notation (4.12) where χo and χm are respectively the pauling electronegativity of oxygen and metal and (4.13) ei in eq. (4.13) is the «ionic energy gap» and eg is the total energy gap between bonding and anti bonding orbitals (ec corresponds to eg for the non-polar covalent bond in the same row of the periodic table, with a correction for inter-atomic spacing) (4.14) with = plasma frequency for valence electrons; and ε∞ = optical dielectric constant. the operational relationship between fractional ionic character of the bond in the oxide and optical basicity is the following: (4.15) ( . .4 6242 4 6702-= f . .r 0 9517i 1 =λ 2) p'~ ( ) e 1 g p 2 1 ' = -f ~ 3 i cg 2 2 2 1 ( ) .e e e= + ( ) expf e e e 1 4 1 i i c i 2 2 2 o m= + = -| |^ h: d based on eqs. (4.12) and (4.15) we get the following approximation: (4.16) which suggests an optical basicity of 0.225 for a purely covalent non polar bond (see also eq. (4.13)). this value compares favourably with the value 0.46 ÷ 0.48 indicated by duffy and ingram (1974a) for sio2 which has a fractional ionic character around 0.5 (0.516 according to pauling, 1960). the reciprocal of optical basicity («basicity moderating parameter» γ m, according to duffy and ingram, 1973) represents the tendency of an oxide forming cation m to reduce the localised donor properties of oxide ions. it is related to the optical basicity of the medium by (4.17) where zm = formal oxidation number of cation m in mo; zo = formal oxidation number of oxide ion in mo; r m = stoichiometric ratio between number of cations m and number of total oxide ions in the medium. although γ m reduces to in a simple single oxide medium, it has the property to describe the additive jorgensen’s h function in chemically complex media (with a, b, ... oxide forming cations) according to (duffy and ingram, 1973): (4.18) moreover, based on eqs. (4.16) and (4.17) the optical basicity of the medium may be expressed as . (4.19) direct estimates of the basicity moderating parameter of the central cation may then be obtained from electronegativies (fig. 1) by appliz r z r 2 2a a a b b b# # f= + + c c λ . h h z z r z z r 1 1 1 1 1 * o a o b a a b b# # $ $ f = c c c c m m < f z z r m m m m m # # =c λ o . e e e 4 6 1 1 i c i 2 2 2 . + λ c m fig. 1. pauling and sanderson electronegativities and basicity moderating parameter. the straight line is the intepolation of duffy and ingram (1974), γ = 1.36 × ( χp − 0.26). 590 roberto moretti cation of the second order polynomials (4.20) (4.21) . ) .0 96=(r.0 1880|+.0 0193|s-.0 7398= s2c 2 .0 97)=(r.0 7404|+.0 6703|p.0 8969= p2c 2 equation (4.20), although conceptually less obvious than (4.16), is operationally more accurate and has been adopted by ottonello et al. (2001) to evaluate basicity parameters, whenever literature values were controversial or lacking. table i summarises the functional relationships among the previously discussed parameters. table i. optical basicity λ and basicity moderating parameter of the central cation γ according to various sources. pauling’s and sanderson’s electronegativities (pauling, 1932, 1960; sanderson, 1967) are also listed. λ, γ, χs: adimensional; χp: ev (from ottonello et al., 2001). oxide λ γ χ p χ s (1) (2) (3) (4) (5) (6) (6) (7) h2o 0.40 0.39 2.56 2.50 2.15 3.55 li2o 1.00 1.00 1.0 0.74 b2o3 0.42 0.42 2.38 2.0 2.84 na2o 1.15 1.15 1.15 1.15 1.15 0.87 0.87 0.9 0.70 mgo 0.78 0.78 0.78 0.78 0.78 0.78 1.28 1.28 1.2 1.99 al2o3 0.60 0.60 0.60 0.61 0.59 0.59 1.69 1.67 1.5 2.25 sio2 0.48 0.46 0.48 0.48 0.48 0.48 2.09 2.09 1.8 2.62 p2o5 0.40 0.40 0.33 0.8 0.40 0.40 2.50 2.50 2.1 3.34 so3 0.33 0.25 0.33 3.03 3.03 2.5 4.11 k2o 1.40 1.4 1.40 1.40 1.36 0.74 0.71 0.8 0.41 cao 1.00 1.00 1.00 1.00 1.00 0.99 1.00 1.00 1.0 1.22 tio2 0.65 0.61 0.61 0.58 1.72 1.54 1.6 1.60 cr2o3 0.70 0.58 1.72 1.6 1.88 mno 0.94-1.03 0.98 0.90 0.59 0.59 1.69 1.69 1.5 2.07 feo 0.86-1.08 1.03 1.00 1.03 0.51 0.48 2.09 1.354 1.8 2.10 fe2o3 0.73-0.81 0.77 1.21 0.48 0.48 2.09 2.09 1.8 2.10 coo 0.51 1.96 1.96 1.7 2.10 nio 0.48 2.09 2.09 1.8 2.10 cu2o 0.43 2.30 2.30 1.9 2.60 zno 0.82-0.98 0.58 1.72 1.72 1.6 2.84 sro 1.10 1.03 0.97 1.0 1.00 sno 0.48 2.09 2.09 1.8 3.10 bao 1.15 1.15 1.15 1.15 1.12 0.89 0.9 0.78 pbo 0.48 2.09 2.09 1.8 3.08 (1) duffy (1992); (2) young et al. (1992); (3) duffy and ingram (1974a,b); (4) sosinsky and sommerville (1986); (5) gaskell (1982); (6) ottonello et al. (2001); eq. (4.19) (note that λ=γ −1); (7) ottonello et al. (2001); obtained by non linear minimization of feo thermodynamic activity data in multicomponent melts. 591 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts 4.1. group basicity since we have established that the experimentally derived concept of optical basicity is related to atomistic properties and first principles such as pauling’s electronegativity and fractional ionic character of a bond (or «bond order») (see eqs. (4.12), (4.15) and (4.16)) we may tentatively extend the concept to formal entities such as the silica polymers (or «structons» in the sense of fraser, 1975, 1977) within binary joins and then to complex melts. to address the problem we may still use eq. (4.19) but now the cation to oxide ratio represents the local coordination present in the structon (duffy and ingram, 1976). for instance, for a monomer sio44− ra = m/o〈−i〉 = 1/4, for a dimer si2o7 6− (or si〈0〉2ο〈0〉ο〈−i〉66− in distributed oxidation state notation) ra = 1/3, and so on. group basicities of the most important polymer units present in silicate melts are listed in table ii. we may note that, with the increase of polymerisation, the group basicity of polymer chains progressively decreases due to the decrease of the ratio o〈−i〉/o〈0〉. we may also note that the presence of foreign cations (i.e. aliii, feiii) in the polymer units changes the group basicity in a linear fashion with respect to the group stoichiometry. we may now inquire if, based on this new concept, the basicity of a complex medium such as a silicate melt or glass may be expected to vary linearly with composition along a binary join. for this purpose the three forms of oxygen present in the melt (i.e. o2−, o〈−i〉 and o〈0〉) (toop and samis, 1962a,b) are related to melt stoichiometry and to the polymerisation reaction constant k2 by mass balance (eqs. (2.4), (2.5), (2.7)). along the polymerisation path (from monomers sio44− to silica), the group basicity of the polyanion (or structon) matrix, λ«structons», may be expressed as a linear function of the ratio between singly bonded oxygen and total oxygen within the structons (see also table ii) (4.22) where , are respectively the group basicity of the sio44– monomer and of the sio2 tectosilicate. the bulk basicity of the medium results expressed as (4.23) where (4.24) and λμο is the basicity of the oxide in the binary join mo-sio2. by combining eq. (4.23) with the mass balances (2.4), (2.5) and (2.7) we may evaluate which part of the bulk basicity of the medium may be ascribed to the effect of the structon mao o i o o0 2= + + -^ ^ ]h h g/ +( )1-λ λ λ = + 2 ( ) . o o o o i o o i o o i o 0 5 1 0 medium mo mo sio sio 2 4 4 # + + λ λ ^ ^ ^ ^ ^ h h h h h / / / / sio2λsio44λ λ+= o i o o i 1 0 structons sio sio4 4 2 + λ λ ^ ^ ^ ^ h h h htable ii. basicities of «structons» along the polymerisation path. group group basicity sio44− 0.50000 0.73923 alo45− 0.37500 0.85227 feo45− 0.37500 0.80443 si2o76− 0.57143 0.70198 al2o711− 0.42857 0.83117 fe2o711− 0.42857 0.77649 sialo77− 0.76658 sifeo77− 0.73924 si3o108− 0.60000 0.68707 si4o1310− 0.61538 0.67906 sio32−; si2o64−; si3o96−; 0.66667 0.65231 si4o128−; si5o1510−; 1-ring si2o52−; si4o104−; si6o156−; 0.80000 0.58278 si8o208−; si10o2510−; 2-rings si3o72−; si9o216−; si12o289−; 0.85714 0.55297 3-rings sio2 1.00000 0.47847 al2o3 1.00000 0.60606 fe2o3 1.00000 0.47847 z r 2 a a# 592 roberto moretti trix (third and fourth terms on the right in eq. (4.23)). in fig. 2 see how the basicity of the structon matrix is affected by the value of the polymerisation constant, which dictates the structons contribution to the basicity of the medium. moles of quasi-chemical species of oxygen are also shown for comparison. table iii lists structural details along the join cao-sio2, calculated adopting k2 . 1 = 0.0017 (toop and samis, 1962a) and λmo = 1.00 (duffy and ingram, 1974b). we may note that the bulk optical basicity of the medium is identical to that obtainable through direct application of eq. (4.19). however, note also that the bulk basicity of the medium may be entirely ascribed to the structon matrix over most part of the compositional join (for xsio2 >1/3). the basicity control operated by the structon matrix is the more extended the more the dissolved oxide in the binary mo-sio2 system is basic (in the lux-flood acceptation of the term) and the lower the polymerisation constant k2.1. we have seen that in solving the various mass balances for different values of the polymerisation constant, toop and samis (1962a,b) showed that the fincham and richardson assumption of a purely anionic contribution to the gibbs free energy of mixing in binaries mo-sio2 (with mo completely dissociated basic oxide) holds true. it must be noted that the toop-samis model accounts for (negative) chemical interactions only and is not able to reproduce the experimentally observed solvi at high silica content even in simple systems. this implies additional excess gibbs free energy terms of mixing which are not fig. 2. basicity of the medium and of the structon matrix (ordinate right-axis) calculated along the binary join cao-sio2 assuming λcao = 1 and k2.1= 0.0017 (toop and samis, 1962a) and k2.1= 1 (guess value put for comparison). abundances of quasi-chemical species of oxygen (ordinate left-axis) are shown as a function of the molar fraction of sio2. table iii. basicity of structons and basicity of the medium in the mo-sio2 binary (mo is cao). λmo=1, kp = 0.0017. nsio2 o 〈0〉 o 〈−i〉 o 2− λstructons λmedium 0.000 0.000 0.000 1.000 0.739 1.000 0.100 0.000 0.399 0.700 0.739 0.905 0.200 0.003 0.795 0.403 0.738 0.826 0.300 0.019 1.162 0.119 0.735 0.759 0.400 0.211 1.178 0.011 0.700 0.702 0.500 0.503 0.993 0.003 0.652 0.652 0.600 0.801 0.797 0.001 0.609 0.609 0.700 1.101 0.599 0.001 0.570 0.571 0.800 1.400 0.400 0.000 0.536 0.536 0.900 1.700 0.200 0.000 0.506 0.506 1.000 2.000 0.000 0.000 0.478 0.478 593 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts accounted by the model itself, as shown by ottonello (2001). these additional terms (mechanical strain) are much smaller than chemical interaction, but are sufficient to open solvi at high silica content (ottonello, 2001, 2005; ottonello and moretti, 2004). nevertheless, the mechanical strain energy contribution is so low that eqs. (2.1) and (2.8) may be used by ottonello et al. (2001) to infer appropriate values of the polymerisation constant k2.1 in mo-sio2 systems from measured thermodynamic activities or gibbs free energy of mixing (fig. 3 and ottonello and moretti, 2004). a simple t-independent exponential relationship linking the polymerisation constant k2 and the basicity moderating parameter of the dissolved cation, based on the estimates of toop and samis (1962a,b), hess (1971), reyes and gaskell (1983), masson et al. (1970) on binary mo-sio2 melts has been proposed by ottonello et al. (2001) (4.25) m m . . ( . ) ln k r 4 662 1 1445 0 997.2 1v v= + + =c c+ + 2 (4.26) since, according to eq. (4.18), the jorgensen h function is a generalised property, accepting the validity of the above discussed relationship which links ∆γ and k2.1 in simple systems, the extent of polymerisation of chemically complex melts and glasses may be readily obtained by a simple mass balance involving oxide constituents and their specific γ values (ottonello et al., 2001) (4.27) where and are respectively atom fraction and basicity moderating parameter of network modifiers and network formers in one mole of the multicomponent melt or slag. we thus have a formal link between acidbase properties of the medium (expressed as a «contrast» between formers and modifier basicities) and polymerisation constants. this equation represents a high t approximation, polymerisation constants on single binaries being defined at a unique t for each binary (see table 1 in ottonello et al., 2001). more precise formulations in the multicomponent space are in progress, based on new t-dependent parameterisations of polymerisation in binary joins (ottonello and moretti, 2004). let us furnish now more details about the calculation of the anionic structure of the melt. to estimate k2.1 for the various binaries, hess (1971) adopted temkin’s model for fused salts, which ascribes the thermodynamic activity of the molten oxide to the activity product of ionic fractions over cationic and anionic matrixes, i.e. (4.28) where terms in brackets denote activities and terms in parentheses number of moles. m m m [ ] [ ] ( ) ( ) a o cations anions o o 2 v 2 v #= =+ + / / txt j j+ +h hcm mx iiv vc+ + exp= # m x m. . k x4 662 1 1445 . , melt i j t 2 1 i j i j vv + c+ + + +h h tc a l : e / / mm ( . ) ( . ) . expk r 4 10 3 8452 0 926 . , sioo2 1 5 2 v#= = c+ 2 fig. 3. linear relationship between logarithm of the constant of polymerization reaction (2.1) in mosio2 melts and basicity moderating parameter of the network modifier mν+. ∆γ = γ mν+ −γ si4+ . 594 roberto moretti since in a mo-sio2 melt (4.29) and since the number of moles of o2− in the melt is related to k2.1 and nsio2 by mass balance (eqs. (2.4), (2.5) and (2.8)), the evaluation of amo rests solely on the estimate of the number of structons present in the anion matrix (4.30) . (4.31) to evaluate σstructons, hess (1971) followed the method devised by flory (1953) (4.32) where nsi are the moles of silicon in the melt, is the number of silicon atoms in the structon of and p〈0〉 is the fraction of silicon bonds that link to doubly bonded oxygens, i.e. (4.33) being dependent on composition in a complex fashion, eq. (4.32) cannot be easily adapted to multicomponent melts. to address the problem quantitatively we must know the acid-base behavior of each dissolved oxide (i.e. the disproportionation between «network formers» and «network modifiers») in order to consider the effect of mixing of both cationic and anionic constituents over the two sulblattices of interest (the activity of the generic oxide mo being now the bulk of eq. (4.27)). to solve the problem toop and samis (1962a,b) proposed a «polymerisation path» of general validity, based essentially on the viscosity data of bockris et al. (1955). as shown by ottonello (1983), the polymerization path of the toop-samis model may be reconducted to or .p i2 o 0 o 2 o 0 0 = + -^ ^ ^ h h h 7 a or $= ! structons ( ) ! ( ) ! n p p p p1 1 2 2 4 3 x n si x x si si 0 0 2 0 0 2 1 $ +o o o = r r r ^ ^h h < < f f / / m ( ) ( ) a o structons o 2 2 = +o 7 a/ anions o onsstruct2= +-] g/ / m( ) cationsv /+ / the simple form (4.34) where the simple power function (4.35) accounts for the mean number of tetrahedrally coordinated cation per structon, p〈−ι〉 is the proportion of singly bonded oxygen (4.36) and accounts for the presence of charge-balanced tetrahedrally coordinated cations other than si (see ottonello, 1983, for details), i.e. nt = nsi + nal + nfeiii. equations (4.32) and (4.34) lead to consistent results, in terms of k2.1 when applied to mo-sio2 melts. however it has been shown by ottonello (1983) that the polymerisation path in chemically complex immiscible liquid portions (watson, 1976; ryerson and hess, 1978, 1980) is better represented by the exponential form (4.37) the amount of experimental data is at presentday large enough to allow a re-estimation of the above parameters. through non linear minimisation techniques we obtained (4.38) such a form allows us to define the extension of the anion matrix in the toop-samis framework along a unique polymerisation path. the generalisations made for complex melts through eqs. (4.27), (4.28), (4.34), (4.36) and (4.38), together with eq. (4.17) and the toopsamis equations constitute the polymeric model. deconvolution of the investigated systems into network formers and network modifiers was carried out by (ottonello et al., 2001) assuming amphotheric behaviour for al2o3 and fe2o3: i.e. al3+ and fe3+ are considered to have a partly acidic behaviour. they are network formers if counterbalanced by basic oxides such as h2o, ( . ) .exp p2 8776 ( )#=o .1 7165. )2 67-( .exp ln p5 i= -o lr n p o 0 o i o i t i = + + ^ ^ ^ h h h 7 a p .i 1 95=o --r n structons t= or / 595 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts na2o, k2o and cao, to form complexes of the type mal4+, mfe4+or m0.5al4+, m0.5fe4+ which polymerise as sio44+ does. a completely acidic behaviour was assigned to p5+ while ti4+ is treated as a network modifier, in agreement with new experimental observations (see later on) and with findings based on quantum chemistry argumentation applied to glass clusters (kowada et al., 1995). a more precise definition of the luxflood character of the various oxides was later achieved by ottonello and moretti (2004) based on the conversion of the pelton and blander (1986) quasi-chemical parameterisation of binary mo-sio2 interactions to the hybrid polymeric model. the new classification does not substantially alter the preceding observations. on the basis of the above considerations, we may now address the problem of reactivity of altervalent oxides (i.e. those oxides which do disproportionate into different valent states and have potentially distinct structural roles) on a thermochemical basis. 5. factors controlling the feii/feiii ratio in silicate melts since iron is the main redox buffer in natural silicate melts, the treatment is specifically developed for this element. equilibrium among dissolved iron in glasses or melts, the anion matrix and the gaseous phase is usually written in the form (johnston, 1964; duffy, 1996) (5.1) however, based on what was previously stated, this form is misleading since it confuses formal oxidation numbers with formal ionic charges. let us assume that we have spectroscopic evidence that ferric iron is only present in polyanionic complexes, and that the octahedral coordination of fe2+ observed in melts is the result of ionic couplings dictated by simple coulomb interactions. we will have in this case the bulk homogeneous equilibrium (5.2) +fe,e+o+( )fe iii o o o 2 7 0 7 2 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt melt 4 5 2 2+ + + .o4+fe4,o4+fe4 ( ) ( ) ( ) ( )melt melt melt gas 3 2 2 2 + + obtained summing up the partial equilibria (5.3) (5.4) (5.5) (5.6) and the corresponding heterogeneous reaction (5.7) adopting the usual polymeric notation it may be easily seen that iron reduction induces depolymerisation of the melt structure (5.8) (5.9) let us now imagine that we have the spectroscopic evidence that ferrous iron form in the melt or glass true octahedral complexes (in the sense of pauling, 1960). we could write the following homogeneous equilibria: (5.10) (5.11) (5.12) 6o e ,++ + ( ) ( ) fe iii o i fe ii o o o3 0 ( ) ( ) ( ) ( ) ( ) melt melt melt melt melt 4 5 6 10 2 , + o5 e ,++ + ( ) ( ) fe iii o i fe ii o o o 2 5 0 2 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt melt 4 5 6 10 2 , + o4 e ,++ + ( ) ( ) fe iii o i fe ii o o2 0 ( ) ( ) ( ) ( ) melt melt melt melt 4 5 6 10 , -+fe,si o 2 .o + + ( )fe iii o sio 2 7 7 2 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 2 7 6 2 4 4+ + +fe,e+si o o + + ( )fe iii o sio 2 7 7 2 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 2 7 6 2 4 4 2+ + ( )fe iii o o fe o o 2 7 0 7 4 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 2 2 ,+ + + + fe fee( ) ( )melt melt 3 2 ,++ + o o o 2 7 2 7 0 7( ) ( ) ( )melt melt melt 2 ,+o o 2 1 e 4 1 ( ) ( )melt gas 2 2, + ( )fe iii o fe o4( ) ( ) ( )melt melt melt4 5 3 2 , ++ 596 roberto moretti or the corresponding heterogeneous reactions (5.13) (5.14) (5.15) the above notations emphasise the fact that we must now produce additional free oxygen ions o2− by polymerisation steps (5.16) (5.17) the iron reduction may be regarded as an internally buffered auto catalytic reaction: production of free electrons through the normal oxygen electrode favours decomposition of ferric iron clusters, making iron ions available to reduction by free electrons. this is true regardless of the fact that octahedral iron clusters may be present as simple ionic couplings (eqs. (5.2) to (5.4)) or as true complexes (eqs. (5.11) to (5.17)). in both instances, the whole process is buffered by the availability of free oxygen in the melt, which ceases at a critical acidity limit, due to polymeric equilibria. the fact that nominal o2− may appear either as reactants or products stresses how misleading it could be to conceive the le chatelier principle in terms of the simple mass action effect of oxide ions. as noted by douglas et al. (1966) the altervalent equilibria in melts and .o+o2+ si( )fe ii o + , , ( )fe iii o sio5 4 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 4 4 6 10 7 6 2 2 o+osi+ + ( )fe ii o , , ( )fe iii o sio5 e 2 5 2 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt melt 4 5 4 4 6 10 7 6 2 +2 +( )fe ii o,o6+ o+ ( ) . fe iii o i 2 1 0 4 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 6 10 2 + o3+2 -o o5 ,+ + ( ) ( ) fe iii o i fe ii o o o 2 5 0 4 1 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 6 10 2, + 4o ,++ + ( ) ( ) fe iii o i o fe ii o o o 2 1 2 0 1 4 ( ) ( ) ( ) ( ) ( ) melt melt melt melt gas 4 5 2 6 10 2, + glasses may be generalised as follows: (5.18) equilibria such as those proposed by johnston (1964); or duffy (1996), (eq. (4.38)) and apparently violating the le chatelier principle demand . if , such as in the holmquist (1966) formulation (5.19) there is no paradox, and when the equilibrium is written in a simple stoichiometric form (i.e. no oxide ions involved) (5.20) in the above equations it is assumed that ferric iron behaves essentially as a network former, although we know that in chemically complex melts or glasses the structural behavior of feiii is a complex function of both bulk composition and feiii concentration. this simply means that the above equation, written for macroscopic melt components, must be coupled with homogeneous speciation reactions defining the structural state of iron in melts and glasses (which is a function of bulk composition and p, t conditions, as shown by experimental evidence). mössbauer observations on quenched melts (mysen, 1990, and references therein) indicate that when feiii exceeds feii (feiii/σfe ≥ 0.3) ferric iron is only present in tetrahedral clusters; for 0.5 ≥ feiii/σfe ≥ 0.3 both tetrahedrally and octahedrally coordinated feiii is present and for feiii/σfe ≤ 0.5 tetrahedral clusters are absent. virgo and mysen (1985) on the basis of spectroscopic and magnetic data suggested that coexistence of feiii and feii leads to formation of units stoichiometrically resembling fe3o4 and composed of 0.33 tetrahedrally coordinated feiii, 0.33 octahedrally coordinated feiii and 0.33 octahedrally coordinated feii (virgo and mysen, 1985). experimental data on melt .feo o feo 4 1 .2 1 5,+ ( / )b a m 2+= ( )fe iii o i fe o o 4 4 6( ) ( ) ( ) ( ) melt melt melt gas 2 2 2 2 ,+ + + + ( / )b a m 22 +( / )b a m 21 + ( ) ( ) ( ) r n m r n m a b m o 4 o o 2 o ba 2 2 ,+ + + + + -: d 597 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts densities at various t, f o2 conditions seem to confirm that feiii is essentially present in tetrahedral coordination, although some discrepancies in the partial molar volumes of molten fe2o3 based on various experiments could be ascribed to the limited presence of a higher coordination state in some of the investigated materials (see bottinga et al., 1983; dingwell et al., 1988, and references therein). the fact that feiii could partly exist in octahedral coordination with oxygen may be ascribed to the partial reaction (5.21) the above equilibria show us that the nature of bonding between central cations and ligands must be attentively evaluated before reaching unwarranted conclusions. although the compositional effect differs in the various investigated systems ( f o2 being held constant), duffy (1992, 1996) has shown that the basicity effect is identical for all binary systems whenever optical basicity is introduced for the compositional axis. duffy (1992) proposed, at t = 1400°c, the following semilogarithmic relationship between the observed redox mass ratio of iron and optical basicity (5.22) although this relationship disregards the effect of temperature on the extent of the polymerisation reaction, it is sufficiently accurate to allow comparative estimates on widely differing systems. nevertheless, this kind of equation cannot be usefully employed on an empirical basis, not because of the chosen parametric scale (i.e. optical basicity) but rather because of the adopted functional form. figure 4 shows that duffy’s reworked expression (see figure) does not reach a good accuracy in reproducing the 1400°c data available from the literature. following fraser (1975, 1977), ottonello et al. (2001) assumed that fe2o3 behaves as an amphoteric oxide in the lux-flood acid-base . . .ogl fe fe 3 2 6 5iii ii = λb l ( ) ( ) . fe iii o o i fe iii o o o 5 2 5 0 2 1 ( ) ( ) ( ) ( ) melt melt melt melt 4 5 6 9 2 ,+ + + + acceptation. its double dissociation in the melt (or glass) may be reconducted to the following homogeneous reactions: (5.23) (5.24) for ferrous iron on the other hand only a basic dissociation is plausible, i.e. (5.25) adopting the temkin model for ionic salts (temkin, 1945) and assuming the fe(iii)o〈−i〉2 clusters to mix ideally over the structon matrix and the fe3+, fe2+ cations to mix ideally over the cation matrix, after some passages one arrives at (ottonello et al., 2001) (5.26) which is analogous to eq. (14) in fraser (1975), k a k a k a k fe fe 1 anions cations cations . / . / . / / . iii ii o o o 5 20 1 4 5 23 1 2 2 5 24 1 2 1 2 5 25 2 2 2 # # = +b l / / / .feo fe o( ) ( ) ( )melt melt melt 2 2 , ++ .fe o fe o2 3( ) ( ) ( )melt melt melt2 3 3 2 , ++ ( )fe o o fe iii o i2( ) ( ) ( )melt melt melt2 3 2 2,+ fig. 4. experimental ferrous to ferric iron ratio versus summation of oxide optical basicities. data from various sources, also included in the database of ottonello et al. (2001), show the need for a more rigorous approach to the functional form based on optical basicity. 598 roberto moretti although here σanions replaces, more correctly, σstructons, since free anions such as o2−, co32−, s2−, so42− etc., are present in the anionic matrix, besides polymeric species. equation (5.26) implies that, due to disproportionation of trivalent iron between the cationic and anionic matrixes, we cannot expect the ratio of rational activity coefficients of feo1.5 and feo (second term on the right) to be 1. in fact the first term on the right side of eq. (5.26) represents afeo/afeo1.5 (see eq. (5.20)), whereas the second term represents γ feo1.5/γ feo. if we compare eq. (5.26) with the functional form (5.22) we would deduce that the intercept term in the equation of duffy (1992) corresponds to the first term on the right in eq. (5.26), whilst the slope coefficient embodies the remaining structural parameters. since the polymeric model allows the calculation of the extension of the structon and the cation matrixes, ottonello et al. (2001) conveniently solved eq. (5.26) on thermochemical grounds, based on the plethora of experimental data concerning ferrous iron solubility and iron redox ratios in melts (and/or glasses) equilibrated at known t and f o2 conditions. nevertheless, this was done only for nominally anhydrous melts synthesised at 1 bar pressure. on this basis we can also investigate the dependence of iron oxidation state under hydrous conditions and therefore at higher pressure. it may be here anticipated that the way iron disproportionates also depends on water speciation in melts, as a consequence of the effect that water carries on polymerisation and then basicity in terms of free oxygen ions activity. 6. iron oxidation state in hydrous aluminosilicate melts: a preliminary model extension water is commonly perceived as the most basic oxide: its presence in the natural systems undergoing melting dramatically affects the solidus temperature and the composition of the incipient melting liquid. nevertheless, its basicity (in the lux-flood sense of the term) seems to be over rated, λh2o being very close to λsio2 (table i). therefore, it is of primary interest to test the model reproducibility at pressure and investigate how both polymerisation and the ferric to ferrous iron ratio in melts are affected. the still few data at present available in literature also involve the presence of water. here i present an exploratory extension of the 1-bar anhydrous model of ottonello et al. (2001). first of all, it is necessary to introduce the effect of pressure on the equilibrium constants for reactions (5.20) and ((5.23) to (5.25)). this is easily done by accounting for volume terms of both ionic species and macroscopic compotable iv. molar volumes employed for macroscopic and ionic species involved in reactions (5.20), ((5.23) to (5.25)), (6.3) and (6.12). for ionic species i also listed the the adopted ionic radius. molar volume @ 298.15 k ionic radius reference 298.15 k (cc/mol) (å) feo 9.64 lange (1994) fe2o3 29.63 lange (1994) fe2+ 0.90 0.78 shannon (1976) fe3+ 0.51 0.645 shannon (1976) o2− 6.92 1.40 shannon (1976) feo2− 75.99 (*) 3.29 (**) shannon (1976) oh− 6.92 1.40 shannon (1976) oh 6.92 1.40 shannon (1976) h+ 0 0 this work (*) see text for details. (**) the value here adopted represents the summation of the radius of four-folded ferric iron (0.49 å) and the diameter of an oxide ion o2−. 599 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts nents. this procedure is consistent with the temkin approach inherent in the toop-samis model, which demands scaling of the activities of liquid components from the standard state of pure melt components at p and t of interest to the standard state of completely dissociated ionic component. the equilibrium constant for reaction (5.20), involving the macroscopic oxides feo and feo1.5 is then recomputed as (6.1) where molar volumes of melt phases, as well as isothermal compressibilities and isobaric thermal expansivities have been taken by lange (1994). for reactions (5.23) to (5.25) i still consider the lange (1994) data for macroscopic oxides fe2o3 and feo, whereas volume of ionic species are calculated on the basis of ionic radii of shannon (1976) assuming that the ‘effective molar volume’ of each ionic species equals that of a mole of spherical molecules each characterised by its appropriate shannon radius. note that for feo2− species i calculated the molar volume from that of a sphere of radius . since the spherical volume associated with this radius should represent, at a first approximation, the ‘effective volume’ of the feiiio45− complex, i subtracted the volume of two oxide ions o2− in order to obtain the ‘effective volume’ of the feo2− compound. this means that i assume the volume change reaction for the association reaction to be zero. values of employed volumes are listed in table iv. in our calculation the ionic radius for each ionic species is fixed for all temperatures (i.e. feo 2o feo2 2 4 5 ,+( / )r r1 2 ( )o fe iv2 3++ ( ) .v dp v v dp, ,melt p feo melt feo melt p 1 1 .1 5 = -∆ c c c# # +ln k ,o gas = v dpmelt∆ + ln k rt rt v dp 1 4 1 . ( , ) . ( , )p t t p p 5 20 5 20 1 1 1 2 c c # # thermal expansivity is zero), therefore i recalculated the thermal expansivity of macroscopic oxides at 298.15 k, obtaining that the variation in the reaction volume change is a constant at any temperature. the model is now ready to investigate the role played by water in the ferric to ferrous iron ratio of melts. data in the literature disclose some controversies about the oxidation state of iron under hydrous conditions. following moore et al. (1995), water does not affect the ferric to ferrous iron ratio, which is a record of other processes having imposed the oxygen fugacity. according to baker and rutherford (1996) and gaillard et al. (2001) water does affect the ferric to ferrous ratio. in some region of the pt-f o2 space it may cause either a decrease or an increase of oxidation. for example water-bearing rhyolitic melts have higher ferric to ferrous ratio than anhydrous melts of the same composition (baker and rutherford, 1996). the same occurs in metaluminous melts, but at higher temperatures (t > 900°c) and around nno, whereas in peralkaline melts such an increase is observed at high t (baker and rutherford, 1996). gaillard et al. (2001) generalise this perspective, observing an increase in the ferric to ferrous ratio of iron in hydrous melts at log f o2 < < nno + 1.5 for all studies compositions, metaluminous and rhyolitic melts and natural peraluminous and peralkaline obsidians. however they find that above nno + 1.5 water does no longer affects the ferric to ferrous iron ratio, controlled by the anhydrous composition in agreement with moore et al. (1995). finally, wilke et al. (2002) investigated tonalitic melts at 850°c, whose ferric to ferrous iron ratio showed a marked decrease with respect to the values computed through the kress and carmichael (1991) and then based on the anhydrous composition. nevertheless, this effect is mainly ascribed to the inaccurate calibration of the kress-carmichael equation at low t rather than to the water content of melts. it is important to remark that wilke et al. (2002) and gaillard et al. (2001) did not observe any effect of the quench rate on the ferric to ferrous ratio of investigated melts. this conclusion cannot be obviously extended to the re600 roberto moretti maining data here discussed, so quench-rate effects may still represent an important source of uncertainty. moreover, the dependencies of the ferric to ferrous iron ration on water amount are contrasting: baker and rutherford (1996) find different explanations about the role of hydroxyl groups (baker and rutherford, 1996), whose complexity is enriched by the t dependency of water speciation (between oh− and h2o for all the authors observing change on the iron oxidation state with the water content). these experimental results are likely to show only apparent controversies. i then considered the database generated by these authors (119 compositions) in order to expand the model of ottonello et al. (2001). it is clear that the parameterisation of the ferric to ferrous ratio must consider the «impact» of water on melt acid-base properties and then polymerisation. in order to match this goal in the widest available p-t-x range, i also considered thirty-seven 1 bar compositions (from fudali, 1965; and shibata, 1967) showing some water content (up to 0.66 wt%) and which were already accounted for by ottonello et al. (2001). consistent with the temkin formalism of complete dissociation of component oxides and on the basis of that «common perception» which requires water to behave as a strong modifier (being a strong basic oxide in the luxflood notation), i first considered water as undergoing uniquely a basic dissociation (6.2) let us recall that this kind of dissociation is accompanied by other homogeneous reactions in the melt phase (fraser, 1975; ottonello, 1997), i.e. the association to nbos’ originating strong hydrogen bonding, and the polymerisation reaction (6.4) (6.3) (6.4) the summation of eqs. (6.2), (6.3) and (6.4) gives (6.5)h o o 2oh2 0 ,+ .o o o22 0 ,+h o oh2 2 2,++ .h o h o22 2 , ++ which well displays the depolymerising effect of water and which has been first discussed by fraser (1975, 1977). according to the temkin notation of complete dissociation, implicit in the toop-samis approach, all the protons were considered to contribute in defining the basicity of modifiers (i.e. the basicity of the cationic matrix). the polymeric constant and then initial (o−) values were computed assuming only the occurrence of reaction (6.2), without any concomitant equilibrium (i.e. eq. (6.3)) leading to bonding with nbo’s. the latter mechanism was considered as a subsequent step involving depletion of both initial o− and h+ to form oh. the following mass balances must be satisfied: (6.6) (6.7) the equilibrium constant for reaction (6.3) may be expressed as (6.8) with some passages, substitution of eqs. (6.6) and ((6.7) to (6.8)) gives the following quadratic equation: (6.9) the equation above has two possible roots, but only the following one provides solutions falling between initial o− and initial h+, and then physically meaningful (6.10) (o2−), (o0), σstructons were then recalculated on the basis of the new o− values. the number of newly formed oh groups, noh, was then included in the quantity σanions. n$n$k4 $n k n k n k k n k n cations 2 cations / / / / / . . . . . . oh 1 2 h in 1 2 o in 1 2 1 2 h in 1 2 o in 2 h in o in 6 3 6 3 6 3 6 3 6 3 6 3 $ $ $ $ = + + + + + + + + -_ i / / . k n k n k n n k n n cations 0 / / / / . . . . 1 2 oh 2 1 2 h in 1 2 o in oh 1 2 h in o in 6 3 6 3 6 3 6 3 $ $ $ $ $ $ $ + + + = + + _ i/ .k n n n cations / . 1 2 o oh h 6 3 $= + / .n n nh h in oh= -+ + n n no oho in= -601 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts furthermore, we should also consider that the theory, based on the lux-flood formalism, gives us an alternative to be evaluated: the amphoteric behavior of water, i.e. the existence of an acidic dissociation, as testified by its relatively low value of optical basicity. the following reaction: (6.11) was first proposed by fraser (1975). moreover the existence of free oh− has recently been reported by xue and kanzaky (2003). reaction (6.11) is actually that normally invoked in literature to explain water dissolution in aluminosilicate melts. nevertheless, in the literature it is not regarded as an acidic dissolution mechanism, neglecting the fact that it leads to melt polymerisation because of the consumption of free oxygens. i then introduced in the model the difference between reaction (6.11) and reaction (6.2), i.e. (6.12) whose equilibrium constant may be written as (6.13) this equation simply recognises the existence of two dissolved species of water in melts, i.e. oh− and h+, consistently with the temkin formalism and the lux-flood notation for oxide solvents. i therefore by-pass the problem of determining the activity of water in melts as well as in the fluid phase, a problem which would be posed by solving eqs. (6.2) and (6.11) separately or by solving their algebraic sum. the system of equations is simply solved through additional mass balance on water (6.14) and the equilibrium constant for water speciation reaction (6.12). equation (6.10) partitions the initial water amount, so that k2.1, polymerisation is no more defined on the total analytical water content. n n n2h in oh h o2$+ =+ .k n n n n cations . tot o oh h oh 6 12 2 $= -+ / h o oh2 ,++ h o o oh22 2 ,+ regression on available experimental data is performed through non-linear minimisation techniques based on steepest descent and gradient migration methods (james and roos, 1977) on both k6.3 1/2 and k6.12. equilibrium constants values and statistics for the extended iron model are given in table v, whereas reproducibility may be appreciated in fig. 5. it is worth remarking that the t dependence obtained for equilibrium (6.12) shows that this reaction becomes more important at higher t. on the other hand, reaction (6.3) is independent of temperature (the entropic term of the arrhenian dependence fig. 5. reproduciblity (calculated versus experimental) of the extended iron oxidation state model. anhydrous and hydrous datasets are distinguished. the whole database consists of 608 compositions. table v. equilibrium constants for water speciation mechanisms and model statistics. logk6.12 1.835 – 1304.65/t 1/2(logk6.3) –1.335 number of hydrous 120 compositions number of all compositions 608 (anhydrous+hydrous) mean error (hydrous dataset) 0.277 standard error 0.357 (hydrous dataset) mean error (whole dataset) 0.187 standard error (whole dataset) 0.264 602 roberto moretti describes the equilibrium constant) and leftward shifted. the effect of pressure was neglected for reactions involving water species as the volume associated with h+ was assumed to be zero, so that vo2− =voh− =voh. the comparatively low precision of the hydrous dataset with respect to the anhydrous one probably reflects model approximations, similarly to what was described for sulphur speciation in moretti and ottonello (2003a). in particular, a more general model based on the assessment of water solubility and speciation should require the flood-grjotheim treatment (flood and grjotheim, 1952) opportunely implemented and already used for sulphur species (moretti, 2002; moretti and ottonello, 2003b). moreover, more accurate data for molar partial volumes are needed, in particular for iron oxides. in principle, we could improve the precision of our model by refining on volume reactions, but i prefer to employ independent experimental volume data. a possible source of error is also related to the t-independent computation of the polymeric extension of the anionic matrix: a more general, i.e. tdependent (ottonello and moretti, 2004, and work in progress) polymerisation equation would represent a step forward in the continuos attempt to ameliorate model results and applications. figure 6a,b shows a comparison between: i) equation (5.26), accounting for water speciation and volume terms; ii) equation (5.26) under the 1 bar approximation and without considering eqs. (6.3) and (6.12); and iii) the kress and carmichael (1991) empirical model. it is evident that both eq. (5.26) under the 1 bar approximation and the kress-carmichael algorithm do not work well in reproducing the observed feii/feiii ratio, which is largely underestimated in the first case. it is important to remark that the fact that we «identify» three water-derived species in melts (h+ cations, oh− free anions and, to a very minor extent, oh groups terminating polymeric units, which can be then ascribed to t-oh linkages) is quite consistent with nmr findings (kohn et al., 1989; schmidt, 2001; xue and kanzaki, 2003). intuitively, it would seem that their combination reproduces the water speciation – and solubility – observed in melts, similarly to what was argued by liu et al. (2002). nevertheless these arguments cannot be pushed further and are purely qualitative: the model here developed is not aimed at reproducing the speciation observed via ftir or nmr since model computations are based on a particular standard state (that of completely dissociated component) which is introduced to describe the acid-base properties of melts and not the structural units detected by spectroscopic tools. for example, the existence of equilibrium (6.2) and (6.11) (and hence equilibrium (6.12)) implies incomplete dissociation of the water component fig. 6a,b. model reproducibility of the hydrous datasets following different approaches (see text). in part a) of the figures data from various sources have been distinguished. h2o-unsaturated data from baker and rutherford (1996) were not considered. a b 603 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts and therefore the presence of molecular water. the existence of this species is not ruled out by the present model, but calculations need not treat it to solve chemical interactions of interest whenever the temkin standard state is applied. model generated distributions of (o−), (o0). (o2−), noh, noh−, nh+ are plotted in fig. 7a,b for the system na5-4x alx si3x (al/si =1/3) containing an arbitrary amount of water (6 wt% in the whole compositional range) at 900°c. we see that h+ and oh− are always the predominant water derived species in melts, whereas oh are subordinated. in particular, abundances of oh− anions and h+ cations are inversely related, obtaining similar values at both low and high alumina contents. the abundance of t-oh groups follows the same trend of h+ cations, although much more smoothed. a slight increase in the concentration of free oxygen (o2−) is computed for compositions with alumina content larger than that of albite (x=1). finally, it is worth remarking that the appreciable occurrence of both eqs. (6.2) and (6.11) suggests a strong similarity with water behaviour in aqueous phase. 7. conclusions and perspectives the superiority of polymeric models in depicting silicte melts and slags reactivity with respect to other conceptual approaches is linked to the following facts: i) it is well recognised that «regular mixture» models fail to reproduce the gibbs free energy of mixing of silicate melts. minima in the gibbs free energy of mixing are badly allocated and badly conformed in the chemical space of interest. for heterogeneous equilibria (solid-liquid or liquid-gas in multicomponent systems) this problem is almost ineffective since internal consistency is achieved with extended databases encompassing model deviations through adjustable interaction parameters. ii) the arbitrary deconvolution of chemically complex melts into fictive components is a path-dependent process eventually complicated by charge-balance considerations whenever amphotheric oxides are involved (this applies to iron and other transitional elements in slags and natural melts). iii) preliminary attempts to parameterise the bulk polymerisation proved satisfactory in deciphering the complex effect of the bulk luxflood acidity of the system on the oxidation state of iron in multicomponent melts and glasses (ottonello et al., 2001). the gibbs free energy of mixing of simple binaries mo-sio2 and of ternary systems (cao-feo-sio2) was successfully simulated (ottonello, 2001). iv) polymeric models carry a minimal set of structural information which can be employed a b fig. 7a,b. relative proportions of oxygens of the fincham-richardson (1954) notation (a) plotted against the compositional parameter in the binary join na5-4x alx si3x o8. water-derived species dissolved in the same compositional range have been plotted in b). note the comparable amount of oh and oh− for the albitic composition (x = 1). 604 roberto moretti for the study of partitioning of elements, viscosity and – plausibly – other transport properties of silicate melts such as thermal and electrical conductivity. the adopted polymeric parameterisation is based on three main previous observations: 1) the basicity of a complex aprotic medium such as a silicate melt or glass is conveniently represented by the «optical basicity», arising from the nephelauxetic effect induced on p-block metals by the ligand field (duffy and ingram, 1971, 1973, 1974a,b, 1976; duffy and grant, 1975). 2) optical basicity is related to atomistic properties of the dissolved oxide components in the melts or glass, such as the pauling and sanderson electronegativities (pauling, 1960; sanderson, 1967) and the fractional ionic character of the bond (pauling, 1960; phillips, 1970). 3) bulk optical basicity of molten silicates, or glasses can be split into two distinct contributions, the basicity of the dissolved basic oxides and the basicity of the polymeric units (or «structon matrix» in the sense of fraser, 1975a,b, 1977). while the optical basicity effect induced by the dissolved oxides varies widely with the type of oxide component, the optical basicity effect ascribable to the structon matrix is virtually unaffected by composition, at parity of silica content in the system, and is dominant at high silica contents. an exploratory application to the modelling of the oxidation state of iron shows that it is possible to extend the model of ottonello et al. (2001) to hydrous aluminosilicate melts. this requires the introduction of volume terms for both ionic species and macroscopic components, together with equilibria relevant to water speciation. these preliminary results are quite satisfactory and promising, especially considering that the polymerisation constant employed represents the high-t approximation (ottonello and moretti, 2004) and that i adopted experimental values for molar volumes, whereas more accurate partial molar volumes should be employed. in particular, data are well explained as long as both basic and acidic dissociations of water are considered. a further slight amelioration to model precision may be introduced by accounting for a subsequent process of association to nbo’s, that may be seen at first approximation as characteristic of strongly hydrogen-bonded t-oh groups. the fact that water also undergoes an acidic dissociation, originating free anions oh−, agrees with the recent findings of xue and kanzaki (2001, 2003), based on density functional theory, inferring the existence of naoh groups in alkaline silicate glasses and confirms the prediction of fraser (fraser, 1975, 2003; and this issue). it is worth stressing that the reliability of calibrated equilibrium constants involving ionic species of water and iron is subjected to i) the quality and p-t-x extension of the reference database; and to ii) accurate estimates of reaction volumes of iron species and of reactions in which they are involved. moreover, it must be clear that the present modelling does not have any straight implication about the geometry of coordination polyhedra in silicate melts, not required for the purposes of understanding polymerisation and the acid-base behaviour of investigated species. therefore, ionic species depicted by the model are not necessarily related to structural units that can be identified by means of current spectroscopic tools. in the light of these results, some future activity may be here planned, both experimental and theoretical. some research lines may be proposed and followed contemporaneously to solve accurately, the mixing properties of silicate melts: i) experimental – in situ measurement of optical basicity (nephelaxeutic parameter) at t and p. ii) experimental – xps measurement of free oxygen (o2−) in silicate melts coupled to the toop-samis modelling of silicate melts (see next point). iii) theoretical – application of the hybrid model of ottonello (2001) to the conformation of liquidus in multicomponent systems, following the guidelines of the flood and grjotheim treatment for the calculation of chemical interactions coupled to strain energy modelling. iv) theoretical – by applying quantum-mechanical codes to simple binaries and ternaries; in order to better assess the nature of nephelauxetic effect in ligand field-related spectroscopic observations. 605 polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts the first task has the objective of improving the toop-samis model, by translating the t dependence of nephelaxeutic parameters into polymerisation constants of the type of eq. (4.27). as required by the second task, this may also be done through xps measurements of oxygen species, which coupled to predictions of the fincham-richardson approach allow a thorough assessment of polymerisation in melts (park and rhee, 2001). the third task has the objective of deciphering the contribution given by the dissociation of every component in chemical systems of increasing complexity. first we should reconstruct the binary sio2al2o3 and then study ternary fields mo-al2o3sio2. with the introduction of alumina it is important to investigate the effect of entropic effects, because of the similitude of acid-base properties with silica. navrotsky (1994) pointed out that in this binary the al3+ cation is forced to occupy the octahedral site. entropic terms, arising from a «competitive» effect of al2o3 and sio2 upon the polymerisation are then expected to come out. entropic terms should also be much more evident in the presence of alkalies in the system, mainly because of the charge compensation of the tetrahedral aluminum. the effects of non-random mixing of some network modifier oxides like na2o and k2o must be carefully evaluated. moreover, a systematic comparison, while creating the thermodynamic database, will allow us to further refine nephelauxetic parameters and their dependence upon intensive variables, especially in terms of temperature. finally, we should also consider the mechanical strain energy contribution to the bulk free energy of mixing, since such a term explains the observed solvi experimentally determined in sio2 rich ranges of binary systems. as shown by ottonello and moretti (2004) the plethora of thermodynamic data emerging from the metallurgical community is, to this purpose, of invaluable help. some queries may be addressed to experimentalists, such as coupling optical basicity measurements with spectroscopic measurements on the rydberg’s and electron transfer emission lines of 3d chromophores. this would allow us to assess better the differential nephelauxetic effects and the structural state of complexes. the fourth research line is devoted to a better comprehension of model clusters and complexes which characterise the speciation state of silicate melts. a feasibility study for the adoption of parallel computing techniques has to be carried out. semi-empirical methods, such as huckel-mo, have to be ruled out as they need a large amount of experimental data, the consistency of which is often doubtful. all the considerations here reported are preliminary to the set-up of an ambitious general thermochemical simulator able to depict the evolution of a complex (but essentially aprotic) system. given such a general polymeric model, we can promote on its grounds thorough studies of water solubility (as well as any other volatile), through recalculation of equilibrium constants for both eqs. 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paleoseismological evidence for historical surface faulting in são miguel island (azores) rita carmo1, *, josé madeira2, ana hipólito1, teresa ferreira1 1 centro de vulcanologia e avaliação de riscos geológicos da universidade dos açores, cvarg, açores, portugal 2 universidade de lisboa, faculdade de ciências, departamento de geologia, and instituto dom luiz (laboratório associado)-idl(la), lisboa, portugal abstract the azores archipelago is located at the triple junction between the eurasian, nubian and north american lithospheric plates, whose boundaries are the mid-atlantic ridge and the azores-gibraltar fault zone. são miguel is the largest island of the archipelago and is located on the eastern part of the western segment of the azores-gibraltar fault zone. the achada das furnas plateau, located in the central part of the island, between fogo and furnas central volcanoes, is dominated by several wnw-ese and e-w trending alignments of basaltic cinder cones. two e-w trending scarps were identified by aerial photo interpretation. transect trenches exposed two active normal faults-the altiprado faults – confirming the tectonic nature of the scarps. several paleoearthquakes were deduced, most of which in historical times, producing 1.38 m and 0.48 m of cumulative displacement. maximum expected magnitudes (mw) determined from slip per event range from 5.7 to 6.7. one of the events probably corresponds to the historical earthquake of october 22nd, 1522, the deadliest in the archipelago. radiocarbon ages are in agreement with this interpretation. 1. introduction the population of azorean islands faces significant geological hazard due to the geodynamic context of the archipelago, including frequent seismicity, active volcanism and landslides. earthquakes claimed more than 6,000 victims in the six centuries since the beginning of settlement in mid-15th century. we present a paleoseismology study that revealed several events of surface faulting in the island of são miguel during historical times, thus contributing to the seismic risk awareness in the most populated island in the azores. the azores archipelago is located in a zone of anomalously shallow topography (the azores plateau), corresponding to the expression of the eurasia, nubia and north america triple junction (figure 1). the mid-atlantic ridge separates the north america from eurasian and nubian plates, while the azores-gibraltar fault zone (terceira rift (s.l.) and gloria fault) is the boundary between eurasian and nubian plates. the archipelago comprises nine islands distributed by three groups. the western islands (flores and corvo) lie on the stable north american plate, while the central (faial, pico, são jorge, graciosa and terceira) and eastern groups (são miguel, santa maria and formigas islets) are located along the western segment of the azores-gibraltar fault zone, the terceira rift (s.l.). this segment is bordered to the south by the inactive east azores fracture zone and to the north by the são miguel-graciosa alignment (terceira rift (s.s.), machado 1959). it corresponds to a diffuse and complex deformation zone, sheared by a dextral transtensive regime [madeira 1998, lourenço et al. 1998, madeira and brum da silveira 2003] due to higher spreading rates at the mid-atlantic ridge north of the triple junction [e.g. laughton and whitmarsh 1974, fernandes et al. 2003, argus et al. 2011] and to the obliquity relatively to the spreading directions [e.g. madeira and ribeiro 1990, madeira 1998, lourenço et al. 1998]. the fault pattern in the azores islands is characterized by three-dimensional strain [reches 1983, reches and dieterich 1983] and is represented by two main fault systems with oblique slip, each composed of two sets dipping in opposite senses: wnw-ese to nw-se normal dextral faults, and conjugated nnwsse to n-s normal left-lateral structures. this geometry and kinematics indicate a stress field with a horizontal maximum compressive stress axis (σ1) trending nw-se, a horizontal minimum compressive stress axis (σ3) in the ne-sw direction and a vertical intermediate compressive stress axis (σ2); permutations between σ1 article history received october 8, 2012; accepted may 10, 2013. subject classification: earthquake geology and paleoseismology, seismic risk, tectonics. special issue: earthquake geology and σ2 may originate from transtensive or tensile tectonic regimes [e.g. madeira 1998, madeira and brum da silveira 2003]. the deformation is accommodated by a large number of active faults along which significant seismic and volcanic activity occur. since settlement, in the 15th century, the azores islands have been severely affected by seismic activity, characterized by usually low to moderate magnitude isolated events or seismic swarms of tectonic and/or volcanic nature [e.g. silveira et al. 2003]. however, high magnitude earthquakes (e.g. m 7.2; hirn et al. [1980]) may also occur, causing thousands of deaths and severe damage [e.g. machado 1949, hirn et al. 1980]. volcanic activity also occurred, both on land and at sea, sometimes with severe consequences [e.g. chaves 1960, weston 1964, madeira 2006]. the temporal distribution of eruptions and earthquakes (at the archipelago scale) suggests alternating eruptive and tectonic phases. volcanism would happen when the stress field becomes largely extensional, due to permutations between the maximum and intermediate stress axes [madeira 1998]. in the azores archipelago, surface faulting is well documented in all islands located along the eurasianubia margin (central and eastern groups) producing fault scarps of variable dimension. most post-settlement earthquakes capable of producing surface rupture occurred off-shore with the exception of two events in terceira island in 1614 and 1841, for which there are coeval texts describing rupture on land. earthquake magnitudes ranging from 6.9 to 7.1 were estimated from slip per event in the islands of faial, pico and são jorge [madeira and brum da silveira 2003]. however, this procedure should be used carefully because slip frequently overestimates magnitude as coseismic, post-seismic and fault creep displacements are very difficult to discriminate in paleoseismological analyses. at são miguel, active faulting is represented by prominent fault scarps and linear volcano-tectonic structures with a main nw-se to wnw-ese trend and normal dextral slip. at achada das furnas area, aerial photo analysis identified the presence of two e-w trending scarps. trenching across these scarps concarmo et al. 2 figure 1. tectonic setting of the azores archipelago (adapted from hipólito et al. 2010). the main morphotectonic features of the region are presented. the shaded area in both the inset and main figure tr (s.l.) represents the sheared western segment of the eurasia-nubia plate boundary, and tr (s.s.) corresponds to são miguel-graciosa alignment. tectonic structures: mar-mid-atlantic ridge; tr-terceira rift; eafz-east azores fracture zone; gf-gloria fault. islands: c-corvo; fl-flores; fa-faial; p-pico; sj-são jorge; g-graciosa; t-terceira; smgsão miguel; fo-formigas islets; sma-santa maria. azores bathymetry adapted from lourenço et al. [1997], and world topography and bathymetry from gebco_08 database [2010]. 3 firmed their tectonic nature revealing two previously unknown normal faults, hereafter named altiprado faults 1 and 2. this work presents evidence of surface faulting on these structures in historical (post-settlement) times. 2. volcano-tectonic setting são miguel island is located on the eastern part of the terceira rift (figure 1). it comprises three active central volcanoes with summit calderas (sete cidades, fogo and furnas), characterized by basaltic (s.l.) effusive activity during the early subaerial stages followed by explosive trachytic volcanism in late pleistocene and holocene. these are linked by volcanic fissure zones (picos region and achada das furnas plateau). two older and inactive volcanoes (povoação and nordeste) form the eastern part of the island (figure 2). volcanism becomes younger westwards. the nordeste mafic shield volcano is the oldest part of the island and its age ranges from 4 to 0.95 million years [abdel-monem et al. 1975]. its southwest flank is cut by the povoação caldera of unknown age. the 100,000 years old furnas volcano [moore 1990] is the easternmost of the three active central volcanoes of são miguel and cuts the caldera of povoação. fogo volcano, located on the center of the island, started its subaerial edification more than 200,000 years ago [muecke et al. 1974]. the western end of the island is dominated by sete cidades volcano whose formation began prior to 210,000 years bp [moore 1990]. the main active fault system trends nw-se to wnw-ese. the surface expression of the faults is represented by fault scarps, forming large graben structures that truncate the flanks of the main volcanoes, and linear volcano-tectonic structures (figure 2). the observable length of these structures is limited by the dimension of the island and by mantling by recent (mostly holocene) volcanic deposits that locally conceal the faults. nnw-sse and ne-sw trending structures are represented by volcano-tectonic alignments and lineaments (figure 2). tectonic structures played a key role in the volcanic evolution of são miguel, controlling the emplacement of major volcanoes and locally acting as conduits: central volcanoes were built at the intersection of the nw-se to wnw-ese and nnw-sse to ns conjugated fault systems, while monogenetic cones lie along them. the achada das furnas plateau, located in the central area of the island between fogo and furnas volcanoes, is a flat area dominated by several basaltic cinder cones, occasional maars and trachytic domes, defining wnw-ese and e-w alignments (figure 3). the area is mantled by thick trachytic fall deposits produced by eruptions on the neighboring central volcanoes. aerial photo analysis revealed the presence of two e-w trending scarps, related to the previously unknown altiprado faults 1 (south) and 2 (north), which interrupt the flatness of the plateau (figures 3,4). 3. the altiprado faults 3.1. altiprado fault 1 the altiprado fault 1 (af1) trace is marked by an 835 m-long, 3 m-high, south-facing scarp, trending n87ºe (figures 3,4). to the east and to the west its historical surface faulting in azores figure 2. volcano-tectonic map of são miguel superimposed on a dtm of the island: scv-sete cidades volcano; pcr-picos region; fgvfogo volcano; afp-achada das furnas plateau; frv-furnas volcano; pvv-povoação volcano; ndv-nordeste volcano. the red box indicates the area presented in figure 3. trace becomes uncertain. an 18 m-long trench was open across the scarp, located at 37.79ºn-25.396ºw, exposing a n87ºe, 65ºs fault. the fault displaces a stratigraphic succession comprising six units separated by paleosols (figure 5). units 1 to 5 are pumice fall deposits produced by eruptions from fogo and furnas volcanoes and unit 6 is a sedimentary deposit (remobilized pyroclasts). the sequence carmo et al. 4 figure 3. main tectonic and volcanic structures of achada das furnas plateau: af1-altiprado fault 1; af2-altiprado fault 2; fgv-fogo volcano; afp-achada das furnas plateau; frv-furnas volcano. the red box indicates the area presented in figure 4. figure 4. vertical aerial photograph of altiprado region, achada das furnas plateau, showing the geomorphic expression of altiprado faults: af1-altiprado fault 1; af2-altiprado fault 2; red dots-location of trenches (aerial photo from direcção geral de planeamento urbanístico 1974). 5 is composed of: 1-olive brown paleosol developed on top of fogo a deposit, produced by a major plinian eruption of fogo volcano at about 4520±90 years bp [wallenstein 1999; table 1]; 2-pumice fall deposit composed of alternating yellowish fine grained lapilli and light olive brown ash beds, probably corresponding to fogo c deposit, the product of a hydromagmatic eruption at fogo volcano; 3-fall deposits, probably from fogo d eruption, composed of three layers, from the base to the top: (3a) dark brown pumice ash fall deposit containing disseminated lapilli; (3b) yellowish lapilli pumice fall deposit; and (3c) brown ash fall deposit; 4-grey pumice ash layer corresponding to furnas c deposit (1870±120 years bp; guest et al. 1999], table 1), which, in this trench, is completely altered to a very dark grey soil (410±30 years bp, 1440-1500 cal ad; table 1); 5-stratified fall deposit of alternating beds of fine to medium greyish white pumice lapilli and ash (5a to 5d) containing sanidine crystals from 1563 ad fogo volcano plinian eruption, topped by a very dark grey paleosol (5e) rich in coal fragments (160±30 years bp, 1660-1700 cal ad, table 1); 6-sediment formed by remobilization of the underlying unit. the marked southward dip of all units suggests that they are draping a fault scarp produced by previous (pre 4520±90 bp) surface ruptures. the af1 affects all stratigraphic units, including the present-day soil, and the scarp is an intact free-face almost devoid of soil that corresponds to the projection of the fault plane to the surface, indicating its youth (figure 5). historical surface faulting in azores table 1. radiocarbon ages of units trenched in altiprado faults. lab. ref. trench field ref. unit material age 2σ calib. date author af1 1 charcoal 4520±90 y bp wallenstein [1999] af/af2 4 carbonaceous layer from the base of deposit 1870±120 y bp guest et al. [1999] 295323 af1/af2 tr1-1 4 paleosol 410±30 y bp 1440-1500 cal ad this work 295337 af1/af2 tr1-2 5 paleosol 160±30 y bp 1660-1700 cal ad this work figure 5. map of the east wall of the altiprado fault 1 trench. legend: 1-fogo a; 2-fogo c; 3a to 3c-fogo d; 4-furnas c; 5a to 5e-1563 fogo volcano eruption; 6-remobilized deposit. several wnw-ese to e-w trending fractures and open cracks, sometimes filled with material from overlying units (3 and 4), and a colluvium (c1) composed of material from units 3 and 4 were also exposed in the trench (figure 5). the exposed geometry shows that units 2 to 4 are displaced by 1.38 m and units 5 (5a to 5e) and 6 by 0.38 m, indicating at least two surface rupturing paleoearthquakes. although there is a time gap between the deposition of unit 4 and the development of the overlying soil, the presence of a single colluvial wedge on top of the soil indicates a major surface rupture producing a minimum displacement of 1 m. units 5 and 6 are deeply eroded, so it is not possible to determine if the remaining 0.38 m of offset were produced by one or more earthquakes. since unit 5 presents an overthickening in the downthrown side when compared to the upthrown block and some branches of the fault apparently do not cut the entire deposit, part of the displacement may have occurred during the emplacement of unit 5, and the remaining displacement after the deposition of unit 6. the 1.38 m accumulated dip-slip in 4520±90 years yields a minimum slip rate of 0.31±0.01 mm/year. if we take into account the age of the soil (410±30 years bp) a minimum slip rate of 3.37±0.25 mm/year is obtained. 3.2. altiprado fault 2 the altiprado fault 2 (af2) trace is marked by an almost imperceptible 1690 m-long and ~40 cm-high south-facing scarp, trending n87ºe (figures 3,4). to the east its expression is lost. a 29 m-long trench, located at 37.796ºn 25.392ºw, exposed the same stratigraphic sequence observed in af1 trench that is displaced by two n75-89ºe trending subvertical faults (af2-1 and af2-2), which display frequent changes in dip direction (generally dipping 62-88ºs), and an e-w trending paleo-channel (filled by unit 6) (figure 6). a colluvium (c2), containing material of unit 3, is deposited at the base of af21 fault scarp (figure 6). open cracks trending ene-wsw to e-w were also filled by the colluvial material. the faults produced different vertical offsets on units 2 and 3, and on units 4 (that in this trench is almost totally altered to soil) and 5. af2-1 exhibits an accumulated dip slip of 33 cm, offsetting units 2 and 3 by 26 cm, and units 4 and 5 by 7 cm. af2-2 displaced units 2 and 3 by 11 cm, and units 4 and 5 by 4 cm, resulting in an accumulated dip-slip of 15 cm. displacement values of 26 cm, 7 cm, 11 cm and 4 cm are thus obtained. assuming that af2-1 and af2-2 are branches of the same fault and that the ruptures in both planes were produced by the same earthquakes, displacement values of 0.37 m (0.26 m + 0.11 m) and 0.11 m (0.07 m + 0.04 m) are obtained, indicating two paleoearthquakes. an alternative interpretation is that four paleoearthquakes occurred in this tectonic structure, each offset corresponding to a separate event. the strong dip of the faults suggests dominant strike-slip component associated to down throw to south, indicating that the offsets may be significantly higher. a dominant strike-slip component could also account for the subdued geomorphic expression of this tectonic structure when compared to af1. considering both branches, the accumulated normal displacement in af2 fault is 0.48 m (0.26 m + 0.07 m + 0.11 m + 0.04 m) in less than 4500 years, which yields a minimum slip rate of 0.11 mm/year. the presence of a paleo-channel trending e-w, perpendicular to the general northward dipping slope and drainage direction, suggests that it may have been developed at the base of a previous fault scarp or along the trace of a fault plane that has not been exposed in the trench. carmo et al. 6 figure 6. map of the west wall of the trench across altiprado fault 2. legend: 2-fogo c; 3a to 3c-fogo d; 4-furnas c; 5a to 5d-1563 fogo volcano eruption; 6-remobilized deposit. 7 3.3. evolution of the altiprado faults geometric analysis of the trenches allowed the reconstruction of the sequence of deposition, faulting and erosion events. as the faults are less than 740 m apart from each other and affect the same stratigraphic succession the evolution of faulting, reconstructed for both locations in figure 7, is the following: a) deposition of units 1 (4520±90 years bp) to 3; b) surface rupture(s) at af2-1 and af2-2 producing normal offsets of 26 cm and 11 cm, respectively; c) erosion truncating unit 3, with the formation of gullies in af1 fault zone, and fault scarp retreat at af2-1 with the formation of a colluvial wedge (c2) composed of material from unit 3; d) deposition of unit 4 (1900 years bp) and development of a soil (1440-1500 cal ad, 410±30 bp); e) surface rupture at af1 producing a normal offset of 1 m; f) erosion with minor fault scarp retreat and formation of a colluvial wedge (c1) in af1; g) deposition of unit 5 (1563 ad, fogo volcano eruption) with probable sin-eruptive ruptures of 7 cm and 4 cm in af2-1 and af2-2 respectively, without surface expression and surface rupture of unknown offset at af1?; h) erosion truncating the top of unit 5 and formation of a paleo-channel in af2 fault zone; development of a soil (1660-1700 cal ad, 160±30 bp); i) erosion truncating unit 5 and deposition of unit 6, probably representing a major storm event that totally filled the gully exposed in af2 trench; j) surface rupture(s) at af1 contributing to an accumulated normal displacement of 0.38 m in units 5 and 6; k) erosion truncating units 5 and 6; development of the present topsoil. 4. discussion the trenches allowed the recognition of several surface rupture events at the altiprado faults, some of which post-date the settlement of the island. in af1, the first rupture (1 m) occurred after the development of a paleosol on unit 4 (1440-1500 cal ad) and before the 1563 ad eruption (unit 5). the presence of a colluvium above the paleosol developed on unit 4 indicates that the rupture occurred after the formation of the soil and has no relation to the underlying volcanic event. a second offset of 0.38 m is measured at units 5 and 6, but the available data are insufficient to determine the number of earthquakes that produced it and the temporal relation with the deposition of those units. volcano-tectonic relations (see villamor et al., 2011) suggest that part of the displacement may have occurred during the 1563 ad fogo eruption, as there is a difference in thickness of unit 5 in both sides of the fault, or after the formation of unit 6 (1660-1700 cal ad). in af2, two hypotheses are possible. at least two earthquakes produced rupture in both branches (af2-1 and af2-2) of the fault zone, with an accumulated offset of 0.37 m (0.26 m + 0.11 m) in the first event, during the 4520 bp to 1900 bp interval, and 0.11 m (0.07 m + 0.04 m) in the second rupture, during the 1563 ad eruption or just after this event, as offsets do not propagate through all the thickness of unit 5. alternatively, it ruptured at least four times. the two earliest earthquakes (0.26 m in af2-1 and 0.11 m in af2-2) occurred in the 4520 bp-1900 bp interval. the latest earthquakes (0.07 m in af2-1 and 0.04 m in af2-2) occurred during the 1563 ad fogo eruption or after it. historical surface faulting in azores figure 7. sequence of depositional, erosional and tectonic events leading to present-day geometry exposed in altiprado faults 1 and 2 trenches. the first surface faulting event(s) is (are) related to af2, generating an accumulated normal displacement of 0.37 m in af2-1 and af2-2 fault branches. it occurred between 4520 bp (unit 1) and 1900 bp (unit 4). the second surface rupture event occurred in af1 after unit 4 paleosol formation (1440-1500 cal ad) and produced 1 m of normal displacement. the third rupture(s) produced an accumulated 0.11 m normal offset of unit 5 in af2-1 and af2-2 fault branches and probably occurred during the 1563 fogo volcano eruption seismic activity. a last offset of 0.38 m of units 5 and 6 is related to af1. 4.1. relation with historical seismicity the first surface rupturing earthquake(s) is (are) associated to af2, producing an accumulated dip-slip offset of 37 cm in af2-1 and af2-2. it occurred in pre settlement times, during the interval 4520±90 bp1870±120 bp (unit 4-furnas c deposit). the second surface rupture event occurred in af1, after the development of the paleosol on unit 4 (410±30 bp, 1440-1500 cal ad) and before the deposition of unit 5 (1563 ad-fogo historical eruption), producing 1 m of normal displacement. considering that the settlement in são miguel occurred in 1439-1443 ad, and taking into account the historical record of earthquakes in the island, this event probably corresponds to the october 22nd, 1522 vila franca do campo earthquake. this was the deadliest and one of the most violent events recorded in the azores, causing about 5000 deaths [frutuoso 1522-1591]. based on macroseismic analysis of historical records, using the european macroseismic scale 1998 (ems-1998; grünthal [1998]), silveira [2002] and silveira et al. [2003] inferred that the epicenter was located inland, nnw of vila franca do campo (a few km southwest from the altiprado faults) and that the maximum intensity x was reached at the central part of the island (figure 8). silveira [2002] drew nw-se trending elliptic isoseismal lines based on structural constraints. the earthquake produced severe damage and triggered numerous landslides mainly in the central-eastern part of the island, in an area located between vila franca do campo, ponta garça, furnas, fenais da ajuda and maia [mendonça dias 1945] (figure 8). a major landslide occurred on a slope above vila franca do campo (the capital town of the island at the time), burying the village a few minutes after the main shock, destroying the houses that were still standing, and killing most of its inhabitants [silveira et al. 2003]. nevertheless, the historical records are not clear enough in what concerns the direct effects caused by the earthquake. although the paleoseismic data of af1 reveals 1 m of surface rupture displacing the paleosol developed on unit 4 (1440-1500 cal ad), historical records do not account for surface ruptures. the documents reveal that “…in the earth entrails frightful noises sounded…” moving the earth “… with furious shakes” [monte alverne 1629-1726] and that there was no hill that did not collapse” [maldonado 1644-1711]. however surface rupture phenomena could easily go unnoticed as the villages are preferentially located in coastal areas and, at the time (only 80 years since the beginning of settlement), the central part of the island was probably still covered by dense vegetation. the third rupture(s) is (are) associated with af2. as it is not clear if the last displacement (11 cm in af21 and af2-2) is affecting only the lower part of unit 5 (1563 ad) or affects the entire unit, these displacements could be associated to the intense seismic activity that accompanied the 1563 eruption of fogo volcano. the earthquakes were felt in a wide region and caused severe damage in ribeira grande and ribeira seca villages [frutuoso 1522-1591, silveira 2002]. the earthquakes were distributed over a broad area, making it difficult to determine an epicentral location. based in the damage levels reported in coeval records and using ems-1998, silveira [2002] deduced that the damage was similar to that caused by an carmo et al. 8 figure 8. isoseismal map for the 1522 earthquake (adapted from silveira 2002): red dot-macroseismic epicenter; red lines-isoseismal lines; dashed black lines-area affected by landslides according to mendonça dias [1945]. 9 earthquake reaching intensities x at ribeira grande and ix-x at água de pau, respectively. historical documents do not describe the occurrence of secondary environmental effects related to seismic activity. on the other hand, other earthquakes occurred after this eruption that could be responsible for the observed offsets; possible candidates are the 1591 earthquake or the seismic activity associated to the 1630 eruption at furnas volcano. the july 26th, 1591 earthquake caused several deaths and produced severe damage in vila franca do campo and água de pau. the lack of information does not allow inferring the epicentral location. silveira [2002] considered intensity viii (or greater) at those localities. the 1630 eruption at furnas volcano was also preceded and accompanied by seismic activity distributed over a wide area, and caused several landslides and a lot of damage, mainly in povoação, ponta garça and vila franca do campo [monte alverne 16291726], from which silveira [2002] estimated an intensity viii or greater. the most recent displacement(s), producing an offset of 0.38 m, is (are) related with af1. however, the data are insufficient to correlate deposition, erosion and faulting events. there is no evidence of a high-magnitude earthquake with epicenter inland in historical or instrumental records justifying the observed offset. part of it could have been produced by the intense seismic activity that accompanied the 1563 ad fogo eruption. another possibility is that part (or the totality) of this offset may have been produced after the deposition of unit 6 (<160±30 years bp, <1660-1700 cal ad). in the central region of são miguel seismicity occurs mainly as shallow seismic swarms of low to moderate magnitude, with focal depths varying between 0 and 8 km [silva 2011]. there is a long time record of seismic swarms in this area since the beginning of 20th century (e.g. 1922, 1967, 1983, 1985, 1989 and 2005). the most recent seismic swarm started on may 10th, 2005 and continued until the end of the year, with more than 46,000 shallow earthquakes recorded (figure 9), 170 of which were felt. the focal depth ranged from 1 to 7 km [silva et al. 2012]. the strongest events occurred on september 20th and 21th and had ml 4.1 and 4.3, respectively, causing several small landslides and e-w trending ground cracks. the larger fissure was 1 km-long and 15 cm-wide [trota 2008]. the landslides were heterogeneously distributed over a 10 km2 area around the epicentral zone, and some formed natural dams [marques et al. 2007]. as the earthquakes were felt with intensity vi at vila franca do campo, using the mercalli modified scale 1956 (mm-56, civisa) which, in turns, corresponds to the same intensity value in ems-1998 [musson et al. 2010], intensity should have been higher (≥vii) at the epicentral zone. the region is unpopulated, but some farming support buildings suffered structural damage compatible with ems-1998 intensity vii. surface rupture could easily pass unnoticed as the main villages are located on coastal areas. gps monitoring data allowed trota [2008] to verify that there was ground deformation associated with the seismic activity. considering the seismicity pattern, the low seismic energy released, and the gps data, this activity must have been related to a magmatic intrusion at 1.8 to 2.4 km depth. the previous seismic swarms were not geodetically monitored and the seismic network was limited at the time, so there are no evidences of previous episodes of crustal deformation. thus, it is more realistic to consider that the 0.38 m displacement observed in af1 trench could have accumulated as a consequence of several moderate shallow earthquakes during periods of increased seismic activity in this area, which may have produced crustal deformation. azzaro [1999], azzaro et al. [2000] and ferreli et al. [2002] report surface ruptures and paleoseismological evidence produced by shallow moderate magnitude earthquakes (mw 3.0-5.0) and creep in faults located on the eastern flank of etna. 4.2. slip rates and recurrence intervals normal slip rates range from 0.11 mm/year in af2 to 0.31-3.37 mm/year in af1. it should be noted that the minimum slip rate at af1 may be overestimated, since a considerable time gap exists between the deposition of furnas c tephra (unit 4) at about historical surface faulting in azores figure 9. epicenter distribution of the 2005 seismic swarm (may 10th to december 31st; data from civisa). more than 46,000 earthquakes, with focal depths between 1 and 7 km, were recorded. the most energetic events reached magnitudes 4.3 and 4.1 ml and triggered several landslides and e-w trending ground cracks. 1900 bp and the development of the overlying soil (410±30 bp). some amount of displacement may have occurred before the formation of the paleosol. the slip rate in af2 is a minimum value, since the age of the oldest rupture is unknown and because the geometry of the faults suggests that the strike-slip component could be significant. recurrence intervals were not estimated because the short period analyzed suggests clustering, as most of the events occurred in historical times and because, between the deposition of furnas c (unit 4, 1900 bp) and the development of its topsoil (410±30 bp), there is a time gap of about 1500 years without any geological record. 4.3. implications for seismogenic potential assessment in the azores, interactions between volcanism and faulting difficult seismic hazard assessment; besides coseismic ruptures, surface faulting can also occur during seismic swarms related to volcanic eruptions (such as during the 1957-58 capelinhos eruption in faial; madeira and brum da silveira 2003). the maximum probable magnitudes of the earthquakes generated by the altiprado faults were estimated based on wells and coppersmith’s [1994] correlations between seismic moment magnitude (mw) and surface rupture length [srl; mw=5.08+1.16log(srl)], rupture area [ra; mw=4.07+0.98log(ra)], and maximum displacement [md; mw=6.69+0.74log(md)], considering the offset produced by each surface rupturing event (table 2). however, one should be aware that the achieved numbers are approximate values, as we are dealing with a volcanic environment, where earthquakes may have non-double couple mechanisms. relatively to rupture area, we used a similar procedure to that used by meletti et al. [2008] to estimate the thickness of the brittle crust (seismogenic crust) for italian territory, by using the maximum depth at which earthquakes occur. according to matias et al. [2007], the moho is located at 12-13 km depth. nevertheless, the majority of earthquakes of são miguel central area are located at depths that do not exceed 8 km [silva, 2011]. this is comparable with other volcanic regions, like the south iceland seismic zone [tryggvason et al. 2002, gudmundsson and brenner 2003] and the taupo volcanic zone [villamor et al. 2011]. fault dip at depth is unknown, so we consider that fault dip does not change significantly. the rupture area was then calculated using a thickness of 8 km for the seismogenic crust and an average fault dip of 65º. the maximum slip method frequently overestimates magnitude, since geological record does not allow distinguishing between coseismic, post-seismic and aseismic slip. on the other hand, the observed offsets may not represent the maximum offset produced by each rupture. considering the total observed length of altiprado faults (af1, 0.8-5.5? km; af2, 1.7-3? km), moment magnitudes in the order of 4.9-5.9 are obtained for surface rupture length and rupture area parameters, contrasting with magnitudes estimated from maximum displacement (5.7-6.7). however, in the two first cases, length and consequently area values are certainly underestimated as the total length of the faults is unknown. the faults must be longer but their traces may be concealed beneath thick fall deposits. for example, the deposits of the 1563 fogo eruption reach a thickness of 2.5 m in altiprado area. we must also considerer that, as this zone is prone to farming activities, morphological evidence may have been totally or partial destroyed by recent anthropic activities. although those magnitude values are usually considered as insufficient to generate surface rupture, in volcanic domains the magnitude threshold for surface faulting may be significantly lower due to existence of shallow seismogenic sources related to carmo et al. 10 table 2. maximum expected magnitudes from surface rupture length (srl), rupture area (ra) and maximum displacement (md) of altiprado faults using relationships by wells and coppersmith (1994). (1) maximum displacement assuming a single paleoearthquake. (2) maximum displacement assuming surface rupture in both branches of af2 (af2-1 + af2-2) in a single paleoearthquake. fault srl (km) mw(±0.24-0.28) ra (km2) mw md (m) mw af1 0.8 5.5 5.0 5.9 7.4 48.5 4.9 5.7 1 0.38 (1) 6.7 6.4 af2 1.7 3 5.3 5.6 15.0 26.5 5.2 5.5 0.37 (2) 0.11 (2) 0.26 0.11 0.07 0.04 6.4 6.0 6.3 6.0 5.8 5.7 11 volcano-tectonic or purely tectonic mechanisms [smith et al. 1996, azzaro et al. 1998, zobin 2012]. there is a long record of seismic swarms in the studied area, probably with features similar to that of 2005 that was related with a magmatic intrusion. several authors [e.g. rubin and pollard 1988, rubin 1992] suggest that a magmatic intrusion at shallow levels can induce brittle deformation of the surface. gravimetric studies performed at são miguel island indicate that in this area, at about 8 km depth, a high-density region extending to shallower levels (still present at 1 km) exists and has been interpreted as an old basaltic shield or a partly solidified magmatic body [camacho et al. 1997]. relocation and fault plane solution analysis of earthquakes that occurred between 2002 and 2008 in the area suggest the existence of a local heterogeneous stress field [silva 2011]: in the upper 5 km depth, stress field is extensional, but at deeper levels (>5 km) fault plane solutions indicate a compressive stress field. according to silva [2011], the extensional stress field may result from a compressive regime at larger depths, which promotes the ascension of thermal fluids to crustal lower levels, weakening pre-existing fault planes which, due to gravitational stress, become potential slip surfaces. the role of hydrothermal fluids is well documented in são miguel. seismic tomography for this area relates the velocity anomalies to hydrothermal activity [zandomeneghi et al. 2007]; the existence of a positive vp anomaly with high vp/vs ratio suggest that this zone is characterized by low vs due to fluids contained in fractures. the possibility for creeping cannot also be disregarded, but the altiprado faults were not subject to precise leveling measurements. for instance, aseismic creep due to volcanic-tectonic processes that cause gravitational instability is well documented in faults at etna volcano [ferreli et al. 2002]. conversely, maximum displacement magnitudes may be overestimated. in af1 there is a time gap between deposition of unit 4 and the development of the topsoil, so the separation of 1 m could be erroneously considered as the result of a single event. on the other hand, part of the displacement could be post-seismic or aseismic. for instance, after the 2009 l’aquila earthquake, boncio et al. [2010 in wilkinson et al. 2012] observed widening in ground cracks. in the same way, in volcanic areas surface ruptures may occur related to moderate earthquakes, and in this case wells and coppersmith’s [1994] correlations should not be used. nevertheless, the correlation between magnitude and surface rupture length, deduced from aftershock distribution, for off-shore instrumental earthquakes in the azores (the m 7.2, january 1st, 1980 and ml 5.8, july 9th, 1998 earthquakes; hirn et al. 1980, dias et al. 2007) are in agreement with those obtained using the correlations from wells and coppersmith [1994], suggesting that they can be used in the azores for tectonic earthquakes [madeira and brum da silveira 2003]. however, the tectonic faulting origin for the observed offsets is not definitively proven. 5. final remarks trenching across the two scarps observed at achada das furnas confirmed their tectonic nature and revealed the presence of two previously unknown normal faults, with a possible strike-slip component. the altiprado faults 1 and 2 trend e-w and cut a volcanic succession younger than 4500 years, the age of the well-known fogo a deposit, undoubtedly showing that the faults are active. the proximity of the two faults and their geometry suggests that they could be synthetic branches of a major fault dipping to the south. it is probable that such a fault would present oblique (normal dextral) slip so we can envisage some amount of strain partitioning between the two branches; the subvertical geometry of the fault planes in af2 is compatible with this branch accommodating most of the strike-slip component, while af1 would accommodate mostly normal slip. af1 exhibits an accumulated dip-slip of 1.38 m that may have been produced by at least two surface rupturing paleoearthquakes (1 m + 0.38 m). the radiocarbon age of the paleosol developed on unit 4 is compatible with the first rupture having been produced by the 1522 earthquake. the event that produced the second rupture remains unclear. nevertheless, this area is subject to frequent seismic swarms of volcano-tectonic origin, and recent geodetic studies during periods of increased seismic activity showed the occurrence of crustal deformation. thus, the normal offset of 0.38 m could be the result of displacements produced by several moderate magnitude events or swarms related or not with magmatic cycles. af2 presents an accumulated dip-slip of 0.48 m, but uncertainties remain about the exact number of paleoearthquakes. assuming that both branches of af2 ruptured during the same events, then two paleoearthquakes occurred (0.37 m + 0.11 m). on the other hand, four events may have occurred and, thus, each displacement be related to individual earthquakes (0.26 m, 0.11 m, 0.07 m and 0.04 m). in both cases, the older displacement(s) (0.37 m, or 0.26 m and 0.11 m) correspond to pre-settlement events, while the following historical surface faulting in azores ruptures (0.11 m, or 0.07 m and 0.04 m) occurred during the 1563 historical eruption of fogo volcano or later. although there is some uncertainty about the seismic parameters, the altiprado faults have the potential to cause surface rupture either related to purely tectonic high-magnitude earthquakes (e.g. 1522 earthquake), or associated to volcano-tectonic seismic swarms of low to moderate magnitude, or even during surface deformation associated to eruptions, complicating seismic hazard assessment in the azores due to the interaction between volcanism and faulting. data and sharing resources maps were made using arcgis 10.0 software, esri®. world topography and bathymetry from gebco_08 database [2010], iho unesco, general bathymetry chart of the oceans, digital edition at http://www.gebco.net/data_and_products/gridded_ba thymetry_data/. aerial photo of achada das furnas plateau supplied by direcção geral de planeamento urbanístico, 1974. epicenter distribution of the 2005 seismic swarm and its macroseismic data was taken from civisa database, centro de informação e vigilância sismovulcânica dos açores, centro de vulcanologia e avaliação de riscos geológicos da universidade dos açores. acknowledgments. rita carmo is supported by the azores regional government, through a ph.d. grant from fundo regional da ciência e tecnologia (m3.1.2/f/016/2007), and through serviço regional de protecção civil e bombeiros dos açores in the scope of the scientific and technical protocols to guarantee the azores seismovolcanic surveillance and the emergency planning studies of centro de informação e vigilância sismovulcânica dos açores (civisa). josé madeira's contribution to fct project shaazores (ptdc/cte-gix/108637/2008).we also acknowledge the contribution of the reviewers (emanuela falcucci and raffaele azzaro) and associate editor salvatore barba to significant improvement of the initial version of the manuscript. references abdel-monem, a.a.; 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(2012). introduction to volcanic seismology, 2nd ed., elsevier b.v., amsterdam, 482 pp. *corresponding author: rita carmo, centro de vulcanologia e avaliação de riscos geológicos da universidade dos açores, cvarg, açores, portugal; email: rita.l.carmo@azores.gov.pt © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. carmo et al. 14 borexino: geo-neutrino measurement at gran sasso, italy annals of geophysics, 60, 1, 2017, s0114; doi:10.4401/ag-7389 s0114 borexino: geo-neutrino measurement at gran sasso, italy borexino collaboration matteo agostini17,8, konrad altenmüller17, simon appel17, gianpaolo bellini1, jay benziger2, daniel bick3, giuseppe bonfini4, david bravo5, barbara caccianiga1, frank calaprice7, alessio caminata9, paolo cavalcante4, alexander chepurnov10, davide d’angelo1, stefano davini11, alexander derbin12, lea di noto9, ilya drachnev8, alexander etenko13,14, kirill fomenko4,15, andrey formozov15,1, davide franco16, federico gabriele4, cristian galbiati7, chiara ghiano9, marco giammarchi1, marianne göger-neff17, augusto goretti7, maxim gromov10, caren hagner3, ed hungerford11, aldo ianni4, andrea ianni7, karol jedrzejczak21, dominik jeschke17, vladislav kobychev18, denis korablev15, gyorgy korga11, didier kryn16, matthias laubenstein4, bjoern lehnert19, evgeny litvinovich13,14, francesco lombardi4, paolo lombardi1, livia ludhova1, georgy lukyanchenko13, igor machulin13,14, szymon manecki5, werner maneschg20, simone marcocci8,9, emanuela meroni1, mikko meyer3, lino miramonti1, marcin misiaszek21, michele montuschi25, pablo mosteiro7, valentina muratova12, birgit neumair17, lothar oberauer17, michel obolensky16, fausto ortica22, marco pallavicini9, laszlo papp5,17, laura perasso9, andrea pocar6, gioacchino ranucci1,*, alessandro razeto4, alessandra re1, aldo romani22, romain roncin4,16, nicola rossi4, stefan schönert17, dmitrii semenov12, hardy simgen20, mikhail skorokhvatov13,14, oleg smirnov15, albert sotnikov15, serguei sukhotin13, yury suvorov4,13,23, roberto tartaglia4, gemma testera9, jan thurn19, maria toropova13, evgenii unzhakov12, alina vishneva15, robert bruce vogelaar5, franz von feilitzsch17, hanguo wang23, stefan weinz24, juergen winter24, marcin wojcik21, michael wurm24, zachary yokley5, oleg zaimidoroga15, sandra zavatarelli9, kai zuber19, grzegorz zuzel21 1 università di milano, dipartimento di fisica, and infn, milan, italy; 2 princeton university, chemical engineering department, princeton, new jersey, usa; 3 institut für experimentalphysik, universität hamburg, hamburg, germany; 4 istituto nazionale di fisica nucleare (infn), laboratori nazionali del gran sasso, assergi (l’aquila), italy; 5 virginia polytechnic institute and state university, physics department, blacksburg, virginia, usa; 6 university of massachusetts, physics department, amherst, massachusetts, usa; 7 princeton university, physics department, princeton, new jersey, usa; 8 gran sasso science institute (infn), l’aquila, italy; 9 università di genova, dipartimento di fisica, and infn, genoa, italy; 10 lomonosov moscow state university skobeltsyn institute of nuclear physics, moscow, russia; 11 university of houston, department of physics, houston, texas, usa; 12 st petersburg nuclear physics institute, gatchina, russia; 13 nrc kurchatov institute, moscow, russia; 14 national research nuclear university mephi (moscow engineering physics institute), moscow, russia; 15 joint institute for nuclear research, dubna, russia; 16 apc, université paris diderot, cnrs/in2p3, cea/irfu, observatoire de paris, sorbonne paris cité, paris, france; 17 technische universität münchen, physik-department and excellence cluster universe, garching, germany; 18 kiev institute for nuclear research, kiev, ukraine; 19 technische universität dresden, fachrichtung physik, dresden, germany; 20 max-planck-institut für kernphysik, heidelberg, germany; 21 m. smoluchowski institute of physics, jagiellonian university, krakow, poland; 22 università di perugia, dipartimento di chimica, biologia e biotecnologie, and infn, perugia, italy; 23 university of california los angeles (ucla), physics and astronomy department, los angeles, california, usa; 24 institut für physik, johannes gutenberg universität mainz, mainz, germany; 25 università di ferrara, dipartimento di fisica e scienze della terra, and infn, ferrara, italy article history received march 11, 2016; accepted october 5, 2016. subject classification: geoneutrinos, solar neutrinos, neutrino oscillations, liquid scintillators, low background. agostini et al. 2 abstract geo-neutrinos, electron anti-neutrinos produced in b-decays of naturally occurring radioactive isotopes in the earth, are a unique direct probe of our planet’s interior. after a brief introduction of the geo-neutrinos’ properties and of the main aims of their study, we discuss the features of a detector which has recently provided breakthrough achievements in the field, borexino, a massive, calorimetric liquid scintillator detector installed at the underground gran sasso laboratory. with its unprecedented radiopurity levels achieved in the core of the detection medium, it is the only experiment in operation able to study in real time solar neutrino interactions in the challenging sub-mev energy region. its superior technical properties allowed borexino also to provide a clean detection of terrestrial neutrinos. therefore, the description of the characteristics of the detected geo-neutrino signal and of the corresponding geological implications are the main core of the discussion contained in this work. 1. introduction borexino at gran sasso is the last player which entered the solar neutrino arena, where, thanks to its unprecedented low background, it provided breakthrough results in the low energy sub-mev regime. having already measured three components of the solar neutrino spectrum over the past years, i.e. 7be, 8b and pep (providing jointly with this component also a stringent upper limit on the cno contribution), borexino has recently crowned its remarkable series of results with the detection of the fundamental pp neutrino flux, coming from the reaction which provides most of the sun’s energy. therefore, borexino is the first experiment able to perform an almost complete spectroscopy of the whole solar neutrino flux, allowing a thorough data-model comparison [serenelli et al. 2011]. the last step of this investigation will be the attempt to measure the tiny cno flux, which borexino will undertake over the next years. moreover, specifically relevant to the present discussion, borexino has also detected an unquestionable geo-neutrino signal, contributing to pave the way to a complete new method to investigate the interior of our planet based on the study of the properties of such elusive messengers. in the following, after an introduction to the underground gran sasso laboratory hosting the experiment, and a general highlight of the features of geo-neutrinos from earth, the experimental characteristics of borexino, which made possible the outstanding accomplishments accumulated so far, will be briefly reviewed. afterwards, the characteristics of the geo-neutrino signal detected by borexino will be thoroughly described and quantified, and its geological implications carefully emphasized. finally, the future perspective of this new and fascinating field will be sketched, pointing to the potential big advantage for the study of the interior of our planet that would stem from a network of powerful geo-neutrino detectors deployed around the world. 2. the hosting laboratory: laboratori nazionali del gran sasso in italy since a fundamental prerequisite for the breakthrough results of borexino is the hosting laboratory, few words for its description are mandatory. the lngs facility (laboratori nazionali del gran sasso; http://www.lngs.infn.it/) under the gran sasso mountain in the abruzzo region of italy, is currently the largest of the underground facilities all over the world. following the proposal in 1979 by a. zichichi, the infn president at that time, construction was completed in 1987, together with two road tunnels through the mountain. these road tunnels provide the horizontal access by road vehicle to the facility, allowing transport container delivery. the underground area comprises three main hallways, with several other experimental tunnels available, including two 90 m hallways used for a michelson interferometer for geological studies. the dimensions of the hall c are 100 m by 20 m by 18 m, with the other two, a and b, only slightly smaller, providing a total area of 17,300 m2, and a total volume of 180,000 m3. a surface set of auxiliary buildings provides offices and administration facilities, computing and conferencing capabilities, laboratories, electrical and mechanical workshops and warehousing facilities. there is a permanent staff of 76 physicists, engineers and technicians, with about 20 non-permanent staff, servicing about 800 users from across 26 countries. the rock overburden is 1400 m, with an azimuthal and zenith distribution, due to the topology of the mountain, which varies between 3100 m.w.e. and 3800 m.w.e. this reduces the muon flux to a measured value of 2.87×10−4/m2/s [ahlen et al. 1990, aglietta et al. 1998]. measured neutron fluxes are 2.93×102/m2/s and 0.86×102/m2/s for thermal and fast neutrons (with a breakpoint defined at 1 kev), respectively [belli et al. 1989]. radon levels in the air range between 50 and 120 bq/m3 with the ventilation system providing one exchange of air in 3.5 h. the science program at lngs is very broad, with neutrino beam studies just completed (opera and icarus), galactic dark matter searches (dama/libra, darkside, xenon, cresst), 0obb decay studies (cuore, gerda, lucifer and cobra), solar and geo-neutrinos (borexino, the topic of this discussion), supernova neutrinos (lvd, borexino) and nuclear astrophysics through a low energy ion accelerator (luna). in addition, low background counting facilities exist, like the stella hpge detector array. 3 in terms of expansion of the facility, there are no current plans to extend the available underground space, with new areas becoming available as the current experimental program evolves and projects are completed. 3. geo-neutrinos geo-neutrinos (geo-o−e) are electron anti-neutrinos (o−e) produced in b-decays of 40k and of several nuclides in the chains of the long-lived radioactive isotopes 238u and 232th, which are naturally present in the earth: 238u → 206pb + 8a + 8e−+ 6o−e + 51.7mev (1) 232th → 208pb + 6a + 4e−+ 4o−e + 42.8mev (2) 40k → 40ca + e−+ o−e+ 1.32mev (3) hence, while the sun shines in neutrinos, the earth shines in geo-neutrinos, with a flux of about 106 cm−2 s−1. it is important to note that the released radiogenic heat is in a well fixed and known proportion with respect to the geo-neutrino flux. therefore, it is in principle possible to determine the urey ratio (defined as the relative contribution of the radiogenic heat from radioactive decays to the earth’s surface heat flux) by measuring the geo-neutrino flux. in particular, by determining experimentally the geo-neutrino signal at different locations through the planet, in different geological settings, and/or by being able to identify the incoming direction of detected geoneutrinos, it might be possible to: a) study the distribution of radioactive elements within the earth, to determine their abundances in the crust and in the mantle; b) determine if there are any radioactive elements in the earth’s core; c) understand if the mantle composition is homogeneous or not; d) test, validate, and discriminate among different bse models; e) exclude or confirm the presence of a putative georeactor in the core of the planet; f ) determine the urey ratio, important for geochemistry and geophysics. g) study the bulk u and th ratio in the silicate earth, an important parameter for geochemistry, which could shed light on the process of the earth’s formation. we can see, hence, that geo-neutrinos can be used as a unique direct probe of the earth interior, not accessible by any other means. all this information could be used as inputs for many geological, geophysical, and geochemical models describing such complex processes as the mantle convection, movement of tectonic plates, geo-dynamo (the process of the generation of the earth’s magnetic field), the process of the earth formation, etc. the energy spectrum of geo-neutrinos extends to about 3.3 mev. they are detected via the inverse neutron b-decay reaction o−e + p → e + + n (4) which has a kinematic threshold of 1.806 mev. the cross section of this interaction as a function of anti-neutrino energy is well known and can be found in strumia and vissani [2003]. unfortunately, all geo-neutrinos produced in the decay of 40k are below this threshold and we are able to detect only the tail of the 238u and 232th geo-neutrinos, as shown by the dashed line in figure 1. geo-neutrinos from the 232th chain have the end point of their energy spectrum at about 2.25 mev, while those from the 238u chain extend up to 3.3 mev. ideally, this spectral feature could be used in order to measure the u and th ratio in the earth. it is important to recall that the relative proportions of the elements abundances are much better known than their absolute abundances. therefore, by measuring the absolute abundances of 238u and 232th, the absolute abundance of 40k can be deduced with a better precision. geo-neutrinos are emitted and interact as flavor states, but they travel as superposition of mass states and therefore undergo flavor oscillations. in the approximation dm231 ~dm 2 32 >>dm 2 21 (the square-mass differences of mass eigenstates 1, 2, 3) the survival probability pee for a o − e in vacuum is: pee = p (o − e → o − e) = sin 4 i13+ (5) . cos sinsin e mev md ev l$ m 1 2 4 1 2674 13 1 2 2 2 21 2 2 i i md + ! ! ! $ l$ $a a dd borexino: geo-neutrino measurement at gran sasso, italy figure 1. the geo-neutrino luminosity as a function of energy is shown for the most important reaction chains and nuclides [enomoto 2009]. only geo-neutrinos of energies above the 1.8 mev energy (vertical dashed line) can be detected by means of the inverse beta decay on target protons shown in equation (4). for completeness, though not mentioned in the text, also the 235u contribution is indicated. in the earth, the geo-neutrino sources span a vast region compared to the oscillation length, given by: (6) for example, for a ~3 mev antineutrino, the oscillation length is of ~100 km, small with respect to the earth’s radius of ~6371 km: the effect of the neutrino oscillation to the total neutrino flux is, therefore, completely averaged, giving an overall survival probability of: (7) according to the neutrino oscillation mixing angles and square-mass differences reported in fogli et al. [2012], pee ~ 0.54. it has been calculated in enomoto [2005] that the so called matter effect contribution to the average survival probability is an increase of about 2% and the spectral distortion is below 1%. to conclude, the net effect of flavor oscillations during the geo-neutrinos propagation through the earth is an absolute decrease of the overall flux by ~0.55, with a very small spectral distortion, negligible for the precision of the current geo-neutrino experiments. 4. characteristics of the borexino experiment borexino [alimonti et al. 2009] is a scintillator detector, which employs as active detection medium 300 tons of pseudocumene-based scintillator. the intrinsic high luminosity of the liquid scintillation technology is the key toward the initial design goal of borexino, the real time observation of sub-mev solar neutrinos through oe elastic scattering, being the 7be component the main foreseen target at the time of the proposal. however, the lack of directionality of the method makes it impossible to distinguish neutrino-scattered electrons from electrons due to natural radioactivity, thus leading to the other crucial requirement of the borexino technology, e.g. an extremely low radioactive contamination of the detection medium, to a degree never reached before. specifically, 10−16 gs/g of 238u and 232th, and 10−18 gs/g of 40k were the initial radiopurity specifications of the experiment. the active scintillating volume is observed by 2212 pmts located on a 13.7 m diameter sphere and is shielded from the external radiation by more than 2500 tons of water and by 1000 tons of hydrocarbon equal to the main compound of the scintillator (pseudocumene), to ensure zero buoyancy on the thin nylon inner vessel containing the scintillator itself. key factors for the accomplishment of the planned measurements of the experiment were also the purification and handling systems, designed and installed to ensure the manipulation and decontamination of the fluids at the radiopurity levels mentioned above [bellini et al. 2009]. when data taking started in may 2007, it appeared immediately that the task of the ultralow radioactivity was successfully obtained, with residual contaminations even better than the design values (u and th at 10−18 gs/g and 40k not detectable), representing per se a major technological breakthrough, opening a new era in the field of ultrapure detectors for rare events search. the achieved ultra-low background implies that, once selected by software analysis the design fiducial volume of 100 tons and upon removal of the muon and muoninduced signals, the recorded experimental spectrum is clean enough to show the feature of the 7be scattering edge, i.e. the unambiguous signature of the occurrence of solar neutrino detection [bellini et al. 2011]. as additional bonus, the better-than-anticipated features manifested by the detector made it also clear since the beginning of the physics run that the geo neutrino detection would have been amply within the reach of borexino, as later on confirmed after the accumulation of enough data statistics. an overall pictorial view of the detector is reported in figure 2. 4.1. detector response several steps are required to extract from the raw data the quantitative information of interest regarding the solar neutrino and geo-neutrino fluxes. the raw signals must be converted into meaningful amplitude variables; of the total accumulated signals only those satisfying the scintillation event acceptance criteria are kept; the data spectrum is constructed by including only events which are reconstructed within a fiducial volume far from the wall of the containment vessel; and, finally, the spectrum is fitted to a global signal-pluscos sin sinp 1 2 1 2ee 1 132 4 13 2 4. i i i+@ c .l m md e 2 48. mdm ev mev 21 2 2 ! ! $ $ agostini et al. 4 figure 2. schematic pictorial view of the borexino detector. 5 background model in which the flux amplitude to be evaluated is a fit parameter. the task to obtain a detailed understanding of the detector response has been accomplished through two coordinated efforts, i.e. a thorough calibration campaign and the development of a full mc able to reproduce the detector performances. 4.2. calibration the calibration of the detector has been accomplished to characterize both the energy and time response of borexino. several sources have been deployed in different locations within the liquid scintillator: gamma sources producing monoenergetic lines spanning the energy range of interest from 0.122 to 1.4 mev, a radon source realized by filling with liquid scintillator taken from the inner vessel a small quartz vial which was successively loaded with radon, and an am-be neutron source, specifically employed to test and characterize the anti-neutrino detection process (see previous equation (4)). the gamma lines and the radon source, positioned in several hundred locations, gave an accurate probe of the energy response of the experiment as function of the event position; furthermore they also provided a way to careful calibrate the capability of the time signals from the array of photomultipliers to identify precisely the vertex of the events. jointly, the energy and spatial measurements obtained throughout the calibration campaign provided a complete map of the detector response, leading to the following estimates of the crucial features of the detector: energy resolution 5%/√ e(mev), linearity of the energy scale of the order of 1%, fiducial volume uncertainty close to 1%, too [back et al. 2012]. 4.3. mc tuning the second ingredient at the basis of the precise measurements accomplished is the accurate monte carlo description of the detector. it required a while to develop a full mc code incorporating all the details of the light generation and transport in the liquid scintillator, of the behavior of the photomultipliers and of the electronics response. at the end of this development, we were able to produce a full simulation suited to be confronted with the many outputs stemming from the calibration campaign. such a comparison has been used to contrast the source data with the corresponding simulated events; in this way it has been possible on one hand to perform a fine tuning of the mc code, so to ensure the best match between the measured and simulated data, and on the other to quantify precisely the crucial, residual uncertainties on the energy scale and on the fiducial volume, already listed before. a thorough account of the effort to characterize the performances of the detector has been published in bellini et al. [2014], where also the full methodology adopted for the solar neutrino analysis is described. 5. geo-neutrino results the borexino geo-neutrino result [bellini et al. 2013] reported here refers to the statistics collected from december 2007 to august 2012, corresponding to 1352.60 days of live time. the corresponding fiducial exposure after all cuts is (613 ± 26) ton × year or (3.69 ± 0.16) × 1031 proton × year. electron anti-neutrinos are measured through the inverse neutron b-decay reaction of equation (4). the positron created in this reaction promptly comes to rest and annihilates. all deposited energy is detected in a single prompt event, with a visible energy of eprompt= eo−e − 0.784 mev. the emitted free neutron is typically captured on protons, resulting in the emission of a 2.22 mev de-excitation c ray, providing a delayed coincidence event. the mean neutron-capture time in borexino was measured with an ambe neutron source, resulting equal to x= (254.5 ± 1.8) ns. the characteristic time and spatial coincidence of prompt and delayed events offers a clean signature of o−e detection, further suppressing possible background sources. the o−e’s from nuclear power plants are the main anti-neutrino background to the geo-neutrino measurement. since there are no near-by nuclear plants, the lngs site is well suited for geo-neutrino detection. the number of expected o−e reactor candidates is nreact= (33.3 ± 2.4) events, after cuts. thanks to the extreme radiopurity of the borexino detector, the non anti-neutrino background is almost negligible, only 0.7 ± 0.18 events, dominated by b + neutron decays of cosmogenic 9li and 8he, accidental coincidences, and (a, n) reactions with a’s from decays of 210po. through the application of the selection cuts, in the period under consideration 46 antineutrino candidates have been selected, whose prompt energy spectr (i.e. the spectrum of the positron in equation (4)) is shown in figure 3. an unbinned maximal likelihood fit of the energy spectrum (expressed in unit of photoelectrons) of the prompt candidates was performed, with the th/u mass ratio fixed to the chondritic value of 3.9, and with the number of events from reactor antineutrinos left as a free parameter. the resulting best fit values are ngeo = (14.3 ± 4.4) geo-neutrino events, and nreact= 31.2 +7. −6. 0 1 reactor events. if converted to the standard unit used in this field, the tnu (defined as 1 tnu = 1 terrestrial neutrino unit = borexino: geo-neutrino measurement at gran sasso, italy 1 event / year /1032 protons), such events correspond, respectively, to the signals sgeo= (38.8 ± 12.0) tnu and sreact= 84.5 +1 −1 9.3 6.9 tnu. from the measured geo-neutrino signal we can infer the overall o−e fluxes from u and th decay, i.e. z(u) = (2.4 ± 0.7) × 106 cm−2 s−1 and z(th) = (2.0 ± 0.6) × 106 cm−2 s−1. from the log-likelihood profile, the null geo-neutrino hypothesis has a probability of 6 × 10−6, which is, therefore, the significance of the present geo-neutrino detection. the signal from the reactors is in full agreement with the expectations of (33.3 ± 2.4) events, in the presence of neutrino oscillations. 6. the bse models the bulk composition of the silicate earth, the socalled bulk silicate earth (bse) models, describes the composition of the primitive mantle, i.e. the earth composition after the core separation and before the crust-mantle differentiation. the estimates of the composition of the present-day mantle can be derived as a difference between the mass abundances predicted by the bse models in the primitive mantle and those observed in the present crust. in this way, the predictions of the u and th mass abundances in the mantle can be done, which are then critical in calculating the geo-neutrino signal. the most recent bse-models classification is that of šrámek et al. [2013], which distinguishes among geochemical bse models, cosmochemical bse models and geodynamical bse models. the earth’s surface heat flux is estimated based on the measurements of temperature gradients along several thousands of drill holes along the globe. the most recent elaboration of these data leads to evaluating such a flux equal to 47 ± 2 tw, consistent with the prediction in davies and davies [2010] and jaupart et al. [2007]. the relative contribution of the radiogenic heat from radioactive decays to this flux (the so called urey ratio) is not known and this is the key information which can be pinned down by the geo-neutrino measurements. the geochemical, cosmochemical, and geodynamical models predict the radiogenic heat of 20 ± 4, 11 ± 2, 33 ± 3 tw and the corresponding urey ratios of about 0.3, 0.1, and 0.6, respectively. the heat producing elements (hpe) predicted by these models are distributed in the crust and in the mantle. the crustal radiogenic power was recently evaluated in huang et al. [2013] as 6.8/+1.4/−1.1 tw. by subtracting this contribution from the total radiogenic heat predicted by different bse models, the mantle radiogenic power driving the convection and plate tectonics can be as little as 3 tw and as much as 23 tw. the determination of this mantle contribution is therefore one of the main goals and potentials of geo-neutrino investigation. 7. geological implications the careful evaluation of the contribution of the local crust (loc) to the total geo-neutrino signal, based on the local 3d geology around the lngs laboratory, has been reported in coltorti et al. [2011] as sgeo(loc) = (9.7 ± 1.3) tnu. on the other hand, the contribution from the rest of the crust (roc), based on the recent calculation by huang et al. [2013], results in the estimate of the geo-neutrino signal from the crust (loc+roc) equal to sgeo(crust) = (23.4 ± 2.8) tnu. subtracting such estimated crustal contribution from the measured borexino geo-neutrino rate, we can infer the contribution of the mantle, sgeo(mantle) = (15.4 ± 12.3) tnu. as we already stressed, the earth releases radiogenic heat, hgeo, together with geo-neutrinos in a well fixed ratio; however, the observed geoneutrino signal depends both on the abundances of the individual radioactive elements and on their distribution inside the earth. the extraction of the radiogenic heat power from a measured sgeo signal is, therefore, model dependent. the calculation of the expected sgeo(u+th) as a function of the radiogenic heat produced by u and th, hgeo(u+th) is reported in figure 4, and it is compared to the borexino results. the allowed region between the red and blue lines in the plane sgeo(u+th) and hgeo(u+th) represents models which are consistent both with geochemical and geophysical data. for each total mass of u and fixed th/u ratio, the maximal geo-neutrino signal (red line) can be obtained by maximizing the radiogenic material agostini et al. 6 figure 3. light yield spectrum of the 46 prompt (positron in equation (4)) anti-neutrino candidates and the best fit. the yellow area isolates the contribution of the geo-o−e in the total signal. dashed red line/orange area: reactor-o−e signal from the fit. dashed blue line: geo-o−e signal resulting from the fit. the contribution of other backgrounds is almost negligible and is shown by the small red filled area in the lower left part. the conversion from photoelectrons (p.e.) to energy is approximately 500 p.e./mev. 7 in the crust and allowing uniform distribution in the mantle. similarly, the minimal signal (blue line) is obtained for the minimal radiogenic mass in the crust with the rest concentrated in a thin layer at the bottom of the mantle. note that the maximum signal is obtained with a uniform distribution and not with the specular situation of a thin concentration layer at the top of the mantle, because the crust originated from the mantle itself, removing from it during its formation the u and th elements. the expected signal from the crust is taken from table v of fiorentini et al. [2012]. we have chosen as a reference the bse model from mcdonough and sun [1995], predicting that the silicate earth contains m(u) = (0.8 ± 0.1) × 1017 kg, with mass ratios th/u = 3.9 and k/u = 12,000. the green region is allowed by this bse model. the arrow ”min” indicates the contribution of the crust only. the arrow for the fully radiogenic model indicates the value of 39.3 tw: it assumes that the total earth surface heat flux of (47 ± 2) tw [davies and davies 2010] is completely due to radiogenic heat from u, th, and k. taking the relative proportions from the bse of mcdonough and sun [1995], we get that, in a fully radiogenic earth, u, th, and k would produce 19.1, 20.2, and 7.7 tw, respectively. the circumstance that in figure 4 the +/− 1a band of the borexino result contains almost completely the portion of plane between the maximal and minimal line indicates that at this stage such a result does not restrict the choice among different bse models. we have performed another unbinned maximal likelihood fit of our 46 candidates, in which the individual contributions from the 238u and 232th chains were fitted individually. in this case the best fit values are nth = (3.9±4.7) events and nu = (9.8 ± 7.2) events, corresponding to sth = (10.6 ±12.7) tnu and su = (26.5 ± 19.5) tnu, or as o−e fluxes (above 0 mev) z(th) = (2.6 ± 3.1) × 106 cm−2 s−1 and (u) = (2.1 ± 1.5) × 106 cm−2 s−1. although our data is compatible, within 1v, either with only 238u signal (and sth = 0), or only 232th signal (and su = 0), we note that the best fit of the th/u ratio is in very good agreement with the chondritic value. a geo-reactor with thermal power < 30 tw and 235u : 238u = 0.76 : 0.23 composition was suggested by herndon and edgerley [2005]. it is assumed to be confined in the central part of the earth’s core within the radius of about 4 km [herndon 1996]. we have produced mc spectra of the expected geo-reactor anti-neutrino. in a similar unbinned maximal likelihood fit of our 46 anti-neutrino candidates we have added another fit component, ngeo-react, while constraining nreact to the expected value of (33.3 ± 2.4) events. all other fit details were as above, including fixed chondritic mass th/u ratio: in this way, we are able to set an upper limit on the hypothetical geo-reactor power of 4.5 tw, at 95% c.l. 8. future perspectives the geo-neutrino measurement of borexino (together with that performed by kamland; araki et al. [2005]) opened a door towards a new field, by not only proving that geo-neutrinos can be detected, but also showing that a new powerful tool to study our planet is available. in order to find definitive answers to the questions correlated to the radiogenic heat and abundances of radiogenic elements, more data is needed. both borexino and kamland will continue to take data in the near future. in addition, it would be important to construct larger volume detectors in order to increase the number of detected geo-neutrinos and so improve the precision of the flux measurement. results from different detector sites placed at different geological settings is a key point for understanding, for example, if the earth mantle composition and heat distributions are homogeneous or not. answers to questions like what is the bulk-earth u versus th ratio, (the same as in meteorites or not) can help in better understanding of the process of earth formation and the distribution of elements in the solar system. a new generation of experiments using liquid scintillators is either under design or even in advanced construction process. sno+ at sudbury mine in canada [chen 2006], having 1000 tons of target, is almost ready to start data taking. the site is located on an old continental crust and the signal from reactor anti-neutrinos is about twice as that at gran sasso. the main goal of the juno experiment in china (formerly called daya bay 2) [wang 2013] is to deterborexino: geo-neutrino measurement at gran sasso, italy figure 4. the signal su+th from u and th geo-neutrinos as a function of radiogenic-heat production rate hu+th in borexino. mine the neutrino mass hierarchy. thanks to a very large mass of 20 kton, it would detect up to 400 geoneutrinos per year. unfortunately, the reactor-neutrino and cosmogenic background would be the dominant sources of the systematic error. an ambitious project to construct a 50,000 ton detector is lena [wurm et al. 2012], proposed for the pyh¨asalmi mine in finland. this experiment could detect of the order of 1000 geo-neutrinos per year, therefore a few percent precision of the total flux measurement could be reached within the first few years. the individual contribution of the u and th geo-neutrino flux could be determined as well. an interesting project of ~10,000 ton underwater experiment is hanohano [learned et al. 2008], planned to be placed on the oceanic crust (hawaii). due to the thin oceanic crust, the mantle contribution to the total geo-neutrino flux should be dominant. therefore, this measurement would provide the most direct information about the mantle. if all realized, these forthcoming and proposed projects, together with the currently running experiments, would be the initial building blocks of a network of sophisticated and powerful instruments, which would greatly improve the understanding of the interior of our planet. acknowledgements. the author wishes to thank the organizers for the invitation to contribute to such an interesting and enlightening workshop. references aglietta, m., et al. 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(2012). the nextgeneration liquid-scintillator neutrino observatory lena, astropart. phys., 35, 685-732. *corresponding author: gioacchino ranucci, università di milano, dipartimento di fisica, and infn, milan, italy; email: gioacchino.ranucci@mi.infn.it. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. borexino: geo-neutrino measurement at gran sasso, italy << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 61,4, 2018, doi: 10.441/ag-7731 1 comment on the paper “recent geoethical issues in moroccan and peruvian paleontology„ by juan carlos gutiérrez-marco, artur abreu sá, diego c. garcía-bellido, césar a. chacaltan joan corbacho geological museum of the seminary, barcelona (spain) association of experts and appraisers of catalonia, sabadell (spain) fosilart@hotmail.com 1. introduction this situation dates back to 2010, when dr. gutiérrez-marco made a trip to morocco, at a time when the study of the new ordovician trilobite faunas that were appearing in that region was practically carried out only by me and collaborators, vela and corbacho [2007; 2009], corbacho [2008; 2011; 2014] corbacho and vela [2010; 2011; 2013], corbacho and kier [2011], lópezsoriano and corbacho [2012], corbacho and lópez-soriano [2012; 2013] and corbacho et al., [2014], with some notable exceptions such as those made by dr. richard fortey [2009; 2010; 2011]. in 2015, at an international congress held in morocco, dr. gutiérrez-marco already presented a communication stating that the trilobites i had described from the bou nemrou deposits at el-kaid errami [corbacho, 2011] had been made with pieces of different specimens [gutierrez-marco et al., 2015]. a similar episode was later repeated at the 6th international conference on trilobites and their relatives held in tallinn (estonia) in 2017 [gutiérrez-marco et al., 2017]. this effort to discredit my work internationally reaches a new phase with the publication of the article mentioned above, which forces me to make the following considerations. 1) it is false that some of the holotypes and paratypes of the new species described in my articles have been sold through the internet. all these specimens are kept in the museums indicated in the corresponding publications, where they remain available to any member of the scientific community who wishes to examine them and verify their authenticity. 2) regarding the certificate of authenticity corresponding to the topotype of uralichas hispanicus tardus vela and corbacho, 2009 mentioned in figure 1 of the article by gutiérrez-marco et al. [2017] and which is said not to be a completely real fossil because it presents epoxy resin in cephalon, thorax and pigidium, a photograph (figure 2) is attached under ultraviolet light of this specimen in which it can be seen that the only resin that appears in it corresponds to where the broken part of the matrix to the level of pygidarticle history receveid april 11, 2018; accepted july 21, 2018. abstract recently, in an article written by gutiérrez-marco et al. and published in annals of geophysics [60, fast track 7, 2017], there are some incorrect affirmations written against my work in the paleontological field that i have been carried out for more than 10 years on new species of trilobites from the ordovician of morocco. they have questioned the quality of my work but also that of my collaborators and the institutions with which i am collaborating, particularly the geological museum of the seminario of barcelona (spain), an institution that has been doing geological research since 1874. for this reason, i will present here some evidences that show the incorrectness of such statements reported in the paper by gutiérrez-marco et al. [2017]. joan corbacho 2 figure 1. part of the specimens used as paratypes and other trilobites in my publications, all them available to any member of the scientific community interested in their study. the pictures were taken with the first page of the article by gutiérrez-marco et al. [2017] to demonstrate that the pictures were taken after the publication of that paper. ium. any paleontologist who has worked with trilobites of this size perfectly knows well that it is very easy to produce this kind of breakage during the extraction process and that their repair is perfectly acceptable. as can be seen in the figure 2, there is no presence of resin neither in the cephalon nor in the thorax of the specimen. any scientist who wants to verify the authenticity of this topotype has it at their disposal. similarly, when it is stated that the genal spines in the specimens of asaphellus cuervoae corbacho and lópez-soriano, 2012 have been “obviously, generated as an ‘improvement’ with epoxy paste, probably based on poorly preserved specimens of asaphellus stubbsi [fortey, 2009]", a false information is again written, that it is reconstruction but, again, without any validation of this assertion. 3) sale of a holotype on the internet. this is a very serious issue and, like the others, false. the holotype of parvilichas marochii corbacho and vela, 2013 is conserved in the collections of the natural sciences museum of barcelona (spain), as can be verified with the certificate issued by the director of said institution (figure 3). figure 1 shows some of the specimens used as paratypes and other trilobites in my publications; in this figure, the first page of the article by gutiérrez-marco et al. is attached in each picture in order to demonstrate that the photographs have been taken after the publication of that paper. again, the specimens are available to anyone who wishes to verify it in the corresponding museums. 4) it is stated in the article that some specimens used in my studies have been acquired from local merchants. but most have been collected by staff who worked on my behalf, and others have been personally collected by myself, such as those of symphysops stevaninae lópez-soriano and corbacho, 2012 or pauxillites thaddei valent and corbacho, 2015 and valent et al., 2013. in fact, some of the holotypes used in my papers and currently kept in geological museum of the seminary of barcelona and národní muzeum, prague, have been bought by me or collected personally by me and then donated by me to these institutions; in the case of those deposited in the natural history museum of london, they have been donated by one of my collaborators. on the other hand, the purchase of fossil specimens from local collectors is a common practice on the part of "professional" paleontologists. in these cases, it is essential to verify the exact origin of the same, which i have always done with all my papers (see references). 5) the association of experts and appraisers of catalonia is a young association but with more than five partners, contrary to what is said in gutiérrez-marco et al. [2017]. accordingly with the association's statutes, there have been several people who have not been allowed to join because they do not have the corresponding degree or a sufficiently accredited experience. the fact that i was one of its founding members and being currently its president does not mean that i am the owner, as is stated in the article. the statutes are in the headquarters of the association, at the disposal of anyone who wants to consult them. 6) justification of my education and qualifications. at the following link you can see the admission requirements for master's degree of the european 3 figure 2. picture taken under uv light of uralichas hispanicus tardus (topotype). only resin present in it corresponds to the junction of the broken part of the matrix at the pygidium level. joan corbacho 4 university miguel de cervantes. to do the master’s is needed a three-year university degree. (taken from the web on february 1, 2018). http://www.escuelacriminologia.com/index.php/tit ulos-propios-universitarios 7) “as an example of the first view, martin et al., [2016] reported a general list of the lower ordovician trilobites identified from the fezouata shale, with no reference of the new taxa described by mr. corbacho from the same formation [vela and corbacho, 2007; corbacho, 2008, 2014; corbacho and vela, 2010, 2013; corbacho and lópezsoriano, 2012], with the single exception of anacheirurus adserai [vela and corbacho, 2007]”. although martin et al., [2016] did not respect the species described [published previously by vela and corbacho, 2007; corbacho, 2008, 2014; corbacho and vela, 2010, 2013; corbacho and lópezfigure 3. holotype sold in internet? the certificate issued by the director (dr. anna omedes) of the museu de ciències naturals of barcelona (spain) states that the specimen is diposited in this museum and accessible to the scientific community. translation of the certificate: “the natural sciences museum of barcelona preserves in its paleontological collections the specimen mgb 58638, both genotype and holotype of the taxon parvilichas marochii corbacho and vela, 2013, collected in the upper fezouata formation of tinzouline (zagora, morocco), of floian age (lower ordovician, paleozoic) which entered in the collection on april 19, 2012 as a part of the j.a. vela collection (specimen number 4962)”. 5 soriano, 2012] in the fezouata formation, it does not mean that they do not exist. other authors have recognized them such as patrice lebrun [2018]. concerning the different species of lehua, the cited authors affirm that they only know one species. the normal procedure would have been to study the different the holotypes and paratypes kept in the geological museum of the seminary of barcelona. 8) “gutiérrez-marco et al., [2017] have used the name megistaspis (ekeraspis) hammondi [corbacho and vela, 2010], but stating that ‘the original description of the species needs revision, because the diagnostic characters are barely recognizable on the holotype, and the purported differences with the subspecies m. (e.) hammondi forteyi could be due to preparation by fossil dealer’s. on the other hand, the same late tremadocian trilobite was cited as m. (e.) cf. filacovi by martin et al. [2016], in the hope of a more complete study with better preserved material”. this is also untrue; the two holotypes of both species are in the nhm, london, in addition to the presence of a considerable number of paratypes that are kept in the geological museum of the seminary of barcelona. the differences can easily be seen between the two species, above all with m. (e.) filacovi by martin et al. [2016]. 9) “about fossil fakes and their recognition and legal expertise in paleon-tology he has written several papers with very elemental and obvious examples [corbacho, 2009, 2015; corbacho and martínez, 2014, 2015; corbacho and sendino, 2015; corbacho et al., 2007, 2011, 2015]”. despite these claims about my articles on fossils and fossil expert reports, the truth is that none of the authors of the paper by gutiérrez-marco et al. [2017] have published any papers on these topics, whist i have published. final remarks in summary, the article by gutiérrez-marco et al. [2017] is full of inaccuracies towards the work that i have been developing on the subject (holotypes and paratypes have not been sold; my title is authentic and awarded at a recognized university; and specimens have not been faked). additionally, they have used this journal to promote their own views on systematic palaeontology whilst missing the truth. references bignon a., corbacho j. and lópez-soriano f.j. (2014). a revision of the first asteropyginae (trilobita; devonian), geobios, 47, 281–289. calzada, s., sendino, c. and corbacho, j. posible inclusión en el género triásico ferreniscus (isopoda) de la península ibérica de una especie del cretácico japonés. bull. soc. hist. nat. toulouse, 147, 2011, 43-45. corbacho j. (2008). tres nuevas especies de lehua del ordovícico inferior del valle de draa (marruecos), scripta mus. geol. seminar. barcinonensis [ser. palaeontol.], 5, 3-13. corbacho j. (2009). falsificación de fósiles del n. de áfrica (no trilobites). batalleria, 14, 53–56. corbacho, j. (2011). upper ordovician trilobites of bou nemrou el kaid errami (morocco). batalleria. 16, 16-36. corbacho j. (2014). kierarges morrisoni: un nuevo género de isotelinae (trilobita) del floiense de portugal y marruecos. batalleria, 20, 21-26. corbacho j. (2015). peritajes y tasaciones en paleontología. trabajo de título propio escuela internacional de criminología, universidad europea miguel de cervantes, valladolid, 19 pp., doi 10.13140/rg.2.1.1872.8489. corbacho, j. and vela, j.a. (2010). giant trilobites from lower ordovician of morocco. batalleria, 15, 3-32. corbacho, j. (2011) trilobites from the upper ordovician of bou nemrou -el kaid errami (morocco). batalleria, 16, 16-36. corbacho, j. and kier, c. (2011). trilobites of a new outcrop of upper ordovician in jbel tijarfaïouine, el kaid errami (morocco), with first mention of genus corrugatagnostus. scripta musei geologici seminarii barcinonensis [ser. palaeontologica], 10, 7-22. corbacho, j. and vela, j.a. (2013). parvilichas marochii: new genus and species of lichidae from the zagora region (morocco); early ordovician (floian). scripta musei geologici seminarii barcinonensis [ser. palaeontologica] 14, 3-13. corbacho j. and lópez-soriano f. j. (2012). a new asaphid trilobite from the lower ordovician (arenig) of morocco. batalleria, 17, 3-12. corbacho, j. and lópez-soriano, f.j. (2013). chattiaspis? budili: una nueva especie de dalmanitidae de marruecos; ordovícico superior (katiense inferior). batalleria, 19, 6-12. corbacho, j. and lópez-soriano, f. j. (2016). platypeltoides hammondi (trilobita, nileidae): a new joan corbacho 6 species from the upper tremadoc of the dra valley, morocco. batalleria, 23, 11-19. corbacho j. and martínez d. (2014). perito judicial en paleontología: una nueva visión. comuni-cats inst. cat. miner. gemmol. paleontol., 38, 71-72. corbacho, j., morrison, s. and ait addi, a. (2014). dionide carlottae: una nueva especie de dionididae (trilobita) del ordovícico superior de marruecos. batalleria,21, 13-21. corbacho j. and martínez bautista d. (2015). sobre peritajes y tasaciones en paleontología. comunicats inst. cat. miner. gemmol. paleontol., 5, 40-41. corbacho j. and sendino c. (2015). fossil fakes and their recognition. deposits magazine, 30, 35-40. corbacho j. and vela j.a. (2013). parvilichas marochii: new genus and species of lichidae from the zagora region (morocco); early ordovician (floian). scripta mus. geol. seminar. barcinonensis [ser. palaeontol.], 14, 3-13. corbacho j., sendino c. and tahiri m. (2011). palaeontological fakes. batalleria, 16, 37-45. corbacho j., vela j.a. and cuevas j.a. (2007). falsificación de trilobites del norte de áfrica. batalleria, 13, 29-36. corbacho j., corbacho i., morrison s. and valent m. (2015). últimas técnicas detectadas en la falsificación de fósiles. batalleria, 22, 22-27. fortey, r.a. (2009). a new giant asaphid trilobite from the lower ordovician of morocco. memoirs of the association of australasian palaeontologists 37: 9-16. fortey, r.a. (2010). trilobites of the genus dikelokephalina from ordovician gondwana and avalonia. geological journal, 46, 405-415. fortey, r.a. (2011). the first known complete lichakephalid trilobite, lower ordovician of morocco. memoirs of the association of australasian palaeontologists, 42, 1-7. fortey, r.a. and edgecombe, g.d. (2017). an upper ordovician (katian) trilobite fauna from the lower ktaoua pág. 16 formation, morocco. bulletin of geosciences, 92,1-12. geyer g. and corbacho j. (2015). the burlingiidae (trilobita): revised generic composition, stra-tigraphy, and the first species from the early middle cambrian of west gondwana. gff, 137, 212-225. gutiérrez-marco, j. c., rábano, i., and garcía-bellido, d.c. (2015). reappraisal on the ordovician trilobites from the bou nemrou assemblage (tafilalt biota, morocco). abstract volume of the international conference the rise of animal life, rali 2015. promoting geological heritage: challenges and issues. marrakesh 05-10th october 2015, p. 48. gutiérrez-marco, j. c., sá, a.a., garcía-bellido, d.c. and chacaltana, c.a. (2017). recent geoethical issues in moroccan and peruvian paleontology. annals of geophysics, 60, fast track 7,1-11. gutiérrez-marco, j.c., rábano, i., garcía-bellido, d.c. and sá, a.a. (2017). selenopeltis longispina (trilobita, odontopleurida) from the ordovician of morocco: a reappraisal based on new skeletal and soft-bodied features. abstract volume of the 6th international conference on trilobites and their relatives, tallinn (estonia), 7-10 july 2017, pärnaste, h. (ed.), 20-21. lebrun, p. (2018). fossiles du maroc. tome i. gisements emblématiques du paléozoïque de l’anti-atlas. fossils from morocco. volume i. emblematic localities from the palaeozoic of the anti-atlas. les editions du piat – glavenat, 304 pp. lópez-soriano, f. j. and corbacho, j. (2012). a new species of symphysops from the upper ordovician (lower ashgill) of morocco. batalleria, 17, 13-20. martin e.l.o., vidal m., vizcaïno d., vaucher r., sansjofre p., lefebvre b. and destombes j. (2016). biostratigraphic and palaeoenviron-mental controls on the trilobite associations from the lower ordovician fezouata shale of the central antiatlas, morocco. palaeogeogr., palaeoclimatol., palaeoecol., 460, 142-154. rábano, i. (1990). trilobites del museo geominero. i. platypeltoides magrebiensis n. sp. (asaphina, nileidae), del ordovícico inferior del anti-atlas central (marruecos). boletín geológico y minero, 101, 21-27. rábano, i., gutierrez-marco, j. c. and garcía-bellido, d.c. (2014). a remarkable illaenid trilobite from the middle ordovician of morocco. bulletin of geosciences 89, 365-374. valent, m., corbacho, j. and martínez, d. (2013). hyolith localities of zagora region (morocco), upper fezouata formation (lower ordovician). batalleria, 19, 20-23. valent, m. and corbacho, j. (2015). new hyolith species pauxillites thaddei (floian, ordovician) from morocco. acta musei nationalis pragae, series b – historia naturalis. 71, 51–54. vela j. a. and corbacho j. (2007). a new especies of lehua from lower ordovician of dra valley of morocco. batalleria, 13, 75-80. vela j. a. and corbacho j. (2009). new trilobites from upper ordovician of el kaid errami, morocco. batalleria, 14, 99-106. layout 6 temperature and pressure gas geoindicators at the solfatara fumaroles (campi flegrei) giovanni chiodini1,*, rosario avino1, stefano caliro1, carmine minopoli1 1 istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5002 abstract long time series of fluid pressure and temperature within a hydrothermal system feeding the solfatara fumaroles are investigated here, on the basis of the chemical equilibria within the co2-h2o-h2-co gas system. the pisciarelli fumarole external to solfatara crater shows an annual cycle of co contents that indicates the occurrence of shallow secondary processes that mask the deep signals. in contrast, the bocca grande and bocca nova fumaroles located inside solfatara crater do not show evidence of secondary processes, and their compositional variations are linked to the temperature–pressure changes within the hydrothermal system. the agreement between geochemical signals and the ground movements of the area (bradyseismic phenomena) suggests a direct relationship between the pressurization process and the ground uplift. since 2007, the gas geoindicators have indicated pressurization of the system, which is most probably caused by the arrival of deep gases with high co2 contents in the shallow parts of the hydrothermal system. this pressurization process causes critical conditions in the hydrothermal system, as highlighted by the increase in the fumarole temperature, the opening of new vents, and the localized seismic activity. if the pressurization process continues with time, it is not possible to rule out the occurrence of phreatic explosions. 1. introduction solfatara fumaroles are located within the homonymous crater in the center of the campi flegrei caldera (figure 1), which is a very dangerous volcanic area because it includes part of the city of napoli (naples), the town of pozzuoli, and numerous densely inhabited villages. the area was affected in the past by numerous large explosive eruptions [orsi et al. 1996], the last of which occurred in 1538 at monte nuovo. this eruption followed a period of ground uplift, which suddenly interrupted a secular subsidence [dvorak and mastrolorenzo 1991, troise et al. 2007], which continued with the same rate after the eruption. a new uplift started in 1950 and reached a maximum of about 4 m in 1985 near the town of pozzuoli [del gaudio et al. 2010]. this process occurred during three major unrest periods (bradyseisms), the first in 1950-1952, the second in 1969-1972, and the third in 1982-1984 [barberi et al. 1984, del gaudio et al. 2010]. in addition, four "mini" uplift periods occurred in 1989, 1994, 2000 and 2006. all of the uplift periods in this area were accompanied by seismicity, and in particular, many thousands of earthquakes occurred during the two major crises in 1969-1972 and in 1982-1984. since the last main bradyseism of 1982-1984, the general trend of ground motion showed a subsidence period until 2004-2005, when the soil started to uplift with a relatively low rate of ~1.0–1.5 cm a−1. this study is aimed to determine whether variations in the temperature–pressure (t–p) conditions of the hydrothermal system feeding the solfatara fumaroles accompanied this oscillating ground motion of campi flegrei. here, we analyze long time series (from 1998 to 2010) of the chemical compositions of the fumaroles of the solfatara of pozzuoli, focusing on the reactive gas species (mainly hydrogen and carbon monoxide) that have been recognized by previous studies as geoindicators of the t–p conditions of hydrothermal systems [giggenbach 1987, chiodini and cioni 1989, chiodini and marini 1998]. 2. analytical methods the three main fumaroles of solfatara (bocca grande [bg], bocca nova [bn] and pisciarelli; figure 1) have been routinely analyzed for their chemical composition by the geochemistry laboratory of the istituto nazionale di geofisica e vulcanologia (ingv) – osservatorio vesuviano since 1998. this dataset, which consists of 348 samples, is reported in the electronic annex 1. for the determination of the major gas species, fumarolic gases were collected in under-vacuum flasks that contained a 4 n naoh solution [giggenbach 1975, giggenbach and gouguel 1989]. the condensates of water vapor and noncondensable gases were sampled by passing the fumarolic gases through a condenser cooled to 20 ̊ c to 30 ̊ c by water. the gas constituents were analyzed following the methods article history received december 21, 2010; accepted march 3, 2011. subject classification: solfatara, campi flegrei, caldera, gas equilibria. 151 of cioni and corazza [1981], modified for the analysis of he, ar, o2, n2, h2 and ch4. the chemical compositions of the non-absorbed gases that were mainly present in the headspace over the naoh solution were also measured, by gas chromatography using a thermal conductivity detector, through a single injection onto two molecular-sieve columns (ms 5å capillary, 30 m × 0.53 mm × 50 m; he and ar as carrier gases). the co2 and sulfur species absorbed in the alkaline solution were analyzed after oxidation via h2o2, by acid–base titration and ion chromatography, respectively (analytical error, ±3%). co was analyzed on dry gas samples by gas chromatographic separation with a ms 5 å 1/8´×50´ column (he as carrier gas), coupled with a high-sensitivity reduced gas detector (hgo). 3. origin of the solfatara fumaroles the solfatara crater is located near the area of maximum ground uplift and of the recurrent earthquakes, and it is affected by an intense process of diffuse soil degassing and fumarolic activity. the total output of hydrothermal co2 of ~1500 t d −1 from the diffuse degassing processes was measured in december 1998 and in july 2000 [chiodini et al. 2001, cardellini et al. 2003]. according to chiodini et al. [2001], the thermal energy release associated with the solfatara gas emission (~100 mw) is the most important term in the energy balance of the whole of the campi flegrei caldera, as this is: (i) orders of magnitude greater than the elastic energy released during the seismic crises; (ii) higher than the energy associated with the ground deformation; and (iii) ~10-fold greater than the conductive heat flux over the entire caldera. the fumaroles discharge at variable temperatures (from 95 ˚c to 165 ˚c), as mixtures of h2o and co2 with minor amounts of h2s, n2, h2 and ch4 (see annex 1). geochemical studies of this hydrothermal system began in the early 1980's. a conceptual geochemical model of the system was first proposed by cioni et al. [1984], and then refined by cioni et al. [1989], chiodini et al. [1992, 1996], chiodini and marini [1998], and chiodini et al. [2000, 2001]. furthermore, on the basis of a large dataset of chemical and isotopic compositions, caliro et al. [2007] investigated the origin of the fumarolic fluids, and they proposed a more comprehensive geochemical model. according to caliro et al. [2007], the system is fed by a mixture between fluids degassed from a magma body and the vapor generated at high temperatures (~360 ˚c) by the vaporization of hydrothermal liquids of meteoric origin (figure 2). the mixing process appears to occur at the base of the hydrothermal system, where a plume is formed that is mainly composed of a gas phase that migrates towards the surface. according to physical simulations [chiodini et al. 2003, todesco et al. 2003], pulses of the magmatic source that arise from magma degassing episodes cause sudden increases in the mass of the fluids stored in the hydrothermal system; this is coupled with fluid pressurization, which triggers the volcanic unrest periods that periodically affect this area. in the following sections, we will consider in detail whether there are any geochemical sign of fluid pressure variations within the hydrothermal system, on the basis of the chemical equilibria within this co2-h2o-ch4-h2-co gas system. 4. the co2-h2o-ch4-h2-co gas equilibria the hydrothermal gas equilibria in this co2-h2o-ch4h2-co system were reviewed by chiodini and marini [1998]. they proposed a method that consists of the comparison of the analytical data with the compositions of both the equilibrium vapor and the equilibrium liquid phases, and of the vapor phases separated in a single-step from the equilibrium liquids at varying temperatures. in particular, the formation reactions of co, h2 and ch4 (equations 1, 2 and 3, respectively) were combined to obtain two reactions (equations 4 and 5), the equilibrium constants of which are independent of the oxygen fugacity. (1) (2) (3) (4) (5) for gas phases that are largely made up of water vapor, the ratios of the fugacity coefficients ch2/ch2o, cco/cco2 and cch4/cco2, do not deviate significantly from 1.0 in the typical p–t range of hydrothermal systems, as 100 ̊ c to 374 ̊ c and 1 bar to 220 bar [chiodini and marini chiodini et al. 152 figure 1. location map and main geological features of the solfatara crater, showing the bocca grande (bg), bocca nuova (bn) and pisciarelli fumaroles. oco co2 1 2 2= + h o h o2 2 1 2 2= + co 2 h o ch 2 o2 2 24+ = + co h h o co2 2 2+ = + 3 co ch 2 h o 4 co2 24+ = + 153 1998, and references therein]. therefore, the temperature dependence of the equilibrium constants of equations (4) and (5) can be expressed as functions of the ratios of the mole fractions in the vapor phase, the xi values, as: (6) and (7) where tk is the absolute temperature. in equation (7), the water fugacity has been assumed to be fixed, by the presence of the liquid (log fh2o =5.510 − 2048/tk) [giggenbach 1980]. the relations of equations (6) and (7) are then used to compute the theoretical l1 and l2 values for the equilibrium vapor phases (figure 3, "vapor" line), and considering the corresponding vapor–liquid distribution coefficients for the equilibrium liquid phases (figure 3, "liquid" line) and for the vapor phases that are separated in a single step from the equilibrium liquids at the varying temperatures (figure 3, "ssvs" lines; see chiodini and marini [1998] for details of the calculations). figure 3 compares these theoretical compositions with the l1 and l2 values measured at the solfatara fumaroles. the analytical values fall outside the field of the vapors separated by the liquid, i.e. outside the field delimited by the "vapor" and "liquid" lines. this behavior was originally interpreted as being caused by the presence of superheated vapor, at temperatures of 200 ̊ c to 240 ˚c, the theoretical compositions of which fall to the left of the "vapor" line in figure 3 [chiodini and marini 1998]. however, more recently caliro et al. [2007] studied a larger dataset, and based on their new 13c data of the co2-ch4 couple, they suggested a different interpretation. by using fractionation factors from horita [2001], the co2-ch4 isotope data indicated isotopic equilibration temperatures of 360 ˚c to 436 ˚c, which were close to the temperatures estimated by the chemical geothermometer based on the ch4/co2 ratio when redox buffers typical of hydrothermal systems were considered (figure 4; see caliro et al. [2007] and chiodini [2009] for further details). these ch4-co2 equilibrium temperatures are much higher than those previously inferred in figure 3 by chiodini and marini [1998], and they can be explained by the different kinetics of ch4 gas geoindicators at solfatara log (x /x ) log (x /x ) l 2.485 2248/tk h2o h2 co co2 1 + = = = 3 log (x /x ) log (x /x ) l 19.605 17813/tk, co co2 co ch4 2 + = = = figure 2. revisited geochemical conceptual model of solfatara, showing the zones of deep mixing, and h2, co and h2s re-equilibration in the vapor phase (after caliro et al. 2007). the gas/liquid mass fractions (xg), and low-temperature isotherms are shown, as arising from the physical–numerical simulations described in detail by chiodini et al. [2003] and todesco et al. [2003]. the gas fractions are indicated by the gray shading scale. with respect to h2 and co. on the one hand, ch4, which is a species characterized by much slower kinetics than h2 and co [giggenbach 1987, chiodini et al. 1993, giggenbach 1997], does not re-equilibrate, which preserves the same concentrations as in the deepest and hottest parts of the hydrothermal system where the methane originates from co2 reduction [caliro et al. 2007, chiodini 2009]. on the other hand, h2 and co re-equilibrate under the decreasing temperature conditions encountered by the ascending vapor phase (figure 2). apart from these theoretical considerations, the different behaviors of h2 and co on the one side and of ch4 on the other are also suggested by examination of figure 5, where log co/co2 is plotted against log h2/h2o (figure 5a) and log ch4/co2 (figure 5b). log h2/h2o positively correlates with log co/co2 (r 2 = 0.66), as expected for a vapor phase. assuming equilibrium conditions in the vapor phase, this correlation was used to derive the oxygen fugacity (fo2)−a temperature function that better describes the redox conditions at the solfatara hydrothermal system: (8) this function was obtained by solving the following set of equations with respect to fo2: (9) (10) (11) where equation (9) is the best-fit regression of the two variables (figure 5a), while equations (10) and (11) are the expressions of the temperature dependence of the equilibrium constants of the h2o and co2 dissociation (reactions in equations 1 and 2). it is worth noting that the derived function is very similar to the fo2-tk function (log fo2 = 8.20 + 23643 / tk) that was proposed by d'amore and panichi [1980] and was derived by investigating the fluids discharged by several hydrothermal systems around the world. excluding an improbable fortuitous coincidence, this similarity suggests that the fumaroles of solfatara are fed by a system for which the h2 and co contents are controlled by typical chiodini et al. 154 figure 3. theoretical values of log(xh2o/xh2) + log(xco/xco2) versus 3log(xco/xco2) + log(xco/xch4) for a single saturated vapor phase and a single saturated liquid phase, shown as the vapor line and the liquid line, respectively. the equilibrium gas contents in a single saturated liquid phase were computed using the vapor–liquid distribution coefficient, bi, defined as bi = (xi/xh2o)vap/(xi/xh2o)liq. the composition of the vapors separated in a single step at varying temperatures (t0) (single-step vapor separation lines, or ssvs lines) and the compositions of superheated vapors equilibrated at different t–ph2o values (solid lines) are also shown. the equilibrium conditions for the high-temperature fumaroles of the solfatara crater (bg and bn) can be inferred for a superheated vapor phase at temperatures between 200 ˚c and 240 ˚c, and between 200 ˚c and about 150 ˚c for the pisciarelli fumarole. the possible occurrence of h2 and co re-equilibration upon cooling of the gas phase (see text) has also been investigated through drawing of the single gas-phase theoretical reequilibration line from 370 ˚c (point a) to 120 ˚c (point b) under the redox conditions controlled by the buffer of d'amore and panichi [1980] (see the text for further details). the solfatara compositions plot along the re-equilibration pathway, which supports the occurrence of such processes. log f 8.20 23725/tko2 = + log (x /x ) log (x /x ) 0.2731 1.8066h2 h2o co co2= # log (x /x ) 2.548 12707/tk log fh2 h2o 1 2 o2= log (x /x ) 5.033 14955/tk log fco co2 1 2 o2= 155 hydrothermal redox conditions. in particular, the solfatara fumaroles fall close to the equilibrium vapor line at temperatures from 130 ̊ c to 240 ̊ c. in contrast, the log ratio ch4/co2 falls far from the equilibrium values expected for the same redox and temperature conditions (figure 5b). this different behavior of h2 and co with respect to that of ch4 in figure 3 explains the trends of the l1 and l2 values of the solfatara fumaroles, as highlighted by the good correlation of the measured data with the theoretical values (figure 3, line a-b) expected for the re-equilibration of h2 and co in a vapor phase that moves from a high-temperature deep zone (figure 3, point a) to colder and shallower levels (figure 3, point b). practically, we considered that the ratios h2/h2o and co/co2 quickly readjust at the equilibrium values for redox conditions fixed by the d'amore and panichi [1980] empirical buffer, while the ch4/co2 ratio maintains its original value of ~ −4 (figure 4). on the basis of these observations, we can conclude that h2 and co are good partners for the derivation of the t–p geoindicators of the shallower parts of the hydrothermal system, while ch4 cannot be used because it reaches equilibrium conditions, if at all, under different conditions to those of h2 and co. in agreement with these considerations, we considered the co2-h2o-h2-co gas system. the equilibrium temperatures were computed by inserting the measured xh2o, xco2, xco and xh2 molar fractions into equation (6), and by solving the equation with respect to temperature. in the computation of the pressure, the fugacity coefficients were assumed to be close to unity, a reasonable assumption at relatively low pressures (<100 bar) [giggenbach 1980]. the pressure of the water (ph2o) was derived using the assumption that ph2o is fixed by the abovementioned fh2o –t relation of giggenbach [1980] that is valid for the coexistence of vapor and liquid phases (the liquid is considered as pure water). this assumption is not based on gas geoindicators at solfatara figure 4 (top right). log xch4/xco2 versus 1000/tk diagram. the analytical ratios for the solfatara fumaroles are plotted against the equilibrium temperatures computed through the ch4–co2 isotopic geothermometer. the theoretical ratios in a single saturated vapor phase under redox conditions controlled by the hydrothermal redox buffers [giggenbach 1987, d'amore and panichi 1980] are shown for reference. the theoretical ratios expected for varying water fugacities and redox conditions fixed by the magmatic so2-h2s buffer [giggenbach 1987] are also shown. the solfatara fumaroles fall close to the equilibrium conditions, which supports the concept that ch4 is formed by co2 reduction in the hydrothermal environment. figure 5 (bottom right). (a) plot of log(xh2/xh2o) versus log(xco/xco2). (b) plot of log(xch4/xco2) versus log(xco/xco2). in the diagrams, the analytical values are compared with the theoretical log ratios computed for the equilibrium vapor and liquid phases, and for the vapor phases separated in a single-step from the equilibrium liquids, at different temperatures. the theoretical grids assume that redox conditions are controlled by the fo2-buffer of d'amore and panichi [1980]. the geochemical interpretation of the data, because the co2h2o-h2-co composition indicates the presence of a vapor phase and not necessarily vapor–liquid coexistence. the vapor line in figure 5 and the re-equilibration trend in figure 3 are actually valid both for vapors coexisting with the liquid and for vapors with ph2o values lower than those fixed by the presence of the liquid. the assumption is based on the results obtained by the physical numerical modeling of the hydrothermal system of solfatara [todesco et al. 2003] (figure 2). the model was able to reproduce some of the main features that characterize the natural system, including the energy budget associated with the ascent and condensation of the hot fluids, and the development of a single-phase gas zone at a depth of a few hundreds of meters and at a temperature of 200 ̊ c to 230 ̊ c [todesco et al. 2003] (figure 2). the physical model returned ph2o values close to the vapor–liquid coexistence within all of the rising fluid plume, including the single-phase gas zone. our assumption of ph2o fixed at any temperature by vapor–liquid coexistence is based on these results. the pressure of co2 (pco2) was estimated on the basis of the following relation: (12) which was derived for the co2-h2o-h2-co gas system by chiodini and cioni [1989]. the temperature and pressure estimations (ph2o, pco2, ptot = ph2o + pco2) are reported in annex 1. the equilibrium temperatures range from 195 ̊ c to 230 ˚c (mean, 209 ̊ c), from 202 ̊ c to 237 ̊ c (mean, 213 ̊ c) and from 122 ˚c to 232 ˚c (mean, 157 ˚c), for the bg, bn and pisciarelli fumaroles, respectively. the total equilibrium pressures (ptot = ph2o + pco2) range from 17 bar to 33 bar, from 19 bar to 37 bar, and from 3 bar to 38 bar, for the bg, bn and pisciarelli fumaroles, respectively. the large variations of the apparent t–p equilibrium values estimated at pisciarelli, which are possibly caused by secondary processes, will be discussed in more detail in the next section. it is worth noting that the average estimation performed for the two hottest fumaroles of the area, as bg and bn (t, ~210 ˚c; p, ~25 bar), practically coincide with the t–p conditions of the "single gas-phase zone" arising from the physical simulations of the system [todesco et al. 2003] (figure 2). we can thus consider that the variations in the time of the equilibrium t–p, which are discussed in the next section, are in some way representative of this large gas zone, which according to the simulation results, is located at a shallow depth (100-300 m) in the subsoil of the area. 5. time series of the equilibrium temperatures and pressures from 1998 to 2010 the time series of the equilibrium p–t values recorded at the pisciarelli fumarole are discussed separately from those computed at the bg and bn fumaroles. 5.1. the pisciarelli fumarole in recent years, the fumarolic field of pisciarelli has experienced an evident increase in activity, which has been marked by temperature increases, by the opening of new vigorous vents and boiling pools, and by the occurrence of seismic activity localized in the area [d'auria et al. 2011] (figure 6). chiodini et al. 156 figure 6. (a) chronogram of the discharge temperature (˚c) of the pisciarelli fumarole with the chronological occurrence of the main events (as indicated) linked to the increasing activity. the temperature of 95 ̊ c is the boiling temperature for the fumarolic fluids. (b) photograph of the new roaring fumarole established on december 20, 2009. log p 3.573 log (x /x ) 46/tk,co2 h2 co= 157 figure 6 shows the fumarolic discharge temperatures together with the chronological occurrences of the main macroscopic events that have occurred at pisciarelli since 1999. the discharge temperature from 1999 to 2005 has fluctuated around 95 ˚c, the boiling temperature of the fumarolic fluids, with the exception of a sample in may 2002 that for the first time showed a temperature of 105 ˚c. after this period, and following an anomalous mud emission from the vent (april 2006), the temperature increased up to 102 ̊ c in june 2006, and dropped down after the occurrence (on 23 october 2006) and establishment of a new boiling pool. an enlargement of the fumarolized area and the opening of a new vent occurred in march 2008. a new increase in the temperature occurred in june 2008 (103 ˚c), and excluding a few short periods, since then the discharge temperature has shown an increasing trend, which reached 108 ̊ c in october 2010. over this period, events have occurred closer in time: the opening of a new boiling pool in march 2009, which was probably accompanied by a small explosion (mud covered the soil slope up to 3–4 m above the emission point), with the co2 flux from this pool estimated at 15 ton/d to 20 ton/d by the injection into the pool of a tracer gas at different (known) flow rates, and the measurement of its relative concentration with respect to the co2 in the plume; the establishment on 20 december 2009 of a new vigorous, roaring fumarole (figure 6), which represents the strongest gas emission of the entire area to date; the recording of a seismic swarm (of about 190 events) in the pisciarelli area on 30 march 2010; and the opening of a new vent on 15 november 2010. in spite of this macroscopic evidence of increasing activity, the t–p geoindicators have not given clear signals, because they have been masked by seasonal effects. in particular, an annual variation affects the fumarolic co content, and consequently also the temperature and pressure estimations (figure 7). as an example, the annual cycle is evident in figure 8, where the equilibrium temperature is plotted against the month of sampling. the lowest temperatures, which are generally from 120 ˚c to 150 ˚c, characterize the winter months, while the highest temperatures, from 150 ˚c to 200 ˚c, are typical of the summer months. similar behavior characterizes the equilibrium pressure values (figures 7b and 6c). these seasonal variations might be caused by different process. a real temperature decrease might occur during the winter because of the arrival of cold meteoric water in the upper part of the feeding system. however, this is improbable, because it would imply concurrent increases in the water contents of the fumaroles and variations in the isotopic signatures of the steam; such variations are not actually observed (unpublished data, ingv – osservatorio vesuviano). another explanation is that the fumarolic co content is in some way affected by the condensation of the gas geoindicators at solfatara figure 7. chronograms of co content (a), ptot (b), pco2 (c), and equilibrium temperature (d) for the pisciarelli fumarole. seasonal effects affect the fumarolic co content and consequently the t–p estimations. it is worth noting the peak co contents and t–p estimates relative to summer 2010, when the values estimated approached those of the bg and bn fumaroles. figure 8. equilibrium temperature of the pisciarelli fumarole versus the month of sampling. maximum estimated values characterize the spring and summer samples, indicating strong seasonal effects. chiodini et al. 158 figure 9. chronogram of vertical ground displacement, as recorded at benchmark 25 (in pozzuoli) and temperature (a) and pressure(b) for the bg and bn fumaroles. the similar shapes of the geochemical signals and the ground elevation suggest a possible relationship between them. figure 10. chronogram of the pco2 estimated at the bg and bn fumaroles. since 2007, an increase in the values has been observed, and this pressurization process is still ongoing. 159 fumarolic fluids, a process that is linked to the ambient temperature and that is consequently characterized by a seasonal cycle. whatever the cause is, the data from pisciarelli that are heavily modified by this seasonal cycle are not useful for investigations into the eventual variations caused by volcanic processes. the only sign that appears to be linked to the increased fumarolic activity is the anomalous peak of co content in the spring to summer in 2010. over this period, the estimated t–p conditions increased to the highest values recorded over the last 10 years (t = 230 ˚c, ptot = 38 bar, and pco2 = 9 bar; figure 7b-d), which are close to the values estimated for the bg and bn fumaroles. 5.2. the bg and bn fumaroles the data from the bg and bn fumaroles are thus more interesting, as they do not show seasonal effects. in this case, the discharge temperatures are above the condensation temperature, and in contrast to the pisciarelli site, the environment is dry, which potentially prevents important secondary processes. the chronograms of figure 9 compare the ground elevation of the area with the equilibrium temperatures (figure 9a) and pressures (figure 9b) recorded from 1998 to 2010. figure 9 highlights the good fit between the geochemical signals and the ground elevation: the initial period of subsidence from 1998 to 2000 was accompanied by a t–p decrease; the crisis of 2000 caused a positive peak in both the elevation and t–p conditions; the subsequent period from 2001 to 2004-5 was characterized by subsidence and a contemporary decrease in the t–p values; and finally, since 2004-2005, an uplift phase has started that has been accompanied by a t–p increase. finally, figure 10 shows the chronogram of the equilibrium pco2 estimated at the bg and bn fumaroles. these values were recorded in both of the fumaroles and they were relatively constant (4–5 bar) until 2006. after this, the values started to increase, and they reached the maximum of ~7 bar for the last samples of 2010. this increase in pco2, which is a process that at the moment is still ongoing, is most probably being caused by the arrival of deep gases that have co2 contents that are higher that those of 1998 to 2006. 6. conclusions the general considerations regarding the kinetics of the redox-sensitive gas species and the mass of analytical data indicate that h2 and co are good partners for the derivation of reliable gas-geoindicators of the t–p conditions of the upper parts of solfatara hydrothermal systems. this method was applied at the two main fumarolic fields of solfatara: the pisciarelli and the bg-bn sites. at pisciarelli, strong seasonal effects and the possibility of re-equilibration of the fumarolic fluids at very shallow depths cover the deep geothermo-barometric signals. however, it is in this area that the signs of the increased activity are more evident. over the last few years, the main evidence has been the increase in the discharge temperature, the opening of new vigorous vents and boiling pools, and the occurrence of localized seismic activity [d'auria et al. 2011] (figure 6). the only geothermo-barometric sign that appears to be linked to this increased fumarolic activity was an anomalous peak of co content that was observed in the samples of summer 2010, when the equilibrium t–p reached its maximum values (t = 230 ˚c; ptot = 38 bar; and pco2 = 9 bar, which approached those of the bg and bn fumaroles. gas geoindicators applied to the bg and bn hightemperature fumaroles generally indicate t–p conditions (t = 195–237 ˚c; ptot = 17–37 bar) that are close to those independently simulated by the application of a physical numerical approach [todesco et al. 2003] for the shallow single-phase gas zone that constitutes the upper part of the solfatara hydrothermal system (figure 2). the variation in the equilibrium t–p values with time has resembled the ground movement at campi flegrei over the entire period of observation (1998-2010). this evident correlation between ground movement and geochemical signals supports the concept that the last uplift episodes occurred with active processes of pressurization of the shallower parts of the hydrothermal system. in particular, this process of uplift and fluid-pressure increases at shallow depths has been evident since 2006, and it is still ongoing. this probably caused the observed macroscopic increases in the hydrothermal activities at pisciarelli, and if it continues with time, this might lead to more critical conditions, including the possible occurrence of phreatic explosions. acknowledgements. the authors are grateful to salvo inguaggiato and to an anonymous reviewer, for their useful comments and suggestions for the early version of this manuscript. references barberi, f., d.p. hill, f. innocenti, g. luongo and m. treuil (1984). on the 1982–1984 bradyseismic crisis at phlegraean fields (italy), bull. volcanol., 47, 171-411. caliro, s., g. chiodini, r. moretti, r. avino, d. granieri, m. russo and j. fiebig (2007). the origin of the fumaroles of la solfatara (campi flegrei, south italy), geochim. cosmochim. acta, 71, 3040-3055; 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(2001). carbon isotope exchange in the system co2ch4 at elevated temperatures, geochim. cosmochim. acta, 65 (12), 1907-1919. orsi, g., s. devita and m. divito (1996). the restless, resurgent campi flegrei nested caldera (italy): constraints on its evolution and configuration, j. volcanol. geotherm. res., 74, 179-214. todesco, m., g. chiodini and g. macedonio (2003). monitoring and modelling hydrothermal fluid emission at la solfatara (phlegrean fields, italy); an interdisciplinary approach to the study of diffuse degassing, j. volcanol. geotherm. res., 125, 57-79. troise, c., g. de natale, f. pingue, f. obrizzo, p. de martino, u. tammaro and e. boschi (2007). renewed ground uplift at campi flegrei caldera (italy): new insight on magmatic processes and forecast., geophys. res. lett., 34, l03301; doi: 10.1029/2006gl028545. *corresponding author: giovanni chiodini, istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy; e-mail: giovanni.chiodini@ov.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. chiodini et al. 160 layout 6 annals of geophysics, 53, 2, april 2010; doi: 10.4401/ag-4717 abstract in the presence of perturbations of the thermospheric auroral region produced by traveling ionospheric disturbances during the propagation of atmospheric gravity waves, an analytical expression of the velocity of the thermospheric plasma is derived through magnetohydrodynamic formalism. the expressions of the joule heating and the viscous heating are derived, and their rates of variation are presented. a threshold height for their transition has been determined from their ratio, which is in agreement with the experimental data. the analysis indicates that the time taken by the thermospheric plasma to reach a steady-state corresponds to the nature of the traveling ionospheric disturbances in the medium. introduction ionospheric convection modulated by solar wind and magnetohydrodynamic wave coupling to the dayside magnetosphere produce atmospheric gravity waves (agws) [hocke and schlegel 1996, kirchengast 1996]. in the auroral zone, agws are associated with the auroral electrojet [millward et al. 1993, prölss and roemar 2000, yuan et al. 2005]. these ionospherically forced agws deposit their momentum and energy in the upper and lower atmosphere, thereby providing a link between the solar wind and the earth atmosphere. coupling of the magnetosphere to the ionosphere is much felt through the precipitation of energetic particles that promote large amounts of local ionization. joule dissipation of these currents is a major ionospheric energy source at auroral latitudes, which appears to be several times greater than that directly associated with particle precipitation. traveling ionospheric disturbances (tids) are frequently observed at high and middle latitudes, and they can be taken as manifestations of middle-scale ionospheric irregularities arising as a response to agws [oliver et al. 1994, bristow et al. 1996, hernandez-pajares et al. 2006, nicolls and heinselman 2007, onishi et al. 2009]. during geomagnetic storms, the atmosphere at high latitudes is greatly affected by the energy deposited in the auroral region. auroral processes, e.g., auroral electric currents and charge-particle precipitation, produce joule heating and gravity waves that introduce tids [hocke and schlegel 1996, afraimovich et al. 2000]. the characteristics of tids change with latitude, longitude, local time, season, and solar cycle [hernandez-pajares et al. 2006, kotake et al. 2006]. currently, different investigations are being carried out to explore the geographical and temporal variabilities of the observed gravity waves, and their relationships with the observed tids, so that the upward energy flow from the troposphere to the mesosphere, through the stratosphere, can be understood. this way of energy coupling from the lower to the upper atmosphere, along with the resulting ionospheric effects, contributes to the upper atmospheric energy balance [kersey and hughes 1989, allen and vincent 1995; fritts and alexander 2003]. auroral heating of the neutral atmosphere is a part of the more general question of the total energy budget. there are large-scale modeling studies of the responses of the neutral thermosphere to ion convection. the local responses of the neutral thermospheric temperature and mechanism for joule and particle heating have been modeled. the propagation of gravity waves is mainly in the e-region of the ionosphere by joule heating and lorentz forcing. the propagation of agws in the neutral atmosphere and their ionospheric signatures (i.e., tids) have been studied both experimentally and theoretically for many years. on the basis of these theoretical and experimental investigations, several models have been developed to describe the main characteristics of the agw–tid relationships on a global scale. the models reflect our level of understanding of the basic physical principles governing article history received september 26, 2009; accepted march 18, 2010. subject classification: ionospheric dynamics, plasma physics, auroral phenomena 33 heating of the auroral ionosphere by traveling ionospheric disturbances initiated by atmospheric gravity waves syam sundar de1,*, bijoy bandyopadhyay1, suman paul1, dilip kumar haldar1, mridul bose2 1centre of advanced study in radio physics and electronics, university of calcutta, kolkata, india 2department of physics, jadavpur university, kolkata, india the atmospheric and ionospheric processes. a comprehensive simulation of the relationships between all of the fundamental parameters of the neutral atmosphere perturbed by agws and all of the fundamental ionospheric parameters was performed by kirchengast [1996], for comparisons with incoherent scatter data. under the stated physical situation, the auroral region of the ionosphere can be characterized by different nonlinear processes that arise from variations in the velocity distribution of the thermospheric constituents, medium temperature, ionizing frequency, effective collision frequency, and recombination coefficients of electrons and ions [schlegel 1982, prölss 2006]. investigations into the perturbations in temperatures and wind in the auroral region of the upper atmosphere have indicated the presence of high latitude heat sources [istomin and leyser 1997]. some studies have suggested that energy transfer by precipitating particles provides the dominant contributions [allan and cook 1974, olson and moe 1974], while joule dissipation and ion drag can take on more important roles, particularly surrounding the auroral oval and within the polar cap [cole 1975]. the non-linear energy transfer from electromagnetic radiation from the auroral magnetospheric zone to the down-flowing electrons is directly associated with the transient auroral disturbances [hanasz et al. 2006, zesta et al. 2006]. auroral electric currents and charged-particle precipitation produce joule heating, gravity waves and traveling ionospheric disturbances that can initiate temperature increases and temperature fluctuations [fagundes et al. 1996, belikovich et al. 1997]. the presence of a fluctuating electric field initiates joule heating, along with viscous heating [brekke and kamide 1996]. magnetosphereionosphere coupling mechanisms also provide information about joule heating rates [yuan et al. 2005], along with various other electrodynamic parameters. this heating provides the largest contribution to the total energy budget in the medium. gravity waves that propagate to thermospheric heights and interact with the auroral ionospheric plasma give rise to tids. knowledge of the intensity of the wave disturbances of the neutral atmosphere and of the ionospheric parameters are important for radio forecasting, and aeroplane and space-craft navigation. to date, tids have had a beneficial impact on field users of the global navigation satellite system [coster and tsugawa 2008]. an analytical expression of the upper thermospheric plasma-flow velocity in the auroral region is derived here through magnetohydrodynamic formalism. the influence of ionization and recombination processes, of density and temperature fluctuations of the thermospheric medium, and of gravity and viscosity are taken into account in this analysis. the spatial distribution of the velocity and its altitude dependence within the auroral zone are explored. the expressions of joule heating and viscous heating are derived. for different characteristic times, the variations in the velocity with the altitude are investigated. the data are presented here and discussed. the height-dependent ratio of the viscous heating to the joule heating rate terms in generating agws in the auroral electrojets at 100 km to 150 km altitudes is derived and the results are compared with those of a previous study [yuan et al. 2005]. mathematical formulations the physical situation can be represented by the following momentum balance, continuity and state equations: (1) (2) (3) where t is the mass density, o the effective electron-neutral molecule collision frequency, n the coefficient of viscosity of the medium, q the rate of ionization, a the electron-ion recombination coefficient, the geomagnetic field vector, and the source term. is the current density, which is heating by tids within auroral region 34 figure 1. variations in the average velocity of the thermospheric plasma with altitude at different characteristic times (x). the different curves show that the thermospheric plasma approaches greater velocities with increasing x, to reach a steady-state at different altitudes. the dashed lines are for functional completeness (these values are not physically valid). 3 1 t g p j b 2 + + = = + +# $ $ 2 2 d d d d d t o t o o too t n o n o ^ ^ h h t q 2+ =$ 2 2 d a t to t-^ h p k tb= t b b j# j 35 chosen according to: (4) where is the auroral electric-field vector, v0, v1 and v2 are the longitudinal, pedersen and hall conductivities, respectively, and the other symbols have their standard representations. for the present purposes, the current density has been chosen in a form of a gaussian distribution, which is specifically in a plane perpendicular to the direction of flow. for simplicity of the computational work, the time development is taken as a function with a rapid build-up, a steady phase, and a subsequent decay. fluctuations in the mass densities and mean temperatures are taken as t = t0(1 + h) and t = t0(1 + i), where h is the fluctuation in mass densities and i is the fluctuation in mean temperatures. when combined with the perturbations, equation (1) can be expressed as: (5) where, the plasma is considered to be bounded within the auroral region −l to +l along the x-direction, where the momentum can be transferred to the surrounding region (|x|> l) through the action of a viscous force. this is used to treat the analysis within a finite domain. thus, the initial and boundary conditions are specified as o (x, z, 0) = 0, o (−l, z, 0) = o (+l, z, 0) ≠ 0, and = 0 and v1 = 0 for |x|> l, with x, z and t as independent variables. the boundary condition is valid for all values of t, although for the mathematical simplification, this is chosen as t = 0. the study covers the altitude range from 90 km to 200 km. the solution of equation (5) can be expressed in the following propagatory form: (6) where, and are the roots of the auxiliary equation, and c1 and c2 are constants. the time constant required for the thermospheric plasma flow to reach a steady-state is def ined by the characteristic time, which is given by . this parameter has been introduced to simplify the problem. also, for the dimensionless time, this characteristic time is adopted to normalize the time. it is considered as , where d (a constant) is the dimensionless time. thus, in terms of characteristic time, the solution for the average thermospheric plasma velocity yields: (7) the solution of equation (7) gives the joule heating (qj) and the viscous heating (qv) through the following expressions: the ratio of the viscous heating rate to the joule heating rate terms can be readily obtained explicitly from these two expressions above, as: heating by tids within auroral region figure 2. changes in the heating rates with dimensionless time at an altitude (z) of 180 km. the total heating is the sum of the joule heating and the viscous heating, which approaches a steady value. j e k k e b e k0 1 1 2= + + +# #$v v v o vt t t^ ^ ^ ^h h h h e a t b cx z2 2 2 2 + = 2 2 2 2 2 2o o o o1 a 0= + h n t ^ h 1 b b b0 1 x y = + +h n ot v^ h 1 c k t e b e b gx 0 0 1 0 0b y z z y= + + 2 2 h n t i n v v n t^ h e , , exp exp x z t c kx pz t c kx pz t b c 1 1 2 2 = + + + + o a ~ a ~ ^ ^ ^ h h h " " , , 2 k p a a b k p4 1 2 2 2 2 2 2 = + + + + a ~ ~^ ^ h h 2 k p a a b k p4 2 2 2 2 2 2 2 = + + + a ~ ~^ ^ h h b1 2=x t v t = dx , , exp exp x z c kx pz c kx pz b c 1 1 2 2 = + + + + d d o x a ~ x a ~ x ^ ^ ^ h h h " " , , exp exp q e e e b e c kx pz c kx pz b c b e1 2 1 2 2 1 1 1 2 2 j z x x zx y z0= + + + + + + + d d v v v v a ~ x a ~ x ^ ^ ^ h h h 8 b " " , , exp exp q k p c kx pz c kx pz 2 2 1 1 2 1 2 2 2 2 v = + + + + + d d no a a ~ x a a ~ x ^ ^ ^ h h h 6 @ " " , , (8) the data of the committee on space research international reference atmosphere (cira) and the international reference ionosphere (iri) ionospheric models [bilitza et al. 1993] are used in the computation. for an 80˚ n latitude at 180 km in altitude, the values of the electron densities, neutral particles, and ion and electron temperatures were taken from these models. fortran subroutine programs were used to calculate the average velocity of the thermospheric plasma, the joule heating and the viscous heating, and their ratio. the dynamics explorer (de-2) satellite observations identified the enhancement as a storm phenomenon [prölss 2006], which supported the results of the present study. summary the variations in the average velocity of the thermospheric plasma flow with altitude for different characteristic times are shown in figure 1. the different curves in figure 1 show that the thermospheric plasma approaches higher velocities with increasing values of the characteristic time to reach a steady-state at different altitudes. the inclusion of the dashed lines in figure 1 is for functional completeness: these values are not physically valid, as the hall conductivity is negligible compared to the pedersen conductivity in this region, although the hall conductivity itself is not negligible. figure 2 shows the heating rate plotted against the dimensionless time at an altitude of 180 km, as the occurrence of tids within the f-layer of the ionosphere almost maintains its peak levels around this region [kirchengast 1996]. the average values of the joule heating and the viscous heating in the disturbed volume are presented separately. the joule heating is effective in the initial stages, while it decreases with time as the volume increases. when the motion is set up due to a dominating electric field, only then is there viscous heating. this increases due to the movement of the thermospheric plasma medium, and it eventually reaches a steady value. figure 3 shows the variations in the values for the ratio of the viscous heating to the joule heating rate terms, r, derived from equation (8), with the altitude ranging from 100 km to 150 km. the present study is shown (figure 3, continuous line) in comparison with a previous study (figure 3, dotted line; yuan et al. 2005), which was based on the european incoherent scatter (eiscat) data during the period from 1989 to 1991. according to this comparison, the method adopted in the present study is shown to be reasonable. between 100 km and 125 km in altitude, the ratio ranges from 0.5 to 10, while between 125 km and 150 km, it is approximately constant, with a value of 0.3. the viscous heating is relatively important below 116 km, while the joule heating dominates above this altitude. as brekke and kamide [1996] indicated, the relative importance of the two sources in generating the agws is critically dependent on the altitude. the present study supports this prediction quite satisfactorily. acknowledgements. the authors acknowledge with thanks the indian space research organization (isro), through s. k. mitra centre for research in the space environment, university of calcutta, kolkata, india, for the financial support to carry out this study. the authors also thank the two reviewers for making important comments. references afraimovich, e. l., e. a. kosogorov, l. a. leonovich, k. s. palamarchouk, n. p. perevalova and o. m. pirog (2000). determining parameters of large-scale traveling heating by tids within auroral region 36 exp exp 1 1 exp exp exp exp 1 1 r q q r e e e b e b e c kx pz c kx pz b b k t x e b e b g k p c kx pz c kx pz c kx pz c kx pz b b k t x e b e b g 1 2 1 2 0 2 1 1 1 2 2 0 1 0 0 1 0 0 2 2 1 1 2 1 2 2 2 2 1 1 2 2 0 1 0 0 1 0 0 j v x y z z x x z x y b y z z y x y b y z z y = = + + + + + + + + + + + + + + + + + + + + + + + + # # # # 2 2 2 2 h h h h d d d d d d v v v v a ~ x a ~ x ot v t i v v t no a a ~ x a a ~ x a ~ x a ~ x ot v t i v v t ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h h h h h > 8 6 > 8 bh @ bh " " " " " " , , , , , , figure 3. variation of the ratio of the viscous heating rate to the joule heating rate with altitude. the continuous curve connecting the filled circles represents the data from the present study, while the dashed curve joining the filled squares represents the data from the earlier study of yuan et al. 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(1996). elucidation of the physics of the gravity wave-tid relationship with the aid of theoretical simulations, j. geophys. res., 101, 13353-13368. kotake, n., y. otsuka, t. tsugawa, t. ogawa and a. saito (2006). climatological study of gps total electron content variations caused by medium-scale traveling ionospheric disturbances, j. geophys. res., 111, a04306, doi:10.1029/2005ja011418. millward, g. h., s. quegar, r. j. moffett, t. j. fuller-rowel and d. rees (1993). a modeling study of the coupled ionospheric and thermospheric response to an enhanced high-latitude electric field event, planet space sci., 41, 45-56. nicolls, m. j. and c. j. heinselman (2007). three-dimensional measurements of traveling ionospheric disturbances with the poker flat incoherent scatter radar, geophys. res. lett., 34, l21104, doi: 10.1029/2007gl031506. oliver, w. l., s. fukao, y. yamamoto, t. takami, m. d. yamanaka, m. yamamoto, t. nakamura and t. tsuda (1994). middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets, j. geophys. res., 99, 6321-6329. olson, w. p. and k. moe (1974). influence of precipitating charged particles on the high latitude thermosphere, j. atmos. terr. phys., 36, 1715-1726. onoshi, t., t. tsugawa, y. otsuka and j. j. berthelier (2009). ground based and satellite simultinous observations of medium scale travelling ionospheric disturbances, geophys. res. abs., 11, egu 2009-2236. prölss, g. w. (2006). subauroral electron temperature enhancement in the nighttime ionosphere, ann. geophys., 24, 1871-1885. prölss , g. m. and m. roemer (2000). heating of the polar upper atmosphere, phys. chem. earth., 25, 547-553. schlegel, k. (1982). reduced effective recombination coefficient in the disturbed polar e-region, j. atmos. terr. phys., 44, 183-185. yuan, z., r. fujii, s. nozawa and y. ogawa (2005). statistical height-dependent relative importance of the lorentz force and joule heating in generating atmospheric gravity waves in the auroral electrojets, j. geophys. res., 110, doi: 10.1029/2005ja011315. zesta, e., l. lyons, c. p. wang, e. donovan, h. frey and t. nagai (2006). auroral poleward boundary intensifications (pbis): their two-dimensional structure and associated dynamics in the plasma sheet, j. geophys. res., 111, a05201, doi:10.1029/ 2004ja010640. *corresponding author. dr. syam sundar de, centre of advanced study in radio physics and electronics, university of calcutta, kolkata 700 009, india; e-mail: de_syam_sundar@yahoo.co.in © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. heating by tids within auroral region o r g a n i z z a z i o n e e u t i l i z z a z i o n e d i u n p r o g r a m m a di a n a l i s i s p e t t r a l e (organisation anil une of a program of spectral analysis) f . m a t t i o l i ( * ) r i c e v u t o il 10 s e t t e m b r e , 1975 riassunto. — si r i a s s u m o n o d i v e r s i p r o b l e m i , sia t e o r i c i c h e p r a t i c i , i n e r e n t i l ' a n a l i s i a u t o e c r o s s s p e t t r a l e d e l l e s e r i e t e m p o r a l i , p e r p o r r e i n e v i d e n z a lo s p i r i t o con il q u a l e t a l i t e c n i c h e v a n n o i m p i e g a t e . si r a g g i u n g o n o p o i c o n c l u s i o n i c i r c a i c r i t e r i s e c o n d o i q u a l i un p r o g r a m m a d i a n a l i s i s p e t t r a l e d o v r e b b e e s s e r e o r g a n i z z a t o ed u t i l i z z a t o . summary. s e v e r a l t h e o r e t i c a l a n d p r a c t i c a l p r o b l e m s on s p e c t r a l a n d c r o s s s p e c t r a l a n a l y s i s of t i m e s e r i e s a r e s u m m a r i z e d in o r d e r t o p o i n t o u t t h e w a y in w h i c h t h e s e t e c h n i q u e s a r e t o b e e m p l o y e d . c o n c l u s i o n s a r e d r o w n a b o u t t h e c r i t e r i o n s t o b e f o l l o w e d in t h e o r g a n i z a t i o n a n d u s e of a p r o g r a m of s p e c t r a l a n a l y s i s . 1 . i n t r o d u z i o n e i programmi di analisi spettrale che si trovano in commercio non sono, di solito, ampiamente utilizzati. i n genere u n u t e n t e o si costruisce da sé il suo programma, oppure utilizza un programma costruito da altri, continuando però ad applicarlo alla stessa r i s t r e t t a categoria di problemi per i quali questo era stato originariamente pensato. la ragione di questo stato di cose risiede nella convinzione, comunemente accettata, che l'analisi spettrale sia un ben preciso susseguirsi di d e t e i m i n a t e operazioni che, applicate alla serie da esaminare, pro(*) a s s e g n i s t a m i n i s t e r i a l e p r e s s o l ' i s t i t u t o d i f i s i c a " a . r i g h i " , (li b o l o g n a . 3 2 2 f. mattioli duce come risultato le funzioni s p e t t r a l i desiderate. questa erronea visione delle cose lia i n d o t t o molti p r o g r a m m a t o r i a costruire dei prog r a m m i e s t r e m a m e n t e rigidi e, di conseguenza, difficilmente a d a t t a b i l i alle esigenze dei diversi u t e n t i . p e r chiarire meglio la n a t u r a del problema proposto facciamo u n esempio in cui esso, al contrario, non esiste. se si vuole invertire u n a m a t r i c e è facile usare il s o t t o p r o g r a m m a relativo: b a s t a i n f a t t i definire pochi p a r a m e t r i e riservare u n ' o p p o r t u n a area di memoria. poiché l'operazione che si vuole eseguire è u n i v o c a m e n t e d e t e r m i n a t a , ne segue che l'applicazione del p r o g r a m m a è del t u t t o a u t o m a t i c a : l ' u t e n t e non deve, né può, f o r m u l a r e alcuna decisione su di essa. p e r l'analisi s p e t t r a l e invece la situazione è differente: ci sono i n f a t t i diverse ragioni che impediscono di utilizzare u n q u a l u n q u e p r o g r a m m a di analisi s p e t t r a l e senza u n a p r e v i a discussione ed u n a susseguente c o n t i n u a interazione con esso da p a r t e d e l l ' u t e n t e . cerchiamo di riassumere i diversi problemi che p o r t a n o a questa i m p o r t a n t e conclusione. 2 . p r o b l e m i c o n n e s s i a l l ' a n a l i s i s p e t t r a l e 2.1. d a u n p u n t o di v i s t a statistico u n a q u a l u n q u e funzione s p e t t r a l e è c a r a t t e r i z z a t a d a u n a certa stabilità ed u n a c e r t a risoluzione in f r e q u e n z a , d i p e n d e n t i a m b e d u e da u n o stesso p a r a m e t r o , il n u m e r o di gradi di libertà del sistema ( b l a c k m a n and tuckey, 1958; j e n k i n s and w a t t s , 1969) (^2). se gli s p e t t r i sono « smootli », allora è conveniente t e n e r e alto tale p a r a m e t r o , o t t e n e n d o così u n ' a l t r a stabilità, u n i t a però ad u n a bassa risoluzione in f r e q u e n z a ; se invece negli s p e t t r i compaiono diversi picchi s t r e t t i e vicini, è conveniente u s a r e u n basso n u m e r o di gradi di libertà, per essere in grado di risolverli in m a n i e r a soddisfacente, anche a scapito della valutazione della loro ampiezza, resa difficile dalla bassa stabilità così o t t e n u t a (2). come si vede, la definizione di questo p a r a m e t r o d i p e n d e dal r i s u l t a t o finale che si o t t e r r à . sarà quindi necessario stabilirlo d a p p r i m a in base alle considerazioni teoriche che di v o l t a in v o l t a si p o t r a n n o p o r t a r e , e poi ricorregerlo in base alle indicazioni s p e t t r a l i r i c a v a t e dal primo t e n t a t i v o . i n generale u n calcolo s p e t t r a l e deve essere f a t t o con 2-3 diversi n u m e r i di gradi di libertà, per poter stabilire, dall'evoluzione o r g a n i z z a z i o n e e u t i l i z z a z i o n e d i u n programma d i a n a l i s i 3 2 3 delle stime da un numero di gradi di libertà all'altro, la f o r m a v e r a degli s p e t t r i da v a l u t a r e (2). i n alcuni casi la procedura p u ò anche essere più complessa. se si sospetta che nelle serie originali siano c o n t e n u t e delle componenti periodiche di frequenza fissa, oppure delle componenti stocastiche di larghezza d i b a n d a molto r i s t r e t t a , è necessario eseguire il calcolo degli s p e t t r i con 2 gradi di libertà. i n conseguenza del risultato o t t e n u t o si p u ò decidere di eliminare quelle eventuali componenti, per esempio m e d i a n t e filtraggio, oppure di lasciarle, tenendone però conto nell'interpretazione degli s p e t t r i finali. analogamente, quando uno s p e t t r o p r e s e n t a u n a n d a m e n t o molto complesso, può essere conveniente filtrare le serie originali in più p a r t i , in modo che la loro somma dia ancora le serie originali, ed eseguire l'analisi spettrale di ogni p a r t e s e p a r a t a m e n t e dalle altre, con dei numeri di gradi di libertà di volta in v o l t a differenti (2). i n alcuni casi, quando gli s p e t t r i h a n n o delle f o r m e molto regolari, può essere preferibile usare la cosiddetta tecnica del « prewhitening », che consiste nell'elaborare le serie originali o p p o r t u n a m e n t e filtrate, in modo che gli s p e t t r i risultanti abbiano u n a n d a m e n t o per q u a n t o possibile costante; conoscendo poi la funzione di risposta del filtro usato, si p u ò risalire alla f o r m a degli s p e t t r i delle serie originali, compiendo errori di calcolo di minor e n t i t à i n definitiva possiamo affermare che u n a valutazione spettrale viene r a g g i u n t a a t t r a v e r s o u n processo iterativo, che p u ò richiedere in alcuni casi delle operazioni intermedie di filtraggio. 2.2. esiste inoltre u n ' a l t r a categoria di problemi, legata alla possibilità di v a l u t a r e le stime spettrali passando a t t r a v e r s o il calcolo delle funzioni di covarianza o, più d i r e t t a m e n t e , a t t r a v e r s o il calcolo della t r a s f o r m a t a di fourier (veloce) delle serie originali. l ' u t e n t e deve quindi v a l u t a r e p r e v e n t i v a m e n t e quale delle due vie si a d a t t a meglio al suo problema. i n genere si sceglie la p r i m a via t u t t e le volte che 11011 sia indispensabile seguire la seconda. i n f a t t i le funzioni di covarianza h a n n o di per sé u n loro significato che si richiama d i r e t t a m e n t e alla teoria in base alla quale le funzioni spettrali sono s t a t e i n t r o d o t t e . esse permett o n o di individuare e v e n t u a l i r i t a r d i f r a u n a serie e l'altra, nonché la presenza o meno di spiccanti a n d a m e n t i periodici. dalla valutazione del massimo dei r i t a r d i per cui esse sono significativamente diverse da zero si può anche ricavare la risoluzione in 3 2 4 f. mattioli f r e q u e n z a più o p p o r t u n a (la tenere nel calcolo s p e t t r a l e seguente. qualora poi u n a funzione di ci-oss covarianza presenti un picco in prossimità dell'origine, traslarla in modo che il picco sia c e n t r a t o nell'origine p r i m a di e f f e t t u a r n e la t r a s f o r m a t a di fourier, p r o d u c e delle stime spettrali migliori (2). bisogna ricordare che le operazioni che richiedono il maggior t e m p o di calcolo sono in p r i m o luogo le funzioni di covarianza, calcolate col m e t o d o diretto, e in secondo la t r a s f o r m a t e di fourier, qualora non si impieghi u n m e t o d o di f a s t f o u r i e r t r a n s f o r m (f.f.t.). nel caso però in cui, p u r a v e n d o scelto la seconda s t r a d a , si sia c o m u n q u e interessati alle funzioni di covarianza bisogna ricordare che queste possono essere calcolate come a n t i t r a s f o r m a t e di fourier delle funzioni spettrali, anche se i risultati così o t t e n u t i non sono molto buoni. le funzioni di covarianza d ' a l t r a p a r t e possono essere o t t e n u t e d i r e t t a m e n t e dalle serie originali a t t r a v e r s o l'uso r i p e t u t o di u n prog r a m m a di f . f . t . : anzi, se si è interessati alla loro valutazione per r i t a r d i molto grandi, q u e s t a è l'unica soluzione possibile. gli s p e t t r i a loro volta p o t r e b b e r o essere calcolati anche a p a r t i r e dalle funzioni di covarianza così o t t e n u t e . bisogna però n o t a r e che questo metodo, anche se non richiede t e m p i di calcolo eccessivi, esige l'impiego di u n a notevole q u a n t i t à di m e m o r i a ausiliare. 2.3. accanto ai problemi citati che sono s t r e t t a m e n t e legati, i primi alla stessa n a t u r a teorica dell'analisi spettrale, e i secondi ai diversi m e t o d i di calcolo numerico esistenti, ne esistono altri ancora, legati più alle esigenze specifiche dei diversi u t e n t i , che riguardo preval e n t e m e n t e le operazioni di l e t t u i a e di p r i m a t r a t t a z i o n e dei dati, nonché i diversi modi in cui i r i s u l t a t i finali possono essere p r e s e n t a t i in uscita. le operazioni di l e t t u r a si diversificheranno a seconda clie i dati si trovino su scheda, su n a s t r o o altro. p u ò darsi poi il caso che nelle serie originali m a n c h i u n a certa q u a n t i t à di dati, in intervalli di v a r i a lunghezza e v a r i a m e n t e disposti. questo problema p u ò essere a f f r o n t a t o seguendo u n a delle due seguenti vie: o si sostituiscono ai d a t i m a n c a n t i dei valori calcolati in base ai d a t i a disposizione, m e d i a n t e o p p o r t u n e formule interpolatorie, oppure si a d o t t a n o per il calcolo delle funzioni di covarianza formule particolari, che tengono conto solo dei d a t i presenti. l a p r i m a soluzione è indispensabile se si deve applicare d i r e t t a m e n t e alle serie in esame u n p r o g r a m m a di f . f . t . , ed è da evitare per q u a n t o possibile, poiché organizzazione e u t i l i z z a z i o n e d i u n programma d i a n a l i s i 3 2 5 q u a l u n q u e sia l'operazione di interpolazione a d o t t a t a , si i n t r o d u c e nelle serie un'informazione arbitraria, i cui effetti sono difficilmente prevedibili. l a seconda soluzione consiste nel calcolare le funzioni (li covarianza mediando soltanto sui p r o d o t t i eseguibili f r a le serie originali e le serie r i t a r d a t e (qui per p r o d o t t o eseguibile si intende u n prod o t t o in cui a m b e d u e i termini siano d a t i noti). le stime che così si ottengono a v r a n n o un minore n u m e r o di gradi di libertà, rispetto al caso in cui t u t t i i d a t i fossero noti, vista l'inevitabile perdita di informazione, e saranno f r a di loro maggiormente correlate; esse però saranno « unbiased », come conseguenza del f a t t o che non è s t a t a immessa alcuna informazione aggiuntiva (mattioli; mattioli, tibaldi and pellizzardi) (3^4). mentre la scala su cui porre le funzioni di covarianza 11011 p u ò altro che essere lineare, non così si p u ò dire pei le funzioni spettrali. poiché i logaritmi delle funzioni a u t o s p e t t r a l i e le a r c o t a n g e n t i iperboliche delle coerenze q u a d r a t e possiedono intervalli di confidenza costanti, spesso si possono t r o v a r e quelle grandezze in queste scale. t u t t e le volte che le più f o r t i variazioni spettrali avvengono nel campo delle basse frequenze, p u ò essere o p p o r t u n o a d o t t a r e u n a scala logaritmica in ascisse, così come la scala doppio logaritmica è p a r t i c o l a r m e n t e a d a t t a per rivelare degli a n d a m e n t i di potenza. u n a scala caratteristica delle stime a u t o s p e t t r a l i è poi quella che dispone in ascissa il logaritmo della f r e q u e n z a e in o r d i n a t a il p r o d o t t o della funzione spettrale per la f r e q u e n z a : u n a tale scala p e r m e t t e di avere u n grafico in cui l'area sottesa dalla c u r v a con l'asse delle ascisse m a n t i e n e , come nel caso di scala doppio lineare, il significato di potenza relativa a quel campo di frequenza. a questo p u n t o però è bene osservare che gli s p e t t r i possono essere posti con u n a scala logaritmica nella f r e q u e n z a , solo nell'ipotesi che siano s t a t i calcolati con il m e t o d o della f . f . t . i n o l t r e possono rendersi necessarie t a n t e operazioni preliminari dei dati, come istogrammi, medie progressive, normalizzazioni, ecc.. . . . , che, se p u r non indispensabili, f a però molto comodo t r o v a r e accanto ad u n p r o g r a m m a di analisi spettrale. 2.4. esiste insomma u n a notevole q u a n t i t à di problemi che non p e r m e t t o n o né di pensare all'analisi s p e t t r a l e come ad un susseguirsi ben d e t e r m i n a t o di operazioni, né di utilizzare u n p r o g r a m m a di analisi s p e t t r a l e come u n sistema che da u n ingresso costituito dalle serie originali, fornisce u n a o più uscite, in un ordine sequenziale prefissato a priori, corrispondenti ai risultati voluti. 3 2 6 f. mattioli 3 . o r g a n i z z a z i o n e d i u n p r o g r a m m a d i a n a l i s i s p e t t r a l e se la b o n t à di un generico p r o g r a m m a , come quello per l'inversione di u n a m a t r i c e viene solitamente v a l u t a t a in base al t e m p o di calcolo e alla memoria richiesti per eseguile le operazioni volute, questi f a t t o r i risultano di secondaria i m p o r t a n z a in u n p r o g r a m m a di analisi spettrale. le differenze di t e m p o di calcolo e di m e m o r i a u s a t i dipendono p r e v a l e n t e m e n t e dal m e t o d o scelto, e t u t t e le possibili v a r i a n t i dello stesso m e t o d o introducono c a m b i a m e n t i di b e n scarso rilievo. il f a t t o r e d e t e r m i n a n t e che p u ò i n d u r r e u n u t e n t e ad a d o t t a r e u n certo prog r a m m a di biblioteca è la possibilità di r i t r o v a r e in esso f r a t u t t e le operazioni che a b b i a m o p r e c e d e n t e m e n t e citato, quelle che gli servono per il suo problema particolare. quindi ciò che bisogna fornire a l l ' u t e n t e non è u n unico p r o g r a m m a , m a u n complesso di s o t t o p r o g r a m m i , ciascuno dei quali esegua u n insieme elementare di operazioni, costruiti in modo da p o t e r essere semplicemente connessi l'uno all'altro secondo la necessità. u n a possibile suddivisione in blocchi e l e m e n t a r i delle diverse operazioni che intervengono nell'analisi s p e t t r a l e è la seguente: 1) l e t t u r a dei dati. 2) operazioni preliminari (interpolazioni di d a t i m a n c a n t i , normalizzazioni, ecc. . . . ) . 3) filtraggio dei d a t i . 1) calcolo delle funzioni di covarianza. 5) calcolo delle funzioni di covarianza a t t r a v e r s o l'uso r i p e t u t o della f . f . t . 6) calcolo delle funzioni a u t o , coe q-spettrali come t r a s f o r m a t e di fourier delle funzioni di covarianza. 7) calcolo delle funzioni, auto, coe q-spettrali col m e t o d o f . f . t . 8) calcolo delle funzioni cross spettrali d e r i v a t e (guadagno, rumore, coerenza e fase) a p a r t i r e dalle funzioni cross spettrali semplici (auto s p e t t r i , coe q-spettri). 9) messa in scala o p p o r t u n a delle grandezze di uscita, con relativo calcolo degli intervalli di confidenza. 10) s t a m p a . nella figura 1 poniamo in rilievo i più comuni tipi di connessione di questi blocchi elementari. o r g a n i z z a z i o n e e u t i l i z z a z i o n e d i u n programma d i a n a l i s i 3 2 7 p i g . 1 s c h e m a a b l o c c h i delle p r i n c i p a l i o p e r a z i o n i r e l a t i v e a l l ' a n a l i s i s p e t t r a l e . 3 2 8 f. mattioli n a t u r a l m e n t e esistono anche altri tipi di connessioni possibili ed altri t i p i di operazioni utili, m a un insieme di s o t t o p r o g r a m m i che eseguisse t u t t e le operazioni elencate lasciarebbe ben poco lavoro all ' u t e n t e che esigesse qualcosa di più. i n p r a t i c a il suo compito consisterebbe nello scrivere u n p r o g r a m m a principale per chiamare, nell'ordine voluto i s o t t o p r o g r a m m i in grado di eseguire le operazioni necessarie al particolare problema in esame. p e r r e n d e r n e p i ù facile la c h i a m a t a , poiché gli stessi p a r a m e t r i compaiono molto spesso in più d ' u n o di essi, si possono trasferire in « common » t u t t i i p a r a m e t r i scalari esistenti in t u t t i i s o t t o p r o g r a m m i . i n questo modo la loro c h i a m a t a avverrebbe esplicitando come p a r a m e t r i formali le sole grandezze v e t toriali, che all'interno di ogni s o t t o p r o g r a m m a dovrebbero avere u n dimensionamento fittizio p a r i a 1. p e r agevolare poi il calcolo del dimensionamento dei vettori nel p r o g r a m m a principale, visto che esso cambia n o t e v o l m e n t e al v a r i a r e del procedimento usato anche per raggiungere lo stesso r i s u l t a t o , è sempre possibile costriúre u n s o t t o p r o g r a m m a che, in base ai p a r a m e t r i scalari, sempre trasferiti in « common » e definiti p r e c e d e n t e m e n t e , sia in grado di dare come risposta i dimensionamenti da a t t r i b u i r e ai diversi v e t t o r i , al variare dei singoli s o t t o p r o g r a m m i in cui essi p o t r e b b e r o apparire. infine, poiché molti p a r a m e t r i vengono per lo più u s a t i con u n certo d e t e r m i n a t o valore, e anche per evitare errori di esecuzione deriv a n t i dalla m a n c a t a definizione di uno di essi, si p u ò costruire u n sottop r o g r a m m a d a chiamare prima di ogni altra operazione, per inizializzare t u t t i i p a r a m e t r i col loro valore p i ù comune. con questi s t r u m e n t i il compito d e l l ' u t e n t e sarebbe g r a n d e m e n t e facilitato e, in definitiva, consisterebbe nella stesura di poche decine di istruzioni, salvo esigenze molto particolari. 4 . c o n c l u s i o n i l a costruzione di u n p r o g r a m m a di analisi s p e t t r a l e siffatto non p r e s e n t a particolari difficoltà e d ' a l t r a p a r t e dipende dalla sensibilità dell'autore aggiungere o togliere alcuni blocchi, sviluppare o meno certi settori: ciò che è i m p o r t a n t e , ed è q u a n t o si è voluto porre in evidenza in questa sede, sono soltanto le ragioni ed i modi a t t r a v e r s o i quali si deve e si p u ò salvaguardare in u n p r o g r a m m a di analisi spettrale u n a s t r u t t u r a che p e r m e t t a a l l ' u t e n t e un « dialogo » con esso. o r g a n i z z a z i o n e e u t i l i z z a z i o n e d i u n p r o g r a m m a d i a n a l i s i 3 2 9 b i b l i o g r a f i a f1) blackman r . b . , t u c k e y j . w . , 1 9 5 8 . the measurement oj power spectra. " d o v e r p u b l i c a t i o n s " , i n c . , n e w y o r k . (z) j e n k i n s c . m . , w a t t s d . g . , 1 9 0 9 . spectral analysis and its applications. " h o l d e n d a y " , s a n f r a n c i s c o . ( 3 ) m a t t i o l i f . , 1 9 7 5 . spectral analysis oj wind and s02 concentration in the venice area. " a t m o s p h e r i c e n v i r o n m e n t " , i n p r e s s . ( 4 ) m a t t i o l i f . , t i b a l d i s., p e l l i z z a r d i p . , 1 9 7 4 . a program for axito and cross spectral analysis. " l ' e l a b o r a z i o n e a u t o m a t i c a " . 1, 5, p . 4 4 . 4 404 not found a technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique laura sandri, warner marzocchi istituto nazionale di geofisica e vulcanologia, sezione di bologna, via donato creti 12, 40128 bologna, italy e-mail: sandri@bo.ingv.it, marzocchi@bo.ingv.it submitted to annals of geophysics version: november, 2005 1 abstract we investigate conceptually, analytically, and numerically the biases in the estimation of the b-value of the gutenberg-richter law and of its uncertainty made through the least squares technique. the biases are introduced by the cumulation operation for the cumulative form of the gutenberg-richter law, by the logarithmic transformation, and by the measurement errors on the magnitude. we find that the least squares technique, applied to the cumulative and binned form of the gutenberg-richter law, produces strong bias in the b-value and its uncertainty, whose amplitudes depend on the size of the sample. furthermore, the logarithmic transformation produces two di!erent endemic bends in the log(n ) versus m curve. this means that this plot might produce fake significant departures from the gutenberg-richter law. the e!ect of the measurement errors is negligible compared to those of cumulation operation and logarithmic transformation. the results obtained show that the least squares technique should never be used to determine the slope of the gutenberg-richter law and its uncertainty. key words: b-value, seismology, least squares technique, syntheticearthquake catalogs. short title for the page headings: bias in the estimation of b-value and its uncertainty through least squares. 2 1 introduction the gutenberg-richter law (from now on gr law) (gutenberg & richter, 1954) is certainly one of the most remarkable and ubiquitous features of the worldwide seismicity. in the most common form it reads log[n (m)] = a ! bm (1) in the so-called binned form of the gr law, n represents the number of events with magnitude m, while in the cumulative form n is the number of events with magnitude larger or equal to m. the constants a and b are the coe"cient of the linear relationship. as discussed in a previous paper (marzocchi and sandri, 2003, from now on ms03), the scientific relevance of the gr law is linked to its apparent ubiquity, and to the theoretical implications and meaning of its possible universality. in particular, the value of b, representing the opposite of the slope of the linear relationship in equation 1, is considered very important. in fact, in spite of a “first order” validity of the gr law with a constant b-value " 1 observed in a variety of tectonic settings, significant spatial and temporal variations in the b-value are found (e.g., schorlemmer et al., 2005; murru et al., 2005; ratchkovski et al., 2004; wyss et al., 2004; del pezzo et al., 2004; mandal et al., 2004; legrand et al., 2004; riedel et al., 2003). these possible variations are very important from a theoretical point of view, and for seismic hazard studies. we refer to our previous paper (ms03) for a deeper discussion about these issues. as a matter of fact, a crucial aspect in inferring variations or 3 constancy of the b-value is represented by the method used to determine the b-value and its uncertainty, given a seismic catalog, as shown in ms03. in that paper, we discussed the maximum likelihood method to estimate the b-value and its uncertainty, showing that incorrect formulae based on this method produce a bias in the b-value and an underestimation of its uncertainty. the latter implies that we might observe a fake variation in the b-value only because we have underestimated the uncertainty. intentionally, in ms03 we did not consider the least squares (from now on, ls) method, because it had already been recognized (page, 1968; bender, 1983) that this technique applied to the problem of the estimation of the b-value and of its uncertainty does not have any statistical foundation. however, the ls technique is still widely employed to estimate these two quantities, both in the binned and in the cumulative form, also in recent literature (e.g. working group on california earthquake probabilities, 2003; gruppo di lavoro, 2004; lopez pineda and rebollar, 2005). the main purpose of this paper is to demonstrate, by means of conceptual issues, analitical formulations and numerical simulations, that the use of the ls technique in the estimation of the b-value and of its uncertainty leads to strongly biased estimates of these two quantities. for the correct estimation of these two quantities, we refer to the appropriate formulae given in ms03 and in references therein. 4 2 estimation of b and !b̂ through the ls technique the ls estimation consists of applying the linear regression analysis to the gr law described by equation 1 in the binned or cumulative form. the parameter b should be the same in the two formulations, while the intercept a is di!erent in the two cases. 2.1 cumulative form of the gr law despite its large use in past and recent papers (pacheco & sykes, 1992; pacheco et al., 1992; karnik & klima, 1993; okal & kirby, 1995; scholz, 1997; triep & sykes, 1997; main, 2000; working group on california earthquake probabilities, 2003; gruppo di lavoro, 2004; lopez pineda and rebollar, 2005), the use of ls technique to estimate the b-value on the cumulative form of the gr law does not have any statistical motivation. in this case, indeed, the strongest assumption of the regression analysis, i.e., the independence of the observations, is clearly violated. in practice, the cumulative form is an integration and, therefore, it represents a filter for the high frequency noise. as a result, the uncertainty of the slope of the gr law is certainly strongly underestimated. in order to evaluate numerically the bias in the regression analysis applied to the cumulative form, as a function of the number of earthquakes contained in the catalog, we simulate 1000 seismic catalogs, for different catalog sizes. the magnitudes mi are obtained by binning, with a fixed bin width of 0.1, a continuous random variable dis5 tributed with a pdf given by equation f (m) = b ln(10) 10!b(m!mmin) (2) which is valid if the catalog contains earthquakes with a completeness magnitude equal to mmin and with a magnitude range of at least three units (see ms03). in this way, mi is the magnitude attached to all the synthetic seismic events with real continuous magnitude in the range mi ! 0.05 # m < mi + 0.05. then, for each synthetic catalog, we estimate the b-value (from now on, we use the notation b̂ for the estimated value, and b for the theoretical value) by means of the ls technique applied to the cumulative gr law described by equation 1, i.e., when n is the number of events with magnitude larger or equal to mi. in figure 1 we report the averages of b̂ calculated in 1000 synthetic catalogs as a function of the number of data, for the cases b = 1 and b = 2. at each average is attached the 95% confidence interval. there is a clear negative bias, that decreases with the number of data and ranges from 5% to 2%. an explanation of this bias will be given in the following when we describe the e!ects of the logarithmic transformation in the regression analysis of the binned magnitudes. as mentioned above, another crucial aspect concerns the estimation of the uncertainty attached to b̂, here indicated by !̂b̂ and estimated as the average error on b̂ provided by the 1000 linear regressions. in particular, we are interested in evaluating if the estimation of !̂2 b̂ is an unbiased estimator of the true variance of the estimation of b̂ around its central value. this can be done by sim6 ulating 1000 seismic catalogs as described before. for each catalog we calculate b̂ and !̂b̂. then, we compare the dispersion of b̂ around its average with the average of !̂b̂, through the fisher test (e.g. kalbfleisch, 1979) f = variance of the estimation of the b!value average of the square of the uncertainty (3) the null hypothesis we test is that the average of !̂2 b̂ is equal to the variance of b̂. the results, reported in figure 2, are very interesting. in particular, the fisher test shows that !̂b̂ strongly underestimates the dispersion of b̂, the former being at least one order of magnitude smaller than the latter. this confirms that the uncertainty on the b-value estimated through the cumulative ls method is strongly underestimated. in general, the potential factors which can bias the estimations made through the ls technique (both in the cumulative and binned form) are the logarithmic transformation of the number of events, and the presence of the measurement errors on the magnitude. by taking into account the criticism at the regression analysis applied to the cumulative form just reported, in the following we study the impact of these factors only on the estimation of the b-value made by means of the regression analysis applied to the binned form. 2.2 e!ects of the logarithmic transformation the expected number of events for a magnitude m is "i = npi = n mi+!m/2! mi!!m/2 f (m)dm (4) 7 where n is the total number of earthquakes in the catalog, #m is the bin width used, and f (m) is given by equation 2. the expected fluctuations around this value can be written as #ni = " npi(1 ! pi) (5) in this frame, the observed number of events ni for a binned magnitude mi can be written as ni = "i(1 + #i), where #i is a random variable with zero mean and standard deviation equal to #ni/"i. by taking into account equations 4 and 5, we can write #ni "i = # 1 ! pi n pi (6) it is easy to demonstrate that #ni/"i is a strictly monotonic decreasing function of pi. this means that the expected fluctuations of the variable # are larger for lower pi, that is for large magnitudes. the estimation of b in the classical linear regression analysis is (see e.g. draper & smith, 1981) b̂ = ! $ mi $ [log(ni)] ! n $ mi[log(ni)] ( $ mi)2 ! n $ m 2i (7) if we substitute ni = "i(1 + #i) we obtain b̂ = ! $ mi $ [log("i)] ! n $ mi[log("i)] ( $ mi)2 ! n $ m 2i ! $ mi $ [log(1 + #i)] ! n $ mi[log(1 + #i)] ( $ mi)2 ! n $ m 2i (8) if we apply the expectation operator at both sides of equation 8, it is possible to verify that the bias is zero only if e[log(1+#i)] = 0. this is certainly not true because e[log(1 + #i)] < log{e[(1 + #i)]} = 0. to summarize, the application of the classical linear regression analysis to the logarithm of real data (therefore a!ected by errors) 8 produces a systematic bias on the estimated slope. this is not due to the regression analysis (because the latter always provides an unbiased estimate), but it is related to its application to this type of data. the logarithmic transformation has also another very important e!ect due to the discreteness of the dependent variable n . we have just shown that the dispersion of the data around the curve increases with magnitude. since we cannot compute log(ni) if ni = 0, at large magnitudes, where " is low (for example " # 1), the ls technique can take into account only the “positive” fluctuations around the mean value, i.e., the fluctuations that increase the expected number of events. this acts as a filter for the “negative” fluctuations, that is at large magnitudes (where it can be computed) log(ni) tends to be overestimated if compared to log("i). the global e!ect consists in the introduction of a systematic negative bias in the estimation of b, that is b̂ is always lower than b. recalling that the amplitude of the dispersion around the curve is higher for small data sets, in these cases the bias should be larger. in figure 3, we report log("i) (see equation 4) and the averages of log(ni) obtained by 1000 synthetic catalogs as a function of the magnitudes mi. for each magnitude mi, the average of log(ni) is computed only on those catalogs for whom ni $ 1. at lower magnitudes, in the left part of the graph (to the left of the intersection point between the two lines in figure 3), the log(ni) tends to underestimate log("i). this bias, due to the second addendum in the right side of equation 8, should lead to an overestimation of the b-value. on the contrary, at large magnitudes (to the right of 9 the intersection point between the two lines in figure 3), there is a turnover in the trend of log(ni) with respect to log("i). here, log(ni) tends to be larger than log("i). as mentioned before, this e!ect is due to the filtering of the negative fluctuations of ni at large magnitudes performed by the logarithmic transformation of a discrete variable. the consequence is an underestimation of the bvalue. the results reported in figure 3 suggest that, when the larger magnitudes are taken into account, this last bias is always predominant. as argued before, this bias is reduced by increasing the size of the data set. instead, if the larger magnitudes are excluded (e.g. karnik & klima, 1993), the plot log(ni) versus mi shows only one bend, and the b-value is overestimated independently on the size of the catalog. in other words, the global e!ect of the logarithmic transformation of a discrete variable (the number of seismic events) introduces two bends in the gr curve. it is worth to note that these discrepancies from a straight line are not linked to any physical process, but only to the mathematical transformation. similar considerations can be done by looking at figure 4 that reports the estimations of the bvalue as a function of the size of the data set. the bias is very strong for small data set and it decreases as the size of the data set increases. the bias is higher in the binned case compared to the cumulative one (compare figures 4 and 1). this is due to the fact that in the cumulative regression there are less zero values in the dependent variable. as shown in figure 2, the uncertainty of the b-value made in the binned regression is still underestimated, even though it is better than the strong underestimations obtained by 10 the cumulative regression. 2.3 e!ects of the measurement errors if we add a measurement error at the theoretical magnitude (m), we obtain the “real” magnitude ( %m ) as a sum of two independent random variables %m = m + $ (9) where m is the earthquake magnitude devoid of measurement errors distributed with a pdf given by equation 2, and $ simulates the measurement errors distributed as a gaussian noise. let us consider the number of the earthquakes with a binned magnitude mi, that is n (mi). by adding a gaussian noise to each magnitude as in equation 9, some events go out of and some come into the considered bin, as shown in figure 5. the final number is n ( %mi). the number of data which go out of the bin is (see figure 5) mol = mor = p!n (mi) (10) where mol is the number of data which go into the adjacent left bin, mor is the number of data that go into the adjacent right bin, and p! is the probability that a randomly chosen event inside the bin goes out of the bin itself when a measurement error is added. p! is independent from mi and it depends on ! 2 ! (p! % 0 when !2! % 0). the number of data which come in from the left bin is (see figure 5) mil = p!n (mi!1) = p!n (mi ! #m) (11) the number of data which come in from the right bin is (see figure 11 5) mir = p!n (mi+1) = p!n (mi + #m) (12) then, we have n ( %m ) = n (m)+p! & n (mi +#m)+n (mi !#m)!2n (m) ' (13) if equation 1 holds, equation 13 can be rewritten as n ( %m )i = n (mi) + p! 10a!bmi (10!b !m + 10b !m ! 2) (14) for b " 1 and #m = 0.1, as in most of the practical cases, the term of equation 14 inside the round brackets is " 0. in these case n (mi) " n ( %mi), therefore the calculation of b is only slightly a!ected by the measurement error. at the opposite, when #m is large and/or b > 1, the term inside the round brackets cannot be neglected and n (mi) &= n ( %mi). this obviously introduces a further bias in the b-value estimation. figure 6 reports the e!ect of the added measurement errors. as argued before, the largest di!erence with the case without measurement errors reported in figure 4 is for b = 2, where the term of equation 14 inside the round brackets is significantly di!erent from zero. since p! depends on the amplitude of the measurement error (!!), equation 14 explains also the proportionality of the bias with !! noted in figure 6. as regards the estimation of the uncertainty of the b-value, the results reported in figure 7 show that the addition of the measurement errors does not produce any further bias. 12 3 final remarks we have studied conceptually, analytically, and numerically the bias introduced by some factors such as the cumulation operation, the logarithmic transformation, and the measurement errors on the magnitudes, on the estimation of the b-value and of its uncertainty by means of the least squares technique. we have found a great deal of shortcomings. first, besides violating a basic assumption of the regression analysis, the use of the cumulative form of the gutenberg-richter law leads to a very strong underestimation of the uncertainty of the b-value, due to the filtering e!ect of the cumulation operation. secondly, the logarithmic transformation of the discrete random number of events produces a significant bias in the estimation of the b-value both for the cumulative and binned form of the gutenbergrichter law. moreover, the bias strongly depends on the size of the data set; this means that two sample of di!erent sizes coming from the same statistical distribution will have a significant di!erence in the b-value estimated. the same logarithmic transformation produces two endemic bends of di!erent signs in the log(n ) versus m plot. these bends are not linked to the physical process. this means that some departures from a straight line in the plot log(n ) versus m do not invalidate the gutenberg-richter law. finally, the influence of the measurement errors appears to be less important than the bias introduced by the logarithmic transformation. the main purpose of this paper was to show that the estima13 tion of the b-value by means of the least squares method is strongly biased and its uncertainty results heavily underestimated. in this view, this work intended to make clear why this method should not be used to determine the slope of the gutenberg-richter law, and to make inferences about its constancy or variations. in practice, any spatial or temporal variation of the b-value obtained by this method has to be regarded with a strong skepticism. on the other hand, unbiased estimates of the b-value and of its uncertainty can be obtained by using the correct formulae, based on the maximum likelihood method, that have been analitically and numerically tested in marzocchi and sandri (2003) (see also references therein). 14 acknowledgement: to be written 15 references bender, b., 1983. maximum likelihood estimation of b values for magnitude grouped data, bull. seismol. soc. am., 73, 831-851. del pezzo, e., bianco, f., petrosino, s., & saccorotti, g., 2004. changes in the coda decay rate and shear-wave splitting parameters associated with seismic swarms at mt. vesuvius, italy. bull. seismol. soc. am., 94, 2, 439-452. draper, n.r., & smith, h., 1981. applied regression analysis, 2nd edition, john wiley & sons, new york. gruppo di lavoro, 2004. redazione della mappa di pericolositá sismica prevista dall’ordinanza pcm 3274 del 20 marzo 2003. rapporto conclusivo per il dipartimento della protezione civile, ingv, milano-roma, aprile 2004, 65 pp. gutenberg, b., & richter, c., 1954. seismicity of the earth and associated phenomena, 2nd ed., princeton university press, princeton, new jersey. karnik, v. & klima, k., 1993. magnitude-frequency distribution in the european-mediterranean earthquake regions, tectonophys., 220, 309-323. legrand, d., villagomez, d., yepes, h., & calahorrano, a., 2004. multifractal dimension and b value analysis of the 1998-1999 quito swarm related to guagua pichincha volcano activity, ecuador. j. 16 geophys. res., 109, b1, b01307, doi:10.1029/2003jb002572. lopez pineda, l., & rebollar, c.j., 2005. source characteristics of the m (sub w) 6.2 loreto earthquake of 12 march 2003 that occurred in a transform fault in the middle of the gulf of california, mexico. bull. seismol. soc. am., 95, 2, 419-430. main, i., 2000. apparent breaks in scaling in the earthquake cumulative frequency-magnitude distribution: fact or artifact?, bull. seismol. soc. am., 90, 86-97. mandal, p., rastogi, b.k., satyanarayana, h.v.s., & kousalya, m., 2004. results from local earthquake velocity tomography; implications toward the source process involved in generating the 2001 earthquake in the lower crust beneath kachchh (india). bull. seismol. soc. am., 94, 2, 633-649. marzocchi, w., & sandri, l., 2003. a review and new insights on the estimation of the b-value and its uncertainty, ann. geophys., 46, 6, 1271-1282. murru, m., montuori, c., console, r., & lisi, a., 2005. mapping of the b value anomalies beneath mt. etna, italy, during julyaugust 2001 lateral eruption. geophys. res. lett., 32, 5, l05309, doi:10.1029/2004gl021545. okal, e. a., & kirby, s. h., 1995. frequency-moment distribution of deep earthquake; implications for the seismogenic zone at the bottom of slabs, phys. earth planet. inter., 92, 169-187. 17 pacheco, j.f., & sykes, l.r., 1992. seismic moment catalog of large shallow earthquakes, 1900 to 1989, bull. seismol. soc. am., 82, 1306-1349. pacheco, j.f., scholz, c.h., & sykes, l.r., 1992. changes in frequencysize relationship from small to large earthquakes, nature, 355, 7173. page, r., 1968. aftershocks and microaftershocks of the great alaska earthquake of 1964, bull. seismol. soc. am., 58, 11311168. ratchkovski, n.a., wiemer, s., & hansen, r.a., 2004. seismotectonics of the central denali fault, alaska, and the 2002 denali fault earthquake sequence. bull. seismol. soc. am., 94, 6, part b, 156-174. riedel, c., petersen, t., theilen, f., & neben, s., 2003. high bvalues in the leaky segment of the tjornes fracture zone north of iceland; are they evidence for shallow magmatic heat sources? j. volcanol. geotherm. res., 128, 1-3, 15-29. scholz, c.h., 1997. size distributions for large and small earthquakes, bull. seismol. soc. am., 87, 1074-1077. schorlemmer, d., wiemer, s., & wyss, m., 2005. variations in earthquake-size distribution across di!erent stress regimes, nature, 437, doi: 10.1038/nature04094. 18 triep, e.g., & sykes, l.r., 1997. frequency of occurrence of moderate to great earthquakes in intracontinetal regions: implications for changes in stress, earthquake prediction, and hazard assessment, j. geophys. res., 102, 9923-9948. working group on california earthquake probabilities (2003), earthquake probabilities in the san francisco bay region: 2002 2031, u.s. geol. surv. open file rep., 03-214. wyss, m., sammis, c.g, naudeau, r.m., & wiemer, s., 2004. fractal dimension and b-value on creeping and locked patches of the san andreas fault near parkfield, california. bull. seismol. soc. am.,94, 2, 410-421. 19 figure captions figure 1: average of b̂ (dashed line) calculated through cumulative ls technique in 1000 synthetic catalogs, as a function of the catalog size, for the cases b=1 (left) and b=2 (right). at each average is attached the 95 % confidence interval. the solid line represents the true b-value. figure 2: f test values (see equation 3) for the case of figures 1 and 4 (see text). the plus signs represent the cumulative regression, and the squares the binned regression. the dotted line represents the critical value to reject the null hypothesis at a significance level of 0.05. note the logarithmic scale also on the vertical axis. figure 3: logarithm of the expected number of earthquakes (log("i); see text) according to the gutenberg-richter law (solid line) and average of the logarithm of the generated number of events in each of 1000 synthetic catalogs (dotted line), plotted as a function of the magnitude, for di!erent catalog sizes (from top to bottom: 100, 1000, 10000 events per catalog). the intersection between the solid and dotted lines represents the point where the bending changes in sign. figure 4: average of b̂ (dashed line) calculated through binned ls technique in 1000 synthetic catalogs, as a function of the catalog size, for the cases b=1 (left) and b=2 (right). at each average is attached the 95 % confidence interval. the solid line represents the true b-value. 20 figure 5: e!ect of the measurement errors on the number of earthquakes with binned magnitude mi. adding the measurement errors, the number of events that come into the bin with central magnitude mi from the adjacent left bin is mil, while those coming from the adjacent right bin is mir. the number of events that go out of the bin with central magnitude mi towards the adjacent left bin is mol, while those going out towards the adjacent right bin is mor. figure 6: averages of b̂ (dashed lines) calculated through binned ls technique in 1000 synthetic catalogs as a function of the catalog size, for the cases b=1 (top) and b=2 (bottom). the magnitudes have measurement errors of standard deviation !! = 0.1, 0.3, 0.5 (from left to right). at each average is attached the 95 % confidence interval. the solid line represents the true b-value. figure 7: f test values (see equation 3) for the case of figure 6 (see text). the dotted line represents the critical value to reject the null hypothesis at a significance level of 0.05. 21 b=1 1 2 3 4 5 log10(# earthquakes) 0.940 0.950 0.960 0.970 0.980 0.990 1.000 b va lu e b=2 1 2 3 4 5 log10(# earthquakes) 1.90 1.95 2.00 b va lu e figure 1: 22 b=1 1 2 3 4 5 log10(# earthquakes) 0 1 lo g 1 0(f te st ) b=2 1 2 3 4 5 log10(# earthquakes) 0 1 lo g 1 0(f te st ) figure 2: 23 100 earthquakes 3 4 5 6 magnitude 0.0 0.5 1.0 1.5 2.0 2.5 3.0 lo g 1 0(n ) 1000 earthquakes 3 4 5 6 magnitude 0.0 0.5 1.0 1.5 2.0 2.5 3.0 lo g 1 0(n ) 10000 earthquakes 3 4 5 6 magnitude 0.0 0.5 1.0 1.5 2.0 2.5 3.0 lo g 1 0(n ) figure 3: 24 b=1 1 2 3 4 5 log10(# earthquakes) 0.50 0.60 0.70 0.80 0.90 1.00 1.10 b va lu e b=2 1 2 3 4 5 log10(# earthquakes) 1.4 1.6 1.8 2.0 b va lu e figure 4: 25 figure 5: 26 b=1 !"=0.1 1 2 3 4 5 log10(# earthquakes) 0.50 0.60 0.70 0.80 0.90 1.00 b va lu e b=1 !"=0.3 1 2 3 4 5 log10(# earthquakes) 0.50 0.60 0.70 0.80 0.90 1.00 b va lu e b=1 !"=0.5 1 2 3 4 5 log10(# earthquakes) 0.50 0.60 0.70 0.80 0.90 1.00 b va lu e b=2 !"=0.1 1 2 3 4 5 log10(# earthquakes) 1.4 1.6 1.8 2.0 b va lu e b=2 !"=0.3 1 2 3 4 5 log10(# earthquakes) 1.4 1.6 1.8 2.0 b va lu e b=2 !"=0.5 1 2 3 4 5 log10(# earthquakes) 1.4 1.6 1.8 2.0 b va lu e figure 6: 27 b=1 !"=0.1 1 2 3 4 5 log10(# earthquakes) 0 1 2 3 4 5 f te st b=1 !"=0.3 1 2 3 4 5 log10(# earthquakes) 0 1 2 3 4 5 f te st b=1 !"=0.5 1 2 3 4 5 log10(# earthquakes) 0 1 2 3 4 5 f te st b=2 !"=0.1 1 2 3 4 5 log10(# earthquakes) 0 1 2 3 4 5 f te st b=2 !"=0.3 1 2 3 4 5 log10(# earthquakes) 0 1 2 3 4 5 f te st b=2 !"=0.5 1 2 3 4 5 log10(# earthquakes) 0 1 2 3 4 5 f te st figure 7: 28 avallone_corretto_layout 6 773 high-rate (1 hz to 20 hz) gps coseismic dynamic displacements carried out during the emilia 2012 seismic sequence antonio avallone1,*, elisabetta d'anastasio1, enrico serpelloni1, diana latorre1, adriano cavaliere2, ciriaco d'ambrosio1, sergio del mese1, angelo massucci1, gianpaolo cecere1 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6162 1. introduction in may-july 2012, emilia romagna (northern italy) was struck by a significant seismic sequence, which was characterized by two moderate-magnitude earthquakes: a ml 5.9 event on may 20, 2012, at 02:03:53 utc, and a ml 5.8 event on may 29, 2012, at 07:00:03 utc, about 12 km to the west of the first mainshock. the earthquake sequence produced a total of 20 casualties and severe and widespread damage, mainly to historical and commercial buildings. a detailed description of the seismic sequence can be found in scognamiglio et al. [2012, this volume]. the largest of the earthquake static displacements were recorded by tens of continuous global positioning system (cgps) stations, as described in serpelloni et al. [2012, this volume]. most of these stations were operating with a sampling frequency of 1 hz, and they belonged to scientific or commercial networks: ring (http://ring.gm.ingv.it); italpos (http://smartnet.leica-geosystems.it); geotop (http://www. netgeo.it); fondazione geometri emilia romagna (http://www. gpsemiliaromagna.it; lombardia [http://www.gpslombardia. it); and veneto (http://147.162.229.63). some hours after the first mainshock, the sampling frequency of the near-field ring stations (sbpo and mode) were switched to 20 hz, thus recording the coseismic displacements produced by the may 29, 2012, earthquake at higher frequency. this sampling frequency was previously used for the detection of coseismic dynamic displacements only for the mw 9 tohoku-oki 2011 event [colosimo et al. 2011b]. thus, the 20-hz-sampling displacements for the tohoku-oki 2011 earthquake and the may 29, 2012, emilia event might represent important recordings to investigate coseismic contributions at frequencies higher than 1 hz with gps. in the present study, after the description of the high-rate gps (hrgps) data analysis, we will show and compare the preliminary results. then, for the two mainshocks, we will compare the displacements recorded by the hrgps (1 hz up to 20 hz) and the strong-motion time histories (100 hz) at mode, where the different instruments were approximately co-located (figure 1, inset, relative distance of ca. 90 m). 2. high-rate gps data analysis strategies and results the available hrgps data were analyzed with two different approaches, using two noncommercial, geodetic-quality, software modules for kinematic processing: the track module of gamit (http://wwwgpsg.mit.edu/~simon/ gtgk/), which was developed at massachusetts institute of technology; and the gd2p module of gipsy/oasis ii (http://gipsy.jpl.nasa.gov), which was developed at the jet propulsion laboratory ( jpl, california, usa). we will briefly describe the computing strategies of these two software packages. then, we will analyze the noise levels of each solution and the consistency of the two different solutions, by comparing the epoch-by-epoch time series of the cgps stations located within 90 km of the epicenter and well placed along the two profiles shown in figure 1. finally, we will describe the peculiar mo05 results. the hrgps data of the ring stations are available at ftp://gpsfree.gm. ingv.it/emilia2012. 2.1. gd2p (gipsy/ oasis) data-processing strategy we used the gd2p.pl (version 6.1) module of the gipsy-oasis ii software package [bertiger et al. 2010], following the precise point positioning approach [zumberge et al. 1997] in kinematic mode. with this approach, single receiver ambiguity resolution is achieved by processing dual frequency gps data together with wide-lane phase bias estimates from a global gps receiver network [bertiger et al. 2010]. the same global network is used to tie in and calculate satellite fiducial orbits and clocks in the current reference frame. thus, we used the jpl final high rate (30 s sampled) article history received july 25, 2012; accepted september 13, 2012. subject classification: satellite geodesy, earthquake source and dynamics, crustal deformations, hrgps, gps data processing, strong motion. 2012 emilia earthquakes fiducial orbits and high rate (30 s sampled) clocks to process the code and phase the gps data, and to obtain the epoch-byepoch time series directly in the international terrestrial reference frame (itrf)08 reference frame [altamimi et al. 2011]. following avallone et al. [2011], the satellite fiducial orbits and clocks were held fixed to estimate the receiver clocks, random walk zenith troposphere delay, and white noise receiver positions. however, with respect to avallone et avallone et al. 774 figure 1. locations of the high-rate (1 hz and 20 hz) cgps stations operating during the emilia-romagna 2012 seismic sequence (squares). red triangle, co-located strong-motion station of mode; dashed boxes, n-s and w-e profiles of the coseismic displacements of figure 2a, b. the two largest events (yellow stars), their focal mechanisms (beach balls), and the spatial distribution of the ca. 2300 events aftershocks (dots) are shown, as updated to july 20, 2012 (data from http://iside.rm.ingv.it and described in scognamiglio et al. [2012]). the color scale indicates the depths of the aftershocks. 775 al. [2011], two main additional aspects were applied in this study: the absolute antenna phase center corrections for the gps receivers and transmitters, and the epoch-by-epoch ambiguity resolution [bertiger et al. 2010]. 2.2. track (gamit) data-processing strategy we used track (version 1.27), the kinematic module of the gamit/globk software [herring et al. 2010], to perform epoch-by-epoch solutions of the hrgps data and to obtain the three-dimensional time-series of the surface dynamic displacements. track uses floating point lc (l3) observations and the melbourne–wubbena wide-lane combination, with ionospheric delay constraints, to determine integer ambiguities at each epoch, adopting kalman-filter smoothing while estimating atmospheric delays. we use the international global navigation satellite systems (gnss) service (igs) final orbits and absolute antenna phase center model. different from the precise point positioning strategy of gipsy, track performs relative kinematic positioning, and it requires at least one reference station and one, or more, kinematic ('moving') station. for the 1 hz data, we chose the ring site of maga as a fixed, 'reference', station, as it was the one with the highest data quality (i.e., low multipaths) outside the epicentral area. having no gps stations available outside the epicentral area with 20 hz sampling frequency, we analyzed the higher-rate data available for mode at 10 hz, using as the fixed station the only 10-hzrecording ring site, ingr. although it has a longer baseline (mode-ingr, 336 km) with respect to the 1-hz solution (mode-maga, 100 km), the noise level of the 10hz solution was comparable to the 1-hz solutions (0.5 cm). the coordinates of the reference points are expressed in the itrf08 reference frame. 2.3. level of noise estimation and comparison of the hrgps solutions to quantify the noise levels of our hrgps solutions, for each station and each solution, we calculated the residuals with respect to a linear fit in the 40-s time window before the earthquake. then, we calculated the histograms of the residuals of all of the sites for the two solutions (figure 2a). for these two earthquakes, the distributions showed that, on average, more than 96% and 72% of the residuals were within 0.5 cm for the horizontal and vertical components, respectively. these values are representative of the noise level of each solution. moreover, to correctly compare different solutions for the same site, we also needed to quantify the consistency of each solution with respect to the other ones. thus, we compared the may 20 and 29, 2012, hrgps time series produced by the two analysis strategies, on two profiles (figure 2b) that were roughly oriented in cross-strike (ca. n-s) and alongstrike (ca. w-e) directions of the causative faults [serpelloni et al. 2012]. these profiles show the variations of the waveform amplitudes at each station with the distance from the epicenter as a traditional seismic array. we considered the hrgps time series recorded at conc as unreliable during the may 29, 2012, event, as discussed in serpelloni et al. [2012] for the static signal. to quantify the consistency between the gd2p and the track solutions, we firstly calculated the epoch-by-epoch differences between the two solutions site-by-site. then, we calculated the distribution of the so-determined residuals for all of the sites (figure 2c). the histograms of these residuals show that the two solutions agree within ±1 cm accuracy. thus, although significantly different, the differences in the parameter tuning and, in general, in the processing strategies did not significantly affect the observations of the coseismic dynamic displacements. this comparison indicates two additional aspects. first, the n-s and w-e profiles clearly show the propagation of the seismic waves generated by the occurrence of the may 20, 2012, ml 5.9 earthquake and the may 29, 2012, ml 5.8 earthquake, the epicenters of which are given in figure 1. the waveform amplitudes reached the noise level (ca. 3 mm) at stations located at a distance greater than 60 km from the epicenter. furthermore, considering the stations located approximately at the same distance from the epicenter, but on opposite sides (figure 2a, serm and pers; figure 2b, guas and fera-fer1), there appeared to be significant differences in the waveform amplitudes for the may 20, 2012, event. this difference might be related to a source directivity effect, and we will investigate this more rigorously and in more detail in future studies. in contrast, it is of note that there were no differences in the waveform amplitudes for the may 29, 2012, event. second, the north component of mo05 for the may 20, 2012, event shows an initial displacement of about 50 cm peak-to-peak before this quickly decreased in amplitude until the noise level (figure 2b). the first preliminary investigations have been performed to understand if this large displacement is reliable. the mo05 antenna was mounted on a 1-m-tall steel rod that was anchored to the top of the southward side of a school building in finale emilia, a village very close (6 km) to the epicenter. the side of the building where the antenna was located was not affected by macroscopic fractures or fissures, and neither liquefactions nor other local effects of the coseismic shaking were observed at its base. minor damage might have been localized in the points where the steel rod was fixed to the building, making the mounting less stable. however, if this were the case, an increase in the noise level in the mo05 time series would be expected. instead, the noise level before and after the mainshock that occurred and was recorded by the gd2p and track solutions spanning a 350-s time interval (figure 2d) remained unchanged on the hrgps dynamic motions of emilia mainshocks three components. the low noise level at mo05 allowed us to also detect the small coseismic dynamic displacements related to the two aftershocks immediately following the may 20, 2012, event. whether the observed displacements at mo05 are directly related to the ground movement or to amplification effects due to the building oscillations is still unclear. this last aspect will be further investigated in the near future. 3. hrgps and strong motion previous studies [larson et al. 2003, bock et al. 2004, ohta et al. 2006, 2012, bock et al. 2011, colosimo et al. 2011a] have shown that gps can detect seismic waves (both body and surface waves) for large earthquakes, and that hrgps position time series are comparable to strong-motion data. in the case of the mw 6.1 l'aquila 2009 earthquake, avallone et al. [2011] showed that the 1-cm accuracy avallone et al. 776 figure 2. a. distribution of the residuals of all of the time series in a pre-seismic 40-s time window for gd2p (red) and track (green). b. north (left) and east (right) components of the 1-hz gd2p (red) and track (green) time series of sites located along the n-s and w-e profiles, respectively, of figure 1, in a time interval of 190 s spanning the may 20, 2012, ml 5.9 (top) and may 29, 2012, ml 5.8 earthquakes (bottom). c. distributions of the residuals obtained by the epoch-by-epoch difference between the gd2p and track solutions on the north (left) and east (right) components. these histograms include the residuals of all of the sites of the two profiles in (b) for the may 20, 2012, event (doy 141, top) and the may 29, 2012, event (doy 150, bottom). d. detailed comparison between gd2p (red) and track (green) solutions for mo05 of may 20, 2012. the time series spans a time interval of 350 s. dashed vertical bars, occurrence of the may 20, 2012, mainshock and of the two immediately successive aftershocks. 777 hrgps dynamic motions of emilia mainshocks f ig u re 3 .c om pa ri so n be tw ee n th e h r g p s so lu ti on s an d th e st ro ng -m ot io n ti m e se ri es a t t he m o d e s it e fo r th e m ay 2 0, 2 01 2, m l 5. 9 (l ef t) a nd m ay 2 9, 2 01 2, m l 5. 8 (r ig ht ) e ar th qu ak es . t op : n or th c om po ne nt of th e st ro ng -m ot io n re co rd , i n ac ce le ra ti on , w it h th e pi ck in g of th e fir st a rr iv al s. c en te r: c om pa ri so n, in d is pl ac em en t ( no rt h co m po ne nt ), b et w ee n bo th th e m o d e g d 2p (r ed ) a nd t r a c k (g re en ) h r g p s so lu ti on s an d 10 0h zsa m pl in g se is m ic d at a (b lu e) . l ef t: 1 -h z h r g p s so lu ti on s. r ig ht : g d 2p a nd t r a c k s ol u ti on s, w it h 20 -h z an d 10 -h z sa m pl in g fr eq u en ci es , r es pe ct iv el y. b ot to m : n or m al iz ed c ro ss -c or re la ti on fu nc ti on s be tw ee n g d 2p a nd t he s tr on gm ot io n di sp la ce m en t (r ed ), a nd b et w ee n t r a c k a nd t he s tr on gm ot io n di sp la ce m en t (g re en ). b lu e lin e, a u to co rr el at io n fu nc ti on o f th e st ro ng -m ot io n di sp la ce m en t. of the hrgps solutions allowed the detection of the first arrivals at near-field hrgps sites also for moderate-magnitude earthquakes. both the may 20, 2012, ml 5.9 and the may 29, 2012, ml 5.8 emilia earthquakes were recorded by co-located gps and strong-motion instruments at the mode station (figure 1, inset: modena). for the first earthquake (figure 3a), the gps station was acquiring data with a 1-hz sampling frequency, whereas, for the second earthquake (figure 3b), the gps was operating with a 20-hz sampling frequency. as mentioned above, for the second mainshock, 20hz and 10-hz solutions were carried out by gd2p and track, respectively. the strong-motion station was equipped with a kinemetric episensor coupled with a gaia2 digitizer (produced in the ingv laboratories), and it was recording with at 100-hz sampling rate. on the n-s component of the strong-motion data, we marked the picking of the first arrivals (p-waves and s-waves) observed at this station (figure 3a, b, top). since the s-wave arrival appeared more uncertain for the second earthquake, we marked an interval within which we would suggest the s-wave was sure to have arrived (t1-t2). to analyze the similarity between the strong-motion time series and the hrgps solutions, we doubly integrated the strong-motion data, to obtain displacements (figure 3a, b, center). in a short time window (20-100 s), we then corrected the strong-motion displacement drifts by removing the linear trend and subtracting the mean of the displacement. the comparison between the displacement waveforms was obtained by cross-correlating both the hrgps solutions with the strong-motion data (figure 3a, b, bottom). for the may 20, 2012, event, the normalized cross-correlation functions revealed correlation coefficients of 0.85 and 0.84 for gd2p and track, respectively. considering the maximum value of 1 (autocorrelation of strong motion), we can affirm that the cross-correlations show a relatively good match between the different solutions. for the may 29, 2012, event, the normalized cross-correlation functions showed even better correlation coefficients (0.95 for gd2p, and 0.96 for track), potentially due to the higher sampling frequency, and then better similarity, of the hrgps solutions. in correspondence with the p-wave and s-wave arrivals on the displacement time series, we can observe a significant variation either in amplitude or frequency in both the hrgps waveforms, in good agreement with the strong-motion data. this observation confirms the possibility that hrgps can detect the arrival of the body waves. in figure 3b, the sampling rate of the hrgps solutions was higher than in figure 3a, and different between each other (20 hz for gd2p, 10 hz for track). however, no additional features appear to be present in the 20-hz solution with respect to the 10-hz solution. 4. conclusions the 1-hz sampling coseismic gps displacements were observed for the seismic wave propagation related to the two mainshocks of the emilia-romagna 2012 seismic sequence. the higher sampling solutions (10 hz and 20 hz), as well as the strong-motion data, are dominated by low frequencies, which is mainly due to two factors: 1) the lithology that generally outcrops in the investigated area, which is characterized by a huge amount of alluvial sediments of the po plain that quickly attenuate the high frequencies; 2) the distance from the epicentral area (35 km). the results of this study confirm previous studies concerning the possibility that hrgps measurements can detect both the coseismic static displacements and the body-wave-arrival dynamic displacements, without the drawback of the bias due to double integration when strong-motion data are used [boore 2001]. the several scientific and commercial cgps stations in italy, operating at high-rate (mainly 1 hz) sampling rates, allowed us to use the gps data as seismic data. a further effort in developing denser cgps networks that operate with sampling intervals ≥10 hz would provide helpful contributions, as well as the strong-motion data, to seismology. this applies from the study of the kinematic rupture processes of causative faults [miyazaki et al. 2004, yokota et al. 2009, avallone et al. 2011, yue and lay 2011] to the estimation of earthquake focal mechanisms [zheng et al. 2012]. furthermore, in the last few years, the real-time-telemetered dense cgps networks have allowed a move towards real-time gps seismology [colosimo et al. 2011a, ohta et al. 2012] for earthquake early warning [bock et al. 2011] or tsunami early warning [blewitt et al. 2006, sobolev et al. 2007] purposes. acknowledgements. we thank all of the agencies that made the gps observations available that are used in this study. particular thanks are addressed to sergio mantenuto and leica geosystems s.p.a. for the rapid distribution of the italpos cgps station 1-hz data. we thank the geophysics groups of the university of siena and the university of bologna for the maintenance of some of the local cgps stations (such as mo05) and for the distribution of the relative data. finally, we thank andre herrero for helpful discussions about the strong-motion and hrgps time series comparisons. we thank also giulio selvaggi for supporting this work. the figures were prepared using the gmt software package [wessel and smith 1998]. references altamimi, z., x. collilieux and l. métivier (2011). itrf2008: an improved solution of the international terrestrial reference frame, j. geodesy, 85, 457-473; doi:10.1007/s00190011-0444-4. avallone, a., m. marzario, a. cirella, a. piatanesi, a. rovelli, c. di alessandro, e. d'anastasio, n. d'agostino, r. giuliani, and m. mattone (2011). very high rate (10 hz) gps seismology for moderate magnitude earthquakes: the case of the mw 6.3 l'aquila (central italy) event, j. geophys. res., 116, b02305; doi:10.1029/2010jb007834. avallone et al. 778 779 bertiger, w., s.d. desai, b. haines, n. harvey, a.w. moore, s. owen and j.p. weiss (2010). single receiver phase ambiguity resolution with gps data, j. geodesy, 84, 327-337. blewitt, g., c. kreemer, w.c. hammond, h.p. plag, s. stein and e. okal (2006). rapid determination of earthquake magnitude using gps for tsunami warning systems, geophys. res. lett., 33, l11309; doi:10.1029/2006gl026145. bock, y., l. prawirodirdjo and t.i. melbourne (2004), detection of arbitrarily large dynamic ground motions with a dense high-rate gps network, geophys. res. lett., 31, l06604; doi:10.1029/2003gl019150. bock, y., d. melgar and b.w. crowell (2011). real-time strong-motion broadband displacements from collocated gps and accelerometers, b. seismol. soc. am., 101, 29042925; doi:10.17855/0120110007. boore, d.m. (2001). effect of baseline corrections on displacements and response spectra for several recordings of the 1999 chi-chi, taiwan, earthquake, b. seismol. soc. am., 91, 1199-1211. colosimo, g., m. crespi and a. mazzoni (2011a). real-time gps seismology with a stand-alone receiver: a preliminary feasibility demonstration, j. geophys. res., 116, b11302; doi:10.1029/2010jb007941. colosimo, g., m. crespi, a. mazzoni and t. dautermann (2011b). coseismic displacement estimation, gim international, may 2011, 25, 19-23. herring, t., r.w. king and s. mcclusky (2010). gamit reference manual, release 10.4, massachussetts institute of technology, cambridge, ma. larson, k., p. bodin and j. gomberg (2003). using 1 hz gps data to measure deformations caused by the denali fault earthquake, science, 300, 1421-1424. miyazaki, s., k.m. larson, k. choi, k. hikima, k. koketsu, p. bodin, j. haase, g. emore, and a. yamagiwa (2004). modeling the rupture process of the 2003 september 25 tokachi-oki (hokkaido) earthquake using 1-hz gps data, geophys. res. lett., 31, l21603; doi:10.1029/2004gl 021457. ohta, y., i. meilano, t. sagiya, f. kimata and k. hirahara (2006). large surface wave of the 2004 sumatra-andaman earthquake captured by the very long baseline kinematic analysis of 1-hz gps data, earth planets space, 58, 153-157. ohta, y., et al. (2012). quasi real-time fault model estimation for near-field tsunami forecasting based on rtk-gps analysis: application to the 2011 tohoku-oki earthquake (mw 9.0), j. geophys. res., 117, b02311; doi:10.1029/2011 jb008750. scognamiglio, l., l. margheriti, f.m. mele, e. tinti, a. bono, p. de gori, v. lauciani, f.p. lucente, a.g. mandiello, c. marcocci, s. mazza, s. pintore and m. quintiliani (2012). the 2012 pianura padana emiliana seimic sequence: locations, moment tensors and magnitudes, annals of geophysics, 55 (4); doi:10.4401/ag-6159. serpelloni, e., l. anderlini, a. avallone, v. cannelli, a. cavaliere, d. cheloni, c. d'ambrosio, e. d'anastasio, a. esposito, g. pietrantonio, a.r. pisani, m. anzidei, g. cecere, n. d'agostino, s. del mese, r. devoti a. galvani, a. massucci, d. melini, f. riguzzi, g. selvaggi and v. sepe (2012). gps observations of coseismic deformation following the may 20 and 29, 2012, emilia seismic events (northern italy): data, analysis and preliminary models, annals of geophysics, 55 (4); doi:10.4401/ag-6168. sobolev, s.v., a.y. babeyko, r. wang, a. hoechner, r. galas, m. rothacher, d.v. sein, j. schröter, j. lauterjung, and c. subarya (2007). tsunami early warning using gps-shield arrays, j. geophys. res., 112, b08415; doi:10.1029/2006jb 004640. wessel, p., and w.h.f. smith (1998). new, improved version of generic mapping tools released, eos trans. agu, 79, 579. yokota, y., k. koketsu, k. hikima and s. miyazaki (2009). ability of 1-hz gps data to infer the source process of a medium-sized earthquake: the case of the 2008 iwatemiyagi nairiku, japan, earthquake, geophys. res. lett., 36, l12301; doi:10.1029/2009gl037799. yue, h., and t. lay (2011). inversion of high rate (1 sps) gps data for rupture process of the 11 march 2011 tohoku earthquake (mw 9.1), geophys. res. lett., 38, l00g09; doi:10.1029/2011gl048700. zheng, y., j. li, z. xie and m.h. ritzwoller (2012). 5-hz gps seismology of the el mayor–cucapah earthquake: estimating the earthquake focal mechanism, geophys. j. int., 190, 1723-1732; doi: 10.1111/j.1365-246x.2012.05576.x zumberge, j.f., m.b. heflin, d.c. jefferson, m.m. watkins and f.h. webb (1997). precise point positioning for the efficient and robust analysis of gps data from large networks, j. geophys. res., 102, 5005-5018. *corresponding author: antonio avallone, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy; email: antonio.avallone@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. hrgps dynamic motions of emilia mainshocks << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice macroseisinie eviilence f o r the fault p i a n e a . g . galanopoulos under the assumption that the earthquakes are the result of faulting under the action of a couple the amplitudes of the longitudinal waves and those of the transverse waves are zero in ali directions situated in the fault piane (honda-emura, 1957). taking this info consideration, it is intuitively evident that the minimum radius of the felt area should occur in the direction of the fault piane. the unsymmetrical energy distribution from the hypocentre proved strong enough not to be masked by the influence of the inhomogeneity of the medium, especiallv of the upper layers, upon the isoseismal pattern (keilis borok, 1956). such being the case, the minimum radius of the macroseismic area should be used auxiliarly in the case it is not possible with the liei]) of the initial motion of transverse waves or in other way to determine which of the two nodal planes for longitudinal waves in the focus was the actual fault piane. the macroseismic evidence presumably fails in case of dip-slip motion and reasonably is a striking one in the transcurrent type earthquakes. the suggestion is illustrated by the data of some well studied shocks. the earthquake of no veni ber 2, 1954 near sumbawa tsland (8°. os, 119°. oe) is a very typical example: strike of the fault piane n 1° e; azimuth of the fault movement n 182° e (eitsema, 1957): diameter of the felt area in the e w direction about 800 km and perpendicular hereto 400 km only (ritsema, 1955). the ivern county earthquake of july 21, 1952 started from the white wolf fault. with the assumption that the strike of the earthquake fault is in azimuth of about 50° from north towards east, the seismic solution led to the following results (gutenberg, 1955): at the deptli of the source (about 10 miles) the fault piane has a dip of about 60° to 66° towards southeast; the slip along the fault was roughly up towards north in the upper block, down towards soutli in the lower; the vertical component of the slip was about 1.4 times that of the horizontal: the horizontal component produced a relative movement northeastward 190 a . g. g a l a n o p o u l o s in the upper block (southeast of the fault), southwestward in the lower (northwest of the fault). a s the motion was much closer to dip-slip motion than to strike-slip the macroseismic evidence is not conspicuous. however, the minimum radius of the felt area occurs clearly in the direction of the fault piane. the evidence is more clear under the asfig. 1. isoseismal map, kern county earthquake of july 21, 1952, after f. neumann and. w . cloud (1955), and tlie direction of faulting in tlie main shock (35° 00' n, 119° 01' w ) after b. gutenberg (1955). m a c r o s e i s m i c e v i d e n c e f o r t h e f a u l t p l a n e 191 sumption of the new solution obtained by g. sutton and e. berg (1958): fault piane striking zst 33° e, dipping 63° se, motion up and north on the se side, and a ratio of dip-slip to strfke-slip motion, r = 0.8. i n the fairview peak earthquake of december 16, 1951 the seismic solution obtained implies a fault striking n 11° w and dipping 62° fig. 2. isoseismal map, nevada earthquakes of deeember 16, 1954, after w. c l o u d (19.57), and the seismic solution for the direction of faulting in the fairview peak earthquake (39° 17' n, 118° 07' w), after c. romney (1957). to the east; motion along this piane was such that the east side of the fault moved toward 155° from north (measured clockwise) relative to the west side, and down at an angle of about 24° measured from the surface; the horizontal displacement should be about twice as large as the vertical component (romney, 1957). the seismic solution was com192 a . g. g a l a n o p o u l o s pletely confìrmed by the results of triangulation and leveling by the u. s. coast and geodetic survey. although the maeroseismic effects of the fairview peak earthquake were disturbed by those of the dixie valley shock (39°. 8 x , 118°. 1 w ) originated more than 4 minutes later, from an epicentèr about 55 kilometers to the north, and the isoseismal map shows intensity distribution for the two shocks without regard to which sliock may have been the responsible agent, the minimum radius of the felt area occurs clearly in tlie direction of the fault piane. fig. 3. intensity distribution in the area most strongly affected by the earthquake of aprii 30, 1954, after a. galanopoulos (1955) and the seismic solution obtained by j. hodgson and j. cock (1958). another example with a large dip component ( — .301) is the sophades earthquake of aprii 30, 1954 (39°.3 x , 22°.2 e ) : the area of strong shaking centered in a point near the village sophades shows clearly the relationship of the origin of the earthquake to the marginai fault of the southeastern side of the faulted basin of karditsa (galanopoulos, 1955). the solution obtained by j. hodgson and j. cock (1958) implies m a c r o s e i s m i c e v i d e n c e f o r t h e f a u l t p l a n e 193 a fault piane striking either x 86° e or n 46° w . the macroseismic evidence favours the b-plane striking n 46° w and dipping 78° towards southwest. diameter of the felt area in the direction of the fault piane 380 km (konitsa-laurion) and in that of the auxiliary piane 140 km (corfou-lemnos). on grounds of geological logie the fault piane for the samos earthquake of july 16, 1955 (37°. 9 n, 27°. .1 e ) is the a-plane of the seismic solution (hodgson-cock, 1958): maximum radius of the felt area in the direction of the a-plane (n 40° e ) 200-270 km (syros, milos) and in that of the b-plane (n 50° w ) 360-440 km (argalasti, domokos); motion was strike-slip (dip component + .105). as another argument may be cited the cephallenia earthquake of august 12, 1953 (di filippo-marcelli, 1954, hodgson-cock, 1958): maximum radius of the felt area in the direction of the fault piane (n 62°. 5 e ) about 520 km ( i i at catania) and in that of the auxiliary piane (n 31° w ) about 600 km ( i i i at foggia). under the above mentioned assumption of the fault mechanism the direction of maximum radiation of transverse waves is approximatelv at right angles to the piane of the fault. in most cases destructiveness produced by sliear waves is greater than that produced by other types of waves (benioff-gutenberg, 1955). since the strike of the fault piane of the great majority of the shocks is directed more or less perpendicular to the seismic and structural zones (ritsema, honda-emura, 1957), it is self-evident why the earthquakes are mostly felt over greater distances parallel to the structural lines than perpendicular thereto (sieberg, 1932-1933). this must be considered one more confirmation of the reliability of the macroseismic evidence for specifying the fault piane and the auxiliary piane normal to the motion vector. abstract since the amplitudes of the longitudinal waves and tliose of the transverse waves are zero in ali directions situated in the fault piane, the minimum radius of the felt area should occur in the direction of the fault piane. the unsymmetrical energy distribution from the hypocentre proved strong enough not to be masked by the influence of the inhomogeneity of the medium, especially of the upper layers upon the isoseismal pattern. such being the case, the minimum radius of the macroseismic area should be used auxiliarly in the case it is not possible with the help of the initial motion of transverse waves or in other ivay to determine which of the two nodal planes for longi194 a. g. g a l a n o p o u l o s tudinal ivaves in the focus ivas the actual facult piane. the macroseismic evidence presumably fails in case of dipslip motion and reasonably is a strildng one in the transcurrent type earthquakes. z usammenfass ung im falle einer erdbebendislokation unter der beanspruchung eines scherungskràftepaars ist die dislokationsebene im hypozentrum eine knotenebene fiir beide vorlauferwellen. die maximale ausstrahlung von sclierungswellen ist ungefahr senkrecht zu der bewegungsebene. die longitudinalwellen haben minimalamplituden in der riclitung der bewegungsebene und senkrecht dazu. fiir die erdbebenwirkungen und besonders fiir die in grossen epizentralentfernungen beobachteten makroseismischen erscheinungen sind die transversalwellen von alien erdbebenwellen in den meisten fàllen weitaus mehr schuldig. so muss die minimale ausbreitung der makroseismischen energie mit der streichrichtung der bewegungsebene zusammenfàllen bzw. der minimale durchmesser des makroseismischen schuttergebietes auf die orientierung des scherungskrdftepaars im hypozentrum hinioeisen. wenn die beivegungsrichtung senkrecht zum streichen der scherungsflache ist, sollen die makroseismischen data mutmasslich versagen. die makroseismischen data sóllten besonders zutreffen in dem zweiten grenzfalle, wo die bewegungsrichtung parallel zum streichen der scherungsflàche ist. das streichen der bewegungsebene verlàuft fiir die meisten erdbeben mehr oder minder senkrecht zu den seismischen zonen bzw. zu den stórungszonen. insoiveit die erdbebenverwerfungen senkrecht zu der faltenrichtung streichen, scheint die maximale ausbreitung der erdbebenenergie in der richtung der gebirgsketten physikalisch gut verstàndlich zu sein. riassunto poiché l'ampiezza delle onde longitudinali e di quelle trasversali è zero in tutte le direzioni situate nel piano di faglia, il raggio minimo della zona deformata dovrebbe verificarsi in direzione dei piani di faglia. la asimmetrica propagazione dell'energia dall'ipocentro si è dimostrata abbastanza forte ma non tanto da essere mascherata dall'inimogeneità del mezzo, e da riflettersi sull'andamento delle isosiste negli strati superiori. in questo caso, il raggio minimo della zona macrosismica dovrebbe essere preso in m a c r o s e i s m i c e v i d e n c e for t h e f a u l t p l a n e 1 9 5 considerazione soltanto quando non sia possibile usufruire dell'ausilio del movimento iniziale delle onde trasversali, o con qualche altro metodo, per determinare quale dei due piani nodali (risultanti dalla registrazione delle onde longitudinali) è stato il piano di faglia effettivo. l'uso di osservazioni macrosismiche cade presumibilmente in difetto nel caso di movimento di slittamento profondo, mentre diviene elemento positivo in un terremoto di scorrimento. r e f e r e n c e s b e n i o f f , h. g u t e n b e r g , b . , general introduction to seismology, in: «earthquakes in kern county, california during 1952 », state of calif., division of mines, bull., 171, 131-135 (1955). — mechanism and strain characteristics of the wlnte wolf fault as indicated by the aftersliock sequence, in: «earthquakes in kern county, ' california during 1952 », state of calif., division of mines, bull., 171, 199202 (1955). cloud, w., intensity distribution and strong-motion seismograph besults, nevada earthquakes of december 16, 1954, in: « bull. seism. soo. ani. », 47, 327-334 (1957). di f i l i p p o , d. m a r c e l l i , l . , uno studio sul terremoto di gefalonia (del 12 agosto 1953) con particolare riguardo alla natura fisica della scossa all'ipocentro, in: «ann. di geof. », 7, 547-561 (1954). galanopoulos, a., seismological institute bulletin, athens, 1954-1957. gutenberg, b., the first motion in longitudinal and transverse waves of the ma/in shock and the direction of slip, in: « earthquakes in kern county, california during 1952 », state of calif., division of mines, bull., 171, 165-170 (1955). — magnitude determination for larger kern county shocks, 1952 effects of station azimuth and calculation methods, in: « earthquakes in kern county, california during 1952 », state of calif., division of mines, bull., 171, 171-175 (1955). h o d g s o n , j . c o c k , j . , direction of faulting in the greek earthquakes of august 9-13, 1953, in: «ann. géol. des pays heuén. », 8, 29-47 (1956). — direction of faulting in some of the larger earthquakes of 1954-1955, in: «pubi. doni. observ. », 19, 223-258 (1958). honda, h . emura, k . , the production of the two-dimensionai elastic waves, in: «sci. rep. of the tóhoku univ.», ser. 5, geoph., 8, n. 3, 186-205, (1957). — m a s a t s u k a , a . e m u r a , k . , on the mechanism of the earthquakes and the stresses producing them in japan and its vicinity (second paper), in: «sci. rep. of the tóhoku univ.», ser. 5, geoph., 8, n. 3, 206-211 (1957). 196 a . g. g a l a n o p o u l o s keilis borok, v. i., methods and resulta of the investigations of earthquake mechanism, in: « pubi. bur. cent. séism. int. », ser. a, trav. scient. fase., 19, 383-394, (1956). neumann, h. cloud, w., strong-motion records of the kérn county earthquake, in: « earthquakes in ivern county, california during 1952 », state of calif., division of mines, bull., 171, 205-210, (1955). r i t s e m a , r . s o e t a d i , r . , the earthquake of november 2, 19-54 near sumbawa island, in: « verk. », 47, 1-35, djakarta 1955. —on the use of the transverse ìvaves in earthquake mechanism studies and the direction of fault displacement in se asian earthquakes, in: « yerh. », 52, 1-31, djakarta 1957. earthquake-generating stress systems in southeast asia, in: « bull. seisni. soc. am. », 47, 267-280, (1957). romnet, c., seismic waves from the dixie volley-fairview peak earthquakes, in: «bull. seism. soc. am. », 47, 301-319 (1957). sieberg, a., untersuchungen iiber erdbeben und bruchschollenbau ini óstlichen mittelmeergebiet, in: « denkschr. med.-naturw. ges. », 18, jena 1932.. erdbebenforschung und ihre yerwertung fiir teclinik, berqbau und geologie, jena 1933. sutton, g. berg, e., direction of faulting from first-motion studies, in: «bull. seism. soe. am. », 48. 117-127, (1958). vol52,6,2009new 739 annals of geophysics, vol. 52, n. 6, december 2009 key words history of seismology – historical documentation – information technology – historical database – tromos – euroseismos 1. introduction over the last few decades, historical documentation, both of a scientific and non-scientific nature, has acquired critical importance in the different sectors of seismology: i.e. historical seismology and historical seismometry. apart from the undisputed cultural and historical value of the testimonies concerning the italian historical earthquakes and the documentation of the long italian seismological tradition (ferrari, 1997), multidisciplinary methodological approaches to the study of past earthquakes have been taking shape, both from the macroseismic and the instrumental standpoints (boschi et al., 2000; guidoboni et al., 2007). in these innovative study sectors, italy has been a very important laboratory for methodological experimentation. the literature on the methodology and the results of these approaches are broad and consolidated. this paper analyses the contribution of the modern technologies to the valorisation and the dissemination of the historical-scientific materials on seismology and the products of their elaboration. among the most interesting materials, the epistolary documentation of a conventional scientific kind stand out together with specialised documents (macroseismic questionnaires). ininformation technology tools for the valorisation of seismology’s historical documentary heritage gabriele tarabusi (1) and graziano ferrari (2) (1) sga storia geofisica ambiente, bologna, italy (2) istituto nazionale di geofisica e vulcanologia, bologna, italy abstract in the past few decades historical documentation of a scientific and non-scientific nature has taken on critical importance in the different sectors of seismology: historical seismology and historical seismometry. modern technologies offer unique opportunities for cataloguing and efficiently distributing the reproductions or digital versions of scientific letters. over the last 20 years sga (storia geofisica ambiente) has set up and promoted, within the scope of the tromos (ingv-sga) and euroseismos (ingv-sga) projects, initiatives for the cataloguing, reproduction, study and dissemination of documentation relating to the history of seismology in both italy and europe. this approach, based on the application of modern technologies, has enabled us to achieve a rigorous processing and study of documents using the most appropriate information technology tools. the techniques we have used encourage the dissemination and study of historical scientific documentation seeking to bring together scholars and research groups within a new disciplinary community dedicated to studying the history of earth sciences. mailing address: dr. gabriele tarabusi, sga storia geofisica ambiente, via fioravanti 59/2, 40129 bologna, italy; e-mail: tarabusi@sga-storiageo.it vol52,6,2009new 2-12-2009 12:40 pagina 739 740 g. tarabusi and g. ferrari fig. 1. simplified scheme of the structure of the tromos database and the relations between the different files. although it can be managed in a relational way, in this case a hierarchical structure of the links has been preferred. the levels of the hierarchy correspond to the respective positions of the files in the figure. in particular, for the observatories there are two files: the one of the observatory’s history, in which all the information is summarised as well as the history of the observatory and the file of its variants (name, location, etc.). as regards the instruments, there are three files connected with one another: the file of the prototypes (the idea and the description of the instrument even if never actually made), that of the instruments actually made and located in the observatories and the file of the variants of the instruments devised (modifications, re-location etc.). one file collects the information related to scholar and the association. two files collect the information relating to the earthquakes: one relating to the whole sequence and one relating to each earthquake that is the reference for the file cataloguing the seismograms. the bibliographic file with the related texts of the sources and the information relating to the archive or the library of conservation, guarantees the documentary support to the information contained in the variation files on observatories, instrument variants, prototypes, scholars, seismic sequences, and seismological glossary. the database is potentially set up for sharing its files with other complementary files such as the cfti4med (guidoboni et al., 2007) is indicated on the right of the figure. different types of interface and interactions with other information technology tools allow for the use of the data in different modalities and a wide range of scientific and cultural users. vol52,6,2009new 2-12-2009 12:40 pagina 740 741 information technology tools for the valorisation of seismology’s historical documentary heritage deed, it is important from the scientific standpoint both in itself and as a complementary document to the seismological data sensu stricto (i.e. seismic recordings, seismological observations, etc.). 2. the tromos project over the last 20 years sga has performed and/or coordinated research and processed data within the scope of historical seismometry studies and the history of seismology in italy (tromos project) and europe (esc working group history of seismometry) (ferrari, 1997). started by ing in 1990, the tromos project has been addressed to the research, recovery, restoration and re-establishment of the historical heritage of italian seismology. the results of tromos (1990-2007) project in italy have led european seismological commission to establish the working group «history of fig. 2. the main consultation window of the tromos database contains four lists with summary information: observatories, instruments, scholars/builders/associations, epistolary correspondents catalogued in the database. a fifth list, at the bottom, is dedicated to collecting a selection of observatories, activated by double-clicking one ach line of interest of the observatories list, following activation of the button: «consultation/selection». a click on the button «map» allows us to create a map with the localisation of the selected observatories. double-clicking on each line of the list activates a specific detail window. vol52,6,2009new 2-12-2009 12:40 pagina 741 742 g. tarabusi and g. ferrari seismometry» (wg hos) in 1992. during its 17 years of activity the project’s chief aims were: – the listing of the centres of meteorological and seismic observation operating in italy (18th-20th century), with details of the current whereabouts of the relevant materials and historical instruments; – the restoration of some of the most important historic seismic instruments in the italian meteorological and seismological observatories; – the recovery and restoration of the historical seismograms of strong italian earthquakes recorded from the late 19th century in italian and european centres; – the dissemination of the historic and scientific results of the research performed. a dedicated indexing system has been set up to store the information gathered from the systematic collection and analysis of both published and unpublished contributions found in the course of the research. a dedicated database (ferrari, 1992), developed by sga on behalf of the istituto nazionale di geofisica e vulcanologia, has been designed for the electronic storage of the indexed information (fig. 1). thanks to the long experience of sga in the research and electronic storage of all kinds of environmental history data, the database, after its first release, was upfig. 3. observatories file, example of record. the following are indicated: name, international code, location, province or region, country, geographical coordinates and quota. the button «bulletins» open the window with the list of all the bulletins present in the data base, relating to the selected observatory (fig. 5). in the lower part of the window there is, only when the data are available, the list of instruments used in the observatory, present in the data base. when available these report, besides the name of the instrument, also the components and the start and end dates of function. with the button ‘constants’, the window instrument constants is accessed, which allows for the visualisation of all the characteristics of the selected instruments, which in turn allows for access to the consultation window of the constant history of the instrument (fig. 7). vol52,6,2009new 2-12-2009 12:40 pagina 742 743 information technology tools for the valorisation of seismology’s historical documentary heritage dated «on the field». the current state of the database has been verified after 16 years of improvement in the rationale and the efficiency of its structure and tools (figs. 2-9). the research numbers can be summed up as follows: 15,000 bibliographic and documentary sources (17th-20th century) have been found; they concern over 1,000 meteorological and seismological observatories, over 600 instruments and 250 instrument-makers; 5,000 of these sources have been entered in the database. over 5,000 letters have so far been individually and partly microfilmed and analyzed. approximately 70 historic centers with an observational tradition from the 1,200 catalogued ones have been visited or examined; about 150 instruments (from 1818 to the 1970s) out of the 600 ones registered in the database have been identified and photographed. apart from valorising the fundamental contribution of the italian scientific community to world seismology, the research performed within the scope of the tromos project and the related specialist and non-specialist scientific products developed (books, data base and internet web site) have allowed us to outline and create a relatively accurate picture of the consistency and the state of preservation of the instrufig. 4. observatories file, example of iconography – valerio meteoric-magnetic observatory in pesaro. fig. 5. bulletins file, consultation window of all the bulletins present in the data base, relating to the selected observatory. by highlighting a record from the list of bulletins it is sufficient to press the button «show text» to open the pdf file containing the digital scan of the original text. vol52,6,2009new 2-12-2009 12:40 pagina 743 744 g. tarabusi and g. ferrari ments and historical scientific documentary materials of the italian seismological observatories that operated from the 1850s until the 1970s. the most recent investigations have shown a worrying increase in the risk of dispersion of these assets. among these materials the station log book, scientific letters, seismograms, particularly rare italian and foreign seismic bulletins stand out. in other cases, documentary files very important for seismology are inaccessible or hard to trace unless they are properly classified, reproduced and transcribed. in fact, it is often a matter of handwritten materials that are quite difficult to read and that require a good knowledge of the contextual historical seismological and meteorological setting for a complete understanding. among these scientific materials produced by scholars in earth sciences an important role is played by the scientific letters in the reconstruction of the development of scientific instruments, the methods of inquiry and the scientific theories. scientific letters between meteorologists and seismologists from the 1850s quite often contain important scientific data that may be potentially useful even today. in the activity of averting the dispersion and incentivising the wealth of unpublished historical scientific documentation from the important historic meteorological-seismic observatories, starting from 2002 a fig. 6. instruments file, example of record relating to the dissemination of the 1,000 kg wiechert astatic horizontal seismograph. by double-clicking on a line of the list you can access the detail of the information on that specific instrument. highlighting a record from the list of bulletins it is enough to press the button constants to access the window instrument constants, which allows the visualisation of all the characteristics of the selected instrument and that in turn allows for the access to the consultation window of the constant history of the instrument (fig. 7). vol52,6,2009new 2-12-2009 12:40 pagina 744 745 information technology tools for the valorisation of seismology’s historical documentary heritage specific project of cataloguing and reproduction of scientific correspondence has been launched: letters in earth sciences (ferrari, 2002). the research, extended to the broader community of scholars and sites preserving historic materials, aims to promote and foster the identification, cataloguing, electronic scanning and dissemination of files relevant to the history of earth sciences and astronomy. at the same time the project also attempts to promote the recovery, in digital format, of everything hitherto published: letter catalogues, registries and whole letter transcriptions: digital document scanning should also support this phase. in this phase the project leaders are making contact with the various institutions that preserve historical correspondence having some relevance for this research, already partly transferred into printed materials, to sound out their willingness to collaborate. cataloguing, scanning and dissemination are carried out in compliance with any copyrights safeguarded by the applicable laws and on the grounds of any agreements signed with bodies or private individuals holding the files relevant to the study. within the scope of this project, we have catalogued and scanned the tacchini correspondence archives held at the ufficio centrale di ecologia agraria (central office of agrarian fig. 7. instruments seismic constants history file, consultation window of the constant history of the instrument. by selecting a line from the list the related bibliographic reference is highlighted in the text field «details» at the bottom. by clicking on the button «show text» the pdf file is opened containing the digital scan of the original text from which the information have been extracted. vol52,6,2009new 2-12-2009 12:40 pagina 745 746 g. tarabusi and g. ferrari ecology, rome) and the grablovitz archive at ischia, only recently made available by the heirs. the tacchini correspondence archives, crucially important for the history of seismology, comprise about 5,100 letters from one of the most important periods for seismology (from 1861 to 1900), during which the italian geodynamic service started taking shape, the forerunners of the geophysical network of the istituto nazionale di geofisica. for this archive the cataloguing has been completed followed by the digital reproduction of the correspondence having a historical seismological interest. the letter archive of giulio grablovitz at ischia was first of all catalogued and subsequently the digital reproduction of the correspondence having a prevalent seismological and meteorological interest was performed, for over 1,000 letters altogether, while the seismograms were restored and made available to the sismos centre for high-resolution digital scanning. a website (http://storing.ingv.it/letters/) has also been set up. the site allows for the selection of project database subsets by the name of the correspondents and by date-intervals (fig. 10). also being set up is a gis interface that fig. 8. scholars and associations file, example of record. apart from the biographical information, the window reports the list of the letters sent or received by the scholar. double clicking on the line of interest opens a window with the details of the related letter, also with the chance to open the pdf file of the integral raster reproduction of the letter. vol52,6,2009new 2-12-2009 12:40 pagina 746 747 information technology tools for the valorisation of seismology’s historical documentary heritage will allow for the geographical querying of the online database. 3. the euroseismos project digital preservation of the unique seismological patrimony consisting of historical seismograms and earthquake bulletins, and of related documentation (e.g., observatory log books, station books, etc.), is of utmost importance to avoid deterioration and loss over time (kanamori, 1988). dissemination of this seismological material in digital form is of equal importance to allow for the re-evaluation of past earthquakes using modern techniques and the re-evaluation of the seismic hazard. in 2002 at the esc held in genoa, a project entitled «saving and studying the seismograms of the strongest euro-mediterranean earthquakes» coordinated by graziano ferrari (sga – storia geofisica ambiente) and nicola alessandro pino (istituto nazionale di geofisica e vulcanologia, ingv) was started. the project was named euroseismos (es) and initially saw the participation of 24 researchers belonging to 15 countries which then grew to more than 40 researchers belonging to 31 countries from the euro-mediterranean area. the goal of the project was to exploit fully fig. 9. bibliographic file, main search window. in this file different kind of printed and manuscript documents (observatories, instruments, scholars, earthquakes etc.) are stored: selected papers from scientific journals, books, station bulletins, station handbooks, letters etc. each chart reports the bibliographic code (codbib) and the progressive order of the document and, if available at the current state of the research, the following information: name, role, geographical and chronological reference of the author; title, geographic and chronological reference of the document; the place and date of the publication; whether it is a publication, a manuscript or a copy of a manuscript. in the case of scientific correspondence, if further information is available for a letter, a button named «letter details» is activated, which, once pressed, opens the window with the details of the related letter, and the chance to open the pdf file of the integral raster reproduction of the letter. vol52,6,2009new 2-12-2009 12:40 pagina 747 748 g. tarabusi and g. ferrari fig. 10. simplified scheme of the process of acquisition and dissemination materials of the project letters in earth sciences. the letters processed in different forms (digital scanning, full transcription, registry) from the original place of preservation are catalogued and all the papers are added to the tromos database. the internet-based dissemination project of all the materials provides for the parametric (i.e. the name of the correspondents, chronological range, type of letter and contents, etc.) as well as geographical querying of the database. the first type of querying has already been set up, while the geographical type of approach is still undergoing completion. at the bottom, an example of the outputs list. the keys to the right of the correspondents’ names refer back to their respective biographies, while the button on the far right refers back to the image of the letter and the correlated information. vol52,6,2009new 2-12-2009 12:40 pagina 748 749 information technology tools for the valorisation of seismology’s historical documentary heritage fig. 11. «collection» section of the es website with progressive updating of the phases that have affected the seismograms coming from the different partner countries involved in the project. fig. 12. from the «seismograms available» section of the es website, it is possible to know for each observatory the details relating to the scanned seismograms available in the sismos web site. from this section it is also possible to have low-resolution preview access of the seismograms, know the mean values of the main instrumentals constants (from the tromos database) and know which earthquakes, among those chosen by the project partners, refer to each recording. vol52,6,2009new 2-12-2009 12:40 pagina 749 750 g. tarabusi and g. ferrari the potential offered by historical seismograms for reappraising historical earthquakes using modern techniques. the necessary steps for pursuing the above goals are: 1) a well organized data-collection, 2) high resolution digital scanning of the historical material, 3) dissemination of the digital images 4) fostering the growth of a euro-mediterranean community on the topic of the processing of the seismograms of the historical earthquakes. as the sismos facility of ingv (see michelini et al., 2005, for a recent update on its activities) had already set up a scanning laboratory with dedicated personnel (http://sismos.ingv.it), it was agreed that the image-processing activities and the data dissemination should be centralised at the ingv headquarters in rome, where the sismos facility is situated. the euroseismos project website (http://storing.ingv.it/es_web/) has been developed and updated by sga storia geofisica ambiente of bologna. thanks to regular updating of the data, all the partners have been able to follow the project stages (fig. 11) and gain a picture of the information contents available in real-time (figs. 12, 13 e 14). in the various section of the website, the pages are enriched with previous information for the study of the seismograms, archived in the tromos database thanks to the recovery and the digitalisation of the complementary documentation (figs.15-17). by the end of 2007, the project had been capable of collecting 30,000 historical seismograms from 132 observatories from a list of 611 earthquakes chosen by the participants (figs. 13 and 14). the scanned digital images have been assembled in dvd volumes and mailed to the contributing partners. similarly, all the historical records are viewable interactively at low resolution and can be requested at very high resolution on the «seismogram request» sismos web site. throughout the four years of the project, financial support was provided by ingv to sga. fig. 13. for each project partner a page is active with the list of chosen earthquakes, which allows us to access, by means of the link of the nterr column the updated situation of the seismograms and the data available for each event. vol52,6,2009new 2-12-2009 12:40 pagina 750 751 information technology tools for the valorisation of seismology’s historical documentary heritage sga was in charge of gathering and identifying the relevant material, organizing the exchange of materials among the participants and ingv, preparing and managing the euroseismos web site, and maintaining contacts with the project partners. in addition, the project has greatly benefited from the expertise of the sismos personnel for archiving and managing the abundance of digital raster images. finally, in september 2005, the «1st workshop on the vectorizazion of historical seismograms» was held in grottaminarda, italy, organized by ingv in collaboration with sga, during which the teseo2 vectorizer (http://sismos.ingv.it/teseo/, pintore et al., 2005) was presented and its use explained. about 20 participants from 10 countries of the euro-mediterranean region attended the workshop. fig. 14. from the «main and regional list» section of the es website (a), it is possible to access the consultation of the data available for each of the 611 earthquakes chosen by the project partners, both on chronological (earthquake catalogue) (b), and geographical grounds (earthquake map) (c), in relation to the country in which the epicentres are localised. from both windows it is possible to select the earthquake of interest and visualise an interactive map all the stations where seismograms scans are available in the sismos database, besides the epicentre of the event (d). by clicking on one of the sensitive points the list of seismograms available in the sismos database for that earthquake is visualised; for each one the main data present in the tromos database relating to the observatory and the recording instrument and its constants (e) are reported. by clicking on the icon of the left-hand column of the latter window (e) it is also possible to access a low-resolution preview of the seismograms. vol52,6,2009new 2-12-2009 12:40 pagina 751 752 g. tarabusi and g. ferrari as explained above, the project has paid special attention to the complementary documentation and has promoted the recovery, digital scanning and dissemination of those materials. seismic bulletins, station books, handwritten notes, scientific correspondence, copies of seismograms amount to a complementary documentation to the seismic recordings have a great bearing on their improved interpretation. this kind of documentation, being the subject of recovery and reproduction within the tromos project in italy, would also be the subject of special attention in euroseismos (figs. 15 and 16). in the various sections of the es web site a lot of precious information is made available to the researchers relating to the obfig. 15. example of consultation through the es website of the complementary documentation relating to the jena observatory, collected and scanned within the scope of the tromos and euroseismos projects and stored in the tromos database. in the main page of the station (a) the following are indicated: name, location, province or region, nation, international code, geographical coordinates and quota. also present, when available, are observatory photos. the tables relating to the instruments that have worked at the station follow, divided into mechanical and galvanometric. for each instrument the main average constants and the recording period are indicated; by clicking on the name a page opens with the image of the single instrument (b); through a link it is possible to access the page relating to the instruments’ seismic constant history (c); for each time-interval the constants refer to, there is a link that enables the viewing of the scan of the single page of the bulletin from which the information has been drawn (d). at the bottom, in the main page of the station (a) the available bulletins are listed in a table, with the indication of the author, the title and the year of the edition; through a link it is possible to open the pdf of the integral raster reproduction of the bulletin. vol52,6,2009new 2-12-2009 12:40 pagina 752 753 information technology tools for the valorisation of seismology’s historical documentary heritage servatories and the instrumental constants and their change in time, coming from such a complementary documentation and archived in the tromos database, both during the project and during the previous 17 years. within the framework of the euroseismos the project bulletins of the international seismological summary (iss) from 1918 to 1963, for a total of over 39,000 pages have also been scanned and collated. the original bulletins of the international seismological summary (iss) from 1918 to 1939 have been obtained thanks to funding provided by the us national science foundation through grant ear-9725140 (villaseñor et al., 1997) and have been scanned and collected by sga storia geofisica ambiente (bologna) thanks to funding provided by the istituto nazionale di geofisica e vulcanologia (rome). the scanned images of the bulletins of the international seismological summary (iss) from 1940 to 1963 have been obtained as part of a global earthquake relocation project (villaseñor et al., 1997) initiated with funding from the us national science foundation through grant ear-9725140 and collected by sga storia geofisica ambiente (bologna) on behalf of the istituto nazionale di geofisica e vulcanologia (rome). a dedicated section of the euroseismos site allows for the online consultation of the single pages of the iss bulletins or, alternatively, the download of each and every whole bulletin in pdf (fig. 17). the updating of the euroseismos site is guaranteed by a complex automatic procedure devised by sga, which starts from the data relating to the scanned seismograms coming from the sismos database. that procedure searches in the tromos database for all the information available relating to the station and the recording instruments and automatically fig. 16. euroseismos website: geographic access by country to the seismic bulletins of the different observatories. by clicking on the map it is possible to ace the list of bulletins collected during the project for each country. from this list it is possible to open the pdf file of the integral raster reproduction of each single bulletin. vol52,6,2009new 2-12-2009 12:40 pagina 753 754 g. tarabusi and g. ferrari generates the html pages for the consultation of the es site and illustrated in this chapter (fig. 18). 4. macroseismic questionnaires in 1985, during its research, sga came across a copious unpublished historical scientific documentation in the 16th century loft spaces of the ucea, which for years had been used merely as stock rooms. this documentation consisted of macroseismic questionnaires, seismograms, meteorological questionnaires and unpublished documentation of seismological and meteorological interest. the documentation of seismological interest was given out on study loan to sga by the ucea management. the operation achieved a dual objective: that of saving this material from the risk of decay and dispersion, and that of making available the scientific contents of the documentation for research into historical seismology and the history of seismology. the macroseismic questionnaires represent an original documentary format, fundamental in the reconstruction of the historical seismic scenarios. these are special pre-formatted questionnaires that were filled in when earthquakes occurred and then centralised at the central office of meteorology and geodynamics (subsequently at the ing). notwithstanding their peculiarity, these documents belong to the genre of scientific letters. in italy there are two important archives of macroseismic questionnaires: the one cited above (1871-1900), preserved at the ufficio centrale di ecologia agraria (central office of agrarian ecology of rome – ucea, now csa-cra), and the other preserved at the ingv that involves the subsequent period up to the present-day, with a few gaps in the 1970s. fig. 17. section of the es web site in which it is possible to consult the iss bulletins from 1918 to 1963 page by page and download them wholly in pdf. vol52,6,2009new 2-12-2009 12:40 pagina 754 755 information technology tools for the valorisation of seismology’s historical documentary heritage in the spring of 2003, on behalf of ingv (istituto nazionale di geofisica e vulcanologia), sga completed the digital scanning of the entire archive containing the macroseismic questionnaires (over 37,000 documents) of the ufficio centrale di ecologia agraria of rome (where the first italian public geodynamic service was founded in 1887). the documents that make up the ucea archive have been submitted to colour scanning at a resolution of 300 dpi. the reproduction has involved all of each document. in order to guarfig. 18. simplified scheme of the procedure for updating the euroseismos site. vol52,6,2009new 2-12-2009 12:40 pagina 755 756 g. tarabusi and g. ferrari antee a user friendly and complete consultation the 76,244 images have been set up in pdf files that maintain the uniqueness of the different documents. so a pdf file with the back and front images of the document identifies a macroseismic questionnaire, while a paper consisting of several pages has been laid out in a pdf file containing all the pages of the report. this processing has produced 37,467 file-documents. as for the other similar reproduction initiatives performed within the scope of the tromos project and in compliance with the rules for the use of archival materials, each pdf page presents a heading (fig. 19) that cite the body owing the documents, the body promoting its reproduction and the terms of use of the documentation reproduced. the single images were transferred to filedocuments in pdf complete with headings by means of a single automatic procedure performed by software, developed in c++ language and specifically designed. the 37,467 pdf files thereby obtained of the digital scans of the macroseismic cards of the ucea archive are all collected in 10 dvds that altogether occupy over 32 gigabytes. the numbering of the single pdf files and their organisation inside the folders largely follows the order present in the paper archive at the time of the scanning which, apart from some exceptions pointed out here, also represents the chronological order. an html index (fig. 20), created by means of automatic procedures specifically developed for this job, allows the chronological consultafig. 19. example of pdf file with the front and back images of a macroseismic questionnaire. the heading reports the institution owning the documents, the institution promoting the reproduction and the use of the documentation reproduced. vol52,6,2009new 2-12-2009 12:40 pagina 756 757 information technology tools for the valorisation of seismology’s historical documentary heritage tion, by year and month, of the whole. the advantage of a consultation by means of an html index is first and foremost its accessibility through any browser, irrespective of the platform and the operating system. this form of data organisation facilitates the online digital publication of the macroseismic questionnaires already reproduced and also opens up interesting applicative prospects or other analogues archives present in the seismological research bodies both in italy and abroad. given the territorial nature of the seismic event, an application has also been developed that allows us to consult the digital files of the macroseismic questionnaires on a geographical basis. this application has so far been tested on some sample events, and not on the whole file, in that for it to work properly it is necessary to carry out a preliminary task of re-coding the single pdf files, which, given the size of the file, calls for a large amount of time. after a thorough job of comparing the cards with the toponomastic catalogues and the earthquake catalogue, the pdf files are renamed on the grounds of a pair of codes that univocally identify the location of origin of the document (nloc) and the seismic event whose effects are being described (nterr). subsequent to such a codification, it becomes possible to automatically create, through a mapinfo professional tool developed using mapbasic language, an interactive map for each earthquake (fig. 21), which visualises all the locations from which the questionnaires had been sent, thus allowing the geographic access to the documents. the cartographic tool developed first of all generates the earthquake list on the grounds of the nterr codes present in the file names, taken only once. for each line of that list it identifies the nloc associated to the nterr and it extracts from the database the basic information relating to the seismic event (intensity and epicentral localisation) and the different locations (geographic coordinates). the individual maps of the distribution of fig. 20. initial html page (home) for the chronological consultation of the digital of the macroseismic questionnaires. in the top frame the project credits are given; in the one on the left there is a general index with the links for the navigation within the archive; in the one on the right-hand side, the history of the archive is concisely told, from its birth to the digital scanning in 2003. vol52,6,2009new 2-12-2009 12:40 pagina 757 758 g. tarabusi and g. ferrari the information for each event are thus created. thanks to the nloc-nterr codification it is also possible, by means of a special and purpose-designed hot-link key to access the documents directly from the map itself. 5. conclusions the methodological approach followed by sga in the valorisation of historical scientific documentary heritage from 1986 to the present, based on the application of information technology tools that have enabled us to achieve a rigorous treatment and study of documents, is always oriented to such techniques and solutions that offer widescale accessibility to the documentation and that allow a modern form of dissemination. the methodological approach followed by sga is also oriented to creating a new disciplinary community dedicated to studying the history of the earth sciences. the cases we have reported here comprise some possible examples of new forms of interaction between research in historical seismology and the techniques used to encourage the dissemination and study of historic scientific documentation. references boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (2000): catalogue of strong italian earthquakes from 461 b.c. to 1997, introductory texts and cd-rom, version 3 of the catalogo dei forti terremoti in italia, annali di geofisica, 43 (4), 609-868. ferrari, g. (1992): the tromos project 1990-1: a laboratory for the history of seismology, in two hundred years of seismic instruments in italy 1731-1940, edited by g. ferrari, (ing-sga, bologna), 14-18. fig. 21. example of interactive map allowing to geographically access the consultation of the digital file of the macroseismic questionnaires. by clicking on the map the documents relating to each location can be visualised and interpreted. vol52,6,2009new 2-12-2009 12:40 pagina 758 759 information technology tools for the valorisation of seismology’s historical documentary heritage ferrari, g. (1997): cultural and scientific value of seismology’s heritage in europe: why and how to preserve, proceeding of the workshop: historical seismic instruments and documents: a heritage of great scientific and cultural value, edited by g. ferrari, may 16th to 18th, 1994 walferdange (grand-duchy of luxembourg), conseil de l’europe, cahiers du centre européen de géodynamique et de séismologie, vol. 13, 130. ferrari, g. (2002): census, filing and elaboration of scientific letters in the earth sciences, nuncius, 17 (1), 307320. guidoboni, e., g. ferrari, d. mariotti, a. comastri, g. tarabusi and g. valensise (2007): cfti4med, catalogue of strong earthquakes in italy (461 b.c.-1997) and mediterranean area (760 b.c.-1500), ingv-sga, available from http://storing.ingv.it/cfti4med/. kanamori, h. (1988): importance of historical seismograms for geophysical research, in historical seismograms and earthquakes of the world, edited by w.h.k. lee, h. meyers and k. shimazaki, (academic press), 16-33. michelini, a., b. de simoni, a. amato and e. boschi (2005): collecting, digitizing, and distributing historical seismological data, eos trans. agu, 12 july 2005, 86 (28). pintore, s., m. quintiliani and d. franceschi (2005): teseo: a vectoriser of historical seismograms, computers & geosciences, 31, 1277-1285. villaseñor, a., e.a. bergman, t.m. boyd, e.r. engdahal, d.w. frazier, m.m. harden, j.l. orth, r.l. parkes and k.m. shedlock (1997): tovard a comprehensive catalog of global historical seismicity, eos trans. agu, 78 (50), 581, 583, 588. vol52,6,2009new 2-12-2009 12:40 pagina 759 vol52,3,2009 255 annals of geophysics, vol. 52, n. 3/4, june/august 2009 key words ionospheric modeling – ionospheric forecasting – ionospheric predictions 1. introduction ionospheric predictions of four different types are essential for mitigation of ionospheric effects on radio systems: (i) long-term ionospheric predictions that supply information on the ionosphere for a particular epoch of solar activity and can be used for planning radio systems operations. their success depends on the long-term solar activity prediction (belehaki et al., 2007). (ii) ionospheric now-casting where realtime observations are used to show the geographical and temporal variations of the ionospheric parameters (zolesi et al., 2008). this type of ionospheric specification is important for management of radio services in near real-tinear-earth space plasma modelling and forecasting hal j. strangeways (1), ivan kutiev (2), ljiljana r. cander (3), stamatis kouris (4), vadim gherm (5), diego marin (6), benito de la morena (7), s. eleri pryse (8), loredana perrone (9), marco pietrella (9), stanimir stankov (10), lukasz tomasik (11), ersin tulunay (12), yurdanur tulunay (12), nikolay zernov (5) and bruno zolesi (9) (1) school of electronic and electrical engineering, university of leeds, leeds, uk (2) geophysical institute, bulgarian academy of sciences (bas), sofia, bulgaria (3) rutherford appleton laboratory, didcot, uk (4) electrical and computer engineering department, aristotle university of thessaloniki, greece (5) department of radiophisics, university of st. petersburg, russian federation (6) university of huelva, huelva, spain (7) atmospheric sounding station el arenosillo, inta, huelva, spain (8) aberystwyth university, aberystwyth, uk (9) istituto nazionale di geofisica e vulcanologia, roma, italy (10) royal meteorological institute, brussels, belgium (11) center for space research, warsaw, poland (12) middle east technical university (metu), ankara, turkey abstract in the frame of the european cost 296 project (mitigation of ionospheric effects on radio systems, miers) in the working package 1.3, new ionospheric models, prediction and forecasting methods and programs as well as ionospheric imaging techniques have been developed. they include (i) topside ionosphere and meso-scale irregularity models, (ii) improved forecasting methods for real time forecasting and for prediction of fof2, m(3000)f2, muf and tecs, including the use of new techniques such as neurofuzzy, nearest neighbour, cascade modelling and genetic programming and (iii) improved dynamic high latitude ionosphere models through tomographic imaging and model validation. the success of the prediction algorithms and their improvement over existing methods has been demonstrated by comparing predictions with later real data. the collaboration between different european partners (including interchange of data) has played a significant part in the development and validation of these new prediction and forecasting methods, programs and algorithms which can be applied to a variety of practical applications leading to improved mitigation of ionosphereic and space weather effects. mailing address: dr. hal j. strangeways, university of leeds, leeds, uk; e-mail: h.j.strangeways@leeds.ac.uk vol52,3,2009 22-09-2009 14:19 pagina 255 256 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi me. (iii) shortterm ionospheric prediction that can be based on extrapolation of past data sets, forecasting of a short-term disturbance index on which the ionosphere critically depends and monitoring of ionospheric propagation conditions in connection with solar-terrestrial parameters (bremer et al., 2006). this is required to improve the quality and reliability of radiocommunication services, including frequency adaptive applications at mf and hf and transionospheric radio determination (wheadon et al., 2006). (iv) prediction of special disturbing factors like scintillations, which disturb gps signals. the terms of reference of working package 1.3 (wp 1.3) «near-earth space plasma modelling and forecasting» comprised a large area of near-earth plasma modelling, supporting basic research, forecasting and radio communications. the structure of this paper follows the main topics of investigations in wp 1.3, viz: 2. topside model development and testing. 3. development of techniques for real-time forecasting. 4. forecasting of fof2, m(3000)f2, muf and tec. 5. tomographic imaging and model validation. 6. modelling mid-scale (meso-scale) inhomogeneities at low-latitude (bubbles) and high-latitude (patches, blobs) to predict their scintillation effects. a catalogue of ionospheric forecasting and predicting models developed in the framework of cost actions 238, 251, 271 and 296 has been compiled. the catalogue and brief description of all 18 models are available at: http://www.cost296.rl.ac.uk/word/cataloguetext.doc 2. topside models development and testing 2.1. topside electron density model triskova et al. (2007), have developed a model, providing electron density distribution at fixed altitudes as functions of invariant latitude and magnetic local time. a system of associated legendre polynomials up to the 6th-order are fitted to the data from four satellites: hinotori, iss b, isis 1, and isis 2. the full model consists of submodels for individual seasons and altitude ranges. triskova et al., 2007 have developed a topside electron density model by using vary-chap functions for the scale height. at each given location, ne submodels provide the anchor points for each altitude level, which are then used to construct the vertical profiles. this new topside presentation avoids the «kinks» in the profiles resulting from applying the booker formulism. an example of the topside model is shown in fig. 1. it is one of options in the international reference ionosphere (iri). 2.2. tsmp-assisted topside reconstruction to improve the accuracy of the real time topside electron density profiles given by the digisonde software, a new model-assisted technique is used (kutiev et al., 2009). this technique uses the topside sounder model (tsm), which provides the plasma scale height (hs), o+-h+ transition height (ht), and their ratio rt=hs/ht, derived from topside sounder data of alouette and isis satellites. kutiev and marinov (2007) introduced the concept of a new profiler «tsmp», using their scale height (hs) and transition height (ht) models. tsmp provides the shape of the topside n(h) profile as the sum of the o+and h+profiles from hmf2 up to gps heights. the o+ profile takes hs as its own scale height (ho+=hs), at the transition height both densities are equal, and the scale height above h+ (hh +) is taken as 16 times the o+ scale height. for the present analysis we use tsmp in the form: (2.1) f2(ht)=no+=nh+ at transition height ht. to calculate the topside density profile, tsmp needs values of nmf2 and hmf2 at the lower boundary (denoted as nm and hm), which are supplied by digisonde measurements. in addition, the digisonde also provides the topside scale 2 ,exp exp exp exp n n h h h h h h n n h h h ne f h f h f h h h h 2 1 1 2 2 m m m h h m h t t h t 0 0 0 = = = = + + + + + + + + b ] b ] ] b l g l g g l ; e' 1 vol52,3,2009 20-09-2009 19:06 pagina 256 257 near-earth space plasma modelling and forecasting height hd. it is assumed that, for a particular moment in time, hd represents the real conditions, and is therefore a better value than the average model value of hs. when hd is specified, the corresponding transition height ht is obtained through the ratio rt, e.g. ht = hd.rt. to illustrate the general concept, we apply tsmp to reconstruct the topside ne profile in two particular cases. figure 2 show two examples of profile reconstruction for athens. 2.3. tsm model development and testing the topside ionospheric scale height (hs) and the o+-h+ transition height (ht) are key ionospheric parameters which are of special interest when studying and modelling the plasma composition and dynamics. recently, the hs (kutiev et al., 2006) and ht (marinov et al., 2004) empirical models have been combined in a unified topside sounder model (kutiev and marinov, 2007). the database for these models has been built upon 170,000 hs and ht values deduced from electron profiles obtained via topside sounding measurements. for validation purposes, stankov et al. (2007) compared tsm model results with those from the well-known models nequick and the parameterized ionospheric model (pim). electron density profiles, in the altitude range 200-2000 km, are obtained from the nequick and pim models for a grid of input parameters, such as month, local time, geomagnetic latitude, and solar activity. the scale height and transition height were extracted from the model profiles in the same way they were extracted from the measured topside sounder profiles. their values are then compared with the respective values provided by the tsm model. comparison of the scale height and the transition height show that, in general, the nequick and pim models provide higher values of both scale height and transition height than those of tsm. fig. 1. examples of ne vertical profiles constructed by using densities at the sub-model altitudes, taken as anchor values, along with corresponding iri models, for two levels of solar activity. vol52,3,2009 20-09-2009 19:06 pagina 257 258 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi briefly, the comparison results for the plasma scale height can be summarized as follows: • pim generally highest values during high solar activity • pim unusually high values during an hsa day at low and middle latitudes • nequick highest values (unusually high) in lsa winter. the comparison results for the ion transition height can be summarized as follows: • pim systematically higher (probably overestimated) values at hsa • nequick relatively weaker dependence on solar activity • nequick discontinuities over the equator occur at night time. 2.4. modeling tec and slab thickness variations considerable efforts have been devoted to quantifying the response of tec to changes in solar activity, location, season and time of day. kouris et al. (2006; 2008) found that tec is highly correlated with solar activity and latitude and also with season. applying a least squares analysis to the vertical tec obtained from gps measurements at different european locations and the corresponding solar flux density data at 10.7 cm, the total electron content relative to the quiet sun level (to ) at each hour/ month/ location is counted. then, these to values are correlated with corresponding values of the seasonal parameter cosχ at noon at each location according to the following least squares equation to = k (cosχ12 )m (2.2) where cosχ12 is the cosine of the solar zenith angle at noon (a seasonal parameter). the calculated values of the factor ‘k’ and the exponent ‘m’ determined by using monthly median vertical tec values measured at noon and/or any other hour, exhibit a latitudinal dependence (kouris et al., 2009) since tec depends on latitude. the slab thickness, an important parameter in the determination of n2max from tec measurements, is also investigated and some specifications are established. it has a diurnal variation (kouris et al., 2006) in winter from about 200 km to 450 km on average, whereas in equinoxes fig. 2. tmsp profile reconstruction for athens: at 04:30 ut (left) and 16:30 ut (right) on 1 october 2000. tsmp-assisted digisonde (tad) profiles are shown the heavy line; solid and dashed curves show o+ and h+ distributions respectively. for comparison, champ-based reconstruction (heise et al., 2002) profiles are given by the grey curves. vol52,3,2009 20-09-2009 19:06 pagina 258 259 near-earth space plasma modelling and forecasting and summer, this ranges from circa 300 km to 450 km depending on the time of day. in winter and equinoxes the nighttime values are much higher than the day-time, whereas in summer the opposite occurs so that the day values are higher than the nighttime ones. the predawn peak is found to occur in winter and equinoxes from about 500 km to 700 km and in summer from circa 400 km to 450 km. the monthly average values of the slab thickness all over europe are practically the same, within a range of circa 20 km to 30 km during day-time. some greater differences exist during nighttime due to disturbed conditions which depend on location and occur predominantly during nighttime (fotiadis et al., 2004; kouris et al., 2005). the equivalent slab thickness is independent of solar activity and latitude (kouris et al., 2008) but depends strongly on season (kouris et al., 2009). a mean value of the equivalent slab thickness for day-time conditions (cosine of the solar zenith angle greater or equal to 0.10) can be determined from the following analytical expression: est. = 370 (cosχ12 ) 0.44 (2.3) for nighttime conditions (i.e. cosine of the solar zenith angle less than 0.10) a mean value can be calculated from the equation: est = 313 (cosχ12 ) -0.24 (2.4) these relationships are valid for locations within the latitudinal range of 30 to circa 60 degrees. higher latitude locations are excluded because of the decreased stability of the f-region in this zone (rawer et al., 2003). neglecting diurnal and seasonal variations, an overall mean slab thickness for europe (circa 30on to 55on) can be taken as 323 km with a standard deviation less than 50 km. 3. development of techniques for real-time forecasting a study has been carried out on those major areas where our current physical understanding and recent advances can lead to positive prediction, forecasting and mitigation of the expected effects of ionospheric activity on the near earth space environment and on technological systems which operate within this environment (cander and mihajlovic, 2005; cop et al., 2008; cander and ciraolo, 2008). the results show some of the key links between solar activity and the various physical processes, which govern ionospheric plasma structure. specific examples during extremely intense solar event as well as during low solar activity illustrate how ionospheric monitoring techniques that have contributed immense data sets and related empirical and theoretical formulations can be incorporated in different models for real-time operational applications (jansen et al., 2006; cander 2009). an example of the long-running services developed in the course of the european ionospheric cost actions is the «space weather web facilities for radio communications users» at rutherford appleton laboratory, uk since 1998. in this on-line service the real-time ionospheric observations have been integrated into suitable methods and models to provide a number of products. an overall extensive validation procedure for these products has shown that the combined subsequent ionospheric predictions are essential for any mitigation of ionospheric effects on radio systems: 1) long-term ionospheric predictions that supply information on the ionosphere for a particular epoch of solar activity and can be used for planning radio systems operations. their success depends on the long-term solar activity prediction (belehaki et al., 2007); 2) ionospheric now-casting where real-time observations are used to show the geographical and temporal variations of the ionospheric parameters (zolesi et al., 2008). this type of ionospheric specification is important for management of radio services in near real-time; and 3) shortterm ionospheric forecasting (stif) that can be based on extrapolation of past data sets, forecasting of a short-term disturbance index on which the ionosphere critically depends and monitoring of ionospheric propagation conditions in connection with solar-terrestrial parameters (muhtarov et al., 2002; bremer et al., 2006). figure 3 shows a comparison between the vol52,3,2009 20-09-2009 19:06 pagina 259 260 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi fof2 produced by the short-term ionospheric forecasting method and the data from chilton (51.50 n, -1.30 e) ionosonde. it can be seen that this forecasting facility performs very well during quiet geomagnetic periods. although it falters at intense geomagnetic events, it still provides a valuable ionospheric predictor over europe, having in mind the storm distribution given in fig. 4. 4. forecasting of fof2, m(3000)f2, muf and tec 4.1. fof2 forecast during severe geomagnetic activity in rome observatory to forecast the ionospheric response to geomagnetic storms, geomagnetic indices have been introduced taking into account their history (wrenn, 1987; wu and wilkinson, 1995; perrone and de franceschi, 1999). one of these is ap(τ) (wrenn, 1987) derived with a time weighted series accumulation from the geomagnetic planetary index ap. the attenuation multiplier τ ranges between 0 and 1 and determines the weights assigned to past ap values. the larger is τ, the smoother are the ap variations. an improved linear correlation was found between transformed data obtained from hourly and monthly median values of fof2 and ap(τ) with τ= 0.8 (perrone and de franceschi, 1999). the transformed data of fof2 that gives the best result is log(nmf2(t)/nmf2m(t)), where nmf2(t) is the hourly value of the maximum electron density at the f2 peak and the suffix m indicates the monthly median value (wu and wilkinson, 1995). instead for the monthly median representing the «quiet» ionosphere, an average is calculated by perrone et al. (2007). it is obtained considering, for a selected hour, fof2 values with ap(τ)�7 in the thirty days preceding the day for which we want to have the «quiet» value. this average is called the «daily» mean. fig. 3. measured and 24 hours in advance forecast fof2 values during quiet and disturbed days in january 1999 at chilton ionospheric station. vol52,3,2009 20-09-2009 19:06 pagina 260 261 near-earth space plasma modelling and forecasting the data utilized are the fof2 hourly values measured at the rome (41.9° n; 12.5° e) observatory. geomagnetic storms with a maximum of ap�132 are classified as strong events. an example of fof2, forecast with the relationship between the fof2 transformed data and ap(τ), is presented in fig. 5, which illustrate the results of forecasting for the period 12-17 july 2000, when a negative ionospheric storm occurred. the r.m.s. of forecast is 1.23 mhz and for the «daily mean» it is 2.0 mhz. the model was further developed to adapt it for different seasons and ap levels. the new model, called ifelmor (ionospheric forecasting empirical local model over rome, pietrella and peronne, 2008) uses different sets of coefficients for each month of the year and several ranges of ap(τ). ifelmor predictions are compared in table i with those of the iri-storm model in terms of r.m.s. error for selected days with very disturbed ionospheric conditions (daily mean of ap(t = 0.9), ap(τ = 0.9) > 70). the best performance is labelled in bold. 4.2. neurofuzzy techniques applied to model and predict the f2-layer critical frequency fof2 a new methodology to predict the ionospheric f2-layer critical frequency, fof2, 1-24 hours in advance has been advanced. the proposed method is based on artificial intelligence techniques, specifically, on neuro-fuzzy modelling, very little used in ionospheric modelling until now. such techniques have a natural capability to model well non-linear and complex systems. neurofuzzy models incorporate from fuzzy models the ability to understand and connection with physical processes and, on the other hand, from neural networks based models, the capability of adaptation and learning. neurofuzzy models allow incorporation of exfig. 4. years count of magnetic storms during solar cycle 23 (1996-2007). 14 12 10 8 6 4 2 0 1996 1997 1998 1999 2000 2001 years n u m b e r o f st o rm s w ith d st < -1 0 0 n t 2002 2003 2004 2005 2006 2007 vol52,3,2009 20-09-2009 19:06 pagina 261 262 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi pert knowledge in different parts of the modelling process. to predict fof2 n hours ahead, the values of fof2 at the current time t, at two previous hours, and 24 and 25 hours in the past are considered, along with the respective ap values. for predictions at 1 or 2 hours ahead, fof2 values from the previous day are not taken into account. in order to test the fof2 neurofuzzy models, the relative mean deviation (rmd) between observations and short-term fof2 predictions (1-24 hour lead times) were calculated for 100 periods of 3 consecutive days. values of fof2 were observed at slough in 1980-82. 62.1% of the selected 300 days are geomagnetic quiet days (ap=10), the other 37.9% correspond to moderate geomagnetic activity (10<ap<40). as can be seen from table ii, best predictions are made for 1-3 hours in advance, where the rmd varies from approximately 4 to 8%. for larger lead times, the rmd stays approximately the same (around 9-10%) and the prediction accuracy does not depend significantly on the lead time. for lead times larger than 3 hours, prediction accuracy decreases significantly, with an rmd around 20-30%. these conclusions were reached by calculating the rmd over 15 periods of 3 days with ap>40. despite this poor accuracy for lead times larger than 3 hours, the great efficiency of the neurofuzzy model to predict 1-3 hours in advance, even under very disturbed periods, should be noted. to illustrate this result, the variations of observed and 1hour predicted fof2 values for three time periods, when effects of strong negative storms were observed, are represented in fig. 6. it can be noticed how both observed and predicted fof2 variations are very close. table i. comparison of predictions between ifelmor and iri-storm. day/month/year ifelmor stormmedians medians stormfof2qt fof2qt ap(t = 0.9) r.m.s (mhz) r.m.s (mhz) r.m.s (mhz) r.m.s (mhz) r.m.s (mhz) 16th july 2000 0.47 0.56 1.90 0.42 2.24 77.8 31th march 2001 1.70 2.81 2.97 4.11 4.48 77.1 1th april 2001 1.54 1.71 1.98 2.02 3.34 78.7 30th october 2003 0.87 1.87 1.89 6.78 6.83 125.1 31th october 2003 0.83 2.33 2.34 6.62 6.65 147.8 1th november 2003 1.88 0.83 0.73 3.03 2.83 81.6 21th november 2003 0.73 0.76 0.80 1.55 1.40 77.8 global r.m.s. error 1.34 1.78 1.94 4.02 4.25 table ii. relative mean and standard deviations between observations and predictions. lead time (hours) rmd(sd) 1 2 3 6 12 18 24 3.92 7.91 8.24 9.11 9.73 9.88 9.92 (1.65) (1.36) (1.45) (1.51) (1.83) (2.06) (1.85) vol52,3,2009 20-09-2009 19:06 pagina 262 a p (τ ) 263 near-earth space plasma modelling and forecasting fig. 5. the behaviour of fof2 observed, forecasted and of the «daily» mean for 12-17 july 2000. fig. 6. 1-hour prediction examples over 3 very disturbed periods. the ap index value is given to indicate the geomagnetic activity level. 0 24 48 72 96 120 2 4 6 8 10 12 14 16 observations 1-hour predictions rmd (%) = 5.30 fo f 2 v a lu e s (m h z) 0 24 48 72 96 120 144 19.4 61.9 42.9 13.1 18.8 7.3 25 26 27 28 29 30 ap value: 39.0 95.8 120.6 26.8 8.1 day: 11 12 13 14 15 observations 1-hour predictions rmd (%) = 3.84 may 1981 time (hours) april 1981 april 1981 0 24 48 72 96 120 11.3 39.4 32.0 60.8 13.3 8 9 10 11 12 observations 1-hour predictions rmd (%) = 4.36 fo f 2 (m h z) 12-17 july 2000 200 150 100 50 0 12 10 8 6 4 2 12 13 14 15 16 17 days vol52,3,2009 20-09-2009 19:06 pagina 263 264 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi 4.3. forecasting of fof2 and m(3000)f2 using the nearest neighbour (nn) algorithm forecasting of ionospheric characteristics has been accomplished by means of the nearest neighbour and modified neural network algorithms. algorithms were used for data gaps correction, as well as the characteristics’ forecast. they are determined for the european area. both models are in operational use at rwc warsaw (space research centre). a full description is given on the cost 296 wg1 home page (http://rwc.cbk.waw.pl/cost296/wg1/) (ionosphere forecasting models). table iii illustrates the quality of forecast through the rms error for prediction 24 hours ahead. 4.4. neural network based forecasting, maps, and process identification the middle east technical university (metu) data driven models have proven to be powerfull in forecasting the parameters of the non-linear processes including the ionospheric processes. these models consist of neural networks (nn), neuro fuzzy networks (nfn), genetic programming (gp) and cascade models. the models are capable of performing nowcasting and forecasting of ionospheric parameters such as ionospheric critical frequancy (fof2), total electron content (tec) values up to 24 hours in advance for single and multistations and also performing forecast mapping over the cost 296 area during disturbed space weather (sw) conditions (tulunay et al., 2004; tulunay et al., 2006; senalp et al., 2008). some illustrative examples for mapping application are presented. in the first example, the metunn-c model has been employed to forecast the gps tec grid values over europe (senalp, 2007). bezier surfaces are used for mapping including the sw events in november 2003 whose tec absolute error map is presented in fig. 7. in the second example, as illustrated in fig. 8, the forecast fof2 values are obtained by using the metu fuzzy-neural network model (metu-fnn) (altuntas, 2007) during the halloween 2003 storm and the mapping is performed using genetic programming (gety) (yapici, 2007). 4.5. tests of muf forecasts in order to test the preliminary results obtained in a previous research (zolesi et al., 2008), the hourly muf measurements were compared with the hourly muf predicted by the longterm models asaps, icepac and sirm&lkw and with the hourly values of muf predicted by the nowcasting models sirmup&lkw and iswirm&lkw. the comparisons in terms of the global r.m.s. error between the long term models and the nowcasting models, carried out for the 33 days selected among different seasons and geomagnetic conditions, quantified in table iv how much better is the nowcasting pretable iii. rms error for prediction 24 hours ahead. name rms fof2 [mhz] rms m3000f2 el arenosillo 1.15 0.2 juliusruh 0.90 0.17 loparskaya 1,2 x sofia 0.90 0.16 tashkent 1,70 x tortosa 0.98 0.17 vol52,3,2009 20-09-2009 19:06 pagina 264 265 near-earth space plasma modelling and forecasting diction of individual days than the longterm model prediction during winter months and equinoctial months, as well as under moderate, disturbed, and very disturbed geomagnetic conditions (pietrella et al., 2009). 5. tomographic imaging and model validation during the evening and nighttime the midlatitude ionosphere is separated from the auroral region by the main ionisation trough (pryse et al., 2006a). the mechanisms that form and sustain the trough have been studied for many years but are still open to debate, in particular in the post magnetic midnight sector. recent studies by the solar system physics research group at aberystwyth university have shown the role of plasma drawn across the polar region in sustaining the poleward wall of the trough (pryse et al., 2007). radiotomography observations by the international ionospheric tomography community (iitc) have been the focus of the work, reinforced by measurements from other experimental techniques and the coupled thermosphere-ionosphere-plasmasphere (ctip) model. in turn, the observations have served to validate and develop the model. several studies have investigated the role that plasma transported from the polar region plays in the nightside ionosphere. one example using observations by radiotomography and the eiscat and superdarn radars concluded that in this instance enhanced ionisation was drawn from the polar region, through the harang discontinuity and into the pre-midnight sector where it was restructured into a boundary blob (pryse et al. 2006b). radiotomography observations in scandinavia and uk (middleton fig. 7. absolute error map for observed and 1 h ahead forecast tec during 16-29 nov. 2003. a b so lu te e rr o r v a lu e ( t e c u ) latitude (°) longitude (°) vol52,3,2009 20-09-2009 19:06 pagina 265 266 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi fig. 8. superimposed are the observed and 1-h in advance forecast values of fof2 for sofia between 12 and 24 nov. 2003. table iv. comparison of rms errors between nowcasting and long term models. asaps sirm icepac sirmup&lkw iswirm&lkw ap daily mean r.m.s. (mhz) r.m.s. (mhz) prof ccir r.m.s. (mhz) r.m.s. (mhz) r.m.s. (mhz) 23 june 2005 3.21 2.89 3.36 2.29 1.61 50 10 july 2005 3.26 2.59 3.33 2.18 1.52 57 12 september 2005 2.51 2.70 2.50 1.55 2.32 75 18 january 2005 3.48 4.06 2.79 2.52 3.11 84 15 may 2005 3.30 2.80 3.18 1.82 0.97 87 8 may 2005 3.77 3.28 3.70 2.05 0.98 91 24 august 2005 3.83 3.22 3.99 2.42 2.65 102 global r.m.s. error 3.13 3.35 3.26 2.12 2.03 fo f 2 (m h z) super storm time period (12/11/2003 24/11/2003) (hour) vol52,3,2009 20-09-2009 19:06 pagina 266 267 near-earth space plasma modelling and forecasting et al., 2008) under imf bz negative revealed a prominent poleward trough wall in the post magnetic midnight sector. using the ctip model and multi-instrument measurements the ionisation of this wall was interpreted as arising from a tongue-of-ionisation (toi) drawn antisunward through the polar cap, through the harang discontinuity and into the dawn sector where it formed the trough wall. other studies have shown dayside ionisation drawn antisunward towards the nightside under conditions of imf bz positive. an example using radiotomography measurements from the scandinavian and greenland sectors (middleton et al., 2005) revealed enhanced density on the periphery of the polar cap in the afternoon and evening. this was interpreted as a toi being drawn around lobe cells in the polar cap, rather than being drawn directly across the polar region. wood et al. (2008) investigated the spatial variation in the nighttime ionisation under bz positive, and revealed enhanced ionisation on the nightside under bz positive, but with a reduction in density in a region where it was likely that the lobe cells were blocking the transit of ionisation from the dayside to the nightside. initial comparisons of the ctip model output with observations (middleton et al., 2008) showed disparities in the detail of the location and density of the poleward trough wall. in a subsequent study, electric potential patterns shaped by superdarn plasma flow observations were used to replace library patterns as input to the model. a significant improvement was obtained between the observed and modelled enhancements forming this wall (pryse et al., 2009; whittick et al., 2009). sample comparisons between the model and observations are shown in fig. 9 under conditions of imf bz negative, where the density enhancement in the equatorward field-of-view of the panels represents a cross-section through a toi swept into the post-magnetic midnight sector to form the poleward trough wall. for conditions of imf bz positive, the ctip model with a superdarn electric potential pattern yielded dayside ionisation drawn antisunward around the periphery of the polar region, in accord with tomography observations under imf bz positive (pryse et al., 2009). the library of electric potential patterns, used with ctip prior to the use of superdarn observations, did not include lobe cells characteristic of imf bz positive required to reproduce a spatial plasma distribution akin to that observed. 6. modelling mid-scale (meso-scale) inhomogeneities at low-latitude (bubbles) and hight-latitude (patches, blobs) for scintillation predictions an analytical model has been developed for mid-scale (meso-scale) ionospheric inhomogeneities in the background ionosphere occurring either as patches in the high-latitude ionosphere or bubbles in the low latitude/equatorial ionosphere. the mid-scale irregularity model is embedded in the large-scale background ionosphere in order to investigate and predict their scintillation effects. separate models were constructed for both ionospheric electron density bite-outs (bubbles) and enhancement (patches) together with the smaller rod-like irregularities contained within them. to construct the mid-scale inhomogeneity model, a basic function was employed of the form: (6.1) the pattern given by function (6.1) is shown in fig. 10. the 3-dimensional structure can be modelled by means of functions of type (6.1) as the product:(6.2) (6.3) (6.4) in the approximation of the «frozen drift» the positions of the «walls» are functions of time whereas the shape of a structure is kept the same. , , , , ,v a b c v a b c1 1 1 1 2 2 2 2= =" ", , , , , , , , , ,r x y z r x y z r x y z1 1 1 1 2 2 2 2= = =" " ", , , , , , , . , , , , , , , , . , , , , r r v r v f y y b y b f x x a x a f z z c z c 1 1 2 2 1 1 2 2 1 1 2 2 1 1 2 2 φ =^ ^ ^ ^ h h h h , , , , , . exp exp f x x a x a a x x a x x x x 1 1 < 1 1 2 2 1 1 1 2 2 1 1 2 = + + ^ b b h l l ; ; e e vol52,3,2009 20-09-2009 19:06 pagina 267 268 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi embedding the mid-scale inhomogeneity into the ionosphere is managed by modulation of the background ionosphere utilizing equation 6.2: (6.5) a is a constant, which is positive for an enhanced level of the electron density in a local structure and negative for a depletion. to model the electron density fluctuations, a fully three-dimensional inverse power law spatial spectrum of the cor, , , , ,ne r t ne r a r r t v r t v1 10 1 2 2$ $ φ= +^ ] ] ]^h g g g h7 a relation function was formulated, including an aspect ratio for two mutually perpendicular transverse (to the geomagnetic field) directions as well as a longitudinal to transverse one. the model of the electron density fluctuations is given by: (6.6) α is the along-magnetic field and ß the across, / / , k r p p r l 3 2 2 1 2 /n n x y z p 0 3 3 2 2 0 2 2 2 2 2 2 2 0 κ π αβ σ κ κ β κ α κ κ π ψ γ γ = + + + = ^ ^^ ^ ] d h h h h g n fig. 9. plasma distributions modelled by the ctip model (left-hand-side panels) at the 18ºe meridian in the northern hemisphere at 00ut and 03ut on 13 december 2001, and corresponding radio tomography observations (right-hand-side panels). the contours are in units of 1011m-3. vol52,3,2009 20-09-2009 19:06 pagina 268 269 near-earth space plasma modelling and forecasting magnetic field aspect ratio. where l is the smallest outer scale of the fluctuations. the variances of the fractional electron density fluctuations along the ray paths are given by; (6.7) details of the propagation model, calculation of the two-point correlation and cross correlation functions of the phase and log-amplitude of the field, estimation of the statistical characteristics of the received field and resultant simulated time series of received amplitude and phase can be found in (gherm et al., 2005; maurits et al., 2008). the parameters of the spectrum of the electron density fluctuations, as well as the meso-scale local polar structures (e.g. bubbles or patches) are chosen empirically. the nequick model [http://www.itu.int/itu-r/study-groups/softwa, , , , , ,r t r t b r r t v r t v1fr fr 2 0 2 1 1 2 2$ $σ σ φ= +^ ^ ] ]^h h g g h7 a 2 k l0 π = re/rsg3-p531-electron-density.zip] was used to describe the low latitude/equatorial ionosphere. (gherm et al., 2007; strangeways et al., 2007) describe using the meso-scale model to predict scintillation effects for bubbles and the good agreement obtained with experimental data taken at douala, cameron (maurits et al., 2008; zernov et al., 2008; 2009) describe the application to modelling propagation through polar patches. the university of alaska, fairbanks eulerian parallel polar ionosphere model was used to represent the large-scale background polar ionosphere. the resultant propagation simulator enabled realistic predictions of the scintillation effects for high-latitude transionospheric propagation at lband (e.g. gps signals) in the presence of polar patches for a number of different propagation scenarios for l-band transionospheric signals. for some more restricted conditions, the propagation model can also produce results for lower frequencies, such as 400 mhz, where the regime of strong scintillation arises. fig. 10. plasma bubble/polar patch model. 1 0.8 0.6 0.4 0.2 0 0 2 4 6 8 10 x1=2 a1=0.25 x2=7 a2=0.5 f (x ) x vol52,3,2009 22-09-2009 14:52 pagina 269 270 h.j. strangeways, i. kutiev, l.r. cander, s. kouris, v. gherm, d. marin, b. de la morena, s. e. pryse, l. perrone, m. pietrella, s. stankov, l. tomasik, e. tulunay, y. tulunay, n. zernov and b. zolesi references altuntas, e. 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(2007): cascade modeling of nonlinear systems, ph-d thesis, (electrical and electronics eng., middle east technical univ., ankara, turkey). senalp, e.t., e. tulunay and y. tulunay (2008): total electron content (tec) forecasting by cascade modeling: a possible alternative to the iri-2001, radio science, 43, rs4016, doi:10.1029/2007rs003719. stankov, s.m., p. marinov and i. kutiev (2007): comparison of nequick, pim, and tsm model results for the plasma scale and transition heights, adv. space res., 39, 767-773, doi:10.1016/j.asr.2006.10.023. strangeways, h.j., v.e. gherm and n.n. zernov (2007): modeling and mitigation of the effect of scintillations on gps, paper at 49th international symposium elmar-2007, (12-14 september 2007, zadar, croatia. conference proc.) pp. 69-73, available online in ieee xplore, doi:10.1109/elmar.2007.4418802. triskova, l., i. galkin, v. truhlik and b.w. reinisch (2007): application of seamless vertical profiles for use in the topside electron density modeling, adv. space res., 39, 774-778. tulunay, e., y. tulunay, e.t. senalp and lj.r. cander (2004): forecasting gps tec using the neural network technique «a further demonstration», bulgarian geophysical journal, 30 (1-4), 53-61. tulunay, e., e.t. senalp, s.m. radicella and y. tulunay (2006): forecasting total electron content maps by neural network technique, radio science, 41 (4), rs4016. wheadon, n.s., r. larsen, a. belehadi, lj. r. cander, b. zolesi, j. bremer, c. juren, i. stanislawska, d. dialetis and m. hatzopouloss (2006): exploitation of the european digital upper atmospheric server (dias) for radio systems applications, in proceedings of the iet conference on ionospheric radio systems and techniques, (18-21 july 2006, london, uk). whittick, e.l., s.e. pryse, h.r. middleton, a.r. lotinga and a.d. aylward (2009): the ctip model with superdarn electric potential patterns as input: first comparison of modelled ion densities with radio tomography observations, earth, moon and planets, 104 (1), 45-49, doi:0.1007/s11038-008-9264-9, (available on line at www.springerlink.com/content/53w6472172p73808/). wood, a.g., s.e. pryse, h.r. middleton and v.s.c. howells (2008): multi-instrument observations of nightside plasma patches under conditions of imf bz positive, ann. geophysicae, 26, 2203-2216, (available on line at http://www.ann-geophys.net/26/2203/2008/). wrenn, g.l. (1987): time-weighted accumulations ap(t) and kp(t), j. geophys. res., 92 (10), 125-10, 129. wu, j. and p.j. wilkinson (1995): time-weighted indices as predictors of ionospheric behaviour, j. atmos. terr. phys, 57, 1763-1770. yapici, t. (2007): influences of interplanetary magnetic field on the variability of the aerospace media, ms thesis, aerospace eng., (middle east technical univ., ankara, turkey, sep. 2007). zernov, n.n., v.e. gherm and h.j. strangeways (2008): the scintillation propagation model for the effects of scintillation on transionospheric paths traversing midscale ionospheric inhomogeneities, cost296 workshop «ionospheric scintillation: scientific aspects, space weather, application and services», (nottingham, uk, 20-22 february, 2008). zernov, n.n., v.e. gherm and h.j. strangeways (2009): on the effects of scintillation of low latitude bubbles on transionospheric paths of propagation, radio science, 44, rs0a14, doi:10.1029/2008rs004074. zolesi, b., g. fontana, l. perrone, m. pietrella, v. romano, g. tutone, a. belehaki, i. tsagouri, s.s. kouris, f. vallianatos, j.p. makris and m.j. angling (2008): a new campaign for oblique-incidence ionospheric sounding over europe and its data application, j. atmos. sol. terr. phys., 70, 854-865. zolesi, b., and lj.r. cander (2008): from cost 238 to cost 296: four european cost actions on ionospheric physics and radio propagation, in aip conference proceedings, 974, 39-46. zolesi, b., lj.r. cander, a. belehaki and i. tsagouri (2008): effective sunspot number: a tool for ionospheric mapping and modelling, xxix general assembly of ursi, (chicago, usa, 07-16 august 2008). vol52,3,2009 20-09-2009 19:06 pagina 271 sul vento prevalente a pavia g. aliverti • e. avico ripetutamente i· cupit'-lto a una ,li noi. neg:li • .!lini scorsi, di ricevere richieste di dati da persone interessate per i più svariati fmi a ,'onoscere le caratteristi"he princil'uli del vento di una determinata zona ,l'italia. per {jllanto rig:uunla pavia ~i è rilevato così che mancava una elahora7.ionc ,lei dati del vcnlo che tenesse conto dei due clementi, direzione cd intcn~itù; per quelle informazioni si poteva l'i· (:orrere soltanto o alla climatolog:ia dci ro~tcr o al lavoro del l'rol eredia, per il ,]c(:ennio ls91-19{)o, lavori che considerano la direzione e non l'illlcnsitù, ora la o~~ervazione quotidiana (ielle carte del vento, ottenute all'osservatorio dopo il 19,12, cioè (iopo l'imtallazione di un ottimo anemoi!rafo meccanico siap, ha fatto nascere il sospetto in una di noi che i ,lati riassuntivi a ,lisposizione e relativi a pavia rispecchiassero una distrillllzione del vento nelle ,"arie ,lirezioni notevolmente diversa ,la quella che ci si poteva aspettare ,l alla elahorazione completa dei nuovi dati. appariva ,lalla semplice ispezione a occhio dclle carte che una direzione, il sw, era di [!ran lunga la preferita. decidemmo perciò la elahorazione ,ii tutti i dati registrati ,lurante le 2-1· ore c la hasammo sulle seguenti considerazioni: ~iccome una delle caratteristiche più interessanti praticamente è la ,lirezione prevalente e siccome il vento ha la tendenza a camhiare hruscamente direzione ma poi persiste nella direzione assunta ahbastanza a lungo, se si suddivide la registrazione ,li ogni i!iornata per direzioni e si contano in corrispondenza i ehilometri di vento filato e il tempo trascorso, si può ottenere facilmente uno specchio ,iella importanza ,lei vari venti. la 5ud,li"isione ,liventa un po' faticosa soltanto in occ'-lsione ,li temporali, durante i quali si hanno variazioni frequenti e ampie; ma altrimenti capita in generale che un vento tiene il campo per ore e,l ore consecutive. a pavia ciò si verifica in modo particolarmente yi~toso per il sw e in misura note"ole anche per il ne. ci siamo accontentate ,li considerare otto direzioni, perché effettivamente, ,lato il ,:arattere del vento, la suddivisione in se,lici parti ci parve eccessiva e un numero intermedio ,li ,lirezioni non era possihile 3 4 g . a l i v e r t i e . avico considerarlo date le carte diagrammate a disposizione. gli anni considerati sono sette e precisamente dal 1943 al 1949; durante quasi tutto quel periodo, per cause contingenti relative alla guerra, l'osservatorio ebbe scarsità di carte diagrammate e di conseguenza ogni carta fu in generale usata almeno due volte e magari anche tre con inchiostri di colore diverso; questo fatto ha reso talvolta un po' penosa la elaborazione e purtroppo ci ha fatto perdere qualche centinaio di ore di registrazione. dell'inverno 1946-47 estremamente rigido si perdettero alcune, giornate di registrazione per cattivo funzionamento dell'orologio; in complesso le ore utilizzate sono state quindi 60474 invece delle 61368 costituenti i sette anni considerati. siccome gli uffici centrali europei di meteorologia usano ancora raccogliere nel « registro delle stazioni » i dati del vento relativi alle tre ore soltanto delle osservazioni (cioè delle ore 8, 14, 19), e si vedono anche in pubblicazioni recenti elaborazioni basate sull'uso di quei dati relativi a tre sole ore fìsse della giornata, abbiamo eseguito anche noi una elaborazione di quel tipo per poterne confrontare i risultati con quelli delle 24 ore. nella tabella i sono raccolti per l'elaborazione estesa alle ventiquattro o r e : i chilometri totali per ciascuno degli otto venti, la corrispondente durata in ore, la velocità media in km/h dedotta dividendo il numero dei chilometri per il numero delle ore, e infine il numero di volte in cui ciascun vento ha soffiato; nella penultima orizzontale sono indicate le ore totali di calma e i l numero delle volte in cui la calma si è verificata; nell'ultima orizzontale ci sono le somme tabella i dalle registrazioni delle 24 ore ( 1 9 4 3 4 9 pavia) dalle tre ore (8,14,19) vento percorso k m durata tot. ore m i n vel. media km/h volte percorso k m volte ( = ore) n ne e se s sw w nw 28 789 59 232 34 333 22 902 6 492 1 1 4 hi 1 32 487 2 4 1 2 5 4662 52 11999 23 6622 38 4188 59 1976 45 180(53 ufi 6541 35 4054 28 6,2 4,9 5.2 5.6 3.3 (ì.4 5,0 5,9 2173 4953 3973 3288 1378 5403 3872 2598 6231 3843 8394 1361 2360 <j<>2u 9410 1345 886 837 1534 235 636 1447 16» 1 229 calma — 2361 06 — 2847 — 79 totale 323 171 60473 52 5,3 (5,6) 30485 42570 7474 srl vento prevale);:re a p.\\'l\ dei chilometri e della durata e la velocità media corrispondente; in parentesi la velocità media calcolata escludendo le ore di calma. nelle ultime due colonne della tahella i sono poi rael:olti per il confronto i risultati della elahorazione relativa alle tre ore de1le osservazioni e precisamente i chilometri totali e il co r rispondente numero di volte (= ore) in cui ciascun vento è stato osservato. a proposito della elaborazione estesa ane 24 ore è da notare che pavia· o!>servatorio ~ j<lnto ,spirito ( anni 194j · 49) geofisico n • ne / / sw nw 1 \ /1 \ / ! i / ...... ...... -.. ' / ......... w-~---l __ _ ~':~ --e \ /;0::/ \ //" i ' / }:/ s 'se ./ j/ " --<km lo/ah (n:gillrilzion l doz lle , 14 ore) pereoo! de/ venia . km lotal/per /e ore delle ojj er_ v"7fom( 8 " · i'i~ . 19") fig. l i risultati di un singolo anno si scostano poco da quelli ottenuti dai 7 anni insieme; così per es. il sw risulta in ogni anno il vento più importante, pelo il numero dei km e il numero delle ore, mentre il s risulta sempre il meno importante. ciò non accade per l'elaborazione dei dati delle tre ore c su sette anni elahorati soltanto tre danno la prevalenza del sw per i km percorsi, e il s soltanto due volte risulta il vento meno importante. si può così concludere che tenendo conto di tutte le 2'1 ore , il cosidetto periodo normale (valevole per il calcolo della distrihuzione normale) è già verosimilmente raggiunto con l'in· tervallo di sette anni, mentre ciò non avviene sicuramente per la ela3 6 fi. a l i v e r t i e . avico borazione delle tre ore che comprende soltanto l'ottava parte del tempo tenuto in conto nella prima elaborazione, e già per questa ragione essa richiederebbe un numero di anni almeno otto volte più grande; inoltre le tre ore scelte, significative per altri elementi meteorologici, non lo sono di sicuro per il vento; l'elaborazione in parola darebbe risultati migliori certamente se le tre ore fossero scelte a caso entro le 24 ore. i l confronto fra i numeri della seconda e della sesta colonna della tabella i mette subito in evidenza la differenza sostanziale fra i risultati delle due elaborazioni; gli stessi numeri sono rappresentati graficamente nella fig. 1 che rende il confronto più efficace; le unità tabella i i vento importanza riguardo a : vento percorso durata volte n 9% 7 % n e 18 20 16 e 11 11 13 s e 7 7 11 s 2 3 5 s w :«> « 0 -< w 10 11 13 n w 7 7 8 calma — 4 9 v * usate per le due rappresentazioni sono naturalmente differenti fra loro. i venti di w, e , n, di moderata importanza effettiva, nella elaborazione delle tre ore assumono importanza pari o quasi al s w che è i l vento nettamente prevalente; perciò non abbiamo elaborato ulteriormente tali dati. nella tabella i i sono riportate le percentuali relative alle otto direzioni rispetto ai km totali, alle ore totali e al numero di volte totali; i numeri di questa tabella, ricavati dalla tabella i, graduano cioè l'importanza di ciascun vento; per la durata e le volte si è tenuto in conto anche la calma. come si vede il s w è il vento più importante per i chilometri percorsi, per la durata e anche per il numero di volte in cui ha soffiato; il s è invece il meno importante. le tabelle i e i i si riferiscono ai risultati complessivi dei sette anni; ma è interessante anche vedere la distribuzione nelle stagioni. la tabella i i i riassume tali dati. suf. \'f:';to l'nh'm.ente a 1'..1\'1,\ tabella iii i inverno primavera e"tate velllo ----km duratn km durata km durala n 7450 1014 hoo'" 9981 1l73 h 17 m 69<18 h52 h4s m ne 9980 1729 33 18715 2915 ,io 16398 4ui'i4ij e 6477 103942 13235 2223 52 7064 1651 h se 2740 4945:; 9247 h53 35 6123 1288 26 5 1838 5.h 50 2005 54·~ ss 1015 335 31 sw 421177 uq!l::l ao ;1:11l1 il 4:\<;1 io :!ulitii l 306!l 32 w 8657 ls33 ~2 illl82 1-\21 20 8621. 15r6 ~9 nw 6599 h700 5935 sli lo _ 67~ 1 u21 5-l ---ralma -l99 13 i 317 26 ;;12 15 ---31 autunno km ... 50 i-h39 7557 -~792 1634 17hun 7lh 4875 durnta 1022 b .17 m 333627 170740 952 03 561 29 :11.;.154 1700 h 1174 2-t 1005 12 anche in questa tahella sono sott-llineati i valori massimi e minimi; si vede hene da essa che per i chilometri percorsi il sw eon~erva la prevalenza (1) in tutte le quattro stagioni, quantunque il di~taeeo dagli altri venti si attutisca man mano passando dall'inverno alla primavera, dalla primavera all'estate e dall'estate all'autunno; il ne nell'estate ha la prevalenza per il numero di ore, ma sta al 2° posto per i chilometri. pure il s per i chilometri percorsi rimane semtabella iv epoca km totali durata total e ore di calma .. elopità media ---------inv" rno f!671s il.186 h.'j2 rn 499 13 m 6,0 km; h primav" ra 100 .119 1-1894. 59 317 26 6,7 e>late 73 462 j.l.71457 542 15 5,0 autunno 62471 l-l1l2 58 100s 12 4,4 ---------------anno 323171 5810946m 2j64 h06m 5.6{5,31km / h --._--pre il meno importante. l'\ella tahella iv sono raccolti i dati complesshri relativi alle quattro stagioni senza distinzione di direzione; da essa si vede come la primavera è la stagione con il numero massimo di ehilometri percorsi dal vento e con la maggiore velocità media: ad essa seguono per importanza di percorso e di velocità media il) pr"val"nza evidentemente tletcrminata dalla circolazione g"nerale dell'almo_ ~("ra. 3 8 c . a l i v e r t i e . avico nell'ordine: inverno, estate, autunno. l'autunno spicca sulle altre stagioni per il maggior numero di ore di calma. per quanto riguarda l'elaborazione di ercdia citata più su, è da tener presente cbe i dati da lui usati erano stati raccolti nell'osservatorio meteorologico dell'università, sulla torretta dell'edificio universitario situato al centro della città. essa dà nell'anno i seguenti valori di frequenza per le otto direzioni : n ne e s e s sw w nw calma 101 204 158 127 81 175 215 98 41 in questa elaborazione (di dieci anni delle tre osservazioni giornaliere), per la frequenza prevale dunque l'w seguito da ne, sw, e , s e , n, nw, s. nella analoga elaborazione dei dati raccolti da noi a santo spirito, e riprodotti nella tabella i, ultime due colonne, i venti per i chilometri percorsi si ordinano cosi: sw, w, e, n, ne, s, s e , nw e i chilometri percorsi dall'w sono di poco inferiori a quelli del sw. l'ordine dei venti per la frequenza risulta invece il seguente: w, e, sw, n, ne, s, s e , n w ; dunque anche a santo spirito l'w risulterebbe il vento più frequente, giusta i risultati della elaborazione dell'eredia. però anche per la elaborazione dell'eredia si possono ripetere le considerazioni già svolte a proposito della nostra elaborazione dei dati delle tre ore. concludendo si può sottolineare cbe per ottenere i valori normali della velocità e della frequenza del vento, l'elaborazione dei dati delle 24 ore consente l'uso di un periodo di tempo non tanto lungo; perciò anche da questo punto di vista è desiderabile di elaborare sempre tutti i dati forniti dalle registrazioni e non soltanto quelli delle tre ore, tanto più che così facendo è più facile conseguire l'omogeneità del materiale elaborato. napoli — istituto superiore navale — gennaio 1952. riassunto nella presente nota si rende conto della elaborazione dei dati del vento registrati a pavia presso l'osservatorio dell'istituto nazionale di geofisica durante sette anni (1943-49). l'elaborazione è stata eseguita annotando per ogni giorno e per le otto direzioni i chilometri percorsi dal vento e la durata di ciascun vento. da questa elaborazione si è ricas u l v e n t o p r e v a l e n t e a p a v i a 35 imta poi la velocità media di ciascun vento e l'importanza relativa per i chilometri percorsi, per la durata e per il numero di volte in cui ciascun vento si è presentato. sono pure stati suddivisi i dati per stagione e si è così dimostrato che il vento prevalente a pavia è sempre il sif per quanto riguarda i chilometri percorsi e quasi sempre anche, per le ore di durata (fa eccezione l'estate). si paragona poi tale elaborazione con i risultati della elaborazione dei dati osservati alle ore 8, 14, 19, per dimostrare la inefficienza di quest'ultimo metodo di elaborazione, che di tanto in tanto viene ancora adoperato. summary account is given tvitli the present paper about the elaboration of ivind data recorded in pavia, istituto nazionale di geofìsica observatory, during seven years (1943-49). the elaboration has been done registering, for every day and for the eight directions, the kilometers the wind travelled and the time eacli ivind bleiv. from this elaboration has been draicn the mean velocity of each ivind and the importance of thern for the kilometers travelled, the time over wliicli tliey blew, and, at last, for the number of times during whicli every one occurred. the data are likewise selected according to the seasons and it has been shcnvn tliat the prevailing ivind in pavia, is always the sw-ivind as what is concerned with the kilometers travelled, and almost always for the duration (the summer is the exception). after ali these results have been compared with those of elaboration of data observed at 8, 14, 19 hours in order to show the inefficiency of this last metliod of elaboration. layout 6 rapid response seismic networks in europe: lessons learnt from the l'aquila earthquake emergency lucia margheriti1,*, lauro chiaraluce1, christophe voisin4, giovanna cultrera2, aladino govoni1,11, milena moretti1, paola bordoni1, lucia luzi3, riccardo azzara2, luisa valoroso1, raffaele di stefano1, armand mariscal4, luigi improta2, francesca pacor3, giuliano milana2, marco mucciarelli5, stefano parolai6, alessandro amato1, claudio chiarabba1, pasquale de gori1, francesco p. lucente1, massimo di bona1, maurizio pignone1, gianpaolo cecere1, fabio criscuoli1, alberto delladio1, valentino lauciani1, salvatore mazza1, giuseppe di giulio2, fabrizio cara2, paolo augliera3, marco massa3, ezio d'alema1, simone marzorati1, monika sobiesiak6,12, angelo strollo6, anne-marie duval7, pascal dominique8, bertrand delouis9, anne paul4, stephan husen10, giulio selvaggi1 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy 2 istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy 3 istituto nazionale di geofisica e vulcanologia, sezione milano-pavia, milan, italy 4 université de grenoble, institut des sciences de la terre, laboratoire de géophysique interne et tectonophysique, grenoble, france 5 università della basilicata, dipartimento di strutture, geotecnica, geologia applicata all'ingegneria, potenza, italy 6 helmholtz-zentrum potsdam deutsches geoforschungszentrum gfz, potsdam, germany 7 cete méditerranée (centre d'etudes techniques de l’equipement), laboratoire régional des ponts et chaussées, nice, france 8 brgm (bureau de recherche géologiques et minières), service aménagement et risques naturels, orléans, france 9 université de nice-sophia antipolis, institut geosciences azur, valbonne sophia antipolis, france 10 eth zürich (eidgenössische technische hochschule zürich), zurich, switzerland 11 istituto nazionale di oceanografia e geofisica sperimentale ogs, sgonico (trieste), italy 12 christian-albrechts-universität zu kiel, institut für geowissenschaften, kiel, germany annals of geophysics, 54, 4, 2011; doi: 10.4401/ag-4953 abstract the largest dataset ever recorded during a normal fault seismic sequence was acquired during the 2009 seismic emergency triggered by the damaging earthquake in l'aquila (italy). this was possible through the coordination of different rapid-response seismic networks in italy, france and germany. a seismic network of more than 60 stations recorded up to 70,000 earthquakes. here, we describe the different open-data archives where it is possible to find this unique set of data for studies related to hazard, seismotectonics and earthquake physics. moreover, we briefly describe some immediate and direct applications of emergency seismic networks. at the same time, we note the absence of communication platforms between the different european networks. rapid-response networks need to agree on common strategies for network operations. hopefully, over the next few years, the european rapid-response seismic network will became a reality. 1. introduction the rapid deployment of temporary seismic stations soon after the occurrence of a moderate-to-large earthquake is an essential action. this serves both to upgrade the resolution of the permanent monitoring systems of the ongoing seismic crisis, and to ensure the availability of high quality scientific datasets for studies related to hazard, seismotectonics and earthquake physics. moreover, temporary improvement of the detection capabilities of local and regional networks during periods of special interest, such as for foreshock/aftershock sequences and for unusual background, swarm or induced seismicity, is becoming a major aim for the whole of the seismological community. the near-real-time availability of high-quality datasets recorded by rapid-response temporary networks also article history received january 20, 2011; accepted june 14, 2011. subject classification: surveys, measurements and monitoring, tectonics, instruments and techniques, rapid response seismic networks, open data archives. 392 provides a crucial source of information for decision makers to evaluate and update the existing situations, and for the support of emergency management during seismic crises. in europe, the temporary deployment of local seismic networks is mainly a national task, and it is generally managed by some of the major public research institutions. the networking of the existing capabilities throughout europe would improve the ability for rapid and more efficient response networks to support the emergency management, with a valuable impact on hazard mitigation. an example of spontaneous and collaborative rapidresponse seismic network coordination in a european framework was triggered by the occurrence of the disastrous l'aquila earthquake (central italy) in april 2009, when groups from different european institutions installed dense local networks to record the aftershock sequences [chiaraluce et al. 2011]. in this report, we will overview this experience, to highlight the positive aspects, and those that require further effort to optimize the outcomes, in terms of coordination between the european seismological communities. 2. the l'aquila earthquake emergency on april 6, 2009, at 03:32 local time, a shock with a moment magnitude greater than 6 (mw = 6.1, scognamiglio et al. 2010; mw = 6.3, pondrelli et al. 2010) woke up millions of people in central italy, including many in rome and in many other towns from the tyrrhenian to the adriatic coast, and through the regions of abruzzo, lazio, umbria, marche and molise. the town of l'aquila and its ~70,000 inhabitants were sleeping right above the fault that ruptured that night. more than half of the buildings in down-town l'aquila were severely damaged, and many collapsed; more than 300 people died under the ruins. the artistic, historic and cultural heritage of this beautiful area disappeared in a handful of seconds. using the data collected by the seismic and geodetic monitoring systems in italy [amato and mele 2008] that are managed by the istituto nazionale di geofisica e vulcanologia (ingv) and funded by the italian national civil protection (dpc), the main parameters of this earthquake were immediately evaluated and made available to the scientific and civil community (web page http://iside.rm.ingv.it/iside/standard/index.jsp). the permanent seismic stations in the region (rsn and mednet) had a mean inter-station distance of 30 km. to improve the monitoring resolution of these seismic and geodetic networks, several italian teams moved into the epicentral area to install temporary monitoring networks only a few hours after the mainshock occurred. two groups, rete mobile ingv-cnt (centro nazionale terremoti; margheriti et al. 393 figure 1.location of the earthquake that occurred near l'aquila on april 6, 2009, at 01:32 utm. the map shows the instrumental seismicity of the ingv italian seismic bulletin (from january 1, 2003, to december 31, 2008, with m≥2.5). 394 national earthquake center) roma (re.mo), and rete mobile telelemetrata from the ingv-cnt grottaminarda (remotel), had already started the installation on the morning of april 6, 2009: by the end of that day, eight standalone and three real-time stations were operating, which reduced the mean inter-station distance to about 20 km. the new geometry of the network was defined to optimize the seismic network coverage while taking into account the main cultural noise sources and the land topography. in less than 30 h from the mainshock, the french seismic crisis committee had made 20 seismic stations available to the ingv. then four scientists from the laboratoire de géophysique interne et tectonophysique (lgit) of grenoble moved from france to arrive in l'aquila on the evening of april 7, 2009. after a coordination meeting with the ingv-cnt personnel, starting on april 8, 2009, all of the 20 stations were deployed in four days through this spontaneous italian-french teamwork. by this time, the spacing between stations had decreased to below 10 km (two stations were also installed by ingv-catania [ct] as part of re.mo); the configuration of the network, devoted to earthquake location was fixed from april 12 to june 26, 2009. the day after the mainshock (april 7, 2009), other groups started installing stations to detect site effects and building responses; the institutions involved in this activity included: the ingv (milano-pavia [mi-pv]; sismologia e tettonofisica, roma1; centro nazionale terremoti [cnt]; osservatorio d'arezzo), helmholtz zentrum potsdam (geoforschungs-zentrum [gfz]), universitàdella basilicata and two french groups (lrpc de nice, centre d'etudes techniques de l'equipement mediterranée [cete], and the bureau de recherches géologiques et minières [brgm]). by april 12, 2009, the deployment in the epicentral area (figure 2; table 1) included a total of 61 stand-alone seismic stations (23 of which were devoted to site and building responses) and nine seismic stations connected in real time to the monitoring room in rome and integrated into the ingv national seismic network (figure 2; table 1). the contacts between the different groups did not follow any pre-defined communication channels or procedures. this aspect resulted in the need for several operational meetings, which slowed down the initial process of station deployment. it was decided that each group (italian, french and german) had to take care of the initial deployment of their stations, while the maintenance was mainly a joint and coordinated effort. in this way we were able to optimize the use of the stations, to accomplish the different tasks necessary. in the following months, the network geometry was reorganized according to different needs. stations devoted to site response investigations were moved to areas of particular interest for the civil protection, to collect data that would be useful for seismic microzonation studies [cultrera et al. 2011]. stations devoted to earthquake location were reorganized according to their availability, to better follow the temporal and spatial evolution of the seismic sequence [margheriti et al. 2010]. some of the real-time stations operated until the end of 2010. 3. the data archives the team (ingv-cnt and lgit) that was running the networks that were mainly devoted to earthquake detection and location decided that it was important to share the continuous recordings. to achieve this aim, the common seed (standard for the exchange of earthquake data) data format was chosen. the continuous data recorded by the 21 mobile stations operated by the ingv-cnt that were deployed during the l'aquila sequence (re.mo networkrm01 to rm32 stations iv network) were checked and converted to the seed data format, and then made available to the scientific community using the eida platform (european integrated data archive, http://eida.rm.ingv.it/). all of the metadata available was also checked and converted to the seed format. these datasets spanned from a few hours after the april 6, 2009, mainshock (first data recorded at 04:43:08 utc) to march 23, 2010, when the last nine stations of re.mo were closed, and they consist of some 270 gb of data. a total of 30 sites where occupied during this period, with the network geometry adapted to the evolution of the seismic sequence. the data recorded by the 20 seismic stations installed by lgit (grenoble, france) between april and june 2009 were distributed in the same seed format (at http://www.fosfore.ipgp.fr/en/ in "fosfore portal" under "stations", network xj), together with the metadata. the eida archive also provides access to the national seismic network data, including the re.mo.tel stations that were deployed after the mainshock and received in real time (t0101 to t0110 stations iv network). some of these stations operated until the end of 2010, and they contributed to the real-time hypocentral locations for many hundreds of local and regional earthquakes, thus significantly lowering the detection threshold of the national seismic network in the region. all of the other data that were recorded to evaluate site effects and to monitor building structures have been archived by their reference institutions. a set of events with magnitudes greater than 3 is now shared by all participants, and some of these waveforms are archived inside itaca http://itaca.mi.ingv.it/, the italian accelerometric archive. further steps should be followed to build upon the automatic procedures, to rapidly have all of the data in the same format and in the same archive. 4. direct applications of the rapid-response networks this large amount of data collected is very valuable for scientific studies related to hazard, seismotectonics and emergency seismic nets: l'aquila 2009 margheriti et al. 395 institute network name number of stations sta. name recording mode main aims of the stations data availability ingv-cnt re.mo.tel./iv 9 t0101 t0110 real time improved detection capacity eida (continuous recordings) ingv cnt-ct re.mo/iv 21 rm01 rm32 stand alone seismotectonics and earthquake physics eida (continuous recordings) lgit /xj 20 lg01 lg20 standalone seismotectonics and earthquake physics fosfore (continuous recordings) brgm 2 stand alone seismotectonics and earthquake physics itaca (main events) nice 3 stand alone site effects itaca (main events) ingv mi rais/iv 5 stand alone site effects itaca (main events) gfz 5 stand alone monitoring building structures itaca (main events) ingv rm1-ar 7 stand alone site effects itaca (main events) table 1.temporary stations deployed in the epicentral area. for abbreviations, see main text. figure 2.deployment of the seismic stations during the l'aquila april 2009 seismic sequence. several european rapid-response seismic networks took part in the deployment. 396 earthquake physics. several studies have already been carried out with the collected data, both for seismotectonic purposes [chiarabba et al. 2009, di stefano et al. 2010, chiaraluce et al. 2011] and for site effect evaluations [mucciarelli et al. 2009, d'alema et al. 2010, picozzi et al. 2010, ameri et al. 2011, bergamaschi et al. 2011, bordoni et al. 2011, di giulio et al. 2011, milana et al. 2011, puglia et al. 2011]. here, we report on two direct applications of the dataset, which helped the decision makers to assess the current situation during the seismic sequence, in terms of indicating villages and towns where checks on building vulnerabilities were urgent, and orienting the definition of the areas for reconstruction. 4.1. earthquake location improvements the deployment of emergency networks during seismic crises temporarily improves the detection capacity of local and regional networks, through lowering the detectable magnitude threshold. the deployment of the re.mo. re.mo.tel. and lgit networks allowed us to detect all of the earthquakes with a magnitude ml >0.7 for the time period from april to june 2009 [valoroso et al. 2011]. moreover, these networks made the hypocentral location approximations more precise. this general improvement in the location parameters is illustrated in figure 3, which shows a map with earthquakes of m>1.9 and their location parameters using only the national network (figure 3a), and combined with the emergency networks (figure 3b). the residuals for both the arrival times and the location errors halved when the emergency network data were considered (figure 4). 4.2. microzonation the data collected for site-response purposes were used for microzonation studies in the months following the mainshock [gruppo di lavoro ms_aq 2010]. site-responserelated parameters were estimated, such as the fundamental resonance frequencies (fo; figure 5), calculated as the first peak in the earthquake and noise spectral ratios (http://www.protezionecivile.gov.it/docs/www.ulpiano11. com/docs/microzonazione_pdf/macroarea1/m1_milana rumorestrumentalemacro1.pdf), and the amplification factors (fa). the fa is here defined as the ratio of the integral of the earthquake response spectra between a soft site and a reference rock site, evaluated in two frequency ranges (0.5 hz < f < 2 hz and 2 hz < f < 10 hz). these fo and fa parameters promoted the easy and fast mapping of the zones that showed different seismic responses. furthermore, the resonance frequency information helped to constrain the thickness of the soft sediments in the geological model used for the one-dimensional simulations of ground motion that were carried out to estimate the theoretical fa after calibration with the observed amplification factors (fa) [gruppo di lavoro ms_aq 2010]. 5. towards a coordinated european rapid response seismic network the experiences gained during this emergency that arose following the l'aquila 2009 earthquake emphasized the necessity to define the communication platforms that are needed to facilitate rapid information exchange among the european rapid-response network community. some efforts to join forces and to establish contacts had already been carried out in the framework of the orfeus working group on mobile seismology (http://www.orfeuseu.org/workinggroups/wg3/mobile-stations-summaryonshore.html), within which several european nations are participating: austria, czech republic, denmark, finland, france, germany, great britain, greece, ireland, italy, the netherlands, norway, portugal, the slovak republic, spain, sweden and switzerland. a further plan of activities for rapid-response-network coordination is included in the nera project (network of european research infrastructures for earthquake risk assessment and migitation) that was approved by the eec funding scheme: combination of collaborative projects and coordination and support actions for integrating activities (cp-csa); this started in november 2010. in this framework italy, germany, switzerland and france are looking to establish a unique european rapid response seismic network (errsn) for optimal intervention after earthquakes. in this context, and despite the difficulties that have been encountered, the scientific results gained with the l'aquila experience underline the importance of wide deployment, and hence of coordination at the european level. rapid-response networks need to agree on common strategies for network operations. decision making must be streamlined and coordinated, and based on the improved knowledge of the potential losses and existing local capacities. partners should agree beforehand on suitable communication channels, and decide also on the need for specialization. these procedures will be defined over the next year. different partners can cover different tasks; e.g. one specializing in very rapid response with light equipment, another in sustained efforts for month-long monitoring, and another in investigating site effects and/or monitoring building structures for engineering purposes. the project will also work to develop the tools necessary to allow optimization of the network geometry, taking into account the main cultural noise sources, the historic seismicity, the building distribution, and the land topography, and will develop and produce tutorials to increase inter-operability between the crews in the field. the data needs to be exchanged using common protocols for data archiving (in terms of waveforms and metadata), and common database formats are needed; the nera project has proposed the eida platform as the emergency seismic nets: l'aquila 2009 margheriti et al. 397 figure 3. (a) hypocentral locations using the permanent network (black triangles) for earthquakes with m>1.9 between april and june, 2009. map above and cross-sections below. the cross sections are reported as two perpendicular grey line in the map. (b) hypocentral locations using the permanent network (black triangles) and the emergency networks (gray triangles) for earthquakes with m>1.9 between april and june, 2009. again, map above and cross-sections below. the cross sections are reported as two perpendicular grey line in the map. figure 4. location parameters for the ml >1.9 earthquakes of the 2009 l'aquila sequence obtained using: (a) data from the italian national network; and (b) data from the italian national network and the local networks (ingv lgit). 398 common archive. hopefully, in the next few years the errsn will became a reality. acknowledgements. we are grateful to many other colleagues at the ingv and at other institutions who contributed to the handling of the l'aquila emergency. this study has partially benefited from funding provided by the nera project and by funding provided by the italian presidenza del consiglio dei ministri, dipartimento della protezione civile (dpc). scientific reports funded by the dpc do not represent its official opinion and policies. references amato, a. and f.m. mele (2008). performance of the ingv national seismic network from 1997 to 2007, annals of geophysics, 51 (2-3), 417-431. ameri, g., a. oth, m. pilz, d. bindi, s. parolai, l. luzi, m. mucciarelli and g. cultrera (2011). separation of source and site effects by generalised inversion technique using the aftershock recordings of the 2009 l'aquila earthquake, b. earthq. eng., 9, 717-739; doi: 10.1007/s10518011-9270-6. bergamaschi, f., g. cultrera, l. luzi, r.m. azzara, g. ameri, p. augliera, p. bordoni, f. cara, r. cogliano, e. d'alema, d. di giacomo, g. di giulio, a. fodarella, g. franceschina, f. galadini, m.r. gallipoli, s. gori, p. harabaglia, c. ladina, s. lovati, s. marzorati, m. massa, g. milana, m. mucciarelli, f. pacor, s. parolai, m. picozzi, m. pilz, s. pucillo, r. puglia, g. riccio and m. sobiesiak (2011). evaluation of site effects in the aterno river valley (central italy) from aftershocks of the 2009 l'aquila earthquake, b. earthq. eng., 9, 697-715; doi: 10.1007/s10518-011-9245-7. bordoni, p., j. haines, g. milana, s. marcucci, f. cara and g. di giulio (2011). seismic response of l'aquila downtown from comparison between 2d synthetics spectral ratios of sh, p-sv and rayleigh waves and observations of the 2009 earthquake sequence, b. earthq. eng., 9, 761-781; doi: 10.1007/s10518-011-9247-5. emergency seismic nets: l'aquila 2009 figure 5. map showing the fundamental frequencies (f0) and the second frequency peak (f1) extracted from the h/v ratios of the seismic noise. the values of f0 and f1 are classified into six groups (circles or triangles; different colors; see legend in figure), with the sizes of the symbols indicative of the amplification amplitude. chiarabba, c., a. amato, m. anselmi, p. baccheschi, i. bianchi, m. cattaneo, g.p. cecere, l. chiaraluce, m.g. ciaccio, p. de gori, g. de luca, m. di bona, r. di stefano, l. faenza, a. govoni, l. improta, f.p. lucente, a. marchetti, l. margheriti, f. mele, a. michelini, g. monachesi, m. moretti, m. pastori, n. piana agostinetti, d. piccinini, p. roselli, d. seccia and l. valoroso (2009). the 2009 l'aquila (central italy) mw 6.3 earthquake: main shock, geophys. res. lett., 36, l18308; doi: 10.1029/2009gl039627. chiaraluce, l., c. chiarabba, p. de gori, r. di stefano, l. improta, d. piccinini, a. schlagenhauf, p. traversa, l. valoroso and c. voisin (2011). the 2009 l'aquila (central italy) seismic sequence, bollettino di geofisica teorica e applicata, 52 (3); doi: 10.4430/bgta0019. cultrera, g., m. mucciarelli and s. parolai (2011). the l'aquila earthquake a view of site effects and building behavior from temporary networks, b. earthq. eng., introduction to the special issue, 9, 691-695; 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doi: 10.1029/2011gl047365. margheriti, l., m. anselmi, a. antonioli, r. azzaro, p. baccheschi, a. bono, b. castello, c. chiarabba, l. chiaraluce, m.g. ciaccio, g.b. cimini, g. colasanti, m. colasanti, f. criscuoli, s. d'amico, p. de gori, a. delladio, m. di bona, r. di stefano, a. frepoli, e. giandomenico, l. giovani, a. govoni, l. improta, v. lauciani, a.g. mandiello, c. marcocci, s. mazza, m. moretti, n.m. pagliuca, n. piana agostinetti, d. piccinini, d. seccia, f.p. lucente, s. pintore, l. pizzino, p.r. platania, m. quintilliani, s. rapisarda, g. selvaggi, a. serratore, m. silvestri, s. silvestri, g. soldati, l. valoroso and l. zuccarello (2010). emergenza "l'aquila2009": la campagna di acquisizione dati della rete sismica mobile stand-alone del centro nazionale terremoti, rapporti tecnici ingv, 151, 54 pp.; available for download at http://portale.ingv.it/portale_ingv/produzione-scientifica/rapporti-tecniciingv/archivio/copy_of_numeri-pubblicati-2010/). milana, g., r.m. azzara, e. bertrand, p. bordoni, f. cara, r. cogliano, g. cultrera, g. di giulio, a.m. duval, a. fodarella, s. marcucci, s. pucillo, j. régnier and g. riccio (2011). the contribution of seismic data in microzonation studies for downtown l'aquila, b. earthq. eng., 9, 741-759; doi: 10.1007/s10518-011-9246-6. mucciarelli, m., m. bianca, r. ditommaso, m.r. gallipoli, a. masi, c milkereit, s. parolai, m. picozzi, m. vona (2009). far field damage on rc buildings: the case study of navelli during the l'aquila (italy) seismic sequence, b. earthq. eng., 9, 263-283; doi: 10.1007/s10518-010-9201-y. picozzi, m., r. ditommaso, s. parolai, m. mucciarelli, c. milkereit, m. sobiesiak, d. di giacomo, m.r. gallipoli, m. pilz, m. vona and j. zschau (2010). real time monitoring of structures in task-force missions: the example of the mw = 6.3 central italy earthquake, april 6, 2009, nat. hazards, 52 (2), 253-256; doi: 10.1007/s11069-0099481-1. pondrelli s., s. salimbeni, a. morelli, g. ekstrom, m. olivieri and e. boschi, seismic moment tensors of the april 2009, l'aquila (central italy), earthquake sequence, geophys. j. int., 180, 238-242; doi: 10.1111/j.1365-246x.2009.04418.x. puglia, r., r. ditommaso, f. pacor, m. mucciarelli, l. luzi and m. bianca (2011). frequency variation in site response over long and short time scales, as observed from strong motion data of the l'aquila (2009) seismic sequence, b. earthq. eng., 9, 869-892; doi: 10.1007/s10518011-9266-2. scognamiglio, l., e. tinti, a. michelini, d. dreger, a. cirella, m. cocco, s. mazza and a. piatanesi (2010). fast determination of moment tensors and rupture history: application to the april 6th 2009, l'aquila earthquake, seismol. res. lett., 81 (6), 892-906; doi: 10.1785/gssrl.81.6.892. valoroso, l., and l'aquila 2009 mobile network working group (2009). the 2009 l'aquila seismic sequence (central italy): fault system geometry and kinematics, eos trans. agu, 90 (52), fall meet. suppl., abstract u12a-04. *corresponding author: lucia margheriti, istituto nazionale di geofisica e vulcanologia,centro nazionale terremoti, rome, italy; email: lucia.margheriti@ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. margheriti et al. 399 the newtonian approch in the meteorological tidewaves forecasting: preliminary observations in the east ligurian harbours the newtonian approach in the meteorological tide waves forecasting: preliminary observations in the east ligurian harbours osvaldo faggioni (1), giovanni arena (2), mauro bencivenga (2), giuseppe bianco (3), roberto bozzano (4), giuseppe canepa (5), paolo lusiani (6), gabriele nardone (2), giovanna lucia piangiamore (1), maurizio soldani (1), luciano surace (6), giuseppe venzano (7) 1) ingv stazione di geofisica marina, villa pezzino, via pezzino basso 2, 19020 fezzano (sp), italy 2) apat dip.to tutela acque interne e marine – servizio mareografico, via curtatone 3, 00185 roma, italy 3) asi centro di geodesia spaziale “g.colombo”, loc. terlecchia, 75100 matera, italy 4) cnr istituto di studi sui sistemi intelligenti per l’automazione, via de marini 6, 16149 genova, italy 5) autorità portuale di genova, ufficio ambiente, via della mercanzia 2, 16100 genova, italy 6) mm istituto idrografico, passo dell’osservatorio 4, 16132 genova, italy 7) sepg servizi ecologici porto di genova, via molo giano, 16128 genova, italy sea level oscillations are the superposition of many contributions, among which the main are astronomic and meteorological low-frequency tides. in ligurian sea meteo-tide components, being most ample than astronomic fluctuations, drive water exchange in harbours. the present note shows first results about port of genoa concerning a coherency study between atmospheric variation and corresponding sea level adjustment (meteorological tide). the newtonian forecasting method of meteorological tides is based on measurements of time elapsing between barometric sea level unbalance (δg) and its meteorological tide compensation (inverse barometer component). meteorological tide component is independent on the earth-moon-sun gravitational relationships, moreover parameters related to the shifted water mass are too many to describe the phenomenon analytically (basin topography, barometric strength position and time, chemical water quality, off-shore sea circulation, etc.); then, meteorological tide can’t be accurately foreseen by atmospheric pressure measurements only. a gravimeter can detect the geodetic unbalance starting time and a tide-gauge can detect the newtonian compensation (tide wave) coming time. the difference between these two times is the meteorological tide delay. an opportune statistic of this delay provides an experimental law typical for each harbour to forecast the meteo-tide compensation wave delay. this paper describes the methodological procedure adopted and first evidences of the phenomenon in genoa harbour. key words: meteorological tide waves, newtonian sea level compensation, tide forecasting, environmental harbours quality. the phenomenon astronomical tide is a regular and periodical sea level up-down motion that depends on the earth-moon-sun gravitational relationship. such a phenomenon can be easily described by a fourier analysis, both in its elementary (fundamental) components band and in the composed harmonics resulting from interference among the fundamental waves themselves. the harmonic sinusoidal tide components have specific amplitude depending on the site of observation, and they are characterized by typical and recurrent frequencies (see tab. i). the rete mareografica nazionale (rmn), managed by the agenzia per la protezione dell’ambiente e per i servizi tecnici (apat) servizio mareografico, provides a coverage of the italian coasts that allows to observing effectively the sea level fluctuations. the harmonic analysis of these data underlines both the presence of the fundamental tide components and the composed ones (also named over-tides), that cover an important role in the sea level fluctuations of shallow water (m4 e ms4; see tab. i). together with these contributions, there are some sea level fluctuations due to a pressure variation of thermic origin (e.g. alternation night-day) and, above all, aperiodic low-frequency sea level fluctuations (due to meteorological pressure variations) representing the background level on which astronomic components overlap. these last fluctuations depend in variable way from the transit of atmospheric fronts or, in general, from the atmosphere meteorological dynamics over the considered sea basin (soldani et al., 2004; cattaneo, 1979), and therefore they aren’t predictable by harmonic analysis. when a high-pressure area moves on a free water surface, it origins an additional weight on it. the isostatic reaction of free water surface is a concave adjustment with respect to the starting surface to compensate, in the local atmosphere-water column, the increase of atmosphere weight with a water out-flow (low meteorological tide; see fig. 1); on the contrary, when a perturbed front produces a diminution of atmosphere weight, the isostatic compensation adjustment will be realized in a bump produced by a flow of incoming tide (high meteorological tide). the time among the pressure unbalance and its newtonian compensation is called “tide inertness”. in a lot of europen harbours, the amplitudes of maxima meteorological tide waves (briefly named meteo-tides) are sensitively greater than astronomical tide waves (astro-tides); this phenomenon is typical in the gulfs of la spezia and genoa (fig. 2a), where the merchant and military harbour structures are subject, under not exceptional meteorological conditions, to low-frequency meteo-tide flows showing an amplitude even four times greater than normal astro-tide amplitudes. therefore, the importance of forecasting the meteo-tide flows is remarkable for safety reasons, environmental protection aspects, and management of nautical traffic in waterways, harbours and in the coastal navigation. as for the environmental question in harbours such as la spezia and genoa, the meteo-tides drive the greatest flow-reflow impulses of the harbour waters, having a primary role i.e. in pollutants dispersion and waters oxygenation balance. the knowledge of such phenomenon, besides defining the capacity of the harbours to resist to environmental impact, is therefore able to plan human activities in the harbour waters. if, for instance, an harbour must be submitted to the dredging of the waterways or to any other heavy interaction activities with shallow water, the statistical knowledge of the seasonal period of maximum water exchange and the ability to forecast the exchange times will result decisive in minimization of environmental impact, because they can provide some indications on the fitter time to works execution. currently, flow times and amplitudes of meteo-tides, unlike the astro-tides, are not easily predictable because the correlation between atmospheric pressure variation and consequent sea surface adjustment (meteo-tide) is generic both in temporal phase-displacement and amplitude, and the stochastic nature of the atmospheric dynamics so it doesn't provide an effective parameter in the harbours environmental management (and nautical traffic). the purposes of this work are the development of a quantitative method to forecast the flow times and amplitude of meteo-tides (based on the joined use of barometers, tide-gauges, gravimeters and clocks), the description of the experimental test to obtain a preliminary law of meteo-tide parameters studying the meteo-tides as adjustment of the geodetic surface (free sea surface), and the presentation of first results about classification of astronomic (high) and meteorological (low) frequency bands in the fluctuations of sea level measured by the meteo-mareographic apat station placed in genoa harbour (fig. 2b). the methodological approach meteo-tides flows can be considered as adjustments of free sea surface resulting from local atmospheric weight variations. when sea level is in isostatic equilibrium with atmosphere weight, tide fluctuations concern only astronomic components. when an atmospheric weight variation occurs, sea surface reacts by a low-frequency fluctuation getting to a new equilibrium state. such adjustment is characterized by a delay time due to the different dynamic responses of atmosphere compared with sea water. this delay cannot be evaluated with a theoretical law on the basis of barometric measurements. meteo-tide flow and reflow times, in fact, feel the effects of many local factors influence: the most important of them are coast and bottom morphology and, in second order, the currents, the wind and the sea water density. on the contrary, if a gravimetric anomaly signal will be detectable, the force producing meteo-tide time of start, related to atmospheric conditions change, will be technically measurable before the sea surface adjustment related to barometric variations (faggioni et al., 2004). an atmospheric weight increase induces a newtonian adjustment in sea surface; when the cause (high pressure) generating such adjustment stops, the sea surface is in geopotential unbalance conditions and produces a gravimetric anomaly inducing a newtonian tide flow to compensate this unbalance (returning to the sea surface geometry preceding the increase of pressure), as a barotropic propagating wave. then, using a gravimeter near the meteo-mareographic station provides an essential preliminary datum, since such instrument (if sensitive to the phenomenon) is able to measure the newtonian signal of geodetic unbalance when it occurs. if a tide-gauge is joined to the gravimeter, the sea level measurement and the meteo-tide wave arrival time can be achieved. the difference between gravimetric maximum time and meteo-tide wave arrival time (that is the delay between the newtonian generating cause and the geodetic reaction) is the meteo-tide time of flow (or outflow). a reasoned statistic of these measurements is able to provide the law of meteo-tide delay based on the entity of the newtonian generating push. such law, characteristic of every harbour, will become the gravimetric measure predictors of meteo-tide delay time. instrumentation the observation of meteo-tides newtonian phenomenon is carried out by multi-instrumental stations composed by mareographic, meteorological and gravimetric instrumentation; such stations are called newtonian meteo-mareographic stations (nms). the data processed in this work come from a meteo-mareographic apat rmn station in genoa harbour, (genoa aquarium station) (latitude φ=44°24’31” n; longitude λ=08°55’33” e; see fig. 3). the main component of this station is the id5793/a mechanical tide-gauge. it measures sea level vertical motion by a craft suspended by means of an anti-torsion steel cable. the tide-gauge is referred to the zero level igm (istituto geografico militare) genoa 1942. the analogical measure is plotted by a nib connected to a drum with useful recording height of 250 mm and rotation speed of 2 mm/h. the signal amplitude is turned also into electric signal by means of a transducer; a converter a/d gives in output the sampled signal digital form. such device has a precision of ± 0.5 cm and vertical run sensitivity of 0.5 cm. in a short time, a nms, with an ultrasounds hydrometer tide–gauges with sensitivity of 0.1 cm, maximum systematic error of 0.1 cm and sampling rate variable from 1 to 0.05 hz, will be installed in la spezia harbour, too. the barometer for atmospheric pressure data acquisition is constituted by a silicon capacitive transducer to guarantee high measure repeatability and stability for long period. this instrument has a measurement field of 500-1100 hpa with repeatability of ± 0.03 hpa, resolution of 0.01 hpa, non linearity of ± 0.05 hpa and it has a serial interface of rs232 or ttl or rs485 or rs422 for the connection, through ascii protocol, to standard acquisition unity. as it regards the gravity acceleration measure, the starting experiment will be achieved through a series of absolute measurements every six hours by decreasing the sampling interval up to 15 minutes in correspondence to the high temporal pressure variation periods start and end. such type of sampling allows getting information of absolute gravity in low atmospheric pressure gradient periods either in high gradient ones. the absolute gravity measurements can be related to geodetic unbalance of the sea surface and to the consequent vertical water mass rearrangements. the scheduled instrument for such type of measure is the fg5 absolute gravimeter of the micro-g solutions, inc. the fg5 uses the free-fall method. its technological solution guarantees an accuracy of 2 μgal, a g measure precision of μgal·(hz)-½ at a quiet site and 10-30 °c of operating temperature range. then, the measures of the three instruments constituting the nms are referred to a clock that provides the independent common reference to the three signals (sea level, atmospheric pressure and gravity acceleration). the clock in the rmn meteo–mareographic stations is referred to gmt (greenwich meridian time). preliminary analysis of mareographic and barometric data we consider now sea level and atmospheric pressure measured between 12/02 and 04/11 2001 from the apat meteo-mareographic station in genoa harbour to evidence qualitative correlations between the two phenomena; both signals have been acquired every hour. sea level power spectral density (also named energy spectrum; see fig. 4) shows the presence of astronomic tide components (see tab. i); the most significant components have periods nearly equal of 24 and 12 hours and a remarkable low-frequency contribution, independent from astronomical phenomena. this low-frequency mareographic component is related to the atmospheric pressure pattern. (le traon et gauzelin, 1997; el-gindy et eid, 1990; garrett et majaess, 1984; garrett et toulany, 1982; mosetti, 1969; crepon, 1965). in particular, at low frequencies, atmospheric pressure energy spectrum (see fig. 5) looks similar to sea level energy spectrum. at high frequencies (astronomic tide components band), on the other hand, atmospheric pressure spectrum components aren’t significant. then, it’s obvious that atmospheric pressure variations drive sea level low-frequency fluctuations (stocchino et scotto, 1970). now, we are studying an appropriate low-pass filter able to exclude astronomic harmonics from mareographic signal and to keep only components depending on atmospheric pressure. we consider now sea level and atmospheric pressure measured between 01/03 and 04/04 2001 (see fig. 6). a first qualitative comparison of this two plots shows that atmospheric pressure maxima (minima) occur shortly before sea level minima (maxima): an increase (decrease) of atmospheric pressure comes before a sea level decrease (increase). especially, we study the time interval between 05/03 and 10/03 2001: in this period there is an entire cycle of rise-drop pressure (see fig. 7) and the consequent cycle of drop-rise sea level (see fig. 8). to evaluate the relative delay between the two signals, they are first interpolated and smoothed by a polynomial; the outcome (with maxima and minima) are shown in fig. 7 and fig. 8. geometrical comparison between low-frequency components of the two signals (see fig. 9) shows that a time ∆t≈13 h elapses between atmospheric pressure maximum time and corresponding sea level minimum (meteo-mareographic reflow) time; such delay is also between atmospheric pressure minimum time and sea level maximum (meteo-mareographic flow). moreover, we note that a sea level raising of |∆h|≈0.27 m corresponds to an atmospheric pressure decrease of |∆p|≈12hpa. we can now calculate the transfer factor jph for genoa harbour, with regard to the period between 05/03 and 10/03 2001: jph = |δp| / |δh| ≈ 12 / 0.27 ≈ 44.44 (hpa / m) this means that an atmospheric pressure increase of 44.44 hpa would induce a sea level decrease of 1 m. this value of jph provides an interesting starting point for the genoa harbour; moreover, it is in agreement with preliminary measurements performed in tuscan and ligurian harbours. conclusions to detect possible relationships (in phase and amplitude) between atmospheric pressure and sea level (starting point to perform a tide forecasting analysis), the two signals, acquired by the apat meteo-mareographic station near genoa aquarium, have been compared. the analysis of mareographic and barometric temporal series measured between 12/02 and 04/11 2001 allowed us to separate the frequency components in two bands: an high-frequency band having astronomic origin and a residual low-frequency band having newtonian origin. the low-frequency components of two energy spectra are related each other. in particular, the observation of atmospheric pressure and corresponding sea level measured in the time period between 05/03 and 10/03 2001 allowed us to relate the low-frequency component of the two signals. a variation of the atmospheric pressure and consequent opposite variation of the sea level are shifted of nearly 13 h. this delay represents the inertia of water mass responding to the newtonian pulse generated by a variation of the atmospheric pressure. finally, a comparison of the two signals amplitude variations during this pressure event (05/03 – 10/03 2001) allow us to evaluate the transfer factor jph for genoa harbour: jph = 44.44 (hpa/m). the meteo-mareographic newtonian transfer factor jph, once statistically defined over many pressure events in genoa and la spezia harbours, will be the starting point to evaluate the target of the gravimetric measures induced by atmospheric pressure variations (if harbour basin volume and water density will be known), that is the water mass moving during the newtonian low-frequency oscillations of sea level in harbours of the ligurian coast. aknowledgments: the authors wish to thank the ap sp (port authority of la spezia), the ap ge (port authority of genoa) and the sepg society (servizi ecologici porto di genova) for their financial support. we also thank apat servizio mareografico for the data shown in this report. references cattaneo, m. 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(1969): oscillazioni del livello medio marino a venezia in rapporto con le oscillazioni di pressione atmosferica, boll. geof. teor. appl., xi, n. 43-44, 264-277. soldani, m., regazzoni, c.s., bozzano, r., faggioni, o. and m. musso (2004): misure ambientali in mare aperto: sviluppo di tecnologie per l’acquisizione e l’analisi di dati meteo-mareografici misurati da una boa oceanografica in mar ligure, 23° convegno gngts, roma 14-16 dicembre 2004. stocchino, c. and v. scotto (1970): il livello medio marino e la pressione atmosferica nel porto di genova, bull. inst. oceanogr. monaco, 69, 1399-1418. captions: ref _ref96764140 \* mergeformat tab. i main astronomic tide components, periods and radian frequencies. fig. 1 picture of the geodetic phenomenon of sea level deformation due to atmosphere weight. fig. 2 a) geographical localization of genoa and la spezia harbours. b) the internal part of genoa harbour with localization of apat-rmn station (graphic base from mapsend bluenav europe v. 1.00d – modified). fig. 3 the meteo-mareographic station (belonging to apat rmn) in genoa harbour. fig. 4 power spectral density of sea level measured by apat station in genoa harbour between 12/02 and 04/11 2001. fig. 5 power spectral density of atmospheric pressure measured by apat station in genoa harbour between 12/02 and 04/11 2001. fig. 6 a) sea level measured by apat station in genoa harbour between 01/03 and 04/04 2001. b) atmospheric pressure measured by apat station in genoa harbour between 01/03 and 04/04 2001. fig. 7 atmospheric pressure measured by apat station in genoa harbour between 05/03 and 10/03 2001 (bold line) and its low-frequency component (dashed line). black triangle shows the time of the pressure maximum, while white triangle shows the time of its minimum. fig. 8 sea level measured in genoa harbour by apat station between 05/03 and 10/03 2001 (bold line) and its low-frequency component (dashed line). black triangle shows the time of the sea level maximum, while white triangle shows the time of its minimum. fig. 9 low-frequency components of pressure (bold line) and sea level (dashed line). x axis represents the time, y axis on the left represents the pressure, and y axis on the right represents the sea level. harmonic period (h) radian frequency (rad/sec) o1 25.819.341 6.7597744 e-5 π1 24.132.140 7.2323884 e-5 p1 24.065.890 7.2522945 e-5 s1 24.000.000 7.2722052 e-5 k1 23.934.469 7.2921158 e-5 j1 23.096.474 7.5560361 e-5 2n2 12.905.374 1.3524049 e-4 η2 12.871.757 1.3559370 e-4 n2 12.658.348 1.3787969 e-4 ν2 12.626.004 1.3823290 e-4 m2 12.420.601 1.4051890 e-4 l2 12.191.620 1.4315810 e-4 t2 12.016.449 1.4524500 e-4 s2 12.000.000 1.4544410 e-4 r2 11,983595 1.4564320 e-4 k2 11.967.234 1.4584231 e-4 m4 6.210.300 2.8103780 e-4 ms4 61.033.392 2,8596300 e-4 tab. i fig. 1 fig. 2 fig. 3 fig. 4 fig. 5 fig. 6 fig. 7 fig. 8 fig. 9 1019 hpa 1007 hpa 0.05 m 0.32 m 06/03/01 h10 08/03/01 h18 06/03/01 h23 09/03/01 h07 atmospheric pressure sea level (ref. 0 igm) sea surface x y z scale units x km e2; y km e2; z km e-4 water flow out high atmospheric pressure period ≈ 24 h period ≈ 12 h δp δh δt δt a b 08° 00’ 00” e 10° 00’ 00” e 44° 00’ 00” n • • la spezia genoa i t a l y l i g u r i a n s e a n the harbour of genoa 44° 24’.5 44° 24’.7 8° 55’.7 8° 55’.5 8° 55’.3 apat rmn station “genoa aquarium” a b page 4 bianchi 697_705 697 annals of geophysics, vol. 46, n. 4, august 2003 signal processing techniques for phase-coded hf-vhf radars cesidio bianchi (1), umberto sciacca (1), achille zirizzotti (1), enrico zuccheretti (1) and baskaradas james arokiasamy (2) (1) istituto nazionale di geofi sica e vulcanologia, roma, italy (2) tril fellow, the abdus salam international center for theoretical physics, trieste, italy abstract hf-vhf radar techniques are easy to employ and commonly used in geophysical applications. they include deep radio soundings, used for probing the ionosphere, stratosphere-mesosphere measurement, weather forecast and radio-glaciology. fast algorithms and powerful processors facilitate the development of several kinds of low power radars, but the reduction of the transmitted power has to be compensated by on-line processing of an encoded signal to maintain a favorable signal-to-noise ratio suitable for detection. moreover, radars have to reconstruct return echoes with different travel times due to various origins (multi-path, adjacent objects, etc.). such needs can be accomplished by means of signal phase coding and one of the most attractive is the reversal phase code. the composite echo signal must be processed to extract the physical information useful for the measurement considered. in this paper some algorithms used for on-line processing of phase-coded signals will be described, both in time and frequency domain. mailing address: dr. cesidio bianchi, istituto nazionale di geofi sica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: bianchi@ingv.it key words phase-coded radar – pulse compression – signal integration the required performances, has become possible. although these algorithms can be implemented both in time and in frequency domain, working in frequency domain has given faster and more powerful results. such on-line processing techniques have been used in radar systems developed at ingv (ariokasamy et al., 2002). the present work illustrates the concepts behind the signal processing techniques implemented in systems like those, though, a more general description and design reference can be found in radar manuals (skolnik, 1990). figure 1 contains the sequence of the most general steps in phase-coded radar signal processing. the essential fi rst step is the quadrature sampling and the analog-to-digital (a/d) conversion. in this way the information is available in digital format, ready for the actual processing carried out in the second step. the quadrature sampling is preferred because it allows all the information included in the received signal to be captured for subsequent processing. these two passages are essential for the following detection 1. introduction the current trend in hf-vhf radars is to simplify the system hardware, reduce transmitted power and develop more powerful techniques of signal processing (woodmann, 1980; hunsucker, 1991). the latter is one of the challenging jobs of these radars (phase-coded included), because it consists of a series of tasks, each involving a heavy iterative procedure. with the advent of powerful digital signal processors (dsps) and very fast pcs, the implementation of these algorithms, that give to hf-vhf radar systems 698 cesidio bianchi, umberto sciacca, achille zirizzotti, enrico zuccheretti and baskaradas james arokiasamy process, to extract relevant information from the received echo (i.e. position, velocity, refl ected energy, etc.). the third step is the operations on the processed signal, such as data display or data storage for further analysis (off-line), worked out by a pc. working in time domain is easier for some aspects, because the changing of domain introduces two more tasks, the fourier transform and its inverse. nonetheless, in frequency domain new processing techniques are possible, so such an approach has become dominant. figure 1 is representative enough of the physical units constituting the hardware of the system. the fi rst step is usually implemented by a digital circuitry, which follows an analog receiver providing an amplifi cation and fi ltering of the received echo (radio frequency). in some cases a good design of the receiver can limit the amount of noise entering the system: the noise produced inside the receiver itself (thermal, due to images, etc.) and those coming from the environment that are not exactly inside the frequency band used by the radar system. in other cases it is diffi cult to discard such external noise, so different methods must be applied to increase the signalto-noise ratio (s/n); they are implemented subsequently. the signal processing is usually done by a specifi c dsp, but, because of the calculation power of modern pcs, could be carried also on a general purpose computer. a tradeoff between cost, speed and system requirements has to be made before choosing the type of computing machine. in the following paragraphs the steps included in the signal processing will be discussed. before this description, it is important to delineate the methods of phase coding used to improve the radar performance. 2. phase coding briefl y, the coding of the transmitted signal is adopted to improve the radar resolution increasing the bandwidth. this is necessary both for increasing some features (e.g., maximum distance covered by the system) and for attaining a greater immunity to disturbing sources. to achieve this, the designer cannot use a simple decreasing of the time length of the transmitted pulse without loosing detection capability or increasing the cost of the transmitter. it is well known (skolnik, 1980) that an increased bandwidth can be attained by means of a proper coding of the signal, now fulfi lled in a digital manner. the easiest way to encode the signal is using a particular type of psk (phase shift keying) that makes the carrier phase change only between two values (0° and 180°) according to a sequence of binary digits. figure 2 depicts what happens when a carrier sin(w t) is multiplied by the sequence of bits c (t) composing the code. the total sequence establishes the length of the pulse. the duration of a single symbol/bit is called «subpulse» and is related to the bandwidth of the encoded signal (the shorter the subpulse, the greater the band). the phase change (0 or p radians) is obtained at each subpulse t or its multiples nt, while the whole pulse duration is t. this linear operation instantaneously gives m (t) = ± 1◊sin(w t) (that fig. 1. signal processing general steps. 699 signal processing techniques for phase-coded hf-vhf radars is m (t) = sin(w t) if c (t) = 1 or m (t) = sin (w t + + p) in correspondence with c (t) = –1). using exponential notation (2.1) where w is the angular velocity of the wave carrier and f is the phase. after the modulation process the signal is amplified, transmitted through the media and a small fraction of energy can be refl ected back. the echo signal (replica) s(t), apart from the amplitude, has the same characteristics of m(t) with a noise n(t) superimposed. in practical cases, the echo signal s(t) spans from several tens of db above the noise to units of db below the noise platform. because of the multipath propagation the echo signal s(t), which is a composite attenuated and time delayed signal, can be written as (2.2) (2.3) where a i is the amplitude of i-th multipath component of the signal, and d i is the propagation delay related to the path i. the information on the phase f i is now contained in d i . of course a decoding is to be carried out when the echo is received and processed to extract the information it contains; this is usually called correlation and will be described later. even in case of optimal reception, correlation gives rise to spurious outputs called «sidelobes», that can be seen as a sort of mathematical noise which can interfere with the detection of other echoes. many types of codes have been developed, one is the barker code, characterized by low and equal sidelobes and easy implementation. codes that reduce sidelobes completely were developed, e.g., the complementary codes (golay, 1961), using a couple of pulses combined together after correlation. such codes are harder to implement, because if the combination is not perfect sidelobes appear again. complementary codes have been used in few radar applications because they rely on a constant radar-target position between code transmission and so they are not particularly suitable for tracking of moving targets. on the contrary, for geophysical application the target distance is in most cases constant. the two mentioned types of codes have been used in the radar systems developed at ingv, giving satisfactory results. when receiving the echoes, the code should be reconstructed by determination of the phase variations of the carrier. given that no information on the delay is known, the sampling canm t c t e j t( ) = ( ) −( )ω φ s t a c t ei i j t i i( ) = ( ) −( )∑ ω φ s t a m t di i ii( ) = −( )∑ fig. 2. bi-phase coding. 700 cesidio bianchi, umberto sciacca, achille zirizzotti, enrico zuccheretti and baskaradas james arokiasamy not be done in correspondence of the maxima of the carrier, with the risk of losing much of the energy entering the receiver. the best solution is to perform a double sampling, with a couple of values of the received signal for each period of the carrier, the second sample being taken a quarter of a period after the fi rst. in this way, beside sampling for the a/d conversion, a baseband conversion is also performed. after sampling, two streams of data are available, the first usually called the in-phase (i ) and the second the quadrature (q). this operation leads to discrete-time domain yielding n couple of values, where the signal amplitude and phase are recovered by a n = (in 2 + q n 2)1/2 and f n = tan–1 (i n /qn). a composite baseseband containing all the multipath delays compatible with the sampling window signal remains. this baseband can be expressed mathematically using eq. (2.3) and remembering the change of time from the continuous to the discrete domain (2.4) a i contains the attenuation information of the single i-th multipath echo. once they have been stored, the actual processing can take place. it is worth remembering that if a multiple echo enters the receiver (due to multipath or to the presence of multiple refl ecting structures) the possible superposition of the long received encoded pulses does not necessarily give rise to loss of information, which can be reconstructed after correlation, assuming the time separation between pulses is long enough to be resolved by the system bandwidth. 3. time domain processing if we choose to perform signal processing in time domain, we can extract the information from the received encoded signal as follows. as stated before, this process is called correlation, it is a sort of weighted moving average of the received signal, the weight being a copy of the encoded transmitted signal stored inside the correlation processor. it is possible to demonstrate that the correlation gives an output shortened in time, or «compressed», reducing the actual duration of the pulse to one subpulse length (neglecting the sidelobes). if the bandwidth of the system is bw, the total code length is t and the subpulse is t the peak power of the compressed pulse can be increased by a compression factor m (3.1) where bw = 1/t. the position of the maximum and its amplitude give information about distance and refl ecting properties of the target. multiple echoes can be resolved if the time separation between two near received signals is greater than the subpulse length. the actual detection of an echo is obviously determined also by other factors: noise, transmitted power, the attenuation due to the path and the refl ecting characteristics of the targets, etc. in discrete time processing the correlation of the baseband signal b(n), that contains the code sequences (echoes), with c(n), that is the discrete-time version of the function c(t), can be written as (3.2) where n is the number of samples of the acquired baseband temporal segment, m is the code length, n is an index that runs from 1 to n and m is an index that runs from 1 to m. neglecting the side lobes of the correlation, it follows: (3.3) where, d i takes into account all the possible delays (all the replicas compatible with the sampled window), d (n – d i ) has value 1 or 0 depending on the lag in the correlation operation. using a complementary phase code (no side lobes), the relation (3.3) represents the exact result. such a time domain process was developed at ingv by means of a dsp. the i and q streams were combined in order to obtain the real amplitude of the received baseband signal. then a correlation was performed, both in a system that used the barker code and in another that used the complementary code (it was a preliminary implementation of the cited ais-ingv). the processing power gain in decic n b n c n b n m c mbc m m n n( ) = ( ) ( ) = +( ) ⋅ ( ) == ∑∑ 11 c n ma n dbc i ii( ) = −( )∑ δ b n a c n di ii( ) = −( )∑ m bw t t= ⋅ = τ * 701 signal processing techniques for phase-coded hf-vhf radars it is important to note that the i and q signals at input are treated as real and imaginary parts of a single complex signal (i + jq), so the actual algorithm to be used to perform a fourier transform is a complex fft (cfft) and its inverse (icfft), having the resulting spectrum different from the real fft. in addition, all calculations performed in frequency domain take into account a single «complex» signal, even though some processes may act on the real and imaginary part separately. figure 4 shows an example of cfft of the acquired data signal. it is well known from signal theory that correlation can be performed in frequency domain, simply multiplying the spectrum values of the received signal by the ones of the complex conjugate of the transmitted pulse. it follows: (4.1) where b(w n ) and c(w n ) are the time discrete fourier transform of the baseband echo and the code. this calculation is simpler than the correlation in time domain; it is true that complex numbers have to be multiplied (fi g. 5), while in time domain the values were reduced to their modulus, but the price of this simplifi cation was a loss of information, now excluded. it is possible to demonstrate that the loss of computing time due to the transform processes (direct and inverse) can be compensated by the simplifi cation of the correlation; this compensation depends on various factors, mostly the algorithm chosen to perform transformations, the number of samples constituting the signal at the input and the length of the code. 5. frequency domain processing: fi ltering once the spectrum of the received echo is available, it is possible to perform a fi ltering in different ways. of course, the classical fi ltering accomplished by an analog band pass fi lter is also possible and it was done. a good analog fi ltering carried out inside the receiver can cut off all the undesired signals entering the system (image frequencies, interfering radio stations, etc.). an analog process is never a perfect one, bels due to this correlation operation is about g mc = 20 log (db) . the extraction of the modulus of the received signal was chosen to make the system easy to implement, even though it does not permit a coherent integration, as discussed in a next paragraph. 4. frequency domain processing: fft and correlation figure 3 shows all the passages included in a typical frequency domain process. in the next paragraphs the filtering and integration will be discussed. the present section summarizes some topics on correlation and fourier transform (rabiner et al., 1975; oppenheim et al., 1999). c n b n c n b cbc n n( ) = ( ) ( ) = ( ) ⋅ ( ) ω ω fig. 3. frequency domain processing schematics. * * 702 cesidio bianchi, umberto sciacca, achille zirizzotti, enrico zuccheretti and baskaradas james arokiasamy fig. 4. spectrum of the complex fft (cfft) of the acquired 1024 i and q samples (the x-axis in arbitrary units). fig. 5. correlation in the frequency domain between the cfft and the spectrum of the code (the x-axis in arbitrary units). 703 signal processing techniques for phase-coded hf-vhf radars because some residual out-of-band signal can pass through the receiver and appear in the computed spectrum in the dsp. it can be cut off easily simply putting to zero some spectral components, those referring to frequencies beyond a specifi ed limit. it is clear that this kind of filtering cannot resolve hard interference problems, which can be treated only by means of a good design of the receiver. the real advantage in using a frequency domain fi ltering is the easy processing of noise lying inside the useful band. in fact, if a strong interfering carrier stays too near to the radar carrier it is not cancelled by a classical fi lter. the method we used consists in putting to zero (or clipping) all the single spectral lines whose energy overcomes a threshold, assuming that a typical spectrum does not have such «peaks». the threshold value is infl uenced mostly by the total energy content of the signal entering the processor, but it was determined after empirical trials. this fi ltering method leads to a slight distortion in the shape of the spectrum, without great consequences. we know it is not the best way to reduce such interfering signals, but it was used for its great ease of implementation. 6. frequency domain processing: integration the minimum detectable signal can be improved by an integration process (north, 1963), a term indicating a sum of n received signals, acquired after n successive transmissions. the amount of improvement depends on the place in the receiving chain where the integration is carried out. in fact, the noise can also increase its power as a consequence of integration, so the fi nal gain in s/n is the result of the combination of the signal and noise integration. if the integration is done at the end of the receiving chain, i.e. after detection so that the modulus of the signal is extracted, the information on the phase of the signal is lost. in this case, the process is called not-coherent integration; both signal and noise sum their amplitudes in a statistical way (rms), so the gain in the s/n (as a power ratio) remains theoretically unchanged. nonetheless, it is possible to prove that the system increases its performance because of the reduction of the variance of the received signal plus noise. in this way, the probability of detection increases. in some cases, this type of integration is the only possibility, and it is effective enough when the amplitudes of the received signals vary rapidly. if the phase of the received signal (as a complex function i + jq) were known, it could be used to sum all k echoes with the same phase, while the noise would continue changing its phase randomly. in this way, the s/n (as a power ratio) would increase by a factor k. this process is called «coherent integration». if a sum of the echoes is performed before detection the information on the phase is maintained and, if the phase of the successive echoes does not change, the process can be as effective as a real coherent integration. equation (4.1), letting k be the index of the integrating echoes (from 1 to k ), becomes (6.1) obviously this ideal condition never occurs, but it is suffi cient for the phase differences between the integrating echoes to remain low. even great phase differences can increase the s/n (not as much as the ideal factor k ), provided they never exceed p /2. in order to avoid the phase changing too much, the maximum number of pulses to be integrated should be determined, but this is a very difficult task for the intrinsic variations experienced in the geophysical environment. the processing power gain due to phase coherent integration is about g ki = 20 log (db). in the radar developed (ariokasamy et al., 2002) the coherent integration was implemented, and the duration of integration was determined empirically. an example of the non integrated signal is given in fi g. 6 while in fi g. 7 a phase coherent integration process for k = 10 is shown. a non-coherent integration process can be performed to reduce the noise and signal + noise variance in the post processing operation. i c n b cbck k n nkkcoherent = ( ) = ( ) ⋅ ( ) ∗∑∑ ω ω . 704 cesidio bianchi, umberto sciacca, achille zirizzotti, enrico zuccheretti and baskaradas james arokiasamy fig. 6. inverse complex fft showing the correlation peak (time delay of the echo). fig. 7. resulting peak after icfft of 10 iterations of the phase coherent integration process. 705 signal processing techniques for phase-coded hf-vhf radars 7. conclusions low power phase-coded hf-vhf radars useful for geophysical application require some particular processing on the encoded signal to extract physical parameters. the reduced transmitting power (200-500 w), instead of the corresponding power of an envelope radar (about 4000-10 000 w), is compensated by an on-line processing gain due to the pulse compression and integration. the described algorithms were implemented at fi rst in a fi xed and a fl oating point dsp board, then also in a pc microprocessor. those processors were used in a couple of systems developed at ingv: a radar employed for electronic density measurement in the upper atmosphere and a radio echo sounder used for radio glaciology measurements. it was found that knowledge of the physical radio pro pagative phenomena such as the particular medium and target nature can be useful to extend as long as possible the integration operation without losing the phase coherence. though the frequency domain signal processing requires more mathematical operations, the speed of the whole process is comparable with the time domain. in fact, operating in frequency domain, the correlation is simply executed by a product of the two spectra and after the integration only a ifft is required. another advantage arises implementing a fi ltering algorithm directly onto the spectrum, clipping and cutting off unwanted frequency components. these issues, besides allowing less peak transmitted power, also improve the overall signal-to-noise ratio of the employed radar systems for geophysical applications. references arokiasamy, b.j., c. bianchi, u. sciacca, g. tutone and e. zuccheretti (2002): the new ais-ingv digital ionosonde design report, ingv internal technical report n. 12. golay, m.j.e. (1961): complementary series, ire transaction on information theory, it-7, 82-87. hunsucker, r.d. (1991): radio technique for probing the terrestrial ionosphere, physics and chemistry in space, edited by m.c.e. huber, l.j. lanzerotti and d. stoffler (springer-verlag, berlin heidelberg), vol. 22. north, d.o. (1963): an analysis of the factors which determine the signal/noise discrimination in pulsed carrier systems, proc. ieee, 51 (7), 1015-1028. oppenheim, v., a. schafer, w. ronald and j. r. buck, (1999): discrete-time signal processing (englewood cliffs, new jersey, prentice hall), 2nd edition, pp. 870. rabiner, l.r. and b. gold (1975): theory and application of digital signal processing (englewood cliffs, new jersey, prentice hall), pp. 762. skolnik, m.i. (1980): introduction to radar systems (mc graw-hill, new york), 2nd edition, pp. 442. skolnik, m.i. (1990): radar handbook (mc graw-hill, new york), 2nd edition, pp. 1200. woodman, r.f. (1980): high altitude resolution stratospheric measurements with the arecibo 430 mhz radar, radio sci., 15, 417-422. (received february 5, 2003; accepted july 11, 2003) 1 annals of geophysics, 65, 3, dm319, 2022; doi:10.441/ag-8817 o p e n acc e s s integrating geological data in europe to foster multidisciplinary research marc urvois*,1, sylvain grellet1, henning lorenz2, rainer haener3, christelle loiselet1, matthew harrison1, matija krivic4, christian brogaard pedersen5, marianne b. wiese5, amelia baptie6, martin nayembil6, james trench7, ivor marsh8, carlo cipolloni9, chiara d’ambrogi9, maria pia congi9 (1) brgm, french geological survey, orléans, france (2) uppsala university, department of earth sciences, uppsala, sweden (3) gfz, helmholtz centre potsdam, german research centre for geosciences, potsdam, germany (4) geozs, geological survey of slovenia, ljubljana, slovenia (5) geus, geological survey of denmark and greenland, copenhagen, denmark (6) bgs, british geological survey, the lyell centre, edinburgh, united kingdom (7) gsi, geological survey ireland, booterstown hall, ireland (8) marsh technology, ireland (9) ispra, italian institute for environmental protection and research, department for the geological survey of italy, roma, italy article history: received march 5, 2022; accepted june 27, 2022 abstract the european plate observing system (epos, www.epos-eu.org) is a multidisciplinary pan-european research infrastructure for solid earth science. it integrates a series of domain-specific service hubs (thematic core service, tcs) such as the geological information and modelling, which provides access to data, data products and services on european boreholes, geological maps, mineral occurrences, mines and 3d models. tcs gim services are hosted by a group of european geological surveys and a couple of national research organizations. this paper presents novel data discovery and integration, facilitated using borehole logging information with on-demand web services to produce 3d geological structures. this domain interoperability across epos was created for the purpose of research, but it is also highly relevant for the response to societal grand challenges such as natural hazards and climate change. european and international interoperability implementation frameworks are well described and used (e.g., inspire, iso, ogc, and iugs/cgi). it can be difficult for data providers to deploy web services that support the full semantic data definition (e.g., ogc complex feature) to expose several millions of geological entities through web-enabled data portals as required by pan-european projects. the tcs gim group implemented and innovatively extended two standardized descriptions, i.e. geosciml-lite and earthresourceml-lite, with an important reuse of content from linked data registries. this approach was applied to design and implement the european borehole index and associated web services (view-wms and discovery-wfs), extended to 3d models, geological maps as well as mineral occurrences and mines. results presented here apply the linked data approach ensuring optimal semantic description and enriching the data graphs, with complex descriptions and contents. in this way, it is now possible to traverse from one borehole index instance to linked richer information such as the borehole http://www.epos-eu.org marc urvois et al. 2 geological log, groundwater levels, rock sample description, analyses, etc. all this detailed information is served following international interoperability standards (observations & measurements, groundwaterml 2.0, geosciml4, amongst others). key words: epos; geological information; borehole; fair; linked data 1. introduction geological data are essential information for different types of users (e.g., scientists, land planners, teachers, construction sector, citizens, etc.) and stakeholders (e.g., policy makers, investors, etc.). integrated data and product services like harmonized geological maps, boreholes and 3d models facilitate their respective tasks, such as: – to better understand the major geological characteristics of soil and subsoil within the area of interest (e.g., stratigraphy, lithology, structural elements, geotechnical and hydrogeological properties, etc.), with several applications, such as the assessment of potential georesources [gourcerol et al., 2019; gloaguen et al., 2020], and evaluation of geological stability at specific sites [perrin et al., 2015] as well as groundwater resource monitoring [vergnes et al., 2020]; – to identify the main factors that have driven the regional geological evolution of the study area in order to point out major active processes that may have implications for the society, e.g., in terms of geohazards [thiery et al., 2019], with the purpose to develop mitigation plans. large amounts of geological data are collected, stored, analyzed and published by the national geological surveys as well as universities and regional/national research centres. through interpretation and processing of the observation data, the value chain produces information, knowledge and predictive models derived from multidisciplinary sources [habets et al., 2010]. these are used to characterize natural processes, locate underground resources and to predict phenomena and mitigate them. such tasks require a well-developed data infrastructure that supports cross-domain science. from this perspective, the european geological surveys established a common information system called egdi1 over recent years [tulstrup et al., 2016]. through a series of thematic and data harmonization projects, european geoscientific data and information are now available from a common location, and complying with widely used interoperability standards. egdi covers onshore and marine geology, mineral resources, geohazards, energy, groundwater and soils [čápová et al., 2021]. depending upon the scope of the supporting projects, the geographical extension ranges from pilot study areas to the european continent. as part of the epos-ip project (2014-2019)2 [cocco et al., 2022; saleh contell et al., 2022], the geological information working group selected a subset of these data to be used as reference knowledge and background by the data and products services of other epos thematic groups, such as seismology, near fault observatories and volcanoes. these are (i) the geological map of europe at 1:1,000,000 scale, (ii) boreholes (geotechnical and scientific drilling, underground water, mineral resources exploration), (iii) mineral occurrences and mines and (iv) 3d geological models. as described in section 4 here after, the corresponding thematic web services and interfaces are implemented by the tcs gim3 group to serve these data to the epos ics-c [cocco et al., 2022] and make them accessible through the epos data portal4. under the umbrella of eurogeosurveys5, the data providers include geological surveys from france, u.k., italy, denmark, ireland, slovenia and czech republic as well as the department of earth sciences, uppsala university, sweden, bringing its expertise in scientific drilling projects and the helmholtz centre potsdam, german research centre for geosciences, germany, renowned for its 3d model expertise. 1 european geological data infrastructure: www.europe-geology.eu (last accessed 21/06/2022) 2 https://www.epos-eu.org/ (last accessed 21/06/2022) 3 https://www.epos-eu.org/index.php/tcs/geological-information-and-modeling (last accessed 21/06/2022) 4 https://www.ics-c.epos-eu.org/data/search (last accessed 21/06/2022) 5 the geological surveys of europe http://www.eurogeosurveys.eu/ (last accessed 21/06/2022) http://www.europe-geology.eu https://www.epos-eu.org/ https://www.epos-eu.org/index.php/tcs/geological-information-and-modeling https://www.ics-c.epos-eu.org/data/search http://www.eurogeosurveys.eu/ geological data to foster multidisciplinary research 3 the mineral occurrences and mines are georeferenced as points whereas the geological maps include points, lines and polygons, all being two-dimensional objects. in case of boreholes, these are 2d points with descriptions along a third dimension axis downwards from the ground level. considering the geographical extension of these national and transnational data sets, it is about managing millions of entities from several data providers and serving them through a one-stop-shop portal. the aim of the tcs gim is to design and implement efficient and sustainable access to geological multi-scale data assets through the integration of distributed infrastructure components (nodes) governed by the epos geology domain (geological surveys and research communities). the processing and use of simulation and visualization tools in virtual research environments will subsequently support the integrated analysis and characterization of complex subsurface structures and their inherent dynamic processes. in addition, faster and more comprehensive data discovery and access, and cross-referencing with third party services e.g., sample access, will also benefit traditional research. altogether, this work will in turn improve research outcomes and facilitate a deeper understanding of complex multi-scale geoscientific questions. 2. background for many years, the w3c has been leading a considerable collaborative effort supported worldwide by many actors from both private and public sectors in providing a set of technologies, standards and best practices that constitute the basis for representing, publishing and sharing the data over the web in a standardized way. the set of technologies and standards proposed by the w3c, also known as the “semantic web” technologies (semantic web layercake), are embodied by the four simple principles, known as the “linked data” principles [berners-lee t., 1989]: 1) use uris to name (identify) things. 2) use http uris so that these things can be looked up (resolvable, “dereferenceable”). 3) provide useful information about what a name identifies when it is looked up, using open standards. 4) refer to other things using http uri-based names when publishing data on the web. the so-called “web of data” results from the adoption of these principles by data providers over the world. it is not intended to be a substitution to the already existing “web of documents”, but rather to complement it to achieve the original vision of the web [berners-lee t., 1989]. “data on the web” working group released its first public draft document in 2015 which became a w3c recommendation [w3c, 2017a]. this set of recommendations aims that ‘data should be discoverable and understandable by humans and machines’. this was then complemented by a second document resulting from the ogc/w3c collaboration initiated in 2014: spatial data on the web best practices [w3c, 2017b]. these include the provision of metadata for the datasets, stable identifier in the form of http uri, and linking data through those http uris. adopting them is essential for the geological community as it enhances the discoverability, reuse, and enrichment of datasets of public interest for education, research, knowledge creation, and support of economic activities. fair principles were defined in 2016 to improve the findability, accessibility, interoperability, and reuse of scientific data both for machines and humans [wilkinson et al., 2016]. presently, those principles extend to any other digital assets such as services and software tools [koers et al., 2020]. the research data alliance also started to work on applying them to virtual research environment6 (vre). open science requires these principles to be implemented. two of them are worth noting for their formal impact. interoperability, both technical and semantic, drove dynamics such as the european inspire directive (directive 2007/2/ec of the european parliament and of the council of 14 march 20077). the data reusability is clearly included in the european commission “directive (eu) 2019/1024 of the european parliament and of the council of 20 june 2019 on open data and the re-use of public sector information”8. 6 https://www.rd-alliance.org/groups/fair-virtual-research-environments-wg (last accessed 21/06/2022) 7 https://eur-lex.europa.eu/legal-content/en/all/?uri=celex:32007l0002 (last accessed 21/06/2022) 8 https://eur-lex.europa.eu/legalcontent/en/all/?uri=celex:32019l1024 (last accessed 21/06/2022) https://www.rd-alliance.org/groups/fair-virtual-research-environments-wg https://eur-lex.europa.eu/legal-content/en/all/?uri=celex:32007l0002 https://eur-lex.europa.eu/legal-content/en/all/?uri=celex:32007l0002 marc urvois et al. 4 the european geological surveys have a long history of engagement with these international initiatives. brgm, france, was represented on the board of ogc 2009-2021 by françois robida. geosciml 1.0.0 was first published in 2005 but importantly brought under the ogc banner at release 4.1. the ogc geoscience domain working group [beaufils et al., 2018] was set up to align some of the geoscience standardisation initiatives around the globe in 2017 under the leadership of ogc and iugs/cgi. in the development of the inspire regulations guidance, the working groups for geology annex ii, mineral resources and natural risk zones [european commission, 2013; tomas et al., 2015] in annex iii were each chaired and largely populated by representatives of the national geological surveys. egdi [tulstrup et al., 2016] was built by the eurogeosurveys to coordinate the fair publication of european commission funded project geoscientific datasets. during the earlier phase of epos [eu h2020; epos implementation phase project, 2015-2019], several european geological surveys teamed up as a continuation of their respective activities in the standardization and interoperability activities (inspire, iugs/gci, ogc). these were national surveys from denmark (geus), ireland (gsi), italy (ispra), and france (brgm). in addition, academia were represented by the department of earth sciences, uppsala university, sweden, and the helmholtz centre potsdam, german research centre for geosciences, germany. the geological information and modelling tcs gim was formally established in 2019 under the umbrella of eurogeosurveys (egs), and through the extension of the initial group. the geological surveys of czech republic (cgs), slovenia (geozs) and u.k. (bgs) joined. the tcs gim consortium is chaired by egs, co-chaired by cgs, and coordinated by brgm. bgs is in charge of the stakeholder panel. the data and product services are provided under the umbrella of egs. three it contact point representing as many web service end points are in place by brgm, geus and geozs. 3. inter-linked data architecture during the epos-ip project, geological data services of interest for the epos geoscientific scope were identified and prioritized in the tcs gim roadmap. the first services delivered to the ics-c were the european boreholes, 3d models, the geological map of europe at 1:1,000,000 scale as well as the mineral occurrences and mines. to follow the path taken internationally by this very same community since 20 years, tcs gim decided early in the epos-ip project that it would base its it work solely on the iso, ogc, iugs/cgi, inspire, w3c and rda published identified standards and best-practices baseline. this philosophy has applied from this point in the following steps of the epos operational phase [cocco et al., 2022; saleh contell et al., 2022]. when needed, this work also contributed to the evolution of these standards. even though interoperability implementation frameworks are well described and used, it proved to be difficult for several data providers in several earlier pan-european projects to deploy the ogc services with full semantic data definition (ogc/iso complex feature) for the discovery and view of millions of geological entities. tcs gim decided to take a complementary route. thus, the way the indices are served is unique compared to previous interoperability projects in which the tcs gim community took part. indeed, the entire data architecture revolve around the idea of serving simplefeature data. fully linked data-oriented simple data payload provide the relevant link to semantically richer ones. the group reused and enriched specifications (e.g., geosciml, earthresourceml) and previous data collection projects when they were available (egdi, minerals4eu). it also complemented the specification (e.g., for models) and data collection (borehole and 3d model data) landscapes where no previous standard or european data set existed. as such, the tcs gim exposes at the central tcs level (i) a metadata description of the available data sets and services, (ii) a number of simplefeature “index services” towards the epos ics-c, (iii) a code list registry and (iv) a linked data resolver. these allow epos end users to search and locate boreholes, geological maps and features, 3d models, etc., based on the information referenced by the index services. information indices have been set up for data types that require data compilation for epos (e.g., model index) as well as for data types where such data were collected during previous european projects (e.g., geologicalunits by egdi, georesources by minerals4eu and mintell4eu). in this linked data approach, the data instances are associated with uris and point to other information resources also using uris. the linked data principles ensure the best semantic description (e.g., uris to shared code list registries entries) and also enrich an initial “information seed” (e.g., a set of borehole entries matching a search) with more content (e.g., uris to richer information in a complexfeature data flow). as a result, this pattern of simplefeature service with linked data has a positive geological data to foster multidisciplinary research 5 effect on the it architecture: interoperable services are simpler and faster to deploy (thus by more data providers) and there is no need to harvest a full ogc complexfeature data set. the architecture is also more sustainable and scalable. the european geological services code list registries were enriched with new vocabularies as part of the broader european geoscience registry9. in compliance with the relevant european inspire rules, this registry is now part of the inspire register federation, the central access point to the repository for vocabulary and resources. the european geoscience registry is available for reuse and extension by other geoscientific projects. as stated above, european geological data are exposed with a linked data approach. the borehole index gives access to the location and a limited number of descriptors whereas the comprehensive data include full information on the borehole and samples for scientific research. the full sample material (drill core) information is available by linked third party services as a reference with an international geo sample number (igsn) [conze et al., 2017]. as an example, consider boreholes used to monitor groundwater levels equipped with sensors, data from measurements are collected over time. these are stored in separate information systems. they can be made accessible to the end users through the linked data architecture being progressively set up by tcs gim data providers (fig. 1). the full semantic consistency of the underlying data was assured with the borehole conceptual model, which is based on geosciml, groundwaterml2.0, observations & measurements and the inspire environmental monitoring facility data specification. figure 1. example of the borehole-related data sets linked with uris through a dynamic graph. each arrow represents the link between instances, e.g., from one borehole description to the environmental monitoring facility (sensor) deployed there, then to the measurements acquired and to the description of the hydrogeological unit(s) being monitored. each instance is associated with a uri and points to other information resources also using uris. another significant input to the user community is achieved by effectively establishing interdisciplinary collaboration across national, domain, and system boundaries. this collaboration between largely heterogeneous geoscientific communities is mainly based on interoperable services that provide standardized data. in addition, these can be seamlessly integrated into scientific workflows or collaborative environments such as jupyter10 notebooks. 9 https://data.geoscience.earth/ncl/ (last accessed 21/06/2022) 10 https://jupyter.org/ (last accessed 21/06/2022) https://data.geoscience.earth/ncl/ https://jupyter.org/ marc urvois et al. 6 4. tcs gim services as mentioned above, the thematic core service implemented by the geological information and modelling group represents a selection of basic geological topics useful for other tcs communities (fig. 2). figure 2. tcs gim services on the epos data portal: boreholes, geological map at 1:1,000,000, mineral resources and 3d models. 4.1 geological map one of the main missions of the geological survey organizations is focused on the knowledge of the subsurface. it is then made available through the geological maps at local, regional, national scales. the pan-european surface geology at 1:1,000,000 scale is a transnational map provided by egdi and harvested from inspire-compliant national web feature services on geologicunit. two services are offered to the users: – map view service (ogc wms): the surface geology entities are described as geological units forming a continuous map across europe on the epos data portal, – feature index discovery service (ogc wfs): enables the user to search for geologicunit by matching a set of criteria such as location and other properties that are defined in the specification based on geosciml-lite (e.g., lithology, age…), inspire code lists are used. geological data to foster multidisciplinary research 7 4.2 boreholes the data set exposed in epos includes onshore and offshore boreholes made available by the tcs gim members (over 2.2 million boreholes as on 31/12/2021). it includes both scientific and commercial boreholes. the regional and national geological surveys expose national data sets as part of their public service mission, often comprising several millions of boreholes. most boreholes drilled for the purpose of scientific research are located in areas that are of particular interest for understanding the structure and composition of the subsurface and which are not sufficiently explored by commercial wells. therefore, data from scientific drillings are of particular interest for the solid earth science community despite their marginal number when compared to the data sets that contain millions of commercial boreholes. the department of earth sciences of uppsala university provides a data set compiled from projects and expeditions of the international continental scientific drilling and international ocean discovery programs (icdp/iodp). index data services are exposed by each data provider according to the tcs gim specifications (epos-geosciml-lite profile11). they are in-turn harvested at the tcs gim central node level for quality check, european consolidation and exposition to epos ics-c. two services are offered to the users: – the map view service enables the user to discover and interact on a map with the available boreholes. a click on a borehole provides the user with a set of descriptors that are defined in epos geosciml-lite profile (e.g., depth, purpose, drilling method, availability of borehole logs, link to monitoring equipment, etc.), – the feature index discovery service enables the user to search for boreholes matching a set of criteria such as location, and many other properties that are defined in the specification based on geosciml-lite (e.g., drilling method, core length, access to the physical core, to logs, to water level, etc.). when code list registries were not available from inspire and/or the commission for the management and application of geoscience information of the international union of geological sciences (iugs/cgi) level, specific entries were added in the european geoscience registry. the data providers expose ogc complexfeature services where such rich information is available. these services are directly interfaced from the index service as linked data, i.e. they are not routed through the epos tcs or ics infrastructures. this is the perfect example of the benefits of a fully linked data approach. those datasets/services are still integrated in epos in the sense that are described using the common metadata semantics agree on in epos while still being hosted as close as possible to the data provider which is important for data maintenance. another benefit of such a fully linked data approach is the capacity to dynamically enrich the data content handled by a software when a user is using it without asking him to go back again in a data catalogue to search for the second dataset that will enrich the first. this is feasible in tools such as qgis gis with its “gml application schema toolbox” plugin12. this feasibility has been implemented as a demonstrator13. 4.3 mineral resources this data set was compiled over more than 15 years in a series of european mineral inventory projects (e.g., promine, minerals4eu, mintell4eu). most geological surveys host data on raw materials, however, data are typically organized in different ways from one country to another based on different geological traditions, legal frameworks, etc. the latest mintell4eu project [flindt jørgensen et al., 2021] built on previous projects to collect a selection of these national/regional raw material data, to store them in a central database, and finally to offer a visualization in a harmonized way at the european geological data infrastructure (egdi). this minerals inventory database [kumelj et al., 2021] includes, among other assets, the location of individual mineral occurrences and 11 https://gitlab.brgm.fr/brgm/epos/epos-tcs-gim/-/tree/master/documents/epos_geosciml_lite_profile (last accessed 07/06/2022) 12 https://plugins.qgis.org/plugins/gml_application_schema_toolbox/ (last accessed 07/06/2022) 13 https://github.com/brgm/gml_application_schema_toolbox/tree/master/docs/presentations/2018_egu (last accessed 07/06/2022) https://gitlab.brgm.fr/brgm/epos/epos-tcs-gim/-/tree/master/documents/epos_geosciml_lite_profile https://plugins.qgis.org/plugins/gml_application_schema_toolbox/ https://github.com/brgm/gml_application_schema_toolbox/tree/master/docs/presentations/2018_egu marc urvois et al. 8 mines, aggregated statistical data at national level on production, trade, resources and reserves compiled in the electronic minerals yearbook. the corresponding harvesting system for collecting and validating these european mineral resources data is operational, and technical routines are in place during the harvesting phase to ensure a rigorous control of the data quality. as of 31/12/2021, the data collection involves 36 providers from 31 countries. it includes 143,537 mineral occurrences and 58,910 mines. two services are offered to the users: – map view service: the european mineral occurrences and mines are displayed using a first set of descriptors (name, status, activities and associated products, citation), – feature index discovery service: in addition to the geographical search provided by the epos data portal interface, the user can interact with the mineral occurrences and mines with dedicated descriptors (e.g., commodity, geologic history, type of mineral occurrence type, etc.). 4.4 3d models this data set is an inventory of 3d models available at the data provider level. presently, the 3d model index includes a first selection of models exposed by the geological surveys of france (vosges-rhine graben), denmark (south jutland-schleswig) and italy (central po plain and fossombrone area). tcs gim developed an ad hoc semantic specification14, as no previous one was available. based on these four examples, the development process already produced two versions of the 3d model and associated representation specifications in conjunction with the ogc geoscience domain working group15. two services are offered to the users: – map view service (ogc wms): to discover the available models through a first set of descriptors, i.e. location, name, description, positional accuracy, online resource, – feature index discovery service (ogc wfs): to interact with the proposed models through the new entries in the new code list registries, i.e. area of interest, purpose, status, modelled feature, discretization type used, etc. 5. vocabulary registry and resolver the tcs gim utilizes the european geoscience registry, which was set up as the reference code list registry to consolidate and share controlled vocabularies throughout the various thematic projects and data infrastructures in which eurogeosurveys members participate. the tcs gim activities aim at avoiding the single use of project code lists while building on the knowledge and enriching reusable european vocabulary registries on boreholes, models and associated representations. each registry and its entries are identified by a uri that can be addressed in data flows when needed. the system provides human and machine-readable representations of these vocabularies. along with the resolver, each information element (e.g., data, service, metadata…) served from tcs gim to the epos ics-c is assigned a uri under the same uri16 base, which allows: – fine grained identification, – external systems to refer to them and dereference the content when needed, – guarantee access to those information resource to external systems without impacting when a resource provider changes within the tcs. 14 https://gitlab.brgm.fr/brgm/epos/epos-tcs-gim/-/tree/master/documents/epos_lite (last accessed 07/06/2022) 15 https://external.opengeospatial.org/twiki_public/geosciencedwg/webhome (last accessed 07/06/2022) 16 https://data.geoscience.earth/ (last accessed 21/06/2022) https://gitlab.brgm.fr/brgm/epos/epos-tcs-gim/-/tree/master/documents/epos_lite https://external.opengeospatial.org/twiki_public/geosciencedwg/webhome https://data.geoscience.earth/ geological data to foster multidisciplinary research 9 6. discussion essential base information for all disciplines in solid earth science, whatever they describe, is geology or is a property that depends on geology or is an interpretation of such properties. thus, geological data are the baseline in all earth sciences research. they are essential for increasing our multidisciplinary knowledge about the planet earth and the transformation of this knowledge into innovation, i.e. value that supports a sustainable society. how this is done or could be approached is discussed with the help of examples from the tcs gim achievements. 6.1 serving multidisciplinary scientific drilling data in-situ subsurface information is the least accessible part of geoscience information. however, it is very valuable for interpreting complex data sets and building relevant models of the solid earth. although drillings can only penetrate the uppermost curst, this is the deepest in-situ information that is accessible, and it is from the part that is of most importance for society. a scientific borehole usually produces substantial sample material (drill core) supplemented by a geological description, image documentation and geophysical downhole logs. in special cases, time series data will be available from downhole sensors (e.g., downhole seismometers installed after drilling an active fault zone; piezometers in aquifers). a wide range of research can benefit from such data, like modelling groundwater flow patterns, heat flow and geothermal potential; rock mechanical testing and modelling for subsurface construction; and estimating the risk for seismic hazards to a settlement, mine or water reservoir close to a drilled fault zone. data discovery for multidisciplinary data sets in the epos portal provides an overview over the scientific (and other) boreholes of the area. already at this stageprovided by the view-wms of the european borehole indexinformation about the availability of data sets is available and the geological description can be dereferenced either directly or via the complex borehole service. this facilitates the scientists’ selection of relevant objects. the complex service also provides information on whether the borehole is accessible for complimentary studies, its dimensions and last known conditions (where specified) and collected drill core samples. references to the international geo sample numbers of the borehole and samples provide the opportunity to interface igsn services from the epos gim borehole service that allow scientists to screen the entire drill core virtually for further assessment of the borehole’s relevance to the topic or require access to the physical samples in the core archive. in this way, for example, seismologists and rock physicists can discover and access sensor data from a fault zone that was penetrated by the borehole, combine it with downhole logging data and compare results with images of the drill core (on an igsn service) before they sample the optimal pieces of core for laboratory measurements and, eventually, add new research data to epos services. in the larger picture, data from the epos gim borehole service would add an additional dimension to the regional model that is based on surface geology and seismological data, insar satellite and gnss data for surface motion and tectonic analysis, and existing laboratory data on rock samples from the area – all discovered and provided by epos community services. 6.2 groundwater monitoring from a borehole index entry, one can access the hydrogeological logs available along with the description of (i) the monitoring facility, e.g., piezometer, (ii) the aquifer being monitored and (iii) the observations/measurements acquired by the monitoring facility (fig. 3). all the elements in this data graph are described according to well-established international interoperability standards (such as ogc/iso observations & measurement, ogc geosciml, inspire data specifications, etc.). figure 3 exemplifies how such a fully linked data approach improves multidisciplinary science. currently in many geological surveys legacy systems, each “box” shown on this figure is “siloed” in its own information system. in such a context, the end user must have an intimate knowledge of where are all the corresponding datasets/services, how to connect to them and search the data of interest within each database. a fully linked data approach allows to go all across those siloes from one object description (e.g., one borehole) to another (one geological log). marc urvois et al. 10 building on this approach that utilizes pre-existing open source libraries, interoperable client tools can be deployed in a generic way. for example, an open source desktop gis software such as qgis17 can be used to ingest and interact with the interoperable data (fig. 4). the linked data approach allows the end user to walk within and across data silos using features provided by a community (e.g., a geologic fault, a borehole, etc.) as a reference, without being the original data provider of that feature and without requiring additional data integration mechanics. this fosters multidisciplinary work as the access to relevant and useful data content is direct and immediate. figure 4. discovery of borehole linked data using qgis gml application schema toolbox. figure 5 illustrates another example of interaction within tcs gim data graph via the interaction of the epos data portal with the borehole index services. starting from the web map service to spatially scope the search (millions of boreholes on the map), it is then narrowed down to a limited data set and the web feature service is used to identify the borehole index entries relevant to a given need. after an entry is selected, more content (code list definition, logs) are accessed by fetching linked data content. 17 https://www.qgis.org (last accessed 07/06/2022) figure 3. simplified representation of the data graph. each arrow represents the link from one instance to another. from one borehole description selected in the index, one can access the environmental monitoring facility (sensor), the observations acquired and the description of the concerned hydrogeological unit(s). in addition, the various geology/hydrogeology logs available at the borehole are also accessible. https://www.qgis.org geological data to foster multidisciplinary research 11 the subsequent steps are either data download for local analysis on the end user’s system or triggering the borehole visualization tools in the epos data portal using tcs gim dedicated libraries. this way, the user interacts with the data served by the epos platform, including the possibility of “bookmarking” the data uris for later use or passing the data payload to a distributed processing system, the epos ics-d [cocco et al., 2022]. 6.3 data integration using borehole logging information for 3d modelling of geological structures beyond the correlation of borehole logs and existing geological models, figure 6 illustrates the application of a web-based fusion of information from 3d modelling, neural network analysis and borehole logs. such integration will enhance the definition of geological horizons, both spatially by control points and physically by information figure 5. working sequence using the epos data portal to access detailed borehole information. figure 6. correlation of borehole logs and geological models. improvement with depth information of geological interfaces (point-clouds) derived from the analysis (e.g., visual or ai-based, evaluation) of borehole-logs and cores. marc urvois et al. 12 about composition and physical properties, and thus, substantially improve 3d models that encompass such horizons and information. the corresponding modelling processes can take place both in a collaborative environment such as a web-based application (fig. 7 and fig. 8) or a jupyter notebooks (fig. 9). the improved models are relevant to a multitude of topics, from subsurface construction and critical infrastructure via groundwater, raw materials and energy to risk assessment of natural and anthropogenic hazards. figure 7. interactive and web-based well log analysis. while visualizing borehole logs, the geologist can classify and distinguish geological structures, units, or formations together in a collaborative environment (e.g., jupyter notebook) that provides all standard domain controlled vocabularies. figure 8. on-demand modelling of the bottom of a geological interface. boreholes with lithostratigraphic logs provided by epos are used to either trigger services that compute (e.g., kriging) a 3d surface from a point cloud of depth information or interactively process the information in a jupyter notebook. geological data to foster multidisciplinary research 13 another prominent example is the on-demand and web-based modelling of geological structures on the basis of logging information from boreholes provided by web services. figure 9 illustrates the web-based modelling (kriging) of a geological horizon based on stratigraphic information from borehole logs provided by web services. as a major objective of the epos research infrastructure, this data mining and fusion process joins the power of data integration and modelling in one tool and provides it to the end user. 6.4 data integration using a notebook in iceland the collaboration between largely heterogeneous geoscientific communities is facilitated by the adoption of interoperable services that provide standardized data. in return, these can be seamlessly integrated into scientific workflows or collaborative environments such as a jupyter notebook. figure 9. snapshot of an animated earthquake visualization related to a plate boundary (observable notebook). figure 9 illustrates a use case on how the temporal development of earthquakes can be related to geological structures (plate boundary, in this case) and outlined by geological modelling. the example shows a working notebook application18 correlating information from different domains. all the required information is searched for and retrieved through the data assets discovered thanks to the epos ics-c metadata catalogue. although this is a simple example, it perfectly illustrates how scientists can collaborate in a common environment by interactively processing heterogeneous information in a cross-domain manner. 6.5 geological information to address societal questions one of the main interests of a research infrastructure such as epos is the high-level disciplinary and cross-disciplinary interoperable services offered to scientists, the public and private stakeholders. beyond a better understanding of the earth system, also societal questions can be addressed such as the impact of anthropogenic hazards and the adaptation to climate change. a recently started research project (geo-inquire) [cotton et al., 2022] aims to complement multi-scale approaches of anthropogenic hazard episodes. this interdisciplinary research will take advantage of the synergy between tcs gim and tcs ah (anthropogenic hazard) [orlecka-sikora et al., 2020] 18 https://observablehq.com/@rhaener/japan-earthquakes-2011 (last accessed 21/06/2022) https://observablehq.com/ marc urvois et al. 14 data and product services through both the epos data portal and the episodes platform19. extended access will then be improved to a wide range of multi-disciplinary data sets, e.g., geology, subsurface structures and hydrogeology information from boreholes, geophysical and broadband monitoring data, as well as technological data describing the cause of the man-induced hazards. in relation to the low carbon emission, a part of this project is about improving the co2 storage. geological characterization, geophysical data and water table monitoring data in combination with data from in-situ equipment (injection wells, laboratory facilities) shall improve underground co2 storage. the result of this research on the gas-water-rock interaction will benefit from epos and eccsel20 research infrastructures and contribute to develop the next generation of ccus21 technologies in an efficient and structured way, and help to enable the low to zero co2 emissions goals of industry and power generation to combat global climate change. 6.6 challenges the fundamental challenge for a research infrastructure like epos is to provide more and richer data, as an ever increasing accuracy is a major trait of the present age. computations can be increasingly complex and thus, include and produce more detail, and observations/analyses are increasing in resolution on the specimen level. however, this cannot be readily transferred to a complex and inhomogeneous system like the earth, where observations on properties are comparatively few and unequally distributed, and with the perspective that this will not change in the foreseeable future. against this background, a first challenge is to increase geographical coverage. this was expected and thus, addressed from the very beginning by including a low cost of entry into the service design, with the intention that a high ratio between added value and costs to participate would encourage potential data providers to join the tcs gim efforts. the second challenge is to increase the granularity of the data where this is possible and viable, because the adaptation and preparation of data becomes more complex with increasing granularity, and the required effort increases. in addition, it has to be ensured that the underlying services, and the standards they rely on, are capable of dealing with the higher data granularity. the third challenge in this context is to include new data types by extending the existing data models and services while keeping them fully interoperable and compliant with standards (e.g. diverse geophysical downhole logs in the borehole service). this is the least controversial challenge, as it follows needs defined by the community and is mainly limited by the available workpower/funding. much legacy data exist, but these are often only available in legacy formats and dispersed across diverse more or less well organized data stores. in general, it is not possible without a major financial and community effort to prepare such data for services like those of tcs gim. however, new data are usually collected in a structured way and could be available without much delay, if this is supported by the communities. 7. conclusion the tcs gim data models and services reuse and enhance well-established semantic and technical interoperability standards (iso, ogc, iugs/cgi, inspire). with a unique linked data approach, consistent geoscience data are served for cross-disciplinary research. as such, this work improves standards and practices of the ogc and rda communities. the consequent utilization of open source software and libraries facilitates the implementation and development of tcs gim services in the community, and provides for easy adaptation to other purposes. the combination of these technologies guarantees that the services can be interfaced and digested by other services and end users, utilizing both, open source and proprietary software. 19 episodes: https://tcs.ah-epos.eu/ (last accessed 08/06/022) 20 https://eccsel.org (last accessed 08/06/022) 21 ccus: co2 capture, use, transport and storage https://tcs.ah-epos.eu/ https://eccsel.org geological data to foster multidisciplinary research 15 tcs gim addresses the challenge of providing more and increasingly detailed data with a continued development towards new data types and with a high added-value to cost ratio, in order to attract new data providers and in this way increase geographic coverage and data density. thus, community outreach is a major task for the coming years. with this approach, epos tcs gim and its services further multidisciplinary earth sciences research, which is essential to maximize the gain in new knowledge and its transformation to value for a sustainable society. acknowledgments. the authors would like to acknowledge the following projects and initiatives for financing part of the activities: epos-ip, european commission h2020 program, grant agreement id 676564; epos-eric (https://www. epos-eu.org/epos-eric) for coordination and management; the national epos initiatives and/or internal research projects in czech republic, denmark, france, germany, italy, ireland, slovenia, sweden, and u.k. the authors also wish to thank françois robida who regularly took the lead to engage the european geological community in the several epos phases. references beaufils m., kemp c. 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https://www.w3.org/tr/sdw-bp/ https://www.w3.org/tr/sdw-bp/ https://doi.org/10.1038/sdata.2016.18 https://doi.org/10.1038/sdata.2016.18 https://www.w3.org/tr/dwbp/ https://www.w3.org/tr/sdw-bp/ mailto:m.urvois@brgm.fr kovacs_final:layout 6 hungarian repeat station survey, 2010 péter kovács*, andrás csontos, balázs heilig, andrás koppán geological and geophysical institute of hungary, budapest, hungary annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-5450 abstract the last hungarian repeat station survey was completed between october 2010 and february 2011. declination, inclination and the total field were observed using one-axial dmi fluxgate magnetometer mounted on zeiss20a theodolite and gsm 19 overhauser magnetometer. the magnetic elements of the sites were reduced to the epoch of 2010.5 on the basis of the continuous recordings of tihany geophysical observatory. in stations located far from the reference observatory, the observations were carried out in the morning and afternoon in order to decrease the effect of the distant temporal correction. to further increase the accuracy, on-site didd variometer has also been installed near the aggtelek station, in the baradla cave, during the survey of the easternmost sites. the paper presents the technical details and the results of our last campaign. the improvement of the accuracy of the temporal reduction by the use of the local variometer is also reported. 1. the hungarian repeat station network (hrsn) the network of geomagnetic observatories is sparse and uneven for providing detailed information on the spatial variation of the geomagnetic field, even in the characteristic wavelength representing the core field. it follows that in order to obtain a comprehensive model of the main field or also the crustal field in regional scale, network measurements have to be carried out filling the gaps in the spatial net of observatories. in hungary, the network campaign activities look back to a long history starting in 1847, that is summarized in szabó [1983] or in kovács and körmendi [1999]. from 1950, the spatial variation of the field is monitored periodically by national survey and repeat station (rs) networks. until 1995, the country surveys have been carried out in 15 year periodicity with the occupation of 300 stations (in the 1950, 1965, and 1980 campaigns) or 195 stations (in 1994-95) distributed evenly over the 93,000 km2 area of hungary. the hungarian repeat station network (hrsn) was established in 1966 [aczél and stomfai 1969] consisting of 15 primary and 22 secondary anomaly–free sites selected from the stations of the 1964-65 country network. because of the increase of the artificial noise around and the demolition of some original hrsn stations, our current rs network consists of only 13 sites, that is thought to be still enough for modeling the main field and updating the magnetic field obtained from country survey campaigns. in 2003, hrsn joined magnete, which was initiated for harmonizing the network activities over europe from the point of view of measurement standards and periodicity of campaigns. in the following, technical details and basic results of our last rs campaign carried out in 2010 are presented. then, the normal models and the normal annual change of the magnetic elements are shown based on the results of the last campaigns. in the vicinity of some stations didd type of portable variometer was operated during the last survey. the improvement in the temporal reduction accuracy of these stations due to the application of local variometer is shown in section 4. 2. technical details and results of the last hungarian repeat station survey the last hungarian repeat station survey was carried out on 12 stations between october 2010 and february 2011 (figure 1 and table 1). the magnetic vector field was determined from the declination and inclination angles and the total field by the application of a di fluxgate magnetometer mounted on zeiss theo020a theodolite and an overhauser type of gsm19 magnetometer, respectively. the angle components were measured according to the traditional zero-field method [jankowski and sucksdorff 1996]. in some stations, where the reference azimuth mark became invisible during the last decades, new reference direction had to be installed with the use of a pair of ashtech mxii type gps receivers. for the majority of the stations the continuous recordings of the tihany geophysical observatory were used for temporal reduction. the distance of the farthest station (nagyszekeres, see figure 1) from tihany is 376 km, while the biggest latitudinal and longitudinal deviations are 1.56˚, and 4.72˚, respectively. to decrease the error of the temporal reduction, a portable, suspended didd type variometer article history received october 17, 2011; accepted february 9, 2012. subject classification: main geomagnetic field, instruments and techniques, data processing, global and regional models. 1113 magnete workshop 2011 was operated near the aggtelek site during the occupation of the stations located in the biggest distances from tihany (north-eastern part of hungary). moreover, to avoid considerable temporal magnetic variations during the observations the stations were occupied in the morning or afternoon periods. both afternoon and morning observations were carried out in consecutive days in the stations located within a considerable distance from the reference observatory and kovács et al. 1114 repeat station nr. geographic coordinates magnetic components on rss, and tihany (2010.5) number of series scatter u (°) k (°) d h (nt) z (nt) i f (nt) d (") h (nt) z (nt) i (") f (nt) nyirád 13 47.01833 17.46333 3° 27.2' 21481.1 43101.1 63° 30.6' 48157.0 4 8 2.0 1.1 7 0.1 csátalja 38 46.03971 18.97224 3° 40.4' 21998.9 42652.1 62° 43.0' 47990.4 8 13 2.3 0.9 10 0.4 bajánsenye 49 46.79853 16.40904 3° 6.8' 21610.2 42901.5 63° 15.9' 48036.2 10 2 1.5 1.2 8 0.5 fertőd 52 47.66483 16.89555 3° 25.2' 21143.1 43431.8 64° 2.6' 48304.2 6 7 1.8 0.9 9 0.2 tápiógyörgye 120 47.31548 19.97342 3° 59.2' 21258.6 43523.0 63° 58.0' 48436.9 11 20 4.2 1.3 19 0.6 szank 127 46.54547 19.69839 3° 55.3' 21711.6 43054.3 63° 14.3' 48218.3 13 51 0.8 0.7 4 0.7 battonya 145 46.31469 21.07999 4° 9.2' 21796.6 42978.8 63° 6.5' 48189.4 11 14 2.7 1.2 11 0.8 furta 159 47.10086 21.46354 4° 16.6' 21393.5 43505.7 63° 48.9' 48480.7 10 22 2.2 0.7 9 0.9 mezőcsát* 224 47.82553 20.83687 4° 18.1' 20985.1 43837.7 64° 25.2' 48601.0 6 28 1.0 1.1 5 0.8 aggtelek* 225 48.45559 20.49015 4° 13.8' 20659.6 44156.4 64° 55.6' 48750.1 9 16 2.4 1.3 11 0.8 karos* 254 48.33807 21.74217 4° 36.8' 20708.9 44161.8 64° 52.6' 48775.9 5 16 0.5 0.8 2 1.1 nagyszekeres* 284 47.96391 22.61887 4° 37.0' 20852.4 44096.2 64° 41.5' 48777.7 7 22 1.0 0.7 5 0.7 tihany (obs) 300 46.90000 17.89166 3° 31.6' 21528.0 43054.0 63° 26.0' 48136.0 figure 1. locations of the hungarian repeat stations and the reference observatory, tihany (thy). table 1. definitive results of the 2010 repeat station campaign of hungary, and the scatters of the observations. the magnetic component and the total field values are reduced to 2010.5. * the temporal reduction of the absolute measurements in the aggtelek, mezőcsát nagyszekeres and karos stations were carried out with the use of temporal variometer record. 1115 surveyed without the operation of portable variometer. most of the measurements were carried out in quiet geomagnetic conditions exhibiting kp index between 0 and +2. this condition was not satisfied at the mezőcsát and aggtelek stations (see figure 1) where the measurements were disturbed by moderate geomagnetic storm of kp = +3 and kp = 4 indices, respectively. however, for these stations the temporal reduction error could be diminished by the record of the portable variometer operating nearby. in accordance with other european repeat station campaigns, each absolute observation was reduced to the epoch of 2010.5. for that, the 2010 annual means of the reference observatory, tihany, were used. the obtained 2010.5 absolute values referred to the surveyed stations are reported in table 1. according to the recommendation of magnete, the errors of the obtained magnetic elements are given in the form of scatter that is defined by the maximum absolute deviation of the individual station observations from the averaged result. the maximum scatters in the 12 stations are 51", 4.2 nt, 1.3 nt, 19", and 1.1 nt, for the d, h, z, i magnetic components and the total field, respectively. the values are also listed in table 1. 3. reference field and normal annual change of the magnetic elements in hungary in the practice of hungarian geomagnetic surveys, the normal reference field is approximated by firstor secondorder polynomials of the geographic coordinates (u,m). in the case of the repeat station campaigns, carried out in a sparse network of 12-13 stations, the normal model of the b i magnetic elements is determined according to first-order polynomial given in the form of: (1) hungarian repeat station survey, 2010 table 2. coefficients of the first-order polynomial normal field models of the geomagnetic elements for hungary obtained for the epoch of 2010.5. table 3. residuals between the 2010.5 model and measured magnetic elements at the repeat stations, and the tihany and hurbanovo observatories. p0 p1 p2 declination (d) 176.40 (') 0.11062 0.22092 horizontal intensity (h) 22331.2 (nt) -9.03251 (nt/') -0.28607 (nt/') vertical intensity (z) 42125.7 (nt) 9.38378 (nt/') 1.39605 (nt/') inclination(i) 3725.8 (') 0.86982 0.06126 total field (f) 47655.9 (nt) 4.44508 (nt/') 1.13774 (nt/') repeat station residuals d (') h (nt) z (nt) i (') f (nt) nyirád -1.3 2.1 2.0 -0.2 3.8 csátalja -1.0 -11.2 26.4 1.9 12.4 bajánsenye 3.6 10.2 -10.5 -0.8 -6.0 fertőd -2.6 -0.5 -12.2 -0.5 -9.8 tápiógyörgye 1.9 20.4 -42.3 -2.8 -25.6 szank -2.9 -10.5 -30.2 -0.3 -31.1 battonya -0.0 5.8 31.1 0.5 30.6 furta 2.9 -23.7 -21.1 0.6 -24.9 mezőcsát -2.1 2.7 2.4 -0.2 5.4 aggtelek 1.8 -7.3 9.4 1.0 0.6 karos -5.5 -14.4 42.8 2.5 29.0 nagyszekeres 3.5 29.8 -28.9 -2.7 -12.8 tihany (obs) -0.8 12.0 18.3 -0.2 22.5 hurbanovo (obs) 2.6 -15.4 12.8 1.2 5.9 , , ,b p p p pi 0 1 0 2 0z m z z m m= + + -^ ^ ^h h h where z0 = 45.5˚, and m0 = 16.0˚. the coordinates are inserted in minute units. the model coefficients, pj ( j = 0,1,2), are obtained by means of least-square fitting method. beyond the magnetic elements of the repeat stations, the modeled data base includes the annual means of the tihany and hurbanovo (slovakia) observatories from the year of the rs campaign. for the epoch of 2010.5, the obtained pj ( j = 0,1,2) model coefficients are listed in table 2, while the differences between the measured and adjusted (normal) magnetic values, i.e. the residuals at the repeat stations and the observatories are reported in table 3. the normal charts of the magnetic d, h, components and the total field are presented in figures 2-4. note, that according to bullard's equivalence rule [bullard 1967] the first-order polynomial approximation over the area of hungary corresponds to maximum spherical harmonic model degree of 128, i.e. the smallest wavelength portrayed by the model is about 312 km. it means that, beyond the core field, our normal model contain contribution from the crust. we have also studied the normal annual change of the magnetic field elements, .. , over hungary between consecutive campaign epochs. similarly to the normal field, the annual change was modeled in the form of the first-order polynomial of the coordinates: (2) the model was fitted to the annual change of the field measured at the repeat stations and the observatories between consecutive epochs of rs campaigns. the obtained qi (i = 0,1,2) model coefficients deduced for the periods of 2000.5-2003.5, 2003.5-2006.5, 2006.5-2009.0, 2009.0-2010.5 are listed in table 4. 4. application of an on-site variometer during the survey of the aggtelek, karos, nagyszekeres and mezőcsát stations (see figure 1) a suspended didd type variometer was operated near the aggtelek site, in the baradla cave. the cave environment ensured the thermal stability of the temporal observation. the variometer record started on november 9, 2010, at 08:05 a.m., and lasted until november 12, 2010, 09:50 a.m. supposing that the geomagnetic variation was the same in the proximity of the variometer site, the bases of the variometer record were determined for any field element in each of the nearby repeat stations by absolute measurements. consequently, temporary absolute magnetic records were referenced to the repeat stations occupied in the operational period of the portable variometer. the temporary observations lasting for more than three days enabled to deduce the field differences between the repeat stations and the reference observatory (i.e. tihany) during quiet time periods [newitt et al. 1996]. this condition could increase the accuracy of the reduction of the station observations to common epoch. the difference x, y, z and f time-series measured between the magnetic variations of tihany and the temporary record referenced to the nagyszekeres station are shown in kovács et al. 1116 figure 2. first order polynomial model of declination in hungary for the epoch of 2010.5. figure 3. first order polynomial model of the horizontal intensity in hungary for the epoch of 2010.5. figure 4. first order polynomial model of the total field in hungary for the epoch of 2010.5. , , .b q q q ql 0 1 0 2 0z m z z m m= + + -o ^ ^ ^h h h b lo 1117 figure 5. we also present the temporal variations of the standard deviations computed in 60 minutes of sliding-window sequences of the difference time-series (figure 6). the two figures evidence that the most stable difference values are exhibited during the evening-night-morning period of november 9-10, and the late afternoon period of november 10. later, a medium geomagnetic storm started with maximum kp index of five (see bottom panel of figure 6) that resulted in stronger variability in the difference time-series of the tihany and nagyszekeres records. the definitive values of the x, y, z, and f differences were computed from those parts of the difference time-series where the measured standard deviations were less than 0.2 nt for each component. the mean difference values computed with this condition are indicated by green lines in the graphs of figure 5. in table 5 the maximum absolute displacement of the difference geomagnetic time-series from the quiet-time mean differences are listed for the three consecutive days represented by different geomagnetic activity. in quiet magnetic hungarian repeat station survey, 2010 2000.5 2003.5 2003.5 2006.5 2006.5 2009.0 2009.0 2010.5 q0 q1 q2 q0 q1 q2 q0 q1 q2 q0 q1 q2 declination 5.09 0.00108 -0.00027 4.78 0.00179 -0.00055 6.13 0.00276 -0.00144 7.39 -0.00133 0.0003 horizonthal int. (h) 7.8 -0.00524 -0.014 13.7 -0.00859 -0.00735 14.4 -0.00614 -0.01087 7.4 -0.01836 -0.01466 vertical int. (z) 37 0.00759 -0.00689 27.3 -0.00618 0.00285 26.1 0.00754 0.00302 31.5 -0.00353 0.01278 inclination 0.75 -0.00013 0.00062 0.04 -0.00018 0.00053 -0.03 0 0.00078 0.61 0.00056 0.00112 total field (f) 36.3 0.00778 -0.0122 30.3 -0.00759 -0.00066 30 0.00288 -0.00192 30.5 -0.00635 0.00527 table 4. coefficients of the first-order polynomial normal annual change of the secular variation of the geomagnetic elements for hungary in periods between consecutive repeat station campaigns. the unit of q0 is min./year and nt/year for the angular and intensive components, respectively. q1, and q2 is given in 1/year for d and i, while in nt/(year*min.) for h, z and f components. figure 5. x, y, z magnetic component and total field difference time series between tihany and nagyszekeres station. nagyszekeres component data were obtained from the variation recorded at aggtelek (baradla-cave) and absolute measurements made at nagyszekeres. green lines represent the quiet time means of the differences, while the red columns show the period of the rs measurements in nagyszekeres station. conditions the biggest possible displacement appear in the y component, while during the storm the three components and the total field exhibit about the same absolute deviations (3.8-4.8 nt) from the means. it is also shown that the medium magnetic storm on november 11 could increase the expected displacements by a factor of about 1.5. note that the difference magnetic time-series referred to the four stations occupied during the operation of the didd variometer differ from each other by constant base differences. it means that the quiet periods, as well as the displacements of the magnetic trends from the quiet-time means (i.e. the values in table 5) are independent from the rs location. thus, it is suggested that the values listed in table 5 can refer to the lower limits of the possible errors that could appear in the time-reduction of the easternmost repeat stations during previous campaigns carried out without the application of on-site variometer. it should be emphasized that in the computation of these errors the difference between the geomagnetic variation at the portable didd site and the nearby repeat stations were not considered. for the case of the last campaign the period of the reoccupation of the nagyszekeres station are indicated by red columns in the graphs of figure 5. the heights of the columns represent the averages of the component differences between nagyszekeres and tihany observatory in the period of the absolute measurements. consequently, the dekovács et al. 1118 figure 6. temporal variation of the standard deviation of the difference time-series plotted on figure 5 (top), and the variation of the planetary kp index (bottom). date r kp x (nt) y (nt) z (nt) f (nt) 2010.11.09. 4 2.6 4.1 2.1 1.9 2010.11.10. 6 2.3 3.7 2.7 3.2 2010.11.11 24 4.1 3.8 4.7 4.0 table 5. maximum absolute deviations of the difference geomagnetic timeseries of tihany and nagyszekeres (figure 5) from the quiet-time means, in three consecutive days represented by different geomagnetic activity. 1119 viation of these values from the quiet-time means (green lines) represent the errors that would have been emerged in the temporal reduction of the magnetic elements in the case of lacking the local variometer records. the potential errors of the time-reductions of the three cartesian magnetic components and the total field are summarized in table 6 for nagyszekeres and the other three stations. 5. summary the technical details and the results of the last hungarian repeat station campaign carried out between october 2010 and february 2011 have been reported. the normal model of the magnetic field and its secular variation was also presented for the epoch of 2010.5 and for the periods between consecutive past hungarian rs campaigns, respectively. the models were defined in plane coordinate system by the first-order polynomials of the geographic latitude and longitude. note that the polynomial fit does not satisfy the curl-free property of the magnetic field in the case of the lack of electric currents, and does not consider the curvature of the earth over an extended area. for this reason, to define the reference field in a regional scale, the polynomial fit has to be substituted by spherical harmonic analyses. currently, the adjusted spherical cap harmonic analysis [de santis 1992] was applied for the determination of joint reference magnetic field of croatia and hungary [kovács et al. 2011]. it is intended to extend this analysis over larger area by inclusion the rs data of neighboring countries. in the 2010 campaign, portable didd on-site variometer was operated in the vicinity of the easternmost hungarian repeat stations in order to increase the accuracy of the temporal reductions. during the variometer record moderate geomagnetic storm occurred. it was shown that the profit of the application of the on-site variometer in the time-correction depended on the geomagnetic activity of the period of reoccupation. it is suggested that the differences between corrections carried out with and without the use of the onsite variometer record represent the potential errors of time-reductions of previous campaigns in the north-eastern part of hungary. acknowledgements. the work was supported by the national innovation office, hungary, and by the ministry of science, education and sport of croatia. we thank the aggtelek national park for the permission to install our portable variometer in the baradla cave. the hurbanovo observatory is also acknowledged for providing their 2010 annual means for the computation of the hungarian normal field models. references aczél, e., and r. stomfai (1969). the variations of the geomagnetic elements according to the secular measurements in the year 1966, geofizikai közlemények, 18 (1/2), 3-11 (in hungarian with english summary). bullard, e.c. (1967). the removal of trend from magnetic surveys, earth planet. sci. lett., 2, 293-300. de santis, a. (1992). conventional spherical harmonic analysis for regional modeling of the geomagnetic field, geophys. res. lett., 19 (10), 1065-1067. jankovski, j., and c. sucksdorff (1996). guide for magnetic measurements and observatory practice, iaga, warsaw. kovács, p., and a. körmendi (1999). geomagnetic repeat station survey in hungary during 1994-1995 and the secular variation of the field between 1950 and 1995, geophysical transactions, 42 (3/4), 107-132. kovács, p., e. vujic, a. csontos, m. brkic, b. heilig and a. koppán (2011). regional magnetic field model for croatia and hungary, 6th congress of balkan geophysical society, ext. abstract published in earthdoc.org. newitt, l.r., c.e. barton and j. bitterly (1996). guide for magnetic repeat station surveys, international association of geomagnetism and aeronomy, boulder, colorado, usa szabó, z. (1983). the magnetic declination and its variation in hungary between 1850-1980, geodézia és kartográfia, 35 (6), 436-442 (in hungarian with english summary). *corresponding author: péter kovács, geological and geophysical institute of hungary, budapest, hungary; email: kovacs.peter@mfgi.hu. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. hungarian repeat station survey, 2010 day of meas. in nov. 2010 kp. ind. diff. in x comp. between thy and rs (nt) diff. in y comp. between thy and rs (nt) diff. in z comp. between thy and rs (nt) diff. in total field between thy and rs (nt) during reocc. quiet time d during reocc. quiet time d during reocc. quiet time d during reocc. quiet time d aggtelek -1 09 0 -886.0 -884.7 1.3 197.2 199.6 2.4 1104.3 1102.4 -1.9 615.4 614.1 -1.3 aggtelek 2 12 4 -886.5 -884.7 1.8 196.4 199.6 3.2 1104.6 1102.4 -2.2 615.9 614.1 -1.8 karos 10 0 -847.1 -846.2 0.9 338.4 341.8 3.4 1107.7 1107.8 0.1 639.7 639.9 0.2 nagyszekeres 11 2-3 -704.1 -703.2 0.9 352.3 354.1 1.8 1041.6 1042.2 0.6 640.9 641.7 0.8 mez csát 11 3 -561.7 -562.0 -0.3 249.7 250.0 0.3 784.5 783.7 -0.8 466.8 465.0 -1.8 table 6. differences of the magnetic elements (x, y, z, f) between the repeat stations and tihany observatory (1) measured during the reoccupations of the stations and (2) referred to quiet-time period with the use of on-site variometer record. d means the difference between the two determinations, i.e. the potential error of the time-reduction carried out without on-site variometer record. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 5, 2004 1635 annals of geophysics, vol. 47, n. 5, october 2004 key words earthquakes clustering – etas model – large earthquakes in italy 1. introduction the spatio-temporal distribution of large earthquakes is a fundamental ingredient for seismic hazard assessment. remarkably, despite the importance of the issue and many efforts made in the past, so far shared conclusions could not be reached and different statistical distributions are still used for large earthquake forecasting. the most striking recent example is the report concerning the seismic hazard assessment for the northern california (working group on california earthquake probability, 1999), where quite different (and opposite) models were used for the calculations (e.g. poisson, brownian passage time, time predictable). in a recent paper, faenza et al. (2003) proposed a nonparametric and multivariate method (the proportional hazard model; hereinafter phm) to estimate the spatio-temporal distribution of large earthquakes, that drastically reduces the a priori assumptions on the temporal behavior. this method was applied to the italian seismic catalog of the last four centuries. the results indicate the presence of a time clustering of large earthquakes. in spite of a first order similarity with the time behavior of the etas model (e.g., ogata, 1988) used to successfully model aftershock sequences (e.g., console et al., 2003 for italian seismicity), it is remarkable to note that the length of the time clustering for large earthquakes (few years) seems to be larger than the clustering time of aftershocks sequence. this difference, if confirmed, could have important theoretical implications. in particular, a difference in the clustering properties may suggest the existence of different physical mechanisms for aftershock and large earthquake occurrences, and/or that the specific etas model used to describe the aftershock sequences is not suitable to model some major features of seismicity. some insights into the time clustering of large earthquakes in italy licia faenza (1), warner marzocchi (1), anna maria lombardi (2) and rodolfo console (2) (1) istituto nazionale di geofisica e vulcanologia, bologna, italy (2) istituto nazionale di geofisica e vulcanologia, roma, italy abstract the aim of this work is to investigate the clustering properties of the large earthquakes which occurred in italy in the last four centuries. in particular, we compare the results of a new multivariate nonparametric model applied to the catalog of large earthquakes in italy, and to a synthetic catalog generated through a specific etas model, successfully applied to describe aftershock sequences. the results disclose a longer clustering time for real large earthquakes, suggesting that the physical process that governs aftershock sequences and the occurrence of large earthquakes may be different. alternatively, the results can be explained by suggesting that the etas model, used to describe aftershock sequences, is not a suitable tool to model seismicity as a whole. mailing address: dr. licia faenza, istituto nazionale di geofisica e vulcanologia, via donato creti 12, 40128 bologna, italy; e-mail: faenza@bo.ingv.it 1636 licia faenza, warner marzocchi, anna maria lombardi and rodolfo console here we compare the clustering properties of large earthquakes found by faenza et al. (2003), with those of the etas model used to described seismicity of small magnitudes (console et al., 2003). in particular, we apply the phm used for large earthquakes in faenza et al. (2003) to a synthetic catalog of large earthquakes generated using the etas model. etas model parameters have been estimated from the spatio-temporal distribution of the events with m ≥ 2.0 in italy in the time interval 1987-2000 (console et al., 2003). this allows us to prove if the hazard function coming from the synthetic catalog is different from the one observed for real large earthquakes. in other words, we check the validity of the specific etas model used for aftershock sequences also to describe the spatio-temporal distribution of large events. 2. proportional hazard model (phm) the model used here (phm) is based on the study of the spatio-temporal occurrence of earthquakes through a non-parametric multidimensional fit of the hazard function. this method (kalbfleisch and prentice, 1980; faenza et al., 2003) presents several advantages compared to other more traditional approaches. in particular, it may account for tectonics/ physics parameters that can potentially influence spatio-temporal variability, and it tests their relative importance. another important aspect is that this model is nonparametric; this means that no a priori distribution is assumed for the interevent time. for a generic time x* since the most recent event, the hazard function of this model can be written as λ(x*, z) = λ0(x*) exp(zβ) (2.1) where z is a vector of covariate, β is a vector of coefficient and λ 0(x*) is an arbitrary and unspecified base-line hazard function. the random variables (from now on rvs) of the system are the interevent time (from now on iet) and the censoring time (from now on ct), i.e., the time interval between present time and the last earthquake which occurred. for a more accurate description of this model we refer to kalbfleisch and prentice (1980) and faenza et al. (2003); here we give a brief explanation of the model we proposed, stressing its main points. first at all, this model is nonparametric because it does not assume any specific form for base-line hazard function λ 0 (⋅); therefore we do not impose any a priori assumption on the earthquake occurrence process. in other words, we do not choose any arbitrary temporal distribution for fitting the events. the technique is robust because it allows us to consider at the same time all the available data coming from different regions. this is possible through the vector of covariate, z, that is attached to each of the rvs and can contain any spatial/tectonic information on the subregion where iets and/or cts are sampled. there is a important assumption below this model. in eq. (2.1) the covariates act multiplicatively on the hazard function and they do not depend on time. this means that the shape of the base-line λ 0 (⋅) versus time is always the same for each area apart for a multiplicative factor that depends on the covariates. so, from a physical point of view, the mechanism of earthquakes occurrence, described by λ0(⋅), is the same for different areas; only the parameters of the system can vary (i.e., exp(zβ)). the goal consists of estimating β and the nonparametric form of λ 0 (⋅) in eq. (2.1). the vector of coefficients gives the relative importance of each covariates; λ 0 (⋅) affords important insights into the physics of the process. a detailed explanation of such estimations can be found in kalbfleisch and prentice (1980) and faenza et al. (2003). 2.1. checking the model we perform the statistical validation of the model using an independent dataset, i.e., data that have not been considered at any step of the modeling. in order to do that the available dataset is divided into two parts, one used to set up the model (the learning phase), and the other to check the model (the validation phase). each iet of the validation dataset is transformed in order to form residuals (see kalbfleisch and prentice, 1980; and faenza et al., 2003). this transformation is a sort of statistical standardization and it 1637 some insights into the time clustering of large earthquakes in italy changes the random non-negative point process into a poissonian process with rate 1 (see, for instance, ogata, 1988). therefore, if the model is appropriate, the residuals are expected to behave like an exponential distribution with λ = 1. the comparison between the cumulative of residuals and the theoretical exponential curve is checked through a one-sample kolmogorov-smirnov test (e.g., gibbons, 1971). this provides a goodnessof-fit test of the model. 3. etas model and simulation of synthetic catalog the epidemic type aftershocks-sequences (etas) model is a stochastic marked point process representing the occurrence of earthquakes of size larger than, or equal to, a threshold magnitude m0, in a region and in a period of time (ogata, 1988). as in other triggering models, it is based on the principle that earthquakes are clustered in time and space because of occurrence of aftershocks; but, unlike those, it solves the debated problem to find the best way to identify clusters and to classify events (between mainshocks, aftershocks, foreshocks, ...). in fact, even if it considers the overall seismicity as the superposition of a background activity and of seismicity induced by previous earthquakes, its application to real data does not require any discrimination of events. we do not discuss the characteristics of this parametric model in details: a complete description of its formulation can be found in ogata (1988, 1998). an application of the etas model to italian seismicity was provided by console and murru (2001) and by console et al. (2003). considering as marks magnitude (mi) and epicentral coordinates (xi, yi), they inferred the following expression of conditional intensity function, based on history of occurrence ht = {(ti,mi, xi, yi); ti < t} ( , )g x y +n $ , , , ( ) ( ) ( ) ( ) t m x y h t t c ke x x y y d q d1( ) ( ) : < ( ) t i p m m i i q q i t t m m 2 2 2 2 1 i i 0 0 + + + r a b + =m .e$b ! _ i 6 r t s s s s v x w ww@ (3.1) where g(x, y) is the spatial density function of background events. for details on formulation of function and on significance of parameters we refer to console et al. (2003). the parameters of the model were estimated by the maximum likelihood method on the national instrumental catalog, collected by ingv (istituto nazionale di geofisica e vulcanologia), for period 1987-2000. the values obtained are: β = 0.997 ⋅ ln(10), µ = 0.0613, k = = 0.0014, c = 0.0068, p = 1.0580, α = 0.9740, d = = 3.07, and q = 1.828. these parameters are used to simulate a synthetic catalog. it is developed following the thinning simulation procedure, outlined by ogata (1981) for the hawkes processes, of which the etas model is an application, and then adjusted by himself to the etas model (ogata, 1998). in every realization events are simulated sequentially: first the time and then the magnitude and the epicentral coordinates are obtained. this method involves simulating the time to the next event, using a rate equal to an upper boundary of the intensity function, and calculating the intensity at this point. the ratio of this rate with the upper boundary is compared with a uniform random number to determine if the time is retained or not. then epicentral coordinates and magnitude are simulated according with density functions chosen. the synthetic catalog (from now on sc) is simulated with the same cutoff magnitude used to estimate parameters by real catalog (m0 = 3.0); then only events with magnitude larger than, or equal to, 5.5 were selected. the interval time is the same of the historical real catalog (16002002) used in faenza et al. (2003) to test the phm model. the events obtained are 131. 4. phm applied to etas catalog we apply phm to sc (see above) to compare these results with those obtained using the real catalog (from now on rc). as in the previous work (see faenza et al., 2003), the italian territory has been divided into a 13 × 13 grid between the latitudes 36-48 n, and longitudes 520 e. each node of the grid is the center of a circle. in order to cover the whole area the ra1638 licia faenza, warner marzocchi, anna maria lombardi and rodolfo console fig. 2. plot of empirical survivor functions for real and synthetic catalogs. the parameter α represents the significance level at which the null hypothesis of a common parent distribution can be rejected. fig. 1. plot of the residuals ε (x) (see equation 23 in faenza et al., 2003) for real and synthetic catalogs as a function of the time elapsed since the most recent event. 1639 some insights into the time clustering of large earthquakes in italy fig. 3a,b. a) empirical and theoretical (solid line) cumulative functions for the learning dataset. the parameter α is the significance level at which the null hypothesis (poisson hypothesis) can be rejected (see text for more details). b) the same as (a), but relative to the validation dataset. dius r of the circle is set about d/ 2 , where d is the distance between two nodes. for such grid, d ≈ 100 km and r = 70 km. in the following step, we select the circles that contain at least 3 earthquakes; 31 areas have been analyzed for sc, and 29 for rc. this choice makes the statistical analysis performed for the etas catalog homogeneous with that done to the real italian catalog in faenza et al. (2003). moreover, we emphasize that this technique is robust because it considers all the spatially inhomogeneous data simultaneously to build the statistical model. for each one of these circles we calculate the iets among the earthquakes inside the circle, one ct relative to the time elapsed since the most recent event, and a two-dimensional vector of covariates z, attached to each rv, bearing the information about the event itself and the area where this is sampled. specifically, z is composed of the logarithm of the rate of occurrence, i.e., the total number of clusters which occurred inside the circle divided by 403 years (1600-2002), and by the magnitude of the event from which we calculate the iet and the ct. by using equations 10 and 11 in faenza et al. (2003), we obtain β1 = =1.2 ± 0.2 and β2 = 0.2 ± 0.2. as for rc (see faenza et al., 2003) β2 is not significantly different from zero, thus the magnitude of the events is not important for the distribution of the iets. on the contrary, β1 is positive and significantly different from zero. figure 1 comparises between the residuals ε (x) (see equation 23 in faenza et al., 2003) for sc and rc, respectively. the trend of ε (x) is comparable to the trend of λ 0 (⋅) (see faenza et al., 2003); in particular, the use of cumulatives to calculate ε (x) make it a filtered version a b 1640 licia faenza, warner marzocchi, anna maria lombardi and rodolfo console of λ 0 (⋅). in both cases a negative trend is shown. in other words, earthquakes tend to be more clustered than in a poisson process, where λ 0 (⋅) is a constant. this is what we expect to obtain from the etas model, since it is constructed imposing a cluster of events. an important result is that for sc the time clustering seems to be shorter than for rc. both catalogs show a negative trend (indicating clustering) that becomes almost flat for larger times as in a poisson process; fig. 1 shows that the flattening occurs before for sc than for rc. we suggest that this longer clustering for rc may be due to the fact that the interaction between earthquakes lasts longer than that imposed by the specific etas model used. note that in both cases there is no evidence of a positive trend as expected for the gap seismic hypothesis. a quantitative test of the difference between rc and sc is performed through a two-sample kolmogorov-smirnov of the empirical survivor functions of the phm reported in fig. 2. the null hypothesis of equal distribution is rejected at a significance level α < 0.01. figure 3a,b reports a graph representing the goodness-of-fit of the phm model applied to sc. as for the study of rc (faenza et al., 2003), the time interval 1600-1950 (109 earthquakes) is used for the learning phase, i.e., to set up the model. the time interval 1951-2002 (22 earthquakes), instead, is used for the validation phase, i.e., to verify the model with an independent dataset. the goodness-of-fit is quantitatively evaluated through a one-sample kolmogorovsmirnov test (e.g., gibbons, 1971). the significance levels at which the null hypothesis of equal distributions is rejected are reported in figure. they are both > 0.95. 5. concluding remarks the main goal of this paper was to investigate on the capability of a specific etas model, successfully used to describe aftershock sequences, to model also the spatio-temporal distribution of large earthquakes (m ≥ 5.5) which occurred in italy in the last four centuries. to this purpose, we compared the hazard functions obtained by the real catalog and by a synthetic catalog generated by an etas model, the parameters of which have been estimated by console et al. (2003) by analyzing aftershock sequences. the two catalogs are both clustered in time, but the length of the clustering seems to be significantly different. for the real catalog it reaches a few years and after this time the distribution of the earthquakes appears to follow a poisson distribution (see also faenza et al., 2003). in the synthetic catalog the cluster is shorter: the negative trend lasts for a few months after the event, and then it becomes flat as in a poisson distribution. thus, the cluster imposed by the specific etas model used (described in section 3) seems to be shorter than the one in the real catalog. a plausible reason might be that the interaction between earthquakes may last for longer than the typical characteristic time of aftershock sequences. references console, r. and m. murru (2001): a simple testable model for earthquake clustering, j. geophys. res., 106 (5), 8699-8711. console, r., m. murru and a.m. lombardi (2003): refining earthquake clustering models, j. geophys. res., 108 (b10), 2468, doi:10.1029/2002jb002130. faenza, l., w. marzocchi and e. boschi (2003): a nonparametric hazard model to characterize the spatiotemporal occurrence of large earthquakes; an application to the italian catalogue, geophys. j. int., 155, 521-531. gibbons, j.d. (1971): non-parametric statistical inference (mcgraw-hill, new york), pp. 306. kalbfleisch, j.d. and r.l. prentice (1980): the statistical analysis of failure time data (john wiley and sons, new york), pp. 336. ogata, y. (1981): on lewis’ simulation method for point processes, ieee trans. inf. theory, it-30, 23-31. ogata, y. (1988): statistical model for earthquake occurrences and residual analysis for point processes, j. am. stat. ass., 83, 9-27. ogata, y. (1998): space-time point-process models for earthquake occurrences, ann. inst. stat. math., 50, 379-402. working group on california earthquake probability (1999): earthquake probability in the san francisco bay region: 2000-2030 – a summary of findings, u.s. geol. surv. open-file rep. 99-517. (received march 5, 2004; accepted may 27, 2004) annals 47, 1, 2004, 01/07def 237 annals of geophysics, vol. 47, n. 1, february 2004 mailing address: dr. yuri m. mikhailov, institute of terrestrial magnetism, ionosphere and radiowave propagation (izmiran), russian academy of science, 142190 troitsk (moscow region), russia; e-mail: yumikh@izmiran.rssi.ru key words atmospheric quasi-static electric field – atmospheric precipitation – atmospheric waves 1. introduction anomalous variations in the quasi-static electrical field ez in the near-earth atmosphere before the m ≥ 4 earthquakes have been recorded in many seismo-active regions of the earth. they usually occur at an interval of a few hours to a few days before the main shock and prove to be either a bay-like field intensity decrease (even to a sign reversal) or an oscillation train lasting a few hours. these observations were summarized and analysed by rulenko (2000) and made it possible to establish that both types of ez anomalies are caused by the deformation processes in the subsurface layers of the earth’s crust during the earthquake preparation phase. the former type of anomalies were observed on kamchatka for a few years and were referred to the radon (rn) emission from subsurface gases into the atmosphere, resulting in variations of atmospheric conductivity and, therefore in ez magnitude (rulenko et al., 1992, 1996; buzevich et al., 1998; smirnov, 2001). these results, as well as the observations in other seismo-active regions, have made it possible to assume that the quasi-static electric field in near-earth atmosphere can be responsible for ionospheric disturbances occurring before seismic shocks (morgunov, 1988; pulinets et al., 1998; mikhailov et al., 1999). research of electrical field intensity in temporary series has shown that the field value is affected by meteorological characteristics (precipitation, winds) (smirnov, 2001; mikhailov et al., 2002). this fact makes it difficult to detect some anomalies in ez behaviour before the earthquake. the object of this paper is the investigation of the power spectrum in the near-earth atmospower spectrum features of the near-earth atmospheric electric field in kamchatka yuri m. mikhailov (1), galina a. mikhailova (1), olga v. kapustina (1), alexandr v. buzevich (2) and sergei e. smirnov (2) (1) institute of terrestrial magnetism, ionosphere and radiowave propagation (izmiran), russian academy of science, troitsk (moscow region), russia (2) institute of space physics researches and radiowave propagation, far-east branch of russian academy of science, petropavlovsk-kamchatski (kamchatka region), russia abstract power spectrum of the diurnal variations of the quasi-electrostatic field ez in the near-earth atmosphere have been presented for the first time. the ez power spectrum variations in the period of fine weather have been shown to exhibit two bands of the periods of natural atmospheric oscillations with t = 1-5 and 6-24 h. these oscillations are the modes of the internal gravity and tidal waves in the lower atmosphere. on the days under atmospheric precipitation, the spectral power of ez increases by an order of magnitude. during the pre-earthquake period, when the diurnal ez variation had an anomaly, the intensity of harmonics with t = 1.8, 2.2, and 3.8 h increased by an order of magnitude or more in comparison with the ez spectra in fine weather. two additional spectral bands with t = 0.6 and 1 h have appeared simultaneously. 238 yuri m. mikhailov, galina a. mikhailova, olga v. kapustina, alexandr v. buzevich and sergei e. smirnov -2000 -1000 0 1000 2000 0 5 10 15 20 ut, hours 0 200 400 600 800 -400 -200 0 200 400 0 100 200 300 400 ez, v/m 16.09.1999 19.09.1999 18.09.1999 17.09.1999 1 2 3 4 fig. 1. typical diurnal variations in a quasi-static electric field ez in the near-earth atmosphere: 1-3 – days without atmospheric precipitation; 2 – a day before the earthquake; 3 – for the earthquake day; 4 – day with heavy atmospheric precipitation. 239 power spectrum features of the near-earth atmospheric electric field in kamchatka pheric quasi-static electric field ez in a period of the fine weather, under atmospheric precipitation and during the pre-earthquake period of anomalous diurnal variations ez. 2. experimental data and processing procedure the pole-2m electrostatic flux-meter, whose output data are transmitted into the information processing centre in the analogue form, is used at paratunka (ϕ = 53°n, λ = 158.3°e) to measure the vertical component of the atmospheric electric field (ez). the ez values are digitized with sampling rate ∆t = 1 min. the results of the investigations of the power spectrums of the near-earth atmospheric electric field in september-october 1999 are presented below. figure 1 shows the typical diurnal ez variations in the fine weather (curve 1), on the day before the earthquake (curve 2, september 17), on the day of the earthquake (curve 3, september 18, 21.28:33.17 ut, ϕ = 51.21°n, λ = 157.56°e, h = = 60 km, mp = 6.0) and on the day with intense precipitation (curve 4, september 19). the character of the diurnal ez variations in the fine weather is described qualitatively by physical processes, occurring in the near-earth atmosphere. the increase in ez magnitude at 19-24 ut (06.00-11.00 lt) is related to the convective processes in the atmosphere due to the air temperature variations in the morning. in the fine weather in the daytime and at night the electrical fields are ∼ 100-120 v/m, varying in the range ∼ ± 20 v/m. at high and durable precipitation, the electric field value decreased to − 1 kv/m. on the rainy day (september 19), which carries to the earth’s surface volume changes, the atmospheric conductivity increases and ez decreases, even to a sign reversal. other examples of the diurnal ez variations in fine weather and at high precipitation were shown in the paper (mikhailov et al., 2002). on the day before the earthquake (curve 2), the bay of decreased ez magnitude was observed at 15.00-24.00 ut against a background of quiet meteorological conditions. similar effects were observed in the paratunka during period 1997-2000 in 1-36 h before earthquakes (smirnov, 2001). by example of 29 september 1999, fig. 2 illustrates the spectral processing of the diurnal ez data. the upper curve 1 represents the diurnal variations ez. the next curve below 2 is the same data obtained by subtracting the daily average ez value from curve 1. curve 2 shows the well-marked semidiurnal wave, whose daytime amplitude is less than the nighttime one. these values have been added by zeros to have 2048 points, needed for using the fast fourier transform algorithm (fft). the power spectrum of the augmented ez time series was computed by the method of periodograms with rectangular time «window» at frequencies of fk = k ⋅ ∆f with a corresponding frequency step of ∆f = 1/2048 ⋅ ⋅∆t = 8.14 ⋅ 10−6 hz and with periods of tk = = 1/fk, where k = 1, 2, …, 1024. this spectrum is shown in the panel 4 (solid line, left ordinate axis). the period of the predominant harmonic is t = 12 h and less intensive harmonics with periods of 1-5 h are also present. to distinguish them we excluded periods longer than 5 h from the spectrum (the dash-dotted line, right ordinate axis). the time form of these filtered series is shown by curve 3. the spectra labelled 5 are shown in the bottom of fig. 2 as a function of the periods varying to 24 h (on the left) and t < < 5 h (on the right). 3. main results the method described above was used to make the spectral analysis of the diurnal variations in ez for september-october 1999. naturally, the spectrum forms varied from day to day. to obtain statistically reliable spectra and to distinguish stable maximums the data have been averaged over days with specific ez behaviour. we used days without atmospheric precipitation and earthquakes, when the spectral variations may be caused by global effects in the earth’s crust in the given region and, consequently, in the near-earth atmosphere. figure 3 shows the control atmospheric precipitation data for the observation period. the resulting averaged spectra for the days without (22 days, curve 1) and with (11 days, 240 yuri m. mikhailov, galina a. mikhailova, olga v. kapustina, alexandr v. buzevich and sergei e. smirnov 0 200 400 600 0 2 4 6 8 10 12 14 16 18 20 22 24 ut, hours -200 0 200 ezez, v/m 0 5 10 15 20 25 30 35 t, hours 0 5 10 15 20 25 30 35 t, hours 0 4e+10 8e+10 s 2, v 2 m-2 hz-1 0.0000 0.0002 0.0004 0.0006 f, hz 0 4e+10 8e+10 0 2 4 6 t = 1/f, hours -100 0 100 ez (t<5 h), v/m 0 4e+9 8e+9 0 4e+9 8e+9 0 5 10 15 20 29.09.1999 1 2 3 4 5 _ ez, v/m s 2, v 2 m-2 hz-1 fig. 2. the method for spectral processing of the diurnal ez variations. 241 power spectrum features of the near-earth atmospheric electric field in kamchatka curve 2) atmospheric precipitation are shown in fig. 4 (t < 5 h) together with the individual spectrum for september 17 (curve 3). the vertical bars in curves 1 and 2 are the mean-square deviations. curve 1 indicates the spectrum weakening from ∼ 2.5 ⋅ 109 v2/m2 hz at t ∼ 4 h to ∼ 2 ⋅ ⋅ 108 v2/m2 hz at t ∼ 1 h without pronounced peaks at a level of 0.5 relative to the maximum value. the character of the spectrum shown by curve 2 has changed insignificantly, namely, the energy enhancements at t ∼ 4 h and ∼ 2 h, but the power level have increased twofold compared to curve 1, with very large mean-square deviations indicating that individual spectra are widely variable. finally, curve 3 represents the individual ez power spectrum for 17 september 1999 without atmospheric precipitation, but with anomalous diurnal variations (see fig. 1). 0 10 20 30 0 5 10 15 20 25 30 october 1999 0 10 20 30 40 0 5 10 15 20 25 30 september 1999 a tm o sp h e ri c p re ci p ita tio n , m m a tm o sp h e ri c p re ci p ita tio n , m m fig. 3. variations in the atmospheric precipitation level measured twice a day during september-october 1999. 242 yuri m. mikhailov, galina a. mikhailova, olga v. kapustina, alexandr v. buzevich and sergei e. smirnov 0 2e+9 4e+9 0 2e+10 0 1 2 3 4 5 6 7 t, hours 0 2e+10 4e+10 1 2 3 s 2, v 2 m-2 hz-1 fig. 4. the averaged ez power spectrum in the band of periods t < 5 h on the days without precipitation (curve 1) and with precipitation (curve 2) and 17 september with the anomalous diurnal variations ez (curve 3). 243 power spectrum features of the near-earth atmospheric electric field in kamchatka the following features are clearly defined in this curve: i) there are the spectral bands (at 0.5 level of the maximum) at t = 0.6, 1, 1.8, 2.2 and 3.8 h. ii) the intensity at maxima of these bands exceeds the corresponding values for the days without atmospheric precipitation by an order of magnitude or more. the t > 5 h harmonics were distinguished by calculating the spectra of longer initial date series with a time step of ∆t = 1 h only for days of the fine weather, i.e. september 1-4, september 20-24, october 20-24 and october 25-29. figure 5 represents the resulting spectrum, averaged over these days, which shows the clear maximums with t ∼ 8, 10, 12 and 24 h. during september-october 1999 (except september 18), two more kamchatka earthquakes occurred, whose preparation zone included the ez measurement point. the earthquakes occurred on october 5 (05.01:35.94 ut, ϕ = 51.21°n, λ = 157.61°e, h = 76 km, m = 5.6) and on october 18 (20.49:47.92 ut, ϕ = = 51.30°n, λ = 157.12°e, h = 138 km, m = 4.8). from fig. 3 it is seen, however, that rainfalls were recorded during these days, which probably screened the ez effects caused by other sources. 4. discussion the continuous measurements of quasi-static electric field ez with the sampling rate ∆t = 1 min have made it possible to study the fine structure of the power spectra with periods from 4 min and longer. two spectral bands with periods t = 1-5 h and 6-24 h have been found by analysing the ez diurnal variations for september-october 1999. under fine weather, clearly defined maximums (at a level of 0.5) were absent, but the intensity trended to weaken when the periods decreased from 5 to 1 h. these periods are known to be the periods of internal gravity waves, which are clearly seen in the seismo-gravitational oscillations of the 0 12 24 t, hours 0 5e+9 1e+10 1.5e+10 s 2, v 2 m-2 hz-1 fig. 5. the averaged ez power spectrum with periods of 2 h < t < 30 h for the fine weather days from 1 september to 29 october 1999. 244 yuri m. mikhailov, galina a. mikhailova, olga v. kapustina, alexandr v. buzevich and sergei e. smirnov earth and related with pressure disturbances in the near-surface atmosphere (garmash et al., 1989; petrova, 1999). the coincidence of the ez power spectra with those of the earth’s seismogravitational oscillations, allows us to state that the ez variations are of seismic nature. if we accept the «piston» mechanism of generation of air pressure fluctuations by the earth’s surface oscillations (garmash et al., 1989), then we may expect similar variations in the radon concentration spectra in subsurface gases. the t = 6-24 h spectral band shows the clear harmonics with t = 8, 12 and 24 h. these are the tidal waves in the lower atmosphere, related to the variations in the temperature of underlying surface. their intensity is of the same order of magnitude as that of internal gravity waves. one day before the earthquake on 18 september 1999, the ez power spectrum showed three distinct maximums (relative to the halfmaximum level) at t = 1.8, 2.2, and 3.8 h. their intensity increased by an order of magnitude and more, especially at t < 3 h, as compared with the ez spectra during the days of the fine weather. moreover, spectral components at t = = 0.6 and 1 h appeared too. the similar spectral structures within the t = 1-5 h band were observed by lin’kov et al. (1990) and their enhancement in the variations in the earth’s seismo-gravitational oscillations and in the simultaneous pressure variations in the near-earth atmosphere before a strong earthquake. the agreement between the ez spectra and the results of those papers indicate that not only the quiet diurnal variations in ez, but also anomalies before earthquakes are of the same seismo-gravitational nature. 5. conclusions the fine spectral analysis of diurnal variations in the quasi-static electric field ez in the near-earth atmosphere has been carried out for the first time and has shown the following: i) in fine weather and on the days with atmospheric precipitation the natural frequencies of atmospheric oscillations have been distinguished in two bands with periods of t = 1-5 h and t = 6-24 h in the ez power spectrum. the former band corresponds to internal gravity wave modes of the lower atmosphere, while the second band to the tidal waves. ii) during the earthquake preparation period, the intensity of bands with t = 1.8, 2.2, and 3.8 h increased by an order of magnitude and more compared with spectra obtained in fine weather. during this period, two additional spectral bands with t = 0.6 and 1.0 h were distinguished. iii) the nature of the ez spectral anomalies from the earth’s seismo-gravitational oscillations has been confirmed experimentally. however, the question has still to be answered as to whether the pre-earthquake bay-like weakening anomalies of ez are caused by enhancement of conductivity in the lower atmosphere as a result of the additional ionization by a radioactive gas, or this weakening is a purely seismic electrical signal caused by the rock failure in a hypocenter. acknowledgements the authors are deeply indebted to l.n. petrova for useful discussions of our report at the conference «problems of geocosmos» held in st. petersburg and to the referees for their kind help and useful suggestions. this work was supported by the russian foundation for basic research, project no. 0005-65020, no. 00-05-79047, and no. 00-0565380. references buzevich, a.v., g.i. druzhin, p.p. firstov, e.f. vershinin, s.e. smirnov and v.i. filimonov (1998): the heliogeophysical effects prior to the kronotsk earthquake of december 5, 1997, m = 7.7, in kronotsk earthquake of the december 5, 1997, in kamchatka: precursors, specific features and aftereffects, edited by e.v. gorgeev (petropavlovsk-kamchatsky, ross. acad. nauk), 177-188. garmash, s.v., e.m. lin’kov, l.n. petrova and g.m. shved (1989): excitation of atmospheric oscillations by sesmo-gravitational oscillations of the earth, izv. akad. nauk sssr, 25 (12), 1290-1299. lin’kov, e.m., l.n. petrova and k.s. osipov (1990): seismo-gravitational oscillations of the earth and atmospheric disturbances as possible precursors of strong earthquakes, dokl. akad. nauk sssr, phys. atmos. ocean, 313 (5), 1095-1098. 245 power spectrum features of the near-earth atmospheric electric field in kamchatka mikhailov, yu.m., g.a. mikhailova and o.v. kapustina (1999): ulf-elf electromagnetic emissions over the fault in kangra valley of india and their relation with radon emanation (intercosmos 24 satellite data), geomagn. aeron., 39 (4), 62-68. mikhailov, yu.m., g.a. mikhailova, o.v. kapustina, a.kh. depueva, a.v. buzevich, g.i. druzhin, s.e. smirnov and p.p. furstov (2002): variations of different atmospheric and ionospheric parameters in preparating phase of earthquakes at kamchatka: preliminary results, geomagn. aeron., 42 (6), 805-813. morgunov, v.a. (1988): to the nature of lithosphere-ionosphere coupling, izv. akad. nauk sssr, phys. earth, 5, 80-87. petrova, l.n. (1999): seismic process in the 0.05-0.5 mhz frequency band: reguliarities and specific features, volcanol. seismol., 4-5, 116-124. pulinets, s.a., v.v. khegai, k.a. boyarchuk and a.m. lomonosov (1998): atmospheric electric field as a source of ionospheric variability, uspekhi fiz. nauk, 168 (5), 582-589. rulenko, o.p. (2000): effective earthquake precursors in the electricity of the near-earth atmosphere, volcanol. seismol., 4, 57-68. rulenko, o.p., a.v. ivanov and a.v. shumeiko (1992): the short-term atmospheric-electricity precursor of the kamchatka earthquake of 6 march 1992, m = 6.1, dokl. akad. nauk sssr, 326 (6), 980-982. rulenko, o.p., g.i. druzhin and e.f. vershinin (1996): measurements of an atmospheric electrical field and natural electromagnetic emission before the kamchatka earthquake of 11 november 1993, m = 7.0, dokl. akad. nauk sssr, 348 (6), 814-816. smirnov, s.e. (2001): anomalies behaviour of electric field intensity before the earthquake according to the observations of magnetic observatory «paratunka» in 19972000, in solar-terrestrial coupling and electromagnetic precursors of earthquakes. thesises of the reps. at the 2nd international meeting at paratunka (kamchatka region), 14-19 august 2001, edited by yu.s. shumilov and g.i. druzhin (petropavlovsk-kamchatsky, ikir, far-eastern branch of ras), pp. 48-49. layout 6 teleseismic magnitude relations markus båth annali di geofisica, vol. 30, n. 3-4, 1977. annals of geophysics, vol. 53, n. 1, february 2010 abstract using available sets of magnitude determinations, primarily from uppsala seismological bulletin, various extensions are made of the zurich magnitude recommendations of 1967. thus, body-wave magnitude (m) and surface-wave magnitudes (m) are related to each other for 12 different earthquake regions as well as world-wide. depth corrections for m are derived for all focal depths. formulas are developed which permit calculation of m also from vertical component long-period seismographs. body-wave magnitudes from broad-band and narrow-band short-period seismographs are compared and relations deduced. applications are made both to underground nuclear explosions and to earthquakes. the possibilities of explosion-earthquake discrimination on the basis of magnitudes are examined, as well as the determination of explosive yield from magnitudes. for earthquakes, relations between magnitudes of main earthquakes and largest aftershocks are investigated. a world-wide station network for more homogeneous magnitude determinations is suggested in order to provide the necessary reference system. 1. introduction at the assembly of the international association of seismology and physics of the earth's interior (iaspei) in zurich in 1967, certain recommendations were adopted in order to create better homogeneity in seismic magnitude calculations. in the past decade, the zurich recommendations have had both a stabilizing and a stimulating effect on magnitude calculations and related research. in the past years, numerous sets of magnitude determinations have been collected which call for more detailed study, both in order to revise earlier magnitude relations and to make any possible extensions. in general, relations between body-wave magnitudes (m) and surface-wave magnitudes (m) have hitherto been given as world-wide averages, a notable exception being bormann and wylegalla [1975]. it now becomes evident that significant regional deviations exist. likewise, the focal-depth correction of surface-wave magnitudes (m), hitherto only known approximately for focal depths less than 100 km, could now be derived for any focal depth. the differences between body-wave magnitudes (m) as derived from different instruments, have also called for more detailed study. there are numerous applications of the magnitude concept which can be subjected to further investigation and possible refinement, such as for discrimination between explosions and earthquakes, for determination of explosive yield, and in the study of aftershock sequences. problems as those just mentioned and related ones, together with the present existence of homogeneous sets of magnitude determinations, prompted the present study. in this paper, i shall attempt to contribute to the solution of the problems mentioned, using primarly observational material from the swedish seismograph network. improved and estended magnitude relations will contribute to our knowledge of the underlying causes, i. e. both source and path properties. naturally, not all magnitude problems have been dealt with. for example, the calibrating functions (distance and depth dependence, attenuation) have been assumed as given in the zurich recommendations. this will not affect the methods used, nor the main results as these are primarily based on relative measurements. in the past decade, numerous valuable papers have appeared in the literature with close relations to one section or another in the following. these papers would rather deserve a special detailed review than brief mentioning here. from such considerations, i have restricted the present paper essentially to my own contributions and limited the references accordingly. to a certain extent, the present paper follows up and extends results derived earlier by me, especially in båth [1956]. 2. list of symbols and abbreviations a, b, a´, b´ parameters in derived solutions; a ground amplitude, microns; ap, ar ground amplitude of p wave and of rayleigh wave, respectively, microns; b benioff instrument, used as subscript; c calculated, used as subscript; d standard deviation (subscripts are used to specify which quantity the deviation refers to, e. g. dm , dm); e seismic wave energy, ergs; e, n, z east-west, north-south and vertical components, respectively, used as subscripts; g depth parameter in the calculation of m; 69 g grenet instrument, used as subscript; h focal depth, km; h horizontal component, used as subscript; log decadic logarithm; l wavelength, km; m body-wave magnitude, average of upp and kir; m (us) body-wave magnitude, as given by neis; m surface-wave magnitude, average of upp and kir; m1 surface-wave magnitude of largest aftershock, average of upp and kir n number of observations or pairs of observations; o observed, used as subscript; p perpendicular distance of a straight line from the origin; q (δ, h) calibrating function for m; t wave period, sec; x,y cartesian coordinates; y yield of underground nuclear explosion, kt; α angle between abscissa and normal to straight line; γ seismic efficiency; δ epicentral distance, degrees; isc international seismological centre; kir kiruna, sweden: 67˚50.4´ n, 20˚25.0´ e; neis national earthquake information service (u. s. geological survey); nts nevada test site; pal palisades, new york, u. s. a.: 41˚00.4´ n, 73˚54.5´ w; ume umeå (wwssn), sweden: 63˚48.9´ n, 20˚ 14.2´ e; upp uppsala, sweden: 59˚51.5´ n, 17˚37.6´ e. 3. methods in short, the zurich recommendations can be stated as follows [båth 1969, p. 131]: body-wave magnitudes m are calculated for p, pp, s from: (1) where q (δ, h) is the calibrating function after gutenberg and richter [1956]. surface-wave magnitudes m are calculated for horizontal-component surface waves (rayleigh) from: (2) i. e. the so-called moscow-prague formula [vanek et al. 1962]. the calculations in this paper will essentially consists in relating various quantities two and two, like m and m. in graphical plots of such quantities, generally quite a large scatter is observed resulting in oval-shaped point clouds. this fact has been of dominating influence on our methods of dealing with the data. as a general notation of the two teleseismic magnitude relations 70 table 1. observational material for m, m and m (us). log / ,m a t q h= + d^ ^h h 16c$d log / 1.66 log 3.3m a thh= + +d^ h 10 30 sec,t 20h = c$d 71 quantities to be related we use y (ordinate) and x (abscissa). with a and b as constants, we can in our cases then best describe the relation between y and x as a linear equation: (3) putting a = 1, i. e. assuming a constant difference b between y and x, proves generally not to be an acceptable procedure. on the other hand, it proves unnecessary to include second-degree terms in x. actual tests on our observational material demonstrate that the scatter is such that a second-degree polynomial on the right-hand side of equation (3) does not improve the accuracy. with the straigh-line representation as in equation (3), there are three different possibilities to place such a line to fit a given point cloud: 1. regression of y on x, which we denote as . this is the common procedure, which by a least squares solution, i. e. minimizing the sum of the squares of the vertical distances from individual points to the line, yields the following expressions for the constants a and b: (4) where the sums are extended over all points, i. e. all pairs of values y, x. 2. regression of x on y, which we denote as . the same procedure applied to the horizontal distances (parallel to the x-axis), leads to a solution of the following equation: (5) 3. major axis solution or orthogonal regression, denoted , reproduces the major axis of the point cloud and is obtained by minimizing the sum of the squares of the distances perpendicular to the line. a least squares procedure leads to the following solution for the major axis: with (6) in the following we have applied all three methods, as they have each their merits. methods 1 and 2 are mainly justified for computational purposes, providing the minimum error of the required quantity. thus, the regression of the form of equation (3), is used for calculating y for a given x. conversely the regression , i. e. equation (5), is used for calculating x for a given y. on the other hand, the major axis solution, which best represents the point cloud as such, is to be preferred for physical teleseismic magnitude relations table 2. relations between m and m for h ≤ 50 km. .y ax b= + a x n x x y n xy 2 2= r r r r r ^ h b y a x n= r r-^ h .x a y b= +l l cos sin 0x y p+ =a atan 2 2 x x n y y n xy x y n 2 2 2 2=a r r r r r r r ^ ^ ^ h h h 6 6@ @ cos sinp x y n .= +a ar r^ h y x" x y" x y) y x" x y" reasons. while the major axis solution can be inverted, i. e. solved for either x or y, an inversion is generally not permitted for the regressions. the relations between the three solutions 1, 2 and 3 depend on the shape and slope of the point cloud. if the point cloud is very narrow, approaching a straight line, the three solutions will be closely similar, whereas they diverge more and more for a broad oval-shaped point cloud. the slope will also influence, an extreme case being a horizontal point cloud, for which the regression and the major axis solution will agree, whereas the regression will be at righ angle to the other two. in general, the three solutions show best agreement with each other in the central parts of the point clouds and they may diverge strongly towards their extremities. the scatter as observed in the point clouds is not due to measuring errors but to the circumstance that quite a number of factors influences the measurements, such as source and path properties. to correct each reading for such influences would be a practically hopeless task. instead we apply statistical averaging and assume that by the mentioned straight-line fittings to a sufficient sample, the various effects will cancel each other. in the regressions, evidently one variable is assumed to be correct and the deviation is assigned exclusively to the other variable, whereas in the major axis solution both variables are assumed to exhibit deviations. in earthquake statistical research, where homogeneous magnitude series over long periods of time are needed, relations of the types mentioned will have to be used. therefore, a good rule for seismograph stations replacing old equipment by new one is to have both running in parallel for such a long time as to permit reliable comparisons. from overlapping series of records, it is possible to relate one magnitude to another magnitude (from a different instrument or a different station, etc.), which will permit reductions of long observational series to one homogeneous magnitude scale. an example of such calculations can be found in a paper by alsan et al. [1975]. it is then important to use the proper regressions, also to use only direct or immediate relations and to avoid the consecutive use of intermediary relations. the latter will rapidly lead to accumulation of errors and becomes increasingly unreliable. the observational scatter is in each case expressed as the standard deviation of the respective formulas. for the regression solutions the standard deviations dy and dx are calculated, respectively, while in the case of major axis solutions, we give three standard deviations, i. e. in addition to dy and dx also d, which refers to the perpendicular distances from the points to the line. it is obtained from the following equation: (7) in dealing with observations of this kind, it is important to test that the sample used is large enough to be representative. an efficient method was found to use successively 5, 10, 15, 20, ... observations, until the whole set is used up, and to calculate the parameters in each case. plotting the respective parameters against the number of observations, we find that after some initial, sometimes large oscillations, the parameters stay within the limits corresponding to the standard deviation of the total sample. the number of observations at the point where the parameters begin to stay within the final limits, is the minimum number of observations needed for a representative sample. 4. relations between m and m the observational material consists of magnitudes published in the uppsala monthly seismological bulletin for teleseismic magnitude relations 72 table 3. correlations and regressions between the parameters a and b, a´ and b´ (earthquakes). cos 90 sin 90d d dy x= =c ca a-^ ^h h y x" x y" 73 the period from january 1970 to april 1975. the body-wave magnitudes m are averages of short-period verticalcomponent p from uppsala and kiruna (grenet), and the surface-wave magnitudes m are averages of long-period measurements from uppsala (benoff ) and kiruna (galitzin). these magnitudes ought to be rather homogeneous, having all been measured and calculately by the present author. the material has been divided into 12 regions according to epicenter location, as listed in table 1. to avoid the influence of local depth h on m, only earthquakes with h ≤ 50 km are used in the present section. the regressions and the major axis solutions are summarized in table 2 for each region and for the total material, together with respective standard deviations. we note that the major axis solution for the total material, that is: (8) agrees almost exactly with the one included in the zurich recommendations: (9) an interesing result is that the parameters a and b are strongly connected to each other, as evidenced by their correlation coefficients and regressions, summarized in table 3. geometrically, this means that all the lines in each group intersect in nearly one and the same point, but have different slopes. without loss of accuracy, it is thus possible to substitute these regressions into those between m and m, with the results given in the last column of table 3. this means that just one parameter, instead of two, is enough to define each relation between m and m. this parameter bears the characteristics of a regional magnitude parameter. it is probably a complicated function of source and path properties, but it does not show any obvious dependence on m. even though calculated magnitudes agree between the different regions within error limits, at least in the central parts of the point clouds, the parameters a and b differ significantly from region to region (db = dm and db´ = dm). this could not be caused by insufficient sampling, because tests as describbed in section 3 reveal that already 20 to 30 observations per region represent a sufficient sample. the different parameters for different regions must reflect some physical differences. a physical explanation, however, can only be formulated in general terms. the differences of the regional magnitude parameters a, i. e. the slope dm/dm, refer to unequal amplitude increase of p and of surface waves, as source energy increases, or in formula: (10) this ratio is less than 1, but significantly different in different regions. source mechanism and radiation pattern could be suggested as factors of significance in this connection. 5. focal depth correction of m amplitudes of surface waves, and hence the calculated m, decrease as focal depth h increases. so far only scanty information is available on this effect on m, and generally restricted to h ≤ 100 km. as a consequence, in nearly all cases, no depth correction is applied to m given in bulletins. for the horizontal amplitude of rayleigh waves in a homogeneous medium, we have the following depth relation [bullen 1963, p. 90]: (11) for h = 0, this expression is = 0.42; it becomes = 0 at teleseismic magnitude relations table 4. depth dependence of surface-wave magnitudes m. 0.55 2.94m m= + 0.5 2.9 .m m6= + log loga dm dm d a d a .p r= = esp 5.34 0.58 esp 2.45a h l h lh + -^ ^h h h/l = 0.19 and attains its (negative) maximum = 0.10 at h/l = 0.46. this result may be strongly modified by the crustal layering. partly due to this effect, partly due to inevitable inaccuracies of h, no clear-cut dependence is to be expected for the depth range h = 0-50 km, as also verified by our observations. below h = 50 km, the second term in equation (11) dominates more and more, which would mean that the depth dependance would be more purely exponential. the magnitudes, i. e. logarithms of amplitudes, could thus be expected to vary linearly with h for h > 50 km. the depth effect on m is calculated from the following relations: (12) where mo = observed m and mc = m calculated from given m using the regional regressions (table 2,) which yield the highest accuracy. for the calculation of g we have used the material for h > 50 km (table 1), and the results are summarized in table 4. we find rather large and apparently irregular variations of g for 50 < h < 100 km, with an average g = 0.0088 (in close agreement with some earlier determinations by båth, 1952, 1956). on the other hand, for h > 100 km, g exhibits a steady decrease with increasing depth, with the remarkable result that g (h – 50) is constant = 0.38 ± 0.03. for h > 100 km, this implies a constant correction to m. depth corrections to apply to m can thus be summarized as in the right-hand column of table 4. certainly, regional variations of this depth dependence are to be expected, but our present material is insufficient to establish this reliably. it is of interest to note that a constant correction for h ≥ 100 km was obtained already earlier for depths down to 300 km [båth 1952, figure 2], but that scarcity of data then precluded its further study. the theory of rayleigh waves in a homogeneous medium is clearly insufficient to explain this result. it could do so for the depth range 50 < h < 100 km, corresponding approximately to the lithosphere. but for greater depths, the rayleigh wave theory would require far greater m corrections than observed. a plausible explanation could be that the asthenosphere occupies the range below 100 km depth and that the deeper earthquakes take place in subduction zones. the downgoing slab may act as a conveyer of the surface waves, leading to the observed effect. it is quite pronounced in deep earthquakes in the japan and bonin islands area, but it is apparently a general property of deep earthquakes. 6. determination of m from vertical-component records where the moscow-prague formula for m, equation (2), was developed only for the horizontal component of surface waves, it is obvious that it would be convenient to have a corresponding farmula for the vertical camponent. partly, this would need measuring only one record instead of two for the m calculation, partly and above all, the vertical component shows pure rayleigh waves with no interference of love waves as the horizontal components. the problem can be stated as to develop a formula for the vertical component giving the closest possible fit in m to the horizontal component, equation (2). this reduces to determining the following regression: (13) where we admit a parameter an the basis of earlier experience from pasadena [båth 1952]. on the other hand, we are assuming the same distance dependence (geametrical spreading, attenuation, dispersion) for the vertical as for the teleseismic magnitude relations 74 table 5. review of observational material for sections 6 to 9. 50m m g hc o =-^ h m a bmc = +l l log / log /a t a a t bh h z z= +^ ^h h m m" 1a ! 75 horizontal component. the resulting horizontal amplitude is in each case calculated as the vectorial sum af ae and an, that is: (14) this will include also love waves in ah, but this effect is found to be relatively insignificant in the total material. th is taken as the average of te and tn. our observational material is reviewed in table 5. in this case i have also included a set of long-period measurements which i made on palisades records (columbia university seismographs) on a visit in 1952. the depth range has been restricted to h ≤ 50 km, even thoug the relations between h and z components are expected to be valid for any depth. least squares solutian of equation (13) yields the following results: uppsala: kiruna: palisades: (15) in these equations the standard deviations refer to each station separately, i. e. they are the deviations of mz from mh for uppsala, kiruna and palisades, separately. in our bulletins, we report the average m for uppsala and kiruna, and for this average the standard deviation is calculated as follows: (16) where we find dm= ± 0.11 (n = 232), which means that the fit of the vertical components to the horizontal ones has been made with a deviation well within errors of any single magnitude calculation. the resulting vertical-component magnitude formulas become: uppsala: kiruna: (17) palisades: two conclusions are obvious from these results: 1. the pasadena result [båth 1952] with in equation (13) is not confirmed. the present result appears more agreeable also from a theoretical viewpoint. 2. formulas for m from the vertical component can be written as for the horizontal component, equation (2), and the deviation can be introduced as a station correction. as the ratio of the vertical to the horizontal component depends on the structural layering around the station, this is a correct procedure also theoretically. a further refinement could be to express this station correction as a function of wave period and of azimuth, considering structural variations with depth and azimuth. however, this proves superfluous, considering the small deviations already achieved. focal depth corrections of m (section 5) are assumed to be the same as for the horizontal component. 7. relations between m and m (us) although the type of short-period seismograph to be used in calculating m is not stated in the zurich recommendations, it soon became clear that there are significant differences in m from narrow-band instruments, like benioff, compared to even slightly more broad-banded seismographs, like grenet. in our bulletins, m from uppsala and kiruna is based on grenet instruments. on the other hand, m (us), closely agreeing with m (isc), is based on benioff instruments. as both magnitudes are in current use, it appears desirable to have clear relations between the two. reg ressions and as well as major axis solutions have been derived using the same material as in table 1, but this time including earthquakes of any depth, as the relations between m and m (us) are expected to be depth-independent. the results are summarized in table 6. as in section 4, it is found that the parameters a and b, a´ and b´ are strongly correlated to each other, as evidenced by the results summarized in table 3. again, we are therefore in a possibility to replace the two parameters by just one parameter in each case, as given in the last column of table 3. as in section 4, these results imply that the straight lines in each group intersect in nearly one and the same point, but have different slopes. as in section 4, we also find that the results in terms of m and m (us), respectively, generally agree between regions, at least in the central parts of the point clouds, but that the parameters a and b´, a´ and b´ show significant deviations between regions. on the other hand, there are no relations between a, b, a´, b´ for m – m (us) and those for m – m (section 4), which means that the underlying factors are different in the two cases. the significant difference between m and m (us) makes it necessary to specify in each case (bulletins, publications, etc.) which kind of m is used or which type of instruments is used. judging from the worldwide relation between m and m, equation (8), it is suggested that m is in better agreement with the definition of gutenberg and richter [1956] than is m (us) or m (isc). it is instructive to compare the different standard deviations for the m – m relations (table 2) and the m – m (us) relations (table 6). due to the larger slope of the point clouds in the latter case, with α approximating 135˚, all four teleseismic magnitude relations a a a .2 2h e n 1 2= +^ h log / 0.97 log / 0.05 0.14a t a th h z z= !-^ ^h h log / 0.9 log / 0. 0.14a t a t9 18h h z z= + !^ ^h h log / 0.9 log / 0.0 0.1a t a t9 2 2h h z z= !-^ ^h h 2d m m n2zm 1 2= ! r -^ ^h h6 @ 2 1 log / log / uppm m a a t b a tz z z h h= + +-^ ^h h6 @ 2 1 log / log / .a a t b a t kirz z h h+ + -^ ^h h6 @ log / 1.66 log 3.2m a tz z= + +d^ h log / 1.66 log 3.m a t 5z z= + +d^ h log / 1.66 log 3.m a t 3z z= + +d^ h 1a ! (us)m m" (us)m m" (us)m m) standard deviations dm and dm (us) are approximately equal. but in the m – m case, the slopes are smaller with the consequence that dm > dm both for regressions and for major axis solutions. comparing the two latter, we find that dm is about equal but that dm is larger for the major axis solutions than for the regressions. proceeding into more detail and comparing the different regions, we find similar relations between standard deviations and point cloud slopes. estimating the average deviation of any single magnitude determination as around ±0.3 units, we find that all d are within or equal to this limit, except dm. while m is derived from records at upp and kir, m (us) is based on a number of various stations reporting to neis. therefore, it would appear desirable to have not only m but also m (us) under more direct control. for this purpose, we operated at umea (ume) for a few months in 1966 a grenet seismograph side by side with the benioff instruments. with the material as specified in table 5, measurements on the two instruments could be related to each other, with the following regressions as result: (18) the corresponding relation between the magnitudes is thus: (19) with sufficient accuracy. this means that there is a constant difference between these two magnitudes. similar results are also found from parallel recordings at uppsala. identifying mg with our m, and mb with m (us), we find approximate agreement with results in table 6 only for the major axis solutions for region 5 and the regression for region 12. but a significant difference is that the correlation between our g and b is much stronger, with considerably smaller standard deviation in the results. we have anticipated the differences between g and b to be due to different bandwidths of their response curves. even though this is at least a partial explanation, it is possibly not the whole explanation. partly do the response curves for g and b not deviate considerably from each other, partly are the recorded periods identical in all the 159 cases investigated at umeå. other instrumental differences, not fully clarified, may contribute to the difference. 8. underground nuclear explosions magnitudes have found extensive application in explaining dynamic properties of seismic events. we shall illustrate this by two examples, in this and the following sections. due to the special source mechanism of underground nuclear explosions, the relation between m and m is generally different from the relation for earthquakes (section 4). for any given m, the corresponding m is generally much higher for explosions than for earthquakes. in order to investigate the m – m and m – m (us) relations for explosions we collected material from the uppsala teleseismic magnitude relations 76 table 6. relations between m and m (us) for all h. log / . 6 log / 0. 0.a t a t0 9 47 05b g= !-^ ^h h log / . log / 0. 0. .a t a t1 03 47 05g b= + !^ ^h h 0.47 0.05m mg b = !(us)m m" 77 seismological bulletins and from the preliminary determination of epicenter monthly listing of neis (table 5). the results are summarized on the bottom lines in tables 2 and 6. a certain trouble arises from the fact that in some instances the period or the distance is below the range stipulated for the moscow-prague m-formula, equation (2). there are quite clear deviations between results for different shot locations. for both the regressions and , the residuals mo – mc are generally negative for nevada, positive for semipalatinsk and the aleutians and around zero for novaya zemlya and the caspian sea area. the relations between m and m (us) have remarkably different slopes compared to earthquakes, but they do not contribute anything to the discrimination problem. on the other hand, the relations between m and m are of interest for discrimination between underground explosions and earthquakes. our results indicate that discrimination based on magnitudes is not so fully reliable as has sometimes been suspected in the past. comparing the major axis solutions, as best representing the point clouds, we find, including the standard deviations, that separation is best for low magnitudes and can be trusted up to about m = 6.3 for earthquakes with h ≤ 50 km but only up to m = 5.6 for earthquakes of greater depth. this holds even though surface waves are usually more important in earthquakes of any depth than in explosions. discrimination on this basis is generally more favourable for semipalatinsk and the aleutians than for nevada. if instead of comparing major axes and respective standard deviations, we compare the point clouds directly, we find even worse results. the point clouds overlap partially from m about 5.5 and up, whence more reliable discrimination on the basis of magnitudes alone would be possible only for magnitudes below this value. this is an argument in favour of the installation of more high-sensitive long-period seismographs. another application of magnitudes to underground explosions is to calculate their yield. the needed information on yields for the derivation of such relations is available only for american shots [bolt 1976]. we find a good relation between our m (average of uppsala and kiruna) and the yields y in kiloton for nts shots (table 5), as follows: correlation between m and log y = +0.95 ± 0.03 regression : m = 0.53 log y + 4.95 ± 0.08 regression : log y = 1.70 m – 8.20 ± 0.16 major axis solution : m = 0.54 log y + 4.92 ± 0.08 log y = 1.84 m – 9.05 ± 0.16 resulting d = ± 0.08 (20) the regression with a standard deviation of ±0.16 in log y, implies that y can be calculated with an error of 37 percent. this error compares favourably with results of spectral calculations, and in fact, by subdividing the yrange, it is possible to decrease the error to 18 percent of the yield for y ≤ 200 kt for nts shots from magnitudes only. tests on other explosions with approximately given yields, not used in deriving equations (20), give in general acceptable agreement. but this is true only for nts shots. caution is required in using these relations for other shot areas. for instance, shots in the aleutians and probably also in the semipalatinsk area give too high magnitudes at our stations. in other words, yields calculated from equation (20) by our magnitudes are too high. this depends probably on structural properties near the source, especially downgoing asthenospheric slab in case of the aleutians and exceptionally good propagation properties in the semipalatinsk case. the regression , equation (20), permits an estimate of the seismic efficiency γ, i. e. the fraction of the explosive energy that is converted into seismic wave energy. from the e – m relation [båth, 1958, 1966]: (21) in combination with the world-wide major axis solution for earthquakes (table 2): (22) and the energy relation: (23) we find: (24) numerical results are given in table 7. values of γ lie generally between 10–2 and 10–1 and exhibit a slight increase with increasing yield. this could be due to a positive correlation between yield and depth of explosion, the greater depths implying better seismic efficiency. 9. main earthquakes and largest aftershocks a dynamic property of aftershock sequences is that the difference in m between the main shock and the largest aftershock averages 1.2 units (the so-called båth's law, cf. teleseismic magnitude relations table 7. seismic efficiency γ of underground nts explosions. log 12.24 1.44 me = + 1.80 .m m 5 29= 4.2 10e y19= $ c y 1800.4=c (us)m m" m m" m y" m y" y m" y m" m y) m m) richter 1958, p. 69). in other words, there is a finite magnitude difference between these two events. when first formulated, it was based on relatively few sequences, but it has got additional confirmation in course of time and been studied by several authors both from statistical and physical sides. with the greater abundance of reliable magnitudes now available, it appears desirable to test this formula on a larger material. from the uppsala seismological bulletins, 116 aftershock sequenze were listed (table 5). the aftershocks have occurred within a few hours up to several months after their respective main earthquakes. sometimes there may be two or several main earthquakes of nearly equal magnitude, likewise there may be two or several about equally large aftershocks. in such cases the main earthquake is assigned a magnitude m corresponding to the total energy release in all main earthquakes, and similarly the largest aftershock is assigned a magnitude corresponding to the total energy of these aftershocks, using equation (21) and following a principle suggested by båth [1965]. if m´, m˝, ... , (m´ ≥ m˝ ≥ ... ) is the sequence of nearly equally large main shocks (or aftershocks), then the magnitude for the main shock (or largest aftershock, respectively) is obtained from the following formula: (25) in spite of this procedure, it is unavoidable that some bias creeps into this work, as it is not always obvious if an earthquake is to be considered as a main earthquake or as an aftershock. cases of more swarm-like characteristics have to be excluded. in a few cases, even large main shocks are not followed by any aftershocks recorded by our long-period seismographs, and these have naturally also to be excluded. still, it is believed that such effects will have only a minor influence on the result. as an average of all magnitude differences m – m1 we find: (26) which is in perfect agreement with the original formulation [richter 1958, p. 69]. in combination with our relation we then get the corresponding relation for m: (27) expressed in energy, this means that the energy of the largest aftershock is only about 2 percent of the energy of the main earthquake. we have further investigated possible relations between m – m1 and other parameters, with the following results. 1. magnitude m. plotting m – m1 versus m we find in spite of large scatter some suggestion for a dependence of the type: (28) as seen, the standard deviation is practically as large as in equation (26) and no improvement in accuracy has been achieved. 2. epicentral region. listing m – m1 for different regions, as defined in table 1, we find clear regional variations, as follows for regions with a sufficient number of observations: region 3 m – m1 = 1.2 normal n = 13 4 1.2 normal 19 5 0.8 low 7 7 1.2 normal 14 8 1.0 normal 18 11 1.6 high 13 kermadec-new zeland 1.0 normal 6 new hebrides 0.9 low 7 3. focal depth. our present material has been restricted to h ≤ 70 km, and within this depth range no dependence exists between m – m1 and h. observations are too scanty to deduce any relation between m – m1 and h for deeper events, even though a dependence is expected, as tentatively suggested by båth [1965]. probably the most extensive use that magnitudes have found in earthquake statistics is represented by magnitudefrequency relations of the type: (29) our bulletin material does not prove very suitable for this kind of study. as a world-wide average for m = 5.5 – 8.0 and h ≤ 50 km, we find the value of b to be as low as around 0.7. it is not excluted that this may be influenced by a gradual tapering off of our material towards the lower magnitudes in the range studied. 10. homogeneous magnitudes complete magnitude formulas should include both source correction and station correction [båth 1973, p. 110]. theoretically this is naturally a step in the right direction, i. e. to correct for radiation patterns from the source, for path properties and for receiver structural influences, etc. but in practice we are quite handicapped in determining reliable corrections because of the lack of acceptable reference magnitudes. usually this is circumvented by using determinations at one or a few stations, considered to be reliable. however, even if they may be very reliable, their readings are also subjected to the same influences as at any other station. such reductions can therefore hardly be teleseismic magnitude relations 78 1.44 1 log 1 10 ...m m 1.44 m m= + + +-l m l6 6 @@ 1.15 0.42m m1 = !0. .m m 631 =. 0. 0.4m m m0 28 93 01 = !log .n a bm= m m) 79 considered to be more than a relative calibration. one way to approach a solution of this problem would be to use a world-wide net of magnitude reference stations. i have earlier [båth 1969, p. 80] suggested such a world system of magnitude stations, numbering about 15, well located and well distributed around the earth. by an even distribution we would eliminate influences of source mechanism, path and station properties, which are unavoidable for individual stations, however good they may be. instead of forming a simple average magnitude from these stations, i like to discuss an alternative procedure. the underlying principle is to deduce a magnitude which corresponds to the energy radiated from the source. in other words, the adopted magnitude should correspond to the average energy. due to the relation between e and m, equation (21), the average energy is not equivalent to the average magnitude. the magnitude corresponding to the average energy is calculated from the following equation: (30) obtained by applying equation (21) and where m´, m˝, ... are the individual station magnitudes. as an example, with only n = 2, m´ = 6.5, m˝ = 7.5, the magnitude average is 7.0, while the magnitude which corresponds to the average energy is 7.3 from equation (30). the latter magnitude would then be considered more representative than the simple average. having this, other stations could calibrate their own magnitudes against this standard. calibration for a sufficient number of earthquakes, well distributed around the earth, would permit calculation of a station correction. then, a further improvement would be to calculate corrections for each seismic region separately, i. e. source corrections. the two corrections can naturally be combined into one value, for each region. along these lines, magnitude residuals could be made equally unbiased as travel-time residuals. a related suggestion for the eurasian region has been given by christoskov et al. [1974]. availability of homogeneous magnitudes would also assist considerably in further development of the formulas for m and m (section 3), especially to bridge the gap between about 10˚ to where local scales (ml) may be applicable, and the lower limits of 16˚ and 20˚ for m and m, respectively. 11. conclusions main results of general validity can be summarized in the following points. 1. relations between body-wave magnitude (m) and surfacewave magnitude (m) depend on the earthquake region concerned and can be expressed in terms of one regional magnitude parameter. 2. the focal deph correcfon to m incereases from zero at 50 km depth to +0.4 at 100 km depth and after that it remains constant = + 0.4 for all greater depths. 3. surface-wave magnitudes (m) can be calculated from the vertical component by the same formula as for the horizontal component, only by including a minor station correction. 4. body-wave magnitudes (m) determined from broadband short-period seismographs show consistent differences from those determined from narrow-band seismographs. this is illustrated by differences between grenet and benioff instruments. 5. relations between m and m for underground nuclear explosions demonstrate that discrimination from earthquakes on the basis of magnitudes alone is trustworthy only for m less than about 5.5 to 6.0. 6. relations between m and the yield of underground nuclear explosions in nevada suggest a seismic efficiency of the order of 10–2 to 10–1, increasing slightly with yield (or depth). 7. main earthquakes and largest aftershocks show an average difference of 1.15 in surface-wave magnitude (m). the difference increases slightly with magnitude of the main shock, it also varies from region to region, but it is independent of focal depth at least down to h = 70 km. 8. a homogeneous world-wide network is suggested for establishing more relable magnitudes, particularly for deducing corrections to individual station determinations and for further development of magnitude scales. references alsan, e., l. tezuçan and m. båth (1975). an earthquake catalogue for turkey for the interval 1913-1970, seismol. inst., uppsala, rep. no. 7-75, 166. båth, m. (1952). earthquake magnitude determination from the vertical component of surface waves, trans. am. geophys. un., 33, 81. båth, m. (1956). the problem of earthquake magnitude determination, publ. bur. centr. séism. int., a 19, 5. båth, m. (1958). the energies of seismic body waves and surface waves, contr. in geophys. (gutenberg vol.), 1, 1. båth, m. (1965). lateral inhomogeneities of the upper mantle, tectonophysics, 2, 483. båth, m. (1966). earthquake energy and magnitude, phys. and chem. earth., 2, 115. båth, m. (1969). handbook on earthquake magnitude determinations, seismol. inst., uppsala, 158. båth, m. (1973). introduction to seismology, basel. bolt, b. a. (1976). nuclear explosions and earthquakes, san francisco. bormann, p. and k. wylegalla (1975). untersuchung der korrelationsbeziehungen zwischen verschiedenen arten der magnitudenbestimmung der station moxa in abhängigkeit vom gerdtetyp und vom herdgebiet, publ. inst. geophys., polish acad. sci., 93, 159. teleseismic magnitude relations 1.44 1 log 10 10 ...m n1.44 1.44m m= + +l m^ h6 @ bullen, k. e. (1963). an introduction to the theory of seismology, cambridge. christoskov, l., i. v. fedorova, n. v. kondorskaya and j. vanek (1974). on a homogeneous system of selected eurasian stations for magnitude determinations, pure appl. geophys., 112, 619. gutenberg, b. and c. f. richter (1956). magnitude and energy of earthquakes, ann. geofis., 9, 1. richter, c. f. (1958). elementary seismology, san francisco. vanek, t., a. zátopek, v. kárník, n. v. kondorskaya, yu. v. riznichenko, e. f. savarensky, s. l. solovyev and n. v. shebalin (1962). standardization of magnitude scales, izv. akad nauk ssr, ser. geofiz., 2. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. teleseismic magnitude relations 80 annals 47, 1, 2004, 01/07def 133 annals of geophysics, vol. 47, n. 1, february 2004 key words slip weakening – accelerated creep – short-term earthquake precursor – strain – spatialtemporal scale – mosaic pattern – teleseismic effects 1. introduction several fundamental problems are still unsolved in the problem of earthquake (eq) prediction. nevertheless, the optimistic point of view is supported by few successfully predicted destructive eqs (raleygh et al., 1977; hui et al., 1997; sykes et al., 1999). on the other hand, the phenomena of self-organized criticality (soc) in the earth’s crust (bak et al., 1988) have no scale length, and hence any reliable evaluation of the place, time, and intensity of an eq is considered to be unlikely. the soc concept actually denies any progress and brings the problem of eq prediction back to its initial state (geller, 1997). further research is required to clarify whether the soc is an insurmountable obstacle or it only indicates the inadequacy of available data. among various prediction problems, the feasibility of short-term prediction is the least studied and depends on the nature and scale of the aseismic nucleation phase of large eqs (sykes et al., 1999). the stage of tertiary creep (or slip weakening), which precedes the brittle fracture of rocks, can be regarded as a mechanism of short-term precursor generation. accelerating strain considerably increases the signalto-noise ratio of measurable geophysical parameters, which is beneficial to the identification of the ductile failure stage in the source by measuring geophysical fields at the earth’s surface. the creep process develops in an avalanche-like manner and acquires an important slip weakening, strain and short-term preseismic disturbances vitali a. morgounov the head of tectono-electromagnetic laboratory, schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia abstract the problem of short-term earthquake precursors is discussed. in contrast to the increasing number of reports on short-lived precursors of various types, direct strain measurements cannot detect clearly expressed preseismic anomalies, as follows from the aseismic nucleation mechanism. based on previously published data and the assumption that the attenuation of the stress-strain field is proportional to r− 3, a possible scenario of the final stage of earthquake nucleation process is proposed on the basis of the slip weakening mechanism in the source and the associated mosaic pattern of precursors on the earth’s surface. the formulas for estimating the maximum distance of precursor detection and minimum duration of the final stage of inelastic deformation preceding brittle failure of rocks are derived. the data of electromagnetic precursors are interpreted in terms of a skin-layer model. a considerable increase in strain rates at the final stage of the earthquake nucleation provides an opportunity to explain teleseismic effects before strong earthquakes in terms of normalized epicenter distance. the modeling results are compared with in situ observations. mailing address: prof. vitali a. morgounov, the head of tectono-electromagnetic laboratory, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: vam@ifz.ru 134 vitali a. morgounov property of irreversibility. this scenario of eq development appears to be logically consistent. however, in spite of the encouraging experimental results of short-term precursor studies (fleischer, 1981; warwick et al., 1982; oki et al., 1988; rikitake, 1988; roeloffs, 1988; fraser-smith et al., 1990; varotsos et al., 1993; wakita et al., 1998; uyeda et al., 2000; meloni et al., 2001a,b; tramutoli et al., 2001; spichak, et al., 2002) and theoretical models of faulting according to which the rupture should be preceded by a gradually accelerating slip (press, 1965; rice et al., 1979; dieterich, 1992; ohnaka, 1993), the strain anomalies have not been reliably established by direct strain measurements before earthquakes in various regions of the earth and in california in particular (johnston et al., 1994; wyatt et al., 1994; abercrombie, 1995). the san andreas sliding fault zone seems to be the most spectacular example of the contradiction. since the problem in question is of crucial importance, a brief review of in situ measurements in this region and the discussion of the related important features (the mosaic pattern of the strain field and teleseismic effects) are presented below. along with controversial arguments, many authors report on preseismic irregular patterns observed in the san andreas zone (o’neil et al., 1981; teng et al., 1981; roeloffs, 1988; ben-zion, 1990; fraser-smith, 1990). leary and malin (1984) observed ground deformation events a few hours before the homestead valley sequence of eqs of 1979 in southern california (4.7 < ml < 5.2); the observations were made at three separate points on the periphery of the mojave block. these events manifested themselves as anomalous water level variations and a creep step measured by a us geological survey creepmeter. dodge et al. (1996) found that foreshocks of the landers eq provide clear evidence for an extended aseismic nucleation process before some eqs. harris (1998) came to the conclusion that aseismic creep controls the rupture process and the creep process is the key idea of eq instability models. wesson and nicholson (1988) emphasized the importance of the creep loading mechanism and suggested, in particular, that the landers foreshock sequence may have been driven by aseismic creep. johnston and linde (2002) reviewed the surface observations of episodic displacements on active faults and explained unsuccessful attempts to detect any indications of exponentially increasing strain (tertiary creep or slip weakening) by the fact that the scale of the initial failure must be small compared to the eventual rupture zone. evidently, the interpretation of the experimental data is complicated due to such factors as the specific role of the surface layer or the mosaic pattern of the stress field. a significant example is the fact that no precursors were recorded by the strainmeters of the pinyon flat observatory (pfo) at a distance of only 70 km from the epicenter of the landers eq (m = 7.3) (wyatt et al., 1994; abercrombie et al., 1995). the pfo and epicenter were located on the opposite sides of the san andreas fault. according to mount et al. (1992), the san andreas fault zone can be considered a weak fault, which in extreme case can be thought of as a free surface and acts as a stress refractor. wesson et al. (1988) consider the california crust not as an elastic half-space but as a mosaic of relatively rigid elastic plates or blocks separated by relatively weak fault zones (this interpretation is similar to the model proposed by bilham and beavan, 1979). 1.1. mosaic pattern of the stress field water level anomalies during the landers eq can serve as a convincing example of a mosaic structure of the strain field. in three of four boreholes in the pfo area, the water level dropped by 0.15 to 0.3 m at the time of the landers event. in the fourth well, located at a distance of 100-250 m from the other three, the water level dropped over 5 m (wyatt et al., 1994); i.e. observations made at points a few hundred meters apart differ by more than 10 times! it is noteworthy in this context that, according to mogi (1981), many precursors tend to appear at singular points of tectonic structures. these facts, as well as many other data, confirm the mosaic pattern of the strain field distribution associated with seismic events. it is 135 slip weakening, strain and short-term preseismic disturbances evident that a uniform model of the crust is generally inadequate. comparing various seismic regions, mogi (1981) characterized central california as a homogeneous straight fault zone without any appreciable precursors, in contrast to japan as a region of complex structures of small dimensions producing noticeable precursors in relatively small areas and some parts of china as a region of complex continental structures of large dimensions where remarkable precursors can be observed in large areas. this is consistent with laboratory experiments showing that the more inhomogeneous the specimen, the earlier the stage at which fracture precursors arise and agrees with the conclusion of dodge et al., (1996) that the fault zone heterogeneity is an important factor controlling the number of foreshocks (i.e. the stronger the heterogeneity, the greater the number of foreshocks). a complicated mosaic pattern of the strain field was revealed before the north palm springs (m = 5.6) eq of 1986 in california. no anomalies were recorded at the pinon flat strain observatory (∼ 33 km from the epicenter) and by the pubs dilatometer (∼ 95 km from the epicenter), whereas the xwr creepmeter (∼ 103 km from the epicenter and ∼ 10 km from the pubs dilatometer) showed a sharp change in the slope of the creep records that leads the water level excursion recorded by the phelan water level monitor (∼ 65 km from epicenter). ben-zion et al. (1990) considered these records as a possible preseismic signal. however, during the landers eq of june 28, 1992 (the largest in california during the last 40 years; mw 7.3), the borehole dilational strainmeter pubs (a sensitivity of less than a nanostrain) installed at the site about 100 km from 85 km-long landers rupture and 135 km from the epicenter exhibited no indications of precursive strain at this location due to either precursive slip on the landers rupture or local slip on the san andreas (johnston et al., 1994). 1.2. short-term precursors at teleseismic distances the mosaic pattern of the strain field creates a specific obstacle to the detection of precursory events and determination of the spatial scale of the preparation zone. obviously, the size of the zone largely depends on the eq magnitude. the first results on anomalous pre-seismic tilts at greater distances (tomaschek, 1955) cannot be regarded as adequate because of imperfections of the instruments used at that time. however, these data cannot be ignored because similar reports on precursors of various types recorded at great distances were published later. wideman and major (1967) recorded strain steps in colorado at great epicentral distances. lomnitz et al. (1978) reported a radon anomaly preceding tangshan event (m = 7.8) and recorded at a distance of 1800 km. fleischer (1981) reviewed the geochemical responses to distant eqs and proposed a dislocation model. preseismic anomalies were recorded by resistivity variometers at distances of more than 1000 km (rikitake 1976; yamazaki, 1983; rikitake and yamazaki, 1985). five observatories in the united states recorded an em emission event a few days before the great chilean (m = = 8.5) eq of 1960 at epicentral distances of about 10 000 km (warwick et al., 1982). based on satellite measurements tramutoli et al. (2001) identified thermal infrared (tir) anomalous effects at a distance of 400 km from the epicenter of the m = 6.9 november 23, 1980 eq in italy. teleseismic effects are recognizable from records of strong eqs. however, it is more appropriate to analyze these effects in relative (r/r*), rather than absolute distances (normalized to the radius of preparation zone r*), i.e. to compare geometrically similar eqs of different magnitudes. then, highand low-energy seismic events can be analyzed in the same terms. this paper is an attempt to consider some consequences of a scenario modeling the final stage of eq nucleation based on the slip weakening mechanism. 2. spatial scale of short-term seismic precursors the problems of the ultimate distance of short-lived precursor detection have been extensively studied and several empirical and theoretical formulas have been proposed for the esti136 vitali a. morgounov mation of the size (radius r*) of the preparation zone. figure 1 illustrates the results of studying the relation r*(m) of various geophysical parameters (resistivity, geochemical and electromagnetic characteristics, water level, magnetic field, strain, tidal variations, etc.) within a wide range of the eq energy (4 < m < 8). wideman and major (1967) made an attempt to find the response of the earth’s crust to the nucleation process at teleseismic distances; they derived the relation for the magnitude dependence r*(m), (fig. 1) of strain steps radius of preparation zone from magnitude of eq 1 1.5 2 2.5 3 3.5 4 4.5 4 5 6 7 8 m log r * wideman et al. (1967), (strain) rikitake (1988), (complex) yamazaki (1977), (resist) ulomov (1987), (geochem, magn. field) yoshino (1992), (eme) morgounov (2001), (em complex) fig. 1. log r*-m relationships for precursory anomalies in various fields: strain steps (wideman et al., 1967); a summary of all available precursory data including strain, tilt, hydrogeochemical variations, microearthquakes, magnetic field, telluric currents, electromagnetic emission, and electrical resistivity (rikitake, 1988); variometer data on the electrical resistivity (yamazaki, 1977); geochemical and magnetic fields (ulomov, 1987); electromagnetic emission (yoshino et al., 1992); and summarized electromagnetic data (morgounov, 2001). measured in colorado during distant eqs (3.5 < < m < 8.5). the results by wideman and major were criticized from various points of view and were generally considered erroneous in spite of the corroboration their results by other authors (berger, 1973; yamada, 1973). note however that, among tens of cases listed in the catalog of strain steps collected by yamada (1973), at least five were recorded few hours before eqs, and rikitake (1976) concluded that these events «were clearly distinct as precursors». 137 slip weakening, strain and short-term preseismic disturbances the solution of this old seismological enigma is beyond the scope of the present paper. wideman’s relation is plotted in the figure to demonstrate that this debatable relation gives distances of the same order of magnitude as the values obtained by other geophysical methods and apparently gives an upper limit of the precursor detection range. wideman and major estimated the amplitude of the observed strain at ε ∼ 10− 9. according to rikitake et al. (1985), such a deformation could be responsible for the generation of precursory anomalies in the resistance of the upper layers of the crust. this value was used in our calculations. the results summarized in fig. 1 suggest that the variations in different parameters have the same origin, namely, the activation of the stress-strain state of the crust. moreover, the similarity between the relations obtained for the so-called «propagating» fields, such as the electromagnetic emission (eme) in the earth-ionosphere waveguide, and the «nonpropagating» fields (radon, electrical resistivity of rocks, groundwater level, etc.) is consistent with the idea that em precursor sources (mechanical-toelectric energy converters) are located near the observation point in the skin-layer of the crust (morgounov, 1985, 1991, 1995). 3. short-term precursors and slip weakening earthquake precursors recorded on the earth’s surface are an indirect response of upper crust rocks to the strain evolution in the focal zone. geological scales of the processes in the crust and its subcritical stress state require that the actual rheology of rocks, characterized by clearly expressed ductile properties, be taken into account. at the final stage of the eq nucleation process, the conditions in the focal zone are favorable for the development of tertiary creep characterized by well-pronounced self-regulation properties. an accelerated inelastic deformation in the focal volume, eventually results in a rupture whose dimension is controlled by its accumulated elastic energy (m). beginning from a certain time of plastic flow, strains and strain rates in surface layers attain values that can be detected by geophysical instruments. many authors considered slip weakening (or creep) as a possible mechanism of eq generation (benioff, 1951; kranz and scholz, 1977; lockner and byerlee, 1978). rice and rudnicki (1979) defined the precursor time as the duration of «self-maintained accelerating creep.» it is evident that a real rock massif is not uniform. a fundamental property of the geophysical medium is the fact that it consists of a system of interacting heterogeneities (blocks and fractures) that constitute a strictly defined, discrete hierarchy of sizes of its elements. due to the inadequate knowledge of the real crustal structure, unknown initial conditions of the deformation process, and the vague mechanism of eq nucleation, an exact solution of the problem cannot be obtained at present. a deterministic description of such a medium is a problem unsolvable even statistically. therefore, the medium should be characterized by generalized parameters in accordance with its scale factors (sadovsky et al., 1987). from this point of view, the heterogeneity of a rock massif does not seem to be an insuperable obstacle for the prognostic tasks. therefore, we use the assumption of a formally uniform half-space, and nominal strain value ε (t, r) keeping in mind that the strain value is a qualitative (measured on the order of magnitude) characteristic of the mosaic secondary response of local dislocations to the general quasi-elastic large-scale strain field. it meets the assumption of benioff (1954) that elastic stresses can have a planetary scale. he substantiated the assumption by the regularity that the strongest earthquakes occur successively in different parts of the world (turkey, taiwan, and greece: 1999; algeria, philippines, taiwan, and japan: may, 2003; etc.). to describe phenomenologically the general relations between spatial and temporal scales of the nucleation, we make the following additional assumptions: a) creep mechanism; b) an r− 3 attenuation of the stress-strain field; and c) calculations in terms of the normalized distance rs /r* for the purpose of obtaining general regularities applicable to eqs of various magnitudes. given constant loading conditions (f = 138 vitali a. morgounov = const), the tertiary creep in rocks of the focal zone can be described by the power function of relative strain εf = ∆l/l t tf f m f 0 = +f f f∆]g (3.1) where εf (t) and ε f0 are the current and initial strain values in the focal zone, t is time, and ∆εf is the strain increment. the deformed volume can be divided into two at least main parts (fig. 2): the nucleation (focal) zone of radius rf in which the nonlinear creep process (governed by eq. (3.1)) develops and the surrounding rocks with quasi-elastic properties described by the equation r r r r n 1 2 2 1=f f^ ^ _h h i (3.2) where r1 and r2 are the distances from the source to the points of measurement. taking into account the rheology of the real medium, the term «quasi-elastic surrounding medium» means here that a rock mass is able to transmit elastic stresses through the preparation region but possesses inelastic properties giving rise to surface precursors such as anomalous disturbances in electrical resistivity (ρ), electromagnetic emission (eme), acoustic emission (ae), and others. given constant loading conditions, the following notation is used below (fig. 3): t fe and tse, the time of eq in the source (f) and at the surface (s); ts0, the onset time moment of the anomaly at the surface; tf 0, the reference time of inelastic deformations in the source; τ, the duration of the anomalous surface signal; t, the duration of the tertiary creep in the focal zone. thus, we have t ts e s 0 = -x and .t t tf e f 0 = (3.3) taking, in the first approximation, tfe = tse and setting tf0 = 0, we obtain .t ts 0 = -x (3.4) also, the following notation is used below: εf 0 and εf e, initial and final strain values in the focal zone; εs0 , initial measurable strain (or strain-defig. 2. a schematic layout of the earthquake nucleation zone: r* is the ultimate distance of the preparation zone; rf is the radius of the seismic source; re is the epicentral distance from the point of measurement; rs is the hypocentral distance, and v is the source volume. fig. 3. diagram illustrating the tertiary creep stage in the focal zone and at the earth’s surface. the following notation is used: tf e and ts e, time moments of an earthquake in the source ( f ) and at the surface (s); ts 0, time moment of the anomaly appearance at the surface; tf 0, reference time of inelastic deformations in the source; τ, duration of the anomalous surface signal; t, duration of the tertiary creep in the source zone; εf 0 and εf e, initial and final strain values in the source zone; εs0, initial surface strain at which anomalies of geophysical parameters can be recorded; rf, characteristic size of the source or the radius of the focal zone; rs, hypocentral distance. 139 slip weakening, strain and short-term preseismic disturbances pendent parameter) at the surface; rf, characteristic size of the source (the radius of the focal zone); rs, hypocentral distance (rs ≥ rf). we have εf 0 = ε0 at the time t = 0 and εf e = = ∆εft m + ε0 at t = t. equation (3.1) yields .t t tf f e m 0 0= +f f f f] _ _g i i (3.5) according to (3.2) and (3.5), the strain at the surface (r = rs) at its initial time t = ts0 is found from the conditions , .t r t t r rs s s f e s m f s n 0 0 0 0 0= +f f f f^ _ _ _h i i i9 c (3.6) then, .t r r ts s s f n m f e m 0 0 0 0 1 1 = -f f f f _ _i i9 c& 0 (3.7) the substitution of (3.7) into (3.4) gives the following expression for the precursor duration τ depending on the epicentral distance and eq energy .t= r rs s f n f e m 0 0 0 1 1 -x f f f f 1 _a _i k i: d' 1 (3.8) 4. ultimate distance of precursor detection the condition τ = 0 determines the ultimate distance rsmax = r* at which the precursor can be detected. this condition and eq. (3.8) yield .r rf f e s n 0 1 ) = f f_ i (4.1) the duration of inelastic deformation in the source is controlled by the initial strain εf 0, which depend on the sensitivity of the method in use and is actually a reference value. according to estimates given in (yamazaki, 1977, 1983; rikitake et al., 1985), preseismic anomalies recorded by an electrical resistivity variometer comply with a surface strain not smaller than εs0 = 10– 9. this value is taken here as an initial value of the tertiary creep process in the focal zone. this provides certain constraints on the duration of the deformation process in the source. taking into account the level of natural noise and the sensitivity of the method in use and dambara’s (1966) way to evaluate the size of the focal zone rf, we assume εf 0= εs0 . numerical order-of-magnitude estimates can be made as follows. according to various authors (in particular, rikitake, 1976), the rupture of rocks occurs at εf e ∼ 10− 4. equation (4.1) at n = 3 gives r*/rf ∼ 50. the source size rf being strongly dependent on the magnitude m, numerical estimates of rf can be derived from the formula (dambara, 1966) .2 27km .r . f m0 5 = 10] g (4.2) using the statistics of data for geophysical precursors of 13 types, rikitake (1988) obtained a general empirical formula relating the maximum distances d (i.e. radius of the earthquake preparation zone, d = r*) within which the precursors are detectable and the earthquake magnitude: m = − 0.87 + 2.6 1og d. dividing this relation by eq. (4.2), we obtain r*/rf = 138, 106, 81, and 62 for the respective magnitudes m = 4, 5, 6, and 7, with the average value being r*/rf ∼ ∼ 96 for m = 4-7. consequently, the maximum range of the precursor detection can be approximately hundred times as large as the source size. data of yamazaki’s variometer (rikitake and yamazaki, 1985) provide the following estimates for the ultimate normalized distances of a short-term precursor (see no. 5, 8, and 21 in table i, and eq. (4.2)): rs / rf = 85.7, 92.7 and 86.5. therefore, the estimate r*/rf ∼ 50, obtained above for εf e ∼ 10− 4 and εs0 ∼ 10− 9, is inconsistent with the experimental data. thus, the value r*/ rf ∼102 is more adequate to in situ measurements .10+= 3 rf f e s 0 2) f f 1 r _ i (4.3) hence, we have (εf e/εs0 ) ∼ 106. this can increase the known estimates of the in situ ultimate strain to the value εf e ∼ 10− 3, which is consistent with data of direct measurements of the breaking strain in rocks under laboratory conditions (kasahara, 1981). (note that the value r*/rf ∼ ∼ 102 with εf e∼ 10− 4 is consistent with εs0 ∼ 10−10). thus, the value r*/ rf ∼ 102 complies with data of the in situ resistivity observations by yamazaki. then, considering eqs. (4.2) and (4.3), 140 vitali a. morgounov table i. 30 earthquakes that were preceded by preseismic resistivity variations (rikitake and yamazaki, 1985) and their calculated parameters. note: m – magnitude; h – depth; re – epicentral distance; rs – hypocentral distance; τ – precursor duration; r* – maximum size (radius) of the preparation zone; rf – characteristic size (radius) of the focal zone; rs /r* – normalized hypocentral distance. no. data m h (km) re (km) rs (km) τ (hour) r* (km) rs /r* rf (km) 1 16.05.1968 7.9 0 935 935 2 4786 0.195 47.9 2 01.07.1968 6.1 50 127 136 3-4 h 45 min 603 0.226 06.0 3 12.08.1969 7.8 30 1094 1094 4 4266 0.330 42.7 4 09.09.1969 6.6 0 320 320 2 h 15 min 1072 0.299 10.7 5 06.01.1971 5.5 40 256 259 0.5 302 0.858 03.0 6 23.07.1971 5.3 10 100 100 5 240 0.417 02.4 7 02.08.1971 7.0 60 1004 1005 7 1698 0.592 01.7 8 12.08.1971 4.8 60 110 125 0.5 135 0.927 01.35 9 06.10.1972 5.5 30 174 177 0.5 302 0.586 03.02 10 04.12.1972 7.2 50 337 341 2-7 2138 0.159 21.4 11 27.03.1973 4.9 60 66 89 3 151 0.588 01.51 12 30.09.1973 5.9 50 147 155 1 479 0.324 04.79 13 03.03.1974 6.1 60 160 171 3 603 0.284 06.03 14 09.05.1974 6.9 10 144 144 4 1514 0.095 15.14 15 27.09.1974 6.4 60 311 317 2-10 851 0.372 08.51 16 16.10.1974 6.1 40 215 219 2 603 0.363 06.03 17 30.10.1974 7.6 420 673 792 4-9 3388 0.234 33.90 18 02.04.1975 5.8 40 194 198 5-11 427 0.464 04.27 19 20.02.1978 6.7 50 461 464 3.5 1202 0.386 12.00 20 07.04.1978 6.1 30 160 163 1 603 0.271 06.03 21 13.08.1978 4.7 80 67 104 1 h 15 min 120 0.865 01.20 22 03.12.1978 5.4 20 50 54 5.5 269 0.201 02.69 23 11.07.1979 5.9 40 223 227 1 479 0.474 04.79 24 28.10.1979 5.5 90 104 138 6 302 0.457 03.02 25 12.03.1980 5.6 80 85 117 1 339 0.345 03.39 26 08.05.1980 5.7 60 104 120 6 h 45 min 380 0.316 03.80 27 29.06.1980 6.7 10 45 46 0.5-5 1202 0.038 12.00 28 25.09.1980 6.1 80 69 106 1 603 0.176 06.03 29 21.02.1982 6.7 0 230 230 8 1202 0.191 12.00 30 23.07.1982 7.0 10 241 241 3.5 1698 0.142 17.00 141 slip weakening, strain and short-term preseismic disturbances the following formula can be obtained for estimating the ultimate radius of the strain sensitivity (preparation) zone (fig. 1): .0 27km .r . m0 5) = 10] g (4.4) for example, eq. (4.4) yields r* ∼ 1700 km for m = 7 and r* ∼ 5400 km for m = 8; i.e. the strain-sensitivity zone can extend over considerable distances from the epicenter. this approach explains, in particular, the long-range effects of strain-related precursors, including «strange» (in terms of rikitake and yamazaki, 1985) precursors in rocks recorded at epicentral distances longer than 103 km. the stereotypes associated with the property of em waves to propagate over large distances dominated the early model concepts of the em precursor range. however, the examples presented above show that the waveguide propagation model of an em signal fails to account for the long-range effect of strain-related resistivity precursors recorded at epicentral distances of up to a thousand kilometers, and the same is true of geochemical, hydrodynamic, and other precursory phenomena (lomnitz and lomnitz, 1978; fleischer, 1981; warwick et al., 1982; tramutoli et al., 2001). in particular, the preparation zone radius of the chilean earthquake of 1960 (m = 8.5) is estimated at r* ∼ 9550 km according to (4.4). this agrees with the data of warwick et al. (1982), who recorded radiowave precursors of this earthquake at distances of up to 104 km (the formula of rikitake (1988) gives r* ∼ 4075 km for this magnitude, fig. 1). note once more that tomaschek (1955) reported special tilts of about 0.1 s that were recorded in japan at an epicentral distance of ∼ 1800 km 20 h before the taiwan earthquake of m = 7. in this case, the relation (4.4) gives a similar value for the ultimate radius of preparation zone (∼ 1700 km). is this coincidence accidental? the answer requires additional in situ data in order to perform independent detailed analysis. such eq parameters as the magnitude, depth and epicentral distance allow the estimation of the nominal strain value at an observation point. at the epicentral distance rs, eqs. (4.1) and (4.2) yield (εf e =10− 3) .9 81.r . s m s 1 5 3 =f -10] g (4.5) table ii presents the values r* and εs estimated by eqs. (4.4) and (4.5). rikitake (1976) reported preseismic strain anomalies that enabled such estimation (case no. 1, 2, and 3 in table ii). case no. 4 illustrates the estimated strain value εs at the observation point and the value of r* in the case of the preseismic em event recorded by warwick et al. (1982) at five us stations before the great chilean earthquake of may 22, 1960 at distances of up to 10 000 km. event no. table ii. examples of calculated values εs, r*, and rs /r* in comparison with parameters of some earthquakes that were preceded by precursory variations. no. m ∆ km h km ε (in situ) ε (calcul.) r* km rs /r* references 1 7.0 94 70 2.5 ⋅ 10− 6 3.0 ⋅10− 6 1700 0.068 rikitake (1976) 2 6.0 250 n 4.0 ⋅ 10− 8 1.0 ⋅10− 8 537 0.46 rikitake (1976) 3 3.0 5 n 3.0 ⋅ 10− 8 3.9 ⋅10− 8 17 0.29 rikitake (1976) 4 8.5 10 000 n 0.9 ⋅10− 9 9549 ∼ 1 warwick (1982) 5 6.6 368 16 1.7 ⋅ 10− 8 1071 0.34 eftaxias (2002) 6 5.9 359 n 2.0 ⋅ 10− 9 478 0.75 eftaxias (2002) 7 6.5 192 n 1.2 ⋅ 10− 7 955 0.20 eftaxias (2002) 142 vitali a. morgounov 5, 6 and 7 represent a few recent eqs in greece, including the destructive earthquake of september 7, 1999 (m = 5.9) in athens. a distinct preseismic eme signal was recorded by eftaxias et al. (2002) at a distance of ∼ 370 km (event no. 6 is yielded a nearly ultimate value of the normalized radius, rs /r* = 0.75). table ii shows that all precursors (including those recorded at great distances) were observed within the preparation zone of radius r* eq. (4.4). obviously, the uncertainty of the precursor detection increases with distances approaching the ultimate value (only 10% of all the events, table i). it is natural that more reliable detection of precursors can be expected closer to the epicentral zone where the strain values are higher. for example, in the case of ε ∼ 10− 6 the «effective» radius of the preparation zone eff ) r is determined by the expression .km10 . .m0 5 1 27 eff ) r = ] g (4.6) 5. creep duration and normalized precursor time undoubtedly, the determination of main invariant relationships in the space-time distribution of short-term precursors is a key point in solving the prediction problem. published data fig. 4a,b. synchronous development of anomalous disturbances in the variometer record of electrical resistivity and eme in japan at points 50 km apart one from the other and 70 km apart from the epicenter. a) example of an electrical resistivity variation before an m = 6.1 earthquake (kanto area) at the epicentral distance re = 69 km (september 25, 1980). the symbol «p» and the dotted arrow indicate the onset of the anomaly. the solid arrow indicates the onset moment of the earthquake. the time scale is shown in the top left corner (yamazaki, 1983). b) eme level at a frequency of 81 khz from observations at the suginami seismic station near tokyo before the same earthquake (gokhberg et al., 1982). a b 143 slip weakening, strain and short-term preseismic disturbances are mainly episodic measurements related to individual seismic events. in this context, welldocumented long-term data recorded by yamazaki’s variometer (ρ) over 30 years (19681998) provide a good opportunity for testing the model described above. yamazaki (1977, 1983) subdivided seismic anomalies into preseismic and coseismic types. the data on preseismic anomalies are still the subject of vivid discussions or mistrust. yamazaki (1977, 1983) and rikitake and yamazaki (1985) presented 14-year data (1968-1982) in the form of records of preseismic ρ anomalies for 30 eqs with magnitudes of 4.7-7.9 that occurred in the region; they also presented the duration of the anomalies, their relative amplitudes, epicentral distances, and focal depths. yoshino et al. (1998) questioned the reliability of these data because, in their opinion, the successive records of 1982-1997 revealed no precursors. this obvious contradiction requires an explanation. undoubtedly, the preseismic anomaly presented in fig. 4a (yamazaki, 1983) is debatable from the standpoint of an independent expert. however, it is important to note that this particular «questionable» anomaly recorded at the aburatsubo observatory (yamazaki, 1983) coincides in the times of arrival, duration, and completion with the eme anomaly independently recorded at the suginami observatory (yoshino et al., 1992) before the same seismic event (fig. 4b). comparative analysis of published data reveals several coincidences of this kind. after the first eme measurements made by morgounov, yoshino and tomizawa in 1980 in japan (gokhberg et al., 1982), yoshino et al. (1992) published the statistics of eme precursors over the period from 1985 through 1990. the joint analysis of data published in yoshino et al. (1992, 1998) showed that at least ten anomalous resistivity disturbances recorded in this period coincided in time with eme precursors (yoshino et al., 1992), and these coincidences cannot be considered accidental. in other words, the resistivity anomalies reported in yamazaki (1977, 1983), rikitake and yamazaki (1985) and rikitake (1988) are actually precursory anomalies due to the earthquake nucleation process and deserve a more detailed analysis. similar to precursory anomalies of other types, the duration of ρ precursors is invariably irregular as a function of the epicentral distance. figure 5a illustrates this with 30 cases recorded in 14 years (see table i) (rikitake et al., 1985). in order to estimate the minimum duration of the rock failure process in the focal zone, only the anomalies that developed monotonically prior to an eq were chosen from table i. by analogy with the notion of «canonical» creep, which is a deformation process gradually developing under a constant load, we introduce the term «canonical» for a precursor that continuously develops until earthquake onset. nearly 80% of all cases from table i meet this condition. in six cases (20%), anomalous disturbances ended before the earthquake onset (no. 7, 10, 15, 17, 18, and 26 in table i). these cases are shown as stars in fig. 5a. as is evident from the diagram (fig. 5b) showing the distribution of the duration times of «canonical» precursors as a function of the hypocentral distance rs normalized to the radius of preparation zone r* (eq. (4.4)), the chaotic set of points in fig. 5a is transformed into a group of points bounded by the straight line fitting the maximum duration values τmax of canonical precursors in ρ recorded at various normalized distances rs/r* from epicenter (fig. 5b). these maximum values are of particular interest. the precursor durations τ that are smaller than the critical value τmax (the points below the straight line in fig. 5b) can be naturally interpreted in terms of an indistinct, inadequate manifestation of precursors due to the mosaic space-time pattern of the deformations, nonuniformity of the stress field, varying strain sensitivity at the observation point, or specific features of a particular earthquake mechanism. the regression line approximating the maximum values τ max (no. 6, 21, 24, and 29 in table i; open circles in fig. 5b) is τ = 9.83 − 9.88 (rs/r*) with λ2= 0.96. within a reasonable accuracy, this gives the following empirical formula: )t r rs0,x 1 -_ i (5.1) where t0 ∼ 10 h. 144 vitali a. morgounov thus, one can evaluate empirically the duration of slip stage of rocks in the source. when distance to the focal zone decreases, i.e. at rs → rf, we have τ → t0. data by yamazaki (1977, 1983) yield t0 = 9.885 ≅ 10 h as an empirical estimate of the ductile failure duration in the source. as seen from fig. 5b, four precursory events (only ∼ 13% of all cases) were observed in the immediate vicinity of the normalized ultimate epicentral distances. empirical formula (5.1) complies with theoretical formula (3.8). since εf e/εs0 ≅ 105− 106 > > 1 1.25 5 6 t =8 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3log rs log x (hours) 6 8 5 1.25 9.8 8.6 9.3 11.05 t = 9.83 9.88 (r s /r*) r 2 = 0.96 0 2 4 6 8 10 12 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1rs /r* x (hour) fig. 5a,b. a) time of the electrical resistivity precursor (log τ) versus the logarithm of the hypocentral distance (log rs) (rikitake and yamazaki 1985). the triangles and circles are precursors that continuously developed until the earthquake onset (i.e. «canonical» cases). the stars are precursors that terminated before an earthquake (no. 7, 10, 15, 17, 18, and 26). the circles indicate the events with τmax in terms of the normalized distance rs / r*. the numbers near circles indicate the duration of the precursors in hours. b) duration of a precursor and a failure process in the focal zone as a function of the hypocentral distance normalized to the size of preparation zone r*. the triangles and open circles indicate the «canonical» cases (see table i). open circles (no. 6, 21, 24, and 29) indicate the events with τmax estimated in terms of the normalized distance rs / r*. solid circles are duration times of the fracture process in the source calculated from eq. (5.1) for the events with the precursors shown as open circles. a b 145 slip weakening, strain and short-term preseismic disturbances and m = n = 3, we have (rs / rf)3 > > 1, and this agrees with the in situ observed distances for which rs / rf > 2-3. substitution of these values into (3.8) gives formula (5.1). the agreement between theoretical formula (3.8) and empirical formula (5.1) suggests that the process of tertiary creep in form (3.1) can be considered, to a first approximation, as a mechanism responsible for the fracture of the source rock mass. it also confirms the validity of yamazaki’s precursors (yamazaki, 1983; rikitake et al., 1985). equations (3.8) and (5.1) can be transformed into the form suitable for estimating the minimum time of the ductile failure t0 in the source (rs ≠ r*) ) .t r rs 1 = x 1 -_ i (5.2) for cases 6, 21, 24, and 29 (table i; solid circles in fig. 5b), we obtain t0 = 9.8, 8.6, 11, and 9.3, respectively. the dotted line in fig. 5b is the regression line approximating the t0 values for τmax at different normalized distances. this regression line indicates that the rate of creep at its final stage apparently has a limiting value (t0 ∼ 10 h) that is independent of the earthquake magnitude (at lease in the range m = 4.7-6.7 of the cases 6, 21, 24, and 29). naturally, the t0 value is determined by the onset time tf0 of the plastic deformation that in turn depends on the sensitivity of the method εs0. therefore, the duration of the fracture monitoring can generally be extended with the use of more perfect instrumentation. it is obvious that there exists a natural threshold of detection of seismogenic anomalies due to natural noises (tides, temperature, local geodynamic processes, etc.). 6. discussion the agreement between the empirical (5.1) and theoretical (3.8) relations is remarkable and suggests that the initial assumption on the creep mechanism can be consistent with the natural process, at least, as regards the events considered. the conclusion on the invariance of t0 complies with the known evidence on the smallness of the scale factor during plastic deformations (mcclintock, 1976). studying the scale factor in rocks under laboratory conditions, mogi (1988) came to the same conclusion that the dimension effect on the strength of rock samples is insignificant. several authors noted that, with the energies of earthquakes covering a range of a few orders of magnitude, the amplitudes of variations in precursors vary insignificantly, and this supporting the fundamental hypothesis of tsuboi (1956), according to which a rock volume is characterized by an ultimate strength, implying that the ratio of elastic energy to the seismic source volume is a global characteristic and, in a first approximation, is independent of rock properties. the inference of the present work stating that t0 is invariant, i.e. that the minimum time of the creep-type failure (under a quasiconstant load) in the seismic source (eq. (3.7)) is independent of the magnitude (m), is also consistent with the tsuboi hypothesis and extends the latter to the duration of the ductile failure stage. within the framework of the tertiary creep under constant load conditions, the strain rate increases up to the fracture value. in practice, an earthquake can occur during the period of relatively smooth development of an anomaly, with no intensity peaks preceding the main shock. moreover, many earthquakes occur during the decay period of a bay-like anomalous disturbance or after its completion, during the so-called quiescence period of the signal. in this case, the usual model of avalanche creep under a constant load cannot serve as a direct analogy of the fracture development. the uniaxial loading of a sample under laboratory conditions differs from in situ conditions in that the load applied to an in situ deformed rock mass does not remain constant during the process of plastic flow but decreases because due to considerable creep strain and the fact that this process proceeds in the environment of surrounding rocks contributing to the load redistribution. the mechanism of creep failure under load relaxation conditions provides an explanation for various precursory signatures, as well as 146 vitali a. morgounov signal-quiescence earthquakes and precursors of the oscillatory type. the deceleration of the failure process, i.e. an increase in the precursor duration (several hours, a few days) and signal instability degree, can be interpreted in terms of the mechanism of stress relaxation during the deformation of the focal zone volume (the relaxation creep model: morgounov, 2001). events 7, 10, 15, 17, 18, and 26 (see table i), which have been excluded from the analysis under the f=const condition, have features characteristic of the failure of the relaxation type. the generation of anomalous disturbances unaccompanied by an earthquake (silent, or slow earthquakes) seems to be a natural element of the nucleation process (false alarm) and can be regarded as a consequence of the relaxation creep mechanism. the quiescent phase of relaxation creep could be the reason why preseismic anomalies could not be detected when measurements were made immediately (a few minutes or hours) before a shock. the creep mechanism is effective under conditions of either pure compression (extension) or pure shear (benioff, 1951). therefore, this model can be applied to the results obtained in seismically active regions dominated by shear deformations (the san andreas and anatolian faults), subduction zones (kuril-kamchatka and japan islands), or continental areas of china subjected to compression. the value t0∼10 h was obtained using data from the subduction zone of japan. analysis of experimental data from other regions can provide constraints on the variability of this value. 7. conclusions johnston and linder (2002) noted that observations of crustal strain in high sensitivity zones near large earthquakes during the final stages before the rupture are unfortunately rare, and the knowledge of the timescale and mechanics of failure is therefore limited. this is largely due to a relatively low recurrence of earthquakes and sparseness of adequate instrumental networks. the discrepancy between the results of the strain measurements, and the other geophysical disciplines, and expected theoretical values suggests that blocks can concentrate regional stresses at a relatively small portion of their perimeters. then, the average stress on block boundaries can be relatively low. thus, the observation of aseismic deformation preceding earthquakes is consistent with the block tectonics (leary et al., 1984). such secondary local mosaic effects could explain why direct stress measurements at a given point have a low probability to detect preseismic anomalies. in other words, this probability depends on the number of stations and/or the detection of a sensitive point of stress concentration. from this point of view, taking into account elements of soc phenomena, the problem of the epicenter location seems to be more unreliable than the determination of the time interval (from the similarity of temporal scales) and the magnitude (from spatial scales) of the pending shock. exact calculations of the strain of a impending earthquake in such a medium are problematic. nevertheless, the calculations in a formal uniform half-space can be useful for the qualitative estimation of the possible (maximal) secondary mosaic response of a fractal rock mass near the surface to the seismogenic strain evolution in the nucleation volume. thus, the main problem is not the absence of precursory anomalies (they do exist!) but the nonadequate response of a fractal crust to the source fracture process. does this mean the insolvability of the prediction task? the response of local dislocations to a field of a larger scale reflects, to an extent, the general nucleation creep process in the source. under favorable conditions, the measurements mirror the nucleation process in the source that resulted in successful predictions in the past (raleygh et al., 1977; hui et al., 1997). a considerable increase in the strain rate before the failure significantly raises the signal-tonoise ratio, which is beneficial to the identification of short-lived precursors and provides an opportunity to explain published data on preseismic anomalous disturbances recorded at ultimate (teleseismic) distances during strong earthquakes. the relative (normalized) epicentral distance is beneficial to the comparison of geometrically similar earthquakes of different mag147 slip weakening, strain and short-term preseismic disturbances nitudes. this concept makes it possible to analyze a «telesesmic equivalent» of seismic events of a lower energy. for example, the ultimate radius of an m ∼ 5.0 preparation zone is approximately r*=150 km (eq. (4.4)), i.e. the distance 150 km for an m = 5 event is equivalent to the telesesmic distance r*= 5400 km of a strong, m = 8.0 eq. in both cases r/r*∼1. thus, the phenomena of teleseismic precursory effects does not seem to be abnormal and can be measured in exceptional cases under favorable conditions (tomaschek, 1955; wideman et al., 1967; lomnitz et al., 1978; warwick et al., 1982; yamazaki, 1983; and others). the real strain field is a superposition of the complicated strain field of the source and heterogeneous geological structures and topography, giving rise to an intricate pattern of stressstrain state controlled by fault structures. the satellite image of thermal infrared anomalies demonstrated by tramutoli et al. (2001) illustrates this natural mosaic distribution of precursors associated with the strain field. these patterns are the possible reason strain-metering instruments fail to record short-lived precursors at a given point, and why more «integral» methods (like eme) are more efficient than pointwise strain measurements. returning to the question whether soc is an insurmountable obstacle for eq prediction, it is appropriate to note the conclusion by sykes et al., (1999) that this issue does not depend on the soc nature of seismicity. it depends on whether there exists a precursory phase of instability that can be reliably detected from instrumental observations. numerous studies performed in various countries, including the positive experience of a «negative results», is a good reason for reserved optimism with respect to future research. acknowledgements the author would like to express sincere thanks to m.j.s. johnston for helpful discussions regarding the problem of the creep-strain measurements and v. lapenna for reviewing the manuscript and valuable comments. references abercrombie, r.e., d.c. agnew and f.k. wyatt (1995): testing a model of earthquake nucleation, bull. seismol. soc. am., 85 (6), 1873-1878. bak, p., c. tang and k. wiesenfield (1988): self-organized criticality, phys. rev. a, 38, 364-374. benioff, h. 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region, russian federation article history: received october 12, 2018; accepted march 2, 2020 abstract the studies completed to-date on a relation of the earth’s seismicity and solar processes provided the fuzzy and contradictory results. the main problem of this research is a lack of physical explanation of a mechanism of earthquake triggering by strong variations of space weather conditions. based on results obtained in the field and laboratory experiments on earthquake triggering by dc pulses injection into the earth crust we may assume that the similar triggering phenomena may occur after the strong electromagnetic impact to the earthquake source due to solar flares or geomagnetic storms. numerical estimations demonstrated that telluric currents induced by geomagnetic pulsations generated by solar flare have the similar density at the depth of earthquake source location (10-6 a/m2) in comparison with the current density generated by artificial power sources (10-7 – 10-8 a/m2) resulted in observed spatiotemporal redistribution of seismic activity in the regions of pamirs and northern tien shan. for supporting the idea of a possible earthquake triggering by solar flares we carried out a statistical analysis of global and regional (greece) seismicity behavior during the solar flare of x9.3 class occurred on september 6, 2017 (the strongest flare over the past thirteen years). we have discovered a new evidence of earthquake triggering due to the sun-earth interaction by simple comparison of a number of earthquakes before and after the strong solar flare. the global number of earthquakes (usgs catalog, m ≥ 4) for time window of ±11 days after the solar flare has increased by 68%, and the regional seismicity (greece, emsc catalog, m ≥ 3) has increased by 120%. keywords: space weather, solar flare, telluric currents, earthquake triggering. 1. introduction it is well known that space weather, which incorporates a set of phenomena in the space including solar flares and magnetic storms, may affect technological systems and activities of modern civilization. the effects of geomagnetic disturbances on telegraph lines, submarine phone cables, and power systems have been noticed for over a century and described in a number of reviews [e.g., lanzerotti and gregori, 1986; boteler et al., 1998]. in addition, a discussion of an impact of strong variations of space weather on the lithosphere and possible triggering of earthquakes is under way. the problem of an influence of solar activity on the terrestrial processes is considered for more than 150 years starting from the study [wolf, 1853] where it was pointed that sunspots may exert some influence on the earthquake occurrence. to-date quite a large number of studies have been completed to find a correlation between the earth’s seismicity and solar processes. results of these statistical studies are contradictory: some researchers found the direct correlation between the energy or number of earthquakes and wolf numbers [mazzarella and palumbo, 1988; sytinsky and oborin, 1997], and others the anti-correlation [sobolev et al., 1998; shestopalov and kharin, 2014]. moreover, there is some skepticism in assessing the existence of such solar-terrestrial relations resulted in statistically significant seismicity variations [love and thomas, 2013]. it should be noted that all the mentioned conclusions (both optimistic and pessimistic) are based on correlation analysis only. even when such analysis yields statistically significant results, the authors only assume that there is a possible triggering mechanism in the case of positive correlation, without attempts of understanding the physics of the possible relations, or provide the most general considerations. for example, it was noted [georgieva et al., 2002] that earthquakes occur more often when the solar activity level drastically changes. during a solar flare the radiation level increases many times, which interacts with the earth’s magnetosphere resulted in its disturbance and occurrence of magnetic storm. in turn, the magnetic storms may influence to the earth’s rotation speed [gribbin, 1971; simpson, 1968] resulted in redistribution of stresses in the earth crust and earthquake triggering. some researches [riccardi, 2007] connect the variations of seismic activity after solar flares with an increase of a number of cyclones that can act as a trigger due to sharp variations in atmospheric pressure during passage of their fronts over the earthquake source. nevertheless, the estimations [sobolev et al., 2012] demonstrate that such variations are substantially lower than the level of stress variations due to impact of numerous local earthquakes of moderate magnitudes (m 4–5) or due to lunar-solar tides whose influence on seismicity is still under discussion [beeler and lockner, 2003]. in the recent years a few papers were published where electromagnetic hypotheses were proposed on the triggering impact of geomagnetic field variations (sudden commencement storms) [sobolev et al., 2001; zakrzhevskaya and sobolev, 2002; tarasov and tarasova, 2002], as well as diurnal sq-variations [duma and ruzhin, 2003]. particularly, results of statistical analysis obtained for different regions (northern tien shan, caucasus, northern america, central europe, china, and japan) demonstrated an evident correlation between geomagnetic field variations and a number of earthquakes both for average diurnal cycle and for a long period of about 150 years. an idea was discussed on a possible generation of additional stresses in the earth crust due to the magnetic field variation [duma and ruzhin, 2003; tren’kin, 2015]. in this paper we make an attempt to validate a possibility of earthquake triggering by solar flare based on electromagnetic triggering phenomena recently discovered in the field [tarasov et al., 1999, 2000; tarasov and tarasova, 2004; chelidze et al., 2006] and laboratory [bogomolov et al., 2004; avagimov et al., 2006; lapshin et al., 2016; chelidze et al., 2002; novikov et al., 2017] experiments. 2. field and laboratory results on electric/electromagnetic triggering of earthquakes as motivation of research in the middle of 90th a statistically significant impact of electrical pulses of dc current injected through emitting dipole into the earth crust on the spatial and temporal distribution of weak seismicity of pamirs [tarasov, 1997] and northern tien shan regions [tarasov et al., 1999, 2000; tarasov and tarasova, 2004; chelidze et al., 2006] has been discovered. the grounded electrical dipole of about 4 km length was supplied by single pulses of electrical current of 1.5 – 2.5 ka, voltage of 1 kv, and duration of 2.5 – 10.0 s provided by geophysical magneto-hydrodynamic (mhd) generator used for deep electromagnetic sounding of the earth crust for monitoring the crust stress state and search of electromagnetic precursors of earthquakes. statistically, by the epoch superposition method, with application of data of regional seismic catalogs and catalogs of firing runs of mhd generator it was found that after injection of electric current into the earth crust the number of weak earthquakes (mb < 3.0) is increased by several times with a time delay of 1-2 days, and then within 5-7 days it returns to the previous background level. a hypothesis on triggering action of electrical pulses in the stressed rocks has been proposed [zeigarnik et al., 1999]. laboratory experiments carried out to date with application of various press equipment and samples of geomaterials under stress-strain state demonstrated a modulation of acoustic emission by electric current pulses applied to the sample victor novikov et al. 2 [bogomolov et al., 2004; avagimov et al., 2006; lapshin et al., 2016]. it was shown that a response of acoustic emission to electric action characterized crack formations in the sample rises as the sample loading and maximally appears at the stress level of 0.95-0.99 of breaking stress of the geomaterial. the laboratory study of electric triggering of macro events (laboratory “earthquakes”) were carried out at simplified slider system [chelidze et al., 2002] and spring-block model [novikov et al., 2017] simulated the seismogenic fault. the both experimental facilities are based on the idea that “stick-slip” behavior of the fault is a mechanism of earthquake preparation and occurrence [brace and byerlee, 1966]. in the both series of experiments the sudden slip of the movable block was initiated by injection of electrical current into a contact between the fixed and movable blocks (simulated crust fault). the triggering of slip of movable block of the spring-block model is possible only under subcritical state of the system, when the accumulated shear stresses are 0.98–0.99 of critical value, when the sharp slip (laboratory “earthquake”) occurs [novikov et al., 2017]. numerical calculations of dc current distribution in the crust from grounded dipole powered by mhd generator [novikov et al., 2009] provide the current density at a depth of earthquake source locations (5-10 km) for northern tien shan at a level of 10-7-10-8 a/m2. keeping in mind that sharp variation of space weather can generate a flash of geomagnetically induced currents in the conductive subsurface structures [lanzerotti and gregori, 1986; boteler et al., 1998] it was supposed that the solar flares can induce telluric currents with parameters similar to artificial dc pulsed power systems [sorokin et al., 2012]. 3. numerical estimations of telluric currents generated in the earth crust by solar flares. for verification of idea on electromagnetic earthquake triggering by solar flares the theoretical model of electric and geomagnetic field perturbations due to changes in the ionosphere conductivity caused by solar flares [sorokin et al., 2019] has been developed. it was assumed that the largest current amplitudes in lithosphere are induced by short-periodical oscillations of geomagnetic field [sorokin et al., 2012]. the simple sketch of the sun-ionospherelithosphere interaction is shown in figure 1. 3 space weather and earthquakes figure 1. diagram of the sun-ionosphere-litosphere interaction during the solar flare. 1 – conductive layer of lithosphere; 2 – conducting layer of ionosphere; 3 – x-rays of solar flare; 4 – electric field in ionosphere; 5 – geomagnetic pulsations; 6 – induced telluric currents in conductive layer of lithosphere. victor novikov et al. 4 one of the mechanisms for generating geomagnetic pulsations is associated with a local change of ionospheric conductivity in the presence of an external electric field. such a mechanism was considered in [bell, 1976] in relation to the problems of generating pi2 pulsations, as well as artificial oscillations of the geomagnetic field line. in these studies the processes with characteristic periods of more than 10s were considered. the earth-ionosphere resonator forms oscillations of the geomagnetic field with periods of 1 to 100s during ionization of ionosphere by flare radiation with a short front of its amplitude rise. theoretical model of these processes has been developed in [sorokin and yaschenko, 1990] in assumption of ideally conductive earth. for a study of parameters of induced electrical current in lithosphere the model was modified taking into account its real crust conductivity. estimates of these parameters may be easily obtained considering the geomagnetic field as vertical. therefore, the horizontal derivatives in equations may be neglected. it corresponds to the case when during the specific period of the electric field variation the ionosphere currents and fields diffuse in horizontal direction to the distance much less of the horizontal scale. numerical calculations demonstrated (see figure 2) that as a consequence of sudden change of integral conductivities due to solar flare radiation during about ten seconds the electric fields of 0.01 to 0.1 mv/m and electric currents of density of 10-6-10-7 a/m2 are generated, that is comparable with the current density generated in the crust by artificial pulsed power sources provided earthquake triggering observed in two regions (pamirs and northern tien shan) with different geological settings. electric current concentrates in the crust layers with increased electrical conductivity. it was shown (figure 2) that if the electrical conductivity in the layer at a depth of 10 km is five times more than in the upper layer, the current density increases there by an order. thus, the telluric current density induced by solar flares at the depth of location of earthquake sources (510 km for northern tien shan) may be similar to current density from the artificial power source affecting the regional seismic activity, that pointed to a reality of existence of solar-terrestrial electromagnetic-seismic relations and a possibility of triggering the seismic events not only by artificial power sources, but perturbations of space weather like short-period geomagnetic pulsations and strong geomagnetic storms as a result of solar flares. these results may serve as the physical background of new approach to solve the problem of short-term earthquake prediction based on electromagnetic triggering phenomena. figure 2. a) a depth dependence of electric conductivity of lithosphere used for numerical calculations; b) dependence of electric current density on the lithosphere depth at different points of time (1 – 5s, 2 – 10s, 3 – 20s , 4 – 50s) for characteristic duration of the front of ionosphere conductivity of 5s and ionosphere conductivity of 1 [sorokin et al., 2019]. 4. new evidence of earthquake triggering by solar flare powerful surge of solar flash activity early in may 2017 during approaching the minimum of 24th solar cycle was accompanied by significant disturbances of space weather. on september 6, 2017 a group of sun spots ar2673 generated a large solar flare of x9.3 class, the strongest flare over the past thirteen years. its explosion produced a coronal mass ejection partially directed towards the earth. we carried out a statistical analysis of the usgs and emsc catalogs of earthquakes for determination of effect of solar flares on global seismic activity. considering the results published in [sobolev et al., 2001; zakrzhevskaya et al., 2002] indicated that the seismic response to electromagnetic impact on the lithosphere appeared during the followed 1-6 days, we choose two periods of time: one with flares of a class x (flares on september 6, 7, and 10, 2017), that is, from september 6 to 16 (blue arrow in figure 3), and the similar background interval one day before, from august 26 to september 5 (red arrow), inclusive. it should be noted that is difficult or practically impossible at this stage of research to apply the sound statistical analysis (e.g. correlation analysis for complete data sets, both global and regional) to clear a presence of electromagnetic triggering phenomena resulted from solar flares. the earthquake faults sensitive to electromagnetic impacts should have specific parameters favorable for earthquake triggering by electromagnetic impact, namely, subcritical stress level [novikov et al., 2017], increased electric conductivity, certain fault orientation [han et al., 2004] for occurrence of geomagnetically induced currents of specific density required for the earthquake triggering. in many cases these parameters are unknown. in this case, we consider the simplified approach of analysis of seismicity behavior during the short period around the x class solar flares of september, 2017. 5 space weather and earthquakes figure 3. magnetic activity during a month from august 20, 2017. red and blue arrows designate periods selected for an analysis. figure 3 shows a magnetogram, i.e. geomagnetic field variations (three components) according to data collected in izmiran observatory during a month (2017.08.20 to 2017.09.20) with the time interval of an analysis denoted by horizontal arrows. the lower part of the graph presents local index k of magnetic activity. figure 4 demonstrates a rise of global seismic activity according to usgs earthquake catalog [https://earthquake.usgs.gov/earthquakes/search/] completeness of which begins from m ≥ 4.0 according to gutenberg-richter law for one year preceded the solar flare of x9.3 class. a comparison of earthquake number during the same period (11 days) before and after the solar flare demonstrates a significant rise (up to 68%, figure 4) of a number of earthquakes with a magnitude of m ≥ 4.0. the observed pattern of earthquake activity is very similar to behavior of seismicity after injection of dc pulses into the earth crust from artificial power systems (figure 5). as mentioned above, we need to find the regions sensitive to electromagnetic impact. according to our study it may be the greece region (latitude of 33.5110° 42.4428°, longitude 18.8439° 29.4344°). when we analyzed emsc catalog [https://www.emsccsem.org/earthquake/] with a completeness starting from m ≥ 3.0, we found that the number of earthquakes has increased by 2.2 times (5 earthquakes before the solar flare, and 11 – after) in the time window of ± 11 days. at the following stage of the research we need to concentrate on a study of the fault properties where the earthquakes occur just after the strong solar flare. thus, based on these results we may suppose that the solar flares can provoke the earthquake occurrence. one of the possible reasons of earthquake triggering is the sharp increase of telluric currents induced in the conductive areas of geological faults matured for earthquake occurrence [han et al., 2004]. victor novikov et al. 6 figure 4. global daily number of earthquakes before and after the solar flare of x9.3 class (vertical red line) of september 6, 2017 (11:53 utc). catalog usgs, m ≥ 4.0 (2016-2017, average annual daily number of earthquakes is 38.37). figure 5. daily number of earthquakes in the upper layer (0-5 km) of the earth crust of bishkek geodynamic al proving ground (northern tien shan) before (t<0) and after (t>0) operation of mhd generator for dc current injection into the earth crust [tarasov and tarasova, 2004]. https://earthquake.usgs.gov/earthquakes/search/ https://www.emsc-csem.org/earthquake/ https://www.emsc-csem.org/earthquake/ conclusion the overview of studies completed to-date on a relation of the earth’s seismicity and solar processes demonstrates the fuzzy and contradictory results. the main problem of this research is a lack of physical explanation of a mechanism of earthquake triggering by strong variations of space weather conditions. based on results obtained in the field and laboratory experiments on earthquake triggering by dc pulses injection into the earth crust we supposed that the similar triggering phenomena may occur after the strong electromagnetic impact to the earthquake source due to solar flares or geomagnetic storms. this idea is supported by results of numerical estimations made with an application of developed physical model of possible mechanism of stimulation of seismic activity by ionizing radiation of solar flares. it was shown that telluric currents induced by geomagnetic pulsations generated by solar flare have the similar density at the depth of earthquake source location (10-6 a/m2) in comparison with the current density generated by artificial power sources (10-7-10-8 a/m2) resulted in observed redistribution of seismic activity in the regions of pamirs and northern tien shan. these results are supported by the simple statistical analysis of global and regional (greece) seismicity behavior during the solar flare of x9.3 class occurred on september 6, 2017 (the strongest flare over the past thirteen years). a number of earthquakes after the strong solar flare increased both for global and regional scales by 68 and 120%, correspondingly. the global number of earthquakes (usgs catalog, m ≥ 4) for time window of ±10 days after the solar flare has increased by 68%, and the regional seismicity (greece, emsc catalog, m ≥ 3) has increased by 120%. based on the results described above we can suppose that the earthquake triggering due to impact of strong solar flares is possible. the physical mechanism of interaction of week electrical current with rocks under critical stress-strain state is not clear today. the effect of seismicity response to electrical action may be explained either by electric current concentration in the narrow highly conductive fault zones [han et al., 2004] or by secondary triggering mechanisms driven by electric pulses, like electromagnetic stimulation of fluid migration into the fault zone resulted in reduction of the fault strength properties [novikov and novikova, 2014]. it should be noted that these hypotheses have only a phenomenological character without a detailed theoretical justification and 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(2015). possible influence of telluric current on seismicity of the earth’s crust in seismoactive zones, geomagn. aeron., 55: 133-138. usgs earthquake catalogue https://earthquake.usgs.gov/earthquakes/search/ wolf, r. (1853). on the periodic return of the minimum of sun-spots: the agreement between those periods and the variations of magnetic declination, philos. magazine, 5: 67. zakrzhevskaya, n. a. and g. a. sobolev (2002). on the seismicity effect of magnetic storms, izv., phys. solid earth, 38: 249-261 zeigarnik, v. a., a. a. avagimov and n. t. tarasov (1999). managing earthquakes? science in russia, 2: 16-21. *co r r e s po n d i n g a u t h o r: victor n ov i kov, joint institute for high temperatures of russian academy of sciences, moscow, russian federation, e-mail: novikov@ihed.ras.ru, © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 9 space weather and earthquakes sondaggi elettrid sul fondo di un lago (*) (ele"'ric"l $olwdirlg ori l/le b"/iom 0/ " l"ke) riecvuto il j.! l<'chhraio 19b riasst:-~<'l·o. si pren(le in ('samc il ea~o ili un suolo stratifi\'ato 1'ieo· perto (la una ('oltre d'a('(!ua e si ('onsidera (li dfettuarc son(lag'g'i clcttriei eon clcttrolli posti sul fonao. vengono riportate akunc \'ui'\'o teoriehc e vengono iil(!ieati i vantaggi ottebihili euehuan(lo i son(laggi d!'ttriei eon elettrodi poggianti sul foih10 inve('e ehe s01\(laggi con elettrodi galleggianti sulla stlljcrlirie ilell'ae(!ua, si ripol'tnno am'lle alcune illihlalit..\ operative. su)diary .. 'l'he ease of vel'tienl eledrieal soilili1illgs earriell out. on a zone ('ovl1rc(1 ljy a lake with c1edrodes placed on t.he lakc bottoill is ron· sillc)'cll, the generai expression or t.hc apparcnt resistivities on the houoill of a basi n cow'l'cll with water, in prcsenee of a layeretl groun(l, is shown, as wdl as the theorclieal eurves ",hen t.he g1'ound is homogcncous (fig. l) or composcd or no first laycr anll of a rcsistivc suhstrat\lul (lig. 2·u). 'l'he a. shows that soulctimes the iilethlhl with ('ketro(\cs laying on thc bottoiil gives morc uscful rcsults than the mcthotl with dedrodes floating on thc watcr surface: in lig~. 7, 9 mul lo are reporle(1 thrce examples in whieh the presenee or fl, thin layel' near the botto m is casil)' rccognizahlc in thc ljuttom methutl cun'cs (fi), ",hile it. is bot in stufaee meutod eurycs (a). informations ahout the cquipl1lcnt ana ilc\,\c('s u~cil in ealt~·ing out. mcasu1'cmcnts in t.hc count.ry, are given (fig. s). (*) lavoro eseguito ('on eontributo ilei consiglio ~azionale delle ri· cerchc. ( .... ) istituto di gcoflsica )iinerai'ìa t 1nì\'ersità (\egli stul1i ai roma. tra le varie allplieazioni aei metmli geoelettrici ed in particolare tra quelle dei sondaggi elettl'iei vertieali eltettuate (lailo ~erivellte ill que~ti ultimi anlli, (.' risultato illtel'essallte, ller le varticolal'itù eseeutive e ili intervretaziollp, il l'aso di una rieerea· eitehuata in una zona nella quale riea(leva un vieeolo lago. per il problema ~peeifieo, oggetto aella l'ieel'ea, rivestiva· ulla lm-rtil'olare imvortn.llza, oltre ehe la rieostruziolle geologiea /lenerale ilei baeino laeustl'e, anche la detpl'minazione ilelle resisti-dtù. degli strati lliù sullel'fieiali costituenti il follllo ael lago. benehé la ]ll"ofonditù. dpl lago fosse esi~"la (dell'ordine (lei 5-10 m), si ilensò ehe fosse pl"efel'ibile, per lllia miglim'e l'ieostruzione e eal'lttterizzazione dei terreni eostituenti il fmulo del lago, eitettuare sia nm'mali sondaggi elettriei vel'tieali eon elettroai galleggianti in suvel'fieie (eollhideralldo quilldi l'aequa eome ilrimo strato), sia solhlaggi elettrici eon eletttodi poggianti slù folldo del lago, poiehé il rilievo e~ef:,'ltito in tal modo ha dato ri~ultati soaaisfaeellti, si ritiene possa essere illteressa·nte desl'rivere ill breve le nlolta.litù. 0llerative ed i meto(li ai intel"]n·etazione adottati, ill quallto llon risnlta. dalla lettemtm'a a eonoseenza aello ~el'ivente l'he sia· stato alil"olltato in modo adeguato un ilroblenm dello stesso tipo, solo giuseplle latmil"al (3) ripol"ta i ri.~lùbti di alc'ulle ilro>'e eftettuate siù fondo di un hl/lo, mentre gaetano latmiral e lorellzo ì\lirahile (~) ril'a:vallo eoll il metodo aelle immagini elettl"iehe aleune espressiolli ilella resistivibì.. app~u'ellte per il easo di due strati (nequa e sub~trato). determinazioni ai resistivib\ sul folldo dd mare sollo sbte desel'itte da aeeerboni t' )iosetti (1) ehe si l'ivol'tnno al raso teol'ìeo (li misure eftettuate al eontatto tm due mezzi semi-indefilliti: ilei' elimillare !'influenza preponderallte dell'aequa marina ~ui valori iielle resistivitù. apparenti, si 11l·o])(llle di ettetbull'p allehe misun:-" della l"esistivib aell'aequa eon 1lisllositivi sospesi :ul lllia eel'ta altezza del foll(lo, xella ilresente nota verranno rieavate le eslll'pssiolli gellerali delle l"psistivitìò apljanmti sul fonao ai un baeino l"ieoverto d'aequa ill ilresenza ai un suolo stmtifil'ato e verrallno fm'nite le eul've teol"iehe nel easo ehe tale ~uolo sia illimitato o i~ostituito da· ull primo stmto e da un substl'ldo. ver!'l't rillm-tato ul) esemllio ili allplieazionp pratica l'on le intlieazioni ilei dihpohitivi e ilegli aeem'gimenti utilizzati nell'e~eeu­ ziolle delle misul'e in eamllagna. so:\lj ,\ggi ei.etthici sui" }'oxdo di ux i . ago 393 l'autore desi41enl l'icoi'(l;\i'e il collega maurizio gralhlinetti con la cui collaborazione il presente stndio em· stato iniziato }loco }lrima che ull mortale infortunio lo ~tra.ppa~~e pei' sem})1'e ad una feconda opei'o~itù .. 2. helm~toxt peh il calcolo della llesn-ittytta ,\ppcire::\te kul si ron~idei'i il easo di uno spazio costituito da 41ne mezzi semiillimitati, omogenei, i~oti'opi, di resistività (li e '!~ separati da lillasliperficie piana :7 sulla quale (' llo~ta mlasol'gente puntiforllle erogante mja. corrente eontinua i; il potenziale ~h1 una 41istanza· h dnila soi'gente è dato dall\,s}li'ehsione: 1"= (jl(j~i :3 :7 ((li + i?~ j b sup})oniamo che siano }lresenti altri mezzi separati da sullerfiei }lialle }jarniiele al pin,no n; assunwlhlo un hi~tema di coordinate eilindrirlle l', rp e z ron il eeiltro llella ~oi'gente e l'a,%e normnle al piano n, il potenziale può af\sulllere la forma, r j (a o + l + ho-l~) j o (l,l') ti}, " [1] cui dovrù, essere aggiunta una ehpressione pa.rt ieoia.re pei' i mezzi ehe contengollo la sorgellte, xci easo ehe iiltei'es~a il pref\ente studio, sia presente uno htrato superjìeiale (l'acqua) con resist,ività 1]", e hilcssore li ", sormontato 41a ull mezzo semi-illimitato (a.l'in.) eon resii\tivitù infinita e }loggiante su di un eomplei\so stratilicato, xel caso 41i un suolo omogeneo illilllitalo (hl -+ =), pcr il 41ispositivo di \veniler, defillen410 la rcf\istività apparente ande in questo ca,<;o con l'e~llresi\ione (la =:3:l: il .a f i 30 4 ~!. be u .... abiki con a distanza, clettrodica, si ha ~ = u' con 1(,0 k .. +1 " \ 1 + 2 ,1 ](,,1 r 1: 'i -'-.' -t:( :"-",;-',,1,",,-)' '''"j '" ( ,, _ 1 1 + "' :,." ( _ j{ .. )h ! 1 12 "" .. \ ' i ,,o 4 + ~ -a' , , pcr il (lispositivo (li schhunbcl'gcr si otticnc invccc con a ugualc alla scmi(list.anza tra gli clcttroai (li corrcntc ar/'2. nella fig. 1 sono riportatc lc curvc pcr vari valori di k,c. [2] si consideri ora un suolo costituito (la, un primo strato (el, hj) c rla 1m substrato ([l2) 8cmi-illimitato. pcr il rlispositivo schhunbcrgcr si ha, pollclhlo h,. = p"ho c hl = p,ho con p,# c pl muncri intcri: i coefficienti h n sono ilei ca80 pl > p" 1 ~ n < p", li,. < n < pi n = pl 111 < n < pl + p,. n = pi + p .. n > pi + p,. con le = (l! 12 1 (l2 + (li b. = o bp .. = 1-kw h" = l{,. b(n-v"j bpi "-" hl (1 + il,) + k", b(p1-"",j h" """' 1{1 l{w h(n-jl i j -i{,. b(.-vwj b(pi+p,oj = 2lù + 1{1 l{ .. h",c le" b pi b" '""" 1{1 b(n-11 -1),.j + iù il... b(n-p : j il... b("-p,, j [3j ~o),i)"gg i t:u:ttuic i !iv .. l'osi>o u i us i.ago i i . i i i \' " i ~i -\ \ :\'\ _ _ o ()or -i ~ . i\~~ i i -l , '. , .-. . . , ~.-_. .., ~. ~ ~ a.-f ~/ ~ ~ ~ , n ... n ~ " o 31)5 o n ro o " , o 5 , , • )1, bj-:w';abi~i .xe! (',ako l'w> j11 si ila: 1 .< 11 < j11 h n /) " = l'i hn ; /(, (1 + !( ", ) l'' <il < l'" h n /( .. /(, iir ft pii 1j. ,= l'" h~,. = 1 le" -:!(i !( 1<' 11(li,. /it! l' ,. < 11, < l'" tl'i " l' ,. "i l'' h(pi+p,. j , 'j/(i il. hpi +/(1 !(,. h p ,. li > l'" -i [ji li n -1\"1 h "-" ,, -p,) -g,. h(n-i',.j /(, ii' ", h1n-i',), l ( ~j~ --->00) rlwl'lido akf!lulwl'{' a ii'" i v alori ± o,h, !. o,ii, /) e al rappol'to tl'a gli sl){'f!sori h.-ihl i yalori 1!'jii, i/l'), 1110, 1/7, ijr;, i/:~, i/'j, 1, 'j, :1,:'), 7, io, iii, :!ii (iigg, 'j, :~, ,1, il, li), il (':d{'olo (~fl{'l!liito 1)('1' h llwgl!ior palte rlei (',asi (',oli il {'al(',olatore llliyiw 110h dl'll'cllivl\l'sita di homa, è stato all(,il(' k(~mplifi{'at.o dalla ('onstataziolw <'il(' lwl (,aso in (~~ame (iii i) il vaiol'(' d"lla f!omma/,ol'ia in ihil'('ii1'('f!i non varia f!f'ambi:l1u!o llluw'i'o di ('ll1'v(~ da. ('alc'o!a]'(', l'appl'ossimaziollt' volata 11('1 ('akolo d.·jlt' ]'(~kifl1ivilù apparenti i~ stat.a di nlnu'llo o,ii";,: si 4\ l'aggillllto talt' limi t.· arrestando il cal" (',010 (lella sommatoria ad llli niulu'i'o di tl'j'lnilli ,<'" l'i('aya1.o da ulla espl'(~flfli01w l'ipol'tata da :\iooih~y"()rt'llalla t'c! altri (5): 8 ,--·1 f! ~ l'. 8 f (}~ iii ('ui l', mppn'sl'nta il illjlflsimo yalol'e assolato t'lw i (~oefli('it'llt.i h n raggiullgollo 1)('1' /i >,'] ec! ~ 1\~j']'o]'(~ ]wi'('elltuah' anlllwsso, l't'l' l'. i, stato assanto il ynlo]'(':!, in ([uallto pt'r i (,a fl i (~onsic!l'l'ati i (',odlkicnti hl! k01io sl'illpl"· risllj1,ati in[{'i'iori (hl uh~tali a 'j. l,c' ('un'(' l'ipol'taj(· ili'lle figui'c :!·h ihmsolio (,ollsid('rursi huflidl'ld,i per giadi(~ar.·, jl(~i singoli (',asi, la t'j'j'el,h"a hl ili/,h di impiego dd ml'lodo proposto, 1'('1' ra]'(~ hll eselllpio l'oil,~idl'l'inlll() il {'aso (li illlo sp.w(']lio il'a('it!la (,on l'csistiyib di iii olllll,m t' spt'hso],(~ di ilo l1l ('h(' l'i{'op1'l1 ullo strato (li slwss()j'i' \o iii e l'i'kisti-"ith di g ohm.m poggiantl' su di un substrato 20 lo b 6 4 2 1 0.8 'w o,~ lo 8 6 ci'5a 4 ~1 2 1 0,8 0.6 0,5 1 10 8 6 <;'a 4~1 2 , 'o i / v , i w 57 ,/ h w ~w = 9 , ~ h, ~, ~ 1 ~~. d h, ~ ~ '?2 = 00 i 0~ ~ , 25 i ~ i k = +08 1 ,2 .4,6 ,8 , 4 6' ~ lo 20 40 60 '101 fi ).; . 2 clll'l'c leo l'kh c \wi' d uc ~ tra li 1'ì<:o jh' l'ti l1a lill o s t ra to ,ra o.;' jli:l (' oli e lett ro di poi.qd :l u t i sul fondo nel rilso ili '!",/(!c = \) (l,-w = o,h) c (!" = 00. i ,1,, 25 hw ~w ~ 3 hw 1., ~ 5 7 )" ;; / h, \/z0'~y/y /// h, ~w = 1 ~z ~ 00 '~~v i i :~~ ~ i //~ i i ~ , v i t k=+05 , .2 .4 .6.8 l 2 46 8 ij 20 4-0 60 100 f i ~ . 3 cnn'c t co ril 'h u l' er dll e .~ t r at i r i,'o p c rl i il a 11ilo blrat n d ' i h:(jua ,'o n c lc t1l'o d i poggiant i sul fondo lwl {':iso ili (j "' /(!! = 3 (t,-", = o,g) () '12 = =. i , -"-w hw l'w ~ 1 h. 1"-, ia+v')) )/) " h, y, = 1 /" ~~~~~~/ // ~2 = 00 ~ % ~ v r; ~ v v , ~ ~ " --0,8 0.6 -, ;j -----v ,/ ,// /~ i 0,4 0,2 .1 '5 a 2 ~, v ./ /--v v /~ i , i i i i i k=o i i .2 .1. .6.8 1 2 468 10 20 40 60 10 0 a bi/h 1 fig. 4 curyc il'orielle ppl' due strati j'ko]lcrti dil uno ~trilto d':wqna {'on (']ejtrodi pog g ianti su l fondo ilt'l,'nso di ",,:'{l , = l (ii," = o) e '.', = =. j 5 l ."l--a r7' " )a c?l ~, 42 , 0,8 0,6 0,4 5 1 0,8 0,6 0,1. u2 .1 5 l. ~a 2 s'l l o,b 0,6 0,1. 0.2 ~ l h, y, ~ 1 y, ~ 00 , " , -è 'o k=o .2 .4 .6.8 l 2 6 8 '0 20 1.0 60 '00 ab/ 2/h, / fig. 4 cul'\'c teoriche l'cr ,lue strati ricoperti da lillo strato ,l'acqua ('un elcttro,li poggianti sul fondo nel caso di (j,,'/flt =, l (li", = o) c o:!: = 00. , i hw \'w~ + ~~ ~,~ 1 , h, 'l' h, _, w 25...... /-t , '~ , , ) 10 15 'i'. ~ 00 ~~ v / //v' / v ~/; ...-c: / / ~/ / / / / / /25 ._/"/ /:: /-: /' // / / / v/ // / 0;' ~v /' v/ // v ~../ // . /' ,/' /' ~ ~ t:::: ../ v.....-::::: /' ...--::::: i ik=-o.51 ---i .2 .4 .6.b 1 2 46 · blo 20 40 60 '00 abj 2/h / 1 l<'ig. r; curve teoriehe per due strati rieoperti .la uno ~trato d'a{'qua poll elettrodi poggillnti sul fondo nel caso di f},"lrh '""" 1/3 (i)~w = -0,5) c f!! = 00. i chw rw· t i i k = -0.8 h, r. ~ 1 hw -.;;-~ "'-, , r~,= oq ~:;j 1.. /'/'/) ~/ , ) ~t ~ v '/1/ 5 7 'o . /' .// // / v / / v ./ v v /// // v // /// 12 / / v v l/ l / ' v / ~v,-i ';r v:: ~ / v / (/ / vj v w//l v / ~ ~k v'v ~ ~ ~ ~ 2%// /' v 0,1 .1 ,2 .4 .6.8 1 2 l. 6 8 lo 20 1jo fig. li cluye tporidw 1)('1' ,lui' strati l'il'operti dll uno ~trllto d"a('lllla nlil elettrodi poggianti sul f01],10 nel easo ,li f! ", /i], = l i \! (e," = -o,s) c '.'~ = =, 9, e e wo 80 so:'idann i eletthici ,~ul roxdo di un lago :lo, resisti1'o; }lrendendo in esame (fig. i) le d1!e em'y(\ tpori('1w (,ol'l-i.~l)(m­ denti a sondal-u6 elettriei efle'ii:niti in super!ieie e w l fondo, ai nota ehe in }lreheilza dello strato a 15 ohm,m non è evidente nella clu'va a l'elati1'a ad mi soildaggio sulla superficie del lago, mentre lo è nella ('m'1'~1 b coltihiioiu!enlp n rnis1!l'e ~ml t'oildo, 3, al'l'licazio~e an us caso ltk\le le clu'1'e flo}ll'a, riportate flono fltate 1!til izz:tte ]lei' l'iilter]li'etnzione dei risultati di un l'ilie1'o efi'ettuato ~nlla,g'o di ycntina nei ]l\'l's,~i di l'iedilu('o ('l'emi). colne si i, detto l'aeqlla ha. mlo hllessore massimo dell'ordine di lo m ed 1!na. reaifltiviti'l (determinata. hia ('011 rnisw'e in 10('0 sia su (%m]lioni ill'elevati a 1'arie }lrofondità) di :~5 olnn.m. si i, 1!tilizzntn fao}lparecehiatura a, col'l'el1te eolllmutata, a, lo iiz de~critta in lina precedente nota ("). l'uso di tale a]l]lare'echiatlll"h, se sl'm}l1ifieava l'eseeuzione delle misure (ii! qllanto eliminava gli pffphi di })(llnrizznzioile degli elet/ 60 / " 20 i a" / t/ p <w / o 8 6 , 2 8 / v \'w o isohm·m ih_,som ,. _ s ohm·m , j h, ,le m " 0_ , , , 2 , ti 8 lo 20 40 50 80 100 200 400 600 1000 m a 8/2 l<'ig. 7 confronto tra ,!ui' (' tu vi' \('ol'ielle, una (a) relativa a,] 1'1('ltnjlli 1li superfteie, l'nl tta (b) r('!a l i"l1 :li! elettrodi lloggi(\nli su! fon,lo nel ('aso indieato in fig-nra. 398 t.l'odi}, aveva, cume incunveniente la nece88itù di dover eunsiderare le mutue iihluziuni t.ra drcuitu energizzante e eirl'uit~) di mislua. per <liminuirc al masflimo t.ali mutue induziuni e<l n.ssieura.re un 8ulftcientemente a,ccurato posizionmuent.o degli elet.t.ro<li alle varie distanze elettrodiehe, si 011erava nel seguent.e modo: si stendeva dapprima un filo di na.ilon della. lunghezza di 24u m sorrettu da ga,lleggianti ed ancoratu alle estremitìt.; al centro della stessa e lmmllelamente a,<l essa veniva w18atu sul fundo un tubo (vedi fig. 8) poco !les~ibile ollpol'tunamente zavonato della lunghezza. di 20 m, sul quale erano stati fissati anelli di rame in eorrisljondenza. delle varie <lista.nze elettl'odiche. t quattro anelli centrali, utilizza,ti eome euppie di elettrodi di potenzia.li (mn = 1 e 2 m), erano eollegati a eavi ehe, fuoriuscendo (hd pentro del tubu, giullgevanu alla barca. centrale slùla quale era.no ista.llati gli appa1'eechi di mislll'a; gli a,itri a,nelli (funzionant.i come elettrodi di corrent.e l)e1' le llrime miflul'e e pome elettrudi di potenziale pelo le maggiori dista,nze elett1'o<liehe) erano collegati a cllovi che useivano dalla estremità del tubo, risalivano vert.iea.lmente, e, lluggiando su llipeule hoe, giungeva,nu anch'essi alla barm. eent.rale. fil.o di nailon i ~ cavi di potenziale li'ig. 8 :':\ehelll[l opcmtivo ntlottat,o in camp[lg-ll[l. per le misul'e a dist.anze e1et.trudiehe ab/2 superiori ai lu lll, venivano utilizzati elettl'odi di eorrente che venivano calati, da due altre barehe, in pol'l'iflpollllenza. di segnali posti sul filo ste80 8nlla 8uperfide del lago. :furuno fatt.e aleune prove per pontrullare l'entità della mut.ua. induzione tra i due drcuiti, energizza,nti e di misura, entro l'aequa, pruve effet.tuate corloeirl'uitando gli elettrodi (li ljut.enzinle: i pot.enziali mismati nelle normali eulhlizioni di utilizzazionc del sihtema. sono semlll'e ri~ultati t.ra~cumhili rispet.t.o ai valori dei llut.enziali che si stabilivano nel terreno o lliù sllesso addirittum inferiori ai normali e 100 80 60 40 e 20 l o e " , , 5 100 80 40 e 20 l o " , , 5 soslh\ggi }:u:tt uic i s u i, f osijo ili us i.ago n a '/ 1 / /' / ~ i t i se v1 1 t t t , 4 6 8 10 " '-o 60 80 100 m 1 l i i ~ ""'i 7 i ~ -:7 i i l ---b , ~ '---i ,/ i se v5 1 t , 4 6 8 10 " 40 60 80 100 m a~ l<' ia', ti eiw lllpi di t'ut\'(' ri lt'v:' i.' in u n 'in.l agine effe ttuh t lt in u n lugo; in ('\u'vh a è l'dann. ntl drt lr(" l i gall('g}:: ianti in l! li j1('t'!\t;", in l' un'a b 1,, 1 i'let h 'o .!i llogg;;l11 t i sn l fmulo, 400 disturbi esistenti ( ~ 0,3 my). gli accorgimenti alloi,tal,i hanllo quindi permesso ulla buona riureita della prosllczionc, 8ia llci' la )),(·pul'atezza ed attendibilità dei valori misurati, sia auehe llci' la buona rapiditù dell'esecuzione delle mùnuc. ".\ titolo di esempio si riportano nella fig. () due cop})ie di sondaggi elettl'iei ese/-,'l.liti, per ciascuna. coppia, il ljrimo sul fondo del lugo e il secon!lo in su}lel'ficie coli elettrodi galleggianti. si riscontra. nel rolhlaggio vi fondo lago (eu!'va b) una }ll'ima diminuzione di l'esistività nella. lja.rt.e più prossima al fondo s('::., .. uiia !la ull orizzont.e leggerment.e più resist.ivo; nel sondaggio y 5 fondo lago (curva b), si nota che il primo stmto conduttivo ò aumentato di spessore: tali l)al'i,i rolarit:~~ non sono chiaramente visibili nelle curve relative ai soll<laggi pon elei.trod i galleggiani,i (pmve a). col\cl~ uslo.1\i l'esecnzione di sondaggi cleurici (e di misure di resi~tivitù in genere) in aree rieoljeri,e a'acqua coli elettrolli poggianti o infl.ssi sul fondo puù presentare vani,aggi anehe notevoli rispelto ai mci,olli di su}lerficie. in molti casi infat.ti strat.i rel",tivamente sottili posti sul fondo o in pros~imiiù di es~o, possono sfuggil'{~ o essere diffidlmente determinabili aa ulm ill<lagine hmmta solo su m isure etteuuai,e sulla superfide dell'acqua. i sondaggi imi fondo permei,tono di ricostruire con maggior dettaglio l'",ndamento e la resistivitù dei terreni più }lrossimi all'acqua, ]lortando un contrihuto notevole alla risoluzione di aleuni }lroblemi geologieo-teeniei (quali ad es. la pre~enza e l'entitù ili livelli argillosi impermeabili). hl.:"; gll\ zi ,\ ì\iet\ti l,o scrivente desillera l'ingraziare il l'rof. aquilina, direttol'e aril'isi,il,uto di geofisica ;\lineraria, che ha reso possibile la ricerca, cd i colleghi e. brizzolal'i 11('1' l'aiui,\) lli'estato per i caleoli ilelle curve e }<'. di fonzo e g. httmiml per le }lrove in nlmllagna. ~o!>[daggi eletthh'i sul 1'oxdo di ex la(;o bibltografta (1) accelwo:s1 e" mo~etti f., 19gg. jle~u1"e~ de l'ùùtll'ilé rl.erlrii{lte in ,~itll ~ur lr fonli ,le la ma. "bol!. geaf. 'l'cm'. app!' >, viii, 42. (' ) bei{"ahi"i ~i., l!}(;:,. ()!,-srrva~ioni .~ul c(j'jllportrt'!llf'nlo ,li '101n appareeehio!ufil f/eoeldh'ùji! l! «(ntenle ("ommldoll! ]1/'1' misi/w ai fe~is!il'iltì. "annali di (h,()ti ~in1 ' , xviii, 4. (') la'dijral g., 19(14. jfi8uj"(" di l'crlslil'i!l) e.~/cll.~i()ni del '!iic/od() di we/mel' l'arle 1. "annuii (lell' 1.\' ,)1. , (li )1 apoli, xxxii. (~) la'l"\rij{,\l g., j\lwabllf. l., lflg4. del me/odv di tycnllct p!1rt,e ti. xxxii. jlisi/h di l"csislivifll j~'slensiljlw "annali (lell't.u.x.» (li )l'ilpoli, (,,) moo"y h. 'i., om,,",,'a e., l'w",'", li., ton'hen, l., logg. a fcsislivi/1i colllplllal!()n mc/lw,z for carlh lii()lids. "ge()]lhy~i('~)l, xxxi, l. o n e d i m e n s i o n a l m o t i o n o f a v i s c o u s fluid c . d e s i m o n e ( * ) e . p u k i n i ( * * ) e . s a l u s t i ( * * * ) received on april 6 t h , 1975 summary. — t h e effect of t h e f r i c t i o n h a s been s t u d i e d on t h e one d i m e n s i o n a l m o t i o n of a viscous fluid. t h i s f r i c t i o n is usually s c h e m a t i z e d in v a r i o u s s e m i e m p i r i c a l f o r m u l a e . in t-liis work t h e d i f f e r e n t s c l i e m a t i z a t i o n s of t h e f r i c t i o n were n o t s t u d i e d s e p a r a t e l y b u t it was shown t h a t a solution e x i s t s for t h e fluid m o t i o n . t h e r e s u l t s give i n f o r m a t i o n on t h e d a m p i n g of t h e fluid m o t i o n in t h e case of t h e seiches. r iassunto . — gli e f f e t t i d e l l ' a t t r i t o nel moto di un fluido non possono essere t r a s c u r a t i nello s t u d i o di molti f e n o m e n i geofisici. le origini di tali a t t r i t i sono riconducibili a v a r i e f f e t t i fisici che n o r m a l m e n t e vengono schem a t i z z a t i in diverse f o r m u l e s e m i e m p i r i c h e . sulla base di u n a s c h e m a t i z z a zione del m o t o di un fluido lungo u n a sola d i m e n s i o n e si è d e t e r m i n a t a la soluzione d e l l ' e q u a z i o n e del m o t o , c o n s i d e r a n d o le varie f o r m e di a t t r i t o a g e n t i c o n t e m p o r a n e a m e n t e . i r i s u l t a t i d a n n o i n f o r m a z i o n e sullo s m o r z a m e n t o del moto del fluido e sul peso r e l a t i v o dei v a r i a t t r i t i . t h e e f f e c t of t h e f r i c t i o n i n t h e m o t i o n of a fluid, e . g . t h e s e a , c a n n o t b e n e g l e c t e d i n m a n y g e o p h y s i c a l p h e n o m e n a . i t s o r i g i n c a n b e r e l a t e d t o v a r i o u s p h y s i c a l e f f e c t s , so o n e u s u a l l y p r e f e r s t o s c h e m a t i z e i t i n d i f f e r e n t s e m i e m p i r i c a l f o r m u l a e . i n a p r e v i o u s w o r k (6) s o m e of t h e p o s s i b i l i t i e s of s c h e m a t i z a t i o n f o r t h e o n e d i m e n s i o n a l m o t i o n of a fluid h a v e b e e n s t u d i e d . p a r t i c u l a r i t y of t h i s w o r k h a s b e e n t h a t t h e s e e f f e c t s w e r e n o t s t u d i e d s e p a r a t e l y b u t i t w a s s h o w n (*) i s t i t u t o di fisica « g. marconi », u n i v e r s i t à degli s t u d i r o m a . (**) c . n . r . i . f . a . r o m a . (***) i s t i t u t o di fisica « g. m a r c o n i » , università degli s t u d i r o m a i . n . f . n . sezione di r o m a . 118 c. d e simone r . j ' u r i n i e. salusti t h a t a solution exists if one considers also a d i f f e r e n t t y p e of scliemat i z a t i o n . i n f o r m u l a e t h e ell'ect was r e l a t e d t o t h e non-linear d i f f e r e n t i a l e q u a t i o n : ~i>2tt u — <j i>2xx [u • li (x)] = v t>-xx u — £ z u\u\a + f (t, x) [1 ] a = o a w h e r e t h e s y m b o l u d e n o t e s t h e v e l o c i t y of t h e fluid a t x = const, section of t h e c h a n n e l , t h e s u r f a c e of w h i c h is h(x) t h e coefficient v is c h a r a c t e r i s t i c of t h e v \ u t y p e f r i c t i o n while y a is t h e o n e r e l a t e d t o y u u \ u \ a f r i c t i o n , / is t h e s y m b o l of a n e x t e r n a l force, in o u r case it could b e t h e w i n d . i n [1] t h e b o u n d a r y c o n d i t i o n a r e t h o s e of an o p e n a n d semi-closed b a s i n . w e h a v e also considered, on t h e basis of comm o n sense, t h a t t h e r o u g h n e s s of t h e b o t t o m m a y b e considered as a origin of a f r i c t i o n t h e w h i c h is n o t possible t o b e seen t h r o u g h t h e u s u a l n u m e r i c a l s t u d i e s t h a t utilize c o m p u t e r s . so we h a v e p r e f e r r e d t o t r e a t a flat b o t t o m case w h i c h allows a n a n a l y t i c a l t r e a t m e n t of t h e f o r m u l a e . i n t h i s w a y we h a v e s u p p o s e d t h a t t h e s u r f a c e of t h e section x=x0 h a s t h e v a l u e : h (,t0) = h (x0) + d siny.r0 [2] w i t h t w o small v a l u e s of <5 a n d y. o n e can i n t u i t i v e l y say t h a t t h i s r o u g h n e s s increases t h e t u r b u l e n c e a n d t h e n it r e n o r m a l i z e s t h e a b o v e m e n t i o n e d coefficient. t h e p u r p o s e of t h i s w o r k is t o p u t i n t o a c o m p a r i s o n t h e s e effects. w e use a n a l y t i c a l m e t h o d s which in some cases a r e r i c h e r with inform a t i o n t h a n t h e p u r e l y ones. t h e r e f o r e we l i m i t ourselves t o a flat s c h e m a t i z a t i o n h0 of t h e a d r i a t i c sea. t h e m e t h o d u s e d , b e c a u s e of t h e smallness of t h e v, d coefficients, can b e t h e first order p e r t u r b a t i o n of t h e f r e e m o t i o n . obviously t h i s m e t h o d gives some i n f o r m a t i o n f o r n o t a long period of t i m e . w e will discuss t h e r e s u l t s in t h e conclusion. 1 . t h e p r o b l e m a n d i t s e l e m e n t a r y s o l u t i o n s i n o r d e r to s i m u l a t e a t o u r b e s t t h e a d r i a t i c sea, d e s p i t e o u r flat h y p o t h e s i s li(x) = h0, we use t h e d e f a n t ' s b o u n d a r y conditions. i t implies t h a t f o r t h e first a n d t h i r d seiches t h e c h a n n e l is closed o n e d i m e n s i o n a l m o t i o n o f a v i s c o u s f l u i d 1 19 a t venice a n d is o p e n e d a t o t r a n t o . f o r t h e second seiche, a t t h e cont r a r y , t h e c h a n n e l lias t o b e considered closed b o t h a t venice a n d a t o t r a n t o . w e t h e n consider t w o m a i n cases: t h e effect of t h e f r i c t i o n on t h e f r e e m o t i o n of t h e w a t e r a n d t h e effect of a s t r o n g w i n d on a q u i e t sea. as f o r t h e w i n d , we h a v e a s s u m e d , in t h i s case, also a n " a d h o c " s h a p e f o r s e m p l i f y i n g c a l c u l a t i o n . w e s u p p o s e t h a t t h e wind s t a r t s s u d d e n l y a t t = 0 a n d t h e f o r c e is m o r e r e l e v a n t a t o t r a n t o t h a n in t h e n o r t h e r n p a r t of t h e b a s i n : ( 0 ' < 0 , r 1 {x> i) = 1 0 sin hl ( > 0 > = i h a v i n g c o n s i d e r e d t h e s e s c h e m a t i z a t i o n , we s t a r t b y discussing t h e first case. 1 . 1 . m o t i o n a n d f r i c t i o n o f t u b s e i c h e s w e r e p e a t h e r e t h e e q u a t i o n 1 a s2« u — g ~i2xx[h (x)-u] = v s« h xx u •—s x ^t u\u\ [1] u a "a if we i m p o s e t h a t t h e b a s i n is closed a t venice a n d opened a t o t r a n t o , t h e s u r é l é v a t i o n is zero, we o b t a i n : u (0, t) = 3* 11 (x, t)x-l = u (l, t) = 0 f o r t h e o d d o r d e r seiches. i n t h e second seiche case we h a v e : u (0, t) = u (l, t) = 0 w e t h e n p r o c e e d b y s u p p o s i n g , as described in t h e first p a r a g r a p h , t h a t s, v a n d y_a a r e small p a r a m e t e r s , of o r d e r e. w e t h e n develop o u r e q u a t i o n in p o w e r of t h i s e p a r a m e t e r . f o r t h e solution, we h a v e : u = uo + £ ml + £2m2 + • • • f o r e = 0 we h a v e f o r t h e o d d o r d e r seiches: s2» uo — gho wxx uo = 0; mo(0, t) = dxuo(l, t) w h i c h solution is also f o r t h e second seiche case: uo = a sin x cos \n (j~ n = 1 , 2 , 3 [2] 1 2 0 c. d e s i m o n e r . j ' u r i n i e . salusti w e now s c h e m a t i z e t h e f r i c t i o n s as a n effect t h a t s t a r t s a t < = 0 in order t o m a k e clearer t h e difference b e t w e e n t h e f r i c t i o n s a n d t h e f r e e m o t i o n . f o r u = ua + eui, we h a v e t o t a k e i n t o a c c o u n t t h e properties of 11 „: d2ttui — (jliov-xx ui = f(u0)\ ui(x, 0) = ~bxiii(x, 0) = 0 where f (me) = g d2xx (u0 (5 sin y x) + v s< ua —• 2 « z { u° ' i u° l" } o " " i n t h e following we p u t a = 1. t h e case a = 0, which is also a m o n g t h e m o s t i n t e r e s t i n g ones, can b e easily d e d u c e d f r o m calculat i o n . we deal now with t h e well k n o w n i n h o m o g e n e o u s w a v e p r o b l e m . w e will follow t h e c o u r a n t h i l b e r t ' s classical s c h e m e (4). w e develop a t t h e first f (uo) = ax sin fix • /*(<) then we m a k e easier our p r o b l e m b y r e m a r k i n g t h a t we can say u = £,tk(t) sin/9 ft» where t h e u n k n o w n t k (t) a r e d e t e r m i n e d b y } a2,, tk(t) + g ho /v tk(t) = fk(t) j ta-(0) = a t tk(0) = o w e t h e n k n o w t h a t t sin | g h018k (t — s) fk(s) d s tk(t) = ßk | g ho a n d it implies t h a t u = £fc sinßkx • tk(t) 1 . 2 . e f f e c t o f t h e f r i c t i o n w h e n t h e w i n d m o v e s t h e w a t e r w e s t u d y t h e case of a semiopen c h a n n e l in r e s t a t t 0, when s u d d e n l y t h e wind p r o d u c e s an e x t e r n a l f o r c e : (0 t < 0 f (x.t) = { r . x ' v ' ' ) 0 sin — t > 0 also in t h i s case we will s t a r t bv leaving aside t h e f r i c t i o n . o n e d i m e n s i o n a l m o t i o n o f a v i s c o u s f l u i d 1 19 tlie motion e q u a t i o n is given b y 32ííwo — gho t>2xx u0 = $ s i n — 8 (t) a t h e solution of which is 0 t < 0 uo ( x , t ) = { , . x . t ^ n v ' ' 1 1 $ sin — sin — í > 0 i a t because of t h e r e m a r k a b l e similarity with t h e previous case, we will use also in this case t h e above described m e t h o d to solve t h e equation a t first order in e. t h e a n a l y t i c a l r e s u l t s f o r t h e first seiche we h a v e f(uo) = j g ô y + y j — y i \ 2 a cos c o s t cos — t y + j ì * + va x , t — sin — sin h a 2 r a r a2 { 2x ) . 21 cos tir zoa . x t 7,1 — { cos —r 1 > sin — —— -j sin — cos — r ( a ) r |cos t¡r\ r a t one m a y r e m a r k t h a t t h e first t e r m is related to t h e roughness of t h e b o t t o m . i t is proportional to <5 and it contains t h e y coefficient. the second p a r t is r e l a t e d t o t h e l a m i n a r friction in t h e a = 0 a n d a — 1 cases a n d provides t h e last two t e r m s . w e r e m a r k here t h a t in this case t h e a = 0 p a r t of t h e friction is proportional to t h e l a m i n a r friction, we will t h e n n o t discuss it a n y more. w e now describe f(u0) as s/t-l/t sin (fikx + (fk) • fk(t) f r o m which it follows: ai = 4" 9 à ( y + t ßi = y + 71 ; «pi = , t fi = cos t a 2 = — — gò\y — ß* = y <p* = t ; a =cos t 1 2 2 c. d e s i m o n e r . j ' u r i n i e . s a l u s t i va a 2 r p 3 = 9?3 = 0 f3 = sin a. = — xi a* a s = xi a2 a x ft = 0 n <p* = 77 n <ps = t /4 sin 2í eos t/x x |cos t¡x\ . 2 í eos t/r h = 8 1 1 1 7 l ^ b i l m yl0 = zo 9?« = 0 f» = sin then m = a sin — sin — + ss-ju sin (fax + <p*) t¡,(í) x t where 1\ ti gho i \2 1 ( t gho y + x c o s — ( t gho ( i \ 2 1 ( t — gho [ y x < c o s — ( v cos v ( y + y j í j v gho [ y — j j < t t x. t j 3 = t eos — + —sin — " t •> r this last result is p r o p o r t i o n a l to t h e effect of xo. the f u n c t i o n 7',, and t5 are more complicated to describe in order to e l e m e n t a r y functions. w e m u s t i n t r o d u c e t h e sk t i m e i n t e r v a l s : s0 0 < i < si 71 t 3 — c — < — 71 so 3 t 5 71 ^ — ^ — 71 5 t 7 s2 — ji < — ^ — ti etc. 2 x 2 then in t h e 7 f t h i n t e r v a l we h a v e t t 4 = , , 1 . 2 1 t 21 l)f c i — sin cos le ti 1 2 x x x 2< . 2 « + sin 2 k n x x o n e d i m e n s i o n a l m o t i o n o f a v i s c o u s f l u i d 1 19 f o r t h e second seiche, we can easily calculate t h e result b y p u t t i n g a r a -> — a n d r -*• — in t h e tk(t) expressions. i n t h e ease of a wind, in analogy with t h e f r e e motion, t h e result is: f (mo) =6 [y + y j cos (y + y j x sin ~ + , &x ( 1 \2 i 1 \ . t 0v . x t d r [ y t ) c o s v ~ i ) x s m v w s m x 0 0 8 ~x + . 21 sin tlx 2x . 21 sin tlx sin — r— — — cos —i— sin x x |sin </r| t 2 a x |sin tjx\ f r o m which we o b t a i n : . _ t / 1 \ 2 . 1 t ai=gò& — \y + — j ; ßv = y + y ; gpi = — ; fx = s i n — = g d o j (y — j j ; ßt = y — y ; <p2 = ; /» = sin 0 1 í vl3 = r— r ; ß3 = -t; 933 = 0 ; f3 = cos — aa t n r . 3t . . 2t sill tlx a i = 0 2 x l x ; & = 0 ; * = ;.,4 = sin ^ j 2 , . 2t sin t/r ' ^ = t = 2 ; / 5 = s m 7 isinw a n d t h e tj.-(i) f u n c t i o n s are 2'i (i) = jgho (r + y ) + 7 sin v<7 äo [y + y ) t + sin — + t 2 *jg ho [y — j gho ( r + j j i t2 vgho (y + t ) ~ sin 2 vi/äo 1 \ 1 1 2 4 c. d e s i m o n e r . j ' u r i n i e . s a l u s t i t 2 0 ) = t1 (t) j j = > t3 (i) = — t sin — 2', ( 0 = ; i 1 . 2< — sin b r 2 r t + t d — ( — ! ) " ) 71 (— 1)* r2 i 1 . 2t t 2t t . w = t ¥ s i n 7 c o s 7c + ( l — ( — 1 ) * w ( — 1)* we n o t e here a difference between t h e effect of t h e roughness of t h e b o t t o m (which gives results limited when t increases) a n d t h e t w o other cases, where t h e velocity increases in norm as t oo. i n our opinion this effect is d u e to t h e m a i n effect of t h e roughness: it gives m a n y small m o v e m e n t b u t it d o e s n ' t decrease t h e kinetic energy. so we are encouraged to t r y and see t h e effect of a l a m i n a r friction vau on this p e r t u r b a t i o n of t h e velocity. this friction should decrease t h e kinetic energy b u t n o t t h e a m p l i t u d e of t h e effect, because t h e wavelength of t h e p e r t u r b a t i o n is 1/y which is v e r y large c o m p a r a t e d with a. w e s t u d y t h e r e f o r e t h e p a r t of u\ which is p r o p o r t i o n a l to <5, ui to which we apply t h e ra operator. w e o b t a i n : m = a g cos — t r + j i cos \y + y i ® cos [y + y ) vi7 t + i \2 cos c o s s \l() ho g-ho [ y j j l t * t h e n c e ui cc •• ft cos y x (jh (t) + c sin ~ sin y x <p2 (t) o n e d i m e n s i o n a l motion o f a viscous f l u i d 1 2 5 , a <7 ô v = . y3 vir/to ß vif /to /t . , r~r. sin 2 v? /to t sm3 vi/ «o y t , , —, h / h — cos 2 vir /to y < + ghy3 x ' a g ôvy3 \ t t2 *jg /t o y •4 (vi/ /to y ) 3 r sin vi/ /to í ? 02 = — " /—-1i z ~ cos jq h0 y t — — : r sin jg ha yt i c \l g ho (2^0/toy 2ghay2 a ) w e r e m a r k t h a t we h a v e obtained velocities which decrease with time. their numerical i m p o r t a n c e will be discussed in t h e following p a r t . w e would like to add t h a t one could intuitively assume t h a t this linear first p a r t of a negative exponential, a t least for t h e l a m i n a r friction t e r m a n d p e r h a p s for t h e others as well. 3 . n u m e r i c a l e v a l u a t i o n a n d f i n a l d i s c u s s i o n i n order t o e v a l u a t e numerically t h e relative weight of these results we h a v e assumed l = 106 m v = 1.4 10"2 m 2 s~l; y = 10~2 n r 1 ho = 3 • 102 m a = 10-! m s"1 g = 10 m s2 xi = 6 • 10"6 n r 1 zo = 10-5 8-i «5 = i m w e t h e n h a v e ad t 1 m l = -2h7v + ; i \2 x — cos ^y + j 1\ ( t x i cos — + cos vir /to [y + y 1 t t3 a ó 2ho 1 \2 y — i 'x2 cos y + 1 \ ( t ,—— i 1 \ ) v a t ( t t x { cos — cos vi/ /to ly + y i t cos y + 7. . t ) . x a21 , ( 1 . 2í t 21 — sin — sin 7i — ; — (— l)fc sin cos t ) i 4 ( 2 t t r , , 2 x a2 x { 4 t ( 2 t — hn } cos y xi — j — — + (— 1)* j — + + • 2 t + sin 2 ten x xo , t t . x — cos — sin — x x a o n e d i m e n s i o n a l m o t i o n o f a v i s c o u s f l u i d 1 19 the smaller scale motions which are strongly related to the bottom irregularities. pig. 2 t h e p o s i t i o n a t venice of t h e f r e e s u r f a c e f d u r i n g t = t = 3 h 2 0 m . i 0 is t h e position c o r r e s p o n d i n g t o t h e velocity v0, | 1 ( t o vt and f t o v. the preceding statement is stressed by the fact t h a t at the second order, the bottom roughness gives an internal friction the value of which can be larger than the corresponding first order quantity. the part of the friction which is proportional to v, the laminar part, gives a very small contribution to the general form of the friction, because the coupling constant is very small. i t has to remarked, however, t h a t its analytical form is very closed to the part proportional to 7.0. this analytical similarity could give origin a bit of confusion in the interplay of these quantities, in any case we consider here the values usually assumed on an experimental field. we can also say t h a t the term proportional to x0, usually called linear bottom friction, describes also the effect of the friction on the boundary layer. on these same ground, moreover, we can say t h a t in the term proportional to 1\ the higher velocities give the more remarkable effect. if, however, one has not a flat bottom, one could expect t h a t this kind of friction will give different effect for different depth h0. we expect this phenomenon to be important for a study of the real adriatic sea which is now in progress. we can also say t h a t all the frictions by us dealt have a linear effect, because we have analyzed the first order of the perturbation expansion of the real friction. if we assume on physical field t h a t these effects 0 . 5 o 1 2 8 c . d e s i m o n e r . j ' u r i n i e . s a l u s t i are exponential, this hypothesis has some experimental evidence and we could also say t h a t these terms are proportional to „, , „ „ , v at ad zia^r, xoar, — , — and at the second order, to a g 6 v y3 g h0 r e f e r e n c e s a c c e k b o n i e . , m o s e t t i f . , 1 9 7 1 . sulla possibilità di previsione del livello marino e dell'acqua alta nella laguna di venezia mediante la misura continua della corrente marina nei canali di accesso col metodo elettromagnetico. " b o l l e t t i n o di geofisica t e o r i c a ed a p p l i c a t a " , x i i i , 49. ( 2 ) a c c e r b o n i e . , m a n c a b . , 1 9 7 3 . storm surges forecasting in the adriatic sea by means oj a uro-dimensional liydrodynamic numerical model. " b o l l e t t i n o di geofisica teorica ed a p p l i c a t a " , x v , 57. ( 3 ) b a j c c . , 1 9 7 2 . seiches in the adriatic sea: a theoretical approach. " b o l l e t t i n o di geofisica teorica ed a p p l i c a t a " , x i v , p p . 53-54. ( 4 ) c o u i ì a n t r . , h i l b e r t i ) . , 1 9 5 3 . methods of mathematical physics, i n t e r s c i e n c e p u b l i s h e r , 1. ( 5 ) d e f a n t a., 1960. physical oceanography, i i . p e r g a m o n p r e s s . ( " ) d e m o t t o n i p . , s a l u s t i e . 1 9 7 2 . on a non-linear equation describing the unidimensional motion of a viscous fluid. " i s t i t u t o di fisica g. marc o n i " , r o m a , n o t a i n t e r n a , n. 401. ( 7 ) m o s e t t i f . , 1 9 7 3 . alcune caratteristiche delle variazioni del livello del mare adriatico. " b o l l e t t i n o di geofisica teorica ed a p p l i c a t a " , x v , 58. ( 8 ) p r o u d m a n j . , 1953. dynamical oceanography. m e t h u e n , l o n d o n . vol52,6,2009new 709 annals of geophysics, vol. 52, n. 6, december 2009 key words scientific letters – earth sciences – grablovitz introduction giulio grablovitz was one of the leading figures in italian and international seismology from the second half of the 1880s up to the 1920s. the documentary assets of his long, multifaceted and intense scientific experience consist of several thousand items, amongst which seismic, meteorological and oceanographic recordings, publications, seismic bulletins, scientific journals, various manuscripts, photographs and photographic plates. the following items survive from the original instrumental equipment of the observatory: the seismic tank, some masses and the precision pendulum time-keeping clock. for a number of years the historical materials had been stored in the rooms of the observatory and the damp conditions and partial decay of the rooms has not been of help in preserving the historical materials, seeing that part of the documentation has been damaged. some partial operations of cataloguing, reproduction and study have so far allowed us to appreciate the great scientific and historical value and utility of the various published and unpublished components, which are individually of great value, but when taken as a whole represent a unique heritage. giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence graziano ferrari istituto nazionale di geofisica e vulcanologia, bologna, italy abstract giulio grablovitz was one of the leading figures in italian and international seismology from the second half of the 1880s up to the 1920s. the documentary assets of his long, multifaceted and intense scientific experience consist of several thousand items, amongst which seismic and instrumental recordings, scientific correspondence, publications, bulletins, various manuscripts and photographic materials. what remains from the observatory’s original wealth of instrumental equipment is: the seismic tank, some masses and the precision pendulum time-keeping clock. some partial operations of cataloguing, reproduction and study have so far allowed us to appreciate the great value and scientific and historical utility of the various published and unpublished components, which are individually of great value, but taken as a whole represent a unique heritage. the cross-matching of this documentation with other italian epistolary archives allows us to sketch a preliminary profile of grablovitz, his scientific and human character, together with his fundamental contribution to the birth and development of the italian geodynamic service and the development of the experimental seismology of his day. mailing address: dr. graziano ferrari, istituto nazionale di geofisica e vulcanologia, sezione di bologna, via donato creti 12, 40128 bologna, italy; e-mail: ferrari@bo.ingv.it vol52,6,2009new 2-12-2009 12:40 pagina 709 710 g. ferrari the long period of abandonment of the documentation and the subsequent interventions have substantially mixed up the historical materials and only a thorough project of reordering, cataloguing and restoration can make this wealth of materials completely usable once more. this contribution uses only part of the correspondence to sketch an initial biographic profile of grablovitz and his activity at the ischia observatories which he himself had designed and set up. along this path we shall bring to life some of grablovitz’s manuscripts and some of the epistolary and administrative documentation of the direzione generale dell’agricoltura (general direction of agriculture), preserved at the central state archives (csa), which the ufficio centrale di meteorologia e geodinamica (ucmg central office of meteorology and geodynamics) depended on. 1. giulio grablovitz and the observatories of the island of ischia giulio grablovitz was born in trieste on 9th december 1846, during the austrian occupation. he was very keen on earth physics and cosmic physics, and soon started dealing with astronomy and oceanography. he obtained italian citizenship in 1885 (1). here are some biographical notes that we have gleaned from his short unpublished diary. his astronomical interests, which were so useful to him also in his seismological practice, as we shall see later, began early in his childhood, as he wrote in his diary: 29th july 1851 partial eclipse of the sun observed from the roof window with smoked glass (2). he came from a family of traders, and started his studies in november 1857 at the academy of commerce and nautical studies (ag, diario). in 1861 (3) with all of his family he moved to milan. shortly afterwards he started to help with his family’s business activities (ag, diario). in 1869 he moved back to trieste. in 1874 he published his first astronomical work on the calculation of the celestial phenomena recorded in trieste and in italy predicting a partial eclipse, a covering and a passage of venus over the sun that indeed occurred on 10th and 14th october and on 9th december of the same year respectively (ag, diario). the following year, when there was an extraordinarily high tide at trieste which grablovitz called «(...) the inspiration for new scientific concepts» (ag, diario), he published tables of tides, suggesting the use of new types of oceanographies. in 1876 he published the new seismic theory of tides and in august of the same year, on a trip to venice, grablovitz met elia millosevich, deputy director of the ucm (fig. 1). these were crucial years when the scientific events in grablovitz’s life were interwoven with those of another great protagonist of seismology of those days: michele stefano de rossi. de rossi was keen on geodynamics starting from 1868, but only embarked on its study in the early 1870s. he wanted to pursue his objective of organising the observations of geodynamic phenomena, and personally committed himself, befig. 1. parts of the pages of the grablovitz diary in which there is mention of a) 17 august 1876 the meeting with elia millosevich, and b) september 4, 1880 with pietro tacchini, deputy-director of the ucm and director of the ucm, respectively. in c) the letter with the offer of the position of director of the ischia observatory arrived on august 8, 1885. vol52,6,2009new 2-12-2009 12:40 pagina 710 711 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence ginning in 1873, to organising a network of correspondents. a year later he launched the publication of the bullettino del vulcanismo italiano (bvi), which was supposed to represent a centre for collecting data and discussing results, as well as the main promotional vehicle for the fledgling organisation (fig. 2). the bulletin was the first real review wholly dedicated to the earth sciences, drafted almost exclusively by de rossi himself. the success of the great adventure he had devoted himself to four years before was already apparent in 1877: the correspondents of the bulletin had risen to 105 – giulio grablovitz, antonio stoppani and torquato taramelli among the new members – and de rossi was able to announce that he had «managed to found a near-society or a new school for the studies of endogenous phenomena in italy». at the end of 1877, grablovitz published a work on the effects of lunisolar attraction (rapporti fra i moti microsismici e l’azione lunisolare delle maree). in 1880 he met pietro tacchini, the director of the central meteorological office (ucm) (ag, diario), by chance on a train journey (fig. 1). after the meeting with millosevich in 1876, this meeting was a turning point in the life of fig. 2. sites of the tromometric observations activated between 1870 and 1888. from 1873 the observatories gradually engaged in regular correspondence with de rossi who collected the data and published them in his bullettino del vulcanismo italiano. vol52,6,2009new 4-12-2009 15:55 pagina 711 712 g. ferrari the trieste scholar as well as for the fledgling italian geodynamic service. as early as 1881 the engineer felice giordano, chief inspector of the mines, recalled people’s attention to de rossi’s studies on «vulcanismo italiano» (letter from giordano to miraglia), with the only outcome, however, of delaying a possible government commitment to de rossi’s field of study. it took two earthquakes, on 4th march 1881 (fig. 3) and 28th july 1883 (fig. 4), both on the island of ischia, to convince the government to support de rossi’s research first of all, with the setting up of the observatory and the geodynamic central archive where de rossi completely transferred his activities and of which he was appointed director, subsequently to launch a full-blown programme of observation of geodynamic phenomena in italy. encouraged by millosevich, with a letter on 3rd november 1882, grablovitz put himself forward to tacchini for a future collaboration within the scope of geodynamic observatories (4). in march 1883 de rossi, with the establishment of the geodynamic observatory and archive at the royal geological committee, wanted to start up «an endogenous meteorology service» in italy, guaranteeing italy the leadership in this new branch of earth physics, «born and bred in the peninsula which was interested in it»: an authoritative centre situated within a governmental scientific body, albeit in a provisional form, which assured pledges of collaboration and cooperation from all over italy and even from abroad. confirmation came from abroad concerning the international success achieved by the italian geodynamic service. in particular, the french ministry of public works sent an inspector of fig. 3. thematic map of the seismic effects of the earthquake on 4th march 1881 in the island of ischia, drawn from the study of de rossi (de rossi 1879-1881). the full red area identifies the destructive earthquake effects; the area dotted in red outlines the primitive crater; the red line indicates the fracture line and perimeter of the primitive crater. vol52,6,2009new 2-12-2009 12:40 pagina 712 713 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence mines, accompanied by another two engineers to study the state of the art. this technical-scientific delegation was particularly appreciative of the italian experience and, in particular, of de rossi’s competencies. the new seismic disaster at casamicciola on 28th july 1883 drove the government to intervene in an even more incisive manner. the royal geodynamic commission was set up by a decree issued on 20th december 1883; it was chaired by giovanni schiaparelli and was composed of pietro blaserna, francesco denza, de rossi, galileo ferraris, giordano, luigi palmieri, francesco rossetti, orazio silvestri and tacchini (5). the commission, which in the meantime changed its president twice – schiaparelli who resigned and his successor quintino sella who died (14th march 1884) – only met for the first time at the end of may 1884 under the chairmanship of blaserna. one of its preliminary aims was to set up three large observatories «at the centre of the main volcanic regions of the country»: that is to say, rocca di papa, the central point of the vulcano laziale, casamicciola, site of the recent disastrous seismic phenomena, and catania. the establishment of observatories in other points of interest was put off to a later date. numerous secondary observatories were supposed to depend on these main observatories. grablovitz accepted the position given to him by blaserna on 15th september 1884, according to his diary (ag, diario). however, according to tacchini’s correspondence from the state central archives, he was to deal with the installation of the geodynamic observatory in casamicciola (6). from 11th to 15th december of the same year, grablovitz accompanied by de rossi, tacchini, l. baldacci, de ferrari, carrara and amato, surveyed the island of ischia to identify the location best suited to setting up the observatory and to come to an agreement with the municipal authorities (7). on 18th december 1884 blaserna presented the report given to him on the same day by grablovitz to the ministry of agriculture (8), concerning the inspection performed on the island of ischia. in the report the hill called grande sentinella was suggested as being the right place to build the observatory. it was situated in the central part of the zone that had suffig. 4. rare photograph of the effects of the earthquake on 28th july 1883 in casamicciola (archives of sga storia geofisica ambiente, bologna). fig. 5. dovecot annexed to the casina reale set up as a weather tower, in a late 19th century photograph (grablovitz archive). vol52,6,2009new 2-12-2009 12:40 pagina 713 714 g. ferrari cepted, making it the opening article of the 4th part of the ucm annals (ag, diario). on the occasion of the second meeting of the geodynamic commission from 14th to 16th june 1886, a proposal was made to the government to set up a full-blown central geodynamic office. the many relations developing between meteorology, geodynamics and the other branches of earth physics had led the commission to suggest to the government that it should also entrust the publication of the geodynamic observations to the central office of meteorology, leaving the directors of the main station with the responsibility to direct the works and transmit the recordings to the central office. in this way the ucm took on the character of a central office for the whole of earth physics, making the office faster and more united. all of this was handed over to the authority of tacchini, director of the office, who hoped to have the italian geodynamic sector gain, in a few years, the development that everyone already acknowledged to italian meteorology. the last meeting of the royal geodynamic commission was held in january 1887, on the occasion of which the organisation of the italian seismic service was established. on 23rd february 1887 a disastrous earthquake hit western liguria and the death and destruction it caused represented a further point pressurizing the government to complete the process of installing a permanent, organised, institutional geodynamic service in italy. a private diatribe between grablovitz and de ferrari, which started in 1886 and went on until 1889 and which cost grablovitz a prison sentence, later turned into internment in rome, and de ferrari’s definitive removal from ischia, slowed down the establishment of the ischia observatories. during grablovitz’s period of absence from ischia, giovanni agamennone (september) and later luchesi (december) (16) (ag, diario) were appointed as assistants for the continuation of the observations at casamicciola. again in 1887 the geodynamic commission was broken up but, as tacchini explained, the management of the geodynamic service was entrusted to him and his ucm as there fered the most damage due to the earthquake on 28th july 1883. it was characterised by its solidity and field of vision: from that point the surrounding mountains, the islands and vesuvius could be seen. among the various ideas expressed in the work, grablovitz added the hypothesis of setting up other subsidiary stations, amongst which one to be situated in proximity to the port of ischia, at «casina reale», licensed by the ministry of war (9). that observatory was certainly ready in a short time, and would have allowed for preliminary geodynamic research to be carried out, together with very useful mareographic and meteorological studies (10) (fig. 5), even before the observatory of the grande sentinella had been completed. on 23rd january 1885, the ministry of public works commissioned gambara, who was the ingegnere reggente l’ufficio speciale del genio civile, with the job of drafting the project for the geodynamic observatory of casamicciola and seeing to the relevant preliminary operations of expropriation, vacating of disused farm buildings and the removal of shacks (11). in november of that year, in spite of the repeated requests, the project for the building had not yet been completed, whereas the work for the organisation of the subsidiary observatory at «casina reale» appeared to be well underway (12). following the royal decree passed on 24th july 1885, king umberto i appointed g. cantoni, t. taramelli and g. grablovitz as members of the royal geodynamic commission and confirmed blaserna as president (13). on 8th august 1885 grablovitz received a letter in trieste from the ministry with the offer of a position as director on the island of ischia (ag, diario), which he accepted and which was followed by the official communication of the royal decree of appointment (14). in a note dated 18th april 1886 grablovitz urged miraglia to see to it that the appropriate geological surveys be carried out to test the stability of the ground, promised by baldacci back in 1884, and required in order to obtain the approval for the project presented to the ufficio del genio civile (15). on 15th june 1886 gg presented the definitive report for the geodynamic observatory of ischia to the royal geodynamic commission which unanimously acvol52,6,2009new 2-12-2009 12:40 pagina 714 715 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence were insufficient economic resources to create an independent service (17). the period of the construction and start up of the observatory was a particularly laboured one for grablovitz. the scientific activities were significantly hampered owing to the poor quality of the building work and the materials used. for grablovitz there started a period of arguments and general disagreement with the civil engineers of naples, entrusted with overseeing the work done by the direzione generale dell’agricoltura. the difficulties he came across convinced him to bring forward the instrumental outfitting of the subsidiary observatory of porto d’ischia which, even after the inauguration of the main observatory of la grande sentinella of casamicciola, remained his favourite workplace. as soon as he had come back from a forced absence (i.e. prison and internment in rome), grablovitz started to process the data that he had collected in the meanwhile at his observatory and he devoted himself to oceanographic studies that would later lead him to the publication of a study on the tides of ischia (ricerche sulle maree d’ischia (ag, diario)). the ministry of agriculture, following the shocks felt and the eruptive phenomena in progress on the aeolian islands, on the grounds of a proposal of the consiglio direttivo di meteorologia, agreed to involve prof. giuseppe mercalli in the study of the lipari volcanic eruption (18). on 1st february 1889, tacchini, after having commissioned grablovitz to go to lipari for study reasons, came to an agreement with the ministry of agriculture to see that de ferrari be removed from ischia, taking advantage of grablovitz’s absence (19), who nonetheless was not going to be absent from ischia for very long. his presence was absolutely necessary to implement a series of initiatives already in progress, and above all «finally», after a very difficult period studded with lengthy absences (20). on his part, grablovitz accepted the job of going «to the lipari islands» to study the phenomena in progress in that period, in turn expressing the need to go back to dealing with the construction of the observatory of casamicciola as soon as he could, something that had always been hampered by a variety of problems (21). after a one-year absence, for the preceding 3 months he had worked hard to make up for the damage and it would have been a pity if a new suspension had made all his work and effort vain (22). on 8th february 1889, grablovitz and mercalli left naples to go to messina and lipari following silvestri’s report of an earthquake in the former location and a volcanic eruption in the latter (23). although it had been understood that grablovitz should rightly deal principally with the ischia project, his successes in the scientific field made him more suited to the study of the volcanic events of the aeolian islands (24). nevertheless, in october 1889 tacchini pointed out in a letter to the direzione dell’agricoltura that grablovitz would only set up the service on the aeolian islands and that subsequently it would have been given the control of catania. the scholar’s intervention was deemed necessary as the intention was to set up a proper transmission service between vulcano and lipari and the automatic equipment that was to be installed had indeed been developed by him (25). on 16th july 1889 (ag, diario), grablovitz went back to ischia on a definitive basis (26). cuts to resources, spiralling costs due to the breaking up of responsibilities and personal squabbles drove the president of the consiglio direttivo di meteorologia e geodinamica blaserna to write, on 1st november 1889, an impassioned letter to the ministry of agriculture, industry and commerce on the history of the geodynamic service in italy, praising the work of grablovitz in particular (27). section i of the congress of genoa in 1892, at the special meeting on 19th september, voted in favour of extending the mareographic research, and that the various central administrations should agree on the plan drafted by grablovitz. on the occasion of this congress, grablovitz took his leave from casamicciola from 6th september until 18th october, going to rome and trieste (ag, diario). from a series of service notes on 23rd june 1892, written before leaving for genoa, the geodynamic observatory of casamicciola had its stable location at porto d’ischia and comprised the following: the management office, the meteorological and geodynamic station, the oceanographic station, the geodynamic station at the grande sentinella and vol52,6,2009new 2-12-2009 12:40 pagina 715 716 g. ferrari the hydrothermal points: at the municipal establishment of ischia, at the stabilimento balneomilitare d’ischia and at the gurgitello (28). on the occasion of the summoning of the consiglio direttivo di meteorologia e geodinamica on 26th june 1893 grablovitz asked tacchini to see that his programme was implemented, emphasizing its ease and convenience (29). in 1894 the geodynamic levels he had built were set up in the observatory of porto d’ischia, whose first recording was the earthquake of ljubljana on 14th april 1895 (fig. 6), and in october of that year he obtained the first recording of a teleseism: the earthquake on 27 october 1894 (ag, diario), m = 8.0 san juan, argentina with the horizontal electrical alarm pendulums. at last, the seismic tank grablovitz had built and perfected at grande sentinella (30). recorded its first diagram in 1895 (18th october) even if it did have some problems owing to water leaks (ag, diario), problems practically solved the following year, by applying bitumen along the walls of the seismic tank (fig. 7). this instrument was one of those that recorded the indian earthquake on 12th june 1897; after this event grablovitz added the smoked paper recording system to the ink-based one. in the meantime, on 29th may 1896 (ag, diario), the fig. 6. seismogram of the lublin earthquake on 14 april 1895 recorded at the porto d’ischia observatory by the continuous recording geodynamic levels and reverse side of the same seismogram with the calculations and the information systematically summarised by grablovitz. (grablovitz archive). vol52,6,2009new 2-12-2009 12:40 pagina 716 717 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence observatory had been equipped at the rooms of casina reale (31) with two more horizontal pendulums with masses of 12 kg. in a letter dated 8th june 1896, grablovitz stressed the great sensitivity and low cost of his invention to tacchini (32). these considerations were in time also confirmed abroad (33). by grablovitz’s own admission in his diary, the construction of these «pendulums with continuous mechanical recording» were the fruit of previous experiences dated in late-march 1895. thanks to these seismographs he also recorded the japanese earthquake on 15th june 1896. the positioning of the new instruments had cast light on the advantages and drawbacks of the different rooms that made up the observatory (fig. 8). grablovitz, concerned about the possibility aired by tacchini concerning the closing down of the station at the port, in a letter dated 1st february 1897, illustrated some defects of the grande sentinella observatory, linked to the environmental non-seismic noise enhanced by the increased sensitivity of the instruments designed by grablovitz and the advantages of the one at porto d’ischia (34). in april 1897, after a series of events involving his assistants in whom he had by then lost all esteem, grablovitz moved his residence to grande sentinella to have the main instruments close to hand and he left one of his assistants at the port. in october, owing to a family bereavement, grablovitz left for trieste and took advantage of his trip to meet albin belar in ljubljana, with whom he discussed the horizontal pendulums (ag, diario). fig. 7. grablovitz’s seismic tank at the grande sentinella observatory in a rare photograph of the end of 19th century (archivio grablovitz). this instrument still exists. vol52,6,2009new 2-12-2009 12:40 pagina 717 fig. 8. seismic instruments hall of the observatory of the grande sentinella of casamicciola (island of ischia) in a rare photograph between the end of the 19th and the start of the 20th century (grablovitz archive). in the foreground we can see the small water geodynamic levels of grablovitz (9), a marine chronometer (10), two brassart seismometrographs with stationary mass on smoked paper (5) and on a smoked glass pane (6), some topographic instruments (2, 3 and 4) and, in the background, a precision pendulum clock (1). the instrument set is completed by two brassart rod seismoscopes (11), in the foreground and in the background respectively, in front of a precision clock. behind the glass bell of the brassart rod seismoscope we can catch a glimpse of a pair of grablovitz electric alarm horizontal pendulums. in the same room, not visible, there was also a meridian, with which grablovitz adjusted the clocks. 718 g. ferrari in 1906 the instruments designed by grablovitz won a gold medal at the milan exhibition. his fame was by then acclaimed and his international collaborations already confirmed, for example, by his correspondence with j. milne or f. omori and the use of his instruments in other observatories, such as those in spain (batlló, 2009). it is grablovitz himself who provided the detailed instructions for the reproduction of his instruments elsewhere, as can be seen from the correspondence with the ebro observatory on the occasion of the recordvol52,6,2009new 2-12-2009 12:40 pagina 718 719 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence ing of the california earthquake on 18th april 1906 (batlló, 2009). another observatory that used his instruments is that of ljubljana. grablovitz also took advantage of his international collaboration to increase the number of oceanographic data at his disposal relating to the mediterranean as in the case of the ebro observatory (tortosa) in spain (batlló, 2009). at the end of the 19th century, the casamicciola observatory was among the best equipped and most efficient in europe. today we still have grablovitz’s extraordinary archive, which comprises thousands of documents, including letters, seismograms, notes, meteorological and oceanographic observations, etc. thanks to his instruments, in particular the seismic tank (fig. 8) grablovitz recorded some of the most powerful earthquakes of the late 19th century, extraordinary recordings still existing with which attempts have been made to determine the moment magnitude (table i) (ferrari et al., 2005). on the earthquake of assam, the first teleseism recorded in italy so clearly (fig. 9), grablovitz wrote an article in bssi and put forward some considerations on the seismograms of the japan earthquake on 31st august 1896, which will be discussed later. in the 1896 seismic bulletin (notizie 1896, pp. 91-95) grablovitz, describing the seismogram of the japanese earthquake on 31st august 1896, m = 7.3 (abe, 1994), recognised three different phases: the first one was characterised by «tremors of negligible size and minimal movements, more accentuated in the meridian than in the parallel and with predominant nne-ssw direction», the period of oscillations are mostly brief: from 2.5 to 4s; the second phase consisted in «more noticeable short-term impulses and consisting of longer-lasting oscillations» with oscillation periods in the range of 3.2 and 20s; the third phase «consisting exclusively of longterm oscillations» up to 41s. about one year later, on 12th june 1897, an earthquake of 8.1 magnitude hit the area of assam (north-east india) and was recorded by numerous italian observatories. richard dixon oldham (1858-1936), of the geological service of india, in studying the propagation velocities on the italian seismograms, recognised three phases in them, but independently from grablovitz and published a weighty article on them in which he analysed a dozen or so earthquakes far from the recording sites. some of the elements that could be gleaned from one of his publications and a clarification at distance between oldham and grablovitz on the paternity of the recognition of the three phases, can help us to understand some aspects of the history of italian seismology between the late 19th and early 20th centuries. moreover, the issues of the recognition of the different phases that make up a seismogram and that of the sensitivity and the definitive power of the seismographs are central to the untable i. final results concerning the moment magnitude of some earthquakes of the end of 19th century calculated from original seismograms recorded at the grablovitz’s ischian observatories. date region uo fc (hz) mo*e+18 mw r (km) stress drop (mpa) june 12, 1897 india 0.86714e-03 0.106 251.39 7.60 (p) 20.94 11.9723 0.29240e-02 0.061 103.09 7.34 (s) 21.01 4.8619 september 4, 1899 alaska 0.14123e-03 0.199 48.37 7.12 (p) 11.16 15.2412 0.88986e-03 0.111 35.45 7.03 (s) 11.60 9.9376 september 20, 1899 turkey 0.95183e-04 0.387 2.66 6.28 (p) 5.74 6.1576 0.20772e-03 0.216 0.61 5.86 (s) 5.93 1.2828 vol52,6,2009new 2-12-2009 12:40 pagina 719 720 g. ferrari derstanding of the internal structure of the earth and the mechanisms generating and propagating the earthquakes, the latter being fundamental aspects in the development of seismology. in his memorandum oldham mainly used the data of the italian observatories and produced the first travel-time curves of seismic waves for distant earthquakes. table ii reports the observatories considered by oldham among those that recorded the seven earthquakes that he examined. he reports the phases detectable by the seismograms and the distances of the stations from the earthquake epicentres, measured in degrees. as can be seen, out of fifteen observatories, seven are italian and cover 74% of the observations made by oldham and with which he constructed the graph of the propagation velocities of the three wave phases. in his weighty memorandum, the irish seismologist often referred to grablovitz, with great respect (he called him «professor», on a par with milne, while cancani and agamennone, just «doctors»), but he hinted to the fact that grablovitz only recognised two of the three phases in the recordings. for his work, oldham is universally attributed as having been the first to recognise these three main phases (davison, 1927; milne, 1900; agnew, 2003). the story does not finish here and has a follow-up that we have been able to reconstruct only by means of the unpublished documentation of the archive of the casamicciola observatory. going back to the aspect of the different phases of the seismograms (grablovitz, 1913), grablovitz did not miss the opportunity to challenge oldham over the record in the recognition of the three phases in the recordings of distant earthquakes, claiming it for himself on the occasion of the japan earthquake on 31st august 1896. on 18th september 1914 oldham wrote to grablovitz (fig. 9), reconstructing the reasons he had mistakenly failed to recognise grablovitz’s record. he admitted that at the time of his study (oldham, 1899) he did not read italian very well with the ensuring consequences. however, oldham stressed that his work was not limited only to the recognition of the three phases, but that it consisted of three distinct forms of waves that propagated along different paths and at different velocities, consequently arriving at different times (35). oldham concluded the letter by stating that, in regard to the three phases of the seismogram, there could be no doubt that he (grablovitz) had established his priority over their recognition. in the years when the confrontation between grablovitz and oldham was unfolding, the italian seismologist was at the height of his scientific maturity, with a substantial experience behind him as a designer and tester of seismographs, but also with an outstanding experience in interpreting the contents of his seismograms, which the instruments installed at the two observatories of casamicciola and porto d’ischia fig. 9. letter from r.d. oldham 1914 to giulio grablovitz on 18th september in which the british seismologist recognised without any doubt grablovitz’s priority in the identification of three different phases in seismograms. vol52,6,2009new 2-12-2009 12:40 pagina 720 721 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence record in large quantities. he makes up for the modest sensitivity and the low speed of the recording (30cm/hour) with his great experience in reading the traces. due to his longstanding interest in the astronomical field he was, in addition, highly skilled in the determination of time with astronomical methods and, therefore, in the correction of the clocks of the two observatories. in this regard he had equipped the observatory of the grande sentinella of casamicciola with a meridian. as can be seen in the table, the recordings of the observatories of the island of ischia are among those that have most recorded the earthquakes analysed by oldham. figure 10 reports the concluding graph of the above-described study by oldham, in which black dots represent the contribution of the recording of the italian observatories, instruments of great mass and inertia. the graph shows, as acknowledged by oldham himself, the great contribution of the italian instruments to this important scientific result that paved the way, with the subsequent extable ii. phases recognised in the seismograms recorded by the different observatories taken into consideration by oldham (1897) for his analyses: 1 = first phase, 2 = second phase, 3 = third phase. starting from these data he published the graph reported in fig. 10. for some of these sequences, some observatories recorded more than one shock, indicated in square brackets. observatory country 22 03 1894 27 10 1894 15 06 1896 31 08 1896 12 06 1897 05 08 1897 17 09 1897 y y japan argentina japan japan japan giappone turkestan catania i 1,3[1] 1,2,3 1,2,3 1,2,3 2,3[1], 1,2[2] charkow pl 2,3 1 edinburgh uk 1,2 3 grenoble f ischia i 3 2,3 1,2,3 1,2,3 1,3 1,3[1] 1,2,3[2] nicolaiew ru 2,3 2 1,2 1,2 2,3[1] padua i 1,3 1,2,3 1 pavia i 3 3 1,3 potsdam d rocca di p. i 1,2,3 3 1,3 [i], 3[2,3] 1,2,3 1,2,3 1,3 3 rome i 1,2,3 1,2,3 3 1,2,3 1,2,3 1,2,3 1,3[1] 1,2[2] shide uk 1,2,3 1 3 siena i 1,2,3 (1) 1,2,3 strasbourg d(f) 1,2 1 tokyo j 1 vol52,6,2009new 2-12-2009 12:40 pagina 721 722 g. ferrari traordinary developments of milne, jeffreys, bullen etc., to the study of the internal composition of the earth and the modelling of the generation and propagation mechanisms of the earthquakes. in 1908, the casamicciola observatory was also equipped with a tromoseismometer, whose first recording was the messina earthquake on 28th december 1908 (ag, diario) the seismograms of which are still existing (fig. 11). the care with which grablovitz continuously made improvements to the observatory is also testified to by the intense correspondence exchanged with b. paoloni, director of the geodynamic and meteorological observatory of montecassino, above all in regard to the intermediate corrections and the definitive correction of the respective meridian lines (36). in 1913 he fundamentally dedicated himself to the works of the oceanographic commission for which he drafted the oceanographic programme of the tyrennian commissioned to him by the royal talassographic committee, also collaborating with the royal hydrographic navy institute. the avezzano earthquake on 13th january 1915 was thus noted by grablovitz avezzano earthquake; provenance intuited, easy to tranquillise everyone (ag, diario). it was the year of the war against austria! fig. 10. time curves of the three phases of the earthquake motion recognisable in distant records (after oldham, 1900 modified). black dots identify phases from italian observatories, 74% of the total number of data used by oldham. fig. 11. particular of the ene-wsw component of the seismogram of the earthquake on 28th december 1908 in the straits of messina (m = 7.1), recorded at the observatory of porto d’ischia by a tromometrograph. vol52,6,2009new 2-12-2009 12:40 pagina 722 723 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence and on 29th december he was appointed cavaliere della corona d’italia (ag, diario) and in 1918 the epilogue of the war and the 5 days of trieste are thus summarised in his diary: «the 5 days of the liberation of trieste october 30th the population declares the austrian government as fallen. october 31st alfonso valerio evicts the austrian governor november 1st destruction of every austrian emblem and radiotelegraphic appeal to the italian fleet. november 2 san giusto anxious wait and at the vespers of the 3rd entrance of the bersaglieri by sea at trieste. long live redeemed. this event closes, somewhat belatedly, but nevertheless in time, the battle-hardy stage of my life, with the crowning of my life’s highest ideal.» before the years that followed, which continue in his diary, the words «new era stand out». in april 1919 he took part in a meeting of the thalassographic committee in pisa and during his absence the observatory lost its caretaker de angelis (suicide by hanging). in october 1925 ischia hosted the hydrological congress and on 21st january 1926 grablovitz was appointed «knight commander», an honour that would be officially conferred to him by the mayor of casamicciola on 25th december while in july of the same year he again moved to grande sentinella. the last entries in his diary are not his own: 1928, 5th may fatal fall of father. 19th september death (ag, diario). 2. conclusions giulio grablovitz was one of the most important figures in italian and international seismology between the late 19th and the early 20th centuries, whose far-reaching and outstanding scientific production overrides the mere consultation of his rich series of publications in seismology, meteorology, oceanography. this first study of the documentation preserved in his scientific and personal archive has been an attempt to make just a preliminary sketch of the trieste scholar’s scientific and human biographical profile, highlighting numerous aspects for future research and analysis. the informational wealth, only briefly touch upon here, is the fruit of the extraordinary unpublished documentary wealth contained in his archive, mostly made up of a substantial epistolary collection, only partially organised. the knowledge of other epistolary collections of seismology, analysed during the tromos project, has allowed us to put together part of grablovitz’s manuscripts and part of the epistolary and administrative documentation of the general management of agriculture and the tacchini archive, preserved at the cracma of rome. notes (1) a letter from nicola miraglia, general director of the ministry of agriculture, industry and commerce to pietro blaserna, president of the royal geodynamic commission, dated rome, 12th september 1884 says «é superfluo poi avvertire che ove al grablovitz venisse offerto il posto di direttore, egli dovrebbe anzitutto rinunziare alla propria nazionalità e prendere l’italiana». translation: «also, it is superfluous to warn you that should grablovitz be offered the position as director, he should first of all waive his own nationality and take up italian citizenship». archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 965, fasc. 4243, minuta di lettera del direttore generale dell’agricoltura n. miraglia a p. blaserna sull’impianto dell’osservatorio geodinamico nell’isola d’ischia, roma 12 settembre 1884, cc. 2; and in another letter dated rome, 5th november 1885: «questo ministero [...] promuoverà la nomina del dr. giulio grablovitz a direttore dell’osservatorio di casamicciola, non appena sia emanato il r. decreto che accorda la naturalizzazione italiana allo stesso dottor grablovitz». translation: «this ministry (...) will promote the appointment of dr. giulio grablovitz as director of the observatory of casamicciola, as soon as the royal decree is issued granting dr. grablovitz italian naturalisation.» vol52,6,2009new 2-12-2009 12:40 pagina 723 724 g. ferrari archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 962, fasc. 4230, minuta di lettera del ministero di agricoltura industria e commercio, direzione generale dell’agricoltura al presidente della r. commissione geodinamica relativa all’organizzazione del servizio geodinamico, roma 5 novembre 1885, cc. 7). (2) giulio grablovitz archive, diary from 1846 to 1928 (hereinafter in the text ag, diario), the archive has been temporarily entrusted to the author by the grablovitz heirs, for purposes of study and future valorisation in an appropriate site. (3) in 1859 with the war of lombardy the region went under the rule of the piedmontese. (4) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di giulio grablovitz a pietro tacchini, trieste 3 novembre 1882. (5) decreto reale di istituzione della commissione di studio sull’ordinamento del servizio vulcanologico, roma 20 dicembre 1883, cc. 5, archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 962, fasc. 4230 (6) archivio centrale dello stato, direzione, generale dell’agricoltura, v versamento, busta 965, fasc. 4243, lettera di g. grablovitz a p. tacchini di accettazione dell’incarico di esaminare il modo d’impianto dell’osservatorio geodinamico di casamicciola, trieste 15 settembre 1884, cc. 2. (7) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 965, fasc. 4243, relazione di g. grablovitz sull’impianto di un osservatorio geodinamico in casamicciola, lettera di p. blaserna di trasmissione della medesima al ministero di agricoltura, industria e commercio, roma 18 dicembre 1884, cc. 14; archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 965, fasc. 4243, comunicazioni relative all’esame del progetto per l’osservatorio geodinamico per l’isola d’ischia, roma 17-26 dicembre 1884, cc. 3. (8) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 965, fasc. 4243, relazione di g. grablovitz sull’impianto di un osservatorio geodinamico in casamicciola, lettera di p. blaserna di trasmissione della medesima al ministero di agricoltura, industria e commercio, roma 18 dicembre 1884, cc. 14. (9) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 962, fasc. 4230, minuta di lettera del ministero di agricoltura industria e commercio, direzione generale dell’agricoltura al presidente della r. commissione geodinamica relativa all’organizzazione del servizio geodinamico, roma 5 novembre 1885, cc. 7. (10) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 965, fasc. 4243, relazione di g. grablovitz sull’impianto di un osservatorio geodinamico in casamicciola, lettera di p. blaserna di trasmissione della medesima al ministero di agricoltura, industria e commercio, roma 18 dicembre 1884, cc. 14. (11) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, lettera del segretario generale del ministero dei lavori pubblici, segretariato generale al direttore generale dell’agricoltura del ministero di agricoltura industria e commercio, roma 3 febbraio 1885. (12) archivio centrale dello stato direzione generale dell’agricoltura, v versamento, busta 962, fasc. 4230, minuta di lettera del ministero di agricoltura industria e commercio, direzione generale dell’agricoltura al presidente della r. commissione geodinamica relativa all’organizzazione del servizio geodinamico, roma 5 novembre 1885, cc. 7. (13) archivio centrale dello stato direzione generale dell’agricoltura, v versamento, busta 962, fasc. 4230, decreto reale di nomina di g. cantoni, t. taramelli, g. grablovitz a membri della r. commissione geodinamica, monza 24 luglio 1885, cc.4. (14) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di giulio grablovitz a pietro tacchini, trieste 30 dicembre 1885. (15) archivio centrale dello stato direzione generale dell’agricoltura, v versamento, busta 965, fasc. 4243, lettera del direttore dell’osservatorio geodinamico di casamicciola alla direzione generale dell’agricoltura relativa alla analisi geologica del sottosuolo della grande sentinella, ischia 18 aprile 1886, cc. 2. (16) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di giulio grablovitz a pietro tacchini, napoli 1 ottobre 1887. (17) tacchini wrote: «[...] per impiantare un ufficio centrale geodinamico occorreva una certa somma, che il ministero dell’agricoltura non riusciva ad aggiungere nel proprio bilancio, così si pensò al ripiego di addossare a me la direzione di questo nuovo servizio con promesse verbali di un compenso che poi non ebbi mai. in conseguenza di questa decisione con decreto del giugno 1887 il servizio geodinamico fu unito all’ufficio centrale di meteorologia, che perciò assunse il titolo di ufficio centrale di meteorologia e geodinamica. a me toccò di provvedere alla sistemazione degli osservatori geodinamici di catania, d’ischia, di rocca di papa e di pavia ed alla stazione sperimentale geodinamica del collegio romano, ed a tutto quanto riguarda il servizio che poco per volta si è andato estendendo e migliorando in tutta italia». vol52,6,2009new 2-12-2009 12:40 pagina 724 725 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence translation: «[...] in order to install a central geodynamic office a certain sum was needed, which the ministry of agriculture was unable to add to its own budget, so we thought of the compromise of giving me the management of this new service with verbal promises of a fee that i later never had. as a consequence of that decision, pursuant to the decree passed in june 1887, the geodynamic service was joined to the central office of meteorology, which thereby took the title of central office of meteorology and geodynamics. it was down to me to see to the setting up of the geodynamic observatories of catania, ischia, rocca di papa and pavia and the experimental geodynamic station of collegio romano, and everything concerning the service that gradually grew and improved across the whole of italy», biblioteca estense di modena, fondo tacchini, 20a, appunti di p. tacchini sulla storia del servizio geodinamico, sl., sd., cc. 2, nn. 3-4. (18) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, 963, fasc. 4239, pratica relativa all’organizzazione dei lavori della commissione di studio sui fenomeni vulcanici alle eolie, roma-lipari 22 gennaio-16 febbraio 1889, cc. 10. (19) archivio centrale dello stato direzione generale dell’agricoltura, v versamento, 963, fasc. 4239, pratica relativa ai problemi suscitati dalla nomina di g. grablovitz a componente la commissione per lo studio dei fenomeni vulcanici alle eolie, roma 1-4 febbraio 1889, cc. 8. (20) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di pietro tacchini al ministro di agricoltura industria e commercio, roma 2 febbraio 1889. (21) grablovitz spoke of numerous serious impediments caused by de ferrari, who out of «personal vendetta» and by way of private manoeuvres had managed several times to remove him from his post, preventing the completion of the scientific programme: «[...] with serious prejudice to the scientific interests entrusted to me by the state and to my particular position, lies in an obstinate and unjustifiable persecution of a person employed by the ministry for the interior and temporarily working in ischia. the direct aim of that individual is to disturb me in performing my duties towards the government [...]». (22) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di pietro tacchini a nicola miraglia direttore generale dell’agricoltura ministero, roma 4 febbraio 1889. (23) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di pietro tacchini a nicola miraglia direttore generale dell’agricoltura, roma 8 febbraio 1889. (24) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, 966, fasc. 4248, lettera del direttore dell’ufficio centrale di meteorologia e di geodinamica alla direzione generale dell’agricoltura sulle spese da sostenersi per l’impianto di un servizio sismico alle isole eolie, roma 29 agosto 1889, cc. 4. (25) archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, 966, fasc. 4248, lettera del direttore dell’ufficio centrale di meteorologia e di geodinamica alla direzione generale dell’agricoltura sulle modalità di impianto di un servizio sismico alle eolie, roma 8 ottobre 1889, cc. 2. (26) archivio giulio grablovitz, fondo g. grablovitz, lettera di pietro tacchini a giulio grablovitz, casamicciola 12 luglio 1889. (27) «[...] il direttore di casamicciola è il solo, dico il solo dei nostri direttori, il quale abbia indirizzo veramente scientifico.» e dununciando i rischi dovuti ai tagli imposti dal governo su un servizio già con poche risorse «[...] quando agli osservatori si danno, a titolo di annua retribuzione, lire 100 o 200, una diminuzione di qualche decina di lire diventa sensibile e li disgusta. [...] in quale crudele imbarazzo doveva trovarsi il consiglio nel respingere tutte le nuove e modeste proposte e nel restringere le modestissime condizioni già esistenti. io non so come si andrà avanti nell’avvenire. lo dico e lo ripeto con triste convincimento, che tutto è compromesso e che se non vi si rimedia, andrà a sfascio un servizio, che era una delle nostre glorie». translation: «[...] the director of casamicciola is the only, i repeat the only one of our directors, who has a truly scientific orientation,» and by denouncing the risks due to the cuts imposed by the government to a service that already had few resources «[...] when 100 or 200 lire are given to the observatories, as a yearly payment, a cut of a dozen or so lire becomes significant and offends them. [...] the council must find itself in cruel embarrassment in rejecting all the new and modest proposal and in restricting the very modest conditions already existing. i do not know how we can get by in the future. i say it and repeat it with sad belief, that everything is compromised and that if there is no remedy, the service will go to ruin, a service that was one of our glories.» archivio centrale dello stato, direzione generale dell’agricoltura, v versamento, busta 967, fasc. 4250, lettera del presidente del consiglio direttivo di meteorologia e geodinamica pietro blaserna al ministro di agricoltura industria e commercio sulle vicende del servizio geodinamico in italia, roma 1 novembre 1889, cc. 6. (28) archivio giulio grablovitz, fondo g. grablovitz, lettera di giulio grablovitz a placido attard, casamicciola 23 giugno 1892. (29) biblioteca estense di modena, fondo tacchini, 20m c. 342, lettera di giulio grablovitz a torquato taramelli presidente del consiglio direttivo di meteorologia e geodinamica, sulla estensione delle ricerche mareografiche nel territorio nazionale, ischia 17 giugno 1893, cc.1 r.v.-2 r. vol52,6,2009new 2-12-2009 12:40 pagina 725 726 g. ferrari (30) biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di giulio grablovitz a pietro tacchini, ischia 21 marzo 1896. (31) archivio giulio grablovitz, fondo g. grablovitz, copia di lettera di giulio grablovitz a pietro tacchini, 20 dicembre 1896. (32) «ho costruito testé un sismografo a pendoli orizzontali con masse alquanto pesanti, in cui gli attriti e le resistenze corrispondono ad 1/30000000 (dico un trentamilionesimo) della massa e con ciò credo di poter emulare i sistemi fotografici, tanto costosi; la corsa della carta e di 30 centimetri all’ora ed a conti fatti il costo è di 30 lire [...]. appena avrò ottenuto una bella registrazione gliene invierò la descrizione». translation: «i have just built a horizontal pendulum seismograph with rather heavy masses, in which the friction and the resistances correspond to 1/30000000 (that is to say, one thirty millionth) of the mass and with this i believe i can emulate the photographic systems, which are so costly; the run of the paper is 30 centimetres per hour and, ultimately, the cost is 30 lire [...]. as soon i have obtained a good recording i will send you the description of it.» biblioteca dell’ufficio centrale di ecologia agraria, fondo tacchini, lettera di giulio grablovitz a pietro tacchini, ischia 8 giugno 1896. (33) in the earliest bulletins of the observatory of ebro (tortosa) which adopted them in 1905 we can read, amongst other things, «costruidos en 1895 por vez primera con 5 kilogramos de masa, que en los modelos actuales asciende à 12 kilogramos, los péndulos del profesor g. grablovitz se distinguen por su sencillez y gran baratura [...] 90 liras las dos componentes con el cilindro registrador, provisto de su mecanismo para marcar el tiempo, costruido, bajo la direcciòn del inventor, por el mecànico del observatorio.» translation: «constructed in 1895 for the first time with 5 kg mass, which in the current models has risen to 12 kg, the pendulums of professor grablovitz are characterised by their simplicity and convenience [...] 90 lire the two components with the recording cylinder, equipped with its time-marking mechanism, built by the observatory’s mechanic, under the inventor’s supervision.». (34) «non debbo però dissimulare l’apprensione in cui mi mette la probabilità della soppressione dell’osservatorio del porto [d’ischia] ch’io per molte ragioni ritengo necessario alla compostezza dell’indirizzo intrapreso. all’epoca dell’impianto non era bene accertata l’influenza del vento e del mare ed anzi i moti sismici venivano attribuiti ad attività sismica. in oggi è provato che il colle della grande sentinella è grandemente soggetto a quelle influenze e gli strumenti delicatissimi che vi si trovano dimostrano questo fatto che io in base a diretta percezione dei sensi, mi rammento di aver accennato sul luogo alla s.v. ill.ma quando era in trattativa l’espropriazione del fondo. ciò non toglie importanza alla località che anzi è viemmaggiormente degna di studio, ma la stazione del porto è un utilissimo corollario adattatissimo alla registrazione dei terremoti lontani e dirò quasi se non esistesse sarebbe conveniente erigerla nell’interesse della scienza. ma di quest’argomento si potrà ragionare quando la s. v. ill.ma verrà sul posto e spero riconoscerà pure dal lato economico e l’opportunità delle mie ragioni.». translation: «however, i must not dissimulate the apprehension that the likelihood of the suppression of the observatory of the porto [d’ischia] causes me, an observatory that for many reasons i believe necessary for the coherence of the orientation undertaken. at the time of the installation the effect of the wind and the sea was not well ascertained, and indeed the seismic motion was attributed to seismic activity. today it is proven that the hill of the grande sentinella is greatly subject to those effects and the very delicate instruments that are to be found there demonstrate this fact that i, on the grounds of the direct perception of the senses, recall having mentioned in situ to your honourable self when the expropriation of the land was being negotiated. this does not reduce the importance of the location which is actually even more so worthy of study, but the station of the port is a very useful corollary highly suited to the recording of far-off earthquake and, i should say, if it did not exists, it would be convenient to erect it in the interests of science. but we may discuss this matter when you come to the site and i hope you too will recognise the economic advantages and the validity of my arguments». archivio grablovitz, fondo giulio grablovitz, minuta di lettera di giulio grablovitz a pietro tacchini, ischia 1 febbraio 1897. (35) letter from r.d. oldham, from horsham, sussex on 18th september 1914 to giulio grablovitz. «dear sir i have been reading with great interest your paper in the bollettino della società sismologica italiana, and regret to note that you seem to consider that i have done you an injustice in not recognizing your priority to the recognition of the three phase character of a distant seismogram. i certainly have overlooked the reference to your papers in which you had pointed out the three phases the seismogram previous to 1897, and most express my regret and apologies for having done so. in excuse i can only say that in 1897 i was still unable to read italian, except with great difficulty and slowness; my knowledge of the language still leaves me unable to appreciate niceties of meaning, and may be that i have partially misunderstood your paper, but it seems to me that you have not fully appreciated the claim which i made in my report on the earthquake of 1897. this was not merely that there are three recognisable phases in the seismogram, but that there was evidence of three distinct vol52,6,2009new 2-12-2009 12:40 pagina 726 references abe, k. (1994): instrumental magnitudes of historical earthquakes, 1892 to 1898, bull. seismol. soc. am., 84, 415-425. archivio grablovitz, fondo grablovitz, diario di giulio grablovitz (1846-1928) ms. batllò, j. (2009): influence of g. grablovitz in spain: instruments and scientific correspondence, this issue. davison, c. (1927): the founders of seismology, pp. 240, (london). de rossi, m.s. 1879-1881: la meteorologia endogena, 2 vols. ferrari, g., j. batllò and r. macià (2005): moment magnitude estimation of great late 19th century 727 giulio grablovitz (1846-1928) and his scientific activity based on personal and institutional correspondence forms of wave motion, which differed in character and travelled in different paths and at different speeds, arriving consequently, at different times. this was the really the important new conclusion which resulted from the investigation of the earthquake of 1897, and i believe i’m right in supposing that the discovery dates from that time. i am not aware of its having been announced at an earlier date, nor could it have been established previous to 1897 when for the first time sufficiency of evidence had accumulated. as regards the three phases of the seismogram there can be no doubt that you have fully established your claim to priority in their recognition. yours sincerely, r.d. oldham». (36) archivio giulio grablovitz, fondo g. grablovitz, lettere di bernardo paoloni a giulio grablovitz del 1911: 29 gennaio; 4, 15 e 24 marzo; 11, 19 e 27 aprile, 11 maggio; 1 e 22 giugno; 1 luglio. archivio giulio grablovitz, fondo g. grablovitz, lettere di giulio grablovitz a bernardo paoloni del 1911: 8 e 13 aprile; 13 maggio; 20 giugno e 7 luglio. earthquakes from the seismograms of geodynamic observatories in the ischia island, poster (abstract 552), (iaspei general assembly, santiago de chile (chile), 2-8 october 2005). grablovitz, g. (1913): sulle varie fasi dei sismogrammi, boll. della soc. sismol. ital., 17, 218-244. milne, j. (1900): british association for the adv. of science, report 1900, 65-67. oldham, r.d. (1899): report on the great earthquake of 12 june 1897, memoirs of the geological survey of india, 29, 135-174. oldham, r.d. (1900): on the propagation of earthquake motion to great distance, phil. trans. roy. soc., a., 194, 135-174. vol52,6,2009new 2-12-2009 12:40 pagina 727 n u m e r i caratteristici dei flocculi d ' i d r o g e n o e di calcio c dei filamenti d ' i d r o g e n o p e r l ' a n n o 1958 g . godoli proseguendo la pubblicazione annuale dei numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno ('), vengono riportati nelle seguenti tre tabelle di immediata interpretazione i valori giornalieri per il 1958. i numeri caratteristici sono stati stimati sugli spettroeliogrammi eseguiti alla torre solare di arcetri con l'immagine di 63 mm; le caratteristiche strumentali e la tecnica delle osservazioni sono state descritte in altra sede (2). durante tutto il 1958 sono stati eseguiti quotidianamente tre spettroeliogrammi successivi nella riga ea3 dell'idrogeno e tre spettroeliogrammi successivi nella riga k2i 3_ 2 del calcio ionizzato. l'esecuzione di spettroeliogrammi successivi ha lo scopo di mettere in evidenza le variazioni di intensità delle zone più attive. tali variazioni di intensità, interessantissime nello studio dell'evoluzione dei fenomeni, non hanno però alcuna influenza nella stima dei numeri caratteristici. per questo abbiamo continuato a stimare i numeri caratteristici su di un solo spettroeliogramma in lì«;l ed uno solo in k2 3> 2. gli spettroeliogrammi in ha3 esaminati erano generalmente i primi eseguiti. solo in qualche raro caso, quando il primo risultava difettoso, veniva esaminato il secondo od il, terzo spettroeliogramma. gli spettroeliogrammi in k2i 3> 2 esaminati erano quelli ridotti in sede di programma agi. conie è noto, l'osservatorio astrofisico di arcetri è centro mondiale c principale (p w d c c) per la raccolta ed elaborazione dei dati relativi ai flocculi di calcio (3). per questo, dal 1° gennaio 1957 l'attività dei flocculi di calcio viene seguita rilevando area e posizione dei singoli fenomeni. sebbene questo metodo di determinare l'attività dei flocculi sia più informativo della stima del numero caratteristico e sebbene il numero caratteristico sia in media una funzione nota dell'area (4), pure abbiamo anche continuato a stimare i numeri caratteristici dei flocculi 19s g. g 0 d 0 l i t a b e l l a i n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d ' i d r o g e n o p e r l ' a n n o 1958 <d ® o fi ro a ® o giorno o "5 d ci o o h & <£> o n jh <d • ih sh ph o se c3 o tì bo •ph o co o bc u & a © ® co ® ii r<2 o +3 a ® t> o o fi ro a ® o & uh § <1 s j <1 u & a © ® co o £ s 1 1.6 1.8 2.2 1.8 2.0 2 2.1 — — — 2.0 1.9 1.9 1.4 — 1.4* 2.1* 3 2.0 1.4 — 1.9 2.4 1.8 — 1.5 1.9 — 2.1* 4 — 1.9 2.0 — — — . 2.1 1.8 1.5 2.0 1.6* 1.8 5 — . — 2.0 — 2.2 — — 2.0 1.6 — 1.5 1.9 6 — 2.3 2.0 — — _ — 2.0 1.5 2.1* 1.5* 1.8 7 — — — — — 2.2 2.6 — — — — 1.9* 8 2.3 — 1.8 1.8 — 2.6 1.6 1.6 1.9 0.9 1.8* 9 2.5 . — — . — 1.9 3.2 2.4 1.6 1.6 1.8 0.9* — 10 2.6 .—. — 1.7 1.9 — 2.3 — • — — — • — • 11 — — — — — — 2.3 2.2 1.7 1.8 — 2.2 12 — 2.3 — — 1.8 . — 2.0 1.9 1.8 — — 2.2 13 — 2.3 — . — 2.0 — — — 1.6 — — — 14 — — — 1.4 1.9 — 1.9 2.0 — 1.7 0.9* — 15 2.5 — 1.3 — — — 1.9 — 1.8 1.9 0.9 — 16 2.3 — — — — 1.6 1.9 2.1 1.9 — 1.0* 1.6* 17 2.6 — — — — — 1.9 — 2.5 1.8 1.0 — 18 2.2 1.9 1.8 2.3 1.7 1.0 1.9* 19 — .—. — 1.8 1.8 — . 1.9 1.8 2.2 — 1.0 2.0* 20 — 1.8 — — . 1.9 — — 1.8 2.1 — 1.2 — 21 1.6 1.9 — 2.2 2.0 • — 1.8 • — — 1.8 1.4* — 22 1.8 — 1.0 — — — — 1.9 2.2 2.0 1.8* 2.1 23 1.9 .—. — — — — — 1.8 1.8 1.8* — — 24 2.2 — . — 2.6 2.1 — — — 2.0 1.7 2.2* 2.0* 25 2.3 — — 2.4 . — 2.0 2.0 2.0 1.9 1.7 — 2.3* 26 — — . — 2.2 2.2 — 2.1 2.4 2.1 1.4* — 2.2* 27 1.5 1.9 . — .—2.2 . — — 2.3 — 1.3 2.8* 2.0* 28 1.4 — — 2.2 2.1 — 2.0 2.1 — 1.4 2.6* — 29 1.5 — — 1.8 2.2 — — 2.0 2.0 1.6 2.0 — 30 1.4 — — 2.0 — . — 2.2 — ' • — 1.6* 2.3* 2.0 31 1.5 ~ 2.3 2.1 1.6* media 2.0 2.0 1.6 2.0 2.0 2.3 2.1 2.0 1.8 1.7 1.5 2.0 media semestrale 1 99 1.85 media 90 amili ale x . 90 n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d ' i d r o g e n o e d i c a l c i o , ecc. 199 t a b e l l a 2 n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d i c a l c i o p e r l ' a n n o 1 9 5 8 giorno g en n ai o f eb b ra io m a rz o ® ft m a gg io g iu gn o l u g li o a g o st o s et te m b re o tt o b re j n o ve m b re d ic em b re 1 2 3 4 5 6 2.9 2.2 3.2 2.7 2.3 2.5 2.5 2.4 — 2.9 3.0 3.1 2.8 3.3 3.2 3.0 3.2 3.1 4.3 4.2 3.8 3.9 4.0 3.2 3.1 3.2 3.3 3.1 2.7 2.8 2.6 3.5 3.5 3.4 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 3.1 3.1 3.3 3.1 2.9 3.0 2.9 2.8 2.6 2.6 2.5 2.4 2.4 2.5 3.2 3.1 2.4 2.1 2.3 2.8 3.2 2.2 2.5 2.6 2.7 3.2 3.1 3.4 3.4 3.2 2.8 2.9 2.6 2.7 2.8 2.6 2.3 2.1 2.3 2.8 3.3 3.2 3.3 3.2 3.1 3.1 2.5 3.3 d.b 3.5 3.5 3.5 3.4 3.0 2.9 3.0 3.1 3.2 3.2 3.2 3.2 3.4 3.8 4.3 4.3 3.7 3.5 3.5 3.4 3.0 3.1 2.9 2.8 3.0 3.4 3.7 3.8 3.8 3.8 3.4 3.2 3.4 3.3 3.3 3.3 3.1 3.2 3.0 2.9 3.0 3.2 3.1 3.0 2.9 2.8 2.6 2.7 2.5 2.9 2.8 2.8 2.8 2.7 2.7 2.4 2.3 2.1 2.2 2.5 2.2 2.6 2.5 2.6 3.0 3.0 3.0 3.6 3.5 media 2.8 2.7 2.5 2.9 2.8 3.1 3.4 3.6 3.1 2.6 2.9 3.5 media semestrale 2.82 3.16 media annuale 3.02 2 0 0 g. g o d o l i t a b e l l a 3 n u m e r i c a r a t t e r i s t i c i d e i f i l a m e n t i d ' i d r o g e n o p e r l ' a n n o 1958 cd ® © giorno o '5 a c5 o '3 u ® o kj m ci & < o fct c3 s o pj bc h s éo te p o rù c te ® +3 -p ® ce ® ch "o o £ ® > o £ u 6 o p 1 1.6 2.3 2.2 2.3 1.8 2 2.8 — — — 2.0 — 2.4 1.7 2.3 1.6* 2.3* 3 2.7 — 2.5 — 1.4 2.0 2.3 — 2.4 2.0 2.3* 4 — 2.2 3.2 — — — 2.2 1.8 2.4 2.2 2.3* 2.4 5 — — 3.0 — 1.9 — — 1.9 2.4 — 1.8 2.3 6 — 2.4 2.8 — — — — • 2.2 2.4 2.5* 2.2* 2.2 7 — — ; — — 1.4 2.2 — — — — 2.5* 8 9 10 2.6 2.7 2.8 — — 2.3 2.4 2.2 2.7 2.8 1.6 2.1 2.3 2.1 2.6 2.3 2.2 2.4 2.3 2.2 1.6 2.4* 2.5* 11 12 13 — 3.1 2.9 — — 2.3 1.9 — 2.2 2.3 2.5 2.4 2.4 2.0 2.1 2.3 2.4 2.6 14 — — 2.8 2.0 — 1.1 2.2 2.2 2.0* — 15 2.4 — 2.4 — — — 1.4 — 1.8 2.3 1.7 — 16 2.4 — — — — 2.4 1.5 2.2 1.7 — 2.0* 1.9* 17 2.5 — — — — — 1.7 — 1.8 1.2 1.9 18 2.4 — — — — — 2.2 1.9 2.3 1.1 2.2 2.6* 19 20 21 2.1 2.3 2.4 — 2.5 2.6 1.9 1.4 2.4 — 2.2 2.6 2.0 2.3 2.2 2.5 1.2 2.1 2.3 2.6* 2.9* 22 23 2.8 2.6 1.0 — — — — 2.4 2.6 3.2 3.1 1.4 1.8* 2.5* 2.3 24 25 2.2 2.3 — 2.6 2.5 2.8 2.3 2.7 2.5 2.9 2.6 1.4 1.4 2.0* 2.4* 2.5* 26 27 28 29 30 31 2.3 2.4 2.5 2.6 1.9 2.5 — 2.4 2.4 2.4 2.4 3.0 2.7 2.5 2.2 2.0 — 2.8 1.8 1.9 1.7 2.5 2.8 3.0 2.5 2.7 2.0 1.4* 1.2 1.3 1.4 1.5* 1.6* 2.0* 2.0* 2.0 2.0* 2.6* 2.5* 2.6 media 2.5 2.4 2.5 2.5 2.2 1.9 2.1 2.3 2.4 1.7 2.1 2.4 media semestrale 2.36 2.15 media annuale z.z3 n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d ' i d r o g e n o e d i c a l c i o , ecc. 2 0 1 di calcio: sembra infatti che in fasi peculiari del ciclo solare, i due diversi metodi diano informazioni di significato diverso (5). dall'ottobre 1958 è stato iniziato ad arcetri un programma di osservazione fotografica sistematica al monocromatore halle in ha che già era in funzione per l'osservazione visuale dall'agosto 1957 (6). vengono giornalmente eseguiti diversi fotogrammi formato 24 x 36 mm ad intervalli di 10-15 minuti. se vengono osservati visualmente fenomeni a rapida evoluzione quali brillamenti o protuberanze eruttive, la frequenza dei fotogrammi viene aumentata adeguandola alla variabilità del fenomeno. l'osservazione fotografica al monocromatore è molto più rapida che allo spettroeliografo cosicché in molti giorni misti in cui non è possibile eseguire spettroeliogrammi alla torre solare, è invece possibile effettuare filtrogrammi al monocromatore. un filtrogramma per ognuno di tali giorni è stato ingrandito a 63 mm di diametro ed esaminato per la determinazione dei numeri caratteristici dei flocculi e filamenti in ha3. i numeri caratteristici così dedotti sono individuati in tabella 1 con un asterisco. sono stati esaminati complessivamente 165 spettroeliogrammi e 28 filtrogrammi in ha3 e 152 spettroeliogrammi in x2,3,2l a stima veniva eseguita, come per il passato, attenendosi ai modelli distribuiti dall'osservatorio di mt. wilson. per evidenti ragioni di omogeneità e costanza di giudizio, è stata determinata prima tutta la serie dei numeri caratteristici per i flocculi d'idrogeno; quindi tutta la serie [ter i flocculi di calcio ed infine tutta la serie per i filamenti d'idrogeno. l e stime sono state eseguite per due volte seguendo un diverso ordine di esame. l o scarto delle singole determinazioni si è sempre mantenuto inferiore a 0.3 unità. nei pochi casi in cui veniva riscontrato uno scarto di maggior entità, un riesame degli spettroeliogrammi metteva hi evidenza errori di stima. i l contributo dei singoli osservatori alla raccolta delle lastre ridotte è stato il seguente: osservatori sa3 ^2, 3, 2 totale m. c. ballario . . 110 104 214 g. godoli . . . . 4 4 8 b. romagnoli . . 51 44 95 2 0 2 g. g o d o l i i l contributo dei singoli osservatori alla raccolta dei fotogrammi ridotti è stato invece il seguente: osservatori g. godoli . . . . . 2 a. lepri . . . . . 20 g. noci . . . . . . 6 l a distribuzione mensile degli spettroeliogrammi e filtrogrammi ridotti è la seguente: mese a 1 2 3 4 5 6 7 8 9 10 11 12 s. e. g. ila, h fot. 19 8 7 12 19 5 21 21 22 16 5 8 12 7 10 k 2 , 3 , 2 15 6 6 11 19 6 20 20 20 17 8 4 riassunto si danno i numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per vanno 1958, secondo le osservazioni eseguite alla torre solare ed al monocromatore halle dell'osservatorio di arcetri. abstract the character figures of ca brighi flocculi and ha brighi and darle flocculi for the 1958 are given according to the observations carried out at the solar tower and ha filter of the arcetri observatory. n u m e r i c a r a t t e r i s t i c i d e i f t o c c u l i d ' i d r o g e n o e d i c a l c i o , ecc. 2 0 3 b i b l i o g r a f i a f1) g o d o l i , g . , numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per gli anni 1950-51-52, « ann. geof. », 6, 199 (1953). — idem per l'anno 1953, 1. c. 7, 135, (1954). — idem per l'anno 1954, 1. e. 8, 325, (1955). — idem per l'anno 1955, 1. c. 9, 299, (1956). — idem per l'anno 1956, 1. e. 10, 133, (1957). — idem per l'anno 1957, 1. e. 11, 235, (1957). ( 2 ) r i g h i n i , g . g o d o l i , g . , riduzione del materiale spettroeliografico raccolto alla torre solare di arcetri nel periodo 1932-1949, « mem. sait », 21, 333 (1949). ( 3 ) g o d o l i , g . , relazione preliminare sull'attività del « principal world data center agi » ad arcetri, « la ricerca scientifica », in corso di stampa. ( 4 ) r i g h i n i , g . g o d o l i , g . , sui numeri caratteristici dell'attività solare, « a n n . geof.», 3, 501, (1950). — — the physical meaning of the character figures of solar phenomena, « j. of geophys. res. », 55, 415, (1950). ( 6 ) g o d o l i , g . , attività dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per il periodo 1° luglio 1957-30 giugno 1958 « boll, di geod. e scienze affini», 17, 487, (1958). ( 6 ) r i g u t t i , m . , il filtro interferenziale dell' osservatorio astrofisico di arcetri, « la ricerca scientifica », in corso di stampa. layout 6 characteristics of flux-time profiles, temporal evolution, and spatial distribution of radiation-belt electron precipitation bursts in the upper ionosphere before great and giant earthquakes georgios c. anagnostopoulos1, efthymios vassiliadis1, sergey pulinets2,3 1 space research laboratory, democritus university of thrace, xanthi, greece 2 fiodorov institute of applied geophysics, moscow, russia 3 space research institute, russian academy of sciences, moscow, russia annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5365 abstract the analysis of energetic electron observations made by the demeter satellite reveals that radiation belt electron precipitation (rbep) bursts are observed in general several (~1-6 days) before a large (m > 6.5) earthquake (eq) in the presence of broad band (~1-20 khz) vlf waves. the ebs show in general a relative peak-to-background flux increase usually < 100, they have a time duration of ~0.5 – 3 min, and their energy spectrum reach up to energies <~500 kev. the rbep activity is observed as one, two or three ebs throughout a semi-orbit, depended on the magnetic field structure above the eq epicenter. a statistical analysis has been made for earthquakes in japan, which reveals a standard temporal variation of the number of ebs, which begins with an incremental rate several days before major earthquakes, and after a maximum, decreases so that the electron precipitation ceases above the epicenter. some earthquake induced ebs were observed not only in the nightside ionosphere, but also in the dayside ionosphere. 1. introduction 1.1 electromagnetic methods of earthquake precursory signals research over the last few decades, significant efforts have been made to detect and interpret electromagnetic phenomena that are related to seismic activity. due to these efforts, characteristic physical changes have been confirmed as precursor signals of earthquake preparation. to this end, several methodologies have been developed that use groundbased instrumentation to detect electromagnetic variations that occur in the lithosphere [kopytsenko et al. 1990, hayakawa et al. 1996a, b, varotsos et al. 1998, hattori 2004, eftaxias et al. 2007]. other studies have shown that before strong earthquakes, there are characteristic electromagnetic interactions in the ionosphere that are observed as plasma variations or electromagnetic (ultra-low frequency – ulf, very-low frequency –vlf) emissions, detected by either ground-based [hayakawa et al. 1996a, b, pulinets and boyarchuk 2004; hattori 2004] or space-based [boskova et al. 1994, pulinets and legen’ka 2003, parrot 2006, ouzounov et al. 2011, pulinets and ouzounov 2011] instrumentation. for instance, rozhnoi et al. [2007] recently studied earthquakes in japan by investigating the ionosphere using vlf broadcasting between a transmitter in japan and a receiver in russia. in addition to the electromagnetic phenomena that can take place in the lithosphere and the ionosphere, electromagnetic processes related to seismic activity affect the trapped population of the radiation (van allen) belts, and several studies have reported satellite measurements that have suggested that there are radiation belt energetic particle flux variations accompanied by vlf electric field activity before earthquakes [ginzburg et al. 1994, galper et al. 1995, pulinets and boyarchuk 2004, and references therein; sgrigna et al. 2005, aleksandrin et al. 2003, fidani et al. 2010, anagnostopoulos and rigas 2009, anagnostopoulos et al., 2010, sidiropoulos et al. 2011]. 1.2 physical and anthropogenic sources that trigger radiationbelt electron precipitation the precipitation of energetic electrons from the van allen belts (radiation-belt electron precipitation; rbep) in the upper ionosphere at middle and low latitudes has been a well-known phenomenon for many years [paulikas 1975, vampola 1983, kuznetsov and myagkova 2002]. the major sources of this process were considered to be the powerful military vlf transmitters [vampola 1983, abel and thorne 1998, green and fung 2005] and natural processes, such as special issue: earthquake precursors 21 article history received august 11, 2011; accepted january 22, 2012. subject classification: electromagnetic earthquake precursors, ionospheric precursors of earthquakes, radiation belt electron precipitation, lithosphere-ionosphere coupling, wave particle interaction. the earthquakes [ginzburg et al. 1994, pulinets and boyarchuk 2004, and references therein] and lightening [inan et al. 2007]. it has been hypothesized that cyclotron-resonant interactions of trapped electrons via scattering by lighteninggenerated whistler waves is a significant process in the loss of electrons from the radiation belts. the most recent concepts concerning cyclotron resonance interactions of vlf emissions with the particles of radiation belts can be found in trakhtengerts and rycroft [2008]. on the other hand, many studies have indicated the powerful military vlf transmitters as the main physical process that causes electron precipitation at middle latitudes. for instance, based on theoretical calculations, abel and thorn [1998] arrived at the hypothesis that manmade transmissions dominate the losses in the inner radiation belt, and vampola [1983] claimed that, «if satellites replace the vlf transmitters for both communication and navigation, we may have a chance to observe the slot (between the inner-outer belt) refill and remain filled». vampola [1983] also suggested that, «we might have to continue to radiate vlf waves at high power levels in order to protect low altitude satellites». we would indicate that this belief is acceptable to a significant part of the scientific community, although we are not aware of any statistical scientific study up to the year 2010 that has supported this hypothesis. a possible reason for this belief is the traditional interface between ionospheric/plasma physics and radio communications, whereas earthquake-induced radiationbelt electron precipitation as another candidate source of rbep is a recently developed research field. to estimate the relative contributions of the two sources, as the great earthquakes and the ground-based transmitters, a recent study presented the first statistical analysis of rbep burst characteristics [sidiropoulos et al. 2011]. first, sidiropoulos et al. [2011] identified two types of electron bursts (ebs) on the basis of the different forms of the related electric field spectrum: vlf transmitter-induced ebs are related to the presence of a narrow band emission that is centered at the transmitter radiating frequency emission, whereas the earthquake-related ebs are related to the presence of broadband emissions (from 2 khz to >~20 khz). secondly, sidiropoulos et al. [2011] demonstrated that although the north west cape (nwc) transmitter (in western australia) is the most powerful military transmitter (1 mw), in that it produces the strongest electron precipitation effects compared with all of the other transmitters over the globe, nwc-induced ebs are observed during only ~2.1% of the detection of electromagnetic emissions transmitted from earthquake regions (demeter) satellite passes above the transmitter-influenced region (25˚-30˚s, 114˚-167˚e), with the nwc-associated ebs (including the conjugate ebs) as low as <~1.5% of the whole number of ebs. sidiropoulos et al. (2001) also provided clear evidence opposing the hypothesis that man-made transmissions dominate the losses in the inner radiation belt. they showed that more ebs were observed when the nwc transmitter was off (449) than during a similar ~6-month interval when the transmitter was on (374); on the contrary, in agreement with the assumption that seismic activity provides the most intense contribution to rbep events, a higher number of great earthquakes were found in the first period (14 earthquakes) than in the second period (5 earthquakes). based on previous statistical analyses, sidiropoulos et al. (2011) suggested that seismic activity dominates the electromagnetic interactions that produce the ~70 kev to 500 kev rbep events that last for ~2 min to 3 min at middle latitudes; therefore seismic activity dominates the losses in the inner radiation belt. in this report, we attempt to provide an advance in the study of rbep events observed before great earthquakes (m ≥6.5). to this end, we classify the flux-time profiles of rbep events in some distinct forms at characteristic seismogenic regions all over the globe by investigating the demeter satellite observations before recent large earthquakes (honshu, japan, 2005; sumatra, indonesia, 2005; andravida, greece; south-easern europe, 2008; haiti, central america, 2010; and chile, south-eastern america, 2010). in addition, we demonstrate the existence of a standard temporal variation in the number of rbep events before the occurrence of great earthquakes (m >6.7) in japan, based on a statistical analysis. furthermore, the detailed analysis of the combined temporalspatial distribution of the rbep events for one of the japan earthquakes (august 16, 2005) included in the statistical sample of a previous study reveals that at around the time of the maximum eb rate, the eb spatial distribution showed a broad dispersion in longitudes, and then the rbep event coordinates were reduced around the region of the earthquake epicenter. a few hours before the earthquake occurrence, a silence in the rbep in the upper ionosphere above the region of the epicenter was recorded. 2. the demeter mission and instrumentation in this report, we present the results from an analysis of the ebs observed by the demeter satellite (http://demeter.cnrs-orleans.fr/) [parrot 2006]. the demeter satellite was designed and built specifically for the research of earthquake precursory signals, and it was launched on june 29, 2004, into a sun-synchronous, almost polar (inclination, 98.3˚), orbit, at ~710 km in altitude, where it performed 16 orbits per day. its orbit was lowered to 660 km in altitude in 2005, and it stopped operation in december 2010. the primary scientific objective of demeter was the detection and characterization of ionospheric and radiationbelt disturbances related to seismic or human activities. 22 anagnostopoulos et al. 23 consequently, the scientific payload of the satellite was composed of several types of sensors that measure waves and particles, which included an instrument for the detection of particles (idp), and an electric field instrument (instrument champ electrique; ice). data from these two instruments were used in the present study and are presented in the next section. the idp electron spectrometer [sauvaud et al. 2006] covers all energies from 0.07 mev to 2.5 mev, across 256 energy bands. in the present study, the idp electron intensities were averaged over three energy bands: 72 kev to 526 kev (band 1), 526 kev to 971 kev (band 2) and 971 kev to 2350 kev (band 3). the ice measurements were made over a wide frequency range of electromagnetic and/or electrostatic waves from direct current (dc) to 3.175 mhz, subdivided into four frequency channels dc/ulf, extremely low frequency (elf), vlf and high frequency (hf). here we use the measurements from the vlf channel in the frequency band of 0 khz to 20 khz. for the idp, there were two modes of operation, the ‘burst’ and ‘survey’ modes. the burst mode had a time resolution of 1 s and it was applied automatically when the satellite was over-flying areas that had been identified as seismically active. when demeter moved away from these seismically active regions, it returned to the survey mode, with a time resolution of 4 s.it is well known that within the van allen belts electrons and ions follow spiral trajectories around the field lines of the earth magnetic field, while protons and electrons drift perpendicular to the magnetic field in the opposite direction. at the same time, ions and electrons moving towards higher latitudes are reflected at sites of strong magnetic field and are trapped within the inner and the outer van allen belts. the ebs associated with an earthquake at low and middle geographic latitudes manifest radiation electron precipitation, and this process has been considered as one of the links in a long chain of electromagnetic processes in the lithosphere-atmosphereionosphere-plasmasphere-magnetosphere coupling [pulinets and boyarchuk 2004, anagnostopoulos and papandreou 2012]. 3. observations 3.1 flux-time profiles of radiation-belt electron bursts we examined anomalous energetic ebs detected by the idp experiment, accompanied by the vlf electric field activity from the ice instrument that was on board demeter, for the whole period of the satellite life from 2004 to 2010. a thorough investigation of idp electron observations for a time interval of 30 days before and 10 days after a large number (>30) of great earthquakes all over the globe reveals a variety of flux-time profiles of radiation belt ebs that can be categorized into certain well-defined forms. we will classify the flux-time profiles of pre-seismic rbep events into some distinct categories. figure 1 presents the observations of the demeter satellite during the upward semi-orbit numbered 5952_1, about half a day before the earthquake of august 16, 2005, in eastern honshu, japan. figure 1 shows, from top to bottom, (a) the dynamic spectrogram of the electric field in the frequency range 0 khz to 20 khz, (b) the intensity of the electrons from the idp experiment in the energy bands 72 kev to 526 kev (band 1), 526 kev to 971 kev (band 2) and 971 kev to 2350 kev (band 3), and (c) semi-orbit 5952_1 projected onto an earth map. the universal and local time, as well as the longitude and the latitude of the position of demeter during the time period examined in figure 1, are noted below panel (b). as we said in the previous section, the idp instrument operated in two modes: the burst and survey modes. the burst mode was applied automatically when the satellite was over-flying areas that are shown in red on the map (figure 1c); when demeter moved away from these seismically active regions, it returned to the survey mode. during the demeter semi-orbit, enhanced electron fluxes are evident on the left as well as on the right edge of figure 1b. two pairs of large-intensity enhancements at the south and north latitudes correspond to the nearest edges of the earth outer and inner radiation electron belts, in the south and north hemispheres, respectively. large flux enhancements at high south (~43˚-54˚s) and north (~61˚-71˚n) latitudes are evident in all of the three electron bands, while the inner enhancements at middle south (~32˚-39˚s) and north (~52˚57˚n) latitudes are evident only in band 1 and band 2. the pairs of flux enhancements at south and north latitudes were very often observed by idp during the demeter semi-orbits from south to north, depending on the longitude. the short sudden flux increases at ~12:48 (~29˚s) ut and ~13:09 ut (~47˚n), which were superimposed on the low-energy (band 1) flux-time profile (figure 1b, purple arrows) that was observed as long as the intensity of the inner large-flux enhancement decreased towards the equator (figure 1b, near the middle of the plot), lasted for ~1 min and ~2 min (including the two peaks in the second case), and they are examples of the rbep events observed earlier by demeter for several days before the strong earthquake in eastern honshu. these two short-lived peaks fall into the set of ebs identified in the studies by anagnostopoulos et al. [2010] and sidiropoulos et al. [2011], as precursor signals of the earthquake that occurred at 02:46:31 ut, on august 16, 2005, near the east coast of honshu, japan. the eb at ~13:09 ut on august 15, 2005, was observed during an anodic (night-time) semi-orbit that passed westwards of the future japan earthquake epicenter (44.5˚n, 125.6˚e). as the energetic electrons travel along the magnetic field lines, they can obviously produce a conjugate eb in the electron precipitation before earthquakes southern hemisphere that is most probably the eb seen at ~12:48 ut, and its presence was already noted above. the position of the conjugate eb (147˚e, 29˚s) appears above eastern australia. in the electric field spectrogram of figure 1a, a strong vlf signal can be seen that was observed almost simultaneously with the eb near the earthquake epicenter. this signal is dispersive and extends in frequencies all over the scale of figure 1a, ranging from 0 khz to 20 khz. on the contrary, the eb observed above australia is not related to a strong vlf emission. this differentiation in the strength of the electric field is self-consistent with the suggestion above: that the eb in the south should be the conjugate (not the original) rbep event formed in the south hemisphere. we also note that a narrow-band signal in the spectrogram of figure 1 is also present around the nwc transmitter radiating frequency of 19.8 khz over a large portion of the time examined in figure 1, while the near japan eb is very well correlated with the time of the intense broadband vlf emission. we believe that the time coincidence of the occurrence of the eb with the intense broadband vlf emission confirms that in this case the earthquake in japan had the main role in the production of the electron precipitation at ~13:09 ut on august 15, 2005. examination of the energy spectra of the anomalous strong eb observed near the earthquake epicenter at ~13:09 shows a type with a hardening toward the equator. the spectrum of this eb extends up to ~500 kev. the eb observed at the conjugate region is a little less intense and lasts for a shorter anagnostopoulos et al. 24 figure 1. observations from the demeter satellite during an upward semi-orbit, about half a day before the earthquake of august 16, 2005, in eastern honsu, japan. (a) spectrogram of the electric field in the frequency range 0 khz to 20 khz. (b) the intensity of the electrons from the idp experiment in the energy regions of 72 kev to 526 kev, 526 kev to 971 kev, 971 kev to 2350 kev. (c) the demeter orbit projected on an earth map. an electron intensity peak at ~12:48 (~29˚s) ut in (b) is well correlated with a strong broadband vlf signal observed near the earthquake epicenter. 25 time, and its spectrum extends up to lower energies (~430 kev). in general, our investigations suggest that the ebs preceding earthquakes that occur at middle latitudes show similar spectra that usually extend up to 200 kev to 500 kev. figures 2-5 are constructed in the same format as figure 1, and they indicate the characteristic rbep events observed before some other famous strong earthquakes that occurred in various seismogenic regions around the globe. figure 2 presents the demeter data obtained one day before the famous recent giant earthquake in chile (m = 8.8; february 27, 2010, 287.1˚e, 36˚s) near the epicenter longitudes. zhang et al. [2010] reported that demeter detected an enhancement of counting rates of low-energy charged particles in the 90.7 kev to ~600 kev region 13 days before the occurrence of the earthquake, during orbits within a range of 10˚ and 0.1 mc-ilwain parameter value l across the epicenter region (287.1e, l = 1.32). zhang et al. [2010] inferred that the bursts examined were likely to be related to the chile earthquake and can be taken as the precursor of this earthquake. zhang et al. [2010] studied charged particle flux increases only during upward demeter semi-orbits, which correspond to night observations. as indicated in the introduction, a major (or the main) source of the electron precipitation observed at middle latitudes has been previously assumed to be ground-based transmitters. ground-based transmitter-induced rbep events are observed only for night-time semi-orbits, almost certainly due to the much lower ionospheric absorption of electron precipitation before earthquakes figure 2. observations from the demeter satellite during an upward semi-orbit, for the day before the giant earthquake of february 27, 2010, in chile (m = 8.8, 287.1˚e, 36˚s). details as for legend to figure 1. two ebs were observed in the dayside upper ionosphere, with one above the future epicenter and a conjugate one in the north hemisphere. the emitting radiation through the night-time ionosphere. to further check the origin of the rbep events observed near the chile earthquake, we wanted to examine the presence of energetic electron activity at local day times, when the vlf radiation emitted from the ground cannot produce strong rbep events [sauvaud et al. 2008]. for this reason, figure 2 shows data obtained during the downward (daytime) demeter semi-orbit 30264_0. the electric field spectrogram shows a distinct difference in comparison to the night-time spectrogram of figure 1. here, the electric field spectrum was quieter; only some low vlf electric field energy was present, and no characteristic ground-based emissions (horizontal signals at a certain radiating frequency) can be seen in figure 1. however, a strong rbep burst with a peak-to-background flux increase as large as p/b c 20 can be seen centered at ~16:01 ut, with a duration time of ~1 min and accompanied by some light broadband vlf activity (figure 2a, normal lines) during the passage of the demeter satellite near the chile earthquake epicenter region. in addition, a distinct peak of lower intensity at north latitudes is evident, which is most probably the conjugate earthquake-induced rbep event. we point out that such an electron burst with a peakto-background flux increase as large as p/b c 20, as detected by demeter before the giant earthquake in chile in the daytime upper ionosphere, has not been reported in the scientific literature as being caused by ground-based emitted vlf radiation (not observed by the satellite), as far as we know. therefore, we infer that the eb of february 26, 2010, almost certainly appeared due to the preparation processes of the february 27, 2010, giant earthquake that occurred in chile. anagnostopoulos et al. 26 figure 3. observations from the demeter satellite on june 5, 2008, about three days before the occurrence of an earthquake in andravida, greece. the electron enhanced activity recorded at a geographic latitude (37.93˚n) similar to that of august 16, 2005, earthquake in japan (38.3˚n), although instead of one peak superimposed on the (high) flux background in japan, a sequence of electron peaks are evident here (b) under a very low background flux level. the eb is well related in time to the presence of a broadband (~2-19 khz) vlf activity (a). 27 in the previous paragraphs, we investigated the form of the rbep bursts observed before earthquakes in eastern asia (japan) and south-eastern america (chile). in both of these cases, a pair of anomalous ebs were observed during each demeter semi-orbit, one near the earthquake epicenter, and the conjugate one in the other hemisphere (north or south). in the next paragraphs, we examine the flux-time profiles in another major seismogenic region of the globe: south-eastern europe. we investigated the electron observations during the most important earthquakes that occurred in the greek territory during the life of demeter (2004-2010): (a) june 8, 2008, m = 6.5, andravida, 37.93˚n/21.41˚e; (b) february 4, 2008, m = 6.7, methoni, 36.22˚�/21.80˚e; and (c) january 8, 2008, m = 6.8, kythira, 36˚n/23˚e. from this investigation, we selected and present in figure 3 a representative example of the flux-time profile of the rbep events observed by demeter above greece throughout several days before each earthquake. in figure 3, after the high fluxes in the first part of upward semi-orbit 20955_1 (figure 3a, b), which were observed in a region influenced by the south atlantic anomaly (saa), demeter passed near the epicenter of the earthquake that occurred in andravida on june 8, 2008 (37.93˚n/21.41˚e) and recorded a low flux background (jg c 2 e/cm 2 s sr kev). this low electron flux background is representative of the fluxes observed by demeter in the greek territory, in contrast to the normal high flux background observed above japan as, for instance, in the case of the japan-earthquake-induced eb shown in figure 1 (jj c 500 e/cm2 s sr kev). the observations were made on june 5, 2008, about three days before the occurrence of the earthquake. electron precipitation before earthquakes figure 4. observations from the demeter saellite on january 12, 2010, about 3 days before the haiti earthquake occurred at 21:53 on january 12, 2010 (18.46˚n/287.47˚e). a pair of short-lived peaks were observed that were separated by a time interval of ~1 min, and were accompanied vlf (~2-10 khz) wave activity. although the earthquake in andravida occurred at a geographic latitude (37.93˚n) similar to that of the august 16, 2005, earthquake in japan (38.3˚n), an anomalous electron activity ‘above’ andravida was observed that was very different to that above japan. here, beside the much lower intensity background (with a flux ratio as low as 2/500), instead of one peak superimposed on the (high) flux background, a sequence of (~5-6) electron peaks are evident, which lasted for a total of ~2 min, with a peak-tobackground flux increase p/b ranging between ~10 and ~20. furthermore, the near andravida eb is well related in time with the presence of a broadband (~2-19 khz) vlf activity (figure 3a). a conjugate burst in a south region cannot be identified in this case, obviously due to the very high flux background (in particular at low energies; band 1). a similar electron flux-time profile to that of the andravida 2008 earthquake was observed on january 12, 2010, about 3 days before the haiti earthquake that occurred at 21:53 on january 12, 2010 (18.46˚n/287.47˚e), and which is presented in figure 4. in this case, when the earthquake occurred at low latitudes, a pair of short-lived peaks were observed that were separated by a time interval of ~1 min, while the electric field spectrum extended in a moderately broad band of frequencies (~2-10 khz), compared to the more broad spectra seen in figures 1 (japan) and 2 (chile). the rbep event observed above chile concerns a unique time interval and a corresponding ionospheric region without a record of a conjugate eb. finally, figure 5 shows the data relative to another well-known giant earthquake (m = 8.8) that occurred in sumatra (2.09˚n, 97.11˚e) on march 28, 2005. for this earthquake, several studies reported variations in electromagnetic field and plasma parameters in the ionosphere [liu et al. 2011]. electron precipitation events anagnostopoulos et al. 28 figure 5. observations from the demeter satellite related to the well-known giant (m = 8.8) earthquake that occurred in sumatra (2.09˚n, 97.11˚e) on march 28, 2005. the observations reveal a flux-time profile with a sequence of short-lived intensity peaks observed above the region of the epicenter and accompanied by vlf wave activity in the range ~3 khz to >20 khz, as well as a pair of ebs, with one in the north and the other symmetric one in the south. 29 were also observed in the ionosphere by demeter, well before (at least 2 weeks) the occurrence of the earthquake (data not shown). the case of the 2005 sumatra earthquake is important from the point of view of the flux-time profile form, because the observations in this case reveal a new type compared to the flux-time profiles seen in the previous cases (figures 1-4). a sequence of short-lived-intensity peaks was observed above the region of the epicenter (~16:26:30-16:27:45 ut), which was accompanied by vlf wave activity in the range of ~3 khz to >20 khz, as well as a pair of ebs, with one burst in the north (16:16-16:17 ut) and another symmetric to the previous one in the south (centered at ~16:38 ut). 3.2 statistical results on the temporal evolution of radiation belt electron precipitation events before the earthquakes in japan spatial redistribution one of the most important results of the present study is the characteristic pattern of the temporal variations of the rbep events we found before the occurrence of earthquakes in japan. to check this pattern, we performed a statistical analysis and the results are presented next in this section. figure 6 (bottom panel) shows the daily number ndg of the low energy band 1 ebs observed by demeter for the time interval of august 2-20, 2005 in the longitude range 107˚177˚; i.e. 35˚ eastwards and westwards of the earthquake epicenter (142˚e). a peak in the ndg 8 days before the earthquake is evident. about 15 days before the earthquake occurrence, the nd started to increase, and it showed a minimum on the day of the earthquake, august 16 (figure 6, short red bar). figure 6 (top panel) shows the daily number ndl of ebs observed by demeter all over the globe for the same time interval as for figure 6 (bottom panel) (august 220, 2008). we see that the distribution pattern of ndg all over the globe is similar to that observed in the area above the earthquake epicenter. at this point, we examined whether the two curves were identical or not. we computed the relative frequencies fdg and fdl over the total number of ebs of each distribution, and we use chi-squared (x2) tests for the goodness of fit. we set the null hypothesis that: the distributions of the daily frequency fdg of ebs observed by demeter all over the globe (figure 6 bottom panel) and the daily frequency fdl of ebs observed by demeter locally in the longitude range 107˚177˚ (figure 6 bottom pane) in the time interval of august 2-20, 2005, are the same. for this reason, we determined the critical value of the x2 distribution with degrees of freedom df = 18 (number of days between august 2-20 = 19; df = 19-1 = 18) from the suitable tables, and it was 37.156 at a = 0.005 [mann 1998]. we computed the test value from 19 pairs of fdg and fdl values x 2 = 7.75. since 7.75 < 31.5, the decision is not to reject the null hypothesis. this result suggests that the temporal variation of the daily frequencies of the ebs fdg and fdl are identical at a 2.5% significance level. in addition, we evaluated the cross-correlation coefficient for the two curves, and there was a positive correlation between the distributions of the daily numbers of ebs ndg and ndl. the crosscorrelation coefficient was evaluated to be as high as r = 0.87 electron precipitation before earthquakes figure 6. temporal distribution of the daily number of ebs across the globe (top) and in a region extending ±35˚ eastwards and westwards of the epicenter longitude for the period august 2-20, 2005 including the day of a strong earthquake in japan that occurred on august 16, 2005 (bottom). the correlation of the distributions is highly significant. for dt = 0 (where dt counts the number of days between the corresponding daily values ndg and ndl). we understand that the similarity of the two distributions of the demeter eb daily distribution suggests a strong dependence of the rbep flux all over the globe on the precipitation caused by the ‘local’ electromagnetic processes of the earthquake preparation region; the opposite interpretation, i.e. a dependence of the local eb temporal evolution (figure 6 top pane) on a global phenomenon, is in contradiction to the fact that the boundary condition at the time of the minimum (august 16, 2008) is defined by the day of the earthquake occurrence. therefore, almost certainly we can infer that the local electromagnetic processes of the earthquake preparation region influenced the rbep all over the globe before the august 16, 2005, earthquake. an extended (40 day) investigation of a large number of strong earthquakes in japan led us to the conclusion that the temporal evolution of energetic precipitation in the upper ionosphere follows the pattern of the august 16, 2005, earthquake discussed above. this pattern is clearer when a strong future earthquake is separated by long times (at least 1 day) and/or takes place at distant places. our statistical analysis was performed for m >6.7 earthquakes in japan in a geographic window with longitudes between 135˚-155˚e and latitudes 36˚-47˚n for the time interval from august 2004 to june 2008. there were 9 earthquake under these restrictions. from these 9 earthquakes, we selected these 9 earthquakes (numbered 1-9) for which complete demeter datasets were available, and table 1 shows the appropriate information for these earthquakes. the first 6 columns of table 1 after the numbering of the earthquakes in the first column show the time, the epicenter coordinates (longitude, latitude), the magnitude, the depth of the earthquakes, and the ratio of the daily number of ebs on the day of their maxima, as compared with their previous minimum values and the minimum values ±1 the earthquake days (see below). we see that earthquake #3 (46.6˚n, 153.3˚e) was the greatest earthquake that occurred in the greater japan area during the time period examined in this statistical study, and after ~35 min it was followed by earthquake #4, at almost the same location (46.6˚n, 154.7˚e) and with exactly the same epicenter (10 km). in the same way, earthquakes #7 and #8 were closely related in space and time. an evaluation of the daily number ndl of the local (±35˚) ebs on the days before and after the seismic events #2, #3-4, #5, #6, #7-8, and #9 gave the following results. earthquake #2: 2, 7, 6, [7], 4, 6, 7; earthquakes #3-4: 5, 7, 10, 12, [6], 3, 11, 10; earthquake #5: 5, 11, 11, 7, [6], 9; earthquake #6: 3, 8, 11, 8, 7, 11, 4, [7], 7; earthquakes #7-8: 3, 7, 9, 11, [8], 5, 3, 3, 10, 10; and earthquake #9: 8, 13, 8, 9, 6, 5, [8], 8. the brackets here indicate the daily number ndl of the ebs on the day of the earthquake occurrence. a careful study of the daily variations suggests that before the earthquakes under examination, ndl starts from low values, and then it always shows a local maximum before the day of the earthquake considered. the ratio of the maximum nd over its initial value is 7/2, 12/5, 11/5, 11/3, 11/3 and 13/8, while the ratio of the maximum ndl over its value on the day of the earthquake ±1 day is 7/3, 12/3, 11/6, 11/4, 11/5, 13/5; these data are shown in the last columns of table 1, and they suggest that in all six of the cases, the form of the temporal distribution of the daily number of ebs follows the pattern of the distribution of the august 16, 2005, earthquake shown in figure 6, in the sense that strong rbep activity precedes each earthquake, which starts 3-6 days before the main shock. a careful examination in the whole set of data suggests that the long duration (13 days) of the intense rbep seen in figure 6 is instead due to 5 earthquakes with magnitudes m >6 that occurred between august 3 and 11; for this reason a 3-day rbep period is also marked in the appropriate site of table 1. in a series of studies under preparation, we will present additional higher timeresolution indices, which we believe will better explain the pattern of the temporal evolution of the rbep phenomenon. anagnostopoulos et al. 30 no date time lat. long. magn. depth duration ebm/ebs ebm/ebeq (utc) (˚n) (˚e) (km) (days) ±35˚ ±35˚ 1 2005/08/16 02:46:28 38.3 142.04 7.2 36 13 (3) 13/1(7/5) 13/1 (7/1) 2 2005/11/14 21:38:51 38.1 144.9 7.0 11 2 7/2 7/3 3 2006/11/15 11:14:13 46.4 153.3 8.3 10 3 12/5 12/3 4 2006/11/15 11:40:55 46.5 154.7 6.7 10 3 5 2007/01/13 04:23:21 46.2 154.5 8.1 10 3 11/5 11/6 6 2007/03/25 00:41:57 37.3 136.6 6.7 8 6 11/3 11/4 7 2007/07/16 01:13:22 37.5 138.4 6.6 12 3 11/3 11/5 8 2007/07/16 14:17:37 36.8 134.9 6.8 350 3(+) 11/3 11/5 9 2008/05/07 16:45:18 36.2 141.5 6.9 27 5 13/8 13/5 table 1. characteristics of the earthquakes with m >6.7 in the greater region of japan. 31 although the statistical results for the temporal evolution of the daily number of rbep ebs are presented in table 1 for japan, we should point out that a similar pattern like that of figure 6 has been confirmed for many earthquakes all over the globe. such statistical results for indonesia, western america and other seismogenic regions, as well as the statistical results for the whole globe at low and middle latitudes, is the subject of a manuscript under preparation. here, we show some statistical results for one of the greatest and most catastrophic earthquakes of recent years at low latitudes, the earthquake of m = 8.6 in north sumatra, on march 28, 2005. as we mentioned above when describing figure 5, this earthquake was presented as an example of a special type of rbep process. in this case, the electron precipitation was not a latitudinal confined event. on the contrary, the rbep activity was observed as follows: (a) a series of spikes above the region of the epicenter; (b) one strong eb at the north latitudes; and (c) another eb symmetric to the previous one at south latitudes. for this earthquake, we show in table 2 the data as for the earthquakes in japan in table 1, but including orbits all over the globe. the evaluation of the daily number ndg on the days before and after the march 28, 2005, earthquake gave the following results: 4, 16, 24, 30, 16, [33], 31. the ratio of the maximum nd over its initial value is 30/4, while the ratio of the maximum ndl over its value on the day of the earthquake ±1 day is 30/16 (table 2). these data are similar to those of table 1, and they suggest that a strong increase in the number of rbep ebs started 4 days before the earthquake occurrence. an examination of more data from demeter (data not shown) suggests that rbeps accompanied by vlf and ulf activity were observable from at least march 12, 2005. figure 7 shows the distribution of the ebs recorded by demeter as a function of longitude for selected days before the japan august 16, 2005, earthquake. in figure 7, the longitude of the earthquake epicenter has been defined as a longitude with a value i = 0˚. distributions are presented for 5 days (figure 7, top to bottom): (a) a day before the beginning of the eb daily rate nd increasedecrease phenomenon (august 2, 2005); (b) the day of the maximum eb daily rate nd (august 8, 2005); (c) a day during the decreasing rate nd (august 12, 2005); (d) the day of the earthquake occurrence (august 16, 2005); and (e) the day after the earthquake occurrence (august 17, 2005). it is obvious that the daily number of ebs of figure 7 varies according to the shape of the temporal evolution shown in figure 6. moreover, figure 7 reveals two important new features concerning both the time evolution and the spatial distribution of the rbep events. to make these electron precipitation before earthquakes table 2. characteristics of the catastrophic earthquake of march 28, 2005, in north sumatra. the electron bursts in the two last columns are considered for demeter orbits at all latitudes. date time lat. long. magn. depth duration ebm/ebs ebm/ebeq (utc) (˚n) (˚e) (km) (days) all lat all lat 2005/03/28 16:09:36 2.1 97.1 8.6 30 4 30/4 30/16 figure 7. distribution of electron bursts recorded by demeter as a function of longitude for selected days before the japan august 16, 2005, earthquake. longitude with a value �i = 00 has been defined as the longitude of the of the earthquake epicenter. characteristics of the temporal evolution and the spatial distributions of the bursts are discussed in the text. features clearer, we draw a rectangle in each panel covering, in longitude, a region from 25˚ westward to 45˚ eastward of the epicenter location (i = 00). we will call this region around the earthquake the ‘er’. we see that: (i) first, on the day of the earthquake, the idp experiment onboard demeter revealed a quiet time period for electron precipitation in the er above the earthquake epicenter. this rbep ‘silence’ above the epicenter on the day of the earthquake occurrence recalls the statistical results of nemec et al. [2009], who saw a decrease in the vlf wave intensity from 0 h to 4 h before the time of the main earthquake. (ii) secondly, the longitude extent of the eb distribution was reduced dramatically from august 8 to august 12 in the er (around the region of the epicenter), with the distribution showing no eb within the er on august 16 (as we mentioned above, from the point of view of the temporal evolution of ndl); we note that the ebs that correspond to the bars to the right of the er in figure 7, on august 12 and 16, were most probably related to a ground-based transmitter in europe, so the seismic-induced rbep events reduction in the er region actually had a very clear effect before the august 16 earthquake. the reduction in the rbep events over the epicenter a few days before the occurrence of the earthquake resembles the concentration of the foreshocks before the occurrence of the main shock found in the case of the l’aquila (italy) earthquake of april 6, 2009 [papadopoulos et al. 2010]. here, we note that the minimum in the number of ebs does not always coincide with the date of earthquakes considered in the statistical sample of table 1 and discussed above. this is sometimes due to the averaging of the numbers of ebs within a day (0-24 h/ ut) independent of the time of earthquake occurrence. for instance, earthquake #6 of table 1 shows a minimum ‘before’ the day of the earthquake occurrence, march 25, 2007, but the earthquake occurred at the beginning of this day, at ~00:42; therefore, the number of ebs has actually shown a minimum closely related to the main earthquake shock. furthermore, the date of earthquake #9 in japan (may 7, 2008) shows an increase in the daily number of ebs ndl, this daily increase in the number of ebs is evidently affected by the preparation of the seismic region in china, which caused the catastrophic earthquake of may 12, 2008; however, fine time resolution data (data not shown) have demonstrated that before the earthquake occurrence in japan, a ~3-h silence in the rbep was observed in the top side ionosphere above the earthquake epicenter. 4. summary of observations, and discussion 4.1 dependence of electron precipitation events on the geographic coordinates. in this study, we have extended our understanding of the earthquake-induced electron precipitation process. we find that earthquake-induced ebs show different forms that are dependent on the geographic location of the earthquake epicenter. we investigated the earthquake-induced ebs at middle latitudes at depth, and in particular the earthquake-induced electron precipitation events above a broad area around japan, and in the south-eastern mediterranean (greece), which were observed at similar (middle) latitudes. for the eb types at middle latitudes, we found systematic anagnostopoulos et al. 32 figure 8. the average intensities of idp/demeter ~200 kev energy electrons for one year (october 2005 to october 2006). the electron distribution is highly affected by the saa. 33 differentiation in the observations above japan and the greece/south-eastern mediterranean: although greece is at approximately the same geographic latitude as japan (white horizontal line at ~ 39˚n in the map of figure 8, top panel), the electron precipitation events show very different forms in the flux-time profiles. the main differences observed by demeter between the rbep ebs before large earthquakes in greece and japan are the following: (1) the observed ebs over japan are superimposed on a permanent high electron flux background jjp, which is often called a ‘pseudo-trapping region’. on the contrary, the electron flux background level in the top-side ionosphere of greece jgr is much lower; the comparison of the corresponding measurements suggests a normal ratio jjp / jj c 10 2. (2) the rbep ebs observed before the earthquakes in japan were usually associated with ebs in the south hemisphere. on the contrary, no conjugate ebs were found in the case of the earthquake precursory ebs observed above greece. (3) the pre-seismic ebs over japan most often showed a single flux enhancement that lasted for ~2 min to 3 min (with over ~80% of the recorded ebs lasting for over 2 min in the statistical sample examined by sidiropoulos et al.) [2011]. the pre-seismic electron precipitation activity over greece lasted for almost the same time period, but the flux-time profile was composed of a sequence of several (3-8) short-lived intensity spikes. from the point of view of the rbep, more work needs to be done in the future to study the possible differentiation between the energy spectra of the electron intensities in these two regions (greece and japan) as a function of latitude/l-shell parameters. the evident reason for this systematic differentiation of the flux-time profiles between these two seismogenic regions that are situated at almost the same geographic latitude is evidently the different structures of the geomagnetic field. the saa results in a north-south asymmetry of the average electron fluxes over a large range of longitudes, which includes south america, the atlantic ocean, europe and africa. this can easily be seen in figure 8, where the average intensity of the ~200 kev energy electrons for one year is displayed (october 2005 to october 2006). therefore, we expect a differentiation of the detected earthquake-induced electron precipitation events over greece and japan, since greece is affected by the saa. in the case of earthquakes in japan, the most frequent conjugate ebs are detected near to eastern australia. a similar example to the appearance of a conjugate eb seen in the japan earthquake (north hemisphere) was seen in the case of the south giant chile earthquake also (conjugate burst in the north). the details of the ways in which the earthquakeinduced electron precipitation events are formed in various geographic regions due to the differentiation of the earth magnetic field over the globe are outside the scope of the present study. here, we attempt only a classification of the various forms of the electron flux-time profiles on the basis of the large-scale variations in the geomagnetic field structure at various characteristic seismogenic regions all over the globe. more detailed theoretical interpretation will be the subject of a future manuscript. as well as the earthquakes in japan and greece, we have presented preliminary results from a study of a few earthquakes that occurred at low latitudes (haiti). the haitirelated rbep event reveals a third type of flux-time profile of the earthquake precursory ebs. we see that in this case, the satellite detected a unique event with a few short-lived intensity peaks in the absence of an observable ambient electron background, as in the case of the ebs observed above the south-eastern mediterranean. the fourth type of earthquake-induced electron precipitation event was revealed in the case of the sumatra 2005 earthquake. in this case, the electron precipitation shows one peak near the epicenter, and simultaneously two other peaks, at north and south latitudes and almost symmetrical to the epicenter. this means that, in this last case, three distinct rbep bursts occurred before a giant earthquake at low latitudes. further investigation is needed to achieve a more detailed description of the rbep events based on additional observational characteristics: namely, the energy spectrum and the energy dispersion of the flux-time profiles. for instance, a preliminary investigation has suggested that earthquakes that occur at very high latitudes might show energy spectra that extend to higher energies (>1 mev) than those observed at middle latitudes. more deep understanding of the observed rbep bursts will be possible after investigations into the physical mechanisms that are characteristic of these seismically induced vlf emissionparticle interactions that lead to precipitation, which is the subject of a following study. in this report, we have continued to provide new material that supports our previous data that strong rbeps precede strong earthquakes by a few days [anagnostopoulos et al. 2010, sidiropoulos et al. 2010]. of note, we have not provided results from a large statistical sample of earthquakes that covers the whole globe. it is our research strategy here to first elaborate upon the characteristic features of the major seismogenic regions around the globe. the possible reasons for the observed differences of longitudinal and latitudinal distributions of ebs for different seismogenic regions of the globe might be based in magnetic declination (large declination, as is characteristic of american sectors, creates the problem of conjugation effects between hemispheres), and also in different l-parameters for different longitudes (this factor determines whether the magnetic flux tube of the seismogenic region is inside or outside the radiation belt). these problems will also be considered in our following studies. electron precipitation before earthquakes in this way, we have accumulated with time more information about spatial differentiation of rbep signals. as a result, we improve our knowledge for the selection criteria of ebs for the performing of an automatic procedure on a very broad statistical sample in the future. 4.2 temporal evolution of electron precipitation before large earthquakes furthermore, our studies have demonstrated that there is a permanent characteristic temporal variation in the behaviors of the flow of inner van allen radiation belt electrons in the upper ionosphere that lasts for several days to a few weeks before strong earthquakes in japan. this phenomenon resembles a ‘rain’ of electrons from the van allen belts, which begins with an incremental rate several days to a few weeks before major earthquakes. these then show a maximum, and finally this rain becomes weaker, with a minimum or a pause in the electron precipitation some hours before the onset of an earthquake. the effect is observed locally, over the earthquake preparation zone, although in cases of strong earthquakes, the precipitating electrons can control the behavior of the drifting electrons over a wide ranges of longitudes. we find that this kind of temporal evolution in the space measurements of energetic electrons has been known for several electromagnetic geophysical, ionospheric and space physics parameters before those of strong earthquakes (pulinets and boyarchuk 2004). for instance, we recall the characteristic temporal patterns of the ulf magnetic field activity detected by ground magnetometers some weeks before an earthquake [hayakawa et al. 1996a, b]. in this case, the ulf polarization ratio z/h of the magnetic field starts increasing gradually, reaches a maximum value, and then around the time of the earthquake it shows a deep minimum. it has been surmised by hayakawa et al. [1996a] that these ulf emissions are related to earthquake preparation processes. it is possible that the magnetic field ulf polarization ratio z/h changes before strong earthquakes and is related to a redistribution of the sq electric current system in the ionosphere [duma and ruzhin 2003]. it is also possible that the ground ulf polarization and the radiation belt electron temporal evolution are related to the sq ionospheric current variations under a series of physical processes that are observed as almost simultaneous changes in ground, ionospheric and radiation belt physical parameters. we found that the temporal pattern of rbep changes, with a maximum flux several days before and a minimum flux around the earthquake occurrence time, is clear in 7 of the 7 m >6.7 earthquakes that occurred in or near japan between august 2004 and june 2008 (we actually found 9 earthquakes with m >6.7, but in 2 cases two m >6.7 earthquakes occurred within about half a day and in the present study are related to the same general precursor signal; more detailed analysis that can separate the signals of these earthquakes will be presented in a future study). our investigation suggests that this pattern of temporal evolution is clear when we study the occurrence of a single earthquake, i.e. when no other strong earthquakes occur in close areas and/or times. furthermore, we found that for a great earthquake in japan (august 16, 2005), this temporal evolution pattern is not only clear above the area of the earthquake epicenter, but it appears almost the same when the daily number ndg of the ebs over all of the globe are averaged (figure 7). this finding suggests that most probably the earthquake preparation process is strong enough in its relation to the electromagnetic phenomena in the atmosphere-ionosphere-radiation belts that they can control the global energetic electron precipitation at low altitudes. furthermore, we have presented in this report a pair of intense rbep bursts with one eb above the epicenter of the 2010 chile earthquake, and its conjugate eb at north latitudes. these two intense bursts were detected by demeter (figure 2) before the giant earthquake in chile in the daytime upper ionosphere under very weak, moderately broad, vlf activity; because of the strong ionospheric absorption of the emitting radiation through the daytime ionosphere, no vlf electromagnetic emissions from the ground were recorded by the ice instrumentation. this finding most probably suggests that the ebs of february 26, 2010, the day before the giant chile earthquake, are almost certainly due to the preparation processes of the february 27, 2010, earthquake that occurred in chile. furthermore, it might suggest that the radiation belt electrons in this case do not interact with vlf radiation emitted from the ground (ground transmitter or seismic vlf radiation), but from an increased registration of vlf noise in the radiation belts that was caused by the influence of the anomalous electric field above the earthquake epicenter [pulinets and boyarchuk 2004]. in the case examined here, it appears that the electrons reach the dayside ionosphere of chile after a strong interaction in the nightside radiation belts. 5. conclusions in this report, we provide significant new information on the phenomenology and some physical processes related to rbep, starting several days before great earthquakes (m >6.5). the main results of our study are the following: (1) earthquake precursory rbep events show flux-time profiles with: (a) two ebs at middle latitudes, with one burst above the future epicenter and a conjugate one in the other hemisphere (japan, chile); (b) only one eb at north middle latitudes, when the saa affects the south hemisphere (south-eastern europe/greece); (c) one eb above an earthquake epicenter at low latitudes (haiti); and (d) one eb above an earthquake epicenter at low latitudes and two symmetric ebs at north and south latitudes (sumatra). (2) for earthquakes in japan, a standard temporal variation of the number of ebs was found, anagnostopoulos et al. 34 35 which begins with an incremental rate several days before major earthquakes, and which after a maximum, decreases so that electron precipitation ceases above the region of the future epicenter a few hours before the occurrence of the earthquake. (3) the analysis of the spatial distribution of the ebs for the earthquake of august 16, 2005, in japan reveals that the rbep bursts were reduced in the space near the epicenter before the earthquake occurrence. (4) strong rbep bursts above the chile earthquake were observed not only in the nightside, but also in the dayside upper ionosphere. the present study was based on results from a systematic study of the demeter satellite measurements at low and middle latitudes before very strong earthquakes (i.e. m >6.5), but the results from an investigation not yet published suggest that such events are also observable before earthquakes with lower magnitudes (m >~5). we understand the radiation belt energetic electron precipitation related with an earthquake as a ring in the chain of electromagnetic processes that can start above the earth surface with the anomalous electric field, which can cause an increase in the vlf electric field wave activity in the magnetosphere. it has been suggested that a cyclotron resonance interaction between vlf waves and radiation belt energetic electrons is responsible for variations in the electron velocity phase space, and as a result, a part of the trapped electrons of the radiation belt change their pitch angles and travel towards the earth, where they are recorded by low orbit satellites in the topside ionosphere. it is important to note that the ionization produced by the precipitation of electrons from the radiation belts in the lower ionosphere produces disturbance in the d region of the ionosphere at night, which is then observed both from ground and space instrumentation [pulinets 2011]. the results of the present study might open a new window in earthquake prediction research. we note that the methodology of radiation belt energetic particle events earthquake precursor signals has the particular advantage of studying distant earthquake preparation regions, because of the fast eastward drifting of the energetic electrons. recent studies have started to use systematic analyses of several physical and environmental parameters, such as temperature and concentrations of electrons, vlf/ulf activities and rbep in the ionosphere, thermal infrared radiation, radon/ion activities, and air temperature/ humidity in the atmosphere. these parameters have been found to be earthquake precursors. we believe that such approaches will achieve high confidence levels in earthquake prediction, at least in some cases, in future years. acknowledgements. the authors thank drs. sauvaud and berthelier for providing the demeter data, and dr i. vesselovsky for useful discussions during the stay of the lead author in the skobeltsyn institute of nuclear physics, moscow university, russia. references abel, b. and r.m. thorne (1998). electron scattering loss in earth’s inner magnetosphere 1. dominant physical processes, j. geophys. res., 103, 2385-2396; doi: 10.1029/ 97ja02919. aleksandrin, s. yu., a.m. galper, l.a. grishantzeva, s.v. koldashov, l.v. maslennikov, a.m. murashov, p. picozza, v. sgrigna and s.a. voronov (2003). high-energy charged particle bursts in near-earth space as earthquake precursors, ann. geophys., 21, 597. anagnostopoulos, g. and v. rigas (2009). variations of energetic radiation belt electron precipitation observed by demeter before strong earthquakes, egu abstracts. anagnostopoulos, g., v. rigas, m. athanasiou, a. iliopoulos, e. vassiliadis and n. iossifidis (2010). temporal evolution of energetic electron precipitation as a promising tool for earthquake prediction research: analysis of idp/ demeter observations, in: proceedings of the 9th international conference of the hellenic astronomical society, proceedings of a conference held 20-24 september 2009 in athens, greece, edited by k. tsinganos, d. hatzidimitriou and t. matsakos, astronomical society of the pacific, san francisco, 424, 67-74. anagnostopoulos, g. and a. papandreou (2012). space conditions during a month of a sequence of six m >6.8 earthquakes ending with the tsunami of december 26, 2004, nat. hazards earth sys. sci. 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(1983). observations of vlf transitter-induced depletions of inner zone electrons, geophys. res. lett., 10 (8), 619-622. varotsos p., n. sarlis, m. lazaridou and p. kapiris (1998). transmission of stressed induced electric signals in dielectric media, j. appl. phys., 83, 60-70. zhang, z., x. li, x. shen, y. ma, h. chen, x. you and y. yuan (2010). demeter satellite observations of particle burst prior to chile earthquake, http://arxiv.org/ ps_cache/arxiv/pdf/1011/1011.3592v1.pdf. *corresponding author: georgios anagnostopoulos, department of electrical and computer engineering, demokritos university of greece, xanthi, greece; email: ganagno@ee.duth.gr. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved anagnostopoulos et al. 36 sull'anomalia di appleton della regione ionosferica e (*) (ai""" fh .. ap"i .. !"" fi"""'flly "f fii" imw .• ph .... ic f"' ""gin,,) ricevnt.o il 20 settem)lte 1972 ru~niany. 'l'he appleton seasonal allonwl,\' or the ìono.~phel"ic e region, ilellucel1 iloth with a constant·x, const,llnt·l: met,holl and a (1., h) correlat.ion illl'tholi from the notiualizcl1 foe dat.a at. roml', isin'icfl,y discussell; in partìl"ullli', thc fads thllt thc alllplit1hle of thc anomaly ìill'!"('ascs with ~olar al"tivit.y antl that. any explieation ilased npon a "~ing-le ljatameter tlwory" is nnsatisfaetoi'y, ai'e emphasi?ed. ri.~s.~u:-;tu. -i~ ljreyeml'n te ill ustrata l'anomnlia stngionalc lli appleton llelln l"eg-ione e, dellotta llai ilati foe llcll"ailllamcnto nol'male ionosfcrìl"o a romn sin con un metollo a x ell il costanti, sia ('on un metodo a l"oltclazione (1., r); sono messi in (>\-idenza l"auml'nto dcll'ampil'zza dcll'anomalia all'aumentare dell'nttil'itl~ solarc e l"ineftical"ìa ili spieg-azioni, (iuali tludle correnti, hnsate sulla variazione di un solo }jnrallletto fra ì molti in gioco. 1. il'\ttlodfjl.io;'i"e {juakhe alllio fa k v. j\}l}lleton (i) sco}ll'ì lln'intenlssa.nt.e anomalia, che }loi da lui ha preso nome, ilell'anda.ment.o annuo della frequenza erit.i(~a onlinaria (iella regione b della ionohfel'a, fob_ prcl'isament.e, è usuaie ajl"enllare che per quc.~ta regione è i)('n verificata la teol"ill. di c1lmpman sulla fotoionizzazione atmosferica ali opera di r:\.!liazioni solari; a. 1l00'ma di tale teoria, per i dati ili un detel"lninato osservatorio ionosferico può seriversi, in condizioni quaf!i f\t.azionarie; \ [oe' ? /t = cosl = gcosl> [1] (*) comunìl"azìone prcsentata al convegno aililuale dell'assodn?ionc geofì~kn italiana, roma 2g-2r aprile 1973. (hl istituto xa?ioilalc di geofisica, roma. dove x è la (1i~blllza wnitale del sole, ila è il tasso di prollllzione di fotoelettrolli ilei' x =, 0, il è il eoeftkielite effettivo di rieomhinazione, orbene, lie ~i col1liidcrallo valori di foe~ riferclltisi a nn valure costante ilella brillallza, dci holc ionizzante, eslii dovrebbero iliilelhlere soltantu da x e quindi per hli assegnato valore di x iloyrehbero essere colilanti dlll'alite l'alino: si t,i'ova. invece un massimo invernale, 2, a:-:alisi sta'l'1~'l'ic'\ nel dati a partire dall'andamento normale dei valori mmliani mensili di foe ricava,ti nell'oss(;l'yatorio ionljhferico di h,oma llell'istit.ut.o na,zionalb ili (;pofisiea nel periodo 1\)48-70 e assnmemlo come indice della brillanza. dci sole ionizzallte il liumero ilelle macchie solari h, ahhiamu estmtto i valori mpdialli mensili ili foe-1 per h = 100, cos x = 0,4 e l)er il ==c 100, eus x ccc0,2, ril)ort.ati lielle cmve a, c ili fig.: il ma~simo illycrnale è hen evidente. l'er confront.o, soli o riilortat.i a.nche i yalori ili foe" l)cr il = o, eus x = 0,4, liella enrva b: l'a.ndament.o è a.ssai mello prollunciato. qlwsto mollo ili ra.pl)resentare l'a.nomalia, di(; ehiameremu "metodo a. x costa.nte », è sostanzialment.e quello segnilo ila appleton (i), salyo il fa,uo che qui la normalizzazione verso assegna.li ya.lori ili r ì\ st,at.a fatt.a mmliallte ret,le di rcgresliione, mentre là manca una vera e 1)j'ol)j'ia normalizzaziollc, avpldon avendo considerat.o medie di valori mediani mensili in un periodo, undeeennale, di variazionc ili r. il prucedimento scgnilu 1)110 esser crilieat,o vcrchè mette a. confronlo dat.i non omogenci, riferenlilii a coihlizioni yariament.e diycrse da.lla staziullariet.à (a homa., cos x = 0,4 ('orrisponde drea. alle ore 12 a llicemhre, circa a.lle ore 7 e alle ore 17 a giugno), cioè a condizioni raijl)resenta.lc generalmellt,e, iliù che dalla [lj, da. uli'equazione ili equilibrio del til)o: d(fo e ) 2 a foe -----d' [2j allo scopo di avere una. ml)l)rescntaziolle esent.e da. ta.le difet,to ahhiamo rieel'(·a.t~), mese ver mese, il legame fra foe'1 c cos x per n = 0, melliant.e rette (ii regrcssione in 1111 rifcrim(li( tu lugaritmico; eome già a itri, a.bbiamo t,ruvato che il legame anzilletto lmò essere espresso dalla relazione: foe· = g'o(cos x)p, [3j sull'a:'>iomalia di appleto:'>i della regio.':}; ionosfmuca e 25 come risulta dalla curva e in tlg-ura, l'e~ponente p non present.a una signiiicativa variazione staj"r1ollale, e ad esso è attribuibile il valor meaio p = 1,27 (seeondo la teoria di chapman, e dunque in ac('mao con la [l], si sarebbe aovuto avere p = 1,(0); invece, l'all(lamenf.o della, quantit.à co (em'ya d), che è il valore (ii foe4 per 1. = o, r = o, rappresellt.a l'anomalia di apl)leton a honm, con un regolare, netto massimo ilrvernale . .molto recentement.e (gennaio 1973) s. s. kouris e l. 1>i. mugglet.on lmnno comunicato (") i l'isultati ottenuti allplicando ai (lat.i ai yari o~seryat.ori ionosferiei un loro met.oao eile sostanzialmente è quello "a col'l'elazione» ora esposto, salvo una maggiore accuratezza, deriyallte aal premlere ili eonsiaerazione ilnelle la. variazi01w durante l'anno della aistanza 'l'erm-sole, e salyo il fat.to cile come indice solare è llreso, anziclu'~ h, il yalme meaio mensile aeu'intemiti. aeua raaioemissione ~olal'(l a 2,8 ghz (equiva,lente, 1)('1' st.atistiehe a lungo t.ermine, alt). helativamente ai dati ai uoma nel perioao 1957 -()7 essi hallno ottenuto l)er 1) il valore 1,~2 e per 0 0 i valori congiunti in fig. eon la linea t.mtt.egj"r1at.a d'; a patte una sistematica piccola dill'erenza, a no;;tro avvi~o derivante principalmente aalla (liversiti. aei periodi eonsiderati, l'aeeol'ao fra gli andamenti il, d', è molto buono: ciò cost.ituisee, fra l'altro, una conferma, della, non influenza della variazione annua della distanza terra-sole sulla aensitù elettronica della regione e. 3. discussione dei risultati l'esame acue curve (! --:c in tlgum porta a t'ilevare i seguent.i fatti: (a) il" metodo a x co;;t.ante ~ (curve a, b, c) si presta bene a mettere ili evidenza evenhmli aipemlenze aell'anomalia da part.icolari eoll(lizioni; per es., il confront.o fra le curve a, b, port.a a riconoseere una netta dipendenza dell'ampiezza· dell'anomalia dall'attiyitù. solare: tale circostanza noli ci llun, sia stata sinora rilevata (il metodo seguito da appleton e poi da molti alt.ri normalizza i dati verso r, come accennato, seml)licemellt.e mediando su 11 anni; il met.odo di kouris e mugglet.on preflenta la stessa limitazione). (b) il ({ metodo a correlazione» (curve d, d', c) realizza un'efficace perequazione st.atistiea. e ben ~i presta l)er studi con dati, non sempre omogenei, di vari osservat.ori; come aceennat.o, kouris e 1>iuggleton lo hanno apl)licato ai dat.i di una quarantina ai osservat.ori ionosferici difltribuiti in latitudine int.orno ai meridiani 5ab. 150"b. 75°'1,7. otte2(; .. , ., ., " ' 0 o____ ~ , , ", [~ i1'] ", h' .. , "::j ;o=--~ " " l' . ijo)i !:'\ l c i ~ 1 111111 illul a -~ b ' __ ..... _ _ -0-__ -<_ _ _ "• e cr-,--r--r~:j ~--,---,--, ,--r, --.-~["/"'jj , '" , 'io , '" , iii , ,o< ." , ,o< , lo, , ." 'l' , '" , "~o , lo, sull'a!>o)iall\ di ai'pll,to!> della ltn;lo!»è !o!>o~feluc,\ e /ic'/hlo (2) una. confprma su sl'ala plane!al'ia delle giù note enmtleristi{']l(' deif:mollla.lia, prineipale fra tuttp il nu\s,~illlo nel1'inyp1'no loealt" ('oli forte effetto (li lalitudine (ampil'zza pi('("ola nplla fa,~eia l'qunlm'iak, massima a me(lia lat.itthline, ('oli asimm('ll'ie fl':l i (hw ('misù'ri) l' nilill'ezzauile effetto (li longitudine" j)nlla liglll':l s('atul'i,,('p nna c'il'('o,~tanza abbastanza interessante, e ejo('. che, ('ome lwn l'i~li1ta <lal ('onfl'olllo fl'h le eurve c ix~78") l'd f, l'nlhlamento dell'es}lolh'ntp}j nelln [:-lj, al qnnlp si shreblw }lortati a. non (1:ii'e impol'tanzh, (' ('ol'relalo ('on lei'arillzir!/d dell'a·nomalia a· grandi (listanzl' zenitali; a ilo~tniori, la. ('osa non sorili"e/hle ove si ponga :ltlenzione al faho (,hl' la i)('1h!pnza (lelk reht, (li regressione log (foe1, ('os x), da eui }i t' ri('avato, (lipp]hle fo1'1l'mente dai yalol'i di foeo! 1'elatiyi a grandi yhlol'i (li x" al hm' di verifi('nre la signifì('atiyib\ dplle rhjl}li'esl'll1:tziolli proposte, abhiamo eonsi(lerato 1)('1' ogni mese, e (!uindi 1)('1' un determinato valore (li q", eoppie di m'e simmetriehe l'ispetto nlmezzogiol'no ]()('n]l', p quilldi per nn (lptel'luin:lto vidon' eli ('os 7.; ri(':rniihlo (lni dati nol'illhlizzati pn r o le quantiu~ fob, d(fob)/dt, foe.!, abbiamo (':tl('olalo il eoptlieienle effeltivo (li rleombina.zione (! eliminando 'l e eohx fra le (,op}lil' di equazioni [2] ('m'l'ispondenti nlle ore scelte; s'è ottenuta la ('nrva j in ligurn., ehe riill'o(lnee, eontl'ofase ('on1(' (ley'ps~pl'e, l'a1l(ihmenlo (lell'esllonente ]i, è illl-el'esflant·e osspitm'p ('ile se si tol'n:l a ('ondizioni l'igorosalllentp stazional'ie, eonsidemndo valoi'i di foe.! hl mezzogiorno im"ale, e ~i riea·ya 'le (laha [l] ntilizzllndo i predetti y:llmi (li il, ~i ouipn{) ]h'r 'l e 111l andamento identieo a quello (lel1a· eul'vh t, eon valori intorno a lo' m 3 see l, 'rnli yhlori num('1'iei (ii 1[0 sono :i('('punhili, mentre l'ajl(lamento l' ineompatihile {'un gli altri l'iwjt.ati. ya lenllto ]ll'esent.e ('hl' dei ite pannuetl'i in gioeo, l' ('iolfoe, il, 'lo, quello elle el'l'thnwn((' ('011t,iene l'anomalia in tel'luini (li quhntità os~prvahili l' f(le; per il (' pohsibile, l'ome il1(lieato, una ynlutazione lm1'zinlnwnt(' indiil('/hll'lll e; pn fio eib ehe si ottiene ò soltant{) di l'iy('r~nre ~u t'sso il ('al'nhp1'p di il, senza. aleun ~ignilieato fisi('o. cib ~ilieg~1 l'inanità delle j)],opoflte (li spiegare l'anomnlin (li ajlilieton l'on temie «a ljal'ametl'o unil'o», basnle gulla eostanza (li ilo e suu'ahsllnzione ehe l'appnrpnll' yal'iazione (li il sia (lovuta. a eorris}lolldenti variazioni di temperatura e (li ('omi)()~i­ ziolw dei gas atmosferici. j~o stesso si puù dim a ]u"ojlosito dplln fljlieg-aziolh', cui illl'lilla altualmenle h mhggioi' ljal'te degli stlj(lio~i, bnsnta su va.ri:lzioni (li lln termine di trasporto yl'l'ti('ale ('()llegato al sislemn (1plle ('or1'enti '''i[, iii tale teoria., espl'imelldo, eome faho sinm'n, la (len~ib'l eleul'olli('a ma~28 l'. j)o~il~icl siml~ ~v)[ in t.ermini di frequenza critica ordinaria e dando al t.ermine di trasporto la. forma div( .. "l)! v), con v velocità verticale di scorriment.o, l'equazione di equilibrio può essere assunta nella forma: d(foe) ", il''co~r. il~a foe4 _[ fob òe_ ~(/. {oe :3 òh + v ò(fo~j. l. òh [4 ] come si vede, il termine di tl'asllort.o viene fio ditlcihlcl'c anche da fob e dal gradiente vertica.le ò(foe)/òh, ciol\ ad esso viene impressa anello una variaziolle anuua che in l'ealtì~, anzicho'\ derivare dalla vc!ocit.ù. di scorrimento, è ih'opria. di fob pel'chè, per es., deriva da a (che invece, al pari di (lq, è assunto co~tallte). "sei citato articolo di kouris e 1'iluggleton (2) è fattl~ un'eccellente rassegna dei lavori basati huua sviega.ziono in discussiolle, c la. conclusione, eome era (la aspet.t.arsi, è che gli argomenti a fa,yore sono soltanto qualit.ativi. i. co::\"clusioni .a questo punto, quel che si può dire (~ che l'anomalia di .appleton rivela una sino a qualche t.empo fa insospettata nat.ura complessa della regione e, cont.ribuendo, con altri fa.tt.i quali la rieonosciut.a non linearitù di resl)on80 a.jl'at,tivitù solare e l'esist.enza di una consistente 'i<j nottul'lla, ad avvarentare quest.a regione alla sovrastante regione f: si viene così a costituire un'ident.ità molfologica fra le aue regioni (cui probabilmente l)artecil)a anche la. regione]) in virt.ù della sua ben not.a il anomalia invernale n), la differenza fra esse consistendo nella diversa evidenza del comvol'blmento i( non alla chal)m~ul n. b dunque, ver dar cont~) dell'a,noma.jia della re,gione e, i)osto che spiegazioni semplici riescono non plausibili, occorrerà trasferire alla regione e i meccanismi, di trasvorto e no, ]ll'ovosti e in via di studio l)er dar conto delle assai più vistose anomalie della regione f. bibliografia (1) appleton e. v., 1963. ,toi't. atm. 'l'erro phys., 25, 577. {'l kùunrs s. s., ::'iruggu:'l"ù~ t,. ::'il, 1973. jour. atlll. t ett. phys., 35, l41. 48,06,2005misc 957 annals of geophysics, vol. 48, n. 6, december 2005 key words lava domes – squeeze of magma – growth rates of domes – hagen-poiseuille law – macroscopic viscosity 1. introduction lava domes are one of the significant features of volcanism at the earth surface and their definitions and classifications have been published by several researchers. however, the processes of their formation have not always been systematically discussed. we know some examples of lava dome formations in historical times, which may give us fundamental knowledge on the movement of magmas in their formations. yokoyama (2004) interpreted the formation processes of the 1909 tarumai lava dome (andesite) and the 1944 usu lava dome (dacite), both in hokkaido, japan, as being squeezed from magma forced to flow through the vents. ascending magmas became lower in temperature, gas and water contents, with increased degrees of crystallization. accordingly their rheologic behavior changed from newtonian viscous flows to bingham plastic flows. in fact, these lava domes assumed different features of effusions such as growth process, its rate and final configurations, due to differences in viscosities of their magmas and other eruption parameters. the stationary magma flows in formation of lava domes are governed by the hagenpoiseuille law which relates the flow rates to size of conduits, driving pressure gradient and viscosity of magmas. considering that measurements of viscosity of juvenile magma in situ are difficult, we will utilize the growth rates of growth rates of lava domes with respect to viscosity of magmas izumi yokoyama higashi 1-17-7-1304, kunitachi, tokyo 186-0002 japan abstract in the discussion of lava dome formation, viscosity of magma plays an important role. measurements of viscosity of magmas in field and laboratory are briefly summarized. the types of lava dome emplacements are classified into two, squeezeand spine-type, by kinetic processes. the squeeze-type is the formation of a dome as a result of squeezes of magma through conduits and the latter is solidified magma forced to ascend by underlying fluid magma. an important parameter in the formation of such lava domes is their growth rates. lava domes of squeeze-type are governed by the hagen-poiseuille law which involves their viscosoties and other eruption parameters. at present, the real viscosity of magmas at the site of lava dome is still inaccessible. in order to avoid uncertainty in viscosity of magmas, a conception of «macroscopic viscosity» is proposed, which involves effects of chemical components, mainly sio2 and volatile material, crystals and temperature, and their changes with time. lava dome formations during the 20th century are briefly examined and their growth rates are estimated. the relationship between the growth rates and the sio2 content of the magma is statistically studied, and the macroscopic viscosity is empirically expressed as a function of sio2 content. the linearity between the two parameters is reasonably interpreted. this means that formation processes of lava domes are dominantly controlled by macroscopic viscosity of magma. mailing address: dr. izumi yokoyama, higashi 1-177-1304, kunitachi, tokyo 186-0002 japan; e-mail: iyokoya@aol.com 958 izumi yokoyama various lava domes to estimate effective viscosity of their magmas from the macroscopic and statistic viewpoint. 2. viscosity of magmas related to dome formation knowledge on the physical properties of magma is fundamental to interpret the formation of lava domes. the mode of formation is influenced by viscosity of magma, which strongly depends on contents of sio2, water, bubbles and microlites and temperature. of these factors, only temperature is determinable at the dome sites under some circumstances. in the following, temperature and viscosity of magmas shall be discussed briefly in relation to dome formation. 2.1. temperature of magma originally the temperature of magmas at reservoirs may be much higher than 1000°c. basaltic magma may keep its original temperature as far as vents due to its fluidity, while dacitic magma is viscous and moves slowly, and accordingly reaches vents at a lower temperature. therefore, we may take the internal temperature of andestic and dacitic magmas during formation of lava domes to be lower than 1200°c, around 1000°c on the average as a whole. field measurements of temperature of lavas are usually limited to those of basaltic lavas. for dacitic lavas, fumaroles on some dacitic domes are helpful. on the 1944 lava dome of usu volcano, hokkaido, whose formation was completed in 1945, there were a few strong fumaroles issuing gases which transmit the deep temperature. some of them became accessible around 1946. the temperature of one of the strong fumaroles has been periodically measured by thermocouples. its secular changes for 50 years (yokoyama and seino, 2000, fig. 13) indicate that the initial temperature of the fumarole was approximately 1000°c. and accordingly, the original lava temperature should be higher than 1000°c. huppert et al. (1982) suggested that the maximum temperature of the 1979 lava (basaltic andesite) of the soufrière of st. vincent was 800 ∼ 1100°c, by petrological studies. murphy et al. (1998) applied the quilf thermometer to the 1995-1997 soufrière hills andesite and obtained a range of temperature of 810 ∼ 880°c. nakada et al. (1999) estimated temperatures of the 1990-1992 unzen dacite lavas at 850 ∼ 900°c with fe-ti oxides in groundmass. 2.2. viscosity of magma in general, magmas at depths are higher than 1000°c in temperature, and may be viscous newtonian fluids. as the magma ascends through conduits, its pressure lowers and its temperature drops gradually, and necessarily volatiles in the magma exsolve and crystallization progresses. both the effects cause increases in magma viscosity. when they cool further, their viscosities increase exponentially and plastic behavior is greatly enhanced; they behave as plastic bingham flows. considering that glaciers, of which ice has a viscosity of the order of 1013 pa⋅s around 0°c (paterson, 1994), flow slowly, we may assume even dacitic lavas (roughly 107 pa⋅s at 1000°c measured in laboratory by goto, 1997) probably flow rather smoothly under endogenous or gravitational forces. in fact, booth and self (1973) measured viscosities of the 1971 basaltic lava of etna volcano along its flows and obtained the viscosity range as 103∼ 107 pa⋅s field measurements – the measurements have usually been made at vents or along lava flows and rarely at lava lakes of basaltic lavas which scarcely form lava domes. their results are more natural than those of dry melts. here, we may mention a unique example of field measurement of dacitic lava flow on santiguito volcano; rose (1973) calculated viscosity of the 1932-1933 lava flow near the vent as 5×107 pa⋅s, based on the equation of nichols (1939). we have no information on the temperature of the lava flow, but it may have been a little lower than 1000°c because the site was near the vent. growth rates of lava domes with respect to viscosity of magmas huppert et al. (1982) theoretically estimated the viscosity of the dome lava of the 1979 eruption of soufrière of st. vincent as 2 × 1011 pa⋅s by observations of its lateral spreading. the lava dome was spread by different temperatures at each part, cooled skin, disrupted blocks and a flow front, and consequently the viscosity was determined to deviate from the simple model. huppert et al. (1982) called it «effective viscosity». laboratory experiments – since the 1930’s, the viscosities of silicate rock-forming minerals and igneous rock melts at the temperature range higher than 1150°c have been measured in the laboratory. goto (1997) studied the viscosities of silicate melts from some japanese volcanoes, including the 1944 lava dome of usu volcano and the 1992 one of unzen volcano (table i). he applied the fiber-elongation method for a viscosity range higher than 108 pa⋅s (roughly temperature range lower than 950°c) and counterbalanced sphere method for a viscosity range lower than 105 pa⋅s (roughly temperature range higher than 1150°c). at present, the viscosity range 105∼ 108 pa⋅s is still left experimentally unsettled and only interpolation methods are applicable. in such viscosity ranges, magmas are half-solidified and usually common to the magmas forming lava domes. when we mention the viscosities of lavas in discussing dome formation in table i, they are reduced to the same temperature of 1000°c by interpolation. in this case, we refer to the experimental results obtained by murase et al. (1985) and goto (1997) that viscosities of melts of igneous rocks increase roughly 101 times with temperature decrease of 100°c in a temperature range around 1000°c. 959 table i. growth rates of the lava domes and viscosities of the lavas measured in laboratory. dome type, lava type, sio2 growth rate viscosity (a) reference volcano dome volume (%) log (m3/day) logη (pa·s) (viscosity meas.) (×107 m3) (d) (e) squeeze-type soufrière s.v. (1979) ba 3.5 55 5.73 7.3 (b), 11.3 (c) huppert et al. (1982) colima (1998) a 0.04 59 5.58 lamington (1951) a 6 59 6.20 (5.7) (6.3) redoubt (1989) a 2 60 6.34 soufrière hills (1996) a 6.8 60 5.30 tarumai (1909) a 1.5 60 6.58 (7.0) 6 kani and hosokawa (1936) bezymianny (1956) a 4.2 60 6.15 (4.6) (5.2) shiveluch (1980) a 1 61 5.27 (5.4) pelée (1902) a 3.7 62 5.50 mt. st. helens (1980) d 1.0 63 5.92 (4.6) (4.6) ca. 6 murase et al. (1985) santiaguito (1922) d 20 64 5.20 (4.9) (4.6) 7.7 (c) rose (1973) popocatépetl (1996) a 1.1 64 5.26 unzen (1992) d 5 66 5.70 6.0 goto (1997) usu (1944) d 4.4 69 5.05 (5) 6.8 goto (1997) novarupta (1912) d 0.5 73 4.40 (6) spine-type lamington (1951) a 0. 07 60 4.82 pelée (1902) a 0.003 62 4.45 a – andesite; d – dacite; ba – basaltic andesite; (a) – reduced to 1000°c; (b) – deduced from petrology (huppert et al., 1982); (c) – field data (temperature probably 1000°c); (d) – after newhall and melson (1983); (e) – after swanson et al. (1987), for long-term formation. 960 izumi yokoyama furthermore, viscosities of natural magmas are controlled by water, bubble and crystal contents which are not always easily determinable in situ. recently viscosities of several volcanic rocks for a dissolved water content ranging from dry to 3 or 4% for wide temperature ranges were studied by romano et al. (2003), giordano and dingwell (2003) and giordano et al. (2004); hydrations remarkably lower the viscosities of dry melts. sparks (1997) discussed large increases in magma viscosity resulting from degassing and microlite growth as causes of pressurisation in lava dome eruptions. it is not easy for us to measure the properties of actual magmas in situ such as viscosity, water and crystal contents, and laboratory measurements of rock viscosities remain approximate for a certain viscosity range. in order to avoid uncertainty about the viscosities of ascending magma, i later propose a conception of «macroscopic viscosity». 3. classification of lava domes and modes of their formations 3.1. classification of lava domes lava domes have been defined in various ways. for example, bardintzeff and mcbirney (1998) define six principal types of domes based mainly on their morphological aspects: a) cryctodomes, b) plug domes, c) peléan domes, d) spines, e) lava domes and f) coulées. the present paper does not always adhere to such definitions. cryptodomes are theoretically possible, but i do not know of any examples geophysically verified. in fact, as discussed by yokoyama (2004), during the 1944 lava dome formation of usu volcano, a 65 m high mound was caused by magma intrusion through a conduit, but the magma top remained at a depth of 100 m. it is not always necessary to assume magma intrusion directly beneath the mounds. peléan domes are defined as collapsed parts of plug domes. some lava domes are formed by the spread of relatively fluid lava over squeezed domes. from the standpoint of formation mechanism, plug domes and spines should be different from the others; they extrude through vents in an almost solidified state, even if the inner part remains soft. the possibility of formation of a lava dome and its morphology change with the viscosity of magmas. lava domes are highly possible with dacitic compositions and rare with basaltic compositions in the order of silica contents. 3.2. types of lava dome formation the substantial mode of lava dome formation is squeezing, or forced extrusion, of magma through a narrow opening similar to toothpaste. such a behavior is a characteristic of the bingham fluids. this mode shows a wide variation, such as the above-mentioned six types defined by final morphology. from the standpoint of magma movements at vents, we may classify two types: squeeze of fluidal or half-solidified magma (squeeze-type), and extrusion of solidified magma (spine-type). these two also include a variation according to the viscosity of magma, endogenous pressure, size of vents and degrees of magma solidification. squeeze-type – usually magmas at high temperature are fluidal. as the magma ascends through a conduit as a viscous flow, it half solidifies due to lowering in temperature and pressure. the boundary viscosity between liquid and solid is approximately 1013 pa⋅s (e.g., huppert, 1982). half-solidified magma behaves like a plastic bingham body and piles up at the vent; this is a lava dome of squeeze-type. we assume magma movements of this type exactly or approximately obey hagen-poiseuille law. spine-type – when magma lifts through a conduit, sometimes it solidifies totally or partly en route beneath the vent and extrudes as a solid spine, the interior of which may remain soft. extrusions of solidified lava or lava spines are not similar to squeezes of fluidal or half-solidified lavas but are displacements of solid lava bodies overcoming friction of the ground around conduits. the solid lavas are driven up by fluid magma beneath, whose viscosity effects their displacements. as will be seen later, their growth rates are far smaller than the squeeze-type because of larger resistance between solidified spines and conduit walls. growth rates of lava domes with respect to viscosity of magmas rare examples of the spine-type in the 20th century were found in the 1902-1903 eruption of mt. pelée and in the 1951 eruption of lamington volcano. both the spines may have been fluid flows at the early stage, and changed to plug flows, in which the central part had a uniform speed, before becoming solidified at the top part. in some cases, during formation of domes of squeeze-type, spinescent extrusives have appeared at the surface — the 1944 usu dome (yokoyama, 2004) and the 1996 soufrière hills dome (watts et al., 2002), for example. such apparent spines may be lava blocks solidified at or near the surface, and deeper parts must remain fluidal. 3.3. alternation of formation and destruction of lava domes the history of volcanic eruptions shows that dome formation and demolition have alternated, often being repeated several times. shiveluch volcano, kamchatska has finished five cycles of production and demolition since the 19th century and the last formed in 1980 (dvigalo, 1988). in the case of tarumai volcano, hokkaido, the 1874 eruption destroyed the pre-existing lava dome (i), and the next eruption in 1909 produced the new lava dome (ii) at the site of the previous lava dome (i) (oinouye, 1909). in the following examples, the last eruptions demolished the preexisting lava domes, and another production may be expected in the future. the 1982 eruption of el chichon volcano, chiapas – no historical eruptions of el chichon volcano were known before 1982. according to duffield et al. (1984), growth of a lava dome following excavation of a crater had occurred at least twice in the past. before the 1982 eruption, there were two internal nested domes within the summit oval crater measuring 1900 × 900 m. the larger central dome was conical with a maximum relative height of approximately 200 m, and the other is the smaller flank dome. the 1982 eruption destroyed the central dome and caused pyroclastic flows at the last phase, leaving the 1982 crater which is approximately 1 km diameter and approximately 240 m depth (macias et al., 1997). the 1982 eruption of galunggung volcano, java – in the history of this volcano, the 1833 eruption produced lava dome (i) which was destroyed by the 1894 eruption, and the 1918 eruption produced lava dome (ii) named mt. jadi, measuring 560 × 440 × 85 m on the crater lake. on april 5, 1982, a new activity took place and the pre-existing lava dome (ii) was destroyed within one month. the activity stopped in january 1983 leaving a cinder cone on the crater floor (katili and sudradjat, 1984). 3.4. examples of lava dome formations in the following, each type of dome formations will be exemplified by the domes formed in the 20th century to estimate their growth rates. later we apply the hagen-poiseuille law to the dome formation under constraint that the dome is driven by liquid. 3.4.1. lava domes of squeeze-type a few papers have compared growth rates of several lava domes. newhall and melson (1983) reviewed the dome growth of more than 70 volcanoes in historical times, and related it to their explosive activity. among their data, duration of dome growth ranges from 0.3 to 2600 weeks. one of their conclusions is that the average rate of dome growth shows no systematic relationship to the timing or character of explosions. swanson et al. (1987) discussed the volumetric growth of a composite dome during 1980 ∼ 1983 on mt. st. helens. nine dominantly nonexplosive episodes of dome growth after 1981 were separately described. furthermore, they compared the growth of four relatively long-lived contemporary lava domes at mt. st. helens for 1980∼ 1983 (3.2 years), lamington for 1951∼ 1952 (1.4 years), bezymianny 1956∼ ∼ 1982 (27.3 years) and santiaguito 1922∼ 1982 (61.5 years). they remark that the dome, its feeding conduit and its magma reservoir are in a delicate balance, able to alternate over short periods of time between dominantly explosive and effusive activity. 961 962 izumi yokoyama during dome formations, magma movements fluctuate sometimes repeating the rise and fall in growth of lava domes. this may be attributed to changes in the driving pressure of magma from the standpoint of the hagenpoiseuille law. on the other hand, melnik and sparks (1999) interpreted large changes in the dome extrusion rate and pulsatory patterns of dome growth observed during the 1995-1999 eruption of the soufrière hills, montserrat by the nonlinear effects of crystallization and degassing in the ascending magma. as far as we assume hagen-poiseuille law for magma flows in the formation of lava domes of squeeze-type, the following conditions are necessary in collecting the data. the flows must be stationary for a certain period, possibly a few weeks, and explosions and collapses of the dome have not intervened in the period. to assume stationary magma flows, it is desirable to select larger domes, such as larger than 107 m3, more or less in the final volume because smaller domes can be formed even by unstable processes. the following examples proved not to satisfy the above conditions because their volumes are not sufficiently large. the 1989 dome of lascar volcano, northern chile – (the dome volume was roughly 106 m3 after matthews et al., 1997), the 1990-1992 dome of galeras volcano, colombia (4 × 105 m3 after calvache and williams, 1997), the 1991 dome of mount pinatubo, philippines (3∼ 6 × 105 m3 after daag et al., 1996) and the 1992 dome of merapi volcano, java (2 × 106 m3 after subandriyo et al., 1992). in the following, lava dome formations for which quantitative data exist to estimate growth rates will be reviewed. the examples shall be mentioned in order of the sequence of eruptions, and growth rates are expressed in m3/day for practice in field observations, not in the si system, and accompanied with the error ranges, not probable errors. the 1902 dome of mt. pelée, martinique – according to lacroix (1904), the 1902 dome formed at the summit during the period from may to october 1902. the beginning of dome formation may be assumed as the day of the catastrophe, may 8, and the dome may be assumed to have been completed on october 4, when a new spine began to extrude above the dome. the duration of dome formation is roughly 150 days and its volume as of october 4 is estimated in the sketches given by lacroix (1904, fig. 28) as 3.7 × 107 m3, on the assumption that the dome is a circular cone. then, the growth rate is q = 3.7× 107 m3/120 days = 3.1× 105 m3/day. the error range in this estimate is derived from the errors in estimation of volume of the dome; it can be erroneous ±10%. both the magmas of the 1902 dome and the 1902 spine of mt. pelée were andesite, but they manifested a large difference in their formation processes because the former was a squeeze flow and the latter was solidified at some depth beneath the vent before extrusion. the sio2 content of the lava dome is 62% (lacroix, 1904, p. 573). the 1909 dome of tarumai volcano, okkaido – the formation of this lava dome is discussed in detail by yokoyama (2004). the lava dome formed in the summit crater in 4 days in april 1909. the andesite magma was squeezed through the vent of 30 m diameter in a half-solidified state. q=1.5 × 107 m3/4 days = 3.8 × 106 m3/day. the error range in this estimate is derived from the errors in estimation of duration and volume of the dome; the duration is 4 ± 2 days and the volume can be erroneous at ± 10%. kani and hosokawa (1936) measured the viscosity of the dry melt from the tarumai lava dome, and the results are extrapolated to approximately 106 pa⋅s at a temperature of 1000°c. the reynolds number of the lava flow in the conduit during the formation of the tarumai dome was very small (2.2 × 10−3) and hence the flow was laminar (yokoyama, 2004). the sio2 content of the dome lava is 60%. the 1912 dome of novarupta dome, katmai, alaska – the 1912 eruption in the valley of ten thousand smokes was discussed in detail by hildreth (1983) and fierstein et al. (1997). in june 1912, magma of approximately growth rates of lava domes with respect to viscosity of magmas 15 km3 erupted from the novarupta caldera, and quasi simultaneously, at mt. katmai, 10 km e of novarupta, a 600 m deep caldera formed. the novarupta dome is blocky and a circular knoll of 380 m diameter and 65 m high is centered within the 2 km diameter basin of subsidence that was the source of the 1912 ejecta. it apparently reached the final level of formation at the end of the eruptive sequence, possibly in several months after the last major pumice eruption. its volume is approximately 5× 106 m3 which barely satisfies the condition upon the minimum volume of lava domes. if we take «several months» as 200 days, the growth rate is q = 2.5× 104 m3/day. the error range in this estimate is derived from the errors in estimation of duration and volume of the dome; duration is 200 ± 50 days and the volume can be erroneous at ± 10%. the sio2 content of this lava dome ranges 65∼77%, and is 73% on the average (hildreth, 1983). the 1922-1925 santiaguito dome, guatemala – according to rose (1972, 1973), santa maria volcano erupted in 1902 for the first time in history, producing pyroclastics of 5.5 km3 and a large and deep crater was formed on its sw slope. in june 1922, a lava dome (santiaguito) rose in the 1902 crater and grew to a relative height of 400 m by the end of 1923. rose (1973, figs. 1 and 7) showed a diagram of the estimate of the magma extrusion rate during the historic activities of the volcano. the highest rate for the period 1922∼1925 is given in annual rate as q = 60 × 106 m3/yr = 1.6 × 105 m3/day. the error range in this estimate can be ± 20% of the value if we consider ambiguity in the original diagram. rose (1973) calculated the viscosity of the 1932-1933 lava flow of santiaguito near the vent as 5 × 107 pa⋅s, as mentioned previously. the sio2 content of the 1967 dome rock is 64% (stoiber and rose, 1969). the 1944 lava dome of usu volcano, hokkaido – the formation of this lava dome was fully discussed in the previous paper (yokoyama, 2004). this dome is unique in that it was formed at the base of a volcano, flat ground, not within a crater. this is a variation of squeeze-type. the upward movement of magma was slow and the uppermost part was solidified in the course of upheaval while the deeper part remained partly solidified and movable. the top part was successively followed by emergence of solidified slablike lavas, forming an onion structure. the volume of the dome including the subsurface part was estimated at 4.4 × 107 m3 by yokoyama (2004). the dome grew up rather uniformly for the early 13 months while the activity continued for approximately 17 months in a combined total. hence, the typical growth rate of the dome is obtained as q = 4.4 × 107 m3/13 months = 1.1 × 105 m3/day. the error range in this estimate is derived from the errors in estimation of subsurface volume of the dome; it can be erroneous at −25%. as mentioned above, goto (1997) measured the viscosity of dry melts from the lava dome as 106.8 pa⋅s at a temperature of 1000°c. the magma is dacite of sio2 content 69%. the 1951 dome of mt. lamington, papua – our knowledge on the 1951 eruption of lamington volcano entirely depends on the paper of taylor (1958). a gigantic eruption occurred at the summit of the volcano on january 21, 1951. a lava dome began to extrude on january 25 and the first phase ended on march 5 when it reached a height of 450 m and the paroxysmal explosion shattered the dome causing pyroclastic flows. the growth curve of the dome in height is given by taylor (1958, fig. 154) and the maximum rate is 30 m/day between february 3 and 9. he states that this rate is probably the highest recorded for the dome uplift. in the present paper, growth rates are defined as increases in volume of domes. then, the growth rate is calculated for the period from january 25 to march 5 under the assumption that the dome was a cone with 730 m basal diameter (taylor, 1958, fig. 4) and 450 m height. then we have q = π /4⋅7302× 450/3/40 days = 1.6×106 m3/day. 963 964 izumi yokoyama the error range in this estimate is derived from the error in estimation of volume of the dome; it can be + 20%. it is noticeable that a knife-edged spine emerged from the center of the dome on august 19, similarly to that at mt. pelée in 1902. this spine will be referred to later. the sio2 content of the lava dome is 59% (after taylor, 1958). the 1956 dome of bezymianny volcano, kamchatka – according to gorshkov (1959), the first historical eruption of bezymianny volcano began on october 22, 1955 after threeweeks’ earthquake swarm. the activity had developed into the paroxysmal explosion on march 30, 1956 and the new crater had the shape of a semi-ring of 1.5 km by 2 km in size. following the explosion of march 30, which produced large pyroclastic flows, an extrusive dome grew in the new crater. by early july, after one month, the dome was mostly completed, approximately 320 m high above the crater floor and 340 m in average diameter. its volume, the sum of the two trapezoids, is estimated at 4.2 ×107 m3. the growth rate of the dome is q = 4.2×107 m3/30 days = 1.4×106 m3/day. the error range in this estimate is derived from the errors in estimation of duration and volume of the dome; the duration can be 40 days and the volume can be erroneous at ±10%. the activity continued till march 1957, and was repeated during march 1965 to march 1970 accompanying pyroclastic flows, and later numerous explosive episodes took place with lava extrusion. the lava of the 1956 dome is andesite and 59.9 in sio2 percentage (gorshkov, 1959). the 1979 dome of the soufrière of st. vincent, west indies – the 1979 eruption of soufrière volcano was first discussed by shepherd et al. (1979). the eruption began on april 13 and a series of strong explosions continued until april 26. the extrusion of lava was observed from about may 6 until its final emplacement after 5 months. huppert et al. (1982) applied a theoretical analysis to the radial spreading of the lava extrusion accumulated in the crater, and estimated the «effective» viscosity at 2×1011 pa⋅s. on the other hand, the 1909 tarumai dome did not deform noticeably after the emplacement (yokoyama, 2004) though its formation was not observed quantitatively. it is probably because the tarumai lava is andesite and was plastic after the squeeze. huppert et al. (1982) also estimated the viscosity at 2.1×107 pa⋅s at 1000°c from the petrological model assuming a phenocryst content of 45% and dacitic liquid phase. huppert et al. (1982, table ii and fig. 11) published the growth data of the soufrière dome during the period may to october 1979 when the growth rate had gradually decreased. they interpreted that the lava ascended under a decreasing hydrostatic driving pressure. the growth rate at the early stage for 65 days (may 7∼july 10) is obtained as q = 3.5 × 107 m3/65 days = 5.4 × 105 m3/day. in table ii of huppert et al. (1982), the highest rate is 6.3 × 105 m3/day for 19 days (may 7 to 25); the error range in the above estimate can be indicated by this value.the sio2 content of the 1979 lava is 55.0% (basaltic andesite, huppert et al., 1982). the 1980 domes of mt. st. helens, washington – the great eruption of mt. st. helens on may 18, 1980 was followed by formation and collapse of lava domes in the newly formed crater 1.5× 3 km across. moore and albee (1981) presume the bulge at the summit just before the outburst as a cryptodome of 0.11 km3 in volume directly beneath the summit. i already remarked that such a bulge could be formed by upward thrust of ascending magma. swanson et al. (1987, table 2 ) obtained the average growth rate as 0.014 km3/yr (3.8 × 104 m3/day) for the period of 1981∼1983. during the period, lavas extruded step by step and a composite dome continued to grow. the present paper selects the relatively short-lived magma extrusions, 4 steps in 1980 and 5 steps in 1981 (swanson et al., 1987, table 1). these domes were partly destroyed by subsequent explosions one after another. the average growth rate of the 9 steps for 34 days is obtained as q = 28.5 × 106 m3/34 days = 8.4 × 105 m3/day. growth rates of lava domes with respect to viscosity of magmas on the other hand, moore et al. (1981, table 58) compared dimensions, volume, growth periods etc. of three domes in june, august and october 1981. the mean growth rate of the three domes is estimated as 9.6 ×105 m3/day. this estimate indicates the error range. murase et al. (1985) measured viscosity of the dome lava (melt) in the laboratory. the viscosity reduced to 1000°c is approximately 106 pa⋅s. the sio2 content of the dome materials on mt. st. helens ranges from 61 to 64% (after swanson et al., 1987). the 1980 lava dome of shiveluch volcano, kamchatka – the volcano has produced lava domes 5 times in the explosion crater since the 19th century. during the 1980-1981 eruption, a new lava dome of approximately 150 m height formed. dvigalo (1988) observed the growth of the dome with a photogrammetric method for the period from july 1980 to march 1982. the growth rate was the maximum for the first survey period (july to october 1980) amounting to q=1.86 × 105 m3/day. in this estimate, the volume can be erroneous at ± 10%. the sio2 content of the lava dome is 61%. the 1989-1990 lava dome of redoubt volcano, alaska – during six months of the 19891990 eruption, the volcano repeated a domegrowth and -destructive phase in which 14 shortlived andesite domes were formed and 13 subsequently destroyed. miller (1994) estimates the cumulative volume of the domes dividing the period of six months into three segments, i, ii and iii, and calculates the growth rate of each segment: segment i produced the largest dome with the highest growth rate. the lava domes in segments ii and iii were not sufficiently large in volume to show stable growth rates. the growth rate in segment i is given by miller (1994, table 3) as q = 20×106 m3/9 days = 2.2 × 106 m3/day. the volume of the dome is erroneous at ± 5×106 m3. the sio2 content of the dome lava is 59.8% on the average (miller, 1994, table 2). the 1990-1992 lava domes of unzen volcano, kyushu – unzen volcano reawoke at the summit crater in november 1990 after 198 years’ quiescence. on may 20, 1991, magma extruded in the summit crater, forming lobes and domes which flowed very gently and showed elongated forms. thereafter, magma was continuously supplied to form successive domes. seven lava domes were formed at the summit during may 1991 to april 1992. new domes pushed and partly overrode the preexisting domes to form an onion structure that resulted from squeeze processes, similar to the 1944 lava dome of usu volcano (yokoyama, 2004): the unzen lava domes belong to the squeeze-type of half-solidified magma. afterwards the domes repeatedly collapsed to form pyroclastic flows along the slope. according to nakada (1996), the magma effusion rate was highest in september 1991 amounting to q = (4 ∼ 6) × 105 m3/day. thereafter it decreased with time before stopping in 1995. the error range in the above estimate is ambiguity of growth periods. goto (1997) measured the viscosity of dry melts of the 1992 unzen lava dome as 106.0 pa⋅s at 1000°c. the sio2 content of the dome lava is 66% on an average (after yanagi et al., 1992). the 1995-1997 lava domes of soufrière hills volcano, montserrat – phreatic explosions began in july 1995 at this volcano, and were followed by a continuous eruption of andestic magma in the form of a lava dome in november 1955. the eruptions have been monitored and discussed from various standpoints. the growth patterns of the lava dome during the period of november 1995 to december 1997 are illustrated and the growth rate are graphically reported: sparks et al. (1998, fig. 2) present the extrusion rates as a function of time and watts et al. (2002, fig. 3) present the change in dome volume. we search the periods of monotonous growth to estimate growth rates of the lava dome. considering the change in dome height given by melnik and sparks (2002, fig. 1), we select two periods from february 17 to september 30, 1996 (stage iii), and october 1, 1996 to may 13, 1997 (stages iv and v). the latter period is selected on the condition that the higher 965 966 izumi yokoyama growth rate is preferable. the growth rate through stages iv and v is graphically determined in the diagram of change in dome volume given by watts et al. (2002, fig. 3b) q = 68 × 106 m3/340 days = 2.0 × 105 m3/day. naturally this roughly agrees with the values given by sparks et al. (1998, fig. 2) and the error range is given by the growth rate in stage iii. the sio2 content of the dome lava is 59.5% on an average (murphy et al., 1998). the 1996 lava dome of popocatépetl volcano, central méxico – the summit of popocatépetl volcano is 5450 m a.s.l. and located approximately 50 km distant from méxico city. popocatépetl had repeated small explosive eruptions in 1920-1922 and produced a small lava plug in the summit crater, but no lava flows. the present activity began in 1994. after the first outburst on december 24, 1994, lava domes formed several times in the elliptical summit crater, which measures 600 m by 400 m, and is approximately 330 m deep, measured from the lower crater rim. according to global volcanism network (1996) and s. de la cruz-reyna (personal communication), dome a in the summit crater was found first on march 25, 1996 from the air and had grown to 1.1 × 107 m3 by may 25 when the growing stopped. then the growth rate is q = 1.1 × 107 m3/61 days = 1.8 × 105 m3/day. the error range in this estimate is derived from the errors in estimation of volume of the dome; it may be erroneous at ±10%. the bulk of dome a had been demolished by repeated explosions by august 1996. the sio2 content of the lava is 64% (robin and boudal, 1987). the 1998 lava dome of colima volcano, western méxico – colima volcano (3850 m a.s.l.) has experienced at least 50 eruptions since 1560. the 1913 summit eruption produced disastrous pyroclastic flows. after that, lava rose through the vent, and block lavas gradually filled the 200-m-wide summit crater by 1957. the latest series of eruptions began in march 1991, and lavas extruded onto the summit crater resulted in block flows and ash flows on the south flank of the volcano. navarro-ochoa (2002) reports on the emplacement of the november 1998-february 1999 lava flows. on november 20, 1998, a lava dome grew rapidly at a rate of 4.4 m3/s inside the summit crater and its volume amounted to 3.8 × 105 m3 in 24 h. on the following day, the lava dome began to collapse, causing the largest pyroclastic flows on november 25-26, and thereafter, block-lava flows went down in three branches. their volume is estimated at approximately 3.9 × 107m3. in this case, the dome is small in volume but may be assumed to have been formed by hagen-poiseuille law because its growth rate is rather high and it was shortly followed by a large amount of lava flows. we take the growth rate as q = 3.8 × 105 m3/day. the error range in this estimate is derived from the errors in estimation of duration and volume of the dome; both of them can be erroneous at ±10%. the sio2 content of the lava is 59% (navarro-ochoa, 2002). 3.4.2. lava domes of spine-type there are only two examples of this type that have been monitored quantitatively, as far as the present author knows. one is the 1912 lava spine of mt. pelée, and the other is the 1951 lava spine of lamington volcano. they were solidified lavas driven by underlying magmas through conduits, and met with resistance at the conduit walls. hence, their growth rates do not obey hagen-poiseuille law. the 1902 lava spine of mt. pelée – this lava dome is characterized as an extrusion of a huge solidified spine. a lava dome formed at the summit of mt. pelée from may to october 1902. during this period, on may 8, pyroclastic flows occurred, devastating the town of st. pierre along the sea. according to lacroix (1904), a spine extruded above the top of the dome after the dome was completed. the extrusion of the spine first began on the night of november 3-4, 1902. it may have ascended growth rates of lava domes with respect to viscosity of magmas through the vent of the preceding lava dome. the 1902∼1903 lava spine of mt. pelée measured 190 m high above the terrace in march 1903 and approximately 60 m across at its base; some parts were partially hollow. it is different from the 1944 lava dome of usu, which grew laterally after the lava reached the surface and completed an onion structure. the spine repeated noticeable growths 3 times, and small collapses with short steps, during the period from november 1902 to september 1903, as illustrated and sketched by lacroix (1904, figs. 29 ∼ 35). the total amount of extrusion is estimated at approximately 620 m. lacroix (1904) reported that the maximum growth rate of the spine was 10 m/day for november 3∼ 4. if we assume the section of the spine as a 60 m-diameter-circle (lacroix, 1904, fig. 55), the rate of magma flow to cause the rise of the spine is approximately q = π /4⋅602×10 m3/day = 2.8 × 104 m3/day. the error range in this estimate is derived from the errors in estimation of sectional diameter and ascending velocity of the spine; both of them may be erroneous at ±10%. the upward movement of the solidified spine through the conduit may have met with more friction than fluidal flows. the lava is andesite with sio2 content of 62% (lacroix, 1904). the 1951 lava spine of lamington volcano – the 1951 eruption of lamington volcano was accompanied by extrusion of a spine. a lava dome began to extrude after the gigantic eruption in march 1951. when the dome reached a height of approximately 450 m in march, it was partly destroyed. in july a lava spine began to extrude at the center of the dome. its growth curve is given by taylor (1958, fig. 4). the uplift rate of the spine is estimated at 83 m/10 days by the growth curve, and the spine is assumed to be a cone with roughly 175 m basal diameter (taylor, 1958, fig. 140) and 83 m height. then we have q = π /4⋅1752× 83 × 1/3 m3/10 days = = 6.6 × 104 m3/day. the error range in this estimate is derived from the errors in estimation of sectional diameter of the spine, and it may be erroneous at ±10%. according to taylor (1958), this monolith showed neither plastic deformation nor slickensides. the sio2 content of the spine is 60% (after taylor, 1958). 4. growth rate of lava domes of the squeezetype and macroscopic viscosity we select the 15 lava domes of the squeezetype formed in the 20th century with reliable observational data, and we can discuss their growth rates statistically. lava flows may be modeled as viscous or plastic fluids. flows of dome-forming lava are usually laminar, as confirmed by the 1909 lava dome of tarumai volcano (yokoyama, 2004). if we know the growth rates of lava domes, it is possible to find a relationship among the physical parameters of lava dome formation, such as viscosity of magmas, conduit dimension and driving pressure, by hagen-poiseuille law which is conditioned for fluids to flow through a narrow space. in the present case, the growth rate q of lava dome is expressed as (4.1) where r denotes the radius of the vent, ∆p driving pressure, ∆p/l pressure gradient, l vertical length of the vent, ρ density of lava, g gravity acceleration and η viscosity of lava. among these factors, ∆p is liable to fluctuate due to degassing and crystallization in the magma. the viscosity η determined by eq. (4.1) involves the effects of chemical components (mainly sio2 and water), crystallization and temperature averaged for the period of dome formation, and their changes with time and space. actually the temperature may be around 1000°c. we may call such η «macroscopic viscosity». from eq. (4.1) we obtain (4.2) where (4.3).logk r l p g 8 4 $= r t ∆ c m log logq k= h q r l p g 8 4 = r h t ∆ c m 967 968 izumi yokoyama the growth rates of the lava domes described in the previous chapter are tabulated in table i where those estimated by newhall and melson (1983) and swanson et al. (1987) are additionally shown for reference. the growth rates are also plotted in logarithmic scale against sio2% of lavas in fig. 1, where some of the growth rates (ordinate) have a rather high degree of uncertainty and their error ranges are calculated and indicated by the bars attached to the plots. the growth rates of the spines of lamington and pelée volcanoes are indicated by star symbols in fig. 1 for reference, and are significantly smaller than those of their domes, respectively, because solidified or half-solidified spines may meet with higher resistance through conduits. their formation processes do not obey hagen-poiseuille law, and they are excepted from the following discussion. as shown in fig. 1, the relationship between the logq and the sio2% is roughly linear though the plots are rather scattered, and classification of the plots into two groups, upper and lower, is not justified because of their inaccuracy. we may assume that logη can be replaced by sio2% in eq. (4.2) and that k does not vary with the volcano. their relationship is determined by the least squares method as (4.4) where the correlation coefficient is − 0.68. from eqs. ((4.2) and (4.4)), we obtain approximately (4.5) hence, we may label 104.3, 105.2 and 106.1 pa⋅s in macroscopic viscosity for 50, 60 and 70 in sio2%, respectively on the abscissa of fig. 1. as the data increase in number, the parameters in eq. (4.4) should change necessarily. at present, macroscopic viscosities calculated by eq. (4.5) are not directly comparable with the lava viscosities experimentally determined at 1000°c. however, the former is usually lower than the latter as shown in table i. this is reasonable if we consider the effects of volatile material and crystallization in real lavas. the growth rate q most strongly depends on r and next on η, ∆p, l and ρ in eq. (4.1). the linearity of the diagram (logq ∼ sio2%) suggests that q changes in logarithmic proportion to sio2 percentage. constant term k in eq. (4.2) proves to be roughly common in logarithmic scale among various volcanoes. this is plausible when we consider that both toosmall-radius and too-large-radius conduits are not favorable for lava dome formation and that too shallow and too deep magma positions are not probable. radii of the conduits feeding lava domes must be a few tens of meters in common. driving pressure is liable to fluctuate as observed in some dome forming eruptions . ( %) .log sio0 087 2]h . . ( %)log sioq 11 04 0 087 2= fig. 1. logarithm of q (growth rate of lava dome) versus sio2%. the two domes of spine-type (star symbols) are excepted from the least squares to determine the best fit line. the bars attached to the plots indicate the ranges of errors. η along the abscissa denotes «macroscopic viscosity» with exponents in round figures. msh – mt. st. helens; souf sv – soufrière of st. vincent; souf hm – soufrière hills, montserrat. growth rates of lava domes with respect to viscosity of magmas (melnik and sparks, 1999) and must have its limit because lava domes have a limit of their heights, a few hundred meters at maximum above the surface. the result shown by eq. (4.5) is simple, but not a result of oversimplification. we should take into account that the growth rates may be correct in order of magnitude and the log-linear diagram is rather limited in resolvability. it means that growth rates in some eruptions may be affected by unsteadiness in driving pressure. 5. concluding remarks firstly physical properties of magmas, temperature and viscosity, are examined in relation to lava dome formation. growth rates of lava domes are expected to undergo the strongest effect by viscosity of magmas. modes of dome formation are classified into two, squeezeand spine-types, from the standpoint of the mechanism of magma movements. the majority of the lava domes mentioned in the present paper belongs to the squeeze-type. squeezes of viscous and plastic magmas through conduits should satisfy hagen-poiseuille law, by which growth rates are related to the viscosity of magmas. the viscosity involved in the law should be different from the normal viscosity of lavas, and is defined as macroscopic viscosity on the assumption that the temperature is around 1000°c during dome formation. growth rates of the 15 lava domes formed in the 20th century are determined with reference to the published quantitative data. the relationship between the macroscopic viscosity and sio2 content of lavas is statistically obtained through the agency of growth rates of the domes. formation processes of lava domes of squeeze-type are strongly controlled by macroscopic viscosity of magma. on the other hand, the parameters in hagen-poiseuille law, i.e. radius and length of the conduits, pressure gradient and density except viscosity, are found to be roughly common in logarithmic scale among various volcanoes. if we know the sio2 percentage of magma, we may have a rough idea of its macroscopic viscosity that controls the growth rate of the lava dome. acknowledgements the author appreciates the assistance of dr. t. l. wright who critically read the first version of the manuscript and provided helpful suggestions for improvements. dr. a. goto kindly commented on rheology of magmas, to whom the author expresses deep thanks. this manuscript benefited very much indeed from the critical comments of an anonymous referee. references bardintzeff, j.m. and a. mcbirney (1998): volcanology (jones bartlett pub., sudbury, mass.), pp. 268. booth, b. and s. self (1973): rheological features of the 1971 mount etna lavas, phil. trans. r. soc. lond. a., 274, 99-106. calvache, v.m.l. and s.n. williams (1997): emplacement and petrological evolution of the andesitic dome of galeras volcano, 1990-1992, j. volcanol. geotherm. res., 77, 57-69. daag, a.s., m.t. dolan, e.p. laguerta, g.p. meeker, c.g. newhall, j.s. pallister and r.u. solidum (1996): growth of a postclimactic lava dome at mount pinatubo, july-october 1992, in fire and mud: eruptions and lahars of mount pinatubo, philippines, edited by c.g. newhall and r.s. punongbayan (phil. inst. volcanol. seismol. and univ. wash. press), 647664. duffield, w.a., r.i. tilling and r. canul (1984): geology of el chichon volcano, chiapas, méxico, j. volcanol. geotherm. res., 20, 117-132. dvigalo, v.n. 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(received february 10, 2005; accepted august 2, 2005) vol52,1,2009 57 annals of geophysics, vol. 52, n. 1, february 2009 key words geostar – tyrrhenian sea – deep seafloor – electrical conductivity – lithosphere 1. introduction variations of the magnetic fields produced in the ionosphere and magnetosphere generate electromagnetic (em) waves that penetrate in the earth’s interior down to the crust and the mantle, inducing electric currents which, in turn, produce their magnetic counterpart at the earth’s surface. magnetovariational (mv) techniques, which make use of the effects of induction magnetic fields from such sources over an appropriate magnetometric stations distribution, disclose some geoelectric properties that characterise subsurface structures (e.g. banks, 1969; parkinson, 1983; gough, 1989; armadillo et al., 2001). during recent years it has also been possible to adapt such land based techniques directly to seafloor observations (e.g. filloux, 1987). geostar (geophysical and oceanographic station for abyssal research) missions belong to a series of european projects, led by the istituto nazionale di geofisica e vulcanologia (ingv), having as main target some long-term deep-sea geophysical investigations. the automatic multidisciplinary station was designed to geostar deep seafloor missions: magnetic data analysis and 1d geoelectric structure underneath the southern tyrrhenian sea sergio vitale (1) (2) (4), angelo de santis (1) (2), domenico di mauro (1), lili cafarella (1), paolo palangio (1), laura beranzoli (1) and paolo favali (1) (3) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) università degli studi «g. d’annunzio», chieti scalo (ch), italy (3) università degli studi «la sapienza», roma, italy (4) now at eni s.p.a., san donato milanese (mi), italy abstract from 2000 to 2005 two geophysical exploration missions were undertaken in the tyrrhenian deep seafloor at depths between -2000 and -3000 m in the framework of the european-funded geostar projects. the considered missions in this work are geostar-2 and orion-geostar-3 with the main scientific objective of investigating the deep-seafloor by means of an automatic multiparameter benthic observatory station working continuously from around 5 to 12 months each time. during the two geostar deep seafloor missions, scalar and vector magnetometers acquired useful magnetic data both to improve global and regional geomagnetic reference models and to infer specific geoelectric information about the two sites of magnetic measurements by means of a forward modelling. mailing address: dr. angelo de santis, istituto nazionale di geofisica e vulcanologia (ingv), via di vigna murata 605, 00143 roma, italy; tel: +39 06 5186 0327; e.mail: desantisag@ingv.it vol52,1,2009 28-05-2009 13:45 pagina 57 58 s. vitale, a. de santis, d. di mauro, l. cafarella, p. palangio, l. beranzoli and p. favali meet the restrictive requirements of a standard land-based observatory, in spite of the extremely harsh environmental operating conditions. in europe, the series of geostar projects is a unique approach and it has been implemented since 1995 in two successive steps (geostar and geostar-2), and then integrated in 20022005 by the orion-geostar-3 (ocean research by integrated observation network) project with the purpose of developing a prototype for a network of seafloor observatories. here, we describe the validation of the prototype in the bathyal plain of the tyrrhenian sea (marsili basin). further information on all geostar projects and other seafloor projects can be found in a recent review (favali and beranzoli, 2006). this work presents the analysis of magnetic data from the two deep-sea floor missions: geostar-2 and orion-geostar-3, with a short description of the geostar observatory and mission plans. 1.1. the magnetic purposes of geostar projects during the geostar missions some of the earth’s tectonic processes (such as seismicity, geomagnetic and gravity fields) and physical, geochemical and biological processes which occur on the seafloor environment with potential impact on geo-hazards and global changes were monitored. the measurements of the geomagnetic field on the sea bottom are fundamental to complement land recordings in order to have a full analysis of the earth’s magnetic field (emf). measuring the emf on the deep seafloor has some evident advantages: 1) the temperature stability over time, since any change of temperature can affect the threecomponent magnetometer performance causing some artificial drifting; 2) the improvement of the knowledge of the emf itself through a better measurements coverage; 3) slowly varying fields are practically unperturbed while rapidly varying external coverage magnetic fields are screened by the seawater layer. the magnetic data acquisition on the seafloor is much more difficult than inland. the main problems arise from the increase in pressure with depth (about 0.1 atm/m), corrosion (especially for long periods of running, such as more than a few months), difficulties on vector instruments orientation according to the geographical directions and possible em disturbances due to dynamo actions of the sea water motion (mean conductivity σw ≈ 3-6 s/m) within the emf. in spite of the above considerations, in terms of emf observations, the geostar projects have given significant contributions to demonstrate: 1) the potential of an almost equal-area distribution of long-term points of observations all over the world to improve the reliability of global (e.g. igrf) and regional magnetic field models; 2) the study of the magnetic field temporal variations from short to long periods (seconds to years), even in marine extreme environment where it is not easy to install a «traditional» observatory; 3) the investigation of the conductivity structures within the earth by means of mv techniques; 4) the study of the emf radial variation in correspondence with oersted (1999 present), champ (2000 present) and future swarm satellite missions. 1.2. structure of geostar observatory the whole idea behind the geostar project’s concept took inspiration from the experience of nasa during apollo and space shuttle missions, where the «two-body» system was a winning approach. analogously, the architecture of geostar observatory includes a mobile docker vehicle (called modus – mobile docker for underwater sciences) and a bottom station. the latter module can run autonomously for long periods (over one year) and it can be employed for abyssal depths (up to 4000 meters). modus, properly manouvred onboard a ship, allows the deployment and the recovery of the bottom station directly from the surface (ship facilities), and it is used for the system check and bi-directional communication between ship and bottom station, when it is connected to the station. the bottom station, with a tubular cube shaped structure in aluminum alloy, was devol52,1,2009 28-05-2009 13:45 pagina 58 59 geostar: magnetic data analysis and geoelectric structure underneath the southern tyrrhenian sea signed to host all the acquiring instruments, acquisition and control systems, the communication system and the power supply of the station. for its multidisciplinary nature, geostar can be equipped with different instruments in relation with to purpose of the mission. the standard configuration foresees a gravimeter, vector and scalar magnetometers, seismometer, hydrophone, acoustic doppler current profiler (adcp), conductivity temperature depth sensor (ctd), transmissometer, currentmeter, and water sampler. the magnetometers installed in two benthic spheres at the ends of two 2-m long booms. these «arms» are positioned at the two opposite corners of the bottom station and they are kept in vertical position during the descent phase. when the bottom station reaches the seafloor, the booms extends along the horizontal position upon operator command. this separation is essential to minimize the effects of induced current of the bottom station in the magnetic records. the communication system is designed to allow three different transmission methods. the first is committed to some special capsules (named «messengers») which are periodically released by the station to reach the sea surface (geostar-2 configuration) and here they establish a satellite communication, to notify their position; the recorded data are stored in these «messengers». data are also fully stored in onsite hard-disks, recoverable at the end of the mission. the second communication method is a bi-directional acoustic transmission. this latter technique provides a real-time communication of the station with the «mother» ship. in this way an operator can periodically download the acquired data and can also change some parameters of the station according the necessities of the mission. additionally, a surface buoy serves as radio/satellite bridge communications between the underwater observatory and an inland station (see e.g. favali et al., 2006). 2. geostar deep seafloor missions 2.1. locations both geostar deep seafloor missions, namely geostar-2 and orion-geostar3, were undertaken in the tyrrhenian sea. locations are shown in fig. 1 and geographical coordinates in its caption. geostar-2 had a duration of about seven months (from september 25, 2000 to april 16, 2001). the station was deployed at 1950 m depth in an abyssal plain sw of the ustica island. orion-geostar-3 had a double duration in comparison to the preceding mission. it lasted about fifteen months in total but divided in two legs: the first from december 14, 2003 to april 24, 2004 and the second from june 13, 2004 to may 23, 2005. the station was deployed in an abyssal plain, nw of the marsili seamount, reaching a depth of 3320 m. this paper uses only the three-component magnetic data acquired during the second part of the mission. 2.2. southern tyrrhenian sea the tyrrhenian sea represents a back-arc basin and its evolution, still in progress, has developed since the upper cretaceous in a complex geodynamic frame within the collisional system between the european and african plates (dewey et al., 1989) and has been increasingly involved in the alpine-apennines orogenesis since the eocene (scandone, 1980; malinverno and ryan, 1986; sartori, 1990; gueguen et al., 1998). the southern part of the sea, of our interest, is characterized by a great stretch process that caused the formation of the two main subbasins: vavilov (7-3.5 ma) and marsili (1.7-1.2 ma) (bigi et al., 1989; cella et al., 1998), with their respective volcanic structures. in the southern tyrrhenian sea we find two main gravimetric anomalies centered in the two major sub-basins (rehault et al., 1986), where the lithospheric thickness is about 50 km and the moho depth reaches about 10 km. in a surrounding regional heat flow of about 120 mw/m2, there are two heat flow anomalies greater than 200 mw/m2 (e.g. mongelli and zito, 1994) placed within the vavilov and marsili basins; also magnetic data taken at sea surface show strong anomalies within the whole basin. vol52,1,2009 28-05-2009 13:45 pagina 59 60 s. vitale, a. de santis, d. di mauro, l. cafarella, p. palangio, l. beranzoli and p. favali the marsili basin is characterized by a stretch process in ese direction, with a seafloor depth of about 3500 m. the rocks types are basalts and andesites, depth of the moho of about 11 km, with lithosphere’s thickness less than 30 km, heat flow rate more than 200 mw/m2 and magnetic anomalies typical of expansion basins (marani and trua, 2002). an important aspect concerns the volcanic structure of the basin, since marsili seamount is a morphologic anomaly that rises from the seafloor for about 3000 m. its shape is lengthened for about 50 km in nne-ssw in an axial direction and the perpendicular minor axis extends for about 16 km in wnw-ese direction. this structure shows the main middle oceanic ridge (mor) features, typical for axial or periaxial zones; magnetic anomalies are positive in the axial zone and negative along the flanks (marani and trua, 2002). 3. magnetometers and preliminary data calibration in both geostar deep seafloor missions, a couple of magnetometers were used: a scalar magnetometer and a vector (three-component) fig. 1. locations of the deployments of geostar bottom station in the thyrrenian sea. geostar 2 geograph. coord.: 38°32’24” n, 12°46’30” e; geostar 3 geograph coord.: 39°29’12” n, 14°19’52” e. vol52,1,2009 28-05-2009 13:45 pagina 60 61 geostar: magnetic data analysis and geoelectric structure underneath the southern tyrrhenian sea magnetometer. the scalar magnetometer was an overhauser proton type. for the purpose of the missions, this instrument was an adaptation of the commercial model gsm-19l by gem system inc. it was characterized by a resolution of 0.1 nt, accuracy of 1 nt, a power consumption of 1 w and a sampling rate of 1 sample/minute. the vector magnetometer was a suspended three-axis fluxgate magnetometer, developed by ingv. it was characterized by a resolution of 0.1 nt, accuracy of 5-10 nt, a power consumption of 2 w and a sampling rate of 6 samples/minute/component. during the geostar-2 mission, vector magnetometer recorded almost 100% of the expected amount of data, but the scalar magnetometer recorded only about 8% because an electronic device failure reduced the sampling rate from 1 sample/minute to only 1 sample every 12 minutes. in orion-geostar-3, the expanding booms were damaged during the deployment operation, preventing storage of x, y, z measurements from the vector magnetometer, while the scalar magnetometer worked properly all the time. in the second part of the mission, the vector magnetometer recorded data for 100% of the time while the scalar magnetometer returned data corresponding to the first 42% of this part of the mission. the recorded data cannot be directly used for analysis, because the acquired magnetic data are affected by magnetic disturbances caused by induced currents from external magnetic fig. 2. apparent resistivity as given by the forward models for geostar-2 (left) and oriongeostar-3 (right) missions. d e p th ( m ) d e p th ( m ) resistivity (ohm-m) resistivity (ohm-m) vol52,1,2009 28-05-2009 13:45 pagina 61 62 s. vitale, a. de santis, d. di mauro, l. cafarella, p. palangio, l. beranzoli and p. favali field variations (mainly due to the geostar structure), and by the non-perfect orientation of the geostar frame. some calibration and orientation corrections were applied on the recorded magnetic data, with respect to a ground station used as reference (de santis et al. 2006a). 4. magnetic data analysis and forward models to consider the ionospheric fields as polarising fields, the usable periods in magnetic data analysis at the sea bottom at depths of 2-3 km are in the band 5h>t>3 min, this because variations with smaller periods are screened while those with greater periods do not satisfy the condition κ2 < 2πµσ/τ, where k is the spatial wavenumber and µ is the magnetic permeability; in addition for greater periods sea tides and water motions can be significant and produce disturbing local magnetic fields. a forward model which takes the behaviour of the electrical conductivity in depth (or its reciprocal, the resistivity) into consideration was obtained by using software named «ix1dv3» by interpex (www.interpex.com). first of all, we calculated the apparent resistivity values by means of the apparent electrical conductivity profiles for each mission. we then imported these values of resistivity in the software and tried to build a more realistic behaviour of the resistivity profiles. at the end we succeeded in developing two models that took into account the conductivity variations. as we can see in fig. 2, a lower resistivity appears under the first 5 km of geostar-3 site, probably due to the more complex tectonic and volcanic processes in the marsili area. regarding the lithospheric bottom under the two sites, it can be located from 15 to 45 km for geostar-2 mission and from 10 to 12.5 km for orion-geostar-3 mission, with values of resistivity of 30 ωm and 10 ωm respectively, clearly less than the values of surrounding resistivity. values of lithospheric depth found by means of the forward models confirm those found from previous magnetic data analyses (de santis et al., 2006b) and from seismic data (calcagnile and panza, 1981). 5. conclusions the deep seafloor geostar-2 and orion-geostar-3 missions have provided an important magnetic dataset, useful both for the definition of conductivity structures underneath the seafloor and for improving the geomagnetic models. starting from the three geomagnetic components, we have been able to provide 1d geoelectric models under the two deep seafloors of geostar-2 and geostar-3 missions in the southern tyrrhenian sea, in particular the identification of the bottom of the em lithosphere under the two sites. moreover, our estimations on the identified depths are in accordance with the literature based on independent data, mostly seismic data. in future, more analyses will be needed to uncover more details and properties of the tyrrhenian crust and mantle to confirm the current results and possibly improve them in time, space and frequency domains. acknowledgements the geostar and orion projects were funded by the ec under the marine science and technology programme. we are ver grateful to christopher turbitt of british geological survey for his suggestions to improve the paper. discussions with patrizio signanini, bruno di sabatino of chieti university helped some parts of the work. we thank also all the people who were involved directly or indirectly in both projects: without them no possible result or work would have been found or realized. references armadillo, e., e. bozzo, v. cerv, a. de santis, d. di mauro, m. gambetta, a. meloni, j. pek and f. speranza (2001): geomagnetic depth sounding in the northern apennines (italy), earth planets space, 53, 385-396. banks, r.j. (1969): geomagnetic variations and the electrical conductivity of upper mantle, geophys. j. r. astr. soc., 17, 457-487. bigi, g., a. castellarin, r. catalano, m. coli, d. casentino, g.v. dal piaz, f. lentini, m. parlotto, e. patacca, a. praturlon, f. salvini, r. sartori, p. scandone and g.b. vai (1989): synthetic structuralvol52,1,2009 28-05-2009 13:45 pagina 62 63 geostar: magnetic data analysis and geoelectric structure underneath the southern tyrrhenian sea kinematic map of italy, scale 1:2.000.000 (cnr, progetto finanziato geodinamica, roma). calcagnile, g. and g.f. panza (1981): the main characteristics of the lithosphere-asthenosphere system in italy and surrounding regions, pure appl. geophys., 119, 865-879. cella, f., f. fedi, g. florio and a. rapolla (1998): gravity modelling of the litho-asthenosphere system in central mediterranean, tectonophysics, 287, 1-4. de santis, a., d. di mauro, l. cafarella, r. d’anna, l.r. gaya-piquè, p. palangio, g. romeo and r. tozzi (2006a): deep seafloor magnetic observations under geostar project, annals of geophysics, 49 (2-3), 681-693. de santis, a., d. di mauro, l. cafarella, p. palangio, l. beranzoli, p. favali and s. vitale (2006b): extending magnetic observations to seafloor: the case of geostar and orion missions in the adriatic and tyrrhenian seas, publs. inst. geophys. pol. acad. sc., c-99, 398, 114-122. dewey, j.f., m.l. helman, e. turco, d.h.w. hutton and s.d. knott (1989): kinematics of the western mediterranean, in alpine tectonics, edited by m.p. coward and d. dietrich, geol. soc. spec. publ., 45, 265-283. favali, p. and l. beranzoli (2006): seafloor observatory science: a review, annals of geophysics, 49 (2-3), 515567. favali, p., l. beranzoli, g. d’anna, f. gasparoni , j. marvaldi, g. clauss, h.w. gerber, m. nicot, m.p. marani, f. gamberi, c. millot and e.r. flueh (2006): a fleet of multiparameter observatories for geophysical and environmental monitoring at seafloor, annals of geophysics, 49 (2-3), 659-680. filloux, j.h. (1987): instrumentation and experimental methods for oceanic studies, in geomagnetism, vol. 1, edited by j.a. jacobs, (academic press.), pp. 143–246. gough, d.i. (1989): magnetometer array studies, earth structure, and tectonic processes, reviews of geophysics, 27 (1), 141-157. gueguen, e., c. doglioni and m. fernandez (1998): on the post-25 ma geodynamic evolution of the western mediterranean, tectonophysics, 298, 259-269. malinverno, a. and w.b.f. ryan (1986): extension in the tyrrhenian sea and shortening in the apennines as result of arc migration driven by sinking of the lithosphere, tectonics, 5, 227-245. marani, m.p. and t. trua (2002): thermal constriction and slab tearing at the origin of a super-inflated spreading ridge: the marsili volcano (tyrrenian sea), j. geoph. res., 107, 2188, doi:10,1029/2001jb000285. mongelli, f. and g. zito (1994): thermal aspects of some geodynamical models of tyrrhenian opening, boll. geof. teor. appl., xxxvi (141-144), 21-28. parkinson, w.d. (1983): introduction to geomagnetism, (scottish academic press ltd.), pp. 433. rehault, j.p., j. mascle, a. fabbri, e. moussat and m. thommeret (1986): the tyrrhenian sea before leg 107, proceedings of the ocean drilling program, national science foundation joint oceanographic institutions inc., (part a initial report 107). sartori, r. (1990): the main results of odp leg 107 in the frame of neogene to recent geology of perityrrhenian areas, proc. ocean drill. program sci. results, 107, 715-730. scandone, p. (1980): origin of the tyrrhenian sea and calabrian arc, bollettino società geologica italiana, 98, 27-34. vol52,1,2009 28-05-2009 13:45 pagina 63 layout 6 the rigidity of the earth's inner core keith edward bullen annali di geofisica, vol. 6, n. 1, 1953. annals of geophysics, vol. 53, n. 1, february 2010 abstract the evidence for the existence of significant rigidity in the earth's inner core is summarised and discussed in the light of recent work. quantummechanical calculations, based on a thomas-fermi-dirac model, suggest that an earlier estimate of 3.6 × 10¹² dyn/cm² for this rigidity may need to be reduced by 0.5 × 10¹² dyn/cm². the theoretical travel-times of the phase pkjkp would then need to be increased by amounts ranging from zero to 40 seconds. it is shown that the use of gutenberg's seismic data in place of that of jeffreys would reduce the estimated rigidity of the inner core by a further 1.6 × 10¹² dyn/cm², approximately. it is suggested that gutenberg's data lead to the lowest likely value for the rigidity of the inner core, namely 1.5 × 10¹² dyn/cm², so that, on all present evidence, it is improbable that the inner core is less than about twice as rigid as steel is at zero pressure. 1. the purpose of this paper is to examine and assess, in the light of recent evidence, the theory that the earth's inner core has a significant rigidity. the presence of an inner core in the earth is revealed from observations of the seismic phase pkp in the «shadow zone» for which the epicentral distance ∆ lies in the range 105˚< ∆ <143˚. miss i.lehmann in 1936, followed by gutenberg and richter in 1938, attributed these observations to the presence of an inner core; and jeffreys in 1939 [1939a] applied airy's theory of diffraction near a caustic to show that the alternative theory of diffraction round the outer boundary of the central core was not capable of explaining the observations in the shadow zone. the existence of the inner core has been fairly generally accepted since this calculation of jeffreys. 2. the work of jeffreys [1939b] on the p-velocity, α say, in the central core favours, the view that the inner core is composite, consisting of an outer region f [bullen 1947] occupying the range 1250 km < r < 1390 km, where r denotes the distance from the centre, and an inner region g of radius 1250 km; (the region e refers to the outer part of the core, for which 1390 km < r < 3470 km; and the region d to that part of the mantle for which 3470 km < r < 5400 km, approx.). in the solution given by jeffreys, the value of α in f decreases with increase of depth from 10.44 km/sec to 9.5 km/sec (the latter figure corresponds to a rate of decrease of a taken arbitrarily to be linear by jeffreys), while, in g, α increases slowly from 11.16 km/sec at the boundary to 11.3 km/sec at the centre; across the boundary between f and g, there is a discontinuous jump in a. the existence of the region f in which the velocity gradient is negative is not accepted by all authors, for example gutenberg [1951]; (the existence of f rests on a hypothesis of reflection at the inner core boundary which jeffreys considers may well be true in the light of various supporting evidence). gutenberg's velocity distribution, on the other hand, gives a steady increase of α from 8.0 km/sec at the outer boundary of the central core to 10.1 km/sec at r = 1250 km, then a sharp increase in the gradient of α, with no discontinuous change in α itself, resulting in a value of α equal to 11.2 km/sec at about r = 1000 km; from here to the centre, gutenberg finds to be nearly constant. 3. following calculations [bullen 1940, 1942] on the pressure and density distributions in the earth, the present writer noted in 1946 that the indicated values of dk/dp, where k denotes the incompressibility and p the pressure, were not sensibly different in the outer part of the central core from the values in the lower mantle down to a depth of 2700 km. (this result is indicated most clearly by consideration of the relation dk/dp = 1 – g¯¹dø/dr , which applies to a homogeneous material, where g denotes the acceleration due to gravity, ø = α² – 4 ß²/3, and ß denotes the s-velocity). moreover, the change in k itself was so small as to be not necessarily significant. all well-established geophysical data could in fact be fitted by assuming that k and dk/dp change smoothly between the mantle and the core. at the same time, it was noted that the materials of the mantle and core are widely different in respect of the density ρ and rigidity µ. further, in 1946, it was widely accepted that the chemical compositions of the mantle and core are distinct, that the mantle is composed of ultrabasic rock, and the outer core of iron or nickel-iron. and, in addition, 1 laboratory investigations at high pressure had shown that, in general, the differences in the compressibilities of different materials diminish with increasing pressure. it these circumstances, the writer proposed as a trial working hypothesis [bullen 1946, 1947, 1949a] that: a) the compressibility of a material and its pressure gradient are independent of chemical composition at pressures and temperatures of the order of those prevailing in the earth's deep interior (say below a depth of 1000 km). 4. when the hypothesis (a) was first set down, it had not been envisaged that there would soon be a serious challenge to the view that the mantle and outer core are chemically distinct. following work of kronig, de boer and korringa [1946], however, ramsey [1948, 1949] and the writer [bullen 1949b], on the basis of planetary data, have held that there is a probability that the region e consists essentially of a pressure modification of ultrabasic rock; (ramsey, but not the present writer, holds that the inner core, also, is caused by pressure). several authors, notably elsasser [1951], urey [1952] and birch [1952], have put forward opposing arguments to the modified-utrabasic-rock core theory, but the opposing arguments themselves have been shown [bullen 1952a, 1952b] to be subject to several weaknesses; the present writer's view is that it is still at least moderately probable that the region e and the mantle are not sharply distinct chemically. in these circumstances, the original hypothesis (a) is possibly too broad a generalisation, so that it becomes necessary [bullen 1950a, p. 58] to consider the less general hypothesis that: b) a phase transition occurs at the boundary between the regions d and e of the earth and leaves the compressibility and its pressure gradient unchanged on the two sides of the boundary. at the same time it needs to be remarked that k, if determined by the relation k/ρ = dp/dρ, would be discontinuous at the transition point itself, so that there are no a priori physical grounds for expecting (b) to hold. thus, on the emprical evidence stated in section 3, (a) may well still be a fair approximation for a class of materials whose representative atomic number is close to that of the region e. 5. on the hypothesis (a), it is to be expected that the values of k and dk/dp in the regions f and g are close to those at the base of e. this would, however, be impossible on the results of jeffreys for the p-velocity α, if the inner core is fluid; (we ignore the extremely improbable happening that the density decreases with increase of depth in this part of the earth). on the jeffreys data, α, increases suddenly at r = 1250 km by 15 per cent, so that if the inner core were composed of fluid for which k = ρα², there would be at least a 32 per cent jump in k across the boundary. if, on the other hand, the inner core possesses significant rigidity µ, with k + 4 µ/3 = ρα² , then k will remain continuous across the boundary if µ has a value equal to about 25 per cent of that of k in the inner core. the hypothesis (a), in conjunction with the data of jeffreys, thus leads to the theory, stated in 1946 and developed in 1949 and 1950 (8, 9, 18), that the earth's inner core is solid in the sense that description of its elastic behaviour under stresses of periods of a few seconds requires the presence of a rigidity parameter of order 3.6 × 10¹² dyn/cm². this conclusion has received support in further work of ramsey [1950]. the diminution of α with increase of depth found by jeffreys in the region f, in conjunction with the relation k = ρα², incidentally entails that, in f, either ρ increases rapidly or else k diminishes rapidly. on the hypothesis (a), the former alternative is preferred, the total increase in ρ in f being 25 per cent. also the relatively low gradient of α inside g entails that the average density gradient in g is more than twice as great as the density gradient in the region e. 6. a consequence of rigidity in, the inner core is that the inner core could transmit s seismic waves. on the basis of the jeffreys data and the hypothesis (a), theoretical tables for a seismic phase pkjkp have been constructed [bullen 1950b], where the symbol j corresponds to s waves in the inner core. companion energy calculation [bullen 1951] show, however, that such a phase could be only on the border of observability. a comprehensive survey by burke-gaffney [bullen and burke-gaffney 1952] of the whole series of seismograms recorded at the riverview college observatory between 1909 and1952 shows that none of these seismograms quite conforms to the theoretical requirements for a firm identification of the phase pkjkp, though a few observations could contribute to a wider statistical examination which would include data from a number of observatories. further, the unknown character of the region f leaves it possible that the energy in any s waves generated in g would be even less than the energy calculated [bullen 1951] on the assumed boundary conditions. it follows that the rigidity of the inner core can at best be established by this direct seismic method only through intensive studies of seismograms followed by the use of searching statistical significance tests. 7. in the meantime, it is desirable to examine the sources of evidence for a solid inner core in some further detail. points on which further investigation can usefully be made at present include: i) a comparison of the effects of using gutenberg's and jeffreys' observational data on α in the inner core; ii) questions arising from the forms that the compressibilitypressure hypothesis might take; the rigidity of the earth's inner core 2 3 iii) the bearing on the problem of quantum-mechanical data at extremely high pressures. 8. in considering (i), it will be sufficient to make an approximate calculation based on gutenberg's velocity data quoted in section 2. we shall represent this data by α² = 11.2² – a (r – 1000) (1) where a = (11.2² – 10.1²)/250² , taking dα/dr to be zero at r = 1000 km, and taking the km and sec as units. the equation (1) yields 0.19 km/sec² for the magnitude of dαµ²/dr when r is just less than 1250 km, as against 0.019 km/sec² when r is just greater than 1250 km, and implies that the magnitude of d { (k + 4 µ/3)/ρ } dr suddenly increases (as the depth increases) by a factor of ten at r = 1250 km. on the hypothesis (a), it would follow that, while µ and its gradient are zero in the fluid region above r = 1250 km,|d (4 µ/3ρ)/dr| changes to the value 0.17 km/sec² just below; i.e. |d (µ/ρ) dr| suddenly rises from zero to 0.13 km/sec². assuming for ß² (= µ/ρ) a form similar to (1), we then obtain for 1000 km < r < 1250 km ß² = ß∘² – b (r – 1000)² , where b = 0.13/500, and ß∘ is the s-velocity at r = 1000 km. since ß is here being taken as zero at r = 1250 km, we then have ß∘ ~ 4.0 km/sec at r = 1000 km. previous work using the data of jeffreys had yielded ß∘ = 4.8 km/sec. the jeffreys data had involved an increase in ρ from 12 to 15 g/cm³ in the region f (see section 5). if we ignore the possibility of an abnormal increase in density in the range 1000 km < r < 1250 km (such increase is neither indicated nor precluded on gutenberg's data), then the relation µ = ρß² would, on the hypothesis (a) and gutenberg’s data, yield a value of µ at r = 1000 km equal to 2.0 × 10¹² dyn/cm², approximately, i.e. 0.6 of the value yielded on the jeffreys data. if an abnormal density increase were present, the yielded rigidity would of course be greater. thus, on the hypothesis (a) and gutenberg's data, the minimum rigidity in the innermost 1000 km of the earth appears to be about 2.0 × 10¹² dyn/cm². this value is still comparable with the rigidity of the mantle, and is appreciably greater than that of surface rocks. between r = 1000 km and. the centre, gutenberg's data would, on the hypothesis (a), yield a slightly higher density gradient than the jeffreys data because of the less velocity gradient, but the density reached at the centre need not be much greater than 15 g/cm³. this is closer to the figure preferred by birch [1952], although gutenberg's data do not preclude an appreciably higher value. a further point is that whereas the main change in µ would take place suddenly on the jeffreys data, it is only the gradient of µ that would change suddenly on the gutenberg data, the change in µ being spread over 250 km. the boundary conditions of the inner core would then be very different from those assumed in the energy calculations [bullen 1951] for pkjkp, and the energy transformed into s waves in the inner core would be very much less. thus, if gutenberg's velocity distribution is approximately correct, the phase pkjkp is quite unobservable, and it would not be possible to establish the rigidity of the inner core directly by observations of these waves. 9. the point (ii) of section 7 on the possible limitations of the hypothesis (a) of section 3 will now be considered in relation to the inner core. if the view of elsasser, urey and birch that the region e consists of iron is substantiated, then the hypothesis (a) remains, as in 1946, an important working hypothesis. it is likely to be reliable at least to the extent that, for atomic numbers between about 18 and 29, k and dk/dp at pressures of 1.3 million atmospheres differ by not more than the uncertainties in k and dk/dp in the writer's earth model a [bullen 1947, 1950a]. in model a, the formal solution gave a reduction (not necessarily significant) of 5 per cent in k at the boundary between the regions d and e. ramsey [bullen 1949b; ramsey 1950] prefers a greater change in k at this boundary, but his argument involves deviation from homogeneity in the region d to an extent that birch [1952] regards as improbable. ramsey [1950] finds dk/dp for the mantle and core to be about the same, namely 3.7 and 3.53.8, respectively, while the writer's estimates yield 3.2-3.3 near the base of the mantle, and the same or a little less at the top of the core. birch [1952], on independent grounds, finds the value in the mantle to be 3.3. in these circumstances, application of the hypothesis (a) to the inner core probably does not lead to errors in k and dk/dp of more than the order of 5 per cent. on the assumption that the region e consists of iron, and in the light of cosmic abundance data for elements, it is strongly probable that any deviation from the hypothesis (a) will involve changes of k between e. and g smaller than between d and e. and therefore much smaller than would be required to bring the value of µ obtained in section 5 down to zero. a 5 per cent jump in k near the boundary of g would still require the rigidity µ in the inner core to be nearly 3.1 × 10¹² dyn/cm² on the jeffreys data. if, on the other hand, the region e consists of modified ultrabasic rock, the remarks in section 4 become relevant. the rigidity of the earth's inner core the rigidity of the earth's inner core 4 on the view of ramsey that the region g is not chemically distinct from either e or d, the expected change in k between e and g would not be greater than between d and e, and the argument favouring rigidity in g is essentially unchanged. but the case favoured by the writer on present evidence [bullen 1949c, 1950c, 1952a], in which g consists of iron or denser materials while e has the same chemical composition as the ultrabasic-rock material of d is more difficult. the remark in the last paragraph of section 4 shows that there is still some probability that the value of k in g is very close to that in the lower part of e, but a less general hypothesis than (a), closer to (b) of section 4, would not lead of itself to any inference on the rigidity of g, and further considerations are desirable. 10. such further considerations come from the point (iii) of section 7; this supplements the argument in section 9 in cases in which an estimate of the rigidity is yielded, and also goes some distance towards filling the gap in the more difficult case. several authors, including jensen [1938], feynman, metropolis and teller [1949], have considered the compressibility-pressure relations for thomas-fermi-dirac gas models. jensen, elsasser [1951] and the writer [bullen 1952b] have discussed the bearing of relevant parts of the results on the composition of the earth's core. elsasser quotes data at pressures of the order of 10 to 30 million atmospheres, according to which k incre₁ases by 8 per cent when the atomic number z changes from 18 to 36. the pressure, p₁ say, at the boundary of the region g is approximately 3.2 million atmospheres, and elsasser's comparison of experimental results of bridgman with the theoretical high-pressure results indicates about the same proportionate change of k with z at the pressure pi as at the higher pressures. the writer has amended certain parts of elsasser's work, and finds that bridgman's results, geophysical data and the feynman data are well fitted together by taking smaller values of k at the pressure p₁ than those assumed by elsasser; this procedure raises the proportionate change in k when z changes from 18 to 36 to about 12 per cent at the pressure p₁. most theories of the composition of the earth's core require the difference in z between the outer part of the region g and the base of e to be nearly zero; the writer's theory, referred to in section 9, gives the biggest difference, of order seven units. thus the quantum-mechanical calculations indicate that any jump in k between e and g is of the order of 5 per cent or less, and that, on the jeffreys velocity data, the rigidity of the inner core is still not less than the minimum value arrived at in section 9. this argument is of course subject to uncertainties in the interpolations between bridgman's data and the quantum-mechanical data, and again falls well short of deductive proof, but, in combination with the earlier discussion, leaves it most probable that the inner core is significantly rigid. 11. gutenberg's data show appreciably smaller changes in α than jeffreys' data in the vicinity of the boundary of the inner core, and may be reasonably taken as indicating the least allowable changes in α. taking then the lowest estimate of µ in section 8, diminished by a possible 0.5 × 10¹² dyn/cm² in accordance with the discussions in sections 9 and 10, we arrive at the value 1.5 × 10¹² dyn/cm² as the least value of the rigidity in the inner core that is reasonably compatible with all present evidence. this value is less onehalf the value 3.6 × 10¹² dyn/cm² involved in earlier calculations of the writer [bullen 1950a, 1950b, 1951], but is still about twice the rigidity of steel at zero pressure. as in earlier papers, temperature effects have been disregarded in all the foregoing discussions. birch's recent work [1952] gives further confirmation that this would not lead to serious error. if the rigidity of the inner core were in actual fact as low as the above suggested minimum value of 1.5 × 10¹² dyn/cm², there would be repercussions on the calculations of the travel-times of the phase pkjkp, and the time of the j rays would need to be increased by a factor of 1.3, approximately. this would entail increases ranging from zero up to about 2½ minutes in the calculated times. if the phase pkjkp arrived as late as this, however, the theoretical traveltime table [bullen 1950b] would be devoid of practical value, since, on gutenberg's data (see section 8), the phase would not then be observable. but from the discussion in sections 9 and 10 it follows that, even on the jeffreys data, the times of the j rays may need to be increased by a factor of 1.08, approximately. this would entail increases in the calculated times for pkjkp, reaching nearly 40 seconds at ∆ = 180˚. it follows that, in seeking evidence on the occurrence of the phase pkjkp, it is desirable to consider readings on seismograms ranging up to perhaps a minute later than the times in the calculated table. the existence of the phase could be established only if sufficient of such readings fitted to adequate precision a travel-time curve possessing the right gradient. this consideration emphasises further that the existence of the phase pkjkp can be established only by statistical examination of a large number of readings. references birch, f. (1952). elasticity and constitution of the earth's interior, j. geophys. res., 57, 227-286. bullen, k. e. (1940). the problem of the earth's density variation, bull. seism. soc. amer., 30, 235-250. bullen, k. e. (1942). the density variation of the earth's central core, bull. seism. soc. amer., 32, 19-29. bullen, k. e. (1946). a hypothesis on compressibility at 5 pressures of the order of a million atmospheres, nature, 157, 405. bullen, k. e. (1947). an introduction to the theory of seismology, cambridge, ch. xiii. bullen, k. e. (1949a). compressibility-pressure hypothesis and the earth's interior, m.n.r.a.s., geophys. suppl., 5, 355-368. bullen, k. e. (1949b). on the constitution of venus, m.n.r.a.s., 109, 457-461. bullen, k. e. (1949c) on the constitution of mars, m.n.r.a.s., 109, 688-692. bullen, k. e. (1950a). an earth model based on a compressibilitypressure hypothesis, m.n.r.a.s., geophys. suppl., 6, 50-59. bullen, k. e. (1950b). theoretical travel-times of s waves in the earth's inner core, m.n.r.a.s., geophys. suppl., 6, 125-128. bullen, k. e. (1950c). venus and the earth's inner core, m.n.r.a.s., 110, 256-259. bullen, k. e. (1951). theoretical amplitudes of the seismic phase pkjkp, m.n.r.a.s., geophys. suppl., 6, 163-167. bullen, k. e. (1952a). cores of terrestrial planets, nature, 170, 363-364. bullen, k. e. (1952b). on density and compressibility at pressures up to thirty million atmospheres, geophys. j. int., 6, 383-401. bullen, k. e. and t. n. burke-gaffney (1952). detection of s waves in the earth’s inner core, nature, 170, 455. elsasser, w. m. (1951). quantum-theoretical densities of solids of extreme compression, science, 113, 105-107. feynman, r. p., n. metropolis and e. teller (1949). equations of state of elements based on the generalized fermithomas theory, phys. rev., 75, 1561-1572. gutenberg, b. (1951). pkkp, p'p' and the earth's core, trans. amer. geophys. union, 32, 373-390. gutenberg, b. and c. richter (1938). p' and the earth's core, m.n.r.a.s., geophys. suppl., 4, 363-372. gutenberg, b. and c. richter (1939). on seismic waves, gerlands beitr. geophys., 54, 94-136. jeffreys, h. (1939a). the times of the core waves, m.n.r.a.s., geophys. suppl., 4, 548-561. jeffreys, h. (1939b). the times of the core waves (second paper), m.n.r.a.s., geophys. suppl., 4, 594-615. jensen, h. (1938). das druck-dichte diagramm der elemente bei höheren drucken am temperaturnullpunkt, zeitsch. f. physik, 111, 373-385. kronig, r., j. de boer and j. korringa (1946). on the internal constitution of the earth, physica, 12, 245-256. lehmann, i. (1936). p', publ. bur. centr. séism. internat., série a, 14, 3-31. ramsey, w. h. (1948). on the constitution of the terrestrial planets, m.n.r.a.s., 108, 406-413; ramsey, w. h. (1949). on the nature of the earth's core, m.n.r.a.s., geophys. suppl., 5, 409-426. ramsey, w. h. (1950). on the compressibility of the earth, m.n.r.a.s., geophys. suppl., 6, 42-49. urey, h. c. (1952). the planets, their origin and development, yale. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the rigidity of the earth's inner core vol50,2,2007 283 annals of geophysics, vol. 50, n. 2, april 2007 key words e-learning – jigsaw method – eduseis web tools 1. introduction the educational seismological project (eduseis) is a research and experimentation project that is aimed at involving research centres, universities, scientific museums and schools in the consolidation and maintenance of a prototype of a seismographic network that is formed of high technology and low cost components (bobbio and zollo, 2000). the project is designed to promote the earth sciences (especially seismology and geophysics) and an to increase awareness of seismic risk at the level of high schools and the public at large through the «città della scienza» science centre in naples. a network of digital seismographs has been installed in a number of high schools in central and southern italy (cantore et al., 2003). through the continuous updating of seismic data, and following its analysis and processing, relevant events are selected and made available on the eduseis website (http:// eduseis.na.infn.it) (fig. 1). the project has many educational, scientific and social goals. the main educational objectives are to train students and teachers in the new tools for scientific learning in the eduseis project: the e-learning experiment antonella bobbio (1), luciana cantore (2), nicola miranda (3) and aldo zollo (2) (1) istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, sezione napoli, italy (2) dipartimento di scienze fisiche, università degli studi di napoli «federico ii», napoli, italy (3) itis «e. majorana», somma vesuviana, napoli, italy abstract the educational seismological project (eduseis) is a scientific and educational project, the main aim of which is the development and implementation of new teaching methodologies in earth sciences, using seismology as a vehicle for scientific learning and awareness of earthquake risk. within this framework, we have recently been experimenting with new learning and information approaches that are mainly aimed at a high school audience. in particular, we have designed, implemented and tested a model of an e-learning environment in a high school located in the surroundings of the mt. vesuvius volcano. the proposed e-learning model is built on the eduseis concepts and educational materials (web-oriented), and is based on computer-supported collaborative learning. ten teachers from different disciplines and fifty students at the itis «majorana» technical high school (naples) have been taking part in a cooperative e-learning experiment in which the students have been working in small groups (communities). the learning process is assisted and supervised by the teachers. the evaluation of the results from this cooperative e-learning experiment has provided useful insights into the content and didactic value of the eduseis modules and activities. the use of network utilities and the «learning community» approach promoted the exchange of ideas and expertises between students and teachers and allowed a new approach to the seismology teaching through a multidisciplinary study. mailing address: dr. antonella bobbio, istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, sezione napoli, via coroglio 156, 80124 napoli, italy; e-mail: bobbio@na.infn.it 284 antonella bobbio, luciana cantore, nicola miranda and aldo zollo analysis and interpretation of seismological data. for this purpose, modules and teaching activities have been designed and tested using computer science and web-oriented tools (cantore et al., 2005). the scientific objective is to introduce into high schools the use of advanced instruments and experimental methods that are usually restricted to research laboratories, with the main product being the creation of an earthquake waveform archive. thus a large amount of this data can be used by the students and teachers for educational purposes. for the social objectives, the project represents an effective instrument for informing and creating an awareness of the seismic risk, for experimentation into the efficacy of scientific communication, and for an increase in the direct involvement of schools and the general public. 2. the e-learning experiment in the itis «majorana» technical high school e-learning has been defined as the use of new multimedia technologies and the potential of the internet to improve the quality of teaching/learning. according to this definition, rather than being a new tool for formal education that can be used in the classroom, e-learning should be considered as a modern method for teaching and learning that integrates and completes the work in classroom. on the «alpi» platform of the edulab cultural association, an e-learning model has been built that is based on the eduseis concepts and its related educational materials. this model is based on computer-supported collaborative learning (ligorio, 1995; trentin, 1998), which is an innovative method for teaching and learning that adopts modern information and communication technologies. the eduseis educational activities (e.g., earthquake location and magnitude estimate, seismogram analysis) are mainly based on web and internet communication tools and are suitable for this kind of experimentation. determination of their teaching value is also one of the main educational objectives of this project. a number of students at the itis «majorana» technical high school (naples) who are following the teaching strategy of the «learning fig. 1. the eduseis website (http://eduseis.na.infn.it). 285 new tools for scientific learning in the eduseis project: the e-learning experiment community» and the «jigsaw» method (clarke, 1994) have been involved in this cooperative elearning experiment. the basic idea is that the students work in small groups (communities) where each member evolves gradually from an initial stage as a «fellow learner». this is followed by an intermediate level as a «teacher», and then finally as a «scientist». the learning process is assisted and supervised by the teachers, who have already improved their use of the internet and their knowledge of earth science. 2.1. the jigsaw method at the majorana school the jigsaw method uses the cooperative concept to promote individual learning, whereby this kind of structure creates interdependence and status equalization (clarke, 1994). this jigsaw method is a very useful tool for helping students to integrate knowledge and understanding that has been drawn from various sources and experts. indeed, the basic idea behind the jigsaw method is very simple: a general theme and a number of related topics are identified, and the application of this strategy is carried through the following steps: – define the general theme that the class will work on; – split the class in small groups of students; – assign to each group a topic related to the general theme. each group member will become an expert in this topic, using didactic material and support from the teachers. – re-mix the students into new groups that include these experts of each topic; – reassemble the original groups where each member teaches the rest of the group what they have learnt about the other topics; – the groups each present their own analysis of the general theme to the entire class. 2.2. trial of the e-learning platform with the eduseis tools ten teachers from different disciplines and fifty students at different educational levels have been involved in the trials around this eduseisbased e-learning platform. the fifty students were divided into five groups, so that students of different levels were present in each of these groups (miranda, 2004). the general theme chosen for this experiment was seismology. this choice was prompted by the collaboration between the school and the research staff of the eduseis project. indeed, during this collaboration, teachers of scientific disciplines trained themselves on and experimented with the tools provided by the eduseis website, which they evaluated as being suitable for this e-learning experiment. at the same time, to test the background knowledge of the students participating in the project, students were asked to initially complete a multiple-choice questionnaire on seismology and earth sciences in general. during the «kick-off» meeting, the teachers showed the «edulab» platform architecture to all of the student groups and illustrated the guidelines for using the tools built on this platform (edulab, 2001). one of each of the five didactic units concerning different topics on seismology that have been set-up on the eduseis website were attributed to each group; these five units relate to the following: – world-wide seismicity and earthquake genesis; – seismometry and seismic networks; – seismic waves and their propagation; – earthquake classification according to epicentral distance; – observation and interpretation of seismograms (complexity analysis). in the first step, each group became the experts on their own topic. this was achieved by their using both the teaching material available from the eduseis website, and advanced searching on the internet to improve their knowledge and to acquire further expertise on the subject (fig. 2). the exchange of data and information among the students within each group occurred through the internet, using the tools within the chat and forum services for document up-loading and down-loading, which were all made available through the platform. for each student the first approach is to consult and use the eduseis material and tools available on the website, improving the knowledge on the topic through an internet research, the results of which have been shared with each member of its own group 286 antonella bobbio, luciana cantore, nicola miranda and aldo zollo using the forum, the e-mail and the chat services. most of student activity for the e-learning experiment was carried out via computer both in the school and at home, only a 30% of all experimentation phases were face-to-face with and without the teachers’ presence. in this phase, all of the students were considered as being apprentices (or «fellow learners»), and they were involved in: – learning new concepts, and being opening to questioning their own knowledge; – accessing new information using various internet communication channels; – discussing their own ideas, doubts and problems, through the chat and forum services available through the platform. as a major result of this phase, a number of documents (e.g., powerpoint presentations, word fig. 2. a scheme of the e-learning experiment at the itis «majorana» technical school, naples. fig. 3. an example of a typical product produced by the students in the 1st phase of the process. the products are included on the website and made available for the community. 287 new tools for scientific learning in the eduseis project: the e-learning experiment documents, excel data and analysis sheets) were produced by each group, and the high quality of the content demonstrated their enthusiastic participation and the success that they had obtained through working in these collaborative communities. a typical starting point for the realization of each product in the e-learning experiment is the generation of conceptual maps which are diagrams showing the relationships between concepts in the form of propositions. concepts are included within boxes whereas the relations between them are explicated by means of connecting lines. these maps can often be used to communicate complex ideas. an example of a conceptual map produced by students is shown in fig. 3. in the second phase, each group was split, and five new groups were formed in such a way that an expert on each seismology topic covered (see above) was present in each new group, so that each student could then be a teacher for the other members of their new group. in this phase, all of the students were considered as «teachers», and this involved: – sharing with each other of their own stock of knowledge; – teaching of their acquired expertise to the other members of their new group; – demonstrating the reasonableness of their own opinions to the other members of their new group. in the last phase, the original five groups were re-assembled, and each member progressed further on the basis of the experience they had gained in the previous phases. having acquired a deeper knowledge of these seismological topics, each group was ready to make full use of the eduseis activities implemented on the website, by using either the web tools supplied or analogue devices in the school laboratories. at the end of the experiment, each group presented its work to the whole community, i.e. to the classes and the teachers involved in the experiment. in this phase all of the students were considered as «scientists», and they were faced with the need to demonstrate: – their acquisition of expertise within a new theme (i.e. seismology) that is not a part of the scholastic curricula; – their ability to promote the diffusion of seismological concepts and an increased seismic risk awareness to a widespread community. 3. the eduseis didactic unit: an example the teaching unit chosen for showing the architecture of a typical didactic unit is: «seismic waves and their propagation». four topics are correlated to this theme: – section 1: seismic wave propagation. – section 2: the interior of the earth. – section 3: the seismic wave. – section 4: travel-time curves. each section contains one or more activities that are designed around the use of web tools. in the following, the classical structure of an activity will be illustrated, with this example relating to the activity: «an analysis of the p and s waves on seismograms». 3.1. objectives measurement of p-wave and s-wave first arrivals with the help of theoretical estimates from the jeffreys and bullen (1940) travel-time curves (these give the travel times of all of the main seismic phases from different depths of hypocentre to surface stations at different distances). 3.2. area of expertise: seismology the activity is subdivided into two main phases: first phase – inferring arrival times from jeffreys and bullen curves. ingredients: an event selected from the eduseis archives; the «distance applet» for the calculation of epicentral distance (fig. 4); jeffreys and bullen tables; a ruler. second phase – estimation of p-wave and s-wave arrival times inferred from seismograms. ingredients: an event selected from the eduseis ar-chives; the «seisgram2k applet» (lomax, 2000) for the visualization and analysis of seismograms; the origin time of the earth288 antonella bobbio, luciana cantore, nicola miranda and aldo zollo quake inferred from national and/or international seismic alerting agencies; the theoretical values of the arrival times inferred from the jeffreys and bullen curves. in this activity, the use of web tools under the supervision of the teachers allows the students to compute the epicentral distance (the distance from the earthquake epicentre to the point of observation) of the chosen event, and to recognize the first arrival of the p and s waves using the jeffreys and bullen curves to provide reference values. the basic idea of each of these activities is to lead the student through a «learning by discovering» process. 4. results and conclusions one of the most interesting factors that has arisen from this e-learning experiment has been to promote the working of the students and teachers within a «community of learners». in this model, the classroom is changed into a real community where everyone has different, but changing, roles, as apprentice, teacher and scientist, with the consequent exchange of tasks and responsibilities. a selection of the products from this experiment have been published on the website of the school, and these have become a useful support for the didactic activities. the involvement of students in this high-social-impact activity represents an opportunity for the increasing of their own awareness of earthquake risk. this is thus seen as improving their understanding of seismic phenomena and the potential damage arising therefrom, along with their knowledge of the measures that can be implemented to mitigate these effects. figure 5a,b shows a comparison between the input and output values of a questionnaire administered at the beginning and the end of this e-learning experiment for two different age-level classes. based on the number of correct answers, a quantitative value can be put on the knowledge that has been gained about the topic under consideration, and therefore the positive effects of this e-learning experiment. fig. 4. a web tool for a didactic activity 289 new tools for scientific learning in the eduseis project: the e-learning experiment the evaluation of the results from this e-learning experiment has also provided useful insights into the contents and the didactic value of the eduseis didactic modules and activities. the success of this e-learning experiment is due to the great involvement and motivation of the students, who have provided a large number of high quality products that testify to their enthusiasm for working in cooperative communities. the use of the network utilities for the elaboration of the final products has provided each student with the opportunity to feel that they are themselves a protagonist of the experimentation, and it has promoted the exchange of ideas and expertise between students and teachers. this innovative method that is based on the strategy of the «learning community» has, moreover, allowed a new approach to seismolofig. 5a,b. a comparison between before and after values of a questionaire administered at the beginning and the end of the experiment for 1st and 2nd leveles (a) and 3rd and 4th levels (b). a b 290 antonella bobbio, luciana cantore, nicola miranda and aldo zollo gy through a multidisciplinary study, promoting the ability to deal with such complex themes as the study of seismology. acknowledgements we are grateful to all of the teachers and students of the itis «majorana» technical school in naples who, with much enthusiasm, took on active roles in this project. we wish also to thank angela petrillo, mariella simini, andré herrero and luca d’auria for their contribution in the realization of eduseis website. we acknowledge the anonymous referee for his fruitful comments and suggestions. references bobbio, a. and a. zollo (2000): the educational broadband seismic network at naples (southern italy), orfeus newsletter, 2 (3), p. 23. cantore, l., a. bobbio, f. di martino, a. petrillo, m. simini and a. zollo (2003): the eduseis project in italy: a tool for training and awareness on the seismic risk, seismol. res. lett., 74 (5), 596-602. cantore, l., a. zollo, a. bobbio, f. di martino and m. simini (2005): il progetto eduseis un triennio di esperienze didattiche in sismologia (cuen, napoli), pp. 50. clarke, j. (1994): pieces of the puzzle: the jigsaw method, in handbook of cooperative learning methods, edited by s. sharan (greenwood press, westport, ct), 34-50. edulab (2001): associazione culturale per la ricerca sulla didattica multimediale education laboratory (http://www.edulab.it). jeffreys, h. and k.e. bullen (1940): seismological tables (british association for the advancement of science, gray milne trust, london), pp. 48. ligorio, m.b. (1995): «community of learners»: strumenti e metodi per imparare collaborando, td tecnologie didattiche, 1 (4), 22-35. lomax, a. (2000): seisgram2k: seismogram visualization software for the internet (anthony lomax scientific software mouans-sartoux, france). miranda, n. (2004): eduseis con l’ambiente alpi, e-learning: formazione, modelli, proposte, in seminario di studio università degli studi di macerata, facoltà di scienze della formazione, april 2004. trentin, g. (1998): insegnare e apprendere in rete (zanichelli, bologna), pp. 263. (received december 18, 2006; accepted february 15, 2007) annals 47, 6, 2004def 1689 annals of geophysics, vol. 47, n. 6, december 2004 key words molise region – climatic change – longterm instrumental series – fractal analysis – the cantor dust method 1. introduction the reconstruction and analysis of past climate is important not only in basic research, but also in land use management. moreover, instrumental meteorological data constitute the reference basis for setting up indirect techniques in such fields as palynology and dendrochronology, which are commonly used to investigate the history of past climate. it is widely held that a combined effect of natural and human causes could underlie recent climatic changes, even if great uncertainty still exists about their quantification (ney, 1959; kelly and wigley, 1992; halgh, 1994; stouffer et al., 1994; lassen and friis-christensen, 1995; mitchell et al., 1995; santer et al., 1995; friischristensen and svensmark, 1997; hegerl et al., 1997; lean and rind, 1999; rozelot, 2001). naturally, analysis which is carried out on a series of this kind requires great effort not only in collecting data, but also in verifying their homogeneity (barriendos et al., 2002; camuffo, 2002a,b; camuffo and jones, 2002; cocheo and camuffo, 2002; demarée et al., 2002; maugeri et al., 2002a,b). this operation ensures that the time variations observed in the meteorological parameters can be identified as climatic signals and it is thus extremely important to collect the so-called metadata, i.e. all the information about the history of the station where measurements have been taken. this study presents the results of the reconstruction and analysis of the daily precipitation and temperature series at agnone, an upland town (800 m a.s.l.) in molise, a small region in central italy. in analysing air temperature, only recent changes in rainfall and air temperature at agnone (molise – central italy) michela izzo (1), pietro p.c. aucelli (1) and adriano mazzarella (2) (1) dipartimento di scienze e tecnologie per l’ambiente e il territorio, università degli studi del molise, isernia, italy (2) dipartimento di geofisica e vulcanologia, università degli studi di napoli «federico ii», napoli, italy abstract an exhaustive daily rainfall and extreme air temperature series (1883-2000) was reconstructed for agnone, a small town in molise (central italy). long-term analysis identified an increasing trend of 1.3 ± 0.4°c per 100 years, statistically confident at the 95% level, only for minimum air temperature, and of a seasonal march, reasonably stationary along the entire investigated interval, explaining more than 50% of the corresponding monthly variance, with maxima in november and july for rainfall and air temperature, respectively. daily clustering analysis evidenced scale-invariant properties, largely dependent on the threshold value, for all the investigated parameters. mailing address: dr. pietro aucelli, dipartimento di scienze e tecnologie per l’ambiente e il territorio, università degli studi del molise, 86170 isernia, italy; e-mail: aucelli@unimol.it 1690 michela izzo, pietro p.c. aucelli and adriano mazzarella the daily maximum and minimum values were considered. extreme air temperature is more suitable than daily mean temperature since: a) due to the quasi-sinusoidal nature of the diurnal temperature cycle, temperatures close to extreme values occur more frequently than those around the mean daily value; b) international methods of recording data across the world have only recently been standardized; therefore, when a historical series is examined, it must be ascertained that the data used for deriving mean values were observed at the same local times throughout. this problem obviously does not hold for extreme temperatures (palumbo and mazzarella, 1984). 2. data collection 2.1. historical outline of the molise climate in molise, there is a substantial lack of climatic studies and this is the first in such a direction. in molise, the earliest instrumental observations date back to the end of the 19th century, when in some areas of the region the first meteorological measurements began under the direction of private citizens who had a natural aptitude for recording such kinds of data, which were then sent to the central office of meteorology (com), as it was then known. the longest daily rainfall and air temperature series concern the following towns: agnone, where instrumental meteorological measurements were started by the de horatiis family in 1875; venafro, where ludovico giannini founded a meteorological observatory in 1883; isernia, where camillo d’apollonio began meteorological observations in 1898; campobasso, the main town of the region, where precipitation measurements began in 1886, and temperature observations in 1918, both carried out by francesco pesce. there were various reasons behind our decision to choose the daily agnone series. first of all, it goes the furthest back in time but, what is much more important, it shows a good completeness of data, a significant continuity in terms of station management and substantial constancy of the environment where measurements were taken. furthermore, it should be recalled that over the years agnone has not experienced a significant change in its urban nucleus, nor has there been human activity which could significantly affect precipitation and temperature. from this point of view, it is reasonable to state that the agnone series does not suffer the so-called heat-island effect which commonly masks the meteorological variables measured in urban areas (palumbo and mazzarella, 1984). 2.2. the agnone climatic station in agnone, meteorological observations were initiated by pier francesco rufino de horatiis, who founded an observatory in 1875, which was soon to become part of the national network of the state central office of meteorology and geodynamics (its name at that time). this observatory continued its activity without interruption until 1977 (fig. 1), when it closed down and the task of carrying out precipitation and air temperature measurements was taken over by the state forest department, on whose premises at agnone a pluviometer and a thermometer were set up. this station works to this day, even after the old instruments were replaced by electronic ones in march 2002. the closing down of the de horatiis station resulted in a huge loss, because it assured not only the collection of precipitation and air temperature, but also of other meteorological parameters (atmospheric pressure, wind speed and direction, state of the sky). most of the documentation concerning the station and many of its instruments donated by the de horatiis family can now be found in the agnone municipal library. the observatory was directed by its founder for 68 years, until 1942, when management passed first into the hands of his son cesare, until 1957, and then of his nephew pier francesco junior, who was in charge of the station until 1977. this great continuity in station management, with well documented changes, makes the quality of the agnone series data really appreciable. we consulted the following sources for the acquisition of daily rainfall and air temperature data: 1691 recent changes in rainfall and air temperature at agnone (molise – central italy) – for the 1883-1921 period, original handwritten registers available about ucea and, in their absence, the daily meteoric bulletins of the com. – for the 1922-2000 period, the annals of the national hydrographic service (nhs) (at first in the department of chieti and then in the department of pescara). according to a posthumous publication by de horatiis (1967), the observations made in the 1875-1882 period were not included in those designated for publication, as they were carried out with old instruments, in different locations and in the early stages of the observer’s experience, who was therefore not particularly confident about their quality. in the analysis of the daily agnone rainfall and air temperature series, problems arise when the original registers were missing. in their absence we analysed the daily meteoric bulletin of the com that attributed the record of the daily rainfall and of maximum air temperature to the previous day. hence, considering the years for which mean monthly and annual data were obtained from this source, the daily rainfall and maximum air temperature series has been shifted back by one day. the accuracy of the daily rainfall data and extreme air temperature is estimated to be equal to 0.1 mm and 0.1°c, respectively (de horatiis, 1967). as regards the period after 1977, when the old station was closed down, no significant influence was exerted on precipitation and temperature measurements, so the whole series was analysed without differentiating between the period prior to closure and the period after it. 3. methodology 3.1. long-term analysis long-term analysis is performed here according to the least square regression line between the n available yearly values of each investigated parameter and time, only when it is found to be confident at a level not less than fig. 1. locations of the agnone climatic station since 1883. 1692 michela izzo, pietro p.c. aucelli and adriano mazzarella ent scales in the same manner, the major variable being the fractal dimension which is used as a measure of the nature of the phenomenon. the available daily rainfall and extreme air temperature events measured in agnone over a period of 80 years provide a wide range of available timescales. this is why the fractal approach to time clustering of daily rainfall and extreme air temperature catalogue is here based upon a boxcounting algorithm, like the cantor dust method. to construct a cantor set, start with the closed interval [0,1]; remove the middle third leaving two closed intervals each with a length 1/3; continue by deleting the middle third of these two intervals, leaving four closed intervals of length 1/9. the process of removing the open middle third of the remaining closed intervals is repeated an infinite number of times and defines the cantor set. this set shows a scale-invariant clustering quantified by the fractal dimension d log logd n n t ti i i i1 1= =+ + / .log log2 3 0 6309= = _ _i i (3.2) where ni is the number of events occurring in the interval of size ti. different cantor-like sets with the other fractal dimension d can be obtained simply by starting from different values of ni and ti. to compute the fractal time clustering of a catalogue, divide the time interval t0, over which the n events occur, into a series of n smaller intervals of length t = t0 / n, with n = 2, 3, 4 ... and compute the fraction r = n/n of intervals of length t occupied by events. if the distribution of events has a fractal structure then )dr ct (1 = (3.3) or, equivalently, on a log-log scaled plane ( ) ( ) ( )log logr c d t1= + (3.4) where c is a constant of proportionality (mazzarella, 1998, 1999). the power law (3.4) is further verified to be significantly different from the relative uniform distribution in which the same number n of events is equally spaced in time (no time clustering). as a mathematical representation, the relation (3.4) 95% (than 99%), i.e. when the following relationship is verified (mazzarella, 1998): ( ) ( ) . ( . )lnn r r3 2 1 1 1 96 2 58$ $+ (3.1) where r is the correlation coefficient between the series of n annual values of each meteorological series versus time. 3.2. seasonal analysis seasonal analysis is performed here according to fourier analysis, that provides the values of mean amplitude (a), phase (σ) of the 12 and 6 months harmonics. the confidence level of each harmonic is computed according to 1 − exp ⋅ ⋅ [− (0.833 a/vpe)2], where vpe is the vector probable error, i.e. the scattering of individual harmonics from the mean a (mazzarella, 1998). the harmonic is found to be confident at 95% (99%) level when a ≥ 2.08 vpe (a ≥ 3.00 vpe). to ascertain the stationarity of the annual harmonic over the entire investigated interval, the same seasonal analysis is here applied to two non-overlapping subintervals (1883-1940, 19412000) and then the results compared. 3.3. fractal daily clustering analysis analysis performed on 120 years of data showed the existence of dishomogeneity between daily values for the period 1883-1921. we therefore decided to postpone the daily analysis of this time interval and limit fractal analysis only to the period 1922-2000. fractals (mazzarella, 1998) are sets that are not topological. for sets that are topological the hausdoff-besicovitch dimension d is an integer (0 for points, 1 for any curve, 2 for any surface). for sets that are fractals d is not an integer but a real number in which the value of the whole number describes the topological nature of the data set under consideration and the size of the decimal fraction represents the irregularity exhibited within the data. the essential feature of fractal sets is their scale invariance. the basic concept of a fractal distribution is that a phenomenon will be repeated on differ1693 recent changes in rainfall and air temperature at agnone (molise – central italy) could be valid over an infinite range; however, for physical applications, there will be upper and lower limits on the applicability of the fractal distribution (scaling region). the fractal dimension d = 1 − s is estimated from the slope s that provides the best fitting of the least square regression line of log(r) on log(t) when it is found to be confident at a level not less than 99%, i.e., when the relationship (3.1) is verified, with r representing the correlation coefficient between log(r) and log(t). it is worth noting that an increasingly clustered distribution of events causes an increase of d up to the limiting value d = 1 that is the topological dimension of a line; on the other hand, a concentration of events at a definite time corresponds to the limit d = 0, i.e., to the dimension of a point. the fractal dimension characterises the data distribution globally and, in general, does not reflect the situation in the different time subsets. thus, to investigate the change in d with time, one can calculate d for catalogue subsets with a fixed number of events or choose a fixed-length time window and calculate consecutive values of d shifting the window through the catalogue. here both options are adopted using windows containing no less than 100 events, the minimum for sound fractal analysis (smith, 1988). 4. results 4.1. rainfall the yearly values of rainfall measured in agnone (1883-2000) show a wide interannual variability (fig. 2) and the least square regression analysis on time shows no significant time trend. seasonal analysis of the available monthly values (table i) shows that the annual and semiannual harmonics are determined at a level greater than 99%, with maxima occurring at the beginning of january for the annual wave and at the beginning of march for the semiannual wave. analysis performed on the two non-overlapping subintervals (1883-1940 and 19412000) shows a small increase in the amplitude of annual harmonic. to compute the daily clustering of rainfall according to the cantor dust method, fraction r (the fraction of time intervals including a rainy day) is computed as a function of the interval t, on a log-log scaled plane, for different intensity thresholds t. the smallest time interval is chosen to be 1 day, gradually increased by a factor of 2. fig. 2. interannual variation of rainfall measured at agnone in the 1883-2000 interval. table i. seasonal analysis of monthly rainfall measured in agnone for the 1883-2000 interval and for the two different non-overlapping subintervals (1883-1940, 1941-2000). a, vpe and σ represent the mean amplitude, the vector probable error and the phase of the harmonic, respectively. interval total mean monthly value period a vpe σ 1883-2000 75.7 mm 12 months 23.6 mm 2.6 81.8° 6 months 12.6 mm 2.2 177.0° 1883-1940 75.1 mm 12 months 21.1 mm 4.0 78.2° 6 months 14.1 mm 3.0 189.9° 1941-2000 76.2 mm 12 months 26.0 mm 3.5 84.6° 6 months 12.0 mm 3.1 162.1° 1694 michela izzo, pietro p.c. aucelli and adriano mazzarella tensity-dependent dimensions. some typical plots of log (r) on log (t) for different intensity thresholds (t ≥ 0.2 mm, t ≥ 5 mm, t ≥ 10 mm, t ≥ 20 mm, t ≥ 30 mm) are reported in fig. 3. table ii shows the relative values of fractal dimension and of specific scaling region, found to be confident at a level not lower than 95% and significantly different from the relative uniform distribution. the time variation of d, computed according to both different time variable windows enclosing successive daily events and to a time fixed windows, show substantial stability over the period 1922-2000 with no significant time trend. 4.2. air temperature the yearly values of minimum and maximum air temperature measured at agnone (1883-2000) show wide interannual variability and least square regression analysis on time table ii. fractal dimension and scaling region values of the rainfall recorded in agnone in the 19222000 interval, found to be confident at a level not less than 95% for different intensity threshold values. intensity fractal scaling threshold dimension region ≥ 0.2 mm 0.5 ± 0.1 1-8 days ≥ 5 mm 0.31 ± 0.08 1-8 days ≥ 10 mm 0.25 ± 0.07 1-16 days ≥ 20 mm 0.16 ± 0.05 1-32 days ≥ 30 mm 0.10 ± 0.03 1-64 days fig. 3. the logarithm of the fraction r of time intervals t (in days) including the rainfall events for agnone observatory, as a function of log (t) (interval: 1922-2000); the values relative to daily rainfall events equal to or greater than 0.2 mm, 5 mm, 10 mm, 20 mm, and 30 mm, respectively, are plotted from the top of the graph downwards. the fractal dimension and the specific scaling region of the daily rainy events is found to be strongly dependent on the chosen intensity threshold: the higher this value, the lower the fractal dimension and the wider the relative scaling region. as a result, the rainfall process is characterised by an infinite hierarchy of in1695 recent changes in rainfall and air temperature at agnone (molise – central italy) shows an increasing time trend equal to 1.3 ± 0.4°c per 100 years, confident at least at 95% level, only for tmin (fig. 4). seasonal analysis of the monthly values of tmax and tmin (table iiia,b) shows that the annual and semiannual harmonics are determined at a level greater than 99%, albeit with a large dominance of the annual wave whose maximum occurs at the end of july. analysis performed on the two nonoverlapping subintervals (1883-1940 and 19412000) shows no significant differences. the continuous daily series of tmin and tmax measured at agnone has been transformed into a daily series of events simply considering daily values of tmin lower than 5°c, 0°c and −5°c and daily values of tmax higher than 25°c and 30°c. hence, the same fractal methodology applied to rainy days is applied to lowest and highest daily minimum and maximum air temperature, respectively. as regards minimum temperature, the lower this value, the lower the fractal dimension; for maximum temperature, the higher the threshold value, the lower the fractal dimension and the wider the relative scaling region (fig. 5). table iv shows the relative fractal dimension and scaling region values obtained at a level of confidence not lower than 99% and significantly different from the relative uniform disfig. 4. interannual variation of minimum air temperature measured at agnone in the 1883-2000 interval. the line represents the least square regression line of tmin on time, calculated at 95% level of confidence. table iiia,b. seasonal analysis of monthly maximum (tmax) (a) and minimum (tmin) (b) air temperature measured in agnone for the 1883-2000 interval and for the two different non-overlapping subintervals (1883-1940, 1941-2000). a, vpe and σ represent the mean amplitude, the vector probable error and the phase of the harmonic, respectively. interval total mean monthly value period a vpe σ 1883-2000 15.7 °c 12 months 9.5 °c 0.1 −113.9° 6 months 0.9°c 0.1 −36.4° 1883-1940 15.7°c 12 months 9.4°c 0.2 −114.1° 6 months 0.9°c 0.1 − 37.2° 1941-2000 15.6°c 12 months 9.5°c 0.2 −113.7° 6 months 0.9°c 0.1 −35.6° 1883-2000 7.9°c 12 months 7.6°c 0.1 −117.4° 6 months 0.4°c 0.1 − 29.7° 1883-1940 7.3°c 12 months 7.5°c 0.1 −118.2° 6 months 0.4°c 0.1 −30.9° 1941-2000 8.4°c 12 months 7.6°c 0.1 −116.7° 6 months 0.5°c 0.1 −28.8° a b 1696 michela izzo, pietro p.c. aucelli and adriano mazzarella tribution. investigation of the time variation of d shows, over the entire 1922-2000 interval, no significant time trends for tmax with a substantial stability of the value of d and a decreasing time trend for tmin, representative of an ever rarer occurrence of lowest values of minimum air temperature. the different trend of the minimum temperature in comparison with the maximum is also confirmed by the time analysis of the daily thermal excursion (the difference between the daily maximum temperature and the daily minimum temperature) averaged over the period of a year. it shows a decreasing rate of 1.3 ± 0.7°c per 100 years, confident at a level just below 99%. 5. discussion long-term analysis shows no time trend for rainfall and maximum air temperature and a statistically significant increase in yearly values of minimum air temperature equal to 1.3 ± 0.4°c per 100 years. such values are more accurate than rainfall and maximum air temperature as they are measured at the end of the night, in the absence of convective motions. monthly rainfall and extreme air temperature show high seasonal variation that remains substantially unchanged over the entire interval of observation. fractal analysis gave results which agree with those obtained in other studies (mazzarella, 1999). variability in d with the threshold may provide insights into the nature of rain and the thermal field over agnone, as the observed catalogue of rain and extreme air temperature also reflects the intermittency of the field, i.e. its three-dimensional structure organised in eddies and subeddies and changeable in time for the same points. research in intermittency (berndtsson and niemczynowicz, 1988; fraedrich and larnder, 1993; lovejoy and schertzer, 1999; mazzarella, 1999) shows that cascade processes, where the large scale multiplicatively modulates the small, when carried out over wide enough ranges of scales with a repeating (scale invariant) mechanism, generally tend to multifractal structures. a b table iv. fractal dimension and scaling region values of daily extreme air temperatures recorded in agnone from 1922 to 2000, found to be confident at a level not less than 99% for different threshold values. intensity fractal scaling threshold dimension region ≤ −5°c 0.34 ± 0.05 1-128 days ≤ 0°c 0.64 ± 0.03 1-128 days ≤ 5°c 0.84 ± 0.01 1-64 days ≥ 25°c 0.74 ± 0.03 1-64 days ≥ 30°c 0.46 ± 0.02 1-128 days fig. 5a,b. the logarithm of the fraction r on t (in days) (interval: 1922-2000) including: a) the daily minimum temperature events for agnone observatory as a function of log (t); the values relative to daily events of tmin lower or equal to 5°c, 0°c and − 5°c respectively, are plotted from the bottom of the graph upwards; b) the daily maximum temperature events for agnone observatory as a function of log (t); the values relative to daily events with tmax greater than or equal to 25°c and 30°c respectively, are plotted from the bottom of the graph downwards. 1697 recent changes in rainfall and air temperature at agnone (molise – central italy) as regards precipitation and maximum air temperature, the higher the intensity threshold, the lower the fractal dimension and longer the relative scaling region. the opposite holds for minimum temperature: the lower the threshold value, the lower the fractal dimension. thus the different intensity thresholds reveal different properties of the physical process: when using a higher threshold, the low-intensity parts are «cut-off» and only the properties of the high-intensity events (the opposite occurs for tmin) are taken into account. this gives evidence of the so-called multifractality in which a multiplicative cascade process is responsible for the concentration of energy fluxes into successively smaller parts of the atmosphere (lovejoy and schertzer, 1990; olsson et al., 1993). the time variation of the fractal dimension of daily rainfall does not reveal any significant trend for any intensity threshold, which suggests that agnone exhibits no trend towards the climatic tropicalization which seems to be characteristic of the coastal areas (mazzarella, 1999; brunetti et al., 2000). the time variation of the fractal dimension shows a decrease, confident at the 99% level, only in minimum air temperature lower than − 5°c. this means that in agnone there has been a significant reduction in the occurrence of lowest temperatures. this seems to be perfectly compatible with a world which is heating up increasingly. according to various estimates, there has been a global increase in surface mean temperature between 0.3 and 0.6°c in the last century (data from ncdc; jones et al., 1991). the asymmetric trend of the minimum and maximum temperatures, underlined, among other thing by the decreasing trend in daily thermal excursion, may have a major effect on local ecosystems, given the physiological effects which the minimum temperature has on living beings. consequently, it would be well worth investigating the contemporary evolution of vegetation at agnone, in order to discover any changes that may be related to climatic evolution. furthermore, given the influence on evaporation and evapotranspiration rates, it cannot be excluded that in future this asymmetry in temperature trends could also affect precipitation. a preliminary analysis conducted on other stations in molise, shows a general tendency towards an increase in temperature, particularly in the minima. however, the situation throughout molise is by no means uniform due to the fact that, besides stations which experienced substantial stability in the period examined, such as chiauci, there are others, such as campobasso (the region’s capital) and guardiaregia which show positive trends, even more so than agnone, both in mean temperature and minimum temperature. 6. conclusions reconstruction of the instrumental climatic series constitutes a fundamental step in understanding climatic evolution. the importance of reconstructing a historical series such as that of agnone lies in the fact that 120 years of climatic history constitutes a valid starting point for comparisons with results from the analysis of other stations, both within and outside the region, and for setting up other analytical techniques which allow us to investigate the history of climatic trends more deeply. in this sense, the agnone series is also an important point of reference for further analysis which will involve other long-term stations in molise: it could be useful to identify the heatisland effect in the campobasso series, as campobasso, more than other stations in molise, should allow us to clearly underline and quantify it. analysis of the agnone series produced important results particularly if they are inserted into a wider context, enabling us to discover similarities and differences with other places. application of fractal analysis confirmed the validity of this method in describing precipitation events and temperature extremes and enhancing our knowledge of them. hence this study is a valid basis for further analysis, with the aim, amongst others, of in-depth investigation. by extending the analysis to other places, within and outside molise, it will be possible to establish whether or not there is a zonal distribution of the change and, in the event of a positive answer, try to determine the causes of the specific pattern. furthermore, one of the main 1698 michela izzo, pietro p.c. aucelli and adriano mazzarella objectives is to go increasingly further back in time, in order to establish whether climatic change has always occurred in the same way and with the same structure. references barriendos, m., j. martín-vide, j.c. peña and r. rodriguez (2002): daily meteorological observations in cádiz – san fernando. analysis of the documentary sources and the instrumental data content (1786-1996), climatic change, 53, 151-170. berndtsson, r. and j. niemczynowicz (1988): spatial and temporal scales in rainfall analysis – some aspects and future perspectives, j. hydrol., 100, 293-313. brunetti, m., m. maugeri and t. nanni (2000): variations of temperature and precipitation in italy from 1866 to 1995, theor. appl. climatol., 65, 165-174. camuffo, d. 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(received may 13, 2003; accepted july 19, 2004) layout 6 evaluation of automatic ionogram scaling for use in real-time ionospheric density profile specification: dourbes dgs-256/artist-4 performance stanimir m. stankov1,*, jean-claude jodogne1, ivan kutiev2, koen stegen1, rené warnant1,3 1 institut royal météorologique de belgique, brussels, belgium 2 geophysical institute, bulgarian academy of sciences, sofia, bulgaria 3 université de liège, unité de géomatique géodésie et gnss, liège, belgium annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-4976 abstract statistical evaluation of the dourbes (4.6˚e, 50.1˚n) digisonde automatic scaling of the more frequently used ionospheric parameters (fof2, fof1, foe, h’f2, h’f, h’e, and m3000f2) was performed using automatically and manually scaled data from the time period of 2002 to 2008. automatic scaling was provided in 92% to 94% of cases for most characteristics, except for fof1 (81%). in terms of the automatic scaling accuracy, the magnitude of the residual error for fof2 and m3000f2 (automatically minus manually scaled values) varied according to local time, season, and solar activity. although geomagnetic storms appear to affect the automatic scaling, the overall results for the influence of geomagnetic activity were inconclusive. based on this analysis, error bounds were determined (95% probability) for each characteristic: fof2 (–0.75,+0.85), fof1(–0.25,+0.35), foe(–0.35,+0.40), h’f2(–68,+67), h’f(–38,+32), h’e(–26,+2), and m3000f2(–0.55,+0.45). 1. introduction real-time ionospheric measurements and updates of empirical models are increasingly sought after due to the requirements for timely corrections to precise satellite ranging systems and to various other space weatherforecasting services [e.g. stamper et al. 2004, zolesi et al. 2004, belehaki et al. 2006, warnant et al. 2007, buresova et al. 2009]. considering the still unsurpassed quality of groundbased vertical incidence sounding and the increased number of digital ionosondes that are being installed around the world, many of these services and researchers will rely on this network of ionosondes for valuable input. the automatic scaling (autoscaling) of ionograms has long been recognized as a priority development considering the ever improving digital ionosonde (digisonde) technology, which increases sounding rates and the cost of manual scaling [mazzetti and perona 1978, reinisch and huang 1983, fox and blundell 1989, galkin et al. 1996, reinisch et al. 2005, pezzopane and scotto 2005, mcnamara 2006, ding et al. 2007]. all of these have led to steady improvements in autoscaling techniques over the years, and modern digital ionosondes now offer real-time autoscaling estimates for the important ionospheric characteristics (figure 1). we developed a technique for the reconstruction of vertical electron-density profiles from digisonde and global positioning system (gps) measurements, which was subsequently converted into a prototype real-time procedure [stankov et al. 2003]. this technique has evolved over the years, and a fully operational system for the monitoring of the vertical electron-density distribution in the local ionosphere has been implemented [stankov et al. 2011] at the site of the dourbes digital ionosonde. as this monitoring service relies heavily on autoscaled digital ionograms to provide input to the profile reconstruction procedure, it is necessary to evaluate the reliability of this input. earlier comparisons of manual versus automatic computer processing have been reported; however, these were for older versions of the autoscaling software [jodogne 1998], for other digisondes/ locations and time periods [bamford et al. 2008], and/or for only some of the characteristics that are of interest to us [pezzopane and scotto 2005, 2007]. the purpose of the present study is to assess the performance of the royal meteorological institute digital ionosonde (lowell dgs-256) and its autoscaling software (artist automatic real-time ionogram scaler with true height v.4) in terms of both their ability to provide autoscaled values (i.e. independent of the accuracy) and the quality of their autoscaling (i.e. accuracy and precision), for the period of 2002 to 2008. this includes the deduction of error bounds for the autoscaled characteristics that are of particular importance to our service. data and experiment descriptions 283 article history received march 28, 2011; accepted august 24, 2011. subject classification: ionosphere, digital ionosonde, automatic ionogram scaling. 2. real-time electron-density profile specification based on concurrent measurements from the collocated digital ionosonde and gps receiver digital ionosonde measurements and concurrently obtained gps total electron content (tec) data, together with the empirically modeled upper o+/h+ ion transition level, are used for determination of the key unknown characteristics: the topside o+ and h+ ion density scale heights. these scale heights are calculated by the solving of a transcendental equation that arises from the incorporation of a suitable ionospheric profiler (chapman, epstein, or exponential) into formulae that describe the basic features of the plasma distribution/processes in the upper ionosphere [stankov et al. 2011]. the system, which has been dubbed as liedr (local ionospheric electron density profile reconstruction), acquires and promptly processes the incoming measurements, computes the fullheight ionospheric electron-density profile using the above technique, and displays the resulting profilograms. the current update rate is 15 min, and the latency is about 10 min. liedr is primarily designed to operate in continuous real-time mode for service applications, and to simultaneously provide historical data/plots for research applications and for further developments of the system. 3. the data all of the measurements were made at the geophysical centre in dourbes (50.1˚n, 4.6˚e) [jodogne and stankov 2002). the dourbes digital ionosonde (ursi code: db049) carries out regular vertical ionospheric soundings with a 284 stankov et al. figure 1. real-time autoscaled ionograms from dourbes. (a) (b) 285 digisonde 256 sounder, which was developed by the university of massachusetts, lowell, usa [reinisch 1996]. the ionosonde/ionogram settings include: frequency range, 1-16 mhz; scale, linear; step, 100 khz; amplitude resolution, 0.25 db; phase resolution, 1.4˚; doppler resolution, 4 hz; range resolution, 5 km; range start, 60 km; ionogram duration, 4 min; and sounding rate, 1 every 15 min. the ionograms are automatically scaled by the artist software, and the values of the key ionospheric characteristics are deduced almost instantly. for the purpose of the evaluation, the hourly data from the period covering the year 2002 through to 2008 was analyzed. this period comprises both solar maximum and minimum conditions, and geomagnetically quiet and disturbed/storm conditions. the analysis will focus on the scaling of seven of the most important ionospheric characteristics: the critical frequencies (fof2, fof1, foe), the minimum virtual heights (h’f2, h’f, h’e), and the propagation factor m(d) (a conversion factor applied to the critical frequency at vertical incidence to obtain the maximum frequency usable for a given oblique propagation over a distance d; in this case, d = 3000 km, i.e. m(d) = m(3000)f2). the residual error, which is defined as the difference between the automatically scaled evaluation of the automatic ionogram scaling: dgs-256/artist-4 figure 2. autoscaled ionograms from dourbes. (a, b) autoscaling failures, as (a) partial, with only the e-layer parameters scaled, and (b) completely unscaled, as an example of the severely depleted ionosphere during a geomagnetic storm. (c, d) gap occurrence (due to interference), where smaller gaps are successfully ignored/ interpolated, and larger gaps are falsely interpolated/extrapolated, resulting in the automatic layer trace being truncated prematurely (d). (e, f) incorrect autoscaling of the h’f2 virtual height. auto, automatic scaling; manu, manual scaling. (a) (b) (c) (d) (e) (f) (autoscaled) and the corresponding manually scaled values from the same sounding, were investigated for each of the above-mentioned characteristics. all of the manual corrections of the autoscaled values were performed by experienced scalers who followed the ionogram scaling rules [piggott and rawer 1972, wakai et al. 1987] and who worked in close cooperation. notwithstanding the possibility of occasional human error, the approach implicitly assumes that the manually scaled values are correct. the data used for this study are based on more than 60000 ionograms. 4. assessment of the digisonde ability to provide autoscaled values in this section, we investigate the performance of the digisonde autoscaling algorithm, in terms of the software ability to provide autoscaled values as output, independent of the accuracy. the autoscaling quality (i.e. accuracy) will be the subject of the following section. for this purpose, we extracted all of the cases when the digisonde failed to deliver autoscaled values for any of the seven characteristics in question (e.g. figure 2a, b). possible reasons for these failures are represented by dlayer absorption, blanketing by the sporadic e-layer, bands of strong interference, among others [e.g. bamford et al. 2008, buresova et al. 2009]. the number of totally unscalable ionograms was insignificant. the remaining cases of failed autoscaling for which it was possible to obtain some/all of the characteristics after manual scaling were analyzed, and the data are summarized in figure 3. the scatter plots in figure 3a show the occurrence of these autoscaling failures with respect to the dst and f10.7 indices. the majority of the cases are clustered around the low dst and f10.7 values, with no obvious dependence on magnetic activity/ storm conditions. more detailed statistics (figure 3b) confirm that there is no significant difference in the distribution of failed and autoscaled values, with respect to the geomagnetic index dst. however, failures appear to occur more frequently during low solar activity conditions for fof1, during mid-level solar activity for fof2, h’f2, h’f, h’e, and m(3000)f2, and during higher solar activity for foe. also, the autoscaling was more susceptible to failure during the daytime. overall, the autoscaled values are provided in 92% to 94% of cases, except for fof1 (81%). stankov et al. 286 figure 3. the digisonde autoscaling ability for different ionospheric characteristics during the period of 2002 to 2008. (a) failed autoscaling, dst [nt] versus f10.7 [10-22 wm-2hz-1]. (b) distribution of the failed autoscaling (black) and the number of autoscaled values (red) with respect to the dst (left) and the f10.7 (right). the percentages of the autoscaled values are given below the label for each characteristic. (a) (b) 287 5. comparisons between the automatically and manually scaled characteristics a more demanding task is to investigate the quality of the autoscaling. for this purpose, the residual error for each characteristic was calculated and analyzed statistically, in search of possible dependency on solar and geomagnetic activities, local time, and season, and of the probable reasons behind these dependencies. first, crude time series of the autoscaling errors were produced for the 2002 to 2008 period. this period comprised all levels of solar and geomagnetic activity conditions. figure 4 initially reveals some obvious patterns in the error variations. for example, the fof2 characteristic appear to be frequently (and on several occasions quite substantially) underestimated (which causes higher residual errors) by the autoscaling procedure during years of high solar activity, compared to the period of minimum solar activity from year 2007 onwards. given the higher number of geomagnetic storms during high solar activity, this indicated the need to also look at a probable influence from the geomagnetic activity. a somewhat opposite effect is seen in the foe errors. there is an obvious seasonal effect in the error for h’f2, with that error showing a substantial increasing in the summer. it appears also that the h’e is systematically underestimated by the autoscaling software. further on, the scatter plots of the autoscaled against the manually scaled hourly characteristics were produced (figure 5a). each set of data was fitted to a line. for the frequency characteristics, figure 5a demonstrates that the fit is generally good, with a slope close to 1 and only small intercepts. nonetheless, rather large scattering is observed for the virtual heights and the propagation factor, meaning that, in general, the virtual heights are determined less reliably than the critical frequencies. also, prominent layers (‘ledges’), extending to the right of the ‘ideal’ line (with a slope of 1), are observed in fof2 (figure 5a). it is obvious that the autoscaling algorithm underestimates the true values of the frequencies at around 4 mhz, 6 mhz, 7 mhz, 9.5 mhz, and 11.5 mhz. this is caused by gaps in the echo trace that lead to the premature truncation of the automatic layer traces. investigations of the raw ionogram data revealed that this phenomenon is most probably due to interference from external radio signals, the presence of which can significantly raise the noise level [bamford et al. 2008]. as the digisonde monitors the noise for each sounding frequency and only retains signals that are larger than a pre-defined noise threshold, an increased noise level would cause a gap in the echo trace (figure 2c, d). while the smaller gaps can sometimes be successfully managed, the larger gaps can cause either false extrapolations (figure 2c) or truncation (figure 2d). this latter can be quite substantial, and can occur more often around certain frequencies, which thus form ledges in the scatter plot. virtual heights can also be difficult to scale properly; sometimes the echo trace is weak or the algorithm is misled by false inflexion point/s in the echo trace (fig. 2e, f). in such cases, the errors can be substantial, especially during storms (figure 2f). for h’f2, autoscaling underestimations are more often observed (cf. figure 5a, with scatter at around 200 km). in addition to the scatter plots, error histograms were produced (figure 5b). ‘perfect’ matches (error within ±0.05 mhz) are achieved in 58% of cases for fof2, 78% for fof1, and 54% for foe. about 56% of the m(3000)f2 errors are within the ±0.05 mhz limit, and more than 68% of the h’f2 and h’f errors are within the ±5 km range. the h’e error histogram confirms that the autoscaling is systematically lower (exceeding 10 km, on average). to assess the quality of the automatic scaling under varying solar and geomagnetic activity conditions, the residual errors in the autoscaled hourly values were compared (figure 6) with the f10.7 and dst indices. with respect to the f10.7 index, the plots show that the fof2 errors evaluation of the automatic ionogram scaling: dgs-256/artist-4 figure 4. residual errors (automatically [auto] minus manually [manu] scaled values) for different ionospheric characteristics during the period of 2002 to 2008. the solar and geomagnetic activity indices, dst [nt] and f10.7 [10-22wm-2hz-1], are plotted in the bottom panel. stankov et al. 288 figure 5. comparisons between the automatically and manually scaled ionospheric characteristics during the period of 2002 to 2008. (a) scatter plots of the residual errors (automatically versus manually scaled values). (b) histograms of the residual errors. figure 6. comparison between automatically (auto) and manually (manu) scaled ionospheric characteristics during the period of 2002 to 2008. scatter plots of the residual errors versus f10.7 [10-22 wm-2hz-1] (a) and the dst [nt] (b) indices. (a) (b) (a) (b) 289 are more often negative than positive, with larger errors observed with high solar activity. the majority of the h’e errors are negative at all levels of solar activity (cf. figure 4). for the dst index, the great majority of the errors are clustered around the crossing point of the zero axes, which suggests that there is no obvious relationship between the dst index and the errors in any of the autoscaled values (figure 6b). examination of the correlation coefficients between dst and each of the characteristics for the entire period reveals that the magnitudes of the coefficient do not exceed 0.2, which indicates that the variables are essentially uncorrelated. this is in agreement with the evaluation of the chilton digisonde autoscaling [bamford et al. 2008]. to analyse the diurnal and seasonal variations in the autoscaling accuracy, contour plots were generated for the residual-error magnitude (autoscaled minus manually scaled values), with respect to the day of the year and the local time (figure 7). the color scale indicates the error magnitude, where blue indicates negative residuals and red indicates positive residuals. visual inspection of these data in figure 7 reveals that there are several patterns. the fof2 data show pronounced underestimation of the soundings in the dusk hours during high and medium solar activity. as an opposite effect, overestimation is observed during the winter nights at high and medium solar activity. these two patterns gradually fade away at lower levels of solar activity, when they are replaced by slight underestimation during the daytime. the autoscaling quality improves noticeably during the solar minimum years of 2007 and 2008. the h’f2 results are characterized by slight underestimation, most of the time. some structures appear in the dusk hours at higher solar activity, although these disappear during the low activity years. substantial negative residuals are seen during the summer daytime in 2003, 2005, and 2008, some of which coincide with periods of geomagnetic storms. the foe error is predominantly positive (with a tendency to increase at lower solar activity), and as already shown, the h’e error is almost exclusively negative. the error variations in m(3000)f2 are almost opposite to those in fof2: overestimation after dusk, and underestimation at night and during minimum solar activity. 6. conclusions a comparison has been made here between autoscaled and manually scaled ionospheric characteristics based on data from 2002 to 2008 from the dourbes digisonde. these data show that that the dgs-256/artist-4 system can provide automatic scaling in 94% to 98% of cases for all of the characteristics, except for fof1 (89%). in terms of the autoscaling accuracy, for some characteristics (most notably for fof2 and m3000f2), the magnitude of the residual error (autoscaled minus manually scaled values) varies according evaluation of the automatic ionogram scaling: dgs-256/artist-4 figure 7. contour plots of the annual and diurnal variations in the residual errors (automatically minus manually scaled values) for the fof2, h’f2, foe, h’e, and m(3000)f2 characteristics during the period of 2002 to 2008. to local time, season and solar activity. although geomagnetic storms appear to affect the autoscaling, the overall results for the influence of geomagnetic activity remain inconclusive. based on this analysis (cf. figure 5), error bounds have been determined (95% probability) for each of the seven characteristics investigated: fof2 (–0.75,+0.85), fof1(–0.25, +0.35), foe(–0.35,+0.40), h’f2(–68,+67), h’f(–38,+32), h’e(–26,+2), and m3000f2(–0.55,+0.45). overall, the automated ionogram processing/scaling has demonstrated sufficiently good performance to allow the use of the instantaneous ionospheric sounding data for the operation of a monitoring system (liedr) for deduction and imaging of the vertical distribution of the electron density in the local ionosphere (http://ionosphere.meteo.be/). the growing availability of global navigation satellite system tec and ionosonde measurements, combined with the demonstrated ability to run liedr in real-time, gives both this approach and this system great potential. acknowledgements. the authors wish to thank g. crabbe, e. van malderen, and l. lejeune for past and ongoing technical support. this study was funded by the royal meteorological institute via the belgian solar-terrestrial centre of excellence (stce). i. kutiev acknowledges the stce for financial support through the stce visiting research fellowship grant. the solar and geomagnetic activity data/ indices were provided by the belgian solar influences data centre, the us national oceanic and atmospheric administration, and the world data centre for geomagnetism, kyoto. references bamford, r.a., r. stamper and l.r. cander (2008). a comparison between the hourly autoscaled and manually scaled characteristics from the chilton ionosonde from 1996 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e-mail: s.stankov@meteo.be © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. evaluation of the automatic ionogram scaling: dgs-256/artist-4 layout 6 porting and optimizing magflow on cuda giuseppe bilotta1,2,*, eugenio rustico1, alexis hérault2,3, annamaria vicari2, giovanni russo1, ciro del negro2, giovanni gallo1 1 università di catania, dipartimento di matematica e informatica, catania, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy 3 conservatoire des arts et métiers, département ingénierie mathématique, paris, france annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5341 abstract the magflow lava simulation model is a cellular automaton developed by the sezione di catania of the istituto nazionale di geofisica e vulcanologia (ingv) and it represents the peak of the evolution of cellbased models for lava-flow simulation. the accuracy and adherence to reality achieved by the physics-based cell evolution of magflow comes at the cost of significant computational times for long-running simulations. the present study describes the efforts and results obtained by porting the original serial code to the parallel computational platforms offered by modern video cards, and in particular to the nvidia compute unified device architecture (cuda). a number of optimization strategies that have been used to achieve optimal performance on a graphic processing units (gpu) are also discussed. the actual benefits of running on the gpu rather than the central processing unit depends on the extent and duration of the simulated event; for large, long-running simulations, the gpu can be 70-to-80-times faster, while for short-lived eruptions with a small extents the speed improvements obtained are 40-to-50 times. 1. introduction modeling and simulation of lava flows is of extreme importance for short-term and long-term hazard assessment in volcanic areas. physical models of lava flows must take into consideration the non-linear, temperature-dependent rheology, the variation in space and time of the rheological parameters, and the irregularity of natural topography. the more physically correct a model, the more computationally intensive it is, a condition that can hinder the applicability of the model to short-term forecasting of lava emplacement during an eruption, as it is essential for scenario simulations to be completed in very short times compared to the actual evolution of the phenomenon. one of the most successful approaches to lava-flow modeling is cellular automaton [crisci et al. 1986, ishihara et al. 1990, miyamoto and sasaki 1997, avolio et al. 2006], in which the computational domain is represented by a (usually regular) grid of two-dimensional or three dimensional cells, each of which is characterized by some properties, such as lava height and temperature, and where the modeling of the phenomenon is described through an evolution function for the properties of the cells. the magflow model [vicari et al. 2007] was developed at the sezione di catania of the istituto nazionale di geofisica e vulcanologia (ingv-catania), and it is one of the few cellular automata that are based on physical modeling of lava flows, including thermal effects, to describe the system evolution. the magflow model has been used successfully both to reproduce past events with welldetermined characteristics (for validation) and to predict mount etna lava flows in real time during the eruptions of 2004 [del negro et al. 2008], 2006 [hérault et al. 2009, vicari et al. 2009] and 2008 [bonaccorso et al. 2011]. the use of this model in scenario forecasting has been possible because of its good performance, which allows simulation of several days of eruption in a few hours. although the original magflow implementation is written for serial execution on standard computer processors, the cellular automaton paradigm has a very high degree of parallelism that makes it particular suitable for implementation on parallel computing hardware. in particular, our choice has been geared towards the use of graphic processing units (gpus), as these offer very high performance in parallel computing with a total cost of ownership that is significantly lower than that of traditional computing clusters of equal performance: a workstation with a modern gpu offers a computing capability in the order of teraflops (1012 floating-point operations per second), while it is priced at about €1,000 and consumes about 600 w. we chose to implement the magflow model using compute unified device architecture (cuda), an architecture that is provided by nvidia for the deployment article history received november 23, 2010; accepted july 15, 2011. subject classification: gpgpu, modeling, high-performance computing, parallel computation, hazard, lava. special issue: v3-lava project 580 of their latest generations of gpus as high-performance computing hardware [nvidia 2008]. as cuda provides a programming environment that is closely related to the c family of programming languages [nvidia 2010], this has allowed us to preserve much of the code and structure from the original serial magflow implementation, while providing a significant benefit in terms of computational speed, due to the parallel nature of the hardware. as a preliminary to a better understanding of the details of the porting of magflow from c to cuda (section 5) and our discussion of the optimization strategies considered (section 6), we will briefly present the fundamental concepts of gpu programming (section 2) and a summary of the automaton structure and evolution functions (section 3). a summary of the overall benefits are presented in section 4, and the conclusions are presented last. 2. hardware platform the videogaming request for ever more realistic gameplay has driven the development of extremely highperformance, dedicated hardware for the real-time rendering of high-quality, realistic, animated, interactive threedimensional scenes. the computing power of gpus has been initially exploited for non-graphic computing tasks (gpgpu: generalpurpose programming on gpu) by going through convoluted transformations that can express the numerical problems in graphical terms. the growing interest for the use of the gpus as general purpose high-performance computing processors has led the two main manufacturers, ati and nvidia, to develop interfaces that allow programming of their gpus with a more conventional approach. for nvidia cards, this has resulted in cuda, an extension of the c programming language, to support the gpu programming model. this has led to a significant breakthrough in performance in their use as high-performance computing devices [berczik 2008]. gpu programming follows the stream processing concept, where a set of instructions (called a kernel) is executed in parallel over all of the items of a dataset. each instance of a kernel is called a thread; threads are grouped in one-, twoor three-dimensional blocks that are themselves arranged in oneor two-dimensional grids. the number of threads in a kernel launch is thus given by the number of threads per block times the number of blocks in the grid. as this is in general much larger than the number of computing cores available on the card, the gpu takes care of the dispatching of the threads to the multiprocessors as they become available. the threads from the same block are always assigned to the same multiprocessor, which will automatically distribute their execution over its cores. the smallest number of threads that can run physically in lockstep on a single multiprocessor is called the warp size, which is 32 for cuda cards. as the thread dispatching and concurrency is handled by the hardware, the programmer can focus on the implementation of the actual algorithm. this has allowed gpu usage for high-performance computing to flourish in a wide range of fields, from medical imaging [roberts et al. 2010], to cryptography [szerwinski and gneysu 2008], and from statistical physics [preis 2011], to computational fluid dynamics [hérault et al. 2010, kuznik et al. 2010]. the details of the thread distribution only become relevant during the optimization phase and in cases where threads have to communicate with each other to solve concurrency issues (separate threads trying to update the same datum). in these cases, the developer can query the driver at runtime for the hardware details of a specific card and tune the kernel parameters accordingly. different hardware generations (marked as the compute capabilities of the device) usually have a different number of processors, as well as a different number of cores per processor and different amounts of resources, such as registers and shared memory per processor. the number of concurrent threads that are executing at any given time depends on the number of computing cores that are available, and also on the complexity of the kernels and on their use of the resources available on the multiprocessors: the overall resource use by a batch of concurrent kernels is called the occupancy, which is an index of the overall efficiency with which the hardware is being used. the occupancy, however, does not tell the whole story about the performance of an implementation, as there are a number of other factors that must be taken into consideration. these include algorithm features such as the presence of conditionals, or the nature and structure of the memory access patterns. some of these issues will be discussed when presenting the gpu implementation of magflow. more information about the cuda platform can be found in the cuda programming guide of nvidia [2010]. 3. automaton structure in this section, we present the key aspects of the automaton underlying the magflow model, while recalling only the aspects that are more relevant to the gpu implementation. a more detailed description of the model can be found in vicari et al. [2007]. the magflow cellular automaton has a twodimensional structure with cells described by five scalar quantities: ground elevation, lava thickness, heat quantity, temperature, and amount of solidified lava. the system evolution is purely local, in the sense that each cell evolves according to its present status and the status of its eight immediate neighbors (the moore neighborhood). the domain (automaton size) is chosen to be large enough to include the prospected maximum extent of the lava flow emplacement, and it is decomposed into square cells such 581 bilotta et al. 582 that their width matches the resolution of the digital elevation model (dem) that is available for the area, which is used to set the ground elevation property of each of the automaton cells. the lava thickness varies according to the lava influx from the vent (for cells corresponding to a vent location), plus any lava flux between neighboring cells. cross-cell lava flux is determined according to the height difference, using a steady-state solution for the one-dimensional navier-stokes equations for a fluid with bingham rheology. the rheological parameters are the yield strength sy and the plastic viscosity h. the bingham flow condition, which requires that the shear stress is greater than the yield strength, is modeled by introducing a critical height hcr and having flux between two adjacent cells only when h > hcr, with h being the lava height in the cell with the higher total height. when this condition is verified, the volumetric flux is given by: (1) where dx is the distance between adjacent cells, and a = h/hcr. the bingham flow condition ensures that a > 1 always when the formula is applied. the critical thickness hcr is computed from the yield strength and the slope angle, to account for both pressuredriven and gravity-driven flows, according to the formula: (2) where t is the lava density, a is the slope angle of the inclined plane, g is the gravity acceleration, dz is the overall height difference (considering ground elevation and lava height), and dh is the difference in lava thickness. following ishihara et al. [1990], the yield strength is computer according to the formula: (3) and the viscosity follows giordano and dingwell [2003]: (4) where t is the temperature in kelvin, and h2o is the water content in weight percent (wt%). magflow assumes a constant (average) water content across the flow, and its value is a user-controlled parameter that is typically in the range 0.02-0.2. the actual amount of lava gained or lost by a cell at each iteration is given by the total flux q of the cell multiplied by the timestep dt for that iteration. to prevent non-physical solutions, the timestep is controlled by ensuring that for each cell qdt < cha, where h is the lava height in the cell and a is the cell area. the constant 0 < c < 1 ensures that only a fraction of the total fluid lava volume is lost at each iteration, and this should be selected o be small enough to ensure that the stationary solution of the navier-stokes equation used to compute the flux remains approximately valid during the next step. the timestep used by the cellular automaton is then the minimum of the dt computed by each cell. 3.1. computational pipeline an iteration of the cellular automaton can be divided into the following steps, each of which can be executed independently by each cell: (i) compute the eruption flux if the cell is a vent cell; (ii) compute the fluxes with the neighboring cells; (iii) compute the maximum allowed timestep; and (iv) update the cell status. the cell-status update itself consists of three steps: (a) compute the new lava thickness; (b) compute the heat radiation loss; and (c) transfer an appropriate amount of lava thickness to the solid lava thickness if there is solidification. there are a few important differences between serial and parallel implementation of such an algorithm. when computing the fluxes between neighboring cells, serial execution can compute the flux between each pair of cells a single time, and then add the result (with opposite signs) to the total flux of each cell involved. on the other hand, a parallel implementation benefits more from totally independent cell evolutions: it is therefore more efficient to let each cell compute all of its fluxes, even though this means that fluxes will be computed twice. similarly, during the serial execution, the maximum allowed time-step is updated by each cell at the end of the flux computation loop, while a parallel implementation would store the maximum time-step of each cell in an array, and then use parallel reduction to determine its minimum. in both cases, the simulation time is updated at the end of the iteration, and a new iteration begins if the simulation time has not passed the given end time. 4. preliminary results 4.1. a test case: the 2001 mount etna eruption to test the performance of our cuda implementation, we compare here the timing for three different nvidia gpus versus the cpu timing. all of these simulations were run on the same machine, which has an intel core2 quad clocked at 2.83 ghz as well as all three of the gpus specified in table 1. the operating system used for the tests is ubuntu 10.04, with the cuda tool-kit version 3.2. to compare the computational performance of the cpu and gpu implementations, we look at the average number of evolutions per second simulated by each hardware choice, and then compute the speed improvements obtained by the cpu over the gpu. the average number of evolutions per second (obtained by dividing the total number of evolutions at the end of the 3 3 2 1 2q s h x a a 2 3 2y cr= +h d -^^ h h (sin / cos ) ( )h pg h x s s pg z h z x2 2 cr y y= + 2 2 a a d d d d . .log s t13 00997 0 0089y10 = log 4.643 499.31 28.74ln (h o) 5812.44 427.04 h o t10 2 2= + + # h porting and optimizing magflow on cuda simulation by the seconds spent in the simulation functions) gives us better grounds for comparison than the simple runtime, because due to hardware differences, the actual number of evolutions needed to complete the simulation differ on the different hardware platforms, as further discussed in section 5.3 (accuracy). in applications such as scenario forecasting, the total runtime of a simulation, i.e. the time from program launch to production of the results, is an important factor to consider, and possibly even more so than the bare computational speed improvements that we have considered. the total runtime accounts for janitorial and maintenance tasks, such as loading the initial data and saving the final results, in addition to the time spent in the actual simulation. it is therefore important to consider that the gpu code must perform some additional tasks with respect to the cpu implementation; namely, the uploading of the initial data to the gpu, the retrieval of the maximum allowed dt at each time step, and finally the download of the final results from the gpu. however, the total cost of these additional operations is barely significant, and they only take a fraction of the overall simulation time: the overall performance of the gpu implementations closely follows the computational performance in terms of the overall speed improvements, as shown by the results presented in the next section. we will present and discuss the timing results from two simulations, both based on the data available for the 2001 mount etna eruption, and which differ only in the size of the cells and the cell numbers, due to their different grid resolutions (dems with 10-m and 5-m cell resolutions, respectively). 4.2. performance comparison by considering only the raw computational power of the cpu and the gpus, and therefore factors such as the clock rates, the number of cores, and finally that on the gpu each flux is computer twice (once per cell, instead of once per pair of cells), we can expect a maximum speed improvement of about 8 times when using the 9500 gt. similarly, the gtx 280 can be expected to be 6-to-8-times faster than the 9500 gt (and thus about 50 times faster than the cpu), and the gtx 480 more than twice as fast as the gtx 280 (and therefore about two orders of magnitude faster than the cpu). the actual performance numbers are presented in tables 2 and 3. we also show the average runtimes over three simulations for each hardware choice, with the offsets for the slowest and fastest execution times (table 4). the results show both the accuracy of the timing throughout the study, and how the runtimes essentially follow the qualitative benefits of the computational speed improvements. the highlights of tables 2 and 3 are the low performance of the oldest hardware generation, which comes out as 3-to-4-times slower than the forecast according to its raw computing power; only minor differences in the speed improvements are seen between the 10-m and 5-m cells. the more modern hardware, on the other hand, comes closer to the expected results for the 5-m cells, but falls short by about 50% with the 10-m cells. these phenomena can be explained by looking at some of the factors that can have a significant (negative) impacts on the performance of a gpu. the first, and probably the most important factor, is given by the very high latency of the gpu global memory: 800 computing cycles are needed to access a single value in the global memory, with the threads stalling in a wait queue until the datum becomes available. the gpu can compensate for these long wait times by coalescing memory accesses and by running some threads while others are stalled. coalescing happens when a single memory access is carried out to fetch or write multiple values. this typically happens when threads with consecutive global indices in the launch grid access values that are contiguous in correctly aligned memory; the actual details about when memory accesses are coalesced depend on the hardware, with that of the more recent generations having improved coalescing capabilities than the older hardware. the thread dispatcher built into the gpu will also take care of the execution of threads that are ready to run while the others are stalled waiting for data: this will allow the bilotta et al. 583 cpu 9500 gt gtx 280 gtx 480 cuda compute capability n.a. 1.1 1.3 2.0 clock rate (ghz) 2.83 1.35 1.30 1.40 cuda cores n.a. 32 (4 × 8) 240 (30 × 8) 480 (15 × 32) table 1. specifications of the hardware used to run the test simulations. cpu 9500 gt gtx 280 gtx 480 10 m dem cell 86.2 127.8 2501.6 4415 5 m dem cell 17.1 31 833.5 1685 cpu 9500 gt gtx 280 gtx 480 10 m dem cell 1 1.5 29 51.2 5 m dem cell 1 1.8 48.7 98.5 cpu 9500 gt gtx 280 gtx 480 10 m dem cell 6:07:21 ± 4s 4:05:26 ± 3s 13:58 ± 2s 8:29 ± 2s 5 m dem cell 89:35:59 ± 35s 43:04:26 ± 21s 1:39:31 ± 2s 52:46 ± 1s table 2. average number of automaton evolutions per second. table 3. speed improvement factors over the cpu. table 4. total runtime (h:m:s). 584 latency of subsequent accesses to be covered by the execution of the other threads, as long as there are enough threads ready to be executed. when all of the gpu multicores are busy running threads, the gpu is said to be saturated. saturation typically needs tens or hundreds of thousands of threads, depending on the complexity of the kernels, and it is also a very important factor in achieving optimal gpu performance. a rule of thumb to detect saturation is a linear increase in the execution time with the size of a problem. indeed, table 2 shows such a trend for the 9500 gt, but not for the gtx cards. a third aspect that can have an impact on the performance is given by the number of divergences in the code. divergences happen when threads in the same block run different branches in conditional code paths. when such a situation occurs, thread execution is serialized by the gpu, losing the benefit of the parallel execution, which is a crucial aspect of the gpu performance. in the magflow case, the first phase of an eruption has conditions that are typically not favorable for the gpu, as there are only a very few active cells, and a number of these are either neighbors to the vents or are boundary cells for the lava flow, both of which represent situations that increase the number of divergences. when the number of such cells is dominant, the gpu will suffer significant performance loss, resulting in little benefit over the cpu execution. the gpu works closer to peak performance when the number of cells covered by the main body flow grows to be significant. this usually happens towards the end of the simulation, if the lava flow grows to cover a large area, or if the resolution of the dem is not too coarse. 5. porting details 5.1. memory structures most of the automaton data is stored in the gpu global memory. to allow easier coalescing, and thus to speed up memory access, we follow the best practice recommended by nvidia: the use of “structures of arrays” rather than “arrays of structures”. while the original serial implementation of magflow defined a cell structure holding the data for that cell, and the automaton was modeled as a list of cells, the cuda version of magflow defines six arrays with the cpu and 10 arrays with the gpu. these six cpu arrays are ground elevation, vent index (which is 0 if the cell has no vent, or the vent number otherwise), lava height, solid lava height, heat quantity, and temperature. each of these arrays has the number of elements equal to the number of cells in the automaton. with the gpu, there are six arrays that mirror those of the cpu, plus four additional arrays that hold lava fluxes, heat fluxes, the maximum time-step, and a marker to denote whether the cell has an erupting vent or not. again, these arrays have as many elements as there are cells in the automaton. the only cpu arrays that get initialized and uploaded to the gpu are the ground elevation array, where the data are read from a specified dem file in ascii grid format, and the vent index array. the other arrays are only used during checkpoints, to download the data from the gpu and to save the results on file. they are also used when loading a previous checkpoint to continue a simulation that was interrupted abruptly. 5.2. cuda kernels in magflow, the cuda computational kernels can be distributed in a natural way by having one thread per cell in the automaton. the iteration structure also suggests a natural way to split the algorithm into computational kernels: erupt, which is executed only on vent cells and which computes the effusion rate at the given time; calcflux, which is executed on each active cell and which computes the ingoing and outgoing lava fluxes with respect to all 8 neighbors, and the maximum timestep for that cell; reduction, from the cudpp library [sengupta et al. 2008], which operates a parallel reduction to determine the maximum allowed timestep; updatecells, which is executed on each active cell to update its status, and thus to complete the iteration. the instant timings for each kernel, i.e. the actual kernel runtime at each iteration, are plotted in figure 1. the instant timings show that the runtime for calcflux, the timestep reduction and updatecells grows with the number of active blocks. the erupt kernel, on the other hand, has an overall constant runtime, which is one-to-three orders of magnitude lower than the runtime of each of the other kernels. a small decrease in the execution time is seen in the erupt kernel due to the actual eruption ending a few days before the end of the simulation. on the gtx 280 and gtx 480, the runtime of calcflux has a slightly negative slope after the maximum number of blocks is reached. this is due to the increasing homogeneity of the status of the active cells as the eruption progresses, and the consequent decrease in the number of divergences for the conditional kernel code. the differences in the runtimes between erupt and the other kernels is particularly visible for the oldest hardware, while with the newer hardware, calcflux and updatecells gain up to an order of magnitude in execution speed. the same is not observed for erupt, which is easily explained by noting that calcflux and updatecells are highly parallel, as they are distributed across all of the active cells, while erupt is limited to the very small number of vent cells. therefore, the erupt kernel does not scale as well as the others with the improvements offered by the newer hardware. on the gtx 480, the erupt runtime is of the same order of magnitude porting and optimizing magflow on cuda (10-2 ms) as the hard lower boundary imposed by the kernel launch time. the same factors also influence the distribution of the runtime percentages at each iteration, as plotted in figure 2. the percentages are obtained by dividing the runtime of the single kernel by the sum of all of the kernel runtimes at the given iteration. these clearly show that calcflux takes more than half of the total iteration runtime, as expected by its high computational density, but with a decreasing weight (from over 75% on the 9500 gt down to 65% or less on the gtx 480) as the hardware improves and the kernel runtimes grow smaller. bilotta et al. 585 0.01 0.1 1 10 0 5 10 15 20 25 1 4 16 64 256 1024 4096 m se c simulation days 10m dem erupt calcflux dt reduction updatecells blocks 0.01 0.1 1 10 100 0 5 10 15 20 25 1 4 16 64 256 1024 4096 16384 m se c simulation days 5m dem erupt calcflux dt reduction updatecells blocks 0.01 0.1 1 0 5 10 15 20 25 2 4 8 16 32 64 128 256 512 1024 2048 simulation days 10m dem erupt calcflux dt reduction updatecells blocks 0.01 0.1 1 10 0 5 10 15 20 25 1 4 16 64 256 1024 4096 16384 simulation days 5m dem erupt calcflux dt reduction updatecells blocks 0.001 0.01 0.1 1 0 5 10 15 20 25 1 2 4 8 16 32 64 128 256 512 bl oc ks simulation days 10m dem erupt calcflux dt reduction updatecells blocks 0.001 0.01 0.1 1 0 5 10 15 20 25 1 4 16 64 256 1024 4096 bl oc ks simulation days 5m dem erupt calcflux dt reduction updatecells blocks figure 1. instant timings for each kernel on the 9500 gt (left), gtx 280 (center) and gtx 480 (right). the kernel timing is on a base-10 log axis, and the block sizes are on a base-2 log axis. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 5 10 15 20 25 0 200 400 600 800 1000 1200 pe rc en t o f e vo lu tio n ru nt im e simulation days 10m dem erupt calcflux dt reduction updatecells blocks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 5 10 15 20 25 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 pe rc en t o f e vo lu tio n ru nt im e simulation days 5m dem erupt calcflux dt reduction updatecells blocks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 5 10 15 20 25 0 200 400 600 800 1000 1200 simulation days 10m dem erupt calcflux dt reduction updatecells blocks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 5 10 15 20 25 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 simulation days 5m dem erupt calcflux dt reduction updatecells blocks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 5 10 15 20 25 0 50 100 150 200 250 300 350 400 bl oc ks simulation days 10m dem erupt calcflux dt reduction updatecells blocks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 5 10 15 20 25 0 200 400 600 800 1000 1200 1400 1600 1800 2000 bl oc ks simulation days 5m dem erupt calcflux dt reduction updatecells blocks figure 2. percent timing for each kernel on the 9500 gt (left), gtx 280 (center) and gtx 480 (right). 586 5.3. accuracy for historical reasons, the first generation of nvidia gpus was strongly optimized for floating-point operations in single precision; the later hardware revisions introduced support for double precision, which was about eight-times slower than single precision. although the support for double precision has greatly improved with the latest generation of nvidia gpus (known as fermi), our cuda implementation of magflow uses single precision to support a wider range of hardware. the existing cpu implementation of magflow can be used with either single or double precision, which is important both for performance comparison with the gpu implementation (in our benchmarks here, we used single precision on both the cpu and the gpu), and to help determine the numerical robustness of the algorithm. indeed, the lack of associativity of the floating-point operations (on both cpus and gpus) can produce slight differences in the computed fluxes, which can depend, for example, on the order in which the neighborhood is transversed (clockwise vs counterclockwise, which neighbor the cycle is started from, etc.) the differences in the flux computation can accumulate, although even on long-running simulations (a month of simulated time), the number of cells invaded by lava and the cell status components (height, heat, solid height, temperature) remain well within 1% discrepancy. the algorithm itself is therefore particularly stable numerically. further details, with a more complete analysis of the sensitivity of the model can be found in bilotta et al. [submitted 2011]. we elected to further improve the accuracy of the gpu code by using kahan summation [kahan 1965] during the flux computation. this incurs a performance penalty of less than 5%, due to the handful of extra summations that need to be computed, but it stabilizes the results with respect to the neighbor access patterns. kahan summation was also considered for the evolution of each of the cell variables, but the idea was discarded due to its inefficiency. using kahan summation for the cell evolution requires one additional array for each cell status variable (height, heat, solid height, temperature), to store the kahan reminders, and this results in the need for twice the access to the global memory during the updatecells kernel. the overall effect on the performance is between 25% and 30%, which is not justified for differences in the results of less than 1%. 6. optimization 6.1. register use the effects of latency hiding that are provided by having more blocks per multiprocessor can be seen by studying the effects of register use. for example, the calcflux kernel with kahan summation compiles by default for the use of 33 registers, which results in a limit of six blocks per multiprocessor on the gtx 280; forcing the compiler to use no more than 32 registers, thus allowing eight concurrent blocks, results in a 5% speed improvement for the kernel. on the 9500 gt, however, a reduction in the number of registers (thus bringing the concurrent blocks from three to four) provides no significant benefits. 6.2. active cells and block structure at each iteration, only the cells that either have lava or have a neighbor with lava can have non-zero thermal or mass flux, and therefore can evolve into a different status at the end of an iteration. in the cpu implementation of magflow, the number of cells for which fluxes were computed was therefore minimized by the tracking of a list of active cells, which was updated at the end of each iteration when a previously empty cell had a positive lava flux. a similar approach with the gpu would result in scattered memory reads and very low coalescing, with a drastic reduction in performance. to favor the regular access patterns that increase coalescing and improve the memoryaccess performance, our choice was to compute at each iteration the smallest rectangular bounding box that covered any cells with fluid or solid lava, plus one additional row/ column on each of the four sides of the rectangle. with the appropriate memory and block layout (see also sections 6.3 and 6.6) the two-dimensional structure of the blocks and the grid allows fast memory access, which on the fermi architecture provides good exploitation of the cache. the bounding box also ensures that all of the active cells (in the sense used for the cpu implementation) are included in the computation, although it also includes a number of additional cells for which fluxes do not actually need to be computed. figure 3 illustrate a fictitious case that represents porting and optimizing magflow on cuda figure 3. bounding box inefficiency. black squares mark cells with lava, yellow squares mark active cells without lava, white squares (inside the red frame) indicate cells for which computations are run but which are known to have zero flux. the worst possible situation, in which the lava flow has a linear emplacement which is at 45˚ with respect to the automaton orientation. an interesting effect of the bounding box covering is that especially during the latter phases of the simulation, a single extra active cell in one direction can cause the number of cells for which computations are run to jump suddenly by a significant factor. this phenomenon can be seen in figure 1, when comparing the behavior of the gtx 480 (right) with that of the gtx 280 (center) for the 5-m dem resolution, around about day 20. the detailed plot of the grid structure with the distinct width and height increases (figure 4) clearly shows the anomalous step caused by the 384 × 4 block grid expanding to a 384 × 5 grid. 6.3. optimal thread and memory layout the optimal block structure for kernel execution depends on the compute capability of the hardware, due to the following differences between the hardware generations: the number of cores and registers and the amount of shared memory available for each multiprocessor, the improved memory coalescing available in devices with a compute capability 1.2 or higher, and the availability of l1 and l2 cache memory in devices with a compute capability of 2.0 or higher. the thread layout can be tuned both in terms of the number of threads per block and with respect to the twodimensional structure of the block. for example, blocks with 64 threads can be distributed as 64 threads in a row (1 × 64), 64 threads in a column (64 × 1), a square block with 8 × 8 threads, or any other combination. the choice can have a drastic impact on performance, as it contributes to the improvement of coalescing of memory access. in particular, it is important that the thread block structure closely follows the memory structure. the linear layout of the two-dimensional array storing cell data in global memory can be either row-major, with the data in the same row that occupies consecutive column-ordered memory locations, or column-major, with the data in the same column that occupies consecutive row-ordered memory locations. with row-major global arrays, a block structure of 1 × 64 is optimal because threads in the same block will access consecutive memory locations, which improves coalescing. conversely, with column-major data disposition, a block structure of 64 × 1 will provide the best performance. the results for row-major data ordering are shown in table 5, with the average number of evolutions per second correlated to the block structure, assuming a constant block size of 64 threads. we can see that in the case of the 9500 gt, the oldest device, the block structure does not have any significant influence (10% variation). in the case of the more recent hardware, however, the correct block structure can provide up to a five-times boost in performance. this can be explained by the very small number of concurrent blocks that can execute on a multiprocessor in the 9500 gt and to its reduced coalescing capabilities, which means that the global memory access latency is the dominant slowing down factor. in the case of the gtx 280, instead, the improved coalescing and the higher number of blocks that can be dispatched to a single multiprocessor result in much more reduced memory latencies in the case of the optimal block structure. these effects are not as obvious in the case of the gtx 480 due to the l1 cache that is provided by its fermi architecture, which helps further to reduce the memory latency. indeed, if we let the block size grow with the gtx 480, we can see how the performance improves until the number of blocks per multiprocessor drops below the level at which the cache system can compensate for this (table 6). in this case, a more two-dimensional block structure helps to improve the performance, as the l1 cache becomes more effective. bilotta et al. 587 1 1.5 2 2.5 3 3.5 4 4.5 5 0 5 10 15 20 25 0 2 4 6 8 10 12 14 16 18 20 bl oc ks (w id th ) bl oc ks /1 00 (h ei gh t, to ta l) simulation days grid structure total width height figure 4. width and height of the computational grid, and resulting number of blocks, during the 5-m simulation with the gtx 480. the left scale tracks the width of the grid in blocks, the right scale tracks the height of the grid and the total number of blocks, in hundreds of blocks. the large step around day 20 is caused by the width of the grid increasing from 4 to 5, when the height of the grid is already at 384 blocks. block size (rows × cols) 9500 gt gtx 280 gtx 480 1 × 64 114 2501 3449 8 × 8 127 1347 3345 64 × 1 120 553 869 1× 64 1 × 128 1 × 192 2 × 64 3 × 64 4 × 64 gtx 480 3449 3634 3151 4285 4326 4210 gtx 280 2501 2150 n.a. 2299 n.a. n.a. table 5. influence of block structure on the average number of evolutions per second, with 64 threads per block. table 6. influence of increased block size on the gtx 480 and gtx 280. 588 table 6 shows that the gtx 480 reaches its peak performance with 192 threads per blocks, with a 3 × 64 structure, while blocks with more than 64 threads are counter productive on the older hardware generations. 6.4. texture versus global memory graphic cards provide a special memory area, called the texture memory, that is optimized for two-dimensional access and that provides caching even on the older hardware generations. although the two-dimensional structure of the cellular automaton suggests that this feature can provide some benefit, experimentation shows that the reads and writes of the coalesced global memory obtained with the highly regular access patterns of appropriate thread/ block structuring leads to a higher data throughput than for the noncoalesced but cached texture memory. indeed, the loss of coalescing due to the use of the texture memory can reduce the implementation performance by two or three times. 6.5. shared memory versus multiple data reads with a fully parallelized implementation, each cell computes the interaction with all of its neighboring cells, with the result that each interaction is computed twice. as flux computations depend on costly operations, such as the exponential, it is possible to halve the number of operations by computing each interaction once and then updating both of the cells involved. to achieve this, each block must load into the shared memory the height, total solid height, and temperature of the whole block, plus a ring of neighboring cells; this is achieved according to the loading pattern illustrated in figure 5. the shared memory also holds the lava and heat flux for each cell in the block, before it is stored back in the global memory. each cell then computes the interaction with its left, bottom left, bottom and bottom right neighbors, while updating both its own flux and that of its neighbor. thread synchronization is used between the updates, to ensure that all of the writes occur correctly. the leftmost, rightmost and topmost cells also compute the interactions with the outer ring that are not computed in the common loop (figure 6). in this case, a block structure with a square shape provides the best benefit, as the amount of shared information is higher. however, the benefits of this approach depend on the compute capability of the device: the enormous increase in shared memory use results in a proportional reduction in the concurrent block execution. additionally, the different behavior of the extremal cells in each block causes divergences at every execution, both during the initial phase of the kernel execution (data loading), and during the computational phase. while a net benefit of 35% has been measured for the older, memory-bound hardware, such as the 9500 gt, due to the reduced computation, the much smaller number of concurrent blocks results in an equal loss in kernel performance for the gtx 280, as the reduced number of computations are not sufficient to offset the reduced number of concurrent threads. given the obsolescence of the older hardware, the structure without shared memory is preferred. the l1 caching introduced in the fermi architecture (gtx 480) provides an automatic way to reduce memory access, which is more efficient than manual shared memory use. porting and optimizing magflow on cuda figure 5. load pattern for shared memory use. each block cell (inside the red frame) loads its own data, and the boundary cells also load data from the outer ring of their neighboring cells. figure 6. flux computation pattern. when all of the cells use the same pattern (highlighted in the central cell), each pair of cells interacts only once, although the cells in the outer rim are missing some neighbors, which requires additional interactions to be computed. 6.6. the fermi l1 cache with the new fermi architecture, nvidia introduced two caching levels for global memory access (l1 and l2). the appropriate use of this capability can bring a substantial benefit to the execution of kernels that require repeated access to a small number of global memory values from multiple threads in the same block. this is indeed the case for our flux computation kernels, where all of the neighboring cells have to access each other’s height, lava height, solid lava height and temperature. as much of the hardware for the l1 cache is common with the shared memory hardware, cuda provides an interface that allows the programmer to choose whether shared memory is preferred (in which case 48 kb will be used for the shared memory, and 16 kb will be used for the l1 cache), or whether the l1 cache is preferred (in which case 16 kb will be used for shared memory, and 48 kb will be used for the l1 cache). a third option (which is the default) informs cuda that the kernel should use whatever setting is currently active, which is typically to prefer shared memory, unless the programmer has set the preference as the l1 cache for a previous kernel. as in our case shared memory does not bring any particular benefit for the more recent gpus, we can set cuda to prefer l1 caching to shared memory without affecting the number of kernels being executed. the performance comparison (table 7) shows a consistent gain of about 2% that derives from the preference for l1. we can get deeper insight into how the cache affects the performance by making use of the visual profiler provided in the latest versions of cuda, and by analyzing the impact that the cache settings have on the cache hit rate (figure 7) and on the average kernel runtime (figure 8). we observe that the effects on the two kernels of the cache preference and the block structure are quite different, as expected. as calcflux needs to access the data for each cell multiple times, the cache hit rate without any specific optimization is already relatively high (>50%), and is limited by the actual cache size, as expressed by the increase in the cache hit rate with the preference for l1 caching. in contrast, the updatecells kernel only accesses data for a single cell in each thread, and therefore it can exploit the cache at a much lower level; this is indicated by the lack of significant increase in the cache hits when l1 is preferred, for the default 1 × 64 block structure. as the block structure becomes more rectangular with the increase in the first dimension, the fermi hardware can make better use of the cache lines, and the cache size becomes an increasingly limiting factor. the average kernel runtime provides a comparison with the older cuda architecture, thereby indirectly showing the influence the new l1 cache can have on performance. in figure 8 there is a growth in the average of the gtx 280 kernel runtimes, as expected by the increased block size and the diminishing number of blocks per multiprocessor. in contrast, for the gtx 480, the calcflux kernel shows a constant decline in the average runtime with the increase in the blocksize, which can be explained by the cache efficiency and the high number of memory accesses required by the kernel. in the updatecells case, on the other hand, we again see a slight increase in the average runtime, due to the very small number of memory accesses and the consequent minor bilotta et al. 589 0 20 40 60 80 100 1x64 2x64 3x64 ca ch e hi t ( % ) block structure calcflux calcflux(l1) updatecells updatecells(l1) figure 7. cache hit rate for the different block structures and the l1 cache preferences. 0 50 100 150 200 250 300 1x64 2x64 3x64 av g ru nt im e (u se c) block structure calcflux calcflux(l1) updatecells updatecells(l1) calcflux(gtx280) updatecells(gtx280) figure 8. average running time for each kernel for the different block structures and l1 cache preferences. 1 × 64 2 × 64 3 × 64 4× 64 prefer shared 3449 4285 4326 4210 prefer l1 3543 4337 4415 4328 table 7. average number of evolutions per second on the gtx 480 that depend on the shared versus l1 preference setting for kernel launch. 590 impact of the caching. the l1 influence can, however, still be seen by considering that the gtx 280 suffered almost a 100% increase in average runtime with a triple block size, while for the gtx 480 this effect was limited to a 30% increase. 6.7. cpu versus gpu vents the eruption phase cannot really exploit the gpu parallel computation capabilities. however, it is more efficient to run the kernel on the gpu directly rather than running it on the cpu and then uploading the new data to the gpu at every iteration, because of the high host-todevice latency. 6.8. vector types versus separate memory array the cellular automaton structure needs four floats to describe each cell status, plus two floats to describe mass and heat flux from cell to cell. this suggests that using the native float4 and float2 vector types might result in more efficient code and better memory accesses. as with the shared memory, however, the actual benefit is only present for the lower-end hardware (9500 gt), where the bottleneck is caused by the memory access, and the lower number of blocks limits the number of blocks running concurrently: switching from arrays of floats to arrays of vector types in this case results in a gain in performance of up to two times. for the gtx 280, however, the same change results in a 10% loss in performance, due to the higher register use of the resulting kernel. this again can be mostly recovered by forcing the compiler to use no more than 32 registers for the kernel, which limits the performance loss to 1%; this can be attributed to the slightly inferior bandwidth that vector types have over scalars. 7. conclusions the magflow cellular automaton has an intrinsically highly parallel structure that can substantially exploit the high-performance parallel hardware of modern gpus. a straightforward conversion of the serial cpu code leads to only minor improvements in the execution speed on the older gpus, while a speed improvement of almost 50 times can be obtained with the last generation of cuda cards during the simulation of lava flows from typical mount etna eruptions. further benefits are harder to achieve for typical eruptions due to the inability for these events to saturate the hardware, although long-running eruptions or lava flows spanning a wider area can provide speed improvements of almost 100 times. we believe that the optimization strategies presented in this study can also be useful for other cuda applications that are based on similar mathematical structures and algorithms. in particular, the same computational approach can be extended to include other hazardous natural phenomena where the physical modeling is close to that of lava flow, with the primary examples being land slides and debris flow. an optimization that has not been implemented yet is the possibility to use different block sizes for the calcflux and updatecells kernels. the profiling conducted on the gtx 480 card appears to indicated that a block size of 1 × 64 is optimal for updatecells, while 3 × 64 is optimal for calcflux; however, updatecells only takes 10% of the total evolution time, and the consequent speed improvement of 30% on its runtime would result in a net impact of less than 1% over the whole simulation runtime; this is actually within the standard runtime fluctuations encountered during our tests. other optimizations might also be possible, in particular concerning the bounding box strategy used to determine the kernel launch grid: the current strategy can launch a significant number of kernels on cell which are far from the lava flow and would therefore not need flux calculations nor updating. a more sophisticated strategy can be used to map execution blocks only on the minimal number of cells surrounding the lava flow that can be covered, while still retaining the efficient memory access patterns that ensure coalescence (and high l1 cache use on the gtx 480). experimentation is needed to determine whether the complexity of such an irregular grid mapping would be balanced out by the fewer blocks needed to cover the active area of the automaton. even without this additional optimization, the increase in simulation speed is already sufficient to provide a 7-day forecast in a couple of minutes, and a month forecast in 10 to 15 min. this thus matches, or indeed surpasses, the speed of more simplistic lava-flow simulation models, while still retaining the reliability of the physics-based evolution. acknowledgements. this study was carried out with financial support from the v3-lava project (ingv-dpc 2007-2009 contract). references avolio, m.v., g.m. crisci, s.d. gregorio, r. rongo, w. spataro and g.a. trunfio (2006). sciara g2: an improved cellular automata model for lava flows and applications to the 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(2008). sph astrophysics – "state-of-the-art", in: spheric 2008 keynotes; url: http:// cfd.mace.manchester.ac.uk/sph/meetings/ 3rdspheric_workshop/berczik_keynote_spheric_ 2008.pdf (last visited: november 09, 2011). bilotta, g., a. cappello, a. hérault, a. vicari, g. russo and c. del negro. sensitivity analysis of the magflow cellular automaton model, (submitted 2011, sub judice). bonaccorso, a., a. bonforte, s. calvari, c. del negro, g. di grazia, g. ganci, m. neri, a. vicari and e. boschi porting and optimizing magflow on cuda (2011). the initial phases of the 2008-2009 mount etna eruption: a multidisciplinary approach for hazard assessment, j. geophys. res., 116, 1-19. crisci, g.m., s. di gregorio, o. pindaro and g.a. ranieri (1986). lava flow simulation by a discrete cellular model: first implementation, international journal of modelling and simulation, 6 (4), 137-140. del negro, c., l. fortuna, a. hérault and a. vicari (2008). simulations of the 2004 lava flow at etna volcano using the magflow cellular automata model, b. volcanol., 70 (7), 805-812. giordano, d. and d.b. dingwell (2003). viscosity of hydrous etna basalt: implications for plinian-style basaltic eruptions, b. volcanol., 65 (1), 8-14. hérault, a., a. vicari, a. ciraudo and c. del negro (2009). forecasting lava flow hazards during the 2006 etna eruption: using the magflow cellular automata model, comput. geosci.-uk, 35 (5), 1050-1060; doi: 10.1016/j.cageo. 2007.10.008. hérault, a., g. bilotta and r.a. dalrymple (2010). sph on gpu with cuda, j. hydraul. res., 48 (extra issue), 74-79. ishihara, k., m. iguchi and k. kamo (1990). numerical simulation of lava flows on some volcanoes in japan, in: lava flows and domes: emplacement mechanisms and hazard implications (iavcei proceedings in volcanology), edited by j.h. fink, springer-verlag, 174-207. kahan, w. 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(2011). gpu-computing in econophysics and statistical physics, eur. phys. j.-spec. top., 194, 87-119. roberts, m., j. packer, m.c. sousa and j.r. mitchell (2010). a work-efficient gpu algorithm for level set segmentation, in: proceedings of the conference on high performance graphics, hpg ‘10, eurographics association, airela-ville, switzerland, 123-132. sengupta, s., m. harris and m. garland (2008). efficient parallel scan algorithms for gpus, nvidia technical report nvr-2008-003, dec. 2008, 17 pp.; url: http://mgarland. org/files/papers/nvr-2008-003.pdf szerwinski, r. and t. gneysu (2008). exploiting the power of gpus for asymmetric cryptography, in cryptographic hardware and embedded systems ches 2008, edited by e. oswald and p. rohatgi, vol. 5154 of lecture notes in computer science, springer, berlin/heidelberg, 79-99. vicari, a., a. hérault, c. del negro, m. coltelli, m. marsella and c. proietti (2007). modeling of the 2001 lava flow at etna volcano by a celluar automata approach, environ. modell. softw., 22 (10), 1465-1471. vicari, a., a. ciraudo, c. del negro, a. hérault and l. fortuna (2009). lava flow simulations using discharge rates from thermal infrared satellite imagery during the 2006 etna eruption, nat. hazards, 50 (3), 539-550. *corresponding author: giuseppe bilotta, università di catania, dipartimento di matematica e informatica, catania, italy; email: bilotta@dmi.unict.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. bilotta et al. 591 s u l g r a d i e n t e a d i a b a t i c o d i t e m p e r a t u r a n e l l ' i n t e r n o d e l l a t e r r a paolo emilio valle 1. — i l gradiente adiabatico di temperatura nell'interno della terra ha una notevole importanza in molte questioni di carattere geofisico. esso è espresso dalla nota relazione termodinamica m r « m 3p /„ q cp nella quale t è la temperatura assoluta, a la dilatazione termica, q la densità, cp il calore specifico a pressione costante ed s l'entropia. se si ammette l'equilibrio idrostatico e si indica con li la profondità e con g l'accelerazione di gravità, risulta dp = q gdh e la [ 1 ] può essere scritta nella forma (3t\ _ tag kdh)s cp [ 2 ] o anche d l o g t= ^dh [ 3 ] cp fino a qualche anno fa, il rapporto « /cp veniva dedotto da misure di laboratorio eseguite su varie rocce ignee. nel mantello della t e r r a il suo valore medio era ritenuto pari a circa 1 . 5 1 0 l s g erg . recentemente j . verhoogen i r) ha proposto un nuovo metodo per la valutazione del suddetto rapporto, allo scopo di eliminare l'inconveniente dell'estrapolazione dei valori ottenuti in laboratorio, alle alte pressioni esistenti nel mantello della terra. i l suo procedimento richiede però alcune semplificazioni, le quali appaiono alquanto arbitrarie. in questa nota viene indicato un metodo per il calcolo del rapporto a/cp, basato sulla teoria di un solido ideale, nell'ipotesi che la temperatura assoluta sia un poco superiore alla temperatura di debye relativa alle onde longitudinali. si am4 2 paoi.o e m i l i o v a l l e mette poi la reversibilità dei processi termodinamici, ciò che del resto è già implicito [ 1 ] , viene effettuato il calcolo del rapporto a/cv e l'integrazione della [ 3 ] per il mantello. l'aumento di temperatura per compressione adiabatica nel nucleo, viene valutato soltanto nello strato e , mediante l'estensione del metodo sopra accennato. 2. — è noto (2"3"4) clie un solido, il quale, in seguito ad una dilatazione o compressione isotropa ed omogenea, assuma il volume v, reagisce alla pressione esterna p con due tipi di pressioni i n t e r n e : l'uno dovuto agli sforzi statici, l'altro all'agitazione termica. mentre la valutazione del primo tipo di pressione interna è piuttosto difficile e richiede una dettagliata conoscenza della struttura del solido, la valutazione del secondo tipo di pressione interna è relativamente semplice. gli atomi di un solido non possono muoversi liberamente, ma oscillano intorno a determinate posizioni di equilibrio, facendo nascere onde elastiche completamente diffuse. l'agitazione termica può essere associata a queste onde, la cui lunghezza però non può essere inferiore al doppio della distanza tra due particelle. ne consegue che lo spettro delle frequenze è limitato superiormente. se si indicano con l'indice 1 le grandezze clic si riferiscono alle onde longitudinali e con l'indice t le grandezze che si riferiscono alle onde trasversali, la frequenza massima è data da nella quale v„, è la velocità, supposta indipendente dalla frequenza, ed n il numero di atomi contenuti nel volume v, che nel seguito verrà assunto come volume specifico. la pressione dovuta all'agitazione termica, coincide con la pressione di radiazione dei due tipi di onde, e può scriversi ( 2 ) um rappresenta l'energia media di oscillazione. l'equazione di stato di un solido ideale, il quale sia sottoposto soltanto a variazioni isotrope ed omogenee di volume, è quindi della forma (m = l, t) [ 4 ] p=f(v)+?mpl 'ai (m — i, t, t) s u l g r a d i e n t e « d i a b a t i c o di t e m p e r a t u r a n e l l ' i n t e r n o d e l l a t e r r a 4 3 la pressione (li radiazione delle onde trasversali deve essere contata due volte in relazione alle due possibili direzioni di vibrazione di queste onde. posto pi=2n, pm ( m = l,t,t) [5] si ottiene facilmente dalla [4] tt l 1 , 3 logva , n tri 1 a log va \ 3 3 log q ) \ 3 3 log q j le energie medie di oscillazione u\ ed u, sono pari all'energia complessiva di n oscillatori, considerati come indipendenti, la cui frequenza non può superare v| e v, rispettivamente. ad una certa temperatura ui ed [/, saranno in generale differenti. si supponga che la temperatura sia alquanto superiore alla temperatura di debye relativa alle onde longitudinali, essa sarà superiore anche alla temperatura di debye relativa alle onde trasversali, dato cbe queste ultime posseggono una velocità minore delle prime. tn questo caso è facile vedere (2"3'4) cbe l'energia media di oscillazione dei due tipi di onde, tende ad assumere il comune valore classico n k t, dove k è la costante di boltzmann, e quindi si potrà scrivere uì = ut = u e l'energia media totale sarà pari a 3 u, poiché l'energia media delle onde trasversali deve essere contata due volte. la pressione interna di radiazione assume allora la forma 3 log ( o t w ) r , , , pì=q u l6 j 3 log e si indichino ora con xt e .r.s rispettivamente la compressibilità isoterma ed adiabatica, con cv il calore specifico a volume costante, da note relazioni di termodinamica, si ha 3 p \ a _ ~st)y xt c, p a. a cv _ a _ _ / 3p,\ [7-| 3 t / v 4 4 paolo e m i l i o v a l l e nella quale tenendo conto della [ 6 ' ] e della [ 8 ] si ottiene dalla [7] 3 log (p vi 3 log( ricordando poi che risulta in definitiva i ' x s = [ 8 ] [ 9 ] [10] v r a cd tv vt3 3 log (p vjvt2)1, 3 iok o [ 1 1 ] è questa una relazione c-lie consente il calcolo del r a p p o r t o r i / c p , mediante la conoscenza della velocità delle onde longitudinali e trasversali e della densità, indipendentemente dalla composizione chimica del solido. pertanto questa relazione si presenta particolarmente adatta per il calcolo di a /'e,, nel mantello della terra. 11 gradiente adiabatico si ottiene sostituendo la [ 1 1 ] nella [ 3 ] e si ha d l o g t= vevx3 3 log (p t|ti!)'/i 3 log p dh [ 1 2 ] si consideri infine uno strato di materiale omogeneo, nel quale la variazione della densità sia dovuta alla sola compressione adiabatica. si avrà allora 1 / 3 p xsqg= [ — e 3h l e quindi dalla [ 1 2 ] , tenuto conto della [ 1 0 ] . risulta f 3 q v,vt~ = c o s t a n ; e [ 1 3 ] come si era già ottenuto in un precedente lavoro ( 5 ) per altra via. s u l g r a d i e n t e « d i a b a t i c o di t e m p e r a t u r a n e l l ' i n t e r n o d e l l a t e r r a 4 5 questa equazione può essere applicata agli strati nei quali l'andamento della densità è stato calcolato supponendo l'equilibrio adiabatico e una costituzione omogenea. 3. — la temperatura di debye relativa alle onde longitudinali alla profondità di 33 km è presumibilmente alquanto inferiore a 1000" k , mentre in prossimità del nucleo vale circa 2000" k . queste temperature non sembrano essere eccessivamente elevate e si può quindi ritenere che l'ipbtesi sotto la quale è stata dedotta la [ 1 1 ] sia verificata. p e r la valutazione del rapporto a/c,„ occorre conoscere la velocità di propagazione delle onde longitudinali e trasversali in funzione della densità. ora la conoscenza dei dati sismici, della massa, della densità media e del momento di inerzia, non è sufficiente a determinare univocamente l'andamento della densità nell'interno della terra. pertanto il calcolo di tale andamento è stato eseguito sotto varie ipotesi ed è stata calcolata più di una distribuzione della densità. in base ai valori della velocità delle onde sismiche calcolati da pi. jeffreys i6"7), k. e. bullen (8ì, allo scopo di eliminare l'indeterminazione del problema, considera due casi: i) clic la densità vari in modo continuo attraverso tutto il nucleo centrale, e cioè nelle regioni e, f e g, ii) che la densità al centro della terra sia più grande di 10 gcm"3 che nel caso i). le due ipotesi conducono ad una distribuzione tabella i ;i km ti i, i> p 10 km sec 'i km sec — 1 g t r a 3 , d i n e c m " cm sec ,—i oys (on v i y3 33 7,75 4,35 3,32 100 7,95 4,45 | 3,38 200 8,26 4,60 3,47 300 8,58 4,76 3,55 413 8,97 | 4,96 3,64 500 9,56 5,28 3,89 600 10,25 5,66 4,13 800 11.00 6,13 4,49 1000 11,42 6.36 4,68 1200 11,71 6,50 4.80 1400 11,99 6,62 • 4,91 1600 12,26 6,73 5,03 1800 ; 12,53 6.83 5,13 2000 12,79 6,93 5,24 2200 13,03 7,02 5,34 2400 , 13,27 7.12 5,41 2600 13,50 7,21 5,54 2800 j 13,64 7,30 5,63 2898 ! 13,64 7,30 5,68 0,009 0,031 0,065 0,100 0,141 0,173 0,213 0,300 0,392 0,49 0,58 0,68 0,78 0,88 0,99 1,09 1,20 1,32 1.37 985 11.7359 989 12.1611 992 12.8150 995 13,4809 998 ' 14,2994 1000 15,9174 1001 17,7590 999 f 20,2738 995 21,6283 991 22,5055 988 23,3112 986 24,1296 985 24,8699 986 25,6448 990 26,3572 998 27,1024 1009 27,8232 1026 28,4550 1037 28,6232 4 6 paoi.o e m i l i o v a l l e della densità lievemente diversa nel mantello, mentre nel nucleo si ha un divario più accentuato. successivamente w. ii. ramsey (") e lo stesso bullen ( 1 0 ) hanno studiato nuove distribuzioni della densità, basate su altre ipotesi. d'altra parte anche l'andamento della velocità delle onde sismiche nell'interno della terra è stato ricalcolato da b. gutenberg ( n ) , partendo da nuovi dati sperimentali, e i suoi risultati differiscono in modo abbastanza sensibile dai valori precedentemente calcolati da jeffreys. l'autore della presente ricerca si è servito delle velocità calcolate da jeffreys e, per quanto riguarda il mantello, delle densità calcolate da bullen in un lavoro precedente alla formulazione delle ipotesi i) e ii) (1 2). i valori delle densità usate sono intermedi fra quelli delle if» o'f jìtm' km >fc ' — — — — — — — j j 1 — 1 —f— —r *p01 afilli : (c ir vf'j ' . 7. fiu, u f 1 tt io! o't! o/ jl"*l ri l eulllm : tasili* i 1 *p01 afilli : (c ir vf'j ' . 7. fiu, u f 1 tt io! o't! o/ jl"*l ri l eulllm : tasili* i | i l 1 | m 1 | | 1 | 1 | 1 f i g . 1 due ipotesi e ne differiscono soltanto per qualche unità percentuale. per quanto riguarda il nucleo sono stati considerati separatamente i due casi sopraccennati. dal lavoro di bullen sono stati tolti altresì i valori della gravità e della pressione, i quali peraltro sono stati accuratamente calcolati anche da g. boaga (13"14), che ne ha inoltre fornito le espressioni analitiche in funzione della profondità. i dati relativi al mantello sono contenuti nella tabella l e i dati che si riferiscono al nucleo nella tabella iv. 4. — nella valutazione del rapporto u/cp, l'operazione più delicata s u l g r a d i e n t e « d i a b a t i c o di t e m p e r a t u r a n e l l ' i n t e r n o d e l l a t e r r a 4 7 è costituita (lai calcolo della derivata che compare a secondo membro della [ 1 1 ] si è notato che se si moltiplica per l'espressione (p vt v,)v3 si ottiene una funzione lineare nella p. pertanto si può scrivere vivls)v3 = a-\-b p [ 1 4 ] eccettuato il valore della funzione in prossimità del nucleo, l'espressione a secondo membro della precedente relazione rappresenta molto bene i valori del primo membro, calcolati con i dati della tabella i. mediante il metodo dei minimi quadrati è stato calcolato il valore delle costanti a e b, e si è ottenuto a = — 12,14000 ; b = 7,21467 e quindi la [ 1 4 ] si può scrivere (o f i f , ) 2 ^ = 7 , 2 1 4 6 7 p ^ — 1 2 , 1 4 p ^ [ 1 5 ] gli scarti f, tra i valori calcolati mediante la [ 1 5 ] e i valori calcolati dai dati della tabella i, sono contenuti nella tabella i i . tabella i i q e e e 3,32 — 0,0768 4,80 + 0,0151 3,38 — 0,0845 4,91 + 0,0272 3,47 — 0,0799 5,03 — 0,0202 3,55 + 0,0088 5,13 — 0.0014 3.64 + 0,1780 5,24 — 0,0201 3,89 — 0,0077 5,34 — 0,0291 4,13 f 0,1024 5,44 — 0,0054 4,49 + 0,0199 5,54 — 0.0061 4.68 + 0,0036 5,63 — 0,0236 lo scarto più elevato si trova in corrispondenza di p = 4,64, cioè alla profondità di 413 km, proprio in corrispondenza della discontinuità del secondo ordine. tale scarto non è però tanto elevato da introdurre seri errori nel calcolo del rapporto a/cp. la forma della [15] non sembra essere del tutto fortuita, almeno per quanto riguarda il primo termine a secondo membro. da essa infatti si ricava ( i m » ) 1 / 3 = 7 , 2 1 4 6 7 p 1 / s — 1 2 , 1 4 p % [16] 4 8 paoi.o e m i l i o v a l l e se si pensasse ora di estrapolare annientando la densità fino a valori per i quali il secondo termine a secondo membro della [16] diventi trascurabile di fronte al primo, tenuto conto cbe la velocità delle onde f i g . 2 trasversali sta in un rapporto press'a poco costante con la velocità delle onde longitudinali, si otterrebbe 1 / 3 m 7 1 r i = c o s t q l17j ora ad altissime densità corrispondono altissime pressioni, alle quali è lecito supporre cbe gli elettroni degli atomi si comportino quasi come un gas di fermi ( 1 5 ). se questo punto di vista è giusto, la [17] deve avere la stessa forma dell'equazione cbe fornisce la velocità delle onde longitudinali in un gas di elettroni. effettivamente, tenuto conto cbe per un gas di fermi, a temperature non altissime, vale l'equazione di stato 51 p= cost q 13 si ottiene »/ vi= cost p 3 che coincide formalmente con la [17] ulteriori considerazioni su questo argomento uscirebbero dai limiti del presente lavoro. s u l g r a d i e n t e « d i a b a t i c o di t e m p e r a t u r a n e l l ' i n t e r n o d e l l a t e r r a 4 9 tabella i i i 11 km 3 log ( o ' v , 2 ) 1 / , 3 log l> — 1 0 » c|. g erg 1 t ts3 33 1,694 4.86 1 100 1,658 4,50 — 200 1,608 4,02 1,0 70 300 1,568 3,61 — 413 1,526 3,20 1,16, 500 1,429 2,63= .— 600 1,354 2,17 1,21, 800 1,266 1,78= — 1000 1,228 1,60 1,31, 1200 l,206r, 1,49 — 1400 1,188 1,39 1.39, 1600 1,169 1,30 — 1800 1,155 1,22 1,46, 2000 1,140 1.14= — 2200 1,127 1,08 1,53» 2400 1.114= 1,03 — 2600 1,103 0,98 ],59„ 2800 1.093 0,95 1,62, 2898 (1,088) (0,95) (1,64.) la seconda colonna della tabella iti contiene i valori di 3 log (qvivts)%/ 3 log p e la terza colonna il rapporto a/cv, il cui andamento in funzione della profondità è mostrato nella fig. 2. nella fig. 3 è riportato l'andamento del rapporto a/cv con la pressione, ma è bene ricordare cbe la temperatura nell'interno della terra non è costante. si può tuttavia ammettere che alle alte pressioni esistenti nel man\ "3 \ "3 \ p io 1 0 2 0 1 0 4 0 5 0 6 0 7 0 » 09 1.1 i l 1.5 u f i g . 3 tello, in prima approssimazione, l'influenza della temperatura sul rapporto a /e,, sia trascurabile, rispetto alla influenza della pressione. sotto questa ipotesi è stato calcolato il rapporto t/t3 3, dove t 3 3 indica la temperatura a 33 km, e i risultati sono contenuti nella ta5 0 paoi.o e m i l i o v a l l e bella i i i . da essi si vede cbe il gradiente adiabatico diminuisce al crescere della profondità. se per es. si pone t33 = 1000° k , si ottengono i seguenti valori medi nei diversi strati. s t r a t o gradiente medio gradi k m 1 b 0,42 c 0,25 s d 0,17„ 5. — a rigore, la teoria precedente non potrebbe essere applicata al nucleo, dato che esso, almeno per quanto riguarda lo strato /?, si presenta come liquido. 2 0 0 4 0 0 600 800 1000 1200 1100 1600 1800 2000 2200 2400 2600 2800 f i g . 4 un liquido come è noto (2"i), è costituito da una struttura cristallina, nella quale vi sono molti posti vuoti o, meglio, molte buche di potenziale. gli atomi non sono quindi del tutto obbligati ad eseguire oscillazioni intorno a posizioni di equilibrio, ma hanno una certa probabilità di muoversi quasi liberamente dalla loro posizione attuale ad un'altra adiacente e momentaneamente vuota. ne consegue che la pressione interna di agitazione termica non coincide esattamente con la pressione di radiazione delle onde longitudinali, le sole possibili in un liquido, né l'energia interna sarà pari all'energia di oscillazione media di queste onde. peraltro sotto le fortissime pressioni esistenti nel nucleo, è da presumere che un liquido conservi una struttura molto s u l g r a d i e n t e « d i a b a t i c o di t e m p e r a t u r a n e l l ' i n t e r n o d e l l a t e r r a 5 1 più vicina a quella del solido da cui proviene, di quanto non accada alle ordinarie pressioni di laboratorio. in altre parole è da presumere cbe la percentuale delle buche di potenziale non sia troppo elevata. non sembra quindi del tutto arbitrario applicare la precedente teoria anche al nucleo, con la condizione vt = 0. procedendo in modo analogo a quello indicato per lo stato solido, si ottiene facilmente la t 2̂898 t 2̂898 t vf u c i / e • tp il ! 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 «00 5000 h f i g . 5 relazione che lega la densità, la temperatura e la velocità delle onde longitudinali in una trasformazione adiabatica reversibile. si ha con qualche passaggio f i ^ c o s t a n t e e * , 3 [ 1 8 ] e non è necessaria la condizione che la temperatura sia superiore alla temperatura di debye. tabella i v h k m e ( i ) g c m 3 0 ( i i ) g c m 3 v i k m s e c — 1 t ~ift ( i ) j.280s t "rt ( ì ì ) ì 2 s 8 s 2 8 9 8 9 , 7 9 , 1 8 , 1 0 1 1 3 5 0 ( 1 1 0 , 5 9 , 8 8 , 9 l , 1 2 s 1 , 1 2 » 4 0 0 0 1 1 , 1 1 0 , 3 9 , 5 1 1 , 2 2 8 1 , 2 2 » 4 5 0 0 1 1 , 6 1 0 , 8 9 , 9 7 l , 3 0 o 1 , 3 0 3 4 9 8 2 1 1 , 9 1 1 , 1 1 0 , 4 4 1 . 3 8 o 1 , 3 7 7 52 paoi.o e m i l i o v a l l e l'andamento della densità nello strato e del nucleo, è stato calcolato da bullen nell'ipotesi che in tutto lo strato il materiale sia omogeneo e l'equilibrio adiabatico. pertanto l'equazione [18] è stata applicata a questo strato e i risultati sono contenuti nella tabella i v . essi differiscono molto poco nei due casi i) e ii). la fig. 5 mostra l'andamento del rapporto t/to.sos nel caso i). roma — istituto nazionale di geofisica — gennaio 1952. riassunto mediante la teoria di un solido ideale si calcola, nel mantello della terra, il rapporto tra la dilatazione termica e il calore specifico a pressione costante, dai dati della velocità delle onde sismiche longitudinali e trasversali e della densità. risulta che tale rapporto decresce con l'aumentare della profondità, passando dal valore di 4.8610"12 g erg'1 a 33 km, al valore di 0,9510"12 g erg'1 in prossimità del nucleo. viene calcolato nel mantello l'andamento adiabatico con la profondità del rapporto t / t 3 3 , dove t:ìm è la temperatura a 33 km, e se ne deducono i valori approssimati del gradiente. l'analogo rapporto t/to898 nello strato e del nucleo, viene valutato con l'estensione della teoria. su m mary on the basis of the theory of an ideal isotropie solid body, and starding from seismic data, the ratio of the coefficient of the thermal expansion to the specific heat at constant pressure in the earth mantle, has been calculated. it appears tliat such a ratio decreases by increasing of the depth, going from 4.8610 12 g erg1 at the depth of 33 km to 0 , 9 5 1 0 1 2 g erg1 near the bottoni of the mantle. the adiabatic behaviour of the ratio t / t : n (t:):i means the temperature at 33 km of depth) by varying of the depth, has been calculated and the approximate values of the gradient have been deduced. through the extention of tliat theory, the similar ratio t/tasos» referring to the e region of the core, has been estimated. s u l g r a d i e n t e «diabatico di t e m p e r a t u r a n e l l ' i n t e r n o della terra 5 3 b i b l i o g r a f i a ( ! ) verhoocen j . : the adiabatic gradient in the mantle. t r a n s . a m e r . g e o p h y s . u n i o n 32, 4 1 4 3 ( 1 9 5 1 ) . ( ) brillouin l . : tenseurs en mécanique et en elastiche. m a s s o n , p a r i s ( 1 9 4 6 ) . ( : ì ) brillouin l . : ifave propagation in periodic structures. me g r a w h i l l , new y o r k ( 1 9 4 6 ) . ( 4 ) slater j . c . : introduction to chemical physics. me g r a w h i l l , new y o r k ( 1 9 3 9 ) . ( " ) valle p . e . : sull'aumento di temperatura nel mantello della terra per compressione adiabatica. a n n . g e o f . , i v , 4 7 5 4 7 8 ( 1 9 5 1 ) . (0) jeffreys h . : the times of p, s and sks, and the velocities of p and s. m . n . r . a . s . g e o p h y s . s u p p l . i v , 498-533 ( 1 9 3 9 ) . ( ' ) jeffrfyes i i . : the times of the core waves ( s e c o n d p a p e r ) . m . n . r . a . s . g e o p h y s . s u p p l . i v , 594-615 ( 1 9 3 9 ) . ( s ) bullen k . e . : introduction of the teory of seismology. c a m b r i d g e , u n i v e r s i t y p r e s s ( 1947). (9) ramsey w . h . : on the nature of the eartli's core. m . n . r . a . s . g e o p h y s . s u p p l . v , 4 1 0 4 2 6 ( 1 9 4 9 ) . ( 1 0 ) bullen k . e . : a ti earth model jìased on a compressibility pressure hypotesis. m . n . r . a . s . g e o p h y s . s u p p l . v i , 50-59 ( 1 9 5 0 ) . ( n ) gutenberg b . : on the layer of relatively low wave velocity at a depth of about 80 kilometers. b u l l . s e i s m . s o c . a m . 38, 121-148 ( 1 9 4 8 ) . ( 1 2 ) bullen k . e . : the problem of the earth's density vuriation. b u l l . s e i s m . s o c . a m . 30, 235-250 ( 1 9 4 0 ) . ( 1 3 ) boaca g . : sopra l'ipotesi di bullen sulla variazione discontinua della densità nell'interno della terra e sulla conseguente variazione della gravità. p o n tificia a s s . s c i e n t . v , n . 2 ( 1 9 4 1 ) . ( 1 4 l boaca g . : sulla variazione della pressione nell'interno della terra con riferimento a variazione discontinua della densità. a c c a d e m i a d ' i t a l i a , s e r i e v i i , i i i , 619-625 ( 1 9 4 2 ) . (15) ramsey w . h . : on the compressibility of the earth. m . n . r . a . s . g e o p h y s . s u p p l . v i , 4 2 4 9 ( 1 9 5 0 ) . t h e r e l a t i o n s h i p b e t w e e n t h e b o d y w a v e a n d t h e l o c a l m a g n i t u d e s f o r h i m a l a y a n e a r t h q u a k e s ir m . p . chhabra (*) e . k . s . chauhan (*) h . n . srivastava ( * * ) h . m . chaudhitry. r e c e i v e d o n d e c e m b e r 4 t h , 1976 summary. — b o d y w a v e ( m b ) a n d l o c a l m a g n i t u d e (ml) w e r e d e t e r m i n e d f o r e a r t h q u a k e s o c c u r r i n g w i t h i n t h e e p i c e n t r a l r a n g e of 2 t o 10° f r o m n e w d e l h i d u r i n g t h e y e a r s 1 9 6 5 1 9 7 4 . t h e f o l l o w i n g r e l a t i o n s h i p s b e t w e e n t h e m w e r e f o u n d t o h o l d g o o d . m l = ( 0 . 9 5 ± 1.4) + ( 0 . 8 1 ± 0 . 0 6 ) m b ( a = 2» t o 5°) ml = ( 1 . 2 6 ± 0 . 7 6 ) + ( 0 . 7 6 ± 0 . 0 2 ) m b ( a = 5° t o 10°) ml = (1-00 ± 0 . 5 7 ) + ( 0 . 8 0 ± 0 . 0 1 ) m b ( a = 2° t o 10») t h e r e g i o n a l v a r i a t i o n of ( mcgs — -mb) h a s b e e n d i s c u s s e d i n t e r m s of t h e f o c a l m e c h a n i s m a n d o t h e r p a r a m e t e r s . i t h a s b e e n f o u n d t h a t (mcgs— mb) is p o s i t i v e n e a r i n d i a n e p a l a n d t i b e t b o r d e r w h i l e e l s e w h e r e i t is g e n e r a l l y n e g a t i v e . riassunto. — l e m a g n i t u d o mb e d ml s o n o s t a t e d e t e r m i n a t e p e r q u e i t e r r e m o t i a v v e n u t i d u r a n t e gli a n n i 1 9 6 5 1 9 7 4 , e n t r o u n ' a r e a e p i c e n t r a l e c o m p r e s a t r a 2° e 10° d a n u o v a d e l h i . l e r e l a z i o n i c h e l e g a n o mb a d ml s o n o r i s u l t a t e v a l i d e e s o n o le s e g u e n t i : m l = ( 0 . 9 5 ± 1.4) + ( 0 . 8 1 ± 0 . 0 6 ) m b ( a = 2" t o 5°) ml = ( 1 . 2 6 ± 0 . 7 6 ) + ( 0 . 7 6 ± 0 . 0 2 ) m b ( a = 5° t o 10°) ml = (1-00 ± 0 . 5 7 ) + ( 0 . 8 0 ± 0 . 0 1 ) m b ( a = 2" t o 10°) è s t a t a d i s c u s s a l a v a r i a z i o n e r e g i o n a l e (mcas — mb) b a s a n d o s i s u l m e c c a n i s m o f o c a l e e d a l t r i p a r a m e t r i . l a d i f f e r e n z a s u d d e t t a h a v a l o r e p o s i t i v o al c o n f i n e f r a i n d i a n e p a l e t i b e t , m e n t r e a l t r o v e r i s u l t a g e n e r a l m e n t e n e g a t i v a . (*) g e o p h y s i c s d e p a r t m e n t , i n d i a n s c h o o l of m i n e s , d h a n b a d , i n d i a . (**) i n d i a m e t e o r o l o g i c a l d e p a r t m e n t , l o d i r o a d , n e w d e l h i . 3, i n d i a . 3 8 2 m. p . ciiiiabra r . k . s. ciiauhan h. n . sri vast a va h . m. c h a u d h u r y i n t r o d u c t i o n the m a g n i t u d e scales derived f r o m body waves and surface waves h a v e been widely accepted a f t e r t h e installation of world wide seismograph system. however while using t h e m for e a r t h q u a k e s occurring within t h e epicentral distance of ten degrees, several difficulties are encountered. i n particular, surface waves of 20 ± 2 seconds period are rarely developed within this epicentral range to allow a c o m p u t a t i o n of ms (surface wave magnitude). on t h e other h a n d , t h e use of t h e body wave m a g n i t u d e , mb has been recommended for epicentral distances larger t h a n 5 degrees. the d e t e r m i n a t i o n of mb is difficult when t h e initial motion within a few cycles of t h e onset of p w a v e s is disturbed due to t h e complexities of t h e crustal s t r u c t u r e as generally observed for t h e h i m a l a y a n e a r t h q u a k e s . also, mb c a n n o t be calculated for strong earthquakes, which r e n d e r t h e record white. for earthquakes occurring within 600 k m , t h e local m a g n i t u d e , ml first derived b y e i c h t e r for southern california based on the records of s t a n d a r d wood anderson seismographs is generally used. since however the magnification of t h e torsion seismometer is q u i t e low, m a n y earthquakes occurring in the h i m a l a y a n region are n o t recorded b y t h e m . thus, t h e only a l t e r n a t i v e is t o use mb for such smaller events inspite of its limitations within 5 degrees. the object of this p a p e r is to s t u d y t h e relationship between mb and ml for t h e epicentral distance range of 2 to 10 degrees so t h a t t h e effects of using mb for a < 5 a n d ml > 5.5 m a y be b r o u g h t out. i n addition, t h e regional v a r i a t i o n of the c.g.s. m a g n i t u d e m, mcas versus mn and ml has also been a t t e m p t e d in t h e light of t h e d o m i n a n t focal mechanism in different regions. d a t a a n d a n a l y s i s for this study, t h e records of all t h e e a r t h q u a k e s with foci within t h e crust which occurred w i t h i n t h e epicentral distance of 2 t o 10 degrees f r o m new delhi were examined for t h e years 1965 t o 1971. a t o t a l of 150 earthquakes were selected out of which mb a n d ml b o t h could be determined for 52 e a r t h q u a k e s only. although t h e focal d e p t h determination were available only u p t o 1973 f r o m t h e bulletin of t h e i n t e r n a t i o n a l seismological centre, we h a v e included those earth382 m. p. ciiiiabra r. k. s. ciiauhan h . n . sri vast a va h. m. c h a u d h u r y quakes occurring during 1974 for which reliable e s t i m a t e s of d e p t h s •were available f r o m i n d i a n seismological n e t w o r k . the d a t a for t h e e a r t h q u a k e s is given in table 1. the b o d y wave magnitudes, mb were d e t e r m i n e d b y using t h e gutenberg e i c h t e r ' s relation, n a m e l y mb = log 10 + q [1] where am*x is one half of t h e t h r o u g h to p e a k t r a c e a m p l i t u d e in microns, k is t h e p e a k magnification in t h o u s a n d s a t t h e period to be read f r o m the i n s t r u m e n t a l response characteristics of w w s s n benioff system, t t h e period of t h e waves within (1 ± 0.3) records of t h e onset of p and q is t h e d e p t h distance f a c t o r . i t m a y be mentioned t h a t t h e values of q h a v e been t a b u l a t e d f r o m 2 degree onwards. the local m a g n i t u d e , ml was determined f r o m t h e t w o horizontal component torsion seismometers aligned in n o r t h s o u t h and east-west directions. since t h e magnification of these i n s t r u m e n t s has been set a t 1000 a t delhi seismological observatory, t h e average of t h e m a x i m u m double amplitudes measured in b o t h t h e components (after allowing for t h e t r a c e thickness and reducing to ground a m p l i t u d e in microns) has been used to c o m p u t e mlthe plot of mb and ml is shown i n fig. 1 where details will be discussed later. r e s u l t s a n d d i s c u s s i o n f r o m t h e definition of mb and ml one m a y n o t e t h a t t h e r e is a v e r y narrow range of epicentral distance around 5 degrees where b o t h t h e m a g n i t u d e scales are theoretically valid. a search for such earthquakes gave us meagre d a t a (fig. 2). however in order t o e x a m i n e t h e effects of using mb for a < 5 and mr, > 5.5, two s e p a r a t e plots of m„ and ml were m a d e (fig. 1). the following relations h a v e been f o u n d to hold good by least squares m e t h o d s : m l = (0.95 ± 1.4) + (0.81 ± 0.06) m b ( a = 2° t o 5°) [2] m l = (1.26 ± 0.76) + (0.76 ± 0.02) m b (a = 5° t o 10°) [3] m l = (1.0 ± 0.57) + (0.80 ± 0.01) m b ( a = 2° t o 10°) [4] the relationship between mb and ml has been deducted earlier [2] as mb = 1.7 + 0.8 ml — 0.01 j f l 2 mb verses ml x a 2* to 5* o a 5* to i o*. depth < 60 kms 4 4 4-8 5-2 5-6 6 0 6-4 6 8 7 0 7-6 mb f i g . 1 t h e r e l a t i o n s h i p b e t w e e n mb a n d ml f o r e a r t h q u a k e s i n h i m a l a y a s a n d n e i g h b o u r h o o d . 6 8 6 6 6-4 6 2 60 5-8 5 6 5-4 5-2 5 0 4 8 4.6 4-4 4-2 m b v e r s e s m l a = 4 8 to 5-5 d e p t h < 6 0 km 4 0 4*4 4-8 5 2 5 6 6 0 6 4 fig2 mb f i g . 2 t h e v a r i a t i o n of jlib a n d ml w i t h i n t h e e p i c e n t r a l d i s t a n c e of 4 . 8 t o 5 . 5 ° . 382 m. p . c i i i i a b r a r . k . s . c i i a u h a n h . n . s r i v a s t a v a h . m . c h a u d h u r y comparison of e q u a t i o n [4] with [5] shows t h a t mb t e n d s t o be equal t o ml a t m a g n i t u d e 5 i n t h e f o r m e r case while this v a l u e is a r o u n d m a g n i t u d e 7 in e q u a t i o n [5]. one of t h e possible causes m a y be t h e use of r e l a t i v e higher m a g n i t u d e e a r t h q u a k e s in deriving e q u a t i o n [5j. the large v a r i a t i o n of mb and ml on t h e basis of these relationships emphasizes t h a t no single relationship m a y be valid in t h e entire magnit u d e ranges. more d a t a is t h u s needed to establish mb versus ml relationship for m i c r o e a r t h q u a k e s i n t h e h i m a l a y a n region. although several wood anderson seismometers h a v e been installed recently along t h e h i m a l a y a n foot lulls, comparison of mb and ml has n o t been practicable d u e to t h e d i s t a n t location of t h e new delhi observ a t o r y using s t a n d a r d i s e d seismograph s y s t e m . f o r such microearthquakes, a n e w m a g n i t u d e scale using t h e t o t a l signal d u r a t i o n is being developed whose results will be reported separately. our results show t h a t t h e operational r e q u i r e m e n t s for r e p o r t i n g t h e m a g n i t u d e s of t h e e a r t h q u a k e s on t h e basis of new delhi observ a t o r y can be a d e q u a t e l y n e t by e x t e n d i n g t h e local m a g n i t u d e scale, ml u p t o t h e epicentral distance of 10 degrees. the c.g.s. m a g n i t u d e is t h e average of t h e b o d y wave m a g n i t u d e s reported b y t h e s t a n d a r d i z e d stations all over t h e world a n d t h u s t a k e s i n t o consideration t h e effects of t h e source mechanism. the difference of mcgs — mb t h u s brings out t h e regional effects (fig. 3). although t h e influence of t h e focal m e c h a n i s m on ml is less understood m a i n l y because of its restricted use, ( m c g s — ml) h a s also been p l o t t e d for t h e scale of comparison (fig. 4). fig. 5 shows t h e focal m e c h a n i s m solution of some e a r t h q u a k e s in t h e same region, which h a v e been t a k e n f r o m t h e results of c h a u d h u r y et al., 1974 (2), nowroozi (4), 1972, t a n d o n and srivastava, 1975 (7). since t h e m a g n i t u d e s depend u p o n t h e focal depths, i t was desirable t o s e p a r a t e t h e e a r t h q u a k e s occurring in t h e g r a n i t i c and basaltic layers. t a n d o n and d u b e , 1973 (6), using t h e body waves h a v e f o u n d t h e thickness of these two layers as 38 km and 13 respectively, for t h e h i m a l a y a n region. however some e a r t h q u a k e s h a v e occurred close t o t h e h i m a l a y a n foot hills where t h e c r u s t a l s t r u c t u r e and t h e g r a n i t i c layers are relatively less. thus t h e e a r t h q u a k e s were grouped i n t o two focal d e p t h ranges n a m e l y 0 to 30 ion and > 30 ion b u t restricted to t h e c r u s t . i t m a y be seen f r o m lig. 3 t h a t n e a r t h e h i m a l a y a n tibet a n d n e p a l border, mcgs — mb is positive for b o t h t h e focal d e p t h s and elsewhere it is generally n e g a t i v e . a l t h o u g h t h e n a t u r e of t h e f a u l t i n g (fig. 5) t a b l e 2 e a r t h q u a k e m a g n i t u d e s ¿\ = 5 ° t o 10°: d e p t h u p t o 60 k m . s . n o d a t e o r i g i n t i m e h m s l a t i t u d e °n l o n g i t u d e °e d e p t h in k m mag. cgs a i n d e g r e e s mb ( d e l h i ) ml ( d e l h i ) 1 12 1.1965 13 32 24 27 6 88 23 6 1 9 . 4 6 . 6 6 . 2 2 29 1. » 20.06 02 4 35 6 73 6 41 5 7 1 0 . 0 5 . 7 5 . 6 3 17 6. » 20 14 48 6 32 87 8 15 5 4 9 . 7 5 . 7 5 . 7 4 11 1.1966 09 12 59 3 34 72 36 5 4 9 . 1 5 . 7 5 . 6 5 24 1. » 07 23 07 6 29 9 69 7 26 5 8 6 . 7 6 . 2 6 . 5 6 24 i . » 15 32 48 1 29 9 69 8 14 5 3 6 . 4 5 3 5 . 4 7 02 2 . » 09 20 07 5 33 9 73 37 5 3 8 . 3 5 . 7 5 . 7 8 09 2. » 08 22 17 9 29 8 69 8 16 5 2 8 . 7 5 . 8 5 . 9 9 13 2 . » 19 09 47 4 29 8 69 7 9 5 1 8 . 7 5 . 8 6 . 3 10 17 2. » 18 26 17 7 29 9 69 8 40 4 4 8 . 7 6 . 5 5 . 6 11 04 3. » 06 01 05 30 0 70 0 33 4 4 8 . 4 5 . 8 6 . 0 12 06 4 . » 01 51 51 8 35 73 54 5 1 9 . 3 6 . 4 6. 1 13 01 8. » 19 09 55 1 29 9 68 8 33 5 8 7 . 3 6 . 3 5 . 9 14 02 8 . » 05 41 37 4 30 68 8 32 5 2 7 . 5 5 . 9 5 . 5 15 02 2.1967 07 37 54 9 39 7 75 5 39 5 3 1 0 . 7 6 . 0 5 . 0 16 30 5. » 18 56 28 7 31 7 30 1 44 4 6 6 . 4 5 . 0 4 . 9 17 11 2.1968 20 38 29 4 34 2 78 6 44 5 1 5 . 5 5 . 1 4 . 8 18 03 3. » 09 31 20 2 34 7 72 3 43 5 2 7 . 1 5 . 4 5 . 3 19 0 4 . 4 . » 01 44 26 4 24 6 66 33 5 0 1 0 . 3 5 . 6 5 . 7 20 26 7. » 20 48 07 2 32 07 70 07 50 4 8 7 . 2 5 . 0 4 . 9 21 26 6. » 00 46 13 8 37 7 69 9 16 5 2 10.0 6. 1 0 . 0 22 18 11. » 05 05 04 3 33 1 71 1 41 5 3 6 . 8 5 . 6 5 . 1 23 22 1.1969 19 42 21 8 32 2 70 23 4 7 7.1 5 . 8 5 . 2 24 03 3. » 14 03 00 5 31 71 8 35 4 5 5 . 5 4 . 6 5 . 3 25 10 4.1970 10 23 58 2 25 3 66 7 33 5 1 9 . 7 6 . 3 5 . 5 26 12 5. » 22 07 39 4 27 5 67 5 7 4 7 8 . 5 4 . 7 5 . 2 27 14 8. » 00 36 34 5 31 2 74 3 44 5 2 5 . 2 5 . 7 5 . 4 28 08 1.1971 23 52 16 3 29 1 69 1 19 5 2 7.1 5 . 7 5 . 8 29 24 3. » 13 39 46 3 30 3 67 8 18 4 9 8 . 0 5 . 4 5 . 0 30 21 12. » 09 54 40 35 7 73 4 15 5 7 1 0 . 0 6 . 2 6 1 31 06 11.1972 10 56 14 26 8 88 4 59 4 4 9 . 8 5 . 6 5. 7 32 20 1.1973 12 34 20 29 3 68 6 19 5 3 7 . 1 5 . 9 5 . 6 33 24 2 . » 21 57 04 26 7 66 3 48 4 2 9 . 2 6 . 0 5 . 2 34 09 2.1974 04 07 37 7 28 7 69 4 33 4 9 8 . 5 5 . 4 4 . 8 35 07 4 . » 16 07 43 8 32 0 69 7 44 4 8 9 . 6 4 . 8 5 . 3 36 03 8. » 04 08 13 9 35 4 71 7 20 5 0 10.3 6 . 0 5 . 4 f i g . 4 r e g i o n a l v a r i a t i o n of mcgs — ml o v e r h i m a l a y a a n d n e i g h b o u r h o o d . t h e r e l a t i o n s h i p b e t w e e n t h e b o d y w a v e e t c . 3 8 9 f i g . 5 f o c a l m e c h a n i s m s o l u t i o n s of s o m e e a r t h q u a k e s in h i m a l a y a a n d n e i g h b o u r h o o d (see r e f e r e n c e ) . is generally of t h r u s t t y p e with different a m o u n t s of strike slip compon e n t in different regions, t h e o r i e n t a t i o n of t h e nodal planes w i t h reference t o t h e new delhi seismological observatory could t h u s be responsible for this t y p e of regional variations. the regional var i a t i o n of ( m c g s — ml) is comparatively less m a r k e d (fig. 4) although this also gives a positive residual n e a r t h e h i m a l a y a n t r i j u n c t i o n . p a d m a n a b h a m u r t l i y , 1969 (5), has f o u n d negative residual f r o m ( m c g s —• ml) a t new delhi f r o m shallow e a r t h q u a k e s , b u t due to lack of d a t a , t h e regional v a r i a t i o n was n o t be a t t e m p t e d within 10 degrees. ignoring t h e regional v a r i a t i o n of (mcgs — mb) or ml t h e following relations hold good (figs. 6 a, b). (mcgs — mb) = ( — 2 . 2 + 0 . 3 ) + ( 0 . 3 8 + 0 . 0 2 ) mcgs [ 6 ] (mcgs — ml) = ( — 2 . 3 + 0 . 3 ) + ( 0 . 4 5 + 0 . 0 2 ) mcgs [ 7 ] comparison of [6] with [7] shows t h a t b o t h t h e scales ml or mb can be used with almost same accuracy u p t o 10 degrees f r o m new delhi observatory. p i g . oa t h e r e l a t i o n s h i p b e t w e e n ( m c g s — mb) a n d macs f i g . ob t h e r e l a t i o n s h i p b e t w e e n (mcgs — i l / t ) a n d 31, t h e r e l a t i o n s h i p b e t w e e n t h e b o d y w a v e e t c . c o n c l u s i o n the above s t u d y brings out t h e following results: 1) the following relationships between mb and ml hold good. m l = (0.95 ± 1.4) + (0.81 ± 0.06) m b ( a = 2° to 5") m l = (1.26 ± 0.76) + (0.76 ± 0.02) m b ( a = 5» to 10") m l = (1.0 ± 0.57) + (0.80 ± 0.01) m b ( a = 2° to 10") the local m a g n i t u d e scale, ml can be used u p t o 10 degrees for t h e crustal e a r t h q u a k e s occurring i n t h e h i m a l a y a n kritliar sulaiman ranges. 2) some regional variation of ( m c g s — m b ) has been n o t e d . ( m b — mcgs) is positive for t h e h i m a l a y a n nepal tibet tri-junction and general negative elsewhere. r e f e r e n c e s (' ) batii, m., 1973. introduction to seismology, p . 112. ( 2 ) c h a u d h u k y , h . m . , s r i v a s t a v a , h . n . a n d s u b i i a r a o , j . v . , 1 9 7 4 . seismotectonics investigation of the himalaya. " h i m a l a y a n g e o l o g y " , 4 , p p . 4 8 1 4 9 1 . ( 3 ) g u t e n b e r g , b . a n d r i c h t e r , g . f . , 1 9 5 6 . magnitude and energy oj earthquakes, " a n n a l i g e o f i s . " , 9 . ( 4 ) n o w r o o j i , a . n . , 1 9 7 2 . focal mechanism oj earthquakes in persia, turkey, west pakistan and afghanistan and plate tectonics oj the middle east. " b u l l s e i s m . soc. a m . " , 6 2 , p p . 8 2 3 8 5 0 . ( 6 ) p a d m a n a b h a m u r t i i y , b . , i 9 g 9 . station magnitude connections for shillong, poona and new delhi. " i n d . j r . m e t . g e o p h y s i c s " , 2 0 , p . 275. ( 6 ) tandon, a . n . a n d dube, r . k . , 1 9 7 3 . i n : " p u r e a n d a p p l i e d g e o p h y s i c s " , 1 0 5 . (7) tandon, a. n . a n d srivastava, h . n . , 1975. focal mechanism oj some recent himalayan earthquakes and regional plate tectonics. " b u l l . s e i s m . soc. a m . " , 65, p p . 9 6 3 9 7 0 . layout 6 annals of geophysics, 53, 2, april 2010; doi: 10.4401/ag-4549 abstract since 2004, a continuous global positioning system (gps) network has been operated by the istituto nazionale di geofisica e vulcanologia (ingv) to investigate active tectonic processes in italy and the surrounding regions, which are still largely debated. this important infrastructure is known as rete integrata nazionale gps (ring) network, and it consists of about 130 stations that are deployed all over italy. the development and realization of a stable gps monumentation, its integration with seismological instruments, and the choice of both satellite and internet data transmission, make this network one of the most innovative and reliable cgps networks in the world. the technologically advanced development of the ring network has been accompanied by the development of different data processing strategies, which are mainly dependent on the use of different gps analysis software. the different software-related solutions are here compared at different scales for this large network, and the consistency is evaluated and quantified within an rms value of 0.3 mm/yr. introduction the plate boundary between nubia and eurasia is characterized by complex kinematics, mainly due to geometrical and rheological features that remain to be described in detail [westaway 1990, serpelloni et al. 2005]. several seismotectonics syntheses have been proposed over the last two decades, both at local and regional scale [anderson and jackson 1987, jackson and mckenzie 1988, ekstrom and england 1989, westaway 1990, serpelloni et al. 2005, d'agostino et al. 2008]. the plate boundary shows a transition from simple deformation at the oceanic plate boundaries of the atlantic, which is characterized by narrow seismic belts, to a broad zone of seismicity and deformation that characterizes the apennines and alpine belts, which results in a complex pattern of crustal stress and strain. the distribution of the instrumental seismicity (figure 1) indicates the high crustal fragmentation of the area and outlines a firstorder picture of the plate boundary mosaic. here, several quasi-aseismic domains, such as the adriatic sea and the tyrrhenian sea, are embedded in domains with higher seismicity where most of the deformation probably occurs. the gps is a fundamental tool for studying the kinematics of continental deformation at diffuse plate boundaries, which allows inter-seismic velocities and velocity gradients at different scales to be measured. the evaluation of the instantaneous velocities of most of the tectonic plates has become increasingly well constrained due to improvements in the global tracking network and the gps infrastructure (i.e. the increasing number of continuously operating stations worldwide, which allows better resolution of the global reference frame). various estimates of gpsderived africa-eurasia euler vectors have been proposed from the analysis of space geodetic data [sella et al. 2002, calais et al. 2003, kreemer et al. 2003, mcclusky et al. 2003, d'agostino et al. 2008]. the short-time span and the lack of density prevent those works to adress the question of the deformation inside the plate boundary itself, or allow just the identification and discussion of the behaviour of relatively large blocks. an example is the definition of an adriatic plate and its southern boundary [calais et al. 2002, oldow et al. 2002, battaglia et al. 2004] and the present-day activity of the calabrian slab [goes et al. 2004, d'agostino and selvaggi 2004, chiarabba et al. 2005, serpelloni et al. 2005]. some recent papers [hollenstein et al. 2003, d'agostino and selvaggi 2004, serpelloni et al. 2005] indicate that strain rates in italy are higher than expected [anzidei et article history received january 18, 2010; accepted april 15, 2010. subject classification: geodesy, seismotectonics, cgps network, gps data analysis, central mediterranean 39 the ring network: improvements to a gps velocity field in the central mediterranean antonio avallone1,*, giulio selvaggi1, elisabetta d'anastasio1, nicola d'agostino1, grazia pietrantonio1, federica riguzzi1, enrico serpelloni1, marco anzidei1, giuseppe casula2, gianpaolo cecere1, ciriaco d'ambrosio1, prospero de martino3, roberto devoti1, luigi falco1, mario mattia4, massimo rossi4, francesco obrizzo3, umberto tammaro3, luigi zarrilli1 1istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy 2istituto nazionale di geofisica e vulcanologia, sezione di bologna, italy 3istituto nazionale di geofisica e vulcanologia, sezione di napoli, italy 4istituto nazionale di geofisica e vulcanologia, sezione di catania, italy al. 2001], with values in the range 0.5÷2*10−7 yr−1 through sicily and the apennines [d'agostino and selvaggi 2004]. recently, d'agostino et al. [2008] and devoti et al. [2008] have proposed more refined models of the active tectonics in the central mediterranean, showing the importance of a dense cgps network, to properly measure strain rates within the plate boundary. the contribution of geodesy to seismological studies is also becoming more important, from the observation of the inter-seismic strain accumulation across active faults, to the constraints of the history of earthquake rupture process. the mentioned scientific topics relating to active tectonics, along with the africa-eurasia plate boundary zone, were the main goals for planning the dense, continuously operating, real-time integrated national gps (ring) network in italy, following the examples of japan (miyazaki et al., 1994) and the usa [zhang et al.1997; or http://pboweb.unavco.org]. after a first description of the aspects related to the implementation of the ring network, the aim of this study is to describe the different software-related gps data processing strategies, and to evaluate the consistency among the different solutions, thus, finally, outlining the results carried out with the present-day gps velocity field along with the africa-eurasia plate boundary zone. the ring network for active tectonics studies 40 figure 1. seismicity distribution in the central mediterranean. rcmt focal mechanisms from pondrelli et al. [2004] are shown together with the 19732006 seismicity from neic (http://neic.usgs.gov). the color scale indicates the distribution of both focal mechanisms and seismicity with depth. 41 the ring network implementation the ring network (http://ring.gm.ingv.it) has been developed in italy since 2005 [selvaggi et al. 2006], and it integrates various experties acquired by the ingv since the early 90s in the management of gps networks and data [anzidei et al. 2008]. this integration is not limited to geodesy and most of the ring stations are co-located with a seismic station including broad-band and very broad-band (40sec÷240sec) seismometers and strong motion accelerometers. the co-location of those different instruments represents a unique scientific opportunity for recording and evaluating the whole range of frequencies of the plate boundary deformation and earthquake cycle at the same site. this will allow a large range of scientific problems to be tackled, from earthquake source studies to regional plate kinematics. moreover, the co-location of different instruments allows the use of the same transmission vector for different data types in real time, with the consequence that there will be strong cost reductions in the management of the network. the ring is thus a multi-sensor network, that transmits in real time, consisting of about 130 stations (figure 2a). these stations are not homogeneously distributed all over italy and their present distribution mainly reflects the results of some important projects that have been carried out (i.e. the cesis project for southern italy, http://www.gm.ingv.it). in addition to the conventional monumentation types used in permanent gps sites devoted to geophysical applications (i.e. concrete pillars on bedrock, vertical steel rod on a building), a significant effort has also been put into the development of a scign-like (southern california integrated gps network, http://csrc.ucsd.edu/howto/scignmonumentinfo.html) short or deep-drilled braced tripod, which is fastened directly to the ground and has the antenna phase center very close to the most stable point of the monumentation, i.e. at the junction of the braces (figure 2b) [d'ambrosio 2007]. the long-term and shortterm stabilities of such a monumentation type is particularly important because of the possibility of detecting low-amplitude deformation rates, as inter-seismic or post-seismic signals. the majority of the gps instruments are leica (sr520, grx1200pro and grx1200ggpro), with chock-ring antennae (at504 or at504gg). the scign or scign-like the ring network for active tectonics studies figure 2. details of the ring network and the ring infrastructure implementation: a) map showing the locations of all the ring network sites; b) example of short-drilled steel rod tripods adopted for use in most of the ring sites; c) details of the antenna-scign mount-scin radome coupling; d) example of a remote ring site (siri) with satellite data transmission and seismic instruments co-location. mounts are important accessories of the ring stations, and they allow a very good coupling between the antenna and the monumentation, and with the scign or leica radomes [figure 2c]. most of the ring sites transmit data by satellite telemetry (nanometrics vsat technology, http://www.nanometrics.ca) [figure 2d]. this transmission system has the following advantages: 1) it assures optimal continuity of the observation acquisitions; 2) it allows installation of stations in remote locations that transmit both seismic and geodetic data in real time; 3) it is reliable, and provides re-transmission of lost packets; 4) it requires low power supply; and finally, 5) it allows an important reduction in management costs. the data are transmitted to three different acquisition centers (located in rome, grottaminarda and catania) to assure redundancy in the satellite data acquisition of all of the ring sites. moreover, further developments in data transmission have been carried out using both mobile phone (gprs/umts) [falco 2008] and wi-fi technology. these last developments are designed to ensure alternative and reliable real-time data transmission for most of the ring sites via the internet. in conclusion, both the satellite telemetry and the internet connections currently allow for real-time transmission from about 90% of the ring sites. the real-time monitoring of the remote ring sites and the relative data acquisition are performed at the grottaminarda ingv acquisition center, where all of the raw data are stored and archived. a quality check of the acquired data (e.g. marker name, cycle slips, gaps, first and last epoch, acquisition percentage) is also performed using a home-made program, known as clinic, based on teqc software [http://facility.unavco.org/software/teqc/teqc.html; estey and meertens 1999]. the resulting quality check summary is continuously stored in a database. to attest the good quality of the whole ring network infrastructure, we define a parameter for the «completeness degree» (named compl) of the acquired data (figure 3). this parameter is an average value calculated for each day, and it is obtained using the following formula: where aobs and eobs correspond to the numbers of acquired daily observations and of expected daily observations, respectively, as they output from the teqc quality check, hr represents the acquired data time-span for each rinex file, and n is the total number of rinex files available each day. the evolution of the compl parameter with time shows a first period that lasted to approximately 2005 (i.e. before the recent developments of the ring network), which was characterized by moderate compl values, with mean and rms values of about 89% and 8, respectively. since 2004, the ring network has guaranteed a better and optimal level of data completeness (mean = 95%; rms = 1.5). this optimal result is mainly related to the improvements obtained using new instrumentation, and the more reliable and technologically advanced data transmission types, with respect to the conventional phone connections. a collaborative environment has been developed to create an advanced technological frame that is devoted to complete managing and sharing of ring rinex data and of the relative information content [cecere 2007]. this uses the recent «knowledge management» techniques [kim et al. 2002] with the database subdivided into three levels: 1) the data, containing the raw and rinex data in a file system structure, and all of the metadata with the information of each site (i.e. logfiles, monographs, sites photos, rinex quality check summaries); 2) the data managing software the ring network for active tectonics studies 42 figure 3. the compl parameter evolution over time (see text) for the ring network infrastructure. 24compl a e hr n1 obs obsi n= )= ^ ^h h6 @/ 43 (i.e. clinic), to check the data and to provide alerts about critical discrepancies or data gaps; 3) the web services (i.e. the ring web site, http://ring.gm.ingv.it, the ring webgis, http://labgis.gm.ingv.it/, and the ring bancadati restricted area, http://bancadati.gm.ingv.it), which allow any user to view and download dynamically created products, such as thematic maps, station log-files, monographs, quality check graphs, and coordinate time-series plots. these web services have been planned to promote data, information and knowledge exchange (know-how), starting from the point of view that the best resource for an institute is represented by the knowledge owned and shared by the people who work in it. gps data processing at the ingv, the gps data analysis is carried out with the three main geodetic-quality softwares used in the gps scientific community: bernese (http://www.bernese.unibe.ch/), which was developed at the astronomic institute of the university of berne (aiub); gamit (http://wwwgpsg.mit.edu/~simon/gtgk/), which was developed at massachusetts institute of technology (mit); and gipsy/oasis ii (http://gipsy.jpl.nasa.gov/orms/goa), which was developed at the jet propulsion laboratory (jpl). the ring network for active tectonics studies figure 4. maps showing the location of all of the cgps sites used by: a) the bernese solution; b) the gamit solution; and c) the gipsy solution. d) map showing the locations of the cgps sites defining the eurasian reference frame in the three solutions: bernese (blue diamonds), gamit (green diamonds) and gipsy (red diamonds). the location of the three eulerian poles of the differently defined eurasia reference frames are also shown with their associated 95% confidence ellipses (details listed in table 1). in this section, we describe the different strategies adopted to obtain the velocity field solutions with the three softwares. the main aspects of these strategies are summarized in table 1, to highlight the choice of similar or different models and/or parameters. bernese data-processing strategy the gps phase data are processed by subdividing the whole network into 12 regional clusters of about 40 stations each, which contain at least 11 common anchor sites, i.e. selected sites used as core sites for the cluster combination, based on station performance and geographical distribution [figure 4a]. the gps observations considered in this analysis span a time interval of 12 years (1998.0-2009.0). the data processing is performed by the bernese processing engine of the software bernese 5.0 [beutler et al. 2007], which forms double difference observables. the gps orbits and the earth orientation parameters are fixed to the combined international gps service (igs) products, and an a-priori error of 10 m is assigned to all of the site coordinates. the pre-processing phase that is used to clean up the raw observations, is carried out in a baseline by baseline mode. independent baselines are defined by the criterion of maximum common observations. the elimination of gross errors and cycle slips, and the determination of new ambiguities are computed automatically using the tripledifference combination. the a-posteriori normalized residuals of the observations are also checked for outliers. these observations are marked for the final parameter adjustment. the elevation-dependent phase center corrections are applied, with the inclusion in the processing of the igs phase center calibrations (relative calibrations). the troposphere modelling consists of an a-priori dry-niell model that is fulfilled by the estimation of zenith delay corrections at 1-h intervals at each site, using the wet-niell mapping function [niell 1996]. the ionosphere is removed by applying the ionosphere-free linear combination of l1 and l2. the ambiguity resolution is based on the quasiionosphere-free baseline-wise analysis [beutler et al. 2007]. the final network solution is solved with back-substituted ambiguities, if they are integer; otherwise, ambiguities are considered as real-valued measurement biases. each daily solution was estimated within a loosely constrained reference frame that is close to the rank deficiency condition, which obtains the so-called loosely constrained solution. each solution is realized in an intrinsic reference frame that is defined by the observations themselves, and which differ from day to day only for rigid network translations, keeping the site inter-distances always well determined. in this way, the constraints to realize the chosen reference frame are imposed only a posteriori at the final stage of the analysis. the daily loosely constrained cluster solutions are then merged into global daily loosely constrained solutions of the whole network, applying a classical least-squares approach [bianco et al. 2003]. the velocity field is estimated directly from the loosely constrained time series of the daily coordinates, obtaining a loosely constrained velocity solution. the velocities are estimated simultaneously, together with annual signals and sporadic offsets at epochs of instrumental changes. subsequently, the loosely constrained velocity field has been transformed into the itrf2005 reference system [altamimi et al. 2007] by applying an eight-parameter helmert transformation (scale, translation, and their derivatives) and the inner constraints to the final solution. the rigid plate motion is estimated directly from the official itrf2005 velocity solution, and it is statistically inferred using simple |2 test-statistics iteratively applied to select the coherent subset of sites that define a stable plate [devoti et al. 2008]. starting from three central european sites (the pilot-triad: wsrt, wtzr and zimm), a total of 24 sites were assigned to a stable plate, with . the absolute itrf2005 eurasia pole and rotation rate are (55.85˚ n, 95.72˚ w) and (0.266 ± 0.003)˚/myr, which are consistent with previous the ring network for active tectonics studies 44 soft. organization orbits & eop elevation cutoff phase centre calibrations troposphere models ztd est. rate time series analysis reference frame transformations bernese clustered fixed 15º relative dry niell + wet niell 1hr velocity+ annual signals + offsets 8 parameters (scale, translations & their derivatives) gamit clustered estimated 10º absolute dry gmf+ wet gmf+ horiz. gradients 1hr velocity + annual and semiannual signals + offsets 7 parameters (scale, rotations & translations) gipsy unique cluster (ambizap) fixed 15º relative dry niell + wet niell+ horiz. gradients 5min vel+ annual and semiannual signals + offsets 7 parameters (scale, rotations & translations) ! table 1. comparison of the main aspects concerning the different gps data-processing strategies. 1.462 =|o 45 the ring network for active tectonics studies figure 5. comparisons among the gps velocity fields carried out by the three different solutions for: a) northern; b) central; and c) southern italian regions. the three solutions include only the common sites (217) with an observation interval longer than 2.5 years. itrf2000 and recent itrf2005 poles [fernandes et al. 2003, kreemer et al. 2003, d'agostino and selvaggi 2004, serpelloni et al. 2005, altamimi et al. 2007]. the mean postfit residuals are 0.25mm/yr and 0.39mm/yr for the east and north components respectively. a few central european sites, including pots, bor1, lama and joze, are rejected, both at the 95% and 99% significance levels. their uncertainties are probably underestimated in the itrf2005, and may need to be reassessed in a future itrf realization. all of the siberian sites are rejected from being rigidly connected to central europe, east of artu (58˚ e), and they have a 2-3 mm/yr w-ward residual velocity. gamit data-processing strategy code and phase data from more than 780 continuous gps stations that operate in the euro-mediterranean and african regions (figure 4b) were analyzed with the gamit software (version 10.33), for the time interval 1998.0-2009.5, and following standard procedures for regional networks [e.g., dong et al. 1998, mcclucsky et al. 2000, serpelloni et al. 2006]. we divided the whole network into twenty subnetworks, using fifteen stations in common to each sub-network. the sub-networking was performed following the original cgps network configuration, to allow for the production of single daily loosely constrained solutions (including sinex files) for each individual network, and to account for differences in the network data availability over time. moreover, considering that in a later step our solutions are combined with global and regional loosely constrained solutions provided by the scripps orbit and permanent arrays center (sopac; http://sopac.ucsd.edu), the original rinex data of stations already included in the sopac solutions are not re-analyzed if they are not among the fifteen common stations. the twenty sub-networks were analyzed on a 16-node linux cluster [serpelloni et al. 2009]. our data analysis scheme to go from raw data to ground velocities, is divided into three main steps: 1) the code and phase data analysis (using gamit); 2) the combination of solutions; and 3) the position time-series analysis, including noise characterization. the post-processing steps, points 2) and 3), are performed with the qoca software [dong et al. 1998, dong et al. 2002; available at http://gipsy.jpl.nasa.gov/qoca] and the cats software [williams 2008]. gamit uses double-differenced, ionosphere-free, linear combinations of the l1 and l2 phase observations, to generate weighted least-squares solutions for each daily session [king et al. 1985, bock et al. 1986, schaffrin and bock 1988, dong and bock 1989]. an automatic cleaning algorithm [herring et al. 2006] is applied to post-fit residuals, to repair cycle slips and to remove outliers. the effects of solid-earth tides, polar motion and oceanic loading are taken into account according to the iers/igs standard 2003 model [mccarthy and petit 2004]. the estimated parameters for each daily solution include the three-dimensional cartesian coordinates for each site, the six orbital elements for each satellite (semi-major axis, eccentricity, inclination, longitude of ascending node, argument of perigee, and mean anomaly), earth orientation parameters (pole position and rate, and ut1 rate), and integer phase ambiguities. the atmospheric propagation delay is modeled by means of the global mapping function (gmf) models of boehm et al. [2006], which introduce longitude, latitude and time-of-year dependence of the older niell mapping functions (nmfs) [niell 1996], and which results in the highest accuracies in the vertical studies [herring et al. 2006]. since the wet contribution to atmospheric delay is poorly modelled using surface meteorological data, hourly piece-wise linear atmospheric zenith delays are also estimated at each station to correct the poorly modeled troposphere, along with three east-west and north-south atmospheric gradients per day, to allow for azimuth asymmetry; the associated error covariance matrix is also computed and saved. the elevation cutoff is set to 10˚ and we use the igs absolute elevation dependent tables for modelling the effective phase centre of the receiver and satellite antennae. a full re-analysis of the whole data-set with the igs05 orbits and the absolute phase center model has been completed from 2000 to the present. in the second step, our loosely constrained solutions (gamit ascii «h-files») are combined with the sopac global and regional loosely constrained solutions using the st_filter software [dong et al. 2002] developed at the jpl. the st_filter is run under the portable batch system the ring network for active tectonics studies 46 id lat lon w smax smin az sw e n be 54.847 -95.724 0.266 0.1 0.0 46.3 0.002 0.25 0.39 ga 54.230 -99.430 0.253 0.1 0.1 42.0 0.001 0.40 0.29 gy 54.875 -98.603 0.257 0.4 0.1 45.3 0.001 0.40 0.36 be = bernese; ga = gamit; gy = gipsy; pole coordinates (lon., lat. in degrees); rotation rate (w, in º/myr) and associated 1-sigma uncertainty (sw); semiaxes of the error ellipses (smax, smin); azimuth of the smax (az). also shown are the rms residuals (in mm) of the horizontal components (e, n). table 2. parameters of the eulerian poles for the eurasia reference frame obtained by the bernese, gamit and gipsy solutions. 47 [serpelloni et al. 2009], thus making daily combinations very fast (about 2-3 min per day). in particular, we combine our solutions with all of the six igs sub-networks, and the euref (eura) and eastern mediterranean (emed) h-files available from sopac. this combination brings to the total to about 800 stations that are available for the final combined solution. the st_filter uses common stations and orbital parameters that are recorded in the gamit h-files, to align each solution to a reference precise global orbit, available from sopac, and to the itrf (or igs), adopting the internal-constraint approach [dong et al. 1998, dong et al. 2002]. a seven-parameter transformation (three network rotations, three network translations, and one scaling parameter) is performed, which aligns each daily solution to the igs05 frame and to the gps realization of the itrf2005 reference frame [altamimi et al. 2007]. all of the igs core sites are initially used to compute the transformation parameters, and a maximum number of five iterations are performed to find the best set of igs sites that better define the global reference-frame. in this step, offsets of the igscore sites due to earthquakes and/or station configuration changes are corrected using values obtained in a preliminary run, using a-priori values from sopac. in the third step, position time series are analyzed to simultaneously estimate the secular term (i.e., the linear velocity), the offsets in the time series, and the seasonal terms (i.e., annual and semiannual seasonal terms). residual time series (where bias, velocity, jumps and seasonal terms are removed) are later analyzed with the cats software [williams 2008], to estimate the noise characteristics of the time series, assuming a white+flicker noise error model. velocity uncertainties are then rescaled based on the analysis of the noise at individual stations. gipsy/oasis data-processing strategy code and phase data from about 730 cgps sites for the time interval 1996.0-2009.7 have been processed with the gipsy-oasis ii software from the nasa jpl [lichten and borders 1987]. these sites are located either in italian regions (about 350 of them) or in the surrounding areas (about 380 of them) (figure 4c). point positioning [zumberge et al. 1997] and precise orbits and clocks from the jpl (http://sideshow.jpl.nasa.gov) were used to analyze the gps data, followed by integer the ring network for active tectonics studies figure 6. left: evolution of the velocity differences obtained between each solution and the avf (dv, see text), with the time-span of each common site. right: dv frequency distributions for the different solutions. ambiguity resolution. gps ambiguity resolution of large amounts of data presents a serious challenge in terms of processing time. this step was performed using the ambizap strategy [blewitt 2008], to obtain unique, self-consistent daily ambiguity fixed solutions for the entire network. the ambizap processing algorithm uses a fixed point theorem to identify linear combinations of network parameters that are theoretically invariant under ambiguity resolution [blewitt 2008]. this strategy allows for very rapid and multiple reanalysis of large networks to assess various models, makes trivial the addition of extra stations of sub-networks to an existing solution, and produces unique, self-consistent daily ambiguity fixed solutions for the entire network. each ambiguity fixed daily solution is aligned to the the ring network for active tectonics studies 48 figure 7. comparisons for some of the ring sites of the residuals time series for the different softwares (left) and the relative rms distributions (right) for which the number of samples is also given: a) grot, grottaminarda; b) malt, malta; c) maon, monte argentario; d) teol, teolo; e) usix, ustica. a) c) d) b) 49 itrf2005 reference frame [altamimi et al. 2007] using a seven-parameter transformation. the realization of itrf2005 is obtained using daily precise-point positioning solutions for about 92 stations in eurasia, nubia and the central mediterranean, which are characterized by the longest observation intervals and minimum hardware changes, along with parameters from the jpl (x-files) that allow daily coordinate transformation into itrf2005. a nonet rotation constraint was applied using a subset of 32 sites within the stable eurasian continent that were deemed to be sufficiently far from tectonic and horizontal glacial isostatic adjustment effects [nocquet et al. 2005]. after application of the no-net-rotation constraint, the rms of the horizontal residual velocities of these 32 sites were 0.40 and 0.36 mm/yr for the east and north components, respectively. the rotational constraint was then applied to the 92-station frame, and transformation files (x-files) were then generated to transform daily non-fiducial solutions into a stable eurasian reference frame. in this case, the temporal variation of the positions thus represents crustal motion relative to the stable eurasian plate in the north (latitude), east (longitude) and vertical components. the time series for all of the stations in the eurasian reference frame (or in itrf2005, and then locally rotated to neu components) are cleaned from outliers using the strategy described in nikolaidis [2002]. they are then analyzed for their noise properties, linear velocities, periodic signals and antenna jumps, using maximum likelihood estimation (mle). this technique allows simultaneous estimations of the noise properties structure together with the parameters of a time-dependent model of the data. the quantities estimated in the mle analysis are linear trend, offsets at designated times, annual and semi-annual periodic signals and spectral law noise index and amplitude [williams 2003]. the mle analysis is performed using the cats software [williams 2008], and all of the parameters are estimated with white+flicker error models. their uncertainties can thus be considered to be realistic estimates based on the analysis of the noise at individual stations. comparisons of the gps solutions in this study, we performed comparisons of the differently obtained gps solutions on two different scales: 1) a regional scale comparison, which considers the discrepancies between each gps velocity field; and 2) a more local scale comparison, which considers the differences in the residual time series for some particular cgps sites. in these comparisons, which were performed only on the horizontal components of the gps velocities, we took into consideration only the stations with an observation period longer than 2.5 years, given that shorter intervals might result in biased estimates of linear velocities [blewitt and lavallée 2002]. regional scale comparison the bernese, gamit and gipsy gps velocity solutions used in this study include different numbers of sites (from ~370 to ~590). also, the definition of the three eurasian reference frames is significantly different (figure 4a-4c): the gamit solution does not use gps sites to the west of the rhine graben, but it uses several stations also in central eurasia, whereas both the gipsy and the bernese solutions are mainly concentrated in europe, from spain to eastern europe. on the other hand, despite these differences, the determinations of the eulerian poles of the differently defined eurasian plates relative to itrf2005 are very similar (figure 4d; table 2). this aspect confirms the good quality of many of the cgps stations in the eurasia region. the gamit and gipsy solutions include points on the african plate. thus, we determined the africa poles with respect to eurasia for these two solutions and we compared them with the previously gps-derived africa-eurasia poles [sella et al. 2002, calais et al. 2003, kreemer et al. 2003, mcclusky et al. 2003, serpelloni et al. 2007, d'agostino et al. 2008]. table 3 shows that eulerian poles parameters of a nubia reference frame with respect to eurasia together with the previously mentioned solutions. in the comparison, we also inlcuded the africa-eurasia pole proposed by nuvel1a [de mets et al. 1994], obtained averaging the plates motion over the last 3ma. the agreement of our solutions with the other gps-derived solutions and their significant discrepancy with respect to the nuvel1a africa-eurasia pole [de mets et al. 1994] support the hypothesis of a post-3ma change in relative plate motion between africa (nubia) and eurasia [mcclusky the ring network for active tectonics studies e) figure 7e. caption on previous page. et al. 2003, calais et al. 2003, d'agostino et al. 2008]. to determine whether the differences in the analyses affect the relative solutions that produce significant differences in the gps velocity fields, we proceeded in three steps: 1) we defined an average velocity field (avf); 2) we compared the three solutions to the avf; 3) we rotated the three solutions with respect to the avf. despite the small number of different velocity solutions (three), and using only the 217 cgps sites that are common to the three solutions, the avf was calculated according to the site by site mean of the eurasia-fixed velocity values for each horizontal component. this mean was weighted by the relative velocity uncertainties, the covariance, and the correlation indexes between the horizontal components. however, as the gps velocity uncertainties have been calculated in different ways, to perform a rigorous comparison these uncertainties and covariances will probably need to be rescaled to avoid miscomputing of the avf. bianco et al. [2003] showed that knowing the factor for each solution i, which corresponds to the ratio between the chi2 value and the number of degrees of freedom (i.e. chi2/dof), it is possible to impose the following equation: to equally balance each solution contribution and to estimate the relative scaling factors. the scaling factors obtained for the bernese and gipsy solutions appear to be closer (5.58 and 5.41, respectively) than that obtained for the gamit solution (8.49). finally, we again determined the weighted mean for the avf, but with weighting also by the scaling factors computed in this way. the original, un-rotated, results of the three independent analyses are plotted in figure 5a-c and listed in table sm1-3 (supplementary material). in the supplementary material also the avf results (table sm4) are reported. it is clearly evident that, at a first order inspection, no important differences in the regional velocity field can be recognized. furthermore, a few sites have gps velocity vectors that are clearly not coherent with respect to the neighbour velocity vectors and it is worth noting that the three solutions show the same incoherent aspects, thus strongly suggesting probable local site instabilities. on the other hand, in a few cases, one solution diverges significantly from the two other solutions. this aspect could be related to slight differences, in the analysis strategies, for the estimations of the offsets occurred at those sites (due to instrumental changes) or of the annual or semiannual signals in the time series (table 1), thus providing differences in the velocities in the visually checked noisiest sites. however, these discrepancies represents few exceptions (< 7%) of a regionally coherent velocity field. the consistency of the three velocity field has been also evaluated quantitatively. for each site and for each horizontal component, we computed the differences between each velocity solution and the avf (dv). the rms values of these residuals are summarized in table 4. then, we first plotted the dv values for both the horizontal components, with respect to the time-span of each avf site (figure 6, left), to determine whether these differences can be amplified, in any way, for young gps sites. indeed, figure 6 shows that the largest dv cluster, which ranges from 0 to 1.5 mm/yr, is observed for the cgps sites with an observation time-span lower than 3.5 yr. for sites with a life-span greater than 3.5 yr, the ring network for active tectonics studies 50 reference lat lon w smax smin az sw nuvel1a 21.0 -20.6 0.120 6 0.7 -4.0 0.020 sel02 -18.2 -20.1 0.062 9.5 3.7 -17.0 0.005 ca03 -10.3 -27.7 0.063 10.3 3.3 52.0 0.004 kr03 1.1 -21.3 0.060 6.9 5.8 21.0 0.010 mc03 -1.0 -21.5 0.060 4.8 4.3 0.0 0.005 ser07 -1.0 -15.9 0.068 2.2* 2.6* n.d. 0.003 da08 -8.7 -30.8 0.049 3.4 2.6 22.4 0.001 gamit -0.9 -26.1 0.062 1.8 1.4 27.3 0.001 gipsy -8.7 -28.5 0.049 1.8 1.3 15.8 0.001 nuvel1a = de mets et al. 1994; sel02 = sella et al. 2002; ca03 = calais et al. 2003; kr03 = kreemer et al. 2003; mc03 = mcclusky et al. 2003; ser07 = serpelloni et al. 2007; da08 = d'agostino et al., 2008; gamit = the present study; gipsy = the present study; pole coordinates (lon., lat. in degrees); rotation rate (w, in º/myr) and associated 1-sigma uncertainty (sw); semiaxes of the error ellipses (smax, smin); azimuth of the smax (az). star (*) indicates pole uncertainty expressed in the north and east components, instead of smax and smin. n.d. = not-determined in the study. table 3. parameters of the eulerian poles for the nubia reference frame with respect to eurasia obtained by the gamit and gispy solutions and their comparison with previous studies. 2 i| 12i =|/ 51 most of the dv values are below 0.5 mm/yr. then, we plotted the residuals frequency distribution to quantify most of the discrepancies between the three different solutions and the avf. this distribution shows that intrinsic discrepancies range between 0.1 mm/yr and 0.5 mm/yr, with an average value of 0.3 mm/yr (figure 6, right). finally, we rotated each velocity solution with respect to the avf. the rms values resulting from each rotation (table 4) for both of the horizontal components are very similar to those carried out in the previous step (i.e. before rotation). from the results from either the simple differences between each pair of solution or the velocity field rotations, we can infer that there are no evident and systematic differences in the three solutions, and that only intrinsic differences among the three velocity solutions are seen. these intrinsic differences can be quantified in values of about 0.250.3 mm/yr for both of the horizontal components. this result is the first important evidence of the consistency of the differently defined gps velocity fields in the area of the central mediterranean plate boundary zone. site-scale comparisons after the regional scale comparisons, we performed a more local comparison on some particular cgps sites belonging to the ring network, in terms of their time series. the time series available from the three solutions were produced in three different ways: the bernese solution used the method proposed by bianco et al. [2003]; the gamit solution adopted the method described by dong et al. [2002]; whereas the gipsy solution used the cats software proposed by williams [2008]. to rigorously compare the time-series results, we used the same type of analysis, using the cats software, for most of the sites that are common to the three solutions. this was performed for the time interval ranging from the first epoch of the sites to their epoch 2009.0, which corresponds to our shortest investigated timespan solution (i.e. the bernese one). we have essentially applied the same approach as that previously described for the cases of the gipsy solution, and we compared for each site the time series of residuals with respect to their nominal position. thus, we have estimated in the mle analysis the linear trend, the offsets at designated epochs, the annual and semi-annual periodic signals and the spectral law noise index and amplitude [williams 2003]. all of the parameters are estimated adopting a white+flicker error model. in figure 7a-e, the comparison of the residual time series is associated with the comparison of the residuals frequency distribution in the three different solutions. the number of samples for each solution is also indicated. on one hand, the residuals time series are very similar and in most of the cases we can recognize the same periodic signals. on the other hand, some interesting discrepancies can be distinguished in the residuals frequency distributions. the residuals distributions for the north components are very similar, whereas for the east component the bernese solution appears to be always more peaked than the others, with the distribution peak very close to the 0 value. about the vertical component, these discrepancies disappear. the gipsy residuals distributions always appear to show a slightly smoother gaussian distribution than the two other solutions. in general, we can infer that the three solutions are very close also at a local scale, i.e. the site scale. conclusions in the present study, we have first described in detail the technologically advanced characteristics of the high-quality ingv cgps infrastructure, as the ring network, and the impact for geophysical studies that is provided by such a dense national-scale cgps network that is enlarged by other more local cgps networks in italy. to really establish and evaluate the robustness of all of these cgps networks in the central mediterranean, we have described the different strategies adopted by the three data analyses. despite the differences between the processing strategies, the comparisons show very good agreement both at the scale of the whole velocity field and at the scale of the particular site. indeed, in the first case, we noted that the velocity discrepancies (rms) among the three solutions do not exceed the rms value of 0.3 mm/yr, whereas, in the second case, the differences in the distributions of the residuals in the position time series appear to be mostly on the east component. the new increased number of cgps stations now available allows more and more detailed spatial and temporal resolutions of the ongoing crustal deformation with respect to previous studies [anzidei et al. 2001, oldow et al. 2002, caporali et al. 2003, hollenstein et al. 2003, nocquet and calais 2003, pondrelli et al. 2004]. this is confirmed by the robust improvements in the velocity field provided by more recent studies [serpelloni et al. 2005, d'agostino et al. 2008, devoti et al. 2008] of plate boundary deformation in the central mediterranean area. the ring network for active tectonics studies cases common sites before rotation rms after rotation rms e n e n be avf 217 0.14 0.15 0.13 0.12 ga avf 217 0.49 0.48 0.49 0.49 gy avf 217 0.48 0.48 0.49 0.49 be, bernese; ga, gamit; gy, gipsy; avf, average velocity field the rms residuals of the horizontal components (e, n) are shown. table 4. comparison of the rms values for the east and north velocity components before and after applying the rotation of each solution with respect to the average velocity field. finally, the present study has highlighted the importance and the richness that the ingv represents, where different analysis groups are working with the three main geodeticquality softwares. acknowledgements. we thank the staff of the ingv grottaminarda observatory and all the other ingv technicians who have tirelessly installed, and currently maintain, the ring network. furthermore, we thank all of the agencies that have made the gps observations that are used in this study available. the ring network was partially funded by the italian ministry for university and research (miur) through the «programma per la sismologia e l'ingegneria sismica» (prosis) and partially funded by the italian civil protection department. r. d. acknowledges the civil defence department project s1-ur1.04. we also thank the associate editor and the two anonymous reviewers for constructive observations, which helped to increase the quality of the manuscript. references altamimi, z., x. collilieux, j. legrand, b. garayt and c. boucher (2007). itrf2005: a new release of the international terrestrial reference frame based on timeseries station positions and earth orientation parameters, j. geophys. res., 112, b09401, doi:10.1029/2007jb004949. anderson, h. and j. jackson (1987). active tectonics of the adriatic region, geophys. j. r. astr. soc, 91, 937-983. anzidei, m., p. baldi, g. casula, a. galvani, e. mantovani, a. pesci, f. riguzzi and e. serpelloni (2001). insights into present-day crustal motion in the central mediterranean area from gps surveys, geophys. j. int., 146, 98-110. anzidei m., p. baldi and e. serpelloni (2008). the coseismic ground deformations of the 1997 umbria-marche earthquakes: a lesson for the development of new gps networks, annals of geophysics, 51, 27-43. battaglia, m., m. h. murray, e. serpelloni and r. bürgmann (2004). the adriatic region: an independent microplate within the africa–eurasia collision zone, geophys. res. lett., 31, l09605, doi:10.1029/2004gl019723. beutler, g. et al. 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(2008). cats: gps coordinate time series analysis software, gps solutions, 12 (2), 147-153, doi:10.1007/ s10291-007-0086-4. zhang, j., y. bock, h. johnson, p. fang, s. williams, j. genrich, s. wdowinski and j. behr (1997). southern california permanent gps array: error analysis of daily positions estimates and site velocities, j. geophys. res., 102 (b8), 18035-18055. zumberge, j. f., m. b. heflin, d. c. jefferson, m. m. watkins and f. h. webb (1997). precise point positioning for the efficient and robust analysis of gps data from large networks, j. geophys. res., 102 (b3), 5005-5018. *corresponding author: dr. antonio avallone, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy; e-mail: antonio.avallone@ingv.it © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the ring network for active tectonics studies 54 annals 47, 1, 2004, 01/07def 65 annals of geophysics, vol. 47, n. 1, february 2004 key words acoustic emission activity – energy impact – rock 1. introduction at the close of the xxth century, the progress of science and technology pointed out new possible ways to approach the problem of how to reduce the hazard of strong earthquakes. this unexpected way is related to power physical fields, which may influence the structures in the terrestrial crust to induce tectonic overstress unloading and thus reduce seismic hazard. initially, the effects allowing us to control deformation processes in seismogenerating zones manifested itself as induced seismicity, which results from underground nuclear explosions, or from fluid industrial waste injection into boreholes located in a seismic area, or from variations in water level in large water reservoirs, or from mining operations, etc. thereafter it was revealed that dynamic actions redistribute the seismicity in the following manner. they decrease the number of major events due to a growth of energy released by weak earthquakes. there are some ways, which are the most acceptable from an ecological viewpoint, for manmade relaxation of tectonic stress in the terrestrial crust. one of them is electromagnetic action by electric current flashing. pioneer results on the effect of power electromagnetic pulses produced by magnetohydrodynamic (mhd) generators to test the seismic activity in regions of acoustic emission response of rocks to electric power action as seismic-electric effect manifestation leonid m. bogomolov (1), pavel v. il’ichev (1), victor a. novikov (2), vladimir i. okunev (2), vladimir n. sychev (1) and alexander s. zakupin (1) (1) scientific station of united institute for high temperatures (ivtran), russian academy of science, bishkek, kyrgyzstan (2) institute for high temperatures, russian academy of science, moscow, russia abstract two parts of the research are distinguished in this paper. the first part is devoted to the structure of signals of acoustic emission (ae) and electromagnetic emission (eme) which accompany the inelastic straining of terrestrial materials. special attention is paid to the similarity of waveform of eme signals at various scale lengths. the second (and main) part of the work involves the investigation of ae responses to the action of additional power fields over strained rocks. our experimental investigations have revealed the interrelation of acoustic emission activity to power actions applied externally (impacts of electromagnetic field). different modes of responses to electromagnetic impact have been specified. the characteristics of such responses and their variations depending on the material of specimens tested and of electric parameters of external pulses during power impacts have been considered. a general conclusion on possible electromagnetic triggering of ae has been drawn, the prospects of further studies being outlined. mailing address: dr. leonid m. bogomolov, scientific station of united institute for high temperatures (ivtran), russian academy of science, 720049 bishkek, kyrgyzstan; e-mail: leonidb@gdirc.ru 66 leonid m. bogomolov, pavel v. il’ichev, victor a. novikov, vladimir i. okunev, vladimir n. sychev and alexander s. zakupin bishkek and garm testing fields were obtained in russia and kyrgyzstan (uipe, oivtran) (tarasov et al., 2001). it is very important that such external impacts always trigger seismic events of minor magnitude (m < 6). such power actions are adequate for civil and scientific purposes only. this paper is related to the concept of seismicity control by means of technological power actions (so-called energy impacts, ei). the phenomenon of acoustic and electromagnetic energy release during the process of structural defects formation and fracture of terrestrial materials is well-known (nitsam, 1977; gokhberg et al., 1982; warwick et al., 1982; kurlenya et al., 2000; manzhikov et al., 2001). it is a reasonable assumption that the effect of electromagnetic impacts over defects accumulation processes (microcracking and so on) in loaded rock specimens is to occur as well. the manifestation of such inverse energy conversion is that the acoustic emission and the rate of rock straining are influenced by electromagnetic impacts excited externally. the aim of our investigations is to study the responses of terrestrial material specimens to electric power actions. the investigation of ae and eme structure based on waveform analysis is significant to reveal the physical origin of emission pulse. besides, the results of structural studies give essential information on the kinetics of fault formation process in a loaded solid. so, for example, the emergence of the second spectral maximum of acoustic signal whose frequency is lower than that of the main maximum implies the microcracking transition from the stage of diffusive accumulation of defects to cluster stage of defects growth (trapeznikov et al., 1997). the experience has demonstrated that the combined analysis of statistic and structural data on ae yields more reliable results and facilitates results interpretation. our studies have been oriented to the works on statistical characteristics of ae and eme events occurrence. 2. experimental results ae is a good indicator of inelastic straining processes and microfracture. so during experiments we focused the measurement system on recording the ae of loaded specimens of terrestrial materials. some characteristics of ae turn out to be keyword parameters revealing correlations between the rate of straining processes up to specimen fracture and power impacts supplied externally. the work on modeling above inverse processes involves the creep test of specimens of rocks and of artificial heterogeneous materials overburdened by uniaxial compression. the experiments were performed on spring rheological installation udi with a maximum compressive load of 100 tons (designed by a.n. stavrogin, vnimi, st. petersburg). we tested a number of samples manufactured from granodiorite, quartzite, granite and halite. some concrete specimens which were prepared by stavrogin routine (stavrogin and protosenya, 1979) and have the sizes of 100 × 120 × 250 mm3 were tested as well. additional electric power impacts produced by external sources took place during a deformation session with constant level of compressive load. the sources of additional power action were as follows: i) square-wave generator g5-54 giving square-wave signals, whose amplitude was close to 50 v and duration was around 5-50 µs, the frequency being 1-3 khz. ii) 10 kv generator of sparks (without wave form control). iii) capacitor discharges supplying electric pulses with parameters: the time of voltage ramping was about 1 µs and the peak voltage was around 1 kv. ae and eme signals were recorded on a wide frequency region 80 khz-5 mhz. this allows signals waveform control. the measuring system operates in a waiting mode. figure 1 shows the experimental set up. we also focused on electromagnetic emission induced by inelastic straining of terrestrial materials. eme from loaded rocks was recorded on the laboratory studies simultaneously with acoustic emission. eme was caused by the conversion of elastic to electromagnetic energy during microcracking or other fast processes on microand mesostructural levels. the comparison of the experimental eme signal recorded during our experiment with the seismic signal is shown in fig. 2a,b. the signal shown in fig. 2a 67 acoustic emission response of rocks to electric power action as seismic-electric effect manifestation fig. 1. the sketch of the experimental set up: 1 – upper cross arm; 2 – upper platen; 3 – specimen; 4 – lower platen with 5 embedded ae sensors; 5 – hemispherical bearing; 6 – insulating base; 7 – lower cross arm; 8 – electrods; 9 – piezotransducer; adc – analog-to-digital converter; gpv – generator of pulced voltage. fig. 2a,b. a) eme signal of granite sample; b) series of eme impulses recorded before the aftershock 27.05.1991 during the period of aftershocks activity of dzhva-rachin earthquake, 29.04.1991 (morgunov, 1994). a b 68 leonid m. bogomolov, pavel v. il’ichev, victor a. novikov, vladimir i. okunev, vladimir n. sychev and alexander s. zakupin was recorded during the experiment with pristine cylindrical sample from kainda granite deposit (kyrgyz republic) with the help of an electrical capacitive sensor sensitive to eme. the sample was examined at «creep test» conditions with fixed uniaxial compression load. according to kuksenko (1997), the origin of signals on capacitive sensors during such experiment is related to the change in specimen polarization due to the straining process. figure 2b demonstrates the series of eme impulses recorded during the period of aftershocks activity after dzhva-rachin earthquake, 29.04.1991 (morgunov, 1994). the waveform of eme signals from specimen (fig. 2a) appears to be similar to that of eme (fig. 2b) accompanying coseismic crust deformation with possible changes in rock polarization. this is a specific manifestation of such a distinctive feature of stressedstrained state of terrestrial materials as its simifig. 3a,b. a) ae activity of halite sample versus time. electric actions took place during time interval 480010 800 s; the parameters of periodic pulses produced by the g5-54 generator were 2 khz, 5 mks, 60 v. b) ae activity of concrete sample versus time. electric actions of periodic electric pulses took place during time intervals: a) 4800-9600 s, b) 15 000-27 000 s; the parameters of pulses produced by rectangular generator g5-54 were respectively: a) 2 khz, 5 mks, 60 v; and b) − 1 khz, 10 mks, 60 v. fig. 4a,b. a) ae activity of granitic sample versus time. the arrows represent the moments of spark gap converter action (the amplitude of charge is not less than 3 kv). b) ae activity of granodiorite sample versus time. electric actions took place during the time interval 7125-13 620 s; the parameters of periodic pulses produced by the g5-54 generator were 2.5 khz, 20 mks, 50 v. a b a b 69 acoustic emission response of rocks to electric power action as seismic-electric effect manifestation larity for different scale lengths (from laboratory sizes of some cm up to natural seismological scale). there are a number of works (for instance: petri, 1994; zapperi, 1997) confirming the similarity of near-critical dynamics from large scale earthquakes down to the microscopic scale of the specimens rheological structures. among them the works implying ae measurements (diodati, 1991; canneli, 1993, etc.) have revealed that the statistical parameters of ae events flow (ae activity, amplitude and duration of ae) reflect the self-organized criticality of the fracture processes. this means the selfsimilarity of emission effects at various scales of length. resuming the first part of our studies one can remark that the well-known multi-scale similarity is valid for electromagnetic emission (eme) during rocks fracture as well. as regards the main part of the investigation the obtained indicative results are represented in figs. 3a,b, 4a,b and 5. in most cases, the activity of ae increases after the start of electric action. this growth occurs with some delay after the instant of action beginning. then ae-activity decreases to background or (in some cases) still below the average level. remarkable results were obtained on a halite sample. figure 3a shows that the enhanced activity of ae due to electric action remains during the session with square-wave generator. figure 3b demonstrates the triggering effect of electric pulses produced by the g5-54 generator on acoustic emission of a concrete specimen with 1 cm granite inclusions whose weight is about 5% of total weight of a solid. the case of long delay of ae activation by electric impacts is represented in fig. 4b showing the temporal plot of ae activity of the granodiorite specimen. the response to the effect of pulses supplied by square wave generator g5-54 was revealed 1000 s after the generator power was turned on. it is seen in fig. 4b that the activity multiplies by 20 times in comparison with background level but thereafter it drops rapidly to the initial level. figure 4a gives the timing dependence of ae activity of granitic specimen with short term ei; the spark gap converter being the source of such solitary few kv voltage pulses. one can see that the mean value of ae activity doubles some time after the first disfig. 5. ae activity of granodiorite sample versus time. electric actions took place during the time interval 7150-14 000 s; the parameters of periodic pulses produced by the g5-54 generator were 2 khz, 5 mks, 60 v. the figure also shows the expanded range of diagram which demonstrated that the ae activity growth follows the temporal drop after power impact. 70 leonid m. bogomolov, pavel v. il’ichev, victor a. novikov, vladimir i. okunev, vladimir n. sychev and alexander s. zakupin charge, the twofold level remaining steady during the long period. the change in the trend of ae activity plot occurs after the second discharge and the lag like the one observed after the first discharge is present. the activity of ae is increased again, the increment being considerably more than before. finally the average level of ae activity does not change during the time up to specimen fracture. in this state each repeated discharge causes an abrupt drop of ae being followed by ae growth in stepwise manner up to steady level before discharge. sometimes the action of external electromagnetic field results in a temporal reduction of ae activity. this is apparent from plots on fig. 5 that demonstrate the dependence of ae activity of the granodiorite specimen versus time. thereafter minor ae falloff is followed by a very rapid outburst of activity. this speaks in favour of a united origin of responses in the cases of fig. 5 and, for instance, of fig. 4b discussed above. 3. discussion it has been found by experimental studies that the effect of the electromagnetic field on strained structures has different modes depending on the source of the em field, the specimen material, the value of the main load, and the time of specimen exposure under this load. the superposition of these factors affects the kind of response to electric impact, particularly the variations of response specific parameters. as a rule, the responses of ae from loaded terrestrial materials represent a growth of ae activity. sometimes (rarely) such growth follows by the temporal drop of ae activity after power impact (fig. 5); the integral effect being an increment of ae events. the parameters describing ae response to power impact are as follows: triggering time (the lag of response), the increment of activity, the length of response (the duration of enhanced level of activity for the steady response case or the typical width of ae activity spike for short-term responses), the presence or absence of aftereffect, the aftereffect duration. meanwhile, the statistical parameters of a flow of ae events can be taken as the parameters of straining process. in our experiments, the main parameters of ae flow which describe the extent of disorder (caused by both natural heterogeneity and forming zones of microcracks concentration; kuksenko, 1977, 1986) are the dispersion of ae activity, the duration and average amplitude of ae signals. it should be noted that during the test of rock specimens by uniaxial compression, the heterogeneity of specimens was increased due to generation of new zones of enhanced defects population. the occurrence of this indicates the transition from the first (diffusive) stage of the fracture process to the second stage (the phase of defects concentration and coalescence; kuksenko, 1977). such a transition may be an explicit example of selforganized criticality observable by the number of measuring tools including ae (swanson, 1984; gershenzon et al., 1989; morgounov, 1999). a very slow decrease of ae activity of the halite specimen after the session with g5-54 generator pulses (fig. 3a) represents the aftereffect of electric action. this is quite similar to the manifestations of vibration effect on aes (bogomolov et al., 2001). because the halite is a highly resistant material, the results suggest that both polarization and conductive effects play an essential role during additional electric action. taking into account the rate of response rise and subsequent drop one can assume that the response arises inside the domain with locally strained structure, and this entails the avalanche defects formation there. generally two types of response to electromagnetic power action may be specified. the first corresponds to observations of short-term increase in ae activity. in this case, the activation front is quite sharp. usually such responses were recorded when the sample is overburdened by compressive load of moderate value. during repeated electric impacts at the same stress, the responses of this type decrease: we have recorded minor or marginal manifestations or did not observe any such repeated response at all. taking into consideration the well-known kaizer effect one can try to explain ae re71 acoustic emission response of rocks to electric power action as seismic-electric effect manifestation sponse suppression during repeated electric impact. usually the kaizer effect is formulated in terms of stress drop and increment: the growth of ae after stress drop requires stress increment by the value exceeding the drop (tang et al., 1997). they assumed microcracking to be controlled by the stress increment. actually the rate of crack growth depends on a number of parameters (atkinson, 1984), the specific surface energy being very important. this parameter of crack surface state is sensitive to electric effects as well as to the fluid or vapour adsorption. change in the value of surface energy is to accelerate or reduce subcritical cracks evolution (the length is much less than griffith’s). such changes are followed by changes in observable ae activity which look quite similar to the kaizer effect. the origin of this formal similarity is as follows. the increment of load as well as all external impacts results in some energy influx which is sufficient for evolution and/or propagation of a certain limited number of structural defects. a spike of ae activity due to external action (electric impact, in particular) may be treated as a manifestation of rapid growth of available microcracks triggered by ei. thus, the repeated ei over given current stressed-strained state of a specimen obtained no result (no triggering) because of the absence of defects which have been prepared for propagation but wait to be triggered. a new population of microcracks arises after load increment to change the specimen state. then the responses of ae to triggering electric impacts should occur again. the second type of responses may be specified by a steady increment of ae activity. the enhanced level of ae remains quasi-stationary for a long time after electric impact. then (in the case of no repeated impacts) it returns smoothly to the initial value. sometimes the activity decreases to the level below the initial background. so the aftereffect takes place for responses of the second type. in the case when repeated electromagnetic impacts occur on the steady phase of response to primary ei, the transition to the new state with still higher activity of ae appears to be possible. as a rule, the second type responses were revealed at high enough values of the main compressive load (close to breaking loads). all this suggest that such ae responses indicate that the considerable part of tested specimen volume is at near critical conditions (close to critical point). typically, the temporal dependence of parameters related to energy release follows power distribution near critical point. we considered the law of ae activity growth during bifurcation induced by ei (i.e. during the transition to enhanced activity level, see fig. 4a). the power relation of ae activity appears to be valid for more than 70% of amount of second type responses under consideration. in the case of seismic data, the relation described near critical dynamics (bowman and king, 2001; malinetski and kurdumov, 2001) has been reduced to polynomial dependence of benioff strain versus time. the authors are planning the subsequent processing of ae data to derive the above dependence. it is surprising that the activation of ae and inelastic straining described as second type response to ei leads to a new steady state rather than fracturing when the compressive load was of order of 0.9 of the maximum value for a given specimen. in this case, the fracture took place a long time after the ae bifurcation, the load being fixed. the comparison of responses of first and second types by their lifetimes and leading edge times suggests that the first type indicates faulting inside localized domains. the second type of recorded responses seems to represent an ae-manifestation that the sample structure bifurcates to the new state. the diffusive (more uniform) distribution of defects controlling the rate of plasticity and/or other inelastic processes makes ae more regular. the results obtained during our investigations demonstrated that the chosen approach is reasonable, further work is very promising. acknowledgements this research was partially carried out owing to the grant yg1-2316-bi-02 of the american fund of civil researches and development for the new independent states (nis) of the former soviet union. 72 leonid m. bogomolov, pavel v. il’ichev, victor a. novikov, vladimir i. okunev, vladimir n. sychev and alexander s. zakupin reference atkinson, b.k. 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(1986): model of transition from micro to macrofracture, in physics of strength and plasticity (leningrad), 36-41 kuksenko, v.s., s.n. zhurkov, v.a. petrov, v.n. savel’ev et al. (1977): on the problem of prediction of fracture of rocks, izv. akad. nauk sssr, fiz. zemli, 13 (6), 11-16. kuksenko, v.s., h.f. mahmudov and a.v. ponomarev (1997): relaxation of electric fields, induced by mechanical load in natural dielectrics, phys. solids state, 39 (7), 1202-1205. malinetski, g.g. and s.p. kurdumov (2001): unlinear dynamics and problems of prediction, trans. russ. ac. sci., 71 (3), 210-232. manzhikov, b.ts., l.m. bogomolov, p.v. il’ichev and v.n. sychev (2001): structure of acoustic and electromagnetic emission signals on axial compression of rock specimens, geol. geofiz., 42 (10), 1690-1696 morgounov, v. (1999): predictability of earthquakes: state of the art, izv. akad. nauk sssr, fiz. zemli, 35 (1), 72-83. morgunov, v.a. (1994): acoustic and electromagnetic emissions, and straining process, in dynamic processes in geophysical medium, edited by a.v. nikolaev, (nauka, moscow), 167-185. nitsam, v. (1977): electromagnetic emission accompanying fracture of quartz-bearing rocks, geophys. res. lett., 4, 333-336. petri, a., g. paparo, a. vespignani, a. alippi and m. constantini (1994): experimental evidence for critical dynamics in microfracturing processes, phys. rev. lett., 73, 3423-3426. stavrogin, a.n. and a.g. protosenya (1979): plasticity of rocks (nedra, moscow), pp. 301. swanson, p. (1997): tensile fracture resistance mechanisms in brittle polycrystals: an ultrasonics and in situ microscopy investigation, j. geophys. res., 92, 8015-8036. tang, c.a., z.h. chen, x.h. xu and c. li (1997): a theoretical model for kaizer effect in rock, pure appl. geophys., 150, 203-215. tarasov, n.t., n.v. tarasova, a.a. avagimov and v.a. zeigarnik (2001): the effect of electromagnetic impacts on seismicity over the bishkek geodymamic test ground, geol. geofiz., 42 (10), 1641-1649. trapeznikov, yu.a., l.m. bogomolov and b.ts. manzhikov (1997): on the origin of stepwise responce of loaded samples to effect of microvibrations, in proceedings of the international conference on earthquake prediction and deep geodynamics, almaty, 167-172. warwick, j., c. stoker and t. meyer (1982): radio emission associated with rock fracture: possible application to the great chilean earthquake of may 22, 1960, j. geophys. res., 87, 2851-2859. zapperi, s., p. ray, h. stanley and a. vespignani (1977): first-order transition in the breakdown of disordered media, phys. rev. lett., 78, 1408-1411. 373_383 meloni .pdf 373 annals of geophysics, vol. 46, n. 2, april 2003 ultraviolet radiation and aerosol monitoring at lampedusa, italy daniela meloni (*), franco marenco and alcide di sarra enea, clim-oss, centro ricerche della casaccia, s. maria di galeria (roma), italy abstract the measurements of uv spectral irradiance, ozone and aerosol load obtained with a double monochromator brewer at the station for climate observations of the national agency for new technology, energy and environment (enea) in lampedusa, italy, are presented. to derive the aerosol optical depth, the direct-sun measurements at the brewer operational wavelengths (302.1, 306.3, 310.1, 313.5, 316.8 and 320.1 nm) were calibrated using the langley method. a radiative transfer model was used to investigate the role of ozone and aerosols in modulating uv irradiance and to reproduce the measured uv spectra. the optical scattering and absorption properties of aerosols input to the model have been derived from measured size-distributions. the modelled and measured uv spectra are in agreement for different atmospheric conditions and allowed us to estimate the radiative impact of the aerosols for two case studies related to the paur ii campaign held in 1999. mailing address: dr. daniela meloni, enea, climoss, centro ricerche della casaccia, via anguillarese 301, 00060 s. maria di galeria (roma), italy; e-mail: daniela.meloni@casaccia.enea.it key words uv radiation aerosol ozone radiative transfer radiative forcing 1. introduction the amount of solar radiation reaching the earth in the uv spectral region is affected by several factors such as solar zenith angle, ozone, cloud cover, aerosols and surface albedo. the observed decrease of stratospheric ozone (harris et al., 1997) at high latitudes and, to a lesser extent, at mid-latitudes is expected to lead to an increase in uvb levels at the surface (madronich, 1992), with potentially harmful effects to marine ecosystems, terrestrial vegetation and humans (tevini, 1993). while the relationship between total ozone and uv irradiance is well established (e.g., mckenzie et al., 1995; zerefos et al., 1995, and others), the trend due to atmospheric parameters like tropospheric ozone, aerosols and clouds still has to be quantifi ed. the increase in tropospheric ozone following photochemical production due to growing emissions of nox in the industrialized regions of the northern hemisphere could mask the effect of the decrease in stratospheric ozone, especially in summer (brühl and crutzen, 1989). besides tropospheric ozone, aerosols should be considered as well. aerosols are a major component of the climate system, though mostly still unknown. they can affect the radiative fi eld both with direct and indirect effects, by absorbing and scattering the incoming solar radiation and by modifying cloud properties and the hydrological cycle acting as cloud condensation nuclei (twomey, 1977; charlson et al., 1992; schwartz and andreae, 1996). aerosol effects on the uv irradiance have been studied by several authors, through both fi eld observations and radiative transfer model (*) now also at : dipartimento di fisica, università «la sapienza», roma. 374 daniela meloni, franco marenco and alcide di sarra calculations. liu et al. (1991) showed that the observed reduction in ground level visibility since the industrial revolution over non urban areas in the eastern united states and europe may have balanced most of the increase in uvb due to stratospheric ozone depletion. studies have been conducted at continental sites in north america (deluisi, 1997; kerr, 1997) and in europe (krzyścin and puchalski, 1998; cañada et al., 2000) which show how aerosol optical depth and aerosol characteristics can affect uv irradiance. in the mediterranean basin, investigations have been carried out at coastal (kylling et al., 1998; cañada et al., 2000), rural (meleti and cappellani, 2000) and urban sites (lorente et al., 1994; repapis et al., 1998). in particular, the mediterranean experiences high mineral aerosol loads due to the transport of dust particles from the sahara, mainly in spring and summer (di sarra et al., 2001a). the sahara is one of the major sources of mineral aerosols worldwide (morales, 1986). mineral aerosols are a major class of tropospheric aerosols and are expected to have a considerable infl uence on the earth radiative budget (schwartz and andreae, 1996). dust is a mixture of various minerals, and its optical constants depend on origin, mobilization properties, physical and chemical transformation during transport in the atmosphere, as well as how minerals are aggregated. sokolik and toon (1996) estimated that, although the key quantities which infl uence the direct radiative forcing are affected by a wide range of uncertainties, the regional forcing due to mineral aerosols can be of the order of 20 w/m2 locally, comparable to the forcings observed for clouds. although desert dust has a big impact on uv radiation through scattering and absorption, only a few studies on this topic have been carried out. since 1997, the italian national agency for new technology, energy, and environment (enea) has maintained a station for climate observations at the island of lampedusa (35.5°n, 12.6°e), in the southern mediterranean, approximately 100 km off the west coast of tunisia. several atmospheric parameters (greenhouse gas concentrations, measured at lampedusa since 1992, meteorological parameters, total ozone, aerosol optical depth and solar radiation) are monitored continuously. a double monochromator brewer mkiii performs measurements of total ozone, uv irradiance and aerosols. in may and june 1999, the paur ii campaign (photochemical activity and ultraviolet radiation modulation factors) took place at lampedusa, with the aim of studying the role of ozone and aerosols on the transfer of ultraviolet and visible radiation through the atmosphere (di sarra et al., 2001b). the island, isolated from continental and polluted sites, is particularly suitable for studies of desert aerosols, which often reach the measuring site, especially in summer, as well as for the observation of the effects produced by aerosols of different origin on the radiative budget. in previous papers the evident effect of the saharan dust aerosols on the uv, erythemal, and visible irradiance has been shown (di sarra et al., 2001b, 2002). in this paper, the measured brewer spectra are simulated by means of a radiative transfer model for cloud free conditions in order to study the dependence of uv radiation on aerosol load and on particle optical properties. the model is initialized with a large set of observations, allowing a detailed characterization of the atmospheric properties. 2. radiative transfer model in this study the uvspec model by mayer et al. (1997) is used. the model is based on the discrete ordinate method (stamnes et al., 1988) for the solution of the radiative transfer equation, and irradiances are computed with the pseudospherical approximation (dahlback and stamnes, 1991) which accounts for the sphericity of the atmosphere. the model can simulate radiance, direct and diffuse irradiance and actinic fl uxes between 200 and 800 nm assuming an extraterrestrial solar spectrum and taking into account the atmospheric structure and composition. the high resolution extraterrestrial spectrum is from atlas 3 shifted to air wavelength between 280 and 407.8 nm, from atlas 2 from 407.8 and 419.9 nm, and from the modtran 3.5 model above 419.9 nm (kylling et al., 1998). 375 ultraviolet radiation and aerosol monitoring at lampedusa, italy temperature dependent ozone cross-sections used for model simulations are from burrows et al. (1999) for the 280-794 nm range. to accurately account for the variation of the optical properties with altitude, the atmosphere was divided in 64 layers from 0 to 120 km with a vertical resolution of 150 m in the lowest 2.1 km, 300 m between 2.1 and 8.1 km, 1 km until 25 km and progressively increasing thickness up to 120 km. the model inputs are: solar zenith angle; total ozone; surface albedo; vertical profi les of pressure, temperature, air and ozone density. aerosols are also characterised through singlescattering albedo w0, asymmetry factor g and optical depth profi le. some of the model input parameters had to be assumed due to lack of measurements. the albedo was estimated as an average of the wavelength dependent vegetation and water albedoes weighted for the respective areas within a region of 20 km radius around the site (ruggaber, 1998). since lampedusa is relatively small (surface area of about 20 km2), the albedo we used is very close to that of sea water (see doda and green, 1980). the surface is assumed to be lambertian in the model. vertical profi les of pressure and temperature at 12:00 ut were obtained from the national centers for environmental prediction (ncep) analyses, and the molecular density profile is calculated in the hypothesis of perfect gas behaviour. the total columnar ozone and its vertical distribution as well as the aerosol optical depth (aod), vertical extinction profi le and size distribution have been directly measured or derived from measured data, as described in the following paragraph. the original model has been modifi ed to take into account the vertical distribution of the aerosol single scattering albedo and asymmetry factor and to include various tropospheric aerosol types, whose spectral optical properties have been calculated applying the mie theory. the calculated spectral irradiances are convoluted with the brewer slit function, assumed to be wavelength-independent and approximated by a triangle with 0.55 nm fwhm bandwidth. 3. measurements measurements of total columnar ozone and uv irradiance have been continuously made by means of the brewer spectrophotometer #123 since january 1998. a description of the instrument and of its operational mode is given in kerr et al. (1985). the brewer was originally designed for total ozone measurements and is able to perform reliable uv measurements. the double monochromator mk iii measures spectral global irradiance between 286.5 and 363 nm at steps of 0.5 nm with a resolution of 0.55 nm through a tefl on diffuser: the suppression of stray light results in an improvement in the uv measurements below 305 nm with respect to the single monochromator brewer mk ii (bais et al., 1996). the brewer #123 was calibrated for ozone measurements in july 1998 by comparison with the travelling brewer #17. global uv irradiance has been absolutely calibrated with a 1000 w lamp traceable to the national institute of standards and technology (nist) and its stability is regularly checked by using 50 w external lamps. aerosol optical depth measurements were made by sun photometric methodology, using the brewer mk iii spectrophotometer #123 and a multi-filter rotating shadowband radiometer (mfrsr). the methodology to derive the aod from the brewer is based on the data of the total ozone measurements and is fully described in marenco et al. (2002). calibration was obtained at sea level with langley plots for a large number of clear days, and averaged to remove residual atmospheric disturbances. this methodology provided an uncertainty of less than 2% on the instrument calibration constant. since during normal operation the instrument takes measurements using different neutral density fi lters, their transmissivity also had to be determined. a description of the mfrsr can be found in harrison et al. (1994). the methodology used to derive aerosol optical depth from mfrsr data has been described in di sarra et al. (2002). this instrument has six channels centered at 415, 500, 615, 671, 868, and 937 nm (10 nm bandwidth). the data used in this work refer to the 415 nm and 376 daniela meloni, franco marenco and alcide di sarra 868 nm bands. the calibration of the mfrsr is also based on langley plots, performed for fi ve selected cloud-free and low aerosol mornings, with an uncertainty on the instrument calibration constant of 2% at 868 nm and 5% at 415 nm. the derived aerosol optical depths measured with the two instruments during paur ii appear consistent (see for instance fi g. 2a,b). during the paur ii campaign, besides the brewer and the mfrsr, other instruments were deployed at the enea station, and were operated during daytime: uv-a and uv-b radiometers, solar pyranometers, a guv-511 band radiometer, a pom-01l aureole sensor, a li-1800 visible spectrometer, an aerosol lidar, and meteorological sensors. additional instruments were installed aboard the ultralight aircraft of the fraunhofer institut für atmosphärische umweltforschung (ifu, germany): a uv photometer for determination of ozone concentration, a fssp-100 optical particle counter, a tsi-3010 condensation nuclei counter, a laser particle counter, an avm iii aerosol scatterometer, two solar pyranometers (upward and downward looking), an uv-b radiometer, two jo(1d) and two jno2 filter radiometers (upward and downward looking), digital cameras, pressure, temperature, humidity and position sensors. profi les of ozone, radiation, and aerosol properties were determined during 16 flights carried out from the lampedusa airport in may, reaching a maximum altitude of 4.5 km. a description of the instrumented aircraft together with some results of the measurements at lampedusa are reported by junkermann (2001). the ozone profi les needed by the model are taken from bhartia et al. (1985) who computed standard middle latitude profi les derived from solar backscatter ultraviolet (sbuv) and balloon observations. in the lower atmospheric layers the standard profi le has been replaced with the profi le measured by the ozone uvphotometer installed aboard the aircraft and the whole profi le is scaled to match the columnar ozone measured by the brewer. the aerosol vertical profi le was observed by lidar (di sarra et al., 2001a). the aerosol optical depth at 532 nm was obtained by interpolation with a power law from the measurements performed by the mfrsr at 415 nm and 868 nm. the vertical extinction profi les at 532 nm were derived from the analysis of the lidar and the particle counters profi les as described in di iorio et al. (2003). from the top of the aerosol layer to 10 km altitude the tropospheric aerosol type was assumed, and background stratospheric aerosol from 10 to 30 km (shettle,1989). the aerosol size distribution was retrieved from the data collected during the aircraft fl ights by fssp and condensation nuclei counter (di iorio et al., 2003), whereas the dry mass refractive indices are the only unknown parameters. the aerosol refractive index for each examined day was determined: this was done by calculating the aerosol single scattering albedo and asymmetry factor using the retrieved size distributions for different values of the real and imaginary parts of the refractive index; then the aerosol refractive index which minimizes the differences between measured and modelled spectral irradiances was chosen. 4. results and discussion spectral uv irradiances obtained on two clear-sky days, may 14 and 28, characterized by different aerosol loadings were selected. table i summarizes some relevant information for the two cases. measured uv spectra were corrected for the sun-earth distance and, together with o3 total columns, interpolated at the 20° solar zenith angle. the interpolation of the spectra was made with a second order polynomial, using the three acquired spectra which are closest to the time corresponding to the given solar zenith angle. linear interpolation is used to derive the total ozone amounts at the given solar zenith angle. figure 1a,b depicts the ultraviolet spectra measured at 20° solar zenith angle on may 14 and 28 and their ratio. it should be noted that at 20° solar zenith angle (around 10:20 ut for may 14 and 10:10 ut for may 28) the aerosol optical depths at 320.1 nm are equivalent within the standard deviations, while they differ at 532 nm, with the aod of may 14 being greater than that of may 28 (see table i). 377 ultraviolet radiation and aerosol monitoring at lampedusa, italy on may 14, the aerosol optical depth at 532 nm is relatively high and the columnar ozone is low, while the opposite occurs on may 28. the negative correlation between aerosol optical depth and total ozone can be explained in terms of transport patterns (di sarra et al., 2002). signifi cant differences in the measured irradiances appear. at short wavelengths, the effect of ozone absorption is dominating, and higher irradiances are observed on may 14 when day solar zenith angle aod (320 nm) aod (532 nm) ozone (du) 14/05/1999 20° 0.384 ± 0.018 0.392 ± 0.007 295.1 28/05/1999 20° 0.375 ± 0.019 0.276 ± 0.007 343.7 table i. model input parameters. fig. 1a,b. a) measured spectral global irradiance on may 28 (solid line) and on may 14 (dashed line); b) their ratio. b a 378 daniela meloni, franco marenco and alcide di sarra the total ozone is low. at longer wavelengths the effect of the aerosol overcomes the effect of the ozone, and higher irradiances are found on may 28, a day characterized by low aerosol optical depth. the ratio is 1 around 320 nm, a wavelength which is only weakly absorbed by ozone, and where the two aods coincide. according to the 10-day isentropic trajectories ending at lampedusa at approximately 2.5 km height, in may 14 the air masses arrived at lampedusa after spending several days over the sahara desert, and the aerosol composition is expected to be mainly of mineral dust. the air mass reaching the island on may 28 had passed over europe, thus suggesting that the aerosol may contain particles of anthropic origin as well as marine. the trajectories were derived with the national aeronautics and space administration trajectory model (schoeberl et al., 1992), which uses the national centers for environmental prediction (ncep) analyses. figure 2a,b shows the daily evolution of the aod at 320.1 nm as derived from brewer direct sun measurements in may 14 and 28. also depicted are the aods at 415 nm and 868 nm from mfrsr measurements. it can be seen that on both days the aods at the three wavelengths show the same behaviour. the aerosol optical depths at 320.1 and 415 nm are always very close; an exception is represented by the time interval between approximately 10:30 and 15 on may 14, when the curve corresponding to 320.1 nm is below the other two. this may be due to the presence of clouds or may refl ect a misalignment of the mfrsr leading to an underestimation of the direct irradiance and thus to an overestimation of aerosol optical depth. the wavelength dependence of the aod can be expressed through the ångström exponent a . (4.1) according to di sarra et al. (2001a, 2002), low and high a values characterize days with saharan and continental-maritime aerosol during paur ii, respectively. the ångström exponent calculated using the aods at 415 nm and 868 nm is 0.11 for may 14 and 1.21 for may 28 (20° solar zenith angle). to evaluate the impact of aerosols on radiative budget, it is necessary to have the best knowledge of the aerosol optical properties that can be obtained from the available information. a retrieval of the aerosol size distribution for may 14 was not possible due to the presence of clouds during the aircraft fl ight, therefore the model simulation is performed using the aerosol size distributions of a day with similar atmospheric conditions, may 18. for the calculation of may 28 the aerosol size distribution of may 27 was available. the complex refractive indices for the aerosols were chosen as the values which give w0 and g in best agreement with global uv irradiance measurements (assuming to have no wavelength dependence): 1.5-0.0042 i for may 14 and 1.5-0.026 i for may 28. the imaginary part of the refractive index for may 14 is compatible with the values reported by patterson et al. (1977) and sokolik et al. (1993) for mineral dust. the airmass that reaches lampedusa on may 28 was over europe the previous days, and the aerosols may reasonably contain anthropogenic absorbing particles including black carbon. d’almeida et al. (1991) disclosed that the refractive index values for black carbon show differences up to a factor of 3 in the imaginary part, and by more than 50% in the real part. this may lead to very different effects on atmospheric radiative budget. aerosols with a high imaginary part have strong absorption properties, thus determining a net warming effect, while aerosols with a low imaginary part have a cooling effect, mainly producing an increase of the scattering of the incoming radiation back to space. table ii displays the single scattering albedoes and the asymmetry factors calculated at 500 nm with the mie code using the above mentioned size distributions and refractive indices; for comparison, w0 and g from d’almeida et al. (1991) for the desert (may14) and the maritime polluted (may 28) aerosol types are reported: the latter values refer to july and for a relative humidity of 0%. the range of variability for the calculated w0 and g are due to the changes of the measured size distribution with altitude. the results of the radiative transfer calculations are shown in fi g. 3a,b as the ratio of the model result and the measured spectrum. the comparisons show an agreement within 8% α λ λ λ λ = ( ) ( )     ⋅     − ln ln aod aod 1 2 2 1 1 379 ultraviolet radiation and aerosol monitoring at lampedusa, italy fig. 2a,b. daily evolution of the aerosol optical depth for may 14 (a) and may 28 (b) at three wavelengths: 320.1 nm (single points with error bars) from brewer measurements, 415 nm (solid line) and 868 nm (dashed line) from mfrsr measurements. also shown is the uncertainty range in the mfrsr aerosol optical depth at 415 and 868 nm. b a table ii. refractive index and single scattering albedo w 0 and asymmetry factor g at 500 nm derived for may 14 and may 28 by mie calculation and comparison with the values reported in d’almeida et al. (1991) for the desert (may 14) and the maritime polluted (may 28) aerosol types on july and for a relative humidity of 0%. day refractive calculation d’almeida calculation d’almeida index 14/05/1999 1.5-0.0042 i 0.75-0.76 0.725-0.777 0.81-0.82 0.778-0.822 28/05/1999 1.5-0.026 i 0.69-0.75 0.960 0.58-0.67 0.692 w0 g 380 daniela meloni, franco marenco and alcide di sarra 1.15 1.10 1.05 1.00 0.95 0.90 0.85 295 305 315 325 335 345 355 365 wavelength (nm) m o d e l /m e a s u r e m e n t r a t io 1.15 1.10 1.05 1.00 0.95 0.90 0.85 295 305 315 325 335 345 355 365 wavelength (nm) m o d e l /m e a s u r e m e n t r a t io fig. 3a,b. ratio of modelled to measured spectral irradiance for may 14 (a) and may 28 (b). the thick solid curve represents a 10 nm-running mean. b a 381 ultraviolet radiation and aerosol monitoring at lampedusa, italy table iii. aerosol radiative forcing (units of w/m2) at the ground and at the top of the atmosphere (assumed here at 120 km) calculated for may 14 and 28 for the brewer operational range (286.5-363.0 nm) and in the uv-b spectral range (280-320 nm). this is different from the usual sw radiative forcing (see text). may 14 may 28 altitude (km) brewer uv-b brewer uv-b 0 4.63 0.57 3.55 0.40 120 (toa) 0.72 0.06 + 0.27 + 0.01 (single wavelength peaks) in the 296-362 nm region for both days; below 296 nm, the differences become slightly larger due to the higher uncertainties in the irradiance measurements (attributed to the decrease of the signal and to the effects of stray light). the peak in the model/ measurement ratio is attributed to the differences between the true slit function of the brewer and the one modelled to reproduce the spectra and to the adopted extraterrestrial spectrum. to remove this effect, a 10 nm-running mean was calculated. if this curve is considered the agreement between model and measurements is within 3%. the model also evaluates the attenuation of irradiance due to aerosols. at 350 nm it is 12.5% for may 28 and 16.7% for may 14 with respect to the aerosol-free case. the model allows the calculation of the aerosol radiative forcing at different altitudes: at the ground it is defi ned as the difference between the downward global irradiance with and without aerosols respectively, while at the top of the atmosphere (toa) it is the difference between upward diffuse irradiance with and without aerosols, so that negative values indicate a greater upward irradiance for the aerosol free case. the values in the brewer (286.5-363.0 nm) and in the uv-b (280-320 nm) spectral ranges are shown in table iii. the negative forcing at the surface for both days is due to the loss of radiation by absorption and backscattering. the aerosol on may 14 attenuates the incoming radiation more than on may 28, due to larger absorption by desert aerosols: the coalbedo (1-w0), which represents the fraction of radiation that has been absorbed, is 0.24-0.26 for may 14 and 0.16-0.21 for may 28 at 320 nm. at the toa we can observe opposite effects for the two cases. in other words, the aerosol layer reduces the atmospheric albedo: although the imaginary part of the aerosol refractive index is not high, the overall effect is a subtraction of upward irradiance with respect to the aerosol-free case. on may 28 backscattering prevails, thereby causing a major fraction of incoming radiation to be refl ected. it must be remarked that calculations of the local radiative forcing are made in the uv portion of the solar spectrum where the brewer operates (286.5-363.0 nm). this is different from the usual shortwave (sw) radiative forcing which is conventionally related to the (0.28-4.3 mm). several studies have been performed to estimate the aerosol radiative forcing due to airborne saharan dust. for example, ackerman and chung (1992) found from the noaa-9 erbe observations that the presence of dust increased the shortwave (sw) exitance at the toa over ocean by 40-90 w/m2. hsu et al. (2000) derived the toa direct forcing of saharan dust using satellite data from toms and erbe; they found values of 20-100 w/m2 in the sw over the ocean. a comparison of our results with previous works is not possible since the radiative model we used performs calculations for a maximum wavelength of 800 nm. the island of lampedusa is small (area of approximately 20 km2) and produces a limited perturbation to the radiative fi eld over the sea: this explains why we simulate the radiative fi eld using an albedo very close to that of water. a change in surface albedo passing from sea to ground can cause important differences in the aerosol radiative forcing (ackerman and chung, 1992; sokolik and toon, 1996; hsu et al., 2000). 382 daniela meloni, franco marenco and alcide di sarra acknowledgements measurements at lampedusa were partly supported by the european union, agenzia spaziale italiana, and ministero per l’ambiente of italy. we would like to thank m. cacciani, t. di iorio, g. fiocco, and d. fuà for providing the lidar data, t. di iorio for the aerosol size distributions, j.j. deluisi for the mfrsr data. contributions at lampedusa by p. chamard, l. de silvestri, p. grigioni, and f. monteleone are gratefully acknowledged. we also thank b. mayer and a. kylling for their helpful suggestions on the radiative transfer model. references ackerman, s.a. and h. chung (1992): radiative effects of airborne on regional energy budgets at the top of the atmosphere, j. appl. meteor., 31, 223-241. bais, a.f., c.s. zerefos and c.t. mcelroy (1996): solar uvb measurements with the double-and singlemonochromator brewer ozone spectrophotometers, geophys. res. lett., 23, 833-836. bhartia, p.k., d. silberstein, b. monosmith and a.j. fleig (1985): standard profiles of ozone from ground to 60 km obtained by combining satellite and ground based measurements, in atmospheric ozone, edited by c.s. zerefos and a. ghazi (reidel publishing company, dordrecht, the nederlands), 243-247. brühl, c. and p.j. crutzen (1989): on the disproportionate role of tropospheric ozone as a fi lter against solar uv-b radiation, geophys. res. lett., 16, 703-706. burrows, j.p., a. dehn, b. deters, s. himmelmann, a. richter, s. voigt and j. orphal (1999): atmospheric remote-sensing reference data from gome: 2. temperature-dependent absorption cross sections of o3 in the 231-794 nm range, j. quant. spectrosc. radiat. transfer, 61, 509-517. cañada, j., g. pedrós, a. lópez and j.v. boscá (2000): infl uences of the clearness index for the whole spectrum and of the relative optical air mass on uv solar irradiance for two locations in the mediterranean area, valencia and cordoba, j. geophys. res., 105, 4759-4766. charlson, r.j., s.e. schwartz, j.m. hales, r.d. cess, j.a. coakley jr., j.e. hansen and d.j. hofmann (1992): climate forcing by anthropogenic aerosols, science, 255, 423-430. d’almeida, g.a., p. koepke and e.p. shettle (1991): atmospheric aerosols global climatology and radiative characteristics (a. deepak publication, hampton, va), pp. 561. dahlback, a. and k. stamnes (1991): a new spherical model for computing the radiation fi eld available for photolysis and heating at twilight, planet. space sci., 39, 671-683. deluisi, j. 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(1989): models of aerosol, clouds and precipitation for atmospheric propagation studies, paper presented at conference on atmospheric propagation in the uv, visible, ir and mm-region and related system aspects, nato adv. group for aerosp. res. and dev., copenhagen. sokolik, i.n. and o.b. toon (1996): direct radiative forcing by anthropogenic airborne mineral aerosols, nature, 381, 681-683. sokolik, i., a. andronova and t.c. johnson (1993): complex refractive index of atmospheric dust aerosols, atmos. environ., 27a, 2495-2502. st a m n e s, k., s.-c. ts a y, w. wi s c o m b e and k. jayaweera (1988): numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media, appl. opt., 27, 2502-2509. tevini, m. (1993): effects of enhanced uv-b radiation on terrestrial plants, in uv-b radiation and ozone depletion: effects on humans, animals, plants, microorganisms, and materials, edited by m. tevini (lewis publishers, boca raton, florida), 125-153. twomey, s. (1977): the influence of pollution on the shortwave albedo of clouds, j. atmos. sci., 34, 11491152. zerefos, c.s., a.f. bais, c. meleti and i.c. ziomas (1995): a note on the recent increase of solar uv-b radiation over northern middle latitudes, geophys. res. lett., 22, 1245-1247. geo 915 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction bruno zolesi (1) and ljiljana r. cander (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) rutherford appleton laboratory, chilton, didcot, oxon, u.k. the cost 271 action («effects of the upper atmosphere on terrestrial and earth-space communications») within the european ionospheric community has the objectives, embodied in the memorandum of understanding (mou): to study the influence of upper atmospheric conditions on terrestrial and earth-space communications, to develop methods and techniques to improve ionospheric models over europe for telecommunication and navigation applications and to transfer the results to the appropriate radiocommunication study groups of the international telecommunication union (itu-r) and other national and international organizations dealing with the modern communication systems. this introductory paper summarises briefly the background and historical context of cost 271 and outlines the main objectives, working methods and structure. it also lists the participating countries and institutions, the management committee (mc) meetings, workshops and short-term scientific missions. in addition, the paper discusses the dissemination of the results and the collaboration among the participating institutions and researchers, before outlining the content of the final report. 1. background and historical context temporal and spatial changes in the upper atmosphere can act to limit and degrade the performance of terrestrial and earth-space radio systems in many different ways. command, control and communication systems involving transionospheric propagation may be disrupted; global positioning networks compromised and surveillance (optical and radar) systems affected. in addition, conditions in near-earth space have other adverse effects on modern technology, ranging from the tracking and the lifetimes of satellites to the induction of damaging currents in power grids and transcontinental pipelines. the vulnerabilities of systems to changes in the upper atmosphere incorporate many different aspects. these include: space-weather effects on communication, navigation and surveillance systems; solar cycle phenomena and their impact on operational systems; modelling, forecasting, and prediction services; ionosonde sounder methods and measurements; applications of ionospheric tomography; transionospheric effects including scintillation; ionospheric propagation for high frequency communication systems; longwave propagation systems and effects; global positioning system (gps) and ionospheric total electron content (tec) studies. the european ionospheric community has long been aware that co-operative research on an international basis is essential to deal with such complex issues. in particular, international co-operation is required for the collection of data, in both real-time and retrospective modes, the development and verification of new methods to improve the performance of both operational and future terrestrial and earth-space communications systems and the exchange of expertise on space plasma effects on global navigation satellite systems (gnss). bruno zolesi and ljiljana r. cander cost (the french acronym for «co-operation in the field of scientific and technical research») is an intergovernmental mechanism that sponsors scientific and technical co-operation and co-ordination between research teams and institutional partners from the 34 cost countries and with noneuropean institutions. cost 271 on «effects of the upper atmosphere on terrestrial and earthspace communications» is an action within the telecommunications information science and technology (tist) technical committee. it is a four-year project that started in august 2000 to work collaboratively on the topics outlined above. the core of cost 271 action has been a dynamic group of more than 100 active participants from 17 cost countries, 3 non-cost institutions and the european space agency (esa), working as a technical community towards a common goal. the primary objective was to examine an area of research that had been given recent impetus by an increased technological interest in terrestrial and satellite radio communications and navigation (zolesi and cander, 2002). cost projects are well suited to developing research co-operation in fields where finance is provided at the national level, because the resultant benefits to the sponsors of work are greater than accrue from the efforts of individual organisations. the success of this approach has been demonstrated in the telecommunications domain in previous cost actions concerned with the prediction of ionospheric propagation characteristics. the earlier cost actions relating to ionospheric radio propagation were particularly useful in creating a critical mass of researchers in projects where the initial emphasis was on long-term ionospheric prediction and retrospective ionospheric modelling over europe. the cost 238 action prime (prediction and retrospective ionospheric modelling over europe) was a four-year research project with objectives to develop regional maps and models of the european ionosphere that are more accurate than internationally available global maps and models (bradley, 1995). it involved the participation of some 72 scientists and engineers from 31 organisations in 17 nations. the work encompassed the production of regional monthly median maps and algorithms for generating ‘instantaneous’ snapshot maps of the standard vertical-incidence ionospheric characteristics from coordinated sounding measurements within europe. additional studies involved height profiles of electron density, the vertical total electron content up to an altitude of 1000 km and research leading to an improved understanding and thus potential for better modelling of ionospheric storm morphologies. work conducted within cost 238 was continued in the follow-on project cost 251 iits (improved quality of service in ionospheric telecommunication systems planning and operation) on the application of prime results in the improved quality of service in ionospheric telecommunication systems planning and operation (hanbaba, 1999). both projects yielded significant results, but they also identified further areas where future development was needed. the topics for more research included: hour-to-hour and day-to-day variability prediction capabilities in the topside ionosphere, upgrading of the current models to include scintillation effects, the prediction of the ionospheric and plasmaspheric effects on navigational systems and the development of methods for calculating the reliability and compatibility of hf radio systems using digital modulation techniques and over the horizon hf backscatter radars (zolesi and cander, 2003). the cost 271 action was then established with a remit to examine a range of relevant research areas within four broad groupings. 2. cost 271 action objectives cost 271 is an action for the promotion, stimulation and co-ordination of the european research in ionospheric and plasmaspheric areas. the memorandum of understanding laid out the main objectives of the cost 271 action as follows: – to perform studies to influence the technical development and implementation of new communication services, particularly for the gnss and other advanced earth-space and satellite-to-satellite applications. 916 917 cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction – to develop methods and algorithms to predict and minimise the effects of ionospheric perturbations and variations on communications and to ensure that the best models over europe are made available to the itu-r. – to collect additional and new ionospheric and plasmaspheric data for nowcasting and forecasting purposes. – to stimulate further co-operation in the domain of ionospheric and plasmaspheric prediction and forecasting for terrestrial and earth-space communications, including interactive repercussions on the corresponding standards in this field, taking into account the present and future needs of users. 3. cost 271 working method and structure at the outset of the project consideration was given to a wide range of technical questions of practical relevance. in addition, a special issue of the journal quaderni di geofisica (cander and zolesi, 2001) was published at an early stage. the aim was assess the current state of knowledge and to outline the activities in each participating country in the work areas of the cost 271 action, with an indication of their significance. the research within cost 271 was then organised within four working groups arranged into at total of 15 work packages that are shown in table i. the activities within each of these areas are documented fully in the papers that follow in this volume that serves to make up the final report of the cost 271 action. table i. cost 271 action structure. wg 1 – impact of wg 2 – assessment wg 3 – ionospheric wg 4 – space plasma variability of space of space plasma effects effects on terrestrial effects on earth-space environment on for satellites applications. communications. and satellite-to-satellite communications. communications. wp 1.1 – impact of wp 2.1 – plasma effects wp 3.1 – effects of large wp 4.1 – effects of space space weather on on gnss applications. scale ionospheric plasma variability and communications. fluctuations on terrestrial irregularities on communications, including earth-space and satelliteremote sensing, radio -to-satellite communication localization and radar. channels. wp 1.2 – database wp 2.2 – assessment wp 3.2 – effects of wp 4.2 – development of and tools for nowcasting, of plasma propagation small-scale ionospheric algorithms and software forecasting and warning. errors in navigation irregularities, interference to treat disturbances in systems and merits and and noise on terrestrial earth-space and satellite-toshortcomings of novel communications. to satellite communications. data sources. wp 1.3 – long term wp 2.3 – investigation wp 3.3 – mid-latitude wp 4.3 – application of trends in the ionosphere of extremes of ionization. ionospheric features theoretical considerations and upper atmosphere in radio propagation to the study of space parameters. models. plasma effects. wp 1.4 – upper wp 3.4 – development wp 4.4 – effects of the atmosphere parameters of methods and algorithms vertical and horizontal monitoring for to minimize the deleterious gradients of the electron nowcasting and effects of the ionosphere density on earth-space forecasting purposes. on terrestrial and satellite-to-satellite communications. communications. bruno zolesi and ljiljana r. cander 918 3.1. working group 1: impact of variability of space environment on communications space weather and its impact on terrestrial and space communications have drawn increasing attention in recent years. four working packages were defined under this heading: impact of space weather on communication – to identify present and future anticipated terrestrial and earth space radio systems, to identify propagation phenomena which can lead to impairments of these radio systems that need to be modelled, to identify those space weather parameters that impact adversely and significantly on propagation conditions and to develop mitigation techniques. real-time satellite and terrestrial measurements for nowcasting, forecasting and warning purposes – to establish a space weather database consisting of both past and new measurements and to use these measurements for the development of now-casting and forecasting propagation procedures and software tools. long-term trends in the ionosphere and upper atmosphere parameters – to investigate and understand the nature of the long-term behaviour of all ionospheric regions and potential effects of the long-term trends on prediction models. upper atmosphere parameters monitoring for nowcasting and forecasting purposes – to develop methods to extract thermospheric parameters using routine ionospheric observations and to develop a version of the self-consistent method which would use routine electron density ionosonde profiles to monitor the upper neutral atmosphere above europe. some initial activities on improved robustness of prediction, which were separated as additional work packages at the beginning of the investigation, were later incorporated into the studies of real-time satellite and terrestrial measurements for nowcasting, forecasting and warning purposes. 3.2. working group 2: assessment of space plasma effects for satellites applications the distribution and dynamics of the ionospheric plasma have a significant impact on gnss applications for navigation, positioning and remote sensing of the earth’s atmosphere. three working packages were included in this area: plasma effects on gnss applications – to explore the amplitude and dynamics of horizontal structures in tec by combining data derived by different measuring techniques (groundand spacebased gps, nnss, satellite altimetry), in particular under perturbed ionospheric conditions, to detect and analyse tid’s and the resultant phase fluctuations that degrade accuracy in gnss applications under various geophysical conditions. assessment of plasma propagation errors in navigation systems and merits and shortcomings of novel data sources – to assess ionospheric effects in non-ionospheric applications of gnss signals, a) ionospheric influences in the use of gnss occultation for stratosphere/troposphere applications, b) the effects of higher order ionospheric propagation errors in advanced ground based applications, like water vapour retrieval. investigation of extremes of ionization – to deal with observations aimed to come up with reasonable occurrence statistics when possible, to guide the data collection, to collect well-documented extremes and to provide a list of criteria to define type and nature of the extreme cases. 3.3. working group 3: ionospheric effects on terrestrial communications additional knowledge of the effects of large-scale ionospheric fluctuations, small-scale ionospheric irregularities, noise and interference with terrestrial communications including remote sensing, radio location techniques and radar is required. four working packages were established within this general area: effects of large-scale fluctuations on terrestrial communications – to determine at regional/global scale the percentage contribution to the variability of main ionospheric parameters. effects of small-scale ionospheric irregularities, interference and noise on terrestrial communication, including remote sensing, radio localization and radar – to give a definition of the classes of irregularities to be taken into account, an analysis of their effects on the performances of the systems, the establishment of a catalogue of the known characteristics and of the available equipment for the studies. mid-latitude ionospheric features in radio propagation models – to assess the role of ionospheric and plasmaspheric irregularities of various dimensions in radio propagation at middle latitudes. development of methods and algorithms to minimize the above-referred effects on terrestrial communications, including remote sensing, radio localization and radar – to identify the most important problems due to the ionospheric characteristics and variability, to develop possible methods, if any, to minimize the deleterious effects and when possible, propose specific algorithms. 3.4. working group 4: space plasma effects on earth-space and satellite-tosatellite communications space plasma variability and irregularities effects are of increasing interest to the practical operation of satellite systems. four working packages were included in this topic: effects of space plasma variability and irregularities on earth-space and satellite-to-satellite communication channels – to develop a database of space plasma variability and irregularities characteristics, using both measurements and results from theoretical models and to review the effects of variability and irregularities on communications, considering different locations of transmitter and receiver. development of algorithms and software to treat with disturbances in earth-space and satelliteto-satellite communications – to forecast tec in time and space from 1 to 24 h in advance by using neural networks, signal processing and other relevant techniques and to obtain quantitative description of the tec variability and develop algorithms for nowcasting and forecasting. application of theoretical considerations to the study of space plasma effects – to study the ionospheric disturbances generated by natural electromagnetic and electrostatic instabilities. effects of the electron density vertical and horizontal gradients on satellite communications – to assess the effect of electron density gradients in the slant to vertical time delay conversion in earth919 cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction bruno zolesi and ljiljana r. cander 920 space communications, to assess the effect of electron concentration gradients in satellite-to-satellite communication, to validate and improve existing topside electron concentration models by using the large russian topside profiles database and to validate models of electron concentration profiles based on instantaneous (nowcasting) maps of basic parameters from vertical soundings data by using igs slant tec data and tomographic reconstruction. 4. cost 271 participating countries and institutions 4.1 cost 271 management committee the 17 cost member countries signatories of the cost 271 mou are indicated in table ii by names and addresses of their representatives. some 34 institutions, listed in table iii, formed the core of the activity. in addition, the institutions from 3 non-cost member countries listed in table iv participated, together with one observer organization (estec-esa). table ii. members of the cost 271 management committee. prof. r leitinger dr. r. warnant prof. i. kutiev insitute for geophysics, royal observatory of belgium geophysical institute bulgarian astrophysics and meteorology ave circulaire 3 academy of sciences university of graz b-1180 brussels -belgium «acad. g. bonchev» str. block 3 universitaetsplatz 1113 sofia bulgaria a-8010 graz austria dr. j. laštovička dr. e. turunen prof. c. goutelard institute of atmospheric physics geophysical observatory universite paris-sud 11 academy of sciences sodankyla finland laboratoire d’etude des of czech republic transmissions ionospheriques bocni ii cp 1401 bat. 214 14131 praha 4 czech republic 91405 orsay cedex france dr. n. jakowski prof. s.s. kouris dr. p. bencze dlr/dfd department of geodetic and geophysical fernerkundungsstation telecommunications research institute neustrelitz aristotelian university hungarian academy of sciences postfach 5123 of thessaloniki h-9401 sopron pob 5 hungary d-17235 neustrelitz germany thessaloniki greece dr. b. zolesi (chairman) dr. i. stanis ⁄lawska prof. a.m. casimiro istituto nazionale di geofisica space research centre fct e vulcanologia polish academy of sciences universidade do algarve via di vigna murata 605 bartycka 18a campus de gambelas 00143 roma italy 00-716 warsaw poland 8000 117 faro portugal mr. m. mihaljcic dr. b.a. de la morena prof. y.k. tulunay geomagnetic institute inta, atmospheric sounding istanbul technical university narodnog fronta 45/vi station ‘el arenosillo’ faculty of aeronautics 11 000 belgrade space science division and astronautics serbia and montenegro fed. rep. mazagon (moguer) ayazağa campus 21130 huelva spain 80626 maslak istanbul turkey 921 cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction table iii. participating institutions from cost countries. insitute for geophysics, astrophysics and meteorology, university of graz, austria geophysical institute, bulgarian academy of sciences, bulgaria royal observatory of belgium, belgium institute of atmospheric physics, academy of sciences of czech republic, czech republic geophysical observatory, finland university of ouly, finland dlr/dfd fernerkundungsstation, germany leibniz-institute of atmospheric physics, germany universite paris-sud 11, laboratoire d’etude des transmissions ionospheriques, france universite de rennes, france cnrs, grenoble, france ieea, france university paris 11, france istanbul technical university (i . tü), faculty of aeronautics and astronautics, turkey middle east technical university, turkey inta, atmospheric sounding station, ‘el arenosillo, spain observatorio del ebro, spain universidade do algarve, portugal space research centre, polish academy of sciences, poland istituto nazionale di geofisica e vulcanologia, italy istituto di fisica applicata, cnr, italy the abdus salam international centre for theoretical physics, aeronomy and radiopropagation laboratory, trieste, italy department of telecommunications, aristotelian university of thessaloniki, greece national observatory of athens, greece geodetic and geophysical research institute, hungarian academy of sciences, hungary rutherford appleton laboratory, u.k. qinetiq, u.k. university of bath, u.k. university of wales, aberystwyth, u.k. university of leeds, u.k. university of leicester, u.k. university of nottingham, u.k. university of sheffield, u.k. geomagnetic institute, serbia and montenegro fed. rep. table iv. participating institutions from non-cost countries. institute of terrestrial magnetism, ionosphere and radiowave propagation, russia university of st. petersburg, russia university of massachusetts lowell, u.s.a. dr. lj.r. cander mrs. a. vernon dr. balodis (vice chairperson) (secretary) latvia rutherford appleton laboratory rutherford appleton laboratory chilton, didcot chilton, didcot oxon ox11 0qx u.k. oxon ox11 0qx u.k. 4.2. participating institutions bruno zolesi and ljiljana r. cander 922 5. mc meetings, workshops, short-term scientific missions management committee meetings 9 october 2000, brussels, belgium (inaugural meeting). 24-27 january 2001, trieste, italy. 25-29 september 2001, sopron, hungary. 6-9 march 2002, graz, austria. 1-5 october 2002, faro, portugal. 27 february-1 march 2003, roma, italy. 23-27 september 2003, spetses, greece. 18-20 march 2004, roquetes, spain. 26-28august 2004, abingdon, u.k. cost 271 workshops 1st cost271 workshop on «ionospheric modelling and variability studies for telecommunication applications», 25-27 september 2001, sopron, hungary. 2nd cost 271 workshop on «cost 271 products for itu-r and other radiocommunication applications», 2-4 october 2002, faro, portugal. 3rd cost 271 workshop on «significant results in cost 271 action», 23-27 september 2003, spetses, greece. cost 271 meetings of the working groups 26 march 2001, egs meeting, nice, france. 23 april 2002, egs meeting, nice, france. 20 august 2002, ursi meeting, maastricht, the netherland. 10 april 2003, egs/agu meeting, nice. 21 april 2004, egu meeting, nice. cost 271 short-term scientific missions a. belehaki from noa to ral, u.k. e.t. senalp from metu to ral, u.k. g. miro from inta to aristotelian university of thessaloniki, greece. v. depuev from izmiran to abdus salam ictp, italy. i. tsagouri from noa to ral, u.k. n. malan from the university of wales, aberystwyth to dlr, germany. d. burešová from institute of atmospheric physics to ral, u.k. m. cuerto from universidad complutense de madrid to dlr, germany. m. materassi from cnr to src, poland. in addition, the following young scientists: p. sauli (czech republic), i. tsagouri (greece), m. cueto (spain), e.t. senalp (turkey) and m. rieger (austria) were invited to attend the 2nd cost 271 workshop held in faro, portugal, 2-4 october 2002. the chairman and vice chairperson attended the e-star meeting on 1-2 december 2003 in strasbourg and the joint esa/cost271/cost274/swwt/e-star/fp6: sw-risk meeting on 27 february 2004 in paris, respectively. 6. dissemination of results 6.1. major publication most of the papers reporting activities of the cost 271 action have been published in the following special issues: – 1st cost 271 workshop cd proceedings on «ionospheric modelling and variability studies for telecommunication applications», 25-27 september 2001, sopron, hungary. – special volume of acta geophysica hungarica devoted to the selected papers from the cost 271 workshop on «ionospheric modelling and variability studies for telecommunication applications», vol. 37, no. 2/3, 2002. – special volume of annals of geophysics devoted to the selected papers from the xxvi egs general assembly session on «ionospheric variability and modelling», vol. 45, no. 1, 2002. – 2nd cost 271 workshop cd proceedings on «cost 271 products for itu-r and other radiocommunication applications», 2-4 october 2002, faro, portugal. – 3rd cost 271 workshop proceedings on «significant results in cost 271 action», 23-27 september 2003, spetses, greece, published on line at the cost 271 web site. – special volume of annals of geophysics devoted to the selected papers from the egs/agu general assembly session on «effects of the ionosphere on terrestrial and earth-space communications» (in press). – supplement to annals of geophysics devoted to cost 271 final report, this volume. 6.2. cost 271 action web site the web site of the cost 271 action has been active since the beginning of the action on the following address: http://www.cost271.rl.ac.uk/. it is maintained by mrs. a. vernon at rutherford appleton laboratory and contains the cost 271 action main information documents with management structure, the minutes of the management committee meetings, the calls for papers for workshops and other relevant documents for the action, in addition to the related web sites. 7. scientific and technical cooperation the research links, originally established under the previous cost 238 and 251 actions, led to bilateral and multi-lateral collaborations that have continued in cost 271. in addition, participants have been very active in many different international projects. there is a significant participation of members from working group 1 in the esa space weather program, in particular in the space weather working team (swwt) and the pilot projects scheme. the geomagnetic indices forecasting and ionospheric nowcasting tools (gifint) pilot project, involving italian, greek and u.k. partners, and the dlr pilot project have been supported financially by the esa framework of the swenet (space weather european network) program. the ursi beacon satellite group is linked closely to the work of the second and fourth working groups of this action, providing important contacts between ionosphere, plasmasphere and upper atmosphere scientists, engineers and users of satellite beacon applications. several cost 271 group members are advisers and observers in the international geodynamics service (igs) and offspring organisations like the gps-iono group. while designed to meet the needs of europe, cost 271 has also made an impact on the international work of itu-r study group 3l, through major contributions to the recommendations of that organisation, the provision of data for the validation of prediction models for europe and by assuming a 923 cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction bruno zolesi and ljiljana r. cander 924 leading role in working party sg3l (ionospheric propagation). further steps are planned to make the most applicable results of the action available to itu-r. close links have also been established with several organizations that deal with gnss (at present mainly gps) applications for navigation and surveying. there is a strong involvement of action members in esa/estec projects: a) concerning egnos and galileosat and b) in connection with assessment studies for the use of gnss occultation for atmospheric and ionospheric research. there is an active collaboration of group members in intas projects with russian and ukrainian participation. another ongoing collaboration involves the international reference ionosphere (iri) working group, with international task force activities being carried out at the abdus salam ictp on improvements to the iri model. participants in cost 271 have made a considerable contribution to the international high rate campaign (hirac) of the international gps and in the validation of champ results. they are also involved in the dias (digital upper atmosphere server) econtent framework activity and the rose project on establishing an international geophysical observatory at gaudos. 8. outline of the report this cost 271 final report, published as a supplement to the international journal annals of geophysics, is organised in five sections. the first section comprises this introduction that contains the cost 271 action background and historical context, the general objectives and priorities agreed in the memorandum of understanding, the working methods and structure, the participating countries and institutions, information about the mc meetings, workshops and short-term scientific missions, dissemination of results and finally the scientific and technical cooperation the second section contains 18 scientific papers concerning the main activities and results achieved in the four working groups of cost 271. each paper is in the form of an independent review of one of the activities of the action with an introduction, discussion of the results and references. four scientific papers, in which each wg leader reviews the main achievements obtained in their respective areas of responsibility, make up the third section of the report, together with an additional paper containing some latest results. the fourth section summarises the main achievements of cost 271 as a whole in the context of the mou requirements and looks to the way ahead. significant achievements have been made, developing out understanding of the issues and relevance of field and offering new knowledge that will serve to increase economic and social development. in addition, new challenges have been identified and a concrete proposal is outlined for a follow-on from cost 271. the fifth section comprises a joint paper by two internationally recognized experts, not directly involved in the action, who provide a final comment on cost 271. in particular, they assess the results within the context of the research activities in the other parts of the world, their importance to itu-r and the iri and consider the future for this area of radio science and its applications. the final report is addressed primarily to those in the mou signatory countries involved in frequency planning, spectrum management and system design; to those who sponsored the work undertaken; to industrial partners concerned with the technology who will learn from the results and to those in national radio regulatory administrations and international planning. in addition, the report is of interest to students of radiowave communication and navigation and others in the propagation research community. references bradley, p.a. (1995): prime (prediction regional ionospheric modelling over europe), in cost action 238 final report, commission of the european communities, brussels. 925 cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction cander, lj.r. and b. zolesi (editors) (2001): ionospheric physics and radio propagation over europe: reports on national activities and co-operations, quad. geofis., 15. hanbaba, r. (1999): improved quality of service in ionospheric telecommunication systems planning and operation, cost action 251 final report, space research centre, warsaw. hochegger, g., b. nava, s.m. radicella and r. leitinger (2000): a family of ionospheric models for different users, phys. chem. earth, 25, 307-310. zolesi, b. and lj.r. cander (2002): effects of the upper atmosphere on terrestrial and earth-space communications: the new cost 271 action of the european scientific community, adv. space res., 29 (6), 1017-1020. zolesi, b. and lj.r. cander (2003): effects of the upper atmosphere on terrestrial and earth-space communications, iee conf. proc. no. 491, vol. 2, 565-568. serpelloni_corretto_layout 6 759 gps observations of coseismic deformation following the may 20 and 29, 2012, emilia seismic events (northern italy): data, analysis and preliminary models enrico serpelloni1,*, letizia anderlini2,4, antonio avallone1, valentina cannelli3, adriano cavaliere2, daniele cheloni1, ciriaco d'ambrosio1, elisabetta d'anastasio1, alessandra esposito1, grazia pietrantonio1, anna rita pisani1, marco anzidei1, gianpaolo cecere1, nicola d'agostino1, sergio del mese1, roberto devoti1, alessandro galvani1, angelo massucci1, daniele melini3, federica riguzzi1, giulio selvaggi1, vincenzo sepe1 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy 3 istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy 4 università di bologna, dipartimento di fisica, settore di geofisica, bologna, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6168 1. introduction in may-july 2012, a seismic sequence struck a broad area of the po plain region in northern italy. the sequence included two ml >5.5 mainshocks. the first one (ml 5.9) occurred near the city of finale emilia (ca. 30 km west of ferrara) on may 20 at 02:03:53 (utc), and the second (ml 5.8) occurred on may 29 at 7:00:03 (utc), about 12 km southwest of the may 20 mainshock (figure 1), near the city of mirandola. the seismic sequence involved an area that extended in an e-w direction for more than 50 km, and included seven ml ≥5.0 events and more than 2,300 ml >1.5 events (http://iside.rm.ingv.it). the focal mechanisms of the main events [pondrelli et al. 2012, scognamiglio et al. 2012, this volume] consistently showed compressional kinematics with e-w oriented reverse nodal planes. this sector of the po plain is known as a region characterized by slow deformation rates due to the northwards motion of the northern apennines fold-and-thrust belt, which is buried beneath the sedimentary cover of the po plain [picotti and pazzaglia 2008, toscani et al. 2009]. early global positioning system (gps) measurements [serpelloni et al. 2006] and the most recent updates [devoti et al. 2011, bennett et al. 2012] recognized that less than 2 mm/yr of sw-ne shortening are accommodated across this sector of the po plain, in agreement with other present-day stress indicators [montone et al. 2012] and known active faults [basili et al. 2008]. in the present study, we describe the gps data used to study the coseismic deformation related to the may 20 and 29 mainshocks, and provide preliminary models of the two seismic sources, as inverted from consensus gps coseismic deformation fields. 2. available gps data in and around the epicentral area, several continuous gps (cgps) stations managed by different private and public institutions were operating before and after the seismic sequence that started on may 20. figure 1 shows the distribution of these cgps stations and their affiliations, together with the instrumental seismicity recorded after may 20. with the exclusion of the cgps stations at concordia sul secchia (conc), for which the last data available before the may 20 mainshock was on april 19 all of these stations recorded data continuously across the may 20 mainshock. this allowed for direct measurements of the three-dimensional coseismic displacement of the earth surface. within two days of the may 20 mainshock, we installed six gps units on existing benchmarks belonging to the igm95 network (http://www.igmi.org/geodetica) of the italian istituto geografico militare (igm; military geographic institute). this choice was related to difficulties in installing new gps stations in an area characterized by unconsolidated soils and by the absence of a shallow rock basement, and the need for the collection of post-seismic data as rapidly as possible. we chose the igm95 points (see figure 1) based on their positions article history received july 25, 2012; accepted august 31, 2012. subject classification: crustal deformations, measurements and monitoring, earthquake source and dynamics, gps, coseismic deformation, emilia sequence. 2012 emilia earthquakes serpelloni et al. 760 f ig u re 1 .d is tr ib u ti on o f in st ru m en ta l s ei sm ic it y th at o cc u rr ed fr om m ay 2 0 to ju ne 5 , 2 01 2 (f ro m h tt p: // is id e. rm .in gv .it ), t og et he r w it h t he d is tr ib u ti on o f th e g p s st at io ns d is cu ss ed in t hi s w or k. c ir cl es sh ow e ve nt s w it h 3≤ m l < 5 an d st ar s sh ow e ve nt s w it h m l > 5. t he m ay 2 0 (m l 5. 9) a nd m ay 2 9 (m l 5. 8) e pi ce nt er s ar e th e bl u e an d or an ge s ta rs w it h th ic ke r lin es , r es pe ct iv el y. s ei sm ic it y is c ol or ed d ep en di ng on h yp oc en tr al d ep th s. t he b lu e to p u rp le c ol or s ca le r ef er s to e ar th qu ak es o cc u rr ed in th e m ay 2 028 ti m e in te rv al , a nd th e ye llo w to b la ck c ol or s ca le r ef er s to e ar th qu ak es o cc u rr ed a ft er m ay 2 9. t he g re y lin es s ho w t he m ai n th ru st fr on t of t he n or th er n a pe nn in es . t he g re en t ri an gl es s ho w t he d is tr ib u ti on o f ig m 95 p oi nt s oc cu pi ed f ro m m ay 2 1. c lo se t o st at io n c o n c (p u rp le s qu ar e) , w e in st al le d a g p s st at io n on a n ex is ti ng b en ch m ar k (c o n 6) . t he g re en s qu ar e sh ow s th e po si ti on o f a ne w s it e in s an p ro sp er o su l s ec ch ia ( sn p r ) in st al le d on ju ne 5 . 761 with respect to the may 20 epicenter and taking into account the monument descriptions from the igm log sheets. unfortunately, the number of relatively good quality igm95 benchmarks in the area was rather limited. the green triangles in figure 1 show the distribution of the igm95 points measured after may 20. with the exclusion of the point at nonantola (5903), which was disinstalled before may 29 because of clear increasing instability of the antenna mount with respect to the geodetic benchmark, the other five igm95 stations recorded continuously across the may 29 ml 5.8 mainshock. after may 29 another two points were installed in the western sector of the aftershock area (figure 1, snpr and con6). it is worth noting that many of the cgps stations were collecting data at higher sampling rates than the standard 30 s (at sampling frequencies from 1 hz to 20 hz), and they recorded the dynamic displacements related to the two mainshocks over a wider area in northern italy [avallone et al. 2012, this volume]. the daily, 30-s-sampling, rinex files of the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology) rete integrata nazionale gps (national integrated gps network) stations and of the igm stations occupied after may 20 described and used in the present study are available at the following anonymous ftp address: ftp://gpsfree.gm.ingv.it/emilia2012/static. 3. gps data analysis the available gps data in the form of 24-h, 30-s-sampling, rinex files, were processed by three different gps data-analysis groups using different software and analysis procedures. it is worth noting that several aspects can affect the level of daily repeatability for different geodetic solutions (e.g., the number of phase ambiguities fixed to integer values, different levels of a-priori constraints on the geodetic parameters, the way the reference framework is realized), thus potentially providing different estimates of the coseismic offsets from separate sets of daily position time-series. this is particularly important when the offsets are small; i.e., of the order of magnitude of the daily repeatability. given the small displacements expected at most of the gps stations (in most cases below the centimeter level), due to the relatively moderate magnitude of the two mainshocks and the distances of most stations from the epicenters, a combination procedure was adopted, with the goal being to minimize any possible systematic processing-dependent errors [see devoti 2012, this volume], and to realize a single consensus dataset of the coseismic deformations related to the may 20 and 29 mainshocks. in particular, in this study, the gps data were analyzed using the bernese (http://www. bernese.unibe.ch), gamit (http://www-gpsg.mit.edu/~ simon/gtgk) and gipsy (http://gipsy.jpl.nasa.gov/orms/goa) software. we refer to avallone et al. [2010] for more detailed descriptions of the procedures of the three data-analysis groups. the bernese analysis was performed using double-difference phase observations with ambiguity resolution, adopting the absolute phase center model, and applying tight constraints on the international global navigation satellite systems (gnss) service (igs) orbits and earth orientation parameters (eops). the analysis of a wider network was distributed on several subnets, realizing the international terrestrial reference frame (itrf)08 position-time series using a set of 10 core stations located in europe. the gamit analysis was performed using double-difference phase observations with ambiguity resolution, adopting the absolute phase center model for receiver and satellite antennas, although in this case the igs orbits and eops were loosely constrained and re-estimated together with the station positions, realizing the itrf08 position-time series using the global igs08 core stations as internal constraint stations. the gipsy analysis was performed using the precise point positioning strategy (module gd2p.pl, gipsy, version 6.1) developed at the jet propulsion laboratory ( jpl, california, usa), which includes absolute antenna phase center modeling and ambiguity resolution [bertiger et al. 2010]. by using the jpl final fiducial orbits and clocks, the position time series was obtained directly in the itrf08 reference framework. the three data-analysis groups provided the itrf position-time series of daily station positions as sinex and stacov files, which were used to independently estimate the three coseismic displacements. thus, the combination of the individual solutions was obtained by solving for the combined offsets (unknowns) in a least squares sense [see devoti 2012]. the weighted root mean squares (wrms) of the differences between the three solutions is 2.1 mm and 4.2 mm for the horizontal and vertical components, respectively, for the may 20 event, and 2.6 mm for both components for the may 29 event. 4. coseismic displacement and fault models figure 2 shows the combined horizontal coseismic displacements for the may 20 and 29 mainshocks. the vertical coseismic displacements are presented in figure 2 only for the may 20 mainshock, where reliable vertical offsets were estimated, while for the may 29 event, no significant vertical patterns were observed. for co-located gps stations, or groups of stations where the short inter-distance does not justify significantly different coseismic offsets, we constrained these stations to have the same coseismic offset during the least-squares estimate. the coseismic displacement fields, with their related uncertainties, for the may 20 and 29 events are available on-line, as ascii tables, at the following addresses: 1) ftp://gpsfree.gm.ingv.it/emilia2012/static/emilia combinedred120520_ver4.dat 2) ftp://gpsfree.gm.ingv.it/emilia2012/static/emilia combinedred120529_ver1.dat gps deformation during the 2012 emilia sequence for the may 20 event, the largest displacements were measured at finale emilia (mo05), which moved to the ssw by ca. 3 cm and uplifted by ca. 7 cm. san giovanni in persiceto (sgip, pers) moved nne-wards by ca. 2 cm, whereas sermide (serm), which was located north of the aftershock distribution, moved southwards by ca. 1.5 cm. the other stations moved horizontally by <1 cm in a radius of ca. 50 km from the epicenter (e.g., bologna and modena moved nnewards by ca. 7 mm). we measured reliable vertical displacements (>1 cm) also at san benedetto po (sbpo, ca. 1 cm) and concordia sul secchia (conc, ca. –2 cm). due to the already mentioned data gap at conc before may 20, 2012, the coseismic displacements at this station were less constrained than for other sites. the errors associated with the coseismic displacements represent the formal 1v uncertainties of the least-squares estimates [see devoti 2012, for more details], and were generally <0.5 mm in the horizontal components, while for the vertical component, they were between 1.5 mm and 4.0 mm. for the may 29 event, we measured coseismic displacements at a greater number of sites, as data was also available from the igm95 stations installed soon after the may 20 event, although the displacements were in general smaller than for the may 20 event. they were also all <1 cm in the horizontal components, with the exception of conc, which was close to the epicenter of the may 29 mainshock. it is worth noting that all of the gps stations discussed in this study were mounted on buildings or on small manmade concrete infrastructures, although of different natures. for the two stations closest to the two mainshock epicenters, mo05 and conc, after visual inspections of the gps monuments, we found significant damage only to the building hosting the conc antenna, which was damaged after the may 29 mainshock, and for this reason this station serpelloni et al. 762 figure 2. horizontal coseismic displacements with 68% error ellipses, as obtained from the combination of the three independent gps solutions (see section 2) for the may 20 (red arrows) and may 29 (blue arrows) events. the seismicity is drawn as in figure 1. upper-right inset: vertical displacements associated with the may 20 event. 763 has not been used in the further modeling. a few hours after the may 20 mainshock, different groups started studying the seismic source using the preliminary estimated gps displacements. in the present study, we report on the results obtained from the three data-analysis groups affiliated with different ingv departments, with the goal of providing consensus fault models for the may 20 and 29 sources, starting from a shared dataset of coseismic gps displacements. it is worth noting that the number and distribution of gps stations, which were particularly missing in the nearsource, did not allow for a reliable study of the variable distributions of the coseismic slip on the fault planes, so we limited our analysis to the inversion of uniform-slip faults. table 1 gives the summary of the results obtained by uniform-slip inversions using rectangular dislocations in a uniform half-space [okada 1985]. after some very preliminary model inversions, the three data-analysis groups adopted similar non-linear inversion algorithms, and particularly those based on simulated-annealing techniques. inversions performed with no a-priori constraints on the geometric and kinematic parameters show the presence of several local minima and strong parameter correlations. in particular, for the may 20 event the stations with the largest displacements were roughly aligned along a direction normal to the fault strike and crossing the center of the fault (derived from the aftershock distribution), making it difficult to separately resolve the fault, strike and rake. for this reason, a-priori constraints derived from seismological data (e.g., focal solutions, aftershocks distributions) and geological information (e.g, fault databases, seismic lines) were used in the nonlinear inversions. in particular, the dip (towards the south) and the fault strike nonlinear search were limited to realistic ranges for both of the sources. table 1 shows the lower and upper bounds for each fault parameter that were adopted in the source inversions. independent inversions of the combined displacement fields for the may 20 and 29 events provided comparable results (see table 1 and figure 3). small differences among the three fault solutions, however, can result from different tuning of the non-linear inversion algorithms, and from the use of different fault boundaries in the inversions (see table 1). we provide consensus fault models for both the may 20 and 29 events, through the averaging of the geometric and kinematic parameters that were inverted independently by each of the three data-analysis groups (see table 1). for the may 20 event, our study indicates that the mainshock occurred on a n-verging, ca. 38˚ s-dipping, ca. 109˚ striking fault plane, with a thrusting mechanisms (rake, ca. 89˚) gps deformation during the 2012 emilia sequence table1. coseismic fault parameters obtained by the three independent uniform-slip inversions for the may 20 and may 29, 2012, events. subscript and superscript numbers are the lower and upper bounds of the geometric fault parameters used in the non-linear inversions and, eventually, the bounds of the kinematic parameters used in the linear slip inversions. depthtop and depthbottom, depth of the top edge and bottom edge of the rectangular source, respectively. cf lon and cf lat, the longitude and latitude of the center of the rectangular source. ingv1 inversion performed by l.a. and e.s. (at ingv, sezione di bologna); ingv2 inversion performed by d.m and v.c. (at ingv, sezione roma 1), and ingv3 inversion performed by d.c. (at ingv, centro nazionale terremoti). the mean solution was obtained by averaging the parameters values obtained from the three independent inversions. the mean depthbottom and mw are derived from the average (mean) values. 20 may length (km) width (km) depthtop (km) depthbottom (km) dip (˚) strike (˚) cf lon (˚e) cf lat (˚n) slip (cm) rake (˚) mw ingv1 8.7 156 6.5 12 6 5.2 9.3 14 6 38.8 50 25 106 110 95 11.293 11.32 11.20 44.866 44.90 44.80 116 (free) 85.7 (free) 6.12 ingv2 17.3 355 4.4 20 2 4.6 15 0 7.2 37.6 50 10 112 130 90 11.299 11.73 10.73 44.853 45.39 44.39 80 100 10 85.8 110 70 6.10 ingv3 12.5 205 5.0 15 5 4.9 15 0 7.9 36.7 60 30 110 110 80 11.294 11.48 11.12 44.857 45.08 44.72 101 102 0 86.5 180 0 6.11 mean 12.8 5.3 4.9 8.1 37.7 109 11.295 44.859 99 86.0 6.13 29 may ingv1 9.6 125 7.1 12 6 2.4 6.3 10 6 33.3 45 25 102 105 90 11.075 11.1 10.9 44.828 44.9 44.8 39 (free) 83.0 (free) 5.86 ingv2 14.2 355 5.0 20 2 3.0 15 0 6.0 37.5 60 20 99.6 125 65 11.068 11.59 10.59 44.806 45.35 44.35 35 100 10 83.1 110 70 5.84 ingv3 14.3 155 5.1 15 5 2.9 15 5 5.8 34.9 50 25 104 105 95 11.069 11.48 11.12 44.793 45.08 44.72 35 100 0 90 (fixed) 5.85 mean 12.7 5.7 2.8 6.1 35.2 102 11.071 44.809 36 85.4 5.85 serpelloni et al. 764 f ig u re 3 .o bs er ve d an d m od el ed h or iz on ta l c os ei sm ic d is pl ac em en ts fo r th e m ay 2 0 (l ef t) a nd m ay 2 9 (r ig ht ) m ai ns ho ck s, o bt ai ne d by th e th re e in de pe nd en t m od el in ve rs io ns (t ab le 1 ). t he r ed a rr ow s an d bo x sh ow t he r es u lt s ob ta in ed b y th e in g v 1 gr ou p, t he g re en a rr ow s an d bo x sh ow t he r es u lt s ob ta in ed b y th e in g v 2 gr ou p, t he b lu e ar ro w s an d bo x sh ow t he r es u lt s of t he i n g v 3 gr ou p. t he b ot to m pa ne ls s ho w c ro ss s ec ti on s co nd u ct ed a bo u t th e no rm al t o th e st ri ke o f fo ca l s ol u ti on s of t he m ay 2 0 an d 29 m ai ns ho ck s (s ee d as he d bo xe s in t op p an el s) , w he re t he b lu e an d re d be ac h ba lls s ho w t he t im e d om ai n m om en t t en so r [s co gn am ig lio e t a l. 20 12 ] a nd q u ic k r eg io na l c en tr oi d m om en t t en so r [p on dr el li et a l. 20 12 ] f oc al s ol u ti on s, r es pe ct iv el y. t he s ei sm ic it y (f ro m h tt p: // is id e. rm .in gv .it ) a nd th e to po gr ap hy a re a ls o sh ow n in th e pr ofi le s. t he c ol or ed tr ia ng le s sh ow th e su rf ac e pr oj ec ti on o f th e di ff er en t c os ei sm ic fa u lt s, w it h re sp ec t t o th e su rf ac e pr oj ec ti on s of th e it c s0 50 (p og g io r u sc om ig lia ri no ) an d it c s0 51 ( n ov i-p og g io r en at ic o) c om po si te s ei sm og en ic s ou rc es fr om t he d is s da ta ba se ( ht tp :/ /d is s. rm .in gv .it ) [b as ili e t al . 2 00 8] . 765 and a slip of ca. 99 cm, confined between about 5 km and 8 km in depth, giving mw 6.1. the derived source model is in agreement with the distribution of aftershocks and focal solutions (see figure 3). for the may 29 event, our study indicates that the mainshock occurred on a n-verging, ca. 35˚ s-dipping, ca. 102˚ striking fault plane, with a thrusting mechanisms (rake, ca. 85˚) and a slip of ca. 36 cm, confined between about 3 km and 6 km in depth, giving mw 5.8. the derived fault model is geometrically in agreement with the focal solutions, although offset sw-wards (by ca. 5 km) with respect to the distribution of the seismicity with depth (see figure 3). 5. discussion and conclusions we have studied the crustal deformation caused by the two mainshocks of the earthquake sequence that started on may 20 in the emilia po plain (northern italy), as measured by continuous and campaign gps stations. we have modeled the observed coseismic displacements, provided in terms of the consensus deformation fields obtained from the combination of independent geodetic solutions, through inversions based on elastic deformation theory, using the standard okada formulation. the availability of gps stations continuously recording across moderate magnitude earthquakes, although of relatively lower data and monument quality, as the ones installed on the igm95 benchmarks after the may 20 event, allows accurate (sub-centimeter) estimates of the horizontal coseismic displacements, and provide valuable data to constrain the source faults. the modeled source faults fit the n-nne-verging blind thrusts of the external ferrara-romagna arc both geometrically and kinematically. inversions of the gps data confirm that the may 20 and 29 earthquakes ruptured two independent segments of this blind structure. the two inverted sources show a gentle counterclockwise rotation of the fault strike, from ca. 109˚ to ca. 102˚, which is consistent with the rotation of the arc and shows mostly pure reverse faulting kinematics, in agreement with focal mechanisms and known seismotectonic and geological features of the area. although preliminary, the source faults inverted from available gps data can be tentatively associated to segments belonging to the itcs050 'poggio rusco-migliarino' (for the may 20 event) and a segment of the itcs051 'novi-poggio renatico' (for the may 29 event) composite seismogenic sources of the database of individual seismogenic sources. however, while for the may 20 event we find relatively good agreement between the model fault plane and the distribution of the aftershocks, for the may 29 event the modeled fault plane imaged by gps is shifted by ca. 5 km to the sw with respect to the crustal volume affected by the aftershocks. the lack of near-source gps stations (especially for the may 29 event, after the decision not to use the displacements measured at the conc station, which was the only near-source station available for that event) was certainly a limit on the source inversion. however, it is worth considering that only when better constrained, precisely relocated, aftershocks will be available, will a comparison between space-geodetically derived source faults and seismicity distribution be meaningful. an integration of the gps coseismic displacement fields with denser insar displacements [eg., salvi et al. 2012, this volume] is needed to solve for the variable slip distribution on the two fault segments activated during this sequence, and to better understand the geometric relationships between the geodetically derived fault planes, the aftershock distribution with depth, and the known geological structures. acknowledgements. we thank all of the public and private institutions that manage and distribute the gps data used in this study, including: euref, asi, ersaf-regione lombardia, regione veneto. we also thank the general commander g. petrosino and g. tomasino of the istituto geografico militare italiano for the data of the igm95 network.we thank the fondazione geometri e geometri laureati dell'emilia romagna (foger) for providing the emilia-romagna gnss network, the department of earth sciences of the university of siena and the department of physics of the university of bologna for providing data from the gps station in finale emilia (mo05), which formerly belonged to the assogeo network. we also thank geotop s.r.l., gruppo topcon-sokkia, for providing data from the conc station, and leica geosystems for data from the italpos gnss network. giuseppe casula is acknowledged for sharing data of the blgn station. fabiana loddo and nicola cenni helped in the update of the hardware of the sgip ring station during the seismic sequence. the manuscript benefited from the thoughtful comments of two anonymous reviewers. references avallone, a., g. selvaggi, e. d'anastasio, n. d'agostino, g. pietrantonio, f. riguzzi, e. serpelloni, m. anzidei, g. casula, g. cecere, c. d'ambrosio, p. de martino, r. devoti, l. falco, m. mattia, m. rossi, f. obrizzo, u. tammaro and l. zarrilli (2010). the ring network: improvements to a gps velocity field in the central mediterranean, annals of geophysics, 53 (2), 39-54; doi:10.4401/ag-4549. avallone, a., e. d'anastasio, e. serpelloni, d. latorre, a. cavaliere, c. d'ambrosio, s. del mese, a. massucci and g. cecere (2012). high-rate (1 hz to 20-hz) gps co-seismic dynamic displacements carried out during the emilia 2012 seismic sequence, annals of geophysics, 55 (4); doi:10.4401/ag-6162. basili, r., g. valensise, p. vannoli, p. burrato, u. fracassi, s. mariano, m.m. tiberti, e. boschi (2008). the database of individual seismogenic sources (diss), version 3: summarizing 20 years of research on italy's earthquake geology, tectonophysics; doi:10.1016/j.tecto.2007.04.014. bennett, r.a., e. serpelloni, s. hreinsdóttir, m.t. brandon, g. buble, t. basic, g. casale, a. cavaliere, m. anzidei, m. marjonovic, g. minelli, g. molli and a. montanari (2012). syn-convergent extension observed using the retreat gps network, northern apennines, italy, j. geophys. res., 117; doi:10.1029/2011jb008744. bertiger, w., s. desai, b. haines, n. harvey, a. moore, s. gps deformation during the 2012 emilia sequence owen and j. weiss (2010). single receiver phase ambiguity resolution with gps data, j. geodesy, 84, 327-337. devoti, r., a. esposito, g. pietrantonio, a.r. pisani and f. riguzzi (2011). evidence of large-scale deformation patterns from gps data in the italian subduction boundary, earth planet. sci. lett., 311, 1-12; doi:10.1016/j.epsl.2011. 09.034. devoti, r. (2012). combination of coseismic displacement fields: a geodetic perspective, annals of geophysics, 55 (4); doi:10.4401/ag-6119. montone, p., m.t. mariucci and s. pierdominici (2012). the italian present-day stress map, geophys. j. int., doi:10.1111 /j.1365-246x.2012.05391.x. okada, y. (1985). surface deformation due to shear and tensile faults in a half-space, b. seismol. soc. am., 75, 1135-1154. picotti, v., and f.j. pazzaglia (2008). a new active tectonic model for the construction of the northern apennines mountain front near bologna (italy), j. geophys. res., 113; doi:10.1029/2007jb005307. pondrelli, s., s. salimbeni, p. perfetti and p. danecek (2012). quick regional centroid moment tensor solutions for the emilia 2012 (northern italy) seismic sequence, annals of geophysics, 55 (4); doi:10.4401/ag-6146. salvi, s., c. tolomei, j.p. merryman boncori, g. pezzo, s. atzori, a. antonioli, e. trasatti, r. giuliani, s. zoffoli and a. coletta (2012). activation of the sigris monitoring system for ground deformation mapping during the emilia 2012 seismic sequence, using cosmo-skymed insar data, annals of geophysics, 55 (4); doi:10.4401/ag-6181. scognamiglio, l., l. margheriti, f.m. mele, e. tinti, a. bono, p. de gori, v. lauciani, f.p. lucente, a.g. mandiello, c. marcocci, s. mazza, s. pintore and m. quintiliani (2012). the 2012 pianura padana emiliana seimic sequence: locations, moment tensors and magnitudes, annals of geophysics, 55 (4); doi:10.4401/ag-6159. serpelloni, e., m. anzidei, p. baldi, g. casula and a. galvani (2006). gps measurement of active strains across the apennines, annals of geophysics, supplement to vol. 49 (1), 319-329. toscani, g., p. burrato, d. di bucci, s. seno and g. valensise (2009). plio-quaternary tectonic evolution of the northern apennines thrust fronts (bologna-ferrara section, italy): seismotectonic implications, b. soc. geol. ital., 128, 605-613; doi:10.3301/ijg.2009.128.2.605. *corresponding author: enrico serpelloni, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, c/o sezione di bologna, italy; email: serpelloni@bo.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. serpelloni et al. 766 << /ascii85encodepages false /allowtransparency false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 4, gd430, 2022; doi:10.4401/ag-8770 o p e n a c c e s s seismic assessment of high-rise buildings through dynamic nonlinear analysis xiuwen wu1 and daoyun lin*,2 (1) hainan hongjing architectural design co., ltd., haikou, china (2) dowsion science and technology engineering (hainan) co., ltd., haikou, china article history: received december 1, 2021; accepted may, 2, 2022 abstract this paper present a seismic assessment of high-rise buildings using dynamic nonlinear analysis. the work touches upon an urgent problem of ensuring the stability of high-rise buildings under seismic load and preventing their destruction. a mathematical model has been built to investigate the effect of seismic activity on high-rise buildings in the sichuan province (china). pearson’s test was used to compare the statistical dependence between variables in the study period. values with a statistical significance of p ≤ 0.05 were considered statistically significant. the probability of earthquakes of 8 magnitude and above was found to be 5% with a frequency of recurrence of 3900 ± 400 yrs, p ≤ 0.001. the paper presents the technique of refractive geophysical research. the seismic reliability index was n = 3.2 at t = 5 s. the standard deviation was 2% and the χ2 statistic was 0.95. the overall (0.59, or 59%) and conditional (0.06, or 6%) risks of failure under seismic load were calculated for high-rise buildings with a service life of 100 years. the coefficients of seismic activity were as follows: 3.203 at the conditional risk level of 5%; 2.97 at the conditional risk level of 10%; and 2.523 at the conditional risk level of 30%. keywords: high-rise building; incremental dynamic analysis; seismic force; soil; structure reliability. 1. introduction seismic and paraseismic tremor shocks represent a type of dynamic load. the seismic-type shocks are associated with earthquakes, the natural, short-term shaking of the ground caused by the propagation of seismic waves through the depths of the earth. earthquakes can be divided into three categories based on depth of the shaking: shallow, intermediate, and deep. seismic tremors are of three types: tectonic (the most common one), volcanic, and collapse. they are characterized by these estimates: the total radiated energy, e (j); the maximum acceleration, amax (cm/s2); radius of action, r (km); the size of the area under risk, a (km2); and the number of protrusions. paraseismic tremors results human activities, such as mining, mine or quarry blasting, pile driving activity (in close proximity of the building), and vibrations generated by industrial equipment and vehicles. the most common cause of paraseismic tremors is mining, but they also can be produced by heavy traffic in large cities. mining tremors can be seen in previously exploited areas many years after exploitation because of disturbances in the geostatic xiuwen wu and daoyun lin 2 balance of rock mass that the exploitation process causes. moreover, scientists highlight a continuous increase in the intensity of these tremors, in china specifically [tan et al., 2020; kumar et al., 2021]. when planning the construction multi-storey buildings in an area with increased seismic activity, project managers have to establish seismic risks and failure criteria. one reason behind vibrations is unbalance [sheng et al., 2020]. in this case, it is best to balance the moving masses. this strategy for vibration reduction is also applied to equipment and different mechanisms, especially their rotary and reciprocating parts. one type of unbalance is a displacement of the gravity center of a load rotating around the eccentric, an unbalance which increases the centrifugal force [an et al., 2021]. to prevent this, one can reduce the amplitude by balancing the mass of the movement so that the rotating mass is close to the axis of rotation of the mass [li, 2021]. the eccentricity is this case does not matter. this solution was employed in some skyscraper constructions [al-kodmany, 2014; contin, 2021]. there are more reasons for unwanted vibration, which are associated with equipment aging and malfunction [beketaeva et al., 2016]. therefore, it is important to keep an eye on the state of machinery in order to immediately alleviate all defects and visible signs of wear [kapasakalis et al., 2021]. thin-walled building structures (such as chimneys) can develop vibrations under the action of air flow. to mitigate the effects of aerodynamic forces on buildings, a special construction device is used, which can resemble a turbo isolator [huo et al., 2021]. architecture engineering under earthquake conditions is a subject of attention, which is considered by all participants in the construction project, especially in earthquake-prone regions [sheng et al., 2020]. in china, such region make up more than 25% of the country’s land. the skyscraper construction projects in the country have recently moved to seismic zones, which were previously poorly populated and not used for permanent construction purposes [jiang et al., 2020]. potentially, these zones can be used to build industrial enterprises, residential complexes, and other structures. yet, given the risk of paraseismic events in the area of high-rise building construction, the structural reliability of the buildings should be significantly improved [lei et al., 2020]. according to seismologists and eyewitnesses, the seismic activity has two distinct phases: (1) the short phase, which lasts a total of several seconds, sometimes preceded by smaller foreshocks [oliveira and correia, 2021]; and (2) the subsequent phase, sometimes followed by aftershocks (rocks destruction during the movement of tectonic platforms) [liu and zahradník, 2020]. the short phase of the shock occurs due to crushing of the rock mass in places of concentration of forces that shift geologic platforms of the earth’s crust. the subsequent phase (oscillations with gradual attenuation) follows a shift of the foundation of the structure. sometimes the frequencies of natural vibrations are close to or even coincide with the frequencies of forced vibrations. in this case, vibration of the buildings increases [wang and huang, 2020]. this can be avoided by changing the vibration frequency of the system itself. for this, the value of the vibrating mass or stiffness of the system must be corrected [xu et al., 2020]. another way is to use special mufflers that help to damp vibration [kaczmarczyk, 2021]. these devices are commonly employed in the construction of tall, slender buildings and are part of a special assembly [bollano et al., 2021]. nonlinear dynamic analysis methods are used to study earthquakes and predict the extent of destruction [forcellini, 2019]. this technique is widely used not only in china (sichuan province, qinghai province, yunnan province, etc.), but also in italy, japan, the philippines, singapore and many other countries [chen, et al. 2021; bollano et al., 2021]. destruction of structures usually occurs under the action of the first shock. it can be due to the impact action of soil on the structure, which results in stress that arises within the structure and leads to cracking and failure. if the building did not collapse during the first (short) phase, then the damage is likely repairable [dalle et al., 2021; wang and shen, 2020]. therefore, when designing a building, it is very important to estimate the level of stress from the shock during its short phase in order to ensure the strength of the structure [qingyun et al., 2021]. the present work aims to conduct a series of numerical tests to determine the sensitivity of nonlinear dynamic systems to seismic conditions. 2. materials and methods 2.1 study location the study was conducted in sichuan province, china, a region which has small (ml <5) but frequent earthquakes. normally, these are earthquakes that follow a major tectonic element – a southwest (sw) trending anticline. an anticline is a system of nappes/folds (3-6 km thick) that slowly slide along the underlying base [liu and zahradník, 2020]. seismic assessment of high-rise buildings 3 2.2 refraction technique a refraction technique utilizes the ‘sliding’ of waves along the boundaries of layers with different wave velocities. this phenomenon is characterized by an increase in velocity with depth. due to this feature, this geophysical method is suitable for the accurate recognition of boundaries. a seismic refraction analysis reveals deep sections with marked layers and changes in wave velocity. using refraction techniques allows a quasi-continuous representation of the structure of the center with accuracy unattainable in geotechnical engineering. 2.3 statistical analysis statistical data processing was performed using pearson’s chi-squared (χ2) test and t-test. pearson’s test was used to compare the statistical dependence between variables in the study period. the 95% confidence interval has been determined. statistical significance was set at p <0.005. standard deviation and variance were also determined. all calculations were performed in ms excell and statistica. 2.4 mathematical modeling of seismic shock based on the shockwave theory the velocity of seismic compression (𝜎) and shear (𝜏) waves propagating through different materials (soil and concrete) after shock depends on the elastic properties of those materials, such as elastic modulus (𝐸) and density (𝜌). the vibration velocity of particles in the wave front was designated as 𝑉. note that the process of wave transmission usually occurs amid the continuous interaction between soil and foundation (before destruction). hence, the above properties can all be considered in the context of continuity. in addition to that, the impact energy from the earthquake is continuously converted to deformation energy of the medium proportionally to its modulus of elasticity. such representation of the soil-foundation interaction process allows calculating the relationship between soil 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" and concrete 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" velocities of vibration for particles in the compression and/or shear wave fronts. this enables the calculation of the normal compressive stress (𝜎𝜎! +𝛥𝛥𝜎𝜎! = 𝜎𝜎" ) and shear stress (𝜏�) acting on concrete [li and su, 2021; wang et al., 2021b]. penetration of shock waves into a building/structure occurs in two ways: through the lateral surface and through the foundation of the construction at the points of contact with the ground (figure 1). due to the impact action of soil on the lateral surface of the foundation, waves propagating through the body of the foundation will create normal stress (𝜎). equations describing the continuity of wave-front stress and velocity at the interface between the soil (𝜎𝜎! +𝛥𝛥𝜎𝜎! = 𝜎𝜎" ) and the lateral vertical face of the foundation (𝜎�) can be expressed as follows: 𝜎𝜎! +𝛥𝛥𝜎𝜎! = 𝜎𝜎" (1) 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" (2) the proportional relationship between the relative deformation and the ratio of soil and concrete velocities can be determined using the following equation: 𝜎𝜎" = 𝜀𝜀𝐸𝐸" = $! $"! 𝐸𝐸" (3) in a similar way to equation (3), we wrote 𝜎𝜎! = $#%# $"# (4) 𝛥𝛥𝜎𝜎! = &$#%# $"# (5) xiuwen wu and daoyun lin 4 through the substitution of equation (5) to equation (1) and with due consideration to equation (4), we get: $#%# $"# + &$#%# $"# = 𝜎𝜎" (6) from which 𝛥𝛥𝑉𝑉! = )𝜎𝜎" − $#%# $"# +$" # %# (7) if we substitute equations (7) and (2) and transform the resulting formula, we will get: 𝛥𝛥𝑉𝑉! = 𝑉𝑉! ,1+ 𝐸𝐸" 𝑉𝑉'" 𝑉𝑉'! 𝐸𝐸! . (8) 𝑉𝑉! = 𝛥𝛥𝑉𝑉! 1+ 𝐸𝐸" 𝑉𝑉'" 𝑉𝑉'! 𝐸𝐸! (9) the shockwave theory holds that the velocity of shock wave propagation in different materials ((v!", v#", v!$, and v#$) , (v!", v#", v!$, and v#$) , (v! ", v#", v!$, and v#$) , and (v!", v#", v!$, and v#$) ) depends on deformation modules (e and g) and density (𝜌) of the material. for concrete, it is equal to: 𝑉𝑉'"/ %! (! 𝑉𝑉)" = / *! (! (10) for soil, it will be: 𝑉𝑉'!/ %# (# 𝑉𝑉)! = / *# (# (11) if we express the concrete and soil velocities of compression waves in terms the properties these two media have, the equation (9) will be written as follows: 𝑉𝑉" = &$# +,$! %# %! $# (12) figure 1. a schematic representation of shock waves propagating into a building. seismic assessment of high-rise buildings 5 the second way of impact energy propagation is through the base of the foundation, where the transfer of energy occurs under the action of shear stress (𝜏�) at the soil-foundation contact. continuity equations for wave velocity (𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" ) and shear stress at the soil-foundation interface can be expressed in a similar way to equations (13) and (14): 𝜏𝜏! +𝛥𝛥𝜏𝜏! = 𝜏𝜏" (13) 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" (14) the proportional relationship between the relative deformation and the ratio of soil and concrete velocities during shear can be expressed as: 𝜏𝜏! = $#*# $&# (15) 𝛥𝛥𝜏𝜏! = &$#*# $&# (16) the stress, in this case, will depend on the adhesion of soil to the foundation, which is characterized by the coefficient of adhesion (c). the values of 𝜏� and 𝛥𝜏� must be adjusted to the ratio between the adhesion coefficient and its largest value, which is equal to the shear strength of the medium (soil or concrete). normally, this property is taken as (1.5 ÷ 2.5)𝑅𝑅%& but this study assumes that it is equal to. in a similar way to equation (7), we thus get: 𝛥𝛥𝑉𝑉! = )𝜏𝜏" −𝑉𝑉! *# $"# +$" # *# .!' / (17) substituting equations (17) and (15) to equation (14) gives: 𝑉𝑉" = 1 +, (!' ) +, (!' ) *!+# *& !*# 2 (18) and if we consider equations (10) and (11), the final formula will be: 𝑉𝑉" = )1+𝑅𝑅"0𝐶𝐶 +𝑉𝑉! 1+𝑅𝑅"0𝐶𝐶 / 𝐺𝐺"𝜌𝜌" 𝐺𝐺!𝜌𝜌! (19) besides shear stress, transmission of impact energy can cause normal stress on structure’s foundation, which will complement normal stress from shock waves that hit the side surface of the building. the shear stress, which results from the force acting on the lateral surface of the foundations can arise in the superstructure at the edge of the foundation and combine with shear stress produced during the transmission of impact energy through the bottom part of the foundation. the superstructure stress can thus be determined using combined velocity (𝑉𝑉1 = 𝑉𝑉" +𝑉𝑉" ), the formula of which is: 𝑉𝑉1 = 𝑉𝑉" +𝑉𝑉" (20) when planning the construction of the underground part of the building, stress values should be computed by the following formula: 𝜎𝜎" = 𝜎𝜎(𝑉𝑉")+𝜎𝜎9𝑉𝑉": (21) xiuwen wu and daoyun lin 6 in formulas (1) to (21) above, the following designations are used: 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" , 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" , and 𝜎𝜎" = 𝜎𝜎(𝑉𝑉")+𝜎𝜎9𝑉𝑉": are the compressional wave-front velocities of horizontal displacement in the soil medium, under the transmission of impact energy through the foundation’s lateral surface, and under the transmission of impact energy through the bottom part of the foundation, respectively; 𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" , 𝛥𝑉�, and 𝛥𝑉� represent the increment of horizontal velocities at the interface of two media on impact; 𝑉𝑉'"/ %! (! 𝑉𝑉)" = / *! (! , 𝑉𝑉'"/ %! (! 𝑉𝑉)" = / *! (! , and 𝑉𝑉'!/ %# (# 𝑉𝑉)! = / *# (# represent the velocities of waves (compressional and shear) propagating within the soil medium and concrete medium, which depend on the elastic properties of those media; 𝜎𝜎! +𝛥𝛥𝜎𝜎! = 𝜎𝜎" , 𝜎𝜎! +𝛥𝛥𝜎𝜎! = 𝜎𝜎" , and 𝜏𝜏! +𝛥𝛥𝜏𝜏! = 𝜏𝜏" are the compressive stress and the tangential (shear) stress on the soil and concrete; 𝜎𝜎! +𝛥𝛥𝜎𝜎! = 𝜎𝜎" , 𝛥𝜎�, and 𝛥𝜏� represent increments of stress at the interface of two media; 𝐸�, 𝐸�, 𝐺�, and 𝐺� are deformation moduli and shear moduli of soil and concrete; 𝜌� and 𝜌� represent the densities of soil and concrete, respectively. the velocity (𝑉𝑉# +𝛥𝛥𝑉𝑉# = 𝑉𝑉" ) of horizontal movement of the soil can be determined by the following formula: 𝑉𝑉! = 𝛾𝛾 ⋅ 𝑡𝑡 (22) where: 𝛾 is the acceleration of ground movement during an earthquake and 𝑡 is the duration of the impulse of the force. based on international standards [wang et al., 2021a], the numerical values of ground surface acceleration (𝛾) were established with regard to the magnitude of an earthquake. these values were expressed in millimeters per second (mm/s2). the impact duration, however, is not normalized in terms of magnitude; therefore, it should be taken based on the results of seismogram analysis. to ensure reliability in calculations, the duration of shock should be taken in the range from 0.25 to 3 seconds, as suggested in literature [sheng et al., 2020; an et al., 2021; kumar et al., 2021]. according to the shockwave theory, the velocity (𝑉) of horizontal displacement in the shock front can help to determine the magnitude of compressive and/or shear stress on a structure: 𝜎𝜎!2 = 𝑉𝑉>𝜌𝜌𝜌𝜌 (23) 𝜏𝜏!2 = 𝑉𝑉>𝜌𝜌𝜌𝜌 (24) when estimating the stress, a building/structure was considered as a complex isotropic system, although it is, in fact, an anisotropic system – a set of structural elements (such as walls, pillars, and floors). stress values can vary from structure to structure (element to element). to consider this feature during calculations, the following equilibrium condition was employed: 𝜎𝜎!2 0.4 ∑6'.-. = 𝜎𝜎!2 2.4.∑6..-. (25) where: 𝜎𝜎'( &.*∑,!.#. and 𝜎𝜎'( (.*.∑,$.#. represent stress at the interface of shock-transmitting structures and shock-absorbing structures, respectively; ∑𝐹𝐹&.*. and ∑𝐹𝐹(.*. are the combined areas of cross-sections, which transmit and absorb the shock, respectively. seismic assessment of high-rise buildings 7 based on equilibrium condition, stress on the shock-absorbing sections can be determined by this formula: 𝜎𝜎!2 2.4. = 𝜎𝜎!20.4. ∑6'.-. ∑6..-. = 𝜎𝜎!20.4.𝐾𝐾0 (26) where: 𝐾� is a coefficient of transition, which takes into account the change in the magnitude of stress at the interface of structures with different areas. the shear stress (𝜏) can be determined in a similar way: 𝜏𝜏!2 2.4. = 𝜏𝜏!20.4. ∑6'.-. ∑6..-. = 𝜏𝜏!20.4.𝐾𝐾0 (27) due to the fact that the shock wave propagating through the building’s structures encounters resistance, it loses energy and gets extinguished. in other words, the horizontal displacement will decrease, resulting in the reduction of stress. since the propagation of stress waves is similar to that of sound waves, one can assume that the decay of stress waves and sound waves occurs in the same way, that is, proportionally to the square of the travel distance. the loss of impact energy during the upward movement of a shock wave along the walls of a building, which attenuates at its top completely, can be taken into account by introducing the coefficient of attenuation to the stress value: 𝐾𝐾7 = 1−) 8/ 9 + (28) where: 𝐻 is the height of a building, m; and ℎ� is the height at which the stress is computed. consequently, equations for stress on the upper structures from a shock wave produced by an earthquake are as follows: 𝜎𝜎!2 2.4. = 𝜎𝜎!20.4.𝐾𝐾0𝐾𝐾7 (29) 𝜏𝜏!2 2.4. = 𝜏𝜏!20.4.𝐾𝐾0𝐾𝐾7 (30) when estimating stress on flat structures (such as floors), it is necessary to consider dissipation of energy in the horizontal direction due to deformation processes, as shown in figure 2. under a linear law of stress reduction within half the span, the coefficient of dissipation for formulas (29) and (30) will be equal to 0.5. figure 2. a representation illustrating energy dissipation of the ceiling. xiuwen wu and daoyun lin 8 3. results since real-life systems are classified as nonlinear systems, the maximum value of the standard deviation is taken as 𝜎 = 0.15 m. to simplify the calculations, the examined system was linearized and considered as a quasilinear system. the dynamic intensity of the shock wave was calculated according to rice’s formula: 𝑈𝑈(𝑡𝑡) = 𝑇𝑇(𝑡𝑡)b− ':0123 :;0(0) c (31) where: 𝑇(𝑡) is the value of the effective period at time 𝑡, 1/s; 𝜎���� is the maximum standard deviation; and 𝐷�(𝑡) represents variance. a conditional risk of destruction (𝐻(𝑡)) was determined by using the following formula: 𝐻𝐻(𝑡𝑡) = 1−𝑒𝑒𝑒𝑒𝑒𝑒)−∫ 𝑈𝑈(𝑡𝑡)𝑑𝑑𝑡𝑡 0 > + (32) the results from solving equations (31) and (32) are depicted in figures 3 and 4, respectively. figure 3. shock-wave intensity u(t) against time t. figure 4. conditional destruction risk curve. seismic assessment of high-rise buildings 9 when calculating the total risk of failure of the system, consideration was given to the estimate of seismic hazard of the territory, which obeys the poisson’s law: 𝐻𝐻(𝑡𝑡) = [1−𝑒𝑒𝑒𝑒𝑒𝑒(−𝛥𝛥𝛥𝛥)] (33) note that there the intensity of seismic activity is likely to exceed the set level at least once. the overall probability of failure under seismic load is determined by this formula: 𝑃𝑃(𝑡𝑡) = 1−𝑒𝑒𝑒𝑒𝑒𝑒)−∫ 𝑈𝑈(𝑡𝑡)𝑑𝑑𝑡𝑡 0 > +[1−𝑒𝑒𝑒𝑒𝑒𝑒(−𝛥𝛥𝛥𝛥)] (34) theoretically, in the sichuan province, the probability of earthquakes of 8 magnitude and above is 5% with a frequency of recurrence of 3900 ± 400 yrs, p ≤ 0.001. the results of the reliability assessment of buildings, the service life of which is projected to be 100 years, are presented in table 1. variable point of time 0 1 2 4 6 8 10 intensity, 𝑈(𝑡) 0 0.003 0.028 0.525 0.377 0.025 0.011 conditional probability of failure, 𝐻(𝑡) 0 0.005 0.015 0.59 0.825 0.831 0.833 overall probability of failure, 𝑃(𝑡) 0 0 0.002 0.067 0.088 0.08 0.079 table 1. seismic reliability of a building at different points of time, s. the results suggest that the study area is subjected to a substantial seismic risk, which is determined by the likelihood of damage to buildings. the conditional probability of failure of a building at the time a standard is reached was up to 59%. at the same time, the overall probability of failure was 6.7%. the seismic reliability index was calculated using this formula: 𝑛𝑛 = /−2𝑙𝑙𝑛𝑛n− ?4 @a(+bc∗) ) o (35) where: 𝑇� is the effective period of the system, 1/s; 𝑃* is the conditional risk level (for the sichuan province, it is equal to 0.05); and 𝜏 is the duration of the seismic activity. the theoretical value of the seismic reliability index was 3.2 at time t = 5 s with the standard deviation of 2% (figure 5). conditional accelerations for a given frequency of the carrier wave(𝜔(𝑡)) were calculated using this equation: 𝑎𝑎∗(𝜏𝜏) = 𝜔𝜔e: (𝑡𝑡)𝜎𝜎f(𝑡𝑡) (36) the maximum shock-wave acceleration at time t = 5 s was 𝑎* = 4.195 m/s. under these conditions, the amplification factor was equal to 5.147. the probability of failure of a building was 59% with the maximum theoretical acceleration of the shock wave of 2.8%. xiuwen wu and daoyun lin 10 figure 5. changes over time in the seismic reliability of a building at the conditional risk level of 𝑃* = 0.05; 𝑃* = 0.1 and 𝑃* = 0.3. 4. discussion the problem of seismic activity is important for many countries in the world. in particular, strong earthquakes occur in italy, indonesia, japan, armenia, etc. the use of seismic risk prediction models is important for such areas. the use of such techniques is quite common in worldwide practice [contin, (2021); kumar et al., 2021]. the results of this study show that in order to achieve the probability of system failure of 5%, the seismic load should be increased by 3.03 times. at the same time, a 30% failure probability is associated with the ultimate value of seismic load should be increased by 2.526 times. these findings coincide with previous research [bollano et al., 2021]. the present calculations show that high-rise buildings are more likely to collapse during the oscillatory phase of an earthquake. according to the literature evidence, however, theoretical stress on loaded structures is greater than that from vibrations. therefore, it is possible that results from the standard calculation can be lower than those from the strength calculations [muscolino et al., 2021]. at the moment, there are no documents standardizing the conditional risk of destruction for high-rise buildings under seismic conditions; therefore, there is a substantial variation in its value between countries [sabadash et al., 2020]. in this study, the conditional risk value was equal to 0.05. the present results suggest that a building may fail either on the first impact or afterwards, during the subsequent oscillatory period. to avoid this, recommendations are given on strengthening or replacing the weak structures. investigating the impact of kinematic excitation on the wall of a three-dimensional building [taraszkiewicz et al., 2021], researchers carried out a series of numerical tests, which made it possible to determine the sensitivity of nonlinear dynamic systems to changes in properties of the medium and density of discretization, obtained with the finite element method (fem). the study was performed on a 2d model of a frame-and-panel wall and a 3d model of a low-rise building, which were exposed to loads (kinematic input data) at time intervals consistent with earthquake accelerograms. the construction materials (masonry and concrete) were described using the barcelona plastic damage model [taraszkiewicz et al., 2021]. the advantage of this model is that it allows depicting the real properties of geomaterials, as well as considering material failure under cyclic loading. the disadvantage of this model is that when applied to integrate motion equations, it requires more computational studies, as the density of discretization, obtained with fem, can affect the accuracy of a solution. 5. conclusions this paper presents a strategy for the probabilistic seismic (or paraseismic) performance assessment of highrise buildings. the linearization of the mathematical model was performed with the help of canonical expansions. the results of seismic load modeling are presented as amplitude-frequency characteristics. the nonlinear system was characterized using a function of spectral frequencies and dynamic characteristics of the system. the maxseismic assessment of high-rise buildings 11 imum value of the amplification factor (at time t = 5 s) was 5.147. the values of these variables were adequate: standard deviation, sse = 0.5; 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doi: 10.4401/ag-5458 abstract the influence of solar activity on magnetic measurements taken during the italian repeat station surveys performed in 1999/2000 and 2009/2010 has been investigated. a method to estimate the difference between the 2000.0 and 2010.0 surveys in terms of the "residual" (i.e. not completely reduced) external contribution is proposed. this method is also based on the removal of the contribution due to the magnetic field of internal origin by means of chaos3 model from magnetic repeat station measurements. the origin of the observed differences is interpreted in terms of the very different level of solar activity between 2000.0 and 2010.0. indeed, results seem to suggest that differences could be attributed to the enhanced ring current intensity during a phase of solar maximum. the investigation of the spatial patterns of these differences suggests that they could be reduced by introducing a larger number of variometer stations, especially during surveys performed under conditions of high solar activity. 1. introduction as many other countries worldwide, italy has a long standing tradition in magnetic repeat station surveys, and since 1979 measurements have been repeated regularly every five years at about a hundred points distributed quite uniformly on the italian territory [molina et al. 1985, meloni et al. 1988, meloni et al. 1994, coticchia et al. 2001, dominici et al. 2007, dominici et al. 2012]. these measurements allow the compilation of magnetic charts that are generally used for navigational purposes, and also representing and modeling the geomagnetic field and its secular variation on the regional scale [de santis et al. 2003] with a detail that would not be achievable only using data from the two italian magnetic observatories of l'aquila (aqu) and castello tesino (cts). moreover, the collection of magnetic repeat station data from many countries makes it possible to model, for instance, time-varying magnetic anomalies over broad regions as proposed by korte and haak [2000] for the case of europe. when measuring the magnetic field at the earth's surface, the instruments record the superposition, at the time of the measurement, of the magnetic fields produced by different sources. these sources, which differ in nature and location, also widely differ in magnitude and spatio-temporal behavior. it is known that a magnetic measurement on the ground includes essentially two magnetic fields: one of internal origin and the other of external origin. the magnetic field of internal origin is the sum of the main and crustal fields. the main field, which is produced within the core through a self-sustaining dynamo process, is the most intense and changes on secular timescales. the crustal field is weaker, on average, and it is mainly due to induced magnetization in the crust by the main magnetic field. induced magnetization results in magnetic anomalies, which to our purposes can be considered practically constant in time, as shown by thébault et al. [2009]. as already stated, the earth's magnetic field arises also from sources outside the earth. these external fields, which result from the interplay between the magnetic field of the earth and that of the sun, are mainly produced by the electric currents flowing in the ionosphere and magnetosphere. these are highly variable: some changing on the timescale of seconds, others on the timescale of days or longer. at ground observatories the typical amplitudes of the disturbance resulting from external fields range between a few and some hundred nt. furthermore, the interaction between temporal fluctuations of the earth's external magnetic field and electrically conductive rocks of the earth generates internal (crust and upper mantle) secondary electric and magnetic fields. the resulting magnetic fields are the so-called electromagnetically induced fields. unfortunately, magnetometers can measure only the field resulting from the superposition of the different contributions, so if we are interested in obtaining an accurate estimation of secular variation using data coming from magnetic repeat station surveys, it is consequently important to remove all external fields from data. to obtain repeat station data the least affected by external fields, a number of recommendations should be followed, both in the execution of measurements and also in their postprocessing [newitt et al. 1996]. however, for a series of practical reasons not all of these recommendations can be strictly followed and this unavoidably leads to data that could partly include the external fields; this contribution could be related, article history received october 21, 2011; accepted february 28, 2012. subject classification: magnetic repeat stations, solar activity, magnetic models. 1137 magnete workshop 2011 for instance, to the level of solar activity. indeed, the external fields can strongly be affected by the solar wind, a stream of electrically charged particles that is constantly emitted by the sun. under certain conditions of interplanetary magnetic field, the enhanced emission of radiation and charged particles associated with solar phenomena, such as for instance coronal mass ejections and solar flares, increases the ionospheric and magnetospheric currents giving rise to rapidly varying magnetic fields. thus, the contribution of the external magnetic fields to the measured magnetic field is modulated by the eleven-year solar activity cycle. the purpose of this paper is to investigate if measurements of the italian magnetic repeat station surveys can be affected by additional external fields due to enhanced solar activity. the paper is organized as follows: in section 2 we briefly describe the data used and our method of analysis then, in section 3 we illustrate the application of this method to the italian magnetic network data together with a discussion of results. 2. method and data as already mentioned, magnetic repeat station data are meant to represent the field of internal origin (main and crustal fields) and, for this reason, all transient external variations must be removed. we emphasize that the external magnetic fields may give a significant contribution to the measured magnetic field even at mid latitudes where the italian magnetic network is located. indeed, although at mid latitudes we are far away from the equatorial as well as the polar electrojects, the occurrence of geomagnetic storms can considerably influence the terrestrial magnetic field. the characteristic signature of a geomagnetic storm is a depression in the horizontal intensity h of the magnetic field lasting normally several hours to a few days. this depression is caused by the growth of the ring current flowing westward in the magnetosphere, and can be monitored by the dst index. it is generally accepted that storms are temporary disturbances of the geomagnetic field during which dst is less than −50 nt. according to this criterion, 20-50 storm events occur annually, depending on solar activity. indeed, the number and intensity of storms vary with the eleven-year solar activity cycle with about one-year lag. the dependence of the total external field intensity on solar activity level can be observed in figure 1 that shows maps of the difference of the total external component of the magnetic field (magnetospheric and ionospheric, both primary and induced) for x, y and z cartesian components between the epochs 1995.0 and 2000.0 (i.e. close to a period of minimum and maximum, respectively). the values of x, y and z at 1995.0 and 2000.0 used to estimate these differences have been obtained by means of the comprehensive model cm4 [sabaka et al. 2004] that separately models all sources of the earth's magnetic field. from figure 1 we observe that the component most affected by the enhanced solar activity is the north component (x) and that this influence is characterized by a marked latitudinal dependence. this result can be understood taking into account that, as already mentioned, a significant contribution to the magnetic field of external origin is given by the ring current which causes a magnetic field perturbation that at the equator is parallel to the terrestrial dipole magnetic field but directed southward. actually, if ionospheric and magnetospheric contributions were mapped separately, the magnetospheric part would result to be one order of magnitude greater than the ionospheric part. so, what figure 1 exhibits practically coincides with the magnetospheric contribution. a way to remove all transient external variations from tozzi et al. 1138 figure 1. difference in the total external contribution, i.e. magnetospheric and ionospheric primary and induced, between 2000.0 (maximum solar activity) and 1995.0 (minimum solar activity) estimated by means of cm4 model for x, y and z magnetic components. 1139 magnetic repeat stations data is to follow the recommendations suggested by newitt et al. [1996]. these include the proper choice of a site where to install the repeat and variometer stations. for what concerns repeat stations, newitt et al. [1996] describe the proper execution of absolute measurements (for instance, during a time with low daily variation and during magnetically quiet days) and also the proper data reduction using records from the magnetic observatories and installed local variometers. we underline that data used in this study have been collected on points of the italian magnetic network following the main indications suggested by newitt et al. [1996]. in particular, two magnetic observatories and four variometers have been used for data reduction (for the location of variometers and observatories, refer to figure 4). indeed, when a repeat station is far from a magnetic observatory, local variometers data are used to establish the real instantaneous variation of the magnetic field. we will focus on the survey of 2000.0 that was performed, as shown in figure 2, during a phase of maximum activity of the last solar cycle. we recall here that to quantify solar activity the so-called relative sunspot number, also known as the wolf sunspot number, is generally used. in order to quantify the additional external fields, we need to compare data taken during the 2000.0 survey (the survey was performed between september 1999 and october 2000) to repeat station data relative to a period of minimum activity in order to fix a sort of "ground state" for magnetic repeat station data. taking advantage of the peculiar features of solar cycle 23, that was exceptionally long and characterized by a very marked minimum that was reached in december 2008, we chose the 2010.0 survey as term of comparison. this survey was performed between september 2009 and october 2010 and, even if it fell during the ascending phase of solar cycle 24, the average number of sunspots during this time interval is comparable to the number of sunspots at minima of previous solar cycles. in fact, even though solar cycle 23 reached its minimum in december 2008, solar activity remained very quiet throughout 2009 and also in 2010 the sunspot activity has been extraordinarily low [pulkkinen et al. 2011]. it's worth underlining that, despite the availability of data from other previous surveys, we preferred to focus on data from the most recent ones, since they are characterized by a higher accuracy, and we believe they could allow more accurate estimation of the additional external contribution. the two surveys consist of measurements of declination d, inclination i and total field f at 113 and 129 points, respectively. d, i and f have been converted to x, y and z components to better interpret results. considering data only from points reoccupied in both surveys, the final size of the magnetic network here considered is of 108 stations, that are quite uniformly distributed on the italian territory with a mean distance of about 60 km. this value of repeat stations spacing guarantees that phenomena such as piezomagnetic effects – typical of tectonically active areas characterized by high magnetic anomalies, as much of the italian territory is [caratori tontini et al. 2004] – and electromagnetic induction effects due to the seas surrounding italy do not significantly distort the true secular variation [newitt et al. 1996]. data used for this study are already reduced for the daily variation recorded at the nearest magnetic observatory or variometer; so they are reduced at 02 ut of the day of measurement. if measurements have been correctly performed and data properly reduced, theoretically the considered datasets are expected not to contain a significant external contribution (the ionospheric effect having been removed by reduction at observatory/variometer, and magnetospheric effect having been avoided by taking measurements during magnetically quiet days) and to be representative of the main and crustal fields. in order to check for the presence of a residual solar activity and repeat station surveys figure 2. values of sunspots number (ssn) between 1961.0 and 2011.0. lilac areas indicate the time intervals during which magnetic surveys of 2000.0 and 2010.0 have been undertaken, the blue horizontal dashed line indicate the average number of sunspots observed between september 2009 and october 2010. external contribution in these datasets, we first need to remove the internal field by means of a global model as igrf [finlay et al. 2010] or chaos3 model [olsen et al. 2010]. some tests, made to verify which of them could better represent the magnetic field on the italian territory, have shown that they are practically equivalent and that the choice of the model does not substantially change the results of this study. however, since chaos3 model estimates secular variation on italy slightly better than igrf, all estimations have been made using chaos3. after all, with respect to igrf, chaos3 model is characterized by a higher resolution both in time and space. while igrf consists of a spherical harmonics expansion up to degree 13 and with a time resolution of five years, chaos3 model is characterized by an expansion up to degree 60 and by a time resolution of 6 months, that is the spacing between cubic splines knots [olsen et al. 2010]. so, we estimated at the location of each of the 108 selected repeat stations, values of x, y and z components from chaos3 model at epochs 2000.0 and 2010.0. then, at each point and for each component, chaos3 value has been subtracted from the corresponding repeat station data for both 2000.0 and 2010.0 epoch. in practice, at epoch t and for each field component, we could define a residual rt as the difference between the value of the magnetic field measured during the survey at the repeat station (rs) with coordinates i and z and the value of the magnetic field modeled by chaos3 at the point with the same coordinates as: we expect that maps of the residuals rt (i,z) obtained for a given epoch represent the sum of the crustal field that is not modeled by chaos3 and the part of the external contribution that it has not been possible to reduce. let now consider, for each value of i and z, the difference dr (i,z) defined as follows: assuming that the portion of the crustal field not modeled by chaos3 that is still contained in the data does not change in time, we can think of dr as representing the difference between the residual external contribution between 2000.0 and 2010.0. we recall here that the time interval covered by the 2010.0 magnetic survey has been a period of very low solar activity characterized by an average number of sunspots equal to 13 and that this value is comparable to the number of sunspots of other solar minima. on this basis it is reasonable to assume the survey of 2010.0 as practically not influenced by the external contribution. at the light of these considerations, the quantity dr (i,z) can be interpreted as the additional external contribution, namely the ionospheric and magnetospheric fields (primary and induced), due to conditions of high solar activity, that affected the 2000.0 survey. 3. results and conclusions following the procedure described above, the spatial distribution of dr (i,z) has been estimated for all cartesian components of the geomagnetic field. then, values of dr (i,z) over the 108 selected repeat stations have been interpolated using the method of gridding with continuous curvature splines in tension [smith and wessel 1990] to obtain maps shown in figure 3. the choice of this particular interpolation method is motivated by its ability to eliminate fictitious tozzi et al. 1140 figure 3. maps of dr for x, y and z components. the average value, estimated over all stations, of dr is of about −16 nt, 2 nt and 16 nt for x, y and z, respectively. black dots indicate the position of magnetic repeat stations. ( , ) ( , ) chaos3 ( , ) .r rst t ti z i z i z= ( , ) ( , ) ( , ),r r r. .2000 0 2010 0i z i z i zd = 1141 inflection points typical of other methods as that of minimum curvature. from figure 3 we observe that the algebraic sign of the residual dr relative to the x component well agrees with the direction of the magnetic field associated to the flow of the ring current. actually, a decrease of the horizontal component h corresponds to an enhancement of the ring current intensity. due to the geometry of the ring current and to the features of the present main field, over the area under investigation, this effect is suffered mainly from the x component as also shown by figure 1. according to figure 3, the residual dr relative to the x component takes only negative values whose average over all repeat stations gives −16 nt. this average is of the same order of magnitude of the analogous mean value found by means of cm4 (−25 nt) but its lower value could be related to the effect of data reduction applied on magnetic measurements, but also to differences inherent to the two datasets. the average found using cm4 has been estimated from 1995.0 and 2000.0 annual means, while the average of the residual dr relative to the x component has been estimated using data for the days of observation and for 2000.0 and 2010.0 epochs. of course it is not possible to distinguish between the primary and induced residual external contribution. the residual relative to the y component is characterized by a very low value of the average over all repeat stations (of about 2 nt). this is not surprising since, as already mentioned, magnetospheric currents are, in general, responsible of a magnetic field in the direction of the h component that, over italy, practically coincides with the direction of the x component. differently, the distribution of residuals relative to the z component is a bit more complex in space and could be interpreted in terms of the presence of discontinuities in the electrical conductivity of the crust and upper mantle that are responsible for electromagnetically induced magnetic fields. at this point, it would be interesting to understand how to improve the procedure used to remove the external contribution from magnetic repeat station data in order to more and more increase the accuracy of next magnetic repeat surveys, especially those performed under conditions of high solar activity. a first step is to understand if the highest values of the residual dr observed in the x component are closely related to the intensity of the ring current at the time of the measure. to answer this question we drawn a map of differences of the daily dst index (left side of figure 4), where the dst index is a measure of the strength of the equatorial ring current. in practice, at each repeat station it has been associated the difference between the daily dst index of the day when that repeat station was occupied during the 2000.0 survey and the dst index of the day when the same repeat station was occupied during the 2010.0 survey. so, left side of figure 4 represents the different conditions of ring current intensity under which magnetic repeat stations have been reoccupied in the considered surveys. since dst is estimated from the horizontal component h, on the right side of figure 4 we reported a map, analogous to those reported in figure 3, but this time relative to the h component. from figure 4 we notice that there are regions very little affected by the residual external field despite measurements of the 2000.0 survey have been taken under conditions of magnetic activity worse than those of the 2010.0 survey (high values of the difference of dst). one of these regions is, for instance, the north-eastern part of italy, near the observatory of castello tesino (11.65˚e, 46.05˚n). also the areas nearby the solar activity and repeat station surveys figure 4. on the left a map of the difference of daily dst index at the times of 2000.0 and 2010.0 surveys, on the right the map of dr for the horizontal component h. black dots indicate the position of magnetic repeat stations, red dots indicate the position of magnetic observatories and variometers. dashed circles have a radius of 230 km, while solid circles have a radius of 150 km. observatory of l'aquila (13.317˚e, 42.383˚n) and the variometer of brindisi (17.922˚e, 40.673˚n) are characterized by a low value of the residual external field, despite measurement in 2000.0 have been made under less favorable conditions of solar activity with respect to those made in 2010.0. the only area where this discussion does not hold is that between 14˚e and 17˚e of longitude and 40˚n and 42˚n of latitude. measurements in this region have been made in 2000.0 and 2010.0 under similar conditions of ring current intensity as revealed by the very low value, in the same area, of the dst differences. nonetheless, measurements of 2000.0 survey seem to contain a much higher residual external contribution than 2010.0 survey. this could be interpreted in terms of the origin of the external contribution. in fact, by selecting dst index we are evaluating external contributions related mainly to the ring current and in minor part also to other large scale currents (ionospheric, fieldaligned and tail currents) [campbell 1996]. however, since dst is evaluated from magnetic observatories close to the magnetic equator, it might not represent the whole external contribution at the latitudes of the italian magnetic network. so, the anomalous external contribution measured between 14˚e and 17˚e of longitude and 40˚n and 42˚n of latitude during the 2010.0 survey could be related to phenomena, either ionospheric or magnetospheric, whose effects are not included in the dst index. a completely different interpretation could be given in terms of long-term magnetic effects that can be observed near active faults and volcanoes [johnston 1989]. actually, from the analysis of a large gps dataset, it seems that the surface of the area we are referring to, undergoes a complex deformation pattern, due to the fact that it is crossed by the adriatic-ionian thrust, also related to a possible upwelling of mantle flow [devoti et al. 2011]. in any case, by a more careful inspection of figure 4 we observe that the regions characterized by the highest residual external contribution are those in the proximity of the boundary of the red dashed circles. these circles are centered on the magnetic observatory/variometer used for data reduction and have a radius of 230 km. regions nearby magnetic observatories and variometers are almost all characterized by low external residual contribution. this evidences the importance of choosing the distance between a repeat station and a variometer so as to guarantee the validity of the assumption that the magnetic variation at the repeat station and at the variometer can be assumed to be the same. this means that especially under magnetically perturbed conditions, that are frequent during a period of high solar activity, the use of a larger number of variometers would be preferable. the maximum distance between the repeat stations and the variometer should be no more than 100-150 km. indeed, the areas inside the small red solid circles, whose radius is 150 km, are all characterized by low external residual. actually, newitt et al. [1996] do suggest that the variometer should be located within a few hundred meters from the repeat station, however this is not always feasible, so it is common to accept distances of more than a hundred kilometers between a repeat station and a variometer. the value of 100150 km for the maximum distance between repeat stations and the variometer is, of course, only a rough indication. in fact, it is difficult to specify a unique value for the validity distance, since the features of electromagnetically induced magnetic fields strongly depends on the internal electrical conductivity structure. large values of this distance can be confidently used wherever the electrical conductivity of the crust is sufficiently homogeneous. for instance, this is not the case of the northern apennines, in particular the area approximately between 11˚e and 14˚e of longitude and 42˚n and 43˚n of latitude. here armadillo et al. [2001] found, at the transition between the tyrrhenian and the adriatic domains, the existence of a sudden discontinuity in the deep conductive layer that underlies the apennine chain at about 20 km depth. in any case, an interesting attempt to reduce repeat station measurements to quiet night time levels without installing variometers, as could be the case of remote areas, has been described by matzka et al. [2009]. in this case, data reduction has been performed using cm4 model to reconstruct the ionospheric and magnetospheric contribution. acknowledgements. the authors are grateful to all the people of the istituto nazionale di geofisica e vulcanologia that have been involved in the magnetic surveys investigated in this study, and to two anonymous referees for their useful suggestions. figures 1, 3 and 4 were drawn using the generic mapping tools [wessel and smith 1998]. references armadillo, a., e. bozzo, v. cerv, a. de santis, d. di mauro, m. gambetta, a. meloni, j. pek and f. speranza (2001). geomagnetic depth sounding in the northern apennines (italy), earth planets space, 53, 385-396. campbell, w.h., (1996). geomagnetic storms, the dst ringcurrent myth, and lognormal distribution, j. atmos. terr. phys, 58, 1171-1187. caratori tontini, f., p. stefanelli, i. giori, o. faggioni and c. carmisciano (2004). the revised aeromagnetic anomaly map of italy, annals of geophysics, 47 (5), 1547-1555. coticchia, a., a. de santis, a. di ponzio, g. dominici, a. meloni, m. pierozzi and m. sperti (2001). italian magnetic network and geomagnetic field maps of italy at 2000.0, bollettino di geodesia e scienze affini, 60, 261-291. de santis, a., l.r. gaya-piqué, g. dominici, a. meloni, j.m. torta and r. tozzi (2003). italian geomagnetic reference field itgrf: update for 2000 and secular variation model up to 2005 by autoregressive forecasting, annals of geophysics, 46 (3), 491-500. devoti, r., a. esposito, g. pietrantonio, a.r. pisani and f. tozzi et al. 1142 1143 riguzzi (2011). evidence of large scale deformations patterns from gps data in the italian subduction boundary, earth planet. sci. lett., 311, 230-241. dominici, g., a. meloni, m. miconi, m. pierozzi and m. sperti (2007). italian magnetic network and geomagnetic field maps of italy at year 2005.0, bollettino di geodesia e scienze affini, 66, 25-47. dominici, g., a. meloni, a. di ponzio and m. miconi (2012). italian magnetic network and magnetic reference fields at 2010.0, annals of geophysics, 55 (6); doi:10.4401/ag5411. finlay, c.c., s. maus, c.d. beggan, t.n. bondar, a. chambodut, a. chulliat, v.p. golovkov, b. hamilton, m. hamoudi, r. holme, g. hulot, w. kuang, b. langlais, v. lesur, f.j. lowes, h. luhr, s. macmillan, m. mandea, s. mclean, c. manoj, m. menvielle, i. michaelis, n. olsen, j. rauberg, m. rother, t.j. sabaka, a. tangborn, l. toffner-clausen, e. thebault, a.w.p. thomson, i. wardinski, z. wei and t.i zvereva (2010). international geomagnetic reference field: the eleventh generation, geophys. j. int., 183, 1216-1230. johnston, m.j.s. 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curvature splines in tension, geophysics, 55, 293-305. thébault, e., k. hemant, g. hulot and n. olsen (2009). on the geographical distribution of induced time-varying crustal magnetic fields, geophys. res. lett., 36, l01307; doi:10.1029/2008gl036416. wessel, p., and w.h.f. smith (1998). new, improved version of generic mapping tools released, eos trans. amer. geophys. u., 79, 579. *corresponding author: roberta tozzi, istituto nazionale di geofisica e vulcanologia, roma, italy; email: roberta.tozzi@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. solar activity and repeat station surveys << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 56, 6, 2013, s0672; doi:10.4401/ag-6276 s0672 recognition of pleistocene marine terraces in the southwest of portugal (iberian peninsula): evidences of regional quaternary uplift paula m. figueiredo1,2,*, joão cabral1,2, thomas k. rockwell3 1 instituto dom luiz, idl, lisbon, portugal 2 lisbon university, science faculty, geology department, lisbon, portugal 3 san diego state university, geological sciences department, san diego, ca, united states abstract southwest mainland portugal is located close to the eurasia-nubia plate boundary and is characterized by moderate seismicity, although strong events have occurred as in 1755 (mw≥8), 1969, (mw 7.9), and more recently in 2007 (mw 5.9) and 2009 (mw 5.5), all located in the offshore. no historical earthquakes with onshore rupture are known for this region. at the coastline, high sea cliffs, incised drainages, emergent marine abrasion platforms and paleo sea cliffs indicate that this region is undergoing uplift, although no morphological features were found that could be unequivocally associated with the 1755 mega earthquake. to better understand the recent tectonic activity in this sector of iberia, it is necessary not only to analyze active structures on land, but also to search for evidence for deformation that may relate to inferred offshore active structures. we thus conducted a study of marine terraces along the coastline to identify regional vertical crustal motions. several poorly preserved surfaces with thin sedimentary deposits, comprising old beach sediments, were recognized at elevations starting at 2 m elevation and rising inland up to a regional abrasion platform situated at about 120 m a.s.l.. we identified distinct paleo sea level references at several locations at consistent elevations. this terrace sequence is likely late pleistocene in age, with individual platforms correlative to mis 5 high stands and is coherent with a long-term slow uplift of the littoral zone for the southwest of portugal. although dating of discrete platforms is an ongoing and difficult task, preliminary correlations of paleo-shoreline elevations suggest that the uplift rate is in the range of 0.1-0.2 mm/yr. 1. introduction the littoral section of southwest portugal corresponds to the onshore area located close to the main known regional offshore active structures and also to the inferred source area of the 1755 earthquake mw ≥ 8 (estimated mw 8.7 by johnston [1996], or 8.5±0.3, updated by martínez solares and lópez arroyo [2004]. vertical movements for the southwestern portuguese coastal region were previously recognized as the result of a pliocene-pleistocene long term uplift [feio 1951], and the first attempt to quantify the vertical rate was conducted by cabral [1995] who estimated it to be in the range of 0.1 to 0.2 mm/yr, using an assumed pliocene or pleistocene age for the displaced geological markers. however, the ages of sediments were not fully constrained nor were any late pleistocene interglacial marine terraces recognized, meaning that there was not enough control on the variability of vertical motions through time. furthermore, no relationship was established between the observed vertical motions and active crustal structures. in what concerns the last major regional tectonic event, the 1755 earthquake, there aren’t any references of littoral vertical motion associated with the earthquake in the historical accounts for the studied area, so it is unknown if there were any co-seismic effects, and if so which kind. following the same reasoning, no recent vertical motions associated with other large historical earthquakes known for the region (such as 382 a.d.) or paleo-earthquakes are described or were identified. in order to understand the vertical crustal motions in this region, and to better constrain the younger movements, detailed field surveys were conducted along both the western and southern coastlines for recognizing and mapping paleo sea level features and correlative beach deposits (marine terraces) that could be associated with specific global euarticle history received december 30, 2012; december 20, 2013. subject classification: marine terraces, coastal geomorphology, mis 5, pleistocene, uplift, active tectonics, portugal. special issue: earthquake geology static high stands (marine isotopic stage mis stages). with this aim, a 60 km-long section along the western coastline and a 40 km-long section along the southern coastline were mapped for terrace remnants and surveyed for precise elevation. two specific high-priority targets were delineated: one was the recognition of geological evidences of recent vertical movements along the coastline that might be associated with historical seismicity, namely with the 1755 earthquake, and the second was the recognition of the last interglacial (mis 5e) marine terrace, which is expected to be locally preserved and emergent, even if the uplift rate is very low, because the terrace was cut at about 6 m above modern sea level. 2. regional setting southwest mainland portugal is located close to the eurasia-nubia plate boundary. this plate boundary can be differentiated into three distinct zones: one, a western segment, starting at the mid-atlantic ridge and characterized by a transtensional wnw-ese fault zone, the terceira leaky transform. this zone continues eastward to a second segment, the gloria fault, a 400 km-long right-lateral transform fault that trends roughly e-w [laughton et al. 1972]. approaching the gorringe bank area, this linear boundary evolves to a third zone, a more complex and diffuse plate boundary [sartori et al. 1994], with deformation being distributed among several structures that span a zone up figueiredo et al. 2 figure 1.main active structures and morphological features in the southwest iberia region according with rosas et al. [2012] and martínezloriente et al. [2013] for the offshore and dias [2001] for the onshore and their localization regarding the eurasia-africa plate boundary. (a): schematic eurasiaafrica plate boundary for the region and localization of figure 1 mar mid atlantic ridge; atjazores triple junction; agfz-azores gloria fault zone; sim-southwest iberian margin adapted from duarte et al. [2013], (b) off-shore structures identified in the map; inland structures: ap-alentejo-placencia fault; stasfs-são teotónio-aljezur-sinceira fault system; bsj-barão de são joão fault; eo-espiche-odiáxere fault; p-portimão fault; smq-são marcos-quarteira fault. the area studied in this work is delimited by the dashed line adapted from rosas et al. [2012]. 3 to a few hundred kilometres in width at the gulf of cadiz (figure 1). several geophysical and geological studies have been undertaken during the past two decades to better understand this area of diffuse deformation, and the primary tectonic elements of this region [e.g. bartolomé et al. 2012, gràcia et al. 2003a, 2003b, gutscher et al. 2012, terrinha et al. 2003, 2009, zitellini et al. 2001, 2004, 2009]. this has allowed the recognition of previously unknown relevant active structures, with some of them showing significant morphological expression, such as the marquês de pombal and horseshoe thrust faults, each with conspicuous scarps that trend nne-ssw to ne-sw. structures in the gulf of cadiz trending e-w, such as those bounding the portimão bank (also named in some references as gualdaquilvir bank), and trending wnw-ese named the southwest iberia margin (swim) faults, which are expressed as very long lineaments in the sea floor and interpreted as strike slip faults [zittellini et al. 2009, duarte et al. 2011, bartolomé et al. 2012]. more recently martínez-loriente et al. [2013] demonstrated that south of the swim faults, in the coral patch ridge and seine hills areas at the south gulf of cadiz region, active ne-sw thrusts (such as north coral patch ridge and south coral ridge faults, seine hill 1 and 6 faults) and wnw-ese dextral strike slip faults (such as lineament south and strike slip 1 faults) deform the holocene sedimentary unit and are therefore active structures. the structural complexity in the gulf of cadiz area is the result of the long term interaction between iberia and nubia, where structures with different geometries and tectonic styles, mainly thrust and strikeslip, have interacted at different scales through time [duarte et al. 2011, duarte et al. 2013, martínez-loriente et al. 2013]. geodetic data along with analysis of focal mechanisms indicate ongoing wnw-ese iberianubia convergence at a rate of 4-5 mm/yr [fernandes et al. 2007, nocquet and calais 2004, serpelloni et al. 2007, stich et al. 2006], while geological models for the last 3 ma point to a nw-se convergence at roughly the same velocity [demets et al. 1994, 2010]. ribeiro [2002] suggested the occurrence of sinistral rotation of the maximum horizontal compressive stress in southwest iberia since the pleistocene, although changes in the on-going deformation in this region are a matter of debate [cunha et al. 2012]. the southwest portuguese margin is located in the northwest sector of the gulf of cadiz, and is characterized by nne-ssw to ne-sw striking thrust systems, such as the previously referred marquês de pombal, horseshoe and gorringe bank faults. some of these structures in this sector extend onshore, such as alentejo-placencia fault system [pereira and alves 2013] where a minor sinistral strike-slip component has been recognized offshore and the são teotónio-aljezur-sinceira fault system [terrinha 1998, lopes 2002], where a sinistral component was also recognized but inland. the regional instrumental seismicity corroborates the occurrence of diffuse deformation: seismicity is moderate, frequently shallower than 40 km and broadly distributed and focal mechanisms are dominantly thrust and strike-slip [carrilho et al. 2004, carrilho 2005, stich et al. 2010]. an ocean bottom seismometer (obs) campaign undertaken in the study region for 11 months (from august 2007 to july 2008) showed that seismicity extends deeper than 40 km, concentrating in this time period at a depth of 50-55 km [geissler et al. 2010], corresponding to three major clusters of earthquakes that occurred in areas coinciding with the location of the three larger instrumental earthquakes for the region [silva et al. 2010]: the 28th february 1969 (mw~7.9), the 12th february 2007 (mw 5.9), and the 17th december 2009 (mw 5.5) events. the focal mechanisms calculated for the three major earthquake clusters and for the obs survey also reveal different deformation behaviours, indicating thrusting at the marquês de pombal fault region, and strike slip associated with the swim fault system in the horseshoe plain [bartolomé et al. 2012], consistent with the location and focal mechanism of the mw 5.9, 2007 earthquake [custódio et al. 2012]. despite the tendency for the occurrence of moderate levels of seismicity, large earthquakes were generated in this area, such as the november 1st 1755 earthquake and tsunami and the mw 7.9 1969 earthquake [zittelini et al. 2009 and references within]. large historical and paleo-earthquakes generated in the gulf of cadiz were also recognized by the identification of correlative tsunami deposits [baptista et al. 2009] and turbidites [gràcia et al. 2010], suggesting a 1300-1800 yr recurrence interval for a regional earthquake with magnitude larger than 8. the marquês de pombal fault is usually considered one of the best candidates for the generation of the 1755 tsunami [baptista et al. 2009], and is therefore assumed to have ruptured in 1755 together with other faults, thereby producing a complex rupture. if so, the 1755 mw > 8 event is likely closely associated with the southwest portuguese continental margin. several faults with evidence of pliocene and quaternary activity have been previously recognized onshore southwest portugal [feio 1951, pereira 1990, cabral 1995, dias 2001, dias and cabral 2002], mainly through geomorphologic analyses. the majority of the sw portugal pleistocene marine terraces active structures in this region trend nne-ssw to ene -wsw, though n-s to nw-se faults are also present (barão de são joão, espiche and portimão faults), are mostly less than 20 km long, and show predominantly reverse and strike-slip component. historical seismicity is known for the past 2000 years (due to roman and arab occupations) [brito 1597, lopes 1842], however no holocene fault surface rupture was ever recognized. the active faults that are recognized have low activity rates, less than 0.1 mm/yr [dias 2001, dias and cabral 2002], and therefore long periods of seismic quiescence are expected. instrumental seismicity is usually of low magnitude, diffuse and hard to correlate with the known quaternary structures [carrilho et al. 2004, carrilho 2005]. the most representative of such active structures in the study region is the nne-ssw, sub-vertical, 50km long são teotónio-aljezur-sinceira left-lateral fault system (stasfs). this fault zone extends north towards the southwestern section of the alentejo-plasencia fault [cabral 1995, villamor et al. 2012], though their spatial and temporal relationships are still not clear from the available data. stasfs trends parallel to the western coast, corresponding to the nearest known inland active brittle structure that may be related to the ongoing plate boundary deformation observed in the offshore. this fault zone deforms a ca.10 km wide regional abrasion platform that is considered to be originally of late miocene age, and which has been reoccupied or recut during the pliocene and the pleistocene, implying that the capping sediments are of quaternary age. dias [2001] estimated a slip-rate of ca. 0.03-0.06 mm/yr for this fault zone, based on the vertical offset of morphological features. however, considering that the sense of slip on this structure is probably mainly strike-slip, those vertical separation values are likely a minimum estimate. some authors suggest that the stasfs extends to the offshore, towards the south or southwest [lopes 2002, terrinha 1998, roque 2007], but there is no unequivocal evidence to correlate the inland fault system with the morphological features inferred as the fault extension in the offshore. detailed geomorphologic and paleoseismological studies along the stasfs[figueiredo et al. 2011, figueiredo et al. in preparation] indicate that its southern sector (south of sinceira basin) does not show evidence of significant plio-pleistocene deformation, the fault system probably having splayed into several minor faults. in fact, several parallel faults do exist in the local mesozoic limestone bedrock, promoting the development of karst pits, which are filled with pliopleistocene sands (faro-quarteira sands) along the coastal section (martinhal-zavial) where the stasfs should intersect the coastline. the spatial distribution and elongated shape of those karst features suggest a structural control by a previous structural fabric on the karst development [dias and cabral 2002]. 3. the southwest portugal coastal geomorphology the southwestern portuguese littoral can be subdivided in two distinct coastlines types: the first is the nne-ssw oriented, west facing coastline which is directly exposed to the atlantic swells, whereas the second is the ene-wsw trending southern coast of portugal (figure 2). the n-s section of coast is exemplified by steep cliffs developed in highly deformed paleozoic schist and greywacke bedrock. the modern abrasion platform is narrow, around 300 m in width based on topographic and bathymetric maps (i.g.e.o.e. and i.h.), and the littoral zone is dominated by a rocky-shore environment, with coarse clastic sediments and thin sandy beaches present at the mouths of the main drainages and in sheltered areas, or along some linear coastal segments that locally reach some kilometers in length, but less than a few tens to a few hundred meters in width. cliffs have steep slopes, and towards the south increase their height, reaching circa 130 m in elevation. several narrow and deeply incised creeks hanging along the cliffs are also recognized. these creeks contrast with larger fluvial drainages that have broad alluvial filled valleys, which are frequent along the portuguese coastline in general and also in this sector. mass movements are common, favoured by the bedrock structure. an exception exists along this coastline in the carrapateira area (figures 2, 7 and 8), where the bedrock is composed of jurassic limestone in the alentejo basin (a n-s elongated mesozoic basin generated from the atlantic extension; pereira and alves 2011). this coastal section, with cliffs reaching only 40 m, is cut on less deformed mesozoic bedrock where karst development has taken place and there are fewer mass movements. cliff retreat along the western coastline varies substantially according to lithology and local structure geometry, with present rates ranging from 2.5 to 11 m/ka in the paleozoic schistose bedrock, to as low as 1 m/ka in the jurassic limestone, based on 50 years of observations [marques 1997]. andrade et al. [2002] have calculated that the average wave energy input for a representative site along this coastline (monte clérigo) reaches 40 to 45 kwm-1/yr, which is significantly higher than at a sheltered location along a south facing coastal zone near lisbon, where a value of only 6 kwm-1/yr was inferred using the same methodology, corroborating that the wave energy input along the sw coastline is significantly high. the approximately e-w oriented southern coastline figueiredo et al. 4 5 is developed on mesozoic limestones in the west and on marls to sandy miocene sediments towards the east. this difference in lithology, and the consequent resulting morphology, lead to an irregular and lower coastline with cliff heights reaching 50-60 m at sagres in the west, and decreasing eastward to heights of about 20-30 m. karstic features are common and sink holes are usually filled with sands. the modern abrasion platform is slightly broader here than along the n-s coastline, also increasing in width eastward. this coastline is sheltered from north and west atlantic swells, but more exposed to the southwest and southeast ones. sandy beaches are common, mostly in sheltered areas in the west, but are continuously present towards the east, where dune systems also exist. historical cliff retreat rates also vary significantly along this coastline, ranging from 0.9 m/ka in the ingrinafurnas sector to about 16 m/ka at boca do riosalema [marques 1997] (figure 2). in the carrapateira area, the limestone promontory in the west coast, coastal morphology exhibits similarities with that along the southern coastline, and retreat rates are similar to the ingrina-furnas sector. inland, the most significant geomorphologic feature corresponds to a broad regional abrasion platform cut across the paleozoic schists. this surface is particularly well represented along the western coast, north of sagres, where it reaches an elevation of 120130 m. capping beach sediments are absent in this sector of the platform, although scattered rounded quartz pebbles are commonly present. no geochronologic information is available for this surface, although the platform has been considered as possibly late miocene in age, and reoccupied and re-trimmed during pliocene and pleistocene times [feio 1951, pereira, 1990, dias, 2001]. it is likely that during the plio-quaternary, an abrasion platform was cut across older miocene sediments, thereby developing a younger erosional abrasion platform, where miocene sediments could be sparsely preserved beneath plio-quaternary capping sediments. the highest surface with cover sediments, assumed to be marine or fluvio-marine, is present at fonte santa (figure 2), located 15 km inland at an elevation of about 350 m. according to feio [1951], these sediments are correlative of a high “summitplain” which corresponds to a remnant of a pliocene planation morphology that surrounds mount monchique, a cretaceous igneous massif that reaches 900 m in elevation and is considered an inselberg. these marine-like sediments are overlain by aeolianites that are strongly consolidated by iron and manganese oxides and considered to be pliocene in age [manuppella 1992], again implying that all lower tersw portugal pleistocene marine terraces figure 2. digital terrain model of southwestern portugal: (a): regional geomorphology with main active structures shaded relief (srtm) and continental shelf bathymetry (0 to 200 m depth, contour lines, each 10 m (ih); (b) digital terrain model from the study area, (igeoe topographic data). red dots indicate marine terrace locations, black dots correspond to toponymy. races have been refreshed or re-cut after this time. towards the southeast, the regional erosion surface transitions to several discrete surfaces cut into the mesozoic sandstone, marl and limestone bedrock of the algarve basin [pereira 1990], which may actually correspond to a sequence of closely spaced, poorly preserved, middle pleistocene marine terraces. this is very similar to the middle pleistocene linda vista terrace set of southern california, which transitions from a very broad (>10 km) abrasion surface at san diego to a sequence of discrete, narrow (50-200 m-wide) marine terraces northward towards los angeles, primarily as a function of bedrock resistance to erosion [kern and rockwell 1992]. 4. identification of references for characterizing vertical movements 4.1 the present reference the present tidal range in portugal is meso-tidal within a 2 to 4 m maximum range, and with the maximum high-tide at circa 2 m m.s.l. for characterization of the elevation of the modern wave-cut platform and associated beach deposits in this region, one must take into account this tidal range, as most terrace cutting likely occurs during storms at high tide. as expected, other features in this area, such as notches and sea caves, will also reflect this tidal range and present larger dimensions [wziatek et al. 2011] than equivalent features that form in regions of micro-tidal ranges, such as in the mediterranean sea, and their mean elevation fluctuates within the tidal range. the coastline is frequently hit by storms generated at long distances in the atlantic [costa et al. 2001], and the wave energy during such storms is commonly quite high, resulting in the formation of storm benches and storm notches. the shoreline angle elevation reflects not only the tidal range, the wave energy impact, and the bedrock resistance, but also the smaller scale coastal physiography (if it is a bay or an exposed cliff) and, therefore, the shoreline angle elevation fluctuates within a range up to a few meters from the mean sea level [wright 1970]. assuming that the resulting effect from all these variables has not changed significantly since the pleistocene, it is therefore crucial to underfigueiredo et al. 6 figure 3 (counterclockwise from down left). the modern abrasion platform is reoccupying an older one at several sites in the southwest portuguese coastline. all of these sites share the same sedimentary sequence, and the paleo-shorelines are at the same elevation. a) perspective at amoreira beach; in the background the abrasion platform is cut into schists. overlying the abrasion platform (p), now reoccupied by the nowadays sea level are basal coarser sediments (bc) and aeolianites (aeol), with a notch (n) developed at the back edge. b) another perspective from amoreira beach, aeolianites (aeol) on top of basal coarser sediments (bc). c) castelejo beach, coarser basal sediment (bc), interbedded with medium to fine sand, overlying an abrasion platform (not visible in this image, covered by modern beach sand) and underlying aeolianites (aeol); d) bordeira beach, basal coarser sediments (bc), underlying several aeolianite (aeol) units. 7 stand the local modern processes involved in the development of a wave-cut abrasion platform, along with the cliff and beach system, to properly interpret the past and correctly interpret the elevations of paleo features. we discuss this further below in the section on field interpretations. as referred above, in this work we will focus on the surfaces and features associated with sea-level that we expect to be the youngest, or attributable to recent vertical movements. secondly, we discuss observations that pertain to a possible uplift or subsidence signal associated with historical seismicity. 4.2 recognition of paleo marine terraces the morphological recognition of paleo marine terraces follows the nomenclature of bradley and griggs [1976]. according to these authors, most platforms can be described as having three different segments: an offshore or outer edge segment with a generally low gradient, a near-shore segment, with an intermediate gradient, and a shoreline segment characterized by a steeper gradient adjacent to the inner edge at the paleo-shoreline. thus, a single marine terrace can exhibit these different surface segments, and distinct morphological elements, all generated with the sea at the same level. this differentiation may vary locally, especially with lithology resistance, wave energy incidence and coastal morphology. in the present study we infer the original elevation of the platform segments and of other morphological elements based upon the elevation of their modern analogues. paleo sea level features related to past sea level high stands, such as raised marine terraces, are present along the western and the southern coastlines, albeit in a discontinuous pattern. in general, the marine terraces or wave-cut platforms are poorly preserved, beach sediments are mostly eroded and, locally, younger aeolianites cover the entire morphology, making it difficult to recognize the terraces and to distinguish them from other features. along the cliffs in the paleozoic schists, lower elevation features that are expected to be correlated with the late interglacial (mis 5) are mostly absent, since they have been mostly or completely removed by the holocene transgression and erosion. in areas of limestone cliffs, which are even more resistant to erosion, paleo sea level features at lower elevations are better represented, although they are still poorly preserved in most areas. to understand the regional marine terrace patterns and to establish a regional sequence, consistent observations from several sites are needed, along with their tentative correlations. we will present a brief description of the mentioned sites, describing each of the recognized surfaces, and discuss their inferred ages later. for detailed geomorphological analyses, topographic surveys were conducted using a dgps-rtk (leica gps1200), with a vertical error smaller than 10 cm. all elevations presented correspond to elevations above mean sea level. 4.2.1 ingrina beach located along the southern coastline (figures 2 and 4), the ingrina beach is located in a small and confined bay, 200 m in width and 300 m in length, cut into jurassic limestone. at this site, several surfaces are interpreted as paleo wave-cut platforms, and are preserved in a sequence along with their respective paleo-sea cliffs. the modern wave-cut platform, t0, is developed at the infra-tidal, tidal, and supra tidal zone, and is mostly recognized in the submarine section and during very low tides. the beach has fine to medium sand, and the inner edge is usually covered by beach and aeolian sediments at an elevation of about 2 to 3 m (white line in figure 4). a bench at 3-4 m elevation corresponds to the modern storm boulder accumulation. at a sheltered section along the eastern side of the bay, paleo sea caves and a possible level of bioerosion, inferred by the presence of round borings that develop perpendicular to the surface and are assumed to be lithophaga borings, occurs at 2.8 to 3.8 m. no correlative sediments are preserved at this elevation. a well-preserved bedrock surface at 6 to 8 m elevation is recognized primarily along the western side of the bay. although this surface is very well preserved, it shows a scarce sedimentary cover. the elevation of the shoreline or inner-edge varies from 6 to 7 m along the southern, ocean-ward section of this surface, and reaches a maximum elevation of 8 m at the cuspate landward back-edge of the bay, similar to the modern shoreline. we refer to this terrace as t1 (blue line in figure 4). another surface cut into the limestone bedrock at circa 12 m elevation is a striking feature at ingrina, and it is visible mainly along small spurs on both sides of the bay. this surface expressed along the spurs is generally flat or shows a low gradient, and smoothly ramps up to a higher elevation farther inside the bay, ranging then between 12 m and 16 m elevation. it has pedogenic carbonate locally capping the abrasion surface along with locally preserved cemented littoral sediments. sandy sediments are scarcely preserved (except where cemented by carbonate), but round quartz pebbles are scattered ubiquitously on the surface. another surface is present at elevations ranging from 16 to 18 m, sw portugal pleistocene marine terraces gently rising to an internal section where an inner-edge is recognized at 20-21 m.along the inner-edge or paleoshoreline, conglomerates of quartz pebbles testify to the presence of the paleo-beach face. no sediments were identified above this level, but scattered quartz pebbles are preserved. we have interpreted these 3 surfaces as morphological segments of a single wave-cut platform. in general, the surface ramps up towards inland as three distinct morphologic sections: a generally flat or lowgradient section near the outer-edge that ranges between about 12 m and 16 m elevation; a moderately sloping middle section at circa 16-18 m elevation, and a higher more steeply sloping surface that ramps up to the inner-edge of the terrace at 21 m elevation. collectively, we designate this sequence as t2, or the second emergent marine terrace at ingrina. the inner-edge, or shoreline angle, which is locally buried by colluvium, reaches maximum elevations of 20-21 m near the shoreward cuspate part of the paleo-bay, similar to the modern shoreline, which reaches its maximum elevation at the shoreward inner-edge of the bay (green line in figure 4). higher surfaces, which correspond to older marine terraces, are also recognized at ingrina, inferred from slope changes at about 26 m, 33 m, 37 m and 45 m, but further work is needed to adequately identify shorelines, characterize their elevations and their degree of preservation. nevertheless, their presence, along with the well-preserved terraces at 8 m and 21 m (figure 9), figueiredo et al. 8 figure 4. map identifying the marine terrace at the ingrina site. a) panoramic view from east and terrace perspective. b) inner edge and terrace map. terraces are mapped in transparent colors and the inner edges in solid line when visible and dashed line when inferred. white-t0 (modern); blue-t1; yellow-t2; green-t3; red-t4. inner edges elevations in meters are provided. topography contours lines each 10 m. 9 and the higher broad abrasion surfaces at elevations higher than 100 m, indicate long-term uplift of a rising coastline. 4.2.2 furnas at furnas, located in the southern coastline 2 km east of ingrina and 10 km to the southeast of castelejo beach (presented in 4.2.3), a narrow raised bench at 2 m elevation is present at the base of a 40 m-high cliff (figure 2 and 5). additionally, at this site, there are two levels of sea caves and associated beach sediments at different elevations that we interpret as additional evidence for the presence of the terrace t1 along the southern coast. of note at this location is that the modern sea cave notch is actively being cut at about 3 m below m.s.l., probably because this is along a prominent section of the coast and inner edges tend to be lower at promontories; and also because locally the rock could be more resistant to erosion due to its hardness. thus, paleo-sea level features need to be interpreted according to this relative reference frame. the raised bench corresponds to a terrace highlighted by a narrow abrasion platform and a sea cave/notch at 2 m, and another sea cave/notch at about 3.8 m elevation (figure 5). both sea caves/notches are filled with cemented coarse marine sediments, with cobles ranging from 5 to tens of centimeters (figure 5b). a cemented sand deposit with marine shells (mussels) is present, covering the coarser sediments, at elevations ranging from 2 m to 4 m. this sand deposit has a depositional shape comparable to a fan, and may be interpreted as a small paleo surge channel. the sands with marine shells indicate a submarine proximity to a paleo beach and a likely paleo intertidal environment presently at the ~4 m elevation. these features are approximately 5-7 m above their modern counterparts, so we infer these to correlate to the t1 terrace surface at ingrina, which is at a comparable elevation above its modern counterpart. remains of beach deposits with small pebbles, shells and cross bedding are also preserved at 8 m elevation (figure 5c) and probably correspond to a paleo beach in the mouth of the paleo surge channel that is present here. for now, we correlate these marine remnants with the highest t1 terrace elements at the ingrina site. in summary, the modern shoreline notch at furnas is located at about 3 m below sea level, and there are elevated notches at 2-4 m, indicating 5-7 m of relative difference for exposed rock erosional features. at the mouth of a small surge channel, littoral depositional features are preserved up to about 8 m. together, we interpret these features to represent different littoral elements of the same sea level high stand and correlate all of these with the t1 marine terrace at ingrina. 4.2.3 castelejo castelejo site is located in the western coastline at the base of a 120 m high cliff (figures 2, 3c and 6). at castelejo beach we only recognized one surface, which was referred by previous authors [pereira 1990, cabral 1995, dias 2001]. this surface corresponds to a wave-cut platform cut into the paleozoic schistose bedrock at 1.5 to 2 m elevation: the modern abrasion platform is therefore essentially reoccupying and retrimming an older surface, as evidenced by the preservation of a wave-cut platform locally capped by sediments. overlying the abrasion platform, there are two conglomeratic layers separated by a thin beach sand layer [figueiredo et al. 2010, 2011] overlain by strongly cemented aeolianites. equivalent remnants of this surface and the capping sediments are also preserved at the amoreira and bordeira beach sites, also along the western coastline, 15 and 30 km farther north, respectively (figure 3). the upper unit, the aeolianite sequence, contains several paleosols and colluvial units (figure 6), and therefore probably represents multiple periods of deposition on top of a paleo marine terrace and against a paleo-cliff during the late pleistocene. the sequence is outcropping as a residual relief located 75 to 100 m seawards of the present active cliff. marques [1997] calculated a maximum cliff retreat of 0.017 m/yr for this specific location based on a 50 year period of observations. this rate is consistent with the average holocene rate of cliff retreat sw portugal pleistocene marine terraces figure 5.marine terrace at furnas site. location is given in figure 2. a) perspective of the terrace: an abrasion platform and an inner edge are recognized at ~2 m elevation; a notch level (~4 m elevation) and sea cave (between ~4 and ~2 m elevation)are also indicated; a small surge channel (sgc) is identified, suggesting that the sand deposit (sd) that covers the coarser deposits is correlated with the beach sand deposit (bs) present at ~8 m elevation. b) detail of beach sand unit; c) detail of sand deposit; d) sea cave filled by coarse deposits. for the last 4500-6000 years, when holocene sea level stabilized. this observation also corroborates that the modern wave-cut platform has retrimmed an older one and has extended it inland. considering ages from other carbonate aeolianites from portuguese coastal areas [moniz 1992, pereira and angelucci 2004, soares et al. 2006, prudêncio et al. 2007], we assume that the consolidated aeolianites are most probably related with the last stages of the würm glaciation (~mis 4-3), or with the last glacial maximum (~mis 2) and consequently, the underlying beach sediments and platform should be older and possibly correlative of the mis 5, most likely the last sub-stage, mis 5a. close to the beach, a small canyon cut into the schistose bedrock is incised in a flat erosional surface at 7-8 m elevation, which we assume to be a fluvial terrace that should be located very close to the mouth of this drainage. the base level correlated with this surface is likely to correspond to the sea level position that has generated t1. we conclude that this fluvial terrace should correspond to the final section of this canyon, located near the shore, and most likely re-activated during the holocene transgression and cliff retreat, and re-incised to its actual position (figure 6c). we consider that the recognized abrasion surface may correspond to a lower (seaward) section of the terrace t1, and the fluvial terrace as secondary evidence for a higher inner edge elevation, and so we tentatively correlate it to the t1 surface at the furnas and ingrina sites. 4.2.4 carrapateira carrapateira is located 10 km north of castelejo beach, and is a promontory cut into mesozoic (mainly jurassic) limestones along the west coast (figure 2). because the cliff retreat rate in the western coast is slower in this area [marques, 1997] than in the rest of the western coast, it is expected to be a reasonably good site to preserve t1 and t2 terraces. plus, it presents lithological and morphological similarities with the southern coast and is therefore the ideal site along the western coastline to compare with the marine terraces identified along the southern coastline. however, because the west coast is more exposed to high energy waves than the southern coast, resulting in a broader zone of wave erosion, t1 along this section is poorly preserved and generally difficult to survey. paleo features, such as raised notches, raised marine sediments, and inner edges were recognized in a discontinuous pattern along a 3 km long section of coast. locally, at the cliff base, remains of small spurs at about 12 m elevation are present that provide additional evidence of a raised abrasion platform. along the south section of carrapateira, the outeredge of a poorly expressed, narrow wave-cut platform with irregular morphology is exposed in a cliff face at 13-14 m elevation. this surface is covered by coarse, round pebbles and sandy beach sediments, confirming its marine origin (figure 7). the irregularity of the wave cut rock surface probably indicates that these are surge channels, similar to those present at many sections of the modern coast. in the same vicinity and at generally similar elevations, paleo-sea caves were also identified. the wave-cut platform ramps up to 17-19 m elevation where a thin layer of beach sediments and cemented aeolianites are also present. notches and sea caves were also recognized at these higher elevations, which generally are larger than the ones at about 14 m elevation. at the northern section of the promontory (figure 8), the 13-14 m surface is substantially eroded, but figueiredo et al. 10 figure 6. marine terraces at the castelejo site. location is given in figure 2. a) perspective of the aeolianites unit which lays on top of a paleo wave-cut platform; b) detail of a colluvium wedge with soil development inter bedded with aeolianites. c) fluvial terrace at 8m; d) sedimentary sequence on top of the paleo platform: 1a-basal coarser unit; 2sandy beach unit; 1bupper coarser unit; 3-aeolianites. figure 7.panoramic view of carrapateira, southern section carrapateira site: a) t2 terrace perspective ; b) notch at 20/21 m elevation c) detail of beach sand with imbricate shells; d) basal section of the sedimentary t2 sequence, laying on top of the wave-cut platform with coarser sediments at 14 m elevation. 11 the 17-19 m surface is very well represented and may reach a width of 20 m to 30 m, likely to correspond to a marine terrace tread area, i.e., to a near-shore zone of marine abrasion. it is locally covered by cemented, gray to white aeolianites. this flat surface evolves into a smooth ramp to a slightly higher section where the inner-edge is well defined at 21 m elevation. we interpret these elements as sections of a marine terrace, and correlate this terrace with the t2 at the ingrina site, which has similar surfaces and an inner-edge at similar elevations (figure 9). other higher surfaces with inneredges at about 26 m and 33 m were also recognized at carrapateira promontory, which culminates in a higher, flat well-formed surface at about 40 m elevation that is capped by paleo and modern aeolianites, similar to what was identified at ingrina site, but as mentioned earlier, they will not be interpreted in this work. as mentioned above, the t1 terrace along the carrapateira site is very difficult to recognize. two benches were identified at elevations of 4 m and 8 m, but both are covered by large boulders that were likely deposited by high energy events (storms and/or tsunamis). however, at bordeira beach, immediately north of carrapateira promontory, cemented beach sediments composed of rounded schist pebbles cover a wave-cut platform cut into the paleozoic bedrock, and are in turn overlain by cemented aeolianites, similarly to what is observed at the castelejo site (figure 3d), so we considered this deposit as likely to be correlative of castelejo terrace and therefore representative of the t1 terrace. this outcrop is usually buried by the modern beach sand and exposed only during extreme storm events. 4.3 historical seismicity and coastal deformation southwest portugal has not experienced any significant earthquakes generated by inland active structures for at least as long as there are historical records, that is, in the past 2000 years. however, this region has been affected by several large earthquakes generated from off-shore sources, the most significant being the 1st november, 1755 event. other relevant historical earthquakes have also been reported for this region, such as the 382 ad [brito 1597] earthquake, referenced to have caused the submersion of three islands offshore of sagres, but descriptions also suggest a macroseismic area extending along all of the mediterranean coast, including palestine, so the reliability of such reports is dubious. there is still no consensus about what structure or structures were the sources for the 1755 earthquake, which was a complex earthquake with three distinct periods of shaking that are considered as sub-events and that possibly reflect a complex rupture pattern with rupture of multiple faults. several structures are considered as plausible major candidates to have ruptured, such as the marquês de pombal, horseshoe or gorringe bank faults. considering their size, geometry, kinematic characteristics and distance to the shore, it is expected that along the coastline the crust would not suffer any significant vertical deformation. however, if some other fault located closer to the shoreline ruptured in this complex earthquake, some vertical movement may actually have occurred along the coastal region. despite all of the detailed descriptions about the 1755 earthquake and resulting effects at many places in southern portugal, there is no specific mention or known description of relative sea level changes along this southwestern shoreline. this may be partially due sw portugal pleistocene marine terraces figure 9. southwest portugal marine terraces topographic section profiles, surveyed with gpsrtk: (a) north carrapateira; (b) south carrapateira; (c) eastern ingrina; (d) western ingrina. t2 inner edges are visible in all profiles at equivalent elevations and t1 inner edges are only visible at ingrina profiles, due to erosion at carrapateira. figure 8.panoramic view of carrapateira, northern section carrapateira site, showing the northern section of the promontory. marine terrace mapping: a) panoramic view and terrace perspective (site where picture was taken marked by “star” at b); b) inner edge and terrace map. for each terrace, the inner edge is mapped, solid line when visible, dashed line when inferred; yellow-t2; green-t3; red-t4. inner edge elevations are provided in meters. t3 and t4 are not discussed in detail in this work. to the fact that within 15 to 20 minutes after the earthquake, the coastal area was hit by a large tsunami, which destroyed all of the anthropogenic structures such as harbors, wharfs and other structures, so that people lost all sea level reference frames. further, the tsunami was so powerful that it generated a large flood and debris field that may have masked any subtle changes. the only relevant report concerns the mention to a harbor at aljezur, located about 5 km inland from the coast, which connected with the ocean through a deep river channel. descriptions from a few years after the earthquake reported a decrease of the navigable depth of the channel [lopes 1842]. however, at the mouth of this river, at amoreira beach, no vertical change was identified in the holocene sea level references (t0 in this work), so that a plausible explanation is that the river may have silted up due to the numerous landslides reported in the region as a result of the earthquake. hindson et al. [1999], based on foraminifera and ostracod assemblages from the sedimentary fill sequence of a fluvial valley at boca do rio, located near the southern coastline (figure 2), inferred an abrupt change from a marine to a brackish environment in this valley at about 1800 to 1300 years ago. they interpreted this change as a consequence of the development of a sand dune barrier, which may have isolated the fluvial valley. however this environmental change may also be the consequence of a minor uplift event, preventing the ocean water from penetrating inland. kortekaas and dawson [2007] in their study of the 1755 tsunami, also studied a sedimentary fill near the coast at sagres, 12 km to the west of boca do rio, and corroborate a change from a marine to a brackish environment, about 1700 years ago. in fact, a large earthquake is inferred for the area in 382 ad, as mentioned above, so an associated vertical deformation is an open hypothesis. kortekaas and dawson [2007] also inferred that after the 1755 earthquake and tsunami, this area was subjected to dryer and higher elevation depositional conditions, suggesting that minor uplift might have occurred. 5. discussion 5.1 marine terraces correlation t1 is the lowest terrace recognized in the region (figure 9). we identified distinct sections or segments for this single terrace at elevations starting from 1.5-2 m for the outer flat section, up to an inner edge at about 8 m elevation, with a similar morphology to the modern marine terrace. we recognized t1 in the west coast at amoreira, bordeira, castelejo and in the south coast at ingrina and furnas, at consistent elevations. t2 is the second terrace present in this region (figure 9), and correlative sections of this terrace were identified along the west coast at carrapateira, and along the south coast at ingrina, exclusively in areas with limestone bedrock. these are areas which are more resistant to erosion and preserved from modern cliff retreat. we have identified the inner edge of this terrace at a consistent elevation of about 21 m. the best-preserved terrace sequence is at the ingrina site (figure 4), where several surfaces with multiple marine features are interpreted as a complete sequence of two marine terraces with shore-line angles at elevations of 8 m and 21-22 m, respectively. at this site, paleo sea cliffs and remains of surfaces with inner edges at about 26 m, 33 m, 37 m and 45 m were also recognized, but details of these surfaces will not be described in this work. ingrina is the key site for the interpretation of the regional marine terraces sequence for this region, as the terraces are well-preserved and morphologically exposed. we interpret their ages to range from late to middle pleistocene, and this interpretation is corroborated by the presence of similar surfaces with the same elevations at other locations along the southwest portuguese coast, such as at the furnas, castelejo and carrapateira sites (figures 5, 6, 7 and 8). 5.2 terrace ages and interglacial interpretation during the pleistocene climatic oscillations, sea level fluctuated between low stands (glacial times) and high stands (interglacial times). only one of the sea level high stands during the past 130 ka is interpreted to have been higher than current mean sea level, which was during the beginning of the last interglacial period, i.e. the mis 5e (or mis 5.5), the oldest of the 3 peaks (5a, 5c, and 5e) of interglacial mis 5 [shackleton and opdyke 1973, bloom et al. 1974, siddall et al. 2006] (figure 10). this higher peak was also a long lasting one, starting from about 128 ka to 116 ka [muhs et al. 1994, muhs et al. 2002a], so it should have generated a broad wave-cut platform and marine terrace. several studies have been concerned with the elevations of mis 5e marine terrace features, and their corresponding age, at different locations around the world. the eustatic sea level at the time of mis 5e has been estimated to be in the range of +2 to +9 m [hearty et al. 2007, sidall et al. 2006, pedoja et al. 2011] with a high probability to have reach +6.6 m [kopp et al. 2009]. these compilations have been derived from many sources and regions, implying the application of various geochronological methods to distinct paleo-sea features at different latitudes and in distinct coastal environments. this is discussed by dutton et al. [2012], who also discuss the impact of glacial-hydro-isostatic figueiredo et al. 12 13 processes induced by the weight of large ice sheets and water volume changes in the ocean basin promoting deformations in the earth surface, and consequently influencing the positions of the paleo-shorelines. these deformations occur with different intensity and rate according to their geographical proximity to the ice sheets and also with different timing accordingly with the ice sheet melting time and hydro-isostatic processes. dutton et al. [2012] state that for mis 5e, the relative sea level was not the same for everywhere on earth, as the sea level response would not be synchronous, and consequently mis 5e could have distinct time frames and different elevations for areas according to different glacial-isostatic responses to the ice sheets weight. rockwell et al. [1989] commented about the correlation between paleo sea level references at near equatorial locations such as barbados and new guinea and their own observations for the southwest californian and northwest mexican coastline at middle latitudes, suggesting a worldwide variation in the relative sea-level of some meters, which if not taken into account then would have an implication on the estimating of vertical motions on a slowly rising coastline, such as portugal. in this work, we will primarily consider the mis 5 sea level references for regions with oceanic rocky coasts at equivalent latitudes that are expected to be similar to the southwest portuguese coast [rockwell et al. 1994, muhs et al. 1994, 2002b]. thus, we will consider a eustatic sea level reference of +4 to +6 m for the mis 5e [rockwell et al. 1989, 1994, muhs et al. 2002b], of -2 to -4 m for mis 5a (80 ka) and of zero to -2 m for mis 5c (105 ka). due to global observations that indicate sea level was 4-10 m above the present mean sea level during the mis 5e, we expect not only a broad wave-cut platform to have been generated, but also that the corresponding paleo morphology would be emergent, unless the coastal region of portugal is significantly subsiding. previous vertical motions studies conducted in portugal have suggested that this area has been subjected to uplift since the pliocene, although with a low rate of less than 0.2 mm/ a [cabral 1995]. it is therefore expected that the more recent interglacial marine terraces should be at least locally preserved, and at elevations that are now higher than elevations at which they were originally formed. it is also expected that they should be well represented and preserved, unless the holocene erosion rates are sufficiently high so as to have removed all lower terraces. although the latter is clearly the case in some areas, the presence of significantly elevated marine terraces to 360 m (fonte santa), along with the deeply incised canyons and valleys in the algarve region all support coastal emergence. thus, the presence of the mis 5e terrace is expected to be preserved in sw portugal, albeit only locally. we have recognized two low terraces that are reasonably well preserved at four locations along the southwestern coast of portugal, the t1 and t2 terraces with inner edge elevations at 8 m and 21 m, respectively. comparing these two terraces, the one which has a broader expression is the t2 terrace, where it was possible to us to identified 3 distinct subsections, interpreted as morphological segments of a single wave-cut platform. also, t1 seems to have been cut into the t2 terrace. with this reasoning, we infer that t2 likely corresponds to mis 5e and that t1 would then correspond to mis 5c or mis 5a. mis 5c corresponds to a very short peak at 103105 ka, which may have later been reoccupied by the longer mis 5a highstand at 80 ka. thus, the 5c terrace level is commonly not distinguishable from the 5a terrace for regions of low uplift rate, as it has most commonly been cut out [hanson et al. 1994, muhs et al. 1994, 2002b, kern and rockwell 1992, rockwell et al. 1994]. we therefore infer that t1 should correspond to the mis 5a highstand, and that t1 terrace likely reoccupied or cut out the slightly older mis 5c terrace section, which is not preserved. another factor that tentatively supports the correlation of the t1 and t2 levels to the mis 5a and 5e global highstands is that their elevations and spacing are nearly identical to the well-dated mis 5a and 5e terraces at localities in central and southern california [hanson et al. 1994, muhs et al. 2002, kern and rockwell 1992] northern baja california, mexico [rockwell et al. 1989], and tangier peninsula, morocco along the strait of gibraltar [abad et al. 2013]. areas undergoing slightly high rates of uplift show that the elevation difference between these two terraces increases as their respective terrace elevations increase, as would be expected if paleo-sea level were similar [hanson et al. 1994, muhs et al., 2002b]. if our correlation is correct, then t2 has been uplifted approximately 13 m since the last interglacial (21 m minus the modern inner edge elevation (+2m) and minus the assumed +6 m at the time of formation). there are no references for the precise time and duration of the mis 5e along the portuguese coast. data compilation analysis suggest that the last interglacial had an average duration from 130 ±2 ka to 119±2ka [hearty et al. 2007 and references within]. however, as abrasion terraces are cut during the entire length of the highstand, terraces ages should correspond to the end of the interglacial period. speed and chang [2004] recognized at barbados abrasion terraces the latest mis 5e peak, that lasted from 127 ka to 120 or to 115 ka; sw portugal pleistocene marine terraces dabrio et al. [2010] through a prograding barrier claims a 10± 2 ka duration for a highstand during mis 5e for the southeast spain, but without presenting accurate ages for the interval; muhs et al. [2011] through a fossil reef complex at florida, identifies a peak that has likely started at 127 ka and lasted until 114 ka, but with some uncertainties due to the reef characteristics. however, muhs et al. [2002b], based upon high precision dating of solitary corals from the mis 5e terrace along the west coast of north america, suggest that the last interglacial peak lasted from ~128 ka to 116 ka (and may have been double peaked). thus, we considered that is very likely that the last mis 5e peak lasted until 116ka and we infer the 13 m of uplift occurred after about 116 ka, yielding a long term uplift rate of 0.11 ± 0.01 mm/ a. if sea level had already begun to drop towards the end of mis 5e time, then this rate will increase accordingly. the sea level reference for the mis 5a, relative to the mis 5e shoreline elevation, was 2 m to 4 m lower than the modern sea level, as determined for abrasion terraces along the west coast of north america [muhs et al. 2002b, 2004]. taking into consideration this 0.11 ± 0.01 mm/ a for the last 80 ka, we should expect 8 to 10 m of uplift, meaning that the 80 ka zero sea level reference should now stand at an elevation of 6 m to 8 m. thus, we consider that the t1 inner edge that is present at approximately 8 m elevation correlates to the mis 5a marine terrace. the lower surface section of t1, recognized mainly at castelejo and furnas, with some remains preserved and reoccupied by the modern wave-cut platform and beach, corresponds to the subtidal section of the marine terrace, exposed during mis 4 and mis 3, when it was probably covered by aeolianites. if the southwest of portugal is rising at this slow rate (figure 10), we expect that other marine terraces, such as those that correspond to the mis 7 and mis 9 highstands, have been uplifted as well. in fact, we have already identified features that might correspond to their inner edges at elevations consistent with this rate, but further work is needed to characterize this sequence. additional marine terraces at higher elevations have been also recognized, although they are typically poorly preserved, towards the regional abrasion surface identified at 120-130 m elevation, which should be middle pleistocene in age. further work is needed, especially to constrain the age of the sediments and the age of the erosional surfaces when sediments are not present, to properly assess the long-term rates of vertical motion. 5.3 historical seismicity and marine terrace vertical deformation efforts were made to identify co-seismic evidence of large earthquakes that have affected this area, but no irrefutable evidence was found. assuming the uplift rate for this area is 0.11 ± 0.01 mm/ a as inferred, we would only expect a little more than 1 m of uplift for the entire holocene, meaning that either the source (or sources) that are responsible for this vertical motion have long quiescence periods and have not ruptured since the modern sea level stabilized or that the resulting effects were so small, centimetric to decimetric scale, that they would not be differentiable in a high energy mesotidal environment coast such as sw portugal. it is possible that a small vertical motion has been figueiredo et al. 14 figure 10.schematic southwest portugal marine terraces correlation with paleo-sea level estimates compiled from different studies: mis 5a, mis 5e, mis 7, mis 9 interglacials are identified (adapted from siddall et al. [2006]). 15 identified through interpretation of abrupt changes in the sedimentary fills of two fluvial valleys near the coastline (boca do rio and martinhal, at sagres), and that this may have happened twice, once following the 382 ad earthquake and again following the 1755 earthquake. however, it is also possible that these environmental changes were partially due to the local perturbations caused by the tsunami flooding, local landsliding, or for other reasons. the closest large active structure known in this region is the marquês de pombal thrust, a 70 km long structure located circa 100 km west-southwest from sagres, and motion on this fault is not likely to have produced vertical motions at the coastline. thus, if there was any vertical motion associated with these two historical earthquakes, it is likely to have been generated by a more local, currently unknown source. 6. conclusions we recognize several marine terraces along the southwest portuguese coastline, with the lower two (t1 and t2) probably correlating to abrasion terraces cut during the last interglacial period of relative sea level highs (mis 5). these two terraces were surveyed, and corresponding paleo sea level features were identified. considering that the mis 5e marine terrace should be the higher and better preserved of the two, we assumed that it corresponds to t2. if correct, this implies an uplift rate of 0.11 ± 0.01 mm/ a, which also corroborates the t1 marine terrace as the result of the mis 5a marine high stand. a sequence of higher marine terraces is present, although they are poorly preserved, towards the higher regional abrasion surface, which according with to this rate, should be middle pleistocene in age (1 to 1,2 ma). this section of the portuguese coastline is rising at rates not recognized for other regions in south portugal, suggesting that an unknown local tectonic source be responsible for the long-term uplift of this region. acknowledgements. this work was funded by fundação da ciência e tecnologia, through a phd scholarship (sfrh/bd/36892/2007) attributed to paula marques figueiredo and research project “paleoseismological study of active faults in mainland portugal” (ptdc/cte-gin/66283/2006), co-financed by feder. the authors also are thankful to filipe rosas for providing figure 1 and to hector perea whose comments significantly improved this manuscript. references abad, m., j. rodríguez-vidal, k. aboumaria, m.n. zaghloul, l.m. cáceres, f. ruiz, martínez-aguirre, t. izquierdo and s. chamorro (2013). evidence of mis 5 sea-level highstands in gebel mousa coast (strait of gibraltar, north of africa), geomorphology, 182, 133-146. andrade, c., f. marques, m.c. freitas, r. cardoso and p. madureira (2002). shore platform downwearing and cliff retreat in the portuguese west coast, in: gomes, f.v. et al. littoral 2002 6th international symposium proceedings: a multi-disciplinary symposium on coastal zone research, management and planning, 2,423-431. baptista, m.a. and j.m. miranda (2009). revision of the portuguese catalog of tsunamis. natural hazards and earth systems sciences, 1, 25-42. bartolome, r., e. gràcia, d. stich, s. martínez-loriente, d. klaeschen, f. l. mancilla, c.i. iacono, j.j. dañobeitia and n. zitellini (2012). evidence for active strike-slip faulting along the eurasia-africa convergence zone: implications for seismic hazards on the sw iberian margin. geology, g33107.1. bloom, a.l., w.s. broecker, j.m.a. chappel, r.k. mattews and k.j. mesolella (1974) quaternary sea-level fluctuations on a tectonic coast: new th/u dates from the huon peninsula, new guinea, quaternary reseacrh, 4, 185-205. bradley, w. c., and g.b. griggs (1976). form, genesis, and deformation of central california wave-cut platforms: geol. soc. of am. bull., 87, 433-449. brito, b. 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lisbon university, science faculty, geology department, lisbon, portugal; pmfigueiredo@fc.ul.pt © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. sw portugal pleistocene marine terraces layout 6 annals of geophysics, 56, 6, 2013, s0677; doi:10.4401/ag-6222 s0677 neotectonics of graciosa island (azores): a contribution to seismic hazard assessment of a volcanic area in a complex geodynamic setting ana hipólito1,*, josé madeira2, rita carmo1,3, joão luís gaspar1,3 1 centro de vulcanologia e avaliação de riscos geológicos da universidade dos açores, açores, portugal 2 universidade de lisboa, faculdade de ciências, departamento de geologia, and instituto dom luiz (laboratório associado)-idl(la), lisboa, portugal 3 centro de informação e vigilância sismovulcânica dos açores, civisa, açores, portugal abstract graciosa is a mid-pleistocene to holocene volcanic island that lies in a complex plate boundary between the north american, eurasian, and nubian plates. large fault scarps displace the oldest (middle pleistocene) volcanic units, but in the younger areas recent volcanism (holocene to upper pleistocene) conceals the surface expression of faulting, limiting neotectonic observations. the large displacement accumulated by the older volcanic units when compared with the younger formations suggests a variability of deformation rates and the possibility of alternating periods of higher and lower tectonic deformation rates; this would increase the recurrence interval of surface rupturing earthquakes. nevertheless, in historical times a few destructive earthquakes affected the island attesting for its seismic hazard. regarding the structural data, two main fault systems, incompatible with a single stress field, were identified at graciosa island. thus, it is proposed that the region is affected by two alternating stress fields. the stress field #1 corresponds to the regional stress regime proposed by several authors for the interplate shear zone that constitutes the azorean segment of the eurasia-nubia plate boundary. it is suggested that the stress field #2 will act when the area under the influence of the regional stress field #1 narrows as a result of variations in the differential spreading rates north and south of azores. the islands closer to the edge of the sheared region will temporarily come under the influence of a different (external) stress field (stress field #2). such data support the concept that, in the azores, the eurasia-nubia boundary corresponds to a complex and wide deformation zone, variable in time. 1. geodynamic and volcanic setting the azores archipelago lies on a complex geodynamic setting on the triple junction between the eurasian (eu), north american (na), and nubian (nu) plates (figure 1). graciosa island is located on the western segment of the eu-nu boundary, which is characterized by diffuse and complex deformation in a dextral transtensile tectonic regime [e.g. lourenço et al. 1998, madeira and brum da silveira 2003, carmo 2004, hipólito 2009] (figure 1). this boundary acts as an ultra-slow (ca. 4.5 mm/a-demets et al. 2010) oblique article history received october 8, 2012; accepted may 10, 2013. subject classification: structural geology, stress, plate boundaries, motion and tectonics, geodynamics, seismic risk. figure 1. main morphotectonic features of azores region. white lines define approximately the morphological expression of each structure; white shaded area represents the sheared western segment of the eu-nu plate boundary, whereas the white shaded area limited by a dotted grey line represents its main structure, the terceira rift (tr). plates: eu eurasia; nu nubia; na north america; tectonic structures: mar mid-atlantic ridge; eafz east azores fracture zone; nafz north azores fracture zone; gf gloria fault; ffz faial fracture zone; afz açor fracture zone; pafz princesa alice fracture zone; pfz pico fracture zone. islands: sma santa maria; sm são miguel; t terceira; sj são jorge; p pico; f faial; fl flores; c corvo. azores bathymetry adapted from lourenço et al. [1997]; world topography and bathymetry from gebco_08 database [2010]. datum: wgs 1984. special issue: earthquake geology spreading centre [vogt and jung 2004] and as a transfer zone accommodating the differential motion between eu and nu plates, due to the higher spreading rates and slightly different spreading direction north of azores [demets et al. 1994, sella et al. 2002, altamimi et al. 2002, fernandes et al. 2003, kreemer et al. 2003, calais et al. 2003, demets et al. 2010]. the stress regime, the tectonic processes, and the segmentation patterns of the western segment of the eu-nu plate boundary are still a matter of debate. this boundary does not correspond to a discrete structure but to a wide (~150 km) large-scale interplate shear zone, and the islands within this region present average motions that are intermediate to those predicted by global plate motion models for eu and nu [fernandes et al. 2006, trota 2008, miranda et al. 2012, mendes et al. 2013]. graciosa and santa maria islands, the emerged areas closer to the north and south edges of the plate boundary zone, display kinematic behaviours close to those predicted for stable eu and nu, respectively [fernandes et al. 2006]. some authors [e.g. schilling 1975, cannat et al. 1999, escartín et al. 2001, gente et al. 2003, madureira et al. 2005, silveira et al. 2006, yang et al. 2006] consider that the intense magmatism in the region results from a ridge-hotspot interaction. shifting of the plate boundary, possibly associated to migration of the hotspot [e.g. vogt and jung 2004], created a topographically disturbed area which has been successively attached to the nu plate as the triple junction jumped northwards [luis and miranda 2008]. this region the azores plateau [needham and francheteau 1974] is characterized by an anomalously thick ocean crust [e.g. hirn et al. 1980, luís et al. 1998, miranda et al. 1998, dias et al. 2007] and shallow depths relatively to the surrounding sea-floor. the neotectonic data [e.g. madeira and brum da silveira 2003] on the islands show a structural pattern indicating three-dimensional strain [reches 1983, reches and dieterich 1983], represented by two main conjugated sets of faults with oblique slip (figure 2): (1) the main system, trending wnw-ese to nw-se, with normal dextral oblique displacement is parallel to the inter-plate shear zone; and (2) the nnw-sse trending conjugate system, with normal sinistral oblique displacement, is oblique to the plate boundary direction. the stress field is characterized by horizontal maximum compressive stress axis (σ1) trending nw-se in the axial area of the shear zone (which rotates to n-s near santa maria island; madeira [1986], figure 2), horizontal maximum tensile stress axis (σ3) trending nesw (rotating to e-w in santa maria island; madeira [1986], figure 2), and vertical intermediate compressive stress axis (σ2). permutations between σ1 and σ2 after stress drop events during transtensile phases, as proposed by reches [1983], account for alternating transtensile and tensile regimes [madeira 1998]. the changing regimes are deduced both from slickensides indicating events in strike-slip, oblique slip, or normal slip in the same fault, and from different focal mechanisms of earthquakes in faults with the same direction (table 1). in the eastern edge of the deformation zone (eastern part of são miguel) a distinct set of wnw-ese to nw-se normal left-lateral faults conjugated with nehipólito et al. 2 figure 2. stress pattern and stereographic plots of the main fault systems of the azores region as inferred from neotectonic data [madeira 1986, madeira and ribeiro 1990, madeira 1998, lourenço et al. 1998, carmo 2004]. the tensile stress field (a) close to the mar passes laterally to compressive (b) as the sea-floor moves away from the ridge. inside the sheared region (c) stress permutation and deviation of stress trajectories occur. red dashed lines separate the different stress field domains; white shaded area represents the interplate sheared region. acronyms as in figure 1. bathymetry adapted from lourenço et al. [1997]. datum: wgs 1984 (modified from madeira [1998], lourenço et al. [1998] with data from carmo [2004]). 3 neotectonics of graciosa island (azores) table 1. seismic events with published focal parameters for the azores region (from 1900 to present). references: cmt: centroid-moment tensor, havard in borges et al. [2007, 2008], buf: buforn et al. [1988], bor: borges et al. [2007]. for the magnitude w and s indicate mw e ms, respectively [borges et al. 2007, 2008]. event n° date (d/m/yr) latitude (°n) longitude (°e) depth (km) m mo (x1017 nm) strike rake dip ref 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 08/05/1939 06/09/1964 04/07/1966 05/07/1966 20/04/1968 23/11/1973 11/12/1973 01/01/1980 02/12/1981 09/09/1984 16/10/1988 21/11/1988 21/01/1989 26/06/1989 23/09/1989 09/12/1991 20/01/1993 09/03/1996 27/06/1997 27/06/1997 28/06/1997 09/07/1998 01/08/2000 30/11/2002 05/04/2007 04/11/2007 37.40 38.30 37.50 37.60 38.30 38.46 38.74 38.81 38.38 36.93 37.38 38.34 37.92 39.11 39.27 37.22 38.39 37.13 38.33 38.26 38.41 38.65 38.79 39.25 37.45 37.40 -23.90 -26.60 -24.70 -24.70 -26.60 -28.31 -28.67 -27.78 -26.13 -24.60 -25.16 -26.27 -25.92 -28.32 -29.24 -23.61 -29.34 -23.85 -26.68 -26.16 -26.64 -28.63 -29.01 -28.45 -24.62 -24.39 15 15 10 18 15 15 15 7 15 12 15 15 15 15 15 15 15 15 7 15 15 7 15 15 12 12 7.1s 5.1w 5.5w 5.0w 4.6w 5.1s 5.0w 6.8w 5.6w 5.3w 5.3w 5.9w 5.7w 5.8w 5.1w 5.2w 5.4w 5.7w 5.8w 5.2w 5.1w 6.0w 5.1w 5.1w 6.2w 6.0w 199 0.54 1.90 0.41 0.09 2.0 0.34 190 3.20 0.95 0.89 7.10 3.40 5.40 0.44 0.82 1.20 3.80 7.0 0.62 0.58 14 0.51 0.52 41 11 41 185 341 180 117 23 329 149 141 178 303 345 131 105 233 330 132 319 290 284 290 153 97 106 129 133 35 62 49 48 42 90 58 85 42 37 90 29 41 32 45 45 33 28 44 27 44 85 62 45 44 44 -154 3 -42 30 89 -179 -20 -2 -80 -79 180 -37 -87 -110 -90 -90 -59 -106 -114 -147 -114 6 -170 -129 -89 -87 buf buf buf buf buf buf buf bor cmt cmt cmt cmt cmt cmt cmt cmt cmt cmt bor cmt cmt bor cmt cmt cmt cmt location figure 3.seismicity in azores region from 1980 to 2012. the red dots mark epicentres of earthquakes. plot includes earthquakes of all magnitudes. acronyms as in figure 1. data from civisa (centro de informação e vigilância sismovulcânica dos açores), 2013. sw normal dextral faults, is incompatible with the regional stress field described above (carmo [2004], figure 2). this set of faults is ascribed to a different, previously unknown, stress field, characterized by horizontal e-w σ1. due to this geodynamic setting, the azores region is subject to active volcanism and frequent seismicity (figure 3). the seismic activity is characterized by isolated seismic events and/or swarms of tectonic and/or volcanic nature, with shallow focal depths and low to moderate magnitudes. moderate to high magnitude earthquakes, up to m=7, have also occurred in modern times (table 1). instrumental data present seismic events with variable focal mechanisms mainly indicating dextral (with wnw-ese and nne-ssw nodal planes) and sinistral, normal slip (with nw-se trending nodal planes), oblique-slip, and a few reverse events [mckenzie 1972, udías et al. 1976, udías 1980, hirn et al. 1980, grimison and chen 1986, udías et al. 1986, buforn et al. 1988, moreira 1991, mezcua et al. 1991, miranda et al. 1998, lourenço et al. 1998, matias et al. 2007, dias et al. 2007, borges et al. 2007, 2008, borges and buforn 2008, silva et al. 2012].the instrumental seismicity recorded since the 1980’s highlights the eu-nu plate boundary (figure 3). graciosa island was affected by a few significant earthquakes since settlement in mid-15th century, causing some deaths and severe damage (table 2). based on the historical descriptions of damages, some authors suggest that the epicentre of the january 21st, 1837 earthquake was on land [madeira 1998, silva 2005]. instrumental seismicity in and around graciosa is presented in figure 4. graciosa has a nw-se trending elliptical shape, 12 km long, 7 km wide and has a smooth relief reaching a maximum elevation of 402 m. as in other volcanic islands of the azores archipelago, the elongated morphology of graciosa reflects the main tectonic trend of the azorean segment of the eu-nu plate boundary and, more specifically, the nw-se direction of the western sector of the terceira rift [machado 1959], a tectono-magmatic structure defined by the alignment of alternating volcanic buildings (graciosa and terceira islands, banco dom joão de castro seamount, and são miguel island) and tectonic basins (west and east graciosa and north and south hirondelle). the tectonic structure controlled most of the volcanic activity responsible for the build-up of the island. that is a common phenomenon that occurs in several oceanic volcanic islands independently of their geodynamic setting, such as the fuerteventura and lanzarote islands (canary islands) in which the elongation clearly hipólito et al. 4 table 2. historical seismicity in graciosa island (modified from gaspar [1996]). references: 1. moniz [1883], 2. arquivo dos açores [1890], 3. matos [1982],, 4. pereira [1986], 5. inmg/lnec [1986], 6. silva [2005]. date (d/m/yr) intensity (ems-98) most affected area damage ref. ?/?/1611/36 ?/?/1717 13/6/1730 ?/3/1787 ?/1/1817 21/1/1837 ?/?/1868 11/12/1883 19/2/1868 vii vii santa cruz (?) northern part of island (guadalupe and santa cruz villages) south part of island (luz and praia villages) central part of the island (guadalupe and santa cruz villages; caminho das fontes and almas) santa cruz church houses destroyed total destruction houses destryed; 3 deaths; probable rupture along the south serra das fontes fault houses damaged; landslides 4 4,3 6 1 1 6.2 1,5 1 1 5 neotectonics of graciosa island (azores) figure 4.seismicity in graciosa region from 2002 to 2012. the red dots mark epicentres of earthquakes and its magnitude. plot includes earthquakes of all magnitudes. g graciosa island; sj são jorge island. data from civisa (centro de informação e vigilância sismovulcânica dos açores), 2012. figure 5. three-dimensional elevation map of graciosa island. main morphological structures and location of volcanic complexes. topography from igeoe [2001]. reflects the nne-ssw trend of the east canary ridge [e.g. camacho et al. 2001, acosta et al. 2003]. the oldest volcano-stratigraphic units occupy the central part of graciosa. they comprise the serra das fontes volcanic complex (620 ± 120 ka, féraud et al. [1980]), a shield volcano composed of basaltic to mugearitic lavas, and the serra branca volcanic complex (350 ± 40 ka, féraud et al. [1980]) a trachytic (sensu stricto) central volcano [gaspar and queiroz 1995, gaspar 1996] (figure 5). on the southeast sector a quiescent basaltic to trachytic (sensu lato) polygenetic volcano with a summit caldera constitutes the central volcano unit (caldeira volcano) and in the northwest part of the island a low altitude plateau (the nw basaltic platform) is formed by alignments of basaltic cinder cones and the correlative lava flows forming the vitória unit [gaspar 1996] (figure 5). this fissural volcanic system extends to the southeast mantling the older volcanic complexes and developed simultaneously with the edification of the caldeira volcano. the two younger volcanic systems (caldeira volcano and nw basaltic system) are upper pleistocene to holocene in age. the most recent volcanic event was a pre-settlement basaltic hawaiian-strombolian eruption at about 2 ka b.p. (walker, unpublished data, in maund [1985]). currently, the volcanic activity is expressed by secondary manifestations, namely by thermal springs, fumarole fields, and diffuse degassing [e.g. ferreira et al. 1993, gaspar 1996]. the older volcanic complexes (serra das fontes and serra branca) are displaced by several nw-se trending faults morphologically expressed by large fault scarps parallel to the elongation of the island. those faults define a graben structure that is crossed in its south-eastern portion by a nne-ssw fault scarp, which separates the older units from the younger volcanic units to the south (figures 6, 7). the faults can be tracked into the younger volcanic areas where they express as volcanic alignments. the lack of surface ruptures displacing the younger units indicates that these structures present low slip-rates during the holocene. this work presents the first detailed structural analysis of graciosa island. the neotectonic data is used to discuss the relationship between geodynamic processes and the stress fields acting in the azores region. they also contribute to define neotectonic parameters essential for seismic hazard assessment in order to estimate the seismogenic potential of active faults. hipólito et al. 6 figure 6. morphotectonic map of graciosa island. main tectonic structures: shf saúde-hortelã fault; ssff south serra das fontes fault; nsbf north serra branca fault; ssbf south serra branca fault; esff east serra das fontes fault. topography from igeoe (2001). 7 2. data acquisition in graciosa neotectonic analysis is limited due to active volcanism that conceals the faults in significant areas of the island, reducing the available outcrops. major tectonic features correspond to fault scarps several tens to hundreds of meters tall located in the central region of the island (figures 6, 7). these structures extend into the younger volcanic regions as blind faults. the absence of detailed bathymetric surveys on the surrounding offshore precludes the reconnaissance of the full length of the faults. the large size of the fault scarps and the absence of recent surface faulting with favourable conditions for trenching preclude paleoseismological studies. the neotectonic data was collected mainly in cinder cone quarries (figure 8) and at sea cliffs (figure 9). the nature of the younger volcanic deposits, low-cohesion coarse lapilli and lava flows, is not favourable for the generation of slickensides, limiting kinematic analysis. the mapped structures are normal faults or present a normal component. the kinematics were deduced from the displacement of morphologic and stratigraphic markers, and wherever possible, from striated fault planes. although in most cases it was difficult to recognize a strike-slip component, a few structures present dextral or sinistral strike-slip components typical of a transtensile tectonic regime. part of the amount of dip-slip offsets may be apparent and be the result of a strike-slip component displacing inclined surfaces. a preliminary structural mapping was based on vertical air photo-interpretation and dem (digital elevation model) and topographic map analysis, allowing recognition of morphologic features with probable tectonic origin (fault scarps) or tectonically controlled (e.g. cone or dome alignments, elongated craters, displaced or linear stream channels, linear sea cliffs). the tectonic nature of the identified alignments was confirmed or infirmed through detailed analysis of the outcrops. a total of 177 fault planes were measured and analyzed, including faults of variable importance cropping out in sea cliffs, small scale faults in cinder cones, and mapneotectonics of graciosa island (azores) figure 8. small faults exposed on wall of a quarry exploiting lapilli from a cinder cone. white arrows specify the dip slip-vector. figure 7. photograph of fault scarps in the central region of graciosa island viewed from the se. continuous white lines indicate fault traces; dashed white lines mark hidden fault traces. fault legend as in figure 6. figure 9. example of a fault trace exposed in a sea cliff south of serra branca. legend as in figure 8. scale structures deduced from geological mapping. the structural map was later converted to a digital format, at the 1:25 000 scale (from igeoe altimetry), using a gis software. the structural data was included in the azoris data base [gaspar et al. 2004]. 3. structural data 3.1. geometric and kinematic fault analysis the geometric analysis shows a main set of faults trending nw-se to nnw-sse, dipping to ne and sw or to ene and wsw, respectively (figure 10). this set of structures includes the larger structures expressed morphologically as fault scarps. the main nw-se trending structures are the south serra das fontes fault (ssff), the saúde hortelã fault (shf), the north serra branca fault (nsbf), and the south serra branca fault (ssbf) (figures 6, 7). the ssff presents a slightly convex, 4600m-long and 150-200m-high southwest-facing scarp, trending ca. n306º. to the north this structure is marked by an alignment of four basaltic cinder cones. the shf trace is marked by a 5000m-long, less than 10m-high south-facing scarp, trending ca. n320º from pico da hortelã to pico da ladeira do moiro cones, producing 50m of dextral strike-slip displacement at the intersection with the east serra das fontes fault (esff) scarp. the nsbf presents a 4800m-long northeast-facing scarp, trending n302º, which is aligned with two cones of the nw basaltic platform and to the southeast with the long axis of the caldeira volcano. the ssbf scarp faces southwest, trends n305º, and may control the location of some basaltic cones to the northwest. these faults define a graben structure in the centre of graciosa, the serra branca serra das fontes graben, south of which the nsb and ssb faults define a horst (figures 6, 7). the total length of these faults is unknown because on land they are locally concealed beneath younger lava flows, pyroclastic and/or epiclastic deposits, and there is no offshore data to assess its extension in the seafloor. a second set of faults trend nne-ssw to ne-sw and dip to the se and nw (figure 10). it is represented by small-scale structures cropping out in cinder cones, by cone alignments, and by a major nne-ssw trending structure, the esff (figures 6, 7). the n20º trending esff trace is marked by a 4600m-long and 185m tall southeast facing scarp. inside the caldeira volcano there is a volcanic lineament trending ne-sw (figure 6). e-w trending faults, although less frequent, also occur. in addition to the observed faults, some morphological features evidence similar trend, such as two ene-wsw cinder cones alignments in the nw basaltic platform, or a rectilinear sea cliff south of serra branca, suggesting an e-w tectonic control (figure 6). gaspar [1995, 1996] proposes that e-w faults are inherited from the oceanic basement on which the islands stand (old transform faults). e-w trending faults were also described by madeira [1998] in the islands of são jorge, faial, and pico. moreover, the presence of e-w trending structures crossing the azores plateau is mentioned by searle [1980]. the geometric and kinematic fault analyses show that the nw-se trending, sw-dipping faults, present normal-dextral or dextral-normal oblique slip. oblique normal-left lateral or left lateral-normal slip was observed in nne-ssw to ne-sw trending, se to esedipping faults, and normal-dextral or dextral-normal oblique slip in nne-ssw to ne-sw faults dipping to wnw and nw. despite the limited number of faults presenting evidence of oblique displacement (strike-slip associated to normal component), the kinematic data obtained from field observations in graciosa island suggest a transtensive tectonic regime evidencing three-dimensional strain [reches 1983, reches and dieterich 1983], in agreement with the tectonic pattern described in previous works in other azorean islands [e.g. madeira 1986, madeira and ribeiro 1990, madeira 1998, madeira and brum da silveira 2003, carmo 2004]. hence, kinematic data from graciosa indicate the existence of two main fault systems (figure 11). system a is composed of two sets of conjugated faults, one trending nw-se, dipping to sw and ne, presenting normal-dextral or dextral-normal oblique slip, and another striking nne-ssw, dipping to ese and wnw, with oblique normal-left lateral or left lateral-normal slip (figure 11a). system b includes nne-ssw to nesw trending faults, dipping to wnw to nw or to ese, presenting normal-dextral or dextral-normal oblique slip (figure 11b). a family of faults conjugated with these structures was not found. the steep fault dips (the 80º to 90º range dominates) also suggest the presence of a strike-slip component (figures 10b-11). 3.2. geometric dyke analysis several dykes, both of basaltic and trachytic nature, were observed at the sea cliffs cutting the serra das fontes and serra branca volcanic complexes and the caldeira volcano. in the first case the dykes’ directions are similar to the main fault systems, with two main trends, nnw-sse and ne-sw, attesting for the tectonic control of the magmatic phases (figure 12). the dyke system exposed on the sea cliffs cut on the caldeira volcano, represented by ne-sw trending dykes on the northeast flank and nw-se dykes on the southeast, although scarce, suggests a radial pattern around the volcanic edifice (figure 13). however, nnw-sse trending dikes occurring in the southeast hipólito et al. 8 9 coastal cliffs are not in agreement with a radial pattern and may represent en echelon dykes consistent with nw-se dextral shearing compatible with the kinematics of fault system a, implying a magmatic phase conneotectonics of graciosa island (azores) figure 10. geometry of all analyzed faults: a) circular histogram of unweighted frequencies of fault plane directions; b) fault poles density contour plot (lower hemisphere; schmidt net); tectonicsfp software® [ortner et al. 2002]. figure 11. stereographic plot of the two main fault systems (lower hemisphere; schmidt net – β diagram): a) system a nw-se to nnwsse faults, with normal-dextral oblique slip, conjugate of nne-ssw to ne-sw normal-left lateral structures; b) system b nne-ssw to ne-sw faults with normal-dextral oblique slip. tectonicsfp software® [ortner et al. 2002]. trolled by ene-wsw crustal extension. note that, because of three-dimensional strain, transtensional faults may act as volcanic conduits, so dykes are not necessarily normal to the direction of extension (i.e. they may open obliquely to the fracture direction). this is attested by the fact that in many azorean islands, alignments of monogenetic centres are mostly parallel to the fault system synthetic with the plate boundary, but alignments along the conjugate direction also occur. 4. discussion 4.1. determining paleostress the two main fault systems (a and b) are incompatible with a single stress field. none of the fault systems is exclusive of a given volcanic system; they affect all stratigraphic units and may occur together. this indicates that the different stress fields are neither related to a given period of time nor related to deformation associated to a particular volcanic centre. the paleostress analysis indicates the presence of two different stress fields. the stress field #1, associated to the fault system a, presents horizontal nnw-sse σ1 (maximum compressive stress axis; anderson 1951; angelier 1994), horizontal ene-wsw σ3 (maximum tensile stress axis) and a vertical intermediate compressive stress axis (σ2). permutations between σ1 and σ2 may occur producing alternating transtensile and tensile tectonic regimes [madeira 1998] (figure 14). the stress field #2, related to the fault system b, is characterized by horizontal wsw-ene σ1, horizontal nnw-sse σ3 and vertical σ2. permutation may also occur between σ2 and σ1 following events of stress drop during transtensile phases (figure 15). because the two different stress fields cannot operate simultaneously, and since they are not local stress fields, they must alternate in time, generating new faults or reactivating pre-existing structures. the spatial-temporal relation between the two stress fields is unclear. the stress field #1 agrees with the findings of madeira and ribeiro [1990] for the azores plateau and with the geodynamic model proposed by madeira [1998] and lourenço et al. [1998] for the azorean segment of the eu-nu plate boundary. it considers the occurrence of a rotation to n-s and e-w of σ1 and σ3, respectively, closer the edges of the shear zone, as observed in santa maria island [madeira 1986], (figure 2). the maximum tensile stress axis direction is also in agreement with the opening direction according the current kinematic plate models for the relative eu-nu motion (e.g. n66ºnuvel-1a, demets et al. 1994; n80º-revel, sella et al. 200; n72º-deos2k, fernandes et al. 2006; n71ºmorvel, demets et al. 2010; table 3). the stress field #2 is probably external to the shear zone and may temporarily affect the areas at the border of the shear zone, since it is also found in the eastern part of são miguel island [carmo 2004]. the width of the shear zone accommodating the deformation between eu and nu may change proportionally to the difference between the na-eu and na-nu spreading rates, north and south of the triple junction, respectively. narrowing of the area under the influence of the regional stress field #1 (i.e. the shear zone) would put the margins of that sheared region temporarily under the influence of the outer transtensile stress field #2. two hypothesis are considered for the origin of the stress field #2: (1) the existence of an intermediate region under transtension, developing parallel to the mar and establishing a transition zone between the pure tensile stress field at the mar and the distal compressive stress field, established as the sea-floor moves away from the ridge (figures 16-ia-16-ib); (2) the existence of an intermediate region, with a transtensile regime, defining a narrow band between the interplate region with dextral transtension (stress field #1) and the external compreshipólito et al. 10 figure 12. geometry of all measured dykes: a) circular histogram of unweighted frequencies of dykes’ plane directions. tectonicsfp software® [ortner et al. 2002]. figure 13.spatial variations of dyke directions around caldeira volcano (circular histograms of unweighted frequencies of dyke directions and dip angles). topography from igeoe (2001). 11 neotectonics of graciosa island (azores) figure 14. stereographic projection of the stress field #1 deduced from structures of fault system a that have kinematic data of both slip components (lower hemisphere; schmidt net – βdiagram). a) transtensile stress field: nw-se to nnw-sse faults with dominant dextral slip component, conjugate of nne-ssw to ne-sw structures with dominant left-lateral slip component. horizontal nnw-sse σ1, horizontal ene-wsw σ3 and vertical σ2; b) tensile stress field after permutation between σ1 and σ2: nw-se to nnw-sse faults, dipping to both sides, with dominant normal slip component conjugate of nne-ssw to ne-sw faults, dipping to both sides, with dominant normal slip component. vertical σ1, horizontal ene-wsw σ3 and horizontal nnw-sse σ2. tectonicsfp software® [ortner et al. 2002]. figure 15. stereographic projection of the stress field #2 deduced from structures of fault system b that have kinematic data of both slip components (lower hemisphere of schmidt net – β diagram). a) transtensile stress field: nne-ssw to ne-sw faults with dominant dextral slip component. horizontal ene-wsw σ1, horizontal nnw-sse σ3 and vertical σ2; b) tensile stress field after permutation between σ1 and σ2: nne-ssw to ne-sw faults, dipping to both sides, with dominant normal slip component. vertical σ1, horizontal nnw-sse σ3 and horizontal ene-wsw σ2. tectonicsfp software® [ortner et al. 2002]. hipólito et al. 12 figure 15. estimated trajectories of the main stress axis in the western segment of the eu-nu plate boundary. above – hypothesis (1): the tensile stress field (a) close to the mar passes laterally to transtensive (b) with the establishment of a transition stress field. eastward, further away from the ridge, permutation of σ3 with σ2 occurs as a region in compression is entered (c). inside the sheared region (d) stress permutation and deviation of stress trajectories occur. ia) stages of widened shear zone; ib) stages of narrowed shear zone: the islands near the edges of the sheared interplate region are subjected to the stress field b. below – hypothesis (2): the tensile stress field (a) close to the mar passes laterally to compressive (c) as the sea-floor moves away from the ridge. inside the sheared region (d) stress permutation and deviation of the stress trajectories occur. during stages of narrowing of the sheared region, permutation between σ2 and σ3 may originate a local intermediate stress field (b). iia) stages of widened shear zone; iib) stages of narrowed shear zone: the islands near edges of the sheared interplate region are subjected to the stress field b. red dashed lines separate the different stress field domains; white shaded area represents the interplate sheared region. trajectories of maximum tensile stress axis near the mar are based on the spreading direction of na-eu according to the nuvel-1a (demets et al. 1994). trajectories of the main stresses within the sheared zone are based on neotectonic data from emerged areas [e.g. madeira and brum da silveira 2003, carmo 2004, hipólito 2009, and the present work]. acronyms as in figure1. bathymetry adapted from lourenço et al. [1997]. datum: wgs 1984. table 3. relative velocities and slip directions for eu, nu, and na plates in azores. global kinematic models: nuvel-1a [demets et al. 1994], revel [sella et al. 2002], deos2k [fernandes et al. 2003], morvel [demets et al. 2010]. plate pairs velocity (mm/a) azimuth (degrees) kinematic model eu-na(stable) ~23 ~25 ~24 ~23 ~97° ~96° ~94° ~96° nuvel-1a revel deos2k morvel nu-na(stable) ~20 ~19 ~19 ~20 ~103° ~151° ~99° ~149° nuvel-1a revel deos2k morvel eu-nu(stable) ~4 ~6 ~4 ~4.5 ~66° ~72° ~80° ~71° nuvel-1a revel deos2k morvel 13 sive stress field established as the sea-floor moves away from the ridge. the intermediate region would be established when the interplate shear stress is weaker (figures 16, 16,2a, 2b). the e-w trending faults may reflect former deep transform faults cutting the ocean floor generated at the mar (gaspar 1995; 1996) that, because of a favourable direction, were later reactivated by the current dextral transtensile stress regime, probably as shear fractures (p fractures, considering the riedel shear model; e.g. petit 1987). 4.2. insights on the seismogenic potential of graciosa faults the available data do not provide enough information to carry out an assessment of seismic hazard associated to the identified structures. nevertheless, the determination of neotectonic parameters such as the slip-rate is essential to define potential seismogenic sources in a given area, which will be helpful for a future detailed seismic hazard analysis. only three tectonic structures allowed slip-rate estimation: the shf, the ssff and the esff (figure 6, 7). however, none of these structures allowed the determination of the total slip (normal slip and strike-slip components) along the fault over the considered time period. in two of them only the normal slip offset was determined, while in the other only the strike-slip could be measured. given that dated stratigraphical markers only exist for the base of the volcano-stratigraphic units, we consider that the observed deformation accumulated during the 620 ± 120 ka to 31.4 ± 0.3 ka interval. this time interval corresponds to the oldest ages obtained for the serra das fontes volcanic complex and for the caldeira volcano / nw basaltic platform complex, respectively [féraud et al. 1980, maund 1985], since the younger volcanic units are not tectonically displaced by those structures. the estimated slip-rates are of the order of tenths of a millimetre to few millimetres per year (table 4). these values are of the same magnitude as those provided by plate motion kinematic models for the whole archipelago, considering the eu-nu relative motion (table 3). the higher slip-rates could be over-estimated because the age of the younger marker may not represent the end of the deformation period. on the other hand, the beginning of the tectonic deformation affecting the older units is unknown and it is probably more recent than the oldest marker used. in addition, the displacements obtained from morphological markers may be underestimated due to infilling of tectonically depressed areas by younger deposits. the deformation accumulated since 350 ± 40 ka bp (or less) in the older units is large (fault scarps as high as 200 m; table 4). considering a constant deformation rate it would be expected that the younger volcanic units (> 31.4 ± 0.3 ka, maund [1985]) would be faulted, neotectonics of graciosa island (azores) table 4. slip-rates for graciosa faults. structure dip-slip separation (m) strike-slip age of marker (ka bp) dip-slip slip-rates (mm/a) strike-slip shf ssf esff 200 185 50 ca. 620-31.4 ca. 620-31.4 ca. 620-31.4 0.3-6.4 0.3-5.9 0.08-1.6 table 5. maximum expected magnitudes from surface rupture length (srl) and rupture area (ra) for graciosa faults. m/srl m=5.08+1.16log(srl); m/ra m=4.07+0.98log(ra) from wells and coppersmith [1994]. structure srl (m) mw (from m/srl) ra (km 2) mw (from m/ra) shf ssff nsbf ssbf esff 5000 4.600 4.800 3.200 4.600 5.9 5.8 5.9 5.7 5.8 75 69 72 48 69 5.9 5.9 5.9 5.7 5.9 with displacements of the order of 1/10 of those observed in the older units. thus, the fault scarps should have surface expression in their projection northwestwards into the nw basaltic platform and southeastwards into the caldeira volcano. this fact suggests slowing down of fault activity in late pleistocene and holocene. the present low seismicity, the large displacement accumulated by the older volcanic units, and the fact that no deformation is observed in the youngest areas of the island suggest that slip-rates must have been higher in the near past. this period with higher sliprates was followed by a phase dominated by important magmatism, during which caldeira volcano was built and the nw basaltic platform developed. this decrease of slip-rates must have increased the recurrence interval between fault rupturing-events. these observations in graciosa are consistent with the occurrence of variations in the deformation rates in the archipelago, with periods with slip-rates higher or lower than the average motion predicted by global plate models, as proposed by madeira [1998]. in that work the author estimated slip-rate values for a set of faults of the central group of azores archipelago based on a detailed neotectonic analysis through tectonic mapping, chronostratigraphic correlations, and paleoseismological studies performed in são jorge, pico, and faial islands [madeira 1998, madeira and brum da silveira 2003]. however, while in são jorge, pico, and faial, neotectonic data [madeira 1998, madeira and brum da silveira 2003] indicates recent slip-rates higher than those predicted by global plate kinematic models [demets et al. 1994, sella et al. 2002, fernandes et al. 2003, demets et al. 2010], in graciosa what is observed is the opposite. the deformation integrated in a longer time frame is larger than what is observed for the last tens of thousands years. maximum expected magnitudes (mw) were estimated for the shf, ssff, esff, nsbf, and ssbf, using the surface rupture length (srl)/magnitude (mw) and the rupture area (ra)/magnitude (mw) correlations of wells & coppersmith [1994], assuming the rupture of the full known length of the faults, a brittle crust thickness of 14 km, based on the depth of the aftershocks from the january, 1st 1980 and july, 9th 1998 earthquakes (hirn et al. 1980; matias et al. 2007), and average fault dips of 70º. the estimated mw values range from 5.7 to 5.9 (table 5). however, these are minimum values for the expected maximum magnitudes because the total length of the faults must be longer than the values used since the faults are concealed by recent volcanism and the small size of the island limits the observation of the full length of the faults that certainly extend to offshore areas. nonetheless, the magnitudes estimated from neotectonic studies are within the range of magnitudes of instrumental seismicity in the archipelago (table 1). 5. final remarks several limitations prevent a more detailed neotectonic survey of graciosa. recent volcanic and epiclastic deposits mantling the topography conceal the surface expression of major tectonic structures. in most faults, exposed in lapilli quarries, basaltic lapilli deposits do not allow the generation of linear kinematic markers (slickensides). no faults were found presenting scarp heights and lithological conditions (e.g. absence of lava flows) favourable to be trenched for paleoseismological studies, thus contributing to seismic hazard assessment. the paleostress analysis suggests that the region is affected by two different stress fields that can alternate in time, which are responsible for the generation of new faults or reactivation of pre-existing structures. the data suggest a diffuse eu-nu plate boundary expressed by a complex deformation zone, evidencing variable width in time, probably caused by changes in spreading rate between the eu-na and nu-na sections of the mar, north and south of the triple junction. thus the islands closer to the edge of the shear zone may become under the influence of different (inner and outer) stress fields. the absence of seismic events producing surface rupture since settlement and the current low seismic activity in graciosa island, contrast with the youthful aspect of its tectonic morphology. the loss of geomorphic expression of the fault scarps in the areas covered by the younger units suggests the occurrence of a period of important tectonic activity before ca. 31 ka, with higher slip-rates than those observed in present times, responsible for the tectonic deformation of the central part of the island. despite the present low seismicity, historical records show that a few significant earthquakes affected graciosa, since settlement in mid-15th century, causing fatalities and damage. maximum magnitude values are probably underestimated due to unknown full length of the faults and as result of the island small size. currently, graciosa seems to present low tectonic activity. nevertheless, as the island is located in an important seismogenic zone, seismic hazard cannot be disregarded. the difficulty in developing paleoseismological studies constitutes a significant problem given that the neotectonic information available from traditional structural field analysis comprises a too long time window of the geological record than what is required hipólito et al. 14 15 for seismic hazard assessment. on the other hand, the historical and instrumental seismic data available for the azores region only portray a very short period of the tectonic history of the analysed faults. further neotectonic work, such as acquisition of bathymetric and seismic reflection surveys offshore graciosa and re-evaluating the possibility of a paleoseismology analysis, will contribute to better understanding the tectonic processes and to characterize seismic hazard of emerged areas at the western segment of the eu-nu plate boundary. 6. data and sharing resources maps were made using arcgis 9.3 software, esri®. topographic map of graciosa island is from igeoe instituto geográfico do exército (2001). st. cruz da graciosa (graciosa-açores) folha 21. in: carta militar de portugal série m889 2nd ed. instituto geográfico do exército, lisboa. world topography and bathymetry from gebco_08 database (2010), iho-unesco, general bathymetry chart of the oceans, digital edition at http://www.gebco.net/data and products/gridded bathymetry data/. seismic map of graciosa region was produced with data from civisa database, centro de informação e vigilância sismovulcânica dos açores, centro de vulcanologia e avaliação de riscos geológicos da universidade dos açores. structural analysis was made using tectonicsfp® software [ortner et al. 2002]. tectonic plate motion parameters calculated using the science product support plate motion calculator from unavco, http://www.unavco.org/community science/science-support/crustal motion/dxdt/model.html. acknowledgments. ana hipólito is supported by a ph.d. grant from fundação para a ciência e tecnologia (sfrh/bd/73664/2010). fieldwork was supported by the azores regional government / serviço regional de protecção civil e bombeiros dos açores through the project “emergency planning studies”, contract ref.fgf/2005. we appreciated and acknowledge the reviewers francesco mazzarini, joão fonseca, and an anonymous reviewer, the associate editor salvatore barba, and the volume editor edoardo del pezzo for their careful reviews, suggestions, and helpful comments which contributed to significantly improve this manuscript. references acosta, j., e. uchupi, a. munoz, p. herranz, c. palomo, m. ballesteros and z.e.e. working group (2003). geologic evolution of the canarian islands of lanzarote, fuerteventura, gran canaria and la gomera and comparison of landslides at these islands with those at tenerife, la palma and el hierro, mar. geophys. res., 24, 1-40. altamimi, z., p. 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(1980). seismic stresses in the region azoresspain-western mediterranean, rock mech., suppl. 9, 75-84. vogt, p.r. and w. y. jung (2004). the terceira rift as hyper-slow, hotspot dominated oblique spreading axis: a comparison with other slow-spreading plate boundaries, earth planet. sci. lett, 218, 77-90. doi:10.10116/s0012-821x(03)00627-7. wells, d. l. and k. j. coppersmith (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seism. soc. am., 84 (4), 974-1002. yang, t., y. shen, s. lee, s. c. solomon and s.-h. hung (2006). upper mantle structure beneath the azores hotspot from finite-frequency seismic tomography, earth planet. sci. lett., 250, 11-26, doi: 10.1016/j.epsl.2006.07.031. *corresponding author: ana hipólito, centro de vulcanologia e avaliação de riscos geológicos da universidade dos açores, cvarg, açores, portugal; email: ana.rc.hipolito@azores.gov.pt © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. hipólito et al. 18 223_234 caputo.pdf 223 annals of geophysics, vol. 46, n. 2, april 2003 diffusion with space memory modelled with distributed order space fractional differential equations michele caputo dipartimento di fisica, università di roma «la sapienza», roma, italy abstract distributed order fractional differential equations (caputo, 1995, 2001; bagley and torvik, 2000a,b) were fi rst used in the time domain; they are here considered in the space domain and introduced in the constitutive equation of diffusion. the solution of the classic problems are obtained, with closed form formulae. in general, the green functions act as low pass fi lters in the frequency domain. the major difference with the case when a single space fractional derivative is present in the constitutive equations of diffusion (caputo and plastino, 2002) is that the solutions found here are potentially more fl exible to represent more complex media (caputo, 2001a). the difference between the space memory medium and that with the time memory is that the former is more fl exible to represent local phenomena while the latter is more fl exible to represent variations in space. concerning the boundary value problem, the difference with the solution of the classic diffusion medium, in the case when a constant boundary pressure is assigned and in the medium the pressure is initially nil, is that one also needs to assign the fi rst order space derivative at the boundary. mailing address: prof. michele caputo, dipartimento di fisica, università di roma «la sapienza», p.le a. moro 2, 00185 roma, italy; e-mail: mcaputo@g24ux.phys. uniroma1.it key words distributed order – fractional order – differential equations – constitutive equations – diffusion – space fractional derivative 1. introduction fractional order derivatives to model physical phenomena have been widely used in the last few decades in basic mathematics (graffi , 1936; oldham and spanier, 1974; ross, 1975; caputo, 1994, 1995; kiryakova, 1994; podlubny, 1994a,b, 1999; flores and osler, 1999; luchko and gorenfl o, 1999; bagley and torvik, 2000a,b) and in applied research (le mehaute and crépy, 1983; jacquelin, 1991; wyss, 1986; caputo, 1989, 2000; körnig and müller, 1989; mainardi, 1996; caputo and plastino, 2002; belfi ore and caputo, 2000). fractional differential equations consisting of sums of fractional order derivatives have been extensively studied by podlubny (1994a,b, 1999). fractional differential equations where the order of differentiation has been integrated over a given range, and therefore there is no single order of differentiation, have been used and studied (caputo, 1967, 1995) to represent phenomena where the order of differentiation varies in a given range. bagley and torvik (2000a,b) studied extensively these distributed order differential equations (dode), developed a very interesting mathematical theory and gave series expansion solutions. in ordinary differential equations, where the derivatives appear with integer order, the order of the equation is given by the maximum order 224 michele caputo of differentiation appearing in the equation. the same could be true also when the orders of differentiations are real; however when this order is smaller than unity it makes no difference on the properties of the solutions. the same applies when the order of differentiation is distributed in a range 0 < a < b < 1 and therefore the name dode emphasises the property that the order of differentiation is distributed in a continuum. caputo (2001a) solved the classic problems of anelastic, dielectric media and of diffusion when the classic constitutive equations of these media are described in the time domain with dode. the scope of this note is to fi nd more fl exible models of the diffusion of fl uid introducing space domain dode in the constitutive equa-tion of diffusion which should allow more fl ex-ible representations of non local phenomena. specifi cally we will use dode in modelling solutions of problems in diffusion which have been already considered in the literature with constitutive equations containing time domain fractional derivatives of a given order (wyss, 1986; mainardi, 1996; caputo, 2000) and also with space domain fractional order differential equations (fode) (caputo and plastino, 2002; mainardi et al., 2001), assuming now that the fractional order of space differentiaton is integrated over a given range generating thus a dode. the solutions to the problems of diffusion are here found in a half space where the boundary values are applied to the plane limiting it; among others is the computation of the green function of the pressure in the case when the pressure in the half space is initially constant with the fi rst and second order space derivative. the problem is solved with the use of the laplace transform (lt) and closed form formulae are obtained in the space-time domain. it has been shown by cisotti (1911) that the memory introduced in the dielectric constant implies that the phenomenology of the medium is irreversible. it has also been shown also that in anelastic media the stress strain relations with fode are causal (bagley, 1991). in an extensive study of energy storage in electric networks, jacquelin (1991) used the complex frequency dependent impedance represented by transforming into a fode the relation between the induced electric current and the electric fi eld. jacquelin’s (1991) discussion is practically extended to almost all possible circuits; he illustrates his results in the frequency domain at steady state, when the fresnell terms of the fractional order derivatives are nil, and his technique, based on the observations at many frequencies, relies on convolution for the retrieval of the response in the time domain. in general, we may state that the formalism of fode is now spread to many linear studies of dissipative and dispersive properties of many anelastic and dielectric media (e.g., caputo, 1967, 1989, 1996; le mehaute and crépy, 1983; körnig and müller, 1989; jacquelin, 1991; mainardi, 1996). the studies of diffusion in porous media are very important in various fi elds of science and also for social needs especially in the case of water and pollutants. the diffusion is currently modelled to represent the diversifi ed forms of deviations from the classic constitutive law and several complex mathematical methods for modelling the phenomena have been introduced, ranging from non linearity to statistical mechanics and memory formalisms. christakos et al. (1995) used stochastic diagrammatic analysis of groundwater flow in heterogeneous porous media, hu and cushman (1991) used a non equilibrium statistical mechanical derivation of a non-local darcy’s law for unsaturated/saturated flow, kabala and sposito (1991) studied the diffusion using a stochastic model of reactive solute transport with time-varying velocity in heterogeneous aquifers. recently, cushman and moroni (2001) presented a theory with statistical mechanics origin to simulate fickian, quasi-fickian, convolution fickian and fractional fickian fl uxes fi nding a dispersive fl ux which is a convolution in space. we may also add that appropriate experimental data are not yet available to verify most theories. in this note, we will consider an extension of the constitutive relation of diffusion to the case when a space memory mechanism operating in the medium is represented by fode whose order covers a continuum in a given range; in 225 diffusion with space memory modelled with distributed order space fractional differential equations which case the constitutive equation becomes a dode. the time fractional order derivative of the pressure represents the local variations and is particularly valid when considering local phenomena. in an infi nite medium it more appropriate to introduce the space fractional order derivative to represent the effect of the medium and its space interaction with the fl uid. therefore, the fl ow is not directly related to the instantaneous pressure gradient in the measurement site but to the spatial fractional derivative i.e. to the pressure gradient investigated by the fl uid in the path from the starting point to the measurement site. we will consider the porous medium fi lling a half space where the initial conditions are assigned and the boundary conditions are assigned in the plane bounding it and fi nd the closed form solution of the initial and boundary value problems. 2. the constitutive equation with space derivative model and the solution of the governing equation in order to fi nd the general solution of the problem, that is the pressure distribution in the porous media, we begin setting the constitutive equations. the fi rst equation is the continuity equation between the time variation of the density and the divergence of the fl ux (2.1) another constitutive equation is that relating the pressure to the variation of the density from its undisturbed condition (2.2) successively, to take into account the observed deviations of the fl ow from those implied by the classic diffusion equation we introduce, as follows, a space memory formalism in the classic darcy’s law: q u t uq t p u t k( , ) ( , ) ( , ) / .− + =0 0 ∂ ∂ ∂ρ ∂q x t x x t t( , ) / ( , ) /+ = 0 p x t k x t( , ) ( , ).= ρ (2.3) q b a a n p x dn p xn n a b = − − [ ] −∫( / ( )) ( ) / /α ∂ ∂ β∂ ∂ q u t b a us p u t us p t us us us up u t p t n b a ( , ) ( / ( ))( ) ( , ) ( ) ( , ) (( ) ( ) ) / log ( , ) ( , ) = − − −[ ] −[ ] − −[ ] −α β 1 0 0 uq t p u t k t b a us p u t us p t us us us up u t p t n b a ( , ) ( , ) / ( / ( ))( ) ( , ) ( ) ( , ) (( ) ( ) ) / log ( , ) ( , ) 0 0 0 1 − = = − − −[ ] −[ ] − −[ ] − ∂ ∂ α β ukq t kp t k b a us p t us us us p u t t p u t k b a us us us us u k b a n b a ( , ) ( , ) ( / ( ))( ) ( , ) (( ) ( ) ) / log ( , ) / ( , ) ( / ( ))( ) (( ) ( ) ) / log 0 0 01− − − ] −[ ] = + − − − −[ ] −β α ∂ ∂ α β lt ukq t kp t k b a us p t us us us k b a us us us us u k v u v p u t v b a n b a , , , / , / log / / / log , , / 0 0 0 0 1 ( ) − ( ) − −( )( )[{ ( ) ( )] ( ) − ( )( )[ ]} − −( )( )( ) ( ) − ( )( )[{ + ]} = ( ) − ( ) − − β α ν α β ν ααk /([{ βb a us us us us u kn b a / log−( )( ) ( ) − ( )( ) + )]} q u t b a us p u tn a b ( , ) ( / ( )) ( ) ( , )= − − +[[∫α us p t dn p x( ) ( , ) /− ] −− β∂ ∂1 0 ukq t kp t k b a us p t us us us k b a us us us us u k p u t p u k b a us b a n b a n ( , ) ( , ) ( / ( ))( ) ( , ) (( ) ( ) ) / log exp / ( ))( ) (( ) ( ) ) / log ( , ) ( , ) exp ( / ( ))( ) 0 0 0 0 1− − −[ ]{ −[ ]} [{ − − + ]} = = − − −β α α β α (((( ) / log us us us u k b a +[{ −( ) ) + ]}β p u t p u k b a us us us us u k ukq t kp t k b a us p t us us us k b a us n b a b a n ( , ) ( , ) exp ( / ( ))( ) (( ) ( ) ) / log ( , ) ( , ) ( / ( ))( ) ( , ) (( ) ( ) ) / log exp ( / ( ))( ) = − +[{ − + }] + − +[{ − − ] − ][ } − − 0 0 0 01 α β β α α (((( ) ( ) ) / log us us us u k b a−[{ + ]}β q u t uq t p u t k t( , ) ( , ) ( , ) /= − =0 0∂ ∂ 226 michele caputo with m = 1, where u is the lt variable, we obtain from eq. (2.4) the following equation: (2.6) where . proceeding to the solution we take the lt of eq. (2.5) with respect to the t variable and obtain (2.6') where n is the new lt variable of the lt with respect to t. from (2.6') follows: (2.7) where v u v p u tt v( , ) ( , ),= lt and p u p xx u( , ) ( , ),0 0= lt . substituting n = iw in (2.7) we see that in the diffusion the pressure is affected by a low pass fi lter. the solution p(x,t) is then obtained by inverting both lt. the ltt v, −1 of eq. (2.7) gives 0 £ n < 1, where clearly the memory mechanism is applied to the pressure and not to the pressure gradient. the defi nition of derivative of fractional order n is (caputo, 1969) where w is the integration variable. the factor a(n), as in the work of caputo (1995), is a weight of the fractional derivative of order n in the range [a,b]. the factor 1/(b-a) normalizes the integration with respect to n and implies that when b æ a the integral in eq. (2.3) gives ∂ ∂a np x t x( , ) / , with 0 < a < 0.5, which is the case studied by caputo and plastino (2002). we note that in eq. (2.3) the fractional derivative (the memory mechanism) operates on the pressure rather than on the pressure gradient (the fl ux), which was done in previous work (caputo and plastino, 2002); in fact it seems easier to conceive the space memory limiting its mechanism to a rate of variation of lower order (order n (0 < n < 1)) of the pressure rather than to a rate of higher order (order 1 + n) as when applying it to the gradient of the pressure. replacing p(x, t)/k from eq. (2.2) in eq. (2.1) and taking into account the derivative with respect to x variable in eq. (2.3) we obtain a single equation in p(x,t) . (2.4) in order to normalize the dimensions, we will here assume a(n) = s n− , where s has the dimension of a length. since 0 < a < n < b < 1/2, choosing s near to unity implies that all fractional derivatives of order in the range [a,b] have practically the same weight factor. in order to solve eq. (2.4) we fi rst take its laplace transform (lt) respect to x variable; using the lt theorem (caputo, 1969) (2.5) ∂ ∂ α ∂ ∂p t k b a a n p x dnn n a b / ( / ( )) ( ) /= − [ ] ++ +∫ 1 1 β∂ ∂k p x/+ 2 2 p u t lt p x tx u( , ) ( , ),= ∂ ∂n np x t x( , ) / = ∂ ∂n x n x w p w t w dw/ ( ) ( ) ( , ) /−[ ] − [ ]−∫1 1 0 γ ltx u m n n m m m m m f x u u f u u f u f f u f , ( ) ( ) ( ) ( ) ( ) ( ) ( ) .... ( ) ( ) + − − − − [ ] = − +[ − − − − ] 1 2 1 1 0 0 0 0' ∂ ∂ α β α ∂ ∂ β ∂ p u t t k us us b a us u u p u t k p t u p t x us us b a us k up t p t b a b a ( , ) / (( ) ( ) ) / ( ) log( ) ( , ) ( , ) ( , ) / (( ) ( ) ) / ( ) log ( , ) ( , ) − −[ − + ] = = − +[ ] − − − + + + − + + 1 1 2 2 1 1 10 0 0 0 // ∂x[ ] vv u v p u k us us b a us u v u v lt u t u t k p t u p t x us us b a b a t v b a ( , ) ( , ) (( ) ( ) ) / log ( , ) ( , ) ( , ) ( , ) ( , ) / (( ) ( ) ) / ( ) log( , − − −[ −( ) ( ) + ] = = − +[ ] − − + + − + + 0 0 0 1 1 2 1 1 1 α β α ∂ ∂ ξ ξ usus k up t p t x ) ( , ) ( , ) / + − +[ ]β ∂ ∂0 0 v u v p u v ku k us us b a us k us us b a us p t u p t x v k us us b a b a t v b ( , ) ( , ) / ( (( ) ( ) ) / ( ) log ) ((( ) ( ) ) / ( ) log ( , ) ( , ) / / (( ) ( ) , = − + −[ − ] − −{ − ) + ][ } − − − − + + − + 0 0 0 2 1 1 1 1 1 β α α ∂ ∂ α lt ++ + + − −[ ] + − + +[ − − − −[ ] a t v b a b a us ku k up t p t x v k us us b a us ku ) / ( ) log ( , ) ( , ) / / (( ) ( ) ) / ( ) log , β β ∂ ∂ α β 2 1 1 2 0 0lt 227 diffusion with space memory modelled with distributed order space fractional differential equations (2.8) where the asterisk indicates convolution and its subscript indicates that the convolution is made with respect to the variable t; performing the ltt v, −1 we fi nd (2.8') as shown in the appendix the ltt v, −1 of eq. (2.8') gives fi nally (2.9) where the asterisks indicates convolution and the subscripts t or x indicate that the convolution is made with respect to the variables t or x respectively. separating initial conditions from boundary conditions we fi nd (2.9') (2.9') is the formal lt domain solution of the problem and includes the boundary condition p(0,t), in the terms of the 3nd, and 8th lines and the boundary condition p(0,t)/ x in the 5th, 6th and 10th lines the initial condition p(x,0) appears in the terms of 1st and 2nd line. the inverse lts appearing in (2.9') are computed in the appendix and give the fi nal solution in the time and space domain expressed in terms of bestimmte integrals. these inverse lt have vague similarities with the mittag leffl er function and stronger similarities with the functions found by caputo (2001a) and appearing in the solution of the diffusion problem when the memory of the corresponding constitutive equation is expressed in the form of a dode in the time domain. it is to be noted that the boundary conditions depend on the pressure on the boundary p(0,t) but also on its fi rst space derivatives p(0,t)/ x which may be arbitrarily assumed. the presence of a p(0,t)/ x in (2.9') is due to the use of the lt in the x domain which is applied to a derivative of p u t v u v p u v k us us b a us ku k us us b a us p t u p t t v t v b a u b a ( , ) ( , ) ( , ) / ( (( ) ( ) ) / ( ) log (( ) ( ) ) / ( ) log ) ( , ) ( , ) / , , = = = − −[{ − − ]} − −{ − + − − + + + + − lt lt 1 1 1 1 1 1 1 0 1 0 0 α β α ∂ ∂xx v ku us us b a us ku t t v b a u [ ]} − −[{ − − ]} + − + + * / ( (( ) ( )) / ( ) log ) ,lt 1 1 11 α β − +[ ] − −[{ − − ]} − + + β ∂ ∂ α β k up t p t x v k us us b a us ku t v t v b a lt lt , , ( , ) ( , ) / * / (( ) ( ) ) / ( ) log 0 0 11 1 1 2 p u t v u v p u tk us us b a us tku t v b a ( , ) ( , ) ( , ) exp ( (( ) ( ) ) / ( ) log( )) ,= = − − +{ } + − + + lt 1 1 1 2 0 α β − − − +[ + ] ( ) − ( )( )[ + + − + + α ∂ ∂ α k us us b a us p t u p t x tk us us b a b a ((( ) ( ) ) / ( ) log ) ( , ) ( , ) / exp / 1 1 1 1 1 0 0 ( ) log , , / exp / log . b a us ku k up t p t x tk us us b a us ku b a − + ] − ( ) + ∂ ( ) ∂[ ] ( ) − ( )( ) −( ) −[ ]+ + β β α β 2 1 1 2 0 0 b a us kp t us us b a us tk u tku us us x u b a b a −( ) } − −{[ − } + −{ − + + + + log ( , ) (( ) ( ) ) / ( ) log exp (( ) ( ) ) / ,α β α 0 1 1 1 2 1 1 lt b a us k p t x u us us b a us tk u tku us us t x u b a b a −( ) }] − − −{ }[ + ( ) − ( )( ){ − − + + − − log ( , ) / * (( ) ( ) ) / ( ) log exp / , α ∂ ∂ β α 0 1 1 1 1 2 lt b a us kp t lt u tk us us b a us t ku x u b a −( ) }] − { − − +[ ]} + − + + log ( , ) exp (( ) ( ) ) / ( ) log ,β α β 0 1 1 1 2 − [{ + − − + ]} − + + β ∂ ∂ α β k p t x tk us us b a us ku t t x u b a ( , ) / * exp (( ) ( ) ) / ( ) log ,0 1 1 1 2 lt p x t lt p u t p x lt tk u u u b a u x u x x u b a , , , exp / log , ,( ) = ( ) = ( ) + −( ) −( ) )({ } + − ∗ − + + 1 1 2 1 1 0 β α − − −[ ] +[{ + ] + −( )([ − + + − + + lt k u u b a u p t u p t x tk u u u x u b a b a , (( ) / ( ) log ) ( , ) ( , ) / exp / 1 1 1 1 2 1 1 0 0 α ∂ ∂ β α ( ) log ) , , / exp / log b a u k up t p t x tk us us b a us ku x u b a − ]} − ( ) + ∂ ( ) ∂[ ]{ ( ) − ( )( ) −( ) +[ ]}+ + lt , -1 β α β 0 0 1 1 2 p x t p u t p x tk u tk us us x u x x u b a ( , ) ( , ) ( , ) exp (( ) ( )) / , , = = + −{ − ∗ − + + lt lt 1 1 2 1 1 0 β α 228 michele caputo order 1 + n and therefore, according to theorem (2.5), implies in (2.9') the presence of the 1st order derivatives of p(x,0) with respect to x. the presence of b p(0,t)/ x in (2.9') is also due to the use of the lt in the x domain which is applied to a derivative of order 2 and therefore, according to theorem (2.5), implies in (2.9') the presence of the 1st derivative of p(x,0) with respect to x. we also note that in eq. (2.9') we have two types of convolution, one relative to the time variable and one relative to the space variable. moreover, we note that the fi rst term in eq. (2.9') takes into account the initial values of the pressure in the medium while the other terms take into account the boundary values. the computation of the term with the initial value implies the convolution relative to the space variable only while the computation of the terms relative to the boundary values imply convolutions relative to the time variable only. as a check it is seen that eq. (2.9'), when b æ a, gives the particular case when the constitutive eq. (2.3) is a space fode which was studied by caputo and plastino (2002); in order to see this in detail it is suffi cient to substitute b = a + e, compute the limit of (2.9') for e æ 0 and fi nd that ua in (2.9') substitutes the term u u b a ub a1 1− −−( ) −( )/ log whose limit, when b æ a, is ua . with the extreme value theorem applied to eq. (2.8') it is verifi ed that p(0,0) = 0. 3. the initial value problem a case of interest is when the pressure and its fi rst respect to the x variable on the boundary (x = 0) are nil for any t while the pressure in the medium p(x,t) has initial (t = 0) constant value p(x,0). we will fi rst assume a 0 and b = 0 and solve later the general case a 0 and b 0. the solution is then readily obtained from eq. (2.9') considering only the fi rst term (3.1) using the expression of (a.2) of appendix for the integrand and substituting the values of c, d, l, m of (a.5) and (a.6) in the appendix in eq. (a.3) of the same appendix; we fi nd (3.2) once more one may verify the computations assuming b = a + e , computing the limit of (3.2) when e æ 0 and fi nding that the exponent of the exponential becomes tk rs aaα π( )( )+1 cos and the argument of the sine becomes tk rs aaα π( ) +1 sin as in the work of caputo and plastino (2002). assuming that the pressure in the medium has initial (t = 0) constant value c 0, performing the convolution and grouping the exponential terms in (3.2) we obtain (3.2') with the extreme value theorem applied to eq. (2.7) it is verifi ed that p(x,0) = c. p x t lt p u t p x tk us us b a us x u x x u b a ( , ) ( , ) ( , ) exp (( ) ( ) ) / ( ) log . , , = = − −{ }[ ] − ∗ − + + 1 1 1 1 0 lt α p x t p x rx tk rs nb rs a rs rs b rs a b a rs tk rs b rs x b a b a b ( , ) ( ( , ) / ) exp( )sin (( ) sin ( ) sin log ( ) cos ( ) cos ) / ( )(log ) exp cos = = − [{( − ) − +( − ] − + } −( ) + ( ∗ + ∞ + + + + ∫ 0 1 0 1 1 1 2 2 1 π α π π π π π π α π )) )([[ + − ( ) − ( ) ]( −( ) +( )]) + + + 1 1 1 2 2 a b a a rs rs b rs a b a rs dr cos log sin sin / log . π π π π π p x t c rx r tk rs nb rs a rs rs b rs a b a rs b a b a ( , ) ( / ) (( exp( )) / )sin (( ) sin ( ) sin ) log ( ) cos ( ) cos ) / ( )(log ) = = − − [{( − − +( − ] − + } + ∞ + + + ∫ π α π π π π π π 1 1 0 1 1 1 2 2 exp ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) / ( )(log ) . tk rs b rs a rs rs b rs a b a rs dr b a b a α π π π π π π − + +[[ − − ] − + ]) + + + + 1 1 1 1 2 2 229 diffusion with space memory modelled with distributed order space fractional differential equations the solution of the problem with b 0, a 0 is found considering that the portion of eq. (2.9') of interest in this section is (3.2'') which may be written and seeing also that in the appendix is found ltx u b atk us us us. exp ( ( ) ( ) ) / (log ) − + +− + ]{[1 1 1 2 2α π and fi nally that ltx u tku, exp( ) − [ ]1 2β is known since it is the solution of the classic initial value case without memory. the effect of the memory is obviously represented by the exponential and sine terms which contain a in eq. (3.2'). an analysis of the integrand of eq. (3.2') shows again that in order to ensure the convergence, in the exponential term containing the time t, it must be 0 < b < 1/2 which implies a negative decreasing value of the exponent when r is relatively large. in (3.2') we see that the increase of p(x,t), with increasing, x is affected by a only indirectly, that is, at a given time p(x,t) increases with x at a rate dependent of a only through the sine and the exponential terms, in the integrand of (3.2'), which are not depending on x. 4. the boundary value problem in order to give a better description of the solution (2.9'), we will specialize it to the case when p(0,t) is assigned and p(x,0) = p(0,t)/ x = = 0. note that p(0,t)/ x = 0 does not imply that the fl ux is nil at the boundary since the fl ux is given by eq. (2.3). equation (2.9') is then (4.1) in order to compute the ltx u, −1 in eq. (4.1) we assume that y(u,t) is the analyitic function of the complex variable u resulting from the product of the two functions in braces of eq. (4.1) and consider eq. (a.2) and (a.3) of the appendix (4.2) and introduce from eqs. (a.7) and (a.8) of the appendix, the values of l, m, c, d (4.3) we obtain p x t p u t p x lt tk u tk us us b a us u x x u b a ( , ) ( , ) ( , ) exp (( ) ( ) ) / ( ) log , , = = + −{[ − }] − ∗ − + + ltx 1 1 2 1 1 0 β α p x t p x tk us us us lt tku x x u b a x x u ( , ) ( , ) exp (( ) ( ) ) / (log ) exp( ) , , = = −{[ + [ ][ ∗ − + + ∗ − 0 1 1 1 2 2 1 2 lt α π β p x t kp t us us b a us tk us us b a us t x u b a b a ( , ) ( , ) (( ) ( ) ) / ( ) log exp (( ) ( ) ) / ( ) log . ,= −{[ − } −{ − }] ∗ − + + + + α α 0 1 1 1 1 1 lt ltx u u t rx c l d m d dr , ( , ) ( / ) exp( ) exp ( sin cos ) − ∞ = = − +∫ 1 0 1 ψ π c tk rs b rs a rs rs b rs a b a rs b a b a = − + +[ − − ] − + + + + + α π π π π π π ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) ) / ( )(log ) 1 1 1 1 2 2 d rs b rs a rs rs b rs a b a rs b a b a = − +[ − − ] − + + + + + (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ) 1 1 1 1 2 2 π π π π π π l rs b rs a rs rs b rs a b a rs b a b a = − + +[ − − ] − + + + + + ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) ) / ( )(log ) 1 1 1 1 2 2 π π π π π π m rs b rs a rs rs b rs a b a rs b a b a = − +[ − − ] − + + + + + (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ) 1 1 1 1 2 2 π π π π π π exp ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) ) / tk rs b rs a rs rs b rs a b a b a α π π π π π − + +[{ − − ] + + + + 1 1 1 1 ltx u u t rx, ( , ) ( / ) exp( ) − ∞ = −   ∫ 1 0 1ψ π 230 michele caputo (4.4) assuming p(0,t) = d (t), we obtain from eqs. (4.2) the green function of the problem. the discussion of the case when b 0, a 0 made in the section 4 may be applied also to the case studied in this section. the solution of the boundary value problem with p(0,t) = c is readily obtained also from eq. (3.2'). in fact indicating with p3(x,t) the solution (3.2'), the solution of the boundary value problem is c – p3(x,t). 5. conclusions the major difference between the diffusion in medium with a constitutive equation of the type (2.3) and that whose constitutive equation contains a fractional derivative of given order (caputo and plastino, 2002), is that the solutions found here are potentially more fl exible to represent more complex systems (caputo, 2001a). the difference between the space memory medium and that with the time memory is that this is fl exible to represent local phenomena while the latter is more fl exible to represent variations in space. b a rs rs b rs a rs rs b rs a b a b a −( ) +( )} − +[{[ ( ) − − ] + + + + log ( ( ) cos cos ) log (( ) sin ( ) sin ) ) / 2 2 1 1 1 1 π π π π π π b a rs kt rs b rs a rs rs b rs a b a rs b a b a −( ) +( )} +[{ − − + − − + } + + + + + log sin (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ) 2 2 1 1 1 1 2 2 π α π π π π π π + − +[{ − − ] − + } ( ) +([{ + + + + + (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ) cos sin rs b rs a rs rs b rs a b a rs kt rs b b a b a b 1 1 1 1 2 2 1 π π π π π π α π − − + − ] − + }]) + + + ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ) . rs a rs rs b rs a b a rs dr a b a 1 1 1 2 2 π π π π π concerning the initial value problem the solution (3.2') may be compared with that of caputo and plastino (2002) who used the constitutive eq. (2.4) with a single fractional order derivative of order a instead of a dode. the solution of caputo and plastino (2002) is (5.1) which is valid for 0 < a < 1/2. the solution (5.1) is similar to (3.2'), it is the presence of the additional free parameter b which possibly makes (3.2') more fl exible than (5.1) to represent more complex types of diffusion. with more detail the two solutions differ for the presence in (3.2') of the terms, (rs)1+b cosbp (rs)1+a cosap, (rs)1+b sinbp (rs)1+asinap and of their weighted differences, which represents the average effect of the memory when the order of differentiation is integrated over the range [a,b]. the discussion made for the initial value problem solution, and its comparison with that of the case when in the constitutive eq. (2.4) a fode is used instead of a dode, is valid also for the boundary value problem solution represented by eqs. (4.1) through (4.5). we conclude that besides the difference in the number of parameters which may be used to represent the more complex types of diffusion, there is no great difference between the solution of the dode constitutive eq. (2.4) used here and the solution found by caputo and plastino (2002) in the case when a fode constitutive eq. (2.4) is used. the introduction of fractional order derivatives in the equations governing economy and fi nance have been proved successful also modeling fi nancial phenomena (caputo and kolari, 2001; caputo, 2001b). p x t c xr r ktr r a ktr r a dr a a ( , ) ( / ) (( exp( )) / )sin( ( cos )) exp( ( cos ) = = − − + ∞ − + ∫π α π α π β 1 2 0 2 1 231 diffusion with space memory modelled with distributed order space fractional differential equations appendix. in order to compute the inverse lt of eq. (2.9') we set u = r exp (iq) (a.1) in the complex plane of fi g. 1 and integrate along the path shown there. we then note that the contribution to the integral from the inner circle of radius r1 when r1 0→ , is nil since the path of integration of the fi nite function is nil; also when the radius of the outer circle r, when r → ∞ the contribution of the integral is nil since the function itself is nil. the integral is then reduced to the integration along the two lines parallel to the negative real axis and to the line in the real plane and parallel to the imaginary axis which give the ltu x, −1 of eq. (2.9'). in the points of the line from – • to 0 (q = p) the integrand is the complex conjugate of the value in the same points on the line from 0 to • (q = -p). in order to compute the ltx u, −1 in eq. (4.1) we assume that y(u,t) is the analytic function of the complex variable u resulting from the product of the two functions in braces of eq. (4.1). glossary a, b limits of the distribution of the fractional order derivatives. k (s-2 m2) ratio of the fl uid pressure to the fl uid density (see eq. (2.2)). n fractional order of differentiation (see eqs. (2.3) and (2.4)) (dimensionless). p(x,t) (kg s-2 m-1) fl uid pressure. q(x,t) (kg s-1 m-2) fl uid mass fl ow rate in the porous medium. r1, r radius of the inner and outer circles, respectively, of the integration path of eq. (a.1) shown in fi g. a.1. s (m) variable normalizing the dimensions. t (s) time. x (m) distance from the boundary plane. a (s mn ) coeffi cient of the darcy’s law modifi ed (see eq. (2.3)). ak (s-1m2+n ) pseudodiffusivity. r (x,t) (kg m-3) fl uid density. w, e imaginary and real parts in the plane of the integral in eq. (a.1) and shown in fi g. a.1. the difference of the two, in all terms appearing in eq. (2.9'), may be reduced to the form (a.2) ∞ ∞ = − − − = − − = − ] =    ∫∫ 0 0 1 2( / ) exp( ) ( , ) exp( ) ( , )ψ ψi rx r dr rx r drπ θ π θ π lt− − ∞ + ∞ = − =∫1 1 2( ( )) ( / ) exp( ) ( )ψ ψu i xu u du i i π ξ ξ = − = − = −[1 2( / ) exp( ) ( , ) ( , )ψ ψi rx r r drπ θ π θ π ]] =    ∞ ∫ 0 232 michele caputo where (l + im)exp(c + id) and (l – im)exp(c – id) represent the values of the integrand of (2.9') when the argument of m in the complex plane is q = p and q = p respectively. the integral along the path of fi g. 1 gives then (a.3)p x t p u t rx l d m d c drx u( , ) ( , ) ( / ) exp( )( sin cos ) exp( ) .,= = − + − ∞ ∫lt 1 0 1 π = − +{ } = ∞ ∫( / ) exp( ) (exp )sin (exp ) cos1 2 0 πi rx il c d im c d dr = − +[ ] ∞ ∫( / ) exp( ) exp sin cos1 0 π rx c l d m d dr = − − −[ ] +( / ) exp( exp (exp exp( ) exp exp1 2πi rx l c id id im c idid id dr+ −[ ]{ } = ∞ ∫ exp( ) 0 fig. a.1. path of integration in the complex plane ltu x, −1 in eq. (2.9'). b and iu are the real and imaginary parts respectively. = − + + − − −[ ] = = − + − −[ ] + + + −[ ]{ } = ∞ ∞ ∫ ∫ ( / ) exp( ( ) exp( ) ( ) exp( ) ( / ) exp( (exp( ) exp( ) exp( ) exp( ) 1 2 1 2 0 π π i rx l im c id l im c id dr i rx l c id c id im c id c id dr o (a.2) 233 diffusion with space memory modelled with distributed order space fractional differential equations we now proceed to the computation of the inverse lt appearing in (2.9') beginning with the fi rst term (the factor convolving p(x,0) in lines 1 and 2) of eq. (2.9') (a.4) we fi nd l = 1, m = 0, (a.5) for the second term (the factor convolving ap(0,t) in lines 3 and 8) of eq. (2.9') we fi nd c, d, l, m (a.7) for the third term (the factor convolving a p(0,t)/ x in lines 5 and 10) of eq. (2.9') we fi nd again c and d as in eq. (a.7) and d tk rs b r b rs a a rs b a rs b a = − +[ − − + ] − + + + α π π π π π π π ( ) (sin log cos ) ( ) ( cos sin log ) / ( )(log ). 1 1 2 2 c tk rs b rs a rs rs b rs a b a rs b a b a = − + +[ − − ] − + + + + + α π π π π π π ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) ) / ( )(log ) 1 1 1 1 2 2 d tk rs b rs a rs rs b rs a b a rs b a b a = − +[ − − ] − + + + + + α π π π π π π (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ) 1 1 1 1 2 2 l rs b rs a rs rs b rs a b a rs b a b a = − + +[ − − ] − + + + + + ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) ) / ( )(log ) 1 1 1 1 2 2 π π π π π π exp (( ) ( ) ) / ( ) logtk us us b a usb aα 1 1+ +− −{ } c tk rs b rs a rs rs b rs a b a rs b a b a = − + +[ − − ] − + + + + + α π π π π π π ( ( ) cos ( ) cos ) log (( ) sin ( ) sin )) / ( )(log ) 1 1 1 1 2 2 m rs b rs a rs rs b rs a b a rs b a b a = − +[ − − ] − + + + + + (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ). 1 1 1 1 2 2 π π π π π π l r rs b rs a rs rs b rs a b a rs b a b a = − − + +[ − − ] − + − + + + + 1 1 1 1 1 2 2 ( ( ) cos ( ) cos ) log (( ) sin ( ) sin ) ) / ( )(log ) π π π π π π m r rs b rs a rs rs b rs a b a rs b a b a = − − +[ − − ] − + − + + + + 1 1 1 1 1 2 2 (( ) sin ( ) sin ) log (( ) cos ( ) cos ) ) / ( )(log ). π π π π π π (a.6) 234 michele caputo references bagley, r.l. 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(1996): fractional relaxation-oscillation and fractional diffusion-wave phenomena, chaos, solitons and fractals, 7, 1461-1477. mainardi, f., yu. luchko and g. pagnin (2001): the fundamental solution of space-time fractional diffusion equation, fractional calculus and applied analysis, 4 (2), 153-192. oldham, k.b. and j. spanier (1974): the fractional calculus (academic press). podlubny, i. (1994a): analytical solution of linear differential equations of the fractional order, in proceedings of the 12th imacs (intern. assoc. for mathematics and comp. in simulation) world congress, atlanta, u.s.a. podlubny, i. (1994b): numerical solution of initial value problems for ordinary fractional-order differential equations, in proceedings imacs (intern. assoc. for mathematics and comp. in simulation) world congress, atlanta, u.s.a. podlubny, i. (1999): fractional differential equations (academic press, new york). ross, b. (1975): a brief history and exposition of the fundamental theory of fractional calculus, lect. notes math. (springer verlag), vol. 457. wyss, w. (1986): fractional diffusion equation, j. math. phys., 27, 2782-2785. g e o c r o n o l o g i a e p a l e o m a g n e t i s m o d e l v u l c a n o r o c c a m o n f i n a (***) (geochronology and paleomagnetism of roccamonfina volcano) m . c o r t i l i ( * ) 1st. r o b e r t i ( * ) e . s c a n d o n e ( * * ) r i c e v u t o il 15 aprile 1975 r i a s s u n t o . — è s t a t o eseguito u n d e t t a g l i a t o s t u d i o geocronologico, col m e t o d o k a r , e p a l e o m a g n e t i c o sulle rocce p i ù a n t i c h e a f f i o r a n t i del v u l c a n o di r o c c a m o n f i n a . i p r i m i episodi v u l c a n i c i sono r a g g r u p p a t i int o r n o a 1 m.a., m e n t r e l ' a t t i v i t à p r i n c i p a l e si è a v u t a t r a 0.4 e 0.5 m.a. il secondo ciclo di a t t i v i t à inizia a 0.25 m.a., dopo la f o r m a z i o n e di u n a c a l d e r a . i d a t i p a l e o m a g n e t i c i i n d i c a n o u n episodio a p o l a r i t à n o r m a l e a 1.10 + 0.05 m.a.; l ' e s i s t e n z a di u n t a l e e v e n t o e r a s t a t a s u g g e r i t a d a w a t kins (14) nel 1968. non si h a n n o p r o v e di altri e v e n t i m a g n e t i c i in rocce pili giovani. s u m m a r y . — a detailed k a r a n d p a l e o m a g n e t i c s t u d y w a s m a d e on t h e oldest o u t c r o p p i n g l a v a s of t h e r o c c a m o n f i n a volcano. t h e v e r y first episodes of volcanism were c o n c e n t r a t e d a t a r o u n d 1 m.y., w h e r e a s m o s t of t h e a c t i v i t y occurred b e t w e e n 0.4 a n d 0.5 m . y . a f t e r t h e f o r m a t i o n of a c a l d e r a a second cycle of volcanism s t a r t e d a t 0.25 m . y . t h e p a l e o m a g n e t i c m e a s u r e m e n t s i n d i c a t e an episode of n o r m a l p o l a r i t y a t 1.10 + 0.05 m.yr. t h e occurrence of t h i s episode w a s suggested b y w a t k i n s (14) in 1968. t h e r e is no evidence of o t h e r m a g n e t i c e v e n t s on y o u n g e r r o c k s . (*) i s t i t u t o di geologia e geofisica d e l l ' u n i v e r s i t à , l a r g o s. marcellino 10 n a p o l i . (**) osservatorio v e s u v i a n o e r c o l a n o (napoli). (***) rie,crea e f f e t t u a t a con i c o n t r i b u t i finanziari del c n r c o n t r a t t o n . 73.000289.05 e c o n t r i b u t o n . 74.00031.05. 1 3 0 m . c o r t i n i x . r o b e r t i i i . s c a n d o n e i n t r o d u z i o n e il vulcano roccamonfina è compreso tra le valli dei fiumi gari<iliano e volturno ed è circondato dai rilievi carbonatici mesozoici della piattaforma campano-lucana. appartiene al vulcanismo alcalino potassico dell'italia centro-meridionale. è un vulcano strato con due principali cicli di attività: il primo ciclo è caratterizzato da prodotti sottosaturi a leucite (tefriti, fonoliti, basaniti); il secondo ciclo, iniziato dopo la formazione di una caldera terminale, ha dato principalmente trachiti, latiti, trachibasalti. mentre l'attività del primo ciclo è stata prevalentemente effusiva, in quella del secondo le piroclastiti predominano sulle cupole e le colate laviche. gli ultimi prodotti del roccamonfina sono ricoperti dalla ignimbrite campana. l'inizio dell'attività del roccamonfina non è chiaramente determinata su basi stratigrafiche (c. bergomi et al., 1969) (2). i terreni sedimentari più giovani affioranti sotto le vulcaniti sono del miocene superiore, ma è chiaro che la maggior parte dell'attività vulcanica si è sviluppata durante il quaternario. le uniche età assolute pubblicate in precedenza (p. gasparini e j . a. s. adams, 1969) (8) danno un'età di 1.26-1.07 m.a. su una leucitite nella zona di s. maria di mortola e un'età di 0.46 m.a. su una tefrite leucitica nella zona di s. carlo. è stata inoltre determinata un'età di 0.368 m.a. da j . f . evernden e g. h. curtis (1965) (7) su una lava del versante orientale della caldera. i n base a questi dati si poteva ritenere che l'attività vulcanica del primo ciclo del roccamonfina fosse stata più o meno continua fra circa 1.0 m.a. e 0.4 m.a., e per tale ragione a d a t t a a determinare accuratamente il passaggio di polarità magnetica f r a le epoche di brunhes e matuyama (0.69 m.a.). è in tale prospettiva che è stato iniziato questo lavoro. come si vedrà dai dati geocronologici risulta invece che l'attività del primo ciclo del roccamonfina si è concentrata principalmente fra 0.56 e 0.4 m.a., e solo alcuni episodi hanno u n ' e t à intorno a 1 m.a. g e o c r o n o l o g i a le datazioni sono state effettuate con il metodo k-ar. l'40ar rad è stato misurato mediante spettrometria di massa con la tecnica della g e o c r o n o l o g i a e p a l e o m a g n e t i s m o d e l v u l c a n o r o c c a m o n f i n a 1 3 1 diluizione isotopica, il k è stato misurato mediante assorbimento atomico. le strumentazioni, la tecnica e le costanti usate per i calcoli sono descritti in dettaglio in l. civetta et al. (1971)(3). le analisi geocronologiche sono state effettuate sui campioni che all'esame microscopico apparivano più freschi, quando possibile è s t a t a separata la leucite. la misura della composizione isotopica dell'aiè stata eseguita in duplicato per alcuni campioni, principalmente rocce intere, quando la percentuale di 40ar rad era bassa. l'ubicazione dei campioni e la loro petrografia sono riportati in tabella 1. tutti i campioni si riferiscono al i ciclo di attività, eccetto rmp 31 il quale appartiene all'attività iniziale del i i ciclo (campione 11. 8 pag. 280 di m. r. ghiara et al. (1973)(9) gentilmente fornito dal t a b e l l a 1 c a m p i o n e u b i c a z i o n e tipo p e t r o g r á f i c o r m p 9 k m 20.7, ss 430 t e f r i t e leucitica r m p 11 k m 15.9, ss 430 (sotto la c a v a ) l e u c i t i t c r m p 12 k m 18.5, ss 430 t e f r i t e leucitica r m p 15 k m 18.2, ss 430 t e f r i t e l e u c i t i c a r m p 17 c a v a t r a i k m 10-18, ss 430 l e u c i t i t e r m p 18 s t r a d a li paoli-corigliano, 200 m dopo il p a e s e di li p a o l i stessa località della l a v a di s. carlo, di g a s p a r i n i e a d a m s ( 1909)(8) t e f r i t e leucitica r m p 19 km 18.0, ss 430 l e u c i t i t e r m p 22 l o c a l i t à p o n t e dei g r o t t o n i , sulla s t r a d a sessa a u r u n c a g r o t t o n i f o n o l i t e l e u c i t i c a r m p 23 s t r a d a r o c c a m o n f i n a s i p i c c i a n o , v e r s a n t e w di collo alto l o c a l i t à n . 48, t a v . i di b e r g o m i e t al. (1969) t e f r i t e leucitica r m p 23' s t r a d a sessa a u r u n c a g r o t t o n i , 1 k m d o p o la l o c a l i t à p o n t e dei g r o t t o n i t e f r i t e leucitica r m p 24 stessa località di r m p 23, m a a valle della s t r a d a f o n o l i t e leucitica r m p 30 c a v a a b b a n d o n a t a , k m 13.8, ss 430 l e u c i t i t e r m p 31 s t r a d a r o c c a m o n f i n a c a i a n e l l o , f r a t a v o l a e c a m p a g n o l a p o m i c i t r a c h i t i c h e 2 g e o c r o n o l o g i a e p a l e o m a g n e t i s m o d e l v u l c a n o r o c c a m o n f i n a 1 3 3 prof. l. lirer). t u t t i i campioni sono stati prelevati sul versante occidentale del vulcano che è quello meglio conservato. i risultati ottenuti sono riportati nella tabella 2, insieme a quelli già citati da j . f . evernden e g. h. curtis (7) e da p . gasparini e j . a. s. adams (8). gli errori riportati sulla determinazione del k si riferiscono allo scarto massimo tra le 3 aliquote usate per la misura; l'errore riportato per l'ar (a), è calcolato con la formula di a. cox e g. b. dalrymple (1967)(n) quando è s t a t a eseguita una sola misura di ar. per le misure in duplicato è lo scarto fra i risultati delle due determinazioni. la riproducibilità delle misure è sempre migliore dell'errore calcolato con la formula di a. cox e g. b. dalrymple (6). le basse percentuali di 40ar rad, ottenute per alcuni campioni di roccia intera sono dovute ad alta contaminazione di ar atmosferico contenuto nella roccia. i campioni rmp i l e rmp 17 sono stati raccolti nella stessa cava da cui proviene il campione rm 1 di santa maria di mortola analizzato da p. gasparini e j . a. s. adams (8). la media pesata dei risultati, calcolata assegnando ad ogni misura un peso proporzionale alla concentrazione di 40ar rad, dà un'età di 1.20 ± 0.08 m.a. p a l e o m a g n e t i s m o t u t t i i campioni analizzati hanno polarità normale e sono stati prelevati dagli stessi affioramenti dai quali provengono i campioni datati. misure di stabilità magnetica sono state effettuate su un campione pilota per ogni formazione por mezzo di smagnetizzazione con campi magnetici alternati (fig. 1). il campione rmp 16 è stato smagnetizzato termicamente presso l ' i n s t i t u t für angewandte geophysik di münchen. per t u t t i i campioni sono state misurate le temperature di curie (fig. 2). t u t t i i campioni sono privi di rilevanti componenti secondarie. la magnetizzazione rimanente naturale è di origine termorimanente ed è attribuibile alla presenza ili titanomagnetite. i risultati di polarità per i campioni più giovani sono in accordo con la scala temporale delle epoche di polarità magnetica (a. cox, 1969(5); d. f . stacey, 1969)(13). i l campione rmp 30 (età 0.34 ± 0.03) cadrebbe nella zona v a polarità inversa ipotizzata da d. f . stacey sulla base di dati non pubblicati. tale campione, anche se ha un'inclinazione magnetica più bassa rispetto a quella degli altri campioni, ha t u t t a v i a 1 3 4 m . c o r t i n i n . r o b e r t i r . s c a n d o n e una polarità normale. si può quindi ipotizzare ohe la zona v, piuttosto che un'evento di polarità magnetica, rappresenti invece una larga escursione del campo magnetico (m. w. me elhinny, 1973)(10). i campioni più vecchi cadono invece nell'intervallo di età attorno all'evento di jaramillo. fig. 1 c u i v a di s m a g n e t i z z a z i o n e con c a m p i m a g n e t i c i a l t e r n a t i del c a m p i o n e k m p 17. fig. 2 c u r v a t e r m o m a g n e t i c a per la d e t e r m i n a z i o n e della t e m p e r a t u r a di curie del c a m p i o n e r m p 15. le frecce i n d i c a n o il ciclo di r i s c a l d a m e n t o e quello di r a f f r e d d a m e n t o . g e o c r o n o l o g i a e p a l e o m a g n e t i s m o d e l v u l c a n o r o c c a m o n f i n a 137 t a b e l l a 3 c a m p i o n e n d ( o) i(o) e « 9 5 ( ° ) ii (oe) e m p 9 3 1 6 5 7 3 4 2 1 2 0 0 r m p 1 2 4 3 5 8 5 2 3 4 9 5 6 0 0 r m p 1 5 4 2 6 4 5 6 8 1 4 1 2 0 0 e m p 1 6 4 2 0 6 4 2 6 5 5 6 0 0 e m p 1 7 4 3 0 6 1 1 2 3 8 6 0 0 e m p 1 8 7 3 4 7 5 5 1 3 8 5 m r n r m p 1 9 3 3 3 7 6 4 2 2 6 8 2 0 0 r m p 3 0 4 7 2 5 1 4 5 0 3 m r n n = n u m e r o di c a m p i o n i a n a l i z z a t i d = declinazione i = inclinazione k = p a r a m e t r o di precisione di fislier a05 = c i r c o n f e r e n z a con il 9 5 % di confidenza ii = v a l o r e massimo del c a m p o m a g n e t i c o a l t e r n a t o in oersted u s a t o p e r s m a g n e t i z z a r e i c a m p i o n i p i l o t a mentre l'età del campione e m p 19 coincide perfettamente con quella dell'inizio dell'evento, l'età dei campioni della colata al km 16.2 della strada del garigliano, è leggermente più alta (1.20 ± 0.08 m.a.) di quella che segna la fine dell'evento di jaramillo 0.93 ± 0.03 (n". d. opdyke, 1969) (12). c o n c l u s i o n i l'istogramma in fig. 3 mostra che l'attività del i ciclo del roccamonfina è caratterizzata da una fase t r a 1.2 e 0.9 m.a. e da un periodo più intenso intorno a 0.5 0.4 m.a. i prodotti più giovani datati per il primo ciclo danno un'età di 0.34 i 0.03 m.a. per il campione e m p 30 ed un'età di 0.368 ± 0.02 m.a. per il campione datato da j . p . evernden e g. h. curtis (7). dopo la formazione della caldera terminale si ha una ripresa dell'attività d a t a t a a 0.25 ± 0.04 m.a. sulle pomici trachitiche (campione e m p 31). 1 3 0 m . c o r t i n i n . r o b e r t i r . s c a n d o n e nella zona presa in esame (versante occidentale della caldera), non sono stati ritrovati campioni nell'intervallo di età del passaggio tra le epoche di polarità magnetica di brunhes e matuyama (0.69 m.a.). n 6 4 0 0.5 1 1.5 m a fig. 3 i s t o g r a m m a delle e t à disponibili per il v u l c a n o di r o c c a m o n f i n a . la polarità magnetica dei campioni delle lave al km 16.2 di età 1.20 i 0.08 m.a., è discordante con la scala di polarità magnetica attualmente adottata. si possono formulare due ipotesi per spiegare questa discordanza: o l'evento di jaramillo ha una d u r a t a maggiore di quella determinata precedentemente sui sedimenti oceanici (ma questo sembra poco probabile dato l'alto numero di campioni esaminati), oppure la magnetizzazione normale di tale lava può essere attribuita ad un episodio di breve durata successivo all'evento di jaramillo. n. d. watkins (1968)(14) aveva suggerito l'esistenza di episodi molto brevi a polarità normale nell'epoca di matuyama, alle età di 0.82 e .1.07 m.a. tali episodi potrebbero essere misurati sui sedimenti oceanici solo in presenza di velocità di sedimentazione assai elevata. non è possibile dire con certezza se l'evento trovato sulle lave al km 16.2 coincide con quello ipotizzato da n. d. watkins (14), oppure si t r a t t i di un altro evento. nel calcolo dell'età media di 1.20 i 0.08 m.a., il peso maggiore è stato attribuito all'età ottenuta sulla fase leucitica da p. gasparini e j . a. s. adams (1909)(8). questa età è più elevata, al di fuori dei limiti degli errori analitici, delle età ottenute sulle corrispondenti rocce intere. ciò potrebbe essere dovuto alla presenza di un leggero eccesso di 40ar nella fase analizzata, fn effetti essa ha una concentrazione in iv di g e o c r o n o l o g i a e p a l e o m a g n e t i s m o d e l v u l c a n o r o c c a m o n f i n a 1 3 7 circa 11%, che fa sospettare la presenza di una notevole frazione di plagioclasi, nei quali sono frequentemente state osservate concentrazioni di 40ar estraneo, dell'ordine di io12 moli/gr. non considerando il dato relativo alla fase leucitica, l'età media diviene 1.10 i 0.05 m.a., praticamente coincidente con l'età dell'evento data da n. d. watkins (1968) (14). è infine da notare che l'attività principale del primo ciclo del roccamonfina si colloca nell'intervallo fra 0.1 e 0.7 m.a., periodo che sembra sia stato di massima attività per t u t t a la fascia vulcanica alcalina tirrenica. i n f a t t i , oltre al roccamonfina, in questo periodo erano attivi il vulture (m. cortini, 1975) (4) e i vulcani del lazio (p. basilone e l. civetta, 1975; m. nicoletti, 1969) (i-"). r i n g r a z i a m e n t i ringraziamo il prof. maino e i dott. bergomi e manganelli per l'aiuto prestato nella raccolta di alcuni campioni e il prof. lirer per aver fornito il campione di pomici trachitiche che data l'inizio del secondo ciclo dell'attività del roccamonfina. b i b l i o g r a f i a ( j ) b a s i l o n e p . e l . c i v e t t a , 1 9 7 5 d a t a z i o n e k-ar dell'attività vulcanica dei mt. emici (latina). « r e n d . soc. i t . min. p e t i . » , 3 1 , (i), p p . 1 7 5 1 7 9 . ( 2 ) b e r g o m i c . , v . c a t e n a c c i , g . c e s t a r i , m . m a n f r e d i n i , v . m a n g a n e l l i , 1969. — note illustrative della carta geologica d'italia, f . 171, c a s e r t a . ( 3 ) c i v e t t a l . , m. c o r t i n i , p . g a s p a r i n i , r . s c a n d o n e , 1971. il laboratorio k-ar dell'istituto di fisica terrestre dell'università di napoli. « r e n d . soc. i t . min. p e t r . », 2 7 , p p . 309-319. ( 4 ) c o r t i n i m . , 1975. età k-ar del monte vulture (lucania). « r i v . i t . geof. », 2, 1, p p . 45-46. (6) c o x a . , 1 9 6 9 . geomagnetic reversal. « s c i e n c e » , 193, p p . 2 3 7 2 4 5 . (°) c o x a . , e g. b . d a l r t m p l e , 1 9 6 7 . statistical analysis of geomagnetic reversal data and the precision of the potassium-argon dating method. « j . g e o p l i y s . r e s . » , 7 2 , p p . 2 6 0 3 2 6 1 4 . 1 3 8 m . c o r t i n i n . r o b e r t i r . s c a n d o n e ( 7 ) e v e r n d e n j . f . e g. ii. c u r t i s , 1965. the potassium-argon dating of late cenozoic rock in east africa and italy. « curr. a n t l i r o p . », 6, p p . 343304. ( 8 ) g a s p a r i n i p . e j . a . s . a d a m s , 1 9 0 9 . k-ar dating of italian pliopleistocene volcanic rocks. « e a r t h p l a n . sci. l e t t . », 6, p p . 225-230. ( 9 ) g u i a r a m. r . , l . l i r e r e d . s t a n z i o n e , 1973. contributo alla conoscenza vulcanologica e petrográfica del vulcano di. roccamonfìna. « p e r i o d . min. », 42, fase. i i i , p p . 267-293. 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( 1 4 ) w a t k i n s n . d . , 1 9 6 8 . short period geomagnetic polarity events in deep-sea sedimentary cores. « e a r t h p l a n . sci. l e t t . », 4, p p . 341-349. layout 6 anomaly disturbances of the magnetic fields before the strong earthquake in japan on march 11, 2011 yuri a. kopytenko1, valery s. ismaguilov1, katsumi hattori2, masashi hayakawa3 1 spbf izmiran, st-petersburg department of izmiran, st-petersburg, russia 2 graduate school of science, chiba university, chiba, japan 3 university of electro-communications, chofu, tokyo, japan annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5260 abstract one of the strongest earthquakes, with magnitude m 8.9, occurred at the sea bottom near to the east coast of japan on march 11, 2011. this study is devoted to the investigation of anomaly disturbances in the main magnetic field of the earth and in ultra-low frequency magnetic variations (f <10 hz) observed before this earthquake. secular variations of the main geomagnetic field were investigated using three-component 1-h data from three magnetic observatories over the 11-year period of january 1, 2000, to january 31, 2011. the esashi and mizusawa magnetic stations are situated northwest of the earthquake epicenter, at distances of around 170 km to 200 km, and the kakioka observatory is situated southwest of the earthquake epicenter, at a distance of about 300 km. during this period, there were four local anomalies in the secular variations. the last anomaly was the biggest, which began around 3 years prior to the earthquake moment. all of the anomalies can be most distinctly recognized, in the form of differences in the corresponding magnetic components at these remote magnetic stations. for investigations of the ultra-low frequency magnetic field disturbances, three-component 1-s data at two magnetic stations (kakioka and uchiura) were used. the uchiura station is situated 119 km south of kakioka, at a distance of about 420 km from the earthquake epicenter. data from the time interval of february 18, 2011 to march 10, 2011 (only at night-time: 01:00 to 04:00 local time) were investigated in a wide frequency range. in the frequency range of 0.033 hz to 0.01 hz, there was the clearest anomaly, seen as a decrease in the correlation coefficients of the corresponding magnetic components at these two stations, from february 22, 2011. differences in the z components showed an increase, and became positive after this date. this might suggest that the ultra-low frequency lithospheric source appeared north of the kakioka station. outside this specified frequency range, the anomalies were not well defined. 1. introduction one of the strongest earthquakes, with a magnitude mw 8.9 (japan meteorological agency classification), occurred at the sea bottom near the east coast of japan on march 11, 2011. this earthquake triggered a devastating tsunami that killed over 15,000 people. we have studied the precursors of this earthquake in the secular variations of the main geomagnetic field and in the ultra-low frequency (ulf) magnetic disturbances. it is now evident that during the preparation period prior to a strong earthquake, ulf (f <10 hz) lithospheric magnetic emissions with a noise-like character originate from around the site of the forthcoming earthquake [kopytenko et al. 1990, 1993, 2001, 2003, 2007, 2009, bernardi et al. 1991, molchanov et al. 1992, hayakawa et al. 1996, kawate et al. 1998, molchanov and hayakawa 1998, 2008, ismaguilov et al. 2001, 2002, hattori 2004, fraser-smith 2009]. the intensities of the lithospheric ulf disturbances are very weak (usually <0.1 nt), but when we can observe these emissions, the epicentral distances can extend up to 100 km or more [fraser-smith 2009, hayakawa et al. 2007] before strong earthquakes (m >6). modern high-sensitivity magnetometers can detect these weak signals from the lithospheric sources, although there is a problem with strong artificial electromagnetic noise, especially in the industrial areas of japan. additionally, there are ulf geomagnetic pulses of ionospheric origin that can have high amplitudes during disturbed geomagnetic periods. therefore, before an earthquake moment, we can usually observe a superposition of ulf emissions from different sources at the earth surface. the natural ulf geomagnetic variations have ionospheric sources. at the earth surface, the recording points are usually at large distances from the ionospheric sources (many hundreds of kilometers), and the ulf disturbance gradients along the earth surface are very small and depend on the distance from the sources [ismaguilov et al. 1992, kopytenko et al. 2000, 2003]. on the other hand, the local lithospheric ulf sources are situated much closer to the recording point, and their gradients are greater than those of the natural pulses [kopytenko et al. 2000, 2003]. therefore, differential methods of data processing are particularly special issue: earthquake precursors 101 article history received june 21, 2011; accepted october 11, 2011. subject classification: earthquake precursor, magnetic field, ulf magnetic field, 2011 japan earthquake. appropriate to detect these lithospheric signals. the phasegradient method [kopytenko et al. 2000, 2001, 2003, 2007, 2009, ismaguilov et al. 2001, 2002] is based on the data at three remote magnetic stations (ulf magnetic gradientometers). these allow us to find local anomalies in the ulf magnetic field gradients and phase velocities before the earthquake moment, and to estimate the direction to a strong earthquake epicenter during the ‘earthquake preparation phase’. 2. experimental results 2.1. secular variations of the main geomagnetic field figure 1 shows a schematic representation of the epicenter of this earthquake (figure 1, largest yellow star). the white triangles in figure 1 indicate the magnetic stations at kakioka (kak; z = 36.23˚, m = 140.19˚), esashi (esa; z = 39.112˚, m = 141.204˚), mizusawa (miz; z = 39.237˚, m = = 141.355˚), and uchiura (ucu; z = 35.16˚, m = 140.20˚). the esa and miz observatories are situated northwest of the earthquake epicenter, at distances of 170 km to 200 km from it. the distance between esa and miz is 19 km, and the distance between esa and kak is 332 km. the epicentral distance to the kak station is about 300 km. three components (h, d, z) of the secular variations of the constant magnetic field at the esa and kak magnetic stations over an 11-year period (january 1, 2000, to january 31, 2011) are plotted in figure 2, as the 1-h data. these data were received from the world data centre (http://swdcwww.kugi.kyoto-u.ac.jp/index.html). the data were averaged using a 24-point window, to delete sq daily variations. the secular variations in figure 2 are very similar at kak and esa at the scales shown in figure 2. it can be seen from figure 2 that the z components at these magnetic stations increased after 2006, the d components decreased, and the h components slowly decreases during the whole of this 11-year period. the vertical line to the right in figure 2 marks the earthquake moment. in figure 2 (top panel), the daily sum of the kp indices reflects the magnetic 102 kopytenko et al. figure 1. location of the magnetic stations (white triangles) and the areas of strong seismic activity (yellow stars) during the period of january 1, 2000, to january, 2011. figure 2. daily sum of the kp indices (top panel) and secular variations of the three components of the main geomagnetic field during the period of january 1, 2000 to january 31, 2011. the vertical line marks the earthquake moment. 103 activity. the kp index is usually calculated using the level of the ‘magnetic quiet days’ as the ‘zero’ level. however, the magnetic quiet days level is the level of the secular variation, so that kp reflects the activity of the ionospheremagnetosphere sources, and these are not connected with the lithospheric sources. evidently, the secular time variations in the six lower curves in figure 2 are not connected with the ionosphere-magnetosphere magnetic activity. it is very difficult to find any precursors of earthquakes using the source data presented in figure 2. we will use differential values for this purpose. the differences in the corresponding magnetic components of the two pairs of the remote magnetic stations (esa-miz, esa-kak) are plotted in figure 3. the bottom curve of figure 3 is the difference dz(em) = zesa – zmiz. the second curve from the bottom in figure 3 is dz(ek) = zesa – zkak, and so on. the magnitudes of earthquakes (as the us geological survey classification) are indicated at the top of figure 3. only earthquakes with m >4 and with epicentral distances <150 km from the esa station were taken into account. anomaly variations with durations of 1 year to 3 years and of 1 nt to 5 nt amplitude are distinctly seen on the six lower curves. more exactly, the anomalies are seen in the second panel from the top of figure 3, which shows the ratio of the vertical component difference to the horizontal component difference: dz(em)/dh(em) = (zesa – zmiz)/(hesa – hmiz). the black arrows under this curve mark the moments of the anomalies, and the red arrows above the curve mark the moments of seismic shocks with m ≥6. it is apparent that the shocks have a delay of 1 year to 3-years relative to the start of the magnetic anomalies. the latest and the most distinct anomaly shows the longest time interval, and it increased before the strongest earthquake. the seismic activity areas during the 11-year period are shown in figure 1 with the yellow stars, with their magnitudes indicated inside the stars. after the anomaly of 2002, there was an earthquake epicenter about 100 km south of the esa station, near sendai city. in 2005 to 2007, the earthquake sites were situated at the sea bottom at a distance of about 150 km to 200 km southeast of the esa station. in 2008, an earthquake epicenter was situated very close to the esa and miz stations (about 50 km southwest of the esa station). after 2009, seismic activity was seen at the sea bottom, approximately as in 2005 to 2007. 2.2. ulf magnetic disturbances in previous studies, anomaly manifestations in amplitudes, gradients and phase velocities of the ulf magnetic disturbances have been found for 1 month to 3 months before strong earthquakes [kopytenko et al. 2001, 2003, 2007, 2009, ismaguilov et al. 2001, 2002]. in these studies we used the data from three magnetic stations situated at the top of a triangle at the earth surface, spaced at distances of 4 km to 6 km (ulf magnetic gradientometer). the gradient vectors were directed to the local lithospheric source of the ulf variations, and the phase velocity vectors were directed to the opposite side. in the present study we used the 1-s data from the uchiura magnetic station (ucu), which is situated at a distance of about 420 km from the earthquake epicenter and from the kak observatory (kak station epicenter distance, ulf disturbance of the japan earthquake figure 3. magnitudes (m ≥ 4) of earthquakes (top panel) during the period of january 1, 2000 to january 31, 2011. the differences (dh, dd, dz) of the corresponding magnetic components of pairs of remote magnetic stations (esashi-mizusawa – em and esashi-kakioka – ek) (six lower curves). ratio dz/dh for the esashi and mizusawa magnetic stations (second from top). black arrows mark moments of the anomalies, and red arrows indicate the moments of seismic shocks with m ≥ 6. about 300 km). the distance between ucu and kak is 119 km, as seen in figure 1. the data from a time interval of february 18, 2011 to march 10, 2011 were used (for the last three weeks before the earthquake). as during the day-time the man-made noise in japan is very high, only the night-time data were used (01:00 to 04:00 local time). the data were initially filtered using a band-pass filter for different period ranges of the ulf variations: 10 s to 30 s, 30 s to 100 s, and 100 s to 300 s. the results of this data processing are presented in figure 4, where the filled blue squares represent the h magnetic component, the black crosses, the d component, and the filled red circles, the z component. the following values are plotted in figure 4, from top to bottom: – the root mean square (rms) of the magnetic components at the kak station; – the differences dh = hkak – hucu, dd = dkak – ducu, dz = zkak – zucu; – the ratios z/h (blue squares) and z/d (black crosses), as the vertical component divided by the horizontal one for the kak station; – cor(h,d,z), as the correlation coefficients of the corresponding magnetic components of these remote stations. the period range of 30 s to 100 s in figure 4 is more interesting than the other period ranges shown. the correlation coefficients of the z components of the kak and ucu magnetic stations sharply decreased 18 days before the earthquake moment (march 11, 2011). the differences in the z components increased and became positive. outside this specified frequency range, the anomalies are not so well defined. ratios z/h, z/d $ z/h, z/d decrease towards the earthquake moment, and the rms values changed according to the variations in the magnetic activity throughout the entire period range. 3. discussion and summary two geomagnetic poles in the secular variations closest to japan are situated in eastern siberia and in the pacific ocean, as based on the computations by demina et al. [2008] using a dynamic model of sources of the main geomagnetic field. a long period displacement of these poles leads to large-space changes in the secular magnetic variations in the japan area. when we compare the six curves shown in figure 2, we can conclude that the space scale of the variations is very large. in addition, the global geomagnetic activity has no influence on the secular variations (see figure 2). therefore, the anomalies observed in figure 3 should be local, and they have to be situated close to the esa and miz magnetic stations. the black arrows under the second curve from the top in figure 3 (ratio of the vertical component difference and the horizontal component difference for esa and miz) mark four anomalies that were discovered. the red arrows indicate the moments of the earthquakes with m ≥6 (usgs classification). apparently, the shocks have a 1 year to 3 year delay relative to the starts of the magnetic anomalies. kopytenko et al. 104 figure 4. rms of the three magnetic components (kakioka observatory) during the period of february 18, 2011 to march 10, 2011 (top curves). differences (dh, dd, dz) of the corresponding magnetic components of a pair of remote magnetic stations (kakioka-uchiura, second from top). cor(h,d,z), as the correlation coefficients of the corresponding magnetic components of the kakioka and uchiura stations. blue squares, h; black crosses, d; and red circles, z component. ratios z/h (blue squares) and z/d (black crosses), vertical component divided by the horizontal one (kakioka, second from bottom). 105 moreover, these earthquakes were located in different zones of seismic activity (figure 1, yellow stars). the seismic zone (m = 7.0) appeared after the magnetic anomaly in 2002. this was located about 100 km south of the esa station, near to sendai city. the average depth of the seismic hypocenter was about 37 km (we considered shocks with m ≥ 4). the magnetic anomaly of 2005 showed before the beginning of the seismic activity at the sea bottom, at a distance about 150 km to 200 km southeast of the esa station (m = 6.9). the average depth of the seismic hypocenters was about 54 km. during 2008, the earthquake (m = 6.0) epicenter was situated very close to the esa and miz stations, and the average depth of the seismic hypocenter was about 18 km. after 2009, the seismic activity was at the sea bottom in a location more eastern than in 2005 to 2007, and the average depth of the seismic hypocenter was 36 km. the seismic activity in 2005 to 2007 and after 2009 was located in the subduction area, and it was probably connected with movement of the pacific plate [simon 2011]. seismic activity at greater depths usually has a tendency to appear earlier in almost all of the subduction zones [molchanov 2011]. we considered only earthquakes with m ≥4, and found that there was no seismic activity in the subduction zone from 2007 to 2009. the start of the anomalies in the total magnetic field vector before the earthquakes was observed earlier [sasai and ishikawa 1980, mogi 1985]. these authors interpreted the effect as a local change in the conductivity in the earth crust. actually, the tectonic movements lead to the generation of an increased temperature and consequently to higher conductivity in the earthquake site region. telluric currents reallocate near to the local high conductivity region and create a magnetic anomaly at the earth surface. the signs of the anomaly depend on the location of the magnetic stations relative to an active seismic area [sasai and ishikawa 1980]. additionally, during the earthquake preparation period, a surface slope can change due to the tectonic processes. a change in the magnetic sensor orientation is another possible reason for the generation of the anomaly. for instance, a tilting of the sensor at 0.01˚ leads to a 6.6 nt change in the horizontal magnetic component. molchanov and hayakawa [1998] assumed that the generation of the ulf seismogenic electromagnetic emissions is a natural consequence of a microfracturing process. according to this theory, the intensity of the ulf emissions increases exponentially with the frequency of the emissions. from the other side, attenuation of the emissions in the earth crust depends on distance and conductivity, and is inversely proportional to the square root of the emission frequency. the kak and ucu magnetic stations are situated at large distances from the earthquake site (about 300 km and 420 km, respectively), and therefore mainly long period disturbances are expected to be observed at these stations. industrial noise is very high in the higher frequency regions of the ulf emissions. the lowest curves in figure 4 show the correlation coefficients of the corresponding magnetic components of the kak and ucu magnetic stations for the period ranges of 3 s to 10 s, 30 s to 100 s, and 100 s to 300 s. the values used are the mean values for the night-time interval of 01:00 to 04:00 local time. we observe a sharp decrease in the coefficients before the earthquake moment of march 11, 2011. kopytenko et al. [2009] found an increase in the correlation coefficients before the moment of an earthquake at the boso peninsula in 2002. nevertheless, the situation was different in that case. the magnetometers at the boso peninsula were spaced at a small distance (about 5 km). the lithospheric emissions thus arrived at the magnetic stations with very close phases, because the phase velocity of the ulf disturbance propagation was very high (tens of km/s), and the correlation coefficients have to increase when we expect an increase in the intensity of the lithospheric emissions. in the present case, the distance between the ucu and kak stations is 119 km, so that the lithospheric source was around 300 km distant [simon et al. 2011]. therefore, the lithospheric emissions arrived at the magnetic stations of kak and ucu with different phases and the correlation coefficients must decrease when the intensity of the lithosphere emissions increases. the h and d magnetic components of the ulf emissions are mostly connected with ionospheric sources. the earth crust has a greater influence on the z component than on the horizontal ones [kovtun 1980]. it can be seen from figure 4 that the clearest anomaly is in the period range of t = 30 s to 100 s, as a decrease in the correlation coefficients that was seen from february 22, 2011. the top parts of figure 4 show the rms of the three magnetic components recorded at the kak observatory. geomagnetic disturbances took place on february 18, 2011, and on march 1-7, 2011, and the rms values reflect the magnetic activity of the magnetosphere-ionosphere origin, as is seen in figure 4. the differential values (bkak – bucu) must be proportional to the rms values if we assume that sources of the ulf magnetic variations are in the ionosphere (db = b1 – b2 = b1(1-k), here b2 = kb1). the amplitudes of the natural magnetic variations is usually higher than those of the magnetic emissions of lithospheric origin, but the gradients are very small because of the very long wavelength. the differences in figure 4 (the second curves from the top) have to be proportional to the rms (for the case of natural variations). however, we observe a more complicated picture, especially in the period range of 30 s to 100 s. it is thus likely that besides the natural sources, there are additional ulf disturbance sources. the positive differences in the z component (z and d components for the period range 10 s to 30 s) signify that the ulf magnetic disturbance sources are situated north of the kak observatory. ratios z/h and z/d decreased toward the ulf disturbance of the japan earthquake earthquake moment (figure 4, second curves from the bottom). similar time variations of the ratios were observed previously [yanagihara 1972, hayakawa et al. 1996]. outside the specified frequency ranges, the anomalies are not so well defined (for f >0.1 hz, due to a high level of industrial noise, for f < 0.0033 hz, due to the high amplitudes of the natural disturbances). in summarizing these results, we can conclude the following: – during the period of 2000 to 2011, there were four local anomalies in the secular variations. the last of these anomalies was the biggest one, and it began about 3 years prior to the earthquake moment. all of these anomalies are most distinctly seen in the form of the differences of the corresponding magnetic components at these remote magnetic stations. the distinctive form of the anomalies in the second curve from the top in figure 3 can be explained by a tilting of the magnetic sensors situated at the esa and miz observatories during the development of the tectonic processes, or by a change in the conductivity of the anomaly zone before the earthquake. at the first stage, tectonic pressure in the subduction zone leads to an increase in the temperature and conductivity in the area of the forthcoming earthquake hypocenter. at the second stage, the increases in the numbers and sizes of microfractures lead to 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(2011). underlying mechanism of precursory activity from analysis of upward earthquake migration, natural hazards earth sys., 11, 135-143. sasai, y. and y. ishikawa (1980). tectonomagnetic event preceding a m 5.0 earthquake in the izu peninsula – a seismic slip of a buried fault?, b. earthquake res. inst., univ. tokyo, 55, 895-911. simon, m., s.e. minson, a. sladen, f. ortega, j. jiang, s.e. owen, l. meng, j-p. ampuero, s. wei, r. chu, d.v. helmberger, h. kanomari, e. hetland, a.w. moore and f.h. webb (2011). the 2011 magnitude 9.0 tohoku-oki earthquake: mosaiking the megathrust from seconds to centuries, sciencexpress, 19 may, 2011, 5 p. yanagihara, k. (1972). secular variation of the electric conductivity anomaly in the central part of japan, mem. kakioka magn. observ., 15, 1-10. *corresponding author: masashi hayakawa, university of electro-communications, chofu tokyo, japan; email: hayakawa@whistler.ee.uec.ac.jp. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ulf disturbance of the japan earthquake layout 6 regularities in discrete hierarchy seismo-acoustic mode in a geophysical field kamel baddari1,*and anatoly d. frolov2 1 laboratory of physics of the earth, university of boumerdes, algeria 2 russian academy of sciences, geophysical division ncg, moscow, russia annals of geophysics, 53, 5-6, 2010; doi: 10.4401/ag-4725 abstract some regularities in seismo acoustic mode have been studied during the preparation and development of dynamic events generated during the deformation at different scales in a geophysical field. the time-space behavior of certain auto similarity parameters: the slope of the recurrence plot c, the fractal dimension of the hypocenters set d, the relationship d-3c and the crack concentration parameter ksr in laboratory and field experiments in algerian seismoactive zone have been analyzed as well. precursory stage and local failure evolution in rock samples and in natural conditions were studied. it is shown that the regularities in the behavior of the parameters under study do not qualitatively depend on the dimension of the object being loaded. the quoted examples above speak of inhomogeneity and self-similarity of seismicity distribution in space-time. this suggests that the evolution of cracking process at different scales, from rock samples to earth crust, is controlled by the same physical mechanism. the further development of these studies consolidates the physical basis of the prediction of the dynamic events. 1. introduction a geophysical field is a set of geophysical properties of rock at a level ranging from a rock sample scale to a planet scale [sadovsky and pisarenko 1991, lockner et al. 1992, baddari and frolov 1997, sobolev and ponomarev 2003, vettegren et al. 2004, thompson et al. 2006, eftaxias et al. 2007, liakopouloumorris et al. 2007, smirnov et al. 2010]. the seismic structure is distinguished by its auto-similarity property in the spatial, temporal and energetic domains [gutenberg 1956, smirnov et al. 1995, baddari et al 1996, baddari et al. 1999, sobolev and ponomarev 1999, zang et al. 2000, kanamori 2004, lei and satoh 2007, kuksenko et al. 2009]. the nature of this structure is not defined clearly in situ because of a lack of reliable experimental data. modeling in a laboratory using the inverseloading technique by acoustic emission has allowed the final phase of source development to be extended and the acoustic regime to be analyzed in detail [lockner 1993, sobolev et al. 1996, baddari 1997, smirnov 1997, lei et al. 2004, smirnov and ponomarev 2004, amitrano 2006, xu et al. 2009, yuan and li 2009, dresen et al. 2010, youn and tonon 2010]. this study analyzes the similarity of the dynamic seismo-acoustic regime at different scales of a geophysical field. 2. parameters of the seismo-acoustic regime 2.1. relationship between the recurrence plot slope c and the fractal dimension d independent of its scale, rock failure is a multi-stage process, and the final stages can take different forms [sadovsky and pisarenko 1991]. the fracture results from shear stress, which accumulates during tectonic deformation. a tectonic earthquake can be considered as a final step in a determined phase of a discrete failure stochastic process in the earth crust. this process leads to total or partial stress release over the fracture area, and continues to be developed in different stress fields. pore pressure and effective normal stress evolution affect the dynamic propagation of the earthquake rupture [bizzarri and cocco 2006]. we consider a seismic source of a size li that gives rise to a stress drop in its neighboring space of linear dimension x [smirnov et al. 1995, baddari and djeddi 2009]: (1) after the earthquake, this creates a seismic quiescence, i.e. no earthquake with the same source characteristics can occur in the same domain, during the period dt: (2) where t, c, }, and g are specific constants [smirnov 1995]. taking into account the fractal property of a geophysical medium, we consider that the space of a seismogenic zone article history received may 13, 2010; accepted october 9, 2010. subject classification: geophysical field, seismo-acoustic regime, auto-similarity, fractal structure. 31 x li c= t t li= }d g of size y can be composed of a subspace set of size ri. the number j of the ri-sized subspaces, which contain earthquakes, will be equal to: (3) where d is the fractal dimension of the seismogenic zone. at the same time, the seismic cycle t is also characterized by a fractal structure, which can be divided, and will contain p nonempty subintervals: (4) where the parameter dt has the meaning of a temporal fractal dimension. the seismic energy, as a function of its source size [sadovsky et al. 1991], is given by: (5) with a=3 on average [kasahara 1985]. therefore, if ri =x, we will have the number of events with source size li that occur in a space of size yduring time t equal to: (6) if we substitute equations (1), (2) and (5) into equation (6), we arrive at the seismic recursive general law of the fractal property of seismicity: (7) where: thus, we can deduce the following relationship between the recurrence plot slope c, the fractal dimension d, and the temporal fractal dimension dt: (8) taking a = 3 in equation (8) and assuming c = 1 [smirnov et al. 1995], we get: (9) experimentally, c ≈0.5 and d≈1.5 on average [smirnov 1993, baddari and frolov 1997, smirnov and ponomarev 2004, liu et al. 2009], and therefore we can deduce that the constant g≈0 in equation (9), since dt>0 (dt=0 means that all earthquakes will occur simultaneously). the equality of equation (9) is equivalent to dt=2cm, where cm is the slope of the recurrence curve for magnitudes (cm =1.5 c) [main et al. 1990]. 2.2. crack concentration parameter ksr the transition from microcracks to macrocracks in a loaded object of volume v0 takes place upon reaching a certain critical concentration of cracks in the failure source of the appropriate dimensions. the crucial factor here is the crack concentration parameter, which is calculated using the formula [baddari et al. 1996, zavyalov 2006]: (10) where q−1/3 =n/v0 is the volume density of the acoustic (or seismic) events that occur in the object, and is the average rupture length. the dimensionless parameter ksr reflects the ratio of the mean distance between ruptures and the average rupture length. the average rupture length in the crack set is: (11) where n is the number of acoustic (or seismic) events of energy class ki ∈ [kmin −kmax] appearing in volume v0 during the time dt, li is the size of the opening of microcracks or the unity rupture length: (12) here, f=0.244, d=−2.266 for the energy classes ki, and f= 0.440, d=−1.289 for the magnitudes m (ki =0.244+1.8 m) [sobolev and ponomarev 2003]. the parameter ksr represents the ability of cracks to interact and develop into larger defects, i.e. when the mean distance between cracks becomes equal to a critical value, a macrocrack is formed that causes the development of damage in the rock. 3. experimental technique and methods of original data analysis acoustic emissions by rock samples under failure reflect the process of crack formation and can be considered as a model for the seismicity in the earth crust [han and yang 2009, senfaute et al. 2009]. to determine the auto-similarity of the seismo-acoustic regime at different scales, an unparalleled series of six rock experiments (bf1-6) were carried out under bi-axial compression conditions on models with two layers, using surfaces of 500×260 mm, with the lower layer of dry granite of 420 mm thickness, surmounted by a layer of dry sandstone (or concrete) of 80 mm (figure 1). seismo-acoustic discrete hierarchy 32 ,j y ri d= ^ h ,p t t dt= d^ h e alis = a .n y r t ti d dt= d^ ^h h log log log log ,n e d y d ts t= + + +c xlog log log , . d d cd d a t t = = + tx c } c a g .cd d 0t =a gc3 .d dt= gc,k q l1 3sr = l n l1 1 i i n = = / log ,l ki i= +f d l 33 a petrographic analysis gave the granite composition as follows: 42% albite, 42% quartz, 39% biotite, and an admixture of muscovite, pyrite and other components. the grain size was 1-2 mm. the lower layer was heated up to 400˚c. the objective was to reconcile this with the natural conditions of the superficial algerian seismic events. the formation of the main rupture during the last stage of the process was extended by the addition of a maximum number of microcracks under quasi-static conditions for a few hours, by decreasing the rate of axial deformation once the crack acceleration started. this allows the rheologic state to be reached during the long period of a deformation process. two stress concentrators introduced in the model at a 35˚ angle, with respect to the direction of intermediate vertical charge h, encouraged the cutting (shear fracture) of the lower layer, which then propagated to the upper layer block at an angle of 30˚ to 45˚. an intense evolution of deformations starting from concentrator extremities was noted, which was due to the interactions of their stress fields. the intermediate load h was maintained practically constant at h=50mpa. the mean load f applied to the granite changed over three cycles: (i) f gradually increased up to a maximum (fmax) of typically (0.4-0.5)t/tf, where t is the experimental time, given with respect to the total life-time tf of the experiment; (ii) f was maintained approximately constant up to (0.75-0.8)t/tf; and (iii) fdecreased in the interval [(0.75-0.8)-1]t/tf and then the main rupture of the block began to appear. acoustic signals were detected by resonance piezoelectric transducers of eigen frequency of 0.2 mhz. a special multichannel system was used to record the wave forms. the block surfaces were covered with eight sensors linked to input amplifiers, grs-6052 oscillographs and a computer. a transducer was fixed in the middle of the sample to serve as a controller of the organization of the inverse commands of the piston pressure, and as a means to measure the loading as a function of the acoustic activity. the loading was realized under the condition of invariability of the acoustic event intensity. the acoustic event activity used in the chain of the inverse command was of the first dozens of events per second. exceeding the acoustic event rate activity immediately led to a decrease in the axial load through a reduction in the motion speed of the piston pressure. the strain-control regime was selected in agreement with the concept relative to the process that occurs in the earthquake focal zones. the constant rate of strain, 5 mpa/s, and the ability to vary this during the experiment, allowed control of the stability of the development of the macrofailure in the sample. the load fwas measured at a rate of 1-s sampling on a personal computer. the recording system of the acoustic events was realized by recording the times of the first arrivals and the first maximum amplitudes of the signals of each transducer, under a numerical format. the resolution of the numerical channel for the determination of the arrival time was of the order of 0.04 µs to 0.05 µs. to pinpoint acoustic event sources, a seismological technique was used to find the hypocenter coordinates, through the differences in the acoustic event signals registered over time. we applied an algorithm using the time of the first arrivals of longitudinal waves to the transducer and the minimum propagation time. acoustic events with propagation times greater than 5 µs were eliminated from the acoustic event catalog. nevertheless, the coordinates of the sources of an acoustic event were determined with an error of 1.0 mm to 1.5 mm, according to the acoustic event value. the calculations were realized in windows of 1,000 events, which were varied by steps of 500. the number of acoustic event coordinates determined for the experiments bf1−bf6 were, respectively, 37396, 48332, 42785, 36120, 44280 and 36648, which represents between 48.4% and 38.4% of the total of the recorded signals. the catalog of the acoustic events comprises time, the cartesian coordinates, and the energy class ki of the event (ki=2lga, where a is the amplitude of the impulse in mv brought to the 10 mm distance from the hypocenter). in this case, ki is similar to the energy class used in seismology. we selected the acoustic events with regard to their representative energy class ki, which was estimated from the analysis of the recurrence plots n(ki)=a10 −c (ki−2) (figure 2). the values of the minimal energy class, kmin, were 1.7, 1.8, 1.9 and 1.5, for the experiments bf1, bf2, bf3 and bf4, respectively. the square of the amplitude of the acoustic pulses (a2) is proportional to the energy of an event, within a certain allowance. the control of the anisotropy failure evolution structure seismo-acoustic discrete hierarchy figure 1. schematic diagram of the experiment arrangement. (1) pattern with artificial stress concentrators; (2) acoustic emission sensors (rectangles); (3) apparent resistivity electrodes (circles); (4) self-potential electrodes (triangles); (5) ultrasonic transducers (squares); f, main load; h, intermediate load. elements was realized by studying the elastic longitudinal and transversal wave velocities (vp and vs), the electric apparent resistivity dta, and the spontaneous electric polarization du. to study the variations in the elastic-wave velocities, each free face of a block had eight piezo-electric compressional transducers with a free resonance frequency from 80 khz to 100 khz. these were used as focuses and receivers of ultrasonic signals. in the experiments, mainly longitudinal wave transducers were used, but these can easily be replaced by s-wave transducers. ultrasonic sounding started under noload conditions, but as the cycled loading progressed, a series of measurements were taken along the routes. the number of monitored paths was 27; this enables the study of velocity variations in measuring the p-wave and s-wave arrival times of about 0.7%, and for an amplitude of 10%. the arrival time of the first break was used to calculate the ultrasonic velocities. the elastic-wave velocities were measured following the ultrasound traces at the angles of 25˚, 45˚ and 75˚, with respect to the vertical axis of the sample. the sensors of electric potential were nonpolarizable chlorine–silver electrodes. they were placed on the free faces of the sample. various combinations of pick-offs served as receiving pairs of electrodes mn, which allowed the study of the structure of the electric field in detail. the intrinsic drift of the electrode potentials did not exceed 0.6 mv for a complete loading cycle. the background values had been estimated prior to the experiment, by carrying out an areal survey of the surface potentials on a 2×2 cm grid on all of the faces of the sample. the variations in apparent electric resistivity were studied using direct current and four-electrode devices under different orientations. the receiving dipoles were various combinations of the nonpolarizable sensors mentioned above. this strength allowed us to evaluate relative changes of resistivity both in the entire block and in separate parts of it. the uncertainty of measuring the variations of apparent resistivity was below 2%. for the seismic events, we considered the catalogs of seismicity of north algeria from 1950 to 2004. the russian caucasus and some other regions in the world were considered for comparison [smirnov 1993]. the autosimilarity parameters were calculated in aftershock zones for the space of the earthquake preparation, and included the determined number of seismic events of m≥2 during the time interval dt=1.5 years. each zone (earthquake preparation area) was divided into elementary volumes of linear dimension dx=(5−7) li and depth dz=20 km. the area of the aftershocks constituted the boundary of the scaling field. the recurrence plot slope c is calculated by the maximum-likelihood method [zavyalov 2006]. a regional earthquake catalog of the zemmouri zone that contained 1,414 events was studied following a check for homogeneity. for the measure of the fractal dimension d of a set of acoustic or seismic events, we used the cellular evaluation and correlation, and we interpreted the correlation dimension as the fractal dimension, assuming that the seismicity was near a homogeneous fractal. the correlation dimension dwas calculated by applying the formula [smirnov et al. 1995, baddari and frolov 1997]: (13) where c (l) is the correlation integral, which is defined as: (14) where n is the number of pairs of seismic events with distances between hypocenters that are less than l, and m is the number of events. evaluation of the correlation dimensions of the hypocenter set was carried out by plotting a histogram c (l) and assessing the function of log c (l) and the regression estimate of the studied crustal area, limited below by the accuracy of the hypocenter evaluation, and above by the boundary of the assumed zone of strong earthquake generation, which was higher by an order of magnitude than the zone of the aftershocks. 4. results 4.1. dynamics of the acoustic regime we consider the recurrence plot slope c, the fractal dimension d, the relationship d-3c, and the crack concentration parameter ksr as parameters of the acoustic regime in these laboratory experiments. the variations in elastic-wave velocities and the electric parameters must, to a certain extent, mirror the stress–strain state of a rock and the development of anisotropy within it. seismo-acoustic discrete hierarchy 34 figure 2. recurrence curves of the acoustic events in the experiments. (a) bf1; (b) bf2; (c) bf3; (d) bf4. log log d lim l c l 0l = " ^ h 1 c l n l r r r l m m l i j= #^ ^h h 35 figure 3 illustrates the loading history and the acousticevent activity in experiment bf1, where the acoustic energy was summed in 50-s intervals. figure 3 shows an acoustic quiescence at t =28650-30400 s, which corresponds to the scenario preceding the transition from the lower to the higher levels of cracking, and consequently to an avalanchelike crushing of barriers in the course of expanding failure. the distribution of amplitudes (a≥25 mv) and zcoordinates of the acoustic event selected in experiment bf2 (figure 4) shows the initial dispersive character, followed by a localization and an increase in the maximum amplitudes over time. phase i recorded a maximum value up to (t=19800s) of the loading process and developed a dispersive distribution, which was sparse with noncorrelated microcracks. the acoustic events generated are random in the rock block, and do not affect one another during this step. in phase ii (t = 19800-27600 s), we have a gradual concentration of the sources of the acoustic event in the small domain of the central part to the left of the block – formation of a fractal structure of acoustic regime that leads to an increasing mutual effect of the acoustic events. phase iii (t=27600-30200s) was characterized by evolution of the microcracks for the triggering of the main failure, which appeared on the surface of the block. the acoustic quiescence corresponded to the expansion of the fractal structure from one scale level to a greater one. figure 5 shows the variations of the recurrence plot slope c, the fractal dimension d and the relationship d-3c during the block deformation in experiment bf1, and the normalized curve of the acoustic activity n/nmax. time t is normalized to its final value (time of macrofailure creation tf in the laboratory experiment). three stages of variations of the acoustic rate can be noted: i) rise in the acoustic intensity, marked by weak variations in c; ii) acoustic quiescence, marked by a decrease in c; and iii) renewed intensity prior to failure, which is indicated by an arrow, and a decrease in c to 0.4. the parameter d changes from a mean value of 2.89 in stage ii, to 1.1 at the final stage iii of loading. d ≥ 2.5 corresponds to a quasi uniform separation stage of the microrupture in the rock. the decrease in d to 1.1-1.5 shows the formation of a fractal structure of defects at the time of stress drop, when the rock loses its reserves in elastic energy. during stage i (d-3c >0) there is a gradual accumulation of microcracks. stage ii (d-3c ≈0) is noted in the area of maximal stress, and takes up most of the microcracks and the beginning of their interactions after reaching a critical concentration in the area of the future macrofracture. stage iii (d-3c < 0) is marked by progression of the main macrofailure. the acoustic-event energy in phase iii is four-fold that in phase i. the number of cracks in phase iii, which is characterized by high acoustic activity and stress diminution, must be three-fold less than that in phase i, according to the recursive law of acoustic events. according to the generalized dependence between the rupture size li and the energy es of an earthquake [riznichenko 1976, sobolev and seismo-acoustic discrete hierarchy figure 5. dependence of the load f/fmax (1), the acoustic emission n/nmax (2), the recurrence plot slope c (3), the fractal dimension d (4), and the relationship d-3c (5) on the normalized time during the bloc deformation in experiment bf1. arrow, time when the factures reached the free surface. i, ii, iii, correspond to the stages of strike-slip fault generation. figure 3 (left). acoustic emission energy (1) and loading-cycle history (2) in experiment bf1. arrow, acoustic quiescence. figure 4 (right). distribution of zcoordinates (a) and signal amplitudes (b) of the acoustic events during the steps of the macrofailure process preparation in experiment bf2. arrows, acoustic quiescence. i, ii, iii correspond to the stages of strike-slip fault generation. ponomarev 2003]: (15) the elastic pulses energy radiated by separate cracks during stage iii is about four-fold higher than that in stage i. the decrease in c and the fractal dimension d are explained by the formation of a fractal structure in an acoustic regime of major rank in the rock. as a rule, stages (i-ii) in all of the experiments that were marked by high c and parameter d values were followed by a period of reduced c and d, when the macrorupture occurs in the last stage. this decrease in the recurrence plot slope c and the fractal dimension d are caused by coalescing cracks and the formation of larger cracks. an anomalous decrease in the slope of the recurrence plot and the fractal dimension can be regarded as one of the precursors of an approaching macrofailure. 4.2. variations in elastic-wave velocities the analysis of the velocity variations in the experiments (bf1, bf2, bf4) shows that if a zone is characterized by a low velocity, then a domain of high velocity will be formed around it, and vice versa. it was found that above about a half of the failure strength, the average velocity became a non-linear function of the load. in particular, the velocity reached a maximum when the loading was half the maximal, possibly due to the developing fractal structure. figure 6 illustrates the values of the mean square velocity deviation v, measured along the xand yaxes at different loads in experiment bf2. the calculation was performed for the 12 paths in each direction. the velocity variance increased significantly for loads increasing from about 0.7t/tf up to the final macrofailure load. anisotropy was recorded in the velocities in the directions x and y, parallel and perpendicular, respectively, to the direction of the main load f. a considerable degree of elastic anisotropy developed prior to failure. this is due to the oriented microcracks that have a major role in the failure process itself. the average velocities before loading were vp(x) =5.36 km/s and vp(y) =4.98 km/s. after the loading, these became 4.72 km/s and 4.51 km/s, respectively. a typical example of the anisotropy of the p-wave velocity is shown in figure 7. the velocity anisotropy observed was associated with changes in the microcrack density and distribution pattern. we noted an increase in these prior to the macrofailure. the maximum of this increase coincided with the dynamic propagation of the fractal structure. the mean square velocity v deviation determined for 12 routes shows that the increase in stress is accompanied by increased scatter in the velocities. figure 8 demonstrates the behavior of the ratio vp/vs velocity variations for sets of elements 2-3 and 6-8 between the stress concentrators. it can clearly be seen that in the part 6-8 of the block, vp/vs increases at t=0.85 t/tf and decreases up to 0.95 t/tf. in the element 2-3, vp/vs decreases at 0.5 t/tf, increases at 0.85 t/tf, and decreases up to 0.95 t/tf , i.e. seismo-acoustic discrete hierarchy 36 figure 6 (left). dispersion in integral wave velocity in the sample measured in the direction x (a) and y (b) as functions of the normalized time through the block deformation in experiment bf2. figure 7 (right). example of p-wave velocity for different sounding of crack routes (9-1) and (4-7), and the corresponding dispersion measured for 12 routes (2) as functions of normalized time in experiment bf2. the sounds 9, 4 were located on side i, and 7 and 1 on side iii. inset: the diagram of the positions of the stress concentrators and the ultrasonic sounds. figure 8. typical anisotropy in the variation of the ratio vp/vs for sets of routes between the stress concentrators in different sounding directions (as indicated) in experiment bf2. inset: the diagram of the positions of the stress concentrators and the ultrasonic sounds. log (km) 0.244 log ( ) 2.266l e ji s= 37 the dynamics of the fractal structure formation are more active than in direction 2-3 of the block. the middle part of the block is a zone of intense cracking, and in the upper left part of the block the cracking is practically negligible. to sum up, the velocity contrast in different regions of the block increases failure, and the rupture occurs differently in the various regions of the intersections of these paths. the s-wave velocity appears to be more influenced by the microfailure process than the p-wave velocity. in addition, the s-wave velocity increase prior to failure is larger, and the decrease after failure is less than that of the p-wave during the microcracking stages. overall, for the ultrasound traces under the angles of 40˚ to 45˚ with respect to the vertical sample axis (figure 8, direction 2-3), vp/vs increases from 1.73 to 1.85 during the first stage of reversible deformation, decreases from 1.85 to 1.62 during the step of dispersed opening microcracks, increases again from 1.62 to 1.90 when the microcracks affect one another, and finally decreases to 1.6 in the last stage of macrofailure expansion. the ratio vp/vs did not show significant changes in the stable zones. the ratio vp/vs allowed the study of the mechanical anisotropy in the strain– stress state of the block over definite intervals. the contrast in the behavior of vp/vs appears to be evidence of numerous minor oriented cracks arising during the earlier stage of failure generation. the decrease in vp/vs in all cases during the final stage testifies to the process of big crack formation. 4.3. electric parameter variations local variations of apparent resistivity dta were detected, and in certain zones, resistivity anisotropy developed over time. the anisotropy detected at the initial stage of loading is associated with the failure of the pre-existing defects. as the stresses increased, the portion of progressively smaller defects among the failed defects increased. in figure 9, it can be seen that the greatest variations are recorded in the first step of the loading cycle, at (0.25-0.45) t/tf, where their relative value reaches 40% of the variations of the apparent resistivity. anisotropy in dta was recorded in the second, at (0.6-0.8) t/tf, and the last, at (0.8-1) t/tf, stages of rock failure, and can be qualitatively explained by the development of oriented sets of cracks. this is confirmed by the increase in the acoustic activity in that region between the stress concentrators during the final stage of macrofailure expansion. the decrease in dta in one direction of the rock block might be due to expansion of a conducting path system in this direction during crack accumulation. the increase in dta in the other perpendicular direction is probably caused by the coalescing oriented cracks and the rupture of the conducting path system during genesis of the macrocrack. as the block was loaded, the spontaneous electric field du changed significantly. we recorded some cases of initiation and disintegration of local electric anomalies when the load varied in the time range (0.5-0.9) t/tf. figure 10 shows one of the anomalies that began at 0.5 t/tf, and reached a maximum of about 30 mv at the beginning of final stage at 0.8 t/tf. its life-time changed from 25 to 55 minutes, the reason probably being due to an electric relaxation phenomenon. after loading, the variations of surface electric field significantly decreased and nonlinear relationship between mechanical and electric fields has been obtained. the disappearance of anomalous regions during load removal occurred with a lag of several tens of minutes. a number of features observed in the electric disturbances appear to be explainable qualitatively within the framework of ionic migration mechanisms. it is apparent that under high strain the dynamics of increase and of relaxation of the electric potential du (in the last stage of the unstable deformation) does not correlate with the variation of the load, which in our opinion reflects the irregular process of failure in the rock. 4.4. crack concentration parameter in the laboratory experiments, the crack concentration parameter ksr diminishes significantly in phase iii in the interval (0.7-1) t/tf, before the macrorupture (figure 11). for important values of ksr, the cracks do not interact among seismo-acoustic discrete hierarchy figure 9. anisotropy registered in apparent electric resistivity measures for different positions of pick-up electrodes pairs (as indicated) in experiment bf2. inset: the diagram of the positions of the stress concentrators and the electrodes. 0, zero electrode; a, bfeeding electrodes. figure 10. the local anomaly of the electric potential registered prior to macrofailure in experiment bf2, for the electrodes going through the barrier. themselves. starting with the second stage in the range of 0.5-0.7 t/tf, a gradual clustering of the acoustic events within a small portion in the core can be detected. when the cracks are concentrated and their density becomes critical (ksr ≈3-7), i.e. when the crack concentration reaches a stage where the mean relative distance between the cracks is 3-7, a rupture is formed, which causes relatively quick failure of the body, and the deformation process moves to the progressive damage phase of the material. this leads to the formation of a fractal structure in the acoustic regime. 4.5. temporal variations in the structure of a seismic regime analogous changes have been recorded for the recurrence plot slope c in studies in various areas of the algerian seismoactive regions. the c decreased from 0.64 to 0.42 18 months before the seismic event of zemmouri (m= 6.9) of 21 may, 2003, and from 0.68 to 0.38 20 months before the seismic event of el asnam (m=7.3) of 10 october, 1980 (figure 12). the decreases are significant at a level of 0.05. the izmit earthquake in turkey (m=7.4) of 17 august, 1999, had a decrease in c from 1.2 to 0.78 with respect to the normal conditions [zavyalov 2006]. the seismic event of vanskoy (m=7.0) of 24 december, 1976, and of spitak (m=6.9), of 7 december, 1988 [smirnov 1993], were characterized by decreases in c to 0.32 before the main shocks. therefore, we can say that c is among the main indices for the creation of a macrorupture at the corresponding scale. the c remains constant or increases at the beginning of the anomaly, and then decreases until the triggering of the macrofailure (block fracture, earthquake). the earthquakes of el asnam and zemmouri showed a decrease in the fractal dimension d from 1.5 to 1.2 and from 1.62 to 1.25, respectively, 15 and 18 months before the main events (figure 12). the average value for the d obtained for two areas was none other than about 1.52±0.05. analogous variations in parameter dpreceded the earthquakes of spitak and vanskoy by 12-18 months, from 1.5 to 1.45 and 1.4 to 1.2, respectively. the decrease in d can be explained by the progressive appearance of local instability in the deformed volume of the materials as these approached macrocracks. 4.6. variations in the crack concentration parameter in the seismoactive regions the crack concentration parameter ksr decreased to 4.35 and 7.6, respectively, 8-10 months before the main seismic events of el asnam and zemmouri (figure 13). ksr reached 4.2 before the earthquake of kamchatka of 5 december, 1997 (m=7.9) [zavyalov 2006]. in other words, when the parameter ksr reached the critical value 4-7, the crack clustering acquired a threshold character that resulted in relatively quick fault expansion. the quasi-invariance of ksr for zones which are far from epicenters was also noted, which confirms the hypothesis of spatial localization of seismic events at the level of the rupture zone. the values of ksr before the macrorupture calculated for the geophysical medium at different scales are in the interval 3-7. seismo-acoustic discrete hierarchy 38 figure 11. variations in the parameter ksr during the block deformation in experiment bf1. the arrows denote amplitudes: a1, a=80mv; a2, a=83mv; a3, a=85mv. i, ii, iii, correspond to the stages of macrocrack formation. figure 12. variation of the seismo-acoustic self similarity parameters b-value (1), fractal dimension d (2), and relationship d-3 b (3) prior to the zemmouri earthquake m=6.9. figure 13. dependence of the parameter ksr on the normalized time in 200×200 km zone that contained the epicenter of zemmouri earthquake m=6.9. ts corresponds to 23 years of the experimental field time. 39 5. discussion and comments 5.1. interactions of acoustic events and formation of fractal structure the chosen experimental technique in the laboratory allowed the extension of the evolution phase of the rock macrorupture. the formation of fractal structures developed during the deformation of the rocks at different scales of change over time. the fractal structure created during the phase of the stress drop, when the rock loses its reserves of elastic energy, is conditioned by the evolution of its elements (the microcracks). it is the precursor of the genesis of the macrofailure. during the first step of rock deformation, microcracks appear to be homogeneously distributed, and as microfailures progress, interactions between them take place. this leads to their coalescence and initiation of the fractal structure, which is marked by the variation of the acoustic regime. the formation of a fractal structure leads to a critical diminution in the auto-similarity parameters (c, d, ksr) and creates anisotropies in the elastic and electrical properties of the rock sample. amitrano [2006] showed that the process of damage localization is related to the passage from isotropic uncorrelated damage to anisotropic correlated damage. the anisotropy of the strength and the distributions of the mechanical stresses in a deformed rock massif are often a reason for local dominance of brittle destruction, which results in catastrophic phenomena (earthquakes, rock bursts, collapses, etc.). for any rock, there is always some strength distribution of the contacts between the grains, which are variable in the massif volume. the process of deformation is accompanied by rupture of some parts of the least strong contacts, and alteration of their distribution in microfracturing systems. both zones of increased stiffness and zones of increased quasi-ductility are created, and the process of deformation of the anisotropy of the elasticity and the strength increases. this transformation is a result of the reorganization in the rock of ionic and electronic physical subsystems, which must lead to characteristic changes in the mechanical and electrical properties, depending on the stress–strain state of the rock. these studies have allowed us to show that this process of failure involves three stages: dispersed deformation, and initiation and expansion of the fractal structure. the fractal dimension d, c and the crack concentration parameter ksr all decrease before the main seismic and acoustic event of the studied geophysical field, at different scales. these decreases are of 20% to 80% for c, d and ksr with respect to their usual values with 6 to 18 months of field observations before the main seismic shock (m≥6) and 30-60 minutes, i.e. (0.9-0.95) t/tf, before the macrofailure in the rock sample in the laboratory experiments. we did not record such an anomaly for seismic events of m<5 in this algerian seismoactive region. the decrease in dpreceded the decrease in cat different scales of the medium. the parameters d, c and ksr of the geometric set of acoustic events or seismic events reflect the variations in the seismo-acoustic regime. the decreases in c, d and ksr are associated with the gradual clustering and the expansion of the fractal structure in the geophysical medium under load. the existence of a critical crack density in the transition from dispersed volumetric to localized failure testifies the rise in the unstable deformation in the failing parts of the medium. 5.2. temporal variation in fractal structures in a seismo-acoustic regime the distribution (c, d) of the seismo-acoustic data can be adjusted by the line d=3c (figure 14). the evolution of the acoustic regime is directed from cluster i to clusters ii and iii. cluster i corresponds to microcrack nucleation when the stresses are increased in the rock. cluster ii corresponds to the phase of the accelerated accumulation of microcracks in different parts along the future rupture band of the rock, and to their critical concentration and stress drop. this transition reflects the appearance of a fractal structure in the acoustic regime, with a simultaneously corresponding decrease in the parameters c and the fractal structure d. cluster iii corresponds to the progressive evolution of damage and the formation of the mean macrofailure when the stress continues to drop. cluster i is absent at the scale of seismoactive regions characterized by low values of fractal dimension d and of c. this is probably due to the formation in the earth crust of a fractal structure that is more developed than that obtained in the laboratory. the relationship d=3c means that g=0 (equation 14). the interruption of this equivalence (g≠0) can be related to the redistribution of the medium stress–strain state as a function of the dimension of the rupture and of the seismo-acoustic regime evolution preceding and succeeding the mean seismic event (acoustic even) [deng et al. 2009, liu et al. 2009]. deviation over time from the relationship d-3c=0 can be seen as a variation in the stable state of the seismo-acoustic regime. seismo-acoustic discrete hierarchy figure 14. dispersion diagram of parameters c and d in experiment bf3; ▲zemmouri; + el asnam; × vanskoy; � spitak; / d=3c, arrows indicate (1) the macrofailure in the block sample; (2) el asnam earthquake (m=7.3); (3) zemmouri earthquake (m=6.9); (4) spitak earthquake (m=6.9); (5) vanskoy earthquake (m=7.0). i, ii, iii, correspond to the stages of seismo-acoustic regime evolution. 5.3. changes in the percentages of the clustering seismo-acoustic events figure 15a, b shows the variation of the variance coefficients of the number of acoustic events and seismic events during the waiting period of major strong events at different scales. we note the analogy in the increasing of vdt until the triggering of this event. therefore, the gradual reinforcement of ruptures in the epicentral zone is accompanied by an increase in the average magnitude of the small seismic events that preceded the mean shock in the zemmouri earthquake area (figure 15c). the acoustic and seismic events have spatial hierarchy similitude. the three stages of the deformation of the geophysical medium mentioned above are manifest in the variation of the clustering coefficient g% (g% is the ratio of the number of clustering small earthquakes of mmin ≥2 in the epicentral area of the zemmouri earthquake, with their total number in the studied seismogenic zone; g%= ng/ ) [baddari 1997, sobolev and ponomarev 2003] (figure 16). we have considered that two successive seismic events are clustered if the critical distance dcr and the critical time tcr between them is less than the critical values corresponding to these two parameters. for algeria, we have used the following empirical correlations [sobolev 1995, baddari et al. 1996): tcr (hours)=a 10 bm and dcr (km)= c li, where a = 5.2 × 104; b = 0.75; c = 3, as the cluster model parameters determined as coefficients of linear regression for the functions tcr and dcr; li is calculated from equation (15), and it is the length of a rupture in the epicenter of an earthquake of a given energy; m is the magnitude of the first earthquake in a pair of seismic events. the increase in g% in stage i (figure 16) is due to an increase in the interaction between fractures in the epicentral area. the decrease in g% during stage ii is caused by the strain and stress relaxation. the increase of g% during stage iii is explained by the coalescing of all of the fractures and the formation of the seismic source of the 2003 zemmouri earthquake. 5.4. hierarchical arrangements of adjacent rupture sizes at different scales after the experiments, the microscopic study of the residual ruptures showed adjacent microcracks of ranks, respectively: 32; 11; 3.5; 1.15; 0.33 mm in experiment bf1, and 42; 12; 5; 2.5; 0.8; 0.35 mm in experiment bf3. the ratio of thetwo basic adjacent sizes in both of these experiments is li /li−1 ≈2-3.5. as a result, the li /li−1 ratio was close to 2-4 in all of the experiments. for the region of zemmouri, a rupture hierarchy was brought out by analogy, i.e. l1 ≈80 km; l2 ≈40 km; l3 ≈10 km; l4 ≈3 km. the presence of active faults in the zemmouri seismoactive region gives rise to an intricate hierarchical structure of the stress field. an ordered hierarchical pattern of active spatial fault distribution in central asia was realized by ulomov [1991]. the ratio of the average geometrical sizes of adjacent fault ranks was close to 3. in agreement with sadovsky et al. [1991] and sobolev, [1995], the average ratio of the adjacent basic dimensions of the seismogenic blocks li =/li−1 ≈ 3.33 constitutes a highprobability real fact of nature, with the value in keeping with the scale, from fractions of a micron to hundreds of kilometers. the deformation of the geophysical field is related to the continuation in the accumulation of energy, which differs from the active zone to another seismic zone according to the motion speed of the block rocks, and the dissipation speed of the energy at different hierarchy levels of the medium. seismo-acoustic discrete hierarchy 40 figure 15. top: changes in the variance coefficient of the number of acoustic and seismic pulses during the expectation period of an important energetic event respectively for the rock block (a) (experiment bf4) and the zemmouri earthquake (b). bottom: average magnitude of small earthquakes in the zemmouri area (c). ts and tf correspond to the experimental field time (23 years) and the total life time of the laboratory experiment, respectively. figure 16. changes in the percentage of the clustering earthquakes g% in the 2003 epicentral area of the zemmouri earthquake, and prior to the zemmouri earthquake m=6.9. i, ii, iii, correspond to the earthquake stages preparation. m n/ n/ m 41 6. conclusion the seismo-acoustic process is a discrete stochastic process that is related to the kinetic accumulation of ruptures. this suggests that the development of the failure process is controlled by the same physical mechanism. the process of rock failure at different scales observed in the laboratory and the field experiments gives rise to a fractal structure that is due to the active interactions of cracks. the genesis and evolution of a newly formed fractal structure leads to anisotropy in the elastic and electric properties of the deformed geophysical medium, and to the appearance of anomalies in auto-similarity seismo-acoustic parameters (the slope of the recurrence plot c, the fractal dimension d, the relationship d-3c, and the crack concentration parameter ksr) during the last phase before the triggering of the mean seismo-acoustic event. the c and the fractal dimension d decrease, their relationship d ≈ 3c is interrupted, and the parameter ksr of the crack concentration decreases to 3-7 during the expansion of the fractal structure. the decrease in the fractal dimensions at the time of the strong algerian seismic events can be interpreted as a decrease in the dimensions of the seismic area, which is in agreement with the concept of failure localization during the period of an earthquake rise. anomalous changes in the parameters c, d, d-3c and ksr, on whatever scale, can be interpreted as one of the precursors of an approaching large dynamic event. the acoustic rate consists of three main steps: dispersive releasing of the acoustic events; critical concentration of the acoustic events; and formation of the fractal structure; finally, there is the evolution of this fractal structure to give birth to the main fracture. the changes in the number of clustering m ≥ 2 earthquakes in the 2003 zemmouri epicentral area showed three corresponding factors to those noted in the laboratory experiments. the rise in the clustering parameter g% during the first stage is due to growing interactions of seismogenic faults that can act as local mechanical-stress concentrators. the second stage is marked by a decrease in g% due to the increasing mechanical instability caused by the influence of the macrofracture nucleation and the attenuation of the interactions of the neighboring stress fields in the epicentral area. the increase in g% in the third stage is caused by coalescing failures and the formation of larger ruptures that generated the 2003 zemmouri main seismic source. the self-similarity of the rock structures and certain aspects of the seismicity show considerable heterogeneity of the cracks, and the intercrack contact properties. the experiments reveal that the deformed rock is divided into separate blocks, according to echelons of cracks of different sizes and orientations. the presence of active cracks gives rise to an intricate hierarchical structure of the stress–strain geophysical field that can be characterized by its fractal properties. the similarity recorded at different scales in the evolution of the fractal structure can be used to monitor the development of seismic fractures and to predict the superficial earthquakes of the algerian seismoactive regions at intermediate terms. references amitrano, d. 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(2006). intermediate term earthquake prediction, nauka, moscow, 254 pp. *corresponding author: kamel baddari, laboratory of physics of the earth, umbb, algeria; email: doyenfs@umbb.dz. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. seismo-acoustic discrete hierarchy 42 annals of geophysics, 55 (3), 2012: matteucci_et_al a hippocratic oath for geologists? ruggero matteucci1,*, guido gosso2, silvia peppoloni3, sandra piacente4, janusz wasowski5 1 università di roma ‘la sapienza’, dipartimento di scienze della terra, rome, italy 2 università statale di milano, dipartimento di scienze della terra ‘a. desio’, milano, italy 3 istituto nazionale di geofisica e vulcanologia, rome, italy 4 università di modena e reggio emilia, dipartimento di scienze della terra, modena, italy 5 istituto di ricerca per la protezione idrogeologica, consorzio nazionale della ricerca (cnr), bari, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5650 abstract we argue here that the introduction of an ethical code of conduct that follows the example of the hippocratic oath of physicians will help geologists to acquire binding awareness of their professional and social responsibilities. the ethical behavior and obligations of modern geologists involve, but are not limited to, the following issues: correct land/environment use and management; respect of truth and science; and protection of the earth systems, on both the local and global scales, and therefore, of our well-being. we believe that for geoligists, the explicit acceptance of an ethical code will help to promote: (i) an awareness of their social role, expertise and sense of belonging to a professional community; (ii) an understanding of the expectations of citizens and society; and (iii) cultural growth, with better use of research and implementation of scientific and professional skills. all this should enhance the public recognition of the social mission of geologists, which is essential for the well-being of society. therefore, we suggest that like in the majority of medical schools, ethical training should be a part of the university curriculum for students in geology. 1. the hippocratic oath the ‘hippocratic oath’, through which young physicians still today express their ethical responsibilities, represents the first written manifestation of the value inherent in the moral obligations that arise from the possession of specific knowledge that has practical consequences. with hippocrates, medicine that had until then been hieratical and theocratic, became rational. for a modern physician, this oath goes beyond the rules of professional conduct that are based on the deontological code, to exalt the moral obligation of the absolute availability to intervene in case of necessity, and to legitimize the expectation of those in need of medical help. the recurrent modernization of the original text by medical organizations of different countries and the relevant literature [e.g., reiser 2003, gruenbaum and jotkowitz 2009] demonstrate the persistent importance attributed to the voluntary recognition by young physicians of the ethical dimension of their actions, also in relation to the expectations of society at large. 2. parallelism between ethical responsibilities of physicians and geologists the relations between physician, illness and the patient is guided splendidly in the hippocratic oath, to guarantee the patient as a person (figure 1a). until today, this has represented the basis of the medical profession. however, in recent times, this relationship has acquired more complexity, as it is linked to its social dimension. in this context, in the triangle of the hippocratic relations, as physician – illness – patient, there needs to be added the collectivity, and more in general, society (figure 1b). while maintaining the rights of the patient as a person, more and more it is the social dimension (and the cost/benefit ratio) of the illness that necessarily governs biomedical research, the physician’s action and physician’s potential to operate. the problem of so-called ‘rare illnesses’ represents a clear example here (with ethical obligation to treat, but very high cost/benefit ratio for society). the social dimension of medicine represents a relatively recent acquisition. in italy, only during napoleon’s period did the ‘district physician’ became a reality in all provinces, which heralded the beginning of generally available medical help in the form of a public service. the truth is that ethics have a fundamental role in the responsibilities of geologists. the earth is our unique home: our well-being, and also our survival, depends on its habitability and resources. thus, the earth systems (geosphere, biosphere, hydrosphere, atmosphere) reprearticle history received january 23, 2012; accepted march 14, 2012. subject classification: hippocratic oath, geologists, geoethics, social responsibility. 365 sent the value of the common weal. furthermore, our planet is not an immobile body; it has its own life in continuous evolution, with processes that are often unavoidable and damaging for man. adequate investigation and timely transfer of information would in most cases imply lower levels of risk to man arising from such processes. it is also evident that modifications of the earth system processes that are potentially damaging for humankind can be, and are being, induced by man, through actions that do not respect our planet’s natural processes and equilibria. the social role of the responsibility and actions of geologists is therefore extraordinarily important. allman’s activity involving land use, and more in general, the environment, should be confronted with the historical perspectives of geologists to long-term threats to our future. geologists have, therefore, relevant responsibilities towards society, from which, in turn, the ethical importance of their actions is derived. a correct and scientifically sound approach can mitigate, or at least help to avoid, many of the serious consequences that continuously arise through the irresponsible use of land by man. even if geologists have limited power to impose the correct choices on decision makers, their ethical obligation is to propose them and to report wrong actions and behavior. therefore, the parallelism between the hippocratic obligations of a physician with respect to society and those of geologists is evident. by analogy, the hippocratic triangle of physician – illness – patient/society corresponds to the relations between geologists, ‘earth’s illness’ and society (figure 1c), where ‘earth’s illness’ is represented by both natural processes that are actually or potentially damaging for man and the similarly damaging effects of man’s impact on our planet. unlike national and international health organizations, those of the earth sciences do not yet have a great impact, or the ability to orient the actions of governments. it appears that only the u.s. geological survey (usgs), which was established in 1879 in north america, has relevant social and political influence. the current relatively limited influence of the earth sciences community on land management practices also relates to the inadequate and volatile perception by public opinion and politicians of the extraordinary significance in the medium-to-long term of the human and economic costs that can derive from the insufficient consideration given to geological constraints. and yet, the 172,359 victims of geo-hydrological disasters and earthquakes in italy in the last 100 years [prestininzi 2011] represents a tragic reality. moreover, if on the one hand the recurrence of the so-called natural catastrophes has not resulted in significant behavioral changes in society, on the other hand, geologists have not yet been able to force public opinion and political decision makers to clearly distinguish between what is natural and inevitable (geologic events, the effects of which can be mitigated via prevention in many cases), from what is not inevitable (abuse of land and lack of respect for geological processes), and is typically due to ignorance or an overwhelming desire for short-term economic gain. 3. a(geo)ethical approach to the profession of geologist the professional role of geologists is already well rooted in the framework of intellectual professions. in italy, the practice of geology is regulated by law and through the professional association of geologists, with matteucci et al. 366 figure 1. factors affecting the hippocratic triangle [inspired by antoniou et al. 2010], in medicine (physician, illness, patient/society) and the geological sciences (geoscientists, planet ‘illness’ – hazards, human imprinting – and society). bottom right: the early relationships according to hippocrates. bottom left: in medicine today. top: the geoscientists equivalent. 367 its specific deontological code that was issued in 2006 and amended in 2010. the code contains a clear and explicit ethical reference to the public and the general interests of the profession (articles 1 and 36). indeed, in most cases, the professional activities of geologists are linked to the general interests of the territory involved as a system, considered in the medium-tolong term rather than in the short term, and in the framework of the legislation in force. these interests might include, for example: assessment of sustainability and of long-term effects of the removal of river-bed material; evaluation of site suitability for storage of contaminated material or of radioactive waste; stabilization of a landslide through expensive structural measures or through ‘natural’, and probably cost-effective, drainage systems; land-use planning; and geological heritage and landscape conservation. while conducting a cost-benefit analysis of any project involving our environment, it is of prime ethical importance to evaluate if, and make sure that, the benefits are significantly greater than the costs, and not only in the short term, but also in the medium-tolong term (figure 2). the longer temporal framework needs to be considered within the economic–patrimonial logic [roda 2011], thus leading to the maximum benefit of man’s activity at the lowest cost to the territory (economic criterion); however, such benefit has to be secured also for the future (patrimonial or hereditary criterion). therefore, geologists have a unique obligation to guide actions aimed at the conservation of the geosphere and its ‘liveableness’ for future generations. moreover, the ethical value of the expertise of geologists has global implications, because it contributes to sustainable management of a very-high-population-density, overly consumeristic, post-industrial world, in which the disequilibrium between demand and offer is growing (e.g. regarding food, energy, water, health), fueled by irrational management and over-exploitation of resources, and by the enormous inequality in the global wealth distribution. 3.1. ethical obligations the importance of promoting ethical behavior within the geosciences community is well highlighted in the conclusions of the final report of the gsa (geological society of america) presidential conference [horten 1997]: “individual integrity is not enough: to be truly ethical, one must have personal integrity as well as on-going awareness and insight into the ethical problems that exist throughout the geoscience profession. in other words, geoscientists must become alert to, and active in, the subject of ethics in order for the practice of geology to be truly ethical”. there are two main fields where the ethical impact of the behavior of geologists is most dramatic: (a) direct actions, when a commissioned study reflects the interests of private or public customers, but also has geoethical implications in terms of the systemic interests of the territory involved and its dynamics, and in general of our planet (often the short-term and long-term interests of the territory and of its population do not coincide); (b) indirect actions, which are aimed at acquiring scientific information that will be useful for public opinion and decision makers, so that all of the actions that involve the equilibrium of the territory and our planet and its natural evolution follow geoethical principles. in both cases, it is also the obligation of geologists, who have a unique and adequate culture and sensitivity, to demonstrate the actual benefits (not necessarily in the short term) of the geoethically sound choice of action, which would be the most favorable for and consistent with sustainable use of the territory of interest. this might need to be confronted with the often relevant immediate benefits of less prudent choices of action. it is through this cost/benefit-based approach that it is possible to achieve respect for the core ethical values and responsible practices of geology that are consistent with the present and future well-being of society. in particular, the hippocratic obligations of modern geologists would involve the following: (a) landmanagement/use and conflicting economic interests the ethics of sustainable land management must prevail, regardless of the type of a commissioned project and the short-term economic interests of a client. this might include, for example: exploitation of groundwater or mineral resources; engineering construction; urban development and territory management; landscape conservation; assessment of and protection from natural hazards; evala hippocratic oath for geologists? figure 2. long-term cost-benefit ratio in the geoethical approach. uation of and protection from risks related to anthropogenic impact; conservation and enhancement of the natural and geological heritage. (b) to respect truth and science the respect of scientific truth and of scientific consensus based on updated evidence (including the recognition of data uncertainty, or of limited knowledge) and rigorous adherence to scientific method is a must in the professional activities of geologists. they are also ethically obliged to request a transparency policy in handling preliminary results of geological studies and in informing the public of actions that might be damaging for the territory; this obligation is even more stringent in cases of professional and scientific associations, which should therefore also adopt their own ethical code. (c) to favor the earth, and therefore our well-being although this may appear as an obvious statement, following hippocrates, the implication is that those equipped with knowledge and specific expertise should act even outside of their professional duties to protect the earth systems, at both the local territory and global scales. although the following specific ‘obligations’ fit into the above more general point (c), their dramatic urgency justifies their separate treatment here: (d) to promote awareness of every citizen’s responsibility for our world there is urgency in the need to provide adequate knowledge, so that people understand and respect the natural processes that control the life of our planet. this is critical to orient well-informed choices that need to be made at present and in the future by individuals, society and politicians. the challenge of effective information transfer and adequate education of new generations must be won, and geologists have the ethical obligation to be at the forefront here. (e) to assure knowledge advance and life-long learning this critical point is also present in the hippocratic oath. the appreciation of the expertise of geologists by the public represents the necessary condition to promote both efficient scientific and professional action, as well as the general recognition of the role of geologists. for this reason, as for physicians, the education of geologists has to be for life. however, in addition, a high quality university education has to be assured: university studies in geology have to be recognized as one of the most complex and comprehensive educational efforts that require the integration of two areas of expertise: the natural and physical mathematical sciences. furthermore, following the example of the majority of medical schools, ethical training should also be introduced into the university curriculum. 3.2. the significance of an ethical obligation how can geologists, and especially the younger ones, be best assisted in their acquisition of a clear and binding awareness of their ethical responsibility in the geosciences? might the introduction of an explicit act, which follows the example of the hippocratic oath of medical doctors, make sense here, and representan an efficient solution [ellis and haff 2009]? how would young geologists react if they were asked to respond on a voluntary basis to an act that they might consider as an anachronistic ritual or an imposition? there are some strengths in the introduction and acceptance of ethical rules, as this implies a commitment to serve, using the expertise gained, for the well-being of society, which depends on land preservation and environmental protection, as well as on due respect for geophysical processes. in the context of a publicly growing demand for ethical behavior by those who, for whatever reason, have the possibility to intervene within the public domain and to act for the public weal, the explicit acceptance of the ethical responsibility by geologists can have the following effects: (a) favoring explicit awareness of the social role of geologists, of their expertise and contribution, to strengthen their sense of belonging the majority of the actions of geologists involve the public domain. in most cases important and perhaps dramatic decisions concerning geo-hazards are often more difficult to take because of possible contrasting interests and opinions; geologists have to act here as protagonists at the forefront of hazard research, analysis, and management. regarding the questions of the global dimension (e.g. water, mineral, soil and energy resources; sea-level changes and climate variations; relations between technological solutions and respect for natural processes; cost-benefit analysis of anthropogenic intervention; assessment and mitigation of natural risks), as well as those on the regional scale (e.g. policy of geo-hydrological risk management and of protection from earthquakes and volcanic eruptions; land-use management), geologists have to have an essential role in the debate and the information transfer. we expect this will happen more often if alongside the technical–scientific dimension of the input of geologists, the public weal clearly emerges as an underlying objective. therefore, geologists always need to act guided not only by their knowledge, but also by their awareness of the ethical implications (they need to act following ‘science and conscience’, exactly like physicians). self-recognition of their role (and, consequently, recognition by others) represents matteucci et al. 368 369 an element of self-enhancement for geologists, and a way to increase their sense of belonging to a part of society that is essential for its survival and well-being. (b) fostering the awareness of geologists of the expectations of citizens and society the significance of the hippocratic oath de facto entered the common feeling: citizens respect physicians and recognize their special status. something similar is slowly starting to emerge for geologists, but their awareness of the expectations of public opinion has to go step by step along with their self-awareness of the geoethical dimension of their role. (c) fostering general recognition of the social mission of geologists, essential for the public weal, and, consequently, of their specific and unique role the common feeling, at least in italy, is that the community of geologists does not yet have levels of authority, respect and expectation that are comparable to those of some other professionals (e.g. physicians and engineers). however, the growing impact of ‘planetary issues’ on the media and on public opinion (e.g. unsustainable exploitation of natural resources, climate variations, pollution) and of specific regional problems linked to seismic, volcanic and geo-hydrological risks (especially in italy, japan, taiwan, and in many developing countries), as well as the incipient awareness of the inadequacy of government interventions that are insufficiently supported by scientific research and by the knowledge of geophysical processes, make the conscious acceptance of responsibility by geoscientists entirely appropriate, if not necessary. therefore, a public and solemn commitment by those who are to become experts in the management of land and of the problems of the earth represents a useful instrument. (d) stimulation of the cultural growth at the individual and community levels, exploitation of research, and implementation of scientific and professional skills the frantic acceleration in the increase of knowledge, which rapidly makes obsolete what we knew only a few years earlier, represents a demanding reality of our times. keeping our knowledge up to date and the continuous increase in our know-how represent the necessary conditions for the practice of any profession today. however, these efforts acquire moral significance when the wellbeing of the individual and of society largely depend on the quality of the knowledge and skills of a practitioner. for geologists, excellent cultural and practical preparation has to take on an ethical dimension, starting from their university studies. through their individual commitment, young geologists can assume the need for cultural lifelong increase as an ethical duty. 4. conclusions the explicit and conscious assumption of ethical obligations for geologists appears opportune and useful with the consideration of the urgent need to enhance the development of the geoethical dimension in the relationships between man and the geosphere. it is suggested that this can be facilitated by following the model of the hippocratic oath of physicians. clearly, the transition to an operational application will need to be preceded by ample discussion and the acceptance by national and international communities of geoscientists and professionals. references antoniou, s.a., g.a. antoniou, f.a. granderath, a. mavroforou, a.d. giannoukas and a.i. antoniou (2010). reflection of the hippocratic oath in modern medicine, world j. surg., 34, 3075-3079. ellis, e.c., and p.k. haff (2009). earth science in the anthropocene: new epoch, new paradigm, new responsibilities, eos trans. agu, 9 (49), 473. gruenbaum, s.e., and a.b. jotkowitz (2009). content analysis of ethical codes written by medical students compared with other codes of medical ethics, eur. j. int. med., 20, 101-104. horten, h. (1997). report on: conference on “ethics in the geosciences”, gsa presidential conference, welches, oregon; available at http://tierra.rediris.es/ geoethics_planetary_protection/gsa_ethics_in_the_ geosciences.pdf. prestininzi, a. (2011). la mancata prevenzione: costi e disagi trasferiti alle future generazioni, geoitalia, 34, p. 3. reiser, s.j. (2003). what modern physicians can learn from hippocrates, cancer, 98, 1555-1558. roda. c. (2011). geoetica, geoitalia, 34, p. 3. *corresponding author: ruggero matteucci, università di roma ‘la sapienza’, dipartimento di scienze della terra, rome, italy; email: ruggero.matteucci@uniroma1.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. a hippocratic oath for geologists? << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ricci_final_layout 6 annals of geophysics, 56, 4, 2013, s0452; doi:10.4401/ag-6458 s0452 vesuvio civil protection exercise mesimex: survey on volcanic risk perception tullio ricci1,*, rosa nave2, franco barberi3 1 istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy 3 università di roma tre, dipartimento di scienze geologiche, rome, italy abstract in october 2006 the european civil protection exercise mesimex (somma vesuvio mesimex – major emergency simulation exercise) on volcanic risk took place at vesuvio, promoted by campania region and coordinated by the italian civil protection department. the exercise was focused on the preparedness phase for a major volcanic emergency in the area of vesuvio. an evacuation of a sample of 1800 inhabitants from the vesuvio red zone was also tested during the drill because the emergency plan ensures the complete evacuation of the population from the higher risk zone before the onset of the eruption. during that event a survey on volcanic risk perception was carried out on the evacuated population in order to compare the results with the ones coming from a previous similar survey, using the same questionnaire, carried out on a wider sample of residents in the vesuvio red zone few months before mesimex exercise. the aim was to point out any differences in population’s attitude towards volcanic risk after having received detailed information on the emergency plan and on the hazards and risk related to the reactivation of vesuvio, and experiencing the exercise. 463 questionnaires were distributed to the population evacuated from the 18 municipalities of the red zone and participating to the exercise. main results in comparing data from mesimex survey with the vesuvio previous one, put in evidence how the general level of vesuvio residents’ trust remains quite low, indicating that a continuous and effective effort has to be done by both scientific community and civil protection department. particular attention should be paid in education and outreach activities and in involving people in risk mitigation procedures, also through more frequent exercises. 1. introduction vesuvio volcano is located in campania region (southern italy) and it is characterized by a very high population density. hundreds thousand people here are exposed to pyroclastic density currents, ash fallout and lahar hazards. the emergency plan for vesuvio, published in 1995 by the italian civil protection department [dpc 1995], defined three zones related to different volcanic hazards in case of a sub-plinian event taken as the scenario for the maximum expected event in case of renewed activity in the short–midterm [rosi et al. 1993, dobran et al. 1993, dobran et al. 1994, barberi et al. 1995]. the red zone, the area exposed to pyroclastic flows and lahars, includes all the 18 municipalities around the volcano, where presently ca. 550,000 people live. the area outside the red zone, exposed to pyroclastic fallout, was called yellow zone and the population at risk varies from approximately 150,000 to 200,000, depending on the sustained column height and the wind conditions at the moment of the eruption. the blue zone, included in the yellow one, is mainly exposed to lahar and flood hazards. the vesuvio emergency plan provides that the 550,000 residents of the red zone will have to be evacuated before the onset of the eruption. the present study was carried out during the mesimex exercise following a more complete research by barberi et al. [2008] on volcanic risk perception in the vesuvio population. volcanic risk perception studies have been carried out in the last decades in order to asses population at risk real knowledge and their risk perception, and the role of communication in peolpe’s preparedness in facing with an emergency [kartez 1982, perry 1990, johnston et al. 1999, dominey-howes and minos-minopoulos 2004, gregg et al. 2004, davis et al. 2005, carlino et al. 2008, gaillard 2008, haynes et al. 2008, paton et al. 2008, perry and lindell 2008, solana at al. 2008, bird 2009, njome et al. 2010]. our research assesses the volcanic risk perception of mesimex vesuvio red zone participants, mainly through the evaluation of their level of knowledge of volcanic hazard and risk, education and preparedness, and level of information received. article history received august 7, 2012; accepted november 28, 2012. subject classification: risk perception, volcanic hazards, civil protection exercise, risk mitigation strategies. special issue: vesuvius monitoring and knowledge 463 questionnaires were distributed on october 22, 2006, at the four checkpoints settled for the exercise, one for each family participating. the totality of the distributed questionnaires was recollected, even though only 427 were suitable to be analyzed. for an inexplicable pitfall in the distribution system no questionnaires were handed out to boscoreale residents. results of the mesimex survey are compared with the ones for vesuvio red zone presented in barberi et al. [2008]. 2. the survey 2.1. survey instrument the questionnaire used was the same developed for the study on volcanic hazards perception at vesuvio [barberi et al. 2008]. it consists in a 46 items questionnaire designed to investigate issues such as salience of the hazard (the tendency to spontaneously mention volcanic hazards as a problem relative to other concerns, and the amount of time spent thinking about the threat); risk perception (ratings of both the likelihood and severity of future eruptions as well as one’s level of anxiety about a potential eruption and feelings of personal vulnerability to the effects of a potential eruption); feelings of self-efficacy (feelings of control regarding one’s ability to protect him/herself and his/her family from the effects of an eruption); knowledge of the hazard and of hazard mitigation strategies (factual information concerning past eruption events and measures taken to protect the public from future eruptions); perceived preparedness of and trust in officials (confidence in the government’s level of preparedness and in the ability of government officials, scientists and the media to provide accurate information about potential eruptions); sources of received information about volcanic hazards and the emergency plan and preferred methods of receiving such information; sense of community (the degree to which residents feel bonded to their community was measured using the 18 item italian scale of sense of community by prezza et al., [1999]); demographic questions (sex, age, highest level of education achieved, marital status, etc.). the questionnaire includes some open-ended questions, in order to get qualitative data, useful to a better elaboration and interpretation of the quantitative responses. data entry was facilitated using the icr (intelligent characters recognition) and the omr (optical marks recognition) methodologies. 2.2. distribution procedures the distribution of questionnaires took place at the arrival of the evacuated population at the registration desk of each checkpoint. the questionnaires were handed out to no more than one selected member for each family or group of people, in order to achieve the most homogeneous sample in terms of age, genre and cultural background. all the procedures were managed by personnel belonging to the civil protection department expressly trained for the distribution of the questionnaires. the distribution and filling processes were completed after the educational activities addressed to the evacuated population. 3. results 3.1. sample characteristics respondents of the 427 analyzed questionnaires are 56.1% men and 43.9 % women. the age of the respondents ranged from 11 to 86 yr, with a mean of 49.7 yr and a standard deviation of 18.1 yr. 18% were single, 59.5% were married, 15.7% were divorced or widowed and 6.8% failed to report their marital status. with regard to the highest level of education completed, 28.7% had an elementary school education, while 31.7% were junior high school graduates, 32.9% were high school graduates, and 6.7% had a university degree. 3.2. hazard salience hazard salience concerns the extent to which the volcanic threat is prominent on the minds of residents. to measure the salience of the hazard, one of the first questions of the survey was an open-ended item asking respondents to list the three greatest problems facing their community. all the different kinds of problems listed by respondents were then grouped in 17 categories. as shown in table 1, problems such as lack of public services, crime, unemployment, trash, and traffic were the issues most frequently mentioned by respondents from each community. vesuvio was mentioned as a problem by 8.4% of respondents (table 1). no sigricci et al. 2 mesimex % barberi et al. [2008] % 1) public services 53 1) public services 55 2) trash/pollution 35 2) trash/pollution 46 3) crime 22 3) crime 43 4) unemployment 20 4) traffic 26 5) traffic 15 5) unemployment 13 7) vesuvio 8.4 7) vesuvio 9 table 1. rankings of the most frequently mentioned community problems. numbers in the table indicate the percentage of residents who mentioned each problem. the rank and percentages of those who mentioned vesuvio as a problem are included for comparison. 3 nificant difference was found in comparison with the previous study. a second survey item that measured the salience of the hazard asked residents to indicate how often they think about the possibility of an eruption using a 5 point likert scale, with 1 indicating “almost never” and 5 indicative of “almost always”. the mean rating for this item was 2.47 (sd = 1.09), indicating relatively low levels of salience regarding the volcanic threat as in the previous survey (m = 2.26, sd = 0.97). 3.3. perception of risk risk perception is a general term that encompasses a number of different aspects of how people may view their risk from a particular hazard. these include the perceived likelihood of a disaster event, perceptions of how serious such an event might be and how personally one might be affected, and how worried one is about a potential threat. a number of items in the survey were designed to measure these different aspects of perceived risk. first, respondents were asked to judge the likelihood that a future eruption would affect their community, as well as how serious the consequences of such an eruption might be for their town in general, and for themselves and their families personally. they were also asked to rate how much they worry about a potential eruption. all ratings were made on 5 point likert scales, with higher numbers indicating greater perceived likelihood and severity of consequences concerning an eruption, and greater worry about such a possibility. on three out of four measures the ratings comparison between mesimex and the previous survey indicate similar high feelings of respondents concern. nevertheless, there is a significant difference in worry about an eruption, lower in the people involved in mesimex exercise (table 2). 3.4. feelings of self-efficacy one of the survey questions asked to rate personal control and ability to protect from the effects of a potential eruption. this item was designed as a domainspecific measure of self-efficacy [bandura 1977]. self-efficacy has been recognized as an important variable related to the likelihood that an at-risk population will take self-protective measures [paton 2003]. responses to this question were made on a 5 point scale, with higher numbers representing a stronger sense of self-efficacy regarding someone’s ability to protect themselves from the effects of an eruption. overall, residents participating to mesimex exercise demonstrated a higher level of self-efficacy respect to the one expressed in vesuvio previous survey (table 3). this increased level of self-efficacy is probably due to the participants’ opportunity to play an active role in risk mitigation measures. 3.5. knowledge of the hazard among the items about the knowledge of the hazard asked to respondents the most significant finding is related to the knowledge about the volcanic activity (presence of active volcanoes and their last eruption, most threatening volcano, and judgments on the severity of six volcanic phenomena). in particular, one openended item asked respondents to name an active volcano in their area and to indicate the year of its last eruption. in response to this question, 89% correctly named vesuvio and 64.4% correctly identified 1944 as the year of the last eruption. while results coming from the two surveys are similar in identifying the active volcano in the respondents’ area (93.5% in barberi et al. [2008]), the knowledge of the last eruption date is quite lower in the previous study (45% in barberi et al. [2008]). volcanic risk perception at vesuvio how likely do you think it is that there will be an eruption affecting your town? mean sd mesimex 3.43 1.06 barberi et al. [2008] 3.64 0.99 if there is an eruption, how serious do you think the effects would be for your community? mean sd mesimex 4.16 1.00 barberi et al. [2008] 4.33 0.84 if there is an eruption, how serious do you think the effects would be for you and your family? mean sd mesimex 3.96 0.94 barberi et al. [2008] 3.96 0.93 how worried are you about the possibility of an eruption? mean sd mesimex 3.42 1.30 barberi et al. [2008] 3.80 1.15 how much control do you feel you have to protect yourself and your family in case of an eruption? mean sd mesimex 3.40 1.20 barberi et al. [2008] 2.95 1.19 table 2. likelihood, severity and concern about future eruptions. ratings were made on a 5 point scale, with higher numbers indicating higher levels of likelihood, severity and worry. table 3. self-efficacy regarding future eruptions. ratings were made on a 5 point scale, with higher numbers indicating higher levels of self-efficacy. 3.6. knowledge of the emergency plan and confidence in emergency measures four survey items asked respondents to indicate whether they were aware of the emergency plan for their city in the event of an eruption and evacuation order, as well as how much confidence they had in the plan and how useful they judge emergency simulations in people’s preparedness in case of a volcanic event. 56.3% of mesimex respondents stated they were familiar with the emergency plan for their city in case of a future eruption of vesuvio. 23.6% were not aware of the emergency plan while 20.1% failed to respond. in comparison the respondent ratings of the previous study were 41%, 55%, and 4%, respectively. respondents who stated that they were familiar with the plan were then asked two additional questions about the evacuation procedures and the location where vesuvio red zone residents will be evacuated. among those respondents who indicated that they were familiar with the plan, 84.3% correctly stated that the evacuation would be organized rather than spontaneous and 89.2% correctly identified the italian region to which they would be evacuated. for comparison, respondents’ ratings in barberi et al. [2008] were 83 and 53% respectively. two follow-up questions asked respondents to rate on a 5 point scale both their degree of confidence in whether a successful evacuation could be completed and the usefulness of emergency simulation exercises by civil protection department that involve the general public. 40.1% of respondents stated that they had “little” or “no” confidence in the plan while 30.6% had “a lot” or “complete” confidence in its success. for comparison respondents’ ratings in barberi et al. [2008] are 67.5% and only 10.9% respectively. the positive respondents’ attitude showing higher levels of confidence in the plan success can be related to people’s involvement in risk mitigation actions, as also testified by the high levels of usefulness of civil protection exercises stated in a specific questionnaire item (table 4). 3.7. confidence in officials a number of survey items asked residents to evaluate the preparedness of local government authorities and civil protection department to deal with a potential eruption, and also asked respondents to rate their confidence in scientists, government, and the media’s ability to provide accurate information about future eruptions. these data are presented in table 5. regarding perceptions of how prepared local and civil protection authorities are to deal with a volcanic crisis, mesimex respondents scores are higher than the corresponding ones in barberi et al. [2008], especially for civil protection preparedness. however the mean ratings for local authorities remained below 3 (m = 2.87) on a 5 point scale. in evaluating the accuracy of information respectively provided by scientific community, local authorities, and media, mesimex respondents confirm scientists as the more reliable source of information, giving anyway higher scores also to local authorities and media in comparison to previous survey. ricci et al. 4 how prepared are the authorities in your town to deal with a potential eruption? mean sd mesimex 2.87 1.14 barberi et al. [2008] 2.19 1.03 how prepared is civil protection department to deal with a potential eruption? mean sd mesimex 3.50 1.08 barberi et al. [2008] 2.58 1.06 how much confidence do you have in scientists’ ability to provide accurate information about future eruptions? mean sd mesimex 3.54 0.94 barberi et al. [2008] 3.34 0.96 how much confidence do you have in local and regional authorities’ ability to provide accurate information about future eruptions? mean sd mesimex 3.11 0.97 barberi et al. [2008] 2.65 0.94 how much confidence do you have in the media’s ability to provide accurate information about future eruptions? mean sd mesimex 3.18 1.00 barberi et al. [2008] 2.87 0.97 mesimex % barberi et al. [2008] % none 2.1 10.1 very little 12.9 26.7 moderate 18.6 18.4 enough 39.9 29.3 completely 26.5 15.5 table 4. usefulness of civil protection exercises involving residents. table 5. confidence in public officials and media. ratings were made on a 5 point scale, with higher numbers indicating higher levels of perceived preparedness and confidence. 5 3.8. information about volcanic hazards some survey items concerned the amount of information that residents feel they have regarding the volcanic hazards they face, and the most common sources of these information. as the data in table 6 show, 36.3% of respondents feel that the information they have is either “very little” or “insufficient”, while 19.5% say it is “enough” or “too much”. overall, even if a significant better response was given by mesimex respondents in comparison with the previous study, the amount of information received is still poor. when asked from which sources they had received the majority of their information on volcanic hazards, the two most common sources cited were civil protection department (45.2%) and television (38.9%). these results are quite different from the ones presented in barberi et al. [2008] (table 7). when asked from which organization they would most like to receive information about volcanic hazards, respondents indicated osservatorio vesuviano (national institute of geophysics and volcanology, naples section), civil protection department, and local government. respondents were also asked to indicate their favourite channel to get information on volcanic risk. as in the previous study, public meetings scored the highest ratings followed by radio/television and pamphlets (table 8). 3.9. sense of community results of sense of community were derived by averaging responses made on each of the 18 items of the italian scale of sense of community [prezza et al. 1999]. scores range from a low of 1 to a high of 4, with higher numbers indicating a stronger bond to one’s community. the mean for the entire sample was 2.86, with a standard deviation of 0.48 and did not differ substantially from the previous study (2.61 and 0.37 respectively). 4. conclusions on the basis of the results coming from the study conducted during mesimex exercise on the red zone residents the most remarkable aspects are: – the high feelings of respondents’ concern about a future eruption must be taken into account in education/information strategies in order to get an appropriate population response in case of a volcanic emergency; – the great acknowledgement of civil protection exercises involving residents; – the improvement of people’s emergency plan knowledge; – the increasing of people’s self-efficacy in facing a future volcanic emergency; – the tripling of the emergency plan success confidence in comparison with the results of the previous study; – the significant increasing of people’s confidence in civil protection rather than the one expressed for local government and media; – the improvement of the amount of information received about volcanic hazards, even though most part of respondents defined it still not adequate; – osservatorio vesuviano, civil protection department, and local government are confirmed to be volcanic risk perception at vesuvio mesimex % barberi et al. [2008] % insufficient 8.8 23.9 very little 27.5 36.7 moderate 44.1 28.9 enough 15.8 8.8 too much 3.7 1.7 mesimex % barberi et al. [2008] % 1) civil protection dep. 45.2 1) television 55.2 2) television 38.9 2) newspapers 31.0 3) local government 20.4 3) civil protection dep. 16.4 4) newspapers 14.8 4) acquaintances 15.9 5) osservatorio vesuviano 13.1 5) osservatorio vesuviano 14.3 6) radio 6.79 6) local government 12.1 7) public meetings 6.09 7) schools 11.9 8) schools 4.45 8) public meetings 6.4 9) acquaintances 3.98 9) radio 4.7 10) provincial government 2.81 10) provincial government 2.9 mesimex % barberi et al. [2008] % 1) public meetings 44.0 1) public meetings 44.0 2) radio/television 26.7 2) pamphlets 35.4 3) pamphlets 23.9 3) radio/television 29.3 4) audio-visually 20.1 4) audio-visually 24.1 5) newspapers 8.7 5) newspapers 14.9 table 6. amount of information received regarding the effects of a possible eruption. table 7. most common sources of information regarding volcanic hazards. numbers indicate the percentage of responses mentioning each source of information. respondents were able to check more than one source; therefore percentages do not total to 100%. table 8. how respondents would like to receive information about volcanic hazards. numbers indicate the percentage of responses mentioning each source of information. respondents were able to check more than one source; therefore percentages do not total to 100%. the respondents’ favourite source of information; – the confirmation of public meetings as people most appreciated way in receiving information about volcanic hazards. the above listed aspects put in evidence the importance of civil protection exercises and the increasing of people involvement request, also in designing risk mitigation strategies. this fundamental outcome must be put in practice through a systematic education and information policies, and making civil protection exercises ordinary events instead of sporadic ones as they are nowadays. references bandura, a. (1977). self-efficacy: toward a unifying theory of behavioral change, psychol. rev., 84, 191-215. barberi, f., c. principe, m. rosi and r. santacroce (1995). scenario dell’evento eruttivo massimo atteso al vesuvio in caso di riattivazione a breve-medio termine (aggiornamento al 20.1.1995), cnr, gruppo nazionale per la vulcanologia, confidential report to dpc, 19 pp. barberi, f., m.s. davis, r. isaia, r. nave and t. ricci (2008). volcanic risk perception in the vesuvio population, j. volcanol. geoth. res., 172, 244-258. bird, d.k. (2009). the use of questionnaires for acquiring information on public perception of natural hazards and risk mitigation – a review of current knowledge and practice, nat. hazards earth syst. sci., 9, 1307-1325. carlino, s., r. somma and g.c. mayberry (2008). volcanic risk perception of young people in the urban areas of vesuvio: comparisons with other volcanic areas and implications for emergency management, j. volcanol. geoth. res., 172, 229-243. davis, m.s., t. ricci and l.m. mitchell (2005). perceptions of risk for volcanic hazards at vesuvio and etna, italy. australas, j. disaster and trauma studies, 1, 1-16. dobran, f., a. neri and g. macedonio (1993). numerical simulations of collapsing volcanic columns, j. geophys. res., 98 (b3), 4231-4259. dobran, f., a. neri and m. todesco (1994). assessing pyroclastic flow hazard at vesuvio, nature, 367, 551554. dominey-howes, d.t.m., and d. minos-minopolous (2004). perceptions of hazard and risk on santorini, j. volcanol. geoth. res., 137, 285-310. dpc (1995). pianificazione nazionale d’emergenza dell’area vesuviana. dipartimento della protezione civile, roma. gaillard, j.c. (2008). alternative paradigms of volcanic risk perception: the case of mt. pinatubo in the philippines, j. volcanol. geoth. res., 172, 315-328. gregg, c.e., b.f. houghton, d.m. johnston, d. paton and d.a. swanson (2004). the perception of volcanic risk in kona communities from mauna loa and hualalai volcanoes, hawai’i, j. volcanol. geoth. res., 130, 179-196. haynes, k., j. barclay and n. pidgeon (2008). whose reality counts? factors affecting the perception of volcanic risk, j. volcanol. geoth. res., 172, 259-272. johnston, d.m., m.s. bebbington, c.d. lai, b.f. houghton and d. paton (1999). volcanic hazard perceptions: comparative shifts in knowledge and risk, disaster prevention and management, 8, 118-126. kartez, d. (1982). emergency planning implications of local governments’ responses to mount st helens, natural hazard research, working paper no. 46, 1-37. njome, m.s., c.e. suh, g. chuyong and m.j. de wit (2010). volcanic risk perception in rural communities along the slopes of mount cameroon, westcentral africa, j. afr. earth sci., 58, 608-622. paton, d. (2003). disaster preparedness: a social–cognitive perspective, disaster prevention and management, 12 (3), 210-216. paton, d., l. smith, m. daly and d. johnston (2008). risk perception and volcanic hazard mitigation: individual and social perspectives, j. volcanol. geoth. res., 172, 179-188. perry., r.w. (1990). volcanic hazard perceptions at mount shatsa, environ. prof., 12, 312-318. perry, r.w., and m.k. lindell (2008). volcanic risk perception and adjustment in a multihazard environment, j. volcanol. geoth. res., 172, 170-178. prezza, m., s. costantini, v. chiarolanza and s. di marco (1999). la scala italiana del senso di comunità, psicologia della salute: quadrimestrale di psicologia e scienze della salute, 3/4, 135-159. rosi, m., c. principe and r. vecci (1993). the 1631 eruption of vesuvio reconstructed from the review of chronicles and study of deposits, j. volcanol. geoth. res., 58, 151-182. solana, m.c., c.r.j. kilburn and g. rolandi (2008). communicating eruption and hazard forecasts on vesuvio, southern italy, j. volcanol. geoth. res., 172, 308-314. *corresponding author: tullio ricci, istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy; email: tullio.ricci@ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 magnitude and energy of earthquakes beno gutenberg and charles francis richter annali di geofisica, vol. 9, n. 1, 1956. annals of geophysics, vol. 53, n. 1, february 2010 abstract discrepancies arise among magnitudes as derived from local earthquake data (ml), body waves (mb) and surface waves (ms). the relation of ml to the others is as yet not definitive; but ms – mb = a (ms – b). the latest revision gives a = 0.37, b = 6.76. pending further research it is recommended that ml continue to be used as heretofore, but ms (and ultimately ml) should be referred to mb as a general standard, called the unified magnitude and denoted by m. tentatively log e = 5.8 + 2.4 m (e in ergs). revised tables and charts for determining m are given. this paper is in continuation of previous investigations [gutenberg and richter 1942, 1956]. the earthquake magnitude has statistical and other uses independent of the relation between magnitude and energy. indeed, it is possible that there is no complete one-to-one correlation between magnitude and energy for large and complex tectonic events. even so, a mean or representative relation is a legitimate object of inquiry. in attempting to refine the magnitude-energy relation it was found [gutenberg and richter 1956] that three imperfectly consistent magnitude scales had been in use: ml determined from records of local earthquakes according to the original definition [richter 1935]; ms from the amplitudes of surface waves for shallow teleseisms, [gutenberg and richter 1936; gutenberg 1945a]; mb from the amplitude/period ratio of body waves for teleseisms, shallow and deep-focus [gutenberg 1945b, 1945c]. the two latter were originally adjusted to coincide near m = 7, but were later found to diverge linearly so that ms – mb = a (ms – b) (1) for a number of years reductions were carried out with a = 1/4, b = 7, converting values of mb into the corresponding ms. the result of this reduction may be designated mb. the final value given for m was a weighted mean between mb and ms. this may be taken as defining m without subscript. the adjustment between ms and mb can now be performed with considerable accuracy, using the relation (1) with revised parameters a = 0.37, b = 6.76. this is equivalent to mb = 0.63 ms + 2.5 = ms – 0.37 (ms – 6.76) (2) the revision is based on a large body of data. magnitudes have been derived by the senior author, from surface waves and from body waves separately, for a selection of better recorded large shallow earthquakes as listed by gutenberg and richter [1954]. those for which there was suspicion of depth in excess of the normal (believed to be about 25 km) were rejected. values of mb were plotted against those of ms, and (1) derived from the plot. the values a = 0.37, b = 6.76 are comparable with those found by bath [1955] as follows: at pasadena, a weighted mean is taken between mb as found directly from body waves, and ms, the corresponding value derived from ms by applying the relation (1), or still better from tables and charts set up to give ms directly from surface wave data. this weighted mean is designated the unified magnitude denoted by m. in figure 1 residuals mb – ms on the basis a = 0.37, b = 6.76 are plotted against m, using amplitude and period data from all available station bulletins, a) for all shocks in table 13 of gutenberg and richter [1954] for which there was no indication of depth exceeding 30 km, excluding all uncertain or doubtful magnitudes; b) using all similar data for table 14 of gutenberg and richter [1954] for 1936-1939 and 1950-1952 (inclusive). there is little indication of systematic deviation from the axis of zero residuals. the slight apparent excess of positive residuals may be due to the use of a few shocks with depths somewhat greater than supposed, which should result in a 7 station body wave used a b uppsala kiruna pz ph sh pz ph sh 0.45 0.46 0.23 0.59 0.50 0.30 6.3 6.4 5.6 6.2 6.5 6.1 decrease of ms. comparable data for magnitudes below 7 are rare. ten of eleven shocks in the california region, with magnitudes near 6, give mb – ms from + 0.1 to – 0.2; the eleventh gives – 0.4. figure 2 is a nomogram prepared by mr. j.m. nordquist for the direct determination of ms from surface wave amplitudes. the corresponding values of ms and of log e from equation (6), are also indicated. the adjustment of ml to m or m cannot yet be determined so closely as that of m to m but can be stated with an error not likely to exceed 0.5 magnitude unit for those shocks (magnitudes 3 to 6) most often rated in terms of ml. representative results are given in table 1. values in parenthesis are outside the observable range. until 1954, the writers generally reported magnitudes for large deep shocks, and for large shallow teleseisms as determined from body waves, effectively in terms of ms, first determining mb and then correcting to ms by applying equation (1) or an earlier approximation to it. the correction was usually applied only to shocks of magnitude 7 or over. it now develops [gutenberg and richter 1956] that many outstanding difficulties disappear if the linear relation (2) is consistently extended to magnitudes below 7. whereas shocks of the largest magnitude record with surface waves relatively large compared with the body waves, shocks of magnitude below 7 show relatively small surface waves when recorded at teleseismic distances. many long-period instruments do not record such shocks clearly; this makes assignment of magnitude from the data of distant stations difficult. short-period instruments in such cases may show a measurable p; the absence of recorded surface waves is then sometimes misinterpreted as evidence for deep focus. when equation (2) is used, and data for both body waves and surface waves are available, two different determinations are in effect available for either m or m. the equation gives mb = ms for a value near 6 3/4. when the magnitude does not greatly differ from this figure, problems of adjustment are minor, and reduce to judgement as to the relative reliability of the two groups of data. although at present many more stations report amplitudes for surface waves than for body waves, some ten years' experience indicates that mb provides the better data in practice as shown by fewer systematic errors and more consistent results, as well as being theoretically preferable. in using station bulletins to determine ms, the maxima of surface waves can be used for magnitude only when the period is near 20 seconds. if the period is not specified, there is risk that the reported maximum amplitude may refer to much longer or shorter waves, which seriously falsifies ms. with some exceptions, magnitudes currently being reported in station bulletins are either ml determined from nearby stations, or ms. there is less general determination of mb, and the relation in equation (1) or (2) is often overlooked. occasionally ms is even given for deep shocks as found directly from surface waves; if the hypocenter is deeper than about 30 km, calculation on this basis gives too low a value. routine station bulletins issued from pasadena continue to list magnitudes m which are either ml or ms; but beginning with 1954 the annual list of large shocks also tabulates m, which is an intermediate step toward a definitive magnitude-energy relation. the practical definition of the unified magnitude m consists in a system of tables and charts for calculating magnitude from the quotient amplitude/period for the maximum waves of the principal wave groups p, pp, and s. this quotient is used in the form q = log u/t or q = log w/t (3) where u and w are respectively the horizontal and vertical components of the ground displacements in microns and t the period in seconds. each table or chart gives for all distances and focal depths a quantity qsuch that for corresponding distance and depth m = q + q + s (4) magnitude and energy of earthquakes 8 figure 1. residuals mb – ms on the basis a = 0.37, b = 6.76 are plotted against m, using amplitude and period data from all available station bulletins. table 1. values of m, m and log e for given values of ml, using m= 1.27 (ml– 1) – 0.016 ml², m= 0.63 m+ 2.5, log e= 5.8 + 2.4 m(e= energy in ergs). 9 where s is a ground correction characteristic of the station used. charts and tables of this type were first given by gutenberg [1945b, 1945c], where q was designated a. those accompanying the present paper (table 2, figures 3, 4 and 5) represent no change in fundamental concept, but only a revision. the statistical processes by which the tables and charts published in 1945 were derived have now been repeated by the senior author using a much larger body of data, and, it is hoped, with greater precision. one effect has been to remove a persistent discrepancy between magnitudes determined from horizontal and vertical components; this discrepancy was discovered independently by bath [1955]. this procedure places the unified magnitude m on a selfconsistent and independent basis as satisfactory for teleseisms as that of ml for local earthquakes, and with the great advantage of being applicable directly to seismograms recorded on instruments of all types and at all stations. if desired, a formal definition for m may be phrased as follows: m – 7.0 = q n (5) at a distance of 90˚ for normal shallow focal depth, where q = log w/t refers to pz, and the station constant s is taken as zero, representing average station ground conditions. since the relation of ml to m is not yet on a definitive basis, the authors suggest that the «richter scale» as defined in 1935 be retained for determining magnitudes of local shocks. for teleseisms, the use of the unified scale m is preferred and strongly recommended. for magnitudes from about 5½ to 7, the departure between the two scales is within the usual limits of error under the now existing conditions of recording and reporting amplitudes. gutenberg and richter [1954] have not assigned magnitudes below 6 to shocks outside the california area (within which ml is reported); such shocks are merely designated by the letter d. above magnitude 7½ the scales diverge significantly; but then determinations from the data of numerous stations scatter increasingly, and it is advisable to distinguish carefully between determinations from body waves and from surface waves. it is urgent that magnitudes determined from seismograms at single stations should not be published unaccompanied by the amplitude and period readings on which they are based. it is hoped that before many years have passed it will be possible to express the entire range of observed magnitudes in terms of the unified magnitude m. since the provisional use of m is especially intended for investigations relating to energy, m is being published together with the energy calculated from it by the relation log e = 5.8 + 2.4 m (6) to be established on a later page. for most types of publication the writers think it preferable to follow a suggestion by dr. l.b. slichter, giving the magnitude and energy of earthquakes figure 2. nomogram for the direct determination of ms from surface wave amplitudes. the corresponding values of ms and of log e from equation (6) are also indicated. table 2. values of q for shallow shocks. value of log e together with equation (6), and so avoiding confusion due to use of numerically different magnitude scales. most calculations of the magnitude-energy relation depend directly or indirectly on the equation for a wave group from a point source [gutenberg and richter 1956] e = 2π³h²vρ (a/t)²t (7) where e is energy, h is linear distance from the source, v is velocity, ρ is density, a and t are amplitude and period of sinusoidal waves, and t is the duration of the wave group (which hence contains n = t /t waves). this applies at the epicenter when h is hypocentral depth, and includes a factor which takes account of the effect of the free surface. taking v = 3.4 km/sec for transverse waves, applying a factor 3/2 to allow for half as much energy in longitudinal waves, and using h = 16 km, ρ = 2.7 gm/cc, this reduces to log e = 12.34 + 2 qo + log to (8) where q = log a/t and the subscript zero refers to the epicenter. a fundamental empirical equation is qo = − 0.6 + 0.8 ml − 0.01 ml² (9) this is a revised result drawn from the plot of qo as a function of m for california shocks (figure 3) [gutenberg and richter 1956]. two further important equations are derived from plotted data (figures 6): log to = − 1 + 0.4 qo (10) and qo = m − 2.3 (11) for the latter result most of the data cover a relatively small range of m. combining these log to = 0.4 m − 1.9 (12) on the other hand, if in (10) we substitute for qo its expression in terms of ml from (9), we obtain log to = − 1.24 + 0.32 ml − 0.04 ml² (13) which differs only slightly from the corresponding equation setup empirically in gutenberg and richter [1956], showing that the derivation of (9) and (10) has been consistent. combining (9) and (11) m = 1.7 + 0.8 ml − 0.01 ml² (14) this is drawn on figure 7; it is not inconsistent with the plotted data. magnitude and energy of earthquakes 10 figures 3, 4, 5. from top: revised values of q for sh; revised values for q for ppz; revised values for q for pz. 11 magnitude and energy of earthquakes if instead of (9) we had used the corresponding equation in gutenberg and richter [1956], which has a larger coefficient of the quadratic term, the resulting equation replacing (14) would lead to calculated values of mb − ml which for large m are systematically too small to suit the observations. this is the chief reason for revising the empirical relation between qo and ml to the form (9). if we apply the relation (2) to (14) we find ms = 1.27 (ml − 1) − 0.016 ml² (15) equation (6) results from substituting in (8) the expressions for qo and log to from (11) and (12). it has also been verified approximately by the following calculation. for a train of n (= t/t) sinusoidal body waves, emerging to the surface of the earth at arc distance θ from a surface source with horizontal ground displacement u, the total energy calculated as radiated from the source is e = 8π³r²ρvt (u/t)²/u²l (16) where u²/fi² = tan i di / sin θ d θ (17) here e = energy, r = radius of the earth, ρ = density, v = velocity, t = duration of wave train, t = period, i = angle of incidence, fi is a factor expressing the effect of the free surface as a function of i (otherwise it depends only on poisson's ratio; see gutenberg 1944), and l is a factor to allow for absorption, scattering, internal friction, effects of discontinuities, etc. there are several simplifying assumptions: the earth is taken as spherically symmetrical, the effect of hypocentral depth is neglected (it is easily corrected for), energy flux is calculated by the ray method as used in geometrical optics, and the use of (16) to calculate total energy implies radiation equally in all directions from the source. in what follows it is assumed that 1/3 of the original energy is radiated as longitudinal waves, and a factor 3 is accordingly applied. analogous equations to (17) and (18) apply to the vertical component of ground displacement, replacing u by w and u² by a similar factor w². we next take ρ = 3 gm/cc, v = 6.3 km/sec (applying to longitudinal waves), r = 6370 km; we also take q = log u/t or q = log w/t, where u and w are expressed in microns. taking the logarithm of (17) with proper attention to the units used, we arrive at log e = 18.8 + log t + 2q − log u − log l (19) we now assume that t = to; this has been confirmed roughly by dr. c. lomnitz from seismograms recorded at pasadena. applying (12) with t in place of to, and putting q = m – q, log e = 16.9 + 2.4 m − 2q − 2 log u − log l (20) comparing this with (6) we should have 2q + 2 log u + log l = 11.1 (21) and a similar equation for the vertical component. here, 2q may be taken from table 2, and log u can be calculated from (17). working this out for the vertical component of p waves, the following values are found: θ 20˚ 42˚ 100˚ log l −2.0 −1.3 −1.5 the calculation cannot be extended reliably to distances less than 20˚. the contribution of absorption to log l should be about 0.4 near 100˚ and 0.3 at moderate distances. loss by refraction at the mohorovicic, conrad, and other discontinuities in the crust may account for a few tenths in log l. this leaves about one unit in log l unaccounted for. if all assumptions are correct, energy flux is reduced to roughly one tenth within the first 20˚ of distance; this must occur within the upper 200 km of the mantle. if this is correct, we should expect a smaller constant term in the energymagnitude relation corresponding to (6) for shocks at greater depths. this would agree with the relatively low energy figure 6. two important equations are derived from plotted data: log to = – 1 + 0.4 to and qo = m – 2.3. figure 7. combining equations (9) and (11), m = 1.7 + 0.8 ml – 0.01 ml ². magnitude and energy of earthquakes 12 calculated by sagisaka [1954] for a shock at a depth of 360 km. however, the constant term 5.8 in (6) and the coefficient 0.4 in (12) are not accurately fixed, and log e calculated from (6) may be in error by as much as one unit. acknowledgements. contribution n. 750, division of the geological sciences, california institute of technology, pasadena, california. references bath, m. (1955). the problem of earthquake magnitude determination (unpublished). gutenberg, b. (1944). energy ratio of reflected and refracted seismic waves, bull. seismol. soc. amer., 34, 85-102. gutenberg, b. (1945a). amplitudes of surface waves and magnitude of shallow earthquakes, bull. seismol. soc. amer., 35, 3-12. gutenberg, b. (1945b). amplitudes of p, pp, and s and magnitude of shallow earthquakes, bull. seismol. soc. amer., 35, 57-69. gutenberg, b. (1945c). magnitude determination for deep-focus earthquakes, bull. seismol. soc. amer., 35, 117-130. gutenberg, b., and c.f. richter (1936). on seismic waves (third paper), gerlands beitrage zur geophysik, 47, 73-131. gutenberg, b., and c.f. richter (1942). earthquake magnitude, intensity, energy and acceleration, bull. seismol. soc. amer., 32, 163-191. gutenberg, b., and c.f. richter (1954). seismicity of the earth, second ed., princeton press. gutenberg, b., and c.f. richter (1956). earthquake magnitude, intensity, energy and acceleration (second paper), bull. seismol soc. amer., 46, 2, 105-145. richter, c.f. (1935). an instrumental magnitude scale, bull. seismol. soc. amer., 25, 1-32. sagisaka, k. (1954). on the energy of earthquakes, geophys. mag. tokyo, 26, 53-82. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. layout 6 1 annals of geophysics, 62, 3, se344, 2019; doi: 10.4401/ag-7981 “nanotechnologies for concrete and composed structuresfor seismic safety„ agostino catalano*,1 (1) dept. scienze umanistiche, sociali e della formazione, università degli studi del molise, campobasso, italy 1. advanced materials the advanced materials employed in building sector often derive by processes of technological transfer from other industrial sectors characterized by strong pushes to innovation (typically aeronautical, car industry and biomedical). in these sectors the search in the field of materials with elevated performances more and more constitutes an essential condition for the development of products and efficient systems. taking into account that, generally, in building, from the point of view of both industrial production and architecture design, the innovations are absorbed in times longer than in other sectors, in order to ensure that these materials are received in ordinary construction processes, adaptations and verifications of the use performances are necessary, practices which, on the other hand, together with the technical difficulty of using advanced materials and the lack of specific regulations, they tend to delay their spread. the recent development of knowledge in the chemical field has radically changed the relationship between man and matter. in fact, through the manipulation of molecular atomic structures it is possible to set up numerous new complexities managed by materials, in which impurities and anisotropies are purposely designed to obtain very punctual performances. the material identity of the object is replaced by performance, changing a reference code that for centuries article history receveid october 16, 2018; accepted february 5, 2019. subject classification: nanotech; concrete; composed structures; earthquakes; jet grouting technology. abstract performance requirements of civil structures are becoming more and more severe in recent years. users pay an increasing attention to safety and functionality of systems and components over time, because of the impact of failure and/or down time from the social and economic point of view. the market globalization and the transfer of technologies among different branches of engineering and mate− rial science are also influencing the concept of safety of civil structures – buildings, bridges and other critical infrastructures. a differ− ent paradigm in the field of building and infrastructure technology has been envisaged and investigated. in particular, issues like degra− dation control and assessment of materials and components in order to enhance durability of structures and overall performances of the constructions are focused; moreover, emergency management, structural safety and performance monitoring of structures exposed to nat− ural risks such as earthquakes are targets of recent researches. the industry of the constructions must face problems related to the main− tenance of structural elements in concrete or to mixed structure. in virtue of such problem list in the last years it has also spread the use of concretes to high technology with particular characteristics of seismic resistance. the use of this concrete allows to overcome the con− nected difficulties with the geometric complexity of the structures and the problems originated from sections to tall percentage of ar− mor. the addition, then, of fibers of reinforcement in partial or total substitution of the traditional metallic armor it allows to get best performances both static that dynamics of existing structures. has helped man to know the surrounding world. the materials of recent generation cannot be classified according to consolidated parameters since they present a continuum of possibilities with unpredictable behaviors. 2. the role of advanced materials in safety design the family of new materials appears extremely heterogeneous and difficult to classify according to traditional criteria. the main element that distinguishes it, does not derive from the fundamental properties of the material itself, previously described by its material content, but rather from the possibility of attributing extraneous and original characters that increase the information content by intervening on different dimensional scales. the levels at which it is possible to intervene on a single product today are many and vary their dimensional scale based on the properties they want to confer. for example, by manipulating the atomic structure of a material it is possible to modify the general characteristics that distinguish the three large families of ceramic, metal and polymeric products. furthermore, by acting on the type of spatial distribution of atoms and on the intensity of their bonds it is possible to change the state of aggregation from solid to liquid or gaseous, in order to create new metal alloys and ceramic materials with high specific performances. the physical-mechanics properties that determines the type of current positions in a polymer, depend from the type of microstructure, while from the macrostructure it is possible to manage the stickiness properties of a composite by modifying the combination of quantity of fibers and matrices present. these materials are generally characterized by properties that, compared to normal building materials, are optimized for the specific use defined. they can provide variable, selectable and controllable performances, in order to modify their physical-chemical properties in relation to the stimulus received, introducing new services that cannot be reached or considered previously. since the principal difference between new and traditional materials resides in the abilities performance and not only in a detail or unpublished conformation physical-chemistry, also traditional materials “innovated” in their performances an be considered “advanced” to all effects. equally particular productive and of synthesis trials can identify some classes of advanced materials (it is thought the nanotech materials to resultant by the joining of two or more materials). with the definition of advanced innovative materials are generally pointed out all those custom designed ceramic, metallic or polymeric materials for satisfying one or more demands. these types of materials differ from traditional ones not so much because they are made in more recent times, but because they introduce a high degree of function or design by intervening on their physical and chemical structure to vary their information content and increase performance levels. the speed with which the world of science offers us new materials and technologies with enormous potential, together with the properties of thermoplastic ones, an integral part of our industrial heritage, already offers a concrete possibility to develop innovative materials with unprecedented performance. • nano fillers: nanotechnology generally consists of studying the change in the properties of the particles as the dimensions vary below 100 nm. the reason why nanocomposite materials are potentially very interesting comes from the high surface interaction that is created between polymer and nanoparticles which gives rise to considerable variations in the physical-mechanical properties of the base polymer. • special fillers: the introduction of fillers such as aramid fibers, carbon or steel, as well as ceramic inserts inside the polymers, improves those specific characteristics that make them particularly suitable for uses in which the operating conditions are discriminating compared to more traditional metals. • advanced polymers: the high degree of reliability and quality achieved by the multinationals in the production of basic polymers, combined with their ability to focus on a few increasingly refined products, constitutes the development base for innovative niche materials. 3. typologies and characteristics among the advanced materials currently used in the construction it is possible to select two main families: agostino catalano 2 • materials with fixed performances, in which the final properties are selected and predetermined through particular chemical-physical conformations and synthesis processes, and intelligent materials capable of varying the characteristics when stressed by the external stimulus. among the materials with fixed performances it is possible to distinguish: • advanced structural materials what fiberenforced composite, concretes with high performances, structural glasses, metallic and polymeric foams, employed in different typologies of applications in which the function is predominantly expressible in terms of mechanical performance; • thermostructural materials, such as flame retardant and flame retardant fibers, thermosetting resins, advanced ceramics, transparent ceramics, high-performance ceramics, ceramic foams and light ceramics, with high thermo-mechanical performance; • materials with surface and interface performance, such as wear-free, anti-corrosion, thermal and photocatalytic coatings and coatings; self-cleaning, selective and lowemission glasses, which when used in buildings wrap must provide a high level of protection against various environmental factors from chemical-physical conformations. on the other hand, it is possible to divide intelligent material into two main categories: • materials that change properties and memory of the materials of form. they can change their chemical, mechanical, optical, electrical, magnetic or thermal properties, based on changing environmental conditions without requiring an external control system; • energy exchange materials; organic materials for photovoltaic conversion, capable of transforming one form of energy into another, according to the first thermodynamic principle. they are used in buildings as devices for energy production and control systems. it is worth noting that this classification includes types of materials whose application in the building is well established (the fiber reinforced composite is, for example, used for the consolidation of existing structures or for the construction of light and resistant structural components). in particular, the single-walled nanotube has a high tensile strength with a breakup tension comparable to the theoretical value corresponding to the carbon-carbon bond in a benzene ring. this makes it the most resistant organic material in the world: it has, in fact, a tensile strength 100 times greater than a steel bar, but with a weight six times lower. furthermore, it is important to remember that nanotubes are not only resistant to tensile failure, but they are very flexible and can be folded without breaking or damage. the extreme resistance combined with flexibility makes them ideal for buckyballs. 4. nanotechnology for seismic protection of infrastructures the maximum seismic action exerted on a built system is well described by the flexible spectrum, which indicates the maximum seismic acceleration of the system based on its vibration period. as it is known in most structures, the maximum amplification is characterized by periods of 1 s or more (> 2 s) and in this case the most deformable structures are characterized by a reduced vulnerability to earthquakes. at the same time the problem triggered by the maximum duration of the vibration period cannot be neglected because it involves the danger of the collapse of non-structural parts of the building [clemente, 2017]. this is the reason that drives the definitive and widespread use of seismic insulators between foundations and elevated structures, this is increasingly valid also for civil engineering works and, in particular, for bridges. the study of the characteristic of the deformation and of the tension of the structure is always carried out through a series of simplifications and hypotheses which make it possible to trace from the real case back to the “theoretical model” of calculation that adapts it to the limit of our knowledge and intuitions, this is particularly true also for bridges. in the most doubtful cases, where the definition of a suitable “theoretical model” is uncertain or the calculation procedures are arduous or non-existent, at the planner level is the experimenter and in “theoretical model” is the “experimental model” which confirms or reveals why not we can specify or know with the theory. a real bridge, for example, is always a spatial system and “the theoretical models” should always belong to such category. actually, the necessity of easy and ex3 nanotechnologies for seismic safety agostino catalano 4 peditious calculation determines a first simplification, i.e., to study the bridge as “one-dimensional plain system”. remembering the limits of extension of de saint venant problem, this is allowed when the following hypotheses subsist: a)the geometric axis of the beam is on a plan; b)the generic cross-section has such geometric characteristics, so that one of its inertial central axes is contained on a plan; c) stresses, strengths and distorsions work in such plan. since, with the exception of oblique and curved bridges, the hypotheses a) and b) are satisfied, the crosssections being always symmetrical, it is to consider the stress analysis only. while the deformations are always symmetrical with respect to the plane of the system and it is therefore legitimate to consider them as agents on this plane, the same cannot be said for the strengths, which can be classified into: a)forces resulting in the system plan; b)normal forces resulting from the system plan; c) bending resulting from transverse forces with respect to the axis line. in summary, the analysis of a bridge is carried out in two separate phases [reithel, 1964]: a)the system is considered simple and the corresponding stress and tension system is described; b)the factors that determine a difference between the previous results from the actual ones (deformability of the cross section, actual application of the strength points) must be taken into consideration. they determine the corresponding deformation system and the effort that overlaps the first one. it seems evident, therefore, that the use of nanotechnologies with characteristics of resistance superior to ordinary concretes including a strong tensile strength, ensures the greater reliability of the model adopted in terms of a more efficient seismic protection. from this point of view, the performances currently required of such delicate and particular structures seem to be achievable entirely with the use of nanostructured concrete that can also be used to consolidate existing structures, as would appear to be apparent after the recent events in genoa: the collapse of one of the bridge spans designed by riccardo morandi and inaugurated in 1967. 5. nanotechnologies for dynamic interaction foundation – structure one of the possible technologies that can be applied in a seismic zone is seismic isolation. the concept of scientific isolation was developed about 150 years ago, in 1868, by scottish engineer david stevenson when the study of seismic engineering was at the initial development. however, references to this anti-seismic protetion systems also exist in the construction of temples or public buildings since the time of ancient greece. the awareness of the application of this anti-seismic technology is typical of the second half of the twentieth century when scientific thought confirmed the interest in this solution aimed at separating two rigid parts of a building to produce an attenuation of the effects on the entire structure [carpani, 2017]. in synthesis, the basic hypotheses of the soil-structure dynamic interaction can be summarized as it follows [lai, 2011]: a it is not significant in case of flexible structures on rigid grounds b it can be important for rigid structures on deformable ground c the fundamental period of vibration of a soil-structure system is longer than that correspondent to a built structure; d the real damping of a soil-structure system is higher than the structural one; e total movements may increase due to interaction effects and may be important for higher structures; f to ignore the interaction is equivalent to assume the structure founded upon rock. the isolation technology involves the development of the column isolation technology (jet grouting technology) which, appropriately assembled and designed from the geometric and performance point of view of the mechanical-physics according to the nanotechnological mixtures, is able to: a) mitigate the seismic actions for existing buildings, preserving the level of safety towards gravitational actions and in operating conditions, without affecting any kind of formal integrity: b)mitigate the effects of vibrations produced by anthropogenic conditions (vibrating cars, trains, etc.); c) realizing highly efficient and reliable hydraulic barriers even in the presence of polluting agents. 5 nanotechnologies for seismic safety the researches developed nowday have, infact, underlined that: • for objectives a) and b): it is possible to weaken the propagation of shear waves (of anthropogenic or seismic origin) by introducing in the mass poles of nanotechnological material with physical-mechanical performances different from those of the soil. in particular, the idea consists in the introduction of a relatively thin layer of material characterized by a dynamic impedance lower than that of the surrounding soil. numerical analyzes and laboratory tests have shown that when the relationship between the impedance of natural soil and the material that makes the barrier to vibrations reaches values of 20-30, geometrically mounted barriers allow a reduction in the propagation of vibrations1. the research activities in the field of technology applied to geotechnical engineering and structural design have made it possible to find the optimal geometrical configurations, which differ in the operation of some considered (seismic risk mitigation or from anthropogenic vibrations). this allowed us to identify some nanotechnological materials suitable for the intended purpose. although research has also been carried out on original material, the existence has been verified eco-friendly products (in such a way that they can be introduced into the ground without polluting and causing environmental damage) it has been verified that it can be used with great simplicity and economic convenience. for this application, we will also consider the possibility of adding the injections in order to make the porous columns, in order to reduce the density of the material (beneficial dynamic effect) and increase the hydraulic permeability of the barrier, to make this application does not disturb the system underground water supply. • for objective c): it is verified that the materials studied to satisfy the objectives a) and b) could also be adapted to the achievement of the same objective. as previously written, in fact, the reduction of vibrations must be obtained with a material that can be light but has, above all, a low speed of shear waves and is therefore very deformable. the materials that have been identified with these characteristics may have implemented a slightly modified composition to give place to the columns with practically zero permeability, very deformable. it is interesting to note that all possible applications are extensive and that jet grouting technology must be considered as a mixture from a basic chemical component, water and earth, possibly with variable operating percentages for specific applications to be solved. in other words, the proposed product will be adaptable to different needs in a simple way through a percentage variation of the basic components and therefore with nanotechnological performances. this technology is absolutely innovative both in the field of defense from vibration and in that of environmental defense. for the first application, in particular, the seismic isolation of existing buildings is an unresolved problem and, in fact, it usually intervenes reducing the vulnerability through interventions that are invasive and that are particularly expensive. with particular reference to the structures of merit, which characterize the italian architectural heritage, traditional technologies for the reduction of seismic vulnerability must be such as not to compromise the historical and material integrity of the structure to be protected and must guarantee the reversibility of the applications. this can make the intervention on the considered structures particularly complex. with reference to risk mitigation, the technology developed can represent a reliable solution for the seismic adjustment to merit or existing strategic buildings in the event of natural events. in this sense, the advantages depend on the durability of the solution’s effectiveness and on the possible controls. it is estimated that the consequent benefits deriving from the application of the insulation of the column, both in terms of life cycle cost and in terms of the characteristics of the overall performance of the structures, can have a 30% reduction in labor costs. with reference to the structures of strategic interest it is possible to obtain a significant reduction in costs if we consider the post-earthquake use. 1 the general characteristics of the dynamic soil-structure interaction through the method of impedance dynamics functions are: general methodology to define the dynamic response of a foundation; assimilation of the earth-foundation system to the assembled mass system; hypothesis of the system of harmonic vibrations; easily generalizable to non-harmonic vibrations with the fourier theorem; development of calculation codes for more common bases. 6. conclusion it appears evident as the introduction of nanotecnologies allows to raise notably the resistance of concrete introducing, of fact, the ductility concrete concept produced by the strong traction resistance and, accordingly, by elevated control of cracking. such consideration appears well work in circle of capacity design together possibility of realization of plastic hinges. it needs however to underline as it still appears relatively missed the behavior in seismic zone of such structures to withdrawal and fluage that constitute a grey zone still within the viscous deformations and on their relapse on maintenance of performances of seismic resistance. the viscous deformations are related to the composition, the loads and the maturation of the concrete. it can be stated how the nanotech concrete, made with a suitable mix-design and matured in predetermined times, can make this negative influence of the rheological factors less important thanks to the high resistance to loads. this is directly influenced by the workability which is the most complex performance to be considered fresh. it is not possible to characterize the workability with a numerical parameter but the studies already carried out allow us to state whether, to frame the phenomena connected to workability, it is necessary to follow the rules of rheology defining the characteristic parameters such as the coefficient of internal friction and cohesion [de sivo, cito, iovino, irace, 1987]. the better workability influences the value of the porosity and, therefore, of the compactness. faury has verified that in a unit volume of porosity of the fresh concrete can be calculated with the expression: + where: r = mean radius of the shuttering dmax = maximum diameter of the inert k = dependent coefficient from the inert nature (table 1) k′ = dependent coefficient from the installation (table 2) therefore, by using nanotechnological concretes, reduced values are obtained, mainly of k value. still within the frame of the rules of technology in seismic areas, and expeially in the context of the local structural interventions, the nanotech concrete coating technique can achieve all or some of the following objectives: • increase in vertical resistance capacity; • increase in flexural or shear resistance; • increase in deformation capacity. the thickness of the local intervention must be such as to allow the positioning of longitudinal and transverse reinforcements. for the evaluation of the strength and deformation of the structural elements the following simplification hypotheses are acceptable [catalano, 2016]: agostino catalano 6 i = k dmax 5 k′r dmax − 0,75 table 1. value of k. concrete rounded edges inerts rounded edges inerts and big inert with sharp edges sharp edges inert soft texure– normal compacting factor 0,370 0,405 0,450 firm texture – very accurate compact factor or normal vibrating 0,350 – 0,370 0,375 – 0,405 0,430 – 0,450 very firm texture – strong vibrating 0,330 – 0,350 0,355 – 0,375 0,400 – 0,430 wet ground texture – very strong vibrating 0,250 – 0,330 0,330 – 0,355 0,350 – 0,400 table 2. value of k′. concrete k’ normal compacting factor 0,003 strong vibrating 0,002 • the structural element involves monolithically with full adherence between old and new concrete; • the fact neglects that the axial load is applied only to the pre-existing element and is considered to act on the entire section; • the mechanical performance of the concrete is considered to be extended to the entire section if the differences between the two materials are not excessive. the performance values to be adopted in the verification are calculated by referring to the structural section in the simplification hypotheses reduced according to the following expression: • cut resistance v = 0,9 v • flexural strength: m = 0,9 m • yeld strength: qy = 0.9 qy • final deformation: qu = qu references carpani b., [2017], “base isolation from a historical perspective”, 16th world conference on earthquake, 16wcee 2017, paper 4934, 9th 13 th 2017 santiago chile. clemente p., [2017], “seismic isolation: past, present and the importance of shm for the future”, journal of civil structural health monitoring, volume 7, isuue 2, pp 217-231, april 2017. catalano a., [2016], “la tecnologia per la buona regola costruttiva antisismica”, ingenio n.49, 14/12/2016. lai c.g., [2011], “interazione dinamica terreno-struttura di edifici con fondazioni superficiali e profonde”, corso di aggiornamento avanzato sulla geotecnica, 23 settembre 29 ottobre 2011, la spezia. de sivo b., cito g., iovino r., irace a., [1987], “appunti di architettura tecnica”, pp 300, cuen editore, napoli, isbn 978-8871460420. reithel a., [1964], “costruzione di ponti”, pp 424, liguori editore, napoli. isbn 978-8820705640. *corresponding author: agostino catalano, dept. scienze umanistiche, sociali e della formazione università degli studi del molise, campobasso, italy email: agostino.catalano@unimol.it © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 7 nanotechnologies for seismic safety layout 6 1 annals of geophysics, 62, 2, vo226, 2019; doi: 10.4401/ag-7745 “crust development inferred from numerical models of lava flow and its surface thermal measurements„ igor tsepelev1, alik ismail-zadeh2,3,*, yulia starodubtseva1, alexander korotkii1,4, oleg melnik5 (1) institute of mathematics and mechanics, ural branch of the russian academy of sciences, yekaterinburg, russia (2) institute of applied geosciences, karlsruhe institute of technology, karlsruhe, germany (3) institute of earthquake prediction theory and mathematical geophysics, russian academy of sciences, moscow, russia (4) institute of natural sciences and mathematics, ural federal university, yekaterinburg, russia (5) institute of mechanics, lomonosov moscow state university, moscow, russia 1. introduction a flow of lava is a common volcanic feature on the earth’s surface. a lava flow can destroy villages or se− riously damage cities as happened in and around catania, sicily, during the eruption of etna volcano in 1669 [branca et al., 2013]. although lavas have been used as a construction material for centuries and have been a source of nutrients in agriculture, lava flows remain a threat to human activity [poland et al., 2016]. the hazard is not negligible as hot lava kills vegeta− tion, destroys infrastructure, and may trigger floods due to melting of snow/ice [e.g., harris, 2015]. lava flow fields are widespread, e.g., the permian−triassic article history receveid april 30, 2018; accepted february 20, 2019. subject classification: lava crust; nonlinear heat flux; lava rheology; numerical modelling; data assimilation. abstract propagation of a lava flow is governed by slope topography, magma rheology, heat exchange with the atmosphere and the underlying ter− rain, and the rate of the eruption. highly viscous crust is formed due to cooling and solidification of the uppermost layer of the flow. we consider here two numerical model problems for lava flows, both based on the fundamental physics of a hot fluid flow: a model problem, where thermal conditions (e.g. temperature and heat flow) at the lava surface are unknown a priori (a direct model problem), and a model problem, where the lava surface conditions are known and determined from observations (an inverse model problem). in both models, the lava viscosity depends on temperature and the volume fraction of crystals. by way of solving the direct model problem, we perform a para− metric study of steady state lava flows to investigate the influence of the heat flux, viscosity, and effusion rate on the lava crust devel− opment. numerical experiments show that a lava crust becomes thicker in the case of the nonlinear heat transfer compared to the case of a linear heat flow at the interface of lava with the atmosphere. also, the crust thickens at lower lava effusion rates, while higher rates re− sult in a rapid lava advection, slower cooling, and development of a thinner crust. moreover, a lava crust becomes thicker with a higher coefficient of conductive heat transfer, or a higher lava viscosity, or the growth of effective emissivity of the lava surface. by way of solv− ing the inverse model problem, we use an assimilation technique (that is, a method for an optimal combination of a numerical model of lava flows with observations) to propagate the temperature and heat flow, inferred from measurements at the interface between lava and the atmosphere, into the lava flow interior and to analyse the evolving lava crust. results of thermal data assimilation illustrate that the physical parameters of lava flows, including the thickness of it crust, can be recovered from measured surface thermal data well enough at least for slow effusion rates. siberian and cretaceous−tertiary deccan large ig− neous provinces, the quaternary yellowstone rhyolitic lava flows or the holocene lava flows in iceland [christiansen, 2001; self et al., 2008; bryan et al., 2010; pinton et al., 2018]. several regions in the world are prone to lava flow hazard including in hawaii/usa [patrick et al., 2017], iceland [pedersen et al., 2018], italy [corsaro et al., 2009], réunion/france [e.g., sol− dati et al., 2018], kamchatka/russia [belousov and be− lousova, 2018], and some other regions of the active or historic effusive volcanism [e.g., chevrelet al., 2016; dietterich et al., 2018]. volcanic eruptions produce a variety of lava flows depending on the chemical composition and tempera− ture of the erupting material, and the topography of the surface over which the lava flows [e.g., griffiths, 2000; rumpf et al., 2018]. eruption (effusion) rates control lava flow dynamics: with higher effusion rates a lava flows the more rapidly and longer [walker, 1973; row− land and walker, 1990; harris et al., 1998; castruccio and contreras, 2016]. the rapid development of ground−based thermal cameras, drones and satellite data allows collection of repeated thermal images of the surface of active lava flows during a single lava flow eruption [calvari et al., 2005; wright et al., 2010; kelfoun and vargas, 2015]. these data require devel− opment of appropriate quantitative methods to link subsurface dynamics with surface observations and measurements. numerical modelling plays an essential role in un− derstanding lava flow patterns as well as their mor− phology and thermal evolution [e.g., costa and macedonio, 2005; cordonnier et al., 2015; and refer− ences herein]. numerical modelling of lava flows was advanced for the last few decades moving from one − (1d) to three−dimensional (3d) flows. cordonnier et al., [2015] provided a recent overview of the volcanologi− cal community’s codes for modelling of lava flow em− placement. we highlight here only those codes which are related to deterministic models (the mathematical models defined by a set of governing equations and boundary/initial conditions) as they are related to the type of modelling we used in our study. the 1d chan− nel lava flow code developed by harris and rowland [flowgo, 2001] is very fast to run as it does not cal− culate dynamic properties of fluid flow; but its limita− tion is that it does not consider vertical variations within a lava flow. numerical codes for depth−average models of lava flow (using a shallow−water approach) were developed by macedonio et al. [2005] and kelfoun and druitt [2005]; the codes provide rather fast compu− tations, but their limitation is that the lava viscosity does not vary vertically. smooth particle hydrodynam− ics codes have been also employed in modelling of lava flows [e.g., herault et al., 2011]. the codes use meshless numerical methods to calculate flow of individual par− ticles, for which physical characteristics are specified. 3d finite−volume or finite−element fluid−dynamics codes allow for studying lava flows on a complex to− pography, using complex rheology, and various bound− ary conditions, although they are slower compared to those mentioned earlier. tsepelev et al. [2016] used the openfoam toolbox (ver. 2.3.0, http://www.open− foam.org) to develop a code for a 3d isothermal lava flow with crustal pieces in a channel and on a real to− pography. isothermal models of viscous flows have demonstrated how the lava generated by effusive vol− canic eruptions would advance in the absence of cool− ing, and these have been supplemented by more sophisticated models including complex rheology, cool− ing, solidification, and dynamic interaction of lava and its crust [e.g., griffiths, 2000; costa and macedonio, 2005; fujita and nagai, 2015; tsepelev et al., 2016]. a mathematical model of lava flow can be described by a set of partial differential equations and boundary and initial conditions defined in a specific domain. a model links the causal characteristics of a lava flow (e.g., lava viscosity, thermal conditions at lava inter− faces with the atmosphere and the terrain) with its ob− served/measured properties (e.g., lava temperature and its flow rate). the aim of the direct model problem is to determine the properties of a lava flow based on the knowledge of its causes, and hence to find a solution to the mathematical problem for a given set of parameters and coefficients. an inverse model problem is consid− ered when there is a lack of information on the causal characteristics but some information on lava flow prop− erties exists. for example, a mathematical problem of determination of the thermal and dynamic characteris− tics of lava flow from measurements of the absolute temperature on its upper surface belongs to a class of inverse problems [e.g., kirsch, 1996; kabanikhin, 2011], and hence, a small error in measured temperature at the lava surface can result in a significant deviation from the true solution of the problem (e.g. lava temperature). to solve the inverse problem, special techniques for data assimilation [e.g., a variational data assimilation; is− mail−zadeh et al., 2016] should be employed to con− tsepelv et al. 2 strain the unknown thermal condition at the lower boundary of the lava flow from observations at the lava surface [e.g., korotkii et al., 2016]. the basic principle of this assimilation in the case of lava flows is to consider a thermal condition (temperature or heat flow) at the lava bottom (the interface between moving lava and the underlying terrain over which it flows) as a control variable and to find the optimal condition at the bot− tom by minimizing the misfit between the measure− ments at the lava interface with the atmosphere and the model solution at the same interface. to numerically model a lava flow, boundary condi− tions should be known. meanwhile the temperature or heat flux at the lava bottom is unknown as it is almost impossible to measure it. however, airborne and space measurements of temperature (heat flux) at the interface of lava flow with the atmosphere allow to determine thermal conditions at the lava bottom using data as− similation. once the boundary conditions at the lava bottom are determined, the lava flow can be modelled to estimate its extent, temperature and flow rate. in this paper, we develop two−dimensional numer− ical models of steady−state lava flows to analyse the development of the lava crust depending on the heat flux into the atmosphere from the lava surface, the lava viscosity, its discharge rate, and the lava front propagation rate (sections 2−4). we assimilate syn− thetic thermal data (a model of thermal measurement at the lava interface with the atmosphere) into the lava interior to determine the unknown thermal condition at the lava bottom (sections 5−8). we develop direct and inverse models of lava flows introducing the fol− lowing complexities: (i) the lava viscosity depending on temperature and the volume fraction of crystals; (ii) nonlinear heat transfer mechanism at the surface of lava with the atmosphere, including the radiant and convective parts of the total heat flux; (iii) varying flow conditions at the lava surface; and (iv) lava flow on a real surface topography. 2. the direct model problem of lava flow to state mathematically a problem of lava flow, we need to consider the following components of a model: computational domain (e.g. topography on which lava flows or slope of the inclined surface or channel as well as vent geometry); basic equations which govern the lava flow; the boundary condition on flow veloc− ity (e.g., effusion rate, free−slip or no−slip or free sur− face conditions); the boundary conditions on temperature (e.g., effusive temperature as well as ra− diative, convective or conductive heat flow at the in− terface with the atmosphere and with the land); and physical properties of the lava (that is, lava viscosity, density, thermal conductivity, rheology etc.). here we present a mathematical statement of the direct problem of steady−state lava flow in model domain ω ⊂ r2 (figure 1). the model domain covers a part of lava flow at some distance from the volcanic vent and the lava flow front. the boundary of the model domain consists of the following parts: г1 is a line seg− ment connecting points a and e (xa = (x1 a, x2 a) = (0.0,1.5), xe = (x1 e, x2 e) = (0.0,2.5)); г2 is a circular arc connecting points a and b (xb=(x1 b, x2 b) = (10.0,0.0)); г3 is a line segment connecting points b and c (xc = (x1 c, x2 c) = (10.0,0.5)); and г4 is a circular arc con− 3 lava crust development figure 1. model geometry. � � � � e d �1 2 4 3 10h 2h h h/2 a b c vent flow front tsepelv et al. 4 necting points c and e. all coordinates are in meters. although the lava rheology can be more complicated, we assume that the lava behaves as a newtonian fluid with a temperature− and volume−fraction−of−crystals− dependent viscosity and temperature−dependent density and thermal conductivity. we note that crys− tallisation is shown to be the primary process, which is responsible for the increase of the viscosity during emplacement of mafic lava flows [chevrel et al., 2013]. the flow is described by the navier−stokes, continuity, and heat equations [e.g., hidaka et al., 2005; ismail− zadeh and tackley, 2010]. although a lava flow can be non−steady state depending on an effusion rate, for a simplicity of the mathematical problem’s description we assume a steady−state lava flow in the modelling [a justification to this assumption is provided by korotkii et al., 2016]. in the case of viscous flows with the small reynolds number (re<1), the governing equations can be represented in the following form: , (1) , (2) . (3) here x = (x1, x2) ∈ ω are the cartesian coordinates; u = (u1(x),u2(x)) is the vector velocity; p = p(x) is the pressure; t = t(x) is the temperature; η is the viscosity; r is the temperature−dependent density; k is the temperature-dependent heat conductivity; g is the acceleration due to gravity; c* (= 1000 j kg -1k-1) is the specific heat capacity; e2 = (0,1) is the unit vector; ∇, t, and 〈·,·〉 denote the gradient vector, the transposed matrix, and the scalar product of vectors, respectively. lava viscosity depends on temperature, the volume fraction of crystals, water content and some other physical parameters [e.g., dragoni, 1989; giordano and dingwell, 2003; harris and allen, 2008; costa et al., 2009]. in this study, we consider two models of viscosity: a temperature-dependent viscosity [dragoni, 1989] , (4) where n = 4 x 10-2 k-1, and tm (= 1333 k) is the typi− cal lava melting temperature; and a volume-fractionof-crystals−dependent viscosity [marsh, 1981; costa et al., 2009] , (5) where , (6) , (7) , (8) f is the volume fraction of crystals; f* (= 0.384) is the specific volume fraction of crystals; θ = (t-ts)/(tm-ts), ts (= 1053 k) is the solidus temperature; b (=2.5) is the einstein coefficient’s theoretical value (it was experi− mentally determined that the einstein coefficient varies from 1.5 to 5; jeffrey and acrivos, 1976); δ = 13-γ, ξ = 2.0x10-4, γ = 7.701 [lejeune and richet, 1995; costa et al., 2009], θ1 = 0.5, b2 = 3/2 [wright and okamura, 1977], and b1 = √30 [marsh, 1981]. finally, we use in the modelling the following lava viscosity restricting its exponential growth with tem− perature by the prescribed value η0 (figure 2): (9) where η* (=10 6 pa s) is the typical lava viscosity, and η0 takes values from 10 3 to 106 in the modelling. in addition to these equations, we consider the fol− lowing relationships for the thermal conductivity and the density: , (10) , , 11) where, t = 1473 k; rm (= 2750 kg m -3) and rc (= 2950 kg m-3) are the typical lava melt density and the typ− ical crystal density corresponding to the crystals composed of 50% olivine and 50% plagioclase, re− spectively. as the effective density of the lava crust with about 20% volume of vesicles is estimated to be about 2200 kg m-3 [kilburn, 2000], we introduce the term f(t)δr (where δr = 750 kg m-3) to reduce the density of the lava crust and to permit crustal pieces drifting with and not sinking into the lava. we consider the following conditions at the bound− ary γ = γ1 u γ2 u γ3 u γ4 of the model domain. the velocity u1 and the temperature t1 are prescribed at the ~ k (t ) = 1.15 + 5.9 ⋅10−7 (t − t )2 , t < t 1.15 + 9.7 ⋅10−6 (t − t )2 , t > t ⎧ ⎨ ⎪ ⎩⎪ ~ ~ ~ ~ r=r m 1−f (t )( ) + rcf (t ) − (t )δrf ∇p = ∇ ⋅ η (t ) ∇u + ∇ut( )( ) − (t )ge2r ∇ ⋅ u = 0 ∇ ⋅ k (t ) ∇t( ) = u, c*∇ t( )r η (ϕ ) = 1+ ϕ δ( ) 1− f ( , , )⎡⎣ ⎤⎦ − b *~ ϕ ξ γ f ϕ = f / f * = 0.5 1− erf b 1 ( − 1 ) / b 2( )⎡⎣ ⎤⎦ / *f θ θ f (ϕ, ξ,γ ) = (1−ξ ) ⋅ erf π 2(1− ) (1+ ) ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ξ ϕ ϕγ η(t ) = * 12 (t ),η η η 12 (t ) = η 1 η 2 , 1 2 < 0 , η 0 , 1 2 > 0 , ⎧ ⎨ ⎪ ⎩⎪ η η η η η η η 2 (t ) = ( (t ) / * )η f f~ η 1 (t ) = exp n tm − t( )( ) 5 lava crust development left boundary γ1: , (12) no slip condition and zero heat flux (no heat lost to the base via conduction) or temperature t2 are pre− scribed at the lower boundary γ2: , (13) where n is the outward unit normal vector at a point of the boundary. we assume at the right boundary γ3: , (14) where σ = η(∇u+∇ut) is the deviatoric stress tensor. at the interface of lava flow with the atmosphere (boundary γ4), zero normal flow and free−slip tangential conditions are prescribed. lava cooling in the atmos− phere is the simultaneous action of radiation and ther− mal convection. although the lava emissivity is such that radiation is the main mechanism of heat flow dur− ing the very first minutes of cooling, convection by the surrounding air becomes very important after just a few minutes [neri, 1998]. a heat flow at the boundary γ4 (the lava surface) can be then either linear: , (15) , or nonlinear (16) , where the first term in the right−hand side of equa− tion (16) is related to the radiant heat flow, and the sec− ond term to the convective heat flow. here ta (= 300 k) is the air temperature; l ~ is the conductive heat transfer coefficient; ς (=5.668108x10-8 w m-2 k-4) is the stefan–boltzmann constant; ε is the effective emissivity of the lava surface; l is the dimensional (w m−2 k−m) constant; and m (= 1 or 4/3 in this study) is the pow− er−law exponent. in the dimensionless form the linear and nonlinear heat flow conditions can be represented as k〈∇t,n〉 = -nu(t-1) and −k〈∇t,n〉 = a1 (t 4 -1) + a2(t -1) m respectively, where nu = l ~ h / k* is the nusselt number, a1 = hk* -1t3aςε, a2 = hk* -1t1/3λ, k* = rmc*ĸ* is the typ− ical heat conductivity, ĸ* (= 10 -6 m2 s-1) is the typical heat diffusivity of lava as it is a very poor thermal con− ductor, and h (= 2 m) is the typical lava thickness. the effective emissivity of the lava surface ε and the di− mensional constant λ vary from 0.6 to 0.95 and from 1 to 10, respectively [neri, 1998; harris et al., 1998]. the problem (1)−(16) is transformed to a dimensionless form assuming that length, temperature, viscosity and heat conductivity are normalised by h, ta, η*, and k*. 3. solution method to solve the problem (1)−(16), we use the numerical approach and code developed by korotkii et al. [2016]. the openfoam toolbox was used to develop a solver for this study. the finite volume method [e.g., ismail−zadeh and tackley, 2010] is employed in the software to solve the numerical models on multiprocessor computers. the model domain ω was discretized by about 104 polyhe− dral finite volumes. the simple method [patankar and spalding, 1972] was used to determine velocity and pressure at a given temperature (the relaxation parame− ters are chosen to be 0.7 and 0.3 for the velocity and pressure, respectively). the use of polyhedral finite vol− umes has some advantages compared to hexahedral fi− nite volumes employed earlier by korotkii et al. [2016]; for example, it enhances the convergence rate of the simple method, in some cases by a factor of two. to implement the simple method, we employ the conjugate−gradient method [ismail−zadeh and tackley, figure 2. dimensionless viscosity as a function of temperature. 1: (dashed−dotted line) temperature−dependent vis− cosity (η1, equation 4); 2: (dashed line) crystal−vol− ume−fraction−dependent viscosity (η2, equation 5); 3: (dotted line) combines viscosity (η1 · η2); and 4: (solid line) the viscosity used in the model (equation 9). u,n = 0, σ n − σ n,n n = 0, − k ∇t ,n = ςε t 4 − ta 4( ) + t − ta( ) m l u = u 1 ( x 2 ), t = t 1 u = 0, ∂t ∂n = 0 σ n = 0, p = 0, ∂t ∂n = 0 u,n = 0, σ n − σ n,n n = 0 k ∇t ,n = − (t − ta )l ~ 2010] to solve a set of linear algebraic equations (slae) with positive−definite and symmetric matrices, which are obtained after the discretization of the stokes equa− tion. all computations were performed using one cpu intel core i3 2.13 ghz with 4gb memory, os kubuntu 14.4. in the case of the heat equation, slae were solved by the biconjugate gradient stabilized method [van der vorst, 1992] with the pre−conditioner of incomplete lu−decomposition. the linear gaussian scheme with a flow control was used to discretize the laplace opera− tor. to approximate the convective operator, we em− ployed the total variation diminishing (tvd) method with the minmod limiter [sweby, 1984; ismail−zadeh et al., 2007]. the nonlinear condition for temperature (16) at the upper model boundary is solved the simple iteration method, and at each iteration m+1 of the simple method the following scheme is used: . the descent step length 0 < γ < 1 is determined ex− perimentally to provide the convergence to the iterative process. this iterative process is stopped when the tem− perature, pressure and velocity residuals become less than 10-4. an average computational time for solution of the problem (1)−(16) was about 3 min for the chosen dimension of the discrete problem. 4. results: evolving lava crust in this section, we present the results of several parametric case studies to analyse the influence of the heat flux at the interface between the lava flow and the atmosphere, and of the effusion rate on the lava flow temperature, viscosity, velocity, and the thickness of the lava’s crust. the crust is defined in the numerical mod− els as the area, where the lava viscosity is greater than η0/2. the parameters of the case study models are listed in table 1, and η0=10 3. numerical experiments have been performed for lin− ear (equation 15) and nonlinear (equation 16) heat flow conditions at the lava surface with the atmosphere for different injection rates |u1| (hereinafter we assume that the injection rate is the rate at which lava enters into the model domain, and the injection rate is proportional to the effusion rate per unit area). it is shown (see supplementary material s1, figure s1) that in the case of the nonlinear heat flow, the lava cools down faster than in the case of linear heat flux, and hence the lava’s crust becomes thicker in the case of nonlinear heat flux. at higher injection rates, the lava is advected rapidly and hence cools slowly; whereas at lower rates, the crust becomes thicker (figure 3a). the higher coefficient of the conductive heat transfer, the thicker the lava crust. when the injection rate is 0.023 m s-1 and the viscosity is 106 pa s, a lava crust does not evolve at l ~ =1 (figure 3b); at higher l ~ , a thin crust (about 6 cm) starts to develop at a certain tsepelv et al. 6 figure 3. thickness of the lava crust with distance from the lava injection into the model domain depending on: (a) the injection rate |u1|, (b) conductive heat transfer l ~ , and (c) the typical lava viscosity η*. see table 1 for case studies m1.1−m3.4. ∇t ( m+1) ,n = (1−γ ) ∇t ( m) ,n − − γ a 1 t ( m)( ) 4 − ta 4⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + a2 t ( m) − ta( ) 4/3⎡ ⎣ ⎢ ⎤ ⎦ ⎥ distance from the place of a lava injection into the model domain (boundary γ1). with the increase of the lava viscosity (figure 3c), the crust becomes thicker, and it reaches about 60−70 cm in the centre of the model domain. we note that the lava flow viscosity varies by several orders of magnitude depending on its composition. the typical viscosity is about 102-103 pa s for basaltic lava flows, and increases up to 1091011 pa s during emplacement of andesitic lava [chevrel et al., 2016]. for all cases, the crust becomes thinner closer to the left model boundary because of a converging shape of the domain in the model. tem− perature at the lava surface with the atmosphere de− creases with the increase in its effective emissivity and the power−law constant m, although the depen− dence on m is weak (figure 4). at higher effective emissivity, the lava crust thickens (figure 5). we have also performed numerical experiments for linear and nonlinear heat flow conditions with vary− ing slip conditions at the upper surface of the lava flow model, particularly, considering no−slip at the right part of the upper surface (a circular arc con− necting points c and d in figure 1, which is about 1/5 of the model domain’s horizontal length). the no− slip condition can model the case, when the front of lava flow covered by a crust prevents lava to flow. the results show that the crust beneath the no−slip area becomes much thicker than beneath the free−slip area at both linear and nonlinear heat flow at the cooling surface of lava flow (see supplementary material s2, figure s2). 7 lava crust development figure 4. dimensionless temperature at the interface between lava and the atmosphere with distance from the lava injection depending on the effective emissivity (case studies m4.1−m4.3) and the power−law exponent m (case studies m4.4 − m4.6). see table 1 for the case studies. model case |u1|, ms-1 η*, pa s l ~ , w m-2 k-1 l, w m-2 k-m m ε m1.1 0.058 106 5.5 m1.2 (reference) 0.023 106 5.5 m1.3 0.012 106 5.5 m2.1 0.023 106 1.0 m2.3 0.023 106 10.0 m3.1 0.023 105 5.5 m3.3 0.023 107 5.5 0 m3.4 0.023 108 5.5 m4.1 0.023 106 5.5 1 0 m4.2 0.023 106 5.5 1 0.6 m4.3 0.023 106 5.5 1 0.95 m4.4 0.023 106 5.5 4/3 0 m4.5 0.023 106 5.5 4/3 0.6 m4.6 0.023 106 5.5 4/3 0.95 table 1. model parameters for case studies 5. the data assimilation (inverse) model problem here we study the problem of determining thermal and flow characteristics of the lava and the thickness of its crust from measurements of temperature and heat flow at the interface with the atmosphere. we modify equations (1)−(3) applying the boussinesq approxima− tion [boussinesq, 1903] to the equation by keeping the density constant everywhere except for the buoyancy term in the stokes equation. in this approximation, the dimensionless stokes, continuity, and heat equations take the form: , (17) , (18) , (19) where and the rayleigh number is determined as ra = α*grm δth 3η* -1ĸ* -1, α* (= 10 −5 k−1) is the thermal expansivity; δt = tm ts is the temperature contrast; ĸ(t) = k(t) / (rm c*) is the thermal diffusivity. the vis− cosity η(t) and the thermal conductivity k(t) are de− termined from equations (9) and (10), respectively. we assume the following conditions for temperature and velocity at the model boundary: , (20) , (21) , (22) (23) . the principal problem is to find the solution to the problem (17)–(23), and hence to determine the velocity u = u(x), the pressure p = p(x), the temperature t = t(x), and hence the viscosity in the model domain ω, when temperature t4 and heat flow φ are known at boundary (measured data). in this study measured data are obtained numerically by computing thermal heat flow and tem− perature at boundary γ4 (see supplementary material s3). in addition to the principal problem, we state an auxiliary problem to find the solution to equations (17)–(19) with the following boundary conditions: , (24) , (25) , (26) . (27) the auxiliary problem is a direct problem compared to the problem (17)–(23), which is an inverse problem, because the thermal condition is overdetermined at γ4 and underdetermined at γ2 .we note that the conditions at γ1 and γ3 are the same in the direct and inverse problems; temperature t2 is known at γ2 and no heat flow is prescribed at γ4 in the direct problem compared to the inverse problem. consider “guess” temperature t2 = ξ at model boundary γ2. solving the auxiliary problem (17)–(19) and (24)–(27), we can determine the heat flow at model boundary γ4 and compare it to the heat flow φ (the known synthetic data) at the same boundary. the fol− lowing cost functional for admissible functions ξ de− termined at t2 is to be then assessed: , (28) where k(tξ) ∂t ξ / ∂n is the heat flow at γ4 correspond− ing to temperature t2 = ξ at γ2; and t ξ is the tempera− ture determined by solving the auxiliary problem. therefore, we reduce the inverse problem to a minimiza− tion of the functional or to a variation of the function ξ at γ2, so that, the model heat flow at γ4 becomes closer tsepelv et al. 8 figure 5. thickness of the lava crust with distance from the lava injection depending on the effective emissivity and the power−law exponent m (case studies m4.1, m4.3, and m4.6). see table 1 for the case studies. γ 4 : t = t 4 , − k ∇t ,n = ϕ, u,n = 0, σ n − n,n n = 0σ j (ξ ) = k (t ξ ) ∂t ξ ∂n − ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ 2 d γ4 ∫ γϕ γ 1 : t = t 1 , u = u 1 γ 2 : t = t 2 , u = 0 γ 3 : t = t 3 , σ n = 0, p = 0 γ 4 : t = t 4 , u,n = 0, σ n − n,n n = 0σ γ 1 : t = t 1 , u = u 1 γ 2 : u = 0 γ 3 : t = t 3 , σ n = 0, p = 0 ∇p = ∇ ⋅ η (t ) ∇u + ∇ut( )( ) + ra t e2 ∇ ⋅ u = 0 ∇ ⋅ κ (t ) ∇t( ) = u,∇t to the “observations” (known synthetic data) φ at γ4. 6. solution method following korotkii et al. [2016], we minimize the cost functional (28) using the polak−ribière conju− gate−gradient method [polak and ribière, 1969]. the gradient of the cost functional , (29) can be found as the solution (z, w, q) to the adjoint problem [see appendix b in korotkii et al., 2016] for the derivation of the adjoint problem): , (30) , (31) (32) , , (33) , (34) , (35) (36) , where σ ≡ ∇w+∇wt; the square brackets [a, b]=∑ aij bij denote the convolution of two m × m matrices a = (aij) and b = (bij); and sign ʹ means the derivation. the so− lution is a triplet (z, w, q) of quasi−temperature (z), quasi−velocity (w), and quasi−pressure q. the solution of the minimization problem is reduced to solutions of series of well−posed (direct and adjoint) problems. the algorithm for solving the data assimilation problem is based on solving iteratively the direct (aux− iliary) and adjoint problems and on the assessment j(ξ (i)), where i is the iteration number [korotkii et al., 2016]. the numerical methods employed are described in sect. 3. an average computational time for 20 itera− tions was about 150 min on a single node; this included the time required for minimization of the cost func− tional by the conjugate gradient method, and the time to solve the auxiliary and adjoint problems. the performance of the algorithm is evaluated in terms of the number of iterations n required to achieve a prescribed relative reduction of the cost functional (figure 6). the cost functional reaches a plateau after about 15 iterations. this behaviour is likely associated with errors in the model solution (see section 8 for more detail). the quality of the gradient of the cost functional with respect to the control vari− able have been verified using the χ −test by navon et al. [1992] (see supplementary material s4). 7. results of the assimilation in this section, two models of lava advancing down the slope on different topographic surfaces are analysed. the following values of the model param− eters are used h = 2 m, λ =5.5 w m-2 k-1, ta = 300 k, ε = 0.95, ĸ* = 10 -6 m2 s-1, η* = 10 6 pa s, and η0 = 1000. initially we consider the model geometry presented in figure 1. at γ1 we prescribe the temperature t1(x1, x2) = 1320 k and the velocity u1(x2) = ϑ1u(x2)n1, x2 ∈ [x2 a, x2 e ], where n1=(√2/2, -√2/2), ϑ1 is a constant, and u(x2) is the parabola passing through the following three points: u(x2 e ) = 1, u(x2 a ) = 0, and u(0.5(x2 a + x2 e )) = 0.725. the cost functional is reduced to about 10−5 after about 15 iterations (fig− ure 6). the number of iterations to get a given accu− racy in reduction of the cost functional depends on the initial “guess” temperature at γ2. the closer the guess temperature is to the target temperature, the 9 lava crust development ~ m i,j=1 figure 6. relative reduction of the cost functional j with the number of iterations at three dimensionless rates of lava injection: ϑ = 10 (solid line), ϑ = 20 (dotted line), and ϑ = 40 (dashed line). γ 4 : z = 2 k (t ξ ) ∂t ξ ∂n − ϕ ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ , w,n = 0, n − n,n n = 0σ~ σ~ ∇j (ξ ) = k (t ξ ) ∂ z ∂n ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ γ2 ∇ ⋅ η (t ξ ) ∇w + ∇wt( )( ) = ∇q + z∇tξ ∇ ⋅ w = 0 γ 1 : z = 0, w = 0 γ 2 : z = 0, w = 0 γ 3 : z = 0, σ n = 0, q = 0~ ∇ ⋅ κ (t ξ )∇z( ) + uξ ,∇z + ra e2 , w = = ʹκ (t ξ ) ∇t ξ ,∇z + + ʹη (t ξ ) (∇w + ∇wt ),∇u ξ ⎡ ⎣ ⎤ ⎦ fewer the number of iterations is needed. the target temperature at γ2 is obtained from the solution of the direct problem (1)−(16). figure 7 presents the target lava temperature, flow velocity, and viscosity as well as the residuals for vary− ing rates of the lava injection at the left boundary of the model domain. here a residual is defined as the difference between the target physical parameter (e.g., temperature, velocity, or viscosity) and that obtained by the optimiza− tion (inverse) model. the results of this modelling show that at smaller rates of lava injection the residuals be− come small after 18 iterations within the almost entire model domain. at lower injection rates, the temperature, viscosity and velocity of a lava flow are well determined from the known thermal data on its surface with the at− mosphere (figure 7a, b). because of the topography over which the lava flows in the model case study, the crust thickens with the flow, reaches its maximal value in the middle of the channel, and diminishes toward the chan− nel’s end (the boundary γ3). at higher rates, the recov− ery of the physical parameters worsens (figure 7c, d); it can be explained by an advection−dominated lava flow with of a thin crust development (see figure s1a), when the temperature of the lava flow approaches the temper− ature of the injected lava, and the thermal surface data are not properly assimilated into the lava flow. we consider a lava flow on an artificial volcanic to− pography in the second case study (figure 8a). the lava flow from a vent on the topography has been modelled in a three−dimensional rectangular domain by tsepelev et al. [2016]. as the optimization model is assumed to be in a steady state (that is, physical parameters of the model do not change with time), we select a portion of the lava flow on a two−dimensional profile along the lava flow; the profile is presented in the lower panel and its loca− tion in the upper panel of figure 8a, where the selected portion of the lava (the model domain) is marked by red. in the model the lava thickness at the left and right tsepelv et al. 10 figure 7. reconstruction of dimensionless viscosity, temperature, and velocity of lava flow in the model domain ω. (a) and (c): target functions at dimensionless injection rates ϑ = 10 and ϑ = 40, respectively; (b) and (d): the relevant residuals after the 1st iteration (the central panels) and several iterations (the right panels). 1 600 800200 400 1000 viscosity 1 600 800200 400 1000 viscosity 4.03.22.8 3.6 4.4 temperature 4.03.22.8 3.6 4.4 temperature 400 20 velocity 400 20 velocity 20 0 10 30 20 0 10 30 velocity residual velocity residual viscosity residual 800 600 200 400 1000 0 800 600 200 400 1000 0 viscosity residual temperature residual 1 2 0 temperature residual 1 2 0 a b c d 1st iteration 1st iteration 18th iteration 21st iteration sides are 14 m and 31 m, respectively, and the length of the model lava flow is 515 m. a lava is injected from the left boundary of the model domain. we solve the opti− mization problem in the selected domain to determine the lava’s viscosity, temperature, and flow rate based on the known thermal data on its surface with the atmo− sphere. figure 8b presents the target viscosity, tempera− ture and flow rate, and figure 8c their residuals. we see that after about 30 iterations the lava’s physical param− eters are recovered well enough from the surface thermal data. in this case study, a thin crust developing at the left end of the model domain becomes thicker toward its right end, and the flow velocity drops by a factor of about 3 with the lava advancement. 8. discussion under relatively steady eruption conditions, a vis− cous lava flow cools at the interface with the atmo− sphere and develops a crust. a nonlinear heat flow at the interface, lava cooling, and crystallization of the uppermost layer of the moving melt results in the crust insulating the lava flow interior. the crust preserves the lava against rapid cooling and permits the lava flow to extend to substantial distances. if the lava supply ceases and the interior of the lava flow cools (so called “vol− ume−limited” flow regime) or a lava flow cools to such a degree that it can no longer moves even at a contin− uous slow supply of lava (“cooling−limited” flow regime), the lava will stop its further advance [harris et al., 2007; harris and rowland, 2009; rhéty et al., 2017]. in this paper, by solving direct problems we have studied how the lava crust forms at different conditions, namely, the changing effusion rates, and various pa− rameters of nonlinear heat flow and lava viscosity. nu− merical experiments show that lava cools faster and its crust becomes thicker in the case of the nonlinear heat flow compared to the case of linear heat flow at the in− terface between lava and the atmosphere. at lower ef− 11 lava crust development figure 8. reconstruction of dimensionless viscosity, temperature, and velocity of lava flow on a synthetic topography. (a): a relief map (6000 m × 4000 m) of three−dimensional lava flow pattern, view from the top (upper panel) and the cross−section ab (lower panel) along the line indicated in the upper panel; the flow was computed by tsepelev et al. [2016]; the red area marks the model domain in the inverse problem; (b): target functions at dimensionless rates of lava injection ϑ = 10; (c): the relevant residuals after the 1st iteration (the left panel) and after 31st iteration (the right panel). 20 1 velocity residual viscosity residual temperature residual 0.2 0.4 0.80 0.6 1st iteration 31st iteration c 6000 m 6000 m 4 0 0 0 m 3 0 0 m a 1 600400200 1000800 0 600400200 1000800 viscosity 4.0 4.23.83.6 4.4 temperature 4 60 2 velocity b vent a a b b fusion rates of lava into the channel (the model do− main), cooling leads to development of a thick crust; the higher rates result in rapid lava advection and in a delay of a thick crustal formation . it has been also shown that a lava crust becomes thicker with either a higher coefficient of conductive heat transfer or higher lava viscosity. at a higher lava viscosity the flow be− comes slower, the lava cools and the crust develops more efficiently than in the case of a rapid lava flow due to its lower viscosity. temperature at the lava sur− face with the atmosphere decreases and the lava crust thickens with the increasing its effective emissivity. if the front of lava flow covered by a crust prevents lava to flow easily, numerical experiments illustrate that the lava crust beneath the immobile region (the stuck area at the front of lava flow) becomes much thicker. if direct problems present models dealing mostly with understanding of phenomena, inverse models are related to finding either initial conditions or boundary conditions or model parameters based on geological, geophysical, geodetic and geochemical observations and measurements. inverse problems in geodynamical models are mostly related to optimization of residuals between observed data and those obtained from rele− vant mathematical models. earth orbiting radiometers can measure spectral ra− diance at a lava surface to be converted into thermal anomalies; lava temperature and heat flow can be then inferred from the detected anomalies. results of nu− merical modelling illustrate that the lava’s physical pa− rameters, including the lava crust thickness, can be recovered from the surface thermal (known/measured) data well enough after a few dozens of iterations be− tween the adjoint and auxiliary problems. however, a spatial resolution of many satellites is coarse enough to not allow for high−resolution monitoring and precise measurements, and this gives a rise to uncertainties in thermal measurement as well as in the inferred param− eters [e.g. zakšek et al., 2015]. hence, if the measured temperature and heat flow data are biased, this infor− mation can be improperly assimilated into the lava flow models. and vice versa, if surface temperature and heat flow data are of high resolution and radiometric accu− racy, the temperature and velocity in the lava’s interior can be determined properly from measured data using the data assimilation approach. the data assimilation approach becomes be important in studies of natural lava flows, especially in the cases of thick lava flow. synthetic aperture radar (sar) satellite observations on lava thickness, volume, and flow extent [e.g., kubanek et al., 2015], together with thermal measure− ment at the lava surface, could facilitate research and data−driven modelling of lava flow. the studied models describe steady−state flow, al− though lava flows are non−stationary. meanwhile, as measurements on absolute temperature are discrete in time in most cases (e.g., depending on the location of landsat satellites), a problem of non−steady state flow can be reduced to a series of steady−state flow problems with varying model domain and boundary conditions assimilating thermal data available at the discrete−in− time measurements. also, airborne and space measure− ments of absolute temperature at the lava interface with the atmosphere, being almost instantaneous compared to the duration of the lava flow, allow searching for thermal conditions at the bottom of the lava flow using the cost functional (28). once the boundary conditions at the lava bottom are determined, the steady−state problem can be replaced by a non−steady state problem, and the lava flow can be modelled forward in time to determine its extent, lava’s temperature and flow rate as well as backward in time using variational [korotkii et al., 2016] or quasi−reversibility [ismail−zadeh et al., 2007] methods to search for the initial temperature of the lava flow and for the evolution of the effusion rate. 8.1 data assimilation limitations the mathematical problem (17) − (23) does not possess the stability property of its solution [korotkii and kovtunov, 2011]. this means that if the problem is solved numerically, small errors in measured input data, e.g., the absolute temperature estimated from remote sensors measurements of thermal (electro− magnetic) radiation, and in the inferred heat flow at the interface of lava with the atmosphere, as well as computational errors, can result in significant errors in determination of temperature, viscosity, and flow velocity. therefore, to solve the problem special numerical methods should be employed. particularly, the prob− lem (17)−(23) is replaced by the optimization of the cost functional (28) which, in its turn, lead to the so− lution of the coupled auxiliary (17)−(19), (24)−(27) and adjoint (30)−(36) problems to find the gradient (29) of the cost functional. the polak−ribière stable iterative conjugate gradient method was used to min− imize the optimization problem. however, the itera− tive method provides the rapid and accurate solution tsepelv et al. 12 to the adjoint and auxiliary problems only when the following condition is satisfied [korotkii and kov− tunov, 2011]: , (37) where c1 and c2 are positive constants. an iterative convergence slows at high rayleigh numbers, and the iterations diverge at the rayleigh numbers greater than 106. as the injection rate of lava into the model do− main increases, the minimization process slows down (figure 6). a rapid injection results in advection of high temperature with flow and, hence, in a decrease of lava viscosity, and in slowing convergence of iterations and their divergence. 8.2 uncertainties and measurement errors the results of numerical modelling of the problem (17)−(23) show that the optimization works effec− tively: the temperature, viscosity and velocity resid− uals are very small already after a few dozens of iterations within the almost entire model domain. normally any measurements (observations) are pol− luted by errors; for example, in the case of lava tem− perature measurements at the surface, the accuracy of the calibration curve of remote sensors and the noise of the sensors can influence measurements and con− tribute to the measurement errors [short and stuart, 1983]. korotkii et al. [2016] performed numerical ex− periments introducing a noise on the “measured” heat flow data φδ(·) = φ(·) + δχ(·) and analysed the sensi− tivity of the model to the noise. here, δ is the magni− tude of the noise, and χ(·) is the function generating numbers that are uniformly distributed over the in− terval [−1, 1]. they showed that the temperature and velocity residuals get larger with increase of the noise of the input data, but are still acceptable at the level of noise (or errors in measurements) of up to 10%. it should be noted that the error analysis of subpixel temperature retrieval from satellite infrared data showed that errors in measurements of the radiant heat flux are within about 5% to 10%, and can be re− duced [lombardo et al., 2012]. in general, the cost functional related to the inverse (optimization) problem with noisy measurements at the surface of the lava flow with the atmosphere can be written in the following form: (38) . substituting the solution ξ* to equation (28) into equation (38), we obtain jδ(ξ*) ∼ δ 2, because the first and second terms of the right hand−side of equation (38) turn to zero at ξ*. therefore, at the minimization of the functional jδ, the functional will be approaching the non−vanishing value equal to the square of the noise’s magnitude (δ2). in the case of synthetic thermal data prescribed at the upper model boundary (instead of real measurements), the ‘plateau’ in the curve illustrating the minimization of the functional (figure 6) is likely to be associated with numerical errors. forcing the solution to the functional to attain zero may lead to an unstable (or erroneous) solution. moreover, some a priory infor− mation assists in solving the problem. for example, the temperature inside the model domain cannot be higher than that at the left boundary of the model domain (where the lava is injected into the model domain). this can serve as a control parameter for the computed tem− perature in the minimization problem. rather accurate reconstructions of the model temper− ature, viscosity, and flow velocity in this study rely on the chosen method for minimization of the cost functional (28). in the general case, a tikhonov regularization term should be introduced in the cost functional as: (39) , where α is a small positive regularization parameter, ᴧ > 0 is the operator accounting for a priori informa− tion on the problem’s solution (e.g., its monotony prop− erty, maximum and minimum values, and the total variation diminishing), and ξ0 is a priori known func− tion close to the solution of the problem. the introduc− tion of the regularization term in the cost functional makes the minimization problem more stable and less dependent on measurement errors. for a suitable regu− larization parameter α = α(δ), the minimum of the reg− ularized cost functional will tend to the minimum of the functional (28) at δ → 0 [tikhonov and arsenin, 1977]. the choice of the regularization parameter is a challenging issue as it depends on several factors, e.g., 13 lava crust development ra η−1 c 1 ϕ 2 d γ γ4 ∫ ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ 1/2 + c 2 ξ 2 d γ γ2 ∫ ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ 1/2⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ < 1 j α (ξ ) = k (t ξ ) ∂t ξ ∂n − ϕ δ ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ 2 d γ4 ∫ γ+ + α λ − 0( ) d γ γ2 ∫ ξ ξ 2δ k (t ξ ) ∂t ξ ∂n − ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ χd γ4 ∫ γ+ 2 2 δϕ χ+ j δ (ξ ) = k ( t ξ ) ∂t ξ ∂n −ϕ +δ χ ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ 2 d γ4 ∫ γ = j (ξ ) + on errors of measured data [e.g., kabanikhin, 2011]. meanwhile, if there is a lack of the information on the solution of the problem, the better strategy is to mini− mize the functional (28) using a stable minimization method (e.g., the polak−ribière method). 9. concluding remarks understanding the mechanisms influencing cooling and solidification of lava is essential in forecasting the lava flow advance. numerical experiments of lava flows at different conditions, presented in this paper, provide an insight into lava cooling and its crust development and assist in bridging the gap to application for natu− ral lava flows. if the temperature and heat flow at the interface of lava with the atmosphere are of high reso− lution and radiometric accuracy, the temperature and velocity in the lava’s interior can be determined prop− erly from the measured data. modelling of direct and inverse problems provides a knowledge of the thermal and dynamic characteristics of lava flow, and it be− comes important for lava flow hazard and disaster risk assessments [e.g., wadge et al., 1994; harris, 2015; cut− ter et al., 2015]. acknowledgements. we thank augusto neri, donald dingwell, and frank schilling for discussion related to nonlinear heat flow and lava rheology. we are grateful to andrew harris and scott rowland for very constructive comments, which improved the initial manuscript. this work is supported by the rsf grant 1917-00027. references boussinesq, j. 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(2015). satellite and ground based thermal observation of the 2014 ef− fusive eruption at stromboli volcano, remote sens., 7, 17190−17211. *corresponding author: alik ismail-zadeh karlsruhe institute of technology, institute of applied geosciences, adenauerring 20b, karlsruhe 76131, germany email: alik.ismail-zadeh@kit.edu © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 17 lava crust development << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 quantitative prediction of strong motion for a potential earthquake fault keiiti aki, michel bouchon, bernard chouet, shamita das annali di geofisica, vol. 30, n. 3-4, 1977. annals of geophysics, vol. 53, n. 1, february 2010 abstract this paper describes a new method for calculating strong motion records for a given seismic region on the basis of the laws of physics using information on the tectonics and physical properties of the earthquake fault. our method is based on a earthquake model, called a «barrier model», which is characterized by five source parameters: fault length, width, maximum slip, rupture velocity, and barrier interval. the first three parameters may be constrained from plate tectonics, and the fourth parameter is roughly a constant. the most important parameter controlling the earthquake strong motion is the last parameter, «barrier interval». there are three methods to estimate the barrier interval for a given seismic region: 1) surface measurement of slip across fault breaks, 2) model fitting with observed near and far-field seismograms, and 3) scaling law data for small earthquakes in the region. the barrier intervals were estimated for a dozen earthquakes and four seismic regions by the above three methods. our preliminary results for california suggest that the barrier interval may be determined if the maximum slip is given. the relation between the barrier interval and maximum slip varies from one seismic region to another. for example, the interval appears to be unusually long for kilauea, hawaii, which may explain why only scattered evidence of strong ground shaking was observed in the epicentral area of the island of hawaii earthquake of november 29, 1975. the stress drop associated with an individual fault segment estimated from the barrier interval and maximum slip lies between 100 and 1000 bars. these values are about one order of magnitude greater than those estimated earlier by the use of crack models without barriers. thus, the barrier model can resolve, at least partially, the well known discrepancy between the stress-drops measured in the laboratory and those estimated for earthquakes. introduction we are interested in predicting earthquake strong motion on the basis of the laws of physics using information on the tectonics and physical properties of the earthquake fault which can be measured at the present time. recent advances in the study of earthquake source mechanisms, plate tectonics, rock mechanics, and approach viable and gation in realistic media have made this new approach viable and promising to supplement the conventional approach based entirely on the past records of strong motion and earthquake statistics. the first successful attempt of theoretical prediction of strong motion was made by aki [1968] and haskell [1969] for the station no. 2 record of the parkfield earthquake of 1966. the earthquake model used by them was a propagating dislocation parametrized by fault length, fault width, rupture velocity, final uniform slip, and rise time. among these five parameters, the first four can be determined relatively easily from various seismological data and their reliable estimates have been obtained for many earhquakes. the results from plate tectonics also gave constraints on these parameters. for the prediction of strong motion, however, the most important parameter is the rise time which controls the high frequency end of seismic spectra. because of difficulties in separating source and path effects on short period waves, the determination of this parameter has been made for only a few earthquakes. to resolve this problem, we have taken two approaches: spectral analysis of small earthquakes and theoretical work on rupture propagation. in the spectral analysis work, we used the coda method developed by aki and chouet [1975] for separating the source, attenuation, and scattering effects on short period waves. the main purpose of the work was to find the scaling law of seismic spectrum which describes how the seismic spectrum grows with earthquake magnitude. the other approach was to find the fault slip as a function of time and location on the fault for given conditions of tectonic stress, material strength on the fault, and the fracture criterion. the results of such work will not only give us the physical meaning of «rise time», but also allow us to relate the physical properties of an earthquake fault with seismic motions generated from the fault. thus, das and aki [1977] arrived at a new versatile earthquake model called the. «barrier model». in this model, the ad-hoc parameter «rise time» is replaced by a more physical parameter, «barrier interval». the new model allows us to estimate the barrier interval from the observed seismogram of an eartihquake, from the scaling law of seismic spectrum for small earthquakes in a seismic zone, and from the direct measurement of fault breaks along an earthquake fault. we have estimated the barrier interval for six earthquakes by direct measurement of the fault slip, for an 81 additional six earthquakes by model fitting with observed seismograms, and for four seismic areas from locally observed scaling laws of seismic spectra. the results suggest that the barrier interval may be reliably determined by knowing the expected maximum slip. on the other hand, the expected maximum slip, fault length, and width are constrained by plate tectonics and the rupture velocity is usually within the range of 2 to 3 km/sec. thus, our barrier model can give all the necessary parameters for calculating strong motion by the laws of physics. an interesting and important aspect of this model is that the tectonic stress is implicitly contained in the maximum slip vs. barrier interval relation but is not an explicitly required parameter for calculating strong motion. in the present paper, we shall first review how the idea of the «barrier interval» has evolved from the analysis of the growth of source spectrum with magnitude. then, we shall discuss the results of numerical calculation of rupture propagation over a fault plane with distributed barriers. we shall show how the barrier model determined seismologically compares with the surface measurements of fault breaks for the san andreas fault. we shall then obtain the barrier interval for many earthquakes and several seismic areas by comparing synthetic seismograms with observed data, and by direct measurements of fault slip. the results will be summarized in a diagram showing barrier interval as a function of maximum slip. observed scaling law of seismic spectrum for small earthquakes the manner in which the seismic spectrum grows with earthquake magnitude is called the «scaling law». a simple scaling law based on the assumption that large and small earthquakes are similar has explained fairly well observations of earthquakes with magnitudes m greater than 6 [aki 1967, 1972]. the similarity assumption implies the constancy of stress drop independent of magnitude which is supported by observations, at least for shallow earthquakes with m > 6 [kanamori and anderson 1975]. the validity of the similarity assumption means that there is no unique physical length quantitative prediction of strong motion for a potential earthquake fault 82 figure 1. growth of the seismic source spectrum with magnitude for earthquakes in the san andreas rift zone near stone canyon, central california. the locus of corner frequencies appears to consist of three distinct segments between magnitudes 1 (m 1) and 5 (m 5). reproduced from chouet et al. [1978]. figure 2. comparison of the corner-frequency vs. moment relation determined by the coda method (solid line), various deterministic methods (for individual earthquakes) and extrapolation from larger earthquakes (dashed lines). reproduced from chouet et al. [1978]. 83 involved in earthquake phenomena except the total fault length. using the coda method developed by aki and chouet [1975], we have studied the scaling law for small earthquakes in tsukuba and oishiyama, japan, stone canyon and san fernando, california, and kilauea, hawaii. as described in chouet et al. [1978], except for hawaii, all of the areas showed some departure from what would be expected under the similarity assumption. for example, if the similarity assumption is valid, the relation between corner frequency and seismic moment must be a straight line in the log-log plot with a slope of – 3. except for hawaii, the loci of corner frequency follow zig-zag paths consisting of segments of two distinct types. one is a vertical path with constant corner frequency and the other is an inclined line with a slope of – 3. a similar zig-zag path was found by street et al. [1975] for earthquakes in the central united states. violation of simple similarity assumption was also observed by tsujiura [1978] for the kanto region, japan, and by rautian and khalturin [1976] for garm region, ussr, using the coda method. let us now examine closely the results from stone canyon, california, to find a clue for the physical meaning of the observed scaling laws. reproduced from chouet et al [1978], figure 1 shows the average source spectra for various magnitudes and figure 2 compares the corner frequency with those obtained for earthquakes in the same area but by various methods based on deterministic models. since our method relies on coda waves, for which the path effect and the effect of azimuthal radiation is presumably average out, we expect our estimate of corner frequency to be more stable. in fact, the points obtained by other methods scatter around the curve obtained by the coda method. in particular, the very weak dependence of corner frequency (near 1 hz) on magnitude observed by johnson and mc evilly [1974] is supported by our results. while the corner frequency is nearly constant at 1 hz the spectral shape changes with magnitude as shown in figure 1. the high frequency asymptote shows a steeper slope for m = 5 than for m = 4. we shall find that all these observations can be explained by considering rupture propagation along a fault plane with distributed barriers. rupture propagation over a fault plane with distributed barriers it is now well established, at least in numerical experiments, that the in plane shear crack an propagate with quantitative prediction of strong motion for a potential earthquake fault figure 3. the value of (1 + s), a measure of material strength relative to tectonic stress is shown as a function of distance, x1 along the path of rupture propagation on the top of the figure. this is the case p – sv – 0 in which no barriers exist on the fault plane. at the bottom, snap shots of the parallel component displacement on the crack surface u1 (x1, o, t) are shown as a function of x1. u1 is normalized by the factor l (τ0 – τf)/μ and the number attached to each curve indicates time t measured in the unit of 0.5 l/α, where l is the length of the fault, α. is the compressional wave velocity, μ is the rigidity, τ0 is the initial tectonic stress, and τf is the dynamic function of the fault plane. reproduced from das and aki [1977]. figure 4. case p – sv – 1 in which one barrier exists on the fault plane. see figure 3 legend for details. the crack tip skips the barrier without breaking it. reproduced from das and aki [1977]. velocities beyond the rayleigh wave velocity, not only for cohesionless cracks [burridge 1973], but also for cracks with finite cohesive forces [hamano 1974, andrews 1976, das 1976, das and aki 1977a]. this is because the dynamic stress associated with p and s waves traveling ahead of the cracktip can exceed the finite cohesive stress. the same mechanism, when combined with barriers distributed over a fault plane, can explain the observed constant corner frequency path in the scaling law diagram described in the preceding section. consider a segment of fault bounded by two strong barriers. a rupture will be nucleated near one of the barriers, propagate over the segment and stop at the other barrier. consider also relatively weak barriers distributed over the fault segment. das and aki [1977b] showed that the following three different interactions between the crack-tip and the barrier on the fault segment can occur depending on the strength of the barrier relative to tectonic stress. 1. if the tectonic stress is relatively high, the barriers on the fault segment are broken as the crack-tip passes. 2. if the tectonic stress is relatively low, the crack tip proceeds beyond the barrier, leaving behind an unbroken barrier. 3. if the tectonic stress is intermediate, the barrier is not broken at the initial passage of the crack-tip; but eventually breaks due to subsequent increase in dynamic stress. das and aki [1977b] calculated the far-field spectrum of body waves radiated from fault planes with various barrier distributions and showed that this model offers a unified theory for a variety of observed seismic source functions. as an example, we shall compare the case psv-0, in which all the barriers are broken as the crack-tip passes with case psv-1, in which one barrier is left unbroken. figure 3, quantitative prediction of strong motion for a potential earthquake fault 84 figure 5. the far-field wave form is shown for p waves for various azimuths on the left for case p – sv – 1. the arrows indicate the arrival of the stopping phase from the moving crack tip. the absolute value of the fourier transform of the far-field wave form is shown on the right in a logarithmic scale normalized to the value at zero frequency. for comparison, the dashed lines on the figure of the spectrum show the curves for case p – sv – 0. reproduced from das and aki [1977]. figure 6. horizontal displacement along the fault of dasht-e bayaz earthquake of 1968. vertical axis is left-lateral displacement in meters. horizontal axis is distance along the fault in kilometers. solid circles indicate north-facing scarp and open circles south-facing scarp. note the gap near 10 e. reproduced from tchalenko and berberian [1975]. figure 7. frequency distribution of the length of fault strands along the san andreas fault. reproduced from wallace [1973]. 85 for case psv-0, and figure 4, for case psv-1, show fault slip as a function of the length of the fault at several time intervals. the wave-forms of the body waves radiated in case psv-1 are shown in figure 5 together with the spectral density (solid line). the dashed lines in figure 5 show the shape of spectral density for case psv-0. the corner frequency is nearly the same for both cases psv-0 and psv-1, but the high-frequency content relative to the low frequency is greater for case psv1 than for case psv-0. since the average slip over the fault plane is smaller for case psv-1 than for case psv-0, the seismic moment is also smaller. thus we can explain all of the observations described earlier for the stone canyon earthquakes with m = 5 and m = 4 by identifying them as cases psv-0 and psv-1, respectively. the above agreement suggests the existence of strong barriers in the san andreas fault near stone canyon with an interval equal to the fault length corresponding to a corner frequency of 1 hz. using madariaga's formula for s-wave corner frequency f from a circular fault with radius a, i. e. (1) the fault length (2 a) is estimated at 1.5 km for f = 1 hz and β = 3.5 km/ sec. we now seek to determine if there is any geologic evidence supporting this value for the barrier interval. geologic evidence for barriers along a fault plane the u. s. geological survey published a series of maps showing recently active breaks along the san andreas and related faults. as pointed out by wallace [1973], the pattern of surface faulting shown in these maps is not a single continuous fracture but consists of multiple breaks, like a long rope made of many shorter strands. the long est individual fault «strands» are about 10 to 18 km long, comparable to the depth of the deepest earthquakes, suggesting that individual fractures may be relatively equidimensional in length and depth. larger earthquakes, in which several tens or hundreds of kilometers of fault length are involved, would then require multiple fractures. earlier wallace [1968] discovered in the carrizo plain streams with no apparent offset lying along fault segments on which tens of feet of offset had occurred during the great earthquake of 1857. he suggests that the pattern of fault slip along the fault during the 1857 earthquake may consist of composite segments, each several miles long, along which displacement reaches a maximum near the midsection of the segment and then dies out at the ends, similar to the theoretical curve shown in figure 4, for the case of psv-1. a similar observation has been made for other earthquakes. one of the most impressive examples is shown in figure 6, reproduced from tchalenko and berberian [1975]. figure 6 shows the pattern of fault slip in the bedrock section (khidbas) and alluvial section (nimbluk) during the quantitative prediction of strong motion for a potential earthquake fault figure 8. fault-plane geometry used to model the san fernando earthquake of 1971 and the site of the strong motion seismograph at pacoima dam. reproduced from bouchon [1977]. figure 9. slip functions obtained as a numerical solution of a crack problem on the left and those approximated by a sequence of ellipses used in calculating the synthetic seismogram at pacoima at dam. reproduced from bouchon [1978]. table 1. 0.21f hz a= b^ h dasht-e bayaz earthquake of 1968. there was no fault break between the two sections for about 4 km. some of the gaps in fault breaks reported by geologists may be due to the properties of the soil near the surface and some may even be due to the erasure of the fault break by erosion and vegetation. others, however, may be a direct expression of the pattern of slip on the fault plane. then, the length of fault strand may be directly related to the barrier interval along the fault plane. the frequency distribution of length l of a fault strand for the san andreas fault is shown in figure 7, reproduced from wallace [1973]. the distribution is exponential for l < 8 km with a tail of roughly uniform height for 8 < l < 20 km. if we neglect the tail part, the probability density function is given by (2) with β = 0.66 km–1. the mean of l is given by (3) thus, the average value of the fault strand is exactly equal to the average barrier interval estimated from the corner frequency for earthquakes with magnitudes 4 to 5. this exact agreement is probably fortuitous. the fault zone from which wallace's distribution function was obtained includes our stone canyon area but extends outside the area for a great distance. since the nature of fault breaks is different between the north and south of middle mountain, we recounted the number of fault strands mapped by brown [1970] with a scale of 1:62.500 for the fault zone between north gabilan range and middle mountain which includes our stone canyon area. table 1 shows the number of fault segments with a given range of length. the numbers in brackets are obtained if we exclude branches of the main fault. nevertheless, we find a certain average value on a map of given exclude them. the above average length of fault segments depends on the scale of map used for mapping. obviously, on the scale of, say, 1:10.000.000, these segments will merge into a continuous line. on the other hand, the average length may become much shorter on the scale of, say, 1:10.000. nevertheless, we find a certain average value on a map of given scale, although if we look at the whole range from grain size of minerals to the length of the entire san andreas fault, there is no physically meaningful average length of fault segments. this situation is similar to the existence of constant corner frequency for a certain range of earthquake magnitudes mentioned earlier. if we look at the whole range of earthquake source size, which covers from a few meters to a thousand kilometers, quantitative prediction of strong motion for a potential earthquake fault 86 figure 10. synthetic surface ground motion at pacoima dam due to slip on the lower segment fault. the top trace corresponds to the case of uniform step-dislocation, the second from the top to the case of a single continuous unilateral crack nucleating at the bottom, the third to the case of a single continuous bilateral crack nucleating at the center of the lower fault segment, and the bottom trace to the case of multple cracks with ten equidistant unbroken barriers nucleated at the bottom. reproduced from bouchon [1978]. figure 11. synthetic surface motion at pacoima dam due to slip on the upper segment of the fault. the top trace corresponds to the case of uniform step-dislocation, the second to the case of a unilateral single crack nucleating at the bottom of the upper fault segment, and the bottom trace to the case of multiple cracks with seven unbroken barriers. dl e dlf l l= b b-^ h ( ) 1 1.5 kml lf l 0 = = =b 3 # 87 there is no physically meaningful average source size. the observed constant corner frequency, however, suggests the existence of a physical condition which has a characteristic average length and controls the occurrence of earthquakes in a certain magnitude range. the same physical condition may also control the length of fault breaks observed on the surface. we propose to call this length «barrier interval». the observed fault breaks along the san andreas fault from north gabilan range to middle mountain show an average barrier interval of 3 to 4 km. on the other hand, the corner frequency for earthquakes with 4 < m < 5, when interpreted by a model of a circular crack, suggests 1.5 km. as discussed later, the barrier interval may not be a constant of a fault zone but vary with the amount of slip and other parameters of the earthquake process. seismic motion due to rupture along a fault plane with distributed barriers; comparison with observations of the san fernando earthquake of 1971 let us now make a direct comparison of synthetic seismograms based on the barrier model with near and far field seismograms observed during the san fernando earthquake of 1971. we find that a single continuous crack propagation over a fault plane with no barriers (such as psv0) cannot explain the observations. interestingly, we find that multiple cracks over a fault plane with distributed barriers is roughly similar to the uniform dislocation source in seismic radiation, especially in the forward direction, and both explain observations better than a single continuous crack. we now summarize the results obtained by bouchon [1978] using the method described in bouchon and aki [1977]. figure 8 shows a simplified geometry of fault planes for the san fernando earthquake consisting of two planes; the lower one dipping at an angle of about 52˚, and the upper one at a lower angle. the rupture originated at a depth of 12 to 14 km and propagated upwards until it broke the free surface. the slip-functions obtained by das and aki [1977b] for various barrier models, such as shown on the left hand side of figure 9, are approximated by a sequence of ellipses as shown on the right hand side of the same figure. figure 10 shows the synthetic surface ground displacement records at the pacoima dam due to slip on the lower segment of the fault. these four records are computed for different slip functions. from the top to the botton, they are a uniform step-dislocation, a single continuous unilateral crack nucleating at the bottom, a single continuous bilateral crack nucleating at the center of the lower fault segment, and multiple cracks with ten equidistant unbroken barriers nucleated at the bottom and propagated in the manner similar to the one obtained for the case psv-1 by das and aki. the results for the uniform dislocation and the multiple crack slip functions are remarkably similar. they both show a sharp change in slope at the arrival of the shear waves from the nucleation point. such sharp arrivals were recorded by a quantitative prediction of strong motion for a potential earthquake fault figure 12. synthetic horizontal particle velocities at pacoima dam obtained from the displacements shown in figure 11. the upper trace corresponds to the case of uniform dislocation and the lower trace to the case of multiple cracks. reproduced from bouchon [1978]. table 2. average barrier interval estimated from surface measurement of fault slip. strong motion seismograph at pacoima dam. on the other hand, single continuous cracks cannot generate these sharp s arrivals in contradiction to observation. the synthetic ground displacement due to slip on the upper segment of the fault is shown in figure 11. for this segment, we find a striking difference between the uniform dislocation and the multiple cracks with seven unbroken barriers. the latter produces a large high-frequency component of 3 to 4 sec duration, while the former does not. this is because the rupture now propagates mostly away from the pacoima dam site. long trains of high-frequency waves are noted also by das and aki [1977b] for azimuths opposite to the direction of rupture propagation in the case of fault planes with distributed barriers. figure 12 shows synthetic horizontal particle velocities at the pacoima dam obtained from the displacement due to the faulting of the upper segment; uniform dislocation at the top and multiple cracks at the bottom. the last sharp arrival is the rayleigh wave from the point of surface break. the model of multiple cracks explains well the major features of the pacoima dam records. the methods used for calculating synthetic seismograms at the pacoima dam are restricted to twodimensions. we need a threedimensional model for comparison with teleseismic p wave data. for this purpose we assumed that the initial crack of the multiple crack model is a circular shear crack expanding at a rupture velocity of 0.9β and stopping suddenly. the slip function for this crack has been calculated by madariaga [1976]. bouchon [1978] again uses an analytic approximation of madariaga's solution. synthetic p wave forms are computed for a circular crack buried at the hypocenter in a half-space. the effects of geometrical spreading, reflection losses, mantle attenuation, and instrument response are taken into account. the synthetic p wave forms for different choices of crack radius are computed for four scandinavian stations and compared with the initial part of the observed short-period seismogram. the best fit of synthetic and observed seismograms was obtained for a crack radius of 1.5 to 2 km and a maximum slip of 2.5 to 3 meters. empirical relations between barrier interval and maximum slip detailed mapping of slip across fault breaks on the surface has been made along the length of the fault for several earthquakes. the distribution of slip as a function of distance is never a smooth curve with one peak at the center such as expected for a crack without barriers, the case psv0, described earlier (figure 3). a remarkably smooth slip distribution reported by matsuda and yamashina [1974] for the izu-hanto-oki earthquake of 1974 is very much like the case psv-1 with one barrier. it consists of two peaks with an apparent node in the middle at which a slight change in the strike direction occurs. in most cases, the slip shows several peaks suggesting that an earthquake fault is usually a composite of faults separated by barriers. a summary of barrier intervals and maximum slip inferred from the observed slip is shown in table 2 for earthquakes studied by matsuda [1972, 1974, 1976], matsuda and yamashina [1974], and tchalenko and berberian [1975]. representative values for the great fort tejon earthquake of 1857 are taken from the description by wallace [1968, 1973] although the slip distribution along the total fault length is not available for this earthquake. in order to estimate the barrier interval seismologically, we must calculate synthetic seismograms using the barrier model and compare with the observed data. bouchon [1978] made such a comparison for the san fernando earthquake of 1971 as summarized in the preceding section. he also showed that the «composite cracks with unbroken barriers» model is roughly equivalent to the dislocation model with uniform slip such as that used by aki [1968] and haskell quantitative prediction of strong motion for a potential earthquake fault 88 table 3. barrier interval and maximum slip estimated from rise time. 89 [1969] for the parkfield earthquake of 1966. from this equivalence, one can make a rough estimate of barrier interval from the rise time such that barrier interval ~ rupture velocity · rise time. (4) table 3 summarizes the barrier interval and maximum slip estimated using the above equation from source parameters given by abe [1974a, 1974b, 1975] and kanamori [1972] for four japanese earthquakes. as discussed earlier, the corner frequency of small earthquakes may be related to the barrier interval of the fault zone. the observed constant corner frequency of about 1 hz for earthquakes in the stone canyon area with magnitude 4 < m < 5 suggests a barrier interval of 1.5 km if interpreted by the use of a circular crack model. assuming that the «highest stress drop» earthquake located at the top of the constant corner frequency path may be modeled by the circular crack (without barriers) of madariaga [1976], we shall apply the following formulas to this earthquake. (5) where m is seismic moment, μ is rigidity, δumax is the maximum slip, a is the radius of the crack, β is shear velocity, f is corner frequency and δσ is stress drop. using the values of seismic moment and corner frequency for the «highest stress drop» earthquake, we find not only its barrier interval, l = 2a, but also its maximum slip, δumax. for the stone canyon area, we find that l = 1.5 km and δumax= 1 m corresponding to the top of the constant corner frequency path (m 5 in figure 1). the corresponding stress drop is about 400 bars according to equation (5). in figure 13, an arrow is marked on the diagram showing the maximum slip and barrier interval. the end of the arrow corresponds to the «highest stress drop earthquake» in the stone canyon area. the arrowhead points to the direction of constant stress drop. we plot, in the same figure, the results obtained by wallace [1968, 1973] for fault breaks of the 1857 earthquake in the carrizo plain and by bouchon [1978] for the initial part of the san fernando earthquake of 1971. these points appear to lie along the extension of the arrow for the stone canyon area, with a slight increase in stress drop for greater earthquakes. although the data are too scanty to draw any definitive conclusion, they suggest that the barrier interval is not a constant for a fault zone, but increases with the amount of slip involved in an earthquake. figure 13 also shows the results for other areas obtained by geologic and seismic methods as listed in tables 2 and 3, together with the results from the scaling law data obtained by chouet et al. [1978]. the data points in figure 13 are scattered over an area between two lines corresponding to stress drops of 100 and 1000 bars. these values are about an order of magnitude greater than those estimated earlier by the use of crack models without barriers [aki 1972, kanamori and anderson 1975]. thus the barrier model can resolve, at least partially, the well known discrepancy between the stress drops measured in the laboratory and those estimated for earthquakes. the results for kilauea, although the data are the least complete, show extremely low stress drop, or a long barrier interval for a given maximum slip. this may explain why only scattered evidence of strong ground shaking was observed in the epicentral area of the island of hawaii earthquake of november 29, 1975 [rojahn and morril 1977]. prediction of strong motion with further studies using both geologic and seismic methods, we hope to establish a reliable relation between the quantitative prediction of strong motion for a potential earthquake fault figure 13. relation between the barrier interval and the maximum slip obtained by various methods. 0.21f a= b 3 2m u a2max= r n d 24 7 u amax=v r nd d barrier interval and the maximum slip for a given seismic region. if this· is done, we shall have a powerfull method for calculating the strong motion seismogram for a potential earthquake fault. among the parameters of an earthquake source needed for calculating the seismogram, the fault length and width may be supplied from maps of geology, seismicity, and plate tectonics. the rupture velocity appears to be roughly a constant. the maximum slip for a fault zone may be estimated from the rate of plate motion and geologic history of slips. then, the diagram shown in figure 13 can supply the final crucial parameter, i. e. the barrier interval, needed for calculating the strong motion seismogram. this approach, at least, offers a framework in which geologic data on past fault breaks and seismological data on small earthquakes can be combined for the purpose of estimating the strong motion of a large earthquake in a given seismic region. acknowledgements. we thank dr. r. e. wallace of the u. s. geological survey and dr. t. matsuda of the earthquake research institute, tokyo university, for providing us with first-hand information and expert opinions on fault breaks observed on the surface. discussions with drs. v. i. keilis-borok, b. v. kostrov, r. madariaga, t. g. rautian, and p. richards were helpful in developing the basic idea of the present paper. this work was supported by the national science foundation under grant 77-23336 pfr. references abe, k. (1974a). fault parameters determined by near and far field data: the wakasa bay earthquake of march 26, 1963, bull. seism. soc. am., 64, 1369-1382. abe, k. (1974b). seismic displacement and ground motion near a fault: the saitama earthquake of september 21, 1931, j. geophys. res., 79, 4393-4399. abe, k. (1975). determination of static and dynamic fault parameters of the saitama earthquake of july 1, 1968, tectonophysics, 27, 223-238. aki, k. (1967). scaling law of seismic spectrum, j. geophys. res., 72, 1217-1231. aki, k. (1968). seismic displacement near a fault, j. geophys. res., 73, 5359-5376. aki, k. (1972). scaling law of earthquake source time-function, geophys. j. roy. astron. soc., 31, 3-25. aki, k. and b. chouet (1975). origin of coda waves; source, attenuation and scattering effects, j. geophys. res., 80, 3322-3342. andrews, d. j. (1976). rupture velocity of plane-strain shear cracks, j. geophys. res., 81, 5679-5687. bouchon, m. (1976). discrete wave number representation of seismic wave fields with application to various scattering problems, ph. d. thesis, mass. inst. of techn., cambridge. bouchon, m. (1978). a dynamic source model for the san fernando earthquake, bull. seism. soc. am., 68, 1555·1576. bouchon, m. and k. aki (1977). discrete wave-number representation of seismic-source wave fields, bull. seism. soc. am., 67, 259·278. brown, r. d. jr. (1970). map showing recently active breaks along the san andreas and related faults between the northern gabilan range and cholame valley, calif ., misc. geol. invest., map i-575, u. s. geological survey, washington d. c . burridge, r. (1973). admissible speeds for plane-strain selfsimilar shear cracks with friction but lacking cohesion, geophys. j. roy. astron. soc., 35, 439-455. chouet , b., k. aki and m. tsujiura (1978). regional variation of the scaling law of earthquake source spectra, bull. seism. soc. am., 68, 49-80. das, s. 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(1978). spectral analysis of the coda waves from local earthquakes, bull. earthquake res. inst., tokyo univ., 53, 1-48. wallace, r. e. (1968). notes on stream channels offset by the san andreas fault, southern coast ranges, california, proc. conf. on geologic problems of san andreas fault system, stanford univ. publ., geological sci., xi, 6-21. wallace, r. e. (1973). source fracture patterns along the san andreas fault, proc. conf tectonic processes of the san andreas fault system, ed by r. l. k. kovach and a. nur, geological sci., xiii, 248-250. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. quantitative prediction of strong motion for a potential earthquake fault more about severe frost as a triggering agent of microseismic activity in the neighbourhood of large dam ami about the geodynamical conditions leading lo il r. caloi (*) ][. :'origa};i (**) m. o. spadea (*) reecive,l on .\ray 28th, l !l73 su~[~['\ry. in this sceond notc it is cvinceneed as t.lle (·ontrflst in the roeky.nh'dinlll downstl"rllili athi upstl"eulii or the pieve di l':u\on' ,\alll dllc to severc frost, is the triggerillg agent of the liiinoseislllieity. 'l'ii(' frozen liqui,1 c](>liient (·:.pansion in thc r01'ky.mcdillm imbuc,\ wit.h wilt{'r (!o\\"ustrrllm or thc dalll brings out the struins into the iiic(!inlll hi(!(!t·n .. \rol"eov('r fhe geo(!yllalllica! con,litions which originate the liiierolshoeks are stlhli(',i, putt.ing forwilr,l; l) t.1!/' lllì('roshoeks repdrildly st.ar up during swlolen chllnges olr tcili]jcrnture; 2) uj{'y espeeially oc('nr dllrilli! the rew llour~ of (lilll·nal heatini!, eharaclerized ii.\' a partial snrfarial thawin).!; \\"ith ('n~ning iihrupt. "filll" of t.hc {!full dolwnstn'illil. the ilfon'sflid jlhenolllp1hlll n'prp· scnt.s one or thc oi"igin-llice!wnislll or thc earthqllukes in sdslllic zolws il.\' lolcal strains eolntrast eharaeterized. 1. on summ>lrizing in n. }lrcviolls note (') thc me>ln behavior of a large (bm (}lart.ieularly of the rieye di cadore jlalll) wit.hin a s}mn of folli" seasons, the exeeptional interest })ertaining to })erio(ls of severe frost· has been reyeu,lel! for thc first t·imc. it eollll! be sho\\"n how the lllediulll ani! thc (jam are a(lversclr aheet.ed by the almormal belmvior of wat.cr if t.emperature falls below zero . .alllong the phenolllenu, brought 1,0 light that of miero8eislllieityi eaused by cont.msting stmins within the roeky mediulll u}lf;trcalll {*l istituto naziolnale di gcofi~icu, rollia. {hl e.k.e.t,. colllpartilll{'nto di venczia. a!h[ i[owniltream oi thc i[am a!h! partiruhuly adiye at tile dum bottom, calllc qllitp llnc.:qwetell. fn the previolls 1i'0rk spedal attention wa,~ giypn to the seahonal i[eyelopmcnt oi tilc phcnomcnoll. il l'l'i'· wl' .~halllook }lal't.ieularly illl0 itil daily co\ll'hc, :!, to poilll ollt lhiil mirroilciilmil'it~, we availel! our~clyei; oi three ~('i~llli(' ~tatioll.~ caril oi whieh wai; in~talled in \ilh!el'grolllld rooms oi centralil,el[ ('ontrol ('ahi11, ollll oll thc l'ighl hand .~hol'e ili'lu' lhe damtop, onc in a sta}lle-}lit. of ash!al' 1\'1' xi\' (('1.1) imide lhe dam (at. an hight of mil m) alh! one in a .~laple-pit. of ailhhl1' ni'. \' (c5), al an hight. of u:-hl m, likewi,~e insil[e lhe i[am (fig. 1). ('l-j i~ in a eentml poililion oi the dam, ('il is nch1' its left l'!hl. tlw seismo;.;mph~ '\'cre of llw "oirla!h!a" typc with the samc in,~tnll11enta! ehal'l\('tcristic',~ cx(,c}lt thc l[mnlstl'eam-u}l~t.l'pam (\'-m) ('ompo11cnt of a hei.~nlogl'a}lh ill,~t,alled i11 cii ,dlo~e dynamir magniliration pnwails oyel' the otlll'l' lwo. i\liel'o~h()('kil triggcrci[ by fro,~1 are l'erol'ded by t,holl~a1l(!h in wintcl' hctwecn ]lecembel' and )iarrh whcn ail' tempeml1u'e ialls ('!eal'ly hclow zero i!egrces or os('illatci; wil!ely 1'0ll!h[ il onl,\' onc sel'ies i.~ ('omidel'ed hcrc, 1'(leo1'(!ed bct,wcc11 janllary lwi2 ani! ,janllary-feln'\u\l'y ln7:'l, ah a ge1l(~ml il'all1l'c oi tlll' mirro.~hoek.~ tl'iggel'ed h~' the fro.~t. it ~!j()1l](l be ob.~crvcl! tlmt ai; a rnlc tiley are rcro]'(!cl! 011/,/1 by thc ~ci~­ mie ~tations ill,~talled withill the dam, rc('o]'(1ing,~ are 110t unifol'lll, ilmnwel', ilot, ey(lll t.ho.~e of the t,wo ahh!al',~; ~ome mi('l'ohho('k,~ ('0111d be l'i'('ol'dl'd l'leal'ly in ('1-~ alh! wca\{ in c:'l, or yi('e ver~a, olhcrh were l!c1ayi'i! in ('5 01' in ('14, still othcrh only in c'h or only in ('5, t!jc rea,~oil of t.his 111lllvelll\('c.~h of l'el'ol'ding",~ may hl' atll'ibllt.ed ahoye ali to thc dijlerent arl'angemcnt oi thc ~cismie ~taliom, onc being" loeatcd in tile ('cntml ashlar oi tile dam (t'h), lhc ot!jcr in a }lcl'i}lilerieal ahhlar (('f)) (hg. 1). among t.he hulh[rel[s oi mic'rohhoek~ rcro]'(!cl! belwcen ,tallllal'y :1 alld 12, 1ni2, t,he olle of 2:'l111:!m of jalluaryi (tig, 21x) wa,~ not rcco]'(lcd in c5. oh"iou~1y, tilc mierosiloe\{ inlel'esled hlol'e ('1·1 thal1 the 11ih!el'lying j'()('k (ah a mattl'r of faet, tlle high rrcqngney vibratioi1r, pcell!inl' to thc lat,tel', arc lllihsi11g): the i!am l'chpo!u!ci[ jìl',~t with lhe 11ninodal oi lheahhlal', a!h! then with lilci11ndamenta! of the entirc i[am ll1 c-01.5·· (a)j ,\'hich !m,~ no a}lpl'cdable alll}llitul!c in ('5 a~ it is llllh1' the 1101!e, fol' \'hc .~amc l'ca.~on.~ :dro the te11,~ of mic'l'osiloeks oi jìg. 2~ do 110t l'earh ('5, 011 the othcr hnlld, fig:, '2,( hhow~ a lnirl'oshock whi('h was j'('('()l'dpd ollly ill ('5: o bviomly, t he lwl'hll'bntion ol'igi11h ted n t. cil \\ hieh l'e~llolhlpll hr~1 with it~ pl'oller uninodnl, followell b~' ih(' hight'r hal'mollil' of the cntire ilam (t = 1ab,), a ~imi]ar ob~e'rynti()n wah madl' fol' ihe' two mierohho('k.~ of fig:, '20 (20h.3~m and 2lh.3.!m of jnnuary ~), t!h' sn'oihi of \\'hirh wns lll'c('l'ded by high fr('qne11('y vihl'ntiom, tltu" lll'oving thl' fad ihat the mirl'ohhl)('k was also al'tivl' ill t!w ]'()('k i)(']ow tllll n~hlnl', fig, 210 ~hows two lllil'l'o~hoc'k~ (thc lnltl>l', at t]h' o,;,) ~e'i'()]ld'~ ili~b\lh'e nh, wa~ ~tl'ong'el' than tll!' fol'mel') \\'hi('h ol'ig:innte'lllll'ubabl,\' in the l'oc'k in ('onlnd with ('1-l whi('h m('r('l,\' l'l'a('t!'(! lo tll!' htte,,~, fnl'thej' tholl~nnllh oi llii('l'o~hoek~ ilue tu im~t we'rc l'e'('urdt·d in l,he dam from ])l'(,l'mhel' 1972 tu marcii 1973. 'l'hi~ ext.rl'lllely n111lwl'on~ heries of mirl'o~iio('k~, too, fentul't'd ('a~cs a~ the ahove oi whil'h wl' ill'l' nll'nt.io11i11g hel'e ollly h'\\'. thc lllil'l'o~l!()('k of jnnuary 8, 1973 (hjhg,jon ... ) wa~ l'('('onl('d llln('h ~t.l'ollgel' in ('5 ihn11 in ('h (iig, :3), mol'eovel', it real'lll'd {'l-l with a delay of ahout o,ul ,~cl'. rrhi~ ]ll'o\'l;h that tllll ~ho('k origillnt.ed llpaj' ('5, at 18 1>20'" of ,jannnl'y 11, 197:3 a mirl'osho('k is l'ceorde'11 both in the dam (lig111'l'~ -ln and -lh) alld ill ulil ('()]ltrol ('ahi n (fig11l'!'~ ~'(' alld ~d), yet with a ]owel' amlllitudc owing to thc lowi'r ~l'miti\·i'n('h.~ oi thl' hei~mographs imtallell. thi~ ih ~llggehtiye of the shoc'k orig:i11atillg ill i ili' l'orky-plain do\\'mt.l'eam of the liam, a~ ih lll'oved hy tlw bu'k of l)j'o]lcl' o~i'ìjjntiolls uf thc ('onl'rele h1..j'lldmc, otht'r frnetme~ in thl' l'ori{ al'e tho~e l'e('(ll'i\l'11 nt hlh20m of ,jallunl'y l~, 'l'll!'il' ol'igin \\'n~ mtlwl' elo~c to cl.1, n~ pl'oved hy tlt(· majul' nlll]llitndt' ohht·rvt·d and by the vel'y sllol't :lilva1h'e ii ('olllllared to himihn l'ec'ol'dillgs made in ('il (!ig'. ;')). 'l'hl' ~t.l'ol1g:e~t of the two mi('l'o~lll)('k~ ('il1l~ed light tl'a('eh in the c'ontl'ol l'ahi il. anothl'l' mi('1"o~lloek originatcii in thc l'oek wn~ that of ,jnllunl'y 13, at l\j il'2-l m (lig. (i), ('lohel' to ('l.! thall t.o ('g. ()ll!pl' exalllplc~ of mirl'ohhorl{h lll'ovoked hy l'1)('k fra('tnl'l',~ )]rl' l'l'pur(('d in lig.7; thl',\' luul \'ariom intcmitie~, nlthough theil' ol'igi11 l'lo~e to (:':') nllllcar,~ l111ique, 'l'hl' ~ome lift,eell mi('l'o~hu('ks l'l'purt('d in lig. 8, on thc othcl' hand, l'e'g:l1'il almo~t tlxl'lll~ively thc dam allii theil' ())'igill mllht he ~ongh( hehind this: the 1ll0l'e ()j' ]e~h long herics of usrillations al'c nnimatc'll, in fart, hy allomn]ollh ili~pe'l'~ion, a stl'ikillg featul'tl of the ilam'h fl'('l' osrillationh, they al'l' l'erol'ded ahuv(' ali in ('1-1 wher(' thc fnndaml'ntnl uf thc elltil'e dam allpl'an! bcttcl' de've]olllle'11. ]t is ob~el'vl'1i that. hl'twecn the ht'ginning oi t.he val'ion~ pl'1'tll1'lmtio\lh and the hl'ginnink' uf 506 l'. caloi -~!. ~[[fla-";i ~1. c. spalle a free o~cillation~ of ch (or of t.h" one~ concel'lling the cntire dam) t.hcre are dihtn"ent t.im" interval~. ']'hi~ mean~ that t.h" (1i~t.ance oi origin oi t.he variolls microshocks to the ~ei~mic ~tation oi cl-! varics. the nearest to ch appears to be, ior in~tanee, t.h" one uf 18 '1 .!7 m oi january 26, followed by that. uf 191>3-1 m of january il; the iarthest is apparently tlmt of 20l\·j,6'" oi j anuary 2ii . . .i\nyway, the order oi magnit.ude of t.he relat.ive dirtanees may be calcuja.ted after the formula, (3). n ./ l,' 1 v~ .jt l i " l' , where v is the pru]lagation velueity of a frn, osdllation of the ]lcrioa 'j' .l e in an ashlar oi the thicknclss b, l! ilhlinltes tlw lh·ollagation vcllu-, city of tlw (1am's ln·ol)el" ordinary longitmlinal waves. 'g poi' l r; "-' 4500 m/sei', b = 16.5 m, one gets v "-' 1 366 l' . thcrciore, the velocity results to be inversely ln·ol)ortional to the l)eriod of the consider"d free "\vave. 'fur a lwriud of l' = l ' , une gets, for instanec, v = 366 m/sei'; for t = 0.6 ' , v = 470 m/sec. ab, for 'j' = 1.5 ' , v "-' 300 mjsel'ab.; fur l' =.03.0 ' , v = 210 m/sei' ab. i<jvidently, t.he~e arn }jurcly indieative valner. 'vith re8]lcrt to the irn, wave of about o.ii ", the secon<l oi the abovc microshoeks appears to be, thcrefore, some one hundred m"ters farthcr, with its oi"igin, than the first. ']'iw miei"o~hol'k of 2()11.j,ti m of janlhll"y 2ii, might have it.s origin at. t.he l)lug·bottom, ete. the microshocks originating near the base of the centrai ashlar (oh) not. only bring ,.bollt the fundamental useillations of the entii"e dam, but oftcn give l'ire ar well to vibrations in thc or(1cr of abollt 3 whieh may be considered vibmtions of thc concretc building fixed at one of it.8 end8 (ti w left) and "fr"e" iwar tlw plug, always bearing in mimi that these are mìeromovenwntr. the exeitation oi t.he~" oscil· lat.ions may be more or less retar<led: consideration should be given, for inrtanec, to mirruslloeks of ngs (le, 11 (01 h07 m of j alh1ary 29, 19(3), 9b, (la, 12b, 12a. the microrhock a]lpearìng in fig. 91' seclms t.o l'omcl irom right. below ch, c:xactly as the first oi fig. lo. at 23 h 35 m oi j alhhlry 28 (fig. 11), them was "vidently a sm·ies of mieroshocks, as mo!\}; anout severe ~ ros'[ as a ,[riggeri~g ag:ext, etc. 507 well as at 14 11 57 m of jannary 29 (hg. 9b). 'l'he mkroshoek of hg. 11 (at 081<02"') evidenlly took its origin irom tbc rock, cven though it exteillle<l fmtber to lhe dam, simi\a.rly as lbe mil'roshoek of fìg. 12b (op.!9 m) caused succc~sive microfrad-nres in thc rode in hg. 13a (f. in. at 15 1126 m and aì 17 11 14 no ) tbc vibrations oi the dam or ils pa.rts, farle in about. 4 sec, while in fìg. 1:3b (at 1jh2sm) they deyelop at (listinct. intervals and persist. for abont. 50 seeonds, which is indieahve oi the far origin oi t.he micro,~hock (possibly at the 1)lng-hol10m). among the fractures originating from t.be roek, that oi 14 11 22"' of january 2!j, 1973 (fig. 9a) whieh wa~ reeorded only at c14 and i~, bence, very clo~e t.o t.bis ashlar, deserves att.ent.ion. contrary t.o t.his, t.he fraetnre of jannary 31 originated on lhe right side, at. t.wo or three bnndre<l metres dist.anee from the eontrol eabin, extelhling from s\v t.o ss\v. unly t.races of it conld be recorded in c5, fig. 1-1. here it should be remembered that the eontrol eahin is spparatcd hom the rocky-l)\a.in by a gorge of about 120m deep in which t.he plng fìts. :fig. 15 shows a microsho('k in t.he rock, reeorded merely at ('5 a11(l neces~arily with its origin rather dose to t.his a~hlar, ami, moreover, in c5 only tbe microshocks oi fig. 16 are record ed. 3. four series of microshoeks due to host bave been exa.mined, between january 6 and 12 ami between january 15 and 1!j, 1972: between january 11 ami 16 and between january 26 and }'ebruary 2, 1 !j73. the variou~ thousands oi mieroshocks thus originat.ing were divided into one hour interva.ls with the fn·st. t.wo series, and into t.wo hour intervals wit.h t.he third ami fonrt.h ~eries, as far as their ehronological order is concerned. fig. 17 refers t.o the lìrst. two series and shows as well the bchavior of temperat.ure and oi the wafer-ioading moreover the ii0wn.~tteam upstream component (v-~i) of tbe clinogmphic station inst.ai1{'d at tbe base of ch of t.he pieve di ('a<lme dam. figs 18,19 are referre<l to lhe otber two series, the first consi<lering t.emperat.ure, waler-ioading alhi clinogral)hic recor<lings, simultaneously measnred at the hights oi 682, 660 alhi 625 metres of c14, t.he ~eeolhl these same llata plus elinographic recordings obtained by the ~lation installed in the ro('k at 625 ms height in a staple-pit. of t.he right supl)ort. one fact emerge.~ immediately irom the figureh: the micro.~ho('ks l)ertaining to severe irost mass logefher espeeia.lly in time periolls oi swlden t.emperat.nre change~. 'l'hm, sinee eooling olr requires much more time than warming up, it is in periods of the lat.t.er that mierol', c,h oi l i. ~f1ga~1 ~l. c. spadea f!hocks occllr with maximlllll f\'equency, rt is not so much the tc'llll)('l'a· ture val'iation in it,~clf whil'h i8 j'(jspon,~ihl!' fol' micl'ofradures wl thc rnpidity at which it takch plnee. dl1l'ing thc day, p\'ovidel1 there is ,~uil8hille, tempemtul'e recovers what it 10ht the night. bcfo\'e. h!nvevet, ~ilwe t!'lllperatu\'e lw'overy oceul'8 much faster than !orhes, (iming the few hours in whil'h the t.em}leratlll'e inc\'ea~es, .iust then t.he cncl'gy whieh is capablc of b\'inging in plasto-elastil' field in'!'a].;:8, more gathers. 't'he fact ir 'vcll ontlincll al.~o by t.hc l'!inognl]lhic \'ecol'l1illgs which allow, besides, to 11ejinc its cnuses. 11: ih known tllat. 1:111' "diu\'nal wave" following the sun rndiat.ions anl1 }larticulal'ly ampie near the llam·toll l'mlnees its alllplitude ns it pnll'eel1s towal'11 the (iam-bottolll when' it vi\'tllally is a,nnlllled. iii tlw \vinte\' nlonths i1: is stl'ongly relluccll al~o near the l]am-toll aihi at. ia.rt dif!aplle~u'f! in the lle\'iod when the sun is 1m\". at pieve 11i ('ai10re, t.oo, wit.h temperature above zero dcgrcer, the '~dimnal wave" nt the 11am-bottmn of 62;) 1m height is reconlcll only ns a tl'll,ce, if an~'. tts belw.viol' in pet'iolls of sevete frust is a.ltog'cthf'l' ditiel'ent, 't'ili' l'(wk 11owllstl'eam of the danl iinbibed with water swc1h! as it fl'cezef! and ]j\lshch t,ile 11am l1j)stl'eam (:\i). rl'lti~ is shown by tigs j 8-21 w11ieh l'eveal also another fact.: the cont.ra~ting iwhavioll\' of the top and the dam-hot.f,om nnllel' t,he sa/il(' aetillg cause, 't'hc ~inkillg of tempc\'atu\'e hmll1.~ the uppc\' lla\'t of the danl downst.l'eam (y), 'vhile the bottom is puf!hcll llllrt.ream (111), aihi viceverf!a. sij('l'e,~sively, wlwn the upper jjal'ì of the da m, exartly t.he side opposi te the lakc, is wal'lned up hy sullnillpss, hends upst.l'eam (:\i), whi1!' at the snme time t.he warmed a.i\' part.ly meltr thc fl'ozen wate\' within the \'ol'ky lllnin, eall8ing the contraction oi the rocky medium with a smal1 downstream (v) bendillg of tlw dam bott.onl. a t the lower }lart of the 11am anl1 in periods oi severe irost t.hif! rewl1:s in the fmmatiml of an i-nvtrtljd iliul'nal n'ave (lue to the anomalous behavior of water when t.he temlle\'a.tlll'e is llll(jm ze\'o degl'ees, the mentioned ligurcs shmv that for the pieve di cadore }lam the height. of mdi l'otn.tion is few metl'e8 below 660m~ whel'e the clinogra}lhic eom]lonent 11owmtl'ealll llpst.i'c~\m (v-:\[l is stili, thollgh slightly, ill pha~e with t.he correspolhlin,l!; eomponent of the !lam-to}l station ({j82ms), ('omillg bfi.ck to the connecting' phenomenon of mi('tofl'aetu\'es, it is observell t.llat 11nring t.lw gnulunl ;;inking oi temjleratl(j'e belo\\' z!'\'o 11egrees the dam is grnllunlly pl(f!llpd llpst\'mun cm) [).!l11 that only in the ked.ionfl wll('i'!' eooling is more rapi!l, t,hi~ 11llsh may hrillg ahont mìc'l'oshocks, fn other terlllh, dlll'illg cooling the mellium general1y molle abov1' severi:> l"1\081' a.~ a tmggehi~g ag1':~t, etc. ;, ofl reacts withiu t1w plastie tìelil. in tlw few houl·s oi sumhine, ,dlcn tem}lerature rapillly nwovers ,dm.t, il hall lost illlriug" the night, thc outer parto of t.hc medium releases rat.hcr suddcnl.\' fllhi allows the dalll to "fa.ll" downst.ream (v) (figs. 17 -1 d) in too l'al)id a wa)" to bc l'outainell oy thc nwilillm wit,hin thc plafltic tìe!ll. so, aftocr overeoming the 8tl'cng"th limit, microshocks ol'cul' \\'it.h highcr frclj1wnl'y. .as is shown b~' the clinographic rccorllings j t.he defiection dcscribell oy tlle ilam-oottom, iu one way or the op1)(),~ite way, is more or less the same; howevcr, ilul'iug t.he st.age of wanl1il1g uv it. is covered much qlliekiier than during' the cooling oli stage which aecounts for the outgrow of the pla~tie }lhhse ajl(1 for t.he follo\dug microfraeturcs as a re~lllt of a morc fll'centuatell monwntllm. this 1jhcllumclluj! s7wuld be secli ([s a '('(ilua/jlc indication o/ thr mcchanism which accomptmies scismicily in ail a/'(([ subjrl't to contraslilly st/'ains, 'l'he ol'der of magnitude of tltc st,l'csscs sltal'ing with tlte mie1'ofracturcs rising in the l'(){'k can oe oest. valued if one comillers that the t.angential st.mim actillg ilowllst.nmm at tlw dam-llottom j the rcservoil' being completely filled j are abollt, 30 kg/cm~. 1'hehe are t.he iorees whieh ad a.gaiust t,he medium downstream when j furt.hcl' t.o the yielding of the rol'ky system whieh lo~es in strength ilue to }lartial t.hawiug of t,he flul'face, the ilam "falls" towan! the roeky-lllain. hllt, thih will be dealt wit.h in dctaii in another pfl,pcr. 4. in ordcr to lllulerflbuli! always hcttcr the behavior of the pieve di callore dam closely linked by seasonal variat.ioufl it, was couflillcl'cd hel}lful to eompare the course of tempemt.llres (and of the water-loalling) to t.lw ~imult.aueous ilevelopment, of 1:he clinogmphic recordings taken in ch at, thc height,fl of 08:j j liotl j ti~5 ms in t,lw months of feoruary ajl(1 )ll1'ch 1!h3 which, from the geodynami('al viewlloint, are t.l'fl,mit,ion months, at t.he height. of 682 ms, and le~her a.t. tillo ms, t,he fir~t groups ai diurnal waves lllh!el' the cnect o[ sunniness begiu to avpcar, while the dam stm-ts ocnding uimt,rcam cm), also t.hrough the ahatement of t.he watel'-loading. this behaviol' c(hlt.inues thc whole of march, a.s [al' as the height of 66t1 ms i~ concerncll j ami until :'\iar('h :20 [or tlle height. of 682m~, t.he ilam-tov which beginning at. that ilate starts to belld ilownstnmm (y) rfl,t,hcl' t,han upstream (:u). 1'his allomaly is explainell by t,he ucllfl,viol' of air temlleratul'e, which meanwhile is ai1ill'ofll'hing 8-10 <>c, and of the watocl'-loadillg limited to heights below tiotl ms, on i:;io p. ca l,j r -)f. )[[ga~r -)f. c. f'padea the reservoir side the part of the liam ('overed by water lws a teml)eraturc or about -i uc which accountr for the ul)stream pressure of the lower l)art of the (lam up to the sai(l heig-ht whieh hah (10\\"11st.ream a mean temperat.ure of over ..1 0e being reaehe(1 by thc sllllniness. }'rom 660 ms upwan1, howcver, alro the part or the (hun on the l'eservoir sidc is sllbjed. to progressive hcating a.s a. consequcnee of the inerease or air tcml)erature, eontrar:,>' to the submerged part whieh is governeli by a lower temperat.ure. this cont.rast inereases wit.h t.he height. thus one has a relative motion of the emerged vart l'ontrast.illg with t.he rubmerged part., that is a (lowllst.ream (y) bcnding, as is elearly dmnonstrated by hg. 21. 'l'hih hhows ah weli the alteration of thc diurnal wave as a· eonsequellec or thc eontraf!ting thermieal ritua.tion. at any rate, the bchavior or the dam-botlom is mueh more interesting: in february the above sa id il1vcl'tcd dilll'l1allwvc (in op})osite phare to that or the top (liunml-wave) colltinues to lievelop regularly allli with uniform succession (ill the mean t.ime t.he microseismieit.y (lue t.o frost., as shown, start.~). in the early part or fliareh, however, wit.h tempcrature rca.ehing gradualiy vahlcr abovc zcro degrccs, the llm1ulation a.t thc hei~ht of ti25 loser eontinuously its unirormity of sueeession, its rhythm ir brcaking uv, and in the serond parto of thc mont.h, when temperaturc variations are exelusively a.bove zero !legrees, it shows only sporadie and inegular perturbations and at the emi slaekells (10wn into a. line without aplh·eeiable movementr. 'l'his mcans that t.he outer layer of the rocky-plain is now warmed up above zero !legrces and that. any valuable eont.rast agaillst. t.he liam 1ms eease(l. 5. 'l'o eonehllie t.hih note, the interest roused by this l)artieular spceimen of invest.igat.ed mieroseismicit.y allli by its unexpec-ted origin ir ulh1crlinc!l. both have bcen broug-ht t.o light t.hrough the valuable hell) of two instruments whirh alrcady permit.ted to look into the geodynamical behavior of a large liam and seismogra·1jh alhl tlle photo-elillograph. its vhysiral surroundillg: t.he the former has revcalc(1 thc phenomenon, t.hc latt.er has eontrilmte(l decisively t.o its exllla.nation. consù1ered in(livi!lualiy, t.he mieroshoeks triggen,(l by the frost are of a very limited intensity (on an averagc in thc order of 101u erg). it shollid not be overseen, however, that. in mountain areas they may originate in eme rvalls of thrce, four mont.hs and by t.ens of thousands. sinee each microshoek lh·oves the cxistcnre of a mierofi"arture in t.he roek, wit.h a very slow but eollt.inuous weakillg or the medium in whieh thc dam har bcen bllilt. )iohe about severe frost as a 'rhiggeiu,>g age:\t, etc. 5 11 an inlcnsificd, accu/'ale contl'ol o/ ihr r/m;/ic rcspoiisc o/ ihc mrdiumto which ille dam i,~ ancho/'etl and o/ ils l'arialionsin ow lime ilj, t1lr/'e/o/'(', an 1//'gcnl ncccsljily. refere~cetì (i) (',.>,t.oi }l., ~i!ga:\1 ::\l, sp.\j)ea::\l c., lui2, uehavior ,,/ ihe large dam al setere frust, "annali oli geofisica", xxv, 4, 5hj·525. (' ) ealm 1'., seade.> 'l c., '''<hl s"il" "'1'0.1" ""1/,,, d.ll, d"h,. "il" loro varia:;ione 'liel icmpu. "allunli (li gl'ujhica", xxii, l. {') caloi 1'., hi5g. sui periodi di oscillazio/le liberi! dei co/lci di to/a diga l' sulle loro re/azioni coli le l'arai/eristiche dar/iclle del calcestruzzo. "l; enprgia elettrica", xxxiii, 12, thc kushmur (tur'shiz) 1903 .!uni torbut-c hcidnriyeh (sollth) 1923 cnrthqllukes in centrnl khornssnn (irun) j. s. '1'chai,e:-ko ('l') receivell on selltembi'r 20th, 1972 su,iuialty. xi'w bibliographieal mlll fielll data ari' prt'st'nh,tl on two little·known earthquakes w]lieh occultpll in thc khora~sml province of iran. the kashmar {turshiz} eai'iltquake of 25 fli'ptember 1003 dpstroyp,1 part of the lli'escnt.day town of ka~hmar anrl some su!tou)lllinp: villap:cs and killell 200 people. the torllat·c lieirlari:reh earthquake of 25 )iny 1923 dcsti'oyctl comilletdy ;) yillagcs south of torllat·e iicidariych and killi'd 2,210 people. both these i'arthquakes and a few o1hcr slljuller on('s after 1900 lli'csent the chal'aderistics of bcing 10cflìcd on or near the nctive do· l'uneh fault, of having moderate magnitudes beiween il and 6. of being very shallow ant1 responsible for high intcnsitics restridctl to arcas less than lo km in maximum tlimension. riassl'nto. in qui'stn nota sono riport.a1i nnovi dati ra('colti bi· bliograficnnjcntc c in loco concernenti due terr(,llloti 110eo ,"onosduti, n\'· venuti nella provimia di khorassan (iran). il te1'l'elljoto (li kashmar (tur· shi?) dci 25 settemhre ion3 tlihtrusse parte della attuale chti~ ili khslllllar e alcuni villaggi circostanti l'ausando la morto ,li 209 lll'rhone. il tenellloto di torhat_e heitlariyeh del 25 j\faggio 1023 (listru~se completamente cinque villaggi n sud dclla eitt:\ llrovocando la morti' ili 2.219 pcrsone. entrllmlli i tenemoti eli alcuni altri d'intensitil minore llopo il 1900, )lrcscntllno ic seguenti cllratteristiehe: sono loeali?zati sulla o vicino alla fllglia attiva di doruneh; hanno una magnitull0 modcrata com)lrpsa fm ;) e 6; sono molto superficiali e di elevnta intensit:\: mcntre l'area mll['j'osismien interessata ò, nella sua massimh dimensione, in[erio!'c ai lo km. {*} engineering seismology. imperial collegc. london sw7 {englnnd}. 31j .1, i:\' t ronl."r:;t l o"ì l,l (;clil'ml \"t'i',\' litut, !ll'cd,,~~ infol'lljation has lx' l'n i1n('ov(ll '~' d t,o dat.e on the ma il,\' i!eli tl'llelive e:wt.lu i11 f1 kt'li k lltl\\' ll t o havc o('e,111'}'t·t l i n ihe k hol'a."fi:ui l'rovi lu'c (lf ti~ tn el. "hi.'i !:wk or in fol'ln u.ti u ll ii; il:utic ular ly ll<' ute fol' tlw l'cginni; fl,\\'ao)' from t ltc lal'i.'l.'s t puliulati o ll (,.,n trcs o r :\ias lt ad a mi ~il; h;j.pill· iii t. lt e no l'lh limi qain :u hi hirj: u u l i n i-it e litlllth , anl l g l'ca l-l." im pt"l los t! 1() a ~ i;(\ss jjumt, oc i-he ('a d .\" r c:.r iona l heihmi ei t y or c as t iran. tn th e na~ht,t~ bayaz :ìlhl gollltlj:ul lh\sinll whie h wel'e ii cy as1atml h,\' ihe c!1l't-hquakt! or hhik, ana il! tlt e ka .. lllnal' anll tol'h at -!' lieillariych l'ogiol!, i t b unly aftlll' ahol1t h)oo tha.t t.he dala 11 l'r'omes ~11{fi('icntly 1)l'li('ist, tll ile illierl ill sei,~mol,(! rt,ollic' anl\ lieill111ieity .'it1hlil',~. in t be latt.cl' l 't~ :..d oll in partieulal', rlc-lipite n, l'ceon!el"1 legelld w hi ch mcllt,io li s fl't' llll l.'lit tl c\, :l i; tatin g eai'u1 1iu :tkc>i ( ~) a lll i t,be iii'i:'sc iu'c ur f,he m :ljlll' lict ivc jh' i ' un~h f n.ult, on l)' i wii ilesll'll('t. ivt'f\: 11't.hq llakc>i arc k llown iii a n)' tlel.,ai! , th e ka s h m a l' (tul'sh iz) e:.ii"t. hj·lu a k c or .l!i03 a ll i i t,he turbat-e_ l1 (': ill;~l'i:r(!h (so ll t h) ea rt hq uake of 192:t thesl~ t-wo evcnt,<; thns t ~j.ke on a sjll' (' ial impol't. ... n<'c il! ~ t,lldit\!j u( l'~g-ion:ù se islllidt~·, : ~11i1 it hn'um~'s e~liential that they shullw he tloc'llme·n ì('ll hs ae('.nratd~· 11$ po,~.<;ible , bliog'l'aphical allrl fl eirl t h e present. llok .<;nmmal'i~efl .'io nw new hi11 atn on t.lw~c two eal'thquaktl,~ anrl on sollle kmhllel' cal'thqlmk(lj) in thc )'eg-ion. 'rl1(~ teet nni('s or tl w k ashmal'-tor hat-e h e ill:n i," c h r egiol i a rl:'ilom illa\cti b,\' th c noruneh fan lt" a majol' (l l~'nl(' l! t. i n t.ht> st,nwtm't> or iran, s e('olhl onl.\' in impol't.alh'e to tl w zag'wij j'thl'mt" in tlle ko llthwest of t,lw couutr y, in t-he regi 011 eoll >iitl el'ec\ h e re, th e nol'llnch :l<'ault il; lm)aiu,\' east·w()ljt, il! 11il'cet-ion anrl eollcave towal'l\fi thc sollt,h (i!'ig, l ), tt liepal'ate>i tl le kul le s11l'kh moulltail! l':\n:.rc i n t he nol't,1t from t,lw !\. avil'-(l x;lillll k o ll.->;in in t !w ijunth a h/l c l'ussc:> ove r most uf it-s le ng th ui:'ci 'li t, a tl ll t'out.c lllponu 'y lit'llinwnt fall fo l'nmt.i o ns. a rccellt fie lll ret'1j nn :daaall('c tl1 t lll' l'tl/!iol l fl1ll11 ll evi ,lcn~e that t ht, la·tt·s t, i li ~l ,la('t'mell ~ oli t.ht,_ fa 11 1t were ]li"etltlllli na llt ly yt'-rt.i ca-i ami iii tll e fl ellflcuc l']e\' :~t.in:.r t.h(~ ilurt.hcnl llluuntain hl o('k li" it.h n 'lsl't'l'ttu t ue " mwll .l.1( (t t:rm iiz) 1903 ,1:'; 0 n' iuuti-: ii klll all l re u (l'oi,;tii) bt. 3 1 l<'ig. ka 9td! ar·torliat.~: ll~iil.\ltl\eii remo:.;. l. rcgivlls ll huve 1500 m: 2. donm cll f:iuit : 3. l i agnitllli c vc i!lii t,fu rnc ntall.\' il cll!j'mi no!i c1lr ulq ullk es: -j . e picl' litl',.1 l'c~rjon o f flcs trud.iv(l 1 ~ :li' l-il'l uakc: lu.tehed, v ill llges nffcetcil ; f'roas· h:lldicii . vill llgcs co mlll ete l! .i estroyt'fl , tho ba,~ in (1 ! ), the readel' is ,,1.<;0 ['eiorre.t! tu wellman (l~ ) awl (bnsser ( ~) f.)]' more informatiull 011 tllo tect o[)icll oi thc region. 2, ic,\sibl\r ('rrnsiliz) eaji'i'llql'a kf. , 1903 2.1 duc'iii/!(' lifn l s7t(}('h iii 19o:! (/1/(1 1.903 a re\\' shoc.ks ll l'e kllo \\"1l fo i" t,he pre-in ll ll'iijuf.'nt~l l l wl'iod a s t!u>.r w\jl'o l"o)jo!'t,ed b y alnll1-1.o\' (1 ), the j'u~~ia.fi militlll'y f{o('f,oi' i·o~ ifl e.nto 32 .1. ~. tcnai.e:\ko in 'l'ol'bat-('· heillal'i.\'eh a.nd in chal'g'e o[ met.ereologieal, seismologie~1l a·nd intrlligmlf'1' information. 20 ,tnl'y, 1!j02: t;wo comeeutiye shoeks damagell ~ome honses n.bont '7 km nol'tlnhst oi tol'bat-e iieilbl'iyeh in un<1-e majan. the sho('ks were not feh, in torbn.t-e iieidariyeh (1); 10 ,tn.nluu'y, hjo:,; an eart.hquake oeenl're<1 n.t 13 10 18m ioeai time and was felt with intensity iv (rossi-forl'eh) in 'forua.t,-e heidal'iyeh. rt wa~ most. seyel't< in }lowietabn.d 28 km east oi '['ol'ha.t,-e l-il'itln.l'ireh, uut did not, ('anse any destnlctions (~); hl .tune, hl03: a slight shoek wlls fdt in tol'bat-e iieillal'iyeh (2); 20 .tune, hj03: a siight. shoek '\"a~ felt. in 'l'orlmt-n iieilla.riyeh ( ~ ); 21 june, 1!j03: two shoeks oeeuned at. ohh 138 m, the fh'st, 0i1e being of intensity hl (h ossi forl'eh) in '['ol'ha.t,-e l-ieillal'iyeh (2); 23 .tune, hl03: n.n 1'n.l'thqnake occurred a,t. :?3 h .t-8 m and was ielt with intensity y (hossi-forreh) in 'l'ol'bat-e heillal'iyeh, tht< inhahitant.s ilell t.o t.he st.l'eet,~ (2); 21 .tune, hl03: n. shopk was felt at 01 lo 03m (2). 2.2 j((!.shma.j" (tw'shiz) ca'i'thqwrj.e, l[jo,~ the ea,l't.hquake oee\1ltell 011 the 25 st<ptember, i!j03 at 02 10 31 m ioeai time u,lhi was tlesel'ilwll in <1etail by aimazov who visitel! the site shortiy after t,lte ev,mt, (:). lus rqlort, given here in c;rt61lso, eonst.itutes the oniy eyewitness a.(·('onut o[ the event. and is prolmuly very a·epurate on most. of it·s points. the })resent-day t,own o[ kashmal' was called '['nrshiz up to t.he lìrst llecades of t,he 20th centnl'y. "turshiz,l'el'sia. "\t 2h :n m an ea.rthqua.ke oeelll'l'ell in the form o[ several abrupt. shoeks o[ whieh up t.o 20 were eount.el\. the llireetion oi the eal'thquake in the generai opinion was from t,he nol't,hwest, it,s lluration 25-30 seeolllls, its intf'nhit,y (i) ix. 'l'lw en.rthqun.ke preeeded an undel'gl'ounll noise. after the ea,l'thqnake t.he water in t.he wells inel'en..<wtl, as wpll a·s in the qanats, espeeill,lly in t.he qanat. at, ]r'ru[nfill, :2 verst.s (2) [rom tlll'shiz, wlwl'e it was saill that the fiow of watel' tlouuled. out of 1200 houses in tlll'shiz (3) lmlf wel'e llestl'oyed, abont ·100 wt<l'e seyerely damaged, ami the l'est. were slight.ly tue ka~ r nran (tui\slliz) l\ld3 a~1l tonnat_jo; hejj)_\hi.y~:1i (~outl[) ecc. 33 rlama,ged, homcs of ulihakrrl anll mixl'd bri('b sul1'erpii mo,~t, h,nd tlrick wahf! malie of kiln hl'ickf! werc fr:wtnr('11 in pla('c~ but not dpst,roycd; in the mud houses generally the donwrl 1'00f8 whit'li wpi'p usually ma,de of unhakerl hl'ickf\ l'ollapf\('d, thr annual layr.'r of ('ia~' 1 81h'ea,r! on t,he roof anll which ilh'rea.'l('(1 ils thi('kncss 1111 lo l ., a['f\hill (4) heing pa,l't.ly l'csponsiblc for thif\ l'ollapse. 'l'ili' sout-hprll pal'tof t.he town anrl the noi't.hea,~tern f\uhur'bs were particularly dama).;pd, a8 well a,s t.he nearesivillages, 'rhe areaoi dcf\lrnption pxtrnr!('11 lo il, rarliuf\ of 20 vel'st,s wit-ir the l'elitre at 'l'111'shiz, in tlll' wp,~t thp dpstruct.ion cxt..enlling 1,0 3i) versts. 'l'lll'l'e i~ infol'mat-ion tlra,t tl\(' ~lrol'k wa-f\ {elt in bijislrul. \val1s eollapspl1 in ali directiollf\ (5), in sonw plar('s in one rlireet-ion, in othel' p!a('es in anotl\('l' rlil'ection, a,nrl pi~('whpl'(' yet in a,not.lrel' rlil'ection, hutncvel't.lrelesh, as if t.lw walls which 'n'l'p spal'ell were orienterl west or nort.hwcf\t, t.hus indicating tlu' dirpc1ion of t.he ea,l'tlrquake f\11ol'k. 'rhe anfile wilh l'(',~peet. to thp horizonlal of the fisf\1\res in the housef\ wcn, ap}lroximat('ly 90°. 'l'hp 1:.0\\'11 ih locaterl in a valley, in t·he north ti\(' monntain,~ are s vprf\t.~ ,hmy, anrl in t.he sout.h 20 verst-s; in t.he northern monntains q('cllltpd roel,falls of severa.! lnllll]retl pui]s (6). 'l'hl' 80il ih loess, in plar('f\ mixpd \\'itll c!ay. 2."; vi11ages were def\t.roy('rl 01' al1"eet~'rl (7), during t.he time tha,t. i stayel] in 'l'mshiz lo collcd informa,lion on thi8 ea,rthquake, from t.he 2flt.h of se]lt,pmber t-o the 7t-h octo!wi', t.here werc up to 40 fi1'oillhi sho('kf\. 'l'lrc f\t.rongcst o('('url'cll on t!w 30th sept.emher liear snnf\et; it. lastell about ti spcolld,~ and causerl thl' dcf\truction of some houses ili the to\\"n ami in f\omc 'villag'rf\; on that sa,me rla.y dnring the lastt.hree honrs there were six mori' shorks, (1) presuljlahly in the rossi folte! st'a!c; (2) 1 versi = 1.06 km; (3) in 1894 the town wil~ rleseribed a8 eovcrillg an areil of ilhout 1/2 km" and eontaining about 1200 hou8er (baumgarten, is!hi); (4) 1 arshin = 71.12 ciii. (5) 'l'hir ohvious!y rcfers to the town 01 'l'lll'rhiz, a!ld liot lo biji~\.[\!l; (6) i ]1\1(1 = ig.38 kg; (7) 'l'hci'e wel"e hetwecn 80 allli 120 villages in thc vkillity ()f turshiz, with an averag-e of ahout go holl~e~ ilei" villag-e awl a tota!ilopulation, ex· clurling: 'l'urshi?, of about 2·1,000 (billlmgarten, is!)()); (8) 'l'he po]julatioll of turshi? was about godo (biluiilgarten, ishg); (9) the iasi rellol'tc(1 aftershoek was feit in 'l'urshiz on ihe 17th ile· ccmhel" 1003 ahout 40 lllinutcs hefol"e 811nrise, .j. !<. tcil\lexko thollgh not strong (int,,'nsiticf\ from [i f t.o y), yet. distinetiy felt. (t.he ohsl'l"ver~ at thi~ tilllp were either in 1l10vement or ~itting on t.lw floor). alt.ogl'/her, tlw ~h()(~k~ during thi.~ (lay oeelllted eitjwr at. slinset. or ~hor1ly aft,er. 100 people were kille(l in 1'mshiz (8), two having dil'(l the 1irst. !lay of their inju1"les; /here ,,"ere more than 100 injtued. in tl)(' villages (7) oiw eountp(l !h.l killed alhl hil injml'd, jmt there ,,"ere surl'ly more. (y. ,\lmazov, direetor of the cent.er for melueai ohsl'tyation in 1'orbat-e fieillariyeh)". "turbl/t-e hei-r1ari!lrh, persia. at 2" :!8 m thcrc o('cultcll an emthqualw whil'h lammi 25 8e('01](ls. 1'lw liiredion of t.hc ca.ithquakc ,,"ai'! in the o]linion of moi'!t of the inhabitant~ from thc nortln\"e~t., and the illh!lli'!ity yi. the earthquake wa~ a.e.eolll}lanied hy a strong undcrgrolln(l ~ouwl ,,"hieh star/ed about 5 seconlls before the earthquake al](l produ('cll at 1irst t.hc impression of a liistant. thlln(lm' a.lul nl1nhle of a stormo ])ooi":"1 aihi "\"induws lmn,!!;"l'(l violently, !w(li'! swaye(l, a·nd the l'to('kery rauled. during this t.ime the car1h wai'! lllovin,!!;" in 1'nrshiz. of the shocks that followe(l in tlil"~hiz thc strongest oeelute(l 011 thc 30th spjjtl'll1[li'r. jt. w['i'! ali'!o fch in torhat-c lieidariyeh, but not. so ~trongl.)". dllring the night of the 10th t.o 11th oct.oher at a·bout 2l1' :-s'" there again occ'ultel! an pa.rthqllake of inteni'!ity ahollt. 1\' (9) (v. alnmzoy, dil"(l('t.ol· of 1:lw centre for ~le!li('al ob~crvation.~)". lt sl'mns likely t.1mt _-\inmzoy did not yi~it }lersonally thc yillagcs a.rol!lhi tlirshiz and that t1le arca of destruction whie1l 1ie mcntions shoulil be intcl1lrcl"ed a·s tlw ,h"ca ov('l" whieh villages were alteetell hy l!ama.ge of some sort allll'it. not. ne('essm'ily ~evere. the hollsl's in the rc',!!;"ion, bcin,!!;" cntirely of a(lobe awl snn-ilried bride, won1ll be nssllrell a.t vcr.)" low intemities, but the small numher of casualties iii tl]('se villages sllggest.s that. com]jlptely (,olhljlsed hollses were t.lw cxccption. 1'he easna.ity hgllre~ given are ]jrobab!y ai'! l'elia.bic ai'! eoulll ]lossibly hc obtained at thc time beeause alnmzov's mission in '['mshiz wa~ t.he oi"ganizatioll of ill('l!icltl relief allll aill from his hos}lital in torbat-e iicida.riyeh. sykei'! who visited the towll ill ]fio.! also note(l t.hat turshiz "wfierell considcrab!y dmin,!!;" thc ea.rthl!1hl.}w of 1!w3, whi('h fol"tunat.ely affected oniy a small arca" (11). ;{. tohtla'l'-e iielllariyelt (f;ot:tlt) eartliqu},ji:e, .i!.l23 tlw earthquakc oeelllte(l on t.he 25t.h may hl23 at 2:!1\ 21'" gmt (2\i may 1923 at 021\ 21 n1 loea! tilllc) a.mi i~ llottribut{;ll a· ma.gnitlllle l \'~il\i, ... ill~ lwtwn'n ,')" illld li (~, i) . tìu" fid(l stufli('~ 1'('110i'tci[ lwlow juwc csta blishcil that t,he cpiecnt l'al l'egioll whs ('(>lll'cl'c.1 about 8 km so llthw (>rt. of tol'bat,· e 1i1!irbll'iyeh. ('()nti'hl'~' to th c iihli('at,i()n~ oi ~cveml eal't.hqllake catalolinll'~ , [bit·he."l'jo( (in), l'clleatpi! h~' h(>zilnoy ("), allll ot.l ll'l's ], thc town it'4(> l! w;l ~ !lot. all'ecte!! allli .!ill ilot .~ ull' el· all~' ca'.m:\i t.ic.<;. !-fowcvcl', l~s 111 1\ e,"c llt. ii; :ùn·lul:.' knowil il ", iiic " torhat-e iieidar iyeh carthqu:lkf''', tilis il li nte w ill lh> rcklilwd ilere with ihe :hi.li tioi! of "80ilth" to itulicatl~ that· f,he earthqu:lkc :l ll't'c tt •• ! :i n'/.dull $tm ui of t,}1t~ towll. fig . .2 rulllòs o f qai .'e ii sow v1f.l.\~e de stroyi!:p l\l" til yo )!) 2 3 e aiitii· q.ua lie. 'l'he villn).:"c wa~ at the holltl1(j:l~t.cnl iimi1. (lf the eljic cutm l rcgioll all(l wa~ only }hll't.inlly <lcstl'oycù. th c hotl~(,11 whith sul'vivc<l and iii'c s till illhahited io.ji:1y ('h ii he seell in t.h o hn c k!{l'otm<l. the min s oi some of uw il clitroycl"1 v iuages are ijwi vi siblc to-day (~'ig. 2) allli man )" or t.ll e lil1i 'vivol'l! a l'e stili livin~ in t h(\ r4'i,.'ion ami cll.11 rcculln t the (11·t~lils 01 t hc evcnt. b.y cllissr c rl'l'(! uce l)(>twll(' n the .lilt cre llt villages :lllli compal'isun lrith the occas iom~1 coi!t<>m]>or.lj]" ljl'c ~s l'hljol't, t,hl' reliahili ty or t.hi ~ information w!\-" gcne11l11y iolllhl .t. s. tc ii.-i. le">ko to be high. the epi('cntral rl'hfìon was iocatcù along) a.nù south of, the j)ol'1ll1eh valllt, a.nù inciuùcd il yillagcf! which werc coml)letely dehtl'oyerl: ifoz sorkh (forme!'ly )iurtazayil'h), gujll (formcrly kuzan), khnrg, 'l'aghiabatl and kaj da.rakht. ("fig. 3). the rlestrllction dccrea.f!cii ]'apidiy fliway [rom thif! region: in buriabad, nlflony houses eollapscù, but the l\losq1h' still stal1lling to-llay suffered pmct.ically no liamage at. all, a.i1ii in qal'eh sow olliy one haif of thc houscs neerled reconhtruetion (fig. 2). in slmdmchr, oniy a. few houscs eollapseù l'ompletply evcn though the village was eventually rcsct. south of its originai l)osit.ion. casllaltics allli approximatc l)opula.tions of the time for thehe villagef!, a8 rememhcrcù by thc sun..-iving inh abitanh, were fios follows: boy. sol'l{1l (:ì\lurt.a.zavich) killci} 170 l)opuia.t.ion 300 guju (kuzan) 300 'too khlll'g 110 :300 ka.,j darakht 'to 175 bul'ìahatl 64 goo ' qai'eh sow 8 100 shaùmchr :37 650 'l'hc fio bove list is not eomplete as some of the .. -illageh al'ollll(1 the cl)icentra.l rpgion wcrc not vi,o,itcd a,ml othcrs l)roùuecd ilh'omif!tcnt ]'esuith whieh werc not considered sufficicntly rciùloblc. furthermorc, a few villages a.re to-ùa.y no ionger inhahiteù (eg. taghiabaù), anrl otllers 'which hea.r multiple na.mes (eg. iloz sorkh-murta.zavieh-sailmha.ll) may indicate tlla,t after 1923 somc villfloges wcre regrollpcd ra,tllcr tlmn rebuilt inrliviùually. in view o[ these considcrations, the offidai figul'e of :3,219 killell a.nd 170 severely injm'erl given at thc time by thc go\'crnor gencral sccmh ren..'jonahie a.11i1 moreover corresponds t.o thc impression gained from contl'mpor flo ry news report.s (l~). 'l'hc cxtent of the area. where ilama.ged occurrcrl can be dehncri quit.e accurateiy a nù docf! not cxcccii lo km fl'om ea.st to west all(i 5 km [rom north to south (figs. 3 al1l1 4). (jutsiùc thi,o, a,rea, damn.gc "\h1f! inf!ignific~mt or nil; z;a.nueh, zarmihr, z;allimbaù ami islmnda,rabad, sihmtell rcspectively liorthwcht, wi'ht, sout.hwest lmù 80uth slmdmehr a,nù ali lcss than lo km a.way irom thi,o, village, were 1)]·a.ctica.lly unaiiected. similarly, .alahahad, hituated about lo km solltheast of • 1 02 !;;i;l3 _ i. _ 5 ...----6 ,.., 7 .. /8 0-4 ' • .. fig. ::i el'ice!'tl\,\l regio", of tiie 'l'orbat-e liejl)ariyeii (~outtr) idz::! }~arrhqua"g. 1) villag() \lest.roycd or dmnagct1: 2) m l hut \lilinhahitpt1 io-rlay: 3) appl'oximate region of tol,hi rlcstrlll'tioll: 4) fault: 5) youngest fault it:lec: 6) ghcam 1wl1: il ~!asllihl-bif(lj:lllt1 ront1: xl 'innat. .j. ~. tciialesko qal'eh :\ow, and, a~ already mentioned, torba.t-e heidariyeh, ,,-ere lllldamage(l. j~xcel)t fol' a· hinj{le unconfirmed l'e]hll'ì. of lihflurefl neal' kaj d a mkht, no ea,~e,~ of w'ou1hl fmetlll'itlg of any kind were notiee(l at t.he time by the inlmbitants, vlow in the (janf\ì,s wah ilowever cotlsistent.ly repol'ted as haying significantiy inrrea,setl, in pal'ticular in burialmtl, 1-' oli i'iorkll, khurg and g-uju, in lskatldamlmd, a qanat ,,-hich !md been dry for some time befol'e t.he eart.hquake, ,~tart.c(! flowing a.gain. 4, ear1'iiquakr of tt-ir i) octmmr, 1962 .. \fter 1923, otlly one eart.hqlla.ke is known t.o haye rf\nsetl home de~tl'urtion in tlle rejlion,~ rro~hrd by the dol'uneh fault. it. orclll'l'ed on tlle 5 october, 1962 aihl the illhtrumelltal el)icentre was locatcd sont.h of the fanlt. and a.bout mid-wa.y between the 1903 ,w(! the 1923 event~ (fig. 1); the m agtlitude was est,imated at 5. 'l'he yillages of alllnatlabu.l, malnnalad aihl rizteh (!) flitllat.cd in a l'egion abont 10 km south of the instl'llmentally tletel'minetl epicentre \vere ~eyel'ely da,mage(!. 6 ]1eo]11e were kille(! and 3 injured, most of tlle polmlation fig. 4 1923 1.;r!cestrai. regiok see'\ fro~[ ~'iie ea~t. 'l'hc ncal'cst villagc (lch) is burialmd ami the furthest, kaj darakht. 'l'hc fauh 7.oilc lm~bc~ cast (l'i;.:ht) of thc hill hchind !caj daral,ht, and ('rossc~ lhe sllast darreh floollljlain towards the ccllll'c-l'ight ellgc o( the photogrnllh. th}: kasidfail. (tursi!iz) 1003 .ind torbat·}: iieidalhyeif (sol'tu) ecc. 3d imving apparently been forewhrnel1 by a fol'l'shock nhout 3 minutes before the lllain shock (a). 3. co::'\oludi::'\g rfdiarkf'> the 1903 all(1 1923 ea.rthquakcs have several points in colllllloli. they ea·eh afiectcii small arcas sihmt.ell a.long, antl illlmetliately south or, the noruneh fil.111t, imi l probably not exceeding ahont 10 km in large~t. ilimension. epieentra.j intensities were high in thefle arehs even though the magnit.utles were only moderate (;).5 to lì in the 1923 earthquake) illllieating very f\hallow hypocl'ntres. 'flw shoeks neal' 'forba.t·e heidariyeh in hi02 and 1903, and the earthquake or 19ti2 alf\o atieded very loealizetl a.rei "<;, anll the other instrl1mentally tletermined earthql1alces in the region are an or ma.gnit.lllle ti or i1ntler, ,dth one exception on 4 may 1940 '\'it,!l magnihuie ij.;) (fig. 1). :more ila.ta on ilre·1900 earthquakes is howeyer reql1irel[ to esta.blish '\'hether tllis tn1e of earthqlla-ke whieh seems to be c'lmmetel'istic fol' the region dul'ing the 20th eentnry alf\o llrevailell illlring eadier times. aokl'owledgeme.:\''l's 'l'he fielll '\'ork re})orted here wi "<; nnllerta.ken i>8 part of li joint. project between the university of mashatl antl the engineering heismology sect.iclll, imperial college, the lattei' being supported for this work hy il gmnt frolll the national environmental research coulh'il, i,ondon. :many tliseussions in the field ami offiee ,dth )1. berberian of the geologieal smvey of ira.n, allll il. belumdi or t.he uniypr.~ity of l\iaf\hall, with whom a generalreeonnaissanee of tlw eastern doruneh fault w11.s underbken il.lld pllhlishetl (f\ee her{')'ences), ami ,\"ith a. li iohajer·asjai, were aithe ol'igin or the st.ndy llresentetl here. reperekceh (i) auiazov v., hlu-l-. _ :;pi~nk zcllllettih~t'liii, h102. izy. l'ost. 'j'sclit.nd. i'ici8111. kolll, l, il. 112, st. l'etcl'sbul'g. e) al~iazov v., 1905. spiso!;: zellllctriasl'nii, 1903. by. posto 'l'spnh'al. :5cism. kolll, 2. pp. 14, 11-1-, 115, 157, 15,'\, 232. m. l'etcrslj1li'g'. 40 .1. s. 'rcilall;;~ko (' l a)iil[{'\~eys x. x., i!hi3. 'l'hl' huyin zarn (iran) l'artl/quakc 01 be})i('mbe/" iv/;:!: (/ jìelll repor/. "bull. r.eislll. sor. am.", 53, 4, pp. 705·740. (' l .bihiuhn:s x. x. awl 'l'cjialexko ,r. s., 1!)(j!j. the dahst·e })ayaz (lmu) cl1rlhqlwkl' dj aligu,r 31, 1905: a fie/ii repart. "13nll. scisill. ~o(·. ,\111,", 59, ,:i. pp. 17,31-17!j2. (~ ) ,\'l'las l';';i:ih, ldh2. ... lllas o/ l'(/tlh.(lu(/ke~ in the fsse. "arml. sci. l'::;::;u'' hhi2 {in russiiln}. (" ) (:a>,;sseh a., illud. 'l'hl' latge l'ar/hq/wkl's 01 iran allli i1i1'i1" ffl'%ljical trii/ne. "edog-ac (:('01. jllcv,", 59, pp. 4.,1-3·46fi, (' ) le i:)tra"gt: g., hlo;j. thc 1,(//1.118 01 lite e(!s(ern ()nliphntc. "fl'ilnk l'asi> !1ih! co. lm.· lund"ll, 31'(] illlpressioll j !j(j(j, pp. 530, (~ ) ):1ajlavi s., h172. .'i'he seihmicily dj iran, m. sco thcsis, l~nivcrsit.y or i,oil!!oil. (' ) ih:za;<;ov l a., lo;;!). 'l"edo/tics anli 80ismici/y 01 iho 'l"urlancllo-jllwra~s(u1 .l/ounllli/is, ",\('11(1. i:ici. u;-;;-;h", )!oseow lo;")!), pp. 246. (iu ) smllem; z., 1032. dii' erdbl'llon, in "b. gutenbcrg's t1an,lhuch ,ici" geophysk", 4, e,l. g. boi"ntrueger. ( li ) i:iylo:s l'. :'1.,101 j. -.t si.rlli journey in persia. "gcogr . .t.", 37, llp. j4!j165. ( i~ j 'l'ciiau::"ko .i. s., belll\erla~ .\1. ali!! belizadi h., 1!)73. hcismic ami geomorphic evidl'licc 01 reco/ii ae/illily di/. ilio ijor1tlleh fmllt (inm) . • 'l'(l('.tonophy.~ks . , ili 1h(lrs. (l~ ) 'ilie 'inles, lolhlon, allahalhul coltespoihlcnt .. 2 .iullc 1!j23 ali!! li .tuili.' l u23. ( lo) \velu1a" ii. \v., 1%6. actiue il"ti'iich ialdls 01 iron. atg/wllùlal/. allli l'akh/tw. "geol. h1i!l(ir(~hau", 55, pp. 716-735. t h e 8 e i s m o t e c t o n i c r e g i m e in g r e e c e a . g . galanopoulos ricevuto il 3 novembre 1966 summary. — evidence is presented which favours the theory that the earthquake activity in the area of greece should bo ascribed to horizontal currents flowing off to the side of the secondary sedimentary arc, from a minor mantle current rising under the primary volcanic arc of the southeastern section of the dinaric alps. riassunto. — vengono portate testimonianze in favore della seguente teoria: l'attività sismica in grecia si deve attribuire a correnti orizzontali che scorrono ai margini dell'arco sedimentario secondario, provenienti da da una corrente minore del mantello che ha origine al di sotto dell'arco vulcanico primario della sezione sud-orientale delle alpi dinariche. a c c o r d i n g to geologists ( a u b o u i n ( a )), the m o u n t a i n ranges of greece, v i z . the southeastern section of the d i n a r i c a l p s , k n o w n under the n a m e " h e l l e n i d e s " . , consist of f o u r m a i n units, i.e. of the eastern inner zones (eugeoanticline-eugeosyneline) and the western outer zones (miogeoanticline-miogeosyncline). t h e a l p i n e orogenesis started in the upper cretaceous proceeded successively f r o m the eastern inner zones to the western outer zones of the a l p i n e geosyneline, being bordered on a c o m m o n f o r e l a n d ( a p u ñ a n r i d g e ) w i t h the a p p e n i n e s and a c o m m o n hinterland ( r h o d o p i a n massif) avith the balkans. a f t e r the a l p i n e f o l d i n g and overthrusting of the surface layers, v i z . at the end of miocene started a dislocation of the eastern m e d i t e r ranean which created large f a u l t zones running in t w o p r e v a i l i n g directions: nnw-sse and wsw-ene. seisinological and geological studies r e v e a l the f o l l o w i n g features: on the faults along the fracture zone running in wsw-ene direction, the m o t i o n is right-lateral and downt h r o w is on the n o r t h side. on the faults along the other zone, running 110 a . g. g a l a n o p o u l o s in nnw-sse direction, the motion is left-lateral, and downthrow is on the southwest side ( a k i i1)). i n the case of intermediate shocks, the motion along the same fault zones seems to be opposite (hodgson and wickens ( u ) ) . a careful investigation of the earthquake history of greece over the 250-year interval, 1710-1959, revealed that 642 earthquake f o c i were active in the area limited b y 34° and 42° latitudes and 19° and 29° longitudes during the period considered. of the 642 active f o c i which released shocks of magnitude 5> 4.7 in the greek area 389 f o c i were active only once, 203 foci were active 2 to 5 times, 29 foci 6 to 10 times and 21. foci were active more than 10 times (galanopoulos ( ' ) ) . i n the interval 1960-1963 the number of earthquake foci which released shocks of magnitude > 4.7 in the area of greece increased b y 146. thus the number of earthquake foci which w e r e active in the area of greece during the period 1710-1963 total 788. t h e vast m a j o r i t y of these foci, i.e. 620 of them, were active in the period 1951 to 1963. this shows that in active regions, such as greece, even a short interval is good enough to reveal w i t h a high degree of readability the pattern of strain release. in v i e w of this and of the f a c t that the strain accumulated in any region might be releaved either suddenly b y almost one m a j o r shock or gradually b y a series of minor shocks, one might get a more realistic idea about the " q u a k e a b i l i t y " of a seismic region f r o m the distribution of the earthquake foci being denoted b y the "cumulat i v e magnitudes", which correspond to the sum of the strain energy released f r o m each one in a g i v e n period. t h e a d v a n t a g e of this technique is that it is v e r y simple and devoid of any assumption. a map showing the distribution of the earthquake foci denoted b y the cumulative magnitudes allows to see i m m e d i a t e l y in which regions the time of recurrence of strong shocks is relatively short. areas with a thin population of earthquake foci of minor importance do not indicate that these are not subject to strong shocks, but merely that the t i m e of recurrence of strong shocks in these regions is probably v e r y long. this holds especially for the borders of the cratons, i.e. of the forelands and hinterlands and of the intermediate crystalline masses. t h e clustering and the delineation of the earthquake foci, on the other hand, permit to draw some conclusions about the trend or the continuation of the deep seismotectonic lines which lack of surface expression. t o the best of m y knowledge no other kind of mapping of the earthquake a c t i v i t y allows such a conjecture. t i i e s e i s m o t e c t o n i c l i e g i m e i n g r e e c e 111 the delineation of the earthquake epicentres in the area of greece indicates that the northern anatolian dextral shear fault system continues through the trikkeri-canal fault into the gulf of patras and the fig. 1 seismotectonic map showing the large conjugate fault-system which presumably is responsible for the major part of the earthquake activity in the area of greece and the distribution of the earthquake foci during the period 1951-1964. shear fault zone of cephalonia-zante (s. p i g . 1). a s known, the cephalonia fault zone entertains one of the t w o permanent centres of higher 112 a . g. g a l a n o p o u l o s earthquake a c t i v i t y being assessed in the area of greece (galanopoulos (')). a d d i t i o n a l corroborative evidence f o r the extension of the northern anatolian fault system into greece is the migration of earthfig. 2 bathymetric chart showing the principal features of the submarine topography of the eastern mediterranean, after 0 . b. michailof (12). quake foci along the big dextral fault zone and the eastward offset of cephalonia, central greece, magnesia and mysia (northern part of asia m i n o r ) over a distance of about 20 km in reference to zante, peloponnesus, euboea and lesbos, respectively. the large shocks of t i i e s e i s m o t e c t o n i c l i e g i m e i n g r e e c e 113 m a r c h 18, 1953, in asia m i n o r and a u g u s t 9-12, 1953, on cephalonia, as w e l l as those of thessaly (1954, a p r i l 30-1955, a p r i l 19 and 21-1957 m a r c h 8) and the recent a c t i v i t y in the n o r t h e r n sporaden (1965, 45,00044,000 43,0000 j 1,000q 2,000m a g n e t i c gravity w + 1 ho t̂ o a> 1-60 £ 6 0 5 -mo î o 0 s o u t h 100 200 miles 300 400 north fig. 3 a north-south profile of magnetics, gravity, and bathymetry along the meridian 26° east from the african coast through crete to the central part of the aegean province (x. lat. 39°). after k . 0. emery et al. (5). m a r c h 4 and 1967, m a r c h 9), f o r m a striking e x a m p l e of migration of v e r y a c t i v e f o c i along the d e x t r a l shear f a u l t zone in question. t h e offset of cephalonia is p r e t t y well e v i d e n c e d b y the sharp e a s t w a r d b e n d i n g of the i o n i a n f a u l t escarpment between z a n t e and cephalonia (s. p i g . 2). this eastward bending is consistent w i t h the p r e v a i l i n g almost horizontal m a x i m u m tension in s 27 ° w direction in the area of greece (scheidegger ( " ) ) . t h e a v e r a g e rate of displacement is presumed to be of the order of 1 to 2 m m per year. t h e association of the ionian centre of higher earthquake a c t i v i t y w i t h the end of the big 114 a . g. g a l a n o p o u l o s shear fault zone and the sharp bending of the ionian fault escarpment indicates the existing relationship among them. t h e second large fault zone responsible for the earthquake a c t i v i t y observed in greece is the cretan furrow. the eastern wing of the furrow entertains the second large centre of higher earthquake a c t i v i t y asserted in the area of greece. the centre is associated with a fairly steep g r a v i t y gradient (s. f i g . 3). in a distance of about 100 km the g r a v i t y falls f r o m + 8 0 mgals to —100 mgals. e a s t w a r d of maleas cap the cretan furrow bends to the nnw direction into the gulf of argolis. morphological evidence and delineation of foci indicates that the fault zone continues through the trichonis l a k e and the gulf of a m b r a k i a into the valleys of drinos-vojussa r i v e r s and reaches into the coast of the a d r i a t i c sea. the t w o fracture zones that cross each other at the entrance of the gulf of corinth f o r m a large conjugate fault system responsible for the m a j o r part of the earthquake energy released in greece (galanopoulos (8)). a similar fault zone parallels the southern coast of rhodes, k a r p a thos and crete and crosses the ionian fault zone w e s t w a r d of crete. the postulation made b y a . philippson ( w ) that the ionian fault zone extends to the north-northwest up to the western coasts of the adriatic sea is invalidated b y the lack of earthquake foci in the structural trend considered n o r t h w a r d of the 38 ° n parallel. t h e relief of the sea-bottom, as revealed b y the isobaths (michailof (12)), shows a bifurcation of the ionian fault zone n o r t h w a r d of the parallel of the strait zante-cephalonia. t h e eastern branch trends parallel to and east of the strait of otranto. t h e western branch seems to be an extension of the east side of the gulf through of taranto. of the t w o branches merely the eastern one appears to be active. the measurement of g r a v i t y in the eastern mediterranean revealed the existence of a large zone of positive isostatic anomalies ( + 1 0 0 mgals) in the southern aegean sea and a long belt of negative isostatic anomalies (—140 mgals) bordering the island arc ivy thera-crete-rhodes. according to v . fleischer (6), the g r a v i t y anomalies in the eastern mediterranean indicate that " the ¡»went morphology of thin urea has probably developed from an uplifting of the sea bottom, of the aegean sea and a depression outside the island are khitera-crete-rhodes ,,. taking into account this notion, the overriding of the sediments of the eugeosyncline furrow of pindus over the miogeanticline ridge of gavrovo-tripolitza and the pattern of strain accumulation, viz. the distribution of the earthquake foci (delibasis and galanopoulos (4)), t i i e s e i s m o t e c t o n i c l i e g i m e i n g r e e c e 115 table i catalogue of maior shocks (m > 7) occurred in the area of greece since 1800. magnitudes prior to 1900 from maximum intensity and felt area. date time location depth magnitude 1805, july 3 36 ° n , 24 °e i 7.6 1810, febr. 16 — 35v2 ° n , 25 °e i 8.2 1846, march 28 15 36 ° x , 25 °e i 8.1 1846, june 10 15 37 ° n , 22 °e 11 7.2 1851, oct. 12 — 4 0 7 , ° n , 1 9 7 2 ° e 11 7.1 1s53, a u g . 18 8 v , 38v4 ° n , 231/j °e n 7.2 1856, oct. 12 3u 3572 ° n , 26 °e i 8.6 1863, a p r . 22 20 v , 3672 ° n , 28 °e i 8.5 1867, febr. 4 381/4 ° n , 207„ °e i 7.9 1867, sept. 20 03:15 361/2 ° n , 2274 "e i 7.6 1874, n o v . 16 — 36 ° n , 28 °e n 7.3 1886, a u g . 27 211/. 37 ° n , 2174 °e i 8.4 1887, july 17 774 36 ° n , 26 °e i 7.7 1897, may 28 2274 37v2 ° n , 2072 ° e i 7.6 1903, aug. h 04:32:54 36.0 on, 23.0 °e 100 8.3 1905, n o v . 8 22:06:12 40.0 ° n , 24.0 °e » 7.8 1910, febr. 18 05:09:18 36.0 ° n , 24.5 ° e 150 7.0 1911, apr. 4 15:43:54 36.5 ° n , 25.5 ° e 140 7.0 1912, aug. 9 01:29:00 40.5 ° n , 27.0 °e n 7.75 1926, june 26 19:46:34 36.5 ° n , 27.5 °e 100 8.3 1926, a u g . 30 11:38:12 36.8 ° n , 23.3 °e 100 7.0 1947, oct. 6 19:55:37 37.0 ° n , 22.0 °e n 7.0 1948, febr. 9 12:58:15 35.5 ° n , 27.0 °e 40 7.1 1953, march 18 19:06:14 40.0 ° n , 27.3 °e n 7.25 1953, a u g . 12 09:23:53 38.3 ° n , 20.8 "e n 7.1 1954, apr. 30 13:02:37 39.0 ° n , 22.0 «e n 7.0 1956, july 9 03:11:40 37.0 ° n , 26.0 °e n 7.8 1957, a p r . 25 02:25:36 36.5 ° n , 29.0 °e 11 7.1 119 a . g. g a l a n o p o u l o s the writer m a y hazard the theory that the earthquake a c t i v i t y in the area of greece should be ascribed to horizontal currents flowing off to the side of the secondary sedimentary arc, f r o m a minor mantle current rising under the p r i m a r y volcanic arc of the southeastern section of 18° 20° 22° 24° 26° 28° 30° fig. 4 pressure direction in the area of greece, as determined by livin constantinescu et al. (3). numbers are the identification numbers of earthquake mechanism solutions in the constantinescu's catalogue. the long heavy line represents the general trend of the alpine folding. the dinaric alps. t h e stress pattern at the foci of about t w o scores of greek earthquakes (s. f i g . 4), as revealed f r o m the fault-plane solutions (constantinescu et al. (3)), the association of the m a j o r i t y of the foci of m a j o r shocks ( m 7) with the p r i m a r y volcanic arc (s. f i g . 5 and 6), and the fact that the earthquake volume of the m a j o r shocks extends to t i i e s e i s m o t e c t o n i c l i e g i m e i n g r e e c e 117 depths f a r below the moho-discontinuity (galanopoulos (9)) g i v e e v i d e n t l y a fair support to the theory. t h e theory is in harmony with the suggestion made b y j. b . hersey (10) " that vertical movements fig. 5 distribution of the earthquake foci of major shocks ( m 5» 7) occurred in the area of greece since 1800 to 1900. gravity contours after v . fleischer («). of the earth's mantle, in addition to horizontal shearing stress, might better characterize mediterranean tectonics than horizontal shears alone •'. t h e spread of low mantle-velocities and " cobblestone areas " i.e. areas of many small blocks of fractured material, found so far in the eastern mediterranean seem to g i v e a f a i r l y strong support to the suggestion that " upwardmoving mantle assimilates some of the crustal 118 a. g. galanopoujjos rock, and causes other parts to move laterally under gravitational forces ''. fig. 0 distribution of the earthquake foci of major shocks ( m > 7) occurred in the area of greece since 1901 to 1966. gravity contours after v. fleischer (6). a c k n o w l e d g m e n t s . t h e research r e p o r t e d in this d o c u m e n t has been sponsored b y the " a i r f o r c e o f f i c e of scientific research " under c o n t r a c t a f 61(052)-803 t h r o u g h the " e u r o p e a n office of a e r o s p a c e research ( o a r ) " , " u n i t e d states a i r f o r c e " , as part of the a d v a n c e d research p r o j e c t s a g e n c y ' s p r o j e c t vela u n i f o r m . t i i e s e i s m o t e c t o n i c l i e g i m e i n g r e e c e 119 r e f e r e n c e s (*) a k i k . , study of love and rayleigh waves from earthquakes with fault plane solutions or with known faulting, part 2, application of the phase difference method, " seism. soc. a m . " , 54, no. 2, 529-558, (1964). (2) aubouin j., esquisse paléogéographique et structurale des chaînes alpines de la méditerranée moyenne, " geol. rundschau " , 53, lieft 2, 480-534, (1964). ( 3 ) constantinescu l . , rupreciitova l . si enescu d., mecanismul cutremurelor mediteraneene-alpine si implicatiile lor seismotectonice, " studii cercetäri de geologie, geofizicä, geografie " , ser. geof., 3, 173-191, (1965). (4) delibasis n . and galanopoulos a., space and time variations of strain release in the area of greece, " ann. geol. pays hellen. " , 18. 135-146, (1965). ( 5 ) emery k . 0., bruce c. heezen and t. d . allan, bathymetry of the eastern mediterranean sea, " dcep-sea research", 13, 173-192, (1966). (°) fleischer u., schwerestörungen im östlichen mittelmeer nach messungen mit einem aslcania-seegravimeter, " deutseh. hydrogr. zeit. " , 17. 4, 153-164, (1964). ( ' ) galanopoulos a., on mapping of seismic activity in greece, " annali di geoflsica", 16, 1, 37-100, (1963). ( 8 ) galanopoulos a . , the large conjugate fault system and the associated earthquake activity in greece, " ann. geol. pays hellen. " , 18, 119-134, (1965). ( 9 ) galanopoulos a . , evidence for the seat of the strain-producing forces, " a n n a l i di geoflsica", 18, 399-409, (1965). (1 0) iiersey b. j., sedimentary basins of the mediterranean sea, " submarine geology and geophysics " , publishers: butterworth & co. ltd., (london, 1965), 75-89. ( u ) hodgson j. h . , and a . j . wickens, coniputer-determined p-nodal solutions for the larger earthquakes of 1959-1962, " publ. dom. obs. " , 31, no. 5, 123-143, (1965). (12) michailof 0 . b., relief of the mediterranean, " basic characteristici of the geological structure, the hydrological system and the biologie of the mediterranean " , edition nauka, 10-19, (moscow, 1965). (1 3) philippson a., la tectonique de vegeide, " ann. geogr. " , 7, no. 32, 112141, (paris, 1898). (14) sciieidegger a . , the tectonic stress and tectonic motion direction in and europe western asia as calculated from earthquake fault plane solutions, " bull. seism. soc. am. " , 54, no. 5, 1519-1528, (1964). r i c e r c h e s u l l ' a g i t a z i o n e m i c r o s i s m i c a i n g e r m a n i a d u r a n t e l a s e c o j n d a g u e r r a m o n d i a l e ( * ) e. hardtwig v o r r e i q u i r i f e r i r e b r e v e m e n t e s u l l e r i c e r c h e e s e g u i t e i n g e r m a n i a , d u r a n t e l a g u e r r a , s u i m i c r o s i s m i , r i c e r c h e c h e a v e v a n o l o s c o p o d i u t i l i z z a r e , p e r q u a n t o p o s s i b i l e , la c o r r e l a z i o n e t r a l a s i t u a z i o n e g e n e r a l e d e l t e m p o e il m o v i m e n t o m i c r o s i s m i c o d e l s u o l o , a g l i e f f e t t i d e l l e p r e v i s i o n i d e l t e m p o . si d e v e c o m p r e n d e r e la s i t u a z i o n e n e l l a q u a l e m e t e o r o l o g i c a m e n t e si t r o v a v a a l l o r a la g e r m a n i a ; e r a l ' i n v e r n o 1 9 4 1 4 2 . t u t t o l ' o c c i d e n t e e u r o p e o , e c c e z i o n f a t t a d e l l ' i n g h i l t e r r a e d e l l a s p a g n a , e r a c o n t r o l l a t a d a f o r z e a r m a t e t e d e s c h e e l e s u e z o n e e r a n o s o r v o l a t e d a a p p a r e c c h i t e d e s c h i , c h e m u o v e v a n o c o n t e m p o r a n e a m e n t e c o n t i n u i a t t a c c h i a l l ' i n g h i l t e r r a . d ' a l t r a p a r t e b i s o g n a v a t e n e r c o n t o , i n m i s u r a s e m p r e c r e s c e n t e , d e g l i a t t a c c h i s f e r r a l i d a l l ' i n g h i l t e r r a c o n t r o l a g e r m a n i a . q u e s t e c i r c o s t a n z e r e n d e v a n o m o l t o p r e s s a n t e il p r o b l e m a d e l l e p r e v i s i o n i d e l t e m p o p e r la m e t e o r o l o g i a t e d e s c a , e s s e n d o n o t o c h e l ' e u r o p a c e n t r a l e d i p e n d e d a l l ' o c c i d e n t e p e r la s i t u a z i o n e g e n e r a l e d e l t e m p o . s i r i c o r s e ai s e g u e n t i e s p e d i e n t i : 1 . s i i s t i t u i r o n o i n n o r v e g i a e i n f r a n c i a d e l l e s q u a d r i g l i e p e r l e i n f o r m a z i o n i m e t e o r o l o g i c h e , i c u i a e r o p l a n i si s p a r g e v a n o o g n i g i o r n o v e r s o o c c i d e n t e e c o m u n i c a v a n o m e d i a n t e t e l e g r a f ì a s e n z a f i l i l e s i t u a z i o n i d e l t e m p o , t r o v a t e n e l l o r o v o l o , a b e r l i n o , al g r u p p o c e n t r a l e p e r i l s e r v i z i o m e t e o r o l o g i c o . q u e s t e com u n i c a z i o n i f o r m a v a n o la b a s e p e r la c a r t a m e t e o r o l o g i c a d a s e g n a r s i s u l l ' o c e a n o . è c h i a r o c h e d a l l ' a t t e n d i b i l i t à di t a l i c o m u n i c a z i o n i e d a l l a l o r o r e g o l a r e e s e c u z i o n e d i p e n d e v a l ' e f f i c a c i a d e l l a p r o g n o s i d e l t e m p o . 2 . s i u t i l i z z a r o n o c o m u n i c a z i o n i d e l l e n a v i . v a d a sé c h e g i u n g e v a n o i n p r i m o l u o g o c o m u n i c a z i o n i di s o t t o m a r i n i a l t i a l l o s c o p o . a n c h e o s s e r v a t o r i , s i n g o l a r m e n t e s t a b i l i t i , p r o v v e d e v a n o , a l m e n o di t a n t o i n t a n t o , la c e n t r a l e di o s s e r v a z i o n i . in q u e l l ' e p o c a il d o t t . b u n g e r s , d o c e n t e p r e s s o l ' u n i v e r s i t à di g o t t i n g e n , f e c e a l l e c o m p e t e n t i a u t o r i t à la p r o p o s t a di u t i l i z z a r e la m i c r o s i s m o l o g i a p e r l e p r e v i s i o n i d e l t e m p o . e g l i a v e v a u l t i m a t o p o c o t e m p o p r i m a l e s u e r i c e r c h e s u l l a d e t e r m i n a z i o n e d e l l a d i r e z i o n e d e l l e o n d e m i c r o s i s m i c h e . d a l p u n t o d i v i s t a di b u n g e r s n o n s e m b r a v a d i f f i c i l e t r o v a r e il p u n t o d ' o r i g i n e d e l m o v i m e n t o m i c r o s i s m i c o , p a r t i c o l a r m e n t e se la d e t e r m i n a z i o n e d e l l u o g o v e n i v a i n t r a p r e s a d a d u e p u n t i m o l to l o n t a n i f r a l o r o . l a p r o p o s t a d e l d o t t . b u n g e r s a v v e n n e n e l n a t a l e d e l 1 9 4 1 , m a i l p r o g e t t o v e n n e a c c o l t o s o l o d u r a n t e il 1 9 4 2 e p r e s o i n c o n s i d e r a z i o n e n e l l ' a u t u n n o d e l 1 9 1 2 . il p r o f . a n g e n h e i s t e r di g o t t i n g e n v e n n e i n c a r i c a l o d e l p i a n o e d e l l a d i r e ( 4 ) t r a d u z i o n e in l i n g u a i t a l i a n a d e l l a n o t a d i p a g . 9 5 . 1 2 8 e. h a r d t w i g /.ione g e n e r a l e d e l l e r i c e r c h e . l o c o a d i u v a r o n o i r e a s s i s t e n t i . f u m m o i n o l t r e c h i a m a t i i l d o t t . m i l l e r e i o , e n t r a m b i a d d e t t i a l s e r v i z i o m e t e o r o l o g i c o d e l l ' a r m a a e r e a . q u a l e d e l e g a t o d e l m i n i s t e r o e n t r ò n e l g r u p p o il d o t t . b e c k e r . i n o l t r e p r e n d e v a n a t u r a l m e n t e p a r t e , e c o n c o m p e t e n z a , a l l e r i c e r c h e il d o t t . b u n g e r s , c h e a l l o r a a p p a r t e n e v a p u r e t e m p o r a n e a m e n t e a l l ' a r m a a e r e a . n e l c o r s o di u n a r i u n i o n e a g o t t i n g e n si s t u d i a r o n o i p r o b l e m i e l a b i b l i o g r a f i a , i n q u a n t o i p a r t e c i p a n t i , p e r l e l o r o p r o f e s s i o n i c i v i l i , n o n e r a n o d e l t u t t o b e n i n t r o d o t t i n e l l a m i c r o s i s m o l o g i a . p o i v e n n e p r o g e t t a t o u n p i a n o d i l a v o r o e s t a b i l i t e d e l l e d i r e t t i v e . m e t a d e l l a r i c e r c a d o v e v a e s s e r e d i s t a b i l i r e se f o s s e p o s s i b i l e s e r v i r s i d e l l a m i c r o s i s m o l o g i a p e r l a c o n o s c e n z a d e l l a s i t u a z i o n e g e n e r a l e d e l t e m p o . i n c a s o f a v o r e v o l e si d o v e v a n o t r o v a r e m e t o d i p e r v a l o r i z z a r e , m e t e o r o l o g i c a m e n t e p e r l a v i a p i ù b r e v e , d a t i d i f a t t o o t t e n u t i c o n l a m i c r o s i s m i c a . 11 p r o b l e m a e r a q u i n d i a n z i t u t t o p u r a m e n t e t e o r i c o , m a d o p o u n p e r i o d o d i p r o v a d i c i r c a d u e a n n i si d o v e v a p a s s a r e a d u n a v a l o r i z z a z i o n e p r a t i c a . s i a d e t t o q u i s u b i t o c h e i l a v o r i e r a n o r e s i p i ù a r d u i d a c e r t e d i f f i c o l t à . a n z i t u t t o n o n c ' e r a a d i s p o s i z i o n e , q u a l i f o r z e a u s i l i a r r i e , p e r s o n a l e a b b a s t a n z a e s p e r t o ; i l a v o r i d o v e v a n o e s s e r e f a t t i , a n c h e n e i l o r o d e t t a g l i t e c n i c i , d a g l i i n c a r i c a t i s t e s s i . i n s e c o n d o l u o g o , n o n si p o t e v a a s s o l u t a m e n t e p r o c u r a r e m a t e r i a l e , p a r t i c o l a r m e n t e g l i s t r u m e n t i p i ù i d o n e i ; ci si d o v e v a a c c o n t e n t a r e di s i s m o m e t r i già e s i s t e n t i , s t r u m e n t i s p e c i a l i n o n p o t e n d o n é v e n i r e a l l e s t i t i n é p r o c u r a t i . q u e s t o f u a n c h e i l m o t i v o p e r c u i n o n si p o t è l a v o r a r e c o n l a s t a z i o n e a t e r n e di s t r u m e n t i , e s c o g i t a t a d a k i n g e i n t r o d o t t a n e l l a l e t t e r a t u r a da r a m i r e z . s i f o r m a r o n o 4 g r u p p i di l a v o r o : 1 ) g o t t i n g e n , c o n a c a p o a n g e n h e i s t e r , e i s u o i a s s i s t e n t i ; 2) s t o c c a r d a , d o v e h i l l e r d i r i g e v a l ' o s s e r v a t o r i o s i s m o l o g i c o d e l w i i r t t e m b e r g e d o v e i o d o v e v o e s e g u i r e l a p a r t e t e o r i c a d e l l a v o r o ; 3) b r u x e l l e s , d o v e i l d o t t . b e c k e r d i r i g e v a c o l d o t t . b o e r n e r l e r i c e r c h e p r e s so l ' o s s e r v a t o r i o s i s m o l o g i c o d i u c c i e ; 4 ) b e r g e n , d o v e i l d o t t . b u n g e r s , q u a l e m e m b r o d i u n o s c a g l i o n e di i n f o r m a z i o n i m e t e o r o l o g i c h e , l a v o r a v a a n c h e p e r i c o m p i t i d e l l a m i c r o s i s m o l o g i a . a d i s p o s i z i o n e c ' e r a n o : a g o t t i n g e n i l s i s m o m e t r o o r i z z o n t a l e c o s t r u i t o d a k i n g c o n gli a p p a r e c c h i d i r e g i s t r a z i o n e p e r p e r i o d i b r e v i ; a s t o c c a r d a i t r e s t r u m e n t i g a l i t z i n ( w i l i p ) c h e e r a n o c o n f r o n t a b i l i f r a l o r o c o n p e r i o d i p r o p r i d i 1 2 s e c . e r e g i s t r a z i o n e o t t i c a ; a b r u x e l l e s t r e c o m p o n e n t i g a l i t z i n di c a r a t t e r i s t i c h e d i v e r s e . b u n g e r s a v e v a p o r t a t o c o n sé a b e r g e n u n s i s m o m e t r o k r u g e u n a p p a r e c c h i o di r e g i s t r a z i o n e . c o n l ' a n d a r d e l t e m p o si a g g i u n s e a g o t t i n g e n u n a p p a r e c c h i o g a l i t z i n , t r a s p o r t a t o c o l à d a s t r a s b u r g o . s t o c c a r d a si d i m o s t r ò la s t a z i o n e m i g l i o r e , p e r c h é g l i s t r u m e n t i c h e v i si t r o v a v a n o e r a n o a d a t t i a i p r o b l e m i d e l l a m i c r o s i s m o l o g i a . i l p r o g r a m m a e r a c o s ì d i s p o s t o : a m p i e z z a e p e r i o d o d e i m i c r o s i s m i d o v e v a n o e s s e r e r e g i s t r a t e c o n t e m p o r a n e a m e n t e . n e i c i n q u e m i n u t i p r i m a e d o p o l ' o r a p r e c i s a t a , v e n i v a n o m i s u r a t e l e a m p i e z z e e i p e r i o d i m a g g i o r i , m e d i a t i e r i p o r t a t i i n u n d i a g r a m m a . d o v e v a c o s ì r i s u l t a r e u n a i m m a g i n e c o n t i n u a t a d e l d e c o r s o d e i m i c r o s i s m i . c o n t e m p o r a n e a m e n t e si d o v e v a d a r e u n ' i m m a g i n e d e l d e c o r s o d e l l a sit u a z i o n e m e t e o r o l o g i c a , c o n f r o n t a n d o i l b o l l e t t i n o m e t e o r o l o g i c o u f f i c i a l e p u b b l i c a t o d a l z/vi'.g. ( u f f i c i o c e n t r a l e p e r i l s e r v i z i o m e t e o r o l o g i c o ) . i n u n g i o r n a l e a p p o s i t o si i n t r o d u s s e r o l e p o s i z i o n i d e i n u c l e i di b a s s a p r e s r i c e r c h e s u l l ' a g i t a z i o n e m i c r o s i s m i c a in g e r m a n i a ecc. 1 2 9 s i o n e , l a d i r e z i o n e d e l l o r o t r a g i t t o , l a l o r o p r o f o n d i t à ( d e p r e s s i o n e ) , e , l à d o v e e r a p o s s i b i l e , l a m a r i n a d a g u e r r a c o m u n i c a v a l o s t a t o d e l m a r e e l ' i n t e n s i t à d e l v e n t o . s i r e g i s t r a v a n o l a s i t u a z i o n e e l ' u l t e r i o r e s v i l u p p o d e i f r o n t i . a b r u x e l l e s b e c k e r i n t r a p r e s e l ' i n d a g i n e d e l l ' i n f l u s s o d e i m a r o s i s u i n i . s. s i a q u i s u b i t o o s s e r v a t o c h e n o n si p o t è s t a b i l i r e u n s e n s i b i l e i n f l u s s o d e i m a r o s i s u i i n . s. a b e r g e n , b u n g e r s c h e f a c e v a i s u o i v o l i m e t e o r o l o g i c i p r o g e t t ò d i r e g i s t r a r e i m i c r o s i s m i — p e r c o s ì d i r e — n e l l u o g o d e l l a l o r o v e r a o r i g i n e , e d i m i s u r a r e c o n t e m p o r a n e a m e n t e d a l l ' a e r o p l a n o i p e r i o d i d e l l e o n d e m a r i n e , p e r t r o v a r e u n a e v e n t u a l e c o r r e l a z i o n e f r a e n t r a m b i i f a t t i . l a q u e s t i o n e , se i m . s. n o n d e r i v a s s e r o f o r s e d a o s c i l l a z i o n i d e i s i s t e m i r o c c i o s i , d o v u t i a l m o v i m e n t o o n d o s o , e r a a n c o r a a p e r t a . q u e s t e i m p o r t a n t i r i c e r c h e di b u n g e r s f u r o n o p u r t r o p p o i n t e r r o t t e e n o n p i ù r i p r e s e i n s e g u i t o a l l a sua m o r t e . i l s u o a p p a r e c c h i o v e n n e i n s e g u i t o , att a c c a l o e g r a v e m e n t e d a n n e g g i a t o d a a p p a r e c c h i i n g l e s i i n u n v o l o d i r i t o r n o d a l l ' i s l a n d a . n e l l ' a t t e r r a g g i o a b e r g e n l ' a p p a r e c c h i o p r e c i p i t ò e l ' e q u i p a g g i o t r o v ò l a m o r t e . b e n c h é b u n g e r s , i n i z i a t o r e d e l l e r i c e r c h e , f o s s e m o r t o , l e r i c e r c h e p r o s e g u i r o n o . a s t o c c a r d a , a c c a n t o a l l e i n d a g i n i s o p r a d e s c r i t t e , si a g g i u n s e r o a n c o r a l e seg u e n t i : a) 11 s i g n o r h i l l e r c o s t r u ì u n d i s p o s i t i v o c h e p e r m e t t e v a d i r e g i s t r a r e t u t t e e t r e l e c o m p o n e n t i s i n c r o n i c a m e n t e e d e s a t t a m e n t e s u u n a s t r i s c i a . f u c o s ì p o s s i b i l e r i c o n o s c e r e , s e n z a t a n t e d i f f i c o l t à , g l i s p o s t a m e n t i d i f a s e , i s o l a r e l e o n d e di r a y l e i g h d o v e a p p a r i v a n o , e d e t e r m i n a r e d a q u e l l e l a d i r e z i o n e d i p r o v e n i e n z a d e i m i c r o s i s m i . q u e s t e b r e v i r e g i s t r a z i o n i v e n i v a n o e s e g u i t e e d e l a b o r a t e d u e v o l t e al g i o r n o ( m a t t i n a e s e r a ) . c o n t e m p o r a n e a m e n t e si e s e g u i v a n o a g o t t i n g e n b r e v i r e g i s t r a z i o n i c o n l ' a p p a r e c c h i o di k r u g e si c o n f r o n t a v a n o c o n q u e l l e d i s t o c c a r d a d u r a n t e l e s e d u t e c h e si t e n e v a n o a l l ' i n c i r c a o g n i sci m e s i . b) l ' a n a l i s i a r m o n i c a d e l l e r e g i s t r a z i o n i d i m . s. c h e si e s e g u i v a n o i n i z i a l m e n t e di p r e f e r e n z a a g o t t i n g e n , si d i m o s t r ò t r o p p o d i f f i c o l t o s a . s i s c e l s e p e r c i ò u n ' a l t r a v i a p i ù b r e v e : l e o n d e p a l e s e m e n t e i n d i s t u r b a t e v e n i v a n o m i s u r a t e s e c o n d o i l l o r o p e r i o d o , c o n t a t e , e p o i di t u t t e e t r e l e c o m p o n e n t i si d i s e g n a v a n o i c o s i d d e t t i « s p e t t r i » . d a l l o s p a z i o di t e m p o d e l l a b r e v e r e g i s t r a z i o n e ( 1 / 4 o r a ) r i s u l t a r o n o d e l l e i m m a g i n i , i n q u a l c h e m o d o c a r a t t e r i s t i c h e , e s u l l a b a s e d e l l e e s p e r i e n ze, a p p r e n d e m m o c h e q u e s t e s o n o d a m e t t e r e i n c o r r e l a z i o n e c o n d e t e r m i n a t e s i t u a z i o n i m e t e o r o l o g i c h e . c) n e l l e t r e u l t i m e a n n a t e d e i b o l l e t t i n i m e t e o r o l o g i c i d e l l ' o s s e r v a t o r i o m a r i t t i m o di a m b u r g o , p u b b l i c a t i p r i m a d e l l a g u e r r a , q u i n d i d o c u m e n t a t i s i c u r a m e n te, si r i c e r c a r o n o c a r a t t e r i s t i c h e e t i p i c h e s i t u a z i o n i m e t e o r o l o g i c h e , c h e v e n n e r o c o n f r o n t a t e c o n l e r e g i s t r a z i o n i s i s m i c h e d i s t o c c a r d a . l ' i d e a f o n d a m e n t a l e e r a d i t r o v a r e d e l l e c o r r e l a z i o n i m e d i a n t e i l c o n f r o n t o t r a b o l l e t t i n o m e t e o r o l o g i c o e r e g i s t r a z i o n e , c h e si m a n i f e s t a s s e r o a n c h e e v e n t u a l m e n t e c o n s e g n i p u r a m e n t e e s t e r i o r i , n o n s p i e g a b i l i c o n t e o r i a a l c u n a . i o v o r r e i c o n s i d e r a r e q u e s t e r i c e r c h e a p o s t e r i o r i , q u a s i p i ù i m p o r t a n t i e f e c o n d e d e l l e i n d a g i n i r i c a v a t e i m m e d i a t a m e n t e d a g l i e v e n t i m e t e o r o l o g i c i a t t u a l i . s i d à a n z i t u t t o l a p o s s i b i l i t à d i i s o l a r e p e r q u e s t a v i a s i t u a z i o n i c a r a t t e r i s t i c h e ( p . e s . u n a d e p r e s s i o n e c h e i n t e r e s s a l ' i s l a n d a ) . 13(1 e . h a r d t w i c a g o t t i n g e n , a n g e n h e i s t e r e i s u o i a s s i s t e n t i i n d a g a r o n o il v e r o m o v i m e n t o d i u n a p a r t i c e l l a di t e r r e n o s o t t o l ' i n f l u s s o d e i i n . s. e t r o v a r o n o c h e d o v e a g i s c o n o p r o b a b i l m e n t e d e l l e o n d e d e l t i p o r a v l e i g h , la d i r e z i o n e d e l l e o s c i l l a z i o n i d e l l a p a r t i c e l l a di t e r r e n o c o i n c i d e c o n la p r o b a b i l e d i r e z i o n e d e l l ' o r i g i n e . a l t r i p r o g e t t i si d o v e v a n o r e a l i z z a r e , se si f o s s e d e t e r m i n a t o u n c e r t o p r o g r e s s o n e l l a « l o c a l i z z a z i o n e » d e i c e n t r i di b a s s a p r e s s i o n e . r i s u l t a t i : il m a t e r i a l e r a c c o l t o d a l l ' a u t u n n o d e l '42 l i n o al p r i n c i p i o d e l ' 4 5 a g o t t i n g e n , s t o c c a r d a e b r u x e l l e s , si p u ò b e n d e s i g n a r e c o m e il p i ù a b b o n d a n t e r a c c o l t o l i n o a l l o r a r e l a t i v a m e n t e ai n i . s. n a t u r a l m e n t e , ci si a t t e n d e r à c h e d a q u e s t o m a t e r i a l e si s i a n o p o t u t e t r a r r e c e r t e c o n c l u s i o n i p e r s c o p r i r e il s e g r e t o d e i n i . s. a l l ' i n i z i o d e i l a v o r i n o n ci si e r a i n t e n z i o n a l m e n t e f i s s a t i su n e s s u n a d e l l e d u e t e o r i e : n é i l q u e l l a di w i e c h e r t , c h e c a u s a d e i i n . s. s i a n o i m a r o s i , n é l a cos i d e l t a i p o t e s i d e l n u c l e o di b a s s a p r e s s i o n e , s o s t e n u t a s p e c i a l m e n t e f u o r i d i g e r m a n i a . p e r t r o v a r e u n ' a l t e r n a t i v a f r a q u e s t e d u e t e o r i e s a r e b b e n e c e s s a r i o p o s s e d e r e u n p r o c e d i m e n t o s i c u r o di l o c a l i z z a z i o n e . m a a p p u n t o q u e s t o m a n c a v a , p o i c h é u n a s t a z i o n e t r i p a r t i t a , c o n la q u a l e al p r e s e n t e si o t t e n g o n o n o t o r i a m e n t e g r a n d i successi n e l m a r e c a r a i b i c o e n e l l ' o c e a n o p a c i f i c o , n o i n o n l ' a v e v a m o , e p r o b a b i l m e n t e n o n s a r e b b e stata m o l t o u t i l e d a t e l e c o n d i z i o n i e u r o p e e a f f a t t o d i v e r s e , c i si i n d i r i z z ò q u i n d i v e r s o a l t r i m e t o d i . a n z i t u t t o si p r e s e n t a v a il p r o c e d i m e n t o di b u n g e r s c o m e p r o c e d i m e n t o di l o c a l i z z a z i o n e . c o m e n o t o , b u n g e r s m u o v e d a l l a p r e m e s s a c h e i m i c r o s i s m i c o n s i s t a n o n e l l a s o v r a p p o s i z i o n e di o n d e di r a y l e i g h c o n o n d e di l o v e e, d a t a q u e s t a p r e m e s s a , p e r v i e n e a d e t e r m i n a r e la d i r e z i o n e di p r o v e n i e n z a d e l l e o n d e . s i d e v e d i r e c h e il p r o c e d i m e n t o di l o c a l i z z a z i o n e di b u n g e r s n o n e b b e s u c c e s s o a l c u n o . t u t t e l e d i r e z i o n i si t r o v a v a n o , n e l l o r o c o m p l e s s o , a n o . a n c h e q u a n d o n o n v i e r a a l c u n m o t i v o d i s u p p o r r e ili t a l e d i r e z i o n e l ' o r i g i n e d e i m i c r o s i s m i . l a p r e m e s s a di b u n g e r s n o n è e v i d e n t e m e n t e e s a t t a . s i d e v e t r a t t a r e ili v e r i t à di t u t t ' a l t r o c h e di o n d e di r a y l e i g h e di l o v e . l ' i n s e r v i b i l i t à d e l m e t o d o b u n g e r s f u c o n s t a t a t a a n c h e r e c e n t e m e n t e da d ' h e n r y e m o r e l l i . è già s t a t o n o t a t o , c o m e si sia t e n t a t o a s t o c c a r d a di o v v i a r e a l l a d i f f i c o l t à d e l l a l o c a l i z z a z i o n e : r e g i s t r a n d o s i n c r o n i c a m e n t e le t r e c o m p o n e n t i e d i s o l a n d o i t r a t t i di o n d e di t i p o r a y l e i g h , c h e c ' e r a n o s e m p r e . l e d i r e z i o n i t r o v a t e c o n q u e sto p r o c e d i m e n t o i n d i c a r o n o b e n e i l c e n t r o d i b a s s a p r e s s i o n e , s e m p r e p e r ò in q u a l c h e p u n t o d e l l a c o s t a b a t t u t a d a l l a r i s a c c a . a s e c o n d a d e l l e c o n d i z i o n i m e t e o r o l o g i c h e si p r e s e r o i n e s a m e , p e r q u a n t o c o n c e r n e la d i r e z i o n e , l a c o s t a n o r v e g e s e , i n g l e s e , i r l a n d e s e , ed a n c h e la b r e t a g n a e b i s c a g l i a . in s p a g n a e d i n i s l a n d a il p r o c e d i m e n t o n o n a p p r o d ò a n u l l a p e r la p i c c o l e z z a d e l l e a m p i e z z e r e g i s t r a t e . t u t t i gli o s s e r v a t o r i e r a n o a l l a fine p r o p e n s i a d a m m e t t e r e ( b e n c h é fin d a p r i n c i p i o si f o s s e r o r i g o r o s a m e n t e i m p e g n a t i a n o n fissarsi su n e s s u n a d e l l e d u e t e o r i e ) c h e q u a n t o n o i r e g i s t r a v a m o c o m e m i c r o s i s m i , a v e s s e la p r o p r i a o r i g i n e n e l l ' u r t o d e i m a r o s i . q u e s t o p r o d u s s e n a t u r a l m e n t e u n a c e r t a d e l u s i o n e ; p e r q u a n t o i n t e r e s s a v a a l l o r a , s a r e b b e s t a t o i n c o m p a r a b i l m e n t e m e g l i o , se f o s s e r i s u l t a t o c h e si d o v e s s e r i c e r c a r e i o r i g i n e d e i m i c r o s i s m i in c o r r i s p o n d e n z a d e l l e z o n e di b a s s a p r e s s i o n e , p a s s a n t i s o p r a il m a r e . c i t o q u i a l c u n i f e n o m e n i c h e si s p i e g a n o f a c i l m e n t e c o n l ' i p o t e s i d e l l a r i s a c c a . i. — i m i c r o s i s m i d i v e n t a n o t a n t o p i ù i n t e n s i , q u a n t o p i ù l ' a n g o l o d ' i n c i d e n z a d e l l ' o n d a t a , c i o è d e l m o t o o n d o s o p r o v e n i e n t e d a l o n t a n o , si a v v i c i n a a d u n a n g o l o r i c e r c h e s u l l ' a g i t a z i o n e m i c r o s i s m i c a in g e r m a n i a ecc. 1 3 1 r e t t o . c i ò r i s u l t a a n c h e d a l l a c a r t a m e t e o r o l o g i c a , p e r c h é l e i s o b a r e s t a n n o p e r p e n d i c o l a r m e n t e a l l a c o s t a c o l p i t a . q u e s t o c a s o l o p o t e m m o r i p e t u t a m e n t e o s s e r v a r e s u l l a c o s t a n o r v e g e s e , e d a n c h e c o n l ' a u s i l i o d e l l e v e c c h i e e b e n a t t e n d i b i l i c a r t e m e t e o r o l o g i c h e d i p r i m a d e l i a g u e r r a . s i d e v e q u i n o t a r e c h e q u a l c h e c o s a n o n c a l z a p e r f e t t a m e n t e . ii m o t o o n d o s o n o n g i r a c o n t e m p o r a n e a m e n t e a l l e i s o b a r e , m a s e g u e c o n q u a l c h e r i l a r d o la r o t a z i o n e d e l l e i s o b a r e . il m a s s i m o d e i n i i c r o s i s m i a p p a r i s c e q u a n d o l e i s o b a r e h a n n o già o l t r e p a s s a t o l ' a n g o l o r e t t o r i s p e t t o a l l a c o s t a . l o s t e s s o f e n o m e n o si p o t è o s s e r v a r e s u l l a c o s t a s c o z z e s e , i r l a n d e s e e f r a n c e s e , b e n c h é l a n o r v e g i a a b b i a s e m p r e a v u t o u n a p a r t e t u t t a s p e c i a l e r i g u a r d o ai n i i c r o s i s m i . c o m e e s e m p i o t i p i c o c i t o la s i t u a z i o n e m e t e o r o l o g i c a d a l 2 8 f e b b r a i o a l 1" m a r z o ' 1 3 . u n c i c l o n e si t r o v a v a c o l s u o n u c l e o d i 9 8 0 m . b . d a v a n t i a l l a c o s t a n o r v e g e s e e si m u o v e v a , e n t r o l e 2 4 o r e , p e r p e n d i c o l a r m e n t e a l l a d i r e z i o n e d e l l a c o s t a , l i n o d a v a n t i a len i n g r a d o . i n i i c r o s i s m i i n i z i a l m e n t e p i c c o l i , a u m e n t a r o n o r a p i d a m e n t e , f i n o a u n v a l o r e m a s s i m o . t a l e v a l o r e m a s s i m o f u p e r ò r a g g i u n t o in u n m o m e n t o in c u i il n u c l e o d e l l a d e p r e s s i o n e si t r o v a v a già a v a n t i s u l l a t e r r a f e r m a , e p r e c i s a m e n t e in q u e l m o m e n t o , in c u i il m o t o o n d o s o d o v e v a b a t t e r e p e r p e n d i c o l a r m e n t e la costa. s e l ' i p o t e s i d e l n u c l e o c i c l o n i c o f o s s e v e r a , a n c h e p e r c i c l o n i e x t r a t r o p i c a l i , ciò n o n si d o v r e b b e v e r i f i c a r e . l ' i p o t e s i c h e si p o s s a t r a t t a r e di m i c r o s i s m i p r ò v e n i e n t i d a l g o l f o t i n n i r ò o d a q u e l l o di b o t n i a , n o n r e g g e , a v e n d o l o s t u d i o d e i m i c r o s i s m i d e l m a r b a l t i c o d i m o s t r a t o c h e q u e s t i , i n o g n i c a s o , s o n o d i p i c c o l o per i o d o e di p i c c o l a a m p i e z z a . q u i p o t r e b b e a v e r e la sua p a r t e l a p o c a i n t e n s i t à d e l v e n t o , c h e n o n p e r m e t t e la f o r m a z i o n e di u n p o d e r o s o m o t o o n d o s o , c o s i c o m e la s c a r s a p r o f o n d i t à in c o n f r o n t o c o n l ' o c e a n o a t l a n t i c o . a n c o r a d e l l ' a l t r o m o s t r ò q u e s t a s i t u a z i o n e m e t e o r o l o g i c a . e s s a si e r a p r e s e n tata q u a s i e s a t t a m e n t e , i l 2 7 2 8 n o v e m b r e 1 9 4 2 . i m i c r o s i s m i n o n r a g g i u n s e r o la stessa i n t e n s i t à , s e b b e n e il p e r c o r s o d e l b a s s o n u c l e o c i c l o n i c o f o s s e q u a s i l o s t e s s o , m a a g r a d i e n t e a n c o r a p i ù f o r t e : m i n . = 9 6 5 m . b . s t u d i a n d o il s i s t e m a d e l l e i s o b a r e , a p p a r v e c h e l e i s o b a r e si e r a n o t r o v a t e p e r p e n d i c o l a r i a l l a c o s t a s o l o p e r b r e v e t e m p o ; c h e q u i n d i l ' o n d a t a s o l o p e r b r e v e t e m p o si e r a s c a g l i a t a p e r p e n d i c o l a r m e n t e c o n t r o la r i p i d a c o s t a n o r v e g e s e , a c a u s a d e l l a c u r v a t u r a d e l l e i s o b a r e . il p e r c o r s o d e l v e n t o , c i o è , p i ù p r e c i s a m e n t e , il p e r c o r s o r e t t i l i n e o d e l v e n t o , e r a q u i n d i s t a t o p i ù b r e v e c h e n e l p r i m o c a s o . q u i . c o m e in a l t r i c a s i , a p p a r v e ( ile i m i c r o s i s m i s o n o t a n t o p i ù f o r t i , q u a n t o p i ù a l u n g o il p e r c o r s o r e t t i l i n e o d e l v e n t o p a s s a s o p r a al m a r e . a p p u n t o q u e s t a c i r c o s t a n z a , c h e si r i p r o d u c e s e n z a o b i e z i o n e di s o r t a s u l l a s c o r t a di c a r t e m e t e o r o l o g i c h e , n o n si p u ò b e n c h i a r i r e c o n l ' i p o t e s i d e l n u c l e o di b a s s a p r e s s i o n e . a t a l e p r o p o s i t o , r i s u l t ò d a l l ' o s s e r v a z i o n e c h e i m i c r o s i s m i si p o t e v a n o n o t a r e p r o v e n i e n t i d a l l ' i s l a n d a , p a r t i c o l a r m e n t e q u a n d o il v e n t o s o r v o l a v a l ' a t l a n t i c o q u a s i r e t t i l i n e a m e n t e p e r m o l t e m i g l i a i a di k m i n d i r e z i o n e n e / s o . c o m e u l t e r i o r e d a t o di f a t t o r i s u l t ò d a m o l l e s i t u a z i o n i m e t e o r o l o g i c h e , c h e z o n e di b a s s a p r e s s i o n e c h e si a l l o n t a n a v a n o , e r a n o a c c o m p a g n a t e d a m i c r o s i s m i , i cui p e r i o d i s e n s i b i l m e n t e i n g r a n d i v a n o . s i p o t è c o n t r o l l a r e c o m e l e p a r t i c o s t i e r e c o l p i t e p i ù v i o l e n t e m e n t e d a l l a r i s a c c a si a l l o n t a n a v a n o d a l l a s t a z i o n e d i s t o c c a r d a . c i ò c o n c o r d a c o n il n o t o f a t t o , c h e l e o n d e s i s m i c h e a l l u n g a n o i l o r o p e r i o d i n e l c o r s o d e l l o r o p r o c e s s o di p r o p a g a z i o n e . i n d a g a n d o q u e s t o f e n o m e n o i o h o s u p p o s t o la t e r r a c o m e u n m e z z o v i s c o e l a s t i c o ; n o i s p e r a v a m o a l l o r a a n c o r a di p o t e r 1 3 2 e . h a r d t w i c t r a r r e d e l l e d e d u z i o n i c i r c a l a d i s t a n z a d e i p u n t i d i o r i g i n e , d a l l ' a u m e n t o o gen e r a l m e n t e d a l l a g r a n d e z z a d e i p e r i o d i . p u r t r o p p o n o n r a g g i u n g e m m o a l c u n r i s u l t a t o , p e r c h é d a l m i o l a v o r o a p p a r v e c h e l ' a u m e n t o d e l p e r i o d o s o l o p e r p i c c o l e d i s t a n z e è s e n s i b i l e c o n l ' a l l o n t a n a m e n t o ; a g r a n d i d i s t a n z e i n v e c e m u t a a p p e n a a p p e n a . y a l e q u i u n a s p e c i e d i l e g g e p a r a b o l i c a . tipi — e s s e n d o r i s u l t a t a l ' i m p o s s i b i l i t à d i i n d i v i d u a r e e s a t t a m e n t e l a p o s i z i o n e d e i n u c l e i c i c l o n i c i m e d i a n t e i m i c r o s i s m i , c o n i m e z z i a n o s t r a d i s p o s i z i o n e , si d o v e t t e r o f o r m u l a r e p i ù m o d e s t e e s i g e n z e . s o c c o r s e q u i i l p e n s i e r o d e l l a tip i z z a z i o n e . è n o t o c h e il c a r a t t e r e d e i m i c r o s i s m i si r i v e l a m o l t o d i v e r s o a s e c o n d a d e l l a ' s i t u a z i o n e m e t e o r o l o g i c a d o m i n a n t e . a p p a r v e c o s ì p . e s . c h e i m i c r o s i s m i l a c u i o r i g i n e e r a p r e s u m i b i l m e n t e l a n o r v e g i a , m o s t r a v a n o o s c i l l a z i o n i r e g o l a r i sin u s o i d a l i c o n g r a n d i a m p i e z z e e p e r i o d i . n e l l e r e g i s t r a z i o n i d i m i c r o s i s m i d i a l t r a p r o v e n i e n z a , t a l i o s c i l l a z i o n i r e g o l a r i n o n c o m p a r i s c o n o a f f a t t o c o s ì d i f r e q u e n t e ; p e r l o p i ù , m a n c a n o . i m i c r o s i s m i d e l l a n o r v e g i a f o r m a n o q u i n d i u n t i p o a s é . d a q u e s t a si d i f f e r e n z i a n o n e t t a m e n t e i m i c r o s i s m i d e l l a s c o z i a e i n g h i l t e r r a . m a n c a n o q u i l e o s c i l l a z i o n i r e g o l a r i , i p e r i o d i s o n o i n g e n e r a l e p i ù p i c c o l i , l e r e g i s t r a z i o n i i n d i c a n o u n c a r a t t e r e m e n o u n i t a r i o . d a q u e s t i d u e t i p i si s t a c c a n o p a r t i c o l a r m e n t e l e r e g i s t r a z i o n i d e i m i c r o s i s m i d e l l a b r e t a g n a : i t r a t t i d e l l e o n d e s o n o q u a n t o m a i i r r e g o l a r i , s p e z z e t t a t i , n o n l i s c i , i t r a t t i s i n u s o i d a l i s o n o r a r i s s i m i . i m i c r o s i s m i d e l c a n a l e si r i v e l a r o n o a s t o c c a r d a c o n p e r i o d i q u a n t o m a i b r e v i . c a r a t t e r i s t i c i p e r l a l o r o p i c c o l a a m p i e z z a e r e l a t i v a g r a n d e z z a d e l p e r i o d o f u r o n o i m i c r o s i s m i d a l g o l f o d i b i s c a g l i a , e q u e l l i d a l l ' i s l a n d a . i n c o m p r e n s i b i l e r e s t a t u t t a v i a in e n t r a m b i i c a s i , i l f a t t o c h e i p e r i o d i s o n o t r o p p o p i c c o l i p e r p o t e r c r e d e r e s o l o a d u n a a t t e n u a z i o n e d e l l e o s c i l l a z i o n i . l a c o n t i n u a t a i n t e r p r e t a z i o n e d e i p i ù s v a r i a t i s i s m o g r a m m i r e n d e a c u t o l o s g u a r d o p e r d e d u z i o n i di c a r a t t e r e g e n e r a l e . l e d e d u z i o n i c i r c a le r i s p e t t i v e s i t u a z i o n i g e n e r a l i d e l t e m p o n o n si p r e s t a n o n a t u r a l m e n t e s o l o a d u n a i n e q u i v o c a b i l e i n t e r p r e t a z i o n e , i n a i n s i e m e ad a l c u n i p o c h i d a t i di f a t t o m e t e o r o l o g i c i , c o m e sing o l i v a l o r i d i p r e s s i o n e o d i v e n t o , p e r m e t t o n o di d a r e u n ' i m a g i n e r e l a t i v a m e n t e e s a t t a d e l l a d i s t r i b u z i o n e g e n e r a l e d e l l e i s o b a r e . è s o l o q u e s t i o n e d i e s e r c i z i o e d h a b e n p o c o a c h e f a r e c o n u n p r o c e d i m e n t o r i g o r o s a m e n t e s c i e n t i f i c o . i n q u e s t a m a n i e r a , i n s i e m e a s i n g o l i d a t i e ai c o s ì d e t t i « s p e t t r i » , a s t o c c a r d a si « i n d o v i n ò » r i p e t u t a m e n t e l a s i t u a z i o n e g e n e r a l e d e l t e m p o . m a g g i o r i p a r t i c o l a r i s u i t i p i d e i m i c r o s i s m i si t r o v a n o i n u n a r t i c o l o d e l c o l l a b o r a t o r e b e c k e r s u l l a r i v i s t a i d r o g r a f i c a t e d e s c a . a l t r i d u e n o t e v o l i f a t t i s o n o r i s u l t a t i d u r a n t e l a r i c e r c a . l ' u n o r i g u a r d a l ' i n f l u s s o d e i f r o n t i . 11 p r o c e d e r e di u n f r o n t e f r e d d o s u l l a t e r r a f e r i n a d e t e r m i n a u n r a f f o r z a m e n t o d e i m i c r o s i s m i . q u e s t o f a t t o si p u ò s p i e g a r e s o l o c o n l ' a z i o n e d e i m a r o s i , e p r e c i s a m e n t e p e n s a n d o c h e d i e t r o a l f r o n t e f r e d d o i l s i s t e m a d e l v e n t o sia d i v e r s o c h e n o n d a v a n t i a d e s s o . n o n si p o t è t u t t a v i a app u r a r e i n q u a l e m i s u r a v i a v e s s e r o p a r t e l a d i f f e r e n z a n e l l a l a b i l i z z a z i o n e d e l l ' a r i a , l a t u r b o l e n z a d e l m o v i m e n t o d e l l ' a r i a ; s a r e b b e r o p e r q u e s t o n e c e s s a r i e m o l t e r i c e r c h e a p p r o f o n d i t e sul l u o g o . s o l o q u e s t o si a p p u r ò d o p o i l 1 9 1 2 : c h e i f r o n t i f r e d d i r a f f o r z a n o i m i c r o s i s m i at l o r o a v a n z a r e s u l l a t e r r a f e r m a . a l t r o f a t t o n o t e v o l e , c h e t u t t a v i a i l s i g n o r b e c k e r p o t è o s s e r v a r e e n o t a r e s o l o u n a v o l t a i n f o r m a m o l t o i m p r e s s i o n a n t e , si r i f e r i s c e a l l ' i n f l u s s o di i r r u z i o n i d i a r i a f r e d d a s u i m i c r o s i s m i . i l s i g n o r b e c k e r h a p r o v a t o c h e u n a i r r u z i o n e d i a r i a f r e d d a d a l l o s p i t z b e r g e n v e r s o s u d s u d o v e s t h a p r o v o c a t o a d u c c i e f o r t i s s i m i m i r i c e r c h e s u l l ' a g i t a z i o n e m i c r o s i s m i c a in g e r m a n i a ecc. 1 3 3 c r o s i s m i . u n a z o n a «li b a s s a p r e s s i o n e i n t e r e s s a n t e c o m u n q u e i m i c r o s i s m i n o n v i e r a n e l l e v i c i n a n z e ; n e m m e n o a l l ' u r l o d e i m a r o s i s u l l e c o s t e , d a t a l a s i t u a z i o n e n e l s u o i n s i e m e , si p o t e v a a t t r i b u i r e t a l e e l l e t t o s u i m i c r o s i s m i . p u r t r o p p o s o n o a n d a t i s m a r r i t i i d a t i r e l a t i v i a q u e s t o f e n o m e n o . v o r r e i t u t t a v i a r i c h i a m a r e l ' a t t e n z i o n e s u l f a t t o c h e q u i s i a m o a l l a p r e s e n z a d i u n c a s o n o t e v o l e , e c h e v a r r e b b e b e n l a p e n a di s t u d i a r e a f o n d o l ' i n f l u s s o d e l l e i r r u z i o n i d i a r i a f r e d d a s u i m i c r o s i s m i . u n a l t r o c a s o , c o n s t a t a t o a s t o c c a r d a r i g u a r d a q u a n t o s e g u e : c o n v e n t o f o r t e d a n n e , s o f f i a n t e t r a s v e r s a l m e n t e s u l l ' a t l a n t i c o , c o n p e r c o r s o l u n g o , r e t t i l i n e o , t r a l ' i r l a n d a e la s c o z i a d a u n a p a r t e e l ' i s l a n d a d a l l ' a l t r a , si f o r m a n o m i c r o s i s m i a l u n g o p e r i o d o , d i i n t e n s i t à u n p ò o l t r e l a m e d i a . d a l p u n t o d i v i s t a d i e n t r a m b e l e t e o r i e , si p o t r e b b e p e n s a r e c h e i l s i s t e m a di o n d a t e b e n s v i l u p p a t o p e r i l l u n g o p e r c o r s o d e l v e n t o , c o l p i s c e i l f o n d o m a r i n o a p r o f o n d i t à d i v e r s e , d o v e o r i g i n a n o m i c r o s i s m i . m i s e m b r a q u i e s s e r c i l a p o s s i b i l i t à d i f a r v a l e r e e n t r a m b e l e t e o r i e . u n a z o n a d i b a s s a p r e s s i o n e c u i si p o t e s s e r o a t t r i b u i r e i m i c r o s i s m i , n o n v i e r a s t a t a n e i c a s i o s s e r v a t i . s o l o a l l a c o n f i g u r a z i o n e d e l l e i s o b a r e , q u i n d i a l s i s t e m a d e l v e n t o , s e m b r a d o v e r s i a t t r i b u i r e i l m o v i m e n t o m i c r o s i s m i c o d e l s u o l o . h o d a t o q u i u n a b r e v e r e l a z i o n e su a l c u n i r i s u l t a t i a p p u r a t i i n g e r m a n i a d u r a n t e l e r i c e r c h e . d a n d o u n o s g u a r d o r e t r o s p e t t i v o d o b b i a m o d i r e : c o n l e n o s t r e i n d a g i n i ci s i a m o a p p r o f o n d i t i n e l l a c o n o s c e n z a d e i m i c r o s i s m i , m a n o n a b b i a m o r a g g i u n t o n o t e v o l i s u c c e s s i p e r la l o r o p r a t i c a a p p l i c a z i o n e . l ' i p o t e s i d e l l a b a s s a p r e s s i o n e , s u l l a q u a l e t u t t i i n f o n d o a v e v a m o p o s t o l e n o s t r e s p e r a n z e , n o n si è d i m o s t r a t a u t i l e p e r l e c o n d i z i o n i e u r o p e e . m a l ' i p o t e s i d e l l ' u r t o d e i m a r o s i , c o n la q u a l e p o t e m m o s p i e g a r e q u a s i t u t t o , è m o l t o m e n o a d a t t a a l l o s c o p o di f a r e p r o n o s t i c i . s i s o l l e v a o r a q u i i l p r o b l e m a r e l a t i v o a l l a n a t u r a d e i m i c r o s i s m i . c o m e t u t t i s a n n o , i n a m e r i c a si s o n o v e n u t i a c o n o s c e r e f a t t i n u o v i . p o c h i m e s i d o p o c h e s i e r a n o i n t r a p r e s e i n g e r m a n i a l e r i c e r c h e s u i m i c r o s i s m i , a n c h e i n a m e r i c a si i n i z i ò l ' i n d a g i n e d e g l i s t e s s i p r o b l e m i q u a l e p r o g r a m m a d e l l a m a r i n a . r e l a t o r e e d i s p e t t o r e e r a g u t e n b e r g , s o s t e n i t o r e d e l l ' i p o t e s i d e l l ' u r t o d e i m a r o s i . g l i a m e r i c a n i f u r o n o p i ù f o r t u n a t i di n o i . s i d e v e l e g g e r e i l r a p p o r t o a m e r i c a n o , p e r v e d e r e in q u a n t a m a g g i o r e c o p i a si s i a n o i m p i e g a t i p e r s o n a l e , m e z z i t e c n i c i e d e n a r o , p e r d e t e r m i n a r e l a c o r r e l a z i o n e t r a m i c r o s i s m i e c i c l o n i . t a l e c o r r e l a z i o n e è m o l t o s t r e t t a n e l m a r e c a r a i b i c o e s u l p a c i f i c o . s i p u ò a m m e t t e r e , c o m e u n i v o c a m e n t e d i m o s t r a t o , c h e l ' o r i g i n e d e i m i c r o s i s m i là r e g i s t r a t i si t r o v a s o t t o a l c e n t r o d e l l a p e r t u r b a z i o n e v o r t i c o s a t r o p i c a l e . u n a l o c a l i z z a z i o n e d e l l a p o s i z i o n e d e l l e t e m p e s t e , c o n l ' a u s i l i o d e l l a t e r n a d i s t a z i o n i , s a r e b b e a l t r i m e n t i i m p o s s i b i l e . t a l i r i s u l t a l i a m e r i c a n i s e m b r a n o e s s e r e i n d i r e t t a c o n t r a d d i z i o n e c o n q u e l l i t r o v a t i in g e r m a n i a d u r a n t e l a g u e r r a . i o n o n v o r r e i p e r ò c r e d e r e a d a l c u n a c o n t r a d d i z i o n e . c r e d o p i u t t o s t o c h e si t r a t t i f o n d a m e n t a l m e n t e di 2 d i v e r s i r i s u l t a t i : da u n l a t o il m o v i m e n t o d e l s u o l o , c h e p a r t e d a q u e i p o s t i d e l m a r e c h e s t a n n o s o t t o a l l ' a r e a d e l l a b a s s a p r e s s i o n e ; d a l l ' a l t r o l a t o i m o v i m e n t i c h e n a s c o n o d a l l e c o s t e e d a l l e z o n e l i m i t r o f e , s o t t o l ' i n f l u s s o d e l l e o n d e m o n t a n t i d a l m a r e . i l f a t t o c h e q u e s t e s e c o n d e ci s i a n o , e d e b b a n o q u i n d i a g i r e , p e r l o p i ù v i e n e t r a s c u r a t o , f o r s e n o n s e n z a i n t e n z i o n e , e p e r c i ò n o n v i e n e n e m m e n o n o m i n a t o n e l l a l e t t e r a t u r a . 1 3 1 e. h a r d t w i g 11 m o v i m e n t o d e l s u o l o p e r e f f e t t o d e l l a r i s a c c a n o n si p u ò i n g e n e r a l e p a s s a r e s o t t o s i l e n z i o e s e n z a d i s c u s s i o n e . s e m a l g r a d o q u e s t o , a m e r i c a n i e t e d e s c h i p e r v e n g o n o a r i s u l t a t i d i v e r s i , a v v i e n e p e r c h é n e i d i v e r s i c a m p i p r e d o m i n a n o c o n d i z i o n i d i v e r s e . l e t e m p e s t e v o r t i c o s e t r o p i c a l i , b e n d e l i m i t a t e , p r o p a g a n d o s i c o n g r a n d e v i o l e n z a p r e v a l g o n o , c o m e c a u s a , s u l l a r i s a c c a , s i c u r a m e n t e s e m p r e p r e s e n t e , a n c h e se c o m e f a t t o r e s e c o n d a r i o . m a i n e u r o p a a v v i e n e i l c o n t r a r i o : l e z o n e d i b a s s a p r e s s i o n e , c h e si e s t e n d o n o su a m p l i s s i m e r e g i o n i , n o n c o n s e n t o n o u n a , e s a t t a l o c a l i z z a z i o n e d e l n u c l e o c e n t r a l e e p r o b a b i l m e n t e i p r o c e d i m e n t i m e c c a n i c i c h e p o r t a n o a l l ' e c c i t a m e n t o , n o n s o n o m o l t o e n e r g i c i . q u i p a s s a i n s e c o n d o o r d i n e l ' a z i o n e d e l n u c l e o c e n t r a l e , m e n t r e r i s a l t a n o m e g l i o gli e f f e t t i d e l l ' u r t o d e i m a r o s i . l a c o s t a e u r o p e a è q u a n t o m a i v a r i a t a , t a n t o v a r i a q u a n t o l ' u o m o e u r o p e o . c i ò c h e i l s i s m o m e t r o r e g i s t r a è u n g h i r i g o r o d e i p i ù d i v e r s i m o v i m e n t i , l e c u i s i n g o l e c a u s e n o n si p o s s o n o s c i n d e r e . s i t r a t t a di u n c o m p l e s s o d i f e n o m e n i c h e f o r m a n o u n t u t t o u n i c o ; d i q u i l a d i f f i c o l t à d i v o l e r t r o v a r e , i n e u r o p a , u n a c o n f e r m a a l l ' i p o t e s i d e l n u c l e o c i c l o n i c o . e n t r a m b i i f e n o m e n i — m o v i m e n t o d e l f o n d o m a r i n o e m o v i m e n t o d e l l a r i sacca — h a n n o p a r t e a l l a p r o d u z i o n e d e i m i c r o s i s m i . m a q u i n e p r e d o m i n a u n o . l à u n a l t r o . e n t r a m b i gli i n d i r i z z i s o n o l e c i t i , c i a s c u n o a m o d o s u o . e q u e s t o è b u o n a c o s a , p e r c h é si v e d e c h e c o n t r a s t i c o n s i d e r a t i p e r l u n g o t e m p o i n s o r m o n t a b i l i , u n b e l g i o r n o n o n a p p a i o n o p i l i t a l i . riassunto nel corso dell'unno 1942, dal ministero dell'aria tedesco del tempo ju presa la determinazione di studiare a fondo le relazioni fra microsismi e andamento del tempo. scopo delle ricerche doveva essere quello di investigare se i dati relativi ai microsismi potevano servire per la conoscenza della situazione meteorologica e, eventualmente, per la prognostica meteorologica. sotto la direzione del prof. angenheister di gottinga, nella primavera del 1942 iniziarono le ricerche in gottinga, stoccarda e bruxelles. non si fece uso del sistema d'investigazione a tre stazioni di ramirez. non furono fatte inizialmente ipotesi sull'origine dei microsismi ; benché, in seguito, tutte le registrazioni di microsismi potessero essere chiarite con l'ipotesi dell'urto dei marosi, dovuta a ii iechert. per la determinazione dell'origine della perturbazione, le tre componenti vennero registrale sullo stesso foglio, in modo da individuare le onde di tipo rayleigh. in base alla loro direzione di provenienza. l'origine venne individuata nell'urto dei marosi contro la costa. le ricerche furono completate con lo studio delle condizioni del vento e del mare. i dati vennero ricavati dulie carte del tempo per il periodo anteguerra. i tipi comuni di situazioni meteorologiche vennero comparati con i corrispondenti microsismi, conseguendo così una prima, anche se non molto rigorosa, classificazione dei tipi. lo studio continuò con l'elaborazione degli spettri dei periodi. le ricerche, a questo proposito, non ancora concluse, lasciano prevedere che a situazioni meteorologiche diverse corrispondono diversi periodi. ricerchi-: sull'agitazione microsismica in germania ecc. 1 2 5 non provala la dipendenza dal centro del ciclone, mentre alcuni massimi dei microsismi furono osservali, in singoli casi, quando il nucleo del ciclone era già nell'enlro-terra. il vento rinforza i microsismi, soffiando a lungo, nella slessa direzione: tale rinforzo si osserva pure quando un fronte freddo raggiunge il continente, quale chiara conseguenza delle condizioni del vento sulla « coda » del fronte, che rinforza i marosi. è opinione che anche l'afflusso d'aria fredda possa portare un rinforzo dei marosi. layout 6 correlation of geomagnetic anomalies recorded at muntele rosu seismic observatory (romania) with earthquake occurrence and solar magnetic storms iren a. moldovan1,*, anica otilia placinta1, angela petruta constantin1, adrian septimiu moldovan2, constantin ionescu1 1 national institute for earth physics, bucharest-magurele, romania 2 azel designing group s.r.l., bucharest-magurele, romania annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5367 abstract the study presents a statistical cross-correlation between geomagnetic anomalies, earthquake occurrence and solar magnetic storms. the working data are from: (i) geomagnetic field records from muntele rosu (mlr) observatory, and from surlari (sua) and/or tihany (thy) intermagnet observatories; (ii) seismic data for the vrancea source zone; and (iii) daily geomagnetic indices from the noaa/space weather prediction center. all of the geomagnetic datasets were recorded from 1996 to the present, at mlr, sua or thy, and they were automatically corrected using a labview program developed especially for this purpose, highlighting the missing or bad data. missing data blocks were completed with the last good measured value. after correction of the data, there were a number of issues seen regarding previous interpretations of the geomagnetic anomalies. some geomagnetic anomalies identified as precursory signals were found to be induced either by increased solar activity or by malfunction of the data acquisition system, which produced inconsistent data, with numerous gaps. the mlr geomagnetic data are compared with the data recorded at sua/thy and correlated with seismicity and solar activity. these 15 years of investigations cover more than a complete solar cycle, during which time the solar-terrestrial perturbations have fluctuated from very low to very high values, providing the ideal medium to investigate the correlations between the geomagnetic field perturbations, the earthquakes and the solar activity. the largest intermediate depth earthquake produced in this interval had a moment magnitude mw 6.0 (2004) and provided the opportunity to investigate possible connections between local geomagnetic field behavior and local intermediate seismicity. 1. introduction large networks of ground-based instruments [yumoto et al. 1995, 1996, 2001, yumoto 2004; e.g. the international realtime magnetic observatory network, intermagnet], and even some satellite-based systems [lagoutte et al. 2006, parrot et al. 2006], have been dedicated to the monitoring of the geomagnetic field over the last two decades. several studies have reported the identification of possible anomalous magnetic signals prior to earthquake occurrences [hayakawa and fujinawa 1994, stanica et al. 2006, stanica and stanica 2007, 2009, moldovan et al. 2009, yumoto et al. 2009, takla et al. 2011], or increased numbers of seismic events after or during magnetic storms [hayakawa et al. 2002, kessel et al. 2006]. anomalous changes in the geomagnetic field can occur before and during seismic events. as the lithosphere deforms, rock properties can change in response to changes in stress piezomagnetism or to changes in the distribution and composition of fluids in the crust [freund et al. 1999, pulinets and boyarchuck 2004]. the reported expected changes are in the range of a few nt. the problem of identification of seismo-magnetic effects in geomagnetic time series is complicated by the presence of disturbances, which are mainly due to irregular transient time variations that are generated in the terrestrial ionosphere and magnetosphere, and which also depend on the geological structure of the area. the purpose of this study is to examine the dynamics of geomagnetic field variations in relation to the vrancea (romania) crustal and intermediate seismic activity and to magnetic storms. the vrancea seismogenic zone is situated at a bend in the eastern carpathians, and it is bounded to the northeast by the east european platform, to the south by the moesian platform, and to the west by the transylvanian basin. the crustal activity located in the depth interval of 10 km to 40 km is weak, with mw <5.9 (3.4 in the study period) and an activity rate of 0.514526 for mw >3.0 [moldovan et al. 2008]. the intermediate depth seismic zone (60 km to 200 km) is concentrated within a very small area, which is 80 km long special issue: earthquake precursors 125 article history received august 14, 2011; accepted january 19, 2012. subject classification: magnetic anomalies; magnetic storms; magnetic and electrical methods; geomagnetic field variations; 04.06.11. seismic risk. and 40 km wide. the activity rate is 1.762380 for mw >5.0, with about three strong events (mw ≥ 7.0) per century. the largest known earthquake had a moment magnitude mw 7.7. the largest intermediate depth earthquake that occurred in vrancea in the study time interval had the moment magnitude mw 6.0 (on october 27, 2004) [moldovan et al. 2009]. this provided the opportunity to investigate possible connections between the geomagnetic field behavior and the local intermediate depth seismicity. the most significant geomagnetic anomaly was recorded at the muntele rosu (mlr) observatory in february-march, 2010, but was only followed by an mw 3.8 intermediate earthquake. as the largest moment magnitude of the crustal events that occurred in vrancea in the study period was mw 3.4 (only 36 events with mw >3.0, with three of these with mw 3.4 during the last 15 years: july 1, 2000, september 6, 2008, and april 28, 2009), we could not investigate any reliable correlation between the geomagnetic field behavior and the crustal earthquake occurrence. even if earthquakes with mw < 3.4 are crustal, they are not of sufficient strength to produce discernible changes in the geomagnetic field variations. for more than 10 years, the geomagnetic field was monitored in romania at only one location, which is situated at the edge of the vrancea seismogenic zone, the mlr observatory, and which has been related to crustal and intermediate depth seismicity. the mlr location was picked in such a way as to ensure the optimum positioning with respect to the vrancea seismic area (figure 1). moreover, this site was chosen so as to be distant from railroads and any other sources of noise, to avoid disturbing signals. this electromagnetic observatory consists of: (i) a three-axis magnetic field sensor (fluxgate; +/–70 nt measuring range; bartington instruments, uk); (ii) a data-logger acquisition module (six channels, 24bit resolution, programmable sample rate; bartington instruments); (iii) a computer for data storage and preliminary processing. the three-axis magnetic field sensor is a low-noise type, which provides superior characteristics; namely, a band larger than 2 khz, which is actually up to 3 khz; 15 pt rms/(hz^1/2) noise, and a lower-than-standard phase error. the parameters of the data-logger acquisition module are controlled by a software program with a sample rate of 0.2 samples per second and which displays the average values every 60 s. the magnetic equipment was placed in a specially designed, vibration-proof, nonmagnetic, thermostatic tunnel. during the first stages of its functioning (1998-2006), a manual system was used for both the data transfer and processing. special programs were created over the last 5 years [moldovan et al. 2010] to automate these processes and for correcting the already recorded data. this provided an opportunity to reconsider some of the conclusions from 126 moldovan et al. figure 1. distribution of the epicenters and hypocenters of the vrancea (romania) earthquakes. blue diamond, the geomagnetic mlr observatory; black square, the national data center from bucharest; red dots, the vrancea earthquakes. 127 previous studies [enescu et al. 1998, 1999a,b, 2001) that had incorrectly classified all geomagnetic variations as seismomagnetic anomalies related to the occurrence of crustal and intermediate depth vrancea earthquakes. indeed, some of these were due to missing data or to solar storms. the main difficulty in achieving an automatic supervision system for zones characterized by high seismic risk consists both in the monitoring of the values for representative parameters over long enough periods, and in the analyzing of their earthquake-preceding values to identify changes, and to calibrate the reading scale according to these changes and to the magnitudes of seismic events. it is important to note here that using one geophysical parameter (the geomagnetic field in our case) is not sufficient to obtain a reliable earthquake forecasting method. only by accumulating data obtained by means of more and more complex monitoring of the environment [gheorghita et al. 2010], as well as by using an adequate analysis method, can this lead to improvements in earthquake prediction activities and to more efficient civilian protection. 2. the data the present study used the following working data: (i) the seismic data for the vrancea source zone, taken from the seismic bulletins of the national institute for earth physics; (ii) the geomagnetic field records (1996-present) made at mlr national institute for earth physics observatory, and at the surlari (sua) and tihany (thy) intermagnet observatories (see table 1); (iii) the daily geomagnetic indices, kp, from the national oceanic and atmospheric administration (noaa)/ space weather prediction center. to discriminate between local (tectonic) and global (solar) phenomena, the geomagnetic data from mlr observatory are compared with the data recorded at the sua and thy geomagnetic reference stations, which are located outside the epicentral region. these recordings were provided through the intermagnet project. the geomagnetic data were also correlated with the global geomagnetic indices, kp, which are represented as the sums of the 3-h kp values for single days (rkp). 3. data corrections in this section, we emphasize and discuss some misinterpretations that were included in the study of enescu et al. [1998], where missing data blocks within datasets and solar storms with geomagnetic signatures were interpreted as anomalous variations in the geomagnetic field, and were consequently falsely identified as seismo-magnetic anomalies. this point of the study is necessary, because the cited studies gave unrealistic correlations between geomagnetic anomalies and earthquake occurrence. these apparent correlations provided a success rate for earthquake prediction through geomagnetic anomalies of about 92%, which is far from a realistic statistic, especially for intermediate earthquakes with magnitudes that do not exceed mw 5.5. the first source of false magnetic anomalies identified prior to an earthquake occurrence was a bad and inconsistent dataset that was affected by many gaps, some of which lasted for whole days. these gaps introduced leaps in the data that can be interpreted as precursory anomalies. this situation occurred because until 2006 the data recording and processing were conducted manually, and were not corrected for missing data. this stimulated us to develop a labview program that automatically corrects the datasets, highlighting the missing or bad data. using this program, we reprocessed all of the recordings, and all of the datasets were automatically corrected by completing the missing data blocks with the last good existing values. the variations in the sum of the geomagnetic indices kp (rkp) were also represented together with the time-variation diagrams of the geomagnetic components (x, y, z) at mlr observatory. the second source of interpretation errors were variations induced by solar storms that were identified by enescu et al. [1998, 1999ab, 2001] as anomalous variations and were then falsely characterized as seismo-magnetic precursors. until 2004 [enescu et al. 2004], the geomagnetic data did not correlated with solar activity or weather conditions, which can strongly affect the data measured. the correlation of the recordings from mlr observatory with the recordings from sua or thy observatories and the kp indices show that many so-called seismo-magnetic anomalies that were previously identified were only normal influences of solar storms on the geomagnetic field. table 2 provides the parameters of the vrancea earthquakes with magnitudes mw ≥3.5 that occurred in the period of 1997 to 1998 and that were correlated with the geomagnetic field by enescu et al. [1998]. these data are reprinted from the romanian earthquake catalogue [see oncescu et al. 1998, updated], which is compiled by the seismological department of the romanian national institute of earth physics. table 2 shows the time and date when these earthquakes occurred, the geographical coordinates of the epicenters (latitude, ̊ n; longitude, ̊ e), the correlation of geomagnetic anomalies, earthquakes and solar storms code of the magnetic observatory latitude (˚n) longitude (˚e) altitude (m asl) mlr 45.49 25.95 1,360 sua (intermagnet) 44.68 26.12 84 thy (intermagnet) 43.10 17.54 187 table 1. the locations of the magnetic observatories used in the correlation study. moldovan et al. 128 no. date (dd.mm.yyyy) hour lat. (˚n) long. (˚e) depth (km) mw magnetic components affected by precursory anomalies tpb (after [1]) qb (after [1]) date of anomaly (after [1]) rkp for 1 day start/finish date of revised anomaly shape of revised anomaly 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 01.01.1997 1:44 45.7 26.6 141 4.9 bx , by , bz 7 v.g. 25.12.1996 no recorded anomaly – there were 5 days of missing data 2 18.12.1997 23:21 45.5 26.3 134 4.1 bx , by , bz 7-8 g 10-11.12.1997 22/20 no recorded anomaly – there were 6 days of missing data 3 14.01.1998 5:02 45.7 26.6 145 4.0 – by , bz 5 v.g 9.01.1998 15 9.01.1998/ 14.01.1998 (by) by v-shaped/bz step-type 4 19.01.1998 0:54 45.6 26.7 105 4.0 – by , bz 9 v.g. 9.01.1998 15 -“-“5 31.01.1998 21:14 45.5 26.3 136 3.6 – by , bz 21 v.g. 9.01.1998 15 22.01.1998/ 28.02.1998 (by) by v-shaped – earthquake possibly triggered by a kp = 19 magnetic storm on 30.01.1998 6 19.02.1998 14:35 45.7 26.7 132 3.7 bx , by , bz 1 v.g. 18.02.1998 25 no recorded anomaly – there were 3 days of missing data – earthquake possibly triggered by a kp = 25 magnetic storm on 18.02.1998 7 28.02.1998 8:37 45.4 26.2 139 3.6 bx , by , bz 9–10 vg. 18.02.1998 25 no recorded anomaly – there were 3 days of missing data 8 06.03.1998 20:28 45.6 26.4 151 3.7 bx – 3 uns. 03.03.1998 20 no anomaly 9 13.03.1998 13:14 45.6 26.4 155 4.7 bx , by , bz 3-4 g. 10.03.1998 26 no visible anomaly. earthquake possibly triggered by a kp = 26 magnetic storm on 11.03.1998 10 09.04.1998 10:25 45.4 26.4 133 3.8 bx , by – 7 s 2.04.1998 <15 12.03.1998/ 9.04.1998 (by) by v-shaped – the earthquake occurred exactly at the end of the anomaly, during a small kp = 15 magnetic storm 11 14.04.1998 1:03 45.7 26.6 147 3.8 bx , by – 12 s 2.04.1998 <15 -“-“12 23.04.1998 6:37 45.8 26.7 90 3.8 bx – bz 5 g 18.04.1998 20 18.04.1998 (by and bz) by and bz step-type – earthquake possibly triggered by a kp = 25 magnetic storm on 23/27.04.1998 13 27.04.1998 9:31 45.7 26.5 155 3.7 bx – bz 9 s 18.04.1998 20 18.04.1998 by and bz step-type – earthquake possibly triggered by a kp = 25 magnetic storm on 23/27.04.1998 14 04.05.1998 16:10 45.7 26.5 139 4.0 bx , by , bz 2 v.g. 2/3/4.05.1998 36/35/3 8 magnetic storm and not a precursory anomaly earthquake possibly triggered by a kp >35 magnetic storm on 2/5.05.1998 15 02.06.1998 4:49 45.6 26.5 110 3.7 bx – 3-4 s 29/30.05.1998 24/23 no recorded anomalythere were 2 days of missing data; the earthquake occurred 4 days after a kp = 24/23 magnetic storm on 29/30.05.1998 with a possible triggering efect 16 03.07.1998 6:14 45.7 26.8 133 4.2 bx , by , bz 8 v.g. 25.06.1998 18 25.06.1998 bx, by and bz step-type changes. the anomaly was recorded during a magnetic storm that occurred on 24/25/26.06.1998 with kp = = 21/18/29 17 27.07.1998 15:02 45.7 26.5 132 4.4 bx , by , bz 4 s 23/24.07.1998 29/25 no visible anomaly the earthquake occurred 4 days after a kp = 29/25 magnetic storm on 23/24.07.1998 with a possible triggering effect 18 24.08.1998 23:27 45.6 26.5 152 4.0 bx , by , bz 4 v.g. 20.08.1998 22 20.08.1998 bx, by and bz step-type changes. the anomaly was recorded during a magnetic storm that occurred on 24/25/26.06.1998 with kp = = 21/18/29 19 03.09.1998 13:42 46.8 26.4 25 3.7 bx , by , bz 5–6 v.g. 26/27/28/29/3 0.08.1998 29/42/2 9/18/20 no visible precursory anomalies; days of magnetic storms [1] = [enescu et al. 1998]; v.g. = very good; g = good; s = satisfactory; uns. = unsatisfactory. table 2. seismological parameters and electromagnetic precursory data of all of the vrancea earthquakes of magnitudes mw ≥3.5 that occurred in the period investigated by enescu et al. [1998]. 129 depths, h, of their hypocenters, and the magnitudes, mw. the components of the geomagnetic field (b), which manifest precursor perturbations, are also given in table 2 for each earthquake, as they were interpreted by enescu et al. [1998]. the approximate values of the precursor time, tpb, were taken from the study of enescu et al. [1998], and table 2 includes our assessments of the quality, qb, of the precursor perturbations, which are of course of a subjective nature. the precursor times were measured from the onset of a magnetic anomaly to the moment at which an earthquake occurred. enescu et al. [1998] showed that significant magnetic anomalies (perturbations) appeared prior to earthquakes of magnitudes mw >3.5 during their period of study from 1996 to 1998. as indicated in table 2 (columns 7-10), and as stated by enescu et al. [1998], for 10 out of a total of 19 earthquakes of mw >3.5 that occurred in their study period, all three of the magnetic components were disturbed by anomalies; in seven of the other earthquakes, only two components were affected, while in the last two of the earthquakes (of threshold or close-to-threshold magnitude, mw 3.5), anomalies were found in only one component, and even these were doubtful. it is important to note that precursor anomalies were very evidently highlighted even before the crustal earthquake of mw 3.1 that occurred within the period of their study, namely the seism of september 3, 1998 (see table 2). enescu et al. [1998] mentioned that probably these anomalous changes were not seen in all cases due to physical-mechanical events produced in areas where the earthquakes were in preparation, and due either to problems of the instruments or to some geomagnetic storms. the final conclusion of enescu et al. [1998] was that it can be considered that in 92% of cases, these vrancea earthquakes were preceded by magnetic anomalies (perturbations) that can be regarded as their short-term precursors. this percentage appears to be well above any realistic and reliable statistic. as shown in columns 11-14 of table 2, during most of the periods when anomalous precursory magnetic variations were mentioned, there were magnetic storms with rkp >20, due to solar effects or to periods with missing data. from the 19 classified precursory anomalies, five were a consequence of magnetic storms, and five were due to days of missing geomagnetic data. only nine of these anomalies were found to be possible precursory anomalies, which thus decreases the probability of correlation of earthquake occurrence with geomagnetic anomalies from 92% to only 45%. this percentage relates to the strict case of the year 1998, under study here. figures 2-5 show the most interesting and representative examples of anomalous variations of the geomagnetic field, as presented in table 2: (a) magnetic storms interpreted as correlation of geomagnetic anomalies, earthquakes and solar storms figure 2. solar storm that took place on may 2, 3, 4, 1998, had a triggering effect on the earthquake that occurred on may 4, 1998 mw 4.0; h = 145 km). in this case, the geomagnetic anomaly was induced by the solar storm (as the kp index reveals) and it is not a seismo-magnetic precursor. moldovan et al. 130 figure 3. kp indices of january 6, 7, 1998, reveal a magnetic storm. two days later, on january 10, 1998, a step-change of bz and by occurred. the by component developed a v-shaped anomaly with a duration of 4 days. an earthquake with magnitude mw 4.0 (h = 143 km) occurred at the end of this anomaly. figure 4. the v-shaped anomaly of by started on january 23, 1998 (see figure 2), and continued until february 15, 1998, when the prior value of by was restored. one day before restoration, a step-change occurred on the bx and bz components. an earthquake with magnitude mw = 3.5 (h = 147 km) occurred on february 15, 1998. between february 17 to 21, 1998, there was a data gap. the same situation is seen for february 23 to 25, 1998. 131 seismo-magnetic anomalies, sometimes followed by earthquakes that might be triggered by the solar storms (figures 2, 5); and (b) missing data that lead to misinterpretations caused by the stacking of non-consecutive data blocks (figures 3, 4, data gaps) and seismo-magnetic anomalies (figures 3, 4, 5). figures 2-5 illustrate the components of the magnetic field, for bx, by, bz and bt, each for two stations: sua or thy (left) and mlr (right). the time (in days) is given on the abscissa, and the values of each of these components (in nt for sua and thy, and in nt for mlr) are given on the ordinate. x is positive pointing north and y is positive pointing east. figures 2-5 also show the times when the vrancea earthquakes of magnitudes mw >3.0 occurred during the period of this study. the mw 3.0 moment magnitude was imposed as a threshold as significant disturbances cannot be detected before the vrancea earthquakes of mw <3.0. (a) there were days, as for may 2, 3, 4, 1998, preceding the earthquake on may 4, 1998 (table 2, line 14), when the solar storm reached more than half, rkp 36, 35, 38, respectively, of the maximum possible rkp of 72 (figure 2). we consider that for the earthquake that occurred immediately after the end of the magnetic storm, the storm had a triggering effect, and there was no geomagnetic seismic precursor. the value of rkp was less than 15 for only two anomalous periods, which is the lower limit for a magnetic storm. (b) periods of missing data are visible in figures 3 and 4, and these are explained in table 2. due to the missing data, in figure 3 there is a misinterpretation of the date of the anomaly from january 9 that was actually on january 10, 1998. in figure 4, the finish date of the v-shaped anomaly that started on january 23, 1998, cannot be specified exactly. from table 2, it can also be seen that five of the stated anomalies were due to the missing data. (c) the anomaly from january 9, 1998 (table 2, lines 3, 4, 5) was considered by enescu et al. [1998] as preceding three earthquakes: january 14, 1998 (mw 4.0), january 19, 1998 (mw 4.0) and january 31, 1998 (mw 3.6). this anomaly was also classified in the present study as a seismo-magnetic precursory anomaly. from figure 3, it can be seen that there was a magnetic storm with rkp >20 just two days before the magnetic anomaly, and there was also a day of missing data. the anomaly, indicated by enescu et al. [1998] as a «a step-type change», appeared on the vertical component of the magnetic recordings. also in figure 3, another type of anomaly can be seen, on the horizontal by component, which we refer to as a «v-shaped anomaly». the v-shaped anomaly corresponds to the step-type change correlation of geomagnetic anomalies, earthquakes and solar storms figure 5. on march 21, 1998 (day 7) another magnetic storm (kp = 25) induced a variation in the geomagnetic flux density. it can be seen that the by component has a v-shaped anomaly, which ends on day 25 (april 9, 1998). two earthquakes (mw = 3.8 h = 134 km, and mw = 3.5 h = 146 km, respectively) occurred on april 9, 10, 1998. in the bz component. nowadays, after 15 years of magnetic recordings and analysis, we can state that after a step-type change or a v-shaped variation in the magnetic components, an earthquake occurs, although not all earthquakes are preceded by these changes. on january 23, 1998, another by v-shaped anomaly started, and lasted until february 16, 1998. on february 14, the v-shaped anomaly was accompanied by a step-type change for two components: bx and bz. an earthquake of mw 3.5 occurred immediately afterwards, on february 15, 1998 (figure 4). instead of these anomalies, in the analysis of enescu et al. [1998], lines 6 and 7 from table 2 show an anomaly recorded on february 18, 1998, when there were no recorded data. in figure 5, it can be seen that on march 21, 1998, there was a solar magnetic storm with rkp = 25, followed by a relatively quiet month, from both the seismic and magnetic points of view. on april 2, 1998, the geomagnetic index reached the smallest value for the months (rkp = 3), and on the magnetic records there is no visible anomaly, as was erroneously presented by enescu et al. [1998] (see table 2, lines 10, 11). however, on the horizontal ew component, there is a large amplitude by v-shaped anomaly of 150nt with a long evolution of approximately 20 days. two earthquakes occurred at the end of this vshaped anomaly. the last two columns of table 2 give all of the revised anomalies. 4. data correlations as the investigated period was 15 years (1996 to 2011), which covers more than a complete solar cycle (the 23rd and the first part of the 24th solar cycles), the solar-terrestrial perturbations fluctuated from very low values (in 1996 and 2009, at the beginning and end, respectively, of the 23rd solar cycle; archibald 2009) to very high values (in 2000 to 2001; the maximum of the 23rd solar cycle; archibald 2006). these provided the ideal medium to observe perfect crosscorrelation of geomagnetic intensity with solar perturbations and earthquake occurrence. in this section we provide the statistical correlations of geomagnetic anomalies recorded over the last 15 years at mlr seismic observatory (romania) with the earthquake occurrences and the solar magnetic storms after the correction and reprocessing of the whole set of the magnetic data. to discriminate local – tectonic and global – solar phenomena, the geomagnetic data from mlr observatory are correlated with the global solar phenomenon using the geomagnetic indices kp and compared with the data recorded at the sua (romania) and thy (hungary) geomagnetic reference stations, which are located outside the epicentral region. these recordings were provided by the intermagnet project (www.intermagnet.org). the possible seismo-magnetic anomalies considered in this study are those presented in section 3, namely the «steptype change», the «v-shaped anomaly» and the «reverse v-shaped anomaly». the step-type change anomaly has a very short period, while the two v-shaped anomalies are long period, minimum and maximum, respectively, anomalies. in future we will look for other types of anomalies with spectral evidence and using different data processing techniques, like the terminator time deviation from the sunset or sunrise hours. during our studies, we considered different causes for the recording of such anomalies, like equipment problems, temperature problems, or displacement of magnetic objects in the vicinity of magnetometers. one after another, these causes were eliminated by isolating the equipment from man-made magnetic influences and by installing a weather stations ws-3600 type with temperature, pressure and humidity continuous monitoring. as mlr observatory is situated near to the carpathian electrical conductivity anomaly (ceca), the long period anomalies (v-shaped and reverse v-shaped) might be linked to electrical conductivity variations along the ceca, which forms not only a tectonic boundary, but which also represents a peculiar conducting channel, as an ‘open gate’ to the intermediate depth seismically active vrancea zone [stanica et al. 2006; stanica and stanica 2007]. the same type of long-period magnetic anomaly was also reported by takla et al. [2011] prior to two crustal mw 5.7 earthquakes that occurred in italy in the molise region on october 31 and november 1, 2002. figures 6 and 7 show examples of the graphs used for the statistical correlations. the graphs refer to the magnetic intensity bx, by and bz components, and to the total magnetic field intensity bt, for two stations: sua/thy (left panel) and mlr (right panel), and to the daily sums of the geomagnetic index kp (rkp). the graphs shown in figures 6 and 7 were chosen as representative examples of a vshaped long-period magnetic anomaly recorded on the horizontal component by the mlr observatory data prior to the mw 4.4 (october 2, 2008, 14:04:48.2, h = 148 km) intermediate vrancea earthquake. the anomaly started on september 16, 2008, immediately after a minor magnetic storm, and ended on october 6, 2008, with an earthquake of mw 3.5. the mw 4.4 earthquake was just at the minimum point of the anomaly. the data from the mlr observatory are also compared with the data recorded at sua, and are correlated with the seismicity and the solar activity. studying the monthly data year after year from 1996 until now, all the magnetic anomalies were tabulated and they were correlated with possible causes as the earthquake preparation stages, missing data or solar storms. table 3 provides a part of this analysis as an example, as it is not possible to present a moldovan et al. 132 133 correlation of geomagnetic anomalies, earthquakes and solar storms figure 6. the v-shaped anomaly recorded on the horizontal component by of the mlr observatory data prior to the mw = 4.4 (october 2, 2008) crustal vrancea earthquake. figure 7. the v-shaped anomaly recorded on the horizontal component by of the mlr observatory data prior to the mw = 4.4 (october 2, 2008) crustal vrancea earthquake. table here containing more than 10,000 lines. thus, table 3 contains the earthquake catalog for mw >3.0, the dates when magnetic anomalies occurred, their durations in days, the shapes and the amplitudes of the anomalies, and the days when solar magnetic storms hit the earth (rkp >20). all of this information was cross-correlated to investigate any statistical relationships among these different phenomena. only 12 years of recordings were synthesized for this correlation study, because during the first two years of the magnetic records there was inconsistent data, and the earthquake catalog for 2011 has not yet been completed. from these studies, we can conclude that: 1. from the 3,022 earthquakes with mw >2.0 recorded over these 12 years, 1,180 earthquakes had mw >3.0 (39%) and were used for the correlation studies. from the 1,180 tabulated earthquakes, 403 had mw >3.5 (34%), 121 had mw >4 (10%), 19 had mw >4.5 (<2%) and only 4 had mw >5(<0.5%). from the 121 earthquakes with mw >4, 57 moldovan et al. 134 table 3. example of cross correlation table between earthquakes with mw >3.0, magnetic anomalies, and solar magnetic storms with kp >20. 135 earthquakes (47%) were preceded by visible magnetic anomalies, and 53 (43%) were not, and 11 of these earthquakes (9%) were inside the missing periods of the magnetic recordings. if we exclude these last 11 earthquakes, then 52% of earthquakes with mw 4.0 were preceded by seismo-magnetic precursors and 48% were not. 2. from the 66 magnetic anomalies, 42 (64%) were followed by an earthquake with mw >4.0 and 24 (36%) by an earthquake with 3.5 <mw <4.0, in a time interval of 20 days, i.e. all of the magnetic anomalies (100%) were followed by earthquakes with mw >3.5; 3. all of the the solar storms with kp >20 produced perturbations of the geomagnetic field, with different amplitudes and frequencies, although to date there is no clear evidence that the geomagnetic storms are triggering earthquakes. further studies are needed to arrive at a reliable conclusion regarding this correlation. 4.1. the largest intermediate depth vrancea seismic event from the past 15 years in this section, the largest intermediate depth earthquake that occurred during the study period is analyzed, along with its corresponding geomagnetic anomalies. figure 8 shows the v-shaped long-period anomaly that was recorded on the horizontal component by of the mlr observatory data prior to the mw 6.0 (october 27, 2004, 20:34:36.4, h = 98.6 km) intermediate depth vrancea earthquake. these data from the mlr observatory are compared with the data recorded at sua and correlated with the seismicity and the solar activity. starting from october 10, 2004, the eastern component of the local geomagnetic field recorded at the mlr observatory deviated from the general pattern (recorded by the other observatories) and started to decrease «on its own». the issue is that after showing a relative steep decay, it reached a lower peak of about –40nt, which is far greater than the expected values of an anomaly that would appear at a hypocentral distance of about 122 km. after this, the value of the eastern component started to increase slowly, following a specific slope, towards a normal mean value. the earthquake occurred when the value of this component was «restored» to its mean value. it should also be noted that two days after the anomaly started (on october 12, 2004), the kp index shows higher values, which denote solar storms, and these are easily visible on the recordings. this example gives us the hope that future large earthquakes will be preceded by such minimum-type or maximum-type, long-period, magnetic anomalies, which would provide the possibility of forecasting the occurrence of extreme seismic events. correlation of geomagnetic anomalies, earthquakes and solar storms figure 8. the v-shaped anomaly recorded on the horizontal component by of the mlr observatory data prior to the mw = 6.3 (october 27, 2004) intermediate depth vrancea earthquake. 5. conclusions the main purpose of the present study was to evaluate the data obtained after 15 years of geomagnetic surveillance, to obtain more robust conclusions on the nature of various irregularities in the geomagnetic field variations. the present study reveals some issues regarding the interpretation of the geomagnetic activity within the vrancea seismic zone prior to earthquakes with mw >3.0 [enescu et al. 1998, 1999a,b, 2001]. some geomagnetic anomalies identified and presented as precursory signals are seen to be induced either by increased solar activity (as the kp index demonstrates) or by dysfunction of the data acquisition system, which produced inconsistent data with numerous gaps. the first part of our study demonstrates that the previously reported precursory anomalies were insufficiently investigated, which led to some regrettable misinterpretations. in our opinion, more careful approaches in the future will be beneficial for these kinds of studies. the whole geomagnetic dataset recorded at the mlr observatory from 1996 to the present was re-evaluated and tabulated. geomagnetic data recorded at other observatories, such as sua and thy, were also studied, to identify the global anomalies and to correlate these with the solar activity. after evaluation of the examined period of 15 years, the following observations were made: 1.two kinds of anomalies are noted: the step-change anomaly (short period), and the by v-shaped anomaly (long period). the step-change anomalies are seen as modifications of the amplitude of the bt vector, while the v-shaped anomalies are seen as changes in the orientations of the components of the bt vector. 2. most of the anomalies were followed by an earthquake with mw >4.0 in a time interval of less than 20 days, although not all earthquakes were preceded by these kinds of anomalies. 3. there is no visible connection between the amplitude of these anomalies, their duration or the precursory time and the magnitude of the earthquakes that occurred afterwards. 4. there is no clear evidence that geomagnetic storms have an earthquake-triggering effect. data and sharing resources – geomagnetic field records from surlari and tihany intermagnet observatories http://ottawa.intermagnet.org/ apps/dl_data_def_e.php; – seismic data for the vrancea source zone, taken from the seismic bulletins of the national institute for earth physics http://www.infp.ro/ro/lista_evenimente/local; – daily geomagnetic index kp from the noaa/ space weather prediction center -http://www.swpc.noaa.gov/ ftpdir/indices/dgd.txt. acknowledgements. as some of the results presented in this study rely on data collected at the surlari geomagnetic observatory (romania), we wish to thank to the geological survey of romania for supporting its operation and the intermagnet project for promoting high standards of magnetic observatory practice (www.intermagnet.org). this study was supported by the national program pnii, under project number 82066/2008. references archibald, d. 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(2004). transport of the hm energy through the magnetosphere-ionosphere coupling systemresults from the ground-based network observations, in: advances in solar-terrestrial physics, edited by h. oya, terra sci. pub. co., tokyo, 175-211. yumoto, k., s. ikemoto, m.g. cardinal, m. hayakawa, k. hattori, k.j. liu, s. saroso, m. ruhimat, m. husni, m. widarto, e. ramos, d. mcnamara, r.e. otadoy, g. yumul, r. ebora and n. servando (2009). a new ulf wave analysis for seismo-electromagnetics using cpmn/magdas data, phys. chem. earth, 34, 360366. *corresponding author: iren-adelina moldovan, national institute for earth physics, bucharest-magurele, romania; e-mail: iren@infp.ro. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights correlation of geomagnetic anomalies, earthquakes and solar storms vol52,1,2009 97 annals of geophysics, vol. 52, n. 1, february 2009 key words south atlantic anomaly – geomagnetic observatory – geomagnetic repeat station – tristan da cunha 1. introduction one of the largest gaps in the current distribution of geomagnetic ground observations is in the south atlantic ocean (fig. 1). however, this is a region of special interest due to its anomalous geomagnetic field behaviour: here exists the largest depression in the earth’s core field, called the south atlantic anomaly or saa (indicated by the total field strength isoline for 30000 nt in fig. 1). the saa originates from inverse magnetic flux patches at the coremantle boundary mainly beneath south america and south africa, which have developed over the last 200 years (gubbins et al., 2006). it is the weakening of the geomagnetic field in the saa that is in large part responsible for the present decline of earth’s magnetic field strength (olsen et al., 2006, hulot et al., 2007). a consequence of the low geomagnetic field intensity is the distortion of the inner van allen radiation belt, causing a strong increase of radiation damage to low-earth-orbiting satellites when passing through the saa (heirtzler et al., 2002). due to the lack of geomagnetic ground observations in the south atlantic (fig. 1), the geomagnetic field in the saa region is presently mainly determined from satellite-based measurements (e.g. hulot et al., 2002; olsen and mandea, 2006). for a geophysical interpregeomagnetic observations on tristan da cunha, south atlantic ocean jürgen matzka (1), nils olsen (2), cathrine fox maule (1), lars william pedersen (1), alan m. berarducci (3) and susan macmillan (4) (1) danish meteorological institute, copenhagen, denmark (2) dtu-space, national space institute of denmark, copenhagen, denmark (3) united states geological survey, national geomagnetism program, golden, usa (4) british geological survey, edinburgh, united kingdom abstract few geomagnetic ground observations exist of the earth’s strongest core field anomaly, the south atlantic anomaly (saa). the geomagnetic repeat station on the island tristan da cunha, located half-way between south africa and south america at 37° 05’ s, 12° 18’ w, is therefore of crucial importance. we have conducted several sets of repeat station measurements during magnetically quiet conditions (kp 2o or less) in 2004. the procedures are described and the results are compared to those from earlier campaigns and to the predictions of various global field models. features of the local crustal bias field and the solar quiet daily variation are discussed. we also evaluate the benefit of continuous magnetic field recordings from tristan da cunha, and argue that such a data set is a very valuable addition to geomagnetic satellite data. recently, funds were set up to establish and operate a magnetometer station on tristan da cunha during the swarm magnetic satellite mission (2011-2014). mailing address: dr. jürgen matzka, remote sensing and geomagnetism, danish meteorological institute, lyngbyvej 100, dk-2100 copenhagen, denmark; e-mail: jmz@dmi.dk vol52,1,2009 28-05-2009 13:45 pagina 97 98 j. matzka, n. olsen, c. fox maule, l. w. pedersen, a. m. berarducci and s. macmillan tation, the geomagnetic data have to be separated into field contributions from the core, crust, ionosphere, and magnetosphere, plus fields due to currents induced in the earth’s mantle and the oceans. this separation can be achieved by comprehensive modelling (sabaka et al., 2004), a process that, however, depends on the availability of simultaneous geomagnetic data from satellites and ground. surface time series are currently not available for the central south atlantic since there are no geomagnetic observatories within a circle of roughly 3000 km radius centred at 42° s, 15° w. only few islands are located within this circle, but the tristan da cunha group of islands with its main island tristan da cunha (37° 05’ s, 12° 18’ w) is close to its centre. geomagnetic measurements from that island will therefore make a significant contribution towards a more even distribution of geomagnetic ground stations in the south atlantic. at present, there is a geomagnetic repeat station on tristan da cunha, for which we report results for the year 2004 here. the purpose of a geomagnetic repeat station is to provide information about the secular variation for an area without geomagnetic observatories (newitt et al., 1996). time variation of the external geomagnetic field contributions should therefore be subtracted from repeat station measurements. assuming that these variations are homogeneous over a larger area, variation data from a nearby geomagnetic observatory can be used to calculate a yearly mean value for the repeat station. alternatively, an on-site variometer can be used at the repeat station to determine the quiet night time value of the geomagnetic field for a certain day at the repeat station (newitt et al., 1996). the current lack of continuously recorded geomagnetic ground station data from tristan da cunha is caused mainly by logistical challenges. the island has no airport and ships call infrequently. an additional problem is the volcanic nature of tristan da cunha, causing a significant local crustal bias and strong spatial gradients of the geomagnetic field. the geomagnetic observatories within the fig. 1. global distribution of geomagnetic observatories for 2008 (dots) and location of tristan da cunha (triangle). the south atlantic anomaly (saa) is indicated by the 30000 nt isoline (a) for total field strength calculated after the chaos model (olsen et al., 2006). the northern (b) and southern (c) track of the solar quiet (sq) current system foci are indicated as lines (computed from cm4, sabaka et al., 2004). the four dst-observatories are shown by squares. vol52,1,2009 28-05-2009 13:45 pagina 98 99 geomagnetic observations on tristan da cunha, south atlantic ocean saa closest to tristan da cunha are hermanus in south africa (iaga code her, approximate distance: 2800 km), keetmanshoop in namibia (kmh, 3100 km distance, see korte et al. (2009) in this issue), st. helena (sth, 2400 km distance, see korte et al. (2009) in this issue), ascension island (iaga code asc, 3200 km distance, see macmillan et al. (2009) in this issue), vassouras in brazil (iaga code vss, 3400 km distance) and port stanley on the falkland islands (pst, 3900 km distance). observatories to the south of the saa are located on antarctica at a distance between 3500 and 4000 km. the dipole (geomagnetic) latitude of tristan da cunha of 32° s is similar to those of the socalled dst-stations hermanus (her), kakioka (kak), honolulu (hon) and san juan (sjg), which are indicated in fig. 1. the dst-stations are used to determine the dst index, an estimate of the strength of the magnetospheric ring current. additionally, the tracks of the equinoctial northern and southern foci of the solar quiet (sq-)current system are shown in fig. 1. these were derived by calculating the focus of the sqcurrent system for each hour of the day from the cm4 model (sabaka et al., 2004). both the dst-stations and tristan da cunha are located close to these foci (fig. 1), where the sq magnetic field variation is most pronounced in the magnetic east-west component. as the variations due to the magnetospheric ring current occur mainly in the magnetic north component, having ground-based stations close to these foci would allow for a better separation of the two contributions. a regional estimate in the saa of both the ring current field (which shows a longitudinal dependency, e.g. le et al., 2004) and the field from the sq-current system would be an advantage for geomagnetic models, especially for comprehensive modelling (sabaka et al., 2004). for this purpose, continuously recorded geomagnetic data from tristan da cunha would be necessary. 2. observational and computational methods we first describe the repeat station measurements obtained in 2004 on tristan da cunha. the repeat station ‘tt-2’, also known as ‘sat tri sta 069’ or ‘the hillpiece’ (riddick, pers. comm.), was occupied on sept. 27 and 29 by one of us (jm). for measurement of the magnetic declination and inclination, a nonmagnetic zeiss theodolite theo 020a with bartington fluxgate probe (di-flux) with 0.25 nt resolution was set up on a tripod 125 cm above the corps of engineers bronze disk «sat tri sta 069 tt-2 1968» on a hill (called the hillpiece) on tristan da cunha. this bronze disk is a geodetic mark (37° 04’ 52.465” s, 12° 19’ 18.8571” w, 225.1 m altitude). we used the azimuth mark rm2 (a previous repeat station occupied in 1986) of azimuth of 18° 03’ 20” and distance of approximately 1.9 km to tt-2 (riddick, pers. comm.). during the declination and inclination measurements, the total field was measured with an elsec 770 proton precession magnetometers (ppm) on a remote site (called rm1, previously occupied in 2001) approximately 2 m above a brass disk located 9.5 m north-east of tt-2. to determine the difference in total field between rm1 and tt-2, a second elsec 770 ppm was used to measure the total field simultaneously at the remote site rm1 and at exactly the same location as the diflux. this was done prior to and after the diflux measurements. measurements with the di-flux followed the scheme for absolute measurements outlined in jankowski and sucksdorff (1996), with readings in four different positions for declination and inclination. note, however, that the formula for inclination i on p. 96 in jankowski and sucksdorff (1996) is only valid in the northern hemisphere and has to be adjusted for the negative inclination on tristan da cunha by subtracting 180° (e.g. newitt et al., 1996). also note the discussion on sensor misalignment and sensor offset described in matzka and hansen (2007). during the final inclination measurement on sept. 29, the theodolite was frequently exposed to changing wind conditions, and vibrations of the automatic index of the zeiss theo 020a (a pendulum that indicates the vertical) made readings of the vertical circle tedious, but with no obvious degradation of accuracy. the results were checked for consistency by calculating sensor misalignment and sensor vol52,1,2009 28-05-2009 13:45 pagina 99 100 j. matzka, n. olsen, c. fox maule, l. w. pedersen, a. m. berarducci and s. macmillan offset between successive measurements; they do not change by more then 1’ and 3 nt, respectively. azimuth mark readings were performed prior to and after declination measurements and do not vary by more than 0.1’. two independent absolute inclination results can be determined from the four-positions measurements scheme (jankowski and sucksdorff, 1996); they differ for the three inclination measurements by 0.3’, 0.4’ and 0.5’, respectively. this suggests a systematic difference of the order of 0.4’, which can be explained by field gradients in the vicinity of the theodolite. the total field difference measured between tt-2 and rm1 was of the order of 250 nt and we estimate from our repeatedly conducted simultaneous measurements on tt-2 and rm1 that this does not change by more than 0.5 nt during one set of di-measurements. both the determination of the field direction and the field strength was conducted to achieve absolute values. from the above mentioned quality checks we estimate that our data have an accuracy of about 0.3’ for the direction and 0.5 fig. 2. declination (d), inclination (i) and field strength (f) observations (diamonds) at tt-2, tristan da cunha, compared to three models (see text). kp-values for the corresponding three-hour intervals are indicated at the top of the graph. campbell’s (2003) sq-model is shown as full lines, geomagnetic variations are calculated from cm4 with dst (long dashed) and without dst (short dashed). vol52,1,2009 28-05-2009 13:45 pagina 100 101 geomagnetic observations on tristan da cunha, south atlantic ocean nt for the field strength. no ground-based, continuously recording variometer was available on-site or within 2800 km for a temporal reduction of geomagnetic variations from external (magnetospheric, ionospheric) sources. how do these observations fit to global geomagnetic field models? three field models were used to investigate this. the first model is the phenomenological model of the solar quiet variation sq by campbell (2003) derived from observatory time series. the second model is the comprehensive model cm4 by sabaka et al. (2004) that is based on geomagnetic satellite data and observatory hourly mean values. this model describes the magnetic fields due to the core, the crust, the ionosphere, the magnetosphere, and due to secondary, induced currents in the mantle arising from ionospheric and magnetospheric currents. for values for the solar flux f10.7 we used the 3-monthly average centred on september 2004. we calculate two different predictions of the field variations using cm4, denoted as ‘cm4 without dst’ and ‘cm4 with dst’, respectively. both contain the daily ionospheric and magnetospheric field variations (and their earth induced counterpart), but in addition the ‘cm4 with dst’ also considers time-changes due to variations of the magnetospheric ring current as monitored by the dst index. for ‘cm4 without dst’ we excluded field variations caused by changes of the dst-index since we at this point focus on the quiet-time daily field variation as predicted by cm4. (note, however, that dst also includes a quiet-time, slowly varying field in the form of the ever present ring-current.) this also makes the model predictions more compatible with those of the sq-model by campbell (2003). ‘cm4 with dst’ was calculated to estimate the influence of the non-regular magnetospheric field changes at tristan da cunha. provisional values for dst, provided by the wdc for geomagnetism, kyoto, were used for the calculation. the location of the observatories used to derive the dst-index are indicated in fig. 1. the third model is igrf-10 (maus et al., 2005), which is a main field model that covers the period 1900.0 to 2010.0. 3. results on sept. 27 and 29, a total of 3 declination measurements, 2 inclination measurements and 12 total field measurements were made (cf. the diamonds in fig. 2). the results are consistent, but the difference of 0.15 degree between successive declination measurements on sept. 29 by far exceeds reasonable measurement errors (fig. 2). the same applies to the total field measurements on sept. 29, which scatter by about 15 nt (fig. 2). these observed differences are believed to represent the temporal variation of the geomagnetic field. in lieu of an on-site variometer or close-by geomagnetic observatory, the temporal variation expected for the location and the given date is calculated from the various global models mentioned in the previous section. note that the offset of the model results (full line for sqmodel by campbell (2003); short-dashed line for cm4 without dst; long-dashed line for cm4 with dst) shown in fig. 2 is arbitrary: all model curves are adjusted to fit the level of the measurements of sept. 27, and the obtained offset was also used for sept. 29. 4. discussion from the planetary index of geomagnetic activity, kp, shown in the top of fig. 2, the geomagnetic field condition for sept. 27 and 29, 2004, is rather quiet, and the geomagnetic variations are mainly due to the solar quiet daily variation (ionospheric sq-currents). kp was 2o during the measurements of sept. 27, kp was 1+ during the time of the measurements on sept. 29, and did not exceed kp = 2+ during those two days. the dst index is between 6 nt and -22 nt on sept. 27 and between 4 nt and 5 nt on sept. 29, which also indicates low geomagnetic disturbance. this can be seen in fig. 2 by the similarity of the variations calculated for ‘cm4 without dst’ (short-dashed line) and ‘cm4 with dst’ (long-dashed line). the greatest difference between the two model curves is observed prior to and after the declination and inclination measurements on sept. 27, but not during these measurements. vol52,1,2009 28-05-2009 13:45 pagina 101 102 j. matzka, n. olsen, c. fox maule, l. w. pedersen, a. m. berarducci and s. macmillan t ab le i . g eo m ag ne ti c re pe at s ta ti on m ea su re m en ts o n t ri st an d a c un ha . ob s. ye ar so ur ce lo ca ti on la t. lo ng . d [ ° ] i [ ° ] f [ n t ] # 19 58 .3 26 da ta ba se @ b g s -3 7. 04 7 -1 2. 29 8 -2 8. 08 3 -5 4. 89 7 27 53 1 # 19 58 .3 26 w ii d an d va n w ij k (1 96 0) f la gs ta ff a -3 7. 05 -1 2. 3 -2 8. 08 3 -5 4. 89 7 27 53 1 # 19 58 .3 26 da ta ba se @ b g s -3 7. 04 7 -1 2. 29 8 -2 8. 17 3 -5 5. 22 3 27 78 1 # 19 58 .3 26 w ii d an d va n w ij k (1 96 0) f la gs ta ff b -3 7. 05 -1 2. 3 -2 8. 17 3 -5 5. 22 3 27 78 1 # 19 60 .2 21 da ta ba se @ b g s -3 7. 04 7 -1 2. 29 8 -2 7. 89 5 -5 5. 50 3 27 62 9 # 19 60 .2 21 w ii d an d va n w ij k (1 96 0) f la gs ta ff b -3 7. 05 -1 2. 3 -2 7. 89 5 -5 5. 50 4 27 62 9 # 19 60 .2 21 w ii d an d va n w ij k (1 96 0) f la gs ta ff a -3 7. 05 -1 2. 3 -2 7. 91 -5 5. 10 5 27 45 2 # 19 60 .2 24 da ta ba se @ b g s -3 7. 04 7 -1 2. 29 8 -2 7. 96 8 -5 5. 25 2 27 56 7 # 19 60 .2 24 w ii d an d va n w ij k (1 96 0) f la gs ta ff a -3 7. 05 -1 2. 3 -2 8. 02 5 (h = 1 57 21 ) # 19 60 .2 24 w ii d an d va n w ij k (1 96 0) f la gs ta ff b -3 7. 05 -1 2. 3 (z = – 22 78 3) ## 19 86 .8 62 da ta ba se @ b g s g ol f c ou rs e r m 2 -3 7. 06 5 -1 2. 31 5 -2 0. 04 ## 20 01 .5 da ta ba se @ b g s g ol f c ou rs e r m 1 -3 7. 06 5 -1 2. 31 5 27 95 6 ## 20 02 .2 89 da ta ba se @ b g s t t -2 * (1 ) -3 7. 08 1 -1 2. 32 2 -2 4. 98 1 -6 7. 15 6 30 13 2 ## # 20 04 .7 4 th is s tu dy t t -2 * (2 ) -3 7. 08 1 -1 2. 32 2 -2 6. 3 -6 7. 44 7 30 17 8 ## # 20 04 .7 46 th is s tu dy t t -2 * (3 ) -3 7. 08 1 -1 2. 32 2 -2 6. 45 3 -6 7. 43 30 18 7 ## # 20 04 .7 46 th is s tu dy t t -2 * (4 ) -3 7. 08 1 -1 2. 32 2 -2 6. 32 5 -6 7. 44 1 30 18 1 ## # 20 04 .7 46 th is s tu dy , da ta ba se @ b g s t t -2 * (5 ) -3 7. 08 1 -1 2. 32 2 -2 6. 36 7 -6 7. 45 30 19 0 le ge nd sy m bo l ob se rv er # w ii d an d va n w ij k fo r h m o , h er m an us ## h m s e nd ur an ce s ur ve yo rs f or b g s , e di nb ur gh ## # m at zk a fo r f u r a nd l m u m un ic h * al so k no w n as s a t t r i s t a 0 69 t im e an d da te ( do y: d ay o f ye ar ) (1 ) u t 9 :3 611 .2 5 do y 10 6 a pr 1 6t h (2 ) u t 1 6: 36 d oy 2 71 s ep t. 2 7t h (3 ) u t 1 3: 06 d oy 2 73 s ep t. 2 9t h (4 ) u t 1 4: 25 d oy 2 73 s ep t. 2 9t h (5 ) qu ie t ni gh t ti m e va lu e vol52,1,2009 28-05-2009 13:45 pagina 102 103 geomagnetic observations on tristan da cunha, south atlantic ocean the measurements of d and i for sept. 27 and 29 are equally well fitted by the sq-model by campbell (2003) and by the two cm4 predictions (fig. 2), confirming that the geomagnetic variation on tristan da cunha at that time is largely described by the solar quiet-time daily variation. note, however, that the model of campbell (2003) and cm4 do not agree well between 1600 and 2400 ut for declination. unfortunately, there are no declination measurements for this time period to decide which model is preferable. the total field f variation is better described by the cm4’s than the campbell (2003) model. adjusting the offset for the campbell (2003) model for the total field measurements of sept. 27, 1600 ut (when dst = -2 nt is small), gives a significant misfit to the data measured between 1200 ut and 1500 ut on sept 29 (when -4 nt ≤ dst ≤ 0 nt is similarly small). the reason for this behaviour is basically that the ‘cm4 without dst’ has its daily minima around 1540 ut, whereas the campbell (2003) model gives the minima around 1400 ut. there is some scatter in the total field measurements on sept. 27, that is qualitatively resembling the variation of ‘cm4 with dst’, but with a higher amplitude (fig. 2). this could be attributed to short term magnetospheric variations that are visible in the total field spot readings, but attenuated in the dst hourly mean values. one possibility to explain the difference of the campbell (2003) model from cm4 and the measurements is the fact that the campbell (2003) model relies solely on geomagnetic observatory data. it is therefore probably biased in the south atlantic region towards geomagnetic observatory data from continental coastal locations. the sq-variation in the z component (that would affect mostly f in fig. 2) from coastal geomagnetic observatories can be significantly influenced by induced electric currents in the ocean and their missing counterparts in the less conductive continental crust so they do not geometrically cancel out (olsen and kuvshinov, 2004; kuvshinov et al., 2007). in return it can be expected that a geomagnetic time series from an island location like tristan da cunha (for which channelling of the induced currents is much less serious than for a coastal station, leading to a smaller anomalous induction field) would be a valuable addition for quantifying and studying the sq-variation in the saa. since cm4 fits the repeat station data better, and since dst was small during the measurements, we use the ‘cm4 without dst’ to determine quiet night time levels for our repeat station measurements. these are d = -26° 22’, i = -67° 27’, and f = 30190 nt. the repeat station results for 2004 are the latest reported repeat station results for tristan da cunha (as of march 2008). the results are compared to those from previous years in table i and fig. 3. previous data were retrieved from original literature as well as from the world data centre for geomagnetism, edinburgh, maintained by bgs. the original repeat station by wiid and van wijk (1960) was destroyed by a lava flow (locations flagstaff a and flagstaff b, which were a few meters apart from each other, see table i). later measurements by surveyors of hms endurance made for bgs were done at three different locations (rm2, rm1 and tt-2, see table i). the measurements from 2004 were conducted on tt-2, the last site occupied by surveyors of hms endurance in 2002 (riddick, pers. comm.). the scatter of the repeat station measurements and the poor agreement with igrf-10 (maus et al., 2005) seen in fig. 3 is easily explained by the different locations of the observations. tristan da cunha is a volcanic island with strongly magnetised rocks and this leads to spatially varying crustal field contribution. it is therefore necessary to conduct geomagnetic measurements at the same location. whilst this is emphasized to the hms endurance surveyors in their training from bgs the existence of several marked sites on tristan da cunha within close vicinity of one another has led to some confusion in the past. another problem is that the local bias field can change with time, for instance due to volcanic activity, erosion, magnetisation of the volcanic rocks due to lightning strikes, or a variation of the rock magnetisation with temperature. the local bias field is also very inhomogeneous and slight positional changes (e.g. in the height of the sensor) have effects. vol52,1,2009 28-05-2009 13:45 pagina 103 104 j. matzka, n. olsen, c. fox maule, l. w. pedersen, a. m. berarducci and s. macmillan 5. conclusions and outlook magnetic repeat station measurements on tristan da cunha in the south atlantic under magnetically quiet conditions from sept. 27 and 29, 2004 have been described, and the results reduced to quiet midnight values by using geomagnetic variations calculated from cm4 (sabaka et al., 2004). these are d = -26° 22’, i = -67° 27’, and f = 30190 nt. significant differences in the calculated solar quiet variations for tristan da cunha were found between the model by campbell (2003) and cm4 and are attributed to induction effects present in the campbell (2003) model. it is argued that tristan da cunha represents an ideal location to continuously record geomagnetic field variations for studying the saa. firstly, the island is central to the present gap in the geomagnetic observatory network. secondly, induction effects are less pronounced for an island than for a continental coastal location. thirdly, the island’s geomagnetic latitude is ideal for the separation of the field due to ionospheric sq-currents and variations from the magnetospheric ring current. we have received funds from the danish natural science research council to establish and maintain a magnetometer station on tristan da cunha. the aim of this project is to provide continuous high-quality time series of the field variations prior and during the swarm satellite mission (friis-christensen et al., 2006), allowing for better investigations of the special geomagnetic conditions of the saa by combining ground and satellite data. the set up of the ground station will receive significant support from the geomagnetism program of the usgs. it is planned to install a temperature controlled and suspended fge three-component fluxgate magnetometer on a concrete pillar for 1 hz recording of the full geomagnetic vector. a hut will be built for weekly absolute measurements with a di-flux and an overhauser magnetometer and baselines will be determined for the fge. if the local crustal field is not constant over the whole period (several years) of observations (e.g. because of geological processes like the volcanic eruption from 1961 (baker et al., 1964)), the secular variation determined from the ground station alone will be affected. it is expected that this possibly time-varying station bias will be better determined with the help of a satellite-derived geomagnetic field model. ground station data from tristan da cunha will still be of great value since they provide information on short-term geomagnetic variations (e.g. solar quiet, magnetospheric ring current), that are essential ingredients for deriving advanced geomagnetic field models. fig. 3. comparison of repeat station measurements on tristan da cunha with igrf-10 (maus et al., 2005). the large scatter of the observations is due to several relocations of the repeat station (see text and table i) in the presence of large spatial gradients of the crustal field. vol52,1,2009 28-05-2009 13:45 pagina 104 105 geomagnetic observations on tristan da cunha, south atlantic ocean acknowledgements we would like to thank the two reviewers for their helpful comments. jm would like to thank james glass and jeremy green for their help with the measurements. the government of tristan da cunha, and especially anne green and mike hentley are thanked for the permit and support of the measurements. the crew and officers of sa agulhas and chc africa are thanked for their logistical support. thanks to martin feller and werner bauer from geophysical observatory fürstenfeldbruck (fur) and to marko leitner for their help with the preparation of the repeat station measurements. the measurements were conducted with funds from fur during a stay of jm on tristan da cunha that was supported by deutsche forschungsgemeinschaft (dfg). references baker, p.e., i.g. gass, p.g. harris and r.w. le maitre (1964): the volcanological report of the royal society expedition to tristan da cunha, 1962, phil. trans. r. soc. lond. a, 256-a, 439-578. campbell, w. (2003): introduction to geomagnetic fields, (cambridge university press, 2nd ed). friis-christensen, e., h. lühr and g. hulot (2006): swarm: a constellation to study the earth’s magnetic field, earth planets space, 58, 351-358. gubbins, d., a.l. jones and c.c. finlay (2006): fall in earth’s magnetic field is erratic, science, 312, 900-902. heirtzler, j., h. allen and d. wilkinson (2002): everpresent south atlantic anomaly damages spacecraft, eos, transactions american geophysical union, 83, 156-173. hulot, g., c. eymin, b. langlais, m. mandea and n. olsen (2002): small-scale structure of the geodynamo inferred from ørsted and magsat satellite data, nature, 416, 620-623. hulot, g., t.j. sabaka and n. olsen (2007): the present field, in treatise of geophysics, edited by g. schubert (elsevier, amsterdam), volume 5, chapter 2. jankowski, j. and c. sucksdorff (1996): guide for magnetic measurements and observatory practice, (international association of geomagnetism and aeronomy, warsaw), pp. 235. korte, m., m. mandea, h.j. linthe, p. kotze and e. ricaldi (2009): new geomagnetic field observations in the south atlantic anomaly region, annals of geophysics, this issue. kuvshinov, a., c. manoj, n. olsen and t. sabaka (2007): on induction effects of geomagnetic daily variations from eej and sq sources at low and middle latitudes, j. geophys. res., 112, b10102, doi:10.1029/2007jb004955. le, g., c.t. russell and k. takahashi (2004): morphology of the ring current derived from magnetic field observations, ann. geophys., 22, 1267-1295. macmillan, s., c.w. turbitt and a. thomson (2009): ascension and port stanley geomagnetic observatories and monitoring the south atlantic anomaly, annals of geophysics, this issue. matzka, j. and t.l. hansen (2007): on the various published formulas to determine sensor offset and sensor misalignment for the di-flux, publs. inst. geophys. pol. acad. sc., c-99 (398), 152-157. maus, s., s. macmillan, t. chernova, s. choi, d. dater, v. golovkov, v. lesur, f. lowes, h. lühr, w. maj, s. mclean, n. olsen, m. rother, t. sabaka, a. thomson and t. zvereva (2005): the 10th generation international geomagnetic reference field, geophys. j. int., 161, 561-565. newitt, l.r., c.e. barton and j. bitterly (1996): guide for magnetic repeat station surveys (international association of geomagnetism and aeronomy) pp. 112. olsen, n. and a. kuvshinov (2004): modeling the ocean effect of geomagnetic storms, earth planets space, 56, 525-530. olsen, n. and m. mandea (2006): a new approach to directly determine the secular variation from magnetic satellite observations, geophys. res. lett., 33, l15306, doi:10.1029/2006gl026616. olsen, n., h. lühr, t.j. sabaka, m. mandea, m. rother, l. tøffner-clausen and s. choi (2006): chaos – a model for the earth’s magnetic field derived from champ, ørsted, and sac-c magnetic satellite data, geophys. j. int., 166, 67-75. riddick, j. (pers. comm.): in the form of a note entitled: magnetic surveys in the south atlantic 2003 (british geological survey), version 1.0. sabaka, t.j., n. olsen and m.a. purucker (2004): extending comprehensive models of the earth’s magnetic field with ørsted and champ data, geophys. j. int., 159, 521-547. wiid, b.l. and a.m. van wijk (1960): geomagnetic observations on marion island, gough island and tristan da cunha, trans. roy. soc. s. afr., 36-2, 107-117. vol52,1,2009 28-05-2009 13:45 pagina 105 t h e mutual relations between solar activity, thunderstorms and vertical gradients of electro atmospheric potential a . m u r r i r . f u s a r i c. scuterini ( * ) r e c e i v e d on december 4th, 1973 summary. — t h e mutual relations are established between solar a c t i v i t y expressed in w o l f numbers, the vertical gradient of electro atmospheric potential measured in the geophysical observatory of macerata, the geomagnetic field measured in the observatory of the nearby t o w n of a q u i l a , and the thunderstorm a c t i v i t y o v e r the whole of the italian terr i t o r y . t h e mutual relationship between the vertical gradient, the geomagnetic field and the frequency of thunderstorms and solar a c t i v i t y are clearly evident. in particular the antiparallel behavior between solar a c t i v i t y and the vertical gradient, as w e l l as thunderstorm frequency lias been made clear b y observations carried out o v e r a period of 22 years. t h e results obtained f r o m a study of the geomagnetic field and the vertical gradients agree w i t h the research on a much shorter period of t i m e recently carried out b y r a o . observational values h a v e been treated as statistical elements ( 2 ) t o enable smoother readings, w i t h o u t h o w e v e r m o v i n g the m a x i m u m s and mínimums of the phenomenon in the g i v e n period of t i m e so that parallelisms and antiparallelisms resulting f r o m theso observational values are kept. t h e average values of the vertical gradients of electro atmospheric potential were derived f o r electrically calm days. f r o m these 5 days per month were chosen b y probability m e t h o d ; the averages were then calculated on these five days chosen stochastically. w e considered an electrically calm day one in which the sky was absolutely clear, w i t h no thunderstorms either in the v i c i n i t y or far f r o m the observ a t o r y , w i t h neither f o g or haze present. riassunto. — v e n g o n o stabilite le correlazioni fra a t t i v i t à solare espressa attraverso il numero di w o l f , il gradiente verticale di potenziale elettroatmosfcrico misurato nell'osservatorio geofisico di macerata, il campo ( * ) geophysical observatory, macerata. 38 a . m u k e i r . f u s a r i c. sctjterini geomagnetico rilevato nel prossimo osservatorio della città dell'aquila, la a t t i v i t à temporalesca di tutto il territorio italiano. l e correlazioni fra gradiente verticale, campo geomagnetico, frequenza dei temporali ed a t t i v i t à solare appaiono evidenti. in particolare l'andamento antiparallelo f r a attività solare e gradiente verticale, come pure con la frequenza dei temporali è messa in evidenza da osservazioni che vertono su di un periodo di 22 anni. l e considerazioni dell'andamento del campo geomagnetico e del gradiente sono concordanti con quanto recentemente t r o v a t o por un periodo assai più breve da rao. i valori grezzi sono stati trattati come elementi statistici ( 2 ) onde avere andamenti più lisci, senza peraltro che i massimi ed i minimi del fenomeno vengano spostati nel tempo, in modo che le condizioni (li parallelismo ed antiparallelismo che risultano dai dati grezzi siano p e r f e t t a m e n t e conservate. i valori medi del gradiente verticale di potenziale elettroatmosferico sono stati dedotti per i giorni elettricamente calmi. fra questi sono stati scelti con metodi probabilistici 5 giorni per mese; le medie sono state calcolate su questi cinque giorni prelevati stocasticamente. p e r giorno elettricamente calmo è stato scelto quello nel quale il cielo si è presentato assolutamente sereno, non si sono verificati temporali né prossimi all'osservatorio né lontani, non si sono avuto nebbie o foschie. 1. p r e l i m i n a r y remarks i n a r e c e n t s t u d y ( 1 3 ), o n e of our g r o u p e x p o u n d e d t h e results of r e s e a r c h on t h e b e h a v i o r of t h e y . g . e . a . p . , also r e l a t i n g i t t o solar a c t i v i t y e x p r e s s e d b y t h e b z i n d e x . o n t h a t o c c a s i o n t h e corr e l a t i o n v a l u e s b e t w e e n t h e t w o p h e n o m e n a w e r e c a l c u l a t e d , a n d i t w a s seen t h a t t h e t w o p r e s e n t e d a d e f i n i t e a n t i p a r a l l e l b e h a v i o r , w h i c h w a s also i n d i c a t e d b y t h e e x p e r i m e n t a l c u r v e s . o n l y d u r i n g t h e p r e s e n c e of d i s t u r b i n g f a c t o r s in t h e l o w e r a t m o s p h e r e , l i k e f a l l o u t caused b y a n u c l e a r e x p l o s i o n , d i d t h e c o r r e l a t i o n i n d e x d e n o t e d e f l e c t i o n w h i c h w a s t o b e a t t r i b u t e d t o m o d i f i c a t i o n s i n t h e e l e c t r i c f i e l d caused b y t h e e n t r a n c e of a l a r g e q u a n t i t y of r a d i o a c t i v e p a r t i c l e s w h i c h r e s u l t e d in a s c r e e n i n g of t h e e l e c t r o a t m o s p h e r i c f i e l d a n d c o n s e q u e n t i n t e r f e r e n c e w i t h i t s i n t e r d e p e n d e n c e on solar a c t i v i t y . f u r t h e r s t u d i e s h a v e b e e n m a d e w i t h a w i d e r r a n g e of v . g . e . a . p . r e a d i n g s as w e e x t e n d e d our research o v e r a p e r i o d of 22 y e a r s inc l u d i n g 3 y e a r s f r o m c y c l e 18 (1944,3 1954,4), t h e w h o l e of c y c l e 19 (1954,4 1964,8) a n d p a r t of c y c l e 20 f r o m 1964,8 up t o a n d i n c l u d i n g t h e w h o l e of 1972 (2 3'2 4). t i i e m u t u a l r e l a t i o n s b e t w e e n s o l a r a c t i v i t y e t c . 39 w e also compared the behavior of the geomagnetic field and storm activity over the whole of the italian territory. regarding the representation of solar activity w-e chose the wolf numbers for sunspots which, at least as a first approximation, offer an effective measurement of this activity, these numbers were supplied by the observatories at arcetri, rome and catania. finally all the observational data was treated statistically to smooth the curves representing the phenomena, without however altering the maximum and minimum positions (2'12). as in previous studies, the v.g.e.a.p. was calculated with values taken on electrically calm days. from these days, five were drawn by date, and from them wre calculated the average hourly and monthly values. w e only intended as suitable days those with fair weather, without any cloud formations, fog or electric disturbances. 2 . c o r r e l a t i o n o f t h e v . g . e . a . p . w i t h s o l a r a c t i v i t y research on the v.g.e.a.p., begun in 1951 with a quadrant electrometer and then continued with a valve electrometer, enabled the examination of the parameter values over a period of 22 years from 1951 up to 1972, and this research is still being continued today. the curves in fig. 1 illustrate the antiparallelism between solar activity and the v.g.e.a.p. solar activity being represented by its log graph. if variations in solar activity are in fact the primary cause of variations in the v.g e.a.p.; the effect, which is very often in logarithmical relation to the cause and therefore the correlation, should be more apparent with the log wm (solar activity) curve. in fact this correlation is represented in fig. 1 and the antiparallelism is, wre think, quite significant. to the minimum solar activity recorded in 1954 there corresponds, with marked adherence in time, the v.g.e.a.p. maximum, the latter being one of the highest recorded during the whole period of research. the solar maximum of 1957 which coincided with the maximum of the 80 years period of solar activity did not apparently determine a major decrease in the v.g.e.a.p., even if comparisons with successive recordings have been rendered quite difficult owing to the fact that during recent years there has been an erosion of daily values and averages of the gradient due to the progressive increase in pol40 a . m u k e i r . f u s a r i c. sctjterini lution in the area where recordings are effected, with consequent alterations in the local electro atmospheric field. this pollution by markedly increasing the relative humidity and thus favouring the formation of fog and nigh mists, has altered the vertical gradient by lowering its average, as is to be expected since the v.g.e.a.p. is inversely proportional to atmospheric humidity (6'9-21). the following minimum in solar activity, which extended over a longer period of time than the previous one, is correlated with a much less evident gradient maximum, as is easily seen by the figures in table 1 and illustrated in fig. 1. pig. 1 the vertical potential gradients smoothing; the geomagnetic field; thunderstorm frequency, and logs of solar activity expressed in w o l f numbers. the following solar maximum of 1970 was preceded by quite a long period of fluctuating activity which presented a decrease in 1968 and 1969 compared with values reached in .1967. t i i e m u t u a l r e l a t i o n s b e t w e e n s o l a r a c t i v i t y e t c . 4 1 i t was possible to note yet once again that these variations in solar activity were accompanied by fluctuations in an antiparallel manner in the y.g.e.a.p. even though these fluctuations were limited. in fact there corresponds to the secondary minimum in solar activity thus formed in 1968, a halt in the gradual decrease in the gradient values with a tendency to return to higher values. i t can also be seen that, while for the solar minimums and maximums recorded in 1951 and 1957 the corresponding minimums and maximums in the gradient appeared during the same period (the time lapse not exceeding 2 months), the following coincidences were slightly displaced in time, as the minimums and maximums of the gradient appeared a few months after the maximums and minimums in solar activity. this fact can be justified by the seasonal variations of the gradient, which sometimes slightly modify its behavior. this is what happened during the solar maximum of july 1970 which was followed by the gradient minimum during the last months of that year. the fluctuations in solar activity also have their influence in establishing a slight time lapse in the coincidence between the maximums and minimums of the two although they still maintain in their general behavior a clearly antiparallel structure. 3 . c o r r e l a t i o n w i t h t h e g e o m a g n e t i c f i e l d during our research we also considered the values of the geomagnetic field and its average behavior, which has been illustrated together with the other values in fig. 1. the values of this parameter were taken from the bulletin issued by the aquila observatory. the field values recorded in aquila were chosen in consideration of the fact that it is the nearest geomagnetic observatory to our research station. the behavior of this parameter is already known, presenting, as it does, its maximums and minimums in agreement with those of solar activity. the values at our disposition start from 1958 and clearly state their antiparallel behavior respecting y.g.e.a.p. values. this behavior had already been illustrated by rao in 1970 (25) for the same period. 42 a . m u k e i r . f u s a r i c. sctjterini i t can be seen that, although the antiparallelism between the two parameters is maintained, the maximums and minimums of the two phenomena do not always coincide in time, but are separated by variable periods, as lias already been noted regarding the solar and v.g.e.a.p. maximums and minimums. on the other hand this observation, limited to the period considered, can also be applied to the very short periods considered by eao in this work which we have already quoted. 4. correlation w i t h thunderstorms i n i t a l i a n region the frequency of thunderstorms over the whole of the italian territory from 1953 until 1970 has also been considered, and is indicated in pig. 1. as can easily be seen, the frequency of thunderstorms presents a minimum in relation to the solar maximum of 1957, a maximum in correspondence with the solar minimum of 1954 and a minimum in relation to the maximum of 1970. therefore we can add that what has already been said regarding the other phenomena examined can also be applied to thunderstorm activity, i.e. the correspondence between thunderstorms and solar activity is marked when the latter is well evidenced, and less noticeable during other periods, as can be seen by the thunderstorm activity 1968/1972. i t is however evident that, during the period considered and in our area the y.g.e.a.p. and thunderstorm activity are correlated in a positive manner with parallel behavior, even if the coincidences in time between the maximums and minimums, as has already been noted, present slight discrepancies. in a significant recent publication sarukhanyan and smirnov (2fl) have determined the correlation between the atmospheric circulation in the northern hemisphere and solar activity over the past 50 years and reach interesting conclusions, which, we think, confirm what we have discovered regarding thunderstorms. in fact the two russian authors determine that the frequency of atmospheric zonal processes in the northern hemisphere decrease with the increasing solar activity during the 11 years cycle, whilst the frequency of these processes increases when the sun's activity diminuishes. t i i e m u t u a l r e l a t i o n s b e t w e e n s o l a r a c t i v i t y e t c . 4 3 since the frequency of zonal processes determines and establishes the formation of thunderstorms, the russian publication would confirm the antiparallel correlation between the solar cycle and thunderstorm activity, as we have said, or at least this has been so for the past 50 years. 5 . d i s c u s s i o n a n d c o n c l u s i o n s from what has been briefly set forth above it can be seen that, in the area of observation from 1951 to 1972, the v.g.e.a.p.; was antiparallel to solar activity, indicated by the number of w„, sunspots recorded at the observatories of arcetri, catania ,and rome. a t the same time, having considered the behavior of the geomagnetic field (measured at the nearby observatory of aquila) it was noticed that the y.g.e.a.p. and the geomagnetic field (in this area and for the given period of time) could be considered as very near antiparallels, thus confirming what has been discovered by other for shorter periods of time. our study of thunderstorm activity, covering the whole of italy for further precision in determining stormy days, has enabled us to point out the parallelism between thunderstorm activity and the v.g.e.a.p., and that both of these are antiparallel to the solar activity curve as defined above. on the other hand it is necessary to point out that the authors are well aware that what has been found during their research appears at least partially in opposition to the findings of others (16.17) as for example bauer, who when considering sunspots from 1902 to 1922 and the behavior of the electric field found a parallelism (19 vol. i i pag. 52) or brooks who discovered that thunderstorm activity and sunspots during a cycle were parallel, whilst in our research we have found them to be decidedly antiparallel. according to eussian research workers (likter i . ya and coll. written communication to the e.a.'s fourth international conference tokyo 1968 quoted by markson) (20) the antiparallel behavior similar to the one observed by us is typical of the equatorial regions. eao, already quoted by us, found antiparallel behavior between the v.g.e.a.p. and the geomagnetic field for alpine stations. the behavior of the v.g.e.a.p. was compared to the polarity of the sunspots (northern hemisphere) and to the sun's poloidal field 4 4 a . m u k e i r. f u s a r i c. sctjterini in t h e v a r i o u s cycles w h i c h w e considered, a n d in our o p i n i o n t h e r e is no i m m e d i a t e c o r r e l a t i o n b e t w e e n these p h e n o m e n a . i n conclusion w e can a d d t h a t , c o n t i n u i n g our s y s t e m a t i c research on t h e y . g . e . a . p . in our o b s e r v a t o r y w e h a v e d u r i n g this p e r i o d , f o l l o w i n g t h e m a x i m u m solar a c t i v i t y of 1970,8, n o t i c e d a c o n s t a n t a n d p r o g r e s s i v e increase, m o n t h b y m o n t h , of t h e v e r t i c a l g r a d i e n t v a l u e s . t h i s leads us t o suppose t h a t also d u r i n g t h e f u t u r e m i n i m u m solar a c t i v i t y , t h e g r a d i e n t v a l u e s w i l l p r e s e n t a r e l a t i v e m a x i m u m , thus p r o b a b l y c o n f i r m i n g f o r t h e solar c y c l e 20 t h e a n t i p a r a l l e l b e h a v i o r a l r e a d y n o t e d d u r i n g p r o c e d i n g cycles. b i b l i o g r a f i a i1) aubrey m.-cerisier je., 1971. le vents solaire. 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" r e v i e w s of geophysics", 4, 3, (febbraio), pag. 3293 6 2 . 48,06,2005misc 973 annals of geophysics, vol. 48, n. 6, december 2005 key words digital photogrammetry – automatic processing – dem – precision – monitoring 1. introduction digital elevation models (dems) play an important role in earth surface deformation studies: eruptive events, gravitative instabilities, landslides and glacier evolution, geomorphological variations and crustal deformations can be detected and evaluated by means of multi-temporal dem comparisons (kaab and funk, 1999; baldi et al., 2002; honda and nagai, 2002; mora et al., 2003; van westen and lulie getahun, 2003). models registered in a common reference system can be directly compared defining height residuals to identify deforming areas, estimate mass movement and physical surface changes. co-registration is a crucial operation which depends on the correct choice and measurement of control points (the final results could be degraded by systematic errors). several techniques are suitable for dem extraction like digital aerial and terrestrial photogrammetry, airborne and terrestrial laser scanning, gps methodology with its different measurement approaches and active and passive remote sensing, with optical satellite imagery systems (fraser et al., 2002). in particular, digital aerial photogrammetry is a powerful tool in surface model generation extracting high resolution dems by means of automated image matching procedures (farrow and murray, 1992; heipke, 1995; kraus, 1998; mitchell and chadwick, 1999; schenk, 1999). several institutes and agencies planned and executed photogrammetric flights over italian regions providing a wide historical data set: in particular, in 1954, 1976 and 1998 an almost automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring massimo fabris (1) and arianna pesci (2) (1) dipartimento di fisica, settore geofisica, università degli studi di bologna, italy (2) istituto nazionale di geofisica e vulcanologia, sede di bologna, italy abstract automated procedures for photogrammetric image processing and digital elevation models (dem) extraction yield high precision terrain models in a short time, reducing manual editing; their accuracy is strictly related to image quality and terrain features. after an analysis of the performance of the digital photogrammetric workstation (dpw) 770 helava, the paper compares dems derived from different surveys and registered in the same reference system. in the case of stable area, the distribution of height residuals, their mean and standard deviation values, indicate that the theoretical accuracy is achievable automatically when terrain is characterized by regular morphology. steep slopes, corrugated surfaces, vegetation and shadows can degrade results even if manual editing procedures are applied. the comparison of multi-temporal dems on unstable areas allows the monitoring of surface deformation and morphological changes. mailing address: dr. arianna pesci, istituto nazionale di geofisica e vulcanologia, sede di bologna,via d. creti 12, 40128 bologna, italy; e-mail: pesci@bo.ingv.it 974 massimo fabris and arianna pesci total coverage of the peninsula territory was achieved. even if many processing steps, starting from image orientation (hellwick et al., 1994; tang and heipke, 1996; heipke, 1997; dowman, 1998;) to dem extraction (ackermann, 1996; gasior, 1996; bacher, 1998; karras et al., 1998) are automated, the final ‘check’ still depends on the operator’s stereoscopic vision: manual editing is required to correct correlation errors where several points do not fit the ground. at present many acquisitions are performed using digital cameras providing high resolution images directly, while in the frame of digital processing, historical images (film photographs) acquired by means of analogic cameras have to be converted into a digital format using high quality photogrammetric scanners. dem internal precision and grid dimension are strictly related to the quality of metric camera, photographs scale, scanner efficiency, resolution at ground, view angles, surface morphology, vegetation, shadows and atmospheric conditions. in this paper, different automated procedures are analysed and compared to provide accurate dems in the shortest time, reducing manual interactions in the frame of the digital photogrammetric workstation dpw 770 helava (dowman, 1990; dowman et al., 1992). several applications were performed to define the realistic dem precision, estimating threshold values for movement detection on different area typologies. 2. aerial photogrammetry overview the photogrammetric technique defines shape, size and position of objects using images taken from different points of view. images can be acquired by analogic or digital cameras; because digital photogrammetry processes numeric images, the available film frames must first be digitized and translated from a continuous to a discrete data set. at the same time the intensity of the signal is given by a grey or colour scale assigned to each pixel. in general, using the metric properties of an image, together with the coordinates of several ground control points (gcp), the geometry and the position of an object can be described in a defined reference frame. the processing of stereo digital images can be automated using image matching procedures, based on well defined techniques relative to shape or grey/colour intensity for the same zones (kraus, 1998). generally, matching algorithms are classified into three principal groups: the abm (area based matching), the fbm (feature based matching) and the rm (relational matching) (forstner, 1986; shapiro and haralick, 1987; hannah, 1989; cho, 1995; heipke, 1996; kraus, 1997; wang, 1998). in abm small windows composed of grey values are used as matching primitives to perform comparisons over homologous zones of images until the best correspondence is reached: cross correlation or least square matching (lsm) methods can be used. in fbm, features (points, lines, edge elements, small regions, or other elements) are extracted in each image individually prior to matching them: the search is performed in terms of coordinates relative to the same objects characterized by a set of attributes (length, size, curvature, average brightness for regions, etc.). the rm concerns global features that are composed of different local features: the relations can be geometric (angle, distance, etc.) or radiometric (grey level between two adjacent regions, etc.). digital photogrammetry is able to acquire many 3d points for high spatial resolution dem generation through semi-automatic procedures, overcoming the problems of processing time and costs of the analytical approach. the accuracy of digital photogrammetry mainly depends on the camera-object distance, image quality and resolution; the latter is related to the digital camera characteristics or to the scanner resolution when the ‘ analogic’ one is used. dealing with terrestrial applications, where the camera-object distances are short (from few to some hundred meters), the precision of extracted dems ranges from millimetric to few centimetres. in the case of aerial surveys, the image scale may range from 1:1000 to 1:50 000 or more; the precisions decrease from few centimetres to several meters (kraus, 1998). obviously, the image correlation method adopted for the dem automatic extraction automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring 975 plays a key role in the minimization of errors of the photogrammetric working process. the standard procedures to generate dems are based on fundamental steps which consist in internal orientation, external orientation (registration into a defined reference system) and point extraction. the internal orientation is performed to define the frame position inside the camera, using the camera calibration certificate (provided by the factory) to correct for distorsion and to assign known coordinate values at specific points, that is fiducial markers (fig. 1). the external orientation is obtained by two subsequent steps: relative and absolute orientation. in the first case the aim is to create the stereoscopic model in an arbitrary relative system using points common to both images (tie points). the absolute orientation is the transformation of the generated stereoscopic model into an external reference frame (for instance utm) defined by the coordinates of several ground control points, recognized on the images (fig. 1); an s-transformation provides the registration of the two systems. the aim of this work is to analyse the automated procedures for dem extraction using the digital photogrammetric workstation dpw 770 helava, and to validate their internal and absolute accuracy. fig. 1. aerial digital image at 1:17 000 scale (northern apennine area): four fiducial markers are visible in the corners; the ground control points are uniformly distributed over the overlapping zone. 976 3. the helava system the digital photogrammetric workstation dpw 770 helava processes aerial photographs or other digital images to obtain maps, orthophotomaps, photomosaics, digital terrain models and 3d images. the system is based on image automatic correlation and the software implemented is the socet set (softcopy exploitation tool set, lh systems, llc, 1999). concerning the automatic measurement of dem from original images, it is possible to operate in a short time and to extract precise three-dimensional terrain models: the measuring speed is strictly related to the terrain configuration and to the quality of the digital aerial images: as stated, the three fundamental processing steps (internal orientation, external orientation and point extraction) are executed and dems are extracted using algorithms especially designed for different morphological characteristics of the study areas. the relative orientation procedure can be automated defining a geometry point distribution (tie point pattern), characterized by a specific point number and their position, and requiring the program to check for each point (fig. 2). naturally, the search for corresponding points is not always satisfied due to the different prospective acquisition of images: the pattern shown in fig. 2 is defined to correctly measure at least 9 points uniformly distributed on the overlapping area, according von gruber (ackermann and shade, 1993). the method was tested over a single stereo pair, over a strip and over a whole block (formed by more strips). results indicate that, in the frame of a single stereo pair or dealing with a strip (images sequences), about 50% of points are correctly measured providing a quite uniform distribution redundant data set (the percentage refers to very high point numbers). a manual interaction can be required when a complete block has to be processed because of a low transversal overlapping between strips but, in general, the automated procedure provides a time effective performance. the socet set software allows the automatic extraction of models using the hierarchical relaxation correlation (hrc) algorithm that consists in a least square correlation method (helava, 1988a,b). the search for the best correlation is performed following ‘pyramidal levels’: the image resolution is reduced (by a 2 factor, 1:2, 1:4, 1:8, etc.) for each level. the image matching starts from the lower resolution level to the original resolution, gradually improving fig. 2. tie point pattern manually defined and used for the processing. the geometry is well suited to cover the stereo pair and to facilitate the measurement of at least one point over each point groups (formed by 4 points). table i. class of the principal correlation algorithms (based on terrain slope). max terrain slope high speed high accuracy high accuracy high accuracy extraction extraction and artifact removal and high speed for very dense dem grids 20° flat flat_1 flat_plus flat_dense 30° rolling rolling_1 rolling_plus rolling_dense 50° steep steep_1 steep_plus steep_dense massimo fabris and arianna pesci 977 automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring the correlation (miller and de venecia, 1992). different approaches are available for different terrain characteristics (table i), (lh systems, llc, 1999) and the program is able to use a strategy sequence well suited for each morphological condition (fabris, 2004). the whole stereoscopic model is composed of several stereo pairs, and so each natural point belongs to more than two images according to the high overlapping percentage usually adopted (60-70% between subsequent images of a strip, 20-30% between adjacent strips). correlation algorithms, working on image grey and colour levels, are very sensitive to prospective differences; for this reason the operator may choose the best data for each area. in particular, three different approaches were used and compared based on different overlapping strategies: using ‘all the possible overlapping areas’, ‘each subsequent single pair’ and ‘the central part only of each single pair’. when the program works in a completely automatic mode, all the possible overlapping areas can be considered, and homologous areas characterized by very different prospectives may produce low correlation values. a manual interaction of the operator is therefore suitable to choose stereo pairs characterized by a correct overlap: generally, the subsequent images in the strip. a further discrimination concerns the choice of areas internal to the overlap to reduce errors over the image borders: the last operations take a long time. in the next, results relative to dem extraction following these criteria are represented, concerning a data set composed of 52 images of stromboli island distributed along 5 strips at a mean scale of 1:5000 and scanned by the wehrli raster master rm2 photogrammetric scanner at 24 µm (1016 dpi) resolution (fig. 3). fig. 3. stromboli island ortophoto (may 2001): coordinate are given into the utm system. 978 massimo fabris and arianna pesci in this case, and in the following examples, all the images used were kept together with their camera calibration certificate. stromboli island is mainly constituted by very steep vegetated zones: buildings and trees are usually distributed over slopes. an adaptive method was used to extract models over a 5 × 5 m grid. as stated, the presence of smoke cause correlation failure so to estimate the realistic accuracies of automatic extracted models, these areas were removed. when a complete images set composed of several strips is analysed, automatic registration can be complicated by a poor or anomalous prospective situation relative to the models used by the stations. figure 4 shows the results of the first completely automated process where ‘all the possible overlapping areas’ are considered and the processing for ‘each subsequent single pair’, respectively. in the first case, the result is satisfactory only in the lower part of the island (steepest zones are badly represented) while, in the last, performed leading to a set of dems which were merged averaging the heights of points in overlapping zones, problems were evidenced only over borders of stereo pairs. fig. 4. dem automatically extracted using the global elaboration of ‘all the possible overlapping areas’ and ‘each subsequent single pair’ respectively (utm system). fig. 5. dem automatically extracted using the ‘central part of each single pair’ and the reference dem obtained by means of editing operations, respectively (utm system). automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring 979 the processing for the ‘central part of each single pair’ was used to reduce these systematic errors: this procedure requires a manual selection of areas to be processed and increases interactive times. figure 5 shows the results together with the reference model obtained after an accurate editing. each result, that is each extracted dem, was compared to the reference model: residuals were analysed calculating their mean and the standard deviation values (table ii). the standard deviation values listed in table ii are not necessary representative of areas where manual editing is required, but indicate how well the computed model differs from the real one. the greater the error of the model is, the longer the time required to correct it; in fact the operator has to perform interactive adjustment of the dem to obtain a visual correspondence between the stereoscopic images and the measured points. in the latter case (c) examined above, the area percentage where editing was necessary, was about 30% of the total surface. 4. validation, precision, repeability of photogrammetric dems and applications as stated, the internal precision of extracted dems is strictly related to the mean scale of photographs, image quality, pixel dimension and, obviously, morphology of the area: empirically it is estimated about 0.01-0.02% of the mean relative flying height. the real accuracy of data can be verified performing direct comparisons between different models (geo-referenced models) relative to the same ‘stable’ areas, that is undeformed areas. concerning models relative to different epochs, the height differences over the grid points can be calculated leading to residuals distribution characterized by a mean and a standard deviation value. the first indicates the goodness of registration in the same reference system and the second the internal precision. the morphology of the area, as previous described, influences the achievable accuracies and so it will be considered in the following. dems generated at different epochs have to be oriented in the same reference frame when a direct comparison is performed to obtain height differences; this may be accomplished by identifying and measuring the same ground control points whose coordinates are known in an external and stable reference system or using a sufficient number of natural control points recognized a posteriori over all images and measured by means of gps surveys. dealing with old images, the absence of a sufficient number of common points, may cause problem in registration introducing systematic effects. an example of the use of aerial photogrammetry for the study of unstable areas, concerns the vulcano island (aeolian islands, italy) (fig. 6) and in particular the «forgia vecchia» slope; in this region many surveys were performed from 1971 up to now. the main technical characteristics of the last four photogrammetric surveys performed on 1993, 1996 and 2001 are listed in table iii, providing information about used calibrated camera and images resolution. using ground control points, images of 1996 surveys were oriented and the models were extracted into the same reference system. these points, that are the centres of targets (especially designed to be well identified a posteriori on the images) were rapidly measured during a gps campaign carried out at the same time as photogrammetric flight (september 1996). the images relative to different years (1993 and 2001), were oriented using 12 natural points belonging to and measured from the 1996 stereoscopic models, recognized on all the other table ii. comparisons of automated model with reference one: residuals distributions indicate that the case c lead to the best value of standard deviation. type mean (m) σ (m) a all the possible 79.71 177.63 overlapping areas b each subsequent 2.18 25.05 single pair c each central part only 0.32 6.11 of each single pair 980 massimo fabris and arianna pesci fig. 6. vulcano island ortophoto (september 1996): coordinate are given into the utm system. table iii. vulcano photogrammetric surveys performed from 1993 to 2001: photogrammetric camera, calibrated focal length, mean image scale, photogram and strip numbers, resolution, pixel and ground pixel dimensions are listed. characteristics 1993 survey 1996 survey 1996 survey 2001 survey aerial photogrammetric camera zeiss rmk a 15/23 wild rc 20 wild rc 20 wild rc 30 focal length (mm) 153.33 153.20 153.20 152.929 mean image scale 1:5000 1:10 000 1:5000 1:5000 no. photograms 2 36 4 4 no. strips 1 4 1 1 resolution (dpi) 1016 1016 1016 1016 pixel dimension (µm) 24 24 24 24 ground pixel dimension (cm) 13 25 13 13 table iv. images orientation report: ground control points number, tie points number, residual of external orientation over the 3 cartesian components are listed. moreover the chosen grid dimension and dtm points number are reported. data no. gcp no. tie points residuals of external orientation dem grid (m) dem points e (m) n (m) h (m) 1993 10 8 0.05 0.05 0.02 1 1 231 782 1996 18 376 0.10 0.09 0.06 10 543 320 1996 5 23 0.07 0.06 0.02 1 1 840 703 2001 9 26 0.07 0.07 0.05 1 2 658 635 automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring 981 images and localized in stable areas, providing the common reference system (table iv). for the oldest surveys (1971 and 1983) only dems derived from stereo pairs at about 1:10000 scale, produced by other operators, were at our disposal; in this case the registration was obtained by a least square surface matching procedure with respect to the 1996 model. comparing models relative to 1983 and 1993 it was possible to measure the landslide collapse effects occurred in 1988: the mass volume involved was estimated about (193 ± 5) × 103 m3. subsequent comparisons (1996-2001) did not reveal any other significant movement (fig. 7). the comparisons of dems were also performed over four areas characterized by different vegetation and morphological features for the models of 1993, 1996 (at mean scale 1:5000) and 2001 surveys (fig. 8a-d). table v lists the mean and standard deviation values of residuals. when the presence of vegetation is relevant, manual editing is required to adapt the model to fig. 8a-d. area characterized by different morphologies and/or presence of vegetation: a) flat area without vegetation; b) steep area without vegetation; c) steep area and sparse vegetation; d) steep area and dense vegetation. fig. 7. residuals computed over vulcano island from 1983 to 1996 (utm system): it is possible to observe the effects produced by the 1988 activation. a b c d 982 massimo fabris and arianna pesci the real surface: this operation is performed using several visible points on the ground to define breaklines. results are in agreement with the expected theoretical values concerning flat areas; obviously the standard deviation increases over zones where vegetation is quite relevant. a further comparison was performed using dems extracted from images acquired during the 1996 survey, and characterized by different flying height (mean scales 1:10 000 and 1:5000) (table vi). in this case the attention was focalized on image scale dependence. precision are still in agreement with expected values. a further check of the accuracy of dems was obtained by a kinematic gps campaign performed over the top of volcano cone, which provided absolute coordinates of points, distributed along independent tracks (fig. 9), whose precision was estimated by a cross-over analysis to be about 4 cm. gps surveying was executed (september 1996) by three independent operators walking over the edge of the crater, keeping the antenna manually vertical over the ground and collecting data at 1 s sampling rate using trimble 4000ssi receivers. baselines of a few kilometres were formed with reference gps stations, fixed on geodetic vertices, and processed by means of the trimble geomatics office (tgo) software. geographic coordinates were obtained for each measurement epoch using especially designed procedures for kinematic processing to solve ambiguities on-the-fly (beutler et al., 1995). results obtained from kinematic applications and traditional static gps measurements of control points were given in the wgs84 reference frame, and converted into the utm cartographic system (32n). the validation of the 1 m × 1 m dem of 1996 survey (1:5000 scale) was assured comparing the gps data to the photogrammetric model: in fact, height residuals distribution was characterized by a mean value of 0.01 m and a standard deviation of 0.18 m (baldi et al., 2000). a second investigated zone concerned the area external to the rocca pitigliana landslide, located in the northern apennines (bologna, italy) (fig. 10): the images data set was acquired in 1976, 1986, 1988 and 1993 at mean table v. results from direct comparisons are listed: mean and standard deviation are estimated over four different areas, comparing about 500 000 data points. vulcano (1 m × 1 m grid) area type statistics over 2001-1993 2001-1996 comparisons (m) (m) max value 0.206 0.233 flat area min value −0.304 −0.221 without mean 0.067 0.028 vegetation standard 0.055 0.052 deviation max value 2.021 1.832 steep area min value −1.495 −2.068 without mean 0.157 0.218 vegetation standard 0.263 0.251 deviation max value 4.516 7.086 steep area min value −6.725 −6.953 sparse mean 0.150 0.068 vegetation standard 0.397 0.421 deviation max value 12.569 13.217 steep area min value −11.692 −12.853 dense mean 0.159 0.158 vegetation standard 0.654 0.714 deviation table vi. direct comparisons between models relative to the same area and to the same epoch but obtained from different surveys at different image scales. vulcano (10 m × 10 m grid-1 m × 1 m grid) area type statistics over 1996-1996 comparison (m) flat area max value 0.323 without min value −1.706 vegetation mean −0.147 standard deviation 0.467 steep area max value 1.850 without min value −2.233 vegetation mean −0.366 standard deviation 0.748 steep area max value 7.205 sparse min value −11.594 vegetation mean −0.482 standard deviation 1.461 steep area max value 11.297 dense min value −12.959 vegetation mean −0.471 standard deviation 1.586 automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring 983 fig. 9. gps kinematic: operator tracks are evidenced using different colours for different surveying. fig. 10. ortophoto of the rocca pitigliana area (1993 survey): coordinates are given in the utm system. 984 scales of 1:17 000, 1:37 000, 1:34 000 and 1:36 000 respectively (pesci et al., 2004). surveys characteristics and image processing reports are listed in tables vii and viii. the area presents a complex morphology, vegetated zones and a small portion affected by active landslide, not included in the comparisons (fig. 10). following the same approach described above, results were obtained relative to the whole area minimizing the effect of vegetation by editing procedures (table ix, fig. 11). there is a systematic effect in the mean values, probably due to the registration strategy which consisted in an a posteriori recognition on all images of natural points measured with a rapid static gps survey; considering the photographs scale, the systematic differences are acceptable. also in this case the residuals map presents anomalous values in correspondence of dip slopes and vegetated area. in the landslide, which reactivated in 1994 (autorità di bacino del reno, 1998), only a significant but restricted mass movement is detectable in the upper part of the active area and in the lower part of the slope (fig. 12). table vii. rocca pitigliana photogrammetric surveys performed from 1976 to 1993: photogrammetric camera, calibrated focal length, mean image scale, photogram and strip numbers, resolution, pixel and ground pixel dimensions are listed. characteristics 1976 survey 1986 survey 1988 survey 1993 survey aerial photogrammetric camera zeiss rmk a 15/23 zeiss rmk a 15/23 wild rc 10 wild rc 20 focal length (mm) 153.15 153.33 153.26 153.20 mean image scale 1:17 000 1:37 000 1:34 000 1:36 000 no. photograms 2 2 2 2 no. strips 1 1 1 1 resolution (dpi) 2116 2116 2116 2116 pixel dimension (µm) 12 12 12 12 ground pixel dimension (cm) 20 45 41 44 table viii. images orientation report: ground control points number, tie points number, residual of external orientation over the three cartesian components are listed. moreover the chosen grid dimension and dtm points number are reported. data no. gcp no. tie points residuals of external orientation dem grid (m) dem points e (m) n (m) h (m) 1976 9 15 0.09 0.10 0.04 1 2 504 166 1986 7 16 0.05 0.07 0.05 1 1 413 192 1988 9 14 0.04 0.05 0.02 1 2 230 452 1993 10 14 0.05 0.06 0.02 1 2 513 332 table ix. mean and standard deviation of residuals relative to the 1976-1993 time span in the external landslide area. rocca pitigliana (1× 1 m grid) comparisons max value (m) min value (m) mean (m) standard deviation (m) 1993-1976 13.136 − 16.617 − 0.403 1.113 1993-1986 12.833 − 17.291 − 0.741 1.367 1993-1988 8.478 − 12.001 − 0.546 1.192 massimo fabris and arianna pesci 985 automated dem extraction in digital aerial photogrammetry: precisions and validation for mass movement monitoring fig. 12. comparison of dems of the rocca pitigliana landslide area obtained from two aerial photogrammetric surveys (1993, 1976). fig. 11. residuals relative to the 1976-1993 time span. the area inside the blue line indicate the active slide body. 986 massimo fabris and arianna pesci 5. discussion and conclusions the aim of this work was to analyse the digital photogrammetric procedures for automatic dem extraction by means of the dpw 770 helava station (software socet set) and to evaluate the accuracy of the models obtainable in areas characterized by different morphology. the images used were acquired in recent years over stromboli island (2001), vulcano island (1983÷2001) and rocca pitigliana landslide (1976-1993); scales range from 1:5000 to 1:37 000. the first step was to check the relative images orientation using a defined tie point pattern: results indicate that, in the frame of a single stereo pair or dealing with a strip (images sequences), about 50% of points are correctly measured providing a redundant data set (the percentage refers to a very high number of points). manual interaction may be required when a complete block (more strips) has to be processed; this may be due to a low transversal overlapping between strips over steep slopes. the efficiency of default strategies used by the socet set software (based on correlation algorithms especially designed for different terrain characteristics) has been demonstrated. moreover the choice of areas internal to the overlap (‘central part of each single pair’) reduced errors in automated extraction, reaching higher precision with minimum operator interaction. however the ‘editing’ is necessary following the automatic procedures to adapt the digital elevation model to the real terrain morphology in about 30% of the whole area. the real internal precision of models was checked by means of direct comparisons over same areas providing the reliability of photogrammetric technique and the achievable accuracy depending on image quality, mean scale, pixel dimension and terrain characteristics. in particular, it was noticed that these values are quite similar to theoretical values (0.01-0.02% of the flying height) when the morphology is regular and the vegetation is sparse. obviously the mean image scales play an important role in the final precisions. the photogrammetric models are repeatable (evidenced in table v and table ix) providing the validation and reliability of such technique. for each application it is necessary to estimate a realistic precision value, selecting areas where there is no evidence of significant mass movements: in this way, the standard deviation of residuals can be representative of model accuracy and can be used as a threshold to detect high deformed zones and estimate morphological changes. comparing high precision and co-registered models, surface monitoring is allowed and provided; the described applications, relative to rocca pitigliana landslide and vulcano flank, confirm the efficiency of the methodology. the 3d topographic observations, acquired periodically over unstable areas like volcanoes and landslides, can be used for deformation monitoring and detection of morphological changes. different studies for modelling processes can be performed based on the quality and resolution of available dems. digital photogrammetry is currently an efficient method for collecting data useful for dem extraction and recent developments, both in recording techniques and data processing, allow an improvement of resolution and a faster result production. moreover, the existing historical images used in this work provided reference models, preceding landslide collapse and volcanic activity and allowing mass movement and surface changes estimation. acknowledgements this research was developed in the context of miur 2001 («controllo di fenomeni di dissesto idrogeologico in aree urbanizzate e di particolare rilevanza storico-artistica mediante l’integrazione di tecniche di telerilevamento e di monitoraggio») and gnv project («sviluppo ed applicazione di tecniche di telerilevamento per il monitoraggio dei vulcani attivi italiani»). the authors thank fabiana loddo (ingv researcher) for assistance during gps surveys and data processing. references ackermann, f. 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(received february 14, 2005; accepted may 20, 2005) layout 6 scalable multi-gpu implementation of the magflow simulator eugenio rustico1,*, giuseppe bilotta1,2, alexis hérault2,3, ciro del negro2, giovanni gallo1 1 università di catania, dipartimento di matematica e informatica, catania, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy 3 conservatoire des arts et métiers, département ingénierie mathématique, paris, france annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5342 abstract we have developed a robust and scalable multi-gpu (graphics processing unit) version of the cellular-automaton-based magflow lava simulator. the cellular automaton is partitioned into strips that are assigned to different gpus, with minimal overlapping. for each gpu, a host thread is launched to manage allocation, deallocation, data transfer and kernel launches; the main host thread coordinates all of the gpus, to ensure temporal coherence and data integrity. the overlapping borders and maximum temporal step need to be exchanged among the gpus at the beginning of every evolution of the cellular automaton; data transfers are asynchronous with respect to the computations, to cover the introduced overhead. it is not required to have gpus of the same speed or capacity; the system runs flawlessly on homogeneous and heterogeneous hardware. the speed-up factor differs from that which is ideal (#gpus×) only for a constant overhead loss of about 4e−2 · t · #gpus, with t as the total simulation time. 1. introduction numerical problems that expose a high degree of intrinsic parallelism can often be mapped to more than one level of parallelism. if it is possible to model a problem as a set of pseudo-independent subproblems, then it is possible to split the set of computations needed to solve it into two or more partitions that can be executed separately. very few numerical problems have completely inter-dependent subproblems, such as astrophysical n-body simulations; sometimes it is nevertheless possible to parallelize such problems by means of numerical approximations or by adding specific constraints to the model. cellular automaton methods are by definition made up of quasi-independent subproblems: the state of every cell of the automaton depends only on the previous state of the same cell and on the state of the neighbor cells. given a cell and its neighborhood, we can compute its next state in parallel and with almost no knowledge about the other cells. the same reasoning applies to sets of cells that are in turn independent from other sets, except for their neighborhood. in the general case, it is possible to exploit a two-level parallelism and to split an already parallel cellular automaton simulation into parts to be executed on different devices, which can include a graphics processing unit (gpu) card, a central processing unit (cpu) core, the node of a cluster, or even a computer in a network. the real feasibility and the benefit of this theoretical possibility are, however, to be evaluated case by case, as there may be model peculiarities that require specific adaptations or that conflict with the technical limitations of the underlying architecture. the most limiting factors are typically the latency and/or the bandwidth of the interdevice communication channel, which is usually orders of magnitude slower than any intra-device communication. the magflow lava simulation model [vicari et al. 2007] is the cellular automaton developed by the sezione di catania of the istituto nazionale di geofisica e vulcanologia (ingv), and it represents the peak of the evolution of cellbased models for lava-flow simulations. the conversion of the magflow cellular automaton from serial single-cpu code to a parallel, gpu-based implementation is discussed in bilotta et al. [2011] in this volume; here, we show how we exploited a second level of parallelism by running a simulation on two or more gpus simultaneously. there are at least two ways to exploit a two-levels parallelism in the evolution of a cellular automaton. the simplest and most intuitive is to spatially partition the automaton into disjointed areas and to assign each area to a separate device. as we need to operate locally and read information about the neighboring cells, these areas can never be truly disjointed; it is therefore more correct to refer to them as subsets rather than partitions. we could instead operate the division in the domain of computations, thus realizing a pipeline: each device is assigned to all of the cells in the domain, but elaborates only for a specific phase of the evolution. unfortunately, this approach presents three major drawbacks: article history received november 23, 2010; accepted july 15, 2011. subject classification: computational geophysics, general purpose gpu modeling, hpc, parallel programming, gpu, multi-gpu, hazard, lava. 592 special issue: v3-lava project 1. it is not possible to scale to an arbitrary number of devices. 2. it is not trivial to balance the computational load among the devices. 3. it requires the complete domain to be constantly transferred from one device to another, to keep every device of the system updated about the current automaton status. for these reasons, we chose the former division and we focus on how the automaton can be split in the spatial domain. 2. the compute unified device architecture architecture a gpu is a specialized microprocessor that is traditionally used to offload and accelerate graphic rendering pipelines from a cpu. while the market of videogames has been more and more demanding of computational power, gpus have evolved to include on-board microchips with direct hardware support for common two-dimensional (2d) and 3d graphics primitives. they have become incredibly faster than cpus in parallel, arithmetic-intensive tasks, like matrix manipulations and coordinate projections. in 2001, nvidia released the first chip capable of programmable shading, i.e. a chip that it was possible to program with a customized arithmetic pipeline for very sophisticated surface renderers. as gpus became programmable and really fast in matrix and vector operations, engineers and scientists started to use them for non-graphical calculations, by mapping a non-graphic problem to a graphic one, and letting a gpu compute it. in 2007, nvidia released the compute unified device architecture (cuda), a hardware and software architecture that has explicit support for general purpose programmability. cuda has enabled programmers to implement generic algorithms on nvidia gpus using an extension of the c programming language. soon after, the main competitor of nvidia, ati, also released a general purpose gpu architecture called stream, which has a slightly different access interface (an assembly-like language). a third, architecture-independent, parallel computing platform, opencl, was proposed by apple in 2008 and released in 2009, as the result of a collaboration with amd, ibm, intel and nvidia. opencl offers an abstraction layer between the application and the parallel computing hardware, which allows an application to be transparently executed on multicore cpus as well as on gpus; fine-tuning of the opencl code is, however, more complex, due to the wider variety of platforms on which it can run. we started porting magflow to gpus in 2007, when cuda was the only platform that allowed the use of standard c language to write kernels. since the opencl interface is similar to the low-lever cuda c api, we are considering the possibility to port the magflow simulator also to opencl. a modern gpu hosts a set of single instruction multiple data (simd) multicore processors, with a few hundred total cores. from the perspective of a programmer, a gpu can be seen as a coupled, external computer: it has its own ram and processors, and the typical workflow consists of sending the data to the gpu, asynchronously requesting a computation, and finally transferring back the resulting output. a sequence of instructions compiled to be executed on a gpu is called a kernel. kernels are instantiated in parallel threads, which for the convenience of the programmer, are grouped into 1d, 2d or 3d blocks; the thread blocks themselves can be organized into 1d or 2d grids. with a slight change in the programming logic, numerical problems that expose a high level of parallelism can achieve speed-ups of two orders of magnitude with respect to the correspondent cpu implementations. the main breakthrough of this phenomenon, however, is the high cost effectiveness of gpu-based solutions. a workstation with a modern gpu easily reaches the theoretical speed of 1.4 tflops (1012 floating-point operations per second) with an expense of less than €1,000 and a power consumption of about 600 w. even dedicated hardware like the nvidia tesla cards, which share the computing hardware with consumerlevel gpus, is still a very cost-effective solution. it is therefore not surprising that in may, 2010, a hybrid cuda-enabled chinese supercomputer, called nebulae, reached the theoretical peak performance of 2.98 pflops, ranking it first in the list of the most powerful commercial systems in the world [meuer et al. 2010]. although cuda is still young and continuously evolving, it is nowadays a de-facto standard among low-end and medium-end parallel computing platforms. 3. magflow on single gpus in this section, we present only a concise summary of the implementation of the magflow numerical model on cuda gpus, as a necessary basis to understand the multigpu structure and the additional problems that arise in the exploitation of a second level of parallelism. for a more detailed description, please refer to bilotta et al. [2011], in this volume. the magflow cellular automaton consists of a twodimensional grid of cells, each of which is described by five scalar quantities: ground elevation, lava thickness, heat quantity, temperature, and amount of solidified lava. the ground elevation is initialized once, and it remains constant throughout the simulation. we compute the state of a cell at time t from the state of the cell and its immediate neighbors at time t−1. the lava thickness varies according to the lava emission of the vents on the cell, if any, and to the amount of cross-cell lava flux, between the cell and its immediate neighbors (eight neighbors for square of cells). the flux depends on the height difference, and is computed using a steady-state solution for the 1d navier–stokes equations for rustico et al. 593 594 a fluid with bingham rheology. the actual amount of lava gained or lost by a cell at the end of each iteration of the automaton is given by the total flux of the cell (vent emissions plus cross-cell flux) multiplied by the time-step dt for that iteration. the higher the dt, the smaller the number of iterations we need to complete the simulation. however, the time-step is upper-bounded by a numerical constraint, to prevent nonphysical solutions. at each iteration, we are thus interested in computing the maximum time-step value allowed by each cell. we choose as the global time-step the minimum of these values, thus ensuring both adherence to reality and the minimal number of iterations to be computed. each iteration is made up of the following steps: 1. for each cell, compute the eruption flux if the cell is a vent cell. 2. for each cell, compute the flux transfer with the neighboring cells and the maximum allowed time-step. 3. find the global minimum time-step among the computed maximums. 4. for each cell, update the lava thickness and the other fields (i.e. solidification, radiation loss), according to the chosen time-step. it is straightforward to implement these steps in four dedicated cuda kernels. steps 1, 2 and 4 are written from scratch, while step 3 is provided by a well-known, highly optimized, array-scan library [sengupta et al. 2008]. the first level of parallelism is represented by the "for each" at the beginning of the statements: one thread on the gpu computes the evolution of a single cell of the automaton, with a 1:1 mapping. note that only in steps 1 and 4 are the cells fully independent of each other: step 2 requires access to the neighboring cells, and step 3 requires a global selection algorithm. this consideration is particularly important when designing a multi-device structure: additional subproblem dependencies amplify the inter-device communication latencies, and can bring the overall performance down if an appropriate synchronization strategy is not implemented. 4. magflow on multiple gpus we now analyze how the cross-cell dependencies in steps 2 and 3 affect the structure of the multi-gpu magflow, and why it is nontrivial to synchronize the available devices efficiently. the 2d arrays holding the automaton-cell data are stored in row-major order, following the c convention. this structure must be taken into account for the optimization of memory transfers, as accessing a burst of consecutive addresses is faster than accessing the same amount of data sparsely distributed. as a consequence, we only split the domain horizontally, so that the synchronization involves transfers of row segments. in special cases of very narrow, rectangular domains, we can load transposed data, depending on which choice leads to shorter rows. let d be the number of available gpu devices on a machine. on a workstation, we usually have 1 ≤ d ≤ 3, while on a dedicated rack, we migh have up to eight devices per pci domain. also, let r be the number of rows and c the number of columns of the automaton. we split our r×c grid in horizontal strips, and we assign bordering strips to consecutive gpu indices; we will refer to these strips as subdomains. if we split the domain into equal parts, assuming for simplicity that r is a multiple of d, the device of index d will be assigned to zero-based rows (r/d) ∗ d, to (r/d) ∗ (d + 1) −1. however, to compute cross-cell lava fluxes of row (r/d) ∗ d, device d also needs to read the cells in row (r/d) ∗ (d + 1) − 1; i.e. the last row of device d − 1. the same reasoning applies to row (r/d) ∗ (d+1)−1, which requires reading access to row (r/d) ∗ (d + 1). devices d − 1 and d + 1 need in turn to read the first and the last rows assigned to device d, respectively; the minimum overlapping between contiguous subdomains is therefore two rows high. we call rows (r/d) ∗ d to (r/d) ∗ (d + 1)−1 internal rows for device d, while rows (r/d) ∗ d −1 and (r/d) ∗ (d + 1) are external rows for the same device. every device computes and writes the status of all of its internal rows; the external rows are just copies accessed for reading only. figure 1 is a visual representation of this scheme. steps 1 and 4 can be computed in parallel, with no communication at all among the devices; no significant changes are required with respect to the single-gpu code. step 2 can be computed independently in different devices, up to a minimal overlapping border. step 3 is easily extended from 1 to d devices: each device provides its own subdomain multi-gpu implementation of magflow figure 1. representation of subdomain overlapping. the internal rows of the strip assigned to device d = 1 are green, the external rows (i.e. one internal to device d = 0 and one internal to d = 2) are light red. minimum, the cpu quickly finds the smallest of the d local values and the global minimum value is passed back to the gpus, to be used in step 4. this requires a barrier in the iteration: after having found the local minimum, all of the gpus must communicate this value and wait for the global one. the barrier consists of a device flushing call (cudathreadsynchronize) followed by a cpu thread barrier (pthread_barrier_wait). let us use a cpu-centered nomenclature and call a download any transfer of data from the gpu to the cpu, and an upload any transfer in the opposite direction. at the end of each iteration, after step 4, every device downloads its first and last internal rows to a shared cpu buffer and uploads from this the external rows, which have just been downloaded by the neighboring devices; another break is required as a synchronizing barrier between the download and the upload, to ensure that each device reads up-to-date values. so far, every device has to stop two times for each iteration: once to obtain the global time-step and once to wait for downloads to complete. with a little design change, however, we can make every gpu stop only once, therefore speeding up the whole process. the "trick" consists of exchanging the amount of flux at the end of step 2 instead of the lava thickness at the end of step 4; this exploits the barrier needed for the time-step selection; the fluxes are already computed when the devices wait for the time-step, and they can be updated later when the time-step is available. figure 2 represents the sequence of operations that every device executes according to the new logic. this was an important breakthrough, as stopping twice was the main source of latency in our early prototypes. 5. implementation details the cuda parts of the magflow implementation have undergone almost no changes since the single-gpu version; the rest of the simulator, on the other hand, has been completely redesigned. in the multi-gpu implementation, we introduced the class gputhread, which encapsulates all of the pointers and primitives that are needed to handle a gpu; one gputhread per device is allocated. every gputhread starts a dedicated thread in constant communication with the associated device. the wait mechanism is based on the barrier primitives of the ntpl pthread implementation [drepper and molnar 2005]; as an alternative, it is possible to choose a busy-wait mechanism, which is more responsive, but far more cpu-consuming. the main thread has a flux exchange buffer to enable the gpus to download and upload their internal and external rows; this is double buffered to keep data consistency over consecutive iterations. the main thread manages the split, balances the workload, tracks the simulation time, and periodically requests a state dump from the gpus according to a user-defined save frequency. an optional auxiliary thread periodically checks the status of the whole system, for debugging purposes. it is possible to specify at the command line several options, like the input files, the possibility to disable asynchronous data transfers, the list of devices to be used, and so on; the devices can be heterogeneous and can even belong to different generations (e.g. fermi and tesla). it is possible to specify a device more than once: this causes two or more gputhreads to use the same physical device, thus emulating a multi-gpu behavior on a single-gpu machine. rustico et al. 595 figure 2. overall structure of the simulator. the main thread launches and synchronizes one gputhread per device. 596 the data is not multi-gpu aware: it is possible to save the state of a single-gpu simulation and to load it into a multigpu environment, and vice-versa. the transfer times for the exchanges of updated overlapping borders are covered by exchanging the internal borders as soon as they are ready, while the rest of the cells are still being processed. thus, the system scales on any number of cuda-enabled devices with compute capability 1.1 or higher, as capability 1.0 does not support concurrent memory transfers and kernel execution. the execution time of each iteration is lower bounded by the slowest device; a dynamic load balancing is necessary to limit the relative time differences and to avoid undesired bottlenecks. a space domain subdivision is currently performed and dynamically updated as the bounding box grows; the domain is distributed proportionally to the number of multiprocessors in each active device. while this approach works reasonably well, there also many unpredictable, run-time factors that can influence the performance of a device (thread scheduling order, optimization of the cuda run-time in kernel executions, pci bus conflicts, lava flow topology, etc.). the only way to take these factors into account is to distribute the computing load according to the execution times of the previous iterations. we are currently implementing a smart balancing policy that analyzes the fluctuations of the execution times in relation to the subdomain sizes, which takes into account all of the explicit and implicit factors in an a-posteriori analysis (i.e. based on the effective execution times for assigned subdomains). however, due to the scattered nature of these timings and the risk of balancing oscillations in local minima, such an analysis is far from trivial to design, and it requires the use of advanced signal-processing techniques. another approach to dynamic load balancing relies on overriding the cuda run-time scheduler policies with a constantly running custom kernel [chen et al. 2010]. this approach is especially useful when the problem modeled has a large number of small, heterogeneous, independent tasks, at the price of additional design complexity and the reduced possibility for a-posteriori balancing. 6. interface the system can provide a graphical display of the state of the simulation (figure 3). this display is not aimed at being a realistic representation of the phenomenon, but at showing the course of the simulation in a concise and immediate way. basic status information are shown, including the gpus being used and the corresponding subdomain split. it is possible to draw the subdomain limits and/or the minimum bounding box that contains all of the cells with non-zero amounts of lava. the window is optimally zoomed to fit onto the monitor with any automaton resolution. it is possible to draw the topography and the lava heights produced, with an adaptive palette related to the thickness of fluid or solid lava. 7. preliminary performance analysis the main bottleneck of multi-device set-ups is the interdevice communication latency. although code refactoring has brought several improvements that are not specific to multi-gpus, like the ability to save state files asynchronously with respect to computations, we needed to exploit all of the advanced techniques offered by cuda to overcome the communication overhead that arose to maintain continuous synchronization. figure 4 shows the average execution times of each step of the automaton evolution during a single-gpu simulation, in milliseconds. as expected, computing the cross-cell fluxes is the most expensive step in terms of computational time, which takes 0.106 ms (68%), as well as the only step that changes significantly during the multi-gpu implementation of magflow figure 3. screenshot of the simulator interface while running a 4-gpus simulation. computation (see also figure 5); finding the minimum timestep takes 0.025 ms; computing the lava heights from the flux takes 0.018 ms, while eruption requires only 0.008 ms. it is more interesting, however, to analyze how these times vary in relation to the number of active cells in the automaton (i.e. the number of threads running on the gpu), as is plotted in figure 5. two steps basically take constant times: the eruption and the minimum scan. the time for updating the cells increases linearly with the number of cells, with very slow growth (0.02 ms from 1 to 60,000 cells). we do not expect to have any performance gain with the distribution of the work of these steps to more than one device. computing fluxes follow a more interesting trend. their execution time is almost constant until about 6,000 cells, where there is a sudden increase, and it grows linearly afterwards. when an increase in the number of active cells does not lead to a higher execution time, we are not effectively using all of the cores of the gpu; the sudden increase corresponds to the hardware saturation point, from which point on we expect a positive linear relationship between the number of threads on the device and the execution time. splitting the automaton before the saturation of all of the active devices would therefore give us no speed-up. our multi-gpu implementation enables one gpu at a time, as long as all of the already active devices are saturated. the profiling of the application with cuda visual profiler showed that a device should not be considered saturated before at least one order of magnitude more blocks are assigned than the number of its multiprocessors. every time a new device is activated, the domain is split again according to the estimated power ratios (i.e. number of multiprocessors) of active gpus. 8. multi-gpu execution times when the automaton has been distributed over two or more gpus, we observe an important change in the proportion of execution times. figure 6 shows the average timings over 5,000 iterations of a 5-m simulation just after the automaton has been split in half. computing the flux and updating the cells take almost half the time, while the eruption and minimum scan are basically unchanged, as expected. however, the total time is only 10% smaller, because we have to take into account the downloading and uploading of the overlapping rows, and together, these transfers take more than 20% the new total rustico et al. 597 figure 4. average execution times of each step during a single-gpu simulation, 5 m dem. figure 5. execution times for each step, with respect to the number of active cells in the domain, 5 m dem. 598 time. making this overhead negligible is the main challenge of any multi-gpu system where continuous inter-device communication is needed. we have covered these latencies almost completely by the transfer of the overlapping borders simultaneously with the computation of the internal cells, with the results shown in table 1. our test-bed was the simulation of lava eruptions on etna volcano in the year 2001, on dems with 2 m and 5 m resolution, loading from a 20% complete state, where the active box had already reached its maximum extension. this was simulated on a tyan-ft72 rack mounting a dualxeon processor, 16 gb ram and 6×gtx480 cards. all of the tests have been numerically validated, and the time comparison refers to the same simulation with the same options (e.g. the stated dumping frequency), as computed by the single-gpu code. the execution times obtained differ from the ideal times only for a constant overhead loss of about 4e−2 · t · #gpus, with t being the total simulation time. figure 7 shows the real execution times, the ideal times, and the differences between the two. in this specific example, splitting the simulation across more than 5 gpus is not convenient, as the cost becomes comparable with the simulation time. in general, the maximum number of gpus across which it is convenient to distribute the computation depends on the total extent of the simulated flow, i.e. on the number of active cells, as well as on the additional cost mentioned above. for example, the same eruption simulated on a 5 m dem can benefit at most from being distribute across 3 gpus. the linear cost is mainly due to the lack of an a-posteriori balancing policy: a lava flow topology can cause two equally shaped subdomains to require slightly different amounts of computations (due, e.g., to different numbers of cells with the actual flow, different memory access patterns, etc.). multi-gpu implementation of magflow 1 gpu 2 gpu 3 gpu 4 gpu 5 gpu 6 gpu real time (s) 14377 7737 5813 4993 4562 4500 ideal time (s) 14377 7189 4792 3594 2875 2396 real cost (2) 0 549 1021 1399 1687 2104 figure 6. average execution times of each step before and after splitting the domain, 5 m dem resolution. table 1. execution times and cost in seconds, for 1 to 6 devices, and 2 m dem computed by the single-gpu code. figure 7. plot of the execution times (~18 h of simulated time; dem, 2 m), ideal times and linear cost, for 1 to 6 devices. 9. conclusions and future work we have presented here a cuda-based multi-gpu implementation of the magflow cellular automaton that transparently scales on an arbitrary number of homogeneous or heterogeneous gpus. although running a physics-adherent simulation on gpus is already faster than real-time, even faster simulations might open new use scenarios and bring higher levels of flexibility in model validation and scenario forecasting. the speed-up obtained differs from the ideal one by a cost function that is linear with the number of gpus, which is due to the slightly different execution times of equally shaped areas with different numbers of cells containing lava and different flow topologies. it is still possible to improve upon these results, both for the model and for the technical implementation sides. we are currently performing further tests to shortly complete a system that has automatic fine-tuning of the execution parameters, to fully exploit the hardware computational power. by interlacing array data, memory transfers can be merged into a single transfer per iteration. although cuda toolkit 3.2 is far more mature than the first release, some features still need to be completed, and some of them are not well documented yet. above all, a more advanced profiling tool that can obtain absolute execution times of gpu operations is essential, to get accurate information about the real overlap among gpu transfers and computations, which is currently not possible with the standard profiling tools delivered with cuda. the recent release of cuda 4.0 has introduced new multi-gpu-specific features, like device-to-device transfers and a unified address space for both cpu and gpu memories. while inter-device transfer costs are already almost completely covered, we plan to test these new features in the very near future. finally, we will use the techniques and know-how developed to date in other computationally expensive numerical problems, such as mesh-free fluidodynamics and more generic task-driven multi-gpu schedulers. references bilotta, g., e. rustico, a. hérault, a. vicari, g. russo, c. del negro and g. gallo (2011). porting and optimizing magflow on cuda, annals of geophysics, 54 (5), 580-591 ; doi: 10.4401/ag-5341 (this issue). chen, l., o. villa, s. krishnamoorthy and g.r. gao (2010). dynamic load balancing on singleand multi-gpu systems, in: parallel & distributed processing (ipdps), 2010 ieee international symposium, 1-12; doi: 10.1109/ipdps. 2010.5470413. drepper, u. and i. molnar (2005). the native posix thread library for linux, february 21, 2005, 17 pp.; url: http://people.redhat.com/drepper/nptl-design.pdf. meuer, h., e. strohmaier, j. dongarra and h. simon (2010). the top500 project; url: http://www.top500.org/lists/ 2010/06. sengupta, s., m. harris and m. garland (2008). efficient parallel scan algorithms for gpus, nvidia technical report nvr-2008-003, dec. 2008, 17 pp.; url: http://mgarland. org/files/papers/nvr-2008-003.pdf vicari, a., a. hérault, c. del negro, m. coltelli, m. marsella and c. proietti (2007). modeling of the 2001 lava flow at etna volcano by a celluar automata approach, environ. modell. softw., 22, 1465-1471. *corresponding author: eugenio rustico, università di catania, dipartimento di matematica e informatica, catania, italy; email: rustico@dmi.unict.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. rustico et al. 599 bounif 625_646 625 annals of geophysics, vol. 46, n. 4, august 2003 seismic source study of the 1989, october 29, chenoua (algeria) earthquake from aftershocks, broad-band and strong ground motion records abdallah bounif (1) (4), mourad bezzeghoud (1) (5), louis dorbath (2) (6), denis legrand (2), anne deschamps (3), luis rivera (2) and hadj benhallou (1) (1) craag, département ess, bouzaréah, algiers, algeria (2) ipgs, umr 7516, strasbourg, france (3) geosciences azur, sophia antipolis, valbonne, france (4) usthb, institut des sciences de la terre, algiers, algeria (5) universidade de évora, departamento de física, colégio luis verney, évora, portugal (6) ird, paris, france abstract the broad-band teleseismics records of the earthquake of october 29,1989 in algeria (m w = 6.0) allow a detailed study of the rupture process of this earthquake. the focal mechanism obtained by p and sh modeling corresponds to reverse faulting with a small amount of left-lateral movement along a fault striking 246° and dipping 56°. the rupture is found to be complex with two sub-events separated in time but occurring on the same plane. the lowfrequency records of an accelerometer located some 25 km to the west of the main shock are also better fi tted when the rupture is composed of a double pulse. in the two cases, there is strong evidence for the rupture to propagate from south-west towards north-east.the relocalisation of the main shock by using a master-event technique and the data from italian and spanish stations led to the same conclusions. soon after the main event, a temporary seimic network was installed in the epicentral area. the aftershock clouds defi ne a sw-ne fault dipping to the nw compatible with the results of the modelisations of the teleseismic body-waves and the accelerogram. the focal mechanisms correspond mainly to reverse faulting. the maximum principal direction of the stress tensor obtained from the inversion is about n-s and the minimum is vertical, typical of a compressive regime. the chenoua earthquake took place on a fault which was not recognized as active. repeated comparable seismic events on this fault and on the fault that borders the massif to the south explain this intriguing topographic feature. mailing address: dr. louis dorbath, ipgs, umr 7516, 5 rue rené descartes, 67084 strasbourg cedex, france; e-mail: louis@sismo.u-strasbg.fr key words north africa seismicity earthquake 1. introduction the chenoua earthquake of october 29, 1989 is the third earthquake during the decade 19801990 that caused extensive damage and human losses in algeria, after the el-asnam earthquake of october 10, 1980 and the constantine earthquake of october 27, 1985. among these three earthquakes, the elasnam earthquake is by far the most important in magnitude, m s = 7.3, as well by the amount of destructions. the el-asnam earthquake has been extensively studied and is one of the best-known earthquakes in the mediterranean area (ouyed et al., 1981; deschamps et al., 1982; cisternas et al., 1982; ruegg et al., 1982; philip and 626 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou meghraoui, 1983; ouyed et al., 1983; meghraoui et al., 1986; yielding et al., 1989; dewey, 1991; avouac et al., 1992; bezzeghoud et al., 1995). surface breaks, with offsets reaching several meters (the maximum displacement was 6 m) were observed and mapped along a discontinuous fault more than 35 km long. the main fault is a thrust due to the regional compressional tectonic stresses, but secondary faults were also activated, mostly on the surface of the hanging wall, presenting any possible situation: strike-slip faulting, inverse faulting and also normal faulting associated with fl exures. the modelisation of p-wave seismograms indicates a complex source consisting of three segments, following closely the primary surface breaks. the point source focal mechanism corresponds to a thrust fault along a plane striking 225°n and dipping 54°. this mechanism is in agreement with the general 340°n direction of the compression between africa and europe deduced from plate tectonic models (chase, 1978; anderson and jackson, 1987). the constantine earthquake, m s = 6.0, is the most important seismic event in northeastern algeria since 1947. the focal mechanism, obtained by modelisation of p and sh waves corresponds to a strike-slip motion along a fault striking 217°n and dipping 84° (deschamps et al., 1991). the spatial distribution of the aftershocks reveals a complex source divided into three segments(bounif et al., 1987; bounif and dorbath, 1998). surface breaks were observed, affecting quaternary deposits. the main segment of breaks was about 4 km long and mostly formed of left-lateral en échelon cracks. the vertical movements did not exceed 4 cm, while the horizontal ones reached 10 cm. although the strikes of the faults involved during the elasnam and constantine earthquakes are close to each other and the horizontal compressive stress has about the same directions in the two regions, the two mechanisms are quite different. deschamps et al. (1991) propose that the motion between africa and europe is accomodated by a uniform deformation along an ew direction. in term of stresses, the shape of the stress tensor is changed between the western part and the eastern part of algeria in such a way that the minimum stress passes from vertical to horizontal (bounif et al., 1987) and, consequently, the stress regime changes from compression to strike-slip (armijo et al.,1982). the chenoua earthquake of october 28, 1989, with a magnitude m w = 6.0, is the largest earthquake instrumentally recorded in the coastal region of central algeria. in recent history, the region of cherchell-tipaza experienced some moderate to strong earthquakes, whose epicenters are diffi cult to locate accurately on the basis of macroseismic observations. in any case, in the catalogs of rothé (1950) and ambrasseys and vogt (1988) two earthquakes have their epicenters close to that of the 1989 event and might be comparable in size: the earthquake of march 17, 1756 (36°30' n, 2°32' e), with a maximum intensity vii mks in the city of hadjout and the earthquake of june 18, 1847 (36°33' n, 2°32' e), with a maximum intensity vii mks along the coast of the chenoua region. the epicenter of the great earthquake of february 03, 1716, which caused extensive destruction in algiers, is defi nitively located eastwards (ambrasseys and vogt, 1988). the most important damage caused by the chenoua earthquake to human settlements was reported on the southeastern side of the chenoua mountain, in nador (fi g. 1) and around sidimoussa (close to station sms, fi g. 6) where most of the old houses and buildings were almost completely destroyed. in this area, the intensity should have reached viii mks. tipaza, cherchell and hadjout, the main towns of the region, suffered severe shaking, but only a few buildings collapsed (yelles-chaouche, 1991). in algiers, 70 km eastwards, the earthquake was strongly felt but no severe damage was reported. landslides and rockfalls were abundant all around the chenoua mountain, particularly along its eastern and northern fl anks, where the coastal road from tipaza to cherchell was cut in several points (meghraoui, 1992). 2. geological and tectonic setting the coastal range of nw algeria, the tellian atlas, is an active fold and thrust belt which is part of the boundary between africa and eurasia. the deformation within the range results from the 627 the 1989, october 29, chenoua (algeria) earthquake convergence of the two plates and mostly occurs along reverse faults dipping 40°-60° to the nw (meghraoui, 1988; yielding et al., 1989). the general direction of the structures is ene-wsw, but several major faults are oblique to this trend and often organized in en échelon systems. the el-asnam fault, for example, strikes sw-ne along its southwestern and central segments, while its northeastern segment trends wswene. the region of tipaza where the 1989 earthquake occurred is part of the tellian atlas range, but presents some specific features, namely the sahel anticline, the mitidja basin and the chenoua massif (fi g. 1). the sahel anticline is a long (70 km) and narrow (5 km) band of hills which runs from tipaza to algiers along the sea. its mean height is about 200 m. it is divided into three 20-30 km long segments and constitutes an active fold structure (meghraoui, 1988). according to meghraoui (1988), this fold is bordered to the south by a blind thrust fault dipping to the north, divided into three segments as well. however, only the westernmost segment of this conjectural fault is widely held: it extends along the southern fl ank of the chenoua mountain where it is exposed and divided, at least near the surface, into two ew parallel branches (meghraoui, 1988; belhai et al., 1990). in spite of its moderate height, the sahel anticline rises above the mitidja basin that lies at an altitude lower than 100 m. the basin, whose maximum width is about 20 km, is fi lled with quaternary deposits and borderered to the south by the high blida range from which it is separated by an en échelon system of reverse faults dipping to the south. the mitidja basin narrows toward the west, and disappears just in the vicinity of the chenoua massif. westward, the morphology of the coastal region changes and there is no longer a basin between the high range and the coast. however a basin exists, but fig. 1. topographical and tectonical features of the region of the chenoua earthquake. isobath 200 m is shown. main faults from meghraoui (1988). isoseist lines from yelles-chaouche (1991). open squares: seismological stations. 628 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou about 30 km inland: the cheliff basin, where the el-asnam earthquake occurred. the chenoua mountain is an isolated structure that appears as a peninsula. it culminates at more than 900 m and has long been an intriguing feature (glangeaud, 1932). its northern fl ank is mostly formed of paleozoic or even older rocks, while on the southern fl ank mesozoic and tertiary units are dominant. since miocene time, n-s compression has prevailed and, consequently, the formation of the massif was thought to be the result of southward thrusting along the nearly e-w faults which border it to the south (belhai et al., 1990). however, other structures were reported, particularly nw-se dextral faults and sw-ne left-lateral faults, among which the main one is mapped from nador to the sea. these faults, and particularly the last one, have been interpreted as transcurrent faults which accomodate the motion of the massif towards the south (belhai et al., 1990). however, the steep topography of the eastern fl ank of the massif, the linearity of the contour lines and the fold drawn by the isobaths are strong evidence of dip slip motion (fi g. 1 and fi g. 6). 3. main shock mechanism available digital records of p and sh body waves from the global networks (iris, gdsn and geoscope), with epicentral distances in the range 30°-85°, have been collected to perform a global inversion of seismic moment, slip geometrical parameters, and source duration. we used the version of mccaffrey and abbers (1988) of the inversion procedure of nabelek (1984). this procedure is based on the minimization, in the least square sense, of the misfi t between observed and synthetic seismograms. synthetic seismograms were computed in a homogeneous half-space and included p (respectively sh ), pp and sp (respectively ssh ) phases. q-factor was defi ned with attenuation time constants of t* = 1 s and t* = 4 s for p and sh waves respectively. the synthetic seismogram at each station is obtained by convolution of green’s function of the propagation with the instrumental response and then by the source time function. the latter is described by a fi xed number of overlapping elementary isosceles triangles of the same duration. due to the non-even azimuthal distribution station d(°) f(°) weight weight weight weight lp p lp sh brb p brb sh obn/iris 29.6 40.8 0.4 0.4 kev/iris 35.6 14.4 0.7 0.6 0.6 aru/iris 41.9 43.7 0.3 0.4 agd/geoscope 44.1 114.2 1 wmq/gdsn 62.7 54.3 0.3 0.4 0.3 0.4 bcao/iris 35.4 151.5 0.7 1 slr/iris 66.8 154.7 0.7 scp/iris 60.8 300.8 0.9 1 lon/iris 83 325.1 0.9 bji/gdsn 82.3 45.2 0.4 0.3 mbo/geoscope 28.2 222.9 1 hia/gdsn 77.9 36.5 0.4 0.4 0.4 lzh/gdsn 77.2 54.5 0.3 0.4 0.3 kmi/gdsn 83 64 0.4 0.5 0.4 table i. list of the stations used in the inversion procedure steps i. d and f are, respectively, epicentral distance and azimuth of the station. 629 the 1989, october 29, chenoua (algeria) earthquake fig. 2a,b. comparison between observed and synthetic signals for p (a) and sh (b) waves for a single point source inversion (step i). thick lines: observed signal; thin lines: synthetic signal. solid and open triangles: p and t axes, respectively. a b 630 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou of the available records, weighting factors have been applied to seismograms, as given in table i. body waves on long period (lp) records have a very simple shape (fi g. 2a,b), as could be expected from the moderate magnitude of the event and the characteristic frequency of the channels (around 25 s). on the other hand, broadband (brb) records (fi g. 3) suggest that two fig. 3. comparison between observed brb and synthetic signals for the fi nal model consisting of two sub-events (step ii). conventions as in fi g. 2a,b. 631 the 1989, october 29, chenoua (algeria) earthquake subevents should be introduced to explain the waveforms complexity. all brb records present about the same pattern: the p waves signal begins by 2 or 3 high frequency (0.5 hz) oscillations followed by a lower frequency phase, 7 to 8 s after, which is interpreted as the sp phase. in a fi rst stage, we inverted the complete data set (lp and brb) with a single point source and a source time function constituted by 5 elementary, 2 s duration, isosceles triangles. the focal solution deduced from this inversion, azimuth = 246°, dip = 56°, rake = 86°, is in good agreement with the cmt solution, the isoseismal map (fi g. 1) and the aftershocks space distribution (fi gs. 6 and 9). the seismic moment is 8.2 ¥ 1017 nm, also in good agreement with the cmt solution (9.6 ¥ 1017 nm). due to the frequency content of the data, there is a large trade-off between the focal depth, 11 km, and the source time duration. the long period and brb waveforms, together with the observed ones, computed for the best solution obtained from the inversion procedure are shown in fi gs. 2a,b and 3. the low frequency p and sh waves are quite well modelized, with a rms of 0.12, and the relative complexity of the source time function may already be noticed. in a second stage, we used only the brb records. the initial one point source model was replaced by a two sub-events model, the fi rst one represented by 2 elementary, 2 s duration, isosceles triangles and the second one by 3 triangles, so that the total source duration is the same as in the fi rst stage. the fault geometry was assumed to be the same throughout the rupture process and the focal mechanisms of the two subevents identical to that obtained in the fi rst stage. these parameters were not changed in the course of the inversion process, nor was the rupture velocity fi xed at 2.7 km/s. the trial depth of each sub-event was 11 km. the second sub-event was initially delayed with respect to the fi rst by 2 s, according to the shape of the time source function obtained at the fi rst step, so that the two events were overlapping in time. two rupture directions corresponding to the strike of the fault were tested: 66°n and 246°n. thus the inversion is performed to resolve 9 parameters: the source depth of each sub-event, the amplitudes of the elementary sources, the origin time of the second source relatively to the fi rst one and the distance between the two sources along the direction of rupture propagation. the results are clearly better in the case of a propagation in the direction 66°n, namely from the continent towards the mediterranean sea. with this direction the rms is about 50% smaller than in the case of a rupture propagating in the opposite direction. however, this result is constrained by only one station, mbo. the best solution is shown in fi g. 3. the fi rst event occurred at a depth of 13 km and the second one 4 km to the ne and at a depth of 8 km. these depths, which are better constrained than the depth obtained in the fi rst (single point source) stage, are in fairly good agreement with the vertical extension of the aftershocks area (fi g. 6). the results of the inversion of body waves of the main shock may be summarized as follow (table ii): the chenoua earthquake mechanism is a nearly pure reverse faulting along a plane striking 246°n and dipping 56°. the main shock is composed of two sub-events of 3 s and 4 s duration respectively, with the same mechanism, and the rupture propagated likely from sw to ne. a large aftershock (m s = 5.4) followed the main shock by about 10 min. the corresponding p waves have small amplitudes and, moreover, arrive in the coda of the main shock; s waves could not be separated from the noise. on the other hand, the number of available brb records is small (7) and the azimuthal coverage is poor. nevertheless, the inversion routine has been attempted to modelize this event with an isosceles triangle of 1.6 s duration as source time function. we tested many models with different values of strike, dip and rake and selected a «best» solution based on the visual quality of fi tting and on the minimum rms value determined after the fi rst iteration of the inversion process. the parameters of this best solution are 220° ± 20°, 55° ± 5°, 25° ± 5°, h = 8 km ± 1 km for the strike, the dip, the rake and the depth respectively (table ii). the seismic moment is m0 = 2.0 ¥ 10 17 nm, 4 times less than for the mains hock. the dip is mostly constrained by the station col, while the rake is constrained by all the stations with the exception of hia. changing the depth by 2 km produces a noticeable degradation of the fi tting of the waveforms. the strike is poorly controlled. the 632 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou fi nal solution for this large aftershock is shown in fi g. 4. it is a left-lateral strike-slip faulting with a large component of reverse movement. although this mechanism is poorly constrained by the available records, it is in agreement with the stress tensor deduced from the aftershocks analysis (see below). 4. modeling of the acceleration a kinemetrics sma1 accelerometer was in operation in the city of cherchell, some 25 km to the west of the relocated epicenter of the mainshock (see below), that is about 2 times the length of the ruptured zone. this instrument was triggered by the p-waves of the main shock, so that the complete s-waveform on the three components is available. using the data from a single station, the contribution of the source cannot be distinguished from that of the propagation and thus fi ne details of the history of the rupture are outside the scope of this paper. therefore our purpose is limited to a broad description of the propagation of the rupture, assumed to be of constant velocity, along a fault plane in a homogeneous model, with the aim of strengthening some features which may not convincingly be resolved by the inversion of the body waves. to achieve this purpose, ground velocity, obtained by integration of the digitalized original record, was subsequently fi ltered through a fi ve pole butterworth low-pass fi lter at 0.3 hz. therefore, most of the high frequency content which is related to the complexities of the rupture and the medium was eliminated from the original accelerogram. the theoretical velocity was calculated at the surface of a homogeneous, isotropic and elastic half-space for an extended source by summing the contribution of point-sources regularly distributed on the fault plane every 200 m. the spatial derivatives of the complete (near and far fi elds) green’s functions were calculated in the time domain using a cagniard-de hoop method (johnson, 1974; hartzell et al., 1978). we chose strike dip rake depth m0 ¥1017 stf (°) (°) (°) (km) (nm) (s) cmt 1 231 50 62 15 9.6 2 91 48 119 step i 1 246 56 86 (± 4) (± 2) (± 4) 2 71 34 94 11 8.2 6.0 (± 4) (± 2) (± 4) (± 0.5) (± 0.6) (± 0.5) step ii a 13 3.4 3.0 (± 2) (± 0.6) (± 0.6) b 8 4.5 4.0 (± 1) (± 0.9) (± 0.9) large 1 220 55 25 (± 20) (± 5) (± 5) aftershock 2 115 70 115 8 2 1.6 (± 20) (± 5) (± 5) (± 1) table ii. source parameters of the main shock and the large aftershock for fi nal solutions obtained respectively from inversions i and ii and the brb modeling. (1) and (2) indicate the nodal planes; (a) and (b) correspond to sources time function obtained from inversion ii. cmt indicates centroid moment tensor solution. the errors are indicated in brackets. 633 the 1989, october 29, chenoua (algeria) earthquake a p-wave velocity of 6.0 km/s, a poisson’s ratio of 0.25, a rise time of 0.3 s and a sampling rate of 50 samples/s. the synthetic seismograms were low-pass fi ltered through the same fi lter as the data. the rupture initiates at one point of the grid and propagates with a constant velocity of 2.7 km/s, so that the rupture front is a circle. the synthetic waveforms were computed for two general models, following closely the way already used in the lp and brb body-waves inversion: in the fi rst model the main shock consists in a single event, while in the second model two sub-events are involved. in both cases, the geometry and the dimensions of the trial fault-plane were the same and drawn from the body-waves modelling and the aftershocks distribution. the dimensions, 11 km long and 10 km wide, were not changed during the modelling process. the modulus of the slip vector is fi xed by the seismic moment and the surface of the fault. in the fi rst model, four parameters are free, the strike and the dip of the fault, the rake and the position of the nucleation point. a trial and error method was followed to adjust these parameters and the fi rst trial faulting mechanism was the fig. 4. comparison between observed brb and synthetic signals for the large aftershock 10 min after the main shock. conventions as in fi g. 2a,b. 634 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou fig. 5. rupture models from the inversion of the accelerogramm. continuous line: observed signal; dashed line: synthetic signal. 635 the 1989, october 29, chenoua (algeria) earthquake mechanism resulting from the single-event inversion of body-waves. the best visual fi t of the synthetic velocity with the observed data is achieved when the strike, dip and rake take the values 230°, 55° and 86° respectively and the nucleation point is at a distance of 1.5 km from the southern tip of the fault and at a depth of 3.8 km (fi g. 5). the rupture is therefore bi-directional, but the main part develops towards ne. the second model consists of the summation of two sub-events, both of which with the same focal mechanism gathered fom the previous single event model. consequently, the free parameters are now the position of the nucleation point and the length of the fi rst sub-event (the second sub-event initiates where the fi rst stops) and the time lag between the two sub-events. in the best model, the nucleation point is located on the southwestern tip of the fault, at a depth of 3.8 km. it broke 3 km of the complete length of the fault. the second sub-event occurred 2.0 s after the fi rst one and broke the remaining 8 km of the fault plane (fi g. 5). its nucleation point is also at a depth of 3.8 km but at 3 km from the southwestern tip of the fault. the two models and the synthetic wave forms of the horizontal components together with the corresponding data are shown on fi g. 5. the one-source model explains reasonably well only the fi rst 3 s of the n-s component, and then diverges largely from the data. the two-source model explains a longer part of the data: the position of the main peaks and troughs and their amplitudes are quite well matched for about 8 s, a time corresponding roughly to the source duration. the direct modeling of the accelerogram leads therefore to results close to those deduced from the inversion of the body waves. of particular importance are the fact that both methods require a two sub-events source model to match data more accurately and favour a rupture propagation from the continent towards the sea. 5. recording and analysis of the aftershocks one day after the main shock, 8 smoked paper recording seismographs (sprengnether meq 800), equipped with 1 hz vertical sensors (mark product l4c), were installed in the epicentral zone by the craag. a few days later, this network was complemented by 8 identical instruments and 4 three components digital recorders from the ipg of strasbourg, the latter equipped with 2 hz seismometers (mark product l22). the three component digital recorders were installed in sites where smoked paper recorders were already working, so that for aftershocks location and determination of focal solution purposes, which involve only arrival times and polarities, the network was composed of 16 stations (fi g. 1 and table iii). this network was operated from november 07 to november 18. after this period, the ipg strasbourg instruments were removed, but the craag network was maintained up to january 06, 1990. the analogic instruments functioned at 60 mm/mn and internal time marks were placed at the second, allowing a time reading precision of 0.05 s for p waves. the drifts of the internal clocks were measured by checking every two sta lat (n) lon (e) alt (m) aek 36°34.44' 2°32.20' 152 bar 36°31.79' 2°18.87' 192 cai 36°33.95' 2°10.16' 168 cal 36°35.68' 2°20.73' 490 crb 36°37.52' 2°23.64' 176 fig 36°32.30' 2°22.78' 92 kar 36°36.53' 2°15.83' 168 mou 36°35.49' 2°23.33' 230 atb 36°32.72' 2°40.79' 45 bdl 36°33.60' 2°25.73' 120 bhn 36°27.27' 2°22.23' 260 chn 36°37.97' 2°19.71' 220 djr 36°25.07' 2°32.45' 460 ham 36°34.74' 2°14.23' 60 mcr 36°29.37' 2°13.92' 280 sdr 36°34.46' 2°33.82' 260 shl 36°27.83' 2°30.73' 220 sms 36°34.51' 2°20.53' 220 tpz 36°34.01' 2°28.72' 240 table iii. list of the temporary stations installed to monitor the aftershocks activity. 636 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou days the internal time against the reference time radio-transmitted by dcf and by the omega system. for most of the stations, the drifts were lower than 0.1 s per day. time signals for the digital equipment were obtained from the omega system. the present section deals with the results obtained from the data collected during the period from november 07 to november 18, when the complete network was in operation. several hundreds of aftershocks were recorded during this period. to insure a good quality to the locations we kept only the events with at least nine p-wave and two s-wave arrival-times. no detailed deep seismic sounding was performed in the region and, consequently, the velocity structure is poorly known. thus we used about 50 well-recorded aftershocks, with more than 12 p-wave and more than 2 s-wave arrivaltimes, to improve the velocity model to be used in the location process. first, we located them using the fi ve velocity models already proposed by ouyed (1981) for the el-asnam region. it resulted that the model labelled 5 by ouyed was the best one, yielding the smallest mean rms. it consists of a 2 km thick layer with a 4.5 km/s p-wave velocity over a half-space with a 5.5 km/s p-wave velocity. then we changed the thickness of the upper layer and/or the p-wave velocities in the two layers. several trials led to the fi nal velocity structure consisting of a 3 km thick upper layer with a 4.5 km/s p-wave velocity over a 9 km thick layer with a 5.5 km/s p-wave velocity and a half-space with a 6.5 km/s p-wave velocity. the insertion of the second discontinuity is not absolutely required, it has only a very weak infl uence on the mean rms value, but improves the residuals for the largest epicentral distances. the infl uence of the v p /v s ratio has been found to be weak on the mean rms whatever the velocity model tested, and thus a v p /v s ratio of 1.73 was adopted. the aftershocks were located using the hypoinverse program (klein, 1978) and we selected «good» locations on the basis of the following two criteria: rms < 0.3 s and conditioning factor < 100. 392 events fulfi l these conditions among which 85% have rms lower than 0.2 s. in previous studies (lyon-caen et al., 1988; dorbath et al., 1992, for example) using dense networks that cover the aftershock zones quite well, it has been shown that the epicentral locations are only weakly dependent on the velocity model. making comparison between the locations of the same events in different velocity structures, they also concluded that the errors given by hypoinverse are realistic estimates of relative location errors of the events. unfortunately, in the present study, these favourable conditions were not met for the complete set of aftershocks, because a large number of them are not inside the network, since they occurred beneath the mediterranean sea. besides the criteria already mentioned, a criterion commonly used to select events on the basis of the quality of their location is to exclude the events whose epicentral distance to the nearest station is greater than the focal depth. nearly one third of the aftershocks we have kept, and which will be presented further on, are in this situation and would be excluded on the basis of this criterion. however, among these events some of them occurred inside the network, at very shallow depths. this is a common situation encountered in studies of local seimicity or aftershocks and, generally, these events are kept because they are actually shallow, even if the computed errors for depth are quite large, and they preserve some spatial homogeneity in the aftershocks distribution pictures. it is with a similar purpose that we kept the events far from the network whose depth is smaller than the epicentral distance to the nearest station. to remove them would lead to a strongly biased picture of the aftershock zone. however, we checked the quality of these locations against that of the events which fulfi l the above-mentioned criterion, using the standard errors for depth and epicentral position as a measure of the quality. it is known that these errors underestimate the true errors but they are convenient to our purpose which is only to compare two sets of data. the mean errors for epicentral position are 0.91 km and 0.75 km and the mean errors for depth 1.30 km and 0.76 km respectively for these two sets of events. it is obvious that the quality is reduced when the aftershocks are far from the network and that the depths are more affected, however not to prevent making use of them, at least to estimate the extension of the aftershock area. 637 the 1989, october 29, chenoua (algeria) earthquake 6. aftershocks distribution and focal mechanisms we present the locations of the selected aftershocks in map view in fi g. 6. they depict an elongated area in a general sw-ne direction, about 15 km in length and 7 km in width. about 60% of this area lies under the mediterranean sea. the remaining area covers the chenoua massif, principally its northeastern fl ank, only few events occurred beneath its southern fl ank. neither on this map, nor on the cross-sections (fi g. 7a-c), any segmentation of the aftershock zone appear clearly, as should be expected in the case where the two sub-events found in the course of the inversion of body wave and the modelisation fig. 6. map view of the epicenters of the aftershocks. open squares: seismological stations. 638 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou c b a fig. 7a-c. vertical cross sections through the hypocenters of the aftershocks along the lines shown in the insets. no vertical exaggeration above sea-level, vertical exaggeration under sea-level by a factor 5. the focal mechanisms are shown in the lower hemisphere equal area projection. 639 the 1989, october 29, chenoua (algeria) earthquake of the accelerogram of the mainshock would have occurred on two different, more or less parallel, fault planes. thus the absence of evident segmentation of the aftershock zone supports the hypothesis of a single fault surface. towards se, the aftershocks are shallower and the aftershock zone ends abruptly along a nearly sw-ne straight line. on the contrary towards nw, the aftershocks are deeper and there is no clear boundary on this side, thus suggesting a nw dipping fault. the relative amount of deep aftershocks seems to be greater towards the northeastern extremity, but this pattern is likely to result from the selection criteria used. we already pointed out that the usual criterion referring to the ratio between the depth and the smallest epicentral distance was not used to select reliable focus parameters. however, the criterion according to which the conditioning factor has to be smaller than a given value acts in a rather similar way, and an inappreciable number of shallow events that occurred at the northeastern extremity of the aftershock area could have been rejected. at the sw extremity, shallow events form an elongated cluster transverse to the general direction of the seismicity, about 3 km long, underneath the chenoua massif. on a crosssection (fi g. 7a), this cluster appears to be concentrated around 2.5 km depth, even if some bias resulting from the presence of a discontinuity at 3 km depth in the velocity model strengthens the effect. a similar pattern has been observed at the extremities of the main segment of the spitak source region along which reverse faulting was present (dorbath et al., 1992). it was then related to transform faulting connecting adjoining segments; in the present case it might be related to shearing at the extremity of the fault. three cross-sections, two kilometers wide, orthogonal to the general trend of the aftershock zone are presented in fi g. 7a-c. the southern one, fi g. 7a, is partly underneath the continent while the two others, fi gs. 7b and 7c, are under the mediterranean sea. they clearly show a nw dipping fault plane. the dip angle is about 55°, and it seems to increase from the ne extremity to the sw one. however, the locations of the aftershocks, particularly their depths which are crucial in this purpose, are not well enough constrained to strongly support the warping of the fault suggested by the cross-sections. the individual focal mechanisms presented on the cross-sections are produced through the inversion scheme developed by rivera and cisternas (1990). the nodal planes are in fairly good agreement with those obtained by eye fi tting or by maximum likehood solutions. only the events with more than 9 reliable polarities were used. a subset of the fault-plane solutions that have been determined is shown in fi g. 7a-c. the parameters defi ning the nodal planes, azimuth, dip and rake according to aki’s convention (aki and richards, 1980) are presented in annex for the complete set. a large majority of the mechanisms show reverse faulting, frequently associated with a small left-lateral component, on planes dipping towards nw and striking about sw-ne, thus in agreement with the geometry of the aftershock area and the focal mechanism of the main shock. fig. 8. orientation of the principal axes of the stress tensor for the chenoua aftershocks. the ellipses correspond to one standard deviation. lower hemisphere equal area projection is used. 640 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou examples of this type of focal solutions are given by the events n. 10, 14, 19 in fi g. 7a, 27 and 31 in fi g. 7b, 35 in fi g. 7c. almost pure left-lateral strike-slip faulting occurred also, for examples the events n. 3, 4, 12 on the cross-section 7a, 29 and 30 on the cross-section 7b. in this case, one of the nodal planes generally strikes ssw-nne, that is slightly rotated counter-clockwise relatively to the general direction of the aftershock area. such a variability of the focal solutions is a common feature in aftershock sequences and it should be argued in favour of a heteregeneous stress fi eld, particularly if the p and t axes are confused with the s1 and s3 axes respectively. on the contrary it has been shown that this apparent inconsistency could be compatible with a unique stress tensor whose parameters, orientation and shape, are obtained by the method proposed by rivera and cisternas (1990). following this method, we performed the inversion of the stress tensor using 62 aftershocks combining 714 polarities. a normalized likehood function of 98.3 per cent was obtained, and 96.5 per cent of the polarities were explained in the hypothesis of a single tensor, thus strongly supporting it. this tensor (fi g. 8) has a nearly horizontal ns maximum principal stress (s1) direction, a vertical minimum principal stress (s3) direction and a nearly horizontal ew intermediate principal stress (s2) direction. in this case, the shape factor is (s3 – s2)/( s1 – s2) and its value, – 0.5, is typical of a triaxial compressive regime. 7. relocation of the main shock the main shock was located by the neic, csem and isc at 36°47.28' n, 2°26.88' e, 36°43.80' n, 2°24.00' e and 36°46.80' n, 2°26.40' e respectively (fi g. 9). all these locations are to the north of the zone defi ned by the aftershock activity and thus they are in confl ict with the swne sense of propagation of the rupture evidenced by the waveform modellings. we attempted to relocate the main shock with a greater precision by using local and regional data. no large aftershock occurred when the complete network was in operation, and only an aftershock that occurred on november 22 was recorded by both the local temporary craag network and the regional stations in spain and italy. using the seven algerian stations that recorded this event, it was located at 36°39.70' n, 2°28.88' e. although the epicenter is far from the nearest stations, this solution is well established by the location routine with a rms of 0.15 s. the reason is that this event was rather deep, 11.5 km. nevertheless, it is outside the network and we have to check the reliability of this solution. to estimate the degree of confi dence we may attach to the location determined with the algerian temporary network only, we plotted in fi g. 9 the locations of some aftershocks computed with the complete local network, craag plus ipgs stations, and the locations of the same events computed with the algerian stations alone. as expected, the two locations are close to each other in the case of events inside or close to the network, that is more or less inland, and may be as far as 5 km away from each other at the northeastern extremity of the aftershocks area. so our «master event» is not the best possible one, since it precisely occurred at that last place and consequently its location may be wrong by 5 km or so. we compared the mean of the differences of the arrival times between the master event and the main shock at two sets of relatively distant stations where p n is the fi rst arrival wave: 11 italian stations constitute the fi rst set and 6 spanish stations the second. the azimuth of the stations of the fi rst group relatively to the aftershock zone is about the same as the azimuth of the fault plane, while the azimuth of the stations of the second group is nearly orthogonal to the direction of the fault. the epicentral distances are large compared with the extension of the aftershocks zone, so that the differences of the arrival times for the spanish stations are nearly equal to the difference of the origin times. subtracting this value to the corresponding one calculated with the italian stations, it remains only the mean difference of travel times to the italian stations. following this procedure, we found a difference of – 0.9 s. thus, if v is the local velocity of p-wave and i is the take-off angle, the epicenter of the main shock is at a distance 0.9¥v/sin(i) from the epicenter of the master event, in the sw direction. with v = 6.0 km/s and i = 45°, this distance is about 7.5 km, and the epicenter of 641 the 1989, october 29, chenoua (algeria) earthquake fig. 9. comparison of the locations calculated with the complete network (craag + ipgs stations) and the locations calculated with the craag stations only. large open circle: epicenter of the master event; large solid circle: epicenter of the main shock. the main shock is located in the southern part of the aftershock zone (fi g. 9) and slightly outside it. this last feature directly results from the position of the master event, outside the aftershocks cloud as well. the uncertainties and the hypothesis make this result poorly constrained by itself, but it strengthens the result suggested through the inversion of teleseismic waves and the modelisation of the accelerogram, namely that the nucleation point of the main shock was at the southeastern edge of the fault and that the rupture propagated from sw towards ne. 642 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou 8. discussion and conclusions different methods have been applied to produce a detailed description of the chenoua earthquake, although it was not a large event, and the agreement between the pictures inferred from these different approaches is quite good. some discrepancies exist, but they are not signifi cant when the error bars are taken into account. though some complexities could appear, the general picture shows a rather simple feature, such as could be expected from an earthquake of magnitude m w = 5.9. the chenoua earthquake occurred likely on a single surface of rupture, plane or nearly plane, striking about 240° and dipping about 55° to the nw. the main shock resulted from the combination of two sub-events, which is the degree of complexity that the data are able to resolve, and the rupture propagated from sw towards ne. from the aftershock study, the fault is 13 to 15 km long and 10 to12 km wide along the dip. with these values, an elastic modulus m = 3.31010 nm-2 and the seismic moment coming from the modelisation of body waves, 8.2 ¥1017 nm, we obtain a displacement of 15 to 20 cm. this value is in the range of what is generally expected for an event with a magnitude 5.9. for example, the displacement was less than 20 cm during the 1986 kalamata earthquake (m s = 5.9, lyon-caen et al., 1988) and about 40 cm during 1983 earthquake of guinea (m s = 6.3, dorbath et al., 1985). the rupture did not reach the surface, at least along the segment observed within the continent. soon after the earthquake, a fi eld survey was conducted to observe surface deformations (meghraoui, 1992). several hundreds of meters of complex and discontinuous surface cracks and fissures network was discovered on the southern side of mount chenoua, southward of the main thrust which was not reactivated. most of them are striking about e-w with a few nesw en échelon left-lateral cracks and localized right-lateral ones. vertical displacements are only noticeable at the western extremity of the fi ssures network where they reach about 10 cm. the southern block was uplifted relatively to the northern one, the opposite of the expected movement along the north dipping main reverse fault which bounds the chenoua mountain to the south. on the other hand, these surface ruptures are parallel to the nearly vertical beds of folded and fl exured neogene formations. these observations lead us to interpret the surface breaks as secondary effects resulting of slipping on bedding planes due to the intense shaking close to the epicenter. this interpretation is supported by the two seismological results. first, there is no aftershock activity along the observed craks, although the area was properly covered by the seismological stations and, second, the attempts made to modelize the body-waves by a single point source, or one of the two sub-events, located there and striking e-w failed to fi t the observations. the earthquake took place on a fault that borders the chenoua massif to the east. the past activity of this fault is evidenced by the topographic pattern of the fl ank of the massif which shows linear and parallel contour lines, and by the bathimetry. the bathimetric curves clearly indicate the presence of a fold, whose axis trends about 60°n (fi g. 6). the fold is asymmetrical, with its steepest side to the se, a fi gure compatible with the presence of a nw dipping fault running along its southeastern fl ank. thus, the chenoua massif is located within the wedge formed by the intersection of two faults, one to the south of the massif and one to the east, which form an angle of about 150°. since the movement on both faults has a dominant component of inverse dip slip, there is a large concentration of the deformation in the wedge. on the other hand, these two faults are rather steep, 50° to 55° from surface observations for the southern fault (meghraoui, 1992) and from the cross-sections in fi g. 7a-c, so that the ns compression results in large vertical motion. the consequence of the two effects, the existence of a wedge and the steepness of the faults, is the formation of a high relief. 643 the 1989, october 29, chenoua (algeria) earthquake appendix focal mechanisms obtained through the joint inversion of the stress-tensor and focal solutions. solid circle denotes compression; open circle denotes dilatation. lower hemisphere equal area projection. 644 abdallah bounif, mourad bezzeghoud, louis dorbath, denis legrand, anne deschamps, luis rivera and hadj benhallou appendix (continued) 645 the 1989, october 29, chenoua (algeria) earthquake references aki, k. and p.g. richards (1980): quantitative seismology: theory and methods (w.h. freeman & co., san francisco), vol. 1, pp. 557. ambraseys, n. and j. vogt (1988): material for the investigation of the seismicity of the region of algiers, eur. earthquake eng., 3, 16-29. anderson, h. and j. jackson (1987): active tectonics of the adriatic region, geopys. j. r. astron. soc., 91, 937-983. armijo, r., e. carey and a. cisternas (1982): the inverse problem 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(received november 26, 2002; accepted march 26, 2003) annals 47, 1, 2004, 01/07def 73 annals of geophysics, vol. 47, n. 1, february 2004 key words natural processes – seasonal variations – precipitation 1. introduction identifying exogenous, in particular seasonal, variations in time series of geophysical data is an important task in the framework of analysis of earthquake precursors (yamauchi, 1987; zhao, 1987; sidorin, 1992). its solution is complicated by the absence of a precise modeling of the seasonal variations that affect geophysical parameters. such a situation is typical in the time series of apparent electrical resistivity (qian, 1985; sidorin, 1992). physically, it may be expected that the strongest exogenous variations in apparent resistivity depend on different hydrometeorological factors, especially atmospheric precipitation. atmospheric precipitation causes variations in parameters of underground hydrosphere condition, influencing many characteristics of the geophysical medium and the behaviour of some animals. these assumptions were proved experimentally (kawada, 1966; barsukov, 1972; searls et al., 1978; mcclellan, 1980; deng et al., 1981; sidorin, 1984, 1986; cornea et al., 1985; shapiro et al., 1985; zhuravlev and sidorin, 1986; fuye et al., 1988; descherevsky and sidorin, 1999a, 2000) and must be taken into account when searching for geophysical, geochemical, and biological precursors of earthquakes. the investigation of data measured at the khazor-chashma station in the garm test site needs to take into account the exogenous variations in apparent resistivity of rocks. this effect, of course, cannot be estimated directly by the precipitation, since the main role has been played by the water penetrating into the soil layer affecting the surface rocks. during winter, precipitation is accumulated in snow, and its melting and penetration of moisture into the soil occurs mainly during spring. in summer, at heavy drainage of the ground water seasonal variations in natural processes and atmospheric precipitation alexey v. descherevsky and alexander ya. sidorin schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia abstract to study the nature of seasonal variations in time series measured at the garm test site, a local model based on the experimental data of atmospheric precipitation penetration into the soil has been proposed. it is intended for filtration of exogenous variations in the data of various time series and a study of statistical structure of different natural processes, including earthquake preparation processes, and the mechanisms of their effect on the biosphere. using this model, we analyze and compare variations in apparent resistivity and properties of rock moistening. it has been shown that at small current-electrode (ab) separations among all the parameters of water regime, only water saturation of the active soil layer reveals a significant correlation with apparent resistivity variations. when increasing the current-electrode separation, the seasonal variation form varies from quasisinusoidal in the upper layer up to quasi-triangular at the largest investigated depths (maximum separations). mailing address: dr. alexander ya. sidorin, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: sidorin@ifz.ru 74 alexey v. descherevsky and alexander ya. sidorin and a small quantity of precipitation the rainfall only slightly dampens the upper (active) layer of the soil and then evaporates and its penetration into the deeper horizons is practically absent. since water penetration into the soil was not measured and the problem of water penetration from the atmospheric precipitation into the soil is rather complicated, some approaches to its solution were proposed in zhuravlev and sidorin (1986), and in more detail in descherevsky et al. (2001), in which a local model of processes of moisture penetration into the soil at the khazor chashma station was developed. the model uses, as input data: 1) quantity of atmospheric precipitation for a day (mm); 2) average daily and minimum night air temperature (oc); 3) the height of snow cover (cm); iv) deficit of relative air moisture (%). the model takes into account the following processes: i) precipitation in a solid and liquid form and their accumulation in a snow cover; ii) melting and freezing of snow cover and moisture evaporation from its surface; iii) inflow of atmospheric precipitation and melted water from snow into an active soil layer; iv) balance of moisture in an active soil layer: accumulation, abundant moisture outflow due to evaporation, penetration into deep horizons and surface runoff. the main result of the model is the determination, only by data of standard meteorological observations, of the quantity of water that practically enters into the soil. this quantity of water is important regarding the estimation of the effect of exogenous factors on variations of geophysical parameters. the paper shows that these two processes (rainfall and snow precipitation and water penetrated into the soil) have a different seasonal course; therefore the processes of moisture accumulation in a snow cover and its consequent melting in spring have to be taken into account to define the problem of assessment of the effect of exogenous factors on geophysical parameters. 2. initial data two sets of time series were used for this analysis: i) the time series characterizing the process of water penetration into the soil were considered (descherevsky et al., 2001): 1) the initial series of atmospheric precipitation; 2) the series of moisture inflow into the soil; 3) the series of moisture store in the soil’s active layer; 4) the series of moisture outflow from the soil’s active layer. ii) variations in apparent electrical resistivity of rocks, analyzed by the monitoring data at the khazor-chashma station of the garm test site. these variations were observed at the set of fourelectrode schlumberger arrays with the following separations of the ab current and receiving mn electrodes (ab/mn): 6/2 m, 18/2 m, 30/10 m, 650/150 m, 3000/500 m (sidorin, 1990). figure 1 shows the data. 3. data analysis since original series contain wide fluctuations and non-stationarity of a different origin (flicker-noise structure of data), we cannot carry out correlation analysis of these data directly. significant noise inevitably occurs at such comparison which excludes the possibility to observe the more delicate effects of relatively small amplitude which we would like to discover. therefore, a method that is stable to random noise was used for the analysis. the method is based on comparison between seasonal variations in apparent resistivity and hydrometeorological parameters. long-period variations – with typical time of the years and over – have not been studied. since these variations are present in the series of apparent resistivity, to avoid seasonal distortions and to suppress additional noise they were first filtered. the trend was estimated by a method of a sliding average, with a smoothing window 1100 days wide. to improve frequency characteristics of a smoothening filter, the gauss weight window function was used. for the hydrometeorological parameter series the trend was not filtered. firstly, an average seasonal function (asf) was obtained for each series. the calculation of asf for the apparent resistivity series is described in (descherevsky and sidorin, 1999b; 2003). for the hydrometeorological parameters a similar technique was used. 75 seasonal variations in natural processes and atmospheric precipitation secondly, coefficients of correlation and regression between asf and initial realization were calculated in a sliding year window for each series. the correlation coefficient used to reveal the stability of a seasonal variation form, and the regression that of seasonal variation amplitude. the asfs were obtained according to the technique described in descherevsky and sidorin (1999b, 2003), and descherevsky et al. (2001). this technique envisages the asf estimation by a method of superposition of epochs. the asf estimate based on the method of superimposed epochs has certain disadvantages for finite length determination. in particular, the asf form and seasonal variation amplitude may be distorted due to various random peculiarities of the original series. to eliminate this effect, we used the method of adaptive asf smoothing with subsequent cycle described in descherevsky and sidorin (1999b, 2003). relative to the algorithm proposed in anderson (1971), this method is simple (only one parameter, namely the smoothing window width, is variable) and provides an unambiguous physical interpretation. the purpose of smoothing is suppression of all the unreliable peculiarities of asf, therefore the size of a smoothing window depends on many factors: initial realization noise, intensity of a seasonal component, the series’ length and others. the degree of smoothing (the smoothing fig. 1. initial series: apparent resistivity at separations a – 6/2 m, b – 18/2 m, c – 30/10 m, d – 650/150 m, e – 3000/500 m (all of them in ω⋅m), f – atmospheric precipitation, g – the moisture inflow into the soil, h – the moisture store in the soil’s active layer, i – the moisture outflow from the soil’s active layer (all of them in mm). a b c d e f g h i 76 alexey v. descherevsky and alexander ya. sidorin window width) was determined for each realization individually by the results of special tests in such a way as to make the decrease in dispersion of asf caused by the smoothing comparable with dispersion of the noise component of asf (descherevsky and sidorin, 1999b, 2003). taking into account the stated considerations, the following procedure of selection of optimal asf smoothing was used. first of all, a non-smoothed asf was calculated by epoch superposition by a whole term and dispersion of the residual component (after the asf extraction) and its high frequency component was also evaluated. besides, «partial» asf by the first and the second part of the term were calculated and dispersion of their difference was determined. besides the dispersion, statistics of average module of difference of the two partial asf calculated by the first and the second part of the observation term was also used. a problem on evaluation of correlation of the partial asf was solved at the same time. it was assumed that significant correlation between them might be considered independent confirmation of reliability of seasonal periodicity. further, the asf calculated by the epoch superposition by the whole term was smoothed by a cycling sliding average; smoothing windows of different width were tested and decreasing of the asf general dispersion and also the dispersion of its high frequency component (periods up to 14 days) was evaluated. to improve the frequency characteristics of a smoothing filter the gauss weight window function was used. the dispersion and standard deviation decrease caused by smoothing was compared to evaluations of the dispersion and standard deviation of a noise component of the non-smoothed asf obtained by the methods described above. the final optimal width of the asf smoothing window was selected taking into account all the enlisted factors and for the series of apparent resistivity the results for the neighboring separations with relatively close amplitude of a seasonal variation were also taken into account. although the described procedure in a way is based on subjective and qualitative evaluations, the final results by an order of a value are rather uniformly determined by calculated statistics, i.e. the human factor effect may bring only small distortions into these results. as a result, the width of a smoothing window was chosen as follows: for the apparent resistivity series (ab/mn) 6/2 m, 18/2 m, 30/10 m, 650/150 m, 3000/500 m as 63, 63, 63, 123 and 63 days, respectively (descherevsky and sidorin, 1999b); for the series of atmospheric precipitation, 121 days; for the series of moisture inflow into the soil, 31 days; for the series of moisture store in the soil’s active layer, 45 days; for the series of moisture outflow from the soil’s active layer, 31 days. however, increasing the size of window by two or three times, the correlation and regression curves remain approximately the same. the asfs are plotted in fig. 2. comparing the form and the phase of apparent resistivity seasonal variation at a maximum 3 km currentelectrode separation (curve e) with moisture inflow into the soil (curve g), moistening of the soil’s active layer (curve h) and the moisture outflow from the soil’s active layer (curve i), we observe a good similarity among the curves. this indicates that the apparent resistivity increases at maximum separation between spring and summer, in concomitance with abundant inflow of melted and rain waters into a sounding volume, due to which decrease of porous solutions’ mineralization and consequently, of their conductivity and increase in apparent resistivity occur. the pattern is more complicated at minimum separations. first, the seasonal resistivity reaches the maximum earlier, at the beginning of march, and then it follows a more complicated law. second, after reaching the seasonal minimum between summer and autumn, the resistivity does not stabilize but it smoothly increases up to february. however, in this case an observed pattern also becomes more clear, if we admit that at these separations along with soil moisture mineralization moisture variations also play an important part. thus, at the beginning of spring simultaneously with moisture inflow, resistivity starts to decrease in accordance with the medium moistening increase. in the middle of spring, moisture inflow reaches a maximum, and resistivity decrease in this period is small, since the coming water is less mineralized and with rela77 seasonal variations in natural processes and atmospheric precipitation tively high resistivity. at the end of spring, inflow of slightly mineralized surface waters decreases, and general saturation remains high, and due to the soil’s moisture mineralization the resistivity continues to decrease rather rapidly. furthermore, during summer mineralization and conductivity of water solutions continue to increase and saturation to decrease, the first process has a stronger effect on resistivity and it gradually decreases. near the end of summer mineralization apparently stabilizes and apparent resistivity slowly increases with the decrease of a quantity of soil and subsoil moisture, continuing up to the beginning of spring. a more complicated pattern of apparent resistivity seasonal variations is observed at intermediate separations (ab = 30 m, curve c; ab = = 650 m, curve d). however, the pattern of these variations fully agrees with the assumption that changes in subsoil moisture and porous solutions’ mineralization cause effect to a significant degree on apparent resistivity variations at these separations as well. thus, the patterns of apparent resistivity seasonal variations in a wide range of separations are mainly caused by variations of parameters of porous water solutions occurring due to precipitation. the correlation and regression coefficients between asfs and original series were calculated sliding an annual window through the period of measurements. the correlation coefficient defig. 2. smoothed asf: apparent resistivity at separations a – 6/2 m, b – 18/2 m, c – 30/10 m, d – 650/150 m, e – 3000/500 m (all of them in ω⋅m), f – atmospheric precipitation, g – the moisture inflow into the soil, h – the moisture store in the soil’s active layer, i – the moisture outflow from the soil’s active layer (all of them in mm). two periods of average seasonal function are given. a b c d e f g h i 78 alexey v. descherevsky and alexander ya. sidorin fig. 3. the correlation coefficient estimated in a sliding annual window between the initial series and its asf (dimensionless): the correlation coefficient for apparent resistivity series at separations a – 6/2 m, b – 18/2 m, c – 30/10 m, d – 650/150 m, e – 3000/500 m, f – atmospheric precipitation, g – the moisture inflow into the soil, h – the moisture store in the soil’s active layer, i – the moisture outflow from the soil’s active layer. table i. correlation between seasonal course stability coefficients of apparent resistivity and hydrometeorological parameters. parameter stabosa stabd_water stabw_ground stabd_w_ground stab006 − 0.10 0.02 − 0.09 − 0.32 stab018 − 0.30 − 0.18 − 0.07 − 0.16 stab030 − 0.45 − 0.12 − 0.18 − 0.10 stab650 0.03 0.34 − 0.00 0.08 stab3000 − 0.05 − 0.23 0.08 − 0.31 a b c d e f g h i tects the stability of the seasonal variation form, and is plotted in fig. 3. we denote the series of time variations of correlation coefficients as stabi, where the index i indicates the original series, taking the values 006, 018, 030, 650 and 3000 for the apparent resistivity series and osa, d_water, w_ground and d_w_ground for the precipitation series, the series of mois79 seasonal variations in natural processes and atmospheric precipitation ture inflow into the soil, the series of moisture store in the soil’s active layer and the series of moisture outflow from the soil’s active layer, respectively. it should be noted that according to construction, successive values of the series stab are heavily correlated, therefore a number of independent data values for each from these sefig. 4. the regression coefficient of asf estimated in a sliding annual window for the initial series (dimensionless): the regression coefficient for the apparent resistivity series at separations a – 6/2 m, b – 18/2 m, c – 30/10 m, d – 650/150 m, e – 3000/500 m, f – atmospheric precipitation, g – the moisture inflow into the soil, h – the moisture store in the soil’s active layer, i – the moisture outflow from the soil’s active layer. a b c d e f g h i table ii. correlation between the series showing the amplitude of apparent resistivity seasonal course and hydrometeorological parameters. parameter regrosa regrd_water regrw_ground regrd_w_ground regr006 0.35 0.37 0.00 0.16 regr018 0.30 0.60 0.22 0.47 regr030 − 0.28 − 0.03 − 0.30 − 0.11 regr650 − 0.24 − 0.22 − 0.27 − 0.29 regr3000 − 0.23 − 0.44 − 0.10 − 0.36 80 alexey v. descherevsky and alexander ya. sidorin ries is much less than a general number of the series’ terms. from below it may be estimated by a number of years during which observations were continued, from above – by double number of the years. thus, to estimate a correlation coefficient between the series stabi, then 95% level of significance will be somewhere between 0.20 and 0.30. table i shows the correlation between the stabi series. an analysis of table i shows that 14 values out of 20 are less than 0.2 and may be related to insignificant ones at a 95% level. a formal 95% level of significance is increased by approximately a quarter of values in the table. it is noticeably more of expected ones by the statistics of the 5% values. however it is not easy to find logic in the obtained results. apparently, there is a relation between disturbances of a seasonal variation form for the comparable parameters, however it is not definitive, variations may be in phase ones and also in antiphase ones. as a whole, the stabi series is not the most optimal instrument to carry on this analysis. figure 4 shows the regression coefficient series, denoted by regri, which informs on the seasonal variation of amplitude. it is worth mentioning that the largest discordance between asf smoothed and the original series occurs usually in case of a shift of seasonal variation phase occurring due to shift of spring heavy showers’ maximum in time. we observe a sharp decrease of correlation coefficient concomitantly with the increase in regression coefficient dispersion. if changes of seasonal variation occur mainly in amplitude then the correlation coefficient continues to be high and the regression coefficient increases. figure 4 shows a good similarity among all of seasonal course amplitude curves of apparent resistivity and the regri series characterizing parameters of water regime. table ii shows the correlation coefficients of series regri between each other. comparing the previous case, absolute values of correlation coefficients have noticeably increased. now, 8 values out of 20 are not less than 0.3 and may be definitely related to significant ones at a 95% level, 5 – to insignificant ones (< 0.2), and the remaining 7 occupy intermediate position. for the series of apparent resistivity at minimum separations, we observe a very good relation between seasonal variation amplitude and precipitation parameters, especially with the quantity of precipitation, penetrated into the soil. at medium and large separations the relation unexpectedly becomes anti-correlation. it may be deduced that during the years with the increased quantity of precipitation the amplitude of apparent resistivity seasonal variation at large separations decreases, or disturbance of a seasonal variation form occurs, since the regression coefficient decreases. 4. conclusions apparent resistivity variations were analysed and comparied with time series of different parameters of rock moistening, calculated using a local model of atmospheric precipitation inflow into the soil developed by the authors. the model showed that at small current-electrode separations from all the parameters of water regime, only water saturation of the soil’s active layer reveals a significant correlation with apparent resistivity variations. with increase in current-electrode separation, the relation is considerably complicated, but, it is fully in accordance with the fact that not only processes of soil moistening but variations of soil moisture mineralization affect apparent resistivity variations. acknowledgements this research was supported by the russian foundation for basic research, grant no. 0105-65503. references anderson, t.w. (1971): the statistical analysis of time series (wiley, new york), pp. 704. barsukov, o.m. (1972): variations of electric resistivity of mountain rocks connected with tectonic changes, tectonophysics, 14 (3/4), 273-277. cornea, i., c. radu and v. winter (1985): correlations between precipitations and the strong earthquakes in romania, rev. roum. phys., 29 (4), 395-404. 81 seasonal variations in natural processes and atmospheric precipitation deng, q., j. pu, l.n. jones and p. molnar (1981): a preliminary analisis of reported changes in ground water and anomalous animal behavior before the 4 february 1975 haicheng earthquake, earthquake prediction edited by d. simpson and p. richards, (am. geophys. union, washington), 543-565. descherevsky, a.v. and a.ya. sidorin (1999a): experimental research of apparent resistivity seasonal variations applicable to problems of seismology, seismic instruments, 32, 62-75. descherevsky, a.v. and a.ya. sidorin (1999b): some problems of estimation technique of average seasonal functions for geophysical data (uipe ras, moscow), pp. 40. descherevsky, a.v. and a.ya. sidorin (2000): correlation of seasonal variations of geoelecrical parameters with hydrometeorological elements’ regime, electrical correlation of geospheric envelopes (uipe ras, moscow), 87-99. descherevsky, a.v. and a.ya. sidorin (2003): anomalous dependence of the seasonal variation amplitude of apparent electric resistivity on the spacing, doklady earth sciences, 388 (1), 110-113. descherevsky, a.v., v.m. mukhin and a.ya. sidorin (2001): a local model of seasonal variations of atmospheric precipitation inflow into the soil at the garm test site (uipe ras, moscow), pp. 36. fuye, q., z. yulin and x. tongchun (1988): an analysis of the seasonal variation of disturbance in georesistivity, acta seismologica sinica, 1 (3), 69-83. kawada, k. (1966): electric resistivity measurement along and across a ground fissure in the matsushiro area, bull. earthquake res. inst. univ. tokyo, 44, 1759-1769. mcclellan, p.h. (1980): preearthquake animal behavior: a closer look for alternative causes, geophys. res. lett., 7 (5), 333-336. qian, j. (1985): regional study of the anomalous change in apparent resistivity before the tangshan earthquake (m = = 7.8, 1976) in china, pure appl. geophys., 122, 901920. searls, c., k.a. poehls, d.d. jakson and e.m. wescott (1978): changes in crustal resistivity near palmdale, california, geophys. res. lett., 5 (11), 928-930. shapiro, m.h., a. rice, m.h. mendenhall, j.d. melvin and t.a. tombrellot. (1985): recognition of environmentally caused variations in radon time series, pure appl. geophys., 122 (1), 309-326. sidorin, a.ya. (1984): electrical resistivity variations of the earth’s crust upper layer, rep. ussr acad. sci., 278 (2), 330-334. sidorin, a.ya. (1986): the results of precise observations of apparent resistivity variations at the garm test site, rep. ussr acad. sci., 290 (1), 81-84. sidorin, a.ya. (editor) (1990): the garm geophysical test site (ipe ussr academy of science, moscowgarm), pp. 240. sidorin, a.ya. (1992): earthquake precursors (nauka, moscow), pp. 192. sidorin, a.ya., v.i. zhuravlev and m.g. ostashevsky (1983): multidisciplinary electrometrical research of geodynamical processes, experimental seismology (nauka, moscow), 149-162. yamauchi, t. (1987): anomalous strain response to rainfall in relation to earthquake occurrence in the tokai area, japan, j. phys. earth, 35 (1), 19-36. zhao, h. (1987): complex space-temporal model of endogen and exogen parameters and it’s use for earthquake forecast, j. seismol. res., 10 (1), 71-82. zhuravlev, v.i. and a.ya. sidorin (1986): a method and results of account of meteorological factors’ effect on regime electrometric observations’ data, rep. ussr acad. sci., 290 (1), 63-66. 235_240 casazza1.pdf 235 annals of geophysics, vol. 46, n. 2, april 2003 intense events of rainfalls in piemonte marco casazza and angelo piano dipartimento di fisica generale «a. avogadro», università di torino, italy abstract the possibility of studying the precipitations recorded in a large number of meteorological stations in piemonte led us to analyse what has happened in the last century. a comparison between periods of 10-20 years, using an arbitrary index of rainfall, shows an increase in intense events in the last 20 years though we can observe a general reduction, in absolute value, of the quantity of rain. it is shown that, in the last 30 years, there has been a change in the granulometric distribution of suspended particulate matter (spm) which, in standard conditions, might become condensation nuclei (cn). this might have infl uenced condensation phenomenon, causing a greater frequency of intense events. mailing address: dr. angelo piano, dipartimento di fisica generale «a. avogadro», università di torino, via p. giuria 1, 10125 torino, italy; e-mail: piano@to.infn.it key words precipitation – intense events – inorganic particulate matter 1. introduction recent natural events of great intensity and their frequency have aroused the attention of atmospheric physicists (diodato, 1999) who, individually, have suggested different hypotheses to explain what has happened. luckily, precipitations have been studied for a long time in italy for three main reasons: the fi rst is linked to fr. angelo secchi of reggio emilia, who lived in the 19th century and promoted, in 1886, the creation of a national meteorological service; the second is due to the easiness of measuring rainfalls; the third is the danger of this phenomenon, that persuaded many people to accept fl oods as unavoidable events. we have a great quantity of data relative to our region in the last century. our «dipartimento di fisica generale» has two meteorological stations in torino. the aim of this work is to obtain an index of pluviometric intensity to characterise different periods and, particularly, the last 20 years. 2. pluviometry in piemonte in our region, rain distribution shows a greater regularity in the plains. the absolute maxima are located in the interface between the mountains and the plains, with a decrease going to inner mountain regions. the mountainous areas having poor rainfalls are, in fact, the inner western alpine valleys: val susa, varaita and maira (with mean precipitations per year not exceeding 900 mm). the same tendency is more evident in the northern valleys (in val d’ossola we go from mean values of 2300 mm to 1200 mm). this phenomenon is due to the obstruction created by the fi rst relieves close to the plain that stops a portion of air masses from the southern quadrant. in the interface with the plains, we can fi nd the places with greatest precipitations. the most important area goes from lago maggiore to valli di lanzo (with a yearly maximum of 2350 mm in cicogna (vb)). the second one is in the se of the region, close to the borders piemonte, liguria, lombardia and emilia romagna (scrivia and curone basins). the third area is close to maritime alps, from the southern plain of the «provincia» of cuneo and from the langhe to the mountains between valle gesso and tanaro. the last one is referred to as the prealps between val chisone and val pellice. in this cases, the abundance of rainfalls is due to 236 marco casazza and angelo piano the air masses coming from the plain, which, rising, discharge their humidity. in the second and third cases, instead, precipitations are linked with the presence of the sea; in fact, having winds from s-se (from genova), humid currents cross this area. the lowest precipitation levels are in the plain areas close to the apennines. the best condition for having rain in our region is to have air currents from s or se; the leeward area of the maritime alps receives the air poor in humidity, so it is characterised by a small amount of rainfalls. all the other plain areas are crossed by more humid air masses. that is why the northern plains are the rainiest ones. 3. number of days of rain and mean intensity the number of days of rain in a year goes from a minimum, typical of the mediterranean region, of 50 to a maximum, of a continental type, of 140. the areas having fewer events (less than 70 per year) are the monferrato and a portion of langhe. in the plain regions we can go, usually, from 70 to 90 days while, in the relieves, we do not exceed 90 days, with a small area of toce basin, where we exceed 110 days per year. the values of daily precipitations, obtained dividing the mean annual value by the number of rainy days, go from 8 to 24 mm/day. usually the mean intensity value in our region goes from 10 to 15 mm/day. areas with lower intensities (less than 10 mm/day) are located around alessandria and the inner alpine valleys. the greatest ones, instead, are in the areas characterised by abundant yearly rain; this is due more to the intensity than to the number of rainy days. that is why it is important to fi nd an index that is not infl uenced by local phenomena. 4. choices because of the distribution in our region and because of the validation of data, we have privileged, the measurement points considered in the annali idrologici. among all the possible stations available, for different reasons (absence of historical data, lack of long periods of registration or poor accuracy of historical records), we had to reject many of them. we did a further reduction of the number of stations considered by grouping (after a comparison of experimental data) some stations (like torino and moncalieri) to obtain a uniform distribution in the region. furthermore, studying long periods, the fi nal choice of the stations (shown in fi g. 1) seemed expressive for our purposes. the analysed periods are 1931-1950 and 1980-2000. the fi rst one was chosen because of the quantity of experimental data (many stations started their activity in this period) and their importance, related to the small immission (in the period 1931-1938) of pollutants in the atmosphere. until 1950, in fact, in torino the heating systems were working using coal or wood, but they were fewer than now. the second fact is that the number of cars or similar means of transport was limited (it was not uncommon to use animal traction). it should be possible, in fig. 1. chart of the stations of «uffi cio idrografi co» chosen to create our rainfall indexes. 237 intense events of rainfalls in piemonte that way, to see the possible effects of pollutants or, anyway, of a different thermal situation of the atmosphere. 5. rainfall index in order to fi nd extreme events, we decided not to study the mean quantity of rain in a period, because it is possible to increase it having a small quantity of rain every day (without fl oods). so, we decided to analyse the accumulated precipitation, in millimetres, for a period of 24 and 48 h. we did not consider greater periods of time, because it is rare (in the chosen periods, extreme events of more than two days of precipitations, as the one of 13-16 october 2000, have been, in total, less than 10) to have great precipitations on the third day. besides, we considered, as intense events, the ones originated by a large quantity of water in a short time. we created an absolute intensity index (aii) for 24 or 48 h, given by . to see only the extreme events, we fi xed a lower limit under which the index was considered equal to 0. the threshold limit was chosen to be 6% (for 24 h) and 7% (for 48 h) of the mean of the annual precipitation quantity along the period considered for each single station. this choice offers at least two advantages: we obtained an optimum number of events for each period and, furthermore, we found an index not linked with local climatological factors. in fact, the threshold values, moving from one station to a different one, cancel the differences that exist among them. we can point out, using the aii, the increase in millimetres of water fallen in each single event, even if it is diffi cult to see how this could be meaningful during the comparison among the different stations. that is why we created a normalised intensity index (nii), referred to the mean annual value of the period considered, calculated as mean of annual means . in this case, if in the last 20 years we have had less rain, the value of nii increases giving signifi cance to the single events. in this case too, the thresholds chosen are equal to those of the aii. 6. conclusions to see what has happened in the 40 years considered and not having all the data of the fi rst 20 years (we had all the data just for the period 1931-1941), we have divided the periods considered into groups of 11 years, superposing them if necessary. in order not to have any problems with the threshold limits in the different periods, we decided to use a new one (equal to the highest of them), that did not create any problem using the nii, because we are referring to percentages. figures 2 and 4 show the number of events with aii with values different from 0 for a period, respectively, of 24 h and of 48 h. furthermore, fi gs. 3 and 5 highlight the number of events with nii different from 0 for periods of 24 h and 48 h respectively. our analysis shows an increase, in the last period considered, in the number of events, particularly intense ones. for nearly all the stations, in fact, mainly in the period 19902000, we can see an increase in intense events, though it rains less than before. it is important to remember here that, during the fl ood of 15-16 october 2000, that provoked so much damage and so many deaths, we had in many areas more than 500 mm of water in 3 days. considering the causes of those types of rainfalls, the interest in the geofit group of «università degli studi di torino» is focused on the study the infl uence of suspended particulate matter (spm) in the events of intense precipitations (bacci et al., 1983). we can say, using the data of 30 years of experimental measures, that the granulometric distribution of inorganic solid spm with diameter of less than 10 mm has changed. aii mm rain 24h 1 = aii mm rain 48h 2 = nii aii mean annual value for the period = 238 marco casazza and angelo piano fig. 2. number of events of 24 h with aii different from 0. fig. 3. number of events of 24 h with nii different from 0. fig. 4. number of events of 48 h with aii different from 0. fig. 5. number of events of 48 h with nii different from 0. 239 intense events of rainfalls in piemonte table i. cut-off diameters for each single stage of non-viable andersen cascade impactor. stage cutt-off diameter ( mm) 0 11 1 7-11 2 4.7-7 3 3.3-4.7 4 2.1-3.3 5 1.1-2.1 6 0.65-1.1 7 0.43-0.65 fig. 6. granulometric breakdown of particulate matter in 1978, 1985, and 2000. figure 6 shows the results of 3 representative measures of the years 1978, 1985, and 2000, mean of several measures done in the same period of the year with stable weather conditions. the instruments used for those measures are 8 stages non-viable andersen cascade impactors (andersen, 1958; vaughan, 1989), whose cutoff diameters are shown in table i. this type of instrument gives us the possibility to study the granulometric repartition of spm between 0.43 mm and 10mm. it is possible to see that the breakdown for the year 2000 (confi rmed by many measures in 2001) is more fl attened than before and it is quite common in outdoor measurements. we think that this might have infl uenced the production of raindrops in such a way to obtain an increase in intense rain events, considering other causes too. among them, it is important to consider the energy involved in the condensation process and the humidity of the air in different climatological conditions. in addition, we are again studying the chemical and physical characterization of solid inorganic particulate matter in urban and rural areas to understand the degree of infl uence of this in the condensation of water vapour in the atmosphere. references andersen, a. a. (1958): a new sampler for the collection, sizing and enumeration of viable particles, j. bacteriol., 76, 471-484. annali idrologici: ministero dei lavori pubblici, servizio idrografi co (istituto poligrafi co dello stato, roma). bacci, p., a. longhetto, g. marcazzan, a. piano, f. prodi, c. sabbioni and a. ventura (1983): aerosol 240 marco casazza and angelo piano characterization in a po valley site, in proceeding of the gesellschaft für aerosolforschung meeting, j. aerosol sci., 14 (3), 222-225. diodato, n. (1999): ricostruzione storica di eventi naturali estremi a carattere idrometeorologico nel sannio beneventano dal medioevo al 1998, boll. geofi s., 22, 5-39. vaughan, n.p. (1989): the andersen impactor: calibration, wall losses and numerical simulation, j. aerosol sci., 20 (1), 67-90. sulla distribuzione in longitudine dei flocculi di calcio g. godoli f . mazzucconi b . c. monsignori fossi ricevuto il 30 giugno 1966 riassunto. — un'analisi statistica dei numeri caratteristici definitivi dei flocculi di calcio relativi all'anno geofisico internazionale ha mostrato per questi fenomeni l'esistenza di longitudini preferenziali in accordo a quanto rilevato da altri autori per altri indici di attività solare. summary. — a statistical analysis of the definitive character figures of the ca bright fiocculi for the international gjeophysical year has shown for these phenomena the existence of preferential longitudes in agreement with the results obtained by several authors for different indices of solar activity. 1. recentemente è stata pubblicata iu questa sede la lista dei numeri caratteristici definitivi dei flocculi di calcio per l ' a n n o geolìsico internazionale (1 luglio 1957-31 dicembre 1958) (x). è stata questa la prima occasione in cui si è potuta raccogliere una serie, praticamente senza lacune, di numeri caratteristici giornalieri dei flocculi di calcio per un periodo sufficientemente lungo. d a t a la sua omogeneità, la serie si presta ad una analisi della variazione giornaliera dell'attività dei flocculi di calcio in un periodo in cui l ' a t t i v i t à generale del sole si è mantenuta praticamente costante. 2. l ' a n d a m e n t o giornaliero dei numeri caratteristici dei flocculi di calcio per t u t t o il periodo d e l l ' a g i è stato riportato nel grafico di f i g . 1. d a l grafico dì f i g . 1 si possono rilevare successioni di massimi e di m i n i m i che si ripetono abbastanza regolarmente per 3-4 rotazioni 396 g. g 0 d 0 l i f . m a z z u c c o n i b. c. m o n s i g n o r i fossi sinodiche del sole. questo fatto è evidentemente da attribuirsi alla persistenza dei più rilevanti flocculi di calcio. d a determinazioni eseguite all'osservatorio di arcetri i fenomeni più persistenti hanno una durata media che si aggira appunto, per gli anni 1957-1958, intorno a 3-4 rotazioni {-}. i l f a t t o più interessante rilevabile dalla f i g . 1 è però che le successioni di massimi tendono ad essere fra loro intervallate di multipli di 27 giorni. questi f a t t o può essere attribuito alla ben nota esistenza di longitudini preferenziali per l'attività solare (=• 5 . 6 . 7 8 ) . attribuendo alle longitudini centrali la maggior attività osservata nei diversi periodi, si vede che il fuso più attivo è quello compreso fra le longitudini di carrington 300°-40°. questo fuso risulta il più attivo anche in base a determinazioni di altri indici di attività solare ( 7 ' s ) . 3. per una migliore analisi statistica del materiale di osservazione, abbiamo calcolato i coefficienti di autocorrelazione dei numeri caratteristici dei flocculi di calcio per sfasamenti j compresi fra 0 e 500 giorni. indicando con xt mia fra le n (549) determinazioni di numeri caratteristici i coefficienti di autocorrelazione r j sono dati dalla relazione si xj x,+j — (zi xt) [et xi+i)/at n e l grafico di fig. 2 i coefficienti di autocorrelazione r} sono stati riportati in finizione di j . p e r j compreso fra 0 e 100 giorni si nota una netta periodicità del coefficiente di autocorrelazione che conferma la persistenza dei più rilevanti flocculi di calcio. per sfasamenti compresi fra 100 e 200 giorni l'andamento dei coefficienti di autocorrelazione è meno netto. successivamente, però, esso riacquista una discreta periodicità con i massimi spostati di multipli interi della rotazione sinodica del sole confermando così l'esistenza di longitudini preferenziali. g-. g-odoli f . mazzucconi b. c. monsignori fossi ia. 1 0 is. 10 10. 10 1 n. 10 1d. 10 1g. i o ì t t o i m ~ ì o i t ~ i o m w i s t i o i l i o i t ~ i o is71o m i o i f t l o i d ~ ì o " 1958 fig. 1 andamento giornaliero dei numeri caratteristici dei flocculi di calcio per il periodo 1 luglio 1957-31 dicembre j958. 1.0 ojb k 0.6 k i i i i 1 i i i i : 50 100 150 200 2 5 0 300 3 5 0 4 0 0 450 suo l'ia;. 2 coefficienti di autocorrelazione dei numeri caratteristici dei flocculi di calcio per sfasamenti j compresi fra 0 e 500. 398 g. g o d o l i f. m a z z i ' c c o n i •• l i . c. m o n s i g n o r i fossi b i b l i o g r a f i a t1) g o d o l i g . , m a z z u c c o n i f . , m o n s i g n o r i f o s s i b . c . , ann. geoj., 1 9 , 95 (1966) 1, 1966. ( 2 ) godoli g., monsignori fossi b. c., memorie lincei, in corso di stampa. ( 3 ) losii h . m., p u b . of. the obs. of the u n i v . of michigan, 7, 127 (1939). ( ' ) kiepenheuer k . 0 . , in the sun, chicago press, chicago, 338 (1953). ( 5 ) yitinskii y c . i., izv. glav. obs. pulkova, 21, 96 (1960). ( ° ) t r o t t e r d . e . a n d b i l l i n g s d . e . , ap. j., 1 3 6 , 1 1 4 0 ( 1 9 6 2 ) . ( 7 ) w a r w i c k c . s . , ap. j., 1 4 1 , 5 0 0 ( 1 9 6 5 ) . ( 8 ) d r a g o f . , t a g l i a f e r r i g. l . , 21em. sait, in corso di stampa (1966). layout 6 annals of geophysics editor in chief edoardo del pezzo associate editors paola albini salvatore barba francesca bianco alessandro bonaccorso mike burton fabio cammarano claudio chiarabba paola de michelis laura de santis roberto devoti giuseppe etiope fabio florindo giampiero iaffaldano jesús m. ibáñez godoy salvatore inguaggiato riccardo lanari mario la rocca andrea morelli giovanni muscari yongxin pan stephen pekar massimo pompilio gideon rosenbaum martin siegert mare yamamoto editorial office ingv, istituto nazionale di geofisica e vulcanologia sezione di bologna via donato creti, 12 40128 bologna, italy tel.: +000 000 000 000; fax: +0 000 000 000 annals@ingv.it web site www.annalsofgeophysics.eu author guidelines submission to ag is understood to imply that the manuscript is original, unpublished, and not under consideration for publication elsewhere. manuscripts must be submitted 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can be obtained from the iris data management center at www.iris.edu (last accessed august 2003). seismograms used in this study were taken from the data base of ingv, centro nazionale terremoti, and can be requested at this address: xxxxxx.com (continues on page ii) [vol. 53, n. 1, february 2010] annals of geophysics plots were made using the generic mapping tools version 4.2.1 (www.soest.hawaii.edu/gmt; wessel and smith 1998). seismograms used in this paper can be found upon request at xxxxxx yyyyyy, xxxxxx@kk.dd.de data of this experiment can be downloaded from the ag server, following the link: xxxxxxxxxxx.xxxxx.it references should be arranged alphabetically by author and should follow the style of examples below: hough, s. e. and s. martin (2002). magnitude estimates of two large aftershocks of the 16 december 1811 new madrid earthquake, bull. seism. soc. am., 92, 3259-3268. gutenberg, b. (1959). physics of the earth's interior, academic press, new york, 111113. laster, s.j., m. m. backus and r. schell (1967). analog mode studies of the simple refraction problem, in seismic refraction propsecting albert w. musgrave (editor), society of exploration geophysics, tulsa, oklahoma, 15-66. tables should be numbered consecutively with arabic numerals. each table must have a title; column heads should be brief and should contain units of measurement in parentheses. present each table on a separate page. when preparing tables, take into account the size of the journal page (layout box: cm 25,7 x 18,6). generally, the tables will be re-formatted in order to follow the journal style. but in some cases they will be reproduced as they are written. so, for particularly dense information, the authors are invited to produce their tables in pdf format, ready for reproduction. figures: submit figures as separate files, numbered sequentially and in the order of first reference in the text. submit captions as a consecutively numbered list embedded in the text of the manuscript. do not include captions on the figures themselves. accepted file formats are tiff and eps, with a resolution as nearly as possible approximating to 300 dpi. given that resolution, figure dimensions should be correlated to the image complexity and legibility, taking into account the following three options: base cm 9, base cm 19, base cm 26. ii iii dear readers and contributors, the present volume of annals of geophysics is the first in the new, online-only format. the journal has been updated for both the editorial committee and the article types that can now be accepted. as well as the usual «research articles», the journal will now accept «letters», «review articles» and «data and experiment descriptions». this last category will include reports on important experiments or databases that will be useful for the geophysics community. together with the descriptions of the experiments, the authors will need to make the data from the experiments available. the journal will be active in providing assistance to conveners of important national and international meetings, for the publication of the papers presented in their meeting, and will publish special volumes on specific topics of broad interest to the scientific community. this present "first" volume is a historical volume. we have decided to celebrate this change into the modern online format by republishing a set of selected papers by authors who, in different ways, in different contexts, and for different reasons, are considered today to have provided us with important "benchmarks" in the development of geophysics. all of these papers were published before 1977 on annali di geofisica, the former name of this same journal. two of these papers are in italian, with an english abstract. the authors are very well known in italy for their outstanding contributions to geophysics research, and they can be considered as excellent examples of geophysicists who have contributed to the transmitting of italy's cultural heritage to the present generation of scientists. edoardo del pezzo, editor in chief february 2010 editorial layout 6 a ground radiometric study of uranium, thorium and potassium in isparta, turkey n. ayten uyanik1, iskender akkurt2,*, osman uyanik3 1 süleyman demirel üniversitesi, deprem ve jeoteknik araştirma merkezi, isparta, turkey 2 süleyman demirel üniversitesi, fen edebiyat fakültesi, fizik bölümü, isparta, turkey 3 süleyman demirel üniversitesi, müh-mim. fak., jeofizik müh. bölümü, isparta, turkey annals of geophysics, 53, 5-6, 2010; doi: 10.4401/ag-4726 abstract the radioelement concentrations of uranium (238u), thorium (232th) and potassium (40k), as their radionuclides eu, eth and k, of cünür hill in the city of isparta (turkey) and its surroundings were measured using gamma-ray spectrometry with a nai(tl) detector. the measurements were performed over an area of 2 km × 8 km around cünür hill, which forms part of the isparta volcanic region that has a trachyandesitic composition, and which included the limestone outcrops east of cünür. the results are mapped as combined radioelement concentrations. the radioelement concentrations of eu, eth and k measured for cünür hill are high because of its trachyandesitic composition, while those measured in the limestone area were low, as would be expected. 1. introduction natural radioactivity has existed since the universe began, and it has survived to date due to the long half-lives of various radioactive element series. these elements are mainly uranium (238u), thorium (232th), and potassium (40k). radiometric methods are one of the most powerful and rapid methods to obtain information about the distributions and concentrations of these radionuclides (eu, eth, k) in any specific areas. moreover, using these methods, it is also possible to carry out geological and geochemical mapping [durrance 1986, aydin et al. 2006, uyanık and akkurt 2010]. the region around the city of isparta (turkey) is an interesting area, where many different tectonic structures are found. this region is rich in trachyandesitic rock and has large limestone exposures [görmüşand özkul 1995, kanbur et al. 2008]. several geological studies have been performed on this region [gutnic et al. 1979, görmüş and özkul 1995, yagmurlu et al. 1997, koçyigit and deveci 2007, plateovet et al. 2008, kanbur et al. 2008]. however, no complete studies for the regional radiometric data on this area are presently available, although there has been a report of measurements of the natural background radiation [akkurt 2004]. in the present pilot study, the concentrations of the eu and ethelement series and kactivities around the cünür hill area of isparta were measured using gamma-ray spectrometry with a nai(tl) scintillation detector. annual effective outdoor doses were also estimated. 2. materials and methods 2.1. geological setting the field measurements were performed at 60 locations on cünür hill and in the surrounding area near isparta. the city of isparta is located between the latitudes of 37˚ 18´ n and 38˚ 30´ n. the height of cünür hill reaches about 1,090 m, and its geological exposure consists mainly of volcanic sand, limestone and alluvium [görmüş and özkul 1995, yagmurlu et al. 1997, nemec and kazancı 1999, görmüş et al. 2001, francalanci et al. 2001]. this isparta volcanic belongs to the post-collisional alkali potassic-ultrapotassic magmatism that has been active since the miocene [plateovet et al. 2008]. figure 1 shows the location of isparta within turkey, and also the geological structure of cünür hill, near isparta. the satellite view of the locations where the measurements were performed in the present study are shown in figure 2. 2.2. activity measurements a portable gf instruments (isparta,turkey) c-ray spectrometer was used to measure the eu (ppm), eth (ppm) and k (%) elemental concentrations. the spectrometer consisted of a nai(tl) scintillation detector connected to a 512-channel pulse-height analyzer. the detector size was ca. 5 cm × 5 cm and it was enclosed in a single integral unit with a photomultiplier tube, a high-voltage supply and a signal preamplifier. the calibration of the detector was carried out article history received may 17, 2010; accepted october 12, 2010. subject classification: isparta volcanics, gamma-ray spectrometry, potassium, uranium, thorium. 25 . a ground radiometric study 26 figure 1. geological map of isparta [adapted from mta 1997]. figure 2. satellite view of the study area and measurement locations. 27 using the 137cs isotopic source that is mounted inside the probe. the natural radioactivity measurements were based on recording the quantities of the three natural long-lived elements: eu, eth and k, based on their positioning at 1,760 kev, 2,610 kev and 1,461 kev respectively, in the energy spectrum. table 1 gives the gamma-ray energies and emission rates for these nuclides [baloguna et al. 2003]. each of the measurements was performed at ca. 50 cm above the ground, for 5 min. 3. results and discussions in-situ measurements were performed to obtain the mean elemental concentrations of eu, eth and k for the different soil types in the cünür hill region. overall, the results showed 13 ppm, 40 ppm and 2.4%, for eu, eth and k, respectively. these data are given in table 2 and illustrated in figure 3, according to the alluvium deposits, and trachyandesitic and limestone soil types. as can be seen from table 2 and figure 3, the elemental concentrations vary across the different soil and rock types. the highest mean elemental concentrations were calculated from locations where there are trachyandesite outcrops, and the lowest mean concentrations were obtained from the limestone locations. the overall absorbed gamma dose rates in air due to the combined activities of these eu and eth element series and k were also measured, using a gf instruments survey meter. the data are shown in table 2 according to the means within each soil type, and in figure 4 for all of the individual locations, with the latter data ranging from 24.4 ngy/h to 218.4 ngy/h. for the trachyandesitic rock, the relative contribution to the dose due to the eth element series is the greatest (73.5%), followed by the eu element series (21.9%) a ground radiometric study figure 4. absorbed dose rates across all of the sampling locations. figure 3. elemental concentrations for the three types of soil. table 1. natural radioactive elements analysed in this study. table 2.elemental concentrations, absorbed gamma dose rates and the annual effective doses determined, according to the soil types in the study area. element nuclide half life gamma-ray energy (kev) emission rate 214pb 26.8 min 295.2 19 238u 351.2 38.9 214bi 19.9 min 609.3 43.3 1764.7 17 208tl 3.05 min 583.2 185.77 232th 2614.7 99.79 228ac 6.13 h 911.1 22.7 40k 1.3×109 yr 1460.8 10.7 eu eth k (ppm) (ppm) (%) (ngy/h) (msv/y) alluvium 10.3 25.8 1.94 148.2 0.18 trachyandesite 11.2 39.3 2.31 191.9 0.23 limestone 2.65 3.4 0.1 25.2 0.03 soil type elemental concentration dose rate aedr and k (4.6%). for the alluvium, this is similar: eth with the greatest contribution for the gamma dose rate (71.7%), followed by the eu element series (23%) and k (4.3%). however, for the limestone, the relative contribution to the dose rate due to the eth element series is lower, at 59.1%. the mapped combined gamma dose rate distribution according to the sample locations for cünür hill and the surrounding areas is shown in figure 5. as can be seen from figure 5, the highest gamma dose rate is at location 8, which is in the trachyandesite area, and the lowest is at location 47, which is in the limestone field. the low values for these combined gamma dose rates are seen especially to the northeast of cünür hill, which again will be due to the large occurrence of limestone in this area. this study area also includes trachyandesitic rock that forms the volcanic structure of cünür hill. it can be seen from figure 4 and figure 5 that the concentrations at the top of the hill are lower than those at other locations around the hill. this could be the result of the spreading of the trachyandesitic structure out of the hill, due to gravitation effects, and it might also due to meteorological effects on the soil characteristics. this could also be the result of using chemical materials on the cultivated land. the overall dose rate results classified according to the soil types on and around cünür hill are given in table 2; as can also be seen from figure 6, the soil that contains trachyandesite has higher dose rates than the other soil types. the annual effective dose rate (aedr) is an important parameter for the evaluation of radiation hazards due to the absorption from these dose rates. using the conversion coefficient from the absorbed dose in air to the effective dose (0.7 sv/gy) and 0.2 for the outdoor occupancy factor, which indicates that on average, 20% of our time all around the world is spent outdoors [unscear 2000], the aedr was obtained from the following formula [cevik et al. 2007, turhan et al. 2008]: (1) the aedr values obtained range between 0.03 and 0.74 msv/y across all of the sampling locations. these data were combined according to the three soil types on and around cünür hill, and these mean values are given in table 2 and illustrated in figure 7. as can be seen, the values obtained are under the recommended limits of 1 msv/y. these annual effective dose rates are also compared with the values obtained for other locations in other studies, as illustrated in figure 8 [orgun et al. 2005, hafezi et al. 2005, kilic and aykamis 2009, bozkurt et al. 2007]; as can be seen, all of these values are under the recommended limits. 5. conclusion it can be concluded from the present study that as well as measurements of elemental concentrations, this radiometric method can be used for the mapping of geological fields. this helps to illustrate sources of the rock minerals in terms of the radioactivity revealed. on the other hand all of the values obtained in this study remain under the recommended values. to determine the radioactivity levels of isparta as a region in turkey, this type of study can be further enlarged from the cünür area. a ground radiometric study 28 figure 5. absorbed dose rate distribution for cünür hill and its surroundings. aedr (msv/y) d (ngy/h) 8760 (h/y) 0.2 0.7 (sv/gy) 10 6 = = # # # # 29 a ground radiometric study figure 6. absorbed dose rate classified according to soil type. figure 7. effective dose rate as a function of the soil types around cünür hill. figure 8. effective dose rates for the trachyandesitic rock in the present study in comparison with other published measurements: tehran, iran [hafezi et al. 2005], the city of isparta (present study), the town of sivrihisar [orgun et al. 2005], the province of osmaniye [kilic and aykamis 2009], and the region of saliurfa [bozkurt et al. 2007]. acknowledgements. this study was supported by the suleyman demirel university research foundation unit (1913 d 09). references akkurt, v. d. (2004). isparta ve çevresinin dogal gamma radyasyonunun ölçümü s.d.ü. fen bil. enstitüsü dergisi 8-2, special issue, 108. aydin, i., m.s. aydogan, e. oksum and a. koçak (2006). an attempt to use aerial gamma-ray spectrometry results in petrochemical assessments of the volcanic and plutonic associations of central anatolia (turkey), geophys. j. int., 167, 1044-1052. baloguna, f.a., c.e. mokobia, m.k. fasasia and f.o. ogundare (2003). natural radioactivity associated with bituminous coal mining in nigeria, in: proceedings of the tenth symposium on radiation measurements and applications, nucl. instrum. meth. a., 505 (1-2), 444-448. bozkurt, a., n. yorulmaz, e. kam, g. karahan and a. erdal osmanlioglu (2007). assessment of environmental radioactivity for sanliurfa region of southeastern turkey, radiat. meas., 42 (8), 1387-1391. cevik, u., n. damla and s. nezir (2007). radiological characterization of cayırhan coal-fired power plant in turkey, fuel, 86 (16), 2509-2513. durrance, e.m., (1986). radioactivity in geology: principles and applications, ellis horwood limited, chichester, uk, 441 pp. francalanci, l., l. civetta, f. innocenti and p. manetti (2001). neogene alkaline volcanism of the afyon-isparta area, turkey: petrogenesis and geodynamic implications, miner. petrol., 70, 285-312. görmüş, m. and m. özkul (1995). gönen-atabey (isparta) ve aglasun (burdur) arasındaki bölgenin stratigrafisi. süleyman demirel üniversitesi fen bilimleri enstitüsü dergisi, 1, 43-64. görmüş, m., s. caran, h. çoban and k. yılmaz (2001). bedrebarla (egirdir gölü batısı) arasında kıyı kenar çizgisi. 1, egirdir sempozyumu, isparta, 387-402. gutnic, m., o. monod, a. poisson and j.f. dumont (1979). géologie des taurides occidentales (turquie), mem. s. geo. f., 137, 1-112. hafezi, s., j. amidi and a. attarilar (2005). concentration of natural radionuclides in soil and assessment of external exposure to the public in tehran, iran, j. radiat. res., 3, 85-88. kanbur, z., m. görmüşand s. kanbur (2008). isparta yerleşim alanı kuzey kesiminin sıg s-dalgası kesitinin çıkarılamsında kırınım-microtremor tekniginin (remi) kullanılması, hacettepe üniversitesi yerbilimleri dergisi, 29, 77-86. kilic, a.m. and a.s. aykamis (2009). the natural radioactivity levels and radiation hazard of pumice from the east mediterranean region of turkey, b. eng. geol. environ., 68, 331-338. koçyigit, a. and s. deveci (2007). a n-s trending active extensional structure the suhut (afyon) graben: commencement age of the extensional neotectonic period in the isparta angle, sw turkey, turk. j. earth sci., 16, 391-416. mta (1997). maden tetkik arama enst., 1997 report. nemec, w. and n. kazancı (1999). quaternary colluvium in west-central anatolia: sedimantary facies and palaeoclimatic significance, sedimentology, 46, 139-170. orgun, y., n. altinsoy, a.h. gültekin, g. karahan and n. celebi (2005). natural radioactivity levels in granitic plutons and groundwaters in southeast part of eskisehir, turkey, appl. radiat. isotopes, 63 (2), 267-275. platevoet, b., s. scaillet, h. guillou, d. blamart, s. nomade, m. massault, a. poisson, ö. elitok, n. özgür, f. yagmurlu and k. yılmaz (2008). pleistocene eruptive chronology of the gölcük volcano, isparta angle, turkey, quaternaire, 19 (2), 147-156. turhan, s., u.n. baykan and k. sen (2008). measurement of the natural radioactivity in building materials used in ankara and assessment of external doses, j. radiol. prot., 28, 83-91. unscear (2000). united nations scientific committee on the effects of atomic radiation. sources, effects and risks of ionizing radiation, report to the general assembly with annex b: exposures from natural sources of radiation, united nations, new york. uyanık, n.a. and i. akkurt (2010). alkali volkanitler yönünden zengin olan isparta-çünür tepesinde dogal radyoaktivite tayini. afyon kocatepe üniversitesi, fen bilimleri dergisi, cilt 9, sayı 2. yagmurlu, f., y. savaşçın and m. ergun (1997). relation of alkaline volcanism and active tectonism within the evolution of the isparta angle, sw turkey, j. geol., 105 (6), 717-728. *corresponding author: iskender akkurt, süleyman demirel üniversitesi, fen edebiyat fakültesi, fizik bölümü, isparta, turkey; e-mail: iskender@fef.sdu.edu.tr. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. a ground radiometric study 30 de santis.pdf 491 annals of geophysics, vol. 46, n. 3, june 2003 italian geomagnetic reference field (itgrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting angelo de santis (1), luis gaya-piqué (1) (2), guido dominici (1), antonio meloni (1), j. miquel torta (2) and roberta tozzi (1) (1) istituto nazionale di geofi sica e vulcanologia, roma, italy (2) observatori de l’ebre, roquetes, spain abstract the updated version of the italian geomagnetic reference field (itgrf) for 2000.0 and its secular variation model up to 2005.0 are presented in this paper. the main field model is based on a simple polynomial approximation in latitude and longitude of the geomagnetic field elements computed from igrf on a 12° ¥ 11° grid centred over italy. the annual means from l’aquila observatory were used to determine the baseline level, imposing a constant observatory anomaly bias. this procedure gives a set of 6 coefficients every 5 years from 1960 to 2005 for the horizontal h, total field f, vertical z and declination d elements of the geomagnetic field. the extrapolation of itgrf to 2005 is based on an autoregressive forecasting of the l’aquila observatory annual means. comparison of the field values computed from the model with those recorded at the other italian observatory (castello tesino) shows that the itgrf improves the fit of the secular variation pattern with respect to the global igrf model by a factor of 3. the itgrf represents a reliable alternative to global models when reducing magnetic surveys to a common reference epoch over the italian region. mailing address: dr. angelo de santis, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma; e-mail: desantisag@ingv.it key words geomagnetic reference fi eld – secular variation – autoregressive forecasting 1. introduction it is well known that regional models are more accurate than global models for describing the main features of the geomagnetic field over a limited region of the earth. global models, like the international geomagnetic reference field (igrf), are developed to represent magnetic features with wavelengths larger than regional dimensions, and they are not able to represent high spatial frequency magnetic features. regional models, on the other hand, are obtained from data measured within the region of interest and they are constrained to represent the behaviour of the field only over this area without considering what is happening outside. they are therefore able to model shorter wavelengths. many countries have developed their own regional magnetic reference model, and several studies have been carried out to represent the behaviour of the geomagnetic field over limited regions, with areas covering from a few squared geographical degrees to continental scales. different methods are used to generate the sets of coefficients that constitute such reference fields (haines, 1990), ranging from simple procedures 492 angelo de santis, luis gaya-piqué, guido dominici, antonio meloni, j. miquel torta and roberta tozzi such as second degree polynomial fitting to more sophisticated techniques like the relatively new method of spherical cap harmonic analysis (scha; haines, 1985) and its derivatives (de santis, 1991a, 1992). in the case of italy, both simple and complex methods have been applied during recent years to create a regional geomagnetic reference field. molina and de santis (1987) proposed a reference model based on a polynomial fitting. this model, valid in its first version for the period 1965-1984, has been called the italian geomagnetic reference field (itgrf). in its last revision (de santis, 1991b), the model was valid up to 1990 with a secular variation model until 1995. the polynomial approximation was chosen because of its simplicity, and a second-degree polynomial was considered adequate to represent the field, following the empirical bullard’s rule (bullard, 1967; see also de santis and meloni, 1991), and taking into account that the mean errors of the approximation were very low with respect to the corresponding igrf model. another model is that created from the measurements taken at the different points of the italian repeat station network. in coticchia et al. (2001), a description of the field measurements and procedures, maps and data for the latest 2000.0 italian survey, can be found. by using the scha technique, de santis et al. (1990) obtained a set of 100 coefficients able to represent the main features of the field with wavelengths down to 330 km. the main disadvantage of this method is the need for a computer with the appropriate software to obtain the values of the field at a given location and epoch, and this is sometimes is not available when carrying out magnetic surveys, where a simple reference model is more useful. in this paper, we present an up-to-date version of the itgrf which is valid for epochs from 1960 to 2000 with a secular variation model valid until 2005. the model, characterised by a set of six coefficients for each element of the field for each five-year interval was produced by taking as the base level the l’aquila observatory annual means and assuming a spatial gradient equal to that given by the igrf. the itgrf’s validity was checked by comparing its generated field values with real data not used during its development, such as the annual means of castello tesino observatory and the values of the italian network of repeat stations. 2. data used and model generation to a latitude-longitude grid of the igrf values for the horizontal h and vertical z components, declination d and total intensity f of the magnetic field for epochs 1960-2005, we fit a second-degree polynomial of the form (2.1) where e is the value of the considered element at latitude f and longitude l, and a0' , a1,..., a5 are the coefficients to be determined by the least squares technique. latitude and longitude, expressed in degrees, are referred to a central point (12°e, 42°n) of the area concerned. the coefficients a1, a2 (in nt/degree) and a3, a4, a5 (in nt/degree 2) determine the spatial gradient for each element, and they are obtained assuming that the variation in the field values is that predicted by the igrf; a0' (nt) represents the base level of the model, and it will be modified to a0 (see below) using the annual mean values for each element at l’aquila observatory (aqu; 13.32°e, 42.38°n), in order to have the same difference between the model and aqu. the spatial gradients of the itgrf model are taken to be the same as those given by the igrf because the latter is a global model based on a global data set and is therefore likely to represent these gradients well, avoiding boundary effects. this is not the case with regard to the absolute values of the field; so, for instance, a comparison of the values predicted by the igrf with those measured at l’aquila shows some clear differences (de santis and molina, 1988). to correct the values at l’aquila for the typical local anomaly of the observatory, we define the coefficient a0 = a0' + e in order to satisfy, for each year, the following equality: (2.2) e a a a a a aφ λ φ λ φ λ φλ,( ) = ′ + + + + +0 1 2 3 2 4 2 5 e eitgrf aqu aqu( ) = ( ) + ε 493 italian geomagnetic reference field (itgrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting i.e. the field value eitgrf(aqu) generated by the itgrf model at the location of l’aquila observatory is equal to the observatory value e(aqu) corrected by an anomaly e. this anomaly is computed as the difference between the values of each element predicted by igrf and those measured at l’aquila for epoch 1979.0, when the global model was partly based on data measured by the magsat satellite and is therefore expected to be one of the most accurate igrfs (peddie, 1982). actually, magsat was in orbit from november 1979 till june 1980, so the chosen epoch was only at the very beginning of the satellite data coverage. however, for the sake of continuity, following molina and de santis (1987), we have retained this epoch as the reference for the anomaly bias determination. the resulting differences between the values computed from the igrf model and l’aquila observatory annual means for 1979.0 were 13.2 nt for f, 32.0 nt for h, – 4.0 nt for z, and – 4.4 arcmin for d. the input data for producing the model are the annual mean values for f, h, z, and d measured at l’aquila observatory from 1960.5 to 2000.5, obtained from the observatory yearbooks. annual mean values from 2001.0 to 2005.0 have been computed by extrapolation assuming an autoregressive secular variation to define the coefficient a0 up to epoch 2005, i.e. we imposed the following condition relating a l’aquila annual mean e t (aqu), taken at time t (in years), with the m previous annual means: (2.3) coefficients k0, k1,..., km have been estimated by least squares. regressions of different order m were made, and m = 4 was finally chosen because the corresponding autoregressive model showed a better root mean square (rms) deviation with respect to the actual data. in the case of epoch 1960.0, we employed the so-called retrodiction, in the sense that we inverted time as we were observing the phenomenon in the opposite direction (i.e. from 2000.5 to 1960.5) and applied (2.3) again. in the same way, a synthetic grid of values from 1960.0 to 2005.0 at five-year intervals constructed from the eighth generation igrf (iaga, 2000) for each element has also been used as input data. this grid extends from 6.5° to 18.5° for east longitudes and between 36.5° and 47.5° for north latitudes at 1° intervals, covering the whole of italy and adjacent areas. the model coefficients are referred to sea level (altitude h = 0). in this sense, the mean values of each element at l’aquila observatory (situated at h = 0.682 km) have been reduced to sea level by applying a correction that takes into account only the dipolar term of the geomagnetic potential (i.e. contributions with degree n equal to 1; e.g., langel, 1987). for any intensive field element with value e at a height h, the correction de for computing its value at h = 0 is expressed as (2.4) where r = 6371.2 km is the earth’s mean radius. in conclusion, the itgrf model provides the values of each e element as follows: (2.5) where k = i – j, and the coefficients are defined as c00 = a0, c10 = a1, c01 = a2, c20 = a3, c02 = a4, and c11 = a5. 3. model analyses and results values of the itgrf coefficients a1,..., a5 computed using the least squares method at five-year intervals with the associated misfit error with respect to the corresponding igrf are shown in table i. the a0 coefficients (table ii) are computed for every year from 1960 to 2005 applying eq. (2.1) and taking into account the local anomaly introduced in eq. (2.2). the value of the field for an intermediate epoch is computed by interpolation between sets of neighbouring epochs. since the itgrf model was developed using independent polynomials for each element, the self-consistency of the model must be checked. for instance, we could check if the total intensity e k k et taqu aqu( ) = + ⋅ ( ) +−0 1 1 k e k et m t maqu aqu+ ⋅ ( ) + + ⋅ ( )− −2 2 ... . ∆e e h r h e h r = ⋅ ⋅ +( ) ≈ ⋅ ⋅3 3 e h t r h r ckj k j j i i φ λ φ λ, , ,( ) = − ⋅ == ∑∑3 00 2 494 angelo de santis, luis gaya-piqué, guido dominici, antonio meloni, j. miquel torta and roberta tozzi m s .4 0 .4 7 .4 8 .5 0 .5 5 .5 7 .6 2 . 64 . 70 . 72 19 60 .0 19 65 .0 19 70 .0 19 75 .0 19 80 .0 19 85 .0 19 90 .0 19 95 .0 20 00 .0 20 05 .0 f ( nt ) a 1 33 3. 55 34 6 33 4. 70 61 5 33 6. 86 76 4 33 9. 74 08 7 33 9. 88 62 7 34 0. 89 51 8 34 0. 22 16 3 33 9. 47 07 2 34 0. 28 56 8 34 1. 85 24 5 a 2 63 .3 24 81 68 .9 66 34 68 .3 14 63 67 .6 52 47 71 .5 98 74 73 .1 28 21 72 .5 17 48 73 .9 74 17 72 .0 04 73 72 .3 94 34 a 3 – 4. 36 02 8 – 4. 23 49 3 – 4. 23 99 7 – 4. 35 81 8 – 4. 49 64 0 – 4. 49 78 3 – 4. 67 21 2 – 4. 74 04 2 4 .9 36 80 – 4. 95 66 0 a 4 2. 06 59 6 1. 91 94 4 1. 87 87 0 1. 66 45 6 1. 48 36 1 1. 34 51 3 1. 10 38 8 .9 60 39 .7 79 14 .7 08 19 a 5 – 1. 27 10 2 – 1. 73 34 1 – 1. 61 64 9 – 1. 76 75 2 – 1. 92 77 3 – 1. 97 27 9 – 2. 01 27 3 – 2. 02 57 4 – 1. 99 56 3 – 1. 91 81 8 m s .9 2 2. 08 1. 94 2. 06 2. 38 2. 52 2. 63 2. 93 2. 67 2. 78 z ( nt ) a 1 73 3. 29 52 1 73 4. 97 78 9 73 9. 42 07 1 74 6. 60 59 0 75 0. 36 99 0 75 2. 55 95 9 75 3. 41 53 1 75 3. 59 83 5 75 5. 74 75 7 75 7. 52 17 2 a 2 59 .6 59 99 68 .6 86 58 69 .9 91 99 72 .1 74 83 80 .1 99 51 84 .9 20 62 86 .6 11 12 89 .9 57 18 89 .6 84 13 91 .9 86 06 a 3 – 14 .9 63 76 – 14 .9 10 31 – 14 .9 84 56 – 15 .2 57 05 – 15 .5 75 27 – 15 .6 18 22 – 15 .8 92 58 – 16 .0 28 37 – 16 .3 12 50 – 16 .3 36 08 a 4 2. 62 26 8 2. 38 15 6 2. 32 09 5 2. 05 60 2 1. 81 55 3 1. 63 43 1 1. 32 17 9 1. 10 57 4 .9 10 92 .7 94 16 a 5 – .6 94 35 – 1. 10 49 9 – 1. 16 05 6 – 1. 46 53 0 – 1. 90 74 0 – 2. 16 63 5 – 2. 40 50 4 – 2. 51 96 7 – 2. 66 32 9 – 2. 74 70 6 m s 4. 94 6. 07 5. 73 5. 79 5. 95 5. 95 5. 96 6. 24 5. 96 5. 98 h ( nt ) a 1 – 53 6. 66 12 7 – 53 6. 15 08 0 – 53 7. 65 15 7 – 54 1. 54 78 3 – 54 6. 99 30 5 – 54 9. 36 36 2 – 55 4. 40 78 0 – 55 7. 77 70 3 – 56 1. 45 26 5 – 56 2. 32 80 9 a 2 23 .8 84 29 20 .0 86 65 16 .9 07 23 12 .3 48 72 7. 20 31 5 2. 66 45 7 – 1. 17 71 2 – 3. 81 45 1 – 7. 09 80 1 – 10 .0 24 43 a 3 .5 91 24 .7 42 96 .7 36 03 .7 11 47 .7 68 95 .7 62 18 .7 87 99 .8 35 67 .8 49 16 .8 44 68 a 4 – .2 83 82 –. 18 24 9 – .1 63 06 – .1 47 44 –. 1 18 69 – .1 03 63 – .0 70 96 – .0 09 03 – .0 41 85 .0 00 69 a 5 –1 .6 80 37 – 2. 17 34 4 – 1. 99 17 6 – 1. 97 53 9 – 1. 89 11 3 – 1. 79 65 2 – 1. 64 57 9 – 1. 63 14 9 – 1. 44 19 1 –1 .2 96 07 m s 5. 89 5. 77 5. 66 6. 00 6. 37 6. 31 6. 47 6. 50 7. 01 6. 90 a 1 .8 19 07 .7 79 59 .5 77 66 .1 67 12 .2 67 50 .3 83 80 .4 04 30 .6 68 02 .9 49 61 1. 34 05 3 a 2 20 .7 10 24 19 .8 13 23 19 .0 92 19 17 .9 64 05 16 .9 32 14 16 .0 33 31 14 .8 44 47 13 .8 74 14 13 .1 10 32 12 .3 99 33 a 3 – .1 06 24 – .1 59 70 –. 1 51 44 – .1 66 66 – .1 62 68 – .1 57 01 – .1 47 64 – .1 42 70 – .1 20 74 – .1 00 02 a 4 – .1 90 87 – .2 10 83 – .2 10 16 – .2 07 76 – .2 30 09 – .2 31 57 – .2 23 55 – .2 23 55 – .1 91 50 – .1 80 82 a 5 .4 84 14 .5 04 36 .5 00 61 .4 86 05 .4 83 20 .4 76 12 .4 66 71 .4 71 49 .4 44 60 .4 45 54 t ab le i. a 1,. .., a 5 co ef fi ci en ts o f th e it al ia n g eo m ag ne ti c r ef er en ce f ie ld f or e ac h fi el d el em en t at fi ve y ea r in te rv al s to ge th er w it h th e as so ci at ed m is fi t e rr or m s (i n nt f or th e f ,z , a nd h el em en ts , a nd a rc m in f or th e d ). f ie ld v al ue s fo r in te rm ed ia te e po ch s ar e ca lc ul at ed b y in te rp ol at io n be tw ee n ne ig hb ou ri ng s et s. d ( ar cm in ) 495 italian geomagnetic reference field (itgrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting year f (nt ) z (nt ) h (nt ) d (arcmin) 1960.0 1961.0 1962.0 1963.0 1964.0 1965.0 1966.0 1967.0 1968.0 1969.0 1970.0 1971.0 1972.0 1973.0 1974.0 1975.0 1976.0 1977.0 1978.0 1979.0 1980.0 1981.0 1982.0 1983.0 1984.0 1985.0 1986.0 1987.0 1988.0 1989.0 1990.0 1991.0 1992.0 1993.0 1994.0 1995.0 1996.0 1997.0 1998.0 1999.0 2000.0 2001.0 2002.0 2003.0 2004.0 2005.0 44866.4 44901.1 44933.1 44960.6 44985.1 45005.2 45023.2 45044.2 45068.7 45094.7 45122.3 45149.8 45178.9 45209.9 45241.5 45274.5 45306.1 45334.1 45360.7 45384.2 45403.7 45423.3 45444.4 45466.0 45487.6 45510.6 45536.5 45563.3 45590.1 45616.4 45641.7 45662.9 45682.2 45701.9 45723.1 45746.9 45772.4 45798.9 45826.0 45854.0 45886.5 45920.4 45953.1 45984.4 46013.4 46039.7 38072.3 38096.7 38118.8 38139.0 38156.1 38166.7 38178.1 38194.9 38213.8 38229.1 38245.0 38260.5 38276.0 38296.0 38319.5 38342.5 38362.8 38381.5 38403.8 38423.5 38435.2 38453.9 38480.6 38503.8 38524.0 38546.7 38574.8 38603.4 38633.6 38668.7 38699.4 38724.6 38745.3 38761.5 38781.7 38803.9 38824.4 38849.4 38881.4 38913.9 38948.4 38983.8 39015.5 39045.2 39074.3 39102.5 23736.5 23761.1 23786.5 23806.5 23825.0 23846.0 23861.9 23873.8 23890.3 23914.7 23941.1 23968.6 23998.6 24024.6 24046.6 24073.1 24099.9 24122.2 24136.9 24149.7 24166.9 24174.3 24172.2 24176.0 24184.4 24190.7 24195.1 24200.5 24202.4 24195.2 24193.1 24192.0 24195.5 24207.9 24215.8 24225.3 24240.4 24250.5 24250.7 24251.3 24257.4 24262.1 24266.0 24271.1 24276.5 24281.8 –127.78 –123.63 –119.29 –114.74 –110.30 –106.45 –102.75 –99.04 –95.64 –92.53 –89.33 –85.70 –82.17 –77.94 – 73.11 –68.08 –62.91 –56.93 –50.46 –43.89 –37.82 –31.69 –24.76 –17.93 –11.80 – 5.87 – .26 5.05 10.15 15.06 19.37 23.61 27.84 32.28 37.61 43.05 48.52 54.29 60.36 65.73 70.50 75.55 80.31 85.26 90.01 94.96 table ii. a0 coefficients of the italian geomagnetic reference field for each field element. 496 angelo de santis, luis gaya-piqué, guido dominici, antonio meloni, j. miquel torta and roberta tozzi f is equal to the square root of the sum of squares of the horizontal and vertical components, that is, if ∆f f h z= − + =2 2 0 ; this is called the geometrical condition. another method is to impose the absence of vertical electric currents in the atmosphere (chapman, 1942), which in terms of the gradients of the horizontal components, has the form . (3.1) this is called the physical condition. figure 1a,b shows both conditions for the itgrf at epoch 2000. as we can see, df values over the italian territory are small, being lower than 8 nt, and lower than 4 nt per degree for the condition (3.1) on the horizontal gradients; so we can conclude that the itgrf is geometrically and physically self-consistent. with the obtained coefficients, the values of the field elements can be computed for a given location in italy and compared with those obtained using the igrf. figure 2 shows the behaviour of the total field intensity at l’aquila from 1960 to 2005 (annual mean values until 2000, values after 2000 are obtained by the autoregressive method) and those predicted by the itgrf and the igrf. it appears that, during the complete period, the itgrf follows the observed f values better than the igrf. quantifying this difference in terms of rms values, we found rmsaquila-itgrf = 13.2 nt (this difference being the same for every epoch because of the way the model has been designed) whereas rmsaquila-igrf = 15.4 nt. this better behaviour of the itgrf was an expected result, because of the determination of the a0 coefficient from the annual values measured at l’aquila. to test the validity of the regional model, it is necessary to compare its values with data not used in the development of the model, for example with the annual means of castello tesino observatory (cts; 11.65°e, 46.05°n). ∂ ∂ − ∂( ) ∂ = x y λ φ φ cos 0 a b fig. 1a,b. a) difference (nt) between total field intensity and the square root of the sum of squares of the horizontal and vertical components at epoch 2000; b) difference (in nt/degree) between the gradients of the horizontal components. the star marks the location of l’aquila observatory. 497 italian geomagnetic reference field (itgrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting figure 3 is analogous to fig. 2 but for castello tesino f values. apparently, in this case, the global model behaves better than the regional one when comparing the two models with the observed values. this is due to the fact that data from castello tesino are directly used in the development of the igrf, whereas they are not for the itgrf. nevertheless, the regional model follows the temporal variation of the different elements better than the igrf does. figure 4a presents the differences between the f annual mean values at castello tesino and the synthetic values obtained from the itgrf and the igrf. while the difference is practically constant with respect to the itgrf, it changes sharply for the igrf because the latter model has constant secular variation over given fiveyear intervals. the itgrf takes into account the changes in secular variation by the inclusion of the a0 coefficient, allowing the regional model to follow accurately the secular variation as illustrated in fig. 4b. this fact is confirmed if we compare the variation of the annual means of the magnetic elements measured at cts with respect to the respective mean values over the interval 1965 to 1995, and the same for the values obtained from the itgrf and the igrf models. for example, if we consider the h values (fig. 5), we obtain an rms of 3.0 nt for the itgrf variations with regard to those of cts, while an rms of 6.4 nt is reached for the igrf. this difference increases when considering the total field intensity, with an rms of 3.7 nt for the itgrf and 12.3 nt for the igrf. we can compare the values obtained from the itgrf with those computed from the ingrf2000 model, a reference field created from the measurements taken at the different locations of the italian repeat stations network (coticchia et al., 2001). this model has the same form as a second-degree polynomial in latitude and longitude, valid at sea level for the whole italian territory except marine areas. differences between a 0.5° ¥ 0.5° grid of values generated from the ingrf2000 and grids with same locations created using the itgrf and the igrf are similar, and in the case of f for epoch 2000 they are less than 20 nt over more than 90% of the italian territory (fig. 6). once the self-consistency and validity of the itgrf have been demonstrated, and it has been shown that the itgrf secular variation follows the values measured at l’aquila and castello fig. 2. annual mean values of f measured at l’aquila (circles) together with the values predicted by the itgrf (solid line) and the igrf (dashed line). fig. 3. annual mean values of f measured at castello tesino (circles) together with the values predicted by the itgrf (solid line) and the igrf (dashed line). 498 angelo de santis, luis gaya-piqué, guido dominici, antonio meloni, j. miquel torta and roberta tozzi tesino observatories better than the global igrf model, we can create maps for italy for the different elements of the geomagnetic field at a given epoch. figure 7a-d presents maps for the f, z, h and d elements for epoch 2005. fig. 4a,b. a) difference between annual mean values for the f component at castello tesino and those obtained from the itgrf (solid line) and igrf (dashed line) models, respectively; b) behaviour of the secular variation for the f component at castello tesino (circles), for the igrf (dashed line) and the itgrf (solid line) models. a b fig. 5. variation of the horizontal field component with respect to its mean value over the interval 1965 to 1995 for castello tesino (circles), the itgrf (solid line) and the igrf (dashed line). fig. 6. differences between the ingrf2000 and the itgrf for f at epoch 2000. 499 italian geomagnetic reference field (itgrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting fig. 7a-d. maps for: a) f; b) z; c) h, and d) d for epoch 2005. units are nanotesla for f, z and h, and arcmin for d. a b c d 500 angelo de santis, luis gaya-piqué, guido dominici, antonio meloni, j. miquel torta and roberta tozzi 4. conclusions in this paper we have presented the so-called itgrf2000, an up-to-date version of the italian geomagnetic reference field model that allows the determination of the different elements of the geomagnetic field over italy from 1960 to 2005. the model is a second-degree polynomial in longitude and latitude and it consists of sets of six coefficients for each magnetic element at five-year intervals. the design of the model, based on the spatial pattern of the igrf global model but with a base level computed from annual mean values measured at l’aquila observatory, makes it an accurate model for representing the secular variation of the field element values in italy. this fact, together with its simplicity, makes the itgrf a useful model for reducing magnetic surveys carried out at different epochs over the italian territory, including future surveys, up to 2005, to a common epoch. the intention of the geomagnetism unit of the istituto nazionale di geofisica e vulcanologia, the body responsible for geomagnetic field monitoring in italy, is to revise the model at five-year intervals and to allow access to the itgrf2000 through the homepage of the institute (http:// www.ingv.it) for the computation of field values at any location and for a given epoch. references bullard, c.e. (1967): the removal of trend from magnetic surveys, earth planet. sci. lett., 2, 293-300. chapman, s. (1942): notes on isomagnetic charts, vi, terr. magn. atmos. electr., 47, 1-13. coticchia a., a. de santis, a. di ponzio, g. dominici, a. meloni, m. pierozzi and m. sperti (2001): italian magnetic network and geomagnetic field maps of italy at year 2000.0, boll. geod. sci. affi ni, 4, 261-291. de santis, a. (1991a): translated origin spherical cap harmonic analysis (tosca), geophys. j. int., 106, 253-263. de santis, a. (1991b): italian geomagnetic reference field (itgrf): aggiornamento al 1990 e modello di variazione secolare fino al 1995, in atti del 10° convegno annuale del gruppo nazionale di geofi sica della terra solida, roma, 679-681. de santis, a. (1992): conventional spherical harmonic analysis for regional modelling of the geomagnetic field, geophys. res. lett., 19, 1065-1067. de santis, a. and a. meloni (1991): sulla scelta del campo geomagnetico di riferimento in italia, in atti del 2° convegno di geomagnetismo e aeronomia, ing 6, 283-300. de santis, a. and f. molina (1988): italian geomagnetic reference field (itgrf): aggiornamento al 1987 e modello di variazione secolare fino al 1990, in atti del 7° convegno annuale del gruppo nazionale di geofisica della terra solida, roma, 1227-1236. de santis, a., o. battelli and d.j. kerridge (1990): spherical cap harmonic analysis applied to regional field modelling for italy, j. geomagn. geoelectr., 42, 1019-1036. haines, g.v. (1985): spherical cap harmonic analysis, j. geophys. res., 90, 2583-2592. haines, g.v. (1990): regional magnetic field modeling: a review, j. geomagn. geoelectr., 42, 1001-1018. iaga division v, working group 8 (2000): international geomagnetic reference field 2000, geophys. j. int., 141, 259-262. langel, r.a. (1987): the main field, in geomagnetism, edited by j.a. jacobs (academic press, london), vol. 1, 249-512. molina, f. and a. de santis (1987): considerations and proposal for a best utilisation of igrf over areas including a geomagnetic observatory, phys. earth planet. inter., 48, 379-385. peddie, n.w. (1982): international geomagnetic reference field: the third generation, j. geomagn. geoelectr., 34, 309-326. annals 47, 6, 2004def 1713 annals of geophysics, vol. 47, n. 6, december 2004 key words nonlinear processes in geophysics – geomagnetic field – earth’s core – fluid dynamics – turbulence 1. introduction the geomagnetic field is sustained within earth’s outer core by dynamo action, which involves vigorous convective motions. it is widely thought that these fluid motions are driven by the archimedean (buoyancy) force and are strongly affected by both the coriolis and lorentz forces. moreover, it is commonly assumed that these motions are also fully turbulent. direct observations of the fluid motions are unfortunately impossible and consequently the picture of the core turbulence is usually painted by theoretical reasoning. actually, our understanding of the effect of turbulence in the core and of its influence on the diffusive processes is rather limited. however, the study of this topic is essential because it could open new ways both to improve the reliability of geodynamo simulations and for a better understanding of all those phenomena which are connected with the dynamics of the fluid core motions. turbulent flows are generally characterized by «universal» scaling features in the so-called inertial range where fluctuations reach a stationary state due to the continuous flow of energy from large to small scales, or vice versa. this energy flow can be viewed in real space as an energy cascading process generated by the breakdown of eddies at different scale lengths or as a coalescence process in the case of an inverse energy cascade. although energy cascading is also expected to occur in many three-dimensional magnetohydrodynamic (mhd) systems, it is not clear if such a cascading process can occur in the earth’s core. stevenson (1969) and braginsky and meytlis (1990) have considered the problem of the turbulence in the core, and attempted to qualitatively estimate the effects of turbulence. in particular, braginsky and meytlis (1990) suggested that the buoyancy instability, locally supercritical, should strongly affect the developtime intermittency and spectral features of the geomagnetic field paola de michelis (1) and giuseppe consolini (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto di fisica dello spazio interplanetario cnr, roma, italy abstract in the field of geomagnetism a number of studies have been devoted to the investigation of turbulence and intermittency in the outer core fluid motions. here, in order to obtain information on such phenomena we study the time spectral and self-similarity features of the main geomagnetic field fluctuations as measured on the earth’s surface. the existence of a power law spectrum, characterised by an exponent α ≈ −11/3, and an anomalous scaling of q-th order structure functions on time scales longer than 5 years, suggests the occurrence of intermittent turbulence rather than classical kolmogorov turbulence in the fluid core motions. these results are briefly discussed in connection with the existence of a strong magnetic field and drift-wave turbulence. mailing address: dr. paola de michelis, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: demichelis@ingv.it 1714 paola de michelis and giuseppe consolini ment of small-scale turbulence inhibiting a significant cascading of energy from the dominant and larger cells to the smaller ones. thus, because of extreme conditions in the terrestrial core the core turbulence could be totally unlike classical kolmogorov turbulence. braginsky and meytlis (1990) have also suggested that this small-scale turbulence should be highly anisotropic because of the strong influence of the earth’s rotation and magnetic field. they conclude their paper by suggesting that the study of data relative to the geomagnetic secular variation may be highly relevant to understand the dynamo processes. as a matter of fact, the characteristic time interval of the local turbulence lies in the range from 10 to 100 years (of the order of decades), and thus data about the decade geomagnetic secular variations may be relevant. here, taking this suggestion into account we attempt to investigate the nature of the earth’s fluid core turbulent motions using ground-based observations of the geomagnetic field. we would like to remark that our observational approach could be very useful in setting constraints on numerical models of the geodynamo. this paper is structured as follows. the mathematical details are largely confined to section 2, in which we review some of the geomagnetic field equations and of the theory of turbulence and intermittency, while in section 3 and 4 we present our analysis and conclusions, respectively. 2. analytical approach 2.1. the geomagnetic field equations the equation, which couples the main geomagnetic field and its secular variation to the fluid core motions, is the magnetic induction equation bb bt 2 #2 d d= + h#v] g (2.1) where v is the velocity of the fluid, η = (µσ)−1 is the magnetic diffusivity (with µ = 4π × 10 −7 hm−1 the magnetic permeability and σ = 3×105 s/m the electrical conductivity of the core fluid assumed constant) and b is the magnetic induction. the magnetic induction equation shows that the time rate of the change of the magnetic field (the secular variation) depends upon both magnetoadvection ∇× ( v× b) and diffusion η∇ 2 b. if we associate time scales with these two different processes we find that, for the earth’s core, the typical time scale of advection is of the order of 60 years, while the time scale of diffusion is about 3⋅104 years (see bloxham and jackson, 1991). this difference between the time scales suggests omission of the diffusion term, at least over time scales short compared with the time scale of diffusion, and consideration of the reduced equation (roberts and scott, 1965) dbb vt # # $2 d= =v b] ]g g . (2.2) this hypothesis, which is called the «frozenflux» hypothesis, is valid for time scales t in the range 10 < t <100 years (backus et al., 1996). for longer time scales the diffusion of the magnetic field throughout the fluid core is instead observable even if, as bloxham et al. (1989) showed, the frozen flux condition is still valid and gives a good representation of core motions on a broad scale. if we assume that the geomagnetic field (b) and the fluid velocity (v) can be decomposed according to their large-scale (macroscale) and small-scale (microscale) parts the former being the average over turbulence (dented by the subscript 0) and the latter being the small-scale fluctuating part b b b v v v 0 0 = + = + . (2.3) from the induction equation we obtain the equation for the turbulent variables (see braginsky and meytlis, 1990) (2.4)d b b v l m b v v b v b t 0 0 0 b b b b $ $ $ # # d d d = + + v bl = .m #d= ^ ] ] ^ h g g h (2.5) (2.6) here dt = ∂t + v ⋅ ∇ . the term lb is small because it is proportional to the gradients of the basic (averaged) quantities which change over a characteristic length that is much greater than the characteristic length over which the turbulent variables change. consequently, the term lb can be neglected. it is also necessary to stress that, 1715 time intermittency and spectral features of the geomagnetic field though the role of the local turbulence in some internal processes is crucial, its influence on ohmic diffusion of the basic field and on the αeffect is negligible. in particular, braginsky and meytlis (1990) have shown that both the coefficient of the α-effect and the β-effect are small in the local turbulence and this makes both these nonlinear effects negligible. as a matter of fact, the α-effect is most active at intermediate to large scales, and not at small scales, as classically assumed. this means that, in first approximation, the term mb can be neglected too. transforming to a moving coordinate system, we can set v0 = 0 and consequently, eq. (2.4) becomes db v v bt 0 0$ # #2 d= =b^ ^h h. (2.7) in terms of fourier components, eq. (2.7) reduces approximately to i ib k v b, ,k k 0##.~ ~ ~^ h (2.8) where ω is the angular frequency and k is the wave number. therefore, in scalar form we obtain b kb v, ,k k0.~ ~ ~ . (2.9) expression (2.9) relates the temporal spectral features of the magnetic field to the spatial spectral features of the velocity fields in the limits imposed by the frozen-field hypothesis. 2.2. a brief overview of turbulence the fundamental dynamical equation, describing the emergence of turbulence, is the navier-stokes equation, and the parameter, which describes the physical state of the fluid, is the so-called reynolds number r = lv/ν, where l, v and ν are a characteristic length scale, the velocity of the flow and the kinematic viscosity, respectively. this number is a nondimensional measure of the nonlinearity effects in the navier-stokes equation. if the reynolds number is not too large (r < 1), the fluid flow is laminar in the sense that it displays regular and predictable variations in both space and time. when the reynolds number increases (r > 1), the fluid flow undergoes a sequence of instabilities until, at some large enough value of the reynolds number (r >> 1), it becomes fully turbulent and the various symmetries permitted by the navier-stokes equations are broken. kolmogorov’s theory (k41) (kolmogorov, 1962; frisch, 1995) gives a description of the main mechanism acting in an incompressible fluid at high reynolds number. according to richardson’s scenario, in a turbulent flow there is a cascade transfer of energy toward small scales where dissipation is due to molecular friction. the energy cascade is hierarchical, i.e. a disturbance on a certain scale receives its energy from a larger-scale disturbance and transfers it to smaller-scale disturbances. the presence of a range of length scales, where inertial forces are dominant and where viscous effects as well as the external forcing can be neglected, suggests the existence of universal scaling laws. as shown by kolmogorov (1962), the main feature of a turbulent fluid flow is the existence of a statistical relation between the velocity differences δνr = |v(x + r)−v(x)| and the energy transfer rate εr , at the scale r = | r | r / r r 1 3 .do f̂ h (2.10) which is generally known as kolmogorov’s refined similarity hypothesis. in terms of the energy spectrum (e) in the inertial range this corresponds to the even more famous scaling law ( )e k k /5 3 " (k being the wave-number) (2.11) the same picture can be extended to magnetohydrodynamic (mhd) turbulence, though the analogous statistical relations between the velocity differences and the energy transfer rate must take into account the alfvén decorrelation effect (kraichnan, 1965). as a consequence of the alfvén decorrelation effect kolmogorov’s refined similarity hypothesis has to be replaced by c r / / r a r 1 4 1 4 .do f^ h (2.12) where ca = b/(4πρ)1/2 is the alfvén velocity (in a medium of density ρ and average magnetic field b) associated with the largest scale. owing to 1716 paola de michelis and giuseppe consolini this revised refined hypothesis in the case of mhd turbulence (kraichnan, 1965), the famous −5/3 energy spectrum reduces to ( )e k k /3 2 " . (2.13) the aforementioned description of fluid and/or mhd turbulence is valid in the case of non-rotating systems and in the absence of a strong applied magnetic field. in fact, recent experimental results and numerical simulations have clearly shown that an intense magnetic field and/or rotation can strongly affect the spectral features of turbulent fluctuations in the inertial regime. some recent experiments (see branover et al., 1999) revealed that an increasing external magnetic field may change the velocity spectral features. for very intense magnetic fields, when the ratio between the hartman number ( /ha bl= v to , where b is the intensity of the magnetic field, l is a characteristic length, σ is the conductivity, ρ is the fluid density, and ν is the fluid kinematic viscosity) and the reynolds number r exceeds some critical thresholds (θi), the velocity spectrum reduces to ( )e k k k k / < / < / / > / ha r ha r 5 3 7 3 11 3 1 2 2 " i i i (2.14) and turbulence becomes anisotropic and strongly intermittent. a steeper energy spectrum is a consequence of the fact that the existence of an intense magnetic field can inhibit the formation of turbulent structures at small scales, thus reducing the energy transfer towards smaller scales. a different framework for the development of turbulent regimes is due to drift-waves (generally known as rossby waves) in quasi-geostrophic flows. in such a case the equation governing the evolution of flows on a rotating sphere is the well-known charney equation (del-castillo-negrete, 2000) qz 0t r 2 2 # $2 d d+ =} q k yd= +} } b _ i8 b (2.15) where z is the radial outward unit vector on the sphere, ψ the streamfunction, kr the rossby deformation wave number, and β the coriolis force in the quasi-geostrophic β-plane approximation. equation (2.15) is formally equivalent to the hasegawa-mima equation, which is an important paradigm for the description of driftwave turbulence in magnetically confined plasmas. recent studies (ottaviani and krommes, 1992; kukharkin et al., 1995) of the drivendamped hasegawa-mima equation revealed the existence of different scaling regions with respect to a characteristic spatial scale l in the case of a strong-turbulent regime. in more detail, for an inverse cascade process one obtains < l > l ( ) < > e k k k k k / / 11 3 5 3 & & " ) (2.16) while for a direct cascade the hasegawa-mima equation leads to < l > l ( ) < > e k k k k k 5 3 & & " ) . (2.17) again different scaling features are obtained depending on the kind of turbulence and on the different dynamical situations. 2.3. a brief overview of intermittency an exact definition of intermittency is somewhat difficult to provide mainly because the term assumes different meanings in different physical frameworks. however, space-time intermittency seems to be a common feature of fluctuations, characterizing most «weakly confined» or «extended» systems in nature, for which the fluctuations are not uniformly distributed in space and/or time. for example, in the case of mhd or fluid turbulence, the term intermittency is generally used to indicate a non-uniform distribution of energy in space at a given length scale, and is associated with a multifractal structure (i.e. a local dependence of the self-similarity features) of the energy cascade in the inertial range. the approach to intermittency has been applied for a long time. although some possible mechanisms have been proposed to explain intermittency in many physical frameworks, it seems that a comprehensive knowledge of this universal phenomenon has not been completely reached, requiring 1717 time intermittency and spectral features of the geomagnetic field a detailed understanding of coupling among micro-, mesoand macroscale fluctuations. in turbulent fluid flows intermittency is usually investigated by looking at the self-similarity features of measurable quantities such as the q-th order structure velocity functions sr(q), defined as the spatial averages of δνrq = | v ( x +r)−v(x)|q. in detail, one can define a hierarchy of scaling exponents ξq through the following scaling relation: s rv ( ) r q r q .= pqd (2.18) where, in the absence of intermittency effects, the scaling exponents ξq are expected to scale linearly as a function of the moment order q. abundant experimental and numerical evidence (frisch, 1995) suggests that the exponents ξq are different from their classical values, and do not scale linearly as a function of moment order q. these departures are the signature of intermittency, i.e. of a non-homogeneous fragmentation of eddies, which leads to a non-homogeneous distribution of energy at the smaller scales. in the case of «standard» fully developed fluid turbulence, starting from kolmogorov’s «refined similarity hypothesis» (kolmogorov, 1962) intermittency effects can be included through a renormalization of the scaling exponents ξq as follows: m q q q= +p x (2.19) where m = 3 or 4 for fluid or mhd turbulence, respectively. several models have been proposed to describe intermittency by deriving an expression for ξq (benzi et al., 1984; meneveau and sreenivasan, 1987; carbone, 1993, 1994; grauer et al., 1994; she and leveque, 1994; politano and pouquet, 1995). taking into account the effects of intermittency, each of these models generally predicts τq > 0 for q/m < 1, and τq < 0 for q/m > 1; i.e. ξ q is a convex function of q. 3. data set and analysis in order to study the nature and/or the occurrence of intermittency and turbulence in the geomagnetic time series and obtain information on turbulence in the fluid core, ground magnetic observatory data were used. in particular, in this paper the annual mean values of the three geomagnetic field components (x, y and z) relative to 18 observatories, mostly in the northern hemisphere, formed the data set. the observatories were selected in order to have more than 94 years of annual mean values available. data come from both the national geophysical data center, boulder (co), and the «summary of annual means at world magnetic observatories» (golovkov et al., 1983). figure 1 shows the disfig. 1. world-map of the location of the geomagnetic observatories used in this work. 1718 paola de michelis and giuseppe consolini tribution of the observatories used in this work. since some of the observatories were re-located during the time interval considered, when this happened, data were corrected shifting their absolute level appropriately adjusting the annual means to the level appropriate to the latest site. data gaps were linearly interpolated only when the number of missing points is equal to one, otherwise the time series was rejected. first of all, the spectral features of the geomagnetic field have been investigated to keep information on the dynamical state of the fluid core motions. in order to do this, from eq. (2.9) we obtained the following relation for the energy spectra: ( ) ( )e k b e kb v 2 0 2 =~ ~a k (3.1) where eb(ω) and ev(k) are the power spectral densities (psd) for magnetic and velocity fluctuations, respectively. figure 2 shows the average psd s( f ) defined as follows: ,y z s f b f , ji x 2 = d =i ! j l k k^ ^ n p o oh h ! + (3.2) where δbji 2 ( f ) is the squared fourier amplitude of the i-th component at the j-th observatory, and ƒ= ω /2π. the average s( f ) was evaluated after normalizing the result of each observatory at the lowest significant frequency f *=0.03 yr−1. due to the existence of long-term trends in the time series considered, we used a raised cosine (hanning) window before performing the spectral analysis. frequencies below f * were not considered since they are affected by the windowing procedure. the psd is characterized by the existence of two different scaling regimes respectively at frequencies below and above a certain crossover frequency fc. in detail, for frequencies below fc ≈ 0.2 yr−1 the power spectrum follows a power-law with a spectral exponent α close to −11/3 (α = − 3.73 ± 0.03) while above the crossover frequency fc the psd seems to agree with a power-law, characterized by a spectral exponent α close to − 5/3. we remark that our psd, on time scales longer than 10 years, agrees with a previous analysis by filloux (1980). the composite spectrum by filloux (1980) has, indeed, a 1/f 4 dependence in fig. 2. the average psd of the geomagnetic field fluctuations evaluated as described in the text. the solid and dashed lines refer to non-linear best fit using a power law. spectral slopes are approximately –11/3 and –5/3 for frequencies lower or higher than the crossover frequency ƒc ≈ 0.2 yr−1. the vertical dotted line denotes to the crossover frequency. 1719 time intermittency and spectral features of the geomagnetic field the range of time scales from ∼1 to ∼100 years tapering to a less steep spectrum on the lowfrequency side (time scales >100 years). actually, the spatial and temporal spectral features of the geomagnetic field have been widely investigated in the past and an interesting schematic power spectrum of the geomagnetic field combining data from filloux (1980), barton (1982, 1983) and the suggestion of f. lowes (in barton 1982) at the longest period is reported in the paper by courtillot and le mouël, (1988). this schematic power spectrum begins in the range of observatory records with a 1/f 4 slope and tapers to a 1/f for the longest paleosecular variation records of lake sediments and marine cores while the central portion is more like 1/f 2. however, the aim of this work is to try to understand the origin of such a spectrum in connection with spatial features of the fluid-core velocity pattern. because of the great difference between the diffusion characteristic time and the advection time we may assume that any local fluctuation of the velocity field can be considered as frozen-in in the range of time scales here considered (∼ 1-100 years). thus, these local fluctuations are essentially advected with a constant velocity without being dispersed. in terms of spatial (λ) and time (τ) scales this is equivalent to assuming a linear relationship between λ and τ, i.e. a fluctuation of scale λ f is advected in a time τf =λ f / vad. this is equivalent to assuming a linear relationship between k and ω. we emphasise that such a hypothesis of inferring spatial scales from temporal scales is generally assumed in fluid turbulence and is referred to as taylor’s hypothesis (frish, 1995). thus, on the basis of eq. (3.1) we recover for the velocity field spectral features ( )e k k / v 11 3 " . (3.3) such a steep spectral slope (α = − 11/3) suggests that small-scale fluctuations are strongly inhibited and that long-range order as well as coherent structures could be present in the velocity pattern. the existence of a steep spectrum could also involve the occurrence of intermittency in the fluid motions. again, assuming the frozenflux hypothesis to be valid, this intermittency should be evident in the geomagnetic field time fluctuations; i.e. geomagnetic field fluctuations should be characterized by time-intermittency. to investigate this behaviour and to show more clearly the existence of this intermittency, we analysed the geomagnetic field structure functions for different values of the order q defined as follows: ( )b t q ( )s ( )q ji ji= -x ( )b t + x (3.4) where bji is the i-th component of the j-th observatory, τ is a time lag, and q covers in the interval [0, 4] with a step of 0.2. figure 3 displays the behaviour of the q-th order structure functions s(q) versus t for a limited set of q in the interval [1, 4]. we limited our investigation up to the 4th order because of the paucity of the available statistics, the total amount of data being about 6·103 points. these structure functions have been obtained by averaging over the three magnetic field components and all selected observatories. figure 3 shows that for each moment order a range of time scales can be found where the structure function can be fitted using a power law; i.e. s ( )q q"xp (3.5) where ξ q is the scaling exponent at the moment order q. this range generally extends over time scales longer than τ ≈ 5 years up to τ ≈ 50 years. figure 4 shows the scaling exponents ξ q versus the moment order q. the scaling exponents seem to deviate from a linear behaviour meaning that the slope varies nonlinearly with increasing q. this result is consistent with a multifractal nature of the geomagnetic field fluctuations and therefore with the occurrence of intermittency, confirming previous results by the authors (de michelis et al., 1998) and by vörös and gianibelli (1998). experimental measurements and direct numerical simulations of three-dimensional turbulent flows have, in fact, clearly shown how the self-similar energy cascade occurs through the repeated fragmentation of the vorticity field and how this process generates elongated spatial structures (vorticity tubes) which form a multifractal set where dis1720 paola de michelis and giuseppe consolini fig. 3. the behaviour of the structure function s q(τ) of the magnetic field fluctuations as a function of τ for a selected number of moment orders. note the nearly linear trend in the double logarithmic plot meaning that geomagnetic field fluctuations are self-similar. sipation eventually takes place. this contrasts with the k41 theory of homogeneous turbulence where a self-similar energy cascade was considered as a homogeneous not intermittent phenomenon. in the case of standard fully developed fluid turbulence, intermittency effects yield ξ q values slightly greater than q/m. in the last two decades, many different models have fig. 4. the structure function scaling exponents ξq versus the moment order q. the solid black line refers to a nonlinear best fit using the p-model. the resulting fragmentation parameter is approximately p ≈ 0.6. the dashed line refers to a homogeneous fractal signal. 1721 time intermittency and spectral features of the geomagnetic field been proposed to describe intermittency by deriving an expression for ξ q. among these models that have been generally built up in the framework of hydrodynamic turbulence and successively extended to mhd turbulence, we note the p-model (meneveau and sreenivasan, 1987). the p-model, which is based on a twoscale cantor set with equal partition intervals but unequal weights, conjectures that in the cascade process two «daughter» eddies are randomly generated from a «mother» eddy with a probability p and (1− p) (where 0 ≤ p ≤ 1) predicting ( )log p p1 1 / / q q m q m 2= + -p 6 @. (3.6) we have used this model to take into account the intermittency effects and a very good agreement is found between the behaviour of the scaling exponents ξ q and the prediction of the p-model with a fragmentation ratio p = (0.61 ± ± 0.01). 4. summary and conclusions in the field of geomagnetism, many studies are devoted to understanding the nature of the physical features of the motions inside the earth’s core as well as to their relation with the maintenance of the main field. although considerable progress has been made, much remains to be understood. here, as a contribution to the knowledge of the earth’s core processes, we have investigated the spectral and self-similarity features of the geomagnetic field recorded at 18 permanent observatories. we related the magnetic field spectrum to the core fluid velocity spectrum via the frozen flux approximation and taylor hypothesis. our findings suggest that above a certain critical scale λ c = kc−1 (related to the characteristic frequency fc ≈ 0.2 yr−1) the spectrum drops off faster than one might expect for a kolmogorov fluid and/or kraichnan mhd turbulence. this result suggests that small-scale fluctuations are strongly inhibited and supports braginsky and meytlis’ (1990) theoretical point of view of a non-traditional turbulence. moreover, evidence of intermittency in the dynamics of the geomagnetic field has been found by analyzing the average scaling properties of the geomagnetic field fluctuations. in particular, the existence of an anomalous scaling of the structure function scaling exponents ξ q supports the occurrence of intermittency in the fluid core motions. a possible origin of the steeper spectrum and of the observed time-intermittency could involve the presence of an intense core magnetic field, which would cause a rearrangement of the topological structure of the turbulent flow, modifying the velocity field spectral features. as already mentioned (branover et al., 1999), there is experimental evidence that under the effect of an increasing external magnetic field the velocity spectrum evolves from kolmogorov − 5/3 exponent towards a steeper one in intense magnetic fields, and that the fluid motions should be strongly intermittent. in other words, the observed spectral slope could be consistent with an intermittent turbulent fluid dynamics in the outer core. a different framework that could take account of the k− 11/3 spectrum, and that could also clarify the origin of the spectral crossover, might be the formation of coherent vortical structures in the fluid core in a drift-wave turbulent regime. in the outer fluid core, the smallscale fluctuations are restricted to the plane perpendicular to the magnetic field and that makes the turbulence essentially two-dimensional. if we take into account that the turbulence regime in the earth’s core could also be the result of drift-waves (rossby waves), we may describe the problem of the turbulence dynamics of the fluid motions in the earth’s core using the hasegawa-mima (hm) equation. in the case of an inverse cascade this equation, which has a structure identical to that of the charney equation, predicts two asymptotic inertial regimes for small or large wave numbers k with respect to a characteristic scale l = 1/kc (see eq. (2.16)). our results on the spectral features of the geomagnetic field show the existence of these two scaling regimes well in agreement with the solutions of hasegawa-mima equation. thus we can expect that while the steep energy spectrum impedes the formation of small-scale coherent vortices simultaneously, as a consequence of 1722 paola de michelis and giuseppe consolini the piling up of the energy at larger scales, there would be a promoting of the formation of longlived coherent vortices at large-scales. we should, however, emphasise that the possible origin of the high frequency –5/3 spectrum could be different. for time scales shorter than 10 years the evaluation of the spectral features of the geomagnetic fluctuations is affected by two fundamental problems. first of all, even though we have used the magnetic field annual means the contamination due to external field sources remains as a major source of noise in this temporal range. second, we must take into account that any magnetic signal with a period less than approximately 4 years, generated inside the core and crossing the core-mantle boundary, is effectively cut off from observation by the conducting lower mantle. in summary, we have shown that it may be possible to obtain information on the outer core fluid motions using ground-based geomagnetic field measurements. clearly, more work is necessary in order to clarify the proposed dynamical models, this will be the subject of future studies. references backus, g., parker, r. and constable, c. 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(received october 14, 2003; accepted march 7, 2004) 401_413 pappalardo.pdf 401 annals of geophysics, vol. 46, n. 2, april 2003 one year of tropospheric lidar measurements of aerosol extinction and backscatter gelsomina pappalardo (1), aldo amodeo (1), salvatore amoruso (2)(3), lucia mona (1), marco pandolfi (2) and vincenzo cuomo (1)(2) (1) istituto di metodologie per l’analisi ambientale, (imaa), cnr, tito scalo (pz), italy (2) dipartimento di ingegneria e fisica dell’ambiente (difa), università della basilicata, potenza, italy (3) istituto nazionale per la fisica della materia (infm), unità di napoli, italy abstract the aerosol lidar system operational at imaa-cnr in tito scalo (pz) (southern italy, 40°36¢n, 15°44¢e, 820 m above sea level) is part of the earlinet project. systematic lidar measurements of aerosol backscatter and extinction in the troposphere have been performed since may 2000. aerosol backscatter measurements were performed at both 355 nm and 532 nm, while aerosol extinction coeffi cient were retrieved from simultaneous n2 raman backscatter signals at 386.6 nm. the observations were performed on a regular schedule of two night time measurements per week (around sunset) and one daytime measurement per week (around 13:00 utc). furthermore, special observations concerning saharan dust outbreaks have been carried out. starting in may 2000 the lidar measurements performed in tito scalo have been collected and analysed. preliminary results regarding the fi rst year of measurements are reported. in particular, the evolution of the aerosol integrated backscatter and extinction as well as of the mean value of the lidar ratio in the whole aerosol layer is reported. results show clear evidence of seasonal variation of the observed parameters, with higher values and greater variability during summertime. mailing address: dr. gelsomina pappalardo, istituto di metodologie per l’analisi ambientale (imaa), cnr area della ricerca di potenza, contrada s. loja, 85050 tito scalo (pz), italy; e-mail: pappalardo@imaa.cnr.it key words lidar aerosol earlinet 1. introduction atmospheric aerosols, originated from both natural and anthropogenic sources, play a crucial role in changes of the global and regional climate as well as environmental pollution. atmospheric aerosols considerably affect the earth’s radiation balance by both scattering and absorbing incoming and outcoming radiation (ackerman and chung, 1992). at present, the impact of the atmospheric aerosols on climate is not a wellknown process because it involves both direct and indirect feedback mechanisms (houghton et al., 2001); this claims for a more accurate characterisation of aerosol optical properties and spatial distribution. moreover, continuous monitoring of atmospheric aerosols is essential in view of their wide variability in space and time. lidar technology is now mature enough to provide quantitative measurements of the optical properties of the aerosols with high spatial and temporal resolution and with a high level of accuracy. in particular, the use of raman lidar allows independent measurements of the aerosol extinction and backscatter as a function of height (ansmann et al., 1990). the lidar system operational at imaacnr in tito scalo (pz) (southern italy, 40°36¢n, 15°44¢e, 820 m above sea level) 402 gelsomina pappalardo, aldo amodeo, salvatore amoruso, lucia mona, marco pandolfi and vincenzo cuomo provides independent measurements of the aerosol extinction and backscatter at 355 nm and backscatter measurements at 532 nm (pappalardo et al., 2001). this system is part of the earlinet project: the fi rst network of advanced lidar stations operating to provide a climatological database for the horizontal and vertical distribution and variability of aerosol over europe (boesemberg et al., 2001). this lidar network is based on a total of 21 stations, as shown in fi g. 1, located to provide signifi cant coverage of the widely varying atmospheric conditions over europe. table i reports the list of all institutions participating in the earlinet project. the observations, almost simultaneous for each station, are performed on a regular schedule of two night time measurements per week and one daytime measurement per week. daytime measurements are made around 13:00 utc, while night time measurements are made around sunset; these times were selected to have one daytime measurement with a well developed boundary layer condition, and two observations with low background light to perform raman extinction measurements and to monitor the dust layer that has developed during the day. in addition to the routine measurements, further observations are devoted monitor special events such as saharan dust outbreaks, forest fires, photochemical smog and volcano eruptions. lidar fig. 1. network of earlinet stations. 403 one year of tropospheric lidar measurements of aerosol extinction and backscatter measurements are supported by a suite of more conventional observations. back-trajectories derived from the operational weather prediction model of the german weather service are used to characterise the history of the observed air parcels. much effort has been devoted to assure data quality for all european stations, and for this purpose intercomparison experiments at system level as well as at algorithm level have been performed. the experimental intercomparison was made using transportable lidar systems. these were compared with each other in a fi rst step, and then used for comparison with the fi xed systems (matthias et al., 2001). the algorithm intercomparison was performed for both backscatter and extinction retrieval by using simulated elastic and raman lidar signals (boeckmann et al., 2001). the earlinet project started on february 2000 with a duration of 3 years but the intention is to continue on a longer time scale. regular measurements started on may 2000 and this is by far the largest data set on the vertical distribution of aerosol worldwide. in addition to the routine measurements, extended observations of saharan dust episodes, mainly in the mediterranean area, have been performed. the coordination of the special lidar measurements is performed by the lidar group of the national technical university of athens, using forecasted saharan dust events data available on the world wide web (http:// forecast.uoa.gr). the use of all these data will contribute significantly to the quantification of aerosol concentrations, radiative properties, long-range transport and budget, and prediction of future trends. it will also contribute to improve model treatment on a wide range of scales and to a better exploitation of present and future data from satellite remote sensing for a variety of parameters. max plank institut für meteorologie (d) universität potsdam (d) defence research establishment (se) university of wales aberystwyth (uk) centre national de la recherche scientifi que délégation régional île-de-france (fr) institut für troposphärenforschung (d) leibniz institut für atmosphärenphysik an der universität rostock (d) national technical university of athens (el) aristotle university of thessaloniki (el) universitat poliytècnica de catalunya (e) istituto nazionale per la fisica della materia, napoli (i) istituto nazionale per la fisica della materia, lecce (i) imaa-cnr and istituto nazionale per la fisica della materia, potenza (i) università degli studi l’aquila (i) ecole polytechnique fédérale de lausanne (ch) observatoire cantonal de neuchâtel (ch) institute of physics of national academy of sciences of belarus (by) instituto superior técnico (p) ludwig-maximilians universität münchen (d) fraunhofer gesellschaft zur förderung der angewandten forschung e.v. (d) table i. earlinet participating institutions. 404 gelsomina pappalardo, aldo amodeo, salvatore amoruso, lucia mona, marco pandolfi and vincenzo cuomo starting since may 2000, the lidar measurements performed in tito scalo have been collected and analysed. preliminary results regarding the first year of measurements are presented and discussed in this paper. 2. experimental two lidar systems are operational at imaa in potenza (southern italy, 40°36¢n, 15°44¢e, 820 m above sea level). the fi rst lidar system is devoted to study tropospheric aerosols, while the second one is a raman lidar devoted to water vapour measurements. in this paper, we present aerosol measurements and for this reason a detailed description of the raman lidar will not be reported here. the aerosol lidar system is based on a nd: yag laser operating on fundamental, second and third harmonics with a repetition rate up to 20 hz. the three harmonics are transmitted in a coaxial mode along the same optical path. the receiver consists of a vertically pointing telescope in cassegrain confi guration with a 0.5 m diameter primary mirror and a combined focal length of 5 m. the collected radiation is split into fi ve channels by means of dichroic mirrors, and interferential fi lters are used to select the elastic backscattered radiation at 1064 nm, 532 nm and 355 nm, and the n2 raman shifted signals at 386.6 nm and 607.4 nm. photomultiplier tubes are used as detectors. signals from photomultipliers are amplifi ed and sampled both in analog and photon counting mode. analog signals are acquired by means of a digital oscilloscope (500 mhz) and a 12 bit transient digitizer board, while the photon counting chain is based on a fast discriminator (300 mhz) and a multi channel scaler (mcs) board with a minimum dwell time of 5 ns. the overall lidar characteristics are reported in table ii. at present, the aerosol lidar system is used to perform two-wavelength tropospheric aerosol measurements. in particular, tropospheric aerosol observations are performed in terms of aerosol backscatter using the second (532 nm) and third (355 nm) harmonic of the nd:yag laser and in terms of aerosol extinction in the uv region. the aerosol extinction profiles are obtained using the nitrogen raman signals at 386.6 nm (ansmann et al., 1990). the other two channels at 1064 nm and 607.4 nm will be operational in the near future. simultaneous measurements of two-wavelength aerosol backscatter and aerosol extinction allow the retrieval of important microphysical aerosol properties, such as size distribution (sasano and browell, 1989). by using the aerosol backscatter profi le at 1064 nm and the aerosol extinction profi le at 607.4 nm, it will also be possible to retrieve the refractive index (both real and imaginary part) (böckmann and niebsch, 1996; müller et al., 1998). the lidar signals are integrated over 1 min, corresponding to 1200 laser shots, and with a vertical resolution of 15 m. so, the system measures the temporal evolution of atmospheric aerosol backscatter with high temporal and vertical resolutions. 3. data analysis and results systematic aerosol lidar measurements in tito scalo (pz) started in may 2000 on a regu lar base of three measurements per week according to the schedule of the earlinet project. the two night time measurements have been performed every monday and thursday in a time window of – 2 h ÷ + 3 h around sunset, while daytime measurements have been performed every monday in a time window of – 1 h ÷ + 1 h around 13:00 utc. daytime measurements provide aerosol backscatter profi les at both 355 nm and 532 nm, while during night time, raman measurements are also possible providing aerosol extinction profi les at 355 nm. lidar measurements have been always performed following this schedule with the sole exception of prohibitive meteorological conditions. at the moment, in the period may 2000 october 2001, we have collected more than 35 000 fi les, corresponding to about 600 h of measurements. minimum duration of a measurement is 60 min. nevertheless, records of several hours, up to 16 h, of continuous measurements have also been carried out to monitor special events or to follow the evolution of the planetary boundary layer (pbl). row data are integrated over 405 one year of tropospheric lidar measurements of aerosol extinction and backscatter 1 min, corresponding to 1200 laser shots, and with a vertical resolution of 15 m; fi nal aerosol backscatter profi les are obtained by averaging 30 single measurements corresponding to about 30 min and with a vertical resolution of 60 m, according to the earlinet demand. aerosol extinction profi les are also obtained on average of 30 single measurements, but in this case the vertical resolution varies from 60 m up to 350 m, depending on the statistics. for daytime measurements, aerosol backscatter profiles at both 355 nm and 532 nm are retrieved starting from elastic lidar signals using an iterative procedure based on the assumption of constant extinction-to-backscatter ratio (lidar ratio) within the aerosol layer (di girolamo et al., 1999). this assumption could be too strong in presence of various layers corresponding to different aerosol types with different optical thickness. in such a case, it is table ii. imaa-cnr aerosol lidar system. laser source: nd: yag laser (continuum-ny60) wavelength and pulse energy 1064 nm emax = 600 mj 532 nm emax = 300 mj 355 nm emax = 170 mj maximum pulse repetition rate 20 hz beam divergence < 0.5 mrad pulse duration 5 ÷ 7 ns receiver: cassegrain telescope primary mirror diameter 0.5 m combined focal length 5 m field of view 0.2 ÷ 2 mrad spectral selection: interference fi lter wavelengths 355 nm, 532 nm, 387 nm, 607 nm, 1064 nm bandwidth < 1.0 nm out of band rejection £ 10-8 transmission effi ciency ~ 35% detectors: photomultipliers thorn emi 9202 qb 355 nm, 532 nm, 387 nm, 607 nm hamamatsu r1767 1064 nm acquisition photon counting analog eg&g ortec turbo mcs imetec 12 bit transient digitizer minimum dwell time 5 ns number of channels 2 bandwidth 150 mhz sampling rate 30 mhz min. sampling time 33 ns phillips scientific tektronix fast discriminator digital oscilloscope bandwidth 300 mhz number of channels 3 bandwidth 500 mhz 406 gelsomina pappalardo, aldo amodeo, salvatore amoruso, lucia mona, marco pandolfi and vincenzo cuomo not possible to assume a constant value of the lidar ratio along all the path and then different values for different layers must be considered. the typical values of lidar ratio used are taken from literature with the support of the direct and independent aerosol extinction measurements at 355 nm performed in tito scalo during night time, when raman measurements are available (pappalardo et al., 2000). in this case, no assumption on the lidar ratio values is required to retrieve the backscatter profi le in the uv. at 532 nm the iterative procedure is still employed, also for night time measurements, but the values of the lidar ratio used in the algorithm are related to those measured at 355 nm. the aerosol extinction coeffi cient is determined from the n2 raman backscattering signals through the application of the algorithm proposed by ansmann (ansmann et al., 1990, 1992). both algorithms for the fig. 2. vertical profi les of the aerosol backscatter at both 355 nm (solid squares) and 532 nm (open circles) measured in tito scalo on 20 september 2000 (19:56-20:27 utc). fig. 3a,b. vertical profi les of the aerosol backscatter (solid squares) and extinction (open squares) at 355 nm (a), vertical profi le of the lidar ratio at 355 nm (b); tito scalo, 16 october 2000 (18:46 19:05 utc). a b 407 one year of tropospheric lidar measurements of aerosol extinction and backscatter aerosol backscatter and extinction retrieval have been successfully compared within the earlinet community (böckmann et al., 2001). typical profi les of the aerosol backscatter at 532 nm and 355 nm obtained in tito scalo are shown in fi g. 2; these profi les are typical for after sunset conditions. the aerosol backscatter profi les at both 355 nm and 532 nm extend up to 8 km of height and correspond to an integration time of about 30 min (36 000 laser shots) with a vertical resolution of 60 m. aerosol backscattering profi le at 532 nm has been derived from the elastic lidar signal (fernald, 1984; di girolamo et al., 1999), while the aerosol backscatter profi le at 355 nm has been derived from the simultaneous elastic lidar signal at 355 nm and the corresponding raman nitrogen signal at 386 nm (ferrare et al., 1998). figure 3a shows typical profi les of the aerosol backscatter and extinction. both profi les extend up to 6 km of height above the lidar station. clouds are present between 3800 m and 5000 m of height above lidar station. the corresponding lidar ratio profi le, reported in fi g. 3b, shows high values, ranging from 80 sr to 20 sr, at low height and up to the cloud base, with a mean value of 48 sr. meanwhile, inside the cloud the lidar ratio decreases to lower values, ranging from 6 up to 20 sr with a mean value of 11 sr, in agreement with values reported by ansmann et al. (1992). the error bars reported in the fi gures refer to statistical errors that are calculated using a montecarlo procedure. the montecarlo procedure calculates the errors also in case of smoothed data, low statistics and extinction retrieval, when an analytical approach is not possible. this procedure is based on a random extraction of new lidar signals, each bin of which is considered as a sample element of a gaussian probability distribution with the experimentally observed mean value and standard deviation. the extracted lidar signals are then processed to retrieve a set of solutions from which the standard deviation as a function of the height is estimated. as shown in the fi gures, backscatter errors are within 10% up to 3.5 km and less than 20% in the cloud; regarding the extinction, errors are within 10% up to 2.5 km and less than 40% in the cloud. as previously mentioned, more than 600 h of lidar measurements have been performed in tito scalo in the first year of systematic observations, but, to start a statistical analysis of the measurements, only 30 min per day, for both daytime and night time measurements, have been processed and analysed. typical lidar profi les start from 650 m above the lidar station, where there is full overlap between the transmitted laser beam and the telescope fi eld of view, and extend up to 8 km of height. in one year of measurements, many different meteorological conditions and presence of special events, such as forest fi res and saharan dust, have been monitored. as an example, fi g. 4 shows a typical lidar profi le of aerosol backscatter, at both 355 nm and 532 nm, in presence of saharan dust. figure 4 reports lidar measurements performed in tito scalo on 31 august 2000, in the case of a large saharan dust event. several distinct dust layers are present extending from 2 km up 5 km fig. 4. vertical profi les of the aerosol backscatter at both 355 nm (solid squares) and 532 nm (open circles) measured in tito scalo on 31 august 2000 (19: 20 19:50 utc). 408 gelsomina pappalardo, aldo amodeo, salvatore amoruso, lucia mona, marco pandolfi and vincenzo cuomo fig. 5. the ep/toms aerosol index data for 31 august 2000. fig. 6. 96-h air mass backtrajectory analysis for air masses ending over potenza on 31 august, 2000 at 19 utc. 409 one year of tropospheric lidar measurements of aerosol extinction and backscatter above the lidar station, with a pronounced peak at 4.5 km of 5.3 ¥ 10-6 m-1 sr-1 at 355 nm and 3.8 ¥ 10-6 m-1 sr-1 at 532 nm. figure 5 shows the image of the total ozone mass spectrometer (toms) for the same day where there is clear evidence of the presence of a large amount of aerosol above southern italy with an aerosol index of the order of 3. the 96-h backtrajectory analysis, provided by the german weather service, for the same day is reported in fi g. 6. this fi gure shows clearly the saharan origin of the dust layers observed in tito scalo; in particular, it is evident that the air masses ending over tito scalo (potenza) at all levels (850-hpa up to 200-hpa) on 31 august, 2000 at 19 utc had originated from the saharan region, where probably they were enriched with large dust loads, explaining the presence of dust layers with the high aerosol backscatter values observed in tito scalo above 2 km of height. in the following, we report the fi rst preliminary results obtained from a statistical analysis of one year of systematic lidar measurements of aerosol backscatter and extinction performed in tito scalo. aerosol is typically present up to about 2.5 km of height during spring-summer period and up to about 2 km during autumn-winter (pbl plus residual layer). in case of special events such as saharan dust outbreaks, forest fi res or etna eruption, dust layers at higher altitudes are observed. in the fi rst kilometer of height above the lidar station, representative of the pbl and of a part of the residual layer, peak values of aerosol backscatter of 1 ¥ 10-5 m-1sr-1 at 355 nm and 5 ¥ 10-6 m-1sr-1 at 532 nm were observed during summertime. aerosol backscatter mean values calculated during the whole fi rst year of measurements are 6 ¥ 10-6 m-1sr-1 at 355 nm and 3 ¥ 10-6 m-1sr-1 at 532 nm during summertime and 3 ¥ 10-6 m-1sr-1 at 355 nm and 2 ¥ 10-6 m-1sr-1 at 532 nm during wintertime. peak values of the aerosol extinction profi le, always in the fi rst kilometer of height above the lidar station, of about 3 ¥ 10-4 m-1 have been observed in summertime, while mean values are around 4 ¥ 10-5 m-1 in summertime and 2 ¥ 10-5 m-1 in wintertime. figure 7a shows the integrated aerosol backscatter (ib) at 355 nm and fi g. 7b at 532 nm for all the night time measurements. the data have been integrated along the whole aerosol layer inclusive also of layers present at higher altitude due to special conditions. the selection of the integral limits for the calculation of ib is decided case by case considering the lowest probed height and the highest height affected by an uncertainty less than 15% for the aerosol backscatter. this choice depends on the fact that we intend to start the analysis of our data by considering the whole amount of aerosol that we can measure in the low troposphere. lidar data with clouds are not considered for this analysis. in fi g. 7a,b, special measurements, related to the presence of saharan dust, are reported in a different colour (black). it must be pointed out that saharan dust events are also observed during the autumn-winter period. the values of the integrated aerosol backscatter range from 2 ¥ 10-4 sr–1 to 1 ¥ 10-2 sr-1 with a mean value of 4 ¥ 10–3 sr–1 at 355 nm and from 1 ¥ 10– 4 sr–1 to 1 ¥ 10–2 sr–1 with a mean value of 2.3 ¥ 10–3 sr–1 at 532 nm. peak values are evident in june for both wavelengths, and summertime period seems to be characterised by higher values with respect to the wintertime period. data related to saharan dust observations seem to be not affected by this seasonal variation and are characterised by mean values of about 5.3 ¥10–3 sr–1 at 355 nm and 3.6 ¥ 10–3 sr–1 at 532 nm. figure 8 shows the integrated aerosol extinction (i.e. the optical thickness t a ) obtained from nitrogen raman measurements at 355 nm. in the fi gure, both regular and special (saharan dust) measurements are reported and also in this case the data are integrated in the whole aerosol layer. the selection of the integral limits for the calculation of t a is again decided case by case considering the lowest probed height and the highest height affected by an uncertainty less than 30% for the aerosol extinction. the values of ta range from 0.02 to 0.75 with a mean value of 0.2. higher values are evident in the period may september 2000, while autumn-winter time is characterised by lower values of aerosol extinction. aerosol extinction profi les are not so numerous as backscatter profi les, because sometimes it is not possible to retrieve the aerosol extinction profi le from raman channel due to low statistics. from fi g. 8 we observe that also for extinction measurements it seems that saharan dust data do not follow the typical 410 gelsomina pappalardo, aldo amodeo, salvatore amoruso, lucia mona, marco pandolfi and vincenzo cuomo 532 nm 355 nm fig. 7a,b. integrated aerosol backscatter at 355 nm (a) and 532 nm (b) for all night time lidar measurements in tito scalo in the period may 2000 april 2001. regular measurements are reported in white, saharan dust measurements are reported in black. b a seasonal variation we observed in tito scalo under normal conditions. moreover, the values of ta recorded during saharan dust episodes are the highest we observed in wintertime. figure 9 reports the mean lidar ratio values obtained for each measurement (both regular and special) in the whole aerosol layer (pbl, residual layer, and higher dust layers when pre411 one year of tropospheric lidar measurements of aerosol extinction and backscatter fig. 8. integrated aerosol extinction at 355 nm for all night time lidar measurements in tito scalo in the period may 2000 april 2001. regular measurements are reported in white, saharan dust measurements are reported in black. fig. 9. mean lidar ratio values at 355 nm for all night time lidar measurements in tito scalo in the period may 2000 april 2001. regular measurements are reported in white, saharan dust measurements are reported in black. sent). lidar ratio values range from 25 to 65 sr, with higher values in summer period. again it seems that saharan dust data are not affected by seasonal variation. the mean value of the lidar ratio for the whole aerosol layer in one year of systematic observations is 43 ± 11 sr. higher values of the lidar ratio, observed in springsummer period, can be attributed to the presence 412 gelsomina pappalardo, aldo amodeo, salvatore amoruso, lucia mona, marco pandolfi and vincenzo cuomo fig. 10. frequency of the monthly averaged lidar ratio values at 355 nm for both spring-summer (april-september) and autumn-winter (octobermarch) periods. of high absorbing particles which results also in the higher extinction values shown in fi g. 8, as well as to the presence of a high concentration of small particles originated primarily by forest fi res, very common in this period of the year in our area (anderson et al., 2000). in fi g. 10, we report the frequency of the monthly averaged lidar ratio values for both spring-summer (april-september) and autumnwinter (october-march) periods. we considered 10 sr intervals starting from 20 sr up to 90 sr. it is evident in fi g. 10 that spring summertime values are higher and more wide spread with respect to the wintertime values. if we assume an almost gaussian distribution for both autumn-winter and spring-summer values, we obtain a central value of 41 sr with s = 8 sr and central value of 44 sr with s = 12 sr, respectively. different factors could contribute to the wider variability observed for the summertime period. among others, the presence of many forest fi res, with the consequent production of small particles of ash and soot in the atmosphere, the more intense rural activity and the more effective air mass transport, all these resulting in a larger variability of the aerosol types with respect to a more stagnant condition characteristic of the wintertime period (anderson et al., 2000; ackermann, 1998; evans, 1988). 4. conclusions systematic lidar measurements of aerosol backscatter and extinction have been made since may 2000 in tito scalo (pz) southern italy in the framework of the earlinet project. backscatter measurements have been performed at both 355 nm and 532 nm, while aerosol extinction measurements, through the application of the raman lidar technique, have been performed at 355 nm. beyond lidar measurements performed three times per week on a regular schedule, further special lidar observations have been devoted to study saharan dust outbreaks. preliminary results obtained starting from a statistical analysis of the fi rst year of measurements in tito scalo have been reported. seasonal variations have been observed for aero sol integrated backscatter and extinction. in the near future these data will be compared with data of other stations of the earlinet project with the main goal of studying vertical and horizontal transport of the aerosol over europe. the use of these data will also contribute to improve model treatment on a wide range of scales and to a better exploitation of present and future data from satellite remote sensing for a variety of parameters. acknowledgements the fi nancial support of this work by the european commission under grant evr1ct199940003 is gratefully acknowledged. the authors also thank the german weather service for the air mass back-trajectory analysis, and nasa/gsfc for the ep/toms aerosol index data. references ackerman, f.a. and h. chung (1992): radiative effects of airborne dust and regional energy budget at the top of the 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[2007] showed that these are modulated by the solar activity, and that the daytime tec seasonal anomaly is most evident at middle to high latitudes in the northern hemisphere, while the semiannual anomaly is seen at all latitudes, with it being more pronounced in the equatorial anomaly crest region (±20° magnetic latitudes). the winter anomaly has been more clearly observed during periods when the sun has been highly active [walker 1971, huang and cheng 1995, mansilla et al. 2005]. the aim of the present study was to investigate the ionosphere variations using gps-tec measurements obtained at comandante ferraz brazilian antarctic station (eacf; 62.1ºs, 58.4ºw) at king george island on the antarctic peninsula. we present the results for yearly, seasonal and mean diurnal vtec variations as observed at eacf under different solar activity levels in the period from 2004 to 2011, and thus during the decay phase of the 23rd solar cycle, and the increase of the 24th solar cycle. 2. observations the data used for this study were obtained with a dual frequency javad gps receiver that has been operating at eacf since january 2004. the gps-tec was obtained from receiver independent exchange format (rinex) data that were acquired every 15 s. the analysis uses the vtec derived from gps-tec observations using the pre-processing stage of the local point interpolation method (lpim) algorithm, which uses a simple mapping function. the lpim algorithm was developed article history received october 15, 2012; accepted march 27, 2013. subject classification: ionosphere, total electron content, seasonal variation, solar cycle. by the argentine la plata group [brunini et al. 2008]. the vtec was computed as the median vtec values of the satellites available above a 30° elevation angle, at each 30 s, with the ionosphere height assumed at 350 km. the analysis covers the period from 2004 to 2011, but there are some data gaps due instrumental problems, especially during the spring months of 2008. the influence of solar activity on the long-term vtec variation was evaluated considering the solar ultraviolet radiation (fuv) and sunspot number as solar proxies (data obtained from the national oceanic and atmospheric administration data center at http://www.swpc.noaa.gov/ftpdir/indices/; last accessed september, 2012). the fuv data refer to the timed see (thermosphere ionosphere mesosphere energetics and dynamics – solar extreme ultraviolet [euv] experiment) and sorce solstice (solar radiation and climate experiment – soar-stellar irradiance comparison experiment) solar spectral irradiance measurements of the he ii 30.4 nm emission provided by the laboratory for atmospheric and space physics (lasp) solar irradiance data center (http://lasp.colo rado.edu/lisird/lya/; last accessed september, 2012) [woods et al. 2000]. 3. observational results and discussion 3.1. long-term variation – solar radiation control to study the solar influence in the long-term variation of the vtec, we considered the daily median value obtained between 15:00 ut and 17:00 ut (~12:00 lt) as representative of the maximum daily vtec, from 2004 to 2011. the comparisons were carried out considering the 31-day running mean of the daily maximum vtec with the sunspot number (figure 1), and the monthly average of the daily maximum vtec with solar fuv intensity (figure 2). the analyses show good temporal correlation between vtec and solar activity, where all of the datasets have a slow variation that was well fitted by a third-degree polynomial (figure 1, dashed lines). the wrapping of the monthly vtec data in figure 2 is the yearly maximum and minimum values. the results clearly show daytime vtec modulation by the solar activity, which decreased from 2004 to 2008 during the decay of the 23rd solar cycle, and starting to increase in 2009 when the 24th solar cycle began. to evaluate the influence of the solar activity on the maximum daytime vtec, we compared the 3-day smoothed vtec data with the daily fuv solar intensity (figure 3). this comparison was carried out considering the months of january and july separately, which are representative of the summer and winter seasons, respectively, in order to filter the solar zenith angle effect. the analysis shows a strong linear correlation between the vtec and the solar fuv intensity in summer, as demonstrated by the correlation coefficient (r) of 0.84 (r2 = 0.71); this means that 71% of the data are well represented by a linear variation with a gradient of 1.70 tecu/(10−3 w/m2/nm). however, in winter, the correlation coefficient is weak, at 0.56 (r2 = 0.31), which means that only 31% of the data can be explained by the linear relationship with a gradient of 0.44 tecu/ (10−3 w/m2/nm). these results mean that the maximum daytime vtec is modulated by the solar radiacorreia et al. 2 no data no data figure 1. variation of sunspot numbers (a) and the daily maximum vtec (b) from 2004 to 2011. both datasets were 31-day smoothed and fitted using a cubic polynomial regression (dashed curves). figure 2. variation of the 31-day smoothed solar he ii radiation (30.4 nm, fuv) (top) and the monthly averaged daily maximum vtec (lower curve) from 2004 to 2011. the yearly maximum and minimum vtec values are also shown. both datasets were fitted using cubic polynomial regression (dashed curves). 3 tion intensity but depends on the solar zenith angle, as shown by the higher gradient in the summer season. the long-term analysis of the daytime vtec measured at eacf from 2004 to 2011 clearly shows that it is modulated by solar activity. this is in agreement with chung et al. [2011], who used vtec measurements carried out from 2005 to 2009 over king sejong station (~62°s, 58°w), which is very near the brazilian station. modulation of the vtec by solar activity was also reported by walker [1971], huang and cheng [1995], zhao et al. [2007] and galav et al. [2010]. these results also showed that the vtec correlation with the solar radiation intensity at a latitude of ~62°s is different between the summer and winter seasons, demonstrating that the ionization processes are lower in winter (figure 3), probably due to low solar flux at high solar zenith angles, as shown by purohit et al. [2011] from observations carried out in the polar region. 3.2. seasonal mean diurnal variation the seasonal mean diurnal variation was obtained by considering the monthly averages of the 1-h vtec for each day for january, march, june and september, as representative of the summer solstice, autumn equinox, winter solstice and spring equinox, respectively. the mean diurnal variation of the vtec for each season for all of the years is shown in figure 4, where there is a clear peak between 13:00 ut and 17:00 ut (lt = ut − 4), but january (summer). the mean diurnal vtec in the antarctic summer shows no clear variation, which suggested higher values in the nighttime than in the daytime. this can be attributed to the weddell sea anomaly, which refers to the larger nighttime electron density than for the daytime that has been observed around the antarctica peninsula [horvarth 2006, jee et al. 2009, chung et al. 2011]. the diurnal vtec shows a semiannual variation (figure 4), with larger diurnal peaks occurring preferentially during the equinoxes, while the smallest peak occurs in the winter solstice for all of the years. the semiannual anomaly of the diurnal vtec observed at eacf with maximum values at the equinoxes is similar to the observations from equatorial [kumar et al. 2012, olwedo et al. 2012], middle [huang and cheng 1995, mansilla et al. 2005, galav et al. 2010, liu et al. 2012, kumar et al. 2012] and high [chung et al. 2011, purohit et al. 2011] latitudes. the diurnal peak of the vtec for each season decreases from 2004 to 2008, to reach its lowest values in 2008-2009 when the solar activity was at a minimum, and then starts to increase after 2009, with the up-coming 24th solar cycle. in summary, the long-term study of the daily and monthly maximum daytime vtec at eacf show annual variation in close association with the solar cycle, which decreased from 2004 to 2008 during the decay of the 23rd solar cycle, and which started to increase in 2009 when the 24th solar cycle began. the solar control of the vtec is also verified by its good linear correlation with the solar flux intensity, which has been shown to be dependent on the solar zenith angles, as demonstrated by the higher vtec gradients in summer than in winter. the results also show the presence of a semiannual anomaly, with the daytime vtec peaks occurring during equinoxes. a winter anomaly was not observed in this study. 4. concluding remarks the ionosphere behavior was investigated using the maximum daytime gps-tec observed at eacf from 2004 to 2011. we have discussed here the annual, seasonal and mean diurnal variations of the vtec at a latitude of ~62°s as a function of the solar activity. the maximum daytime vtec (~12:00 lt) shows an annual variation that was clearly controlled by the solar activity and by the solar zenith angle, which was greater in summer than in winter, and was greater near the maximum solar activity than near the minimum. these data reinforce the idea that the ionosphere ionization processes are controlled by solar flux intensity and solar zenith angle, especially at higher latitudes [purohit et al. 2011]. the vtec shows a clear semi-annual anomaly with peaks during the equinoxes, similar to what has been obgps-tec in antarctica figure 3. variations of the 3-day smoothed daily maximum vtec for january (summer) and july (winter) with respect to the solar fuv intensity used as a proxy for the solar activity. both datasets were fitted with linear regression (r, correlation coefficients). served at all latitudes; on the other hand, a winter (seasonal) anomaly was not observed, probably because it has the least chance to be observed in the south pacific areas in the south america sector [zhao et al. 2007]. ionosphere studies covering the large spatial distribution of the vtec are important to better characterize the longitudinal and latitudinal dependence on the anomalies. as the ionosphere can be controlled by an internal dynamic/ chemical process that is driven by solar radiation and an external process, such as magnetospheric disturbances or atmospheric waves from below, further studies are needed to investigate the major causes that are responsible for the anomalies in the ionosphere variations. correia et al. 4 figure 4. seasonal variability of the vtec for each year from 2004 to 2011. each curve refers to the mean diurnal variation that is representative of each season, as observed at eacf (~62°s). 5 acknowledgements. this study was supported by cnpq/ proantar (proc. 0520186/06-0 and 556872/2009-6) and partially by inct-apa (instituto nacional de ciência e tecnologia antártico de pesquisas ambientais, cnpq proc. 574018/2008-5 and faperj proc. e-16/170.023/2008). ec thanks the cnpq (proc. 300710/2006-2) and the secirm, and the brazilian antarctic program (proantar) for logistic support at comandante ferraz brazilian antarctic station. we also thank the referee for suggestions that helped to improve our manuscript. references aarons j., b. lin, m. mendillo, k. liou and m. codrescu (2000). global positioning system phase fluctuations and ultraviolet images from the polar satellite, j. geophys. res., 105, 5201-5213. brunini, c., a. meza, m. gende and f. azpilicueta (2008). south american regional ionospheric maps computed by gesa: a pilot service in the framework of sirgas, adv. space res., 42, 737-744; 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(1971). electron content and topside plasma temperature studies at low latitudes during a period from sunspot minimum to sunspot maximum, j. atmosph. terrestr. phys., 33, 1041-1053. woods, t.n., w.k. tobiska, g.j. rottman and j.r. worden (2000). improved solar lyman irradiance modeling from 1947 through 1999 based on uars observations, j. geophys. res., 105, 27195-27215. zhao, b., w. wan, l. liu, t. mao, z. ren, m. wang and a.b. christensen (2007). features of annual and semiannual variations derived from the global ionospheric maps of total electron content, annales geophysicae, 25, 2513-2527; doi:10.5194/angeo-252513-2007. *corresponding author: emília correia, centro de rádio astronomia e astrofísica mackenzie (craam), universidade presbiteriana mackenzie, são paulo, brazil; email: ecorreia@craam.mackenzie.br. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. gps-tec in antarctica << /ascii85encodepages false /allowtransparency false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice pizzi_corretto_layout 6 751 the may 2012 emilia (italy) earthquakes: preliminary interpretations on the seismogenic source and the origin of the coseismic ground effects alberto pizzi*, vittorio scisciani università di chieti-pescara 'g. d'annunzio', dipartimento di ingegneria e geologia, chieti, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6171 1. introduction on may 20, 2012, a ml 5.9 earthquake (t1) occurred in the emilia-romagna region of northern italy. this was preceded by a ml 4.1 foreshock on may 19, 2012, and followed by several aftershocks, including two ml 5.1 events, both on the same day. on may 29, 2012, a second strong event of ml 5.8 (t2) hit the same region, with its epicenter ca. 12 km to the wsw of the first mainshock, t1. the epicentral area of the seismic sequence covers an alluvial lowland that is occupied by both agricultural and urbanized areas, and there were 17 casualties and about 14,000 people left homeless. for both of the mainshocks, the focal mechanism solutions indicate low-angle reverse faulting with ca. wnw-ese-trending ruptures that originated at a depth of 6.3 km for the t1, and 10.2 km for t2 (iside, http://iside. rm.ingv.it). these events occurred along the ca. e-w-striking outermost active sector of the northern apennines thrust front that is currently buried by a thick miocene–quaternary foredeep and thrust-top siliciclastic succession (mainly turbidites and alluvial deposits) that fill the basin of the po plain. the presence of seismogenic thrust structures was already known for this area, based on historical seismicity records, subsurface geological data, and geomorphological observations [burrato et al. 2003, cpti working group 2011, diss working group 2010, scrocca et al. 2007]. however, the occurrence of the 2012 seismic events provided new input for the characterization of the seismogenic sources and gave rise to new questions. for example, the occurrence of two close events in space and time of similar magnitude raise the need to further understand the relationships between the two seismogenic sources at depth (single segmented fault, or two faults). geological surveys soon after a mainshock can provide crucial information about the occurrence (or absence) at the surface of coseismic deformation features either of primary (i.e., surface faulting) and/or of secondary (created by seismic shaking) origin. although the magnitude of the two mainshocks (5.9 and 5.8) were just above the magnitude limit of surface-faulting earthquakes [e.g., wells and coppersmith 1994], several reports, as well as the present study, have indicated the absence of surface faulting (i.e., blind faulting). in contrast, extensive evidence of secondary effects were reported, which were mostly represented by earthquake-induced liquefaction features [e.g., report dpc_1 2012, report regione emilia romagna and dpc 2012, and other reports available on-line]. although not spatially associated with coseismic surface faulting, liquefaction distribution can provide important information about liquefaction hazard assessment for future earthquakes, in terms of maximum distance estimation of expected liquefaction effects, once the potential seismogenic source and its expected magnitude and liquefaction susceptibility at the epicentral area are known [e.g., obermeier 1996, galli 2000]. furthermore, the magnitude threshold for producing liquefaction (i.e., 5.0-5.5) is similar to the magnitude threshold for producing significant earthquake damage. in italy, indeed, several dense populated areas next to seismogenic sources with potential magnitudes ≥ ca. 5.0 lie on sediments with moderate to high liquefaction susceptibility [galli 2000, diss working group 2010]. moreover, field observations of liquefaction distribution can contribute to better definition of the magnitude/farthest liquefaction distance relationships used for magnitude estimation in paleoseismological studies [obermeier 1996, galli 2000]. in the present study, we provide a preliminary model of the seismogenic source(s) responsible for the two mainshocks, by comparing the seismic reflection profile interpretation with the available seismological and interferometric data. furthermore, we show the coseismic ground effects that were observed in the epicentral area during two field survey campaigns: the first conducted after the may 20, 2012, event and the second soon after the may 29, 2012, earthquake, when several sites were revisited to observe the occurrence of newly formed or 're-activated' liquefaction article history received july 27, 2012; accepted august 31, 2012. subject classification: seismic methods, earthquake geology and paleoseismology, structural geology, earthquake source and dynamics, tectonics. 2012 emilia earthquakes features. hence, we discuss the origin and location of the coseismic features observed in the context of the local geological–geomorphological setting and with respect to the epicentral distance. finally, we provide our interpretation for the question: "why did the mainshock ruptures not break the surface?" 2. tectonic and structural setting the emilia 2012 seismic sequence activated the buried central sector of the northern apennine outer thrust fronts. this arc-shaped part of the apennines is composed of several n(ne)-verging compressive structures (mainly folds and blind thrusts), which developed in the miocene–quaternary foredeep basin of the po plain – north adriatic. the basin infill consists of quaternary near-shore/continental deposits and siliciclastic neogene hemipelagic, foredeep and shallow-water sediments resting on top of the mesozoic passive margin succession, which is mainly composed of carbonates, with triassic dolomites and evaporites at the base. the top of the carbonate sequence generally corresponds to the upper part of the pelagic scaglia fm. or to the shallow-water carbonates of the bagnolo fm., which is limited to the sw part of the study area [fantoni and franciosi 2010]. the main facies and thickness variations of the po basin sedimentary infill mainly derive from the foredeep fragmentation operated by the syn-sedimentary growth of thrust related folds, which has been active from messinian to present times [ghielmi et al. 2010, and references therein]. the quaternary activity of the compressive structures is testified by geological [regione emilia romagna and eni-agip 1988, boccaletti et al. 1985, scrocca et al. 2007, toscani et al. 2009], geomorphological [burrato et al. 2003], geodetic [devoti et al. 2011], and seismological [diss working group 2010, cpti working group 2011] data. moreover, borehole breakouts from deep wells and stress inversions from focal mechanism solutions [montone et al 2012] indicate that shmax trends near perpendicular to the attitude of the thrust faults in the subsurface. 3. preliminary interpretation of the seismogenic sources the emilia-romagna seismic sequence affected an area that is characterized by three major buried structural highs that are delimited to the south by the so-called 'pedeapenninic thrust front', as defined by boccaletti et al. [1985]. the wnw-ese striking occhiobello-ficarolo anticline represents the northernmost ridge, which is a north-verging thrust-related anticline, and is delimited to the south by a minor back-thrust. the displacement achieved by the southdipping thrust (figure 1a, b, th1) is low and the fault remains blind at depth. towards the south, a prominent ridge runs from ferrara–bondeno to immediately south of poggio rusco, and this strongly deforms and uplifts the entire siliciclastic and carbonate succession. in the western sector, two near-parallel anticlines that trend wnw-ese comprise the structural high. the northern anticline is buried and limited to the south by a shallow back-thrust (figure 1a, b), and it affects the quaternary sediments, as suggested by the interpretation of seismic reflection profiles. the second thrust-related anticline (figure 1a, b, th3) clearly truncates and deforms the shallow siliciclastic succession, including the quaternary strata, and disappears to the east of finale emilia. the thrust fault shows low displacement near to the top of the carbonate succession and gently dips towards the hinterland (figure 1a, b, th3). however, at a depth of more than 5 km, the continuity of the thrust is unclear on the seismic profiles, possibly due to interference with the innermost and more complex structure corresponding to the cavone– mirandola–san felice sul panaro structural high. in the western sector, this latter structure is arc-shaped in the map view, and shows an apex located immediately north of mirandola. a set of (at least two) relatively small, north-verging detachment anticlines involves only the neogene fine-grained sediments and are detached above the top of the carbonates, corresponding to the scaglia fm. (figure 1b, c, ths5). the scaglia fm. reflector is affected, with a small offset, by the thrust th4 and back-thrust faults (figure 1b, c), and is involved in a thrust-related anticline. both the thrust fault and the anticline are well constrained by several wells tied to the seismic line (camurana 2, san felice sul panaro 1, spada 1, bignardi 1, 1dir, and the wells in the cavone oil field) (figure 1c) and were described in fine detail in the hydrocarbon storage potential evaluation of the rivara field (www. ergrivarastorage.com). the second earthquake (figure 1, t2) clearly occurred underneath the mirandola– san felice sul panaro structural high. ground deformation provided by dinsar data (figure 1a) also supports this interpretation, and in addition, the fringes distribution might suggest that two separate sources are responsible for the two main events (fig. 1a, t1, t2). this hypothesis, however, does not fit with the subsurface geology of the area that was reconstructed by seismic reflection profile interpretations and well data. indeed, the thrust fault that affects the carbonate sequence and is responsible for the mirandola–san felice sul panaro anticline remains shallower, as at least 3 km above the t2 hypocenter (10.2 km in depth), even though possible errors and uncertainties might derive from the regional velocity model for the hypocentral locations (iside database) and from the time-to-depth conversion of the seismic profiles. moreover, the curved attitude of the buried thrust fault is oblique with respect to the distribution of ground deformation achieved by the insar data, which are commonly elliptical for thrust faults with the long axis parallel to the trend of the fault. in light of this evidence and these preliminary results, we suggest that the t1 event (6.3 km in depth) originated along the th2 buried thrust, and slipped along this fault (probably pizzi and scisciani 752 753 origin and coseismic effects of the emilia earthquakes figure 1. (a) structural map of the western sector of the northern apennines fold-and-thrust belt affected by the emilia 2012 seismic sequence. the main events (m >4) are located along the two main central ridges corresponding, from north to south, to the ferrara-bondeno–poggio rusco and the mirandola–san felice sul panaro structures, respectively. gray lines, fringes pattern obtained from the radarsat-1 (l = 5.6 cm) interferogram [atzori et al. 2012], showing the cumulative deformation from both the may 20, 2012, and the may 29, 2012, earthquakes (preseismic image of may 12, 2012, and postseismic image of june 12, 2012). the numbers indicate the progressive fringes used to calculate the total uplift across the epicentral area of the two main shocks (t1 and t2). (b, c) schematic geological cross-sections (line 1, line 2) obtained from time-to-depth conversion of industrial seismic reflection profiles. the traces reported in (a) as line 2 is modified from the erg gasstorage study. the hypocenters of the earthquakes within a strip of about 20 km of width and centered on the line trace are reported. blue curves on top of the cross-sections, uplift in centimeters, obtained from the dinsar data interpretation reported in (a). the deformation data were calculated along cross-sections where the ground deformation was maximal (see the traces sb and sc in (a)) and are projected parallel to the attitude of the thrust faults in the subsurface. with a prominent ese-wards direction), but did not break the surface. this interpretation is also supported by the dinsar fringes pattern, which are located in the back-limb/crestal zone of the thrust-related anticline (figure 1a, b). for the second event, we propose two alternative hypotheses: 1) the t2 event originated on a more westerly segment of the same th2 thrust that underlies the ferrara– bondeno–poggio rusco anticline. in such a case, it is possible that the entire th2 structure is separated into two segments by a transverse fault zone that is nne-ssw-striking, and is likely to be localized close to the t1 hypocenter. this hypothesis is in agreement with the bi-elliptical pattern of the coseismic ground deformation provided by the dinsar analysis. moreover, the occurrence of transverse mesozoic structures in this zone has also been suggested by studies of subsurface data [fantoni and franciosi 2010]. this hypothesized transverse element, therefore, might represent a pre-existing cross-structure (inherited from prequaternary tectonic phases) that acted as a segment barrier (in the sense of pizzi and galadini [2009]) during the rupture of the single fault segment. 2) the t2 event could be related to the th3 thrust which runs parallel to the outermost thrust, but which ends towards the ese, close to finale emilia. in this case, the thrust fault reported in figure 1b should deepen abruptly up to 10 km in depth in its southern part where, unfortunately, the seismic profile has poor resolution. 4. coseismic ground effects two field survey campaigns conducted after the may 20, 2012, event (on may 24 and 28, 2012) and soon after the may 29, 2012, event allowed the detailed mapping of the coseismic ground effects observed in the epicentral areas. although special care was taken to find evidence of primary coseismic thrust faulting, no conclusive evidence for surface faulting was observed in the field. this lack of evidence, however, is in agreement with the blind nature of the seismogenic source and it is supported by seismological and dinsar analysis [atzori et al. 2012]. however, although the main fault rupture did not reach the surface, the epicentral area showed secondary structures (induced by seismic shaking). these mostly consisted of a variety of liquefaction-related features (figure 2), including fissures through which sediment was vented, and isolated sand-blows and fractures, and rarely craters. excluding the particular case of the sant'agostino–mirabello alignment (discussed later in this section), liquefaction features were extensively observed in very flat agricultural areas, which represented the holocene alluvial lowlands. they generally consisted of long fissures, i.e., dike intersections with the ground surface, through which sand vented, forming beltlike linear sand bodies (figure 2a). individual fissures were commonly several meters long, but the alignment of enechelon fissures several tens of meters long was not unusual. sediment vented to the surface typically consisted of bluishgrey silty-sand, sometimes with pieces of organic matter (a pizzi and scisciani 754 figure 2. (a) typical fissure with vented sand along the s. martino spino–bondeno alignment. (b-e) liquefaction features observed at the san agostino cemetery, here interpreted as formed by lateral spreading along the nw-slope of a paleochannel/levee. (f ) possible interpretation of this (the schematic vertical section is not to scale). the stratigraphic sequence and depth to the water table are modified from report regione emilia romagna and dpc [2012]. 755 collected sample that we analyzed in the laboratory provided: silt 15%, fine sand 70%, and medium sand 15%). thicknesses of vented sediments of 0.15 m to 0.2 m were not unusual along the fissures, from where they tapered laterally with a slope angle of about 6˚ to 10˚. statistical analysis of the fissure trend did not show any clear preferential orientations, although more than 70% of the strike data fall within the sector ranging from n90˚e to n140˚e (again, excluding the sant'agostino–mirabello alignment). however, it is of note that the earthquake-induced liquefaction phenomena were not radially distributed around the epicentral areas, but rather, they followed narrow, linear bands of en-echelon fissures and sand blows that corresponded very faithfully to the courses of prehistorical and historical–paleochannels (see figure 3). moreover, even though similar paleochannels have already been mapped in the whole epicentral area, most of these liquefaction alignments were concentrated relatively far from the epicenters (e.g., figure 3, san martino spino–bondeno alignment, mirandola, cavezzo). a very clear n50˚ to 60˚ orientation, however, characterizes the ca. 10-12 km long sant'agostino–mirabello alignorigin and coseismic effects of the emilia earthquakes figure 3. map showing the distribution of the observed liquefaction features in the epicentral areas and their relationships with the courses of the paleochannels and the patterns of coseismic deformation by dinsar analysis. also shown: the liquefaction/ epicentral distance relation evaluated for the two main shocks (see text for explanation). paleochannels pattern from r. ferri (http://www.centuriazione.it/quaderni_win.asp?id=69) and manicardi [1991] (available at: http://www.valledeidossi.it/territorio/geomorfologia.asp); dinsar data [atzori et al. 2012]. ment, where the most impressive liquefaction phenomena originated; i.e., the widest fissures, longest fractures, and largest volume of vented deposits. these occurred after the may 20, 2012, event, which also produced some severe damage to buildings and infrastructure in the urban areas. there were tensto hundreds-of-meters-long linear fissures (figure 2c) parallel to the relief, and very large quantities of vented sand locally (figure 2d), commonly showing several decimeters of downwarping accommodated by grabentype (extensional) structures near the crest of the relief (figure 2e) and reverse (contractional) structures at the base of the slope (figure 2b). these suggest that part of the shallower nonliquefiable units (sandy/silty alternations) moved to the nw, probably above the liquefied layer made of fine to medium sand of a deeper paleochannel facies, in response to lateral spreading (figure 2f ). in this case, therefore, liquefaction susceptibility was greatly enhanced and mostly gravity driven, due to the relief with a very gentle slope (2% to 3%), which corresponded to the paleochannel and levees of the reno river. soon after the ml 5.8 event of may 29, 2012, the revisiting of the sites that had liquefaction after the may 20, 2012, mainshock allowed the identification of those where 'reactivation' of liquefaction occurred and those where it did not. the measuring of the distance between (i) the may 20, 2012, epicenter and the related liquefaction features, and (ii) the may 29, 2012, epicenter and both the newly and reactivated liquefaction, allowed a better evaluation of the liquefaction/epicentral distance relationship for each main event, as schematized by the radii of the circles in figure 3. the farthest liquefaction distances obtained (excluding the sant' agostino–mirabello alignment) for the may 20 and 29, 2012, events are in good agreement with those provided by obermeier [1996] for shallow (<50 km) earthquakes with magnitudes around 5.8 to 6.0. including the farthest distance of the sant'agostino–mirabello alignment for the may 20, 2012, event (figure 3, larger gray circle), instead, the value falls at the boundary of the curve limiting the field of the shallow earthquakes provided by obermeier [1996]. 5. discussion and conclusions although a preliminary interpretation of the seismogenic sources has been discussed previously (see section 3), we provide here a further discussion based on the results of coseismic ground effects presented above. the first point to note is: why did the rupture not break the surface? in agreement with the results provided by the available reports, indeed, the present study indicates the lack of evidence that the coseismic thrust faulting reached the ground surface, even though the magnitude values of the two main shocks (5.9 and 5.8) were just above the magnitude limit for surfacefaulting earthquakes [e.g., wells and coppersmith 1994]. although detailed studies show that – in the mid-to-long term – high subsidence and sedimentation rates are among the main factors that control the blind setting of the outer thrust fronts in the po plain [e.g., carminati et al. 2003], these factors do not fully explain why during a single earthquake large enough to produce surface faulting, primary coseismic evidence is not observed at surface. therefore, we suggest that two further possible causes can help to find an answer to this question. first, the plio–quaternary sequence of the po plain that overlies the mesozoic–cenozoic rocks consists of unconsolidated to poorly consolidated sediments with a thickness ranging from several hundreds of meters to a few kilometers. therefore, such a geological setting provides strong attenuation as the displacement propagates upwards from seismogenic depths, as a result of strain absorption by intergranular shear. secondly, if we consider the low-angle geometry of the seismogenic thrust source (as suggested in section 3), then it is clear that a much larger ruptured area is required for the fault to reach the surface with respect to a high-angle fault (e.g., a high-angle reverse, back-thrust or normal fault). based on the coseismic evidence for recent italian earthquakes, we argue that similar magnitude earthquakes might be capable of surface faulting, like the m 6.0, umbria-marche 1997 earthquake, if the fault dip is ≥50˚ to 60˚ and a bedrock (lithified) fault plane was already exposed at, or very close to, the surface. similarly, following the m 6.3, l'aquila 2009 event, clear evidence for surface faulting away from the central sector of the paganica fault was seen only at the colle enzano and mount stabiata localities [e.g., falcucci et al. 2009, boncio et al. 2010], just in correspondence with the local (along-strike) bedrock fault-plane exposure. the second point comes from the evidence that the earthquake-induced liquefaction fissures (excluding the lateral spreading at the sant'agostino–mirabello alignment) were: (i) frequently striking between e-w and nw-se; and ii) not radially distributed away from the epicenters, but rather concentrated quite far from the epicenters. based on this evidence, we suggest that although it is clear that there was primary control through the paleochannel pattern, also 'remote' tectonic control might have had a role in the location of the liquefaction. moreover, it can be noted that most of these liquefaction alignments (e.g., san martino spino–bondeno, mirandola and cavezzo) roughly corresponded to the outermost fringes detected by the interferometric analysis (figure 3) at the boundary of the coseismically uplifted areas, and overlaying the crestal zone of the buried th2 thrust-related anticline (figure 1b). in this view, it can be argued that fissuring might have been favored by bending-moment extensional 'fracturing' [see belabbès et al. 2009, and references] that developed coseismically by thrust-generated flexure above the growing anticline. clearly, at present, these interpretations are only preliminary for which more detailed studies are definitely needed in the near future. pizzi and scisciani 756 757 acknowledgements. we thank francesca cannarsa for fieldwork assistance. we thank the two anonymous reviewers and the journal editors for providing useful comments that helped us to improve the final manuscript. seismic interpretations were carried out using move and kingdome suite softwares. this study was funded by the miur (ex 60% pizzi and scisciani) grants. references atzori, s., j. merryman boncori, g. pezzo, c. tolomei and s. salvi (2012). secondo report analisi dati sar e modellazione della sorgente del terremoto dell'emilia; ftp://ftpserver.rm.ingv.it/pub/cristiano.tolomei/report_sar_ ingv_emilia_2.pdf belabbès, s., m. meghraoui, z. çakir and y. bouhadad (2009). insar analysis of a blind thrust rupture and related active folding: the 1999 ain temouchent earthquake (mw 5.7, algeria) case study, j. seismol., 13, 421-432. boccaletti, m., m. coli, c. eva, g. ferrari, g. giglia, a. lazzarotto, f. merlanti, r. nicolich, g. papani and d. postpischl (1985). considerations on the seismotectonics of the northern apennines, tectonophysics, 117, 7-38. boncio, p., a. pizzi, f. brozzetti, g. pomposo, g. lavecchia, d. di naccio and f. ferrarini (2010). coseismic ground deformation of the april 6th 2009 l'aquila earthquake (central italy, mw 6.3), geophys. res. lett., 37 l06308; doi:10.1029/2010gl042807. burrato, p., f. ciucci and g. valensise (2003). an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting, annals of geophysics, 46 (5), 865-882. carminati, e., c. doglioni and d. scrocca (2003). apennines subduction-related subsidence of venice (italy). geophys. res. lett., 30, 1717; doi:10.1029/2003gl017001. cpti working group (2011): http://emidius.mi.ingv.it/cpti devoti, r., a. esposito, g. pietrantonio, a.r. pisani and f. riguzzi (2011). evidence of large-scale deformation patterns from gps data in the italian subduction boundary, earth planet. sci. lett., 311, 230-241; doi:10.1016/j.epsl. 2011.09.034. diss working group (2010). database of individual seismogenic sources (diss), version 3.1.1: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas; http://diss.rm.ingv.it/diss/ erg gasstorage study: www. ergrivarastorage.com falcucci, e., s. gori, e. peronace, g. fubelli, m. moro, m. saroli, b. giaccio, p. messina, g. naso, g. scardia, a. sposato, m. voltaggio, p. galli and f. galadini (2009). the paganica fault and surface coseismic ruptures due to the april 6, 2009 earthquake (l'aquila, central italy), seismol. res. lett., 80, 940-950; doi:10.1785/gssrl.80.6.940. fantoni, r., and r. franciosi (2010). tectono-sedimentary setting of the po plain and adriatic foreland, rend. fis. acc. lincei, 21, s197-s209; doi:10.1007/s12210-010-0102-4. galli, p. (2000). empirical relationships between magnitude and distance for liquefaction, tectonophysics, 324, 169-187. ghielmi, m., m. minervini, c. nini, s. rogledi, m. rossi and a. vignolo (2010). sedimentary and tectonic evolution in the eastern po-plain and northern adriatic sea area from messinian to middle pleistocene (italy), rend. fis. acc. lincei, 21, s131-s166; doi:10.1007/s12210-010-0102-4. iside (italian instrumental and parametric data-base): http://iside.rm.ingv.it/iside/standard/index.jsp montone, p., m.t. mariucci and s. pierdominici (2012). the italian present-day stress map, geophys. j. int. 189, 705-716. obermeier, s.f. (1996). using liquefaction-induced features for paleoseismic analysis, in: j. mccalpin (ed.), paleoseismology, new academic press, 331-396. pizzi, a., and f. galadini (2009). pre-existing cross-structures and active fault segmentation in the northern-central apennines, tectonophysics, 476, 302-319. regione emilia romagna and eni-agip (1988). riserve idriche sotteranee della regione emilia romagna, g. di dio (ed.), s.e.l.c.a., firenze. report_dpc_1 (2012): http://www.protezionecivile.gov.it/ resources/cms/documents/ report_dpc_1_emilia_ eqsd.pdf report regione emilia romagna and dpc (2012). primo rapporto sugli effetti della liquefazione osservati a s. carlo, frazione di s. agostino (provincia di ferrara); http://ambiente.regione.emilia-romagna.it/geologia/ temi/sismica/liquefazione-gruppo-di-lavoro/rapporto_ sancarlo.pdf scrocca, d., e. carminati, c. doglioni and d. marcantoni (2007). slab retreat and active shortening along the central-northern apennines, in: o. lacombe, j. lavè, f. roure and l. verges (eds.), thrust belts and foreland basins: from fold kinematics to hydrocarbon systems, frontiers in earth sciences, 471-487. toscani, g., p. burrato, d. di bucci, s. seno and g. valensise (2009). plio-quaternary tectonic evolution of the northern apennines thrust fronts (bologna-ferrara section, italy): seismotectonic implications, ital. j. geosci., 128, 605-613. wells, d.l., and k.j. coppersmith (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, b. seismol. soc. am., 84, 974-1002. *corresponding author: alberto pizzi, università di chieti-pescara 'g. d'annunzio', dipartimento di ingegneria e geologia, chieti, italy; email: pizzi@unich.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. origin and coseismic effects of the emilia earthquakes << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice la stazione sismo-dinofl"rafica di sollillla«o o " (udine) a lunghissimo periodo « l. p. 2» (1'11" "l.p. 2" ~ei~mic·clinogr(jfj"ic ~t(j/iotl al soml'l(jgo (l'dine). provid"d lvi/h /n'n iwrizontul "omfj"nfl"l~ id/h u ver\" long nutllral periodi . .:\l liiwa'xi (*) ri~cvllio il 4 ottobre, 1972 riasscsto. si espongono \lcttagliabullcntc ](' rnrf\t[.('l'istirlh' 1lit'('rnniche 11e11n nuova postazione sislllo.l·li!lografie[\ installa(a in lilla ('a. l'cl'nn nei ]llesili \1clla l'entrale idl'oelcl.trira di solllplngo (cdine) -, c'osti· tllita da ulla ('oppi a di 11clhluli orizzontali avputi 11cl'indo proprio di o8l'illnzionc 81l]lcl'iol'c ngli 8-10 minuti. si riportmlo alellni esempi di registrazioni elinog-rafi('hc c sisillogmfirhc ottenute ('oli in nuova apllu!'e('('hif\tul'l!. su)diarl thc illcclwnieal ehamet.el'ist.i('s of thc new seismir-dinog-rnphi(' stntion sitllated in a gl'otto hy the ihlwcr sfation of solllplago (udine) nre expo8ed in (letnil. 'l'he 8et is ('olliposf'd by n f'ouple of iwl'j!zontal seismogra}jhs lwving a natuml pel'ìod of lliore t.han 8-10 illinut.es. sollle dinogm}jhie and 8eislllogmphic. l'e('olhlings are also {·ndosed. tutte le caratteristiche meccaniche (lei nuovo strument.o sono am]lliament-e illustrate nelle figg. 1, 2, 3, e nella tabella a. si trat.ta in sostanza (li una eopl)ia (li clinomet.ri (li gralhli (limensioni (10 yolte più grande dei clinomet.ri comunemente ukat,i ]lei' le osservazioni geo(linamiche (ielle gralhli (lighe di sharramento), dot.ati inoltre di smorzat.ore ad olio e di aml)liflcazione otticomeccanica. j~a necessità (li far layora.re tale appa.reechiatum in amhiente a t.emperat.ura. costante e non influenzat.o dalle variazioni termiche '" e.n.e.l. compartimento (li vcnezin, ~iestlte. ~ ')i t,i' i'ih' ( .. i a ,ii "nll' du' s loaj!i ull a-li l, d 1m i u d o t ti al i iih i, '!m : l a f'ota ziu lw liililll i l·a n l l'i ll l.l'rllu ili 1111 :\ c:uv(\ r lla l"ie:\ va,l';t ih'ii:. mnn t:\ g n a, i li c'_ili il silltata la cc'ji1.mlc' idrop ]pu,ri,'a di n(jmpla~o (fig. 1 ), (~lti, l.:tlil\ l< li\u, "'md l .oiigjl,li.! i ll\' ehi. <l u ulll ~ln )';ìoll(' ( ir i.' " ( :1 ""'11 1 il tahc ll :, ,\ eu: 'n;~ tl 1:",~ 'fi(j1t 'r i v i p i';i sr.~~ln(:r(.\n jo:m' ij<·m,;'.nc a ) i\ lah · .. ia l., .1..1 li)" iiha l h lul1glu'u,h .!1li fil" ( li( u() ]li:tlnl ' ll'" di,l lilo (mll.) ~l"dlll " .li l'igilliui (dilll,i"m') ilihi:li1y,u at l,:i"('" lil (i · :l 'iiw (ii ]'(,iu . y, i ..... ( /h m) s"~i"'n~;o"r lo ) ~iat!'l'i:th ' d !'l ii i " u~al , ,, 1,1111 1.(11<'1.1.« di,i lilo ( li.li !) iliallll'll''' .ipi filo (111m) ;\100111 10 oli r i l-(illi( , i~ (.! i ll(' ."· iii~ ) i l i ~ i :t il )';: 1 li 11: .. ·.·o l i1" .:/i\iii' .i i 1'" l a· zi ... ,, · ( 11 11 " ) . ;i ii/ii p o "' i /lill' iii uwhilll /' ~laj(, .. ia'" u~alo lull g li':l,1. :t 1'lil1ljll"mil"il ( /hm) il i 1l 111('(,ro (111 111 ) 1',"1( ' ( kg) .... suppllrl .. l 'w''~ti}' luliglo f'1.1.n ( 111111) 1"'sii ("i:) j/ii i1"" n li llle ti) :11111 ,, 1/' dl'i1l i·" i,i ( ).i"'>i_ro d . i h,, ! ridli'.~],eaatll'" ii.os 111 m ,lia . llh'('i"" :!;h ill ill ) ili (1'1"1'11 da :! l,p:. l" ~ l nl!1l(' il[.(1 )lu;' j"' !'lal"l' /ilill li j o jlia~i , 1"t, ilisl:\i11. 1\ l. ad"('!llro mll m t' ), ,inl . 1' 11_' . ii ru t m: iu llt: ( 111m ) ~(i" :lo' :lo " j 3u 1)3 ' .jo" i!i s tu 8.l. m. !'{/,i i'. j.su oi ('0111 ji , n· f:j ctlillp. j'; . \\, 1'1i1j~6 1.' "" 0 70 .l lo oaci :i: 0,01 ( t ,~ :1: 0,(1)10" 211(1 ::c 2 'fuii;r~h'lil1 000 ± io o,~(j ± n,i)! (l , ~ ± 1) ,01)10" 1'uhn f"rro flli ti ;i.~ j n 22 ::i-: 2 004(1:) ..:.. 0.u4 2fl. i 0 ± 0,(12 0, l 112 ~: oji~ (io(; ± io , , • • • • • ! i i l i t a )ii'lificazioxe sis~roguafo ]listanza attuale pllllta spingente ,iall'asse ,li rot, ,lei ]lc'n,jol" (mlll) (si pnò port"re lino" 550 llllll) distanza a tt uale punt" spinp;ente ,lall'assp ,li !"ota1.ione dello sj)('c, chietto {llllll} {si può portare tino a 3ii 1ll111} distanza attualo> spp('ehic'tto·rep;i. stratore (llllll) a~iplil'icazio:çe cl.i~og1{afo distanza spl'(·e]lietto (posto s1lli'"sse ,li 1'01.) l'ep;. (mm) . s~iorz.niexto ,\11 ouo rapporto di slilorz. nh., (~i puù por· tare f " .. ilmentc a \'alori pii! elevati) altbe c,\iutteiustlciie 100 ± 2 20 -r 0,2 1660 ..,.. 20 1140 ± 20 registrazione s1l ell'ta ~ensibile (veloeit:\ ,li seonimcnto ,ii 1 clll :'lllin pc!' i sismografi e di 2 clll,igiol"110 pcr i dinogmli). )!arcatelllpi o segnale r,\! ('ollegati alla locale sta1.ione sismira. 1','so colllples~ivo ,li op;ni struillento (tdaio pen,lolo) 500 kp; ea. infatti, (lat a b not.evole copertura eflterw\ della roccia oltre .j()() m è liullo ijlwlfliafli fi]hjflt.mnento ilei suolo dovuto all'insolazione. l'er i nuo"\·-i llc'ihloli, ('oillc' ri~ulta dalla teoria ,lei l)('ll(loli orizzontali (1) con sospensioni (ii tipo ziillncr, il massimo perio(lo ])!'ollrio ,]i oflcill a zione raggiun gibile è ili 2":i3"', fn realt à , non" si l'iel\ce ~~,l ottenere mia]'{'i!ist.razione venunente i\tabile qu a ndo il periodo proprio ,]i oscillazione (lei pell(lol i è ~ulleriore agli h-io minuti. tale limite oltre all'aziolle ili torsione ,ielle i\oi\])(,ii.~ioni è ,ktpi'mina·t.o da·lle seguent.i cauhe: (i) movimellti ([ovuti a·lle m a l'ee tel'l'el\t.ri, /j) vicinanza (ii ull a grande faglia ilei pressi ilei lago ili cavazzo [in movimento ~ollra·ttntto in eorrifl]hjllllenza a prel'ipitaziolli a tmosf('riehe (' )] , cl }ll'el\enza ilei la centmle idroelett.rica (i\i imnllo notevoli vari a zioni (ii ('arieo sull a !'occia. ,lelllia llo ha·se <lella centrale dovute 50 .\c, )!iuaio all'owilla.zione (ld livello dell'~\cqll a nel i)ozzo piezomdrieo ogni qllhlvolta si elteltm\ lina manovm ilelle ma.cchine), il) impos~ibi1it-à. ili eseguire i cambi ilei fogli (lei regislra tori senza ilifltlll'hare gli ~tnllnenti (l a stessa pre,~enza dell'ovemlore nei vre~si della fltazione (ietermina 111m indinazione ilei terreno e quindi uno spoflta.mento dei pen(loli, i qll ali "flegllono» l'operatore) . . ~ [ , ~ -id '" o, ' _.~ ~p o,~ r----d, ~ ,. ,.00' .~. n ,. ", o,' ", .• ,. ",. ~. ,.~. fig. 2 ljbicuzionc strumenti nella star-ionc scala l : 25. mentre le ('anse (li distl1l'bo h) e c) si possono elimin a re hemvli· eemente ubiea.jhlo la stazione in un luogo i)iù ad a tlo e la ea lls a d) (lotando la sta zione di un aplmmto ili regifltrazione a ili~tanza , la mmht\ a) ì~ ('ertamente la viù diffieile d~\ eliminare. come riwlta chi~u"o anelli.' (lalle fotografie (figure 4a, .1h, ,le, 'l(l), si ì~ fa·tto in mollo che la stazione funzion a sse eontempora ncamcntc flia ila sifllllogtafo ('he (la dinografo; inoltre tutta l'attrezzatura· ì' stata {'o~trui1,a in mo(lo d a i)oter cambiare, con molta faeilità e r a pillikl, le earatteristiche meccaniche più imvorla nti (lella eoppia (lei pelllloli orizzontali. j . • \ .~1· .\z1 0~e si~nn · ui.t~ogra!'ic.\ di ~o~ii' l.\ (.lo (ljdi ~e), rec. .'51 .: h . l "i1''' y • 01 p a ~, , ~ 'o=j \ (l'-: ~ ~ . re 1 11 o " • ; • ~~ ., 1 ~ ii.-°' 'o .11 j ~ i a ) se%. a a 1\ ., , i , .. :1 ;i \ vista d"w i1 jro b) \ ·0 ~ i~ \ 6 fig. 3 l':lr~ icol:lrj costl"llt tivi: a) sc !. oill .\ llcl per il1-\lo s{'.:lln l : 5 ; h) flehclli a st ruillen t o l.i'. 2 6" u111 l : 25. ii:! 'lo 111 (;'\s i f ig. ·ia f ig. "il f i g. -m xi>!la lig, :i ,~oilo rip0i'i ,ati aleulli e,~mnpi di l'egihtl'a:"jolll' hihlllogl'ali(:a, l' (:lillogm.!i(:a, oltt'lluti dalla hta:"jolle alw01'a iii fll~c hjj(,l'inwii lall', ili il li 0(, rafi a (') c,\!.oi l'., 1\1:.0. 11 pl'/h/olo orizzonlalf' colllc di/hjmdro, • annali ili geonhi('h >, iii, 4. ( ' ) (',\ih! l'., .\1](;,\:-;1 .\1., ioj2, ,1/ovi1lllmli fidia faglia del lago di cfwazzo, i,. trlaziu!ll' alht localf' cm/llftt di pioyyin, • annali {li geofisir·[\. " xxv, t. ---• "'" • -, \ o , , ! a " i i ' , • ,i i il ' ! oro \ rv riy' js o i f \ ,.j /'. o ! i i ~oo l la • r !!~;~!!('-:je~ ~ ~ ~ ! ~ ~ ji ~ " ,'f,. 5 c.." c,,:!. v j ,i i i i i j i = = -------------~~-----------------------------------------t h e a f t e r s h o c k s e q u e n c e o f t h e n o r t h w e s t k a s h m i r e a r t h q u a k e o f s e p t e m b e r 3 , 1 9 7 2 v . k . srivastava (*) e . k . s . c h o u h a n (*) e a j i v n i g a m (*) r e c e i v e d on a p r i l 2 5 t h , 1975 s u m m a r y . — t h i s p a p e r is a n a t t e m p t t o s t u d y t h e a f t e r s h o c k seq u e n c e of t h e n o r t h w e s t k a s h m i r e a r t h q u a k e of s e p t e m b e r 3, 1972. b v a l u e of t h e s e q u e n c e is 1.59. t h e a r e a of a c t i v e s t r a i n z o n e is a p p r o x i m a t e l y 2.9 • ]01 3 s q . c m . t h e t o t a l a v e r a g e s t r a i n , a v e r a g e e l a s t i c e n e r g y a n d a v e r a g e s t r e s s of t h e r o c k b e f o r e slip a r e 3.3 • 10~5, 3.2 e r g s / c m 3 a n d 19.8 k g / c m 2 . t h e s t r a i n r e b o u n d c u r v e of t h e s e q u e n c e h a s been c o n s t r u c t e d w h i c h s h o w s a d u a l t y p e of r e c o v e r y w h e r e t h e c o m p r e s s i o n a l e l a s t i c c r e e p is followed b y t h e s h e a r c r e e p r e c o v e r y . t h e r e l a x a t i o n t i m e of t h e s e q u e n c e is a b o u t 0.7 d a y , s h o w i n g t h e k e l v i n b o d y like b e h a v i o u r of t h e u p p e r m a n t l e . r i a s s u n t o . — q u e s t o s t u d i o è u n t e n t a t i v o di i n d a g i n e sulle r e p l i c h e del t e r r e m o t o del k a s h m i r n o r d o c c i d e n t a l e del 3 s e t t e m b r e 1972. il v a lore b della s e q u e n z a è 1.59. l ' a r e a della z o n a di t e n s i o n e a t t i v a è a p p r o s s i m a t i v a m e n t e 2.9-io 1 3 s q . c m . l a t e n s i o n e m e d i a t o t a l e , l ' e n e r g i a m e d i a t o t a l e e lo s t r e s s m e d i o della r o c c i a p r i m a dello s l i t t a m e n t o sono 3 . 3 i o 5 , 3.2 c r g s / c m 3 e 19.8 k g / c m 2 . l a c u r v a di r i s p o s t a della t e n s i o n e della s e q u e n z a c h e è s t a t a c o s t r u i t a m o s t r a u n d o p p i o t i p o di r i p r e s a in cui la d e f o r m a z i o n e e l a s t i c a c o m p r e s s i o n a l e è s e g u i t a d a l l a r i p r e s a di d e f o r m a z i o n e t r a s v e r s a l e . il t e m p o di ril a s s a m e n t o della s e q u e n z a è circa 0.7 g i o r n i , m o s t r a n d o c o m e il c o r p o di k e l v i n si c o m p o r t i s i m i l m e n t e al m a n t e l l o s u p e r i o r e . (*) d e p a r t m e n t of a p p l i e d g e o p h y s i c s , i n d i a n school of mines, d h a n b a d 826004, i n d i a . 1 4 0 v . k . s k i v a s t a v a r . k . s . c i i o u i i a n r a j i v n i g a m 1 i n t r o d u c t i o n the relocated epicenter of the n-w kashmir e a r t h q u a k e which occurred on sept. 3, 1972 at 16»48m31.64s gmt was at 35.95°n, 73.29°e. the unified magnitude of this shock was 6.2 and focal depth was 64 km. the aftershock which followed the main shock were distributed over an area extending from latitude 35.5°ii to 36.5°lsi and longitude 73°e to 74°e respectively. all the d a t a are recalculated with the help of computer. the unified magnitudes of the aftershocks used in the present study ranged from 4.0 to 5.8. 2 s e i s m i c i t y o f ist-w k a s h m i r r e g i o n kashmir region is very much seismically active due to syntaxial structure bend of the himalayas. the epicenter of earthquakes are considered to coincide with the main boundary faults and any movement along this fault may cause earthquake. fig. 1 shows the spatial distribution of the aftershocks of the september 3, 1972 with dislocation line and isostatic gravity anomaly map. the epicenters of the aftershocks of higher magnitude in the range of 4 to 5.9 are clustered in and around the main shock. a little north to main shock, the epicenters of the aftershocks are of magnitude below 4.0. but the distribution pattern of epicenters does not show any regular trend. however, the map clearly shows t h a t the epicenters of the aftershocks can be correlates with the dislocation line, presumably, the main boundary fault of the region, and they are concentrated in positive isostatic anomaly region. this association of positive anomaly shows t h a t the area is in isostatically over compensated. thus this may be a cause of shallow shocks in this region, antonio marussi ( u ). 3 f r e q u e n c y m a g n i t u d e a n a l y s i s . a well known relation between cumulative frequency of shocks and its magnitude is given by gutenberg and richter (9) in the form log10a 7 = a — b m [1] t i i e a f t e r s h o c k s e q u e n c e o f t h e n o r t h w e s t k a s h m i r 149 e p i c e n t r a l m a p f o r n w k a s h m i r l o n g i t u d e e f i g . 1 e p i c e n t r a l m a p of n w k a s h m i r of s e p t e m b e r 3, 1972 e a r t h q u a k e . the constant b of this equation is related to the tectonic structure of the seismic region. mogi (13) showed experimentally t h a t this constant b depends on the homogeneity of the material in the seismic region and on the distribution of applied stress. the value of b increases as the degree of heterogeneity increases and as the degree of symmetry of applied stress decreases. however, scholz (10) has shown t h a t value of b depends upon the state of stress rather than the heterogeneity of the material in a region. gutenberg and ricliter (9) have studied in detail the frequencymagnitude relation for shallow, intermediate and deep focus earthquakes. this relation has been also used by chouhan (5) to study regional characteristics. utsu (19), suychiro (17), chouan (6) etc., have shown t h a t the above relation is applicable to the aftershocks sequences also. utsu (10) has summarised the magnitude distribution of the aftershocks in and 140 v . k . s k i v a s t a v a r . k . s . c i i o u i i a n r a j i v n i g a m near j a p a n and some other regions. he found t h a t the value of b lies between 0.5 and 1.5. choulian et al (7) have also studied the magnitude distribution of " 6 " value for the aftershocks of the assam earthquake of august 15, 1950, and some other regions of india. they have found t h a t the b value lies between 0.5 to 1.1. papazaclios et al. (14) have observed t h a t the value of b decreases for the aftershock sequence in the vicinity of greece systematically with increasing focal depth of the main shock. i t is important to note t h a t there is practically no difference in the value of b obtained for the aftershock sequence and the earthquakes of the same region. a frequency-magnitude analysis for the aftershock sequence of the earthquake september 3, 1972 of nw-kashmir has been carried out here. the resulting graph is shown in fig. 2 and the values of a and b are computed using univariate least square procedure. the following relation is obtained log n = 8.97 — 1.59 m [2] b value has been also calculated by using utsu (21) maximum likelihood method, (m — mm i n ) where m is the average magnitude for the series and mmin is the minimum magnitude. the b value is found to be 1.5 by this method. the value of b for this sequence is rather large and this may be attributed due to low stress level in the region, scholz (10). f u r t h e r this high value of b is also due to the small magnitude range taken here. papazachos (i5) has shown t h a t the reliable b values are determined only if this range is large enough for seismic sequence. 4 s t r a i n r e l e a s e i n t h e a f t e r s h o c k s e q u e n c e to study the elastic energy release behaviour in this sequence a most popular method of benioff (2) has been applied. he has shown t h a t the potential energy j„ of a volume of rock v, possessing a coefficient of shear ¡x, strained an average amount s immediately before the earthquake is given by t i i e a f t e r s h o c k s e q u e n c e o f t h e n o r t h w e s t k a s h m i r 149 m a g n i t u d e <f r e q u e n c y m a g n i t u d e r e l a t i o n pig. 2 f r e q u e n c y m a g n i t u d e r e l a t i o n c u r v e for s e p t e m b e r 3, 1972 e a r t h q u a k e . and the energy of released seismic wave (j) is j = r,/x v es/2 [5] where rj is the fraction of energy released as seismic waves, the rest being consumed in heat and displacement work. if 7-j is one, then entire potential energy is released as seismic waves. thus for equation [5] strain s can be calculated if seismic energy j, the volume v and the coefficient of shear ¡a, are known. for calculating the energy, gutenberg richter's (10) formula is used log j = 5.8 + 2.4 m or log jw2 = 2.9 + 1.2 m [6] 140 v . k . s k i v a s t a v a r . k . s . c i i o u i i a n r a j i v n i g a m or j i / 2 = 10 (2.9 + 1.2 to) [7] fig. 3 is the elastic strain rebound characteristics curve for the aftershock sequence of sept. 3, 1972 earthquake. there the time t of the aftershocks have been reckoned from the time of the main shock. then the cumulative values of the calculated strain rebounds s — 2 ¡ j11'2 are plotted against "z" which gives the accumulated strain rebound (times a constant c) increments of the aftershock sequence. this sequence consists of two segments of recovery, one linear which can be represented by the equation li ji/2 = a + b log t [8] a and b being constant followed by a non-linear segment which can be represented by the equation 2 ¡ ji/2 = a' + b' ( l — e~a í l / 2) + c t? [9] where a', b', a, c and /3 are constants and t is wave period. this is a case where the compressional creep recovery is followed by a shear creep recovery. compressional phase commences immediately after the main shock and the shear phase starts a few hours after the main shock. these interpretations are based on the empirical results obtained by griggs (8) for rooks under compression and those of michelson's (12) for rocks under torsional stresses. accordingly compressional creep recovery is caused by t h e elastic after working of the compressional stresses and the shear creep recovery also results by the elastic after-working of the shearing stress. the release of strain in two phases, compressional and shear has also been noticed by benioff (3) bath and duda (') and chouhan (°). the character of the strain rebound increments with time in fig. 3 strongly resembles with the strain release behaviour of kelvin solid. the maximum s reached in fig. 3 corresponds to the unstrained portion of the kelvin body, the zero value of s to the loaded position. eventually the curve is not expected to come to a complete levelling off, as in addition to the strain release, it must be expected t h a t a constant build up of stress occurs at a slow rate which, presumably, never comes to stop. thus after a few years one would expect the same pattern to repeat itself. thus the present behaviour of strain release can be explained in terms of elastic afterworking of kelvin solid. t i i e a f t e r s h o c k s e q u e n c e o f t h e n o r t h w e s t k a s h m i r 149 3 0 0 x 1 0 e r g s 2 0 0 w 100 1.0 t i m e i n d ay s 10.0 5 0 . 0 f i g . 3 a c c u m u l a t e d elastic s t r a i n r e b o u n d i n c r e m e n t s (time c) of t h e s e p t . 3, 1972 e a r t h q u a k e of n w k a s h m i r . the time in which the strain drops to i j e times the initial value, called relaxation time, in this case is about 0.7 day. this relaxation time is also equal to the ratio v/fi, where ¡i is the coefficient of rigidity and v is the coefficient of viscosity. 5 s t r a i n characteristics of the focai, region wilson (22) plotted aftershock epicentres to delimit the area of the strained zone involved in the nevada earthquake of 193g. since 1 4 0 v . k . s k i v a s t a v a r . k . s . c i i o u i i a n r a j i v n i g a m then the procedure lias been utilised by many workers such as benioff (3), tocher (18), chouhan (5) etc. further, by multiplying the area so obtained by the depth range of aftershock foci, the approximate volume of the focal region can be obtained. other parameters can also be calculated from a knowledge of the energy released in the main shock and its sequence as follows. the spatial distribution of the aftershocks of the northwest kashmir earthquake of t h e september 3, 1972 is shown in fig. 1. the distribution of aftershocks define a rectangular p a t t e r n which is perhaps t h e strained zone having the fault in the middle of the zone. benioif (4) has described m a n y such cases where the distribution of aftershocks have lead to the delineation of the faults. the length of fault break in the present investigation is about 00 km and a width of the strained zone is about 44 km after applying a correction of the location of aftershocks. thus the aftershock area is about 2.9 • 1013 sq.cm. we assume t h a t the average depth of the strain region is 50 kms, since the foci of many aftershocks were found down to this depth. the total volume of the strained rocks is v — 1.45 • 1020 cm3. the seismic wave energy of the principal shock is j = 4 . 7 • 1020 ergs. this gives an average elastic energy density of 3.2 ergs/c.m3. assuming /i = 6 • 1011 dynes/cm2 at this is focal depths the value of the coefficient of viscosity (v) or better the mobility of the medium at this focal depth is of the order 3.6 • 1017 gm/cm/sec. from formula [5] of section 4 i.e., £2 = 2 j / f i v , [ 1 0 ] assuming rj = 1 and putting a = 6 • 1011 dynes/cm2. the elastic strain preceeding the principal earthquake is thus e = 3.3 • 10-5. the total strain release upto the end of the sequence is proportional to 2 j112 = 2.86 • 1010. for principal earthquake corresponding quantity is j 1 ' 2 = 2.1 • 1011. the elastic stress just before the fracture is roughly a = e[i = 6 • 1011 • 3.3 • 10"5 dynes/cm2 = 19.8 • 10° dynes/cm2 = = 19.8 kg/cm2. if "z" = width of the aftershock region, the total relative slip during the principal shock is x = e • i = 3.3 • 10~5 • 4.4 • 1011 cm = 145.2 cm = 1.45 meter. t h e a f t e r s h o c k s e q u e n c e o f t h e n o r t h w e s t k a s h m i r 1 4 7 this displacement is a rough approximation since e as used here represents the shear strain only very approximately. 6 c o n c l u s i o n . the study brings out the following results: 1) the magnitude of the aftershocks are distributed according to the gutenberg-richter magnitude frequency relationship with value of b equal to 1.59 which is rather large in comparison to main shock. 2) the release of strain occurs in two phases, the compressional and shear. the remarkable feature of this sequence is t h a t the recovery is almost shear creep recovery with a small amount of compressional creep recovery. the relaxation time for the shear phase is found to be 0.7 days. assuming rigidity /.i = 6 • 1011 dynes/cm2, the coefficient of viscosity, v, in the region is of the order of 3.6 • 1017 gm/cm/sec. 3) a) the aftershock area is 2.9 • 1013 cm2 and the aftershock volume is 1.45 • 1020 cm3. b) the elastic strain just before the main shock is 3.3 • h)-5. c) the strain release by the main shock is 2.1 • 1 0 n . d) the elastic stress just before the fracture is 19.8 kg/cm2. e) the displacement along the fault plane is 1.45 meters. a c k n o w l e d g e m e n t the authors are grateful to profs. j . sing'h and r. k. verma for discussions from time to time and for providing all the facilities to carry out the present work. junior author acknowledges the financial support from the c.s.i.r. scheme in the form of junior research fellowship. r e f e r e n c e s f 1 ) b a t i i m . , d u d a s . j . , 1 9 0 4 . earthquake volume, fault plane area, seismic energy, strain deformation and relation quantities. " a n n a l i <li geofís i c a " , 1 7 , p p . 3 5 3 3 0 8 . 3 140 v . k . s k i v a s t a v a r . k . s . c i i o u i i a n r a j i v n i g a m b e n i o f f h . , 1 9 4 9 . seismic evidence for the fault origin of oceanic depth, " b u l l . gool. soc. a m . " , 60. b e n i o f f h . , 1 9 5 5 . mechanism and strain characteristcs of the white wolf fault as indicated by the aftershock sequence. " c a l f . d i v . mines. b u l l . " , 177, p . 199. b e n i o f f h . , 1962. movement on major transcurrent faults. " c o n t i n e n t a l d r i f t b y s. k. r u n c o r n c h a p t e r 4, p . 103. a c a d e m i c p r e s s . c i i o u h a n r . k . s . , 1 9 6 8 . seismicity of india and some recent aftershock sequences. i'll. d . thesis, u n i v e r s i t y of r o o r k e e . c i i o u h a n r . k . s., g a u r v . k . a n d r a t i i o r h . s., 1 9 7 0 . aftershock sequence of the solomon island earthquake of june 15, 19g6. " p u r e and a p p l i e d g e o p h y s i c s " , 162. c i i o u i i a n r . k . s . , s r i v a s t a v a v . k . , 1 9 7 0 . global variation of b in the gutenberg richter's relation, log n = a — b m with, depth. " p u r e a n d applied g e o p h y s i c s " , 8 2 , p p . 124-132. g r i g g s d . , 1939. creep of rocks. " j o u r . geol.", 4 7 . g u t e n b e r g b . , r i c i i t e r c . f . , 1 9 5 4 . seismicity of the earth and associated phenomena. " p r i n c e t o n u n i v e r s i t y p r e s s " . g u t e n b e r g b . , r i c i i t e r c . f . , 1 9 5 6 . magnitude and energy of earthquakes. " a n n a l i di geofisica", i x . m a r u s s i a., 1904. geophysics of the karakorum. scientific r e p o r t , " g e o p h y s i c s i i " , i, i s t i t u t o di geologia, u n i v e r s i t à di milano ( i t a l y ) . m i c i i e l s o n a . a . , 1 9 2 0 . elastic viscous flow, part i i . " j o u r . geol.", 28. m o g i k . , 1902 b . study of elastic shocks caused by the fracture heterogeneous materials and its relation to earthquake phenomena. " b u l l . e a r t h . res. i n s t . " . 40, p p . 125-174. p a p a z a c i i o s b . , d e l i b a s i s n . , l i a p i s n . , m o u m o u l i d i s g . , p u r c a r u g., 1907. aftershock sequences of some larger earthquakes in the region of greece. " a n n a l i di geofisica", r o m e , x x , p p . 1-93. p a p a z a c i i o s b. c., 1974. dependence of the seismic parameter b on the magnitude range. " p u r e a n d applied g e o p h y s i c s " , 112, p p . 1059-1005. s c h o l z c . m . , 1 9 6 8 . the frequency-magnitude relation of microfracturing in rock and its relation in to earthquakes. " b u l l . seism. s o c . a m . " , 5 8 , p p . 3 9 9 4 1 5 . s u y e i i i r o s . , 1964. an example of fore and aftershock sequences and difference in the relation between magnitude and frequency of occurrence between two sequences. u.s. j a p a n c o n f e r e n c e on e a r t h q u a k e predict i o n , 53. t o c h e r h o n , 1 9 5 9 . seismographic results from the 1957 san francisco earthquake. " s p e c i a l r e p o r t , calif. d i v . m i n e s " , 61. u t s u t . , 1961. a statistical study on the occurrence of aftershocks. " g e o p h y s i c s m a g . " , 30, p p . 521-605. t i i e a f t e r s h o c k s e q u e n c e o f t h e n o r t h w e s t k a s h m i r 1 4 9 (20) utsu t . , 1964. characteristics of aftershocks in space, time and magnitude. u.s. j a p a n conference on e a r t h q u a k e p r e d i c t i o n , 59. (21) l r , r s u t . , 1965. a method of determining the value of b in a formula log n = a— bm showing the magnitude frequency relation earthquakes (in japanese with english abstract). " g e o p l i y s . b u l l . " , hokk a i d o u n i v . , 13, p p . 9 9 1 0 4 . ( l ! ) w i l s o n j . t . , 1 9 3 6 . foreslioclcs and aftershocks of the nevada earthquake of december 20, 1932 and the park field earthquake of june 7, 1934. " b u l l . seis. soc. a m . " , 26, p . 189. r i s u l t a t i d i u n a n n o d i m i s u r e e s e g u i t e a p a v i a , a c i e l o s e r e n o , c o n i l d o s i m e t r o p e r l ' u l t r a v i o l e t t o g . aliverti g . pannocchia dall'ottobre 1944 al settembre 1945 è stato possibile eseguire a pavia, presso l'osservatorio geofisico, una serie di osservazioni con il dosimetro per l'ultravioletto, osservando ogni ora circa, durante le giornate perfettamente serene verificatesi in quell'epoca; in totale durante 41 giorni. il dosimetro, di proprietà dell'istituto di fisica terrestre e geodesia dell'università di torino, è un apparecchio della i. g. farbenindustrie aktiengesellscbaft a tubo di quarzo, ultimo modello, ed è molto diffuso per scopi climatici e bioclimatici; le sue indicazioni si possono ritenere esatte entro il 10%. lo scopo che volevamo raggiungere era appunto quello di precisare uno dei caratteri del clima di pavia che finora non era stato preso in considerazione. per ogni misura eseguita è stata calcolata la corrispondente altezza di sole e tutte le misure di un mese sono state rappresentate, in funzione dell'altezza del sole, sopra un unico foglio millimetrato, così clie per ciascun mese si è poi potuto tracciare, f r a i punti osservati, una linea rappresentante la media intensità della radiazione uv (diretta -fdiffusa) segnalata dal dosimetro, in funzione dell'altezza del sole. una unica ascissa è stata attribuita a intensità osservate nella mattina o nel pomeriggio con la medesima altezza di sole; e ciò in considerazione del fatto che il grado di precisione delle misure non è molto elevato come già si è detto. l'insieme dei risultati è sintetizzato dai numeri della tabella i, numeri letti sulle curve medie suddette. dalla tabella i si ricavano gli andamenti annui per altezza di sole costante, leggendo i numeri delle colonne verticali, e gli andamenti mensili per altezza variabile, leggendo i numeri delle righe orizzontali. ciascuna curva media mensile è stata planimetrata per calcolare l'energia diurna totale, relativa a ciascun mese, della radiazione uv 2 0 8 c . a l i v e r t i c . p a n n o c c h i a giungente al suolo a p a v i a ; la fig. 1 rappresenta in unità arbitrarie i risultati del calcolo in questione; essa indica un valore massimo, della energia uv ricevuta, in corrispondenza al mese di maggio. ma a proposito di questo andamento della curva dell'energia è da notare clic al variare dell'altezza del sole si sposta il baricentro dello spettro uv giungente al dosimetro. e allontanandosi di poco da a = 0,3 |i verso valori maggiori di lamda, l'energia portata dalla radiazione solare diventa anche 100 e più volte maggiore e quindi il massimo dell'enert a b e l l a i intensità della radiazione uv in unità arbitrarie (anno 1944-45, pavia) a l t e z z a d e l s o l e 1 0 ° 1 5 » 20" 25" 30" 3 5 ° 4 0 " 4 5 " 50" 55" 60" 65" g e n n a i o 2 , 7 3 , 9 5 , 2 6 , 3 f e b b r a i o 1 , 4 2 , 4 3 , 6 5 , 3 7 , 4 8 , 4 m a r z o 1 , 0 1 , 8 3 , 0 4 , 6 6 , 6 1 0 . 1 1 5 , 8 2 1 , 4 a p r i l e 1 . 0 1 , 6 2 , 6 4 , 4 6 , 6 9 , 6 1 4 , 1 2 0 , 4 2 8 , 4 3 2 , 8 m a g g i o 0 , 8 1 , 2 2 , 4 4 , 0 6 , 8 9 , 4 1 3 , 1 1 7 , 6 2 2 , 6 2 8 , 7 3 4 , 4 3 7 , 7 g i u g n o — 1 , 3 2 , 4 3 , 6 5 , 9 8 , 6 1 2 , 2 1 6 , 4 2 0 , 7 2 4 , 4 2 7 , 0 2 8 , 6 l u g l i o 0 , 5 1,4 2 , 8 3 , 9 5 , 8 8 , 3 1 1 . 1 1 4 , 6 1 8 , 2 2 1 , 8 2 5 , 5 2 7 , 4 a g o s t o 0 . 8 1 , 8 2 , 8 3 , 8 6 , 0 9 , 2 1 3 , 4 1 7 , 8 2 2 , 8 2 7 , 4 2 9 , 6 s e t t e m b r e 1 , 4 1 , 7 2 , 6 4 , 7 7 , 8 1 0 , 8 1 3 , 1 1 4 , 5 1 5 , 4 o t t o b r e 1 , 4 2 , 1 3 , 7 5 , 4 7 , 6 9 , 3 1 0 , 4 n o v e m b r e 1 , 8 2 , 2 3 , 3 4 , 3 m e d i a 1 , 3 1 , 9 3 , 1 4 , 6 6 , 6 9 , 3 1 2 , 9 1 7 . 5 2 1 , 3 2 7 . 0 2 9 , 1 3 1 , 5 già dedotto dalle misure in questione è di complicata interpretazione; inoltre va tenuto presente anche l'andamento della curva di sensibilità del dosimetro che cade rapidamente verso zero nei pressi di 1 = 0,32 u. dai dati sperimentali non è possibile il calcolo di un fattore di trasmissione non soltanto per le ragioni ora citate ma anche perche i l dosimetro non misura soltanto la radiazione diretta ma anche la diffusa e per piccoli valori di h soprattutto il calcolo non ha senso; tuttavia, per le altezze maggiori, per le quali è da ammettere che l a r i s u l t a t i di u n anno di m i s u r e e s e g u i t e a pavia 2 0 9 radiazione diffusa sia piccola in confronto della diretta, questo calcolo indica chiaramente un massimo della radiazione uv di lunghezza d'onda molto corta nei mesi primaverili, soprattutto in maggio poiché soprattutto in quei mesi e non d'estate si ottengono, per altezze del sole sopra i 40°, i valori più bassi del fattore di trasmissione e precisamente dell'ordine di 0,05. a proposito di questi valori è da ricordare che per washington, coblenz e stair danno 0,06 per x = 0,300 (x e 0,12 per a= 0,305 [i. nella figura 2 sono rappresentati graficamente gli andamenti lun200 1 5 0 100 5 0 0 < / \ / / / / / / \ \ \ \ s n s s / / / / / n \ \ \ \ \ v / / / / / / \ \ * \ \ \ \ \ / f \ \ < \ \ s \ n meìi 6 f m a m g l a 5 0 n d f i g . l go l'anno della intensità della radiazione uv a pavia per altezze di sole 20°, 30°, 40°, 50° e 60". come si vede la intensità della radiazione misurata con il dosimetro, per altezze superiori ai 40°, ha un minimo marcato in corrispondenza al mese di luglio (1945). per i valori elevati dell'angolo di altezza, per i quali evidentemente la radiazione diffusa è meno importante in confronto di quella diretta, quel minimo di intensità non ha certamente relazione con i l massimo del contenuto di ozono degli alti strati atmosferici; infatti se si guardano gli andamenti annui del contenuto di ozono misurati dal 1939 al 1943 (*) nelle stazioni di arosa, potsdam, aalirus e tromso, una simile cor2 1 0 g. a l i v e r t i g. p a n n o c c h i a rispondenza non si trova poiché i l massimo nel contenuto di ozono si verifica secondo quelle misure, come pure secondo quelle di dohson, nei mesi primaverili. a spiegare il disaccordo in questione probabilmente valgono ancora le considerazioni esposte prima. è tuttavia da segnalare il fatto che i risultati di recenti misure di radiazione solare uv diretta, fatta a locamo monti, con cellula fotoelettrica al cd (a = 0,288 —0,366 n), f r a l'ottobre 1945 e la fine del 1947, danno pure andamenti delle intensità uv con l'altezza del sole che poco 3, u:a 3 0 25 2 0 15 10 0 mesi 6 \ \ \ \ \ t \ \ \ \ \ \ \ / / / / / 6 0 ° \ \ n s / / \ \ \ \ s n / \ \ \ \ \ \ 50° * / / s' v n \ > 4 0 ° 3 0 ° * * 2 0 ° * * 2 0 ° m m n f i g . 2 hanno a che fare con gli andamenti del contenuto di ozono citati prima. d'altra parte anche altre misure di intensità dell'c/f solare al suolo eseguite con la cellula a cd (campionata a davos) ed elabor a t e da f. lindhohn ( 2 ) indicano per ciascuna altezza del sole fino a 50° (misure svizzere) un minimo della radiazione durante l'estate, come appunto risulta dalle nostre misure con il dosimetro. le misure calcolate da lindliolm sono relative ai periodi 1916-18 e 1921-28. nella sensibilità, dei diversi strumenti in questione, per lunghezze d'onda sensibilmente maggiori di 0,30 li si dovrà verosimilmente trovare la r i s u l t a t i di u n anno di m i s u r e e s e c u i t e a p a v i a 2 1 1 spiegazione dell'andamento annuo della intensità uv misurata sia dal dosimetro sia dalla cellula a cd. napoli —• istituto universitario navale. istituto naz. di geo fisica — osserv. di pavia — febbraio 1951. riassunto si pubblicano i risultati di un anno di misure eseguite a pavia, a cielo sereno, con il dosimetro per l'ultravioletto e si mette in rilievo come le nostre misure, d'accordo con quelle analoghe di altri sperimentatori, denuncino un andamento annuo della radiazione uv che non si accorda con l'andamento annuo del contenuto in ozono osservato dalle stazioni che seguono regolarmente tale contenuto. si mettono pure in evidenza alcune condizioni sperimentali che potrebbero intervenire nel determinare il disaccordo. b i b l i o g r a f i a (!) bericlite des deutschen wetterdienstes in der us-zone. n . 1 1 , p a g . 3 5 , fig. 2 9 , 1 9 4 9 . ( 2 ) gutenberg v . : handbuch der geophysik. v o i . v i i i , fig. 2 , p a g . 3 1 3 , tal). 4 5 , e d i z . 1 9 4 3 . on a possible mechanism of alpine debris flows g . breitfuss a . e . scheidegger ( * ) received on december 7th, 1973 summary. — t h e phenomenology and previous mechanical theories of alpine debris flows are reviewed. a new model for the mechanics of such debris flows is proposed which is based on the notion of dispersive pressure occurring in shear flows introduced b y bagnold. i t is shown that the values of the dynamical variables required b y this model are of the order of magnitude of those observed in nature. riassunto. — d o p o aver passato in rassegna e riesaminato la fenomenologia e le precedenti teorie meccaniche sul trasporto di detriti alpini, gli a . a . propongono un nuovo modello per la meccanica di tale trasporto basato sul concetto di pressione dispersiva introdotto da bagnold. viene dimostrato che i valori delle variabili dinamiche richiesti da questo modello sono dell'ordine di grandezza di quelli osservati in natura. 1. phenomenology alpine debris flows (in german "mnrgang") are mixtures of water, fine materials, sand, gravel, large blocks and pieces of vegetation which move downstream with great velocity in an existing river channel. they develop only under special conditions after particularly severe rainstorms. a region may be spared the experience of a debris flow for a hundred years or so, then suddenly an ordinarily tame mountain streamlet becomes "activated" by such a flow and causes great damage. individual debris flows have been described on many occasions starting with stiny (5) in 1910. the present writers also had the opportunity to inspect several debris flows themselves. ( * ) institut f ü r geophysik, technische hochschule, w i e n , austria. 48 g. i î r e i t f u s s a . e . sc h e i d e g g e r thus, a typical profile of a mountain stream prone to conducting debris flows consists of three regions: a region where the debris material is collected, a region where it moves downhill in a gorge, and a region where it is deposited in form of an alluvial blanket. as an example, we show here the drainage area of the dürnbach in the pinzgau in austria (pig. 1) and the corresponding profile (fig. 2). fig. 1 drainage area of a typical mountain stream prone to debris flows (diirnbach in the pinzgau in austria). similar conditions are found in other cases. thus, the region where the material is collected, is usually formed by steep alpine meadows intersected by single rock spurs ending in a gentle bottom; unstable slide areas contribute the debris material (fig. 3). the gorge area is narrow and steep (fig. 4), the area of deposition is generally conical on a p o s s i b l e m e c h a n i s m of a l p i n e d e b r i s f l o w s 4 9 (alluvial cone) where successive "blankets" of debris are deposited by the individual debris flows (fig. 5). the total amount of material transported in a single debris flow may exceed 100,000 m3 (3). the velocity is of the order of several m/sec and may reach 20 m/sec after overcoming an obstacle i5-3'1). the concentration of solids transported in the water has been estimated as 50-80 per cent, of volume. the bulk density of the debris flow h t m ) f i g . 2 p r o f i l e of the course of a mountain stream prone t o debris flows (dürnbach in the pinzgau in austria). during motion is thus about 2.6 g/cm3. the grain size distribution reaches from the finest of materials to rocks several meters in diameter. the material in the deposited blankets is barely or not at all graded; large blocks are usually embedded in fine materials. the orientation of the debris, however, is preferentially with the largest diameter of the individual grains (blocks) in the direction of the flow. 2. previous theories the theories purporting to explain the mechanics of debris flows have generally been based upon analogies with bed load transport 4 5 0 g. i î r e i t f u s s a . e . sc h e i d e g g e r in rivers. a review of these theories has been given by scheidegger (4). however, it should be noted that it is somewhat doubtful that such analogies are pertinent for alpine debris flows, inasmuch as bed load transport appears to be primarily a drag phenomenon whereas alpine debris flows seem to represent the movement of the mixture of solids and water by itself. thus, an analogy with turbidity currents at sea or powder snow avalanches rather than one with bed load transport may therefore f i g . 3 unstable slide area in the collection region of a mountain stream prone to debris flows (raseck bach in eastern t y r o l , austria). be more pertinent for alpine debris flows. however, such analogies presuppose a rather low density of the solid material inasmuch as it is sustained turbulence of the carrying medium which keeps the solids in suspension. evidently, the mechanism of debris flows must be somehow intermediate between bed drag and suspension. a mechanism that has been but little explored in this connection is the motion of fluidized solids. 3. bagnold's dispersive pressure a type of fluidization mechanism that may be operative in alpine debris flows was described by bagnold (2). thus, assume that a dison a p o s s i b l e m e c h a n i s m of a l p i n e d e b r i s f l o w s 5 1 persion of spheres is being sheared (fig. 6). under these conditions, a "dispersive pressure" wi]l become operative normal to the shearing direction because of the collisions between the grains (inertial regime) or because of the influence of the grains on the flow patterns of the fluid around neighbouring grains (macroviscous regime). in the inertial regime, this pressure p is p = cu a x f{x) d 2 (d u/dy)2 cos a where a is the density of the grains, d their diameter, and dtjjdy is the shearing velocity. the quantity x is a measure of concentration (co/c)1'3 — 1 fig. 4 gorge region of a mountain stream prone to debris flows (niedernsiller muhlbach in the pinzgau, austria). 5 4 g. b r e i t f c s s a . e . s c i i e i d e g g e r i t is the dispersive pressure p which counteracts gravity and thus holds a mixture of water and debris in a fluidized state. here, at least, is a mechanism which can explain the phenomenon of suspension. i n turbidity currents, the suspension is provided by the prevailing turbulence; in the " d i s p e r s i v e " flow case, by the grain collisions and the effect of neighbouring grains upon the viscous flow regime in their vicinity. turbulence, if any is present, is rapidly damped away and has no effect in high grain concentrations. 4. a p p l i c a t i o n to a l p i n e d e b r i s f l o w s a n y avocation of bagnold's dispersive pressure as a possible mechanism of suspension and fluidization in alpine debris flows must entail an analysis of the orders of magnitudes of the dynamical quantities involved. the debris flow, accordingly, would encompass two types of materials: the continuous phase consisting of the water including the fine suspensions of viscosity rj and density p; — and the dispersed phase consisting of the large debris of median diameter d, density a and volume concentration c. then, the motion occurs as sketched in fig. 7. the shearing stress g parallel to the slope at distance y above the bed, due to gravitational forces is the dispersive pressure p must overcome the underwater pressure if of the grains a bed f i g . 7 g e o m e t r y of the motion in a debris flow. g =[g + (a—q) c] g (li — y) sin ,3. w = [a — q) eg (h — y) cos /?. on a p o s s i b l e m e c h a n i s m of a l p i n e d e b r i s f l o w s 55 motion is possible if the ratio g/w clue to gravitational forces equals the ratio tjp clue to the dispersive stress. thus t . g q + (a — q)c — = tan a — — = tan p. p w (a — q) g setting the shearing stress t due to the dispersive stresses equal to that due to gravitational forces g yields in the inertial regime 0.0128 a (ad)2 2 = [q + (a — q) c] g (h — y) sin 0 and in the macro-viscous regime (1 + a ) ( l + r, = [q + ( a q ) c]g (h — y) sin £ . this can be checked against the observations. w e use o = 1.6 g/cm3 (of the basic slurry), a = 3.0 g/cm3 and c = 0.70 i.e. a = 53.4. w e obtain in the inertial range au d y q + ( a — é?) c x • o 1 — q (h —• v ) sin ti —r — 0.0128 a ' a dyq + (or — q)c (a d y 0 . 0 1 2 8 ^ ••<*-« v t r i cot2 /? q + (a —q) c g {h—y) . / 0.0128 or (a d y / 1 + q + (a —q)c (a — q)c cot2 a which yields with the values for q, a and c given above and tan a = 0.32: = 0.053 ay d a t the surface, this yields for u after integration i/n 2 u = 0.0533 — hw. u ó 5 6 g. i î r e i t f u s s a . e . sc h e i d e g g e r l e t us take h = 3 m, d = 10 cm, g = 9.81 m/sec2. then one obtains for the surface velocity u u = 5.78 m/sec which shows that the range of velocities observed in nature certainly falls into the theoretical range. one can check whether the flow regime is indeed the inertial one. the condition is given by n > 150. with the values found above one obtains n = (300 — 2/)i/2. rj the question, then, is to insert a reasonable value for the viscosity 77. for water, rj = 0.01 cgs units; however, the viscosity is required not of the water but of the slurry of density o = 1.6 g/cm3 which is supporting the coarse debris. i t is difficult to make meaningful estimates of this viscosity, but since the fine material in the slurry moves essentially in suspension, its viscosity cannot be much greater than about one order of magnitude above that of water. setting r/ = 0.5 cgs yields n = 732 (300 — yyi\ i t turns out that n > 450 for y < 299.62 cm. this indicates that the assumed flow regime is indeed realized over (almost) the whole depth of the flow. thus, the model of alpine debris flows proposed here is a reasonable one. on a p o s s i b l e m e c h a n i s m of a l p i n e d e b r i s f l o w s 57 r e f e r e n c e s (!) aulitzky h., 1970. der enterbach am 26.7.1969. wildbachu. lawinenverbau, h e f t 34, s. 31-66. (2) bagnold r. a . , 1954. experiments on a gravity free dispersion of large solid spheres in a newtonian fluid under shear. " p r o c . r o y . soc. lond o n " , ser. a . , 225, p. 49-63. (3) hofmann f., 1972. die rochwasserlcatastrophen im oberpinzgau in den jahren 1970 und 1971. wildbachund lawinenverbau, sonderlieft (märz), p. 3-51. (4) scheidegger a . e., 1974. physical aspects of natural catastrophes. elsevier, amsterdam. (6) stiny j., 1910. die muren, verlag der wagner'schen univ. bibliothek, innsbruck, p. 139. 48,06,2005misc 989 annals of geophysics, vol. 48, n. 6, december 2005 key words shift doppler – ionosphere – plasma drift 1. introduction the measurement techniques of drift doppler follow the ionospheric vertical sounding where the return signal is analysed in frequency domain. a wave carrier of angular frequency ω is sent toward to the ionosphere where it is reflected. in the frequency range of 2-15 mhz the solid beam angle of the employed antenna is very wide and the gain as low as 1-3 db depending on the frequency so that an area of hundreds of square kilometers is illuminated. because of the rippled ionospheric surfaces and the volume inhomogeneities the signal is reflected back from various point sources that satisfy the reflection law. these point sources if moving are considered doppler sources that superimpose a doppler shift (∆ω1, ∆ω2, ... ) on the signal. in first approximation the angular pulsation shift ∆ω is (1.1) where v is the radial velocity of the reflecting point, c is the light velocity. dividing by 2π both side of the above equation and replacing ω with c⋅k and 2π f we obtain (1.2) where, f is the frequency, ∆fd is the frequency shift due to the doppler effect, and k wave vector (bibl et al., 1975; hunsucker, 1991). the moving source (s) assigns a shift in frequency according with the above equations and in the antenna we receive a composite signal with different ∆ωs that are the contribution of the s significant sources. in case of a single receiving antenna the spectral analysis (typically a complex fft) furnishes all the spectral components and the related phases and cannot discriminate the spatial distribution of the different sources. in case of 3 or more antennas it is posd∆ /kf v= r /v c2= -~ ~∆ ionospheric doppler measurements by means of hf-radar techniques cesidio bianchi (1) and david altadill (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) observatori de l’ebre, roquetes, spain abstract studies of the dynamics of the ionosphere and its related phenomena are mainly based on doppler drift measurements. the time variation (ionisation/recombination) of plasma density, thermospheric wind and others can be observed by means of hf-radars. the technique of doppler drift measurements is a quite complex technique that is now affordable by means of an advanced ionospheric sounder. the combination of vertical sounding and interferometric doppler detection discloses the doppler sources. the echo signal contains the doppler shift in frequency imposed on the wave carrier by each point source where the signal is reflected. other phenomena like environmental noise and the intrinsic error of the measurements that, together with the change in time of the refractive index, affect the measurements in various ways impeding to better quantify the results. mailing address: dr. cesidio bianchi, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: bianchi@ingv.it 990 cesidio bianchi and david altadill sible with a interferometric doppler technique to resolve the spatial distribution of the significant sources (davies, 1990). 2. principle of doppler interferometry in principle the interferometric doppler techniques rely on the frequency and phase analysis of the signal picked-up from spaced antennas lying in a plane at appropriate distance. in our description we refer to 4 antennas placed in the geometric barycentre and in vertexes of an equilateral triangle as in fig. 1. in order to better discriminate the phase contributions it is important to maximize the triangle size (more than 50 m) and to eliminate the phase ambiguity the distances between the antenna in the centre of the triangle must not be more than the wavelength λ employed in the sounding system. now, as in ionospheric vertical sounding, suppose a radio frequency (rf) pulse is sent to illuminate a large area, the various ionospheric moving isodensity surfaces reflect the rf signal with a superimposed shift doppler. a single reflector s contributes with an echo signal that in the receiving antenna 1 has an amplitude a given by (2.1) where, a01 and φ1s are respectively the maximum amplitude and the phase at the receiving antenna 1. for the generic antenna a the time varying amplitude will be (2.2) the phase term due to the source s in the antenna 1 is (2.3) where k is the wave vector, r1s is the oriented vector for the antenna 1 and the source s, and δ is the phase value at the level of the source s. if we assume as reference point antenna 1, the phase for the generic antenna a is (2.4) where la is the oriented vector from antenna 1 and antenna a. it means that the phase differences between antenna 1 and the generic antenna a is ks ⋅ la. it must be also noted that because of the distance the r1s compared with the distance between the antennas the vector ks has the orientation in all the antenna points. referring to fig. 2 the phase difference between antenna 1 and the generic antenna a is (2.5) where θ is the angle between ks and la. for a given source that the phase difference between the antenna 1 and the generic antenna a, according with the above equation, is a function of the angle θ between the vector k and l. it is worth noting that different doppler sources are distinguishable by different ∆ωs values (where s is the number of sources). for a given doppler source ∆ω, once the phase difference is measured knowing the range r, the wavelength λ and ( ) / ( )cos cosk k l ll 2as s a s a1 $ $= = =z i r m i k r k k llas s s s a s s a1 1$ $ $= + + +=z d z k r1s s s1 $= +z d ( ) .cosa a ta a s as0= + +~ ~ z∆7 a ( )cosa a ts s1 01 1= + +~ ~ z∆7 a fig. 1. the receiving antenna at the vertexes and at the geometrical barycentre of an equilateral triangle in an horizontal plane. all the antennas are a the distance l. 991 ionospheric doppler measurements by means of hf-radar techniques the distance l of separation between the considered antennas it is possible to calculate the angle θ and consequently the horizontal position in the ionosphere. each antenna a, neglecting the environmental electromagnetic noise, will receive a composite signal of amplitude a given by (2.6) where s indicates all possible sources. after the quadrature sampling the signal the two following discrete-time sequences ia (in-phase) and qa (in-quadrature) will be obtained (2.7) (2.8) where τn is the sampling time interval. if the sampling is performed at exactly the time period τn of the carrier wave ω, the sampling acts ( ) ( )senaq ( )a n a s s n as s 0= +x ~ x z∆7 a/ ( ) ( )cosai ( )a a s s n as s n 0= +~ x zx ∆7 a/ ( ) ( )cosa t a t( )a a s s as s 0= + +~ ~ z∆7 a/ like a filter rejecting the carrier ω and the two sequences will contain only the doppler shift ∆ωs. amplitude a(τ 1) and phase φ (τ 1) of the signal at a given time τ 1 are (2.9) . (2.10) the two discrete-time sequences ia and qa are the input of the algorithm the performs a complex fast fourier transform, fft (oppenheim, 1999). the fft of the n samples (where n is a power of 2) can be written as (2.11) where n is an index that runs from –n/2 to n/2, d is a dummy index to perform the operation, and fa(n) is . (2.12)( ) ( ) ( )f n i iqa a n a n= +x x ( ) ( )f d f n e / / a a i d n n n n 2 2 1 n 2 = $= r/ ( ) ( )arctan q i1 1a a= x xz 7 a ( ) ( )a i q1 1a a 2 2= +x x fig. 2. the signal from the same source is received by the two antennas with a phase difference depending on the source position. 992 cesidio bianchi and david altadill the spectral analysis (complex fft) furnishes d = n couple of values representing the discrete frequencies and the related phases. the frequency resolution is δω, being δω equal to 2π/t and t the time length of the segment of the signal considered (typically less than 20 s according with the time coherence of the ionosphere). the highest frequency is related to nyquist sampling theorem and the d ⋅ δω is the highest doppler shift range. it is worth applying a tapering function (hanning or others) to the discrete and finite time sequences to avoid the ringing six (x)/x after the spectral analysis. so as a result of the complex fft for each of the 4 antennas n spectral density values a and an associated time independent phase φ will be obtained. even if negative values of the frequencies have no physical meaning, in this case they represent sources that are moving in the direction of r while positive values of the spectral components are related to the incoming sources. these measurements are generally affected by environmental radio noise or interferences produced by other hf stations. so to evaluate the direction of k vector from eq. (2.5) a statistical anlysis must be performed (scali, 1993). 3. radial velocity and determination of the doppler sources in general the observed shift for a sounding performed at a known angular frequency ω, apart from a constant factor, is the time derivative of the phase path p of the radio wave . (3.1) the time derivative of the phase path depends on two terms (dyson, 1975) (3.2) where µ is the refractive index and α is the angle between the wave normal and the ray direction (null for isotropic medium), v is the real velocity of the reflector and pr the unit vector of the wave normal at the reflection point. hence the quantity observed is the apparent velocity, cos dt dp pr v t dr r 0 $ 2 2 = + n a# c dt dp 2 = ~ ~ ∆ vapp, because the time derivative of the refractive index has to be considered. if the electron density changes in time because of production or loss (especially during down and dusk) it is not possible to neglect the second term in the above equation. for that the measured velocities are not real, i.e. not only dependent on the moving reflector. when performing ionospheric drift measurements the second term contribution on the relation (3.2) must be considered and only in stationary electron density condition are the observed velocities of the doppler point sources related to the plasma motion. in this case, the radial velocity of each point sources comes directly from relation (1.1) if a uniform plasma motion is considered by means of statistical method (scali, 1995; bibl, 1998) it is possible to determine the bulk motion of the plasma (drift). the lowell digisonde 256 uses a least-square fit procedure described in (dozois, 1983) in which it is possible to determine the vector velocity v representing a plasma drift velocity. a similar procedure is also described in the digisonde drift analysis manual dda (scali, 1993) and makes it possible to solve the position of each point source and plot the results in a so-called sky-map. 4. conclusions in this paper the theory on which the interferometric technique relies is briefly described. the transmitting antenna has a wide beam that illuminates a large area of hundreds of square km in the ionosphere and the signals coming from different reflection points are resolved by combining fourier transform and interferometric techniques. the echo signal has a superimposed frequency doppler shift, hence the information of each point source where the signal is reflected. this technique is now affordable by means of the advanced ionospheric sounders that perform both on-line analysis and post processing on the recorded signal. the doppler drift analysis dda implemented as post analy/ .v c 2rs s= ~ ~∆ ionospheric doppler measurements by means of hf-radar techniques sis processing in the digisonde 256 and dps-4 determines the doppler shifts of signals arriving from different directions and, in some particular stationary conditions, measures the plasma drift velocity (bibl and reinish, 1978). references bibl, k. (1998): evolution of the ionosonde, ann. geofis. 41 (5-6), 667-680. bibl, k. and b.w. reinisch (1978): the universal digital ionosonde, radio sci., 13, 519-530. bibl, k., w. pfister, b.w. reinisch and g.s. sales (1975): velocities of small and medium scale ionospheric irregularities deduced from doppler and arrival measurements, adv. space res., xv, 405-411. davies, k. (1990): ionospheric radio (p. peregrinus london). dozois, c.g. (1983): a high frequency radio technique for measuring plasma drift in the ionosphere (university of massachusetts lowell center for atmospheric research, lowell, u.s.), rep. no. 6. dyson, p.l. (1975): relationship between the rate of change of the phase path (doppler shift) and angle of arrival, j. atmos. terr. phys., 37, 1151-1154. hunsucker, r.d. (1991): radio technique for probing the terrestrial ionosphere (springer verlag n.y.). oppenheim, v.a., w.r. schafer and j.r. buck (1999): discrete-time signal processing, 2nd edition (new jersey). scali, j. (1993): a quality control package for the digisonde drift analysis (dda), version 2.0 (university of massachusetts lowell, center for atmospheric research). scali, j., b. reinisch, c. dozois, k. bibl, d. kitrosser, m. haines and t. bullett (1995): digisonde drift analysis (university of massachusetts lowell center for atmospheric research). (received february 23, 2005; accepted august 29, 2005) 993 annals of geophysics, 55 (3), 2012: ferrari the mallet project: from an earthquake laboratory to laboratories on the territory graziano ferrari istituto nazionale di geofisica e vulcanologia, bologna, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5654 abstract on december 16, 1857, an earthquake wreaked havoc across a huge area of the regions of basilicata and campania (val d’agri and vallo di diano). more than 180 villages in an area of over 20,000 km2 suffered very severe damage. more than 6,000 houses collapsed or became uninhabitable, and 19,000 people were killed. in the months of february and march 1858, the irish engineer robert mallet undertook a mission to study the earthquake with a grant from the royal society of london. in 1862, he published his extraordinary study diary, in which the technical and scientific observations mingled with descriptive and literary details. this represents, from different angles, a unique ‘observatory’ over the landscape and the social, economic and cultural aspects of val d’agri and vallo di diano. in 2003, the provinces of salerno and potenza started up a joint venture with the storia geofisica ambiente of bologna and with the scientific partnership of the istituto nazionale di geofisica e vulcanologia, with a view to setting up provincial laboratories dedicated to environmental education, and named after robert mallet. the mallet project proceeded up to 2009 on three agreements. within the framework of the idea of a ‘mallet laboratories’ project, there has been the opportunity to set up a network of study centers in the territories of these two provinces (salerno and potenza), which can promote research with great national and international breadth, starting with the world of schools and local communities. the study entitled viaggio nelle aree del terremoto del 16 dicembre 1857 was published through the period from 2004 to 2009. 1. introduction on december 16, 1857, at around 10:15 pm, an earthquake devastated a vast area of basilicata and part of campania (southern italy). the present-day province of potenza and the central-eastern zone of the province of salerno were hit in particular. the most serious damage occurred in the mountain areas, and mainly in val d’agri and vallo di diano. the most deaths were recorded for the villages of montemurro, grumento nova, viggiano, tito, marsico nuovo and polla. as a result of this shock and the numerous aftershocks, including destructive ones that followed, more than 180 villages over an area of more than 20,000 km2 suffered very serious damage to buildings, to the extent that most of the houses had to be abandoned. in this area, more than 30 centers suffered disastrous damage; whole towns and villages spread out across an area of 3,150 km2 were razed to the ground. altogether, 3,313 houses collapsed and 2,786 became uninhabitable, and about 19,000 people died, with 90% of these in the potenza province (figure 1) [ferrari and mcconnell 2005, guidoboni et al. 2007, burrato and valensise 2008]. the irish engineer robert mallet was a witness and an extraordinary ‘scientific chronicler’ of the effects of this earthquake. on january 27, 1858, mallet left england to carry out research in the ruins of the mountain villages of basilicata, supported by a grant worth 150 pounds from the royal society of london. from february 12 to march 1,1858, he undertook a scientific mission to the areas most badly hit by the earthquake, drafting out an extraordinary diary in the process, in which his technical-scientific observations are combined with those that were more descriptive and literary (figure 2). an earthquake such as this one was the natural laboratory that mallet had been waiting for, for over 10 years, to be able to test his theory on earthquake dynamics that he had presented at the royal irish academy in 1846 [mallet 1848]. the maturity of this research is indicated by some of his conclusions, which gave advice to deepen the studies in three fields: 1) surveying of damage caused by earthquakes according to a new series of criteria, to arrive at the origin of the disturbance; 2) study of the velocity of the elastic waves; and 3) installation of observatories for the study of earthquakes in some of the more exposed areas, so as to obtain “complete data relating to the moment of transit of the wave, and the breadth and height of the waves in particular regions”. the originality of the scientific method implemented by mallet consisted in the systematisation and accurate fiarticle history received march 16, 2012; accepted april 16, 2012. subject classification: 1857 basilicata earthquake, environmental education, historical landscape, history of seismology, robert mallet. 461 nalization of observations of the earthquake effects, which had previously been used only partly with the aim of quantitative assessment. upon his return to england, mallet put his observations down in a weighty report, which he published in london in 1862, with the title: the great neapolitan earthquake of 1857. the first principles of observational seismology. with this title, mallet gave a name to the nascent science of earthquakes: seismology. however, this earthquake had another peculiarity, which was not a seismological one: it was the first earthquake where the effects were documented by photographic reportage, which consisted of over 200 photographs, most of which are graziano ferrari 462 figure 1. summary map of the area struck by the earthquake and involved in the mallet project. coloured patterns, black lines and black triangles represent the intensity distributions of the seismic effects (mcs scale), the path of mallet’s journey, and the locations of the geosites mapped during the project, respectively. 463 stereoscopic. between december 20, 1857, and may 1858, at least two photographers ventured into the territories devastated by this earthquake: alphonse bernoud, a famous french photographer who worked in naples from july 1856, and claude grillet. mallet commissioned photographic documentation of his study mission from grillet (whom he calls grellier in his report): 156 photographs (figure 3), 120 of which are stereoscopic and 36 monoscopic, which were used by mallet to document his own scientific speculations when they were published in his large report cited above. from different standpoints, this report by mallet on the 1857 earthquake also represents a unique ‘observatory’ on the landscape and the various social, economic and cultural aspects of the areas that were worst hit by the earthquake. in spite of the dramatic impact that this earthquake had on the territory, the contact with the population leaving there showed that the memory of this event was poor, or almost nonexistent, within the population of this region. in contrast, the irpinia earthquake of november 23, 1980 (m 6.9), was an emphatic reminder of this terrible event, even if it only ‘lapped’ the northern part of the area that had already been affected by the earthquake of 1857, with some further damage and destruction. the part of the territory of the provinces of salerno and potenza that was hit by the earthquake had been one of the few parts of the mediterranean area that had kept its natural and cultural legacy intact and integrated into the economic and social structure. as a result of this ancient ‘harmony’, this area offered a unique opportunity to create a privileged ‘observatory’ over its natural environment, as well as its historic and present-day anthropology, which was finalised to fostering the conscious participation of the inhabitants of the area in the territorial context. in this territorial context, towards the late-1990’s, new challenges started to take shape between two apparently irreconcilable demands: development and quality of life. for the two provinces of salerno and potenza, among the many practical cases, there was also that of the tragic flood of river sarno on may 5, 1998, and that of the development of the extractive oil industry in val d’agri. only recently has the dynamic equilibrium between these two aspirations encountered the prospect of sustainable development. within the framework of the idea of a ‘mallet laboratories’ project, there has been the opportunity to set up a network of study centers in the territory of the two provinces, to promote research characterised by great national and international breadth, starting with the world of schools and local communities. 2. the mallet project at the beginning, the main objective of the mallet project was to increase the sensitivity of the resident population of the salerno and potenza provinces, and in particular of the areas affected by the destructive effects of the mallet project figure 2. particulars of the original drawings by mallet to document the directions of the fractures in buildings having a rectangular plan hit by seismic waves (left), and the trajectories of the seismic-wave particles of different types (right). figure 3. church of the trinity at trinità di sala consilina (province of salerno). left photogram of one of the stereoscopic photos commissioned by mallet. the pen lines superimposed by the irish engineer contribute to the attribution of the photograph to claude grillet. the 1857 earthquake. it was also designed to prevent the risks associated with natural catastrophes, such as earthquakes, landslides and floods, and to educate a new culture of security. despite many bureaucratic difficulties, the project officially started in 2003, as a concrete sign that strategies promoting research and environmental education were being achieved. these aspects are indeed of great importance in the drafting and the enforcement of plans for safeguarding the territory, in which the regional administrations of the italian state, first and foremost the regions and the provinces, are increasingly called upon to operate. the project has involved 52 municipalities of the provinces of salerno and potenza (all of the alta val d’agri, the vallo di diano, the valle del bradano, and the vulture), and since its beginning there have been three main phases envisaged: (i) the creation of a publication as a bibliographical point of reference and awareness; (ii) the realization of initiatives of dissemination at all levels of society; and (iii) the design and creation of the mallet laboratories, to provide education in sustainable development. the first two phases have continued over the duration of the project, as described here (2003-2009), while the third phase was launched in 2007, and is still in progress. at the beginning of the project, there were no predefined guidelines to achieve the main goal, but priority was given to the knowledge of the territory, which was followed as the same journey that mallet made 150 years ago. we tried to be in ‘contact’ with the local people, to understand the different cultures, and to find the best language in which to communicate. so, indirectly, and sometimes directly, the recipients of the objectives of the project became involved in the construction of the project itself, and in the construction of the communication process. graziano ferrari 464 figure 4. scheme of the work phases that led to the realisation of the virtual historical landscapes for the three-dimensional navigational software geoviewer. 465 2.1. publications the publishing activities started in 2003, with two volumes and a multimedia dvd-rom, which have since been extended to a further four volumes and two updates of the multimedia product. the great narrative value of mallet’s work produced the idea of a virtual journey into the territories hit by the earthquake. to achieve this, it was necessary to create a high-immersion environment by means of multimedia instruments, while rigorously adhering to the historical reality of the mid-eighteenth century in the area of lucania. thus, a thorough research phase was started, for the territorial and historical environmental data for the reconstruction of the historical landscape of the area, on a scientific basis. this reconstruction, which at the beginning appeared to be a sub-product of the project, soon became one of the main development sectors of the project itself. the true novelty was not the reconstruction of realistic virtual landscapes, a practice already in use for some years and that has been undergoing development in italy in several professional and scientific sectors. rather, it was that this reconstruction occurred on the grounds of a complex global information system (gis) that was filled with historical data that were retrieved and processed, starting from the cross-matching of different types of sources (figure 4). during the second phase of the project, in 2005-2007, an original interactive system was also developed in the historical landscape thus constructed. a thematic library was built in the area that comprises around 800 original texts, and territorial reconnaissance was performed for over 5,000 hours across an area of some 3,200 km2, that is populated by over 350,000 people. in the most operational part of the project, the irish scholar’s itinerary was retraced several times not only ‘in the territory’, but also in the stories of the land, the water and the people. here we sought the persistence and the changes with respect to what mallet had described or reasonably seen in these lands. following in the tracks of mallet and the two photographers, bernoud and grillet, we looked for, and where possible found, the places described and photographed by these particular travellers. through the collaboration of technicians, scholars and private citizens, we identified the points where the photographs were taken, and we reproduced the same shots. this enabled us to highlight the changes and the persistence in the natural and anthropised landscape (figure 5) as well as to correct some errors made by mallet in identifying the places that were photographed. by his own testimony, the irish engineer received grillet’s photographs very late, without any indication of the subjects and the places photographed, and so he found it very hard to reconstruct them from memory. interviews were carried out using both audio and video, into aspects of the territory, the history, and the popular culture. the project is immersed in the cultural identity of these populations. the guiding idea is that the project should represent a broad cultural catalyzer, without however forgetting the pivot on which mallet’s scientific activity revolved: the earthquake. therefore, by analyzing the earthquake as part of an environment, both natural and anthropic, from these effects of the earthquake, a mature society can draw useful elements for a new culture based on safety. overall, the publication is made up of six volumes and three multimedia dvd-roms [ferrari 2004-2009]. volumes 1, 3 and 5 collected together 44 original contributions that deal with issues and aspects of the natural and inhabited territory, and that are also very different. volumes 2 and 4 collected the reproductions of the historical sources, such as the unabridged italian translation (vol. 2) of mallet’s work [mallet 1862] or the facsimile reproductions, with transcripts of the official handwritten documentation of the kingdom of naples relating to the expeditions of mallet, bernoud and grillet (vol. 4). volume 4 also contains the facsimile reproduction of the original handwritten manuscript of the lavas collected by mallet in his subsequent mission to study the italian volcanoes in 1864. the decision to publish reproductions of these handwritten documents that come from italian and foreign public archives, was to draw in nonspecialist readers, as well as to reveal the value of the unpublished documentary heritage of which italy is so rich. the fact that the historical documentation is elaborated within an important seismological discipline, historical seismology, contributes in a major way towards the mapping of the seismic hazard of the italian territory. this has enabled us to underline that apart from being an operation of great historical and cultural value, the saving of our documentary assets is rather like a sort of ‘life insurance.’ lastly, volume 6 re-elaborates in a summary and thematic manner the contents of the first five volumes, and it represents an iconographic index of the rest of the series. many references in the text refer both to the other five volumes for further analysis, and in a sort of hypertextuality, to specific parts of the same five volumes. the dvd-rom is now in its third release, and it has developed a complex virtual journey in the territory and in time, following the tracks of robert mallet, in the area most damaged by the 1857 earthquake: the lands, the waters and the peoples of these two valleys. we chose to deal with the aspects relating to the project topic in the form of microhistories, suggestions, details of an imaginary trip, short yet intense in the history, the environment and the culture of the val d’agri and the vallo di diano. this is a journey realised with the involvement of government institutions, and those of the coordination, i.e. the village and mountain communities administrations. however, the mallet project above all, it involves the people, many of whom were eager to collaborate on this project, which they experienced so intensely. several schools of the area have also contributed to the contents of the stories on the dvdrom, with school projects performed on various aspects of the natural and inhabited territory, thereby taking part in a direct way in the cultural production. the dvd-rom thus tells the story of this territory, its geology, its geodynamics, its natural risks, its traditions and its cultural identity and history. over 2,500 stories of the water, the land and the people, as 3 hours of a virtual journey like that made by robert mallet and by the other witnesses of the effects of the earthquake. there are also over 10,000 images, mostly from territorial reconnaissance, 450 animations and films, and 45 thematic pathways. 2.2. realization of the dissemination initiatives the mallet project has kept constant contact with the various components of civil society of the area, both for the research, study and cultural production phase, and for the specific moments of dissemination of the results of the project. all of the project activities have been promoted and documented through a website (www.malletlab.it), which soon became the point of reference for the schools and cultural associations that are involved in the project and that have been affected by its repercussions. the targeted distribution of the volumes of the publication to the libraries of all of the municipalities involved in the project has been one of the strengths of the dissemination module of the project. these dissemination initiatives have involved both the inhabitants of the different villages and the school-going population. furthermore, each year, close to the date of the earthquake (december 16) and for the whole duration of the project, dissemination activities have been organised on the various aspects dealt with in the volumes, in both alta val d’agri and vallo di diano (in general, in the certosa of padula), and in vulture. in december 2007, on the occasion of the 150th anniversary of the earthquake, a series of events were orgraziano ferrari 466 figure 5. comparison between the landscape of the vallo di diano today (above) and the virtual one of the middle 19th century (below). 467 ganised with educational, scientific and cultural natures, along with an opening concert of nineteenth century music on the theme of nature. in 2006, the project was presented in dublin, upon invitation, at the italian institute of culture and at the dublin institute of advanced studies. the latter initiative fostered the knowledge of mallet in the irish geophysical world in which he was practically unknown. with the publication of the last box-set with volumes 5 and 6, the first phase has been completed and the dissemination module has entered its more mature phase. volume 6 has been printed as over 2,500 copies in a paperback version, for distribution to the secondary school students of the territory involved in the project. the numerous hypertext links from volume 6 to the other five volumes, as mentioned previously, stimulate the students to analyse in greater depth the topics that are summarised in volume 6. 2.3. design and creation of the mallet laboratories for education in sustainable development the aims that we wanted to achieve were clear from the start, at least on a macro-scale, with the realization of the mallet laboratories for education in sustainable development. however, it was only in the course of the project that it was possible to outline more accurately the structural, operational and communication model of the laboratories that were to be set up. this was accomplished through the analysis of the specificities of the territory studied and described, with an understanding of its environmental and cultural vocation. during the intense activity of territorial reconnaissance, the great wealth of the cultural producers was assessed: e.g. of individual scholars and associations. most of these protagonists of the local cultural production operate in isolated ways, and so the idea has been to bring them together within a sort of cultural network. thus, along the lines of the far more famous academy that funded mallet’s original expedition, this network can function as a cultural catalyst and as a point of reference for the mallet laboratories, as the ‘mallet virtual academy’. the mallet laboratories were first of all intended as a center of cultural aggregation for the environmental, historical and ethno-anthropological issues of the area. it would have its own center of documentation and point of consultation for the publications of the mallet project. the activities of the laboratories are oriented towards developing training and information. indeed, the central fulcrum around which the individual actions provided for in the mallet laboratories project is the involvement of the local population in the environmental education and information processes. specific collaboration was planned with the schools in the territory, both primary and secondary, as well as with the cultural associations, where their specific aims are to address sustainable development and environmental protection. after a first start-up phase, the users themselves became the protagonists of the actions of the mallet laboratories: the teachers and the cultural workers, such as leaders of study groups consisting of students and the local population. then there are the students, teachers, cultural operators, researchers and local population as the producers of the content that contributes to the enriching the digital archive. the sectors of activity in the training and informational fields include: – environmental research into the different territorial ecosystems and biodiversities; – enrichment of the contents of the web portal for the mallet laboratories, on the cultural, environmental, historical and ethno-anthropological aspects of the territory; – setting up and endowment of a library and a specialised media library; – monitoring artistic and natural emergencies; – didactic syllabus design; – production of teaching and dissemination materials; – dissemination and training on seismic risk, the earthquakes, and the constructions in seismic areas; – creation and updating of the archives of knowledge and memories; – periodic meetings, seminars, congresses and exhibitions and events; – observatory on alternative energy; – network with the school institutions, and with public and private institutions; – promotion of the mallet project, of its activities, and its products. the foundation of the first two laboratories, one in sala consilina (in the province of salerno) and the other in potenza, has allowed us to work actively on the increase in value of the geological sites of special value, and to develop educational courses (teaching methods) for their study for high school students. some of these geosites were published as the geosites in tarabusi and lizza [2009]. 3. conclusions this all started from one of the most destructive earthquakes in the seismic history of southern italy that occurred on december 16, 1857, and the subsequent study mission undertaken by robert mallet. then, almost 150 years later, the idea was born to develop an educational project for sustainable development in the present-day territories that at that time were devastated by the earthquake: the mallet project. as well as the issue of the earthquake and its economic and social implications, and of the aspects linked the mallet project to seismic risk and its prevention, a broad range of cultural, historical and ethno-anthropological issues have been developed. the project has involved 52 municipalities of the provinces of salerno and potenza, over an area of over 3,200 km2, populated by over 350,000 people, and it has been carried out with the financial support of the two provincial administrations. a complex and articulated editorial project has also been carried out, consisting of six volumes and three multimedia dvd-roms that represent the knowledge base of the project. these contain 44 contributions on historical, scientific and environmental matters, as well as a substantial corpus of published sources and manuscripts. the mallet laboratories have subsequently been developed with the involvement of the world of schools and all of the individual and associated cultural producers. all of these activities have been accompanied by numerous dissemination activities that have involved a very large part of the population that resides in the territory of the project. the mallet project started with the target of prevalently spreading the issue of natural hazards, and it has gone on to characterize the rising involvement of the population, of the administration, of the scholars, of the cultural producers, and of the schools to achieve these objectives. it has also been possible to develop and transfer to different citizens at different levels the analysis of the scientific, cultural and environmental identity, which are the peculiarities of the identities of these populations. in particular, we found that the results obtained have contributed to enhancing a sense of cultural identity between the two valleys, which has been recalled on a number of occasions by the scholars. furthermore, as the project has been followed very closely by the political administrators of these territories, this has also served to stimulate the citizens’ knowledge, with a view to asking the politicians the right sorts of questions. all too often, the politicians ask themselves questions and give themselves their own answers: answers that are often distant from the real needs of the citizens, or that the citizens, being unprepared, often fail to understand. references burrato, p., and g. valensise (2008). rise and fall of a hypothesized seismic gap: source complexity in the mw 7.0 16 december 1857 southern italy earthquake, b. seismol. soc. am., 98, 139-148; doi:10.1785/0120070094. ferrari, g., ed. (2004-2009). viaggio nelle aree del terremoto del 16 dicembre 1857, (six vols. and three dvdroms), bologna, sga storia geofisica e ambiente. ferrari, g., and a. mcconnell (2005). robert mallet and the ‘great neapolitan earthquake’ of 1857, notes and records of the roy. soc. of london, 59, 45-64. guidoboni, e., g. ferrari, d. mariotti, a. comastri, g. tarabusi and g. valensise (2007). cfti4med, catalogue of strong earthquakes in italy (461 b.c.-1997) and the mediterranean area (760 b.c.-1500), website: storing.ingv.it/cfti4med/. mallet, r. (1848). on the dynamics of earthquakes; being an attempt to reduce their observed phenomena to the known laws of wave motion in solids and fluids, trans. roy. irish acad., 21, 50-106. mallet, r. (1862). the great neapolitan earthquake of 1857, the first principles of observational seismology, 2 vols, london, chapman and hall. tarabusi, g., and c. lizza (2009). i geositi del vallo di diano e dell’alta val d’agri, in: g. ferrari (ed.), viaggio nelle aree del terremoto del 16 dicembre 1857, (six vols. and three dvd-roms), bologna, sga storia geofisica e ambiente, vol. 5, 285-322. corresponding author: graziano ferrari, istituto nazionale di geofisica e vulcanologia, bologna, italy; email: graziano.ferrari@bo.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. graziano ferrari 468 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice applkation of the finite element luethod to a microzonation study of the salonica region a. a. i{.ol'i';soi'oul()~ ("') c .. a. syl{}l\i\ez1s (u) rcccived on .inne l,ith, 1!li3 su)diary. 'l'he main ohjcrt. of the present ljaper is an iltt('mlij to ljl"ehent u mo(ld for t.h(· estilllation o[ the g"l"onnll mow'ments ilnring an t'urthi]nalw, nsinl; thc }'initc .element )let.hod. sollle ljr('liminar.l' re8ults for t.hc suhsoi! o[ the halonica l'q~ion ilh' sullscqu('ntly culeulat('d, nsingthis iliodel, accol"lling to thc cxisting geological anll scislllologkill ilata of thc rcgion. }'inally, home generill conclusions are ilerived. riassunto. -scollo ljrincipale ili qllcsto lann'o 1.' iin t('ntajjvo ili llrcscntilrc nn lllollello per la valutazione ilei movill)('nti 11('1 suolo dllrante un tcrremoto wmfrm'ihlo ilei ,\[dollo ildl'elclllcnto finii.o. csi1lulo 1]111)· st ' uitilllo eli in hasc ili ilati gcologiei c sismici giù esistenti, si hono ]lrimil 1'illcolati alcnni 1"ìsllltati 1)('1' il sott.osllolo della regillne ili "alonirl'o traen. ilone, infinc, alcune conehlhioni di camtterl' generale. 1. tntrodl:ction among the variou8 exi8ting denuitiol18, r-.licrozoning can be (ielined as the stndy of small iam] areas ulldcr ioral gcoiogy eonditions and earthqnakc rharartcristirs, scrvinga·f\ a guide for safcr land use ami safci' ronstruction, areonling to the s])ecilied ]oca] diftereuces iu enrt.hquake attack on stl'uct.nres. ju november 1971 dnrillg thc fil'st mceting of thc "\i"orking g-i'oup on )iirrozoning in belgmde, s~~lonica was chosell as a site for the piloto )iicrozoning study according to t.he ]liall of o}lcratiou or thc usnpunbsco projert for the seismicity of 1.he ballmn regioli. (.) civil eng.t:"nesco pmject on the heihmieit.y of the balkan reg-ion . :-rational 'l'echnicul fnivc1"sit.y o[ atlwns. (**) dr. civ. eng, lednrer, institllte o[ strllctural analysis allll aseislllic research. national 'l'cchniral vnivel'sit.y of athens. ;'14 a. a. lwussopoulos c. a. sylnrakezrs the }l1ll'}iose of the l'l'esent ]la]lc'l' on 3iipl'ozonation is an attem}lt by the authol's to present a moael [01' the estinmtion of the ground movements during an earthquake, using the finite element j\iethod. u~ing t,his model, some earthquake eharaet,el'isties of t,he sllbsoil o[ saloni('a al'l~ sllbsequently {'alclllated ac'(,ol'ding to tlw i~xisting gl~olo­ gical allll seislllologi('al ilata for the l'egion allll some generai conclusions are derived, as a contl'iblltion to the undp-unesco pl'ogramme. the mai n idea of the model pl'oposed in this l)al)er was dis{'msed dul'ing tlw "seminar on ::\iicl'ozonm,ion" held by ui'lesco in 'l'hess(l·loniki, on a]ll'il 9-13, 1973. '. .mod:el cil\il\cteristics for the aevciopment of the modcl, an clastic layel' o[ unifol'm thickness h and width equal to unity is considerea, based on the bedl'opk (snblayer) which is assumed to be l)la.ne {fig. 1 l. i " ~~ .... • -fig. 1 the l'l'esent, st,udy is coneent,rated un the ehects alln to sheal' waves "\vhieh arn of vl'imary im]lortan{'e in tlw micl'omning ]ll'oblem. ::\iol'l~oyel' it is assumell that the llil'ection of wave ])j'o]jagation in the l'ock is il'l'elevant ami so the bedl'ock is tramiatel1 boi1ily. though, the motion of tlw bndroek snrfa{'e between points b a,nll c is assnmed as a l'igid body lllotion allll the aceeleration is t,(lkml {'onstant along the surfa{'e be of t,he bedl'o{,k, this is possible as the wavn length ab in the beal'ock is ex]leetnll t~) bn mu{'h grea,tel' than the wave length aa in alluvium (fig. 1). al'l'licatio~ or l' in }'i~jte };i.e"e~t "metllon, l,l'c, 515 it is al.~o assumed t.hat. thc main contribution for the exeitation of soil at. a ]ioint -4' (fik. 1) of t.lte sul'facc is trom thc l'egion h-c of hmgth l. points out of the rckion b-c, surh as ]loint d, hll"vp not signiiieant. clieet on ]loint a' since the enerky at d will be refl'aete(l at t.hc rock-soil interface ana ]ll'o]mgated vertieally. for the lml'posc of detel'mining the displacements of lite grouml surfacb dul'ing the earthquakc thc lenght l is tnken as a varying ]iat'amet.er. during t.hc present st.1hly t.he distribuhon of accclcrat.ion along t.hc hcight. of t.he alluv ium fo!' thc regi011 b-c consiabl'e!l is assumcd varabolic with a law of tlistribution: é(x, ji) = 10 0 + c(x)· y: whcre cix) is a funetion of x. in the abovc exl)ression thc first. t.crm 10 0 is t.he c'onstnnt aecclcra.hon or t.hc alluvium lnyel' due t.o tllc betlroek movcment.. 'l'he 8n'011(1 t.crm ba '-= c(x)!i~ desc'ribcs thc acceleration at cacl! voint. of ihc allu·vium laybl', auc t.o l'elative 1ll0"vpment of t.he layer 1.0 the betll'ock. a eosinmoi(la.l distribution of nc'celernt.ions along t.he hmgth of thc layer: is assum('.(l with k(y) vary ing from zero at. thc be(lrork surfnce 1.0 it~ mnximum value at tlw groumi smface. 'lihis maxim\1m val\1c is the aiffel'encc lwtwecn thc maximum ac'celerations llt thc ground surface alhl thc bctlroek surfaec, on fig. :2 th is cosinusoitlnl d istl'ibut.ion of sections, b-b' i · oo1g +od12 g , , ,l e +0.000; -. f l 9 2 a. a. llogs~oporlos ~. a. sy101akezih acc'elemtions is shown for tile subsoil of salonica eomi(lerell in tlle }lresellt }laver, for whieh the groulld fhufaee aeeelemtioll all(l tile belll"oc"k surfaee aeeeleration are foulld equal to o.lìg and o.log res}lectively, ah sho\\"l1 ili the liext }laragravh~. ijl1ring the }lre~ellt stud~a ~eetioll i1cc'l1' of the layer is eoilsielered at a llloment where the eosillllsoiclal aeeeleration wave hah the shave of lig. 2. tile ~e}laration of tlli~ fwf't.ion i~ }lermitted heeause the axes c'c', hh' are axef\ oi antimetl'y oi loalling and ~o 110 horizolltal for('e~ lwt along them after se}laration. por tile ilet,ermination of the eharac'teristies oi tlle gl"ol1ncl mo\'elllentf\, tlle finite blemellt 3iethod is a}lvlied (a). a suitable computer }ll"ogramme wah uhed for the eahe coiihidered. 1'he ~urfaee of the layer exalllilled of length l and height h i~ divieled by horizolltal and yi'l"tipallilles into 1'i]ualredangi11ar l'lements "" " " -~,_o._",_~. " , " ~ ~: ( • .• j ' . 0-----'-' : " r of lenght c:lc anll heigilt 1111, orelerecl in llines and ii! polllmm. tllese reetangular elements are subf\equently ~ubdi"il1ecl into ttiangular elemelltf\ (fig. 3) for l'ijllveniellee of vrogramllling alli! eomvllting. tlle lllash of t11e examinell layer is 1'0mic1ered 1'01h'entratell on tile joints of the finite elelllent system. namely all joints are comillerell a~ llh1sf\ }lointf\ witb lllass ili (,oite~wjlle1ing to the mash of one redangular element lb:-lly (fig. 3) exce}lt for tbc jointf\ of the bolll1llaries wllere the ma~s i~ ili (2 allei the eol"ller~ ,dwl'e the lllass is .jl (j. the wl'ight h oi the mass .jl if\ h =!1x·!1y·y wilere y i~ the mean ~jleeitic ,,-eight of the layer at the regimi of the joint c'omillerel!. the ilorizontal seif\lllil' force af'tillg statienlly on eaeh joint is ir .= 6('1', li) 11 ,dlcte 6('1', li) is the funl'tion ilef\eribing the di~triblltion of tlle f\eislllic ael'cleration over the hlli"faee of the la;rer. appli('at10x 01<' t lll f1xite ille~iext ~1}:t110j), }:t(', ,i17 3, sbjp.)tictty of tue re(ttox 'rhe m'ban area, of saloniea is not aseiilmif'. }ially htl'ollg (,:\l'thqllakes have shaken the regioli ,dth (le8hudive results in the pagt, 'l'hl' moht deht l'ildive emt,hquake ioi' tlw ('ity of sa]ollica 8eems io have hecn the ~l\()('k of ,july 5, 190~ with nmximlllll illtmlhity "iii-ix of the l\1ercalli-sieberg h(':l1e. 'rhe lllaoximum int,enhil,y ohhci've11 in t.he area of salollka ov('i' the }lel'iod 1800-1!)72 is, according t{) the cxihting map of isolines of maximllm int,ensit,ieh of t.he l'egion, ix (legreeil oi the mercalli scale. an'oi'l1ing t,o t,he elllpil'ieal fol'nlula fol' t.he gl'oumi sei8mie a('f'del'ation ag for t,he re/(ion of greef'e: a. = o.~w-o.1·[ +0.01 . i2 ,\'\wre 1 i~ the }laximulll ex}lect,e<1 int,ensity of the mel'calli scale, t.he maximum aceelel'at,ion ex}lederi at the gl'olllh] sul'hwe fol' tlw rcgion of salonica is o.17q. 'l'lw pl'0dominant j}criod t ai an cxj}ecteri ea.rthquake is given by t.he fol'mllla: log '}' = o,2hm-1.:)2 givcn by mummut,su (1 ()ho) and foillld equal aj}j}roximat.ely io o.·h) hee, for a magllibh]p .ili = ii.5. salonica is ncar in t.l10 cent.el' of the arca bollndc<1 by tlle }lamllels .j(h'ì-4-1°n an(l t,hl' lnt'l'il1i:llls ~2,gob-23.5ok 'rhe lw~u'{\st, fmi around salonica ,\'!lich getwmtcl] lar/(e cal't.hquakp8 al'p at (listances j5km30km, 1mt. t,hc str011/(er neal' l1estl'uc'l,iyp eart,hquakeg al'0 frum !io km epic'ent,ml di~tances, 1'he l'ctmn j}crio(]s far slmllrn\' hl!mkh whif'h 01'c'mcd in t.]1c' regi 011 of salonica, are 18-2:) ~' ('al's for .:11 = 5.5, -l5 ·i)5 yml-l's for il! = h, 110-1:)f) ycarh ior.:ll = h,5, awi ~75-335 yeal's fol'.ili =7 c'), 4-. gbol.oglcal .h'v trc'toxlc chahactrrtrttc~ of 'l'he lmmox 'l'hp sllhsoil oi t.lw l'egion of salollinl is not homogen('()1is an<1 val'ips irom j}lace to piace. alluvium c'omolida,t.ed sand l1uncs, gra,nit.ic metamol'j}hies ek, are t.h(' 1is1ial tyj}es of soil whi(,h com}l08p the sl1bsoil of t.hc re/(ion. ,:; ) r .a • • \. roui'li'lopou i.os c. a . i'l l"io iakezh! t iic surfaee layer may be assumed to have a mean thicklless of 20m nbove the 1ied1'o('1, . i ii l'ig. j are sh owll sollw t.ypi ca l hoi'(;hole8 of the regimi of salolli ca. l'rom thcse cham.cte1'istie boreholes an d .... ,~ ." .. eor eh o l es , , ... , ... " . _., • • _"-"c.".. ... .,'" .... 0' ... . "00 • 00 . "" , .... . "" • ... , .. <..,.,.. ..... , .... .. ~ .. , . ........ " « .. , • ••• • ot·he1'8 exist illg we emi a]f;u :l,cc',c]lt an avel'age water leve l a.t 1\ depth of brn. alth ough t,he m l.-'(: hallical char:\ct.e i·istics or th e !huface l aycr ma y va!')' from pia ce to l liace, t hc smaii ~lua.ll t i tics of the cxist.ing sand, pe1'mi t u~ to consiùer t hc layer ùuring thc pl'esent l)1'elim in l\ ry stwly as allllvi lll11. ' l'h (' ('hlll':wt(>1'i81i(.1) o r 111i ); allu\" inm an' ('s lima1 (',j ai! ftlllowing: bl' lnw t lll' walcl ' icri' l t lll' alhlyiulll is l'om}l ll'h.i.\' ~:ll\l l 'att'd allil l he fi ]!cdjjt wi·iglit. i,~ gi v i'n h,i' the col'mula: y = r ,-li(y.,.-) wl1l'1'i' -ji ifl t.hl· ]!uroilil~', y. il; th(' s!le>e>if ì<' wi' ighl of 111(' ~olid par ti('h'il, nlld y", is the spe eilie \h'iglit ui tllc' li'a!pr. 1·'01' tlip yahw~ or ì" :!.7:1, ji -= tuo alld y", = l, th(' ynhw (ii tlie ~ahmllpd ~]le<'iti(' \\'(' i~ht or lh(' i;lyel' i.~ found y = :!.o ii t / m j , .. \bon' t!te wat{'!' 1('''1' ] 1]1(' :d lll"inl1l i .~ l' :l l'i ly isl.1 t u n ll ('i i r lld lit e ex iij lilig illl'li l) lirl'llwlii s s ho l\' il ilwa n ":llu(' o( r = 1 .80tjm 3 • thl' labora i oj'~' t 1's11) f rom exa lllill l'l l sii(>('i m(' 11 1$ ,<; 1111wt'l l a illi ih' t>al isla cttu 'y s 1rl'ilg1h llt'j'lllif.t i ng n .~ t u an'{' llt a illlli ll lli 1i\ iir {'iil st i"ily h 300kgj('m~. thc p(li~~ojl '~ l~l t io ii' lakl'll l'qual t o ihl' l'alul' 1'-oa5. vimdlr, thl' vplueily (lf s·wn\'('s ili lhp ailu\'inm is as~t1m('d l'qllal tu v, = o.2i'i km/hl'(·. the afol'c llwlltiollpl1 nlhl\'itllll ih']losit i ~ ('uilsil]l'\'l'd to be i'tl'oll g ('nu tlgh cnablill/.t thc npplk:l t,iol1 of tht' l'la .~tit tli('oi'~· hilii mkl'oli'('moi' tec hu iqul' s. at'i'ol'd i ll g" lo thl' ('x i!;1 in$! j.('('ultlj.('icalil a ta , 111 (' bl' llnwk of i hl' l'l'g ioii u( s a lonka ('u ll s ifits ilmill ly 01 li 1i 1('s1ot1('1) a lll i 111:ii'hl('i$. 'l'il{' nwd lallic' al ch ;u";lc1<'i'is l il'" of lilìlc:'lto)1i' ai'<.' ,!t' l1 foii ty f! = :? ,:111, iilul11l 11l i; uf (' iasl il'i t.y ... 1e~ entl !!c<l m + ----~---------------------b eo ii oc ~ fq:l " '5 r =r:~,"x 1 () io kg!('lllalld uf mlu'b ll' " = :! ,ao, r ::.... :-; 7 ,ii x )1)10 k,g! ('l\l", thc ld g-h "a l ll('." of tll e ll1of l lll11 ~ of c lasl i('ily al'e 1111 (' !ii 111(' ii mall ih· rol"lllab ility or t"l1l' l'o(· k , 'l' lic vi' lo('itic,<; t· p of l he <iihllaliolla ll'wa"{'h :111(1 1", of !he dil'òl m·· litllla l 8-\\"; 1\' (>1) i1l'(' chli llmtt'l l fili' l11 c bt', l l'(wk u f thl' 1"('j.(' ioll u r ~alo11iell l'q\la l to t'" = :!.5 k m fs c(', l'. 1.0 k m f iil'(' , '[, i ip~ {' 1'$1ii1i';; al'(' hlljj i'oxia. a. roussopoljlos c. a. syidfai,ezis mate estimations, since there are no yet accurate measuremenh, oi the wave veloeities of the region. as mentiolled before the maximum aeeeleration 011 the ground sul'faee is ea,kulated aeeording to thc (,xiflt,illg data, for an inttlll~ity i = ix allll follnd ('qual to 0.17 g. 'rhe corre~})olllling intemity lo at thc sllriacc nf the hedrock i~ caleulatcd from thc )inlvedcv iormula ("l: i o =l-lli whcl'tl i is i,lw ~ul'face int.cnsity and lli is givcn by the exlll'cssion: (y"'") . lll = ljt~ -+,1 i", = 1.67 log + j' · e u'ou-y!1>l whcrc yovo and y11'1 are the values oi seismic impedance or l'igidit.y ior uic bedroek and the alluvium respedively, 71 is the depth of lhe '''ater level in met(m; ami v is a, eonstani, eoeffieient taken pqual lo 0.8 for the regi 011 eonsi!1cred. li'nr thc value~ y l = 2.0 l!m~ vi = 0.25 km!ser l'o = 2.30 tjm3 , v o = l.0 kmjsee and li = 8.0 m, ,,'e have i,ha,t lli = i and hcnce lo = vttt, corresponding to a bedrock sul'faee accelemtion equal to 8 0 = 0.10g. 'l'his avproximat,e vn.lllc fnl' the bedl'ock a,ccel(, ratioll is uf\ed in the present l)apcr bccausc, thcre are no olhcr, more a.ccumtc data , available ior the regimi. from the s(,ismnlpdonie ma,p of the l'tlgion of salolliel\ we cali o!)fwi'v(, thc following zones having common bol'ders: a) pnf\i,-teetonie del)osit.; bl eoml)lex geosynclìne of yan111r; c) serbomaccdonian ma,',,~if (ridge). 5. j{,esut,té\ col'\(:t,l;si01'\s four eases 01 iree zones 01 a del)i,h 20m are cnnsidered with values 01 the paramei,er l equal to 25m, 50m, 75m, 100m, respeet.ivcly. the st.el)f\ llx, .i1y along the axcf\ o,,~, oy arc taken equal to 5m anei ·1m rcf\pectivdy for all ca,'!cf\. in figs. 6(1. , ob, 6e, 6d t.he horizontalrelativc to thc hedl'ock displacements ò~ along uw depi,h ti of the hlluvium layer considercd are shown. namely, ior ea,eh case of iength l, tlw horizolltal displaeements along vertieal sedions nf the alluvium layer, taken cvery 5m, are presented. the ordina,tes x oi each scctinn arc written on the corresponding curvp. ,\l'1'j.1 cat 1qx o ~' tut: "l xln: ~:u:.\n:xt met ll 0 1>. t:t('. 52 1 l·2!im l . ~om " . . ~. f io 6 . ~ , ~ .. l doom '" 1;22 ln fig. t tllc yltriation of thc ilol"izontal dis}llacemcnts (5~ witll tlle lpngtlt l is sllown for tlle grollnd slufaee. ..\s s(,l'n from tlle 1'ig-\11'e t tlle nmximum displa('('ml'nls do not nu·.v significantly f01' rl'lativdy ilig-il values or tlle lt'ng-th [., ~.(cm) " • • '-----",_----.oo,-----~, .,_----.,,_----l (m ) flg. 7 h is to be notcd finally that lite ealcnlal.ion of the dispiacelllents of tllc alluvium layel" ace()1'i1ing 1.0 i.he finite blement. :illethod 11111ht tnke int.o eonsidemtion the elabi.ic ol" anelastie }h'opm"ties of tlle allnvi11111 i~o1lhidered and illainly its anisot.ropie st.renghl., witlt \.he best possible accun\ey. on fig-ml',~ h, in the shaded regioni'! j.}\c cul"yes whieh are not inll11em'ei1 ,~ignifi(,:l1lllr by i.lte boulhlary ('owlitions of thc cdgcs be', gu' (iig. ~) are shown. lt is suggeste<l to lake into account t.hese cluves <lming eale111ations for the hnbhoil of thc rcgion collhiilered, referexce~ (l) a~lbha~ey~, x. ~., 10iu. pac!o)"s colllmllillf} ih/· }>,'mlhquakc hi'rj!olige oj foulldaliol. '//hi/criaih. "l'l"ol'eediili!:h of the 3n\ elll"opeall :-iylll]lohilllll oil earthqllake eug-iil('('l"iug-", :-iotia, pp. 3()!j·31i. (2) bub"oy, :-i. ('i. hl., ihiv. seismic :;oll;'nlj (lf zagrcb. "l'l"oe(>(~jlilll-,'1ò of the :1\\ eliropean sylliposilllll on earthqllake eugineel"iug-", :'\ofia, pp. !j5·ju2. aj'i'licatio~ 01' tue fixit}; ele~iext _\ie1'lloil, etc. (~) llll ,\hol'oui.os, .1., ioil. (fellnal ('ollsilleralion8 al"'lll a .ljiero::oi;ill[j siilii!1 (11111 'j'lirshrdolliki iih a sile fo)" such a slnll!}. b,'lgl":ul. (-l) gala~opc)ulo~, a. g., ioil. , ... ·rismu!0!j.i!. ath('ii~. (~) gaus, :\t. l'., kiieilif, :il a., ioi2. zil/wlioll (imi .1firro:rj//alion. "l'n)(,l'eiiìllg-~ of liti' lni,01"nntionnl confel"('!l('l' oli :i[il'l'm:onation", \yahlting-ton, s('atup, i. (") lljhi~~, l. :\1., :-iej':j), il. b., ifhii. h'i's}joii.w of hurlh !fo///,"h dllrijlij 8(rj"lh,/,w/," ~ . "l'ro('('c,linl-(s of tltc ,\.:-:;.c.e.", 93, r:\13. l'i kax,u, h" 1%(;, l"'l,m",,1 li"'p;,';",,1 f""",[" l''' "" c'1"",,,"",';,ii,, of h/roll[j /<,'arlllljllll!;c .1l"lionh. ''l'1"o{'{'eiiìllgs of .11l]hli1 elil'thqnnke ]~lll-(inccring rylll]joi'-inlll", (8) }\oil ,\yasiii, il., }\afi ,\)ii, il., lf)i2. a mdlwll for loeal8ri8mic inll'jlsi/y :::onill[jilirlf!s oliih{' imsis of nub80il condi/iolls. "l'wl·cl',lin.!:!;s of 1"he intcmntìollnl ('ollferclirc on :\jìen,7.onntion", \vnhhillg-ton, rl'attlc, il, l'l'. 513-528. (") :llejj\-ejjev, k_, 1f)0:;. 8n.rjilll'erin!j 8ei8111oi0[j!i. "al'. of :-kìe!l('c c'ksr". ('o) l'ap.l~ta~l\ttou, d .. 1., ]flil. (;l'''unrl liiolion 01111 ri'hiioiis{' of barlh 81)"lidllrf r subire/nl /0 s/rojlij 8rulhljuukl's. l'h. ll. tltehì~ ]lil\}(>l'inl l'o\\f'i-(c, t.'lll,l'lll. (il) seell, il. b., iolii. _ h/opi' s/abilil!} liurin.rj eartllljllllke. "l'l"ol'el'liillgs of tllc a.s.c,e.", 93, s'14. (lo) stefa~ou, g., idio. _ ali upaimelllalulhl/hrorrlicrd inres/i!jalirl/l illl" /111' perfora/ed :::o!len fil)" ilitderl)" )"l'liclor pre~s!!re unse/s. l'h. d. 'l'hesih, lml,erial {'olleg-l', lowlon. (l") :-:;'l'el'lm~so~, \y. r" ]uil. 8dsmic jliero:::ollilllj iii s~ ii' zealu/ld. "bnllptin of 1"he ne\\" t.l'nlllih1, so{'id.,y for earth(lllllkc enl-(ilwprìllg",4, l, :\ian'h. ('-') \yli, t, h., idlii. soil jii'clwllic8, allyn alll\ ba{'oll, im', b,,~ton. scandone_final_layout 6 annals of geophysics, 56, 4, 2013, s0455; doi:10.4401/ag-6461 s0455 volcanic precursors in light of eruption mechanisms at vesuvius roberto scandone*, lisetta giacomelli università di roma tre, dipartimento di matematica e fisica, rome, italy abstract vesuvius entered a quiescent stage after the eruption of march-april 1944. the eruption was not much different or larger than other before, like for example the one of 1906, but it occurred at the end of a long period during which it was observed a decreasing trend of explosivity of eruptions [scandone et al. 2008]. the parallel increase in the frequency of slow effusive eruptions, with respect to violent strombolian eruptions, point out to a process of average slower rate of magma ascent possibly due to a progressive sealing of the ascent path of magma to the surface. the small caldera collapse following the 1944 explosive phase effectively sealed the upper conduit, and since then the volcano entered a quiescence stage that was unusual with respect to the pattern of activity of the previous 300 years when the maximum repose time had been of 7 years (after the eruption of 1906). most of the uncertainty on the duration of the present stage and character of a future renewal of activity derives by the basic questions regarding the nature of the current repose: due to a diminished supply of magma, related with structural condition or a sealing of the upper ascent path to the surface? possibly the variation of structural conditions caused average slower ascent rates of magma favoring its cooling in the upper part of the crust, and effectively sealing the ascent path. introduction mount vesuvius is presently in a quiescent stage that lasts since the eruption occurred 68 years ago, in march 1944. the current repose is the longest since 1631 when the volcano ended a centuries long quiescence, even if dubious reports of activity refer to an eruption in 1500 [alfano and friedlander 1929] or some kind of activity or unrest in 1571 [guidoboni and boschi 2006]. the majority of sources suggest that the repose may have begun as back as 1347 or 1139. during the period 1631-1944, the longest repose period (7 years) had occurred after the eruption of 1906. significantly, the longest reposes occurred immediately after violent strombolian eruptions as defined by macdonald [1972] and classified by walker [1973], which, accordingly, were termed “final” by alfano and friedlander [1929] and carta et al. [1981] (figure 1). the peculiarities of such eruptions were the rapid emission of a fastmoving lava flow, which eventually attained the nearby villages, a following explosive phase with high incandescent lava jets and fountains transitional to a sustained buoyant column, and the collapse of the summit cone with the formation of a small caldera of hundreds of meters depth [scandone et al. 2008] (figure 2). this occurrence was noted several times, and the formation of a small caldera was correctly interpreted already by breislak and winspeare [1794] during the lateral euption of 1794. they explained the decapitation of the summit of the volcano, following the explosive phase of the 1794 eruption, as caused by a collapse due to the fast emission of lava flows because in the proximity of the cone there were not enough detrital deposits justifying an erosion of the crater. such an explanation put forward much of the debate on caldera formation of the following two centuries. it is worth mentioning that during the lateral eruptions of 1760, 1794, and 1861, the explosive phases occurred at the summit cone, and were followed by the caldera collapse thus suggesting that, even in these cases, the eruptions were fed by a central feeder dyke. we emphasize that the caldera collapse is triggered by the rapid drainage of magma residing at very shallow depth (<2 km) [marianelli et al. 2005] under the cone of vesuvius. the magma resided at shallow depth for period ranging 0-10 years [morgan et al. 2006], before being erupted either with slow effusion or during a violent strombolian event. the sequence of the violent strombolian eruptions are driven by the arrival of fast ascending, and gas-rich magma batches that act as a piston on the shallow reservoir, giving rise first to the fast lava flow, and the following explosive activity [morgan et al. 2006, scandone et al. 2008]. as mentioned before, after the most violent such episodes, the volcano entered a complete quiescence, eventually lasting for several years, and the renewal of activity, usually occurred with mild strombolian activity at the botarticle history received may 9, 2012; accepted november 21, 2012. subject classification: eruption, precursors, vesuvius, earthquakes. special issue: vesuvius monitoring and knowledge tom of the caldera and slow lava effusion, that eventually filled the caldera. these mild renewals of activity were possibly due to an incomplete sealing of the ascent pathway following the “crater” collapses after the violent strombolian eruptions. the volcano structure and the current repose the stratigraphy below vesuvius is characterized by a sharp density transition at 2 km b.s.l. from limestone (with a density of 2500-2700 kg/m2) to loose volcanic, alluvial and marine sediments (with an average density of 2300 kg/m2) [berrino et al. 1998, bruno et al. 1998]. the sedimentary fill of the campanian plain represents a lowdensity barrier to the ascent of dense magma and there are strong evidences that shallow magma accumulation below this barrier provides the source for medium-sized reservoirs, which fed the plinian and sub-plinian eruptions [delibrias et al. 1979, barberi and leoni 1980, belkin and de vivo 1993]. hydrothermal circulation, mineral deposition and repeated intrusions of dykes and their rapid cooling tend to reduce the density barrier and build up a structure favourable to magma ascent. bouguer gravimetric anomalies of the volcano, nowadys, provide evidence of a shallow denser structure without very deep roots [cassano and la torre 1987]. a prominent highdensity core has been also identified by seismic tomography [zollo et al. 1996] concentric with the caldera structure. seismic tomography evidenced also an extended low velocity layer at about 8-10 km depth, interpreted as the top of a magma reservoir, having a surface area of at least 400 km2 [auger et al. 2001]. gravity inversion [berrino and camacho 2008] suggest the the upper part of vesuvius is characterized by an high density body (density = 2450 kg/m3) similar to that of cooled lava. such body has also a strong magnetization [fedi et al. 1998], thus confirming the idea of a cooled magma intrusion. a shallow reservoir residing near the surface fed the persistent activity between 1631 and 1944 [marianelli et al. 2005]. during this period, violent strombolian eruption occasionally caused small caldera collapses, and a following short repose. the caldera, formed after the eruption of 1944, was smaller than that formed after the eruption of 1906 (25×106 m3 versus 80×106 m3, reduced to 50×106 m3 after the landslides occurred before the renewal of activity in 1913). also the erupted volume was smaller than that of 1906 [imbò 1949], even if recent estimates suggest a much larger estimate [cole and scarpati 2009]. there is however a strong suggestion that in both cases, the shallow reservoir, that had fed the strombolian activity in the previous period, was almost completely evacuated [scandone et al. 2008]. the reservoir was located above 3 km depth from the summit (between 10 and 100 mpa pressure) [fulignati et al. 2004] that means immediately above the limestone basement that floor the sedimentary cover of the campanian plain. it is tempting to suggest that the caldera collapses cause a density decrease in the central part of the volcano with respect to the surrounding rocks, thus preventing the buoyancy ascent of degassed magmas until the condition for magma ascent are reestablished by repeated intrusions and the deposition of hydrothermal minerals. after the 1944 eruption, signs of possible renewal of activity were observed in two occasions. the first significant anomalies were observed from 1952 to 1954 with an increase in the temperature of fumaroles within the crater from 350 to 600 °c [imbò et al. 1964a] and on may 11, 1964, an earthquake swarm with a widely felt shock (imax = 5-6 mercalli scale) and the formation of a funnel depression within the crater. [imbò et al. 1964b] since then, the temperature of the fumaroles has decreased [nazzaro 1997], but occasional earthquake swarms have been observed in 1978-80, when there was a swarm of moderate earthquakes (m<3). the most significant earthquake since 1964 ocscandone and giacomelli 2 figure 1. scheme of the eruptive activity between 1631 and 1944. the horizontal bars indicate periods of permanent activity. the height of the vertical bars indicate the vei of the eruptions; f indicates the major violent strombolian eruptions eventually causing a small collapse of the “crater” and a subsequent short quiescence (modified after scandone et al. [1993]). 3 curred in 1999 with md = 3.6, accompanied by a relatively small number of earthquakes with md>1.8. caliro et al. [ 2011] detected a significant variation in the chemical composition of fumaroles over the 19992002 period, which accompanied the seismic crisis of autumn 1999, with a continuous increase in the relative concentrations of co2 and he and a general decrease in the ch4 concentrations which were interpreted as the consequence of an increment in the relative amount of magmatic fluids in the hydrothermal system. since 1985-86, there has been a progressive decrease of the b-value of the earthquakes of the gutenberg-richter law, an alternating sequence of seismic quiescence and activity, and an increase in the observed earthquake magnitude [de natale et al. 2004]. the source mechanisms of the major earthquakes are compatible with isotropic components that indicate volumetric expansion [de natale et al. 2004]. the average depth of the earthquakes is comprised between 2 and 6 kilometers [de natale et al. 1998, lomax et al. 2001, de natale et al. 2004]. the recurrence rate of seismic swarms and other unrest phenomena in the past 50 years is = 0.14 y-1, similar to the average rate of paroxistic eruptions evaluated until 1944 (0.13 y-1) [scandone et al. 2008]. scandone et al. [2008] suggested that the violent strombolian eruptions were caused by fast ascending magma batches, which unplugged the upper conduit of the cooler and crystal-rich magma, and caused the following collapse of detrital material into the evacuated conduit. the number of violent strombolian eruption decreased with time and gradually made this process less effective. the volcano finally entered a quiescent stage because the slow ascending magma was not able to reach the surface and crystallized extensively before the arrival of the next energetic magma batch (1964). the crystallized magma and the mineral deposition caused by the hydrothermal circulation in the upper part of the volcano formed an extensive barrier [bons et al. 2001] which further grew because of subsequent intrusions (e.g. 1999-2002). collectively these data indicate the lack of any relevant shallow magma chamber in the first two kilometers depth, like the ones feeding the persistent activity in the period between 1631 and 1944, and at the same times suggest the existence of a rigid structure with density higher than the surrounding rocks, that represent a physical, unfractured barrier to the ascent of magma. we suggest that below such structure, magma has continued to intrude, occasionally with batches rising with higher velocity thus causing episodic seismic swarms. the eruptable magma depends on the ratio between the intrusion rate and the cooling rate which varies with depth and the surrounding temperature. precursory signals for an eruption at vesuvius eruptions are commonly preceded by a number of precursors signalling an “anomalous” state of the volcano. precursors are generally observed in the last stages of processes that lead to an eruption when magma finally approaches the surface. according to dsurisin [2003], the absence of measurable precursors until magma begins its final ascent, is mainly a consequence of: (1) the brittle-ductile transition relatively shallow beneath most volcanoes; (2) the lack of monitoring in the types and amounts of volcanic gases emitted over time; (3) the most classical geodetic techniques are not sensitive enough to detect subtle ground deformation that might occur when magma accumulates near the brittle-ductile transition, especially if the intrusion occurs gradually or episodically in a series of small events. in particular, it is highly probable that the processes leading to the occurrence of an eruption of a quiescent volcano, may occur on time scales of the order of hundred of years, as has been proved, for example, for the monte nuovo eruption of campi flegrei in 1538, where the early signs of unrest dated back more than a century [giacomelli and scandone 2012]. it is otherways true, that we are more capable of detecting signs of unrest in calderas than in central volcanoes. based on this assumptions it is likely that we are: either uncapable of detecting slow magma accumulation in a crustal reservoir, or we are overlooking the weak signals that the volcano may display. as mentioned in the previous paragraph, the occasional seismic swarms observed at vesuvius in the last 50 years may represent a weak signal of magma accumulaprecursors at vesuvius figure 2. oblique view of the vesuvius crater (gran cono) formed during the eruption of 1944. the steep walls of the crater suggest that it was formed by a collapse inside the evacuated portion of the shallow magma reservoir (<2 km) feeding the permanent activity between 1913 and 1944. on the background, the border of the somma caldera, formed as a result of the collapses following the main plinian eruptions of the last 17,000 years. (photo scandone). tion. within this frame, the renewal of volcanic activtiy may only be signalled by the fracturing of the rocks above an inferred magma accumulation zone 3-5 km deep, similar to that which fed the 1631 eruption after the last prolongued period of repose [scaillet et al. 2008]. in this respect it is unlikely that we may observe significant changes of ground deformation or seismicity until the process is well underway and unlikely to stop (like for example in campi flegrei). in some respect, the start of precursory earthquakes may be not much different from a “normal” seismic crisis like that observed, for example in 1999, with the main difference that it may continue with a swarm of magnitude 4 earthquakes. roman and kashmann [2006] suggests that two main processes dominate the seismic swarms preceding an eruption: – failure within the walls of inflating dikes with a local 90° stress field rotation with respect to to the axis of maximum compressive stress, – failure at the dike tip with hypocenter propagation, although in some cases dike propagation may be effectively aseismic and in other cases dike inflation may be effectively aseismic. in the case of vesuvius, bianco et al. [1998] suggest that there are two main nne–ssw and ese–wnwtrending extension directions with the first one related with the regional stress field and the second, local, acting inside the somma caldera. basing also on the last renewal of activity of vesuvius in 1631, when the early phase of the eruption was fed by a lateral vent we may envisage a precursor scenario possibly into two stages: 1) magma ascent filling a dike along the central axis of the volcano perpendicular to the local ese– wnw-trending extension direction 2) possible migration of hypocenters toward the ssw flank of the volcano with a possible opening of a lateral vent. within this frame, we may expect an early swarm of vt earthquakes with mostly strike-slip mechanism oriented along an ese–wnw trending direction, followed by a possible migration of hypocenters toward the ssw flank of vesuvius. the depth of the earthquakes should be in the uppermost 3 km of crust. the duration of such swarm may be as short as a few hours, as the one occurred in the night of december 16, 1631, and with an high seismic destructive potential, expecially in the environment of torre del greco as this city lies along one of the main fault systems feeding the activity of vesuvius a short duration of the swarm and lack of lp events are likely in the case of a sudden rupture of a shallow magma reservoir and rapid magma ascent. the lack of lp earthquakes may be due to an insufficient time to allow efficient degassing from the ascending magma batch [scandone et al. 2007]. in such case it is likely an explosive eruption with kilometers high lava-fountain and the formation of a plinian eruption cloud. rapid rupturing of a crustal reservoir and the lack of efficient degassing make unlikely early detection of extensive geochemical anomalies within fumaroles and water wells. a longer duration of the swarm and the occurrence of lp earthquakes and possible lateral migration of hypocenters could be suggestive of very shallow residence of the ascending magma and of extensive degassing. this could possibly result in an early lateral effusive eruption, eventually followed by an explosive phase from the central vent, as soon as conditions for fast magma ascent are attained (e.g. 1631 or 1794 eruptions). extensive degassing may allow detection of geochemical anomalies in fumaroles. shallow intrusion may be also evidenced by deformation of the flank of the volcano. this picture however, does not ensure that the outbreak may be only effusive, as the opening of the ascent path may be operated by several ascending magma batches each one utilizing the path of the previous one and displacing or intruding the stationary early degassed magma (like in the case of pinatubo eruption where the early degassed dome was rapidly diplaced by faster ascending batches) [hattori and sato 1996]. conclusions the analysis of the past pattern of activity, as well as recent unrest episodes at vesuvius, suggest that the volcano is in a sort of dynamical repose where episodic input of new magma in the shallow crust (3-5 km) are not allowed to reach the surface because of a solidified plug. the lack of evidence of an extensive magma chamber in the shallow crust, suggests that most of the new magma arrivals cool rapidly. relevant accumulation of enough hot magma, capable of eruption may eventually occur with an increase of the episodic supply rate. such occurrence should be eventually detected by a higher frequency of seismic crises and possible increase of magmatic gas emission. we do have a relative good record of the phenomena preceding the last renewal of activity of 1631, after a long repose period [alfano and friedlander 1929, bertagnini et al. 2006, guidoboni 2008, scandone and giacomelli 2008, guidoboni and mariotti 2011]. the suggestion that a two-step scenario occurred at vesuvius, in 1631, is suggested by the sequence of observed precursors, which records a months-week long, succession of “low intensity precursors”, not raising parscandone and giacomelli 4 5 ticular alarm, and then a climax of seismic activity in the day before the outbreak of the eruption [see for example: braccini 1632, guidoboni and mariotti 2011]. the idea that high intensity, short-term precursors may be followed by an explosive eruption is based on the suggestion that short ascent times are required to prevent extensive degassing from the ascending magma batches [scandone et al. 2007]. this requirement may be obtained also late, in the development of the eruption process, if precursory magma batches prepare the ascent path for later fast ascent and eventually full connection between the magma chamber and the surface. in this case, it is likely that a sustained plinian eruption may develop like the one of 1631 or even 79 a.d., if a large enough magma chamber formed at depth below the volcano and became connected to the surface. such case has been observed for the eruption pinatubo, in 1991. in that case, a slow ascending magma produced months-long precursors to the extrusion of a dome. the following explosive climax was preceded by a sequence of of subplinian eruptions, which started three days before the cataclysmic event. the short term precursors to the climactic phase, as often occurs in many explosive eruptions, generally causes a self-evacuation of the area, and reduces the impact of the eruption. a self-evacuation of the area occurred during the 1631 eruption, but the relative, high number of casualties was due to the prohibition to the evacuees to enter naples because the fear of plague [giuliani 1632]. we are conscious of the multiple possibilities that may develop when a volcano come backs into activity, so that, risk scenarios cannot be based only on a single case, but must take into account also an unlikely succession of events. we suggest that the population should be aware of this potential, and be made capable of reacting to unforeseen events. acknowledgements. we acknowledge the useful 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(1973). exlposive volcanic eruptions – a new classification scheme, geol. rund., 62, 431-446. zollo, a., p. gasparini, j. virieux, h. le meur, g. de natale, g. biella, e. boschi, p. capuano, r. de franco, p. dell’aversana, r. de matteis, i. guerra, g. iannaccone, l. mirabile and g. vilardo (1996). seismic evidence for a low-velocity zone in the upper crust beneath mount vesuvius, science, 274, 592-594. *corresponding author: roberto scandone, università di roma tre, dipartimento di matematica e fisica, rome, italy; email: scandone@uniroma3.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. scandone and giacomelli 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice articolo ran.pdf key words daqingshan piedmont fault (china) – hohhot segment – paleoearthquakes – seismic history – completeness of paleoseismic record 1. introduction the daqingshan piedmont fault (dpf) is one of the major active normal faults in hetao late quaternary history of paleoseismic activity along the hohhot segment of the daqingshan piedmont fault in hetao depression zone, north china yongkang ran, peizhen zhang and lichun chen institute of geology, china seismological bureau, beijing, china abstract the daqingshan piedmont fault (dpf) is one of the major active normal faults in the hetao depression zone in the northern part of ordos block, north china. it extends in nee direction along the daqingshan piedmont zone in the eastern part of the depression, dipping to the south, for a length of 223 km. the fault formed in the eocene and underwent strong movement during the cenozoic time. its vertical displacement amplitude has exceeded 2400 m since the quaternary. the fault can be divided into 5 active segments. paleoseismological studies were concentrated on its western part from baotou to tumdzuoqi whereas the hohhot segment to the east was scarcely studied. to fill this gap of knowlegde, the authors carried out in-depth study on the daqingshan piedmont fault during recent years. excavation of trenches at kuisu, ulanblang, and bakouzi sites on the hohhot segment of the daqingshan piedmont fault and study of geomorphic surfaces allow us to identify and date paleoearthquakes and to evaluate the completeness of paleoseismic activity history. this was done both for the individual sites and for the entire segment since the late quaternary using the «method for displacement confining» along the fault and «method for correlation between multiple trenches». in this paper we present the geological loggings of two trenches at kuisu site, provide the evidence for 6 events since 19 ka bp and the cumulative displacement amount produced by them is around 7 m. but the cumulative displacement amount obtained from difference in heights of geomorphic surfaces is 5. .5.5 m. results of tests using the method of displacement confining show that the event sequence revealed at this site can be considered complete. the data supplemented with information obtained in the ulanblang and bakouzi trenches show that 7 paleoseismic events occurred on the hohhot fault segment since 19 ka bp, i.e. they occurred at 18.75 ± 0.75 ka, 16.97 ± ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 6.83 ± 0.26 ka, and 4.50 ± 0.23 ka bp, respectively, and the average recurrence interval is 2.375 ± 0.432 ka. these results basically reflects the history of paleoseismic activity on the fault segment in this period of time. depression in the northern part of ordos block, north china (fig. 1). a m 7.5 earthquake occurred on the fault in 849 a.d. an important industrial-agricultural economic zone and two major cities of inner mongolia autonomous region, baotou and hohhot, are distributed basically along the fault zone. because the fault is a typical active normal fault and its movement can seriously threaten people’s lives and properties in the region, many scientists have studied its paleoseismic activity, fault movement behaviors, and recurrence intervals of strong 1053 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. yongkang ran, institute of geology, china seismological bureau, beijing 100029, china; e-mail: ykran@263.net earthquakes along the fault (research group on active fault system around ordos block, 1988; li et al., 1994; wu et al., 1995; nie et al., 1996; jiang et al., 2001). a large number of data have been accumulated and provided a good basis for new studies. however, there exist some weakly studied problems for the fault, for example, the previous paleoseismological studies were largely concentrated on its western part from baotou to tumdzuoqi and less on the other segments of the fault. are the other fault segments potentially seismogenic too? can the revealed paleoearthquakes be considered representative of a complete history of paleoseismic activity on the fault in the late quaternary? if it is incomplete, how can the seismic hazard be evaluated using the available paleoseismological data? these questions led the authors to conduct a detailed study on the daqingshan piedmont fault during recent years, in particular, on the hohhot segment. this paper deals with the paleoseismic events revealed along the segment since the late quaternary and discusses the completeness of its paleoseismic activity history. 2. brief geology and geomorphology dpf is one of three major active normal faults in the hetao depression zone in the northern part of ordos block, north china. it ex1054 yongkan ran, peizhen zhang and lichun chen fig. 1. map showing active faults in north china and the position of study area: 1 active faults in late pleistocene and holocene; 2 active faults in early and middle pleistocene; 3 hidden active faults in quaternary; 4 normal fault; 5 reverse fault; 6 strike-slip fault; 7 quaternary sedimentary basin; 8 quaternary uplifted area. tends in a ene direction, dip to the south, along the southern piedmont of daqingshan range for a length of 223 km. the daqingshan range in the north of the dpf has been uplifted to an elevation of 1000 m above the hohhot basin, a vertical displacement of more than 2400 m since the quaternary was estimated (research group on active fault system around ordos block, 1988). the range, consisting mainly of achean metamorphic rocks and jurassic debris rocks is formed by a tilted block. the southern slope of the range is far steeper than the northern slope. the tertiary planation surface on the range top dips to the north. as the daqingshan range intermittently uplifted due to the fault movement, gorges were formed by creeks flowing southward, along which at least 3-4 terraces are well developed. the most striking geological-geomorphic features along the daqingshan piedmont fault are three surfaces, which are widely distributed on the footwall of the fault. the first-step surface, formed ca. 8000 10.000 years ago (li et al., 1994), consists mostly of the raised alluvial fan deposits controlled by fault scarp. its height is several to over ten meters above the present surface of the basin. the secondand third-step surfaces are mostly covered with late pleistocene alluvialdiluvial or alluvial-lacustrine sediments (the youngest lacustrine sediments were dated to be 23.0 26.0 ka bp; li et al., 1994) and are located at 120 170 m above the present basin. the piedmont tableland along the hohhot segment is not well preserved, but the spur surface 60-90 m high was found. the elevation of surface in the west of baotou gradually decreases down to 15 m. the fault scarps usually appear to cut the surfaces. the dpf consists of step-like normal faults along its strike and the spatial distribution of their movement appears to have been uneven since the late pleistocene (research group on active fault system around ordos block, 1988). li et al. (1994) claim that the dpf can late quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china fig. 2. principal characteristics of the daqingshan piedmont fault: 1 active faults in late pleistocene and holocene; 2 pre-pleistocene active fault; 3 buried fault; 4 depth contour of the quaternary deposits; 5 segment boundary, dotted lines were decided by predecessors, but given up now. 1055 be divided into 5 segments according to its geometry, kinematics, and distribution of paleoseismic events, i.e. yellow river-xuehaigou, xuehaigou-tumdyouqi, tumdyouqi-meidaiqiao, meidaiqiao-tumdzuoqi and tumdzuoqi-kuisu segments. jiang et al. (2001) divided the dpf into three segments, i.e. west baotou, baotoutumdzuoqi, and tumdzuoqi-hohhot segments. the main evidence for this interpretation is that the 849, m 7.5 earthquake rupture extended from xuehaigou to tumdzuoqi, and thus the three segments in the li et al. (1994) model can be considered as a single segment. we prefer the division of the fault into 5 segments, but by introducing some changes in the the boundary locations and lengths of the segments. according to the distribution of cumulative displacement amounts reflected by the isopach of quaternary deposits along each segment (fig. 2), we consider that the dpf should be better divided into 5 segments, the yellow river-xuehaigou (baotou) segment 37 km long, xuehaigou-tumdyouqi (west tumdyouqi) segment 35 km long, tumdyouqi-tumdzuoqi (west tumdzuoqi) segment 56 km long, tumdzuoqiusutu (bikeqi) segment 49 km long, and usutukuisu (hohhot) segment 46 km long. according to li et al. (1994), the displacement rate since the late pleistocene is 4.75. .6.46 mm/a along the west tumdyouqi, west tumdzuoqi and bikeqi segments, 2.4. .3.5 mm/a along the baotou segment, and 2.4-3.5 mm/a along the hohhot segment. 3. trench logs at several sites and paleoseismological analysis three main trenches and their supplementary trenches were excavated at the sites of kuisu, ulanblang and bakouzi (fig. 3) to disclose evidence for paleoseismic events in adjayongkan ran, peizhen zhang and lichun chen fig. 3. geological sketch showing the hohhot segment of daqingshan piedmont fault: 1 upper holocene; 2 lower holocene; 3 upper pleistocene to holocene; 4 upper pleistocene; 5 cretaceous; 6 jurassic; 7 paleozoic; 8 archean; 9 granites; 10 late pleistocene active normal fault; 11 section with clear fault scarp; 12 quaternary fault; 13 geologic boundary; 14 recent alluvial fan; 15 location of trench; 16 boundary between fault segments. 1056 cent trenches at the same site and along segments to identify the whole rupture extent. 3.1. kuisu site 3.1.1. analysis of deposits and evidence of paleoseismic events exposed in the main trench (tc3) the excavated trenches are located in the west of kuisu village, where the kuisu gully runs from north to south and resulted in the formation of a huge alluvial-diluvial fan accumulated on the mountain spur (fig. 3). the alluvialdiluvial fan was offset by the dpf and its surface was abandoned on the upthrown side of the fault (fig. 4). the surface was partially destroyed by the gully of regressive erosion. it can be seen on the section cut by the gully that the tableland on the upthrown side is composed of boulders or large-sized cobbles covered by eolian or secondary loess. the downthrown side is more complicated, as some deposits are from gully terrace sediments and others are loess or secondary loess with several meters in thickness. we attempted to reveal as many paleoseismic events as possible on the one hand, and to collect samples for 14c dating on the other. a large trench (20 m long, 3 m wide, and 5 m deep in maximum) was excavated in front of the tableland, where the fault scarp was less disturbed by recent human activity. a supplementary trench (tc4) was excavated very close and along the gully (20 m long and 4 m deep) to collect samples for 14c dating where the most recent sediments were accumulated. the sediments exposed in trench tc3 were divided into 9 units (fig. 5). unit 1, exposed on the upthrown side of the fault, consists of gravel layers from the stream, they show clear bedding and sorting. the grain size reaches maximum 50 cm and visible thickness is 4.2 4.6 m. unit 2 is found only on the downthrown side of the fault on the western wall of the trench. it is composed of yellow silt and sandy soil, lithologically similar to eolian loess, weakly pedogenic, corresponding to sediments on the surface. units 3, 4, 5, 6 and 7 are only exposed on the downthrown side of the fault and lithologically are not significantly different, composed of loess and secondary loess. all these units are commonly characterized by a higher content of gravel close to the fault, variegated, and loess or brown loess clumps found near the fault and at the bases of the units, but they gradually change into pure loess or secondary loess at a distance from the fault. obviously, they are the mixed products of accumulation in front of the scarp after faulting and during eolian sedimentation. unit 8 covers the top of the fault and consists mainly of blackish sandy soil near the fault, but pale due to the concentration of calcium, gradually changes into loess or secondary loess at a distance from the fault. unit 9 is composed of yellow-black sandy soil with gravel. five offset scarp-front deposit units possibly representing 6 paleoearthquakes can be distinguished in accordance with the above-described lithological characteristics and the distribution of strata in combination with the disturbance and dislocation of the strata by the fault (fig. 5). unit 2 represents an ancient land surface, a sample (kt1) dated by thermoluminescene (tl) method to be 18.80 ± 1.45 ka bp late quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china fig. 4. topographic map of the area west of kuisu village: spacing between topographic contours is 1 m; square shows bore pit for sampling, at 56 cm below the surface. 14c sample (kc5) from thin-layer carbonaceous sandy clay was dated to be 19.990..±..395 years bp; rectangle shows location of excavated trenches. 1057 1058 yongkan ran, peizhen zhang and lichun chen fig. 5. geological logging of the kuisu large trench (tc3): 1 gravel; 2 yellow sandy soil; 3 loess or soil lumps; 4 surface soil; 5 stratigraphic unit and its number; 6 fault and its number; 7 tl sample site and its number. stratigraphic sequence: unit 1 gravel layer, layered and sorted; unit 2 yellow silt and sandy soil, weakly pedogenic; unit 3 mixed sediments of yellow sandy soil with gravel and loess or brown loess clumps, gradually changing into loess or secondary loess with distance from fault; unit 4 yellow sandy soil, content of gravel increases near fault and at base of the unit, variegated, containing brown loess clumps and gradually changing into loess or secondary loess; units 5, 6, and 7 mixed sediments of yellow sandy soil with gravel and loess or brown loess clumps, content of gravel increases near fault and at their base, variegated, containing loess or brown loess clumps, which gradually change into pure loess or secondary loess with distance from fault, similar to units 3 and 4. unit 8 mainly pale sand, rich in calcium; unit 9 yellow silt with small-sized gravel. (table i). it reflects the oldest paleoseismic event occurring some time later than the age value. then unit 3 was deposited. unit 3 was offset by fault f2, providing clear evidence for a second event. the subsequent unit 4 covers fault f2. tl dating of samples from the top part of unit 3 and the base of unit 4 yielded age values of 17.92 ± 1.38 ka bp (kt2) and 16.01 ± ± 1.20 ka bp (kt3), respectively. thus, the second paleoseismic event occurred between these two age values. the third paleoseismic event caused fault fl to offset unit 4 and resulted in formation of a filled wedge near the fault, as found on both eastern and western walls of the trench. then, unit 5 was accumulated and covers the filled wedge. tl samples from the top part of unit 4 and the base of unit 5 were dated to be 15.31 ± 1.06 ka bp (kt4) and 13.77 ± 1.06 ka bp (kt5), respectively. the fourth event led unit 5 to be in direct contact with the fault on the western wall of the trench and a filled wedge to be developed. it expresses an offset of unit 5 by the fault on the eastern wall of the trench and unit 6 covers a branching fault. tl sample from the top of unit 5 was dated to be 12.51 ± 0.96 ka bp (kt6). afterwards, unit 6 was accumulated. tl sample from the base of unit 6 was dated to be 11.13 ± 0.85 ka bp (kt7). thus, the fourth event occurred between 12.51 ± 0.96 and 11.13 ± 0.85 ka bp. the fifth event is expressed as a possible dislocation of unit 6 and led to accumulation of unit 7 on both walls of the trench. this event is less contrained than the others. it is known from age values of tl samples (kt8 and kt9) from the top part of unit 6 and the middle part of unit 7 that the event occurred between 9.72 ± 0.75 ka bp and 7.09 ± 0.55 ka bp. the sixth event has 1059 late quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china table i. age values confining paleoseismic events. trench# sample# material dating measured age calib. age method (years bp) (years bp) tc1 qc7 humus 14c 9820 ± 160 10 990 ± 150 qc8 humus 14c 9270 ± 130 10 230 ± 150 qc6 humus 14c 10 140 ± 190 qc5 humus 14c 5780 ± 85 6570 ± 110 tc2 tw9 silty soil tl 19 750 ± 1520 tw7 silty soil tl 15 320 ± 1180 tw8 silty soil tl 13 890 ± 1070 tw1 silty soil tl 10 710 ± 820 tw2 silty soil tl 8110 ± 620 tw3 silty soil tl 7720 ± 590 tw4 silty soil tl 6490 ± 490 cw1 humus 14c 4030 ± 10 tc3 kt1 silty soil tl 18 800 ± 1450 kt2 silty soil tl 17 920 ± 1380 kt3 silty soil tl 16 010 ± 1200 kt4 silty soil tl 15 310 ± 1060 kt5 silty soil tl 13 770 ± 1060 kt6 silty soil tl 12 510 ± 960 kt7 silty soil tl 11 130 ± 850 kt8 silty soil tl 9720 ± 750 kt9 silty soil tl 7090 ± 550 tc4 wc2 humus 14c 13 570 ± 140 wc3 humus 14c 9210 ± 180 bore pit kc5 humus 14c 19 990 ± 395 notes: tl samples and 14c samples were dated in the neogeochronological laboratory of institute of geology, china seismological bureau. dislocated unit 7 and then the unit 8 was accumulated. at present, only the time of the sixth event is known to be later than 7.09 ± 0.55 ka bp. moreover, the traces of three later events can be found in trench tc4 too. we attempted to estimate coseismic displacement amount by analysis of thickness and lithological composition of scarp-front deposits. in a simple model of normal faulting, the thickness of fault-scarp colluvial wedges near the normal fault is commonly assumed to be half of the initial scarp height (mccalpin, 1996). but if the scarp-front deposits were not completely the colluvium from initial scarp, then their double thickness should be regarded as a maximum displacement amount. we measured the maximum thickness of scarp-front deposits formed by every event (measurement was done perpendicularly to bed plane and the thickness of deposits on the upper breaking point was measured for the last event). the obtained thickness of them is 0.50 ± 0.03 m, 0.85 ± 0.05 m, 1.1 ± ± 0.1 m, 0.65 ± 0.03 m, 0.8 ± 0.3 m, and 0.6 ± ± 0.02 m, respectively. we know from the above that all the scarp-front deposits formed by events disclosed in the tc3 contain some eolian loess, except for the last event, so they are the mixed deposits of colluvial and eolian materials. therefore, we tend to regard the thickness of scarp-front deposits formed by events i v as the minimum coseismic displacement amount, i.e. 0.50 ± 0.03 m, 0.85 ± 0.05 m, 1.1 ± ± 0.1 m, 0.65 ± 0.03 m, and 0.8 ± 0.3 m, respectively. whereas, double thickness of scarp-front deposits are regarded as the maximum coseismic displacement amount; these are ca. 1 m, 1.7 m, 2.2 m, 1.3 m, and 1.6 m, respectively, and their mean values represent the inferred displacement amounts produced by the events i v at this site, i.e. 0.7 ± 0.25 m, 1.28 ± 0.43 m, 1.65 ± ± 0.55 m, 0.95 ± 0.35 m, and 1.2 ± 0.4 m, respectively. because the deposit by event vi does not contain eolian loess, double thickness of the deposit covering the fault is used to represent the inferred coseismic displacement amount, i.e. ca. 1.2 m. thus, the minimum cumulative displacement amount by the paleoseismic events revealed at this site is ca. 5.1 m and the maximum is ca. 9 m, mean value is ca. 7.03 m. 3.1.2. paleoseismological analysis of supplementary trench (tc4) a supplementary trench was excavated on the eastern wall of the gully east of the main trench (fig. 6). it mainly revealed part of a small graben formed by fault movement on several faults. as the trench was excavated along the 1060 yongkan ran, peizhen zhang and lichun chen fig. 6. geological log of kuisu supplementary trench (tc4): 1 sandy gravel; 2 brown-black clayey sand; 3 yellow clayey sand; 4 fault; 5 fissure and fillings; 6 stratigraphic unit and its number; 7 site of 14c sample; 8 analyzed part of the profile in detail. gully wall, the relation between the exposed strata and displacement is more complicated, a correlation between fault scarp and collapse wedge is not clearly found, but some characteristics of deposits in the small graben in the front of scarp can be distinguished. if the gravel layer-brown-black sandy soil-yellow sandy soil sequence is regarded as a sedimentary unit, and other sequences are stratified in terms of their lithological characteristics. a total of 10 stratigraphic units are identified. units 1 3 are the gravel layer-brown-black sandy soil-yellow sandy soil sequence. unit 5 is composed of about 85 cm thick brown sandy soil. unit 7 is relatively pure eolian loess. units 9 and 10 are recently ditch-filling yellow sandy soil and sediments on the ditch surface. the units 2 5 are characterized by deposition in a small graben in the front of the scarp and well correlated with fault movement traces. in the trench log the top part of fault cutting unit 2 is covered with unit 3 and the fault cutting unit 3 is covered with units 4 7, which were also offset. it indicates that at least 3 events can be distinguished in the log. 14c sample (wc2) from the top part of brownblack soil of unit 2 was dated to be 13.570 .±.140 years bp. 14c sample (wc3) from the top part of brown-black soil of unit 3 was dated to be 9210 ± 180 years bp. it follows that 3 events occurred since 13.57 ka bp, two of them between earlier than 9.21 ka bp and 13.57 ka bp, the third after 9.21 ka bp. these appear to correlate well with the fourth, fifth, and sixth events revealed in trench tc3. 3.2. ulanblang site (tc2) a large trench (tc2) was excavated (20 m long, 3.5 m wide, and 4-4.5 m deep) at about 400 m east of ulanblang village. at this site the fault scarp is well developed and two steps of scarp can be distinguished because of gully erosion. moreover, there is a clear change in the slope grade on the recent wide gully bottom. here actually there are three fault scarps f with different heights. they are about 7.2 m, 3.6 m and 1.2 m high, respectively. the trench tc2 was excavated across the highest step of scarp. the strata exposed in the ulanblang large trench (tc2) can be divided into 16 units (fig. 7). units 1 3 are exposed only on the footwall of the fault. unit 1 consists of cretaceous conglomerate and mudstone and units 2 and 3 consist of late pleistocene fluvial-lacustrine sediments. units 4 11 are characterized by scarpderived deposits caused by fault movement. unit 4 was deposited near fault f1 and consists of brown-black sandy clay with gravel in a fault contact with unit 5, which is gray-yellow sandy clay with gravel and then gray-yellow sandy soil with massive structure was accumulated and formed unit 6. tl sample (tw9) from the upper part of unit 6 was dated to be 19.75 ± 1.52 ka bp (table i). this stratigraphic contact relation indicates that the first event revealed in the trench occurred before the deposition of unit 6, earlier than 19.75 ± 1.52 ka bp. unit 7 is a wedge-shaped calcium-rich pale deposit near fault fl. unit 8 is gray-yellow sandy soil with gravel in its lower part and upward becomes pedogenic brown sandy soil. it indicates that the unit was exposed at the surface for long time and then has offset and buried. thus, it is conceivably the result of a paleoseismic event. tl sample (tw7) from the top part of the unit 8 was dated to be 15.32 ± ± 1.18 ka bp. later, unit 9 was accumulated, tl sample (tw8) from its base was dated to be 13.89 ± 1.07 ka bp. it indicates that the second event disclosed in the trench occurred between 15.32 ± 1.18 ka bp and 13.89 ± 1.07 ka bp. unit 9 has covered the paleosol buried by event 2 and is a chaotic soil and gravel deposit, with more gravel in lower part, and calcium-rich pale deposit in middle part. unit 9 represents a scarp-derived deposit mainly formed of colluvial material and was offset by event 3. soon after the event, the wedge-shaped unit 10 was formed (near faults f1 and f5). tl samples (tw8 and tw1 ) from the base of unit 9 and the top part of unit 10 were dated to be 13.89 ± ± 1.07 ka bp and 10.71 ± 0.82 ka bp, respectively, indicating event 3 occurring between the two age values. event 4 caused fault f5 to offset unit 10, as found on both eastern and western walls of the trench, which was covered with unit 11. age value of tl samples (tw2 and tw3) from the base of unit 11 and the top 1061 late quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china part of unit 10 are 8.11 ± 0.62 ka bp and 10.71 ± ± 0.82 ka bp, respectively. thus, the event occurred between 10.71 ± 0.82 ka bp and 8.11 ± ± 0.62 ka bp. unit 11 was offset by event 5, as found on the west wall of the trench. unit 12 was mainly accumulated near fault f5. age values of tl samples (tw3 and tw4) from the top part of unit 11 and the base of unit 12 indicate the event occurring between 7.72 ± 0.59 ka bp and 6.49 ± 0.49 ka bp. unit 12 was off1062 yongkan ran, peizhen zhang and lichun chen fig. 7. geological log of large trench (tc2) in ulanblang village: 1 interlayers of conglomerate and mudstone; 2 sand-gravel with clear layering; 3 sandy soil with dispersed gravel; 4 buried soil; 5 loess or soil lumps; 6 surface soil; 7 stratigraphic unit and its number; 8 fault and its number; 9 location of tl sample and its number; 10 location of 14c sample .and its number. stratigraphic sequence: unit 1 weathered and semi-weathered jurassic conglomerate with grayish mudstone containing carbonized matters; unit 2 lacustrine gray-yellow and orange-yellow silt clayey sand with interlayers of coarse sand-small-sized gravel and clayey sand; unit 3 red clayey sand with gravel lenses; unit 4 brown-black sandy clay with gravel; unit 5 gray-yellow sandy clay with dispersed gravel; unit 6 gray-yellow sandy soil with massive structure; unit 7 gray-yellowish calcium-rich sandy soil; unit 8 gray-yellowish sandy soil with gravel in lower part and upward becomes darkbrown sandy soil (pedogenic); unit 9 yellow sandy soil with gravel; unit 10 yellow sandy soil; unit 11 sandy soil with gravel; unit 12 brown-yellow sandy soil with dispersed gravel, brown-black and black southward; unit 13 thin black soil; unit 14 deposits in ditch (may be man-dug ditch); unit 15 brown-yellow and brownblack sandy soil with dispersed gravel; unit 16 layering and well sorted small-sized gravel bed in lower part and brown-black soil rich in humic matter in upper part. set by event 6 and faults f3. .f5 and was covered with sediments of unit 13 and later. 14c sample (cw1) from unit 13 was dated to be 4030 ± ± 10 years bp. part of unit 13 filled the cracks in unit 12, reflecting its accumulation following event 6. it indicates that the event occurred slightly earlier than 4030.±.10 years bp. 3.3. bakouzi site (tc1) the bakouzi trench (tc1) was excavated at about 1 km east of bakouzi. correlative geomorphic surfaces at this site are clearly different in elevation, though the fault scarp is poorly preserved due to recent human activity. at a mountain spur, terraces on a series of gullies flowing toward the basin were offset. elevation of the highest terrace is more than 10 m above gully beds, the third-order terrace is 5-6 m, and that of the second-order terrace is 3-4 m. the sediments on the second-order terrace were offset by fault at many sites. we analyzed the exposure of the gully east of bakouzi (fig. 8). the strata exposed on the gully wall can be divided into 7 stratigraphic units. unit 1 is red clay, earlier sediment. units 2 4 are gully deposits . 14c sample (qc8) from the base of unit 4 was dated to be 10.230 .± .150 years bp (table i) after dendrochronological correction. units 5. .7 represent scarp-derived deposits mainly formed of colluvial material. unit 5 is a wedge-shaped deposit formed in a fissure in front of the scarp and is composed of mixed gravel and sandy soil. 14c sample (qc7) from its top brown-black soil was dated to be 10.990 .±.150 years bp after dendrolate quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china fig. 8. geological log of the trench (tc1) excavated on gully wall east of bakouzi: 1 gravel; 2 soil lumps; 3 stratigraphic unit and its number; 4 location of 14c sample and its number. stratigraphic sequence: unit 1 red clay locally containing gravel; unit 2 gravel bed intercalating a brown-yellow sandy clay layer; unit 3 interlayers of small-sized gravel and sandy clay with brown-black soil clumps; unit 4 gravel and sandy soil; unit 5 wedge-shaped deposit of mixed gravel and sandy soil; unit 6 colluvial wedge, more gravel in lower part and brown-black soil with gravel in upper part; unit 7 brown-black sandy soil intercalating a gravel layer, more cobbles in base part and black soil containing more humic matter between gravels. 1063 chronological correction. unit 6 is a scarp-derived colluvial wedge, with more gravel in its lower part and brown-black soil with gravel in the upper part. 14c sample (qc6) from its top part was dated to be 10.140.± .190 years bp (without dendrochronological correction). unit 7 is brown-black sandy soil intercalating a gravel layer and covers the fault. cobbles are more concentrated at the base of the unit and, meanwhile, black humic matter was more found between the cobbles. 14c sample (qc5) from black soil at the base of the unit was dated to be 6570.±.110 years bp after dendrochronological correction. the above-described lithologic characteristics of the stratigraphic units and their structural relations with faults indicate that 3 paleoseismic events can be distinguished in the trench. the earliest event offset the deposits up to unit 3, then unit 4 was accumulated in the gully. the second event offset unit 4, resulting in the formation of mixed deposit of colluvial gravel and sandy soil in the lower part (or cleft in filled wedge), i.e. unit 5, and deposit in upper part of the colluvial wedge, i.e. unit 6. the third event offset unit 6 and led unit 4 on footwall of the fault to be in a fault contact with unit 6 on the hanging wall. age value of 14c sample from the base of unit 4 is slightly higher than that from the upper part of unit 5, the colluvial material of which might come from the older strata. therefore, it is inferred from dating results of the described statigraphic sequence that the first event occurred earlier than 10.990 ± ± 150 years bp, the second event was later than 10.230.±.150 years bp, and the third event was earlier than 6570.±.110 years bp. 4. the completeness of paleoseismic activity history the completeness of paleoseismic activity history along a fault is crucial for understanding the sequence of a fault and for a correct estimation of future seismic hazard. at present, no tested method for the evaluation of the completeness of paleoseismic activity history has been presented. but according to the characteristics of surface ruptures produced by normal faults and their evolution, we propose to use methods for «confining displacement amount» and for «correction between events in many trenches» to determine the completeness of paleoseismic activity history along the fault segment. 4.1. method for confining displacement amount many studies (wallace, 1977; bucknam and anderson, 1979; hanks, et al., 1984; mccalpin, 1987; deng and liao, 1996) highlighted that in addition to the direct determination of amount of displacement of the same marker horizon on both sides of a fault, the difference in elevation of the correlative geomorphic surface on both sides of a fault is a good indicator reflecting cumulative displacement. thus, both observations can be used as a measure for cumulative displacement amount along the fault in a given time period. the method for confining displacement is based on the measurement of the cumulative displacement produced by the paleoseismic events recognized in the trench and that cumulated the same site for the same time interval. if the results obtained using both the methods fit, the paleoseismic history for this time interval can be considered complete. the difficulty in using this technique is mainly related to the approach to correctly estimate the different types of displacement. detailed quantitative studies of elevation difference of geomorphic surfaces produced by normal fault (wallace, 1977; deng et al., 1984; ran et al., 1992, 1996) indicate that in most cases, the elevation difference of geomorphic surfaces obtained in field surveys is equal to (when the geomorphic surface is in a stable environment, the upper and lower primary surfaces are intact) or less than the real cumulative displacement amount along a fault since the formation of geomorphic surfaces (at least only 2/3 of the real displacement; in fact the geomorphic surface on the upthrown side may have been eroded in the long-term whereas on the downthrown side partial filling may have occurred. moreover, it should also be considered that except for the coseismic displacement directly obtained from offset sediments, the displacement amount obtained by other methods has uncertainties, especially when the coseismic displacement is obtained by measuring the double thickness of 1064 yongkan ran, peizhen zhang and lichun chen the colluvial wedge or different height of gully terraces. considering these uncertainties, whatever method is used to estimate the cumulative displacement of a geomorphic surface, this is expected to be a minimum. figure 9 presents a graph of time versus slip accumulation. an area with fluctuating value enclosing the range of the measured height of the geomorphic surface is shown. if this fluctuating displacement of geomorphic surfaces truly reflects the real displacement amount along the fault, comparing it to the coseismic displacement can provide insights on the completeness of the paleoseismic record. when the cumulative displacement is approximately equal to or larger than the displacement in the fluctuating area, then the paleoseismic activity history can be considered complete (fig. 9 case a and b). on the contrary, if the coseismic cumulative displacement is lower than that in the fluctuating zone, then the paleoseismic record appears to be incomplete (fig. 9 case c). 4.2. method for correlation between events in many trenches the method for correlation between events in many trenches is another useful method for decreasing the potential incompleteness of recognized paleoseismic events on the same segment caused by uneven distribution and different preservation degree of surface rupture traces at a single site produced by paleoearthquakes. the essence of the method is principally consistent with that of the «progressive confining method» for correlation between paleoearthquake occurrence times (mao and zhang, 1995). the basis for the method is that a single trench site is not enough to define a complete paleoseismic record and, as far as it is possible, it is recommended to investigate more than one site. even if the ruptures produced by some events did not extend to one site, or were removed by erosion, evidence of the missing event can be derived from different sites. it is also possible to use many trenches for gathering more information supplementing each other, so that any event which occurred along the segment was not missing. for this method, it is assumed that several trenches were excavated along a fault segment and the paleoseismic events revealed in the trenches are different in number, but some events found in different trenches overlap each other in their age values. thus, by using a progressive confining method we can constrain in age some events having only their upperor lower-limit age value determined or having their upperand lower-age limit within their corresponding time intervals and then can better determine the age of these paleoseismic events. 4.3. completeness of paleoseismic activity history identification of the completeness of paleoseismic activity history using the two abovedescribed methods follows the principle that the first is to use the method for confining displacement to determine whether the events revealed in each trenched site are complete and then to use the method for correlation between events in many trenches for supplementing and correc1065 late quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china fig. 9. conceptual scheme for determination of the completeness of paleoseismic activity history from displacement amount. stepped line with arrow indicates an assumed revealed paleoseismic activity history. line a, extending into the completeness area, shows a complete activity history; line b, into the uncertainty area, remains to be further determined by other methods; line c, into the incompleteness area, indicates a revealed incomplete paleoseismic activity history. ting the paleoseismic activity history along the fault segment. 4.3.1. test of the completeness of paleoseismic activity history revealed in kuisu trenches critical in the method for confining displacement amount is that the time of the oldest event should be equal to the time when the geomorphic surface was formed and offset. study on the regional geomorphic surfaces disclosed that a lacustrine tableland formed 26-23 ka bp is widely developed in hetao region (li et al., 1994; wu et al., 1995). then another two or three diluvial tablelands developed. in the boundary zone between shanxi, hebei and inner mongolia, the late quaternary diluvial tablelands were formed in the ranges 3 0 ka, 10 7.5 ka, 13.5-10.3 ka, and 18.7-15.6 ka bp (xu et al., 1996). stratigraphic correlation of the deposits on the tablelands along the hohhot segment suggests that the geomorphic surfaces at the kuisu trench sites were formed later than the lacustrine tableland surface, approximately in 18.7 15.6 ka bp. the age of 14c sample from a thin-layer carbonaceous sandy clay at 56 cm depth below the geomorphic surface at the kuisu site is 19.990 ± 395 years bp, representing basically the formation time of the geomorphic surface. it corresponds roughly to the time when the oldest event were revealed in the trenches. therefore, the identification of the completeness of paleoseismic events at the two sites by using the method for confining displacement amount is feasible. six paleoseismic events were revealed in the trenches (tc3, tc4) at kuisu site on the hohhot segment. the measured geomorphic surface shows displacement of 5 5.5 m, while the cumulative displacement obtained from colluvial wedges is 5.1 m in minimum and 9 m in maximum, and is taken to be 7.03 m produced by paleoearthquakes. the cumulative displacement produced by the paleoearthquakes is larger than that measured from geomorphic surface. the paleoseismic activity history revealed at this site by using method for confining displacement amount is complete (fig. 10a-c). 1066 yongkan ran, peizhen zhang and lichun chen c a b fig. 10a-c. scheme showing the application of the displacement confining method for identifing the completeness of paleoseismic activity history at kuisu site. stepped line with arrow indicates the paleoseismic activity history: a) shows confining result by using the minimum displacement; b) shows confining result by using the maximum displacement; c) shows confining result by using the mean displacement. figure 10a-c shows that even the minimum cumulative displacement does not fall inthe incompleteness area, and both the mean and maximum cumulative displacement fall in the completeness area. on this basis the paleoseismic events revealed at kuisu site are considered to be complete. event does not have its confined age value, but it can be deduced by correlation from the upper-limit age value of the third event and chronological information obtained in kuisu trench tc3, i.e. the age values of the two earliest events in the two trenches are approximately equal. the difference is that the latest event in ulanblang trench occurred in little earlier than 4000 years bp. this event was not found in kuisu trench tc3 and other trenches. the events revealed in kuuisu trench tc4 and in bakouzi trench tc1 are fewer in number, but these events and their age values supplemented those in kuisu trench tc3 and ulanblang trench tc2. in general, the paleoseismic events reveallate quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china 4.3.2. correlation between paleoseismic events along the fault segment it is clear that the above-described three sites on the fault segment are different in tectonic positions and geomorphic surfaces where the trenches were excavated, thus, the number of paleoseismic events revealed in the trenches and age values of dated samples fluctuate (fig. 11). it can be found in fig. 11 that 6 paleoseismic events revealed in kuisu trench tc3 (kiusu 2 in fig. 11) yielded well confined their age values. 6 paleoseismic events were revealed in ulanblang trench tc2, but the second from the last fig. 11. correlation between paleoseismic events at different sites on the hohhot segment of the daqingshan piedmont fault. 1067 ed in 4 trenches and their age values are confined within 7 time windows. it indicates that in all 7 paleoseismic events occurred on the hohhot segment since about 19 ka bp, i.e. they occurred in 18.75 ± 0.75 ka, 16.97 ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 7.41 ± 0.315 ka, and 4.50 ± 0.23 ka bp, respectively. this would suggest an average recurrence interval of 2375 ± 432 years. it is clear that by using correlation between events found in many trenches, the events and their age values revealed in 4 trenches can be corrected and supplemented to each other, so we can eventually determine 7 paleoseismic events which represent the seismic activity history along the hohhot fault segment since about 19 ka bp. moreover, the latest event may be missing at kuisu site, as the site is located near the boundary of the studied fault segment. paleoseismic traces in the bakouzi trench were removed by erosion from gully, so the loss of seismic events is expected. 5. conclusions excavation of trenches at kuisu, ulanblang, and bakouzi sites on the hohhot segment of the daqingshan piedmont fault and detailed study of geomorphic surfaces permitted us to identify the completeness of paleoseismic activity history at single sites and on the fault segment using methods for confining displacement amount and method for correlation between events in many trenches and provide an improved dataset. two trenches at kuisu site have mutually confirmed the revealed 6 seismic events since 19 ka bp. the cumulative displacement amount produced by these events is around 7 m and that obtained from difference in height of geomorphic surfaces is 5. .5.5 m. the result of the test using method for confining displacement amount shows that the paleoseismic events revealed at this site is complete. the ulanblang and bakouzi trenches revealed 6 and 3 events, respectively. the second event from last in ulanblang trench has no age value limit, so its age can be determined from the upper-limit age value of the third event and chronological information obtained in kuisu trench. the latest event in ulanblang trench occurred a little earlier than 4000 years bp, but this event was not found in kuisu and other trenches. events revealed in bakouzi trench are fewer in number, but the events and their age values in this trench well supplement those in kuisu and ulanblang trenches. the integration of data from different trenches at 3 sites shows that a total of 7 paleoseismic events occurred on the hohhot segment since 19 ka bp, i.e. they occurred in 18.75 ± 0.75 ka, 16.97 ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 7.41 ± 0.315 ka, and 4.50 ± 0.23 ka bp, respectively, and the average recurrence interval is 2375 ± 432 years. it basically reflects the paleoseismic activity history on the fault segment during this period of time. acknowledgements this work is supported under one of the national major basic theoretical research projects mechanism and prediction of strong earthquake of china (g1998040701). we are especially grateful to dr. daniela pantosti for her valuable suggestions about the paper. thanks to prof. defu shen, dr. zhujun han, prof. fa yang, wengshen guo, bo hu, and zhiming liu for favorable discussions during this project. references bucknam, r.c. and r.e. anderson (1979): estimation of fault scarp age from a scarp-height-slope-angle relationship, geology, 7, 11-14. deng, qidong and yuhua liao (1996): paleoseismology along helanshan piedmont fault, north-central china, j. geophys. res., 101 (b3), 5873-5893. deng, qidong, wang yipeng, yuhua liao, zhang weiqi and li mengluan (1984): collapse wedges by fault scarps and holocene history of movement along helanshan piedmont fault, chin. sci. bull., 9, 557-560 (in chinese with english abstract). hanks, t.c., r.c. bucknam, k.r. lajoie and r.e. wallace (1984): modification of wave-cut and faulting-controlled landforms, j. geophys. res., 89 (b7): 5771-5790. jiang, wali, xiao zhenmei, wang huanzhen and gong fuhua (2001): segmentation character of seismic surface ruptures of the piedmont active fault of mt. daqingshan, inner mongolia, seismol. geol., 23 (1), 24-34 (in chinese with english abstract). 1068 yongkan ran, peizhen zhang and lichun chen li, ke, wu weimin, yang fa, nie zongsheng, guo wensheng and ma baoqi (1994): a study on segmentation of daqingshan piedmont active fault, in research on active faults in china, edited by seismogeological subcommission of chinese seismological society (seismological press, beijing), 102-113 (in chinese). mao, fengying and zhang peizhen (1995): method for progressive confining of paleoseismic events and paleoseismological study of major fault zone in northern xinjiang, in research on active faults (seismological press, beijing), 4, 153-164 (in chinese with english abstract). mccalpin, j.p. (1987): geologic criteria for recognition of individual paleoseismic events in extensional environments, in directions in paleoseismicity, proceedings of conference xxxix, edited by a.j. crone and e.m. omdahl, u.s. geol. surv. open-file rep. 87-683, 102-114. mccalpin, j.p. (1996): paleoseismology (academic press, new york), pp. 583. nie, zongsheng, jiang wali and wu weimin (1996): study on holocene paleoearthquakes in large trenches on the western segment of daqingshan piedmont fault zone, inner mongolia, in research on active faults (seismological press, beijing), 5, 125-135 (in chinese with english abstract). ran, yongkang, fang zhongjing, wang jingbo, li zhiyi, li rucheng, zhang chongli and zhang dawei (1992): late quaternary movement behavior along shenzhuanghaopjiapo segment of northern border fault of huai-zhu basin, in research on active faults (seismological press, beijing), 2, 152-162 (in chinese with english abstract). ran, yongkang, fang zhongjing, duan ruitao, li zhiyi, wang jingbo, yang zhe, ji fengju and li jianping (1996): recurrence model of paleoearthquakes along baying segment of the northern marginal fault of fanshan basin in hebei province, earthquake res. china, 10 (1), 87-100. research group on active fault system around ordos block (1988): active fault system around ordos block, state seismological bureau (seismological press, beijing), (in chinese). wallace, r.e. (1977): profiles and ages of young fault scarps, north-central nevada, bull. geol. soc. am., 66, 1267-1278. wu, weimin, li ke, ma baoqi, yang fa and guo wensheng (1995): study on holocene paleoearthquakes in large composite trench on the daqingshan piedmont fault, in research on active faults (seismological press, beijing), 4, 123-132 (in chinese with english abstract). xu, xiwei, nobuyuki yonekury, yasuhiro suzuki, qidong deng, yipeng wang, akira takeuci and cunyu wang (1996): geomorphic study on late quaternary irregular favelting along the northern piedmont of liulengshan range, shanxi province, china seismol. geol., 18 (2), 169-181 (in chinese with english abstract). 1069 late quaternary paleoseismicity along the hohhot segment of the daqingshan piedmont fault, north china l l l l l l l l 241_242 casazza2.pdf 241 annals of geophysics, vol. 46, n. 2, april 2003 some remarks on pm2.5 marco casazza and angelo piano dipartimento di fisica generale «a. avogadro», università di torino, italy abstract since 1970, the general physics department of «università degli studi di torino» has carried out a project research, on inorganic solid particulate matter. the special issue of annals of geophysics, published for professor giorgio fiocco’s 70th birthday, gives us the possibility to make some important remarks on this topic, focusing on pm2.5. this has been possible using all the old and new experimental data of the measures made by the authors of this paper since 1970. mailing address: dr. angelo piano, dipartimento di fisica generale «avogadro», università di torino, via p. giuria 1, torino, italy; e-mail: piano@to.infn.it key words pm2.5 – solid inorganic particulate matter 1. introduction since 1970, the general physics department of «università degli studi di torino» has carried out a research project on inorganic solid particulate matter (pm) (longhetto et al., 1975; bacci et al., 1983). the purpose was to obtain not only knowledge about the mass concentration of pm2.5, but also to understand the correlations with climatological and meteorological factors. recently, we devoted more attention to indoor measurements and on outdoor-indoor infl uences. the special issue of annals of geophysics, published for professor giorgio fiocco’s 70th birthday, gives us the possibility to make some important remarks on this topic. this has been possible collecting the experimental data of measures (old and new ones) made by the authors of this over during more than 30 years. the particles, dispersed in the atmosphere, with an aerodynamic diameter of less than 2.5 mm (pm2.5) have been studied with great accuracy because, being considered the respirable fraction of suspended particulate matter (spm), they can reach the inner parts of the respiratory apparatus and hence the alveoli. 2. dimensional spectra and associated chemical elements around 1970 we started to measure the concentrations of inorganic particles, for the purpose of characterising the emissions of sources of great intensity and the immission in the environment in the neighbourhood of the studied sources. at that time, the granulometric distribution of the dimensions of particles, expressed as dn/d (logr) (% of total), was in good agreement with the theoretical one of noll and pilat (1971), for dust of continental origin. we saw, analysing more than 2 106 samples collected both in urban and rural sites, using the technique of atomic absorption spectrophotometry (aas), that the chemical elements associated to the suspended particulate matter (spm), like pb, cd, v, are associated with particles of smaller diameter (fi g. 1), while al, cu, fe are not so much linked to pm2.5 (fi g. 2). after 25 years, the granulometric distribution has changed and, today, particularly in the urban environment, a typical representation of its trend can be shown using fi g. 3. 242 marco casazza and angelo piano this distribution of mass concentration was obtained using an andersen 2000 inc. (1977) non-viable cascade impactor (andersen, 1958; vaughan, 1989), which is composed of 8 different stages and permits the collection of pm with aerodynamic diameter of less than 11 mm. the fraction with aerodynamic diameter of less than 0.8 mm is much increased respect to the particles of larger dimensions. this is confi rmed by the growth by two orders of magnitude, of the concentration of number per unit volume, of aitken nuclei (diameters from 0.01 mm to 0.1 mm), that, nowadays, in urban environment, reach some units multiplied by 105 per unit volume (cm3). 3. correlation with meteorological factors we considered it useful to have continuous measures of mass concentration. those were made using a dusttrak 8520 aerosol monitor, produced by tsi (2001). many temporal sequences, during the last 6 years, and the knowledge of the contemporary measures of the values of the meteorological parameters, such as solar radiation, wind speed and direction, relative humidity, temperature and quantity of rain, give us the possibility to understand the infl uence of many phenomena on the mass concentration trend. the meteorological measures were carried out in several sites of northern italy and, in particular, in the turin cnr area, where rass, sodar and other devices are present to measure vertical profi les of wind, relative humidity and temperature. we can see, as an example, in fi g. 4, a fragment of a temporal sequence collected during march 2001. fig. 1. means of 256 measures of pb, cd and v granulometric distributions in several different places in the po valley. fig. 2. means of 384 measures of al, cu and fe granulometric distributions carried out in northern italy. fig. 3. current and old (25 years ago) granulometric distributions of inorganic particulate matter with aerodynamic diameter from 0.43 mm to 11 mm compared with noll and pilat’s (1971) theoretical continental granulometric aerosol distribution. 243 some remarks on pm2.5 the data used for this graph are automatically collected, with different programmable sampling times, by the instrument and analysed by its software, that furnishes a graphic representation of the mass concentration trend, like the one shown above, with an error of 2 mg m-3. we showed this measure of 6 days because it starts and fi nishes with a rainfall and because it is visible how, in urban areas during winter, an accumulation phenomenon is present that can lead to aerosol concentrations higher than 0.5 mg m-3. this is evident from friday 9th and tuesday 13th (note that the increasing trend is present on sunday too, when it can be referred only to road traffi c or to domestic heating). the spikes that are present during the different days are linked with diurnal, meteorological or local factors. using some other fi gures, we will give an interpretation to some recurrent phenomena, making a qualitative comparison between pm2.5 and some meteorological variables. figure 5 shows the pm2.5 concentration trend compared with the variations of wind speed. in this case wind is responsible, moving air masses, of the visible spikes and of the dilution of the pm2.5 concentration in the examined area. figure 6 shows a period characterized by the presence of a rainfall. pm2.5, considering its dimensions, is not reduced directly and immediately by rain itself but, more easily, by its associated wind. a deeper analysis of this phenomenon becomes necessary considering that, in every rainfall event, a variable period of stable concentration is present, visible under the form of a more or less pronounced bend in the graph. figure 7 helps us to explain this particular concentration trend. after the beginning of the rainfall, after a fall in pm2.5 mass concentration, both relative humidity and pm2.5 show an fig. 4. temporal sequence in urban area obtained using dusttrak 8520 in march 2001. a e ro so l ( m g /m -3 ) 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 march 2001 thu 8 fri 9 sat 10 sun 11 mon 12 tue 13 wed 14 244 marco casazza and angelo piano fig. 5. qualitative comparison between wind speed and pm2.5 mass concentration. fig. 6. qualitative comparison between rain trend and pm2.5 concentration. fig. 7. qualitative comparison between relative humidity and pm2.5 trends. increase, while, after that, a further decrease of the two parameters considered is visible. this might be explained considering that the aerosol might be composed of condensation nuclei too, that can actively participate in the precipitation thereby making a contribution to the reduction of relative humidity. figure 8 shows the frequently found, in the case of very fi ne weather conditions, correlation between the pm2.5 concentration trend and the variation of the solar radiation during a day of measures. using different (more than 2.5 106 measures of 1 min each) of this kind and the related meteorological data, we can affi rm that: – pm2.5 are not immediately removed by rainfalls. – they are taken away by wind or taken around the point of measure from areas with different rate of pollution. – they have a diurnal modulation, linked with traffi c and industrial production. – they refl ect particular local emissions. – in conditions of dynamic equilibrium, they characterise the area in which the measures take place and the activities done in that particular place. – they are a witness more of the path followed by different air masses than their peculiar characteristics. 245 some remarks on pm2.5 fig. 8. trends of pm2.5 and solar radiation. 4. indoor measures the last two characteristics found experimentally are of great interest for indoor measures, considering, in particular, environments without particular sources and without smokers, like an offi ce. if particular sources are present, like the smoke of one cigarette, as shown in fi g. 9, it is possible to see a rapid and marked increase in pm2.5 concentration; the time needed to return to the original values of concentration can be some days. the measure presented here was taken in an offi ce of about 70 m3 in conditions of thermal equilibrium. in terms of aitken nuclei, we can say that the inspired and expired smoke contains, in number concentration, on average, 7 105 particles cm-3. the other characteristics, deduced observing the results of many (more than 103) indoor meafig. 9. pm2.5 mass concentration induced by cigarette smoke in 70 m3 room in thermal equilibrium. 246 marco casazza and angelo piano sures (domestic too), can be summarised saying that: – household appliances intaking air (vacuum cleaners and so on ...) do not retain, through the fi lter, pm2.5. – common domestic activities lift or generate pm2.5 (mainly during cleaning or cooking or frying). – the exchange with outdoor air depends on pre existing concentrations (indoor and outdoor) and on differences of temperature. often, the operation of changing the air of an indoor environment has the effect of inducing a worse air quality. 5. conclusions using the data of our measures, we can affi rm that pm2.5 concentrations are greatly increasing. we are studying the chemical proprieties of this fraction of spm, including its mutagenesis to defi ne its danger. pm2.5 concentrations are more poisonous indoors than outdoors, especially if there are some intense local sources, among which we can consider a smoker or some common household appliances. a better knowledge on pm2.5 can lead to important interventions, useful to reduce the risks for human health. references andersen, a.a. (1958): a new sampler for the collection, sizing and enumeration of viable particles, j. bacteriol., 76, 471-484. andersen 2000 inc. (1977): operating manual for andersen 2000 inc. 1 acfm ambient particle sizing samplers (andersen 2000 inc., atlanta, georgia). bacci, p., a. longhetto, g. marcazzan, a. piano, f. prodi, c. sabbioni and a. ventura (1983): aerosol characterization in a po valley site, in proceedings of the gesellschaft für aerosolforschung meeting, j. aerosol sci., 14 (3), 222-225. longhetto, a., p. pavese and a. piano (1975): comparison among granulometric distributions of pb and cd on urban, bottom valley, and mountain atmospheric aerosols, in proceedings of the 13th international meeting on alpine meteorology, riv. ital. geofi s., 1, 95-96. noll, k.e. and m.j. pilat (1971): size distribution of atmospheric giant particles, atmos. environ., 5, 527540. tsi (2001): aerosol monitor dusttrak 8520 operating instruction. vaughan, n.p. (1989): the andersen impactor: calibration, wall losses and numerical simulation, j. aerosol sci., 20, 67-90. canoer 719_724 719 annals of geophysics, vol. 46, n. 4, august 2003 ionospheric storm forecasting technique by artifi cial neural network ljiljana r. cander (1), milan m. milosavljević (2) and saša tomasević (2) (1) rutherford appleton laboratory, chilton, didcot, oxon, u.k. (2) faculty of electrical engineering, university of belgrade, belgrade, serbia and montenegro abstract in this work we further refi ne and improve the neural network based ionospheric characteristic’s fof2 predictor, which is actually a neural network autoregressive model with additional input signals (nnarx). our analysis is focused on choice of x parts of nnarx model in order to capture middle and long term dependencies. daily distribution of prediction error suggests need for structural changes of the neural network model, as well as adaptation of running average lengths used for determination of x inputs. generalisation properties of proposed neural predictor are improved by carefully designed pruning procedure with additional regularisation term in criterion function. some results from the nnarx model are presented to illustrate the feasibility of using such a model as ionospheric storm forecasting technique. mailing address: dr. ljiljana r. cander, rutherford appleton laboratory, chilton, didcot, oxon ox11 0qx, u.k.; e-mail: l.cander@rl.ac.uk key words prediction and forecasting – neural networks – ionospheric storms modelling – space weather 1. introduction there are a number of methods available for prediction and forecasting characteristics relevant to ionospheric telecommunication applications (kutiev et al., 1999; fuller-rowell et al., 2000; muhtarov et al., 2001; muhtarov et al., 2002; cander, 2003 and references therein). the user usually requires ionospheric forecasting that are near real time for radar and surveillance applications and over 1-24 h ahead for pointto-point and mobile communications (cander, 1998). the neural network non-linear techniques are based on finding more complex correlation over a greater number of steps. successful attempts to build different artifi cial neural networks models for the critical frequency of the ionospheric f2 layer, fof2, prediction and forecasting have already been made at monthly (lamming and cander, 1998, 1999) and daily time scales (williscroft and poole, 1996; altinay et al., 1997; wintoft and cander, 2000a,b; francis et al., 2001). the aim of the paper is to provide a concise summary of the application and development of non-linear neural network techniques to improve ionospheric storm forecasting capabilities. it is shown that the nnarx neural network based autoregressive model with additional inputs (x) can provide forecasting of the hourly variation of fof2 from 1 h ahead, as the shortest time scale over which the ionosphere can be predicted forwards extremely well. 2. fof2 data source the data used for fof2 forecasting 1 h ahead are values taken from the rutherford appleton laboratory cd-rom produced within cost 251 project (http://www-cost251.rcru.rl.ac.uk/) 720 ljiljana r. cander, milan m. milosavljević and saša tomasević for a few european stations listed in table i. the forecast periods were chosen to be february 1986, september and december 1990 representing low and high solar activity as well as geomagnetically disturbed and quiet periods. table ii gives the details concerning the solar-terrestrial conditions during selected periods. the nnarx model was trained using hourly fof2 values from 01 january to 31 december during years of 1986 and 1990 excluding months for which forecasting has been made: february in case of 1986, september in case i on 1990 and december in case ii on 1990. in table ii r i is the international relative sunspot number and a p is an averaged planetary geomagnetic index based on data from a set of specifi c geomagnetic observatories. it serves to classifi ed the storms as follows: 1) minor geomagnetic storm: a storm for which the a p index was greater than 29 and less than 50. 2) major geomagnetic storm: a storm for which the a p index was greater than 49 and less than 100. 3) severe geomagnetic storm: a storm for which the a p index was 100 or more. the d st index is an index of geomagnetic activity derived from a network of near-equatorial geomagnetic observatories that measures the intensity of the globally symmetrical equatorial electrojet (the «ring current»). peaks dst values are between – 20 and 20 nt during quiet geomagnetic condition and decrease to below – 100 nt during highly disturbed periods. in the case of severe geomagnetic storms, dst values are below – 250 nt. 3. nnarx model and training procedure nnarx neural network based autoregressive model with additional inputs (x) is one possible approach that use the hybrid timedelay multi-layer percepton neural network with only critical frequency of the ionospheric f2 layer as input parameter to produce one output fof2 value at hour t + 1. inputs (x) include fof2 value at time t, seven days mean mfof2 values and ionospheric stations ursi codes latitude (n) longitude (e) rome ro041 41.9 12.5 sofi a sq143 42.7 23.4 grocka be145 44.8 20.5 poitiers pt046 46.6 0.3 slough sl051 51.5 – 0.6 julisruh jr055 54.6 13.4 moscow mo155 55.5 37.3 uppsala up158 59.8 17.6 table i. list of european ionospheric stations with their ursi codes and geographical coordinates. months days with storm commencement min d st geomagnetic index max a p geomagnetic index february 1986 with monthly mean sunspot number r i = 23.2 06 at 1312 ut – 307 nt on 9 february 208 on 8 february september 1990 with monthly mean sunspot number r i = 125.2 01 at 1024 ut – 48 nt on 1 september 26 on 1 september december 1990 with monthly mean sunspot number r i = 129.7 21 at 1724 ut – 47 on 24 december 15 on 24 december table ii. solar-terrestrial conditions during selected periods used in this study. 721 ionospheric storm forecasting technique by artifi cial neural network 4. nnarx model results accuracy of prediction, clearly stating range and scope of test conditions, i.e., show the breakdown between training and test data, indicate any comparisons made with other or reference models such as persistence for example. for comparative purposes the accuracy of the nnarx model is quoted in terms of both the root mean square (rms) error and the normalised rms error defi ned as where x are the actual fof2 values, y are the predicted fof2 values, n = s i is the total number of comparisons between actual and predicted values and s is the standard deviation of the actual values from the mean value given by the predictive accuracy of the nnarx model is summarised in the tables iii and iv. in these tables all errors are quoted with respect to the one step ahead nnarx predictive model, for the entire test set in each instance. the persistence model results are included in table iii as an effective reference in each instance. it can be seen in these rms and nrms errors appropriate differences dmfof2 at particularly selected hours (t, t – 1, t – 23, t – 47) as well mfof2 at forecast time (t + 1) calculated using only the learning set of data to generate the background daily variations of fof2. detailed description of this type of the neural network as far as its architecture, fi rst and second hidden layers, learning and test data sets are concerned can be found in cander et al. (1998a,b). the nnarx is trained using cost function with a weight-decay term (bishop, 2000). our experimental experience showed that only the weight-decay scheme is not enaugh to overcome the overfi tting problem. namely, it would have been possible to increase the weight-decay parameters, which have also been tried, but when increasing the weight-decay parameters the quadratic error term in the cost-function increase. this is an undesirable effect, as it is this part of the cost function that is especially interesting and should be minimized as much as possible. therefore a pruning scheme is used to remove superfl uous parameters from the network architecture (le cun et al., 1990). the so called akaike’s final prediction error (fpe) estimate of the test error is used to point out which structure is the optimal (akaike, 1969). additionally, we then tried to retrain the optimal network structure to see if it is possible to overcome the problem of overtraining when optimal network structure already has been found. the experimental results showed that retrained optimal networks possess very good generalyzation properties. all results presented in sequal are obtained by the such retrained optimal networks. σ 2 2 = −( )∑ x x n i i . ionospheric stations rms error (mhz) nrmse february 1986 persistence nnarx model persistence nnarx model ro041 0.6482 0.4456 0.4740 0.3258 sq143 0.6178 0.4460 0.4469 0.3226 pt046 0.8097 0.4745 0.3568 0.2091 sl051 0.7213 0.5258 0.5285 0.3853 mo155 0.6121 0.3778 0.4288 0.2647 table iii. rms and nrms errors at different stations for february 1986. nrmse = −( )        ∑ x y ni i i 2 2σ 722 ljiljana r. cander, milan m. milosavljević and saša tomasević values that there is a substantial improvement in nnarx model over persistence results for all stations during severe geomagnetic conditions in february 1986. table iv gives only rms errors for nnarx model results in case of moderate and quiet geomagnetic conditions in september and december 1990. with the exception of the sofi a ionospheric station rms errors, it is clear that the nnarx model results are as good as these in table iii with a slight tendency of becoming worse during the more or less quiet geomagnetic activity in december 1990. encouraging results in tables iii and iv of comparisons between actual and 1 h ahead ionospheric stations rms error (mhz) nnarx model september 1990 case i rms error (mhz) nnarx model december 1990 case ii ro041 0.49 0.57 sq143 0.96 0.88 pt046 0.45 0.60 sl051 0.44 0.58 up158 0.44 0.60 table iv. rms errors at different stations for september and december 1990. stations/nnarx model input parameters ro041 mape (%) sq143 mape (%) pt046 mape (%) sl051 mape (%) jr055 mape (%) mo155 mape (%) up158 mape (%) fof2 only 8.86 10.22 9.14 8.94 10.66 8.95 9.42 fof2 + r i + a p 8.71 10.07 8.63 9.10 10.79 9.88 10.25 fof2 + r i + d st 8.93 9.86 8.89 9.18 10.19 9.59 10.29 fof2 + r i + a p + d st 8.70 9.90 8.60 9.26 10.46 9.97 11.03 table v. mape (%) at different ionospheric stations for february 1986. stations/nnarx model input parameters ro041 mape (%) sq143 mape (%) be145 mape (%) pt046 mape (%) sl051 mape (%) jr055 mape (%) up158 mape (%) fof2 only 4.16 8.85 4.97 4.26 4.62 4.52 6.20 fof2 + r i + a p 4.04 8.88 4.98 4.23 4.60 5.05 6.27 fof2 + r i + d st 4.18 8.94 4.99 4.24 4.57 4.60 6.05 fof2 + r i + a p + d st 4.07 8.82 4.99 4.30 4.57 4.79 6.08 table vi. mape (%) at different european ionospheric stations for september 1990. stations/nnarx model input parameters ro041 mape (%) sq143 mape (%) pt046 mape (%) be145 mape (%) sl051 mape (%) jr055 mape (%) up158 mape (%) fof2 only 7.05 11.41 6.48 7.62 6.82 8.23 11.53 fof2 + r i + a p 7.22 11.27 6.23 7.34 6.63 8.60 10.26 fof2 + r i + d st 7.13 11.65 6.32 7.40 6.82 8.81 10.61 fof2 + r i + a p + d st 6.73 12.40 6.54 8.53 6.72 8.62 9.03 table vii. mape (%) at different european ionospheric stations for december 1990. 723 ionospheric storm forecasting technique by artifi cial neural network forecast fof2 values show that there is a predictive advantage to neural network modelling which make it a useful tool to be introduced into the current ionospheric weather applications. for these purposes, it is shown here an interesting effect of different solar-terrestrial indices on 1 h ahead fof2 forecasting by nnarx model. this time the prediction accuracy has been examined in terms of the mean absolute percentage error (mape) defi ned as a more in depth investigations of neural network forecasting of the key ionospheric characteristic fof2 at different european ionospheric stations shown in tables v, vi and vii clearly demonstrate that 1 h ahead forecasting performances are not signifi cantly improved by adding solar activity r i and/or geomagnetic a p and d st indices as nnarx input parameters. it should be emphasised that these data and comparisons are specifi cally for a few stations but the results are applicable to the entire mid-latitude region at european longitudinal domain. 5. conclusions a neural network method has been developed for 1 h ahead forecasting of fof2. comparisons with observations are very encouraging because of a very small errors and a high consistence between diurnal morphologies exhibited by forecasting method and observed trends. furthermore, the effects of including various solar and geomagnetic indices as input parameters to the neural network were investigated, and it was found that performance was not improved by adding an index. indeed, the nnarx model does not require neither predicted nor past values mape abs % .( ) = −( ) ⋅ = ∑ 1 100 1n x y x i i ii n fig. 1. measured and nnarx model forecast fof2 values for a 8 day period in february 1986 at slough ionospheric station. 724 ljiljana r. cander, milan m. milosavljević and saša tomasević of solar and geomagnetic indices to forecast 1 h ahead fof2 values even during geomagnetically very disturbed periods as those in february 1986 (see fi g. 1). it remains to be seen to what extent the artifi cial neural network performance, such as the nnarx model, can be improved in predicting fof2 values up to 24-h ahead. nnarx model is implemented in matlab 5.2.1 and requires its installation on pc pentium ii platform. it has matlab type of friendliness with low level of interaction and requires training data for fof2 for recent 365 days. this work could be extended to space weather forecasting with the application of predicting disturbance effects. this will clearly involve prediction techniques based on the access and manipulation of real time ionospheric data. references altinay, o., e. tulunay and y. tulunay (1997): forecasting of ionospheric frequency using neural networks, geophys. res. let., 24, 1467-1470. akaike, h. (1969): fitting autoregressive models for prediction, ann. inst. stat. mat., 21, 243-247. bishop, c.m. (2000): neural networks for pattern recognition (oxford university press, oxford), pp. 504. cander, lj.r. (1998): space weather effects on telecommunication, in ai applications in solar-terrestrial physics, edited by i. sandahl and e. jonsson, esa wpp-148 proceedings, 35-42. cander, lj.r. (2003): toward forecasting and mapping ionospheric space weather under the cost actions, adv. space res., 31 (4), 957-964. cander, lj.r., s. stanković and m. milosavljević (1998a): dynamic ionospheric prediction by neural networks, in ai applications in solar-terrestrial physics, edited by i. sandahl and e. jonsson, esa wpp-148 proceedings, 225-228. cander, lj.r., m.m. milosavljević , s.s. stanković and s. tomasević (1998b): ionospheric forecasting technique by artifi cial neural network, electron. lett., 34 (16), 1573-1574. francis, n.m., a.g. brown and p.s. cannon (2001): predictionn of the hourly ionospheric parameter fof2 using a novel nonlinear interpolation technique to cope with missing data points, j. geophys. res. space phys., 106 (a12), 30,415-30,428. fu l l e r-row e l l, t., e. ar a u j o-pr a d e r e and m. condrescu (2000): an empirical ionospheric stort-time correction model, adv. space res., 25 (1), 138-148. lamming, x. and lj.r. cander (1998): monthly median ionospheric frequencies model using neural networks, in ai applications in solar-terrestrial physics, edited by i. sandahl and e. jonsson, esa wpp-148 proceedings, 229-234. lamming, x. and lj. r. cander (1999): monthly median fof2 modelling cost251 area by neural networks, phys. chem. earth (c), 24 (4), 349-354. le cun, y., j.s. denker and s.a. solla (1990): optimal brain demage, in advances in neural information processing systems, edited by m. kaufman, vol. 2, 598-605 muhtarov, p., i. kutiev, lj.r. cander, b. zolesi, g. de franceschi, m.f. levy and m. dick (2001): european ionospheric forecasting and mapping, phys. chem. earth (c), 26 (5), 347-351. muhtarov, p., i. kutiev and lj.r. cander (2002): geomagnetically correlated autoregression model for short-term prediction of ionospheric parameters, inverse problems, 18, 49-65. kutiev, i., p. muhtarov, lj. r. cander and m.f. levy (1999): short-term prediction of ionospheric parameters based on auto-correlation analysis, ann. geofi s., 42 (1), 121-127. williscroft, l.-a. and a.w.a. poole (1996): neural networks, fof2, sunspot number and magnetic activity, geophys. res. lett., 23, 3659-3662. wintoft, p. and lj.r. cander (2000a): twenty-four hour predictions of fof2 using time delay neural networks, radio sci., 35 (2), 395-408. wintoft, p. and lj.r. cander (2000b): ionospheric fof2 storm forecasting using neural networks, phys. chem. earth (c), 25 (4), 267-273. (received may 2, 2003; accepted july 10, 2003) geo 929 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 1 the impact of space weather on communication yurdanur k. tulunay (1) and peter a. bradley (2) (1) faculty of aeronautics and astronautics, istanbul technical university (i̇tü), maslak, istanbul, turkey (2) pandora, scotlands drive, farnham common, slough, berks, u.k. as a preliminary to the studies conducted by the individual working groups, a review has been undertaken of topics which are relevant to the space-weather phenomena that can occur. the ways in general that these can influence the state of the ionosphere are discussed. the various changes in propagation parameters that arise on earth-space and terrestrial radio paths via the ionosphere are listed. international procedures for frequency allocation and assignment are explained and the extent to which these are influenced by propagation factors is addressed. the role of mitigation techniques to improve radio-system performance is also identified. 1.1. introduction this overview was prepared (tulunay, 2001) in response to the action objectives to examine briefly those present and future anticipated terrestrial and earth-space radio systems that can be affected by space-weather phenomena. throughout the duration of the action emphasis has been placed on characterizing these phenomena and their associated effects. in this respect the radio systems have been taken as including all those associated with communications, navigation and surveillance. this distinction is particularly important because some phenomena, such as for example those concerned with varying signal group delays, influence the performance of navigation and surveillance systems, but are of lesser significance to systems concerned only with the transmission of intelligence. propagation features are restricted to those arising when a radio wave travels via the earth’s ionosphere between terrestrial locations, between earth and space, or on paths entirely within the space segment. here the ionosphere is regarded as arising over heights from about 50 km upwards to several thousand kilometers and, in particular, tropospheric phenomena have been specifically excluded from consideration. «space weather» is a generic term involving a number of separate magnetospheric phenomena and in recent years it has become a fashionable topic for study within many branches of solar-terrestrial physics (stp) (hapgood, 2001, pers. comm.). as a result, there is a risk that the proper and necessary distinctions between space weather and stp are becoming blurred to the ultimate detriment of both topics. space weather is concerned with understanding and where necessary mitigating the impact of timevarying phenomena in the space environment on human beings and technological systems. many problems in ionospheric propagation clearly fall within this regime. in contrast, stp addresses the scientific understanding of the various physical regimes (e.g., ionosphere, magnetosphere and solar wind) that make up the whole space environment. it is fundamental science, much of it concerned with basic processes in plasma physics. the relationship between the two topics is perhaps best expressed by stating that stp is the body of scientific knowledge that underpins the study of space weather. throughout 930 yurdanur k. tulunay and peter a. bradley the action, stp has been considered only insofar as it has aided the understanding of propagation impairment effects. likewise, optimum mitigation techniques, which are examined, can differ significantly for true two-way communication systems with the options of re-transmittal, space, frequency, time and route diversities, and other forms of error-correction procedures based on the use of intelligent modems, and for one-way navigation and surveillance radio systems, where the objective has to be to develop and deploy near-real-time propagation impairment correction algorithms. in this section the primary types of radio systems in use in different frequency bands are surveyed within the context of international regulations (itu, 2004). the separate propagation phenomena which can lead to system impairments, and so need to be modeled, are in turn considered. those space-weather parameters which impact adversely and significantly on propagation conditions are identified. mitigation techniques to overcome such problems are also addressed. figure 1.1 provides a schematic diagram showing the relationship between space weather and radiosystem performance (bradley, 2003). to assess the reliability and compatibility of a radio system, i.e. whether it will achieve its intended purpose for the necessary time in the presence of the natural noise background, and whether it is compatible with co-channel and adjacent channel transmissions, appropriate propagation models are needed. these in turn depend on the relevant ionospheric models, which may be formulated in terms of either long-term predictions for system planning, or near-real-time forecasts for operational assessments. these near-real-time forecasts call for a space weather real-time database. fig. 1.1. chart showing the chain of dependence of radio-system performance on space weather (bradley, 2003). 931 the impact of space weather on communication the separate sections of this final report confirm the progress that has been made within the action in these respective areas, and the extent to which this information is valuable to radio users, technical commentators and decision makers. 1.2. present and future anticipated terrestrial and earth-space radio systems 1.2.1. present usage international use of the radio spectrum is regulated by the geneva-based international telecommunication union (itu), a specialised agency of the united nations in which all countries that have signed the international telecommunications convention of that body may participate. in so doing they undertake to introduce national legislation governing the licensing agreements of radio operating organisations within their territories which are in accord with the itu radio regulations (itu, 2004). in particular, the radio regulations, updated from time to time to reflect changes in system usage and requirements, lay down the conditions under which radio systems may operate. they define a number of types of radio service for varying applications, and allocate different blocks of frequencies to these individual services on the basis of operational need and propagation characteristics. best available propagation models therefore form a key component of this process, and an important objective of cost 271 has been concerned with the refinement and enhancement of such existing models. article 1 of the radio regulations defines the separate types of radio service and article 8 splits the world into geographical regions and zones over which separate service and frequency allocations apply. article 8 also presents tables showing the breakdown of the spectrum into a series of frequency allocation bands for the different services, within which individual circuits having approved technical characteristics may be assigned. at the present time frequency allocation bands are specified for the overall frequency range 9 khz-275 ghz. signals at frequencies up to 20-30 mhz, depending on conditions, rely on ionospheric reflection with potential for near-worldwide coverage after successive reflections. at vhf on frequencies that can extend to more than 100 mhz on occasion, ionospheric support of signals radiated from ground-based transmitters is sometimes possible via scattering from ionospheric irregularities such as those responsible for the phenomena of sporadic e and spread-f. such signals only rarely can be used to provide a reliable service because of their intermittent incidence and the distortions that they are subjected to, but they can be an important source of interference to other co-channel services. these interference aspects have also been addressed within cost 271. higher-frequency signals of particular interest here are mainly those involved for earth-space paths. the principal types of radio service defined in article 1 which are allocated the majority of the available radio spectrum are as follows: – the fixed service: a service of radiocommunication between specified fixed points. – the aeronautical mobile service: a service of radiocommunication between aeronautical stations (on the ground) and aircraft stations or between aircraft stations. – the maritime mobile service: a service of radiocommunication between coast (land) stations and ship stations or between ship stations; – the land mobile service: a radiocommunication service between base stations and land mobile stations, or between land mobile stations. – the broadcasting service: a radiocommunication service intended for direct reception by the general public and which embraces both sound broadcasting and television transmissions. – the radiodetermination and radionavigation services: services using the propagation properties of radio waves to determine position, or for the purposes of navigation, etc. 932 yurdanur k. tulunay and peter a. bradley additionally there are a number of services with lesser spectral allocations serving the needs of specialist user communities: amateur; meteorological aids; radio astronomy; radio beacons; space research; standard frequency and time signals. the above principal services are sub-divided into component services for which different frequency allocation and technical performance specifications apply. for example, there are separate maritime and aeronautical radio-navigation services. many of the higher frequencies are allocated to the various satellite services with specific separate bands for aeronautical mobile satellites, broadcasting satellites, earth-exploration satellites, maritime mobile satellites, meteorological satellites, radionavigation satellites and space operations. distinctions are made between services involving fixed (geostationary) and mobile (orbiting) satellites, and between space-to-earth and earth-to-space services. there are also bands available for the amateur satellite service and for inter-satellite communication and radio-navigation services. the article 8 geographical regions over which different regulations and frequency allocations can apply are as shown in fig. 1.2: region 1 covers europe and africa (including the former ussr), region 2 embraces north and south america and region 3 represents the asian mainland together with the australian area. in addition there is a separate tropical zone between latitudes of around ± 30ºn and s of the equator where additional frequency bands between 2300-2495 khz, as well as in three other special sub-bands in the lower part of the hf-band, are allocated to permit sound broadcasting by short-distance sky-wave signals in the presence of the large amounts of ionospheric absorption and the high thunderstorm atmospheric noise intensities encountered at such latitudes. an example of a frequency allocation table for the frequency range 14 990-18 030 khz is given in fig. 1.3. for this frequency range, because of the worldwide nature of propagation conditions, the same allocations are adopted for all regions. primary services are indicated with capital letters and secondary services (not accorded the same interference protection) are shown in lower case text. the fig. 1.2. the separate itu geographical regions (hatched) and the tropical zone (dark shading). 933 the impact of space weather on communication f ig . 1. 3. e xa m pl e of i t u r ad io r eg ul at io ns f re qu en cy a ll oc at io n t ab le f or t he 1 4 99 018 03 0 kh z ba nd . f ig . 1. 4. e xa m pl e of i t u r ad io r eg ul at io ns f re qu en cy a ll oc at io n t ab le f or t he 1 1. 712 .7 5 g h z ba nd . 1. 3 1. 4 934 yurdanur k. tulunay and peter a. bradley several numbers relate to footnotes not reproduced here detailing specific applied transmission conditions. at higher frequencies, in this case illustrated by the example of fig. 1.4 for the 11.7-12.75 ghz band allocated mainly to the satellite and fixed satellite broadcasting services where simultaneous worldwide coverage is not achieved, important differences in the bands allocated to the separate services are evident. despite the detailed changes that are introduced in such tables from time to time, some general remarks can be made regarding spectral usage for different broad frequency ranges. radiocommunication at vlf is limited by the available bandwidth, but since ionospheric attenuation is very low, near worldwide coverage can be achieved. unfortunately the radiation of energy is difficult at such frequencies and complex transmitting antenna systems coupled with large transmitter powers are needed to overcome the high received atmospheric noise background. because of the stability of propagation, very low frequencies may be used for the transmission of standard time signals and for continuous wave navigation systems which rely on direction-finding techniques, or on phase comparisons between spaced transmissions as in the omega system operating in the 10-14 khz frequency range (pierce et al., 1964), though nowadays more and more such systems are giving way to more reliable satellite services providing the same types of information. at lf increased propagation losses limit area coverage, but simpler antenna systems are adequate and lower transmitter powers can be employed because of the reduced atmospheric noise. low-frequency systems are used for communication by on-off keying and frequency-shift keying. propagation conditions are more stable than at higher frequencies because the ionosphere is less deeply penetrated. other lf systems relying principally on the ground wave, but which are sometimes degraded by the sky-wave at night, include the decca continuous wave navigation system (70-130 khz) now little used, the loran c pulse navigation system (100 khz) and long-wave broadcasting (hall, 1947; bradley, 1984; davies, 1990). at mf daytime absorption is so high as to completely suppress the sky wave. some use is made of the sky wave at night-time for broadcasting, but generally medium frequencies are employed for ground-wave services (bradley, 1984). despite the decline in use of hf by the fixed service since the early 1970’s with the advent of communication satellites and wideband submarine cables, high frequencies continue to be employed predominantly for broadcasting, fixed and mobile point-to-point communications via ionosphere reflection. table 1.i shows the relative fractions of spectrum allocated to the fixed, sound broadcasting and mobile services. maritime elements include coast stations, ship-station radiotelephone working to coast stations and inter-ship working. aeronautical systems include single-sideband hf radiotelephone links between aircraft and the ground in the aeronautical mobile channels of the 2-22 mhz band and radioteletype links between ground terminals in the aeronautical fixed channels in the 2.5-30 mhz band. the majority of long-distance fixed circuits nowadays rely on satellites and cables for both civilian and military applications. likewise, there has in recent years been a growth in the deployment of satellites for aeronautical, maritime and land-mobile communications and an explosion in their use for personal mobile phone and location determination applications. nonetheless, increases in the numbers involved mean that there are actually more hf circuits in use than, say, 20 years ago. partable 1.i. approximate fractional usage by the different radio services of the hf-bands. fixed service 55% land and maritime mobile services 15% sound broadcasting service 15% aeronautical mobile service by the amateur service, 10% the standard frequency and time service and space research services 5% 935 the impact of space weather on communication ticularly for military purposes, hf systems are regarded as providing a necessary backup service to fixed links primarily established by other means. vhf communication by ionospheric scatter propagation between ground-based terminals is possible. two-way error-correction systems with scattering from intermittent meteor trains can be used at frequencies of 30-40 mhz over ranges of 500-1500 km. bursts of high-speed data of about 1 s duration and duty cycles of the order of 5% can be achieved with transmitter powers of about 1 kw. forward-scatter communication systems at frequencies of 30-60 mhz, also operating over ranges of about 1000 km, rely on coherent scattering from field-aligned irregularities at a height of about 85 km in the d-region. they are deployed principally at low and high latitudes. signal intensities are somewhat variable, depending on the incidence of irregularities. directional transmitting and receiving antennas with intersecting beams are required. frequency-modulation techniques involving timedivision multiplex are used to combat doppler effects. systems with 16 channels, automatic error correction and operating at 100 words per minute, now exist. inspection of the article 8 tables confirms that on earth-space paths there is an evident dominance of broadcasting, communication and navigation and earth-surveillance satellite channels. as specific examples of such usage the l-band navigation service, the land-mobile communication satellite service at land s-bands, the low frequency remote sensing satellite service at pand l-bands and altimetry at sand ku-bands all experience ionospheric propagation effects which are important to characterize for system design and operation. 1.2.2. likely future usage as the nations of the world become more industrialised, as populations grow and as expectations of standards of living rise, demand for radiocommunication services will increase. travel delays too mean that future trends will see more personal point-to-point communication contacts as journey times rise and traffic grid-locks become more common. all the world is seeking access to a mobile phone, a television set and an internet link. at the same time there must be increases in levels of surveillance and civil protection. so the radio spectrum is an asset of growing importance to industry, personal communications and civil protection. yet it is of finite size, and despite the moves towards ever-increasing frequencies, the next decade will see significant increases in radio usage. therefore to avoid or minimise congestion, spectrum management techniques must keep abreast of demand. current procedures for frequency allocation and the associated national assignment arrangements within the terms of the itu radio regulations are known to be grossly inefficient. demand and available spectral bands adopted at radio conferences, held from time to time to review and update these regulations, are generally poorly matched despite the earnest endeavours of those involved to produce equitable solutions. there is little grasp of the capacity of the spectrum. attempts at devising procedures to ensure service reliability and compatibility protection from co-channel and adjacent channel services are usually subservient to meeting all stated requirements. the inertia of the system mitigates against optimising spectrum utilisation and there is lack of flexibility to respond to changing requirements. many assignments are under-employed with operators demanding unrestricted full-time use of these, whereas in practice they are required for only a fraction of this. existing itu frequency allocations are too restrictive and inhibit technological changes. at the same time many transmitters perform with poor characteristics: grossly too high powers, inefficient antennas with too wide beams or sidelobes and unnecessary emission characteristics such as bandwidths and spurious emissions. multiple radiation of the same information, on separate frequencies, duplication in time or from separate sites is common. to respond to this situation, moves are afoot in many countries to develop alternative approaches to frequency management (bradley, 2002). these aim to bring a sense of awareness to 936 yurdanur k. tulunay and peter a. bradley radio operators to optimise their requirements and emission characteristics by means of financial incentives. for example the switch from an analogue to a digital television service, besides the improvement in reception quality, means that 6 digital channels can be accommodated within the same bandwidth. charges are being introduced for licences, in principle set at levels required to achieve the desired effect, rather than to optimise exchequer input. there are three ways in which licenses can be awarded: sold at pre-determined costs; auctioned, or acquired by secondary trading. as examples of the auctioning approach, in the year 2000 several european governments auctioned frequencies for the introduction of 3g mobile licences. secondary trading can enhance efficiency and flexibility, aiding time sharing within a regulatory framework. future licenses should be of finite duration of just a few years, rather than open-ended as has usually been the case with past procedures. at this stage it is impossible to anticipate what new radio services the next decades will bring, except to suppose in general terms that there will be more demand for communications within the home: greater automatic control of functions with remote monitoring and over-ride, and provision of additional data services. the use of personal positioning devices seems likely to become much more widespread, so that the locations of individuals are always known. european companies have invested huge fortunes in licenses for the so-called «next generation» 3g mobile communication systems, in the anticipation that new requirements will need far greater bandwidths than has hitherto been possible. but if agreement is reached on how to re-organise spectral utilisation on a more dynamic, flexible and cost-effective way, the door will be open for such innovative procedures to be developed matched to user demands. a naive viewpoint is that future adaptive radio systems will probe the propagation medium in real-time and then optimise their transmission characteristics; hence there is no need for further propagation data. but just as spectrum usage has to be internationally controlled, so the latest adaptive systems must be engineered to operate within defined bounds that themselves have to be specified from a knowledge of the radio medium. more advanced systems require propagation information that has hitherto not been necessary. whilst understandably greatest interest centres on uhf and higher frequencies with the greater available bandwidths, where tropospheric impairments dominate, there remain large numbers of terrestrial radio services still relying on the ionosphere for support. as examples, at hf it was hitherto usual to seek to model received signal-noise ratios and their statistical variations, but the importance to digital systems of being able to characterise likely time spreads and doppler dispersions has been clearly demonstrated. ways of short-term forecasting of ionospheric changes, particularly those concerned with ionospheric storms remain needed. earth-space links up to shf too can be significantly affected by the ionospheric presence (itu-r p531-7, 2003). a priority must be the improved modelling of propagation effects on earth-space links. many of these effects are related to the columnar total electron content (tec) between spacecraft and ground receiver, including refraction, dispersion, angular deviation, change of plane of polarisation and group delay. existing international models of median tec and its day-to-day variability are grossly inadequate in meeting requirements of latest generation communication and surveillance systems. at the same time ionospheric scintillation imposes a major problem to satellite systems, particularly for equatorial and high-latitude paths. scintillation models need to be much improved by extension to provide a broader temporal and spatial coverage than hitherto. 1.3. propagation phenomena which under some circumstances can lead to radio-system impairments and that have been modelled within the cost 271 work programme the later sections of this report discuss the various models that have been generated within the action. here we summarise the individual propagation impairments that can arise under 937 the impact of space weather on communication some circumstances. a radio wave is specified in terms of five parameters: its amplitude, phase, direction of propagation, polarisation and frequency. the principal effects of the ionosphere in modifying these parameters are considered as follows (bradley, 1996; cannon and bradley, 2003). 1.3.1. refraction the change in direction of propagation resulting from the traverse of a thin slab of constant ionisation is given approximately by bouger’s law in terms of the refractive index and the angle of incidence. a more exact specification including the effects of the earth’s magnetic field is provided by the haselgrove equation solution (haselgrove, 1954). the refractive index is determined from the appleton-hartree equations of the magnetoionic theory as a function of the electron density and electron-collision frequency, together with the strength and direction of the earth’s magnetic field, the wave direction and the wave frequency (ratcliffe, 1959; budden, 1961). the dependence on frequency leads to wave dispersion of modulated signals. since the ionosphere is a doubly-refracting medium it can transmit two waves with different polarisations (see below). the refractive indices appropriate to the two waves differ. refraction is reduced at the greater wave frequencies, and at vhf and higher frequencies it is given approximately as a function of the ratio of the wave and plasma frequencies, where the plasma frequency is defined in terms of a universal constant and the square root of the electron density. table 1.ii lists estimates of the magnitude of the refraction and of other propagation parameters for signals at a frequencv of 1 ghz which traverse the whole ionosphere one way with an elevation angle of 30º. 1.3.2. change in phase-path length the phase-path length is given approximately as the integral of the refractive index with respect to the ray-path length. ignoring spatial gradients, the change in phase-path length introduced by passage through the ionosphere to the ground of signals at vhf and higher frequencies from a spacecraft is proportional to the columnar total-electron content. table 1.ii. estimated maximum ionospheric effects at 1 ghz for elevation angles of about 30º one-way traversal (itu-r p531-7, 2003). effect magnitude frequency dependence faraday rotation 108º 1/f 2 propagation delay 0.25 µs 1/f 2 refraction < 0.17 mrad 1/f 2 variation in direction of arrival 0.2 min of arc 1/f 2 absorption (polar cap) 0.04 db ∼1/f 2 absorption (auroral + polar cap) 0.05 ∼1/f 2 absorption (mid-latitude) < 0.01 db 1/f 2 dispersion 0-4 ns/mhz 1/f 3 scintillation (see itu-r p531-7, 2003, sec. 4) 938 yurdanur k. tulunay and peter a. bradley 1.3.3. group delay the group and phase velocities of a wave differ because the ionosphere is a dispersive medium. the ionosphere reduces the group velocity and introduces a group delay which for transionospheric signals at vhf and higher frequencies, like the phase-path change, is proportional to the total electron content. 1.3.4. polarisation radio waves that propagate in the ionosphere are called characteristic waves. there are always two characteristic waves known as the ordinary wave and the extraordinary wave; under certain restricted conditions a third wave known as the z-wave can also exist. in general the ordinary and extraordinary waves are elliptically polarised. the polarisation ellipses have the same axial ratio, orientations in space that are related such that under many conditions they are approximately orthogonal, and electric vectors which rotate in opposite directions. the polarisation ellipses are less elongated the greater the wave frequency. any wave launched into the ionosphere is split into characteristic ordinary and extraordinary wave components of appropriate power. at mf and above these components may be regarded as travelling independently through the ionosphere with polarisations which remain related, but continuously change to match the changing ionospheric conditions. the phase paths of the ordinary and extraordinary wave components differ, so that in the case of transionospheric signals when the components have comparable amplitudes, the plane of polarisation of their resultant slowly rotates. this effect is known as faraday rotation. 1.3.5. absorption absorption arises from inelastic collisions between the free electrons, oscillating under the influence of the incident radio wave, and the neutral and ionised constituents of the atmosphere. the absorption experienced in a thin slab of ionosphere is given by the appleton-hartree equations and under many conditions is proportional to the product of electron density and collision frequency, inversely proportional to the refractive index and inversely proportional to the square of the wave frequency. the absorption is referred to as non-deviative or deviative depending on whether it occurs where the refractive index is close to unity. normal absorption is principally a daytime phenomenon. at frequencies below 5 mhz it is sometimes so great as to completely suppress effective propagation. the absorptions of the ordinary and extraordinary waves differ, and in the range 1.5-10 mhz the extraordinary wave absorption is significantly greater. 1.3.6. amplitude fading if the ionosphere were unchanging the signal amplitude over a fixed path would be constant. in practice, however, fading arises as a consequence of variations in propagation path, brought about by movements or fluctuations in ionization (itu-r, 1990). the principal causes of fading are: a) variations in absorption; b) movements of irregularities producing focusing and defocusing; c) changes in path length among component signals propagated via multiple paths; d) changes in polarisation, such as for example due to faraday rotation. these various causes lead to different depths of fading and a range of fading rates. the slowest fades are usually those due to absorption changes which have a period of about 10 min. the deepest and most rapid fading occurs from the beating between two signal components of comparable am939 the impact of space weather on communication plitude propagated along different paths. a regularly reflected signal together with a signal scattered from spread-f irregularities can give rise to so-called flutter fading, with fading rates of about 10 hz. a good general survey of fading effects, including a discussion of fading statistics, has been produced. on operational communication circuits fading may be combated by space diversity or polarisation-diversity receiving systems and by the simultaneous use of multiple-frequency transmissions (so-called frequency diversity). 1.3.7. frequency deviations amplitude fading is accompanied by associated fluctuations in group path and phase path, giving rise to time and frequency-dispersed signals. when either the transmitter or receiver is moving, or there are systematic ionospheric movements, the received signal is also doppler-frequency shifted. signals propagated simultaneously via different ionospheric paths are usually received with differing frequency shifts. frequency shifts for reflections from the regular layers are usually less than 1 hz, but shifts of up to 20-30 hz have been reported for scatter-mode signals at low latitudes. 1.3.8. reflection, scattering and ducting the combined effect of refraction through a number of successive slabs of ionisation can lead to ray reflection. this may take place over a narrow height range as at lf, or rays may be refracted over an appreciable distance in the ionosphere as at hf. weak incoherent scattering of energy occurs from random thermal fluctuations in electron density, and more efficient aspect-sensitive scattering from ionospheric irregularities gives rise to direct backscattered and forward-scatter signals. ducting of signals to great distances can take place at heights of reduced ionisation between the eand f-regions, leading in some cases to round-the-world echoes (fenwick and villard, 1963). ducting can also occur within regions of field-aligned irregularities above the maximum of the f-region (nielson, 1968). 1.3.9. scintillation scintillations are variations of amplitude, phase, polarisation and angle-of-arrival produced when a radio wave passes through electron density irregularities in the ionosphere (ratcliffe,1956). ionosphere scintillations present themselves as fast fluctuations of signal level with peak-to-peak amplitude fluctuations from 1 db to over 10 db and lasting for several minutes to several hours. the phenomena are caused by one of two types of ionosphere irregularities: sufficiently high electron density fluctuations at scale sizes comparable to the fresnel zone dimension of the propagation path, or sharp gradients of ambient electron density, especially in the direction transverse to the direction of propagation (itu-r p531-7, 2003). either type of irregularity is known to occur in the ionosphere under certain solar, geomagnetic and upper atmospheric conditions, and the scintillations can become so severe that they represent a practical limitation for communication systems. scintillations have been observed at frequencies from about 10 mhz to about 12 ghz. for systems applications, scintillations can be characterised by the fading depth and period. a useful index to quantify the severity of scintillation is the scintillation index s4, which is defined as the standard deviation of received power divided by the mean value of the received power. the fading period of scintillation varies over quite a large range from less than one tenth of 1 s to several minutes, as the fading period depends both upon the apparent motion of the irregularities relative to the ray path, and in the case of strong scintillations, on its severity. the fading period of gigahertz scintillation ranges from approximately 1 to 10 s. long period (of the or940 yurdanur k. tulunay and peter a. bradley der of tens of seconds) components of saturated scintillation (s4 approaching 1) at vhfand uhfbands have also been observed. ionosphere scintillations exhibit a wide range of variations in frequency dependence, morphology patterns, and diurnal, seasonal and solar-cycle dependence. typically s4 varies as frequency raised to the inverse power of 1.5, though on different occasions inverse power frequency dependences in the range 1 to 6 have been encountered. during an ionosphere scintillation event, the nakagami density function provides a close match to the statistics of the instantaneous variation of amplitude. for zenith angles of the propagation path up to 70° the s4 2 is proportional to the secant of the zenith angle. scintillation depth also depends on local time, season and longitude as defined by the angle between the sunset terminator and the local magnetic meridian at the apex of the field line passing through the line-of-sight at the height of the irregularity slab. there are also important latitudinal differences in the incidence of scintillations, with these being particularly strong at equatorial latitudes where they are caused by plasma bubbles, and at high latitudes, related to auroral and geomagnetic activities. at mid-latitudes, ionosphere scintillation tends to be associated with the incidence of the phenomenon known as spread-f. 1.3.10. examples of operational problems within the european theatre (bishop, 2003) from the foregoing it is evident that a series of different propagation phenomena are important to the different radio services, ranging from the requirement to be able to predict, both in the long-term and in near-real-time, the conditions under which ionospheric support is possible and the intensities of the resulting signals for communications at hf and below, to more stringent requirements on earth-space links where the needs of navigation and surveillance dictate that changes from free-space values of group path, angle-of-arrival, polarisation and scintillation all be quantifiable. the dominant ionospheric impairments to l-band navigation systems are considered to arise from delay effects, with scintillations giving secondary problems. land mobile communication satellite systems are principally affected by scintillations and also to some extent by faraday rotation effects. low-frequency earth observation sensors are mainly influenced by faraday rotation effects, but also by delay uncertainties. delay uncertainties are understandably of greatest importance to altimetry systems such as the european ra-2 system for determining ocean surface heights using a pulsed radar, with also resolution limited by dispersion effects. mobile satellite communications using the land s-bands are subject to the variable effects of phase and amplitude scintillations as well as to a lesser extent to faraday rotation problems, though in practice most systems avoid the latter difficulties by using circularly polarised signals. their main need is for accurate global scintillation models. the european space agency (arbesser-rastburg, 2001) is implementing a satellite navigation system egnos (european geostationary navigation overlay system). this involves a network of ground-based stations measuring slant pseudo-range errors from satellites in a 20 000 km orbit. tec values and error budgets determined from multi-band reception are broadcast so that mobile users can correct positioning errors, even when employing single-frequency reception. at the present time though there are needs to know the maximum ionisation gradients in space and time that can arise and the degree of «obliquity errors». this points to the need of having reliable 4d electron-content models, including a fast integration tool to determine slant tec. «worst-case» statistics are needed for input to an end-to-end simulator, to be derived from ionospheric conditions under storm and tid scenarios. what are the dynamics of the ionosphere and can scintillations cause receivers to loose lock? there are longer-term needs to be able to predict trans-ionospheric propagation through planetary ionospheres, such as that of mars. the requirements of the various listed services for ionosphere characterization, not otherwise met, are seen to be many. the work that has been carried out within cost 271 was responsive to many of these various needs to the 941 the impact of space weather on communication maximum extent that it was possible to do so within the available project resources and designated time-scales. 1.4. space weather parameters that create impact adversely and significantly on radio propagation characteristics in this section brief consideration only is possible to identify the different phenomena that arise under separate conditions with different time scales to affect the ionosphere, that are pertinent to the specific discussions of the remaining seventeen other work packages. 1.4.1. solar flares these are sudden, relatively short-lived brightenings of localized «plage» regions of the solar chromosphere. they have life-times ranging from a few minutes up to several hours, with an average duration of around half an hour. the enhanced electromagnetic and associated particle radiation emitted during a flare can affect the terrestrial and interplanetary environment. not all flares yield particle streams, consisting mainly of fast moving protons and electrons, which are detectable on the earth; this depends on the position on the sun where the eruption occurs relative to the earth’s direction. flares are classified according to their importance, or area at maximum brightness, and by a qualitative assessment of brightness. associated simultaneously with a flare are radio noise bursts and sudden ionospheric disturbances. delayed effects detectable at the ground anything from 15 min to 72 h afterwards are the onset of polar cap absorption, magnetic storms, ionospheric storms and aurorae. 1.4.2. x-ray enhancements increased radiation at radio and x-ray frequencies associated with a flare is often of sufficient intensity to affect the earth’s environment. in general effects are most pronounced with major solar flares, but sometimes likewise pronounced effects can occur with sub-flares. increases in solar x-ray and also ultra-violet radiation associated with a flare lead to sudden ionospheric disturbances on time scales ranging from minutes to several hours, during which there is enhanced absorption and frequency deviation of hf signals, while at the same time there is amplitude enhancement and phase change of vlf and lf signals. a number of other features are also associated with sudden ionospheric disturbances, including short-wave fadeouts and sudden enhanced cosmic noise absorption events. ionospheric storms are closely associated with geomagnetic storms and lead to reduced f2-region maximum ionization densities and associated hf circuit maximum usable frequencies. 1.4.3. coronal mass ejections and magnetic effects extremely high energy solar particles emitted during a disturbance travel at velocities approaching the speed of light and may arrive at the earth after delays of from 15 min to an hour. there they cause polar cap absorption events which, though relatively infrequent, can persist typically up to 20 h. polar cap absorption arises from the deflection by the earth’s magnetic field of the incident particle streams to the polar regions of the earth where they cause enhanced ionization in the ionospheric d-region. this gives rise to severe absorption of hf waves propagated across the polar regions which can persist for several days. so hf circuit lowest usable frequencies are increased, with the consequence that 942 yurdanur k. tulunay and peter a. bradley the usable frequency band is reduced, or even propagation is not possible at all. these events are normally followed by disturbances in the geomagnetic field. more common low to medium energy protons and electrons cause magnetic and ionospheric storms and the onset of the aurorae. geomagnetic storms arise from low energy plasma emitted from the sun. normally a geomagnetic storm comprises three phases: a) an initial phase in which a sudden initial disturbance (sudden commencement) quickly subsides and is followed by a quiet period lasting several minutes or hours; b) a main phase with an increase in the disturbance which persists for several hours or days, and c) a recovery phase with a gradual return of the field intensity to normal. various indices are used to describe the magnitude of the disturbance, the most common of which are the k index and the ap value. auroral activity is a visible indication of disturbances in the earth’s upper atmosphere. it occurs most frequently in the auroral zones in the northern and southern hemispheres. these are roughly circular bands around the geomagnetic poles at the centre of which there is a maximum of intensity. auroral induced ionization is associated with increased radio-wave absorption and scattering of hf waves. it occurs quite commonly, particularly during periods of enhanced solar activity during the upper part of the solar cycle. 1.4.4. solar radio burst emissions radio emission from the sun has three distinct components, originating from the quiet sun, from bright regions and from transient disturbances, such as flares. most flares are accompanied by increased emission at radio frequencies. the intensity and complexity of the radiation generally increase with the flare importance. the eruption of particle streams through the sun’s atmosphere is believed to be the cause of this increased emission. radio burst data are an important pre-cursor and indicator of possible solar particle emissions. 1.5. work directed towards improved propagation models various propagation models are available for different frequency bands and geographical theatres as described in current itu recommendations. for the most part they are based on a monthly median ionosphere and provide estimates of monthly median propagation parameters, together with some measure of the statistical probabilities of variations about these conditions. cost 271 has worked to contribute to state of the art with a number of specialist workshops and an impressive list of publications. following from earlier studies carried out within actions cost 238 (bradley, 1999) and cost 251 (hanbaba, 1999) aimed at developing and testing different longand short-term ionosphere and propagation models for varying applications relevant to radio usage and itu long-term service planning, topics examined relating to the impact of space weather on communications have embraced, among other aspects, «nowcasting» and forecasting techniques. specifically, web-based services for disseminating near-real-time space weather, ionospheric model and radio propagation predictions have been established by a number of participating organizations. these are described in greater detail elsewhere in this report. an important aspect of the work has been the review of existing propagation models, as well as the development of new models. the different studies that have been undertaken may be summarized as investigations of linear and non-linear ionospheric predictions; radio occultation techniques (using leo satellites to determine height profiles of electron density in remote areas); tomographical techniques (using signals from orbiting spacecraft to give 3d reconstructions of electron density, also including in remote areas; ionisation changes at different heights, including above the height of peak density, using various probing methods; horizontal gradients and structures of tec; geomagnetic storms and their impact on the ionosphere; ionospheric irregularities with various scale sizes; ionos943 the impact of space weather on communication pheric variability; extreme case scenarios; amplitude and phase scintillations, ionospheric coherence on earth-space links; propagation errors and noise on terrestrial and space-space communication systems and on earth-space navigation systems. future radio system design, spectrum utilisation strategies and spectrum management procedures require propagation information not currently available. collaborative european research project cost 271 has come some ways to meet these needs. 1.6. mitigation techniques within the context of cost 271 «mitigation techniques» are interpreted as means of reducing propagation impairments to radio systems. the basic general approaches possible are well known (bishop et al., 1996). these include routing via other propagation paths to avoid spatially limited sources of impairment, transmitting at a different time, use of a totally different radio system probably with a markedly separate operational frequency, frequency agility to avoid narrow-band interference – either changes of carrier frequency or in-band frequency variability, use of digital as opposed to analogue systems to minimise overall bandwidths, frequency, time, polarisation and/or spatial diversity systems and use of adaptive systems in any of the above regards, as well as with varying data transmission rates. a radio user with a particular system seeks guidance on how to optimise his system (bishop, 2003), and in many cases the optimum solution will be specific to him, depending on precisely what are the operational scenario, constraints, importance and resources available so that a tailor-made solution, perhaps involving a combination of the above approaches is what is needed. from a user’s standpoint there are three separate aspects: i) how to engineer a particular system to meet a defined requirement, ii) how to best operate an established system on any given occasion, and iii) to receive advance warning of abnormal propagation conditions and advice on what to do regarding these. space-weather parameters can be classified as direct if they are regularly monitored and are an established indicator of ionospheric state and indirect if they are part of the chain of events, but do not in themselves characterise the state of the ionosphere. other cost 271 wp’s have made use of indirect parameters in their studies and used direct parameters to generate corresponding same epoch and forecast ionospheric maps and models and the associated propagation models. evidently user synergy is needed to ensure that these propagation models provide the types of information that are wanted for the implementation of the most appropriate mitigation techniques. 1.7. feedback from the user community inevitably, work of the nature of that undertaken within cost 271, by its great diversity and range of topics considered, will be incomplete. interaction with as representative and extensive as possible a group of informed radio users is seen as essential to assessing the practical improvements that can be achieved with the use of its products. to identify areas of continued incomplete knowledge is also important to scientists and engineers working within allied research programmes. in other words, this section is an overview to help the reader appreciate and understand the scope of the work undertaken. references arbesser-rastburg, b. (2001): user requirements for trans-ionospheric propagation modelling, draft paper presented at cost271 first management committee meeting, abdus salam ictp, trieste (italy), 24-28 january 2001. 944 yurdanur k. tulunay and peter a. bradley bishop, g.j., a.j. mazzella, e.a. holland and g.s. rao (1996): an overview of ionospheric effects and mitigation in rf communication, navigation and surveillance, in ionospheric effects symposium, ies-96, 7-9 may 1996, alexandria (va). bishop, g.j. (2003): space environmental impacts on radio system operations, in cost 271 action, 23-27 september 2003, spetses, greece (published on line at the cost 271 web site). bradley, p.a. (1984): the ionosphere and radio communications, in links for the future – science, systems and services for communications, edited by p. dewilde and c.a. may (ieee/elsevier science), 171-174 . bradley, p.a. (1996): ionospheric effects on radio signals, in principles of radio communication, edited by f. mazda (butterworth-heinemann, oxford), 8-12. bradley, p.a. (1999): prediction and retrospective ionospheric modelling over europe, in cost action 238 prime final report, commission of the european communities, u.k. bradley, p.a. (2002): spectrum utilization, spectrum management and cost 271, acta geod. geophys. hung., 37 (4), 419-423. bradley, p.a. (2003): 3rd cost 271 workshop proceeding on significant results in cost 271 action – a review, contribution to discussions at 5th cost 271 management committee meeting, 27 february-1 march, roma. budden, k. (1961): radio waves in the ionosphere (cambridge university press). cannon, p. and p.a. bradley (2003): ionospheric propagation, in propagation of radiowaves, edited by l.w. barclay (institution of electrical engineers, london), 2nd edition, 335-355. davies, k. (1990): ionospheric radio, in electromagnetic wave series 31 (peter peregrinus ltd., institution of electrical engineers, london). fenwick, f.b. and o.g. villard (1963): a test of the importance of ionosphere-ionosphere reflections in long distance and around-the-world high frequency propagation, j. geophys. res., 68, 5659-5666. hall, j.s. (1947): radar aids to navigation, in radiation laboratory series 2 (mcgraw-hill, new york). hanbaba, r. (1999): improved quality of service in ionospheric telecommunication systems planning and operation, in cost 251 prime final report, commission of the european communities (space research center, warsaw, poland). haselgrove, j. (1954): ray theory and a new method for ray tracing, in reports on conference on the physics of the ionosphere (physical soc., london), p. 355. itu (2004): radio regulations (international telecommunication union, geneva). itu-r (1990): ionospheric propagation and noise characteristics pertinent to terrestrial radiocommunication systems design and service planning (fading), report 266-7 (international telecommunication union, geneva). itu-r p531-7 (2003): ionospheric propagation data and prediction methods required for the design of satellite services and systems (international telecommunication union, geneva). nielson, d.l. (1968): the importance of horizontal f-region drifts to transequatorial vhf propagation, in scatter propagation of radio waves, edited by e.v. thrane, agard conference proceedings 37, nato, neuilly-sur-seine, france. pierce, j.a., w. palmer, a.d. watt and r.h. woodward (1964): omega a worldwide navigational system (pickard and burns electronics), p&b publication no. 886. ratcliffe, j.a. (1956): some aspects of diffraction theory and their application to the ionosphere, in reports on progress in physics (physical soc., london), 19, p. 266. ratcliffe, j.a. (1959): the magnetoionic theory (cambridge university press). tulunay, y.k. (2001): wp 1.1. objectives-minutes of the first management committee of cost 271, 24-28 january 2001, abdus salam ictp, trieste (italy). annals 47, 1, 2004, 01/07def 83 annals of geophysics, vol. 47, n. 1, february 2004 mailing address: prof. minakshi devi, department of physics, gauhati university, gopinath bardoloi nagar, guwahati, 781014 assam, india; e-mail: md555@sify.com key words earthquake – tec – fof2 – anomaly 1. introduction earthquake-induced effects are diverse in nature and their manifestations are observed right from the earth crust to thousands of kilometres up in the atmosphere. conventional seismic monitoring along with knowledge of earthquake-associated phenomena like electromagnetic emissions, lithosphere-ionospheric coupling processes (gokhberg et al., 1982; oraevsky et al., 1994; larkina et al., 1989; hayakawa, 1999) are now widely utilized to disclose an association between the processes. anomalous behavior of the ionosphere prior to an earthquake was reported as early as 1929. since then, observations on the changes in ionospheric parameters like fof2, foes, total electron content (tec) and characteristics of turbulences have been examined, to understand if there is any earthquake induced information and to devise some predictor parameters (wolcott et al., 1969; parrot and mogilevsky, 1989; blaunstein, 1999; devi et al., 2001) within these variables. however, analyses and co-relative works for identifying earthquake precursors remain a challenging problem because of the complexity of their very nature. in this paper ionospheric parameters tec and fof2 of anomaly crest stations like guwahati (26.2°n, 91.75°e) and ahmedabad (23.01°n, 72.36°e) are examined, associating a number of earthquake cases. as physical and dynamic processes of the high, mid, low and equatorial latitude ionosphere are of different nature, we will restrict our analysis to the low latitude equatorial anomaly region only. association of total electron content (tec) and fof2 variations with earthquake events at the anomaly crest region minakshi devi (1), ananda k. barbara (1) and anna depueva (2) (1) department of physics, gauhati university, assam, india (2) institute of terrestrial magnetism, ionosphere and radiowave propagation (izmiran), russian academy of science, troitsk (moscow region), russia abstract the paper attempts to identify ionospheric parameters in association with earthquake at anomaly crest station through vhf radio beacon data and ground based ionosonde measurements while the total electron content (tec) parameters from rb observations are based mainly on data taken over guwahati (26.2°n, 91.75° e), fof2 data used in the analysis were collected at ahmadabad (23.01°n, 72.36°e). the paper describes methods and techniques adopted to examine modifications on these parameters if any, due to earthquake preparatory processes at equatorial anomaly crest stations. the mechanism of inducement of density changes in the ionosphere is sought through the generation of strong fountain effect possibly by the development of electric field during the earthquake preparatory process. 84 minakshi devi, ananda k. barbara and anna depueva 2. background some of the strongest earthquakes of the world occurred on this region in the years 1548, 1596, 1642, 1663, 1897, 1875, though their exact magnitudes are not known. some events of known intensity are those of the years 1897 (m = = 8.7), 1918 (m = 7.6), 1930 (m = 7.1), 1950 (m = 8.7) and 1988 (m = 7.3). because of the high seismic propensity of this zone, prediction of earthquakes in this region has become a necessity. though a few short-term predictions based on statistical study have been made, these were observed to be false alarms. fig. 1a,b. tec variations during low solar activity period over guwahati: a) a quiet day feature; b) a disturbed day tec character. note: the large enhancement in ionization density after sunset during the d-day. fig. 2a,b. same as in fig. 1a,b but for high solar activity periods: a) the usual quiet day diurnal variation and (b) one of the typical disturbed day tec pattern. note: the initial decay rate on d-day is arrested by post sunset enhancement. a b a b 85 association of total electron content (tec) and fof2 variations with earthquake events at the anomaly crest region maximum and minimum excursions from the base are determined. we have taken any deviation in tec over this excursion limits as an index of anomaly. thus, deviations in tec from the base during and pre-earthquake days are evaluated. attempts are then made to associate such excursions with earthquake events. further, extensive study on magnetic storm-induced tec behaviour over this station has been made (not discussed here) to omit the magnetic storm afflicted tec data while making this selection. figure 3a,b shows two representative cases for low solar activity period. the two cases are so selected that they represent two basic features of earthquake-time density variation. considering the type i behavior, we take the december 6, 1975 earthquake event. a clear increase in tec prior to the earthquake days is noted in this case. this earthquake occurred at 11:15 h (92° east meridian time, emt) with epicentre at 17.42°n (119.68°e). the figure also shows maximum and minimum excursion values of q-days from its mean value, along with diurnal tec variations of december 6. magnitudes of daytime electron content on the two days show a clear high, relative to average quiet day tec from the 30 day period. as an example of type ii, fig. 3b represents features of the july 21, 1976 earthquake, when a significant depletion in tec values are noted prior to and during the event. this earthquake occurred at 21:10 h (92°emt) on that day with its epicenter at 24.78°n (98.70°e). the tec pattern was the reverse of what was received on 6th december. in the pre-event time, a decrease of density is noted at the latitude of guwahati and the density dips far below the minimum excursion bar of quiet day tec mean. however, such strong changes may not be clear when solar activity and background density both are high. as an example, we take the tec values say for july 29, 1980 (fig. 4), when two consecutive earthquakes are recorded at 18:20 h (92°emt) and 20:58 h (92°emt) with epicenters at 29.33°n (81.26°e) and 29.60°n (81.09°e). on this occasions increase in density at guwahati though appreciable from early morning hours, the large changes in tec as observed in the low solar period are not so appar3. observations 3.1. tec and earthquake events our attempts to understand the association between columnar electron content behaviour and earthquake events are discussed here through a few observations. this work is based on faraday rotation (fr) data collected through satellite radio beacon (rb) signal from ats-6 (frequency 144 mhz, position 35°e) for low solar activity periods and from ets ii (frequency 136.11 mhz, position at 130°e) for high solar activity periods. before attempting to identify earthquake-induced imprints on tec profiles, as it is necessary to have a knowledge on the relation of tec with solar and geomagnetic parameters, we show in figs. 1a,b and 2a,b a few profiles observed over guwahati at different solar and geomagnetic situations. it is seen that during high solar activity periods (1979-1981) tec is at least 2 fold higher (fig. 2a,b) than at low solar epoch of 1975-1976 as in fig. 1a,b, indicating the need to realize the role of background density while assessing possible earthquake-induced features. in fig. 1a, the tec profile of june 12, 1976 while represents quiet day (q-day), the profile of july 1, 1976 features a disturbed day (dday). a large increase in ionization density even after sunset hours is a significant point to be noted in the d-day, indicating processes other than normal ones are active on such days. depending on the solar-geomagnetic conditions, it is observed that the tec profile shape of d-day changes (devi et al., 2002). in fig. 2a,b, we present one q-day event (may 25, 1980) and a d-day tec profile (24 august 1980) of high solar periods. here too, on the d-day the initial decay rate is arrested by injection of extra ionization. keeping these features in mind, we take the following approaches while examining possible seismo-ionospheric effects though this parameter. five most quiet day tec values are first identified and their average figure is taken as a base of tec free from geomagnetic disturbances. we select a period of 30 days preferably keeping earthquake day as the centre while selecting the q-day base. further, as q-day tec also show day to day fluctuations, their 86 minakshi devi, ananda k. barbara and anna depueva a fig. 4. same as in fig. 3a,b but for high solar activity earthquake event. the event shown is for 29 july, 1980. earthquake occurrence times are shown by arrowheads. the vertical bars give excursion of standard deviation of q-day from the base. b fig. 3a,b. plots showing variations in tec (taken over guwahati) from the q-day base, during low solar activity period. earthquakes occurred on (a) december 6, 1975 and (b) july 21, 1976. earthquake commencement times are shown by arrowheads in the figure. the vertical bars give the excursion of standard deviation of q-day tec from the base. 87 association of total electron content (tec) and fof2 variations with earthquake events at the anomaly crest region ent probably because of high background density conditions. in this case, the epicenter lies at 5° away from the sub ionospheric point of the satellite which is at 24.12°n, 95.71°e. results of this nature are presented elsewhere (devi et al., 2001). the analysis therefore indicates that both positive and negative ionospheric effects can be seen prior to an earthquake, and an association between ionospheric effects and its epicenter position cannot be ruled out. 3.2. fof2 as precursor of earthquake the changes in critical frequency of f-layer received from ground-based ionosonde at ahmedabad are next examined to disclose variations in fof2 if any, during the earthquake preparatory process. for this purpose, hourly fof2 data for 15/10 days prior to an event are considered. further, we examine the magnitude and development of bite out, as a result of a fountain effect another input along with temporal changes in fof 2 prior to an earthquake. a few representative cases in fo f 2 features and generation of anomaly before earthquake events are described below. two events presented in fig. 5 are for april 6 (epicenter at 26.19°n, 96.87°e) and april 27 (epicenter at 13.07°n, 119.54°e), 1994. the dark shades indicate strong density zones, prior to an earthquake and a signature of bite out is seen as two black areas around noon. it is also to be noted that the background density during this month is fairly large and ionization density needs to be sufficiently strong to exhibit the signature of bite out effect. arrowheads in the figure indicate time (78°emt) of occurrence of the earthquake. the absence of data is also marked. figure 6 presents a representative event of a summer case where the earthquake occurred on july 30, 1996 at 23:08 h (78°emt) with its epicenter at 14.51°n (119.95°e). here, strong density regions and anomaly (i.e. bite out) were not seen many days earlier, but a high density zone is formed two days before the event and anomaly effect is detected 6 h before earthquake. interestingly, all these cases (shown as representatives of different seasons) indicate generation of strong density along with bite out, which must have sources other than the normal ionizing fig. 5. fof2 variation pattern over the anomaly crest station of ahmedabad before and during earthquakes of april 6 and 27, 1994. the arrowheads give the time of commencement of earthquakes. the density magnitude is defined by gray shades. black zone is for frequencies > 11.00 mhz. early morning data are not shown in the figure. high density zones and bite outs are present before and during the events. 88 minakshi devi, ananda k. barbara and anna depueva process. however, these features may not be well reflected in all cases especially when we have a succession of tremors at frequent intervals as shown in fig. 7 for may 1997, where there were three earthquakes of magnitudes of 5.5-6.0, on 5th, 8th and 21st day. the 5th may quake was observed at 11:15 h (78°mt), with epicenter at 14.91°n (119.89°e). high density zones are seen prior to the quake day but due to non availability of data at 13, 15 and 16 h on day 4, it is difficult fig. 6. one more case showing fof2 variations, prior to and during the 30.7.1996 earthquake. in this case the darkest region is for frequencies > 10.50 mhz. presence of bite out is to be noted before the earthquake event. fig. 7. same as in figs. 5 and 6 but for three consecutive summer earthquake events for may 5, 8 and 22 of 1997. the arrowheads indicate the occurrence time of each earthquake event. here the black zone is for frequencies > 10.00 mhz. white circles indicate absence of data. though bite out is not prominent, high density zones up to day 23 are significant. 89 association of total electron content (tec) and fof2 variations with earthquake events at the anomaly crest region to make a statement on bite out. however, on the day of the event a bite out is observed. on may 8, another tremor was detected at 08:23 h (78°emt) with epicenter at 24.89°n (92.25°e). strong density zones in noon and afternoon hours, prior to the earthquake day are clearly seen in the ionogram. but, we cannot say if bite out was present because of want of data during noon hours, however bite out was seen after the earthquake. so, is the case with the event of may 22, when the tremor occurred at about 04:00 h. however, we cannot make a statement on bite out on may 19, 20 and 21, as there were no continuous data. the high density regions in noon and afternoon periods as noted almost on all days from may 3 to 23rd, subside after may 23rd. the overall pattern described above seems to be maintained during major earthquakes too, one such event is shown in fig. 8. 3.3. major earthquake on 26th january 2001, ahmedabad region experienced a very strong earthquake of scale 8.1, and was followed by a number of tremors. it is interesting to note the high densities indicated by strong fof2 values prior to the event (fig. 8). bite outs are also visible before quake days but high density contours disappeared shortly before the onset of earthquake. this decrease in density and the position of the epicentre of the earthquake (23.36°n and 70.33°e), is to be noted. due to mass destruction, no ionospheric data were available just after the onset of earthquake, which continued for 3 days. 4. discussion although ionospheric parameters behave in a complex manner, scientists have been attempting to associate some of their features with seismic events (wolcott et al., 1969; hayakawa, 1999; depueva and rotanova, 2001). one of the approaches is by analysing the development and inhibition process of anomaly by mapping the magnitude and spread of its crest and trough, before and during earthquake events using topside ionospheric data (depueva and ruzhin, 1993, 1995). in this exercise too, we are trying to see the anomaly effect during the earthquake preparatory process. increase in magnitude of tec, which is observed to be fig. 8. a case of very strong earthquake of m = 8.1 experienced around ahmedabad on 26 january, 2001. the large density contours followed by depletion in density prior to the event (at 09:00 h, 78°emt) are clearly seen. here the black region corresponds to frequencies > 11:50 mhz. due to mass destruction no fof2 data are available just from the onset of the event for 3 days. 90 minakshi devi, ananda k. barbara and anna depueva higher than maximum excursion limits in day to day tec fluctuations (figs. 1a,b and 2a,b), indicate that an inflow of ionisation might have taken place in the earthquake preparatory region from an additional source. similarly, a decrease in tec suggests that an outflow of ionization has taken place from the afflicted site. such changes in tec variations for a number of strong earthquake events (devi et al., 2001) also indicate that high-density tec contours are often associated with earthquakes having their epicenters near to the equator or away from the observational site. the study further indicates that tec depletions (like the event of july 1976) are often observed when the epicenter lies very near to the observational site. it is also observed from fof2 variations over ahmadabad that enhancement as well as biteout are fairly well detected phenomena prior to an earthquake. but unlike the case of tec, decrease in density is not clearly seen even if the epicenter lies near to the receiving site. however, for the major earthquake of january 2001, when its epicenter was at ahmedabad, a decrease in density was detected prior to the event. the depletion features as observed in tec are prolonged in comparison to what is seen through fof2. these enhancements (as well as depletion) in density values along with development of anomaly if considered as earthquake-induced perturbations in the atmosphere, an extra agent (besides the normal ionizing agencies) associated with the earthquake preparation processes in shaping density profiles has to be active. as e× b drift is one of the significant contributors in controlling density at anomaly crest stations like ahmedabad and guwahati (devi et al., 2002), it is necessary to invoke electric field triggered by earthquake preparatory processes that generate e× b drift (depueva and ruzhin, 1993, 1995). in fact, emission of elf fields prior to earthquakes was also detected near the epicenter (larkina et al., 1983; parrot and magilevsky, 1989). depletion and enhancements as seen on density profiles, may be the result of earthquake-associated e× b drift when electron density may flow into or out of the observing station depending on its location, if an electric field of sufficient magnitude is developed at the ionospheric heights. there are reports related to possible effects of earthquake generated electric fields at ionospheric heights (gokhberg et al., 1984; parrot, 1995). considering that such electric field does develop during earthquake preparatory processes and it penetrates to e-layer heights resulting in enhanced electrojet strength, the upward e× b drift will then be amplified. we therefore examine the effects of such field, reflected as bite outs in density profiles while dealing with fof2, as one of the inputs for identifying seismo-ionospheric effects. however, the anomaly effects generally being very regular and strong during equinoxial months, we have taken summer and winter events only, to avoid ambiguities in analysis and interpretation. in fact, the development or inhibition of anomaly during earthquake preparatory processes have also been examined by earlier workers (depueva and rotanava, 2001). while analysing topside electron density over a dip latitude zone of ± 30°, they showed that considerable changes in the development processes of the anomaly do take place prior to an earthquake and also that the position of the epicentre is critical in controlling the magnitude of anomaly. however the magnitude of such electric fields is still a question. references blaunstein, n. (1999): remote sensing of short term ionospheric phenomena, indicators of precursors of earthquakes by use of radiophysical methods, in progress in electromgnetics research symposium proceedings, taipei, vol. 1, pp. 132. depueva, a. and n. rotanova (2001): low latitude ionospheric disturbances associated with earthquakes, ann. geophysics, 44 (2), 221-228. depueva, a.h. and yu.ya. ruzhin (1993): the equatorial earthquake preparatory stage as a reason of «fountain» effect in the ionosphere, preprint izmiran, n82 (1029), p. 10. depueva, a.h. and yu.ya. ruzhin (1995): seismoionospheric «fountain» effect as analogous of active space experiment, adv. space res., 15 (12), 151-154. devi, m., m.k. barman, a.k. barbara and m. depueva (2001): total electron content near anomaly crest as precursor of earthquake, ind. j. radio space phys., 30, 209-213. devi, m., m.k. barman and a.k. barbara (2002): iden91 association of total electron content (tec) and fof2 variations with earthquake events at the anomaly crest region tification of quiet and disturbed days through tec profile features over anomaly crest region, j. atmos. solar terr. phys., 64, 1413-1423. gokhberg, m.b., y.a. morgunov, t. yoshino and t. tomizawa (1982): experimental measurement of electromagnetic emissions possibly related to earthquakes in japan, j. geophys. res., 87, p. 7824. gokhberg, m.b, n.i. gershenzon, i.i. gufel’d, a.v. kustov, v.a. liperovskiy and s.s. khusameddimov (1984): possible effects of the action of electric fields of seismic origin on the ionosphere, geomagn. aeron., 24, p. 183. hayakawa, m. (editor) (1999): atmospheric and ionospheric electromagnetic phenomena associated with earthquakes (terra scientific publishing co., tokyo), pp. 996. larkina,v.i., a.v. nalivayko, n.i. gershenzon, m.b. gokhberg, v.a. liperovsky and s.i. shalimov (1983): observations of vlf emissions related with seismic activity, on the interkosmos-19 satellite, geomagn. aeron., 23, 684-687. larkina, v.i., v.v. migulin, o.a. molchanov, i.p. kharkov, a.s. inchin and v.b. schvetcova (1989): some statistical results of very low frequency radio wave emissions in the upper ionosphere over earthquake zones, phys. earth planet. int., 57, 100-109. parrot, m. (1995): use of satellite to detect seismo-electromagnetic effects, adv. space res., 15 (11), 1,271,35. parrot, m. and m.m. mogilevsky (1989): vlf emission associated with earthquakes and observed in the ionosphere and the magnetosphere, phys. earth planet. int., 57 (1-2), 86-99. oraevsky, v.n., yu.ya. ruzhin and a.h. depueva (1994) seismoionospheric precursors and atmospheric electricity, turkish j. physics., 18, 1229-1234. wolcott, j.h., d.j. simons, d.d. lee and r.a. nelson (1969): observations of an ionospheric perturbation arising from the coalinga earthquake at kurile islands on august 11, 1969, nature, 226, p. 1239. land s u b s i d e n c e i n v e n i c e a n d p o r t o c o r s i n i (*) m . c a p u t o l. p i e r i p. rossi t e s i ( * * ) r e c e i v e d o n j a n u a r y 22, 1972 summary. — t h i s p a p e r is a c o n t r i b u t i o n t o t h e s t u d y of t h e p r o b l e m of s u b s i d e n c e in t h e p o d e l t a a n d in t h e v e n e t i a n l a g o o n . w e h a v e c o m p a r e d t h e d a t a s u p p l i e d b y t i d e g a u g e s i n t r i e s t e , v e n i c e a n d p o r t o corsini ( f i g . 1) d u r i n g t h e p e r i o d 1896-1966, a n d i n g e n o a i n t h e p e r i o d 1894-1966. a l t h o u g h d a t a w e r e r e c o r d e d in s l i g h t l y d i f f e r e n t p e r i o d s , t h e s e p e r i o d s a r e s u f f i c i e n t l y o v e r l a p p i n g t o a l l o w f o r a s i g n i f i c a n t s t a t i s t i c a l c o m p a r i s o n . w e n o t e t h a t t h e m o v e m e n t of t h e a n n u a l m e a n s e a l e v e l in g e n o a a n d t r i e s t e is g r e a t l y d i f f e r e n t f r o m t h a t of p o r t o c o r s i n i a n d v e n i c e a n d , i n o r d e r t o s e p a r a t e t h e c o m p o n e n t d u e t o e u s t a t i c e f f e c t f r o m a n y p o s s i b l e m o v e m e n t of t h e c o a s t , w e calc u l a t e t h e polynomial c o e f f i c i e n t s , t h a t b e s t f i t t h e r e a l m o v e m e n t s . a p p r o x i m a t i n g p o l i n o m i a l s a r e of f i r s t d e g r e e f o r g e n o a a n d t r i e s t e a n d of s e c o n d d e g r e e f o r p o r t o c o r s i n i a n d v e n i c e . w e h a v e e v a l u a t e t h e c o e f f i c i e n t s of c r o s s c o r r e l a t i o n firstly o n o r i g i n a l d a t a a n d t h e n s u b t r a c t i n g f r o m o r i g i n a l d a t a t h e b e s t fit p o l i n o m i a l s . t h e c o r r e l a t i o n is v e r y s i g n i f i c a n t a n d t h e t r e n d is s i m i l a r in t r i e s t e a n d g e n o a . t h e p o w e r s p e c t r u m a n a l y s i s w i t h t h e d i r e c t f o u r i e r ' s m e t h o d s h o w l o n g p e r i o d s w i c h a r e s h a r e d b y t i d e g a u g e s . w e c o n c l u d e e s t i m a t i n g 1,35 m m / y e a r t h e i n c r e a s e of t h e m e a n s e a l e v e l f o r t h e e u s t a t i c e f f e c t in t h e m e d i t e r r a n e a n 3,9 m m / y e a r t h e s u b s i d e n c e in p o r t o corsini a n d 1,4 m m / y e a r t h e s u b s i d e n c e i n v e n i c e . riassunto. — allo s c o p o di c o n t r i b u i r e allo s t u d i o d e l l a s u b s i d e n z a nel d e l t a p a d a n o e n e l l a l a g u n a v e n e t a a b b i a m o c o n f r o n t a t o i d a t i m a r e o g r a fici di t r i e s t e , v e n e z i a e p o r t o corsini nel p e r i o d o ( 1 8 9 6 1 9 6 6 ) e quelli di g e n o v a nei p e r i o d i (1884-1910) e (1928-1966). l ' a n a l i s i di t a l i d a t i m o s t r a u n a n d a m e n t o s e c o l a r e cui si s o v r a p p o n g o n o v a r i a z i o n i p e r i o d i c h e ; r i s u l t a i n o l t r e c h e i m o v i m e n t i del livello m e d i o del m a r e a p o r t o corsini e a v e n e z i a d i f f e r i s c o n o n e t t a m e n t e d a quelli di g e n o v a e t r i e s t e c h e i n v e c e s o n o in o t t i m o a c c o r d o t r a di loro. a b b i a m o poi c a l c o l a t o le linee di r e g r e s s i o n e c h e si a d a t t a n o meglio ai v a l o r i o s s e r v a t i : t a l i l i n e e r i s u l t a n o delle r e t t e p e r g e n o v a e t r i e s t e e delle c u r v e di i i g r a d o p e r p o r t o corsini e v e n e z i a . i c o e f f i c i e n t i (*) t h i s w o r k h a s b e e n c a r r y o u t b y t h e c o n t r i b u t i o n of c . n . r . ( l a v o r o e s e g u i t o c o n il c o n t r i b u t o del c . n . r . ) . (**) i n s t i t u t e of g e o d e s y a n d t o p o g r a p h y u n i v e r s i t y of b o l o g n a ( i t a l y ) . 5 0 m . c a p u t o l . p i e r i f . r o s s i t e s i (li c o r r e l a z i o n e t r a i v a l o r i o r i g i n a l i e t r a q u e l l i c o r r e t t i p e r l ' a n d a m e n t o s e c o l a r e d i m o s t r a n o l ' o t t i m a c o r r e l a z i o n e t r a i d a t i r e g i s t r a t i i n t u t t e le l o c a l i t à . l ' a n a l i s i s p e t t r a l e , e f f e t t u a t a c o n il m e t o d o d i r e t t o d i f o u r i e r , m o s t r a c h i a r a m e n t e i s e g u e n t i p e r i o d i a n n u i p e r t u t t i i m a r e o g r a f i : 2 0 . 0 ; 1 2 . 5 ; 8 . 3 ; 0 . 7 ; 4 . 0 ; 3.4. si c o n c l u d e o s s e r v a n d o c h e l ' i n n a l z a m e n t o a n n u o e u s t a t i c o d e l l i v e l l o m e d i o m a r i n o n e l m e d i t e r r a n e o r i s u l t a d i m m 1,4 n e l l ' u l t i m o s e c o l o , m e n t r e l a s u b s i d e n z a a p o r t o c o r s i n i è di m m 3 , 9 e, a v e n e z i a , d i m m 1,4 a n n u i . 1 . s t a t e m e n t o f t h e p r o b l e m an analysis of long time records of tide gauges located in differents stable areas should give t h e secular sea level variation. u n f o r t u n a t l y it is not easy to prove t h e stability of stations where mareographs are located. w e h a v e collected d a t a on four italian stations located in geologically different areas covering a b o u t 70 years a n d a t t e m p t e d t o estim a t e t h e secular variation of t h e sea level a n d also t h e subsidence of some of the stations. to h a v e more accurate results t h e analyses of a larger n u m b e r of stations should be made; however we feel t h a t t h e d a t a t h a t we analyse give consistent and accurate results. 2 . a v a i l a b l e d a t a d a t a are supplied by t i d e gauges in trieste, venice, p o r t o corsini and genoa, t h e y are yearly averages of observations carried o u t in t h e periods listed in table 1. f i g 1 l a n d s u b s i d e n c e i n v e n i c e a n d p o r t o c o r s i n i (il iii t h e period 1922-1034 in p o r t o corsini occasional recordings were m a d e which allowed for a n interpolation t h a t covers the whole lapse of time 1896-1966. i t was impossible to perform the same operation in genoa, because t h e local tide gauge h a d been damaged in t h e course of various war events and remained inactive between 1911 and 1927. the tide gauges of genoa and trieste are located on solid rocks those of venice and p o r t o corsini are on thick sediments which are known to subside. table 1 summarizes t h e periods for which we have information. t a b l e 1 p l a c e p e r i o d t r i e s t e 1896-196(5 v e n i c e 1 8 9 6 1 9 6 6 p o r t o c o r s i n i 1 8 9 6 1 9 6 6 g e n o a 1 8 8 4 1 9 1 0 1 9 2 8 1 9 6 6 3 . data analysis. pigs. 2, 3, 4, 5, concerning t h e recorded tide gauge d a t a , clearly show t h a t the m o v e m e n t of the m e a n a n n u a l sea levels in p o r t o corsini a n d venice greatly differs f r o m t h a t observed in genoa and trieste. a survey of t h e d a t a suggest t h a t in each s t a t i o n there is a secular t r e n d on which periodic oscillations are placed on. i n order to separate the periodic m o v e m e n t of the sea level and the component due to glacial eustatic effect f r o m t h e secular trend, we computed best fit polynomials of 1st degree for genoa a n d trieste and 2nd degree for p o r t o corsini and venice. the correlation coefficients between t h e raw d a t a and t h e d a t a after subtracting t h e secular t r e n d are given in tables 2 and 3. i t is 50 m . c a p u t o l . p i e r i f . r o s s i t e s i f i g . 3 m e a n a n n u a l s e a l e v e l i n p o r t o c o r s i n i . l a n d s u b s i d e n c e i n v e n i c e a n d p o r t o c o r s i n i (il f i g . 4 mean a n n u a l sea level i n v e n i c e . 350 s-j1 1 1 , -gj 1 ' ' 1 í 1 1 1 s 1 1 s 1 1 s 1 1 a 1 ' l — i f i g . 5 m e a n a n n u a l sea level in g e n o v a . obvious t h a t the correlation is very significant a f t e r removal of t h e secular effect which proves the physical significant of the d a t a for t h e long periodic variations of t h e sea level. t a b l e 2 c o r r e l a t i o n c o e f f i c i e n t s f o r t h e r a w d a t a p c v e g e t s p c 1 v e 0,92 1 — — g e 0,78 0,89 1 — t s 0,75 0,90 0,91 1 50 m. c a p u t o l . p i e r i f . r o s s i t e s i t a b l e 3 c o e f f i c i e n t s o f c o r r e l a t i o n a f t e r s u b t r a c t i o n o f l i n e a r e f f e c t f o r t r i e s t e a n d g e n o a a n d o f 2 n d d e g r e e e f f e c t f o r v e n i c e a n d p o r t o c o r s i n i . p c v e g e t s p c 1 — — v e 0 , 8 3 1 — — g e 0 , 8 1 0 , 8 4 1 — t s 0 , 8 3 0 , 9 1 0 , 8 9 1 4 . s p e c t r a l a n a l y s i s . the power spectrum was also obtained, by f o u r i e r ' s direct m e t h o d . the plot of t h e power s p e c t r u m clearly show the following periods (in years) which are shared by all tide gauges: 2 0 . 0 1 2 . 5 8 . 3 6 . 7 5 . 6 4 . 5 4 . 0 3 . 4 t h e statistical t e s t of w a l k e r on t h e power spectrum show t h a t t h e peaks r e l a t e to period of 20.0 a n d 12.5 years are highly significance (97,5% confidence level); t h e others h a v e a confidence level lower t h a n 6 0 % . i t should be noted t h a t some periods related to t h e genoa recordings are slightly different f r o m those recorded by t h e other tide gauges, f r o m this it is p o i n t out also a period a t 25 years which does n o t appear in t h e other records. this difference is m o s t probably due to t h e different situation in t h e sea basins. also t h e power spectrum concerning genoa d a t a is based on a less complete time series. 5 . s e c u l a r t r e n d a n a l y s i s . as we mentioned t h e tide gauges in venice a n d p o r t o corsini are located on thick sediments in areas wich are known to subside. the linear secular t r e n d in trieste a n d genoa which are located in stable rock of geological different areas are: 1.40 m m / y e a r a n d 1.36 m m / y e a r in perfect agreement. this linear t r e n d is very p r o b a b l y due to eustatic sea rise. l a n d s u b s i d e n c e i n v e n i c e a n d p o r t o c o r s i n i ( i l s u b t r a c t i n g this trend f r o m the q u a d r a t i c secular t r e n d of p o r t o corsini and venice we should o b t a i n the subsidence in these stations; rrvqutncy ciclm/vt*r fi<. 6 we see t h a t t h e lowering of the land level during t h e last 70 years has had the average rate of 3.9 m m / y e a r in p o r t o corsini and 1.4 m m / y e a r a t the tide ganges in venice. layout 6 1 annals of geophysics, 63, 5, pa545, 2020; doi:10.4401/ag-7982 investigation of the characteristics of geoelectric field earthquake precursors: a case study of the pingliang observation station, china zhanghui an*,1,2, yan zhan3, yingying fan1,2, quan chen1,2, jun liu1,2 (1) gansu lanzhou geophysics national observation and research station, lanzhou 730000, china (2) gansu earthquake agency, china earthquake administration, lanzhou 730000, china (3) state key laboratory of earthquake dynamics, institute of geology, china earthquake administration, beijing 100029, china article history: received october 17, 2018; accepted may 7, 2019 abstract an increasing number of earthquake precursors have been observed by geoelectromagnetic stations; however, the statistical characteristics of the earthquake and geoelectric field (gef) are not well known. based on the sliding correlation method (scm) and its validity test, we analyzed 125 months of continuous data from the pingliang (pl) gef station in china. two factors, seismic energy and epicentral distance (d), were used to select the earthquake events. when the lower limit of the magnitude energy is set as 106 j/km2, the number of earthquake events in area a (d < 200 km) is 36, and the number of seismic events in area b (200 km ≤ d < 365 km) is 34. in the analysis of the calculated results, the mean value of 1000 times random test scm calculation is taken as the anomaly standard. in addition, it is found that the anomalies occurring north-south, and north-east of pl station are the most significant in area a, and the anomalies are concentrated approximately 8-26 days before the earthquakes. there is no obvious correlation east-west of pl station in area a, and there is no similar abnormal phenomenon in area b. the reasons and probably mechanism of the investigation results are analyzed and discussed in combination with the fault belt features of the study area, numerical simulation, and laboratory rock fracturing experiments. the uniqueness of the results is verified by reducing the energy standard of a single earthquake. this investigation supports the statistical point of view of the microcosmic interpretation of gef abnormalities, provides significant suggestions for the selection of electrodelayout orientations used for gef observations and also provides a reference for the gathering of the seismic precursor information. keywords: the geoelectric field (gef), statistical characteristic, shifting correlation method, correlation coefficient, earthquake precursor. 1. introduction before and during earthquakes, seismic electromagnetic signals can be produced. these signals have been widely recognized by researchers [e.g. varotsos et al., 1981; varotsos and alexopoulos, 1984a; park et al., 1993; huang, 2002, 2005, 2011; tang et al., 2010; uyeda et al., 2009]. the geoelectric field (gef) is mainly used to observe and study anomalous phenomena related to earthquakes. under normal conditions, for example no man-made disturbance or space electromagnetic activity etc, spectrum analysis results show that the gef not only reveals the tidal components of different periods, e.g., full day and half days [e.g. huang and liu, 2006; tan et al., 2012], it also reveals the characteristics of seasonal variations [cui et al., 2013]. after earthquakes have occurred, we can compare their occurrence with the normal conditions to conduct a retrospective investigation of the earthquake events. it is suspected that the anomalous variation phenomenon is related to the earthquake, e.g., the distortion of the waveforms of the gef before the earthquakes and the fact that the low frequency components of the gef increase in power (dc-ulf) before earthquakes [uyeda et al., 2002; eftaxias et al., 2001; fan et al., 2010; an et al., 2011, 2013]. related analysis of the retrospective test of typical earthquake events has been conducted. the number of suspected anomalous gefs related to earthquakes is limited by the number of earthquake samples and lacks universality or statistical significance. in particular, the statistical correlation between the timing of the gef anomalies and that of earthquakes is still unclear. the process of earthquake occurrence caused by the rupture of underground medium is a very complicated geodynamic process, which is accompanied by various physical phenomena, one of which is the electromagnetic anomaly that occurs before the earthquake [huang, 2005; thanassoulas, 2007; orihara, 2012]. at present, because the physical mechanism of the anomalous variation of the gef before the earthquake is still unclear, statistical analyses may be another feasible method to study the statistical correlation between the gef and an earthquake event. earlier statistical studies [thanassoulas and tselentis, 1993; ifantis et al., 1995, 1997; ifantis, 2002] show that the observation stations a certain distance from the epicenter may detect the low frequency (24±8h) component enhancement before an earthquake. recent statistical studies [sarlis, 2018] have also shown that the anomalous gef phenomena are statistically significant earthquake precursors with lead times in four distinct periods: 3 to 9 days, 18 to 24 days, 43 to 47 days and 58 to 62 days. in this paper, a sliding correlation method (scm) [jiang et al., 2016] based on statistical significance is introduced and used to investigate the correlation between the variation in the diurnal energy of the gef and the energy of the earthquake events. we use data from the pingliang (pl) gef station in the gansu province of china. the statistical characteristics of the anomalous occurrence time of gefs during earthquakes are analyzed. first, the methods of gef observation, the performance of the observational instruments, and the principles of seismic event selection are introduced. then, the scm is briefly introduced, and the validity of the method is verified using a random signal. last, the results of the investigation are analyzed and discussed in conjunction with the structural characteristics of the fault zone in the investigation area, a numerical simulation of the gef anomalies, and laboratory rock fracturing experiments. 2. gef observation and earthquake selection 2.1 introduction of gef observation according to the greek van method [varotsos et al., 1981], it is possible to capture the precursor information for earthquakes by measuring the gef. the pl station (35o54’n, 106 o56’e, elevation 1357 m) is located in the gansu province of china, the small structures around the station are very well developed, mainly in the liupanshan active fault zone. the strata mainly contain continental rocks and igneous rocks are exposed in some areas. the observation system used by the pl station adopts an l-shaped pattern (figure 1a), and the observation channels are aligned in the north-south (ns), east-west (ew), and north-east (ne) directions. along each of these directions, long and short electrode tracks were installed (figure 1b) with a long polar distance of 300 m and a short polar distance of 200 m. the measuring electrode is a solid nonpolarized electrode (figure 1c), and its related parameters are presented in table 1. zhanghui an et al. 2 the burial depth of the electrodes is approximately 3.5 m. the data acquisition instrument is a special digital observation instrument (zd9aii digital gef instrument, as shown in figure 1d) developed and produced by the institute of earthquake forecasting, china earthquake administration. the relevant parameters for this instrument are presented in table 2. 3 characteristics of gef earthquake precursors figure 1. the electrode layout pattern (a), o1/o2 electrode position and partial external circuit (b), electrode (c), and observational instruments (d) used by the pl station. polarization potential difference of a pair of electrodes frequency response range electrode internal resistance short-term error over 10 seconds internal range drift (24 h) temperature range < ±1 mv 0-10 khz < 500 ω < ±0.01 mv < ±0.5 mv –10°c –40°c table 1. the main parameters of the solid impolarization electrode (lgb-3), (5% nacl solution at room temperature). measurement accuracy measurement resolution band range measurement range dynamic range sampling rate < ±(0.1% reading+ 0.02% full scale) < 10 μv dc ∼0.005 hz ±1000.000 mv ≥ 100 db 1 min table 2. the main parameters of the digital gef instrument (zd9a-ii) zhanghui an et al. 4 the observation was interrupted taking into account device maintenance, instrument faults, power failures, etc. under normal conditions, the observation station operates non-stop year round. when the observation curves exhibited step or irregular variations, the relevant personnel conducted an inspection and recorded the results in accordance with regulations. the monthly correlation coefficients of the long and short polar distances of pl station are all greater than 0.85. the observation area of the station has a good environment, which provides high quality observation data for our investigation. 2.2 gef data processing (e series) the data collection started in january 2008 and ended in may 2018 (3804 days). in cases where the original observation data were deleted due to lightning, human activities, etc., based on the pre-processed observation data, the daily mean energy een of the gef was calculated. all of the daily mean energies constitute the gef time series (e series) used in the analysis with the length of 3804 days. the corresponding formulas are as follows: (1) where ei is the measured gef value at the ith minute, in mv/km. n is the number of data per day (local time), at the normal condition n equal to 1440. after the preprocessing and calculations were conducted, the daily energy curves of the three channels at pl station were plotted (figure 2). the abscissa represent time, the ordinate represent energy, and different colored curves represent different channels, i.e., pink represents ns, blue represents ew, and green represents ne. based on the remote reference station theory of magnetotelluric sounding, the correlation between two stations in the same direction was analyzed using the shandan (shd) gef station (the reference station) in gansu province, which is approximately 610 km from the pl gef station (figure 3). the results show that the correlation coefficients of the ns and ew directions of two stations are 0.851 and 0.777, respectively. the strong correlation show the observational data of pl gef station have high accuracy in normal condition, the main proportion of data may be dominated by the same external source, while the local electric field induced by the streaming-electrokinetic phenomena [corwin and morrison, 1977; fitterman, 1978], piezostimulated phenomena [varotsos, 2005; varotsos and alexopoulos, 1984a,1986], or rock fracturing phenomena [ogawa et al., 1985], etc., has a small proportion. 𝐸�� = log10� �𝐸��2₁��� 𝑖 = 1, 2, 3, ..., 𝑁 𝑁 ≤ 1440 1 𝑁 � figure 2. the daily mean energy curves for the pl station. 2.3 seismic event processing (m series) whether a gef station can observe earthquake anomalies is dependent on the magnitude of the earthquake’s energy and the epicentral distance (d) [zhuang et al., 2005; zhao et al., 2009]. in the process of selecting earthquake events, two factors, seismic energy and d, are considered, and the calculation method of determining seismic energy presented by gutenberg and richter [1956], hattori et al. [2006], and han et al. [2014] is adopted. equation (2) is used to calculate the energy of a single earthquake event, and equation (3) is used to calculate the sum of the seismic energy of each day. (2) (3) where men,i is the energy released by the i-th earthquake in j/km2; mi is the magnitude of the ith earthquake; di is the epicentral distance of the ith earthquake in km, n is the total number of earthquake events, and men and nday represent the total energy released and the number of earthquakes in one day, respectively. in order to improve the reliability of the results, the energy limit of a single earthquake event should be 106 j/km2. that is to say, the minimum magnitude considered for an epicentral distance of 100 km is 3.5. we will fill it with ‘0’ when do not have earthquake event in someday, after the logarithm of the daily energy (men) calculation results in all time periods is taken, the earthquake event sequence (m series) used in the study is constructed. because we want to study the characteristics of gef before and after earthquakes for two months (total 121days), so, the maximum length of calculated series is 3683 days. in this paper, the distributions of seismic events in two regions (a and b) take into account the fact that the seismic energy should exceed the energy limit of 106 j/km2, and the number of seismic events in the two areas should be similar in order to facilitate the comparison about the results. in accordance with these requirements, the final number of qualified earthquakes in area a is 36 and the number in area b is 34. the selected information for seismic events is shown in tables 3 and 4. the original earthquake catalogue was obtained from the china earthquake datacenter (http://data.earthquake.cn). the locations of gef stations, earthquakes, and faults are shown in figure 4. 𝑀��‚� = 𝑖 = 1, 2, 3, ..., 𝑁.104.8+1.5 𝑀𝑖 𝑖 = 1, 2, 3, ..., 𝑁��� . 1𝑑𝑎𝑦 𝑀��‚� = �𝑀��‚� 5 characteristics of gef earthquake precursors figure 3. the correlation between pl and shd stations for the ns and ew channels. the red line is the fitting line; the blue dots represent the correlation coefficient. zhanghui an et al. figure 4. map showing the gef station, epicenters, and faults in the target area. the red dots represent seismic activity in area a (d < 200 km); the green dots represent seismic events in area b (200 km ≤ d < 365 km); the triangle represents the gef station; the black circles represent cities; and the blue curves represent faults. characteristics of gef earthquake precursors no. dd-mm-yy lat. (°n) long. (°e) depth (km) dist. (km) mag. (m) men’ (lg) 1 14-feb-08 35.33 106.85 27 63.54 3.7 6.74 2 11-mar-08 37.18 106.37 25 150.00 3.8 6.15 3 06-apr-08 36.22 106.40 30 57.32 4.3 7.73 4 06-apr-08 36.25 106.33 15 64.33 3.5 5.50 5 24-may-08 36.33 106.40 10 65.60 3.3 6.12 6 12-jun-08 35.75 106.63 12 29.51 3 6.36 7 02-oct-08 35.99 106.18 6 65.58 4.2 7.47 8 03-oct-08 35.99 106.12 24 70.93 3.6 6.50 9 24-nov-08 36.25 106.19 14 74.74 4.5 7.80 10 09-feb-09 35.29 106.62 8 72.40 3.7 6.63 11 01-apr-09 36.56 106.21 7 96.00 3.6 6.23 12 05-apr-09 35.21 106.74 8 78.08 3.4 6.11 13 25-may-09 37.20 106.52 10 148.48 3.7 6.01 14 03-aug-09 35.25 106.61 7 76.89 3.4 6.13 15 19-nov-09 35.32 106.62 7 69.28 3.3 6.07 16 20-nov-09 37.15 106.06 8 157.96 4.2 6.70 17 14-jan-10 35.28 106.59 7 74.42 3.4 6.16 18 06-feb-10 35.25 106.68 6 74.97 3.4 6.15 19 26-apr-14 37.13 106.02 6 157.77 4 6.40 20 04-feb-15 36.70 106.09 8 114.81 4.2 6.98 21 23-mar-15 35.95 106.26 9 57.90 3.2 6.07 22 09-apr-15 34.65 105.57 17 184.10 4.3 6.72 23 28-may-15 35.81 106.54 15 33.95 3.1 6.39 24 01-jun-15 36.02 106.08 5 75.00 3.6 6.45 25 19-jul-15 35.62 104.80 12 192.02 3.9 6.08 26 12-jan-16 36.37 106.28 6 76.36 4.1 7.18 27 11-apr-16 37.26 105.76 6 182.29 4.3 6.73 28 23-apr-16 35.83 106.19 6 64.45 4.1 7.33 29 27-apr-16 35.81 106.52 6 35.68 3.4 6.79 30 24-apr-17 36.37 106.05 6 92.51 4 6.87 31 14-may-17 36.37 106.03 5 94.00 3.6 6.25 32 01-sep-17 36.28 106.02 8 89.65 4.9 8.24 33 25-sep-17 36.63 106.22 8 101.52 3.6 6.19 34 06-nov-17 35.97 106.13 6 69.76 3.4 6.21 35 12-nov-17 36.22 106.30 10 64.61 3.3 6.13 36 02-feb-18 36.72 106.08 7 117.10 3.9 6.51 table 3. list of major earthquakes in area a. zhanghui an et al. 8 no. dd-mm-yy lat. (°n) long. (°e) depth (km) dist. (km) mag. (m) men’ (lg) 1 04-mar-08 35.70 103.55 23 302.93 4.6 6.74 2 12-may-08 32.93 105.48 0 355.00 4.5 6.45 3 27-may-08 32.87 105.50 0 360.57 7.7 8.29 4 10-jun-08 32.87 105.80 8 351.71 4.8 6.85 5 23-jul-08 32.83 105.50 4 364.74 5.9 8.53 6 24-jul-08 32.83 105.50 0 364.74 5.3 7.61 7 11-sep-08 32.95 105.62 6 348.39 5.8 8.45 8 19-sep-09 32.90 105.56 8 355.51 5.6 8.04 9 27-oct-09 37.13 103.70 4 316.97 4.3 6.25 10 04-nov-09 34.48 109.14 5 257.60 4.7 6.99 11 21-nov-09 38.21 106.55 6 258.73 4.6 6.85 12 24-jan-10 35.57 110.76 8 350.31 5.2 7.52 13 22-jun-10 38.27 106.20 6 270.75 4.9 7.21 14 09-oct-10 32.95 106.19 8 334.42 4.2 6.05 15 23-feb-11 34.25 103.84 8 333.45 4.7 6.76 16 01-nov-11 34.54 104.22 7 286.58 4.9 7.29 17 20-nov-12 38.43 106.34 21 285.66 5 7.43 18 24-jan-13 38.44 104.50 6 353.51 4.2 6.00 19 21-jul-13 34.54 104.21 15 287.35 6.9 10.21 20 22-jul-13 34.55 104.23 6 285.21 4.3 6.34 21 22-jul-13 34.56 104.21 14 286.16 5.9 8.75 22 22-jul-13 34.55 104.20 6 287.53 4.2 6.18 23 28-jul-13 34.55 104.20 10 287.53 4.6 6.76 24 20-oct-13 34.49 104.25 7 287.32 4.5 6.63 25 14-nov-14 37.12 103.74 9 313.29 5.3 7.75 26 02-dec-14 34.01 105.43 17 249.22 4.2 6.31 27 21-dec-14 34.40 105.36 10 217.76 4 6.12 28 15-apr-15 35.36 104.03 9 266.23 5 7.49 29 15-jul-15 37.12 103.71 14 315.71 4.5 6.54 30 18-dec-15 34.10 105.12 20 257.57 4.2 6.28 31 17-jan-16 34.27 104.88 13 258.12 4.4 6.58 32 07-may-16 34.77 104.86 14 223.67 4.4 6.70 33 29-jul-16 37.04 104.32 13 263.22 4.1 6.11 34 31-oct-17 34.67 103.37 10 348.35 4.9 7.07 table 4. list of major earthquakes in area b. 3. data analysis method the occurrence of an earthquake event is a very complicated geodynamic process, and the anomaly phenomenon before the earthquake varies. however, earthquakes are caused by tectonic movement with similar geodynamic process. for all earthquakes, or all earthquakes of a specific type, the corresponding anomalies related to earthquakes may share similar temporal distribution characteristics[fan and che, 2002; zhang et al., 2005]. correlation analysis is a basic statistical tool. gef anomalies may occur before or after an earthquake, i.e., there may be synchronous or asynchronous correlation between earthquake events and gef observations. in this paper, the scm is used to investigate the statistical characteristics of seismic anomalies in the gef. 3.1 method introduction in this paper, the earthquake energy sequence (m series) relative to the gef energy series (e series) sliding by day and calculate the correlation coefficient (r series) between them. in the process of calculation, ei derived from the e series and it’s length is same with m series, we make ei series fasten and then make m series slide to the left or right, and calculate the correlation coefficient (ri) each step. when the earthquake and the gef anomaly occur simultaneously, the r of the two series (m and ei) can be calculated without relative sliding. when the gef anomaly occurs before the earthquake, it is necessary to slide the m series to the left, and the calculated r represents the correlation before the earthquake, otherwise, it is necessary to slide the m series to the right, and the r represents the correlation after the earthquake. the correlation coefficient of the two series is as follows: (4) where m and ei are the same length (ei is derived from the long sequence, and m is the short sequence), i is the serial number. ei,k and mk are the kth sampling values in the ei and m series, respectively, and m and e are the corresponding average values. in this paper, m is the length of the two equal time series in the calculation, m =3683. 𝑅��� is the ith correlation coefficient. through the relative sliding of the two series, we can obtain the continuous change of the correlation coefficient. 3.2 validity verification a long sequence containing 301 samples is randomly generated, with a minimum value of 1 and a maximum value of 8, as shown in figure 5a. two subsequences, e1[50:90] and e2[180:220], were selected, and the m time series (figure 5b) was obtained through the average calculation, so the information in subseries e1 and e2 is included in m. according to the theory and calculation of the scm, the m time series slid to the left or right of e. when m slides to the location of e1 or e2, the calculated correlation is more significant than for other positions. figure 5c shows the results of the 90 data points of the left and right slipping data and demonstrates that the correlation coefficient increases relative to the position of e1 and e2. therefore, this method can accurately determine the position of correlated signals in different time series, which verifies the validity of the scm. �(𝑀� – 𝑀 )(𝐸�‚� – 𝐸� ) 𝑖∊ [–60, 60]. ��(𝑀� – 𝑀 )2��(𝐸�‚� – 𝐸� )2 𝑅���= � 𝑘= � 𝑘= � 𝑘= 9 characteristics of gef earthquake precursors 4. random test and discussion all stages of an earthquake, preseismic, coseismic and postseismic, may be accompanied by an abnormal gef. the r series reflects the dynamic correlation characteristics of the two time series (m and ei) and shows the time difference between them. by using the time difference of r series, the statistical characteristics of the gef observation and the earthquake occurrence time response can be obtained. this can aid in the application of the statistical characteristics to actual investigation and work. 4.1 random test the scm was used to analyze the daily mean energy of the pl gef from january 2008 to may 2018. the method of earthquake selection is described in section 2.3. figures 6 and 7 show the results for areas a and b, respectively. the abscissa indicates the relative number of sliding days. the “0” value represents the day of the earthquake. the correlation coefficient at that time reflects the synchronous correlation between the gef and the earthquake. the negative values of the abscissa indicate the correlation (left slip) before the earthquake, while the positive value indicates the correlation (right slip) after the earthquake. the ordinate is the sliding correlation coefficient of the two time series. the different color curves represent the sliding correlation results of the gef in different directions, and the corresponding color lines represent the average value of 1000 times random test. in this paper, the random test is performed, that is, the random disorderly operation of the catalogue and the scm calculation are carried out, and the mean value of the correlation coefficient obtained by repeating this process 1000 times is taken as the criterion for the deterministic anomaly. when three or more consecutive points exceed this criterion, they are regarded as significant abnormal points. as can be seen from figure 6, the seismic events located in area a have a good correlation with the gef. the anomalous correlation coefficient phenomena are concentrated along the ns and ne directions; the anomalies occurred approximately 8-26 days before the earthquake and approximately 20 days after the earthquake. however, the anomalies are not continuous before or after the earthquake in the ew direction. compared with area a, the characteristics of the correlation anomalies of the three channels in area b are not significant. however, approximately 16-26 days before the earthquake and 40-55 days after the earthquake, a suspected abnormal phenomenon occurs in the ns and ew directions, but there is no abnormal phenomenon in the ne channel at this time. zhanghui an et al. 10 figure 5. results of recognition of the random sequence using the scm. 4.2 discussion from the correlate coefficient result of pl and shd station, we can concluded that the external source is the main proportion of data, so when analyzing the gef data special attention should be paid to the magnetic influence. usually, the magnetic influence will be record in all channels of gef station [zhang et al., 2006], if the results above are affected by the magnetic storm, then the abnormal characteristics of three channels should have some similarity. in fact, the anomalies only appeared in the ns and ne, and there is no similar phenomenon in the ew channel. 11 characteristics of gef earthquake precursors figure 6. the scm correlation coefficient curves for the pl station in area a. pink represents the ns channel; blue represents the ew channel; and green represents the ne channel. figure 7. the results of the scm method for the pl station in area b. pink represents the ns channel; blue represents the ew channel; and green represents the ne channel. therefore, it is deduced that our results obtained in this paper do not influence from the same external source (such as magnetic storm). compared with area a, there is no similar concentrated anomaly in area b, which may be due to the distance from the observation station to the epicenter of the seismic event, i.e., the seismic event is far away from the epicenter of the observation station, or due to the earthquake events located in different faults. for 16 out of 34 earthquakes, d is greater than 300 km, and the tectonic stress induced by these earthquakes is not enough to cause anomalies in the gef at a long distance. alternatively, because the property of the seismogenic fault in area b is different [zhang et al., 2010; wu et al., 2016], which leads to a lack of correlation between the gef and the earthquake. although the results for area a show that the gef has a concentrated anomaly with the seismic events, but the anomaly appears significant directional differences. the correlation features of the ns and ne channels are very similar, while the ew channel finding has almost no correlation features. combined with the analysis of the geological structure of area a, it is concluded that the geological structure of this area is very well developed [xie et al., 2000], and the selected seismic events are concentrated on active faults such as the haiyuanliupanshan fault, the xiangshan-tianjingshan fault, and the yancongshan fault. the strikes of these fault zones are primarily ns or ne and the maximum principal compression stress direction is nee-sww [dai and xu, 2016]. the existence of these faults makes the surface inhomogeneity of resistivity, or forms a particular channel for conducting electricity, which leads to the direction selectivity characteristics of the anomalous gef before the earthquake [huang and lin, 2010]. in terms of the microcosmic mechanism, one explanation is that when the medium of the seismogenic body is close to rupturing, the formation and expansion of the microcracks in the medium and the migration of charged ions in the fluid cause the change in the local electric field along the direction of the rupture of the medium [varotsos and alexopoulos, 1984a; corwin and morrison, 1977]. another explanation is that the changes in the tectonic stress affect the formation and migration of the lattice defects in the rocks [varotsos, 1977] and the fluidity of these free and bound lattice defects [varotsos and alexopoulos, 1978]. when the tectonic stress reaches the critical value, the relaxation time of these electric dipoles becomes very small which leads to the directional arrangement of the electric dipoles and the emission of transient electrical signals [varotsos, 2005; ma et al., 2009]. recent studies by means of the analysis in a new time domain termed natural time [sarlis et al., 2013] reveal that a series of transient electric signals start when the fluctuations of the order parameter of seismicity become minimum [varotsos et al., 2013 ]. laboratory rock fracturing experiments show that electromagnetic radiation is produced during the fracture process, and the frequency ranges from very high frequencies to ultra-low frequencies. the long period radiation signals appear before short period signals [qian et al., 1998], and these long periodic signals may lead to pre-, co-, and after-seismic correlation coefficient anomalies. in actual observations, many strong earthquakes have recorded electromagnetic radiation signals [karakelian et al., 2002] during the microcracks and expansion of rocks in the late stage of earthquakes. moreover, the problem puzzling the researchers, why is 8-26 days before earthquakes and 20 days after earthquakes of gef anomalous variations occurred. until now, these questions have not yet been answered clearly or fully. all of these require us to carry out more in-depth research and analysis in the future. finally, in order to exclude the magnetic influence and verify the uniqueness of the results for area a, the method of increasing or reducing the single seismic energy standard (105.5 j/km2) was used to compare the results of the calculation. when the energy standard was raised to 106.5 j/km2, the number of qualified seismic events was only 15, which was not considered. when the energy standard was reduced to 105.5 j/km2, the number of seismic events increased to 59. the sliding correlation results are shown in figure 8. in the two cases, the results of each test are very similar, and the higher the energy standard of a single earthquake, the more significant the correlation of the calculated results, which is consistent with the fact that earthquakes with larger magnitudes are more likely to cause anomalies. the comparative analysis enhances the reliability and uniqueness of the correlation features of area a. zhanghui an et al. 12 5. conclusions by using the improved scm and 125 months of data from the pl gef station of china, the statistical characteristics of the gef and earthquakes in two areas, a and b, were investigated and analyzed. in the earthquake processing part, we take into account the influence of d and earthquake’s energy (≥106 j/km2) 36 earthquakes in area a and 34 in area b are considered. for obtaining reliability anomaly, the average results of 1000 times random test were chosen as the anomaly criterion, the following results were obtained: for the area a, the abnormal phenomena are appeared along the ns and ne directions, that’s centralized appearance approximately 8-26 days before the earthquakes and about 20 days after the earthquakes. but, that’s interesting, the similar phenomena are not appeared in the ew direction at the same station. compared with area a, the anomaly features are not concentrate and synchronous at three channels in area b, that’s appeared seemingly 16-26 days before the earthquakes and 40-55 days after the earthquakes, these abnormal phenomenon occurs in the ns and ew directions, but there is no abnormal phenomena in the ne channel this time. the fault features and the earthquakes location in the two areas may be the most reasons, and the reasons worth to investigate deeply. in addition, it is necessary to note that the limitations of the investigation results obtained in this paper. the statistical characteristics of stations located in different tectonic regions will be different, and the statistical characteristics of each gef station and the earthquake events will be determined gradually, which can help to improve seismic information capture. acknowledgements. we would like to thank the china earthquake networks center for providing observation data for the gef and earthquake catalogue. we also thank prof. chen xiaobin and dr. jiang feng (institute of geology of cea) for providing the original program, the reviewers for their helpful suggestions, and the editors for their hard work. this work has been supported by the science for earthquake resilience, cea (grant no. xh16037) and the basic scientific research special fund of the institute of earthquake prediction, cea (grant no. 2012ieslz04). we would like to thank letpub (www.letpub.com) for providing linguistic assistance during the preparation of this manuscript. 13 characteristics of gef earthquake precursors figure 8. a comparative chart of area a. pink corresponds to 106 j/km2, and blue corresponds to 105.5 j/km2. references an z.h., x.b. du, y.y. fan, j. liu, d.c. tan, j.y. chen, t. xie (2011). a study of the electric field before the wenchuan 8.0 earthquake of 2008 using both space-based and ground-based observational data with spectral element method simulation (in chinese), chinese j. geophys., 54, 6, 818–827, https://doi.org/10.1002/cjg2.1665. an z.h., x.b. du, d.c. tan, y.y. fan, j. liu, t.f. cui (2013). study on the geo-electric field variation of sichuan lushan ms7.0 and wenchcuan ms8.0 earthquake (in chinese), chinese j. geophys., 56, 6, 721–730, https://doi.org/10.1002/cjg2.20065. corwin r. f., h. f. morrison (1977). self-potential 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(in chinese with english abstract). zhao g., y. zhan, l.wang, j. tang, q. xiao, x. chen (2009). electromagnetic anomaly before earthquakes measured by electromagnetic experiments, earthquake sci., 22, 395–402, https://doi.org/10.1007/s11589-009-0395-5. zhuang j., d. vere-jones, h. guan, y. ogata, l. ma (2005). preliminary analysis of observations on the ultra-low frequency electric field in a region around beijing, pure appl. geophys., 162, 1367–1396, https://doi.org/10.1007/s00024-004-2674-3. *co r r e s po n d i n g a u t h o r: zhanghui a n, gansu lanzhou geophysics national observation and research station, lanzhou 730000, china, e-mail: anzhanghui5@hotmail.com © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved zhanghui an et al. 16 https://doi.org/10.1007/s00024-004-2674-3 on the distribution of energy and number of earthquakes in accordance with the focal depth p . t i e d e r v a r i ( * ) r i c e v u t o il 30 marzo 1964 summary. — a n investigation is presented here concerning the distribution of earthquake energy and number of shocks in accordance with the focal depth. t h e global distribution of these quantities according to the depth of hypocentres can be approached by an exponential curve between 0 and about 500 km under the earth's surface. riassunto. — viene presentato d a l l ' a . , in questa nota, uno studio sulla distribuzione dell'energia dei terremoti e il numero di scosse nei conf r o n t i della p r o f o n d i t a focale. l a distribuzione nel mondo di queste quant i t y nei confronti della profondita ipocentrale, puo essere rappresentata da una curva esponenziale compresa f r a lo 0 e i 500 km al disotto della superflcie della terra. eitsema (') emphasized the importance of studies concerning the connection between strain(or energy-)release and focal depth of earthquakes. in his quoted paper he mentioned that the strain-release/' depth diagram of the sunda arc and the same diagram of the earth as a whole show surprisingly similar characteristics in the period of 1904-1946. this extraordinarily interesting recognision induced the author of the present paper to extend the investigations for a larger scale of time. gutenberg and bichter (s.3) published the magnitude-data of the greatest shocks between 1896 and 1904; on the other hand the data of all shocks of magnitude 6,9 and over it are at our disposal from 1904 till about the middle of 1962, due partly to the common work of gutenberg and bichter (4) and partly to the bulletin of the seismological society of america (5). altogether we have the data of 1389 earthquakes occured between the beginning of 1896 and about the middle of 1962. w e calculated the energy-release instead of the strain-release. ( * ) f . r . a . s . vice-president of international lunar society. 3 8 2 p . h l s d e r v a r i table i i i . depth, km number of shocks depth, km number of shocks 0 49 5074 7599 100-124 125-149 790 181 73 94 30 400—124 425-149 450-174 475-199 500-524 6 1 4 2 4 150-174 175-199 200-224 225-249 250-274 43 18 37 11 13 525-549 550-574 575-599 600-624 625-649 2 3 6 26 6 275-299 300-324 325-349 350-374 375-399 2 6 6 9 4 650-674 675-699 700-724 725-749 750-774 11 0 1 0 0 table i v depth, km total energy, 1020 ergs ' depth, km total energy, 1020 ergs" 0 49 5074 7599 100-124 125-149 3915110,9 485432.5 178412.6 254351,6 10265,5 400-424 425-149 450-474 475-499 500-524 16515,4 158,5 2488,0 268,1 1046,2 150-174 175-199 200-224 225-249 250-274 161739,9 38446.5 50046.6 23893,3 3221,0 525-549 550-574 575-599 600-024 625-649 588,2 5174,7 1807,0 17480,7 4195,9 275-299 300-324 325-349 350-374 375-399 219,2 1465.4 34469,5 6992.5 905,2 650-674 675-699 700-724 725-749 750-774 7280,5 0 109,6 0 0 t o t a l energy: 5,222085.1026 ergs. d i s t r i b u t i o n of e n e r g y a n d n u m b e r of e a r t h q u a k e s 3 8 3 10 e r g s b.) = n r i g . i the distribution of eartliquake-energy in accordance with the focal depth, between the period 1896-19(33. all over the world, for shocks between magnitude range of 6,9-8,9 (according to the present author). a) between the depth of 0 and 175 km; b) between the depth of 125 and 750 km. 3 8 4 p . i i e d e r v a r i the table i shows the number of the above mentioned earthquakes according to their magnitude and focal depth respectively (see on tho next pages together with its continuation, table i i ) . the total number of shocks in the different levels may be seen in table i i i . according to the table i i i , we may state the fact that the number of earthquakes, regardless of their magnitude, reached a maximum between the depth of 0 and 49 km and the second greatest maximum is between 50 and 74 km under the surface of the earth. further maxima are between 100 and 124 km, 150 and 174 km, 200 and 224 km, 600 and 624 km and between 650 and 674 km respectively. fig. 2 strain release depth diagram of the earthquakes of the sunda arc for the period 1904-1953, after ritsena. for the relation between earthquake-energy and magnitude we used the following expression: where e is the energy in ergs and m is the magnitude. using this formule, we got for the energy-distribution according to depth, the results of table i v . w e state the fact that the total energy of shocks as a function of depth reached a maximal height between 0 and 49 km under the surface o 1 0 o zbo 5 o o 4 0 0 6 do tfoo too log e = 11 + 1,6 m d i s t r i b u t i o n of e n e r g y a n d n u m b e r of e a r t h q u a k e s 3 8 5 and the second greatest maximum could be found immediately under this level, that is between the depth of 50 and 74 km. there are several other maxima, too, namely between the depth of 150-174 km and of 325-349 km respectively. the global distribution of total energy according to the focal depth can be approached by an exponential curve between 0 and about 500 km under the earth's surface. between 450 and 550 km we can experience a wide minimum-zone in the total energy of shocks. throughout the deeper layers in the mantle of the earth the total energy of shocks begin slowly to grow again. a newer, smaller maximum is to be seen about the depth of 600-675 km. this drawing reminds us to the distribution of strain release according to the depth on the territory of sunda arc, as ritsema showed (figure 1 and 2). r e f e r e n c e s ( ' ) ritsema a . r., the seismicity of the sunda arc in space and time. « indonesian journal for natural science », 1-3, january-june (1954). (2) gutenberg b., great earthquakes 1s90-1903. transactions of american geophysical union, 37, 5, (1956). (3) richter c. f., elementary seismology. san francisco, 1958. ( ' ) gutenberg b., riciiter c. f., seismicity of the earth. second edition. princeton, 1954. (5) bulletin of the seismological society of america. 43-52, ( 1 9 5 3 1 9 6 2 ) . ( 6 ) eaton j. p . , takasaki k . j., seismological interpretation... ((bulletin of the seism. soc. a m . », 49, (1959). (7) gutenberg b., major and great earthquakes of 1957 and 1958. « bulletin of the seism. soc. a m . », 49. 50, [(1959, 1960). (8) rotiie j. p., les seismes du chili. « revue pour l'etude des calamites », 37, 1961. (') preliminary determinations of epicentres, 1962 and 1963. u.s. dept. of commerce coast and geodetic survey. layout 6 the july 17, 2011, ml 4.7, po plain (northern italy) earthquake: strong-motion observations from the rais network marco massa*, paolo augliera, gianlorenzo franceschina, sara lovati, maria zupo istituto nazionale di geofisica e vulcanologia, sezione di milano-pavia, milan, italy annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-5389 abstract on july 17, 2011, at 18:30:23 utc, a ml 4.7 earthquake occurred on the east side of the po plain (northern italy), between the towns of ferrara and rovigo. the epicentral coordinates provided by the national earthquake center of the istituto nazionale di geofisica e vulcanologia (national institute of geophysics and volcanology, ingv) were 45.01˚n and 11.41˚e (http://iside.rm.ingv.it/iside). the depth of the hypocenter was constrained at 8.1 km, corresponding to a buried active source that existed in the area. the source of the event was characterized by a predominant left-transverse focal mechanism, even if there was also an important reverse component. although it did not produce relevant damage, the earthquake was clearly felt in an area of about 50 km radius around the epicenter. the maximum observed intensity was v on the mercalli-cancani-sieberg (mcs) scale, with a predominant distribution of damage towards the north-west. this study provides an overview of the strong-motion waveforms of the mainshock as recorded by the rais (rete accelerometrica italia settentrionale) strong-motion network, in particular focusing on the recordings provided by the stations located in the central part of the basin, which were installed in correspondence with hundreds of soft sediments. the preliminary results show the relevant influence of the basin on the seismic wavefield, highlighting in particular a possible site-amplification phenomena, and also affecting the ground motion at long periods (t >1 s). the systematic underestimations provided by the empirical ground-motion predictive models calibrated for italy in terms of acceleration response spectra up to 2.0 s support this hypothesis. the sharing of the 24 waveforms (in raw sac and ascii formats) recorded by rais is assured by the availability of the data at the ftp site: ftp://ftp.mi.ingv.it/download/rais-fr_rel01/. 1. introduction the july 17, 2011, ml 4.7 earthquake (http://www.ingv. it/primo-piano/2011/07180818/) occurred in a low seismicity area of northern italy. figure 1 shows the instrumental and historical seismicity (top and bottom panels, respectively), which indicate that in this region the earthquake epicenters have mainly been distributed in the foothill areas of the central-eastern po plain, with major seismicity associated with the apennine chain. over the last 30 years, the po plain and its surroundings have been characterized by a rate of seismicity of about 200 small events per year, which have mainly been localized at the edge of the alluvial basin (http://csi.rm.ingv.it/; http://iside.rm.ingv.it/iside). the period between 1982 and the end of 2011 was characterized by only two earthquakes with ml >5.0: the november 24, 2004, ml 5.2, salò event, located at the northern edge of the basin, and the december 23, 2008, ml 5.1, parma event, located at the southern edge of the basin. in particular, the area that included the epicenter of the july 17, 2011, earthquake (figure 1, top panel, black dashed inset) is located inside the basin, and it has been characterized by the occurrence of 91 earthquakes with ml higher than 3.0. the strongest events were the december 6, 1986, ml 4.1, polesine earthquake, and the may 8, 1987, mantova, ml 4.0, earthquake. in the black dashed inset in the bottom panel of figure 1, the red squares indicate the more relevant historical events that have occurred in this area: 1. february 22, 1346, maw 5.81 (io 7.5); 2. march 17, 1574, maw 5.12 (io 7.0); 3. march 20, 1234, maw 5.17 (io 7.0); and 4. november 17, 1570, maw 5.48 (io 7.5) (maw, moment magnitude derived from macroseismic data; io, mercalli-cancani-sieberg [mcs] epicentral intensity; gruppo di lavoro cpti [2004], http:// emidius.mi.ingv.it/cpti/). the july 17, 2011, ml 4.7, event was located 25 km north-west of ferrara, about 8 km from the december 6, 1986, ml 4.1, polesine earthquake. it did not produce particular damage, as the maximum observed intensity was v on the mcs scale, with a felt area of about 50 km around the epicenter and a damage distribution extending to the northwest (http://www.ingv.it/primo-piano/2011/07180818/). in this area, the reference seismic hazard map for italy calculated for hard rock sites and considering a return period of 475 yr [gruppo di lavoro 2004, ordinanza pcm 3519/2006] shows predictable horizontal acceleration peaks ranging from 0.050 g to 0.125 g. article history received september 5, 2011; accepted march 20, 2012. subject classification: july 17, 2011 earthquake, rais strong-motion network, po plain, site effects, ground-motion prediction equations, italian seismic code. 309 data and experiment descriptions from a geological point of view, the po plain represents a syntectonic sedimentary basin that forms the in-fill of the pliocene-pleistocene apenninic foredeep, and is characterized by sediment thickness ranging from 1,000 m to 1,500 m [picotti et al. 1997, scardia et al. 2006]. according to the italian database of individual seismogenic sources [basili et al. 2008, diss working group 2010], the central-eastern part of the basin is bordered by several composite seismogenic sources, the structures of which are generally inferred based on regional surface and subsurface geological data. the july 17, 2011, event was characterized by a left-transverse focal mechanism (with a nonnegligible inverse component; http:// autorcmt.bo.ingv.it/qrcmt-on-line/e1107171830b.html), and by a hypocentral depth that is, unfortunately, illconstrained by the available seismological data. however, both the focal parameters and the epicentral coordinates can be considered reliable enough to be able to associate this event to the "itcs050 – poggio rusco-migliarino" composite source of diss 3.1.1, an extended seismogenic area that is 70 km long and is characterized by a maximum depth of 8 km massa et al. 310 figure 1. seismicity of the po plain and surrounding areas considered. top: instrumental seismicity for the last 30 years (http://csi.rm.ingv.it/; http://iside.rm.ingv.it/iside). white triangles, rais stations (http://rais.mi.ingv.it). red and blue squares, ferrara and rovigo, respectively. inset: location of the study area in italy. bottom: historical seismicity. green circle, epicenter of the july 17, 2011, ml 4.7, po plain earthquake. see keys for details. 311 the july 17, 2011, ml 4.7, po plain earthquake figure 2. strong-motion recordings at the rais stations analyzed. left: north-south components of the july 17, 2011, event. recordings are shown in relation to their arrival times. gray dashed insets, portions of the signals (15 s of s and coda phases) considered during the spectral analysis. right: particular of coda windows indicated by the gray solid inset in the left panels. code city latitude (˚) longitude (˚) elevation (m) sensor recorder time eurocode8 leon capriano c. 45.4582 10.1234 92 episensor reftek 130 gps c ctl8 castelleone 45.2763 9.7622 66 episensor gaia2 gps c conc concesio 45.606 10.217 126 episensor gaia2 gps c mant mantova 45.1495 10.7897 36 episensor gaia2 gps c mera merate 45.6725 9.4182 350 episensor gaia2 gps b mila milano 45.4803 9.2321 125 episensor gaia2 gps c euct pavia 45.2026 9.1349 82 episensor gaia2 gps c sand sandrigo 45.6399 11.6099 51 episensor reftek 130 gps c table 1. main features of rais stations considered in the present study. and a maximum magnitude of 5.5. from the engineering point of view, it must be considered that the superficial quaternary alluvial deposits present everywhere in the po plain are expected to strongly modify the seismic wavefield, in terms of both signal duration and amplification. as a consequence, a detailed study of the waveforms recorded during small to moderate events that can occur in the area might be useful to validate the capability of predictive models that can not usually be tested in such a complicated geological setting. these are recorded with broad-band strong-motion sensors installed in the central part of the basin. in such an area, the estimation of the long-period local site effects is a fundamental issue. indeed, the disaggregation of the italian probabilistic hazard [barani et al. 2009] highlights that the expected ground shaking for the area is stronger for events with relevant lowfrequency content, with magnitudes in the range 6.0 to 6.5 and epicentral distances between 100 km and 150 km. it is also worth noting that the po plain represents a high exposure area, as it is characterized by the highest density of industrial facilities and municipalities in italy, some of which have invaluable artistic relevance. moreover, in this region there are decommissioned nuclear power plants, the italian high-speed railway, waste oil reservoirs, and important skyscrapers and bridges. in the present study, 24 three-component recordings collected by eight rais (rete accelerometrica italia settentrionale) strong-motion stations are analyzed (figure 1, white triangles; http://rais.mi.ingv.it), paying particular attention to stations installed in the central part of the basin. preliminary results are presented in terms of spectral ratios (both horizontal-to-vertical spectral ratios [hvsrs] and massa et al. 312 station component epicentral distance (km) pga (cm/s2) pgv (cm/s) sa 0.3 s (cm/s2) sa 1.0 s (cm/s2) sa 3.0 s (cm/s2) arias intensity (cm/s) housner intensity (cm) mant e 51.98 2.46981 0.20883 5.44374 3.38349 0.17664 0.015767 0.810978 n 51.98 2.87259 0.269297 4.52401 3.16835 0.271577 0.015912 0.905395 z 51.98 2.22253 0.130075 4.8086 1.37493 0.066882 0.010562 0.39714 sand e 71.73 3.96209 0.085448 3.63173 0.571226 0.03167 0.02038 0.202407 n 71.73 3.35872 0.089847 4.74157 0.421781 0.038124 0.018657 0.231281 z 71.73 1.11557 0.028732 1.01985 0.224593 0.019009 0.002352 0.090029 leon e 114.3 1.25589 0.124055 3,85825 1.70462 0.084326 0.004625 0.460844 n 114.3 1.16181 0.105747 3.14127 1.54128 0.090736 0.003828 0.427069 z 114.3 0.622325 0.044272 1.52349 0.811842 0.043697 0.001151 0.176712 conc e 116.1 2.29351 0.107125 6.30756 1.18898 0.044852 0.007953 0.321172 n 116.1 2.9453 0.146983 7.52268 0.634902 0.040865 0.010303 0.300469 z 116.1 1.87299 0.054024 2.01732 0.309395 0.016135 0.003414 0.123725 ctl8 e 135.1 3.31139 0.177278 8.64181 3.50759 0.158861 0.007339 0.69093 n 135.1 2.55576 0.183904 6.09347 2.80033 0.177472 0.005986 0.647067 z 135.1 0.511999 0.0312 1,34122 0.378218 0.020032 0.000699 0.101747 mera e 175.5 1.3146 0.064391 3.32264 0.599027 0.026341 0.002542 0.179364 n 175.5 1.29529 0.080054 2.77246 0.517417 0.03342 0.003345 0.221608 z 175.5 0.774228 0.027724 1.57142 0.338607 0.017968 0.00078 0.101602 mila e 181.9 1.0497 0.043298 2.7326 0.375714 0.014724 0.001272 0.130091 n 181.9 0.756287 0.038253 2.05313 0.417281 0.26605 0.000933 0.137395 z 181.9 0.392454 0.013322 1.0888 0.121308 0.005855 0.000414 0.045442 euct e 183.3 0.753942 0.059794 2.18877 0.648491 0.032238 0.00128 0.20693 n 183.3 1.09421 0.060059 2.39014 0.942975 0.059796 0.001482 0.260645 z 183.3 0.377386 0.017494 0.97581 0.360262 0.022404 0.000319 0.071847 table 2. ground-motion parameters calculated from the collected recordings. 313 standard spectral ratios [ssrs]) and comparisons with the empirical ground-motion prediction equations (gmpes) that are available at present for italy. these also consider the new italian seismic code for building, with comparisons made to investigate the predictive capabilities of existing ground-motion models. 2. the rais network and data processing the data presented and analyzed in this study were recorded by eight strong-motion stations (see figure 2 for the north-south components) that belong to rais [augliera et al. 2010, 2011]. the stations considered are equipped with kinemetrics episensor fba es-t force balance accelerometers coupled with 24-bit digital recorders (reftek 130 or gaia2; see table 1). at present, these stations send data to the ingv acquisition center in milan in real-time using tcp/ip protocol or wi-fi links. the management of the data, which is recorded in miniseed format at a sampling frequency of 100 hz, is achieved using the seiscomp package with the seedlink protocol. the geological conditions of the rais station sites were investigated on 1:25,000 geological maps provided by the lombardia region [carg project 2003] or 1:100,000 italian geological maps [sgi 1984]. considering the relationship between the superficial geology and the vs30 (the shear-wave velocity estimated in the first 30 m of depth), as reported in bordoni et al. [2003], the stations can be classify following eurocode8 [cen 2003]: as shown in table 1, two stations (conc and mera) are classified in the b soil category (360 m/s <vs30 <800 m/s), while the other six are classified in the c soil category (180 m/s <vs30 <360 m/s). the july 17, 2011, ml 4.7, po plain earthquake figure 3. velocimetric waveforms obtained after integral of the acceleration time series shown in figure 2 for the eight rais strong-motion stations analyzed. left: north-south components of the july 17, 2011, event. the gray dashed inset define portions of the signals (15 s of s and coda phases) considered during the spectral analysis. right: particular of coda windows indicated by the gray solid inset in the left panels. each strong-motion waveform is processed using a standard procedure, as described in massa et al. [2010], which includes: baseline correction, performed by least-squares regression; mean removal, considering the whole signal; application of a 5% cosine-taper function; and application of an acasual 4th order butterworth digital filter, selecting both low and high pass thresholds by visual inspection of the recorded data. while at high frequencies a good signalto-noise ratio is assured up to 35 hz, considering both the available instrumentation (accelerometric sensors) and the characteristic levels of the background noise of the sites (in particular in the central part of the po plain), at low frequencies the low-cut threshold for filtering was fixed at 0.4 hz. for all of the processed waveforms, peak ground acceleration (pga) and acceleration response spectra (sa, 5% damped) for periods of up to 2 s were calculated, together with the arias intensity [arias 1970] and the housner intensity [housner 1952] (see table 2). finally, after the integration of the acceleration time series, velocity waveforms were obtained (see figure 3 for north-south components) and the peak ground velocities (pgvs) were determined. 3. ground-motion parameters the ground shacking produced by the july 17, 2011, earthquake in correspondence with the eight rais and 13 ran (italian accelerometric network; http://www.protezionecivile. gov/) strong-motion stations was compared with the empirical gmpes at present available for italy, on both the national [bindi et al. 2010] and regional [massa et al. 2008] scales. the comparisons were performed in terms of amplitude, and spectral and duration parameters. for the ran, due to the waveforms not being available, the comparisons were possible only in terms of the pga (available at: http://www.protezione civile.gov.it/jcms/it/view_rst.wp?contentid=rst26698). at the national scale, new gmpes were recently calibrated by bindi et al. [2010], considering all of the italian strong-motion data collected in the italian accelerometric archive (itaca) [pacor et al. 2011] recorded from 1972 to 2009 by ran, and from 2006 by rais. considering data with 4.0 ≥mw ≥6.9 and with distances (joyner-boore or epicentral) of up to 100 km, bindi et al. [2010] developed empirical relations for prediction of the maximum horizontal and vertical pga, pgv and sa (5% damping) from 0.04 s to 2 s. massa et al. 314 figure 4. comparisons between the recorded pga (top) and sa at 2.0 s (bottom) related to the july 17, 2011 event (considering the maximum horizontal component) and the empirical gmpes available for the area. left: comparisons considering the italian gmpes calibrated by bindi et al. [2010] (ita08). right: comparisons considering the regional relationships calibrated by massa et al. [2008] (ma08). solid black lines, dashed grey lines, models (median ±1v) calibrated for hard rock and soft soil, respectively. for both empirical models, the explanatory variable for distance was valid up to 100 km, and was extrapolated up to 200 km. labels for the rais stations are shown. 315 massa et al. [2008] calibrated the regional gmpes for northern italy using both weak-motion and strong-motion data. in this case, considering the local magnitude (3.5 ≥ml ≥6.3) and the epicentral distance (up to 100 km) as explanatory variables, massa et al. [2008] developed a set of empirical relationships in terms of amplitude (pga, pgv), and spectral (sa, up to 2s) and duration (arias and housner intensities) parameters. figure 4 shows the comparison between the experimental and predicted pga (top panels) and sa at 2.0 s (bottom panels), considering in all cases the maximum horizontal component; in the left panels the july 17, 2011, data are compared considering the bindi et al. [2010] empirical relationships, while in the right panels the comparison was performed considering the massa et al. [2008] regional relationships. in all cases, the predicted median values are reported together with the related ±1 standard deviations, both for hard rock (figure 4, black solid lines) and soft soil (figure 4, gray dashed lines). the median gmpe calibrated for the soft soil reflects the italian technical standards for construction [norme tecniche per le costruzioni, ntc 2008] and/or the eurocode8 [cen 2003] c soil category (360 m/s <vs30 <180 m/s) for bindi et al. [2010], while it groups b (800 m/s <vs30 <360 m/s) and c soil categories for massa et al. [2008] (for details about soil classifications adopted in each gmpe, see bindi et al. [2010], and massa et al. [2008]). in general, figure 4 shows that both at high (pga) and at low (sa, 2.0 s) frequencies the median gmpes underestimate the experimental values. for the rais stations, the bias increases moving from high to low frequency, in particular for the stations located in the central part of the po plain (mant, leon, mila, euct, ctl8); on the contrary, the values recorded by the stations located near to the edge of the basin are included in the standard deviation range. the same results are obtained for the regional gmpe calibrated by massa et al. [2008] for the duration parameters (the arias and housner intensities), which are, as already demonstrated by masi et al. [2006], the ground-motion parameters that capture the potential destructiveness of an earthquake well, as they are integral parameters of a time history. the comparisons between the observed and predicted ground-motion parameters in terms of the arias [arias 1970] and housner [housner 1952] intensities are shown in figure 5. in both cases, the underestimation of the predictive models is evident with respect to the observed data for stations mant, leon, euct and ctl8. in particular, the underestimation is more evident for the housner intensities (figure 5, right panel), in agreement with the comparison performed considering the sa at 2.0 s of the period (figure 4, bottom panels). figure 6 shows the sa (5% damping; gray lines) for the four rais stations located in the central part of the po plain, as obtained from the recordings of the july 17, 2011, event, together with the predicted sa from bindi et al. [2010]. especially in the low frequency band (t >1s), the sites where these stations are installed appear to amplify the ground motion with respect to the gmpe predictions. for ctl8, the model underestimates the experimental data also at high frequencies. the other four stations that are not reported in figure 6 agree better with the trend of the empirical model, with, in any case, the real sa included in the ±1v interval. 4. spectral ratio analyses the knowledge of the resonance frequency of a soil, coupled to information about the predominant period of a structure, can give us an idea of the potential damage that we can expect for a site in the case of an earthquake. to analyze earthquake data recorded at a single station, the most commonly used technique at present is the so-called single station hvsr for earthquakes [lermo and chavez the july 17, 2011, ml 4.7, po plain earthquake figure 5. comparisons between the arias (left, ai) and housner (right, hi) intensities calculated from the july 17, 2011, recordings (considering the maximum horizontal component) and the related relationships calibrated for northern italy by massa et al. [2008] (ma08). solid black lines, dashed gray lines, models (median ±1v) calibrated for hard rock and soft soil, respectively. for both empirical models, the explanatory variable for distance was valid up to 100 km, and was extrapolated up to 200 km (black and gray dots). labels for the rais stations are shown. garcia 1993]. in figure 7, the results obtained for all of the analyzed rais stations are presented in terms of the rotated hvsrs, considering 15 s of s-phase, starting from the s onset. in each panel of figure 7, a single amplification function represents the ratio between the rotated northsouth horizontal component and the vertical component, considering 36 directions between 0˚ and 175˚ (steps of 5˚): each color groups an interval of 20˚ as the results obtained for four consecutive directions. all of the computed fourier spectra were smoothed using the konno and ohmachi [1998] smoothing algorithm, fixing the b parameter at 20. in the top panels of figure 7, the results obtained for the conc (left) and mera (right) stations are presented, both of which are located at the edge of the po plain. in these cases, the spectra highlight the presence of amplified peaks for frequencies higher than 1 hz, without any evidence of a preferential direction of amplification (i.e. the hvsrs obtained for each of the 36 analyzed directions show similar amplification values). for the other stations, and also in agreement with the previous considerations, while mila and sand do not appear to suffer particular effects of amplification at low frequencies (between 1 hz and 2 hz for mila, and between 2 hz and 3 hz for sand), stations mant, euct, leon and ctl8 show relevant amplification effects for frequencies lower than 1 hz. the common peaks observed between 0.5 hz and 1.0 hz lead us to assume the presence of a regional site response that affects the central part of the basin. in this case, no particular polarization effects were observed. as previously highlighted, the signal-to-noise ratio at low frequencies for these stations does not allow considerations for frequencies lower than 0.4 hz. to support these results, the same analyses were performed considering a portion of the 15 s selected on coda (starting from a time equal to twice the s travel time from the origin time), the part of the seismogram that is less affected by radiation pattern effects. the results of the hvsr performed on these coda are presented in the top panels of figure 8 for one station located at the edge of the basin (conc, top panel) and for one station located in the central part of the basin (ctl8, bottom panel). as previously seen for the s phase, for both of the stations, the results reflect well the presence of amplification, at high frequencies for conc (between 1 hz and 2 hz) and at low frequencies for ctl8 (at around 0.5 hz). also for the other stations not reported in figure 8, the hvsr on the coda agree well with those obtained from the s phase. massa et al. 316 figure 6. comparisons between the sa (5% damping) obtained from the recordings of the july 17, 2011, event (gray lines) for the stations located in the central part of the po plain and those predicted by bindi et al. [2010] (ita08). solid black lines, dashed black lines, median and ±1v of the model, respectively. 317 the july 17, 2011, ml 4.7, po plain earthquake figure 7. directional (36 rotations, steps of 5˚, between 0˚ and 175˚) hvsrs for the stations considered, computed considering 15 s of s-phase. different colors indicate consecutive directional hvsrs, grouped according to 20˚ intervals. a preliminary estimate of the amplification factors was achieved using, when possible, the ssr technique [borcherdt 1970] at the regional scale. the basic concept is: if the hypocentral distance of the selected event is large compared to the considered station inter-distances, then the seismic wave field can be considered uniform with respect to the heterogeneity of the area. in this case, it is possible to assume that the differences in the observed waveforms are only correlated to the differences in the local structures. considering the location of the rais network with respect to the epicenter of the july 17, 2011, event, we performed directional ssr selecting on the basis of the source-to-site distances, for two groups of stations: the first composed of ctl8 (135 km), leon (114 km) and conc (116 km), and the second composed of euct (183 km), mila (181 km) and mera (175 km). as the present study is particularly focused on possible anomalous low frequency site responses, conc and mera were chosen as the reference sites, massa et al. 318 figure 8 (left). directional (36 rotations, steps of 5˚, between 0˚ and 175˚) hvsrs for conc, at the edge of the basin, and ctl8, in the central part of the basin, computed considering 15 s of coda for these two representative stations. different colours indicate consecutive directional hvsrs, grouped every 20˚ interval. figure 9. directional (36 rotations, steps of 5˚, between 0˚ and 175˚) ssrs computed for the stations located in the central part of the po plain, considering as reference the stations located at the edge. different colors indicate directional ssrs grouped by according to 20˚ intervals. 319 considering that: 1. they are classified in the eurocode8 b soil category; and 2. they show flat hvsrs (amplification factors <2) for frequencies in the range of 0.5 hz to 1.0 hz. it is worth noting that the ssrs in the range of 1.5 hz to 3.0 hz might be biased by the amplified peaks observed at conc and mera (see figure 7, top panels, hvsrs). for the analyses, 15 s of s waves were selected. from the s phase onset. the results here are reported in figure 9. considering the first group of stations (ctl8, leon and conc), for both of the sites located in the central part of the basin, there is a clear amplification peak between 0.5 hz and 1.0 hz. in this case, the amplification factor appears to increase if we move from the north side (amplification up to 6 for leon) to the central part of the basin (amplification up to 9 for ctl8). considering the second group of stations (euct, mila and mera), while euct reflects the behavior of leon and ctl8 (amplification between 0.5 hz and 1.0 hz, with an amplification factor of up to 5.5), station mila does the july 17, 2011, ml 4.7, po plain earthquake figure 10. comparisons between desing elastic sa as provided by the new italian seismic code for building [ntc 2008, ntc08, gray lines] and the sa (5% damping) obtained at each station considered, from the july 17, 2011, recordings (considering the maximum horizontal component, black lines). all of the spectra were normalized considering the related values at the zero period. not show particular amplification peaks. also in this case it is worth mentioning that the amplification increases moving towards the center of the basin. 5. discussion and conclusions in this study, the strong-motion recordings were analyzed from the july 17, 2011 (18:30:23 utc), ml 4.7, earthquake, as recorded by eight rais stations. the 24 waveforms are downloadable in both the raw sac and ascii formats, from ftp://ftp.mi.ingv.it/download/rais-fr_rel01/. the earthquake analyzed was located in the central part of the po plain (northern italy), between the towns of ferrara and rovigo, in an area characterized by a low degree of seismicity, both historical and instrumental (see figure 1). the presence of the rais stations, and in particular those installed in the area of the alluvial basin (table 1, mant, leon, ctl8, euct, mila, sand), gave us the opportunity to test the capability of empirical predictive models for areas where particular amplification, and in particular periods greater than 1 s, can be expected due to the geological setting (very deep sedimentary basin). these preliminary analyses show relevant differences in the site responses considering the stations located at the edge of the basin (table 1, mera, conc) with respect to those in correspondence to the basin. the comparisons with the available gmpes highlight that the empirical models underestimate the true conditions. moving from the stations located at the edge of the basin to those installed in the central areas, this underestimation increases: this evidence involves, in particular, the measured sa at high periods (up to 2.0 s), and the measured duration parameters (the arias and housner intensities), which are highly correlated to possible structural damage. these considerations can generally have relevance for the construction standards for buildings in these areas. at present, the design sa are defined by the new technical italian regulations for buildings [ntc 2008]. in particular, the elastic sa (5% damping) is defined through a spectral shape that is multiplied by the maximum horizontal acceleration, ag, for a generic horizontal hard-rock site. both ag and the spectral shape vary as a function of the probability of exceeding the reference period. to consider the seismic site amplification, the italian regulations, the ntc, include five soil categories defined on the basis of the vs30 [ntc 2008, chapter 3]. to compare the ntc elastic sa and the sa calculated from the july 17, 2011, event, the calculated spectra were normalized by the related pga values for each station. in this way, it is possible to perform a comparison between the spectral shapes, but not between the amplitudes. the comparisons in terms of the normalized sa are reported in figure 10. as reported in table 1, conc and mera are included in the b soil category, while the other six stations are in the c soil category. as can be noted, the flat part of the ntc elastic sa (figure 10, gray) agrees well with the actual sa (figure 10, black) for many of the stations, and in particular for mera and conc, which are located at the edge of the basin. the normalized sa calculated for the ctl8, mila and sand stations are well covered by the shape of ntc spectra. on the contrary, for euct, and in particular for the mant and leon stations, the flat part of the ntc spectra underestimates the normalized sa in the period range of 0.5 s to 1.0 s. references arias, a. (1970). a measure of earthquake intensity, in: r. hansen (ed.), seismic design of nuclear power plants, m.i.t. press, cambridge, 438-483. augliera, p., e. d'alema, s. marzorati and m. massa (2010). a strong motion network in northern italy: detection capabilities and first analyses, b. earthq. eng., 8, 1091-1104. augliera, p., m. massa, e. d'alema and s. marzorati (2011). rais: a real time strong-motion network in northern italy, annals of geophysics, 54 (1), 23-34; doi:10.4401/ag-4855. barani, s., d. spallarossa and p. bazzurro (2009). disaggregation of probabilistic ground-motion hazard in italy, b. seismol. soc. am., 99 (5), 2638-2661. basili, r., g. valensise, p. vannoli, p. burrato, u. fracassi, s. mariano, m.m. tiberti and e. boschi (2008). the database of individual seismogenic sources (diss), version 3: summarizing 20 years of research on italy's earthquake geology, tectonophysics, 453, 20-43, doi:10.1016/j.tecto. 2007.04.014 bindi, d., l. luzi, m. massa and f. pacor (2010). horizontal and vertical ground-motion prediction equations derived from the italian accelerometric archive (itaca), b. earthq. eng., 8, 1209-1230; doi:10.1007/s10518-009-9130-9. borcherdt, r.d. (1970). effects of local geology on ground motion near san francisco bay, b. seismol. soc. am., 60, 29-61. bordoni, p., v. de rubeis, f. doumaz, l. luzi, l. margheriti, f. marra, m. moro, d. sorrentino and p. tosi (2003). geological class map, in: terremoti probabili in italia tra l’anno 2000 e 2030: elementi per la definizione di priorità degli interventi di riduzione del rischio sismico, annex 1, task 3.2, 3-4 pp., gndt proj., rome. carg project (2003). progetto di cartografia geoambientale (regione lombardia) alla scala 1:25000, versione 1.0, base cartografica ctr 1:10000. cd-rom (in italian). cen (comité européen de normalisation) (2003). eurocode 8: design of structures for earthquake resistance part 5: foundations, retaining structures and geotechnical aspects, brussels, belgium. diss working group (2010). database of individual seismogenic sources (diss), version 3.1.1: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas; http://diss.rm.ingv.it/diss/. gruppo di lavoro (2004). redazione della mappa di pericolosità sismica prevista dall’ordinanza pcm 3274 del 20 massa et al. 320 321 marzo 2003, rapporto conclusivo per il dipartimento della protezione civile, ingv, milano-roma, 65 (in italian). gruppo di lavoro cpti (2004). catalogo parametrico dei terremoti italiani, versione 2004 (cpti04), ingv, bologna; http://emidius.mi.ingv.it/cpti04/ housner, g.w. (1952). spectrum intensities of strong-motion earthquakes, in: c.m. feigen (ed.), proc. symp. on earthquake and blast effects structures, university of california, los angeles, 21-36. konno, k., and t. ohmachi (1998). ground-motion characteristics estimated from spectral ratio between horizontal and vertical components of microtremors, b. seismol. soc. am., 88, 228-241. lermo, j., and f.j. chavez-garcia (1993). site effect evaluation using spectral ratio with only one station, b. seismol. soc. am., 83 (5), 1574-1594. masi, a., m. vona and m. mucciarelli (2006). selezione dell’input sismico per la determinazione della vulnerabilità su base meccanica di edifici in c.a., xxv gngts conference, book of abstracts, 263-267 (in italian). massa, m., p. morasca, l. moratto, s. marzorati, g. costa and d. spallarossa (2008). empirical ground-motion prediction equations for northern italy using weak and strong motion amplitudes, frequency content and duration parameters, b. seismol. soc. am., 98, 1319-1342. massa, m., f. pacor, l. luzi, d. bindi, g. milana, f. sabetta, a. gorini and s. marcucci (2010). the italian accelerometric archive (itaca): processing of strong motion data, bulletin of earthquake engineering, 8 (5), 11751187; doi:10.1007/s10518-009-9152-3. ntc (nuove norme tecniche per le costruzioni) (2008). parte 3: categorie di sottosuolo e condizioni topografiche, gazzetta ufficiale, n. 29 del 4 febbraio 2008 (in italian). pacor, f., r. paolucci., g. ameri, m. massa and r. puglia (2011). italian strong-motion records in itaca: overview and record processing, b. earthq. eng., 27-43; doi:10.1007/ s10518-011-9295-x. picotti, v., g. bertotti, r. capozzi and a.m. fesce (1997). evoluzione tettonica quaternaria della pianura padana centro-orientale e dei suoi margini, il quaternario, 10, 513-520 (in italian). scardia g., g. muttoni and d. sciunnach (2006). subsurface magnetostratigraphy of pleistocene sediments from the po plain (italy): constraints on rates of sedimentation and rock uplift, bull. geol. soc. am., 118, 1299-1312. sgi (società geologica italiana) (1984). carta geologica d'italia 1:100,000. base cartografica igm 1:25,000. *corresponding author: marco massa, istituto nazionale di geofisica e vulcanologia, sezione di milano-pavia, milan, italy; email: marco.massa@mi.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the july 17, 2011, ml 4.7, po plain earthquake layout 6 flash sourcing, or rapid detection and characterization of earthquake effects through website traffic analysis rémy bossu*, sébastien gilles, gilles mazet-roux, fréderic roussel, laurent frobert, linus kamb european mediterranean seismological centre (emsc), earthquake information website (www.emsc-csem.org) annals of geophysics, 54, 6, 2011; doi: 10.4401/ag-5265 abstract this study presents the latest developments of an approach called ‘flash sourcing’, which provides information on the effects of an earthquake within minutes of its occurrence. information is derived from an analysis of the website traffic surges of the european–mediterranean seismological centre website after felt earthquakes. these surges are caused by eyewitnesses to a felt earthquake, who are the first who are informed of, and hence the first concerned by, an earthquake occurrence. flash sourcing maps the felt area, and at least in some circumstances, the regions affected by severe damage or network disruption. we illustrate how the flash-sourced information improves and speeds up the delivery of public earthquake information, and beyond seismology, we consider what it can teach us about public responses when experiencing an earthquake. future developments should improve the description of the earthquake effects and potentially contribute to the improvement of the efficiency of earthquake responses by filling the information gap after the occurrence of an earthquake. introduction rapid characterization of earthquake effects is essential for timely and appropriate responses to aid victims. in the immediate aftermath of damaging earthquakes, any field observations of their effects can fill the information gap and contribute to more efficient rescue operations. a small magnitude shallow earthquake below an urban area can be widely felt and become a major event from a public and media perspective. a rapid assessment of their impacts helps seismologists dealing with public information to timely respond to questions about these concerns. this study presents the latest developments of a method called ‘flash sourcing’, which addresses these issues [bossu et al. 2007, 2008, 2011]. flash sourcing relies on eyewitnesses, the first people who are informed of, and hence the first who are concerned by, an earthquake occurrence. more precisely, the use by these eyewitnesses of the european–mediterranean seismological centre (emsc) earthquake information website (www.emsccsem.org) is analyzed in real time to map the area where an earthquake is felt, and to identify, at least under certain circumstances, the zones of widespread damage. this approach is based on the natural and immediate responses of eyewitnesses, who rush to the internet to investigate the cause of shaking that they have just felt, converging on the emsc website and thus increasing the website traffic [bossu et al. 2007]. the area where an earthquake has been felt is mapped simply by locating the internet protocol (ip) addresses of the visitors to the emsc website during these website traffic surges [bossu et al. 2008]. in addition, the presence of eyewitnesses browsing our website within minutes of an earthquake occurrence is evidence excluding the possibility of widespread damage in the localities they originate from; in the case of severe damage, the networks would probably be down [bossu et al. 2008]. the validity of the information derived from this website traffic analysis is confirmed by comparisons with macroseismic maps of the european macroseismic scale ems98 [grünthal 1998] obtained from online questionnaires [bossu et al. 2011]. the name of this approach, flash-sourcing, is a combination of ‘flash crowd’ and ‘crowd-sourcing’, which reflects the rapid collection of data from the public. for computer scientists, a flash crowd indicates a traffic surge on a website. crowd-sourcing refers to work being done by a ‘crowd’ of people, and it is also used to characterize internet and mobile applications that collect information from the public, such as online macroseismic questionnaires. like crowd-sourcing techniques, flash sourcing is a crowd-toagency system, although unlike crowd-sourcing techniques, flash sourcing is not based on declarative information (e.g. answers to a questionnaire), but on implicit data. in our case, this is the real-time analysis of the website traffic observed at the emsc website. in the first part of the report, we present the main improvements of the method, the improved detection of article history received june 28, 2011; accepted september 6, 2011. subject classification: monitoring, seismic risk, algorithms and implementation, methods, public issues. 716 citizen empowered seismology/special section edited by r. bossu and p.s. earle website traffic surges, and a way to instantly map areas affected by severe damage or network disruption. the second part describes how the information derived can improve and speed up the delivery of public earthquake information, and beyond seismology, what it can teach us about public behavior when an earthquake is experienced. finally, the discussion focuses on future developments and how flash sourcing can ultimately improve earthquake response. detection of website traffic surges resulting from felt earthquakes a website traffic surge following a felt earthquake is a result of the rapid influx of site visits by eyewitnesses who were not present on the website before the event occurrence. rather than relying on the classical metrics of website traffic, such as the rate of loaded pages or number of visitors, which can be estimated through the number of ip addresses, we compute the number of new visitors who have arrived in the last minute. this is defined as the number of different ip addresses that have loaded at least one page within the last minute, and that have had no activity (i.e. no loaded pages) on the website in the previous 30 min (figure 1). these arbitrarily defined time values are aimed at detecting fast increases in website traffic. the metric has little sensitivity for the level of baseline traffic or for more gradual increases observed every morning or as a result of the propagation of the news of an earthquake. as the first step, the raw website traffic is systematically filtered to exclude identified indexing robots, ip addresses from seismological institutes [bossu et al. 2008], and visitors coming from http referrers; i.e. visitors linking from an external website. the exclusion of this last component of the website traffic avoids false triggers caused by links to the emsc website that can be published on popular forums or websites [bossu et al. 2011]. there is currently no discrimination between mobile and land-line internet access (see future developments section for more details), and all visitors to the website are treated equally. real time monitoring of the number of new visitors in the last minute allows the automatic detection of felt earthquakes generating a significant traffic increase. this monitoring is performed every second in real time. the latency due to the different steps of the computation is about one second, including for removal of referrers and robots, and determination of the number of new visitors in the last minute. emsc-specific trigger criteria have been defined following an analysis of our website traffic characteristics, to reduce false triggers rather than to optimize the number or rapidity of the automatic detections. the system triggers when the number of new visitors in the last minute exceeds the observed average over the previous 30 bossu et al. 717 figure 1.the different traffic metrics for the m 4.5 vrancea (romania) earthquake of june 6, 2010. the number of loaded pages (black line) characterizes the activity that results from all of the website visitors. the number of unique website visitors (red line) is the number of different ip addresses that loaded at least one page in the previous minute. the number of new website visitors in the previous minute (green line) is a new metric defined to reliably detect the convergence of eyewitnesses following a felt earthquake. http requests from robots, referrers and pre-identified seismological institutes are systematically removed as the first step of these website traffic measurements. the vertical blue line represents the origin time of the earthquake. the first location and magnitude for this earthquake were reported on the emsc web site 4 min after its occurrence, while the website traffic surge was detected in 112 s. 718 min by more than 40. these felt events are automatically detected with delays ranging from 15 s to 284 s of their occurrence, with a median value of 88 s (figure 2). only one of the 63 detected events in the period from january 2009 to may 2011 was outside the european–mediterranean region; namely, a m 6.7 earthquake in chile. this illustrates that such a system can only detect earthquakes felt by the normal users of the website [bossu et al. 2008], and that the emsc users originate mainly from the european– mediterranean region. the magnitudes of the automatically detected earthquakes range from m 2.1 to m 6.7. some smaller magnitude earthquakes can potentially generate a website response. however, they do not meet the current trigger criteria defined to optimize the reliability of the emsc feltearthquake detection. smaller shocks appear to be detected faster than larger events (figure 2). in practice, the key factor is not the magnitude, but the order of the shock in a sequence of earthquakes, with aftershocks detected faster than a mainshock. for example, on november 3, 2010, a m 5.4 earthquake occurred at kraljevo (serbia), and it was detected in 250 s. the next day, a m 4.4 aftershock was detected in 80 s, and the majority of the subsequent shocks were detected within less than 1 min, irrespective of the magnitudes (figure 3). the explanation is probably the following: the first shock enlarges the visibility of the emsc website in the rapid characterization of earthquake effects figure 2 (top). repartition of the time delays of the 63 automatically detected felt earthquakes as a function of their magnitude for the period january 2009 to may 2011, with black symbols for 2009 events, green for 2010, and red for 2011. the delay is measured from the earthquake occurrence. the chilean earthquake, an aftershock of the m 8.8 earthquake on february 27, 2010, is the only earthquake detected outside the european–mediterranean, region where the majority of the emsc website visitors come from. figure 3 (bottom). evolution of the detection delay over the 20 days following the m 5.4 earthquake in kraljevo (serbia). nine earthquakes were automatically detected during this period. the detection delay is not related to the earthquake magnitude but decreases with time. the audience of the emsc website dramatically increased in the epicenter region in the hours following the main shock, easing the detection of the following aftershocks regardless of their magnitudes. epicenter area, making the subsequent triggers much more rapid due to the more rapid convergence when new shocks occur of eyewitnesses who might have already bookmarked the url of the site [bossu et al. 2011]. this would also explain the detection of the m 6.7 chilean earthquake on march 16, 2010, which was an aftershock of the m 8.8 chilean earthquake of february 27, 2010. television programs that mention the emsc website can create a dramatic surge in the website traffic and result in a false trigger; i.e. a trigger that is not associated with a felt earthquake. this has been the cause of the 2 false triggers in the last 29 months. with 63 correct triggers in this period, this gives a false trigger rate of 3%. the first case of television-generated website traffic occurred in may, 2009, following a program on a romanian television channel [bossu et al. 2011]. in another case, a night-time scientific program on a spanish television channel, on march 21, 2010, caused an immediate and massive increase in the website traffic (figure 4) when the emsc url was displayed (e. carreno, private communication). in both cases, the traffic originated from the google search engine is significantly larger than during true triggers: 70% for the television program in romania, 50% for the television program in spain compared to less than 30% for surges associated to an earthquake. we believe that television programs drive on our site primarily persons who have never visited it before, increasing the use of search engines. if this is confirmed, this ratio could help in the automatic discrimination of television-generated triggers. theoretically, a group of well-organized internet users can also generate false triggers by launching concurrent website visits. this has not been observed yet. similarly, we have also never observed coordinated false answers to our online macroseismic questionnaires. this suggests that if such events cannot be excluded, at least their occurrence should remain rare. mapping the felt area the emsc uses the digital element ip intelligence software to identify the geographic location of each website visitor through the ip address. once a website surge is detected, the area where the earthquake was felt is mapped by the geographic origins of the statistically significant increased website traffic [bossu et al. 2008, 2011]. this map is obtained within a couple of minutes of the website surge detection, and it is updated after the origin time and location of the earthquake have been determined by the seismic networks. maps can also be produced for earthquakes that have not generated a detected website traffic surge, and therefore have not triggered the system. in practice, the number of observed visitors at each ip location since the origin time of an earthquake is compared with the average website traffic from the same place, during the same days of the week, and at the same hour of the day, during the previous 4 weeks. increases are considered significant when they reach a 99% confidence level. locations showing significantly increased website traffic delineate the felt area (figure 5a). bossu et al. 719 figure 4. the mention of the emsc website during a television program in spain on march 21, 2010, led to a website traffic surge that was not related to an earthquake. the blue curve represents the website visitors originating from the spanish google search engine. they represent about 50% of the website visitors, which is a significantly higher fraction that what was observed during a website surge as a result of an earthquake, which is typically less than 30%. 720 rapid characterization of earthquake effects f ig u re 5 . ( a) g eo gr ap h ic al o ri g in s o f th e w eb si te v is it o rs t o t h e e m sc w eb si te w it h in 5 m in o f th e o cc u rr en ce o f th e m 5 .4 e ar th qu ak e (s ta r) in s er bi a o n n ov em be r 3, 2 01 0. r ed c ir cl es r ep re se n t ge og ra ph ic al o ri g in s o f st at is ti ca lly s ig n ifi ca n t in cr ea se d tr af fi c o bs er ve d du ri n g th is 5 -m in t im e w in do w . ye llo w c ir cl es r ep re se n t th e ge og ra ph ic al o ri g in s o f w eb si te t ra ffi c w it h n o s ig n ifi ca n t va ri at io n s. b la ck t ri an gl es re pr es en t th e o bs er ve d ge og ra ph ic o ri g in s o f w eb si te v is it o rs in p as t 12 m o n th s. t h is c h ar ac te ri ze s th e re g io n al a u di en ce o f th e e m sc w eb si te a s w el l a s th e m ax im u m s pa ti al r es o lu ti o n o f th e ip lo ca ti o n s. c ir cl e si ze is a f u n ct io n o f th e di ff er en ce b et w ee n t h e ex pe ct ed a n d th e o bs er ve d nu m be rs o f u n iq u e ip s, w it h t h e sm al le st c ir cl e eq u al t o 0 a n d th e la rg es t gr ea te r th an o r eq u al t o 1 00 . t h e st ar r ep re se n ts t h e ep ic en te r lo ca ti o n . i p lo ca ti o n s ar e de te rm in ed b y d ig it al e le m en t. t h er e is a n a bs en ce o f vi si to rs o ri g in at in g fr o m t h e ep ic en te r ar ea , w h er e da m ag e w as o bs er ve d (e pi ce n te r in te n si ty in k ra lje vo : e m s9 8 v ii fr o m o n lin e qu es ti o n n ai re s co lle ct ed b y th e e m sc ). ( b ) m ac ro se is m ic m ap s ba se d o n o n lin e qu es ti o n n ai re s co lle ct ed b y th e u sg s fo r th e sa m e ea rt h qu ak e. t h e ar ea w h er e th e ea rt h qu ak e w as r ep o rt ed ly fe lt is c o m pa ra bl e w it h t h e m ap d er iv ed fr o m fl as h -s o u rc in g an al ys is in ( a) . t h e da ta se ts u se d to d er iv e th es e tw o m ap s w er e in de pe n de n tl y co lle ct ed b y tw o d is ti n ct a ge n ci es . the static maps of the felt area (figure 5a) use a 5-min time window. a time-sequenced map from 0 to 5 min after an earthquake occurrence and with a 5-s time resolution is provided as an electronic supplement. the reduction in the time window from 10 [bossu et al. 2011] to 5 min since 2009 has been made possible by the increased visibility and website traffic on the emsc website. a shorter time window reduces the possibility that the information about the earthquake occurrence has spread beyond the felt area through telephone calls or social networks. assuming that the information has already spread through these channels, and so beyond eyewitnesses, this is very unlikely to affect the felt area. first of all, because the number of these visitors should be large enough (after the filtering of referrers) to generate a statistically significant increase in the website traffic from the locations they originate from. even if they are in sufficient numbers in a few locations, they will not be spatially correlated with the epicenter location. a live television broadcast during the shaking caused by an earthquake would, however, irremediably affect the mapping of the felt area, by attracting visitors to the emsc website from the broadcasting sphere of the program. an example of a map of a felt area is shown in figure 5a, for the m 5.4 kraljevo (serbia) earthquake mentioned above. the overall felt area compares well with the macroseismic maps that were independently produced by the us geological survey (usgs; figure 5a, b). the comparison with the emsc macroseismic maps might be influenced by the intrinsic overlap between the eyewitnesses who have filled in the questionnaires and those who have caused the website traffic surge. another website traffic increase was observed to originate from tirana, albania (figure 5a). in this case, no questionnaires were collected in this country either by the usgs (figure 5b) or the emsc. the possibility that the shaking had been felt in northern albania is consistent with the macroseismic pattern going south to the epicenter (figure 5b). although we have not been able to find evidence to confirm this possibility, the absence of collected questionnaires might be due to the low number of respondents; bossu et al. [2011] showed from tests on several earthquakes that on average, less than 1% of the eyewitnesses visiting the emsc website actually complete the online questionnaire. detection and mapping of earthquake damage a striking feature illustrated by figure 5a is the absence of website visitors who originated from the epicenter area, where the shaking was strong and only light damage occurred. the same phenomenon was observed for the l’aquila (italy) m 6.3 earthquake on april 6, 2009 [bossu et al. 2011]. we believe that this lack of website visitors during strong shaking might result from the eyewitnesses initially being more concerned about personal safety, and/or from network disruption. however, while the presence of website visitors excludes the possibility of severe damage, with network collapse, an absence of website visitors is only an indication of the possibility of damage, and is not on its own concrete evidence of damage. to improve the identification of damaged areas, we developed a system based on the fact that in cases of severe damage, the existing website sessions that originate from an affected area will end instantaneously and simultaneously, due to power and/or network failures. the ajax-based tracking system detects the arrival and departure of each unique visitor to the emsc website using a periodic client ‘ping’ to track visitor presence. nominally, every 30 s, the browser of a website visitor makes an http request to the emsc servers, which updates the tracking information. the arrival of a new website visitor creates a new record in the tracking system, which is subsequently updated each time their browser pings the server while the website visitor remains on the emsc website. departures from the emsc website are determined by the detection of tracking records that have not been updated within the last 30 s. a massive power failure on march 14, 2010, that plunged part of chile into darkness was easily detected by the sudden increase in the number of sessions that originated from chile that ended at 23:43 gmt (figure 6). this is the exact timing for this power failure that is reported on wikipedia (http://es.wikipedia.org/wiki/apag%c3%b3n_de_chile_ de_2010). according to the same source, in santiago the power failure affected 6 of the 52 districts, which represents approximately 27% of the city population. the number of sessions that originated from santiago dropped by a similar proportion (31%), from 147 to 101 (figure 6). these similar ratios might be coincidental, however, as the majority of the sessions that originated from santiago at that time remained active, this shows that it was not a total blackout for this city. when applied to an earthquake, this approach can provide vital information on earthquake damage within 30 s of its occurrence. flash sourcing for faster public earthquake information in april 2011, and under the username @lastquake, the emsc set up an automatic service for twitter, the wellknown, real-time, information-sharing website, to distribute the flash-sourced information. the first twitter message, or ‘tweet’, is published immediately after the automatic detection of a website traffic surge. this indicates the regions where an earthquake appears to have been felt (figure 7). the names of the regions follow the flinn– engdahl naming convention [young et al. 1996], and they are determined by the regions where the locations with traffic increases are. once the causative earthquake has been located, it is automatically associated with the website surge; a second bossu et al. 721 722 tweet, that indicates the magnitude and origin time of the event, is then published (figure 7). the same information is published in parallel on the emsc website as a scrolling banner, with a second notice replacing the first. earthquakes causing a website traffic surge are automatically labeled as felt in the earthquake list published on the emsc website. the services derived from the detection of website traffic surges primarily improves the speed at which the earthquake information is made available to the public via the emsc website and twitter. the time benefit is defined as the delay between the automatic detection of the website traffic surge and the publication of the first earthquake location. in a few cases, this can be null, if an earthquake is located before the website surge is detected. the median value is about 2 min (figure 8). this can be much larger (figure 8), especially for small magnitude events, which are often reported less rapidly by networks than larger shocks. in the vast majority of cases (95% in the period of january, 2009 to may, 2011; figure 8), the automatic detection of website traffic surges speeds up the information for the eyewitnesses at a time when they are actively looking for information on the shaking that they have just felt. information on public behavior website traffic analysis provides some insight into both the event that has caused a website traffic surge and the public behavior when facing this event. the geographic origin of the television-generated website traffic in spain (figure 4) depicts the national extension of the broadcasting sphere of the program, with no website traffic increase from portugal or france (figure 9). the rise time was 1 min, which indicates that the majority of the website visitors shifted from their television to their browser very rapidly. browsing the internet while watching the television might also be quite typical, at least for some sections of the population. the 5 min width of the website surge (figure 4) is much shorter than the width when the website surge is caused by an earthquake (e.g. figure 1), which is on the order rapid characterization of earthquake effects figure 6.change over time in the number of sessions (blue) and the number of session closures observed on the emsc website for a 30 s window on march 14, 2010. the sudden increase in the number of closures (black and red) at 23:43 utc was due to a massive power failure which affected chile. at the time of the blackout, the number of sessions originating from santiago (blue) dropped by 31% which is similar to the estimation of 27% of the affected population in the city. figure 7. example of tweets published by the emsc under the username @lastquake for the m 5.8 earthquake in western turkey on may 19, 2011. the first tweet (bottom) was automatically published once the website traffic surge was detected 3 min and 25 s after the earthquake occurrence. the origin of the website visitors identified western turkey as the area where the shock had been felt. the first location and magnitude were published 6 min and 44 s after the earthquake, with a preliminary magnitude of m 5.9. this triggered the publication of the second tweet (top). the same information is published in a scrolling banner at the same time on the emsc website, pointed to by the shortened url in the tweet. in the first tweet, the 2 min is estimated from the previous trigger, which had a median time of 88 s (figure 2). bossu et al. 723 figure 8.distribution of the time benefit of using flash-sourcing for earthquake information. the time benefit is defined as the delay between the detection of the felt earthquake through the website traffic surge it generates and the publication of the first preliminary location and magnitude on the website. the time benefit is null for earthquakes that were first reported by the seismological networks. the blue line is the cumulative distribution since the initiation of the twitter service, and the red line is for the period january 2009 to may 2011. the median time benefit is around 2 min. this can be much longer, especially for small magnitude earthquakes: it exceeds 8 minutes (480 s) for 30% of earthquake below m 3.5, as compared to 10% for larger shocks. figure 9. the television program in spain that led to a website traffic surge (see figure 4) generated a significant increase in the number of spanish visitors of the emsc website from all over spain (see figure 5a for caption). 724 of 90 min [bossu et al. 2011]. we believe that the website viewers who did not switch immediately to the emsc website simply did not visit it following the program. on january 22, 2010, two earthquakes of m 5.3 and m 4.8 occurred within 3 min and 38 s of each other in the region of patras, greece (figure 10). in agreement with the other greek networks, the bulletin of patras university indicates an inter-epicenter distance of less than 3 km for these two shocks, and a similar focal depth. as the two shocks were co-located, and with the first event being the stronger one, eyewitnesses who felt the second earthquake are then very likely to have also felt the first one. surprisingly, the two shocks are clearly visible on the website traffic curve (figure 10). therefore, some of the eyewitnesses who visited the emsc website only visited it after the second shock. we believe that some of the website visitors might not have consider it worth getting out of the bed after the first shock, but were then sufficiently troubled by the succession of two shocks to finally check what was going on. if this hypothesis is correct, this is probably a more common behavior in areas where felt earthquakes are frequent, than in regions of low seismic hazard. these two earthquakes occurred in the middle of the night (first shock, 02:46 local time). nonetheless, the first eyewitnesses reached the emsc website within 50 s of this first earthquake (figure 10). for comparison, this delay was of the order of 8 min for the first studied earthquake in 2004 [bossu et al. 2011]. the increasing internet penetration rate among the public, as well as the rapid rise in the use of smart phones and other connected devices, are the most likely explanations of these ever-shrinking reaction times. future developments the automatic detection of felt earthquakes is based on significant website traffic surges. therefore, the number of eyewitnesses who converge to the emsc website has to be significant compared to the total website traffic. with a doubling of the website traffic every year [bossu et al. 2011], the number of eyewitnesses required to trigger the automatic detection also needs to double every year, which will impede the detectability threshold of the system. the analysis of the website traffic at different geographic levels, such as per country, region, or city, can dramatically improve the detection performance. this improves the signal-to-noise ratio for more rapid detection (e.g. figure 10), and can isolate several felt earthquakes that occur only a few minutes apart. on may 1, 2011, a m 4.7 earthquake in romania was followed 7 min later by a m 5.4 earthquake in eastern kazakhstan. the first shock was detected, although the website traffic surge caused by the second shock remained unnoticeable in the global website traffic curve. a simple after-the-fact website traffic filtering at the country level rapid characterization of earthquake effects figure 10. website traffic surge as a result of two co-located earthquakes of m 5.3 and m 4.8 that occurred 3 min and 30 s apart in the patras region (greece) on january 22, 2010. the black curve shows the number of new website visitors in the last minute; the red curve shows the new website visitors originating from greece, i.e. the country where the event was felt. while both shocks are clearly visible, the automatic detection of the second shock by seismological networks proved tricky, as its seismic waves were mixed with the waves of the mainshock. these shocks happened at about 03:00 local time, and the increase in the number of visitors from greece is clearly visible within less than 50 s of the earthquake occurrence. geographic filtering (red curve) improves the signal-to-noise ratio and can lead to faster detection of website traffic surges. demonstrates that both of these earthquakes could have been equally detected (figure 11). the ratio of the number of eyewitnesses visiting the emsc website to the regional population might be an indication of the relative strength of the shaking. it might be expected that for a given earthquake, the number of eyewitnesses normalized by the number of inhabitants will be lower for a city where only a fraction of the population felt the ground shaking, when compared to another city where the event was widely felt. the exclusive use of mobile internet access, which can be discriminated through the user-agent of the http requests, might also characterize areas where the citizens have rushed out of the buildings in panic, leaving land-line access unused. when all sessions using land-line access close down at the same time, it can be inferred that the electric grid might be down. the full potential of analyzing mobile versus land-line access remains to be evaluated. this will require more data, which should be available in the coming year on the basis of the rapid rise in smart-phone use in the european– mediterranean region. in summary, the joint analysis of the absence, the partial or total loss of sessions at the time of an earthquake, the increase of the number of visitors in relation to the local number of inhabitants, and the type of internet access is expected to deliver at least in some cases a more progressive classification of local earthquake effects from ‘not felt’ to ‘heavy damage’. discussion flash sourcing provides automatic detection of widely felt earthquakes within an average of 2 min of their occurrence. earle et al. [2010] reported similar performances through monitoring the number of twitter messages that contain keywords such as “earthquake” or “quakes”. these two examples demonstrate that social networks can offer fast detection of rapid-onset events. flash sourcing can currently map the area where an earthquake has been felt in less than 5 min. it can exclude the possibility of widespread damage, and in certain cases it can detect and map regions affected by network failures in near real time. this makes flash sourcing the fastest tool for the gathering of information on earthquake damage. for comparison, the very first online questionnaire that related to the kraljevo earthquake (figure 5) was collected within 7 min at the usgs (quitoriano and wald, personal communication), and within 9 min at the emsc. twenty minutes after the earthquake, the usgs had collected 50 questionnaires and the emsc had collected 3. as the website traffic analysis is based on implicit data, as opposed to declarative data such as questionnaires, it is complementary to crowd-sourcing approaches, such as online macroseismic questionnaires [e.g. wald et. al. 1999], or collection of geolocated pictures of earthquake damage [bossu et al. 2011], each of which provide different constraints on the actual earthquake impact in a different time frame. if successful, the planned improvements should reduce the trigger time to bossu et al. 725 figure 11.superposition of website traffic surges caused by two distant felt earthquakes. on may 1, 2011, a m 4.7 earthquake in romania was followed 7 min later by a m 5.4 earthquake in eastern kazakhstan. the observed website traffic (black line) is the addition of the website traffic originating from romania, bulgaria and moldova, the three countries where the first shock was felt, according to the emsc macroseismic map (red line), and the one originating from kazakhstan, and the baseline website traffic (green line). this demonstrates that the traffic can be broken down in its distinct components by geographic filtering. 726 below a 1 min threshold and characterize up to 7 different levels of effects: not felt, weak, moderate or strong shaking, panic, light or heavy damage. ultimately, we aim to integrate flash-sourced information, answers to online macroseismic questionnaires, and collected geo-located pictures of earthquake damage into a single and time-evolving map of earthquake effects. this will allow us to fill the information gap in a time window ranging from the first minutes to the first few hours following the occurrence of an earthquake. reliability assessment will be based on consistency between the different types of collected information, as well as on consistency with ground-motion prediction derived from earthquake parameters. flash sourcing is cheap and preserves the privacy of the website visitors. no individual behavior is identified or tracked. an analogy would be a road traffic study that records at the geographic origin of the drivers through their license plates. the uniqueness and personal nature of the plate number is thus disregarded, by considering only the state indication of the plate. flash sourcing has one strict constraint: it can only be implemented on a website where eyewitnesses naturally converge after an event. they do not visit the emsc website to fill in a questionnaire or to share their pictures, but rather to find the information they are looking for about their felt event. this implies that the website must be a reference information point for the population for the type of events considered. this approach can potentially be extended to other rapid-onset phenomena, such as flash floods [bossu and walker 2009], as long as the time for the eyewitnesses to reach the website is much shorter than the spread of information about the causal event through the media. although the first website traffic surge that resulted from an earthquake was reported for the 1999 hector mine event [wald and schwartz 2000], we are not aware of other studies that are aimed at characterizing an event through the analysis of the flash-crowd characteristics that it generates. seismologists also benefit from flash-sourcing information. in the case of the two earthquakes in patras (figure 10), many real-time processing tools implemented in the seismological networks that contribute to the emsc realtime services [mazet-roux et al. 2011] failed to detect the second shock, as its waves were mixed with those from the first event. flash-sourcing can be a heads-up to initiate interactive processing in such tricky cases. the benefit for the public is first more rapid information about earthquakes that they consider to be significant, regardless the magnitude and the use of alternative communication channels, such as twitter. this shifts the traditional policy of delivering earthquake information when available to delivering this information when requested. the amplitude of a website traffic surge is a direct measurement of the public desire for information. a large public information request that is ignored or simply not promptly answered by the seismologists might alter the public trust and the scientific credibility in the operating organization, and hence turn eyewitnesses to less-reliable websites; an official agency or an authoritative organization just cannot afford to be the last one informed when a significant event strikes. flash sourcing sheds light on the public reactions following an earthquake. it opens ways to evaluate the influence of culture and of the level of seismic hazard on the way people react during an earthquake. website traffic surges identify clear teachable moments. they can trigger timely geographically targeted online information on seismic hazard and prevention measures, to complement the existing public-information tools. flash sourcing is part of an overall scientific evolution that promotes the engagement of the citizens and the development of community-based environmental monitoring and information systems. in seismology, this started with online macroseismic questionnaires [e.g., wald et al. 1999]. the emsc has been collecting geo-located pictures of earthquake effects on its website since 2006 [bossu et al. 2011]. more recently, in cases of damaging earthquakes, the emsc makes available online the richter (rapid geo-images for collaborative help targeting earthquake response) application for android mobile phones, which was developed by ansur technologies and which allows earthquake eyewitnesses to easily and freely share their pictures with the emsc. the quake catcher network [cochran et al. 2009] was the first citizen-operated seismic network; based on cheap microelectromechanical systems sensors, this is well suited for the deployment of dense networks [cochran et al. 2009]. as mentioned above, the real-time analysis of the use of social networks, such as twitter, offers rapid detection of felt earthquakes [earle et al. 2010]. such initiatives (and the others contained in this special issue) can change the way the seismological community interacts with the public, from simple recipients of information to a more active role. the understanding of the public demands and motivations, and clear communication on project benefits and outcomes are a prerequisite to citizen engagement. in turn, improved public communication can contribute to more efficient risk awareness programs and develop public ownership of risk reduction policies. the benefit for the scientific community is a cheap and complementary approach to document earthquake-related phenomena that can contribute to both scientific knowledge and improved earthquake response. in the longer term, rapid two-way exchange of information between the scientific community and earthquake eyewitnesses through internet and mobile applications or social networks will open the way rapid characterization of earthquake effects to resident-to-resident assistance in the critical first hours of a damaging earthquake. desirable or not, such a possibility raises many complex issues, especially on liability aspects. these cannot be addressed by the scientific community alone, but it will have to contribute to these discussions. conclusions flash sourcing is the fastest tool for the gathering of information on earthquake damage. information is derived from the analysis of the use of the emsc website by the earthquake eyewitnesses, who are the first informed and the first concerned by an earthquake occurrence. flash sourcing offers an inexpensive alternative to dense real-time accelerometric networks, the implementation of which is limited due to high operational costs, and it complements crowd-sourcing techniques, such as online questionnaires. flash-sourcing improves public earthquake communication by taking into account the public desire for information, rather than by focusing solely on the magnitude of an earthquake. if successful, future developments should offer a more detailed picture of earthquake damage, which in turn can contribute to filling in the information gap in the immediate aftermath of an earthquake, and thereby contributing to improved earthquake response. acknowledgements. the authors express their acknowledgements to the french ministry of the environment for its early support in the developing the flash-sourcing approach, and thank digitalelement for offering the ip location software. this work has been partially funded by the ec project reakt fp7-282862. we acknowledge the constructive comments from an anonymous reviewer and from michael blanpied, who both contributed to the improvement of this article. geographic maps were prepared using gmt software [wessel and smith 1998]. references bossu, r., v. douet, s. godey, g. mazet-roux and s. rives (2007). on the use of internet to rapidly collect earthquake impact information, emsc newsletter 22, 31-34. bossu, r., g. mazet-roux, v. douet, s. rives, s. marin and m. aupetit (2008). internet users as seismic sensors for improved earthquake response, eos, transactions, 89, 225-226. bossu, r. and a. walker (2009). new tools for alerts and capturing immediate effects of rapid-onset events across europe, chemical hazards and poisons report, health protection agency, 14, 35-37. bossu, r., s. gilles, g. mazet-roux. and f. roussel (2011). citizen seismology or how to involve the public in earthquake response, in: d.m. miller and j. rivera (eds.), comparative emergency management: examining global and regional responses to disasters, auerbach/taylor and francis publishers, 237-259. cochran, e.s., j.f. lawrence, c. christensen and r.s. jakka (2009). the quake-catcher network: citizen science expanding seismic horizons, seismol. res. lett., 80, 26-30. earle, p., m. guy, r. buckmaster, c. ostrum, s. horvath and a. vaughan (2010). omg earthquake! can twitter improve earthquake response?, seismol. res. lett., 81, 246251. grünthal, g., ed. (1998). european macroseismic scale 1998 (ems-98), cahiers du centre européen de géodynamique et de séismologie 15, centre européen de géodynamique et de séismologie, luxembourg, 99 pp. mazet-roux, g., r. bossu, e. carreno and j. guilbert (2011). report on 2010 real-time activities, emsc report, 38 pp.; url: http://www.emsc-csem.org/doc/emsc_docs/ emsc_rt_activities_2010.pdf (last accessed june 23, 2011). wald, d.j., v. quitoriano, l. dengler and j.w. dewey (1999). utilization of the internet for rapid community intensity maps, seismol. res. lett. 70, 87-102. wald, l.a. and s. schwartz (2000). the 1999 southern california seismic network bulletin, seismol. res. lett., 71, 401-422. wessel, p. and w.h.f. smith (1998), new, improved version of the generic mapping tools released, eos trans. agu, 79, 579. wikipedia report: apagón de chile de 2010 (in spanish); url: http://es.wikipedia.org/wiki/apag%c3%b3n_de_ chile_de_2010 (last accessed june 23, 2011). young, j.b., b.w. presgraveb, h. aichelec, d.a. wiensd and e.a. flinn (1996). the flinn–engdahl regionalisation scheme: the 1995 revision, phys. earth planet. interiors, 96, 221-300. *corresponding author: rémy bossu, european mediterranean seismological centre (emsc), c/o cea, bât. sâbles, centre dam île de france, bruyères le châtel, france; email: bossu@emsc-csem.org. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. bossu et al. 727 n u m e r i caratteristici definitivi dei flocculi di calcio per gli anni internazionali di q u i e t e solare g . g o d o l i f . m a z z u c c o n i b . m o n s i g n o r i f o s s i r i c e v u t o il 1 f e b b r a i o 1 9 6 7 riassunto. — si d a n n o i n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i flocculi d i c a l c i o p e r g l i a n n i i n t e r n a z i o n a l i d i q u i e t e s o l a r e s e c o n d o il m a t e r i a l e d i o s s e r v a z i o n e r a c c o l t o a l w o r l d d a t e c e n t e r d i a r c e t r i g r a z i e a l p r o g r a m m a d i c o l l a b o r a z i o n e i n t e r n a z i o n a l e . q u e s t i n u m e r i c a r a t t e r i s t i c i s o s t i t u i s c o n o q u e l l i p r e l i m i n a r i , g i à p u b b l i c a t i n e l b o l l e t t i n o a n n u a l e d e l l e o s s e r v a z i o n i e s e g u i t e a l l ' o s s e r v a t o r i o a s t r o f i s i c o d i a r c e t r i ( i 2 . " ) , i q u a l i v e n n e r o s t i m a t i i n b a s e a l l e s o l e o s s e r v a z i o n i e s e g u i t e a l l a t o r r e s o l a r e d i a r c e t r i . summary. — t h e d e f i n i t i v e c h a r a c t e r figures of t h e c a b r i g h t flocculi f o r t h e i n t e r n a t i o n a l q u i e t s u n y e a r s a r e g i v e n . t h e o b s e r v a t i o n a l m a t e r i a l u s e d h a s b e e n c o l l e c t e d a t a r c e t r i w o r l d d a t a c e n t e r f o r t h e c a b r i g h t f l o c c u l i , a s a p a r t of t h e i q s y i n t e r n a t i o n a l p r o g r a m m e . t h e s e c h a r a c t e r figures t a k e t h e p l a c e of t h o s e a l r e a d y p u b l i s h e d ( 1 2 . l 3 ) a c c o r d i n g t o t h e o n l y o b s e r v a t i o n s c a r r i e d o u t a t t h e a r c e t r i s o l a r t o w e r . 1. d e t e r m i n a z i o n i di n u m e r i c a r a t t e r i s t i c i giornalieri dei flocculi solari di calcio sono s t a t e p u b b l i c a t e , p e r il periodo 1950-1960, in q u e s t a sede ( 1 9 ) e p e r gli a n n i 1961, 1963, 1964, 1965 nel b o l l e t t i n o a n n u a l e delle osservazioni eseguite a l l ' o s s e r v a t o r i o astrofisico di a r c e t r i (10-13). p e r q u e s t e d e t e r m i n a z i o n i è s t a t o u t i l i z z a t o il m a t e r i a l e spettroeliografico r a c c o l t o alla t o r r e solare di a r c e t r i nella r a d i a z i o n e della comp o n e n t e k2.j,2 della riga k del calcio u n a v o l t a ionizzato. d a l 24 o t t o b r e 1961 al 9 o t t o b r e 1963, n o n sono s t a t e eseguite d e t e r m i n a z i o n i di n u m e r i c a r a t t e r i s t i c i dei flocculi di calcio d a t o che le osservazioni alla t o r r e solare sono s t a t e sospese p e r p r o c e d e r e alla s o s t i t u z i o n e della cupola e dei s e r v o m e c c a n i s m i di c o m a n d o . 1 8 4 g . g o d o l i f . m a z z u c c o x i b . m o n s i g n o r i f o s s i 2. come è noto, sin dall'inizio dell'anno geofisico i n t e r n a z i o n a l e , a g i (i o luglio 1957-31 dicembre 1958), l'osservatorio astrofisico di a r c e t r i è centro mondiale (world d a t a center, wdc) per la r a c c o l t a e l'elaborazione dei d a t i relativi ai flocculi solari in radiazione k del ca i i . grazie a questa collaborazione internazionale, il m a t e r i a l e di osservazione copre t u t t i i giorni r e l a t i v i alla c a m p a g n a a g i con la sola eccezione del giorno 8 dicembre 1957 e t u t t i quelli relativi alla c a m p a g n a degli anni internazionali di quiete solare, i q s y ( i o gennaio 1961-31 dicembre 1965) con la sola eccezione del giorno 26 dicembre 1964. le copili e gli originali i n v i a t i dai vari i s t i t u t i sono s t a t i r i p o r t a t i a negativi su l a s t r a r i s p e t t i v a m e n t e con u n a o due inversioni fotografiche. i l d i a m e t r o dell'immagine solare è s t a t o sempre r i p o r t a t o alla misura di 63 m i n . d a l m a t e r i a l e di osservazione così elaborato, è s t a t a costituita u n a collezione, il più omogenea possibile, di uno spettroeliogramma al giorno. 3. questa collezione era già s t a t a utilizzata per la compilazione delle m a p p e dei flocculi solari in radiazione k del ca i i per l ' a g i (14-16) e gli i q s y (17-18). successivamente, la collezione relativa all'agi è s t a t a utilizzata anche p e r u n a determinazione dei n u m e r i caratteristici in q u a n t o le n u o v e stime, eseguite su m a t e r i a l e che p r e s e n t a v a soltanto u n a lacuna di u n giorno, sarebbero s t a t e senz'altro più complete ed anche più precise di quelle già eseguite sul solo m a t e r i a l e di arcetri (19). col presente lavoro ci siamo proposti, nello stesso spirito, di eseguire sulla base della collezione internazionale, u n a n u o v a s t i m a anche per il periodo degli i q s y . a b b i a m o già n o t a t o ('•10) che, sebbene dal i o gennaio 1957 l ' a t t i v i t à dei flocculi di calcio venga seguita all'osservatorio astrofisico di arcetri rilevando a r e a e posizione dei singoli fenomeni in q u a n t o questo m e t o d o di d e t e r m i n a r e l ' a t t i v i t à dei flocculi è più i n f o r m a t i v o della s t i m a del n u m e r o caratteristico e sebbene il n u m e r o caratteristico sia u n a funzione n o t a dell'area p r o i e t t a t a , t u t t a v i a è opportuno c o n t i n u a r e n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i 1)1 c a l c i o 1 8 5 t a b e l l a i . n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i c a l c i o p e r g l i a n n i i n t e r n a z i o n a l i d i q u i e t e s o l a r e . ( p e r i o d o 1 g e n n a i o 1964 30 g i u g n o 1964). m e s e > g e n n a i o f e b b r a i o m a r z o a p r i l e m a g g i o g i u g n o g i o r n o n e. 0 n . c . 0 n . c . 0 n . c . 0 il c. 0 n . c . o 1 0 . 1 k 0 . 0 k 1 3 m m 0 6 k 0 2 m m 0 . 4 m m 2 0 2 a 0 . 0 k 1 1 m m 0 8 k 0 2 m m 0 4 m m 3 0 3 a 0 . 0 a 1 1 k 0 8 k 0 2 m m 0 4 mm 4 0 3 m m 0 . 0 k 1 0 k 0 9 m m 0 3 m m 0 3 a 5 0 3 a 0 . 0 k 1 0 k 1 0 m m 0 4 mm 0 3 a 6 0 4 m m 0 . 0 a 0 9 k 0 8 k 0 4 a 0 3 a 7 0 3 a 0 . 0 a 0 7 k 0 8 k 0 5 m m 0 3 a 8 0 3 a 0 . 1 a 0 6 k 0 7 k 0 5 m m 0 2 a 9 0 4 a 0 . 2 a 0 4 k 0 6 a 0 5 m m 0 4 m m 10 0 5 k 0 . 2 a 0 3 k 0 6 a 0 6 m m 0 5 a 11 12 13 0 0 0 5 7 7 k a k 0 . 2 0 . 0 0 . 0 k a l i 0 0 0 4 5 8 k f m m ¡?^ mai, 0 0 0 6 7 6 m m k a 0 0 0 5 3 3 v" " ' "ti mm i a--* a • slv a 0 0 0 6 7 8 a a a 14 15 0 0 8 8 k k 0 . 2 0 . 4 mm mm 0 0 7 7 ifi k s 1 x k ] 0 0 6 6 k a 0 0 3 4 "a"' £*» iffa 0 0 9 9 mm k 16 0 8 k 0 . 5 k 0 7 m m 0 5 a 0 4 ì5c' a 1 0 a 17 0 7 a 0 . 7 k 0 6 m m 0 5 a 0 5 a 1 0 a 18 0 6 a 0 . 9 k 0 5 m m 0 4 k 0 5 a 1 0 mm 19 0 7 a 0 . 9 k 0 5 m m 0 5 k 0 5 k 0 9 mm 20 0 6 a 1 . 1 m m 0 5 k 0 5 k 0 5 k 0 8 m m 21 0 5 a 1 . 3 m m 0 5 m m 0 6 k 0 5 k 0 7 mm 22 0 7 a 1 . 3 k 0 7 m m 0 6 a 0 4 a 0 6 a 23 0 7 a 1 . 4 k 0 9 k 0 6 k 0 4 mm 0 5 a 2 4 0 8 a 1 . 5 k 0 9 k 0 5 k 0 4 m m 0 4 mm 25 0 7 k 1. 5 k 0 8 k 0 5 a 0 4 mm 0 2 mm 26 0 7 k 1 . 5 k 0 8 k 0 5 a 0 3 a 0 0 a 27 0 6 k 1 . 5 k 0 7 k 0 4 a 0 5 mm 0 0 mm 28 0 6 k 1 . 5 k 0 6 k 0 3 a 0 4 mm 0 0 mm 29 0 7 k 1 . 5 mm 0 5 k 0 2 k 0 4 a 0 0 a 30 0 6 a — 0 5 k 0 2 ic 0 5 mm 0 1 a 31 0 5 k — 0 6 mm 0 4 m m — m e d i a m e n s i l e —> 0 . 55 0 . 63 0 . 70 0 . 58 0 . 41 0 . 49 m e d i a s e m e s t r a l e 0.56 18g g . g o d o l i f . m az z u c c o n i b . m o n s i g n o r i f o s s i t a b e l l a i i . n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i c a l c i o p e r g l i a n n i i n t e r n a z i o n a l i d i q u i e t e s o l a r e . ( p e r i o d o 1 l u g l i o 1904: 31 d i c e m b r e 1964). .mese l u g l i o a g o s t o s e t t e m b r e o t t o b r e n o v e m b r e d i c e m b r e g i o r n o u . c . 0 n c. 0 n c. 0 n . c . o n c. 0 n . c . 0 1 0 1 a 0 3 a 0 3 a 0 . 6 a 0 9 m m 0 2 i t 2 0 2 a 0 4 m m 0 4 mm 0 . 7 m m 0 8 m m 0 4 m 3 0 3 a 0 4 a 0 5 a 0 . 8 m m 0 8 a 0 4 i t 4 0 4 m m 0 4 a 0 5 a 0 . 9 m m 0 8 i t 0 5 a 5 0 4 mm 0 3 a 0 5 m m 0 . 9 a 0 7 a 0 7 i t 6 0 4 k 0 2 a 0 6 m m 0 . 9 a 0 4 a 0 9 m m 7 0 4 k 0 3 a 0 6 a 1 . 0 a 0 4 k 0 9 i t 8 0 4 k 0 3 mm 0 6 a 0 . 9 k 0 4 m m 1 0 i t 9 0 5 m m 0 3 a 0 7 m m 0 . 7 a 0 4 i t 1 1 m m 10 0 5 mm 0 4 m m 0 8 a 0 . 4 mm 0 5 m m 1 0 a 1 1 0 5 m m 0 4 a 0 8 m m 0 . 4 m m 0 5 m m 1 2 i t 12 0 5 mm 0 5 mm 0 7 m m 0 . 3 k 0 5 k 1 2 i t 13 0 4 a 0 6 a 0 7 a 0 . 4 k 0 4 a 1 1 i t 14 0 4 mm 0 7 k 0 6 mm 0 . 4 m m 0 4 a 1 1 k 15 0 3 mm 0 9 mm 0 4 a 0 . 5 t 0 4 i t 1 1 m m 16 0 3 a 0 9 a 0 2 m m 0 . 5 k 0 5 i t 1 0 k 17 0 3 m m 0 8 a 0 1 m m 0 . 5 mm 0 6 m m 1 0 i t 18 0 4 mm 0 7 k 0 0 a 0 . 6 m m 0 7 m m 1 0 i t 19 0 4 m m 0 6 a 0 1 a 0 . 6 a 0 7 a 1 1 i t 20 0 4 mm 0 5 k 0 1 m m 0 . 7 k 0 6 m m 1 2 i t 21 0 3 m m 0 5 a 0 1 a 0 . 8 m m 0 5 t 1 2 m m 22 0 2 a 0 4 k 0 2 m m 0 . 8 k 0 5 m m 1 0 a 23 0 2 mm 0 2 a 0 2 a 0 . 9 i t 0 4 a 0 9 a 24 0 1 a 0 2 k 0 2 a 0 . 9 k 0 5 m m 0 9 p 25 0 2 mm 0 3 a 0 1 a 0 . 8 i t 0 5 m m 1 0 t 20 0 2 a 0 3 a 0 1 a 0 . 9 mm 0 4 mm — 27 0 1 a 0 3 a 0 1 a 0 . 9 i t 0 4 i t 1 0 m m 28 0 0 m m 0 2 a 0 2 m m 1 . 0 i t 0 3 i t 0 8 i t 29 0 0 m m 0 2 a 0 3 a 1 . 0 a 0 3 i t 0 7 k 30 0 0 m m 0 2 m m 0 5 a 1 . 0 a 0 4 i t 0 6 i t 31 0 2 a 0 2 a — 1 . 0 mm — 0 6 i t m e d i a m e n s i l e 0 . 29 0 . 42 0 . 37 0 . 73 0 . 52 0 . 90 m e d i a s e m e s t r a l e 0,54 m e d i a a n n u a l e 0 . 5 5 n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i 1)1 c a l c i o 1 8 7 t a b e l l a i i i . n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i c a l c i o p e r g l i a n n i i n t e r n a z i o n a l i d i q u i e t e s o l a r e . ( p e r i o d o 1 g e n n a i o 1 9 6 5 30 g i u g n o 1 9 6 5 ) . m e s e g e n n a i o f e b b r a i o m a r z o a p r i l e m a g g i o g i u g n o g i o r n o n . c . o n . c . 0 n c . o li c. o n c . o n c . o 1 0 8 k 1 2 k 1 1 m m 0 6 l 0 2 m m 0 8 m m 2 0 9 k 1 4 k 1 0 m m 0 6 m m 0 3 m m 0 9 k 3 1 2 m m 1 2 a 1 0 m 0 4 m m 0 4 m m 1 . 0 k 4 i 4 k 1 0 a 1 0 t 0 2 m m 0 5 m m 1 1 k 5 1 3 k 1 0 k 1 1 t 0 0 m m 0 5 m m 1 1 k 6 i 0 a 0 8 k 1 0 m 0 1 m m 0 6 m m 1 0 k 7 0 9 a 0 8 k 0 9 t 0 3 m m 0 7 m m 1 2 m m 8 0 9 k 1 0 k 0 6 k 0 5 k 0 8 m m 1 3 m m 9 0 9 k 1 0 a 0 3 l 0 6 k 0 8 m m 1 2 m m 10 1 0 k 0 6 a 0 4 t 0 7 k 0 7 m m 1 0 m m 11 1 2 k 0 6 a 0 4 m m 0 8 m m 0 8 m m 1 0 a 12 1 0 k 0 5 a 0 4 m m 1 0 m m 0 9 m m 1 2 m m 13 0 8 k 0 6 a 0 6 m m 1 0 m m 0 9 m m 1 2 m m 14 0 7 k 0 6 m m 0 5 t 1 2 m m 0 9 m m 1 3 a 15 0 5 k 0 6 m m 0 6 m m 1 2 l 1 0 m m 1 3 a 16 0 1 k 0 5 a 0 6 m m 1 3 k 1 2 m m 1 3 k 17 0 5 k 0 4 a 0 6 l 1 3 k 1 3 k 1 4 m m 18 0 7 k 0 4 k 0 7 m m 1 2 m m 1 4 k 1 4 a 19 0 7 m 0 3 k 0 7 m m 1 1 m m 1 5 k 1 4 a 2 0 0 7 a 0 4 k 0 7 m m 1 0 m m 1 5 k 1 3 m m 2 1 0 9 k 0 4 k 0 6 m m 1 0 m m 1 6 k 1 2 a 22 0 9 k 0 4 k 0 6 k 0 9 m m 1 5 k 1 0 a 2 3 0 8 k 0 5 m m 0 3 k 0 8 m m 1 4 k 0 8 a 2 4 0 8 k 0 6 k 0 2 k 0 6 m m 1 4 a 0 8 a 2 5 0 9 k 0 8 k 0 4 k 0 5 t 1 a 0 9 a 26 0 9 k 1 0 m m 0 6 m 0 7 k 1 4 a 1 1 m m 27 0 9 k 1 1 m m 0 8 m m 0 6 m m 1 2 m m 1 1 m m 28 1 0 k 1 2 m m 0 8 m m 0 4 m m 1 0 k 1 1 m m 29 1 2 k — 0 7 ic 0 4 m m 0 9 k 1 1 m m 3 0 1 3 k — 0 6 k 0 2 k 0 8 m m 1 0 a 3 1 1 3 k — 0 6 m m — 0 8 m m — media mensile 0 . 91 0 . 7 5 0 . 66 0 . 71 0 . 9 8 1 . 12 — > m e d i a s e m e s t r a l e 0 . 8 6 18g g . g o d o l i f . m az z u c c o n i b . m o n s i g n o r i f o s s i t a b e l l a i v . n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i c a l c i o p e r g l i a n n i i n t e r n a z i o n a l i d i q u i e t e s o l a r e . ( p e r i o d o 1 l u g l i o 1965 31 d i c e m b r e 1965). m e s e l u g l i o a g o s t o s e t t e m b r e o t t o b r e n o v e m b r e d i c e m b r e g i o r n o n . c. 0 n . c . o n . c . 0 n . c . 0 n . c. 0 n . c . 0 1 1 1 a 0 6 mm 0 . 6 m m 1 . 3 a 1 2 m m 0 7 a 2 1 2 a 0 6 m 0 . 7 k 1 . 4 mm 1 2 m m 0 7 a 3 1 2 m m 0 7 m m 0 . 7 1c 1 . 6 mm 1 3 mm 0 7 t 4 1 2 a 0 8 a 0 . 7 k 1 . 8 a 1 2 mm 0 8 11 5 1 1 k 0 9 a 0 . 7 k 1 . 9 a 1 0 m m 0 7 m m 6 1 0 a 1 0 a 0 . 6 ic 1 . 7 a 0 9 m m 0 8 t 7 0 9 a 1 1 a 0 . 7 k 1 . 3 a 1 0 m m 0 8 mm 8 1 0 mm 1 1 a 0 . 8 k 0 . 8 a 1 0 a 0 8 mm 9 1 0 mm 1 2 a 0 . 8 k 0 . 6 a 1 1 mm 0 9 mm 10 1 1 mm 1 2 a 0 . 8 a 0 . 5 a 1 1 m m 0 9 mm 11 1 1 m m 1 2 a 0 . 9 mm 0 . 5 a 1 1 mm 0 8 k 12 1 2 a 1 2 a 0 . 9 m m 0 . 5 a 1 1 t 0 7 k 13 1 2 a 1 2 a 0 . 8 a 0 . 5 a 0 9 mm 0 6 t 14 1 2 a 1 3 m m 0 . 7 f 0 . 5 a 0 9 m m 0 6 ß 15 1 1 m m 1 0 mm 0 . 6 a 0 . 5 a 0 8 t 0 5 a 16 0 9 m m 0 7 mm 0 . 6 a 0 . 5 t 0 6 a 0 6 t 17 0 9 m m 0 4 a 0 . 6 a 0 . 5 a 0 5 m m 0 7 m m 18 0 8 a 0 4 mm 0 . 6 mm 0 . 5 m m 0 4 m m 0 7 m m 19 0 8 a 0 5 a 0 . 7 mm 0 . 6 m m 0 3 m m 0 7 mm 2 0 0 5 m m 0 6 a 0 . 7 a 0 . 6 a 0 4 m m 0 5 k 21 0 3 m m 0 7 m m 0 . 7 mm 0 . 7 a 0 5 m m 0 4 mm 22 0 1 a 0 7 m m 0 . 5 a 0 . 7 a 0 6 m m 0 3 m m 2 3 0 1 a 0 7 m m 0 . 5 a 0 . 8 a 0 6 k 0 3 w 24 0 1 a 0 7 m m 0 . 4 k 0 . 9 a 0 6 m m 0 2 t 2 5 0 3 mm 0 8 m m 0 . 4 a 1 . 0 a 0 6 l 0 2 t 26 0 3 mm 0 8 a 0 . 5 k 1 . 1 a 0 6 t 0 4 mm 27 0 2 a 0 8 a 0 . 7 k 1 . 1 mm 0 6 mm 0 6 t 28 0 2 m m 0 8 a 1 . 0 mm 1 . 1 mm 0 6 m 0 7 a 29 0 3 mm 0 7 a 1 . 1 k 1 . 0 mm 0 7 mm 0 8 a 30 0 4 mm 0 7 k 1 . 1 a 1 . 1 mm 0 7 mm 0 8 a 31 0 5 mm 0 6 k — — 1 . 1 mm 0 7 a m e d i a m e n s i l e 0 75 0 . 83 0 . 70 0 93 0 . 80 0 . 6 3 m e d i a s e m e s t r a l e 0.77 m e d i a a n n u a l e 0.82 n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i 1)1 c a l c i o 52 a stimare i numeri caratteristici dei flocculi di calcio d a t o che vi sono elementi per ritenere che, d u r a n t e fasi peculiari del cielo solare, i due diversi m e t o d i diano informazioni di significato diverso (20). 4. le stime dei n u m e r i caratteristici sono s t a t e sempre eseguite all'osservatorio astrofisico di arcetri attenendosi ai modelli distribuiti dall'osservatorio di mount wilson. i modelli definiscono i p u n t i 0, 1, 2, 3, 4, 5 della scala dei n u m e r i caratteristici. ad arcetri è sempre s t a t o considerato conveniente stimare anche il decimo del numero c a r a t t e ristico, m e n t r e altri osservatori preferiscono dare solo i cinque decimi. i modelli originali di m o u n t wilson sono costituiti da positive su c a r t a del d i a m e t r o di 100 m m . r e c e n t e m e n t e ad arcetri, per migliorare la qualità della stima, si è d e r i v a t a dai modelli originali di mount wilson u n a serie di modelli costituita da negative su lastra del diametro di 63 m m . d a l 1962 i n u m e r i caratteristici sono sempre s t a t i s t i m a t i con l ' a i u t o di questi modelli. 5. l a n u o v a stima sistematica di n u m e r i caratteristici è s t a t a f a t t a i n d i p e n d e n t e m e n t e da due osservatori (f.m. e b.m.f.); un terzo osservatore (g.g.) h a s a l t u a r i a m e n t e controllato le stime. q u a n d o lo scarto medio delle singole determinazioni non s u p e r a v a 0.1 u n i t à , veniva f a t t a u n a media delle valutazioni dei due osservatori; nei pochi casi in cui v e n i v a riscontrato uno scarto medio di maggior e n t i t à lo spettroeliogramma veniva riesaminato. nelle tabelle i y i sono r i p o r t a t e le nuove stime che, d a t o il loro c a r a t t e r e di completezza, costituiranno, d ' o r a in poi, la serie definitiva per gli anni internazionali di quiete solare. nelle tabelle è i n d i c a t a anche la f o n t e dello spettroeliogramma analizzato secondo il codice: a = arcetri f = f r e i b u r g k = ivodaikanal l = l o c a m o mm = me m a t h h u l b e r t m = meudon l ì = r o m a t = tokio w = wendel stein. 18g g. g o d o l i f . m az z u c c o n i b . m o n s i g n o r i f o s s i notiamo che lo scarto medio f r a le nuove stime e quelle già p u b blicate r i e n t r a generalmente nel limite a c c e t t a t o non s u p e r a n d o il valore 0.1. nella seguente tabella sono r i p o r t a t i i giorni per cui lo scarto medio supera il valore 0.1. s c a r t o m e d i o g i o r n o 0.1 < s m < 0 . 2 0.2 < s m < 0 . 3 1 9 0 4 : i v 22; v 13, 29; v i 10, 11, 12, 13, 17, 22, 2 3 ; v i i 13; v i i i 4, 10, 16, 17, 19, 2 1 ; i x 3, 4, 7, 8, 10, 13, 30; x 5, 6, 9, 19, 30; x i 5, 19; x i i 10, 22, 23. 1965: i 11, 12; i i 2 ; vi 25, 30; v i i 6, 7, 14, 19; v i l i 4, 5, 7, 9, 10, 11, 12, 13, 26, 27, 28, 29; tx 10, 13, 15, 16, 20; x 1, 21, 23. 1 9 6 4 : v i 16; x 7, 29; x i 3. 1965: i 5 ; v i 26, 27; v i i 1, 2, 4 ; i x 5; x 4, 5, 6. n o t i a m o che, in generale, le nuove stime tendono ad essere più elevate di quelle eseguite utilizzando il solo m a t e r i a l e spettroeliografico raccolto alla torre solare di arcetri. questa differenza sistematica, che si m a n t i e n e e n t r o gli scarti medi a c c e t t a t i , è p r o b a b i l m e n t e da attribuirsi al maggior contrasto rispetto agli spettroeliogrammi di arcetri, delle lastre della collezione internazionale, d o v u t o ai procedimenti fotografici. p e r l ' a g i questa differenza non si è r i s c o n t r a t a d a t o che, in periodi di massima d e l l ' a t t i v i t à solare lo scarto medio assoluto r i s u l t a superiore, p u r m a n t e n e n d o s i costante lo scarto medio relativo. b i b l i o g r a f i a (') godoli g., numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per gli anni 1950-51-52, « a n n . geof.>, 6, 199 (1953). (-) i d e m p e r l ' a n n o 1953, l.o., 7, 135 (1954). (3) i d e m p e r l ' a n n o 1954, l.c., 8, 325 (1955). (4) i d e m p e r l ' a n n o 1955, l.c,, 9, 299 (1956). (5) i d e m p e r l ' a n n o 1956, l.c., 10, 133 (1957). ( i n c o l l a b o r a z i o n e c o n m . p . s t r o c c i i i ) . n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i 1)1 c a l c i o 191 (6) i d e m p e r l ' a n n o 1957, l.c., 10, 2 3 5 (1957). (') i d e m p e r l ' a n n o 1958, l.c., 12, 197 (1959). (8) i d e m p e r l ' a n n o 1959, l.c., 13, 231 (1960). ( 9 ) i d e m p e r l ' a n n o 1960, l.c., 14, 319 (1961). (10) osservazioni eseguile air osservatorio astrofisico di arcetri durante vanno 1901, « o s s . e m e n i . d e l l ' o s s . a s t r o f . di a r c e t r i » , 7 7 (1962). ( " ) i d e m p e r l ' a n n o 1963, l . c . , 7 9 (1964). (i 2) i d e m p e r l ' a n n o 1964, l . c . , 8 0 (1965). ( n ) i d e m p e r l ' a n n o 1965, l.c., 8 4 (1966). (14) godoli g., mappe dei flocculi solari in radiazione k del ca ii per l'anno gcofisico internazionale, i. semestre 1 luglio 1957 31 dicembre 19-57, « o s s . e m e n i . d e l l ' o s s . a s t r o f . di a r c e t r i » , 7 3 (1961). ( , 5 ) i d e m p e r il s e m e s t r e 1 g e n n a i o 195s 30 g i u g n o 1958, l . c . , 7 5 (1961). ( , 6 ) i d e m p e r il s e m e s t r e 1 l u g l i o 1958 31 d i c e m b r e 1958, l . c . , 7 6 ( 1 9 6 2 ) . (17) godoli g., mappe dei flocculi solari in radiazione k del ca ii per gli anni internazionali di quiete solare, i. anno 1904, « oss. e m e n i . dell ' o s s . a s t r o f . d i a r c e t r i » , 8 2 (1966). (18) i d e m p e r l ' a n n o 1965, l.c., 8 7 (1967). ( 1 9 ) g o d o l i g . , m a z z u c c o n i f . , m o n s i g n o r i f o s s i 1 3 . , numeri caratteristici definitivi dei flocculi di calcio per l'anno geofisico internazionale, « a n n . g e o f . », 19, 95 (1966). (20) godoli g., attività dei flocculi d'idrogeno e di calcio e dei filamenti di idrogeno per il periodo 1 luglio 1957 30 giugno 1958, « b o l l , di g e o d . e s c i e n z e a f f i n i » , 17, 487 (1958). un nuovo tipo di magnetometro per le misure magnetiche sulle rocce e . cla i . d e a ricevuto il 6 luglio 1966 riassunto. — l ' a . descrive un nuovo tipo di magnetometro, realizzato nell'istituto nazionale di geofìsica, particolarmente indicato per il rilevamento della direzione dell'intensità di magnetizzazione, eventualmente presente nelle rocce ignee. nella nota viene elaborata la teoria dell'apparato e ne vengono descritte le caratteristiche tecniche e la sensibilità; si suggerisce infine la tecnica da seguire per il rilevamento della direzione dell'intensità di magnetizzazione delle rocce. summary. — the a . describes a new magnetometer realized in the " istituto nazionale di geofìlica particularly appointed to determine the direction of the magnetization intensity, eventually present in the igneous rocks. in the present paper the a. elaborates the theory of this apparatus and describes its technical characteristics and its sensibility; at last the a. proposes the technical method in order to determine the direction of the magnetization intensity. l e misure sul magnetismo delle rocce presentano delle notevoli difficoltà dato il valore generalmente piccolo della loro magnetizzazione. i m a g n e t o m e t r i costruiti a t u t t ' o g g i (*) risultano particolarmente delicati e solo in alcuni casi e con particolari accorgimenti essi possono essere usati in un normale laboratorio di una grande città. p e r questa ragione ho cercato di realizzare un nuovo tipo di magnetometro che m i permettesse di raggiungere sensibilità e precisione elevate, pur essendo d o t a t o di n o t e v o l i caratteristiche di semplicità e di robustezza: esso è particolarmente indicato per il rilevamento della direzione dell'intensità di magnetizzazione. i l magnetometro da m e realizzato consiste in ima base di legno vincolata ad oscillare in un determinato piano verticale per m e z z o di due sospensioni bifilari. u n telaio in bachelite si t r o v a al disotto della 400 e . c i a l d e a baso ed in esso il campione di roccia in esame può essere bloccato in una determinata posizione prefissata. i l telaio con il campione può essere ruotato intorno ad un asse verticale (nella posizione di riposo del pendolo), mentre un'alidada indica la rotazione eseguita. nella sua oscillazione il campione passa sopra a due bobine e b2 in modo che il piano di oscillazione ti contenga il punto di mezzo m della congiungente i centri gx e 02 delle due bobine e sia ad essa normale (v. f i g . 1). l e due bobine, opportunamente dimensionate, sono poste in opposizione in modo che sia nulla ogni f.e.m. generata da eventuali variazioni di campo magnetico purché esse possano essere considerate uguali in gt e in 02. i l che è certamente verificat o per le variazioni del c.m.t. e quasi sempre per le variazioni del c.m. dovute a correnti varie nel laboratorio, a meno che non si ponga l'apparato molto vicino a cavi percorsi da correnti. se i due campi (in gx e in c2) sono variabili nel tempo e diversi, all'uscita delle due bobine si avrà un segnale, che, passando attraverso l'amplificatore a, viene inviato ad un oscillografo. a d ogni oscillazione il campione g, se è magnetizzato, genera un campo magnetico, che assume in genere valori diversi in g1 e in c2. con una opportuna rotazione del campione g si può fare in modo che i due campi in c t e in c2 siano praticamente uguali istante per istante: in questo caso il segnale in uscita dalle due bobine sarà ovviamente nullo. dopo questa rapida descrizione del magnetometro passerò a studiare le sue caratteristiche principali: nel paragrafo a ) tratterò della fig. 1 u n n u o v o t i p o d i m a g n e t o m e t r o 4 0 1 forma e delle dimensioni delle due bobine, eon le loro caratteristiche elettriche e magnetiche; nel paragrafo b) calcolerò il campo j3 nei due centri delle due bobine; nel paragrafo c) studierò la forma del segnale; nel paragrafo ci) parlerò della tecnica, seguita per il rilevamento della direzione di magnetizzazione delle rocce. a ) l e bobine l e bobine devono concatenare al massimo il flusso del vettore b, ma nello stesso tempo devono ovviamente lasciare libero dello spazio per permettere il passaggio del campione. la prima'idea è stata quella di costruire delle bobine sferiche, che godono della proprietà che il flusso del vettore b ad esse concatenato è proporzionale al valore che la componente del vettore b lungo l'asse della bobina assume al centro della bobina stessa. si è preferito invece costruire delle bobine cilindriche di opportune dimensioni, che godono di analoga proprietà. si è trovato infatti (2) che per una bobina cilindrica delle seguenti dimensioni (v. fig. 2) l = 0,72 d e d < 0,33 b il flusso concatenato del vettore b è proporzionale al valore che assume al centro della bobina la componente del vettore b lungo l'asse della bobina stessa. fig. 2 si trovano così le seguenti caratteristiche per una tale bobina: numero totale delle spire d 2 n = 0,24 — 402 l i . c i a 1,1) i: a resistenza elettrica d 3 e = 0,64 q — coeff. di autoinduzione l flusso concatenato a ( b ) dove si è indicato con 3 e con q rispettivamente il diametro del filo e la resistività elettrica del materiale costituente il filo stesso. . l e bobine da me realizzate lianno i seguenti valori l = 56,8 min d = 79 mm d = 26,5 min <5 = 0,115 mm e le sue caratteristiche teoriche sono 113.280 spire 33.900 q 3,71 • io11 u.e.m. = 371 e 2,68.io-6 bi u.e.m. 2,68 • io-2 weber/gauss . i n realtà le bobine da me costruite presentano le seguenti caratteristiche leggermente diverse da quelle teoriche: 91.300 spire 31,5 e q 300 e 2,17 • io-2 weber/gauss . a i capi delle due bobine si ha quindi una f.e.m. data da 0,214 ••—nel sistema assoluto e.m. <54 d i 9,1 • io-2 — bt n = b = l = «i>(£) = b n = b = l = b e = 4 j [«m^i) —4>(b.)] un m o v o t i m i di m a ( i n k t o m k t r i ) 4 0 3 e sostituendovi i valori del m a g n e t o m e t r o e = 2,17 • io2 j t (bh-bh) d o v e e è espresso in volt, b in gauss ed il tempo t in secondi. b) calcolo di {bh—bh) si stabilisca anzitutto una terna di assi di riferimento 0, x, y, z, sinistrorsa con l'origine nel centro geometrico del campione di roccia (supposto abbastanza piccolo ed uniformemente magnetizzato), quando si t r o v a nella posizione di equilibrio, con l'asse £ lungo la verticale verso il basso, con l'asse x parallelo alla congiungente i centri gx e c2 delle due bobine e con l'asse y ad essi normale; in questo m o d o il piano di oscillazione coinciderà con il piano yz. i n un generico istante il campione ha le coordinate 0, y, z, e se si introduce l'angolo a che l'asse del pendolo passante per il campione f o r m a con la verticale, si ha anche ( f i g . 3) y vi-' fig. 3 y = l sen a z = l (cos a — 1 ) [2] 404 k. c i . m . d k a dove l è la lunghezza del pendolo, cioè la distanza del campione dal centro di sospensione. i centri delle due bobine hanno invece le coordinate g1 = x = a y = o z --= il co x = — a y = o z -'-= ii si consideri ora una nuova terna di assi cartesiani q, rj, f solidale alla base oscillante e coincidente con la terna 0, x, y, s nella posi—> zione di riposo del pendolo. sia m il momento magnetico del campione fiu in esame orientato come in fig. 4: le componenti di questo momento rispetto agli assi f , rj, £ sono m.„ m sen xp cos % m sen ip sen % [3] « f = m cos ij> i n una generica posizione del pendolo carati erizzata da un generico valore a dell'angolo formato dal pendolo con la verticale (v. f i g . 3) le componenti del momento magnetico m rispetto agli assi x, y, z sono m x = nig = m sen yj cos % m,j = m cos ip sen a + » ' sen xp sen. ^ cos a rriz = m c o s xp c o s a — m s e n ip s e n s e n a. pertanto il campo magnetico generato dal campione in un punto generico di coordinate a;, y, z è dato dalle seguenti espressioni, dove u n n u o v o t t p o dt m a g n e t o m e t r o 405 0, y, z, sono le coordinate del centro del campione, cioè le coordinate dell'origine q della terna solidale con la base jj ftix^ —xmx + (y— y)my + (z— z) mz •tt x — ^ ~" ó x es e* „ _ % „ x mx + (y — y) mv + (z — z) m2 u y ~ e* ~ ó y ~ 2? „ wiz _ — x mx -f ( y — y) my + (z — z) mz xiz = — ó ~ e3 e 5 dove si è indicato con e la distanza di q dal punto considerato, cioè e = [x* + (y — yy+ (z — zy-fl. delle tre componenti di h interessa solo la componente parallela all'asse delle bobine, cioè la hz. nei due punti gx e c2 la ii z assume i seguenti valori: „ toz „ _ mx a -f y my + (z — h) mz ^ = ^ r 3 7 ' w mz —mx a+y my + (z—h) mz i l o . = ~~— o il zz v jr3 e 5 ma per calcolare la f.e.m. si dovrà fare la differenza tra questi due valori: 6 li a mx questa espressione diventa, introducendovi le [2] 6 h a m x ei. — h2 2,2 = ~~ [a+ fe2 + 2 z2 + 2 h l — 21 (h + l) cos df>* che può essere messa sotto la forma 6 h a m x dove a = a2 + li+ 2 r+ 2 hi e g = 2 l{h + l) sono delle caratteristiche geometriche dell'apparato. 406 l ì . ci a l d e a c) l a forma del segnale l a f.e.m., considerando delle piccole oscillazioni, per le quali si può porre a = ciò sen co t diventa, esprimendo b nel sistema assoluto c.m. / ^ li a co a„g sen a l 1 — — a = 3,2 • 1 0 ^ [ 5 ] si può constatare quindi clic il segnale è proporzionale alla componente del m o m e n t o magnetico normale al piano di oscillazione. d a l l a [5] si v e d e inoltre clic la e è funzione del t e m p o attraverso l'angolo a; tale f.e.m. d o v r à o v v i a m e n t e presentare almeno un massimo che potrà essere calcolato annullando la derivata della funzione i ^ or sen a l l f ( a ) = h " v c o t ^ " [ 6 ] si può ora osservare che l'angolo a m per il quale si d o v r à a v e r e il v a l o r e massimo della e d o v r à essere m o l t o piccolo: i n f a t t i tale massimo d o v r à aversi presumibilmente quando il campione si t r o v e r à ad una distanza dalla posizione di riposo circa uguale al raggio della bobina. n e l m a g n e t o m e t r o da m e realizzato si ha ào ~ 1 0 2 p e r t a n t o sarà possibile sviluppare in serie le funzioni trigonometriche che appaiono nella[6], trascurando le potenze superiori ad a 2 ; si ottiene così: 2 « 2 + h2 n ~ " m — 6 z2 p e r questo valore la / (a) diventa ' ( < x m ì ~ ~l (a2 + tjx n u o v o tipo d i m a u x e t o m e t k o 407 i l valore massimo della f.e.m. diventa quindi dove ho sostituito il valore della co del pendolo. l a grandezza tra parentesi dipende dalla posizione delle bobine rispetto al campione e, data la simmetria della funzione, tale grandezza assumerà il valor massimo per a = ir, in questo caso si ha per il magnetometro da me realizzato si ha, essendo j = 4 • io2 era, a = 8 em, a0 = 0,7, lm = 2,1 mx = 2,1 v mx (m v) dove v ed mx sono rispettivamente il volume e la componente dell'intensità di magnetizzazione del campione in esame nella direzione dell'asse ìk; poiché si possono adoperare campioni aventi un volume di circa 30 cm3, si può avere, se m ha la direzione dell'asse x l a f.e.m. assumerà quindi il valore massimo e m dato dalla [7], ma è opportuno vedere la sua variazione nel tempo; a questo scopo sarà sufficiente fare il rapporto tra la f.e.m. per un generico valore di a e la em. si ottiene cosi con la solita approssinmzione; ma, essendo inoltre a0 » a m si può porre anche 1,92 • io-2 jgla0 • ~ em = 70 m [8] e . / ( « ) . " / («a/) am 408 li. ci a l i ) h a i l generico valore di a si può ora esprimere in unità a m , cioè si può porre a — ìc am, da cui a g + g ^ = (a2 + h2) -7~ a2 6 4le2 a — g + g = (a2 + / i 2 ) il rapporto ejem risulta così molto semplificato k yk 6 + k°xella f i g . 5 ho riportato la forma del segnale che si ottiene; in ascisse è riportato il tempo in unità r, equivalente al tempo necessario al pendolo per percorrere l'angolo a m , eie è 1m a0 w ed in ordinate è riportato il l'apporto e/cm. i l segnale consiste quindi in una serie di impulsi della forma indicata nella f i g . 5, intervallati di un tempo at dato da n nel mio magnetometro si ha zi t ~ 2 " . fig. 5 m a ciascun impulso può essere ritenuto in una prima approssimazione come una sinussoide di periodo uguale al doppio di r, cioè al doppio della distanza tra il massimo positivo e quello negativo: nella f i g . 5 ho riportato a puntini la sinussoide così calcolata. ux s c o t o t i p o r.i m aox htom ktho 409 l a f r e q u e n z a di un t a l e segnale, supposto indeiìnito, è allora 1 u0 | ' g j 3 a0 r— v 2 t , < „ j / ^ n e l n o s t r o caso si ha v = 7 2 h z d) rilevamento bella direzione di magnetizzazione si supponga che il campione sia m a g n e t i z z a l o u n i f o r m e m e n t e di m o d o che il m o m e n t o m a g n e t i c o sarà rappresentato da un v e t t o r e passante p e r il centro g e o m e t r i c o del c a m p i o n e . i n tal caso si tracci sul c a m p i o n e una terna di assi q , f . 7?, j," comunque collegata con l'orient a m e n t o g e o g r a f i c o della località dalla quale è stato p r e l e v a t o . si ponga ora i l c a m p i o n e nel telaio del m a g n e t o m e t r o in m o d o che gli assi f , ?/, 4 siano coincidenti r i s p e t t i v a m e n t e con l'asse •>'. y e 3 ( v . f i g . 1). n e l m a g n e t o m e t r o allora si a v r à un segnale proporzionale a si ruoti allora il telaio e quindi il c a m p i o n e , i n t o r n o all'asse g iino a che i l segnale sarà n u l l o : ciò a v v e r r à quando il piano ih contenente il v e t t o r e m ( f i g . 6), che nella posizione iniziale f o r m a v a un angolo % con l'asse x, sarà r u o t a t o di un angolo at in v e r s o antiorario, d a t o da ax = 9 0 ° + xsi p o n g a ora il campione nel telaio, in m o d o che gli assi v\ e 'q coincidano r i s p e t t i v a m e n t e con gli assi x, z e — y . n e l m a g n e t o m e t r o si a v r à ancora un segnale p r o p o r z i o n a l e a mx = m t = m scn ip cos y. mx -mt = m sen ip cos •/. \ f i g . 6 410 i;. ciai.nr.a si ruoti ora il telaio e quindi il campione intorno all'asse z, cioè intorno all'asse rj fino a rendere nullo il segnale; ciò avverrà quando il piano tiz contenente il vettore m (fig. 7) sarà di nuovo normale all'asse x) cioè si dovrà avere mt tg /?! = — = tg1/)cos^ m-dove è l'angolo di cui si è dovuto ruotare il telaio. f i g . 7 queste due misure sarebbero già sufficienti a determinare la direzione di magnetizzazione del campione relativamente alle coordinate geografiche, poiché con esse si sono misurati gli angoli f e %. ma è o v v i o che il campione potrebbe orientarsi nel telaio in un modo qualunque; nella tabella ho riportato i 24 modi principali di orientamento del campione, indicando le direzioni ed i versi che possono avere le tre componenti del momento magnetico m e l'angolo a (fìoy) di cui deve ruotare il telaio per annullare il segnale. quesl i angoli sono espressi in funzione dei tre angoli principali al, /)\ e yx tra i quali esiste la seguente relazione: t g u l t g r i t g p \ . [9] inoltre essi sono collegati alle caratteristiche magnetiche del campione secondo le seguenti espressioni m£ tg « ! = ctg ; m nit m [10] tg/?x = — s — cos x t-gy ; tg yl = = sen ^ t,gy> un' nl'uvo ti po 1)1 al a < » n et o m et ko 411 bell'ultima colonna ho riportato il valore della componente del momento magnetico nel piano orizzontale (nella posizione di riposo del pendolo); dal suo valore dipende la sensibilità dell'apparato. infatti sia nu il valore di tale componente: il segnale avrà il valore massimo em = j. »li sen a [11] dove a è una costante dell'apparato (v. la [7]) e l'indice i individua uno dei tre gruppi di orientamento possibili del campione. l'angolo a è quello formato dalla componente riu con il piano di oscillazione ti. sia ora a la sensibilià dell'apparato, cioè il valor minimo apprezzabile della f.e.m. e, si ha a = a mi /min dove amiti è il minimo angolo di rotazione del telaio, della posizione di zero, che può essere percepito dall'apparato. mg mn m-r <= ang alo : : rotazione a; y 2 ai x — y z a2 = ai y x z «3 = ai — 71ì2 — y x z a4 = ai — 71ì2 x — y a5 = 71 — « i x y — z «6 = 71 — a! y x — z a. = 3^/2 — ai — y x — z a8 = 3te/2 — ai x z — y pi x z y = pi y z x ft = 71 ¡2 + pi — y z x ft = 71/2 + pi x — z y ft = 71 p i x — g y p* = 71 p i y — « x p, tz/2 — pi — y — z x a = 71,12 p i z x y r i z x — y y2 = ri — 7t/i z y x y3 = yi — 7t/i z — y x = vi — ti/5 — z x — y ys = 71 — yi — z x y yo = 71 — vi — z y x y-i = 3n/2 — 71 — z — y x ys 3jr/2 — yi m, v rn\ + mr to, v mi + m? m3 « 412 ii. ('iai.i)f.a così la massima sensibilità per l'angolo a è o / m i n " . a nn i n questa espressione appare la. componente orizzontale nn, certamente inferiore al momento magnetico m del campione; ma si vede subito che, essendo sufficienti solo due orientazioni possibili del campione, si potranno scegliere quelle orientazioni che rendono le componenti orizzontali di m più grandi possibile. i ] caso peggiore si avrà quando m è orientato in modo da rendere uguali le tre componenti nit, m,? e m^ di m. i n questo caso si ha. m^ — w — m^ ~ 0,8 m, da cui ~ 0,8 a m dove am è il valore massimo del segnale, quando m è normale a n: esso è dato dalla [8] e quindi, nel nostro caso km = m)m con una intensità di magnetizzazione di i o 3 ed una sensibilità di 5 ¡u v si ha a„, = 4° -10' ; mentre con una intensità di magnetizzazioni1 di 10~2 la sensibilità può raggiungere i 28' circa. queste sensibilità, possono essere notevolmente migliorate se invece di cercare la posizione, di segnale nullo, si determina il valore massimo della e per vari angoli a, in modo <la rilevare la. curva em = em (a),che risulta essere la sinussoide [11], o, contando gli angoli della posizione iniziale, cioè i>onendo a = cp — fi e m = anli sen [cp — ¡x) dove a è l'angolo che m» forma con il piano di oscillazione ti, <p è l'angolo di cui bisogna ruotare il campione per annullare il segnale (cioè uno degli angoli a, (ì oy della tabella) e pi è l'angolo di cui si è ruotato un n u o v i » l'lpo 1)1 maon'litomf.tro 4 1 3 il campione della posizione iniziale (v. f i g . 8). si può così applicare il m e t o d o dei m i n i m i quadrati e si t r o v a , determinando per es. la em, corrispondente a diversi valori di // intervallati di 10°, 1 , 2 6 t g ( p ilmj^s ! 26 [eu «en f¿l [ c m cos fi] d o v e [em sen ¡x] — 2 % sen ¡x e [em cos = x cos fi . con questo sistema l'errore su <p viene notevolmente diminuito. a t t u a l m e n t e con questo apparato si sta procedendo, in collaborazione con l ' i s t i t u t o di geologia dell'università di e o m a , al rilevamento della direzione dell'intensità di magnetizzazione delle colate laviche dei vulcani laziali. istituto nazionale di geofisica. 4 1 4 u i a l d k a b i b l i o g r a f i a (*) nagata t., bock magnetism. maruzen tokio, 1953. koenigsberger j. g., terr. magn., 43, 119, 229 (1938). f i s h e r r . a . , p r o c . r o y . s o e . l o n d o n a 2 1 7 , 2 9 5 ( 1 9 5 3 ) . runcorn s. k . , magnetization of bocks, in « handbuch der physik ». b. x l v i i, 470, springer-verlag-, berlin 1950. nagata t., magnetic properties of bocks and minerals, in «handbuch der physik », b. x l i x / 1 , 24s, springer-verlag, berlin 1966. ( 8 ) h e r z o g r . f . k . t i s c h l e r 0 . , rev. sei. lustrum., 2 4 , 1000 (1953). engineering gcoplty8ics a new conccpt a. e. scheidf.ggeh * su)diarl in :lll:ll\) g~' to "('ugi n(ll'l'ing j,h,y sil·ii". "cn"in c(, l'iul{ gco· ph ,'"i>ìc's" is ,]('fiued a!! th c cli gi lll'e l'ing uppli('atious of gpophysir:l l plu;: uo!llcua. 'l'hl 's(j ìue!m](' ]ll'(){'rs scs ucc' u1'l'ing all,vwhrl'r il'o!ll t lh' f.nrth's ('01'(\ to the il]l]l rr at!llo~pll('rr: tlm s, thpl\i i ~ \i va!!t l'angc oi po~liihl., :l}lp jìc'n tiol)s. lu th i ~ pllpcr a llis('lls sioll i~ j:h'cll (ii cngint'cri nl-( a l' plinl tin l1 8 (l i ow pllj~ ì ('~ ot ihe s o li,i j:a r th (i lwl wh 1\jl s(,is lllll]ng~", "o!t-:ulfll ngy 11114 \ gc"liu'('}la u il's), ,, ' 111(, i.ii)'8i('s 01 tjli~ h y ,l r,u'l'lwrtl (i n f llltlin g hjtl ml()~.\· ) ai .. 1 (.1 il,.' l,h.\"il il'8 (l i il, c a lll >osj.h cm (i i1 i'1ulli ll;; inl'nll ,l ocii rlmlligh l n ing), 1~ ' '''ssuk i:0 , anll ]ogthl1énto a l tcl'lii iué "1i6i(':\ illgej.tucrislicn", "gcn fl llil~a ingrgncl'ì sticu " ~i d ('fl n i~ ('c co iii(' ap!,!il' l\ zinn c all' ingcgllc l' ìn ,li fe n n!!\cni gc ofisici. qucsti l'oill]u', 'nll nn o ì 1'i'o('('ssi l'llti ,n"n'ng(li\o in illl 'l11alulil]u c j,unto tlalnudllo il ell .\ '1' en ll all'altn al'liiollfl'nl.; ili lin i l'enor jllc ('a!lljjo i] i i''' s!;il) ìli al'plìl'azinni. in '1 \1eal(l 1[1\'01'0 "ieue fatta unii ,!i s('u8sio llt: sull'ap. pl il'l\zi ouli all'ingl'gn cri,\ ,iella fil>i<'n ,iella terra ~nlilla (<'o!!\ih' elhl en tc sisjii(ilogia \· ulcnnn logia c gcu !llc('culli (' u), lid i" fisil'a tlel l'i ,il'( i ~ fci'[i (indudcllilo j'iol rn l"g ia ) e infine .iell a l:i~i l'a ii llj\' a t mlll,f" )'a (in l'hlt\l! nt e i h.rum ] .. c i inlmi .. i). 1. l'xt1wduc'tio!'i". if ono tr a mlates tllo wol'd ugeophysies" from ol'(!tl k inliu bngli.~h) it signilies uthe na.ture (lf th o eal'th". meant is (lf e(h\rse the ,~tully of the nature of thc eal' th. in the motlerll conno ro t io n, the \l'(ml uphysi es" ,1oc,'1 not mc... .. n " na ture" in gt'>uel'al ali)" more, huìt he worki ll gs o [ t he ma·tcri als in v ol ved oxc hulin g changcs i n compos itio ll . • t cc hn ilw hc hod, sc lml e \yil~n , jw;iìl ut flil' (: l'o jlii ~'si ]i a i o~jl \rien kl\rl!;],lat z 13, au st r ia, .-\. l: .. ~cheideggf.]( by its (1l'jinition, "geophy~i(ls" ih therefore the ~t.ndy of the physies of the ii:al't.h. c'ommonly, thi~ inelndes no\\' the gamut fnnll t.he eart.h's (10m lo tlw f!olat-teltestriai l'elat.ionship~. app!itèatùm8 of geophysies to the economie fiehl have, t.hlls far, mostly been coldìlwd to geophysiea.l explomtion. it is dpar, llo\\'cver, that the~e applieations could be vll-~tjy extend('d, ina~mneh as many as}jeet-s of gcophysies iend themflcl"es 1.0 engineering nses. tn a,ualo!!y with "engineering }jhysies", i like t.o (iesignat", the systematie flhhly of engineering applieatiom of geophysks as "engiucering g'co}jhysie.,;". some pngineerini{ applicatioufl of gpollh.\·flies are, of eonl'~e, weii known, ~ueh as engineering seismology, bnt on t.he whoie, geophy~ie~ ha~ not. as sy~tenlll.til'aiiy hecu ap}jlied to engiueeriu!! a~ it might. have been. tn this impcr, i only }jlan to diseuss some of t.he engiueerini{ alliiiit'atiom of geo}jhysie.,; tha.t might 1m usdul in pra.et.ieal \\'ork. 'l'o give a eomprehemiye revip\\, woiù(! be (luitc impossible as t.he subjeet is vaflt" 'l'hl' physies of thc eart.h is eonutlotdy di"ide(! into the ilhysies of t.he solid earth, the ilhyhic~ of the hy(lrosphere a,thl the phy~ks of t.he atmosphere. i n \\'hat fouol\'s, r wiii keep t,]w sani(' (!i"ision. ~_ k:\gi"xeert:\g geopilysics oi' 'l'ile solid ear'ril fol' man, t.he ~ill'faee of the solid parth is ilrohably t.he itlost. important l'l'gioii of the rnivel'f!c becanse hl' !ives thereon. fn t.his ponneetion, the fast or siol\' ehanges of t.his surfa,ee are of pa'l'tienja.l' signilil'anee, ~inee the:.re}jre~ent. a,n alteratiou in hi,~ habitat, these ('haltg'es are gcncrally ('ause(! by "gl'o"-llltysieai pro('psses i\'ithin an(! wi!hont. the ii:arth. pl!,~l ('hhng{',~ l'an oeeul' a~ veritable ea.ta~tl'ophe~, siow ehanges, 110\\'evel', ean be jll~t. lui serious althollgh thcy may l:wk the illlttieularly spee.ta,enja.l' a~pect.s of a wddenly oeeulting disaster. if (1)(' thinkfl of (lisasters, some of t.he most. feal'ed ones a,l'e earthqnake~, ii:art.hquake~ are flhoeks that. muaiiy oeclli' without waming; they l'u!!clhlel' sndden disphwement." 011 the earth's sllrfaee that. ea.n eanse bnildiugs to {'ollap~e_ the stndy of eal'th(iuakes is ealletl "seishjology"j "engineering seisnlology" is 0l!p of t.he few hm,nehes of l'n i{ineering geophysies \\'hieh has all'eatly reeei"ed sonw at.t.ention. thus far, i\'hat. t.rigg(\l's an e:wtltquake is nut knol\'n, lt 1m .. ,; bepll fonntl that engi s eerl )o:g geq piiy s i cs a se\\" cq~c~; i't carthqllakeli du )\t.er slllldll ll y in ccrblin rcgiun li (n ot.'\bly thc l'il't'lim.p~i{'i!1c belt) , bllt. whnt act.\1l1lj~l'amc~ an cal'thql\ake t.o oc('l!r, is not kllo\\"n. thus, thc gelhwallsoismk l'ihk l'an be tlet ermino ll fol' a (·t;lt.ain area, but a pl'edicti(m of ellrthqllake~ has no1. ~'ct h(~e l1 n('llieved. in('.itlolltally, thi~ is a rathcl' "hotl! ~ilbject jmt. no\\". 1;:art.hquakes are gco.l.rll a mi ('. phenomen:io; i.c. il li1'iiollloj!ll bahicall~' call!)!!l l by j)]'o('{' s~es tnkinj,! plaee in t.hc eal't.h's interi!)!'. of a similal' orijriu :11'0 volcanic erlll't. ioil s. sudi el'uptioll ;; 1i:\\'c \'; 111 1$(\\1 sl)c('tm' ijlar d hi:ui tce l'~ in hi6torìcnl timeli; one necii onl~' i.hink or t.he diha.~t.c .. s ai. he l'cii liilciiiii ami p ompe ii ,\1111 (i{ ml.. pell'e. tbu l'\ , lli:ion y (·olllll.ri eli ('ollid makc gootl engillccrin~ ulie o[ gt'ol)h~'sieal ohsen'al.i olls on yolcanocs ilnd fl'om an ilntlerlitnnding' t)f thei .. nu'('ilh-ni~lll. of dircet apillieability tu t!1lgineering are geoljh ysicnl events ocnil'ing quitegencmlly on "~i(lljes ". i mean any kill11 or 1i1i1p~lj: ex('ayat,i(lj1 ij, motlntain si.-ies, embanklllt'nts etc. qllestiom o[ tlt e litability (if mic.h slopcs hayi' hoe.11 stlld ie ll l'xl.enslvel.y by ciyil 01l ~ilh'('.1'8: t.lte $!"co}lhysieist , howevcr, wii'lh\!s hl go hll'tilcl', ina~mtlclt lis tllo entil't'. p hys ics or tlw mo t ioll, illdwlill~ the ]>1iysicli or t1l c vi!i): l'o ll a,p!ie, is or intce l"q!it to{) hi.m. contral'iwise, thc (·ii /..>1l1oor ili liot {'olleerncti \\"illt t.iw la t,l.cr, bc(~ausc his i1hli li a im is to prevfll t iiiici! a li oc('(ln "q i\('('" nevi··rt. hclc~s, t hc mcehallil'k nf i~ la·lllllslide, if known, {'nulll almi be imt tn engineerin g me, e.g. [or th o e",' lilllation of t·lle safety (il n site or in t.he lle~ign of ill'oteetive mollsuros. stahility cf;timah!~ on ~ ioijcs oftcn de polltl on a kjl()\\'lell~o lif the physics of a ilotential ~ lid e. stran~dy e uough, t.hc physics or a m,lj(l[' ~lido is stili largely a ill!l tln (il spe('ulnti(lil. the otllillal'~fri(,t,ioll hct\\"ccn t.1w j'(wks is sim pl.\" too gl'cat. fol" li ilmjol' sl itl c to o(:c ut i~t l\1 1. one sta·mls h el'c be[ol'e il g l'ca t ll u:nlt! . f:hhuu s cases a re tlte s1 ideli al. pmnk, ai1j{'rl.~l in 1903 :liul th e iilil1 e at v.ljont in ui)iicl' lla l ~' in l !)(i;i. t!t(, 1111.1.1' " \hul li fla .. l.icu liu· i~' d isas troll )\ e ven t .. a tlam \\"a<! hll ilt nl'wss :io 11~p valley 1.0 c'reate a· !al t· hcllilhl it fo r a hytlroele('tl'it· pow ci" il cve!olj!lltmt.. wih'll thc lake \\"as lillillg' up, thc pol'e pl'c~sure ri~e (·ii·usetl t:hcrcby apljarentl y madt! one of t;!le 1i1njles llnst.ablc. jt. starte!! to crcep: t,his in it heh wn.~ no ('anse or co llc.crn to the engi1lt~i!i'h, becl\use ali .~lopcs cl'eep ovt~i".\· "· here an)"\\"l.~·. 1' 110 c.nw}>ing mot.lon !u·c'('jcmtell eventually t u :w celltilllt<tl'l'li lh'1' day on the ila)' hefore tlt e cat.l ss t.i'o})he, hut lioblllly [onmw t·ile d isas t.et· t.h llt then fo ll o\\"ed in side of :\. shol'li !.ime: 2011 !llillion c' ubi c'. nwt.cl'~ or !'od: mater ia l slid illt() i.he l:\.ke, wit,h a velo{'ity or a bollt :!5 m {r:;c<" {':hl li in~ li iire mendolls fi()(klw i ~ve i (i.he d llm hel~ l !) 1,0 "l,ili illl'v lite va lle)" bi'low wit h a dca.t.h toll or fiomc 2(}ou pcol,le. 44 a. e . scul'\idegger similariy large rpccds have been observed in other ca.ta~tl'ollhic irmd slides. by all struhlan[ ilhyricf11 theory, snch uvcnt.<; shollld bc mechanitally impo.~8ible. thc f!iiding surfaees are not steep, }icrha-})s iu'olln([ :!oo, weli below thc a ngle oi repof!e of the materia.l in question ana the rock frietion should m\llf!e auy moving ma·ss to stop rather quickly. how then cau the almo~t tota.lloss oi friction be ex})l<due([ ' pore watpr prerf!lll'c a nd thixotropy has certainly something to do with it, bllt a.lonc it is !mrdly sullicient to reduce the friction to llhnost zcro. thc la.test conjectnres are that the hlù[e movcf! ou an a.ir or steam cllshion, }icrhaps caused by the cva-})oration oi the pore water duc to thc hea.t created by the initial hietion. the accurate followiug u}i of such conjecture8 is, however, a difficult thermo([ymunie ilroblem whieh ha~ by no mcauf! beeu ~oive(l. evidently, the f!im}iic iaws oi meehanics are no ionger suffieient and othcr iields of }ihysies, like thermodynamics, !w.ve to be intl'oduced. who was at iault in the yajont disasted the engineers cau hanlly be bimnm[, beca1l8e the event could not be foref!eeu. iiowever, a bit more "engincering gco}lhyf!ical" rescareh of the behavior (and not just of the ,~tability) oi slopes before the event might have }h'ovided the imight that l'onl([ have given the engineer8 the mea,nf! to foresee the vossible im}lliea,tiom of hih n.etionf!, an([ to mo([ifyis ([esiglls aeeor([ingly. great disastcrs with ilw.ny cwma,lties temi to ma,ke news, becausc thcy are s}lectacnlar. minor events, like rock iai18, the motions of singie boul!lerf! ([owu a f!io}le etc. are just as f!erionf! for tllo.~e affected by it ah a great dis a.ster to the vietim~ oi it. .a motorist killcd by a rock 1'01ling outo the highway is just a,r dea.d as any oi those peol)ie killcd by the vajont 1100([ wa:ve, although he will sca.rceiy make the l)apers. similarly, many slow phenomeua ma.y uot cause ([eath, lmt great 1i011lcrty ([amage. one oniy ha.s to think of thc city of venice sinking into the sea,. a.8 uote([ alxrve, ah siopes creep, "\vith sometimcs sedous el1"ects on blidge abutmcnt.'! ctc. much of the meeha,nics of suc11 events is oniy inl}lerfectly known, in spito of ma,ny stlj([ie~ by civil eugineers. :{. l<ji\ginerlu:'i"g geol'llyslcs oe 'l'1m llydrosphere engiueering apl)liea,tions of hy([rology are nw1wrolls. i oniy need to reeau thc }ihysicf! of flow through 110roilf! me([ia with its appiica,tion to groundwater stu![ie~. in the undersatumtc([ zouc, aga.iu uot ks"gl~eeri~g gl;opiiysics _ a "se\i' co~cei't olliy meehanics, but, ai.~o thcrm()(l~-lla.lllicfl anil the moieeular behaviour oi air ajl(1 moisture ims to be il~('d . . as on the solid eart,h, t,l\!' h:plrosi)herp also ma~give rise to spect:wular disasters: flooilwaves, t,ilial \ntves, tsunami~. sudi eyents iead t,o iliffieult matllpmaticai lh'ohiems, sinee all tlw ilifferent,ial eqnations are llonlinea-r. floods may oeeur hy a. combination of meteol'oiogica.j rollditiolls. att-empts havc beell m:hle to 1)l'ellict tlw orcurrencp of sueh 1iood:'>. thus iar, oniy statist,ica.j prognosticat,iom can be mallc. one tn'ats the "hyllrogravh", that is the t-emvorai,~pquenee of wat.er leyel readings, a.s a "timi' seric~". 'fhe flt,atistiell.1 lll'operties oi sueh time series can be analyse(l ailil describell. from thi.~, prel1ictiolls of the generai frequency of the oeeurrence of a jm-rtieular size tlooll, if not the time thereof, can he maile. less spectaculil'l', but of great importa.nee, is the meehanic~ of erosion: the near-shore circula,tion ~~-stem on a coa~t, is resvollsible ior the evolut,iou oi beaches ek., meajl(lpl's create river termees, and gllllying the iiegra.iiation oi a sioi)('. glaeiers are also vart oi the hyiirosvhere. for engineering ihu'l)oses, they haye reeently gaine<l mm'e and morc imvortance, because develovmeut~ a.l'e 1l11she(1 to thc limit,s of the hahitahle areas. 'l'ims, ncar mattmark in the cant,on oi wallis in swil,zcrla'llil, a workman'~ eamp (in eonneetion with the huililing oi a dam) was locate(l below a glaeicl'. oue aftel'lloon in 1965 a iarge ma..~s oi ice, ].6 to :!.o . l o~ m", hl'oke oji that glarier aihi killml 88 meno an unexi)e(·ted "surge" in the ice imd takcn piace which lell to the brcak-oit when it reacheil t,ile giaeier tongue. needless to say, t,hl' a{'·cidellt gave some imllllt,us t,o t,he ~t1hly of the ther11l0dymuuics of glaeier :mrges; but this wa.~ iike locking the barn aftcr the horse had heen st,oleu. many more exn.mples of ellgilleoring glaciology and engineering hydrology eoulll he eited, but the aboye will have t,o :mhire . .1. .enginbrrikg geopiiysics of ttte a'r:j.losphrrr. l'inally, a. fe\\' words ahout, the l)hysics of the atmosvhere. bilgilleeriug alllilications thercof a re mo:,>uy col]('(',]'ih'11 wit,h wind stress. the willil stress ean 1m substa.ntiai, a.s is eviilenced by t,hl' ilefltructioll eause(l by torua<loes. l\luch research nee(l~ still lo be (ione 1:0 \lllilerstaihi this vheuomenoll, t,o lu'('(liet, it allll to attive at, design criteria for 4[; a. e. sc !i }~ldl':gger constnwhng houscs wich will noto collll.psc whcn hit by a tornll·do. uri to thc pl"cscnt, 'l('ry lit.tlc is done iu t·his l'cr"anl. in tornaao arel1s, ()]le ~iml'ly trust.s c;·od. perha})s onc ha..~ pl'oceeded to pro'liac a w~lorning systelll of imlllineut torna.aocs, but this is noto of too lllueh use if one does not reu,lly ha'le a .~afc plaee in a.n ol'diual'ily built ihm.~c to g-o to. fn conncetion ",ith a.t.mos})herie vheuolllcua, one should al.~o mention the physies of lightniug a.s ]jossibiy leading to some engineeriug a})]jiicu.tions for the rll'otcction of li'lcs and proljel'ty. one lllight pcrlmps think that lii{htnini{ causes only miuol' distl'ess, but. at t.he ena of augu.~t, 1!j72, an oh tankel' bie\\" up when stl"uck. l\iueh minor dmnage is 1i.lso (~u.llsml by lightninl( ",hich reftcct~ it~elf in insumnce rates ajl(l such like. 5. co:\"cl1:sio?\ the above l'cmarks were int.ended t.o jll'esent, afe", typical ar1l1lica,tions of gcophysies to engiueel'ing problellls. i like to eu.n this "engineering geo]jhysies". thc cngincc!' necds to know t,he basie jlhyllies of the phenomena oecul'ing on the eal'th's sul'fa,cc with ",hich hc has t{) dea.j. 'rhc l'equil'ed 'lalues of the ex})eeted stre.~se~, fl'cqucncics, 'lelocities etc. ea·u be ]jl"oviaed by the gcor1hysicist who, as f\ fun(la.ment,al seientist., is rlcl"harl.~ best qualifiea to study the pcrtiuent })hencjlneuf\ per ile without being under t,he pl'essnre t,o finish ,iob aft.el' job. layout 6 the effects of zoning methods on forecasting of the next earthquake occurrences, according to semi-markov models ramin sadeghian1,* 1 bu ali sina university, hamedan, iran annals of geophysics, 53, 5-6, 2010; doi: 10.4401/ag-4558 abstract earthquakes are natural phenomena that can be viewed in three dimensions: time, space and magnitude. earthquakes can be investigated not only physically, but also mathematically. in this study, semi-markov models are applied, which can be considered as useful methods to analyze and forecast the occurrence of future earthquakes based on previous earthquake data. in the present study, the target region, iran, is divided into zones, and each zone is examined as one of the semi-markov model states. several methods to determine the levels of forecasting error are then introduced and applied to the target area. the results of the application of these semi-markov models to investigate and forecast the occurrence of future earthquakes are obtained and analyzed mathematically. a new zoning method is developed and compared with that of karakaisis, through the proposed forecasting method. moreover, the effects of the type of zoning and the number of zones on the forecasting error of the next earthquake occurrences are investigated using several algorithms. editor's note. the present paper revisits and completes the paper by sadeghian and jalali-naini [2008b] cited by the author in the references. introduction earthquakes can be examined both mathematically and physically [sadeghian 2007]. stochastic processes represent one of the branches of mathematics that can be applied to probabilistic investigations of these phenomena, and one of the stochastic process models that has been frequently applied over recent years is the markov model, and especially the semi-markov model. semi-markov models belong to those that can model events that have a relationship with previous events [jalali-naini 1997, minh 2001, rice 2007, sadeghian and jalali-naini 2008a]. for analyzing of the three temporal, spatial, magnitudinal dimensions of earthquakes, markov models, including semi-markov models, can be applied [patwardhan et al. 1980]. markov models have already been applied for earthquake occurrence analysis [di luccio et al. 1997, console 2001, console et al. 2002]; moreover, in this regard, a few studies have recommended the use of semi-markov models [patwardhan et al. 1980, altinok and kolcak 1999, sadeghian and jalali-naini 2008b, jalali-naini and sadeghian 2009]. one of the advantages of the model applied in this study is that it can forecast this triad of dimensions simultaneously, which has not been provided in previous studies [kelleher et al. 1973, nanjo et al. 2006, papazachos 1992, rikitake 1976]. for every semi-markov model, and the inspection of each event, several states have to be defined. in this study, the iran area was chosen as the target for study. seismologically, this area was divided into various parts by karakaisis [1994] (k94), and also in the present study. each part is called a zone, and each zone can be considered as a state of a semi-markov model. a similar classification has to be used for the magnitudes of the earthquake occurrences too. using the proposed semi-markov models, later earthquakes can be forecast probabilistically. then, to demonstrate whether the forecasting results obtained are mathematically valid or not, several methods are introduced to calculate the differences between the probabilities of the occurrences of actual earthquakes and those forecast, which from now on is called the forecasting error. nava et al. [2005] presented another method for validation of their model. the forecasting error of the algorithms presented is obtained, and the results are investigated and analyzed. moreover, a deterministic forecasting algorithm, based on the provided probabilistic one, is presented, and results related to deterministic forecasting are also mentioned. modeling applying semi-markov models in the forecasting of the dimensions of earthquakes was completely examined by altinok and kolcak [1999] and jalali-naini [1997]. one of the most important elements of the semi-markov models is the interval transition probability matrix. the probability of a transition from state i to state j in the interval [0,n] requires the process to make at least one transition during that article history received february 2, 2010; accepted october 4, 2010. subject classification: semi-markov model, transition probability matrix, probabilistic forecasting, deterministic forecasting, forecasting error, k94 zoning. 83 interval. the interval transition probability matrix can be determined in matrix form as follows: (1) where n is the number of time intervals, wg (n) is a diagonal matrix where the i-th element is equal to , and is the probability that the waiting time in state i is greater than n, and g is a transition matrix, so that gij is the probability that if the last step is in state i, the next step will be in state j. in other words, gij is the probability of transition from state i to state j. also t(m) is the holding time matrix, which is defined as follows: the interval transition probability f(n) is obtained by a recursive procedure. since t(0) is equal to zero, f(n) is just obtained for the interval 1≤m≤n. in the case n=0, f (n) is equal to the kronecker delta or identity matrix, which is defined as follows: also, c (m) is the core matrix, which is defined as follows: where n is the total number of states in the system [jalali-naini and sadeghian 2009]. for the earthquake phenomenon, f (n) can be used to study earthquake hazards and to evaluate their risk. the two interval transition probability matrices that consist of fr(k) n (for region-to-region transitions) and fm(k) n (for magnitude-to-magnitude transitions) can be determined through equation (1). if the last earthquake occurred in region r0 with a magnitude m0, the matrix of probabilistic forecasting after k; n time periods (i.e. n) is obtained by the following formula: (2) where r is the number of zones of the assumed area considered, m is the number of classes for all magnitudes considered, and k specifies the number of time periods. therefore, by computing , all of the three dimensions of any earthquake can be taken into account and specified. as in any region, any earthquake with any magnitude can occur, the region and magnitude can be considered as two independent random variables. consequently, based on probability theory, if xand yare two independent random variables, f(x,y)=f(x).f(y), where f is a probability density function [jalali-naini 1997, rice 2007]. the same relationship is also demonstrated by equation (2). also is the probability that an earthquake will occur in the region ri with the magnitude mi after the time period k. figure 1 illustrates this more clearly. in this way, forecasting the dimensions of following earthquakes is possible, through determining the probability forecasting matrices (i.e. ). in this study, the probabilistic forecasting matrix and deterministic forecasting matrix are demonstrated by the and frm symbols respectively. for model validation, assume that is a probability forecasting matrix, where its elements are the estimated probabilities of the next earthquake occurrences according to our proposed model, during the next k time periods, and fmr (k) is a deterministic forecasting matrix defined as follows: there are three common ways for calculating the forecasting error: mean square error (mse), mean absolute deviation (mad) [rice 2007], and mean absolute percent error (mape) [haj shirmohammadi 2003]. let fi be the forecasting value for period i, and di be the real data for period i, then the forecasting error (ei) is calculated by ei = di − fi. these three attributes for comparing forecast precision can be used according to the following relationships: using semi-markov models for zoning 84 ! ! ! ! figure 1. the ways that the matrix is determined [from sadeghian and jalali-naini 2008b]. ( ) ( ) ( ) ( ) ( ) ( ) ( ); 0, 1, 2, ... f n w n g t m f n m w n c m f n m n 0 0 g m n g m n = + = = + = 7 = = / / ( )w ni g ( ) pr ; 1, ..., .t m t m m nij ij= = =" , (0) 1 0 f i j i jij = = ! ' ( ) ( ); , 1, ..., , 1, ..., .c m m i j n m ng tij ij ij= = = 1, ...,k =6 1, ...,k =6 1, ...,k =6 ( ) 1, ...,frm k k =6t 1, ..., ; , ..., ; , ..., frm ( ) ( ) ( ) k r j m k n k fr k fm k 1 1 r mr m m m0i ji j j =# 6= = = t ( ) 1, ..., ; 1, ..., ; 1, ...,frm k i r j m k nr mi j = = =6 t ( ) 1, ..., ; 1, ..., ; 1, ...,frm k i r j m k nr mi j = = =6 t ( ) 1, ...,frm k k n=6t frmt ( )frm kt with a magnitude in th time periodm k otherwise ( ) ( )frm k frm k i jij r mi j =/ if an earthquake occurs in region r1 0 * frmt mse e 2i i= n / mad ei i= ; ; n / 85 where n is the number of historical data for the forecasting. note that the mse and mad are not scale less, while the mape is scale less. therefore mape is more suitable than mse and mad to compare several forecasting errors together. here, two fundamental questions are posed: the first question relates to the probable estimation of earthquake occurrences. in other words, can forecast the next earthquake occurrences successfully? the next question is to find out whether can be used for forecasting the next earthquake occurrences not only probabilistically, but also deterministically. the response to the first question can be found in methods 1, 2 and 3, and the response to the second question is explained in method 3, as: method 1: mse and mad [rice 2007] method 2: mape [haj shirmohammadi 2003] method 3: an innovative plan that can be used for determining probabilistic forecasting, deterministic forecasting and their forecasting errors (in this study, this is called the zero and one (0-1) method technically). in methods 1 and 2, if the total number of the past data is equal to n, the first n1 data will be used for forecasting the next n2 data, which will be used to determine the forecasting error (i.e. n=n1+n2). with all of the n data having already occurred and being available, the forecasting errors can be calculated. in method 3, the first n1 data are used for forecasting the next n2 data, which is applied for the making of a pattern, and the first n1+n2 data are used for forecasting the next n3 data, which is applied to the determination of the forecasting error (i.e. n=n1+n2+n3). the whole ndata have already occurred and are available; therefore, the forecasting errors can be calculated. in these methods, calculation of the forecasting error can be performed in two forms: by using successive data or by using random data within the set of whole data. in the first form, the first n1 data are used for forecasting the next n2 data, (i.e. n=n1+n2), while in the second form, the n2 data within the main n data are eliminated randomly and then they will be forecast by their past data. method 1 in this method, the relationships between mseand mad are used. for this goal, an algorithm is presented for mse, which can be used with some changes for both mad and method 2. algorithm i: this algorithm can be used to determine the forecasting error by mse as follows: step 0: begin step 1: n2 (0)=0 step 2: i=0 step 3: use the first n1+n2 (i) data to determine , frm (n1 is the number of the first data that can be used for forecasting the next data, and n2 (i) is the number of data that occurred during i time periods after the first n1 data). step 4: determine , frm step 5: (3) step 6: i=i+1 step 7: if then go to step 3; otherwise go to step 8 (n is the number of total data) step 8: step 9: end. for calculating the forecasting error by mad, use algorithm (i); just be careful to substitute steps 5 and 8 in algorithm (i) by considering the following steps: step 5: (4) step 8: for each forecast, if the past data are more complete, the forecast results are more accurate. therefore applying after gives more accurate information than . accordingly, only (1) and frm (1) are used in equations (3) and (4), and in the remaining equations. one of the disadvantages of this method is that neither msenor madhave any mathematical interpretation, nor is there any benchmark for their degree of goodness. in other words, if an amount is obtained for them, there is no common mathematical interpretation for every one. for example, by mse=2, regarding the forecasting error, it is not clear whether 2 is a suitable forecasting error or not. method 2 in this method, a similar equation to algorithm (i) is used, yet steps 5 and 8 in algorithm (i) are substituted by the following intended steps: step 5: using semi-markov models for zoning 100mape d e i i i= # ; ; n / frmt frmt frmt frmt ( 1) ( (1) (1)) mse i frm frm 1 2 1 ij j m ij i r + = == r m# t// ( )n n j n1 2 1j i + 1 = / ( ) mse i mse k 1k i = = / ( 1) (1) (1) mad i frm frm 11 ij j m ij i r + = ; ;== r m# t// ( ) i k mad mad 1k i = = / ( )frm k1t( )frm k2t ( )frm k k1 2+t frmt ( 1) (1) (1) (1) 100mape i frm frm frm 11 ij ij ij j m i r + = # ; ;== r.m t // (5) step 8: where, as indicated above, mape is the mean absolute percentage error. mape is a useful equation, as it specifies the error percentage, which is both comprehensible and interpretable for every one. therefore, this method will be concentrated upon in this study. method 3 definition: the i-th maximum order the i-th element in a descending sorted list, where none of the elements are equal to each other, is named the i-th maximum order. this method is an innovative plan that can be used for two goals: a) determining the forecasting error b) deterministic forecasting of earthquake occurrences. a) determining the forecasting error in this section, an algorithm to determine the forecasting error is presented. this algorithm has two parts. the first part of the algorithm is devoted to making a pattern, and the second part is assigned to determine the forecasting error. this algorithm consists of the following steps: algorithm ii: step 0: begin step 1: n2 (0)=0 step 2: i=0 step 3: use first n1 +n2 (i) data for determining , frm step 4: determine , frm step 5: step 6: i= i+1 step 7: if then go to step 3; otherwise k1 = i and go to step 8 (n3 is the number of data used to determine the forecasting error) step 8: for i=1 to k1 do step 10: i=1 step 11: mij={i-th maximum order in } step 12: ([a] obtains the greatest integer number smaller than the real number a) step 13: all of the elements greater than or equal to 1 in , are replaced by 1 step 14: step 15: step 16: i= i+1 step 17: if i-th maximum order is available in then go to step 11 otherwise go to step 18 step 18: if mape(t+1) is the first element greater than the mape obtained in method 2, the i-th maximum order is the most correct one to be considered as a benchmark to calculate the errors. step 19: n3 (0)=0 step 20: i=0 step 21: use the first n1 +n2 +n3 (i) data to determine , frm (n1 is the number of the first data which can be used to forecast the next n2 data, which can be used in making a pattern, and n3 (i) is the number of data that occurred during i time periods after the first n1 +n2 data) step 22: determine , frm step 23: step 24: i= i+1 step 25: if then go to step 21; otherwise k2 = i and go to step 26 step 26: mj={t-th maximum order in } step 27: ([a] obtains the greatest integer number smaller than the real number a) step 28: all elements greater than or equal to 1 in are replaced by 1 step 29: using semi-markov models for zoning 86 ( ) mape i mape k 1k i = = / frmt frmt 1 ( 1) (1), 1 ( 1) (1)frm i frm frm i frm+ = + =t t t t ( )n n j n n1 2 1j i 3+ 1 = / 2 ( ) ( is a zero matrix)frm i o o=t 2 ( ) 1 ( ) 1 ( ) 2 ( ) 100mape i frm i frm i frm i 11 ij ij ij j m i r = # ; ;== r.m t // (0) 2 ( ) mape k mape k 1 1k k1 = = / step 9: 1 ( ), 1, ...,frm j j k1=6t ( ) 1 ( ) , 1, ...,frm j m frm j j k2 1 ij = =6t t; e ( ) 1, ...,frm j j k2 1=6t 2 ( )mape j = 1 ( ) 1 ( ) 2 ( ) 100, 1, ..., frm j frm j frm j j k 11 1 pq pq pq q m p r = =# 6 ; ;== r.m t // ( ) 2 ( ) mape i k mape j 1 1j k1 = = / 1 ( ),frm j j =6t 1, ..., k1 frmt frmt 1 ( 1) (1), 1 ( 1) (1)frm i frm f m i frmr+ = + =t t ( )n n n j n1 1j i 2 3+ + 1 = / 1 ( ), 1, ...,frm j j k2=6t 2 ( ) 1 ( ) , 1, ...,frm j m frm j j k j 2= =6t t; e 2 ( ),frm jt 1, ...,j k2=6 1 ( ) 1 ( ) 2 ( ) 100 , 1, ..., frm frm frm j k j j j 11 2 pq mr pq pq qp = =# 6 ; ;== r.m t // 2 ( )mape j = 87 step 30: step 31: end. note that in algorithm (ii), mape can be substituted by either mse or mad, but because of the aforementioned reasons, mape is preferred. b) deterministic forecasting of earthquake occurrences section (a) of method 3, along with the probabilistic forecasting matrixes, such as , can be used for deterministic forecasting of earthquake occurrences. considering algorithm (ii), and using the following steps, this makes it possible to forecast the earthquakes occurring in the next several time periods, deterministically (i.e. one and zero; so that one specifies the occurrence and zero specifies the lack of occurrence of any earthquake). algorithm iii: step 0: begin step 1: use the past ndata to determine (n is the number of total data) step 2: determine step 3: (k is the number of predictable time periods in the future) step 4: mj={t-th maximum order in } step 5: ([a] obtains the greatest integer number smaller than the real number a) step 6: replace any element(s) greater than or equal to 1 in by 1 and name the resulting matrices step 7: if frmdrm(j)=1 then an earthquake in region rwith a magnitude m in j-th time period will occur, otherwise any earthquake in region rwith a magnitude m in j-th time period will not occur step 8: end. application in this section, the earthquake occurrences are forecast probabilistically and deterministically using the actual data collected from the iran area. then, two zoning methods of the iran area are introduced and considered, consisting of zoning by k94 and a proposed zoning in the present study, and the forecasting error for these zoning methods is determined. the iran area was selected as the area of investigation. this area is bounded by longitudes 44.23˚e, 63.33˚e and latitudes 25.05˚n, 39.78˚n. the data were collected from the united states geological survey website (http://neic.usgs.gov/neis/epic/epic.html). after filtering and removing the unsuitable data according to table 1 [gardner and knopoff 1974], 3,179 data related to earthquakes that occurred during 1973-2007 are used. the maximum time interval between the times of the earthquake occurrences is 45 days, so by considering each 10 days as one time unit, forecasting during the next time units in each time will be possible. also, each zone in each zoning method is considered as a state in the region-toregion transitions of a semi-markov model, and the magnitude of total occurrences is classified into 5 classes for each of the two zoning methods, and each class of magnitudes is considered as a state in the magnitude-tomagnitude transitions of a semi-markov model. zoning by k94 was carried out according to occurrences or epicenters, while the zoning in the present study was carried out by fault lines. in other words, k94 considered the historical data of earthquake occurrences according to their longitudes and latitudes, and classified them into 22 classes; while the present study considered the historical data of earthquake occurrences according to the longitudes and latitudes of their fault lines, and obtained their linear regressions, and then classify these linear regressions into 10 classes. since the epicenters are positioned on the fault lines, zoning by fault lines can be more accurate than by epicenters. a) zoning by k94 (based on epicenters) karakaisis [1994] divided the iran area into 21 zones. in this section, his zoning method is applied. k94 does not considered the center of iran as a zone; hence the zoning is used to divide iran into 22 zones, i.e. 21 zones by k94, plus one central part, as shown in figure 2. also, the magnitudes of the past occurrences were classified into 5 classes, as follows: (6) using semi-markov models for zoning 2 ( ) mape k mape j 2 1j k2 = = / frmt frmt frmt ( ) ( ) 1, ...,frm frm i i ki1 = =6t t 1 ( ), 1, ...,frm j j k=6t 2 ( ) 1 ( ) , 1, ...,frm j m frm j j k j = =6t t; e ( ), 1, ...,frm j j k2 =6t ( ), 1, ...,frm j j kd =6t 10 45 5=` j table 1. a reference for distinguishing foreshocks and aftershocks from main-shocks [from sadeghian and jalali-naini 2008b]. : 3.6m mb1 # : 3.6 .m mb 4 82 1 1 : 4.8 .m mb 5 43 1 1 : 5.4 .m mb 6 34 1 1 : 6.3m mb5 1 these classes were obtained through the agglomerative nesting (agnes) method, which is a technique for data clustering [ghazanfari and rezaei 2006, jalali-naini and sadeghian 2009]. the minimum of the considered magnitudes is 3.1 mb (body wave magnitude) and their maximum is 7.1 mb. with respect to the k94 zoning in figure 2 and equations (1), (2) and (6), the transition probability matrix for both the magnitude-to-magnitude and the region-toregion transitions are obtained as in tables 7, 8a and 8b, which are included in appendix 1. by applying the data in tables 7, 8a and 8b and equation (1), interval transition probability matrices in both magnitude-to-magnitude and region-to-region transitions are determined. by using these and equation (2), the probabilistic forecasting matrix for the next 5 time periods (i.e. the next 50 days) after normalizing are obtained (see also tables 9a and 9b in appendix 1, and more comments in the modeling section mentioned above). considering n = 3179, n1 = 3000, n2 = 179, hence, in zoning by k94, the values of the forecasting error of the algorithms mentioned are calculated as follows: in method 3, according to algorithm (ii) in the validation section, all of the data are divided into three classes. the data ranging from 1 to 3,000 are used for forecasting the next 104 data, which are used for making a pattern. the data ranging from 3,001 to 3,104, which is equal to 28 time periods (each time unit is equal to 10 days), are used for deterministic forecasting of the data ranging from 3,105 to 3,179, which are the data later used for determining the forecasting error in the case of deterministic forecasting. in this way, according to algorithm (ii) in the validation section, t is equal to 5. this value means that the element(s) greater than 5th maximum order in forecasting matrixes are replaced by 1 and the other elements are replaced by 0, then the deterministic forecasting is the nearest forecasting to the real occurrences and its forecasting error is the least. however by considering t= 5 in this zoning method, its mape is 2.398%. b) the proposed zoning (based on fault lines) in this zoning method, the estimated linear equations of fault lines (according to their longitudes and latitudes) were obtained. next, the fault lines and their linear equations were classified into 10 zones according to specific attributes, including the provinces of iran, their importance, the length of the fault lines, etc. therefore, figure 3 was drawn approximately. the above steps were completely run on the matlab software. in this way, the fault lines in the iran area (shown in figure 4) [sadeghian 2007] are divided into 10 zones, as shown in figure 3. the classes of the magnitudes are considered similar to the k94 zoning method. the matrices gr (i.e. probability matrixof region-to-region transitions) and are determined through a method similar to the k94 zoning method, as with table 10, in appendix 2. by using tables 7 and 10, the interval transition probability matrices in both region-to-region and magnitudeusing semi-markov models for zoning 88 figure 2. the siesmogenic source zones of iran, as proposed by k94 [from karakaisis 1994; re-edited by sadeghian and jalali-naini 2008b]. figure 3. the siesmogenic source zones of iran proposed by sadeghian (present study). 0.022mse = 0.0mad 5= .0 %mape 5 0= ( ) 1, ..., 5frm i i =6 89 to-magnitude transitions are determined; in addition, using these and equation (2), the probabilistic forecasting matrices for the next 5 time periods (i.e. the next 50 days) after normalizing are determined. (see tables 11a and 11b in appendix 2). in this zoning method, the forecasting errors are determined through a method similar to the k94 zoning method, as follows: in method 3, similar to the zoning by k94, all of data were divided into three sections. the value of tobtained was 4 and its mape in deterministic forecasting error is 3.552%. discussion with respect to the two zoning methods investigated, the percentages of earthquake occurrences in each of the zones are shown in figures 5 and 6. moreover, the percentages of earthquake occurrences in each class of magnitudes, which are similar in both methods, are presented in figure 7. the transition probability matrices in the magnitude-tomagnitude transitions are the same with both of the zoning methods. table 7 shows that the maximum and minimum probabilities in the magnitude-to-magnitude transitions are related to the transitions from m5 to m2 (1.00) and from m2 to using semi-markov models for zoning figure 4. the fault lines in the iran area [from the website of the international institute of earthquake engineering and seismology (tehran, iran), http://www.iiees.ac.ir/iiees/english/seismology/eng_seis_faults.html]. 0.0 2mse 11= 0.0 5mad 44= . %mape 4 45= m5 (0.0026), respectively (except for none-zero elements). these results are illustrated in figure 7. with respect to the zoning by k94, presented in tables 8a and 8b, it is clear that the maximum and the minimum probabilities in these tables are related to the r8 →r18 (0.333) and r22 → r6, r22 → r8 (0.0028) transitions, respectively. moreover, in the proposed zoning (table 10), the maximum and minimum probabilities are related to r2 →r7 (0.355) and r2 →r10 (0.0059) transitions, respectively. a summary of the minimum and maximum transition probabilities is shown in table 2. the results forecast for the probabilities of earthquake occurrence in the future 5 time periods with the zoning by k94 and the proposed zoning are shown in tables 9a and 9b, and tables 11a and 11b, respectively. these tables show that in zoning by k94, the maximum probability of earthquake occurrences pertains to r22m2 in the first time period, and r18m2 in the second, third, fourth and fifth time periods. similarly, in the proposed zoning, the maximum probability of earthquake occurrences pertains to r5m2 in the first time period, and r7m2 in the second, third, fourth and fifth time periods. ultimately, the forecasting errors in both of the zoning methods show that both the number of zones and the type of zoning affect the forecasting probabilities and the forecasting errors. mape in the proposed zoning is 4.45%, while in zoning by k94, it is 5.0%. therefore, it can be concluded that the proposed zoning will be more successful and precise, although both of these methods have rather low forecasting errors. a summary of the maximum probabilities in the two types of zoning is shown in table 3. in the deterministic forecast method, for different t's, the minimum errors of the forecasting are calculated. so, by considering the forecasting errors obtained through method 3, it can be easily determined that the minimum error of forecasting is related to t=5 in the zoning by k94, and to t= 4 in the proposed zoning. the mapes in the deterministic forecasting error of zoning by k94 and by the proposed zoning can be obtained through the aforementioned method (i.e. algorithm iii). these values are 2.398% in the zoning by k94, and 3.552% in the proposed zoning. these values show that in comparison to the proposed zoning, the k94 zoning method has fewer errors. of course, this priority has a rational reason, and that is because in method 3, although the sum of the absolute percentage error (sape) is the same for both methods, to find the mape in the k94 zoning method, the sape has to be divided by 22×5, while, in the proposed zoning, it has to be divided by 10×5 (since the number of the classes of magnitudes is five, the number of the k94 zones is 22, while in the proposed zoning this is just 10); therefore, the mape obtained through the k94 zoning method is less than that obtained through the proposed method in the present study. of course, this priority exists in using semi-markov models for zoning 90 figure 5. the percentage of occurrences in each region, with the zoning by k94 [from sadeghian and jalali-naini 2008b]. figure 6. the percentage of occurrences in each region, with the zoning by sadeghian (present study). figure 7. the percentage of occurrences in each class of magnitudes [from sadeghian and jalali-naini 2008b]. 91 using semi-markov models for zoning table 6. summary of the results of the forecasting. table 2. summary of minimum and maximum transition probabilities. table 3. summary of maximum probabilities in the two types of zoning. table 4. deterministic forecasting matrix during the first to fifth time periods for zoning by k94 and by the proposed zoning. table 5. actual earthquake occurrences during the following 5 time periods after the last earthquake occurrence. transition minimum probability maximum probability magnitude-to-magnitude transition probability m2 ! m5 (0.0026) m5 ! m2 (1.00) region-to-region transition probability for the k94 zoning r22 ! r 6 , r 22 ! r 8 (0.0028) r 8 ! r 18 (0.333) region-to-region transition probability for the proposed zoning r2 ! r 10 (0.0059) r2 ! r 7 (0.355) zoning type period1 period2 period3 period4 period5 maximum probabilities in k94 zoning r22 m2 r18 m2 r18 m2 r18 m2 r18 m2 maximum probabilities in the proposed zoning r5 m2 r7 m2 r7 m2 r7 m2 r7 m2 zoning type future time periods 1 to 5 k94 zoning method r16 m2, r18 m2, r19 m2, r20 m2, r22 m2 proposed zoning method r5 m2, r6 m2, r7 m2, r8 m2 period k94 zoning method proposed zoning method 1 r16 m2, r20 m2, r21 m2 r6 m2, r8 m2, 2 r2 m2, r13 m2, r14 m2 r4 m2, r5 m2, r9 m2, 3 r7 m2, r17 m2, r18 m2, r19 m2, r22 m2 r2 m2, r4 m2, r7 m2, r8 m2, 4 r12 m2, r18 m2, r18 m2, r19 m2, r20 m3 r5 m2, r6 m3, r7 m1, r7 m2, 5 r20 m2, r21 m2, r22 m3 r6 m2, r6 m3, r7 m2, zoning type forecast (%) completely correct region correct, magnitude wrong adjacent region correct, magnitude correct not forecast k94 zoning 42 16 16 26 proposed zoning 56 19 19 6 deterministic forecasting just due to the greater number of zones in the k94 zoning method, so it does not exist for probabilistic forecasting. conclusion in this study, first, the method of applying the semimarkov model in forecasting these triad dimensions of earthquakes was mentioned. then, the method of calculating the probabilistic and deterministic forecasting was elaborated on. moreover, several algorithms for validating the proposed model and obtaining the forecasting errors are introduced. finally, while introducing two zoning methods, as those of k94 and of the proposed zoning, a case study on the iran area is carried out and the forecasting errors of the proposed models are obtained for both of these zoning methods. in conclusion, it can noted that both the number of zones and the type of zoning affect the forecast probabilities and the forecasting errors. the forecasting errors show that the mape for the k94 zoning is 5.0%, while it is just 4.45% in the proposed zoning. these levels show that the forecasting error can be reduced through manipulating the numbers of zones or the types of zones. with respect to method 3, in calculating the deterministic forecasting errors, this method can also forecast the next earthquake occurrences deterministically. with respect to the collected data in the case study, the last earthquake considered in this study occurred on march 26, 2007, with a magnitude of 4.9 mb, m3, and in the regions of r22 and r5 (in the k94 and the proposed zonings, respectively). accordingly, the deterministic forecasting during the future 5 time periods (the future 50 days) are determined in the k94 and the proposed zoning by algorithm iii, as shown in table 4. during the 50 days after carrying out the proposed model in the iran area, some earthquakes occurred for both of the zoning methods. these are presented in table 5. these results obtained show that in the zoning by k94, 42% of the earthquakes were correctly forecast. the areas of 16% of the earthquakes were correctly forecast, but not their magnitudes. then 16% of the earthquakes occurred in areas adjacent to the forecast earthquakes, with the correct magnitudes, and 26% of the earthquakes were not forecast at all. in the proposed zoning, 56% of the earthquakes were correctly forecast in all of the three dimensions. the areas of 19% of the earthquakes were correctly forecast, but not their magnitudes. then 19% of the earthquakes occurred in areas adjacent to the forecast earthquakes, with the correct magnitudes, and 6% of the earthquakes were not forecast at all. the summary of these results is given in table 6. in addition to the errors obtained and explained above, the statistics mentioned here show that in comparison to the k94 zoning, the proposed zoning provides more accurate forecasting. one of the reasons for the priority of the proposed zoning over that of k94 is that the k94 zoning was not performed correctly (i.e. it did not take into account the center of iran, which is a very extended area, while this was part of the zones considered in the present study). references altinok, y. and d. kolcak (1999). an application of the semimarkov model for earthquake occurrences in north anatolia, turkey, j. balkan geophys. soc., 2 (4), 90-99. console, r. 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(1976). recurrence of great earthquakes at subduction zones, tectonophysics, 35, 335-362. sadeghian, r. and g.r. jalali-naini (2008a). a new probability density function in earthquake occurrences, j. ind. engineering int., 4 (6), 1-6. sadeghian, r. and g.r. jalali-naini (2008b). applying semimarkov models for forecasting the triple dimensions of next earthquake occurrences, dynamics of continuous, discrete and impulsive systems, series b: applications & algorithms, 15, 395-416. sadeghian, r. (2007). whatever you don't know about earthquakes, shooka press, tehran. *corresponding author: ramin sadeghian, bu ali sina university, hamedan, iran; e-mail: rsadeghian@basu.ac.ir. © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. using semi-markov models for zoning using semi-markov models for zoning 94 0. 00 0 0. 02 8 0. 00 0 0. 08 3 0. 00 0 0. 00 0 0. 00 0 0. 02 8 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 04 5 0. 09 1 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 06 9 0. 00 0 0. 03 4 0. 03 4 0. 00 0 0. 00 0 0. 03 4 0. 00 0 0. 00 0 0. 00 0 0. 13 8 0. 01 2 0. 00 6 0. 01 8 0. 07 9 0. 06 7 0. 01 2 0. 02 4 0. 00 6 0. 01 8 0. 00 0 0. 01 2 0. 00 0 0. 00 0 0. 00 0 0. 12 6 0. 26 4 0. 00 0 0. 01 1 0. 00 0 0. 00 0 0. 01 1 0. 01 1 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 04 3 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 04 3 0. 00 9 0. 00 9 0. 00 0 0. 01 8 0. 05 3 0. 00 0 0. 03 5 0. 00 0 0. 03 5 0. 01 8 0. 02 6 0. 00 0 0. 00 0 0. 05 6 0. 05 6 0. 00 0 0. 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01 3 0. 01 6 0. 00 9 0. 00 3 0. 00 3 0. 03 5 0. 00 9 0. 01 5 0. 04 7 0. 00 9 0. 02 6 0. 01 7 0. 03 2 0. 00 4 0. 00 7 0. 00 4 0. 06 7 0. 02 6 0. 00 7 0. 04 9 0. 00 0 0. 01 1 0. 00 7 0. 02 2 0. 01 1 0. 01 1 0. 00 8 0. 05 4 0. 01 7 0. 00 3 0. 03 4 0. 00 3 0. 02 5 0. 01 1 0. 02 3 0. 00 0 0. 11 1 0. 00 0 0. 00 0 0. 08 3 0. 11 1 0. 22 2 0. 08 3 0. 08 3 0. 05 6 0. 11 1 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 04 5 0. 09 1 0. 09 1 0. 09 1 0. 22 7 0. 13 6 0. 18 2 0. 00 0 0. 10 3 0. 03 4 0. 00 0 0. 03 4 0. 06 9 0. 10 3 0. 06 9 0. 10 3 0. 03 4 0. 13 8 0. 01 8 0. 02 4 0. 01 2 0. 01 2 0. 09 1 0. 06 1 0. 14 0 0. 11 0 0. 07 3 0. 06 1 0. 14 0 0. 00 0 0. 02 3 0. 02 3 0. 00 0 0. 08 0 0. 08 0 0. 06 9 0. 09 2 0. 05 7 0. 08 0 0. 06 9 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 04 3 0. 04 3 0. 17 4 0. 00 0 0. 21 7 0. 21 7 0. 21 7 0. 03 5 0. 06 1 0. 00 0 0. 00 9 0. 04 4 0. 07 0 0. 16 7 0. 08 8 0. 11 4 0. 08 8 0. 12 3 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 16 7 0. 05 6 0. 33 3 0. 16 7 0. 11 1 0. 00 0 0. 00 0 0. 01 5 0. 06 0 0. 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01 91 0. 00 17 0. 03 01 1. 00 00 0. 14 24 0. 02 92 0. 00 25 0. 02 16 0. 72 02 0. 10 26 0. 02 12 0. 00 21 0. 00 34 0. 11 32 0. 01 61 0. 00 34 0. 00 04 0. 00 30 0. 09 50 0. 01 36 0. 00 30 0. 00 04 415_421 santangelo.pdf 415 annals of geophysics, vol. 46, n. 2, april 2003 continuous photometric observations at enea base in lampedusa to estimate precipitable water renato santangelo, sergio pugnaghi, stefano corradini, luca lombroso and sergio teggi dipartimento di ingegneria dei materiali e dell’ambiente (osservatorio geofi sico), università di modena e reggio emilia, modena, italy abstract water vapour is a variable component of the atmosphere both in space and time. it is one of the most important components because of its effects in many fi elds: meteorology, climatology, remote sensing, energy-budget, hydrology, etc. this work compares radiometric (sun photometer) readings, global positioning system (gps) data and a meteorological model forecasted data. the aim is to understand if gps measurements may help numerical weather prediction (nwp) models. it is well known that gps measurements are affected by the so-called tropospheric delay. part of it, the so-called wet delay is related mainly to the amount of water vapour along the path of the gps signal through the troposphere. precise knowledge of the abundance of water vapour, in space and time, is important for nwp model because water vapour is the predecessor of precipitation. despite the high variability of water vapour compared to other meteorological fi elds, like pressure and wind, water vapour observations are scarce, so that additional measurements of water vapour are expected to benefi t meteorology. a new sun photometer, which is part of the aeronet (aerosol and robotic network) program, has been installed at the enea (ente per le nuove tecnologie, l’energia e l’ambiente) base of lampedusa island. the sun photometer is quite close (less then 4 km) to an asi (agenzia spaziale italiana) gps permanent receiver. a long record (summer period of the year 2000) of sun photometric measurements is available for the station at lampedusa. we found that the gps and sun photometric data are better correlated (std. dev. about 10 mm for the wet delay) than are the gps measurements with the nwp model predictions. this is an indication that gps delay data may contain information useful for weather prediction. mailing address: dr. renato santangelo, dipartimento di ingegneria dei materiali e dell’ambiente (osservatorio geofisico), università degli studi di modena e reggio emilia, via vignolese 905, 41100 modena, italy; e-mail: santangelo.renato@unimo.it key words water-vapour – aerosols – gps – sunphotometer – nwp-model 1. introduction water vapour is a highly variable component of the atmosphere and plays a crucial role in many atmospheric phenomena (see all emanuel et al., 1995; dabberdt and schlatter, 1996). water vapour is itself a major greenhouse gas, but it also takes part in the formation of aerosols affecting in different ways the energy budget of the atmosphere (russel et al., 1999). water vapour is a key variable in atmospheric motions on a wide range of scales and the lack of water vapour observations, both temporal and spatial, is one of the problems in nwp models; in particular for clouds and precipitation (binder and schär, 1996). in space geodesy, water vapour is seen as a source of error (dodson et al., 1996), as fl uctuations of the tropospheric water vapour content produce spatial and temporal variations of the refraction properties of the troposphere. 416 renato santangelo, sergio pugnaghi, stefano corradini, luca lombroso and sergio teggi the integrated water vapour (iwv), also called, the precipitable water (pw), is the mass per area of water vapour contained in an atmospheric vertical column. it is often converted to a thickness, in which case 1 kg/m2 corresponds to 1 mm of pw. the precipitable water is usually derived from the atmospheric radiosoundings; it is the integral of the absolute humidity rn (or water vapour density) along the vertical direction z or the integral, using the atmospheric pressure as the vertical coordinate, of the specifi c humidity q (the density of the water vapour relative to the total density of the air). alternatively, the precipitable water can be estimated using a microwave radiometer (england et al., 1993), or using solar transmittance methods (ingold et al., 2000). this paper compares three different integrated water vapour estimates. this work derives from the research activities performed by different teams in the framework of the eufunded magic (meteorological applications of gps integrated column water vapour measurements in the western mediterranean) project. for further information about magic and the data in the magic data sample, see http:// www.acri.fr/magic/index.html. the three types of data we compare are from gps measurements, sun photometric readings, and from forecast data produced running the meteorological model hirlam. to distinguish the hirlam and the sun photometer water vapour estimates: iwv and pw respectively have been used in the following. the magic project utilised the data of about 40 gps-sites, mainly in spain, france and italy; in this paper the permanent asi station of lampedusa (a small and fl at island located between tunisia and italy: sicilian channel) is considered. no meteorological measurements are available at the gps site (capitaneria di porto). 2. ztd and pw estimations in the magic project (haase et al., 2001), the zenith total delay (ztd) rather than the precipitable water is used as the key gps tropospheric measure. ztd is the atmospheric measurement provided by the gps processing software. ztd is the sum of two components, the zenith hydrostatic (or dry) delay (zhd) and the zenith wet delay (zwd) (see eqs. (2.1) and (2.2) for defi nitions). (2.1) (2.2) q is the specific humidity; p s is the surface pressure; r d = 287.04 (j kg-1 k-1) is the gas constant for the dry air; g is the acceleration due to gravity and non inertial forces acting upon a particle at rest with respect to the earth; e = m v /m d ª 0.622 is the ratio of the molecular weight of the water vapour (m v ) relative to the molecular weight of the dry air (m d ); t is the air temperature and fi nally: k1 = 7.76 10 –7 (k/pa), k2 = 7.04 10 –7 (k/pa), k3 = 3.74 10 –3 (k2/pa) are empirical constants (bevis et al., 1994). zhd is mainly a function of surface pressure, which may be obtained either from a barometer at the gps site or from a meteorological model. most of the gps sites of the proper type are set-up for geodetic purposes and not equipped with extra instruments like barometers. zwd is related to pw, the precise conversion factor requires knowledge about the temperature profi le of the atmosphere (askne and nordius, 1987), which may come from a meteorological model or radio sonde observations valid at the time of the measurement. alternatively a more approximate conversion factor, based on empirically derived relations, may be used. the scale factor between pw and zwd empirically determined by bevis et al. (1992) is 0.15. at all the italian asi-gps stations the data analysis is done using gipsy-oasis ii (webb and zumberge, 1997) software with the precise point positioning (zumberge et al., 1997) mode. asi ztd products are available on a daily basis and with a delay of 15 days, through anonymous ftp on ftp://geodaf.mt.asi.it. the results described in the following have been obtained using all the available data (for the considered station) pertaining to the year 2000; the time step is 15 min. zhd = ⋅ ⋅∫ r g k dpd ps 0 1 zwd = ⋅ ⋅ ⋅ − ⋅( ) +   ∫ r g q k k k t dpd ps ε ε 0 2 1 3 417 continuous photometric observations at enea base in lampedusa to estimate precipitable water the high resolution limited area model (hirlam) is an nwp model developed jointly by a number of european meteorological institutes. in particular, this model uses a so-called hybrid coordinate in the vertical (i.e. not directly related to altitude or atmospheric pressure); it has 31 levels, from the surface up to about 10 hpa. as boundaries for the hirlam model analyses and short range forecast were used from the global model run at the european centre for medium range weather forecasts (ecmwf). for the purpose of magic a special domain, centred on the mediterranean sea was utilised. for each magic gps location hirlam provides, using eqs. (2.1) and (2.2), the ztd, the zwd, the integrated water vapour (iwv) and the surface pressure; these quantities are provided both at the hirlam surface and at the altitude of the gps antenna. furthermore the 2 m temperature and the relative humidity are provided at the hirlam surface. a sun photometer is an instrument that measures the direct solar irradiance at the earth’s surface. the measurements are taken at chosen wavelengths in the visible and near infrared part of the electromagnetic spectrum to determine the atmospheric transmission and scattering properties. the sun photometer used in this study is the model ce-318 produced by cimel electronique (france). like all sun photometers of this type, it has the same main set of fi lters (see table i), plus three added fi lters not described here. all the fi lters have a bandwidth of 10 nm. sun photometer like ce-318 provides the water vapour transmittance from measurements in spectral channel (936 nm) and in adjacent channels (870 nm and 1020 nm). the problem is then to convert the water vapour transmittance into precipitable water. the fi rst attempts to quantify the pw from water vapour absorption in the near infrared date back to 1912, but this subject continues to be studied (ingold et al., 2000; schmid et al., 2001). figure 1 shows (thick line) the water vapour transmittance in the range 700-1100 nm computed using the modtran3.5 code (kneizys et al., 1996) (assuming a mid latitude summer atmosphere) with a spectral resolution of 50 cm–1 (i.e. about 5 nm, depending table i. sun photometer nominal centre wavelengths. channel# 1 2 3 4 5 wavelenght (nm) 1020 870 670 440 936 fig. 1. modtran midlatitude summer water vapour transmittance (thick line) and 870, 936, 1020 nm fi lter response functions (thin line). 418 renato santangelo, sergio pugnaghi, stefano corradini, luca lombroso and sergio teggi on wavelength); the 0.72, 0.82 and 0.94 microns water vapour absorption bands are visible. together with the water vapour transmittance, the spectral transmittance of the 936 nm and the two adjacent fi lters (870 nm and 1020 nm) are shown in fi g. 1 (thin line). the 870 and 1020 nm channels (window channels) are located outside of molecular absorption bands (the water vapour bands shown on the fi gure and the oxygen band located at about 0.76 mm, not shown); they are usually used to determine the aerosol optical thickness (aot). for the 0.94 mm water vapour band the bouguer’s (lambert, beer) law (used to compute aot) is not valid since it is valid for monochromatic radiation only (see fi g. 1). equation (2.3) is the equation used for the three quoted channels. in this equation jl is the sun photometric measurement (instrumental units) and represents the normal solar irradiance at the surface, j0,l represents the normal solar irradiance at the top of the atmosphere at the mean sun-earth distance (instrument calibration constant), r is the square of the inverse of the sun-earth distance in astronomical units, m ds dz= ⋅ ⋅∫ ∫ρ ρ/ is the relative optical airmass (s is the slant path of the solar ray, z is the vertical path) to compute m the aeronet protocol uses the kasten and young formula (1989), τ τ τλ λ λ= + r a is the total optical thickness corresponding to the sum of the molecular (rayleigh) and aerosol optical thickness, fi nally t wλ is the water vapour transmittance. (2.3) in the window channels, the water vapour transmittance is equal to one so eq. (2.3) enables us to compute aot (the rayleigh optical depth is known). the interpolation at 936 nm of the aot values obtained for the adjacent window channels permits us to estimate, again using eq. (2.3), the water vapour transmittance. the relationship used to estimate the pw from the water vapour transmittance is (halthore et al., 1997) (2.4) where the two constants a and b are related to the considered channel and w = m pw. 3. results figure 2 shows the difference between the zenith total delay estimated from hirlam at the gps antenna altitude and the value obtained from the gps measurement. the ztd mean difference (hirlam-gps) is –11.8 mm and the standard deviation 22.4 mm. the hirlam zenith wet delay is obtained using eq. (2.2). the gps zenith wet delay is obtained subtracting the hirlam zenith hydrostatic delay, computed from eq. (2.1), using the hirlam surface pressure estimated at the antenna altitude. the ztd differences shown in fi g. 2 have the same variability as the zwd (not reported). that is to say that the hydrostatic delay is well computed from the estimated surface pressure. the mean and standard deviation of the zwd differences (hirlam-gps) are exactly the same values (reported above) for the ztd. the iwv is computed from the hirlam atmospheric profi le (above the gps site). the same quantity is derived from the sun photometric measurement inverting eq. (2.4). the sun photometric pw data show values higher than the iwv estimated by the hirlam model; the mean systematic difference is about 10-15 %. on the basis of the recent literature (callies et al., 2000; giver et al., 2000; schmid et al., 2001), the reason for this difference seems due to the underestimated h2o absorption line intensities used to date (hitran database). according to giver et al. (2000), a 14.4% increase in the line strengths for the 0.94 mm band is suggested; this would mean an 8-13% decrease (depending on the considered atmospheric radiative transfer model) in the sun photometric pw estimates (schmid et al., 2001). schmid et al. (2001) suggest, in particular during the period with high pw, to use the following eq. (3.1) instead of eq. (2.4) (3.1) the hirlam data of year 2000, for lampedusa and also for others considered stations results in a ratio between iwv and zwd of about 0.16 (i.e. 1 mm of iwv corresponds to about 6.3 mm of zenith wet delay). the calibration constants of the t ew a w b = − ⋅ . t ew a w b w = − ⋅ − ⋅β . j r j e t m w λ λ τ λ λ= ⋅ ⋅ ⋅− ⋅0, . 419 continuous photometric observations at enea base in lampedusa to estimate precipitable water sun photometers have been modifi ed to account for this factor between pw and zwd. figures 3 and 4 are the scatter plots of the gps zwd versus the pw sun photometric readings and the hirlam iwv estimates respectively. the standard deviation of the difference between the zwd estimated using the linear regression shown in the two scatter plot are 10.5 mm (sun photometer) and 26.9 mm (hirlam). the confi dence interval indicated in the fi gs. 3 and 4 is 95 % (i.e. 2 sd). 4. conclusions one year of forecasted (each 15 min) vertical atmospheric profi les at the gps site of lampedusa and one year of gps data were considered and the ztd computed. the iwv was computed using the hirlam forecasted profi les. three months of sun photometric measurements in lampedusa are available and were considered. the pressure forecast by hirlam for matera agrees very well with local pressure measurements (test not shown here); this is important because the gps zwd was obtained subtracting from the gps ztd the hirlam zhd, for which the predicted pressure at the antenna altitude was used. the results are similar for forecasts of age 0-6 and 6-12 h. the pw sun photometric results are systematically higher than the hirlam iwv. this is likely due to a spectroscopic problem, which results in a calibration problem (schmid et al., fig. 2. (hirlam gps) ztd differences. lampedusa 300 250 200 150 100 50 0 0 5 10 15 20 25 30 35 40 45 pw_sp (mm) z w d _g a ( m m ) fig. 3. gps zwd and sun photometer pw scatter plot. fig. 4. gps zwd and hirlam iwv scatter plot. 420 renato santangelo, sergio pugnaghi, stefano corradini, luca lombroso and sergio teggi 2001). nevertheless, the sun photometric pw data correlate more strongly with the gps zwd data than with the hirlam iwv, indicating that such measurements contain information not fully known by the nwp model. for the hirlam results it should be noted that lampedusa is a very small island (10 km long and 4 km wide); this means that it is not well resolved by the hirlam model (spatial resolution 0.3 deg.), which could explain the large scatter found for the hirlam zwd to gps measurements (fi g. 4). the reported standard deviation of about 10 mm agrees with many other experimental results performed with different instruments in different regions (westwater et al., 1998, 2000; betti et al., 2000; pacione et al., 2000; schmid et al., 2001; vedel et al., 2001). the next problem is to understand how the gps pw or ztd estimates can be well assimilated into nwp models and to study whether such data improve the forecasts, for example by a better prediction of local, severe rainfall. such work is under way inand outside the magic project (cucurul et al., 2001; haase et al., 2001). acknowledgements this work was supported in part by the ec dgxii environment and climate rtd project magic (env4-ct98-0745). the sun-photometer is part of aeronet and we thank nasa for the instrument calibration and in particular brent holben for his suggestions. we thank enea (ente per le nuove tecnologie, l’energia e l’ambiente divisione ambiente globale e mediterraneo) for their hospitality in the new base of lampedusa. freferences askne, j. and h. nordius (1987): estimation of tropospheric delay for microwaves from surface weather data, radio sci., 22, 379-386, betti, b., r. prini, v. tornatore, m. crespi, s. di paola, v. baiocchi, f. radicioni, p. basili, s. bonafoni, r. ferrara, p. ciotti and e. fionda (2000): determinazione del contenuto di vapour acqueo precipitabile con misure gps in reti locali e misure radiometriche: primi confronti, in atti della 4a conferenza nazionale asita, genova, 3-6 ottobre 2000, vol. 1, 453-461. bevis, m., s. businger, t.a. herring, c. rocken, r.a. anthes and r.h. ware (1992): gps meteorology: remote sensing of the atmospheric water vapour using the global positioning system, j. geophys. res., 97, 15,787-15,801. bevis, m., s. businger, s. chiswell, t.a. herring, r.a. anthes, c. rocken and r.h. ware (1994): gps meteorology: mapping zenith wet delays into precipitable water, j. appl. meteorol., 33, 379-386. binder, p. and c. schär (editors) (1996): the mesoscale alpine programme map: design proposal, second edition, pp. 77 (available from map programme offi ce, c/o swiss meteorological institute, ch 8044 zürich, http://www.map.ethz.ch/proposal.html). callies, j., m. eisinger, r. schermaul, r.m. learner, d.a. newnham, j. ballard, j. tennyson and m. wickett (2000): new results of water vapour spectroscopic measurements in the visible and nearinfrared, esa, eoq, 67, 1-4. cucurull, l., j. vila and a. rius (2001): zenith total delay study of a mesoscale convective system: gps observations and fi ne-scale modeling, tellus, 54a, 138-147. dabberdt, w.f. and t. w. schlatter (with contributions from the rest of pdt-2) (1996): research opportunities from emerging atmospheric observing and modeling capabilities, bull. am. meteorol. soc., 77, 305-323. dodson, a.h., j. shardlow, l.c.m. hubbard, g. elgered and p.o.j. jarlemark (1996): wet tropospheric effects on relative gps height determination, j. geod., 70, 188-202. emanual, k., d. raymond, a. betts, l. bosart, c. bretherton, k. droegemeier, b. farrell, j. m. fritsch, r. houze, m. le mone, douglas lilly, r. rotunno, m. shapiro, r. smith and a. thorpe (1995): report of the fi rst prospectus development team of the u.s. weather research program to noaa and the nsf, bull. am. meteorol. soc., 76, 1194-1208. england, m.n., f.j. schmidlin and j.m. johansson (1993): atmospheric moisture measurements: a microwave radiometer-radiosonde comparison, ieee trans. geosci. remote sens., 31, 389-398. giver, l.p., c. chackerian, jr. and p. varanasi (2000): visible and near-infrared h2o line intensity corrections for hitran-96, j. quant. spectrosc. rad. transfer, 66, 101-105. haase, j., e. calais, j. talaya, a. rius, f. vespe, r. santangelo, x.-y. huang, j.m. davila, m. ge, l. cucurull, a. flores, c. sciaretta, r. pacione, m. boccolari, s. pugnaghi, h. vedel, k. mogensen, x. yang and j. garate (2001): the contributions of the magic project (meteorological applications of gps integrated column water vapor measurements in the western mediterranean) to the cost 716 objectives of assessing the operational potential of ground-based gps meteorology on an international scale, phys. chem. earth, 26 (6-8), 433-437. halthore, r.n., t.f. eck, b.n. holben and b.l. 421 continuous photometric observations at enea base in lampedusa to estimate precipitable water markham (1997): sunphotometric measurements of atmospheric water vapour column abundance in the 940-nm band, j. geophys. res., 102, 4343-4352. ingold, t., b. schmid, c. matzler, p. demoulin and n. kampfer (2000): modeled and empirical approaches for retrieving columnar water vapour from solar transmittance measurements in the 0.72, 0.82, and 0.94 mm absorption bands, j. geophys. res., 105 (d19), 24,327-24,343. kasten, f. and a.t. young (1989); 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orsi et al., 1999). the interground tilt monitoring at phlegraean fields (italy): a methodological approach ciro ricco, ida aquino and carlo del gaudio istituto nazionale di geofisica e vulcanologia – osservatorio vesuviano, napoli, italy abstract among geodetic methods used for monitoring ground deformation in volcanic areas, tiltmetry represents the most rapid technique and therefore it is used by almost all the volcanological observatories in the world. the deformation of volcanic building is not only the result of endogenous causes (i.e. dykes injection or magma rising), but also non-tectonic environmental factors. such troubles cannot be removed completely but they can be reduce. this article outlines the main source of errors affecting the signals recorded by phlegraean tilt, network, such as the dependence of the tilt response on temperature and to the thermoelastic effect on ground deformation. the analytical procedure used to evaluate about such errors and their reduction is explained. an application to data acquired from the tilt network during two distinct phases of ground uplift and subsidence of the phlegraean fields is reported. mailing address: dr. ida aquino, istituto nazionale di geofisica e vulcanologia – osservatorio vesuviano, via diocleziano 328, 80124 napoli, italy; e-mail: aquino@ov.ingv.it 1298 ciro ricco, ida aquino and carlo del gaudio fig. 1. location of the volcanic area. est in this area is due to its intense geodynamic and geo-thermal activity, with strong ground uplifts and subsidence episodes (bradyseismic phases), earthquake swarms, and fumarolic emissions. the last eruption dates back to 1538, but major uplifts occurred in 1969-1972 and 1982-1984 when the ground displacements reached a maximum value of 174 cm and 179 cm respectively; these movements refer to the benchmarks (belonging to a phlegraean levelling line) located in the town of pozzuoli (fig. 2). the deformation recorded is characterized by an almost bell-shaped form and covers an almost circular area with a radius of 6 km centered in the town of pozzuoli (orsi et al., 1999). the subsidence phase, starting at the end of 1995 and interrupted by three small uplifts (fig. 2), is continuously monitored by the tilt network of the osservatorio vesuviano (ov). 2. historical data the first clinometric measurements carried out in the italian volcanic areas config. 2. vertical displacement of benchmark n. 25 (located in the town of pozzuoli) over the period 1985-2002 . 1299 ground tilt monitoring at phlegraean fields (italy): a methodological approach fig. 3. levelling line and tilt stations belonging to ov phlegraean surveillance array. cerned mount vesuvius and date back to 1935 (imbò, 1939). the spirit levels placed by imbò in 1935 on the seismic pillar of the ov historical building (by recording on smoked paper) and oriented n70w-s70e and n20es70w measured the apparent deviation of the vertical from october 1935 to january 1939. in 1939, imbò observed five eruptive intervals (strombolian activity on the crater bottom) characterized by a fracturing of the intracrateric cone and by lava flows. he noticed an inverse parallelism between the direction of lava flows and the deviation of the vertical and evidenced a direct correlation between the ssw apparent tilt of the volcano and a lowering of the eruptive column, and nne tilt and the uplifting of the column (imbò, 1939). in 1968, two verbaandert-melchior horizontal pendulums were installed in the earth physics institute of naples university (lo bascio and quagliariello, 1968). these were used to compare the clinometric data obtained by other stations located at the base of the vesuvius cone (imbò, 1959). in april 1970, the institute of geodesy and geophysics of the university of trieste set up a network of four clinometric stations (equipped with two optical tilt sensors with recording on paper, oriented ns and ew) to study the ground deformation caused by the 1970-1972 phlegraean fields bradyseismic crisis (manzoni, 1972) (fig. 2). 3. tilt network ov – institute de physique du globe (ipg) in 1987, a tiltmetric network was set up at the phlegraean fields (fig. 3) in cooperation with the ipg, which provided the instruments. each station was equipped with two tiltmeters sensitive along two orthogonal directions, using zöllner bifilar suspension model, made by p.a. blum at paris ipg (fig. 4a,b) (aste et al., 1986; briole, 1987). the first station of this type, called dmb, was installed at the end of 1985 in pozzuoli, and located in a gallery aligned ns, at about 15 m below the surface (figs. 3 and 5). the second station (bai) was set up in april 1980 in a cistern of baia castle; later another two stations were set up in october 1987 in pozzuoli (dma and dmc) (fig. 3). the analog signals were digitized through an a/d converter on a canon x-07 computer (fig. 4b). the stations dmb and bai initially worked with an 8 bit acquisition program allowing data accumulation in the ram memory with a sampling rate between 1 and 60 cph. the data were recorded in both graphic and numerical form. later, a 12 bit acquisition program was used to control the operation of the whole system, i.e. switch the bulb on and off, reception analogical signal, digital conversion and data accumulation. the fundamental difference from the first 8 bit acquisition program was that the latter program turned on the tilt station only in the time period of sampling and therefore the electric absorption of the installation was reduced. data in memory were periodically transferred onto a cartridge directly in the field, through the same program and later were sent to a pc through an rs232 serial interface (luongo et al., 1988). 4. automatic tiltmeter network between 1991 and 1992, the stations equipped with the pendulums were replaced by stations with electronic sensors, easier to 1300 ciro ricco, ida aquino and carlo del gaudio fig. 4a,b. a) section, view from the top of blum’s optic clinometer. all the pendulum mobile parts e, which are made of silica (with a coefficient of thermal dilatation β = 0.54 ppm/°c), are contained in a pyrex bell b under vacuum. the pendular part is formed by a concave mirror f joined to the silver plate g, which dampens the oscillations through foucault currents excited by a magnet near a. the pendulum is tightly anchored to the soil through a silica cone d fixed in the hole on the base c. the pendulum rotation is linearly transduced into a variation in voltage through the coupling lamp i, mirror, photocell h. the main limit of this apparatus in highly dynamic areas is due to the regular check of the pendulum oscillation period since it is a function of the pendulum tilt (aste et al., 1986, modified). b) tiltmetric station ov-ipg installed at pozzuoli (dmb) and working from the end of 1985 to 1992. the picture shows the pair of transducers and the acquisition unit. b a handle and to install. these sensors were biaxial, bubble-type, short baselength platform tiltmeters, manufactured by applied geomechanics (agi)-mod. 702 a (agi, 1997). these instruments are made of steel and contain two transducers (one for each axis) formed by a glass case closed at the ends by three electrodes and containing an electrolytic liquid. the case is enclosed in a bridged electric circuit; the inclination of the ground produces a voltage proportional to the amount of resistance imbalance. the tiltmeter is also equipped with a temperature sensor. the signals recorded by each sensor are a/d converted and recorded automatically at a sampling rate between 6 and 60 cph. the station also acquires the supply voltage before the telematic connection with the monitoring center at ov in naples. in 1993, another station (ovo) of this type was lodged on a pillar of armed cement inside a gallery at 25 m depth from the surface, near the ov historical building at ercolano (mount vesuvius). ovo is 2.5 km wnw from the crater of the volcano. in 1996, another two stations were set up at trecase, in the forestry barrack (trc) and at torre del greco (cmd). trc and cmd are located 2.5 km sse and to 5 km ssw from the crater, respectively. today, the whole network is formed by the vesuvian (fig. 6) and phlegrean arrays (fig. 3). 5. tiltmeter applications and instrumental characteristics the long-period tilt monitoring of a volcanic region may help to derive the geometry and the speed of the deformation of the studied area. obviously, it is important to compare the acquired tilt data with those obtained from other techniques. the possibility to use electronic tiltmeters is closely connected to the mechanic and electronic features of the sensor (resolution, accuracy/repeatability, range of measure and drift), and is influenced by the dependence on environmental factors (coupling to the surface, temperature, atmospheric pressure, rainfall, oscillation of the water table, freeze-thaw cy1301 ground tilt monitoring at phlegraean fields (italy): a methodological approach cles) (dzurisin, 1992; braitenberg et al., 2001). the surface sensors agi have an angular range of ± 800 microradians (µrad) in high gain (hg) and of ± 8000 µrad in low gain (lg), a resolution of 0.1 µrad, a sensitivity of 10 mv/µrad, a repeatability of 1 µrad, a maximum non linearity of 1% (hg) and 3% (lg). the resolution represents the minimum angular variation measured by the device (µrad) while the sensitivity is the ratio between the instrumental to the angular response (mv/µrad). the sensitivity setting in high gain enhances the quality of the information on tilt variations even if the available angular range is reduced. however, in our case, this is not a limitation since the monitored volcanic areas are not affected by deformations large enough to require the larger angular range. repeatability shows the deviation among several readings referred to the same angular magnitude. repeatability can be considered the absolute error of the measurement itself. maximum non linearity is the ratio between the maximum residual y ymaxi it_ i recorded during the calibration of the sensor (that is the recording on a calibration plate at a steady temperature tcal of the output voltages of the sensor subject to a range of known tilts) to the whole calibration angular range. the angular coefficient of the calibration line gives the scale factor sfcal (expressed in µrad/mv; agi, 1998). fig. 5. topographical section of subsurface gallery in pozzuoli and the dma, dmb, dmc position. fig. 6. ov vesuvian tiltmeter array. 1302 ciro ricco, ida aquino and carlo del gaudio 6. dependence of the device response on temperature and compensation the installation of tiltmeters to derive the ground deformation pattern must be done with great care, as bubble-tiltmeters are very sensitive to temperature changes. the transfer of heat from the air to the soil and the solar radiation heating the ground, during the day, both on the surface and close by. these temperature variations cause a first order effect on the surface tiltmeters, mainly if the medium is heterogeneous. this effect is proportional to the thermal dilatation (β) of the screws made of invar mounted at the base of the sensor; these screws have to be therefore of the same length. instead, borehole tiltmeters have a radial symmetry and therefore are affected by volumetric strain changes. these tiltmeters are installed at depth, where thermal oscillations are minimum. even the electrolytic transducer is subject to thermal fluctuations since the liquid is subjected to contraction and expansion with the consequent change of sfcal and with zero shift (agi, 1995). if we do not take into account these effects due to the difference between the environmental temperature (te) and the calibration temperature (tcal), the measured tilt angle will be different from the real one. the thermal compensation is calculated through the two coefficients of correction ks and kz experimentally measured in the laboratory k sf sf sf te t 1 s cal cal cal $= ^ ^ h h (i.e. the change in slope of the calibration line per unit te change) k te t sf v tilt z tcal cal app$ = ^ ^ h h (i.e. the zero shift per unit te change) where tcal is the calibration temperature, sfcal is the scale factor at tcal; te, the environmental temperature; sf, the scale factor at te; vt, the output voltage when the sensor is perfectly horizontal; tiltapp, the apparent tilt angle. the coefficient ks and kz of the surface sensors agi (in high gain) are respectively equal to 0.04%/°c and to 1.5 µrad/°c. the reduction of the error is determined by the following equation: sftilt $= k te t v k te t 1 s z cal cal cal$ + ^ ^ h h 8 b (6.1) where v is the output voltage and tilt is the compensated tilt. 7. thermal damping in the soil the compensation of the tilt signal to te does not correct the thermoelastic strain of the ground. this effect is present in the signals recorded by surface tiltmeters and may hide the ground tilt linked to volcanic activity, thereby modifying the spectral content of the signals. in order to dampen the thermal excursions, the surface sensors are generally installed in caves. the best installation is achieved positioning the borehole tiltmeters in holes deeper than 10 m (jentzsch et al., 1993; weise et al., 1999). the phenomenon of thermal damping concerns the modality of propagation of the thermal wave in the ground, which, on the surface, follows the oscillation of the solar radiation according to two main periods, a daily and a yearly period. the surface temperature oscillation can be represented as a sinusoidal and stationary wave coste a t= ~^ h (7.1) where t2=~ r is the angular frequency; t, the period of the wave; t, the time and a, the amplitude of the wave. hypothesizing the subsurface as a homogeneous half space in which the isotherms are parallel, the heat conduction takes place according to fourier’s equation (persico, 1962) z te d t te1 2 2 $ 2 2 2 2 = (7.2) 1303 ground tilt monitoring at phlegraean fields (italy): a methodological approach fig. 7a,b. theoretical thermic attenuation of an annual wave with parameters: a) d ≅ 0.0018 m2/h, α = 0.451 m−1, λ = 13.935 m, λ/t ≅ 0.038 m/d (the phase opposition occurs at a depth of 7 m); b) d ≅ 0.0018 m2/h, α = 8.614 m−1, λ = 0.729 m, λ/t ≅ 0.03 m/h (the phase opposition occurs at a depth of 0.37 m). a b 1304 ciro ricco, ida aquino and carlo del gaudio where d k c= t is the thermal diffusivity (≅ 0.0018 m2/h in the soil); k, the thermal conductivity; c, the thermal capacity; ρ, the density; and z, the depth. replacing eq. (7.1) with eq. (7.2) and successively integrating fourier’s equation, the attenuation and delay of the thermal wave in function of the depth and of the characteristics of the medium crossed is determined by coste ae t zz z = -~ aa] ]g g where d dt2= =a ~ r is the attenuation factor. the rate of thermal wave propagation is . t t d 2 $= m r the thermal wave is therefore a sinusoidal function of time and decreases exponentially with depth according to α. the reduction of the amplitude will be higher for a shorter period of the wave or for a longer wave path. moreover, the thermic wave is also subjected to a time delay, which causes a phase shift in comparison to the incident wave. the phase shift of 180° occurs at the depth z = a r . in a ground with thermal diffusivity of 0.0018 m2/h, a yearly thermic excursion of 20 °c would be reduced to 0.2 °c at a depth of 10 m. the thermic wave would have a speed of propagation of 0.038 m/d (fig. 7a); the same diurnal excursion would be reduced to 0.002 °c at a depth of 1 m. the wave would have a speed of propagation of 0.03 m/h (fig. 7b). 8. thermoelastic deformation the heat propagation causes a deformation within rocks. according to the bidimensional model proposed by berger (1975), in an elastic infinite homogeneous half space, a traveling temperature wave on surface is represented by + te ae e( )z z i t x = c ~ l] g where dl1 2+=c l i~ is the attenuation factor, 2=l r m, the horizontal wave number and i, the imaginary unit; causes the dilatation of the medium teij ij=f b d where β is the linear thermal dilatation coefficient; δij is the kronecker delta i j i j 0 1 ! = =∆ and i, j are the indexes referred to the coordinated axis. the strain εij is the second term of the following equation related to the plane xz and describes the infinitesimal displacement of the around of a point in a continuous medium u du u z u x u dz x u z u dz u dz dz 2 1 2 1 . x x x x z z x x xz xz $ $ + = + + + = + + d d d d d d d d f ~ d d n n (8.1) the partial derivatives in eq. (8.1) are the elements of the strain tensor e and of the rotation tensor ω (zadro and braitenberg, 1999). berger’s (1975) solution allows to calculate the thermoelastic tilt in the following way: + x u i z e e ae 1 1 1 2tilt z z z z i t x $ $ $ $ = = + + d d v v c l v l bl c ~ l ] b b ] ] g l l g g 6 ?# , where σ is the poisson’s ratio. it can be noticed how the tilt is mainly formed by two terms exponentially decreasing with depth, e−κz, which is related to the existence of superficial tractions caused by thermal stresses in the layer (whose depth is 2π/γ ), and e−γz, which is due to a body force resulting from the gradient of temperature at depth. since, in practice, the most important variations of temperature occur at period of the order of the hours, assuming λ = 103 m or longer and d = 10–3 m2/h, i d1 2, +c ~^ h . a more complete solution, which takes into account the surface topography (valid only for 1305 ground tilt monitoring at phlegraean fields (italy): a methodological approach slopes that are uniform over a long distance), was found by harrison and herbst (1977) . cos sin ae t z 0 5 1 1 2 tilt z z$ $ $ $ = + v v b ~ a { a ] b ] _ g l g i where ϕ is the surface slope in degrees. this model, however, does not take into account the berger (1975) traction term e−κz and therefore underestimates the theoretical tilt. 9. other sources of error in the tiltmetric signal other sources of perturbations on the tiltmetric signal are water table variations (kümpel, 1983) and rainfalls (wolfe et al.,1981; kümpel, 1986; braitenberg, 1999), which tend to modify hydrostatic pressures acting in permeable rocks and therefore cause deformations, but also the height variations of the sea-level, when the stations are near the sea. also, the strong atmospheric perturbations can disturb on the signal but their non-periodic effects are generally superimposed on those produced by the periodic tides. other non-tectonic signals recorded by tiltmeters are the atmospheric pressure loading (dal moro and zadro, 1998), ocean loading (davis et al., 1987) and earth tides (sleeman et al., 2000). all these causes can create a systematic component in the measurement of the ground inclination that masks the deformation induced by the volcanic activity. another source of systematic error is the site effect, which includes lateral inhomogeneities in rock/soil properties and the cavity effect (harrison, 1976; zadro and braitenberg, 1999). for the surface tiltmeters, it is necessary to realize a stable coupling to the rock. the ground coupling is not quantifiable; testing consists in the periodic verification of setting that is effected by slightly compressing the ground in proximity of the sensor alternatively along the sensitivity directions of bubbles and verifying the recorded tilt values. after this test, the sensor will acquire its new position of balance, which must coincide with the one previously taken. 10. data acquisition and processing for each station of the ov tilt network, four series of signals acquired in real time are available; the first two are the signal acquired by ns and ew components, the third one concerns air temperature at the soil level and the last is the supply voltage. the acquisition frequency is currently of 6 cph at bai, cmd and ovo, of 2 cph at dma, dmb and dmc, and of 360 cph at trc (table i). the signal is analized using equal criteria for each station, but the software used differs according to the age of the station itself as well as to the format of the raw data and to the noise present in the monitored site. the final format of all the data acquired is defined by standard criteria. the most recent station dates back to 1996 and the oldest one to 1991. the following phase of elaboration uses graphic-analytical procedures implemented through the software dadisp by dsp development corporation (dadisp, 1996). the signal is cleaned by detecting possible offsets and spikes connected to differences in supply voltage of the stations; spikes are immediately removed through polynomial detrending and anomalous values residuation, while the offsets and the possible interruption in acquiring data compel us to operate on the signal replacing gaps by null values or constant values. the thermic compensation of the tilt recorded by the two components and the conversion from mv to µrad are performed with eq. (6.1). afterwards, the ew and ns components are combined to a vectorial plot of the tilt variation over time. every point recorded by the plot is defined by a pair of values (tiltew t , tiltns t ) referred to time t. each variation (tiltt+n − tiltt) represents the ground tilt recorded over the nth time period characterized by both the modulus and the direction. the adjustment and the orientation of the biaxial sensors is pursued in such a way that positive and negative values indicate ne sinking and uplift respectively. the tilt variations are such that northward down if n tilt+ >tiltt tns ns eastward down if n tilt>+tiltt tew ew . table i. ov tilt network: description and technical features. station sensor number latitude longitude depht a.s.l.(m) depht (m) place description installation date dma agi 702 1219 40°49′57′′ 14°06′47′′ 30 −18 pozzuoli gallery march 1991 dmb agi 702 1158 40°50′06′′ 14°06′46′′ 50 −13.5 pozzuoli gallery march 1991 dmc agi 702 419 40°50′12′′ 14°06′46′′ 48 −20.5 pozzuoli gallery march 1991 bai agi 702 1593 40°48′29′′ 14°04′26′′ 20 0 baia cistern june 1992 of castle ovo agi 702 1592 40°49′36′′ 14°23′52′′ 608 −25 ercolano historical february 1993 building ov trc agi 702 2539 40°47′47′′ 14°25′22′′ 150 0 trecase forestry march 1996 barrack cmd agi 702 2537 40°46′40′′ 14°24′23′′ 120 −2 torre surface well june 1996 del greco station sampling a/d sfcal (µrad/mv) high gain sensitivity tcal tsf ks kz rate (cph) conv. (bit) (mv/µrad) (°c) (°c/mv) (%/°c) (µrad/°c) dma 2 12 0.09960 x(ns) 0.10018 y(ew) 10.0-09.9 21 .1 0.05 1.5 dmb 2 12 0.09936 x(ns) 0.10005 y(ew) 10.1-10.0 27 .1 0.05 1.5 dmc 2 12 0.10000 x(ns) 0.09615 y(ew) 10.0-10.4 22 .1 0.05 1.5 bai 6 12 0.09964 x(ns) 0.10030 y(ew) 10.0-10.0 21 .1 0.05 1.5 ovo 6 12 0.09965 x(ns) 0.10000 y(ew) 10.0-10.0 20 .1 0.05 1.5 trc 360 12 0.10000 x(ns) 0.10010 y(ew) 10.0-10.0 17.3 .1 0.05 1.5 cmd 6 12 0.09964 x(ns) 0.09947 y(ew) 10.0-10.1 16 .1 0.05 1.5 1306 ciro ricco, ida aquino and carlo del gaudio tilt direction is calculated clockwise moving from n over time, whereas the percentage of counts of each recorded azimuth is shown by a histogram. 11. spectral content of the acquired signals up to now, we considered the most important steps by which we work out the tilt temporal tendency in a certain station. however, the above mentioned ground tilt still present part of deformation due to the thermoelastic response of the rocks that, as we have already seen, may be disregarded only if we use sensors placed in a very deep well. spectrum bands, where thermoelastic effects are more likely to be produced, coincide with the diurnal and annual components. these bands may be very easily identified, analysing the thermal and ground tilt data in the frequency domain using the fast fourier transform (fft) algorithm (oppenheim and schafer, 1975) into the dadisp environment. figure 8a,b compares the spectrum of the tilt signal (ns component) from station bai to the spectrum of the temperature recorded by the sensor. the interval considered is four years (from 1998 to 2001). the prevailing of 24 h component (jentzsch et al., 2002) is noticed in both spectra while the tilt signal gives the main semi-diurnal constituents of the tidal spectrum (melchior, 1978; ricco et al., 2000). the removal of the effect caused by thermoelastic soil deformation is a much debated problem, because the action of filtering the spectrum components of solar provenance through the tilt signal inevitably modifies the signal itself, particularly at periods of the order of months. this can even interfere, in some way, with the understanding of the pattern of ground deformation in volcanic areas. on the other hand, we have experimentally proved that, as concerns tiltmeter data acquired by ov, frequency filtering is likely to bring more reliable results at least regarding diurnal and se1307 ground tilt monitoring at phlegraean fields (italy): a methodological approach fig. 8a,b. spectral analysis of tilt and te signals recorded by bai station over the years 1998-2001 (210384 data points with a sampling frequency of 6 cph). thermal components given by the two signals are to be ascribed to the thermoelastic ground deformation. the semi-diurnal components, not given by the te signal, are related to the influence of sea tide in the baia gulf but also to that of earth tide components: a) fft of tilt data (µrad) and (b) fft of the te data (°c). mi-diurnal components. otherwise, for longer periods, other analysis techniques must be implemented. the procedure adopted to remove periods lasting more than 8 h is shown in fig. 9a-n for the signals from trc station. 12. tilt recording of superficial earthquake waves the filtering procedure shown in fig. 9a-n concerns the recording made on the 18th april 2002. some surface waves caused by the earthquake that hit the lucano appenines (latitude 40.612, longitude 15.594, depth 4.5 km, local magnitude 4.1) on that date are recorded in the tilt components. figure 10a,b depicts 20 min of the signal shown in fig. 9a-n. a large number of regional earthquakes of magnitude larger than 4 and a good number of teleseismic events have been recorded by the trc station and the ovo station, although the latter has a sampling frequency 60 times less than that of trc. 13. application of a statistical procedure of thermal correction to a tilt data we have quantified tilt dependence on the temperature through a linear model of type y a a x0 1= +! , but this can also be done applying a polynomial model of type y a a x a x0 1 2 2= + + +! a xn n f+ + where n is the polynomial order. assuming te as the independent variable, the relation between the recorded thermal data and the average of the ground tilt data can be worked out through a linear least square regression model. assuming that every tilt value has a gaussian distribution, it is possible to obtain, by a dispersion diagram of the pair of values (tei, tilti), the a b 1308 ciro ricco, ida aquino and carlo del gaudio a d g l b e h m c f i n a b fig. 9a-n. example of how components lasting more than 8 h and concerning the signals recorded by trc station on the 18th april 2002 are filtered. in (a), (b) and (c) the two tiltmeter signals and the termic one, acquired every 10 s, are shown respectively, while in (d), (e) and (f), the signals undergo a 1 h moving average procedure, which allows us to reduce noise. in (l), (m) and (n) the inverse fast fourier transform (ifft) of the smoothed signals are computed, highlighting only harmonic content having a period of time lasting more than 8 h. in (g), (h) and (i) harmonic having a period of time lasting less than 8 h are shown: a) ns signal, d) smoothed data, g) high pass data, l) low pass data; b) ew signal, e) smoothed data, h) high pass data, m) low pass data; c) te signal, f) smoothed data, i) high pass data and (n) low pass data. 1309 ground tilt monitoring at phlegraean fields (italy): a methodological approach ‘best’ interpolating curve containing the necessary data to minimize the residual sum of squares y a a x y yi i i0 1= t (davis, 2002). from the solution of the so-called normal equations, a0 and a1 (which define the parameters of the linear model we were looking for) are obtained. in this case, the intercept of the best straight line is a0 and its slope or regression coefficient is a1. we have verified that the assumption of the linear dependence tilt = f (te) can be held as effective only in those intervals in which te, previously filtered at high frequencies, shows an increasing or decreasing trend in time. the yearly te signal recorded is decomposed in a table with n subseries having te values and whose trend is highly increasing or decreasing, according to a spectral band having frequencies lower than 1 cpd. then, n matrices with 2 columns are developed, each of them containing couples of ordered values (te, tilt), which allowed us to calculate the n best straight lines. afterwards, the difference between the n subseries of the original tilt data and n best straight line will be calculated and the whole table so unravelled creating a single vector; obviously, this is done after having deducted the relevant offsets. the new signal so computed is very much less correlated to temperature than the original one. moreover, for each subseries, the following statistical indicators are calculated: i) the bravais-pearson linear correlation coefficient r (equal to the ratio between the covariance of two variables to the product of their standard deviations), which indicates the degree of interrelation between variables te = x and tiltns = y. ii) the regression coefficient a1 (already defined). iii) the standard error of the estimate σ (equal to the square root of the ratio between the residual sum of squares of y to n − 2). these indicators immediately verify the goodness of fit. in this procedure, that we call thermal decorrelation, a linear model is assumed to simplify the calculus, but linearized or more complex models, may be also assumed. 14. monitoring of ground movement at phlegraean fields, tilt monitoring allowed us to detect from 1988 a deflation phase confirmed by levelling data. the maximum vertical displacement observed between 1988 and 2001 was − 405 mm in pozzuoli (fig. 2). the main uplift which interrupted the ground subsidence occurred during the first six months of 1989 (74 mm uplift at pozzuoli) and induced a 90° tilt rotation clockwise at dmb station (ricco et al., 1991, 1994) (fig. 11). unfortunately, the other stations did not record remarkable variations of ground inclination. the tilt network recorded the following ground lowering and, in the year 2000, a new fig. 10a,b. tilt variations induced by an earthquake with epicentral distance of 1.2°. in this figure, 20 min (120 data points) of the ns (a) and ew (b) signals recorded at trc station are shown. the followings numbers close to the peaks point out the main variations of the signals: 50th acquisition at 20:56:56 utc; 51th at 20:57:06 utc; 52th at 20:57:16 utc; 53th at 20:57:26 utc; 55th at 20:57:46 utc; 56th at 20:57:56 utc; 60th at 20:58:36 utc. fig. 11. ground tilt and te recorded by the network ov-ipg in the period 1987-1992: every signal includes more than 100.000 data points with sample rate of 1 cph. the zoomed windows concern the uplift phase. tilt decrease must be interpreted as sinking (southward and westward down) otherwise as uprising. trend inversion over the march-august period (confirmed by levelling data showing an uplift of 28 mm centered at pozzuoli); this is quite clear for both the signals acquired from 1998 to 2001 at dmb (fig. 12a-c). in this figure it is possible to observe that the tilt components show, during the middle months of 2000 and in correspondence with a similar thermal trend, an opposite trend with respect to that of 1998, 1999 and 2001 (achilli et al., 2001). in these three years, the ground tilt average occurred in the se sector, in correspondence with the first 8-9 months of each year (corresponding to the ground te increase) and in the nw sector during the remaining months (corresponding to the ground temperature decrease). otherwise, in the year 2000, the uplift phase due to the bradyseism affects the tilt vector trend, reversing its direction. we can note that the annual component on tilt signals decreases during 2000 because the inversion of the tilt direction is verified. in quantitative terms, the linear regression and correlation coefficients between te and tilt calculated during 2001 (− 4.49 µrad/°c and − 0.92 for ns component, 2.92 µrad/°c and 0.86 for ew component respectively), during 1999 (0.67 µrad/°c and 0.32, 3.59 µrad/°c and 0.79) and during 1998 (− 1.91 µrad/°c and − 0.66, 3.59 µrad/°c and 0.76), appreciably result different from those calculated during 2000 (− 0.63 µrad/°c and − 0.21 for ns component, 0.35 µrad/°c and 0.27 for ew component). it is plain that the comparison of the signals in the different years is very useful because it reveals the possible anomalies in comparison to a «normal» trend that, in this volcanic area, is represented by the subsidence phase. however, when we have few data, or when the geometry of the deformation is not known or is complex, or when we want to know only the variations of the inclination induced by volcanism, it is necessary somehow to eliminate the thermal effect. for thermal correction we chose the ns component of the dmb station because it recorded the phlegraean deformation in the best way. the test concerned the years 2000 and 2001 with the intent to obtain both a sufficiently filtered signal but mainly to verify if the tilt record in the year 2000 succeeded in discriminating the uplift. the 1310 ciro ricco, ida aquino and carlo del gaudio te signals were decomposed into 87 subseries, (each of which ended with a peak or valley) and the application of statistical procedure to a ns tilt component produced a corrected signal showing an inclination towards n beginning by the middle of march (fig. 13a-d). the statistical indicators r and σ related to the subseries indicate a good fit (fig. 13a-d). the same application was performed on te data recorded in 2001; in this case the sequence was decomposed into 124 subseries and the decorrelated ns signal underwent a contraction in comparison to original one but still showed an inclination towards s throughout the year (subsidence) (fig. 14a-c). also r and σ were fig. 12a-c. this figure shows a comparison between tilt ns (a), ew (b) and te (c) recorded at dmb station over the years 1998, 1999, 2000 and 2001. note the opposite trend in the ns (a) and ew (b) components recorded in 2000. fig. 13a-d. application of a statistical procedure of thermal decorrelation to the ns component of the tilt recorded by dmb in 2000 through a linear dependence model. in (a), (b) and (c) the correlation coefficients, the regression coefficients (µrad/°c) and the standard errors of the estimate (µrad/ n 2) are displayed, respectively. the corrected signal (d) shows an inclination towards n (ground uplift). a b c d 1311 ground tilt monitoring at phlegraean fields (italy): a methodological approach a b c 1312 ciro ricco, ida aquino and carlo del gaudio fig. 14a-d. the thermal correction is applied to the ns tilt component recorded by dmb in 2001. the decorrelated signal undergoes a contraction in comparison to the original signal showing an inclination towards s (subsidence). a b c d computed and indicated a good global fit of the data (fig. 14a-c). in both uplift and subsidence, the statistical decorrelation provided good results, in accordance with the other geodetic data (from levelling and gps). 15. conclusions the correct estimate of the variations of ground inclination induced by volcanic activity implies a knowledge of the many causes that influence the tilt signal recorded, such as environmental temperature, atmospheric pressure, rainfall, oscillation of the water table, pressure and ocean loading, earth tides, ground coupling, etc. for this reason, tilt data must be compared with those acquired by the other geodetic methods such as levellings, gps and sar and, in the case in which the stations are near to the sea, it is also necessary to consider the tide-gauge data. almost 15 years of data acquired in different sites have yielded useful information on ground deformation, and the data have disclosed two uplifts of modest entity, but that give the idea that a net of this type shows traces of the inversions of ground displacement. besides we have tried a quantitative approach to estimate the thermal effect, applying a statistical method to data recorded in recent years; 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(1962): introduzione alla fisica matematica (zanichelli), 309-328. ricco, c., c. del gaudio, f. obrizzo and g. luongo (1991): misurazioni delle variazioni delle inclinazioni del suolo ai campi flegrei, in atti del 10° convegno annuale gngts, 1003-1012. ricco, c., c. del gaudio, f. obrizzo and g. luongo (1994): monitoraggio dei movimenti lenti del suolo in aree di vulcanismo attivo: campi flegrei, boll. soc. it. fotogramm. topogr., 3, 33-68. ricco, c., c. del gaudio, f. quareni and w. marzocchi (2000): spectral analysis of the clinometric data at the phlegraean fields from 1992 to 1998, ann. geofis., 43 (5), 939-950. rosi, m., l. vezzoli, p. aleotti and m. de censi (1995): interaction between caldera collapse and eruptive dynamics during the campanian ignimbrite eruption, phlegraean fields, italy, bull. volcanol., 57, 541-554. scandone, r. and l. giacomelli (1998): vulcanologia. principi fisici e metodi d’indagine (liguori editore, napoli), pp. 608. change in strain comes mainly from the rain: kipapa, oahu, bull. seismol. soc. am., 71, 1625-635. zadro, m. and c. braitenberg (1999): measurements and interpretations of tilt-strain gauges in seismically active areas, earth sci. rev., 47, 151-187. (received june 5, 2003; accepted december 19, 2003) 1314 ciro ricco, ida aquino and carlo del gaudio sleeman, r., h.w. haak, m.s. bos and j.j.a. van gend (2000): tidal tilt observations in the netherlands using shallow borehole tiltmeters, phys. chem. earth a, 25 (4), 415-420. weise, a., g. jentzsch, a. kiviniemi and j. kääriäinen (1999): comparison of long-period tilt measurements: results from the two clinometric stations metsähovi and lohja, finland, j. geodyn., 27, 237-257. wolfe, j.e., e. berg and g.h. sutton (1981): the articolo galadini2.pdf key words paleoseismology – active fault – holocene – central italy 1. introduction recent works summarising data on the geometry and the kinematics of the active faults show that the central apennines are affected by two parallel active fault sets characterised by predominantly normal movements (barchi et al., 2000; galadini and galli, 2000). the identification of the active faults mainly followed geomorphologic criteria and paleoseismological data are available for few faults only (i.e. the campo imperatore fault, giraudi and frezzotti, 1995; the aremogna-cinquemiglia fault, d’addezio et al., 2001; the ovindoli-pezza fault, pantosti et al., 1996; the fucino fault, galadini and galli, 1999). a comparison between the active faulting framework and the distribution of the seismicity, as derived from the available historical seismic catalogues (e.g., working group cpti, 1999; reporting earthquakes since 217 b.c.), indicates that earthquakes with m 6.5 can be related to the westernmost fault set. paleoseismological data on the ovindoli-pezza fault (which is part of this 815 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. fabrizio galadini, istituto di geologia ambientale e geoingegneria, area della ricerca di roma «tor vergata», via del fosso del cavaliere 100, 00133 roma, italy; e-mail: f.galadini@igag.cnr.it paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults fabrizio galadini (1) and paolo galli (2) (1) istituto di geologia ambientale e geoingegneria, cnr, roma, italy (2) servizio sismico nazionale, dpc, roma, italy abstract paleoseismological analyses have been performed in the central apennines along faults showing geomorphological evidence of late quaternary activity and characterised by the absence of historical seismicity. three trenches were made along the mt. vettore fault, across a scarp on a late pleistocene-holocene alluvial fan. the youngest displacement event (e1) occurred after 4155-3965 years bp and before the 6th-7th century a.d., a previous event (e2) occurred between 5940-5890/5795-5780 years bp and 4155-3965 years bp, while the oldest event (e3) occurred between 18.000-12.000 years bp and 5940-5890/5795-5780 years bp. one trench was excavated across the laga mts. fault which gave evidence for two displacement events after 8320-8150 years bp. the minimum vertical slip rate estimated through the paleoseismological analysis of the mt. vettore fault is 0.11-0.36 mm/yr, while the minimum slip rate along the laga mts. fault is 0.12 mm/yr. the paleoseismologically inferred recurrence interval is not longer than 4690 years for the mt. vettore fault and not longer than 7570 years for the laga mts. fault, while the minimum elapsed times since the last activation are 1300 and 800 years for the two faults, respectively. the evaluation of the former elapsed time was based on paleoseismological data, while the estimation of the latter was based on the absence of historical earthquakes which may have been caused by the laga mts. fault and on the completeness of the historical catalogues for the large magnitude events in the last eight centuries. based on the length of the fault at the surface, earthquakes with m 6.5 and 6.6 may be expected from the activation of the mt. vettore and laga mts. faults, respectively. fault set) suggest that also this fault activated during the historical period (the youngest event has been radiocarbon dated at the middle ages; pantosti et al., 1996). on the other hand, the active faults of the eastern set show geomorphic evidence of past activation related to strong earthquakes, but no significant historical earthquakes may be related to them (galadini and galli, 2000). for this reason we usually define these faults as «silent». the silent state of the eastern faults suggested to adopt a paleoseismological approach to investigate two faults of this set which display clear evidence of recent movements, i.e. the mt. vettore and the laga mts. faults. investigations through the excavation of trenches were made during 1998 and 1999. the following sections will be dedicated to the recognition of the paleoseismologically inferred displacement events. tectonic data are supported by the stratigraphic analysis related to the sedimentary units detected in the trenches. further sections will be dedicated to the definition of the parameters describing the fault behaviour (slip rate, recurrence interval, elapsed time since the last activation, maximum expected magnitude). 2. seismotectonic framework the apennine chain is affected by active normal faulting (galadini et al., 2001 and references therein). moderate-to-large-magnitude earthquakes reported in the available catalogues (e.g., working group cpti, 1999) define a seismically active belt in the inner portion of the apennine chain, following the apennine axis. active faulting has been documented for the abruzzi sector of the central apennines (e.g., barchi et al., 2000; galadini and galli, 2000; valensise and pantosti, 2001; roberts et al., 2002). some works define two main parallel sets of prevalently normal-to-oblique faults (barchi et al., 2000; galadini and galli, 2000; d’addezio et al., 2001; valensise and pantosti, 2001). in contrast, the works by ghisetti and vezzani (2000) and roberts et al. (2002), mainly dedicated to structural aspects, define a larger number of active normal faults. other works at a regional scale report active fault sets made of master faults showing strike-slip movements and shorter faults characterised by normal movements with a style typical of the strike-slip tectonics (cello et al., 1997). all the available literature indicates, however, that the main active faults of the central apennines are located along the borders of intermontane extensional basins, they are several tens of kilometers long and have been responsible for some of the strongest earthquakes which occurred in peninsular italy in the last few centuries (e.g., barchi et al., 2000; galadini and galli, 2000; valensise and pantosti, 2001). taking into account that: 1) the evidence of recent tectonic activity along some of the faults described by ghisetti and vezzani (2000) and roberts et al. (2002) appears debatable (e.g., compare the avezzano-bussi fault in the former work and the salto valley fault in the latter with the data reported in galadini and messina, 2001), and 2) the role of strike-slip movements in the central apennines seems definitely secondary (as indicated by the great number of works available on the recent faulting of the apennines: see the literature mentioned in galadini et al., 2001 and valensise and pantosti 2001) we decided to adopt the more widely accepted structural scheme (fig. 1) for the active tectonic regime (barchi et al., 2000; galadini and galli, 2000). the apennine sector reported in fig. 1 has been affected by several earthquakes with m > 6.5. in particular, the catalogue by working group cpti (1999, from which all the magnitude values and the epicentral locations have been taken) reports the 1349, 1456, 1703 (january 14), 1703 (february 2), 1706, 1915 events as the strongest earthquakes in the investigated region. galadini and galli (2000) gathered all the available information on active faulting and paleoseismology of the region. data show that the recurrence interval for surface faulting events in the central apennines is always longer than 1000 years and usually plurimillennial. slip rates are lower than 1 mm/yr and the length of the active faults ranges between 16 and more than 30 km. galadini and galli (2000) compared the data on the active faults with the long-term seismicity; this procedure gave rise to the hypothesis that almost the entire westernmost set of normal faults experienced activation after 1000 a.d. 816 fabrizio galadini and paolo galli (only the sangro valley fault seems to be silent) while, in contrast, the available seismic catalogues do not indicate the occurrence of large earthquakes which may be consistent with the activation of faults of the easternmost set. the mt. vettore and laga mts. faults are part of this set of silent faults. 3. paleoseismological analysis the main purpose of our work was the definition of the chronology of the fault activity and the characteristics of the displacements (amount of offset, type of movement, ...). the chronological aim conditioned the choice of the 817 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults fig. 1. seismotectonic framework of the investigated area. the investigated faults are reported in red. earthquake epicenters and the highest intensity datapoints distribution (hidd; size proportional to intensity, i > 8 mcs) were taken from magri and molin (1984) for the 1456 earthquake, boschi et al. (1995) for the 1349 and 1706 earthquakes, monachesi and stucchi (1998) for the 1703 earthquake, molin et al. (1999) for the 1915 earthquake; all the epicenter locations were derived from working group cpti (1999). mvef mt. vettore fault; nfs norcia fault system; lmf laga mts. fault; uafs upper aterno valley fault system; af assergi fault; cif campo imperatore fault; cvf mt. cappucciata-mt. san vito fault; cfcf campo felice-colle cerasitto fault; mavf middle aterno valley fault; opf ovindoli-pezza fault; mmf mt. morrone fault; ff fucino fault; mrpf mt. rotellamt. pizzalto fault; acf aremogna-cinquemiglia fault; usfs upper sangro valley fault system. trench sites. four trenches were excavated across those splays affecting the younger deposits and landforms and samples collected for radiocarbon dating (table i) or mineralogical analysis. radiocarbon dating was generally done on bulk sediments. dated units are related to colluvial (cast-6, -9, -10, camp-21) or alluvial (cast-5, camp-1, -7) deposition. dating of colluvial deposits generally gives, however, only the lower chronological limit for the deposition. paleosols were dated in the trench excavated across the laga mts. fault (camp-15, -17). in this case, sampling was done on the a horizons of the soils. one wood fragment (camp-11) was also collected from displaced deposits along the laga mts. fault. samples for radiocarbon dating were analysed by beta analytic inc. (miami, u.s.a.). two samples (camp-4, -11) gave evidence of an age older than the limit of the method. radiometric counting was generally employed. in three cases (camp-1, -7, -11), due to the lower sample size, ams counting was used. the conventional radiocarbon ages obtained (all standard analyses; table i) were calibrated by beta analytic (if conventional ages were younger than 19.000 years bp) using the intcal98 database reported in stuiver et al. (1998). ages reported in the text represent the 1 calibrated results (68% of probability). table i, however, also reports the 2 values, thus permitting the reader to define the chronological constraints of the displacement events based on the 2 chronology. further chronological constraints were derived from the late pleistocene-holocene stratigraphic setting of the investigated region which is particularly well known, for the moun818 fabrizio galadini and paolo galli table i. radiocarbon dates of samples collected in the investigated areas and in the trenches excavated across the mt. vettore and laga mts. faults. r radiometric technique; blc bulk/low carbon material procedure; ams accelerator mass spectrometry technique. sample lab. code analysis c13/c12 measured conventional calibrated age calibrated age sample beta ratio age.bp age.bp 1 bp 1 bp description cast5 125045 r, blc – 25.5 ‰ 5150 ± 60 5150 ± 60 5940-5890/5795-5780 6005-5745 alluvial deposit (silt) cast6 125046 r, blc – 25.6 ‰ 3740 ± 70 3730 ± 70 4155-3965 4275-3870 colluvium (silt) cast9 125048 r, blc – 25.6 ‰ 1850 ± 60 1840 ± 60 1840-1705 1885-1600 colluvium (silt) cast10 125049 r, blc – 25.7 ‰ 1670 ± 60 1660 ± 60 1600-1510 1705-1400 colluvium (silt) camp1 106260 ams – 28.1 ‰ 10.020 ± 70 9970 ± 70 11.637-11.608/11.5969824-9045 charcoal 11.592/11.580 fragments -11.329/11.323within 11.297/11.272-11.261 alluvial deposits camp4 118167 r, blc – 26.2 ‰ > 39.640 > 39.630 / / colluvium (clayey silt) camp7 106261 ams – 27.1 ‰ 7490 ± 50 7460 ± 50 8320-8150 8345-8125 charcoal fragments within alluvial deposits camp12 128977 r, blc – 28.7 ‰ 33.180 ± 470 33.120 ± 470 / / lacustrine organic silt camp15 142196 r, blc – 24.4 ‰ 1230 ± 40 1240 ± 40 1245-1135/1110-1095 1270-1065 buried soil (silty sand) camp17 163998 r – 25.0 ‰ 2430 ± 120 2430 ± 120 2730-2340 2770-2290/2270-2160 buried soil (silty sand) camp20 120600 ams – 25.1 ‰ 7650 ± 60 7650 ± 60 8425-8365 8500-8330 organic silt (colluviated paleosol?) camp21 142197 r, blc – 24.2 ‰ 9200 ± 230 9220 ± 230 10.685-10.190 11.150-9730 colluvium (silt) tainous area, through investigations made with a paleoclimatological perspective since the end of the 80 s (e.g., frezzotti and giraudi, 1989, 1990a,b; frezzotti and narcisi, 1989, 1996; giraudi, 1994, 1995, 1998a,b, 2000; giraudi and frezzotti, 1997). in these works, the authors define the stratigraphy of continental late pleistocene-holocene deposits of various facies (glacial, fluvioglacial, alluvial, aeolic, etc.) also reporting fundamental information about the paleosols which have formed for about 30.000 years bp. detailed chronological constraints are also given. 4. mt. vettore castelluccio plain 4.1. geomorphic analysis the mt. vettore normal fault is nnw-sse to nw-se trending and is about 18 km long (figs. 1, 2a,b and 3a,b). it is located in a high mountainous environment, since mt. vettore reaches 2476 m a.s.l. only one major intermontane basin formed along the fault, i.e. the castelluccio plain (fig. 3a,b). according to coltorti and farabollini (1995), deposition in the piedmont area is related to the abundant clastic material produced by 819 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults fig. 2a,b. panoramic views of the two areas paleoseismologically investigated. a) view of the western mt. vettore slope, characterised by two bedrock fault scarps (marked by the arrows); the trench sites (not visible in the photograph) are located in the flat area at the foot of the slope (see next figures for further details). b) view of the sw slope of the laga mts.; the arrows mark the scarps presumably related to the holocene activity of the fault (the trench site is located to the right of the photograph, see next figures for further details). a b 820 fabrizio galadini and paolo galli fig. 3a,b. a) map of the mt. vettore fault area; the trenches were excavated across the minor segment located in the prate pala area. b) geomorphological map of the castelluccio plain: the prate pala scarp affects the large late pleistocene-holocene alluvial fan fed from the valle delle fonti area. b a 821 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults the mt. vettore sw slope (fig. 3b). these authors mapped slope deposits and landslide accumulations in the most elevated areas, while they reported alluvial deposits and landforms in the piedmont area. we analysed a large alluvial fan (fig. 3b) fed by the valle delle fonti creek (castelluccio fan), through the use of aerial photographs, and recognised five different depositional phases (fig. 4a). these phases are known throughout the central apennines (e.g., the majelama alluvial fan, frezzotti and giraudi, 1992; the campo imperatore alluvial fans, giraudi, 1994). available works related the 1st depositional phase to the last glacial maximum (lgm, about 22.600 years bp in the central apennines, i.e. the campo imperatore stade in giraudi and frezzotti, 1997), while the 4th and the 5th phases are related to the boundary late pleistocene-holocene and to about 3800-3200 years bp, respectively. no chronological constraints are available for the 2nd and 3rd alluvial phases. the castelluccio plain is, therefore, partly filled by an alluvial fan which probably formed between about 23.000 and 3200 years bp. two fault splays are easily detectable along the mt. vettore western slope, since they formed impressive limestone scarps. in particular, the easternmost scarp is several metres high and shows a fault plane displacing only the carbonate bedrock. the westernmost fault, in contrast, places the bedrock in contact with late pleistocene slope deposits. based on this geomorphic evidence, the fault was considered active by calamita and pizzi (1992) and cello et al. (1997). more recently, galadini and galli (2000) confirmed the recent fault activity by reporting the evidence of displacements affecting recent deposits in the piedmont area. in particular, during 1997-1998, the authors made analyses on the mt. vettore fault and investigated a scarp (prate pala scarp; fig. 4b,c) in the castelluccio alluvial fan, not reported in the previous works. the prate pala scarp formed into the gravel of the alluvial fan and, in particular, it affects the top surfaces related to the 4th and 5th alluvial phases. we interpreted it as the result of recent fault activity, since it is completely divergent from the drainage which recently developed on the alluvial fan and small creeks appear to be displaced across the scarp (fig. 4a). two topographic profiles were made to estimate the height of the prate pala scarp (fig. 4b,c). this height is about 3.3 m in the area of the 4th alluvial fan, while it is about 2 m in the area of the 5th alluvial fan. the trenches made across the scarp in the area of the 4th alluvial fan (see below) showed that these values result from correlation of different surfaces on both sides of the scarp. the topographic surface of the footwall area has an erosional origin and is carved into the gravels, while the surface in the hangingwall is a top depositional surface of colluvial units (see sections dedicated to the trench analysis). if we consider that about 1 m of colluvial units deposited in the hangingwall (as derived from paleoseismological data), the vertical offset affecting the 4th alluvial fan is 4.3 m. since the 4th alluvial fan can be related to the boundary late pleistocene-holocene (about 12.000 years bp), the vertical slip rate is 0.36 mm/yr. the 5th alluvial fan can be related to about 3800-3200 years bp. in this case, considering the vertical offset of 2 m, the slip rate is 0.52-0.62 mm/yr. since the mt. vettore fault zone is made of at least three parallel splays, the reported values define minimum slip rates for the entire structure. in short, the minimum slip rate for the mt. vettore fault ranges between 0.36 and 0.62 mm/yr. three trenches were dug across the prate pala scarp and due to logistic problems (the castelluccio plain is intensely cultivated) we had the opportunity to make the excavations only in the area of the 4th alluvial fan. the depth of the trenches was always conditioned by the presence of the gravels, generally showing a massive texture, which are not useful for the definition of the faulting history, due to their homogeneity across the fault. moreover, the trenches showed the same stratigraphic succession and structural setting. for this reason, we report a detailed description of trench 1 and the few differences observed in trenches 2 and 3. 4.2. paleoseismology 4.2.1. trench 1 the castelluccio alluvial fan is entirely made of gravels and for this reason we decided to focus our attention on the most superficial units. the origin of most of these units (2, 3, 4 in fig. 5a,b) is colluvial and therefore the radiocarbon ages (1600-1510 years bp, 1840-1705 years bp, 4155-3965 years bp, 1 cal ages for units 2, 3, 4, respectively) define lower chronological limits for the deposition. however, the colluvial events to which units 2 and 4 are related have not only a local importance, since these units were found in all the trenches investigated. they were also found in a small excavation we made 300 m north of the trench area for stratigraphic purposes. it is therefore probable that these colluvial phases affected a large part of the castelluccio alluvial fan and that the re-deposition of former sediments/soils occurred during periods of climatic instability. some constraints about the deposition of unit 2 may derive from the climatic history of the investigated region. a colluvium such as that of unit 2 is generally related to the mobilization of surficial soils which are no more in equilibrium with the environment of formation. this lack of equilibrium is due to the modification of the climatic conditions which permitted the pedogenesis. the climatic change chronologically close (and subsequent) to the reported radiocarbon date was identified in the central apennines as having occurred during the 6th-7th century a.d. in particular, since in this period a glacial advancement occurred in the gran sasso chain area (see fig. 1 for location), defined as calderone 3a stade (giraudi, in press). this glacial advancement is consistent with the increase in the lacustrine level in the fucino lake (see fig. 1 for location) during the 7th century a.d. (giraudi, 1998a) and probably represented the effect of a widespread climatic change towards cooler and drier conditions, as suggested by similar chronological constraints for the advancement of alpine glaciers (orombelli and porter, 1982; strumia, 1997). unit 2 may have been deposited, therefore, during the 6th-7th century a.d. unit 3 is lithologically similar to unit 2, as for the fine fraction. the carbonate pebbles are, however, finer and sparser than in the unit pre822 fabrizio galadini and paolo galli fig. 4a-c. a) geomorphological map of the castelluccio alluvial fan: the fan resulted from the deposition during five different alluvial phases. b) panoramic view of the scarp on the alluvial fan (note the slope of the road when it crosses the scarp). c) 3d view of the scarp (vertically exaggerated), location of the trenches and scarp height as derived from the profiles a-a and b-b reported in fig. 4a. c a b viously described. the maximum thickness is about 50 cm. an origin similar to that of unit 2 can be defined for unit 3. the sandy fraction gave a date of 1840-1705 years bp (1 cal age). due to the colluvial origin of unit 3, the same problems defined for unit 2 about the implications of the obtained date may be outlined for unit 3. as for unit 4, the high content of organic matter and the monogenic origin of the deposit (containing very few pebbles) suggest that the colluvium originated from re-deposition of an exposed a-horizon of a soil. for this reason the obtained radiocarbon date cannot be far from the age of colluviation. paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults fig. 5a,b. a) log of the n wall of trench 1 excavated across the prate pala fault scarp (mt. vettore fault); the analysis of the wall defined the occurrence of three displacement events. b) panoramic view of trench 1 (n wall). 823 a b finally, the sedimentological characteristics (faintly stratified silty sands) of unit 5 (radiocarbon dated at 5940-5890/5795-5780 years bp, 1 cal age) define an alluvial origin. major (f1) and minor (f2 to f6) faults place the carbonate gravel (unit 6) in contact with the colluvial and alluvial units 4 and 5 (fig. 5a,b). considering the whole geometry, f3 is probably the antithetic fault of f1. among these two faults, a minor shear plane (f2) displaces units 4 and 5. due to the reduced height of the trench, the fault planes are exposed for a few tens of centimetres, but show a high-angle dip (70°-80°). the main component of the movement is vertical, as indicated by the sinking of the gravels (unit 6) in the central part of the trench, the preservation of unit 5 and the thickness increase of unit 4 in the same area. moreover, the dip towards west of the unit 6 layers (fig. 5a,b) (while the alluvial fan deposition occurs towards south), together with the pebble imbrication related to similarly directed paleocurrents, probably results from gravel deposition controlled by the n-s tectonically-induced depression. the youngest displacement event (e1) occurred after the deposition of unit 4 which is the uppermost unit displaced by the faults f1 and f2. this event occurred, therefore, after 4155-3965 years bp (1 cal age, which is close to the deposition age of unit 4) and before the deposition of unit 2 which seals the fault planes, that is before the 6th-7th century a.d. the preservation of unit 4 only in small sectors of the footwall suggests areal erosion event after the displacement event. this erosional event erased the scarp-derived colluvium which possibly formed after the displacement. the vertical displacement of the base of unit 4 across fault f1 can be roughly estimated in 0.45 m, if an average line representing the base of this unit in the hangingwall is projected on the fault plane and the vertical separation with the same line at the footwall is calculated. this value is a minimum estimate, since the progressively higher elevation of the unit 4 base towards ne in the footwall is probably due to the fault-related displacement. a previous event (e2) affected units 5 and 6 before the deposition of unit 4. this is suggested by the lack of unit 5 in the footwall, between units 6 and 4. this means that unit 5 was displaced and eroded before the deposition of unit 4. further evidence for the occurrence of event e2 is represented by the minor fault planes f4, f5 and f6, which affect units 5 and 6, being sealed by unit 4. the event e2 occurred after 5940-5890/5795-5780 years bp (1 cal age, unit 5) and before 4155-3965 years bp which, based on the considerations about the origin of unit 4 (see section dedicated to the stratigraphy) may represent an age close to the deposition of this unit. due to the irregular depositional surface and the limited exposure of unit 5, the vertical offset related to e2 cannot be defined. a previous event or phase of events (e3) may be indicated by the faint layering of unit 6, dipping westwards and an imbrication of the pebbles indicating similarly directed paleocurrents. the deposition and therefore the attitude of unit 6 have been conditioned by the presence of a lowered area west of the fault f1. this lowered area suggests the occurrence of displacement events along the fault f1 before the deposition of unit 5. therefore the fault activity occurred before 5940-5890/5795-5780 years bp and after the deposition of unit 6 which, based on the geomorphological setting and the correlation with the succession of other central apennine alluvial fan (see previous section), may have occurred about 18.000-12.000 years bp. 4.2.2. trench 2 the trench was excavated 40 m north of trench 1. unit 7 is made of gravels in abundant sandy matrix. the texture is matrix-supported, the pebbles are angular or sub-angular, with long axes highly-dipping towards w. the described attitude suggests a deposition over a west-dipping scarp. since these deposits can only be detected in the fault zone and show typical colluvial characteristics, we interpret unit 7 as a colluvial wedge resulted from the retreat of a scarp carved into the gravels of unit 6. numerous shear planes affect the investigated section (n trench wall) and have been responsible for the displacement of units 4, 5, 6 and 7 (fig. 6a,b). some shear planes (f7, f10) are sealed by unit 4, other (f6 and f11) 824 fabrizio galadini and paolo galli are sealed by unit 2, while the shear planes f1 to f5 are sealed by unit 1. the youngest displacement event (e1) occurred after the deposition of unit 4 and was detected along the fault planes f2 to f6 and the fault plane f11. the available date for unit 4 (derived from trench 1, 4155-3965 years bp) represents the lower chronological limit. the upper chronological limit, similarly to trench 1, may be defined by the 6th-7th century a.d. (see previous section). the presence of unit 4 between fault planes f3 and f4 at depth (fig. 6a) suggests that faulting of unit 4 was also responsible for the formation of an open fissure between the mentioned fault planes (subsequently filled by the same faulted material of unit 4). since the bottom of unit 4 is very irregular and this unit is not present in the footwall, the vertical offset cannot be estimated. the interpretation of unit 7 as a colluvial wedge due to the retreat of a former scarp defines an older event (e2), as this unit was faulted during e1. since unit 4 deposited over this colluvial wedge (see the sector between the fault planes f1 and f2), the scarp which fed the colluvium formed – and the related event occurred – before 4155-3965 years bp (approximate deposition age of unit 4). this event may be the same as that responsible for the displacement of unit 5 in the western portion of the trench. in this case, the occurrence of an event before the deposition of unit 4 is suggested by the fact that this unit seals the fault planes f7 and f10. the lower chronological limit for e2 along f6 to f10 is defined by the age of unit 5 (5940-5890/57955780 years bp). due to the lack of correlative deposits in the eastern and western parts of the trench, the vertical offset cannot be defined. also in trench 2, a previous event (e3) may be defined by the gentle dip of unit 6 towards west, i.e. towards the area lowered by the fault activity. the dip of the gravel is probably due to the presence of a tectonically subsiding area also before the deposition of the younger units 825 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults fig. 6a,b. a) log of the n wall of trench 2 excavated across the prate pala fault scarp (mt. vettore fault); units are the same of fig. 5a,b; the paleoseismological analysis defined the occurrence of three displacement events. b) panoramic view of trench 2 (n wall). a b (5 to 1). this event or phase of events has, therefore, the same chronological constraints defined for trench 1, i.e. e3 occurred between 18.00012.000 years bp (probable age of unit 6) and 59405890/5795-5780 years bp (age of unit 5). 4.2.3. trench 3 trench 3 was excavated between the previously described trenches. also in this case we investigated the n trench wall (fig. 7a,b). the stratigraphic succession (units 4, 5 and 6) is affected by 8 shear planes defining fault activity before the deposition of unit 2, sealing the detected faults. a pervasively sheared area was detected between fault planes f1 and f4; shear planes were responsible for the warping of units 4 and 5 and the re-orientation of the pebbles of unit 6. we only reported the major shear planes (f1 to f3) affecting this highly deformed area (fig. 7a). the most recent event (e1) is indicated by the displacement of unit 4 along fault planes f1, f4 and f7 and by the continuous deformation between fault planes f1 and f4, responsible for the gentle dip towards w of the limits between units 4, 7 and 6. the displacement occurred after the deposition of unit 4, i.e. after 4155-3965 years bp. also in this case, on the basis of the discussion of trench 1 (see previous sections), the upper chronological limit of this event is represented by the colluvial deposition (unit 2) probably related to the 6th-7th century a.d. since unit 4 is almost absent in the footwall, it is not possible to precisely define the vertical offset. an estimate of the minimum vertical offset may be obtained by computing the vertical separation (1.2 m) between the base of unit 4 in the hangingwall and the limit unit 6/unit 2 (indicating the occurrence of erosional activity responsible for the erasure of unit 4) in the footwall (fig. 7a,b). the assumption of a subhorizontal attitude of the unit 4 depositional surface is necessary to consider this estimate as a reliable minimum vertical offset. we believe, however, that a local topography gently dipping westwards characterised the investigated area also before the deposition of unit 4 (as suggested by the fact that such kind of topography characterised the area during the deposition of unit 6, gently dipping towards w, fabrizio galadini and paolo galli fig. 7a,b. a) log of the n wall of trench 3 excavated across the prate pala fault scarp (mt. vettore fault); units are the same of fig. 5a,b; the paleoseismological analysis defined the occurrence of three displacement events. b) panoramic view of trench 3 (n wall). a b 826 and by the present topography). the estimate of the minimum vertical offset has to be taken, therefore, with caution. as already observed for the other trenches, a previous event (e2) affected units 5 and 6 before the deposition of unit 4, as suggested by the presence of fault planes (f5 and f6) sealed by the latter unit and by the lack of unit 5 in the footwall (east of f1), between units 4 and 6 (indicating erosion of unit 5 after its displacement and before the deposition of unit 4). the displacement occurred, therefore, after 5940-5890/57955780 years bp (age of unit 5) and before 41553965 years bp (approximate depositional age of unit 4). since unit 5 is scarcely exposed in the hangingwall and is absent in the footwall, it is not possible to give an estimation of the vertical offset of this unit. the presence of unit 5 between the fault planes f4 and f5 suggests that during e2 a fissure opened and was filled by the sediments of unit 5. as in the case of the previously described trenches, older displacement events (e3) are suggested by the gentle western dip of unit 6, indicating a deposition conditioned by the tectonically subsiding area bounded by the faults f1 to f6. the chronological constraints are the same as those defined for the previously described trenches, i.e. the lower chronological limit is 18.00012.000 years bp, while the upper limit is 59405890/5795-5780 years bp. 4.3. summary of the displacement events the three trenches excavated across the fault scarp in the castelluccio alluvial fan showed the same displacement succession, with at least two events occurred during the holocene. the youngest event (e1) was chronologically constrained between 4155-3965 years bp and the 6th-7th century a.d. an older event (e2) occurred between 59405890/5795-5780 years bp and 4155-3965 years bp. while e1 and e2, easily detectable in all the investigated trenches, were interpreted as related to individual displacement events, e3 is probably related to a number of events which occurred between 18.000-12.000 years bp and 5940-5890/ 5795-5780 years bp. 5. laga mts. campotosto basin 5.1. geomorphic analysis the nw-se-trending laga mts. normal fault is 30 km long and bounds two intermontane basins: the amatrice and campotosto basins, located along the northern and southern portions of the fault, respectively (fig. 8). this fault was already considered active by paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults fig. 8. map of the laga mts. fault. clear evidence of late pleistocene-holocene activity can be detected along most of the fault, apart from the northern portion (dashed) which was considered not active by galadini and messina (2001). 827 828 fabrizio galadini and paolo galli fig. 9a-e. a) geomorphological map of the campotosto area; the trench was excavated at site 3; b,c,d) topographic profiles across the laga mts. fault made at sites 1, 2 and 3, respectively; e) panoramic view of the trench site. c d e a b 829 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults calamita and pizzi (1994), cello et al. (1997), galadini and galli (2000). more recent studies showed that late pleistocene-holocene activity can only be related to the southern portion of the fault (galadini and messina 2001). these authors reported evidence of recent activity along a segment about 20 km long. lacustrine and alluvial deposits not older than the late pleistocene were found in the campotosto basin (fig. 9a-e), directly overlying the miocene flysch. deposits of an alluvial terrace (charcoal fragments within sand levels, sampled at two different sites, fig. 9a-e) gave radiocarbon ages of 11.637 11.608/11.596 11.592/ 11.580 11.329/11.323 11.297/11.272 11.261 years bp (1 ) and 8320-8150 years bp (1 ). in the area of the campotosto basin, the fault is made of three parallel splays affecting the laga mts. sw slope at different height (see fig. 9a for the central part of the fault). the western splay is the most interesting for the analysis of the recent faulting, since evidence of displacements of late pleistocene-holocene deposits and landforms was found (galadini and galli, 2000). evidence of recent activity is represented by fault scarps on the arenaceous bedrock and deposits related to terraces which formed along the incisions perpendicular to the slope (fig. 9a). erosional flat landsurfaces in the piedmont area were also displaced by the westernmost splays of the fault zone (fig. 9a). some scarps can be detected on holocene terraces. complete information on the vertical offset was derived from the displacement of a terrace formed in a valley perpendicular to the laga mts. slope (site 1 of fig. 9a). this terrace is affected by all the main splays which form the fault zone. the topographic profile made along this terrace (fig. 9b) shows a total vertical offset of 21 m (galadini and galli, 2000). the displaced terrace is younger than a large erosional terrace carved into a lacustrine succession radiocarbon dated at 33.120 ± 470 years bp (sampling site indicated in fig. 9a) and older than alluvial deposits radiocarbon dated at 11.637 11.608/11.596 11.592/11.580 11.329/11 323-11.297/11.272 11.261 years bp (1 ). based on the regional data about the chronology of the deposition in the piedmont areas of the central apennines (e.g., dramis, 1983; blumetti et al., 1993; giraudi, 1996), a probable age of 20 000-30 000 years bp can be hypothesised for the displaced terrace. this led galadini and galli (2000) to estimate a vertical slip rate of 0.7-0.9 mm/yr. a terrace formed over alluvial sandy deposits radiocarbon dated at 8320-8150 years bp (1 .; galadini and galli, 2000) is affected by a fault scarp at site 2 (fig. 9a). an abandoned creek carved into this terrace (and therefore more recent than the reported date) was also displaced, as suggested by the presence of a fault scarp. the topographic profile made across the scarp (fig. 9c) defined a vertical offset of about 3 m (galadini and galli; 2000). another topographic profile (fig. 9d) was made on the top of the displaced terrace, in the area chosen for trenching (site 3 in fig. 9a). this topographic profile defined a vertical displacement of 3 m occurred after 83208150 years bp (1 .). this represents, however, a minimum offset since the trench (see below) uncovered about 1 m of colluvial deposits in the hangingwall. this means that the vertical separation between the two correlative surfaces (top of the debris flow unit 11 in the paleoseismological section of fig. 10a,b, see next section) across the fault zone may be of about 4 m. considering that the footwall area experienced continuous erosion, this represents a minimum value for the vertical offset which defines a minimum vertical slip rate of 0.48-0.49 mm/yr. this slip rate is slightly higher than that (0.3-0.36 mm/yr) defined by galadini and galli (2000) who did not consider the data subsequently derived from the trench. the mentioned displaced terrace (site 3 of fig. 9a) was incised by the presently active torrent fed by the laga mts. after 8320-8150 years bp (age of the terrace alluvial sands). the trench site was located on the terrace remnant of the left valley flank (fig. 9e). 5.2. paleoseismology the trench was 32 m long and not deeper than 2.5 m (maximum depth reached in the fault zone; fig. 10a,b). the presence of deposits main830 fabrizio galadini and paolo galli fig. 10a,b. a) log of the nw wall of the trench excavated across the laga mts. fault at site 3 in fig. 9d; the paleoseismological analysis defined the occurrence of two displacement events after 8425-8365 years bp. b) panoramic view of the investigated trench wall. a b ly made of sandstone boulders in sandy matrix (lowest stratigraphic levels) limited the depth of excavation. fault planes within this kind of deposits are not easily recognisable and therefore we limited the excavation in the central portion of the trench at the top of the blocky unit. since the two trench walls showed the same stratigraphic features and defined the same history of displacement, only the nw trench wall will be described (fig. 10a,b). sandy units and deposits made of boulders, cobbles and pebbles in sandy matrix were exposed in the nw trench wall. stratigraphy is definitely more variable than in the case of the mt. vettore fault trenches. figure 10a,b shows alternating coarse debris flow episodes (units 6, 11 in fig. 10a), fine-sediment colluvial (2, 4, 5, 10 in fig. 10a) and alluvial (7, 9 in fig. 10a) deposits. the origin of the colluvial units 2 and 10, i.e. the colluviums which deposited close to the fault zone (fig. 10a,b), is not clear. indeed we have no data to understand if colluvial deposition is related to a process of scarp retreat or re-deposition in the piedmont area during phases of climatic instability. two different pedogenetic levels were identified (units 3 and 8). chronological constraints are available for unit 3 (1245-1135/1110-1095 years bp, 1 cal age), unit 8 (2730-2340 years bp, 1 cal age) and unit 10 (10.685-10.190 years bp and 8425-8365 years bp, 1 cal age). since the latter two dates are related to a sediment of colluvial origin, they define only a lower chronological limit for the deposition of unit 10. part of the stratigraphic succession exposed in the nw trench wall (units 10, 11a and 11b) is affected by five fault planes (f1 to f5 in fig. 10a). faults f1 and f5 bound a small depression filled with the colluvial unit 10. this structure shows a mainly normal component in the fault movement and extension roughly parallel to the trench direction. fault f1 places the debris flow deposit (unit 11b) in contact with the colluvial unit 10. f2 only affects unit 10, while the other fault planes place unit 11a in contact with unit 10. f5 does not seem to displace unit 10. two displacement events were recognized through the analysis of this trench. the youngest event (e1) is indicated by the faults (f2, f3 and f4) affecting unit 10. since unit 10 has a colluvial origin, its deposition occurred after the youngest radiocarbon date obtained for this unit (8425-8365 years bp, 1 .). e1 occurred, therefore, after this date. the upper chronological limit cannot be reliably defined, since f2, f3 and f4 are only sealed by the present soil. the lack of correlative deposits across the fault zone does not permit to define the vertical offset. a previous event (e2) was responsible for the displacement of units 11a and 11b. in particular, it formed the depression between f1 and f5 filled by the colluvial unit 10. the formation of this depression is, therefore, preceding the deposition of unit 10. the chronology of e2 cannot be, however, precisely defined, since the deposition age of unit 10 is poorly constrained (generically after 8425-8365 years bp, 1 .). therefore, if unit 10 is the product of colluvial deposition from the scarp formed during e2, this event may have occurred at about 8425-8365 years bp (youngest age of the presumed colluvial wedge). if unit 10 resulted from deposition during a period of climatic instability, i.e. the deposition was not conditioned by e2, the age of this displacement event cannot be defined. in the previous section, however, we discussed a vertical offset of about 4 m of the terrace dated at 8320-8150 years bp (1 .). since it is improbable that such a large offset is due to one displacement only (see sections dedicated to the fault behaviour), it may indicate that both e1 and e2 occurred after 8320-8150 years bp. the analysis of the trench wall reported in fig. 10a defined a minimum value for the cumulated vertical offset from the stratigraphic data. if the base of unit 10 (exposed in the hangingwall, fig. 10a) had an almost regular attitude, the vertical separation between the top of unit 11b in the footwall (where unit 10 was probably present before an erosional episode erased it) and the projection of the unit 10 base from the area sw of fault f5 on fault f1 gives a minimum vertical offset of 1 m. 6. fault behaviour paleoseismological data corroborate the previous hypothesis of repeated holocene activation of the mt. vettore and laga mts. faults 831 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults (galadini and galli, 2000 and references therein) mainly based on geomorphologic investigations. the implications of the available data, in terms of slip rates, elapsed time since the last event, recurrence intervals and expected magnitude, will be discussed in the following sections. 6.1. slip rates the paleoseismological trenches were excavated across single splays of complex fault zones. for this reason, the data obtained on the vertical offsets from the trench analysis could only provide information on the minimum displacements which may be related to the entire structures. as for the mt. vettore fault, we consider the prate pala scarp (across which the investigated trenches have been made) as the surficial expression of the westernmost splay within the fault zone. therefore, we estimated a minimum slip rate. the topography of the prate pala scarp gave evidence for a minimum vertical slip rate ranging between 0.36 and 0.62 mm/yr (see section dedicated to the geomorphic analysis). a slip rate can also be obtained from the displacement estimated through the trench analysis. a minimum vertical offset of 0.45 m affects the base of unit 4 of trench 1 (fig. 5a,b). based on the age of the displacement (younger than 4155-3965 years bp and older than the 6th-7th century a.d.), a minimum vertical slip rate of 0.11-0.36 mm/yr can be defined. the trench made across the westernmost segment of the laga mts. fault showed a minimum vertical offset of 1 m in the past 8425 years. this offset gives a minimum vertical slip rate of 0.12 mm/yr. in this case, however, more reliable information can be derived from the topographic profiles described in the section dedicated to the geomorphic analysis of the laga mts. fault (fig. 9a-e). by considering these topographic data, galadini and galli (2000) obtained a vertical slip rate of 0.7-0.9 mm/yr. since this rate was obtained through the evaluation of the surface displacement across the entire fault zone, it can be considered a more realistic estimate of the fault behaviour. 6.2. elapsed time since the last activation the paleoseismological analysis did not define precise upper chronological limits for the displacements observed across the laga mts. fault. faulting can be observed throughout the entire stratigraphic succession below the present soil. the latter is, however, a very thin layer of humic material developed over a colluvial deposit. radiocarbon dating of this layer is meaningless, since the obtained date would derive from the mixture of organic matter originated from the colluviated parent material and from the present humic matter. it is clear, therefore, that only the historical seismological information can cast light on the most recent activation of the laga mts. faults. availableseismic catalogues (boschi et al., 1995, 1997, 2000; camassi and stucchi, 1997; working group cpti, 1999) give reliable information on the occurrence of large magnitude earthquakes since the first centuries of the 2nd millennium a.d. historical data suggest that no earthquakes of m > 6 affected the central apennines which may be related to the activation of the laga mts. fault (castelli et al., 2002). for this reason, we define in this paper a sort of «conventional» minimum elapsed time (eight centuries) derived from: 1) the lack of earthquakes in the seismic catalogues which may be related to the mentioned faults, and 2) the hypotheses about the completeness of the historical information for the largest magnitude events reported in stucchi and albini (2000), indicating a low probability of lost historical information for destructive earthquakes in the period 1200 a.d.-present. the investigated splay of the mt. vettore fault is sealed by a colluvial unit (2 in fig. 5a,b). the age of 1600-1510 years bp gives little information on the deposition age of this unit which, due to the colluvial origin, may have occurred many centuries after the reported date. as mentioned in the section dedicated to the stratigraphy of trench 1, the presence of this colluvial unit over a wide area of the alluvial fan suggests that colluviation was not a depositional phase of a mere local importance. significant colluviation following the reported date can probably be related to the 832 fabrizio galadini and paolo galli climatic change which occurred during the 6th-7th century a.d. this means that a paleoseismologically inferred minimum elapsed time of 1300-1500 years can be defined. 6.3. recurrence intervals radiocarbon dates permit hypotheses about the time span between the defined displacement events. however, available chronological constraints do not define very narrow time intervals of event occurrence. first of all, in the case of the mt. vettore fault the event e3 probably represents more than one episode of surface faulting. as for e2 and e1, we have no data to hypothesise that the two events are consecutive. moreover, the upper chronological limit of e2 coincides with the lower chronological limit of e1 (41553965 years bp); therefore, the minimum time span between these two events would be zero. the maximum time span can be evaluated by first defining the uppermost chronological limit of e1. based on the discussion in the previous section, this limit may be the 6th-7th century a.d. this chronology should be compared with the lower chronological limit of e2 (5940 years bp). therefore, the maximum time span between e2 and e1 is 4690-4490 years, representing the maximum recurrence interval for surface faulting events along the mt. vettore fault. the recurrence interval of the laga mts. fault is even less defined than that of the mt. vettore fault, since we only have evidence for two events, both after 8320-8150 years bp (1 .). these events are probably consecutive (i.e. no events are lacking in the investigated trench between e2 and e1) since unit 10 is a colluvium deposited in a wide open fissure formed during e2, while e1 was responsible for the displacement of the same colluvial unit. as reported in the previous section, the upper chronological limit for e1 is 1200 a.d. the possibility that e1 occurred on 1200 a.d. and e2 occurred at 8320-8150 years bp gives a maximum time span between the two events of 7570 years. in short, assuming that the investigated fault segments are representative of the entire fault zone, paleoseismological data may define time intervals not longer than 4690 years and not longer than 7570 years for the mt. vettore and laga mts. faults, respectively. 6.4. expected magnitude hypotheses on the magnitude of the earthquakes which may originate along the investigated faults can be derived from the surficial fault length, by using the equation linking the surface rupture length (srl) with the moment magnitude (wells and coppersmith, 1994). we consider, indeed, that the length of the fault surficial expression (along which geomorphic evidence of recent activity can be detected) is comparable to the srl, as the mentioned surficial expression results from the repetition of earthquakes which caused surface faulting. starting from this assumption we mapped the studied faults through the observation of aerial photographs and subsequent investigations in the field. therefore, we were able to precisely estimate the fault surficial expression which resulted from cumulated episodes of surface faulting. the estimated length of the fault surficial expressions for the mt. vettore and laga mts. is 18 and 20 km, respectively. based on the equations linking the rupture length with the magnitude (wells and coppersmith, 1994), the expected magnitude for the mentioned faults is 6.5 and 6.6, respectively. 7. conclusions previous works on the central apennine active tectonics showed that two parallel sets of nw-se trending active normal faults affect this portion of peninsular italy. a comparison between the active faulting framework and the spatial distribution of the strong historical earthquakes (as derived from the available italian catalogues) plus paleoseismological data indicated that the faults of the westernmost set activated during historical times. in contrast, no strong 833 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults earthquakes (m 6.5) occurred in the areas where the faults of the easternmost set are located. the lack of historical earthquakes which may be related to this set defines a silent state for the eastern faults which, based on considerations about the completeness of the catalogue for the largest magnitude events, lasted for at least eight centuries. considering the historically silent behaviour, paleoseismological analyses were performed along two adjacent faults (mt. vettore and laga mts. faults) of the eastern set, in order: 1) to corroborate the hypotheses of recent activity previously derived from geomorphological investigations, and 2) to define quantitative parameters describing the fault behaviour. as for point (1), the trenches made across the three mentioned faults showed the occurrence of repeated holocene displacement events, thus confirming previous geomorphologically inferred hypotheses. as for point (2), results of the paleoseismological analyses can be summarised as follows: 1) number of events: three events were identified along the mt. vettore fault, occurring between 4155-3965 years bp and the 6th-7th century a.d. (e1), between 5940-5890/5795-5780 years bp and 4155-3965 years bp (e2), and between 18.000-12.000 years bp and 59405890/5795-5780 years bp (e3) respectively; two events have been identified along the laga mts. fault, both occurred after 8320-8150 years bp. 2) slip rate: the minimum vertical slip rates available for the mt. vettore fault are 0.11-0.36 mm/yr (paleoseismologically inferred) and 0.360.62 mm/yr (geomorphologically inferred), while for the laga mts. fault the minimum rate is 0.12 mm/yr (paleoseismologically inferred) and the geomorphologically inferred vertical slip rate (based on the displacement of late pleistocene landforms) is 0.7-0.9 mm/yr. 3) recurrence interval: the time interval between subsequent events is not longer than 4690 and 7570 years for the mt. vettore and the laga mts. fault, respectively. 4) minimum elapsed time: paleoseismological data defined an elapsed time of 1300-1500 years since the last mt. vettore fault activation; for the laga mts. fault paleoseismological data on the elapsed time are sparser, due to the lack of reliably datable units sealing the investigated faults; for this reason the elapsed time was fixed in eight centuries, on the basis of the available historical data showing no evidence of earthquakes which may be related to the investigated faults and on considerations about the completeness of the seismic catalogues for the largest magnitude earthquakes. 5) maximum expected magnitude: based on the length of the fault surficial expressions, and by means of the equations linking the surficial rupture length with the moment magnitude, the investigated faults may cause earthquakes with m 6.5 (mt. vettore) and 6.6 (laga mts.). considering the silent behaviour of the faults during at least the past eight centuries and the related high level of seismic hazard which may be associated to them, we believe that future research should focus on the production of damage scenarios related to the activation of the investigated faults. although the faults affect high mountainous regions, villages with 5000-10.000 inhabitants are located in the hangingwall areas and at distance not larger than 15 km from the surficial expressions of the investigated structures. acknowledgements p. messina (cnr, istituto di ricerca sulla tettonica recente, roma) and r. basili (istituto nazionale di geofisica e vulcanologia, roma) participated in the analysis of the trench excavated across the laga mts. fault. we are grateful to c. giraudi for the discussions on the chronological framework of the late pleistocene-holocene continental stratigraphy of the central apennines. one of the two anonymous referees suggested modifications which improved the paper. references barchi, m., f. galadini, g. lavecchia, p. messina, a.m. michetti, l. peruzza, a. pizzi, e. tondi and e. vittori (editors) (2000): sintesi delle conoscenze sulle faglie attive in italia centrale: parametrizzazione ai fini della caratterizzazione della peri834 fabrizio galadini and paolo galli colosità sismica, gndt, gruppo nazionale per la difesa dai terremoti, spec. publ., roma, pp. 62. blumetti, a.m., f. dramis and a.m. michetti (1993): fault-generated mountain fronts in the central apennines (central italy): geomorphological features and seismotectonic implications, earth surf. proc. and land., 18, 203-223. boschi, e., g. ferrari, p. gasperini, e. guidoboni, g. smriglio and g. valensise (editors) (1995): catalogo dei forti terremoti in italia dal 461 a.c. al 1980 (ing, roma sga, bologna), vol. 1, pp. 974. boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (editors) (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990 (ing, roma sga, bologna), vol. 2, pp. 644. boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (editors) (2000): catalogue of strong italian earthquakes from 461 b.c. to 1997, ann. geofis., 43 (4), 609-868. calamita, f. and a. pizzi (1992): tettonica quaternaria nella dorsale appenninica umbro-marchigiana e bacini intrappenninici associati, studi geol. cam., 1 (spec. vol.), 17-25. calamita, f. and a. pizzi (1994): recent and active extensional tectonics in the southern umbro-marchean apennines (central italy), mem. soc. geol. it., 48, 541-548. camassi, r. and m. stucchi (1997): nt4.1, a parametric catalogue of damaging earthquakes in the italian area (release nt4.1.1), http://emidius.mi.ingv.it/nt/ consnt.html. castelli, v., f. galadini, p. galli, d. molin and m. stucchi (2002): caratteristiche sismogenetiche della sorgente della laga e relazione con il terremoto del 1639, in 21th national congress of the gruppo nazionale di geofisica della terra solida, rome, november 19-21, 2002, abstract volume, 13-16. cello, g., s. mazzoli, e. tondi and e. turco (1997): active tectonics in the central apennines and possible implications for seismic hazard analysis in peninsular italy, tectonophysics, 272, 43-68. coltorti, m. and p. farabollini (1995): quaternary evolution of the castelluccio di norcia basin, il quaternario, 8, 149-166. d’addezio, g., e. masana and d. pantosti (2001): the holocene paleoseismicity of the aremognacinque miglia fault (central italy), j. seismol., 5, 181-205. dramis, f. 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(1998a): late pleistocene and holocene lake level variations in fucino lake (abruzzo, central italy) inferred from geological, archaeological and historical data, in esf workshop palaeohydrology as reflected in lake-level changes as climatic evidence for holocene times, paläoklimaforschung, 25, 1-17. giraudi, c. (1998b): i laghi effimeri tardopleistocenici e olocenici di campo imperatore e del massiccio del gran sasso d’italia (abruzzo italia centrale), il quaternario, 11, 217-225. giraudi, c. (2000): le oscillazioni oloceniche del ghiacciaio del calderone, gran sasso d’italia (abruzzo italia), il quaternario, 13, 31-36. giraudi, c.: le oscillazioni del ghiacciaio del calderone (gran sasso d’italia, abruzzo italia centrale) e le variazioni climatiche degli ultimi 3000 anni, il quaternario (in press). giraudi, c. and m. frezzotti (1995): paleoseismicity in the gran sasso massif (abruzzo, central italy), quat. int., 25, 81-93. giraudi, c. and m. frezzotti (1997): late pleistocene 835 paleoseismology of silent faults in the central apennines (italy): the mt. vettore and laga mts. faults glacial events in the central apennines, italy, quat. res., 48, 280-290. magri, g. and d. molin (1984): il terremoto del dicembre 1456 nell’appennino centro-meridionale. enea report rt/amb, 8 (83), pp. 180. molin, d., f. galadini, p. galli, l. mucci and a. rossi (1999): sismicità della zona del fucino. in 13 gennaio 1915, il terremoto nella marsica, edited by s. castenetto and f. galadini (servizio sismico nazionale e cnr istituto di ricerca sulla tettonica recente, roma), 249-271. monachesi, g. and m. stucchi (1998): dom 4.1 an intensity database of damaging earthquakes in the italian area, http://emidius.mi.ingv.it/dom/home.html. orombelli, g. and s.c. porter (1982): late holocene fluctuations of brenva glacier, geogr. fis. dinam. quat., 5, 13-37. pantosti, d., g. d’addezio and f.r. cinti (1996): paleoseismicity of the ovindoli-pezza fault, central apennines, italy: a history including a large, previously unrecorded earthquake in the middle ages (860-1300 a.d.), j. geophys. res., 101, 5937-5959. roberts, g.p., a.m. michetti, p. cowie, n.c. morewood and i. papanikolau (2002): fault slip-rate variations during crustal-scale strain localisation, central italy, geophys. res. lett., 29 (8), 10.1029/ 2001glo013529. strumia, g. (1997): oscillazioni glaciali precedenti la piccola età glaciale documentate da un suolo sepolto in una morena del ghiacciao del lys, il quaternario, 10, 153-158. stucchi, m. and p. albini (2000): quanti terremoti distruttivi abbiamo perso nell’ultimo millennio? spunti per la definizione di un approccio storico alla valutazione della completezza. in le ricerche del gndt nel campo della pericolosità sismica (19961999), edited by f. galadini, c. meletti and a. rebez, (gndt, gruppo nazionale per la difesa dai terremoti, roma), 333-343. stuiver, m., p.j. reimer, e. bard, j.w. beck, g.s. burr, k.a. hughen, b. kromer, g. mccormac, j. van der plicht and m. spurk (1998): intcal98 radiocarbon age calibration, 24.000-0 cal bp, radiocarbon, 40, 1041-1084. valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger than m 5.5 in italy, version 2.0, ann. geofis., 44 (suppl. to n. 4), 797-964. wells, d. l., and k.j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, seismol. soc. am. bull., 84, 974-1002. working group cpti (1999): catalogo parametrico dei terremoti italiani (ing, gndt, sga ssn, bologna), pp. 92. 836 fabrizio galadini and paolo galli filippini.pdf 513 annals of geophysics, vol. 46, n. 3, june 2003 tdem for martian in situ resource prospecting missions roberto filippini, cinthya ottonello, sergio pagnan and giorgio tacconi istituto di studi sui sistemi intelligenti per l’automazione, cnr, sezione di genova, italy abstract this paper presents a tdem (time domain electromagnetic methods) application, addressed to the search for water on mars. in this context, the opportunities for a tdem system as payload in a future mission are investigated for different in situ exploration scenarios. the tdem sounding capability is evaluated with respect to the expected martian environment, and some considerations are made about the many unknown variables (above all the background em noise and the subsoil composition) altogether with the limited resources availability (mission constraints in mass, time and power) and the way they could represent an obstacle for operations and measurements. mailing address: dr. sergio pagnan, istituto di studi sui sistemi intelligenti per l’automazione, cnr, sezione di genova, via de marini 6, «torre di francia», 16149 genova, italy; e-mail: pagnan@ian.ge.cnr.it key words tdem – in situ remote sensing – planetary exploration 1. introduction the large amount of data and images recorded from mars has given a detailed knowledge of planet environment (surface and atmosphere) thus permitting the scientific community to seek matters of fundamental scientific relevance. first in order of importance, research into any elementary (past or present) form of life on the planet. since life is mainly correlated to the existence of water, whatever its physical state (liquid, ice, permafrost, other…), the scientific efforts have been turned towards the study of new methodologies that could better support the search for water. above all, the methodologies deployed for remote sensing observations by means of hf (high frequency) radars, and high resolution cameras aboard satellites orbiting around the planet (e.g., the mars global surveyor-mgs). secondly, the in situ exploration of the surface and subsurface. the study concerns the analysis of a tdem (time domain electromagnetic method) system for in situ martian geophysical exploration. tdem is a controlled source time domain method operating at lf (low frequency) regime of propagation. it is able to discriminate the geologically layered structure and detect the signature of water by means of subsoil conductivity estimation (as a function of depth). the tdem employment, for an in situ survey on mars, is advisable for many reasons: it is fully controllable in position and strength, it can undergo notable variations due to water presence and the instrument does not need to be in contact with the ground, thus respecting the mission planetary protection and cross contamination requirements. tdem (originally developed for detecting massive sulphide deposits) is beginning to emerge due to its high sensitivity that has the best potential for deep hydro-geophysical mapping. this makes it preferable to other electromagnetic methods such as ip (induced polar514 roberto filippini, cinthya ottonello, sergio pagnan and giorgio tacconi ization), ves (vertical electric sounding), and mt (magnetotelluric) that could in principle accomplish the same task. the purpose of the paper is to investigate tdem performances when employed in the martian environment and also to investigate the opportunities for a tdem system to be housed as scientific payload aboard in situ investigation missions. the structure of the paper is the following: section 2 introduces the tdem survey basic concepts (functioning principles, modelling and data interpretation). in section 3 there is a brief overview of the most recent discoveries concerning the main issue «search for water on mars», the space programs dealing with it and some mission scenarios well suited to a tdem survey. in section 4 the sounding capability, that we could expect performing a tdem survey on mars, is estimated for a simple case study. some final remarks will conclude the paper. 2. tdem generalities tdem is a controlled source, remote sensing, time domain electromagnetic (em) method operating at the diffusion regime of em propagation and sensitive to the conductivity gradient. unlike the frequency method (fdem), it is possible to retrieve information from different depths with a single measurement (west et al., 1984; west and macnae, 1991; nabighian and macnae, 1991; zhdanov and keller, 1994; parasnis, 1996). the system consists of a transmitting antenna (tx) and one (or more) receiving antenna (rx) placed on the surface with respect to a chosen configuration. for example, in the dual loop, separated and horizontal coplanar configuration, the tx and the rx are placed with the axes perpendicular to the surface. the tx is a single or multi-turn loop (vertical magnetic dipole, vmd) while the rx may be a coil or a loop of wire, but a sensitive wideband magnetometer can be used as well. the functioning principles of a tdem survey are here briefly summarized (west and macnae, 1991): the tx is energized with a square wave current for a time in the order of milliseconds then the current is abruptly interrupted. this step-like impulse generates a sharp magnetic field variation which, according to faraday’s law of induction, causes eddy currents in any conductive region of the medium. in their turn, such currents determine a secondary magnetic field that gradually decays in time (see fig. 1). the rx measures the secondary magnetic field (in the case of a magnetometer) or its time derivative (i.e. the induced voltage, in the case of a coil). the signal is recorded as a function of time and processed according to proper sampling/stacking criteria. the amount of delay, strength and shape of the signal characterize the geoelectrical structure of the medium completely. 2.1. modelling a tdem survey the main survey parameters are the distance from the source to the receiver coil, r, and the time of observation t. other parameters concerning the system design are the current square wave shape i(t) (an impulse of amplitude i0, and duration t starting from the instant t = 0 ) and the size of the source and receiver coils in terms of area (a s , a r ) and number of turns (n s , n r ). the following formula expresses, in a very concise way, the dependency of the recorded measure (the transient voltage observed at the receiver) on the parameters, under the assumption that the tx and rx are sufficiently small so that the magnetic field measurement is considered as point measurement and the tx is considered a magnetic dipole: (2.1) where f is the magnetic flux; m0 is the vacuum permeability; h z is the vertical component of the magnetic field at distance r from tx; m= a s a r n s n r i0 ; σ r p( ) is the medium conductivity v t r t n h r t t dst r z ar , , ( ) = − ∂ ∂ = − ∂ ( ) ∂ ≅≥ ∫0 0 φ µ n a h r t t mf p r tr r z , , ,≅ − ∂ ( ) ∂ = − ( )( )0µ σ r 515 tdem for martian in situ resource prospecting missions and r p x y z= ( ), , is the position of the conductive medium with respect to tx. equation (2.1) is affected by errors due to the non-homogeneities of the measurements (geometric errors) and inaccuracy of the model (e.g., deviations from constant parameters assumption). also, it is affected by the em background noise (spies and frischknecht, 1991). these are possible sources of misinterpretations and need to be considered when formulating the model, in order to assess the desired level of robustness. the choice of the system parameters is strongly related to the required inspection depth (i.e. the scale of the survey). the multiplying factor m contains the system parameters and acts as amplifier of the received signal. the dependency on the distance r and time of fig. 1. tdem experiment. 516 roberto filippini, cinthya ottonello, sergio pagnan and giorgio tacconi observation t are most important for data interpretation (geometric sounding). this fact shall be treated later on. the medium geolectric profile s( p) is the unknown parameter of the experiment and it is estimated as a function of depth (laterally uniform model) or more generally as a function of the distance from the source coil (3d models). when a layered medium is assumed, the first modelling approach is computationally less demanding. for the laterally uniform models, the parameters are the thickness and the conductivity of each layer. the rank of a model, namely the number of its layers, determines the accuracy and resolution of the subsoil mapping. whenever the ground is approximately horizontally layered, the diffusion of the em field has a qualitative interpretation which helps understanding of the phenomenon (nabighian and macnae, 1991). the em primary field diffuses downward as a plane wave because of the high air-surface impedittivity, which implies the electric field to lie in a horizontal plane. the eddy currents are horizontal and concentric below the transmitter loop and diffuse away through the medium like smoke rings. if the ground were perfectly insulating, the waves could penetrate without attenuation, not returning back any scattered secondary field. on the contrary, in the presence of a conductive medium, the incident energy is partially absorbed and spread. this is due to the low pass filter behavior of the medium (the lower frequencies are the most penetrating). the quantity accounting for the diffusion phenomenon is the diffusion depth which relates the time t (from the source impulse) to the depth of penetration d and is defined as the distance over which the primary field amplitude is reduced by a factor of 1/e (keller and zhdanov, 1995) (2.2) as the wave penetrates the ground, a scattered vertical magnetic field may arise from any conductive layer to the surface with a delay proportional to the depth of the layer itself. the flow of energy into the ground and back from the conductive targets (by induction) resembles the flow of energy in conventional hf radars (by reflection) so that it is also possible to give a radarlike interpretation of the above phenomenon: a conductor subjected to a time varying em field returns back a secondary em perturbation (i.e. the echo). the strength of the echo and the time it takes to reach the receiver depend in a very complex way on the conductivity, depth and thickness of the target, and also on the conductivity of the above layers (similar to a screen). the em background noise is the most important disturbing factor for any active em sounding system. this is due to the large number of interferences which range from ultra low frequency (ulf) up to ultra high frequency (uhf) bands. basing on the earth experience, the noise is originated by the contribution of two different sources: the instrumental noise, due to the tdem equipment and the carrier, and the em natural background noise, due to the natural magnetic field variation, to the interaction with extraneous sources (solar wind) and to the atmospheric phenomena (lightning). in order to carefully characterize the statistical model of the martian em background noise, it is necessary to take into account the different contribution of noise in terms of gaussian and non-gaussian components. this a priori knowledge is useful to maximize the signal to noise ratio (snr), for the detection/estimation process. the following formula expresses a possible statistical noise model: (2.3) a(t) represents the contribute due to stationary harmonic components (micropulsations like the earth’s schumann resonances), g(t) denotes gaussian background noise and, c(t) represents randomly occurring interferences (lightning and instrument power switch-on/off ). such noise characteristics allow one to adopt the most advanced filtering and noise rejection techniques (macnae et al., 1984; nikias and mendel, 1993). once the data have been recorded and filtered, they have to be interpreted. the interpretation relies on the solution of the forward and inverse n t a t g t c t( ) = ( ) + ( ) + ( ) d t = 2 0 π σµ . 517 tdem for martian in situ resource prospecting missions problem. the inverse problem says: «given an em transient signal, determine the geo-electric structure of the medium that generated it » (zhdanov and keller, 1994). it consists of estimating the geoelectric structure of the earth that generates the recorded signal. the dual problem is the forward problem which says: «given a geo-electric structure of the earth, determine the em transient signal at the receiver» (zhdanov and keller, 1994). the forward problem must be solved in order to evaluate the response of the model for a given parameter setting (in the chosen model parametric space). the inversion problem consists of finding the model that best fits the recorded data. both problems are represented in the block diagram of fig. 2, where the tdem experiment has been modelled in accordance with a signal processing point of view. in the laterally uniform model, the main advantage is that data inversion is unique (zhdanov and keller, 1994). if needed, it could be even possible to provide a more detailed subsoil mapping solution by processing the same data with reference to a 3d model, but remembering the intrinsic computational complexity in managing the data inversion in such a context. moreover, in this, the interpretation becomes ambiguous and the problem of the data inversion is said to be «ill posed». an alternative approach is based on probabilistic tomography (patella, 1997; mauriello and patella 1999). the data interpretation is no longer bounded to any preconceived subsoil model (model free approach) and the results have to be interpreted in a probabilistic sense. it has worked successfully in the mapping of complex geoelectric profiles as well as in the localization of water. thanks to a lower computational cost (with respect to the canonical 3d modelling) and potentially no limit to the range of application (tdem, ip, magnetotelluric, etc.), it represents a valid alternative to the more traditional approaches, and also for subsequent refinement and result validation. 3. insights into water on mars and some future mission scenarios many hypotheses about the presence of water on mars have been formulated thanks to the scientific data provided by some past and more recent missions, like the viking (1975), the pathfinder (1996) and the mars global surveyor (1996) and more recently the mars odyssey (2001). an outline of the results is briefly reported here (briggs, 2000; heggy et al., 2001): the martian atmosphere – martian atmosphere is extremely dry and water exists as ice or vapor in trace quantities (0.03%). the martian surface – a dust layer of a mixture of conducting (iron oxides, hematite, etc.) and less conducting material (silicates and basaltic rocks) covers almost the whole planet. water exists as ice at high latitudes (on the poles) from which it sublimates during the martian summer. the martian subsurface – due to the evidence of superficial water-related processes (crustal erosions and deep valleys), water has been supposed to exist even in the subsurface. the presence of water on the martian surface is theoretically possible as a temporary phenomenon, at certain latitudes and times fig. 2. block diagram of the tdem measurement process. n(t) hz (t,r )¤hz(w,r ) 518 roberto filippini, cinthya ottonello, sergio pagnan and giorgio tacconi (due to a heating of the crust or to a superficial geothermal activity), and the major expectations rely upon the subsoil exploration, where very likely it should exist in the form of a mixture of ground and ice, similar to the terrestrial permafrost. 3.1. mission scenarios the different exploration programs developed by the major space agencies (nasa, esa, asi, …) in the latest few years have agreed on splitting the martian exploration into two distinct phases: a first exploration phase including a large scale mapping of the martian surface from satellites, followed by a second phase of in situ investigations. at present, there exists a great expectation on the martian in situ exploration and a large variety of exploration scenarios have been conceived, from the missions consisting of a single rover or a lander equipped with a heavy package of scientific instruments (the single station approach), to more ambitious missions (multi-station approach) consisting of a set of rovers, landers and robots (brack et al., 1999; heggy et al., 2001). although single as well as multi-station scenarios are well suited to a tdem experiment, the multi-station approach (see fig. 3) would offer the best platform for a geophysical distributed tdem experiment, that means: a single transmitter and a certain number of receivers (one or n) which could be housed aboard each carrier fixed or moving (lander, rover, robots) with respect to a chosen configuration. a distributed tdem experiment will allow: 1) simultaneous measurements with respect to a proper geometric sounding approach (time-space correlated observations); 2) a higher resolution in the measurement (qualitative improvement), by clustering in groups a certain number of stations; and finally, 3) exploration of a larger area recording a significant amount of data for the fixed slice of time available (quantitative improvement). other advantages of the distributed configuration, table i. mission scenarios for a tdem survey. single station multi-stations general features multi-purpose, heavy special-purpose, low cost and mass packaged, high cost and mass. survey scale local survey: restricted area global survey: larger area of of exploration. exploration; geometric sounding. survey fixed lander: traditional single multi-lander: scenarios survey. simultaneous multi-sites survey. mobile rover (msr-nasa): micro-robots (the next future) multi-survey approach. the same as above plus variable, task adaptive survey configuration. tdem system coincident or concentric separated transmitter-receiver(s); (1:1) configuration transmitter-receiver or (1:n). fig. 3. rover distributed configuration. 519 tdem for martian in situ resource prospecting missions with respect to the single station approach, is the possibility to employ a less expensive carrier (smaller and even technologically less complex), experiment flexibility (possibility to perform multi-sites survey by means of the system reconfiguration facility) and increased reliability (in term of probability of mission completion). in table i, some of the mission scenarios are classified with respect to some very general features, pointing out the single capabilities and facilities for a tdem survey. 4. tdem experiment design and expected performance in a planetary mission context: a case study within a planetary mission context, the payload scientific experiments are subjected to more or less evident limitations. the major constraints deal with mass, dimension, power, mobility and other key resources depending also on the type of mission. these limitations together with mentioned em background noise and first layer screening effect, compromise the system performance. the tdem performance is the combination of two quantities, the depth of investigation plus the resolution (spies and frischknecht, 1991). the former is the maximum depth of penetration (generated by the source impulse), scattering a detectable signal back to the surface (echo to the receiver), for a given maximum threshold of sensitivity (i.e. receiver sensitivity). the latter represents the ability to detect the conductivity and thickness of a layer in the subsoil. system sounding capabilities are evaluated here for a simple case study wherein a saltwater deposit lying at a certain depth beneath the surface is the target of the investigation. the following assumptions hold: tdem system – separated transmitter-receiver configuration (1:1). tx and rx are coils. – ideal receiver. – ideal square wave (step function) current source. martian subsoil and em background noise – laterally uniform subsoil and a thin sheet model for target resolution measurement. – em background noise (after filtering and stacking) under the threshold of sensitivity. system and model parameters are shown respectively in tables ii and iii. when needed, a different range of values will be specified. the time t and the spacing r are the free parameters of the study. 4.1. screening effect and depth of investigation a very conductive layer causes the em field to loose a large amount of energy (the ohmeffect), remaining confined within the shallow subsurface. at the same time, the penetration velocity (depending on the conductivity) decreases so that the em wave reaches any potential target later and with not enough energy to scatter back a detectable signal. this effect is called the table ii. system parameter settings. system parameters values a s , a r 1 m2 n s , n r 112 turns i0 1 a dipole moment m 12544 a m4 (n s n r a s a r i0 ) threshold of sensitivity µmv table iii. conductivity profile range of values. martian subsoil conductivity expected values thickness w-1 m-1 (m) first layer 0.001 silicates ÷ 0.1 iron-oxides 1 ÷ 100 target water (saline) 4 10 520 roberto filippini, cinthya ottonello, sergio pagnan and giorgio tacconi screening effect and it represents the most serious limitation for the detection of any buried target. a rough estimation of the screening effect could be obtained by considering the attenuation of the plane wave diffusing downward an uniform medium of conductivity s. in this case, the voltage time response at the receiver (see eq. (2.1) ) can be analytically expressed as follows (zhdanov and keller, 1994): (4.1) for simplicity, (4.1) may be approximated with the following asymptotic expression holding at late time: (4.2) the time t can be made explicit in (4.2) and substituted in eq. (2.2) to obtain the expression of the maximum depth of investigation as a function of the threshold of sensitivity, the medium conductivity and the gain m (spies and frischknecht, 1991) (4.3) the expression gives an approximated information on the maximum depth of investigation. figure 4 shows the results obtained for the two first layer conductivity values assumed in table iii, confirming how a more conductive first layer limits the penetration. 4.2. target resolution when the conductivity difference between a conductive layer and the surrounding medium is more than 100, then there exists a well suited approximation for the analytical evaluation of a tdem experiment: the thin sheet model (nabighian and macnae, 1991; zhdanov and keller, 1994). herein, the medium is approximated by an insulator while the target layer is modeled as a thin sheet lying at a certain depth d from the surface. its conductance is the product of the conductivity s t for the thickness t. the voltage time response at the receiver (see eq. (2.1) ) can be analytically expressed as (4.4) this model is mostly useful to highlight the echo scattering and the importance of relating together the recording time, the spacing between the coils and the depth of the target. let us assume a target of conductance s = 4 ¥ 10 w-1 (salt water), a fixed time t = = 0.01 ms and evaluate the expression (4.4) for different values of d. the results (see fig. 5) show there exists an optimal receiver spacing in order to detect the strongest signal, once the v t r t,( ) =≥0 m r erf u e u u uu = ( ) − + +           − 5 2 2 49 2 2 1 3 9 2 πσ π u r d u= ( ) 2π ; erf error function. lim , . t tv t r m t>> ≥( ) ≅ 0 0 3 2 0 5 2 5 2 4πσ µ fig. 4. diffusion depth of a transient em plane wave versus threshold of sensitivity. d m vth = ( )     4 7 10 1 5 π σ . v t r tcoil ,( ) =>0 u t d t s s tt( ) = + =µ σ 0 ; . − ( ) − + ( )[ ] + ( )[ ] m s u t r u t r u tπ 9 24 4 2 2 2 2 7 2 521 tdem for martian in situ resource prospecting missions recording time window has been selected. by definition, a time window t w is the time interval within which we expect to observe the signal scattered by a conductor lying at a certain depth. the dependency of the time window t w on the depth of penetration d and spacing r between the coils is a strong result of the experimental evidence. the following simple linear equation, which relates the optimal spacing r, the depth of the target d and the t w (see also fig. 6), holds: (4.5) where the constant c depends on the time window. this expression resumes the properties of the geometric sounding. it says that it is possible to control the depth of investigation by simply moving the receiver coil and setting the time of observation to a certain time window so as to retrieve the maximum response signal from the target. this way, just repeating the same measurements at different locations, it is possible to achieve an accurate 2d map of the subsoil conductivity. figure 7 shows the maximum voltage at the receiver as a function of the target depth and for different time windows. of course the spacing must be adjusted as a function of depth. given the threshold of sensitivity of the receiver, it is possible to assess the limit of investigations in terms of maximum signal detectable and therefore maximum reachable target depth. for instance, let us assume a 1mv threshold, the maximum detectable target should lie at almost 20 m (time window = 0.01 ms), and if the threshold was 0.01 mv it could be possible to detect the target even at 100 m deep. due to the model simplification (absence of screening effect) and noise free experiment assumption, the results must be considered the upper limit for the system performance. relaxing optimal spacing_ .≅ +0 4 d ctw fig. 5. voltage response at the receiver for different target depth and fixed time window = 0.01 ms. fig. 6. tx-rx optimal spacing r in function of the target depth. fig. 7. maximum voltage response at the receiver in function of target depth, for different time windows. 522 roberto filippini, cinthya ottonello, sergio pagnan and giorgio tacconi the assumptions means to consider at least a twolayered model for the subsoil, to add noise to the signal and to model the dynamical behavior of the tx and rx with the respective transfer functions. in so doing, a significant spreading and lowering for the time response is expected, the amount of attenuation and delay depending on the first layer conductivity. the dynamics of tx and rx, which depend on the number of turns and area, could affect the selection of the recording time window of the experiment. namely, the experiment requirements consist of: 1) a very fast transient response for the tx (the loop working better than a coil), so that the primary field does not interfere directly (by tx-rx mutual coupling) with the secondary field, and 2) a rx cut-off frequency not too low (i.e. > 1000 hz) in order to achieve the desired level of resolution. advanced design solutions exist in order to counteract the above potential limitations. some of these have already been implemented in commercial systems. for instance, a very early recording time is feasible by spacing opportunely tx and rx or alternatively arranging them in a null-coupled configuration (zero mutual impedance). 5. conclusions the paper has investigated the feasibility of a tdem system for martian subsurface geophysical exploration. the operational environment has been defined and some of the potential obstacles, either of technical or environmental nature, have been highlighted. a simple case study showed how mission constraints could limit the system sounding capability. in such a context, even a few tens of meters of penetration should be a quite remarkable result. this is quite satisfactory for the exobiology research program, where the role of a tdem system could be to target a drill to a suitable site for sampling activity. on the contrary, greater depths are required for water search task. indeed, this kind of problem is usually encountered whenever a methodology, well assessed on the earth, must be adapted for a space application. thus, a very special attention in designing the system in each single part would be required: the transmitter should generate the greatest source impulse with respect to the low maximum peak of current available, the receiver has to be sensitive to very low signals, the em noise characterization and data acquisition strategy will be necessary in order to improve the s/n ratio. altogether with these design recipes, it is worthwhile also thinking of a drastically different approach to the tdem experiment. for instance, just considering the more futuristic mission scenarios, let us suppose having a network of carriers available. the tdem survey might be arranged as follows: a rover could unroll and stand a wire on the surface where the survey is planned and, at the same time, a set of receivers could be arranged in a networklike configuration on the surface. this way, the area of the transmitter would be increased with a resulting advantage in a larger energizing moment, and the data could be acquired in different places, simultaneously, also increasing the s/n ratio by means of the multisensor data fusion. this latter approach would represent the common framework for many of the in situ scientific experiments. a package of instruments, cooperating for the same objective, represents the most promising solution in a such extreme environment. just referring to the earth experience, there exist many examples where the combination of two or more sounding methodologies have increased the overall performances (meju, 1996). this approach is also recommended to obtain the best opportunities of employment or/and merging of the tdem with other sounding methods and represents, in our opinion, together with the distributed configuration approach, the most promising way to successfully perform a tdem geophysical survey on mars. acknowledgements the presented study has been conducted in the context of teseo, a project supported by the italian space agency (asi), in collaboration with cnr-issia, dibe-university of genoa, dsf-university «federico ii of naples», irspsuniversity of pesaro and officine galileo. we are also mostly grateful to prof. domenico patella for fruitful discussions on the subject. 523 tdem for martian in situ resource prospecting missions references brack, a., p. clancy, b. fitton, b. hoffman, g. horneck, g. kurat, j. maxwell, g. ori, c. pillinger, f. raulin, n. thomas and f. westall (1999): an integrated exobiology package for the search for life on mars, adv. space. res. 23 (2), 301-308. briggs, g. (2000): follow the water, meteoritics planet. sci., 35 (5), 892-893. heggy, e., p. paillou, g. ruffie, j.m. malezieux, f. costard and g. grandjean (2001): on water detection in the martian subsurface using sounding radar, icarus, 154 (2), 244-257. macnae, j.c., y. lamontagne and f.g. west (1984): noise processing technique for time-domain em systems, geophysics, 49 (7), 934-948. mauriello, p. and d. patella (1999): principles of probability tomography for natural-source electromagnetic induction field, geophysics, 64 (5), 1403-1417. meju, m.a. (1996): joint inversion of tdem and distorted mt soundings: some effective pratical considerations, geophysics, 61 (1), 56-65. nabighian, n. and j.c. macnae (1991): tem prospecting methods, in electromagnetic methods in applied geophysics, edited by n. nabighian (seg), vol. 2, 427-479. nikias, c.l. and j.m. mendel (1993): signal processing with higher-order spectra, ieee signal process. mag., 10, 10-37. parasnis, d.s. (1996): principles of applied geophysics (kluwer academic publishers, dordrecht), pp. 456. patella, d. (1997): self-potential global tomography including topographic effects, geophys. prospect., 45, 843-863. spies, b.r. and f.c. frischknecht (1991): electromagnetic sounding, in electromagnetic methods in applied geophysics, edited by n. nabighian (seg), vol. 2, 285-386. west, f.g. and j.c. macnae (1991): physics of the electromagnetic induction exploration methods, in electromagnetic methods in applied geophysics, edited by n. nabighian (seg) vol. 2, 5-46. west, f.g., j.c. macnae and y. lamontagne (1984): a tdem system measuring the step response of the ground, geophysics, 49 (7), 1010-1026. zhdanov, m.s. and g.v. keller (1994): the geoelectrical methods in geophysical exploration (elsevier, amsterdam-new york-tokyo), vol. 31, pp. 873. statistics o f indian earthquakes frequency energy distribution r . k . s. ciiouhan ( * ) y . k . shrivastava ( * * ) r e c e i v e d on december 20th, 1973 summary. — frequency-magnitude and energy-magnitude distribution of the indian earthquakes have been studied f o r both shallow and intermediate earthquakes using data f r o m 1910 to 1969. secular energy released b y the indian earthquakes has been studied by considering cumul a t i v e energy release and it is found that these results m a y be used to estimate the size of largest conceivable earthquakes f o r shallow and intermediate f o c a l depths. riassunto. — dai dati reperiti dal 1910 al 1969, è stata studiata la distribuzione della frequenza-magnitudo e dell'energia-magnitudo di terr e m o t i a carattere superficiale e intermedio avvenuti in india, nonché l'energia secolare liberata da questi ultimi considerando l'energia totale liberata. si è t r o v a t o che questi risultati possono essere usati per valutare l'intensità, per quanto grande essa possa essere, di terremoti a f u o c o superficiale e intermedio. 1. introduction g u t e n b e r g a n d r i c h t e r i n t h e i r m o n u m e n t a l w o r k " s e i s m i c i t y of t h e e a r t h a n d a s s o c i a t e d p h e n o m e n a " ( 5 ) h a v e s h o w n t h a t t h e r a t e of e n e r g y r e l e a s e d b y e a r t h q u a k e s is e x t r e m e l y i r r e g u l a r f o r t h e w h o l e w o r l d . t h e s i n g l e y e a r 1906 a c c o u n t s f o r a b o u t o n e n i n t h of t h e g l o b a l e n e r g y r e l e a s e , so t h a t t h e a v e r a g e a n n u a l r e l e a s e b e t w e e n 1907-1952 ( * ) d e p a r t m e n t of geophysics, indian school of mines, dhanbad826004, i n d i a . ( * * ) d e p a r t m e n t of geophysics, banaras hindu university, varanasi-5, i n d i a . ()0 k. k. 8. chouhan v. k. shkivastava is only 11.0 x 1020 ergs while the average between 1906-1952 is 12.4 x 1026 ergs, using energy magnitude relation log e = 12.0 + 1.8 m [1] where e is the energy in ergs and m is the richter's magnitude. intermediate shocks show that the maximum energy was released in 1910 and 19.11 owing to two large earthquakes in these years. the ratio of average annual energy release in shallow, intermediate and deep earthquakes is 31:4:1 according to gutenberg and richter (5). but in the year 1920 this ratio was 265:2:1 i.e. about 98% of the entire energy released in shallow earthquakes. similarly in the year 1969, chouhan and das (3) reported that about 98% of the energy was released by shallow earthquakes. the energy released in different magnitude ranges for shallow, intermediate and deep shocks is found to decrease with the value of magnitude. the number of shocks decreases with depth. according to gutenberg and richter (5) the number of shocks decreases to a minimum at a depth of about 450 km, there being a clear rise to a minor maximum at a depth of about 600 km, beyond which the number falls off very rapidly. ritsema (8) while considering the distribution of intermediate and deep shocks of varying magnitudes with depth, found that the curves showing variation of earthquake number show maxima at depths of 80 to 90 kms and 220 to 280 kms with a minor maxima at a depth of about 180 kms. these general results can be confirmed by use of small agreegate numbers in the range of times and magnitudes considered statistically. thus, there is no general agreement on the distribution of earthquakes with depth, however, these results are expected to be different in different geographic regions of the world. 2 . o b s e r v a t i o n a l d a t a for the frequency-energy distribution of shallow and intermediate earthquakes of india, the data has been extracted from gutenberg and richter (5), seismological bulletins of india meteorological department, i.s.s. bulletins and u.s.c.g.s. bulletins in addition to other sources; the span of time being sixty years (from 1910 to 1969). the magnitudes used here are all richter's magnitudes 31 ^ 5. s t a t i s t i c s of i n d i a n e a r t h q u a k e s f r e q u e n c y e n e r g y e t c . ()1 for calculations of energy from the magnitude values m , the following relation of gutenberg and eichter (") have been used. log e = a + b m [2] where a = 1 1 . 8 and b = 1.5. 3 . f r e q u e n c y e n e r g y d i s t r i b u t i o n annual frequencies of all the earthquakes of different magnitudes m in the range of 5 to 8.6 have been calculated using all the earthquakes that have occurred since 1910; for magnitudes 5 to 5.2 the time span is 13 years. thus, the frequencies of shallow (n.) and intermediate earthquakes (nt) have been determined and given in table 1. annual energy released by shallow (es) and intermediate (et) earthquakes of different magnitudes has been calculated using equation [2] and is also tabulated in table 1. figures 1 and 2 show the frequency-energy distribution of earthquakes against magnitude, the general shape of the figures being approximately parallelogram defining the boundaries of the frequency and energy of different magnitudes in the range of 5.0 to 8.6. these figures clearly show that the frequency of earthquakes is maximum when the energy release is minimum or vice versa, a well known fact in earthquake seismology. in seismology these facts are expressed by representing logarithmic dependence of e the seismic energy and n the earthquake frequency, on magnitude m. these relations are known as energy-magnitude (eq. 2) and frequency-magnitude relations as given by gutenberg and eichter (5) in the form log n = a — bm [3] where a and b are constants. however in the present analysis the energy magnitude relation has been assumed logarithmic and then the energy values are calculated as given in the table 1. the slope of the two sides of the approximate parallelogram (not shown in the figures 1 and 2) may take any shape depending upon the values of b and b. i t has been observed by utsu (10), chouhan (4), page (7) that the value of b varies from 0.4 to 1.5 and bath ( l ) has also shown that the value of b may be taken as 1.44 also, apart from gutenberg-eichter's value of 1.5. thus we can say that the slope of e changes from 1.44 to 1.50 while 6 2 r . k . s. c h o u h a n v . k . s h r i v a s t a v a t a b l e 1 a n n u a l f r e q u e n c y a n d a n n u a l e n e r g y r e l e a s e d b y s h a l l o w a n d i n t e r m e d i a t e e a r t h q u a k e s of d i f f e r e n t m a g n i t u d e s d u r i n g t h e p e r i o d 1 9 1 0 to 1 9 6 9 w i t h m ^ 5 . annual frequency of annual energy released by earthquakes magnitude earthquakes ( x l o 2 0 ergs) shallow (n.) intermediate (ni) (m) shallow (e*) intermediate (ei) 12. 3. 4. 5. 0.0167 8.6 1182.00 0.0333 — 8.3 592.00 — 0.0333 — 8.0 210.00 — 0.0667 0.0167 7.7 149.30 37.39 0.1334 — 7.6 211.40 — 0.0667 0.0333 7.5 74.80 37.36 0.0333 — 7.4 26.45 — 0.0833 0.0167 7.3 46.10 9.30 0.0667 0.0167 7.2 26.54 6.60 0.0500 0.0167 7.1 14.45 4.70 0.0667 0.1167 7.0 13.31 23.28 — 0.0167 6.9 — 2.35 0.0833 — 6.8 8.30 — 0.1167 0.1500 6.7 8.20 10.62 0.0167 0.0833 6.6 0.83 4.17 0.1500 0.3167 6.5 5.32 11.24 0.0167 0.0500 6.4 0.42 1.26 0.2167 0.2167 6.3 3.86 3.86 0.1000 6.2 1.26 — 0.1834 0.0667 6.1 1.60 0.59 0.3167 0.2344 6.0 1.99 1.47 0.1334 0.0500 5.9 0.60 0.23 0.2334 0.1167 5.8 0.72 0.36 0.3000 0.4834 5.7 0.66 1.06 0.2334 0.1334 5.6 0.37 0.21 0.3334 0.4000 5.5 0.37 0.44 0.5834 0.3500 5.4 0.46 0.28 1.3669 0.2500 5.3 0.78 0.14 3.6900 0.8330 5.2 1.47 0.33 3.7600 1.0000 5.1 1.06 0.28 2.6900 2.0700 5.0 0.54 0.41 s t a t i s t i c s of i n d i a n e a r t h q u a k e s f r e q u e n c y e n e r g y e t c . ()1 a n n u a l f r e q u e n c y ( n s ) a n n u a l e n e r g y r e l e a s e ( e s ) i n e r g s 10 10 ió1 . 20 21 22 23 ,„24 10 10 10 10 10 10 10 -7.5fig. 1 frequency-energy distribution of indian earthquakes against magnitude m ^ 5 for a period of 60 years, from 1910 to 1969. the figure shows the distribution for shallow earthquakes. a n n u a l f r e q u e n c y ( n ¡ ) a n n u a l e n e r g y r e l e a s e ( e i ) l n e r g s 10' 10" id1 i o 2 i o 1 9 i o 2 0 io21 i 0 2 2 7.5fig. 2 frequency-energy distribution of indian earthquakes against m 5 for the same period of fig. 1. the distribution is for intermediate earthquakes. 0 4 r . k . s. chouhan" v . k . s i i r i v a s t a v a the slope of n changes from —0.4 to —1.5. hence, when the values of b are 1.4 (approximately) to 1.5 and b 1.4 to 1.5 then the shape of figures 1 and 2 becomes parallelogram. for all other values of b the general shape of figures 1 and 2 is quadrilateral tending to a triangle for small values of b. these observations may be interpreted in support of energy magnitude relations of gutenberg-ricliter (°) and bath where they have used the upper limit of the slope of frequency magnitude relation. these relations can be expressed mathematically by combining the equations [2] and [3] in the form n (e) = c.e.-i»"»-1 [4] where 10 {a+ab/b) ° = ¿ i n f o where n(e) de is the number of earthquakes with energy between e and e + de. here it may be mentioned that in u.s.s.b, the frequency-energy relation is used in the form log n(k) — g — v k [5] where k = log e and v = bjb which is similar to relation [4], 4. secular energy release in the years 1913, 1919 and 1953 the energy released by earthquakes was less than 1019 ergs and in the years 1934 and 1950 the maximum energy exceeding 1024 ergs was released. the average annual energy released by shallow earthquakes is 2.58 x 1023 ergs and for the intermediate earthquakes 1.57 x 1022 ergs respectively; the annual total release being 2.74 x 1023 ergs. there are only eight years in which the energy release was more than the average energy release per year. the ratio of the average energy release in shallow and intermediate earthquakes is 10.5:1. gutenberg and richter (5) have shown that ratio for the shallow intermediate and deep earthquakes is 31:4:1. each plotted points in figures 3 and 4 represents cumulative energy released by earthquakes since the beginning of 1910 until the s t a t i s t i c s of i n d i a n e a r t h q u a k e s f r e q u e n c y e n e r g y e t c . ()1 t i m e in y e a r s fig. 3 cumulative sum of annual energy plotted against time for the same period of figs 1, 2. the figure is a plot of shallow earthquakes. end of 1909, for a period of 60 years. the points are bounded in between parallel straight lines of which the upper line, in fig. 3 is expressed for shallow earthquakes. 5 x = (3.56 t + 2.56) 1023 ergs [6] 66 r . k . s. c i i o u i i a n v . k . s h r i v a s t a v a where 2-es is cumulative energy for shallow earthquakes, and for intermediate earthquakes s-bi = (2.23 t + 0.85) io22 ergs [7] where is the cumulative sum of energy for intermediate earthquakes. in equations [6] and [7] t is counted from 1910. the distance between the two parallel lines in fig. 3 corresponds to about 2 x 1024 ergs which is very close to the energy of the largest conceivable earthquakes. similarly in fig. 4 the distance between the two parallel lines corresponds to 1.81 x 1023 ergs which is very close to the largest intermediate shocks observed in the past 60 years. 0 ' -i—i i ! , 1910 1930 1950 1970 t i m e in y e a r s fig. 4 see the caption of fig. 3. the plot is for intermediate earthquakes. s t a t i s t i c s of i n d i a n e a r t h q u a k e s f r e q u e n c y e n e r g y e t c . ()1 the slope in equations [6] and [7] gives approximately the rate of accumulation of energy in the crust and in the upper mantle upto a depth of about 300 km, respectively. similar study carried out by tsuboi (9) shows that the rate of accumulation of energy in japan is 2.24 x 1023 ergs per year and this rate of energy accumulation is 2.5 times higher than the rate of accumulation arrived at here which, after normalising in area, comes to 0.89 x 1023 ergs per year. these interpretation of secular seismic energy release is quite in line with the observations of bath and duda (2). the rate of energy accumulation for shallow and intermediate earthquakes is always greater than the average energy release in these shocks and at the most they may be equal. distribution of number of earthquakes having m > 5.3 with depth is shown below in table 2. depth range in kms. 0-50 50-100 100-150 150-200 200-250 250-300 number of shocks. 279 51 35 39 76 1 the distribution of earthquakes does show a maximum at a depth of 200-250 kms which is in agreement with bitsema's (8) second maximum and also agrees with gutenberg-richter's minimum in the depth range of 250-300 kms. 5 . c o n c l u s i o n s 1) frequency-magnitude and energy-magnitude distribution of shallow and intermediate earthquakes shown in figures 1 and 2 show logarithmic dependence of n and e on m and relation between n and e may be written in the form n (e) = g.e. -<w«>-1 2) secular studies of energy release clearly show that the average energy release is in general less than rate of accumulation of energy. i t also follows from figures 3 and 4 that the maximum size of conceivable shallow earthquake is m ~ 8.6 and of intermediate earthquake is m ~ 7.6. 3) distribution of number of earthquakes with depth shows a maxima at a depth of 200 to 250 kms and a minima in the depth range of 250 to 300 kms. 66 r . k . s. c i i o u i i a n v . k . s h r i v a s t a v a a c k n o w l e d g e m e n t s the authors are grateful to profs. j. singh, h . s. rathor, e . k . yerma for discussions from time to time and for providing all the facilities to carry out the present work. r e f e r e n c e s (') bath m., 1958. the energies of seismic body waves and surface waves. "contributions in geophysics in hon. of b. gutenberg", 1-16. (-) ba'rn m. and duda s. j., 1964. — earthquake volume, fault plane area, seismic energy, strain, deformation and related quantities. " a n n . di geofis.", x v i i , 353-368. (3) ciiouiian r. k . s. and das u. c., 1971. preliminary report on global seismicity, frequency energy distribution of earthquakes. " p u r e and applied geophysics", 89, 98-108. (4) ciiouiian r. k . s., 1970. on the frequency magnitude relation log n = a — bm. " p u r e appl. geophys.", 81, 119-123. ( 6 ) gutenberg b . a n d richter c. f . , 1954. seismicity of the earth and associated phenomena. "princeton university press". (8) gutenberg b. and riciiter c. p., 1956. magnitude and energy of earthquakes, " a n n . di geofis.", i x , 1-15. (7) page r., 1968. aftershocks and microaftershocks of the great alaskan earthquake of 1964. "bull. seis. soc. a m . " , 58, 1131-1168. (8) ritsema a . r., 1954. a statistical study of seismicity of the earth. 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"geophys. mag.", 30, 521-605. priolo_fc_corretto_layout 6 591 the ferrara thrust earthquakes of may-june 2012: preliminary site response analysis at the sites of the ogs temporary network enrico priolo1,*, marco romanelli1, carla barnaba1, marco mucciarelli1,2, giovanna laurenzano1, lorella dall'olio3, nasser abu zeid4, riccardo caputo4, giovanni santarato4, luigi vignola5, carmine lizza5, paolo di bartolomeo1 1 istituto nazionale di oceanografia e di geofisica sperimentale (ogs), centro di ricerche sismologiche (crs), udine and trieste, italy 2 università della basilicata, dipartimento di strutture, geotecnica, geologia applicata all'ingegneria (disgg), potenza, italy 3 comune di ferrara, servizio ambiente, ferrara, italy 4 università di ferrara, dipartimento di fisica e scienze della terra, ferrara, italy 5 mallet s.r.l., marsicovetere (potenza), italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6172 1. introduction following the ml 5.9 earthquake that struck the emilia area in northern italy on may 20, 2012, at 02:03:53 utc, and in co-operation with the personnel of the municipality of ferrara and the university of ferrara, a team of seismologists of the istituto nazionale di oceanografia e geofisica sperimentale (ogs; national institute of oceanography and experimental geophysics) deployed a temporary seismographic network. this consisted of eight portable seismological stations, to record the local earthquakes that occurred during the seismic sequence. the ogs intervention was integrated into the broader action of the emergency response to the earthquake sequence, which was promoted by the national department of civil protection and the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology) [moretti et al. 2012, this volume]. the aim of the ogs intervention was on the one hand to extend the seismic monitoring area towards the east, to include ferrara and its surroundings, to be ready in case of migration of the seismicity in that direction, and on the other hand to evaluate the seismic response at the instrumented sites. some days later, another team of researchers coordinated by the university of potenza carried out some investigations at a number of sites, and in particular at three of the sites instrumented by the ogs temporary network. these investigations consisted of passive seismic noise measurements: 1) recorded by single stations and processed by the horizontal to vertical (h/v) spectral ratio (hvsr) method; and 2) recorded by arrays and processed by the extended spatial autocorrelation (esac) method [okada et al. 1987, ohori et al. 2002]. in this report, we first describe in outline the seismic sequence and the geological framework of the area. then, we describe the ogs temporary network that was deployed. finally, we show the results obtained in the seismic noise analysis and the three esac surveys at these sites, and discuss them in the light of the information available to date on the geological structure of the area. 2. outline of the seismic sequence and the geology of the area on may 20, 2012, at 02:03:53 utc, the area of emilia, which is located in northern italy, was struck by an earthquake of magnitude ml 5.9, with the epicenter at few kilometers from the towns of mirandola, finale dell'emilia and bondeno, and about 30 km west of the city of ferrara (figure 1). the main earthquake sequence was followed by a new sequence of strong earthquakes nine days later, i.e., on may 29, 2012, the most relevant of which was the ml 5.8 earthquake that occurred at 07:00:03 utc and was located nearly 12 km wsw of the main event. the seismic sequence involved the frontal sector of the northern apennines, and in particular the buried front of the romagna and ferrara northward-verging active thrust belt (figure 1). the frontal-most sector of the northern apennines is represented by a fold-and-thrust belt that underlies the po plain and is buried by a thick wedge-like pliocenequaternary depositional succession. due to the overall geodynamic conditions, subsidence prevails on the locally growing anticlines, therefore generating a laterally variable sedimentary thickness with a depth to bedrock that varies from between a few hundred meters and several kilometers article history received july 27, 2012; accepted august 27, 2012. subject classification: surveys, measurements and monitoring, ground motion, emilia earthquake, seismic monitoring, seismic site response. 2012 emilia earthquakes (figure 2). the geometry of this hidden chain has been relatively well defined by numerous seismic reflection profiles. the most important tectonic structures are represented by the ferrara arc and the minor adriatic and romagna arcs (figure 1). recently, toscani et. al. [2009] showed that some of these blind thrusts are probably still active. priolo et al. 592 figure 1. simplified structural map of the eastern sector of the po plain, showing the major ferrara arc and the two minor adriatic and romagna arcs. triangles, ogs temporary seismographic stations installed around ferrara following the earthquake sequence that started on may 20, 2012. stars, events of the sequence with ml ≥5 (modified after [pieri and groppi 1981] and [cnr-pfg 1991]). figure 2. hydro-geological sections running in the wnw-ese direction, through (a-a') the casaglia structure (ferrara); and (b-b') the mirandola structure (modena) (modified after pieri and groppi [1981]). both sections show the main aquifer bodies corresponding to the middle-upper quaternary lithostratigraphic units that mainly consist of alluvial and shallow-water marine deposits that progressively infilled the po river foreland basin (modified after regione emilia-romagna and eni-agip [1998]). 593 for the superficial geology of the area where the temporary seismographic network was installed, the outcropping materials are represented by alluvial deposits of different environments, like channels and proximal levees, and inter-fluvial, meander and swamp deposits (figure 3). as a consequence, the outcropping deposits are holocene in age and everywhere they are substantially loose or poorly compacted in the first meters to decameters, and granulometrically they vary from clay to coarse sand. two preliminary reports prepared by the national department of civil protection in collaboration with other institutes describe the data recorded by the national accelerometric network and complemented by additional data recorded by a number of temporary stations [dolce et al. 2012a, b]. these reports bear witness to the strong ground-motion values with an acceleration peak of about 0.9 g in the vertical component that was recorded during the ml 5.8 earthquake of may 29, 2012, by the mirandola station, which was located about 2 km from the epicenter. the analysis of the seismic noise recorded at some of the stations shows a relatively pronounced peak of the hvsr in the frequency range of 0.6 hz to 0.9 hz, common to all stations. finally, strong evidence of liquefaction phenomena were reported at several sites (e.g., san carlo, sant'agostino and mirabello), most of which were attributed to the occurrence of saturated sandy layer(s) at shallow depths deposited along an abandoned reach of the reno river [papathanassiou et al. 2012, this volume]. 3. the ogs temporary network and passive-noise measurements the ogs temporary network consisted of eight stations that were deployed on may 21, 2012, the day after the mainshock. a ninth site (og009) was set-up one month later, on june 25, 2012, as a replacement for station og003, the location of which was no longer available. the removal of the stations was programmed for the end of july 2012. details on the sites and the instruments used are given in table 1, while their locations are shown in figures 1 and 3. all of the stations were set at sampling rates of 125 hz, and they recorded data in continuous mode on temporary memory. none of the stations were equipped with data-transmission devices. the locations for the temporary stations were chosen in the area surrounding the city of ferrara (figures 1 and 3, table 1). detailed information on the temporary network ferrara thrust: site response analysis figure 3. simplified geological map of the western portion of the ferrara province showing the locations of the ogs temporary stations (squares), the permanent stations (triangles) of the local microseismic network of the ferrara municipality that is managed by ferrara university (http://servizi. comune.fe.it/index.phtml?id=2593). stars, principal shocks of may 20, 2012. legend of stratigraphic units: (a) medium to fine sand (channel and proximal levee deposits); (b) silty clay, clay and clayey silt (interfluvial and swamp deposits); (c) sandy silt, fine sand and silty clay (distal levee deposits); (d) medium-to-coarse-grained sand (alluvial plain and meander deposits); (e) medium to fine sand (distribution channels and levee deposits); and (f ) silt, clayey silt (swamp deposits). the age of the outcropping sediments is holocene everywhere. modified after banca dati regione emilia-romagna (http://geo.regione.emilia-romagna.it/geocatalogo/). can be found in the ogs archive system of instrumental seismology (oasis; oasis.crs.inogs.it) [priolo et al. 2011], under the network code zr. here, we provide a short overview of the sites: og001 was located east of ferrara, in the basement of the new city hospital located at cona. og002 was inside a warehouse of the ferrara municipality, in front of the chemical industry of ferrara, on a floor made of a concrete slab. the building is quite large, and it has a rectangular shape. the sensor was at the base of a nsoriented wall. og003 was deployed on farmland near the town of sant'agostino, a few meters from a small sand-liquefaction boil (about 3-4-m wide) that occurred during the mainshock of may 20, 2012. this accelerometer was buried in sandy soil in a 50-cm-deep hole. og004 was deployed in ficarolo village, in the garden of the building of a branch of the law courts. the lennartz 5s sensor was buried in sandy soil at a depth of about 70 cm. og005 was located in poggio renatico, in the courtyard of a private house. og006 was located in a private box-car in vigarano pieve. og007 was in the utility room of a private house in aguscello. og008 was in mirabello on the ground of a box near to a free parking area of a restaurant, where several cracks in the ground were observed after the mainshock. og009 was deployed in the basement of the church of san carlo, where diffuse liquefaction phenomena were induced by the mainshock. here, the whole floor, which was a concrete slab in a ca. 10 m × 10 m hall in the church basement, was uplifted by about 50 cm to 80 cm, due to sand injection. as already indicated, this site replaced og003, and it was set-up one month later than the others. unfortunately, there were some problems when the first dump of data was analysed, such that some data were lost or not usable (e.g., saturation for the largest events). however, this is not relevant for the aims of the present report, and will be considered in a later report. the group of potenza university took passive noise measurements at sites og003, og006 and og008. single station measurements were performed using the tromino® instrument, while for the array measurements, the geode 24-channel modular acquisition system by geometrics was used, equipped with geospace 4.5 hz geophones. some tests were performed to verify the consistency of the noise measurements performed by the two different kinds of instrument. as a result, excellent agreement was found between the hvsrs calculated from the passive noise measurements using the ogs and the tromino® instrumentation in the frequency band of 0.3 hz to 15 hz, of interest for this study. 4. hvsr and esac results figure 4 shows the hvsrs calculated from the single station measurements. the ratios were aligned along two hypothetical transects that run sw-ne (transect 1: with stations og003, og005 [as a lateral extension], og008, og006) and se-nw (transect 2: og001, og007, og002, og006, og004), as shown in figure 3. note that transects 1 and 2 are orthogonal and normal, respectively, to the main geological structures. the hvsrs shows relatively good coherence; e.g., an evident, although spread, low frequency peak, at a frequency between 0.5 hz to 1.5 hz. in some cases, this peak is sharper, as for og002, og006 and og007, while for other sites, such as og004 and og008, it is much broader. the amplitude of these peaks is always weak, and the hvsr ranges from 2.0 to 4.2. figure 5 and table 2 give the vertical profiles of the shear-wave velocity (vs) inverted for sites og003, og006 and og008 through the esac method, from passive noise measurements of the array and with the additional constraint of the hvsr calculated from single station noise. the details of the array geometries are shown in figure 6. the actual array length and the resulting investigation depth were, respectively: 345 m and 120 m for og003; 330 m and 110 m for og006; and 340 m and 120 m for og008. in all priolo et al. 594 code sito epicentral distance (km) start recording lat lon acquisition unit sensor og001 cona ospedale nuovo 40.9 2012/05/25 44.78921 11.727598 nanometrics orion lennartz 1s 3d light og002 comune di ferrara lavori pubblici 29.5 2012/05/25 44.85249 11.59847 nanometrics orion lennartz 1s 3d light og003 sant'agostino 16.7 2012/05/25 44.786163 11.38337 reftek 130 episensor og004 ficarolo (ro) 17.6 2012/05/25 44.952037 11.433883 reftek 130 lennartz 5s og005 poggio renatico 24.3 2012/05/25 44.766983 11.48494 nanometrics orion lennartz 1s 3d light og006 vigarano pieve 22.8 2012/05/25 44.861937 11.514676 nanometrics orion lennartz 1s 3d light og007 aguscello 35.6 2012/05/25 44.806519 11.663715 nanometrics orion lennartz 1s 3d light og008 mirabello 18.1 2012/05/25 44.812673 11.431863 nanometrics orion lennartz 1s 3d light og009 san carlo 16.9 2012/06/25 44.804401 11.408932 reftek 130 episensor table 1. detail of the ogs temporarty network 595 ferrara thrust: site response analysis figure 4. single station seismic noise hvsrs estimated along the two transects shown in figure 3. transect 1 (right) runs in the sw-ne direction and includes stations og003, og005 (about 6 km out of line), og008 and og006. transect 2 (left) runs in the se-nw directions and includes stations og001, og007, og002, og006 and og004. the h/v peak value is specified for each curve. figure 5. vs estimation by the esac method at the og003, og006 and og008 sites. table 2. velocity and depth estimates using the extended spatial autocorrelation method at the og003, og006 and og008 sites. site depth (m) vs (m/s) og003 0.0 – 5.86 106 5.86 – 22.76 152 22.76 – 83.24 305 > 83.25 556 og006 0.0 – 11.5 146 11.5 – 29.0 250 29.0 – 101 337 > 101 645 og008 0.0 – 11.30 120 11.30 – 30.20 215 30.20 – 80.80 337 > 80.80 515 cases, a four-layer model was used in the inversion. sites og006 and og008 features relatively similar structures: the vs started with very low values at the surface, which ranges from 120 m/s to 150 m/s in the shallow-most 10 m, and increases with depth, with values of about 215 m/s to 250 m/s at a depth of ca. 20 m to 30 m, and then of 337 m/s for a further ca. 50 m. while the vs appears to increase uniformly for og008 also at greater depths, with a value of 515 m/s, a velocity contrast is imaged for site og006 at a depth of 95 m to 110 m, where the velocity jumps to 645 m/s. the hvsrs of those two sites (figure 4) conform very well to this interpretation, with og008 showing a much broader and smoother peak than that of og006. site og003 has even lower velocity values than the previous sites. we note here that some particular events of sand liquefaction occurred here. the vs ranges from ca. 100 m/s at the surface, for the shallow-most 6 m, and progressively increases at about 150 m/s from 6 m to 22 m, and 305 m/s from 22 m to 83 m. again, relatively weak contrast is seen at 80 m to 90 m of depth, where the vs reaches 550 m/s. priolo et al. 596 figure 6. array geometry (yellow lines) of the esac surveys performed at the og003, og006 and og008 sites. for each site, a map and a list of the geophone coordinates are shown. the location of the temporary station is indicated by a white square in each map. the array geometry consists of two nearly orthogonal branches. the array length specified in the text is the total length of the two branches. 597 5. discussion and conclusions all of the eight sites of the ogs temporary network analyzed in this study features common hvsr characteristics; i.e., a weak and broad peak of fundamental resonant frequency in the low frequency band from 0.5 hz to 1.5 hz. the esac investigations performed at three sites shows that this peak was caused by a weak s-velocity contrast at depth of ca. 80 m to 100 m. above, the s-velocity progressively increases with depth, starting from very low values of about 100 m/s to 150 m/s at the surface, to 300 m/s to 340 m/s at a depth of 80 m to 100 m. beneath that depth and down to the investigation depth of about 110 m to 120 m, the s-velocity settles at 500 m/s to 550 m/s, although site og006 has a higher velocity, of 650 m/s. coherently, the sharpest hvsr curve is estimated at this site. the very low velocities at the surface corresponds to cohesionless soils that are characterized by very low shear-modulus values, such as the sandy and silty-sandy soils that are commonly found in that area. this study provides just the preliminary results: for instance, it does not provide the actual spectral amplification of the investigated sites. more thorough data will follow from the analysis of the recorded earthquakes. finally, at site og0004, which is the northern-most site of transect 2 (figure 2), the hvsr features a very broad, low-frequency lobe, with two distinct peaks at 0.3 hz to 0.4 hz and at 0.9 hz (figure 4). while the latter is well correlated to that of the other transect stations, the peak at the lowest frequency might indicate the presence of a deeper, structural s-velocity contrast. this hypothesis agrees relatively well with the thickening of the sediments previously documented in the area of ficarolo (figure 2), where station og004 was located. acknowledgements. we thank elisa zambonelli of the national department of civil protection, and lucia margheriti of the istituto nazionale di geofisica e vulcanologia (ingv), who gave us many important pieces of information and helped us to keep tightly coordinated with the interventions of their institutes. references cnr-pfg (progetto finalizzato geodinamica) (1991). synthetic structural-kinematic map of italy. structural model of italy, sheet n. 5, s.el.ca., firenze. dolce, m., m. nicoletti, a. ammirati, r. bianconi, l. filippi, a. gorini, s. marcucci, f. palma, e. zambonelli, g. lavecchia, r. de nardis, f. brozzetti, p. boncio, d. cirillo, a. romano, g. costa, a. gallo, l. tiberi, g. zoppé, p. suhadolc, f. ponziani and a. formica (2012a). the emilia thrust earthquake of 20 may 2012 (northern italy): strong motion and geological observations report 1, national civil protection department (dpc); http://www.protezionecivile.gov.it/jcms/it/ran.wp dolce, m., m. nicoletti, a. ammirati, r. bianconi, l. filippi, a. gorini, s. marcucci, f. palma, e. zambonelli, g. lavecchia, r. de nardis, f. brozzetti, p.boncio, d. cirillo, a. romano, g. costa, a. gallo, l. tiberi, g. zoppé, p. suhadolc, f. ponziani and a. formica (2012b). the ferrara arc thrust earthquakes of may-june 2012 (northern italy): strong-motion and geological observations report ii, national civil protection department (dpc); http://www.protezionecivile.gov.it/jcms/it/ran.wp moretti, m., et al. (2012). rapid response to the earthquake emergency of may 2012 in the po plain, northern italy, annals of geophysics, 55 (4); doi:10.4401/ag-6152. ohori, m., a. nobata and k. wakamatsu (2002). a comparison of esac and fk methods of estimating phase velocity using arbitrarily shaped microtremor arrays, b. seismol. soc. am., 92, 2323-2332. okada, h., k. matsushima and e. hidaka (1987). comparison of spatial autocorrelation method and frequencywavenumber spectral method of estimating the phase velocity of rayleigh waves in long-period microtremors, geophys. b. hokkaido univ. 49, 53-62 (in japanese with english abstract). papathanassiou, g., r. caputo and d. rapti-caputo (2012). liquefaction phenomena along the palaeo-reno river caused by the may 20, 2012 emilia (northern italy) earthquake, annals of geophysics, 55 (4); doi:10.4401/ag-6147. pieri, m., and g. groppi (1981). subsurface geological structure of the po plain, italy. consiglio nazionale delle ricerche, progetto finalizzato geodinamica, sottoprogetto modello strutturale, roma, 414, 13 pp. priolo, e., g. laurenzano, c. barnaba, p. bernardi, l. moratto and a. spinelli (2011). oasis – the ogs archive system of instrumental seismology, in: m. mucciarelli (ed.), tecniche speditive per la stima dell'amplificazione simica. studi teorici ed applicazioni professionali, roma, aracne editrice, 445 pp. regione emilia-romagna and eni-agip (1998). riserve idriche sotterranee della regione emilia-romagna, edited by g. di dio, firenze, s.el.ca., 120 pp. toscani, g., p. burrato, d. di bucci, s. seno and g. valensise (2009). plio-quaternary tectonic evolution of the northern apennines thrust fronts (bologna-ferrara section, italy): seismotectonic implications, b. soc. geol. ital., 128, 605-613. *corresponding author: enrico priolo, istituto nazionale di oceanografia e di geofisica sperimentale (ogs), centro di ricerche sismologiche (crs), udine and trieste, italy; email: epriolo@ogs.trieste.it. © 2012 by the istituto 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice hecent investigations of a bronze age eruprion (*) p. f-féderva.lti (**) rieevntu il 25 agustu 19;0 su~u[alty 'l'he firht part of the paper i8 a sllort account about. tllc «int.ernational seientific congn'ss on thf', volcano of thera >, organized by the greek arehaeological serviee. in t.he seeond part, the author deal~ with the proljlcm uf the so-called minoan tsnnami of vulcano santurin. kome ('alculatiolls are ma(le concerning the energy of this thullami in c,ase of (liifer('nt waye_heights. 'l'he noj.jon of free tcetonie fmergy is introduccd and treated. the aut.hor states that the relationship betwcen the height ami the energy of a tsnnami, expressed mathematically by ii(la i8 nsefnl in tllc eahe of t he santorin minoan '·tupt.ion, too. riassunto. la prima parto dell'articulo è una breve relazione hnl congrehmi keientil1co internazionale sul vnleauo ,ii thera, organizzato ,lal 8ervi1.io arcllflologko gre('·o. ~ella ~econ(la parte l'autore tratta il problema del cosiddetto minoan tsunami dci vuleano santorin. sono st.ati bsf',guiti alcuni l'alcoli snll'enci'gia di qnooto t8unami nel caso ,ii varie altezze ,1'ulhla. si introdw.'e e si tratta il concetto (li energia liljera tettonica. l'antore prccisa che il rapport.o tra l'altezza e l'energia di un tsunll.mi, cspn~s~o matt·. liiaticalllente da lilla, è utile anche nel easo dell'eruzione )finoan di santorin. volcano sa.nt.ol'in (sa.ntorini, thera.) is situa.ted in the aegea.nsea., :north from centra.i cl'etc, 36°24' n and 25°24' }j abont. ft is a h'toup of three volcanic border islands and (at prescnt) two (*) report aljont the intemational scientific congres,'1 on the voleano of 'l'hera. (**) .\l se., f. r.a.s., geographci', vice·presi(lent uf internatiunal a,;soeiation of planetolugy (iugs-unesco); se(uetary of cumlllihsiun of planetary physics uf iap; president of international lunar society. vi. lenin·bly,l. k2. iii. 6a, llllliapest, (1inngary). 1!j~ [jfwlm".·\i\1 l'. centrai ihlandh, one of them .~till l101v-a-days is aetive, in a cl~l(hml. . 'filo tlimcil~ions of the (mldpm l~re !(,s follows: db~nwhll' in h". tlirection is -;,ti km, in \' -f; 11.0 km; its ,!...'l·pl ~t~;~t (h'pth l1t iti; northem p a ri ii! ahoht :l!)1) ml'1j·('.~ lllll!el' tlw fwl ~-levd; 11m iwight or tl1(\ walli! ti1erasia c? aspron isi o l o .. • p alaea j(ameni .hrotiri ",i l~ • pi1era • me s s ar j :l. z ood ochos' t k e r a / vii!. .\ial' "r t ii(' j"lawl .. or sa nlorin. \ ( • in ~onw phceh exeeedi! 300 l1lelrllh. 'fhe nmnes of t.lle l'entra] i.~]allds an', l'a bea lmd xea kalllllni; the lub1lles or the ho!'de1'('.1' i.~]alld.~ th a l l~!'c t.lw wall ... or tlte (',alderal~l'o 'l'llllm (t.lw h l.rgeht p a rt), th(jnl .. ~i<1 !(,nd ahpronii'li (1:he sl1mlle.~1: part.) respl'd,ively. 'fhe highest ]loint or t.he t;;:mnellae-ish l.ll(l~ is !(,hollj, 131 metres l1hove t]w sel l.-]eyel. 'l'hl' vo!nme of t.lw lia]i!i'.]'u, liihh'r t11e leve! of t.he wm,(\]' ii! eslilllmi',rl lo he re('ekt ikvestwations 01' a iljwnze .ioe elwl"'l'lon hl::! n,hout 40-·15 k1ll3, tlmt ifl lllur.h biggel' t.lw.ll the yohuno of kra katoa ealcler: l. . the surface of the wat.cr imide t·he wall~ of the c:.lclem i~ 83 k1ll2 • 1t is generally accept0d that tl1ll'ing the formatioli of t.he fig. 2 .'iiodf'l of .':iallt.orill, liia.!\' oll !ile h:.t8is of lile brh.il'h ai!liiindity dmrt {aftllr (jalalwl)(li,i".~. atlullti8. thollla~ ki·l~llll alli! :-;oll~ ltd, l"jllloll, j l)h\j) santorin ealdem about 7:! km 3 of material "disappeal'etl n from the sul'f: l.ce of the barth, clue in !.in ver eent to the collapse phenonu\lloil awl in 1pel' el111t. to (,fi'nct.ivl1 cxploflions. the layas of pabea lfl..jiit:l>jm\'.\iu t'. kmueni as well li .. 'l t.hose of t.he lllol'l'. i·e(lent. er·li]ltioll.'l were fil"sl elm1"l1ctel'ized a.h irlll/(';;it('8 with hypt'rslhl'lit ii/id (tu.yi/,' . now geol'gala~ (1) prefer.'l t.o call the layas "d,wiloidts". coucer'nilll-{ 11m lllosl an(,ient el'u}lt.ionh, l~s [al' al> wl', cl1n see for them !mck in t.he t.ime, r.nhi l'eceutly stated (heezen(~)) that a gnh~t. mu}ltùm oeclll'l'ed a,ljout. 80.000 ymws ag-o. ,\eml1"llilll-{ t.o heezeu a,llii kinkovich (a) the ~e(h)llii eata,~t.rophie erll]llioll of yoleallo sl~ntoriu (on it..'l fo1"ltwl" tmille: slronghyle) had taken }lbl.<~.e in the bl'onzo .age, m. l1hollt. 1·10\)·1500 y(%~i's rc. this lattei' et'u}lt.ion generally is ealled l1.'l t.he ".:\linoan" one, .'lince a(lcol'dillgt,o t,he al'chaeological resel~l'ehes, cal'l'ied ollt hy .:\bl'iuatos (~.5.".7) this g-iwmt.ie ollthul'st de.'lt.1"oybli l111 the .:\linoan sett.lellwnt.s on st,l'onghyle itmlf and lllany ::\iilhlltn set.tlelllent,,> on ('l'ete, as \\·(\1j l ~s ou the neig-ithoul'illl-{ ishnds, too. 'l'he f()]·ltmt.iou of t.lte ltlight.y m.lllen~ wa~ the direet eouseqllellee of thi~ .\linol1u (;j"li]ltion. ..\t. lhe sl~llle t.ime ot.lh'r m~t.lil"al eveut.s al.'lo oeellrl"ed: elwt.hqllakes, t.i>ulmmis and even t.ol"nadoes, t.oo [see foi" instanee (blanopoulos (~), van bellllnelen (~), l\jlwinos (1")]. ('oncel'uingit.s illtensity, the l·es]lp(l1.ive sl ~ut()]'iu ('rllptiou ih eolllparahle with that of hollw iudouesia.n "olefluoph; kmka,tm~ in hn:1, 'l'l~lttbom in 1815, el(,. ("). the lollr/ 1'!lt1'y:l! relel~sed hll.<l t.he onlol" o[ 10 27 et"j.('s (12). i should like t.o rt,j;urn lat.er t,o a, part.iellbr prohllllll of t.lw ll1wi·gy-calelilati(lu. siuee t.lw l\lino a u el"li]lt.ion hall au exeelltioual role in uw life l~lhl hil>t,ol'y or t.he lmtiollh in t.he }lroximit,y, li .. '> [al' l~s ali the cy(',bl.<lie islands t,o the ::"'ol't.h, cret.e to t,he &mt,h, ef,(y}lt, and syl'il~ t,o t.he sollthea,,,t allli r';l.~t re.'lpeet.ively, it l'l lindel'stalllbhle that 01(1 gl'eek at"cillwolog-i('l~1 sol"viee ile(lilled t.o o1wmize l~ g1'(mt eoug-n',s.'l wit.1t the lml"tieipat.ion of al"clmeologist.s, hisl oria,lts, oem~iio)..,"rallllers, l-{eol-{rallhen" voicl11101ogist.s, geo}lhysicist,,> and otlll'l' ex}lert,'1. tlw t.ime or lhe eongnl.'ls wa .. 'l (llto~eu as septeltlllol', 1 \hw, siltee t.his eal'ly-allt.ultln hltle is very [ayourl~hle in orl\oee to b.ke a jonnwy froltl atheu~ t.o sant.uriu lhhl to ('ret.e hy ship. rel111y, t,he ('.ongtchs had heen b~ken l'bel', hetwelln tlw im,h l11hl 2.3rd, sllp1.mnhpr, 1\100. all 1.111'part.ieipauts wm'o glle.'lt,,> of t.he glw>,k governllllwnt lllld of l he hret'k ardmeologi(',l~1 seryiee whieh is linder the dil'e(~tionshi}l of professor dr. h. {'o sllyrido!t ::\larinat,os, llwln!wr of the ,\ealle1ny. 'l'here 'were l1hollt. 120 seieltti.'lt.s [rom lg countries. tlw 1lj(~tnre:-; hall taken 111l><:h\ pl.rhy iu .\t.hmts athi lll.rtly on the .'lhi}l "knoshos" (10.000 t,on). all t.he pallcl's al\(l t.lw limt.!\l'i a l of liisells~ious l ~re iltterllied t.o hl.', pnhlished as l1 .'l}lecial " aet,l~" or t.he (long-ress. 'l'1m partl(lilmnt.s imd opllol't.linit.iph tu yisil ! !l;'; t he ~~et,ive yo[(mllic ('ellt,ei's of t he 8ant oriu, khc["e l'l"ofl'~~ol" ì: oko.\";tilla of l1okkal~lo (',al'1'ied oui" gnl.yl111ot,l"ie ()hs~'tya.tion~, eontinlliihg" lti~ l'eheal'"tl('~ on t.ltih hubjel\t" i~~ l'(\ga.rd~ t,lw gn~yi1-y hollomalil's of yol('~~nil\ ca]llpj'a.h; ful't1wnnon' t.o b~lw visit,s oll tlle sitoh or l'cl',ent, i1l'(',hapo]ogienl l'eheat""hes of .:\ia.riuh.t,os i~t akrotiri (sil.n1orin) holhl a.t -,\lllnisho~ (c'l'et l'}. \ve have fweu the worl~l-falllo\l~ )linm~n pa!alw h.t kno~so~, ('l'(\te a~ kell a,,~ honle inilse\lln.~, t 00. pil-!:_ 3 'l'hp xi':l 1\:all1l'lli ,"oh'allil' i~lalld, yip,kp,l frolli ea,.t (]llwto of 11[(' authol'). '['ih1 [e('.t\ll"o.~ ~~nd the di~e1l.~hioilh ~l('alt wi1h t,he' mos1 (iiffel"('ut pl'ohlcllls. foi" ('xltlllp[(\: ~'x~w.t, tinw of tlw .:\lilloa n ol\th\ll".~i": a.l"eha('o]ogi"al (·x}jla.lmt.ion~ of 11w til)(lillg.~; 11w tc('toll1es of 1 [l{' ì\le~li­ t.ctta u(·a.n: (·ollljlh.l"isou ,d1h yo[eauie l']"(lptiollh ou o11wr p~wts of the wor!d: 1>,ited,~ of t!w ::\linoan yoleani(' ('m a stl'o}jhe on eg.,"pt, 01l c'rd.l> aihl oh otllel" bi.(\(b; 1>iwl'gi('.~ of 1"1w el"uptioll: }j1"(·.~(·ut pl'oblt'lll of inmw vole a uislll iu the ligh1 of the ",\pollo-11 e'xperillwnt, elt·,. thel'e wa·s ~~ debm.e couceming lire height 01 /ii(' jlùwan t.'lwwmi. 8iuee it, 196 uf:ijeltvaitl p, is very i1ll1)ortant fl'om t1hl ]joiut of view of my ealculatiolls of euer.!,'"}', i f;hould iike to deal with t,his illt.erest,ing l)roblem hen) iu it.f; !llltails, aecordillg to g11lauoijonlo~ (8): '''l'here is uo\\' ijillut.y evidencll that the co11aijs(\ of tite ccii trai part of the fomwr st.l'onghyie isiand wa~ abrul)t, ellough t,o set lilj slla-w aves of pl'odigiouh height tlmt swc]jt clean 30 11 the coaf;t~ of the ca.st.ern }iediterranea n, fumice catticd by the ~ea wave~ waf; found in 8cdimcntf; of a ijof;t-gi:\ciai tenace ti m fig, -j. kamcni. islalhl thcra, in tlte for!1gnl1llll1 tltere is t.hc snrfacc of ncn. thc ship i~ thc "kno~s(),," ()f ]()()()() t.on, tlte t.own on t.hll {'hldem-wall i;; p]lc'l'lt (photo or thp allthol'), allove sello levlll n ort.ll of jaffa-te1 ~\viv, according' to }i, pfannenstilll, tlhl ]jnmice ca.me from t.he santorin crul)tion, fll1'thcr, arclmeological evidcncc l)oints ont t.hat the l)ol't aud imlf of the citr of ug11rit, in syria, were !l(i.'li;l'oy(\d abollt 140() re. br a tf!iul,òmi. .. \ssmhiug 11h ensta,tic rise of thc bea-ievel b,v 2 in in t.he time interval of abont 3 ~2 thonsand ycars, t.hc height or the sca wa.ves in ja.ffa tel avi" should have been at least 7 m, disreglwding now the ios8 from abr~;ces1' inves1'iga1'ioss of a bron7.e a(;e erllp1'los i!j, horiltion and taking into account that lhe kea wavei> produced during thc bil'th of the caldcra imd at a distancn of a bout 900 klll a height of more than 7 m, we may casily calculate that their height, at the start,ing poillj; sllould have been at lea st 210 m, the ca lrml atioll of the initial height of t.lll:l tslllmmi is in accorda ncc with )ia.rinos' rccellt estimation (lo). g. ì\larinos alld his associa.te havc found layers of pumice in thrtle illac(~~ on the island of anaphi, situated at a distmlctl of about 25 km east of sant,orino ali thcse la:rers wcrc folllld a l t.he lwads or mvincs or v alleys on tlw ishmd. ì\larinos and hig a~· fil:\". :; til(' ",igo of ihé t!aldcfll· wa ll, \"i c wed from thu 1'uuth. 'l'ile towll is ljhcra (piloto of the anthor). wciate h avc good reasons to believe that t./w puntice on a.naphi was carried by the tsmmmi following the format,ion of t,he volc.'lllic cavity or santorill ". it is 1i0t.ewol"th~that one of thcse three laynrs were found 250 met,res above t,he level of t,he sea , at, a distanctl of 1650 met,res [rom the coast. 1lme d. vitaliano and prof. ch. vitaliano (13) have argued a gainst this st,atmnent, writ.ing a.s follows: " fnasmuch as ana phi lies close enough t,o santorin to have received airborne ilumice in the climatic explosions, tlw anailhi tlillniee lllust. be viewcd ·with rcservatiolls as evidence or thc hcight or the }iinoan tsunami ". this may be also true, of course, however i do not sharc the m.me and prof. vh,a liano's opinion. as h, was ment,ioned above, namel)', 11m 1i~;ih~jt\'a]{1 l'. all ti/ree 1(/.iier.~ of jlnmice were fvu,ud al uw }wad,~ of /'(u'iile.~ or nllley.~. su}l}lo~inl! t.ha.t, thc plunicc 1w.d been ca.\ticd by thc air-shockh, throughollt the ail', t.hc }lumice would dist.ributcc1 ovcr a i!rel ~t.er pal't of uw i.~lanli an~~l'hi, by allli large in the sa.me i!eneml dihtl'iblltùm, bnt ~illcc tlw t}ii'/'(: la.yers were follnli jw;t a.t. t.lw lma ds of vallers, thcy 1mc1 to caltied really by ow wa1.er of 11m ::;ea alonu, this wa.tc1' ha li heeil l ~(\eturl1!la1(\{i a.t t.hc illouth of thc v a llcy ~~jl(1 rm1chcc1 hig-he1' all(i higlw1' luvcls linrilll! it~ fu1'thcr wa.y th1'oul!hout the nalto\\' vallcy to its u}l}lcr cnel owinl! to t.lwse (~on~i(lemtions i i)l'efel' t.lw valne of 21 () metl'eh a.~ the init.bl heil!ht. of t!hj mino a n t.sllnami, in a~\(lo1'da.jlec with t.he ~llgg():';tion of l'l·of. cbla.jlopolllos ~md p1'ol ma1'inoh, 1'cs}lcctivdy. in sonw of my own na.lcub1tioll8 i mcc1 thi.~ va.lue. however i dcebrcd tlmt i mn willinl! to relwat the (\alcnl a tiol!h on the basis of othe1' v~.hlcs for the height of the tsllila.mi. i shonld like t.o maue h here , in t·he pre::;ent, paper, '['lle followinl! v,blues werc c1losun: 30 m eoltcs}loilllinl! roul!hly to the t.:-milami-height aft·m' the kmka.toa. eruption ('l') 60 in, 90 in, 1:20 in, 1fio m, a,nd 1mo m, l"i\,~pcd,ivdy, i il the eoul .. ~e of pnwtim11 nh,lmliation i was for/lcù to llsc tlw l'cl~1tionsltip bct.wccn thc hcig-ht ~md cnc1'gy of tsunmnis, csta.blishec1 by iic1~1 (14) sinec ~1s far a.~ [ know h}leeial mathmllatiea.1 exprpssion f()]' the tsulmlllih of volc:mie-(\()lia.pse ol'igin is not. uxist a,::; ret. i think tlmt tlw ::;iillplu l'ebtion, cxill'csscd a." ~1 t~.blc in rida.'s (h) }laper i~ mctul in evc1'y m~~cs wltcn there is a tsulmmi in t.lte sea, t.he nmximal hcig-ht of which is kilown, independent.ly trom t.he origin (mlollse) of the respedive ,~(\a wa,ve, tlmt. is independen1 from t.1w p1'ohll'.m wllptlwl' it is a. w~1vc (lall,~(\{i by a m'll,~ta,1 iicfo1'mation of t!w sea bottom c1uc to ~. tcct.onic ca.1't.hquakc or it is a tsulmmi of volm~nic-collapse orig-in. (sec later, too). (.) ilnrillg ulc krakat.oa,l~nllailli ill 1.883 uio hpighl of sca wavc~ readlell a vallle of aholll 30-31i jilet,re~ oli lhe (:oa~lal area. acconlillglo li,la (l') t.he pllergy of a t,sllnami, (:ansfhi by /'art.hqllake, wit,h a [wig-hl of 32 mp_t,re,~ on the eoa~l i~ aboll1 2.50· 1(j!~ crgs. il is vcry probable 1ha1.. a "/'tt,ain parl of lhe ollcrg-'y ill lhc ~ea wave \\'a~ t.rall~ferrcd lo t.l1c waler from t,iw air wave (10). 'l'he ellerg-y of t.he air hla~l in t.he krakaloa ea~1' was e~tilllalcd also hy !larkridllr ami pr/'h~ (15) ao; hig-h ah the tot,al ilxplosive ellerg'y of a lludear homh, eqllivat/'nt lo [oo-l;'i() megatmlh of 'l'kt. it is e(!ual lo 4,is,[()"-(i,27·[o~~ ergs, sim'/' al'('(,nlillg to imho (lo) _ [ megaloll coi'r/'hpolhis t,o 4,18·1u" erg,,;. il i~ very reaflollable to assllmc t,iwt t11e energ-y of the milloan l~lljla!lli of salllorill imd a g-reat.er heighl alld greater cncrgy lhall llml of t.he l~\ljlallli of krakatoa allll lhe clicrg-y of air wa\'cs was at. lea~t. a~ high a.~ t,hat. of tl11' air hlaht, of krakatoa. hecest [>;yl>;stt(httosf< of a bro"'\ze a(;e erl:ptio>; 199 by thc cxt.mpobl,tion of t.ltc data of lida (14) ~~nd b~king int.o ac.eount thc [wcmgc vallle~, i ohtained thc followiug re:mlth as reg~1l'd the ellergy of ts1!w\lllis of diffe1'(mt illaxiuml lwight 011 thp (i(ja'/;t: thllll:tmi-lh'ight t~llllajlli-ljiugllitlld(' t~ili111mi-nll!:'rgy (h r. ) liw-trl'~ ('rgo 30 -i-,!i-l1 ,ii2){ ·10" fio ",uu u,252'io" (hl (i.23 1.37-1'102' 120 (ui!) 2,-1-55 ·i02s 1::;0 7,00 3,usi ·io·s 180 -" 1,_5,39" '10" 210 7,-1-;, 7,-i-13·10!' !og 1~'i~ = 21,-i + 0,0 m [lj wlwre "in ih the s()-(\[~lied tslllmllli-uu~gnihlde, h is golwr ally amwptml tll1l,t. a~ a (hjnseljlh'.nee of cel"t. aiu vo!e~~nie l'i·llpt.ions largc caldcn\s lw\y foruh-ld d1l(~ t.o thc ellltltyillg tli'oecs~ of t1w 1ll;1gllla "cl mmhcl" illllicl' thc vokano (il). sueh }lhenolllcnon 01)('.u!"r('d in thc m\sc or liiolint. .?iiaz;\llia (oregou), too. owing t.o sllch il proccss migltty cncrgics ;wc rc1e;\singin dilicrcnt fol"lll~. one of t.hehe encl'gies ih thc tot~~l collapse cncr,!,.']', ge, whieh n1n hc (lollltllited h~' t.he following w~~~': iu expr. [2j ..li is thc total mll.'!s collapscd, fj t.hc accejcmt.ion due to ,!,.'1·iw ity ,i,ml h thc toh~1 way of thc lllovelll(wt of t.he um~h "lf in t.he vm-ti('al dircolion. if thc volmmo t.lte collap:;c or which is ullde,r considemtion is situatcd on the open sea or oeean (e.g. kmbl,h m. , s,l,ut.o1"iu), it. lllll.y o(,eui" llmt thc (nlden~ will lili up with watm' ;md li.'! a (mnscqllcncc of thc eolla}lsc-pltcnolllcnon and suddcn clmngch in thc topogmvhy of thc sb~~ ocea.n hottolll, ,!,.'1·e~~t w~~ves, t1mt ih tsllnalllih origillalcd from t:.lw lll"oxilllity of the vo!nloili(\ i.~h\nd. ael)ol"ding t.o scltuiling ;1nd yink (,a) fili' fo/al ('ollarme ellerfi./i i.~1tserl for curfhquf/kcb, t,~j/,/wmill alhi 200 iitlll;nvant p. for other geophysical phenomena. on t.his basis wc can introduce thc not.ion of free tectonic energlj: [3] where 1118 is the ellergy of t.sunami. let. uh calculatc the y>blue of free t.ectonie enel'gy in case of sa.nt.ol'ill :\linmm etllption, using' di1tel'ont va.lucs for thc depth of lllagmaclmmbul' and coi' the height of the tsmmmi. 've used the follo\\'illg suppositiolls: a) total energy of wa.t.er, filling the ]ntsin, llordel'e!! by the wa lls oc t.he caldera: 0,2,10 25 ergs (h), is ilulicafed with ew; h) the total way or colla}lse in thc vcrtiwl .~(\llsc is h = " + 1 \cm, [.t] where 11 is the deljth of the l'oof of m agma -elmmh(\l' lindel' t.he sea level and h is the depth of the lillal magma-iey(1\ illside tlw llmgnmchamhel' (a t t.he moment of the beginnin g or the colla}li'ie), me>bhill'{'d holll t.he se>b level (the chosen value of 1 km is ,i, rather arbitra,r~' one, oc coul'sc); c) a.ccclcration of gra.vity 9 roughly is 10:1 cm see-!; d) f,he m ass of the sll!jvolca.nic eolumn is j[, its demity ih 2,8 9 clll,si e) t.he ma;!s of t.he colbpsed m>bterial, inclllding the }lreexisted volcallic mollnt is ~1fl, its den.~ity is 2,g fj em-3 , its total volume is 72 km3 and thus jf1 = j}[ -11,8,1017 g; [5j and lasuy the hu,s!l of t.he preexist.ed vole>bllic moutlt. 1m!! 1m a.rea oc s = jo km2 , ]ll these ni.ses we obtain the following vl~iilcf;; talolfl tt h /i n, i e< = (m 1yll) + e,. km klll [on 'i io'· (',l'!;h ~ l 2 ::l,olio fi.32 2 , 4,020 12,2li :j , li,080 21>,12 , 5 8,070 45,0:; :; " 1o,!j30 ti:),78 lo il 20,700 227,00 15 lo 31,200 499,40 20 i 21 i 40,200 844,40 colll11ininl! tlwfw (!: ~ \.a, :'lll! the (!;1.t: ~ , ])l'esmllnl iii '1':.111" i, hu'tiil'i'1ll00'e t l\king into :~ceoun1., c'xll1'. [:~j, wc rccpiyl'd ih(' fulluwìli1! v i.lnes fol' !.lic frc'c tcdonie cnol'j.,':\': 'l'al,l(' iii " bi! ;jo (jo \)(1 izo l 50 iho 2111 fn·.e'. t('('t()nì(~ f'iwl'gy in l ()2~ 1'1'1\'~ , 0,lij7 li. (j\ji, .j-.n·w ;uw5 z.:l:lll 0,llz5 < l> 2 12,107 i 1,(,;j;'i 10,,'1,% !i,soii k,z7ll (ulo5 .j-,k.j-7 " 27.%7 27 ,.j-~j5 21).7.j-i) 25,1)1)5 z.j-.l ;:jj) 22.7z,') zo,707 , h,r!j7 h.,j-:!i, .j-;u,i(j .j-z,5!1,') .ji • i)(j~j :llj,(;[i[i :n.(j;j7 5 h.';,(,27 mi. i i;fi o.j-,.j-o(j 0;1.;iz5 (j i , ,!jll (i(j.:jr5 5s.:i(j7 'l> 227,h7 227,275 221),52(, 22,;.,j-.j-,; zz:i.~llll 222.505 zzo,.j-s7 i,; .j-!hi,2.j-7 .j-!lr,7i,; .j-bs.ozo .j-~j().n.j-5 .j-n,j.,j-l!j .j-b.j-.oo;; .j-bl,mn 20 sh,2.j-7 k.j-3,iii; k.j-;i.oz(, s.j-i.b.j-,; k.j-o.,j-i!j k:l~j.()()5 s;jh.~js7 vel''y ,~illlihl' valnes m a y !w ohl:~ illed if 8 =80 klll\ ijl.~1ea(1 of io' 'l'u eal(\nh1.~l tlw~e v:~lncs howcvcr is nut iwcc.'ls a r'y, siww t,i te (ima 1ll'l\sonlc(1 in 'l'a,ble iii shuw dca,rly that, bc).:'innìn).:' frolll a ecrtain v alnc uf h, hw fre(\ tc(·,tonie cm>.i'gies :>l'e :l,llllo,~t t,lte s:l,lilc tlmn 111{'. re, th:l,t is onl~, i> small ilei' (\1'-111, uf ec is nsni fol' 1.,shiiu,lili. rl'he,~e ellel'gies, e:qln>.ssoll in 'l' ahlc lll, a rc c'norlllum; unc.'l, li'ol' colllij!wisoil we f! a n l'dci' lo thc fa,et tlmt the tot a l scislilio cnor).:'y, l'cleaohcd on thc wholc surfa,ec of tlw e a rl-h !wl\hw.1i .1i>llna,i'y 1, lk9h :l,thl thc cnd of ma y, 1 !w~ w:i,s sollwwh a t ,~mallcr 1 h a n ho ,l o~~ cr).:'s ('9). whi>t lmll h:i,]i])('iilh! wi111 thc frco tcntonie o1wl'g:\'1 lt ih :1, vl'ohh·.l\l of ca pit a i illlliurtannc, 'l'hc }ll'cscnt :lnthol' i.~ l',ollyilwm! th a t, a l: le:l..'\t a 1m!'! of lhifl free t,cetonie enel'gy, h:l,ll tn 1'll111 a m>,d :\.'<; c:~rthqu:~ke.~, lllure cxaetl~·: in tlll' foj'lil of il loil!! iil'1'il'8 o/ m/ha !jtrong (,1ri1hfj/wkc8. a sìlilih ~i' c: l.'le 1m(! hecn exllcrience(1 durillg all(! i ~ftm' tlw eolla,]lse of fel'n a ll(lìlia, caldcr:" of t.he (hlall:1go~ yoh:a,nie gl'ohl1 ili 10h8 ("~) w1lcrc m any sltoekii h:l,ll bee!l oec\li'\'{>.ll a ll(! flollw of l11cm readwd the lll:.gnitulle ;";,0 (h.i(\h 1,1'1'). 1 t i~ v(\)'y (!ifli(mll tu illmgi!le a liligh ly eolla]l~e-]lh('iiui\wlion uf ).:'iga ntie llla~~ witltuut :ony slul,ke of t,lw gl'uljihl iloweyc1' it i,~ }lrolmblc th a t unly a relmiyely ~lili .ll ]lei' ecnl uf frcc ter:1uni(~ ellel'g~' \\·i\.~ ljkt,r! ful' ).:'(\jl(·ti>1ìllg ,~twh i. scc]lwih',c of ,~ho(·.k~. [limy ,~ljgge~t 202 iledbr\' ari p. that iho rclildinilly l'mi 01 il'ee leclonic elleryy wasused lo cause certain ellu,n!le,~ in the lerel 01 ihe bollom 01 ille aeyean-sea around the rolniiw san/orino sorry, at proscllt that is about 3400 years after the event, \\'0. are ilot abie to mea.snre the dimensions of the crusta.l tlefol'llla1iom, lmppened a~ a diroct consequence of the collapsephellomelloll. however if the clmnges in thc sea-hottom topogral)hy wero high enough, bot,h in the horizonta l a ntl vert,iea l tlireetion, t,hen thc mwrgy, lleee~sary to lll'mince uwm might lul.ve l)eell in th{t oj'(lej of 1()2;j_i()~6 mg's 01' l)erlmps 10 27 ergs, tha.t is in the ortler oi 1he free tectollic mwl'g'y. a~ prof. milla ka mi of java.n imd expre~sctl on the ocea~ion 01 the eongress, ille t.~ul/amis fife duc lo sudrlell topo,fjl'lipld(,lil c1/1lnycs 011 the sea-bot/om, 'l'hese tol)ogmphh: al changes (crustal deformations) are the dil'cr:t 4lo1uwqtlenee8 of strong carthqllake~, a~ it, occllltetl for il1~tance ili ('a~(\ of tlw great, 'l'okro-~hoek on tlw 1 ~t, sc1jtmnber, 1023, but :tccol'tling' to the ('onflitlel'atiolls 01 the lll'eflent, a ([thol', di~cuss('d nbove, ,~om(' {dnd 01 ('mslal rlclonnaliolls may bc aio ('o/(scrluci/cc 01 raldr'}"ii-liur!.iil!l collap.~o-proe(,lil1, /00. it i~ now quite denr awl evitlent that s'n('h tyl'r; 01 ('1'1tst,,1 dolonnatiol/s may al,~o cau,w! ts'lt/uonill, 'l'herefore jitla's fol'mula (l~), t,hat is ex1ji' , [ij, ean 1)(: used in ca,se 01 /sullamill 01 'vol('((nic ('ollapso-odyin and hot only in ('((se 01 t.nw(l,mi,~ 01 earlhquflkc-ol'iyin. aceordingjy to use thc l'espect,ivo formula in the eaicnlu.tion of the energy or sfllltorini jtino1t1l t~nllami i~ quite rig'htfll1 q,ltod (!j'lit dclnol/.l'trawlum, refe rences (i) geor(;ai.as, h. c., oalalog!ll~ of .-[dive l'o/cruw('8 of l,h,' wor/d fncl!hling .','olfa/arll fie/tin, l'art xit, gn'll<'e', roma (1!hi2). l') il,,,,,, il. c., a 'l'{,,,, ",,,,/, /",. w,'",y . • ',t,,,",y il,,,,,,,", d,,,. o (idon). (l) ileeze:\ h, c. nixkov!c(( d., .','un/orini 1'cphra, "col.,loll papen,,,. xvii (iun,i), (4) marinatos 81'., ai,iùl'lti/y, xiii. pp, 425_43u (1\13\1), (~) i\ialunatos 81'" o-n the legcnrl of allanl-i:;, 'l'l'all~lated frum, • critica chrunica . i v (i (0), (g) }'l<\.!h~atos 8('" 80me wo/'l/:; uljoul 0/6 lcgrnd of al-llllll-is, atlwn8 (id09). (1) j'.iahil'<atos ~l'., h':cclwa/.ions at '1'hl'l'li, 19g8 season, alhens (19gb). (a) galanol>oij1,o,~ a, g., 'p/w t~a8/.c1'/~ .lfetlilurunl'l/"n 'l'ri/()gy iii ihe bronzr age, [,/l'd,nr!', oli t8cv'l' (1%9). l{ece~t l"'ivestlgatio:'>~ of a bro"':t.e .hm eruption 203 (' ) va:'> he~niele:'>, r. w., fol1r l"olcrwic ()lil1mrsls 1.11.(11 injllm!!leed 11ulll11ll jjistory. "l(~l"~ll!"{'. oll the lllteml).t.iollui seic'uiific collgn~~h oll t.he \·oletlllo of tlwi"a n. iscv'l' (1!hi!l). (i") \r,\luxos g., un i./ic !:ii:;c 01 lhc sca !vtlt'e ('l'~ullwni) /)lirilil/ the l're_ hùloriv eri/più)!! uj 8all/orilli . • lecllii"e ou [i;cyt> (lll6u). (11) xi'~u1ia"" va" paila:-;o:.: .:\1., 1'ii"u {)altp,·tr())lhic eruptiolls iii ilidolle~ia, culii-jhlrablelrilh ihr flinian oullllll"8l. ()j i1w volcrl/lo ()j thera (salilorini) in jlinow. 'l'ime. " lettllre oli iscvt> (1!l0!l). ('") lié1>el\va1u l'., eile/"ycuca./ {)a/eu/aliolls crhlcci"ninylhe "lfilwnn j<,'ruptioll oj !:iclllt()rini (lillal ver~ioll) . .: locllii"o on rscvt» (iuon). ('") vitalia"'io u. il. vl'r,\.lia"'o cii. j., l'lili lan erl/puol,~, erulhfjlulkes (!/id ,)'(!i,lorin a r(~yiew . .:\iallu8ci"ipt for l.he < acta of iscvt j) (1 !l70). (") ilda k., .jlaynitlldc, j<,'ncl"[}!j and generalioll .lire/lnlti:,;//; uj 'l'sunamis ... " 'l'lw t(,ll~h pal."ific sciellcc cull~l"ess . l'apei" (1!l61). (") llahkhllllm d. l'ri"ss f .. the krakaloa air-seawnve:,; ... "tlw geophy~ìi."ul journal» 13. 1·3 (1!l67). (1ft) [)fiiù g., eruplirc 1~lirrl/ies . • allllaii (leli"osservu,[.orio veomyi;l.110 > se~[.a :-;orie, vii (1u05). (l') va" n 1';11 11 1"1.1"" r. "t., 1ie/. (!aldern l'rublcem. de .:\lijnill,!l,"l'nienr (1!l2!l). (l") scrwili:'>g r. d. yl:'>k n. \\'., es/iilw{cò' 010 i1w jtrhiou., 'l'ypes oj enaqy neleascrl by /.itc erl/pllon uj 'l'h era·. grecce, (it a/wul 1100 r. c. .. jppendi.c tu r. iv. va·n 1jelll lnell~n's paper -. < lectme un isc\''l' . (hj6n). ('") liéjlelty.lrl l'., lllvl'~tiljation.~ rnjarrunl/ iho earlh n seismicuy. l'art ili . • g(~rjall<]r nllitrilgl' 7.n!" gllophy,;ik . , 74, il (l!l(ji5). ("") s li l'i'iiso:'>ia 'il j 'i/s'i'i'l'c".i·ion, clllltcr ior shoi"t-li \'0(1 pllolluijlolla: < allllnal pi"o,l(rc~~ hepoi"t iu08 >. publishe(l: i i"ebl"nar.y (lf)(jfj). annals 47, 6, 2004def 1771 annals of geophysics, vol. 47, n. 6, december 2004 quakes such as the 1356 basle event could be more frequent than previously thought. for the prediction of strong ground motion during potential future earthquakes, it is therefore of considerable interest to gain knowledge about wave propagation properties in the unconsolidated sediment layers. in this study we present results from an ambient vibration measurement campaign for a region north-east to the city of basle extending the work of kind (2002) and fäh et al. (1997). kind presented h/v results from microtremor recordings together with a geological model for the urban area of basle. the geology of the measurement area outside of the city border of basle must be seen as not verified due to lack of available data but can be extrapolated from the geological profiles shown in kind’s paper. basle is situated within the rhine rift between key words h / v ratio – ambient vibration – array analysis – fundamental frequency – rhine rift valley 1. introduction the southern rhine rift valley is among the most active seismic regions within central europe. re-evaluation of the seismic hazard based on paleoseismological studies (mayerrosa and cadiot, 1979; meghraoui et al., 2001; becker et al., 2002) showed that larger earthambient vibration measurements in the southern rhine graben close to basle andreas köhler (1), matthias ohrnberger (1), frank scherbaum (1), stefan stange (2) and fortunat kind (3)(*) (1) institut für geowissenschaften, universität potsdam, germany (2) erdbebendienst des landesamts für geologie, rohstoffe und bergbau baden-württemberg (lgrb), freiburg im breisgau, germany (3) swiss seismological service, eidgenössische technische hochschule zürich, hönggerberg, zürich, switzerland abstract this study presents results of ambient noise measurements from temporary single station and small-scale array deployments in the northeast of basle. h/v spectral ratios were determined along various profiles crossing the eastern masterfault of the rhine rift valley and the adjacent sedimentary rift fills. the fundamental h/v peak frequencies are decreasing along the profile towards the eastern direction being consistent with the dip of the tertiary sediments within the rift. using existing empirical relationships between h/v frequency peaks and the depth of the dominant seismic contrast, derived on basis of the λ /4-resonance hypothesis and a power law depth dependence of the s-wave velocity, we obtain thicknesses of the rift fill from about 155 m in the west to 280 m in the east. this is in agreement with previous studies. the array analysis of the ambient noise wavefield yielded a stable dispersion relation consistent with rayleigh wave propagation velocities. we conclude that a significant amount of surface waves is contained in the observed wavefield. the computed ellipticity for fundamental mode rayleigh waves for the velocity depth models used for the estimation of the sediment thicknesses is in agreement with the observed h/v spectra over a large frequency band. mailing address: dr. matthias ohrnberger, institut für geowissenschaften, universität potsdam, karl-liebknechtstr. 24, 14476 golm b. potsdam, germany; e-mail: akoehler@rz.unipotsdam.de (*) now at: converium ltd., general guisan quai 26, ch-8022 zürich, switzerland. 1772 andreas köhler, matthias ohrnberger, frank scherbaum, stefan stange and fortunat kind the allschwil fault zone and the eastern masterfault. near the masterfault, the st. jakob tüllingen synclinal structure consisting of 100400 m thick tertiary sediments forms the boundary of the rift fill. the analysis of ambient seismic vibration recordings for site effect estimation is considered a low-cost alternative to expensive investigation methods such as drilling or active seismic experiments (aki, 1957; milana et al., 1996; bard, 1998; ohmachi and umezono, 1998). it has become very popular to estimate the natural frequencies of soil columns from horizontal to vertical (h/v) spectral ratios at single stations (kudo, 1995; bard, 1998). furthermore, the array analysis of surface wave dispersion characteristics from ambient vibration data allows to derive shallow shear-wave velocity profiles (aki, 1957; lacoss et al., 1969; asten and henstridge, 1984; horike, 1985; tokimatsu, 1997). nakamura and ueno (1986) and nakamura (1989, 2000) suggested that taking the h/v ratio reduces the effect of the source, and thus the spectral ratio represents the stationary amplification function of the soft rock due to «shear wave resonance» (λ /4-hypothesis). the condition for this interpretation is that the wavefield contains a significant portion of body waves and that there is a sufficiently high impedance contrast between sediment layer and underlying bedrock. under the assumption that these conditions are met, empirical correlations between sediment layer thicknesses h and peak frequencies f 0 can be determined, if independent information about sediment bedrock interface depths is available. on the basis of phase velocity analysis results from microtremor array measurements (asten and henstridge, 1984; tokimatsu and miyadera, 1992) many authors argued, in contrast to nakamura’s interpretation of the h/v ratio, that the ambient vibration wavefield is often dominated by surface waves (lachet and bard, 1994; fäh et al., 2001; cornou and bard, 2003; scherbaum et al., 2003). assuming that a significant portion of the wavefield is due to the propagation of fundamental mode rayleigh waves, the fundamental peak in the h/v ratio can be explained as an effect of the frequency dependence of the ellipticity of rayleigh waves as a response to the site structure. in general, the peak frequencies of rayleigh wave ellipticities (fell) differ from those caused by sh-resonance effects (fsh). a quantitative characterization of this problem was given recently by malischewsky and scherbaum (2004) for a simple single layer over halfspace model. the ratio fell / fsh varies from 0.5 to 1.5 for swave velocity contrasts below 3.5. for s-wave velocity contrasts above 3.5, however, the peak frequencies fell and fsh match closely. 2. measurement and data in april 2002, the institute of geosciences of the university of potsdam, the landesamt für geologie, rohstoffe und bergbau (lgrb) and the eth in zürich carried out a field experiment in the area around weil and lörrach using a homogeneous set of lennartz le3d-5s sensors connected to marslite stations with gps timing and mars88 station with dcf77 time receivers. two dense array configurations (a, b) were realized, consisting of 22 (a) and 21 (b) sensors with an aperture of about 900 m and 600 m, respectively. the site of these measurements was located at the southern border of the city of weil, close to the location of a deep drillsite in otterbach (fig. 1). the total recording time (continuous mode) was 4 to 5 h for each array setting. the sampling rate fs was set to 125 hz. in addition, single station measurements were performed along profiles as shown in fig. 1. in the western part of the profile, measurements started at the rhine river. towards eastern direction, crossing the city of weil in the north of the array sites, the profile traversed a topographic height, the «tüllinger berg». from there, the profiles extend through the city center of lörrach in northern and eastern directions, crossing the eastern masterfault of the rhine rift valley. the average station distance in between the 108 individual measurement points was around 100 m along the profile. the recording time (continuous mode) for each site was about 30 min each (at fs = 62.5 hz). 1773 ambient vibration measurements in the southern rhine graben close to basle all measurements were carried out during working hours at day time. 3. analysis of h/ v spectral ratios in accordance to the recommendations described in bard’s review paper (1998), summarizing the experiences of different authors with respect to h/v processing, we selected transientfree time windows for further analysis. time window selection was performed visually and the resulting time windows had lengths of 3 to 10 min. for the single station h/v analysis along the profile, two independent data windows were used for the processing, while 3 to 7 data windows were selected from the array measurement data. for the computation of h/v spectral ratios, we removed the overall mean from the individual records in a preprocessing step. we calculated the single component amplitude spectra for overlapping segments using a standard fft algorithm. the selected segment length of 2048 fft points at a sample rate of 62.5 hz (125 hz for array stations) corresponds to a frequency resolution of 0.0305 hz (0.061 hz). the spectra were smoothed by a logarithmic shaped taper window as introduced by konno and ohmachi (1998) (bandwidth parameter b = 30) and the h/v spectral ratio was computed for each segment. for the horizontal component (h) in the h/v spectral ratio computation, we combined the individual horizontal component spectra of n and e by using the geometric mean ( h ne= : tokimatsu, 1997). finally, we calculated the mean and variances of the h/v ratios from all segments under the assumption of a log-normal distributed amplitude spectra. depending on the window length 10 to 56 segments were used for averaging. for further quantitative interpretation of the fundamental h/v peak frequencies, we introfig. 1. location of the single station measurements (filled triangles) and array settings (open triangles) in the study area in the rhine rift valley near basle. the dashed lines indicate the city borders and the cross shows the location of the deep drillsite in otterbach. 1774 andreas köhler, matthias ohrnberger, frank scherbaum, stefan stange and fortunat kind clear h/v spectral ratio peaks meeting our criteria defined above for 82 sites. from these spectra we determined fundamental peak frequencies between 0.5 to 0.9 hz for sites lying in the western profile part from the rhine river to the city of weil including the two arrays located south of weil. at some of those sites, especially at the stations from the array deployments, secondary peaks between 7 to 10 hz were found and documented. this observation is supported by the results of kind (2002) for his sites more southernly. within the urban center of weil only one spectral ratio fulfilled our criteria. several peaks could be identified for the profile part crossing the «tüllinger berg» between weil and lörrach showing intermediate frequencies between 3.5 to 8 hz. again, no peaks were identified within the urban center of lörrach, but well resolved spectral peaks ranging from 1 to 10 hz appeared in the adjacent areas to the east and north of lörrach. figure 3 summarizes the spatial distribution of all determined fundamental peak frequencies. considering the geological model fig. 2a-f. averaged spectral h/v ratios for representative stations; fundamental frequency peak lower than 1 hz (a), higher than 1 hz (b, c, d), lower than 1 hz and secondary peak (e, f). the dashed lines indicate the uncertainty estimate as one standard deviation. a b c d fe duced consistent criteria for the visual determination of h/v peaks. we considered only peaks within a frequency band from 0.3 to10 hz. the lower frequency limit was chosen to stay away from inevitable long-period disturbances of the data caused by instrumental effects (stabilizing time of the feedback sensor electronics) in rapid mobile experiment setups or weather conditions (cara et al., 2003). the upper limit was specified due to the temporal instability and physical limit for a sensible interpretation of h/v ratios within the context of site effect characterization for peak frequencies above 10-20 hz. to select the peak frequencies, a log/log diagram was chosen. shown in fig. 2a-f are six typical h/v ratio examples averaged from the h/v spectra of all processed recordings being representative for particular profile segments. only the first (lowest frequency) peak was analysed. it was checked if the same peak was identified consistently in the h/v spectra of all available recordings for the site before averaging. analysing all sites along the profile (108) and the individual array stations (43) we found 1775 ambient vibration measurements in the southern rhine graben close to basle fig. 4. spatial evolution of spectral h/v ratios along profile a-a′ (profile stations and stations from array a and b) and b-b′. the averaged h/v spectral ratios were plotted for each individual site where the abscissa of the diagrams corresponds to the relative distances between the stations projected onto the profile axis. the geological profile was modified after kind (2002). the length and position of the boxes in the profile show the approximate h/v measurement locations. fig. 3. fundamental peak h/v frequencies found in the study area, location of profile a-a′ and b-b′ (see fig. 4) and the position of the eastern masterfault of the rhine rift valley. the cross indicates the location of the deep drillsite in otterbach. 1776 andreas köhler, matthias ohrnberger, frank scherbaum, stefan stange and fortunat kind (kind, 2002) it turned out that the distribution of peak frequencies apparently correlates well with the variation of the thickness of the sediment fill within the western part of the study area. in order to allow the identification of the spatial relation between the fundamental peak frequencies, we created fig. 4, showing averaged h/v spectral ratios for two profile segments a-a′ and b-b′ as indicated in fig. 3. the h/v spectral ratios were plotted together with their relative position with respect to the geological profile after kind (2002). 4. fk analysis of array data the use of array analysis methods allows us to decompose the ambient noise wavefield and to derive frequency dependent phase velocities of individual wavefield components. in particular, it is possible to derive the dispersion properties of the surface wave contribution at a particular site. this allows us to invert for shallow s-wave velocity structures (horike, 1985; herrmann, 1987; malagnini et al., 1995; tokimatsu, 1997; scherbaum et al., 2003; wathelet et al., 2004). for the analysis we selected the vertical component data of the same time windows as have been used previously for the determination of h/v ratios at the array station locations. as stations with both gps timing and dcf77 time receivers had been used in the experiment it was necessary to apply a time shift correction for the dcf77 receivers before computing the fk spectra. the time shift delays were determined by crosscorrelation techniques during a laboratory experiment (kind et al., 2002). we applied two different fk methods for the estimation of the wavefield properties. first we computed conventional fk spectra in a sliding window analysis for narrow frequency bands. the dispersion curve information was then derived by averaging the slowness values obtained from the peak location for each single analysis window as well as by visualizing the histogram statistics as shown in fig. 5a,b. beside the mean fig. 5a,b. source azimuth and dispersion curve of array a (a) and b (b). the results of the fk methods are shown in a histogram-contour plot of slowness maxima for each frequency taken from the fk spectra of individual analysis windows. the median (black diamond with the 75% and 25% quantile as given by the error bars) and mean (black circle) are plotted. the white triangles are the slowness estimates calculated by capon’s method. the black curves represent the dispersion curve of the site obtained by forward calculation. the solid one was determined from the geological model after kind (2002) and the dashed one was calculated based on the power law velocity depth distribution. a b 1777 ambient vibration measurements in the southern rhine graben close to basle value (black circle) we computed the median (black diamond) and the 25% and 75% quantiles (represented by black error bars) of the overall distribution. additionally, we used capon’s high resolution method (1969) for single target frequencies and, following the underlying assumption of wavefield stationarity, used the block-averaging technique proposed in capon’s original work for stabilization. as result we obtained a single slowness estimate for each target frequency (white triangles in fig. 5a,b). in fig. 5a,b we observe a clear dispersion relation in the frequency band between 0.8 and 2 hz allowing the conclusion that a considerable part of the wavefield is composed of rayleigh waves. above 2 hz, significant alias effects are recognized indicating that the spatial sampling of the array settings were insufficient for the corresponding wavelength-ranges. in the upper panel of fig. 5a,b, we show the directional distribution of the fk results. for the temporary array deployments, we observed a rather narrow source azimuth region at around 240° corresponding to the direction of the railway station in basle. 5. discussion and conclusions in order to allow a quantitative interpretation of the relation between peak frequency and sediment thickness cover, we derived a depth estimate from the measured h/v peak frequencies. assuming that the origin of the h/v spectral ratio fundamental peak is related to s-wave resonances in a single sediment layer over halfspace (λ /4-hypothesis for shear waves: nakamura, 1989), the layer thickness h can be related to the h/v fundamental peak frequency f as f h nv 4 s = where n are uneven integers n = 1, 3, 5, and vs is the shear wave velocity of the sediment layer. in sedimentary basins, the depth-dependence of the s-wave velocity can often be described by a power law relation of the form vs(z ) = vs0 (1 + z ) x (budny, 1984). here, z is the depth, vs0 the s-wave velocity at the surface and x the exponent of the power-law relation. then, table i. different model parameters for the empirical relation between peak frequency and depth of the dominant seismic contrast. study a b ibs van seht and 96 −1.388 wohlenberg (1999) parolai et al. (2002) 108 −1.551 hinzen et al. (2003) 137 −1.190 by integrating the velocity function with fixed parameters vs0 and x over the depth range from surface to the depth of the seismic contrast h, it is possible to derive a modified relation between h/v peak frequency f and h of the form h af /h v b = . (5.1) this relation can be used to obtain parameters a and b (related to vs0 and x) empirically by fitting data sets of h/v frequencies f at sites where independent information about the sediment thickness h is available (e.g., borehole data). lacking accurate depth information for our study region, we used relation (5.1) to determine the depth of the main seismic contrast from the observed peak frequencies using empirical estimates of a and b derived for the lower rhine embayment (lre) in nw-germany by ibs van seht and wohlenberg (1999), parolai et al. (2002) and recently by hinzen et al. (2004). the parameters a and b found by these authors are given in table i. finally, we compared the depth estimates with the geological model after kind (2002; details are given in table ii). along the profile a-a′, as shown in fig. 6, we found that the values specified by hinzen et al. (2004) are in very good agreement with the geological model. both the depth and the dip of this seismic contrast is in good correspondance to the lithological boundary described in kind (2002) between the oligocene meletta layers and the lower tertiary unit forming the st. jakob tüllingen synclinal structure in the basle area. from west to east the estimated depths of the interface increase from 155 m to 280 m. 1778 andreas köhler, matthias ohrnberger, frank scherbaum, stefan stange and fortunat kind seem to be too shallow considering the geological background information. a more detailed comparison between the individual studies is discussed by hinzen et al. (2004). we further observed clear secondary peaks at higher frequencies around 7 to 10 hz along the profile a-a′, which convert to interface depths of 9 m to 14 m. these shallow layers can be attributed to the quaternary low velocity layers overlaying the tertiary sediments, as suggested by kind (2002) and recently validated by results of s-wave seismic experiments (u. polom, pers. comm.). for the eastern part of the h/v profile (see b-b′ in fig. 4b) the relation between the geological structure and the depth estimates from h/v frequency peaks is apparently more complicated. a first order interpretation allows us to divide the results in three major regions. to the west of and above the eastern masterfault, as supported by the geological model, a shallow inclined structure dipping towards western direction is hypothesized. however, to the east of the fault, due to the existence of parallel «subfaults» and probably complicated geologic patterns, no systematic spatial correlation between the h/v ratios is observed along the profile in table ii. depth and velocity models for location a (p001) and a′ (p019). the models for p019 and the array site are identical. model 1 model 2 layer thickness vs model avg. vs layer thickness vs power avg. vs after kind (2002) after kind (2002) derived from fh/v law model p001 10 m 450 m/s 590 m/s 155 m power law: x = 0.225 534 m/s vs0 = 215.5 m/s (hinzen et al., 2003) 165 m 600 m/s halfspace 2000 m/s 2000 m/s halfspace 2000 m/s 2000 m/s p019 10 m 450 m/s 607 m/s 280 m power law: x = 0.225 595 m/s vs0 = 215.5 m/s (hinzen et al., 2003) 70 m 650 m/s 200 m 600 m/s halfspace 2000 m/s 2000 m/s halfspace 2000 m/s 2000 m/s fig. 6. calculated interface depths (black triangles) h from fundamental peak frequencies f0 (10% uncertainty: black error bars) after hinzen et al. (2003) based on the «shear wave resonance» and a power law velocity depth distribution leading to an empirical relation of the form: h af /h v b = . the solid line represents the depth of the dominant seismic contrast after kind (2002) for profile a-a′ (p001-p019). using the alternative power-law depth models given by ibs van seht and wohlenberg (1999) and parolai et al. (2002), the depth estimates (110-220 m and 130-270 m, respectively) 1779 ambient vibration measurements in the southern rhine graben close to basle eastern direction. finally, to the east of the fault zone two dominant peaks at high frequencies around 8 hz suggest the existence of a shallow dominant seismic contrast, which was also recently validated by the s-wave seismic experiments (u. polom, pers. comm.). considering the observed dominant energy contribution of rayleigh waves in the wavefield derived from the array measurements we tested additionally whether the observed peak frequencies are in accordance with the rayleigh wave ellipticity assumption and the velocity structure. therefore we computed fundamental mode rayleigh wave dispersion curves and ellipticities (herrmann, 1987) for the two outermost stations (p001, p019) from the profile section a-a′ using two types of velocity models (table ii). the first model (model 1) is based on the velocity model given by kind (2002), with interface depths approximated from the geological model. the alternative model (model 2) is an approximation of the power-law distribution by a stack of thin layers over bedrock, where the parameters a and b are selected according to hinzen et al. (2004). in both models the bedrock depth was set to the previously determined value from the h/v fundamental peak frequencies at the station locations. for the bedrock velocity we used the value given by kind (2002). the models parameters are summarized in table ii. in fig. 7a-d we superposed the observed h/v spectral ratios and the theoretical rayleigh wave ellipticities (absolute value). we find that all computed peak frequencies of the ellipticity curves for both models and stations overestimate the observed fundamental peak frequencies. the relative peak frequency mismatch ranges from 10% to 30%, depending on velocity model assumption and station. however, for station p019 (fig. 7b) we observe remarkably similar broadband shapes for the ellipticity curve and the h/v ratio for the power-law velocity depth model (model 2). considering the results of malischewsky and scherbaum (2004) who demonstrated the strong dependence of the ratio between fsh and fell on the velocity contrast in a single layer over fig. 7a-d. measured h/v ratio (solid line) and theoretical ellipticities for station p001 (a) and p019 (b). the ellipticities were determined for the power-law model (dashed line) of hinzen et al. (2003) and a three-layer over halfspace model (dotted line) after kind (2002). model parameters are found in table ii. additionally, the individual component spectra (e, n, z) for both stations are plotted (c and d). the vertical arrows point to the h/v peak frequencies in panels (a) and (b) and relate those to the individual component spectra in panels (c) and (d). a b c d 1780 andreas köhler, matthias ohrnberger, frank scherbaum, stefan stange and fortunat kind halfspace situation, the discrepancies of peak frequencies could be explained by shear wave velocity contrasts in the range of 2.5 to 3.5. using the model parameters from table ii, we derive velocity contrasts ranging from 2.6 to 3.35. interestingly, although the velocity models used are more complicated in our case, this value range is consistent with the theoretically derived statement made in malischewsky and scherbaum (2004). due to the lack of independent information on the interface depth, the question regarding the physical cause of the observed fundamental frequencies f0 remains ambiguous. in favour of the fsh hypothesis is the fact, that for station p019, both the power law model and the constant model after kind (2002) reproduce the observed fundamental peak frequency and independently show the same sediment layer depth. in favour of the rayleigh wave hypothesis are the results of the dispersion analysis from the array measurements and the spectral shape of the individual component spectra as shown in fig. 7c,d. there we recognize a trough in the vertical component spectrum around the frequency of the observed fundamental h/v peak. the horizontal components do not show any amplification at these frequencies as would be suggested from the alternative interpretation as a resonance effect ( fsh). most likely we observe some mixed contribution from both resonance and ellipticity effects. although, at present, we cannot validate this hypothesis, we want to point out the consequences for a possible misinterpretation of fundamental peak frequencies from ambient noise h/v measurements. without knowing the physical cause of the observed fundamental peak frequency f0, we would interpret this frequency as the dominant frequency of site amplification during strong ground motion. however, for the case of observing fell at a site with weak impedance contrast, the energetically most important frequency of site amplification during strong ground motion – fsh – would differ significantly from our estimate. beside the possible mismatch of fundamental resonance frequencies for certain impedance contrasts and its implications for hazard related site effect studies, the question to what degree f0 measured from h/v spectral ratios is controlled by resonance of body waves fsh or by rayleigh wave ellipticities fell contains another important consequence. given that f0 is related to fsh, we could interpret the amplitude of the resonance peak as an estimate of the amplification factor for moderate to strong motion (as long as nonlinear effects are not concerned). however, if fell is the physical cause of f0, the peak amplitude would carry no direct relation to the site amplification factors, as the amplitude solely depends on the relative p-sv energy partioning on horizontal and vertical components. in order to obtain further insight into this problem, we need quantitative estimates of the energy partitioning between body and surface waves in the ambient vibration wavefield. this makes it necessary to use densely deployed three component array measurements to reliably distinguish the individual wavefield components one from another. acknowledgements we thank all participants in the field work, especially w. brüstle, c. cornou, a. kristen, j. woessner, j. kirsch and the administrative staff of the lgrb. our special thank goes to d. vollmer for his continuous efforts in station maintenance, technical improvements and field participation. this work was realised within the scope of the sesame-project (eu-grant no. evg1-ct-2000-00026 sesame); m. ohrnberger was funded by sesame. references aki, k. 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(received january 9, 2004; accepted july 27, 2004) layout 6 moving least-squares corrections for smoothed particle hydrodynamics giuseppe bilotta1,2, giovanni russo1,*, alexis hérault2,3, ciro del negro2 1 università di catania, dipartimento di matematica e informatica, catania, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy 3 conservatoire des arts et métiers, département ingénierie mathématique, paris, france annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5344 abstract first-order moving least-squares are typically used in conjunction with smoothed particle hydrodynamics in the form of post-processing filters for density fields, to smooth out noise that develops in most applications of smoothed particle hydrodynamics. we show how an approach based on higher-order moving least-squares can be used to correct some of the main limitations in gradient and second-order derivative computation in classic smoothed particle hydrodynamics formulations. with a small increase in computational cost, we manage to achieve smooth density distributions without the need for post-processing and with higher accuracy in the computation of the viscous term of the navier–stokes equations, thereby reducing the formation of spurious shockwaves or other streaming effects in the evolution of fluid flow. numerical tests on a classic two-dimensional dam-break problem confirm the improvement of the new approach. 1. introduction smoothed particles hydrodynamics (sph) is a meshless lagrangian method where a fluid is discretized using virtual interpolation nodes (particles) that are not fixed in any predefined mesh and where their evolution is described by the equation of motion of the fluid itself. the advantages of sph include direct computation of all required physical quantities and their gradients, implicit tracking of surfaces (such as the free surface of the flow or internal solidification fronts in the case of thermal evolutions with phase transition), and the ability to cope with large deformations without any need for remeshing (which is typically needed, for example, with the finite elements method). sph was originally developed in the late 1970s by gingold and monaghan [1977] and lucy [1977], with applications relating to astrophysics. recently sph has gained a lot of attention in other fields of computational fluid dynamics, with important applications ranging from coastal engineering [hérault et al. 2009, dalrymple and hérault 2009] to geophysics and volcanology [hérault et al. 2010b, 2011]. these applications rely heavily on the ease with which sph can model both the mechanical and thermal evolution of free-surface fluids. these extensive applications have also highlighted some of the limitations that standard sph formulations encounter, in terms of accuracy or when fluids with high viscosity are modeled. the purpose of this paper is to present an alternative approach that while keeping close to the spirit of sph, improves on the general accuracy of the method, and provides a stronger foundation for better evaluation of the higher order derivatives that are necessary for the viscous term of classic fluid-modeling equations. 1.1. standard smoothed particles hydrodynamics interpolation for fields the mathematical foundation for sph lies in the properties of convolutions. for any scalar field u in a domain x �⊆ rd , by definition of the dirac delta distribution d we can write, with a typical abuse of notation: we can approximate d with a family of smoothing kernels w(·,h), parameterized by their smoothing length h, satisfying: where the limit is intended in the sense of the distributions. we then have: physically, the domain x represents the body of the fluid, which we want to discretize with a finite set of particles with given masses mi, densities ti and volumes vi = mi/ti. formally, let a density function t: x → r and a set of points article history received november 21, 2010; accepted july 15, 2011. subject classification: algorithms and implementation, smoothed particle hydrodynamics, moving least-squares, mesh-free. 622 special issue: v3-lava project .x y x y yu u d= d x ^ ^ ^h h h# , 1 lim , x xw h d h 0 r h d = $ " w = d ^ ^ h h # , .x y x y yu u w h dx ^ ^ ^h h h# at locations {xi}⊂ x be given. we approximate t in the weak sense with a discretization: where the summation is extended to all of the particles and the values mi are called the masses of the particles. the weak approximation is read to mean that for any field u defined in x, we can write: hence: where ti = t(xi) and vi = mi/ti. this gives us the standard sph interpolation for field values: the smoothing kernels are usually chosen with compact support, so that the summation is extended only to particles in a neighborhood of x. we note that there are two errors that contribute to the sph interpolation. the first is related to the approximation of d with the smoothing kernel w with length h, and goes to zero as h → 0. the second source of error in the sph interpolation is given by the space discretization, and therefore it depends on the particle density or equivalently from the average inter-particle spacing h. again, the continuous limit must be obtained for h→0, and to ensure that the entire sph method is consistent, we should have h/h→0 as h→0, although some different approaches have been devised to circumvent this issue, such as the renormalizing mesh-free scheme of lanson and vila [2008]. in applications, the average inter-particle distance h is usually a fixed fraction of the smoothing length h; typically h = ah with 2 < a < 4, with the actual value depending on the smoothing kernel used. the accuracy of the interpolation is related to the number of moments of w that are zero; this is always guaranteed for first moments as long as the kernel is chosen center-symmetric, such that w(·, h) only depends on |x − xi|. to simplify the notation in what follows, we will write ri = |x−xi|, rij = |xi −xj|and wij as a short form for w(rij, h). kernel symmetry ensures that wij = wji. we observe that for center-symmetric kernels, we can compute their gradient as: (1) when choosing an sph kernel, it is often convenient to select one for which the factor: (2) can be computed analytically without an actual division by r. this ensures that the kernel gradient does not suffer from singularities from overlapping particles. sph kernels are also usually chosen as positive, so that the interpolated density: is always strictly positive. this condition, however, prevents the second moments of w from being zero, and effectively limits the accuracy of the sph interpolation to first order at best. an important limitation of sph is that when the interpolation is computed on the location of a particle, the interpolated value is generally different from the interpolating field value. we have: for an arbitrary field u. we say that a numerical interpolation scheme has order k consistency when the interpolated values and the interpolating values match on the interpolation nodes for all of the polynomials of degree at most k. a significant limit of sph is that in its standard formulation, it does not guarantee even order 0 consistency, meaning that even constant fields are not reconstructed exactly. this is usually remedied by introducing a corrected kernel: (the shepard correction) that ensures the exact interpolation of constant functions. it should be noted, however, that while the shepard correction gives a lower minimum and average relative error, it can give a higher maximum error than standard sph for nonconstant fields. we shall see in the next section that a correction based on moving leastsquares (mls) can be used to guarantee a higher order consistency of sph. 1.2. smoothed particles hydrodynamics for first derivatives for gradients, sph takes advantage of the properties of convolutions and the divergence theorem to transfer the ∇ operator on the kernel. we have: bilotta et al. 623 , , x y x y y y x y y x x d w h d m w hi i i =t t t d x x ^ ^ ^ ^ ^ ^ h h h h h h/ # # , x x y y x y x x x u u d m u x w hi i i i =t t d x ^ ^ ^ ^ ^ ^ ^ h h h h h h h/ # , , x x x x x x x x u u u x m w h u x v w h i i i i i i i i i = = = t t t^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h / / < > ,x x xu u x v w hi i i i = -^ ^ ^h h h/ , w , x x x x w h r r h .rx i i r rii =d 2 2 = ^ ^h h , 1 w , f r h r r r h = 2 2^ ^h h < > ,x x xw hmi i i =t -^ ^h h/ < >xu u w v uj i i j i i j= !^ h / , , w r h w v w r h j ij ii j =u ^ ^h h/ 624 where r = ^x and n is its outer normal. if the compact support of w does not intersect r, the first integral is zero, and this leads us to the classic first form of the sph interpolation of the gradient: (3) that is first order accurate in h as long as the point x is far from the boundary of x. near the boundary, the surface integral ∫r u(y)d(y−x)dr is nonzero. a number of strategies have been devised to compensate for this term for particles near the physical boundaries of the domain [liu and liu 2003]. however, near the free surface of the fluid, the term is usually simply discarded, which introduces an additional error in the evaluation of the gradient. we will see later on that this issue does not affect our proposed mls approach. the gradient sph formula is rarely used in the form of equation (3). it is indeed possible to derive other formulations, which are usually preferred. from ∇u = ∇u ± u∇1 we derive: (4) whereas from ∇u = (∇(tu) − u∇t)/t, we get: (5) finally, we can consider (∇u) = t(∇(u/t) + (u/t2)∇t), to obtain: (6) the preference of one expression over another is usually driven by considerations with regard to stability or conservation. for example, equation (4) (with the plus sign) and equation (6) ensure that the influence of particle j over particle ī is opposite to the influence of particle īover particle j. on the other hand, the forms of equation (4) (with a minus sign) and equation (5) guarantee that the gradient of a constant function evaluates to zero; the latter expression is computationally more efficient, but the former is more stable in the case of large differences in the density values between particles, such as in the case of multiple fluids. further details about the ‘golden rules’ of sph can be found in monaghan [1992], for example. 2. moving least-squares in smoothed particles hydrodynamics the low consistency order of sph and the errors introduced by the surface term in gradient computations can lead to highly irregular field values during the evolution of a system. this can create spurious shocks, isolated particles, potential wells, streaming and other similar artifacts. this is usually compensated for by post-processing the field values, either before or during the integration step. this can be achieved with techniques such as periodic reinitializations of the pressure or density fields (see, for example, colagrossi and landrini [2003]) and the xsph method for the velocity field [monaghan 1992], where a weighted neighborhood velocity is used to move the particle, instead of the simple particle velocity. although the shepard kernel correction can be used during pressure-field initialization, this only guarantees order zero consistency, and the resulting field smoothing is often excessive. an alternative approach is based on mls, and it can be shown that it allows sph to achieve first-order consistency [belytschko et al. 1996, 2000]. this is equivalent [belytschko et al. 1998] to correcting the kernel with an affine function b(x, p) = b0(x) + ra ba(x)(p −x)a where a goes from 1 to the space dimension d (i.e. the summation is extended to all of the components of the vector p − x). to derive ba(·) (a = 0,1,... d), we impose: for the constant function u = u0(x) = 1 and for the affine functions u = ua(p) = pa − xa (one for each space dimension). this leads to a linear system with d + 1 equations in d + 1 unknowns, which takes the matrix form: where b(x) = (b0(x),..., bd(x)) t, and: (7) formally, the vector function b(x) is the first column of the inverse of the symmetric matrix a. if a is singular, to obtain b(x) we can compute the pseudo-inverse of a, using, for example, singular value decomposition. more details about the inversion of mls matrices can be found in section 5. 2.1. first derivatives and the müller approach for a scalar field u we can write the taylor expansion in a neighborhood of xj as (8) with the shorthand notation uj = u(xj) and truncating this at first order. writing this for x = xi and comparing this with mls for sph , , x y x y y y x y y x y y y x y n y x y y y x y n y x y y u u d u d u d u d u d u w h d u w h d n y x y x = = = = = = + + d d d d d r r d d d d d r x r x r x r x ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h h h h h h h h # # # # # # # ,< > ,xu w r hu v i i i i =d d^ ^h h/ ,< >u u u wx v ii i ij ij = !d d^ ^h h/ < >u u u wx m1 ij ij j i ij =d dt -^ ^h h/ u < >u u w .x m2 2 ii j j j j i i ij = +d dt t t ^ ch m/ , ,x x xu u w r h v bi i i i i=^ ^ ^h h h/ 1, 0, ..., 0x x t=ba^ ^ ^h h h , 1, 1,x x x x xw r h v .i i i i i= 7a -^ ^ ^ ^h h h h/ ...x x x x x xu u u u ux x jj j jj j= + + +d d--^ ^ ^ ^h h h h the actual field value ui = u(xi), we can evaluate the truncation error as: where xij = xi − xj and uij = ui − uj. hence, the (weighted) mean square error over a neighborhood can be evaluated as: where wij are given weights, the values of which will be discussed later on. assuming the components of ∇u|xj are unknown, we can find values for them that minimize <ej > 2. by setting the derivatives of < ej > 2 with respect to each component of ∇u|xj equal to zero, we obtain the linear system: (9) where mj = ri xij ⨂ xijwij. this approach can be used with a variety of possible weights, but by setting wij = viwij for an sph kernel w, we can create a clear relationship between the mls approach and the sph method. this is for example the choice that was made by müller et al. [2004], and in this case the mj matrix is equal to the cofactor of the a11 element of the reinitialization matrix a as defined in equation (7). with this choice, the mls gradient formula becomes: (10) where is the (pseudo) inverse of mj (see also section 5), which shows a distinct formal similarity with equation (4), with ∇wij replaced by: (11) with such a choice, the term can be used as a correction term for the kernel gradient in any sph formula. this has been shown to work correctly [narayanaswamy 2008], although this use does not have the mathematical foundation of equation (10). we note that the general gradient formula of equation (9) is invariant to weight scaling, in the sense that if weights = mwij are used (with m as a given constant), it produces the same estimate for ∇u(xj). in particular, standard sph kernels and shepard-corrected kernels produce the same results as from equation (10). additionally, equation (9) returns exact results for constant functions and first-order polynomials, which is proven as follows. let u(x) = k · x + c for some constant vector k and constant real number c; equation (9) then becomes: and since (k · xij) xij = k · (xij ⨂ xij), the right-hand side is exactly k · mj, and the solution of the system gives ∇u|xj = k (which is exactly the gradient of u) due to the symmetry of mj. this shows that the mls approach shown so far has first-order consistency. moreover, the first-order consistency is preserved even close to ^x, as the mls gradient formula is not affected by missing surface integral terms. a final remark is warranted concerning the neighborhoods in mls and sph. as discussed in section 1.1, the accuracy of sph is affected by both the smoothing length h of the kernel w and the average inter-particle distance h; indeed, improving accuracy in sph requires that both h and h tend to zero, with h going to zero faster than h. this equivalently means that the density of particles in a neighborhood must increase as the neighborhood grows smaller. in contrast, the mls accuracy is only affected by the distance between the point where gradients are to be evaluated and its farthest neighbor. when using sph kernels as mls weights, this effectively means that the accuracy is only affected by h, and there is no requirement for the density of the particles in a neighborhood to go up when h → 0. indeed, the compact support for kernels used as mls weights can in general be taken much smaller than the support chosen for sph, since the gradient will still be computed accurately as long as the point has as many nonaligned neighbors as there are components in the gradient. for comparison, consider that in a typical application to two-dimensional fluid dynamics, the ratio h/h is such that the kernel support includes about 30 or 40 particles, whereas for an mls gradient, three or four particles (and thus effectively a kernel with h/3 smoothing length) would be sufficient. in practice, to reduce the chance of a singular mj, the kernel support for mls can be taken such that each particle has at least 10 neighbors [müller et al. 2004]. 3. higher-order derivatives one of the long-standing issues in sph is the computation of higher-order derivatives, which is required when modeling the flow of a viscous fluid such as lava. consider for simplicity the case of an isotropic, incompressible fluid with constant viscosity. the navier– stokes equation for the velocity is then given by: (12) where t is the density, v the velocity, p the pressure, and o the kinematic viscosity coefficient, and f collects the external forces per unit mass (typically, gravity). to compute the laplacian of a field with sph, it is possible to apply the same mechanism used for the gradient, obtaining: bilotta et al. 625 xu ue xjj iji ij= d 2< > ee w2j i ij ij = / xu u wm x ij ijj ij ij =d / x xu u m v w , i ij ijij j ij =d +^ h / xw m v w .iijij j ij=d +u x xm u wkx ij ij ijj ij = $d ^ h/ v v fdt d p 2= + +d dt om j + w ij du ijwl 626 once again, under the additional assumption that the support of w does not intersect r = ^x, we would obtain the sph interpolation: however, the error induced by discarding the surface integrals for points near r is now much higher than in the gradient case, and finding correcting terms to take this into consideration is even harder. a different approach to discretize the laplacian was first used by cleary and monaghan [1999] to model thermal conduction, and it was adapted by morris et al. [1997] for the dynamic case. this is derived from a taylor expansion of the sph gradient, and it takes the form: (13) where f ≃ 0.01 and h is the smoothing length. the fh2 term is included to prevent singularities in the case of overlapping particles. in the case of symmetric (non-corrected) kernels, equation (1) can be used to simplify equation (13) to: (14) where fij = f(rij, h) with f defined as in equation (2). this expression has the benefit of being computable even in the case of overlapping particles, provided that f can be computed analytically. 3.1. moving least-squares for second-order derivatives it is possible to use the first-order mls gradient described in section 2.1 as a kernel correction, replacing the kernel gradient in equation (13) with the corrected kernel gradient of equation (11). since we have: where is the unit vector (direction) of xij , this allows us to simplify equation (13) to: (15) where however, this approach does not take full advantage of the power of mls, and suffers from all of the drawbacks of the sph method. instead, we can extend the first-order mls method from section 2.1 to allow direct computation of higher derivatives. for simplicity, we illustrate the approach in the two-dimensional case. the vector components will be (x,y) and the derivatives will be written as u,x for ^u/^x, and so forth for the other derivatives. the taylor expansion for u around xj can then be written up to the second error: and the truncation error when evaluating the expression for x = xi is now: once again, we want to minimize the mean square error <ej > 2 = with respect to the partial derivatives. for simplicity, we can write: then equating the derivatives of <ej > 2 to zero with respect to the components of gives us the linear system: (16) where the new formulae are formally identical to the first-order formulae, with an "extended" relative position vector that includes higher powers and an "extended" differential operator that includes higherorder derivatives. a similar approach to the one described here can be found with the finite pointset method (fpm) that was developed by tiwari and kuhnert [2001]. in the fpm, second-order mls are used to locally and iteratively solve the poisson equation for the pressure in an incompressible fluid. in this case, it is also necessary to include boundary conditions in equation (16), both on the free surface and on the physical boundaries of the fluid. in particular, this requires the detection of the surface particles of the fluid, which loses one of the main benefits of particle methods such as sph, which is the automatic and implicit tracking of surfaces. mls for sph , , , , , x x y y n y x y y x y y n y x y y n y x y y u w h u d u w h d w h u d u w h d d u w h d .2 y y x y x x = + + + + + $ $ $ d d d d d d d r r r r x r r x ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h h h h # # # # # < > ,xu u v w r h .2 2i i i i =d d-^ ^h h/ < > x x h v vm w 2 2 2 i i j i ij j i ij j ij ij = + + $ d d t t t f t ^ ^ h h / < > fv v m2 i i j i ij j ij ij = + d t t tt^ h / x x x xw m w d m d w2ij ij ij j ij ij ij ij j ij ij= =$d + +u t t < > fv v m2 i i j i ij j ij ij = + d t t tt u ^ h / dijt f d m d w .ij j ij ijij = +u t t 2 2 ... x xu u u x x u y y u x x u x x y y u y y , , 2 , , 2 j ,x j y j xx j xy j j yy j = + + + + + + + + ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h 2 2 .u u y u x u x y u ue x y , , 2 , , 2 ,x ij y ij xx ij xy ij ij yy ij ijij = + + + + , , 2 , , 2 , , , , x x y x x y y x y x x y y 2 2 2 2 ij ij ij ij ij ij ij = 2 2 2 2 2 2 2 2 2 2 2 .=d r r c c m m xm u u wj ij i ij ij=dr r r/ e w2iji ij/ udr x xm w .j ij ij iji= 7r r r/ xijr dr instead, we propose to use second-order mls in an approach that fits better with the standard sph modeling of fluid dynamics. in this respect, our approach follows the method described by dilts [1999, 2000] more closely, with interpolants that are centered on the particle for which the derivatives are being computed. as noted in belytschko et al. [1998], this improves the accuracy and stability of the computations, and brings our approach within the general framework described in schrader et al. [2010]. the use of mls over the standard sph discretization gives us higher accuracy for first-order derivatives, as well as the possibility to directly compute second-order derivatives. for example, the laplacian in two dimensions can be evaluated from equation (16) with: (17) where denotes the sum of the third and fifth components of the vector, i.e. the components corresponding to the pure derivatives of order two. a similar formula applies with more than two dimensions. for second-order mls, we can make considerations similar to those for first-order mls. for example, with the appropriate choice of the weighting functions, we can use mls as a gradient correction for sph, effectively replacing ∇wij in any sph equation with the first components of: (18) second-order mls shows second-order consistency (the proof follows that for first-order consistency of firstorder mls), and it is not affected by the surface integral terms that affect sph derivatives of any order, so that the accuracy is preserved even near the boundary of x, as long as there are enough neighboring particles. the minimum number of particles is now higher, with five neighbors as the bare minimum for the computation of all of the derivatives in two dimensions. additionally, the particles must not be laid out on a line (or in a plane in three dimensions), like in the first-order case, nor in a cross shape, as otherwise this might prevent mixed derivatives from being computable. in general, the neighborhoods for second-order mls must be larger than those for first order mls, although the number of neighbors in the compact support of an sph kernel is still sufficient. 4. an example to illustrate the benefits of our approach, we compare the results for a classic two-dimensional dam-break problem, where a mass of fluid with an initial rectangular configuration h0 = 2 m high and 1 m wide is left free to flow within a 4 m × 4 m box. the fluid is assumed to be isotropic and quasicompressible. its motion is driven by the navier–stokes equations paired with an equation of state: (19) (20) where g = (0,−g) is gravity (g = 9.81 m2/s), t0 = 1,000 kg/m 3 is the density at rest, c = 7, and the speed of sound cs is chosen to be an order of magnitude higher than the maximum speed expected during the flow, which in our problem is boundary conditions are implemented using an approach similar to the one described by liu et al. [2002]. near a physical boundary, a particle interacts not only with its own neighbors, but also with mirror images of itself and of all of its neighbors. these ghost particles have material properties that match those of their original, except for the velocity. if no-slip conditions are wanted at the border, the mirror velocities are opposite to their original ones, whereas free-slip conditions are implemented by giving the ghost particles a mirror velocity, where only the velocity component normal to the boundary is rejected. additional ghost particles with opposite velocities are also added at corners where two linear components of the containing box meet. as we want to simulate a viscous fluid, we use no-slip conditions in this example. as ghost particles are not sufficient to guarantee that the boundary will not be penetrated [liu et al. 2002], particles that exert a short-range repulsive lennard–jones force are also added. in contrast to liu et al. (2002), however, we do not establish these particles a priori, but they are generated at runtime as a projection of the particle itself on the boundary. this strategy is particularly efficient in our case, because the lennard–jones particle position can be found quickly as the midpoint of the original particle position and its ghost, which reduces the number of particles that need to be tracked during computation. additionally, the force exerted by these virtual lennard–jones particles is constant for a particle traveling parallel to a border, which solves one of the famous issues with lennard–jones boundary particles [monaghan and kajtar 2009]. our choice for a smoothing kernel both in sph and as weighting function in mls, is based on the }3,1 function by wendland [1995]. we have w(r,h) = w(r/h)/h2, where: we can also define f from equation (2) as f(r,h)=f(r/h)/h4, bilotta et al. 627 xu m w2 ij i j ij xx yy ij =d + + r r^ h/ xw m w .ij j ij ij=d + r r xmj ij xx yy + + r r^ h xmj ij +r r , 1 v v g v dt d p dt d c 2 0 2 0 s = + + = $ d d d t o t t c t t t c ,p = `` j j 2 6.3 m/s.gh0 + 4 7 1 2 2 1 0 2 2 ,w q q q q q 4 = + 2 #r -^ ` ^h j h) 628 where: so that ∇w(xij,h) = xij f(rij,h). the fluid has a kinematic viscosity o=10−6m2/s (water), so we apply the viscosity formula of equation (14). the discretized equations of motion can then be written as: where pij is an artificial viscosity that takes the form: where is the average speed of sound computed at the particles i and j, and their average density. the artificial viscosity coefficient is a = 0.2 in our case, and the correcting factor fh2 (with f = 0.01) prevents singularities in the case of overlapping particles. more details on the artificial viscosity in sph can be found in monaghan and gingold [1983]. in this test, we compare with second-order mls used as a gradient correction, with xijfij replaced by from equation (18), except for the viscous term that relies on equation (17) to compute the laplacian of the velocity. a second comparison is carried out with second-order mls derived from the differential equations (19) and (20); in this case, the discrete formulation becomes: with equations (16) and (17) to compute the gradients, divergences and laplacians. the viscosity is obtained by adding acsh to o when ∇ · vi < 0. in this case, cholesky decomposition is used to solve the linear systems, as detailed in section 5. in all three cases, integration is carried out using an explicit predictor/corrector scheme described later, with a dynamic mls for sph figure 1. dam-break two-dimensional evolution, at t = 0.01 s; 0.4 s; 0.8 s; 1.6 s, using standard sph. color coding by density. 2 0 2 2 ,q q q q f 32 35 3 = 2 #r -^ ^h h) v x v x v dt d f r h m dt d p p f r h m m f j i j i j i j ij i j i j ij i ij ij ij i ij ij i i i ij i = = + + + + + $ p t t t t t o t t t t , , , ^ c ^ ^ h m h h / / / 0 0, otherwise x v x v c h r h2ij s ij ij ij ij ij+ $ $ 1 p a t f r * v v v dt d p dt d g 2 i i i j i i i = + + = $ d d d t o t t t , , u csr tr wijdu ou time-step that is limited by standard courant-friedrichs-lewy conditions: where fm denotes the maximum particle force per unit mass, and we take k = 0.3. with h = 0.02, this gives us dt ~ 10−4. from the particle positions xn and velocities vn at time tn, we compute the particle accelerations (xn, vn) and the maximum timestep dt*; the predicted new positions and velocities are then given by: the correction step evaluates and a new maximum time-step dt**. we then compute the maximum time-step as dt = min(dt*, dt**), and the accelerations as fn+1 = , and integrate with: the evolution of the dam break with sph (figure 1) shows typical issues in sph-driven simulations, with a noisy density field that leads to streaks of high-density particles that infiltrate the main body flow. in addition, the absence of correction terms for the tensile instability [monaghan 2000] leads to particle overlapping in the lower area of the flow, with a consequent reduction in precision and an apparent rarefaction of the particle positions. in contrast, the mls-driven simulations (figures 2 and 3) show much cleaner behavior, with the density field evolving smoothly without any need for post-processing phases or reinitialization. some noise is still present, particularly in the region closer to the physical boundary; this effect does not have a strong influence on the main body flow, although it allows a few particles to detach when the fluid layer is very thin. we can also compare our results with the experimental timing obtained by martin and moyce [1952], which match our case with a = 1 m and n2 = 2. for the wavefront z, the timing is scaled by ~ 4.43 s and the wavefront z is scaled by a; the results are shown in table 1. the column height h is expressed as a fraction of the original column height h0, and the timing is scaled by ~ 3.13 s; these results are shown in table 2. we observe that mls gave the same timing (within two digits of accuracy) used both as a kernel correction and directly, and the results tend to be closer to the experimental bilotta et al. 629 figure 2. dam-break two-dimensional evolution, at t = 0.01 s; 0.4 s; 0.8 s; 1.6 s, using second-order mls as a kernel correction in sph. color coding by density. min , ,t k c h f h h2 s m # od ' 1 ,v v t f x x t v . 1 1 1 1 n n n n n n = + = + d d + + + + ) ) ) ) ) ) ,v v t f x x t v . 1 1 1 1 n n n n n n = + = + d d + + + + f 1n + ) ,f x v1 1 1n n n+ + + )) ) )^ h /2f f1 1n n++ + ) ))^ h n g a n g a 630 values than sph. the major exception is for the lower values of h, for which both particle methods, and mls in particular, tend to lag behind the experimental data. this appears to be related to the observable small peak adherent to the left boundary and clearly visible in figures 2 and 3, an artifact of boundary conditions that appears to be exerting excessive drag in this case. if this issue is taken into account, the corrected timing for mls fall back within the experimental range. 5. implementation notes the mls matrices are symmetric and positive semidefinite. rather than computing the inverse, it is therefore generally more efficient to compute the cholesky decomposition in the non-radical form; i.e. to find a lower triangular matrix l with unitary diagonal, and a diagonal matrix d such that m = ldlt, and then to solve the linear systems associated with any derivatives we need to compute, rather than using the matrix dot vector notation synthetically used throughout this report. such an approach, however, is only possible when the matrix is strictly definitely positive, as otherwise the standard constructive cholesky decompositions fail when elements of d are null or very close to zero. in such a case, a better approach is to compute a pseudo-inverse of mand then using mls for sph figure 3. dam-break two-dimensional evolution at t = 0.01 s; 0.4 s; 0.8 s; 1.6 s, using second-order mls from the analytical formulation. color coding by density. h t range (exp) t (sph) t (mls-corr) t (mls) 0.89 0.76 − 0.79 0.75 0.79 0.79 0.78 1.05 − 1.11 1.10 1.07 1.07 0.72 1.28 − 1.33 1.32 1.29 1.29 0.67 1.44 − 1.47 1.48 1.54 1.54 0.61 1.66 − 1.68 1.73 1.82 1.82 z t range (exp) t (sph) t (mls-corr) t (mls) 1.11 0.30 − 0.48 0.40 0.40 0.40 1.89 1.56 − 1.67 1.46 1.51 1.51 2.33 1.87 − 2.02 1.90 1.92 1.92 2.78 2.21 − 2.38 2.31 2.31 2.31 table 2. time at which column height h reaches a given fraction of the original height in experiments as compared to timing achieved with sph, second-order mls as a kernel correction, and mls from the analytical formulation. scales as detailed in the text. table 1. time to reach wavefront distance z in experiments as compared to timing achieved with sph, second-order mls as a kernel correction, and mls from the analytical formulation. scales as detailed in the text. the formulae detailed in this report in their actual forms. any symmetric, positive, semi-definite matrix can be written [press et al. 1992, golub and van loan 1996] in the form m = gteg, where g is an orthogonal matrix (gtg= ggt = i) and e is a diagonal matrix where its diagonal elements are the singular values of m. to find g and e, we use the iterative jacobi method [golub 2000], which writes g as a product of givens rotations. the pseudo-inverse e+ of e is then computed as the diagonal matrix: and the pseudo-inverse of m is obtained as m+ = gte+g. in practice, both during the cholesky decomposition and for the pseudo-inverse creation, the zero test is replaced by |aij|<f where aij is the matrix element to be tested and f is an appropriately small number. picking too small an f can lead to disproportionately large reciprocals in m+, with a consequent excess force being exerted on the corresponding particles, which can result in large instabilities during a simulation; on the other hand, too large an f will increase the numerical error in the pseudoinverse computation, with a consequent perturbation of the particle motion. our implementation, which is based on the work by hérault et al. [2010a], exploits the parallel computing power offered by nvidia graphics processing units (gpus) to achieve two orders of magnitude in speed-up over the equivalent code for standard central processing units (cpus). however, it is also limited by the single precision native to the hardware by the gpus themselves. in this case, we found that the best value for f lies in the range dmfm/2 ≤ f ≤2dmfm where dm is the dimension of the mls matrix (e.g. dm = 5 for a second-order mls with two space dimensions) and fm is the machine epsilon for single precision. 5.1. performance with respect to standard sph, the mls approach has additional computational cost that is associated with the generation and use of the mls matrix for each particle. the impact of the extra computations on our gpu implementation running on a gtx 280 is shown in figure 4. while the average execution time of the integration (euler) bilotta et al. 631 figure 4. code performance for sph versus mls implementation of the dam break. the average euler integration, neighbor list construction, forces computation times, and number of iterations are plotted against the total simulated time. 1/ 0 0 , 0 e e e eii ii ii ii = = ! + ' 632 and the neighbor list construction (neibs) kernels are essentially the same with both sph and mls, the force computation (forces kernel) is relatively consistently about 4 times greater. the additional cost per iteration is off-set by a slightly larger time-step in mls that leads to a lower number of total iterations to cover the simulated time. although this result might appear a little surprising, it is easily explained by the highly irregular fields that cause spuriously large forces in sph. as a consequence, the total run-time for mls (158 s) is only about 60% longer than for pure sph (98 s). 6. conclusions mls are typically used in sph in the form of either a post-processing smoothing filter for the density, or a kernel correction for gradient evaluation. in both cases, their use is based on a first-order taylor series expansion. we propose a method to extend the kernel correction approach by using second-order mls, which provides improved approximations for gradients as well as a robust formula for second-order derivatives. the new approach can be used both as a kernel correction in sph, which preserves the classic sph formalism and replacing the ∇w terms with a matrix/vector product, and also directly, using discrete equations that are specific to mls. the application of the method described to the solution of the navier–stokes equations for a viscous fluid shows visible benefits over the use of standard sph, which reduces the need for post-processing filters, such as xsph, shepard or mls itself. the computational cost of our method is, however, about 60% greater than that of standard sph. a more thorough analysis of the influence of the weighting functions chosen during the mls process remains to be undertaken, together with an analysis of the opportunity to choose different forms for the derivative computation by taking advantage of the standard gradient identities from which the distinct sph gradient formulae are derived. such an analysis should identify the most appropriate form to guarantee that the discrete system will preserve quantities such as total momentum and angular momentum when they are conserved at the analytical level. acknowledgements. this study was undertaken with financial support from the v3-lava project (ingv-dpc 2007-2009 contract). references belytschko, t., y. krongauz, d. organ, m. fleming and p. krysl (1996). meshless methods: an overview and recent developments, comput. method. appl. m., 139, 3-47. belytschko, t., y. krongauz, j. dolbow and c. gerlach (1998). on the completeness of meshfree particle methods, int. j. numer. meth. eng., 43, 785-819. belytschko, t., y. guo, w. kam liu and s. ping xiao (2000). a unified stability analysis of meshless particle methods, int. j. numer. meth. eng., 48 (9), 1359-1400. cleary, p.w. and j.j. monaghan (1999). conduction modelling using smoothed particle hydrodynamics, j. comput. phys., 148, 227-264. colagrossi, a. and m. landrini (2003). numerical simulation of interfacial flows by smoothed particle hydrodynamics, j. comput. phys., 191, 448-475. dalrymple, r.a. and a. hérault (2009). levee breaching with gpu-sphysics code, in: proceedings of the 4th international spheric workshop (nantes, france, may 2009), available at: http://www.ce.jhu.edu/dalrymple/gpu/ sphericivdalrympleherault.pdf. dilts, g.a. (1999). moving-least-squares-particle hydrodynamics i. consistency and stability, int. j. numer. meth. eng., 44 (8), 1115-1155. dilts, g.a. (2000). moving least-squares particle hydrodynamics ii: conservation and boundaries, int. j. numer. meth. eng., 48 (10), 1503-1524. gingold, r. and j. monaghan (1977). smoothed particle hydrodynamics – theory and application to non-spherical stars, mon. not. r. astron. soc., 181, 375-389. golub, g.h. and c.f. van loan (1996). matrix computations, johns hopkins studies in mathematical sciences, johns hopkins university press, 728 pp. golub, g. (2000). eigenvalue computation in the 20th century, j. comput. appl. math., 123 (1-2), 35-65. hérault, a., a. vicari, c. del negro and r.a. dalrymple (2009). modeling water waves in the surf zone with gpu-sphysics, in: proceedings of the 4th international spheric workshop (nantes, france, may 2009), available at: http://www.ce.jhu.edu/dalrymple/gpu/spheric herault.pdf. hérault, a., g. bilotta and r.a. dalrymple (2010a). sph on gpu with cuda, j. hydraul. res., 48 (extra issue), 74-79. hérault, a., g. bilotta, c. del negro, g. russo and a. vicari (2010b). sph modeling of lava flows with gpu implementation, in: l. fortuna, a. fradokv and m. frasca (eds.), from physics to control through an emergent view, world scientific series on nonlinear science, series b, 15, 183-188. hérault, a., g. bilotta, a. vicari, e. rustico and c. del negro (2011). numerical simulation of lava flow using a gpu sph model, annals of geophysics, 54 (5), 600-620 (this issue). lanson, n. and j.p. vila (2008). renormalized meshfree schemes i: consistency, stability, and hybrid methods for conservation laws, siam j. numer. anal., 46 (4), 1912-1934. liu, m., g. liu and k. lam (2002). investigations into water mitigation using a meshless particle method, shock waves, 12 (3), 181-195. liu, g. and m. liu (2003). smoothed particle hydrodynammls for sph ics: a meshfree particle method, world scientific publishing company, 449 pp. lucy, l. (1977). a numerical approach to the testing of the fission hypothesis, astron. j., 82, 1013-1024. martin, j. and w. moyce (1952). part iv. an experimental study of the collapse of liquid columns on a rigid horizontal plane, philos. t. r. soc. a, 244 (882), 312-324. monaghan, j. and r. gingold (1983). shock simulation by the particle method sph, j. comput. phys., 52 (2), 374-389. monaghan, j. (1992). smoothed particle hydrodynamics, annu. rev. astron. astr., 30 (1), 543-574. monaghan, j. (2000). sph without a tensile instability, j. comput. phys., 159 (2), 290-311. monaghan, j. and j. kajtar (2009). sph particle boundary forces for arbitrary boundaries, comput. phys. commun., 180 (10), 1811-1820. morris, j.p., p.j. fox and y. zhu (1997). modeling low reynolds number incompressible flows using sph, j. comput. phys., 136 (1), 214-226. müller, m., r. keiser, a. nealen, m. pauly, m. gross and m. alexa (2004). point based animation of elastic, plastic and melting objects, in: r. boulic and d.k. pai (eds.), sca '04 symposium on computer animation 2004 (grenoble, france, 2004), eurographics association aire-la-ville, switzerland, 141-151. narayanaswamy, m.s. (2008). a hybrid boussinesq-sph wave propagation model with applications to forced waves in rectangular tanks, ph.d. thesis, johns hopkins university. press, w.h., s.a. teukolsky, w.t. vetterling and b.p. flannery (1992). numerical recipes in c: the art of scientific computing, 2nd edition, new york, 994 pp. schrader, b., s. reboux and i.f. sbalzarini (2010). discretization correction of general integral pse operators for particle methods, j. comput. phys., 229, 4159-4182; url: http://dx.doi.org/10.1016/j.jcp.2010.02.004. tiwari, s. and j. kuhnert (2001). finite pointset method based on the projection method for simulations of the incompressible navier-stokes equations, berichte des fraunhofer itwm, 30, 1-18; available at: http://www. itwm.fraunhofer.de/fileadmin/itwm-media/zentral/ pdf/berichte_itwm/2001/bericht30.pdf. wendland, h. (1995). piecewise polynomial, positive definite and compactly supported radial functions of minimal degree, adv. comput. math., 4 (1), 389-396. *corresponding author: giovanni russo, università di catania, dipartimento di matematica e informatica, catania, italy; email: russo@dmi.unict.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. bilotta et al. 633 rapid diapir growth, a triggering agent of a 6 1/2 magnitude earthquake on october 29, 1966, in acarnania (greece) a. g. galai\op01:los a. l~koxo)ijdwj " rece iyea on jul y 28t.h, 19i3 ru'niar..... 'l'he]"(j exists guud evillence that iliapir gr owing occurh at a rate similal" to j·he rate of sedinh'ntation. taking this into aeroun l , il is rel\.fjonable to assume tlmt aifterential, rnpid lualling, dnring the filling of tlw krmllasta !"eseryoir, colllhinel1 wit.h a ileeper pend.ration uf liuills, may have affertell the large.scale ddormation of the tl"iassie stl"ata of gypsllm in acarnania allil prubahly inereasea iliapir gruwt.h at a cat:1strophie rate; cllnsequently long·tel"ln stahility of the diapiric formahuns of g;qlsum ilishuhell hy exi~essiye ildul"lllniiun mhy hnve resuhel1 in n premature re· lease of t.he regional strains in tlw fol"ln of a yery shallow muuiij1wke on oetober 2d, 1%6. ri assu:'iito. esist.ono huone t estimonianze che l'acrresdllwnto ilei di:1pil·0 ayyenga cun una velocità pari a iluplla di sedimlmi"azione. pllr!{'llilo da lj1wsbl premessa, è mgionevulc ]j1"esumere che l" impruyyiso eal·ico, a seguito del riempimento ild hadno ili kl"cmasta, sumllwto illl una più ill"ofondn penetrazione ai liquilii, possono aver contrihuito alla ildonllazione ili granlli proporzioni s u hit.a ilagli strati trinssiei ael gesso in acarnania e pro\labilmente ayer aumentato in llllllju catastroficu in velueitil. ili aeereseimento ael aiapiro; di conseguenza la lunga stahili t il delle formazioni diajliriche ili gesso i\isturbate aa eccessiya ddonll:1zione, possono avei" pro· voc:1to una prematul"a liherazione delle t.ensiuni regional i sfoei:1t.a quest ' ult ima, il 29 ott.ubre hi6(i, sotto fonua ili un t.eltmlloto molto sui1(jjiiriale. rt is rat.her well establihhecl ancl generaliy aeeeptecl that many triggering forees of relatiyely small magnit.nde, in pomparison with the ma.in clee})-seatecl strain producing forees, ma~t ehange nncler eertain fayourable eonditiolls the mte of strain release in li st.ressed regioll being ciose t{) breaking strengt.h ('l. (*) kational ohservatory of a t hens. scislllologil~al insti t ute , athl'ns (greece). "\ft"l'a ioil,!! !:w]; ()]' ('omplet(· ab.~cih'e or ilefltl'lll'tiyl' ,~ho('kfl ill illl' wl'st"i'll ]ih1't of n'!ltral gl'ce('c :ln 111l11~11a1 l':lrt,llquhkl' hetiyity fltal'tl'd ili th" kl'l'mh~hl tlam hn'h hbont thl'ee mljlttlls h l't {'l' impounlling or the mtijj('iallake (l!wi'i, ,jn1~' 21) in the fln'a or tlle canoni y.olll'. tjj(~ aetivity ('\llmi!lat('d ili tlll'l'f' o l/j to ti 1/2 nmgnitllde fl111jf'b 011 fl'bl'llhi'.\' ;'), idoli, of'toher 2!l, l!)oo hlltl ~"h.\' 1, hhi7. 'l'hl' ghvi'oyo massir fol'm,~ a fallltl'd alltk1i11(\ with sx"\y-i-!i-!1!; nllllling axih lw1011gillg to thl' chntoyo tripolifl zolle: it ('ollhists of cp]lpl' ('l'l'hh'l'llus n'l'f liml'sto!ll's alli l tntial'y lim(\~tollcs ~1l1'l'ou11111'tl by llysf'h dl'posit~, oll ('"vitlell('(\ of thl' "lllstitut: f1'l\!l~ai,~ du pf'tl'o!t'" (1.f.p., l!wo), qlloll'tl hy hi('htn hiii! )ial'iolakos (' ), lite l'a, h/oo m thil'k 1:"ppcr ('l'l'tal'l'ouf! liml'stolll's oy('1'lil' l'h, 1500m garly ('l'da·l'l'oilf! hnd junlssie limefltollef! h11d h 'rl'ihflsie fol'lllatioll or a salt ('u.~jjioll (g~'psum, anh~'lll'itc alili dolomitl') of ullkllowll thieklll's.~ (s, fig'. i), .at thc lwginllillj.{ of ihe llym'jj dl'1jo.~itioli i11 ihl' t011ihll hl111 uhy1'oyo wlie ai: thc cppel' 1!;ocelle tllifl area 1'i'pl'cscnted ah uplifl tlmt wa~ ('on'l'l'd wilh thl' 1l~'m'h not lwrol'c the olij.{of'e!le . ..1f'l'ol'ding to n,l{il'hi in ,,z~ i --~ e ---'" 1 . . .. _ rr------~"----~_------~:_tl---_ -o-_~ ___ ..--~_ ~ _o~_'_'"_-r~ _. ~ r · i ' · " ·' . . -terti.iir "-.... ,..-. .. ~ .... , ... v., ... " .... ob",i kreide t i -. , -, trias fig, l t4!mtigraphy of (~a\'l'ovo zolle aml illlwr !oniall zone afte!' dietel' hil'htel' ani! llia~ ~iariolak()s( · ). {)1!7 tel' hihl r. ;\[~\1'iolnkok (') 11](' iiplift luo\'l'llwlil w,ik ('amwd h,'" linlokilh'~is (s, fig, 2), p1'o(1l1('ill~ liulu('ml1s fnlllt~ tltnt ~ttikl' lipal'i,'pamllel to ihe (1in'diol! of tlte (hl"l'o"n allti\'lim', 'l'hl' f(nluc 1l'!'inni\' ('\'o]u1io1i wai'! 11111 fonnml by ,\,g. ya~ii(' n fui' the t'amarilh\k ~()l'~(' il! t':\pain. , b , ~ fly~ 'l'' 'ii i . . . . . . . . . .~ ... . . '.' . . . . '.' , , , , , ~ kl'llide-kek , , , , , ~ '. ' . ju 01l _k a lk , , q' , , , , , , ". ~ , ... " .. ', .... siilnr-seoian del" ttia5 }'ig, 2 _ :--;('!wli,alie dia!!;!":!!" showilll!; 1111' uplilt l'rol'p~~ ,,( ganol'o 'l<1s~if ah('i' dip({'l' riellh'l' alld ilia;; 'lariojakos ( ~ ): a) '1<1i'gi"<11 1ll0\·piii('llt 01 ~alt jow<1l'ds 1111' (;<1\"i'o\"" 1i1'pa durill!!; 1111' high('i' i·pw'i' \'l'('ta('l'ou.~, 1'11111('0("'11<' allol eoepllc; l,) fol'lwd,ioll o l il. ~ah (·likhioll iii t.lw (;al"l'ol'o an'li il1ld :l('(','jpl'atr'd uplilt a 1 i hl' ln'!!;illllill/! o( fiyse h ,kpo.~i t ioll, 1'i'l'akillg o il' 01 h li!!;(' fl'aglllj'llt.~: i") dilllillutioll or ul'lift il1ld ('(jualizatioll or i!w l'('lid ill ihl' ()lig(l('('ilc. <0 8 a. ~ . gai.,u.iol'oul.os a. eko!oi(hu])es thcl'c cxists go()(l cvj(lcllec tlmf, diapil' gl'owing oceul's at. a rate similar to the rate of sedimentation (~). taking j,llis ili lo account, it is l'easonable to assume t.ha,t, iliftcl'ential, l'ftl)id ]oading, !lul'ing thc filling of tite kl'ernnsta l'l'sl'l'yoir, eombined with a deepel' l)cnetl'ation oi flui(1f;, m,t)' ha ve aitertcd tite hu'ge-sca.le defol'mntion of t.he tl'jassie strata oi gypsnlll in acarnania aihl lli'obably inel'eased diapil' growth ht a catash'ol)hic rate; c'ollsequently long-tcl'lll f\tability oi t.hc (liavil'ic fol'mahons of gyllsum disturbeil by excessive ileformatioll mfly lmvc resnltcd in a. ]ll'cl1latnrc l'elense of the r('giolln,] stl'ains in the fonn oi [\, yer,\' shallow earthqwtkc oh oet~)i)c1' 2d, ] d60 (:{8.f)o~, 21 j ob; h = 1 ± 5.i) lcm, j.1i. = 01/2). the restrielion of the highest intensity ("viii) to jmt one locnlit.y (kn.tomm) sl)etl,ks well for t.lte shallowness of t.he focus. a ,;imilar ea,se, well documented, was observed at the ilum site in lhe canuhillrl,,~ go!'ge, svain, lh-wember 8, 1961, ami p!'esumably at. the dam f!itef! of ca,nelles·sa,nla, anna, spnin, june 9, 1962 (p) and also a,t the ilam site of oued fodda, in algeri,t, from jalhlary t,o l\iay 193:1 (g). there is eviilenee in speeta.eular fashion that movcment of the salt is even now a.l-tecting t.he ul)permost f!ediment layers at the sea tloor of west.ern l\lmlit,eltanean (7), ba,rt,hquake,~ triggered by ira,te!' loading and/o!' lulll'ication a.re in generai af!sociated with normal faulting; thif! wa .. ~ the eaf!e of the krema· sta earthquake on february 5, 190g (:19.1°n, 21.7 ne; h = 1g ± 4.7 km , .m. ,.--c o 1/4). the ka,lonna, earthquake of oetober 29, 19gg, was associateil with thruf!t faulting (,~. fig. 3); this is in aecordanee with tlmt one shoulil expect if rapid dial)ir growth has aded as triggel'ing mecha,nimn (f!. fig. 4). the el)icenter of the second late 'lft,el .. ~hoek of :!ilay 1, 1967 (39.6°n, 21.:1°b, h = :h ± :~,2 km, .m . = 61j2) ami t.hat of october 29, 1fj66, are almof!t eqni(lista.nt from lhe epieenter of the kremasta earthqmlke oi fcbruary 5, 19g6. the fault meeh,tllif!ms of both late aitershoeks wel'e simiia.r, le, of reverse type; it is then l)osf!ible the evapol'ites to exten<l northw anls ami t.he same triggering agent. to have also act.c<l in the case of the ])r(w1ilcghc earthquake of liiay l, 19g7. the l'ate of (liapil'ie proeesses is rclativcly grcatel' when dial)il's are al)proaching or rcaching the wria.ce (s. fig, 4); t.hus the rat.e of movement of evapol'ites a,t fa!' grea,t.el' dcl)th should be ma1'iwdly less tha.n the ra,te of growth of a well-devclopc(l diapir apih'oaching the surface. thif! might cxl)lain, at leaf!t to some extent, why thc l)l'og!'essive, jc1'ky l'elief of t.he regiollul stress fielil was manifesle<l disastrously " :' ,', ,', ,' , ,', ,', ,', " 1:1: ~ l2l'til ~ ~ c::j iii,' ,' ,' ', ii i,. ' ', ','1 ' 1',', ,i, ', ' i li :':': :1, ', ' ,iii , ,i:!:' ' 1111 ',l, ii' ,' iii' , ",1 ,' 1,111 :' 1 ,111, l ,ii ,', " ' i :':1:' , ' ',' l'l' i ' 'i: " ']f l ' :1:1' ,', :' i :! ~ , ,':' 'i: tonian geosynclin e tripolis ridge olonos-pindos g eosyncline post-tectonic deposits pela!;lonian rid!;le o " ~·: .. .'.'::;:i t:·:·:·:·:·:·:.:.; @] ~ ~ east siop e 01 p la t e apullon west si de al pelagonian ridg e outcropin!;ls 01 dìapyric tr,oss'c gypsum front lì ne 01 upthrusts, dash e d when probobl e normal faults, doshed wh e n probobl e " so " .. l<'ig. 3 stl'lw!urnl lineallhl1lh, (l) alhl fanh. . p!allt' so!ntiolls iur tho llwill eartll1l'jake of 10uu, }'ebl'. 5, and tlw two late afh~l'rhoeks of 101i1i (jet.. 20 and ioni, :ltay 1 (~) . 'l'ho iucnl lilcchanis1ll8 arc shown as cqnal . al'ca pl'ujections of t.hc, low(~r foea! helllisphel'c. 'l'he upell cil'clcs arc (.hc axes of colllprcssiull, p, anù the e!ohed dl'deh are t.lw axes of tension, '1'. , b' 6 5 :; ;:; == ....•. 4 : ;; :: 3 " b' ::; ::; 2 at katouna, in (hl' uo)"111 {'liti o[ (hl' ht ']1 of t.he on1el"opini!~ o[ ~iia]lil'it 'l'\'ia,~~it' gyp.~lilll (~. fii!. ;.~), ea.i lllon1h~ {';ll'lh'\' (han al 11l'omlp{'g-ii('. 'l'he .~tratig-[·a]lb.\01' gny\'oyo zone antl inne\' louiall zone nntl th" u]llift l)j·o("p.~~ of gayl"oyo iljah~if (~), lllnke the sil!!'g-,,~1 ion of n mpitl tliapir !!'j"ow1h 1willg l'e~ponsihlt' fol' 11l(' l·l·[jpj' o[ (be n'!!'iollnl .~ti'pss til'ld mon' ('edaiu. 'l'ii{' ùlen i~ ('()]"]"o1!ol'a1ed b~1hp pn'~l'n('{' of ilihil~­ 'vhi'1li .~nlplhll·{'lm~ s]ll'in!!'~ in tbe al'{'a "oil~i~i{·i·l·d; tbe {'.\i~1pnep of . -'\----1'" " '--.. ,...,.. ~ '---'l-_ _ ~~~~ ___ _____ __' i·'ig'. 5. \i;qj sholl"ing ('nl"tlll[1wke d('n~it,\' 01'1'1" a "yi'ar ]wriod, i !w.j..]!lil), nftl'l" two i1('ralioll.~. lh'll!<it~i" 1il('n~lll'('d :i" 111(' 1l1l1ll1wl' or ('\'l'nt~ pn 1).1 0 ihli111l11'-longil mk j'(·dhllgh·. sn11lhnl'eom springs iihlil',l1,('~ tbnt inno\\' oj' wl"fa('{) watet·,~ l'xlpnds ah de{'p ns lbe 'l'l'i;l.~sie gyp~lilll la.n'1' al h'a~t. it sboultl he atltlell that .~lilphllj'('ou" hpl'ings hayt> hppll do.~{·11 dw·in).!.' the eolls1nh.:tioll 01' the kl'l'llla~(:.a ilalll. 'l'11i~ also 1li11~1 han' l'onj\'ilm1{'~1 jo (h" (,bang-e of t.he stn's~ di,,1:rilmlion path'['n in 1be l'egion {·onsill('l'l,tl. 'l'hl' o l!:!. nmgnitude ehi'1hqunk<'s oli ()elo!w\' :!u, iutio, nnd 011 ::uay 1, luoi, lhwillg o('elll'l'ed in the a[(el .. ~lll)('k pel'ioll sta'l'tell hy th{' g12 a. g. galaxoi'oijlos a. eko:s-o\fij>es krenmsta cnrthl]ha-ke on fcbrnary 5, 1960, oll thc ontf\kirts o[ tlle 19h-l-19j"o nnrthl]nakc dcmit.y zone (h. fil!. !'i), rc"eal thcir illt.crplay 1111(1 dose eolllleclioll; tlllls the lnlwl "lat.(~ aher~ho('ks" wc hn"c pnt on them is well justified. the ('ellter of the enrthl] nakc dcmitr pahem oycr t.hc "7 -ycar pcrio(l, hhi-l-19j"o, i.~ do.~e lo thc infltrnmcntal cpi('cnt.cr of tlle krenulsta earthl]ualw, i.c. aboht 18 km lo t·he :north o[ the kremasta (lam. r~oeat.iolls nnd [oeal (leljths foi" all ea-rthquakes are tho~e giyell by ls.e. acj(xoìyledg.ìllex'l's 'l'he autllors wi.~h t.o a.cknowle(lge tlleir gnltithdc t.o pro[essor .1. hothe for ea.nillg" thcir attentioll to tlle yagiic's "'ork all(1 pro"iding them with photo('opicf\ of gourinanl'r; an(1 yagiie's paper.~. (l) boll~ 0\' ,\~, .t., g ,\ la ;<>opou los, a. all(i di,li iia~ is, n., i !l71. s('i'l-/l/ol<>c/o/l-ic j/i1}j o! (ireccr. "l'uhl. 8ds\li. i11st. ~<lt. ohs .. o\t!tl'lls illl(1 illht. (il'ol. ~llhsll)"fll('(' r{'h.'·, (i n'l'l'l'. ('i j)"",opouw.~ •. 1. ",,,i dew,",,«. s., jm. 0". ilo .1[,d"",i,,,, "i ""'" l<jarlhlfl/rlkcx in i111~ ..!ufi ()! we"'/an oreeci' limi/hl' slreisis producin(j thcm. l'n'sl'llteti ill tlte eu1"ljp. ~i'islll. colli. ~ii'i·tillg iii. b1"ilso'", rolliallia 1!l72; ill pl'l'hs ill '''l\·e!llli{"al i'otl\(lit's", lo, bul"ul"l·st.. (3) galan()r()n.()~, a., ihgg. 'l'he inf/nence o! l/rc friuii/w/i()1i o! j/llrr//h/!11 i,r/~i' h'l~ t'r/lion on l,oca/ harlhqaa!.-r .idil'il!! in ihe .lili<'1i hfl8ill _[rn/. "'\1111. g(\o1. l'il)'s liellell.", 18, 2hl·30h. ("') (iella, i.'., 1\)72. hl'deh' o{ sfili '1'1'e/ollicis iii hcll1/ioll lo ihc l)i~flii8ll/0! [(/(i/iol/dil'c l!"(ls!cs iii "'''1111 fonlllrlion.·. "(il'(,]. ~oe. alli. bull.", 83, 12, 3551·3574. (~) ricii'i"jo;ii, n. awl .'ir'\hiolako~, i., 1!173. ()1i.~lholl/r.llmlllll, ~inllislrl'l" ilich! bdil/lln/eo' 1<>!.-10,o·('dllllcllla(oflixehe8 p/wcllolllcni l(. f(!lrrh-_1 /il1/fll'!"l/ nrf~ n. "n. ,fh. n{·oj. l'aliiollt. ahlt.", 142, 2, 1iii'·170, 8tllt.tgart .. ( ~ ) hotiilè, l'. ,1., h170. s~iism"x .irli/icie!~. '''l'ed.ollopltysi(·s'', 9, 21!i · 23h. (1) 8tasu:y, .i. d., h173. j[odc!"1(. fiaisin 8cdimel(.1 de!orlll.(llion //.11 srrll 1'edonic~, il' ~ '''i ~ rll ]1"dil('i"ri/i(.('iiii. ahhtra("toi, eul"opc'<lll geo]jltysìt'nl t',)('iety, ;l,iil"i('h, ~l'ptelllh{'i' 2,;-2!l, ]1. 07. (~) la\li:r-:, g. a., i!w!). ~ jficl"ii.nllrlhqlwk,,~ in hi'si'it()irs. "l'ro{"ectiiugs oc oc the ,ioilll; ~yil'li()silllilor ecce/aris, ke('olld eli)"o]lea11 kylllp08illlll oll earth(lun](e ell!-(illeel"ill!-(", .'ifiilll'og"l"aphc(l, ?lf[/(l1"i,l, pp. 211·3,;. (") 1",iiicjo;, g. e., 1073. 'j'er!"l~liiolox!j nm/jf/(o·eis. "r{lvi.~11l ,le olll"<lsl'llbliens", pp. 7,l3·7iio. vol48/04/2005def 719 annals of geophysics, vol. 48, n. 4/5, august/october 2005 key words volatiles – magmatic rocks – basaltic glass – degassing – hawaiian lava – remelting – black sand 1. introduction the analysis of volatiles in natural whole rocks has historically attracted little attention from igneous petrologists, in contrast to major-, trace-elements and isotope analysis. as caroll and holloway (1994) wrote (1.1): «the words of bowen 1928 are still broadly applicable … to many petrologists a volatile component is exactly like a maxwell demon; it does just what one may wish it to do». the loi (loss on ignition) is in many current publications the only information about volatiles in natural samples. special analytical methods as well as kft, sims or ftir have been developed for several volatile species. degas was developed in the last 20 years for a simultaneous determination of complex mixdetermination of volatiles in volcanic rocks and minerals with a directly coupled evolved gas analyzing system (degas) – part i: interpretation of degassing profiles (degas-profiles) of minerals and rocks on the basis of melting experiments klaus heide and christian m. schmidt institute of geosciences, university of jena, germany abstract volatile components in magma strongly influence many physical properties of melts and minerals. the temperature resolved degassing analysis of volcanic crystalline and vitreous rocks gives detailed information about volatile compounds in the melt. a special high-temperature mass-spectrometry device in combination with a thermo-balance allows a quantitative determination of different volatile species. it enables a differentiation between the primary gas content in the magma and the gas released from decomposition of secondary alteration products. the gas release profiles give the following indications: i) during the littoral explosions of pahoehoe lava the content of volatiles is not changed by interaction with air or sea water; ii) the degassing profiles of vitreous black sand verify the primary content of volatiles in the erupted melt, only co2 was detected; iii) the oxygen release profile gives significant indications for oxygen undersaturation of the erupted magma; iv) remelting of black sand in air at 1450°c for 0.45 h causes an oxygen saturation of the basaltic melt; v) remelting of black sand in argon atmosphere confirms the oxygen undersaturation of the melt; vi) remelting of black sand-black shale mixtures affects a significant change in the degassing profiles, especially in co2-release. with the first investigations we can demonstrate that gas release curves of volcanic rocks are qualified for a) detection of the primary gas content of erupted magma; b) detection of alteration processes of the igneous glass; c) detection of contamination of the magma with adjacent rocks. mailing address: dr. klaus heide, institute of geosciences, university of jena, burgweg 11, d-07749 jena, germany; e-mail: ckh@rz.uni-jena.de 720 klaus heide and christian m. schmidt tures of volatiles. this method has been used with success to solve problems of quality in glass production by uncontrolled formation of bubbles in the melt (heide et al., 2000). it was demonstrated by systematic studies of industrial and laboratory produced specimens that a quantification of volatiles dissolved in industrial glass as well as in the matrix of magmatic rocks is possible. this allows the discrimination of residual volatiles in whole rocks. for individual gasses like water the results are in agreement with other analytical techniques such as sims, ftir, leco or kft (leschik et al., 2004). unlike these volatilespecific techniques degas allows us to determine the amount of all volatiles evolved from the glass or the minerals in a single run. this contribution presents preliminary results of degassing experiments performed on remelted basaltic glasses. the aim of the paper is to show the technical potential of degas in the study of magmatic degassing and to draw the attention of the geological community to the possibilities of our analytical technique. it is well known that the degassing behaviour is related to solubility properties of different volatile species, depending on factors such temperature, pressure and composition. it has been demonstrated that the degassing behavior of a silicate melt in vacuum conditions is a complex process of evolution of different volatile species (heide, 1989; stelzner and heide, 1992). a systematic study of vitreous rims from specific positions in the borehole hsdp2 (hawaiian scientific drilling project ii) by heide and schmidt (2003) gave detailed information about temperature ranges of degassing and the concentrations of volatile species. the degassing rate and the degassing mechanism (formation of bubbles or/and diffusion from the bulk) is different for different species. a new aspect is the phenomenon of oxygen emplacement in the melt. such a decrease of the partial pressure of oxygen during the degassing experiment was frequently observed by degas (heide and schmidt, 2003). the amount of material needed is in the order of 10 to 100 mg, i.e. about the size of synthetic products obtained in experimental petrology (geiger et al., 2001; leschik et al., 2004). fig. 1. black sand beach, kaimu, big island hawaii, u.s.a. 721 determination of volatiles in volcanic rocks – part i: degas-profiles at present we are far from having a complete understanding of the degassing curves, but experiments have shown that the degassing profiles are sensitive regarding the following parameters: i) the melting temperature; ii) the starting materials for the melt, meaning the «raw materials» for the glass forming melt e.g. silicates, carbonates, sulfates, hydroxides, hydrates etc.; iii) the «fining agents» in the melt, meaning polyvalent ions which form oxygen during a redox reaction. in case of natural systems the main process is the reduction of iron oxide (1.1) this knowledge coming from industrial use allows us to ask the questions: – which results can be obtained from the degassing behavior of a natural volcanic glasses or rocks? – which effects result from the smelting of sedimentary rocks from the lithosphere in the extrusive volcanic melt? 2. experimental procedures 2.1. samples and treatment the volcanic glasses studied here were sampled in 1999 from the black sand beaches of kalapana (kaimu), big island, hawaii. the glassy volcanic debris was formed during the littoral explosions of pahoehoe lava flows at kaimu in 1990 and 1991 (fig. 1). as shown in fig. 2 the single black particles consist of a brownish fresh glass with euhedrial crystals of olivine and magnetite. these vitreous fragments are similar to glassy rims on the basaltic lava (heide and schmidt, 2003). .fe o fe o o2 2 1iii ii 2 3 2" + fig. 2. optical micrograph of vitreous particles from the black sand. ol – olivine phenocrysts; m – magnetite; vi – vitreous matrix. table i. chemical composition of hawaiian basaltic lava. tholeiitic hawaiian basalt hawaian shield kaimu black remelted black sand (average, macdonald et al., 1983) basalt (bevens and sand (xrf) (kaimu), vitreous wright, 1992) matrix (edx) sio2 49.4 48 51.0 47.6 tio2 2.5 3 2.5 2.2 al2o3 13.9 14 13.0 11.2 fe2o3 3.0 3 12.8total feo 8.5 8 12.7total mno 0.2 0.1 0.2 0.5 mgo 8.4 7 8.7 15.9 cao 10.3 9 11.0 8.6 na2o 2.1 3 2.2 1.7 k2o 0.4 1 0.4 0.4 p2o5 0.3 0.5 0.2 722 klaus heide and christian m. schmidt the chemical composition of glassy volcanic debris is characterized by tholeiitic lava type (table i). particles between 2 and 6 mm size were separated from the bulk for remelting experiments. a black melt with a volume of 10 cm3 was obtained by heating in an alumina crucible at 1400°c. the melting process was carried out for 0.45 h in an oxidizing (air) or inert (argon) atmosphere. as shown in fig. 3 a characteristic phase separation occurs in the melt. olivine crystals from the starting material were concentrated on the bottom of the crucible. the bulk material is a brownish glass (table i) with inclusions of olivine and pyroxene crystals and magnetite dendrites. olivine and pyroxene crystals show the characteristic spinnifex structure. to demonstrate the influence of a remelting of sedimentary rocks in the basaltic melt 10 wt% of a powdered black shale was added to the starting mixture. 2.2. experimental techniques evolved gas analysis was carried out using a special high-temperature mass-spectrometer (heide et al., 2000). measurements were performed under a vacuum of 10-3 to 10-4 pa using a constant heating rate of 10 k/min from room temperature (rt) to 1450°c. the ion current for different mass/charge ratios (m/z = 1 … 200) was recorded in a multiple ion detection (mid) mode as a function of time by a quadrupole mass-spectrometer (qma-125 balzers ag). temperature was controlled by a type s thermocouple at the bottom of the alumina crucible. the degassing process occurs under highly non-equilibrium conditions. volatile species are rapidly removed from the sample and hence the probability for reverse reactions between the volatiles and the melt can be avoided. furthermore, interactions between the evolved gas species are minimized and a determination of the primary evolved species is possible. for degassing experiments we used carefully separated single particles with a total weight of about 20 mg. in the following the gas release profiles of hydrogen, hydrocarbons (methane), water, carbon monoxide, carbon dioxide, oxygen, hydrogen chloride and sulfur dioxide are discussed. 3. results the degassing profiles of original black sand from kaimu (i), remelting products in oxidizing (ii) and argon atmosphere (iii), and melting products from black sand mixed with 10 wt% black shale in oxidizing (iv) and argon atmosphere (v) are summarized in figs. 4 to 9. h2 (m/z2 = h2+= molecule ion of hydrogen) – samples (i) and (iii) show a hydrogen-release above 1150°c with a broad maximum at 1310°c (fig. 4). sample (ii) shows an increase of hydrogen evolution above 1200°c up to 1450°c without a maximum. samples (iv) and (v) are similar to (i) and (iii). hydrocarbons (fragment ions from ch4+= = 16 like ch3+= m/z = 15) – no significant release of hydrocarbons, here characterized by fragment ion ch3+ (m/z 15) was detectable in any samples (fig. 5). h2o (m/z = 18 = h2o+= molecule ion of water) – only very small quantities of water evolved fig. 3. optical micrograph of remelting product from black sand. melting condition: 1400°c, 0.45 h in air; ol – residual olivine phenocrysts; sol– skeletal olivine 723 determination of volatiles in volcanic rocks – part i: degas-profiles from the original black sand sample (fig. 6). the small spikes at approximately 900°c and 1110°c result from bursting of bubbles. the decrease of partial pressure in the degassing profiles from black sand and black shale mixtures is due to chemical reactions in the melt. co, co2 and n2 (m/z = 28: molecule ions of co and n2 as well as fragment ion of co2 m/z = = 4 4 − 16 = 28) – for determination of co-degassing a more complex analysis of degassing profiles is necessary (schmidt and heide, 2001, fig. 7). the intensity of molecule ion co+ (m/z 28) can be superposed by the molecule ion of nitrogen n2+ (m/z 28) and by the fragment ion co+ from the co2 molecule. by comparison of the degassing profiles of m/z = 14 (fragment ion n+ or co++, fig. 8) with the runs of m/z 28 and m/z 44 (molecule ion of co2, fig. 9) a separation of n2, co and co2 is possible. most of the carbon dioxide evolved from the black sand by bubbling from 1050 up to 1130°c. the degassing behavior is completely changed by remelting in air. carbon dioxide evolved mainly by a diffusion process with a maximum diffusion rate at 1240°c. remelting in inert atmosphere shows a degassing by diffusion and by bubbling. addition of black shale to the black sand shows similar degassing profiles as the samples from inert remelting. only in these mixtures are traces of nitrogen observed in the melt, which escaped by bubbling at approximately 1200°c (see fig. 8). fig. 4. hydrogen (m/z = 2) release profiles from black sand and remelting products. fig. 5. degassing profile of hydrocarbon fragment of methane ch3+(m/z = 15) from black sand and remelting products. 4 5 724 klaus heide and christian m. schmidt o2 (m/z = 32 = molecule ion of 16o2+ and 32s+) – especially interesting are the oxygen release curves (m/z 32). as shown in fig. 10 during degassing of black sand the oxygen partial pressure decreases above 1100°c up to 1300°c. after remelting in air still above 1200°c, oxygen releases from the melt by diffusion with a maximum at 1210 °c. gas release profiles of non-oxidizing melting products are similar to degassing of the natural sample. mixtures with black shales molten in oxidizing as well as non-oxidizing atmospheres show similar trends. the discrimination between oxygen and sulfur is possible using specific fragment ions as well as m/z = 64 for so2+ or m/z = 48 for so+ and by correlation of intensities of molecule ions with fragment ions (more details see in heide, 1982). hcl (m/z = 36 = h35cl+ molecule ion) – there is no indication of hcl-release in the case of the starting material from the beach (fig. 11). the small degassing peak from the in air remelted sample can be explained by a contamination of the sample from the air during the remelting process (2 h) in the laboratory furnace or by an inhomogeneity in the starting material. from the igneous sample hcl does not escape in the range of detection limit (1×10-12 ion current is approximately equivalent to 100 ppb using 50 mg of starting material). so2 (m/z = 64 = molecule ion of so2+ or fragment ion of so3+= m/z 80−16) – no indication fig. 6. water release (m/z = 18) from black sand and remelting products. fig. 7. change of partial pressure of the mass number m/z = 28. 6 7 725 determination of volatiles in volcanic rocks – part i: degas-profiles of so2-release is observed in the original sample from the beach. very small quantities of sulfur oxides (detected as so+ (m/z = 48)) evolved from the remelted black sand samples only by bubbling (fig. 12). the small spikes in the temperature range between 1100°c and 1200°c result from bursting of bubbles. to check the influence of olivine phenocrysts on the gas release, olivine crystals were separated from the glassy matrix. as shown in fig. 13 the described technique allows a simultaneous characterization of escaped gases from the matrix glass as well as from the crystal inclusions. the olivine phenocrystals show a spiky evolution of co2 in the range at 1000°c, which can be explained by the decripitation of fluid inclusions in the xenocrysts. 4. discussion 4.1. gas release from the black sand samples the degassing profiles from the littoral vitreous lava fragments give significant indications that the content of volatiles of magma was not changed by an interaction with air or sea water. from the magma co2 only evolved by bubbling above 1000°c (fig. 9). as shown in fig. 13 small quantities of co2 evolved from olivine as spikes above 1000°c in the same temperature range as from the matrix glass. the formation of fluid inclusions in the olivine crystals and the solution of co2 in the glass forming melt take place in the same temperature – pressure range. this is an important indication that the high temperature degassing fig. 8. change of the partial pressure of the mass number m/z = 14. fig. 9. co2 release of black sand and remelting products. 8 9 726 klaus heide and christian m. schmidt gives valuable information about the interaction of volatiles with the melt during the igneous process. furthermore the main co2 volume evolved from the vitreous matrix in the temperature range between 500 and 600°c. this is an unequivocal indication of a carbonate decomposition. a definite discrimination between carbon dioxide solved in the melt and from secondary carbonates is possible. the non-detectable interaction of erupted lava with air or sea water was confirmed by the oxygen degassing profile. this agrees with the observation by burkhard (2001) that no oxidation of kilauea basalt glass is detectable. furthermore dixon and clague (2001) describe a relatively dry plume component in the basaltic glasses from loihi seamount. in addition, hcl release profiles give no indication for a contamination by sea water. the so2 release is an indication of a subsequent devolatilization of the magma during the eruption process. subaerially erupted hawaiian magmas are characterized by sulfur contents of 100 ppm at a δ 34s < 0. this may indicate loss of total sulfur and δ 34s compared to submarine hawaiian magmas (700 ppm, δ 34s > 0), due to syn-eruptive degassing. this fractionation pattern also indicates that the degassing phase must be oxidized sulfur. so2 commonly is the dominant phase at melt temperatures, although aside from so2 and so3, reduced species may occur (gerlach and graeber, 1985). during the melting of the black sand particles under high vacuum conditions, the affinity 10 11 fig. 10. change of the partial pressure of mass number m/z = 32 (o2+). fig. 11. change of the partial pressure of mass number m/z = 35 (35cl+). 727 determination of volatiles in volcanic rocks – part i: degas-profiles 4.2. gas release from fresh erupted etna lava as fig. 14 demonstrates the fresh erupted subaeric lava from etna (eruption at 31.12.2002 south flank) shows a different oxygen release profile. after a minor decrease of the oxygen partial pressure above 1100°c the oxygen is released from the melt by bubbling. further systematic studies are necessary to confirm this difference between the island-arc and the ocean island basalts. the strong oxygen affinity of the melt of both lavas also causes oxygen incorporation by remelting in air under normal pressure. from the maximum in the gas release curve (fig. 10) a significant change occurring in the oxidation state of the remelted samples can be concluded. the results are confirmed by the remelting experiments under inert atmosphere. from our studies it is clear that the basaltic lava of hawaii volcanism is strongly under-saturated with oxygen. fig. 13. co2 release curves from black sand samples. a – vitreous matrix with olivine crystals; b – vitreous matrix without olivine crystals; c – olivine crystals, separated from the vitreous matrix. fig. 12. change of the partial pressure of mass number m/z = 48 (so+). of the melt to the oxygen concentration in the vacuum furnace is very strong. oxygen was dissolved in the melt and by that the partial pressure of o2 in the furnace decreased. thus the oxygen from the furnace devises was solved in the melt (4.1) from these preliminary results we suggest that the oxygen fugacity in the vacuum of the igneous glass should be lower than fayalite magnetite quartz (fmq)-buffer (see gerlach, 1993). conclusions for the oxygen fugacity of their mantle source require more detailed investigations with materials from experimental petrological studies (leschik et al., 2004). .feo o fe o6 22 2 3in the melt from the sorroundings in the melt"+ 728 klaus heide and christian m. schmidt crust contaminations by pelagic sediments can be excluded from our first results. for a final interpretation further systematic study is necessary. with these first investigations we can demonstrate that gas release curves of volcanic rocks are qualified to i) characterize primary complex gas content of erupted magma in a range of several order of magnitude (between 0.1 ppm and several %); ii) characterize the formation temperature of fluid inclusions in xenocrysts and to determine simultaneous the complex composition; iii) detect and characterize alteration processes of the igneous glass and the secondary phases; iv) detect and characterize contamination of the magma with adjacent rocks; v) of particular interest is the detection of hydrogen as degassing product in a temperature range above 1000°c with a maximum at 1307°c. compared to morb-lava degassing (heide et al., 2000) we do not have indications for an inorganic formation of hydrocarbons (fig. 5). from different studies we know that the pressure, temperature and compositionally-related dependence of volatile solubility is far from being simple (behrens et al., 2001). the solubility of h2o, s, cl, and f increases with increasing sio2-content, these volatiles are thus enriched in residual melts (carroll and holloway, 1994; dixon et al., 1995). while the solubility increases with increasing pressure, it decreases with increasing temperature (williams and mcbirney, 1979), whereby the effect of pressure is minor (behrens and jantos, 2001). surprisingly, oxygen frequently remains neglected in geochemical studies, although it is known from industrial glass melting processes that oxygen plays a crucial role during degassing of silicate melts (jebsen-marwedel and brückner, 1980). chemical equilibriums exist between volatiles and ions with polyvalencies in the melt. different volatile molecules, like co, ch4 or h2, are thus formed depending on oxygen fugacity, temperature and pressure and may be resorbed again in the melt under changing conditions. thus it may be seen that the change in the redox equilibrium with temperature is an important kinetic factor during the degassing process of the melt. reactions with polyvalent ions, especially fe2+, yield oxygen bubbles in the high temperature range. the following reaction could occur between iron-rich olivine (fayalite) and the oxygen in the magma during heating under vacuum conditions in an open system, that means under extreme non-equilibrium conditions (4.2) the oxygen bubbles act like a «fining» agent of a melt for all volatiles; thereby, oxygen effectively promotes the degassing of magma. acknowledgements the authors wish to thank r. moretti and b. scaillet for helpful discussions and review of . fe o sio fe sio o 2 3 3 2 ( ) magnetite in the melt fayalite gas bubbles 3 4 2 2 4&+ + + fig. 14. comparison of the oxygen release from ocean island basalt (hawaii) and island arc basalt (etna 2002). 729 determination of volatiles in volcanic rocks – part i: degas-profiles this report. s. lenk assisted in obtaining the degassing profiles presented in this report. d. schoeps and g. voelksch are thanked for their assistance with xrf and edx analysis. references behrens, h. and n. jantos (2001): the effect of anhydrous composition on water solubility in granitic melt, am. mineral., 86, 14-20. behrens, h., m. meyer, f. holtz, d. benne and m. novak (2001): the effect of alkali ionic radius, temperature and pressure on solubility of water in malsi3o8 melts (m = li, na, k, rb), chem. geol., 174, 275-290. bevens, d. and t.l. wright (1992): hawaii volcanoes national park (hawaii natural history association), pp. 34. burkhard, d.j.m. (2001): crystallization and oxydation of kilauea basalt glass: processes during reheating experiments, j. petrol., 42, 507-527. carroll, m.r. and j.r. holloway (editors) (1994): volatiles in magmas, rev. mineral., 30, pp. 517. dixon, j.e. and d.a. clague (2001): volatiles in basaltic glasses from loihi seamount, hawaii: evidence for a relatively dry plume component, j. petrol., 42, 627-654. dixon, j.e., e.m. stolper and j.r. holoway (1995): an experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids, part i. calibration and solubility models, j. petrol., 36, 911-939. geiger, c.a., n.-s. rahmoun and k. heide (2001): volatiles in cordierite, beih. eur. j. mineral., 13 (1), pp. 60. gerlach, t.m. (1993): oxygen buffering of kilauea volcanic gases and the oxygen fugacity of kilauea basalt, geochim. cosmochim. acta, 57, 795-814. gerlach, t.m. and e.j. graeber (1985): volatile budget of kilauea volcano, nature, 313, 273-277. heide, k. (1982): dynamische thermische analysenmethoden (leipzig, deutscher verlag für grundstoffindustrie). heide, k. (1989): thermogravimetric and mass spectrometric investigation of the decomposition and volatilization of raw materials by heating of a glass batch, j. therm. anal., 35, 305-318. heide, k. and c.m. schmidt (2003):volatiles in vitreous basaltic rims, hsdp 2, big island, hawaii, j. noncryst. solids, 323, 97-103. heide, k., k. gerth and e. hartmann (2000): the detection of an inorganic hydrocarbon formation in silicate melts by means of a direct-coupled-evolved-gas-analysis-system (degas), thermochim. acta, 354 (1-2), 165-172. jebsen-marwedel, h. and r. brückner (1980): glastechnische fabrikationsfehler (springer-verlag), pp. 640. leschik, m., h. behrens, m. wiedenbeck, n. wagner, k. heide, h. geissler, h.g. reinholz and g.-h. frischat (2004): determination of h2o and d2o contents in rhyolitic glasses using kft, nra, ega, ir spectroscopy, and sims, phys. chem. glasses, 45 (4), 238-251. macdonald, g.a., a.t. abbott and fl. peterson (1983): volcanoes in the sea – the geology of hawaii (honolulu, university of hawaii press), pp. 517. schmidt, c.m. and k. heide (2001): thermal analysis of hydrocarbons in paleozoic black shales, j. therm. anal. calorim., 64, 1297-1302. stelzner, t. and k. heide (1992): degassing behavior of optical glasses, glastechn. ber., 65, 150-156. williams, h. and a.r. mcbirney (1979): volcanology (san francisco, freeman, cooper), pp. 397. annals 47, 1, 2004, 01/07def 93 annals of geophysics, vol. 47, n. 1, february 2004 mailing address: dr. anatoly n. kamshilin, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: kamshilin@ifz.ru key words self-oscillations – mechanoelectric self-oscillator – transformations – rocks 1. introduction it is known that a geological medium which has large resources of energy is a complicated multiphase system in which transformations and interactions of geophysical fields of different origin proceed. there are experimentally detected parametrical, resonance, non-linear properties of rocks. a strong response to weak impacts was observed (nikolaev and vereschagina, 1991; tarasov and tarasova, 1995). the recently detected activation of seismic regime in seismoactive zones after powerful artificial electromagnetic pulses and geomagnetic storms (tarasov et al., 1999; tarasov and tarasova, 2002) should be especially noticed. the investigation of relations between geophysical fields and the investigation of properties of the lithosphere, as a converter of energy, as well as the possibility to use this knowledge in practice have aroused increasing interest. this paper is dedicated to these problems. 2. conditions of experiment figure 1 shows the skeleton diagram of a usual laboratory device for the investigation of mechanoelectric phenomena in rocks. the electric generator (1) is connected to the piezoelectric transducer (2), which generates acoustic signal in the sample (3). the energy of the acoustic oscillations is transformed in the sample into the energy of the electric oscillations. electrodes (4) receive electric signals, which are amplified by the amplifier (5) and directed to the recorder (6). the authors designed the device, the skeleton diagram of which is shown in fig. 2. the signal from the electrodes (3) is directed to the special electronic unit (4). from the output of the electronic unit the signal is directed to the input of the piezoelectric transducer (1). the set-up of the components of the electronic unit allows for a positive feedback in a closed circuit and provides the conditions of stimulation of the self-oscillations. the self-oscillations arise because of the mechanoelectric transformations in rock samples and the positive feedback and not because of an external forcing oscillator. the authors self-oscillations in rocks, results of laboratory experiments anatoly n. kamshilin (1), elena n. volkova (1), oleg r. kuzichkin (2) and mikhail a. sokolnikov (2) (1) schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia (2) muromsky institute, vladimir region, russia abstract the method of generation of self-oscillations in rocks is developed here. self-oscillations arise as a result of direct and inverse mechanoelectric transformations without an external generator. laboratory experiments were executed with different samples. a relation between self-oscillation parameters from samples’ humidity and direct electrical field applied to samples was detected. 94 anatoly n. kamshilin, elena n. volkova, oleg r. kuzichkin and mikhail a. sokolnikov fig. 1. the skeleton diagram of the laboratory device for the investigation of mechanoelectric phenomena in rocks: 1 – electric generator; 2 – piezoelectric transducer; 3 – sample; 4 – electrodes; 5 – amplifier; 6 – recorder. fig. 2. the skeleton diagram of the laboratory device for the investigation of self-oscillations: 1 – piezoelectric transducer; 2 – sample; 3 – electrodes; 4 – electronic unit; 5 – recorder. fig. 3. the scheme of the self-oscillator (mso) of the first type (s-e): 1 – sample (box with sand); 2 – measuring transducer; 3 – emitting piezoelectric transducer; 4 – electronic unit; 5 – recording device; 6 – additional electrodes; 7 – measuring electrodes; 8 – source of constant voltage. 95 self-oscillations in rocks, results of laboratory experiments have termed the device the mechanoelectric self-oscillator (mso). various materials were used as samples in the experiments: sand, clay, loam, concrete, bricks. in this paper, some results of the experiments with sand samples are presented. the sample was made in the form of a plastic box (450 × 140 × 180 mm) filled with 19 kg of sand. a piezoelectric transducer was located on the sand surface. the resonant frequency of the transducer was 30 khz. the system of electrodes was located inside the sample’s body. two types of mechanoelectric self-oscillators were investigated. in the self-oscillator of the first type (fig. 3) a transformation of acoustic oscillations (s) into electric oscillations (e) takes place. in the self-oscillator of the second type (fig. 4) a transformation of electric oscillations into acoustic oscillations takes place. the piezoelectric transducer (3) in the first device functions as an emitter. measuring electrodes (7) are connected to the input of the electronic unit (4). the signal from the measuring electrodes is amplified by the electronic unit and is then directed to the input of the piezoelectric transducer. additional electrodes (6) are connected to the constant voltage source (8). the recording device (5) records two signals: the signal ue ∗ k from the output of the preamplifier of the electronic unit (4) and the signal us from the output of the measuring piezoelectric transducer (2). ue denotes a potential difference between the measuring electrodes (7), k denotes the amplification factor of the preamplifier. us corresponds to the displacement of the sample surface in the relative units. the analog-digital converter l-761 of the «l-card» russian corporation (14 bits, signal processor ad-2184, analog devices inc.) was utilized as the recording device. fig. 4. the scheme of the self-oscillator (mso) of the second type (e-s): 1 – sample (box with sand); 2 – measuring piezoelectric transducer; 3 – piezoelectric transducer-receiver; 4 – electronic unit; 5 – recording device; 6 – emitting electrodes. 96 anatoly n. kamshilin, elena n. volkova, oleg r. kuzichkin and mikhail a. sokolnikov fig. 5a,b. mso of the first type (e = 0): a) electric (ue) and acoustic (us) signals; b) spectra of the signals ue and us. the distinction between the first and the second devices (fig. 4) is that the piezoelectric transducer functions as the receiver of acoustic signals. the electric output of the piezoelectric transducer (3) is connected to the input of the electronic unit (4). the output of the electronic unit is connected to the emitting electrodes (6). ue denotes the potential difference between the emitting electrodes (6). 3. results of the experiments 1) the experimental results for the mso of the first type (fig. 3, transformation s-e) are shown in figs. 5a,b, 6a,b, 7a,b and 8a,b. electric and acoustic signals, generated in the sample during self-oscillations, are shown in figs. 5a, 6a, 7a and 8a. the corresponding spectra are shown in figs. 5b, 6b, 7b and 8b. figures 5a,b and 6a,b illustrate the relation between the parameters of self-oscillations and the value of the external direct electric field e. figure 5a,b: e = 0, fig. 6a,b: e = 9 v/cm. change in the phase shift between the signals us and ue by almost 180° and change in the self-oscillations frequency by 680 hz is caused by the influence of e. it is possible to observe a small decrease of the oscillations amplitude and the q factor, comparing figs. 5b and 6b. figures 7a,b and 8a,b show a relation between the parameters of self-oscillations and the sample humidity. figure 7a,b corresponds to the room-dry sample; fig. 8a,b corresponds to the sample, whose humidity is increased by 3%. the phase shift between ue and us is changed by several tens of degrees. the frequency of self-oscillations is changed by 290 hz. the amplitude of oscillations is decreased to half the value of the room dry sample. 2) results, obtained with the use of the mso of the second type (fig. 4, transformation e-s) are shown in fig. 9a-d. a self-oscillations mode was generated in the dry sample (fig. 9a). after that 100 ml of water was poured onto the surface of the sama b 97 self-oscillations in rocks, results of laboratory experiments fig. 6a,b. mso of the first type (e = 9 v/cm): a) electric (ue) and acoustic (us) signals; b) spectra of the signals ue and us. fig. 7a,b. mso of the first type, dry sample: a) electric (ue) and acoustic (us) signals; b) spectra of the signals ue and us. a b a b 98 anatoly n. kamshilin, elena n. volkova, oleg r. kuzichkin and mikhail a. sokolnikov fig. 9a. mso of the second type. change in selfoscillations after sample damping. measurements were executed with 30-40 s intervals: dry sample, electric (ue) and acoustic (us) signals. fig. 8a,b. mso of the first type, sample’s humidity is increased by 3%: a) electric (ue) and acoustic (us) signals; b) spectra of the signals ue and us. ple and three more measurements were taken (fig. 9b-d). all measurements were executed with 30-40 s intervals. comparison of the presented oscillograms traces the process disturbing the self-oscillations mode as a result of sample damping. oscillations become unstable, and at the end of the experiment, the amplitudes of oscillations in both channels have decreased by approximately one order of magnitude. 4. discussion only part of the obtained results are presented in the paper, other results are now being processed. the presented material allows the following conclusions. 1) self-oscillations based on direct and inverse mechanoelectric transformations could be created in rocks. the sample of rock is the frequency defining element; frequency of self-oscillations, q factor and amplitude are determined by the properties of the sample (object). a b b c d 99 self-oscillations in rocks, results of laboratory experiments external influence (direct electric field) also changes the parameters of self-oscillations. this result is indirectly confirmed by a relation between seismoelectric transformations and direct electric field, as was detected earlier (cherniak, 1987). 2) a change in condition of the sample causes a change in phase shift between electric and acoustic signals in the sample. 3) preliminary results show that mechanoelectric self-oscillators of the first and second type have different sensitivity to changes in condition of the samples. 5. conclusions investigation of the self-oscillation processes in rocks in general, and processes based on mechanoelectric transformations in particular, are interesting from the point of view of both fundamental and applied science. there are some possible directions: 1) development of new methods of rock investigation as mechanoelectrical transformer, as well as development of methods of investigation of different physical properties of rocks (material composition, humidity, fluid quality, porosity, anisotropy, etc.). 2) development of a phenomenological model of the mechanism of earthquake initiation under impacts of different origin. 3) natural modeling will open ways of development of new methods of control condition and purposeful influence on rocks and other objects. acknowledgements the authors are sincerely grateful to prof. s.d. vinogradov, dr. c. anastasiadis, dr. a.b. fig. 9b-d. mso of the second type. change in selfoscillations after sample damping. measurements were executed with 30-40 s intervals: damped sample, electric (ue) and acoustic (us) signals. 100 anatoly n. kamshilin, elena n. volkova, oleg r. kuzichkin and mikhail a. sokolnikov maksimov, and mr. v.a. terentjev for discussion and for their valuable comments on the essence of the work. the investigation was executed with the support of grant no. 304 of competition-expertise no. 6 of young scientists of ras. references chernyak, g.v. (1987): electromagnetic methods in hydrology and engineering geology (nedra, moscow), 166-167 (in russian). nikolaev, a.v. and g.m. vereschagina (1991): on initiation of earthquakes by underground nuclear explosions, dan, 319 (2), 333-336 (in russian). tarasov, n.t. and n.v. tarasova (1995): influence of nuclear explosions on seismic regime, rep. russ. acad. sci., 343 (4), 543-546 (in russian). tarasov, n.t. and n.v. tarasova (2002): the effect of geomagnetic storms on the seismicity, in proceedings of the third international workshop on magnetic, electric and electromagnetic methods in seismology and volcanology (schmidt united institute of the earth, russian academy of sciences, moscow), 206-208. tarasov, n.t., n.v. tarasova, a.a. avagimov and v.a. zeigarnik (1999): influence of powerful electromagnetic pulses on seismicity of central asia and kazakhstan, volcanol. seismol., 4-5, 152-160 (in russian). popova_correctedok:layout 6 annals of geophysics, 56, 3, 2013, r0328; doi:10.4401/ag-6224 r0328 neural network approach to the prediction of seismic events based on low-frequency signal monitoring of the kuril-kamchatka and japanese regions irina popova1,*, alexander rozhnoi1, maria solovieva1, boris levin2, masashi hayakawa3, yasuhide hobara3, pier francesco biagi4, konrad schwingenschuh5 1 institute of physics of the earth, ras, moscow, russia 2 institute of marine geology and geophysics, ras, yuzhno-sakhalinsk, russia 3 university of electro-communications, chofu, tokyo, japan 4 university of bari, department of physics, bari, italy 5 space research institute, austrian academy of sciences, graz, austria abstract very-low-frequency/ low-frequency (vlf/lf) sub-ionospheric radiowave monitoring has been widely used in recent years to analyze earthquake preparatory processes. the connection between earthquakes with m ≥5.5 and nighttime disturbances of signal amplitude and phase has been established. thus, it is possible to use nighttime anomalies of vlf/lf signals as earthquake precursors. here, we propose a method for estimation of the vlf/lf signal sensitivity to seismic processes using a neural network approach. we apply the error back-propagation technique based on a three-level perceptron to predict a seismic event. the back-propagation technique involves two main stages to solve the problem; namely, network training, and recognition (the prediction itself ). to train a neural network, we first create a so-called ‘training set’. the ‘teacher’ specifies the correspondence between the chosen input and the output data. in the present case, a representative database includes both the lf data received over three years of monitoring at the station in petropavlovsk-kamchatsky (2005-2007), and the seismicity parameters of the kuril-kamchatka and japanese regions. at the first stage, the neural network established the relationship between the characteristic features of the lf signal (the mean and dispersion of a phase and an amplitude at nighttime for a few days before a seismic event) and the corresponding level of correlation with a seismic event, or the absence of a seismic event. for the second stage, the trained neural network was applied to predict seismic events from the lf data using twelve time intervals in 2004, 2005, 2006 and 2007. the results of the prediction are discussed. 1. introduction low-frequency (lf) signals (range, 10-50 khz) propagate between the earth and the ionosphere as in a spherical waveguide. the bottom boundary of the waveguide is the earth, and the top boundary is the lowest part of the ionosphere. the propagation of low-frequency signals is determined on the one hand by the electrical conductivity of the earth surface, and on the other hand by the conductivity of the lower ionosphere and upper atmosphere. the analysis of the behavior of the amplitude and phase signals from verylow-frequency (vlf)/lf transmitters have shown the possibility for their use as precursors of earthquakes. a night disturbance of the signal amplitude and phase for the long paths has been observed before several strong earthquakes, as described by gokhberg et al. [1987, 1989] and gufeld et al. [1992] over two decades ago. more recently, the changes in the position in minima of the phase and amplitude daily variations during sunset and sunrise for a few days before strong earthquakes in japan were describe [hayakawa et al. 1996, molchanov and hayakawa 1998]. the usefulness of the sub-ionospheric vlf/lf signal propagation method for the detection of seismoionospheric perturbations from observations of ground stations has been demonstrated recently in japan [maekawa et al. 2006, muto et al. 2009, hayakawa et al. 2010], italy [biagi et al. 2007, 2008] and russia [rozhnoi et al. 2006, 2007a, 2009]. this method was used for the analysis of both ground-based transmitter signals detected onboard the demeter satellite above seismic regions, and ground observations [muto et al. 2008, rozhnoi et article history received october 9, 2012; accepted april 23, 2013. subject classification: wave propagation, seismic risk. al. 2007b, solovieva et al. 2009, rozhnoi et al. 2012]. a statistical analysis was realized by rozhnoi et al. [2004] for the purpose of determining the sensitivity threshold of lf signals to the magnitude of an earthquake, and unearthing probable periods of observation of anomalies caused by seismic activity. they showed that the sensitivity of the lf signal to seismic processes becomes apparent for m ≥5.5. the signal anomalies for earthquakes with such magnitudes were observed in 20% to 25% of cases. in this study, we propose a method for the estimation of the lf signal sensitivity to seismic processes using a neural network approach. the trained neural network is applied in forecast mode for the automatic detection of abnormal changes in the signal, relating to seismic activity above a certain threshold. 2. methods historically, the first successful applications of the neural network method were implemented for a pattern-recognition problem; namely, the problems of recognition of printed text, image compression, and image recognition in the field of computer vision. eventually, the properties of neural networks proved to be useful to other areas of knowledge. the essential difference between traditional computing and neurocomputing is that neural networks can produce their own rules from incomplete and noisy data. when it is hard to find a traditional algorithm for the solution to a problem, the ability of a neural network to extract the ‘rules of exit’, to effectively solve nonlinear tasks, and to perform interpolation and extrapolation of an available database can be helpful for many tasks in geophysics. an excellent review of neural network paradigms and a detailed analysis of their application to various geophysical problems was given by poulton [2002]. neural network methods have been used in geoelectrics for inversion of electromagnetic data in threedimensional geoelectric structures [spichak and popova 2000]. interpretation of multidimensional geophysical data generated during a geological exploration was carried out using the neural network method known as self-organizing mapping [klose 2006]. a neural network application for seismic data processing was developed for first-break peaking and trace editing [mccormack et al. 1993]. neural network technology has been used in various fields of geophysics for parameter estimation, filtering, classification and prediction. a backpropagated and associative neural network was applied to predict magnitudes of earthquakes from seismic network signals, electric preseismic signals, and average magnitudes of previous earthquakes [dutta 2011]. the variation of the geomagnetic field declination, horizontal component, hourly relative humidity, ground temperature, rain rate per day, mean number of rainy hours per day, and ground temperature, were used to predict the magnitude of an earthquake two days before its occurrence by means of a neural network [suratgar et al. 2008]. we applied the back-propagation technique [rumelhart and mcclelland 1986], based on the three-layer perceptron to estimate the sensitivity of the vlf/lf signal to seismic processes, as illustrated on figure 1. this type of a neural network is known as ‘supervised’. this involves two main stages of solving a problem: the training of the network, and the recognition (the prediction itself ). in the supervised scheme of teaching, the network is taught the relationship between the input and output pairs, which is called the training set. to train the neural network, we created a teaching database that included both the catalog of seismic events from 2005 to 2007 and the corresponding data (amplitude and phase of the lf signals), measured in the regime of monitoring at the receiving station in petropavlovsk-kamchatsky from the japanese transmitter jjy (see figure 2). the seismic events were excluded from the database for the days when the index of the magnetic field activity, dst, and the flux of relativistic electrons exceeded the given thresholds. the optimal properties for the formation of the teaching database were derived after many experiments on teaching and testing of the neural networks. as a result, the training samples included the features calculated from the amplitudes and phases of the signals, popova et al. 2 figure 1. three layer perceptron of back-propagation neural network applied to prediction of seismic events. the input signal represents the mean and dispersion values of the phases (mph,t-n and dph,t-n, respectively) and amplitudes (ma,t-n and da,t-n, respectively) in the nighttime for five days before the seismic event (n varies from 1 to 5). the corresponding level of correlation with the seismic event ct is used as the output data. 3 which were measured for five days before 40 seismic events of m ≥5.5 that occurred at a depth (h) of less than 150 km. the ratio of the radius (r) of the zone of precursors displayed to the distance (d) of the epicenter from the axis of the propagation path between the transmitter and receiver (r/d) was selected as >0.7, because the preparation zone intersected the sensitivity zone in this case. thus, the preparation activity can influence the lf signal propagation. the radius of the zone of the precursor display was calculated using the relationship r = 100.43m, where m is the magnitude of an earthquake [dobrovolsky et al. 1979]. the teaching database also included 40 examples of a lack of seismic events, because the neural network had to learn to distinguish between the seismic events of m ≥5.5 and their absence. thus, the total teaching database contained 80 examples. each example contained the input and output data for the teaching of the neural network. below, we refer to the seismic events of m ≥5.5 as the ‘seismic event’, and to the rest as a ‘lack of the seismic event’. the mean and dispersion values of the phases and amplitudes in the nighttime for five days before the seismic event (or the lack of it) were used as the input data. thus, the number of the input neurons in the threelayer perceptron was 20. the mean and dispersion values of the amplitude are marked in figure 1 as ma,k and da,k, and those of the phase, as mph,k and dph,k, respectively. the index k refers to the number of days before the seismic event (or lack of it) and this varies from t-1 to t-5, where t is the number of the day of a seismic event (or lack of it). the input signal x consisted of these values. the corresponding correlation level with the seismic event (where the correlation value was 1) or with a lack of the seismic event (where the correlation value was 0) were used as output data u. thus, the value of the output neurons in the three-layer perceptron was equal to unity. the level of correlation is marked as ct on figure 1. after the preparation of the input-output pair, the neural network was trained using 80 examples of the teaching database. the input signal x that is represented by the means and dispersions of the phases and amplitudes in the nighttime for five days, propagated forward from one layer to the next layer. thereby, each neuron i of a next layer received the total signal from all of the neurons j of a previous layer: (1) where ui l is the output signal of neuron i of layer l, g is the neuron response function (e.g., hyperbolic tangent), wi j l are the weight connections between the neurons of layers l 1 and l, and xj is the value of the neuron j of the layer l -1. at the training state, we must obtain the output signal ui on the third layer that minimizes the total standard error: (2) the summation was carried out for each training example p over all of the neurons i of the output layer. the ‘target’ value ui t represented the sample value of the correlation coefficient for the corresponding training example. the real value ui represented the value of the output neuron formed as a result of signal propagation (equation 1). weight connections w1 and w2 between the layers of the neural network were the parameters that defined the value of an error (equation 2). therefore, the essence of the teaching process was the search for the weight connections wij for the error minimization. the weights were assigned random values within a certain range at the beginning of the teaching procedure. the teaching procedure was based on the gradient descent technique of the error minimization (equation 2) between the target values of the outputs specified by the ‘teacher’ and those produced by the neural network: (3) where dwij (n) is the increment of the weight connection at step n, dwij (n −1) is its increment at the previous step, and a and b are internal parameters of the neural network. this procedure was fulfilled for all of the teaching examples, and finished when it reached the neural network prediction of earthquakes figure 2. map showing the position of the receiver petropavlovskkamchatsky (ptk) and transmitter jjy (40 khz). solid circles, positions of the earthquake epicenters with m ≥5.5, d ≤150 km and r/d ≥0.7 for the period 2004-2007 (from usgs/neic http://neic. usgs.gov/neis/epic/epic_global.html); ellipse, projection of the fifth fresnel sensitivity zone on the earth surface. ,w w er w( ) ( 1)ij n ij ij n 2 2 a bd d=+ .er u ui i t ip 2= -^ h// ,u g w xi l ij l j j = ` j/ accuracy threshold, eps, such that er <eps. the procedure of the recognition (prediction) uses the interpolation and extrapolation properties of the neural network. unlike the training procedure that requires many steps of the iteration process, the prediction involves only one passage of the recognizable signal from input to output and uses the weight connections specified at the teaching process. the recognition procedure is performed quickly. the final result formed at the output can be treated as a correlation level with the seismic event, or lack of it. the procedure of the recognition (prediction) was performed as follows. we chose the five days before the day of the seismic event and formed the input vector for the neural network. its dimension was equal to the number of input neurons of the trained neural network (there were 20 input neurons in our case). we obtained the value of the correlation level for the day of the seismic event (sixth day) at the network output after one passage of the input signal from the input to the output of the trained neural network. this value can vary from zero to unity. if the correlation level is more than 0.5, a seismic event of m ≥5.5 is possible. we can consider not only one day of a seismic event, but also the time interval, which includes the day of the seismic event, since we can determine the behavior of the correlation level a few days before the seismic event. therefore, we should form the input vector from the five days before the first day of the time interval and recognize the corresponding correlation level for the first day of the interval using the trained neural network. afterwards, we formed the next input vector from the five days before the second day of the interval, and also recognize the corresponding correlation level for the second day of the interval. thus, we used a sliding window of five days. the recognition (prediction) procedure was performed step by step with a shift of one day for all of the days of the chosen time interval. 3. results we chose 12 time intervals in 2004, 2005, 2006 and 2007 to predict a seismic event from the lf data. the corresponding seismic events were not used in the training set for the teaching of the neural network; therefore, they can be applied to the prediction. the time intervals varied from 6 days to 8 days, including the days of the seismic events. table 1 shows the corresponding seismic events of m ≥5.5. the data in table 1 were taken from an earthquake catalog (http://earthquake.usgs. gov/earthquakes/eqarchives/epic/). table 1 contains the following parameters: the year, month, day and time of the seismic event, the latitude and longitude location of the earthquake, the depth of the epicenter position, the magnitude m, the distance d from the epicenter of the earthquake to the axis of the ‘transmitter–receiver’ line, and the ratio r/d. we suggest that neural network indicates changes in the lf signal due to an earthquake of m ≥5.5 if the values of the correlation coefficient recognized by the network are >0.5 for several days in a row before the earthquake. for six of the 12 time intervals (see table 1 for the seismic events numbered from 1 to 6), the neural network detected changes in the lf signal for several days in a row (2-3 days) before an earthquake of m ≥5.5 and on the day itself. these examples are shown in figure 3. for the next three time intervals (see table 1 for popova et al. 4 n year month day time latitude (°) longitude (°) depth (km) m d (km) r/d 1 2006 9 28 1:36 46.5 153.3 11 5.9 154.7 2.22 2 2004 9 13 3:0 44 151.4 8 6.1 223.7 1.87 3 2004 7 21 0:11 40.9 143.1 30 5.5 127.9 1.81 4 2007 1 11 20:34 43.5 147.1 10 5.5 45.7 5.07 5 2005 3 11 18:47 43.1 144.7 54 5.5 187.7 1.23 6 2005 3 16 13:23 43.5 146.9 39 5.6 59.4 4.3 7 2004 5 29 3:47 37.7 141.9 29 5.8 51.9 5.9 8 2005 8 1 4:40 46.9 153.9 16 5.7 156.8 1.8 9 2004 11 2 13:4 38.8 142.8 23 5.6 26.1 9.8 10 2004 11 26 22:42 42.4 142.9 58 5.7 257.8 1.1 11 2004 5 6 13:43 42.5 145.0 28 5.6 116.8 2.2 12 2004 7 8 10:30 47.2 151.3 128 6.4 49.4 11.4 table 1. the 12 seismic events of m ≥5.5 for considered twelve time intervals in 2004, 2005, 2006 and 2007. 5 the seismic events numbered from 7 to 9), the neural network detected changes in a signal that indicated an earthquake on the second or third day before the earthquake, excluding the day itself. these results can also be considered as positive. for the remaining three time intervals (see table 1 for the seismic events from 10 to 12), there were no correlations between the seismic events of m ≥5.5 and changes in the signal. we discuss the results of the prediction on the example presented in figure 3a. the magnitudes of the earthquakes that occurred during the period of analysis are shown in the upper section of figure 3a. if a magnitude is ≥5.5, the following parameters are given for the corresponding column: magnitude m, depth h, distance d from the epicenter of the earthquake to the axis of the transmitter–receiver line, and the ratio r/d. the dashed line represents the threshold at which m ≥5.5. the results of the prediction from january 6-11, 2007, are shown on the bottom section of figure 3a. these results are formed as the output (single neuron) of the previously trained neural network. this output represents the correlation coefficient. in this way, we find the degree of the correlation with a seismic event of m ≥5.5. a procedure for recognition (prediction) is performed step by step with a shift in one day, up to the day of the seismic event of m ≥5.5. the dashed line in figure 3a represents the threshold value of the correlation coefficient of 0.5. one can see from the bottom section of figure 3a that the correlation coefficients are more than 0.5 for two days in a row before the earthquake and on the day of seismic event itself. such behavior of the correlation coefficient indicates an earthquake of m ≥5.5. 4. discussion and conclusions for nine of the twelve time intervals, the neural network successfully recognized changes in the lf signal that indicated an earthquake of m ≥5.5 a few days before the earthquake. these results confirm that shortterm prediction of seismic events based on changes in the lf signal is possible. a mean value and dispersion calculated from the amplitudes and phases of signals for the night period can be considered as indicators of seismic events. the lf signal sensitivity to seismic processes is seen for seismic events of m ≥5.5 that occur at a depth of <150 km. in addition, the ratio r/d should be >0.7. as an outlook, we suggest the use of several wave paths as input data of a neural network, as well as the additional output parameters, such as a magnitude value and a forecast probability. acknowledgements. the study was supported by the russian foundation for basic research, under grant no. 11-05-00155-a. data and sharing resources – the earthquake catalog used in this study can be found at: http://earthquake.usgs.gov/earthquakes/eqarchives/ epic/ – dst data were taken from the site: http://wdc.kugi.kyoto-u.ac.jp/dstdir/index.html the electron fluxes are on the site: http://www.swpc.noaa.gov/ftpmenu/lists/particle.html neural network prediction of earthquakes figure 3. the neural network prediction. (a) result of prediction for january 6-11, 2007. (b) result of prediction for july 16-23, 2004. (c) result of prediction for september 8-14, 2004. each column on the upper section represents the magnitude of the earthquakes that occurred on certain days of the time period considered. if m ≥5.5, the corresponding column is marked with the following parameters: magnitude m, depth h, distance d from the epicenter of the earthquake to the axis of the ‘transmitter–receiver’ line, and the ratio r/d. dashed line, threshold at which m ≥5.5. the corresponding values of the correlation coefficients are presented in the bottom section: dashed line, threshold of the correlation coefficient of 0.5. references biagi, p.f., l. castellana, t. maggipinto, g. maggipinto, a. minafra, a. ermini, v. capozzi, g. perna, m. solovieva, a. rozhnoi, o.a. molchanov and m. hayakawa (2007). decrease in the electric intensity of vlf/lf radio signals and possible connections, nat. hazards earth syst. sci., 7, 423-430. biagi, p.f., l. castellana, t. maggipinto, d. loiacono, v. augelli, l. schiavulli, a. ermini, v. capozzi, m.s. solovieva, a.a. rozhnoi, o.a. molchanov and m. hayakawa (2008). disturbances in a vlf radio signal prior the m=4.7 offshore anzio (central italy) earthquake on 22 august 2005, nat. hazards earth syst. sci., 8, 1041-1048. dobrovolsky, i.r., s.i. zubkov and v.i. myachkin (1979). estimation of the size of earthquake preparation zones, pure appl. geophys., 117, 1025-1044. dutta, a.k. 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seismo-ionosphere perturbations revealed by vlf signals collected on ground and on a satellite, nat. hazard earth syst. sci., 7, 617-624. rozhnoi, a., m. solovieva, o. molchanov, k. schwingenschuh, m. boudjada, p.f. biagi, t. maggipinto, l. castellana, a. ermini and m. hayakawa (2009). anomalies in vlf radio signals prior the abruzzo earthquake (m=6.3) on 6 april 2009, nat. hazard earth syst. sci., 9, 1727-1732. rozhnoi, a., m. solovieva, m. parrot, m. hayakawa, p.f. biagi and k. schwingenschuh (2012). ionospheric turbulence from ground-based and satellite popova et al. 6 7 vlf/lf transmitter signal observations for the simushir earthquake (november 15, 2006), annals of geophysics, 55 (1), 187-192. rumelhart, d., and j. mcclelland (1986). parallel distributed processing: explorations in the microstructure of cognition: mit press. solovieva, m.s., a.a. rozhnoi and o.a. molchanov (2009). variations in the parameters of vlf signals on the demeter satellite during the periods of seismic activity, geomagn. aeron., 49, 532-541. spichak, v., and i. popova (2000). artificial neural network inversion of magnetotelluric data in terms of three-dimensional earth macroparameters, geophys. j. int., 142, 15-26. suratgar, a.a., f. setoudeh, a.h. salemi and a. negarestani (2008). magnitude of earthquake prediction using neural network, natural computation, icnc ’08. fourth international conference, 2008, 2, 448-452. *corresponding author: irina popova, iinstitute of physics of the earth, ras, moscow, russia; email: ipopova@trtk.ru. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. neural network prediction of earthquakes << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice de michelis 661_670 661 annals of geophysics, vol. 46, n. 4, august 2003 some new approaches to the study of the earth’s magnetic fi eld reversals paola de michelis (1) and giuseppe consolini (2) (1) istituto nazionale di geofi sica e vulcanologia, roma, italy (2) istituto di fisica dello spazio interplanetario, cnr, roma, italy abstract paleomagnetic studies clearly show that the polarity of the magnetic fi eld has been subject to reversals. it is generally assumed that polarity intervals are exponentially distributed. here, the geomagnetic polarity reversal record, for the past 166 ma, is analysed and a new approach to the study of the reversals is presented. in detail, the occurrence of 1/ƒ-noise in the power spectral density (psd), relative to geomagnetic fi eld reversals, the existence of a zipf’s law behaviour for the cumulative distribution of polarity intervals, and the occurrence of punctuated equilibrium, as shown by a sort of devil’s staircase for the reversal time series, are investigated. our results give a preliminary picture of the dynamical state of the geomagnetic dynamo suggesting that the geodynamo works in a marginally stable out-of-equilibrium confi guration, and that polarity reversals are equivalent to a sort of phase transition between two metastable states. mailing address: dr. paola de michelis, istituto nazionale di geofi sica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: demichelis@ingv.it key words earth's magnetic fi eld geomagnetic reversals time series analysis 1. introduction at the beginning of the last century, the french scientists p. david and b. brunhes made the earliest demonstration that the geomagnetic fi eld has changed its polarity in the past. the idea that geomagnetic polarity could change was controversial, and for many years sceptics sought alternative interpretations. the hypothesis became widely accepted only in the early 60s and we can say that this is probably the major phenomenological discovery of the twentieth century in the fi eld of paleomagnetism. however, geomagnetic polarity reversal remains one of nature’s most enigmatic phenomena. from a theoretical point of view the possibility that the geomagnetic fi eld could change its polarity was well known. indeed, the magnetohydrodynamic description of the geodynamo is insensitive to the sign of the magnetic fi eld: i.e. the induction equation is invariant under magnetic fi eld parity transformation (b æ – b). therefore, the geodynamo can have a polarity opposite to the directly observable normal polarity as an equally viable solution. if a mechanism for transition between these two states exists, we should expect normal and reversed polarity states with equal probability. from dynamo theory we can conclude that the geodynamo appears to operate in two regimes, one in which reversals occur and one in which they do not. although the conditions that give rise to these two regimes are not completely understood, it is known that the reversal sequence of the earth’s magnetic fi eld contains information about earth’s core processes. in particular, from the analysis of the length distribution of the polarity intervals it is possible to try to model the reversal mechanism while from the analysis of the rate of occurrence 662 paola de michelis and giuseppe consolini of these reversals it is possible to try to gain information on gradually changing conditions at the core-mantle boundary. indeed, it is generally accepted that changes in the frequency of fi eld reversals refl ect the dynamic evolution of the fl uid core (jones, 1977; mcfadden and merrill, 1984) and that the analysis of long-term trends in geomagnetic reversal frequency might provide important information on core-mantle coupling (e.g., mcfadden and merrill, 1984, 1986; courtillot and besse, 1987; gubbins, 1987; marzocchi et al., 1992). thus it is important to analyse the reversal sequence and to extract all the available information. modern reversal chronologies show more than 300 successful reversals in the past 166 ma. common estimates for the duration of a polarity transition are around a few thousand years (1000-8000 years). however, remarkably different values for the duration of a magnetic fi eld reversal are often advocated even if there are reasons to think that these values are artefacts. in geological terms the duration of a polarity transition is a very short time and is short relative to the typical length of polarity interval. so, in fi rst approximation, it is possible to think of a reversal as a rapid, almost instantaneous event. this then naturally leads to the concept that the reversal process is a renewal process. this idea was originally suggested by cox (1968) who was the fi rst to notice the irregular lengths of stable polarity intervals and to suggest that one might model reversals as a renewal process triggered by instabilities in fl uid motions in earth’s outer core. in the hypothesis in which the reversals are generated by a renewal process, the duration of polarity intervals must be independent and consequently the reversal process must be a poisson process, i.e. a process in which the probability of a future event does not depend on the time since the last event. under the poisson model, one expects that although the lengths of individual stable polarity intervals are highly variable, there is a characteristic rate of reversal if the process is averaged over a suffi cient time interval. however, the observed distribution of reversals exhibits two departures from the basic poisson model. firstly, there is a paucity of short intervals in the magnetostratigraphic records, which could be a consequence both of our inability to detect very short intervals and an inability of the core to generate new reversals during some short time interval immediately following a transition (mcfadden and merrill, 1993). in the second place, the occurrence of geomagnetic reversals appears to be time-dependent. the average rate at which reversals occur has varied signifi cantly over time scales on the order of 108 years. in particular, the reversal rate appears to have decreased from just over 3 per million years at about 160 ma ago down to zero at about 118 ma ago and then increased from zero at about 83 ma ago to around 5 per million years in recent times (mcfadden and merrill, 1997). this could mean that the geomagnetic reversal process is a non-stationary process at least over such time scales. it is important to emphasize that only the existence of this nonstationarity, not its details, has been well established. indeed, the origin of the nonstationarity in reversal rate has been interpreted in different ways in the past decade. the nonstationarity could be, for example, a manifestation of deterministic chaos within the geodynamo or a consequence of gradual changes in the core mantle boundary conditions. recently, longer-term changes in reversal rates have been linked to plume activity at the core-mantle boundary (courtillot and besse, 1987; loper, 1992) or to the arrival at the core mantle boundary of cold material from a mantle event (gallet and hulot, 1997) or to changes at the inner-core/outer-core boundary (kent and smethurst, 1998). nowadays the favoured conjecture is that the change in reversal rate refl ects the evolution in spatial variability of some parameter, such as heat fl ux, at the coremantle boundary (cmb) (merill et al., 1996; glatzmaier et al., 1999). taking into account that reversal sequences are not stationary and that a reversal requires a fi nite amount of time, one can think that there has to be some statistical correlation between the lengths of successive polarity intervals. in this case, the process cannot be truly poisson and it has been shown (naidu, 1971; phillips, 1977) that a gamma distribution could provide a better fi t to the observed intervals. it has also been suggested (mcfadden and merrill, 1984; lutz and watson, 1988) that the process of geomagnetic reversals may be consistent with a 663 some new approaches to the study of the earth’s magnetic fi eld reversals nonstationary poisson process, which is a special case of gamma process where there is no memory of how long it has been since the previous event. we remark that a gamma distribution generally occurs in out-of-equilibrium dynamical systems (sornette, 2000). it has also been found (gaffi n, 1989, seki and ito, 1993) that the cumulative distribution of the polarity intervals follows a power law. in particular, seki and ito (1993) interpreted their results as indicating that the geodynamo is marginally stable and that the geomagnetic polarity reversal is a kind of critical phenomenon. in particular they presented a simple model (a dissipative version of the q2r ising model) in which the geodynamo was assumed to be a system of magnetic spins in a critical phase-transition state. by numerical simulation, they found that intervals of polarity reversals in the model follow a power law at a critical state. they also improved their model (seki and ito, 1999) to make it more realistic and to obtain a power exponent closer to the observed value. the revised model is a coupled map lattice where the elements themselves evolve and reverse polarity autonomously according to the lorentz map obtained from rikitake dynamo dynamics. analysing the behaviour of the system and the distribution of polarity reversals for various values of parameters of the model they suggested that the dynamo in the earth’s core is in a chaotic turbulent state. similar conclusions were arrived at by consolini et al. (2000) by investigating the multifractal features of the polarity interval time series. in this paper, an attempt is made to study the geomagnetic reversal record from the viewpoint of dynamical complex systems theory. reversals of the earth’s dipole fi eld are in fact associated with magnetohydrodynamic processes in the convecting outer core and for this reason it is important to apply any new ideas which can give additional insight into this fundamental aspect of planetary magnetism. moreover, there is general agreement that the dynamo equations are sufficiently nonlinear to allow reversing solutions and that simple analogue equations, such as those of the coupled disk dynamo of rikitake, exhibit spontaneous and apparently random polarity reversals. consequently one possible answer to the geomagnetic fi eld time reversals debate is to describe these events as typical nonlinear behaviour of a highly complex dynamical system especially if one takes into account that nonlinear systems are capable of a vast range of responses. in this picture we investigated the possible occurrence of zipf ’s law, 1/f noise, and punctuated-equilibrium in the geomagnetic polarity reversals, analysing all reversal data back to about 166 ma ago. 2. the rank-ordering statistics and the zipf’s law in the last three decades, growing interest has been devoted to many natural phenomena that are characterised by power-law distributions (plds). although the asymptotic existence of power-laws is a well-established fact in statistical physics and critical phenomena, powerlaw behaviours are also considered to be one of the most striking features of complex systems, i.e. of those systems, which consist of several interacting subunits, and whose phenomenological laws, describing the global behaviour, cannot be surmised from the elemental laws that regulate the evolution of its elementary parts. power-law distributions are commonly taken as a signature of an underlying self-similarity. as a matter of fact, in contrast to exponential distributions, a pld indicates the absence of a characteristic size: i.e. there is no upper limit on the event size. we remark that in the case of a phenomenon whose events follow a pld, the largest events completely dominate the physical processes (see for example what happens in the case of earthquakes). from a practical point of view, a pld is represented by a linear trend in a log-log plot of the frequency or cumulative number as a function of size. one of the main and more debated questions on pld is the validity of such a distributions for very large events, which are generally undersampled. one way to solve such a problem when only a limited statistics of largest events is available is to use the rank-ordering technique (sornette et al., 1996; sornette, 2000). george kingsley zipf of harvard university initially introduced this technique in his book (zipf, 1949), where he made many striking 664 paola de michelis and giuseppe consolini observations of simple regularities in human systems. the most known regularity observed by zipf was the distribution of how often a given word appears in a piece of literature. in any case, rank-ordering statistics was subsequently applied to a wide range of research fi elds. given a set of numbers {n} upper limited, the rank-ordering technique consists in evaluating how many (rank order r) of these numbers are smaller than a certain cut-off value n*, and then in plotting n* as a function of r. thus zipf’s plot is a log-log plot of the cumulative distribution with an interchange of axes [n* = ƒ(r) instead of r = ƒ(n*)]. although the statistical analysis of the cumulative distribution and of the rankordering statistics seems to be equivalent, they are signifi cantly different, the analysis being completely determined by the uncertainties of the quantity plotted in the ordinates. in other words, the rank-ordering statistics provide a different emphasis on the extreme and rare events. the existence of a power law behaviour (n* ª ra ) in the zipf’s plot is the indication of a certain amount of correlation and selforganisation. the original zipf’s law for words is characterised by a scaling exponent a ª 1.0. anyway, different values of a may be recovered in different physical systems meaning a different degree of correlation and self-organisation. for example in the case of dna sequences it was found a ª 0.3 (mantegna et al., 1994). other examples of the zipf’s laws come from rankordering statistics of stock-market variations, of city distributions (marsili and zhang, 1998), of large earthquakes (sornette et al., 1996), etc. here, we will make use of rank-ordering statistics to investigate the existence of zipf’s law in the geomagnetic chron distribution. 3. data description and analysis the geomagnetic polarity time scale compiled by cande and kent (1992, 1995) represents a great improvement in defi ning the chronology of reversals from the late cretaceous to the present. because the time scale of cande and kent (1995) does not go back further than 83 ma, we have merged it with the portion of the scale for the time interval 83-166 ma (ogg, 1995) to investigate the statistical properties of reversals from the present to 166 ma ago. the whole sequence contains 332 intervals of constant polarity, 144 intervals of constant polarity before the cretaceous superchron and 188 afterwards. the sequence of polarity intervals according to the time scale of cande and kent (1992, 1995) and ogg (1995) is presented in fi g. 1, where the polarity interval lengths dt(x) are plotted as a function of the order of occurrence (x). ages (dates) of geomagnetic inversion that indicate the borders of intervals with different main fig. 1. the time series of geomagnetic polarity interval duration (dt(x)) for both polarities as a function of order of occurrence (x). the y-axis was limited to 10 ma. 665 some new approaches to the study of the earth’s magnetic fi eld reversals geomagnetic field polarity are presented in this scale with an accuracy of 0.001 ma. this value is of the same order of magnitude as the process of polarity change, which is around 3.0-3.5 ka. in this analysis we have not made any distinction between excursions, subchrons, chrons, and superchrons. all these events can be considered manifestations of the dynamics of the earth’s dynamo. also, due to the inherent uncertainties in the determination of the reversal ages, we have set the average conventional scale resolution to be about 0.01 ma, even though the accuracy of determination of individual events may be one order higher. although we know that the uncertainties which remain in the precise absolute dates of the geomagnetic polarity time scale make their detailed analysis delicate, we believe that they would not notably modify the broad description we intend to present hereafter. as already noted, there is a clear nonstationarity in this record, which becomes evident when the distribution of the geomagnetic reversal rate on a 5 ma time scale is analysed (mcfadden and merrill, 1984). the obtained trend in fact suggests that the reversal rate has gradually increased from the end of the cretaceous superchron to the present and this is evidence of the nonstationary behavior of the geomagnetic polarity time scale. however, we must take into account that this interpretation, even though it is compatible with the data, is strongly guided by the way the data is presented and by the choice of the strong smoothing made on the series (as also noted by gallet and hulot, 1997). the nonstationarity of the reversal process could cause some diffi culties in defi ning the statistical properties of the record. gaffi n (1989) and marzocchi (1997) have suggested that some results obtained in previous papers could be artefacts of the non-stationarities present in the series. nevertheless, we believe that the problem of the nonstationarity of the reversal process is not relevant for our analysis. first of all, in order to study the nature of the polarity reversals we analysed the power spectral density (psd), and investigated the occurrence of scaling laws (zipf’s law) in the cumulative distribution of the chrons. to evaluate the psd of the chron time series we generated a surrogate two-state (± 1) time series covering the entire period length of 166 ma with a resolution of 0.01 ma. the state + 1 was associated with normal polarity and – 1 with the reversed polarity. figure 2 shows the obtained power spectral density (that is, the squared magnitude of the fourier transform) for the analysed polarity reversal time series. two different scaling regions and a spectral break can be identifi ed. indeed the power spectral density shows a 1/f spectrum for times longer than t0 = 1/f 0 ª 0.4 ma, while for time scales shorter than t0 it scales as 1/f 2, which is reminiscent of a stochastic motion. the physical meaning of the 1/f 2 region should be limited to the random nature of the polarity sequence (i.e. of the random distribution of the ± 1 æ °1 transition) in the surrogate time series. the occurrence of a 1/ƒ spectrum on longer time scales is evidence of a certain amount of longtime correlation in the sequence of polarity reversals, and of a complex time evolution of the geodynamo. we stress that 1/ƒ-noise spectra represent one of the most relevant features of out-of-equilibrium and complex systems whose evolution is characterised by fi rst-order transitions between metastable confi gurations (bak, 1996). because f0 (which represents the value of the spectral break) is the characteristic frequency for a sinusoidal signal, we must conclude that the typical time interval between two polarity reversals is (3.1) this value is in good agreement with the mean length of a polarity interval given in the literature which is about 0.3 ma even though its value depends on the number of short events present in the magnetic polarity time-scale. lowrie and kent (1983) have in fact discussed the effect of adding short events to the magnetic polarity timescale and they have shown that this has a dramatic effect on the polarity intervals distribution and on the mean length of a polarity interval, which is reduced from 0.314 ma to 0.175 ma. in order to study the possible occurrence of zipf ’s law for the cumulative distribution of chron duration, we denote by qdt the number of δτ 0 0 01 2 2 0 22= = ≈f t . ma. 666 paola de michelis and giuseppe consolini polarity intervals having a length equal to dt, and we defi ne the rank r as (3.2) this means that the rank r (dt ) is the number of polarity intervals which last longer than dt. figure 3 shows the rank order statistics of the polarity reversal time intervals obtained. a power-law δτ α= −a r0 applies for ranks r £ 100 with a characteristic scaling exponent a of a ª ª (0.76 ± 0.01). this indicates that the polarity reversal dynamics obeys zipf’s law behaviour for r £ 100. the existence of a zipf’s law means that for time scales longer than 0.25 ma the geomagnetic polarity reversal phenomenon exhibits scale invariance and/or self-similar features, and that a certain amount of selforganization must occur in the geodynamo evolution. in terms of the distribution of polarity chron durations zipf’s law is equivalent to a power law distribution. as a matter of fact our result is in good agreement with the analyses of seki and ito (1993, 1999) and gaffi n (1989), which found a power law distribution with a fig. 2. the psd, in arbitrary units (a.u.), of the surrogate polarity time series. the solid and dashed lines are nonlinear power-law best fi ts. the dotted vertical line indicates the position of the spectral break. r q d δτ δτδτ δτ ( ) = ∞ ∫ ' ' . 667 some new approaches to the study of the earth’s magnetic fi eld reversals scaling exponent near – 1.5 for chron durations longer than 0.23 ma. moreover, the existence of such a power-law statistics could be related to the occurrence of «punctuated equilibrium». the concept of punctuated equilibrium was introduced by gould and eldredge (1993) in the fi eld of evolution in natural history, and refers to a system which is characterized by periods of relative quiescence, interrupted abruptly by rapid change (the crisis). this phenomenon may be taken as an indication of metastability in the dynamics of a system. punctuated equilibrium has been shown to be a common feature of several complex systems (bak, 1996). in terms of polarity chron duration punctuated equilibrium could be paraphrased as: «the existence of periods of stable polarity punctuated by crises where a polarity inversion occurs». this phenomenon may be visualized by the existence of a devil’s staircase for the accumulated activity. the devil’s staircase was introduced by g. cantor in the 19th century as a special mathematical object generated by a multiplicative process and characterized by self-similarity. the name devil’s staircase comes from the existence of very large and very small steps alternating in a random way. although for a long time it was thought that physical and real systems cannot share the mathematical features of the devil’s staircase, in the last 3 decades it has been clearly demonstrated that several complex system are able to show behaviours that may be represented in terms of a devil’s staircase (bak and bruisma, 1982; jensen et al., 1983; bak, 1996). in order to check the occurrence of punctuated equilibrium for geomagnetic reversals we introduced a surrogate «accumulated activity» aa(t) as (3.3) where x (t' ) is the binary time series of polarity aa t x t dt t t ( ) = ∂ ( )∫ ' ' ' 0 fig. 3. the ranking distribution relative to the polarity time intervals. the solid line is a nonlinear power-law best fi t. the horizontal dotted line indicates the characteristic time-scale evaluated on the basis of the psd (see text). 668 paola de michelis and giuseppe consolini reversals. figure 4 shows the result obtained for the accumulated activity. the plot of the defi ned accumulated activity looks very similar to a devil’s staircase. plateaus of stasis (i.e. the steps of the staircase) do not follow one another in a regular way. this result which is a signature of the occurrence of punctuated equilibrium, may be read as an indication of metastability and can be linked to a complex confi gurationally space for the geodynamo. moreover, the irregular character of the stasis sequence confi rms our previous fi ndings of a multifractal and intermittent character of the geomagnetic polarity reversal series (consolini et al., 2000). 4. summary and conclusions in this paper, we have presented a study of the geomagnetic chron time length sequence to infer information on the dynamical state of the geomagnetic dynamo. in order to do this we performed power spectral analysis of polarity reversal time series using the generally accepted geomagnetic polarity time scale compiled by cande and kent (1992, 1995) and ogg (1995) which covers the last 166 ma. we found that the power spectrum shows a 1/f spectrum, where f is the frequency, for times longer than t0 ª 0.5 ma while for times shorter than t0 it scales as 1/f 2. the most striking feature of this result is the existence of a crossover point that divides the total time interval into two different parts with different scaling properties. we also evaluated the possible occurrence of zipf ’s law for the cumulative distribution of chron duration and we have found that the polarity reversal dynamics obeys zipf’s law for time scales longer than 0.25 ma. this result permitted us to conclude that the geomagnetic polarity reversal phenomenon exhibits scale invariance and/or self-similar features. finally, we investigated the possibility that the phenomenology of the polarity reversals in the geomagnetic dynamo is that of punctuated equilibrium. we evaluated the accumulated activity of the polarity reversal time series and obtained a trend quite similar to a devil’s staircase with plateaus of stasis (distributed according to a power law) punctuated by short periods of rapid activity. these results give a preliminary picture of the dynamical state of the geomagnetic dynamo. fig. 4. the devil’s staircase for the geomagnetic polarity time series resulting from the accumulated activity aa(t). 669 some new approaches to the study of the earth’s magnetic fi eld reversals the fact that the geomagnetic dynamo is characterized by punctuated equilibrium means that the geodynamo dynamics is characterized by metastability. combining this result with the observed scale invariance in the polarity interval distribution, we can consequently assume that the geomagnetic dynamo might be in a marginally stable out-of-equilibrium confi guration and that the polarity reversals are equivalent to a sort of phase transition between two metastable states. we suggest that the dynamics of the geomagnetic dynamo could be equivalent to that of a stochastic system near criticality and consequently that the geomagnetic dynamo evolves as a system near a dynamic critical point. we believe that our findings strongly support previous work by ourselves (consolini et al., 2000) and by seki and ito (1993, 1999). we would like to remark that this hypothesis has been also suggested by mcfadden and merrill (2000) who considered the existence of a mechanism for punctuated equilibrium within the core. in particular, they suggested that, for example, phenomena such as the sudden breakup of a stratifi ed layer in the outermost core could lead to a rapid change in boundary conditions that could be responsible for the punctuated evolution manifested in the sudden jumps in the reversal rate. here, reversals would be equivalent to noise-induced cooperative phase transitions. this last picture fi ts well with our latest result of the possible occurrence of stochastic resonance in geomagnetic polarity reversals (consolini and de michelis, 2003). the possible relevance of our fi ndings to a possible critical dynamical state of the geodynamo calls for further investigation, particularly from a theoretical point of view. references bak, p. (1996): how nature works: the science of selforganized criticality (copernicus) (springer-verlag, new york, ny, u.s.a.), pp. 336. bak, p. and r. bruinsma (1982): one-dimensional ising model and the complete devil’s staircase, phys. rev. lett., 49, 249-251 cande, s.c. and d.v. kent (1992): a new geomagnetic polarity time scale for the late cretaceous and cenozoic, j. geophys. res., 97, 13,917-13,951. cande, s.c. and d.v. kent (1995): revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic, j. geophys. res., 100, 6093-6095. cox, a. (1968): lengths of geomagnetic polarity intervals, j. geophys. res., 73, 3247-3260. courtillot, v. and j. besse (1987): magnetic field reversals, polar wander, and core-mantle coupling, science, 237, 1140-1147. consolini, g. and p. de michelis (2003): stochastic resonance in geomagnetic polarity reversals, phys. rev. lett., 90, 058501-4. consolini, g., p. de michelis and a. meloni (2000): multifractality and punctuated equilibrium in the earth’s magnetic fi eld polarity reversals, gephys. res. lett., 27, 293-296. gaffin, s. (1989): analysis of scaling in the geomagnetic polarity reversal record, phys. earth planet. int., 57, 284-290. gallet, y. and g. hulot (1997); stationary and nonstationary behaviour the geomagnetic polarity time scale, geophys. res. lett., 24, 1875-1878. glatzmaier, g.a., r.s. coe, l. hongre and p.h. roberts (1999): the role of the earth’s mantle in controlling the frequency of geomagnetic reversals, nature, 401, 885-890. go u l d, s.j. and n. el d r e d g e (1993). punctuated equilibrium comes of age, nature, 366, 223-227. gubbins, d. (1987): mechanism for geomagnetic polarity reversals, nature, 326, 167-169. kent, d.v. and m.a. smethurst (1998): shallow bias of paleomagnetic inclinations in the paleozoic and precambrian, earth planet. sci. lett., 160, 391-402. jensen, m.h., p. bak and t. bohr (1983): complete devil’s staircase, fractal dimension and universality of mode-locking structure in circle map, phys. rev. lett., 50, 1637-1639. jones, g.m. (1977): thermal interaction of the core and mantle and long-term behaviour of the geomagnetic fi eld, j. geophys. res., 82, 1703-1709. loper, d. (1992): on the correlation between mantle plum fl ux and frequency of reversals of the geomagnetic fi eld, geophys. res. lett., 19, 25-28. lowrie, w. and d.v. kent (1983): geomagnetic reversal frequency since the late cretaceous, earth planet. sci. lett., 62, 305-313. lutz, t.m. and g.s. watson (1988): effects of long-term variation on the frequency spectrum of the geomagnetic reversal record, nature, 334, 240-242. mantegna, r.n., s.v. buldyrev, a.l. goldberger, s. havlin, c.-k. peng, m. simons and h.e. stanley (1994): linguistic features of noncoding dna sequences, phys. rev. lett., 73, 3169-3172. marsili, m. and y.-c. zhang (1998): interacting individuals leading to zipf’s law, phys. rev. lett., 80, 2741-2744. marzocchi, w. (1997): missing reversals in the geomagnetic polarity timescale: their infl uence on the analysis in constraining the process that generates geomagnetic reversals, j. geophys. res., solid earth, 102 (b3) , 5157-5171. marzocchi, w., f. murlagia and p. paruolo (1992): the correlation of geomagnetic reversals and mean 670 paola de michelis and giuseppe consolini sea level in the last 150 m.y., earth planet. sci. lett., 111, 383-393. mcfadden, p.l. and r.l. merrill (1984): lower mantle convection and geomagnetism, j. geophys. res., 89, 3354-3362. mcfadden, p.l. and r.l. merrill (1986): geodynamo energy source constraints from paleomagnetic data, phys. earth planet. int., 43, 22-33. mcfadden, p.l. and r.l. merrill (1993): inhibition and geomagnetic fi eld reversals, j. geophys. res., 98, 6189-6199. mcfadden, p.l. and r.l. merrill (1997): asymmetry in the reversal rate before and after the cretaceous normal polarity superchron, earth planet. sci. lett., 149, 43-47. mcfadden, p.l. and r.l. merrill (2000): evolution of the geomagnetic reversal rate since 160 ma: is the process continuous?, j. geophys. res., 105, 28,455-28,459. merrrill, r.t., m.w. mcelhinny and p.l. mcfadden (1996): magnetic fi eld of the earth: paleomagnetism, the core and the deep mantle (academic, san diego), pp. 531. naidu, p.s. (1971): statistical structure of geomagnetic field reversals, j. geophys. res., 76, 2649. ogg, j.g. (1995): magnetic polarity time scale of the phanerozoic, in global earth physics: a handbook of physical constants, edited by t.j. ahrens (amer. geophys. union, washington, dc), 240-270. phillips, j.d. (1977): time variation and asymmetry in the statistics of geomagnetic reversal sequences, j. geophys. res., 82, 835-843. seki, m. and k. ito (1993): a phase-transition model for geomagnetic polarity reversals, j. geomagn. geoelect., 45, 79-88. seki, m. and k. ito (1999): a coupled map lattice model for geomagnetic polarity reversals that exhibits realistic scaling, earth planet space, 51, 395-402. sornette, d. (2000): critical phenomena in natural sciences. chaos, fractals, self-organization and disorder: concepts and tools (springer-verlag, heidelberg), pp. 450. sornette, d., l. knopoff, y.y. kagan and c. vanneste (1996): rank-ordering statistics of extreme events: application to the distribution of large earthquakes, j. geophys. res., 101, 13,883-13,893. zipf, g.k. (1949): human behavior and the principle of least-effort (addison-wesley, reading, mass). (received january 8, 2003; accepted june 20, 2003) observed travel times for multiply-reflected scs waves from a deep-focus earthquake a . p . espesosa ( * ) ricevuto il 26 agosto 1966 summary. — tlie deep-focus argentinean earthquake of december 8 1962, generated multiply reflected scs phases which were recorded very clearly at stations of the igy and the tjsc&gs standardized worldwide networks and at canadian stations. the data gathered from this earthquake for the multiply-reflected scs and sscs were used to construct the travel times and to extend them to shorter epicentral distances. these new data brought to light an error in published travel times for the 2(scs) phase. riassunto. — 11 terremoto profondo avvenuto in argentina l'8 dicembre 1962, ha originato onde scs riflesse più volte, che sono state registrate molto chiaramente dalle stazioni sismiche della rete mondiale standard dell ' i g y e d e h ' u s c & g s e dalle stazioni canadesi. 1 dati raccolti mediante lo studio di questo terremoto per le onde scs e sscs, sono stati usati per costruire la curva dei tempi di tragitto ed estesi a distanze epicentrali più brevi. questi nuovi dati, inoltre, hanno messo in luce un errore relativo ai tempi di tragitto già pubblicati per le onde 2(scs). introduction multiply-reflected scs and their surface images f r o m the december 8, 1962, deep-focus earthquake in argentina were unusually well recorde d b y long-period seismographs at stations of the igy and the tjsg& gs standardized network, and at canadian stations. this investigation describes a study made on the identification of the multiply-reflected ( * ) environmental science services administration, u.s. coast and geodetic survey. rockville, maryland. fig. 1. map showing location of epicenter, stations, and a great circle path through palisades. obs kr v kd t l i a v k l t i m e s j,\>k m u l t i p i . y r f.flf.cted. etc. 4 1 7 phases, and the construction of their travel-times as recorded on longperiod instruments. t h e observations of the travel-times of the 2(scs) at shorter cpicentral distances have permitted the extension of the travelt i m e curve f o r this phase down to an cpicentral distance of 7°. previously in the literature, the travel-time curve f o r 2(sc8) has been given f o r an cpicentral distance greater than 90°. t h e n(8cs) f a m i l y of phases has been identified without any doubt b y the presence of the n(s8cs) phases on the recordings. identification of phases traveling in the m a j o r circle path is shown. i n the process of plotting the observed t r a v e l times of the 2(scs) phase, an error in the published travelt i m e curve f o r this phase has been found, f o r a depth of focus of 600 k m . interpretation of the data t h e epicenter, origin time, and depth of focus f o r the december 8, 1962, earthquake were located b y using bolt's (1960) program f o r the 7090 ibm computer. one hundred fifty readings of impulsive p phases, .14 pp and 10 pkp readings, were used in the relocation. t h e pertinent data are listed in t a b l e i . table i epicenter origin time (gmt) depth magnitude h m s km 25.8" s 63.4° w 21 27 22.2 620 7 pas usc&gs 25.78° s 62.15° w 21 27 18.0 580 6 3/4 pal t h e data used in this study are f r o m the stations shown in figure 1. t h e solid line in this figure is a great circle path through palisades, n . y . , and the station code i d e n t i f y i n g each of the stations corresponds to the three letter code assigned b y the u8g&g8. readings of the phases which constitute the data were done f r o m long-period recordings having a natural period of 30 sec and 100 sec f o r the seismometer and galvanometer, respectively. f i g u r e 2 shows a graphical description of the phases which have undergone four reflections at the core-mantle boundary, denoted as 418 a . f . e s p i n o s a 4(scs), through the minor circle path and recorded at sgh. a l s o in the same figure is shown a phase which travels through the m a j o r circle path and has been reflected three times at the core-mantle boundary, denoted as 3(scs)~, and also recorded at the same station. pig. 2 graphical example of ray paths and notations used for the multiplyreflected phases which have traveled along the minor and major circle paths. f i g u r e 3 shows a tracing of the e-w component seismogram at gab, w i t h an epicentral distance of 36.24° and an azimuth f r o m the station to the epicenter of 174.61°. t h e original recording was done on a recorder w i t h a 30 mm/min. revolution rate. i n this figure, the tracing has been reduced b y a factor of four. t h e portion of this recording is between the arrival t i m e of the s phase and approximately a minute after the arrival t i m e of the 4(8c8). on the original seismogram, the peak-to-peak amplitude of the scs phase is 357 m m . t h e n(scs) and n(sscs) are indicated on the seismogram. t h e identification of the n(scs) series has been made certain b y the arrival of its surface images on the recordings. o 6 3 7 0 k ( s h ) s s c s 2(scs) 3(scs) 4 ( s c s ) 2(sscs) 3(sscs) 4(sscs) min c a r e w ( 3 0 i 0 0 ) f i g . 3 seismogram f r o m g a r , december 8, 1962, e w long-period component. (sh) s scs s s c s 2(scs) 2 ( s s c s ) 3 ( s c s ) 3 ( s s c s ) 4 ( s c s ) 4 ( s s c s ) 1 • i min b e c e w ( 3 0 i 0 0 ) t i g . 4 seismogram from beg, december 8, 1962, e-w long-period component. 4 2 0 a . f . esim n o f j a a n o t h e r typical sei.sinogr.un tracing is shown in figure 4. this figure is the e-w component, at, beg with an epioentral distance of 57.86° and an azimuth of 17*.6" and shows an especially clear recording of the n(scs) and the nlsscn) series. on this record, the two series are indicated from n 1 to an order i; however, at, the end portion of this seismograni, the 5(kcs) can be .s<-<-n very clearly. this phase was also well recorded at st.it.ions close to the epicenter, but there was difficulty in picking its arrival time since the background noise is high in that portion of the seismograni. on inspection of the n-is' component seismograni, the, horizontally polarized shear phases are v e r y small in amplitude, and in the vertical component these phases were not discernible. f i g u r e 5 shows a composite of seismograms for nine stations. e i g h t are due, n o r t h of the epicenter, and the ninth is due east. these recordings were made; on long-period instruments; the e w component is the one shown in this figure, liiiforfiinatcly, t he variation of speed in the revolution of the drum is different i'or sonic of these stations; some have a 15 irim/min. and others 30 inm/niin. revolution rate. w h e n reduction was done, on each of the seismograms in this composite figure, a one-to-one correspondence was lost. t h e picture length is made up of four parts put together, which can be seen in the breaking of the time axis. this f a c t also has introduced distortion, and because of these distortions, no a t t e m p t is made to draw a travel-time curve which connect one phase with another for a given series. t h e ant, baa, and abe seismograms were omitted in this figure since their locations are too close, to the time, axis. t h e rdj, gab, beg are tracings of the originals, and the pal, hal, sgtt, vic, bes, and ale are reduced pictures of the original seismograms at these stations. t h e t i m e axis is in minutes, with an interval of 2 minutes, and the, distance axis is in degrees. travel-time curves. t h e seismograms obtained f r o m 38 of the igy, usc&gs, and the canadian stations w e r e read (sec f i g u r e 1) and a total of 23 station records were used in the filial gathering of the data which is used to construct the travel-time curves f o r the multiply-reflected phases. t o interpret the phases on the recordings, the travel times of impulsive phases were picked, tabulated, and plotted. these readings were done on all three 130 3 5 0 3 4 0 3 3 0 3 2 0 310 3 0 0 2 9 0 2 8 0 2 7 0 2 6 0 2 5 0 2 4 0 o k 120 110 100 90 80 70 c 6 0 e c o l l ) 50 > < < r 40 30 20 10 0 0 2 0 3 0 j 4 0 5 0 . > s x } v u i < 2 < < cd q oc cr < o u -j ^ lli < < m qi 1 6 0 7 0 8 0 9 0 100 110 120 x o co distance ( d e g r e e s ) ai to o ^ cj c! <v o a a s a £ * % © ® s 3 a* .2 ^ ft d o c3 s 2 o 00 a h © a o b s e r v e d t r a v e l t i m e s f o r m u l t i p i . y r e f l e c t e d , e t c 4 2 1 4 5 0 0 4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 4(sscs) o o0 4(scs) 3(sscs) , o o ü -h . o-o-o"" o n o — o 2000 2(sscs) 3(scs) 0° <8 —o-oo—cd" --oct" -<t<fp" i ..-cm o 1500 1 0 0 0 2(scs) t r a v e l times (sec) december 8 , 1 9 6 2 h= 5 8 0 k m 5 15 25 35 4 5 55 65 75 85 95 105 115 125 135 145 155 distance (degrees) — • fig. 6 observed travel times for the n(scs) and the n(sscs) series. time in seconds; distance in degrees. 4 2 2 a. f. jsspinosa components. a f t e r identification of the phases was certain, ellipticity corrections w e r e applied to each of the readings. f i g u r e 6 shows the t r a v e l times f o r the n(scs) and n(ssas) series obtained f r o m the deep< — d i s t a n c e (degrees) 3 4 5 3 2 5 3 0 5 2 8 5 2 6 5 2 4 5 2 2 5 2 0 5 ( 8 5 d i s t a n c e ( d e g r e e s ) — • fig. 7 observed travel times for the 3(scs)~ and 3(sscs)~ phases. time in seconds; distance in degrees. o b s e r v e d t r a v e l t i m e s for m u l t i p l y r e f l e c t e d , etc. 4 2 3 fig. 8 travel-time curves (after richter). the dash-line travel-time curve corresponds to the correct curve for the 2(sos) phase. 424 a. p . k s p i x o s a focus earthquake of december 8, 1962. t i m e is given in seconds and distance in degrees. t h e criterion used in the identification of the n(scs) series is t h a t of the presence of its surface images on the recordings (see figures 3, 4, and 5). t h e dashed curves in this figure correspond to the traarel times f o r 2{ses), 3(sc8), and 4(sc8) f o r a depth of focus of 580 k m . some of t h e observed points on these curves w e r e checked against the j e f f r e y s and bullen tables (1948), using the times of 8c8 f o r a surface foci and the corresponding depth allowances. a t first, gutenberg and kichter travel-time curves f o r h — 600 k m wen» used; h o w e v e r , the observed t r a v e l times f r o m this earthquake at epicentral distances of 90° to approximately 125° were earlier than the times given b y their curves. i n this figure, the n(scs) and the -»(sscs) series are p l o t t e d up to order n = 4. t h e identification of order n — 5 was possible in many of the records, but the scatter of the data readings was large; hence, these readings w e r e omitted. t h e recently installed long-period instruments b y the usc&gs in south america made possible the gathering of the data at close epicentral distances, and hence, the extension of the t r a v e l time observations of the 2(8cs) down to an epicentral distance of 7°. i n f i g u r e 7 are shown the t r a v e l times of some of the phases avhich have a r r i v e d through the m a j o r circle path. t h e solid lines correspond to the travel-time curves for the 3(8c8)~, and the 4(scs)~, and the dashed lines to their corresponding surface images. t h e observations of the earlier arrivals in this figure f i t those of the 3(8cs)~ and the latter ones fit those of their surface image. a f t e r the arrival times of the 3(ssc8)~, there are a number of observations which do not fit the 4(8c8)~ travelt i m e curve, and the latter arrivals scatter v e r y much, so one cannot i d e n t i f y , w i t h certainty, these phases as being of the 4(8c8)~ series. these phases in question are clearly recorded on some of the seismograms (see f i g u r e 5) with amplitudes larger than those phases which have a r r i v e d through the minor circle path. f i g u r e 8 shows the travel-time curves given in eichter's book, " e l e m e n t a r y seismology on page 684, and on i t a dashed travelt i m e ciu've has been drawn which corresponds to the correct travel-time curve f o r the 2(8cs) phase obtained in this study. t h e travel-time curve published earlier b y gutenberg and richter (1937) f o r the same phase, f o r a depth of focus of 600 k m , should be redrawn as the dashed travel-time curve shown in figures 6 and 8. o b s e r v e d t r a v e l t i m e s for mu lti i ' l y r e f l e c t e l j . etc. 4 2 5 r e f e r e n c e s bolt b. a . , the revision of earthquake epicenters, focal depths and origin times using a high-speed computer. geophysical journal, vol. 3, 433-440 (1960). gutenberg b. and richter c. f., données relatives a l'étude des tremblements de terre a foyer profond. publications du bureau central séismologique international, ser. a , travaux scientifiques, 15, 1-70 (1937). jeffreys h . and bullen k . e., seismological tables. london, british association (1948). richter c. f., elementary seismology. freeman & co., san francisco (1958). indice (le f o r m a para identificar a las pulsaciones geomagneticas (*) (form index for the identification of geomagnetic pulsation) a . c. f . b i n a g h i pages ( * * ) e . g . l i n z u a i n ( * * * ) r i c e v u t o il 3 a p r i l e 1964 summary. — t o make a complete study of pulsation g i v e n the planetary characteristics of his shapes is theorically obtain all the w o r l d magnetograms. b u t this work it's impossible to do, because the number of i n f o r m a t i o n to meet is v e r y rich. w e thought to save these difficulties in that manner; first, choosing only 26 or 30 observatories distribuites conveniently around the w o r l d ; second, b y the f o r m index that appears in the usual bulletins it w i l l permit to solicit only the magnetograms of our interest. t h e index is f o r m e d b y signs, numbers and letters, c o n v e n i e n t l y grouped and built in according to a k e y . i t ' l l be able to g i v e a complete and syntetic information to rebuilt the phenomena. this index is f o r m e d b y : only one first sign who g i v e tlie direction of initial d e f o r m a t i o n ; next, t w o letters that i d e n t i f y the t y p e of pulsation: i m m e d i a t e l y three numbers, the first g i v e the average period, the second the average amplitude, and the third the amplitude or deformation coefficient. t h e last term is named " the configuration t e r m " and at last a number who estimates the frequence deformation gives b y a key. sumario. — p a r a poder realizar un estudio exaustivo de las pulsaciones, dadas las caracteristicas planetarias de algunas de sus formas, es indudable que teoricamente debemos disponer de todos los magnetogramas que mundialmente hayan campado una deter mi na da presentation. ( * ) n o t a presentada a la 2a reunion cientifica de la asociacion argentina de geofisicos y geodestas, reunida en san juan, n o v . 1962 e al symposium international de berkeley, comision 10, a g o s t o 1963. ( * * ) direttore dell'gsservatorio geofisico hudson ( a r g . ) ; i n v i a t o speciale della commissione scientifica del governo di buenos aires all'osservatorio dell'ebro (spagna). ( * * * ) del personale scientifico dell-osservatorio hudson ( a r g . ) . 408 a. c. f . b i n a g i t i paoils e . g. l i n z u a i n n o cabe la menor rluda que tal labor se v e enormeineute difieultada, por la gran cantidad de copias a reujiir. i l e m o s creido salvar en parte esas diflcultades, primero seleccionando acertadamente solo 20 a 30 observatorios en adeeuada distribucion planetaria; segundo, por medio de un indiee de forma que, apareciendo en los boletines habituales, nos permitira solicitar solamente, co])ia del magnetograma en el dia y liora que nos interesare. el indice esta formado por: signos, numeros y letras, agrupados convenientemente y se armara de acuerdo a una clave e instrucciones y tiene por fin inmediato, dar al entendido, una rapida idea de la pulsacion que se trate, en su f o r m a general, y con euyos datos puede reconstruirse la curva aproximadamente, e identificar el fenomeno con mucho acierto. este indice viene f o r m a d o por un unico signo, que iiulica el sentido de la deformacion inicial, seguido de dos letras iiulicadoras del tipo de pulsacion que se trate; continuan tres numeros que indican respectivamente, el periodo medio, la amplitud media y el coeficiente de amplitud o deformacion a longitud de oiula constante. t o d o esto v a seguido de una ultima letra a la que llamaremos « termino de configuration » de acuerdo a una clave dctermiiiada, y finalmente un numero que va a indioar estimativamente, la derfomacion en frecuencia. kiassunto. p e r compiere uno studio completo delle pulsazioni, dando le caratteristiche planetarie delle loro forme, e necessario, in teoria, possedere t u t t i i magnetogrammi del mondo. questo lavoro e i m p o s s i b l e , poiclie il numero dei dati da raccogliere e molto vasto. a b b i a m o pensato di superare dette difficolta in questo m o d o : prima scegliendo solo 20 o 30 osservatori distribuiti convenientemente in t u t t o il mondo, poi basandoci sull'indice di f o r m a pubblicato nei normali bollettini; il che ci permette di richiedere solo i magnetogrammi che risultano per noi di interesse. l ' i n d i c e (di f o r m a ) e f o r m a t o da segni, liumeri e letterc convenientemente raccolti secondo un codice. cio serve a dare la possibility di ricostruire completamente e sinteticamente i fenomeni. questo indice e f o r m a t o da: un segno che dft, la direzione iniziale della d e f o r m a z i o n e — d u e lettere che rappresentano il tipo di pulsazione — t i e humeri, il primo dei quali indica il periodo medio, il secondo l'ampiezza media ed il terzo il coefficiente di ampiezza o deformazione — segue una lettera chiamata « t e r m i n e di conflgurazione » — inflne un numero che rappresenta approssimativamente la deformazione in frequenza. a manera de intltoducoion. las pulsaciones geomagneticas que venimos estudiando, corresponden a los ultimos fenonienos clasificados entre las variaciones r4pidas del magnetismo terrestre, que como tal ha catalogado el comite internacional, y de las cuales, estas. son justamente las menos estudiadas. f o r m a p a r a i d b n t i f i c a r a l a s p u l s a c i o n e s g e o m a g n e t i c a s 4 0 9 lo que caracteriza a las pulsaciones, es su presentation marcadamente universal en la generalidad de los casos, aunque con intensidad variable mayor generalmente en zonas aurorales y menor en la zona ecuatorial magnetica; por lo tanto se impone un estudio que contemple la posibilidad de disponer de todos los reglstros rapidos, provenientes de la totalidad de los observatorios magneticos. es evidente, por razones practicas, que la realization de tal compilation de magnetogramas, puede tener caracter de imposible; primero por la gran cantidad de papeles a reunir anualmente y luego por las dificultades del transporte. pero el indice que proponemos en el presente trabajo, nos permitira elegir unicamente los dias de interes, limitando asi enormemente la cantidad de registros a reunir. l o s estudios actuales. generalmente lo que en la. actualidad prevalece en el estudio de las geopulsaciones magneticas, es un estudio estadistico y cualitativo, muy generico, de sus presentaciones; estudios de sua frecuencias de oscilacion, amplitudes, frecuencia de presentation, y en forma muy limitada se va encarando el estudio de la configuration de la curva, que es justamente lo que nos interesa a nosotros, de cuyas caracteristicas podremos tal ves inferir o ratificar, o modificar las teorlas que tienden a explicar su genesis. es aun prematuro recomendar un analisis armonico de la curva registrada, debido ello a la ambigiiedad que presenta, pero no seria raro que algunas componentes presenten caracteristicas locales, cuando por lo general, definen caracteristicas planetarias, como ya se dijo. la clarification actual de las pulsaciones, aprobadas en el congreso de utrecht (1959), va siendo antigua en la actualidad, ya que se va dcflniendo una nueva clasiflcacion, la « l o n g pulsation». tambien va quedando muy deflnidas las caracteristicas planetarias de las pulsaciones continuas y trenes de pulsaciones, encambio las pulsaciones en perlas y las pulsaciones gigantes, sobre todo estas ultimas presentan caracteristicas marcadamente locales; como asi bien lo indica el prof. veldkamp refiriendose al estudio reahzado por whitham y loonier, donde estas ultimas aparecen probablemente en un area no mayor de 3.000 km de radio estando en parte de acuerdo con el prof. yosliio kato, que explica estas grandes pulsaciones como causadas por una oscilacion de 4 1 0 a . c . f . b i n a g h j p a g e s e . g . l i n z u a i n " earacter diganios liidromagnetica del toroide supuesto en la parte exterior de la alta atmosfera sobre las zonas aurorales. los estudios de las pulsaciones continuas (pc) y irenes de pulsaciones (pt), ha sido acertadamente encarado por el grupo ruso de estudiosos: bolshakova, y u zybin, maltseva, afanasieva, troitkaya, etc., pero todos ellos se refieren a registros de un limitado numero de observatorios que solo abarcan la parte norte de eurasia. mas acertada distribucion dispone en grupo de college (alaska), pero se hallan distribuidos en solo poeos usos horarios, muchos en el hemisferio norte y solo uno en el hemisferio sur. la distribucion de nuestros observatorios a consultar contempla una distribucion planetaria mucho mas satisfactoria. en cuanto a las caracteristicas planet-arias de los pc. y pt. nos adherimos a la idea de otros colegas, donde su probable genesis radica en oscilaciones del «hueco magnetico » que rodea e la tierra en el espacio exterior, oscilaciones producidas por todos los fenomenos solares que con earacter fluidoplasmatieos, inciden sobre la tierra. n u e v a distribucion para un estudio planetario. es indudable que los grupos de trabajo que hemos mencionado, solo disponen de datos provenientes de lo que podemos 11a mar «zona de inflnencia », ya por razones de comunicacion, ya por razones politicas o sensillamente por no haberse establecido aiin un sistema de intercambio definitivo, netamente funcional. lo ideal seria disponer diariamente de los registros de todos los observatorios, pero eso representa justa-r a-nualmente cerca de 30.000 magnetogramas a revisar, cosa practicamente imposible con nuestros medios actuates de transporte y estudio. se impone entonces la inmediata reduction de esa cifra y eso lo haremos en dos formas: la primera reduciendo a un nunimo la cantidad de observatorios a consultar; y segundo, por medio de un indice de forma, nos permitira unicamente pedir los magnetogramas de los dias que interesan. como ejemplo, diremos que, si nos remitimos a 20 observatorios para un promedio de diez dias de interes para anos de maxima solar, la cantidad de magnetogramas a estudiar se reduce a solo 3.000; guarismo ya mas razonable, que aun se puede reducir mucho mas para anos de minima actividad solar, donde a veces bay meses que no se registran mas que dos, una o ninguna pulsacion. f o r m a p a r a i d b n t i f i c a r a las p u l s a c i o n e s g e o m a g n e t i c a s 411 indudablemente la poda propuesta ya no controla el caracter estadistico, pero justamente ese no es nuestro fin, sino que solo nos interesa la configuration general en cada caso. pasemos ahora al indice que nos va a simplificar nuestro trabajo. ixdice de forma. los indices e indicaciones que flguran en la actualidad en los boletines magneticos de cada observatorio, son por lo general estadisticos, ilustrativos de la actividad geomagnetica local, ya diarios o trinorarios; los datos de los ssc, si, y b son acompanados de suticiente aclaracion. pero justamente las pulsaciones p, solo viene controlada su presentation, tampoco, no todos los observatorios las traen, pero ya las van agregando paulatinamente a sus boletines. por ejemplo el observatorio de l'aquila (italia) recien las trae desde enero 1960. de todas formas, estos datos sobre pulsaciones unicamente deben provenir de los registros lentos, y lo que a nosotros nos interesa son los provenientes de los registros rapidos tipo la cour o similares. yeamos algunos antecedentes sobre este asunto, antes de entrar a nuestro indice. indudablemente liay algunas nociones sobre la idea (le clasificar a las pulsaciones; por ejemplo, bolshakova, y u zibin y maltseva en su trabajo present ado al symposium de utrecht, mencionan algunas leyes caracteristicas y proponen una fijacion de ideas respecto a la actividad de las pulsaciones continuas; ellos indican la medida de esa actividad tomando en lineas generales las caiacteristicas de la actividad geomagnetica planetaria; su graduation 0-1-2 estima la amplitud de las oscilaciones de 10 a 50 segundos de periodo, tornados cada hora. el rango de amplitud para cada una de esas determinaciones, estan tomadas apreciativamente para cada periodo. de las determinaciones horarias de las 24 lioras es estimado un cierto grado representative del dia, correspondiente a la maxima amplitud del grupo de oscilaciones que le corresponden al niaximo grado. hasta aqui los autores rusos; pero discrepamos en casi todo con ellos, por que se basan en un maximo-maximorum y eso puede corresponder a una dia de reducida actividad y solo pocos minutos de gran amplitud. es decir que son ideas que no nos sirven para reconstruir la forma de la curva o tener por lo menos una idea de ella; tampoco podemos admitir, que eso represente una idea del valor de la actividad diaria, horaria tal ves. 4 1 2 a . c . f . b i n a g h j p a g e s e . g . l i n z u a i n " otros investigadores, como el prof. angenheister de gottingen, proponen un indice caracteristico, mucho mas acertado que el anterior; lo llama pz, indice de pulsation y es de la forma: _ n.a l z n donde: n = cantidad de oscilaciones en 1/8 de liora (7,5 minutes). a = amplitud media de oscilacion. el numero detinitivo, solo indica un guarismo, que representa muy bien la actividad, pero tampoco es ilustrativo para dar idea de la reconstruction de la curva; antes estos antecedentes, podemos convenir que el coeficiente angenheister representa con mucho acierto, la actividad de las pulsaciones, interpretadas en caracter generico, y sin explicar su elasifi cation. es asi como se nos ocurre la confection de un nuevo indice p fac.il de componer y de cuya lectura podamos interpretar casi completamente el tipo de fenomeno es estudio; por lo menos tomar nota e idea de el, como para entonces si pedir copia del magnetograma para tal y cual dia y bora. f o r m a c i 6 n d e l i n d i c e p . llamaremos asi al conjunto de signos, letras y numeros, indice, para hacer consonancia con sus similares correspondientes a los fenomenos geomagneticos. la information tipo, similar a la ya existente, constara siempre de una serie de columnas y una fila para cada fenomeno, uno a mas en el dia ya sean separados o juntos, sueesivamente y en uno o varios dias consecutivos. yease el croquis final. la primera columna indica el dia; la segunda la hora, minutos y segundos, del comienzo del fenomeno; la tercera indica la duration del fenomeno pulsatorio, en horas, minutos, segundos, si correspondiere. la cuarta columna esta dividida en tres columnas secundarias, una para cada componente del registro rapido, y en ellas se anoter4 el indice p, caracteristico e identificatorio del fenomeno a catalogar, cuya composicion pasamos a. esplicar. la primera letra sera un p mayuscula que siempre va a indicar el fenomeno generico a estudiar las pulsaciones; antepuesta a ella el signo de la derivada primera con respecto al tiempo, que viene a indicar pulsaciones , r s 2, a7 <-7 ihdicz p c o e f i c i e h t e de forha dia t. v a t u i c e p y z ogsiirl'/icion 16 zi^toii t p c p i o t i oes pit• m4££j tor*. h ksj'v 2* is* 21s sibuhoo nvhslo ssmmot term ) / i ' , p t . 8 3 . 1 . 0 a 3 _ f m^n' r»c//e rbjn£*a asnn cimtl. hvnex. cobf de t\ mp 0 j-iti/d fa lonfi. ti/o 3 d£ oho a to hs7a j s 8 n < b o, l/ § c to i* q d qs $ p £ t ^ m a m / i j ^ ^ m a a / ^ : ' a f v v-n < b o, l/ § c to i* q d qs $ p £ ^ w l i w v l / ^ n < b o, l/ § c to i* q d qs $ p £ ^ v w w w v — n < b o, l/ § c to i* q d qs $ p £ v . \ a) n < b o, l/ § c to i* q d qs $ p £ p£"s o « £3 £. k puiiaso oc i^ticne. a £>0 zc&l/l-a k. hl/t'h-o n < b o, l/ § c to i* q d qs $ p £ ^ i / i f w m / k -hli^/^wvvi^ —ffjm^/v—w/vu/vn < b o, l/ § c to i* q d qs $ p £ sian o ichuoo f rumten ierufi p 5cc.unon ijtrr prlhsu hurtfro: hi la ohoq 1 s/cho ihd1cq ulccrtidtshtiltc hchc ca/uctwsoco pe puisaaonfs tpj s cl net> oe tulstaon . -p (rceeisy t(nmj pchlooo hcoid i/f siivbdoj [85) two ri/d hepm ih/mois x (2) colficlcnr£u[ /ihpurvd . fo~) (a) c*) ^ ( a e m t j c(unrmf<uj c t ] sflvhdo humtho rrg.tr hvmfjio t^rct^unfi^ . torino t>£ cohfilvuiciom. cuhhjo hum£lo , jctotmum to fakvpicm (csrihtooj (3j (toi cor.c/icm ihvicth flhmgi dcifo j hudson <cjrhis,oli 19(1 4 1 4 a . c . f . b i n a g h j p a g e s e . g. l i n z u a i n " el sentido de giro del vector en estudio, en otras palabras, el mas/menos nos indica si se trata de un crecimiento o un decrecimiento de la curva en el instante de comienzo del fenomeno pulsatorio. la ausencia de tal signo va a indicar incertidumbre en la determination del comienzo, en la forma de la curva y en la bora. la segunda letra que sigue, sera en mimiscula y estara de acuerdo con la clasificacion aceptada internacionalmente; el indice armado asi sera: pc. pulsacidn continua (continuous pulsation) pt. tren de pulsaciones (train of pulsations) . pp. perlas de pulsaciones (pearls of pulsations) pg. pulsaciones gigantes (giant pulsations) pi. pulsaciones largas (long pulsations). podemos decir que hasta aqui no existe una innovation, pero las siguientes cifras, su significado y forma de presentacion, vienen a dar una idea que de buena fe creeinos nos es original. no debe estranar, nuestra presentacion, por que todo el presente estudio se encuentra aun en el periodo inicial de (lesarrollo y por lo tanto no bien se van aclarando los conceptos, se van ampliando los borizontes de interpretation y por lo tanto, tarde o temprano, ban de relucir nuevas formas de estudio y tal ves nuevos coeficientes o indices. la tercera cifra indica el periodo medio de las oscilaciones expresadas en segundos. separada por un punto, sigue otra cifra que es la amplitud media en unidades gammas. y tinalmente, separada por otro punto viene una cifra o termino de configuration, presedido de un numero y seguido de otro numero. veamos: el primer numero expresa en decimas el porcentaje, de deformacion en amplitud; 2 indica 20%, 4 el 40%, etc., de deformation con respecto al valor medio. en el grafico que se adjunta al final, se ve claramente que se ha utilizado cuatro rangos para esta clasificacion: 0-3-5 y 8, se puede ir vieiulo por estimation la deformacion a medir, podiendo el operador apreciar intervalos si lo creyere conveniente. la letra qiie sigue, representa la configuration general, de acuerdo al cuadro que flgura en el dibujo, es el termino de configuration, como le llamaremos. i)e su lectura se puede ver que n , corresponde a una configuration tal, que la oscilacion presenta amplitud constante en toda su presentacion, con solo variaciones expresadas por el coeficiente de amplitud; b, corresponde a un «creciendo » y g, por lo contrario a un «decreciendo». f o r m a p a r a i d b n t i f i c a r a l a s p u l s a c i o n e s g e o m a g n e t i c a s 4 1 5 la forma a, es prototipo de aumento y disminucion. cuando la pulsation se presenta muy desordenada, la clasificaremos con d, y el coeficiente de amplitud que le corresponde es solamente: dudoso, poco regular, y mucho; no vale la pena estimar mas. para el caso de las pulsaciones en perlas, indicaremos con p , y la clasificacidn que la tabla indica, donde el coeficiente de amplitud viene a indicar lo mismo, pero con respecto a la onda modulada y la portadora fundamental. y sin querer, ya nos encontramos hablando del ultimo numero, unicamente para las «pearls pulsation»; el lector ya habr4 adivinado su significado en las demas pulsaciones, es decir: este ultimo numero viene a representar en decimas de porcentaje y en caracter estimativo, la variacidn de frecuencia con respecto al periodo medio antes mencionado. la aplicacidn de este indice p, va a ser sencilla en el caso de las pulsaciones gigantes, que en la mayoria de los casos se presenta como una onda senusoidal bastante pura, con formas n b g o a; pero con coeficiente de amplitud reducido, por su uniformidad, es decir de 0 a 3 en el peor de los casos; tambien aqui el ultimo numero que va a representar la variation de frecuencia o periodo, ser& muy reducido, de 0 a 3, no m&s. en el presente indice el termino de configuration, deja habierta la puerta a mayor numero de formas a clasificar, para poder identificar asi, nuevas presentaciones que querramos tener en cuenta. inconvenientes. probablemente el inconveniente mayor con que cuenta el presente trabajo, es que aun no todos los observatorios, que planetariamente se hallen bien dispuestos, tengan aun instalaciones de registros rapidos. en principio, el presente indice p va a aumentar el trabajo del personal de los observatorios, cosa logica que trae aparejada el progreso de investigation; pero, no lo creemos muy laborioso ya que tiene mucho caracter estimativo. indudablemente, tambien es necesario la confection de una carta international, anual o no liasta nuevos cambios, donde figuren las formas tipicas del coeficiente de configuration; de acuerdo a lo que resolviera el comite international. 4 1 6 a . c . f . b i n a g h j p a g e s e . g . l i n z u a i n " yentajas. la primera es evidente; si caracter eminentemente sintetico. es de por si, suficiente para explicar e informalsomeramente sobre las caracteristicas fundamentales de la pulsation geomagnetica en estudio. cumple el fin, que como creedores le bemos dado; es decir, nos facilitara la information y en base a el, se har&n los pedidos de copias de los magnetogramas, instrictamente indispensables. y para terminal', diremos que el presente indice simplifica mucho el an&lisis de la configuration, dando una idea general del fenomeno pulsacional en estudio y estamos en un todo de acuerdo con el prof. dr. angenheister, en que, el analisis visual inmediato, satisface momentaneamente y reemplaza al analisis armonico del tipo clasico. observatio hudson (argentina), 1963. articolo sugiyama.pdf key words blind thrust – fault-related fold – flexure – high-resolution seismic reflection profiling – continuous coring study of blind thrust faults underlying tokyo and osaka urban areas using a combination of high-resolution seismic reflection profiling and continuous coring yuichi sugiyama (1), kiyohide mizuno (2), futoshi nanayama (1), toshihiko sugai (3), hiroshi yokota (4), takashi hosoya (5), kenichiro miura (6), keiji takemura (7) and naoko kitada (8) (1) active fault research center, geological survey of japan, national institute of advanced industrial science and technology, tsukuba, japan (2) active fault research center, geological survey of japan, national institute of advanced industrial science and technology, osaka, japan (3) department of natural environmental studies, university of tokyo, japan (4) hanshin consultants co. ltd., osaka, japan (5) chuo kaihatsu corporation, tokyo, japan (6) kiso-jiban consultants co. ltd., chiba, japan (7) institute for geothermal sciences, kyoto university, beppu, japan (8) geo-research institute, osaka, japan abstract we acquired high-resolution seismic reflection profiles and continuously cored boreholes to evaluate active flexures produced by major blind thrust fault systems within two densely populated neogene-quaternary sedimentary basins in japan: the fukaya fault system near tokyo in the kanto basin and the uemachi fault system in the osaka basin. the high-resolution seismic reflection survey made clear the length, geometry and growth history of fault-related folds, or flexures formed above the two blind thrusts. continuously cored boreholes linked with high-resolution seismic profiles enabled us to estimate the uplift rate as defined by shallow stratigraphic horizons and constrain the age of the most recent growth of the flexures during earthquakes on the fukaya and uemachi fault systems. even with the high quality of the data we collected, it is still not possible to exactly constrain the age of the most recent blind thrust earthquake recorded by flexure of these fault-related folds. data presented in this paper form the basis for future efforts aimed at mechanical and kinematic models for fault growth to evaluate the activity of blind thrusts underlying urban areas. 1. introduction in japan several neogene-quaternary sedimentary basins have developed in close association with active faults along their margins. as the basins are composed of plio-pleistocene deposits more than one thousand meters thick, some of the active faults do not appear at the surface, but are expressed at the surface as a flex1071 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. yuichi sugiyama, active fault research center, geological survey of japan, national institute of advanced industrial science and technology, higashi 1-1-1, tsukuba, 305-8567, japan; e-mail: sugiyama-y@aist.go.jp ure or monocline usually one to several hundred meters wide. these flexures are commonly concealed beneath young holocene deposits and show no geomorphic expression. high-resolution seismic reflection profiling is useful for studying this type of active blind fault with no topographic expression but with a wide subsurface deformation zone. continuously cored boreholes coupled with high-resolution seismic reflection profiles are especially useful for obtaining data on fault activity such as uplift rate. we applied this combination of methods to flexures that have grown in response to repeated earthquakes of two major blind thrust fault systems in the densely populated kanto and osaka basins, japan. 2. fukaya fault system in the kanto basin 2.1. kanto basin the kanto basin is situated where the pacific and philippine sea plates are subducted beneath eastern japan respectively from the east and south (fig. 1). the basin is approximately 150 km wide and 130 km long in e-w and n-s directions. greater metropolitan tokyo with a population of more than 30 million lies within this basin. neogene-quaternary deposits are 3000 m deep in the central and northwestern parts of the kanto basin. active faults are recognized within the basin around its northwest and western peripheries. geodetic surveys in the past 100 years show that the northwestern part of the basin is subject to compression in ene-wsw direction and extension in nnw-sse direction (e.g., geographical survey institute, 2002). 2.2. fukaya fault the fukaya fault is located in the northwestern part of the kanto basin (fig. 2). this nwtrending west-side-up fault is identified by a 12 km long flexure or monocline, 4 to 14 m high and about 100 m wide, extending across pleistocene terraces. nw-trending east-side-up back thrusts such as the kushibiki and hirai faults also exist as flexures on pleistocene terraces about 5 to 10 km southwest of the fukaya fault (fig. 2). the longterm uplift rate is estimated to be 0.3-0.4 m/ky for the fukaya fault and around 0.1 m/ky for the kushibiki and hirai faults based on measured displacements and the inferred age of terrace surfaces (yamazaki, 1984). although a very shallow earthquake of magnitude 6.9 occurred in 1931 close to the fukaya fault, no surface rupture was recognized along the fault zone. the focal mechanism determined by abe (1974) for the 1931 earthquake is consistent with ne-sw compression and left-lateral strike slip on an n70°w-trending nearly vertical source fault. 2.3. high-resolution seismic reflection profiling since the mid 1990’s, the national institute for earth science and disaster prevention (ka1072 yuichi sugiyama et al. fig. 1. simplified tectonic setting of japan and location of the kanto and osaka basins. mtl: median tectonic line. open arrows show relative convergence direction between japan and the pacific and philippine sea plates, respectively. sahara et al., 1993; kasahara, 1996) and the geological survey of japan (yamaguchi et al., 1996, 1998, 2000) have carried out comprehensive seismic reflection surveys to reveal the subsurface structure of the kanto basin (fig. 2). saitama prefecture (1996, 1999) also undertook acquisition of seismic reflection profiles to evaluate the fukaya fault and its southeastern extension in the omiya terraces (fig. 2). these studies revealed that the fukaya fault extends for about 35 km southeastwards as a blind thrust and is expressed in the shallow subsurface as a fault-related fold. no clear evidence was obtained, however, for its extension towards the northwest except for indi1073 study of blind thrust faults underlying tokyo and osaka urban areas fig. 2. simplified geologic map of the central to northwestern part of the kanto basin. simplified from sugiyama et al. (1997). the basin is bounded by the kanto and ashio mountains on the west and north, respectively. seismic reflection surveys shown by numbered survey lines were conducted by the following authors: 1 saitama prefecture (1999); 2 yamaguchi et al. (1996); 3 yamaguchi et al. (2000); 4 ikawa et al. (1998); 5 inazaki et al. (1988); 6 yamaguchi et al. (1998); 7 kasahara (1996); 8 saitama prefecture (1996); 9 kasahara et al. (1993). rect geomorphic evidence such as lineaments along the mountain foots. we conducted high-resolution p-wave seismic reflection surveys in the suburbs of takasaki (fig. 2) to confirm whether the fukaya fault extended to the northwest. we acquired pwave seismic profiles on 3 lines shown in fig. 2: kaburagawa line (1700 m long), negoya line (800 m long), and satomi line (2150 m long). an oil hydraulic impactor was used as a seismic source, and both the shot points and receiving points were set at 5 m intervals for all the three lines. main parameters for the seismic reflection survey are shown in table i. figures 3, 4 and 5 show depth-converted seismic reflection profiles of each survey line. a flexure or fault-related fold is clearly imaged on each profile. a concealed flexure in the kaburagawa profile illustrates detailed stratigraphic 1074 yuichi sugiyama et al. table i. main parameters for the seismic reflection survey of the fukaya fault system. seismic source impactor (jmi-200) shot point interval 5 m standard times of shot at each shot point 10 receiver 6-geophone array natural frequency 30 hz receiving point interval 5 m standard receiving channels 120 spread end-on maximum offest 600 m standard cmp fold 60 cmp interval 2.5 m recording system g daps-4 record lenght 2 s sampling interval 1 ms fig. 3. depth-converted seismic reflection profile for the kaburagawa line to the south of takasaki (vertical exaggeration × 2). see fig. 2 for the location of the survey line. growth architecture and cumulative deformation of the reflectors (fig. 3). the negoya profile (fig. 4) is characterized by a kink fold with a hinge line dipping 50 to 60 degrees to the southwest. an angular unconformity is also clearly imaged in shallow strata (around 30 to 50 m below sea level) in the northeastern half of the profile. although the satomi profile (fig. 5) is rather low in resolution due to high traffic noise level in the study area, it extends across the entire monocline, which is around 600 m in width. these results show that the fukaya fault extends for about 80 km in an n60°w direction, and forms a fault system, the fukaya fault system, which is composed of several left-stepping en échelon faults. 2.4. continuously cored boring 2.4.1. 145 m long boring we collected a 145 m long core (gs-tk1) from the downthrown side of the fukaya fault system in the takasaki area. the borehole was located close to the cmp (common mid point) number 65 of the negoya seismic line (fig. 4). we identified two tephras named te 5 (ca. 400 ka) and ks 5 (ca. 450 ka), which are widespread in central and western japan, at a depth of 70 m (+ 7.5 m in altitude) and 103 m (– 25.5 m in altitude), respectively (sugai et al., 2000). the te 5 tephra also crops out on the hillside at about 90 to 96 m 1075 study of blind thrust faults underlying tokyo and osaka urban areas fig. 4. depth-converted seismic reflection profile for the negoya line in takasaki city (no vertical exaggeration). both the location of borehole array and tephras recognized by 145 m deep boring and field survey are also projected. see fig. 2 for the location of the survey line. in elevation on the upthrown side of the fault. on the negoya profile, the exposure of the te 5 tephra is projected just on the southwestward extension of the stratigraphic horizon where this tephra was found in the gs-tk1 core (fig. 4). based on the age of the te 5 tephra and its measured vertical offset across the flexure, the long-term uplift rate of the fukaya fault system in the takasaki area is estimated to be more than 0.2 m/ky. 2.4.2. boring array we acquired an array of continuously cored boreholes to define the recency of displacement on the fukaya fault system. the 200 m long boring array was projected onto the flexure between cmp number 120 and 200 of the negoya seismic profile (fig. 4). figure 6 illustrates late quaternary strata defined by the boring array, which is composed of 13 (a1. .a-13) cores ranging in length from 12 to 24 m. both volcanic mudflow deposits (stratigraphic unit e in fig. 6) of around 50 ka and conformably overlying silt-rich formation (unit d) are deformed by the flexure. vertical displacement of the unit e/d boundary is estimated at more than 10 m. this value is consistent with the long-term uplift rate (> 0.2 m/ky) across the fukaya fault system in the takasaki area. the holocene terrace deposits (units c and b) that unconformably overlie the units e and d, however, do not show evidence of deformation. from these data, we conclude that the fukaya fault system in the takasaki area was reactivated in the past 50 ky, although it remains uncertain whether the last faulting event occurred in holocene time. 1076 yuichi sugiyama et al. fig. 5. depth-converted seismic reflection profile for the satomi line west of takasaki (no vertical exaggeration). see fig. 2 for the location of the survey line. 3. uemachi fault system in the osaka basin 3.1. osaka basin the osaka basin is situated in the easternmost part of the seto inland sea immediate north of the e-w-striking median tectonic line (mtl), which is here an active right-lateral fault system (figs. 1 and 7). the central and western parts of the basin are occupied by osaka bay (fig. 7). the northwestern margin of osaka bay is bounded by active faults extending from awaji island to the foot of the rokko mountains on the island of honshu. the 1995 kobe earthquake with over 6000 fatalities occurred on ne-striking faults in this region. the eastern onshore region of the basin is divided into the kawachi plain and coastal lowland by the n-s-trending uemachi fault system and adjoining pleistocene uemachi terrace. the osaka metropolitan area with a population of more than 10 million is contained in the eastern osaka basin. the basin is infilled with over 1000 m of plio-pleistocene deposits. earthquake focal mechanisms, the strike of compressive folds and results of geodetic surveys indicate that the osaka basin is being actively shortened in an e-w to wnw-ese direction. 1077 fig. 6. correlation of continuously cored boreholes within the flexure zone of the fukaya fault system in the takasaki area. see fig. 4 for the projected location of the survey area. 5 stratigraphic units a-e are recognized, and each unit except for the unit e is divided into several subunits. after mizuno et al. (2001). study of blind thrust faults underlying tokyo and osaka urban areas 3.2. uemachi fault system recent acquisition of seismic reflection profiles (e.g., osaka city, 1996; sugiyama, 1997) and mapping of deformed terrace deposits with air photos (e.g., nakata et al., 1996; okada et al., 1996) suggest that the uemachi fault system extends for about 45 km, and branches repeatedly towards the south (fig. 7). along the southern part of the uemachi fault system, fluvial terraces of the last glacial stage are vertically offset about 6 m in a fault-related fold formed above blind thrusts of the fault system. based on borehole data, the long-term uplift rate since the middle pleistocene is estimated to be 0.3-0.4 m/ky for the central part of the fault system (e.g., osaka prefecture, 1999). the cumulative vertical displacement of pre-neogene basement rocks by the uemachi fault system is more than 500 m. 3.3. combination survey of high-resolution swave reflection profiling and continuous coring for the uemachi fault in central osaka we conducted a combination survey of highresolution s-wave reflection profiling and continuous coring across the uemachi fault in central osaka to constrain the rate and recency of move1078 yuichi sugiyama et al. fig. 7. simplified geologic map of the osaka basin. the basin is bounded by awaji island on the west and the rokko mountains on the north. it is also bounded on the south by the izumi mountains along the median tectonic line and by the ikoma mountains on the east. table ii. main parameters for the seismic reflection surveys of the uemachi fault system. survey area central osaka south osaka south osaka survey line length 790 m 4300 m 310 m applied seismic wave s-wave p-wave s-wave seismic source plate hitter mini vibrator plate hitter shot point interval 1 m 10 m 1 m standard times of shot at each shot point 15 20 10 15 40 60 receiver 3-geophone array 9-geophone array 3-geophone array natural frequency 30 hz 27 hz 30 hz receiving point interval 1 m 10 m 1 m standard receiving channels 120 70 100 spread end-on off-end end-on maximum offset 120 m 730 m 100 m standard cmp fold 60 35 50 cmp interval 0.5 m 5 m 0.5 m recording system g daps-4 strata view 60 g daps-4 record length 2 s 2 s 1.5 s sampling interval 1 ms 1 ms 1 ms fig. 8. depth-converted s-wave reflection profile of the uemachi fault in central osaka (vertical exaggeration × 3). see fig. 7 for the location of the survey line. reflection surfaces x and x are correlative to the base of the holocene nanba formation on the upthrown and downthrown sides of the fault, respectively. reflection surface y is probably correlated to the base of the tenma formation on the upthrown side, and y is supposed to be the equivalent horizon on the downthrown side. locations of three continuously cored boreholes are also shown on the profile. study of blind thrust faults underlying tokyo and osaka urban areas 1079 ment along it in holocene time. a 790 m long survey line was located along the north bank of the shin-yodo river, where the uemachi fault had been previously identified by a p-wave seismic reflection profile (yamamoto et al., 1992). acquisition and processing parameters for the s-wave reflection survey are shown in table ii. the uemachi fault is clearly imaged as a flexure, or fault-related fault (fig. 8). a sharp angular unconformity shown by reflection surface y was 1080 yuichi sugiyama et al. fig. 9. summarized stratigraphic correlation between boreholes 1, 2 and 3. the elevation of the three borehole sites is 2.5 m above the sea level. see fig. 8 for the location of each boring site. a clay bed intercalated at the depth of 33 m of borehole 3 is correlated to the tenma formation because it yields a pollen assemblage of the first half of the last glacial stage. after miura et al. (2002). 1081 study of blind thrust faults underlying tokyo and osaka urban areas fig. 10. depth-converted p-wave seismic reflection profile across the suminoe and oriono flexures in south osaka (vertical exaggeration × 2). see fig. 7 for the location of the survey line. locations of s-wave reflection survey line and six boring sites are also shown on the profile. horizons in the three boreholes is of sedimentary origin, and that unit sy-3 is not deformed by the flexure. correlative horizons in units sy-4 and sy-5 were identified at nearly the same level between boreholes 1 and 2, while they show gradual upward decrease in height difference between boreholes 2 and 3 across the flexure. this gradual upward decrease in height difference across the flexure implies burial of a flexure scarp formed by a faulting event. therefore we can point out a possibility that the latest faulting event occurred either around horizon c, or during the deposition of the under lying lower part of subunit sy-5, although the ex act stratigraphic horizon of the latest event is not known because of the lack of correlative horizons within this subunit. based on the above seismic and boring survey results, we conclude that the most recent displacement of the blind uemachi fault occurred between about 9.5 ka and 25 ka, after deposition of the tenma formation, and before deposition of the stratigraphic unit sy-4. the vertical surface displacement associated with the last faulting event might attain 3 m, assuming that the height diffeimaged in the eastern (right) part of the profile. based on boring data, this unconformity is correlated with the boundary between the uemachi formation of the last interglacial stage and the overlying tenma formation dated as ca. 50 to 25 ka. although the flexure deformation is interpreted to reach the lower part of holocene deposits (nanba formation), it is difficult to constrain the timing of the last faulting event only by the seismic reflection results. we therefore acquired three continuous cores, all about 40 m long, from the downthrown side, the flexure foot, and the upthrown side (fig. 8). more than 15 correlative horizons were recognized in these cores by detailed analysis of sedimentary facies, pollen and volcanic ash as well as measurement of physical properties such as electric conductivity, ph, and magnetic susceptibility. correlation of shallow deposits (fig. 9) shows that the vertical separation of correlative horizons in the stratigraphic unit sy-3 between boreholes 1 and 2 is nearly the same as that between boreholes 2 and 3 across the flexure. this result suggests that the difference in height between these correlative yuichi sugiyama et al. rence of horizon c between boreholes 2 and 3 directly reflects the flexure deformation during the last faulting event. from the minimum elapsed time of 9.5 ky since the last faulting event and long-term uplift rate of 0.3 0.4 m/ky, about 3 m or more vertical surface displacement is expected for the next faulting event. this value is comparable to the above-discussed vertical displacement during the last event. 3.4. combination survey of high-resolution seismic reflection profiling and continuously cored borehole array for the suminoe flexure in south osaka we also studied the suminoe flexure, a major component of the southern part of the uemachi fault system, using a combination of seismic reflection profiling (table ii) and an array of con1082 fig. 11. correlation of continuously cored boreholes across the suminoe flexure in south osaka. see fig. 10 for the location of each boring site. modified from nanayama et al. (2000a). tinuously cored boreholes. we first acquired a 4300 m long p-wave seismic reflection profile (fig. 10) across both the locally named oriono flexure along the western hillside of the uemachi terrace and the suminoe flexure, which is completely concealed beneath holocene deposits in the coastal plain. the profile clearly imaged both the oriono and suminoe flexures as typical fault-propagation folds with compressive growth architecture (e.g., suppe et al., 1992; allmendinger, 1998). on the basis of the seismic profile and existing borehole data, the long-term uplift rates of the oriono and suminoe flexures were estimated to be 0.25 m/ky and 0.15 m/ky, respectively. in addition, holocene deposits around the suminoe flexure were inferred to be more than 15 m in thickness from existing borehole data. consequently we chose the suminoe flexure as the target fault for clarifying the recent activity by high-resolution s-wave seismic reflection profiling coupled with an array of continuously cored boreholes. we acquired a 310 m long s-wave reflection profile across the suminoe flexure and obtained six continuous cores ranging in length from 21 to 55 m. correlation of late quaternary deposits among the six cores (fig. 11) is quite consistent with the p-wave and s-wave reflection profiles. the s-wave profile, coupled with the stratigraphy defined in the cores (fig. 12), shows that the stratigraphic unit sm-4 unconformably overlies the tilted unit sm-5, and onlaps the flexure scarp made up of the older unit sm-6. it is also clearly shown that unit sm-4 1083 study of blind thrust faults underlying tokyo and osaka urban areas fig. 12. depth-converted s-wave seismic reflection profile across the suminoe flexure combined with the boring survey result (vertical exaggeration × 4). see fig. 10 for the location of the survey line. and overlying units are not involved in the deformation. based on our interpretation of the s-wave profile linked with the borehole array, we conclude that the most recent activity of the suminoe flexure occurred after deposition of unit sm-5 whose age is estimated at 70-50 ka based on its pollen assemblage (nanayama et al., 2001a), and before deposition of unit sm-4 dated as 15-10 ka. nanayama et al. (2001b) estimated an average uplift rate of 0.2 m/ky for the last 130 ky based on the assumption that the original depositional dip of the base of unit sm-7 is the same as that of the base of unit sm-3. this uplift rate, together with the minimum elapsed time of 15 ky since the last faulting event, implies that 3 m of vertical displacement has been stored on the suminoe flexure. 4. concluding remarks – problems and future research our work in the kanto and osaka basins has demonstrated the effectiveness of combining acquisition and analysis of high-resolution seismic profiles and continuously cored boreholes to assess the paleoseismic record of blind thrusts underlying densely populated and urbanized areas. it is, however, still difficult to constrain the exact timing of the most recent faulting event on blind thrusts because they are expressed at the surface as a wide deformation zone of flexure. this mainly stems from a difficulty in detecting differences between deformed layers and nondeformed beds, or differences between subtle tectonic tilting and primary depositional dip. use of a newly developed device termed a geoslicer (nakata and shimazaki, 1997; atwater et al., 2001) may improve paleoseismic records on blind thrusts by defining and dating liquefaction or subaqueous slides produced by paleoearthquakes. we have focused on the slip rate as a target fault parameter in our blind fault study. it provides a basis for estimating moment release rates and constraining future earthquakes on blind faults (e.g., oskin et al., 2000). it also can be coupled with structural models of fault-related fold growth (e.g., schneider et al., 1996; cannon et al., 2001). our estimates of slip rates for the fukaya and uemachi fault systems, however, are based only upon data obtained from the subsurface several tens to several hundreds meters deep. the slip rates in such a shallow part might be smaller than those at depths where seismic energy is substantially released (e.g., stein and king, 1984). the effect of compaction on thickness of deposits also must be taken into consideration for estimating slip rates (e.g., schneider et al., 1996). we need to make further efforts to clarify the slip rate of the deeper part of these fault systems for acquiring more plausible earthquake scenarios for tokyo and osaka. acknowledgments we thank robert s. yeats and karl mueller for their helpful and constructive comments by which this paper was significantly improved. references abe, k. 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(1984): on the relationship between active faults and basin-forming movement, geogr. rept. tokyo metropolitan univ., 19, 67-78. 1085 study of blind thrust faults underlying tokyo and osaka urban areas l l l l l l layout 6 long-term variations of fumarole temperatures on vulcano island (italy) iole serena diliberto istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5183 abstract fumarole temperatures are the ultimate results of many processes that are encountered by deep fluids during their passage to the surface. here, the time variations of high-temperature fumaroles acquired by continuous monitoring are presented, to show the effects of the forces that act on the system. data acquired by continuous monitoring of fumaroles and the time relationships with the different parameters related to the activity of the volcanic system are discussed. from 1998 to 2010, the temperature and compositional changes of fumarolic gases were monitored at the same time as variations in the number of volcanoseismic events, which indicate frequent variations of energy release (heat and mass flow, and seismic strain release). geochemical modeling applied to the volcanic system of vulcano island suggests that the overall expansion of magmatic gas through the fractured system is an almost iso-enthalpic process at depth, which shifts to an adiabatic process at shallow depth, where the rock permeability increases. thus, the time variations of the fumarole temperatures reflect various physical variations of the system that can either occur at depth or close to the surface. the temperature monitoring performed in the fumarolic area of la fossa cone showed short-term effects related to rain events, and negligible effects related to other external agents (ambient temperature and atmospheric pressure variations). at the same time, the long-term monitoring highlighted some mean-term and long-term variations. these last are the main characters observed in the time-series, and they both appear to be related to endogenous forces that perturb the equilibrium of this complex geochemical system. 1. introduction the aeolian islands are characterized by seismic energy release that is higher than the regional background [montalto 1996, mattia et al. 2008], and by active magmatism that drives the volcanic activity of stromboli volcano and the hydrothermal activities of panarea, lipari and vulcano islands (figure 1). the time-series of geochemical and geophysical parameters acquired at vulcano island allow us to follow in real time the local effects of the different forces acting in the complex geodynamic system of southern tyrrenium. the fumarolic field at la fossa cone is fed by hydrothermal systems located beneath the lipari–vulcano volcanic complex. the main scopes of long-term monitoring are the evaluation of geological risks associated with the forces acting on the system, and the deterministic forecasting of the dynamics observed. in both cases, it is necessary to register the effective dynamics of any measurable variables related to the mass and energy flow, in order to verify the reliability of theoretical models. at vulcano island, continuous monitoring of the temperature begun in 1984 in the framework of geochemical studies aimed at modeling the processes governing the variations observed for seismic and hydrothermal release. the time relationships between the geochemical and geophysical variations have been highlighted in previous approaches to the system [badalamenti et al. 1986, barberi et al. 1991, montalto 1996]. however, fumarole temperature measurements for active volcano monitoring have not yet obtained an appropriate level of attention. often studies dealing with this argument have expressed the need for the gathering of more data and highlighted relevant effects of external disturbances on the temperatures monitored [badalamenti et al. 1986, connor et al. 1993, richter et al. 2004]. among other sites, continuous monitoring of temperature has been carried out for three particular high-temperature fumaroles on the active cone of la fossa (figure 2, fa, f5, f5at). the specific locations of these monitored sites are illustrated in figure 2b. the present study deals with data from the f5at fumarole, which among all of the monitored sites, is the fumarole vent that is most representative of deep processes, and through all the years of monitoring, it has supplied the series of data that has been the least perturbed by external agents. data coming from other sites have been useful to verify by comparison the general characters of some of the variations recorded, as well as occasional external disturbances that have been caused by either technical failures or dramatic changes in atmospheric pressure and temperature. in the present study, a few examples of time-series from the other fumaroles are included. the high temperatures of the fumaroles of the la fossa cone should reflect the thermal equilibrium between higharticle history received january 5, 2011; accepted may 27, 2011. subject classification: volcano monitoring, instruments and techniques, heat release, geochemical data. 175 enthalpy fluids (separated by magmatic sources) and lowenthalpy fluids (separated by hydrothermal sources), the mixtures of which feed the fumaroles. moreover, the outlet temperatures are greatly influenced on the one hand by the ascent processes of fluids from their sources to the surface [nuccio et al. 1999], and on the other hand by exogenous factors, like atmospheric pressure, wind and rain. with the need to take into account so many variables, it is difficult to extrapolate endogenous variations just according to changes in the temperature records. however, the variations observed by long-term monitoring can offer a way to understand the deep processes affecting the system if they are interpreted as comparative evaluations with the other parameters. specifically, the separation of fluid phases from magmatic and hydrothermal reservoirs, and their expansion from their sources to the surroundings, are the processes invoked by geochemists to fit thermodynamic theories to volcanic dynamics. the scientific models published to date to fit the empirical data have not yet been depicted on a multidisciplinary basis that takes into account the complex relationships caused by the thermodynamic laws governing every geochemical system. for vulcano island, previous studies have linked the surface variations in the geophysical and geochemical parameters to active magmatism [barberi et al. 1991, nuccio et al. 1999] or to tectonic processes related to the dynamics of this mediterranean region [montalto 1996, ventura et al. 1999, mattia et al. 2008]. only recently has a multidisciplinary approach been published [cannata et al. 2011], which showed the comparison of consistent sets of data that were acquired by continuous monitoring. some of the data from this last study are included here in more detail (see below). 2. measurement methods since the 1980's, the crater fumaroles have been monitored by the network of the istituto nazionale di geofisica e vulcanologia, sezione di palermo. the temperatures have been measured using thermocouples that were installed at a few fixed points. monitoring stations for high-temperature fumaroles are dangerous to install and also problematic to maintain, because they require close approach to an area where there is copious discharge of hot and toxic fluids. thus, the time series of the temperatures obtained show some gaps due to the temporal failure of the system. however, the attention paid to the maintenance of the monitoring system allowed us to observe the time variations of the surface temperatures at the different monitoring stations, and to follow the recent evolution of the volcanic area. real-time records of high-temperature fumaroles are now being carried out at the f5, f5at and fa fumaroles. the outlet temperatures are measured by chromel-alumel thermocouples (sensitivity, ca. 41 !v/ºc; resolution, 0.025 ºc), these sensors are equipped with stainless-steel probes that diliberto 176 figure 1. structural sketch map of the aeolian archipelago, redrawn from mattia et al. [2008]. atl, aeolian-tindari-letojanni fault system; vf, vulcano fold, cm capo milazzo. figure 2. (a) photograph of the fumarolic field of the la fossa cone, taken from the southern side of the outer edge. the location of the f5at, f5 and fa monitoring points are indicated. (b) mapped locations of the temperature monitoring sites (with 1-h sampling rates) installed at the la fossa cone by the istituto nazionale di geofisica e vulcanologia, sezione di palermo. 177 are made to resist the corrosive acidic gases. for monitoring purposes, the measuring instrument is kept away from the fumarole emissions, while the metal probe is inserted into the steaming vent to a depth of 0.5 m, to improve the contact between the sensor and the steaming ground, and to reduce external disturbance. to take accurate measurements, the technique of cold junction compensation has been applied, and the maintenance work ensures good thermal contact between the input connectors and the measuring instrument. the cold junction compensation is provided by an electronic process that takes into account the voltage produced by the temperature variations at the cold joint. in this configuration, «cold» refers to the ambient temperature, as compared to the hot fumarole temperature. however, sometimes good thermal contact at the cold junction is lost due to the extreme weather conditions of fumarole areas, and disturbances can appear in the time series. in these cases, we recognize erroneous data by comparing the ambient temperature variations with the fumarole records, and as these disturbances can hide the real temperature variations of the fumaroles, we usually erase them from the time series until the technical personnel have been able to do the maintenance field-work. over these years, the sampling frequency of this continuous monitoring has changed: from 1998 to 2008 the data were sampled every 2 h, then later on the data were acquired with at an hourly frequency and are transmitted via radio once a day to the acquisition center in palermo. from the acquisition center, is also possible to program the acquisition, for changes to the sampling frequency and the download frequency. episodic temperature surveys have also been carried out over the fumarolic field using noncontact infrared sensors, known as pyrometers, as well as using these chromel-alumel thermocouples. at these hightemperature fumarole vents, comparisons between the values obtained using these two different sensors show the same data, which confirms that the multiphase system is in thermal equilibrium with the pressurized steam. thus, direct measurements made below ground level produce the same data as indirect measurements, made above the ground surface, that are based on the radiative transfer processes. the main differences between the data acquired by the direct measurement (thermocouple inserted in the vent) and by indirect measurements (non-contact infrared sensor) may occur only in cases where different targeted surfaces are compared. 3. temperature time series 3.1. external disturbances on the temperatures recorded the high-temperature fumaroles of the la fossa cone are located where heavy rain quickly runs off, instead of seeping through the scoria, because they are mainly on the crest (figure 2, f5, f5at) and along the inner slope (figure 2, fa). an example of the temperature variations that are related to rainfall events is reported in figure 3. in autumn, some intense rainfall occurred during october 2010, and minor rainfall thereafter. generally, the effects of intense rainfall on fumaroles is a prompt decrease in temperature, which can range from 20 ºc to 40 ºc across the three different points, with the background temperatures recovered in less than 2 days. however, on one occasion (october 19-20, 2010), the heavy rain fell in a very restricted area (on the active cone) and had a major effect on the fumarole temperatures. after this rainfall, the normal temperatures were recovered over 4 to 7 days. the following minor rain events caused minor disturbances to the timeseries. this period shows that a systematic method for the removal of such external effects by temperature variations is difficult to propose, because these external effects on the fumarole temperatures depend on many factors. besides the total amount of rain, the disturbances caused by rainfall depend mainly on the surface run-off and the ambient temperature, and even more so on the steam flux. these represent too many parameters to be continuously monitored with the resources available for geochemical surveillance. the meteorological influences on the fumarole emissions have also been analyzed at merapi (indonesia) [richter et al. 2004] for fumarole temperatures ranging between 320 ºc and 400 ºc. altough the outlet temperatures are similar to those of vulcano island, the effects of rainfall at merapi are hardly comparable, for various reasons. first, there are different amounts of rainfall (merapi: 1400 mm of rainfall in 6 months; vulcano: generally less than 650 mm of rainfall in 1 year). then there are different sources of thermal energy that feed these two different volcanic systems: at merapi vulcano, an active lava dome was degassing during fumarole temperatures on vulcano island figure 3. short-term effects of rainfall on the fumarole temperatures. continuous monitoring data from the fumaroles on the northern edge of the crater and rain recorded at the village of vulcano porto and at the top of the cone. the period analyzed [richter et al. 2004], while at vulcano, the fumaroles are fed by a hydrothermal system that receives fluids from a magmatic source. what we registered at vulcano suggests a behavior that can be similar to merapi only at times during short-term variations, while the meteorological impact is negligible on the mean or longterm variations of the fumaroles temperatures. moreover, in contrast to what was observed at merapi, at vulcano island no clear relationship was noted between rainfall and occurrence rate of the volcano-seismic events (vses) [cannata et al. 2011]. data from local meteorological stations sited at vulcano porto, and regional data published by the servizio informativo agrometeorologico siciliano (sias, regione siciliana), indicate an average of 650 mm of rainfall over a year in the aeolian archipelago. taking in consideration a set of data covering the last 7 years, most of time (81% of the days), the amount of rain was negligible (<1 mm/day) and only for 19% of the days was more rain received, reflecting a seasonal regime typical of the southern mediterranean region. as well as the main rainfall events, the ambient temperature variations and barometric pressure changes have also been considered in the investigation of external influences on the fumarole temperatures. data recorded by the meteorological stations located in the region are reported in figure 4a,b, as from january 2003 to december 2009. the ambient temperature recorded on salina island (an aeolian island) show seasonal variations on a stationary trend (figure 4a), with the rainfall also showing a stationary trend and cyclic variations (figure 4b). over the same period, the fumarole temperatures clearly showed increasing trends (figure 4c), but no relationships with the ambient temperature or rainfall can be highlighted over the long term. the atmospheric pressures recorded in this mediterranean region show that from may to october there is a stable mean with minor variations, although the variations become more intense in the winter months. the data in figure 5a refer to the year 2008: when the pressure data showed intense short-term variations (from january to april), the fumarole temperatures showed a stationary mean (372 ºc) with minor short-term variations. from may to october 2008, the pressures showed a stable mean with minor variations, while the fumarole temperatures increased from ca. 370 ºc to more than 410 ºc. then from september to december 2008 the fumarole temperatures were affected by some technical disturbances. figure 5b is a scatter diagram of the fumarole temperatures versus the atmospheric pressures, and the dispersion of these data shows that the fumarole temperature is not dependent on the pressure conditions, with both the covariance and the square correlation coefficient very low (cov = –1.54; r2 = 0.010). short-term comparisons can be discussed considering the data reported in figure 6a, which shows the barometric pressures and the f5at temperatures (acquired every hour) from september 9-15, 2010. the quadratic polynomials that fit these two datasets are shown. in the short term, the fumarole temperatures and barometric pressures show an inverse correlation, which was disturbed by rain events that occurred on september 10 and 11. from september 9-12, the atmospheric pressure showed a cyclic variation on a stationary trend. over the same period, the fumarole temperatures showed a cyclic variation that was inversely related to the atmospheric pressure and was superimposed on an increasing trend. so even over this short period, the main feature observed for the fumarole temperatures was a net increase in temperature (of more than 20 ºc in 4 days). figure 6b shows the scatter diagram of the fumarole diliberto 178 figures 4. (a) atmospheric temperatures recorded in the region. the linear trend calculated over 7 years is shown, with the relative equation. data from servizio informativo agrometeorologico siciliano (sias, regione siciliana). (b) rainfall recorded in the region from 01/01/2003 to 31/12/2009. data from servizio informativo agrometeorologico siciliano (sias, regione siciliana). (c) time-series from the continuous monitoring of the fumarole temperatures (rough data from the f5at station). the linear trends calculated over 7 years are shown (see figure 7b for details). 179 temperatures versus atmospheric pressures for this period, and the dispersion of the data shows low correlation between the two variables. to filter short-term rain effects out of data series, the days with heavy rainfall have been removed. comparing the two temperature series (unfiltered and filtered), the main variations are unchanged (figure 6). at la fossa, the cone temperatures showed short-term effects related to rain events, and negligible effects related to atmospheric temperature variations; moreover, no clear relationships have been highlighted between the fumarole temperatures and the barometric changes. 3.2. main variations the intense variations observed in the temperatures of the fumarole vents of the la fossa cone have been variously documented. the highest value measured in the past was 670 ºc, which was recorded in january 1993 for the fa fumarole located on the inner flank of the la fossa cone [nuccio et al. 1999]. afterwards, until march 1995, this site showed temperatures higher than 500 ºc, while the other fumarole vents ranged between 350 ºc and 500 ºc (database of the istituto nazionale di geofisica e vulcanologia, sezione di palermo). after 1995, the temperatures of fa started to decrease, but the surface of active vents increased, and many new vents opened at the edge of the active cone [bukumirovich et al. 1997, harris and maciejewski 2000; pecoraino, personal communication]. nowadays, the fumarole with the highest temperature is located on the edge of the active cone (figure 2, f5at), while the outlet temperature of fa, on the inner flank (figure 2), has continued to decrease. during the summer of 2010, the temperature of f5at fell from 430 ºc to 410 ºc, while the temperature of fa was reaching its minimum value of 260 ºc. it is worth noting, however, that the active exhaling area of the fa fumaroles is still growing, showing a behavior that cannot be interpreted as the result of decreasing heat flow from the underground system. the time-series of temperatures recorded at the f5at station from 1998 to 2009 is shown in figure 7 and discussed here. to show a more homogeneous set of data, just the means of the recorded values have been considered for each day. starting from august 1998, the main characteristics of the temperature variations for f5at are the negative trend fumarole temperatures on vulcano island figure 5. (a) variations in the atmospheric pressure and the fumarole temperatures recorded over one year (2008). (b) fumarole temperatures versus atmospheric pressures, using the data from (a). figure 6. (a) short-term variations recorded for atmospheric pressure and fumarole temperatures (september 9-15, 2010). the linear trends for both of the parameters are shown, as well as the best-fit polynomial curves of the third order, with the relative equations and regression coefficients. (b) fumarole temperatures versus atmospheric pressures, using the data from (a). from august 1998 to january 2001 that was followed by a positive trend that started in december 2000 and lasting until december 2009 (figure 7b). during this period, many temperature variations were superimposed on these trends. on the occasions of the most intense temperature increases, the local seismic activity also increased [aubert and alparone 2000, alparone et al. 2010, cannata et al. 2011]. from october 1998 to november 2009, at least eight temperature variations were observed. these cyclic pulses project out of the major trends (figure 7b) and show comparable intensities. table 1 gives the times of occurrence, the temperatures reached by each peak, and the rates of variation. these temperature variations are discussed further below, with reference to concurrent anomalous variations recorded through different geochemical and geophysical parameters. the first peak of temperature occurred in autumn 1998 (figure 7b, 1 ft). during this period, many geophysical and geochemical parameters suggested an increase in the rate of energy released from the volcanic system. a general increase in temperature, which started on october 3, 1998, reached its maximum values at the beginning of november (figure 8a). the high temperature was maintained until november 20, 1998. in this period, all of the fumaroles sampled for geochemical surveillance showed peaks of carbon dioxide content (figure 8b; data sourced from the database of the istituto nazionale di geofisica e vulcanologia, sezione di palermo) as well as other variations in their chemical compositions that suggested a temporary increase in the magmatic component in the fumarolic gases. from november 17 to 19, 1998, significant seismic activity was recorded at la fossa (table 2): «five events, focal depths ranging 1-4 km, with seismic signals of typical faulting earthquakes, triggered by mechanism of shear fracturing were recorded» [aubert and alparone 2000]. moreover the same paper shows an increase in the heat flux from the soil, occurred at the top of the la fossa cone a couple of days after the seismic swarm. the second peak (august 1989; figure 7b, 2 ft) was similar to the previous one in terms of intensity and duration, but there was no evidence from other parameters relating to increases in the exhaling activity or seismic release in this period. however, both of these first two peaks (november 1998, august 1989) lay on a decreasing temperature trend that stopped in january 2001. after a further 5 years, when the output temperature diliberto 180 table 1. temperature peaks from the fumarole vent f5at. the peaks are labeled according to figure 7b. temperature variations start date peak date start (°c) peak t (°c) peak rate (°c/day) max temperature variation (°c) duration (days) 1 ft 03/10/1998 20/11/1998 373 422 1.02 49 48 2 ft nd 08/08/1999 341 384 nd 43 nd 3 ft 06/11/2004 13/01/2005 351 398 0.69 47 68 4 ft 08/10/2005 31/10/2005 389 415 1.13 26 23 5 ft 10/07/2006 16/07/2006 372 387 2.50 15 6 6 ft 09/07/2007 30/08/2007 360 397 0.71 37 52 7 ft 03/09/2008 26/10/2008 400 428 0.53 28 53 8 ft 23/09/2009 01/11/2009 394 434 1.03 40 39 figure 7. (a) time-series from continuous monitoring of the fumarole temperatures, as the rough data from the f5at station (january 1998 to december 2009). the horizonal line indicates the maximum temperature recorded over this period. (b) as for (a), after removing short-term rain effects. the equations under the linear plots correspond to the trends for the best-fits of the time variations. the peaks due to the fumarole temperature variations are labeled from 1 ft to 8 ft. 181 fumarole temperatures on vulcano island table 2. main characteristics of the seismic swarm that occurred in november 1998 [diliberto et al. 2007]. md: magnitude; rms: root mean square error of residual travel time (s); erh: horizontal error; erz: vertical error; h: focal depth (km). n° event date time md lat long h rms erh erz 1 17/11/1998 10:38:24.76 1.3 38.409 14.956 1 0.28 1.5 0.2 2 18/11/1998 07:39:45.08 2.3 38.409 14.963 4.1 0.1 1.1 0.9 3 18/11/1998 15:49:32.08 1.4 38.397 14.959 2.7 0.09 0.4 0.4 4 19/11/1998 02:12:54.62 2.4 39.409 14.97 4.3 0.2 1.1 1.2 5 19/11/1998 05:34:34.20 1.4 38.407 14.952 1.6 0.18 1.6 0.9 figure 8. (a) cyclic variations recorded at the high-temperature fumarole field from october 1998 to december 1998 (1 ft). the 3 data series come from the fumaroles located on the edge of the active cone (t1, t2, from f5at; t3 from f5). the temperature data were recorded every 2 h, and missing data are due to technical failures caused by the heavy rains that fell after november 20, 1998. the best-fit polynomial curves are shown, with their relative regression coefficients. red lines, time of the faulting seismic sequence with focal depths ranging from 1 km to 4 km. (see table 2). (b) co2 content and cyclic temperature variations (1 ft) of the fumaroles sampled at the la fossa cone from january 1998 to november 1999. the 4th order polynomial curves which best fit the recorded data are given, with their relative square regression coefficients (r2(t)). the t values were normalized for easier comparisons according to the equations: and where represents the background reference for each monitored site, here as, respectively: = 360ºc; = 380ºc; = 270ºc. / m tx tt t '= r t t n n 1 '=r / tr t 1r t 2r t 3r figure 9. (a) temperature variations of the f5at fumarole from 2004 to 2007, and the cumulative number of volcano seismic events (cvses) registered at the la fossa cone (modified from cannata et al. 2011). the major slope changes of the cvse occur when the fumarole temperatures increase. the 3 ft to 6 ft peaks in temperature are shown (as in figure 7b and table 1). dashed lines are the threshold values for the f5at temperatures for (b). (b) probability plot of the fumarole temperatures and the selected thresholds between different samples of values (dashed lines): 366 ºc is the limit between the two main samples interpreted as the background (25% of the values) and the anomalous temperatures (about 60% of the values). fumarole temperatures <300 ºc represent background values that are generally disturbed by heavy rain (less than 8% of the data). fumarole temperatures >420 ºc represent less than 4% of the recorded values, and occurred during the 4 ft geochemical anomaly (november 2005; see (a)). redrawn from cannata et al. [2011]. appeared to be perturbed essentially by heavy rains, a new series of temperature peaks occurred, between november 2004 and july 2006 (figure 7b, 3 ft, 4 ft, 5 ft). again these events coincided with other geochemical and seismic anomalies that were highlighted on their respective timeseries [diliberto et al. 2007, alparone et al. 2010], and that were sometimes interpreted as increases in the magmatic component in the fumarolic gases [granieri et al. 2006]. from december 2006 to july 2007, the outlet temperature was lower than would be expected according to the positive trend started in august 2000, although afterwards the average temperatures increased again, and three new peaks were recorded up to november 2009 (figures 7b, 6 ft, 7 ft, 8 ft). these three peaks ran parallel to the increasing temperature trend, and the last one reached an outlet temperature comparable with that of november 1998 (figure 7b). a general positive correlation between temperature variations, and seismic release and deformation was discussed by cannata et al. [2011]. the results from a comparison of the fumarole temperatures and the rate of vses is reported here over a period when complete and comparable data were available for both time-series (august 2004 to december 2007; figure 9a). diliberto 182 figure 10. (a) daily rates of vses recorded at vulcano island from 2004 to 2007 (redrawn from cannata et al. 2011), and cumulative vse curve (cvse). dashed lines, threshold values separating the background seismic rate from the anomalous seismic rates. thresholds were chosen using the probability plot in (b). (b) frequency distribution of the vses for the threshold selection of the background rates(<12 vses a day) and the anomalous rates (>19 vses a day). figure 11. details from figure 9a for the onsets of each temperature peak. the black triangles on the temperature curves (ft) and the open triangles on the cumulative curve for the volcano-seismic events (cvses) indicate the starting dates for the increases in temperature. see table 1 for dating and details. (a, b) for 3 ft and 4 ft, where the slope changes for the cvse occur after the onset of the temperature peak. (c) for 5 ft, where no slope change is seen for the cvse. (d) for 6 ft, where the slope change for the cvse occurs at the same time as the fumarole temperature increases (july 2007). 183 the thermal parameter is directly related to the thermal energy released (the heat flow) by the underground system, while the rate of vses is directly related to the local seismic energy released (work performed by the multiphase system). the high temperatures of the fumaroles and the rate of vses are expressions of two different energy forms, so their time variations can be somewhat different, for two reasons: 1. the heat flow is an energy form that is continuously released by the hydrothermal system at vulcano island, and it feeds the fumaroles as well as the thermal springs and thermal wells of vulcano porto village. on the contrary, the seismic energy release is discontinuous, whereby the system needs a physical threshold to be passed before registering a vse. 2. the upper limit of the temperature for the fumaroles can be referred to the thermodynamic conditions of the sources, while the upper limit of vses rate depends on the geophysical interpretations, which are beyond the scope of this study. we expect that the maximum output temperature is mainly influenced by the pressure and temperature conditions of the hydrothermal system beneath the active cone, since the fluid is the result of mixing between hydrothermal sources and the deepest magma sources. however, the data show that from january 2004 to december 2007, there were three periods of major vses. in these periods, more than 14, up to 57, events were registered in a day (figure 10a), with a maximum rate in november 2005. figure 10b shows the probability plot for threshold selection between the background and the anomalous rates. during the same interval of time, the fumaroles showed four intense variations in their output temperatures (figure 9a, 3 ft to 6 ft). these peaks of temperature corresponded to a net slope change, except for july 2006 when the vse rate did not exceed the threshold of the anomalous release (see figure 9a). the correlation between the anomalous heat flux from fumaroles and the anomalous volcano-seismic release has only been focused on the data at the onset of an anomalous heat flux (figure 11). the filled triangles plotted along the temperature curve (ft) and the open triangles plotted along the cumulative curve of vses (cvses) (figure 11) indicate the starting dates of the increases in temperature. the anomalous periods registered by geophysical and geochemical data indicate that the heat flow and the seismic release of local origin show a direct correlation. the time lag between the onsets of the different time-series is of the order of days, and different delays are seen for each period among the fumarole temperatures and the vses. the crosscorrelation between the fumarole temperatures and the vses, as applied to the subsets of data shown in figure 11, give a maximum squared correlation factor (r2) ranging from 0.88 to 0.97 for delays of a few days, even though they are not homogeneous. in particular, the vses registered variable delays with respect to the fumarole temperatures, delays lasting from ten days, as in the autumn of 2004, to zero days, as in july 2007. the sources of these vses were located below the la fossa crater area, at depths of 0.5 km to 1.1 km [alparone et al. 2010]. the small delays observed from 2004 to 2007 suggest that the dynamics of the hydrothermal fluids are the cause of most of the vses [cannata et al. 2011]. 4. conclusions gas flux, composition and temperature have been routinely and sometimes continuously determined at active fumarole fields. these parameters have allowed geochemical evaluations of the hydrothermal systems and the interpretation of the changes observed [barberi et al. 1991, tedesco 1995, bukumirovich et al. 1997, nuccio et al. 1999, nuccio and paonita 2001, di liberto et al. 2002, aubert et al. 2008]. during a period of quiescent activity, the steam released through fumarolic fields is an important component in the energy balance of volcanic systems. while steam flow measurements are not yet available for long-term monitoring, continuous monitoring of fumarole temperatures accounts for the evolution of exhaling activity and contributes to the evaluation of the risk level associated with this. results from thermal monitoring of fumarolic fields indicate that the thermo-physical properties of the hydrothermal systems actually feeding the active cone are not stable and are subjected to frequent departures from stationary regimes. in the fumarole area of the la fossa cone, external agents, such as rain and barometric perturbations, have only minor influences and short-term effects on the fumarole temperatures. the main characters of variations observed for a fumarole located on the northern edge of the active cone was a trend for decreasing temperature that was interrupted at the end of 2000, to be followed by new trends of increasing temperatures, which were still active during 2009. some cyclic pulses are superimposed on these trends (figure 7b). these cyclic pulses were observed over eight periods, starting from october 1998 to november 2009. temperature cycles have been often associated with coherent changes in the molar fraction of co2 in fumaroles [aubert et al. 2008, alparone et al. 2010; paonita, personal communication]. the geochemical interpretation of the observed time relationships is that the continuous monitoring of high-temperature fumaroles can reveal increased enthalpy input from a deep magmatic source to the brine-type biphasic hydrothermal system located between 1 km and 2 km in depth [chiodini et al. 1995, nuccio et al. 1999, di liberto et al. 2002]. moreover, the direct correlation observed between the time-series of temperature and seismic release indicates that both parameters give expression to the internal forces acting on fumarole temperatures on vulcano island the geodynamic system of southern thyrrenium. in particular, the surface thermal variations reveal changes in the heat flow from the underground system, while the rate of vses reveals the work performed by the multiphase system, probably as a local response to the regional geodynamics. by paying attention to a parameter directly related to the heat flow (like the maximum temperature of fumaroles), we have been able to follow the active system during periods of fumarolic activity, when the evolution of the equilibrium can be revealed only by instrumental systems. further investigations are necessary for modeling the effective behavior of the natural system and they will be aimed at defining any relationships with other energy forms released by the underground system, like the seismic activity and mass flow. the exchange of the acquired data among different scientific disciplines involved in surveillance, coupled to long-term comparisons, will be necessary to have the surveillance programs working properly and to obtain reductions to the cost-benefit ratio. acknowledgements. the seismic data compared to the fumarole temperatures were provided by the istituto nazionale di geofisica e vulcanologia, sezione di catania, and were published in cannata et al. [2011]. i am grateful to marcello liotta, who has been supplying data since 1998, and to the technical staff of istituto nazionale di geofisica e vulcanologia, sezione di palermo, for the field-work and the technical solutions adopted along the way. moreover, i thank antonio paonita for scientific support and helpful discussions. references alparone, s., a. cannata, s. gambino, s. gresta, v. milluzzo and p. montalto (2010). time-space variation of the volcano-seismic events at la fossa (vulcano, aeolian islands, italy): new insights into seismic sources in a hydrothermal system, bull. volcanol., 72, 803-816. aubert, m. and s. alparone (2000). hydrothermal convective flux variation related to a seismo-tectonic crisis in the fossa of vulcano (italy), c.r. geoscience, 330, 603-610. aubert, m., s. diliberto, a. finizola and y. chebli (2008). double origin of hydrothermal convective flux variations in the fossa of vulcano (italy), bull. volcanol., 70, 743-751. badalamenti, b., s. falsaperla, g. neri, p.m. nuccio and m. valenza (1986). confronto preliminare tra dati sismici e geochimica nell'area di lipari, vulcano bollettino gnv 1986, 37-47. barberi, f., g. neri, m. valenza and l. villari (1991). 19871990 unrest at vulcano, acta vulcanol., 1, 95-105. bukumirovich, t., f. italiano and p.m. nuccio (1997). the evolution of a dynamic geological system: the support of a gisfor geochemical measurements at the fumarole field of vulcano, italy, j. volcanol. geotherm. res., 79, 253-263. cannata, a., i.s. diliberto, s. alparone, s. gambino, s. gresta, m. liotta, p. madonia, v. milluzzo, m. aliotta and p. montalto (2011). multiparametric approach in investigating hydrothermal systems: the case of study of vulcano (aeolian islands, italy), pure appl. geophys.; doi: 10.1007/s00024-011-0297-z. chiodini, g., r. cioni, l. marini and c. panichi (1995). origin of the fumarolic fluids of vulcano island, italy and implications for volcanic surveillance, bull. volcanol., 57, 99-110. connor, c.b., b.m. clement,x.d. song, s.b. laue and j. west-thomas (1993). continuous monitoring of hightemperature fumaroles on an active lava dome, volcan colima, mexico: evidence of mass flow variation in response to atmospheric forcing, j. geophys. res., 98, 19713-19722. di liberto, v., p.m. nuccio a. and paonita (2002). genesis of chlorine and sulphur in fumarolic emissions at vulcano island (italy): assessment of ph and redox conditions in the hydrothermal system, j. volcanol. geotherm. res., 116, 137-150. diliberto, i.s., s. alparone, m. liotta and p. madonia (2007). relationship between surface temperatures and seismic activity at vulcano (eolian islands), geophys. res. abstr., 9, 08553. granieri, d., m.l. carapezza, g. chiodini, r. avino, s. caliro, m. ranaldi, t. ricci and l. tarchini (2006). correlated increase in co2 fumarolic content and diffuse emission from la fossa crater (vulcano, italy): evidence of volcanic unrest or increasing gas release from a stationary deep magma body?, geophys. res. lett., 33, l13316; doi: 10.1029/2006gl026460. harris, a.j.l., l. lodato, j. dehn and l. spampinato (2009). thermal characterization of the vulcano fumarole field, bull. volcanol., 71, 441-458. mattia, m., m. palano, v. bruno, f. cannavò, a. bonaccorso and s. gresta (2008). tectonic features of the lipari-vulcano complex (aeolian archipelago, italy) from 10 years (1996-2006) of gps data, terra nova, 20, 370-377. montalto, a. (1996). signs of potential renewal of eruptive activity at la fossa (vulcano, aeolian islands), bull. volcanol., 57, 483-492. nuccio, p.m., a. paonita and f. sortino (1999). geochemical mixing between magmatic and hydrothermal gases: the case of vulcano island, italy, earth and planetary science letters, 167, 321-333. nuccio, p.m. and a. paonita (2001). magmatic degassing of multi-component vapors and assessment of magma depth: application to vulcano island (italy), earth planet. sci. lett., 193, 467-481. richter, g., j. wassermann, m. zimmerb an m. ohrnbergera (2004). correlation of seismic activity and fumarole temperature at the mt. merapi volcano (indonesia) in 2000, journal of volcanology and geothermal resediliberto 184 185 arch, 135, 331-342. tedesco, d. (1995). fluid geochemistry at vulcano island: a change in the volcanic regime or continuous fluctuation in the mixing of different systems?, j. geoph. reas., 100 (b3), 4157-4167. ventura, g., g. vilardo, g. milano and n.a. pino (1999). relationships among crustal structure, volcanism and strike-slip tectonics in the lipari-vulcano volcanic complex (aeolian islands, southern tyrrhenian sea, italy), phys. earth planet. inter., 116, 31-52. *corresponding author: iole serena diliberto, istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy; email: s.diliberto@pa.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. fumarole temperatures on vulcano island ag_56.05.13_bizzarri_finalonline:layout 6 annals of geophysics, 56, 5, 2013, s0560; doi:10.4401/ag-6279 s0560 calculation of the local rupture speed of dynamically propagating earthquakes andrea bizzarri istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy abstract the velocity at which a propagating earthquake advances on the fault surface is of pivotal importance in the contest of the source dynamics and in the modeling of the ground motions generation. the rupture speed (vr ) is one of the results provided by spontaneous dynamic models of ruptures, in that it is a part of the solution and it is not imposed a priori, like in non spontaneous models or in kinematic models. since vr is numerically retrieved from the spatial distribution of the rupture times (tr ), a well– constrained value of vr in a given fault node is important. in this paper we focus on the numerical determination of vr. by comparing different numerical schemes to compute vr from tr we show that, in general, central finite differences schemes are more accurate than forward or backward schemes, regardless the order of accuracy. overall, the most efficient and accurate algorithm is the five–points stencil method at the second–order of accuracy. these conclusions hold for homogeneous and heterogeneous configurations and for different constitutive models, such as the slip– weakening law and the rate– and state–friction governing equations. it is also shown how the determination of tr can affect vr; numerical results indicate that if the fault slip velocity threshold (vl ) used to define tr is too high (vl ≥ 0.1 m/s) the details of the rupture are missed, for instance the jump of the rupture front eventually occurring for 2–d supershear ruptures. on the other hand, for vl ≤ 0.01 m/s the results appear to be stable and independent on the choice of vl. finally, it is confirmed that in the special case of the linear slip–weakening friction law the computation of vr from the threshold criterion on the fault slip velocity and from the achievement of the maximum yield stress are equivalent. 1. introduction 1.1. scientific rationale one of the most important physical observables of an earthquake rupture is represented by the rupture speed, which quantifies how fast a rupture front is propagating on a fault surface. it is well known that the variations in the rupture speed (and thus the earthquake acceleration) control the radiation of the seismic waves in the surrounding medium, which in turn are responsible of most damage from an engineering point of view [madariaga 1983]. many studies [bizzarri and spudich 2008, dunham and bhat 2008, bizzarri et al. 2010] clarified that there are systematic differences, in terms of amplitude of the motion and of the frequency content, between ruptures which propagate at a speed lower than the s wave speed, vs (i.e., subshear events) and ruptures which accelerate above vs (i.e., supershear earthquakes). numerical models of earthquakes are powerful tools to investigate the behavior of faulting in realistic ambient conditions and provide the opportunity to simulate different shaking scenarios, in the framework of a deterministic approach. spontaneous dynamic models simulate, as a part of the solution, the expansion through time of the broken region of a fault structure, because the rupture speed is not prior imposed — as in non spontaneous models — but it is completely controlled by the adopted fault governing model and by the initial conditions. indeed, spontaneous dynamic models provide the spatial distribution of the rupture times (tr), which namely define the location of the rupture front during the dynamic propagation of the ruptures. the rupture speed vr has then to be retrieved numerically from tr. this poses a relevant numerical problem, which is the main focus of the present study. in seismology, determination of vr is very important. from a purely theoretical point of view pure mode ii, 2–d singular cracks are known to possess only limited values of allowable rupture speeds, namely from 0 to the rayleigh wave speed (vr) and from vs up to the limiting speed of mode ii (which is the p wave speed, vp). in this case they then predict [freund 1979, broberg 1989, 1999] the existence of a so–called forbidden zone (between vr and vs). recently, bizzarri and das [2012] show that in 3–d, non singular ruptures this forbidden zone disappears and that the rupture can advance with rupture speeds continuously distributed bearticle history received january 9, 2013; accepted september 6, 2013. subject classification: earthquake dynamics, rupture speed, supershear earthquakes, rheology and friction of fault zones, computational seismology. tween 0 and vp. this result of course requires a very accurate estimate of vr. moreover, a well–constrained value of vr is important not only in spontaneous dynamic rupture models (as such considered by bizzarri and das [2012] and in the present study), but also in the case of kinematic models of earthquakes. indeed, in these models the analytical evolution of the fault slip velocity is a priori specified and this requires the exact determination of the rupture times (see for instance ide and takeo [1997], among many others). finally, it is worth mentioning that the knowledge of the rupture speed from numerical models is very relevant because recently it has been developed an analysis on possible spatial correlations existing between the various dynamic variables, such as rupture speed, total developed slip, fracture energy density, peak fault slip velocity and stress drop [guatteri et al. 2004, song et al. 2009, schmedes et al. 2010, song and sommerville 2010, bizzarri 2010b, 2012a]. these correlations are extremely important, because they could be inserted as constraints in kinematic modeling of faults, on which current practice in seismic engineering relies. 1.2. methodological approach in this paper we consider the simplified problem of a single, isolated, vertical, planar, strike–slip fault, embedded in a perfectly elastic medium. the fault geometry is the same as that used bizzarri and spudich ([2008]; see their figure 3). the elastodynamic problem is solved numerically, as described in details in bizzarri and cocco [2005], and the parameters adopted in the present study are tabulated in table 1. we hypothesize that the fault obeys the linear slip– weakening constitutive law [ida 1972], which postulates that the shear traction on the fault drops from its maximum, yield level xu down to the residual value xf, over a characteristic slip distance d0 (see equation 25 in bizzarri [2011]). there is no need to overemphasize that this kind of friction model can be merely regarded as one possibility to describe the non singular stress decay during a seismic failure, and that more elaborated governing equations can be employed (see the discussion in bizzarri [2009, 2011]). indeed, in section 4 we will consider the ruina–dieterich law (ruina [1983]; see also equation 35 in bizzarri [2011]) in order to generalize the conclusions obtained in the slip–weakening case. moreover, we disregard here the effects on the dynamic rupture propagation of the variations of the effective normal stress, due for instance to the thermal pressurization of pore fluids, which has been already discussed elsewhere [bizzarri and cocco 2006a, 2006b]. the idealized geometry and the simplified friction law adopted here do not affect the results presented and discussed in the present study, in that they can be exactly extended to other, andrea bizzarri 2 parameter value medium and discretization parameters lamé’ s constants, m = g 35.9 gpa s wave velocity, vs 3.464 km/s rayleigh velocity, vr 3.184 km/s p wave velocity, vp 6 km/s eshelby velocity, ve = vs 4.899 km/s fault length, lf 16 km fault width, wf 11.6 km spatial grid size, dx 5 m (a) time step, dt 1.2 × 10−4 s (b) coordinates of the hypocenter, h ≡ (x1h, x3h) (8,7) km fault constitutive parameters magnitude of the effective normal stress, vn eff 120 mpa magnitude of the initial shear stress (pre–stress), x0 73.8 mpa static friction coefficient, nu 0.677 ( ↔ xu = 81.24 mpa ) dynamic friction coefficient, nf 0.46 ( ↔ xf = 55.20 mpa ) characteristic slip–weakening distance, d0 0.4 m table 1. parameters adopted in the present paper. (a) the spatial discretization ensures a good resolution of the breakdown zone length (xb). on average we have: <nc> = <xb>/dx = 100. (b) for the adopted parameters the courant–friedrichs–lewy ratio, ~cfl vsdt/dx, equals 0.083 and a conservative estimate [e.g., archuleta and frazier 1978] of the critical frequency for spatial grid dispersion, facc (s) = vs/(6dx), equals 115 hz. as required by explicit time stepping schemes we satisfy the condition dt ≤ dx/(2 vp). 2 df = 3 more complicated configurations. in sections 2 and 3 we will consider faults with homogeneous properties, while in section 4 we will discuss the case of heterogeneous rheology. once the fault starts to propagate from the imposed hypocenter h (the nucleation strategy is thoroughly described in bizzarri [2010a]), it spreads bilaterally over the fault releasing energy and emitting seismic radiation. in each node (x1, x3) of the fault plane we define the rupture time tr(x1, x3) as the first instant when the l2–norm of the fault slip velocity in that point, v(x1, x3), exceeds a threshold value vl. we will discuss the choice of the value of vl and its effects of the computation of the rupture speed in section 3. once the rupture times are known, the definition of the rupture tip c at time t is straightforward: (1) which has the following numerical counterpart: (2) where the symbol denotes the discrete equivalent of a generic quantity q, the doublet (i,k) identifies a fault node ( = idx and = (k−1)dx, being dx the spatial grid size) and m defines the time = m dt, being dt the time step. from tr one can also compute, in each node of the fault, the rupture speed vr simply as the inverse of the slowness [e.g., bizzarri and spudich 2008]: (3) in order to explicitly know at which rate the rupture tip c is enlarging in a given direction, we introduce the rupture speed vector vr, defined as it follows [see also pulido and dalguer 2009, hok and fukuyama 2011]: (4) where (5) where are the unit vectors along the strike and depth directions, respectively. the angle u mathematically defines the direction of enlargement of the rupture front c. in particular, when u = 0, we are in the pure mode ii (i.e., inplane) direction; u = r/2 corresponds to the pure mode iii (i.e., antiplane) direction. (note that in equations 4 and 5 we have omitted the explicit dependence of variables on the spatial coordinates x1 and x3 for brevity of notations.). in this study we will explore the different possible computations of vr from equation (3) in order to provide a clear indication of the most accurate and stable computational method of the rupture speed. we will also discuss the determination of the tr, which affects, by definition, the calculation of vr. 2. computation of the rupture speed 2.1. numerical schemes in figure 1 it is imaged the fault slip at t = 1.92 s resulting from a numerical experiments; the white portion of the fault represents the unbroken area and the rupture front c is also indicated as the black line marked with label “0.00”. this is the curve which separates the fault nodes that are slipping and releasing energy from those that are still at rest at this time. the calculation of the rupture speed vr requires the numerical computation of the spatial derivatives appearing in equation (3). in full of generality, we can write calculation of earthquake rupture speed figure 1. distribution of the fault slip developed by a 3–d synthetic earthquake at time t = 1.92 s. the adopted parameters are listed in table 1. due to the symmetry with respect to the hypocenter h, we report only one half of the total fault length lf. , , for the first time,t v x x t vx x l1 31 3 ; $c =^ ^ ^h h h" , ,, i ki k t m( )m ;c = =r^ ^h h" ,l i , , .v x x x xt 1 ( , r x x r 1 3 1 3)1 3 d< < =^ ^ h h ,v cos sinvr r u u= ^ h arctg arctg x x t t x t x t r r r r 3 1 $ $ d d 2 2 2 2 /u = 1 3 p r q q q q e t v u u u u ov v qj x1k x3k t( )mi andx x1 3v v (6) and (7) ( just for an example, for the central finite difference scheme at second order of accuracy we have: lstart = − 1, lend = 1, c−1 = − 1/2, c0 = 0 and c1 = 1/2, so that in order to see how much the values of vr is affected by the choice of the numerical algorithms we test several finite differences schemes, at different orders of accuracy and of different type, either forward or central. we also consider a modification of the central scheme at second order of accuracy, which comes from the so–called five–points stencil [e.g., lapidus and pinder 1999]: (8) and (9) a compendious summary of the considered approximations is reported in table 2. these numerical schemes, as well as the coefficient reported in table 2, are well–known standard approaches to estimate derivatives and they have been introduced in a quite vast body of literature. however, a systematic comparison of these schemes in the computation of the rupture speed in the source dynamic community is presently missed. 2.2. numerical results: example of a 3–d homogeneous rupture to compare the results obtained by adopting the different numerical approaches described in section 2.1, instead of comparing the spatial distribution of the resulting rupture speed over the whole fault plane, we select four profiles, which are at the hypocentral depth and aligned along the strike direction (x1), at the strike coordinate of the imposed hypocenter and aligned along the dept (x3) and along two mixed–mode directions, having azimuth angles of 45° and 30° with respect to x1. the two former profiles represent the situations where the propagation is basically mode ii and mode iii, respectively. the two latter profiles are important, in that both the two non null components of slip, slip velocity and traction coexist (recall that the geometry of the problem is truly 3–d). the first and fourth profiles are indicated by black and red dashed lines, respectively, in figure 2a of bizzarri and das [2012]. the results are presented in figures 2 to 5, respectively. we exclude the values of vr computed within the initialization patch, where the nucleation strategy is imposed (see section 1.2 and bizzarri [2010a] for further details). first of all, we observe that it is not true that an increase of the order of accuracy always produces more stable results; indeed, the level of fluctuations andrea bizzarri 4 i type of the scheme order of accuracy coefficients of different terms, cl – 4 – 3 – 2 – 1 0 1 2 3 4 1 forward 1 0 0 0 0 – 1 1 0 0 0 2 forward 2 0 0 0 0 – 3/2 2 – 1/2 0 0 3 central 2 0 0 0 – 1/2 0 1/2 0 0 0 4 forward 3 0 0 0 0 – 11/6 3 – 3/2 1/3 0 5 forward 4 0 0 0 0 – 25/12 4 – 3 4/3 – 1/4 6 central 4 0 0 1/12 – 2/3 0 2/3 – 1/12 0 0 7 central 8 1/280 – 4/105 1/5 – 4/5 0 4/5 – 1/5 4/105 – 1/280 8 central, from 5– points stencil 2 n/a; see equations (8) and (9) table 2. different numerical schemes used to discretize equation (1). the various multiplier coefficients {cl } of equations (6) and (7) are also reported. for each type of approximation the first and the last non null coefficients determine lstart and lend of equation (6), respectively. , ,x t x x x c t i l k 1 r l l l l l r 1 1 3 start end 2 2 , d + = = ^ ^h hk/ , ,x t x x x c t i k l 1 r l r l l l l 1 3 3 start end 2 2 , d + = = ^ ^h hk/ , , , .)x t x x x t i k t i k1 2 2 11 r r r 1 1 32 2 , d + + ^ ^ ^ h h hc mk k , , , , , x t x x x t i k t i k t i k t i k 1 4 1 1 4 1 1 1 1 1 1 r r r r r 1 1 32 2 , d + + + + + ^ ^ ^ ^ ^ h h h h h k k k k , , , , , x t x x x t i k t i k t i k t i k 1 4 1 1 4 1 1 1 1 1 1 r r r r r 3 1 32 2 , d + + + + + ^ ^ ^ ^ ^ h h h h h k k k k 5 calculation of earthquake rupture speed 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode ii itype = 1 acc. = 1 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode ii itype = 2 acc. = 2 itype = 3 acc. = 2 itype = 8 acc. = 2 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode ii itype = 4 acc. = 3 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode ii itype = 5 acc. = 4 itype = 6 acc. = 4 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode ii itype = 7 acc. = 8 0 5 10 3 3.5 4 4.5 5 5.5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) v p v s v r vs vr forward central 0 2 4 6 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode iii itype = 1 acc. = 1 0 2 4 6 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode iii itype = 2 acc. = 2 itype = 3 acc. = 2 itype = 8 acc. = 2 0 2 4 6 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode iii itype = 4 acc. = 3 0 2 4 6 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode iii itype = 5 acc. = 4 itype = 6 acc. = 4 0 2 4 6 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) mode iii itype = 7 acc. = 8 0 2 4 6 3 3.5 4 4.5 5 5.5 6 distance from the hypocenter (km) r u p tu re v e lo c it y ( k m /s ) v p v s v r vs vr forward central figure 2. comparison between different numerical computations of the rupture speed, as described in the main text (see section 2.1). the profile is taken at the hypocentral depth and parallel to the strike direction (therefore the propagation is basically mode ii). blue and red curves refer to forward and central schemes, respectively. the legends report the type of the schemes, as listed in table 2. for a better comparison, each plot reports numerical schemes having the same level of accuracy. values of vr and vs are also reported as purple and green lines, respectively. the parameters are those listed in table 1. figure 3. the same as in figure 2, but now for a profile along the x3–direction (i.e., the propagation is basically mode iii). strongly depend on the type of the adopted numerical approach. in other words, high–order methods tend to have larger oscillations. interestingly, the central schemes produce less fluctuations with respect to the forward schemes (compare red and blue curves in figures 2 to 5). we can also conclude that the five–points stencil (equations 8 and 9; black curves in figures 2 to 5) gives the best results in absolute terms; it is even better than the central schemes at fourth–order. compared to the central scheme having the same order of accuracy (the second), we can also note that the five– points stencil is more stable, especially in the cases of mode iii propagation (figure 3) and mixed–modes (figure 4 and 5). indeed, it is able to fit the overall trend, but oscillations are smaller than in the other cases. by looking at figure 2 and 5 it emerges that, contrarily to 2–d singular cracks, for 3–d non singular ruptures (for which the stress release is not abrupt, but it is accomplished over a process zone of finite width), the so–called forbidden zone of rupture speeds disappears. indeed, as already discussed in more details by bizzarri and das [2012], in 3–d all the values of rupture speed in between 0 and to the limiting speed of p wave speed are admissible. the results discussed above do not depend on the specific spatio–temporal discretization, but only on the ratio between dx and dt. this ratio basically describe the numbers of the discrete levels of vr (clearly visible as horizontal ensembles of dots in figure 6). remarkably, the previous results hold also for 2–d geometries; in figure 6 we report the results pertaining to a 2–d, pure mode ii rupture, which develops on a fault still obeying the linear slip–weakening friction law. the solution of the elastodynamic problem is performed by using the finite difference scheme described in bizzarri et al. [2001] and references cited therein; the parameters are the same as for the 3–d case (see table 1); the only exception is the spatio–temporal discretization (now dx = 1 m and dt = 2.5 × 10−5 s). note that in this case the values of dx and dt are different with respect to the 3–d simulation. nevertheless, also in this case the forward schemes give more oscillating estimates of the rupture speed, compared to the predictions of the central schemes. overall, the second–order accurate central difference approach produce the smaller oscillations (note that in the case of a 2–d rupture we are able to compute only the schemes 1 to 7 of table 2). in conclusion, the five–points stencil method at the second–order of accuracy appears to be to most efficient (for a purely computational point of view) and accurate (in terms of oscillations) method to compute the rupture speed from the rupture times. andrea bizzarri 6 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 45° itype = 1 acc. = 1 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 45° itype = 2 acc. = 2 itype = 3 acc. = 2 itype = 8 acc. = 2 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 45° itype = 4 acc. = 3 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 45° itype = 5 acc. = 4 itype = 6 acc. = 4 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 45° itype = 7 acc. = 8 0 5 10 3 3.5 4 4.5 5 5.5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) v p v s v r vs vr forward central figure 4. the same as in figure 2, but now for a mixed–mode profile, having an azimuth angle of 45° with respect to the x1–direction. 7 calculation of earthquake rupture speed 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 30° itype = 1 acc. = 1 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 30° itype = 2 acc. = 2 itype = 3 acc. = 2 itype = 8 acc. = 2 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 30° itype = 4 acc. = 3 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 30° itype = 5 acc. = 4 itype = 6 acc. = 4 0 5 10 0 1 2 3 4 5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mixed mode 30° itype = 7 acc. = 8 0 5 10 3 3.5 4 4.5 5 5.5 6 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) v p v s v r vs vr forward central 0 1 2 3 4 5 6 0 5 10 distance from the hypocenter ( km ) r u p tu re v e lo c it y ( k m /s ) itype = 1 acc. = 1 0 1 2 3 4 5 6 0 5 10 distance from the hypocenter ( km ) r u p tu re v e lo c it y ( k m /s ) itype = 2 acc. = 2 itype = 3 acc. = 2 0 1 2 3 4 5 6 0 5 10 distance from the hypocenter ( km ) r u p tu re v e lo c it y ( k m /s ) itype = 4 acc. = 3 0 1 2 3 4 5 6 0 5 10 distance from the hypocenter ( km ) r u p tu re v e lo c it y ( k m /s ) itype = 5 acc. = 4 itype = 6 acc. = 4 0 1 2 3 4 5 6 0 5 10 distance from the hypocenter ( km ) r u p tu re v e lo c it y ( k m /s ) itype = 7 acc. 8 vs vr forward central figure 5. the same as in figure 2, but now for a mixed–mode profile, having an azimuth angle of 30° with respect to the x1–direction. figure 6. the same as in figure 2, but now for a 2–d, purely mode ii rupture. due to the 2–d nature of the rupture we are able to compute only the schemes 1 to 7 of table 2. the nearly vertical clouds of points appearing at about 2 km from the imposed hypocenter are numerical noise, due to the jump of the rupture front occurring in 2–d. 3. effect of minimum detectable slip velocity on rupture time estimates 3.1. the importance of vl in addition to the need to accurately compute the rupture speed from the rupture time array tr (discussed in section 2.2), an accurate estimate of vr strongly relies on the accuracy of the determination of tr. as stated in section 2.1 and in agreement with other studies [e.g., day et al. 2005, rubin and ampuero 2005, bizzarri and spudich 2008], tr is defined in each node of the fault surface as the time level at which the fault slip velocity at that node first exceeds a threshold value (vl). to quantitatively evaluate the effects of the choice of vl in the determination of tr we perform several numerical experiments by varying the value of the threshold velocity and keeping the other parameters unchanged. we perform the simulations in 2–d, with the same parameters as in table 1. in figure 7a we report the resulting values of the ruptures times, as depending on the choice of vl. in general, we observe a dependence of tr on vl especially in the correspondence of the bifurcation of the rupture tip, i.e., when the rupture jumps to the supershear regime. indeed, in 2–d the rupture speed does not cross the so–called forbidden zone (i.e., the interval between the rayleigh wave speed and the s wave speed) and correspondingly the rupture front exhibits a jump [e.g., bizzarri et al. 2001 and references cited therein]. the “mother” front remains sub–rayleigh, while the “daughter” front accelerates from vs up to the compressional wave speed, which represents the limiting speed for this rupture mode [burridge 1973, das and aki 1977, rosakis et al. 1999, among others]. from figure 7a it is clear that for vl = 0.1 m/s (green circles) the separation of the two rupture fronts are quite different with respect to the prediction at lower vl. in particular, for vl = 0.5 m/s (purple symbols) we can not see the bifurcation, as the rupture front appears to be a continuous line. correspondingly, if one looks at the resulting rupture speed (see figure 7b), it is possible to see that the forbidden region disappears for vl ≥ 0.1 m/s. this is physically reasonable, because it indicates that when the threshold velocity is too high some details of the daughter rupture are not captured by the criterion used to define tr. we are aware of the fact that 0.5 m/s is a relatively high value to detect the rupture front; we have considered it just to complete the exploration of the parameter space. moreover, that value is not improper; in figure 8 we plot the time evolution of the fault slip velocity for the 2–d numerical experiment in different locations of the fault. we can clearly see from that figure that even in the first stages of the rupture, when it is still subshear, the peaks of v significantly exceed 0.5 m/s. this becomes even more true as long as the rupture propagates; after birth of the supershear rupture front the peaks are even larger, as already known since bizzarri et al. [2001]. in spite of the relative infrequency of large values of fault slip velocity (namely greater than 1 m/s) obtained in kinematic modeling of faults, it should be noted that large v is routinely found in dynamic models of fault, not only when the fault is very unstable (due for instance to the incorporation of the thermal pressurization of pore andrea bizzarri 8 figure 7. sensitivity of the determination of the rupture times tr from the choice of the threshold velocity vl. the simulations pertain to 2–d ruptures having the parameters of table 1, but different values of vl, as listed in the legends. (a) distribution of tr as a function of the distance from the imposed hypocenter, with an inset reporting a zoom in the correspondence of the rupture tip bifurcation. at a given rupture time, the fault nodes at distances larger than the location of the symbols are not rupturing (unbroken parts). (b) resulting rupture speeds, compute by adopting the central scheme at second order of accuracy (which has been proved to be the best computational approach). note that for vl ≥ 0.1 m/s the bifurcation ( jump) of the rupture front is not well resolved and correspondingly the forbidden region is penetrated (values of vr and vs are also reported dashed horizontal lines). the results are independent on the choice of the threshold velocity for vl ≤ 0.01 m/s. 9 fluids; e.g., bizzarri and cocco [2006b] or of the mechanical lubrication; e.g., bizzarri [2012b]), but also in presence of spatial heterogeneities (e.g., bizzarri et al. [2010]; their animation s3). on the other hand, for small values of vl the results are quite similar. in particular, for vl ≤ 0.01 m/s the rupture times are indistinguishable and independent on the choice of the threshold velocity (figure 7a). remarkably, the same holds also for the rupture speeds (figure 7b). this is not surprising, because the slip velocity histories in the case of the slip–weakening friction law are characterized by an abrupt onset (i.e., a fast accelerating phase). as an overall conclusion, we have found that the value of vl = 0.01 m/s to define the occurrence of a rupture in a given node of the fault is optimal. the results presented here pertain to 2–d ruptures, but the same conclusion holds also in the case of 3–d ruptures. 3.2. comparison between two criteria to identify the rupture front as well–known [bizzarri et al. 2001, among others], the slip–weakening friction gives a more abrupt fault slip acceleration in the early stages of the rupture compared to the more elaborated rate– and state–dependent friction laws [e.g., ruina 1983]. in particular, within the framework of the linear slip–weakening constitutive model the fault slip velocity quickly increases from zero to non zero values at the rupture onset. this is intimately due to the sliding logic; when the traction first reaches the upper yield stress xu the slip commences. the fact that the fault slip velocity v becomes non zero simultaneously when the traction reaches xu is peculiar of the slip–weakening constitutive models; with the rate– and state–dependent friction laws v is non null also before the rupture onset. this is important, because for the slip–weakening law we have an exact estimate of the occurrence of the rupture (and thus an exact determination of tr) against which we can compare the estimates arising from the threshold criterion in terms of v. to quantitatively compare the rupture times defined from the vl criterion (with vl = 0.01 m/s) and those defined when x first reaches xu we compute the misfit m: (10) the spatial distribution of m is reported in figure 9 from which it is possible to see that the difference between the rupture times defined in the two different ways are in general very small (at maximum we have a difference of 0.20 %). the differences are mainly concentrated near the boundary discriminating between the sub– and the supershear regime (black line in figure 6) and in the neighboring of the imposed hypocenter (where the nucleation is imposed and the linear slip– weakening law is not yet operating; see bizzarri [2010a]). from the inspection of figure 9 one also has that tr (from xu) ≤ tr (from v l ) (because of m ≥ 0); this is physically reasonable, because in general first x reaches xu and subsequently v exceeds vl. 4. results for heterogeneous configurations and different friction laws in the previous sections (2.2 and 3) we have considered the simplest case of a fault having homogeneous properties. indeed, we know that real earthquake calculation of earthquake rupture speed 1.50 1.25 1.00 0.75 0.50 0.25 0.0 2.5 5.0 7.5 10.0 12.5 15.0 time ( s ) f a u lt sl ip v e lo ci ty ( m /s ) x 1 = 7 k m x 1 = 6 k m x 1 = 4 k m x 1 = 7 .5 5 k m x 1 = 2 km v l = 0 .5 m /s figure 8. time evolution of the fault slip velocity v at difference spatial coordinates (indicated near each slip velocity peak). the results pertain to the 2–d simulation of figure 7. the horizontal gray dashed line indicates the greater threshold value of v employed in figure 7 to detect the location of the rupture front (vl = 0.5 m/s). , , , , .m x x t x x x x t x xt 100 (from ) (from )(from ) r v rr v 1 3 1 3 1 3 1 3 l ul = x ^ ^ ^ ^ h h h h are heterogeneous in terms of dynamic parameters and frictional properties. a relevant body of literature deals with the numerical simulation of the dynamic propagation of ruptures on heterogeneous fault surfaces [far of being exhaustive, tinti et al. 2005, lavallée et al. 2006, bizzarri et al. 2010, andrews and barall 2011], so that it is very important to quantify the reliability of the rupture speed computed in these situations. to illustrate how the different numerical schemes described in section 2.1 perform, we select a heterogeneous configuration in which the initial shear stress is still aligned in the strike direction, but it is now heterogeneous. in particular, we assume that its magnitude x0 decays a k –1 at high wavenumbers k, namely it has a power spectral density described by equation (21) in bizzarri [2010b]. the spatial distribution of the x0 is reported in figure 10a of bizzarri [2010b] and the corresponding vr is in figure 10d of the same paper. in figure 10 we compare the computations of vr with the different numerical schemes considered in this work (see section 2.1) along the mode ii profile. these results basically confirm and reinforce what we have previously found in homogeneous conditions (see section 2.2); in general higher orders of accuracy do not correspond to small fluctuations in the resulting values of the rupture speeds and the five–points stencil average (black line in figure 10) gives more stable estimates (small oscillations) of vr. the parameters adopted in this heterogeneous model are listed in table 1 of bizzarri [2010b]; it is apparent that it is characterized by different values of dx and dt (greater than those used in homogeneous conditions; see table 1 of the present paper), different elastic constant (lamè constants, p and s wave speeds), a different courant–friedrichs–lewy ratio, different values of the frictional levels and different characteristic slip–weakening distances. we have deliberately chosen a rather different configuration in order show that the conclusions reached in the present study do not depend on the specific configuration we use. as thoroughly discussed by bizzarri [2011], there is no a general consensus on the most appropriate constitutive model to be applied to describe the traction evolution during an earthquake failure. in addition to the linear slip–weakening law considered so far, we also consider the ruina–dieterich friction law [e.g., ruina 1983], in which the dependence of the fault traction is on the slip velocity and on one state variable, accounting for the memory of the whole history of the seismogenic fault. this governing model (see equation 35 in bizzarri [2011]) is inherently different with respect to the linear slip–weakening law, but has received a lot of attention in the modeling of the seismic source processes (again, far of being exhaustive, bizzarri and cocco [2005], lapusta and liu [2009]). we have then considered this model to test the different numerical schemes for the computation of vr. the considered model has the same heterogeneous initial shear stress as the previous model; the parameters are listed in table 1 of bizzarri [2010b] and the resulting spatial distribution of the rupture speed is imaged in figure 10c of that paper. it is clear that the heterogeneities cause a very complicated evolution of the rupture, which is now characterized by some accelerations and further decelerations of the rupture front, which in turn has a very complex shape (compared to the homogeneous configuration). figure 11 reports the results of the resulting rupture speed computed with the different numerical schemes. it is apparent that now the absolute level of fluctuations is even larger than in the previous heterogeneous case (figure 10), where the rupture is governed by the linear slip–weakening law. this difference can be easily explained by considering the different global rupture behaviors in the two cases; in the simulation with the ruina–dieterich law the rupture heals in many patches of the fault surface, while in the slip–weakening case, it andrea bizzarri 10 figure 9. misfit between the rupture times defined from the vl criterion and those defined when x first reaches xu. the quantity m reported in the figure is defined in equation (10). the simulation pertains to a 3–d rupture with the parameters are tabulated in table 1. the transition between sub– and supershear regime is indicated with the black line. 11 calculation of earthquake rupture speed figure 10. the same as in figure 2, but now for a heterogeneous configuration, which is described in details in section 4. figure 11. the same as in figure 10, but now for a heterogeneous configuration obeying to the ruina–dieterich friction law (see section 4 of the main text for further details). 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 1 acc. = 1 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 2 acc. = 2 itype = 3 acc. = 2 itype = 8 acc. = 2 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 4 acc. = 3 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 5 acc. = 4 itype = 6 acc. = 4 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 7 acc. = 8 0 2 4 6 2.5 3 3.5 4 4.5 5 5.5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) v p v s v r vs vr forward central 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 1 acc. = 1 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 2 acc. = 2 itype = 3 acc. = 2 itype = 8 acc. = 2 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v e lo ci ty ( km /s ) mode ii itype = 4 acc. = 3 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v el o ci ty ( km /s ) mode ii itype = 5 acc. = 4 itype = 6 acc. = 4 0 2 4 6 0 1 2 3 4 5 distance from the hypocenter (km) r u p tu re v el o ci ty ( km /s ) mode ii itype = 7 acc. = 8 0 2 4 6 2.5 3 3.5 4 4.5 5 5.5 distance from the hypocenter (km) r u p tu re v el o ci ty ( km /s ) v p v s v r vs vr forward central does not heal. a direct comparison of these behaviors comes from figures 10c and 10d of bizzarri [2010b], where it is plotted the spatial distribution of the rupture speed over the whole fault plane. finally, we mention that in extreme cases of heterogeneities in frictional properties, to better constrain the resulting rupture speed we should improve the spatial and temporal discretization of the model. the results presented in figure 11 confirm that central schemes perform better than forward schemes (at a given order of accuracy). we note that at high orders of accuracy (4 and 8) the oscillations of the central schemes are greater than in the previous case (compare figures 10 and 11), but they are generally lower than those emerging in forward schemes. moreover, we also confirm here that an increase of the order of the accuracy does not improve the results. also in this case the five–points stencil method can be regarded as the best candidate to compute the rupture speed. 5. conclusions in this paper we have considered the problem of the determination of the rupture speed vr characterizing a fully dynamic, spontaneous earthquake. in spontaneous models the position, as well as the shape, of the rupture front, separating the broken from the unbroken portions of a discontinuity interface, is a part of the solution and it is totally controlled by possible geometric irregularities, rheological properties of the fault structure, heterogeneities, etc. the computation of the rupture speed is performed quite routinely by the modellers of the seismic source. however, a systematic, quantitative exploration of the different numerical method to retrieve it from the results of a numerical experiments is currently missed and this is the primary goal that the present study aims to fill. we analyze the simplistic case of a single fault, governed by the linear slip–weakening friction and with homogeneous and heterogeneous rheological properties (see section 2.2 and 4, respectively), in order to provide some hints for the determination of the rupture times tr and for the computation vr. we also consider a rate– and a sate–dependent friction law (the ruina–dieterich model; see section 4) to generalize the conclusions of the present study. we consider a test case of a supershear event with both 2–d (pure mode ii) and truly 3–d (with mixed mode ii and mode iii, with rake variation allowed) geometries. the results clearly indicate that the way to define tr is very important. in particular, if the fault slip velocity threshold (vl) used to define tr is too high (vl ≥ 0.1 m/s) some details of the rupture process are missed. for an example, in the case of a 2–d, pure mode ii rupture, for which it is expected to have a rupture jump from sub–raleigh to supershear speeds [andrews 1976, bizzarri et al. 2001], the rupture front bifurcation is lost and the jump of the front is not properly captured (see figure 7a). this has relevant consequences when one computed the resulting rupture speeds. on the other hand, for vl ≤ 0.01 m/s the results appear to be stable and, more importantly, independent on the choice of vl. we recall that in the framework of the linear slip– weakening governing law we have an exact estimate of the rupture time, which is formally defined as the instant when the traction reaches the upper yield stress and this exact estimate has been checked against those resulting from the slip velocity criteria with different threshold values. we can therefore regard the value of vl = 0.01 m/s as the optimal candidate to the used also with other constitutive models, such as the rate– and state–dependent friction laws, for which the rupture time can not be intrinsically determined by the governing model itself. the choice of vl = 0.01 m/s, indicated as the preferred one from the present study, is in agreement with previous papers on dynamic modeling of faults [day et al. 2005, bizzarri and spudich 2008, bizzarri et al. 2010, bizzarri 2011]. we also emphasize that in heterogeneous conditions some patches of the fault can potentially produce small stress drop, due to the frictional heterogeneities, and therefore exhibit there relatively small values of fault slip velocity. at the same time we can also expect rapid changes of rupture times on small scales, when the rupture accelerates or it slows down due to the local stress inhomogeneities. as a consequence high values of vl can in principle produce some bias in the determination of tr and thus of vr. (incidentally, it is worth to mention that the adoption of vl = 0.1 m/s has been used in other previous papers [e.g., belardinelli et al. 2003, antonioli et al. 2006, bizzarri and belardinelli 2008] to discriminate between aseismic and seismic regimes of a fault subjected to an external stress perturbation and not to define the rupture front.) moreover, we show that, in general, central finite differences schemes are more accurate than forward or backward schemes, regardless the order of accuracy. overall, the most efficient and accurate algorithm is the five–points stencil method at the second–order of accuracy, because it produces small oscillations. remarkably, these conclusions hold both for simplistic homogeneous and for more realistic heterogeneous configurations, and do not depend on the particular choice of the parameters of the model (expressed in terms of properties of the elastic medium in which the fault plane is embedded, spatial and temporal discretization, constitutive parameters, ratio between the andrea bizzarri 12 13 spatial grid size and the temporal step size). more importantly, these results are general and hold not only for the linear slip–weakening law, but also for different constitutive model, such as the rate– and state–dependent friction laws. the time required by the different algorithms used to compute the rupture time (post–processing time) is in all cases negligible with respect to the total cpu time required to solve the elastodynamic problem. we remark that these conclusions hold for the finite difference, conventional grid method employed here [bizzarri and cocco 2005], and a similar test would be possible also for other numerical algorithms, such as finite elements, spectral elements, discontinuous galerkin, etc. and this comparison in those cases would be also beneficial. acknowledgements. we would thank s. das for intriguing discussions and two anonymous reviewer for their comments which contribute to improve the paper. the editor responsible of this paper was f. bianco. references andrews, d.j. 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(1983), slip instability and state variable friction laws, j. geophys. res., 88, 10359-10370; doi:10. 1029/jb088ib12p10359. schmedes, j., r.j. archuleta and d. lavallée (2010). correlation of earthquake source parameters inferred from dynamic rupture simulations, j. geophys. res., 115, b03304; doi:10.1029/2009jb006689. song, s.g., a. pitarka and p. sommerville (2009). exploring spatial coherence between earthquake source parameters, bull. seismol. soc. am., 99, 2564-2571; doi:10.1785/0120080197. song, s.g., and p. sommerville (2010). physics-based earthquake source characterization and modeling with geostatistics, bull. seismol. soc. am., 100 (2), 482-496; doi:10.1785/0120090134. tinti, e., a. bizzarri and m. cocco (2005). modeling the dynamic rupture propagation on heterogeneous faults with rate– and state–dependent friction, annals of geophysics, 48 (2), 327-345. *corresponding author: andrea bizzarri, istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy; email: bizzarri@bo.ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. andrea bizzarri 14 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true /maxsubsetpct 100 /optimize false /opm 1 /parsedsccomments true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice l ' e l a b o r a t o r e e l e t t r o n i c o n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i f i l t r a g g i o n u m e r i c o ai m i n i m i q u a d r a t i (electronic computer in the study of time series. numerical filtering to the last squares) e . p a s q u a l i l a r i p . b a n d i ( * ) r i c e v u t o il 24 g e n n a i o 1972 riassunto. — i n q u e s t o l a v o r o v i e n e d i s c u s s a l a o p p o r t u n i t à d i u t i l i z z a r e il filtraggio n u m e r i c o ai m i n i m i q u a d r a t i n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i . si d à l a p o s s i b i l i t à ai r i c e r c a t o r i d i p o t e r u t i l i z z a r e q u e s t a t e c n i c a a n c h e s e n z a s p e c i f i c h e c o m p e t e n z e s u l l a p r o g r a m m a z i o n e di c a l c o l a t o r i e l e t t r o n i c i . il r i c e r c a t o r e p u ò a l t r e s ì r i s t r u t t u r a r e a p i a c e r e u n a p a r t e d e l p r o g r a m m a d e s c r i t t o p e r i n t r o d u r r e c o n d i z i o n i c h e gli p e r m e t t a n o d i s f r u t t a r e c o m p l e t a m e n t e l a s u a s e r i e di d a t i t e n e n d o c o n t o d e l l e c a r a t t e r i s t i c h e d e i l a b o r a t o r i d a c u i e s s i d a t i p r o v e n g o n o . summary. — d u r i n g t h i s w o r k w e a r e c o n c e r n e d w i t h t h e e x p e d i e n c e t o m a k e u s e of n u m e r i c a l filtering t o t h e l a s t s q u a r e s i n t h e s t u d y of t i m e s e r i e s . t h e u s e r s c a n u t i l i z e t h i s t e c h n i q u e a l s o w i t h o u t s p e c i f i c k n o w l e d g e a b o u t p r o g r a m m i n g of e l e c t r o n i c c o m p u t e r s . t h e u s e r s c a n a l s o m a n i p u l a t e a t p l e a s u r e a p a r t of p r o g r a m i n o r d e r t o m a k e t h e m o s t of t h e i r d a t a s e r i e s , b e i n g k n o w n t h e f e a t u r e s of t h e l a b o r a t o r i e s w h i c h t h e d a t a c o m e f r o m . 1 . tecnica di calcolo e possibilità. il ricercatore può trovarsi nella condizione di avere una serie di dati molto grande e voler vederne le caratteristiche globali, ad esempio estraendo da essa informazioni relative alla esistenza di particolari bande di frequenza. alcune tecniche numeriche relative a questa impostazione del problema, prendono il nome di filtraggio numerico (2. 3. o, 7, s, 9) e n e s o n o n o t e di diverso tipo e con diverse finalità: (*) c e n t r o d i c a l c o l o i n t e r u n i v e r s i t a r i o d e l l ' i t a l i a n o r d o r i e n t a l e , c a s a l e c c h i o d e l r e n o , b o l o g n a ( i t a l i a ) . l i . p a s q u a l i l a k i p . b a n d i gli a u t o r i h a n n o scelto d i e f f e t t u a r e il calcolo dei coefficienti del filtro n u m e r i c o come d e s c r i t t o d a (2) i n t e g r a t o d a i1). le t e c n i c h e esposte p e r m e t t o n o di e s t r a r r e b a n d e di f r e q u e n z a u t i l i z z a n d o i coefficienti nel n u m e r o m i n o r e possibile e con u n a r i s p o s t a in f r e q u e n z a e s t r e m a m e n t e v i c i n a a quella ideale; n a t u r a l m e n t e essa è in f u n z i o n e del p r e d e t t o n u m e r o di coefficienti. a d ogni modo, poiché il p r o b l e m a a v o l t e i m p o n e u n a s e p a r a z i o n e t r a d u e t i p i d i i n f o r m a z i o n i a l t r i m e n t i indistinguibili, la t e c n i c a s e g u i t a d à q u e i coefficienti che sono s t i m a t i i migliori s o t t o le condizioni richieste. le relazioni su cui si b a s a il calcolo dei coefficienti sono: — p e r il filtro di t i p o passa-basse f r e q u e n z e s o t t o la condizione c h e la s o m m a dei coefficienti sia 1, si h a : 1 / 2 k + 1 a . 2k — l 2 2n + 1\ 1 q* = n \ t sl7l~~t wtt 8t * ] + — p e r il t i p o di filtro p a s s a b a n d a s o t t o la condizione che la s o m m a dei coefficienti sia 0, si h a : 1 / 2 k + 1 2 k + 1 2k — 1 2fc — 1 \ (jk = ~ i al 71 — — a i t i — bi ti 1/si ti — — — 71 \ jl t 1 ! 2 / 2n + 1 2n+1 ti (2.v 1 ) 1 7 1 t ~ ' s i : t r — p e r il t i p o di filtro p a s s a a l t e f r e q u e n z e s o t t o la condizione che la s o m m a dei coefficienti sia 0, si h a : 2 ti 2 2n + 1 1 ho = 1 — _ ^ -rf + /o ut i n . s l 71 71 t (2n + 1) ti t 2n 1 / 2k — 1 2k + 1 \ , 2 2k + l 1 (2k+l)ji t 2n + 1 p e r la discussione sulla scelta d a i m p o r r e alla s o m m a dei coefficienti si r i m a n d a al lavoro (2) già c i t a t o . p e r r e n d e r e accessibile il p r o g r a m m a di calcolo a qualsiasi ricerc a t o r e per qualsiasi p r o b l e m a di t a l e n a t u r a , gli a u t o r i h a n n o c o n c e n t r a t o in p o c h e schede p a r a m e t r i t u t t e quelle i n f o r m a z i o n i che sono necessarie al calcolatore p e r la elaborazione dei d a t i . vengono eseguiti in successione: calcolo dei coefficienti del filtro, r i s p o s t a del filtro alle f r e q u e n z e l ' e l a b o r a t o r e e l e t t r o n i c o n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i , e t c . 6 5 interessate, valutazione statistica dei d a t i filtrati sulla base della deviazione s t a n d a r d dei d a t i in ingresso, calcolo della serie residua e preparazione dei dati da graflcare. l a graficazione è ottenibile mediante un programma (5) che f a p a r t e della biblioteca del centro di calcolo. i d a t i in ingresso vengono quindi t r a t t a t i a seconda delle opzioni espresse dai ricercatori (v. descrizione delle schede p a r a m e t r i più sotto) sì da costituire un insieme omogeneo: le assenze di dati, inevitabili in lunghe registrazioni per mancanza di f u n z i o n a m e n t o delle apparecchiat u r e o alterazione delle stesse, possono essere o sostituite col valore della media della serie o con u n a interpolazione lineare f r a gli estremi validi della interruzione (in questo caso la coincidenza f r a il valore estremo finale calcolato e quello reale viene fissato dal ricercatore, in f o r m a percentuale, a piacere), o con qualsiasi altra f o r m a che il ricercatore ritenga opportuno. i n questo ultimo caso, il ricercatore dovrà però scriversi u n a subroutine a d a t t a col nome di c h i u s e con i p a r a m e t r i descritti più sotto che sostituisca quella inserita nella versione s t a n d a r d del programma. i 6 p a r a m e t r i della subroutine c h i u s devono avere il seguente significato: p t reale valore dell'ultimo d a t o precedente al gap p 2 reale valore del primo d a t o successivo al gap p 3 intero numero d'ordine nella serie elaborata del d a t o il cui valore è p 4 intero numero d'ordine nella serie elaborata del d a t o il cui valore è p 2 p 5 reale percentuale d'errore (vedi c h i u s x nella sezione «utilizzazione del programma») p„ reale p a r a m e t r o di uscita: matrice dei d a t i da sostituire all'interno del gap (dimensione massima 500). la subroutine viene c h i a m a t a all'interno del p r o g r a m m a ogni qual volta si incontra un gap. 2 . d e s c r i z i o n e t e c n i c a d e l p r o g r a m m a ii p r o g r a m m a consta di u n main e (li 5 subroutines, che sono: c o e f i l , s i n t . r d w r . c h i u s . move. k. pasquali lari p . r a n d i ti main legge i p a r a m e t r i necessari alla elaborazione, li controlla e dispone le c h i a m a t e alle subroutines a seconda dei problemi che sono posti ed infine s o v r a i n t e n d e alle s t a m p e ed alla preparazione dei d a t i su nastro da utilizzarsi per la graficazione. c o e f i l : calcola i coefficienti del filtro numerico con le m o d a l i t à richieste dal ricercatore; e f f e t t u a il controllo sulla somma degli stessi (2) e calcola la risposta del filtro alle frequenze richieste. si nt: è c h i a m a t a da c o e f i l per il calcolo dei coefficienti del filtro numerico f1). e d w r : p r e p a r a i d a t i della serie temporale che debbono essere elab o r a t i : elimina d a t i r i t e n u t i non validi e p r o v v e d e alla sostituzione degli stessi nelle forme previste. c h i u s : viene c h i a m a t a da r d w r q u a n d o è richiesta u n a interpolazione lineare s o s t i t u t i v a di d a t i m a n c a n t i o r i t e n u t i non validi dal ricercatore. l'interpolazione è giudicata o t t i m a q u a n d o la differenza f r a il primo d a t o valido, successivo ai d a t i m a n c a n t i , e quello calcolato dalla interpolazione sono uguali a meno del valore dichiarato dal ricercatore (v. « utilizzazione del p r o g r a m m a »). move: ripristina le condizioni di lavoro, m a n t e n e n d o la sequenza temporale, in q u a n t o il p r o g r a m m a elabora u n a serie qual u n q u e di dati, però a g r u p p i di 2000 per volta. 3 . u t i l i z z a z i o n e d e l p r o g r a m m a . le schede contenenti i p a r a m e t r i e t u t t e le altre informazioni necessarie alla elaborazione della serie dei d a t i sono 7 e debbono essere sempre presenti. le prime 4 sono relative al calcolo dei coefficienti del filtro e alla risposta del filtro alle frequenze di prova. le ultime 3 sono legate al processo di elaborazione dei dati. altre schede sono richieste se si verifica la condizione g a p i x ^ 0 come è scritto nella n o t a relativa a questo p a r a m e t r o nella 5a scheda p a r a m e t r i . l ' e l a b o r a t o r e e l e t t r o n i c o n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i , e t c . 6 7 l a s c h e d a d e s c r i z i o n e del t i p o di f i l t r o . f o r m a t o di l e t t u r a (213, 3 f 1 0 . 4 . ) c o l o n n a n o m e d e l l a v a r i a b i l e d e s c r i z i o n e 1 3 k t f s e = 1 filtro p a s s a b a s s o se = 2 filtro p a s s a a l t o se = 3 filtro p a s s a b a n d a •4-6 k c o e n u m e r o dei c o e f f i c i e n t i del filtro. e s s e n d o s i m m e t r i c o , o c c o r r e i n d i c a r e s o l t a n t o q u a n t i t e r m i n i s t a n n o d a u n a p a r t e r i s p e t t o al t e r m i n e c e n t r a l e ; se si v u o l e un filtro di 21 t e r m i n i k c o e s a r à = 10. 7-16 t g v a l o r e del p e r i o d o s u c u i v i e n e f a t t a la finestra. 17-26 t p v a l o r e del p e r i o d o a c u i v i e n e c h i u s a la fines t r a ( q u e s t o v a l o r e è ^ 0 solo q u a n d o k t f = 3). 27-36 sg v a l o r e a c u i l a s o m m a dei c o e f f i c i e n t i d e v e e s s e r e u g u a g l i a t a . i n g e n e r a l e : s e k t f = 2 o 3 s g = 0 s e k t f = 1 sg = 1 2 a s c h e d a d e l i m i t a il c a m p o dei t e s t di f i d u c i a . f o r m a t o = (13) c o l o n n a n o m e d e l l a v a r i a b i l e d e s c r i z i o n e 1 3 n t è il n u m e r o d i p e r i o d i s u c u i si f a il t e s t . 6 8 r . p a s q u a l i l a r i p . r a n d i 3 a s c h e d a e l e n c o di p e r i o d i s u c u i f a r e il t e s t di fiducia. f o r m a t o = ( 8 e 1 0 . 2 ) c o l o n n a n o m e d e l l a v a r i a b i l e d e s c r i z i o n e 1-11 t v a l o r e dei p e r i o d i su c u i si c h i e d e il t e s t ( q u e s t a s c h e d a a n d r à r i p e t u t a fino a d e s a u r i m e n t o dei p e r i o d i r i c h i e s t i , m e t t e n d o n e 8 p e r s c h e d a ) . 4 a s c h e d a c h i u d e l a p r i m a s e r i e di o p e r a z i o n i . e s s a deve e s s e r e u n a s c h e d a s e n z a p e r f o r a z i o n i . 5 a s c h e d a s c e l t a d e i t i p i d i c a l c o l o e d e v i a z i o n e s t a n d a r d d e l l a s e r i e . f o r m a t o = ( i i , f 1 0 . 0 ) c o l o n n a n o m e d e l l a v a r i a b i l e d e s c r i z i o n e 1 2-9 i p a r s i g m a se = 1 v i e n e c a l c o l a t a solo l a s e r i e filtrata se # 1 v i e n e c a l c o l a t a a n c h e l a s e r i e r e s i d u a d e v i a z i o n e s t a n d a r d d e l l a s e r i e o r i g i n a l e . 6 a s c h e d a c o n t i e n e i p a r a m e t r i di l e t t u r a e di e l a b o r a z i o n e d e l l a s e r i e . f o r m a t o = (a3, 212, i i , i x , 12, 2 x , 12, 2 x , 11, i x , f 1 0 . 0 . ) c o l o n n a n o m e d e l l a v a r i a b i l e d e s c r i z i o n e 1 3 4-5 6-7 1 d i s p n n m t d e v e e s s e r e x x x n u m e r o d e i d a t i p e r r e c o r d ( m a x . 80) n u m e r o di u n i t à l o g i c a di l e t t u r a d e i d a t i s e = 5 v e n g o n o l e t t e s c h e d e s e = 66 i d a t i s o n o s u u n filo di q u a l u n q u e n o m e d i c h i a r a t o s u l l a s c h e d a l g o (v. d e s c r i z i o n e d e l l ' i n p u t p i ù s o t t o ) l ' e l a b o r a t o r e e l e t t r o n i c o n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i , e t c . 6 9 s e g u e 6a s c h e d a . c o l o n n a 10-11 1 4 1 5 20-29 n o m e d e l l a v a r i a b i l e i i z e r x d e s c r i z i o n e g a p i x (*) se # 0 s e = 0 s e # 0 g a p e x (*) se = 0 se jf 0 c h i u s x se = 0 s e # 0 p e r c x gli zeri d e l l a s e r i e i n i n p u t s o n o r e a l i gli zeri (o l ' a s s e n z a di p e r f o r a z i o n e ) d e l l a s e r i e i n i n p u t s o n o d a t i m a n c a n t i e v e r r a n n o s o s t i t u i t i nel corso del p r o g r a m m a n o n ci s o n o g a p s in l e t t u r a i n d i c a il n u m e r o d i v o l t e c h e si d e b b o n o s a l t a r e d a t i i n l e t t u r a , d a t i c h e v e r r a n n o s o s t i t u i t i s u c c e s s i v a m e n t e ( m a x . 99) la s u c c e s s i o n e dei d a t i o t t e n u t a d o p o la l e t t u r a e l ' e v e n t u a l e s o s t i t u z i o n e ( g a p i x ^ 0) è c o m p l e t a la s u c c e s s i o n e o t t e n u t a è i n c o m p l e t a , se a d e s e m p i o g a p e x = 1 q u e s t o s i g n i f i c a c h e e s i s t e u n g r u p p o di d a t i fisicamente m a n c a n t i ; la serie l e t t a v a i n t e g r a t a p e r r i p r i s t i n a r e la s e q u e n z a t e m p o r a l e nel g a p v i e n e i n s e r i t a l a m e d i a g e n e r a l e d e l l a s e r i e n e l g a p v e n g o n o i n s e r i t i i d a t i elab o r a t i m e d i a n t e i n t e r p o l a z i o n e lin e a r e u s u f r u e n d o d e l m e t o d o di c h i u s u r a d i f f e r e n z a p e r c e n t u a l e r i c h i e s t a d a l m e t o d o di c h i u s u r a (*) n . b . a q u e s t i d u e p a r a m e t r i è a s s o c i a t o u n c e r t o n u m e r o di s c h e d e c h e d e v o n o s e g u i r e l a s c h e d a p a r a m e t r i s u c c e s s i v a a q u e s t a . su c i a s c u n a s c h e d a c o m p a i o n o d u e v a l o r i i n t e r i , c o n f o r m a t o (2110), r a p p r e s e n t a n t i il n u m e r o ( l ' o r d i n e , n e l l a s e q u e n z a t e m p o r a l e d a e l a b o r a r e , d e i d a t i v a l i d i i m m e d i a t a m e n t e e s t e r n i a d u n g a p s d a i n t e g r a r e . r e l a t i v a m e n t e a d u n g a p i x = 1 c o m p a r i r à u n a s c h e d a c o n t e n e n t e a d e s e m p i o 25 n e l l e c o l o n n e 9-10 e 42 n e l l e c o l o n n e 19-20; q u e s t o signif i c a c h e i n l e t t u r a i d a t i d a l 26.ino al 4 1 . i n o n o n s o n o s i g n i f i c a t i v i e d e b b o n o e s s e r e o p p o r t u n a m e n t e s o s t i t u i t i . q u e s t e s c h e d e d e b b o n o e s s e r e o r d i n a t e p e r v a l o r i s c r e s c e n t i dei l i m i t i d e i g a p s . 7 0 r. pasquali lari p. ran di 7 a s c h e d a c o n t i e n e il f o r m a t o d i l e t t u r a d e i d a t i . f o r m a t o = ( 8 a 1 0 ) colonna nome della variabile descrizione 1 8 0 i f o r m v i e n e s c r i t t o il f o r m a t o d i l e t t u r a n e l n o r m a l e l i n g u a g g i o f o r t r a n , c o m p r e s e l e p a r e n t e s i e s t e r n e 4 . d e s c r i z i o n e d e l l ' i n p u t e output. il p r o g r a m m a è a disposizione dei ricercatori sotto f o r m a di compilato ed è richiamabile m e d i a n t e la seguente sequenza di schede: j o b . r e q u e s t cic1. r e w i n d cic1. copyn (a, lgo, cic1) lgo. 7 / 8 / 9 f i l t r o , 6, cic1 7 / 8 / 9 schede p a r a m e t r i e d a t i 6 / 7 / 8 / 9 l ' u s c i t a su t a b u l a t o è costituita dalle seguenti informazioni: a) tipo di filtro e sue caratteristiche b) coefficienti del filtro c) t e s t di fiducia d) valori della serie filtrata e) deviazione s t a n d a r d della serie filtrata /) serie residua (se richiesta). l ' e l a b o r a t o r e e l e t t r o n i c o n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i , e t c . 7 1 risposta de! filtri f i g . 1 7 2 r . p a s q u a l i l a r i p . r a n d i serie residua / serie filtrata / serie originale p i g . 2 nelle figure 1 e 2 si m o s t r a n o r i s u l t a t i di elaborazioni o t t e n u t e u t i l i z z a n d o il l a v o r o già c i t a t o (5). l ' e l a b o r a t o r e e l e t t r o n i c o n e l l o s t u d i o d e l l e s e r i e t e m p o r a l i , e t c . 7 3 b i b l i o g r a f i a f1) d a l l ' o l i o l . , g u e r r i e r i v . , r a n d i p . , 1 9 6 9 . coefficients and frequency response computing for every numerical filter. « l e t t e r e al n u o v o cim e n t o », 1 , p p . 2 7 1 2 7 8 . (2) galli m., randi p . , 1967. on the design of the optimum numerical filter with a prefixed response. « a n n a l i di g e o f i s i e a », x x , 4. (3) hollowat j . l . , 1958. smoothing and filtering of time series and space fields. « a d v a n c e s in g e o p h y s i c s » a c a d e m i c p r e s s . i n c . n e w y o r k , 4, p p . 3 5 1 3 8 9 . (4) kendall m. g., stuart a . , 1969. the advanced theory of statistics. c. g r i f f i n a n d c., l o n d o n . ( 5 ) l a n z a r i n i m . , p a s q u a l i r . , r a n d i 1 ' . , r o n c h i m . e . , programma generale di graficazione per un benson 111 of]¡line (di p r o s s i m a p u b b l i c a zione). (6) lee v. w . , 1960. statistical theory of communication. « j . w i l e y & s o n s ». n e w y o r k . (') peyret b . g., 1964. éléments calcul des probabilités à usage des ingénieurs. e y r o l l e s , p a r i s . (8) robin son e . a . , 1967. — statistical communication and detection. c. g r i f f i n a n d c., l o n d o n . (9) wainstein l . a . , zubakov v. d . , 1962. extraction of signals from noise. p r e n t i c e h a l l , i n c . n e w y o r k . articolo pucci.pdf key words tectonic geomorphology – normal fault – seismogenic sources – umbria-marche apennines – 29 april 1984 gubbio earthquake 1. introduction the gubbio basin is one of the most prominent intramountain basins in the central apennines. it is bounded to the east by an impressive normal fault known as gubbio fault that is part of the seismogenic umbrian fault system (barchi, 2002). even though the basin overlies inferred seismogenic sources including that responsible for the 1984 m 5.3 earthquake (valensise and pantosti, 2001a,b), seismicity (both historical and instrumental; cpti, working group, 1999; ingv, 1985-2000) is relatively infrequent and not sufficient for a full understanding of the seismogenic potential of the area. to assess the seismic potential of the gubbio basin with confidence we must first answer these questions: 837 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. stefano pucci, istituto nazionale di geofisica e vulcanologia, sezione di roma 1 sismologia e tettonofisica, via di vigna murata 605, 00143 roma, italy; e-mail: pucci@ingv.it geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise istituto nazionale di geofisica e vulcanologia, sezione di roma 1 sismologia e tettonofisica, roma, italy abstract the gubbio basin is a 22 km long, 4 km wide depression located within the north-central apennines fold-andthrust belt. the basin is bounded to the east by the gubbio fault, a w-dipping, normal fault dissecting a large jurassic-oligocene anticline. although located along one of the main seismogenic zones of the peninsula, both historical and instrumental is seismicity is limited with the only exception for the 29 april 1984, ms 5.3 earthquake, which occurred about 10 km southwest of the basin. most of the literature attributes this seismicity to the gubbio fault. new geomorphic and geologic investigations based on field and aerial photo surveys and dem analyses provide new insights on the active faulting in the area and are used to infer potential seismogenic sources. limited evidence of ongoing deformation along the surface expression of the gubbio fault was found, possibly because of low rates of deformation versus fast erosional processes. the western side of the basin appears to be controlled by an east-dipping normal fault, antithetic to the gubbio fault. standard dislocation modeling was used to understand the role played by the gubbio fault and its antithetic. the gubbio fault was divided into a high-angle section above 3.5 km and a low-angle section between 3.5 and 6 km depth. based on different tests we conclude that both sections of the gubbio fault as well as the antitethic fault contributed to the present setting of the basin. at present the antithetic fault appears to be the most effective in producing a geomorphic signature and controlling the basin width. the high-angle gubbio fault played a major role in the basin growth but now its activity rate appears minor. because of the characteristics and location of the 1984 earthquake, the low-angle gubbio fault is assumed to be presently active and seismogenic. based on the integration of geologic, geomorphic and seismological data we suggest that the low-angle gubbio fault is formed by two individual sources capable of m 5.3-5.9 earthquakes. the southern source ruptured in the 1984 earthquake while the northern source did not rupture recently nor historically. 838 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise normal faults thrusts c. di castello gubbio g. tadino assisi perugia colfiorito norcia tiber basin tiber basin altotiberina fault umbrian faults system plio-quaternary basins study area n sansepolcro d e p th ( km ) tiber valley gubbiowsw ene 1984 gubbio eq. ms 5.3 20 km altotiberina fault a 04/29/1984 ms = 5.3 09/26/1997(11.40) mw = 6.0 09/26/1997(02.33) mw = 5.7 10/14/1997 mw = 5.6 fig. 1. structural sketch of the umbria-marche domain (from barchi et al., 2001, modified) showing the alignment of the intramountain basins along the umbrian fault system (sensu barchi, 2002), which coincides with the main seismogenic zone of central apennines defined by valensise and pantosti (2001a). focal mechanisms and magnitudes are for the 1997, colfiorito sequence from ekström et al. (1998) and for the 1984 gubbio earthquake from dziewonski et al. (1985) and isc (2001). the lower left inset includes a schematic regional geological section integrated with the interpretation of seismic profiles and showing also the 1984 gubbio earthquake mainshock location (after boncio et al., 1998, modified). how many seismogenic sources lie beneath the basin? what is its (their) geometry? what is the largest earthquake expected for the area? what is the expected seismic behavior of this (these) source(s)? notwithstanding a rich literature describing various aspects of the basin geology and structure, limited attention was devoted to active tectonics and particularly to the study of landscape as a direct response of repeated coseismic surface deformation (selvaggi and sylos labini, 1989; menichetti, 1992; boncio et al., 1996). to contribute to the understanding of the local earthquake potential we investigated the relationships between seismicity and tectonics by carrying out new geomorphic studies of the gubbio basin based on field and aerial photo surveys and dem analyses. our approach is based on the awareness that landscape evolution may contribute to describing the seismic history of a region. we also compared the deformation patterns of different combinations of active faults based on standard dislocation modeling. in the following we summarize the present knowledge on the area, present the new geomorphic data, and discuss different alternatives of seismogenic source geometry and their possible implications for defining the activity of the gubbio basin. 2. the gubbio basin 2.1. geological and structural setting the study area is located in the inner part of the umbria-marche apennines (central italy) (lavecchia and pialli, 1980). a thick skinned style of compressional deformation, characterized by a system of multiple superposed detachments, generated a set of nw-se trending, neverging folds and thrusts during langhianserravallian (barchi, 1991; barchi et al., 1998). the result of the eastward migration of the deep detachment layers (base of the carbonatic multilayer) is the deformation of the more external, partially grown structure such as the upper detachment layers located within miocene flysch formations. the bedrock of the area is composed by the upper part of the umbria-marche sequence: a pelagic carbonatic multilayer (jurassic-oligocene) overlain by a miocene flysch sequence (bonarelli et al., 1952; moretti and perno, 1968; damiani et al., 1983; menichetti and pialli, 1986). the gubbio basin is bounded to the east by one major box-shaped, ne-verging carbonatic anticline (referred to as gubbio anticline), whereas to the west it is bounded by folded flysch deposits (bonarelli et al., 1952; moretti and perno, 1968) (fig. 2). starting in lower pleistocene, extension took over the compressional tectonics producing structures with a main nw-se direction. west of the gubbio basin, high-angle normal faults both sw and ne dipping are recognized (menichetti and minelli, 1991; menichetti, 1992). however, the most impressive extensional structure bounds the basin to the east and is likely the nw-striking, sw-dipping, gubbio fault which dissects, parallel to its axis, the gubbio anticline down-throwing the sw flank well below the basin sediments. it consists of a 120°to 150°-striking, 50°to 70°-dipping fault planes, which, as a whole, show a gentle bend (collettini, 2001) (fig. 2). the clear result of the long-term activity of the gubbio fault is the topographic low, coinciding with the gubbio basin, filled by continental deposits (ge.mi.na, 1963) (fig. 2). they consist of a sequence of three main sedimentary units, resulting from the evolution of the deposition from lacustrine toward fluvial environment: 1) clayey-lignitic basal complex, characterized by meter-thick lignite layers interbedded with organic gray clay, locally with sand and conglomerates; 2) clayey-sandy complex comprising sand, sandy clay interbedded with sandstone conglomerate lenses; 3) upper alluvial complex, composed by organic sandyclayey matrix with calcareous conglomerate. on the basis of palinological analysis, the unit 1 is referred to the interglacial period günzmindel (500-400 ky) (ge.mi.na, 1963) or to the lower pleistocene (lona and bertoldi, 1972), this latter estimate is in agreement with the fossil fauna (lacustrine facies) found in this layer (bonarelli, 1891). data to attempt a reconstruction of the subsurface setting of the basin are available too. deep boreholes by ge.mi.na (1963) were used to infer the geometry of the basin and of the filling deposits. by correlating their stratigraphy ge.mi.na (1963) imaged: 1) ne-dipping deposits, with dips generally increasing toward the gubbio fault; 2) a deepening toward ne of the bedrock/basin filling deposit contact; 3) a pleistocene depocentre, up to 500 m below the present basin surface, located close to the gubbio fault. however, because of the uneven distribution of the boreholes in the basin (mainly located along the western side and in the southern part) this reconstruction contains large uncertainty. menichetti (1992) re-interpreted 839 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential these data integrating them with more recent boreholes stratigraphy and new electrical and seismic surveying parallel and perpendicular to the basin (giaquinto et al., 1991), reconstructing the geometry of the pre-pleistocene bedrock. he describes the basin as the typical result of the evolution of an half-graben, with a wedge geometry, and progressive thickening of the lacustrine deposits controlled by a sw-dipping normal fault: the gubbio fault. however, this reconstruction shows a peculiar bathtub shape, about 400 mdeep, with flat bottom and long steep facing flanks. the pre-pleistocene depocentre, deeper in the northern part of the basin, appears to be centered within the valley, with a geometry that is not expected for a typical half-graben (fig. 3a). recently, commercial reflection profiles allowed barchi et al. (2000) to perform a detailed reconstruction of the gubbio fault isobaths. the resulting picture of the fault plane at depth shows an high-angle dip of the upper fault portion and a low-angle dip below ~ 3.5 km, due to the reactivation of a pre-existing thrust plane. the interpretation of these seismic profiles images the gubbio fault as an antithetic of the e-dipping, low-angle, altotiberina fault (fig. 1; barchi et al., 1998; boncio et al., 1998). the altotiberina fault is a major extensional discontinuity in the umbria-marche apennines. it affects the upper crust, at least up to 15 km depth, beneath the thrust and fold belt. the altotiberina fault follows a staircase trajectory 840 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise ponte d'assi gubbio gubbio basin watershedwatershed mocaiana gubbio padule ponte d'assi branca regional divide mt. urbino study area chiascio river4 km carbonates (jurassic-oligocene) carbonates (jurassic-oligocene) carbonates (jurassic-oligocene) alluvial deposits (holocene) alluvial deposits (holocene) alluvial deposits (holocene) fluvio-lacustrine deposits (pleistocene) fluvio-lacustrine deposits (pleistocene) fluvio-lacustrine deposits (pleistocene) turbidites (miocene)turbidites (miocene)turbidites (miocene) thrust faultsthrust faultsthrust faults normal faultsnormal faultsnormal faults alluvial fan (holocene) and slope debris (late pleistocene-holocene) alluvial fan (holocene) and slope debris (late pleistocene-holocene) alluvial fan (holocene) and slope debris (late pleistocene-holocene) n fig. 2. simplified geological map of the gubbio basin. thick normal faults are discussed in this work (after bonarelli et al., 1952; moretti and perno, 1968; menichetti and pialli, 1986; collettini, 2001, modified). and shows an offset as much as 5 km. the isobaths of the gubbio fault confirm that the left bending of the structure, shown by its surface trace, is well recognizable even at depth (barchi et al., 2000). the isobaths of the gubbio fault also display a rise of the branch line with the altotiberina fault from 6 km in the southern part to 4.5 km in the central part to get deeper again toward the north. the analysis of denser seismic profiles array allowed mirabella (2003) to derive additional characteristics of the gubbio fault. the fault boundaries are quite consistent with the end of the basin to the north, whereas, to the south the fault appears to continue even outside the basin for a total length of about 30 km although with minor evidence (fig. 4). the total throw of the gubbio fault along strike reaches a maximum of about 2500 m near the town of gubbio and shows a sharp decrease toward the northern and southern tips (mirabella, 2003). 841 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential fig. 3a,b. a) 3d reconstruction of the pre-pleistocene basement (after menichetti, 1992, redrawn); b) 3d view of the northeastern side of the gubbio basin derived by 5-m resolution dem. hypothesized gubbio fault splays are reported along with to the location of the northern part of the pre-pleistocene depocentre suggested by menichetti (1992). slope debris carbonates gubbio fault trace alluvial complex gubbio mocaiana alluvial fan left step artificial cut (fig. 14) pre-pleistocene depocentre paleosoil sample location fanhead entrenched paleosurface n 500 0 ele vat ion (m ) a. s.l. mocaiana gubbio left step n splay pre-pleistocene depocentre a b 2.2. seismicity the gubbio basin is located within the main nw-se apennines seismogenic belt, which extends from tuscany to calabria (valensise and pantosti, 2001a). in the umbria-marche region, most of the historical and instrumental seismicity is related to the activity of the nnw-sse trending sw dipping normal faults alignment (umbria fault system, sensu barchi et al., 2000). this system extends through the intra-mountain basins of norcia, colfiorito, gualdo tadino, gubbio and sansepolcro (fig. 1). historical earthquakes of i > vii mcs (mercalli-cancani-sieberg scale) are located both to the north (citerna-monterchi, april 1917 imax = = ix; cagli, june 1781 imax = ix-x; monterchi, december 1352 imax = ix; città di castello, april 1458 imax = ix; valtiberina, september 1789 imax = viii-ix; bocca serriola, october 1389 imax = ix; cpti, working group, 1999) and to the south (gubbio, april 1593 imax = vii; gualdo tadino, july 1751 imax = x; fiuminata, april 1747 imax = = ix; camerino, april 1279 imax = x; cpti, working group, 1999) of the gubbio basin. instrumental seismicity (md > 1.5), recorded in the region by the istituto nazionale di geofisica e vulcanologia in the period 1985-2000 (ingv, 1985-2000), is mainly located south of the gubbio basin and it is related to the 19971998 colfiorito earthquake sequence characterized by normal faulting events (fig. 5). on april 29, 1984 the study area was struck by a ms 5.3 (neic) earthquake (mo 3.4 1024 dyne. .cm; dziewonski et al., 1985) located at mt. urbino, about 10 km south of the town of gubbio at 7 km depth (isc, 2001). a local network, set in the epicentral area two days after the mainshock, recorded 300 aftershocks, 4 of which had ms > 4.0. the aftershocks occurred in an area of about 20 × 6 km elongated in a n140° direction. their distribution appears to delineate two linear parallel clusters, the southernmost of which is denser and includes also the mainshock (see inset in fig. 4). these clusters describe two high-angle, west-dipping planes, the northernmost being characterized by superficial events with hypocentral determinations between 3 and 5 km (haessler et al., 1988). these planes are located 7-12 km west of the gubbio fault and seem to be not directly related to this major structure (collettini et al., 2003). the cmt (dziewonski et al., 1985) and first-motion (westaway et al., 1989) focal mechanism solutions of the mainshock agree substantially on fault strike and kinematics but not on fault dip (see inset in fig. 5). the 1984 gubbio earthquake instrumental data has been used in the definition of three seismogenic sources recently published in valensise and pantosti (2001b). they solved the discrepancy between aftershock distribution and cmt focal mechanism, which better fits the gubbio fault at depth, as possibly due to the fact that aftershocks were recorded a few days after the mainshock and thus may not directly be related to the main fault but to stress readjustment within the fault hangingwall. in may and june 1987 a local network was installed again in the area (deschamps et al., 1989) and four hundreds events with magnitude between 0.8 and 3.1 were recorded. these were 842 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise jurassic-oligocene fm. miocenic fm. pleistocenic fm.pre-pleistocene depocentre 1 2 3 4 5 6 7 8 9 10 11 12 3000 2500 2000 1500 1000 500 0 0 5 10 15 20 25 fault lenght (km) d is p la ce m e n t a lo n g t h e f a u lt p la n e ( m ) t o p o g ra p h ic t h ro w (m ) 2500 2000 1500 1000 500 250 0 measure points along topographic profiles and seismic lines nw se geologic displacement topographic throw footwall hangingwall basin lenght (km) fig. 4. comparison between geologic displacement and topographic throw along the gubbio fault strike. geologic throw has been calculated by using seismic lines on the base of the marne a fucoidi beds (115-95 my) offset (from mirabella, 2003). topographic throw is measured as difference between highest elevation and surface of the basin. the position of the prepleistocene depocentre, as reconstructed by menichetti (1992), the basin length and the lithologies cropping out in the fault footwall are reported too. organized in two clusters nw-se trending located near gubbio and valfabbrica towns. 58% of the hypocentral determinations concentrated between 3 and 7 km depth, with a clear deepening toward se. a selected group of 33 well-constrained focal mechanisms indicates a minimum horizontal principal stress tensor ( .3) oriented 219 (boncio et al., 1996). 843 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential fig. 5. instrumental and historical seismicity of the gubbio basin area. historical seismicity with intensity larger than vii mcs is reported for the period 217 b.c.-1991 a.c. (cpti, working group, 1999). the inset shows the 4/29/1984 gubbio earthquake sequence; gray star shows the mainshock, white stars show aftershocks with m > 4 and rectangles show aftershocks with m < 4 (modified after haessler et al., 1988). focal mechanism parameters: strike 143 dip 21 rake – 72, strike 304 dip 70 rake – 97 (dziewonski et al., 1985); strike 135 dip 55 rake – 90, strike 315 dip 35 rake – 90 (westaway et al., 1989). boncio et al. (1998), interpreting all the available instrumental seismic data, propose that the altotiberina fault controls the location of the local seismicity, all concentrated in its hanging-wall, close and east of the branch-line with the gubbio fault. consequently, the historical damaging earthquakes are located east of gubbio basin, where the altotiberina fault deepens (fig. 1). on the basis of the substantial agreement between the kinematics of the major active faults, the quaternary tectonics, paleostress computations and the seismologic extensional tensor boncio et al. (1996) suggest that the stress field in the area is constant at least since upper pleistocene. these observations are not definitive in the understanding of the present activity of the gubbio structure but, on the contrary, raise important questions about the true role of the gubbio fault in the seismogenesis of the region. 3. geomorphic evidence of active tectonics to analyze the geomorphic, recent tectonic and geologic evolution of the gubbio basin we performed a field survey focused on geologic mapping at 1:25.000 scale, supported by 1:33.000 scale aerial photos analyses (igm, years 1955/1956, 1991 and 1994). we mapped drainage, erosional and terrace surfaces, scarps, peaks, saddles, sharp ridges, deep linear erosions, structural lineaments, faults, alluvial fans, etc. (fig. 6). at the same time we used digital cartographic data at a 1:5000 scale, provided by the umbria region, to build a 5 m resolution dem (digital elevation model). by using a gis program we geo-referenced all the data collected and performed morphometric analyses of the topography and of the drainage pattern. we used the ge.mi.na. (1963) stratigraphic scheme as a reference for the classification of the continental units outcropping in the study area (fig. 2). 3.1. physiography the alluvial complex crops out over most of the gubbio basin (fig. 2). its sedimentary facies has been interpreted as the final stage of the pleistocene lacustrine infill (ge.mi.na., 1963). it is composed by low gradient alluvial fans that have been fed by transversal streams flowing through the carbonatic anticline from ne to sw (fig. 6). four principal fans can be distinguished along the eastern side of the basin: one isolated is near mocaiana village, while the other three coalesced north and south of gubbio town. the alluvial complex, which hides the lower lacustrine sequence (ge.mi.na., 1963; giaquinto et al., 1991), appear to control the sw dip of the basin floor (fig. 7, cross sections 3 to 9). in general, the alluvial complex does not experience strong erosion, with the exception for the alluvial fan near mocaiana, the northern part of which has been eroded by the assino river (fig. 6). moreover, the alluvial fan, just north of gubbio (fig. 7, cross section 13), acts as local divide for the drainage basin (see section 3.2.). the gubbio basin is a 22 × 4 km, nwtrending elongated depression. the slopes bounding the basin exhibit a different morphology depending both on their lithology and structural setting. the basin floor lies at about 400 m a.s.l. whereas the eastern and western ranges reach elevations of about 950 and 650 m a.s.l., respectively. the eastern flank of the basin has the steepest slope (fig. 7) and presents an important debris deposition at its foot. the declivity of its northern section shows a typical regularized slope replacement profile (bartolini, 1992) with a homogeneous inclination of about 30° (fig. 8, cross sections 5 and 6). differently, the southern part exhibits a higher inclination (35°) and profile irregularities, such as scarps, can be observed on the slope debris, but not in correspondence of the bedrock fault plane (fig. 8, cross sections 1, 2, 3 and 4). the eastern slope, carved by wide triangular facets also detectable from dem (see inset of fig. 8), is modeled on back-dipping jurassic-oligocene bedding of the footwall formations. it represents the source of high-angle young fans developed over slope debris and older fans of the alluvial complex (fig. 6). this flank appears strongly controlled by the activity of the gubbio fault and, because of the significant slip accommodated, only the eastern limb of the gubbio anticline crops out. the resulting occurrence of strong erosional 844 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise processes is testified by the larger amount of continental sediments, which buried most of the down-thrown anticline back-limb. we dated a paleosoil, found inside the slope debris deposits north of gubbio by means of radiocarbon analysis (fig. 3b for location), this yielded an age of 39 ky bp. between gubbio and mocaiana (figs. 2 and 3b), where two sections of the fault overlap, this slope debris has the largest surface extension and reaches the highest elevations. to the northwest, the surface slope of these deposits is higher than their structural dip (about 15°) indicating that its dynamic development is predominantly governed by a strong erosional activ845 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential fig. 6. map of quaternary geology and geomorphology of the northern and central portion of the gubbio basin from aerial photo and field survey. the lower left inset shows the location of the study area (see also dashed area in fig. 2) on the 5-m resolution dem. alluvial deposits (holocene) eluvial-colluvial deposits (holocene) alluvial fan deposits (holocene) slope debris deposits (holocene-late pleistocene) alluvial complex (late pleistocene) clayey-sandy complex (middle pleistocene) clayey-lignitic complex (early pleistocene) bedding saddle peak 400-420 m a.s.l. 420-430 m a.s.l. 430-440 m a.s.l. 440-450 m a.s.l. 450-480 m a.s.l. 480-540 m a.s.l. 540-640 m a.s.l. fan strong linear erosion scarp tectonic lineament sharp ridge paleosurfaceslitology ity interpretable as due to the regional tectonic uplift or as caused by local lowering of the basal level. anti-dip streams participate in the modeling of this slope by removing debris deposits and by forming minor entrenched holocene alluvial fans (figs. 3b and 6). sinuosity of the mountainpiedmont contact is quite high, typical of slopes controlled by low activity faults (bull and mcfadden, 1977). an along strike comparison between the geologic displacement on the gubbio fault and the topographic throw (difference between the maximum elevations of the gubbio anticline and the elevations of the basin floor, measured along transects perpendicular to the gubbio fault) is shown on fig. 4 (collettini et al., 2003). the location of the two maximums does not coincide, suggesting that the differential erosion has a strong influence in the landscape evolution. it is also to be noted that the prepleistocene depocentre (menichetti, 1992) is located exactly where the geologic displacement reaches its maximum. in conclusion, along this flank of the basin the action of the surface processes appears to be in strong competition and mostly prevailing over tectonics. the western flank of the basin has a completely different morphologic expression with respect to the eastern one. it does not show any important folding structure and has low relief energy. in fact, at the contact mountain-piedmont only colluvial deposits and low gradient alluvial fans are found, as a consequence of the range consumption (fig. 6). the bedrock shows basinward, gently dipping beds, generally with oblique strike with respect to the contact bedrock/basin infill. gravitational processes have a strong control on the landscape evolution, the bedrock being constituted by flysch deposits. in the field, relevant fault mirrors do not crop out, probably they are not preserved because of their rheology. in addition, the long-term deformation appears to be accommodated by minor tectonic structures. the competition of the landscape modeling with tectonics is minor and thus the potential for the preservation of surface evidence of recent faulting is higher. the mountain-piedmont junction is noticeably dissected, but the contact between the 15 and the 30 percent grades of the slope shows a clear straight nw-se trend, typical of fault-controlled slopes. 846 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise 20 1 3 500 600 700 800 900 4 400 4 20 1 3 4 5 6 500 600 400 1 700 500 600 400 20 1 3 4 5 2 500 600 700 400 800 900 20 1 3 4 5 6 3 500 600 20 1 3 4 5 6 5 500 400 nw distance (km) 0 2.5 5 7.5 12.5 15 17.5 20 22.510 13 500 600 400 20 1 3 4 5 6 7 10 500 600 700 400 20 1 3 4 5 6 7 9 500 600 700 400 20 1 3 4 5 6 7 8 8 500 600 700 900 800 sw 0 1 2 3 4 5 6 7 8 9 10 7 ne 500 600 700 800 900 20 1 3 4 5 6 6 500 600 400 20 1 3 4 5 6 7 11 0 500 600 400 21 3 4 5 6 12 e le v a ti o n ( m ) se fault trace 1 2 3 4 5 6 7 8 9 10 11 12 gubbio location map of the topographic profiles 13 3 km n fig. 7. topographic profiles across and parallel to the gubbio basin performed by means of the 5 m resolution dem. the position of the gubbio fault trace is shown in the profiles. 3.2. drainage network the complex drainage network of the gubbio basin is shown in fig. 9. it is formed by two secondary hydrographic networks, connected with the assino and chiascio rivers. these streams, located at the northern and southern end of the basin, respectively, flow perpendicularly to the nw-se trending compressional structures. the whole gubbio drainage basin has a watershed that does not follow the main elevations of the area, which coincides with the carbonatic structure but, anomalously, it runs always on the highest crests of the flysch formations (fig. 9). as a consequence the northeastern part of the gubbio basin watershed is peculiarly shifted to the east with respect of the carbonatic anticline and coincides with the regional divide. to the west the watershed is more irregular and enlarges north of mt. urbino. in the northeastern part of the gubbio drainage basin, a trellis water-network developed with a ne-sw and a nw-se trending patterns (fig. 9). while the latter is organized in alongstrike valleys, emphasized by selective erosion, the ne-sw creeks fed the major fans of the alluvial complex, cutting perpendicularly the carbonatic structure. some of these streams evolved in deep watergaps, which incised the slope, shaping smoothed triangular facets (see inset of fig. 8). on the contrary, in the western flank of the basin, composed by flysch formations, the drainage is organized as a dendritic water-network (fig. 9). the gubbio drainage network shows a peculiar diverging trend, where the main fan (fig. 6), close to the gubbio town, acts as a water divide between the north saonda creek and the south saonda creek flowing nw-se with opposite directions (fig. 9, anomaly d). 847 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential 1 trace of the gubbio fault trace of the topographic profiles paleosurfaces 2 km n gubbio 0 400 800 1200 1600 400 800 1200 distance (m) e le va tio n a .s .l. ( m ) carbonates fluvio-lacustrine complexes alluvial complex slope debris gubbio fault slope rupture 1 sw ne 400 800 0 400 800 1200 1600 distance (m) e le va tio n a .s .l. ( m ) alluvial complex slope debris gubbio fault slope rupture 2 sw ne fluvio-lacustrine complexes carbonates 0 400 800 400 800 distance (m) e le va tio n a .s .l. ( m ) 0 800 400 800 400 distance (m) e le va tio n a .s .l. ( m ) 0 400 800 1200 400 800 distance (m) e le va tio n a .s .l. ( m ) 0 400 800 1200 400 800 distance (m) e le va tio n a .s .l. ( m ) gubbio fault slope rupture 5 slope rupture sw ne sw ne sw ne sw ne 4 6 3mocaiana fig. 8. detailed topographic profiles, across the gubbio fault. all the profiles show the location of slope ruptures, the bedrock fault trace and a schematic geological setting. these main creeks show a clear linearity, flow parallel to the maximum basin elongation, and are located close to its western flank. thus they are in asymmetry with respect to the axis of the basin and also in disagreement with the prepleistocene depocentre (fig. 3a,b). this asymmetry may be explained with a progressive migration of the rivers westward as the fans prograded. however, the rivers linearity and low fan dip, coupled with the observation that the south saonda creek flows close to the western edge of the basin, even where large alluvial fans are not present, are suggestive of a tectonic control. within the gubbio basin, the south saonda creek incises the lacustrine sedimentary sequence up to the lowest units, both along the ne-sw gradient direction and perpendicular to it. on the contrary, the north saonda creek is less developed, in fact middle pleistocene deposits are incised only near the confluence with the assino river and the alluvial complex is still well preserved. south of mocaiana, the gentle geomorphology between two coalescent alluvial fans, deposited at the end of lacustrine cycle, presents a local convergence of drainage (fig. 9, anomaly c). most stream anomalies have been recognized in the southern part of the basin, where the south saonda creek appears to be left stepped and composed by two main sections, nwse trending (fig. 9, anomaly e). south of ponte d’assi, where this creek reduces the amplitude of its holocene deposition and assumes a degradational behavior, its sinuosity index 848 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise 4 km gubbio gubbio basin north watershednorth watershed gubbio gubbio basin south watershedsouth watershed mocaiana gubbio padule ponte d'assi branca regional divide local divides mt. urbino north saonda creek south saonda creek chiascio river assino river c d b f e a carbonates (jurassic-oligocene) carbonates (jurassic-oligocene) carbonates (jurassic-oligocene) alluvial deposits (holocene) alluvial deposits (holocene) alluvial deposits (holocene) fluvio-lacustrine deposits (pleistocene) fluvio-lacustrine deposits (pleistocene) fluvio-lacustrine deposits (pleistocene) turbidites (miocene)turbidites (miocene)turbidites (miocene) drainage anomaliesdrainage anomaliesdrainage anomalies normal faultsnormal faultsnormal faults alluvial fan (holocene) and slope debris (late pleistocene-holocene) alluvial fan (holocene) and slope debris (late pleistocene-holocene) alluvial fan (holocene) and slope debris (late pleistocene-holocene) n a fig. 9. drainage map of the gubbio area. thick dashed line divides the gubbio area in two smaller drainage basins: the north and the south saonda creek basins. simplified geology is reported. 849 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential 0 0.4 0.8 400 440 480 520 560 2 0 0.4 0.8 380 420 460 500 540 1 0 0.4 0.8 420 460 500 540 3 0 1 2 400 440 480 88885 0 1 2 400 440 480 520 96 0 0.4 0.8 1.2 1.6 380 420 460 500 540 107 0 1 2 420 460 500 540 4 lenght (km) 0 4 8 12 16 20 320 360 400 440 south saonda creek a b 0 4 8 12 360 400 440 480 520 north saonda creek assino river c holocene deposits location of knickpoints 1 streams normal faults area of increased sinuosity area of increased degradation 3 km 1 km a mocaiana gubbio assino river ponte d'assi padule north saonda creek south saonda creek ponte d'assi b a 1 2 3 4 5 6 7 c n n fig. 10a,b. a) longitudinal profiles of the western basin streams performed from 5 m resolution dem with knick-points location; b) detail of the south saonda creek showing changing behavior running from the footwall to the hangingwall of a fault section. a b 850 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise decreases from 1.6 to 1.1 (fig. 10b). two knick points have been detected along the south saonda creek (fig. 10a), one is located near ponte d’assi (knick point b) and seems to be related to regressive erosion, while the southernmost (knick point a), being in line with a tectonic lineament (mapped through aerial photos interpretation), has been tentatively interpreted as controlled by the activity of the structure. in the same area, west of ponte d’assi, the hierarchic organization of several ne-sw oriented creeks may suggest, again, a recent tectonic control on the development of the drainage network. in the northern part of the gubbio basin, the longitudinal profile traced along the north saonda creek shows the presence of a knick point at the northern threshold of the gubbio plain (fig. 10a, knick point c) and consequently testifies the capture of the north saonda creek by the assino river. the location of this capture through a threshold located in an area of local topographic high, induced us to consider a possible tectonic control by a nw-se trending lineament bounding the northwest side of the basin (fig. 6). similar analyses have been performed along the tributaries of the north and south saonda creeks. several of these drainages, incised on the flysch deposits of the western side of the basin, show non-equilibrium profiles and knick points, which align in a nw-se direction, this is also suggestive of tectonic control (fig. 10a). 3.3. paleosurfaces within the gubbio basin we have mapped several relict surfaces (fig. 6), which are evidence of paleolandscapes; these include both erosional surfaces, carved on the local bedrock, and lacustrine depositional terraces. the majority of these surfaces are found in the northwestern part of the basin at different elevations. because of the lack of direct dating, we attempt their analysis by sorting them by height in seven groups, which are reported in the fig. 6 with different colors. a correlation among all this surfaces being very difficult, we looked for their possible relations with other geomorphic elements such as scarps, lineaments and streams. the majority of these surfaces cannot be unequivocally attributed to variations of the pleistocene lake level or to recent tectonic dislocations. along the western slope of the basin it is possible to observe a large number of surfaces, widely distributed, and, in particular, the bestpreserved sequence is located near ponte d’assi (fig. 6). differently, the eastern slope of the basin does not show any comparable sequence of surfaces nor indented on the slope debris or on the alluvial complex. a series of topographic profiles, both parallel and perpendicular to the western basin flank, are shown in figs. 11 and 12a-d to evidence the relative height of the surfaces. at the northern termination of the basin, the principal surfaces, both erosional and depositional, progressively decrease in elevation toward south. in fact, the highest depositional surfaces, belonging to the sandy-clay complex, lie at a maximum height of 540-560 m a.s.l. in the northern area whereas, appear, at an elevation of 450-480 m a.s.l. near ponte d’assi (fig. 11, cross-section 5 and 6; fig. 12a-d, crosssection 12). in order to analyze the distribution and relation of the different surfaces with respect to the present topography, we simulated a draining sequence of the gubbio basin by subsequently setting the lake level at 535, 480, 440 and 420 m a.s.l. (fig. 15a-f). when we fix the lake level at the highest elevation (535 m a.s.l.) (fig. 13) only few surfaces appears. at the northern termination of the basin, depositional surfaces topping the lacustrine sequence can be seen where the geologic throw of the gubbio fault tapers to zero (figs. 4 and 7, between cross sections 1 and 2). along the central part of the eastern flank of the basin, some nw-se elongated small surfaces, indented on the debris deposit, show up. taking into account their position, shape and substratum, these surfaces have been interpreted as slope ruptures, probably related to secondary splays of the gubbio fault (fig. 3b). on the western side of the basin, surfaces carved in bedrock up to 640 m a.s.l. determine a staircase topographic profile about 3 km nw of ponte d’assi and present their outer edges bounded by tectonic lineaments. the western flank 851 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential fig. 11. detailed topographic profiles, across and along the western slope of the basin, performed from 5-m resolution dem. the location of the main paleosurfaces (dark gray) together with the traces of the observed tectonic lineament is shown. profile 1, 2 and 3 are shown in overlap to highlight the elevation decrease of the paleosurfaces between north and south. 852 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise b) 8 a) 12 0 200 400 600 800 380 400 420 440 460 480 12 0 200 400 600 380 400 420 440 460 480 8 distance (m) distance (m) e le va tio n ( m ) e le va tio n ( m ) 12 8 n n fig. 12a-d. 3d view of some paleosurfaces and scarps, from 5 m resolution dem, along with their aerial photograph view (a and b) and their topographic profiles (12 and 8). c) landscape photograph detail of a paleosurface of fig. 12a. d) landscape photograph of a scarp crossing a recent alluvial fan (see fig. 11 for the location). c d b a assumes an almost rectilinear, nw-se trending attitude only when we decrease the water level at 480 m a.s.l.; conversely, the eastern side appears strongly controlled by the gubbio fault since the earlier stages (fig. 13). going on with the draining sequence simulation up to 420 m a.s.l., we observed: 1) the presence of several surfaces at the northern end of the basin, mainly on lacustrine deposits; 2) a discontinuous distribution of surfaces, both on bedrock and continental deposits, often organized as staircase flats included within parallel tectonic lineaments along the western flank; 3) the absence of analogous surface sequence along the 853 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential gubbio padule mocaiana ponte d'assi gubbio padule mocaiana ponte d'assi gubbio padule mocaiana ponte d'assi gubbio padule mocaiana ponte d'assi 3 km 3 km lake level 535 m a.s.l. 3 km 3 km lake level 480 m a.s.l. lake level 440 m a.s.l. lake level 420 m a.s.l. n n n n fig. 13. draining sequence simulation of the gubbio basin obtained by subsequently setting the lake level at 535, 480, 440 and 420 m a.s.l. see legend of fig. 6 for the reported paleosurfaces. topographic base acquired from 5-m resolution dem. eastern side, where alluvial deposition dominates. we also noted that the lower surfaces along the western slope of the basin are formed only on pleistocene continental deposits, with the outer edge scarps generally parallel to the north and south saonda creek. 3.4. morphotectonic observations morphostructures such as tectonic lineaments, escarpments and bedrock fault planes have been mapped in the gubbio basin (fig. 6). different authors suggested the existence of active tectonic in the gubbio basin. boncio et al. (1996) describe deformed slope debris directly in contact with the fault plane, at a location about one kilometer se of the town of gubbio. however, we did not notice any clear deformation possibly linked to recent activation of the gubbio fault at the site. selvaggi and sylos labini (1989) observed a 2 km long scarplet displacing the deposits of the gubbio fans. archeological literature (branconi and manconi, 1982-1983) describes in this site a buried ancient roman wall. although the published evidence appears to be highly questionable, the signature of active tectonic processes in the area can be found through detailed analysis. along the eastern flank of the basin, we noticed that the gubbio fault zone exhibits a clear geometric segmentation with a left step of the fault plane exposed between gubbio and mocaiana. the same zone of stepover is also recognized at depth, from the reconstruction of the bedrock/basin infill contact (menichetti, 1992). as a whole, the gubbio fault zone has a concave shape with dip directions from n240° to n220° moving northward. topographic profiles show several irregularities on the slope debris identified as morphological scarps (fig. 8, cross sections 1, 2, 3 and 4). even though part of them may be related to human activities, it is interesting to note that they are often rectilinear and aligned for a maximum length of two kilometers. calcareous bedrock has been exhumed from the debris cover in the hangingwall of the outcropping gubbio fault, due to erosion and to extensive quarrying. such bedrock appears to be located between slope ruptures and the bedrock fault planes, suggesting the existence of at least two main fault splays below the continental deposits (fig. 3b). at the northeastern edge of the basin, several shear structures, affecting the debris polycyclic depositional horizons, can be recognized on the walls of large artificial cuts. one of these cuts was studied in detail (1/20 scale) (fig. 3b for location). in fig. 14a,b, vertically re-oriented clasts and offset, of the order of few centimeters, of distinct layers indicate the presence of small nwtrending and sw dipping normal faults. considering the steep slope related to this side of the basin, the discrimination between tectonic and gravitational origin of these structures remains unclear. along the western flank of the basin two new morphologic lineaments have been recognized (fig. 6). these two nw-se striking lineaments are parallel. the western one is defined by several linear sections, arranged in a right stepping system, and is entirely located on bedrock. several geomorphological features contribute to the identification of this structure: saddles, tops, escarpments and staircase surfaces included within parallel sections (fig. 6). the eastern lineament is located at the bottom of the slope and is identified by aligned scarps, both in colluvium and bedrock, and by paleosurface outer edges (fig. 11, topographic profiles 7, 9 and 11). although taking in consideration the possibility for a fluvial-lacustrine origin for the alignment of these outer edges, we propose a tectonic origin, mainly because of a clear elevation change in their along strike topographic profile (fig.11, topographic profile 13). in general, the two lineaments have been recognized and mapped in detail for more than 15 km, from the northern termination of the basin to the south. these lineaments are interpreted as the expression of an east-dipping normal fault, which describe, as a whole, a concave geometry mirroring that of the gubbio fault (fig. 2). 3.5. defining active faults in the gubbio basin the gubbio basin is clearly bounded to the east by the prominent nw-striking, sw-dip854 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise ping, about 22 km long gubbio normal fault. this structure is clearly responsible for the basin inception and for a maximum total throw of the carbonatic multilayer (jurassic-oligocene) of about 2500 m (mirabella, 2003). the detailed reconstruction of the gubbio fault isobaths by barchi et al. (2000) shows an abrupt change in the plane dip from 50° to 20° below 3.5 km. all the geomorphic elements used to discuss the gubbio fault activity should refer to the steeper and more superficial part of the fault: no obvious geomorphic evidence for the characterization of the low-angle part of the fault exists. therefore, in the following we will discuss the gubbio fault as composed of a high and a low-angle portion. simple topographic profiling across the fault trace shows a gently dipping regularized profile with the section on the calcareous bedrock having a grade similar to that built on debris deposits (fig. 8). even if the gubbio fault strongly controls the eastern flank of the basin, separating two different lithological sectors, no evidence for a recent important activation of the bedrock fault plane has been observed. the predominance of the erosional and depositional processes with respect to tectonic activity along the main gubbio fault plane is shown by the presence of young entrenched alluvial fans (figs. 3b and 6) and by the misfit between the curves of topographic and geologic throw along the fault strike (fig. 4). here differential erosion dominates due to the presence of the pre-existing carbonatic anticline. however, field and airphoto investigations of the debris deposits along the fault hangingwall are suggestive of minor recent displacements located west of the main bedrock plane. the presence of slope ruptures and elongated small suspended benches (fig. 8, cross section 2), associated with small 855 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential 20 cm fig. 14a,b. a) photo of the nw wall of the artificial cut affecting the slope debris near the gubbio fault. in figure are highlighted normal faults (continuous line) and main depositional horizon (dot line); b) detail showing the re-oriented clasts and the offset amount. a b normal faults affecting slope deposits (fig. 14a,b), and the unconformity of the debris layers dip with respect to the present erosional surface slope, suggest the existence of two parallel fault splays located about 500 m west of the gubbio fault between gubbio and mocaiana (fig. 3b). some of these variations of the topographic gradient represent landslide crowns, although their linearity and continuity (up to 1.5 km) would support a tectonic origin. the clear predominance of the erosional processes over tectonics, partially controlled by the apennines regional uplift (doglioni et al., 1998), indicates low or negligible rates of tectonic activity of the high-angle gubbio fault. the activity of the low-angle portion of the gubbio fault can be inferred on the basis of the location of the april 29, 1984 earthquake. the main shock location (fig. 5) is close to the branch line of the gubbio fault with the altotiberina fault (see inset in fig. 1). the cmt solution for the mainshock (dziewonski et al., 1985) indicates the low-angle gubbio fault as the probable seismogenic source. the gubbio basin is the long-term evidence of the activity of a ca. 22 km long fault. it is well known that geometry and size of a fault together with its slip amount and distribution, are the major factors controlling the evolution, length, and depth of a fault related basin, coupled with the regional uplift context. with a fault of a such length, which produced 2000 m of displacement, and assuming that both sections of the gubbio fault (high and low-angle) were active simultaneously, we would expect a fault related basin 8 to 10 km wide. this is in clear contrast with the present setting of the basin which is peculiarly narrow, not exceeding a width of 4 km. our investigations highlighted that the western flank, although without an associated impressive topographic relief, exhibits a peculiar geomorphology which suggest a tectonic control on the basin. looking in detail, two mains, nwtrending, parallel lineaments appear to dominate all along this flank (fig. 6). the westernmost is clearly arranged in a right stepping system with several creeks, flowing across it, showing aligned knick-points suggesting normal faulting with east side down (fig. 10a,b). the second lineament is located close to the contact with the basin infill (figs. 6 and 11), where it affects both miocene and quaternary deposits. this lineament is composed by a series of aligned scarps, bordering several fluvial-lacustrine surfaces at different elevation and intersecting recent alluvial fans (fig. 12d). the interpretation of these lineaments as e-dipping normal faults is supported also by the location and linearity of the north and south saonda creeks, flowing close and parallel to the basin western flank. thus, we propose that a normal fault antithetic to the gubbio fault exists along the western side of this basin, close to the contact bedrock/infill. this antithetic fault has an average strike parallel to the gubbio fault and may explain the reduced 4 km width of the basin. 4. modeling of faults in relation with landscape evolution in the previous sections we suggested that the present setting of the gubbio basin is the result of long-term activity of two w-dipping normal fault (a low and high-angle sections confined between depth 6 and 3.5 km and above 3.5 km, respectively) and of a secondary antithetic high-angle normal fault located above 2.8 km (fig. 15b). to test this hypothesis with an independent approach, we tried to verify the influence of these fault sections on the landscape evolution of the gubbio basin. the idea is to compare the deformation expected to be produced at the surface by each fault, or by a combination of faults, with the observed basin setting. to achieve this goal we modeled the expected deformations by using a code, based on standard dislocation theory developed by ward and valensise (1989). consequently our calculations assume uniform slip on planar, rectangular faults embedded in elastic half-space. to establish the fault parameters of our model, we adopted for the low and high-angle portions of the gubbio fault a simplified fault geometry (fig.15b, faults 1 and 2), derived from the detailed reconstruction of the fault isobaths (barchi et al., 2000). to determine the fault length of the antithetic structure, we used the data derived from field survey and air photo 856 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise 857 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential 20° 50° 60° gubbio fault antitethic fault 3 2 1 6.0 5.0 4.0 3.0 2.0 1.0 0 0 2 4 6 8 10 12 14 distance (km) a a' d e p th ( km ) basin e le va tio n ( m ) 500 600 700 800 900 0 2 4 6 8 10 12 14 a a'topographic profile distance (km) basin unitary slip unitary extension unitary extension 3 km n antithetic fault gubbio fault a a' distance (km) e le va tio n c h a n g e 1 2 3 -1.4 -1.2 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0 2 4 6 8 10 12 14 0.0 -1.0 basin a a' a a' distance (km) e le va tio n c h a n g e distance (km) e le va tio n c h a n g e -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0 2 4 6 8 10 12 14 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0 2 4 6 8 10 12 14 basin basin 1 3 2 1+2+3 combined contributions distance (km) a a' e le va tio n c h a n g e a a' distance (km) e le va tio n c h a n g e -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0 2 4 6 8 10 12 14 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0 2 4 6 8 10 12 14 basin basin 1 2 3 1+3 1+2 individual contributions combined contributions individual contributions individual contributions fig. 15a-f. modeled elevation changes due the activity of the two sections of the gubbio fault and its antithetic structure. a) location map of the modeled transect. b) sketch of the faults geometry adopted. c) individual contributions of the faults, assuming a unitary slip for each of them. d) topographic profile along the modeled transect. e) individual and combined contributions of the fault sections. applying a unitary horizontal extension, nesw oriented, and assuming faults 2 and 3 accommodate alternatively the extension. combination of faults 1 and 2 or faults 1 and 3 is shown in the lower part. f ) individual and combined contributions of the fault sections applying a unitary horizontal extension, ne-sw oriented, assuming the three faults work simultaneously. each fault 2 and 3 has to accommodate half of the unitary extension. the combination of the three sections seems to fit well the topographic setting of the basin. c d a b e f interpretation assuming a constant 60° dip down to the intersection with fault 2 to obtain its width (fig.15b, fault 3). for faults 2 and 3, we assume they are both surface-breaking and with a pure normal dip slip movement. thus, we adopted a length of 22 km and a total width of 11.5 km for the two portions of the gubbio fault, a length of 18 km and a width of 2.8 km for the antithetic fault (fig. 15b, fault 3) and a general strike of 130°. all the elevation changes obtained by the dislocation modeling are calculated along the same transect (fig. 15a) for which a topographic profile is also plotted in fig. 16d. in the following, on the basis of our previous discussion, we consider that the lowangle portion of the gubbio fault (fig. 15b fault 1) is definitely active. in order to test the role played by the geometric parameters of the modeled faults, we assigned to each fault the same unitary slip (fig. 15c), being aware of the fact that this is not a realistic model for their long-term activity. from this model it is possible to note that: fault 1 generates a gentle, almost symmetric wide signal at the surface with its maximum subsidence in correspondence with the western edge of the basin; fault 2 produces an asymmetric sharp depression, about 6 km wide deepest in the eastern part of the basin; fault 3 produces an asymmetric sharp depression, deepest in the western part of the basin, the only completely confined within the basin. the location of the antithetic fault in the area of maximum subsidence produced by fault 1, could be interpreted as the consequence of the deformation pattern resulting from the activity of fault 1. in order to make the expected elevation changes more realistic and because of the lack of direct measurements of extension across the area, we apply a unitary horizontal ne-sw extension on the system (fig. 15e,f) with fault 1 always active. in particular, in the model shown in fig. 15e, we assume that the whole crustal extension above 3.5 km is accommodated only by one of the high-angle faults: thus only by fault 2 or by fault 3. whereas, in fig. 15f faults 2 and 3 are active simultaneously accommodating half of the extension each. we are aware of the fact that probably this is a reasonable model only for the short-term activity of these faults. an important implication of this test is that, summing the contribution of fault 1 alternatively with that of faults 2 or 3 in fig. 15e, no satisfactory match with the basin setting is found. in fact, looking in fig. 15e at the combined contribution of faults 1 and 2 (forming the whole gubbio fault), we remark that the width of the modeled depression is quite larger with respect to the actual one (fig. 15d), and that the resulting asymmetry toward ne is in disagreement with the present drainage network within the basin, being clearly asymmetric toward the sw (fig. 9). only the gentle dip toward ne of the continental deposits filling the basin (ge.mi.na., 1963; menichetti, 1992) seems to fit this model. on the other hand, the deformation distribution produced by modeling the combination of faults 1 and 3 (even if this setting is mechanically unlike) in fig. 15e, shows a better fit with the basin width, linearity, and position of the north and south saonda creeks, but is in strong contrast with the basin quaternary sedimentary infill geometry (ge.mi.na., 1963; menichetti, 1992). the lack of fit of the model shown in fig. 15e, which assumes only one of the faults 2 or 3 to be active along with fault 1, and the geomorphic results discussed above, highlights the need for a combined contribution of the three faults in a graben-like setting (fig. 15f ). an immediate consequence of this test is that the relative importance of the signal produced by fault 1 at the surface increases. we can also clearly note how the actual basin width is in good agreement with the modeled one (fig. 15d,f). moreover, the overall shape of the computed elevation changes for the three faults combination (fig. 15f ) is well comparable with the topography as well as with the prepleistocene basement shape according to menichetti (1992) (fig. 3a). finally, the fig. 15f model would fit better also the gentle dip of the basin infill deposits (fig. 3a) and admits some complexity in the near fault (e.g., high dip angle of the clayey-lignitic basal complex, ge.mi.na., 1963). 4.1. the gubbio fault system we have concluded that three main fault sections can be imaged in the gubbio basin, however, no unequivocal data exist to place some 858 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise constraints on the age of activity of the individual fault sections, nor on their slip rates even on a relative basis. for the gubbio fault the authors (selvaggi et al., 1989; menichetti, 1992) set an inception of the activity between late pliocene and early pleistocene. during this period a maximum throw of 2500 m (mirabella, 2003) has been accumulated, which would translate to a maximum slip rate higher than 1.4 mm/yr. no constraints for the antithetic fault (fault 3) age of inception exist. the long-term evidence of this fault does not suggest an activity lasting since early pleistocene and producing kilometric displacement like the gubbio fault. the fact that the antithetic fault should have a smaller role than the gubbio fault in the basin development can be also inferred by comparing figs. 15f with 15d and 3a. we can notice a discrepancy between the modeling, where faults 2 and 3 are given the same weight (i.e. each accommodating the same amount of extension) and the present basin setting. to obtain a better fit we should reduce in the model the role of the antithetic fault with respect to the gubbio fault. it is hard to define whether this minor role of the antithetic fault is due to its lower slip rate, its more recent activation, or by a combination of these two. even though the antithetic fault has had a minor role in the basin development, at present this appears to be the fault section most effective in producing a landscape signature and thus, to be considered active. on the basis of the dislocation modeling we have argued that the combined activity of the two sections of the gubbio fault and its antithetic, is needed to explain the present setting of the gubbio basin (fig. 15f ). however, we have also to consider erosion, driven by the regional uplift, as an important contributor to the present basin shape. the rates of uplift of the apennines are expected to be as much as 1 mm/yr (doglioni et al., 1998); if these exceed the rate of subsidence in the basin, sedimentation in the hangingwall would not probably be enough to compensate the tectonic offset. at present, both footwall and hangingwall undergo erosion and is possible that sedimentation was never able to entirely fill the basin. however, this does not seem to be enough to assume the basin narrowness could be derived only by the activity of the gubbio fault (fig. 15e, faults 1 and 2). in fact, the western edge of the basin is linear and not shaped according the distribution of displacement on the gubbio fault (wider in correspondence of the maximum throws). summarizing, on the assumption that the low-angle gubbio fault is the main active fault in the area below a depth of 3.5 km, it is still unclear if today the antithetic fault takes over most of the extension across the upper few kilometers of crust, with an essentially inactive highangle gubbio fault, or if this latter is active too. the short-term geomorphic signal seems to suggest the gubbio fault to slip at a very low rate or to be inactive, whereas the antithetic fault to lead the present landscape evolution of the area. on the other hand, the maximum slip rates inferred for the gubbio fault (>1.4 mm/yr) on its long-term activity appear to be too high in relation to its poor present geomorphic signature even in consideration of the expected erosional rates for the area. this clear discrepancy points out that the gubbio fault activity changed through time, allowing at an unknown time the inception of the activity of the antithetic fault. 5. seismogenic sources of the gubbio basin based on the considerations discussed in the previous sections we conclude that the gubbio basin contains active and potentially seismogenic faults. however, the extent of the gubbio fault is quite large with respect to the presumable rupture length of the 1984 earthquake (ms 5.3). at the same time, geomorphic data from the basin and the long-term evidence for pleistocene fault activity do not suggests the existence of permanent fault boundaries. the database of potential seismogenic sources for earthquakes larger than m 5.5 in italy (valensise and pantosti, 2001b) proposed a preliminary scheme for the seismogenic sources of the gubbio area based on available literature. the new data and considerations presented in this work allow us to propose an updated version of this scheme. in the previous sections, we defined the gubbio fault system as formed by three 130°859 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential striking normal fault sections: a w-dipping, low-angle fault located below 3.5 km depth and two high-angle faults arranged in a graben-like setting located above 3.5 km depth (fig. 15b). this structure is limited by sharp northern and southern boundaries which limit its length to about 22 km. the northern boundary is set on the basis of: a) the position of the northern tip of the gubbio fault as shown by geophysical data (fig. 4); b) the northern end of the basin as defined by the presence of several depositional paleosurfaces located at the highest elevation within the basin (fig. 6), indicating an area of lower subsidence typical of a fault tip, and 860 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise mocaiana gubbio padule branca mt. urbino chiascio river assino river c e a d b ponte d'assi gubbio seismogenic sources fossato di vico-valle dell'esino tectonic lineament projected sources cut-off transversal alignment intersection with the altotiberina fault gubbio seismogenic sources parameters width strike carbonates (jurassic-oligocene) carbonates (jurassic-oligocene) carbonates (jurassic-oligocene) continental deposits (pleistocene-holocene) continental deposits (pleistocene-holocene) continental deposits (pleistocene-holocene) turbidites (miocene)turbidites (miocene)turbidites (miocene) anomaliesanomalies normal faultsnormal faultsnormal faults a n4 km length dip thrust faultsthrust faultsthrust faults divides n 8 km 1 2 1 2 bottom depth top depth 7 km 130° 10 km 20° 4 km 6 km4.5 km 130° 7 km 10 km 20° 2.5 km fig. 16. hypothesized seismic sources in the gubbio area. simplified geology and basin divides are reported. the thick normal faults are the high-angle sections of the gubbio fault system. the sources are represented as boxes, which are the projection of the low-angle plain of the gubbio fault to the surfaces. dashed gray line is the hypothetical transversal lineament highlighted by structural and geomorphological anomalies. the inset shows the position of the sources with respect to the reconstruction of the gubbio fault isobaths, its intersection with the altotiberina fault and the 1984 seismicity (after barchi et al., 2000, modified). c) some assino river captures (cattuto, 1973). seismic reflection profiles suggest that the gubbio fault probably continues south-eastward outside the basin (mirabella, 2003). because of the total lack of geomorphic evidence we assume this southern part of the fault is the witness of an early, longer structure, not active today. thus we infer the length of the gubbio fault as directly derived from the basin size. the southern boundary is less constrained and coincides with the southern end of the basin marked by the fossato di vico-valle dell’esino transverse tectonic lineament (cencetti, 1988; dramis et al., 1991 and references therein) and with some chiascio river captures (cattuto, 1973). on the basis of the 1984 inferred seismogenic source and of similarities with the geometry of the 1997, colfiorito earthquake sources, we may approximate the main seismogenic source of the gubbio area with the low-angle section of the gubbio fault. this corresponds to a 22 km long, 7 km wide, 20° dipping fault plane which, according to wells and coppersmith’s (1994) empirical relations could produce earthquakes in the magnitude range 5.9 to 6.4. the geomorphic evidence of the gubbio fault suggests that this structure has been active for a long time over its entire 22 km length. however, no strong basis exists for deciding whether the fault will rupture as a whole segment or in shorter independent sections. in both cases the gubbio basin would be evidence for long term activity of the fault but in the second case it would be the result of the coalescence of smaller basins. understanding which of these two scenarios is more realistic has a critical impact on seismic hazard issues. in the middle part of the basin there is evidence for a ne-sw alignment highlighted by the following anomalies (fig. 16): a) abrupt southern termination of the carbonatic anticline; b) lateral termination of miocene thrust fronts (menichetti et al., 1986); c) low hierarchic grade of some ne-sw creeks; d,e) divide and watershed flexures (cencetti, 1988). this alignment is also consistent with discontinuities within the altotiberina fault and the bending of the gubbio fault plane at depth (see inset fig. 16; barchi et al., 1999, 2000) and may suggest internal complexities of the gubbio fault system that could act as geometric barriers in the propagation of earthquake ruptures. this potential segmentation of the gubbio fault system is also in agreement with observations on the 1984 earthquake. as discussed in the previous part of the paper, this ms 5.3 earthquake is interpreted to have ruptured the southern part of the low-angle gubbio fault. moreover, the 1984 aftershock distribution shows a peculiar step over zone (collettini et al., 2003) (inset in figs. 5 and 16) occurring along the nesw alignment. based on these considerations we suggest that the gubbio area may comprise two smaller seismogenic sources whose boundaries are represented by the basin fault system boundaries and by the hypothesized ne-sw transverse alignment (fig. 16). the maximum magnitude expected for these sources can be estimated both by assuming the 1984 earthquake as the typical size earthquake for the area, and also on the basis of the fault length and width using empirical relations (wells and coppersmith, 1994). on this basis, these sources may generate m 5.3-5.9 earthquakes. however, the activation of the 22 km long fault for its entire length cannot be completely ruled out. no information on the long-term seismic behavior of these sources exists. taking into account that the historical record of seismicity can be considered complete for 5.0 < m < 5.6 in the past 400 years and for m > 5.6 in the past 800 years (zonno et al., 2002) and that there is no evidence for 1984-type earthquakes during this period, we expect recurrence intervals longer than four centuries. 6. conclusions we used geomorphic analysis coupled with dem analysis and dislocation modeling to understand the present tectonic activity of the gubbio basin and infer the potential seismogenic sources threatening the area. the present setting of the gubbio basin appears to be controlled by three main fault sections: two of them form a w-dipping fault known as the gubbio fault (fig. 15b, faults 1 and 2), while the third one is a e-dipping high861 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential angle fault (fig. 15b, fault 3) antithetic to the gubbio fault. the antithetic fault has a clear surface expression and several lines of evidence suggest its recent activity: aligned scarps in young deposits and bedrock, drainage anomalies, basin asymmetry, staircase-like uplifted erosional and depositional surfaces. the existence of this antithetic fault contributes to explain the narrowness of the gubbio basin. the gubbio fault, which certainly played an important role in the basin construction, does not show sizable ongoing activity. no stratigraphic data are available to decide conclusively if the high-angle section of the gubbio fault is: 1) totally inactive; 2) slipping at very low rates (easily compensated by erosional processes), or 3) slips at a substantial slip rate (> 0.5 mm/yr). neverthless, we believe that the 1984 earthquake provides living evidence that the lowangle portion of the gubbio fault (fault 1) is active. after testing different combinations of the fault sections of fig. 15b, we conclude that only the joint action of all three faults can fully explain the present setting of the basin (fig. 15f ). the antithetic fault and the low-angle section of the gubbio fault appear to be the main active features although their long-term contribution is smaller than that of the highangle gubbio fault. no firm constraints on the relative age and rates of activity of the different fault sections exist at present. rupturing of the whole gubbio fault could produce a m 5.9-6.4 earthquake. however, based on the existence of a transverse lineament in the middle part of the basin that may act as a fault boundary, we hypotesize the existence of two smaller, independent seismogenic sources, each one capable of a m 5.3-5.9 earthquake (fig. 16). these sources are represented by low-angle, w-dipping normal faults that may propagate to the surface through the adjoining high-angle sections. the southern source is thought to have ruptured entirely in the 1984 earthquake, but no evidence for earthquakes rupturing the northern source exists in the historical record. under these circumstances this latter source may be the cause of a future damaging earthquake. the peculiar geometry of these sources and their relatively small size, which sets an upper bound to the size of the expected earthquakes, appears to be a typical feature for this part of the central apennines seismogenic belt. this observation may explain the frequent but moderate magnitude earthquakes reported for this region (cpti, working group, 1999) and has important implications on seismic hazard evaluations. acknowledgements we thanks c. collettini, f. mirabella, r. basili and m. moro for the constructive discussions and suggestions, t. tamashvili, m. marchetti and his group for their field support, and the countess torlonia-borghese for permission for working in her land. we are grateful to m. barchi and m. meghraoui for their valuable comments and suggestions, which helped substantially in preparing the final version of the paper. this research was funded by the gndt project «terremoti probabili in italia tra l’anno 2000 ed il 2030: elementi per la definizione di priorità degli interventi di riduzione del rischio sismico» and ingv «seismology and tectonophysics» department. references barchi, m. 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(2002): lithological and structural controls on the seismogenesis of the umbria region: observations from seismic reflection profiles, boll. soc. geol. it., 1 (spec. vol.), 855-864. barchi, m., a. de feyter, m.b. magnani, g. minelli, g. pialli and b.m. sotera (1998): the structural style of the umbria-marche fold and thrust belt, mem. soc. geol. it., 52, 557-578. barchi, m., s. paolacci, c. pauselli, g. pialli and s. merlini (1999): geometria delle deformazioni estensionali recenti nel bacino dell’alta val tiberina fra s. giustino umbro e perugia: evidenze geofisiche e considerazioni geologiche, boll. soc. geol. it., 118, 617-625. barchi, m., m. cardinali, p. chiraz, c. collettini, c. federico, f. guzzetti, m.b. magnani, g. minelli, f. mirabella, c. pauselli, g. pialli, s. pucci and e. troiani (2000): integrazione di dati geofisici e geologici per la caratterizzazione delle strutture sismogenetiche di colfiorito e di gubbio, in le ricerche del 862 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise gndt nel campo della pericolosità sismica (19961999), edited by f. galadini, c. meletti and a. rebez (cnr-gruppo nazionale per la difesa dai terremotiroma), 149-156. barchi, m., s. pucci, c. collettini, f. mirabella, d. massoli, f. guzzetti, p. reichenbach, m. cardinali, n. vergoni, e. troiani, p. chiraz and l. giombini (2001): a geologic map of the colfiorito area, in geoitalia, 3° forum fist poster session, 333-334. bartolini, c. 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(1891): il territorio di gubbio, notizie geologiche (roma), pp. 38. bonarelli, g., t. lipparini, a. moretti, c. pillotti, p. principi, f. scarsella and r. selli (1952): carta geologica d’italia 1:100.000, italian geological survey, 1st edition, sheet 116, gubbio. boncio, p., f. brozzetti and g. lavecchia (1996): state of stress in the northern umbria-marche apennines (central italy): inferences from microearthquake and fault kinematic analyses, ann. tectonicae, 10 (1/2), 8097. boncio, p., f. brozzetti, f. ponziani, m. barchi, g. lavecchia and g. pialli (1998): seismicity and extensional tectonics in the northern umbria-marche apennines, mem. soc. geol. it., 52, 539-555. branconi, p. and d. manconi (1982-1983): gubbio: nuovi scavi a via degli ortacci, ann. fac. lett. fil., studi classici (perugia), xx, 81-159. bull, w.b. and l.d., mc fadden (1977): tectonic geomorphology north and south of the garlock fault, california, in geomorphology in arid regions: binghamton, n.y., edited by d.o. doehring, state university of new york at binghamton, 115-138. cattuto, c. 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(1992): evoluzione tettonico-sedimentaria della valle di gubbio, studi geologici camerti, 1 (spec. vol.), 155-163. menichetti, m. and g. pialli (1986): geologia strutturale del preappennino umbro tra i monti di gubbio e la catena del m. petrano-m. cucco, mem. soc. geol. it., 35, 371-388. menichetti, m. and g. minelli (1991): extensional tectonics and seismogenesis in umbria (central italy) the gubbio area, boll. soc. geol. it, 110, 857-880. mirabella, f. (2003): seismogenesis of the umbria-marche region (central italy): geometry and kinematics of the active faults and mechanical behavior of the involved rocks, ph.d. thesis, university of perugia (italy). moretti, a. and u. perno (1968): carta geologica d’italia 1:100.000, italian geological survey (2nd edition), sheet 123, assisi. selvaggi, g. and s. sylos labini (1989): analisi sismotettonica del bacino di gubbio, in proceedings of the 8° meeting gngts, rome, 67-72. valensise, g. and d. pantosti (2001a). the investigation of potential earthquake sources in peninsular italy: a review, j. seismol., 5, 287-306. valensise, g. and d. pantosti (editors) (2001b): database of potential sources for earthquakes larger than m 5.5 in italy, ann. geofis., 44 (suppl. to n. 4), pp. 180 (with cd-rom). 863 geomorphology of the gubbio basin (central italy): understanding the active tectonics and earthquake potential ward, s.n. and g. valensise (1989): fault parameters and slip distribution of the 1915, avezzano, italy earthquake derived from geodetic observations, bull. seismol. soc. am., 79, 690-710. wells, d.l. and k.j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area and surface displacement, bull. seismol. soc. am., 84 (4), 974-1002. westaway, r., r. gawthorpe and m. tozzi (1989): seismological and field observations of the 1984 lazio-abruzzo earthquakes: implications for the active tectonics of italy, geophys. j., 98, 489514. zonno, g., v. montaldo and i. leschiutta (2002): valutazione della pericolosità sismica in alta valtiberina, ing. sismica, 1, 33-43. 864 stefano pucci, paolo marco de martini, daniela pantosti and gianluca valensise t e c t o n i c stress field in central e u r o p e g. b a n a l i ^ a . e . s c i i e i d e g g e r ( * ) k i c e v u t o il .6 m a r z o 1 9 6 7 summary. — s c l i e i d e g g e r ' s s t a t i s t i c a l m e t h o d f o r t h e t e c t o n i c i n t e r p r e t a t i o n of e a r t h q u a k e f a u l t p l a n e s o l u t i o n s is a p p l i e d t o r e c e n t l y p u b l i s h e d s o l u t i o n s of e a r t h q u a k e s w h i c h o c c u r r e d i n t w o a r e a s of c e n t r a l e u r o p e . t h e r e s u l t s of t h e c a l c u l a t i o n s s h o w t h a t i n s o u t h w e s t e r n g e r m a n y , e a s t of t h e e a s t e r n b o r d e r of t h e r h i n o t r e n c h , t h e t e c t o n i c s t r e s s s t a t e is a t e n s i o n a l o n e , w h i l e i n b e l g i u m i t s e e m s t o b o c o m p r e s s i o n a l . t h e p a n d t d i r e c t i o n s , h o w e v e r , a r e v e r y s i m i l a r i n b o t h r e g i o n s . riassunto. — i l m e t o d o s t a t i s t i c o d i s c h e i d e g g e r p o r l ' i n t e r p r e t a z i o n e t e t t o n i c a d e l m e c c a n i s m o d e i t e r r e m o t i a l l ' i p o c e n t r o ( " e a r t h q u a k e f a u l t p l a n e s o l u t i o n s " ) è q u i a p p l i c a t o a r e c e n t i i n d a g i n i di s i s m i o c c o r s i i n d u e d i f f e r e n t i r e g i o n i d e l l ' e u r o p a c e n t r a l e . i r i s u l t a t i d e i c a l c o l i m o s t r a n o c h e n e l l a g e r m a n i a s u d o c c i d e n t a l e , a d e s t d e l b o r d o o r i e n t a l e d e l l a f o s s a r e n a n a , l o s t a t o t e t t o n i c o p r e d o m i n a n t e è a c a r a t t e r e t e n s i o n a l e , m e n t r e i n b e l g i o s e m b r a e s s e r e a c a r a t t e r e c o m p r e s s i o n a l e . t u t t a v i a le d i r e z i o n i d e l l e c o m p r e s s i o n i e d e l l e t e n s i o n i s o n o m o l t o s i m i l i i n a m b e d u e l e r e g i o n i c o n s i d e r a t e . i n t r o d u c t i o n . i t is a well known f a c t t h a t t h e crust of t h e e a r t h is subject to a " tectonic " stress s t a t e which m a y v a r y in its c h a r a c t e r f r o m region t o region. i t r e m a i n s one of t h e more i m p o r t a n t problems in geophysics t o ascertain t h e precise n a t u r e of this stress s t a t e inasmuch as it is n o t easily possible to measure u n d e r g r o u n d stresses b y direct means. thus, m o s t of our knowledge of tectonic stresses has come f r o m indirect evidence, such as analyses of surface f a u l t p a t t e r n s , etc. (*) u n i v e r s i t y of i l l i n o i s , u r b a n a 1 9 4 g. r a n a l l i a . e . s c i i e i d e g g e r one of the most promising alleys for investigating t h e regional tectonic stresses is based on seismic evidence. a statistical analysis of f a u l t plane solutions of e a r t h q u a k e s yields directly t h e direction of greatest compression (p-axis), t h e direction of least compression (taxis) a n d t h e axis of relative motion (normal t o ' t h e i n t e r m e d i a t e principal stress |\b] axis) in a n y one a r e a (see sclieidegger, °). this statistical t r e a t m e n t of e a r t h q u a k e f a u l t p l a n e solutions has already given i n f o r m a t i o n a b o u t t h e tectonic stress s t a t e in various areas of t h e world [scheidegger (9), (10), ( n ) , (12), (13)]. recently, new sets of f a u l t plane solutions h a v e been calculated. these news solutions available m a k e now possible a detailed s t u d y of some region previously n o t v e r y a v e l l k n o w n f r o m t h e p o i n t of view of seismotectonics. i n this instance, t h e present p a p e r deals with t w o areas of c e n t r a l e u r o p e n o r t h of the alps, n a m e l y s o u t h w e s t e r n g e r m a n y a n d belgium. as to southwestern g e r m a n y , eleven e a r t h q u a k e s which h a v e occurred in t h e southern black f o r e s t , in t h e southwestern p a r t of t h e swabian j u r a , a n d in t h e western p a r t of t h e molasse basin h a v e been studied. the region in question is roughly couflned between 47°-49° if and 7°-10° e . the references in t h e l i t e r a t u r e h a v e been given b y h o r d e j u k (8), hiller (5), (6), a n d schneider, schick a n d berckhemer ("). the directions of t h e p , t, a n d b axes h a v e been calculated for each one of these e a r t h q u a k e s . the f a u l t p l a n e solutions are listed in table i ¡in t h e s t a n d a r d f o r m 'employed b y f a r a (4). the k e y to t h e l i t e r a t u r e references is as follows: 1: h o r d e j u k (8); 2,3: hiller (5), (6); 4,5,6,7,8,9,10,11: schneider, schick a n d berckhemer ( u ) . 'for t h e first t h r e e e a r t h q u a k e s , planes i a n d i i h a v e also been calcul a t e d . i n seven o u t of t h e eight e a r t h q u a k e s studied b y schneider, schick a n d berckhemer (14), t h e b axis has b e e n a s s u m e d to be vertical, a n d accordingly, t h e p a n d t axes h a v e been assumed to be horizontal, since t h e a u t h o r s give no i n f o r m a t i o n a b o u t t h e dip of t h e b axis, b u t only t h e projections of t h e t w o nodal lines on t h e surface of t h e e a r t h a n d t h e horizontal directions of t h e p and t axes. sometimes t h e nodal lines are n o t perpendicular t o each o t h e r ; this can be explained b y a slight t i l t of t h e force system. a n o t h e r case in which t h e n o d a l lines were n o t perpendicular has been studied b y caloi (2). this e a r t h q u a k e occurred on october 18, 1936, a n d its epicenter was located n e a r t h e t o w n of belluno, in n o r t h e a s t e r n i t a l y ; its epicentral t a b l e i f a u l t p l a n e s o l u t i o n s o f e a r t h q u a k e s i n s o u t h e r n g e r m a n y d a t e e p i c e n t e r p t b p l a n e i p l a n e i i n o . 1 6 . 1 1 . 1 1 4 8 . 3 n 0 0 9 . i e 00 n 7 5 w 3 8 s 6 0 e 5 1 n 2 1 e 0 7 n 6 8 w 8 4 n 6 3 e 10 *d 1 1 1 0 8 . 0 2 . 3 3 4 8 . 8 n 0 0 8 . 2 e 00 n 2 5 w 0 0 n 6 5 e 0 0 n 0 0 e 9 0 s 7 0 e 9 0 n 2 0 e 90 s 2 2 7 . 0 6 . 3 5 4 8 . i n 0 0 9 . 4 e 00 s 0 5 e 0 0 n 8 5 e 0 0 n 0 0 e 9 0 n 5 0 w 9 0 n 6 1 e 90 s 0 1 3 0 2 . 0 5 . 4 3 4 8 . 3 n 0 0 9 . 0 e 00 n 4 0 w 0 0 n 4 6 e 0 0 n 0 0 e 9 0 4 2 8 . 0 3 . 6 0 4 8 . 3 n 0 0 9 . 0 e 00 n 4 3 w 0 0 n 4 3 e 0 0 n 0 0 e 9 0 5 19.04.61 4 8 . i n 0 0 9 . 0 e 00 n 2 8 w 0 0 n 6 3 e 0 0 n 0 0 e 9 0 6 2 8 . 0 4 . 6 1 4 7 . 7 n 0 0 7 . 9 e 00 n 3 8 w 0 0 n 5 1 e 0 0 n 0 0 e 9 0 7 2 7 . 0 3 . 6 5 4 8 . i n 0 0 9 . 5 e 00 n 0 2 w 0 0 n 8 8 e 0 0 n 0 0 e 9 0 8 3 0 . 0 3 . 6 5 4 8 . i n 0 0 9 . 5 e 00 n 0 3 w 0 0 n 8 7 e 0 0 n 0 0 e 9 0 9 2 5 . 0 5 . 6 5 4 8 . i n 0 0 9 . 5 e 00 no 1 woo n 8 9 e 0 0 n 0 0 e 9 0 10 19.09.65 4 7 . 9 n 0 0 8 . 2 e 00 s 2 3 w 1 0 n 2 3 e 8 0 s 6 7 e 0 0 11 i!)f> g . k a n a l l i a. e . s c h e i d e g g e l l coordinates are a p p r o x i m a t e l y 46° n a n d 12° e . i t is interesting to n o t e t h a t t h e horizontal projections of t h e p a n d t axes are n 29° w and n 61° e respectively, i.e. v e r y similar to those of southwestern g e r m a n y a n d belgium. i n e a r t h q u a k e (11) t h e b axis is horizontal, a n d t h e dips of t h e p a n d t axes h a v e been calculated on t h e basis of a m e t h o d proposed b y holler (7). f i g . 1 p d i r e c t i o n s f o r e a r t h q u a k e s i n s o u t h w e s t e r n g e r m a n y . fig. 1 shows t h e p directions for each of these e a r t h q u a k e s which h a v e occurred in s o u t h w e s t e r n g e r m a n y . t h e second region considered is belgium. the a r e a u n d e r consideration is roughly confined between 50°-52° n a n d 4 ° 6 ° e . the d a t a h a v e been calculated f r o m i n f o r m a t i o n given in p a p e r s b y ahorner and v a n gils (*), a n d v a n gils (16). three e a r t h q u a k e s h a v e been t a k e n into account, b u t , since in t h e e a r t h q u a k e of december 15, 1965, t h e foreshock and t h e m a i n shock showed a completely reversed ground motion direction, f o u r cases are listed in table i i . the k e y to literat u r e references is: 12: ahorner & v a n gils (x); 13,14,15: v a n gils (16). cases listed as (13) a n d (.14) refer respectively t o t h e foreshock and t o t h e m a i n shock of t h e above m e n t i o n e d e a r t h q u a k e of december 15, 1965. f o r e a r t h q u a k e (12), planes i a n d i i h a v e also been calculated. the b axis has been assumed t o be vertical in (13), (14), v t a b l e 11 f a u l t p l a n e s o l u t i o n s o f e a r t h q u a k e s i n b e l g i u m . d a t e e p i c e n t e r p t b p l a n e i p l a n e i | n o . 2 5 . 0 6 . 6 0 5 1 . 2 n 0 0 5 . 7 e 00 4.2 n 0 0 e 0 0 n 9 0 e 3 2 n 9 0 w 5 8 n 9 0 e 77 nooe 00 * 12 1 5 . 1 2 . 6 5 5 0 . 2 n 0 0 4 . 0 e 00 4.4 n 57 woo n 3 3 e 0 0 n 0 0 e 9 0 13 15.12.65 5 0 . 2 n 0 0 4 . 0 e 00 4.4 n 3 3 e 0 0 n 5 7 w 0 0 n 0 0 e 9 0 14 2 1 . 1 2 . 6 5 5 0 . 4 n 0 0 5 . 3 e 00 4.4 n 6 6 w 0 0 n 2 4 e 0 0 n 0 0 e 9 0 15 198 g. r a n a l l i a . e . s c i i e i d e g g e r (15). since these solutions are based upon recordings f r o m belgian stations only, t h e y have no exaggerate claim of accuracy. fig. 2 shows t h e p directions for each case. t r e a t m e n t of data . the f a u l t plane solutions listed in tables 1 a n d 2 have been t r e a t ed b y a statistical m e t h o d developed b y scheidegger (9); t h e best f i t t i n g p, t, a n d p n o r m a l axes h a v e been calculated b y an eigenvalue t e c h n i q u e w i t h t h e help of t h e university of illinois i b m 70941401 computer system. the p n o r m a l axis has the sense of direction of m o t i o n ; if it is more or less in t h e same direction of t h e best p axis, t h e tectonic area is a compressional one, in t h e other case, we are dealing with a tensional tectonic region. t a b l e 111 r e s u l t s o f c a l c u l a t i o n s . a r e a n o . of q u a k e s b e s t p b e s t t m o t i o n a x i s s o u t h w e s t e r n g e r m a n y b e l g i u m 11 4 n 1 9 w 0 1 n 3 3 w 0 0 n 7 0 e 0 4 n 5 1 e 10 s 6 0 w 0 1 n 0 0 e 0 0 t e c t o n i c s t r e s s f i e l d i n c e n t r a l e u r o p e 109 the results of t h e statistical c a l c u l a t i o n s are shown in table i i i . the m e a n stress systems in southerwestern g e r m a n y a n d belgium are p l o t t e d in fig. 3. b e s u l t s . w e shall consider separately t h e two geographical areas under consideration. i n southwestern g e r m a n y , all e a r t h q u a k e s under consideration h a v e occurred in t h e region immediately east of t h e eastern border of t h e b h i n e trench, namely in intensely f a u l t e d areas (as t h e southern black forest, a n d t h e western p a r t of t h e swabian j u r a near t h e hohenzollern trench). i n d i v i d u a l f a u l t plane solutions show t h a t b o t h strike-slip and dip-slip motions are present in this zone [schneider, schick a n d berckhemer (")]. the result of the statistical calculations points out t h a t the stress system is a tensional one (the motion axis nearly coincides a v i t h t h e best fitting t axis). this is n o t unexpected in a border zone of a trench, a n d therefore this result is consistent with w h a t one m a y reasonably infer f r o m p u r e l y geological considerations. as regards t h e belgian region, it is to be again pointed out t h a t t h e f a u l t plane solutions are based on d a t a f r o m few seismic stations, 2 0 0 g . r a n a l l i a. e . s c i i e i d e g g e r a n d , moreover, t h a t t h e n u m b e r of e a r t h q u a k e s is v e r y small. a n y w a y , the result indicates t h a t t h e area is one of compressional tectonic stress (the m o t i o n axis is closer to t h e p axis t h a n t o t h e t axis). an interesting f e a t u r e is observed in t h e f a c t t h a t in b o t h , southwestern g e r m a n y and belgium, t h e p a n d t directions are almost identical. this seems to indicate t h a t t h e tectonic stress field is more or less t h e same in all of central e u r o p e n o r t h of t h e alps, a n d t h a t it is only t h e i n t e n s i t y (i.e. t h e predominance) of t h e p or t direction t h a t is different. very recently, new light has been b r o u g h t , f r o m seismic evidence, into t h e tectonic f e a t u r e s of t h e m e d i t e r r a n e a n region [constantinescu, r u p r e c h t o v a , and enescu (3)], a n d into t h e present stress s t a t e of t h e alpine r a n g e [scheidegger (")]. the present s t u d y complements these other investigations with regard to t h e remaining p a r t of e u r o p e . a c k n o w l e d g m e n t s . t h e calculations r e p o r t e d here were performed on t h e university of illinois i b m 7094-1401 c o m p u t i n g system operating u n d e r a g r a n t f r o m t h e n a t i o n a l science f o u n d a t i o n . the writers are i n d e b t e d to professor b e r c k h c m e r of f r a n k f u r t for l e t t i n g t h e m h a v e a copy of t h e p a p e r 011 f a u l t plane solutions of e a r t h q u a k e s in b a d e n w ü r t t e m berg prior to its publication. r e f e r e n c e s (1) ahobner l . , van gils j . m . , das erdbeben vom 25 juni i960 im belgischnederländischen grenzgebiet. a c a d . r o y a l e d e b e l g i q u e , " b u l l . cl. s c i e n c e s " , s e r i e g e o p l i y s . , 6 7 , ( 1 9 6 4 ) . (2) caloi p . , bicerche su terremoti ad origine vicina. scosse del cansiglio dell'ottobre 1930. " l a r i c e r c a s c i e n t i f i c a " , 9 , 2, 1 4 1 , ( 1 9 3 8 ) . (:l) constantinescu l . , r u p r e c h t o v a l . , e n e s c u u . , mediterranean alpine earthquake mechanisms and their seismotectonic implications, " g e o p l i . j . r . a s t r . s o c . " , 1 0 , 3 4 7 3 6 8 , ( 1 9 6 6 ) . (') fara h . d . , a new catalogue oj earthquake jault plane solutions, " b u l l . s e i s m . s o c . a m . " , 5 4 , 1 4 9 1 1 5 1 7 , ( 1 9 6 4 ) : (6) hiller w . , der herd des bastatter bebens am s februar 1933, " g e r ì . b e i t r . z u r g e o p l i . " , 4 1 , 2, 1 7 0 1 8 0 , ( 1 9 3 4 ) . t e c t o n i c s t r e s s f i e l d i n c e n t r a l e u r o p e 201 (6) hiller w . , das oberschwäbische erdbeben am 27. juni 1935. " w ü r t t . j b . f ü r s t a t i s t i k u . l a n d e s k d e " , j n g . 1 9 3 4 / 3 5 , 2 0 9 2 2 6 , ( 1 9 3 6 a ) . (7) iiiller w . , das erdbeben im nördlichen schicarzwald am 30. dezember 1935, " s e i s m . b e r . w ü r t t b g . e r d b e b e n w " . j g . 1 9 3 5 , (1936ft). (8) hordejuk j . , charakterystyka dynamiczna ogniska trzesienia ziemi z dnia 1(1. 11. 1911, " a c t a g e o p l i . p o l o n i c a " , 5, 2, 103-109, (1957). (") scheidegger a. e . , the tectonic stress and tectonic motion direction in europe and western asia as calculated from earthquake fault plane solutions, " b u l l . s e i s m . s o c . a m . " , 5 4 , 1519-1528, (1964). (10) sciieidegger a. e . , the tectonic stress and tectonic motion direction in the pacific and adjacent areas as calculated from earthquake fault plane solutions, " b u l l . s e i s m . soc. a m . " , 5 5 , 147-152, ( 1 9 6 5 a ) . (11) sciieidegger a. e . , grosstektonische bedeutung von erdbebenherdmechanismen, " z. g e o p l i . " , 3 1 , 300-312, (19656). (12) sciieidegger a . e . , tectonics of the artic seismic belt in the light of faultplane solutions of earthquakes, " b u l l . s e i s m . s o c . a m . " , 5 6 , 2412 4 5 , (1966). (13) sciieidegger a. e . , the tectonic stress in the vicinity of the alps, " z. g e o p h y s i k " 3 3 , 167-181, (1967). (1 4) s c h n e i d e r g . , schick r . , b e r c k i i e m e r i l , faultplane solutions of earthquakes in baden-württemberg, " z. g e o p h y s i k " , 3 2 , 383-393, (1966). (15) van gils j . m., les tremblements de terre des 15 et 21 décembre 1965 en belgique. a c a d . r o y a l e d e b e l g i q u e . " b u l l . cl. s c i e n c e s " , s é r i e g e o p h . 7 4 , 101-107, (1966). 14toppozada.qxd 509 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words california – historical earthquakes 1. introduction california’s written history is very short. it started in 1769 with the diaries of the first spanish explorers. that year they felt earthquakes in the vicinity of los angeles during their travel from san diego northward. the spanish priests established 21 catholic missions near the coast, from san diego in 1769 to san francisco de solano in 1823 (fig. 1). many of the missions’ annual reports are available in the santa barbara mission archives. these reports documented the numbers of births, deaths, baptisms, livestock, agricultural harvest, and mentioned earthquakes when they damaged the buildings. the missions were secularized in 1834, ending the annual reports and leading to general abandonment and disrepair. for the next 15 years, until the 1849 gold rush and united states statehood, the sources of information were mostly the diaries of travelers and the records of ships’ captains. in 1849 regular newspaper publishing started in san francisco and in the gold country east of sacramento. felt earthquakes were noted in the newspapers, which continued to spread across california with time as the population increased. the descriptions of the earthquakes were cataloged by various contemporary researchers, and were later interpreted in terms california earthquake history tousson toppozada and david branum california geological survey, sacramento, ca, u.s.a. abstract this paper presents an overview of the advancement in our knowledge of california’s earthquake history since ~ 1800, and especially during the last 30 years. we first review the basic statewide research on earthquake occurrences that was published from 1928 through 2002, to show how the current catalogs and their levels of completeness have evolved with time. then we review some of the significant new results in specific regions of california, and some of what remains to be done. since 1850, 167 potentially damaging earthquakes of m ~ 6 or larger have been identified in california and its border regions, indicating an average rate of 1.1 such events per year. table i lists the earthquakes of m ~ 6 to 6.5 that were also destructive since 1812 in california and its border regions, indicating an average rate of one such event every ~ 5 years. many of these occurred before 1932 when epicenters and magnitudes started to be determined routinely using seismographs in california. the number of these early earthquakes is probably incomplete in sparsely populated remote parts of california before ~ 1870. for example, 6 of the 7 pre-1873 events in table i are of m ≥ 7, suggesting that other earthquakes of m 6.5 to 6.9 occurred but were not properly identified, or were not destructive. the epicenters and magnitudes (m) of the pre-instrumental earthquakes were determined from isoseismal maps that were based on the modified mercalli intensity of shaking (mmi) at the communities that reported feeling the earthquakes. the epicenters were estimated to be in the regions of most intense shaking, and values of m were estimated from the extent of the areas shaken at various mmi levels. mmi vii or greater shaking is the threshold of damage to weak buildings. certain areas in the regions of los angeles, san francisco, and eureka were each shaken repeatedly at mmi vii or greater at least six times since ~ 1812, as depicted by toppozada and branum (2002, fig. 19). mailing address: dr. tousson toppozada, california geological survey, 801 k street, sacramento, ca 95814, u.s.a.; e-mail: ttoppoza@consrv.ca.gov 510 tousson toppozada and david branum of shaking intensities, and ultimately earthquake epicenters and magnitudes (table i). the routine seismographic determinations of earthquake epicenters and magnitudes (m) started in southern california in 1932. by comparing the shaking intensities of the modern instrumentally determined earthquakes to those of pre-1932 events, the epicenters and magnitudes of the pre-instrumental earthquakes could be estimated with some consistency. the short earthquake history has shown that most regions in california are susceptible at various levels to earthquake damage. this information has been applied to earthquake hazard assessment and mitigation at critical structures such as nuclear power fig. 1. index map showing the california missions and the dates the missions were established. all missions were secularized in 1834 (modified from toppozada et al., 2002). 511 california earthquake history table i. significant california earthquakes (m ~ 6 to 6.5 and/or destructive). date (*) latitude longitude m region loss of life and property 1812.12.08 34.37 (?) –117.65 (?) ~ 7.3 orange county, los angeles, wrightwood 40 deaths at san juan capistrano 1812.12.21(**) 34.75 (?) –118.60 (?) ~ 7.1 los angeles, ventura, santa barbara 1 death 1838.06.00 37.30 (?) –122.15 (?) ~ 7.4 san francisco to san juan bautista damage from san francisco to monterey 1857.01.09 36.20 –120.80 7.9 great fort tejon earthquake 1 death, damage from monterey co. to san bernardino co. 1865.10.08 37.20 –121.90 6.5 santa cruz mountains $500 000 in property loss 1868.10.21 37.70 –122.10 7.0 hayward fault 30 deaths, $350 000 loss 1872.03.26 36.70 –118.10 7.4 owens valley 27 deaths, 56 injuries, $250 000 loss 1873.11.23 42.00 (?) –124.20 (?) 6.9 crescent city regoin damage in california oregon border area 1890.02.09 33.40 –116.30 6.8 san jacinto fault? little damage 1892.02.24 32.55 –115.65 7.3 laguna salada, baja california damage san diego to imperial valley 1892.04.19 38.40 –122.00 6.6 vacaville 1 death, $225 000 loss 1898.03.31 38.20 –122.50 6.4 mare island $350 000 loss 1898.04.15 39.20 –123.80 ~ 6.7 fort bragg mendocino damage from fort bragg to mendocino 1899.12.25 33.80 –117.00 6.7 san jacinto and hemet 6 deaths, $50 000 loss 1906.04.18 37.70 –122.50 7.8 great 1906 earthquake 3 000 deaths, $524 million loss (counting fire damage) 1918.04.21 33.75 –117.00 6.8 san jacinto 1 death, several injuries, $200 000 loss 1923.01.22 40.40 –124.90 7.2 off cape mendocino destructive in humbolt co., strongly felt to reno 1925.06.29 34.30 –119.80 6.8 santa barbara 13 deaths, $8 million loss 1927.11.04 34.60 –120.90 7.1 40 km west of lompoc damage in santa barbara and san luis obispo counties 1933.03.11 33.70 –118.00 6.4 long beach 115 deaths, $40 million loss 1940.05.19 32.73 –115.50 7.0 imperial valley 9 deaths, $6 million loss 1952.07.21 35.00 –119.02 7.3 kern county earthquake 12 deaths, $60 million loss 1954.12.21 40.93 –123.78 6.6 east of arcata 1 death, several injuries, $2.1 million loss 1971.02.09 34.41 –118.40 6.6 san fernando 65 deaths, 2,000 + injuries, $505 million loss 1979.10.15 32.61 –115.32 6.5 imperial valley 91 injuries, $30 million loss 1980.11.08 41.12 –124.67 7.4 west of eureka 6 injuries, $2 million loss 1983.05.02 36.23 –120.31 6.4 coalinga $31 million loss, 1 death, 47 injuries 1984.04.24 37.31 –121.68 6.2 morgan hill $8 million loss 1987.10.01 34.07 –118.08 6.0 whittier narrows 8 deaths, $358 million loss 1987.11.24 33.01 –115.85 6.6 superstition hills $3 million loss 1989.10.18 37.04 –121.88 6.9 loma prieta 63 deaths, 3,757 injuries, $6 billion loss 1992.04.25 40.33 –124.23 7.2 cape mendocino area 356 injuries, $48.3 million loss. two m 6.6 aftershocks next day 1992.06.28 34.20 –116.44 7.3 landers 1 death, 402 injuries, $91.1 million (losses include big bear, m 6.5, earthquake 3 hours later) 1994.01.17 34.21 –118.54 6.7 northridge 57 deaths, 9000 + injuries, ~$40 billion loss 1999.10.16 34.60 –116.27 7.1 hector mine minimal injuries and damage due to sparse population loss from stover and coffman (1993). (*) the date is given as year, month, day (greenwich). (**) proposed epicenter on or near san andreas fault (toppozada et al., 2002). 512 tousson toppozada and david branum plants, dams, schools and hospitals. the historical earthquake information is also an important component of the probabilistic seismic hazards assessments that are part of the modern building codes. 2. earthquake catalogs in california 2.1. before 1970 the most comprehensive catalog of historical california earthquakes available before 1970 was that of townley and allen (1939). this provided mostly narrative descriptions of historically reported felt earthquakes, from the 1769 spanish exploration of california to 1928. it did not generally provide epicenter coordinates or magnitudes for the earthquakes. the sources of information were mostly catholic mission documents before the 1849 gold rush, and newspapers and varied correspondence thereafter. the bulletin of the seismological society of america (bssa) started publication in 1910, and has included many articles on various california earthquakes. the bssa section titled «seismological notes» describes the shaking and felt effects of contemporary earthquakes, with some seismographic information generally for post-1932 events. since 1928, the annual publication united states earthquakes (use) has provided descriptions of felt effects for earthquakes in the united states, and available seismographic information, such as epicenter coordinates and magnitude. since 1950 use has included rough isoseismal maps of some of the most widely felt earthquakes. special publications were also available for numerous significant events, including the major san andreas fault earthquakes of 1857 (wood, 1955) and 1906 (lawson, 1908). 2.2. the 1970s hileman et al. (1973) published the california institute of technology (cit) seismographic catalog of earthquakes from 1932 to 1972 for southern california, roughly south of the 36th parallel. this catalog lists instrumentally determined epicenter coordinates and magnitudes, and provides annual and multi-year epicenter maps of earthquakes of various magnitudes, generally of m > 3. online updates of the cit catalog are available at: http://www.scec dc.scec.org/catalogsearch.html bolt and miller (1975) published the university of california, berkeley (ucb) catalog of earthquakes from 1910 to 1972, for northern california, roughly north of the 36th parallel. their estimates of pre-1942 earthquake location and size were approximate, and generally not determined from seismographs. these estimates were largely based on townley and allen’s (1939) descriptive catalog, except for a few seismographic values of epicenter and magnitude (m) quoted from gutenberg and richter (1949) and cit. starting in 1942, ucb seismographic records were used to determine epicenters and m for generally m > 3 earthquakes. online updates of the ucb catalog are available at: http://quake.geo.berkeley.edu/nced c/catalog-search.html toppozada (1975) analyzed the size of the areas shaken by earthquakes in california and western nevada at various modified mercalli intensity levels (mmi), and related them to richter local magnitude. these relations were subsequently used in the 1970s and 1980s to estimate the magnitudes of preinstrumental earthquakes. toppozada et al. (1978) interpreted the felt effects and estimated epicenters and magnitudes for the most strongly felt california earthquakes occurring from 1900 through 1931. they based their estimates on the felt effects and size of areas shaken at various levels of intensity as reported by townley and allen (1939) and use. supplemental information came from felt shaking effects described in pre1932 newspapers, bssa, and available seismographic information including the ucb bulletin of the seismographic stations. they selected 517 events of probable m ~ 4 or larger based on toppozada’s 1975 relations, having modified mercalli intensity (mmi) of v or larger or felt area ~7000 square km or larger. they determined that at least 229 earthquakes were of m ≥ 4, but the available mmi and felt 513 california earthquake history area data were not sufficient to allow robust estimates of magnitude for the remaining events, that were probably of m ~ 4. real et al. (1978) combined this pre-instrumental 1900-1931 catalog with a 1932-1975 catalog that they created by merging the cit and ucb catalogs, and the usgs catalog for central coastal california since 1969. their resulting 1900-1975 catalog was available on computer tape, and on an epicenter wall map of 3600 earthquakes of m~4 or larger, or mmi v or greater if m was unknown. the catalog lists the event dates, epicenter coordinates, and m, and includes 452 events of m ≥ 5. this catalog is essentially pre-instrumental before 1932 in southern california, and before 1942 in northern california. 2.3. the 1980s toppozada et al. (1981) analyzed in detail the felt effects of the most strongly felt california earthquakes between 1769 and 1900. they selected earthquakes from townley and allen (1939) of probable m ~ 5 or larger based on toppozada’s 1975 relations, having mmi at least vi or felt area at least 30 000 square km. they constructed isoseismal maps for 100 earthquakes, showing the reporting points and their mmi values. they supplemented the published intensity information by researching the reports of spanish missionaries from 1769 to 1840, mainly in the santa barbara mission archives. after the advent of regular newspaper publishing with the 1849 gold rush, reports of feeling earthquakes became available beyond the coastal zone where the missions were located (fig. 1), and extended inland to the goldfields east of sacramento (fig. 2a). the newspaper coverage gradually expanded to cover most of california by the 1880s (fig. 2b). pre-1932 earthquakes were best known in the areas that had catholic missions before the 1840s, and newspaper coverage after 1849. the state epicenter map (fig. 3) employs three symbol shapes, xs, circles and triangles, to identify fig. 2a. locations of newspapers in print, 18501859 (modified from toppozada et al., 1981). fig. 2b. locations of newspapers in print, 18801889 (modified from toppozada et al., 1981). eureka sacramento san francisco los angeles eureka sacramento san francisco los angeles 514 tousson toppozada and david branum the three approximate thirds of the 200-year record. it shows that before 1869, earthquakes indicated by xs were relatively well known in the central and southern coast and in the gold and silver fields northeast of sacramento, but not in the northwestern or southeastern regions of california. after 1868 when the newspaper coverage expanded to these remote regions their high seismicity became apparent, as indicated in fig. 3 by the circles and triangles in these areas. the san andreas fault region between the 36th and 37th parallels was much more active before than after 1932, as illustrated by the xs and circles greatly outnumbering the triangles in this region (fig. 3). this will be discussed below under the «parkfield region». fig. 3. magnitude greater or equal to 5.5 california earthquakes, 1800-2000 (modified from toppozada and branum, 2002). 515 california earthquake history most of toppozada et al. (1981)’s data resulted from searching about 12 000 newspaper issues for towns that might have felt the earthquakes, for several issues after each event. about a quarter of the issues searched provided some earthquake descriptions, although many descriptions were duplicated in various newspapers. a bibliography of the newspapers searched for each earthquake was appended to the 1981 report. also, a summary of the reported felt effects for each earthquake and the corresponding mmi value at each reporting town was appended to the 1981 report. toppozada and parke (1982) completed a similar study of the areas damaged by california earthquakes that caused mmi vi to vii or greater effects in california, from 1900 to 1949. this study provided isoseismal maps and summaries of reported effects, which were not available in use (1928-1949) or in toppozada et al. (1978)’s study of pre-1932 events. a newspaper bibliography and a summary of the reported effects were also appended to the 1982 report. 2.4. 1990 to 2003 ellsworth (1990) reviewed the history of major california and nevada earthquakes from 1769 to 1989, and compiled a comprehensive catalog of m ≥ 6 earthquakes. this included the known values of the various magnitude types for each earthquake, derived from mmi, short and long period seismic records, seismic moment, and a summary or preferred m. stover and coffman (1993) summarized the information from use, toppozada et al. (1981), and other available sources, and provided a catalog and felt descriptions for historical earthquakes of m ≥ 4.5. they also provided isoseismal maps for some of the most significant california events, generally of m ≥ 6.5. toppozada et al. (2000) compiled a wall map of epicenters of m ≥ 5 earthquakes from 1800 to 1999. this map lists the date, co-ordinates, and magnitude of the 383 events of m ≥ 5.5. an inset map shows the areas damaged at mmi vii or greater by these earthquakes, which is the threshold of damage to weak buildings, and the number of times the various areas have been damaged. certain areas in the regions of los angeles, san francisco, and eureka were shaken repeatedly at mmi vii or greater at least six times each since ~1800. a generalized subset of this wall map is available on the web at: http://www.consrv.ca.gov/cgs/rghm/quakes/ms49.htm toppozada and branum (2002) summarized the information on the m ≥ 5.5 historical earthquakes, and the areas that they damaged. for each earthquake they listed all known values of m, and identified a preferred m. they obtained instrumental magnitudes generally for post-1932 events from the earthquake catalogs of cit and ucb, ellsworth (1990) for m ≥ 6.0 events, hutton and jones (1993), stein and hanks (1998), and bakun (1999, 2000). the preferred magnitude was moment m when available. otherwise, the order of preference was generally mg-r (gutenberg and richter, from 20-sec surface waves); ms (from surface waves, before 1935 generally from un-damped milne seismographs according to abe, 1988); ml (local m of richter, 1935); ma (based on the size of the areas shaken at or above mmi v, vi, vii); mi (from bakun 1999, 2000, using toppozada et al., 1981, mmi data). they also showed the spatial and temporal evolution of the seismicity and the areas damaged at mmi vii or greater from 1800 to 2000. 2.4.1. updated relations between areas shaken and m toppozada and branum (2002) developed new relations between the moment magnitude of modern california earthquakes of m 5.5 to 7.3 and the areas in square km that they shook at or greater than mmi v, vi, or vii (fig. 4). the standard deviations of the three linear least squares fits range from 0.11 magnitude units for m derived from mmi3v, to 0.17 for m derived from mmi3vii: mv = 0.83log10 areav + 2.43 st. dev. = 0.11 mvi = 0.73log10 areavi + 3.44 st. dev. = 0.13 mvii = 0.67log10 areavii + 4.29 st. dev. = 0.17 516 tousson toppozada and david branum toppozada and branum (2002) and toppozada et al. (2002) used these relations to derive estimates of magnitude of pre-instrumental earthquakes from the areas shaken at these intensities or greater. for example the area shaken at mmi v or greater by the 12 april 1885 earthquake is that enclosed by the innermost two contours in fig. 5. that is the whole area from visalia to santa cruz (e to w) and from oakdale to santa maria (n to s). the values of mmi are determined at the points that described the shaking effects, and the contours are drawn to distinguish between areas having different values of mmi. the new equations result in magnitudes that are approximately 0.3 to 0.4 units higher than those resulting from toppozada (1975)’s relations, which were based on local magnitude values and partly on nevada earthquakes. ellsworth (1990) independently found that toppozada (1975)’s relations underestimated m by about 0.2 to 0.3 units for modern california earthquakes. the 2002 relations are superior to the 1975 relations because: i) they are based on california earthquakes only whereas the 1975 relations included data from several nevada events; ii) they are based on modern events up to the year 2000 whereas the 1975 relations included no data after 1971; and iii) they are based on moment m whereas the 1975 relations were based on local m. 2.4.2. selected regions the earthquake history of selected regions of california has also been studied. for example, agnew (1991) studied the completeness of the historical earthquake record in southern california, in light of the distribution of newspapers and other available sources of information. bakun (1999, 2000) studied the seismicity of the san francisco bay region, and of california’s north coast, using the method of bakun and wentworth (1997) to determine epicenters and magnitudes for pre-instrumental earthquakes. uhrhammer (2003) is currently studying the early seismographic record (1910-1950) in the san francisco bay region, using the ucb seismic records and notebooks to revise the early epicenter and m determinations. this should distinctly improve the earthquake record before 1942 when ucb started to routinely determine epicenters and magnitudes using seismic records. fig. 4. moment magnitude as a function of the areas, in km2, shaken at or greater than mmi v, vi, vii (modified from toppozada and branum, 2002). 517 california earthquake history fig. 5. 12 april 1885 isoseismal map. numbers at towns are the mmi values based on the felt effects. when felt effects did not indicate a mmi value we use a letter: f, felt; s, severe; l, light; h, heavy. the triangle represents the epicenter. the three dashed lines represent the areas of mmi greater than or equal to ii-iv, v or vi used in the magnitude-area relations of toppozada and branum (2002) (modified from toppozada et al., 2002). 3. significant results 3.1. parkfield region the parkfield earthquake prediction by bakun and lindh (1985) followed, and was based largely on the historical earthquake catalog work available at that time. the prediction was based on the observation that m ~ 6 events had occurred quasi-regularly near parkfield in 1857 (identified by sieh, 1978), 1881 (identified by toppozada et al., 1981), 1901, 1922, 1934, and 1966 (bakun and mcevilly, 1979, 1984). this simple model predicted that the next m ~ 6 earthquake would occur before 1993. the predicted earthquake has not yet occurred as of this writing (september 2003). toppozada et al. (2002) undertook a detailed study of the earthquake history of the parkfield region and adjoining segment of the san andreas fault, based on exhaustive research of newspapers and local diaries. this study showed that the pre-1932 seismicity of parkfield and the adjoining segment of the san andreas fault to the north was much greater than previously thought, and has been decreasing steadily from 1857 to the present time. the simple 1985 model of bakun and lindh does not include the high pre-1900 san andreas fault seismicity within 70 km nw of parkfield. this seismicity brackets the northern end of the major 1857 san andreas fault rupture, which is at the largest 1857 epicenter symbol (x) in fig. 6a. epicenters of the strong 1885 earthquake of m ~ 6.5 (fig. 5) and other pre-1932 m ~ 6 earthquakes in this zone are shown in fig. 6a. the epicenters were estimated at the centers of strongest mmi. relative locations of neighboring earthquakes were determined by comparing their mmi values at the same towns (e.g., parkfield, salinas, etc.). the earthquakes in fig. 6a have isoseismal maps or descriptions of mmi in toppozada et al. (2002). their rate of occurrence indicates a decaying earthquake sequence (fig. 6b). this suggests post-earthquake stress decay at the northern end of the 1857 fault rupture, and helps explain the delay of the predicted earthquake. figures 6a and 6b show that m ≥ 5.5 earthquakes have occurred in the san andreas fault zone within 70 km northwest of parkfield ten times from 1857 to 1932, but only twice after 1932. ben-zion et al. (1993) modeled a decreasing stress at the terminus of the 1857 rupture that is generally compatible with that indicated by our observed decrease in earthquake rate. they concluded that the decreasing stress could delay the predicted parkfield earthquake from ~1988 to ~1995. 3.2. december 1812 southern california earthquakes toppozada et al. (2002) re-evaluated the major earthquakes that occurred on 8 and 21 december 1812, damaging the missions from san juan capistrano to purisima concepcion (fig. 7), in orange, los angeles, ventura and santa barbara counties. they found that the 1812 san andreas faulting indicated by jacoby et al. (1988), extending ~170 km from near cajon pass to near fort tejon, could have ruptured in two events and generated both earthquakes (fig. 7). tree ring studies by jacoby et al. (1988) placed the first event that damaged mostly san juan capistrano and san gabriel, on the wrightwood segment that includes cajon pass and pallett creek. toppozada et al. (2002) concluded that the second faulting event 13 days later could have started from the west end of the first, with rupture directed to the west toward fort tejon and purisima concepcion. it damaged mostly santa barbara, santa inez and purisima. missions san fernando and san buenaventura were damaged by both the 8 and 21 december 1812 earthquakes (fig. 7). the damage at purisima concepcion was stronger than that at santa inez. this was apparently due to mission purisima’s location on sloping ground at the edge of a marsh (toppozada et al., 1981). also, rancho de san antonio, 17 km north of purisima, was not damaged (fig. 7). the adverse site effects at mission purisima were recognized by the padres and noted in their 1813 written communications. they consequently rebuilt mission purisima on flat dry ground, 5 km north of the 1812 ruins. if the 1812 faulting extends westward to fort tejon, or beyond to mil potrero as suggested by 518 tousson toppozada and david branum 519 california earthquake history fig. 6a,b. a) epicenters of m ≥ 5.5 earthquakes in the parkfield-bitterwater region and surrounding areas. symbol size is proportional to m and shape indicates time period. the dashed region is that considered in fig. 6b (modified from toppozada et al., 2002). b) cumulative seismic moment released in the parkfield-bitterwater zone of the san andreas fault from 1857 to 2001 (modified from toppozada et al., 2002). a b 1 8 5 0 1 8 6 0 6 c u m u la ti v e m o m e n t (1 0 2 6 d y n e -c m ) date 5.5 5 4.5 4 3.5 3 2.5 1 8 7 0 1 8 8 0 1 8 9 0 1 9 0 0 1 9 1 0 1 9 2 0 1 9 3 0 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 1 9 8 0 1 9 9 0 2 0 0 0 520 tousson toppozada and david branum fig. 7. missions damaged by the december 1812 earthquakes. we divide the 1812 rupture proposed by jacoby et al. (1988) into 2 segments to account for the damage observed on 8 and 21 december, and on both dates (modified from toppozada et al., 2002). davis (1983), it cannot all have ruptured on 8 december. faulting that extends that far westward would damage santa barbara, which was not damaged on december 8th but on the 21st. thus the total 1812 faulting suggested by jacoby et al. (1988) and depicted in fig. 7 would suggest that the western segment ruptured on the 21st. definition of the western extent of the total 1812 faulting is complicated by the major 1857 earthquake rupture that extended from near parkfield to cajon pass, and re-ruptured the 1812 faulting after only 44 years. the conventional santa barbara channel epicenter of the 21 december earthquake was based largely on a tsunami that resulted from the event. however, it is becoming clear that tsunamis are often generated by submarine landslides. lander et al. (1993) interpreted the description of the 1812 sea wave as «a submarine landslide-generated wave and not one generated by a (submarine) fault movement source». such landslides are mapped frequently in the santa barbara channel (greene and kennedy, 1989), and could have been triggered by the 1812 earthquake shaking (greene et al., 2000). 3.3. 1830s san francisco bay area earthquakes the destructive 1989 loma prieta earthquake provided the impetus for toppozada et al. (1996) to study the san francisco bay area earthquake history in detail. this lead to a bet521 california earthquake history ter understanding of the major 1838 san andreas fault event and to a radical re-evaluation of the 1836 «hayward fault earthquake» by toppozada and borchardt (1998). they concluded that the 1838 earthquake was of m ~ 7.4, significantly larger than previously thought. the 1906 san francisco earthquake of m 7.8 occurred 68 years later as a result of major san andreas faulting that overlapped and reruptured the 1838 faulting. this pair of events is similar to the 1857 central san andreas fault earthquake of m 7.9 whose major faulting overlapped and reruptured the faulting associated with the1812 earthquakes of m ~ 7.3 and ~ 7.1. thus the 1857 and 1906 m 7.8 and 7.9 san andreas faulting events have each overlapped a m ~ 7.4 faulting event that occurred 44 to 68 years earlier. earthquake ruptures that occur 44 to 68 years apart are difficult to differentiate in fault trenches because of the limitations in stratigraphic resolution and paleoseismic dating. this suggests that overlapping ruptures occurring a few decades apart can be more common than paleoseismology would indicate. toppozada and borchardt (1998) also found that the 1836 event was not a m ~ 7 event on the hayward fault as was previously believed, but rather a m ~ 6 1/2 event ~ 100 km to the south, in the general vicinity of monterey bay. the idea of an 1836 hayward fault earthquake had been based on successive false assumptions and misinterpretations of information, which toppozada and borchardt summarized in their table 2 : ‘evolution of the myth concerning the 1836 «hayward fault earthquake»’. 4. current work we are building a database of the felt effects of m > 5 historical earthquakes. for each earthquake there are various towns that reported felt effects. the database will have a separate entry for each earthquake and each town that reported the event. we have digitized the data previously analyzed by toppozada et al. (1981) and toppozada and parke (1982). we will supplement this with unpublished data that we have analyzed, and also add information for post1950 events from use. the database will be searchable for event dates, towns, geographic regions, and levels of mmi shaking. this will allow us to determine how many times specific towns, as well as regional areas, have felt different levels of mmi shaking. the database will be linked to digital isoseismal maps of the corresponding earthquakes. these maps and data will be published on a website to allow comparing the felt effects of current earthquakes to those of neighboring historical events. this will permit a rapid assessment of the potential mmi shaking and damage from a current strong earthquake from the shaking of neighboring historical events of similar magnitude. it will also allow recalibrating the epicenters and magnitudes of preinstrumental earthquakes by comparing their intensities of shaking to those of nearby well-instrumented modern earthquakes. references abe, k. (1988): magnitudes and origin times from milne seismograph data: earthquakes in china and california, 1898-1912, in historical seismograms and earthquakes of the world, edited by w.h.k. lee, h. meyers and k. shimazaki (acad. press, san diego, ca, u.s.a.), 37-50. agnew, d.c. (1991): how complete is the pre-instrumental record of earthquakes in southern california, in environmental perils san diego region, edited by p.l. abbott and w.j. elliott (association of geologists, san diego). bakun, w.h. (1999): seismic activity of the san francisco bay region., bull. seismol. soc. am., 89, 764-784. bakun, w.h. (2000): seismicity of california’s north coast, bull. seismol. soc. am., 90, 797-812. bakun, w.h. and t.v. mcevilly (1979) earthquakes near parkfield, (california): comparing the 1934 and 1966 sequences, science, 205, 1375-1377. bakun, w.h. and t.v. mcevilly (1984): recurrence models and the parkfield, california, earthquakes, j. geophys. res., 89, 3051-3058. bakun, w.h. and a.g. lindh (1985): the parkfield california earthquake prediction experiment, science, 229 (4714), 619-624. bakun, w.h. and c.m. wentworth (1997): estimating earthquake location and magnitude from seismic intensity data, bull. seismol. soc. am., 87, 1502-1521. ben-zion, y., j. rice and r. dmowska (1993): interaction of the san andreas fault creeping segment with adjacent great rupture zones and earthquake recurrences at parkfield, j. geophys. res., 98, 2135-2144. bolt, b.a. and r.d. miller, (1975): catalogue of earthquakes in northern california and adjoining areas, 1 522 tousson toppozada and david branum january 1910 31 december 1972, seismographic stations (university of california, berkeley, california), p. 567. cit (california institute of technology), southern california seismographic network, weekly earthquake report for southern california. davis, t. (1983): late cenozoic structure and tectonic history of the western «big bend» of the san andreas fault and adjacent san emigdio mountains, ph.d. dissertation (university of california, santa barbara), pp. 580. ellsworth, w. l. (1990): earthquake history, 1769-1989, in the san andreas fault system, california, u.s. geol. surv. profess. pap. 1515, edited by r. wallace (us government printing office, washington, dc), 153-188. greene, h.g. and m.p. kennedy (editors) (1989): central california continental margin map5a, california division of mines and geology. greene, h.g., n. maher and c.k. paull (2000): landslide hazards off of santa barbara, california, in american geophysical union fall meeting program, p. 291. gutenberg, b., and c. f. richter (1949): seismicity on the earth and associated phenomena (princeton university press, princeton, new jersey), p. 273. hileman, j.a., c.r. allen and j.m. nordquist (1973): seismicity of the southern california region, 1 january 1932 to 21 december 1972, seismol. lab. contrib. 2385 (calif. institute of technology, pasadena), pp. 81. hutton, l.k. and l.m. jones (1993): local magnitudes and apparent variations in seismicity rates in southern california, bull. seismol. soc. am., 83, 313-329. jacoby, g., p. sheppard and k. seih (1988): irregular recurrence of large earthquakes along the san andreas fault evidence form trees, science, 241, 196-199. lander, j. f., p. a. lockridge and m.j. kozuch (1993): tsunamis affecting the west coast of the united states 1806-1992, ngdc key to geophysical records doc. 29 (us dept. of commerce), p. 242. lawson, a.c. (1908): the california earthquake of april 18, 1906 (carnegie institute, washington, dc). real, c.r., t.r. toppozada and d. l. parke (1978): earthquake epicenter map of california, div. mines geol. map sheet no. 39. richter, c.f. (1935): an instrumental earthquake magnitude scale, bull. seismol. soc. am., 25, 1-32. sieh, k.e. (1978): central california foreshocks of the great 1857 earthquake, bull. seismol. soc. am., 68, 1731-1749. stein, r. s. and t. c. hanks (1998): m ≥ 6 earthquakes in southern california during the twentieth century: no evidence for a seismicity or moment deficit, bull. seismol. soc. am., 88, 635-652. stover, c. w. and j.l. coffman (1993): seismicity of the united states, 1568-1989, u.s. geol. surv. prof. pap. 1527, pp. 418 (revised). toppozada, t.r. (1975): earthquake magnitude as a function of intensity data in california and western (nevada), bull. seismol. soc. am., 65, 1223-1238. toppozada, t.r., d.l. parke and c. t. higgins (1978): seismicity of california 1900-1931, cal. div. mines. geol. spec. rep. 135. toppozada, t.r., c.r. real and d.l. parke (1981): preparation of isoseismal maps and summaries of reported effects for pre-1900 california earthquakes, calif. div. mines geol. open-file rep. 81-11 sac, pp. 182. toppozada, t.r. and d.l. parke (1982): areas damaged by california earthquakes, 1900-1949, calif. div. mines geol. open-file rep. 82-17, pp. 65. toppozada, t.r., g. borchardt, c.l. hallstrom, and d. ransom (1996): using 1800’s earthquake sequences to elucidate the increased san francisco bay area seismicity since 1979, annual technical report to usgs/nehrp, pp. 32. toppozada, t.r. and g. borchardt (1998): re-evaluation of the 1836 hayward and the 1838 san andreas fault earthquakes, bull. seismol. soc. am., 88, 140-159. toppozada, t. r., d. branum, m. petersen, c. hallstrom, c. cramer and m. reichle (2000): epicenters of and areas damaged by m ≥ 5 california earthquakes, 1800-1999, calif. div. mines geol. map sheet 49. toppozada, t.r. and d. branum (2002): california m ≥ 5.5 earthquakes, history and areas damaged, in international handbook of earthquake and engineering seismology, edited by w.h. lee, h. kanamori and p. jennings (international association of seismology and physics of the earth’s interior). toppozada, t.r., d.m. branum, m. s. reichle and c. l. hallstrom (2002): san andreas fault zone, california: m ≥ 5.5 earthquake history, bull. seismol. soc. am., 92, 2555-2601. townley, m.d. and m.w. allen (1939): descriptive catalog of earthquakes of the pacific coast of the united states, 1769 to 1928, bull. seismol. soc. am., 29, 1-297. uhrhammer, r.a. (2003): historical earthquake reanalysis project: san francisco bay region, seismol. res. lett., 74 (2), p. 217. ucb (1910-1931): bulletin of the seismographic stations (bssuc), semi-annual publication, university of california at berkeley (ucb). use (1928-1985): united states earthquakes (use), annual publication, department of commerce, coast and geodetic survey; department of the interior, geological survey (washington, dc). wood, h.o. (1955): the 1857 earthquake of california, bull. seismol. soc. am., 45, 47-67. in8uence of rivers od geotemperatures f. monurlli (*) ri"evut.u il lo sdtemhre 1970 ,"'u,n[ary. ali eht.imat.ioll i.~ lila(le of the thel"mal disturhancc thal lhe eventll a l differerl('e between the mean annual lcmperut.ure of a rivh]' aliti that ur lhe groujl<i would cause un t.he lio!"!lial gl' othnnnai li l' ld. itiassunto. viene eseguita una stima del di.~turho tel'mi("o che l'cven· tuale differenza t.ra temperatu!':l me,ua annua di illi fiumc e quella dl.l suolo polrdhhe cau~are sul l'ampu geotermieo normale. 1. tn'l'iwduction. during a rcflcarch on thc gcothcrma.1 fiow in thc fo~fl'. bra.da.· nica (1) it h a.s been necessary to investigate under what collditiom a lld to what extent a river influellces the norllml geotlh-ll'lliai l1em, luuler the hypothesis of heat condllction in st8f~dy sh~te. in ordmth a t thc rivcr lh'ovokhfl a tlwrmalllcrturbation, it sumee~ merei.v for thc river t.o have a rne<òll annual temperature dilre1'8ut from that. for the b'1.'oulul '1'lwrefore, with the aim of cal'l'yiug out t.hifl type of invest.ig,~tiou, it wouid bh ju~t lwcllhflat'y to kno\\" thc mc an rl nlllml tcmvcratures of t.hc rivcr ,.nd of thc floil in thc area under examinatiojl. avaiiable litcrature provides no direct comp,~rihollh betweeu t.he~e two t.cmpen~tu1'es. how,wer, they may he ohtained hy (lonq)~u·iwll bet· w8ml t.he tmnplll'a turc of thc river w,1tcr and t.lw air tempemturc (investigatiom run by hydrologist8) aud by comparisou lw,tween a.il' temllemt.urb alld that of the gl'oluld (ma de by clim,~tologiflt8 a.nd, (,flpcei,.lly, by agronomists). (*) jkt.ituto di ge,)(lesia e geofisica, università. bari (ilali a ). 2(j(l :i[onoelu l'. 2. co'ii'alason oli rrver a:t\d a.lr 'l'el\fpera'l'urrs. h,nlrologihh< haye not paili ltlllch auenhon to variat..iollf! in rivpr t.mnlll\r a. ll1re (') ,t1ul ho preflent knowlmlge is incomplctc, howevcr, from llw few paller~ l~vaih"ble (for a b1"icf bibliog'l'allhy ,~('l' (l. j, ')) on medinm-high latitude are ali, t;lwre emerge, inter alia, two illlllortant fad,orh uearing on the llresent inve~hgation: l°) menn iinmu/.l l'i-vili' temperll/~o'c diffcrg fl'om tfml of 11/0 a.ir, ffcncrall~1 ùcinff hi!l'wr :lo) the temllemtl!i'c gmdient of rivers (with bleyation) ifl a1.~o difterent aut! genemlly above that o[ the air. fn the case or ltaly, 'ronini (') maintains tlmt the mean annua! t.ellllh'l'atlll'e of riv('.r waters in this eliltlate is flome 200 higher thall tlmt of l.iw l ~ir. visentini (&) ob~cl'yerl a telllllemture gradient of abollt [oc .foi' every 200 m in the m~se of slirfaee waters in the po basin. on eompn,ring this wit.lt that of tlw a ir (0.4-1 gc/.l00m) il will be ~ecn th a l tlh' dilr(\l'('u(~(1 inerea.ses with e1eva1,ion. 3. co~ll'.-\ln:::;on 01' am and glwund tempehature". 'l'hc reslllt,~ o[ tlwsc invesliga1.-ionfl are gmwrally iiwi:wd iii geothermn,l ~tudies fol' nmking thr. topogml)11ie col'metion ('j, if local eliltmtic colulitions llre1òent no lmrticular rea! llre1ò 'md if thc!'c are no fllmllo\\" gl'ollndwatms in movclllent and thel'('. is no {\vl~vo­ mtion, it is genel'ally held tha.t, in thc ea.he or clev a. 1ion.'! whieh arc not too high, the mean al1mm! ah and i:,'l'ound tcill]lcratures can be con~idel'ed (~qllal 01' (iirlel' hh no mori! than a few t(jnthll of il d/!ff/"{j(j ol!/ltiff/"lldl', tlte gl'oulhl f!l(rface f..emvcl'alurc bcing highcr tha.n tlu"t of tlw l"il'. in mohnl,tin l'cgions, on thc othcr halhl, thifl aifil\i'cnoo incl'cafles 'vith elevalion, since the air tcmllemtul'(' gmdient ih greater thn,n th a t of the ground. exhallstive r(·j"el'ence1ò on eolllval'isolls of l~ir-i:."'(lul\(1 tblllveratlll'es in i taly a l'c givpll iu wol'ks hy tlw agl'onomists seottoll (8) al\(l ::\il ~echia ('). ì\!(\ilt,ion o[ f·lle l'nmhh o[ a il' ana gl'ound t(\mpemt.h1'il grn,dienls in tltc ai1)s ajl(1 thel,'ogs,t brwbnica a re iu t.hc work1ò or hi]'(~h (') a na ì\[ougdli a jl(l hieeh<jtti (1). !)\fl1jence 010' !{jveit.~ on gf.(j're~ii'el'a'rulll·;s :!u7 1 t is ~\lllla,rent. tlmt in generai the mean temperature of !'iypi' water is higher tha,n tlmt or the air , ~\ihl tlmt the n10~m wm])(\mture of the lbir is lowe1' tl mn timi of the ground, since it is unlikel,\' tha.1" tlw~e lwo will bl ~lance out ex a cti.)', it ii! to bi) exped-ml tlmt thel'e wili be a dijierence bet\\'tlhll the tmnl'el'atul'ti of river wa tl'.]' l\1ul tlmt of tlw a'l'ound, a,nd thl\t thi8 lll>\y be of the ardei' of a few degrl'e~. in 1.1i p1'(}b~\bility this ia ct will h ave no inllu(\1l1:{' on /:.l1e h'mpl\l'ature rea.dings taken in bores ~~ considemble di,;1a1i!\p fl'olll rivers, hut the dist·1ll'lmnce in holes birl,y nearby lllust be ~1i!s(' .. %ilii. since tlw nhsolute v:blue of the disturimnl\(j cannot be ealliubled, we est·illlate its iicrclmtagli y:bhw , th,\ t is the mt·io bl'/:.wccn il~ yahw and t·lw lliffhrench betwhtm the me a n a nnw\ 1 telllpe1'i ~ture 01' the river allll thm, of tlw soil. -lo ']'neoll.y. '['he telllllemturo umler st.l~tiona,ry collditions at a ll,\' giylln ]loillt oi a ,~(',ini-intìnile solidi whoso sul'face coincides with piane ,1:11 (axi:-; z bl,ing ol'i{\nll'd tmnwlh! tlw inner l'art of the holly) anll with a surbce t,l.\l1ll'l'1'i \ turh is p(.v', 11'), is ginm boy the fo[[owing expression (lo); 'l' (x, !i, z) = 2 ~ r d,"c ' -o exprhsi!ion [11 (mn al.~o btl 'wl'lie(l wlw1i ihe :ml'face telllll('·mture is p(m', ,l') in ~\ giycn zone 8 and nil aver the \'(·.nmilhler of the ,~urfaee, 'l'his given zone may l'epl'esent the area occullied hy a river ha,ying a,nr forin; in whieh case: 'l' (x, 11j z) 1 2n j s ::. }' (,$', ?t') d,l" il!!, [(x -x'p + (11 -,l'p + z~]"i~ [21 j~i~t hs ,\ hsllllle t,imi, nelh a hol'ehole p (fig. 1) the l'iyol' l'1m he collsidnl'nd ,\ 8 being stl' aight, is l metms wide ,\ llil (1li1", ~\ fii\\' nwtres deep, aml t,hat, the river-ground lllean annual tempemtu1'e dill'el'ence is t o • :!08 mongelli f. in a referencc system iike tha t in fig. l, if ji'(x', l/l is nil over the w·hole of the piane xv, excillding thc fìcld given by o<x<l, where is t" then [l] m ay be written: o z 1'. t (x, y, z) = _.2jt j~ dx' j" [;c(x:---ccc;--;-,',-"l~,-' --;c;;-;-:;c x')2 + (y !l'f + z2j3/, "> 1. x p o y l<'ig. l .coordinatf'.-x ~j'htelll for plott.illg t.ijc illfluelll"e af il rivpt oll tbc temper ature distributioll of thc sail. which, on intr.gmting gives: l' = .! (arctg y + l :1", 7t \ z 'i) arctg ~ [3j whcre y is thc di.~tance from the bore to the nea rcst bank, z is the depth. 8 • l ' hl" way 01' ex ampie, fig. ~ 6rlv ('s thc pcl 'colltage fli ll turbanc(l . l'. cau sali in r\ flcct.ion of g' l'ouml at. l'ight :'ingl ;;fi tu ~\ 11vl'll' wit.h widt.1! l = 100 m . i / / i i i / i / o 5 • :: e. , o,t-----------~--~~_r----~----------.' ~ ~ n , o 2111 fo' i/,,,,. 3 ", b ~\ihl c wlt iclt sltow tlte pel'coll t;~ge clis l.ul'hh.ih \(\ muli6(1 down a horo ilot. di~tllonces y = 100, 1ouo ami 50uu m tl'om a riye l' 100 tu 1000 m wid e, indica.te more clearly l , f l' , t. lc ox1~ t./mce u a max u11ulll to wltich shifts to e vo!' gl'ealer !lept.h a~ 11 alhi l increase . • ' . •• '. • > • , • • 110_ , .. -, .00 •• fig. :i a, /j, c 'l'olll pl2'ru.lul'o p(\reelllagl\ ,lis lli l' htlllco cu.udc,l duwll a lj() i'cl al di~tallc"~ !i ... il)(> . if)l)() ajl(1 al)o/llll frolli n dr!,\, 1()() j.o !()()() iii ",id (\. indecd , [rom [.,i ] wc obtain: '( l') il !i ! i. ì)z 7'. = jt (11 + l p +::-: [5] which ca.jlcùl s ohi. fo r z ';y (l + y) . 2 11 "" '00 ~--~,~.::-----'00 ,,~ pig. :1 /. ,., 212 ~iox(1}:u.r l'. !'i. conclusionr. from fig'8. 3a, b ami c it may be deduced ;tlmt: wheh y i~ al'o\lnd 100 ln the therltw.1 di~turbance is always pl'escnt allli vl11'ie~ (jonsidemhly with t.lm width of the river when :'i is al'o\lml 1000 ln, the distul'h:mce i~ negligiblc (lesh tll<l.n 1 %) at a 8hallow dcpth (lc88 tlmn ] 00 hl), whil!l it he(johles :~hnost constant at g"l'cat dcpths alid ims no offcet on tlw gl'l\tlient; tlw widtlt oe tlw riv(\]' i~ not v(lj'y impol'tallt iwl'!l when y is larj!c, bcillg' al'ound 5000 111, thc distul'bance is alw a ys ncg1igiblc. itefe rence:'; (l) afong};lll p. :l.lhl ($outhern ltaly) , ;\ll[(lritl, (hi6d). ricciietti g., jjrat fimi' -in thc fossi! ja:';pei ~ylltposiullt oh c:cotltel"lltal hr[!(l{~nic{l pl'oblellti!, (!) zumlerge j. h. :11[(1 , vtm th chow (et!.) ( l \lii4). j. c. '\'yeil.s, ifyrlrology of lakes llallllhollk of applit'll hytlrolo)/."y", a 1111 ~u'(u!.pn. :\it'!:l":\w-llill. e) ~\[[tj[ k .. 8rn/u thermul c1l1lraderintics of tu'o rivers in the lw/ii/i!!c {~rea ()f northern rliglwul . • j. jly<ll'ology . , vi. 4, (i968). (l) pl,asllcllgv a. v., 'l'hetl/wl ref/ime of ()rillle(!n rifers !l1/(! i!!fllil'/we 011 it karsi in/ter feedillg . • sovict jly<ll'ology., 2, (hi66). (' l to~ixi n., elemenli ili lllrogl'afia e idrologia, l'()l. l e il . • lilh'el"ia. uniyci''lital'ia», veltezia, (id5d). (o ) yis1.;nt\;>;\ :\l,j,(~ lem.pel"lllltm delle acque superficiali ile! baci/io ilc/ l'o . • ahlt. lavori pnhhlid . , fa;;c. :1, (ida7). (7 ) bircl1 p., plolv of heut ìli the flronl hange, golotl/d() . • bnll. geu!' suc. am . • , 61. (ld5u). (8) scotto:" ~r., t,{~ h'/llflemlltm del terrei/o nelle ricerche i/uuuiit' dell'ldtimo quiluluclwio . • altlt. pac. agraria . vniv(jr;;itil pl'rugia, xiv (ih5}l). (' ) ~iacciiia p., un 'p()~8ibile 1n~l()do imlirdlo di ileter-/il. ina:;ione del livcllo dcll'(icifu,a nel tcrrelw duranle il jlcriodo di ~iccilù e8tilja. \vehhia, 20. ( l \lii;;). (lo) c.-\il.sr.aw h. s .. a.ltil j. c. jaeoeil., oondlw/.ion of heut iii ~olid8 . • soc. etlit... oxfunl \jlliver.~it . .r" pwks. lojl(lolt, (19.'hi). layout 6 non-permanent gps data for regional-scale kinematics: reliable deformation rate before the 6 april, 2009, earthquake in the l'aquila area arianna pesci1,*, giordano teza2, giuseppe casula1, nicola cenni3, fabiana loddo1 1 istituto nazionale di geofisica e vulcanologia, sezione di bologna, italy 2 dipartimento di geoscienze, università degli studi di padova, italy 3 dipartimento di scienze della terra, università degli studi di siena, italy annals of geophysics, 53, 2, april 2010; doi: 10.4401/ag-4740 abstract a gps-based geodetic study at a regional scale requires the availability of a dense network that is characterized by 10 km to 30 km spacing, typically followed in a few continuous gps stations (cgpss) and several nonpermanent gps stations (npss). as short observation times do not allow adequate noise modeling, nps data need specific processing where the main differences between npss and cgpss are taken into account: primarily time-series length and antenna repositioning error. the gps data collected in the 1999-2007 time-span from non-permanent measurement campaigns in the central apennine area (italy) that was recently hit by the mw 6.3 l'aquila earthquake (april 6, 2009) are here further analyzed to compute a reliable strain-rate field at a regional scale. moreover, areas characterized by different kinematics are recognized, and a complete characterization of the regional-scale kinematics is attempted. these new data can be interpreted as indicators from the viewpoint of seismic risk assessment. introduction global positioning system (gps) measurements obtained from continuous or campaign-style surveying allow the definition of surface deformation patterns at both regional and local scales. the availability of a continuous gps station (cgps) network provides an accurate estimation of the strain-rate field, although the actual internal network spatial resolution is not always adequate for the definition of a dense strain-rate pattern in highly complex tectonic settings [gasparini et al. 1985, galadini and galli 2000, d'agostino et al. 2001, anzidei et al. 2005]. a significant improvement in the mean cgps network spacing is expensive in terms of human and financial resources, which are often not available. to increase the geodetic observation in the italian territory, several research institutes and universities are working on the installation of new cgpss and several non-permanent gps stations (npss). a nps is a stable structure on which a gps antenna is mounted for short periods of time, which can range from a few days to a few months per year, depending on the campaign organization. the observations arising from both nps and cgps networks are currently used to investigate geological and geophysical phenomena at a regional scale, e.g. to study deformation caused by tectonic movements [gomez et al. 2007, cenni et al. 2008, d'agostino et al. 2008, baldi et al. 2009], or at a local scale, e.g. to monitor gravitational macroscopic effects due to rock-mass collapse, landslide activation, or other instabilities [pesci et al. 2005, pesci and teza 2007, baldi et al. 2008]. the daily coordinates from npss and cgpss processed using the currently available scientific packages that are optimized for cgps data are generally characterized by an error of a few millimeters for the horizontal components, and about twice that for the vertical coordinates. the station velocity is generally computed using sequential least-squares adjustment procedures, and are also quantified taking into account white noise and time-correlated noise [langbein 2008]. the data processing procedures based on the hypothesis of equi-spaced coordinate time series can model the noise of cgps, but are not adequate for nps. therefore, the velocity error estimates obtained from the standard processing of nps data can be too optimistic. also, the effects of irregularly repeated antenna installations, which often result in significant point-position instability [leonard et al. 2007], are neglected in standard gps data processing. pesci el al. [2009] showed a method that was intended to provide reliable estimates of velocity uncertainties of a nps using subsampling of data related to a nearby cgps, taking into account the nps occupation plan, i.e. the set of start and stop survey dates. in this way, the quantities that are article history received november 18, 2009; accepted may 13, 2010. subject classification: gps, non-permanent station, subsampling, velocity, strain field 55 critically dependent on velocity uncertainties can be reliably computed, e.g. the strain. the algorithmic aspects of this method, and the package that implements it, are described in teza et al. [in press]. the present study shows the results from a further analysis of data belonging to the central apennine geodetic network [cageonet, italy; anzidei et al. 2005] using a cgps subsampling-based method. moreover, a new statistical approach is used to obtain more stable solutions, and therefore to allow stronger interpretation of the results. finally, emphasis has been placed on interpretation of the data in light of the recent mw 6.3 l'aquila earthquake (april 6, 2009). nps simulation from cgps data the velocities of nps and cgps are usually estimated through interpolation of the daily or weekly position time series, computed within a suitable reference frame (e.g. itrf 2005), and using methods mainly based on least-squares adjustments or similar procedures. in particular, the velocities of the studied sites are generally computed from daily position estimates using the forward kalman filter ability of globk, or of other gps analysis packages, with the combining of the nps and cgps observations [mcclusky et al. 2000, mcclusky et al. 2003, battaglia et al. 2004, bendick et al. 2006, casula et al. 2007, aktug et al. 2009, d'agostino et al. 2008, alchalbi et al. 2010, buble et al. 2010, weber et al. 2010]. the gps coordinate time series are affected by significant colored noise, i.e. time-correlated noise, which includes a large predominance of flicker noise. ray et al. [2008] showed that a continuum flicker noise, i.e. noise with a spectrum of the form 1/f (where f is the frequency), describes the background spectrum down to periods of a few months. moreover, if longer time-spans are considered, the signal-to-noise ratio is not improved. as indicated by previous studies [langbein and johnson 1997, dixon et al. 2000, williams et al. 2004, king and williams 2009], flicker noise components of the colored noise are commonly observed in a wide variety of environmental phenomena, e.g. atmospheric loading, sunspot variations, wobble of the earth axis, and undersea ocean currents. however, they also appear as bias in estimations of satellite atomic clocks, and in semidiurnal, diurnal, annual and semi-annual frequency bands. moreover, random-walk noise, the spectrum of which has the form 1/f 2, has also been seen in long gps time series. this is probably related to the instability of the structure [mao et al. 1999]. these effects are observable in long time series, but cannot be detected if nps data are used. a numerical experiment was performed to evaluate the effects of white, flicker and random-walk noise in velocity estimations from nps datasets. in particular, three synthetic 20-year-long time series were generated. let nw (t), nf (t) and nr (t) be the normalized white, flicker and random-walk noise, respectively (the distribution of a normalized noise is 0-centered in the range [−1, 1] m). three 20-year-long simulated daily time series (7,306 days) were generated: (i) a 12 mm/y linear trend slt (t) with 3 mm amplitude white noise, i.e. slt (t)+ 0.003 nw (t), as 3 mm is a typical noise level for real observations [see, e.g. cenni et al. 2008]; (ii) the signal slt (t) + 0.003 [nw (t) + nf (t)] obtained by adding 3-mmamplitude flicker noise to the previous signal; and (iii) the time series slt (t) + 0.003 [nw (t) + nf (t) + nr (t)]. the power spectra of the added noise were deterministic, with phases uniformly distributed in the range [−r, r]. it should be noted that some hypotheses about the phase are needed to obtain a time-domain signal from its power spectrum. indeed, no information about the phase is provided by the power spectrum [little et al. 2007]. the continuous time series were subsampled to simulate 2 weeks of yearly non-permanent observations, and therefore with 280 simulated acquisition days over 20 years. for each year, the data were extracted corresponding to the same time periods, according to the typical occupation plan of real campaigns, which are quasi-periodical. moreover, the procedure was repeated 10 times, with the extraction of data on similar epochs, although with time-shifting to enlarge the velocity dataset, making the next statistical analysis more stable. for each simulation, the estimated velocity was compared to the reference velocity obtained using the whole dataset (the best velocity), which led to a set of velocity differences. the root mean square (rms) velocity differences were computed for each synthetic time series, with respect to the number of simulated campaigns; the results are shown in figure 1. it is clear that the colored noise affects the velocity estimates, and that more measurement campaigns are needed to make the rms on small values stable. the noise cannot be modeled if time series that are too short are considered. therefore, to obtain reliable quantities of geodetic interest from the estimated velocities (e.g. the reliable deformation rate from gps nps data 56 figure 1.empirical relationship between the rms of the velocity differences and the campaign numbers using synthetic data from three signals: white noise; white + flicker noise; white + flicker + random-walk noise. 57 strain-rate field), the velocity uncertainties must be analyzed appropriately. using cgps or nps data changes the results significantly in terms of velocity estimates, as shown by applying the described subsampling approach to aqui cgps, a station in the central apennines and in the middle of the cageonet network (table 1). this area is characterized by high seismic risk, and it was recently hit by the mw 6.3 april 6, 2010, destructive earthquake that caused extensive damage to buildings and resulted in hundreds of human casualties [anzidei et al. 2009]. several simulations were carried out with different settings to study significant cases. the first simulation series was performed to explore the effects of campaign frequency. in particular, the cases of 1, 2 and 3 campaigns per year were considered. the campaign length was 14 days in all the cases. also in this case, 10 simulations were generated, which led to the velocity difference rms shown in figure 2. the second simulation series was performed by varying the campaign duration. in particular, yearly campaign series of 3, 7, 14 and 30 days in length were simulated. as expected, the rms decreases with increasing campaign number (figure 3). all these results show that as at least five campaigns are performed, the velocity rms lies below 0.5 mm/y, and it rapidly decreases with campaign number until negligible values are reached. in particular, two 7-day sessions per year provide 4 y results that are practically the same as those obtainable with one 30-day measurement session per year, since the difference is 0.5 mm/y (absolute value), even if the corresponding instrument days are 56 and 120, respectively. at present, each nps is generally equipped with a highaccuracy (sub-millimeter) structure for the gps antenna. nevertheless, even if the antenna installation/removal operations are carefully performed, some operator-induced errors can affect the results. these can depend on several factors, such as level calibration, a different antenna (even if reliable deformation rate from gps nps data figure 3. mean rms of the velocity differences for the east and north components based on the campaign numbers. figure 2.rms of the velocity differences obtained by simulating 14-day campaigns characterized by different frequencies (campaigns per year). bottom right panel: summary of the data showing the means of east and north velocity rms. the antenna model is the same, there can still be small differences), and other factors. the third simulation series was performed by imposing the effects of antenna installation/removal operations for campaign-style measurement simulations. a synthetic operator-induced error, i.e. a random distribution of horizontal steps of ~1-2 millimeters was imposed on the aqui station coordinates (positioning offsets), and the above analysis was repeated. yearly campaigns were extracted with 7-day durations, and 10 simulations were performed that considered different epochs for data extraction (figure 4). the rms of the two horizontal components goes to zero, increasing the number of occupations. a similar behavior appears if the antenna replacing errors are taking into account, even if the convergence to zero is slow. also, the effects of the stepwise error for the vertical component are similar, although wider amplitudes are involved. these results show that nps data require careful analysis and that short coordinate time series that are well distributed in time are a lot better than relatively long time series with too long a returning time. moreover, the millimeter uncertainty of nps data estimated using standard gps analysis software appears to be highly underestimated. therefore, a valid method for uncertainty evaluation is needed. a possible solution for the estimation of reliable velocity errors of a nps was shown in pesci et al. [2009], where a method based on subsampling of data provided by one or more cgpss neighboring this nps was described in detail, together with discussion and validation. this method was mainly based on the hypothesis that similar noise sources should affect the data provided by gps stations inside a welllocalized area (e.g. 10-50 km), depending on the size of the gps network. if gps stations are installed in areas characterized by high topographic heterogeneity, the atmospheric effects might not be completely modeled and removed, which would lead to degradation of the results in terms of coordinate time series obtained by standard operations of data processing [wdowinski et al. 1997, zhang et al. 1997, dong et al. 2002]. actually, there is nounambiguous definition of the characteristic distances between the reference cgpss and the npss that will allow modeling of the nps behavior from the cgps data. however, a reasonable solution appears to be the definition of the area containing the npss used and the choice of all of the available cgpss placed inside and around the area as reference stations for the velocity uncertainty modeling. clearly, the smaller the area is, the more representative the neighboring cgpss are expected to be [king and watson 2010]. for a specified real nps, the method follows three steps: 1) the search for one or more neighboring cgpss where the time series can be modeled as linear trends plus noise, and the computation of the corresponding reference velocities and related standard deviations. it must also be noted that the cgps time series used are already postprocessed data, that have been correctly analyzed, checked and corrected of evident anomalies (e.g. steps and outliers caused by earthquake or human damage, antenna changes, electronic problems). 2) for each cgps, performing several simulations via cgps data subsampling, taking into account the defined occupation plan and antenna replacement effects. if the occupation plan consists of ns surveys where the start and stop dates and corresponding duration are tim, tfm (tfm= tim + tdm) and tdm, respectively (m = 1, 2,..., ns), the simulated m-th survey of each simulation series has a start date randomly obtained from (or, alternatively, regularly shifted from) tim and duration tdm. in this way, for each number of simulated occupations, a set of nps velocities is obtained. 3) the statistical analysis of the distribution of differences between the reference velocities and the velocities of the simulated npss. for each number of simulated occupations (on the basis of the specific occupation plan), the rms of these velocity differences can be considered as thresholds of the real nps velocity uncertainty. if for a velocity component of the nps, the standard gps postprocessing provides an error smaller than this threshold, the threshold should be used as an estimate of the velocity error. clearly, to have meaningful results from this subsamplingbased approach, the requirement that the standard deviation of the reference velocity is significantly smaller than the threshold obtained must be satisfied. the results can be used to both analyze the available data (the aim of this study) and optimize the survey plans according to the accuracy needed and the human and financial resources available. velocity pattern and strain-rate estimation the cageonet was designed to measure both the subregional and near-field strain rates across the main seismogenetic structures and faults, which are believed to reliable deformation rate from gps nps data 58 figure 4.mean velocity rms from the simulation of 7-day yearly campaigns with antenna installation errors (antenna offsets). 59 reliable deformation rate from gps nps data t ab le 1 .v el oc it y pa tt er n an d ve lo ci ty e rr or s fr om s ta nd ar d (g l o b k -b as ed ) d at a pr oc es si ng , i nc lu di ng s ta ti on n am es , s ta ti on c oo rd in at es (l at it u de a nd lo ng it u de ), o cc u pa ti on s, v el oc it ie s an d ve lo ci ty e rr or s. if c g p s st at io ns a re c on si de re d, t he la be l « pe rm » is u se d in st ea d of t he in te ge r nu m be r of t he r el ev an t ca m pa ig n. n um n am e la t ( °) lo n (° ) c am p v e (m m /y ) v n (m m /y ) !! v e (m m /y ) !! v n (m m /y ) 1 a c c i 13 .7 44 42 .1 78 3 -1 .2 3. 4 1. 0 1. 3 2 a n a t 13 .5 49 42 .1 82 3 0. 7 0. 9 0. 6 0. 7 3 b a n o 13 .5 82 42 .3 37 3 1. 0 1. 7 0. 7 0. 6 4 b s p i 13 .6 50 42 .3 06 3 1. 3 1. 4 0. 8 0. 7 5 b s s i 13 .8 15 42 .2 14 3 0. 8 3. 2 0. 9 0. 8 6 c d a q 12 .9 91 42 .5 90 3 2. 7 2. 7 1. 1 1. 1 7 c id a 12 .9 07 42 .5 16 3 1. 1 3. 4 0. 8 0. 6 8 c in c 13 .4 05 42 .0 08 3 -0 .1 -0 .1 0. 6 0. 6 9 c n t a 12 .9 16 42 .4 76 3 1. 3 0. 6 1. 2 1. 3 10 c o c a 13 .3 75 42 .3 02 3 0. 9 2. 1 0. 8 0. 8 11 c p p d 13 .2 69 41 .9 98 3 -0 .3 1. 7 0. 5 0. 4 12 c p s e 12 .9 07 42 .6 04 3 0. 1 2. 8 0. 7 1. 0 13 c u m u 13 .0 28 42 .5 69 3 -0 .9 1. 5 0. 7 1. 0 14 c v a l 13 .8 11 41 .9 84 3 -0 .2 3. 3 0. 5 0. 5 15 c v s e 13 .7 45 42 .1 31 3 -0 .6 3. 8 0. 5 0. 6 16 f c lm 13 .4 59 42 .1 11 3 -0 .4 2. 5 0. 6 0. 7 17 f r c a 13 .6 78 42 .0 59 3 1. 3 1. 1 0. 7 0. 5 18 f r lc 12 .9 75 42 .6 96 3 -1 .6 3. 2 1. 5 1. 7 19 f t t i 13 .3 34 41 .8 92 3 -0 .2 0. 9 0. 7 0. 6 20 g r e c 12 .7 31 42 .4 45 3 -0 .8 -0 .1 0. 8 0. 6 21 m a m a 13 .2 53 42 .2 09 3 -1 .1 -0 .6 0. 9 0. 8 22 m e r 2 13 .0 18 42 .6 87 3 -0 .9 3. 8 0. 9 1. 0 23 m ln n 13 .7 05 41 .8 22 3 -1 .1 1. 8 0. 9 0. 8 24 m p e t 12 .8 33 42 .6 00 3 -0 .1 1. 3 0. 8 0. 6 25 m r p n 13 .6 85 41 .8 86 3 -0 .7 1. 7 0. 6 0. 8 26 m s lv 13 .7 42 42 .0 51 3 1. 9 0. 3 0. 6 0. 5 27 m v ip 13 .3 77 41 .9 29 3 1. 1 1. 9 0. 6 0. 6 28 n u r i 13 .0 61 42 .3 75 3 5. 0 -2 .4 1. 0 0. 8 n um n am e la t ( °) lo n (° ) c am p v e (m m /y ) v n (m m /y ) !! v e (m m /y ) !! v n (m m /y ) 29 o c r e 12 .9 71 42 .5 97 3 -2 .3 -0 .4 1. 6 1. 5 30 o r t o 13 .7 25 41 .9 99 3 -1 .2 3. 4 0. 7 0. 6 31 p o g b 12 .8 73 42 .5 15 3 0. 5 -0 .1 1. 0 1. 1 32 p p e z 13 .4 26 42 .1 83 3 0. 8 2. 0 0. 8 0. 7 33 p r c a 13 .4 94 41 .7 82 3 1. 1 2. 7 0. 6 0. 6 34 p s c a 13 .1 25 42 .1 28 3 -1 .2 1. 8 0. 8 0. 6 35 p s ir 13 .5 93 42 .1 80 3 -1 .1 1. 4 0. 7 0. 7 36 p s m a 13 .5 81 42 .1 27 3 1. 1 1. 6 0. 7 0. 6 37 q r t o 13 .3 99 42 .2 84 3 -1 .3 0. 4 0. 8 0. 6 38 r d c a 13 .4 89 42 .2 28 3 2. 3 0. 5 0. 6 0. 9 39 r e n d 13 .4 65 41 .8 34 3 1. 1 3. 6 0. 8 0. 6 40 r id o 13 .6 05 41 .7 99 3 0. 8 2. 5 1. 0 1. 0 41 r if p 13 .1 76 42 .7 63 3 0. 5 3. 2 1. 3 1. 4 42 r o c a 13 .6 97 42 .3 28 3 0. 9 2. 2 0. 5 0. 5 43 s e c i 13 .6 70 42 .1 48 3 0. 5 1. 5 0. 8 0. 7 44 s e t c 12 .7 96 42 .4 68 3 0. 5 -0 .2 1. 2 1. 4 45 s ie r 13 .6 68 41 .9 25 3 0. 0 1. 3 0. 5 0. 5 46 s ll i 13 .1 21 42 .7 27 3 0. 3 2. 7 1. 4 1. 0 47 s o r b 13 .3 17 42 .0 82 3 -0 .9 1. 5 0. 6 0. 6 48 s s 83 13 .7 80 41 .8 42 3 -0 .2 1. 0 0. 7 0. 8 49 s s e b 13 .7 52 41 .9 34 3 -1 .3 1. 0 0. 7 0. 5 50 s s t s 13 .6 51 42 .3 60 3 1. 3 1. 0 0. 5 0. 8 51 t e r r 12 .7 79 42 .4 26 3 -1 .1 4. 8 0. 9 1. 2 52 t n e r 13 .5 32 42 .2 37 3 -1 .1 -0 .6 0. 8 0. 7 53 t r a s 13 .5 43 41 .9 54 3 0. 1 2. 3 0. 5 0. 7 54 t r m t 13 .2 01 42 .0 96 3 -0 .5 2. 3 0. 6 0. 6 55 t v io 12 .9 88 42 .6 56 3 1. 8 1. 5 1. 6 1. 0 56 v c a r 12 .9 96 42 .5 66 3 -3 .2 0. 2 1. 8 2. 4 n um n am e la t ( °) lo n (° ) c am p v e (m m /y ) v n (m m /y ) !! v e (m m /y ) !! v n (m m /y ) 57 v la g 13 .8 29 41 .9 38 3 0. 9 -0 .5 1. 0 0. 9 58 v r c e 13 .2 40 42 .0 39 3 0. 1 1. 6 0. 8 0. 4 59 v t f u 12 .9 18 42 .5 64 3 2. 0 2. 4 0. 7 0. 8 60 c la c 12 .9 76 42 .5 46 4 -0 .9 0. 1 0. 6 0. 9 61 p r e t 13 .3 16 42 .3 82 4 -1 .0 -1 .6 1. 5 1. 3 62 b o r b 13 .1 62 42 .5 11 5 0. 1 0. 7 1. 1 1. 1 63 c a s b 12 .8 49 42 .3 90 5 -0 .3 0. 7 1. 3 1. 0 64 c r o g 13 .4 85 42 .5 86 5 0. 2 2. 8 1. 0 1. 2 65 la c u 13 .1 12 42 .4 88 5 1. 5 0. 0 1. 1 1. 2 66 m o s p 12 .9 49 42 .6 47 5 0. 1 0. 4 0. 8 0. 6 67 m s a n 13 .1 54 42 .7 61 5 -0 .1 2. 7 1. 5 1. 3 68 p o c a 13 .3 26 42 .5 71 5 0. 5 4. 0 1. 2 1. 3 69 r o f a 13 .5 41 42 .3 97 5 -0 .3 1. 0 0. 7 0. 6 70 r o io 13 .3 86 42 .3 27 5 -0 .1 1. 5 1. 5 0. 9 71 s c u o 13 .3 59 42 .6 29 5 -0 .9 1. 5 1. 3 0. 9 72 s f r a 13 .4 08 42 .4 60 5 1. 0 3. 1 2. 0 1. 5 73 s r o t 13 .1 42 42 .6 28 5 -2 .7 1. 9 1. 2 1. 5 74 v p e z 13 .4 85 42 .1 82 5 -1 .3 0. 5 1. 0 2. 1 75 c e p p 12 .8 55 42 .5 30 6 -0 .8 1. 5 0. 6 0. 9 76 f v io 13 .0 87 42 .5 35 6 -0 .1 0. 3 0. 9 1. 6 77 t r lo 13 .0 10 42 .4 69 7 -1 .6 0. 8 1. 1 1. 1 78 a q u i 13 .3 50 42 .3 68 pe rm 0. 6 2. 0 0. 6 0. 5 79 in g p 13 .3 16 42 .3 82 pe rm 1. 9 3. 8 0. 6 0. 4 80 in g r 12 .5 15 41 .8 28 pe rm -0 .2 1. 7 0. 5 0. 5 81 ip r a 12 .7 05 42 .4 84 pe rm 1. 6 0. 2 0. 8 1. 3 82 r s t o 14 .0 01 42 .6 58 pe rm 1. 5 2. 5 0. 5 0. 5 83 u n p g 12 .3 56 43 .1 19 pe rm -1 .0 1. 0 0. 5 0. 6 84 v v lo 13 .6 23 41 .8 70 pe rm -1 .8 2. 5 0. 6 0. 6 respond to the crustal dynamics of the area. this network comprises npss measured for the first time during a session over a few days in the 1999-2001 time-span. the first estimation of the strain rate of the area was computed over a regular 10 km × 10 km grid using velocity data processed over three years. these preliminary results showed both extensional and compressive behavior at the same values along the chain axis, ranging from about −10−7y−1 to +10−7y−1 [anzidei et al. 2005]. in this section, the main results of the processing and related analyses of the cageonet data obtained for the timespan of 1999-2007 are presented. the gps data were processed using the gamit 10.3 software [herring et al. 2006a], and included 22 permanent stations located over the whole italian territory. the registration in the igs05 external reference frame was performed using about 26 international global navigation satellite systems (gnss) service (igs) stations located both in italy and europe [bruyninx 2004, kenyeres and bruyninx 2004]. the igs stations of the global permanent network included in the regional computation were used, together with positions and velocities from the scripps orbit and permanent array center (sopac) facility ftp site [sopac 2009]. robust combinations of our regional solutions with the global ones computed by sopac for the igs gps network were obtained using the globk package [herring et al. 2006b]. a distributed session approach was applied that was similar to sequential least-squares minimization, using sequential kalman filtering and considering the |2 parameters to estimate the levels of data fitting [dong et al. 1998, casula et al. 2007]. finally, the total adjusted solution was obtained, in terms of the coordinate time series and velocities in the same reference frame. the inherently rigid rotation of the eurasian plate was removed by applying the general definition of the euler pole given by altamimi in the itrf2005 reference frame [altamimi et al. 2007], and using the plate module of the globk package. the resulting intra-plate velocity vectors that ranged between zero and a few millimeters per year are listed in table 1, and the residual velocities with respect to a fixed eurasian frame [altamimi et al. 2007] are shown in figure 6. the velocity uncertainties were estimated using the statistical method proposed by williams et al. [2004]. moreover, the time series were iteratively edited by means of the tsview matlab toolbox [herring 2003], to estimate/remove outliers and offsets, and subsequently verified by means of the cats package [williams 2008]. taking into account the non-permanent nature of the measurements, the short session lengths (about 2-3 days), and the results provided by previous simulations, the statistical values obtained appeared to underestimate the errors. the cgps subsampling-based approach presented by pesci et al. [2009] was applied to the time series of five cgpss located inside and around the area investigated, i.e. aqui, ingr, rsto, unpg and vvlo. the typical measurement reliable deformation rate from gps nps data 60 campaigns ve rms (mm/y) vn rms (mm/y) 2 7.6 11.7 3 1.8 2.1 4 1.4 1.2 5 0.7 0.7 6 0.5 0.5 7 0.4 0.3 8 0.3 0.4 9 0.2 0.4 table 2. east and north velocity error thresholds, defined from the subsampling procedure. the velocity thresholds for the relevant campaign numbers are given. figure 5.errors of the east and north velocity components related to the cageonet stations, including both the standard and the modified errors corrected with thresholds. since the npss measured using three campaigns show standard errors lower than the computed thresholds, their errors are replaced by the corresponding thresholds. in contrast, npss measured more than three times are characterized by errors that overcome the thresholds, and their final errors (blue) coincide with the standard errors. the station numbers along the abscissa are as listed in table 1. the ordinate axes are labeled as «v error» to highlight the corrected errors dataset (blue line). 61 sessions of a cageonet nps characterized by 2-3 day duration were considered. unfortunately, this was not the best choice for the stability of the velocity estimation, as shown in the previous section, but these were the only data available. moreover, the campaigns were typically irregularly spaced in time. for this reason, for each real nps, the uncertainty analyses were carried out on the basis of the occupation plan; in order to reduce the number of calculations, these npss were grouped with a criterion of similarity in the occupation plan. the thresholds of velocity errors were therefore obtained for each campaign number and for an assigned occupation plan. moreover, table 2 shows the results in the special case in which almost regularly spaced yearly campaigns are simulated in the whole network, to show how the errors change with respect to those provided by standard cgps processing applied to the nps time series. as shown by pesci et al. [2009], the procedure is based on a regular time sequence of campaigns that provides the worst case for timemeasurement distribution. for this reason, the threshold cannot be underestimated. the velocity uncertainties evaluated using the subsampling-based thresholding method are shown in figure 5 (blue), where it can be seen that the statistical values obtained by standard data processing were not significantly underestimated if more than three campaigns were available. clearly, if a subsampling-based threshold does not exceed the velocity error provided by the standard processing software, this threshold is not used in further computations. the velocity pattern and the corresponding uncertainties reliable deformation rate from gps nps data figure 6.velocity patterns and associated 95% confidence error ellipses obtained with standard (a) and threshold-modified (b) uncertainties. strain-rate fields were computed using the same velocity patterns, but different associated errors, as standard (c) and threshold-modified (d). estimated using both the standard data processing, where the npss are processed like cgpss, and the subsampling-based thresholding method are shown in figure 6, where it can be seen that in the latter case, the errors are increased. these velocity patterns were used to compute the strain rate on the nodes of a 10 km × 10 km regular grid distributed in the area. a modified least-squares method was used, with rescaling of the covariance matrix of velocity data (observables), and considering a weighting function that took into account the distances between the grid node and the gps stations [shen et al. 1996]. this method was implemented in the grid_strain package [teza et al. 2008], and together with the estimation of the strain rate and the corresponding uncertainty, it also provides a geometrical significance to the results. the strain rate in a grid node is highly significant only if the stations are well distributed all around this node (requirement for angular distribution) and if their distances do not exceed about three times the mean inter-station distance (requirement for radial distribution). for each nps station, the errors associated to the velocity solutions in the 1999-2007 time-span were checked on the basis of the thresholds obtained as a function of the number of end epochs of the campaigns performed. the scale factor considered in modified least-squares computations was 25 km, according to the mean spacing between the gps stations. the deformation appears to be in agreement with the estimations by anzidei et al. [2005]. the computed strain rates ranged from 1.2 × 10−8y−1 to 1.6 × 10−8 y−1 and from −1.4 × 10−8 y−1 to −0.3 × 10−8 y−1 along the normal and parallel directions to the chain axis, respectively, with a 1.1 × 10−8 y−1 mean error. figure 7 shows the effects of the thresholding-based method on computation of the principal strains. in particular, the maximum of the strain-rate tensor field computed from the standard and threshold-modified velocity fields are shown together with their differences, as also for the minimum eigenvalues. these differences are not zero centered, which indicates that the deformation tensor is globally changed as an effect of error thresholding, even if the values are small with respect to the mean computation error of the order of 10−8. regional velocity field the velocity field with the subsampling-based error thresholding (figure 6d) shows a high heterogeneity for both vector modules and directions. the cageonet was designed to measure the regional and near-field strain rates across the main seismogenetic structures and faults. thus, the aim was to highlight possible differences in the regional and subregional velocity fields. for this reason, a simple statistical method was applied to highlight areas with similar kinematics. the method was based on an iterative search in a confined area, to compare the direction and module of the gps station velocity. simple criteria were used to select stations with similar trends, in terms of direction and module. for the agreement level between the velocity directions of two stations, a scalar parameter, dij, is defined by the relationship: (1) where are the normalized east and north velocity components of the i and j stations, respectively. the discrepancy parameter dij ranges from 0 (perfect agreement) to 1 (total disagreement). figure 7 shows the dij parameter as a function of the angular differences between the directions of two normalized vectors. in particular, the reference vector vi is fixed (unit vector of y-axis, i.e. where t indicates the vector transposition), while the vector vj is rotated, providing all of the possible directions in the [0,360˚] interval. reliable deformation rate from gps nps data 62 figure 7. principal strain rates computed using the standard velocity patterns and the threshold-modified patterns. the differences between the modified and original eigenvalues (thick black lines) have means of 0.8 × 10−8y−1 (standard deviation 0.5 × 10−8y−1) for the maximum eigenvalues (left panel), and means of 0.4 × 10−8y−1 (standard deviation 0.3 × 10−8y−1) for the minimum strain (right panel). 2 1 ,d v v v v2 2ij e= +-in ijn je^ ^h h , , andv v v vein i jn je / ;v v v v v( n n e= + =; ;n e /v v v )e n e= +; ; 1 0v ji t= =t 6 @ 63 the discrepancy between the velocity modules of the station are also estimated by comparing the single modules: (2) to select the stations characterized by similar trends, for each i-station, a circular area of radius r (search radius) is defined, and the set of the j-stations inside this is recognized. the velocity of the i-station is then compared to all of the jstation velocities, defining the dij and dij parameter dataset. subsequently, to obtain a unique pair of values representative of the discrepancy level for each i-station, the mean and the rms of the dij and dij distributions are considered. the parameters di and di are defined as follows, in order to characterize the angular and modular distributions by means of a single value, while taking into account both the mean and the rms: di = (di0+ di1)/2, di = (di0+ di1)/2. the computation was performed using the total dataset, and varying the search radius from very local (10 km) to regional (90 km), as shown in figure 9. figure 10 shows the dm and dm that are representative of the gps velocity in the whole network with respect to the radius (10-90 km). the lower value that corresponds to the 10 km radius indicates a more scattered data distribution. in particular, changes in the discrepancies parameters occur going from 10 km to 40 km. longer ranges provide values of about 0.33 (di distributions) and 0.97 (di distributions). reliable deformation rate from gps nps data figure 9.distributions of di and di computed on different search radii from 10 km to 90 km. gray shading: number of j-stations around each i-stations based on the search radii. for station numbers, see table 1. figure 8.the dij parameter shows the discrepancy level between vector vi and vj direction in the [−1, 1] interval (red squares). abscissa: angle between the reference velocity vi = [0 1] t and vj. the vector components, i.e. cosine and sine of the angle, are shown by solid and dashed lines respectively. v v v v2 2ij ie in e nj j= + +d -^ ^h h this indicates that the sizes of the sub-areas affected by different kinematics are probably lower than 40 km. this statistical method characterizes velocity discrepancies, which depend on the chosen search-radius and threshold values for di and di, and it is aimed at extracting some kinematicsrelated information, and in particular to recognize possible different velocity patterns in the area, without the use of a tectonic or a geodynamic model. in this way, a-priori hypotheses that might force the results are avoided. the discrepancy parameters shown above that characterize the network were used together with a search radius of 20 km, to select a reduced velocity dataset that was representative of a regional pattern (figure 11, left panel). the results are superimposed on a map that shows the topographical information of the area, and they provide a detailed view of the 1999-2007 sub-regional and near-field velocity patterns of this region. figure 11 right panel shows the complete velocity pattern and in both cases the strain rate tensors are shown. discussion although a geological interpretation of these data is not within the scope of the present study, which mainly concerns the methodological procedures for data treatment, some aspects can be observed and discussed (see figure 6d and figure 10). it has to be noted that a large portion of the velocity vectors are characterized by errors at the same level as the velocity magnitudes, which makes unambiguous data interpretation very difficult. the eastern part of the network, and in particular the gran sasso area, presents a nne trend, which is in agreement with the extrusion pattern of this sector of the belt that has been suggested by some studies [e.g. mantovani et al. 2006], and with the activity of the assergi and campo imperatore faults [anzidei et al. 2005]. the sites located on the umbria-marche apennines (uma) and in the north-western sector of the fucino plain might be compatible with the sinistral transtensional tectonics observed in the area [galli et al. 2008], although more information cannot be obtained using the data processing proposed. a n-ne trend is seen in the north-eastern part of the fucino valley, although also in this case the results are relatively limited by the errors. the more significant station velocities in the liri valley (south-western part of the fucino plain) are characterized by a n-ne-ward trend, and they are responsible for the compressive deformation pattern computed in the fucino plain (figure 6d and figure 11). this result is clearly in disagreement with the extensional pattern expected in a basin area, but this is only a preliminary reliable deformation rate from gps nps data 64 figure 10. the dm and dm values computed as the means of the di and di parameters shown in figure 9, for each search radius. 65 analysis, where the necessary geological and geophysical information about the local movement for each site is neglected. some stations that belong to cageonet that were included in the data analysis are in zones affected by large gravitational effects that can lead to movements that are not related to tectonics. indeed, a preliminary study that was also based on an analysis of photogrammetric images reported the stations affected by local effects, e.g. seci, acci and cvse [galvani 2009]. despite this partial disagreement related to local movements, the general results for the strain rate that were obtained using this thresholding-based approach agree substantially with the known regional kinematics, whereas the strain-rate field computed without the application of this method does not. this confirms that the standard gps data processing optimized for cgps data treatment cannot provide reliable velocity uncertainty estimates if nps data are used. clearly, another important result of the velocity uncertainty analysis is that new surveys need to be planned and performed in the coming years, to improve the velocity, and therefore the strain-rate, patterns. as well as the thresholding of velocity uncertainties, a simple but efficient method was developed and applied that was intended to recognize areas that are characterized by similar velocity vectors. in this way, the subareas that are kinematically homogeneous at the regional scale were easily recognized, and in each subarea the displacement vector is shown. on the other hand, the border between two subareas should be characterized by an abrupt spatial change in local strain rate, whereas possible changes in strain rate within an area should be relatively low. figure 11 shows the strain-rate field from selected velocities and that from the whole errormodified velocity field, which allows a direct visual comparison. the first characterizes a regional trend, and therefore provides global information. this shows two main zones that are positively and negatively deformed. the second provides relatively local information because all of the velocity vectors are used. both of these results need to be taken into account in the data interpretation. figure 12 summarizes the main result using a contouring map of the computed strain rate from the threshold-corrected velocity field (using the whole velocity dataset according to figure 6d). the rate of change in an area, i.e. the sum of the principal strain rates, is used to recognize possible lines that are characterized by high-strain gradients, which in extreme reliable deformation rate from gps nps data figure 11. left panel: velocity field obtained using the statistical method proposed, with a search radius of 20 km and the matching parameters di = 0.33 mm/y and di = 1.0 mm/y. cyan blocks, sites with similar trends. the strain rate field estimated using a modified least-squares approach is also shown. right panel: previous velocity fields (bold black arrows) and trends (blue arrows) of the stations ruled out of the analysis, together with modified errors. umbriamarche apennines (uma) region. figure 12. change-in-area contour map from the 1999-2007 corrected velocity fields and the displacements caused by april 6, 2009, destructive earthquake. computed velocities (cm/y; blue arrows) and data published by anzidei et al. [2009] are also shown (gray arrows). cases might be related to the existence of faults or other discontinuities. the data used in this study were derived from nonpermanent gps campaigns that were performed from 1999 to 2007. additional information relating to the displacements caused by the mw 6.3 l'aquila earthquake (april 6, 2009) was obtained by processing cgps data from the weeks before and after these shocks, and this is superimposed on the map (figure 12). the aqui and ingp stations, which were affected by the largest displacements, are located in the area that is mainly characterized by a different strain-rate pattern, which coincides with the line of the apennine chain. moreover, the results from anzidei et al. [2009] are included in figure 12, providing further information. in figure 12, the area in which the strain-rate computation is characterized by a high significance level is highlighted. conclusions a cgps subsampling-based method was applied to the cageonet data to obtain reliable errors in nps velocity estimates, and therefore to allow computation of a reliable strain-rate field at a regional scale. the results show that: (a) the strain rate computed using this method can account for the existence of seismogenetic features, and it appears to be more regular than the strain-rate field computed without the specific treatment; (b) only velocities obtained from more than three measurement campaigns that were characterized by session lengths of a few days led to errors lower than 1 mm/y; (c) the method provides operative suggestions for the design of a nps network and related observation planning, and for the inclusion of nps velocities in the tectonic study. moreover, a simple statistical method was developed that was intended to detect possible regular patterns in the velocity field related to a network with a large number of stations. although this procedure is strongly user-dependent because of the thresholds imposed on the module for the direction variations and search radius, its application to the cageonet network allowed the robust recognition of some areas that are characterized by different kinematic patterns. in this way, these local-scale and regional-scale behaviors can be studied and compared. these data agree with those from previous studies, and they provide very useful information about kinematics at different scales. moreover, the displacements due to the l'aquila earthquake of april 2009 are in areas that are characterized by the main strain-rate variations, suggesting that reliably corrected nps data can be effectively used for territory monitoring. finally, these procedures indicate that the currently available information from cageonet is still too poor to provide reliable constraints to geological and geodynamical interpretations. references aktug, b., j. m. nocquet, a. congöz, b. parsons, y. erkan, p. england, o. lenk, m.a. gürdal, a. kilicoglu, h. akdeniz and a. tekgül (2009). deformation of western turkey from a combination of permanent and campaign gps data: limits to block-like behavior, j. geophys. res., 114, b10404, doi:10.1029/2008jb006000. alchalbi a., m. daoud, f. gomez, s. mcclusky, r. reilinger, m. abu romeyeh, a. alsouod, r. yassminh, b. ballani, r. darawcheh, r. sbeinati, y. radwan, r. al masri, m. bayerly, r. al ghazzi and m. barazangi (2010). crustal deformation in northwestern arabia from gps measurements in syria: slow slip rate along the northern dead sea fault, geophysical journal international, 180 (1),125-135. altamimi, z., x. collilieux, j. legrand, b. garayt and c. boucher (2007). itrf2005: a new release of the international terrestrial reference frame based on time series of station positions and earth orientation parameters, journal of geophysical research, 112, b09401, doi:10.1029/2007jb004949. anzidei, m., p. baldi, a. pesci, a. esposito, a. galvani, f. loddo, p. cristofoletti, a. massucci and s. del mese (2005). geodetic deformation across the central apennines from gps data in the time-span 1999-2003, annals of geophysics, 48 (2), 259-271. anzidei, m., e. boschi, v. cannelli, r. devoti, a. esposito, a. galvani, d. melini, g. pietrantonio, f. riguzzi, v. sepe and e. serpelloni (2009). coseismic deformation of the destructive april 6, 2009 l'aquila earthquake (central italy) from gps data, geophysics research letters, 36, l17307, doi:10.1029/2009gl039145. baldi, p., n. cenni, m. fabris and a. zanutta (2008). kinematics of a landslide derived from archival photogrammetry and gps data, geomorphology, 102 (3-4), 435-444. baldi, p., g. casula, n. cenni, f. loddo and a. pesci (2009). gps-based monitoring of land subsidence in the po plain (northern italy), earth and planetary science letters, 288 (1-2), 204-212, doi: 10.1016/j.epsl.2009.09.023. battaglia, m., m. h. murray, e. serpelloni and r. bürgmann (2004). the adriatic region: an independent microplate within the africa-eurasia collision zone, geophysical research letters, 31, l09605, doi:10.1029/2004gl019723. bendick, r., s. mcclusky, r. bilham, l. asfaw and s. klemperer (2006). distributed nubia-somalia relative motion and dike intrusion in the main ethiopian rift, geophysical journal international, 165 (1), 303-310. buble, g., r. a. bennett and s. hreinsdóttir (2010). tide gauge and gps measurements of crustal motion and sea level rise along the eastern margin of adria, journal of geophysical research, 115, b02404, doi:10.1029/2008jb006155, 2010. bruyninx, c. 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d.j. johnson (2004). error analysis of continuous gps position time series, journal of geophysical research, 109, b03412, doi:10.1029/2003jb002741. williams, s.d.p. (2008). cats: gps coordinates time series analysis software, gps solutions, 12 (2), 147-153. zhang, j., y. boch, h. johnson, p. fang, j. genrich, s. williams, s. wdowinski and j. beh (1997). southern california permanent gps geodetic array: error analysis of daily position estimates and site velocities, journal of geophysical research, 102(b8), 18035-18055. doi: 10.1029/97jb01380. *corresponding author: dr. arianna pesci, istituto nazionale di geofisica e vulcanologia, sezione di bologna, italy; e-mail: pesci@bo.ingv.it © 2010 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. reliable deformation rate from gps nps data 68 pavlov ic cs s layout 6 comparison between low-cost and traditional mems accelerometers: a case study from the m7.1 darfield, new zealand, aftershock deployment elizabeth s. cochran1,*, jesse f. lawrence2, anna kaiser3, bill fry3, angela chung2, carl christensen2 1 university of california, department of earth sciences, riverside, ca, usa 2 stanford university, geophysics department, stanford, ca, usa 3 gns science, natural hazards division, lower hutt, new zealand annals of geophysics, 54, 6, 2011; doi: 10.4401/ag-5268 abstract recent advances in micro-electro-mechanical systems (mems) sensing and distributed computing techniques have enabled the development of low-cost, rapidly deployed dense seismic networks. the quake-catcher network (qcn) uses triaxial mems accelerometers installed in homes and businesses to record moderate to large earthquakes. real-time accelerations are monitored and information is transferred to a central server using open-source, distributed computing software installed on participating computers. following the september 3, 2010, mw 7.1 darfield, new zealand, earthquake, 192 qcn stations were installed in a dense array in the city of christchurch and the surrounding region to record the on-going aftershock sequence. here, we compare the ground motions recorded by qcn accelerometers with geonet strong-motion instruments to verify whether low-cost mems accelerometers can provide reliable ground-motion information in network-scale deployments. we find that observed pga and pgv amplitudes and rms scatter are comparable between the geonet and qcn observations. closely spaced stations provide similar acceleration, velocity, and displacement time series and computed response spectra are also highly correlated, with correlation coefficients above 0.94. introduction while many seismological and engineering applications would benefit from very dense strong-motion observations, the costs of deploying and maintaining dense seismic networks has limited their implementation. however, recent improvements in micro-electro-mechanical systems (mems) sensor technology as well as development of new communication and processing techniques can greatly reduce per-station costs. over the past decade, several studies have shown that low-cost triaxial mems accelerometers provide sufficient sensitivity levels and reliability for use in geophysical and earthquake engineering applications [e.g. holland 2003, evans et al. 2005, hons et al. 2008, zhao and xiong 2009]. furthermore, infrastructure costs can be reduced by transferring data over existing wireless networks [e.g. evans et al. 2005, fleming et al. 2009, picozzi et al. 2009] or even by taking advantage of underutilized personal computers for monitoring sensors and initial data processing [e.g. cochran et al. 2009a]. one such effort to develop a low-cost strong-motion network is the quake-catcher network (qcn), which uses mems accelerometers either internal to laptops or connected via usb to desktop computers [e.g. cochran et al. 2009a, cochran et al. 2009b, chung et al. 2011]. qcn utilizes data from triaxial mems accelerometers of varying resolution (currently 8-14 bit) that cost less than us$50 per sensor. qcn runs on the berkeley open infrastructure for network computing (boinc) open-source platform [anderson 2004]. boinc enables scientists to create a distributed computing project to which a volunteer participant can donate their unused central processing unit time. the qcn station hardware includes a mems sensor connected via usb-port to a computer or a sensor internal to a laptop; however, no datalogger or gps unit is used. this data collection scheme greatly reduces the cost of installing an individual seismic station. the mems sensor is monitored through boinc software that is installed on a participant’s computer and communicates with the central qcn server. timing and station location must be determined without a local gps unit. we utilize network time protocol (ntp; http://www. ntp.org) [mills 1990, frassetto et al. 2003] to ensure accurate timing at each station. each station requests a timing ping from the server every 15 minutes and internally tracks the timing drift. the offset (in seconds) between the internal article history received june 29, 2011; accepted october 25, 2011. subject classification: seismology instruments and techniques, ground motion, seismic risk, seismological data, citizen science. 728 citizen empowered seismology/special section edited by r. bossu and p.s. earle computer clock and ntp time is used to correct the data. frassetto et al. [2003] showed that ntp accuracy is typically within +/– 20 msec, with only 9 out of approximately 17,000 clock locks having errors larger than 0.1 seconds. a qualitative comparison of timing at nearby qcn and geonet stations as well as the observed moveouts across the network suggests adequate timing control on most stations. however, we noted a few stations (<5) that had consistently poor timing control (arrivals were several seconds early or late). in these cases, the individual computer’s network settings may be affecting the ntp pings, but further analysis is required to verify the source of these timing errors. a station’s location is determined using either an ip look-up table or via participant entry into a google maps application programming interface (api). we get an initial, low-resolution station location using the ip lookup table, which utilizes the computers ip address to estimate the station location. this location method is 83% accurate to within 40 km in urban areas, but location errors can be much larger (>100 km) especially if the participant logs into a vpn account (http://www.maxmind.com). to improve the station location accuracy, participants can enter their location via a google maps api during the initial software installation. participants are directed to a google maps that is centered on the best estimate of their location from the ip lookup. participants then mark their location on the map and can achieve a location that is accurate to sub-building scale if they utilize the ‘satellite’ view. the location type, either ip or user-input, is included in the station’s metadata. during installation the three components of the sensor are aligned north-south, east-west and vertical; the orientation, if known, is also recorded in the metadata. teams from qcn and gns science installed most of the stations used here, so over 95% of the stations had locations entered using the google maps api and sensors aligned to north. each station monitors the sensor accelerations in realtime and produces both triggers and waveform records. a trigger is generated when the current acceleration is significantly larger than the previous 60 seconds of acceleration, as determined by a modified short-term average/long-term average (sta/lta) algorithm [cochran et al. 2009a, 2009b]. when a trigger occurs, a minimal amount of information is rapidly transferred to the central server [chung et al. 2011]. waveforms are collected in either triggered mode or continuous mode. in triggered mode, 180 seconds of time-series data are saved around the trigger time, with a 60 second pre-trigger window and a 120 second posttrigger window. triggered waveforms are only uploaded to the server if correlated with an earthquake reported by the u.s. geological survey national earthquake information center (neic) catalog (http://earthquake.usgs.gov) or by the qcn event detection algorithm [chung et al. 2011]. in continuous mode, ten-minute-long records are transferred to the server when the qcn monitoring software is running. all time series data are recorded at 50 samples per second. new zealand aftershock data were recorded in continuous mode. data collected by qcn may be used to augment existing seismic networks, essentially filling in the gaps between higher-quality sensors [e.g. evans et al. 2005]. these data may provide valuable observations for use in practical applications such as earthquake early warning as well as seismic hazard mapping and analysis. for example, increasing station density may reduce the time needed to detect and characterize an earthquake, potentially providing a few critical extra seconds of warning to mitigate the effects of damaging ground motion [kanamori 2005]. chung et al. [2011] discuss a retrospective test of rapid earthquake detection using triggers observed by a qcn aftershock deployment following the 2010 m8.8 maule, chile earthquake. and, the observed peak ground accelerations (pga) could be integrated into real-time, high-resolution maps of shaking intensity, e.g. shakemap [wald et al. 1999a]. dense observations improve our understanding of how ground motions vary over small spatial scales (<5 km) and provide fundamental information about wave propagation through complex geologic material. evans et al. [2005] concluded that station densities should exceed one station per 1 km2 to produce coherent ground-motion maps capable of resolving realistic variations in ground motions such as those observed in affected urban areas. in order to utilize the qcn data in the applications mentioned above it is important to first verify that the data collected in a typical deployment provide reliable records of ground motion. here, we compare aftershock data collected by traditional geonet strong-motion stations and qcn stations that were deployed following september 3, 2010, mw 7.1 darfield, new zealand, earthquake [gledhill et al. 2011]. we analyze peak ground-motion measurements collected by strong-motion stations installed in the city of christchurch and the surrounding region to assess whether the rms scatter in the observations are comparable. then, we examine data from a small-scale array of six qcn stations installed within 1 km of a geonet station to compare full waveform characteristics and response spectra. strong-motion data we use data recorded by 33 geonet strong-motion stations located near christchurch, within the study region (figure 1). these stations are part of the national strong motion (network code: sm) and canterbury regional strong motion (network code: sc) networks. the strong-motion stations include accelerometers installed inside structures as well as free-field sensors; the main instrument types include kinemetrics etna and canterbury seismic instruments (csi) cusp-3 and cusp-m accelerographs. while there are also cochran et al. 729 730 several building arrays deployed in christchurch, here we include only the stations that are located on the ground floor of structures or free field installations. sensors are installed in a range of building types from small sheds to large, multistory structures. these sensors are generally bolted either to a concrete floor or the foundation of the building. data are recorded in triggered mode with 5-20 seconds preevent memory at 200 samples per second. 192 qcn sensors were installed in the christchurch region following the september 3, 2010, mw 7.1 darfield mainshock to augment the existing strong-motion networks as part of a rapid aftershock mobilization project (ramp) (figure 1). the sensors installed were codemercenaries joywarrior jwf14f8 triaxial accelerometers, a 14-bit sensor that connects via usb to a computer. during the ramp, all stations recorded continuous waveform data that were transferred to a central server every 10 minutes; in addition, stations sent real-time trigger information when significant ground motion was detected. approximately 140 sensors were distributed in the urban sections of christchurch region low cost mems strong motion data figure 1. map of the study area showing qcn (circles) and geonet (squares) station locations. small blue stars indicate m≥4.5 earthquakes that occurred between september 15, 2010, and january 31, 2011, used in this study (table 1) and large red star indicates the epicenter of the september 3, 2010, m7.1 darfield mainshock. red box indicates the zoom region shown in figure 4. event id year day hour min sec lat lon depth mag geonet qcn 3373925 2010 260 22 3 47.703 –43.5962 172.3814 7.28 4.6 12 78 3374803 2010 262 12 30 3.627 –43.5602 172.3892 8.71 4.5 17 130 3376639 2010 265 18 22 26.236 –43.5660 172.4258 9.12 4.5 13 131 3382676 2010 277 9 21 50.283 –43.5627 172.4031 10.33 5.2 24 120 3388384 2010 286 3 42 30.338 –43.5899 172.4131 10.35 5.1 21 123 3389582 2010 288 9 31 40.549 –43.6390 172.4859 7.91 4.7 21 120 3391440 2010 291 22 32 15.901 –43.5946 172.5676 5.00 5.1 19 44 3394581 2010 297 2 13 29.396 –43.4594 172.7221 9.12 4.7 12 42 3400620 2010 305 6 19 55.764 –43.5862 172.4068 8.05 4.6 15 19 3403219 2010 310 13 52 3.665 –43.2113 172.0630 8.72 4.6 4 25 3406713 2010 317 12 34 6.907 –43.5805 172.4154 10.39 4.8 23 76 3407078 2010 318 6 21 5.496 –43.5838 172.4153 10.70 4.8 21 77 3436956 2010 359 13 32 9.924 –43.5335 172.6393 6.66 4.6 3 16 3437105 2010 359 21 30 15.952 –43.5493 172.6271 5.12 4.9 21 25 3450113 2011 19 17 3 20.828 –43.6076 172.5279 5.00 5.1 22 38 table 1. m ≥ 4.5 earthquakes: september 15, 2010, to january 31, 2011. with station spacing of approximately 1.6 km2. the remaining stations were installed outside of the main city limits at greater station spacing. the qcn sensors were installed in homes, schools, and businesses of participants who volunteered to host a sensor. almost all qcn sensors were installed on the ground floor of the building and, in some cases, sensors were placed in arrays around existing geonet stations. basic information about the building, including building type (wood frame, steel, concrete, brick) and the extent of damage, if any, suffered during the m7.1 mainshock was collected at the time of installation. sensors are typically securely fastened onto the floor of a building using either screws, glue, or tape and are located within approximately 5 m of a desktop computer. sensor hosts were required to agree to at least a three-month hosting period. following those three months, many of the temporary stations were removed; by february 2011, approximately 40-50 qcn stations remained in the christchurch region and the removed sensors were returned to gns science for deployment in other regions. the participant’s computer must be on to monitor the sensor and we don’t require participants to modify their computer usage in order to participate in the project (e.g. leave the computer on 24 hours per day). therefore, roughly 50% of the stations are monitoring and reporting data at any given time in a typical deployment [chung et al. 2011]. using data recorded by the geonet and qcn stations, we examine aftershocks recorded between september 15, 2010 and january 31, 2011. qcn stations recorded earthquakes as small as m2.6 locally, but these events tend to have poor signalto-noise ratios. we limit our analysis to m ≥ 4.5 earthquakes, which are well recorded by both networks. within the study area and time period of interest, fifteen earthquakes occurred ranging in size from m4.55 to m5.15 (figure 1; table 1). here, we compare the data recorded by the geonet and qcn stations to determine if qcn data provide reliable records of ground motion. comparison of peak ground motion peak ground motions recorded by strong-motion stations can be used to determine earthquake magnitude [e.g. campbell 1981, wu et al. 1998], map shaking intensities [e.g. wald et al. 1999a, wu et al. 2003], and to examine the response of structures [e.g. nigam and jennings 1969, wald et al. 1999b, wu et al. 2004]. thus, it is important to first examine whether qcn stations provide reliable observations of peak ground acceleration, velocity, and displacement. accelerograms are integrated to velocity, and then velocity time series are integrated to displacement. for all traces, the mean is removed, a 5% taper is applied, and accelerograms are high-pass filtered at 1 hz before data are integrated. the static displacements at very low frequencies are expected to be negligible due to the small seismic moments of the earthquakes examined. peak ground motion is defined here to be the maximum acceleration, velocity, or displacement that is observed within two minutes of the earthquake origin time. we examine the peak ground motions recorded during a m5.1 earthquake that occurred on october 18, 2011 (event id: 3391440; table 1). the aftershock was recorded by 19 cochran et al. 731 figure 2. peak horizontal ground acceleration (pgah) recorded by qcn (circles) and geonet (squares) stations during a m5.1 aftershock on october 18, 2010 (blue star). the stations are colored by observed pga. note that a few (<3) of the qcn stations have ip-based locations that are not very accurate. we included those data here for completeness; although the location quality is included in the metadata so the data could be excluded. 732 geonet and 44 qcn stations located within 25 km of the hypocenter. figure 2 shows the spatial distribution of observed peak horizontal acceleration (pgah) across the city of christchurch and outlying regions for the m5.1 aftershock. pgah is spatially variable, but with a clear decay in amplitude with distance as expected. in general, closely located geonet and qcn stations have similar reported pgah. to further examine the ground-motion variation, we plot the peak horizontal acceleration and velocity (pgah and pgvh) versus distance for the the october 18, 2011 m5.1 aftershock (figure 3a). the observed ground motions at geonet and qcn stations have comparable amplitudes at a given distance from the earthquake hypocenter; although the qcn accelerations and velocities are slightly higher, on average, than the geonet ground motions. the observed ln(pgah) and ln(pgvh) versus distance are fit to a leastsquares linear trend determined for the qcn and geonet datasets, as well as for both datasets combined. we then detrend the observed pgah and pgvh to compare the scatter in observed ground motions (figure 3b). the number of geonet and qcn observations and their rms scatter are given in table 2. rms values are slightly lower for qcn data, but this difference is not significant. local variation in pgah can reflect either spurious effects related to the instrumentation or real variation related to the source, path, or site conditions. low cost mems strong motion data figure 3. (a) pgah versus distance observed on qcn (circles) and geonet (squares) for the october 18, 2010, m5.1 aftershock. dashed line shows the linear least-squares fit to all of the data. (b) detrended pgah showing scatter in the observed accelerations. (c) pgvh versus distance observed on qcn (circles) and geonet (squares) for the same event shown in (a) (d) detrended pgvh showing scatter in the observed velocities. average rms scatter is given in table 2. network # obs pga rms pgv rms geonet 19 0.475 0.638 qcn 44 0.466 0.518 table 2. rms scatter for event 3391440. small-scale array comparison at the time of the ramp deployment, several qcn stations were installed within 1 km of existing geonet stations. we examine records from an array of six qcn stations installed near geonet station shlc, located in the christchurch suburb of shirley (figure 4). all of the qcn stations were located between 0.3 km and 1 km away from station shlc. shlc is installed on the ground floor of the shirley library, which is located in a single story building constructed in 1996. the six qcn stations were installed primarily in singlefamily residences constructed of either wood or brick (table 3). at the time of station installation any damage related to the september 3, 2010, m7.1 darfield mainshock was noted; these residences reported little to no structural damage. cochran et al. 733 network station latitude longitude dist. (m) installation description geonet shlc –43.507 172.663 n/a ground floor, shirley library, single story, commercial qcn 407 –43.505 172.661 318 ground floor, wood construction, residence qcn 313 –43.510 172.664 319 ground floor, wood construction, residence qcn 365 –43.503 172.671 817 ground floor, brick with wood frame construction, residence qcn 390 –43.510 172.672 827 ground floor, brick construction with elevated floor, residence qcn 321 –43.508 172.652 886 ground floor, concrete, commercial qcn 326 –43.515 172.660 982 ground floor, wood frame on concrete pile, residence table 3. location and installation data for the qcn array near the geonet station shlc. figure 4. map of the small-scale array of six qcn stations (circles) installed near the geonet station shlc (square). the location of the map is shown in figure 1. additional information about each of the stations is given in table 3. 734 figure 5 shows acceleration, velocity, and displacement records from one geonet and the two closest qcn stations that recorded the october 18, 2011, m5.1 aftershock. acceleration records are integrated to recover velocity and displacement time series, as outlined in the section above. acceleration, velocity, and displacement waveforms are qualitatively similar in amplitude and frequency content. some small-scale variability exists, with qcn stations 313 and 390 reporting somewhat higher accelerations than observed at geonet station shlc (figure 5). we note that the vertical displacement records of all three stations are noisy and include spurious signals after the main p-wave and s-wave phases. this noise is likely because the vertical accelerations are approximately an order of magnitude lower than the horizontal accelerations. the displacement records of the lower-resolution qcn sensors are not obviously degraded compared to the geonet shlc displacement record. next, we compare the response spectra computed for shlc with closely located qcn stations in the small-scale array; the response spectrum is an important metric in engineering applications to estimate effects of earthquakes on the built environment [e.g. nigam and jennings 1969, campbell and bozorgnia 2008] and to explore the spatial variability in ground motions [e.g. field and hough 1997]. we compute the pseudo acceleration response spectra at each station in the small-scale array for four earthquakes (events 3374803, 3376639, 3382676, and 3388384, see table 1) recorded by at least 4 of the 6 qcn stations in the array. response spectra were computed for periods between 0.05 and 20 seconds, with 5% critical damping, for 15-second time series segments starting one second before the p-wave arrival and extending well into the s-wave coda. for each event, we compare the response spectra at each station, averaged over the two horizontal components (figure 6). a simple gain correction was applied to the data but full instrument response corrections were not applied. the instrument responses of the qcn stations are unknown, but the response is expected to be flat in the frequency range examined [e.g. holland et al. 2003, hons et al. 2008, zhao and xiong 2009]. while there is some variation between events, the horizontal component response spectra for all stations tend to show peak accelerations for periods between 0.1 and 5 seconds. and, when we cross-correlate the qcn response spectra with geonet station shlc we find crosslow cost mems strong motion data figure 5. (a) acceleration, (b) velocity, and (c) displacement time series for the north-south, east-west, and vertical components for stations shlc, 313, and 390. note that the vertical component ground motions are approximately one order of magnitude smaller than the horizontal ground motions. correlation coefficients between 0.95 and 0.99. the pseudo accelerations appear to differ most significantly at longer periods (>10 seconds), with shlc reporting lower accelerations than qcn stations for these periods. discussion we compare peak ground motion and full waveform data recorded by existing geonet stations and quake-catcher network stations deployed following the september 3, 2010, m7.1 darfield earthquake. the peak ground accelerations, velocities, and displacements observed by the networks are comparable and show a clear decay with distance due to attenuation and geometric spreading. there is significant scatter in the observed ground motions at a given distance from the source, which could reflect actual variability in the ground motion, or may be due to spurious effects. source effects (e.g. radiation pattern), propagation through different geologic material, site amplification, or building response can result highly variable ground motion. but, scatter could also be due to poor instrumentation performance (e.g. incorrect calibration or sensor malfunction) or inadequate coupling to ground motion (e.g. sensors incorrectly affixed to the ground). we find that observed pga and pgv amplitudes and rms scatter are comparable between the geonet and qcn observations, suggesting that the qcn installations provide adequate sensor performance and coupling to ground motion. using data from a local qcn array installed near geonet station shlc, we show that the acceleration, velocity, and three-component displacement time series were comparable (figure 5). we show the response spectra computed for qcn and geonet stations are also similar suggesting the data could be used to explore spatial variability in ground motions and for use in analysis of structures. and, across the seven-station array the horizontal component pseudo acceleration response spectra were cochran et al. 735 figure 6. horizontal component average pseudo acceleration response spectra for the geonet and qcn stations shown in figure 4. response spectra were computed for earthquakes (a) 3374803, (b) 3376639, (c) 3382676, and (d) 3388384 (see table 1 for more information). cross-correlation coefficients are computed between the qcn stations and geonet station shlc and reported in the keys. 736 highly correlated, with average cross-correlation coefficients of 0.94 or higher for station pairs located at distances up to 1.7 km apart. however, the responses diverge somewhat for periods above 10 seconds (< 0.1 hz) with the geonet station shlc showing consistently lower spectral accelerations than qcn stations. this difference may either be the result of structural response convolved into the accelerogram, different instrument responses at these periods, or because of poor reliability of the low-resolution qcn sensors at lower frequencies. further study is needed to determine which of these factors has the largest influence on the computed response spectra. we have shown that the quake-catcher network stations perform comparably to the traditional geonet strong motion stations in a large-scale aftershock deployment in christchurch, new zealand following the m7.1 darfield mainshock. by utilizing mems sensors and distributed computing techniques it is possible to provide reliable records of strong ground motion at greatly increased station densities. data and sharing resources • geonet strong-motion time-series data used in this study are available at ftp://ftp.geonet.org.nz/strong/processed/ proc. ‘vol1’ data was used in this analysis. • qcn strong-motion data are available by request (http://qcn.stanford.edu, ecochran@usgs.gov). • qcn seismic monitoring software is run on the berkeley open infrastructure for network computing (boinc) (http://boinc.berkeley.edu) [anderson 2004]. • data processing was completed using seismic analysis code (sac) version 101.1 (www.iris.edu/software/sac) [goldstein et al. 2003, goldstein and snoke 2005]. • some data analysis was completed using mathworks matlab version 7.9.0 (r2009b). • figure 1 was generated using generic mapping tools version 4.2.1 (www.soest.hawaii.edu/gmt) [wessel and smith 1998] with topographic data from geographx (www. geographx.co.nz). • figure 5 was generated using google earth version 6.0.3 (earth.google.com). references anderson, d.p. 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now at: u.s. geological survey, usa; email: ecochran@usgs.gov. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. cochran et al. 737 annals n.6/2003 ok 23/04 1325 annals of geophysics, vol. 46, n. 6, december 2003 key words geomagnetic survey – sub-surface structures – aeromagnetic data – seismic lines 1. introduction the studied area lies in the southern part of the sinai peninsula, gulf of suez and northern part of the eastern desert (fig. 1) between latitudes 27°50′ and 30°05′ north and longitudes 32°48′ and 34°23′ east covering an area of about 24 000 km2. it is a tertiary cratonic rift south of egypt’s sinai peninsula between north-eastern africa and the arabian peninsula. the rifting essentially ceased (early-middle miocene, 18-14 myr) when continental separation became more oblique because of the dominant movement on the left-lateral transform fault that extends through the gulf of aqaba north-eastward to the dead sea (patton et al., 1994; lindquist, 1998). the tectonics of this area has been constructed using different geophysical tools (e.g., aeromagnetic, land magnetic gravity, and seismic). these tectonics were analyzed from deep depths to show depths along the studied area. 2. geology of the area the red sea, gulf of suez basins consists of a high faulted block more than 100 000 km2 in size that contains block-faulted trapping configurations in the gulf of suez. this unit instructural set-up of southern sinai and gulf of suez areas indicated by geophysical data taha rabeh national research institute of astronomy and geophysics (nriag), geophysical department, helwan, cairo, egypt abstract this work deals with the evaluation of the structural set-up of the southern part of sinai peninsula, gulf of suez and western part of gulf of suez from magnetic, gravity, and seismic data. the utilised techniques including the least squares separation method suggest nw, ne, and e-w trends. the trend analysis shows north 35°-45° west, north 15°-25° east and e-w which may be related to the gulf of suez and red sea stresses.the euler deconvolution illustrates that the area is highly affected by these trends. depths range from 1 km to more than 3 km below sea level and its magnetic susceptibility ranges between 1 to 3 si units. the 2.5d magnetic modelling and analytical signal techniques confirm the depths to the magnetic sources deduced by the euler method, whereas the depth to the basement rocks ranges between 0 km to about 3 km indicating that it is subjected to strong tectonic activities. in addition, two seismic sections (eg-31 and mp-70), compiled by the egyptian general petroleum cooperation (egpc), were interpreted together with a geologic cross section. the studied area may be divided into several major blocks along the gulf of suez area. it can be concluded that the sedimentary was affected by basement tectonics as revealed by the two seismic sections. mailing address: dr. taha rabeh, national research institute of astronomy and geophysics (nriag), geophysical department, helwan, cairo, egypt; e-mail: taharabeh@yahoo.com 1326 taha rabeh cludes coastal areas of the red sea and the southern gulf of suez. the geology of the area ranges from precambrian basement rocks to the quaternary deposits. according to the egyptian geological survey map (egsma, 1993) (fig. 2), the quaternary deposits cover the basement rocks along the gulf of suez and gulf of aqaba. the basement rocks (precambrian) occupy the southern part of the studied area, along the gulf of suez and gulf of aqaba. on the other hand, the palaeozoic rocks are present in the middle part while the mesozoic rocks covered the eastern and western parts of the area. furthermore, a geologic cross section was constructed (fig. 4), using the most updated borehole data. this section was performed crossing gulf of suez taking a northeast-southwest direction. it indicates that the depth to the basement rocks varies from 0 km to more than 3 km. the basement rocks consist of majorfig. 1. location map of the studied area. fig. 2. geological map of the studied area (egsma, 1993). 1327 structural set-up of southern sinai and gulf of suez areas indicated by geophysical data fig. 3. location of the drilled wells, the two seismic lines and profiles of land magnetic survey (l-l′). fig. 4. geologic cross section along profile g-g′ crossing the gulf of suez. 1328 taha rabeh fig. 5. rtp aeromagnetic anomaly map of the studied area (egpc, 1990). fig. 6. bouguer anomaly map of the studied area (after iprg, ca. 1982). fig. 7. least square residual rtp aeromagnetic map (2nd order). fig. 8. least square residual bouguer anomaly map (2nd order). 1329 structural set-up of southern sinai and gulf of suez areas indicated by geophysical data the input magnetic and gravity data. this method was applied using the software surfer v. 7.00 (surfer, 1997). the correlation coefficients between successive residual maps were computed to determine the optimum order of the regional surface to be used. results were r21 = 0.854, r21 = 0.96 and r34 = 0.546. (4.1) according to davis (1973), a correlation factor higher than 0.7 indicates a good similarity between maps, the regional field in this area can be represented by a second-order surface, and the polynomial used has the form (x, y)2 = − 7.947 − 0.4778y + 0.065x + + 0.08144y − 0.03575xy − 0.002528x. (4.2) the resulting least-squared residual maps of second-order (figs. 7 and 8) show that most of the anomalies are the same trends trending north west-south east, north east-south west and eastwest directions. also, it can be concluded that most of these anomalies are related to different blocks with different polarities and structures. furthermore these maps represent the best fitted maps to the original rtp aeromagnetic and bouguer anomaly maps. 5. trend analysis technique a two lineaments map (fig. 9) was constructed from application of the trend analysis technique (grant et al., 1965; parasnis, 1997) for the rtp aeromagnetic and bouguer anomalies maps. this map was subjected to statistical calculating for the lengths percentages of the faults measured clockwise from the north. the deduced lineation of different directions are grouped every 10° around the north for their length percentage (l%). a statistical procedure was used to illustrate the predominant fault trends affecting the studied area. the results of azimuth distribution of both surface and subsurface elements are presented in the form of a rose diagram (fig. 10). the results show that the most predominant direction is north 35°-45° west, north 45°-65° east, eastwest and aqaba trends. it is clear that the main faulted blocks along the gulf of suez. the depth to the carrier reservoir bed (belayim fm.) varies from about 2 km to about 3 km. 3. data analysis and interpretations the land magnetic survey was performed using two magnetic protons of geometric type. one instrument is used as a base station for diurnal corrections for each profile, and the second is used to measure the observed magnetic data. the land survey was performed in the form of profiles along the gulf of suez area, (fig. 3). the distance between the stations ranges from 200 to 300 m according to the changes in the recorded geomagnetic data. the aeromagnetic map was prepared by the egpc (1990) with a flight elevation of 1 km. the aeromagnetic data was digitized into a computer using a suitable surfer program v. 8.00 (surfer, 2002). both land magnetic data and the aeromagnetic data were reduced to the north magnetic pole (hansen and pawlowski, 1989), to obtain the rtp magnetic anomaly map (fig. 5). both the rtp aeromagnetic (fig. 5) and bouguer anomaly maps (fig. 6) indicate that most anomalies are aligned to nw, ne, and ew which may be related to the gulf of suez, and gulf of aqaba. the negative anomalies in the central part may be due to lithology variation of the basement. the bouguer anomaly map (iprg, ca. 1982) was digitized and contoured at 1 mgal (fig. 6). this map conforms to the observed rtp anomalies (fig. 5). it reveals a gravity lows in the central and southern parts. these alternative negative and positive anomalies along the gulf of suez may be due to the faulted blocks or presence of different basins with different thickness of sedimentary sequence in the area. this leads to the conclusion that the shallow parts are extending along the two gulfs and southern part of sinai where the basement rocks are outcropping (fig. 2). 4. least squared residual technique the least square method of henderson (1966) was applied to the rtp magnetic bouguer maps using first, second, and third order fitted to 1330 taha rabeh fig. 10. rose diagrams illustrate the distribution of the lengths percentage (l%) of faults against azimuth. fig. 9. tectonic lines as deduced from the rtp aeromagnetic and bouguer anomaly maps. tectonic trend is north 35°-45° west prevailing for the large scale in both the bouguer and rtp aeromagnetic maps. this may be due to a very strong and active force resulting from the opening of the gulf of suez. the second predominant tectonic force is north 45°-65° east that is probably due to the syrian arc system. the least predominant is the north 15°-25° east due to tectonics related to the gulf of aqaba. 6. euler deconvolution method the two-dimensional euler technique is a method that uses the horizontal and vertical derivatives of the total magnetic field to estimate the location and depth to magnetic sources (thompson, 1982; reid et al., 1990). this method has been applied to the magnetic profiles along the area (figs. 11 and 12). applying euler deconvolution to the vertical gradient of the total magnetic field data provides improved source resolution (stavrev, 1997; hsu, 2002) . x z f x x x z f z z z f bsi n n n n n n z 0 0 2 2 2 2 2 2 2 2 2 2 + = = +∆ j l k k e ^ e ^ e n p o o o h o h o (6.1) where n is the order (which is not necessarily an integer (cooper and cowan, 2003)) of the gradient used. it may similarly be applied to the horizontal gradient . x z f x x z x f z z x f bsi n n n n n n x 0 0 2 2 2 2 2 2 2 2 2 2 + = = +∆ j l k k e ^ e ^ e n p o o o h o h o (6.2) in general the magnetic field falls off as r – n with r = (x2 + z2)1/2. the parameter x is the location of the magnetic source along the magnetic profile and z is the depth to the top of the magnetic source (thompson, 1982). the value of n, called the structural index, depends on the shape of the magnetic source. for example, n = 3 corresponds to a three-dimensional body such as a sphere while a 1331 structural set-up of southern sinai and gulf of suez areas indicated by geophysical data fig. 11. euler deconvolution along l1-l1′ of the land magnetic data. fig. 12. euler deconvolution along l3-l3′ of the land magnetic data. 1332 taha rabeh two-dimensional dike corresponds to n = 2. values of n between 1 and 0.5 correspond to two-dimensional faults and contacts. the depth to the structures depends on structural index n and the size of the applied sliding window. the euler deconvolution method has been applied to the aeromagnetic using 0 magnetic contact index: to determine the depths to the irregular magnetic bodies, 0.5 magnetic step index in order to indicate the depth to the basement rocks and their structures and 1 dike index for delineating the depth to the anomalies caused by vertical thin dykes. most the results derived from the euler deconvolution method indicate that area is highly affected by the regional forces related to the gulf of suez, and red sea tectonics. it is indicated by the different position and depths of the sources of the subsurface magnetic bodies (figs. 11 and 12), which consist of dissected bodies along these profiles. also, it can be stated that these magnetic sources are restricted by tectonic trends have nw-se and ne-sw directions (fig. 13). these structures were confirmed by the analytical signal applied to the same profiles. the magnetic susceptibility of the rocks ranges between 1 to 3 si units whereas their depths range between 1 km to about 4 km. these wide ranges in the depth to the subsurface basement rocks are due to the highly faulted areas. 7. application of the analytical signal and depth estimation methods the analytical signal was performed along the same profiles interpreted by the euler method by reid et al. (1990) using nabighian’s technique (nabighian, 1972, 1984). it is applied to confirm the results deduced from the euler method (figs. 12 and 13). the spikes of the analytical signal profile are an indication for the subsurface structures. the presence of one spike indication of subsurface fault structure, and the presence of two closed spikes are due to the presence of subsurface dike, the high amplitude spike for the near edge of the dike and the small amplitude spike is due to the lower edge of the dike. also, the depth to the magnetic sources was applied to the analytical signal using (atchuta et al., 1981; roest et al., 1992). the depth to the magnetic sources equals half of the amplitude of the analytical signal. this method had been applied to amplitudes along the profiles. the results confirm the parameters deduced from the euler method. the depth to these bodies ranges between 0.5 km to more than 3 km. 8. application of 2.5d modelling computations of the magnetic effects for 2.5d models with complex geometry were carried out using the gm-sys computer program produced by northwest geophysical associates inc. (gm-sys, 1995), for the arbitrary two-dimensional polygon. this type of modeling is a reverse model that depends on the changes in the magnetic susceptibility, or the density contrasts of the sedimentary rocks and the depth parameters till complete matching between the calculated and observed profiles (fig. 14). the resulting parameters represent the main target of this method (e.g., depth, magnetic susceptibility, etc.). this 2.5d model confirms the depths to the magnetic sources deduced by the euler method. it can be noticed from (fig. 14) that the depth to the fig. 13. applications of euler deconvolution to the aeromagnetic map. 1333 structural set-up of southern sinai and gulf of suez areas indicated by geophysical data fig. 14. 2.5d modelling along l1-l1′ of the land magnetic profile. fig. 15. the depth to the basement rocks along the gulf of suez area, derived from the wells drilled in the area. 1334 taha rabeh basement rocks ranges between 0 km to about 3 km while the magnetic susceptibility reaches about 0.0005 cgs unit. also, it is obvious that this result is correlated with the results deduced by the euler deconvolution and analytical signal method. 9. depth to the basement rocks using well logging data the depth to the basement rocks along the gulf of suez from north to south direction was traced, using the most recent wells data (egpc, 2000) drilled to the basement rocks in the studied area. figure 15 indicates that the depth to the basement rocks varies widely from less than 2 km at the north western part to more than 3 km at the eastern part of the gulf. this may be due to the controlling structures related to gulf of suez tectonics. these results conform with the rtp magnetic, gravity and geologic crosssection showing the wide variations of the depth along the gulf of suez. thus it is clear that the gulf of suez consists of different faulted blocks forming the uplifted basement, and the deep basins. this emerges from the results fig. 16. interpretation along seismic line eg-31 western side of gulf of suez. 1335 structural set-up of southern sinai and gulf of suez areas indicated by geophysical data of the depth estimation methods, negative and positive gravity anomalies. these structures have a great importance for oil production and exploration processes. 10. seismic interpretations two seismic sections eg-31 and mp-70 were prepared by egpc trending north westsouth east and north east-south west directions respectively (figs. 16 and 17). these lines were correlated with the vertical velocity logs to determine the exact horizons (marker beds) and their depth in the studied area. these seismic sections were used to illustrate the upward continuation and extension of the previously interpreted subsurface structures in the sedimentary sequences. 10.1. interpretation along line eg-31 this line (fig. 16) lies in the western side of the gulf of suez and extends for about 20 km in a northwest-southeast direction. it can fig. 17. interpretation along seismic line mp-70 along gulf of suez. 1336 taha rabeh be noticed that most of the interpreted structures are older than south gharib fm. where they extended upward and crossed the older formations (rudies and kareem formations), whereas the older structures did not extend upward to cross the younger formations (belayim and south gharib formations). the presence of these structures in large numbers indicate that the studied area is affected by active tectonic forces. these structures have been rejuvenated as indicated by their extensions in different time rock units. 10.2. interpretation along line mp-70 the length of this line (fig. 17) reaches about 12 km and extends in a north east-south west direction. this line is affected by four fault lines. it seems that zeit fm. (top of the sedimentary sequence) is affected at its lower surface by the interpreted faults, while the older formations (rudeis, kareem, belayim and south gharib formations) are crossed by these faults. it is clear that the studied area was affected by the tectonics related to opening of the gulf of suez, and the red sea. the faults divide the studied area into several numbers of major blocks. these faults are very important in sealing oil traps (e.g., middle miocene formations). the depth to these blocks ranges from 0 km to about 3 km. 11. discussion and conclusions this study was carried out using different geophysical tools (aeromagnetic, land magnetic, gravity and seismic methods). the data were analysed using the most advanced and suitable techniques. the results of tectonic trend analysis indicate that most of the predominant directions are north 35°-45° west direction that related to gulf of suez and red sea tectonics, and the least predominant direction is north 10°-25° east, that related to gulf of aqaba tectonics. the application of euler deconvolution with a suitable sliding window to estimate the position of the subsurface intruded magnetic bodies indicates that the depth to these bodies ranges from 0 km to about 3 km and the magnetic susceptibility ranges between 1 to 3 si units. furthermore, application of the analytical signal method confirms the result deduced by the euler deconvolution technique. the 2.5d technique prints out a complete view for the geometry of the basement surface where the depth ranges between 0 km to more than 3 km. for the upper investigations of the sedimentary sequence, a geological cross-section was constructed using the well logging data drilled in the studied area as well as two seismic lines. the results indicate that the structures of the area are related to gulf of suez, and red sea tectonics. these structures extend from the basement rocks upward into the sedimentary sequences and divide the area into several major faulted blocks that have great important in sealing the oil traps. this work is considered a guide line for further exploration process and geophysical works. references atchuta rao, d., h.v. ran babu and p.v. sanker narayan (1981): interpretation of magnetic anomalies due to dikes. the complex gradient method, geophysics, 46, 1572-1578. cooper, g.r.j. and d.r. cowan (2003): application of fractional calculus to potential field data, explor. geophys., 34, 51-56. davis, j.c. (1973): statistical and data analysis in geology (jhon wily and sons inc.), pp. 550. egpc (egyptian general petroleum cooperation) (1990): aeromagnetic map for sinai peninsula (scale 1:100 000). egsma (egyptian geological survey and mining authority) (1993): geologic map sinai, egypt (scale 1:100 000). gm-sys (1995): gm-sys®, gravity and magnetic modelling version 3.6 (northwest geophysical association inc., corvallis, oregon, u.s.a.). grant, f.s. and g.f. west (1965): interpretation theory in applied geophysics (mcgraw-hill book co., new york), 179-191. hansen, r.o. and r.s. pawlowski (1989): reduction to the pole at low latitudes by weiner filtering, geophysics, 54, 1607-1613. henderson, r.g. (1966): a comprehensive system of automatic computer in magnetic and gravity interpretation, geophysics, 25, 569-585. hsu, s.-k. (2002): imaging magnetic sources using euler’s equation, geophys. prospect., 50, 15-25. iprg (institute for petroleum research and geophysics) (ca. 1982): bouguer anomaly map of sinai (scale 1:100 000). 1337 structural set-up of southern sinai and gulf of suez areas indicated by geophysical data lindquist, s.j. (1998): the red sea basin province: sudr-nubia(!) and maqna(!) petroleum systems, u.s. geol. surv. open-file rep. 99-50-a. nabighian, m.n. (1972): additional comment of the analytic signal of two-dimensional magnetic bodies with polygonal cross-section, geophysics, 39, 85-92. nabighian, m.n. (1984): toward the three-dimensional automatic interpretation of potential field data via generalized hilbert transforms. fundamental relations, geophysics, 53, 957-966. parasnis, d.s. (1997): principles of applied geophysics (chapman and hall publisher, london, u.k.), 5th edition, 21-25. patton, t.l., a.r. moustafa, r.a. nelson and s.a. abdine (1994): tectonic evolution and structural setting of the suez rift, in interior rift basins: tulsa, ok, edited by s.m. landon, am. ass. pet. geol. mem., 59, 9-55. reid, a.b., j.m. allsop, h. granser, a.j. millett and i.w. somerton (1990): magnetic interpretation in three dimensions using euler deconvolution, geophysics, 55 (1), 80-91. roest, w.e., j. verhoef and m. pilkington (1992): magnetic interpretation using 3d analytic signal, geophysics, 57, 116-125. stavrev, p.y. (1997): euler deconvolution using differential similarity transformations of gravity or magnetic anomalies, geophys. prospect., 45, 207-246. surfer (1999): surfer® version 7.00, feb 28, 2000 (golden software inc., golden, co, u.s.a.). surfer (2002): surfer® version 8.00, feb 11, 2002 (golden software inc., golden, co, u.s.a.). thompson, d.t. (1982): euldph – a new technique for making computer-assisted depth estimates from magnetic data, geophysics, 47, 31-37. (received june 17, 2003; accepted december 3, 2003) sulla trasformazione delle curve di sondaggio di resistività differenziale trasversa in curve di schlumberger (on the transformation of transverse differential resistivity to schlumberger sounding curves) d . p a t e l l a ( * ) d . schiavone ( * * ) r i c e v u t o il 22 dicembre, 1973 riassunto. — si descrive un metodo per trasformare una curva di sondaggio differenziale trasverso, ottenuta con un dispositivo elettrodico differenziale, nella curva schlumberger relativa alla stessa situazione stratigrafica. p a r t e n d o dall'espressione generale che lega la resistività apparente differenziale a quella schlumberger, è possibile, con alcune approssimazioni, derivare un facile procedimento numerico per poter compiere la trasformazione. v e n g o n o inoltre f a t t e alcuno considerazioni sull'applicabilità del m e t o d o . summary. -— a method is described to transform a transverse differential sounding curve to the f o r m of a schlumberger sounding curve. starting f r o m the general expression which relates the transverse differential apparent resistivity to the schlumberger apparent resistivity, it is possible, with some approximations, to derive an easy numerical procedure in order to carry out the transformation. some considerations are g i v e n about the applicability of the method. 1. introduzione i l d i s p o s i t i v o d i f f e r e n z i a l e p u ò essere e f f i c a c e m e n t e a d o p e r a t o q u a n d o s o n o r i c h i e s t i , i n p r o b l e m i d i p r o s p e z i o n e g e o e l e t t r i c a , u n m a g g i o r e p o t e r e r i s o l u t i v o e d u n a m a g g i o r e p r o f o n d i t à d ' i n d a g i n e ris p e t t o a i c o n v e n z i o n a l i d i s p o s i t i v i e l e t t r o d i c i . ( * ) osservatorio di geofisica e fisica cosmica università di bari. ( * * ) istituto di geodesia e geofisica università di bari. 70 i). patella d. sciiiavone a questo scopo sono stati proposti due metodi di sondaggio differenziale: a) il sondaggio differenziale longitudinale (lds longitudinal differential sounding) (3); b) il sondaggio differenziale trasverso (tds transverse differential sounding) (4). i l dispositivo elettrodico in entrambi i metodi è lo stesso (fig. 1), ma è differente la tecnica di misura per ottenere le corrispondenti resistività apparenti. l e relazioni che definiscono le resistività apparenti longitudinale q e trasversa <?( ( sono date da (4) _ = d l qi d (l/qs) qs — lìq./dl l ' j d(p,£) i eta = f i = & + l ^ s l ò l [ 1 " 2 ] dove q, è la resistività apparente schlumberger ed l la semidistanza tra gli elettrodi di corrente nel dispositivo di schlumberger. i valori di qj ( e n^ possono essere espressi in quantità che sono facilmente misurabili in campagna usando l'espressione generale che definisce gs. dalle [1.1] e [1.2] si ottiene (v. appendice) ^ kl a via vz e la a zi v2 l 1 d j qìa ~ ^ (avz-avr) [1.4] essendo kl e kt i fattori geometrici rispettivamente per l ' l d s ed il t d s ; a vi la d.d.p. generata dalla corrente h che fluisce tra gli elettrodi interni a e b ; zi v2 quella generata dalla corrente 72 tra gli elettrodi esterni a ' e b ' (v. fig. 1). dall'analisi della [1.3] risulta evidente che per valutare è / j ® n e " cessario misurare separatamente a vi e a v2, ottenendo così anche due valori di 0, relativi alle disposizioni a m n b e b ' m n a ' . in pratica, quindi, quando si effettua un sondaggio l d b (*), automaticamente si porta a termine un sondaggio schlumberger. ( * ) l ü r longitudinal differential resistivity. t u r transverse differential resistivity. s u i . l a t r a s f o r m a z i o n e d e l l e c u r v e d i s o n d a g g i o e t c . 71 per valutare invece dalla [1.4] è sufficiente la sola misura di a(av) = a vi — a vi che può essere effettuata direttamente mediante l'invio simultaneo nel terreno delle correnti h e i2. con questo modo di procedere si riducono gli errori introdotti dalla misura separata di a vi e a v2 e si ottiene inoltre un notevole risparmio di tempo nelle operazioni di campagna. pertanto un sondaggio t d r non è necessariamente accompagnato da un sondaggio schlumberger. f i g . 1 dispositivo elettrodico differenziale. 2. interpretazione quantitativa dei sondaggi differenziali l'interpretazione quantitativa dei sondaggi l d r e t d r è stata sinora condotta col metodo usuale di confronto con curve teoriche. per la costruzione di queste ultime sono utilizzabili tre metodi (4): a) calcolo delle curve teoriche sulla base delle espressioni matematiche che forniscono g .̂-i e qta f u n z i ° n e di l. questo metodo è il più naturale ma, come è noto, richiede una gran mole di lavoro e l'uso del calcolatore elettronico; b) calcolo delle curve teoriche l d r e t d r trasformando in esse quelle di schlumberger sulla base delle relazioni [ 1 . 1 ] e [ 1 . 2 ] , tale trasformazione è possibile solo per quelle situazioni per le quali le curve schlumberger sono state calcolate; e) determinazione di gl / 1 e gt t combinando opportunamente i valori di gs, gr, gy e gx, essendo gr, g,j e qx le resistività apparenti per il dispositivo dipolare radiale, perpendicolare e parallelo. a parte la scarsa praticità, quest'ultimo metodo è in realtà possibile solo per un numero molto limitato di modelli stratigrafici per i quali sono disponibili le curve teoriche dipolari. 7 2 i). p a t e l l a d . s c i i i a v o n e da quanto detto segue che sarebbe di grande interesse disporre di un metodo che permetta di trasformare una curva di resistività differenziale nella forma che si sarebbe ottenuta con il dispositivo di schlumberger, senza introdurre variazioni alla normale tecnica di campagna. i l vantaggio derivante da una tale trasformazione è notevole: in tal modo sarebbe possibile in più utilizzare direttamente tutti i metodi attualmente esistenti per l'interpretazione delle curve di schlumberger. questo problema non si pone nel caso di sondaggi l d r . infatti poiché, come si è detto, si ottiene necessariamente la corrispondente curva schlumberger, è sufficiente interpretare quantitativamente quest'ultima sulla base delle informazioni qualitative che si deducono dall'analisi della curva l d r . la nostra attenzione è perciò rivolta solo alla possibile trasformazione della curva t d r nella corrispondente curva schlumberger. i l metodo che descriviamo presenta analogia con quello già usato per la trasformazione delle curve dipolari in schlumberger (2). 3. t e o r i a della trasformazione delle curve t d r i n curve schlumberger riconsideriamo la relazione [1.2] questa può essere trattata come un'equazione differenziale lineare del prim'ordine la cui soluzione generale è data da la costante e deve essere identicamente nulla. pertanto la [3.1] diventa in pratica, affinché si possa eseguire facilmente l'integrazione al secondo membro della [3.2], procediamo come segue: ì t a = e> + lì>qsi~ì>l. qs = 6 i ì l i l ( i etdl/l ùljl cy affinché sia sempre soddisfatta la condizione lim qs ^ oo, l-> o [3.1] [3.2] s u i . l a t r a s f o r m a z i o n e d e l l e c u r v e d i s o n d a g g i o e t c . 73 a) dividiamo l'intero intervallo 0 < l < oo in una serie di intervalli parziali contigui, di ampiezze in generale differenti tra di loro, del tipo h < l < lni [3.3] con j = 1, 2, 3, ,n — 1; b) interpoliamo in ogni intervallo parziale la funzione con l'espressione algebrica quj (l) = a,lbi [3.4] che, in scala logaritmica, rappresenta l'equazione di una retta. imponiamo ora la seguente condizione: in ogni punto lj sull'asse positivo l il valore q (lj) sia noto. tale condizione è, in campagna, ovviamente soddisfatta qualunque sia il numero di punti lj, poiché gt | (lj) è la resistività apparente misurata ad ogni predeterminata spaziatura lj. nessuna condizione restrittiva viene invece imposta sulla scelta degli intervalli parziali del tipo [3.3]; è, comunque, consigliabile sceglierli i più piccoli possibili, compatibilmente con le difficoltà di campagna, in modo da migliorare l'approssimazione data dalla [3.4]. ponendo la [3.4] nella [3.2] ed integrando, si ottiene in ogni intervallo parziale è' = ( j 1 bj ) aj lb' + aikjl [3.5] dove li è una nuova costante derivante dall'integrazione e necessaria a soddisfare la condizione che in ogni punto comune a due intervalli parziali contigui qs deve essere continua. se poniamo cj = djk [3.6] si ottiene = ( i + b , ) a i l b l + c l l l [ 3 , 7 ] che più concisamente può essere riscritta = ? a t + cj/l [3.8] valida per lj < l < lj+i e con j = 1, 2 , . . . ., n — 1. nel punto lj si ha qs(lj) = c 1 ^ b] ] eta (h) + cj/lj [3.9] 72 i). patella d. sciiiavone nella [3.9] li e gt (lj) sono entrambe quantità note; per ottenere, quindi, il valore di q,(ii) basta determinare bj e cj. riferendoci a bi, è sufficiente considerare che, dalle ipotesi di partenza, risulta log o m (h) = log ai + bi log l, [3.10] log ota (h+1) = log ai + bj log h+i risolvendo il sistema [3.10] nelle incognite log ai e bh si ottiene qn (h+0 log b, = e,a ( l l ) log l, [3.11] riferendoci ora a ci, procediamo come segue: consideriamo l'intervallo li-1 < l < li, contiguo e precedente lj < l ^ lj+i, dove è [3.12] per la condizione di continuità di q, in li segue che ( 1 + j a ' l ' b j + c'll> = ( , l ' b 1 1 + ^ da quest'ultima si ottiene la seguente formula di ricorrenza a = cj-i — y i + (si-1 [3.14] dove >" = ( t w ) ^ [3.15] [3.16] mediante la [3.14] è possibile ottenere ci conoscendo ci-1 e così via di seguito, essendo fti-1 e yi quantità note per ogni j. in pratica è sufficiente trovare il primo valore c0 per ottenere tutti i successivi cj. 4. d e t e r m i n a z i o n e delle costanti c] poiché è lini q. (l) = gì. [4.1] s u i . l a t r a s f o r m a z i o n e d e l l e c u r v e d i s o n d a g g i o e t c . 7 5 con qi la resistività del terreno superficiale, è sempre possibile trovare un punto h, sufficientemente piccolo, per cui si possa ritenere e t a (m pi[4.2] ne consegue che qta ( l ) ^ qi [4.3] nell'intero intervallo 0 ^ l ^ h. ponendo la [4.3] nella [3.2] ed integrando, si ha qs(l) = q l + c„/i [4.4] per 0 < l < h. dalla [4.4] segue che lim q,(l) — gì [4.5] se e solo se co = 0. [4.6] perciò, partendo dalla [4.6] è possibile ottenere tutti i successivi cj usando la formula di ricorrenza [3.14]. in pratica, per ottenere una curva di campagna di trasformabile nell'equivalente in gs, è necessario determinare direttamente in campagna l'andamento asintotico a sinistra della curva, o, quando ciò è difficoltoso, eseguire un'estrapolazione a sinistra della curva che rappresenti il più probabile andamento asintotico. 5 . e s e m p i t e o r i c i d i t r a s f o r m a z i o n e mostriamo ora alcuni esempi di trasformazioni eseguite su curve teoriche di . riportate da zohdy (4) e tutte riferite a modelli di terreni a tre strati orizzontali e lateralmente omogenei. a) sezione geoelettrica di tifo h la lig. 2 riporta la trasformazione della curva teorica di . per una sezione di tipo h con i seguenti parametri qì = 1 , 02 = 0.4, £>3 = 1; ^ = !> ^ = 1 57 6 i>. p a t e l l a d . s c i i i a v o n e t a b e l l a i ( * ) h qta (h) b, pi n c¡ q, (l¡) 0.3 1.000 0.0000 0.4000 0.3000 0.0000 1.000 0.4 1.000 —0.2299 0.6168 0.5194 —0.1194 1.000 0.5 0.950 —0.2965 0.7676 0.6752 —0.1778 0.995 0.6 0.900 —0.2815 0.9241 0.7516 —0.1618 0.983 0.8 0.830 —0.5144 1.5239 1.3674 —0.6051 0.953 1 0.740 —0.8827 6.4450 6.3086 —5.3898 0.919 1.2 0.630 — 1.2477 —2.8421 —3.0521 4.1073 0.879 1.6 0.440 — 1.3576 — 1.8177 — 1.9687 3.2339 0.791 2 0.325 — 1.0870 —7.3276 —7.4713 8.8875 0.708 2.5 0.255 —0.3325 1.0787 0.9551 0.6048 0.624 3 0.240 0.4478 0.7542 0.4973 1.1862 0.561 4 0.273 0.5696 0.9875 0.6957 1.2447 0.500 5 0.310 0.5867 1.3046 0.9769 1.2553 0.446 6 0.345 0.4262 2.1876 1.4514 1.1085 0.427 8 0.390 0.4376 2.9911 2.1703 1.1258 0.412 10 0.430 0.3699 4.0295 3.1389 0.9780 0.412 12 0.460 0.4262 5.8337 3.8704 1.1371 0.417 16 0.520 0.5660 7.5351 5.3129 1.6579 0.436 20 0.590 0.5698 10.6701 7.5169 1.6761 0.460 25 0.670 0.5450 14.3689 10.8414 1.5048 0.494 30 0.740 0.4817 22.9466 14.9828 0.8909 0.529 40 0.850 0.3546 33.9584 25.0997 —1.2622 0.596 50 0.920 0.4573 41.1720 31.5652 1.1310 0.654 60 1.000 0.1027 74.7257 54.4119 — 12.1089 0.705 80 1.030 0.0862 96.6673 75.8608 — 13.2440 0.783 100 1.050 —0.1322 141.7377 120.9956 —37.5723 0.834 120 1.025 —0.0341 168.1333 127.3424 —23.1770 0.868 160 1.015 —0.0221 206.5651 166.0702 —21.1 139 0.906 200 1.010 —0.0446 261.6705 211.4298 —25.9786 0.927 ( * ) esempio numerico di trasformazione di una curva teorica di resistività differenziale trasversa nella corrispondente curva di sehlumbergcr. l e colonne riportano i valori di (l¡) in corrispondenza di prefissati valori l) della spaziatura l-, quindi i dati di b¡, fij, y¡, c¡ e qs (l¡) ottenuti rispettivamente dalle relazioni [3.11], [3.16], [3.15], [3.14] e [3.9]. s u i . l a t r a s f o r m a z i o n e d e l l e c u r v e d i s o n d a g g i o e t c . 75 i dati di g,(li), ottenuti dalla trasformazione e riportati nella tab. 1, sono visibili nella stessa fig. 2 congiuntamente alla curva teorica di schlumberger, relativa allo stesso modello e prelevata da orellana e mooney f1). come si può notare, i punti trasformati si dispongono perfettamente su quest'ultima. l i . t a fig. 2 esempio di trasformazione per una sezione di tipo e . ( • ) valori di ô ̂ ; ( • ) valori di qs ottenuti dalla trasformazione; ( ) curva teorica di schlumberger. b) sezione geoelettrica di tipo e la fig. 3 mostra la trasformazione della curva teorica di g ^ per la situazione qi — 1, qa = 5, q3 = 1; hi = 1, 7(2 = 25. c) sezione geoelettrica di tipo a la fig. -1 mostra la trasformazione della curva teorica di gf ì per la situazione o! = 1, g, = 5, g3 = 20; hi = 1, h = 25. fig. 3 esempio di trasformazione per una sezione di tipo k. per il significato dei simboli v. fig. 2. in più ( • ) rappresenta valori negativi di qf j. ps fig. 4 esempio di trasformazione per una sezione di tipo a . per il significato dei simboli v. fig. 2. s u i . l a t r a s f o r m a z i o n e d e l l e c u r v e di s o n d a g g i o e t c . 79 d) sezione geoelettrica di tipo q infine la iig. 5 mostra la trasformazione della curva teorica di qt/i con parametri oi = 1 , q2 = 0 . 4 , o3 = 0 . 2 ; hi = 1 , h2 — 1 5 . l o . i i fig. 5 esempio di trasformazione per una sezione di tipo q. p e r il significato dei simboli v . fìg. 2. 6. conclusioni a) da quanto si è detto, risulta che la trasformazione di una curva di ¡5 nella forma che si sarebbe ottenuta con il dispositivo di schlumberger, è sempre possibile purché si faccia una determinazione accurata della resistività vera dello strato superficiale. per il suo elevato potere risolutivo, un sondaggio di resistività differenziale trasverso dà la possibilità di ottenere una buona ed esauriente interpretazione qualitativa della struttura del sottosuolo. la susseguente trasformazione in curva di schlumberger permette di compiere, senza dover costruire apposite curve teoriche di /() quella interpretazione quantitativa che meglio è in accordo con la precedente analisi qualitativa. 80 i ) . p a t e l l a d. s c i i i a v o n e b) rispetto al dispositivo di schlumberger, quello differenziale ha lo svantaggio (4) di richiedere una maggiore energia, a parità di stendimento, per ottenere un valore misurabile di a(av). ciò può provocare serie difficoltà durante un intero sondaggio. se, comunque, la necessità di ottenere informazioni dettagliate è ristretta a strutture non molto profonde, questo metodo di trasformazione permette di iniziare un sondaggio col dispositivo differenziale e di finirlo con lo schlumberger. la trasformazione della prima parte del grafico è ancora possibile con il risultato di ottenere un'unica curva di schlumberger. a p p e n d i c e se il rapporto l ì j l i (v. fig. 1) è sufficientemente prossimo ad uno, allora le relazioni [1.1] e [1.2] possono essere approssimate come segue = 1_ „ i n i n ^la a{liqs) l2iqs2-—li/qh al-+o ò.. = lim a (osl)ial ~ (os2£2— qsili)l(lo— li) [a.2] dove qa e qs2 sono le resistività apparenti schlumberger per le configurazioni a m n b e b'mista' rispettivamente (v. fig. 1) e date da 7j.i = eia vi ih [a. 3] 0 , 2 = a v j v2ii2 [ a . 4 ] sostituendo le [a.3] e [a.4] nelle [ a . l ] e [a.2] si ottiene ki(l2 — li) av1av2 " " ~ldi i2 l2k1 av — [a"5] r „ av1—av2 h l1k2 k1l1 / k2l2 h m i " ™ ) ^ {l2—l1) il \ kili i2 quando il rapporto delle correnti h e i2 è scelto in maniera tale che per l ' l d r sia h ¡li = l2k1il1k2 [ a . 7 ] sui.la t r a s f o r m a z i o n e d e l l e c u r v e d i s o n d a g g i o e t c . 81 e per il t d r hju = ljûilzez [a.8] le [a.5] e [a.6] diventano kx(l2 — li) 1 a vi a fa e laly h a f i — a f 2 [ a . 9 ] 0 ~ k i l l 1 'la jj2 _ li l 1 (a vi. — a f i ) h [a.10] ponendo, infine, kl = ki(li — l,)jlì kt = kilii(l2 — i i ) [a. 11] [a. 12] si ottengono dalle [a.9] e [a. 10] le [1.3] e [1.4] riportate precedentemente. ( ' ) orellana e., mooney i f . m., 1960. master tables anil curves for vertical electrical sounding over layered structures. madrid, interciencia, 150 p p . and 00 tables. (-) patella i)., 1973. on the transformation of dipole to schlumberger sounding curves. p a p e r presented at the thirty-fifth m e e t i n g of the european association of exploration geophysicists, held at brighton, 5t h-8th june, 1973. ( 3 ) rabinovicii b . i.. 1905. fundamentals of hie method of field difference. « p r i k l a d n a y a geofizika », 43, p p . 47-59. ( 4 ) zoiidy a . a . r . , 1909. a new method for differential resistivity sounding. « geophysics », 34, p p . 924-43. b i b l i o g r a f i a g annals 47, 6, 2004def 1783 annals of geophysics, vol. 47, n. 6, december 2004 mailing address: dr. michael pezzopane, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: pezzopane@ingv.it key words ionograms – automatic scaling – ionosonde 1. introduction ionospheric observations are performed by a high frequency radar know as an ionosonde. the ionosonde sends short pulses of radio waves vertically into the ionosphere. these pulses are refracted back towards the ground and the ionosonde records the time delay between transmission and reception of the pulses. by varying the frequency of the pulses from 1 to 20 mhz, a record is obtained of the time delay at different frequencies. this record is referred to as an ionogram and is usually presented in the form of a graph. from the ionogram an experienced operator is able to obtain the most important ionospheric parameters, in particular the critical frequency of the f2 layer (f0f2) and the maximum usable frequency on a distance of 3000 km (muf(3000)f2). because of the growing interest in real time mapping and short term previsions, in recent years efforts have been made to develop software to achieve real time scaling of ionograms (e.g., fox and blundell, 1989; igi et al., 1993; tsai and berkey, 2000). the artist system developed at the university of lowell, center for atmospheric research, is an automatic scaling program widely used and tested (reinisch and huang, 1983; gilbert and smith, 1988). the national institute of geophysics and vulcanology (ingv) developed a low power pulse compressed ionosonde to be used for standard ionospheric soundings. this instrument – called an advanced ionospheric sounder (aisingv) designed for minimally attended, remotely controlled operation – was installed in the ingv ionospheric observatory of gibilmanna (37.9n-14.0e). a program (called autoscala) for the automatic scaling of f0f2 and muf(3000)f2 was developed to provide scaled software for the automatic scaling of critical frequency f0f2 and muf(3000)f2 from ionograms applied at the ionospheric observatory of gibilmanna michael pezzopane and carlo scotto istituto nazionale di geofisica e vulcanologia, roma, italy abstract software for the automatic scaling of critical frequency f0f2 and muf(3000)f2 from ionograms was developed and applied to the ionograms produced by the new advanced ionospheric sounder installed at the ionospheric observatory of gibilmanna. a test based on 1124 ionograms recorded in normal ionospheric condition was performed comparing the automatically scaled parameters with the corresponding ones obtained by the standard manual method. the results of a preliminary test performed during a strong magnetic ionospheric storm are also presented. 1784 michael pezzopane and carlo scotto data within a few minutes the ionogram being produced. to date, these data are available real time on the internet at the site <http://www.ingv.it/~zscotto/latest.html>. 2. the ingv software for automatic scaling of ionograms the ingv software is based on an image recognition technique and can operate without polarization information. hence it can be used both with single antenna systems and crossed antenna systems. using a family of functions with the typical f2 layer shape, and applying a maximum contrast technique, a particular element of this family is then selected and defined as representative of the f2 layer trace. the vertical asymptote of the selected function corresponds to the critical frequency f0f2; the muf(3000)f2 is calculated numerically establishing the transmission curve tangent to the selected function. compared to the previous version of the ingv software (scotto and pezzopane, 2002) for automatic scaling of ionograms some improvements have been made. 1) the main routine has been redesigned, with improvements in cpu time required and parametrizing the girofrequency, which means that this version will be able to scale ionograms recorded in any location. 2) in a significant percentage of the recorded ionograms it is not possible to scale a value for the considered ionospheric characteristic (muf(3000)f2 and f0f2). this can be due to different reasons, that are well known and classified by the standard ursi. the most common cases are reported below. a) no f2 trace is observed owing to blanketing by the e sporadic layer. in these cases the standard ursi recommends the use of the descriptive letter a on bulletins substituting the value of ionospheric characteristic f0f2. b) the trace near the critical frequency is not clearly recorded for different reasons. in these cases the standard ursi recommends to scale the most reliable value of f0f2 by extrapolation, hypothetically extending the traces up to the most probable value of the critical frequency. if the extrapolated frequency range is greater than 10%, it is recommended to report the highest frequency of the recorded trace, followed by the qualifying letter d (greater than) and the descriptive letter associated to the reason why the trace is not clearly recorded (s interference, r absorption, c equipment). the software for automatic scaling of ionograms recognizes the trace and outputs both the values of the characteristics. this version has been improved and a method for identifying ionograms without sufficient information has been introduced and tested. if an ionogram is considered to lack sufficient information it is discarded by the program and neither the f0f2 nor the muf(3000)f2 are given as output. 3. test: ingv software compared with the manual method during normal ionospheric conditions the test was performed using 1124 ionograms recorded from december 1 to december 15, 2002 by the ais-ingv installed in the ionospheric observatory of gibilmanna. the period selected is composed by magnetically quiet days in order to test the performance of the software in normal ionospheric conditions. 3.1. test of the method to identify and separate ionograms with sufficient or insufficient information the set of 1124 ionograms was divided into two subsets. the subset s, containing the ionograms scaled by the program, and subset n containing the ionograms discarded by the software because of insufficient information. for each subset we considered: a) the number of ionograms for which the operator was able to scale neither the f0f2 nor the muf(3000)f2. b) the number of ionograms for which the operator was able to scale f0f2 only. c) the number of ionograms for which the operator was able to scale muf(3000)f2 only. d) the number of ionograms for which the operator was able to scale both f0f2 and muf(3000)f2. 1785 software for the automatic scaling of f0f2 and muf(3000)f2 from ionograms recorded at gibilmanna the results of this analysis are reported in table i and they confirm the general robustness of the overall system. the ionograms not discarded by the software and for which the operator was not able to scale any value constitute the most critical cases. these cases are very few (9 out of 1124, equal to 0.8%) and are reported in table ii where a comparison between the output of autoscala and the corresponding manual scaling performed by a very experienced operator is shown. this comparison highlights that 8 out of 9 critical cases occurred for ionograms characterized by a truncated trace. on the other hand, it is also important to underline that the software does not discard ionograms that present a trace sufficiently defined for an operator. this can be inferred observing that the cases discarded by the software in which the operator was able to scale both f0f2 and muf(3000)f2 are very few (7 out of 1124, equal to 0.6%). table i. the behavior of the method to identify and separate ionograms with sufficient or insufficient information. scaled by the ingv software discarded by the ingv software no. of cases [%] no. of cases [%] the operator scaled neither the f0f2 9 0.9% 46 62.1% nor the muf(3000)f2 the operator scaled both f0f2 847 80.7% 7 9.5% and muf(3000)f2 (subset a) the operator scaled muf(3000)f2 only 182 17.3% 21 28.4% (subset b) the operator scaled f0f2 only 12 1.1% 0 0.0% (subset c) total 1050 100.0% 74 100.0% table ii. the few cases (9 out of 1124, equal to 0.8%) in which the program scales values but the operator is not able to read a value. a comparison between the output of autoscala and the corresponding manual scaling performed by a very experienced operator according to the standard ursi is reported. day and time muf(3000)f2 muf(3000)f2 f0f2 f0f2 (software) (operator) (software) (operator) [mhz] [mhz] [mhz] [mhz] 5 december 2002 22.00 8.3 a 3.1 a 6 december 2002 13.00 26.1 r 8.1 8.4 dr 7 december 2002 07.45 30.4 r 9.1 8.9 dr 7 december 2002 08.45 29.0 s 8.6 8.2 ds 7 december 2002 12.45 18.1 s 4.7 9.7 ds 8 december 2002 09.00 32.1 r 10.6 9.8 dr 8 december 2002 11.00 31.2 s 9.6 9.6 ds 11 december 2002 14.00 30.2 s 9.8 9.4 ds 12 december 2002 03.30 30.3 s 9.8 8.6 ds 1786 michael pezzopane and carlo scotto 3.2. test of accuracy and acceptability of the automatically scaled parameters with reference to the ionograms scaled by the ingv software, the following three subsets were considered (see table i): 1) subset a, containing the ionograms in which the operator was able to scale both the critical frequency f0f2 and the muf(3000)f2. 2) subset b, containing the ionograms in which the operator was able to scale the muf(3000)f2 only. 3) subset c, containing the ionograms in which the operator was able to scale the f0f2 only. table iiia-c. acceptable and accurate values (with corresponding percentage) for the test carried out on the ionograms recorded in gibilmanna from december 1 to december 15, 2002. in (a) the 847 ionograms for which the operator was able to scale both f0f2 and muf(3000)f2 are considered (subset a). in (b) the 182 ionograms for which the operator was able to scale only the muf(3000)f2 are considered (subset b). in (c) the 12 ionograms for which the operator was able to scale only the f0f2 are considered (subset c). f0f2 muf(3000)f2 no. [%] no. [%] total 847 100.0 847 100.0 acceptable 833 98.3 847 100.0 accurate 638 75.3 827 97.5 muf(3000)f2 no. [%] total 182 100.0 acceptable 179 98.5 accurate 170 93.5 f0f2 no. [%] total 12 100.0 acceptable 9 75.0 accurate 6 50.0 fig. 1a,b. a) differences (δ) between the values of f0f2 scaled by autoscala and by the standard manual method considering ionograms for which the operator was able to scale both f0f2 and muf(3000)f2 (subset a). out of 847 cases it results: for 638 cases (− 0.1 mhz ≤ δ ≤ 0.1 mhz); for 139 cases (0.1 mhz < δ ≤ 0.3 mhz); for 24 cases (− 0.3 mhz ≤ δ < < − 0.1 mhz); for 26 cases (0.3 mhz < δ ≤ 0.5 mhz); for 6 cases (− 0.5 mhz ≤ δ < −0.3 mhz); for 7 cases (δ > 0.5 mhz); for 7 cases (δ < − 0.5 mhz). b) differences between the values of muf(3000)f2 scaled by autoscala and by the standard manual method considering ionograms for which the operator was able to scale both f0f2 and muf(3000)f2 (subset a). out of 847 cases it results: for 827 cases (− 0.5 mhz ≤ δ ≤ 0.5 mhz); for 11 cases (0.5 mhz < δ ≤ 1.5 mhz); for 3 cases (−1.5 mhz ≤ δ < − 0.5 mhz); for 4 cases (1.5 mhz< δ ≤ 2.5 mhz); for 2 cases (− 2.5 mhz ≤ δ < −1.5 mhz); for 0 cases (δ > 2.5 mhz); for 0 cases (δ < − 2.5 mhz). a b a b c 1787 software for the automatic scaling of f0f2 and muf(3000)f2 from ionograms recorded at gibilmanna a quantitative comparison between the values scaled automatically and manually was performed and the results of the comparison are reported in table iiia-c. in this work an accurate value is considered to lie within ± 0.1 mhz of the value obtained by the operator for f0f2 and ± 0.5 mhz for muf(3000)f2. an acceptable value is considered to lie within ± 0.5 mhz for f0f2 and ± 2.5 mhz for muf(3000)f2. limits of acceptability have been adopted in line with the ursi limits of ± 5∆ (∆ is the reading accuracy). for the subset a and b (constituted by a sufficient number of elements), the results of the comparison are presented in form of a histogram in figs. 1a,b and 2. this analysis shows that the ingv software for automatic scaling gives acceptable values in more than 98% of cases both for f0f2 and muf(3000)f2. the ionograms belonging to subset b show the ionogram trace abruptly truncated, but the software considers the information sufficient to identify the trace, and a value of f0f2 is given. the test gives us an evaluation of the capability of the software to scale these cases. an example fig. 2. differences between the values of muf(3000)f2 scaled by autoscala and by the standard manual method considering ionograms for which the operator was able to scale only the muf(3000)f2 (subset b). out of 182 cases it results: for 170 cases (− 0.5 mhz ≤ δ ≤ 0.5 mhz); for 1 case (0.5 mhz < δ ≤ 1.5 mhz); for 6 cases (− 1.5 mhz ≤ δ < − 0.5 mhz); for 0 cases (1.5 mhz < δ ≤ 2.5 mhz); for 2 cases (−2.5 mhz ≤ δ < −1.5 mhz); for 0 cases (δ >2.5 mhz); for 3 cases (δ < −2.5 mhz). fig. 3. an example of ionogram belonging to the subset b. the ionogram trace is abruptly truncated, so the operator is not able to scale the critical frequency f0f2. in this case the software identifies the trace correctly and so the muf(3000)f2 is correctly scaled (33.0 mhz the value automatically scaled, and 33.0 mhz by the operator). the value of f0f2 = 10.3 mhz given by the program is reasonable. 1788 michael pezzopane and carlo scotto fig. 4a-d. with reference to the days from 1 to 7 december 2002: a) comparison between muf(3000)f2 manually scaled (black square) and automatically scaled data (red line); b) comparison between f0f2 manually scaled (black square) and automatically scaled data (red line); c) plot of ∆ = ( f0f2measured + − f0f2median ) / f0f2measured, used as an index of the ionospheric disturbance; d) plot of the geomagnetic activity index kp. fig. 5a-d. with reference to the days from 8 to 15 december 2002: a) comparison between muf(3000)f2 manually scaled (black square) and automatically scaled data (red line); b) comparison between f0f2 manually scaled (black square) and automatically scaled data (red line); c) plot of ∆ = ( f0f2measured + − f0f2median ) / f0f2measured, used as an index of the ionospheric disturbance; d) plot of the geomagnetic activity index kp. of an ionogram from subset b is shown in fig. 3. the ionogram trace is abruptly truncated, so the operator is not able to scale the critical frequency f0f2, but the software identifies the trace correctly. the muf(3000)f2 is correctly scaled (33.0 mhz is the value automatically scaled, and 33.0 mhz scaled by the operator) and the value of f0f2 = 10.3 mhz given by the program is reasonable. figures 4a and 5a report the plot of the muf(3000)f2 values obtained by autoscala and the plot of corresponding values obtained by a well experienced operator. figures 4b and 5b report the same plots for f0f2. figures 4c and 5c report the plot of the parameter ∆ = ( f0f2measured − f0f2median) / f0f2measured (where f0f2measured is the value obtained by an experienced operator), that is considered an index of the ionospheric disturbance. figures 4d and 5d report the plot of the geomagnetic activity index kp. from figs. 4a-d and 5a-d one can observe qualitatively that the plots of the automatically scaled data are very close to the data obtained by an operator. therefore we can conclude that the performance of autoscala is stable for not disturbed magneto-ionospheric conditions (kp < = 4). a b c d a b c d 1789 software for the automatic scaling of f0f2 and muf(3000)f2 from ionograms recorded at gibilmanna 4. performance of ingv software for disturbed ionospheric conditions: a case study on october 29 2003, the sudden commencement of a strong magnetic storm was observed. approximately at 6 ut the arrival of a large high-speed solar wind shock front was detected by the solar wind monitoring satellite. the geomagnetic field disturbance was observed in some regions until 7 ut. the ais-ingv of gibilmanna recorded the start of the initial phase of the associated ionospheric storm at 8.45 ut by an increase in the critical frequency f0f2. figure 6a,b reports the plot of the f0f2 values obtained from the ionograms recorded in gibilmanna; the values obtained by autoscala are compared with the corresponding ones scaled by a well experienced operator. figure 6a,b also reports the plot of the f0f2 values obtained from the ionograms recorded in rome; the values obtained by artist system are compared with the corresponding ones scaled by a well experienced operator. comparing the two plots it is observed that during the morning the commencement of the storm causes the deterioration of the ionograms trace for both digisondes. autoscala is able to detect this condition and does not give data that would be wrong. the system ais-ingv detects the ionospheric storm as soon as the ionogram trace is sufficiently clear. on the contrary, artist does not detect this condition so the real time data of the digisonde of rome are wrongly given in output. as a result the real time data of rome between about 9 and 11 ut are highly underestimated and consequently the plot dissemble the ionospheric effect of the storm. 5. conclusions the results of the test performed can be summarized observing that considering the subset a (constitued by good quality ionograms, for which the operator is able to scale both f0f2 and muf(3000)f2) the percentage of acceptable values automatically scaled is very high (> 98%). for medium quality ionograms (for which the operator is able to scale only muf(3000)f2) the percentage of acceptable values automatically scaled is also good (> 98%). hence we can conclude that for normal ionospheric conditions autoscala is able to provide good quality data. for disturbed ionospheric conditions a preliminary test has been performed showing good results. a study performed during the strong ionospheric storm of october 2003 showed that autoscala better demonstrated the evolution of the ionospheric storm than the artist system. 6. further developments the tests performed are not able to give information on the performance of autoscala for spread f conditions as this case was not observed in the period considered at the ionospheric station of gibilmanna. a separate study on this item would be necessary. autoscala has definitively been applied at the ionospheric station of gibilmanna since april 2003 and to date more than 20 000 ionograms fig. 6a,b. a) plot of f0f2 values, from the ionograms recorded in gibilmanna on 29 october 2003, obtained by a well experienced operator (black square) compared to the corresponding plot obtained by autoscala (red line); b) plot of f0f2 values, from the ionograms recorded in rome on 29 october 2003, obtained by a well experienced operator (black square) compared to the corresponding plot obtained by artist (red line). a b 1790 michael pezzopane and carlo scotto have been produced. tests based on larger datasets comprehensive of different seasons and solar conditions are necessary. extensive comparative studies of autoscala with other automatic scaling software (with particular reference to artist system) would be also interesting. references gilbert, j.d. and r.w. smith (1988): a comparison between automatic ionogram scaling system artist and the standard manual method, radio sci., 23, 968-974. fox, m.w. and c. blundell (1989): automatic scaling of digital ionograms, radio sci., 24, 747-761. igi, s., k. nozaki, m. nagayama, a. ohtani, h. kato and k. igarashi (1993): automatic ionogram processing systems in japan, in proceedings of session g4 at the xxivth general assembly of the international union of radio science (ursi), august 25-september 2, kyoto, japan. reinisch, b.w. and x. huang (1983): automatic calculation of electron density profiles from digital ionograms 3. processing of bottomside ionograms, radio sci., 18, 477-492. scotto, c. and m. pezzopane (2002): a software for automatic scaling of f0f2 and muf(3000)f2 from ionograms, in proceedings of ursi 2002, 17-24 august, maastricht, the netherlands. tsai, l.c. and f.t. berkey (2000): ionogram analysis using fuzzy segmentation and connectedness techniques, radio sci., 35, 1173-1186. (received january 28, 2004; accepted may 12, 2004) faggioni.pdf 525 annals of geophysics, vol. 46, n. 3, june 2003 a realistic inversion algorithm for magnetic anomaly data: the mt. amiata volcano test osvaldo faggioni (1) (2), nicolò beverini (1) (3), fabio caratori tontini (1) (3), cosmo carmisciano (1) and iacopo nicolosi (4) (1) istituto di geofi sica marina, università di pisa, consorzio universitario della spezia, italy (2) dipartimento di scienze della terra, università di pisa, italy (3) dipartimento di fisica and infm, università di pisa, italy (4) istituto nazionale di geofi sica e vulcanologia, roma, italy abstract the aim of this work is the formulation of a 3d model of the mt. amiata volcanic complex (southern tuscany) by means of geomagnetic data. this work is shown not only as a real test to check the validity of the inversion algorithm, but also to add information about the structure of the volcanic complex. first, we outline briefly the theory of geomagnetic data inversion and we introduce the approach adopted. then we show the 3d model of the amiata volcano built from the inversion, and we compare it with the available geological information. the most important consideration regards the surface distribution of the magnetization that is in good agreement with rock samples from this area. moreover, the recovered model orientation recall the extension of the lava flows, and as a last proof of validity, the source appears to be contained inside of the topographic contour level. the credibility of the inversion procedure drives the interpretation even for the deepest part of the volcano. the geomagnetic signal appears suppressed at a depth of about 2 km, but the most striking consequence is that sub-vertical structures are found even in different positions from the conduits shown in the geologic sections. the results are thus in good agreement with the information obtained from other data, but showing features that had not been identified, stressing the informative power of the geomagnetic signal when a meaningful inversion algorithm is used. mailing address: dr. osvaldo faggioni, istituto di geofisica marina, università di pisa, consorzio universitario della spezia, via vailunga 1, 19125 la spezia, italy; e-mail: of@castagna.it key words magnetic methods mt. amiata volcano potential fi eld inversion 1. introduction geophysical data, and above all geomagnetic data, are particularly useful in volcanology to define the characteristics of a volcanic complex. a geomagnetic data set is very informative, in fact, if connected to magmatic rocks or lytotypes that show a particular intense value of the magnetic susceptibility. using adequate inversion software, one can find solutions that seem to correspond to the tectonic-structural model of the subsurface, but additional information can be obtained over areas that cannot be investigated directly. practically, the geomagnetic data are used to find solution about an equivalent source that should be responsible for the observed data, through the inversion process. the transfer from the observed data to a meaningful set of information on the generating source is not obvious, and has a fundamental relevance. this is particularly true for the geomagnetic field, that because of its di526 osvaldo faggioni, nicolò beverini, fabio caratori tontini, cosmo carmisciano and iacopo nicolosi polar nature is far from being interpretable «by eye». it is well known moreover that any inversion process suffers from non-uniqueness problems. we know from gauss’ theorem that when the magnetic field is known only on a bounding surface, there exists an infinite number of source distributions inside the volume that can generate this field. we call this a theoretical ambiguity. however, another kind of ambiguity has an algebraic origin: the number of available data is often under-sampled so that the description is not complete to fully describe the field. many different source distributions can reproduce a particular discrete data set with the same degree of accuracy. in practice, it can easily be shown that any magnetic anomaly measured on the surface of earth, might be reproduced by considering a superposition of (even negative) magnetic dipoles in a thin layer just beneath the surface. today there are two principal strategies in order to develop an algorithm able to overcome these ambiguities. one possibility is given by parametric inversion. knowing the anomalies generated by a simple causative body, it is possible to approximate the source finding the «best one» (bhattacharyya, 1980; wang and hansen, 1990; zeyen and pous, 1991). 475 7 475 5 475 3 475 1 474 9 474 7 fig. 1. topography of the mt. amiata complex (gauss-boaga projection). 527 a realistic inversion algorithm for magnetic anomaly data: the mt. amiata volcano test non-uniqueness is avoided by restricting the number of degrees of freedom to be fitted. in the second methodology, the earth is divided into a large number of prismatic cells with constant magnetization inside and an algorithm minimizes some functional representing a suitable objective function of the model. for example, it is possible to avoid dispersion of the source by minimizing its total volume (last and kubik, 1983) or its momentum of inertia (guillen and menichetti, 1984). realistic models are obtained contrasting the depth decay of the kernel function with an appropriate weighting function (li and oldenburg, 1996) or introducing a multi-layer depth resolution in field measurements (fedi and rapolla, 1999). in this work, the result is achieved using an inversion scheme that has been developed by caratori tontini et al. (2003) and in the following section an outline of this inversion scheme will be shown, to provide the formal context of the performed analysis. the data used for the inversion procedure come from the eni-agip division, and are used according to the scientific collaboration between eni and the istituto di geofi sica marina of la spezia (eni explor. & prod. div. and igmar, 2002). the geologic map and sections used for the interpretation come from the work by enel «direzione studi e ricerche, centro di ricerca geotermica» and cnr «istituto internazionale per le ricerche geotermiche» (calamai et al., 1970; gianelli et al., 1988). figure 1 shows the topographic contour level of the mt. amiata complex projected onto a gauss-boaga reference system. the mt. amiata shows two major peaks, la vetta (1735 m), and la montagnola (1571 m) aligned along a wsw-ene direction. it is a moderate size silicic volcanic complex, whose main activity had two principal phases at about 300 000 and 200 000 years ago (ferrari et al., 1996). the first period is characterized by the emplacement of a trachi-dacitic basal complex (btc), made of two separate sub-units (fig. 3). the whole btc complex lies over a 90 km2 area with an average thickness of about 150-200 m, and probably it is the result of a light explosive activity. the second phase happened after 100 000 years and its main products were domes and lavic flows with composition variable from trachi-dacitic, trachitic and latitic (dlc). these domes and flows, where visible, are aligned along a wswene direction as the btc complex. at the end of the eruptive epoch, two smaller olivine-latitic lava flows (oll) were emitted. the detection of fault systems and their origin in this area is difficult because of the vast erosional cover surrounding the domes and flows and for the dense vegetation. however the mt. amiata is oriented wsw-ene and two faults, with a length of about 2.5 km, seems to be the origin of the btc collocation. a certain number of other smaller faults were activated during the second period of activity (marinelli et al., 1993). 2. outline of the inversion algorithm in this section, we show briefly the method used to perform the inversion, its constraints and the way it works. a full treatment of the topic can be found in caratori tontini et al. (2003), together with some tests, based on both synthetic and real data that gives reliability to the method. here we report the formalism and the basic information about the initialization and the utilization of the algorithm, to fully understand the inversion of the mt. amiata geomagnetic data. let us suppose that we have partitioned the data set into a 2d grid that we call d (i, j) (observations). we want to interpret this data set in terms of the magnetization of the generating source. we adopt a mapping of the region presumably containing the source, dividing it into a certain number (n x , n y , n z ) of prismatic cells with unitary magnetization. obviously, the magnetic anomaly field generated by a particular subsurface configuration is obtained multiplying the unitary cell contribution for the magnetization of the cell, and summing over the whole number of cells (2.1) where df(i, j ) is the grid of the predicted data and m(l, m, n) is the cell magnetization. the ∆ ∆f i j f i j l m n m l m n l m n n n nx y z , ˜ , , , , , , , , , , ( ) = ( ) ( )∑ 528 osvaldo faggioni, nicolò beverini, fabio caratori tontini, cosmo carmisciano and iacopo nicolosi analytic expression of the contribution at one cell ∆ ˜ , , , ,f i j l m n( ) is well-known and here is not reported for conciseness, but can be found in bhattacharyya (1964); rao and babu (1991). the inversion procedure works minimizing the difference between the observed data set d (i, j ) and the prediction grid df(i, j ) in the least squares sense, provided that some constraints over the magnetization are imposed, otherwise it is well-known that the problem has multiple solutions. the algorithm adopts a smoothness constraint and the bound of positivity imposed by using a gaussian approximation. practically the source is expanded using a predetermined number of 3d gaussian functions that are positively constrained. this particular choice introduces a non-linear relationship among the parameters defining the source, but has the fundamental relevance of drastically reducing the degrees of freedom of the problem, permitting however the reconstruction of the most articulated sources. the smoothness constraint moreover is imposed naturally in this way. within this choice, the magnetization is parameterized as (2.2) where the set of 7 parameters (a k , x k , y k , z k , sx k , sy k , sz k ) define completely the k-th gaussian function, while the number n g of functions used as the basis of the envelope is chosen by the user according to some empirical considerations. if the algorithm starts with some unnecessary function, i.e. n g is redundant, the amplitude of these superfluous functions is compelled to a null value through a process of annihilation that is explained in detail by caratori tontini et al. (2003). it is a valid geological observation that often the spatial variations of physical properties are smooth. this is particularly true for some geological scenarios: for example smooth magnetization variations can be seen near magmatic intrusions, and smooth density variations may apply to sedimentary formations under different level of compaction. the positivity constraint, that is of fundamental relevance to reduce the ambiguity domain, in the case of geomagnetic data is obvious because the paramagnetic effects of matter are usually stronger than the diamagnetic ones. this further limitation is imposed through the use of a positivity mapping function for the amplitude parameters a k (oldenburg and li, 1994; li and oldenburg, 1996). the algorithm proceeds by means of a modified levenberg-marquardt iteration scheme for non-linear least squares minimization to determine the best-fit source, together with the statistical confidence level for the parameters. depending on the number n g of functions involved, even the most composite models can be reconstructed by the inversion algorithm to achieve a fine resolution. 3. the mt. amiata aeromagnetic data inversion the data used for the inversion were collected during the survey performed by agip in 1978-1979 in a large area, including tuscany and the tyrrhenian sea, using a cfs cesium magnetometer. the upper part of fig. 2 shows the magnetic anomaly using gauss-boaga kilometric coordinates reference system. the total field data (time corrected) were levelled and filtered through standard procedures, and then a polynomial regional field (order 2) was residuated to obtain the magnetic anomaly. the survey in this area was made through lines (direction sw-ne) with an average spacing of 2 km, orthogonally intersected by tie lines (direction nw-se) with an average spacing of 5 km. the sampling pass along each profile was about 500 m, at an altitude of 1600 m. the data were gridded through a minimum curvature method with a cell size of 750 m, for a spectral resolution of 1.5 km. the magnetic anomaly is very articulated, showing different positive local maxima with the associated negative lobes, suggesting the idea of a very articulated source for geometry and depth. for these reasons, these 22 ¥ 16 data are inverted using a prismatic region defined by the east interval (1708; 1716) km and north (4747; 4755) km with 4 km depth, divided into 6912 cubic m l m n ak k ng , ,( ) ≡ ⋅ = ∑ 1 x l x x y m y y z n z z k k k k k k exp⋅ − ( ) −( ) − ( ) −( ) − ( ) −( )        2 2 2 2 2 2σ σ σ 529 a realistic inversion algorithm for magnetic anomaly data: the mt. amiata volcano test fig. 2. magnetic anomaly and recovered model compared with the topographic contour level (gauss-boaga projection). the broken lines represent the geologic sections of fig. 4. 530 osvaldo faggioni, nicolò beverini, fabio caratori tontini, cosmo carmisciano and iacopo nicolosi cells (24 ¥ 24 ¥ 12). the starting condition for the algorithm employed 30 gaussian functions. the lower part of fig. 2 shows the recovered model plotted by means of horizontal crosssections. the amiata topographic contour level and the trace of two interpretative geological sections are superimposed on the model (fig. 4). apart from very small magnetization echoes of the deepest source in the first cross section, due to the smoothness constraint, the model appears to be fully contained inside the mt. amiata topography. the model has highlighted a shallow distribution of sources with a thickness of about 2000 m in the central area, decreasing towards the lateral margins. the overlap of known dome locations with the inversion model gives a key to understand the geological meaning of the model itself; there is a good correlation between the surface distribution of domes and the recovered model inside the amiata fig. 3. surface distribution of the recovered source compared with the geologic map of the mt. amiata area and the geologic sections (gauss-boaga projection). 531 a realistic inversion algorithm for magnetic anomaly data: the mt. amiata volcano test volcanic edifice. we suppose that the highest magnetized recovered bodies belong to the dlc and oll volcanic complexes. two interpretative geological sections (calamai et al., 1970; giannelli et al., 1988) show two main conduits located in proximity of «la vetta» and «la montagnola» that represent the locations of the shallower portion of magma ascent during the emplacement of domes structures. probably those vertical structures reach a depth of 5-7 km, where a magmatic intrusive granitic body (giannelli et al.,1997) that supplied the dome pile up is supposed to be located. a magnetic signature is given only by the upper portion of those vertical structures, because of the very shallow curie isotherm of the amiata geothermal region (baldi et al., 1995). in fig. 3 the recovered model has been spread over the topographic surface for comparison with the rock magnetization data sampled during field measurements that the istituto di geofisica marina performed in the area of the mt. amiata, using a gms-2 susceptibility meter. the magnetic susceptibility of different lytotypes coming from rock samples of btc, dlc and oll was measured, and an average magnetization was calculated from the different samples to avoid statistical errors coming from geometrical irregularities of the sample surface. the first three horizontal cross-sections of the recovered model have been fused into a unique map, in order to overlap the data found during the inversion procedure on the topographic surface of mt. amiata. the high magnetic behaviour of the rocks coming from dlc and oll, with respect to the lowly magnetized complex of btc, is confirmed by the recovered model. the model shows the presence of the generating source in the central part of the area as arguable from the geologic map. the source geometry appears aligned along the wsw-ene direction in good agreement with the orientation of the principal fractures that caused the emplacement of the volcanic complex; moreover the magnetization peaks seems to confirm the presence of localized sources that should be referred to the volcanic conduits system mentioned before. the inversion has highlighted that the magnetization of subvertical sources located inside amiata volcanic edifice is stronger than that measured on the surface; the deepest part of the domes that have a surface topography expression like «la vetta» and «la montagnola», show a higher degree of magnetization before it reaches the shallow curie isotherm: this is a new fundamental constraint to be considered to understand the petrographic evolution of the amiata volcanic system, imposed only by the recovered inversion model from magnetic anomaly data. fig. 4. geologic sections of the area of the mt. amiata. 532 osvaldo faggioni, nicolò beverini, fabio caratori tontini, cosmo carmisciano and iacopo nicolosi 4. conclusions from the data obtained by eni-agip division we have evaluated the magnetic anomaly of the mt. amiata volcanic complex. from the inversion of the extracted data set we have shown a 3d model of the magnetic source inside the apparatus. the model shows the geometry of the magnetic sources of the volcanic edifice from the altitude of 1.5 km to the depth of 2.2 km, where the geomagnetic signal loses its informative capability. the recovered model is in good agreement with the information coming from the geologic knowledge and the susceptibility measurements of the area of interest, showing however characteristics and features that were not still found. in particular, we were able to suppose the presence of sub vertical structures that should be connected to intrusions or localized magma ascent conduits characterized by a magnetization greater than the surface portion one. the surface distribution of the magnetization is in good agreement with the geologic map and the rock samples measurements. this study is thus fundamental in the definition of the geologic submerged structures of the mt. amiata apparatus, otherwise not accessible by direct methods. acknowledgements we thank eni-agip division for permission to use the amiata geomagnetic data. we are obliged to angelo de santis for his helpful and competent review. we are also grateful to paolo stefanelli (igmar) for his help in measurements of amiata rock samples susceptibility. references baldi, p., s. bellani, a. ceccarelli, a. fiordelisi, g. rocchi, p. squarci and l. taffi (1995): geothermal anomalies and structural features of southern tuscany (italy), in proceeding of the world geothermal congress, florence, 18-31 may 1995, 7-11. bhattacharyya, b.k. (1964): magnetic anomaly due to prism-shaped bodies with arbitrary magnetization, geophysics, 29, 517-531. bhattacharyya, b.k. (1980): a generalized multibody model for inversion of magnetic anomalies, geophysics, 45, 255-270. calamai, a., r. cataldi, p. squarci and l. taffi (1970): geology, geophysics and hydrogeology of the mt. amiata geothermal fields, geothermics, special issue 1, 1-9. caratori tontini, f., o. faggioni, n. beverini and c. carmisciano (2003): gaussian envelope for 3d geomagnetic data inversion, geophysics, 68, 9961007. eni explor. & prod. div. and igmar (2002): aeromagnetic anomaly map of italy and magnetic provinces, scale 1:500 000, r & d project 4700000254/mr 1 final report. fedi, m. and a. rapolla (1999): 3d inversion of gravity and magnetic data with depth resolution, geophysics, 64, 452-460. ferrari, l., s. conticelli, l. burlamacchi and p. manetti (1996): volcanological evolution of the mt. amiata, southern tuscany: new geological and petrochemical data, acta vulcanologica, 81, 41-56. gianelli, g., m. puxeddu, f. batini, g. bertini, i. dini, e. pandeli and r. nicolich (1988): geological model of a young volcano-plutonic system: the geothermal region of mt. amiata (tuscany, italy), geothermics, 17, 719-734. gianelli, g., a. manzella and m. puxeddu (1997): crustal models of geothermal areas of southern tuscany (italy), tectonophysics, 281, 221-239. guillen, a. and v. menichetti (1984): gravity and magnetic inversion with minimization of a specific functional, geophysics, 49, 1354-1360. last, b.j. and k. kubik (1983): compact gravity inversion, geophysics, 48, 713-721. li, y. and d.w. oldenburg (1996): 3d inversion of magnetic data, geophysics, 61, 394-408. marinelli, g., f. barberi and r. cioni (1993): sollevamenti neogenici e intrusioni acide della toscana e del lazio settentrionale, mem. soc. geol. ital., 49, 279-288. oldenburg, d.w. and y. li (1994): subspace linear inversion methods, inverse problems, 10, 915-935. rao, d.b. and n.r. babu (1991): a rapid method for three-dimensional modeling of magnetic anomalies, geophysics, 56, 1729-1737. wang, x. and r.o. hansen (1990): inversion for magnetic anomalies of arbitrary three-dimensional bodies, geophysics, 55, 1321-1326. zeyen, h. and j. pous (1991): a new 3d inversion algorithm for magnetic total field anomalies, geophys. j. int., 104, 583-591. velocity ami directiod of plate displacemeuts by latitude observations e. piw\'erbio y. q,uesada ("') reeeived on january 8th. 1973 rtassu'>:'l'o. con riferiillento ad 11il pre{·e{h·nte lavoro ( ~ ) eon il quale sono st.ate dedoue le velol"itil ili s("onimeuto (lei hloechi allierieaaumsia eli allieri{·a-l'al"ific o, secolhlo la ricost.ruzioue (li le l'i("hon, utilil':l':au(lo la lungfl scrie (70 flnni) ai os;;ervazioni (li lat.itlllliue (ielle stal':ioni dcl sl1., gli autori prelulouo in esame il ]hohlenlfl ilell 'cfl'e!t.o eh e la indeter lliinal':ioue sulla conoscenza llelle coonliuate dei ljoli di robll':iollp ilei hlo('chi, lletermiua 8ul calcolo (tella "e!o("itil di ~(·oltilliento. fal'enllo rìl'or so lhi un ]lro("eaimento ai ot.tilllizzazione (tei (tati v('u;!"ono ill'terlllinak poi le vcloeitil ili scoitiillent.o e le eoonlinat.e 1101 ]1010 ili rota· zioue dei hl o("ehi am(,tìl'a·l~umsia. t risultat i si aecon!ano eon quelli 1il pre("edenza ilet.e\'lllinflti (t ) e risultano nello stesso or dine di gmjl(!ezza di (!u(·lli llellotti mi rilcvailleuti g-eolog-iei e ~eofi8ki. su~dtahy. -the reslilts oljt.aine(1 ljy lhe l1utho l's in a fol'cgoing papc!' ("), ijl"oviding eon1ljlcte!y indepcli\!ent. evidencc ou the plate tedouie8 hypothe~is, 11\'(\ ilis('uss('(1. [low ltitìl'al ih t.he l'lloiec of t1w rotat.ional pole ljosition of t ile platcs is carcfully anfllys('11. 'l'he ]o("atiou of t.hc ("enhe of rotnt.ion awl the l'ate of rot.ation of allicriea·elll'flsiu plate has ll<'en sul"("cssivoly ol, t.ain('(l lly the last ·sr}llfll'e fit ilicthod. 'l'hc8e l'e8 11it.8 al'e in good agl'ccment. with gcophysi('i1\ 1111\! geolo· gical moasill'elllents ani] ('onfi r lll t.he possibility of using ust.ronoilliefll (lat.fi in the stjlil,l' of ]llak tedonìl' s . introduc'l'lo':-< usillg le pieh(}]l ~ ~ (!.) rccollstl'llctioll, the .. l . ..ti. (g) comput.ed the l'ot.at.ioll rate of thc eumsia n-amel'ic a n ami pacific-amcl'ican (*) lnt.emationfll lutitude station, cagliari, " 2ho b. proveiwio v. quesada }llates from m!tronolllical latitudes observed in the five int.ernational astronomical ~tations oì 1lizusawa· (jall:ln), kitab (ussi{), ca·rloforte (haly), gaithersburg a·lli} ukiah (usa) ~ituated on the + 3h"s' }lara11c1. 'l'he data analp!cd cover a period of 70 yea·rs. from t.his ana.jysis it reslùts that in generai, di~reganling loeal dist.orsions whieh undoubt.ediy occur, the kitab al\(l carlofortc stations can be comiderml as iying on t.hc eumsian platc. )iizusawa al80 lie~ on this plate, although close to the boumlary. 'l'ile fourth ami fiìth st.utions, gait.herslnu·g and ukiah, must be on diffcrent. plates. gait.lmrslnu-g licf! near tlw center of tlm american }llat.c a·lli} ukiah muf!t be con· sidered as llloving with thc east pacifie plate. 'l'his plate is almost cntirely eons1idled by t.he ,\merican plate ami onlr a· sma11}jart remainl>, ,,·hose lllot.ion if! essent.ially nort.h-west.. 'l'o deterllline the valuc of the rotation ratc ('h of diffcrcnt ljlatcf! (k) by t.he seeuiar vmiations oi the latitude b! oi the stat.ion (i) af! well as the sccular motion of the earths pole (n), condition equatiolls of the tn)e [1] wcrc used, in whieh thc first. tcrlll of t.hc second member re}h·esents t.he eiiect oi sccuiar }lolar motion on obrerve!l latitude variations, while t.he second tcrm represent.s the c1feet oi the rotation of the station (j) of iollgit.ude ai aroullil the rotation jlole (k) oì eoordinates l k and 1>/: . 'l'he rcsulting absolute rates of the different })late~, averaged on thc ba·sis oì various cases treated (") are iound to be -o" .0037jyear ±o" .0002 +0" .00lbjyear ±o" .0002 +0" .001sjycar ±o" .0002 (emasia.n plate) (amcrican pjn.t.e) (nb pacific pla.f.e) 'l'hcf!c resuit.s are in cxeclhmt. agreement 1vith thc }llate t.eet.onics theorr ami silow that iong-tcrm variations in t.hc motion of tile f!tations can be iound by carehù analyf!is of vcry long serics of astronomical !lat.a. yeloclty a~d dlrection 01" pr,atl'~ j)j.~l'l'\cjn!e~ts etc, 2!j! in'l':eiu,al a:-id extbrnal :eluwrs by ljb'l':eidu.:->i.:->g l'ol:e a':->lj a.:->g"cl,\r ratf-s the volar eoordinat.c~ q)~ anil l i< i whieh arn obtaille!l b:r delcl' mining t.he re lative motions oi dilierenl }1ia.le~, are generali:r alieet.ed by observation errol's 'whieh lmve rel)ercussions on the determination oi t.he absolut.e rates eakula.ted by mea,ns of l'ela.t,ion [ij. the polar eooriliuates of !liffer ent. }llates are usuall:r ,,~tablishe!l clil'ect,]y irom ~})]'efl(lillg rates ami il'aelw'e zone azimut.hs (azimuths of the t.ramiorm iaults). in pa.rtieular, the l)ositions oi the ceuter of rola.lion adol)ted by i~e pkhon ej are obta,ined h om the azimlllhs oi the fradure :>;ones. the sla.ndarcl ~lc-viations oi measlu'ed irom computeil az imlllhf! given by le pichon (2) are ±ijo./ ior the amerieanpadfìe vole ancl ± 9°.1 ior the ameriean-huraf!iflll vo](,. these q ult.ntities only indil'edly give an idea of lhe im}lreeision wit.h 'which rot.a,t.ion l)ole coordinalns are !letermined. because oi the l)rocedul'e of ('aleulat-ing the eool'!linates oi the pole used by le pkhon e), lhe aceul'ae:r oi these coordinates ('all be eoffi}lllled only "a l)osteriori". from t.he sl)herieal tl'iallgle (fig. 1) having as vertices the geographica.l uorth }lole 1'", t.he rot.at.ion pole l'k of the }1la.ln (1.:) on whieh p. s " , o "o , ~ '" a , go a, p, "-----d-, -----"--;:a, fig. 1. spherical t.rianglo p n l't .ai. lo tho borth geographkal polo and 'l'he poinh t' ~ ani} l't coltpspond tho rotat.ion polo o( thc plì,te. 2fj2 e. l'rov mullo v. quesalja the point ai, at which the azimuth~ (t, of t.he fracture zone were taken, lies, ami the point ai itself, we ean write the following sl)herical relations: sin rft~ __ o sin rpi t··o~ d, + cos ti sin di sin (ti _~=(4-a,)~_~~~-~.,_"~.' e08 (jj~ 8ill (l~ }.tl = sin di cos (li in which !i(o is t.he allj.çular distance pkai, whilc ({i(o and àli) are the gcographic coordinates of poillt. ai. from these, ·we can easily obtain t.ag rft~ pos (l~ }.,) = sec cf, ~in (l~ à,) tago (li from whit'h, in tleveloping [uhi ncgleet.ing thc secontl order tcrm, wc havc cos cf' cosec (l~ }.i) (l + tag! (jj k ) ,1(jj + { cos cpi cot (l~ }.i) tag (t,} ,1l = (l + t.ag! al) ,1al [2] by means of this relation it is possible to calcllh1te the tlworetieal values of t.he uncertaint.jes .1(jj ami ,1l of the coortlinates of thc 1"0t,atton cent,cr p eorrespunding to the absolut,ell value of the st.·llldal"ll rleviat.iolls ,1a, = ±!)o.l of fradure zone azimuths. utilizing the coonlina.tes ami a.zimllths of fradure zones ai given by le picholl (2) referring to the center of rotat.ion of the american em'asian plate anrl sup})()sing .1(jj = .1l, we obta,in, by formuh1 [2], l.1rftl = i.1lj :::' 20°. 'rhe~e vahw8, which agree with thc sizes of t.lw elli}j8e of error comlmted by le pichon (!), repre~ent. the amount oi accuracy to be expeet.ed in t.he detennination of the rotation eenter coonlinatcs of the amcrican-eili"fl.sian }jlate from fradure zone azimuths. it is illteresting to note that t.lw vl1hw8 tlms dcterminetl of the imictcrminations of rotation pole coonlinatcs are al80 in agreement with the values oi the external })]'ee1sions detcrmine{\ on t.he basis of eenter of rot.ation coordinates compllt.erl by difiercnt allthor8. in the jir~t folli" lines of 'rable 1, thc valucs of the rotation polc coonlillate8 and the relative rate of rot.ation of the americall-eurasian l)late rleterminel[ by rlitferent author~ are givell. veloulty a~d directio~ oio' l'l.ate displace~!e~ts etc. 2d3 the ,o,tandarll deviatiol1s of t,he eooi'llinates observecl from the average givcn in table l are: alo = +ldo t [\ b l c l rl';).at!vj'; vj~ctor of a~ierica-eiji,as!an pla'l'e rotat!on m i l i 107 dcgirr i authors 78"n lo2 oe 2.8 le pichon (') 63°n 137"f, le l'iehon (~) 60 0 n l 35°}<; 2.1 ~forgan l') 48 0 n 155°f, 2.4 chase (l) 8u on 57°,," pro\'crbio & qucsada (6) }c'rom l'elation [ij, ncglceting thc errcct of secular polar motion u on secular latitude vmiations, for the a,ngula.r rate of the stations lying on the america,n-j<jul'aosia.n pla.te 'we fìnd wher€ and j = 1,2,3,4 rcfel'fl to forte ami gaitherflburg. the rom statiol1s of millllsf.wa, kitab, ca,rlofrom the foregoing we inmlccliately finel [3] "\vhere the coe1ficients a and bare a = [bj sin r[jj; sin (iok ai)] llw" [cos r[jk cos (lt )'i}] llio b = [sin 2 rpk sin "{l" ,l,)] i]{p k [sin 2 (l l' ai) co::;2 w.i..] lllt utilizillg thc va.llles for ,o,eeubr va.ria.tiollf\ bi (g) a,jhl collsidering t,he rota,tion center coordìlmtes (2) we fimi (10 -3 see of al'c(yr). 2!j-1. t:. p1w\'e b bio v. qu};s.~j}.~ altllougll thc rcliability of this sollltion is mthcr scarse (sincc the computed weight of the quantity l1w is oniy o.oog) in any case we have, keeping in mind the maximum uncertainties in ealclllating tllc coor(linates wl: ami lk (order of 10 0 ), an uncertainty in tlw computed anguiar mte of thc same rrmgnitwle of the angular rate by the .a. a. (fi) determined. "\s the {'hoice of ]jole ]josition in rotation calculation by means of thc cquation [lj sccms to bc mther criticai, we {'onsidercd it opportune to reconsider the whole llroblem by turning to a difierent criterion of data proeessing. de'l'eidii'xa'l'io.:\ 01,' iw'l'atiok ce~ter a'xd rate f1w11 awl'iwk01iical lati'ltdes 'rhe loeation of the center of rotation ami thc l'ate of rotation of thc american-ellraf\ian plate \vas dctermined by t.he lea.st.-squal'e fit mcthod, similar to the onc med hy l,e pichon for the determination of l'otation parametel's of rigid hlock8. thc numcricaimethq(1 of litting minimizcs the 8um of the squares or the sbndard deviations of the residuals (i = 1,2,3,4) between the observed values of secutar lat.it.ude varia.tions a.ml those ealelllatc(l by t.he expression [l]. 'ro calcuiate this expression, the fìxed pammeters n = o" .0030 ami v = gdo.g, detcrmincd b'y the a.a. (~) with sufficient accumey, which clmracterizc the secuiar motion of thc eai'th llole, \vere introduced. the valucs of boa l \vere t.hcr!!fol'c calcllbt.wl by means of [1] as a fullctioll of the pa.ra.tneter wk, changing thc coonlinatcf\ wl: alld lk by steps of fìve degrucs over the range o ~ w", ~ 85 0 o ~ l" ~ :155" the values of t.he stalhlanl duviation ak.m ... (wk, w." l.) = illc/ 1 'i, • ( m,ji (i = 1,2,3,4) [41 are plot.ted in dijiel'ent diagrams .. as a11 example, in fig. 2 we reprcscnt the valncs of ak,m,!!, corref\ponding to w l: = ± 0".0012, with the plus -, l a ab 06 04 (5 1\.n ep . o' ep .lo" ep ' 20' ep ' 30' ep ' 40' ep . 50· ep . 5 0 · 0200 , o· 50' 100" 200" 250· 300' 350· l"i~. 2. r c p l'cllcn t n t.inll nf the valu(' 8 (lf t,h o st ll llllanl lieviatioll (j" t ,," , .. c(ll'!'(ls}iolh ling to w~ = ± ()".{)0 12, wit.h t,ho po~i t i vo sig ll l'd cn ing lo lliot,ion o{ th ~ .a lil cl'k all pl il t.c ;\1\(1 n ega t iv e 5i~1l l o lli ot.i on of i lio e u r;l si illl p la t e. 2!)fj e. l'rov.i!h bio v. que.~ada sign l'cierring' tu mution of t.he americfl.ll blocie ((::aithcl'sb1ll'g ~tfl.tion) and the minus sign to motion uf thtl ellra~ia,n biocie (lliizuf!'''wll, kit,ab, carlofurte stat,iuns). 1<'ur par~h {!le value we thus obtain a, curvp rtlprcscnt.ing' a n abwllltc minimmll of t.he stalldanl dcviation in corrpspondence t.o a determined pail' of w." ln valups. oli the ot,ihlr lmnd, ill l<'ig. :{ wc fina the clll'vpf! of tlip absoiutf, minima uf the htfhhlm·a arwil"tion a" k ,m a.s a iunction of latitwlp ln col'l'esponding to the va.iucs oi w k and rfj/n given in table 2. tahle 2 cude i ± wk i ~, (0".001) ( l) ± 12 -:75° 12 (2) ± " + 8(]o 14 (3) ± 16 , 800 16 (4) ± 12 + 8 ll" 20 (5) ± 15 + 85° 35 (h) ± 12 -, 85° 50 (7) 12 + 85° o ana,loguuh cul'vch a,re given in fig. -i. thc lattei' wpm caiclùatell keeping t.he parameter rfj k = 820 collsta,nt a,nd va.rying tlhl vahips of oh, ,1..<; avppal'h [1'om 'l'able 3. tu.ble ::i code i :l: wk (0".001) ( l) o 30 (2) + 5 30 (3) + lo 30 0: 0 o 6 4 1 2 .1 .6 2 ~ __________ 77~~------------__ ~7~ __________ ~~~4 '5 go 180 270 3 60 l 'il:(. 3. _ d ingmlll of thc cu rvcs 0 1 tho aboolutr mi nim a o f t lu; s(.fi il.~1an) deviatioli a~." :ls a hlllf' tion 0 1 la.ti tud c c~ eorrcspoll ù ing to !.ho \'alucs oi wl; ::m d 'p", givcn in t ahle 2-. 2g8 e. rnovlmbio y. quesada cr 0,12 0,10 0 ,08 0,06 0,04 0,02 (1) w (glo) ~ 0 .0000 (~w (gao)0.0005 (3) w (6 .. ) :0.0010 3 à o,ool~--~o---?o--~o---..,--",o--'",---"'--,,,,--~, o 10 20 30 40 50 60 70 80 90 fig. 4. niflgram of the cm·ves of siamlanl devifltion ('aleulatc(l by kceping thc pamlllctcr <p~ = 820 {·onstant am] varying thc valnes or ili" as appears from tablc 3. ygl.ocl'l'l' a:'o nirectlox of plate diepl\ce)fkkte etc. 2\)(1 fl'om the avemge of the values alm antl l o ohtaùwtl by nlluirruzing thc sta·ntlartl deviations in the variou,~ cascs, thc follow ing va.lucs ior the rotation ccntcl' eoordinates of the .\merican-eurusian pla.t.c were obt,ained: the avera.gctl value oi thc rotation eeuter eool'dinates obtainetl by lc piehon a,nd other authors by utiliziug spre:hling rates and a zimuth oi the trausfol'ln f:mlts given in table 1 is, on t.llc othcl' h a utl, al = g2"n, l = 132°b, comparison oi t.lte v alues oi these latt.cl' eo{mliuater to those 'we calclùated sho,'rs that the two l'otat,ion cent,ers are situatctl approximately ou the sa.me mel'idian lmt ou opposite sidcs 'with respeet to the e:u'th rotatiou polc. 'r he vahwr wc fountl ior thc eoortlina.t,cs ali: antl l k a·re, iu a.ny ca.sc, i nchlcletl within the ellipse oi crror given by lc pichon (2). this ra,thel' cornforting result once again demonst,mtes t,he possihilities of a.fìt.ronomical d ata iu st1hlier ou the sceuhu motion of thc eal' th's el' usl. iu big. 5 we fillli re]lrescutctl in a diagram t.llc valucs oi the st,antla.l'tl tlevi ations a l: corresponding to t.lle values <pi: = 82o~ a·nd l i: = mo\v anel ca.}culateel h y keeping fixwl thc va.luc 01 angular veloeit,y (jj t i 2 ) = + 0".0012 of thc amcrica.n p lat-c a,nel changing the va.1uc (ù k i 1 ) (hav iug a· minus sign) oi t.lle angtùfh veloeity of the sta.tions oi the eurasi a n plate. ]<~l'om the cmve interpolatetl from j!~ig. 5 i tis seen tlmt the most prolmhle value fol' a.ugul: u vclocit,y of thc eurasian plate seemr to be w k (1 ) = o".00295jyear. successively using this valuc and varying the value of o.lt 121 , the m iuimum values of sta.nclarcl eleviatious relativc to t-he various valucs 01 ol k (2) introtluccd 'were cletel'lniuctl. tn fig. 6 the tremi of thc values oi w j; ( 2 ) a·rc l'cpresentecl. 'fhe curve presents a m inilllmll in col'l'espojl(lcnee to the value wk (~) = + 0".00125 . this value cali be consiclcrecl as being thc mostpraba.blc for thc rota.t.ion speetl of thc u .s. amel'icil-n plat-c. it is most int,erestiug to observe t,hat the most pl'ob able v alues of the ful!:,'1llar veloeiticr 2.0 . 1,5 . 1,0 . 0,5 . o aoo e. phove ii bio y. quesada (1),,1 1 ) = 0".00295 (emaflian plate) (0,,(2) = + o" .00125 ( .. a.lllel'ican plate) are in exeellenl agreclllent witll those obtaincd by an entircly imlepeudcnt. way ( ~ ) and rc]lorted in the inll'oduction oi this llhol)er. the optimization melhod ll~ed in tllis paper is efficaciouf\ in t.lle hypolheflifl t.hat lhc nulllbcr of residuals .d<p ! is greater than l this llas allowcd t.he determiimtion, as wc have seen, oi f.lle most lltob0', •. <> • .;_<..:1:1) l o 20 30 40 50 l"ig. 5. diagmm of tl\c valucs or thc sbmiard dcviation corrcsponding to thc valucs <p" = 820n and l" '=-' 570 w and cakuia.tcd by kecping nxerl thc value or angular vclo(~ity or the arncrican platc antl changing thc value or thc eurasian platc. 0'" "" 40 35 . 30 25 5 lo 15 20 25 pig. 6. hcprcscntation or thc trenll o[ thc valncs of w,, {l). t he eurvc prcscnts a rninimum in corrispondence tu the valuc w,,(i) + 0".00125. u::/.2 ) 30 vislocity a~d directio'\ 01" plati, nisplace~m"ts istc, 301 able values of rot.a.t.iun cent.er coor(lin ates and or angulal' vclocities wt relative t.o the two ~\merican and eurhsian blocks, in the case or the se l'acific plate, we have only une value or the quantities l11fi, coltesl)olhiing to the ukiah sta.t.ion (u,s,a,), in this circulllst.ance it. is tlms comllletrly imvossible t o obtain minima fui' the sta,1li1anl errol's a, however, it is possible even in this case to carry out an llnalysis of lllll'rly qualitative interest by !lctcrmining the m,nge of variability of the varametel's fjj, l and (j) which annui t.he resi!luals: [5j in fig. 7, in bom-faced type, the area,s of variabiiity of t.lw pa.i'lk llleters w, l and w am rcpresented, for which the lh'evious condition o 40 80 120 o 30 80 90 10 ~ li 10 h cf 14 i \ / \ \ 14 18 i 8 22 22 w \ w fig. 7. rcpl'cscntatioll of thc a.i'clls of variability thc ]htl'amctcl's 'j;, j, ]l!atc, all,i w rorrespolldill.!; tll thc lllotion of thc amcrica.·pacific. is actually veril1cil. \ve consider it most significant. that also in this case the values of the rotation center c'ool'dinatcs of t.hc americanl'ac'ilic viat.e iletel'mincil by lc pichon (2): rp = 530~, l = 47 o w, cleal'ly indicatcil in the sn,me f ifi. 7, fall within the areas or variability which, in a sense, repmsent t1w optimal areas of t.lw values of the parameters cf> alhl l. 302 e. rnovl;; h jlio v. quesava co::-;-clljsio::'\s to (;onelude: a) ll.~ing a lh'oee!lu1'e completely ilu1epcndcnt or the one used by .a. a. (g), the most llrobable value~ oi thc cool'(lina,tes a,nd veloeity oi l'otation of the america.ll-bnrasian plate wel'e determined; b) thefic vailles are in full agreement with tllo.~e lh'eviomly !lcterrnine(1 by a. a. (") and a,re on the sante o1'l1er a,i'! those ca,lcuiated with geophysicul alhi geological measul'ernenti'!; cl this connrms the possibility of ming astronomical (la.t.itude a,llil longitn!le) obseryation !lata ill the stn!lr oi the secular motion oi thc earth'.~ cl'uflt. referekces (1) cliasl~ c. g., 1!j72. tiic n. pla/.c prol!lcm 01 plale tec/ollics. "geoph. j. r. asti'. soc..", 29, pp. ll7·122. (2) l}~ piclion x., 1068. seu·f!ool' sln'ea~ling allil contilujntal drijt. "j.g.r.",73, 12, pp. 3(iul·3u97. (3) le piciio:s x. anti fox p. j., 1071a. jlargina,! offse/s, frac/ltrc zoi . cs, and 7:j'arly openi),g oj t/te no/,t" a.tlantic. "j.c.r.", 76, 2ij, iip. 62!j4· 6308. (~) le 1'lcllon x., hj7l. in press (private communicn.tion by dr. j. frn.nchcten.u). (~) ~loj\ga:s j., hl7!. in press (private communicn.tion by dr. j. fran· chetcn.u). (' ) provlmmo e. and qul'-sada v., 1!j72. analysis o{ sccnlar polar motion and contincntal {lrift. in presso "bull. ccod.". layout 6 1 annals of geophysics, 62, 5, se568, 2019; doi: 10.4401/ag-8020 “significance of fractal correlation dimension and seismic b-value variation due to 15th july 2009, new zealand earthquake of mw 7.8„ saroj kumar mondal *,1, paresh nath singha roy2, joshi k. catherine1, auchitya kumar pandey3 (1) csir−national geophysical research institute, hyderabad−500007, india (2) department of geology and geophysics, iit, kharagpur 721302, india (3) department of applied geophysics, iit (ism), dhanbad−826004, india 1. introduction the occurrence of high frequency earthquake in any region reveals the development of stress in the crust that leads to deformation. such areas must be focused for understanding the mechanism of earthquakes and hence to find out the probable potential zone for future large size earthquakes. several workers have applied various methodologies to find out the precursory signature for future earthquakes, but a working model is yet to be achieved. puzzling earth scientist has the quest to understand these complex tectonic patterns due to the heterogeneity of the crust and non-availability of any such model for precursory study. however, globally earthquakes rarely seem to be having surface outcrops of active faults. most of the proposed models are of geological rather than actual physical base. thus, with a view to have a more valuable perceptive of the physical model of earthquake occurrence, the fractal statistics have been used in which crustal article history receveid november 6, 2018; accepted april 12, 2019. subject classification: clustering, coulomb stress, earthquakes, fractal correlation dimension, seismic b-value. abstract new zealand earthquake that occurred on 15th july 2009 (mw 7.8) was analysed using fractal correlation dimension (dc) and seismic b-value. we have analysed the earthquakes catalog of thirty-five years with a magnitude (mb ≥3.7), in order to observe a crucial information in terms of dc value fluctuation for the event. the event is preceded by fall and anomalous change in dc value in the year 2007 about two years prior to the mainshock. a sudden decrease in dc value with highly clustered events is observed before the mainshock. the low value of dc is an indicator of clustering and it shows how intermediate size events correlate with one another in the preparation process of this event. here the low dc value may be the indicator for high stress developer along the fault to produce large size earthquake. moreover, we also observed abnormal fluctuation in b-value from 2003. the fractal clustering and scaling of earthquakes are indicated by b-value change prior to strong earthquake as a harbinger of stress correlation in various scales. the event is also marked for that occurred in the periphery of the positive coulomb stress development, as obtained from three low dc time windows' events. the drop in dc value is not a single observation prior to this large event, but such pattern is also seen for other strong events in the study zone. one such well identified strong event is mw 7.2 (2003) along with low dc value prior to the event. thus, stress correlation measured along with these indirect statistical tools gives the clue of self-organization of long wavelength of stress, which was not measured earlier with classical approaches. this type of study may provide a very useful information for hazard mitigation. deformation pattern may be studied using fractal dimension. mandelbrot [1983] was the first to put the idea of fractal geometry and dimension to describe the scale invariance in natural phenomena. latter, the concept of fractal structure has been applied by several researchers [hirata, 1989; feng and seto, 1999; dimitriu, et al., 2000; kayal, et al., 2012; roy and mondal., 2012; han, et al., 2015] to understand the complex mechanism of occurrence of earthquakes. moreover, distribution of earthquakes and its pattern of evolution in different parts of the globe were also analysed [legrand et al., 1996; oncel and wilson., 2002; roy et al., 2012; roy and gupta, 2015; mondal et al., 2016; mondal and roy, 2016]. recently michas, et al., [2015] have shown the significance of earthquake study using fractal analysis. scientific community [hirata et al., 1987] has also reported the laboratory experiment of decrease in fractal dimension with the development of rock fracturing process. similarly the temporal variation of fractal dimension (dc) of seismicity gives the idea of preparation process for the large size earthquakes. several large size earthquakes in different parts of the world have been studied statistically and have been found to be associated with some clustering of earthquakes before the main shock earthquake (roy and nath., 2007; roy and mondal., 2010). the scaling nature in distribution of earthquake occurrences in space and time domain are the general characteristics, which are required for understanding the physics of earthquake leading to seismic hazard assessment [sobolev et al., 1991; tomilin and voinov, 1995; knopoff et al., 1996; telesca et al., 2003a, 2003b; mukhopadhyay et al., 2004; zamani and agh-atabai, 2009; teotia and kumar, 2011; özturk., 2018]. the seismicity distribution in new zealand (figure 1) shows that the deformation is widespread, diffused and persistent. here the scattered nature of seismicity is evident on both regional and local scales (figure 1). new zealand is located along a zone of contact between pacific and australia plates. the motion of these plates leads to the major seismicity in this country. the 15th july 2009 earthquake was the largest in the past 80 years. this thrust-related earthquake occurred within the fiordland subduction zone of southwestern south island at 9:22 utc. the mw 7.8 earthquake ruptured the plate boundary interface between the subducting australia plate and the overlying pacific plate [beavan et al., 2010]. mahesh, et.al [2011] reported that this quake has significantly increased the coulomb stress on the overriding plate particularly on the offshore portion of the alpine fault. here we have applied two statistical parameters dc and seismic b-value fluctuation with respect to one another for more than thirty years to see the anomalous behaviour before the new zealand earthquake (mw 7.8). further, coulomb stress change prior to event has been studied to show the significance of dc and b-value study. 2. geo-tectonic setting of the study area intense seismicity is experienced by the people of new zealand, due to its tectonic plate motion (fig.1). australia and pacific plate’s boundary run along the puysegur fault, puysegur trench, the alpine fault and the hikurangi trench. the country is adjacent to the subducting oceanic crust of pacific plate below the continental crust of australia plate to the northeast at hikurangi margin. on the other hand, australia plate is subducting below the pacific plate to the southwest of the country, at puysegur margin. a very long alpine fault of 650 km crosses the country and link the two subduction margin. during the early miocene, displacement of about 480 km was estimated due to strike slip deformation along the alpine fault. new zealand”s southern alps is considered to be formed due to the estimated shortening of about 70-90 km across the alpine fault, during the last 6.4 myr. the plate motion is varying between 35-40 km/myr from northern puysergur trench to southern hikurangi margin [de mets et al., 1994]. the intense seismic activity in the fiordland region was marked with both shallow and intermediate earthquakes. shallow earthquakes of dip-slip and strikeslip focal mechanisms are found, distributed in a narrow zone along the fiordland margin [smith and davery, 1984; reyners, 1989; anderson and webb, 1994]. tectonically and geographically the region is very complex. 3. data set for the fractal analysis of the new zealand earthquake, we prepared the catalog using the global catalog of usgs and catalog of geo-net, considering earthquakes of (mb≥ 3.7) during the period 1973-2009. in our analysis epicentral distance based fractal analysis was considered but not the hypocentral location due to the presence of higher error values in hypocentre. the completeness magnitude plays a vital role in finding out the significance of stress analysis. considering the significance of statistical analysis, we merged the two catalogue mondal et al. 2 3 significance of dc and b-value analysis before large earthquake to make our data set complete for this type of analysis. we estimate the completeness magnitude using classical gutenberg-richter [1944] relation as given in fig. 2a. as we know that the earthquake scaling and frequency of occurrence relations requires that small earthquakes will be just as important as larger ones in redistributing the forces that drive relative displacements across active faults of any dimension, including plate boundaries [hanks, 1992]. so we have taken care of that in our analysis. while considering the catalogue of usgs and local catalogue of geo-net, for the entire periods from 19732009, we felt that the earthquakes which are missing in one agencies are filled up by the other agency which are very much necessary to give better idea about the stress pattern / seismicity distribution. here we focussed in finding the pattern of dc value only before the main event of 15th july, 2009. so the data set after 2009 has not been taken for the analysis. the temporal variation of dc and b-value are estimated by taking successive 100 events” windows in the latitude range 40°s 50°s and longitude range 162° e 178° e (figure1). we have taken all parameters from global centroid moment tensor (gcmt) catalog for coulomb stress analysis. 4. methodology 4.1 correlation fractal dimension (dc) correlation integral method [grassberger and procaccia, 1983; roy and ram, 2006; roy and padhi, 2007; roy and mondal, 2012a, 2012b] is applied for this set of earthquakes to determine correlation dimension. correlation function is defined as: (1) where n = total number of vector pairs, 𝑟𝑖𝑗 = distances between the points of a set, 𝐻 = heaviside step function, 𝑟 = length scale. fractal structure of the epicentral distribution is obtained by the relation given below: c(r) ~ rdc (2) 𝐶(𝑟) = � � 𝐻(𝑟–𝑟𝑖𝑗) 2 𝑁(𝑁–1) � � 𝑗=1 𝑖=𝑗+1 3 figure 1. tectonic map of study area with significant fault system and seismicity in and around the main event that occurred dur− ing the study period. mondal et al. 4 figure 2. a) cumulative frequency magnitude distribution of seismicity for new zealand earthquake for the present study. b) plot for log c(r) versus log r is shown for low dc value before the main event. c) plot for log c(r) versus log r is shown for high dc value for the window of main event a) b) c) 5 significance of dc and b-value analysis before large earthquake the plot of c(r) versus r in logarithmic coordinates is used to get correlation dimension (dc).two plots for log c(r) versus log r are shown in fig. 2b for low dc value before the main event and fig. 2c for high dc value for the window of main event. 4.2 seismic b-value the b-value has been computed using the classical guttenberg richter relation [gutenberg and richter, 1944], and the equation is given as: log10 n(m) = a‐bm (3) where n(m) is the number of earthquakes with magnitude larger than or equal to m, “b” is scaling parameter and “a” is a constant. the plot for data set is shown in fig. 2a and the power law is valid. self-similar property of earthquakes occurrence is considered in this law. the constant “a” depends on the overall seismicity of the region. the b-value is normally 1, but it varies from 0.5 to 1.5 depending on the seismicity and tectonic setting of a seismically active region [reasenberg and jones, 1989; kisslinger and jones, 1991]. here we have considered sampling of 100 events for estimation of b-value with standard deviation of ~ 0.1b. the error value increases as the number of sampling earthquakes decreases. we can estimate b-value with minimum of 25 events having standard deviation as low as ~ 0.25b [bernice, 1983]. the estimation of b-value is more reliable if the sampling subset contains minimum of 100 earthquakes. the higher the size of subset more reliable it is [utsu, 1965]. 4.3 coulomb stress change coulomb stress change may give the idea of stress change in the earth crust due to occurrence of the earthquakes prior to large size event and hence the pattern of seismicity in the study region. observation of increased stress due to an event may explain how other events may be induced by the previous one in the fault zone. the earthquakes that occur in the increased static stress are supposed to have been advanced toward failure by the positive stress increment, where the negative stress region are considered to be more relaxed. static stress change may be the cause of rupturing processes due to previous events. it has been seen that the small, medium and large aftershocks generally do occur in the region of “stressed-up” by a mainshock or damaging earthquakes and that generally do not occur in the “stress-shadowed” region [ellsworth et al.,1981; simpsons and reasenberg, 1994; jaume and sykes,1996]. researchers have also shown the relation between stress change and seismicity rate using some studies on the coulomb stress relative to some global large earthquakes [reasenberg and simpon; 1992; stein et al. 1992; king, et al., 1994; stein, 1999]. here, we have applied in this study to see how the stress is correlated with the low dc value patches which are considered as the possible highly stressed area. here, we have computed the stress changes for entire study region by considering the 2009 earthquake (mw 7.8) as mainshock earthquake source, whose strike, dip and rake values are 26°, 24° and 140° respectively [mahesh et al., 2011]. coulomb failure stress, δcfs has been computed in the elastic half-space associated by mainshock on the assumption of poisson’s ratio of 0.25 and shear modulus values of 32 gpa [okada, 1992]. furthermore, we have utilized the equation (4) to examine the coulomb stress changes given by king et al., (1994), which is related to the shear stress changes and normal stress changes [king et al., 1994; stein, 1999; shan et al., 2013; liu et al., 2017]. δcfs = δτ +µ′δσ (4) δcfs > 0 fault plane loaded δcfs < 0 fault plane relaxed where, δτ is shear stress changes, µ′ is the friction coefficient and δσ is normal stress changes (positive for unclamping). for δcfs calculation, the constant friction coefficient value has been assumed for fault µ′ = 0.40 and also uniform slip model has been considered to estimate the δcfs for the entire study region. we checked the result through two different tests for entire region by using different values of coefficient of friction 0.5 and 0.6 to check the reliability of results and it has been found that very small changes are existing and that can be neglected. previously, several authors [ellis et al., 2006; boulton et al., 2012; barth et al., 2013] utilized the low coefficient of friction in the range of 0.12-0.37 but it was found that these values show the weak results. moreover, the value of coefficient of friction (µ′ = 0.40) has been considered as standard value by several authors [gahalaut et al., 2011; toda and enescu, 2011; xiong et al., 2017; pandey et al., 2019] for different tectonic setting to compute the coulomb stress changes that’s why we have used the coefficient of friction value 0.4 for this study region. generally, the coefficient of friction is dependent on the category of rocks [byerlee and brace, 1968; byerlee, 1978] therefore entire result is based on rock type. we have used this parameter to observe the change in stress in two different plots, one with three time windows events that produced low dc value and second with the entire data set used in the study region to see the pattern of stress transfer. we have estimated coulomb stress changes by utilizing all data available from gcmt catalogue for three low dc time windows and the entire dc time windows from gcmt catalog. 5. results and discussion fault systems for a seismically active region are multifaceted natural arrangement unveiling scale-invariant or fractal deformation. the manifestations of earthquakes follow the fractal distribution property both temporally and spatially. the distribution of fault systems and earthquakes manifestation may be quantified by a single value called fractal dimension (dc) which follow power law distribution. the deformation in general obey power-law scaling of fault length or earthquake distribution. we can measure both the heterogeneity and the clustering of earthquakes evolution using fractal dimension of earthquakes scattering [oncel et al., 1996]. a highly stressed region is associated with dense clustering of smaller and medium size earthquakes which indicates strain release of the accumulated energy due to tectonic plate motion. these highly clustered region of earthquakes may be the most possible weak zones for potential site for future large size earthquake. high clustering of events in a region has been observed with low fractal dimension value leading to information of seismicity before large earthquakes [telesca et al., 2001; roy and padhi, 2007; roy and mondal, 2010]. significance of occurrences of low fractal dimension associated with high clustering before the large earthquake have also been reported by many researchers (hirata et al., 1987; roy and ram., 2006; roy and nath., 2007; roy and mondal., 2010 ). the degree of clustering of earthquakes is inversely proportional to the dc value. so the lower value is associated with high clustering and vice-versa for a region [hirata et al., 1987; kagan et al., 2006]. fractal correlation dimension (dc) of seismicity of a region is utilised to find out the estimated clustering of earthquakes. 6. correlation fractal dimension (dc) analysis the present study for the new zealand earthquake (mw 7.8) shows a very good and clear indication of highly clustered events before the mainshock. we observed densely clustered events both in the case of using entire data set for this study (figure 1), as well as more prominently in the case of three low dc value contributing events (figure 3). these low dc values are observed before the mainshock (figure 4). diagnostic pattern is revealed with highest dc value in the year 2003 and one lowest dc value in the year 2007 prior to the 15th july 2009 earthquake of mw 7.8 as shown in fig. 4 and table 1. this anomalous variation in dc value occurred just few years before the event. in the process of the mainshock three consecutive low dc value were observed, which are the most clustered pattern of seismicity (figure 3). these windows have the dc values viz. 0.98 during april 2006, 0.94 during march 2007 and the third being the lowest is 0.66 during september 2007 (table 1). dc value of 100 events windows clearly depicts the fluctuating nature of statistical value called dc with time (figure 4) and table 1. however it may be noted that the low dc value characteristics is unique with space and time. the reason behind is particularly due to the possible approach of algorithm to quantify actual relative clustering spatiotemporally. further it is known to us that the physical or geological facets change with space and time. especially the geochemical, thermodynamic etc. processes controlling the elasto-dynamics of the entire seismogenic study zone. in fact in support of these we can see the how the earthquakes cluster may easily be observed with the data set as shown in figure 3 and figure 1. similarly the dc patches are plotted using the above data set and we find the lowest dc patch happened to be close to the mainshock (figure 5) and (figure 6). the low dc contour as observed in figure 5 gives clear indication of highly stressed zones [roy and padhi, 2007; oncel et al., 1996; roy and mondal, 2010] and the main event occurred close to it. essentially the coulomb stress study also shows that the main event has occurred in the “stressedup” region (figure 7). here the clustering pattern of intermediate size events prior to main event is an example of strain liberation in self-organized fashion. stress wavelength especially the longer one i.e regional one tries to get correlated with the smaller one in local scale. but this correlation can be observed only by seeing the clustering of earthquakes in fractal scale invariance or statistically self-similar fashion. we observed highly anomalous variation in dc value for this large size earthquake. the study area has also experienced other strong events. the strong events in different time windows is listed in table 1. relative low dc value for the earthquake mw 7.2 (2003) is also observed in fig. 4. the clustering of earthquakes prior to this event is also seen (fig.8). the clustering of events prior to other strong earthquakes are also noticed (figure 9). other two strong earthquakes of magnitude mw 7.1 (2004) and mw 7.4 (2007) are observed with clustering pattern figure 9 and mondal et al. 6 7 significance of dc and b-value analysis before large earthquake figure 4. temporal variation of dc value for 100 events window with error bar. figure 3. spatial distribution of seismicity that contribute to three low dc value. these events have induced low dc value win− dows prior to main shock. mondal et al. 8 figure 5. three low dc value contour for the new zealand earthquake of hundred events window. the lowest dc values patch rep− resents possibly stressed region and clustering of events. mean time (100 events windows) dc value b-value magnitude (maximum in each window and source of data) 92.22 1.21 0.66 7.2 (1981) (usgs) 209.50 1.07 0.66 6.5 (usgs) 250.90 1.29 0.69 7 (usgs) 295.10 1.15 0.69 6.6 (geo-net) 341.00 1.03 0.76 6.2 (usgs) 362.58 1.13 0.63 7.2 (2003) usgs 372.63 1.36 1.24 5.9 ( usgs) 372.70 1.65 1.29 5.4 (2003) (geo-net) 373.08 1.16 0.47 6 (geo-net) 374.71 1.20 0.39 6.4 (geo-net) 382.39 1.29 0.37 7.1(2004) (usgs) 393.84 1.15 0.88 5.7 (geo-net) 404.77 0.98 1.32 5.4 (usgs) 415.67 0.94 1.15 7.4(2007) (usgs) 422.01 0.66 0.76 6.8 (usgs) 427.56 1.21 0.71 6.1 (usgs) 438.24 1.22 0.82 7.8 (15th july 2009) (usgs) 444.43 0.87 0.66 6.2 (geo-net) 444.55 0.87 0.99 5.6 (geo-net) 445.80 1.04 1.05 6.1 (usgs) table 1. mean time of 100 events windows for determination of dc and b−values. figure10 mw 7.2 (2003) earthquake occurred after two decades of event mw 7.2 (1981). on the other hand events mw 7.1 (2004) and mw 7.4 (2007) occurred during the first and third years respectively after the event of mw 7.2 (2003). the occurrence of these two events of magnitude 7.1 and 7.4 within a very short period indi9 significance of dc and b-value analysis before large earthquake figure 7. coulomb stress change due three low dc time windows’ events. figure 6. dc value contour for the new zealand earthquake of hundred events window of all the data set used. the lowest dc val− ues patch represents possibly highly stressed region and clustering events. mondal et al. 10 figure 9. spatial distribution of seismicity that contribute to three low dc value for the earthquake of mw 7.1 in 2004. figure 8. spatial distribution of seismicity that contribute to three low dc value for the earthquake of mw 7.2 in 2003. 11 significance of dc and b-value analysis before large earthquake figure 11. temporal variation of b−value for 100 events windows. figure 10. spatial distribution of seismicity that contribute to three low dc value for the earthquake of m 7.4 in 2007. mondal et al. 12 figure 13. b−value contour for the new zealand earthquake of hundred events window of all the data set used. figure 12. three low b−value contour for the new zealand earthquake of hundred events window. cate the probable reason for the fracturing process of the main event of mw 7.8 in 2009. we do not have the sufficient data to see the dc value variation for the event of mw 7.2 (1981). low dc value variation prior to strong events can be observed in figure 4. 6.1 seismic b-value analysis similar to the dc value anomaly, we are able to see such significant changes in b-value before the mainshock. changes in the b-value are seen in the plot of frequency versus magnitude as shown by figure 11. the plot of b-value clearly depicts the highly abnormal changes in the b-value since august 2003 figure 11 and table 1. the event is observed to have occurred after the abnormal increase and decrease of b-value. there were rapid variations in dc and b-value almost six years before the mainshock. the b-value of each 100 events windows were determined and listed in table.1. the lowest b-value is 0.37 and highest is 1.32. highest dc value of 1.65 corroborates with the b-value of 1.29. this may be supported with the power law validity for both the methods with the dispersed seismicity. three low b-value patches of fig.12 show the anomalous changes near the main events, which is not reflected by b-value of all the data set used (fig.13). further, coulomb stress change for three low dc time windows clearly indicate the event has occurred in the positive δcfs region (figure 7), which is again not reflected in the entire data set (fig.14) that used for coulomb stress study. the change or variation in bvalue from usual value of 1 is dependent on other related factors of the study region. high b-value was reported by mogi [1962] due to increase in the material heterogeneity or crack density. similarly decrease in bvalue was noticed due to increase in applied shear stress [scholz, 1968a, 1968b], or an increase in effective stress [wyss, 1973]. the b-value gradually decreases before a major fracture, even under a constant load [mogi, 1981]. so, there is a possibility of gradual decrease in the bvalue before a large size earthquake. we observed increase and decrease in b-value prior to this event as in fig.11. no matter whatever, the mechanism may be, it is generally recognized that there is a tendency for the bvalue to decrease prior to a large earthquake. so undoubtedly this provides a clue in foreshadowing large size earthquakes. but this does not necessarily mean that a large earthquake will occur when there is a decrease in the b-value. essentially this observation will help us to understand the crustal activities for future study. 6.2 coulomb stress analysis we have calculated the coulomb stress changes of 126 earthquakes dataset for the entire region of the study area, as depicted in the figure 14. from this figure, it has been noticed that the highest stress changes are ob13 significance of dc and b-value analysis before large earthquake figure 14. coulomb stress change using the entire time windows’ events. tained in the northern part of the study region i.e. towards the australia plate. on the other hand pacific plate region is dominated by low-stress changes. low or average stress changes have been observed in the rest of the study area. the coulomb stress change ranges lie between 0.05 to -0.05 bar. fig.8 depicts the stress change due to three low dc time windows. low to high stress variation is noticed in different parts of the study region. distributions of coulomb stress estimated using the entire data set, which shows that the event mw 7.8 has occurred in the negative values of δcfs region figure 14. then the area should be considered as stable due to low stress and hence no large earthquake should be expected. on the other hand, coulomb stress obtained by using the three low dc time windows clearly indicates that the event has occurred in the positive values of δcfs region. (figure 7). thus the low dc patches (figure 5) which is considered the possible highly stressed region corroborates well with stressed region those obtained by coulomb stress change, as depicted by figure 7 for three low dc time windows. not only this, distribution of low b-value patches showed the abnormal pattern of the events that has occurred in that region as given by figure 7 and figure 12. we observed that the low dc patches which are possibly highly stressed region found to correlate well with other seismotectonic parameters. 7. conclusions the aim of this study is to observe how the variation of dc and b-value for new zealand earthquake of mw 7.8 event behave prior to mainshock and its possible correlation with coulomb stress. we observed highly anomalous fluctuation of these two statistical parameters prior to the main shock. on the other hand, we don’t find such high fluctuation in the values of these parameters in the region with other strong events, though we have observed drop in dc value. this may be the possible reason for instability in the area for large size earthquake. correlation of dc/b-value with coulomb stress shows that the mw 7.8 event has occurred in the positive value of δcfs region as observed from three low dc time windows, as shown by figure 5, figure 7 and figure 12. this result also show that the possible highly stressed region, as seen in figure 5 corroborated well with the coulomb stress change figure 7 for the considered mw 7.8 event. these two seismotectonic parameters that have an important role to play in providing information of pattern of seismic activity of the region. we may come up with very good relation with gps based crustal deformation observation for longer period, and hence segregate the highly hazardous zone for future large size earthquake. but the important thing is to be noted that for this type of analysis, the data set should be of large quantity and should be complete even for lower magnitude earthquakes. authors would like to express their gratitude to dr v. m. tiwari, director, csir-national geophysical research institute, hyderabad for his kind permission to publish the work. first author is deeply thankful to dr v. k. gahalaut, director, national centre for seismology, ministry of earth sciences, india, for his valuable suggestions and kind help. we acknowledge global cmt, usgs and geo-net for the earthquakes catalog data. we thank the two anonymous reviewers and editor, barbara orecchio for their constructive suggestions. references anderson, h. and t. webb (1994). new zealand seismicity: patterns revealed by the upgraded national seismograph network, new zealand, j. geol. geophys., 37, 477-493. barth, n. c., c. boulton, b. m. carpenter, g. e. batt and v. g. toy (2013). slip localization on the southern alpine fault, new zealand, tectonics, 32, 1-21. beavan, j., s. samsonov, p. denys, r. sutherland, n. palmer and m. denham (2010). oblique slip on the puysegur subduction interface in the 2009 july mw 7.8 dusky sound earthquake from gps and insar observations: implications for the tectonics of southwestern new zealand, geophys. j. int., 183, 1265-1286. bernice, b. 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1. introduction seimoelectric and seisomomagnetic effects in rock were first measured and described in the late 1930’s, and since that time have been the subject of various analytical investigations. the electromagnetic effects are explained by tectonic processes that occur in rock prior to an earthquake (see, e.g., stacey, 1964; mogi, 1985). in this article, another possible model of electromagnetic excitation over an active volcanic zone is proposed. consider magma as a two-phase liquid (liquid and gas) with viscosity of approximately ν = 5 m2/s and a density ρ ≈ 2500 kg/m3 (marchinin, 1985). initially, magma is found at a depth of approximately 100 km. when volcanic activity increases, magma moves upward toward the earth’s surface through volcanic channels, filling intermediate volcano capacities whose depth can vary from 2 to 20 km and whose radii vary. for example, the capacity under the kluchevskii volcano is estimated to be 4 km deep with a radius of 3600 m (fedotov et al., 2000) and the capacity under the shiveluch volcano is 20 km deep with a radius of 6000 m. just prior to an earthquake, the movement of magma becomes more intense, and waves and vortices develop. in the presence of the earth’s constant magnetic field, these movements induce the formation of variable magnetic fields with different periods which then contribute to the common electromagnetic emission that is present before the earthquake starts (ismaguilov et al., 2001.) we will now discuss the possible mechanisms of excitation of magnetic fields in magma and estimate their magnitude. 2. hydrodynamic processes in magma consider three kinds of hydrodynamic processes in magma that can lead to electromagnetic emission. a possible model for initiation of ulf oscillation in magma yuri a. kopytenko and lidia v. nikitina st. petersburg department of institute of terrestrial magnetism, ionosphere and radiowave propagation (spbfizmiran), russian adademy of science, st. petersburg, russia abstract during the period just prior to an earthquake, an electomagnetic emission develops over seismic zones. in this paper, a model of excitation of magnetic fields over zones of volcanic activity is proposed. movement of magma along volcanic channels precedes an earthquake, hydrodynamic processes in the moving magma can lead to formation of waves and vortices in the flow which, in turn, can cause development of magnetic fields in conducting magma. during this period, the movement of the magma intensifies leading to a corresponding intensification of the magnetic fields. in this paper, different possible sources of ulf pulsation in magma are examined, and the variable geomagnetic fields induced by this pulsation are estimated. mailing address: dr. lidia v. nikitina, st. petersburg department of institute of terrestrial magnetism, ionosphere and radiowave propagation (spbfizmiran), russian adademy of science, p/b 188, muchnoi 2, 191023 st. petersburg, russia; e-mail: lida@mail.spbnit.ru 102 yuri a. kopytenko and lidia v. nikitina 2.1. lifting of magma in magma channels prior to the seismic event, magma moves in volcanic channels toward the ground’s surface. magma velocity is approximately 1-5 m/s (marchinin, 1985; slezin, 1999). the electromagnetic fields can be induced by magma flow in the presence of the earth’s constant magnetic field. these fields are quasi-constant, and their values depend on the velocity of rising magma and the radius of the volcanic channel. 2.2. oscillations at the magma surface the possible sources of the ulf electromagnetic pulsations are the oscillations that can appear at the magma surface (fig. 1). inertial waves appear on the surface of liquid between two media with different densities. the frequency of waves on the magma surface depends on the relative change of density between the gas and the magma and on the diameter of the channel. f gk 2 2 1 2 1= +t t t t_ _` i ij (2.1) where k is the wave number, ρ1 is the gas density, ρ2 is the magma density. relative changes in the density can be estimated as (ρ2 − ρ1)/ (ρ2 + + ρ1) ≈ 0.5. if the magma surface is in the intermediate volcanic capacity with diameter d ≈ 103 m (k = = 10−3 m−1) then according to (2.1) oscillations exciting in magma have frequencies of approximately f = 10−2 hz (t = 100 s). if magma is in a channel with a diameter of approximately 100 m then oscillations with period ranging 10-30 s are excited in the magma. bubbles of gas disturb the magma surface constantly and support the oscillations on this surface. increasing volcanic activity leads to the appearance of new gas bubbles. gas moves in a channel with a velocity greater than the upwelling velocity of the magma (proussevitch et al., 1993; slezin, 1999). step by step, a large volume of bubbles accumulates at the magma surface where they create a foam structure. this leads to a smaller difference in density between the magma and the gas, and inertial waves become weaker. the decrease in wave intensity must be typical for the duration of the preearthquake period when a large amount of gas accumulates in the magma. after the magma becomes free of the «foam cap» (for example, when new cracks appear), the density difference between the gas and the magma is restored, and inertial waves are initiated at the magma surface again. so, at the frequency of inertial waves we can observe that an increase in ulf magnetic pulsation is replaced by a decrease in oscillation corresponding to increased volcanic activity. 2.3. influence of coriolis force to the magma movement another cause of the ulf movements in magma is related to vortex structures that appear in magma flow (see fig. 1). when magma is filling a large intermediate volcano capacity, vortex flow appears in magma according to coriolis force v2= ωrv r 2 { where r is the vortex radius, ω is the angle velocity of the earth’s rotation, ω ≈ 7⋅10−5c−1, vϕ is the rotation velocity of liquid, vr is the radial velocity. if we assume that vr is approximately fig. 1. oscillations and waves in a volcano capacity. the magnetic field bz induced by the vortex in the volcano capacity. 103 a possible model for initiation of ulf oscillation in magma equal to the upwelling velocity of magma in the channel and the radius of the volcanic center is approximately r = 1000 m, then the liquid rotates with angle velocity 5.10− 3 rad/s and rotation velocity vϕ = 5 m/s. 3. electromagnetic emission induced by hydrodynamic processes in magma let us estimate the magnetic fields induced by the processes in magma described above. the most difficult problem connected with the estimation of electromagnetic fields is estimating the conductivity of magma. conductivity of basalts is estimated to be 10− 3 sm/m at the earth’s surface, but their conductivity depends strongly on temperature. magma temperature in volcano channels reaches 1000-1300°c. if temperature increases to the values 500-1000° then conductivity increases on 2-3 fold (van’yan, 1983). according to these data we assume that conductivity of hot magma is approximately σ = 10− 1 sm⋅m−1. 3.1. magnetic fields induced by rising magma flow assume lifting magma in a channel is similar to the cylindrical flow of a conducting liquid. the magnetic field induced by the cylindrical flow can be estimated as (parker, 1979) b b rvz x z0. nv where b0x is the horizontal component of the earth’s magnetic field, r is the radius of the cylinder. if we assume that σ = 10−1 sm⋅m−1, b0x = = 2⋅104 nt, vz = 1-5 m/s and the capacity radius is r = 1000 m then bz ≈ 2-10 nt (fig. 2). the increase in the magnetic field during early seismic activity can be initiated by the acceleration of the flow. 3.2. electromagnetic emission induced by inertial oscillations the vertical component of the magnetic field bz induced by inertial oscillations can be estimated from the induction equation nvrot v b0# = b∆^ h where σ is the media conductivity, v is the oscillations velocity, b0 is the earth’s magnetic field. the magnetic field induced by the wave is approximately b b k v kz z x x z0 2 = nv (3.1) where kx and kz are horizontal and vertical components of the wave vector. for inertial oscillations with t = 100 s according to (3.1) the induced magnetic field is bz ≈ 0.1 nt, if kx = 10−3 m−1, vx = f / kx = 10 m/s, kz = 10−2 m−1. for inertial oscillations with t = = 10-30 s the induced magnetic field is approximately bz ≈ 0.01 nt if kx = 10−2 m−1, vx = = 1/( f⋅kx) = 6 m/s, kz = 10−1 m−1. as the period of oscillation increases, the magnitude of the induced magnetic fields increases too. 3.3. the magnetic fields induced by the vortex in intermediate volcano capacities a vortex in a conducting liquid in the presence of a constant magnetic field induces a fig. 2. the magnetic field bz induced by the lifting flow of magma. r is the radius of the channel or magma capacity. 104 yuri a. kopytenko and lidia v. nikitina constant azimuthal magnetic field (parker, 1979) b v b d 2z0= nv{ { where vϕ is the rotation velocity, d is the depth of rotating liquid. the value of the magnetic field induced by magma movements in intermediate volcanic capacity is approximately bϕ = 2.5 nt for d = = 1000 m, vϕ = 5 m/s, s = 10−1sm⋅m−1. the vertical magnetic field bz over the vortex is calculated as a field of circular current jϕ = = σ (vr × b0 z) b j r ds2z s = n ## _ i where s is the magma surface. then the induced magnetic field is approximately bz = µσπ⋅ · b0z r0vz. the qualitative estimation gives a value bz ≈ 10 nt (see fig. 1). the value of the magnetic field that can be measured on the earth’s surface, at distance r from the magnetic field source is approximately .b r b r v r2z z z0 0 3 2 = nvr] g at distance r = 100 km from the volcanic center the measured magnetic field must be approximately 0.05-0.1 nt. the corresponding gradient of the magnetic field is approximately ∇bz ≈ 1 pt/km. these theoretical results are in accordance with the measurements. in practice, the vertical component of the magnetic fields decreases as r−n where n ≈ 1.2 (ismaguilov et al., 2001) and the gradient of the magnetic field is 1-5 pt/km. 4. comparison with observed data let us compare the described mechanism of magnetic field excitation with the results of actual measurements. measurements of ulf electromagnetic disturbances were carried out in japan before and during a period of seismic activity. (ismaguilov et al., 2001). behavior of the magnetic field prior to the earthquake has the following stages: 1) the magnitude of the magnetic field starts to increase ≈ 1.5 months before the seismic activity in the frequency band 0.001-0.01 hz. 2) on the background of the common increase oscillations of the magnetic field value are observed with the period of approximately some days. 3) the magnetic field decreases sharply (1.5-2 days) prior to the earthquake. according to the mechanism described above, the increase in the magnetic field just prior to an earthquake is related to the lifting of magma to the earth’s surface. oscillations of the magnetic field can be related to changes in magma levels in volcanic channels and capacities and with inertial waves at the surface of magma. sharp decrease of the magnetic field 1.5-2 days before the earthquake can be explained by slowing of all movements in magma before the beginning of the earthquake, by the great amount of gas bubbles and/or hard particles that accumulate at the magma surface and difficulties in upwelling of magma and also vortex and wave movements in it. the observed values of the magnetic fields are about 0.01-0.4 nt, that corresponds to the estimations made. 5. conclusions we can assume that hydrodynamic processes in magma are one of the possible causes of electromagnetic emission. in this work, three different possible mechanisms for excitation of magnetic fields were examined. these are: i) magma flow in volcano channels; ii) inertial waves at the magma surface that contribute to the magnetic field at the frequencies 0.01-0.1 hz; iii) vortex flows that appear in magma when magma fills intermediate capacities. a general increase in seismic activity leads to an intensification of the hydrodynamic processes in magma and to changes in electromagnetic emission. 105 a possible model for initiation of ulf oscillation in magma acknowledgements the authors are very grateful to dr. richertboe for her help and useful discussions. references fedotov, s.a., i.s. utkin and l.i. utkina (2000): the estimation of scales of core volcano centers, volcanol. seismol., 3, 3-14. ismaguilov, v.s., yu. a. kopytenko, k. hattory, p.m. voronov, o.a. molchanov and m. hayakawa (2001): ulf magnetic emissions connected with under sea bottom earthquakes, natural hazards earth sys. sci., 1, 23-31. marchinin, j.k. (1985): volcanism (nedra, moscow), pp. 285 (in russian). mogi, k. (1985): earthquake prediction (academic press. tokyo), pp. 355. parker, e.n. (1979): cosmical magnetic fields. their origin and their activity (claredon press, oxford), pp. 858. proussevitch, a.a., d.l. sahagian and v.a. kutolin (1993): stability of foams in silicate melts, j. volcanol. geotherm. res., 59 (1-2), 161-178. slezin, yu.b. (1999): the mechanism of volcanic eruption (a stationary model), p. 127 (in russian). stacey, f.d. (1964): the seismomagnetic effect, pure appl. geophys., 58, 5-22. van’yan, l.l. and p.p. shilovskii (1983): the deep electroconductivity of oceans and continents (nauka, moscow), (in russian). geo 1295 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 impact of cost 271 peter a. bradley (1) and bodo w. reinisch (2) (1) pandora, scotlands drive, farnham common, slough, berks, u.k. (2) environmental, earth, and atmospheric sciences department, center for atmospheric research, university of massachusetts, lowell, ma, u.s.a. this article discusses the significance of the achievements of the cost 271 action on science and technology for space weather and telecommunications in europe and the world. the action’s work has impacted national and international projects and the decision processes. the key words encompassed in the title of cost 271 are «space weather». but as the reader of this final report will appreciate, many more topics were addressed during the action by the large team of workers from a wide range of countries and organisations than this wording would suggest. relevant to the performance of telecommunication systems that rely on the presence of the ionosphere for propagation support, or that are affected by transmission through it, there have been investigations among other items of solar and magnetosphere disturbances on the ionosphere, satellite and ground-based measurements of the ionosphere, assembly of near-real-time databases of ionosphere information on the web, studies of planetary and gravity waves in the ionosphere, ionosphere modelling, mapping and forecasting, long-term changes, ray-path deviations in the presence of irregularities, channel-scattering functions, and scintillations on earth-space paths. the impact of all this work on the outside communities can be considered within three broad headings as follows. 1. the final report and published material the final report by its limited size has to be, of necessity, restrictive of content, and to include just a selection of topics with only brief comments for each. in some senses then it serves rather like an encyclopaedia giving a range of headings, where the reader is referred elsewhere for the fuller stories. indeed an extensive series of lists of references are included, and it is in the details of these that much information on what has been done may be obtained. throughout the lifetime of cost 271 there have been, as previously noted in the report, three very successful workshops with specific themes, at which in total 172 papers were presented. the written versions of these papers appear on two cd’s plus a dedicated web site. there have too been a number of stand-alone papers tabled at management committee meetings. there is also an impressive list of single-topic papers presented at multi-nation conferences and published in established peer-reviewed international journals. the body of all this information, considered collectively, can be regarded as a legitimate product of cost 271 achievements. with so much information it is understandable that there is no single body or organisation as the natural recipient of these results. on the one hand there are international groups that can benefit from cost 271 findings as discussed below. there are also individuals and single radio-user organisations that may find solutions to their problems of the moment in the texts of cost 271. undoubtedly this final report is a first step in learning what has been done and is available, but these reviewers feel that consideration should be given to making this information more generally available through a wider publicity medium, probably via a permanent general access web-based source. this is most likely a problem encountered by all completing cost actions that should be addressed on a commission basis. 1296 peter a. bradley and bodo w. reinisch nevertheless, throughout the lifetime of the action the management committee has been conscious to ensure the greatest liaison with those outside groups that can best take account of the work accomplished. the chairman and vice-chair have taken every opportunity to publicise and draw attention to cost 271 activities at many relevant and related meetings of outside bodies. the action has been fortunate in including among its membership a number of key-players who hold office posts within other groups, and this has helped in a two-way synergy in passing on products, in learning of requirements and in obtaining suggestions from the international community at large for the studies necessary for the solution of the problems of the day. an obvious benefactor of cost 271 results is the international telecommunication union (itu), which body serves the role of laying down standards for the operation of the world’s telecommunication systems, with in particular its radiocommunication sector (itu-r) examining technical aspects of radio systems and making recommendations to the itu on best engineering practises for radio-service planning. specifically the work of cost 271 falls almost entirely within the area of study group 3 (propagation in ionised media) of the itu-r. the study groups of the itu-r, consisting of national delegates from member countries, meet periodically to consider formal proposals for additions or amendments to standing texts. cost 271 has been fortunate in that several of its participants have served as national delegates, thereby providing an avenue for the transmittal of products and the presentation and discussion of these at the appropriate meetings. of particular mention are the submission and adoption, with associated computer programs being maintained by the itu-r secretariat, of the median electron-density model height profile nequick for the determination by integration of the associated total electron content, and the scintillation model gism, generated under esa auspices but with cost 271 involvement, for estimating scintillation amplitude and phase fluctuations on earth-space paths. at the same time cost 271 has been made aware of itu needs through the standing formal questions of the itu-r. the international union of radio science (ursi) does not have a direct role in the control of radio usage, but through its periodic general assemblies and ongoing working groups that both correspond informally and hold occasional limited-agenda meetings, it is able to address some of the topics of an intricate and perhaps more scientific element that cannot be considered within the formal nature of itu-r sessions. commission g of ursi has as its remit «ionosphere radio and propagation». several cost 271 participants are or have been office holders of ursi working groups and have attended assemblies and group meetings. ursi is a formal member of the itu and cost 271 participants have served on a special joint ursi-itu scientific committee on telecommunications with the goals of seeking to steer the ursi studies to address the itu needs and to ensure that the relevant outcomes of ursi activities are transmitted to the itu. it has to be admitted that, admirable though the goals of this committee are and the enthusiasm of its members, the outcomes have not been entirely successful, a matter of some concern to the ursi boards. the involvement of cost 271 within ursi has been quite positive, and the interchange of ideas with the wider scientific community fruitful in both directions. a separate organisation, but also operating like ursi as a loose grouping of primarily un-sponsored scientists within the umbrella of the international council of scientific unions (icsu), is the committee on space research (cospar). this addresses all aspects of research within the area of the earth’s environment, the neighbourhood of other planets and within outer space. its commission c has the remit: «space studies of the upper atmospheres of the earth and planets, including reference atmospheres», and it also has a «panel on space weather». cost 271 participants take part and present results to general assemblies of the cospar. a joint cospar commission c-ursi standing organisation is the international reference ionosphere (iri), and cost 271 participants are actively involved as members of this group. the iri is concerned with producing reference models to define on a worldwide basis the geographical and temporal variations of the height distributions of electron density, ion composition, temperatures, and drifts. the iri holds a wide number of meetings as it seeks to extend and refine its models in the light of the latest observational data. a number of 1297 impact of cost 271 cost 271 participants are also members of the iri, where for them the greatest interest lies in the electron density component, with particularly the determination of total electron content (tec), which is the height-integrated profile. cost 271 has made a major impact in the specification of the topside model above the height of peak electron density, which is responsible for much of the contribution to total electron content, and so to ionosphere influences on earth-space radio propagation. besides attendance at these international meetings, cost 271 participants have hosted such meetings including an annual event at the abdus salam international centre for theoretical physics in trieste where in recent years attention has been paid to the quantification and specification of ionosphere variability. the iri model has also been forwarded to the itu-r. one of the authors of this review is the chairman of iri and thankfully acknowledges the helpful consultation the cost 271 action provided for the definition of the iri ionosphere-plasmasphere electron density distribution. there are a range of other regional and international organisations that address specific topics through their periodic meetings that encompass aspects within the purview of cost 271, and where its participants have made significant contributions to proceedings. we specifically like to mention the annual european geophysical society (now union) meetings, where sessions have been convened by the cost 271 chairman, with wide attendance of other cost 271 participants. critical action issues were thus presented to a broad scientific audience inviting critique and inputs. mention should be made too of links with the northern european incoherent scatter facility (eiscat), data from which have been valuable to the studies conducted within the action. there are other limited-agenda conferences organised on an ad-hoc basis in which cost 271 participants have taken the lead and provided the hosting logistics. in all this work the name of cost 271 and the range of its studies have received wide recognition and respect within the international scientific community. there is no doubt that propagation plays a vital role in controlling the ultimate performance characteristics of radio systems. the evolution of ever-more sophisticated navigation, communications and surveillance systems calls for data needs to planning and operation that do not currently exist, or can even be envisaged at the present time. traditionally where radio planners identify a requirement that cannot be met with current information, they establish specific groups to address the topics in hand. particular examples are where proposals for changes in broadcast planning have been formulated via regional groupings such as the european broadcasting union (ebu), or the equivalents in other parts of the world like the asia pacific broadcasting union (abu) and the inter-american association of broadcasters (air). the same applies within other disciplines, as for example space navigation with the esa initiatives noted above. but such groups are short-lived, have a particular remit, and are disbanded when they have met their target goals. we know of no other co-ordinated regional groupings like cost 271 and its former actions to have been established in north and south america, asia, the far east, or australia and to have maintained a continuity of existence, in the case of cost for more than a decade, to provide an ongoing forum and level of expertise of state-of-the art in ionosphere studies. the wealth of information about the ionosphere that has been derived over nearly a century cannot be learned over-night and it is important that this information is not lost, lest it fail to be applied to ongoing problems that arise. in some senses it can be argued that to maintain this general level of expertise is more important than the precise work of the moment that is carried out, and it is encouraging to note the numbers of young workers who have participated in this action in recent years. the ionosphere knows no geographical bounds and it is important to europe that there should be a co-ordinated group that can be called upon to formulate and defend proposals that affect the whole world when an independent need is identified. 2. activities within host organisations cost is a framework for european collaborations within areas specified within the memorandum of understanding of each action and the level of support provided by the commission is primarily limited to secretarial and meeting expenses. funding of the work carried out by individual participants is 1298 peter a. bradley and bodo w. reinisch the responsibility of employer organisations. in principle a conflict could arise where the needs of those organisations differ from what activities the management committee decides to carry out. in the case of cost 271 though this problem has never arisen, with individuals selecting those work packages to join where the objectives most closely match their stated needs. good examples are wp 1.2 for the creation of a space-weather database and for now-casting and forecasting propagation conditions, and wp 2.1 for the investigation of plasma effects on global navigation satellite system (gnss) applications, particularly by using the radio-occultation technique in conjunction with low earth-orbiting satellites (leos) to study vertical ionosphere structure. inevitably, there has been some adapting of the work in certain cases to bring these into line as the studies have progressed, and inevitably too there have been a few topics initially conceived as to be investigated which have not been addressed, e.g., to gain better knowledge of effects of noise and interference on terrestrial communications. this is seen as a natural consequence of the cost framework and is here of only minor significance. unlike some actions, participants in cost 271 have been drawn mainly from universities and government-sponsored organisations in the separate countries, with only limited industrial involvement. it is not then possible to quantify how valuable the cost 271 work has been to each of these organisations, or to the wider interests of the numerous other organisations within their separate countries that these groups in turn serve. to determine this information would require an in-depth survey that would probably be very difficult to conduct. what can be cited though is that cost 271 is a natural successor of former actions cost 238 and cost 251 with many participants common to all three actions. their continued presence can be seen as justification of the value their sponsors place on this work. on the other hand we notice that more than 20 young scientists, both female and male, have joined the action from a range of countries including spain, italy, united kingdom, czech republic, and greece. the important role women play in science and engineering in europe is reflected in the large number of female active participants in cost 271. besides the specific work package achievements, mention should also be made to the valuable synergy that has undoubtedly resulted for all those involved. teams have been assembled to tackle jobs that were too extensive for single organisations to embark upon, the opportunities to meet scientists and engineers from other countries with like interests but differing ideas, and the value of informed opinions of topics to pursue, particularly in the case of those working in isolation within their organisations or without wide background experience, must have been considerable. 3. collaborative partnerships in other initiatives throughout the duration of cost 271 participants have become aware, in some cases without having prior knowledge, of other groups in other countries with the same interests, complementary resources and levels of expertise. this has led to a number of bilateral and multi-national collaborations that most probably would not have taken place without the existence of cost 271 and the former two ionosphere cost actions. these collaborations have undoubtedly been strengthened by the short-term missions initiatives, which have enabled participants from one organisation to work for a short time alongside colleagues in the host organisation on common limited-objective tasks. at the same time to see the peripheral work in progress, the teams involved, aspirations and future goals are strong incentives for extended common activities. such collaborations are not specifically mentioned within cost 271 publications, but there are a good number of these. joint italian-greek organised meetings in crete can be cited. other bulgarian-belgium, u.k.-serbia, russia-poland, germanygreece, u.k.-france, u.k.-russia, u.k.-greece, turkey-italy and spain-italy collaborations either already have or are continuing to take place. management committee meetings have always included an agenda item for those present to be made aware of forthcoming meetings and opportunities for future collaborations. undoubtedly the subject of space-weather is important within a range of disciplines, and it is timely that many cost 1299 impact of cost 271 271 participants have also joined the new companion action cost 724: «space weather applications and services», for which the vice chairperson is also a work package leader in cost 271. others have associated themselves with the european space agency’s exploratory navigational satellite system initiative egnos as part of the galileo research project. cost 271 participants are actively involved in the new initiative estar (european solar-terrestrial and atmospheric research) proposed to the european science foundation to investigate the contributions of the sun to long-term global climate change. all these initiatives benefit from the existence of cost 271, even if that benefit is hard to quantify. 4. summary it is hoped this short article will serve to confirm that the contributions from action cost 271 are considerable. they cannot be measured solely in terms of the contents of this final report, nor by the extensive number of publications that have ensued during the project lifetime. besides the formal contributions to itu-r and the contributions to international organisations such as ursi and cospar, cost 271 has provided materials that it would be hard to quantify to participants’ host organisations by way of both consultancy and final products. it has provided a forum for the establishment of collaborative european initiatives, a centre of expertise in ionosphere knowledge when none other equivalent in europe or elsewhere exists, and a motivation for the continuation and the longterm archiving of synoptic soundings of the state of the ionosphere. it seems entirely unthinkable to allow the team of experts that composes the cost 271 action to dissolve after the successful completion of the action. the continuation into the future of such a group of dedicated workers is highly desirable and will assure success in the work that needs to be done next: development of mitigation techniques. completo_def.qxd 1301 annals of geophysics, vol. 47, n. 4, august 2004 key words aftershocks – bhuj earthquake – omori’s law – gba array – lg magnitude 1. introduction the destructive bhuj earthquake of mw 7.7 on january 26, 2001, which ravaged through the kachchh and adjoining regions of india, caused a death toll of 30 000 and loss of property worth more than 10 million us dollars (gupta et al., 2001). the india metrological department (imd) gave the epicentre of this earthquake as 23.40n and 70.28e with focal depth of 25 km. the magnitude of this earthquake was similar to that of an earlier earthquake, which had hit the northern part of kachchh, india, in 1819 (oldham, 1926). the epicentre of this bhuj earthquake and the area of occurrence of aftershocks are shown in fig. 1. the aftershocks following this major earthquake were recorded at gba, india, at distance of around 1300 km from the epicentre. the layout of seismometers at gba station is also given in the in-set of fig. 1. in this paper the temporal behavior of the aftershocks in the initial 144 h is analysed. 2. theory the stress energy following a main shock is slowly released as a sequence of aftershocks. according to omori (1894), the rate of aftermailing address: dr. g. jayachandran nair, electronics and instrumentation group, seismology division, bhabha atomic research centre, trombay, 400085 mumbai, india; e-mail: nairgj@apsara.barc.ernet.in the oscillatory behaviour of the aftershocks rate of the 2001 bhuj earthquake, india: observation and interpretation g. jayachandran nair, m.m. abdul razak, a.g. venkatesh prasad and erulankatil unnikrishnan electronics and instrumentation group, seismology division, bhabha atomic research centre, trombay, mumbai, india abstract a damaging earthquake of mw 7.7, which struck the bhuj region of india on january 26, 2001, was followed by a large number of aftershocks. the aftershock data available at gauribidanur seismic array station (gba), india, till 869 h following the main shock were compiled. the plot of the aftershocks rate with time was found to be oscillatory decay. there was a sharp decrease of the aftershocks rate in the initial 144 h from the main shock and this paper presents the analysis of the temporal characteristics of aftershock activity during this period. a statistical best fit for the rate of aftershocks is performed using the generalised omori’s law and the exponential decay law. the statistical errors for the exponential fit are found to be lower than that of the generalised omori’s fit. the superimposed oscillations present in the aftershock activity are extracted by differencing the observed aftershock activity from the statistical fits. the frequencies of these oscillations are found to be 0.062 h–1, 0.078 h–1, 0.102 h–1, 0.118 h–1, 0.141 h–1, 0.164 h–1, 0.233 h–1 and 0.476 h–1. some of the plausible causes for this kind of oscillations present in the aftershock activity are also discussed in this paper. 1302 g. jayachandran nair, m.m. abdul razak, a.g. venkatesh prasad and erulankatil unnikrishnan shocks decays with time as 1/t, where t is the time from the main shock. a generalization of omori’s law was proposed by utsu (1961), in which the rate of aftershocks decays with time as 1/(t + t0)n, where t0 is a constant and n is the exponent. from this generalized omori’s relation, the number of aftershocks, n, occurring per unit time is proportional to 1/tn, when t >> t0. the aftershock activity of the california earthquakes gave the value of n lying between 0.5 and 1.5 (reasenberg and jones, 1989), which is also observed in most of the aftershock activity in the rest of the world. a similar power law for the rate of aftershocks is also obtained due to viscoelastic creep (schaff et al., 1998) and stress transfer mechanism (stein, 1999). in the above mechanism there is an implicit assumption that the coulomb stress change is proportional to the sum of normal stress (∆σd) and pore pressure change (∆p). the coulomb stress change (∆σf) is defined as (stein, 1999) ∆σf = ∆τ + µ(∆σd + ∆p) (2.1) where ∆τ is the shear stress on a fault and µ is the frictional co-efficient. the rate of aftershocks n, for such a process at a time t, n = n0⋅(t + t0)–n (2.2) is obtained from the solution of the following differential equation, for the decay of the stress s (shaw, 1993), ds/dt = –a⋅s (n+1)/ n. (2.3) the nature of the solution of eq. (2.3) takes the form as eq. (2.2), under the assumption that s and n are linearly related. as the exponent n is empirically observed to be less than 1.5, an exponential relation for the rate of decay of aftershocks will not arise as a solution of eq. (2.3). the scaling and translation of time ordinate is permitted in equation (2.2). thus n0 ⋅t0–n denotes fig. 1. the geology and neotectonics of kachchh region, india, affected by the bhuj earthquake. the star indicates the imd epicenter. the circle around the star shows the area of occurrence of aftershocks. the inset box shows the lay-out of gba seismic array, at a distance of ~1300 km from the epicentre, india. 1303 the oscillatory behaviour of the aftershocks rate of the 2001 bhuj earthquake, india: observation and interpretation the rate of aftershocks at time t = 0. the generalized omori’s law is here in after referred as the hyperbolic law in this text. 3. instrument and ground motion data the ground motion data of the aftershock sequence of this bhuj earthquake were recorded at gba, (fig. 1) india, in digital format and used for the analysis. the recording instruments were vertical component seismometers of natural frequency 1 hz. the sampling frequency was 20 samples per second. there were a large number of overlapping events in the first three hours following the main shock and it was very difficult to separate them. hence the aftershocks which originated later than three hours following the main shock were only considered for the analysis. the most prominent arrival from these aftershocks was the lg phase and it was used to quantify the lg magnitude. all the aftershocks used in this analysis had lg magnitude varying from 5.9 to 3.5. the minimum detectable lg magnitude at gba from bhuj was ~ 3.2. a few examples of observed digital waveforms of aftershocks corresponding to usgs mb 5.5 and 4.7 are shown in fig. 2. the nature of the rate of aftershocks recorded till 869 h from the main shock is shown in fig. 3 as trace ‘a’. a total of 320 aftershocks were recorded during this period. from fig. 3, it is observed that there is a sharp decrease of rate of aftershocks in the initial 144 h, an increase in activity from 144 h to 300 h and a slowly decreasing activity from 300 h onwards. in order to confirm that the recorded aftershocks originated from the bhuj region, the difference between the time of arrival of s and p phases and lgand p-phases were checked and found to match the expected times from the bhuj region. digital array tuning was also done for some of the selected events to further confirm whether their origin coordinates matched the expected area. 4. statistical analysis the plot of rate of aftershocks against time for 869 h, following the main shock, is shown in fig. 3 as trace ‘a’. traces ‘b’ and ‘c’ of fig. 3 indicate the exponential fit, 10.95.e–t/36.53, and the hyperbolic fit, 39.72.t–0.76, respectively. the fig. 2. typical aftershock digital waveforms of bhuj earthquake, 2001, observed at gba for usgs mb 5.5 and 4.7. the date and time of arrival at gba are shown in the figure. 1304 g. jayachandran nair, m.m. abdul razak, a.g. venkatesh prasad and erulankatil unnikrishnan fig. 3. trace ‘a’ shows the plot of rate of aftershocks of bhuj earthquake, 2001, against time, observed at gba, for 869 hour following the main shock, the trace ‘b’ shows the exponentially fitted curve, 10.95e–t/36.53, for the trace ‘a’. the standard deviation (sd) and χ 2 are 0.37 and 0.16 respectively for the trace ‘b’. trace ‘c’ shows the hyperbolically fitted curve, 39.72.t–0.76, for the trace ‘a’. the sd and χ 2 are 0.58 and 0.37 respectively for the trace ‘c’. all parameters and errors are rounded off to two decimal places. function constants standard deviation (sd) χ 2 n0.exp (–t/tc) n0 = 11.05 tc = 35.91 0.73 0.54 n0.(t) –n n0 = 31.93 n = 0.65 1.18 1.44 table i. fitted parameters and statistical errors (rounded off to two decimal places) in the initial 144 h of rate of decay of aftershocks of the bhuj earthquake, 2001. fig. 4. trace ‘a’ shows the plot of rate of aftershocks of bhuj earthquake, 2001, against time in the initial 144 hour. trace ‘b’ shows the exponentially fitted curve, 11.05.e – t/35.91, for the trace ‘a’. the sd and χ 2 are 0.73 and 0.54 respectively for the trace ‘b’. trace ‘c’ shows the hyperbolically fitted curve, 31.93.t – 0.65 for the trace ‘a’. the sd and χ 2 are 1.18 and 1.44 respectively for the trace ‘c’. all parameters and errors are rounded off to two decimal places. standard deviation (sd) and levenberg-marquardt χ 2 values (levenberg, 1944; marquardt 1963; more, 1977) are 0.37 and 0.16 respectively for the exponential fit and 0.58 and 0.37 respectively for the hyperbolic fit. the levenberg-marquardt χ 2 value for the exponential fit is denoted as χe2 while it is denoted as χh2 for the hyperbolic fit. for 5χe2 deviation value of the exponential fit, a confidence level of 95% is obtained and the corresponding hyperbolic fit gives 94% confidence level. the rate of decay of aftershocks in the initial 144 h is smoothed by a three point adjacent average method and is shown in fig. 4 as trace ‘a’. this curve is analytically fitted by an exponential law of the form n = n0.exp (–t/tc) and hyperbolic law of the form n = n0⋅(t)–n as shown in fig. 4 as traces ‘b’ e ‘c’ respectively. for the exponential fit the amplitude n0, the time constant tc, sd and χe2 values are found to be 11.05, 35.91 h, 0.73 and 0.54 respectively. for the hyperbolic fit, the amplitude n0, the exponent n, the sd and χh2 and are found to be 31.93, 0.65, 1.18 and 1.44 respectively. for 4 χe2 deviation value of the exponential fit, a confidence level of 95% is obtained while the hyperbolic fit gives 89% confidence level. the exponential fit gives greater confidence level than the hyperbolic fit. the results of these statistical fittings are given in table i. the plot of logarithm of the aftershocks rate against time (here-in-after called log-linear plot) during this 1305 the oscillatory behaviour of the aftershocks rate of the 2001 bhuj earthquake, india: observation and interpretation period is shown in fig. 5 as trace ‘p’ and its linear fit is given as trace ‘r’ in the same figure. the plot of logarithm of rate of aftershocks against logarithm of time (here in after called log-log plot) is shown in fig. 5 as trace ‘q’ and its linear regression fit is given as trace ‘s’ in the same figure. the statistical errors of these log-linear and log-log fits are given in table ii. the linearity of the log-linear curve is more prominent than that of the log-log curve as found from the statistical errors given in table ii. it can be concluded from the trends of these curves shown in fig. 5 that the instantaneous slope of trace ‘q’ is time dependent. the large sd for the exponential and hyperbolic fits can be attributed to the superimposed oscillations present in the aftershock activity. such oscillatory behavior of aftershocks over hyperbolic trend was observed for cumulative benioff strain by sornette and sammis (1995). 5. extraction of oscillatory periods the superimposed oscillations present in the rate of decay of aftershocks are analytically extracted by subtracting the exponential and hyperbolic fits, traces ‘b’ e ‘c’ of fig. 4, from the observed decay curve, trace ‘a’ of fig. 4. these subtracted curves are smoothed separately by a three point adjacent average method to avoid sharp discontinuities. these smoothed curves are filtered through a high pass filter with a cut-off frequency at 0.05 h–1. the filtered curves for the exponential and hyperbolic fits are shown in figs. 6 and 8 respectively. the filtered curves, figs. 6 and 8, are fourier transformed to obtain the amplitude and phase spectrum which are shown in figs. 7 and 9 respectively. the amplitude and phase spectra for the function standard deviation (sd) χ 2 log-linear 0.39 0.15 log-log 0.47 0.22 table ii. statistical errors (rounded off to two decimal places) for linear fits of log-linear and log-log plots of rate of decay of aftershocks (initial 144 h) of the bhuj earthquake, 2001. fig. 5. the log-linear plot of observed rate of aftershocks of bhuj earthquake 2001 is shown as trace ‘p’ and trace ‘r’ represents the linear fit, –0.02.t + 2.16, for the trace ‘p’. the sd and χ 2 for are 0.39 and 0.15 respectively for the trace ‘r’. the log-log plot of the rate of aftershocks is shown as trace ‘q’ and trace ‘s’ represents the linear fit, –1.07.ln(t)+4.84, for the trace ‘q’. the sd and χ 2 are 0.47 and 0.22 respectively for the trace ‘s’. all parameters and errors are rounded off to two decimal places. fig. 6. the plot of the difference between the observed rate of aftershocks, trace ‘a’ of fig. 4, and its exponential fit, trace ‘b’ of fig. 4, after filtering through a high pass filter with a cut off frequency at 0.05 h–1. this curve shows the oscillatory part present in the rate of aftershocks. 1306 g. jayachandran nair, m.m. abdul razak, a.g. venkatesh prasad and erulankatil unnikrishnan fig. 7. the fourier amplitude and phase spectra of the oscillatory part shown in fig. 6. the amplitude spectrum shows prominent peaks at 0.062 h–1, 0.078 h–1, 0.102 h–1, 0.118 h–1, 0.141 h–1, 0.164 h–1, 0.182 h–1, 0.213 h–1, 0.233 h–1 and 0.476 h–1 (rounded off to three decimal places). h–1 and 0.476 h–1. in order to find the nature of aftershocks it is necessary to examine the phase spectra of figs. 7 and 9, whether these oscillations correspond to random time series or deterministic series. runs test (bradley, 1968) is performed to check whether a given sequence of data is random or not. the result of this test is called as z score or test statistic value. if z score is greater than 1.96, then the data under consideration are not random at a confidence level of 95%. this runs test is explained in appendix. using this test, the z score of the phase spectra of figs. 7 and 9 are found to be – 7.487 and – 9.635 respectively. since the z score  for the phase spectra are greater than 1.96, the phase spectra of figs. 7 and 9 are not random at a confidence level of 95%. hence the oscillations present in the aftershock activity are not random. 6. discussion and conclusions generally the fourier spectra obtained from random signals will show either the amplitude and/or phase of oscillations is random. in most of the geophysical signals, the fourier phase spectrum is observed to be random. in order to give an fig. 8. the plot of the difference between the observed rate of aftershocks, trace ‘a’ of fig. 4, and its hyperbolic fit, trace ‘c’ of fig. 4, after filtering through a high pass filter with a cut off frequency at 0.05 h–1. this curve shows the oscillatory part present in the rate of aftershocks. fig. 9. the fourier amplitude and phase spectra of oscillatory part shown in fig. 8. the amplitude spectrum shows prominent peaks at 0.062 h–1, 0.078 h–1, 0.102 h–1, 0.118 h–1, 0.141 h–1, 0.164 h–1, 0.185 h–1, 0.217 h–1, 0.233 h–1 and 0.476 h–1 (rounded off to three decimal places). exponential fit are given in fig. 7. similarly the amplitude and phase spectra for the hyperbolic fit are shown in fig. 9. from the amplitude spectra of figs. 7 and 9 the periods of prominent common spectral peaks are picked and the frequencies are found to be 0.062 h–1, 0.078 h–1, 0.102 h–1, 0.118 h–1, 0.141 h–1, 0.164 h–1, 0.233 1307 the oscillatory behaviour of the aftershocks rate of the 2001 bhuj earthquake, india: observation and interpretation example, a synthetic signal is generated, fig. 10 trace ‘a’, for a deterministic case. the fourier amplitude and phase spectrum of this deterministic signal are shown in fig. 10 as traces ‘b’ and ‘c’ respectively. using the fourier amplitude spectrum of fig. 10 trace ‘b’, another synthetic signal is generated using random phases and is shown in fig. 11, trace ‘a’. the fourier amplitude and phase spectra of this signal, fig. 11 trace ‘a’, are given as traces ‘b’ and ‘c’ respectively in the same figure. runs test is performed on the data of phase spectra of figs. 10 and 11 to check for randomness. the z scores (test statistic values) of the figures are found to be –3.34 and 0.54 respectively. as the z score for the phase spectrum of fig. 10 is greater than 1.96, this phase spectrum is not random at 95% confidence level while that of fig. 11 is random. though the fourier amplitude spectra of figs. 10 and 11 are the same, their phase spectra are different since they have different z scores. the phase spectrum of fig. 10 is deterministic, while the phase spectrum of fig. 11 is random. thus by observing either the fourier amplitude or the phase spectrum one can ascertain whether a given signal under consideration is deterministic in time or random. in this analysis of the aftershock activity of bhuj earthquake, the fourier phase spectra of figs. 7 and 9 do not show random characteristics. hence it is concluded that the oscillations present in the aftershock activity do not correspond to random time series. from the analysis of the initial 144 h of aftershock activity, the exponential decay fit gives lower statistical errors than that of the generalized omori’s fit. the superimposed oscillations present in the aftershock activity during this period are extracted by differencing the observed rate of decay and the statistical fits. the prominent common frequencies of these oscillations are found to be 0.062 h–1, 0.078 h–1, 0.102 h–1, 0.118 h–1, 0.141 h–1, 0.164 h–1, 0.233 h–1 and 0.476 h–1. it is also shown, using synthetic signals, that the oscillations present in the aftershock activity do not correspond to random time series. a few plausible causes which can generate this kind of oscillatory decay of aftershocks are (a) the elastic or visco-elastic oscillations of the crustal blocks following the main shock and/or (b) gravitational perturbations, tidal waves, etc., fig. 10. trace ‘a’ shows a synthetic signal obtained by superposition of two decaying sinusoidal curves. traces ‘b’ and ‘c’ show the fourier amplitude and phase spectra respectively for the trace ‘a’. a b c fig. 11. the synthetic signal generated from the fourier amplitude spectrum of fig. 10, trace ‘b’, and random phases is shown as trace ‘a’. traces ‘b’ and ‘c’ show the fourier amplitude and phase spectra respectively for the trace ‘a’. compare the phase spectra shown in figs. 10 and 11. a b c causing modulation in the energy release through the aftershocks. in order to relate the above causes to the oscillatory behavior of aftershock activity of this bhuj earthquake, a correlation should exist between the rate of aftershocks and measurements from tidal gauges, tilt meters and gravity meters. however, during the occurrence of aftershock sequence of this bhuj earthquake no such measurements were available. acknowledgements the authors express their sincere thanks to the staff at gba for their efficient operation of the gba seismic array during the aftershock sequence of bhuj earthquake and the excellent quality of digital data provided by them. the authors express their sincere gratitude to shri b.v. dinesh for his valuable assistance in obtaining high quality of data. 1308 g. jayachandran nair, m.m. abdul razak, a.g. venkatesh prasad and erulankatil unnikrishnan appendix. runs test. runs test (bradley, 1968) is performed to check whether a given sequence of data is random or not. according to this test, the average value of the data is computed. each member of the data whose value is above this average is assigned as +1, and those that are below are assigned as –1. the total number of +1 is taken as n1 and that of –1 is taken as n2. the number of consecutive changes of +1 to –1 and vice versa is taken as the length of the run r. then compute n n n n1 1 2 1 2)= + +a ^ h (a.1) n n n n n n n n n n2 2 1 2 1 2 1 2 1 2 1 2 2 1 2) ) ) ) ) )= + + -v ^ ^ ^`h h hj (a.2) .z r= -a v^ h (α.3) this z value is called the z score or test statistic value. it is compared with the cumulative standard normal distribution curve given by / /expz p z z u du1 2 2 z 2) )g= = -z r 3#_ ` b _i i k i . (a.4) a z score with absolute value greater than 1.96 indicates non-randomness at the confidence level of 95%. references bradely, j. (1968): distribution-free statistical tests, ch. 12 (on line: http://www.itl.nist.gov/div898/handbook/ eda/section3/eda35d.htm). gupta, h.k., n.p. rao, b.k. rastogi and d. sarkar (2001): the deadliest intraplate earthquake, science, 291, 2101-2102. levenberg, k. (1944): a method for the solution of certain problems in least squares, quart. appl. math., 2, 164-168. marquardt, d. (1963): an algorithm for least-squares estimation of non-linear parameters, siam j. appl. math., 11, 431-441. more, j.j. (1977): the levenberg-marquardt algorithm: implementation and theory, in numerical analysis, edited by g.a. waston (springer verleg), lecture notes in mathematics, 630, 108-116. oldham, r.d. (1926): the cutch (kachh) earthquake of 16th june 1819 with a revision of great earthquake of 12th june 1897, mem. geol. surv. india, 46, 71-146. omori, f. (1894): investigation of aftershocks, rep. earthquake inv. comm., 2103-39. reasenberg, p.a. and l.m. jones (1989): earthquake hazard after a mainshock in california, science, 243, 1173-1176. schaff, d.p., g.c. beroza and b.e. shaw (1998): postseismic response of repeating aftershocks, geophys. res. lett., l25 (24), 4549-4552. shaw, b.e. (1993): generalised omorìs laws for after1309 the oscillatory behaviour of the aftershocks rate of the 2001 bhuj earthquake, india: observation and interpretation shocks and foreshocks from simple dynamics, geophys. res. lett., 20, 907-910. sornette, d. and c.g. sammis (1995): complex critical exponents from renormalization group theory of earthquakes: implications of earthquake predication, j. phys. i, 5, 607-619. stiein, r.s. (1999): the role of stress transfer in earthquakes occurrence, nature, 402, 605-609. utsu, t. (1961): a statistical study on the occurrence of aftershocks, geophys. mag., 30, 521-605. (received september 19, 2003; accepted march 26, 2004) pettenati_fc_correttoereimpaginato_ok_layout 6 673 rapid scenarios and observed intensities franco pettenati*, livio sirovich istituto nazionale di oceanografia e di geofisica sperimentale (ogs), sgonico (trieste), italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6175 1. introduction – applicative interest after a destructive earthquake, national governments need to know the approximate amount of damage, the number of casualties, and the financial losses as soon as possible. rapid scenarios are also used to inform the general public; see the widely used shakemap package [wald et al. 1999, 2006] of the us geological survey (usgs) and the one modified by the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology), which is reproduced for figure 1. the general matter of the use of intensities in damage scenarios was discussed in a special session at the 2008 annual meeting of the seismological society of america (http://www.seismosoc.org/meetings/2008/ specialsessions.html), and was also discussed in the nis-1 session of the european congress in moscow, in august 2012 (http://www.esc2012-moscow.org/esc_thematicareas.html). the purposes of the present report are to: (i) compare different types of intensities; (ii) check two rapid scenarios of intensity; and (iii) understand whether the kf formula [sirovich 1996, sirovich et al. 2009] can be used as a new 'attenuation' relationship to improve rapid scenarios. here, we focus on the meizoseismal area of the may 20, 2012, emilia (po valley, north-eastern italy) earthquake, which includes all of the damage and was the immediate target of the italian department of civil protection (dipartimento protezione civile, dpc). in this area, the dpc needed to estimate the amount of damage, the number of casualties, and the financial losses in the first tens of minutes after the event. strictly speaking, none of the pictures we are going to present can fulfill all of the requirements of the dpc, but we hope that this analysis is useful to improve the available algorithms for rapid scenarios. our reference for comparisons are the intensities observed in the field by seismologists and engineers. the surveyed sites are shown as black dots in figures 1 and 2. we used the natural neighbor approach (n-n; based on the voronoi polygons) to draw isoseismals. let us recall that the n-n isolines [sirovich et al. 2002] are obtained without any explicit or implicit assumption about the observed phenomenon, and thus they strictly honor the data. therefore, in figures 1c-f the intensity observed at each site can be read simply by looking at the color of the area where the black dot is, because each site is inside its correct isoseismal area. with the same data, everybody obtains the same n-n isoseismals; the use of these isolines helps in the understanding of the phenomenon better than the italian tradition of plotting dots in colors. let us remember that any collection of field data (no matter if from field intensities or web answers) samples the regional seismic response. the sampling must obey the spatial version of the nyquist principle along the coordinate axes [press et al. 1992, pp. 494-495]. we will see that this is crucial to avoid ill-posed comparisons of contoured isoseismals. we want to stress that we take as reference the field intensities produced only by the main shock of may 20, 2012 (not the cumulative damage due to the subsequent shocks). finally, three warnings about the figures. first, figure 1a-f shows an area of the po valley between ferrara (right side, middle) and mantova (top left corner; both not shown); this can be checked from the longitudes and latitudes on the x and y axes. second, we apologize for the quality of some figures, which were prepared in a hurry, by different institutes, with different tools, and at different scales, and they were greatly enlarged. third, it also needs to be considered that different institutions adopt different color conventions for intensities. some of them use soft-shade colors; the istituto nazionale di oceanografia e di geofisica sperimentale (ogs; national institute of oceanography and experimental geophysics) uses full colors, in agreement with the traditional definition of intensities as degrees of a scale. when reviewing the new interpolation scheme of shakemap [worden et al. 2010], we suggested adding thin black isoseismal lines that correspond to the full intensity degrees. indeed, soft-shade colors call to mind more continuous variations of real numbers than degrees of a scale, and render the understanding of the shape of the regional intensity pattern difficult. galli et al. [2012a, b] (see later) and the 'www.haisentitoilterremoto.it' group of the ingv added these isoseismals, but use their own color scales. the full colors of the ogs try to follow the article history received july 28, 2012; accepted september 7, 2012. subject classification: ground motion, historical seismology, seismic risk, statistical analysis, algorithms and implementation, seismological data. 2012 emilia earthquakes pettenati and sirovich 674 figure 1. (a) close-up of the ingv shakemap (mcs scale) [michelini et al. 2008] (version #7 june 2, 2012). (b) close-up of the ingv map www. haisentitoilterremoto.it (mcs) by tosi, de rubeis and sbarra, from 10,294 web answers. note that the color scale is different from (a): red represents vii. (c) the rapid scenario sent from the ogs to the dpc and the ingv one day after the event; resampled in the web answers of www.haisentitoilterremoto.it [de rubeis, written communication 2012]. (d) as for (c), but resampled at the sites surveyed by the dpc. (e) mcs intensity map obtained by the dpc (52 sites) [galli et al. 2012a, b]; n-n contours, same color convention as in (c, d, f) (see text). (f) ems98 intensities by quest [arcoraci et al. 2012] for the main shock of may 20, 2012; same contour and color as in (c, d, e). central shades of the colors adopted by shakemap and other packages (see the figures for clarification of the situation). 2. source parameters used we used the source parameters available on may 20, 2012, soon after the earthquake to calculate our scenario in quasi–real-time. seven source determinations were available, from the ingv (first report relazione sul terremoto e la sequenza in corso agg. 20/5 ore 8 and the time domain moment tensor, usgs centroid moment solution, body-waves solution and w-phase, emsc-csem agency, ogs udine). we discarded two of these (usgs body-waves and ogs) for seismological reasons and also because they were discordant. table 1 lists the ranges of values explored parametrically and the sampling steps used to build our scenario; a barycentric position for the nucleation was used. the limits of the ranges coincide with the extreme parameters available. in this way, 108 sources were obtained. 3. rapid forecasts, and web intensities figure 1a shows a strong enlargement of the mcs intensity map produced by the ingv using its improved version [michelini et al. 2008] of the shakemap software package [wald et al. 1999, 2006]; only the meizoseismal area is shown. see http://shakemap.rm.ingv.it/shake/archive for the map we used, which is version #7 of june 2, 2012. note that the ingv can consider the influence of the extended source (figure 1a, small rectangle). a slight tendency towards w-e elongation is seen; the early versions of the ingv shakemap of the may 20, 2012, event had a more rounded concentric aspect. the mcs intensity map produced shortly after the earthquake from web answers (see www.haisentitoilterremoto.it) by p. tosi, v. de rubeis and p. sbarra of the ingv is shown in figure 1b. note that the complete map available at the time used 10,294 answers (sites not shown in figure 1b). it is worth commenting that the ingv procedure resembles the well-known "did you feel it?" reports of the usgs, but it adopts its own forms [see details in sbarra et al. 2010]. note that in figure 1b the red signifies vii; the area of degree vi is also marked by a black line; regarding this, figure 1b is selfexplanatory, however. both isoseismals show e-w orientations, with a slight tendency toward wnw-ese. figure 1c repeats the tentative rapid scenario of the ogs that we sent to some colleagues of the dpc and the ingv just after the may 20, 2012, earthquake, for a totally blind test; in other words, we offered them the opportunity to check in the future on the quality of our results by comparing them with the intensities that were still to be collected via the web, or observed in the field. at that time, we sent the median scenario, which was calculated on a regular grid. in figure 1c, the scenario already sent to the dpc was resampled in the sites used to build figure 1b. usually, we compare field results and forecasts by calculating the summed squares of the differences in the same sites, and their distribution. here, we can propose only qualitative comparisons. as said, geographical sampling is crucial to draw contour lines of any geophysical more or less regular field [see also sirovich et al. 2002]. thus, the comparison between field intensities (observed in sparse sites) and intensities calculated on a regular grid would be ill-posed. for this reason, in view of an advanced blind test, with the purpose of a well-posed graphical comparison between figure 1b and 1c, we plotted again our rapid scenario using the coordinates of the web answers collected by the www.haisentitoilterremoto.it group [de rubeis, written communication, 2012]; obviously, the parameters used on may 21, 2012, remained fixed. the median values minus one absolute deviation are shown in figure 1c (the deviation is a few tenths of a degree); only the coordinates with at least three web answers were used. the ogs scenario was obtained with the kf formula model, which simulates the horizontal component of the body-wave radiation from an extended source. let us recall that kf uses 11 source parameters, which identify a fault of unit width, with the three coordinates of its nucleation, the fault-plane solution, the seismic moment, the rupture velocities, and the shear-wave velocity in the half space. these parameters are available from agencies in the first hours after a destructive earthquake; the four further parameters used (see these and the kf in sirovich and pettenati [2009]) are not rapidly available, and thus they must be fixed. in the study case, the rupture was kept symmetric, and the fixed parameters are in table 1. for the description of the direct, parametric, use of kf to build scenarios, we refer to sirovich and pettenati [2009]. the scenario of figure 1c used the data available on may 21, 2012, but could have been produced in some tens of minutes after the event. similar kf scenarios were tested for the los angeles 2008 m 5.4 earthquake [sirovich et al. 2009] and for the l'aquila event of 2009 [pettenati et al. 2011]. figure 1d repeats the scenario of figure 1c, with the only difference being in the geographical sampling. indeed, 675 rapid scenarios and observed intensities table 1. source parameters explored parametrically to calculate the scenarios of figures 1c and 1d. parameter range, or fixed value step nucleation latitude (˚) 44.85 n // nucleation longitude (˚) 11.22 e // strike angle (˚) 100 – 120 4 dip angle (˚) 45 // rake angle (˚) 89 – 111 4 nucleation depth (km) 8 // source length (symmetric) (km) 10.8 – 14.0 0.8 mach number 0.6 // vs (km s-1) 3.5 // mo (nm) 1018 0.8 – 1.58 0.34 we used the sites surveyed by the dpc with the purpose of a well-posed graphical comparison with figure 1e. note the importance of the sampling, which in this case was inadequate (see section 6). 4. field macroseismic surveys a group from the dpc [galli et al. 2012a, b] (http:// www.protezionecivile.gov.it/) and the quest group of the ingv [arcoraci et al. 2012] (http://portale.ingv.it/real-timemonitoring/quest/rilievi-macrosismici) performed two separate macroseismic surveys. figure 1e shows the dpc results (52 sites, mcs scale). we stress that the intensities shown were produced by the main earthquake of may 20, 2012 [galli et al. 2012a, b]. the n-n contour was used, with the same color convention as in figure 1c, d and f. most likely, the uniform green area (v) in figure 1e is due to the absence of peripheral sampling points (sites surveyed by the dpc). the intensities from quest [arcoraci et al. 2012] are shown in figure 1f (in the ems98 scale). we found the geographical coordinates of 38 intensity values and two 'd' classifications (for hamlets with a few buildings; we put d = vi for buonacompra and reno centese). the same comment of the green area applies as for figure 1e, but the absence of sampling points is also responsible for the yellow (vi) area that opens eastwards. it can be reasonably assumed that all of the damaged sites eastwards were visited (and thus the vi isoseismal should close), but the v degree was not sampled there. 5. preliminary comparisons figure 2a compares the ems98 intensities by quest with the mcs values by the dpc in the 26 localities visited by both. there is a slight discrepancy around the damage/ no damage limit (twelve vi degrees for quest, four for the dpc), but no more systematic differences are seen. figure 2b shows the same comparison as figure 2a, but in the emilia territory (see longitude and latitude in figure 2b). the black dots are the 26 sites visited by both the ingv and the dpc. figure 2b shows the dpc intensity minus the ingv intensity. the voronoi polygons help to understand the local differences. the residuals can be seen to be scattered. there are only three dark gray polygons (dpc ingv = +0.5), which confirms that the ems98 estimates were slightly higher than the mcs estimates. figure 2c compares the mcs intensities of www.haisen titoilterremoto.it with the mcs field values of the dpc in the 24 localities they have in common. the numbers added to the dots indicate the number of the localities in common; consider that each locality of www.haisentitoilterremoto.it corresponds to one or more web answers. for example, in figure 2c there is one locality (the inhabited center of bondeno) where the dpc obtained degree 5.5, while www.hai sentitoilterremoto.it received six answers, which produced pettenati and sirovich 676 figure 2. (a) scattergram of the ems98 intensities by quest versus the mcs values by the dpc in the 26 localities visited by both groups. the numbers added to the dots indicate the number of the localities in common. (b) same comparison as (a), but in the emilia territory. black dots, 26 sites visited by both quest-ingv and the dpc. the gray scale shows the difference (dpc intensity minus the quest ingv intensity). (c) scattergram of the mcs intensities of www.haisentitoilterremoto.it versus the mcs field values by the dpc in the 24 localities in common. the numbers added to the dots indicate the number of the localities in common (see text for the web answers). 677 degree viii. this strong discrepancy remains to be understood. however, what is more important is that there is some dispersion around the v and vi degrees, without systematic effects (the misfit of high intensities with the web data is not surprising as people are frightened and have many difficulties in the most damaged areas). so, figure 2c confirms the acceptable performance of www.haisentitoilterremoto.it for degrees v and vi, as already commented on in figure 1b. 6. discussion and comments we have verified beyond any doubt that the black dots in figure 1c are more than sufficient to describe the intensity pattern in this figure. instead, spurious effects are seen in figure 1d, e (sampling by the dpc), and in figure 1f (sampling by quest), due to scarce peripheral sampling, and thus peripheral divergence of the voronoi polygons. this is the reason why degree v remains constant with distance in these figures, which is not realistic; the same holds for the open shape of vi in figure 1f. insufficient sampling in figure 1f might have applicative fallout in the definition of the area that was most stricken by the earthquake. still on sampling, the differences between figure 1c and d (same model, same source parameters) are due only to the denser sampling in the central part of figure 1d than in figure 1c. on the contrary, the almost total disappearance of degree iv in figure 1d is the spurious effect of the lack of surveyed sites in peripheral areas. the isolated point of degree v to the north of figure 1d is one more piece of evidence that the n-n approach honors the data. then we assume that the reference intensity data for validations are the intensities observed by seismologists in the field. from this point of view, the existence of two surveys in two different scales complicates our attempt at validation of rapid scenarios and web answers. first of all, for the use of different scales, we believe that the uncertainties brought into our problem by the mcs-ems98 scale differences are less important than the many uncertainties in the present comparisons. as a very first approximation, we are satisfied by the authoritative opinion by three renowned experts [musson et al. 2006]: "ideally, direct conversion between intensity scales should never be made", but, "these values are likely to vary more between two seismologists using the same scale than between two scales used by the same seismologist", and thus the "relationship between the major 12-degree scales, such as msk, mmi and mcs, and ems-98 is more or less 1:1". we are aware that the matter is questionable, however. then, figures 1e, f and 2a, b show that 26 localities were visited by both the dpc and the ingv, and that different groups often gave different interpretations of damage. both references in figure 1e, f show that the area of vi is elongated in the wnw-ese direction, which is the orientation of the fault source. the dpc gives two sites with vii, quest gives four (one is in common). the five sites are also approximately aligned in the wnw-ese direction. we conclude that the heaviest damage is there, even if the two groups of experts diverged a little bit with their final interpretations. we conclude that figure 1e, f can be used together as reference. thus, our comments are as follows. notwithstanding some difficulty in the interpretation of the shaded colors in figure 1a, its area of vi appears to be overestimated; the influence of the recording site (figure 1a, triangle) needs to be better understood. the web intensities of figure 1b behave well; also degree vii is approximately in the correct position (this is not trivial). the ogs rapid scenario overestimates the dpc and quest references, very roughly by half a degree in the central area. the kf rapid scenario was able to match the wnw-ese orientation of the meizoseismal area of the reference, which follows the orientation of the known causative fault at depth. in figure 1d, the limit between vii and vi is not well constrained northwards by sampling. in the kf model, this wnw-ese orientation is the combined effect of the orientation of the extended fault, its directivity, and the radiation pattern of the horizontal component of shear waves produced by the parameters of table 1. as said, kf uses many sources parametrically (e.g., different positions, mechanisms), and some overestimation is intrinsic. both references are not reliable close to the borders, although it can be said that figure 1c (ogs, with good sampling) and figure 1b are in reciprocal agreement on degrees v and iv, at least northwards. finally, let us stress that the rapid kf scenario is the only forecasting procedure mentioned in this report, which starts only from source information. in other words, our scenarios cannot take advantage of any rapid information from the field. thus, they cannot compete with shakemap. more appropriately, the kf formula can be used to improve shakemap or other packages by using kf as a new 'attenuation' relationship for intensity. references arcoraci, l., m. berardi, f. bernardini, b. brizuela, c.h. caracciolo, c. castellano, v. castelli, a. cavaliere, s. del mese, e. ercolani, l. graziani, a. maramai, a. massucci, a. rossi, m. sbarra, a. tertulliani, m. vecchi and s. vecchi (2012). rapporto macrosismico sui terremoti del 20 (ml 5.9) e del 29 maggio 2012 (ml 5.8 e 5.3) nella pianura padano-emiliana, ingv, quick earthquake survey team (quest; http://quest.ingv.it/it/), 15 pp. galli, p., s. castenetto and e. peronace (2012a). rilievo macrosismico mcs speditivo, rapporto finale, 15 june, 2012, dipartimento della protezione civile, 26 pp. galli, p., s. castenetto and e. peronace (2012b). may 2012 emilia earthquakes (mw 6, northern italy): macroseismic effects distribution and seismotectonic implications, alpine and mediterranean quaternary, 25, 105-123. rapid scenarios and observed intensities michelini, a., l. faenza, v., lauciani and l. malagnini (2008). shakemap implementation in italy, seismol. res. lett., 79, 688-697. musson, r., g. grünthal and m. stucchi (2006). conversions between older intensity scales and ems-98, in: first european conference on earthquake engineering and seismology (3-8 september, 2006, geneva, switzerland), abstract ecees cd id542. pettenati, f., l. sirovich and d. sandron (2011). rapid simulation of seismic intensity for civil protection purposes; two recent cases in italy, seismol. res. lett., 82, 420-430; doi:10.1785/gssrl.82.3.420. press, w.h., s.a. teukolsky, w.t. vetterling and b.p. flannery (1992). numerical recipes in fortran; the art of scientific computing, cambridge university press, cambridge (2nd ed.), 678 pp. sbarra, p., p. tosi and v. de rubeis (2010). web based macroseismic survey in italy: method validation and results, nat. hazards, 54, 563-581; doi:10.1007/s11069009-9488-7. sirovich, l. (1996). a simple algorithm for tracing out synthetic isoseismals, b. seismol. soc. am., 86, 1019-1027. sirovich, l., f. pettenati, f. cavallini and m. bobbio (2002). natural-neighbor isoseismals, b. seismol. soc. am., 92, 1933-1940. sirovich, l., and f. pettenati (2009). validation of a kinematic and parametric approach to calculating intensity scenarios, soil dyn. earthq. eng., 29, 1113-1122. sirovich, l., f. pettenati and d. sandron (2009). sourceand site-effects in the intensities of the m 5.4 july 29, 2008 earthquake in south los angeles, seismol. res. lett., 80, 967-976; doi:10.1785/gssrl.80.6.967. wald, d.j., v. quitoriano, t.h. heaton, h. kanamori, c.w. scrivner and c.b. worden (1999). trinet 'shakemaps': rapid generation of peak ground motion and intensity maps for earthquakes in southern california, earthq. spectra, 15, 537. wald, d.j., b.c. worden, v. quitoriano and k.l. pankow (2006). shakemap manual; technical manual, users guide, and software guide, version 1.0 6/19/06, 1-156. worden, c.b., d.j. wald, k. lin, g. cua and d. garcia (2010). a revised ground-motion and intensity interpolation scheme for shakemap, b. seismol. soc. am., 100, 3083-3096. *corresponding author: franco pettenati, istituto nazionale di oceanografia e di geofisica sperimentale (ogs), sgonico (trieste), italy; email: fpettenati@ogs.trieste.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. pettenati and sirovich 678 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice r i c e r c h e s u l « c o m p o r t a m e n t o » u m a n o i n s o g g e t t i s o t t o p o s t i a « c o n f i n a m e n t o » (*) m . s t r o l l o ( * * ) r i c e v u t o il 19 f e b b r a i o 1 9 6 3 riassunto. — 11 « c o n f i n a m e n t o » c o n o s e n z a i s o l a m e n t o , s p e c i e q u a n d o le e s i g e n z e d e i n u o v i p r o g r a m m i c o m p o r t e r a n n o p r a t i c a m e n t e u n a u m e n t o d e l l a d u r a t a d e i v o l i s p a z i a l i , r a p p r e s e n t a u n o d e i f a t t o r i s u s c e t t i b i l i di i n f l u e n z a r e l ' e f f i c i e n z a e l ' i n t e g r i t à d e l c o s m o n a u t a a l l a s t r e g u a d e l l e a c c e l e r a z i o n i , d e l l a s u b g r a v i t à , d e l l e r a d i a z i o n i n o c i v e , p e r d u e p r i n c i p a l i m o t i v i : — l a r i s t r e t t e z z a d e l l ' a b i t a c o l o r i s e r v a t o a l l ' u o m o ; l ' a l l o n t a n a m e n t o d a l g l o b o t e r r e s t r e . t a l i r a g i o n i h a n n o s u g g e r i t o il r i c o r s o a l l a p r e l i m i n a r e s p e r i m e n t a z i o n e o n d e p o t e r d o c u m e n t a r e o b i e t t i v a m e n t e le p o s s i b i l i t à e d i l i m i t i d i t o l l e r a n z a u m a n a n o n c h é m e t t e r e i n e v i d e n z a e s t u d i a r e le p r e m i n e n t i c a r a t t e r i s t i c h e c h e f a v o r i s c o n o u n a m a g g i o r e a d a t t a b i l i t à . a t a l e s c o p o l ' a . h a c o n d o t t o , p r e s s o il c e n t r o s t u d i e r i c e r c h e d i m e d . a e r . e s p . d i r o m a , d u e g r u p p i di i n d a g i n i , u n o p r e c e d e n t e e l ' a l t r o s u c c e s s i v o a l l ' a p r i l e 1 9 6 1 , m o m e n t o c i o è c h e i n a u g u r a il v e r o v o l o s p a z i a l e u m a n o . s e r v e n d o s i n e l p r i m o g r u p p o d i 10 v o l o n t a r i , p e r u n a d u r a t a m a s s i m a d i 2 g i o r n i , e n e l s e c o n d o di 70 n o n v o l o n t a r i , p e r u n a d u r a t a m a s s i m a d i 6 o r e , e g l i h a s t u d i a t o , c o n p a r t i c o l a r i a c c o r g i m e n t i t e c n i c i , le m a n i f e s t a z i o n i d e l c o m p o r t a m e n t o e i t r a t t i d e l l a p e r s o n a l i t à . d a t a l i s t u d i è r i s u l t a t o q u a n t o s e g u e : i. l a tolleranza ( e s p r e s s a i n b a s e a l l a d u r a t a d i p e r m a n e n z a n e l l a s i t u a z i o n e c o n f i n a t a ) è a p p a r s a i n v e r s a m e n t e p r o p o r z i o n a l e a l l a q u a n t i t à e d a l l a i n t e n s i t à d e l l e l i m i t a z i o n i r e s t r i t t i v e s o m a t i c h e e c o r r e l a t a p o s i t i v a m e n t e c o n i t r a t t i d i p e r s o n a l i t à c o i n c i d e n t i c o n lo c o s i d d e t t e « s t a b i l i t à a f f e t t i v o e m o t i v a », « c a p a c i t à , r i c c h e z z a e flessibilità i n t e l l e t t i v a », « f o r z a d e l l a d e t e r m i n a z i o n e »; (*) n o t a p r e s e n t a t a al 2° c o n g r e s s o i n t e r n a z i o n a l e t e c n i c o s c i e n t i f i c o d e l l o s p a z i o . r o m a , 1 9 2 3 g i u g n o 1 9 6 2 . (**) i s p e t t o r a t o di s a n i t à a e r o n a u t i c a , c a p o d e l l ' i s p e t t o r a t o : t e n . g e n . m e d . c. s. a . p r o f . t . l o m o n a c o . 608 m. s t r o l l o 2. l e manifestazioni comportamentali s o n o s t a t e d o m i n a t e d a ins o f f e r e n z a e d i r r i t a b i l i t à (sul p i a n o a f f e t t i v o ) , d a f r a m m e n t a r i e t à e r a l l e n t a m e n t o d e l l ' i d e a z i o n e (sul p i a n o i n t e l l e t t i v o ) d a a c c e n t u a t a i r r e q u i e t e z z a gen e r a l e s p e c i e p e r le e s t r e m i t à s u p e r i o r i (sul p i a n o p s i c o m o t o r i o ) . summary. — c o n f l n e m e n t , w i t h o r w i t h o u t i s o l a t i o n , is o n e of t h e f a c t o r s whicli a r e l i k e l y t o i n f l u e n c e t h e e f f l c i e n c y a n d i n t e g r i t y of t h e c o s m o m a u t , in t h e s a m e w a y a s a c c e l e r a t i o n s , s u b g r a v i t y , h a r m f u l r a d i a t i o n s , e t c . t h i s is e s p e c i a l l y so w h e n t h e n e c e s s i t i e s of t l i e n e w p r o g r a m s i m p l y l o n g e r (lying t i i n e s in s p a c e . t h i s is d u e t o t w o m a i u r e a s o n s : — t h e n a r r o w s p a c e r e s e r v e d t o m a n on t h e a i r s h i p ; — t h e i n c r e a s i n g d i s t a n c e f r o m t h e e a r t l i . s u c h r e a s o n s b a v e s u g g e s t e d t l i a t p r e l i m i n a r y e x p e r i m e n t s b e r e s o r t e d t o , i n o r d e r t o o b t a i n a n o b j e c t i v e d o c u m e n t a t i o n of t h e p o s s i b i l i t i e s a n d l i i n i t s of h u m a n t o l e r a n c e . t h i s w o u l d also allow t o e n l i g l i t e n a n d s t u d y t h e m a i n c h a r a c t e r i s t i c s t h a t m a k e s u c h a d a p t a b i l i t y e a s i e r . t o t h i s p u r p o s e , t h e a u t h o r c a r r i e d o u t — a t t h e s t u d y c e n t r e f o r a e r o n a u t i c a l a n d s p a c e m e d i c i n e i n r o m e — t w o s e t s of i n v e s t i g a t i o n s o n e of w h i c l i p r e c e e d e d a p r i i 1961, w l i e n t h e f i r s t r e a l h u m a n s p a c e flight t o o k p l a c e , w h i l e t h e o t l i e r o n e f o l l o w e d s u c h d a t e . i n t h e first s e t , 10 v o l u n t a r i e s w e r c u s e d f o r a m a x i m u m of 2 d a y s , w l i e r e a s t h e s e c o n d s e t w a s c a r r i e d o u t w i t l i 70 n o n v o l u n t a r i e s f o r a m a x y m u m of 6 h o u r s . t h r o u g l i s p e c i a l t e c h n i c a l d e v i c e s , ali m a n i f e s t a t i o n s of b e l i a v i o u r a n d p e r s o n a l i t y f e a t u r e s w e r e s t u d i e d . t h e r e s u l t s of s u c h s t u d i e s w e r e a s f o l l o w s : 1. tolerance ( e x p r e s s e d o n t h e b a s i s of t h e t i m e s p e n t in t h e c o n f i n e d s i t u a t i o n ) a p p e a r e d t o b e i n v e r s e l y p r o p o r t i o n a l t o t h e q u a u t i t y a n d i n t e n s i t y of s o m a t i c r e s t r i c t i v e l i m i t a t i o n s . on t h e o t h e r h a n d , i t w a s d i r e c t l y l i u k e d t o t h e p e r s o n a l i t y f e a t u r e s e o i n c i d i n g w i t h t h e so c a l l e d " a f f e c t i v e e m o t i o n a l s t a b i l i t i e s " , " i n t e l l e c t u a l c a p a c i t y a n d flexibility " , " d e t e r m i n a t i o n s t r e n g t l i " ; 2. beliaviour manifestations w e r e c h a r a c t e r i z e d b y i m p a t i e n c e a n d i r r i t a b i l i t y (on t h e e m o t i o n a l level), b y s c a t t e r e d a n d s l o w e d f o r m a t i o n of i d e a s (on t h e i n t e l l e c t u a l level), b y i n c r e a s e d g e n e r a l r e s t l e s s n e s s , e s p e c i a l l y f o r t h e u p p e r l i m b s (on t h e p s v c o m o t o r y l e v e l ) . d e l l ' i n q u a d r a r e i p i ù i m p o r t a n t i p r o b l e m i p s i c o f i s i o l o g i c i d e l l ' u o m o l a n c i a t o n e l l o s p a z i o il l o m o n a c o p o n e il « confinamento » t r a i f a t t o r i s u s c e t t i b i l i d i i n f l u e n z a r e l a e f f i c i e n z a e l a i n t e g r i t à d e l l a p e r s o n a u m a n a a l l a s t r e g u a d e l l e a c c e l e r a z i o n i , d e l l e r a d i a z i o n i n o c i v e , d e l l a s u b g r a v i t à . l e r a g i o n i c h e g i u s t i f i c a n o l ' i n t e r e s s e v e r s o i l p r o b l e m a d e l « c o n f i n a m e n t o » s o n o p r i n c i p a l m e n t e d u e : a) l a ristrettezza d e l l ' a b i t a c o l o r i s e r v a t o a l l ' u o m o i n s e n o a l l a c a p s u l a ; b) l ' a l l o n t a n a m e n t o d a l g l o b o t e r r e s t r e . r i c e r c h e s u l « c o m p o r t a m e n t o » u m a n o i n s o g g e t t i , e c c . (ili d o p o i p r i m i d a t i o t t e n u t i a seguito delle n o t e i m p r e s e spaziali u m a n e si è p o t u t o osservare che il p r o b l e m a del c o n f i n a m e n t o a t u t t ' o g g i è secondario r i s p e t t o a d a l t r i m a è suscettibile di a c u t i z z a r s i allorq u a n d o le esigenze p r o g r a m m a t i c h e i m p o r r a n n o il p r o l u n g a m e n t o della d u r a t a dei voli. i n t a l senso p a r l a n o i v a r i p r o g e t t i resi di p u b b l i c a ragione d a cui è p r e v e d i b i l e , a s c a d e n z a non l o n t a n a , l ' a t t u a z i o n e di imprese più s p i n t e q u a l i a d esempio il viaggio p e r i l u n a r e e l ' a p p r o d o sul satellite. tali f a t t i l e g i t t i m a n o l ' o p p o r t u n i t à di n o n a c c a n t o n a r e il p r o b l e m a e di p e r s i s t e r e nelle ricerche s p e r i m e n t a l i a v v i a t e d a i v a r i studiosi di medicina a e r o n a u t i c a e spaziale. p o i c h é il c e n t r o s t u d i e r i c e r c h e di m e d i c i n a a e r o n a u t i c a e spaziale di r o m a già agli albori, s o t t o la g u i d a di l o m o n a c o , si è a f f i a n c a t o agli a l t r i i s t i t u t i i n t e r n a z i o n a l i nello s v o l g i m e n t o di i n d a g i n i in q u e s t a direzione, la. p r e s e n t e comunicazione vuole essere u n a sintesi del lavoro c o m p i u t o in r a p p o r t o a n c h e a l l ' o p e r a s v o l t a d a a l c u n i a u t o r i s t r a n i e r i . ci s o f f e r m e r e m o p e r t a n t o su due p r i n c i p a l i m o t i v i di s t u d i o : a) le m a n i f e s t a z i o n i del c o m p o r t a m e n t o ossia le esteriorizzazioni o b i c t t i v a t o l i ; b) i t r a t t i di p e r s o n a l i t à p i ù o m e n o c o r r e l a t i con u n a m a g g i o r e a d a t t a b i l i t à a l c o n f i n a m e n t o . i l t e r m i n e « c o n f i n a m e n t o » n o n h a m a i figurato n e i dizionari medici c o m e p r o b l e m a a sé ed è c o m p a r s o solo d a p o c h i a n n i nella l e t t e r a t u r a m e d i c a specializzata quale quella a e r o n a u t i c a e spaziale. quello di « i s o l a m e n t o » che gli è f r e q u e n t e m e n t e collegato figura invece come voce r e l a t i v a alle m i s u r e p r o f i l a t t i c h e di polizia s a n i t a r i a c o n t r o la diff u s i o n e delle m a l a t t i e i n f e t t i v e . n e l n o v e r o dei n o s t r i p r o b l e m i la fisionomia del c o n f i n a m e n t o (con o senza i s o l a m e n t o ) si delinea come u n a condizione imprescindibile di lavoro e q u i n d i si iscrive nel r a n g o dei f a t t o r i connessi con le esigenze t e c n i c h e : non già perciò s t r u m e n t o di profilassi m a p o t e n z i a l e s t r u m e n t o p a t o g e n e t i c o di cui l ' u o m o d e v e subire l'azione. f i n dove t a l e f a t t o r e è s o p p o r t a b i l e ? d a q u a l e m o m e n t o e d a quale i n t e n s i t à esso c o m i n c i a a c o s t i t u i r e u n carico fisico e psichico? quali sono i p r i m i s i n t o m i che d e n u n c i a n o l'azione lesiva? q u e s t i sono i p i ù u r g e n t i i n t e r r o g a t i v i che si p r e s e n t a n o a d u n a a t t e n t a d i s a m i n a del p r o b l e m a e che h a n n o sollecitato il ricorso alla s p e r i m e n t a z i o n e . p r i m a di r i a s s u m e r e s o m m a r i a m e n t e i n o s t r i c o n t r i b u t i r i c o r d e r e m o che sul p i a n o storico, p e r q u a n t o si riferisce a l c o n f i n a m e n t o , si t r o v a 6 1 0 m. t r o l l o u n a c o r r i s p o n d e n z a s o l t a n t o con q u e l l a p e n a p a r t i c o l a r e c o n t e m p l a t a dal d i r i t t o p e n a l e c o n c r e t a n t e s i nella c o s i d e t t a reclusione q u a l e m i s u r a r e s t r i t t i v a della l i b e r t à p e r s o n a l e p i ù o m e n o s e v e r a a seconda delle epoche. circa l ' i s o l a m e n t o invece si r i c h i a m a n o le v a r i e p r a t i c h e di vita di chi v o l o n t a r i a m e n t e a b b a n d o n a il consorzio degli u o m i n i p e r v i v e r e solitario come gli a n a c o r e t i in cui, però, la s o l i t u d i n e è s o l t a n t o f o r m a l e ed esteriore. p u r e essendo e n t r a m b e m i s u r e r e s t r i t t i v e , i t e r m i n i isolam e n t o e c o n f i n a m e n t o non sono n é s i n o n i m i né simili: n e l l ' u n o v ' è assenza del « socius », n e l l ' a l t r o p r e s e n z a del « finis ». l ' e v o l u z i o n e della civiltà con l ' a m p i o uso della m a c c h i n a h a sott r a t t o tali condizioni a l n o v e r o d e c i s a m e n t e religioso o p u n i t i v o config u r a n d o loro u n a n u o v a d i m e n s i o n e q u a l ' è quella creatasi r e c e n t e m e n t e in seno alla medicina spaziale. q u i si s a r e b b e quasi i n d o t t i a r i t e n e r e che q u a n t o più la m a c c h i n a a c q u i s t a l i b e r t à nello spazio t a n t o p i ù l ' u o m o viene ad essere c o n f i n a t o nella m a c c h i n a . n o n è superfluo r i c o r d a r e in ogni caso che il c o n c e t t o di restrizione legato a quello di c o n f i n a m e n t o non è assoluto bensì r e l a t i v o ed in f u n zione sia d e l l ' i n t e n s i t à dello stimolo sia della d u r a t a di p e r m a n e n z a in c o n f i n a m e n t o sia infine delle c a r a t t e r i s t i c h e dei singoli soggetti. se ci s o f f e r m i a m o p i ù a t t e n t a m e n t e sul c o n f i n a m e n t o s p e r i m e n t a t o d a l l ' u o m o in u n a cap su la spaziale e r i c o r d i a m o che a f f i a n c a t o vi è in t a l e situazione il f a t t o r e della s u b g r a v i t à , v e r r e b b e quasi di p e n s a r e che le d u e condizioni ( c o n f i n a m e n t o e i m p o n d e r a b i l i t à ) siano f r a loro i n v e r s a m e n t e proporzionali. m e n t r e i n f a t t i nel c o n f i n a m e n t o g r a v a il peso della restrizione a l c o n t r a r i o nella s u b g r a v i t à g r a v a , p e r così dire, il peso della e s t r e m a l i b e r t à . ce ne d à m o t i v o lo stesso s c r i t t o di t i t o v q u a n d o nel libro «17 a u r o r e cosmiche» nel t r a c c i a r e u n r i t r a t t o delle sue v a r i e impressioni, t r a cui quella di f a t i c a , p e r l ' i m p o n d e r a b i l i t à che gli sollevava le b r a c c i a e le m a n t e n e v a sospese nell'aria dice: « p e r p o t e r d o m a r e le b r a c c i a le infilai s o t t o le cinghie della p o l t r o n a »; s o l t a n t o allora p o t e i p r e n d e r e sonno ». l ' i m m a g i n e di questo a l t e r n o gioco di cost rizione e di l i b e r t à è u n argoment o di v i v o interesse scientifico a n c h e se a p p a r e t i n t o d a u n a v e n a di i l a r i t à . d i a m o ora u n o s g u a r d o p a n o r a m i c o alle n o s t r e ricerche. a titolo di s o m m a r i a esemplificazione esse possono riunirsi, secondo u n p u n t o di v i s t a cronologico, in d u e g r u p p i : a) quelle a n t e c e d e n t i il p r i m o volo cosmico (aprile 1961); b) quelle successive. r i c e r c h e s u l « c o m p o r t a m e n t o » u m a n o i n s o g g e t t i , e c c . ( i l i la d i f f e r e n z a f r a essi si b a s a sulle s e g u e n t i fasi f o n d a m e n t a l i : a) dur a t a del c o n f i n a m e n t o ; b) n u m e r o dei p a r t e c i p a n t i ; c) m o t i v a z i o n e vol o n t a r i a o n o n . i n o l t r e il p r i m o g r u p p o si c a r a t t e r i z z a p e r u n a maggiore s e v e r i t à nelle registrazioni i m p o s t e d a a l c u n i r i c e r c a t o r i , severità giustificata dal c o n c e t t o di r e n d e r e g r a v e il carico s o m a t i c o e psichico p e r meglio evidenziare i limiti di t o l l e r a n z a e la f o r z a stessa delle m o t i v a z i o n i , c r e a v a qualcosa di simile a d u n v e r o supplizio. p a r l a n o in q u e s t o senso i r i s u l t a t i degli e s p e r i m e n t i c o n d o t t i d a e v r a r d e d a w u l f t e n p a l t e . q u e s t ' u l t i m o d e n u n c i a la c o m p a r s a di d i s t u r b i che lasciano v e d e r e la possibilità di p r o v o c a r e u n a v e r a e p r o p r i a disintegrazione della person a l i t à a n c h e in m o d o irreversibile. c o n t e m p o r a n e a m e n t e c o n d u c e v a m o a n c h e noi u n g r u p p o di ricerche di cui si d a v a c o m u n i c a z i o n e a l congresso di med. a e r o n . e spaz. 1959 r o m a . tali ricerche, c o n d o t t e su dieci e l e m e n t i v o l o n t a r i e di livello m e n t a l e e l e v a t o , i m p l i c a v a n o u n carico r e s t r i t t i v o m o l t o m e n o severo p e r m e t t e n d o così di a n d a r e oltre la d u r a t a m a s s i m a di d u e giorni di c o n f i n a m e n t o d a noi s t a b i l i t o . u n secondo p i ù r e c e n t e g r u p p o di i n d a g i n i è s t a t o d a noi ripreso presso la stessa sede del c e n t r o s t u d i e r i c e r c h e di medicina a e r o n a u t i c a e spaziale di r o m a dopo u n lasso di t e m p o che ci h a p e r m e s s o d i v e d e r e i voli u m a n i d a g a g a r i n a g l e n n e q u i n d i f o n d a r c i sugli e v e n t i reali d e t e r m i n a t i s i i n seguito a d essi. tali indagini si discostano dalle p r e c e d e n t i p e r u n a p r o g r a m m a z i o n e la q u a l e p u r t e n e n d o i n a l t e r a t a le linee essenziali h a messo a f u o c o il p r o b l e m a , al livello p e r cosi dire s t a t i s t i c o , m e d i a n t e le s e g u e n t i i n n o v a z i o n i : a) a u m e n t o notevole dei p a r t e c i p a n t i ; b) d i m i n u z i o n e della d u r a t a . i n f a t t i i soggetti sono saliti a s e t t a n t a u n i t à r i s p e t t o ai dieci preced e n t i e la d u r a t a c o n t r a t t a ad un m a s s i m o di sei ore c o n t r o le q u a r a n t o t t o di p r i m a . p e r t a l i ricerche a b b i a m o u t i l i z z a t o u n n u o v o a m b i e n t e e p r o p r i a m e n t e u n a c a m e r a silente in cui il soggetto era o b b l i g a t o alla •posizione s e d u t a con divieto di leggere, f u m a r e , d o r m i r e ed a m b u l a r e . u n a i m p o r t a n t e c a r a t t e r i s t i c a connessa con t a l e serie di ricerche è quella che lascia i n t r a v e d e r e il p r o b l e m a del c o n f i n a m e n t o s e m p r e p i ù collegato con quello della selezione psico-fisica i n t e g r a n d o p e r t a n t o la delineazione p r e l i m i n a r e dei f o n d a m e n t a l i requisiti r a p p r e s e n t a t i v i del profilo ideale p e r u n c o s m o n a u t a . altro a s p e t t o i n t e r e s s a n t e d a r i c o r d a r e è quello r e l a t i v o alla osservazione e v a l u t a z i o n e del g r a d o di efficienza m a n t e n u t o dai p a r t e c i p a n t i nel corso della p r o v a di c o n f i n a m e n t o quale segno i n d i r e t t o delle ripercussioni d e t e r m i n a t e sulla p e r s o n a l i t à dei singoli. 6 1 2 m. s t r 0 l l 0 nel r i a s s u m e r e i r i s u l t a t i r e l a t i v i ai (lue g r u p p i di ricerche r e p u t o o p p o r t u n o riferire s o m m a r i a m e n t e sulla n a t u r a delle limitazioni i m p o s t e ai soggetti allo scopo di r e n d e r e la situazione p i ù a d e r e n t e al c o n c e t t o f o n d a m e n t a l e della restrizione. p o s s i a m o d i s t r i b u i r e t a l i limitazioni su t r e p i a n i : a) senso percettivo; b) intellettivo; 0 ) psico-motorio. nel p r i m o caso la d e p r i v a z i o n e è s t a t a s o l t a n t o p a r z i a l e e m i t i g a t a . ad es. le m o d a l i t à senso-percettive, visiva ed a c u s t i c a , non sono s t a t e a l t e r a t e m e d i a n t e l ' e v e n t u a l e uso di b e n d e oculari, t a m p o n i auricolari o a l t r i espedienti; nel secondo è s t a t a i n t r o d o t t a u n a f o r t e l i m i t a z i o n e v i e t a n d o a i soggetti l'uso di q u a l u n q u e m a t e r i a l e a t t o a soddisfare il bisogno di occupazione m e n t a l e (libri, giornali, ecc.). nel t e r z o le l i m i t a zioni sono s t a t e m a g g i o r i nel corso delle p i ù r e c e n t i i n d a g i n i poiché non occorreva s o d d i s f a r e gli essenziali bisogni fisiologici (alimentazione, sonno evacuazione). i r i s u l t a t i conseguiti si r i a s s u m o n o secondo u n doppio ordine di considerazioni: r e l a t i v o alle c a r a t t e r i s t i c h e comuni ( p e r t i n e n t i alla situazione c o n f i n a m e n t o in generale) e a quelle particolari. v a p r e m e s s o che essi sono il f r u t t o di u n a t t e n t o e s a m e c o n d o t t o m e d i a n t e t r e sussidi: l'osservazione e s t e r n a (realizzata con l ' e s p e d i e n t e del v e t r o unidirezionale), la v a l u t a z i o n e o b b i e t t i v a del r e n d i m e n t o (realizzata con i t e s t s f a t t i eseguire), infine lo s t u d i o i n t r o s p e t t i v o (realizzato con colloquio e protocolli). p e r q u a n t o r i g u a r d a le c a r a t t e r i s t i c h e c o m u n i sono d a r i m a r c a r e le d u e seguenti n o t e essenziali: a) t u r b e di o r d i n e a f f e t t i v o e m o t i v o r a p p r e s e n t a t e d a u n o s t a t o d ' i n s o f f e r e n z a e di disagio o i n q u i e t u d i n e in direzione aggressiva; b) t u r b e di o r d i n e psico-motorio c a r a t t e r i z z a t e d a i r r e q u i e t e z z a g e n e r a l e (ipercinesia). volendo riferirci p e r ò a i d u e diversi t i p i di c o n f i n a m e n t o , quello p r o t r a t t o e v o l o n t a r i o e quello l i m i t a t o e p r i v o di m o t i v a z i o n e , ricorder e m o i s e g u e n t i d a t i p r e m i n e n t i . p e r il p r i m o t i p o di c o n f i n a m e n t o : 1. l a t o l l e r a n z a , espressa s o t t o f o r m a di d u r a t a nella p e r m a n e n z a in c o n f i n a m e n t o , è s t a t a t a n t o m i n o r e q u a n t o maggiore era l'int e n s i t à delle limitazioni r e s t r i t t i v e s o m a t i c h e (evidente s o p r a t t u t t o nelle ricerche di e v r a r d e w u l f e t e n p a l t e ) . nelle n o s t r e ricerche m a g g i o r e t o l l e r a n z a si è a v u t a nei soggetti d i s t i n t i d a n o t e di « estroversione » ; 2. le m a n i f e s t a z i o n i c o m p o r t a m e n t a l i d a noi r e g i s t r a t e sono stat e quelle a carico s o p r a t t u t t o del bisogno di o p e r a t i v i t à (manipolazione degli oggetti e c o m p i m e n t o di a t t i m o t o r i senza scopo). è seguito il bisogno r i c e r c h e sul « c o m p o r t a m e n t o » u m a n o i n s o g g e t t i , e c c . (ili di esplorazione della r e a l t à e s t e r n a con t u t t i i mezzi senso-percettivi c o m p a r e n d o finanche u n a specie di e s a l t a z i o n e della sensibilità e della sensorialità ( s t a n d o al r i f e r i m e n t o dei soggetti); 3. l ' a t t i v i t à del pensiero h a m o s t r a t o u n a flessione verso u n a s o r t a d i v u o t a m e n t o m e n t a l e con r a l l e n t a m e n t o della ideazione e p o v e r t à d i t e m i (questi u l t i m i poi a s f o n d o s e n t i m e n t a l e e in senso depressivo). ciò coincide con le osservazioni di f e l d e m a n in ricerche s u l l ' i s o l a m e n t o c o n d o t t e p e r a l t r i scopi. circa il secondo t i p o di c o n f i n a m e n t o : 1. l a t o l l e r a n z a è s t a t a p i ù d i r e t t a m e n t e collegabile con la fisionomia c a r a t t e r o l o g i c a del s o g g e t t o ; 2. le m a n i f e s t a z i o n i c o m p o r t a m e n t a l i sono s t a t e di o r d i n e psicom o t o r i o e l e m e n t a r e con l ' a c c e n t u a z i o n e d i f f u s a (ipercinesia), s o p r a t t u t t o n e l l ' a m b i t o d e l l ' e s t r e m i t à superiore, t r a d u c e n t e s i nel m o v i m e n t o cont i n u o delle m a n i e delle d i t a , con visibile a c c r e s c i m e n t o dopo la p r i m a ora di p e r m a n e n z a . si è p o t u t o c h i a r a m e n t e a p p r e z z a r e c o m e t a l e mov i m e n t o rispondesse a d u n i m p u l s o c o n t i n u o e non s p o n t a n e a m e n t e dom i n a b i l e . ciò è s t a t o a c c e r t a t o a t t r a v e r s o il colloquio finale che h a permesso così d i riferirli all'azione di d i n a m i s m i inconsci; 3. l ' a t t i v i t à di pensiero h a s e g n a t o , a n c o r a u n a v o l t a , u n deciso r a l l e n t a m e n t o con u n a s o r t a di f r a m m e n t a r i e t à e (li e v a n e s c e n z a dei t e m i . i l c o n t e n u t o è s t a t o più f r e q u e n t e m e n t e s e n t i m e n t a l e e di t i p o r i p r o d u t t i v o , a n c o r a t o , in genere, a d episodi b a n a l i . n o t a t a , infine, u n a c e r t a t e n d e n z a alla sonnolenza. c o n c l u s i o n i . volendo m e t t e r e a f u o c o l ' e l e m e n t o c e n t r a l e che d o m i n a il q u a d r o r e a t t i v o del c o m p o r t a m e n t o nella situazione t r a t t a t a , non p u ò p a s s a r e i n o s s e r v a t o come esso si identifichi in u n o squilibrio a c u t o sul p i a n o a f f e t t i v o e m o t i v o e si t r a d u c a in u n senso di sgradevolezza (sotto f o r m a di i r r i t a b i l i t à e di insofferenza) cui f a cornice u n a a c c e n t u a t a i r r e q u i e t e z z a sul p i a n o psico-motorio ed u n a f r a m m e n t a r i e t à e r a l l e n t a m e n t o i d e a t i v o sul p i a n o i n t e l l e t t i v o . a spiegarci la genesi di tali m a n i f e s t a z i o n i è forse sufficiente richiam a r c i ai p r i n c i p i o r m a i a c q u i s i t i a l s a p e r e u m a n o . b a s t a r i c o r d a r e inf a t t i che la n a t u r a u m a n a , q u a l e d e r i v a d a l lungo c a m m i n o storico, sembri: c o n f o r m a t a p e r v i v e r e e s v i l u p p a r s i in m o d o consono alle esigenze specifiche di c o n v i v e n z a nel g r u p p o e di l i b e r t à nel m o n d o dello spazio. 0 1 4 m. s t r o l l o p e r t a n t o , q u a n d o sono soppresse le relazioni con il g r u p p o e sono r i d o t t i gli a p p o r t i m u l t i f o r m i della r e a l t à , a f f l u e n t i e r i n n o v a n t i s i a t t r a v e r s o le vie s ens oper cet t iv e, e infine sono c o a r t a t i gli a t t i m o t o r i che stabiliscono il r a p p o r t o con la r e a l t à stessa, si e n t r a in pieno in u n a condizione glob a l e d i squilibrio nelle singole s f e r e dello psichismo in m o d o più o m e n o a p p a r i s c e n t e e precoce. connesso a t a l e squilibrio n a s c e il d i s a d a t t a m e n t o e il r e l a t i v o s t a t o d i t e n s i o n e che si svolge in u n a g a m m a di esperienze i n t e r i o r i a d i v e r s a g r a d a z i o n e . al di là, però, della i n t e n s i t à e della d u r a t a della « situazione-stimolo », p e r cui è lecito s u p p o r r e l ' i n t e r v e n t o d i u n a l e n t a e p r o g r e s s i v a assuefazione, occorre d o m a n d a r s i se sussista u n e l e m e n t o tipologico, l e g a t o q u i n d i a l soggetto, che c o n s e n t a un p i ù facile s u p e r a m e n t o delle conseguenze t r a u m a t i z z a n t i dello « stress » in questione. q u e s t o o b b i e t t i v o è s t a t o u n o degli scopi p r i n c i p a l i delle n o s t r e ricerche e i r i s u l t a t i n o n ci h a n n o deluso poiché si è p o t u t o d e c i s a m e n t e o s s e r v a r e l ' i m p o r t a n z a delle q u a l i t à « c a r a t t e r i a l i » come f a t t o r i essenziali nel d e t e r m i n a r e u n a m a g g i o r e o m i n o r e t o l l e r a n z a al c o n f i n a m e n t o . i n connessione con q u e s t a è a p p a r s a i n f a t t i la p r e s e n z a di u n q u i d , p r o f o n d o e complesso, e l e m e n t o c e n t r a l e della p e r s o n a l i t à u m a n a riassumibile nel t r a t t o c o m u n e m e n t e definito « t e n a c i a o p e r s e v e r a n z a ». esso n o n v a p e r ò confuso con quello della s t a b i l i t à a f f e t t i v o e m o t i v a poiché t r a t t a s i s o p r a t t u t t o di e l e m e n t o a t t i v o e s t r a n e o ai s e g m e n t i i n t e l l e t t i v o o a f f e t t i v o e m o t i v o ed inserito in u n a p i ù v a s t a cornice di c a r a t t e r i u m a n i i quali a t t i n g o n o alla s f e r a etica e spirituale. tali sono d a consid e r a r e i n f a t t i il c o s i d d e t t o s e n t i m e n t o del dovere, la f e d e l t à al c o m p i t o , l ' i m p e g n o e l ' a d e s i o n e ai valori l i b e r a m e n t e a c c e t t a t i . t r o v a n d o s i al polo o p p o s t o di quelle q u a l i t à g e n e r i c a m e n t e definite come leggerezza e v o l u b i l i t à si è r i v e l a t o d e c i s a m e n t e p r e s e n t e , nel corso dei n o s t r i esper i m e n t i , nei soggetti che h a n n o r a g g i u n t o la m a s s i m a p r e s t a z i o n e ; il che ci suggerice di f a r e su (li esso u n a d e g u a t o a s s e g n a m e n t o p e r considerare la p r o v a d i c o n f i n a m e n t o come u n n u o v o s t r u m e n t o di v a l u t a z i o n e e di discriminazione nell'ordine dei requisiti psico-fisici richiesti al p o t e n z i a l e c o s m o n a u t a . i n c o n c o r d a n z a con l'asserzione f a t t a in p r o p o s i t o d a alc u n i a u t o r i a m e r i c a n i (holt e goldberger), i quali p a r l a n o di f o r z a d e l l ' e g o ( e g o s t r e n g h t ) come di u n a s t r u t t u r a f o n d a m e n t a l e della p e r s o n a l i t à c o m p r e n d e n t e sia la m a t u r i t à a f f e t t i v a che la ricchezza e flessibilità i n t e l l e t t i v a i n s i e m e con la f o r z a della d e t e r m i n a z i o n e , si configura il « t r a t t o » p r o f o n d o e complesso della p e r s o n a l i t à affiorato a t t r a verso le n o s t r e i n d a g i n i di cui a b b i a m o d a t o p r e s e n t e m e n t e u n resoconto s o m m a r i o . 404 not found influence of shell structure on the equation of state of matter at high pressures* e . boschi ( * * ) received on february 21st, 1972 summary. — w e develop an equation of state of matter in the pressure range of geophysical interest introducing shell structure in the thomasf e r m i d i r a c atom model. riassunto. — viene sviluppata un'equazione di stato della materia nell'intervallo di pressioni d'interesse geofisico, introducendo la struttura a strati nel modello atomico di tliomas-fermi-dirac. the thomas-fermi model, in its original formulation, cannot give account for the existence of discrete energy levels for bound electrons or, in other words, is not sensitive to the effects of shell structure (2). i t is also obvious that these effects are the principal cause of the inapplicability of the thomas-fermi-dirac model in the pressure range of geophysical interest (4). up today, these effects have been considered only from a qualitative point of view and the influence of them on the equation of state has not been determined. w e believe that it is possible to solve consistently this problem by means of an appropriate modification of the normalization of the thomas-fermi-dirac electronic charged cloud. as it is well-known, in the thomas-fermidirac theory, the number of the possible states per volume unit ( * ) p a p e r presented at the " f i r s t european eartli and p l a n e t a r y physics c o l l o q u i u m " , reading ( u . k . ) , 30 march-2 a p r i l 1971. ( * * ) istituto di fisica, università degli studi di bologna ( i t a l y ) . 3 3 4 e . b o s c h i of the electrons, whose momentum is between p and p + dp, is 8 n p2 dp¡h3. their density is given by the fermi-dirac statistics: , 8 np2 dp dp = . , ekin — ew 7i exp i it + f l where eun is tlie kinetic energy, cp the thomas-fermi potential, k the boltzman constant, t the temperature, and /t the chemical potential. if ave consider an atom of atomic number z, the integral of do over the entire phase-space atomic volume must satisfy the condition: 3 2 n 2 h3 r2 dr p2 dp exp e tin — e cp kt z furthermore the poisson's equation gives another relation between the electron density and the potential. now we assume that only a part of the electrons in tlie atom can satisfy to the thomas-fermi-dirac requirements. the assumption means that there is, for a given value of the pressure and of the temperature, some "critical radius", bc, which divides the atom volume into two parts: an outer part containing electrons which can be actually considered as a charge cloud and an inner part containing electrons whose behaviour needs a quantum mechanical description. so, for given external conditions, we have b " b o u n d " electrons and f " f r e e " electrons, where b and f are linked by the obvious relation: f + b = z. i n more precise words we introduce a cutoff in the momentum phase space of the atom that means to change the lower limit of momentum integration, 0, which appears in tlie normalization equation, to some value p. i n this manner we obtain the actual number of electrons, the so-called free electron, which can be treated statistically. since the free electrons are contained in a continuum of energy levels, the use of the thomas-fermi-dirac theory appears well justified (3). the way to determine the counting of bound electrons is to use a trial potential v in sclirodinger's equation, to obtain the energy i n f l u e n c e of s h e l l s t r u c t u r e on t i i e e q u a t i o n e t c . 3 3 5 levels corresponding to y , and calculate tlie number of bound electrons from tlie relations: exp (— eni/kt + n) + 1 and b = sn, where n and i are tlie principal and orbital quantum numbers, n„i is the average number of electrons in the subshell (nl) and b is the total number of bound electrons. after these considerations, the thomas-fermi-dirac normalization condition becomes: 32 n? f r2 dr p2 dp h3 / / (e*in — ev ~ 6xp i îct +-") + 1 0 rc + v 2(21 + 1) = z / ni exp (—eni/kt + n) + 1 searching the appropriate potential v which appears in the last equation, it seems natural to take into consideration the ionization phenomena and to represent the passage from bound electrons to free electrons as some kind of ionization induced by pressure. this makes possible the task of finding an appropriate potential function 0): v = r \r 2 r2 2/ ^ f e f r 2 a x 0 r (1 + ar)2 r \2 r where ax is chosen to make v continuous at r — rc: _ z r 5 , 3 ' 1 ~ rc (1 + a rc)2 f rc 2 " by means of poisson's equation: 2 z a rc z (1 + a rc)3 (1 + a rc)2 8 e . boschi where a is a constant for a given value of z. bc divides the atomic volume into two regions: an inner region around the nucleus where the electron density is high and due to bound electrons, and an outer region where the electron density is low and due to free electrons. to perform the calculations, we use an iterative process starting from a trial value of the electron density. the figure shows the results obtained for an atom of atomic number 26 and puts in evidence the discrepancy between the thomas-fermi-dirac model, represented by i n f l u e n c e of s h e l l s t r u c t u r e on t h e e q u a t i o n e t c . 3 3 7 a continuous curve line, and the results of these considerations, represented by a broken and dotted line. the dotted line, near to the continuous one, represents a thomas-fermi-dirac calculation with the potential v made to check the internal consistency of the work. acknowledgements i wish to thank prof. m. caputo for valuable discussions on this work. r e f e r e n c e s t1) a r m s t r o n g b . h . , j o h n s t o n r . r . , k e l l y p . s . , 19(37. progress in high temperature physics and chemistry, edited b y carl. a . rouse, p e r g a m o n press. ( 2 ) boschi e., caputo m., 1969. « n u o v o c i m e n t o » , 1, 441. ( 3 ) boschi e., caputo m., 1971. mantle and core in planetary physics, edited by j. coulomb and m. caputo, academic press, milano. c ) t a k e u c h i i l , k a n a m o r i h . , 1 9 6 6 . « j o u r n . g e o p h y s . r e s . » , 7 1 , 3 9 8 5 . e a r t h m e a s u r e m e n t s v i a s a t e l l i t e s (*) a . l . c o t c h e r r i c e v u t o il 19 f e b b r a i o 1903 s u m m a r y . — geodetic p a r a m e t e r s a n d i n t e r c o n t i n e n t a l ties c a n be d e t e r m i i i e d using a n e w t e c h n i q u e whicli combine» eleotronic d i s t a n c e m e a s u r i n g e q u i p m e n t (dme) a n d artiflcial satellites. [il initial field t e s t s , a v e r a g e r a n g e r e s o l u t i o n s of 30 cm b a v e been o b t a i n e d a t d i s t a n c e s g r e a t c r t b a n 3000 k m . r i a s s u n t o . — nella n o t a l ' a . m e t t e in e v i d e n z a l ' u s o di n u o v e t e c n i c h e per d e t e r m i n a r e i p a r a m e t r i geodetici e i c o l l e g a m e n t i intercont i n e n t a l i , a b b i n a n d o l ' e q u i p a g g i a m e n t o elettronico per m i s u r a d i s t a n z e (dme) ai satelliti artificiali. the purpose of this paper is to introduce t h e concept of using artiflcial earth satellites for measuring the size and shape of the earth, and the relative position of places on its surface. an advanced electronic system which has been built to implement this eoncept will be described in general terms, with a re vi e w of the experimental data obtained to date. this system, called secor, provides a new and powerful tool for extending man's knowledge of t h e planet on which he lives. before discussing secor itself, i should like to talk about the basic building blocks of which it is eomposed . . . electronic dme (distancemeasuring equipment). dme operates on a very simple principle, which says tliat t h e pliase shift experienced by any propagated electromagnetic wave is directly proportional to the propagation p a t h length. therefore, if this phase shift can be measured by comparing t h e wave at the beginning of the p a t h with the wave at the end, the p a t h length (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. 3 h 2 a . l . c o t c h e r can be determined. making such a measurement if flit' beginning a n d end of t h e patii are f a r a p a r t poses serious problems, which m a y be easily avoided, however: the beginning and end are a t t h e sanie place, b u t in traveling f r o m one to the other, t h e wave is propagatori via t h e point whose distance is of interest. then this point's distance is, of course, exactly half t h e patii length indicated by t h e phase dift'erence measurement a t the common beginning-end. for example, a c-\v signal is t r a n s m i t t e d f r o m a ground station to some airborne veliicle carrying a transponder; the transponder receives t h e signal and retransmits it, back to the ground station, where it is compared in phase with t h e originai transmission to determine t h e distance from the ground station to t h e airborne veliicle. techniques have been developed in recent years which m a k e this kind of distance measurement, using phase comparison of continuous-wave signals, extremely accurate. or, more precisely speaking, extremely accurate whenever t h e phase dift'erence measured is less t h a n one wavelength (360°); phase differences greater t h a n t h a t raise measurement problems t h a t are not readily solved. in general, t h e accuracy of m e a s u r e m e n t , expressed as a certain small fraction of a wavelength, depends only on t h e equipment used, but does not depend 011 the wavelength; in other words, equipment t h a t can measure a phase dift'erence as small as one thousandth of a wavelength can m a k e p a t h length measurements with a resolution of 1 k m if t h e wavelength is 1000 k m , b u t with a resolution of 1 cm if the wavelength is 10 meters. in s u m m a r y , then, t h e two f u n d a m e n t a l f a c t s abont phase-comparison dme are (1) e s t r e m e accuracy inversely proportional to wavelength and (2) b u t only a t distances less t h a n one wavelength. thus, we m u s t choosé a short wavelength (high frequency) to ensure high accuracy, and a t t h e same time choose a long wavelength (lovv frequency) if we w a n t to measure long distances. here is t h e basic dilemma. i t is solved, in cubie dme, by temporarily ignoring t h e a c t u a l transmitted frequency (carrier) and considering modulation signals impressed upon it. these modulation signals undergo exactly the same sort of pathlength-dependent phase shift i previously described, and in cubie dme t h e r e are usually four or live of t h e m , covering the range between t h e longest distance ever to be measured (say, 20,000 km) and t h e d e s i m i accuracy (say, 1 meter). the lower-frequency modulating signals provide t h e unambiguous d a t a revealing less and less roughly how f a r away t h e transponder is, wliile t h e highest-frequency modulating signal pinpoints the distance, much as the numerals in a i-digit n u m b e r teli e a r t h m e a s u r e m e n t s v i a s a t b l i . i t e s 3 8 3 you, one by one from t h e left, t h e thousands, hundreds, decades, and fìnally t h e units involved. so much for a simple dme, measuring even very long ranges with very liigh accuracies. b y measuring the distarne to some airborne object, it tells us in effect t h a t the object is somewhere on a hemisphere whose center is at the ground station and whose radius is the measured distance. if the distance to t h e sanie airborne object is simultaneously measured f r o m a second ground station somewhere else on t h e e a r t h ' s surface, a second hemisphere is established, and the object m u s t lie on this second hemisphere as well. therefore, it must lie on t h e semicircle which is t h e intersection of t h e two hemispheres. a distance measurem e n t from a t h i r d ground station provides a hemisphere which intersects the semicircle a t only one point, thereby uniquely determining the location of t h e object in space. secor measures spatial positions in just this way, nsing t h r e e or more ground stations, except t h a t t h e measurements are not iliade quite simultaneously. i n s t e a d , to p e r m i t using only a single transponder in t h e airborne vehicle, where space and weight are nsually a t a premium, this transponder is rapidly time-shared among t h e ground stations; system accuracy is not appreciably affected. secor is being used today on m a j o r test ranges throughout t h e world for tracking and guiding both long-range and short-range missiles. now let's consider an a c t u a l e a r t h measurement problem, like determining t h e distance between n o r t h america and hawaii. because of certain difflculties associated with celestial observations, such distances are now known to within perhaps the nearest kilometer. w i t h secor, it is anticipated t h a t this u n c e r t a i n t y can be reduced to as little as 30 meters. the f u n d a m e n t a l technique is as follows: (1) place three dme ground stations a t accurately surveyed points, a few lmndred miles a p a r t , in n o r t h america; (2) place a f o u r t h dme ground station in hawaii; (3) orbit a n artificial satellite carrying a secor transponder; (4) determine the satellite's orbit (with respect to n o r t h america) precisely from t h e north american ground stations; (5) determine t h e hawaiian station's position (with respect to t h e orbit) f r o m t h e satellite. perhaps t h e last step requires a brief explanation. as it moves along its orbit, t h e satellite can measure t h e distance to the hawaiian station whenever t h e latteiis " in sight " , which might be for several thousand orbitai miles. measurements at merely t h r e e points, perhaps 500 miles a p a r t , however, are enough to pinpoint t h e hawaiian site; we bave three intersecting hemispheres, just as before, except t h a t now 3 8 4 a . l . c o t c h e r their centers lie along t h e orbit while their only point of intersection is on t h e e a r t h ' s surface. at present, the secor earth-measurement program is preparing for large-scale evaluation. a number of helicopter-transportable ground stations bave been built and tested against secor transponders carried aboard three e a r t h satellites; in tliese tests, average range resolutions of 30 cm were obtained a t distances up to 2000 miles. more ground stations, of advanced a n d ruggedized design, as well as additional transponders and satellites, are built and building. i t is anticipated t h a t t h e first intercontinental d a t a will be available witliin less t h a n a year, thus m a r k i n g the beginning of a long and useful life for secor as a unique scientific i n s t r u m e n t for exploring the earth from t h e newest of its moons. 173_184 benedetti.pdf 173 annals of geophysics, vol. 46, n. 2, april 2003 giorgio fiocco: a jolly good fellow and his research glauco benedetti-michelangeli (*), marco cacciani (1), anna capasso (2), fernando congeduti (3), tatiana di iorio (1), alcide g. di sarra (1) (4), daniele fuà (1) (5), gerald grams (6), george magyar (*), giandomenico pace (1) and guido visconti (7) (1) dipartimeno di fisica, università di roma «la sapienza», roma, italy (2) uffi cio stampa, cnr, roma, italy (3) istituto di scienze dell’atmosfera e del clima, sezione di roma, italy (4) enea, clim-oss, s. maria di galeria, italy (5) disat, università di milano-bicocca, milano, italy (6) atlanta university, atlanta, ga, u.s.a. (7) dipartimento di fisica, università dell’aquila, italy abstract in celebrating the 70th birthday of giorgio fiocco we recall that his scientifi c production includes some pioneering work in a number of fi elds where he set important milestones. in particular, several of his papers represent the fi rst published evidence in certain fi elds and triggered such a large fl ow of research, new developments and new paper production that the original source is almost forgotten. here the topics where his contribution was of pioneering importance are briefl y recalled and his original works cited. mailing address: prof. daniele fuà, dipartimento di fisica, università di roma «la sapienza», p.le aldo moro 2, 00185 roma, italy; e-mail: fua@g24ux.phys.uniroma1.it key words atmosphere lidar aerosol 1. introduction since his thesis at the faculty of engineering of the university of rome, giorgio fiocco has been extremely active in several fi elds of both engineering and physics. here we do not need to acknowledge his already high visibility in the italian and international academic and scientifi c community: his authority is well recognized by his participation in many committees and panels, as well as his affi liation in academies, scientifi c societies and, last but not least, his past positions as head of important research institutions (director of the former institute of atmospheric physics, ifa, of the national research council, cnr, and president of the italian space agency, asi). affi rming that most people involved in atmospheric physics all over the world know who giorgio fiocco is is not an overstatement. according to one of fiocco’s basic teachings that «quality is more important than quantity», his paper production is not as heavy (in a literal sense) as that of other colleagues in the physical sciences although it still numbers an outstanding 300 or so. here we recall that among his scientifi c production one can fi nd some pioneering work in a number of fi elds where he set important milestones. in particular, several of his papers represent the fi rst published evidence in some fi elds and triggered such a large fl ow of research, new developments and new paper production that the original source is almost forgotten.(*) «retired». 174 glauco benedetti-michelangeli et al. the main aim of this paper written for celebrating his 70th birthday is to settle this point: to recall briefl y the topics where the contribution of giorgio fiocco was of pioneering impor tance and cite his original works which confi rm his primacy in order to reaffi rm the true paternity of some ideas and techniques accepted worldwide. 2. great britain fiocco had just graduated as an electrical engineer in 1956; what could he become if not an employee of one of the «diamond tips» in the electronics industry at that time: the marconi company. the fact that i) going abroad for a job at that time was not common (during the italian economic «boom» of the late fi fties, job opportunities at home were abundant), ii) the choice of the u.k. was even less common for italians and iii) the choice of working for the fi rm founded by a very famous and brilliant italian inventor, may or may not be relevant. surely it was the fi rst (and, probably not easy) step forward in view of his next big jump overseas. the facts are that he was right away involved in advanced research in radar and navigational aid systems and his contribution was such that some of his discoveries were patented for marconi (fiocco,1958a-e, 1959a-e; adam and fiocco, 1959). being in private industry, his contributions at that time are published as internal reports. moreover, most of the discoveries and developments which were the ultimate thing at that time, are probably nowadays outdated by any of the computer chips that control our kitchen appliances (on the relationship between the scientifi c knowledge of different generations of students, in this same issue, there is an interesting essay by l. smullin, a teacher and good friend of fiocco’s). of his «science» there, we, friends and atmospheric physicists, do not have much record. what we believe is that marconi’s, like england, was the training entrancehall where fiocco fi rst came in touch with the true, «dirty» world and its fascinating applied sciences after his book-based university years. since then the enchantment of such devices as radars, able to sense far objects, never left him and always spurred his knack for practical and brilliant solutions to new and challenging problems. in particular, he was interested in a deeper understanding of what professional radar people consider noise but is indeed the atmospheric signature. at that time it probably occurred to giorgio fiocco that studying the source of the radar noise could become a new branch of the geophysical sciences. 3. united states we must face it that to be successful in scientifi c research it is not often suffi cient to be talented, one needs certain amount of luck: to be at the right place at the right time. this was the case in the laser fi eld in the early sixties. the fi rst laser was constructed in 1960 in the u.s. during the next 2-3 years various types of lasers, laser effects and applications were discovered or invented, each of which became the foundation of a whole new research fi eld. two of these were associated with the name of giorgio fiocco. he, like many young european researchers, went to the united states in order to perfect his research skills and learn the latest techniques. one of the outstanding professors of the electrical engineering department at mit was louis smullin, formerly a member of the radiation laboratory of the same university where the microwave radar had been developed. smullin chose young fiocco to set up an active optical instrument able to measure the distance from the moon: one of the fi rst optical radar or lidar (acronym for light detection and ranging, an obvious readjustment of the acronym radar, radio detection and ranging). when one considers how diffi cult is was to observe a few photons of laser light refl ected from the lunar surface without the benefi t of the retro-refl ector placed much later on the moon by astronauts, everybody can appreciate the sense of humor in the chosen code name used for the lunar ranging program «project luna see» sounds 175 giorgio fiocco: a jolly good fellow and his research like «project lunacy». after struggling with electronics that nowadays can be seen only in science museums, they got the fi rst light pulses echoed from a half moon during three nights of may 1962. the news was soon highlighted in newspapers all over the world (it made the fi rst page in the new york times); in particular, the scientifi c report was published in the outstanding journal nature (smullin and fiocco, 1962). next came the application of laser pulses to diagnose a plasma by thomson scattering (fiocco and thompson, 1963). the advantages of laser light are: negligible disturbance of the plasma, high spatial and temporal resolution (a very short laser pulse is used), remoteness from the plasma. at the same time, the laser beam is suffi ciently powerful to produce observable effects. in the case of the thomson scattering the originally narrow spectrum becomes broadened after interacting with the hot electrons of the plasma. the broadening is directly proportional to the temperature and the amount of light scattered depends on the plasma density, thus providing information on the two most important parameters of a plasma while the centroid of the spectrum provides a measurement of the radial bulk velocity of the medium. lasers became the standard diagnostic tools for fusion plasmas all over the world, fi nding many other applications, but fiocco’s was indeed the pioneering paper. in the experiments by fiocco and smullin (fiocco and smullin, 1963), laser echoes from the mesosphere were tentatively attributed to incoming micrometeorites (fiocco and colombo, 1964). the presence of aerosol layers near or below the mesopause has been established by numerous visual observations of noctilucent clouds. fiocco and grams (1966, 1969) conducted lidar measurements at high latitudes where noctilucent clouds are observed. observations of transient features of a noctilucent cloud were also obtained during that experiment. fiocco and grams (1971) related latitudinal and seasonal changes in the vertical distribution of extraterrestrial particles below 100 km to the general circulation of the upper atmosphere. estimates of the mass fl ux of extraterrestrial dust based on lidar data obtained at high latitudes (fiocco and grams, 1969) are in agreement with results obtained by other techniques (summarized, for example, by parkin and tilles, 1968). it is interesting to recall that volcano agung was timely enough, in 1963, to explode and inject into the atmosphere dust and gases, that would form in the lower stratosphere the liquid and solid targets to be detected by giorgio fiocco’s lidars. the first application of lidars to study the stratospheric aerosol layer was the work of fiocco and grams (1964). the lidar could rapidly acquire backscattering profiles of the layer of aerosols found in the interval from 15 to 25 km. these observations permitted the study of shortterm temporal variations in the aerosol layer. stratospheric aerosol observations were made with a lidar operated at lexington, massachusetts, during 1964 and 1965 (grams and fiocco, 1967). some observations were also conducted at college, alaska, in the summer of 1964. vertical profi les of aerosol concentration were obtained by dividing the backscattering coefficients measured with the lidar echoes by the expected return from a molecular atmosphere. the lidar «scattering ratios» introduced by fiocco and grams have been used extensively by other lidar groups since that time. other large volcanoes (st. helens, in 1980; el chichón, in 1984, and pinatubo, in 1991) exploded during the following years, all at the right time to be monitored by lidar, and to prompt important research on stratospheric composition, dynamic, chemistry and radiation. the work at marconi’s with doppler radars probably also triggered the idea that such technique applied to lidars would have made it possible to detect the velocity of the small light scatterers present in the atmosphere. in case such scatterers were large and dragged by the air, their average velocity would be equal to the wind velocity; if the scatterers were air molecules, their velocity distribution would be directly related to the gas temperature. the theoretical treatment of such phenomenon is extensively developed in an often cited paper of fiocco and a bright young student: fiocco and de wolf (1968). at the same time he started working at the way to measure such properties in the laboratory. the fi rst devices were built in the u.s. but soon he was called by esrin in the laboratory of frascati and he was allowed to carry with him many of the already constructed devices. 176 glauco benedetti-michelangeli et al. it is worth mentioning that in those fi rst years at mit, fiocco conceived and literally designed for the laboratory a sort of optical «meccano» that all his co-workers know very well. with the aid of a score of different modular pieces (a compromise between metric and anglo-american sizes but always perfectly fitting) precisely machined in anticorodal, bronze and steel on the original drawings of fiocco, it has been possible, since then, to construct any kind of optical system. such versatility is hardly found even in catalogues of important optics producers. several of his former co-workers, who have continued their work in atmospheric optics by themselves, are still using fiocco’s «meccano». 4. italy, frascati as mentioned above, in 1969 fiocco was called back to italy to set up a group of atmospheric physics at esrin, a child laboratory of esro, born with great auspices that soon faded under the weight of diplomatic-political questions. the fact that esrin was in the neighborhood of rome, famous for an inexplicably too abundant production of low-quality wine, has probably nothing to do with what happened next, but the laboratory eventually turned out to be quite short lived. one of its merits is that its wealth of funds, space and facilities, was the defi nitive bait that brought giorgio fiocco back to italy. if it were not for that outstanding laboratory, fiocco would probably still be in the u.s. the doppler effect on visible radiation had been known by astronomers since the beginning of the 20th century but the involved velocities were very high and the famed «red shift» of the expanding universe was easily studied. conversely, the velocities involved in atmospheric physics requires a quite high instrument resolution not easily achieved. the apparati like radars which use radio frequencies that have the advantage of keeping their coherence, solve the problem by mixing the signal with a reference oscillator (homoor hetero-dyne technique). such technique shifts the frequency of the signal to be analyzed to a much lower range where the required resolution is more easily reached. applying the same technique to optical frequencies is possible (and indeed is done) but has several limitations. giorgio fiocco at that time took the «ultimate decision» that put him for good on one of the two exclusive parties of the lidar community: on one side the «coherent» doppler lidar guys who use the mixing technique and on the other, the «incoherent» doppler lidar guys who perform the analysis of the raw signal without any mixing. the effort from the second party to discourage being nicknamed «incoherent» by demonstrating that their method utilizes somehow light coherence as well, has gone unheard to date. here, for sake of impartiality (which is not the case because some of us belong to this very side), the denigrating defi nition «incoherent technique» is called «direct detection technique». the application of the direct detection technique for measuring the atmospheric temperature and wind, proposed and published for the fi rst time by fiocco, is based on the spectral scanning of the lidar echo signal with a very high resolution interferometer. fiocco’s idea was original also in proposing the use of an interferometer known in the hyperfi ne spectroscopy community but otherwise seldom used: the fabry-perot etalon. typically a fabryperot used to study hyperfi ne transition lines in laboratory experiments where there is abundance of signal has a diameter of 2'' or less; giorgio fiocco had special 4'' plates made and coated for him with very high precision by a u.k. glass manufacturer. he designed from scratch also the mechanics that held the plates and made possible the frequency scanning either piezoelectrically or by pressure change. in a search for the optimum light transmittance (although fiocco, talking to his junior co-worker would often say that «optimum is the enemy of good»), with g. benedetti-michelangeli, he dug out of the books an even more obscure spectroscopic device: the spherical fabry-perot invented and studied by connes (1958). at very high resolutions needed for the doppler technique it can be demonstrated that confocal mirror cavities have a great advantage over at mirror cavities. he and his team made a whole series of different spherical fabry-perot cavities with impressive resolutions that allowed the measurement of the wind velocity with such accuracy to compute 177 giorgio fiocco: a jolly good fellow and his research the turbulent eddy diffusion coeffi cient in the boundary layer. the papers written in those years are the fi rst record of utilizing the lidar doppler technique in the atmosphere (fiocco et al., 1971; benedettimichelangeli et al., 1972, 1974). many years later when the lidar doppler technique became a fashionable sort of topic in the international scientifi c community, the main problems involved were still the same and fiocco had some solutions to suggest for following the drift of a laser line (zuccheretti et al., 1992). as with the two kinds of fabry-perot interferometers, he took another little known idea from the books and set it into use. an astronomer from bologna observatory, g. horn-d’arturo, had thought that instead of having a very expensive large area telescope built, he could use an array of properly made small telescopes (horn-d’arturo, 1935). the idea comes from the observation that due to the extreme diffi culty of making large precise optical surfaces, the cost of many small mirrors was by far less than the cost of a large mirror with the same total area. at that time, mirror arrays were already used for solar furnaces or other non-optical grade applications, here the point was to obtain a telescope good for astronomers and, in the case of fiocco, good for a lidar. incidentally this idea is still valid and seems one of the better solutions for large telescopes in space. in 1973, the 3 m mosaic telescope made of 36 hexagonal mirrors in a 12 m high tower adjacent the esrin laboratory started to collect the fi rst lidar echoes from the stratosphere. a unique facility in the world. after going through few lifting jobs and passing from esrin to the italian national research council (cnr), it worked and produced a fair amount of scientifi c results up to a few years ago (1999) when the laboratories were moved to a new site and the relocation of the tower was not considered economically viable. the building and some of the steel innards are still there like an old pharaoh’s monument. as previously mentioned, in 1980 mt. st. helens exploded, and in 1984 so did the volcano el chichón. in the period following the eruptions, most of the lidars all around the world made measurements of the aerosol distribution and evolution in the stratosphere. the lidar in frascati made measurements (adriani et al., 1983), that were also used as ground truth for the stratospheric aerosol and gas experiment ii (ackerman et al., 1988). studies on the interactions between aerosol and ozone in the stratosphere were also conducted (adriani et al., 1987). in the early to mid-1970s, fiocco was a frequent visitor at the national center for atmospheric research (ncar) at boulder, colorado, where he worked with the «ol’ good times» and great friend gerry grams on a series of papers involving the energetic equilibrium of small particles in the earth’s upper atmosphere. aerosols absorb energy from the solar and planetary radiation fi elds, exchange energy by collisions with the ambient gas, and emit thermal radiation. interactions with the atmospheric radiation fi eld is a function of size and refractive index of the particles and of the spectral density and anisotropy of the radiation fi eld. the heat exchanged through collisions is a function of the temperature and density of the ambient gas and of the temperature of the particles. equilibrium conditions for any given particle size and composition are thereby functions of the altitude, time of day, season, and planetary albedo. equilibrium particle temperatures can be substantially different from the ambient gas temperature; fiocco et al. (1975) showed that the effects could be very large above 60 km where differences in excess of 100 k could be obtained in daytime. these differences could account for substantial increases in the sublimation rates of certain species (fiocco and visconti, 1973) and for a reduction in the altitude interval where ice particles could exist in the mesosphere (grams and fiocco, 1977). the identifi cation of the role of aerosol on the atmospheric heating and on the photodissociation rates treated in those years with different colleagues (fiocco et al., 1974, 1976, 1978; mugnai et al., 1978, 1979; petroncelli et al., 1980) constitutes one of the many fundamental contributions to the understanding of the atmospheric processes. at the same time, he dedicated some efforts to the development of acoustic sounders (mastrantonio and fiocco, 1982), to the characterization of the atmospheric boundary layer (fiocco and mastrantonio, 1983), and to the combination of different techniques (congeduti et al., 1981). 178 glauco benedetti-michelangeli et al. in his neverending search for literally «pioneering» challenging measurements, he deployed in person and with other adventurous co-workers an acoustic sounder on a ship cruising the red sea and, later, on one of the highest possible sites in europe: capanna margherita on top of mt. rosa (4400 m a.s.l.). in the same years of esrin fiocco, using his expertise in optical spectroscopy, studied the properties of few minor atmospheric components such as oh (fiocco et al., 1970; visconti and fiocco, 1970, 1973; fiocco and grams, 1971; visconti et al., 1971; fiocco and visconti, 1973, 1974). 5. italy take 2: rome in 1984, fiocco started a new laboratory in the department of physics of the university of rome. the fi rst experiment in the laboratory was dedicated to the measurement of the ozone absorption cross section in the ultraviolet spectral range (cacciani et al., 1989), and, later, in the visible (amoruso et al., 1990). of particular interest was the dependence on temperature. these measurements had been carried out by few groups previously. the small differences among the published measurements had a signifi cant impact on the ozone values retrieved from the surface and from space: the change of the ozone absorption, mostly in the huggins bands, with wavelength, is used to derive total and vertical profi les of ozone by remote sensing. the apparatus developed at the university of rome used a long absorption path and high ozone concentrations to measure very low absorption cross sections. one of the most challenging aspects of the experiment was the ability to produce a relatively large amount of ozone: it was produced by a high voltage discharge in a pure oxygen gas, and stored in the liquid phase at the liquid nitrogen temperature. even the smallest impurity could start a fast reaction, leading to noisy explosions famous throughout the department of physics. the ozonizer had to be rebuilt several times before all the measurements could be completed. later on, measurements were extended also to no2, a gas easier to handle than ozone (amoruso et al., 1993). at the same time, experiments on the detection of small changes of the refractive index of air were carried out with a michelson interferometer (crescentini and fiocco, 1988). the spectroscopic measurements were then moved to the gran sasso laboratory. this large laboratory, built in the inner part of the gran sasso mountain, 100 km east of roma, shielded by approximately 1000 m of rock, is mainly dedicated to experiments in nuclear physics and astrophysics. at the time of the construction of the laboratory it was possible to foresee the use of some of the service tunnels for geophysical experiments (fiocco et al., 1986). one of such experiments had to do with the deformation of rocks (crescentini et al., 1997) and the detection of relatively slow phenomena like tides. for this purpose a michelson 90 m long interferometer was ideal. the interferometer beams were to travel in a vacuum system. in addition to the geophysical metrology, the existence of a 90 m long pipe was useful for spectroscopy. to allow detailed measurements of very low absorption cross sections, a multi-pass cell was developed, and measurements of the o2 absorption in the herzberg continuum were carried out (amoruso et al., 1995, 1996). at the university, of rome, giorgio fiocco started to develop the lidars that were later deployed to different parts of the world forming something half-jokingly dubbed as the «small but global lidar network». these projects will be described later in this paper. at the same time, different experiments, including measurements on the boundary layer with a combination of different techniques (sodar, lidar, radiometers, etc.) were carried out (rao et al., 1995; casa-dio et al., 1996a,b; rao et al., 2002). in one of his pioneering studies, he suggested that clouds of different nature, possibly containing chemical compounds other than water (namely nitric acid), may form at the tropical tropopause (crescentini and fiocco, 1983; hamill and fiocco, 1988). some of these studies were done well before the discovery of the ozone hole, and of the different properties and compositions of polar stratospheric clouds. the possible role of the condensation of nitric acid in cloud particles in the tropical regions was then evoked much later. 179 giorgio fiocco: a jolly good fellow and his research 6. south pole when the italian government signed the antarctic treaty and offi cially joined the international program for research in antarctica setting forth a call for proposals, giorgio fiocco did not wait long to deliver another original and challenging idea: the installation of a lidar at the amundsen-scott south pole, station to monitor the presence of polar stratospheric clouds (psc’s) during the austral winter. the antarctic «ozone hole» phenomenon had been just confi rmed by satellite measurements and theories implying the role of such clouds in the modifi cation of the chemical reactions in the stratosphere were being proposed. the project was not very simple; it implied the design of a reliable lidar system able to run without failures and as little as possible human attendance for several months. in particular, during the winter closure of the south pole base, when the 10-20 personnel are mainly involved in staying healthy and safe, the only thing that can be required of the person in charge is throwing off and on a couple of switches. any more technical intervention is out of the question. such a system needs to be thoroughly tested in the laboratory, disassembled, transported to the site, reassembled, optically aligned and re-tested. the project started at the beginning of 1987 at the university of rome with the aim of deploying the lidar around the following december; the lidar system was completely built from scratch, starting from the acquisition of the laser with a special cooling unit, to the construction of the telescope and the assembly of the electronics. in particular, the electronics, as was usual at that time, was made of a chain of different units originally designed for research in particle physics; the adjustment to a lidar system was not always simple. no need to say that it was a «tour de force» and not all the desirable tests were performed within the deadlines. fiocco himself and his co-workers, within a collaboration with the u.s. national oceanographic and atmospheric administration (noaa) and the national research council (nrc), brought along 400 kg of equipment to the south pole just before christmas 1987, spent memorable holidays in the «southernmost place on earth» and set up the lidar ready for measurements during the austral winter of 1988. flying to the amundsen-scott station with the well maintained but quite old c-130 propeller airplanes with skis as landing gear is itself a pioneering deed and, even if well sheltered, whoever experienced the antarctic katabatic winds knows it well. beside the extreme cold, the south pole is remarkable also for its height above sea level: more than 2600 m of geometrical altitude but, at those temperatures, equivalent to more than 3000 m according to ambient pressure. giorgio fiocco liked the adventure very much but did not like the height feeling and in his worries he enjoyed, in particular, the friendly reassurance of the base doctor. such a close and somehow «interested» reassuring friendship with a doctor was seen again in another remote base where fiocco went to deploy another of his lidars: u.s. air force base of thule, greenland. the fi rst antarctic campaign was mainly a test for the instrument but still a few data were collected. in the following years the instrument was maintained and updated and was able to work till 1995 when the project was discontinued and the lidar was brought back to rome. the collection of a rich and unique data set on psc’s allowed the publication of a fair amount of papers on several related topics (fiocco et al., 1989, 1991, 1992, 1996; fuà et al., 1992; cacciani et al., 1993, 1997a,b; panegrossi et al., 1996). in particular the paper of 1991 that encountered a strong criticism from the referees but was eventually published, presented a rather original and controversial way of interpreting the temperature dependence of psc’s to infer their chemical composition. following the eruption of mt. pinatubo (1991), it was possible to study the impact of the presence of a large amount of sulfate aerosol in the lower stratosphere on the formation and properties of the psc’s (cacciani et al., 1997b). 7. thule in 1990, within a collaboration with the danish meteorological institute a lidar was installed at thule, greenland. the lidar was designed to observe the upper troposphere and 180 glauco benedetti-michelangeli et al. the stratosphere. later the capability to make daytime observations, and to measure temperature profi les in the middle atmosphere, was added. the observations at thule allowed the increasing and subsequent decaying phases of the stratospheric aerosol load formed by the eruption of volcano pinatubo to be studied (di girolamo et al., 1994; larsen et al., 1994; neuber et al., 1994; di sarra et al., 1998a). the aerosol backscattering shows an evident annual cycle, induced by the polar vortex but also the infl uence of the large scale stratospheric circulation, mainly the quasibiennal oscillation. following previous studies conducted after the eruptions of mt. agung (grams and fiocco, 1967) and el chichón (adriani et al., 1987), an investigation of the relationship between volcanic aerosol and ozone in the lower stratosphere at thule was also carried out (di sarra et al., 1992, 1995). the arctic vortex strongly modulates the ozone and aerosol profi les, that display sudden changes with height, and an anti-correlated layered structure. a similar behavior had also been observed between psc’s and ozone at the south pole (fiocco et al., 1989). psc’s appeared occasionally also at thule (di sarra et al., 2002). starting from 1993, temperature measurements were also collected at thule (marenco et al., 1997) and later a study was dedicated to the effects of small cirri at the summer arctic tropopause (benedetti et al., 1997). 8. a lidar in the island of lampedusa in 1999, the group led by giorgio fiocco installed a lidar for tropospheric aerosol studies at the station for climate observations of the «ente nazionale per le nuove tecnologie, l’energia e l’ambiente» (enea) on the small and pleasant island of lampedusa, placed between south italy and africa. the main objective of the measurements was the detection of desert dust transport over the mediterranean. the desert dust strongly modifi es the atmospheric structure, with large loads of crustal particles, that may reach altitudes of 7-8 km. the lidar observations (di sarra et al., 2001a) were carried out in combination with an extended set of other instruments, leading to the identifi cation of the radiative effects (mainly in the ultraviolet spectral range) induced by the aerosol (di sarra et al., 2001b). 9. an airborne lidar as the south pole project was approaching its completion, giorgio fiocco, in his search for new and challenging experiences, began to develop an airborne lidar which would be installed onboard the scientifi c version of the myasishchev stratospheric aircraft m-55, geophysica. the lidar was named able (airborne lidar experiment). able is a completely automated, medium-power lidar capable of operating in an unpressurized, cold environment such as that of geophysica, observing in the nadir and in the zenith direction. the development of this kind of instrument that works in very extreme ambient conditions requires considering different problems and constraints: weight, strength, thermal stresses, low pressure, humidity condensation are just few of the problems faced in the process. giorgio fiocco was totally at ease spending hours in front of the drawing board trying all kind of solutions or discussing with the ever present machinist on the way to fi nish a part of the lidar. little by little, a jewel of aluminium looking like a fancy space object blossomed in the laboratory. we should mention also the frequent, tiring, neverending and babilonian (languagewise) meeting with the team of russian engineers in charge of the aircraft. in summer of 1996, the mock-up of able was installed for the fi rst time on the m-55, which took off from zukhovsky airport, russia, in order to check how able would behave in real fl ight conditions. in fact, the geophysica usually fl ies at 20 km forcing able to work at pressure of 50 mb (one twentieth of atmospheric pressure at sea level) and at a temperature ranging between 190-200 k (approximately 80° below freezing point) (fiocco et al., 1999). in winter 1996-1997, able was fl own to rovaniemi, finland, which had been selected as the base for the fi rst measurement campaign of geophysica. the campaign was named apepolecat (airborne polar experiment-polar ozone, lee waves, chemistry and transport) (stefanutti et al., 1999; pace et al., 2003a). 181 giorgio fiocco: a jolly good fellow and his research then other campaigns followed: ape-theseo (airborne platform experiment third european stratospheric experiment on ozone) based at the airport of victoria-mahé, seychelles (pace et al., 2003b), ape-gaia (geophysica aircraft in antartica) based at the airport of ushuaia in terra del fuoco, argentina, euplex (european polar stratospheric cloud and lee wave experiment) based at kiruna (sweden) and the envisat (environment satellite) validation campaign based at forlì (italy) and at kiruna (sweden). the location of ushuaia permitted the aircraft to fl y across the polar vortex down to the antarctic peninsula approaching 70° south. it is noteworthy that during the campaign in the seychelles, in order to support the operation of able and acquire independent ground based lidar observations of high equatorial cirrus clouds also a twin lidar housed in a container was deployed: cable, a self contradicting acronym originating from the words «container» plus «able». the «small but global lidar network» still rising! fiocco is now participating in the proposal to fl y a lidar in the international space station in a complex venture with livio scarsi called euso and described in one of the papers of this issue. recently, the external and internal structure of the receiver of able has been totally redesigned and built to reduce its weight and make it resemble more a submarine robot. we all believe that the fi nal goal of this small but complex lidar system is to fl y sometimes in a space payload, but nobody is offi cially stating it because changing the «a» with another meaningful utterable letter is too diffi cult. the able project is still going on and absorbs much of giorgio fiocco’s time (and research funds): it is indeed the present enjoyable toy and needful thing of such a good ol’ fellow. references ackerman, m., c. brogniez, b. diallo, g. fiocco, g.p. gobbi, m. herman, m. jaeger, j. lenoble, c. lippens, c. megie, j. pelon, r. reiter and r. santer (1988): european validation of sage ii aerosol profi les, j. geophys. res., 94, 8399-8411. adam, a.m. and g. fiocco (1959): automatic device for following an assigned course, marconi’s co., baddow res. lab., itm 598. adriani, a., f. congeduti, g. fiocco and g.p. gobbi (1983): one-year lidar observations of the stratospheric aerosol layer following the el chichón eruption, geophys. res. lett., 10, 1005-1008. adriani, a., g. fiocco, g.p. gobbi and f. congeduti (1987): correlated behaviour of the aerosol and the ozone content of the stratosphere after the el chichón eruption, j. geophys. res., 92, 8365-8372. amoruso, a., m. cacciani, a. di sarra and g. fiocco (1990): absorption cross sections of ozone in the 590to 610-nm region at t = 230 k and t = 299 k, j. geophys. res., 95, 20,565-20,568. amoruso, a., l. crescentini, g. fiocco and m. volpe (1993): new measurements of the no2 absorption crossection in the 440-to 460-nm region and estimates of the no2-n2o4 equilibrium constants, j. geophys. res., 98, 16,857-16,863. amoruso, a., l. crescentini and g. fiocco (1995): longbase-line multiple refl ection cell for uv and visible spectroscopy, rev. sci. instrum., 66, 4824-4826. amoruso, a., l. crescentini, m.s. cola and g. fiocco (1996): oxygen absorption cross-section in the herzberg continuum, j. quant. spectrosc. radiat. transfer, 56, 145-152. benedetti, a., a. di sarra, g. fiocco and d. fuà (1997): cirrus clouds at thule, greenland, during summer: lidar observations and infl uence on the planetary radiative budget, in irs ’96, current problems in atmospheric radiation, edited by w.l. smith and k. stamnes (a. deepak, hampton, va), 23-26. benedetti-michelangeli, g., f. congeduti and g. fiocco (1972): measurements of aerosol motion and wind velocity in the lower troposphere by doppler optical radar, j. atmos. sci., 29, 906-910. benedetti-michelangeli, g., f. congeduti and g. fiocco (1974): determination of vertical eddy diffusion parameters by doppler optical radar, atmos. environ., 8, 793-799. cacciani, m., g. di sarra, g. fiocco and a. amoruso (1989): absolute determination of the cross sections of ozone in the wavelength region 339-355 nm at temperatures 220-293 k, j. geophys. res., 94, 8485-8490. cacciani, m., p. di girolamo, a. di sarra, g. fiocco and d. fuà (1993): volcanic aerosol layers observed by lidar at south pole, september 1991-june 1992, geophys. res. lett., 20, 807-810. cacciani, m., g. fiocco, p. colagrande, p. di girolamo, a. di sarra and d. fuà (1997a): lidar observations of polar stratospheric clouds at the south pole. 1. stratospheric unperturbed conditions, 1990, j. geophys. res., 102, 12,937-12,943. cacciani, m., p. colagrande, a. di sarra, d. fuà, p. di girolamo and g. fiocco (1997b): lidar observations of polar stratospheric clouds at the south pole. 2: stratospheric perturbed conditions, 1992 and 1993, j. geophys. res., 102, 12,945-12,955. casadio, s., a. di sarra, d. fuà, m.p. rao and g. fiocco (1996a): raman lidar and doppler sodar observations of the nocturnal boundary layer at rome, italy, during 1994 and 1995, in advances in atmospheric remote sensing with lidar, edited by a. ansmann, r. neuber, 182 glauco benedetti-michelangeli et al. p. rairoux and u. wandinger (springer-verlag, berlin), 27-30. casadio, s., a. di sarra, g. fiocco, d. fuà, f. lena and m.p. rao (1996b): convective characteristics of nocturnal urban boundary layer observed with doppler sodar and raman lidar, boundary layer meteorol., 79, 375-391. congeduti, f., g. fiocco, a. adriani and c. guarrella (1981): vertical wind velocity measurements by a doppler lidar and comparisons with a doppler sodar, appl. opt., 20, 2048-2054. connes, p. (1958): l’etalon de fabry-perot sphèrique, j. phys. radium, 19, 262. crescentini, l. and g. fiocco (1983): possible effects of stratospheric aerosol layers on the vertical transport of water, nuovo cimento, 6c, 337-349. crescentini, l. and g. fiocco (1988): automatic fringe recognition and detection of subwavelength phase perturbations with a michelson interferometer, appl. opt., 27, 118-123. crescentini, l., a. amoruso, g. fiocco and g. visconti (1997): installation of a high-sensitivity laser strainmeter in a tunnel in central italy, rev. sci. instrum., 68, 32063210. di girolamo, p., m. cacciani, a. di sarra, g. fiocco and d. fuà (1994): lidar observations of the pinatubo aerosol layer at thule, greenland, geophys. res. lett., 21, 1295-1298. di sarra, a., m. cacciani, p. di girolamo, g. fiocco, d. fuà, b. knudsen, n. larsen and t.s. jørgensen (1992): observations of correlated behaviour of stratospheric ozone and aerosol at thule during winter 1991-1992, geophys. res. lett., 19, 1823-1826. di sarra, a., m. cacciani, g. fiocco, d. fuà, t.s. jørgensen, b. knudsen, n. larsen and i.s. mikkelsen (1995): ozone and aerosol correlated observations at thule, greenland, in the period 1991-1994, j. geophys. res., 100, 25,965-25,977. di sarra, a., l. bernardini, m. cacciani, g. fiocco and d. fuà (1998a): lidar observations of the pinatubo aerosol at thule, in greenland: relationships with the polar vortex, in atmospheric ozone, edited by r. bojkov and g. visconti, parco. sci. tecnol. d’abruzzo, l’aquila, italy, 197-200 di sarra, a., l. bernardini, m. cacciani, g. fiocco and d. fuà (1998b): stratospheric aerosol observed by lidar over northern greenland in the aftermath of the pinatubo eruption, j. geophys. res., 103, 13,873-13,891. di sarra, a., t. di iorio, m. cacciani, g. fiocco and d. fuà (2001a): saharan dust profiles measured by lidar at lampedusa, j. geophys. res., 106, 10,335-10,347. di sarra, a., m. cacciani, m. campanelli, p. chamard, c. cornwall, j. deluisi, l. de silvestri, t. di iorio, p. disterhoft, g. fiocco, d. fuà, p. grigioni, w. ju n k e r m a n n, f. ma r e n c o, d. me l o n i, f. monteleone and b. olivieri (2001b): radiation, ozone, and aerosol measurements at lampedusa during the paur ii campaign, in irs 2000: current problems in atmospheric radiation, edited by w.l. smith and yu.m. timofeyev (a. deepak publishing, hampton, virginia), 1193-1196. di sarra, a., m. cacciani, g. fiocco, d. fuà and t.s. jørgensen (2002): lidar observations of polar stratospheric clouds over northern greenland in the period 1990-1997, j. geophys. res., 107 (aac6), 1-16. fiocco, g. (1958a): comparison of some published data on sea and land reectivity, marconi’s co., baddow res. lab., itm 299. fiocco, g. (1958b): notes on integrated doppler-inertia navigational systems, marconi’s co., baddow res. lab., itm 342. fiocco, g. (1958c): a method of evaluating and correcting for calibration errors in doppler navigational systems, marconi’s co., baddow res. lab., itm 352. fiocco, g. (1958d): a method of range determination using fm, marconi’s co., baddow res. lab., itm 391. fiocco, g. (1958e): echoes from rain in doppler navigational systems, marconi’s co., baddow res. lab., itm 394. fiocco, g. (1959a): a note on the use of harmonic bands of high order in fm doppler systems, marconi’s co., baddow res. lab., itm 434. fiocco, g. (1959b): fm altimeter; a preliminary investigation, marconi’s co., baddow res. lab., itm 555. fiocco, g. (1959c): british patent 871656, improvements in or relating to doppler radar systems, british patent offi ce. fiocco, g. (1959d): british patent 869809, improvements in or relating to radar systems, british patent offi ce. fiocco, g. 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[1994] proposed a probabilistic approach that was based on the historical record of past eruptions, to constrain a series of monte carlo simulations. favalli et al. [2009] applied a similar approach to obtain a hazard map, with the simulation of the inundation areas for a large number of possible future eruptions using an empirical relationship for the maximum length of the lava flows. recently, crisci et al. [2010] elaborated a lava-invasion susceptibility map that was based on the results of numerical simulations of flows that erupted from a grid of 393 hypothetical vents that are located on the eastern sector of mount etna. to validate the results of the proposed methodology, crisci et al. [2010] simulated a set of 13 documented actual eruptions on the present-day topography, and evaluated the overlap between the affected areas and the susceptibility classes. in recent years, we have made significant progress in hazard assessment at mount etna through the development of accurate and robust physical–mathematical models that can forecast the spatial and temporal evolution of lava flows. in particular, during the 2001, 2004, 2006, 2008 and 2011 effusive eruptions at mount etna, we obtained encouraging results using the magflow cellular automata (ca) model [vicari et al. 2007, del negro et al. 2008, hérault et al. 2009, vicari et al. 2009, vicari et al. 2011]. magflow was developed at ingv-catania to simulate lava-flow paths and the temporal evolution of lava emplacement. more recently, it has also been applied to more sophisticated issues, including prospective hazard mitigation on mount etna [scifoni et al. 2010], and for compiling a new lava-flow invasion hazard map for mount etna volcano [cappello et al. 2011]. in the present study, we present a new validation procedure for the evaluation of the accuracy of lava-flow invasion hazard maps. the validation approach consists of splitting the dataset that contains the information about eruptions at mount etna volcano into two parts: eruptions article history received november 29, 2010; accepted june 30, 2011. subject classification: lava-flow paths, magflow simulator, fitting scores, temporal validation. 634 special issue: v3-lava project that occurred before 1981, which was chosen as the reference time, and eruptions that happened after 1981. the first information was used as the learning dataset, to develop a lava-flow hazard map up to 1981, which was then discretized into ten classes. the second dataset represents the testing dataset: the actual shape of the lava flows that occurred after 1981 were superimposed on the hazard map, and two scores were calculated to measure their percentages of overlap. lava-flow hazard map by a magflow model the lava-flow invasion hazard map at mount etna was developed by considering the main volcanological structures and the information on past eruptions. the methodology used is based on the following steps: (i) computation of the susceptibility map that provides the spatio-temporal probabilities of vent opening; (ii) characterization of the expected eruptions; (iii) simulations of lava-flow paths by the magflow model; (iv) computation of the lava-flow invasion hazard map; and (v) discretization of the resulting map into a number of fixed hazard levels. the computation of the susceptibility map was based on the characterization of the zones with the highest probabilities of the opening of future eruptive vents. we defined a grid of potential vents that was spaced at regular intervals of 500 m, in both the vertical and horizontal directions, and that assigned a probability of activation (pa) to each potential vent, as follows: (1) where mt and mxy are the temporal and spatial recurrence rates, respectively, dt is the time interval of 50 years, and dxdy is an area of 0.25 km2. for the temporal recurrence rate mt, we used the results obtained for the power intensity function that was applied to the tanguy et al. [2007] catalog by smethurst et al. [2009]. for the spatial recurrence rate mxy, we used the most common spatial point process model, which is based on computation of a kernel function. a kernel function is a probability density function that is symmetric about the origin and that spreads the probability away from the structure considered. we decided to use the gaussian kernel, although other different kernel functions could have been used, including the cauchy kernel [martin et al. 2004] and the epanechnikov kernel [lutz and gutmann 1995]. connor and hill [1995] and lutz and gutmann [1995] demonstrated that the shape of the kernel function chosen in this type of analysis generally has a trivial impact on the probability calculations, compared to other parameters. the formula of the gaussian kernel is given by: (2) where di is the distance from the point (x,y) to the i-th volcanic structure location, h is a smoothing parameter that controls the size of the zone to which each data point contributes an increased intensity, and n is the number of volcanic structures that are considered in the calculation. as n occurs in the denominator, the integral mxy across the whole area will be unity. the smoothing parameter was calculated by comparing the observed nearest-neighbor distances of the main vents with the expected distribution of the nearestneighbor distances [weller at al. 2006]. as discussed in cappello et al. [2011], plots for h = 750 m and h = 2,200 m gave the upper and lower bounds, respectively, to the cumulative nearest-neighbor distances of the volcanic events of mount etna. hence, a medium value (1,500 m) between the lower and upper bounds was chosen and used for equation (2). for the characterization of the expected events, a statistical analysis of flank eruptions that have occurred on mount etna in the last 400 years was performed, and five eruption classes were obtained by combining three different values of the emitted lava volumes (30, 100 and >100 × 106 m3) with two times of eruptions (30, and >30 days). a sixth type of event that was characterized by a duration of 30 days and a lava volume of more than 100 × 106 m3 was introduced as an extreme event, as the past cannot represent the future perfectly. the event probability pe, represents the probability that the e-th class of eruption occurs at the point (x,y), and this was estimated as follows: (3) where ne is the total number of events in the e-th class, and dj is the distance between point (x,y) and the j-th vent if dj is less than or equal to he. therefore, the value of pe(x,y) depends on the number of main vents within a distance he of the point. the estimate pe for each point (x,y) satisfies the following: (4) lava flow simulations were performed with the magflow code, which has been extensively used in lavaflow hazard applications for mount etna. the magflow model is based on the ca structure, in which the states of the cells are the thickness of the lava and the quantity of the heat. the states of the cells are synchronously updated according to local rules, which depend on the values of the cell and the values of the neighbors within a certain proximity. in this way, the ca can produce extremely complex structures from the evolution of rather simple and local rules. the evolution function of magflow is a steady-state solution of navier-stokes equations for the cappello et al. 635 , , t 1 1p x y e ea t x yt xy=d -m md d d-^ ^ ^h h h 2 1 nh e2 1xy i n 2h d 2 2 i r = =m / ,p x y d 1 1 e j j ne = = -^ h / , 1p x y 1 6 e e = = ^ h/ 636 motion of a bingham fluid on an plane that is subject to pressure force, in which the conservation of mass is guaranteed both locally and globally [vicari et al. 2007]. the rheological properties were modeled using a variable viscosity relationship according to giordano and dingwell [2003], which was parameterized in terms of the temperature and water content. for each potential vent of the grid, we launched six simulations that correspond to the six expected eruptions. the effusion rate functions that are required to run magflow were established by following the information relating to historic eruptions. the short-medium time and the long time were set to 30 and 90 days of simulation respectively, and 30, 100 and 200 million cubic meters were fixed as the erupted lava volumes. from the combination of these values, we obtained six possible functions that represent the variations of the flux rates in relation to the times of the eruptions. the curves were considered to be a kind of bell shape, in which the eruption starts from a null value of flux rate, reaches its peak after a ¼ of the entire time of simulation, and then gradually decreases until the end of the eruption. the lava-flow invasion hazard map was computed for each point (x,y) of the area covered by the flow simulations, by taking into account the information on the lava-flow overlap, the activation probability of vents vi, and the event probability of eruptive classes cj, as follows: (5) if (x,y) is invaded by a simulated lava flow that erupted from vi, or else. the value assigned to each point represents the probability of it being affected by a lava flow during the time interval considered. finally, to make the map more readable, we discretized it into ten hazard intervals. validation of lava-flow hazard at etna parameter value unit density of lava 2600 kg m−3 specific heat 1150 j kg−1 k−1 emissivity of lava 0.9 − temperature of solidification 1143 k temperature of extrusion 1360 k , ,hazard x y t p v p v* 1 6 1 a i i ei n e=d == ^ ^ ^h h h6 @// figure 1. susceptibility map at mount etna, as updated using dikes, faults and eruptive fractures until 1981. the colors are associated with different levels of vent opening probability. table 1. typical parameters for mount etna lava flows. validation of the hazard map the validation of the lava-flow hazard map is the last crucial step of the whole process, as this provides a quantitative evaluation of its reliability. as our knowledge only includes the historical information, we propose a retrospective approach that uses the more recent past eruptions as the potential future events. our strategy is based on the following concept: if the invasion probabilities reflect the past lava flows, then it is reasonable that they will also predict the areas that will be most likely to be invaded in the future. hence, the comparison can evaluate the probability distribution of lava-flow hazards at the reference time against the historical lava-flow paths that occurred successively. the first step of the validation consists of dividing the available dataset into two parts that are based on the ages of the past eruptions: eruptions that occurred before 1981, and eruptions that happened after 1981. this time limit was chosen for two reasons. from a mathematical point of view, as the reference time, the year 1981 determines two statistically meaningful sets, which divides the dataset into 49 events used for learning and 14 used for testing. from a volcanological point of view, starting from 1981, the activity of mount etna volcano showed a temporal trend (an increase cappello et al. 637 hazard class area (km2) probability level (pi) [0 0.0001] 247.12 0.0001 [0.0001 0.0005] 203.54 0.0005 [0.0005 0.001] 109.27 0.001 [0.001 0.002] 129.20 0.002 [0.002 0.004] 139.43 0.004 [0.004 0.008] 119.64 0.008 [0.008 0.015] 81.38 0.015 [0.015 0.03] 38.41 0.03 [0.03 0.06] 11.15 0.06 [0.06 0.012] 1.67 0.12 figure 2. lava-flow hazard map at mount etna, constructed using the learning dataset (reference time 1981). the colors represent different levels of lavaflow invasion hazards. table 2. probabilities and areas of the ten hazard intervals into which the lava-flow invasion hazard map was discretized. 638 in the number of eruptions), which is seen in particular by the activity over the last 20 years or so [salvi et al. 2006]. the learning dataset is used to calculate for each point of the study area the probability of a vent opening pa for a time period of 25 years (equation 1), and the event probability pe (equation 3). the susceptibility map obtained is shown in figure 1. then we ran the lava-flow simulations through the magflow model, using the typical material properties of etnean basaltic lava (table 1). the basis for the numerical simulations was a digital elevation model of the volcano area updated to 2005, which was represented as a 10 m cell grid and a regular vent grid (of 36 km × 32.5 km) that was spaced in regular intervals of 500 m, in both the vertical and horizontal directions. using equation (6), we developed the lava-flow hazard map for mount etna volcano up to 1981. a logarithm scale was then applied (figure 2) to discretized it into ten levels of hazard (table 2). the testing dataset includes 14 eruptions that occurred after 1981 (table 3), and this was superimposed on the hazard map (figure 3). the two scores were then calculated to measure the overlap. the first validation index vi1 represents the percentage that the observed values hit the highest hazard class, and therefore the best fit corresponds to 100. this was calculated as: validation of lava-flow hazard at etna eruption year onset end 1983 march 28, 1983 august 6, 1983 1985 march 12, 1985 july 13, 1985 1986-1987 october 30, 1986 march 1, 1987 1989 september 27, 1989 october 9, 1989 1991-93 december 14, 1991 march 31, 1993 2001 (2700 m) july 18, 2001 august 9, 2001 2001 (2100 m) july 18, 2001 august 9, 2001 2001 july 26, 2001 august 2, 2001 2002 october 27, 2002 november 4, 2002 2002-2003 october 27, 2002 january 28, 2003 2004-2005 september 7, 2004 march 8, 2005 2006 october 27, 2006 november 27, 2006 2006 september 4, 2006 december 7, 2006 2007 april, 2007 may, 2007 table 3. eruptions that occurred after 1981, as used for the validation. figure 3. lava-flow hazard map for mount etna constructed using the learning dataset (reference time 1981) with superimposed actual lava-flow paths from eruptions that occurred after 1981 (in black). the colors represent different levels of lava-flow invasion hazards. (6) where c is the total number of pixels invaded by the lavaflow simulations, ci and pi are the number of pixels and the probability level, respectively, for the i-th hazard class, pmax is the highest observed value of hazard, and n is the total number of hazard classes. we obtained a value for vi1 of 17.3, which is not significant as it only depends on the extension of the highest hazard class, not on the expected distribution of all of the classes. for this reason, we introduced another validation index, vi2, that represents the mean distance between the observed and expected values for each class of probability. this is 0 if the values provide a perfect fit, while it increases if the discrepancy increases. this has been calculated as: (7) where c is the total number of pixels invaded by the lavaflow simulations, ci and pi are the number of pixels and the probability level, respectively, for the i-th hazard class, and n is the total number of hazard classes. we obtained a vi2 of 0.09. conclusions a lava-flow invasion hazard map defines the probability that a specific area will be reached by a lava flow during a given time window. many different approaches have been developed for the generation of lava-flow hazard maps at mount etna volcano [wadge et al. 1994, favalli et al. 2009, crisci et al. 2010, cappello et al. 2011], which have confirmed that the main difficulties regard specifically not the elaboration of the map, but also the verification of its consistency. ideally, the validation should include a further independent experiment that is run forwards in time and that uses a significant number of events to make the statistical tests valid. since further forward studies are unworkable from a scientific point of view, the only chance to validate the map is to re-use the datasets from which it was derived. the strategy described in the present study represents a possible approach that is based on a retrospective analysis of the hazard, as it looks at what happened in the past to predict what will happen in the future. we believe that this a plausible approach, because it exploits the only knowledge we have, which is the eruptions that occurred in the recent past. nevertheless our study must be considered as a preliminary study of how lava-flow invasion hazards can be validated in volcanic areas. the application of the retrospective validation shows that the two validation indices provide a quantitative evaluation that relates not really to the hazard map, but to the methodology developed to elaborate it. moreover, the simulations used to generate the lava-flow invasion hazard map up to 1981 were performed by using a digital elevation model updated to 2005. from a statistical point of view, this change should not cause any large variation in the results of the two validation indices. however, for some specific eruptions, it is possible that the solidified lava represented a limiting factor, or even a barrier, and therefore changed the time-space evolution of the simulated lava flows. acknowledgements. this study was performed with the financial support from the v3-lava project (ingv-dpc 2007-2009 contract). the authors thank the anonymous reviewer for the helpful and constructive comments. references andronico, d. and l. lodato (2005). effusive activity at mount etna volcano (italy) during the 20th century: a contribution to volcanic hazard assessment, nat. hazards, 36, 407-443. behncke, b., m. neri and a. nagay (2005). lava flow hazard at mount etna (italy): new data from a gis-based study, geol. s. am. s., january 2005, 396, 189-208. cappello, a., a. vicari and c. del negro (2011). assessment and modeling of lava flow hazard on mt etna volcano, b. geofis. teor. appl., 52 (2), 299-308. connor, c.b., and b.e. hill (1995). three nonhomogeneous poisson models for the probability of basaltic volcanism: application to the yucca mountain region, j. geophys. res., 100 (b6), 10107-10125. crisci, g.m., m.v. avolio, b. behncke, d. d'ambrosio, s. di gregorio, v. lupiano, m. neri, r. rongo and w. spataro (2010). predicting the impact of lava flows at mount etna, italy, j. geophys. res., 115, b04203; 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doi: 10.1016/j.cageo. 2007.10.008. lutz, t.m. and j.t. gutmann (1995). an improved method cappello et al. 639 1vi n c c p2 1 i i i n = = / 100vi c p c p* *1 1max i ii n = = ^ h/ 640 for determining and characterizing alignments of pointlike features and its implications for the pinacate volcanic field, sonora, mexico, j. geophys. res., 100 (b9), 17,65917,670. martin, a.j., k. umeda, c.b. connor, j.n. weller, d. zhao and m. takahashi (2004). modeling long-term volcanic hazards through bayesian inference: an example from the tohuko arc, japan, j. geophys. res., 109, b10208; doi: 10.1029/2004jb003201. salvi, f., r. scandone and c. palma (2006). statistical analysis of the historical activity of mount etna, aimed at the evaluation of volcanic hazard, j. volcanol. geoth. res., 154, 159-168. scifoni, s., m. coltelli, m. marsella, c. proietti, q. napoleoni, a. vicari and c. del negro (2010). mitigation of lava flow invasion hazard through optimized barrier configuration aided by numerical simulation: the case of the 2001 etna eruption, j. volcanol. geoth. res. 192, 16-26. smethurst l., m.r. james, h. pinkerton and j.a. tawn (2009). a statistical analysis of eruptive activity on mount etna, sicily, geophys. j. int., 179 (1), 655-666. tanguy, j.-c., m. condomines, m. le goff, v. chillemi, s. le delfa and g. patanè (2007). mount etna eruptions of the last 2,750 years: revised chronology and location through archeomagnetic and ra-230th dating, b. volcanol., 70, 55-83. vicari, a., a. hérault, c. del negro, m. coltelli, m. marsella and c. proietti (2007). modeling of the 2001 lava flow at etna volcano by a cellular automata approach, environ. modell. softw., 22, 1465-1471. vicari a., a. ciraudo, c. del negro, a. hérault and l. fortuna (2009). lava flow simulations using effusion rates from thermal infrared satellite imagery during the 2006 etna eruption, nat. hazards, 50 (3), 539-550; doi: 10.1007/ s11069-008-9306-7. vicari, a., g. ganci, b. behncke, a. cappello, m. neri and c. del negro (2011). near-real-time forecasting of lava flow hazards during the 12-13 january 2011 etna eruption, geophys. res. lett., 38, l13317; doi: 10.1029/2011 gl047545. wadge, g., p.a.v. young and i.j.mckendrick (1994). mapping lava flow hazards using computer simulation, j. geophys. res., 99 (b1), 489-504. weller, j.n., a.j. martin, c.b. connor, l.j. connor and a. karakhanian (2006). modelling the spatial distribution of volcanoes: an example from armenia, in: h.m. mader, s.g. coles, c.b. connor and l.j. connor (eds.), statistics in volcanology, special publications of iavcei no. 1, geological society of london, 77-88. *corresponding author: annalisa cappello, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy; email: annalisa.cappello@ct.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. validation of lava-flow hazard at etna la magnitudo calcolala mediante sismo~ .. afi a hreve periouo y, ,al'ju~llo ]{. ('o:\so l e c. oasi'aluxt (*' ) hi('('vllto il 2;; ~(,\i(,llihn' i!},:l hl\.~ s(l,,'i'o. e stato l'll'eh ll a t o un ('oll f i'onto spcrimcntulc tra n'.!.:i· "trazioni ,li cyl'h(.i "isllli('i otkhllli ('oh in ('oppi a ('allipioll(' "\\·ood·anderhoh" (', ('on uhri tipi (li "i"'lliollletri u hn've j)('riodo. :-\i l", l'i('avala ulia ('urva di ('olt('la~ion(' tra i logaritmi dell\' allljji('zw delle f asi mas..~illl(' l'pgislratc eon i vari sttullwnli iii fuliziolie di'ila dishlll7.a cpieclllra1e. il risultalo 0((('hulo ha ('ohfcl'ma t o la possihili ti\ di utilizza n" pci' il ealeolo ,ldla lilu.!.:llillldo, hlrullh'hti divel'hi (lullu ('o]!]!ia ('um]!iollc, ]!ur"('ih~ lui e h"u l'if ('rili immit(· urla l'elaziolle s]l('l'ill,ellt"h·. ru ~dl\ry. ah ex peri meni al ('omjlal'isoll hdwccli n'eoni" of s('i s mie (' v('nl,h ohlaill(,(\ hy thc "\vood·alidcl'soll" stuwlar,l ('ollpk all(1 hy ilil\'(')"('lil, ];iwls of shmt pc'rioll s('i~lilomdel'" 1y"8 ('ani('11 on1. wl' ohluirw,j a ('olt(·ialioll ('lll'l'(' ('olllll'eting the loga1'ìthm k of t ll('. m:lximlllli awplitlllks h' ('ol'l[(o,1 hy diff ('n'lit ill"trlllll('llts with l'cfen'1l('c to thl' epì{'en ttal distalwc. l'hp l"esult, allinuell th(· possihility o l usillg ihstl'\llllclit s dijl'ei'cllt, frolli thc stululani ("hlpje to ('aklll:ll(' the maghitude, oll eollllitioh tlwt tll('~' uri..' ('omjlah'd with il hy all exjlerillwhlal rdatioll. l la defi niz ione opll1'htivh, deha nmg-llitlldo pm' i 1-citpmot,i vieilli l'), illlvliehndo la diveu(lellza (lal f\ i.~lllonwil'o l1f\a,l;o per la. regi s t,razione, ha impok1-o la m'l'ha, (li ullo ~tnllll('nto eampione; qui:'~to è il sismompl;ro:\ tor~ ione \yoo(l-a1h!erwll poll in ki'g'ih\l11;i (~l-l'hlj:el'iktil'lli': l 'o = 0,8 ki'i'; l'o = 2.800; li = 0,8 ciò eompol'!;;1 elle gli osservatori ~ 111'ovv i ~ t,i (lella, eopp ia, eamvione potl'e1)ber o snperam 1-all' i neonvelliellt.e se rinseis8g1'o a dej:el'fllill:ll'e ulla (*) jstitllt-o naziollale ili gpo118i('h. 088 ('l'v ntol'io celltrale di rollla mollle porzio. v ,\vili-:j.10 h co).:,~oj.i': c. nasl'aih'\f l'dar.ione 1m la lilag-liill1do ('aleolala ('on g-li sll'ullwllli iii 101'0 dolar.ione (' 1]1w[[a di(' avi'{'hl){'l'o del{,i'minal0 usalhlo ,~i.~monh'll'i \\"ood-~\lidpl'­ soli. coilt;elll1alnwnte ('iò t~ pus>iihile (lato ('he, ('uno~('endo k ('araul'l'islidle di l'isposla di l'mi s11'1ulh'nti, si 1hlò risalire (lalla rpgistrar.ium' di 1111 evenlu da pmte di uno di essi alla regislmr.ione ('he"i "al'ehhe ottenula ('on tut,ti gli altl'i. q1h'hhl lll'o('pdllra ]wl'ò è j)al'li1'olal'lilpllte lahul'io.~a e 11011 ù uli[izr.ahile ili lilla liol'mllle l'oliliiip. ]li l<'ililllhj p )lal'('l'lli ("), aft'i'olltalh[o questo j)l'obkma i){'i' la l'l'le liaziollall', dolata di sismogl'afi '\"in'h('1"i ('ome stl'umenli base, slippusel'o ('11(' la frequenza ddla fase massima (li due regis1raziuni di lino stesso ('vento, ut.tenute una ('on un sismometl'o "'iedwrt, e l'altra ('on un ,rood-~\ndel'"un fosse iu "te"sa. ciò ('01l"{'1i1iva di delenniwu'p, una volta t'atto[ati lo "iio"hwh'njo ]'(ll![e dpl ~liolo e [a il'eqllellza della fase lilas~iljla n1('diante l'e~allw di uii ~i~lllog-l'amnm uttenllto ('oli 1111 ,ri,>(']\('l't, l'amlliezza massinm ebe avi'ehbero uttenuto ('on un '\"ood-~\ii­ (lp!"son e quindi la magnitu(lo (li un evelltu .~i.~mi{'o. ]"t da notare elle hl ~eelta falla da ili l<'ilip1l0 p )laj'('elli fii dplpl'millala dal fatto tlw 11011 psi::;tl'vallo iii f[ii{,[ lh'l'iodo iii jla[iu sismometl'i \\"ood-1\lid{'i'soli. ila ('il'('a dlle anni i)('i'ò, pl'es~o l'o~sel'vatorio si~mic'o c'entra1e deli'ln.n., ò in funr.ione oltre a sislllometri a ('orto, me(lio e lungo l)('l'iu(lo, una ('u1lpia di ,"vuo(l-j\ndel'sun. uiù ha suggel'ito !'idea (li vel'ifkare se era iio",~ibi[p ottellere ulla l'elazioli{' "1)(,l'illl{'1i1alr', fllllzioli{' della (li"tanza epi('elltl'lllp, ('he [eg-usm' 1m 101'0 le lila~sillle alllj)iezze di l'eg-i~1:l'llziolle ottenllte ('on i \'ari tipi di stl'limeliti a breve periodo. si noti ('be il metodo mato 111'eseinde da ugni ('omiderazione relativa alle em've di ris]losta dpg-li ~ll'ii11wllti p alla (,olllposiziollp iii fref[llpllr.a dp][e hh~i lilas~illl(' de[le registrazioni. l'litilitù, di tale \'el'ili('a· enl g-illstifi('ata, ohi"(' ('be dalle (·oll"idem·zioni ('he potevano (lerivarne ('in'a il filtraggiu (ielle oi\(le ('oli la distanza, uli('he dalla po"sihilit.à, di utilir.wl'e 1lmticamente i risultati 1)('1' [a detel'miuazio1le della maguit,lido iii o~set"vatol'i 1011tani (lalla sede dell'o.s.c. iii pul'ti('o[al'e ,~i j)l'e\'edp di litilizzal'e i l'i.~u[tati; il) 1lel' re('u]lerare inful'maziuni sul materiale l'eg-i~tl'ato in ilata ~l1lte('edplltt\ alla elllmta in funr.iune della ('oppia ('alllpione; h) 1lel' fo.~spi'\'atol'io di gihihml1lna (8idlia), attl'ezw10 con iina h'i'lul a ill'e\' e 1lel'iodo lelwiipi'-griflith, ide1l1ieh· a qup[ht utilir.zata 1)('1' il eollfronto; c) per la slazione )!ohi[e iii doluziolw all'os~el'valol'io si;;m](oo centrale ('011 sismomet,ri 8 -j;j e he]i('onler. li .ilio;>;itu.ilo calcolata )iei>la;';'['.io; f'.is.iiu(:llafi a bln:n,: 1'1':1(1<>.1)1' mb ~ sono ,~tnti eseg'uiti due di~tinti {'onfronti 11'n in ('oppia e:llllpione e, l'i.~]ld1ivamentc, ii' ('op}lie ,~ii'lll()]lwtridw ol'ir.zontali a breve ppl'iollo tipo lehenel"-griijillt ed s-l:.t ije ('arattl'ris1khe ,~tl'ulllentali di qm'ai ultillli ~0110: il) st,azione eletltonmg;neti('a ,i l'l'g"i~tj'ur.ione [otog"ralil'a tipo leltcner -(.triffith; t . = 1,;") re('; t" = l/i scl'; la ('l1i"vn di illg;nl.nllimento (' dnta, ila: i· (i il , t. l'u t h) hblzio11c ,id alllplill('nzioilp el]'! 11'olli('a e l'l'~i.~!tnziolle su c,\l'ta, termo~pllhihile "geolc(,lt"; 1'. = l,o ,~i'e; f1'equcnz~\' ili taglio illfel'iol'e lh1i'i a 0,1 hr.; [l'pqu('nz~\' ili taglio ,~upl'riorp pnl'i a 5 hz; la i:un'il di i11gra11dillil'1110, enkolhtn ppi' ulla attenllar.iojje di (lo db, (' riporblbl in fig. l le l'eg;i~b'uzioni ~0110 state }ll'ese in c'onsiderar.ioll!' rolo qun11do l'irlllt-availo (']lianlllll'nle h'ggihili s\1 eiltl'illllbe le ('ompollcnti ili l'iahcllna {lellc iluc ('oppie poste ,i i:onfl'onto. in o/.wi (:aho h' llln~,~illle nlllpier.ze noll doveva ilo es~el'e iilfel'iol'i a e]n'n ~ llllll, cnkolnte le llledie d, delle rellli,llllpiezze dpllu fn,~e lllassinm ili t'ìureuml regi~fnlr.iollc, ,~e ne ò il l'terminato il1og;arilmo. li: ,~fntn quiildi pl'c,~a in l'oll.~ideraziolw b ilil1"e1'('nzn: g = log di log b dove hl l' l'ifl'l'ho ~\'llo htl'umento (li ('(jllironta,rc l'oli il \vood-.\lldptson c d a iji!ert 'llltimo. i rihllltati dei due eoill'ronti sono ripol'lat,i 11elll' fab. 1 e~; in qucste, }lei' ogni t,e1'1'ellloto, ~olio iildi('nft': lu dat,a; l'ora (t,l\lg,); lu distunzn e}lieentl'nle /1, detel'lllilmt,a preferibillllente llwdiujj1e la l:onoseeilzn dell'epi('enll'o, l'il i ntlol'i ili 11 1 , 11 e c. 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' o !.~ '!. 7, 1 ' (: !j-t 6' si i • ' f i u" !! !h!.!;:; ' l) u!."i, !.f: ' i (1(. 1 il: tll • ' ,-_o hol) t ' 1) !.~ ' " (:1 ' 1 0;:;[ hii 0(1 o " t" ttoi: ' o mi ' s; lf: ' (: '1(:1 l !.! 1: 1 " 1:(; ltoo "o 0'1 ' (:(: !.s ' t '1(:1 si: " " 0 (: 1(:!!-t ' o {'' , ~(, il!.!' ,, giq !.!t ~(i " hl f:!!(; f: ' o i. l " ;: 1)1 ) ' 1 !!(,f sç l. 1 " !. l l f:t.!. ' o u(i " le; !h ' t oli: " ,o " !?[ !)()'1t ' u !?z ' e;1 ~,(, ' t 1)(;1 m: o, " !i h'u il: ' il !!.!. ' ii i u -' t , -( , 1)(; i (: 1: o, " '1 iw1: , n 0'1 ' (: !i;; ' 1 t' t 1> ' " 11.\ " (; 10(:'1 ' 11 iili ' lj !:!. ' c, li t e; " 911 1.\ " !ii fl t t t ' u !.!!. ' t !i.!.' , 00 ' !io hl • "ur. nnl t 'o !.!(; ' (; !.!l ' fi 111:(; or. n, • " 07, hf. l!" !) 7,u ' t (; , ' 1 ~(;(; r.o tc; • • " i; li (, i '1 ' il iih ' i:\: (:, ' 01 ocr. stcn ,\ "o (;fl'1'1 ' 11 ge; ' t ! l ' l oll!) li!! (:0 • • " or [sih! ' o ~ie; ' !i i) g ' ;; 1)11 (i!! 1:1 " (, sl!l'1 " o (j !.! ' !)f: !~ ' ii oo[ 1:1: ,o " (: !! f:ot " o oo ' !.! ~ !1! ' [(: o[ i tt l" a' " (: oolil ' o tl ' !! l !i !. ' 1\ , 1:1 " l!(i! " iii " h(: '" 'i n "~i 'ii 111'1 (" ~-:r l ":-, i ,l l'l ll i i , \1.10 ..... ,,1:1 i i i : ) " iii1 x il li :t ' l "c, \· ... l 'al ' ",i. la \iau:-;itudo calcolata \ieijlaxtg l'i>'\iomul'l a iil\e\'}; l'eriodo (i!j!) segue tabella 2 (ll'o1 , nata (t,~1.(t,) km b, 111 c h m 20 . ]\-' ih"i2 o;j .,--, ;~oo 0 , 50 , ,li2 0,:1100 20 . , , o,; 17 :w :1 , 7,; '; , 00 o. i 2i)o 27 , " , 00 17 "'o 2 . 7,; (i . 2,; il , ;j,")h7 , \" , '" 00 " 2 . 12 2, ,;0 0,o"i1l7 ". , , oh 02 ;\0 ;~ , 12 h,2i) 0,4,12 7 . " , " h ;14ii o , s7 2,2,") o,411hl 17 . " , 'o 2;~ 2,; 2 , 7,; -1-. li2 o,225"i 2~) . " , ih 5h 2ho 2 , 12 ';, s7 il.-1--1-17 ;:1o • , " 2:1 iili 2·h) i . so 4, ;:17 il , :ls..j.7 il \., " 17 ,;:1 ii,; ;:1 . :'17 '.l, "ii) 0,1707 " . " , " i)l) 220 1:1 , 7,; ·h) ,,")0 o,5i)li"i 21 " " l,") oli 2241 2 . 2,; <. 12 o, ,"),;hs , \. [[ " oli 42 iio ". 12 h,oo o, ;u"'h h . , " " iq li'i 2 . 7,; ". 12 o. il,'' 21 , , 0:'1 o,") ii) 2 , ,io ;j, ,,; o. h",; 7 . yiji. , o, :w 2,0 1 , :i"i 4,12 0,4,(i7 , , " , il); "" 235 '. 12 ;j. 25 o,4h1j!l 2:1. , " 12 4(i iii :1. :17 -1-, 12 o il:'11i7 7 . xl , 2;:1 o"i 'o , . 12 iii, ,io o. i (i:'l2 2-l " , o" "o 1;:10 4,87 l!l, ,io o. (i021 2-1. , , 08 oli i ;:10 " ,i)o ". 2i) o. i)oo", 2-1. , , 08 28 i ;~,; ". s-. , 1:1, 2,; o . -1-;~-1-1 21i. " , ih 07 1:10 , ,00 , ,00 o 3010 .,--, . , , , , 4,i i;w o. 7,i , ,li2 o. 8:1:l\j 27. " , i,") 'o 2k ., 7,") -1-, ,j)) o, 2 l ;j"i 2'" . " , 211 ;j7 1311 ". 7,; , , ,1)1) o, ;jus-1;~o . , " , , "' 2711 :1.37 l"i . ,;11 0.71-1-s 22 . xll , 00 :14 12.j4, :17 , , , ,;0 0.42110 selle ligg. ~ e a sollo l'ipol-tati i vullti ,~pl'l'iln(,llhdi e, l'i~iwhiya­ mclltc i gl'nhei dpile fullzioni [1j, [~j e [aj, [-1-j. bi:-wg'l\a os8l'1'\'~\l'c elw, mcll1te la [1] e la [:n ,~i irhmhficnllo ell1to gli eltol'i statistiei, lo stesso lloli si phù dire 1)('1' la [:1] e la [11; ill questo caso infatti il cocllicicllt"e dd tcl'lllille quadl'atieo l'iw!1-a molto viù gnlll(le rlcll'cltoi'c nd ehso at.tribuibile. pertanto è stato aeeiko rli utilizzal'e, ('·ome fhnzioni rli t"oi'i'claziolle, ih[1] e la [-1-]. le ligg. ~ c :3 mosh'nllo ehe i gl'alici delle fhllziolli di t"oi'i'elaziolie sec11-c hh-nllo ulla ]lelhleliza fra 101'0 opposta; ciò è spiega bile se si 11el18a l'hl' i lllllssimi ddil> curve (li l'isposhl della i-;lnr,iolw1\lobilc e dci },e]wllcl'1 do 008 006 004002 o do -002 ~ ~ u ~ ~o o o o o 50 o o o o o o o o b § o o 100 150 200 o fig. 2 funzioni di correlazione pcr la stazione mobile. o 250 300 "" o o ~ il> d ~ 1;:1 t-< :5 i ?j o o z u1 o t-< 1;:1 p q il> u1 'ti il> i:d h z h o o o '" i)1[o!zlin./ il! orb]i ti bo! l! !11,) li! il fio! h );'o[ = il' .h1lll')!'h !]lq!,l1lh i~ 1:.7i0jl:m~ h!nuuoj hlin o]luh;l!phill hihu!lli,[,)i,l]l ;l.1ah~,l (/11d o]llll!1[);'hlu 1~! ,)i!,) 'l]llq,)lio;:! l\<: auo!zh.hl]l!\<:uo,) 11] !ha,u! !ollll]l !,)qiw! !hh,) !,)s:}l' ~: o' ~: (.'t o't o'i, il'(: !i'(, s'(: t , -,. ;:;';:; o' (: l'l s' l (:' i " , o' i o '(i o')) l:l ;.: <'i ~o['1'\: o't !:'\: il 'l: l ' ~: !i't r'-, ,. f: '~: , ,-,. o'!: , '-,. o'l: 0 ' (: o' (: !i'" }l'" s'(. }l'(. f" -, , 0'(. l'i, t' -,. 1;'1 , '" l'l (i' l s'i l'l t' , ~)' [ f: ' i s'i (.' i t' , (.' l t , ':)-"[ 'v-',\\ «1)3°1'i noj~: v'l'i:'iuvj. oot o'ji, oui, u!il 001 olj o}l ol 00 il!i of0\; !i(: 0(. si '" '1 " (m'l) i" ',l!,ii:i[,lajt:lll).,)!!,t ,)i[,)!i[li')liia !1[o!zliuuoj 1i~ o)hlll!~ 'o!,!oih.u;w~o o,qh01[ lall a)lqo')lh,1 al!anh lio,) ['o !]i olll!~>:i~lli 0i,il:,)h 01111 lio,) 01[u')!l!)l\'1111[!,~' 'd1i')!1],):)()a !1[o!zi~hl,ioj i\~ 0llnlp>: ',)l>:')!'[,!' !]i di h)lh)1ii!,i.1ii'r o,)!~'li0an o!,[ow,\.i,l>:~(), llhp ,qm[!h!,i,) i,l]l o!lli l! ustwli <>1 .11[,1 a,lh)\qsilo,) o:pqod ,? !~ ,1ll!l1llhlio,) 'o]1i,illil\,1)~ o! ol~'oli,~ !n,) il>: !1],).[.i'li !a]lll,)!.no[o,»);' h,nqhli i:uli]i a]lll,ld!]i ;l!ullb l:] ',1ll0!zlq~ !]i a,)!liqho,) ,llt:liiil,)a:),ll,)]l 01110,) ,l11.1h uoli 'iwlpulq iq\<:a11h all]'lllljiuil)h.,\: '!]i!]]()!;f,hll.hllll! !hl~!l1h.n,i() !l,71lql o]llll!ubliiii il[ ui!1[,io.i ,)i\,l!a !li,) !lo,) h;f,z,l).!,),)l[!,]i !phl!l !jml>:l1 nst,)u iuiu,)!,i .1uo!;f,'hu !uu.1.!alll[! al hj, 'i 't) h,),i!,) ! l) oli li 1:,i!,i')ll! l) ()111!>:>:mu l h ! ,ll[dll[l\.[ 1 ~ '),]1 ! ilo,) ,):pqo,)[i~,) o]ln,pll.nmu al ,11[,) 1:')!ldlll! (!!,i 'to'o !ji ,lli!1i,io.!j,i]l ,) o')!l~ -qlqh .1,io,i,h),[ .11[,) a,llij1a]l!~ [l'],) [i] ;l!!h(1 '!ii:l,),1.i(j.) o~~,l ]li: !11]')1ii1)'])>: ,hi]i !,l]l ou11 ,1110!liuw,) o,ll[')h!ll.ll~ 0ilh ojl1[alll!j~'or ,)qq,),h)llailluio,1 !~ ,11[,1 ,1,10,1.1,1,11')]1 1,qqlj,) 1~ilh lia!)\qd,1 1i1l,l\lh ,) a1[o!ziu"ldr~o 1i,l)[h,li ~1 'lio~,['l]lllv-]loo.\\ 1,)\1 0!l-ntq !]l i:zlhl1lh,l,[./qlh oiplir!,1 a,)ho,ilio q,wd li! !liml!>: ou()~ 1[111/!,io l'i))\'d»\'1i '.) :,l'io~,''''.) 'h ui'[,illl\' '.\ (.iil ~lella disth,llza ya diminuito o aumenlalo dpl nliol'(' di (' l'i('avnto ('on lc fol'liinl(' [1] n [..j-] l'ispehiyament-e. hl magnihhlo (' ilata quindi da: jf = log bl log b (' = log u 1 log h i n c u i h \':liol'e ('a1eolato da iliehter per il \yood-,\nderson: hl ~'oltispondente va lore calrolat-o per lo sl l' umento lamto: (' fat.tore (li coltezione gi:'~ detcl'minato , hipol'tiulllo in fab. 3 i valol'i di log h 1'ispehivamentc ve1' il \\'oo~l­ .all!lel' ~on e pci' i due tipi di ,~i1'l1nwllti da iloi tal'ali. 3 ln pl'i!h'ipale ('o!h'lnsioll(' (']1(' si poh'('hbe t-1':\1ti' dnl layol'o elt(·ttnato ò la ('onfel'ma della pos~ihiliti~ (li ('o1tc!a1'e tra loro, dallj\llllo di yi~ta del ('al('olo (lclla magnitlhlo, stl'umcnti (liycl'~i efl'ehualhlo solo dellc s('mpli('e ollemzioni ili ('on[1'ont-o tm ~lati hllerilll('llbdi. l'l'obahilmente ve1'ò i l'iwltati ottenuti ,~ono shll;i agevolnl i dal fai lo di aycl' prcso in ('oii~id('l'nziolln pyl'nti ~i~llli('i eamtl('1'izzali da nwec:ulismi all'ipocentro non molto din,~illlili frn loro. quc~t{) detel'mincl'cbhe una ('{>l'la ollwgeneilù, nella eomlhj~izione ~pettl'ale (lellc olhl(' enh'sse ~lai ynl'i tcl'remoti e quindi ne l'ihulterebbe un mpllol'to eamttel'istleo, funzionp della distanza, ha 1(' nllllliezze 1'cgistrat-!\ ~ia vari ni l' llllle!jti. bibllij{,rafia (il hlc'lltl<:h l'. f., ih:l;,. _iii ill ~ ll'illiielllal {'al'ih'/iwh' iiwljlliiiu/(, $1'(1/(', "bull. of (he :o;('ìsill,,1. :0;0('. nf aiil.", l. ("l ])1 filll'l'n ]l. l' ~luiceli.1 l., ih;,o. .l/aljllilutl!! (',{ ('//('/"!iia di'i /(,/"i"('iiio/i iu l/alia. "ailil. oli gc,,/ìs"·, iii, :1. 26camassi.qxd 645 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical seismology – earthquake catalogue 1. italian earthquake catalogues up to 1980 1.1. from roman paradoxographers to bonito’s terra tremante (1691) the tradition of earthquake studies in italy is very long; its origins go back to a classical literary genre known as «paradoxography» or collection of marvels and curiosities (pauly and wissowa, 1949). the earliest listings of italian earthquakes were compiled by paradoxographers such as julius obsequens, a roman of uncertain date but generally placed about the middle of 4th century a.d., who gave an account of the prodigies and portents that occurred in rome between 249-12 b.c. the first «modern» italian earthquake lists were included in treatises written in the aftermath of large (or even less than large) earthquakes, following a vogue set by humanist giannozzo manetti in the wake of the catastrophic southern italy earthquake of december 1456 (manetti, 1457). many such works were written in the 16th and 17th centuries (table i), culminating in the best and largest of them all, terra tremante (bonito, 1691), a worldwide compilation including about one hundred european earthquakes (seventy of them italian) and some tens of reports of earthquakes from canada, china, japan, panama, peru and the philippines. these are based on large selections from original sources, often transcribed complete. the originals of some of these sources were later destroyed, which increases the usefulness of this great compilation. 1.2. the age of enlightement and national states (18th-19th centuries) from the 18th century to the first half of the 19th century, earthquake listings go out of fashion. there is no lull in italian seismicity (far from it, in fact) but with a few remarkable exceptions such as mongitore (1743), on sicilian earthquakes, and soldani (1798) on siennese ones – treatise writers seem more interested in debating whether earthquakes are caused by catalogues of historical earthquakes in italy romano camassi istituto nazionale di geofisica e vulcanologia, sezione di milano, sede di bologna, italy abstract a complete survey of historical earthquake investigation in italy cannot be compressed into a few pages, since it would entail making a summary of widely different phases of research (performed by past scholars and by contemporary scientists and historians) and taking into account the widely different historical contexts, methodological assumptions and critical awareness of each of them. this short note only purposes to chart the main stages of the progress made by italian historical seismology, from the late 17th century compilation by bonito (1691) up to the latest parametric catalogue (working group cpti, 1999). mailing address: dr. romano camassi, istituto nazionale di geofisica e vulcanologia, sezione di milano, sede di bologna, via d. creti 12, 40128 bologna, italy; e-mail: camassi@bo.ingv.it 646 romano camassi electricity or not, than in piling up macroseismic data. this trend was reversed again in the second half of the 19th century, thanks to the europeanwide birth of national institutions for the study of meteorological and seismological phenomena (camassi, 1991; ferrari, 1992). in italy, the creation of the earliest observational networks in the 1870s went hand in hand with the resumption of large scale collection of macroseismic data, first advocated by alessandro serpieri, antonio malvasia and michele stefano de rossi. in 1874 de rossi founded the bullettino del vulcanismo italiano, that only three years later could boast of an 85-strong national network of correspondents. the italian central meteorological office (ucm) was created in 1876, and in 1887 it branched out with a specialised geodynamic service, becoming the central office for meteorology and geodynamics (ucmg). by 1890 the geodynamic service network numbered more than 900 stations and observatories, whose output (macroseismic questionnaires and reports) featured regularly (under the heading news of earthquakes observed in italy) in the bulletin published from 1895 to 1913 by the italian seismological society (founded in 1895). 1.3. rounding up past knowledge: baratta’s i terremoti d’italia (1901) in the second half of the 19th century, along with the development of state-sponsored earthquake-monitoring, the ancient tradition of earthquake listing flourished anew in italy (fig. 1) as in the rest of europe, thanks to the groundbreaking efforts of people such as hoff (1840-1841), perrey (1848), mallet (1853, 1854, 1855), volger (1857), schmidt (1879), mercalli (1883), kispatich (1891-1892), baratta (1901) and many others. the huge compilation i terremoti d’italia (baratta, 1901) was the result of an italy-wide research project carried out in 1896-1900 by the geographer mario baratta with the help of a large network of collaborators (mainly librarians and seismologists). baratta personally conducted research in the ucmg library and the main libraries of rome, naples, milan and pavia, while his collaborators sifted through local histories relevant to their locations to compile chronological earthquake lists from which baratta would select the items to be included in his compilation. baratta’s personal archive was lost shortly after his death in 1935; the full list of his collaborators is unknown, and it is hard to tell how much each contributed to the general effort. the reference list appended to baratta (1901) shows more than 25% of the listed earthquakes to be derived from previous earthquake compilations (pilla, 1846; perrey, 1848; guarini, 1880; mercalli, 1883; piovene, 1888, tommasi, 1888, de rossi, 1889; baratta, 1897; mercalli, 1897a,b; baratta, 1899; benassi, 1899, etc; for a complete list see guidoboni, 2000), and the rest from historical compilations, standard collections of printed historical sources, scientific literature and – in a few cases – seismological bulletins. in other words, baratta (1901) is almost entirely based on historical and seismological compilations, mostly of a local and regional nature. providing as it does a sort of rounding up of past earthquake knowledge, compiler earthquake time span giannozzo manetti (1457) 1456 southern italy antiquity-1456 colanello pacca (1563) 1561 val d’agri antiquity-1561 pirro ligorio (1574-1577) 1570 ferrara antiquity-1570 filippo da secinara (1652) 1646 l’aquila antiquity-1646 marcello bonito (1690) 1688 campania antiquity-1688 table i. main italian compilers of earthquake lists in the 16th and 17th centuries. 647 catalogues of historical earthquakes in italy geodynamic service and started the slow but inexorable decline of state-sponsored earthquake monitoring activities. from 1913 to 1943 the geodynamic service continued to publish summaries of its seismic data (lists of affected localities and related intensities) in a succession of scientific magazines (table ii) but this activity did not survive the upheaval of the second world war. the original data collected by ucmg (seismic questionnaires and related papers, letters, newspaper cuttings etc.) are today held partly by the national institute for geophysics and vulcanology (ingv), and partly by the central office for agrarian ecology (ucea, former ucmg). baratta (1901) stands as a watershed in italian seismology, and played a key role in the establishment of italy’s «earthquake consciousness». more than a century after its publication, and despite having been found wrong on very many occasions, this volume remains an authority italian historical seismologists have still to reckon with and still cannot do without. 1.4. the decline of the practice of systematic observation the first world war put an end to the practice of collecting reports of earthquakes by the fig. 1. geographical distribution of italian seismological compilation from bonito (1691) to baratta (1901). 648 romano camassi 1.5. the decade of parameters (1970s) in the early 1970s italian seismologists and geologists started compiling parametric earthquake catalogues for hazard estimation purposes, on a regional (carrozzo et al., 1975; bernardis et al., 1978; iaccarino and molin, 1978; magri and molin, 1979; barbano et al., 1980; dell’olio e molin, 1980; molin, 1981; margottini and molin, 1983; ogs, 1987) and national scale (carrozzo et al., 1973) (fig. 2). some of these catalogues were unpublished, their use being restricted to the various italian institutions that sponsored their compilation. their focal parameters were mostly derived from previous seismological compilations, barring a few instances of slightly more complex studies funded by the italian national nuclear energy committee (cnen, afterwards enea) for selected areas (friuli, latium, basilicata). generally these studies consisted in a «critical» re-assessment of information miscellaneously derived from seismological compilations, local historiography, 19th century newspapers and seismological bulletins. 2. the first modern published italian catalogue (1985) one of these catalogues, compiled on behalf of the italian electricity agency (enel, 1978), provided the starting point for the first modern systematic revision of italian earthquakes, undertaken after the destructive irpinia earthquake of 1980 by the «catalogue working group» of the progetto finalizzato geodinamica (pfg) of the national research council (cnr), under the chairmanship of daniele postpischl. the working group’s aim was producing a revised national parametric earthquake catalogue and detailed studies of the major italian historical earthquakes. this experience was seminal in introducing the involved researchers to the complexities of historical research and fostering their appreciation of its practical uses. some contented themselves with retrieving and appraising a few original sources quoted by seismological compilations; others attempted wider-ranging bibliographical searches; a few enlisted the help of professional historians to make the first attempts at full-blown historical earthquake investigation, applying the methods of historical criticism to primary historical sources. the variety of these experiences was reflected in the broad spectrum of quality shown by the output of the pfg project, which led to the publication of 81 monograph studies comprising in all about 6300 intensity datapoints (postpischl, 1985a), and the postpischl (1985b) catalogue. the postpischl (1985b) catalogue was compiled by carefully collating the enel time span bullettino del vulcanismo italiano, voll. i (1874) xx (1897), roma 1874-1894 notizie sismiche pervenute..., supplemento al bollettino meteorico giornaliero, ufficio centrale di meteorologia, roma 1887-1894 notizie sui terremoti osservati in italia durante l’anno..., appendice al bollettino della società sismologica italiana, roma, modena 1895-1913 macrosismi avvertiti in italia nell’anno..., bollettino della società sismologica italiana, roma 1916-1926 bollettino sismico, ufficio centrale di meteorologia e geofisica, roma, fasc. ii macrosismi 1917-1936 bollettino sismico settimanale, ufficio centrale di meteorologia e geofisica, roma 1917-1943 notizie sismiche, bollettino della società sismologica italiana, xxxiii(1935) xxxvii(1939), roma 1935-1939 table ii. main seismological bulletins published in italy from 1874 to 1943. title 649 catalogues of historical earthquakes in italy catalogues inserted in the postpischl catalogue used the medvedev-sponheuer-karník (msk) scale. the postpischl catalogue generally takes its macroseismic magnitudes from the enel catalogue, which derived them from intensity using karník’s (1969) relation. for several recent earthquakes the postpischl catalogue also reports ml magnitudes, derived either from bulletins or from one of the main quoted sources. the postpischl (1985b) catalogue (again, like all parametric catalogues of its generation) is derived largely from the parameterisation of 19th century seismological compilations, and primarily from baratta (1901). in fact, an analysis of the postpischl (1985b) catalogue for the time-window prior to 1900 and the epicentral intensity threshold i0 ≥ v-vi mcs (table iii) shows that more than 50% of the entries derive from baratta (1901). as most of the parametric catalogues merged in the postpischl cat(1978) catalogue with several other national/local parametric catalogues and the postpischl (1985a) studies (fig. 3). it has the paramount merit of being a published catalogue (the previous ones were all meant for restricted use only, or available only to scientific institutions), making available for general use no less than 37 000 earthquake records. on the reverse side, like most early parametric catalogues, it is very inhomogeneous (the increase in quantity of records having been paid for in terms of lowered quality), it includes many duplications and quite a few events which further research has later shown to be fictitious. finally, the procedures used for assessing earthquake parameters are far from transparent. for example, the «intensity» parameter is intended to be imax, but sometimes it is derived as i0 from isoseismal lines. the declared scale is mercalli-cancani-sieberg (mcs), but in fact several of the parametric fig. 2. main characteristics of italian parametric earthquake catalogues from 1948 to 1985: the turning point is the publication of the karník catalogue which introduced a magnitude parameter. the type of macroseismic scale used (mcs or msk) is not clearly reported, so in the postpischl catalogue it comes out mixed. 650 romano camassi alogue derive from baratta (1901) too (or from similar compilations), it can be concluded that approximately 96% of the entries are supported by baratta (1901), and only about 2% by recent studies based on the critical revision of historiography and primary historical sources. 3. the growth of historical seismology (1985-2000) the first project of extensive historical investigation dealing with some thousands of earthquakes and involving academically trained reference entries percentage baratta (1901) 1195 51% other compilations known and quoted by baratta (1901) 137 6% other parametric catalogues 922 39% postpischl (1985b) 52 2,2% personal comm. 43 1,8% table iii. analysis of the background of the postpischl (1985b) catalogue (time window 1000-1899, i0 ≥ 5.5); the reference code analysed is the main one when multiple roots are available. fig. 3. simplified filiation scheme of the postpischl (1985b) catalogue. 651 catalogues of historical earthquakes in italy historians was carried out in 1983-1987 in the frame of an enel-sponsored feasibility study for the study of sites for the building of nuclear power plants. this project, led by the former coordinating unit of the pfg catalogue working group (established as sga), and supported by about a hundred historians, focused on low seismicity areas (southern piedmont, the po valley and the salento peninsula). from 1988, the national institute of geophysics (ing) assumed the sponsorship of the project, setting as its goal the revision of all strong italian earthquakes. this study was carefully planned, taking into account the historical features and historical source potential of all areas potentially shaken by the earthquakes to be investigated. though carried out, in the main, by earthquake, extensive investigations by source were also made in several archives and libraries (mainly for the ancient and medieval periods). this allowed the acquisition of detailed knowledge of many kinds of sources, essential for the correct interpretation of historical records. all investigations should have been performed according to a preset standard and were co-ordinated by a single managing team. the collected historical records were compiled according to a pre-set pattern that allowed them to be inserted in a databank. the main outcomes of this project were a collection of studies on mediterranean earthquakes before 1000 a.d. (guidoboni, 1989; guidoboni et al., 1994) and the first edition of the catalogue of strong italian earthquakes (boschi et al., 1995), a modern database (on cd-rom) providing studies on 346 major earthquakes, supported by a large bibliography and (in some cases) by transcriptions of original sources. the second edition (boschi et al., 1997) provided studies for 559 earthquakes (including some updates on the 1995 issue). the third edition (boschi et al., 2000), published as a special issue of the journal annali di geofisica, provides studies for fifty more earthquakes (mostly located in the northern and umbromarchesan apennines) and several updates of studies included in the previous issues, but unfortunately no transcriptions of original sources. in the last few years the research group responsible also published monographs dealing with the seismic history of italian regions (boschi et al., 1998) and towns (boschi and guidoboni, 2001, 2003). a sister project was undertaken in 1986 by the national group for protection against earthquakes (gndt), then attached to the italian council for national research (cnr), now a branch of the national institute of geophysics and vulcanology (ingv). this was the launch of a five-yearly «macroseismic project» involving a large number of academically-trained historians, some as full-time investigators, and others as members of university research units provided with ad-hoc grants. the project aimed to prepare: i) in-depth studies of a selection of earthquakes each of which was a «key event» for hazard assessment in a given moderately seismic area; ii) standard and homogeneous preliminary studies of all earthquakes listed by the postpischl (1985b) catalogue with i0 ≥ viiviii mcs; iii) a database of macroseismic data and a parametric catalogue specifically designed for hazard purposes. some of the earthquakes concerned had already been studied by various agencies after 1980, and intensity points had been assessed for them. however, no less than 600 of these earthquakes had never been studied and no intensity points were extant for them. all the available earthquake studies were retrieved and the intensity data they provided were re-compiled in a standard format. the 600 earthquakes without intensity points were studied for the first time ever. at the end of the work, a careful analysis of the macroseismic procedures employed was also made, to point out weaknesses and suggesting possible solutions (stucchi and albini, 1991). the resulting «nt» catalogue was first released as a working file in 1993 (stucchi et al., 1993) and published in 1997 (camassi and stucchi, 1997a). in 1998 an updated version of the catalogue was made available online (http://emidius.mi.ingv.it/nt/) together with the whole database of macroseismic observations «dom» (monachesi and stucchi, 1997), which includes more than 36 000 datapoints relating to 1000 damaging italian earthquakes (http://emidius.mi.ingv.it/dom/). the nt4.1 and dom web pages, still existing and widely used by researchers, allow users to download data, to query the catalogue and the database in ground of any. the matchless wealth of the italian historical documentary patrimony and the frequency with which destructive earthquakes have affected the italian territory in historical times are the two mainsprings for the age-old italian tradition of historical-seismological studies (guidoboni and stucchi, 1993; guidoboni et al., 1994). as guidoboni et al. (1994) points out, it is possible to find comparatively well documented earthquakes as far back in time as the 6th century b.c., thanks to evidence available in several kinds of sources, epigraphical, literary and archaeological (guidoboni, 2000). between the 8th century a.d. and the 15th century the main sources of information are at first somewhat limited, and mostly of a narrative kind (annals and chronicles). but with the passing of time, they steadily grow more detailed, varied in kind, and abundant in numbers, embracing the administrative records of city-states and the church, and numerous serial sources of remarkable consistency, such as chronicles and diaries. during the first half of the 16th century the earliest newsletter networks come into being, in the shape of manuscript reports («avvisi») exchanged between several italian and european courts. the first half of the 17th century sees also the birth of the modern printed gazettes (camassi and caracciolo, 1994; camassi and castelli, 2004), connected in a network whose main nodes are rome, florence, bologna, mantova, genoa, venice and milan. selling news becomes a profitable activity in the 18th and 19th centuries and the popularity of printed gazettes is testified by the preservation of large collections in italian libraries and archives. in the second half of the 19th century, as was previously mentioned, the creation of national meteorological and seismological networks lead to the building up of a large amount of macroseismic observations, which to this day has still to be exploited fully. after the drastic decline of earthquake investigation caused by the second world war, and perpetuated during the 1960s and 1970s, the activities started by the pfg project in the 1980s represents a «new birth» of the italian tradition of seismological studies. 652 romano camassi different ways and to plot the observed seismic history for about 2000 localities. from 1989 to 1992 the gndt research group was also the leading team of the ec project rhise (review of historical seismicity in europe), co-ordinated by m. stucchi (stucchi, 1993b), whose output is available both in book form (stucchi, 1993a, albini and moroni, 1994) and online (http://mi.ingv.it/rhise/ home.html). from 1995 to 1998 the same team led the ec project beecd (basic european earthquake catalogue and data base), which led to the preparation of an european macroseismic database and to a detailed analysis of the problems to be expected when merging together different kinds of catalogues or intensity data sets produced by several authors and in variable contexts (camassi and stucchi, 1997b; albini and stucchi, 1997). during this period, valuable studies of historical earthquakes have also been performed by other teams and institutions. the vesuvian observatory (now an ingv department) published an outstanding study of the 1456 earthquake (figliuolo, 1988), one of the largest that ever affected the italian peninsula, and whose main characteristics (as reconstructed by meletti et al., 1988) stimulated seismologists to speculate on its possibly being a sequence of multiple, sub-contemporary earthquakes, the assessment of whose focal parameters requires special care. the same team also recently published a study of southern italy early medieval earthquakes (figliuolo and marturano, 2002) the national seismic survey (ssn) published monumental monographs of some major 19th-20th century earthquakes (castenetto et al., 1998; castenetto and galadini, 1999; castenetto and sebastiano, 2002). in 1999 the «cpti» catalogue (working group cpti, 1999) was published, which is an updated version of nt4.1, compiled with the agreement of the whole italian seismological community as embodied in gndt, ing(v) and ssn. 4. the historical background of the current italian catalogue the italian seismic catalogue can boast of what is probably the best historical back653 catalogues of historical earthquakes in italy however, the bulk of the current parametric catalogue still remains to some extent a reflection of the complex historical-seismological tradition of earthquake compilations summarised by baratta (1901). most of the cpti catalogue entries for the time window up to 1899 are those known to baratta (1901), and in quite a few cases recent research – however careful and well-balanced – has not been able either to alter or to improve the pattern of information already made available by baratta (1901). these facts have an evident bearing on the assessment of catalogue completeness, which is a significant question for hazard estimation. as already mentioned, baratta (1901) was a huge collector of data, mostly derived from previous historical and seismological 5. what next? in the last twenty-odd years, the contribution made by italian historical-seismological studies to general earthquake knowledge has taken two main directions. advanced historical investigations have been made available for a large number of earthquakes (1070 in the cpti catalogue, out of a total of 2480 entries). all the strongest italian earthquakes are now supported by recent historical investigations and intensity data (fig. 4). furthermore, approximately 250 fictitious earthquakes have been detected and «put away» (such fakes are present in quantity both in the «classical» earthquake compilations and in the first generation of parametric earthquake catalogues). fig. 4. distribution of entries in cpti catalogue (working group cpti, 1999) without a supporting dataset: all 22 earthquakes with m ≥ 5.5 are located in the border part of the space window covered by the catalogue. 654 romano camassi compilations of a local character. the geographical distribution of the main historical and seismological compilations from which baratta drew his raw data (see fig. 1) gives at least a rough idea of the coverage they afford for the different parts of italy. their density is remarkably higher in northern and (up to a point) central italy than in southern italy. another factor which deeply impaired the level of completeness achieved by baratta’s work was the lack of a systematic approach to the study of serial sources, especially gazettes and newspapers. this is true equally of baratta and the 19th century compilations on which he relied most heavily (perrey, 1848; de rossi, 1889; baratta, 1899). in this regard, it is interesting to note that these studies were doomed from the start by their use of incomplete collections of gazettes and newspapers in the first place (fig. 5). it is a legitimate assumption that if the current catalogue is (and it is), on the whole, a faithful mirror of baratta (1901), and through it of the «seismological tradition» embodied in the pre-baratta compilations, it cannot avoid being affected by the same faults and lacunae. it is therefore advisable to devise specific strategies aimed at detecting those earthquakes that have remained until now «unknown» to the seismological tradition (camassi and castelli, 2004). 6. conclusions after twenty years of historical research a large amount of historical-seismological data is now available in italy, most of it published or made available in public databases. the database which supports the cpti catalogue contains more then 50 000 intensity data points (≈ 11 000 prior to 1880, table iv) assessed in a rather homogeneous way on the basis of historical sources, most of them primary. recent italian earthquake catalogues can rely on a good basis of macroseismic data and have adopted clearer and more transparent fig. 5. time-coverage of the gazette de france collection perused by alexis perrey (1848) compared with the collection actually available. time period cpti catalogue cpti database < 1000 24 54 1000-1099 5 16 1100-1199 15 127 1200-1299 28 131 1300-1399 51 229 1400-1499 70 385 1500-1599 103 489 1600-1699 130 1732 1700-1799 262 3146 1800-1850 185 2064 1851-1899 450 8603 1900-1963 878 19 917 1964-1992 279 16 640 total 2480 53 533 table iv. number of earthquakes in the cpti catalogue (working group cpti, 1999) and number of intensity data points in the corresponding database. 655 catalogues of historical earthquakes in italy procedures in the parameterisation of historical records. the nt4.1 catalogue (camassi and stucchi, 1997a) derives its parameters from intensity data points for about 1000 earthquakes; the procedures adopted for deriving parameters from intensity data are not particularly sophisticated, but they are at least clearly expressed. the main magnitude provided is ms, which can be observed or calculated from io or from other kinds of magnitude (ml, mb). the cpti catalogue (working group cpti, 1999), adopting the gasperini and ferrari (2000) parameterisation method, represents a significant advance in the formalisation of the procedures for the definition of parameters, which, for the first time ever, are reproducible. today seismologists are submitting new questions to historical seismology and new research perspectives are in progress, on catalogue completeness, apparently silent areas, minor earthquakes, unknown earthquakes, etc., involving also new disciplines, as in the fields of archaeoseismology and paleoseismology. acknowledgements the author wish to thank v. castelli, the two reviewers and the editors for their careful revision of the manuscript and their suggestions. references albini, p. and a. moroni (editors) (1994): historical investigation of european earthquakes, materials of the cec project «review of historical seismicity in europe» (cnr, milano), vol. 2, pp. 254. albini, p. and m. stucchi (1997): a basic european earthquake catalogue and a dtabase for the evaluation of long-term seismicity and seismic hazard (beecd), in seismic risk in the european union, edited by a. ghazi and m. yeroyanni (brussel-luxembourg), vol. i, 53-77. baratta, m. 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(1891-1892): potresi u hrvatskoj, rad. jugosl. akademje znanosti i umjetnosti, cvii, 81-164; cix, 1-79. ligorio p. (1574): libro, o trattato, de diversi terremoti, raccolti da diversi autori, per pyrro ligorio cittadino romano, mentre la città di ferrara, è stata percossa et ha tremato per un simile accidente dal moto della terra, archivio di stato di torino, antichità romane, cod. 28. magri, g. and d. molin (1979): attività macrosismica in basilicata, campania e puglia dal 1847 al 1861, cnen rt/amb (79) 5, pp. 104. malaroda, r. and c. raimondi (1957): linee di dislocazione e sismicità in italia, bollettino di geodesia e scienze affini, 16 (3), 273-323. mallet, r. (1853): catalogue of recorded earthquakes from 1606 b.c. to 1850, brit. ass. rep., 1853, 1-176. mallet, r. (1854): catalogue of recorded earthquakes from 1606 b.c. to 1850, brit. ass. rep., 1854, 118-213. mallet, r. (1855): catalogue of recorded earthquakes from 1606 b.c. to 1850, brit. ass. rep., 1855, 2-326. manetti, g. (1457): de terraemotu libri tres (edited by c. scopelliti and d. molin, roma, 1983). margottini, c. and d. molin (1983): risultati preliminari delle ricerche di sismica storica condotte nell’appennino tosco-emiliano, enea, pas-isp br, (83) 2, roma. meletti, c., e. patacca, p. scandone and b. figliuolo (1988): il terremoto del 1456 e la sua interpretazione nel quadro sismotettonico dell’appennino meridionale, in il terremoto del 1456. storia e scienze della terra, edited by b. figliuolo (edizioni studi storici meridionali, nocera inferiore), i, 71-108. mercalli, g. (1883): vulcani e fenomeni vulcanici in italia, milano (reprint, bologna 1981), pp. 374. mercalli, g. (1897a): i terremoti della liguria e del piemonte, memoria con tre tavole in litografia, napoli, pp. 146. mercalli, g. (1897b): i terremoti della calabria meridionale e del messinese, mem. soc. ital. sci. (detta dei xl), iii, (11), roma, 117-266. molin, d. (1981): sulla sismicità storica dei colli albani, cnen/rt/amb (81) 11, roma. monachesi, g. and m. stucchi (1997): dom4.1, un database di osservazioni macrosismiche di terremoti di area italiana al di sopra della soglia del danno, gndt, internal report, milano-macerata, (on line: http://emidius.mi.ingv.it/dom/ home.html). mongitore, a. (editor) (1743): istoria cronologica de’ terremoti di sicilia, in della sicilia ricercata nelle cose più memorabili, 2, 345-445. morelli, c. (1948): contributo allo studio dei microsismi (ing, roma), pp. 32. ogs (1987): alpor: catalogo delle alpi orientali, trieste (computer printout). pacca c. (1563): discorso del terremoto, biblioteca della società napoletana di storia patria, sismica ms. 7/a3 pauly, a. and g. wissowa (1949): realenzyclopaedie der classischen altertumswissenschaft, 18.3, «paradoxographoi», 1137-1166. perrey, a. (1848): mémoire sur les tremblements de terre de la peninsule italique, mémoires couronnés et mémoires des savant étrangers, academie royale de belgique, xxii, 1-144. 657 catalogues of historical earthquakes in italy pilla, l. (1846): principali tremuoti avvenuti in tempi storici in toscana, in istoria del tremuoto che ha devastato i paesi della costa toscana il dì 14 agosto 1846, pisa, 195-217. piovene, g. (1888): cronaca dei terremoti a vicenza, roma, ann. uff. cent. meteorol. geodin. ital., ii (8), 46-57. postpischl, d. (editor) (1985a): atlas of isoseismal maps of italian earthquakes, quad. ric. sci., bologna, 114 (2a). postpischl, d. (1985b): catalogo dei terremoti italiani dall’anno 1000 al 1980, quad. ric. sci., bologna, 114 (2b), pp. 239. schmidt, j.f.j. (1879): studien über erdbeben, leipzig, pp. 360. soldani, a. (1798): relazione del terremoto accaduto in siena il dì 26 maggio 1798 divisa in sei lettere, siena, pp. 98. stucchi, m. (editor) (1993a): historical investigation of european earthquakes (cnr, milano), vol. 1, pp. 258. stucchi, m. (1993b): through catalogues and historical records: an introduction to the project «review of historical seismicity in europe», in historical investigation of european earthquakes, edited by m. stucchi (cnr, milano), vol. 1, 3-14. stucchi, m. and p. albini (1991): new developments in macroseismic investigation, in proc. cenapred-ec international workshop on «seismology and earthquake engineering», edited by e. faccioli, and r. meli, mexico city, 22-26 april 1991, 47-70. stucchi, m., r. camassi and g. monachesi (1993): nt: un catalogo «di lavoro» del gndt, milano, pp. 85. tommasi, a. (1888): i terremoti nel friuli dal 1116 al 1887, ann. uff. cent. meteorol. geodin. ital., ii (8), 1886/4, 183-205. volger, o. (1857): untersuchungen über das phänomen der erdbeben in der schweiz, gotha, pp 367. working group cpti (1999): catalogo parametrico dei terremoti italiani (ing, gndt, sga, ssn, bologna), pp. 92 (on line: http://emidius.mi.ingv.it/cpti/home.html). layout 6 annals of geophysics istituto nazionale di geofisica e vulcanologia [special issue] the lava flow invasion hazard map at mount etna and methods for its dynamic update editors ciro del negro istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy stefano gresta università di catania, dipartimento di scienze geologiche, catania, italy 462 preface annals of geophysics, 54, 5, 2011; doi: 10.4401/ag-5517 this special issue of annals of geophysics contains seventeen peer-reviewed papers that cover a wide variety of topics related to the v3-lava project funded by the italian dipartimento della protezione civile in the framework of the 2007– 2009 agreement with the istituto nazionale di geofisica e vulcanologia (ingv). the frequent eruptions of mount etna can produce lava flows that can cover distances long enough to invade vulnerable areas on the flanks of the volcano. these require improvements to our forecasting tools for the effective assessment of lava-flow hazards, to help the local authorities to make the necessary decisions during a volcanic eruption. the lava project aims to develop, validate and unify methods for mapping the areas around etna that are threatened by lava invasion within the next 50 years, and also within the immediate days after an eruption has begun. both timescales of lava-hazard mapping call for estimations of the probabilities of vent openings – using geological evidence over the long-term, and monitoring data over the short-term. the application of numerical models to the simulation of lava-flow paths represents the central part of the extensive methodology for long-term hazard assessment at etna. these numerical simulations of lava-flow paths are based on our knowledge of past etnean eruptions, and are derived from the integration of historical and geological data, and from adopting an updated digital representation of the topography on which the lava flow will be emplaced. the short-term maps are dynamic instruments that can be rapidly modified by consideration of the signals collected by the monitoring networks, the temporal evolution of the eruption, and the weighted opinions of the experts. probabilities of vent opening can be estimated using a bayesian approach and expert elicitation (bet-ef). once vents are assumed (or known), three flow models for simulating lava paths have been compared – downflow, sciara and magflow. a fourth modeling approach, known as smoothed particle hydrodynamics, has also been introduced. its implementation on graphic processing units poses significant mathematical and computer-science challenges. it is not yet ready for side-by-side comparisons with the three other models, but the method has gained much favor in other applications, and it will probably be competitive in the not-too-distant future. since physics-based models to simulate lava-flow paths require their effusion rates as an essential input parameter for real-time scenarios, much effort has also been applied to the estimation of effusion rates in real time. the primary tools are infrared remote sensing (seviri, avhrr and modis, with seviri providing the best time resolution, and modis the best spatial resolution). satellite techniques are focused on hot-spot detection and effusion-rate estimation, to drive numerical simulations of lava-flow paths during ongoing eruptive events. the various images are compiled in the lav@hazard web-gis framework, from which information can be fed directly into magflow, and presumably into other models as well. as satellite-derived lava eruption rates can be obtained in real time and simulations of several days of eruption can be calculated in a few minutes, such a combined approach has the potential to provide timely predictions of areas likely to be inundated with lava, which can be updated in response to changes in the eruption conditions. the interactive capability of this architecture appears to be excellent – indeed, it is leading the way in dynamic hazard mapping. in summary, we believe that the lava project has made major progress towards reliable long-term and short-term lava-hazard maps. all of the methods in this project – for the estimation of vent-opening probabilities, effusion rates, and flow scenarios – will undoubtedly continue to evolve. however, they are already useable and are providing a great service to the population around mount etna. many parts of the lava project are at the cutting edge of volcanology worldwide. it is an outstanding project, and we thank and congratulate all of the contributors. ciro del negro istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy stefano gresta università di catania, dipartimento di scienze geologiche, catania, italy acknowledgements. we would like to express our sincere gratitude to enzo boschi, who has always encouraged and supported our work. we wish to thank the overall coordinator, paolo papale, and the members of the international evaluation committee, donald b. dingwell, jörg keller and chris newhall, for their many and fruitful discussions. we also express our appreciation to the editorial board of the annals of geophysics, for accepting and supporting our proposal for a special issue on the lava project, to the editor-in-chief, edoardo del pezzo, for assisting the guest editors and authors, and for all of the work that has been done to make the publication of this volume possible. special thanks also go to the referees, for their efforts to ensure and improve the scientific quality and consistency of the manuscripts: prof. nikolaus a. adams, institute of aerodynamics and fluid mechanics, university of munich, germany prof. mark bebbington, institute of fundamental sciences, massey university, new zealand dr. matthew blackett, department of geography, environment & disaster management, coventry university, uk dr. alfonso brancato, dipartimento di scienze geologiche, università di catania, italy dr. sonia calvari, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, italy dr. long chen, qualcomm incorporated, san diego, ca, usa prof. corrado cigolini, dipartimento di scienze mineralogiche e petrologiche, università di torino, italy prof. pierfrancesco dellino, dipartimento geomineralogico, università di bari, italy prof. rosa di maio, dipartimento di scienze della terra, univeristà di napoli, italy dr. alicia felpeto, observatorio geofísico central, instituto geográfico nacional, madrid, spain prof. fabrizio ferrucci, institut de physique du globe de paris, france dr. marilena filippucci, centro di ricerca per il rischio sismico e vulcanico, università di bari, italy dr. sonia giovinazzi, natural hazards research centre, university of canterbury, new zealand dr. gerard gorman, department of earth science & engineering, imperial college london, uk prof. gianluca groppelli, dipartimento di scienze della terra, università di milano, italy dr. barbara hirn, ies consulting ltd., intelligence for environment and security, rome, italy prof. matthew d. jones, center for computational research, university at buffalo, usa dr. valerio lombardo, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, italy dr. giovanni macedonio, istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, italy dr. francesco marchese, istituto di metodologie per l'analisi ambientale, cnr, potenza, italy prof. alexander r. mcbirney, department of geological sciences, university of oregon, usa dr. gianluca norini, dipartimento di scienze geologiche e geotecnologie, università di milano, italy dr. massimo pompilio, istituto nazionale di geofisica e vulcanologia, sezione di pisa, italy prof. sergio pugnaghi, dipartimento di ingegneria dei materiali e dell´ambiente, unimore, italy prof. rocco rongo, dipartimento di scienze della terra, università della calabria, italy prof. laurent royon, département génie thermique et energie, iut de marne la vallée, france prof. roberto scandone, dipartimento di fisica, università roma tre, italy dr. david r. sherrod, u.s. geological survey, hawaii, usa prof. william spataro, dipartimento di scienze della terra, università della calabria, italy prof. roberto sulpizio, dipartimento geomineralogico, università di bari, italy prof. jean-claude tanguy, institut de physique du globe de paris, france dr. simone tarquini, istituto nazionale di geofisica e vulcanologia, sezione di pisa, italy dr. sudarshan tiwari, fraunhofer institut für technound wirtschaftmathematik, kaiserslautern, germany prof. giuseppe a. trunfio, dipartimento di architettura design e urbanistica, università di sassari, italy dr. renato umeton, massachusetts institute of technology, cambridge, usa dr. frans van den berg, meraka institute, csir, pretoria, south africa dr. nick varley, facultad de ciencias, universidad de colima, colima, mexico prof. leonid f. vitushkin, d. i. mendeleyev institute for metrology, st. petersburg, russian federation prof. peter webley, alaska volcano observatory, university of alaska fairbanks, alaska, usa. special issue: v3-lava project 463 185_196 argentini.pdf 185 annals of geophysics, vol. 46, n. 2, april 2003 characteristics of the boundary layer at ny-ålesund in the arctic during the artist fi eld experiment stefania argentini (1), angelo pietro viola (1), giangiuseppe mastrantonio (1), alberto maurizi (2), teodoro georgiadis (2) and marianna nardino (2) (1) istituto di scienze dell'atmosfera e del clima (isac), cnr, roma, italy (2) istituto di biometeorologia (ibimet), cnr, bologna, italy abstract a preliminary analysis of boundary layer data acquired during the arctic radiation and turbulence interaction study experiment (artist) at ny-ålesund (spitzbergen) in 1998 is presented. as expected, the wind fi eld and the thermal structure of the boundary layer are strongly infl uenced by the katabatic fl ow blowing along the kongsfjorden. in particular, if the large scale circulation has a component along the same direction, the resulting low level wind reaches velocities comparable with geostrophic wind. stable to neutral conditions occurred most of the time, sustaining inversion or spiky layers respectively, depending on the wind intensity. the behaviour of some micrometeorological parameters was examined in relation to the general fl ow pattern and the characteristics of this area. the highest values of surface sensible heat fl ux (with negative sign) were observed during daytime, in presence of forced convection, due to the transfer of warmer upper layer air into the surface layer. mailing address: dr. stefania argentini, istituto di scienze dell’atmosfera e del clima (isac), cnr, via fosso del cavaliere 100, 00133 roma, italy; e-mail: s.argentini@isac.cnr.it key words arctic planetary boundary layer sodarturbulence 1. introduction a fi eld experiment was carried out in svalbard area in spring 1998 in the framework of the ec program artist (arctic radiation and turbulence interaction study). the objective of artist was to improve the knowledge of the atmospheric properties during the sea-ice melting season and evaluate the relationship between the local dynamics, some micrometeorological parameters and the general fl ow pattern. the experiment was carried out by several european scientifi c institutions that provided airborne and ground-based measurements of wind, surface measurements of turbulence and radiation, as well as satellite and conventional meteorological data. a detailed description of the whole fi eld programme, with the complete data inventory of artist is given by hartmann et al. (1999). the present work highlights some observational results obtained from the analysis of ground-based and turbulent measurements at ny-ålesund in the kongsfjorden in spitzbergen island. in particular, some aspects of the arctic boundary layer structure, and the behaviour of the atmospheric parameters governing the boundary layer evolution are discussed in the general meteorological and geographical context. 2. site and instrumentation the svalbard islands are situated in the east part of the arctic basin, 1000 km from the north pole at an average latitude of 80°n. 186 stefania argentini, angelo pietro viola, giangiuseppe mastrantonio, alberto maurizi, teodoro georgiadis and marianna nardino the main island of svalbard is spitzbergen where the climatic infl uence of the gulf stream allows small communities to live there. the northernmost community lives in ny-ålesund (78.933°n, 11.933°e, 11 m a.s.l.) (fi g. 1a,b) and principally cares about the arctic station. due to the site characteristics and thanks to the logistic support of the ny-ålesund community, this place was chosen to carry out the turbulence and ground-based boundary layer measurements during the artist fi eld experiment that took place between march 15 and april 16, 1998. the instrumentation employed in the fi eld work included: a three-axis doppler sodar, a tethersounding station, a micrometeorological station, a sensor to measure the ground fl ux, and three subsoil thermometers. the sodar was located along the coast of the kongsfi orden in the zeppelin bay (s in fi g. 1b) to avoid the noise due to the human activity of the ny-ålesund village. the tethersounding station was set up along an old mine road at the foot of a small coal hill at about 20 m a.s.l. (t in fi g. 1b) to avoid interference with the sodar measurements and to have free space to release the balloon. an iso 20 container was also placed nearby to recover the balloon during strong wind episodes. the fl ight control and the data acquisition were done at the italian base (ib in fi g. 1b). the mast with micrometeorological sensors was located between the sodar and the tethersounding station (r in fi g. 1b). the sodar, similar to the one used in some antarctic campaigns by argentini et al. (1992, 1996) is a monostatic three-axis doppler system with 1.2 m diameter antennas that simultaneously emit three acoustic tones, one from each antenna, at 1750, 2000 and 2250 hz. two of the antennas were tilted 20° off the vertical, while the third one pointed vertically. the length of the acoustic pulse was 100 ms. the harmonic analysis of the signal to retrieve the radial wind was made with 0.16 s length samples, corresponding to approximately a 27 m depth layer, the fi rst of which was at 50 m. the height reached by the sodar profi les ranged between 200 m and 900 m depending on the signal to noise ratio, and the wind conditions. a cross-beam confi guration was chosen for the antennas; the tilted antennas were positioned at the acute vertices of a rectangular triangle and oriented towards the vertically pointing antenna. as the vertical and tilted antennas were 25 m apart, they crossed at a height of 67 m. an angle of 30° was chosen between the y axis and the north to avoid the noise from the ny-ålesund village. the harmonic analysis of the received signal was then performed to derive the wind profi le and to visualise the dynamic evolution of the airfl ow crossing the antenna’s beam. the spectral analysis used to retrieve the doppler shift and the echo intensity for each channel was carried out through a two-step procedure as explained by mastrantonio and fiocco (1982). for each channel, the processing of the returned signal provided the profi les of the radial wind velocity and echo intensity. the data were retained when a parameter related to the signal-to-noise ratio s/n exceeded the threshold of 0.75 (mastrantonio and fiocco, 1982). the tethersounding system included a receiving ground station, and an airborne sensor package consisting of dry and wet bulb thermometers, pressure, wind speed and wind direction sensors and a 403.5 mhz transmitter. a 5 m3 balloon, properly shaped to facilitate orientation upwind and retrievable by a winch with 1000 m of tetherline was used to fl y the tethersonde. the temperature sensors have an accuracy of ± 0.5 °c. the wind speed, obtained by a three-cup anemometer, has an accuracy of ± 0.25 ms–1, whereas the wind direction is measured with a magnetic compass that relies on the orientation of the balloon within an accuracy of ± 5°, when the balloon is suitably stable. an aneroid barometer measures the pressure with an accuracy of ± 1 hpa. during flight, the electronics of the tethersonde scanned the sensors sequentially and transmitted raw data to the ground station every 6 s. the altitude of the sensor package above the surface was estimated from the pressure and temperature profiles. the balloon could be released only when the ground wind speed was less then 6 ms–1. before each launch a pre-fl ight control was conducted by measuring the temperature and pressure with ground sensors. the correct operation of the system (including battery check) and data quality were checked on-line to allow for any real time modifi cation of the balloon fl ight. 187 characteristics of the boundary layer at ny-ålesund in the arctic during the artist fi eld experiment a b fig. 1a,b. a) map of the kongsfjorden spitsbergen island; b) map of ny-ålesund and location of the experiment sites: 3-axes doppler sodar (s), radiation and fast response sensors mast (r), italian base (ib), tethersounding station (t). 188 stefania argentini, angelo pietro viola, giangiuseppe mastrantonio, alberto maurizi, teodoro georgiadis and marianna nardino the micrometeorological station was installed in a fl at zone with little obstruction nearby. the instrumentation consisted of a metek sonic thermo-anemometer (mod. usa-1) mounted on top of a 3 m mast, a fast response campbell kripton hygrometer (mod. kh20), and a kipp and zonen net radiometer (mod. cnr1). the sonic thermo-anemometer was oriented to the north with an accuracy of 1.5°, and its vertical alignment was assured by two optical levels allowing a precision of 1°. the fast hygrometer was used to estimate the latent heat fl ux by means of the eddy-covariance technique. both sonic anemometer and fast hygrometer data were acquired at a frequency of 10 hz and then collected by a meteoflux acquisition system (sozzi and favaron, 1996) to provide the real-time values of the three wind components and sonic temperature. using three axis rotations (sozzi and favaron, 1996) the data were processed to derive all the turbulence variables, as well as, the sensible and the latent heat fl uxes. the net radiometer was used to determine the solar radiation budget. the incoming and outgoing components of the shortwave and longwave radiation were separately measured to derive the net radiation r n . the shortwave net radiation s wn was obtained by two pyranometers: one facing upward, to measure the incoming solar radiation and the other facing downward to measure the radiation refl ected by the snow surface. the longwave net radiation l wn was obtained by two pyrgeometers to measure the incoming and outgoing components separately. the self-heating of the instruments was controlled by a platinum-wire thermistor incorporated in the cnr1’s body. the sub-soil temperatures were measured using two termocouples buried in the snow at depths of 3 cm, 8 cm and 17.5 cm respectively. 3. wind fi eld observations according to the climatology of the spitzbergen (førland et al., 1997), the prevailing winds in the area blow from the ne-se sector, except during the summer. however, the lowlevel circulation is strongly infl uenced by the orography and by fl ows of local origin. these fl ows, which move along valleys and fi ords, are drainage winds transporting cold and heavy air from the inland glaciers to the warmer sea. these considerations refer to winds measured at 10 m by the weather stations located in the area. since the sodar supplies vertical profi les, sometimes up to several hundred meters, it is possible to inspect whether the observed three-dimensional wind fi eld is in agreement with the climatology and how it varies with height. for about 12 days during the fi eld experiment (from march 19 to april 1), the wind speed often reached peaks of 18 ms–1 with average values higher than seasonal observations. the noise created by high wind velocities caused strong limitations in the range of the sodar measurements. fig. 2a-c. scatter plot of the wind speed versus the wind direction during artist at 65 m (a) and 224 m (b) for sodar and at 3 m for sonic data (c). a b c 189 characteristics of the boundary layer at ny-ålesund in the arctic during the artist fi eld experiment figure 2a-c reports the scatter plots of the wind speed against the wind direction at 65 m and 224 m (sodar), and 3 m (sonic anemometer), respectively. above 224 m the statistics is too poor to yield an accurate description of the wind speed behaviour. as expected, the sonic anemometer data present a larger scatter due to the infl uence of local roughness. two main wind directions can be recognized: one around 100°-150°, along which the highest wind speeds are recorded, and one with a lower intensity peak at 300°-360°. these two directions correspond to the se-nw geographic orientation of the kongsfjorden, whose orographic characteristics are responsible for the forcing of the wind direction along this axis. the fact that the strongest winds are observed from se sector mainly depends on synoptic circulation that often has a southern component. from the nw sector only fl ows with velocities greater than 6 ms–1 seem to overpass the mountains ridge north of the kongsfjorden and reach ny-ålesund. the wind dynamics in the fjord area of nyålesund is very complicated. figure 2a-c shows that the wind speed at 65 m often exceeds that at 224 m. this is due to the existence of drainage fl ows, with a peak below 224 m, moreover a deviation of about 10°-20° from the mean direction for velocities higher than 8 ms–1 is evident at 65 m (fi g. 2a) because the fl ow is less infl uenced by the local orography. figure 3 plots the direction and speed at 1100 gmt of the geostrophic and low level wind as a function of time. the geostrophic wind was obtained by using radiosoundings while the sodar data at 65 m were used for the low level wind. a difference of about 30°-40° is observed from march 24 to 30 when the wind speed at both level is about 10-14 ms–1. in this case the geostrophic wind enhances the katabatic fl ow. between april 6 and 12 the geostrophic wind rotates to the northern sector with a decline in the wind intensity although the low level wind still persist from e-se. a large deviation (ranging fig. 3. time series of the low level wind given by sodar at 65 m, and the geostrophic wind by radiosounding during artist at 1100 gmt. 190 stefania argentini, angelo pietro viola, giangiuseppe mastrantonio, alberto maurizi, teodoro georgiadis and marianna nardino between 50° and 180°) is observed between the two fl ows. it would appear that in this case the geostrophic wind does not infl uence the fl ow along the kongsfjorden which will be simply gravity driven downslope the glacier. this large rotation between the surface and geostrophic fl ows is determined by the position of the high-low pressure system, that causes a coupling of the low level and geostrophic circulation when the low pressure centre is located sw of svalbard and a de-coupling when an opposite situation occurs. an example of the three dominant flow patterns encountered at ny-ålesund is presented in fi g. 4a-d. in order to show the role of the orography in this area, the wind direction measured at the research station on mount zeppelin at an altitude of 473 m (full dot) south of ny-ålesund is also plotted. as shown by beine et al. (2001), south-east fl ows are the most common situation at ny-ålesund. in fi g. 4a a rotation of the wind direction is observed around 600 m. the depth of this layer is determined by the orographic influence on the large scale wind fi eld and the presence of the katabatic wind in the lower level. several superimposed fl ows may be present within this layer with the intensity of the katabatic fl ow determining the height of the katabatic layer. in other cases (fi g. 4b,c) the katabatic fl ow was confi ned to the fi rst 300-400 m while a superimposed fl ow blowing from 0-30° could be observed above 400 m. in both cases the sodar and the zeppelin station observed, within a deviation of ± 10°, a northerly fl ow while the wind in ny-ålesund was from 120°. in the case of northerly fl ow at ny-ålesund, fi nally, the fl ow was from the same direction at the zeppelin station (fi g. 4d). 4. thermal structure and stability of the atmosphere the sonic anemometer data, even if they refer to point measurements, can provide quantities such as the monin obukhov length (l) which indicate the stability conditions of the boundary layer. the histogram in fi g. 5 shows the statistics of z / l (z = 3 m is the height of the sensor) as a function of wind direction. these values were divided into three classes: according to unstable (z /l < – 0.05), neutral (– 0.05 < z / l < 0.05), and stable (z / l > 0.05) conditions. the plot shows two main peaks in the sectors centred around 120° and 310°, corresponding to the two predominant wind directions. most of the time neutral conditions occur in these two sectors. stable conditions with thermal stratifi cations at different levels are mainly observed when the wind blows in the sector centred at 120°. unstable situations, occurring under very light wind conditions, can span from 90° to 270°, with a peak at 245°. the predominant structures observed by the sodar during the experiment were spiky layer, caused by forced convection, in the fi rst period of the campaign until april 1 and stable stratifi cations, often modulated by internal gravity waves, generated by shear of the fig. 4a-d. typical 1 h averaged wind profi les (circle) observed at ny-ålesund, the wind direction measured at the zeppelin research station (full dot) is given for comparison. a) april 7 between 2000-2100 st; b) april 6 between 0900-1000; c) april 5 between 1900-2000; d) april 5 between 0200-0300. a b c d 191 characteristics of the boundary layer at ny-ålesund in the arctic during the artist fi eld experiment basic fl ow. these features generally alternated depending on the wind intensity, cloud cover, sunshine factor and large scale fl ow. forced convection occurred because of the presence of a large wind shear in the fi rst 100300 m generating anisotropic turbulence primarily in the horizontal direction. the resulting spiky structures are similar to those observed in antarctica by gera et al. (1998). figure 6a shows an example of these structures on march 26. the wind had velocities up to 15 ms–1, and the direction was mainly from 120°. unfortunately, the noise generated by the wind did not allow us to derive the profi les above 300-400 m. large instantaneous values of the vertical velocity of the order of 2 ms–1 (not shown in this paper) were also associated with these high wind velocities. the vertical mixing due to the forced convection produced an almost neutral layer with the stability parameter z / l very close to zero as shown in fi g. 6b. this process was responsible for the neutral or lightly stable cases presented in fi g. 5. stable stratifi cations were observed when the wind velocity was lower than 5-6 ms–1. in these conditions, the sodar measurements could range up to 1000 m. an example of the observed stratifi cations as well as the horizontal wind profi les are shown in fi g. 7a,b for april 7. three inversion layers topped by a zone of enhanced thermal stability and relevant wind shear at about 150 m, 400 m and 700 m are present together with kelvin-helmotz waves of different frequency. the horizontal wind shear increases at thermal turbulent layer height (fi g. 7a) as was also observed by overland (1985) and curry et al. (1988). during a fi eld experiment in the fram strait north-west of the svalbard, brummer et al. (1994) also observed four sublayers below 1700 m topped by moisture inversions (zones of enhanced moisture). busak and brummer (1988), using vertical soundings from one fl ight mission over the north sea, also showed that despite the stable temperature stratifi cations, the wind shear at certain levels is suffi ciently large for the richardson number to be below its critical value of 0.25 and coherent motions – in the form of kelvin helmoltz waves – may be generated under these conditions. the presence of the kelvin-helmholtz waves makes the determination of the inversion height diffi cult because of the strong oscillations. the stability parameter z /l calculated for april 7 (fi g. 7b), oscillates between positive and negative values. although the waves develop in fig. 5. histogram with the distribution of the observed cases for three different stability classes, unstable: z /l < – 0.05; neutral: – 0.05 < z /l < 0.05; stable: z /l > 0.05 during artist. 192 stefania argentini, angelo pietro viola, giangiuseppe mastrantonio, alberto maurizi, teodoro georgiadis and marianna nardino b fig. 6a,b. a) sodar facsimile and horizontal velocity recorded at ny-ålesund on march 26, 1998 during forced convection; b) stability parameter z /l as a function of solar time. solar time (h) z / l 0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 a b fig. 7a,b. a) sodar facsimile and horizontal velocity recorded at ny-ålesund on april 7, 1998 showing a stable layer with waves; b) stability parameter z /l as a function of solar time. a stable boundary layer, these oscillations may be attributed to upward and downward oscillations of air masses with different thermal and dynamic characteristics. although these values are not representative of the «real» stability conditions, they are suffi ciently indicative of the thermal structure of the atmosphere to be used as an indicator of the presence of waves. a 193 characteristics of the boundary layer at ny-ålesund in the arctic during the artist fi eld experiment use of the sodar for measuring the turbulent components of the wind is still problematic (wilczak et al., 1996), but some quantities such as the variance of the vertical velocity give values that are correct if the data are properly analysed. in these cases, as shown by angevine et al. (1994) and argentini et al. (1999), these values are more representative than in situ surface measurements, since they are less sensitive to surface conditions than a point sensor. 5. surface fl uxes the surface fl uxes have been calculated from the covariances of the temperature, humidity and vertical fl uctuations, using sonic anemometer and fast hygrometer measurements. the behaviour of the sensible heat flux (h0), turbulent kinetic energy (tke), and of vertical momentum fl uxes (u'w'), for the whole measurement period are presented in fi g. 8 a,b fig. 8a-c. a) behaviour of the sensible heat fl ux; b) turbulent kinetic energy; c) vertical momentum fl ux, during artist. a b c 194 stefania argentini, angelo pietro viola, giangiuseppe mastrantonio, alberto maurizi, teodoro georgiadis and marianna nardino and c, respectively. until the end of march the sensible heat fl ux is, most of the time, negative as expected under stable atmospheric conditions. the highest values of sensible heat fl ux, tke and momentum flux are reached for strong wind speeds. when the mechanical turbulence is high, the strong vertical mixing produces a remarkable sensible heat fl ux transfer from the atmosphere to the surface (negative sign). during the last measurement period (early to mid-april), characterised by weaker winds and a low cloud coverage (as pointed out later), the sensible heat fl ux tends to assume positive values (convective atmospheric conditions). it is noted that the behavior of tke and momentum fl ux are negatively correlated. the radiative budget at the surface, obtained through the radiometric measurements, is shown in fi g. 9. the low values of the net radiation r n indicate that very low energy is available at these latitudes. the shortwave net fl ux s wn has an increasing trend due to the increase in the daytime period. the straight lines in the s wn and r n plots are due to the shadow of the mast on the radiometer. the fi rst measurement period was characterised by intense cloud coverage days with very low values of s wn and irregular behaviour. low cloud coverage was present after april 4. finally, the value of the sub-surface fl ux through the measurements of the snow temperature at different depth was calculated. the value of the fl ux in the soil, if no waterphase changes occur at the surface, is given by two terms: one due to the heat transferred by conduction and the other caused by transmission of the shortwave radiation into the snow (bintanja and van den broeke, 1995). this parameterisation was applied to our measurements using two snow temperatures to obtain the conduction heat, and the shortwave radiation component for the radiative heating. figure 10 a-c shows the values of the fl uxes into the snow obtained using three different temperature gradients; glow using the difference between t2 and t3, gup fig. 9. behaviour of the net radiation, longwave net radiation and shortwave net radiation for the whole measurement period. 195 characteristics of the boundary layer at ny-ålesund in the arctic during the artist fi eld experiment between t1 and t2 and, gthick between t1 and t3. the general behaviour of glow is smoother than the corresponding gup and gthick calculated in the uppermost and thickest layers respectively. this is a direct consequence of the smaller fl uctuations at greater depths. ground fl uxes minima are observed when the surface temperature reaches a minimum (generally during the night). under these conditions the snow temperature in the deeper layers is higher than at the surface and the heat fl ux is directed upwards. in interpreting these results we should bear in mind that gup is surely over-estimated because the upper part of the temperature sensor tends to heat itself during clear days. indeed, the last days of fig. 10a-c. time evolution of the fl ux into the snow calculated with three different temperature gradients: a) t2 t3 for glow; b) t1 t2 for gup, and c) t1 t3 for gthick, being t1 at 3 cm, t2 at 8 cm and t3 at 17.5 cm below the surface respectively. a b c the measurement campaign show a higher value of the fl ux into the snow. moreover, during night time in clear days the glow values were almost 10 wm–2 lower than the corresponding gup values. this can be explained considering that during the night the surface temperature and the snow temperature at 8 cm decreased following almost the same slope, whereas the snow temperature at 17.5 cm decreased more gently, so that the difference between the decreasing rate of t2 and t3 was larger than between t1 and t2. these temperature differences indicate that the sensor t2 is not deep enough to account for all heat stored in the snow or released toward the surface during the day. the sensor t3 is probably at the most appropriate reference depth for calculating heat exchanges between the surface and the deepest layers. 6. summary we have presented some preliminary observational results from the ground-based measurements during the field experiment artist in ny-ålesund. neutral conditions were observed from the two predominant wind directions centred in the 120° and 310° sectors. a minor peak with the 20% of unstable cases was observed at 245° while stable conditions were mainly distributed around the sector at 120°. the thermal structure of the atmosphere was recorded with a sodar system. two main features were observed: spiky layers and stable stratifi cations. spiky layers, due to the presence of forced convection, were observed in correspondence of high wind speed, stratifi ed fl ows with waves occurred for low wind velocities. the two features mostly alternated with each other depending on the wind intensity. to further characterise wavy and well mixed layers, the vertical velocity variances were measured over time, using the sonic anemometer and the sodar. a general consistency was observed between the two sets of data during forced convection when the atmosphere, under the infl uence of strong mechanical mixing along the vertical, presents air masses with similar properties at the two sensors. a comparison between the wind in the boundary layer and the geostrophic wind estimated by radiosoundings 196 stefania argentini, angelo pietro viola, giangiuseppe mastrantonio, alberto maurizi, teodoro georgiadis and marianna nardino was undertaken. for moderate to high wind speed a deviation of about 30° is observed between the two directions; in this case the geostrophic wind contributes to enhance the katabatic fl ow intensity and persistence. a deviation (ranging between 50° and 180°) was observed for weak winds. the highest values of the sensible heat fl uxes were observed during day time under forced convection because of the transfer of warmer upper layer air into the surface layer. the radiometric and temperature measurements were utilised to compute the net radiation and subsurface heat fl ux. the fl ux into the snow depends on several factors such as the depth of the snow layer, the thermal conductivity and the fraction of solar radiation absorbed by the surface. an analysis of gradient fl uxes g, obtained for different snow layers, showed that the most representative values are those of the layer between the surface and the deepest temperature sensor. however, further studies are necessary to arrive at more complete parameterisation schemes of the conductive and radiative components of the subsurface heat fl ux. acknowledgements this research was supported by the ec environment and climate research program n. ct97-0487. funding for the logistics in nyålesund was provided by the italian «progetto strategico artide». the authors wish to thank «piano nazionale ricerche in antartide» for providing the instrumentation used in the fi eld work, as well as mr. a. conidi for his help during the fi eld experiment. references angevine, w.m., r.j. doviak and z. sorbjan (1994): remote sensing of vertical velocity variance and surface heat fl ux in a convective boundary layer, j. appl. meteorol., 33 (8), 977-983. argentini, s., g. mastrantonio, g. fiocco and r. ocone (1992): complexity of the wind fi eld as observed by a sodar system and by automatic weather stations on the nansen ice sheet, antarctica, during summer 1988-1989: two cases studies, tellus, 44b, 422-429. argentini, s., g. mastrantonio, a. viola, p. pettré and g. dargaud (1996): sodar performances and preliminary results after one year measurements at adelie coast, east antarctica, boundary layer meteorol., 81, 75-103. argentini, s., g. mastrantonio and f. lena (1999): the convective boundary layer during winter in the urban aerea of milano: a few cases study, boundary layer meteorol., 93, 253-267. beine, h.j., s. argentini, a. maurizi, g. mastrantonio and a. viola (2001): the local wind fi eld a ny-ålesund and zeppelin mountain at svalbard, meteorol. atmos. phys., 78, 107-113. bintanja, r. and m.r. van den broeke (1995): the surface energy balance of antarctic snow blue ice, j. appl. meteorol., 34, 902-926. brummer, b., b. busak, h. hoeber and g. kruspe (1994): boundary-layer observations over water and arctic seaice during on-ice air fl ow, boundary layer meteorol., 68, 75-108. busak, b. and b. brummer (1988): a case study of kelvinhelmholtz waves within an off-shore stable boundary layer: observations and linear model, boundary layer meteorol., 44, 105-135. curry, j.a., e.e. ebert and g.f. herman (1988): mean and turbulence structure of the summertime arctic cloudy boundary layer, q. j. r. meteorol. soc., 114, 715-746. førland, e.j., i. hansen-bauer and p.ø. nordli (1997): climate statistics and long-term series of temperature and precipitation at svalbard and jan mayen, den norske meteorologiske institutt, oslo, norway, rep. omni 39/90 klima, pp. 40. gera, b.s., s. argentini, g. mastrantonio, a. viola and a. weill (1998): characteristics of the boundary layer thermal structure at a coastal region of adelie land, east antarctica, antarct. sci., 10 (1), 89-98. hartmann, j., f. albers, s. argentini, a. bochert, u. bonafè, w. cohrs, a. conidi, d. freese, t. georgiadis, a. ippoliti, l. kaleschke, c. lupkes, u. maixner, g. mastrantonio, f. ravegnani, a. reuter, g. trivellone and a. viola (1999): arctic radiation and turbulence interaction study (artist), alfred-wegener-institute für polar meeresforschung, bremerhaven, germany, rep. on polar research 305/ 1999, pp. 81. mastrantonio, g. and g. fiocco (1982): accuracy of wind velocity determination with doppler sodar, j. appl. meteorol., 21, 820-830. overland, j.e. (1985): atmospheric boundary layer structure and drag coeffi cients over sea ice, j. geophys. res., 90 (5), 9029-9049. sozzi, r. and m. favaron (1996): sonic anemometry and thermometry: theoretical basis and data-processing software, environ. software, 11 (4), 259-270. wilczak, j.m., e.e. gossard, w.d. neff and l. eberhard (1996): ground-based remote sensing of the atmospheric boundary layer: 25 years of progress, boundary layer meteorol., 78, 321-349. layout 6 monitoring active volcanoes: the geochemical approach salvatore inguaggiato1,*, agnes mazot2, takeshi ohba3 1 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy 2 gns science, wairakei research centre, department of volcanology, taupo, new zealand 3 tokai university, school of science, department of chemistry, kanagawa, japan annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5187 volcanic systems: shallow magmas located beneath active volcanoes release volatiles both during eruptive activity and during inter-eruptive periods (passive degassing). the fluids in active volcanic areas rise directly from magma, and their composition is characterized mainly by h2o, co2, so2, h2s, hf and hcl (condensable gases), and by some non-condensable gases (e.g. he, h2, n2, co, ch4). the chemical composition of fumarolic gases can reflect the pressure, temperature and oxygen fugacity conditions of the deep magmatic source, provided that during their rise towards the surface, the gases do not undergo re-equilibration processes [giggenbach 1980, giggenbach 1996, nuccio and paonita 2001, paonita et al. 2002]. as the equilibrium kinetics of several chemical reactions is much slower than the rising velocities of the gases, the gas molecular compositions often undergo quenching phenomena, so that the gases show temperature and pressure equilibria higher than their outlet values. the concentration of magmatic species or their molecular ratios can be determined by means of direct sampling of fumarole gases or by using telemetric methods of observation. the extensive parameters (mass output) of volcanic fluids, coupled with the intensive parameters described before, provide basic and useful information for the formulation of volcanic fluid degassing models [italiano et al. 1997, brusca et al. 2004, inguaggiato et al. 2011]. the first step in the framework of the geochemical investigation of a volcanic system aimed at surveillance is the chemical and isotopic characterization of the fluids, and the putting forward of a geochemical model [inguaggiato et al. 2011]; within this geochemical model, it is possible to interpret the observed changes in any single investigated parameter. the geochemical approach is to identify the following topics: • the main end-members involved in the studied system; • the possible type and degree of interaction processes: e.g. water-rock and gas-water interactions; • the mixing among the individual end-members; • the chemical and isotopic characterization of a possible hydrothermal system; • the formulation of a geochemical model. the geochemical surveillance of an active volcano aims to recognize possible signals that are related to changes in volcanic activity. indeed, as a consequence of the magma rising inside the volcanic "plumbing system" and/or the refilling with new batches of magma, the dissolved volatiles in the magma are progressively released as a function of their relative solubilities. when approaching the surface, these fluids that are discharged during magma degassing can interact with shallow aquifers and/or can be released along the main volcano-tectonic structures. under these conditions, the following main degassing processes represent strategic sites to be monitored: 1) fumaroles [gerlach and nordlie 1975, giggenbach 1980, taran et al. 1986, gerlach 1993, giggenbach 1996, capasso et al. 1997, giammanco et al. 1998, nuccio and paonita 2001, paonita et al. 2002, liotta et al. 2010]; 2) diffused soil degassing [chiodini et al. 1996, chiodini et al. 1998, favara et al. 2001, 115 inguaggiato, mazot, ohba 116 diliberto et al. 2002, cardellini et al. 2003, brusca et al. 2004, chiodini et al. 2005, pecoraino et al. 2005, werner and cardellini 2006, inguaggiato et al. 2011, mazot et al. 2011]; 3) degassing associated with geothermal waters [capasso and inguaggiato 1998, taran et al. 1998, inguaggiato et al. 2000, inguaggiato and rizzo 2004, inguaggiato et al. 2004, inguaggiato et al. 2005, liotta et al. 2006, rouwet et al. 2008, inguaggiato et al. 2010, inguaggiato et al. 2011]; 4) plumes [oppenheimer et al. 1998, bruno et al. 1999, galle et al. 2003, aiuppa et al. 2004, aiuppa et al. 2005, mcgonigle 2005, burton et al. 2007a, burton et al. 2007b, aiuppa et al. 2009]. the main purpose of this special volume is to collect papers that cover a wide range of topics in volcanic fluid geochemistry, which include geochemical characterization and geochemical monitoring of active volcanoes using different techniques and at different sites. moreover, part of this volume has been dedicated to the new geochemistry tools. in particular: tassi et al. show a classic approach to characterize a volcanic system (chilean volcanoes) based on the chemistry and isotope composition of fumaroles, discussing the mantle, slab and atmospheric contributions to volcanic emissions; caliro et al. report on a long time-series of chemical and isotopic compositions of vesuvius fumaroles that were acquired in the framework of a volcanic surveillance period (1998-2010), and that allow the identification of processes that occur at shallow levels in the hydrothermal systems, and the variations induced by deep changes in volcanic activity; chiodini et al. report on a long time-series of fluid pressure and temperature within the hydrothermal system feeding the solfatara fumaroles, investigated here on the basis of chemical equilibria within the co2-h2o-h2-co gas system; rouwet and tassi present a study of a generalized box model approach that can be useful for geochemical monitoring of active crater lakes, as highly dynamic natural systems. the mass budget of a lake is based on observations of physical variations over a certain period of time: lake volume (level, surface area), lake water temperature, meteorological precipitation, air humidity, wind velocity, input of spring water, and overflow from the lake; diliberto presents temperature data acquired from 1998 to 2010 by continuous monitoring of fumaroles, and discusses the time relationships of the different parameters related to the activity of the volcanic system. during the study period, temperature and compositional changes in the fumarolic gases took place at the same time as variations in a number of volcanoseismic events, indicating frequent variations of energy release (heat and mass flow, as well as seismic strain release). geochemical modeling applied to the volcanic system of vulcano suggests that the overall expansion of magmatic gas through the fractured system is an almost iso-enthalpic process at depth, which shifts to an adiabatic process at shallow depths, where the rock permeability increases. this interpretation encourages fumarole temperature monitoring and suggests that only a multidisciplinary approach that considers the geochemical and geophysical parameters should be considered to explain the recorded variations. ohba et al. indicate that in 1998, many volcanic earthquakes were observed at iwate volcano, japan. however, although an eruption was anticipated, it never occurred. fumarolic gases were sampled at the volcano on six occasions during 1998 and 1999, and were analyzed for their chemical compositions and isotope ratios. fumarolic gases were again sampled in 2004 and 2006, after the period of seismic activity. the hcl concentrations and isotope ratios of h2o in the gas samples collected from an active geothermal area of the volcano were high in 1998 and 1999, but decreased significantly after 2004, irrespective of the relatively stable geochemical monitoring approach concentrations of co2 and sulfur-bearing gases. a notable feature of the fumarolic gases is the high isotope ratio of h2o, which is similar to that of the pure magmatic gas that is typical of andesitic volcanoes, although the temperature at the outlet was less than 146 ˚c in the present case. the above features can be explained if the enthalpy of the magmatic gas was equivalent to that of water vapor at 252 ˚c to 370 ˚c under a pressure of 1 bar. the correlations observed among co2/h2o, hcl/h2o, and d 18o values of the fumarolic gases suggest the partial condensation of h2o vapor during the ascent of the volcanic gas to the surface. the estimated co2/h2o molar ratio of the magmatic gas is 0.008, which is less than the value of 0.03 reported previously for magmatic gas sampled during an effusive eruption at unzen volcano, japan. tamburello et al. indicate that measurements of volcanic so2 emission rates have been the mainstay of remotely sensed volcanic gas geochemistry for almost four decades, with significant contributions to our understanding of volcanic systems and their impact upon the atmosphere. the last ten years have brought step-change improvements to the instrumentation applied to these observations, beginning with the application of miniature ultraviolet spectrometers deployed in scanning and traverse configurations, with doas evaluation routines. this study catalogues the more recent empirical developments, such as: ultraviolet cameras; wide angle field-of-view doas systems; advances in scanning operations, including tomography; and improved understanding of errors, in particular concerning radiative transfer. furthermore, the outcomes of the field deployment of these sensors over the last decade are documented, with respect to improving our understanding of volcanic dynamics and degassing to the atmosphere. the last part of this special volume collects articles together that describe new instrumentation and tools to monitoring active volcanoes. in particular: inguaggiato et al. show new home-made equipment designed to measure the continuous partial pressure of co2 in the natural waters, along with its application to the thermal aquifer of stromboli island; and finally: tamburello et al. report on a new tool to measure and analyze the so2 flux of the volcanic plume using an so2 camera. this study describes vulcamera, a stand-alone program for the determination of volcanic so2 fluxes using ultraviolet cameras. the code enables field image acquisition, and provides all of the required post-processing operations. references aiuppa, a., m. burton, f. muré and s. inguaggiato (2004). intercomparison of volcanic gas monitoring methodologies performed on vulcano island, italy, geophys. res. lett., 31, l02610; 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of thermal springs,fumaroles and bubbling gases at tacaná volcano (mexico– guatemala): implications for volcanic surveillance, bull. volcanol., 59, 436-449. taran, y.a., a.d. esikov and a.l. cheshko (1986). deuterium and oxygen-18 in waters of mutnovsky geothermal area, geochem. int., 4, 458-468. taran, y., t.p. fisher, b. pokrovsky, y. sano, m. armienta and j.l. macias (1998). geochemistry of the volcano-hydrothermal system of el chichón volcano, chiapas, mexico, bull. volcanol., 59, 436-449. werner, c. and c. cardellini (2006). comparison of carbon dioxide emissions with fluid upflow, chemistry, and geologic structures at the rotorua geothermal system, new zealand, geothermics, 35, 221-238. 119 a n c o r a s u l l a d i s t r i b u z i o n e i n l o n g i t u d i n e d e i flocculi d i c a l c i o g . g o d o l i f . m a z z u c c o n i b . c . m o n s i g n o r i f o s s i r i c e v u t o il 17 a p r i l e 1967 riassunto. —• v i e n e e s t e s a a i n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i flocc u l i d i c a l c i o r e l a t i v i a g l i a n n i i n t e r n a z i o n a l i d i q u i e t e s o l a r e u n ' a n a l i s i s t a t i s t i c a g i à e f f e t t u a t a s u i n u m e r i c a r a t t e r i s t i c i r e l a t i v i a l l ' a n n o geofìsico i n t e r n a z i o n a l e . l a n u o v a a n a l i s i h a c o n f e r m a t o l ' e s i s t e n z a d i l o n g i t u d i n i p r e f e r e n z i a l i . i l f u s o d i l o n g i t u d i n i p r e f e r e n z i a l i p e r il 1965 è s f a s a t o di c i r c a 180° r i s p e t t o a q u e l l o p e r il 1964. summary. — t h e s t a t i s t i c a l a n a l y s i s of t h e d e f i n i t i v e c h a r a c t e r fig u r e s of t h e c a b r i g h t flocculi p e r f o r m e d f o r t h e i n t e r n a t i o n a l g e o p h y s i c a l y e a r is e x t e n d e d t o t h e i n t e r n a t i o n a l q u i e t s u n y e a r s . t h e n e w a n a l y s i s h a s c o n f i r m e d t h e e x i s t e n c e of p r e f e r e n t i a l l o n g i t u d i n e s . t h e p r e f e r e n t i a l l o n g i t u d e b e l t f o r 1965 is o u t of p h a s e 180° w i t h r e s p e c t t o t h e l o n g i t u d e b e l t f o r 1964. 1. recentemente è s t a t a pubblicata in questa sede (*) la lista dei numeri caratteristici definitivi dei flocculi di calcio per gli anni internazionali di quiete solare (iqsy, 1 gennaio 1964-31 dicembre 1965) la quale completa un'analoga lista per l'anno geofìsico internazionale (agi, 1 luglio 1957-31 dicembre 1958) (2). come già era stato f a t t o per il periodo agi (3) si è ritenuto opportuno eseguire un'analisi statistica della variazione giornaliera dell ' a t t i v i t à dei flocculi di calcio anche per il periodo i q s y . anche per questo periodo, infatti, si è p o t u t a n u o v a m e n t e raccogliere u n a serie, praticamente senza lacune, di numeri caratteristici giornalieri di flocculi di calcio. 2. l ' a n d a m e n t o giornaliero dei numeri caratteristici dei flocculi di calcio per t u t t o il periodo iqsy è stato riportato nel grafico di fig. 1. 2 0 0 g . g o d o l i f . m a z z u c c o n i b . c . m o n s i g n o r i f o s s i come già è s t a t o osservato per il periodo a g i (3), anche per il periodo i q s y si possono rilevare successioni di massimi e di minimi che si ripetono, a b b a s t a n z a regolarmente, p e r circa 3 rotazioni sinodiche del sole. si è già n o t a t o (3) che questo f a t t o p u ò essere a t t r i buito alla persistenza dei più notevoli floccnli di calcio. ri. 0,70 0,60 0,50 0,40 100 200 300 l f i g . 2 d i s t r i b u z i o n e i n elassi d i 2 0 ° d i l o n g i t u d i n e d e l l e m e d i e d e i n u m e r i c a r a t t e r i s t i c i d e i flocculi d i c a l c i o p e r l ' a n n o 1964. sempre a n a l o g a m e n t e a q u a n t o è s t a t o osservato per il periodo a g i , si può rilevare dalla fig. 1 che le successioni di massimi tendono a d essere t r a loro i n t e r v a l l a t e di multipli di 27 giorni, confermando così l'esistenza di longitudini preferenziali anche per i flocculi di calcio. g . g o d o l i f . m a z z u c c o n i b . c . m o n s i g n o r i f o s s i f i g . 1 a n d a m e n t o g i o r n a l i e r o d e i n u m e r i c a r a t t e r i s t i c i d e i flocculi d i c a l c i o p e r i l p e r i o d o 1 g e n n a i o 1 9 6 4 3 1 d i c e m b r e 1965. s u l l a d i s t r i b u z i o n e i n l o n g i t u d i n e d e i f l o c c u l i d i c a l c i o 2 6 7 mentre però per t u t t o il periodo a g i il flusso di l o n g i t u d i n i ' p r e ferenziali interessato è risultato quello compreso f r a circa 300° 40°, per il periodo i q s y invece si sono a v u t i due f u s i di longitudini preferenziali: per il 1964 ancora quello compreso f r a circa 300° e 40°; per il 1965 quello quasi d i a m e t r a l m e n t e opposto. 1,00 0,90 0,800,700,60i i i 100 200 300 l f i g . 3 d i s t r i b u z i o n e i n c l a s s i d i 20° d i l o n g i t u d i n e delle m e d i e d e i n u m e r i c a r a t t e r i s t i c i d e i flocculi d i calcio p e r l ' a n n o 1965. 2 0 8 g . g o d o l i f . m a z z u c c o n i b . c . m o n s i g n o r i f o s s i l'esistenza di f u s i di longitudini preferenziali e la loro diversa localizzazione è meglio illustrata nei grafici delle figg. 2 e 3, in cui sono r i p o r t a t e s e p a r a t a m e n t e per il 1964 ed il 1965, le medie dei n u m e r i caratteristici rilevati nei giorni in cui la longitudine del meridiano centrale era compresa nella classe di longitudine i n d i c a t a in ascisse. n o t i a m o che ogni m e d i a si riferisce a circa v e n t i valori giornalieri. w 100 200 300 f i g . 4 d i s t r i b u z i o n e i n e l a s s i (li 20° (li l o n g i t u d i n e d e l l e i n e d i e d e i n u m e r i r e l a t i v i d i w o l f p e r l ' a n n o 1904. r e c e n t e m e n t e wilcox e s c h a t t e n (4) h a n n o messo in evidenza la piccola significatività statistica di distribuzioni del tipo di quelle rip o r t a t e nelle figg. 2 e 3, ed il f a t t o che l'epoca dei minimi di tali distribuzioni dipende dal valore a t t r i b u i t o al periodo sinodico di rotazione solare. n o t i a m o però che, nel nostro caso, le distribuzioni si riferiscono ad un campione circa v e n t i volte più numeroso di quello studiato da wilcox e s c h a t t e n e a due soli anni anziché a d u n intero ciclo; nel nostro caso la significatività statistica è quindi molto maggiore e l'influenza di errori nel periodo sinodico di rotazione solare p r a t i c a m e n t e trascurabile. stilla d i s t r i b u z i o n e i n l o n g i t u d i n e d e i f l o c c u l i d i c a l c i o 2 6 9 si p o t r e b b e pensare che i r i s u l t a t i illustrati nelle figg. 2 e 3 fossero da a t t r i b u i r s i a d errori personali periodici sulla s t i m a dei n u m e r i caratteristici. p e r questo a b b i a m o voluto estendere l'analisi i l l u s t r a t a nelle figg. 2 e 3 anche ai n u m e r i relativi di wolf (=•g). i r i s u l t a t i r i p o r t a t i nelle figg. 1 e 5 confermano p i e n a m e n t e quelli già conseguiti per i flocculi di calcio. sembra soltanto che per le macchie il fenomeno sia meno a c c e n t u a t o . w 250 100 200 3 0 0 [_ f i g . 5 d i s t r i b u z i o n e i n e l a s s i d i 20° d i l o n g i t u d i n e delle m e d i e d e i n u m e r i r e l a t i v i d i w o l f p e r l ' a n n o 1965. 2 7 0 g . g o d o l i f . m a z z u c c o n i b . c . m o n s i g n o r i f o s s i 3. p e r u n a più a c c u r a t a analisi statistica del m a t e r i a l e di osservazione a b b i a m o calcolato, a n a l o g a m e n t e a q u a n t o è s t a t o f a t t o per il periodo a g i , il coefficiente di autocorrelazione dei n u m e r i caratteristici dei flocculi di calcio (jv = 731) per sfasamenti j compresi f r a 0 e 550 giorni. tale calcolo è stato eseguito per mezzo di u n calcolatore i b m 1620. i l p r o g r a m m a di autocorrelazione, e v e n t u a l m e n t e a d a t t a b i l e anche a calcoli di correlazione, è a disposizione dei ricercatori interessati. nel grafico di fig. 6 i coefficienti di autocorrelazione r j sono s t a t i r i p o r t a t i in funzione degli sfasamenti j. si n o t a u n a n e t t a periodicità del coefficiente di autocorrelazione. p e r sfasamenti j compresi f r a 0 e 250 giorni si rivela u n a b u o n a periodicità del coefficiente r j con periodo uguale a quello della rotazione sinodica del sole; successivamente t a l e periodicità persiste, m a s f a s a t a di mezza rotazione. si conferma quindi il f a t t o che d u r a n t e il 1965 si è a v u t o u n o sfas a m e n t o di circa 180° del fuso di longitudini preferenziali. b i b l i o g r a f i a ( x ) g o d o l i g . , m a z z u c c o n i f . , m o n s i g n o r i f o s s i b . c . , ann. geof., i n c o r s o d i s t a m p a . ij ( 2 ) g o d o l i g . , m a z z u c c o n i f., m o n s i g n o r i " fossi b . c . , ann. geof., 1 9 , 1 ( 1 9 0 6 ) . ( 3 ) g o d o l i g . , m a z z u c c o n i f „ m o n s i g n o r i [fossi b . c . , ann. geof., 1 9 , 3 9 5 ( 1 9 6 6 ) . ( 4 ) w i l c o x j . m . a n d s c h a t t e n k . h . . ap. j . 1 4 7 , 3 6 4 ( 1 9 6 7 ) . ( 5 ) w a l d m e i e r m . , bull. soc. a str. trance, 7 9 , 3 3 4 ( 1 9 6 5 ) . (6) waldmeier m . , bull. soc. astr. france, 8 0 , 339 (1966). g . g o d o l i f. m a z z u c c o n i b . c . m o n s i g n o r i possi v xp i g . 6 c o e f f i c i e n t i di a u t o c o r r e l a z i o n e d e i n u m e r i c a r a t t e r i s t i c i d e i flocculi d i c a l c i o p e r s f a s a m e n t i j c o m p r e s i f r a 0 e 5 5 0 . completo_def.qxd 1455 annals of geophysics, vol. 47, n. 4, august 2004 key words volcano monitoring – volcano plumes – ir and uv spectoscopy 1. introduction volcanoes emit gases (principally h2o, co2, so2, hcl, hf and h2s) and aerosols to the atmosphere, both during and between eruptions. measurements of the chemistry, isotopic composition, and flux of these emissions, and their temporal variations are crucial in several applications (oppenheimer et al., 2003). 1.1. «imaging» volcanic plumbing systems and predicting volcanic activity volcanic gas composition and flux relate to subsurface magmatic conditions and higher-level fluid-rock interactions, providing information on magma composition, volume, storage, and transport. for instance, temporal variations in the ratios of emitted co2/so2 and hcl/so2 have been used to infer changes in magmatic systems feeding volcanoes (e.g., noguchi and kamiya, 1963; gerlach and casadevall, 1986), whilst gas flux measurements have been used to constrain the masses of degassing magmas (symonds et al., 1996). such data are valuable for hazard assessment because volcanic activity is strongly controlled by the dynamics of degassing (sparks, 2003). in the broader context of volcano monitoring efforts, the links between seismic, geodetic and gas geochemical signals can be strong. the degassing process may be closely associated with, even responsible for, observed seismicity and ground deformation. 1.2. understanding atmospheric and radiative effects quantification of volcanic volatile emissions is also important in order to evaluate their hemispheric to global scale atmospheric and climatic impacts. the sulfur emissions from major explomailing address: dr. clive oppenheimer, cambridge volcanology group, department of geography, university of cambridge, downing place, cambridge, cb2 3en, u.k.; e-mail: co200@cam.ac.uk exploiting ground-based optical sensing technologies for volcanic gas surveillance clive oppenheimer and andrew j.s. mcgonigle cambridge volcanology group, department of geography, university of cambridge, u.k. abstract measurements of volcanic gas composition and flux are crucial to probing and understanding a range of magmatic, hydrothermal and atmospheric interactions. the value of optical remote sensing methods has been recognised in this field for more than thirty years but several recent developments promise a new era of volcanic gas surveillance. this could see much higher timeand space-resolved data-sets, sustained at individual volcanoes even during eruptive episodes. we provide here an overview of these optical methods and their application to ground-based volcano monitoring, covering passive and active measurements in the ultraviolet and infrared spectral regions. we hope thereby to promote the use of such devices, and to stimulate development of new optical techniques for volcanological research and monitoring. 1456 clive oppenheimer and andrew j.s. mcgonigle sive eruptions can perturb the chemistry, dynamics and radiative transfer in both the troposphere and stratosphere, influencing surface climate (e.g., mccormick et al., 1995). passively degassing volcano emissions must also influence tropospheric dynamics and chemistry, as recent investigations suggest that between 15 and 40% of the global tropospheric sulphate burden is volcanogenic (chin and jacob, 1996; graf et al., 1998; stevenson et al., 2003). these figures exceed the percentage source strength of volcanic sulphur by a factor of 2-4, reflecting the generally higher altitudes of atmospheric entrainment from volcanoes compared with biogenic or anthropogenic sources, and highlighting the importance of better understanding the tropospheric chemistry of volcanic emissions. 1.3. evaluation of environmental and health impacts from local to regional scales, volcanic emissions can result in serious environmental and environmental health consequences, including destruction of agricultural crops, contamination of pasture, and human respiratory morbidity and cardiovascular mortality. large lava eruptions, such as that of laki (iceland) in 1783/1784, which released ≈ 120 tg of so2, 7.0 tg of hcl and 15 tg of hf, have resulted in major pollution episodes responsible for regional-scale extreme weather, agricultural losses, and elevated human morbidity and mortality (thordarson et al., 1996; witham and oppenheimer, 2004). individual passively degassing volcanoes can also represent major pollution sources. for example, mt. etna continuously emits of order 2 kg s–1 of hf, and > 8 kg s–1 of hcl (francis et al., 1998). its so2 flux (≈ 70 kg s–1) is comparable to the total industrial s emission from france, and substantially elevates tropospheric sulphate in southern italy (graf et al., 1998). adverse environmental and health impacts are observed downwind of many degassing volcanoes, including masaya (nicaragua), k1̃lauea (hawai`i), poás (costa rica), miyakejima (japan) and popocatépetl (mexico) (e.g., baxter et al., 1982; mannino et al., 1996; delmelle, 2003; fujita et al., 2003). at masaya volcano, which has been degassing strongly for over a decade without significant eruption, boundary layer so2 concentrations are elevated over a downwind area > 1000 km2, resulting in substantial economic impact from the loss of coffee crops, and exposing ≈ 50 000 people to levels exceeding who air quality standards (125 ppb over 24 h; 50 ppb over 1 yr). downwind of popocatépetl volcano, so2 quadrupled, and sulphate concentrations doubled in mexico city when fumigated by the volcanic plume (raga et al., 1999). the aims of this paper are to review briefly some of the latest developments in ground based optical sensing of volcanic gas and aerosol emissions. we focus particularly on those instruments that have come to the fore over the last ten years that are capable of augmenting or superseding the correlation spectrometer (cospec) device, which has seen widespread use over the last thirty years for volcanic so2 flux measurements. we also discuss the challenges that remain in implementing gas-monitoring networks, and in interpreting their data streams. it is hoped that this article will stimulate development of further optical sensing technologies, characterised by low cost, low maintenance, low weight and bulk, high temporal resolution, and multi-component sensitivity, that will meet key volcanological requirements. 2. general methodological background and challenges the conventional way to measure volcanic emissions is by direct sampling, either by closerange collection of samples from fumarole vents and active lava bodies using «giggenbach bottles», filter packs and condensing systems, or within atmospheric plumes from aircraft using various kinds of sampling apparatus and onboard analysers. a range of spectroscopic, gravimetric, isotopic and chromatographic techniques is available to determine chemical concentrations in real time or subsequently in the laboratory (symonds et al., 1994). whilst direct sampling is capable of delivering very detailed and accurate analyses, it is difficult to sustain routine surveillance in this way, or to 1457 exploiting ground-based optical sensing technologies for volcanic gas surveillance provide data of sufficient temporal resolution to compare meaningfully with seismic and geodetic data streams. arguably, the primary reason for this is the risk involved in the field, which generally restricts investigations to low temperature, subordinate vents (that probably do not characterize the bulk emission or represent the parts of the magmatic-hydrothermal system most sensitive to change in the event of renewed magmatic activity). additionally, chemical reactions between the container material or reagents and the collected gas sample may mask the original chemical composition. whilst the use of telemetered electrochemical sensors for continuous gas monitoring avoids many of the above complications (mcgee and sutton, 1994), this approach is not yet widespread and it suffers the problem that sensors may be destroyed in the event of an eruption. in this section, we look at the general background to volcanic gas sensing using optical methods, highlighting some of the key issues in adapting and developing remote sensing instruments for volcanological purposes, and interpreting the data they yield. much of the discussion applies to volcano monitoring devices in general. 2.1. spectroscopy background the sensing methods described in section 3 are based on the spectroscopic observation of molecular species of interest from their fingerprint rotational, vibrational and electronic transitions, usually seen in absorption (i.e., the attenuation of a source of radiation behind the gas cloud; fig. 1) but sometimes in emission. specific gases can be identified by their characteristic absorption spectra, and their abundances derived from the strength of the absorption, following the beer-lambert formula: i(λ) = i0 (λ)exp(– σ(λ)nl) (2.1) where i(λ) is the observed intensity of radiation at wavelength λ, i0(λ) is the original intensity of radiation, before interaction with the sample, σ(λ) is the absorption cross section, of the absorbing molecule at wavelength λ, and n is the mean concentration of the species over the pathlength l of the sample. measurements of more than one species provide gas ratios, in an analogous manner to direct sampling; traverses or scans of volcanic clouds combined with plume fig. 1. diagram of radiative transfer problem in absorption spectroscopy. from horrocks (2001). 1458 clive oppenheimer and andrew j.s. mcgonigle transport speeds can be used to derive fluxes of gases. the remote sensing measurements may be considered «passive» if the source of radiation is natural, and «active» if it is artificial. examples of passive measurements include use of natural terrestrial radiation (e.g., from active lava surfaces), diffuse sky radiation, or direct solar radiation (i.e., occultation measurements). active sources include lamps (usually used for broad spectral band observations), and lasers (sometimes multi-wavelength or «tuneable» for differential spectral measurements). the spectrometers may be dispersive, if the incoming light is separated spatially into its component wavelengths, or non-dispersive if the wavelength discrimination is achieved by other means (e.g., by interferometry). passive measurements are invariably «open-path» observations through the free atmosphere. active measurements may be open-path, or «closed-path» wherein the gas sample is admitted to a cell, of given path-length, through which the beam is passed. an advantage of closed-path observations is that the cell’s internal pressure may be lowered to reduce pressure-broadening effects on the spectral lines, permitting high-spectralresolution measurements of trace species even in noisy regions of the spectrum. multi-path cells of modest dimensions (e.g., < 1 m) can simulate substantial atmospheric paths (e.g., > 100 m) via multiple internal reflection. for quantitative applications, it is usually necessary to separate out the spectral features of the volcanic gas species of interest from those of the background atmosphere (which are dominated by spectral lines originating from h2o and co2, and molecular scattering at shorter wavelengths). this can be achieved by modelling the total atmospheric path (e.g., using a radiative transfer code, atmospheric and meteorological data, and known absorption coefficients for all relevant species), or ratio-ing spectral observations acquired for the same path with and without the volcanic plume present (iλ and i0,λ , respectively). from inspection of (2.1), it can then be seen that the column amount of a given species (i.e., the product of concentration and path length) can be obtained if σ(λ) is known. 2.2. instrumental specifications: data acquisition initial considerations in the design or exploitation of a gas sensor include the species to be targeted, and the dynamic range and sensitivity of the measurements. this requires, firstly, an appreciation of the molecules and/or isotopes worth observing from the perspective of what volcanologically-useful information they can provide, and the purpose of the investigation (volcano monitoring for activity prediction, quantification of concentrations of harmful gases, etc). the mixing of volcanic gases with the atmosphere and subsequent plume transport and dispersion act to dilute concentrations of species, making it particularly difficult to detect and measure the two principal volcanic gas components, h2o and co2. the significant and rapid changes in humidity of the background atmosphere add to complications of sensing volcanic h2o. h2o and co2 also hamper spectroscopic observations of other gas species because of their abundant absorptions across the fingerprint region of the infrared spectrum, which can mask more subtle features of other trace gases. because of the budgetary constraints on volcano observatories and institutes, it is certainly true that it helps if instrumentation is comparatively cheap. decisions always have to be reached in respect of costs of personnel, field vehicles and running costs, as well as equipment. however, given the potential cost benefits to society of effective volcano monitoring and accurate activity forecasts, volcanologists should be prepared to defend even expensive equipment purchases where a case can be made that the instrumentation will significantly enhance capabilities. observatories are often stretched not only in financial terms but also because of the many demands on the time of their personnel. this is especially the case during volcanic crises when operations are increased, and other parties (local officials, the public, the media, etc.) demand access to information. field equipment should therefore be, generally speaking, easy to operate and maintain; data should be available in real time or near-real time, preferably with high 1459 exploiting ground-based optical sensing technologies for volcanic gas surveillance time-resolution, and ideally with minimal postprocessing required. (recording data from the cospec instrument – see section 3.1.1 – has traditionally been achieved using chart recorders, demanding time-consuming, and potentially inaccurate, visual processing of the rolls of paper. this can often result in data not being processed for several days, despite their potential importance for hazards assessment). internet-ready, digital data-streams have obvious advantages for remote access, and multi-parameter data interpretation. equipment should be robust enough for sustained operation, even in hostile conditions (acid gases, ash deposition, etc.), and generally low in weight in bulk, especially if its use relies on being moved around regularly and transported in vehicles (clearly, autonomous operation is the ideal since it frees up personnel for other activities). power requirements should be modest as grid supplies are seldom an option on remote volcano locations. 2.3. error budgets and error reduction in order to interpret measurements, and the significance of any observed changes, it is crucial to understand sources and magnitudes of errors in the data. a number of data fidelity issues can be addressed in an automated fashion through various error checking regimes. instrument response functions should be monitored regularly where necessary, with calibration and intercalibration issues addressed as appropriate. this may include careful examination of radiative transfer models, assumptions concerning atmospheric profiles, plume distribution (especially in the vertical), and mie scattering processes. while budgeting and reducing errors are essential exercises, they should be approached in the context of the necessary accuracy and precision for the volcanological task in hand, and the non-volcanological influences that can modify observations. for example, imagine a volcano where instantaneous so2 gas fluxes at source may change by an order of magnitude on timescales of days. it may be sufficient to discriminate changes in flux of a factor of 5 in order to identify reliably the source signal, and therefore be unnecessary to go to great lengths to reduce errors. in respect of gas flux measurements, considerable attention has to be given to the estimation of plume transport speeds. it is widely acknowledged that uncertainty in plume speed (typically 20-40%) represents the major contribution to errors in measurements of so2 and other gas fluxes. a consistent approach to constraining plume speed is therefore essential. if plume speeds are poorly constrained there is little point in engineering spectrometer sensitivity to improve retrievals by only a few per cent. absorption-correlation methods using spatially distributed instruments provide one means to track plume velocity accurately (williams-jones et al., 2003). 2.4. data interpretation having secured a geochemical data-stream, the next job is to interpret it. the basic tasks are to identify volatile sources, magma-hydrotherless soluble volatiles exsolve and form separate vapour phase more soluble volatiles exsolve fresh input cooling and crystallisation fluid-rock reactions; sealing; water-soluble gases scavenged in hydrothermal system diffuse (soil) degassing h2o, co2 and so2 hydrofracturing gas escape if magma and/or conduit walls permeable meteoric water seawater rn, co2 d ecom pression fig. 2. potential physical and chemical processes occurring in a magmatic-hydrothermal system, including the influence of magma dynamics in the chamberconduit plumbing system, and interactions between magmatic fluids and the crust. these can strongly modulate the speciation and flux of various magmatic components emitted into the atmosphere, complicating the interpretation of geochemical measurements of surface emissions (from oppenheimer et al., 2003). 1460 clive oppenheimer and andrew j.s. mcgonigle mal system interactions, the dynamics of degassing, and changes in these through time. unfortunately, interpretation of the observations is far from straightforward because of the many factors that control magmatic volatile content (mantle melting, slab contributions, wall-rock assimilation, etc.), exsolution and gas separation of different volatile species from magma, and the subsequent chemical and physical interactions of the exsolved fluids, for example, with crustal rocks and hydrothermal fluids, as they ascend to the surface (fig. 2). although thermodynamical codes enable «restoration» of observed gas analyses to equilibrium compositions (e.g., gerlach, 1993; symonds et al., 1994), identifying precise magmatic and hydrothermal conditions remain real challenges. indeed, a casual perusal of the literature will reveal conflicting interpretations of similar observations for instance, decreasing so2 fluxes could be due to i) depletion of volatiles in a magma body, or ii) a decrease in the permeability of the plumbing system. process i) might indicate decreased eruption likelihood, while ii), perhaps induced by sealing of bubble networks, which would act to increase overpressure (e.g., edmonds et al., 2003c), could increase the chance of an explosive eruption. thus the same observation can be interpreted in different ways with contradictory hazard implications. advances in this area will benefit from development and validation of comprehensive physico-chemical models for volcanic degassing based on the integration of results from experiments on the controls on distribution of volatiles in synthetic and natural melts, analysis of dissolved volatiles preserved in melt inclusions, and observed volcanic gas geochemistry. ultimately, such models can be applied to integrated geophysical, geodetic and geochemical monitoring data to support eruption forecasting. 3. instrumentation over the last thirty years, ground-based optical remote-sensing techniques have been increasingly used for volcanic gas (and aerosol) monitoring (table i). such techniques possess many of the advantages of seismic and deformation monitoring over direct sampling, such as the ability to obtain measurements in reasonable safety, allowing semi-continuous montable i. overview of spectroscopic methods for ground-based optical sensing of volcanic gases. instrument transport methods volcanic gas species detectable flux measurements (yes/no) cospec car, aircraft, boat so2 yes uv grating spectrometers (e.g., ocean optics usb 2000) on foot, car, aircraft, boat so2, h2s, bro yes ftir (e.g., brucker opag 22, midac am series) car co2, co, ocs, ch4, so2, h2o, hcl, hf, sif4 yes (with sun-tracker) other ndir (e.g., li-cor co2 analysers) on foot, aircraft co2, h2 o yes (by plume profiling or ground flux surveys) dial (i.e., using atmospheric backscatter to return signal) truck, ship so2, other species feasible yes laser spectroscopy (short path, extractive) car as ftir plus isotopes no 1461 exploiting ground-based optical sensing technologies for volcanic gas surveillance itoring even through violent eruptive periods. additionally, gas concentrations may be retrieved non-invasively in near real-time, obviating the need for subsequent laboratory analysis, and eliminating the possibility of sample contamination. because remote sensing techniques measure integrated gas concentrations through cross sections of the plume, in contrast to in situ sampling, they can potentially yield a more representative picture of bulk plume composition and flux. in the following subsections, we briefly review some recent developments in optical sensing applications to volcanic gas surveillance. with the exception perhaps of the aster sensor on nasa’s terra platform, spaceborne remote sensing is unlikely, in the near future, to provide a capability for frequent measurements of weaker tropospheric volcanic plumes, e.g., sustained by quiescent degassing. we focus, therefore, on ground-based approaches that are suitable for routine monitoring applications. for a fuller review of the field, including spectroscopic determination of volcanic aerosol concentrations and properties, see mcgonigle and oppenheimer (2003). 3.1. uv spectroscopy 3.1.1. correlation mask instruments: cospec arguably, the most widely applied instrument for ground-based remote-sensing of volcanic plumes is the barringer research cospec, which was originally developed to measure industrial so2 and nox emissions (moffat and millán, 1971). over the last thirty years, openpath ultraviolet (opuv) cospec so2 measurements have been performed at numerous volcanoes worldwide (e.g., casadevall et al., 1984; stoiber et al., 1986; caltabiano et al., 1994; gerlach et al., 1998, and references therein). the cospec is typically operated by measuring the absorption of ultraviolet (uv) zenith skylight, by overhead so2 (millán and hoff, 1978). this is achieved by dispersing the collected skylight with a grating and then imaging this spectrum on to the radial length of a spinning disk, behind which is a detector. the disk has segments with etchings at radii that block light «correlating» either to wavelengths of low or high absorption (in the spectral region 300-315 nm), such that the output signal from the detector is modulated according the absorption of overhead so2. (a development of this form of correlation spectroscopy is to mount cells containing the gases of interest on a rotating chopper disk, in an analogous fashion to the cospec’s etched masks. this gas correlation filter spectrometry method has been implemented to sense volcanic co and ocs concentrations but operating in the infrared spectral region (stix et al., 1996). internal electronic processing yields so2 concentrations (in parts per million meters ppm m; see gerlach, 2003, for discussion of the problems inherent in use of these units), and calibration is achieved by placing quartz cells containing known amounts of so2 in the internal optical path of the spectrometer. fluxes are obtained by traversing underneath the plume, approximately perpendicular to its axis, in a road vehicle, boat or aircraft, and recording so2 concentration as a function of position (from a gps receiver). by integrating the concentrations across the plume and multiplying by plume speed, so2 fluxes are derived (typically expressed in tonnes per day, t d−1, or kg s−1). the cospec possesses many advantages as a tool for volcanic surveillance. as this device operates using scattered skylight, alignment is trivial and measurements are possible in overcast conditions. however the accuracy of the derived fluxes is very dependent on the adequacy of the plume speed data (stoiber et al., 1983). typically, the wind speed is obtained from distant radiosonde data, visual observations of the moving plume, or ground based anemometry. more accurate wind data can be obtained by videography, or from aircraft navigational equipment in the case of airborne traverses. a further source of error is that the diffuse skylight can be scattered into the cospec’s field of view from above, below or within the plume (millán, 1980; moffat and millán, 1971), in contrast to the flux calculation’s assumption that light passes vertically through the entire plume. scattering effects in ash-laden 1462 clive oppenheimer and andrew j.s. mcgonigle plumes can introduce further scattering related errors (andres and schmid, 2001). the cospec was originally applied in a volcanological context to assess whether changes in so2 gas fluxes could be associated with changes in eruptive activity of open conduit volcanoes (i.e. rising or falling of magmas, and sealing or opening of magma chambers). positive correlations of increasing so2 flux with activity were observed during measurements at mt. etna (malinconico et al., 1979). in 1991, the so2 flux of mt. pinatubo was observed to increase by an order of magnitude over two weeks, in parallel with seismic unrest, providing evidence for a shallow intrusion of magma, and prompting a civil evacuation (hoff, 1992; daag et al., 1996). decreasing cospec so2 fluxes in parallel with decreasing post-eruptive activity have also been observed on many volcanoes, notably at mt. st. helens from 1980 to 1988 (mcgee, 1992), following the 1980 eruption. through cospec a catalogue of fluxes from actively and passively degassing volcanoes worldwide has been obtained, from which the total volcanic so2 flux to the atmosphere has been estimated at ∼ 20 tg yr–1 (stoiber and jepsen, 1973; berresheim and jaeschke, 1983; stoiber et al., 1987; andres and kasgnoc, 1998). cospec measurements have also revealed that many volcanoes emit so2 in excess of levels that could be sustained by degassing of their erupted magmas, highlighting the socalled «excess sulfur» issue (wallace, 2001). however, interpreting cospec data can be complicated. for instance, in the past both increasing and decreasing flux signatures have been found to pre-empt changes in volcanic activity (symonds et al., 2001). furthermore, so2 fluxes are modulated by the action of hydrothermal systems (doukas and gerlach, 1995; oppenheimer, 1996; symonds et al., 2001) and during transport in the atmosphere through deposition and chemical transformations (malinconico, 1979; oppenheimer et al., 1998a). this is partly why monitoring gas ratios can be so informative. a final, more practical problem is that the cospec is no longer in routine production, and servicing and sourcing replacement parts is becoming increasingly costly and difficult. 3.1.2. broad band measurement and spectral analysis a potential drawback of the cospec is its «black box» nature. the original design of the cospec was very much engineering-oriented, with the goal to build a system capable of minimizing all «noise» (i.e., other atmospheric absorptions, etc.) to deliver information on just one species (e.g., so2). the result is an instrument that yields an estimate of the column amount of so2 in the field of view. while this greatly simplifies data retrieval and processing, it makes it difficult to assess potential errors that arise from wavelength shifts, scattering and solar elevation effects, thermal and mechanical distortions, etc. also, the cospec response depends on the mask used and the concentration of gases present. an alternative approach is to measure broad band spectra with sufficient spectral resolution to be able to model trace gas concentrations. this apfig. 3. simple configuration of instrumentation for opuv measurements using mimniature spectrometer (a) connected by fibre optic cable (b) to telescope (c). gps receiver (d) provides continuous tracking to locate all spectra saved to laptop computer via usb cable. the ensemble could be further reduced in size and weight by replacing the laptop computer with a palm top unit. a reflective screen is desirable for visibility and enhanced battery life. a) b) c) d) 1463 exploiting ground-based optical sensing technologies for volcanic gas surveillance proach is often termed differential optical absorption spectroscopy (doas; platt, 1994). the first volcanological doas observations were performed at sea from 1992 to 1997 (edner et al., 1994; weibring et al., 1998), to measure the so2 emitted by etna, stromboli and vulcano. in general, the scattered skylight is collected using a vertically pointing telescope, and coupled into the spectrometer with an optical fibre, i.e., the same opuv approach used for cospec. direct solar uv (i.e., sun occultation) is also used in doas observations, which has the advantage of simplifying the radiative transfer problem. light is generally spectrally dispersed using a grating, and spectra are collected using a ccd array (or photomultiplier tube and scanning mechanism) outside the plume, through the plume, and with light blocked from entering the spectrometer (to read the «dark current»). in the retrieval process the latter spectra are subtracted from the former two types in order to reduce instrumental noise effects. all the «plume spectra» are then divided by an out-of-plume «background» spectrum, in order to reduce interferences caused by background atmospheric absorption and the solar spectral structure (fraunhofer lines). the logarithm of the result is taken (along with highand low-pass filter stages), following beer’s law (2.1), and then so2 concentrations are derived by scaling a reference spectrum of known column amount over the ∼ 303-315 nm fitting region to match the observed spectrum. the instrument used on the italian volcanoes was comparatively bulky. more recently, in 2001, volcanological field tests were carried out with a commercially available, miniature ultraviolet spectrometer (fig. 3; galle et al., 2003). this instrument is considerably smaller, lighter, cheaper, and lower in power consumption, than the cospec (galle et al., 2003) but the performance is comparable. side-by-side intercomparisons with a cospec at soufrière hills volcano (montserrat) and mt. etna revealed good correspondence between the two instruments (galle et al., 2003; mcgonigle et al., 2003). given the favourable characteristics of the smaller device, it is already attracting interest in the volcanological research community as a replacement technology for cospec. because of its genuine portability (rather than «transportability», which better describes many other devices sold on the market with claims to be «portable»), so2 flux measurements can even be obtained by traversing beneath the plume on foot (fig. 4). this opens up the possibility of measurements at vol0 50 100 150 200 250 300 350 400 450 500 1:59 2:01 2:04 2:07 2:10 2:13 2:16 2:19 ut /hh:mm 1 2 3 cell 3 4 5 6 fig. 4. example of raw data collected in real-time by walking traverse at meakan-dake volcano, japan, using equipment shown in fig. 3. the first excursion shows a so2-filled quartz cell placed in front of the telescope. six backand-forth traverses beneath the plume, a few tens of metres from the source, follow. the timescale is subsequently corrected using the gps log to a distance scale perpendicular to the plume transport direction. this provides the column cross section of so2, which is then multiplied by plume speed to yield flux. in this case the so2 emission amounts to around 40 g s-1, highlighting the capability to measure very low fluxes from individual fumaroles. 1464 clive oppenheimer and andrew j.s. mcgonigle canoes that lack suitable roads or other forms of vehicular access, with a minimum of logistical support, and avoiding the high costs of airborne operation (mcgonigle et al., 2002). the miniature uv spectrometer can be readily adapted for scanning measurements, whereby the field of view is manually turned through the plume (e.g., from horizon to horizon; mcgonigle et al., 2002. it has also been configured for automated fixed position flux measurements, by way of a 45º turning mirror rotated by a stepper motor (galle et al., 2002; mcgonigle et al., 2003; edmonds et al., 2003a). a particularly promising development is the installation of a number of automated and telemetered plume scanners by the montserrat volcano observatory (mvo) for soufrière hills volcano. the use of multiple instruments in different locations helps in observing the plume under varying wind directions, and to permit crude, tomography-style retrieval of range-resolved so2 concentrations. this system is now delivering high temporal resolution (every few minutes) sustained so2 flux data during daylight hours (edmonds et al., 2003a). such data will provide unprecedented opportunities for the cross-correlation of timestamped geodetic, seismic and geochemical data streams, promising new volcanological insights into degassing and magma dynamics. by virtue of recording spectra, broadband uv spectroscopy has the capability of detecting multiple gas species. measurements of h2s/so2 ratios have recently been accomplished at vulcano, italy using the miniature uv spectrometer in a closed-path configuration (i.e., with a fixed path-length cell and artificial uv source; o’dwyer et al., 2003). the same instrument, applied to sky opuv measurements at montserrat, yielded so2/bro ratios in the volcanic plume (the bro being the oxidation product of volcanic hbr; bobrowski et al., 2003). 3.2. non-dispersive infrared (ndir) spectroscopy as discussed, it is desirable, in many cases, to measure not only so2 but other volcanic gas species. this has been one of the great benefits of the conventional direct sampling approach to gas geochemistry, since many species can be measured down to ppt levels using laboratory analytical techniques. of particular relevance, the availability over the last ten years or so of field portable, robust fourier transform infrared (ftir) spectrometers has extended the capabilities of remote sensing of volcanic gases in this direction. these devices operate across the socalled fundamental region of the infrared spectrum, providing access to the rotation-vibration absorption features of, among other species, hcl, h2o, so2, hf, co2, sif4, ocs and co. ftir spectrometers are based on michelson interferometers, in which incoming light is split into two beams using an optical beam-splitter, which also recombines these beams after they are reflected at mirrors. one of the mirrors in scanned back and forth along the axis of its beam, introducing a variable path difference between the two beams, resulting in a time-variable signal from the single broad-band detector due to interference of recombined beams. application of an inverse fourier transform to the temporal signal yields spectra, which are analysed (using radiative transfer models or by ratio-ing in-plume and out-of-plume spectra) in order to determine the concentrations of volcanic gases absorbing in the optical path. a range of ir light sources: direct sunlight, fire fountains, artificial ir lamps, and hot rocks have been used in volcano ftir surveillance highlighting the flexibility of the approach to adapt to the circumstances of activity, access and terrain. love et al. (1998, 2000) have shown it is also possible to measure volcanic gases in emission (i.e., their emission lines rather than their absorption lines) against a cold sky background. at popocatépetl volcano, mexico, love et al. (1998) observed a steady increase in sif4/so2 ratio prior to an eruption in february 1997, followed by a tenfold decrease within a few hours. these results suggested a cooling of the gas prior to the eruption, attributed to adiabatic gas expansion on release of a conduit plug. the first ftir volcanic gas spectroscopy was carried out in 1991 at asama volcano, japan, (notsu et al., 1993), by japanese researchers who have subsequently reported measurements at unzen, japan (so2 and hcl; mori et al., 1993), aso, japan (co, ocs, co2, so2 and 1465 exploiting ground-based optical sensing technologies for volcanic gas surveillance hcl; mori and notsu 1997), and vulcano, italy (so2 and hcl; mori et al., 1995). the co/co2 ratio obtained at aso was used to constrain fumarole temperatures. further developments have been undertaken by a uk-based group (francis et al., 1995; francis et al., 1996; oppenheimer et al., 1998c, 2002; francis et al., 2000). the last of these works permitted plume temperature estimates based on hf/sif4 ratio obtained at vulcano. later work on mt. etna resulted in the first ftir spectroscopy of volcanic plumes by solar occultation (francis et al., 1998). since 1998, this group has carried out annual campaigns at masaya volcano, nicaragua, where high volcanic gas concentrations permitted measurements of volcanic co2 and h2o, in spite of these species’ high ambient concentrations (burton et al., 2000). measurements at masaya indicated consistent so2/hcl and hcl/hf molar ratios of 1.6 and 5, respectively, during 1998-2000, indicating steady-state, open system degassing (horrocks et al., 1999). in contrast, in 2001 a so2/hcl ratio of 4.5 was found, coinciding with reduced so2 fluxes, descent of the magma column in the vent on the crater floor, and preceding a small explosive eruption on the 23rd april (duffell et al., 2003). traverse flux measurements of hcl have also been obtained by operating the ftir spectrometer with a solar tracker, in contrast to the usual method of combining opuv-derived so2 fluxes with so2/hcl ratios from ftir spectroscopy or direct sampling (duffell et al., 2001). ftir measurements at soufrière hills volcano, montserrat have indicated that hcl/so2 molar ratios of 1-5 typify dome building episodes, and that lower ratios (down to 0.1) characterise non-eruptive periods (oppenheimer et al., 1998d, 2002; edmonds et al., 2001, 2002, 2003b). this behaviour has been explained in terms of an andesitic hcl source that exsolves on ascent from the magma chamber, and a deeper so2 reservoir (probably derived from intruded mafic magma) that degasses to the atmosphere discontinuously, depending on the plumbing system’s permeability. based on this interpretation, edmonds et al. (2002) have identified the potential degassing signals that might herald the end of this eruption. oppenheimer et al. (1998b) demonstrated the application of ftir spectroscopy from a helicopter at distances of ≈ 100 m from the lava dome. the first routine ftir spectroscopic volcano surveillance programme has been running since april 2000, at mt. etna, under the auspices of the istituto internazionale per geofisica e vulcanolgia (ingv; burton et al., 2003). the results obtained to date serve as an excellent advertisement for the value of ftir spectroscopy in complementing other surveillance efforts (e.g., geodesy, seismology and petrology), and in supporting monitoring efforts during volcanic crises (calvari, 2001). in particular, a doubling of the so2/hcl ratio was observed prior to the 2001 mt. etna eruption and clear geochemical trends in so2/hcl, co2/so2, and hcl/hf for different active vents at different elevations through the course of the eruption. these kinds of data would have been impossible to collect in real time by any conventional technique. while the operation of ftir spectrometers is relatively straightforward, processing the spectra and retrieving gas column amounts requires some expertise. representing another class of ndir spectrometer in volcanological use are the instruments manufactured by li-cor. these are generally dual-wavelength closed-path analysers, and have been used to measure both diffuse co2 emissions from the ground (mcgee et al., 2000), and co2 fluxes by in-plume sampling (gerlach et al., 1998). a third application of li-cor is based on eddy correlation or covariance, which involves measurements of vertical windspeed and co2 concentration (e.g., anderson and farrar, 2001). 3.3. laser techniques to date, the most commonly applied laser technique for volcano measurements is lidar, in which a pulsed laser beam is directed towards the plume. recording the temporally varying intensity of backscattered light provides information about the atmospheric composition as a function of propagation distance along the beam’s path. lidar has been used to measure concentrations and fluxes (via traverses) of sulfate aerosol (casadevall et al., 1984; edner 1466 clive oppenheimer and andrew j.s. mcgonigle et al., 1994; porter et al., 2002), and ash (hobbs et al., 1991). parallel gas sampling and aerosol measurements can enable estimation of gas to particle conversion rates (e.g., for so2 to so4 2stith et al., 1978; radke, 1982; rose et al., 1986). although studies of anthropogenic plumes indicate so2 to so4 2conversion rates of a few % h–1 in the lower troposphere (eatough et al., 1994), very few comparable investigations of volcanic plumes (which can have widely varied gas, ash and liquid water contents) have been undertaken (e.g., oppenheimer et al., 1998a; rose et al., 2001; horrocks et al., 2003). a variation on lidar, known as differential absorption lidar (dial) involves rapid switching the frequency of laser pulses onand off-resonance of an absorption feature of the gas of interest. by dividing the lidar curves (returned signal versus height) obtained at the two wavelengths and applying the beer-lambert law, range-resolved gas concentrations (ppm) may be derived providing 2d or 3d plume structure, in contrast to the pathlength integrated concentrations (ppm m) obtained from ftir, cospec and doas. the technique has been applied to the southern italian volcanoes using uv lasers (edner et al., 1994), in parallel with cospec and uvdoas instruments, revealing so2 concentrations up to 50% higher in the former case as a consequence of scattering-induced errors in the passive techniques. however, this dial apparatus was costly, heavy and bulky. whilst dial offers unique capabilities for volcanology, it requires further innovation in order to become a suitable tool for routine observatory use. alternative sensing strategies using near and mid-infrared diode based lasers (gianfrani et al., 2000; de natale et al., 2001; richter et al., 2002) have been evaluated by monitoring the laser’s absorption following numerous transits of a multipass cell, into which the volcanic gas sample is pumped. due to the very narrow linewidths lasers can provide, the most promising application of these techniques is in spectrally resolving between isotopes to provide infield isotope ratios. whilst only gas ratio measurements have been realised to date in the field, richter et al. (2002) and weidmann et al. (2003) have described a mid-ir laser system capable of measuring all isotopes of co2, while gianfrani et al. (2003) have presented a diode laser spectrometer able to measure water isotopes in the near-ir. an additional laser based technique that shows potential for field isotope measurements is photoacoustic spectroscopy, in which the sound waves, generated by resonant laser light exciting the target species, are measured (meyer and sigrist, 1990). 4. concluding remarks surveillance of gas composition and flux are essential for interpretation of volcanic activity, since the nature of degassing exerts a strong control on eruption style, and is closely associated with volcano seismicity and ground deformation. new optical remote sensing techniques are emerging for the monitoring of volcanic emissions such as the miniature ultraviolet spectrometers described by galle et al. (2003). these tools and their data-streams have the potential for full automation and telemetry, fast processing and evaluation, closer integration with other monitoring data-streams (i.e., seismic, geodetic, etc.), and more sophisticated modelling and interpretation. while instrument developments are always to be encouraged, we believe that the state of the art is already adequate to justify the proliferation of both ir and uv spectroscopic tools amongst the volcano observing community. this is beginning to happen, with ftir spectroscopy routinely applied to monitoring of the southern italian volcanoes by ingv, and uv methods used on montserrat by mvo. in addition to the immediate monitoring and hazard evaluation goals of such institutes, sustained surveillance programmes promise the most significant science gains in the future, as geochemical, geodetic and seismic datastreams are further integrated, cross-correlated, and modelled. as we introduce tools that will replace or supplement other methods, it is crucial that we maintain continuity between old and new datasets. this demands the direct comparison and intercalibration of the various gas geochemical tools via laboratory and field experimentation, 1467 exploiting ground-based optical sensing technologies for volcanic gas surveillance as carried out periodically by the iavcei commission on volcanic gases. additionally, it helps to conform to internationally recognized data standards. one attractive development in this respect is the «wovodat» concept of the world organisation of volcano observatories, which aims to establish standardised units and formats for surveillance data so that they may be brought together in a unified, digital database, accessible via the web and linked with the smithsonian institution’s unique database of historical eruptions. a further important benefit of proliferating optical sensing techniques capable of gas flux measurements is that this will lead to more accurate, and time-resolved estimates of the global emission of volcanic volatiles to the atmosphere. acknowledgements we gratefully acknowledge research support from the gruppo nazionale per la vulcanologia (project «development of an integrated spectroscopic system for remote and continuous monitoring of volcanic gas»), the european commission 5th framework programme (projects «multimo; http://earth.leeds.ac.uk/~aj/multimo/» and «dorsiva»), and the uk natural environment research council project «field laser spectroscopy of volcanic gases and their isotopes» and research fellowship awarded to ajsm. this work originates from the tremendous workshop-short course on volcanic systems held in seiano (napoli) in september 2002 «geochemical and geophysical monitoring: melt inclusions: methods, applications and problems» organized by b. de vivo, r.j. bodnar, e. boschi and g. macedonio. we thank especially our colleagues in italy with whom we have worked over the past years: mike burton, tommaso caltabiano, sonia calvari, giovanni chiodini, salvatore inguaggiato, filippo murè, paolo de natale, pino de natale, alessandra rocco and giuseppe salerno. references anderson, d.e. and c.d. farrar (2001): eddy covariance measurement of co2 flux to the atmosphere from an area of high volcanogenic emissions, mammoth mountain, california, chem. geol., 177, 31-42. andres, r.j. and a.d. kasgnoc (1998): a time-averaged inventory of subaerial volcanic sulfur emissions, j. geophys. res., 103, 25251-25261. andres, r.j. and j.w. schmid (2001): the effects of volcanic ash on cospec measurements, j. volcanol. geotherm. res., 108, 237-244. baxter, p.j., r.e. stoiber and s.n. williams (1982): volcanic gases and health: masaya volcano, nicaragua, lancet, 2, 150-151. berresheim, h. and w. jaeschke (1983): the contribution of volcanoes to the global atmospheric sulphur budget, j. geophys. res., 88, 3732-3740. bobrowski, n., g. hönninger, b. galle and u. platt (2003): detection of bromine monoxide in a volcanic plume, nature, 423, 273-276. burton, m.r., c. oppenheimer, l.a. horrocks and p.w. francis (2000): remote sensing of co2 and h2o emission rates from masaya volcano, nicaragua, geology, 28, 915-918. burton, m., p. allard, f. murè and c. oppenheimer, (2003): ftir remote sensing of fractional magma degassing at mt. etna, sicily, in volcanic degassing, edited by c. oppenheimer, d.m. pyle and j. barclay, geol. soc., london, spec. publ. 213, 281-293. caltabiano, t., r. romano and g. budetta (1994): so2 flux measurements at mt. etna, j. geophys. res., 99, 12809-12819. calvari, s. 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(in review). a p r o p o s a l f o r c o m b i n e d e f f o r t s r e g a r d i n g g e o p h y s i c a l r e s e a r c h a n d d e t e c t i o n o f g r a v i t a t i o n a l w a v e s v . d e s a b b a t a p . f o r t i n i c . g t t a l d i s . p e t r a l i a ( * ) r i c e v u t o il 28 f e b b r a i o 1970 summary. — t h e possibility of s t u d y i n g t h e e a r t h f r e e oscillations w i t h a laser i n t e r f e r o m e t e r is considered. i n p a r t i c u l a r t h e p r o b l e m of d e t e c t i o n of g r a v i t a t i o n a l w a v e s is a n a l y s e d in detail. t h i s p r o b l e m seems t o be a v e r y p r o m i s i n g m a t t e r owing t o t h e f a c t t h a t w i t h a t w o m o d e he-no laser one m a y resolve c h a n g e s in l e n g t h d o w n t o 10~u cm. o t h e r m e t h o d s t o r e v e a l g r a v i t a t i o n a l w a v e s are m e n t i o n e d . riassunto. — vengono prese in e s a m e le p o s s i b i l i t à offerte d a un i n t e r f e r o m e t r o laser p e r lo s t u d i o delle oscillazioni libere della t e r r a . i n p a r t i c o l a r e viene considerato il p r o b l e m a della rivelazione delle onde gravit a z i o n a l i ; la cosa s e m b r a assai p r o m e t t e n t e s o p r a t u t t o se si tiene c o n t o che con u n laser h e n é a d o p p i a f r e q u e n z a , si possono a p p r e z z a r e spostam e n t i dell'ordine di io-11 c m . vengono a c c e n n a t i a n c h e altri possibili m e t o d i di rivelazione delle o n d e g r a v i t a z i o n a l i . w e b e r f1) c l a i m s t o h a v e d e t e c t e d g r a v i t a t i o n a l r a d i a t i o n i n t h e 1 6 0 0 h z b a n d ; b e c a u s e of t h e e x t r e m e i m p o r t a n c e of t h i s e x p e r i m e n t a n d t h e s i g n i f i c a n t c o s m o l o g i c a l i m p l i c a t i o n s , i t is i n d i s p e n s a b l e t o c h e c k t h e e x i s t e n c e of g r a v i t a t i o n a l w a v e s i n d i f f e r e n t c o n d i t i o n s a n d w i t h d i f f e r e n t m e t h o d s a n d t e c h n i q u e s . h a v i n g t h i s i n m i n d w e p r o p o s e d a t t h e b e g i n n i n g of 1 9 6 8 t o c . n . r . a n o u t l i n e of a n e x p e r i m e n t t h a t , b e s i d e s g i v i n g v a l u a b l e i n f o r m a t i o n s o n t h e soil p r e s e r v a t i o n , a l l o w s u s t o s t u d y t h e e a r t h f r e e o s c i l l a t i o n s a n d t h e p o s s i b i l i t y t h a t s o m e of t h e s e m a y b e d u e t o t h e i n t e r a c t i o n w i t h g r a v i t a t i o n a l r a d i a (*) i s t i t u t o nazionale di fisica nucleare i s t i t u t o di fisica u n i v e r s i t à di bologna. sezione di bologna v . d e s a b b a t a p . f 0 r t i n i c . g u a l d i s . p f . t r a l i a tion. this project has been a t last approved and we hope to s t a r t doing practically as soon as possible. our a p p a r a t u s consists of a laser interferometer which iillows to measure the soil deformations and which, in t h e ultimate model, should have an e x t e n t of the order of a kilometer. as previously said, this instrument, besides observing the microseismic activity, the deformation of the e a r t h crust, slower movements due to earthly tides a n d f a u l t y movements, would be also utilized to measure free quadrupole oscillations of the e a r t h connected with t h e detection of gravitational waves. to discriminate f r o m seismic waves one needs to accumulate d a t a over a long interval of time and to look for fourier components a t t h e presumed source frequencies. also t h e s t u d y of the decay t i m e of the oscillations constitutes a tool to discriminate between waves of seismic and gravitational origin. the i n s t r u m e n t m a y be a ile-ne laser operating a t a wave length of 6328 a . w i t h this kind of an i n s t r u m e n t yali a n d bostrom ( 2 ^ 4 ) have been able to reach a sensitivity of al/l~ 6-10 1 2 and to reveal microseismic activity of a period of 6 sec and amplitude from 500 to 1500 a. the instrument noise on this t y p e of measurements arises f r o m : a) optical p a t h length changes by variation of v a c u u m in pipe, length change of pipe due to atmospheric pressure changes or temp e r a t u r e changes. b) drift in the i n s t r u m e n t piers. c) laser frequency instability induced by changes of our f a b r y p e r o t reference cavity, as usual disposed for frequency stabilization. d) fluctuations of the number of photons entering t h e detectors. with a laser interferometer we can hope to measure extremely small strains e (where e = al/l) a n d we hope to increase the sensitivity of the i n s t r u m e n t with the use of a double frequency laser. recently eichler and wiesemann (5) have prospected the possibility of appraising a length shift of 10~3 a with a two-mode he—ne laser in an optical resonator coupled to the laser, a n d herzinger and lindner (6) have proposed a laser device to resolve changes in length down to 10~n cm. besides this we are planning to m a t c h the experiment with other experiments in coincidence, carried out with totally different methods. dyson (') gives an estimate of the displacement al which would be measured on the e a r t h crust if t h e flux of incomimr eravitational a p r o p o s a l f o r c o m b i n e d e f f o r t s r e g a r d i n g g e o p h y s i c a l , e t c . 2 3 waves, iii t h e 1 h z f r e q u e n c y b a n d , is of t h e order of c3 w2 h 2 f = — — — 7 = 2 • 10-5 erg c n r ! sec-1 . 64 7ig e h e gives f o r t h e d i s p l a c e m e n t al: al ~ 2 • 10~17 cm . this m e a n s t h a t a t t h e p e a k d i s p l a c e m e n t t h e strain is al s ~ 2 — = 1 0 ^ , a q x = s being t h e w a v e l e n g h t c o n n e c t e d w i t h t h e s o u n d velocity i n v e a r t h i n t e r i o r t h a t is cs = 4000 m / s e c . this figure m a y b e i m p r o v e d b y m a n y o r d e r s of m a g n i t u d e : as a m a t t e r of f a c t we m a y t a k e f o r t h e e a r t h a q of ~ 400 as g i v e n b y geophysical e s t i m a t e s (see f o r i n s t a n c e c a p u t o (8)) i n s t e a d of q = 1 implied b y d y s o n ' s e s t i m a t e , w h e r e q = e/ae is t h e q u a l i t y f a c t o r . moreover t h e flux of gravit a t i o n a l w a v e s on t h e 1 h z b a n d m a y b e g r e a t e r t h a n t h a t a s s u m e d b y d y s o n . i n f a c t , f o r i n s t a n c e , t h e flux revealed b y w e b e r (!) in t h e 1 k h z f r e q u e n c y b a n d , t h a t is f ~ 3 • 10 1 erg cm-= sec;-1 , is m a n y order of m a g n i t u d e g r e a t e r t h a n t h a t foreseeable on t h e g r o u n d b o t h of a n y k n o w n m o d e l of t h e u n i v e r s e a n d of e s t i m a t e s which t a k e i n t o a c c o u n t possible sources of g r a v i t a t i o n a l w a v e s . i f , f o r e x a m p l e , we a s s u m e a flux of 3 1 0 1 erg errr 2 sec 1 also in t h e 1 h z b a n d , t h a t is f o u r o r d e r of m a g n i t u d e g r e a t e r t h a n t h a t a s s u m e d b y d y s o n , r e m e m b e r i n g t h a t e ~ g2 b'oto qjm", where k'oio is t h e c u r v a t u r e t e n s o r c o m p o n e n t which drives a gravit a t i o n a l w a v e d e t e c t o r a n d is p r o p o r t i o n a l t o t h e s q u a r e r o o t of t h e flux b'oto oc \jf , t h e n t h e s t r a i n , w i t h t h e s e new e s t i m a t e s of q a n d f, is e ~ 7-101 8 , a n d f o r t h e p e a k d i s p l a c e m e n t we h a v e : al ~ 1.4-10~12 cm . w e t h i n k of i m p r o v i n g f u r t h e r these figures using v e r y large masses, well isolated f r o m t h e e a r t h , a n d t h e laser as a t r a n s d u c e r t o 24 v . d e s a b b a t a p . f o k t i n i c. g u a l d i s . p e t r a l i a detect their relative motion or, alternatively, t h e oscillations of a single large mass. i n this case we m a y h a v e a quality f a c t o r q ~ 106 a n d t h e strain would be e ~ 2 1 0 " , with al ~ 4-109 cm. b u t , as we h a v e said, t h e best would be to h a v e some other device which, exploiting t o t a l l y different principles, gives a measure of t h e g r a v i t a t i o n a l waves flux: in t h i s case we can look for coincidences between t h e two a p p a r a t u s . this new a r r a y will be f o u n d e d on t h e i n t e r a c t i o n between gravit a t i o n a l waves a n d a s t a t i c m a g n e t i c held. l u p a n o v (9) has calculated t h e change of a s t a t i c electric field of a capacitor in t h e field of a gravit a t i o n a l wave. w e h a v e m a d e a calculation (10) along t h e same line, considering a m a g n e t o s t a t i c field parallel to t h e direction of propagation of t h e g r a v i t a t i o n a l wave direction). between t h e surfaces of t h e polar expansions on t h e m a g n e t i we h a v e a s t e a d y electrom a g n e t i c wave: ey = — ez = e^x a cos s sin <5 e<*°+~/2> sin h x where e^°>x is t h e static m a g n e t i c held, a is t h e a m p l i t u d e of t h e incident g r a v i t a t i o n a l wave, linked to the flux b y : c 3 w 2 u | 2 f — ---64 n g d is t h e angle between t h e « a x i s a n d t h e direction of oscillation of t h e source of t h e g r a v i t a t i o n a l wave. the ratio between t h e change of t h e m a g n e t i c field and t h e static m a g n e t i c field, in t h e case of a flux f ~ 3 1 0 1 erg c m 2 sec 1 is: ez ey = = 10 10 . e^x e^x the case of a g r a v i t a t i o n a l wave i n t e r a c t i n g with a dipole m a g n e t i c field is being studied. this case seems to be promising to see if t h e r e is a n y relation between periodic micropulsations of t h e geomagnetic field and incident g r a v i t a t i o n a l waves. a p r o p o s a l foll c o m b i n e d e f f o r t s r e g a r d i n g g e o p h y s i c a l , etc. 2 5 r e f e r e n c e s ( ! ) w e b e r j . , " p h y s . r e v . l e t t e r s " 22, 1320 (1909). ( 2 ) v a l i v . b o s t r o m r . c., " e a r t h a n d p l a n e t a r y science l e t t e r s 4 . 4 3 6 ( 1 9 6 8 ) . ( 3 ) v a l i v . b o s t r o m r . c., " n a t u r e " , 2 2 0 . d e c e m b e r 1 9 6 8 . ( 4 ) v a l i v . b o s t r o m r. c., " r e v . of scient. i n s t r . " 3 9 , 1304 (1968). ( 5 ) e i c h l e r h . w i e s e m a n n w . , " z e i t s c h r i f t a n g e w . p h y s . " , 2 8 , 129 (1969). ( 6 ) i i e r z i n g e r g r . l i n d n e r i i . , " p h y s i c s l e t t e r s " , 2 4 a , 6 8 4 ( 1 9 6 7 ) . ( ' ) d y s o n f . j . , " a s t r o p h . j o u r n . " , 1 5 6 , 5 2 9 ( 1 9 6 9 ) . ( 8 ) c a p u t o m., " e l a s t i c i t à e dissipazione " , zanichelli (1969). (») l u p a n o v c. a., " z u r n . e k s p . teor. fiz. " , 52, 118 (1967); " sov. pjiys. j e t p " , 2 5 , 7 6 ( 1 9 6 7 ) . ( 1 0 ) b o c c a l e t t i d . d e s a b b a t a v . f o r t i n i p . g u a l d i c., " n u o v o cim e n t o " ( 1 9 7 0 ) in press. geo 1109 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 10 planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability david altadill (1), emil m. apostolov (2), josef boška (3), jan laštovička (3) and petra šauli (3) (1) observatorio del ebro (url-csic), roquetes, spain (2) geophysical institute, bulgarian academy of sciences, sofia, bulgaria (3) institute of atmospheric physics, academy of sciences of czech republic, prague, czech republic the aim of this work within the wp 3.1 of the cost 271 action is the characterization of the variability introduced in the f-region ionosphere by ‘planetary wave signatures’ (pws) and ‘gravity wave signatures’ (gws). typical patterns of percentage of time occurrence and time duration of pws, their climatology and main drivers, as well as their vertical and longitudinal structure have been obtained. despite the above characterization, the spectral distribution of event duration is too broad to allow for a reasonable prediction of pws from ionospheric measurements themselves. gws with a regular morning/evening wave bursts and specific gws events whose arising can be predicted have been evaluated. as above, their typical pattern of occurrence and time duration, and their vertical structure have been obtained. the latter events remain in the ionospheric variability during disturbed days while additional wave enhancements of auroral origin occur. however, both types of disturbances can be distinguished. 10.1. introduction interest in the ionospheric f-region variability has increased recently (forbes et al., 2000; rishbeth and mendillo, 2001; mendillo et al., 2002), partly due to the role that the ionosphere plays in the earth’s environment via coupling processes from above and below, the former being a significant part of the space weather. the variability of the ionospheric f-region ranges from time-scales of minutes (e.g., tid’s) to long-term changes (solar cycle variations and even secular). table 1 from rishbeth and mendillo (2001) summarizes the possible causes of the ionospheric f-region variability. it is widely accepted that the geomagnetic activity is the main cause of the ionospheric variability, although meteorological causes transmitted from below may contribute comparatively. many of the «meteorological influences» on the ionosphere arise from upward propagating gravity, tidal and planetary waves (kazimirovsky et al., 2003). here we discus the main results on ‘planetary wave signatures’ (pws) and ‘gravity wave signatures’ (gws) as sources of the variability of the f-region ionosphere obtained within the cost 271 action. it should be mentioned that we also studied the effects of gravity and planetary waves on the lower ionosphere (boskova and laštovička, 2001; laštovička, 2001; laštovička et al., 2003c). the pws in the f-region with an emphasis on their pos1110 david altadill, emil m. apostolov, josef boška, jan laštovička and petra šauli sible sources, time duration and time occurrence patterns, their typical longitudinal size, and their climatology are presented in section 10.2. section 10.3 deals with the gws of non-auroral origin in the f-region with an emphasis on their regular occurrence and on other specific events. the last section highlights the most relevant conclusions of our work within the cost 271 action. 10.2. planetary wave signatures on the f-region the planetary waves (pw) in the mesosphere/lower thermosphere (mlt) (salby, 1984) may indirectly reach the f-region ionosphere and drive pw effects there. the pw modulation of upward propagating tides or gravity waves, the interaction of pw with the strong diurnal/semidiurnal dependence of the ionosphere, and the transmission of pw activity via vertical plasma drift or mean vertical mass transport may explain the arising of pw effects in the f-region from the pw activity in the mlt (laštovička and šauli, 1999; pancheva et al., 2002; altadill and apostolov, 2003; and references therein). experimental results show also the vertical propagation structure, from above and below, of such oscillation activity in the f-region (altadill and apostolov, 2001; altadill et al., 2001b). the latter case studies assess that pw effects in the f-region originated from the pw activity in the mlt propagate upwards. however, these investigations stated that a «quasi-periodic» sequence of geomagnetic storms and sub storms and/or large storms can generate wave-like oscillations in the ionosphere as well. hereafter we call ‘planetary wave signatures’ (pws) both effects, the oscillation activity of the ionosphere driven by the pw activity in the mlt and geomagnetic activity. the pws discussed here have periods of about 2, 5, 10, 13.5, and 16 days, these pws being an important source of variability of the ionosphere – they may contribute up to 50-60% of the variability of the f 0f2 in the periodic range 2-35 days (apostolov et al., 1998). 10.2.1. time duration and time occurrence patterns an important input to cost 271 is the evaluation of the persistence of pws in f 0f2 over europe (laštovička et al., 2003a). they found a typical duration of 4 cycles for the 5-day events. the typical duration being of about 3.5 and rather 3 cycles for the 10-day and 16-day events, respectively. so, in terms of numbers of cycles pws with longer period of oscillation have shorter duration, but they have longer duration in terms of time. however, the spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration (fig. 10.1a), which means that pws-type oscillations remain to be unpredictable noise in ionospheric predictions until a reasonably reliable combination of external predictors will be found. laštovička et al. (2003b) found quite similar results for stations at middle latitudes in japan and u.s.a. by taking into account the average values of event duration and numbers of events found by laštovička et al. (2003a), one can conclude that the 5-day, 10-day and 16-day events manifest themselves during about 16%, 26% and 34% of the time respectively. altadill and apostolov (2003) confirmed the latter results at midlatitude global scale and that 2-day and 13.5-day events manifest themselves during about 12% and 22% of the time, respectively (fig. 10.1b). 10.2.2. longitudinal size pattern altadill et al. (2001a, 2003) evaluated the zonal structure of pws in some case studies, and they showed that pws are large-scale phenomena in the f-region. the typical longitudinal size of such events in the mid-latitude f-region of the northern hemisphere has been obtained recently (altadill and apostolov, 2003). they did not distinguish between traveling and standing waves and did not 1111 planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability fig. 10.1a,b. the top plot shows an example of the typical duration of pws in terms of number of cycles for the 5-day events (after laštovička et al., 2003a). the bottom plot depicts the typical percentage of occurrence of pws events according to their dominant oscillating periods (after altadill and apostolov, 2003). fig. 10.2. the plots shows the percentage of occurrence of pws events (y axes) referred to the number events found to exist in sector 1 as function of their longitudinal size (x axis). the horizontal error bars mean the longitudinal size of the labeled midlatitude sector. the horizontal lines of each plot indicate the assumed threshold for the typical longitudinal size (after altadill and apostolov, 2003). 10.2 consider the possibility that standing oscillations would not necessarily be seen at a given single sector if the minima in the wave were present in that sector. they compared the coherent pws at consecutive midlatitude longitudinal sectors and their result may be summarized as follows (fig. 10.2). the pws with periods of 2-3 and 5-6 days have a typical longitudinal size of 80°. although they can be observed on a larger zonal scale, it is quite unlikely. pws with periods of 10 and 13.5 days have a typical longitudinal size of 100°, they being likely observed on a larger zonal scale. pws with a period of 16 days have a typical longitudinal size of 180°, they being likely observed at global scale. to some extent the above results contradict the estimation of the scale size of the ‘meteorological influences’ reported by mendillo et al. (2002), who find that these variations are coherent some 2500 km apart only. by considering pws as a part of the ‘meteorological influences’ on the ionospheric variability, the above results indicate that pws in the f-region ionosphere are large scale phenomena (note that a longitudinal scale of 80° at latitude of 50° means a distance of 6000 km), having larger zonal scale for pws with longer oscillating periods. 10.2.3. climatology and sources the typical climatology of pws in the f 0f2 at midlatitudes has been also evaluated within cost 271 action (altadill and apostolov, 2003). from their results one senses that the occurrence of pws a b 10.1 1112 david altadill, emil m. apostolov, josef boška, jan laštovička and petra šauli in the f 0f2 is better expressed during the summer half of the year (fig. 10.3). the latter is better expressed for pws with shorter period. this is not surprising because the contribution of the pws to the variability of the f 0f2 is higher also at that time (apostolov et al., 1998). however a quite singular pattern is obtained when the seasonal occurrence of pws in the f 0f2 is considered in relation to their dominant ‘driver’. according to altadill and apostolov (2003) these drivers may be the pw activity in the mlt, the geomagnetic activity, and pws events ‘independent’ from the above two, there being a very small contribution of solar flux variations to the ionospheric variability at periods of 2-30 days, see also forbes et al. (2000). these results are summarized in table 10.i. the pws related with the pw activity in the mlt tend to occur according to the climatology of the latter, pws with shorter period occur during summer half year and those with longer period occur during winter half year. an exception to that is the pws with a period of 13.5 days, which are related to the geomagnetic activity variations. the pws related with the geomagnetic activity tend to occur during summer half year. the so-called independent pws events tend to occur during summer half year also. the evaluation of the possible ‘main driver’ of the pws in the ionosphere suggests that geomagnetic activity variations play the most important role (altadill and apostolov, 2003). this is especially clear for the 13.5-day events. the geomagnetic activity variations practically drive 100% of the latter events in the ionosphere. the geomagnetic activity variations can drive at least 20% of the 2-3day events, at least 30% of the 5-6-day events, 20-70% of the 10-day events, and 25-65% of the 16day events. a likely candidate for the ‘independent’ events may be the non-linear interaction or the amplitude modulation between different pws (e.g. altadill et al., 2001b; pancheva et al., 2002). the pw activity in the mlt can drive about 20-30% of pws in the ionosphere with periods near 2-3, 56, 10, and 16 days. note that the above percentages agree with those reported by forbes et al. (2000), which refer to the ‘meteorological influences’ on the ionospheric variability. 10.3. gravity wave signatures on the f-region the atmospheric gravity waves (agw) can generate disturbances in the ionosphere, the latter being mainly driven by heating produced by electric currents, energetic particle precipitation and mechanical forces in the auroral ionosphere. other sources of agw disturbances in the ionosphere are from lowerand middle-atmosphere associated with regions of turbulence and wind shear (hunsucker, 1987; hocke and schlegel, 1996), and strong tropospheric events (šauli and boška, 2001). there are a variety of studies that show the moving solar terminator as a source of such type of disturbances (somsikov, 1995; galushko et al., 1998). moreover, several studies showed the arising of agw in the ionosphere as a consequence of solar eclipses. the latter can contribute to a gravity wave field and build up a wave front that may produce ionospheric disturbances (chimonas and hines, 1970; liu et al., 1998). solar eclipses can generate temperature disturbances in situ in the thermosphere and electron density changes in situ in the ionosphere and such disturbances will propagate as a wave (müller-wodarg et al., 1998). hereafter we call gravity wave signatures (gws) to the above discussed disturbances in the f-region with characteristics of agw. our results were obtained mainly during quiet periods of geomagnetic activity. however, some important results were obtained during campaign «hirac/solarmax» (feltens et al., 2001). as an input to the cost 271 action we will emphasize mainly on regular occurrence of gws and on specific events whose arising can be predicted. 10.3.1. specific gws events specific gws events arising in the f-region have been evaluated by preparing ionospheric campaigns of rapid sequence sounding in order to study the source-response relationships between production and 1113 planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability fig. 10.3. percentage of occurrence of pws events ( y axes) as function of month during the time interval 19832001 (adapted from altadill and apostolov, 2003). table 10.i. percentages of seasonal occurrence of the different pws in the f 0f2 according to their possible drivers. win means from november to february, equ means march, april, september and october, and sum means from may to august. mlt means pws driven by pw activity in the mlt, geo by the geomagnetic activity and ind means pws events ‘independent’ from the above two (after altadill and apostolov, 2003). pws 2-3 5-6 10 13.5 16 win 5 4 2 0 3 ind equ 4 11 0 0 3 sum 15 13 5 0 3 win 5 8 11 0 10 mlt equ 14 6 5 0 11 sum 13 7 4 0 5 win 9 3 10 25 11 geo equ 12 21 30 38 21 sum 23 27 33 37 33 loss mechanisms, and dynamics. these events are related to solar eclipses and the passage of cold tropospheric fronts (e.g., altadill et al., 1999, 2001c; boška et al, 2000; boška and šauli, 2001; šauli, 2001a,b). the main results on these gws events are related to their vertical structure, time life, degree of variability and their dominant oscillating periods (fig. 10.4). although the source of disturbances are quite different, the ionosphere reacts at very similar periods (between 60 to 80 min), their amplitudes in the plasma frequency range from 0.2 to 1.2 mhz (showing a high degree of variability), their life time is of about 4-6 cycles of oscillation (up to 6 h), and their energy propagates vertically at an average speed of fig. 10.4. vertical structure of a gws event caused by a cold front passage during 2 november 1997 as observed above ebro station (40.8°n, 0.5°e). the left plot shows the vertical phase and group velocities of the wave packet with dominant period of 75 min as function of the altitude. the right plot shows the disturbance of this event caused in the plasma frequency as function of time and altitude, where the black line simulates the energy progression of the wave with an averaged velocity of 8 ms-1 and the white lines simulate their phase progression with an averaged velocity of − 47 ms-1 (adapted from altadill et al., 1999). fig. 10.5. vertical structure of a gws event observed during solar eclipse 11 august 1999 above pr °uhonice observatory (49.9°n, 14.5°e). the plot shows the vertical phase and group velocities, time occurrence of central period of the wave packet with dominant period of 85 min as function of the altitude (adapted from šauli, 2001b). 1114 david altadill, emil m. apostolov, josef boška, jan laštovička and petra šauli about 5-10 ms-1. however, their vertical structure is quite different. whereas the gws caused from cold fronts propagates upwards in the f-region (fig. 10.4), the gws caused by solar eclipse originates at transition region between the f1and f2-layers and it propagates upwards and downwards simultaneously from that level (fig. 10.5) – see altadill et al. (2001c), šauli (2001a). thus the vertical structure of gws displays different patterns according to their sources of origin. the gws caused by tropospheric fronts are due to dynamics from lower atmospheric levels, that is why gws propagate upwards. the gws caused solar eclipses are due to the cooling/heating processes linked with the decreasing/increasing solar radiation that leads to a reduction/augment of the scale height for both plasma and neutrals. this fact causes a downward/upward motion of plasma and neutrals as well as of f1 production peak and of the transition region between f1and f2layers, that is why the source of disturbance is located in that region from where its energy propagates upwards and downwards. 10.3.2. regular occurrence of gws it was already mentioned that solar terminator acts as a source of gws in the f-region, and this is a regular phenomenon occurring at sunrise/set times. that is why the diurnal variation of the activity of gws related with that has been evaluated (boška et al., 2003). they found systematic occurrence of these events during sunrise time and lasting several hours (fig. 10.6). diurnal variations of agw activity during periods of low solar and geomagnetic activity (1996, 1997 years) show a clear enhancement during and several hours after sunrise. the observed internal agw (with main periods between 60 and 75 min) propagates with significant vertical component through ionosphere from a source lo10.4 10.5 1115 planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability 10.6 10.7 fig. 10.6. time variation of the probability of existence of a gws event with dominant period of 61 min observed above pr °uhonice station (49.9°n, 14.5°e) during 31 october 1997 (adopted from boš ka et al., 2003). fig. 10.7. altitude variation of the amplitude (left), phase velocity (middle) and group velocity (right) obtained for the wave event with dominant period of 72-min observed during october 12, 1996 in the pr °uhonice data (adopted from boš ka et al., 2003). fig. 10.8. wavelet power spectra of electron concentration variation on 24 april 2001 at 230 km with substantial intensification of wave-like activity in dawn and dusk hours at period range 70-100 min. cated at altitude of 180-220 km. vertical components (amplitude, phase and group velocity) of the typical structure with dominant period 72 min are shown in fig. 10.7. similar effects of gws were observed in a period range 50-100 min during the period of high solar activity (2000-2001). particular wave enhancements corresponding to solar terminator movement on 24 april 2001 are shown on the wavelet power spectra of the electron concentration at altitude 230 km (fig. 10.8). amplitudes of the observed oscillations reach the maximum usually above 200 km and below f-layer peak, with typical value in electron concentration of 0.3-0.5 [1011m-3], in the plasma frequency range from 0.2 to 1.2 mhz, their time life is of about 4-6 cycles of oscillation, and their energy propagates vertically at an average 1116 david altadill, emil m. apostolov, josef boška, jan laštovička and petra šauli speed of a few ms-1. their vertical structure (fig. 10.7) recalls that described for gws caused by solar eclipses (fig.10.5). one senses that gws originates at altitudes of about 180-200 km, i.e. near the transition region between the f1and f2-layers and it propagates upwards and downwards simultaneously. as a consequence of the above discussion, the heating processes linked with the rapid increase of the solar radiation during sunrise/set can act as a source of turbulence in the ionospheric f-region and generate gws in the electron density (see also somsikov, 1995). effects of the morning and evening wave bursts were observed during all campaigns and seem to be a regular wave enhancement in the f-region ionosphere. results show that gws that could be related to the solar terminator remains in the spectra during day of storm and they show similar vertical structures as during previous days while additional strong wave enhancements of auroral origin occur in the electron concentration. during the periods of high solar and geomagnetic activity (1991, 2001), a substantial intensification of agw activity, associated with the response of magnetosphere to a sharp increase of solar wind, was observed. the amplitudes of the gws during the latter periods increase twice to three times compared to the gws detected during geomagnetic quiet time. campaign hirac/solarmax (23 april29 april 2001) started during a period of low geomagnetic activity and finished during a magnetic storm. the clear difference between gws and disturbances of auroral origin was studied and presented (šauli, 2001a,b; šauli and abry, 2003; šauli et al., 2003a,b). wave structures occurring during a geomagnetic storm do not have a vertical component of phase and group velocities. onset time of the particular wave structure at distant observatories confirm the horizontal propagation of the structure. 10.4. summary and conclusions the research work carried out under cost 271 action as refers to the pws in the periodic range 2-16 days and gws leads to the following conclusions. pws in the midlatitude f-region ionosphere may contribute up to 50-60% of the total variability in the period range 2-35 days. the latter is better expressed during summer half year. the typical time of occurrence of pws with periods from 2 to 16 days is from 12 to 35% respectively, and their typical duration ranges from 4 to 3 cycles of oscillation, respectively. however, the spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration, which means that pws remain to be unpredictable at present from ionospheric measurements themselves. pws in the midlatitude f-region ionosphere are large-scale phenomena. their typical longitudinal size ranges from 80° for pws with periods of 2-3 and 5-6 days to 180° for pws with periods of 16 days, those being observed on a larger zonal scale and even global. one senses that geomagnetic activity variations are the main drivers of pws in the midlatitude fregion ionosphere. however, pw activity in the mlt may contribute comparatively in some cases, there being a very small contribution of solar flux variations to the ionospheric variability at periods of 2-30 days. the pws related with the geomagnetic activity tend to occur during summer half year. the so-called independent pws events tend to occur during summer half year also. the pws related with the pw activity in the mlt tend to occur during summer half year for pws with shorter period, and those with longer period tend to occur during winter half year. the latter events display a distinct vertical structure with upwards energy propagation. gws in the midlatitude f-region ionosphere with a regular occurrence and specific gws events whose arising can be predicted have been evaluated. the results were obtained mainly during quiet periods of geomagnetic activity. however, some important results were obtained during disturbed periods as well. the specific gws events are those caused by meteorological cold fronts passages and by solar eclipses. despite the different source of disturbance, the ionosphere reacts at very similar periods, their amplitudes in the plasma variations show a high degree of variability, their time life is up to 6 h, and their energy propagates vertically at an average speed of a few ms-1. their vertical structure however is 1117 planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability quite different, the gws caused cold fronts propagates from below in the f-region and the gws caused by solar eclipse originates at transition region between the f1and f2-layers and it propagates upwards and downwards simultaneously from that level. morning and evening wave bursts are regularly observed in the f-region ionosphere during all ionospheric sounding campaigns, showing that these gws could be related to the solar terminator. their typical duration is about 4-6 h, their amplitudes in the plasma variations show a high degree of variability, their vertical structure is similar to the gws caused by solar eclipse, it originates at transition region between the f1and f2-layers and it propagates upwards and downwards simultaneously from that level. the gws related to the solar terminator remain in the spectra during day of storm and shows similar vertical structures as during quiet days while additional strong wave enhancements of auroral origin occur. the latter however do not have vertical energy propagation. so that sunrise terminator-related gws events are a good candidate for being included into ionospheric prediction algorithms. acknowledgements the work of j.l., j.b. and p.š. has been supported by the ministry of education, sports and youth of the czech republic via grant oc.271.10, and by the grant agency of the czech republic via grants 205/01/1071 and 205/02/077p. the work of d.a. and e.m.a. has been carried out under the international cooperation project between spain and bulgaria 2001bg0012 and under project bte2000-0825 (mcyt, spain). the authors would like to acknowledge the contribution of l.f. alberca and j.g. solé to the wp 3.1 of cost 271 action. references altadill, d. and e.m. apostolov (2001): vertical propagating signatures of wave type oscillations (2and 6.5-days) in the ionosphere obtained from electron density profiles, j. atmos. sol.-terr. phys., 63, 823-834. altadill, d. and e.m. apostolov (2003): time and scale size of planetary wave signatures in the ionospheric f-region. role of the geomagnetic activity and mesosphere/lower thermosphere winds, j. geophys. res., 108 (a11), 1403, doi:10.129/2003ja010015. altadill, d., j. boška, j. g. solé and l. f. alberca (1999): short-time fluctuations in the f-region 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(1984): survey of planetary-scale traveling waves: the state of theory and observations, rev. geophys. space phys., 22 (2), 209-236. šauli, p. (2001a): wave-like effects in the ionosphere connected with the passage of the gravity 1119 planetary and gravity wave signatures in the f-region ionosphere with impact on radio propagation predictions and variability waves of tropospheric origin, d. thesis (faculty of mathematics and physics, charles university of praha, czech republic), (in czech). šauli, p. (2001b): wave-like effects in the ionosphere connected with the passage of the gravity waves of tropospheric origin, auto report of d.thesis (faculty of mathematics and physics, charles university of praha, czech republic). šauli, p. and p. abry (2003): detection of the wave-like structures in the f-region electron concentration, in proceedings of 2nd workshop cost 271, ral. šauli, p., and j. boška (2001): tropospheric events and possible related gravity wave activity effects on the ionosphere, j. atmos. solar terr. phys., 63, 945-950. šauli, p., p. abry and j. boška (2003a): wave structures observed in the ionospheric f-region during low and high geomagnetic activity, in symp. gaii. 06, iugg, sapporo, japan. šauli, p., p. abry, d. altadill and j. boška (2003b): height dependence of gravity wave activity in the f-region ionosphere, in symp. gaii. 06, iugg, sapporo, japan. somsikov, v.m. (1995): on mechanisms for the formation of atmospheric irregularities in the solar terminator region, j. atmos. terr. phys., 57, 75-83. microsoft word sapia_et_al_new.doc annals of geophysics, 60, fast track 6, 2017; doi: 10.4401/ag-7406   1   fast geophysical prospection to map the archaeological site of cocciano: preliminary results vincenzo sapia*, federico florindo**, marco marchetti *, maria di nezza * *istituto nazionale di geofisica e vulcanologia, roma, italy **gruppo archeologico latino “latium vetus”, monte porzio catone, roma, italy vincenzo.sapia@ingv.it abstract we present preliminary results of a multidisciplinary geophysical investigation applied to the subsoil imaging of the archaeological site of cocciano, near rome. we acquired capacitive coupled resistivity data along two parallel profiles and we performed a magnetic survey over a small subset of the survey area. the recovered resistivity models suggest the presence of a shallow, sub-horizontal, resistive layer (ρ > 350 ωm), of slightly variable thickness (2 – 3 m), which we interpret as the response of ancient substructions overlying a relatively low-resistive layer, which we ascribe to the geological substratum. processed magnetic data show a clear magnetic signature aligned to form a curve-shaped anomaly right at the prosecution of a nearby, partially exposed, ancient wall. i. introduction the study area (latitude n41°49'11", longitude e12°41'02") is located in cocciano, hamlet of the town of frascati, inside the colli albani volcanic district about 20 kilometers southeast of rome. the investigation area regards the north-west-facing terrace in the property called spinetta, which is most notably characterized by the presence of a sequence of substructional spaces opening on its front. the whole spinetta area was place of a series of archaeological findings, which are generally recognized as the remains of a vast roman villa of the imperial age, notably attributed to emperor tiberius (valenti 2003a). as already reported from valenti (2003b), some relevant archaeological features characterize the site: • lower substructions which constitute the north-western front of the spinetta terrace, composed of a sequence of 36 vaulted rectangular spaces (figure 1); • upper substructions which consist of further substructural spaces with vaulted ceilings located at the base of a late medieval (?) towerhouse; • caementicium wall, about 0.6 m thick, only partially emerging from the soil and vegetation and clearly showing a curved trend (figure 2). with the aim to discover new buried archaeological features, non-invasive, fast and easydeployable geophysical measurements were performed using the magnetic and the capacitive coupled resistivity (hereinafter cr) methods (see figure 3 for surveys location). annals of geophysics, fast track 6, 2017; doi: 10.4401/ag-7406   2   archaeological-related magnetic surveys are frequently reported in the literature of the past 50 years (weaver, 1961; bevan, 1994; schmidt, 2007; di mauro et al., 2014). conversely, cr measurements applied for similar studies, are far less used and only few scientific articles are available in the recent literature (bottacchi et al., 2009; alfouzan et al., 2016) two cr profiles were acquired over the spinetta terrace, while the magnetic survey was conducted in an area where, an ancient wall, interrupt its circular trend and still, presumably, below the ground surface. figure 1. example of the substructions located at the northwestern side of the survey area and forming the front part of the spinetta terrace. figure 2. ancient wall with a clear curved trend and an average thickness of about 0.6 m thick, only partially emerging from the soil and the vegetation. preliminary geophysical results are encouraging. we observe a high resistivity anomaly, of nearly constant thickness, throughout the acquired electrical data, which overlay a subhorizontal (3 m thick) relatively conductive layer. the extension of the surficial resistive layer is comparable to the observed substructions forming the front side of the spinetta terrace as shown in figure 2. the conductive layer is interpreted as fallout pericalderic scoriacone deposits of the final stages of the villa senni eruption cycle (madonna degli angeli succession, from 365±4 to 351±4ka) which crop out in the investigated area (giordano et al., 2006; marra et al., 2009). these deposits, especially along the alban hills caldera walls are made by alternating lava, scoria and welded scoria fall deposits. as for the magnetic data, the vertical gradient magnetic map highlights the presence of a clear curve-shaped magnetic anomaly resembling the trace, at depth, of the exposed wall structure trend. ii. methods a scalar magnetometer g-858 by geometrics (sensitivity 0.05 nt) in gradiometer configuration was employed with sensor separation of 0.8 m and lower sensor placed at about 0.3 m above the ground. data were acquired in bidirectional mode along parallel paths about 2 m spaced, using a sampling rate of 1 hz. acquired magnetic data were geo-referenced by means of an integrated gps receiver (utm projection, wgs84 coordinate system). the vertical magnetic gradient is computed by subtracting the measured bottom sensor magnetic data to the top sensor response. since magnetic investigations depend on the contrast in the magnetic properties between the target and the enclosing soil matrix, a rapid magnetic susceptibility survey was performed. we used portable magnetic susceptibility meter (sensitivity 1 x 10-3 si units) to complement the magnetometer survey and help with the interpretation of the recovered magnetic anomalies. ground resistivity data were measured using a geometrics ohmmapper system, using a multireceivers array and 5 m transmitter and receiver dipole. to increase subsoil investigation annals of geophysics, 60, fast track 6, 2017; doi: 10.4401/ag-7406   3   depth the two cr profiles, approximately west-east oriented, 15 m spaced apart and 150 m long, were collected using three different rope lengths of 5 m, 10 m and 15 m respectively. raw data are corrected using the effective dipole lengths for line antennas (oldenborger et al., 2013). despiked and corrected apparent resistivity data are inverted with res2dinv software using a smooth constrained leastsquared inversion algorithm (loke and barker, 1996). figure 3. a) location map of the area. the red dots indicate the survey area near the village of frascati, rome, italy. b) close up view of the survey area with the location of the geophysical measurements. the red lines show the cr profiles and the yellow box indicate the magnetic survey area. iii. results the magnetic survey provided us valuable indications to speculate on the presence of a subsurface trace of the wall structure as the subsurface prosecution of the exposed wall. data processing consisted of the removal of data spikes and other errors introduced during the survey, such as striping effect between lines. among high intense, sparse and small-scale magnetic anomalies, the presence of a distinct magnetic signature in the vertical gradient magnetic map is evident. as shown in figure 4, this vertical magnetic gradient anomaly reaches an intensity of -200 nt/m and is arranged to form an approximatively east-west oriented curve pattern. due to its lateral continuity, this magnetic anomaly is probably related to the presence of a continuous magnetic sources, with less magnetic susceptibility than the surroundings, buried at very shallow depth. in fact, magnetic susceptibility measurements were carried out to characterize the magnetic properties of the exposed wall and the ground. data were collected along the exposed wall and to the adjacent soil with a 2 m mark spacing. measured data revealed that the ancient wall exhibits magnetic susceptibility values ranging from 12 – 17 x 10-3 si while the surrounding ground ranges from 5 – 9 x 10-3 si units. although very small, the difference between these values evidences a magnetic susceptibility contrast between the exposed wall and the soil. inversion results for the despiked cr data along each profile are shown in figure 5. annals of geophysics, fast track 6, 2017; doi: 10.4401/ag-7406   4   figure 4. 2-d contour map of the magnetic vertical gradient. data are pre-processed using the magmap2000 software and then interpolated by the kriging method using a regular grid of 3 × 3 m cells. in general, the recovered resistivity models clearly show two distinct electrical units. from the surface down to about 3 m depth, we observe a relative higher resistivity response of the subsurface materials. in particular, profile p1 (figure 5a) shows a resistive layer of slightly variable thickness with resistivity values above 400 ωm throughout its entire length. this layer overlays a sub-horizontal, 3 m thick low-resistive layer with minimum resistivity values of about 60 ωm. below this layer, the p1 cr resistivity model hints to a high resistive layer at depth. similar results come up also from the cr profile p2 (figure 5b). the shallower portion of the subsoil clearly shows a high resistive layer, about 3 m thick, which overlays a sub-horizontal 3 m thick lowresistive layer up to a depth of 7 m. again, the recovered cr model indicates that the resistivity increases at depth, although we are aware that the resistivity features at the bottom of each cr profiles are in a lower sensitivity region and they could reasonably be explained as numerical artifact of the inversion. iv. discussion this short paper presents the preliminary results of a geophysical survey conducted in the cocciano-spinetta archaeological site. the aim of this work was to check the feasibility of two fast and easy-deployable geophysical methods as a tool for detecting the presence of subsurface archaeological targets. the success of any magnetic investigation strictly depends on the existing magnetic contrast between the feature of interest and the soil. in this case, the ancient wall is made of leucitite shards, an effusive volcanic rock type, mixed to a fine concrete matrix. the magnetic susceptibility contrast between the volcanic rock and the surrounding soil supports the recorded magnetic data and facilitates the interpretation of the observed magnetic anomalies. the magnetic results allowed us to confirm a circular trend of the wall, in accordance to what proposed by valenti (2003a). the recovered negative vertical magnetic gradient anomaly, right at the prosecution of the wall, can be due to: a) the magnetic properties of the wall are different at depth, b) the wall has been entirely replaced with materials characterized by different magnetic properties. the first option will involve the partial substitution of the wall with soil of significantly lower magnetization. the second option implies that the wall has been entirely removed from its original position and replaced with less magnetic susceptibility soils. both options would justify the link between the recovered negative vertical magnetic gradient anomaly and the lack of the wall (which showed higher magnetic susceptibility), thus ascribing the recovered magnetic gradient response to the presence of its trace at depth. annals of geophysics, 60, fast track 6, 2017; doi: 10.4401/ag-7406   5   figure 5. cr subsurface resistivity inversion models for the a) p1 profile with rms = 0.8% (with left being south-west and right north-east), b) p2 profile with rms = 1.1% (with left being south-west and right northeast), same resistivity colour scale is adopted for the two cr profiles. dashed white line hypothesizes the substructions/bedrock boundary. we considered the cr survey as a very useful tool for a preliminary investigation of the subsoil prior to set up a more laborious electrical resistivity tomography measurements (sapia et al., 2017). in fact, the ohmmapper instrument was found to be very fast and accurate to map the subsurface resistivity up to about 7 m depth. we were able to acquire two parallel electrical profiles, each 150 m long, using three rope lengths for a total path of almost 800 m in less than one hour (see hra-p1 and hra-p2 in figure 6). the overall resistivity distribution evidenced the presence of a high resistive layer up to a depth of 3 m, which is coincident with the thickness of the north-western substructions (see figure 1). in our opinion, the high resistivity could be the response of such a mixture of bricks and concrete as observed from the exposed substructions which is in contrast with the hypothesis of decorative front wall as reported from valenti (2003a, b). the spinetta terrace, although taking advantage of the natural slope of the hill, is clearly artificial. facing north-west it overlooks a very impressive landscape (today partly covered by constructions and the rome-frascati railway artificial slope) ranging from rome to the monti tiburtini, visually dominating the whole eastern “ager romanus”. this preliminary study indicates that both magnetic and cr techniques can be used for archaeological investigation purposes in cocciano. in the near future the geophysical survey will be extended to other sectors of the archaeological area. in particular, electrical resistivity tomography (ert) will be acquired either over the terrace or to image the subsoil below the circular wall structure. the ert data collected over the same profile as the cr can be also used to cross-check both recovered resistivity models. in addition, new cr profiles will be performed parallel to hra-p1 and p2 sections in order to cover the entire terrace area. by this way, cr and ert data could then be used to build up a 3d resistivity model of the whole spinetta terrace up to a depth of about 20 m. a combined use of magnetic and ground penetrating data (gpr) will be planned over two, infrastructure-free, subset areas located both at the eastern and western side of the archaeological site with the aim to image the presence of further buried targets (i.e. buried walls, floorings, roads). annals of geophysics, fast track 6, 2017; doi: 10.4401/ag-7406   6   figure 6. general planimetry of the study area adapted from valenti (2003a) showing the substructional structures, with schematization of the high resistivity areas (hra) resulted from the two cr profiles (p1 and p2) and results from the magnetic survey carried out in the area of the caementicium circular wall. acknowledgements the authors kindly thanks enrico devoti and giovanna cappelli for their valuable assistance during the fieldwork. we also thanks the reviewer for the constructive comments and helpful suggestions which helped us to improve the manuscript. references alfouzan, f., zhou, b., bakkour, k., and alyousif, m. (2016). detecting near-surface buried targets by a geophysical cluster of electromagnetic, magnetic and resistivity scanners. journal of applied geophysics, 134:55-63. bevan, b. w. (1994). the magnetic anomaly of a brick foundation. archaeological prospection, 1, 93. bottacchi, m. c., colonna, t., mantovani, f., and medri, m. (2009). application of the ohmmapper resistivity-meter to detect the theatre of sentinum roman town by using 3d resistivity model. archeosciences. revue d'archéométrie, 267-269. di mauro, d., alfonsi, l., sapia, v., and urbini, s. (2014). a neighborhood revealed by geophysical prospection: an example of urbanization at the phoenician–punic settlement of mozia (western sicily, italy). journal of applied geophysics, 104:114-120. giordano, g., de benedetti, a. a., diana, a., diano, g., gaudioso, f., marasco, f., miceli, annals of geophysics, 60, fast track 6, 2017; doi: 10.4401/ag-7406   7   m., mollo, s., cas, r. a. f., and funiciello, r. (2006). the colli albani mafic caldera (roma, italy): stratigraphy, structure and petrology. journal of volcanology and geothermal research 155:49-80. doi:10.1016/j.jvolgeores.2006.02.009. loke, m. h., and barker, r. (1996). rapid leastsquares inversion of apparent resistivity pseudo-sections using a quasi-newton method. geophys. prospect. 44:131-152. marra, f., karner, d. b., freda, c., gaeta, m., and renne, p. (2009). large mafic eruptions at alban hills volcanic district (central italy): chronostratigraphy, petrography and eruptive behavior, journal of volcanology and geothermal research 179:217-232. doi:10.1016/j.jvolgeores.2008.11.009. oldenborger, g. a., and leblanc, a. m. (2013). capacitive resistivity inversion using effective dipole lengths for line antennas. journal of applied geophysics, 98:229-236. sapia, v., baccheschi, p., villani, f., and marchetti, m. (2016). multidisciplinary geophysical approach to map disposal site: the ponza island case study. journal of applied geophysics, 138:264-274. schmidt, a. (2007). archeology, magnetic methods. in encyclopedia of geomagnetism and paleomagnetism, springer netherlands 23-31. valenti, m. (2003a). ager tusculanus, leo s. olschki editore, firenze., forma italiae serie i, 41:428 pp. valenti, m. (2003b). gli scavi nella vigna lucidi a frascati, in ghini g., lazio sabina 2, de luca editori d’arte, roma, 187-192. weaver, g. h. (1961). magnetic dating measurements. archaeometry, 4:23-28. mitchell 687_696 687 annals of geophysics, vol. 46, n. 4, august 2003 a three-dimensional time-dependent algorithm for ionospheric imaging using gps cathryn n. mitchell and paul s.j. spencer department of electronic and electrical engineering, university of bath, u.k. abstract global positioning system (gps) satellite receivers provide a world-wide network of phase and group delay measurements. the combination of two-frequency measurements can be used to derive the integral of the electron concentration along each satellite-to-receiver path, a parameter known as the total electron content (tec). at this stage these slant tec data are diffi cult to interpret as they originate from a combination of a temporally changing ionosphere and spatially changing observation geometry. in this paper tec data are inverted to evaluate the underlying distribution and time evolution of electron concentration. accordingly, a new three-dimensional, time-dependent algorithm is presented here for imaging ionospheric electron concentration using gps signals. the inversion results in a three-dimensional movie rather than a static image of the electron-concentration distribution. the technique is demonstrated using simulated ground-based gps data from actual measurement geometry over europe. mailing address: dr. cathryn n. mitchell, department of electronic and electrical engineering, university of bath, claverton down, bath, ba2 7ay, u.k.; e-mail: c.n.mitchell@bath.ac.uk key words ionosphere – imaging – gps – inversion – tomography 1. introduction the ionosphere results from the interaction of solar emissions with the earth’s atmosphere. its evolution is influenced by changes in solar emissions, atmospheric dynamics and the interplanetaryand geo-magnetic fields. ionospheric morphology results from the interaction of these drivers and isan interesting topic for scientifi c study. special instrumentation devoted to such studies has been developed and used extensively for many years. the ionosphere affects electro-magnetic waves by refraction and the magnitude of this is frequency dependent. the result of this is that signals are bent and delayed in a manner that may be either advantageous or disadvantageous to radio systems. for example, hf communication beyond the horizon is possible only because oblique signals can be refracted and returned to the ground.in this case the user can determine the path of a signal by ray-tracing through a map of the electron concentration. l-band signals used in satellite navigation systems based on the time-of-fl ight of the signal are put in error by the unknown ionospheric delay. for the user needing to compensate for this error, the requirement is to be able to know the delay introduced by the total electron content (tec), along specifi c satellite-to-receiver ray paths. again, this requires accurate knowledge of the distribution of electron concentration. recently, dual-frequency satellite signals have provided a new source of ionospheric information. the global positioning system (gps) satellites are monitored by a network of dual-frequency receivers, recording the phase and time delay of each signal. these measure688 cathryn n. mitchell and paul s.j. spencer ments provide a valuable source of information about the ionosphere in the form of ray-path integrations of electron concentration. these measurements are rather uneven in distribution and coverage but have some similarities to polarorbiting low-earth orbit satellite tecs derived from the navy ionospheric monitoring system (nims) satellites. in contrast to gps tecs, nims satellite tecs map easily into a plane and thus can be inverted into images of electron concentration using tomographic methods. tomography is the inversion of networks of line-integral measurements to yield the twodimensional spatial distribution of the integrated parameter. this is very important for ionospheric data combination since many measurements of the line integral of the electron concentration (i.e. the tec) are easily obtained, but the parameter of interest is often the electron concentration itself. in the case of conventional ionospheric tomography, tec measurements are made from dual-frequency trans-ionospheric signals along many intersecting raypaths between a leo satellite and a chain of ground-based receivers. these tec data are subsequently analysed and inverted using a suitable mathematical algorithm to produce two-dimensional images of electron concentration. a review of the experimental results from ionospheric tomography has been published by bernhart et al. (1998). the application of the tomographic technique to ionospheric imaging was fi rst proposed in 1986 by austen et al. leo satellite-to-earth measurement geometry only allows observations over a limited number of viewing angles and yeh and raymund (1991) investigated some of the theoretical limitations this would impose on ionospheric tomography. in fact the orientations of the satellite-to-receiver ray paths are biased in a vertical sense with no ray paths running horizontally through the ionosphere. consequently the vertical electronconcentration gradients are poorly defi ned by the tec measurements alone. this geometrical constraint means that the reconstruction of the tomographic images is not straightforward and the reconstruction cannot be implemented directly using conventional inversion algorithms alone. there have been several solutions to the problem and the one described by fremouw et al. (1992) has been widely used. this algorithm, adapted from another geophysical application, uses a set of vertical orthonormal functions, created from ionospheric models, to image the vertical profi le and a power law spectrum to select the horizontal structures from a fourier basis. the algorithm presented in this paper is an extension into three-dimensions of that presented by fremouw et al. (1992) for two-dimensional tomographic imaging. in addition, a time-dependence has been incorporated into the inversion, allowing for changes in electron concentration throughout the imaging period. this allows the algorithm to invert multi-directional slant tec data derived from gps signals over a period of typically one hour into movies of electron concentration. 2. theory in this section the theory relating directly to the inversion problem is outlined. a detailed description of the propagation of electromagnetic waves through an ionised medium and the effects of the ionosphere on radio systems can be found in davies (1990). total electron content is defi ned as the line integral of the electron concentration along a path from a satellite, s, to a receiver, r. the tec (b s ) may be expressed as (2.1) in which n is the electron concentration, r the radial distance from the centre of the earth, q is the latitude, f the longitude and s the distance along the satellite-to-receiver ray path. tec can be measured using faraday rotation or differential doppler techniques. in the latter case, dualfrequency radio signals that propagate through the ionosphere are subject to a differential phase change due to the dispersive nature of the plasma. as a fi rst order approximation the change in the differential phase shift is directly proportional to the change in total electron content (tec) between the transmitter and receiver. in addition to the phase technique, gps signals also contain information about the signal delays. the measurement of differential delay is subject b n r dss s r = ( )∫ , ,θ φ 689 a three-dimensional time-dependent algorithm for ionospheric imaging using gps to systematic errors known as interfrequency biases associated with the satellite and receiver hardware and to random and systematic errors due to multipath. nevertheless, it is still useful to utilise differential delay measurements for tec calibration as has been done by others, for example see ciraolo and spalla (1997). the fi rst stage of the inversion is to set up a three-dimensional grid of voxels (i.e. volume pixels), each bounded in latitude, longitude and altitude, and to compute the length of each element of a satellite-to-receiver signal propagation path though each intersected voxel. for i tec measurements and j voxels the unknown electron concentration is defi ned to be constant within each voxel and contained in the column vector x of j components. the problem may now be expressed as ax = b (2.2) where a is an i ¥ j matrix of the path lengths within each voxel and b are the i observed tecs. this cannot be solved directly as the matrix a is rectangular, highly singular and incorporates no prior information as to the likely solution. to overcome this diffi culty a mapping matrix, x, is used to transform the problem to one for which the unknowns are n coeffi cients of orthonormal basis functions, the combination of which will give the fi nal image of electron concentration. the choice of orthonormal basis functions is critical in the determination of the fi nal solution to this underdetermined inversion problem. here the basis functions (x ) were generated using a spherical harmonic expansion to represent the horizontal variation and empirical orthonormal functions (eofs) for the radial variation in electron concentration. the spherical harmonics provide a fl exible basis to determine the horizontal distribution of ionisation which should be well defi ned by the measurements. the eofs form a constraint to the vertical profi le, only allowing a certain range of possible solutions. this is now expressed mathematically as axw = b (2.3) where the matrix x contains the basis functions, such that ax defi nes a basis set of line integrations of electron concentration. since the raypath integrals derived from differential phase along the same satellite-to-receiver paths are subject to an unknown cycle offset adjacent rows of the matrices ax and b can be differenced in a manner to negate the effect of this on the solution. the n unknowns, w, now represent the relative contribution of the basis functions where w = (ax)-1b. (2.4) here (ax)-1 is a generalized inverse matrix such that w is the maximum likelihood solution. in this case singular value decomposition (svd) has been chosen to solve the inversion. applying singular value decomposition to the matrix ax returns two orthogonal matrices u and v and a diagonal matrix of singular values, w, thus . (2.5) the solution to the inverse problem is then given by (2.6) where the reciprocal of the terms in w that are suffi ciently small, typically 10–7 of the dominant singular weight, are zeroed to account for degeneracy in the ax matrix. a description of the use of svd can be found in golub and van loan (1989). finally, the electron densities within each voxel, j, are recovered using x = xw . (2.7) gps satellites have an orbital period of just under 12 h and are inclined at 55∞ to the equatorial plane. from a fi xed location on the earth they appear to move slowly across the sky and for high-elevation passes take several hours from rise to set time. a series of tec measurements from any single gps-to-ground link consist of both spatial and temporal changes in the ionosphere. the time-independent algorithm described is based on the assumption of stationarity of the medium and for the ionosphere this generally requires a short measurement time of a few minutes. this short measurement period would result in limiting the data quantity and spatial ax v w u t( ) = ( )( )−1 1diag / w v w u b= ( )( )( )diag 1 / t 690 cathryn n. mitchell and paul s.j. spencer coverage when using gps. extending this algorithm into a time-dependent inversion allows a great increase in the quantity and angular coverage of measurements that can be used in each inversion. in addition, since there is no implicit stationarity on the ionospheric electron concentration distribution it allows moving features to be imaged in a consistent manner rather than being averaged out. the algorithm can be extended into a time-dependent inversion by incorporating a priori information about the evolution of the electron concentration during a specifi ed period of time. assuming that the change of the electron concentration within a voxel with time is linear, which makes sense if short enough time periods are dealt with, then it is possible to write the same system of equations to solve for the change in the relative contributions of each basis function. a relatively short period is chosen for the time-dependent inversion, for example one hour, and data collected at typically 30 s intervals are considered. the change in the ray path geometry, defi ned in the d matrix, multiplied by the unknown change in electron concentration ( y) is equal to the change in tec (c) dy = c. (2.8) thus the matrix, d, is formed from the difference in ray-path geometry at successive time intervals, y is the unknown change in electron concentration between each time interval and c are the measured changes in tec. the mapping matrix, x, is again used to transform the problem to one for which the unknowns are the linear changes in coeffi cients (g) of a set of n appropriately selected ortho-normal basis functions d(xg) = c . (2.9) the matrices can be re-written such that the ray path geometry is multiplied directly by the mapping matrix to create the basis set dx(g) = c (2.10) and the change in the unknown contributions of each of these line integrations of electron concentration is solved for g = (dx )-1c (2.11) the time-dependent solution to the inverse problem is then given by y = xg (2.12) and this can be combined with the static solution of (2.2)-(2.7). critical factors in the solution are the choice of the vertical profile set to form the basis functions and the total time over which linear changes can be assumed. linear changes are generally enforced over time periods of onehour. 3. results in this section simulations of twoand threedimensional static and time evolving inversions are demonstrated. for the initial simulation a twodimensional latitude versus altitude model of the electron concentration was created. the model was not intended to represent any particular ionosphere but to contain features that might be diffi cult to image using a static solution. this is to demonstrate that a time-evolving ionosphere can be imaged with the algorithm presented here. the vertical profi le of the ionisation in the model was formed from chapman functions (chapman, 1931). in the initial model the ionosphere had a constant peak height at 285 km and scale height of 110 km, with a peak electron concentration of 4 ¥ 1011 m-3. in the fi nal model the peak height to the north of the image was increased to 370 km, the scale height to 140 km and the peak electron concentration was increased to 20 ¥ 1011 m-3. a large depletion in ionisation was superimposed onto the images to represent the night-time mid-latitude structure known as the main trough. the centre latitude of the trough was 48º in the fi rst frame and 45ºn in the fi nal frame. thus the changes to the ionosphere included a 3º southward movement of the trough, an increase in electron concentration and an increase in the peak and scale height to the north of the trough. the 691 a three-dimensional time-dependent algorithm for ionospheric imaging using gps a b c fig. 1a-c. two-dimensional maps of electron concentration. a) initial frame of the model ionosphere. b) middle frame of the model ionosphere. c) final frame of the model ionosphere. the contours are shown every 2.0 ¥ 1011electrons m3. a b c fig. 2a-c. two-dimensional maps of electron concentration resulting from the time-dependent inversion. a) initial frame of the inversion. b) middle frame of the inversion. c) final frame of the inversion. the contours are shown every 2.0 ¥ 1011electrons m– 3. 692 cathryn n. mitchell and paul s.j. spencer modelled electron concentration varied linearly between the extremes of the two images shown in fi g. 1a,c. six ground-based receivers, evenly spaced every 5∞ between 35∞ and 60∞n, were used in the simulation. two counter-orbiting nims type satellites at 1200 km altitude were modelled. nims were chosen since their polarorbits are applicable to two-dimensional imaging. a minimum of two satellites is required to separate temporal and spatial changes in this type of imaging. tecs were simulated between each satellite and receiver for 1∞ steps in satellite latitude. thus a single set of twelve (two satellites and six receivers) simulated tec measurements were found by integration through a single frame of the ionosphere. subsequent sets of tecs were found by integration through subsequent frames of the ionosphere according to changes described above, relating to the time-evolution. the time-dependent inversion results are shown in fi g. 2a-c. the three images are taken from the fi rst, middle and fi nal frames of the inversion. comparing fig. 2a with fig. 1a it can be seen that the image represents the peak height, scale height and electron density quite well. the latitudinal location of the trough and its depth are in agreement with the model. comparing fi g. 2b with fi g. 1b it can be seen that the electron concentration is slightly low to the south of the trough and high to the north. the contour to the far north of the image is closed, indicating that the inversion has produced a slightly low peak density to the northern edge of the image. nevertheless, the scale heights are well reproduced by the inversion and the step in peak height across the trough is clearly seen. the latitudinal location of the trough has also been reconstructed well. the fi nal image (fi g. 2c compared to fi g. 1c) shows that the features are well replicated but the electron concentration is slightly higher than that of the model to the north of the trough. there is a slight gradient in the peak height in the reconstructed image that is not in the model. nevertheless the overall features and the changes in the ionosphere have been very well accommodated in this timedependent inversion. in order to make a comparison, the inversion was also performed using the time-independent algorithm. the result is shown in fi g. 3. surprisingly, the resulting image has a lower peak concentration than that found in the middle frame of the model. some artifi cial structuring has appeared in the imaging, caused by the inconsistencies between the measurements. this shows that it is not possible to rely on the static imaging if extreme density gradients and moving features could be present in the ionosphere. the time-dependent algorithm in threedimensions is now demonstrated using a onehour simulation. the ionosphere over europe was modelled using the iri model (bilitza, 1990) at 2 min intervals from 06 to 07 ut on 20 april 2002. the iri model was chosen because of its ability to represent the ionosphere globally in three-dimensions without any discontinuities, thus allowing smooth integration with which to simulate tec. actual gps satellite orbits and 15 receiver locations from the international gps service network were used. the slant tecs were simulated by integration along the satellitereceiver path every two minutes throughout the hour through high-resolution grids of iri electron densities. the high-resolution grid, ensuring no discontinuities in the simulated tecs, was 1º latitude by 2º longitude and 25 km altitude extending from 35ºn to 65ºn in latitude, 10º w to 40º e in longitude and 100 km to 1200 km in altitude. the inversion was based on the same grid. figure 4a,b shows the vertical tec from the iri model at 06 ut (fi g. 4a) and 07 ut (fi g. 4b). a smooth gradient in tec is seen rising fig. 3. electron concentration determined using the time-independent (static) inversion. the contours are shown every 2.0 ¥ 1011electrons m-3. 693 a three-dimensional time-dependent algorithm for ionospheric imaging using gps in magnitude toward the south-east. over this one-hour period the model shows an overall increase in tec of around 15%. the lower two panels of the fi gure show the vertical tec obtained by integration through the fi rst and last frames of the three-dimensional time-dependent reconstruction. comparing fi g. 5a with fi g. 4a, it can be seem that the inversion has produced a very accurate representation of the zonal and meridional gradients in tec. the reconstruction shows a slight underestimation of tec towards the south of the image and overestimation in a b fig. 4a,b. vertical tec from the iri-95 model for (a) 06 ut and (b) 07 ut on 20 april 2002. contours are shown every 2 tecu. a b fig. 5a,b. vertical tec resulting from integration through the time-dependent inversion of model phase data from ground-based receivers and gps satellites for (a) 06 ut and (b) 07 ut. contours are shown every 2 tecu. 694 cathryn n. mitchell and paul s.j. spencer the north. there is also some evidence of an artifi cial change in the zonal tec gradient to the north of the image, resulting from the use of spherical harmonics and a limited number of discrete voxels in the image reconstruction. the last frame of the inversion (fi g. 5b) shows the vertical tec at 07 ut. again the image shows an accurate representation of the zonal and meridional gradients and a slight underestimation of tec. this underestimation may be the result of the limited number of eofs used in the inversion. twenty-four simulations were run to test the inversion method over an entire day. one three-dimensional time-dependent inversion was done for each one-hour period of the 20 april 2002. each reconstruction was evaluated by comparison of vertical tec between the reconstruction and the iri model at the start of the hour, to determine the mean and the mean absolute error in both tec units and as percentages of the model tec. the mean errors in tec were found using (3.1) where biri are the 750 (nb) vertical tecs found by integration through the iri model represented on the 30 latitude ¥ 12 longitude grid used for the inversion and binv are the corresponding values found by integration through the electron concentrations in the inversion. the mean absolute errors in tec were found using the following equation: . (3.2) the mean errors in tec as percentages were found using (3.3) and the mean absolute error in tec, as a percentage, has been found using . (3.4) figure 6 shows the errors, i.e. the differences between the vertical tec in the image and the vertical tec in the model, over the twenty-four hour period. it can be seen from the graphs that there is a slight underestimation of vertical tec of the order of 1%. slight variations occur for each reconstruction, but the accuracy is no worse than 1 tecu. the absolute values indicate that the tec may be underestimated in some areas of the image and over in others, since at 4 ut, for example, the error is 2.5% in absolute but only – 1.5% in mean error. it is important to note that in this test a general set of basis functions were generated from spherical harmonics and orthonormal vertical functions. these basis functions happen to fi t the vertical profi les of certain images better than others and the choice of them remains a critical factor in the accuracy of the images. 4. conclusions a new algorithm has been presented which could use carrier-phase observations to image the electron concentration in the ionosphere. 1 nb binv birii i i nb −( )∑ 1 nb binv birii i i nb −∑ 100 nb binv biri biri i i ii nb −( )∑ 100 nb binv biri biri i i ii nb − ∑ fig. 6. errors found between the inversion and the model over twenty-four, one-hour runs. the errors were calculated at the start of each hour using eqs. (3.1)-(3.4). 695 a three-dimensional time-dependent algorithm for ionospheric imaging using gps the algorithm can be applied to twoor threedimensional spatial imaging either with or without a time evolving solution. the advantage of using the time-evolving solution has been demonstrated by means of a two-dimensional tomographic simulation. it was shown to be appropriate to use the time-dependent approach to image an ionosphere with a moving trough and increasing electron concentration. in particular, the time-independent algorithm can prevent imaging artefacts caused by moving structures and changing layer heights. watermann et al. (2002) have already recognised this limitation in ionospheric imaging and suggest that ionospheric imaging in the polar cap can be relied upon when the magnetometers suggest a slower plasma convection. the time-dependent algorithm shown here could be useful for imaging the polar ionosphere, provided that suffi cient satellite data are available to separate the spatial and temporal variations. it has already been established that gps data can be used in ionospheric imaging by using a short measurement period and combining with other measurements or models. bust et al. (2001) have shown such results from their combined ionospheric campaign in the caribbean region. the work presented here opens up the possibility to produce images of the ionosphere using gps data on its own. the key to this is the incorporation of the time-dependence in the imaging process and this has been demonstrated with an iri-based simulation with realistic satellite and receiver geometry taken from an actual case study. the work demonstrates the potential of the technique to use gps data for applications requiring ionospheric mapping and for geophysical research. in the work presented here only the differential phase technique is utilised to provide relative tec from ground-based receivers. a future development of the work will involve the incorporation of many different data sources into the image. it is already feasible to extend the input matrix to contain any approximately linear ionospheric measurement such as information derived from ionograms. hajj et al. (1994) suggested using the satellite-to-satellite transmission of gps to leo satellite measurements in a tomographic framework to provide the so-called «missing horizontal rays» and improve the vertical resolution. in addition, leos provide measurements over the oceans and into remote polar caps, thus enabling the ionosphere to be studied continuously in both space and time. thus this technique to create global image of the ionosphere will soon be used to solve practical radio propagation problems and to study the earth’s ionosphere on a global scale. acknowledgements the authors would like to thank the uk epsrc for supporting this research and the international gps service for information used in the simulations. they also are very grateful for the use of the international reference ionosphere. references austen, j.r., s.j. franke and c.h. liu (1988): ionospheric imaging using computerised tomography, radio sci., 23, 299-307. bernhardt, p.a., r.p. mccoy, k.f. dymond, j.m. picone, r.r. meier, f. kamalabadi, d.m. cotton, s. charkrabarti, t.a. cook, j.s. vickers, a.w. stephan, l. kersley, s.e. pryse, i.k. walker, c.n. mitchell, p.r. straus, h. na, c. biswas, g.s. bust, g.r. kronschnabl and t.d. raymund (1998): two dimensional mapping of the plasma density in the upper atmosphere with computerized ionospheric tomography (cit), phys. plasmas, 5, 2010-2021. bilitza, d. (editor) (1990): international reference ionosphere, national space science data centre/ world data centre for rockets and satellites, lanham, md, report n. nssdc/wdc-a-r&s, 90-22 bust, g.s., d.s. coco and t.l. gaussiran (2001): computerized ionospheric tomography analysis of the combined ionospheric campaign, radio sci., 36 (6), 1599-1606. chapman, m.a. (1931): the absorption and dissociation ionisation effect of monochromatic radiation in a rotating earth, proc. phys. soc., 43 (1), 26. ciraolo, l. and p. spalla (1997): comparison of ionospheric total electron content from the nnss and gps, radio sci., 32 (3), 1071-1080. davies, k. (1990): ionospheric radio (peter peregrinus ltd., london, u.k.), pp. 580. fremouw, e.j., j.a. secan and b.m. howe (1992): application of stochastic inverse theory to ionospheric tomography, radio sci., 17, 721-732. golub, g.h. and c.f. van loan (1989): matrix computations (johns hopkins university press, baltimore), pp. 642. 696 cathryn n. mitchell and paul s.j. spencer hajj, g.a., r. ibañez-meier, e.r. kursinski and l.j. romans (1994): imaging the ionosphere with the global positioning system, int. j. imag. syst. technol., 5, 174-184. watermann, j., g.s. bust, j.p. thayer, t. neubert and c. coker (2002): mapping plasma structures in the high-latitude ionosphere using beacon satellite, incoherent scatter radar and ground-based magnetometer observations, ann. geophysics, 45 (1), 177-189. yeh, k.c. and t.d. raymund (1991): limitations of ionospheric imaging by tomography, radio sci., 26, 1361. (received february 4, 2003; accepted july 28, 2003) t h e seismic activity in the area of greece between 1966 and 1969 p . a . giannakopoulos * received on march 28'", 1972 summary. — t h e earthquake a c t i v i t y in the area of greece for the years 1966, 1967, 1968 and 1969 is investigated. this earthquake a c t i v i t y as a rule follows the general pattern found b y the use of data f o r a much longer period. t h e r e is a clear tendency for the focal depth of the earthquakes to increase f r o m the convex side of the aegean arc to the inner volcanic arc. t h e value of the constant b of the recurrence curves is much smaller for the intermediate earthquakes than for the shallow ones. riassunto. -— è stata studiata l ' a t t i v i t à sismica in grecia durante gli anni 1966, 1967, 1968 e 1969. d e t t a attività, come di consueto, segue il modello generale t r o v a t o usando i dati per un periodo di tempo piuttosto lungo. l a profondità focale dei terremoti ha una chiara tendenza ad aumentare dalla parte convessa dell'arco e g e o verso la parte più interna dell'arco vulcanico. 11 valore della costante b delle curve di ricorrenza è molto più piccolo per i terremoti intermedi che per quelli superficiali. introduction t h e a r e a of g r e e c e is a n a c t i v e p a r t of t h e a l p i d e h i m a l a y a n m e l a n e s i a n b e l t . i t s t e c t o n i c a c t i v i t y is d e m o n s t r a t e d b y s e v e r a l g e o p h y s i c a l a n d g e o l o g i c a l e v i d e n c e . t h e seismic a c t i v i t y of t h e a r e a is c o n s i d e r a b l e , t o o . i n v e s t i g a t i o n of t h e s e i s m i c i t y of t h e a r e a w a s s t a r t e d i n t h e e n d of t h e n i n e t e e n t h c e n t u r y ( 4 ) . t h o s e studies, h o w e v e r , w e r e b a s e d o n i n l i o m o g e n e o u s m a c r o s e i s m i c d a t a . a f t e r 1911, w h e n t w o h o r i z o n t a l c o m p o n e n t s of m a i n k a s e i s m o g r a p h w e r e p u t i n r e g u l a r o p e r a t i o n i n ( * ) national observatory of athens, seismological institute, athens, (greece). 3 6 0 p . a . g i a x x a k o p o u i . o s athens, systematic collection of microseismic data lias been made, too. on the basis of the data, collected between 1900 and 1962, and of some historic data as well galanopoulos (*) has investigated in detail the region bounded by the 34° n and 42° n parallels and by the 19° e and 29° e meridians. papazachos and comninakis (2) based on revised homogeneous data, concerning the period between 1911 and 1969, were led to some interesting conclusions in respect to the spatial distribution of the earthquake foci. the purpose of the present work is to investigate the seismic activity in the area of greece between 1966 and 1969 and compare this short term activity with the results published previously by other authors. t h e data i n 1965 and 1968 the seismological institute of the national observatory of athens established a network of six stations at several places in the area of greece. with the data from these stations and the central station in athens it is possible to determine with good accuracy the epicenters of all the shocks with magnitude m > 4.4 which occurred in the area bounded by the 34° n and 42° n parallels and by the 19° e and 29° e meridians. with this network of stations we can distinguish the intermediate (ii > 60 km) from the shallow earthquakes. this network helps much to estimate the focal depth of the intermediate earthquakes. these data are regularly published in the monthly bulletins of the seismological institute. i n the present work, data of earthquakes with magnitudes m ^t 4.4 have been used. information for a few intermediate earthquakes with smaller magnitude has also been used. the amplitudes of the two horizontal components of the woodanderson seismograph in athens have been used to calculate the magnitudes of the shallow earthquakes. the following relation has been used to calculate the magnitude of the intermediate earthquakes ml = log a + 0.40 d + 2.80 where d is the distance between the focus and the station of athens, and a is the mean value of the maximum recorded amplitudes by the t h e s e i s m i c a c t i v i t y i n t h e a r e a of g r e e c e b e t w e e n 1 9 6 6 a n d 1 9 6 9 361 two components of the wood-anderson seismograph. the a is measured in mm and j) in hundreds of km. to find the magnitude m the value 0.14 was added to the magnitude ml (3). t h e shallow earthquakes figure 1, is an index map of the location of the epicenters of the shallow earthquakes which have been occurred in the area of and near f i g . 1 i n d e x map of the epicenters of the shallow earthquakes which occurred in the area of greece between 1966 and 1969. 3 6 2 p . a . g i a x n a k o p o u l o s greece in the years 1966, 1967, 1968 and 1969. in the same figure the trend of the main mountain range in the area, and the hellenic trench are shown. i t is observed that the main seismic belt of the shallow earthquakes parallels the alpine folding and lies on the seaward slope of the folding, that is, in the convex side of the aegean island arc. most of the epicenters in the western, southern and southeastern part of the region, lie in the landward slope of the hellenic trench. except the above mentioned seismic zone, which is directly related to the aegean island arc, there are two other groups of epicenters; one in the northwestern turkey and the other in the northern aegean sea. figure 2, shows the cumulative frequency function of the magnitude of these earthquakes. the data are fitted by the equation: log n = 7.18 — 1.01 m f i g . 2 cumulative frequency function of the magnitude of the shallow earthquakes which occurred in the area of greece between 1966 and 1969. earthquakes of intermediate depths figure 3, is an index map of the location of the epicenters of intermediate earthquakes of magnitudes 3.7 < m < 4.8, which oct h e s e i s m i c a c t i v i t y i n t h e a r e a of g r e e c e b e t w e e n 1 9 6 6 a n d 1 9 6 9 3 6 3 cured in the area of greece between 1966 and 1969. in the same figure the isodepths (lines of equal focal depth) of 150 km and 100 km, determined by papazachos and comninakis (2) by the use of data of intermediate shocks with magnitudes m > 4.9 are shown. i n the map of figure 3, three different symbols have been used to denote the earthquakes with focal depths between 50 km and fig. 3 index map of the epicenters of intermediate earthquakes occurred in the area of greece between 1966 and 1969 and having magnitudes between 3.7 and 4.8. the isodepths of 100 km and 150 km determined by papazachos and comninakis (2) on the basis of independent data are also shown. 3 0 4 p . a . g i a n n a k o p o u l o s 80 km, between 80 km and 120 km, and between 120 km and 170 km. tliese symbols have two sizes to represent two magnitude ranges. the epicenters of all the earthquakes with focal depth larger than 120 km, lie on an arc which coincides with the volcanic arc. the epicenters of almost all the earthquakes with focal depth between 80 km and 120 km are located in the inner slope of the sedimentary arc. the epicenters of most earthquakes with focal depth between 50 km and 80 km, are located in the crest line of the main mountain range or in the outer part of the arc. i t is observed that the seismic zone of the intermediate earthquakes is deeping to the inner part of the arc. the data for the intermediate shocks are not homogeneous in order to calculate with accuracy the parameter b. however tliese data show clearly that the value of this parameter is much smaller for the intermediate shocks than for the shallow ones. s u m m a r y and conclusions the seismicity in the area of greece between 1906 and 1969 has been investigated. most of the shallow earthquakes, in this period, occurred along the seismic zone which follows the dynaric arc and lies in the seaward slope of the main mountain range in the area. considerable shallow seismic activity has also occurred in northwestern turkey and northern sporades. the value of the constant b of the magnitude frequency curve of the shallow earthquakes has been found equal to 1.01. the same constant for the intermediate earthquakes has a much smaller value. the epicenters of the intermediate earthquakes show the well known distribution of these shocks in island arcs. that is, thore is a general tendency for the focal depths to increase from the convex to the concave side of the arc. in general, the distribution of the shallow as well as the intermediate earthquake activity has a similar pattern with the distribution derived by the use of data of longer periods. this distribution agrees with the theory that an african lithospheric plate underthrusts the eurasian plate under the aegean island arc and that the african plate moves from the mediterranean sea towards europe. t h e s e i s m i c a c t i v i t y i n t h e a r e a of g r e e c e b e t w e e n 1 9 6 6 a n d 1 9 6 9 3 6 5 acknowledgements t h e a u t h o r e x p r e s s e s his t h a n k s t o d r . b . c. p a p a z a c h o s f o r c r i t i c a l r e a d i n g of t h e m a n u s c r i p t of t h e p a p e r . r e f e r e n c e s ( 1 ) galanopoulos a . , 1963. on mapping of the seismic activity in greece, « a n n a l i di geofísica », x v i , 1. ( 2 ) papazachos b . c., comninakis p . e., 1971. geophysical and tectonic features of the aegean arc, « .t. geplivs. res. », 76, 35. ( 3 ) romney c. f., 1964. an investigation of the relationship between magnitudes scales from small shocks, y e s i a c r e p . 4410-71-x. ( 4 ) schmidt i., 1879. studien iiber erdbeben, 2nd aufl., leipzig. a c o m p u l a t i o n p r o g r a m f o r n u m e r i c a l f i l t e r i n g m . a t t o l i n i ( * ) l . d a l l ' o l i o ( * * ) v . g u e r r i e r i ( * * ) p . r a n di ( * ) ricevuto il 9 aprile 1968 riassunto. — viene descritto u n p r o g r a m m a , scritto in linguaggio f o r t r a n iv, per il filtraggio numerico di serie t e m p o r a l i . tale prog r a m m a è s t a t o p r o v a t o con u n calcolatore del tipo ibm 7094/7040 dcs. l ' o p e r a z i o n e di filtraggio numerico p e r m e t t e di e s t r a r r e o sopprimere, in u n a serie t e m p o r a l e , onde di f r e q u e n z a prefissate i n d i p e n d e n t e m e n t e da variazioni di fase ed ampiezza con cui esse si possono presentare. summary. — a c o m p u t a t i o n p r o g r a m , w r i t t e n in f o r t r a n iv language, for n u m e r i c a l filtering of time series is described. i t was tested with an i b m 7094/7040 dcs c o m p u t e r . the numerical filtering operation allows to e x t r a c t or suppress in a time series prefixed f r e q u e n c y waves i n d i p e n d e n t l y by t h e change of phases and wideness w i t h which t h e y can a p p e a r . m e t h o d . i t h a s b e e n d e s c r i b e d , i n a p r e v i o u s p a p e r ('), a filtering o p e r a t i o n of t i m e series o n e q u i s p a c e d d a t a , c o r r e s p o n d i n g t o a p r e f i x e d r e s p o n s e . i n t h e f o l l o w i n g p a p e r is d e s c r i b e d a c o m p u t a t i o n p r o g r a m b a s e d o n t h e m e t h o d d e v e l o p e d i n ('). a n u m e r i c a l filtering o p e r a t i o n o n a n e q u i s p a c e d d a t a series ni(i = 1,2, ) is o b t a i n e d b y c o m p u t i n g a n e w s e r i e s : +s n'k = sj aj nk+i , k^r + 1 , [1] — r (*) i s t i t u t o di fisica università di bologna. gruppo i t a l i a n o di fisica cosmica. consiglio nazionale delle ricerche. (**) centro di calcolo di bologna. comitato nazionale per l ' e n e r g i a nucleare. 2 0 k m . a t t o l i n i l . d a l i . ' o l i o v . g u e r r i e r t p . r a n d i w h e r e t h e (r + s + 1) q u a n t i t i e s a j a r e called coefficients or w e i g h t s of t h e n u m e r i c a l filter, as d e s c r i b e d in ('). t h e series n ' k c o n t a i n s t h e a l l o w e d f r e q u e n c i e s c o m p u t e d b y m e a n s of t h e used filter. f o r o b t a i n i n g a new series n " * w h i c h does n o t c o n t a i n t h e s a m e f r e q u e n c y b a n d s , one m u s t r e v e r s e t h e coefficients sign of t h e filter, e x c e p t t h e c e n t r a l one w h i c h is r e p l a c e d w i t h t h e d i f f e r e n c e b e t w e e n n u m b e r 1 a n d t h e c e n t r a l coefficient, a n d t h e so o b t a i n e d filter m u s t b e u s e d on t h e original d a t a series. t h e t w o o p e r a t i o n s , n u m e r i c a l f i l t e r i n g a n d t h e d i f f e r e n c e f r o m t h e original series, a r e e q u i v a l e n t t o t h e r e s u l t i n g series n " k — v ' k . t h e choice of t h e coefficients of t h e filter is s t r i c l y c o n n e c t e d t o t h e k n o w l e d g e of t h e p h y s i c a l p r o b l e m a n d i t d e p e n d s on t h e r e s t r i c t i o n s i m p o s e d 011 t h e f r e q u e n c y b a n d t o s e a r c h a n d also on t h e i n d e p e n d e n c e of t h e r e s u l t s . d e s c r i p t i o n o f t h e p r o g r a m . t h e p r o g r a m h a s b e e n c a r r i e d o u t a t " c e n t r o di calcolo del on e n " in b o l o g n a u s i n g t h e f o r t r a n iv l a n g u a g e , v e r s i o n 13, u n d e r t h e i b s y s 7094/7040 dcos m o n i t o r c o n t r o l . t h e d a t a set t h a t c a n b e e l a b o r a t e d is p r a c t i c a l l y u n l i m i t e d ; t h e m a x i m u m allowed set of coefficients of t h e filter is 99. t h i s d a t a set is e x t r e m e l y s u f f i c i e n t f o r m a n y p r o b l e m s b u t i t c a n also b e i n c r e a s e d b y m o d i f y n g t h e a p p r o p r i a t e s t a t e m e n t s . s o m e t i m e s i t m a y b e n e c e s s a r y t o c o m p u t e series of filtering o p e r a t i o n s on t h e s a m e d a t a , a n d , i n t h i s case, i t will b e s u f f i c i e n t t o follows t h e s u g g e s t i o n s d e s c r i b e d a f t e r f o r o b t a i n i n g series of comp u t a t i o n s a t a l o w e s t t i m e . i t h a s also f o r e s e e n t h e e l a b o r a t i o n of s e v e r a l d a t a series w i t h t h e s a m e coefficients or w i t h d i f f e r e n t coefficients f o r g a i n i n g t h e setu p t i m e , i n t h e c o m p u t e r , of t h e r u n n i n g p r o g r a m . b e s i d e s , i t h a s b e e n c o n s i d e r e d t h e n e c e s s i t y of g r o u p p i n g t h e i n i t i a l of filtered series d a t a b e f o r e a n u m e r i c a l filtering: i t allows t o r e d u c e t h a t r a n d o m noise t h a t s h o u l d give u o i n f o r m a t i o n a b o u t t h e t e s t i n g series. t h e p r o b l e m t o t r e a t will s u g g e s t , t i m e a f t e r t i m e , t h e r i g h t choice f o r g r o u p p i n g . t h e p r o b l e m c o n s i s t s i n k e e p i n g t h e t i m e p o s i t i o n of t h e o r i g i n a l d a t a , n a t u r a l l y p r o v i d i n g t h a t t h e n u m b e r of t h e coefficients (r -f s + 1) a c o m p u t a t i o n p r o g r a m f o r n u m e r i c a l f i l t e r i n g 21 5 will b e o d d . t h e c o m p u t e d coefficient n'k is set in t h e m i d d l e of t h e i n t e r v a l only if t h e c o n d i t i o n r = s is satisfied. t h e c o n d i t i o n s r = s m u s t b e satisfied in o r d e r t o use t h i s p r o g r a m c o r r e c t l y . a f u r t h e r e x p e d i e n t , t o a v o i d over-excess of figures f o r s u b s e q u e n t e l a b o r a t i o n s a n d w i t h no d a m a g e t o t h e clearness of t h e p r o b l e m a n d to t h e a m o u n t of t h e g a i n a b l e i n f o r m a t i o n , consists i n s e l e c t i n g t h e s i g n i f i c a n t figures, in t h e r e s u l t s of e v e r y o p e r a t i o n s , f o r t h e considered p r o b l e m . b e s i d e s , t h i s e x p e d i e n t allows t o rescale t h e r e s u l t s as desired so as t o b e seen i n a n a p p r o p r i a t e scale. f u r t h e r e x p e d i e n t s h a v e b e e n described below. o r d e r o f d a t a c a r d s d e c k s e t x j p . all c a r d s m u s t b e i n c l u d e d in t h e o r d e r s h o w n b e l o w . [a] = t i m e series d a t a p a r a m e t e r c a r d . [b] = v a r i a b l e f o r m a t c a r d f o r t i m e series i n p u t d a t a . [c] = t i m e series d a t a . [d] = e n d p u n c h e d in col. 1-3. [e] = c o m m e n t c a r d . [f] = p a r a m e t e r c a r d f o r a filter o p e r a t i o n . [g] = v a r i a b l e f o r m a t c a r d f o r o u t p u t d a t a t o b e p u t on logical t a p e n t p o . [ii] = v a r i a b l e f o r m a t c a r d f o r o u t p u t d a t a t o b e p r i n t e d . [i] = filter coefficients. r e p e a t [e] t h r o u g h [i] as desired. [j] = b l a n k c a r d . [k] = b l a n k c a r d . r e p e a t [a] t h r o u g h [k] as desired. [l] = b l a n k c a r d (to n o t i f y t h e p r o g r a m t h a t t h e e n t i r e j o b s is c o m p l e t e d a n d c o n t r o l m u s t b e r e t u r n e d t o t h e m o n i t o r ) . 20k m. attolini l. dali.'olio v. g u e r r i e r t p . r a n d i e x a m p l e o f d a t a d e c k s e t u p . 1. r e p e a t e d as desired (see page 3) 2. repeated as desired (see page 3) c a r d p r e p a r a t i o n . card columns corres p o n d i n g variable [a] card 1 2 n d s 3 4 5 6 7 8 n t p i k e i t d e s c r i p t i o n f o r m a t ( 4 1 2 ) — n u m b e r of t i m e series d a t a c o n t a i n e d i n e a c h i n p u t c a r d . —l o g i c a l t a p e n u m b e r of i n p u t (5 if i n p u t is f r o m p u n c h e d c a r d s ) . — n u m b e r of d e c i m a l figures if c o n v e r s i o n is u s e d (*) — = 0 if i n p u t c o n v e r s i o n is r e q u i r e d (*) ^ » » » » n o t r e q u i r e d (*). (*) one m u s t use conversion wlien t i m e series i n p u t d a t a are p u n c h e d as integer c o n s t a n t s h a v i n g t h e sign (hole zone) over t h e less significant figure of t h e n u m b e r . i n this case each field of w i d t h w, c o n t a i n i n g t h e n u m b e r , is considered like it were f o r m e d of two sequential fields: t h e 1st of i n t e g e r t y p e of wliidth w — i, t h e 2nd of a l p h a m e r i c t y p e of w i d t h 1. i.e.: if n u m b e r 12.125 is p u n c h e d according to t h e f o r m a t specification p 7 . 3 no conversion is required, being a f o r t r a n n u m b e r (in this case it = 0). the c h a r a c t e r string 12121^, (where n means holes 5 a n d 11, m i n u s sign overpunched) columns of t h e card p u n c h e d in a field of w i d t h w = 5 for having t h e same value as t h e above one, m u s t be read according t o t h e f o r m a t specification (14, a l ) and must be ke = 3 (decimal figures of t h e number) a n d it ^ 0. a c o m p u t a t i o n p r o g r a m f o r n u m e r i c a l f i l t e r i n g 21 5 c a r d ' c o l u m n s c o r r e s p o n d i n g d e s c r i p t i o n v a r i a b l e [b] card format (13a6,a2) 1-80 f r 1 — f o r m a t of t i m e series i n p u t d a t a m u s t b e like (a3, s p e c i f i c a t i o n s f o r i n p u t d a t a ) . c o l u m n s 1-3 c a n n o t b e u s e d . t h i s r e s t r i c t i o n is u s e d t o allow a n i n d e f i n i t e n u m b e r of t i m e series i n p u t d a t a c a r d s , t h e e n d of w h i c h is r e a c h e d w h e n t h e p r o g r a m r e a d s t h e [d] c a r d . [g] card f r 1 f o r m a t (see [b] c a r d ) 4-n e l (n < 80) ( k a , — t i m e series i n p u t d a t a (those d a t a will b e t h e k b ) 1 s t file of a t a p e w h e r e logical n u m b e r is 3. t h i s file is a d d r e s s e d b y m e a n s t h e v a l u e 1 a s s i g n e d t o t h e n f e l v a r i a b l e ) . t h e i n i t i a l t i m e series d a t a a n d successive filtered ones a r e r e c o r d e d on logical t a p e 3 t o f o r m a s e q u e n c e of files. e a c h file c o n t a i n s a s e q u e n c e of r e c o r d e d d a t a . a s d e s i r e d , t o filter d a t a r e c o r d e d i n t o o n e of t h o s e files, one m u s t assign t o t h e n f e l p a r a m e t e r t h e v a l u e of t h e file a d d r e s s (its p h y s i c a l p o s i t i o n on t h e reel) a s s u m e d p u r i n g t h e file r e c o r d i n g . [/)] card format ( a 3 ) 1 3 k e n t ) — t h e w o r d e n d must b e p u n c h e d in t h e corr e s p o n d i n g c o l u m n s . t h i s c a r d n o t i f y t h e p r o g r a m t h a t all t h e c a r d s c o n t a i n i n g t i m e series i n p u t d a t a h a v e been r e a d . [£] card format (13a6,a2) 1-80 f c o m — t h i s is a c o m m e n t c a r d (i.e. i t m a y c o n t a i n s a c o m m e n t a b o u t t h e n u m e r i c a l filter o p e r a t i o n ) . [i1] card format ( 9 1 2 , e l l . 6 ) 1-2 n f e l — specifies t h e a d d r e s s of t h e file of logical t a p e 3 c o n t a i n i n g t h e i n p u t d a t a f o r t h e filter o p e r a t i o n ( n f e l = 1 a d d r e s s e s i n i t i a l i n p u t d a t a ) (see [c] c a r d ) . 3-4 n — n u m b e r of filter coefficients. 5-0 n d p — n u m b e r of d a t a p e r r e c o r d t o b e p u t on logicel t a p e n t p o . (must a l w a y s b e n d p > 0). 20k m. a t t o l i n i l . d a l i . ' o l i o v . g u e r r i e r t p . r a n d i c a r d c o l u m n s [jp] car 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 3 2 c o r r e s p o n d i n g v a r i a b l e d (cont.) n e n t p o m s t a m p e e i c k a 2 a l d e s c r i p t i o n — n u m b e r of i n p u t d a t a t o b e g r o u p p e d b e f o r e e x e c u t i n g filter o p e r a t i o n . — logical t a p e n u m b e r on w h i c h (if m p e e ^ 0) o u t p u t d a t a a r e p u t (i.e. f o r p u n c h i n g off-line). — 1 t o p r i n t o u t p u t 0 n o t t o p r i n t o u t p u t — 1 t o p u t o u t p u t on logical t a p e n t p o 0 n o t t o p u t o u t p u t on logical t a p e n t p o — 1 if o u t p u t c o n v e r s i o n f o r r e c o r d i n g on logical t a p e n t p o is r e q u i r e d 0 if o u t p u t c o n v e r s i o n is n o t r e q u i r e d . — n u m b e r of figures f o r o u t p u t d a t a . — rescale f a c t o r . t h e rescale f a c t o r ( a l ) allows t h e p r o g r a m t h e rescaling of t h e o b t a i n e d r e s u l t s . k a 2 allows t h e user t o fix t h e n u m b e r of o u t p u t figures of t h e rescaled r e s u l t s . i.e.: if t h e filter o p e r a t i o n h a s given t h e n u m b e r 23456 a n d u s e r desires t o o u t p u t o n l y t h e n u m b e r 34 (2nd a n d 3 r d figures of t h a t r e s u l t ) h e m u s t fix k a 2 = 2 a n d a l = 0.01. [g] card f o r m a t ( 1 3 a 6 , a 2 ) 1 8 0 f e 2 — f o r m a t of o u t p u t d a t a t o b e p u t on logical t a p e n t p o (see [ i f ] c a r d ) if t h e s e o u t p u t a r e n o t desired ( m p e e = 0 in [ f ] c a r d ) t h i s c a r d c a n also b e a b l a n k c a r d . if m p e e ^ 0 t h e f o r m a t specified must b e l i k e (13, s p e c i f i c a t i o n s f o r o u t p u t d a t a ) . c o l u m n s 1 3 of o u t p u t r e c o r d s t o b e p u t on logical t a p e n t p o c o n t a i n a c a r d s e q u e n c e n u m b e r . « s p e c i f i c a t i o n f o r o u t p u t d a t a » m u s t b e like , i w , al, w h e r e w = ka2-1 if i c = 1, or like i w w h e r e w = k a 2 if i c = 0 f o r each o u t p u t d a t a . [h] card format (13a6,a2) — f o r m a t of o u t p u t d a t a t o b e p r i n t e d . if t h i s o u t p u t is n o t desired (msta = 0 in [h] c a r d ) t h i s c a r d c a n also b e a b l a n k c a r d . a computation program for numerical f i l t e r i n g 21 5 d e s c r i p t i o n if m s t a # 0 t h e f o r m a t specified must b e like (15, specifications f o r o u t p u t d a t a ) . t h e 1st 5 c h a r a c t e r s of e a c h p r i n t i n g line a r e t h e line s e q u e n c e n u m b e r . " s p e c i f i c a t i o n s f o r o u t p u t d a t a " m u s t b e like i w , w h e r e w = k a 2 f o r each o u t p u t d a t a . f o r m a t ( 8 x , 1 2 f 6 . 5 ) — filter coefficients. (these m u s t b e p u n c h e d i n t h e following o r d e r : c e n t r a l coefficient a n d t h o s e a t i t s r i g h t ) . [j], [ k ] a n d [l] m u s t b e b l a n k c a r d s . r e p e a t [e] t h r o u g h [i] c a r d s t o do o t h e r n u m e r i c a l filter o p e r a t i o n or using i n i t i a l d a t a ( n f e l = 1) or u s i n g o u t p u t d a t a o b t a i n e d f r o m t h e initial ones (see [c] c a r d ) . c o m m e n t . i n t h e f o l l o w i n g p a g e s it lias b e e n r e p r o d u c e d t h e list of t h e d e s c r i b e d p r o g r a m . a s a n e x a m p l e of u s a g e of t h e r e s u l t s i t h a s b e e n also r e p r o d u c e s a p l o t t e r (see f i g . 1) o b t a i n e d h a n d l i n g t h e 7091/7010 d c s o u t p u t s b y a n a d d i t i o n a l p r o g r a m r u n n i n g on a n i b m 1620 c o m p u t e r . t h e a u t h o r s h a v e m a i n l y c o n t r i b u t e d each in t h e i r a c t i v i t y field. a c k n o w l e d g m e n t s . p r o f . m. galli g a v e t h e o p p o r t u n i t y of v e r y i n t e r e s t i n g discussion. p r o f . e . c l e m e n t e l , d r . a. chiarini, d r . c. lolli m a d e e a s y t h e i b m 7094/7010 d c s c o m p u t e r s u t i l i s a t i o n a t t h e " c e n t r o di calcolo del c n e n " of b o l o g n a ( s h a r e i n s t a l l a t i o n b i ) . correcard s p o n d i n g colums . . . v a r i a b l e [e] card (cont.) [i] card 9-80 co e f r e q u i r e d r o u t i n e s : r f 5 k 0 1 s d a n . 3 0 6 1 a n d r f 5 k 0 3 s d a n . 3057 j a n u a r y 1 9 6 6 2 3 2 5 2 7 2 9 u.t. f i g u r e 1 c a p t i o n : a) original d a t a ; b) f i l t e r e d d a t a ; c) f i l t e r e d d a t a b y r e v e r s e d l>) filter coefficients. a c o m p u t a t i o n p r o g r a m f o r n u m e r i c a l f i l t e r i n g 2 1 5 dimension fcoml 1u1.fr1 ( 1 u ) . f r 2 ( 1 u i , f r 3 i 1 u ) . e l i 5 0 3 0 ) . f i l i 5 0 0 0 ) , 1 k f il ( 5 0 0 0 ) , n r e c i 1 0 0 ) , c o e f l 1 0 0 ) dimension k a ( u 0 ) , k 3 i u 0 ) ecuivalence(fil,<fil) data kend/3hend/.icift/10737u182u/ 530 read!5,203 ) nds,ntpi,ke,it ifintpi.eq.o.or.nds.eq.o) go to 1530 2 0 3 f 0 r m a t ( 9 i 2 , e 1 4 . 6 ) ke =1 0*»ke rewind 3 re ad( 5 , 2 0 2 ) (fr1 ( < ) . k. = 1. 11+) 2 0 2 format(13a6,a2) c c end file 3 nfiie=1 *jtdt = 0 i f t i t . l e . 0 1 g 0 to 5 0 0 50 3 r e a d i n t p i , f r 1 ) k f i n , ( k a ( j ) , k b ( j ) , j = 1 ,nds) ifikf1n.ec.kend1g0 to 501 do 5 0 2 j = 1« nds kb!1)= mod(kb 1 j ) / k i ft , 1 6 ) 50 2 e l l j ) = i s i g n t k a ( j ) » 1 0 + i a b s i m o d ( k b ( l ) , 1 0 ) ) , k b l l ) ) / k e nt ot =nt 0t + 1 writel31nt0t,i eli j ) , j = 1 ,nds) go to 503 c 50 0 r e a d ! n t p i . f r 1 ) k f i n , ( e l ( j ) , j = 1 , n d s ) ifikfin.eq.kendjgo to 501 ntot=ntot+1 w r i t e i 3 j n t 0 t , ( e l ( j ) * j 1 , n d s ) go to 500 c 501 end file 3 nrec(nfile)=ntot»nds 536 read! 5 , 2 0 0 ) (fcomlk) t k=l, 1 u) 200 format!13a6,a2) write« 6 , 2 0 1 ji fcomlk),k = 1,11») 20 1 format 1 1 h 1 , 6 1 / ) , 2 8 x , 1 3 a 6 , a 2 / / 2 8 x , 8 0 < 1 h ) ) c c 506 read« 5 , 2 0 3 ) nfel,1,ndp.nr,ntpo,msta,mper,ic,ka2,a1 ifinfel.eq.o.or.nfel.gt.nfile.or.n.eo.o. or. nrec(nfel). eq. 0)got0530 read! 5 , 2 0 2 ) ( fr2(k),k = 1 , 1 l » ) f ( f r 3 ( k ) , k * l , 1 l » > rewind l* mfil=nfile-nfel+2 call bakfilt3.mfil) c c m1 =n/2 m2=m1•1 read! 5 , 3 3 7 ) ( c o e f l i ) , i = m2,n) 337 format» 8 x , 1 2 f 6 . 5 ) do 50 1=1,ml m3=n-i•1 el( i ) = 0 . 50 coefli)=coef(m3) ka2=10»»ka2 2 1 (i m . a t t o l i n i l . d a l l ' o l i o v . g u e r r i e r i p . r a n d i nn=ntot-nvr+1 do 8 j = 11 nvr el(j)=el(nn) 8 nn=nn+1 i2=nvr c nsp=k3/ndpgo to 513 17 nsp» 1 if(k3« eq.ndp) go to 513 k4=k3+1 do 514 mm=k4,ndp 514 f il{mm)=0» 513 do 15 j 1 = 1 » k3, ndp j2=j1+ndp-1 write141 ( f i l i j ) , j = j 1 , j 2 ) 505 ifi(msta+mper1.le.0) go to 301 do 16 h j = j 1 , j 2 i fifilimj l . n e . o . ) go to 416 kfil(mj)=0 go to 16 416 kfil(mj ) =mod( i f i x ( f i l « m j ) » m » s i g n ( . 5 , f i l í h j ) j ) , ka2 ) 16 continue i f ( m s t a . g t . 0 ) w r i t e ( 6 , f r 3 ) j n p , ( k f i l l j ) , j = j 1 , j 2 ) ifimper.lt.1) go to 301 i f( i c . e q . l ) go to 613 write(8.fr2) j n p . i k f i k j ) , j = j 1 , j 2 j go to 301 613 do 300 j = j 1 , j 2 j j = j j 1 + 1 k a ( j j ) = i a b s ( k f i l ( j ) / 1 0 ) 300 k b ( j j ) = m o d ( k f i l ( j ) , 1 0 ) » k i f t writeintp0.fr2) j n p , ( k a ( j ) , k b < j ) , j = l . j j ) 301 jnp=jnp+1 15 continue go toi 511> 1101,k<k 511 nre = nre-ntot«-ig-l ig=nvr*1 ifinre.le.o i go to 104 go to 11 104 if(nvr.lt.nr»n!g3 to 110 kkk=2 go to 105 1 10 hfil«mfil-1 call skpfil(3»hfil) nf ile=nfil e +1 rewind 4 c c jnp=jnp-1 i np = 0 if{jnp.lt.nds)g0 to 507 k 1 = j np/nds do 508 mm=1 « <1 j l = 1 j 2=ndp do 509 mj = 1 «nds a c o m p u t a t i o n p r o g r a m f o r n u m e r i c a l f i l t e r i n g 21 5 jnp = 1 nvr=0 ml = 1 kkk = 1 ifinr.gt.1)g0 to 13 nr= 1 go to 111 13 m 1=2 m 1 = m 1*nr do 600 i=m2,m1 600 e l i i ) = 0 . m2=m1+ 1 111 ig=m2 c c k 1=4800 nre=nrec(nfel) 11 if(k1.gt.nre)m=nre k1=k1+1 g— 1 do 1 11 = 1 6 , k 1 , nds 12=11+nds-l 1 r e a d t 3 ) n u m , ( e l ( j ) , j = i 1 , 1 2 ) 105 nt0t=i 2 w r i t e ( 6 , 7 6 7 ) i e l ( j ) , j = 1 , n t 0 t ) t67 format t 2 4 f 5 . 0 ) nrag=ntot/nr k3 = nrag-n<-1 if(kkk.eq.2)go to 18 kres=m0d(k3,ndp) k3=k3-kres 18 go to < 7 0 1 , 7 0 2 ) , n 1 c c 702 nr 1 = 1 nr2=nr nrag=k3+n-1 frag=nr do 20 kra=1,nrag el(kra)=el(nr 1 ) nr1=nr1+1 do 21 ira=nr1,nr2 21 el(kra)=el(kra)+el( ira) el1kra)=el(kra)/frag nr1=nr2+1 20 nr2=nr2+nr c c 701 do 12 kk=1,k3 f il(kk)=el(kk)»coef(1 ) kz = kk do 14 mk = 2 « n kz = kz + 1 14 f1l(kk)=fil(kk)+el(kz)»c0ef(mk) 12 continue c c ifikkk.eq.2) go to 17 nvr=nt0t-k 3»nr 2 ik m. a t t o l i n i l. d a l l ' o l i o v. g u e r r i e r i p . r a n d i read i 4 ) ( f i l i j ) « j = j1> j 2) j 1 =j2 + 1 509 j2=j2+ndp j l = l j 2 = nds do 510 mj=1,sdp inp=inp +1 writ e ( 3 ) i n p » ( f i l i j ) » j = j 1 , j 21 j 1 =j 2+1 510 j2=j2+nds 508 continue 507 km=modljnp.nds) ifikm.eq.oígo to 561 j 1 = 1 j2=ndp do 5 1 2 hj=1 « km read!u)ifili j ) , j * j 1 , j 2 ) j 1=j 2+1 512 j2=j2+ndp if{kkk.ne.21k3=n0p ntra=km»ndp-ndp+k3 km=nds-modintra,nds) ifikm.eq.ojgo to 5 1 5 do 516 mj=1,km mm=ntra+mj 516 fili mm)=0. 515 km=ntra/nds+1 if(modtntra.nds).eq.o) km=km-1 j 1 = 1 j 2 = nds do 517 mj=1,(m inp=inp+1 w r i t e ( 3 ) i n p . ( f i l i j ) . j = j 1 . j 2 ) j 1 =j 2+1 517 j2=j2+nds 561 nt0t=inp go to 501 c c 1530 stop end t i m i n g : 1) t o t e s t 960 i n p u t d a t a of a t i m e s e r i e s w i t h 2 n u m e r i c a l filtering e a c h of 69 c o e f f i c i e n t s g r o u p p i n g i n i t i a l d a t a 2 b y 2, i t t a k e s 1 ' 1 2 " ( i n c l u d i n g t h e m o n i t o r t i m e a n d t h e p r o g r a m c o m p i l a t i o n , t h e r e f o r e t h e t i m e of c o m p u t a t i o n is o n l y 2 0 " ) . 2) t o t e s t 3 4 5 6 i n p u t d a t a of a t i m e s e r i e s w i t h 3 n u m e r i c a l filtering of 9, 69, 3 5 c o e f f i c i e n t s r e s p e c t i v e l y g r o u p p i n g 2 b y 2 o n c r o s s i n g f r o m i o t o 2° i t t a k e s 1 ' 4 3 " ( i h c l u d i n g t h e m o n i t o r t i m e a n d t h e p r o g r a m c o m p i l a t i o n : t h e r e f o r e t h e t i m e of c o m p u t a t i o n is o n l y 3 2 " ) . r e f e r e n c e s (*) galli m., randi p . , on the design of the optimum numerical filter with a prefixed response. " a n n a l i di geofisica " , x x , 4, (1967). annals n.6/2003 ok 23/04 1315 annals of geophysics, vol. 46, n. 6, december 2003 key words microseismicity – low-angle normal fault – seismic gap – seismic hazard 1. introduction the northern apennines (italy) undergo active crustal extension of about 0.5 × 10−7 yr–1 (hunstad et al., 2003) accommodated by moderate to large normal faulting earthquakes along a nw-elongated seismic belt (fig. 1). a microseismic study in a low seismicity area of italy: the città di castello 2000-2001 experiment davide piccinini (1), marco cattaneo (1), claudio chiarabba (1), lauro chiaraluce (1), martina de martin (1), massimo di bona (1), milena moretti (1), giulio selvaggi (1), paolo augliera (2), daniele spallarossa (3), gabriele ferretti (3), alberto michelini (4), aladino govoni (4), paolo di bartolomeo (4), marco romanelli (4) and julius fabbri (4) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) istituto nazionale di geofisica e vulcanologia, milano, italy (3) dipartimento per lo studio del territorio e le sue risorse (dip.te.ris.), università di genova, italy (4) istituto nazionale di oceanografia e di geofisica sperimentale – centro di ricerche sismologiche (inogs-crs), udine, italy abstract recent seismological studies contribute to better understand the first order characteristics of earthquake occurrence in italy, identifying the potential sites for moderate to large size earthquakes. ad hoc passive seismic experiments performed in these areas provide information to focus on the location and geometry of the active faults more closely. this information is relevant for assessing seismic hazard and for accurately constraining possible ground shaking scenarios. the area around the città di castello basin, in the northern apennines (central italy), is characterized by the absence of instrumental seismicity (m > 2.5), it is adjacent to faults ruptured by recent and historical earthquakes. to better understand the tectonics of the area, we installed a dense network of seismic stations equipped with broadband and short period seismometers collecting data continuously for 8 months (october 2000-may 2001). the processing of ∼ 900 gbyte of data revealed a consistent background seismicity consisting of very low magnitude earthquakes (ml < 3.2). preliminary locations of about 2200 local earthquakes show that the area can be divided into two regions with different seismic behaviour: an area to the nw, in between sansepolcro and città di castello, where seismicity is not present. an area toward the se, in between città di castello, umbertide and gubbio, where we detected a high microseismicity activity. these findings suggest a probable different mechanical behaviour of the two regions. in the latter area, the seismicity is confined between 0 and 8 km of depth revealing a rather well defined east-dipping, low angle fault 35 km wide that cuts through the entire upper crust down to 12-15 km depth. beside an apparent structural complexity, fault plane solutions of background seismicity reveal a homogeneous pattern of deformation with a clear ne-sw extension. mailing address: dr. lauro chiaraluce, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: chiaraluce@ingv.it 1316 davide piccinini et al. in the past 30 years, three seismic sequences following mw 5.0-6.0 mainshocks, struck a 70 km long section of the northern apennines from norcia (1979, ms = 5.8) to gubbio (1984, ms = 5.3). for all these sequences, seismological data show that earthquakes occur on nw-trending normal faults the lengths of which are usually less than 10 km and dipping towards the southwest (deschamps et al., 1984; haessler et al., 1988; westaway et al., 1989). during the most recent sequence that struck the region in 1997 and 1998, seven mainshocks (5.0 < mw < 6.0, ekström et al., 1998) ruptured adjacent sw-dipping fault segments composing a 50 km long portion of the seismic belt (chiaraluce et al., 2003). at the northwestern end of this «seismically active» belt lies a «silent» region located near the town of città di castello (hereafter cdc). the southern portion of the study area had no significant earthquakes for the past 1.0 kyr (see the historical catalogue of boschi et al., 1997) and an impressive lack of instrumentally recorded seismicity ml > 2.5 earthquakes, which represent the lower threshold magnitude for the located earthquakes detected by the italian network (rsnc). to investigate the cdc seismically silent region, we acquired digital, mostly broadband, data with a seismic array consisting of 30 12 00′ 12 24′ 12 48′ 42 48′ 43 12′ 43 36′ 0 5 10 km 1192 1352 1458 1768 1789 1799 1832 1917 1918 1919 1741 1751 1781 1279 1328 1353 sansepolcro città di castelloarezzo gubbio perugia umbertide 2001/11/25 1998/03/04 1984/29/04 1997/09/26 tiber valley fig. 1. historical earthquakes (empty squares, from cfti-cpti) and instrumental seismicity (ml > 2.5) of the area (from earthquake bulletin 1990-1999): in black the focal mechanisms of gubbio (1984), gualdo tadino (1998) and sansepolcro (2001); in red the mainshock of colfiorito seismic sequence (1997). the dashed box enclose the studied area. 1317 a microseismic study in a low seismicity area of italy: the città di castello 2000-2001 experiment three-component stations. the experiment lasted for 8 months. the experiment aims at defining possible fault location, geometry and kinematics by using the recorded microearthquakes. in this study, we present the recorded data with preliminary hypocentral determinations and focal mechanisms of the recorded seismicity. 2. the cdc experiments between october 2000 and may 2001, a dense temporary local network of 30 digital three-component stations was installed in an area extending 70 × 40 = 2800 km2, from sansepolcro (in the north) to 20 km south of gubbio (fig. 2). we installed reftek, nanometrics and lennartz digital stations equipped with three-component sensors (lennartz le3d-5s, guralp cmg40t, lennartz le3d-1s, mark l22). the geometry of the network varied during the experiment in order to fulfil the aims of the project. 2.1. the test in the first week of october 2000, a data acquisition test was performed to provide a first look at the ongoing local seismicity. this initial part of the 12 00′ 12 15′ 12 30′ 12 45′ 43 00′ 43 15′ 43 30′ 43 45′ ingv ogs-trieste dipteris-genova other permanent network rsnc perugia gubbio città di castello sansepolcro arezzo 20 km t iber v alley fig. 2. map of the temporary seismic network operating in the area. 1318 davide piccinini et al. experiment was also helpful to optimize the network geometry. the test network consists of 10 stations positioned around the nodes of a regular grid spaced at about 10 km. the stations continuously telemetered the data to a seismic acquisition center located in a mobile van. after optimizing the trigger and the coincidence parameters of the acquisition system, it was possible to monitor the seismicity of ml ≤ 1 in real-time. a total of 181 events (143 both local and regional, and 38 teleseisms) were recorded during the month-long experiment. 2.2. the main experiment as the test confirmed the presence of a significant level of background seismicity, we distributed the remaining 20 stations around the 10 stations of the test network (see fig. 2). from the beginning of november 2000, the network was composed of «stand alone stations» only recording continuously. the radio transmitters were replaced by systems recording on hard disks (lennartz digital uher). at a sampling rate of 125 sps per channel, the memory capacity of the stations lasted for more than 1 month. the instruments were powered by high capacity batteries (200 ah) and solar panels. stations were located along secondary country roads, mainly in woodland, far away from any anthropic noise. sites characterized by solid rock outcrop, good insulation for solar panels and visible sky for gps were favoured. the seismometers were buried, when possible, and covered with a heavy cap to avoid wind and rain disturbances. the stations were visited once a month for data collection and servicing. after processing the data of the first 4 months, a cluster of shallow (from 2 to 4 km depth) hypocenters was identified roughly 10 km to the north of gubbio. thus, we decided to move some of the stations from the peripheries of the network to the gubbio area to improve the network geometry around the hypocentral cluster. 2.3. event detection in order to extract seismic events from the continuous raw data, we developed a detection algorithm, denominated detector hereafter. detector is based on the same principles as the automatic detection systems used by centralized networks. we applied an sta/lta (short time average/long time average) trigger algorithm to the continuous data. the values for sta, lta, ratio threshold and other trigger parameters were optimized by performing numerous test runs: sta = 2 s, lta = 40 s, threshold = 3, duration = 2 s, coincidence interval = 40 s, coincidence level = 5. this resulted in an earthquake detection level less than ml ≈ 1 and minimum number of false events. the detection procedure was composed as follows: firstly we selected intervals with sta/lta (i.e. short versus long time averages) greater than a chosen threshold on each station data. then a trigger coincidence of five stations was chosen to declare an event. event time windows were extracted from the continuous data of all 30 stations based upon the selected triggers. we also find the best combination for the time length of sta/lta required to be greater of the threshold, the time interval during which triggers have to be considered coincident and finally the number of coincident triggers necessary to define a possible seismic event. 3. data analysis despite the low magnitude of the events, a high signal/noise ratio allowed us to read pwave and s-wave onset on digital waveforms with the accuracy of few samples for the p onset and less than 0.1 s for s-wave readings. the phase readings has been weighted according to reading accuracy and the following «01234»hypo71 (lee and lahr, 1975) scheme was adopted: 0 (≤ 0.03 s), 1 (0.03-0.06 s), 2 (0.060.1 s), 3 (≥ 0.1 s), and 4 (ignore). a total of 2200 earthquakes were located by using hypoellipse (lahr, 1989) and a gradient 1d velocity model obtained by inverting 1994 earthquakes for a total of 14245 p-phases and 14105 s-phases by using velest (kradolfer, 1989; kissling et al., 1994) code. the starting velocity model was derived from studies performed on adjacent region (deschamps et al., 1984; chiaraluce et al., 2003). the best 1d velocity model was reached after 4 1319 a microseismic study in a low seismicity area of italy: the città di castello 2000-2001 experiment iterations, with a final unweighted rms of 0.04. the p-wave velocity values found are consistent with those derived by logs and laboratory experiment (bally et al., 1986). inversion results (fig. 3) show the presence of a well constrained, low velocity layer (5.7 km/s) at a depth of 20 km. although the distribution of seismicity is mainly located in the first 15 km in depth, the crustal volume affected by the low velocity layer is sampled by a good number of seismic rays (> 500 rays and about 20 hypocenters). the deepest earthquakes recorded were localized around 60 km at depth. the vp / vs ratio computed by velest is also well constrained around the value of 1.84. similar values were found in adjacent regions (haessler et al., 1988; ripepe et al., 2000). finally, a total of 2215 events were located by using hypoellipse (lahr, 1989). to determine ml, synthetic wood-anderson seismograms were calculated by removing the instruments response of each recording and convolving the displacement ground motion with the standard wood-anderson torsion seismograph response (anderson and wood 1925; richter, 1935). observed ns and ew amplitude values were averaged to constitute a single measurement (i.e. sqrt (ns2 + ew2)). following the method originally used by richter (1935,1958), ml is given by m a a slog logl 0= (3.1) where a is the maximum trace amplitude in millimetres measured from a wood-anderson seismogram, a0 is a distance dependent attenuation curve, and s is a station dependent ml adjustment. we assume a parametric form of log a0 proposed by alsaker et al. (1991) and derived by bakun and joyner (1984) as a r n r r r r r log log ref ref ref 0 = + -k v^ ` ^ ^ h j h h (3.2) where r is the hypocentral distance (in km), n and k are parameters related to geometrical spreading and attenuation of s-waves. rref is the reference distance and v (rref) is the reference magnitude value corresponding to rref. for instance, rref = 100 km corresponds to v (rref) = 3 whereas rref = 17 km corresponds to v (rref) ~ 2. to establish the new magnitude scale, we combined eqs. (3.1) and (3.2) to give an over determined sparse system of equations. under the constraints that station corrections sum to zero, the inversion of the linear system was performed by a least-squares procedure based on lsqr algorithm (paige and saunders, 1982). bootstrap resampling techniques (e.g., efrom, 1979) were used to estimate the inversion stability; station corrections, attenuation parameters and magnitude values are computed by averaging the results of 5000 repeated inversions on randomly resampled data sets. bootstrap results are also used to estimate the standard deviation of the inverted parameters. inversion was performed normalizing the local magnitude scale to motions at ref = 17 km (vref ∼ 2.0). we obtain n = (1.420 ± 0.03), k = = (0.009 ± 0.0009) and station corrections ranging between 0.46 and − 0.30. 0 1 2 3 4 5 6 7 8 9 100 90 80 70 60 50 40 30 20 10 0 velocity (km/s) d e p th ( km ) p wave (km/s) s wave (km/s) fig. 3. best 1d velocity model for the study area. 1320 davide piccinini et al. fig. 4a-d. statistical plots for the localized events. a b c e d 1321 a microseismic study in a low seismicity area of italy: the città di castello 2000-2001 experiment 11 48′ 12 00′ 12 12′ 12 24′ 12 36′ 12 48′ 13 00′ 13 12′ 42 48′ 43 00′ 43 12′ 43 24′ 43 36′ 43 48′ 44 00′ 0 5 10 km arezzo perugia umbertide 0 10 20 30 40 50 60 -30 -20 -10 0 10 20 30 0 10 20 30 40 50 60 -30 -20 -10 0 10 20 30 gubbio città di castello sansepolcro depth(km) 0 60 a b t iber v alley ba fig. 5. map and two vertical sections of located earthquakes. the dashed box enclose the hypothesized seismic gap discussed in the text. white dots represent quarry blasts localized at very shallow depth. 1322 davide piccinini et al. finally some statistical analyses were performed on the temporal distribution of seismicity. the main results can be summarized as follows: i) the distribution of the local magnitude for the recorded events is complete down to m = = 1.2, while ranging between 0 and 3 (fig. 4a). ii) the number of phases per events spans from a minimum of 8 phases (about 30 events) to 60 phases per event (including pand s-wave readings from the ingv-national seismic network), indicating the good design of the acquisition experiment, the choice of low-noise sites and minimum failure of the instrumentation (fig. 4b). iii) the rms mean value around 0.08 s indicates an overall high quality of phase data and of the velocity model adopted (fig. 4c). this is also shown by the final locations depicted on the plan view map and along the vertical sections. the number of events versus hour of the day is almost constant. the cumulative number of events (fig. 4d) shows a constant increase between october 2000 and february 2001 while between march and april 2001 this trend shows an evident tilt-up (fig. 4d,e), corresponding to the period when the network was densified around the main clusters of seismicity. we also used blasts produced by some quarries to test the goodness of earthquake location (a in fig. 5). for the best located events, we computed focal mechanisms by using those events which has at least 12 p-wave polarities (fig. 6). here we present the solutions for 150 events computed by using fpfit code (reasemberg and oppenheimer, 1985) which have quality factors equal to a. 12 30′ 43 30′ 0 5 10 km città di castello perugia umbertide gubbio fig. 6. 150 focal solutions for the best located earthquakes. 1323 a microseismic study in a low seismicity area of italy: the città di castello 2000-2001 experiment 4. data interpretation figure 5 shows the epicentral map of the located earthquakes with horizontal and vertical errors less than 1 km and an azimuthal gap less than 240° (80% of the seismicity has a gap less than 180°). earthquakes concentrate on a nw-trending structure elongated for 20-30 km to the north of gubbio and toward the south. background seismicity in the area close to cdc town is scarse, leaving the hypothesized seismic gap almost silent (a in fig. 5). this behaviour is neither referable to the network geometry nor to the detection method which was applied to the data using the same trigger parameters. this is granted by the presence of some microearthquakes in the north-western part of the network (see a in fig. 5) which were localized also using data from the rsnc network. from the vertical sections across the fault system (fig. 5), we observe an east-dipping cloud of hypocenters that, from around 2 km depth beneath the western side of the tiber valley, reach about 12-15 km of depth to the east, cutting the upper crust beneath the apenninic belt. we interpret this seismicity to occur on a gently east-dipping fault, possibly related to the alto tiberina fault previously recognized comparing available seismological data with seismic reflection profiles and geological investigation (barchi et al., 1998; boncio et al., 2002; collettini et al., 2002). in the central part of the study area, the cluster of hypocenters located to the north of gubbio is confined at depth by the east-dipping fault intersecting the west-dipping seismicity at a depth of about 6 km. a few earthquakes occur at midcrustal depth and down to 50 km in the uppermost mantle. the occurrence of this sub-crustal seismicity could be related to the flexured subducting adriatic lithosphere as previously observed by selvaggi and amato (1992). the focal solutions for a very high number of events (> 80%) show a general ne-trending direction of extension, with nw-trending both sw and ne dipping normal fault planes, consistent with the regional stress observed by large earthquakes and borehole breakouts (mariucci et al., 1999; chiaraluce et al., 2003). some inverse solutions are found for the deepest earthquakes. 5. conclusions the microseismicity distribution, revealed by the high detection capability of the local array, let us define two different zones: one in the se, where low magnitude background seismicity is present and well organized, and the other to the nw characterized by the lack of microseismicity. consistently with larger magnitude events, background seismicity occurs on small nw-trending normal fault segments governed by the regional ne trending extension. in the southern and central portions (from gubbio to cdc), the microseismicity defines a seismic volume elongated for 50 km and from 0 to 15 km in depth, confined on the hanging-wall of the low angle east-dipping normal fault (alto tiberina fault). this latter fault seems to play a dominant role in controlling the deformation style of the area, cutting the upper crust almost completely. in the northern part, background seismicity is nearly absent defining a ≈ 20 km long area northward limited by an area (sansepolcro) where three small seismic sequences (ml < 4.5) occurred in the past 15 years (braun et al., 2003). these sequences are referable to the northern segment of the atf fault, or to the antithetic sw dipping normal fault. in the silent area, no earthquakes occur on the east-dipping low angle normal fault hypothesized by active seismic data. the presence of historical earthquakes (1458, mcs = ix; 1789, mcs = viii-ix; 1917, mcs = ix) suggests that this portion of the fault could be presently locked, supporting the hypothesis of a seismic gap. acknowledgements this experiment was partially founded by gruppo nazionale per la difesa dai terremoti within the project: «development and comparison among methodologies for the evaluation of seismic hazard in seismogenic areas: application to the central and southern apennines», coordinated by m. cocco. we thank all the people who contributed to the field work and in particular the technical staff of the ingv mobile network laboratory. 1324 davide piccinini et al. thanks to m. ibs-von seht for the discussion useful to the improvement of the manuscript, and to salvatore barba and massimiliano rinaldo barchi for the useful comments in reviewing the manuscript. references alsaker, a., l.b. kvamme, r.a. hansen, a. dahle and h. bungum (1991): the ml scale in norway, bull. seismol. soc. am., 81 (2), 379-398. anderson, j.a. and h.o. wood (1925): description and theory of the torsion seismometer; 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(received june 16, 2003; accepted october 20, 2003) layout 6 community seismic network robert w. clayton1,*, thomas heaton2, mani chandy3, andreas krause3, monica kohler2, julian bunn4, richard guy1, michael olson3, mathew faulkner3, minghei cheng2, leif strand4, rishi chandy3, daniel obenshain3, annie liu3, michael aivazis4 1 california institute of technology, seismological laboratory, pasadena, ca, usa 2 california institute of technology, earthquake engineering, pasadena, ca, usa 3 california institute of technology, computer science, pasadena, ca, usa 4 california institute of technology, center for advanced computing research, pasadena, ca, usa annals of geophysics, 54, 6, 2011; doi: 10.4401/ag-5269 abstract the article describes the design of the community seismic network, which is a dense open seismic network based on low cost sensors. the inputs are from sensors hosted by volunteers from the community by direct connection to their personal computers, or through sensors built into mobile devices. the server is cloud-based for robustness and to dynamically handle the load of impulsive earthquake events. the main product of the network is a map of peak acceleration, delivered within seconds of the ground shaking. the lateral variations in the level of shaking will be valuable to first responders, and the waveform information from a dense network will allow detailed mapping of the rupture process. sensors in buildings may be useful for monitoring the state-of-health of the structure after major shaking. introduction the most important information that can be provided to emergency responders in the minutes to hours following an earthquake is an assessment of damage, and the best proxy we have to produce estimates on this time scale are measurements of ground shaking. products such as shakemap [wald et al. 1999a] do this with near real-time sensors that are available from traditional seismic networks such the southern california seismic network (scsn). however, the sensors are sparsely distributed and the resulting maps have low-resolution and require model-driven interpolation, which means they need to know the earthquake hypocenter and magnitude. sensors are typically separated by several kilometers (approximately 10 km in the case of the scsn). increasing the density of seismic networks beyond that needed for their basic function of earthquake location is cost prohibitive under their current paradigm of operation of closed networks with high-quality (expensive) sensors. the capital and maintenance costs, permitting and data analysis are the usual limiting factors. another approach to providing this information is to use crowd-sourcing to obtain the measurements. the “did you feel it” (dyfi) product of the us geological survey (usgs) [wald et al. 1999b] does this with a simple post-earthquake web interface, where users enter intensity observations with postal codes providing the location information. the sheer numbers of reports largely overcome the simple and subjective measurement scale and crude location mechanism. recent mild but widely-felt, earthquakes in the los angeles region have produced over 40,000 entries. when the inverse-distance dependency is removed from these maps such as in figure 1, there is surprising level of detail revealed about the lateral variations in shaking intensity. one drawback to this form of sensing is the human responders in the areas of heavy shaking usually do not make the data entry their first priority, and hence information from the most critical areas is usually late in arriving. in this paper we describe an alternative way to achieve the goal of providing detailed and rapid assessment of ground shaking in urban areas. the method is based on an open-network of low-cost sensors that are hosted by volunteers and the telemetry is provided by the internet. the community seismic network (csn) (http://www. communityseismicnetwork.org) described here can be viewed as a quantitative version of dyfi. the primary product of the csn is a map of ground shaking that can be delivered within seconds of major shaking. with a dense network this can be generated before accurate estimates of the location and magnitude are obtained. the measurement of moderate earthquakes will provide maps of anomalous ground amplification. the waveforms from the network will provide the information to determine the dynamic slip on the fault. article history received june 30, 2011; accepted october 26, 2011. subject classification: seismology/ground motion, instruments and techniques, seismic risk, computational geophysics/algorithms and implementation, data dissemination/seismological data. 738 citizen empowered seismology/special section edited by r. bossu and p.s. earle clayton et al. 739 f ig u re 1 . a c ro w d -s o u rc in g ex am p le .t he e ve nt is m ag ni tu de 5 .4 o n ju ly 2 9, 2 00 8, in t he l os a ng el es r eg io n. t he “ d id y ou f ee l i t” in te ns it y m ap is c on st ru ct ed b y th e u sg s fr om o ve r 40 ,0 00 r es po ns es . w he n th e 1/ r fa ct or s ho w n in t he lo w er r ig ht is r em ov ed , t he r es id u al in te ns it y m ap s ho w n in t he u pp er r ig ht is o bt ai ne d. t he d y fi is a va ila bl e fr om t he u sg s w eb si te , a nd t he r es id u al m ap w as c on st ru ct ed b y su sa n h ou gh o f th e u sg s. 3 4 ° 3 0 ’ 740 the concept of basing a seismic network on microelectromechanical systems (mems) sensors was proposed by evans et al. [2005]. in the time since that suggestion, the sensors have become much more sensitive (and cheaper). the quake-catcher network [cochran et al. 2009a, cochran et al. 2009b, chung et al. 2011] and the home seismometer network [horiuchi et al. 2009] are also based on low-cost mems sensors. the oil-industry has been increasing the use of mems accelerometers in their surveys, driven by the need for low-cost compact three-component sensors [hons et al. 2008, mougenot et al. 2011]. the csn described herein is under development with approximately 100 sensors deployed, and thus far no felt earthquakes have occurred. consequently, the system has not been tested under real event conditions. the csn is embedded in the reporting region of the scsn, which is jointly operated by caltech and the usgs. the csn is not intended as a replacement for traditional networks, but rather as a supplement to increase the resolution of ground shaking measurements. the mems sensors that are currently used by the csn are not sensitive enough to detect regional or small local earthquakes. community seismic network figure 2. effect of a dense array. the upper row shows the output of simulated motion in southern california, and a perfect point source. the middle row shows the current seismic network density (the scsn) and the reconstruction of the point source. the bottom row shows the results for a dense array of 1000 stations. advantages of a dense network with the csn, the quality of an individual sensor is traded-off against the density of the network. the tradeoff is common in the seismic exploration industry, where the goal is to measure the wavefield in an unaliased manner. for earthquake monitoring, a dense network is important when there are significant lateral variations in the intensity of ground shaking [field and hough 1997]. this is likely the case when there are near surface structures such as micro-basins or when there are variations in soil conditions. in figure 2, a synthetic example shows the effect of a sparse and dense network on an interpolated wavefield. analysis methods that utilize sampled wavefields will not work with sparse networks. one advantage of a dense network is that the realtime processing is generally simpler. the first group of responding stations provides a fairly accurate location of the hypocenter of the event if it occurs within the network. also a map of shaking can be produced directly from the observations rather than from a model-based interpolation that depends on knowing the epicenter of the event. in figure 3, a real example of a dense network is shown. in this case, it is an exploration network that happened to capture a nearby earthquake. the recorded wavefield is unaliased, and it is clear that there are significant and rapid lateral variations in the peak accelerations. clayton et al. 741 figure 3. a real example of a dense array. the array consists of over 5,000 sensors in a 7 ×10 km area. it was deployed by nodalseismic inc. on behalf of signal hill oil co. and was designed for active source imaging of the signal hill oil field. the upper panels show the location of the stations. the lower left panel shows a time-slice of the s-wave wave field due to a magnitude 2.5 earthquake located approximately 5 km to the west of the array (red is positive and the numbers refer to the frame and time of the slice). the lower right panel shows the variations in peak acceleration due to this earthquake. 742 low-cost sensors the technology change that has produced low-cost sensors is mems. the accelerometer version of these ‘sensor-on-a-chip’ devices uses capacitance variations induced by motions and were developed in 1960’s. their development as low-cost devices was driven by their widespread use in air-bag systems, disk-drive protection devices and computer game controls. they are now commonly included in smart phones and mobile computers. mem accelerometers vary from very low-cost lowresolution high-noise devices to sensors that have performance that is comparable to expensive force-feedback sensors [holland 2003]. sensors with 70 mgal sensitivity at 1 hz are available for us$100. cell phones are typically equipped with sensors that are about ¼ of this sensitivity. in the csn, the initial deployment of phidget sensors (www.phidgets.com) have a sensitivity of 70 mgal with a 16-bit digitizer and a dynamic range of ± 2g. the packaged sensor is shown in figure 4, along with its noise curve. for comparison, the response of a smart phone sensor is also shown. communications with the sensors communications with the sensors is generally over the internet. this reduces costs but does introduce some security and robustness issues. to connect the sensors to the network, there are a number of options, but for most sites a host computer is used. the sensor package connects to the host via a usb port, and a client program analyzes the samples and communicates with the central server. the client program has minimal impact on the performance of the host, but it does require the host be functioning all the time to provide real time monitoring. the advantage of using a host computer is that network connectivity problems are solved by others. this allows the client installation program and process to be fairly simple. ideally, the sensors would not use a host computer, but would rather directly connect to the internet. we have successfully ported the client software to a small single board computers (sbc) but this does not solve the internet connectivity problem. the sbc’s, whether using wired or wireless connections, need to negotiate a variety of protocols that often require passwords. this makes volunteer (i.e. nonexpert) installation problematic. using the cell-phone infrastructure would obviate a number of these problems, but with the current price structure for this type of communication, this is not practical. it may be suitable in countries outside of the usa. some form of the sbc-based sensor will likely become the preferred configuration as always-on desktop computers become less popular. software design the csn software is divided into client and server components. at the moment, the server part is also subdivided into real-time and archiving sub-systems, although our longer-term plan is to join these. client software the purpose of the client software is to retrieve the sampled data from the sensor system, perform a limited set of processing on the data, and send the results (and community seismic network figure 4. low-cost mem sensor used in the csn. the sensor used is a phidget 1056. the response of this sensor and that of a typical smart phone are compared to a standard high-quality force-balance accelerometer in the right panel. the response for various sized earthquakes at two distances is shown for reference, which is adapted from clinton and heaton [2002]. possibly the data) to the central server. in the current implementation, the data samples are decimated to a data rate of 50 samples per second (sps) and placed in a ring buffer. a detector algorithm that uses a variation of the standard ratio of short-term-average over long-term average [earle and shearer 1994] is used to pick events. the adjustable parameters in the detector are the lengths of the shortand long-term averages and the threshold of detection. we plan to dynamically adjust these parameters through machine learning, which will be discussed later. time for the hosted sensors is determined by a local network time protocol (ntp) server [mills 1990, frassetto et al. 2003]. we tested the local clocks on the host computers, which were supposed to sync to an ntp server and to generally available ntp servers, but found that neither were sufficiently accurate. we are currently running our own ntp server and this has stabilized most of the clocks, but we still occasionally have 1-10 second jumps that appear to be introduced by some older operating systems. when the client has detected an event, it measures the peak amplitude in the next second and sends this information along with the time of the detection to the server. we believe that it is important that this initial information be sent as quickly as possible in order to precede a possible (maybe likely) failure of the network infrastructure. the client continues to look for the peak amplitude and send updates as the event proceeds. the clients will send their raw data to the server when they are requested to do so. for some sensors, the parameters are set such that all of the data are sent every few minutes, forming a continuous stream of data. these data streams are very important for research on the detection and processing algorithms and for scientific research on the earthquakes themselves. in other cases, when bandwidth is an issue, the sensors will send a time window of the data when requested by the server. the request is communicated as part of the ‘call-home’ procedure. when initially downloaded, the client software performs the installation task and requests certain information from the volunteer such as location as determined by a google map, floor of the building, building type, and contact information. an identification key is also obtained from the server to authenticate future data exchange. the final task for the client is to contact the server at regular intervals (e.g. hourly) to report its state of health and request any software or parameter updates. with this ‘call-home’ or ‘heartbeat’ mechanism, the entire client code can be changed, and various parameters updated. this is also the mechanism whereby the server can request that the client send the raw waveform data for a particular time segment. this task also fills the important role of letting the server know which sensors are functioning at any given time. server software the role of the server is to receive detection and parametric data from the sensors and to process it to produce a map of the peak ground motion, along with other related products. this information will then be broadcast to the general public and emergency responders. to handle the very impulsive load generated by earthquakes, the server needs to be able to dynamically add computational and i/o resources. it also needs to be robust with respect to failure by the very event that it is attempting to report on, which means it probably needs to be located outside the region of concern. to achieve the dynamic and robust qualities, it was decided to use distributed cloud computing [armbrust et al. 2010, mell and grace 2011] for the server. in our initial configuration, we are using the google cloud, with the google app engine as a development platform. the robustness is achieved through the global distribution of redundant ground servers, and the dynamic loading is through the structure of the database of the google app engine. the database is highly denormalized (i.e. one table instead of several separate tables) which is a change for most seismic software. tutorials on the google database (big table) and programming the app engine can be found at http://labs.google.com/papers/. in theory the cloud will automatically handle the redundant storage of data, so we only need to deal with one system (i.e. no backup system). in measurements provided by google engineers, but otherwise undocumented, the redundant storage will occur very rapidly. one other features of the cloud is the ability to create additional networks anywhere in the world by simply replicating the particular instance. the downside of the cloud is that some aspects of the environment change with time as is to be expected with an evolving system. in standard (sparse) network software the picks are normally associated with events with a fairly complex code that is often termed an ‘associator’. with the csn, the density of the network allows us to use a simpler system based on ‘geo-cells’ (latitude–longitude boxes). the system simply counts the numbers of detections within the geo-cells within time slices, and when that number exceeds a threshold number, the geo-cell declares an event. when a sufficient number of geo-cells detect an event, a map of the ground motion for the whole region is produced. thresholds are determined in order to maximize the detection probability, while minimizing the number of false alarms. since this map is not dependent on the location of the event, only that its effects are within the area of interest, there is no need to initially do the association and location steps. the geo-cells can be used in a telescoping multi-scale approach to determine how widespread the shaking it is. the size of the geo-cells and time slices are tunable parameters of the network. in the initial pasadena test the geo-cells are the size of a few blocks, and the time slice is the transit time of a clayton et al. 743 744 p-wave across the cell. the threshold count is also a parameter that is likely to vary laterally across the network. the system can respond very quickly, and the goal is produce the map within seconds of the shaking, so it has the potential of reaching emergency responders before the network connectivity fails. however, even if this is not the case, the use of the cloud environment should allow the information to be sent to outside responders. a schematic of the server implementation is shown in figure 5. the server also requests and receives the waveform data, which is archived. presently a conventional land-based server handles this, because cloud-based storage is too expensive. however, it is expected that this will evolve so that the waveform archive is also maintained in the cloud. data archive the waveform and other parametric data are sent to a standard archive shortly after they are received and processed. the waveform data are entered into an archive wave pool and the metadata into a database. they are then available to scientists working on the csn project for further analysis. due to privacy concerns of the sensor-hosting entities, these waveform data are not generally available, however, the waveforms for detected events will be made available to the scientific community. one challenge of the archive is the mobility of the sensors and the ‘naming’ of stations. with the use of mobile devices as the sensing platform, we need to be able to handle rapidly changing station coordinates, which also has implications for the naming of stations. in traditional networks, the stations are re-named when they are moved, but that is not practical where the stations are frequently on the move, such as the case with cell phones with mems sensors that are discussed below. at the moment, the station location is kept as an attribute of a particular waveform and not the station (i.e. the database is denormalized), but the station name is kept fixed. this approximates the station location, by its position at the start of the waveform. keeping consistent names for stations in a network where the sensors move around (and disappear) is a challenge that we have not completely solved. the solution may be to abandon names and use the coordinates as the station tag. community seismic network figure 5. a schematic of the client/server interaction. the functionality of the server is shown. the event detection is done by geo-cells as described in the text. the heartbeat is database of regular check-ins by the sensors. the datastore is the cloud database. the advantage of a ‘cloud-based’ server is robustness and extensible load capability. security issues there are a number of security and privacy issues that are not normally encountered in a closed seismic network. the first is ensuring that the host computers are not compromised by entities spoofing as the server. to minimize this issue, all communications in the csn system are initiated by the clients themselves and only with a trusted host. this means that software updates and request for the waveforms wait until the client makes a regular state-ofhealth contact. to minimize spoofing the server with bogus earthquakes, the server only accepts information from clients that have a key, which is assigned at the time of registration (software installation) of client. privacy issues the privacy of information on the sensor host is increasingly becoming a major issue. it is relatively straightforward to keep the contact information separate from the seismic database and in a secure place available only to the administrators of the network, but the precise location information is a necessary part of the meta data for the seismic system. to minimize the broadcast of this information, public displays of the network show only sensor locations at the resolution of a geo-cell (about one block in area), and within a geo-cell only the number of sensors is shown, not the individual locations. this still gives an overall impression of the distribution of the network, but doesn’t give precise locations. note that we chose this procedure after experimenting with adding a small random component to the displayed locations, which seemed to confuse people more than it helped. the geocell display concept is further enhanced for mobile devices such that only geo-cells that have two or more mobile devices will be displayed, which should make tracking of people through their sensors practically impossible. to minimize the issues of distributing the real time waveform (with locations), we plan to only make available extracts of the data for felt earthquakes, and only after a delay. this appears to be an unfortunate consequence of an open network in a privacy-concerned environment. prototype network a prototype of the csn network is currently being installed in the vicinity of pasadena, california, usa. the purpose of this network is to demonstrate the viability of the community-hosted sensors and the open network design of the csn. to date, approximately 100 sensors have been distributed. a map of the distribution and the current size of the network is at http://map.communityseismicnetwork.org. the sensing, detecting and archiving aspects of the network have been implement and are being tested. in figure 6, one of the few events recorded by the csn is shown. the event is a magnitude 4.1 at a distance of 38 km from the center of the network. the data show the general noise level on the sensors that can be expected on this type of network. the timing error that is evident on one of the stations is due to the problems mentioned above. the sensors did not detect the april 4, 2010, 7.2 el mayor cucapah earthquake in baja, mexico some 300 km away. future directions the prototype csn network represents a small part of the envisioned scope of the planned csn network. the clayton et al. 745 figure 6. sample recording from csn. the earthquake is a ml 4.1 near newhall, ca, usa, approximately 38 km from the center of the csn network. the east component of data is shown. the left panel shows the data in distance from the earthquake, while the right panel shows a blowup of the region denoted by arrows. the traces in the left panel are self-normalized, while those in the right panel have a single scale factor. the data are band-passed at 1-10 hz. the amplitude variations in the right panel are the main effect that the csn is trying to measure. there is clearly one station in the right panel that has a timing error of about one second. 746 future plan is to expand over the entire urban los angeles region, and eventually other urban areas that are subject to significant seismic hazard. in terms of developing the network software and sensors, we are working on tuning the network parameters with machine learning, detecting the state of health of buildings, and adding the sensors in cell-phones. machine learning the sensors for the csn are installed by the volunteers themselves and as a result it is expected they will be placed in a wide variety of noise and vibrational environments. the sensors themselves have some self-configuration ability for orientation. they can detect the vertical component because they can sense the acceleration of gravity and do a software rotation to align it with the z-component. currently, the horizontals are not oriented, other than they are in the horizontal plane and are perpendicular to each other. in the future, if an onboard magnetic compass is available (as it is with the phidget sensor), it can be used for determining the compass directions of the accelerometer horizontal axes. the network will use machine learning in order to optimize the detection parameters. this will be implemented when the network has densified. the algorithm for setting the thresholds or sensitivity of picking takes into account the detection history for a particular sensor. it adjusts the threshold up or down to bring the sensor in line with the average for the whole network. this could even be made time-of-day dependent as its performance characteristics become more evident with time. adjusting the parameters of the picking algorithm (such as the short and long-term filters) will be more complex because these will depend on the performance relative to the background noise and on the history of detecting earthquakes of different sizes (hence frequency content). a first approach towards using machine learning to optimize detection performance in the csn is described in faulkner et al. [2011]. in addition to tuning the sensors themselves, machine learning can also be exploited to optimize the network parameters such as the size of the geo-cells and the thresholds within each cell. one planned feature of the network is to incorporate the ability of the clients to initiate a test earthquake to perform end-to-end test of the system; something that is not possible with most seismic networks. to accomplish this, the details of the event, including synthetic waveforms will be downloaded into the clients the day before the scheduled time as part of their regular call-home communications. then at the scheduled time the synthetic data will replace the real data stream, and the test earthquake will be simulated. the entire system including the detection functions on the clients and the capacity of the server will be tested. networks in buildings one of the proposed applications of the csn is in providing dense instrumentation of buildings. this is to measure the level of acceleration that the individual floors have experienced in an earthquake, and also to monitor the state of health of the building by looking at the variations in the modes of the building before, during and after an earthquake [clinton et al. 2006]. this is a new area of research that will require a dense array such as the csn to provide the necessary observations. we have installed such a network in the millikan library (a ten story building at caltech) for structural monitoring. the sensors are capable of detecting the fundamental modes of the building with both forced vibrations and ambient noise. communications within buildings can be challenging for a variety of technical and practical reasons, so we are currently investigating using the electrical wiring system as a communication network (ip over power), but at the moment, noise appears to significantly limit the range. if this can be made to work, it will greatly simplify the placement of sensors, and their communications in buildings. cellphones and mobile devices smartphones are ubiquitous in today’s society, and most are equipped with a motion sensor. while the quality of this sensor is not as good as that of the stationary sensors, its use for gaming and other applications is demanding more precision. the newest generation of phones and mobile devices allow programs to be run in the background, which allows a modified version of the client described above to function on these devices. the obvious problem with mobile devices is that they routinely generate signals that are much larger than earthquakes through regular activities. one straightforward solution is to detect when the phone is at rest, and only use data from the sensors when the phone is in this state. this is the approach of the ishake project [dashti et al. 2011]. a more challenging problem is to separate the humangenerated motion from the earthquake signals.we have developed a prototype algorithm to make this separation and discriminate between earthquake motion and acceleration due to regular activities. the algorithm and its evaluation are described in faulkner et al. [2011]. a csn app for google android phones implementing these algorithms is available as a free download through the android market store (https://market.android.com/details?id=edu.caltech.android). examples from a dense network during the first half of 2011, nodalseismic inc., installed a dense seismic network in long beach for the signal hill oil company. this network consisted of approximately 5,000 autonomous sensors distributed over a 7 × 10 km area. this network recorded a few small community seismic network earthquakes that were within a few kilometers of its center. while the sensors on this network are the standard seismic exploration velocity sensors, this deployment gives some indication of what might be observable with a dense earthquake network such as csn. figure 3 shows the distribution of receivers and shows a snapshot of the s-wave as it crosses the network. the peak accelerations are also shown. it is the variability that we see in this figure that confirms the premise of the csn – that there is significant lateral variability of ground shaking and that it needs to be measured on a fine scale. this network has also been an excellent test bed for the picking algorithms and the geo-cell concept for detecting events. acknowledgements. the development of the community seismic network is supported by the betty and gordon moore foundation. this research is also supported by nsf award cns0932392. we thank nodalseismic inc. for the samples of data used in figure 3. the manuscript was significantly improved by suggestions from the associate editor, paul earle, and a reviewer, jesse lawrence. references armbrust, m., a. fox, r. griffith, a.d. joseph, r. katz, a. konwinski, g. lee, d. patterson, a. rabkin, i. stoica and m. zaharia (2010). a view of cloud computing, communication of the acm, 53 (4), 50-58; doi: 10.1145/1721654. 1721672. clinton, j. and t. heaton (2002). potential advantages of a strong-motion velocity meter as opposed to a strong motion accelerometer, seismol. res. lett., 73, 332-342. clinton, j., s. bradford, t. heaton and j. favela (2006). the observed wander of the natural frequencies in a structure, b. seismol. soc. am., 96 (1), 237-257; doi: 10.1785/ 0120050052. chung, a.i., c. neighbors, a. belmonte, m. miller, h.h. sepulveda, c. christensen, r.s. jakka, e.s. cochran and j.f. lawrence (2011). the quake-catcher network rapid aftershock mobilization program following the 2010 m 8.8 maule, chile earthquake, seismol. res. lett., 82 (4), 526-532. cochran, e.s., j.f. lawrence, c. christensen and r. jakka (2009a). the quake-catcher network: citizen science expanding seismic horizons, seismol. res. lett., 80, 26-30. cochran, e.s., j.f. lawrence, c. christensen and a.i. chung (2009b). a novel strong-motion seismic network for community participation in earthquake monitoring, ieee instru. meas. mag., 12 (6), 8-15. dashti, s., j. reilly, j. bray, a. bayen, s. glaser and e. mari (2011). ishake: using personal devices to deliver rapid semi-qualitative earthquake shaking information, usgs final technical report, g10ap00006, february 28, 2011. earle, p.s. and p.m. shearer (1994). characterization of global seismograms using an automatic-picking algorithm, b. seismol. soc. am., 84, 366-376. evans, j., r. hamstra, c. kundig, p. camina and j. rogers (2005). tremor: a wireless mems accelerograph for dense arrays, earthq. spectra, 21 (1), 91-124. field, e.h. and s.e. hough (1997). the variability of psv response spectra across a dense array deployed during the northridge aftershock sequence, earthq. spectra, 13 (2), 243-257. faulkner, m., m. olson, r. chandy, j. krause, k. chandy and a. krause (2011). the next big one: detecting earthquakes and other rare events from community-based sensors, ipsn’11, acm 978-1-4503-0512-9/11/04. frassetto, a., t.j. owens and p. crotwell (2003). evaluating the network time protocol (ntp) for timing in the south carolina earth physics project (scepp), seismol. res. lett., 74, 649-652. holland, a. 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(1990). on the accuracy and stability of clocks synchronized by the network time protocol in the internet system, acm computer communication review, 20, 65-75. mougenot, d., a. cherepovskiy and l. junjie (2011). memsbases accelerometers: expectations and practical achievements, first break, 29 (2), 85-90. wald, d.j., v. quitoriano, t.h. heaton, h. kanamori, c.w. scrivner and c.b. worden (1999a). trinet “shakemaps”: rapid generation of peak ground motion and intensity maps for earthquakes in southern california, earthq. spectra, 15 (3), 537-556. wald, d.j., v. quitoriano, l.a. dengler and j.w. dewey (1999b). utilization of the internet for rapid community intensity maps, seismol. res. lett., 70 (6), 680-697. *corresponding author: robert w. clayton, california institute of technology, seismological laboratory, pasadena, ca, usa; email: clay@gps.caltech.edu. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. clayton et al. 747 annals of geophysics, 55 (3), 2012: amato_et_al the ingvterremoti channel on youtube alessandro amato*, luca arcoraci, emanuele casarotti, raffaele di stefano and the ingvterremoti team† istituto nazionale di geofisica e vulcanologia, rome, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5546 abstract in february 2010, we launched an experimental scientif ic video channel on youtube (http://www.youtube.com/ingvterremoti), to improve our communication strategy for earthquake risk and preparedness. the main goals of this initiative were to inform people of the ongoing seismic activity in italy and around the world, to communicate the results of scientific research in seismology, and to increase the knowledge of seismic hazard of the people. to date, after almost two years from the start, we have published 52 original videos on youtube, through the collaboration of many researchers at the istituto nazionale di geofisica e vulcanologia (ingv). the videos are organized into eight play lists: (i) earthquakes in italy; (ii) earthquakes worldwide; (iii) the 2009 l’aquila, italy, earthquake; (iv) ongoing seismic activity; (v) tsunami; (vi) earthquake prediction; (vii) seismic hazard; and (viii) may 11 2011 (when a major earthquake was predicted to hit rome). to date, the total number of views is over 466,000, with two peaks of more than 20,000/day after the tohoku, japan, earthquake (march 2011) and before the presumed prediction of a major earthquake to hit rome on may 11, 2011. the most popular videos (6) have been viewed more than 20,000 times, with a maximum of over 67,000. we think that this initiative has increased people’s knowledge and awareness of seismic risk, although at the moment the outreach is limited only to a specific (but growing) target of citizens. in the future, we will try to improve the technical aspects of our video communication, and we will try to broaden our audience. we have learned that when specific earthquakes occur (or even when there are unfounded predictions about upcoming seismic events) this is when the attention is highest, which represent the best occasions to move forward towards improved risk communication strategies. 1. introduction seismological and seismic hazard studies have strong social impact. most of what seismologists accomplish is used in seismic hazard maps and then in seismic risk reduction. in this perspective, one of our main goals as research seismologists is to transfer our knowledge to the relevant authorities and to the general public. geoethics implies that we will do our best to increase the earthquake awareness and preparedness of society, to face future hazards in the right way. civil protection officers benefit from seismological research because seismic hazard maps are used for seismic zonation and for the definition of the correct building codes. in addition, we know that one of the main steps to reduce earthquake risk is to increase the level of knowledge and awareness of the public, including teachers, students, local authorities, civil protection volunteers, and others. for this reason, the istituto nazionale di geofisica e vulcanologia (ingv) is involved in many educational activities with schools, as well as in information transfer efforts through a variety of tools. these include continuous and detailed information on seismic activity and hazard in italy through our website (http://www.ingv.it), through school programs, and more recently through social media, such as twitter, an iphone app, and youtube. in recent years, several universities and research institutes have started to release scientific information through social media, including youtube, and more in general, scientific videos [grime 2011, woolley 2011]. in geophysics, the american geophysical union is expanding science communication through videos [agu 2011] and scientific blogs, while new ways of science video communications are offered today (see for instance symphonyofscience.com). in addition, social media are becoming a powerful tool to involve people in geophysical sciences and earthquake monitoring [allen 2012]. in this contribution, we will describe briefly the ingv youtube channel (youtube.com/ingvterremoti), with the focus on the history of this initiative and with discussion of its social impact. 2. motivation and history of the ingv youtube channel the need for the better improving and communicating of information on seismic activity in italy and on seismic hazard reduction increased significantly after the 2009 l’aquila earthquake. although the seismic hazard of the article history received january 3, 2012; accepted february 8, 2012. subject classification: seismology, educational, history of science, public issues, earthquakes, seismic awareness, seismic hazard. 403 area was well known even before the earthquake (see www.ingv.it/terremoti/pericolosita-sismica/ and www. protezionecivile.gov.it/jcms/it/classificazione.wp), many people living there were not aware of the risk and were not prepared for a strong earthquake. this emphasizes the importance of correct communication to the media and to the people, not only during a crisis (such as an ongoing seismic sequence), but also in ‘quiet’ times. the main goal of the youtube/ingvterremoti channel is to increase the quantity and quality of the information about ongoing seismic activity and seismic hazard in italy, and in other regions of the world, with the final goal of increasing the awareness and preparedness of the people. the birth of the channel youtube/ingvterremoti dates back to february 2010. since then, 52 videos have been uploaded, with an increasing number of views, that has reached today a total of more than 466,000. in the first year of activity, we had about 100,000 total channel views, while in the following period (march-december 2011) the audience increased significantly (by a factor of four). this was mostly due to strong events that raised the attention on earthquakes, but also to a general increase in our audience. to give a better idea of what this number means, we can compare it with the youtube channel of the u.s. geological survey (usgs) (http://www.youtube.com/usgs), which started in 2006 and which has a total number of videos almost five times greater than the ingv youtube channel (261). at the time we write, the total number of views for the two channels is ~264,000 versus ~467,000 for the usgs and ingv channels, respectively. the usgs channel contains videos on a broader variety of topics, from climate to earthquakes, from volcanoes to floods, and of animals, and much more. the number of views of the ingv youtube channel has increased over time, with two obvious main peaks in the history of the views (figure 1a). these two peaks (both over 20,000 per day) correspond to the tohoku, japan, earthquake on march 11, 2011, and the presumed rome earthquake prediction for may 11, 2011. in the first case, we prepared a quick video from the ingv seismic monitoring room in which the senior seismologist on duty (franco mele) explained what had just happened in japan. the sudden increase in views (more than 30,000 in four days; see figure 2) showed us the importance of being able to post a new video quickly, when attention is high. besides the immediate response in terms of number of views, we found that people looking for some rapid information after a specific event can be attracted by the other contents of the channel. in the case of the second peak [see also nostro et al. 2012, in this volume], the increase was due to the presumed prediction of a strong earthquake to hit rome on may 11, 2011. in this case, we prepared and published a video explaining the story of this fake prediction one month before the predicted date of the earthquake. the video was viewed by several thousand people in the month before the earthquake, with an average rate of more than 1,000 views per day (figure 3), thus reaching more than 30,000 views by may 8; then it increased to several thousand/day on may 9, 10 and 11 (7,186, 17,458, and 7,216, respectively), and rapidly became our most viewed movie, with more than 67,000 clicks. even several days afterwards, the video was still being viewed 20-30 times per day (figure 3). apart from these two peaks described above, figure 1b shows the level of views from july 1 to december 31, 2011, a period without any really strong seismic events. the current rate ranges between 250 to 1,000 views per day. more recently, we registered another ‘double’ peak after two moderate earthquakes in northern italy in january 2012 (figure 1a), with more than 6,400 views on the day of the m 5.4 parma earthquake ( january 27, 2012). 3. contents and numbers the videos that we have produced can be basically grouped into four main categories: 1. periodic descriptions of ongoing seismic activity in italy; 2. reports of large earthquakes worldwide; 3. specific scientific seismological topics; and 4. radio and television interviews with seismologists. in addition, some videos have been added as preferred for their relevance to earthquake science or science in general, with links to other channels. the videos are at present organized into eight play lists: (i) earthquakes in italy; (ii) earthquakes worldwide; (iii) the 2009 l’aquila earthquake; (iv) ongoing seismic activity; (v) tsunami; (vi) earthquake prediction; (vii) seismic hazard; (viii) may 11, 2011 (the major earthquake predicted to hit rome). the play lists are allowed to have some partial redundancy. these productions were all made ‘in house’ (except for one video on tsunamis that was previously commissioned to a professional company and then used for the ingv youtube channel), with cheap hardware and software, and internal resources used for the movie making, postproduction and editing. the main cost of the channel consists of the salaries of the people involved, who dedicate a limited part of their working research time to film making. the participation of many researchers (about 30) who have been willing to make themselves available to explain their research or to comment on some specific ongoing event is considered very important. in general, the number of appreciations (the so-called ‘i like’) are much larger than the ‘i don’t like’. figure 4 shows the relative comparison of the two, with 990 positive and 26 negative votes, which correspond to 97.4% and 2.6% of the total, respectively. moreover, we consider the number of comments posted by users on the ingv youtube channel or on specific videos as an indication of amato et al. 404 405 the ingvterremoti channel on youtube figure 1. top: history of daily ingv youtube channel views from february 2010 to the beginning of february 2012. the two main peaks corresponding to the tohoku, japan, earthquake on march 11, 2011, and the rome presumed earthquake prediction on may 11, 2011. the third double peak at the end of january 2012 was due to some moderate shocks felt in northern italy. bottom: detail for the number of ingv youtube channel views for julydecember 2011. even without strong events, the daily number of views remains relatively high (around 300-400), with some peaks around 1,000. figure 2. history of daily views for the tohoku, japan, video prepared on the afternoon of march 11, 2011, which was viewed by more than 30,000 people in the first four days. figure 3. number of daily views for the video terremoto a roma l’11 maggio 2011? (earthquake in rome on may 11, 2011?), which scored a total of more than 67,000 views. the peak was reached the day before the prediction (may 10), with about 18,000 views. the interest and involvement of the public (figure 5). to date, we have had 379 comments concerning the channel and specific videos. most of the comments have been positive (acknowledgements), with many containing requests for additional information; a few have been critical. most of these latter refer to the impossibility for us to predict earthquakes, which is often compared with reference to some people who claim to be able to make such predictions. we are not able to answer to all of the questions, requests and complaints posted on the channel, but we try to do this as much as possible, when the questions are fair and motivated by scientific interest. we believe that the interactions with the public are an essential part of our work, and for this reason we are planning to increase this side, not only through the ingv youtube channel, but also through a specific blog site and other social media (ingvterremoti.wordpress.com). another measure of the channel activity can be obtained by looking at the video sharing in the community. figure 6 shows the monthly number of sharings, with values around zero in the first year and a strong increase in the period march-may 2011, after the japan earthquake and the rome earthquake prediction. it is meaningful that in the following months ( june-december 2011) the level of sharing was low, but higher than before, ranging between 10 and 25 shared videos per month, and with a peak of 106 in january 2012. the number of people registered to the channel (subscribers) has increased relatively constantly over time (figure 7), reaching 1,721 today, and increasing at about 20-40 per month in the last period, with 42 in december 2011, and then 273 in january 2012 due to the m 4-5 earthquakes in northern italy. although our channel is dedicated to the italian population, and therefore all videos (except one) are in italian, we also have a significant public abroad, and particularly in usa (about 8,000 views), spain and thailand (more than 3,000 each), brasil, uk, switzerland, france, germany (more than 2,000 each). most of these are related to one specific video on tsunamis, which is dubbed in english and has been seen by more than 34,000 people, and also by italians living outside italy. some of the most clicked videos obtained their success several weeks or months after their publication. this happened, for instance, for the videos on tsunamis that were posted a few months before the tohoku disaster of march 11, 2011. immediately after the earthquake and tsunami struck japan, many people looked on the internet for videos, images and information on large earthquakes and tsunamis. our videos were originally prepared after the sumatra tsunami of 2004, and they were pubamato et al. 406 figure 4. history of the ‘i like’ (blue) versus ‘i don’t like’ reports per month. the total numbers are 990 vs. 26. most comments were posted in march and may 2011, and january 2012. figure 5. number of monthly comments on the ingv youtube channel from the opening* to date*. the peaks generally coincide with the publication of videos on relevant events. the first peak followed the publication of a video on the l’aquila earthquake, with a reconstruction of its presumed prediction. 407 lished on dvd for schools, and then posted on the ingvterremoti channel in 2010 for a broader audience. after march 2011, they were seen and appreciated by thousands of people, because they explained the origin of tsunamis in a scientific, but simple and effective, way. the numerous videos produced for the may 11, 2011, fake prediction of an earthquake in rome deserves a specific mention [see also nostro et al. 2012, this volume]. we used the ingv youtube channel to communicate with people who were afraid of, or just curious about, the presumed impending earthquake. besides the over 3,000 inhabitants of rome who came to visit the ingv laboratories in rome on the day, several tens of thousands of people accessed our information through the ingv youtube channel. we published 12 movies during that day, starting from 1:00 am on may 11, and finishing at 1:00 am on may 12. we decided to alternate updates from the seismic monitoring room every two hours, each time focusing on a specific aspect of earthquake monitoring (e.g. how we locate earthquakes, how we measure their magnitude, among others), with more general videos on seismic hazard, site effects, active faults, and so on. besides a large number of views of many of these movies during may 11, most of them were seen several times also in the following days, and some of them are still very popular. 4. conclusions the ingvterremoti channel on youtube was inaugurated on february 2010, to increase the level of information about earthquakes in italy, which represents a basic step for seismic risk reduction. the ingv researchers involved in seismology and seismic hazard studies are aware that this is a very important issue for society, particularly in italy, where the frequent seismicity, and more importantly high building vulnerability and low preparedness, raise the risk. over 50 videos have been posted to date, most of which have been produced with ‘in–house’ resources and non-professional equipment and software. although this sometimes results in low quality technical content (e.g. audio, lights), we have preferred to focus on the scientific contents rather than spending too much time on the various aspects of film making. however, we are trying to improve our techniques, without losing immediacy or reducing the amount of information that we post. the audience numbers described in this report are encouraging, with more than 466,000 total views, a constant growth rate, and continued growing interest, as is witnessed by the number of shared videos and the number and comments of subscribers. we will continue to use this medium as a tool for informing and increasing awareness of seismic risk of the people. we intend to increase the quality and quanthe ingvterremoti channel on youtube figure 7. monthly number of subscribers to the ingv youtube channel from the opening* to date*. as well as peaks related to specific events, there is a constant rate of 20-40 new subscribers every month. in january 2012, we had the second highest peak (from the time of the opening of the ingv youtube channel), with 273 new subscribers. figure 6. number of shared videos per month for the ingv youtube channel from the opening* to date*. note that before the march-may 2011 period, the monthly averages were near zero, while in the following months they were around 10-25, with a relevant peak in january 2012 (106 sharings). tity of information both on ongoing seismicity and on the hot research topics in earthquake science. acknowledgements. we thank all of our colleagues who have participated in the film making, for their dedication of some of their time to the dissemination of scientific knowledge: andrea tertulliani, franco mele, massimo cocco, carlo meletti, daniela pantosti, stefano salvi, francesca cinti, salvatore mazza, lauro chiaraluce, alberto michelini, anthony lomax, paolo marco de martini, paola vannoli, valerio de rubeis, patrizia tosi, paola sbarra, diego sorrentino, licia faenza, claudio chiarabba, calvino gasparini, paola montone, antonio meloni, graziano ferrari, lucia margheriti, luigi cucci, alberto frepoli, elisa trasatti, nicola d’agostino, antonio piersanti, pio lucente, alberto basili, and aybige akinci. we also thank domenico sisti for the video on the may 11, 2011, open day. references allen, r.m. (2012). transforming earthquake detection?, science, 335 (6066), 297-298; doi:10.1126/science.1214650. american geophysical union (agu) (2011). the s-factor, http://agusfactor.org/sfactor_blog/. grime, j. (2011). how youtube is popularising science, the guardian (science blog), http://www.guardian.co. uk/science/blog/2011/may/17/youtube-popularisingscience (last accessed, february 3, 2012). nostro, c., a. amato, g. cultrera, g. margheriti, g. selvaggi, l. arcoraci, e. casarotti, r. di stefano and the may 11 team (2012). turning the rumor of the may 11, 2011, earthquake prediction in rome, italy, into an information day on earthquake hazard, annals of geophysics, 55 (3), 413-420; doi:10.4401/ag-5559. woolley, s. (2011). youtube: a science communication world at your finger tips, university of melbourne, scientific scribbles, permalink: http://blogs.unimelb.edu. au/sciencecommunication/2011/05/19/youtube-ascience-communication-world-at-your-finger-tips/. † the youtube ingvterremoti team includes: concetta nostro, corrado castellano, lucia margheriti, giovanna cultrera, maurizio pignone and giulio selvaggi. *corresponding author: alessandro amato, istituto nazionale di geofisica e vulcanologia, rome, italy; email: alessandro.amato@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. amato et al. 408 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice m i s u r e s p e t t r o s c o p i c h e d i r e t t e d e l l a c o n c e n t r a z i o n e d e l l ' o z o n o a t m o s f e r i c o (*) n . m a t t a n a r i c e v u t o il 19 f e b b r a i o 1963 r i a s s u n t o . questa b r e v e relazione, p r e s e n t a t a d u r a n t e la sessione « o t t i c a e s p e t t r o s c o p i a d e l l ' a l t a a t m o s f e r a » al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio, concerne alcuni sistemi spettroscopici fin qui u t i l i z z a t i p e r m i s u r e d i r e t t e dell'ozono atmosferico. s u m m a r y . — tliis is a brief relation, r e p o r t e d d u r i n g t h e session " optics a n d s p e c t r o s c o p y of t h e u p p e r a t i n o s p h e r e " of t h e 2ntl i n t e r n a t i o n a l t e c h n i c a l scientifìc meeting on space, concerning some spectroscopical s y s t e m s h i t h e r t o utilized for direct m e a s u r e m e n t s of a t m o s p e r i c ozone. come è noto, se t u t t o l'ozono contenuto in una colonna verticale dell'atmosfera venisse isolato e portato al livello del mare, alle condizioni di temperatura e pressione normali, assumerebbe uno spessore «ridotto» di circa 3 m m ; cioè, pochi decirnilionesimi dell'altezza dell'intera atmosfera omogenea. benché questa quantità sia t a n t o esigua rispetto agli altri costituenti l'aria atmosferica, è comunque sufficiente, a causa del suo forte potere assorbente sulle radiazioni luminose dell'ultravioletto, per intercettare t u t t a la radiazione proenivente dal sole di lunghezza d'onda inferiore ai 2900 a, e quindi risulta sufficiente per proteggere gli esseri viventi sulla superficie terrestre da un eccesso di tali radiazioni. senza questo schermo naturale le radiazioni ultraviolette raggiungerebbero al livello del mare u n a intensità superiore a cinque volte quella che si registra in alta montagna con il sole allo zenit. (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. 4 2 4 x . m a t t a n a se, invece, per uno strano evento, si dovesse verificare u n abnorme incremento della q u a n t i t à di ozono, la vita sulla t e r r a potrebbe diventare impossibile, come se il globo terrestre venisse avvolto da una t e t r a ombra biologica. le misure sinora e f f e t t u a t e delle q u a n t i t à di ozono atmosferico hanno dimostrato la notevole variabilità, t a n t o nel tempo che nello spazio, delle concentrazioni di questo gas ed in particolare hanno messo in evidenza l'esistenza nella stratosfera, intorno ai 25-30 k m , di uno s t r a t o di massima concentrazione. i n questo s t r a t o le concentrazioni variano n o r m a l m e n t e intorno ai valori di 2-3 p a r t i per milione in volume d'aria, con un tenore cioè già abbastanza fastidioso per la respirazione; se si tiene conto che in qualche caso sono s t a t e misurate concentrazioni anche di 9 p . p . m . , e cioè tali da risultare probabilmente dannose per organismi superiori, e che in un prossimo f u t u r o la stratosfera diventerà il normale ambiente dei voli umani, ne deriva i m m e d i a t a m e n t e la dimostrazione dell'importanza che può assumere il problema dell'esatta valutazione delle concentrazioni atmosferiche dell'ozono anche nel campo pratico del volo stratosferico. già da qualche anno, molti ricercatori hanno dimostrato l'esistenza di notevoli correlazioni t r a le variazioni dell'ozono e diversi fenomeni meteorologici. l'analisi delle fluttuazioni delle distribuzioni orizzontali e verticali dell'ozono si sta rivelando sempre più un essenziale mezzo per lo studio e la soluzione dei problemi relativi alla termodinamica e dinamica della stratosfera ed allo scambio energetico t r a l'alta atmosfera e la troposfera, cioè un interessante metodo di aerologia indiretta. il prof. gotz, l'insigne studioso che ha dedicato gran p a r t e nella sua v a s t a a t t i v i t à scientifica al problema dell'ozono, h a tracciato u n a sintesi felice, un vivo quadro, dell'interdipendenza t r a l'ozono e la. feno111 enologia atmosferica : « l'ozono ha origine, negli alti s t r a t i dell'atmosfera, dalla radiazione del sole, f a t t o r e primo di ogni fenomeno terrestre. a questo gas, pint a n t o esiguo rispetto agli altri costituenti l'aria atmosferica, sono dovuti i contrasti termici che h a n n o inizio nella mesosfera, e quindi le onde ed i vortici che si producono alle quote mesosferiche dello strato caldo, al limite dell'ombra della n o t t e polare. il vortice polare, a sua volta, è la causa dell'abbassamento dell'ozono nella bassa stratosfera (la cosidetta « regione p r o t e t t a », in quanto l'azione fotolitica sull'ozono della radiazione solare, di a < 11000 a, è ivi quasi trascurabile) e successivamente, per subsidenza e scambio turbolento, l'ozono si abbassa sino alla tropom i s u r e s p e t t r o s c o p i c h e d i r e t t e d e l l a c o n c e n t r a / . . d e l l ' o z o n o 4 2 5 sfera, ove si realizzano i più vistosi fenomeni meteorologici; infine, diffuso sino agli s t r a t i più bassi ed in prossimità del suolo si reinserisce nel metabolismo dell'ossigeno ». il sintetico quadro tracciato da gòtz non può fornire che u n a parziale visione dell'insieme dei complessi fenomeni e dei vasti problemi meteorologici connessi con le determinazioni delle concentrazioni dell'ozono nell'atmosfera, comunque la necessità di estendere, intensificare e rendere sempre più precise le determinazioni della concentrazione dell'ozono è oggi t a l m e n t e sentita t r a i meteorologi, che la stessa commissione per l'aerologia, anche recentemente, nella sua u l t i m a riunione tenutasi l'anno scorso a e o m a , ha ribadito questi concetti, formulando sull'argomento diverse raccomandazioni e definendo come « vitale » il ruolo che i d a t i relativi alle distribuzioni verticali dell'ozono h a n n o nei problemi della dinamica stratosferica ». (race. 7, 8, 9). sino a pochi anni fa l'unico metodo utilizzabile per la determinazione delle distribuzioni verticali era quello indiretto basato sull'effetto « unkehr-gòtz », che comporta u n a lunga serie di misurazioni spettrofotometriche, e f f e t t u a t e al suolo, del rapporto delle intensità di due particolari radiazioni della b a n d a di ilartly-huggins, diffuse dal cielo allo zenit, d u r a n t e il corso semidiurno del sole. per applicare tale metodo necessitano osservazioni e calcolatori specializzati, e benché i calcoli siano molto laboriosi e complessi i risultati che si ottengono sono molte volte imprecisi e sempre poco dettagliati. p r a t i c a m e n t e , con il metodo indiretto si può ottenere soltanto u n a valutazione qualitativa e generale della distribuzione verticale dell'ozono, ed i d a t i ricavabili non sono utilizzabili per uno studio sinottico delle correlazioni t r a l'ozono ed i fenomeni atmosferici verificantisi a quote particolari. soltanto con i metodi di misura diretta, per mezzo di ozonosonde t r a s p o r t a t e da palloni o da razzi, si possono ottenere d a t i sufficientemente dettagliati e precisi, confrontabili con i fenomeni meteorologici a scala sinottica di specifici s t r a t i dell'atmosfera. la p r i m a strumentazione con la quale si son p o t u t e effettuare misure dirette delle concentrazioni dell'ozono in quota è s t a t a quella realizzata da regener, nel 1934, che, all'uopo, a d a t t ò un leggero spettrografo in modo da renderlo trasportabile da palloni sonda. n a t u r a l m e n t e , il recupero dello s t r u m e n t o dopo l'effettuazione del volo era la condizione necessaria per l'esame spettrofotometrico delle lastre fotografiche impiegate per la registrazione dello spettro nell'ultravioletto. le stesse difficoltà connesse al recupero, permangono per le registrazioni e f f e t t u a t e per mezzo di spettrografi trasportati da razzi-sonda. 4 2 6 x. m a t t a n a uno dei sondaggi con razzi-sonda meglio riuscito è stato quello realizzato dal naval research laboratory degli u.s.a. nel giugno 1949 con il lancio di un razzo aerobee, munito di spettrografo utilizzante un reticolo di diffrazione concavo, in alluminio, di 40 centimetri di raggio con 15.000 t r a t t i per pollice. al fine di allargare al massimo il campo di visione, in modo da poter ricevere la luce solare anche in condizioni di volo p e r t u r b a t o da f o r t e rullio e deriva, è s t a t a adoperata, invece della normale f e n d i t u r a dello spettrografo, una lente sferica di fluoruro di litio di 2 m m di diametro, che forniva un'immagine del sole di 13 /i di diametro. d u r a n t e il volo del razzo fu e f f e t t u a t a u n a serie di esposizioni, su film particolarmente sensibile all'ultravioletto, della d u r a t a di 1 secondo, intervallate di due decimi di secondo. contemporaneamente allo spettro veniva fotografato nel film un marcatempo. p e r determinare la q u a n t i t à di ozono esistente lungo il percorso atmosferico dei raggi solari, in corrispondenza (li ciascuna esposizione fotografica, veniva comparata la curva d'intensità di ogni specifico spettro con le curve d'intensità degli spettri o t t e n u t i a quote superiori ai 100 k m , ove si ritiene che non vi è più alcuna traccia di ozono. tenendo conto dei coefficienti di assorbimento dell'ozono pei le varie lunghezze d'onda dello spettro u.y., si poteva cosi risalire alla q u a n t i t à t o t a l e di ozono esistente lungo la traiettoria obliqua dei raggi solari, e conoscendo le coordinate del razzo all'istante della registrazione fotografica, si poteva risalire al corrispondente angolo zenitale del sole e quindi alla valutazione della q u a n t i t à di ozono esistente sulla verticale al disopra del razzo. alla quota di circa 20 km l'assorbimento dell'ozono risultò t a l m e n t e forte che poteva, essere utilizzata soltanto u n a s t r e t t a regione dello spettro u.y., da 3200 a 3400 a. alle quote superiori ai 60 km era invece utilizzabile l'intero spettro a p a r t i r e da 2500 a. al disopra dei 70 k m , le concentrazioni dell'ozono risultarono così deboli che la loro valutazione divenne troppo difficoltosa in q u a n t o le misure rientravano nell'ordine dell'errore sperimentale. l'analisi dei dati ricavati d a questo volo permise di ottenere import a n t i conferme di deduzioni teoriche sull'irraggiamento solare nell'ultravioletto e sull'equilibrio fotochimico dell'ozono stratosferico. dal rapido cenno che ho esposto si può arguire quale complessità di operazioni e quale costo comporti la realizzazione di u n a determinazione della distribuzione verticale dell'ozono per mezzo di spettrografi registratori trasportati da razzi-sonda; un tale sistema può p e r t a n t o essere utilizzato soltanto per specifiche osservazioni connesse allo studio m i s u r e s p e t t r o s c o p i c h e d i r e t t e d e l l a c o n c e s t r a z . d e l l ' o z o n o 4 2 7 di particolari i m p o r t a n t i problemi, quali per esempio quelli dell'interdipendenza t r a le fluttuazioni della radiazione ultravioletta solare, i fenomeni ionosferici e l'equilibrio fotochimico dell'ozono nell'alta atmosfera. p e r la valutazione delle distribuzioni verticali dell'ozono in quota, da utilizzare nelle normali ricerche (li c a r a t t e r e sinottico, si rende necessario p e r t a n t o l'uso di strumentazioni più economiche e meno complicate. un notevole passo a v a n t i in questo senso f u f a t t o per merito di coblenz e stair, che nel 1939 realizzarono uno spettrofotometro ottico, che connesso ad un radiotrasmettitore t r a s p o r t a t o da un pallone, perm e t t e v a la ricezione a t e r r a dei d a t i di misura delle intensità di due particolari lunghezze d'onda, differentemente assorbite dall'ozono, m a n mano che il pallone si elevava in quota e per mezzo dei quali potevano essere ricavati gli spessori dell'ozono a t t r a v e r s a t i dalla traiettoria obliqua dei raggi solari. in questi ultimi anni il perfezionamento dei filtri ottici e l'uso di filtri interferenziali, a t t i ad isolare limitate e ben definite regioni dello spettro, ed inoltre, i recenti perfezionamenti dei sistemi radioelettrici di telemisura, specie con l'adozione di circuiti transistorizzati, h a n n o permesso di realizzare leggere ozonosonde ottiche trasportabili da normali palloni sonda, di sufficiente affidamento per un preciso e dettagliato computo delle concentrazioni dell'ozono alle varie quote atmosferiche. u n a delle strumentazioni di questo tipo, p r o g e t t a t a e realizzata da paetzold e kulcke, f u adoperata anche d u r a n t e l'ultimo anno geofisico internazionale, in un limitato numero di stazioni, con discreto successo. l'ozonosonda ottica di paetzold utilizza due filtri ottici, t r a s p a r e n t i in u n a s t r e t t a regione spettrale, ampia circa 150 a, la p r i m a c e n t r a t a intorno ai 3100 a e la seconda intorno ai 3600 a. il primo filtro è costituito dalla combinazione dei due filtri schott u.g. i l e gg 19 rispettiv a m e n t e di 6 e 1,6 m m di spessore, m e n t r e il secondo filtro è composto dallo stesso u.g. i l e da u n w g 1 di un m m . quale ricevitore della luce veniva adoperata u n a sfera di quarzo resa translucida per mezzo di u n a p a t t i n a di ossido di magnesio. i due filtri vengono f a t t i r u o t a r e di f r o n t e ad u n a cellula al selenio la cui fotocorrente di uscita risulta proporzionale all'intensità delle radiazioni incidenti. le fotocorrenti vengono amplificate convenientemente e t r a s f o r m a t e in radiosegnali registrabili a t e r r a . poiché le radiazioni della regione spettrale c e n t r a t a intorno ai 3100 a hanno un coefficiente medio di assorbimento dell'ozono molto più alto rispetto a quelle della regione spettrale c e n t r a t a intorno ai 3600 a, dal rapporto delle due intensità, previa t a r a t u r a , si può determinare, cono4 2 8 x. m a t t a n a scendo le coordinate geografiche e le quote del pallone sonda, lo spessore ridotto dell'ozono s o v r a s t a n t e l'ozonosonda. l'ozonosonda ottica di paetzold è s t a t a in varie riprese sottoposta a diverse prove sperimentali e di comparazione con altre strumentazioni di tipo diverso, b a s a t e su sistemi elettrochimici di misura delle concentrazioni dell'ozono. i risultati delle prove comparative hanno confermato che le misure sono abbastanza precise e risultano sufficientemente dettagliate per la loro utilizzazione negli studi di c a r a t t e r e sinottico. l'elaborazione dei d a t i dell'ozonosonda ottica risulta però molto più complessa e laboriosa rispetto a quella dell'ozonosonda elettrochimica, che t r a l'altro ha il pregio di fornire d i r e t t a m e n t e le concentrazioni dell'ozono, in corrispondenza delle varie quote raggiunte d u r a n t e il volo del pallone. p e r ovviare alla complessità della elaborazione dei d a t i dell'ozonosonda ottica, recentemente è stato p r o g e t t a t o un sistema di valutazione dei d a t i stessi per mezzo di una calcolatrice elettronica. presso l'osservatorio di meteorologia aeronautica di cagliari si stanno eseguendo delle prove sperimentali per la realizzazione di un'ozonosonda economica e di agevole uso anche da p a r t e di personale non a l t a m e n t e specializzato, di tipo elettrochimico, e la sua realizzazione è già a buon punto. i n un secondo tempo si cercheià di realizzare, se i mezzi che potranno essere messi a nostra disposizione lo p e r m e t t e r a n n o , anche un'ozonosonda ottica da adoperare per scopi speciali e possibilmente razzo-trasportabile. si spera così di poter inserire, in un prossimo f u t u r o , il servizio meteoroligico dell'a. m. italiana nel campo delle misure d i r e t t e dell'ozono atmosferico, onde fornire un sostanziale contributo anche in questo r a m o della ricerca scientifica, da cui la meteorologia generale e i problemi della circolazione della stratosfera potranno t r a r r e essenziali elementi chiarificativi. sul calcolo dell'ampiezza originaria di un'onda presente in una serie temporale causale e stazionaria di dati e(lui. spaziati attraverso lo studio di uno spettro di potenza (a fec1mique 01 power ifpecfra analp;" to evaluate originai amputude 01 lwrnwn;c componentlf ;n iftat;ollary ralldom time iferielf al e'lu;"pace,l data) p. r.andi 111. e. r.o:>\clll selva (· l riee"uto il 2lì gennaio l nn ltiassu)i"to. in un precedente lavoro ('l gli autori hanno descritto un programma che permette il cakolo tlello spettro di ijotenza di una scrie temporale ca8uale e stazionaria ili llati equispaziati. nel]}l'{'s{mte lavoro viene es]losta una tecnicfl. che permette di ris,1iire al valore llell'ampiezza originaria di un'onda presente nella serie di llati e viene offcrta la }lossibilibì. al rieercatorc ili cseguire un'analisi automatica tldle earatteristichc dellc onde piì! 8ig-nifieativc estraendone le coltisponllenti ampiezze, sotto determinate condizioni da lui stesso poste. 11 modo di utilizzo e la specifica tleue condizioni sono descritte nd pitntgrnfo "l:tilizz[t.zionc ilei programllla inl'ut-ol:tpv'l' " esposto più sotto. su~dl\ry. _ a prevision work (4) dcscribctla progi'flm which allows the eolll}lutation of powcr s}jectrum [oi' stationary randolll tilllc·scries of equispaced data. in the prcsent work we llescribc a methotl to evaluate the originai wave mnplitllde of hanuollie tenns in these series. this te('niqlle allows authomatical analysis oi the most signifieant features of prinejpal hanllonic components in the serie, 1.1nder some usci' conllilions. in the 8ection "ptilizzazionc ilei }ll'ogralllma inpu'l'-o vl'l' l.·t" wc dosl'l"ìhe these contlitions itnd how the }ll'ogralll sl'ec'l'ra is aceessihle alllllltilizabie. (t) centro ili cakolo intcrllniversitario dell'italia nord -oricninlc. cfl.8alccehio ili reno (bologna) italia. l'. !l\ ~vl ,i. e. no~clli sel.\',\ il l']wgila.ìi.\l\ hpectil\. il proj.!mmma spectil\. permette al rit'erealore di fare l"analisi spettmle su una serie tem}lorale stnzionaria (ii dati ea~uali ed equispaziati di qualunque lunghezzn dopo aver fatto preeedentemenie ~ulb steh.~n queste operazioni: il) si esegue un esame preliminare allo seopo di ri}lristimu·e la st'quenza temponde conetta qualora, o per la pre~enz:l di d:lti ritennti hilllri, rolllulh!1w originati, o per in maneanza (li dati, la serie in tnpi~rl' non ]-i.~ulia e([uisljaziata. h) si riporia detta serie nelle condizioni di me(lia zero. :\ila. serip x(t} qua.le risulta. dalle o}lerazioni sopra (lescj"itte viene applieata la linestra: l 1),(') ~ , eon: t! < t ~t1io .~copo di eliminare le deforliwzioni sulle frequtmze dovute alla· limitatezza (lella s(~rie dei dati. viene ([uindi ealroh1tl1 la funzione di :mloeo!telazione: f(t) ~ v "'\ ])2 (i) .x(i} ])2 (i + t) x(i + t} .n -t l l eol rit.ardo t elle va da zero :ii ilmssimo fis~ato da·1 rieel"ea.tol·e. inline viene ealeolato lo spettro (li potenza tra·mite la: thi"" 'f (w) = ~ j.:. ]i' (,) cos w t t o non iutte le e:ll":ltte1"ìstielle e disponibilità esposte nellnvoro giù citato. r }ll"oblemi su cui gli .autori hanno fis~ato la 101"0 attenzione 8ono i sl'l!lwnti: data l'altezza (lei piceo dello spettro, eolti~110lhlente n una certa frequenzn, quali, l'am}liezza eliettiva dell'onda ori~f"jn:lria clle ha generato quella hand:l slleitrale; qu:lfè inoltre la ilipen(lenza dell'ordinnta hpettrale, ossia (lell'altezza dei pieelli dello ~ll1.'ttl·o di potenza, dal riiardo ma~siilw '''''' .~eelto llai rieercaton~ e infine come è pohsihile eseguire automa.tipamente una ana·lisi di 'f (w) per ripay:hne infonl1:1zioni hulle ampiezze, dopo aver ~iahilito lui opportuno eritel·io di signilicatività per i ma~simi i·elativi clle si pre~eniano nello spettro. sul calcolo in:u:.ull'iezza olugil'>allla di cs'osda ecc. 32\l t ruwt.'l.'a'l.'l dell'lxdagl:"\e :\jerli:l1ltp l'uso dpl pl'ognnnma spectil\. gli ~\utori hanno iihlagat.o sulle variazioni di a1tpzza dei })icehi dellu s]lettro (li })utenza in fuuziune delle caratterihtiehe della serie data, e eioè al variare del llumero (lei dati e dell'ampiezza reale (ielle (1)(lp o1'iginarie, la cui 8onapposizionp eostituisce la serie in esame, fissato che la funzione (li autocorrelazione è stata caleolata lino al massimo ritardo eumpatibile eon la luuglwzza dplla serie, i risulblti dpll'illllagine sono mostrati in fig. l, o '" o ! ,., fig, l 330 l'. hax1jl -:\i. e. roxcl!i "}~lva coilltl si lluò yedere, esiste mm tliptlntlenza lineare fra l'alt,(lzza dei llicchi fp (wl) corrispondenti a lillacerta frequenza e la. lunghezza della, serie, acl aml)ìezza originaria, costante, del tipo: rp (0)1) = (li ].l essendo n il numero degli individui coill}jollcmti la serie eli al un ol)pol'tuno coefficient.tl. 'rut.t.e queste reue, passant.i vei' l'origine, hanno il codficicmtc angolare al che è funzione dell'a,mpiezza, dell'oncia originaria, della, estensione temporale della serill data· llol1chè api ritardo massimo (tmox), scelto dal l'icmcatm'(l a s(lconda api problema, tra-t.t.ato; i)ei' cui si può affermare clltl: (li t (ai, tm,x, n) . ora· poichè ciò che int.eressa, è ricava,l'c l'ampiezza, una volta dtltmmillate le l),ltezze dei l)icchi dello si)et.tro l)er le varie frequenze presenti, conoscendo il numm"o di inclividui coml)ollcnti la serie data e noti come al = ((! (wl) i n i coellìcienti angolari delle rette di allllartenenza eli ciascuna clelle ordinate spilttrali, si è cercato di eslllicitare la funzione (li = t (al, 'mox, n) come quando 'm,x è il massimo conscmtito dalla lunghezza della serie data, cioè quando '':'~~ ~ 1, la· migliore interpolazione dei va,lori ot.tcnut.i jr l)er ti è una llarabola passant.e l)er porigine con assc parallelo a,ll'asse delle a.scisse, il cui ramo per y llositive (che è quanto interessa,) è descritto da,ll'equazione al • ..f kl + k. al xl dove al sono suddetti coefficienti angola,rì, rìeavahili come soljl'a esllosto, e le costant.i numeriche valgono rispclttivamente ki = 0.0002020 k 2 = 12.06 xl = 0.01421 in figura, 2 si most.ra. l'a·ihiament{) dillla· funzione sopra scritta, qua.ndo, come detto, 'mox è il massimo conscmtito dalla smie clei dati. :r sul calcolo ln:i,i,'a)ll'iezza orlgr~arla di u~'oxlla ecc. 33l una lùì-eriore indagine i:~ htata f~1tta per individuam il legame tra le o]'dinaì-p spettmli e i rit.ardi milhhimi, tm ax , fissati l)er il r.aleolo. , , , coeff l c i ente :'ngolare fig. 2 si (~ trova.t.o che, indipendentr.mente dall'ampiezza dell'onda che 1m generato quella banda spettrale, il variare del rapporto ,,,,,x/n gpnera. u1ja. variaziol1p r.oncol'(le 1/{' dell'ordinata 8pettrale con un legame del t.ipo mostra.lo in fig. 3. , 8 n o o o , ........ o o~ ______ ~ ______ ~ 0.0 •• , , •• massimo ritardo/nu me ro dei punti fig 3 in 'l';lbella 1 "ono es]ll'eflse ilel'cent.ualmente quest.e val'in·zioni in funzione di tma~/n. t/n ]/0' " ;;% u,15 10% 0.28 1;3% 0.37 20% 0.48 25% 0.58 30% 0.li8 35% 0.75 40% 0.83 45% 0.88 50% 0.!)2 ,;,;~,(, 0.% lio% 0.d7 g;;% 0.n8;; 70% 0.!)!)3 75% 0.flfl7 80% o.ddd 85% 0.flfjfl8 flo% 1.0000 d,;% 1.0000 100% 1.0000 inline si è voluta l'ell<lel'e eoml)let,amente automa.t.ica· l'n·nn·lisi di uno spettro di potenza. t,a tecnica· "el:.'l.lita b st.a.t.a· quella. che tl'adizionahnent.e b aillliiea.t.a. fllutnao si h a a che ve~lel'e con ulm serie ai aati iihli]len~lent.i e ca.~lmlì: sono considerati signilicfltivi t,uui quei l)icchi dello sl)et,tl'o di pot,enzn. corrisl)olhlenti a· rp (wd il cui vn·lore risultn· maggiore ai in con elusione il rìeercatore, qluj.l"ia~i sia· la lunghezzn. ~lella· serie, qualsi~tsi sia il ritanlo t n\" " ll..'\;1.to, illlò risalire al v a lore !lell'am]liezza ~lell'onda origin ale mediante la : rel:tzionc ('hc è fitata pro/p'animata e inserita come implemcnt.azionc ncl progmmma spec'i'h,\, qualora si prcicrisc,[. ~oflt:ituirl! al criterio di ~iglli(-\eatiyità dei }licchi dello ~pettl'o ai potcnza utilizza/o aal programma lui alt r o l'itelluto }liù eollsono (ad es, il criterio ai \yalkcr) ò sufficicntc scriverc una routinc flj~ctio~ 'l'es'i' (l'hl) forzandone la compilazione cii il caricamento prima del caricament.o del }lrogl'ltmma spec'l'll:\, questa fullzione è diye!'sa da zero }le!' i yalori ili l'iii signifieatiyi) zi'l'o ncgli altri ca~i , il }lrogramul<l chùuna questa fusctiox }lc!' tut.ti i massimi relativi aello spet.tro. quella della f u k(!'i'iox 'l'i<js'i' è l'unica ya!'iaziolle lecita alb ~t!'uttura del }lrogrannna spectn~\, :ufinchè i !'i~ultati !'elatiyi alla stima aelle am}liezze siallo validi. "c'tilizz,\zlox"," del prog-lt.\1nia ixp1?l'-ol?1'p'fl' yiene qui de~critta la scquenza (li flchelle }mnunetti necessaric all'e~ecuzionc l11'i 111'ognulllll<l. il contenuto ili questo paragl'ltfo in partc !'ipctc ciò che è stato detto nellayo!'o giù, ('itato inl11'('('('t!cnza (~) in p,ut.e dipelhle l\alla in1:rodllzionc ili nuoye implementazioni nel progmmma spec'i'il\. et! ò quilhli una novità ri~petto al precedentc scritto. i~c flcheae contenenti i paramctri relativi aua eyentuale ri~t!'uttu­ l'azione dei dati ictt i c all'elaborazione ilella ~e!'ie ill ingresso suno d/w e dcyollo cflscrc scm}lre presenti. l~a }ll'cflcnza ,li altre flcheae può essere neccsflaria sot,to partieob!'i coih!izioni come scritto sotto. la scheda for:\l\ 'i' (212, il, 212, tj, 21<'5.0,15, :u<'10.0,il, 18x, tii1) colonlln ],2 i x ollle (ielln i \'nrinhile :\['1' tipo inte!'o descrizione l\ulllcru ,!r'li'ullità log-il'a ['ontellellt.e i ,lati ill lculu'a: ;) i dnti soilo su s(,][('\le (w i ,lat.i sonu su ull filo bei> (li ,pllllull(lllc ilo me diehinrntu sul in s['he(lll lgu. (y. (ies('rizio]w dell'ixl'ct), l (int.i soilo letli lul,ti fillo nll'cll(lof,file. 334 c .. lo nnn. ' -4 • o-i s-, i o il ili l', i1asiji .\1. e. ronciii sei"'..!. nome d ella l \'mil\b ilc 'l'ilio nn in tero 1'l,er ' in tero gap ' (*l intero intero c ili us intero l' l: re l'cale u cs<:t'iz io no n llill ero d c i (l ati p or sd le!!a o i"l~nnl (lli :lj!si mo 80 ), se ~ o i th,ti nll il i soilo s i,!: uifì cath' i se "" o i tl ati nlllli i"iip ihcscnl:ln" llati iii ft.l u' all ti il crt ati c verranno so~ ~it u ì ti nel cor so il ei pl ogn ll u mll, se = o 1l()88 un gnl' in le ltura nei tlati . se "" o ind ica il nume!'o ,li \' oltc ehi) si il ebbono salt, al'c i ilati in lettum, ,l ati eh c ycl'l'nnn o sos tituiti succes· sivam en te ol)[le !'i]ll'isti n fl' re in sequenzn. temporalc della sen o (mass im o \1\1), se = o la succ ()8sionc ilci dnt i iln jlo la lett-um c l'cvcuilllllo so stitll~iollo (ga!'l *" o) è colllplet a se i= o b sili:cc$sinnc otte nut a ,) illcnmp lda, se a d es. ga pe = i si <r nifi e 1"\eh o esist e u n 1;1' 1111110 ,li ,fat i fi sie1"\' me nf.c m 1"\i1 eall t i ; la se rie lett l\ v a in l.cgtab ile r l'ip ri s til)nrc la so· il"e nza i.c mllorale ( 11i !'is8imo 99 ). se _ o nci ; a])s \'ic nc ì n!lt~ l"i t:~ i :~ m c,ha il e l lt~ lw l";c lotta se =io ilei ~:~ps vengo n o ill se rit i ibt i elahor'iti me,hal\ t~ iiitctljolaziu llc line a re lisuiruelhlo il e i met od o ili chius ur a. differenz n l)orl:entunlo richiest.a ,inl mctllll o di djius ul'n, (.) ai l1al'fl lll tlt ri ga1' i, gare il assodato un nulllcro 10 _ gap ' + gai'e di ijohc\le ch e devono 8pguil'e la sdlell n para motri suc('essì\' n ulln. !\cbclb in e!ll"ulle, ciascuna il i queste schede eontil'n c duo \' (l llir i interi i:o n lo'or)'at (2110) l'aj111l'llllentanti i) nulllero 11'01'1linc , nella 8oqllcn~a tempomie ,la claborare, ilei ùati validi illlmcdial.fl.!nento esterni nd og n i ,.;ap ,l a inl~gmj'c, r cla.lì\' all llllllc a ga1'l = l eompnrir:~ lilla ~dle lln i:onblll!lllto 1111 oli , 25 (1:0 1. \1 · 10) e 42 (co l. 1\1-20); ques l.o sia"uili.c:l che in lettu r:\ i ilat i \1:1 1 260 al 41 " sono non significat ivi c aa,lnmuo oppoi'tuna ,,, entc solltil. u it ì. ques to sch ed e tl e \' olln umcre ordilla te pcr va lo ri c l'c8('e n t i ,l e i lilu iti 11 1\; ga pll. sul c.\lcolo detr '\:'iii'i elt.\ orig-i~ '\rb j)t u~·oxd.\ ecc. .'l3/i ( bt" 1j1u 1~ sc1urln) colonna ig·20 21·2:; 36·.tr. 46·55 56 76 77 7s 80 i nume d(~lin i \'llrì l.hile 'l' 'l'au1'ilax xj.:iiax dnt dk2 ifl'ri x1' if .. ' uxc u' x l'''''' n ''l'au jl<' p hl 'l'ijjo ink.rn reale n~nlc real e in tero int.<l l'() intero in tero int er o int.c i'o int ero descrill ione se mil unglj c)\zl\ ,iella fi llestra 1) 2 (in un ità rli nscissa). )rassimo l'ittu',lo nel calcolo rhllln funzione ili .\ntn('rll'l'(·1allione (mn s· .l!imo dodo). valol'c minim o il('lle frcljul.'lize j:o n· sitlcra.te nel ('n lco lo d e llo sl'dlro .ii pot.<lnm. valorc massim o ti ene frequen:w , passo di frequcnza, ossia diffc· l'cnza in nsdssn fra 2 punti ron· sc(:utivi dolio .l!pettro di potenz a. tnllien qu an te \'olte si vuole a.ppli. ('are ai ti mi la fìn ej>t rn d2 . = l vicne stam ],ata in tnhe lll\. con tc nen te, per o~ni riga, la coppia ili ascissa e onlinat.n. spet.trali (li tutti i ]jullti dello spet.t r o il i pote nza. = o la ]u'cce, lente tnbella noli vienc sla mpatn, = l viene sta ulpa l.:l la. tabe ll a ('_onl.en e nl.cl. "ci' ogn i r iga, asc iss.'l., e or~ inat a sp~tt.rll;li nonrl!c a mp le zza onglil:ll'la dci punti massimi l'dativi signiu cati\'i dello s]wttro, = o la pl'ccc lhllllc tabella non viene st<'lmpala nè calcol a la. = o non ,'engo ll o eseguit-i nè il :!rafico ne la. perforazi o ne <iella 5c r ic .iat<'l. = l viellc elw[;uito solo il gra!ìl:oj = 2 vielle eseguita il-olo li\ per· fol'azioll c; = ::i vengon o (i~()~uiti sia il gl'afi co sia la p cl'f un\liione. analogo a ii" x l'ili' la ,<;cll c ll opo 1:. ])2. a nalo~(i a il'' x per la fu nz io no ,ii ltutoc oltelll)\i one, analogo a ii" x per lo spettro di potenza. 331; l'. lta"'-di ~r. 1':. iw:-;cjll s.t-;l,va ::!" .~("hcda l<'oh1\iat (8..'1..10) colonna xolllc (iella tipo i !lesniziolw yal"iahiic i·ho ifor:\j. alfa contiene ;[ formalo (li let.llli"a uel nnlllcl"i('o uol"lllale liuj!naggio !-'ortrau, com· prese le panmlesi eslerue. il pl'ogmmma spectra è disponibile gi~\ compiin.t.o su un pel'mallenl j!'ile pd (' 1'iehiamubile e ht.ilizzabile nwllin.nte in. fwghente sequenza (li schelle: al nel ea~() in ehi i d~~t.i sollo eollt.eilllt.i sh lla~t.l"o: 7/8/9 7/s/!j h/7/8/9 job,c1\i130000,1\it2,t ... a'l"l'ach(lib,coli b,id =as1<',.:\1 h = l) copyn 0,lgo,lib) hgtuhn lib. heques'i' plo'i"i'i<jit. nast.l'o plotte1' re'vind plo'l"i'eh. req"cest da'l'i. iuj\\t\"n nati, lgo(da'i'i) spiwtha,l1 sellede parametri na~l,l'() in lellnl'a (conte .. llent,e i ila t,i) b) nel m~so in ehi i tlat.i r()no 8h scheda: '7/8/9 job,cl\i130000,l\it1,t. a'i"i'ach(lib,coli b,id=asf,1ir = 1) copyn(0,lgo,lib) iuj'i'u itn li b. reques'l' plo'l"i'git. nast.l'o }llotlel' itl<jwlxn pl,o'l'ter. lgo. i/8/u ti/i /b/d spec'i'h,\,ll ~e1ll'de }mral11etri ~rlh'!l1' ilali ]~ op}loi'hl1lo fol'he a qu('h!-o pu1l10 aggiungere una ofw'itazio11(1 ~ui llomi (li tile risp1taii ilal progralllma sp};c'l'ha e hulla ~l1('re~­ siont' ilei ]jal'hnu'tri dpila ~e11eila ikhl mani]jolahili pvl'lltualnwnte per 1'('heeuzione ilei programma, spec'l'ha ut.ilizim inte1'llamenj-{' i fill'~ t,\pei, rl\pe~, 'l'ape:i, 'l'apri, 'l'ape;), tapiw, t,\pei, 'l'.\petw i (jlmli 11011 ill'yollo eh~(,],(' nhrinwllti u,~a1i in 1111 joh e1ll' pheg-ua spec'l'l-l\, i ]mrametri de!illihili all'atto 11pu'ese('uzione me!liallh' la ,~('hpila ijgo ~ono: uhl (lfl, lf2, ici, hj) lf.1 1l0lue dpi file ('ollh'llpllh' i 11ati ill lettura (quando quehti iloll sono hu hrheda) (default nhlue = liati) if2 llome ilei lile ('ont,ellente ru~eita binaria (le] plo1t!'r (ildault, 11hnw = p],(yl'teh) lf:i 11efau11 llalue rsfct lf,l, !lefault nallh' = ou'i'pu'i'. l'!{()(:l{a.\l.\ia l'eh il ('al('(llo e il l'lo'!' dello ,"::'l'e'!'tril .ll! 1'1)texz,\ co.~tallti n.~atl' pe]' l"elahurazioll(, xumero dl'i dati ktti (;000 x'umpro (lt'i puliti ,jplla d2 5!j!w .;\[assimo rihlnlo 5\hj5 fl'l'q. min. (), prp(j. jjlhx. :1.()(i(i(i()()ooe·(11 l'assu 1.(1(1(io()()()(ie-o:l valori massimo (' minilllo (1p1le sl'ril' yul. 1i1a~8lj1io = .j-,05·i{)o.j-j()e---()0 v,li. \"al. massilllu = .j-.oo.j-:?h32.j-e +u() ya1. lllillhllo = -:1.701 j\j27se uu minilllo = .j-.ll1i:w70ie ou \"a1. massilllu minilllo = .j-.5\j:?711j:l7e~-()1 \"'11. jiia.~silllo -= 2..j-2.j-o(j.j-s2e i-o:? val. minimu = l.l.j-.f!l.j-o\j7e+()1 fig . .ja 33!l p. ra~lll ~1. b. ho~ciit set.ya freqllenza 0.000000 0.001000 0.00.2000 0.003000 frequenza .020000 .02i30fw .12.'5000 ordinat.a dello spettro ordinat.a dello spettro 2.j.2 . .j.oo.j.8173 127.00.j.03fj80 .j.i;.!l0002!lfjfj fig. .j.b .7612!l070 .75623720 .7·hhl87!.l3 .750g.j.731 ampiezza. .o!l ..j.fj .20 è posf\ibile ot-tenere, su scelta dell'utente, a.ltre 2 tabelle, l'una contenente i valori di a .. 'jcifma e ordinata s}wtt-mle di t-uui i punti dello spett-ro, l'altra contenente ascissa e ontina·t-a spett-rale e ampiezza originaria rebtive ai }mnti (e massimo relativo, signifiea.tivi secondo criteri espost-i nei varagrafi }lreeeclenti (ve(e es. in fig. ci a, h). per qua.nto concerne l'useib su tabulato, essa· eontiene sem}n'e una· tabella. con i f\egucnt-i (lati: n) numero e valori massimo e minimo della serie dci (li\ti da elaborare; b) numero e valore minimo e nla-f\simo dci dati (lelb serie dopo la d2; c) valore del massimo rit-ardo e valori massimo e minimo della. funzione (li autoeorrela.zione; d) frequenza minima., m~\ssima· e pa.sso di frequenza. usa.ti per il calcolo dello spettro di potenza., va.}ore m~\ssimo e minimo dell'ordinata s}lett-mle. bibliografia (i) be"ila1' j. s. ami pu:ll.sol a. g., l%o. j[cakurr/li·cnt and .analysis al hal/da/ll da/fl . • john '\·ilcy & son, inc .• , xcw york. scl c,\lcolo ln~ll'a111'1l'~z;:.\ ohigi~ahl\ di u'\'oxd'\ lx'('. :l3n e) black~ia"!> r. b. hilil 'l'vlie, ,l. \\"., ib;;8. t/w mea8urcmelll oj l'ower spt'dra . • dov()j" l'nhl. 111('. ' , xel\" yorl (") ](~~i'i)ar.r. ,t. g. hllil s·i'cahtj\., ibgs. t/w_·ldt'ullced thcor!j oj sialislics, • clwrles gt'illin .\: co. l[.(\. " loililoll. (') raxni l'., ro~cjli s-";ly.\ ;\l e., 1!j'2. sul/o -"}ld/ru ,ii jlu/pii:a, • l ' elaburflzione autollmliea " l, 2. (h) kolodovnikoy v. v., inij5. _ ljillflmiqul' 81alisliqlw dl's ,,'y6/lmi'b lilll'aircs dl~ cumm_linde uulo-mriuquc . • dnn\.h1 " l'aris. ( ' ) yaglo~1 a. ;\l, 1!jij2. an ln/rutlucliull lu ihe 'l"hrory uj slaliu/lllry ramlom fi'unctiolls . • i'nmti(:c·llall, tne, englcwood cliiis ' , xci\" jcrscy, nuzzo.pdf 533 annals of geophysics, vol. 46, n. 3, june 2003 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques luigia nuzzo osservatorio di chimica, fisica e geologia ambientali, dipartimento di scienza dei materiali, università di lecce, italy abstract ground penetrating radar (gpr) is a geophysical method increasingly used in numerous shallow applications. unfortunately, electronic or acquisition problems can cause the presence in the radargrams of coherent noise interfering with the useful signal. a commonly observed phenomenon, especially for not-shielded antennae, is the surface-scattering effect, due to reflection or diffraction from above-surface objects. these noise events appear with a characteristic hyperbolic moveout in the usual common-offset sections. other frequent problems are related to the presence of horizontal or dipping features due to system-ringing or other non-geological causes. several methods have been tried to overcome these problems, most of which involve time domain or fourier domain filtering. this work presents an attempt to reduce some of these noise modes by an original adaptation of filtering techniques implemented in the radon domain. the radon transform (rt), both in the linear (or t-p) and in the parabolic version (or t-q), has been widely used in seismic processing, especially for multiple removal, but is still quite unfamiliar to gpr practitioners. the results achieved by different rt based methods for coherent noise attenuation in a gpr field example, compared to those of more conventional techniques, are quite encouraging. mailing address: dr. luigia nuzzo, osservatorio di chimica, fisica e geologia ambientali, dipartimento di scienza dei materiali, università di lecce, via per arnesano, 73100 lecce, italy; e-mail: luigina.nuzzo@unile.it key words gpr – coherent noise – radon transform 1. introduction the ability of ground penetrating radar (gpr) to provide, almost in real-time, very impressive images of the shallow subsurface makes this geophysical method increasingly used in geological, engineering, environmental and archaeological applications. unfortunately, this method suffers from some electronic or acquisition problems that, in particular conditions, can cause the presence in the radargrams of coherent noise interfering with the useful signal. a commonly observed phenomenon, especially for not-shielded antennae, is for example the surface-scattering effect. in fact, reflections or diffractions from above-surface objects, such as trees, fences, power lines, buildings, could be present in the radar sections and could obscure subsurface reflections, since, due to the much higher electromagnetic wave velocity in the air than in the ground, their arrival time could be in the time window of interest. the problem of surface-scattering is not a marginal one. as pointed out by several authors (young and sun, 1994; sun and young, 1995; bano et al., 2000), it can severely degrade the visibility of interesting reflections or even, if not recognised, can be the cause of interpretation pitfalls. the gpr antennae are generally towed directly above or very near to the ground surface. the radiation pattern of a dipole at the boundary of a dielectric halfspace has always a non-negligible lobe in the air, though generally 534 luigia nuzzo considerably smaller than the main lobe oriented toward the ground. the back-lobe can be fairly energetic if the dipole antenna is placed above a lossy halfspace. since shielding is more diffi cult to accomplish for low-frequency antennae, surface-scattering phenomena can be particularly troublesome for low-frequency gpr surveys. surface-scattering noise has generally a hyperbolic moveout on gpr constant-offset profiles, so that this problem closely resembles that of multiple attenuation on common shot or cdp gathers in seismic reflection. sometimes, the relative position of the scattering sources with respect to the survey lines causes only the diffraction tails to be visible on the sections as (generally) steeply dipping lines. linear noise features can be successfully removed by standard 2d fourier transform ( f-k) methods (grasmueck, 1996) or the «domain filtering», i.e. a «local» f-k procedure (young and sun, 1999). also methods based on the linear radon transform (rt), or t-p, or on the less known wavelet transform (nuzzo, 2001; nuzzo and quarta, submitted) proved promising for linear noise feature attenuation, with performance comparable, if not superior, to that of time-domain methods. in the most frequent case of hyperbolic noise features, unless using particular tricks, approaches based on the linear rt change the problem of removing hyperbolic trajectories into that of removing elliptical trajectories. it is clear that the complexity of the problem could not be decreased in such way. more suited strategies, as in the seismic analogue, seem those based on the hyperbolic rt or, due to its computational suitability, on the parabolic rt or t-q transform. in the following sections, after a closer look at some case histories evidencing the seriousness of the surface-scattering problem, the early results of an attempt of coherent noise attenuation by t-q methods are presented and compared to those obtained by means of t-p and time-domain ones. 2. some surface-scattering case histories the first example is probably the most emblematic case of surface-scattering phenomena experienced by our research group. it was observed during a gpr survey carried out by a 35 mhz antenna near nociglia (lecce, italy), shortly fig. 1. example 1: nociglia, lecce, italy – surface-scattering from trees. location map of the radar profiles (13 and 37-38) and of the pine-trees (dots). 535 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques after the collapse of the roof of an unknown karstic cavity, only a few tens of meters away from a provincial road (nuzzo, 1997). because of the high density of collapse sinkholes and known karstic cavities or burrows in the neighbourhood, the presence of other unknown cavities in the uppermost 10-20 m deep layer was suspected, that could pose a serious hazard for human life. the early gpr survey concentrated mainly along and near the provincial road, oriented almost perpendicularly to the axis connecting the old and recent known sinkholes. at the intersection of the main road with a pathway bordered by seven secular pine-trees (dots in fig. 1), a large, fig. 2a,b. example 1: nociglia, lecce, italy – surface-scattering from trees. a) radar section relative to profile 13 (fig. 1); b) the corresponding kinematic model. a b 536 luigia nuzzo complicated, intense anomaly was noted on the radar section (fig. 2a) that at first sight could suggest the presence of a large cavity. the suspicion that the anomaly could instead be due to surface-scattering from the nearby trees was originated by the observation that, after migration at the air velocity, the hyperbolas collapsed to two zones closely corresponding to the position of the two rows of trees, and was subsequently confirmed by the close agreement between the observed traveltimes and those estimated by a kinematic model of diffraction from the base of the foliage (fig. 2b). moreover, a different anomaly pattern was observed in the a b fig. 3a,b. example 1: nociglia, lecce, italy – surface-scattering from trees. a) radar section relative to profile 37-38 (fig. 1); b) the corresponding kinematic model. 537 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques radar section relative to an almost orthogonal profile (fig. 3a), whose general aspect was also confirmed by the corresponding kinematic model (fig. 3b). in this situation, the surface-scattering problem affected zones of the radar sections free of interesting features, so that its removal was not needed, but only its recognition in order to avoid interpretation pitfalls. surface-scattering can be particularly evident also in urban environments where the presence of buildings can cause both inline and off-line reflections and diffractions. an exemplary case 0 20 m fig. 4a-c. example 2: nardò, lecce, italy – surface-scattering from buildings. a) location map of the gpr survey; b) radar section relative to the profile evidenced by a heavy line in (a); c) cdp gather centred at the position marked by a dot in (a). a b c 538 luigia nuzzo occurred during a gpr survey carried out in the historical centre of nardò (lecce, italy) to assess whether the cracks observed in the cathedral apses and the damage affecting some historical buildings could be due to artificial causes or to local hydro-geologic conditions (carrozzo et al., 1999). in the radar section presented (fig. 4b), which refers to the central profile (heavy line in fig. 4a) acquired in piazza pio xi by a 100 mhz antenna, clear diffraction hyperbolas at the air velocity are easily identifiable: the horizontal positions of their apexes match well those of the buildings at the opposite sides of the square. on the other hand, numerous events asymptotically approaching the direct air wave, and crossing the direct and reflected ground waves, are also visible on the cdp gather (fig. 4c) acquired moving the two 100 mhz antenna along the same line symmetrically with respect to the position marked by a dot in fig. 4a. being not flat, these events are unlikely to be caused by inline objects; rather, being well fitted by reflection hyperbolas at the air velocity, they appear due to off-line reflections from buildings. also in this case, it was not necessary to remove these spurious signals, since they did not interfere heavily with the interesting features. however, in other cases, surface-scattering problems can reduce the visibility of useful reflections so dramatically as to make necessary both the careful recognition of the spatially correlated noise modes and the development of proper approaches for their reduction. this was, for example, the situation during a gpr survey carried out in the archaeological site of the roman ships near the s. rossore station in pisa, italy (carrozzo et al., 2001, 2003b). the investigation, having mainly stratigraphical purposes, was performed with a 35 mhz antenna on the plan of the excavation, located about 5 m below the ground level and protected on every side by a 6 m high iron sheet-piling. surface-scattering from the highly-reflective iron enclosure and from other metallic objects near or around the partly recovered boats, was so severe as to require the development of specific processing strategies to help the recognition and removal of the coherent noise in order to uncover useful reflections (nuzzo, 2001, 2002). this experience represented an invaluable tool for understanding the seriousness of surfacescattering phenomena and the importance of filtering procedures to attenuate them. 3. radon based filtering techniques for hyperbolic coherent noise removal the parabolic rt (appendix), or t-q, has been intensely employed in seismic processing for attenuating multiple reflections, showing, as well known, a hyperbolic moveout on common shot or cdp gathers. since the t-q transform maps hyperbolas into pieces of hyperbolas, this transform can be a useful (and more efficient) approximation of the hyperbolic rt only in the small-offset approximation, where the hyperbola is fairly well approximated by a parabola, or if applied after modifying the data (e.g., after a partial normal-moveout (nmo) correction) so that hyperbolas become more similar to parabolas. this has been, indeed, the way most frequently followed in seismic processing: in this way an almost parabolic trajectory is mapped into a point-like zone in the t-q domain, where it can be more easily removed. the same argument can be valid for hyperbolic diffractions in zero-offset profiles, provided the offset is replaced by the common sourcereceiver abscissa relatively to the apex position and the velocity is replaced by an apparent velocity equal to half the true medium velocity (appendix). a straightforward extension of this quite diffuse seismic procedure could therefore be envisaged for hyperbolic coherent noise attenuation in zero-offset (or monostatic) gpr profiles, and especially for surface-scattering removal, where the medium velocity is that of the electromagnetic waves in the air (0.30 m/ns). unlike the most common seismic situation, a full hyperbola (instead of a half one) is usually present on gpr profiles; moreover, the function inside the summation in the discrete t-q is an even function, so that this transform cannot distinguish contributions coming from one or the other side of the hyperbola. to correctly handle the problem, it is necessary to split the data matrix into two sub-matrices with respect to the apex position, and to process them separately. if applied on the uncorrected data, the procedure 539 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques is expected not to work perfectly, because of the already outlined departure of the diffraction path from the parabolic one at larger distances. for this reason, the t-q filtering can be applied after correcting the data so that hyperbolas become more similar to parabolas, such as by means of a t 2-stretching procedure (eq. (a.6)). an alternative approach can take advantage of the linear rt. conceptually, the surface-scattering hyperbola after being flattened by means of a pseudo-nmo correction, maps to a point at zero slope in the t-p domain, where it can be easily muted, and therefore, after undoing the nmo, it would be removed from the radar section. to save computer time and to avoid distortions (as the stretching at large distances from the apex), an extraction procedure can substitute the pseudonmo correction: a sub-matrix can be extracted from the data in a hyperbolic window, tailored tightly along the air diffraction (using the air velocity and having the height of the estimated radar wavelet), transformed and filtered in the t-p domain, inverse transformed and reinserted in the original section (nuzzo and quarta, submitted). 4. field example results the radar section used to test the various radon based approaches is a 510 × 460 matrix extracted from a profile carried out to assess the presence of underground cavities or fractured zones along a piece of the coastal street s. isidoro-torre inserraglio (lecce, italy), where a karstic cavity collapsed in 1992 (fig. 5a) (carrozzo et al., 2003a). the profile, carried out with a 200 mhz antenna, passed above the refilled collapse sinkhole (intense diffractions at the leftmost side in fig. 5b) and below an aerial telephone cable (diffraction hyperbola at the air velocity centred at the abscissa 512 m). the fact that the hyperbola fades away far from its apex and the amplitude variation between the left and right branches make this real case a near ideal one for testing the various methodologies. the radar section has already been used to test other filtering methods (time-domain, wavelet and t-p). in each case, only a hyperbolically windowed data matrix has been filtered and subsequently reinserted in the original section (nuzzo and quarta, submitted). as an example, fig. 5a,b. example 3: s. isidoro-torre inserraglio street (lecce, italy) – surface-scattering from a cable. a) waterfilled karstic cavity collapsed in 1992. b) radar section showing a diffraction hyperbola from an aerial telephone cable crossing the street near the debris-refilled collapse sinkhole. a b 540 luigia nuzzo fig. 6a-f. example 3: surface-scattering removal by means of the linear radon transform (t-p transform) on a hyperbolically windowed data matrix. fig. 7a-f. example 3: surface-scattering removal by means of the parabolic radon transform (t-q). a e b c d f a e b c d f 541 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques fig. 6a-f outlines the filtering process using the linear rt: after extracting a sub-matrix in a hyperbolic window (fig. 6a,b) and transforming it to the t-p domain (fig. 6c), only the data near p = 0 ns/m have been preserved (fig. 6d) and inverse transformed to model the air diffraction (fig. 6e), which has been subsequently subtracted from the original to yield the residual noise-free data (fig. 6f ). for the present work, using the parabolic rt, the full matrix has been transformed, either for the original or the t 2stretched domain. as previously outlined, after splitting the data matrix with respect to the apex position (vertical line in fig. 7a), the two sub-matrices have been processed separately. for computational effi ciency and to preserve the high-frequency content of the data, instead of a muting process a modelling and subtraction procedure has been developed, as previously carried out for the t-p process. in the t-q process, to reduce aliasing problems the maximum allowable sampling interval and range in the q parameter (curvature), have been selected compatibly with the empirical formulas (a.15) and (a.16) derived by kabir and verschuur (1995). both quantities depend on the maximum frequency of the band-limited signal. in the present situation, the dominant band was between 0.1 and 1 ghz. after setting the above parameters in the forward parabolic rt, the t-q panels shown in fig. 7b,c were obtained: not surprisingly, the hyperbola did not map to a point-like region; moreover some diagonal and horizontal features caused by the more intense anomalies disturbed the whole t-q sections. the weaker amplitude of the right branch of the hyperbola made the surfacescattering anomaly practically unrecognisable in the corresponding t-q panel. however, knowing the time location of the apex (t ) and having estimated the air diffraction velocity (v = 0.296 m/ns), the curvature at the apex could be predicted using formula (a.5), valid in the small-offset approximation. in this case, it yields a value of about 0.5 ns/m2. on the other hand, the filtering window choice can be guided by the slightly better visible anomaly in the left panel. selecting the same t-q window for both panels and muting all the external zone (fig. 7e,f), inverse transforming and recombining the two sub-matrices yields the result in fig. 7d. despite a minor edge effect at the junction of the two matrices, and some rather weak artifacts at greater distances from the apex, an acceptable reconstruction of the diffraction hyperbola is achieved. this is, however, allowed by the lucky circumstance that the hyperbola fades away far from its apex, so that only a small portion near the vertex, where the parabolic approximation holds, effectively contributes to the summation. as a further test, instead of applying a (pseudo) partial nmo correction, the alternative approach proposed by yilmaz (1989) involving a t 2-stretching procedure was followed. by squaring the time axes (t' = t 2 and t' = t 2), the hyperbola in the t'-x plane becomes a parabola in the t'-x domain, so that the so far exposed filtering technique can be applied in a similar way in the time-stretched domains (t'-x and t'-q'). this non-linear mapping, while stretching later events, compresses the earlier ones, so it is hazardous for filtering shallow events. moreover, it needs to resample the data at a constant dt' increment, generally taken as the total stretched time divided by the original sample number. a finer sampling, and thus a higher computational cost, could be useful to avoid shallow event distortion. on the other hand, in the stretched domain the curvature, q', is independent from the intercept time, t', of the diffraction parabola apex (eq. (a.8)), and equal to around 45 (ns/m)2 for the air velocity. after estimating (by a 1d fourier transform) the useful (pseudo) frequency range, and after setting a useful sampling interval in q' (for instance in base of eq. (a.15)), the discrete parabolic rt can be performed over a range of curvatures generally much narrower than the maximum one allowed by eq. (a.16), provided it is sufficiently greater than the just obtained 45 (ns/m)2 value. in fig. 8a-f the results of the application of the parabolic rt based approach in the t 2-stretched domain are shown: apart from a better visibility of the diffraction anomaly through the slant features in the t'-q' panels, an almost comparable quality in the reconstructed data as in the previous test is obtained. this can be better appreciated from fig. 9a-f, where the results of the three radon based techniques so far applied (linear rt in the hyperbolically windowed domain, parabolic rt 542 luigia nuzzo fig. 8a-f. example 3: surface-scattering removal by means of the parabolic radon transform (t'-q' ) in the t 2-stretched domain. fig. 9a-f. example 3: comparison of the results obtained for surface-scattering removal by means of different radon based methods (linear rt in the hyperbolically windowed domain, parabolic rt in the original and in the time-stretched domain). a e b c d f a e b c d f 543 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques in the original and in the time-stretched domain) are compared. the t-q filtered section (fig. 9c) is obtained by subtracting from the original section (fig. 9a) the t-q reconstructed hyperbola (fig. 9b), whereas to yield the t'-q' filtered one (fig. 9f ) the reconstructed hyperbola (fig. 9e), obtained after undoing the t 2-stretching on the t'-q' reconstructed parabola (fig. 8d), has been subtracted from the original section. the results of the two t-q based techniques are comparable to each other and to those previously obtained by means of the modified t-p approach (fig. 9d). moreover, they are also quite similar to those produced by more conventional time-domain methods applied in the hyperbolically windowed domain, such as those involving the subtraction of a running average computed in a horizontally sliding window, provided a suitable horizontal size (expressed in number of traces, n ) is chosen (fig. 10a-d). in the present case, the conventional techniques worked well for n varying between about 40 and about 80 (fig. 10c), whereas they failed for smaller or larger values (fig. 10d and b) or when subtracting the average trace computed in the whole (extracted) section (fig. 10a). 5. conclusions the gravity of the surface-scattering problem, appearing as coherent noise with either linear or, more frequently, with hyperbolic moveout on the zero-offset radargrams, has been disclosed by some case histories derived from the fig. 10a-d. example 3: comparison of the results obtained for surface-scattering removal by means of timedomain methods in the hyperbolically windowed domain (background removal and subtraction of a running average computed over different number of traces, n ). a b c d 544 luigia nuzzo recent experience of gpr investigation for environmental and archaeological applications. these examples highlight the need for a physical and/or geometrical comprehension of the origin of the spurious signals in order to avoid interpretation pitfalls, and the need for modelling and processing strategies as an aid for their understanding and, eventually, for their removal. the radon transform could be a useful tool for coherent noise reduction in gpr data. filtering procedures based on either the linear or the parabolic discrete rt can produce results comparable to other classical methods, but allow more flexibility. on the other hand, the effectiveness of radon based methods depends on the parameters selected for the forward and the inverse transformation and the accuracy of the selection of the passor reject-zones. in the example proposed, the useful frequency (or pseudofrequency) band, and in particular the upper limit upon which the choice of the sampling and range of q in the radon domain depends, was selected as narrow as possible, on the basis of the average 1d fourier spectrum of the whole section, to limit the computational cost. moreover, as usual, the damping factor was chosen as a compromise between stability and resolution: studying the trend of the rms difference between the original and reconstructed section (without filtering) versus the damping factor (in the range 10-5-10-1), a value of 10-3 was selected, corresponding in all cases to the beginning of a steeper increment in the data misfit. more problematic is the selection of the zones to be muted (or preserved) in the radon domain: in the present situation it was carried out heuristically by visual inspection of the filter results after various trials. through a real data example it has been demonstrated that surface-scattering can be attenuated by means of either a modification of the t-p filtering technique or by t-q methods: the first allows a reduced data matrix to be processed, but needs an accurate estimation of both the coordinates of the apex of the hyperbola and the propagation velocity; the second (t-q) and the third (t-q with t 2-stretching) need to split the data matrix in two sub-matrices for a proper amplitude handling of the left and right branch of the hyperbola. the last method seems the most effective from a computational point of view, since in the stretched domain the q-range can be restricted to a zone around the (fixed) curvature corresponding to the air velocity. the independence of the curvature from the time of the hyperbola apex could be exploited to filter all at once diffraction hyperbolas whose apexes have the same abscissa but different ordinate, as for the case of diffraction from buildings or vertical (metallic) fences. acknowledgements i wish to thank my ph.d. tutors, prof. maria teresa carrozzo and prof. tatiana quarta, for valuable suggestions and critical remarks during the doctoral activity where i developed the present work. i am grateful to my colleagues g. leucci and s. negri, and to the technicians s. corriero, g. fortuzzi and m. luggeri for their valuable collaboration during data acquisition. i also thank prof. m.d. sacchi from the university of alberta, canada, for making the matlab codes on the radon transform (for_taup.m and inv_taup.m) available on the internet (at the website http://rubble.phys.ualberta.ca/%7esacchi/ seismic_lab/) and the reviewers for the helpful suggestions. appendix. the radon transform. the radon transform (rt), originally introduced by radon in 1917, is an integration procedure that maps a curve in the space (or time-space) domain to a point in the radon domain. the rt and in particular the linear rt, has been widely used in geophysics, and especially in reflection seismic, for various purposes: plane wave decomposition, velocity analysis, multiple suppression and directional filtering (stoffa et al., 1981; schultz, 1982; treitel et al., 1982; yilmaz, 1987; zhou and greenhalgh, 1994; pawlowski, 1997; sykes and das, 2000). the linear rt (t-p transform or slant-stack) is a particular 545 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques case of the generalized radon transform (beylkin, 1987), where the integration path is a line a straight line (a.1) having intercept t and slope p in the t-x plane maps to a point in the t-p domain. if t represents time and x space, p has the dimensions of the inverse of velocity and is usually named slowness (or rayparameter). in the parabolic rt the integration path is described by (a.2) where q is the curvature of the parabola centred at zero offset. the parabolic expression (a.2) is the taylor expansion, truncated at the second order, of the hyperbolic traveltime curve (a.3) equation (a.3) could represent either a reflection in cmp or common-shot gathers, for a homogeneous medium with velocity v, or a diffraction in zero-offset gathers, providing v is an apparent velocity equals to half the true medium velocity. in other words, for a reflection (a.4) while for a diffraction . (a.5) usually the parabolic rt has been applied on partially normal move-out corrected gathers, where the reflection events become approximately parabolic (centred at zero-offset). an alternative approach has been proposed by yilmaz (1989), involving a t 2-stretching procedure. by setting t' = t 2 and t' = t 2, eq. (a.3) takes the form (a.6) which represent a parabola in the t'-q' plane. using the same notations as before, it follows that for a reflection: (a.7) while for a diffraction . (a.8) in the t 2-stretched domain the curvature is independent from the intercept time. u p d x t px dx, , .τ τ( ) = = +( ) −∞ ∞ ∫ t px= +τ t qx= +τ 2 t x x= + ≅ +τ ν τ ν τ 2 2 2 2 21 2 . q = 1 2 2ν τ q = = 1 2 2 2 2ν τ ν τapp t x' ' + 1 ' + q' x 2 2= =τ ν τ2 q' = 2 1 ν q' = 1 appν ν 2 2 4 = 546 luigia nuzzo for digital signals the discrete forward (drt) and inverse radon transform (idrt) are defined respectively as (a.9) (a.10) where d (x,t ) and m (q,t) represent data in time-space and in the radon domain respectively, and n = 1 for the linear rt while n = 2 for the parabolic rt. indeed, from an amplitude point of view a correction term is required in eq. (a.10). computationally, it is easier to implement the transform in the temporal frequency domain and to obtain the forward transform as a least-squares solution of the inverse rt. in this way, for each frequency (or angular frequency w), a set of linear equations has to be solved that in matrix notation can be written as d = am (a.11) where d = d(w) and m = m(w) are the data and model vectors in the fourier domain and a alk= [ ] is a matrix of elements a i x qlk l n k= −( )exp .ω the (unconstrained) least-squares solution of (a.11) is (a.12) where ah is the hermitian (complex conjugate transpose) of a and (ah a)-1 ah is the generalized inverse of a. to avoid singularities in the matrix aha and thus numerical instability, a new matrix is built by adding a small positive number g, known as the regularization parameter or damping factor, to the diagonal elements: ah a + g i, where i is the identity matrix (lines and treitel, 1984). in general g is a function of w, and its choice depends on the noise content of the data. finally, an inverse fft is performed to obtain m (q,t). the choice of the discretization parameters is very important to avoid (or at least to limit) aliasing problems. whereas the sampling in t normally is the same as that in t, more crucial is the choice of the sampling in p (or q) as well as in x (dp or dq and dx). for the linear rt, in case of band-limited signals, with maximum frequency fmax and x-range, xr, good rules for selecting dp and dx are (turner, 1990) (a.13) and the analogous ∆ x p fr≤ ( )1 / max , which leads to a constraint for the maximum allowable p-range, pr, or the maximum value of p, pmax, assuming a symmetrical range of slopes pr = 2 pmax (a.14) by analogy, kabir and verschuur (1995) derived similar formulas for the parabolic rt. for bandlimited signals, with maximum frequency fmax and assuming that x ranges from 0 to xmax with sampling interval dx, whereas the curvature q ranges from 0 to qmax with sampling interval dq, these formulas are respectively (a.15) . (a.16) m q d x t qx n x , ,τ τ( ) = = +( )∑ d x t m q t qx n q , ,( ) = = −( )∑ τ m a a a d= ( )−h h1 ∆q x f≤ ( )1 2/ max max q x x fmax max max/≤ ( )1 2 ∆ ∆p x fr< ( )1 / max p f xmax max/ .≤ ( )1 2 ∆ 547 coherent noise attenuation in gpr data by linear and parabolic radon transform techniques references bano, m., g. marquis, b. nivièr, j.c. maurin and m. cushing (2000): investigating alluvial and tectonic features with ground-penetrating radar and analyzing diffraction patterns, j. appl. geophys., 43, 33-41. beylkin, g. 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(1996): 3d ground-penetrating radar applied to fracture imaging in gneiss, geophysics, 61, 1050-1064. kabir, m.m.n. and d.j. verschuur (1995): restoration of missing offsets by parabolic radon transform, geophys. prospect., 43, 347-368. lines, l.r. and s. treitel (1984): tutorial a review of least-squares inversion and its application to geophysical problems, geophys. prospect., 32, 159-186. nuzzo, l. (1997): indagini georadar in zona interessata da dissesti idrogeologici (nociglia), university of lecce, m.sc. thesis (unpublished). nuzzo, l. (2001): acquisition, processing and visualization of ground penetrating radar data by means of classical and novel techniques – some applications to archaeological and environmental studies, university of messina, ph.d. thesis. nuzzo, l. (2002): identification and removal of above-ground spurious signals in gpr archaeological prospecting, in european geophysical society scientific program, nice, april 2002, egs02-a-02468, cd-rom. nuzzo, l and t. quarta: improvement in gpr coherent noise attenuation using t-p and wavelet trasforms, geophysics (submitted). pawlowski, r.s. (1997): use of slant stack for geologic or geophysical map lineament analysis, geophysics, 62, 1774-1778. schultz, p.s. (1982): a method for direct estimation of interval velocities, geophysics, 47, 1657-1671. stoffa, p.l., p. buhl, j.b. diebold and f. wenzel (1981): direct mapping of seismic data to the domain of intercept time and ray parameter – a plane-wave decomposition, geophysics, 46, 255-267. sun, j. and r.a. young (1995): recognizing surface scattering in ground-penetrating radar data, geophysics, 60, 1378-1385. sykes, m.p. and u.c. das (2000): directional filtering for linear feature enhancement in geophysical maps, geophysics, 65, 1758-1768. treitel, s., p.r. gutowski and d.e. wagner (1982): plane-wave decomposition of seismograms, geophysics, 47, 1375-1401. turner, g. (1990): aliasing in the t-p transform and the removal of spatially aliased coherent noise, geophysics, 55, 1496-1503. yilmaz, o. (1987): seismic data processing, edited by s.m. doherty (seg, tulsa), pp. 526. yilmaz, o. (1989): velocity-stack processing, geophys. prospect., 37, 357-382. young, r.a. and j. sun (1994): recognition and removal of surface scattering in gpr data, in proceedings of the fifth international conference on ground penetrating radar, kitchener, ontario, 735-746. young, r.a. and j. sun (1999): revealing stratigraphy in ground-penetrating radar data using domain filtering, geophysics, 64, 435-442. zhou, b. and s.a. greenhalgh (1994): linear and parabolic t-p transform revised, geophysics, 59, 1133-1149. annals 47, 1, 2004, 01/07def 107 annals of geophysics, vol. 47, n. 1, february 2004 key words crustal exploration – magnetotellurics – geothermal areas – southern tuscany 1. introduction the magnetotelluric (mt) method is used to estimate the electrical characteristics of earth structures using naturally occurring electromagnetic fields. it is particularly suited to the task of exploring areas of high heat flow since these areas are commonly associated with dynamic activities such as magma emplacement, crustal fracturing, and the circulation of hot, electrically conductive fluids. southern tuscany, which is characterized by a high heat flow and the presence of two of the most important geothermal areas of the world, is an excellent test site for resistivity characterization. indeed, larderello has been studied in detail by mt soundings since 1973 (fiordelisi et al., 1998; and references therein). for a long time mt surveys in southern tuscany region have encountered significant difficulties that mainly stem from three factors: i) the high level of electromagnetic signals from industrial and cultural sources, which interfere with the natural fields used in the method; ii) the presence of a very conductive shallow formation, and hence the need to acquire data for long periods; and iii) the naturally-occurring structural complexity commonly found in these areas, which requires twoor three-dimensional mt modelling for interpretation purposes, thus significantly increasing the amount of effort required by modelling. apart from the uncorrelated noise that mainly affects the high frequencies linked to the wide variety of cultural noise common to all densely populated countries, an extremely coherent noise was found to be pervasive in the period resistivity and heterogeneity of earth crust in an active tectonic region, southern tuscany (italy) adele manzella cnr-istituto di geoscienze e georisorse, pisa, italy abstract southern tuscany, belonging to the inner zone of the northern apennines, italy, is an ideal laboratory for observing the physical features of the lithosphere and their evolution in tectonically active regions. here the crust is very thin, with a thickness of less than 25 km, and heat flow is very high, hence only very shallow depths of exploration are needed to investigate many of the middle-deep crustal features that are common to many other parts of the world. the magnetotelluric (mt) surveys performed in this region have provided information on the resistivity structure, which is related to the extent and distribution of free fluids and to the partial melts in the crust. the picture emerging from these mt surveys is that of a resistivity structure that is only partly related to the heat flow regime of the area. a very low resistivity was found below the vapour-dominated geothermal system of larderello and below areas that have no clear connection to any geothermal system, whereas this reduction of resistivity is less conspicuous below the water-dominated geothermal system of mt. amiata. mailing address: dr. adele manzella, cnr-istituto di geoscienze e georisorse, via moruzzi 1, 56124 pisa, italy; e-mail: manzella@igg.cnr.it 108 adele manzella range of 1-1000 s. this noise, produced by the currents discharged by the direct-current electrified railways at the passage of trains, required considerable study and data processing (fiordelisi et al., 1995; larsen et al., 1996). the most recent mt surveys, however, have shown that in most areas this noise can be effectively removed and the data interpreted without distortion by using appropriate data acquisition and processing techniques. the continuous profiling technique was also used in the most recent surveys to increase the resolution of the shallower structure. the mt surveys in southern tuscany were undertaken primarily for geothermal and deep crustal exploration (crop, deep crust exploration projects) (de angelis et al., 1998; fiordelisi et al., 1998; manzella et al., 1999). the study area described in this paper is crossed by the crop 03 and crop 18 profiles (fig. 1). a total of 232 mt sites were acquired over the last 10 years, 108 of which were acquired for geothermal purposes and were referenced to a remote site on the island of capraia in order to remove the extremely coherent electromagnetic noise. the remainder were single-site or locally referenced and many are affected by noise. this paper describes the overall results obtained by modelling the entire set of data recorded after 1991. this gives a broad view of the deep electrical structure of the region, showing a conductive medium-low crust of regional scale but with some important exceptions that are not simply related to the geothermal systems. the interpretation of the mt data could provide some indication of the role played by fig. 1. location of the magnetotelluric sites in a schematic geological map. the crop profiles and the main electrified rail tracks are shown, and the main survey profiles are indicated by letters for reference in the text. 109 resistivity and heterogeneity of earth crust in an active tectonic region, southern tuscany (italy) electrical conductivity in the middle-lower crustal regimes. 2. geological background and geophysical outline southern tuscany belongs to the inner part of the northern apennines, and originated from the collision between the corsica-sardinia and apulia microplates (gianelli et al., 1997; and references therein). the region is characterised by nnw-sse faults linked to the alpine orogeny (fig. 2). after the post-tortonian, prevalently extensional tectonics, the region underwent a general uplift, so that older units crop out in some places. its extension was accompanied by anatectic magmatism whose ages decrease in an eastward direction. shallow intrusive bodies are the heat sources of the two most important geothermal areas, larderello and mt. amiata, which have undergone intensive exploratory and production drilling for almost half a century. direct exploration by means of hundreds of shallow and deep wells has permitted to achieve a detailed reconnaissance of the first 4.5 km of the crustal structure. at larderello drill-holes within the first 4.5 km of depth encountered sequences of sedimentary, metamorphic and igneous rocks. below the jurassic-to-cenozoic ophiolite and flysch units (ligurids) and a triassic-to-tertiary allochthonous unit (tuscan nappe), there is a zone of tectonic slices, involving mesozoic rocks and paleozoic formations. the deepest metamorphic units are micaschist and gneiss. buried dykes, subvolcanic bodies of granite and thermo-metamorphic minerals were frequently crossed by deep drillings. the mt. amiata area, with respect to larderello area, is characterised by the presence of lava flows covering the sedimentary sequence and by the particular nature of its basement, which is mostly made up of graphitebearing metasedimentary rocks. the vapour-dominated geothermal system of larderello and the water-dominated geothermal system of mt. amiata both consist of two reservoirs: a shallow reservoir within the carbonate units at 0.5-1 km depth, and a deeper, more extensive reservoir within the metamorphic basement, at depths of more than 2 km beneath the known geothermal fields, at pressures of up to 70 bar and temperatures between 300 and 350°c (barelli et al., 2000; fiordelisi et al., fig. 2. location map of southern tuscany, showing the geothermal areas and the main faults. 110 adele manzella 2000). the geothermal fluids circulate through the larger fractures and faults, whose main orientation is the regional apenninic one. permeability is usually low and heterogeneously distributed. the results of many studies on contact metamorphic and hydrothermal minerals suggest that there were two main stages of hydrothermal activity: an early, high-temperature stage related to the intrusion of the granites, which produced contact metamorphic and metasomatic processes in the basement rocks, and a second stage characterised by precipitation of lower-temperature mineral assemblages filling veins at shallow-intermediate depths or, in certain places, replacing early-stage contact metamorphic or igneous minerals (magro et al., 2003; and references therein). since the structure is not widely fractured and the alteration minerals are localized around the faults and fractures, the alteration is not widespread. seismological studies have shown that the area extending from ne of corsica to the inner part of the northern apennines is characterised by a thinned continental lithosphere and a large anomalous structure within the upper mantle (gianelli et al., 1997; and references therein). the thin, elongated tuscan moho (depth ∼ 25 km) lies directly over a soft asthenosphere that has risen by 40-50 km. the effects of this thinning and the subsequent magma intrusions at shallow depth are evident in the present heat flow regime, which shows a regional value of 120 mw/m2 and higher values in the geothermal areas (fig. 3a). the thermal regime leads to a lower seismic velocity and density in the crust and in the upper mantle, as revealed by seismic tomography and gravity data (batini et al., 1995; bernabini et al., 1995). minimum bouguer values in the geothermal areas (fig. 3b), filtered for shallow structures and mantle effects, have been modelled as a roughly dropshaped low-density body between 7-8 and 20 km depth, and interpreted as shallow intrusive and still molten bodies (bernabini et al., 1995; fiordelisi et al., 1995). seismic data provided valuable indications as to the shape and depth of the anomalous body in the larderello region. inversion of local earthquake arrival times and teleseismic travel-time residuals revealed the presence of a low velocity anomaly, 20 km fig. 3a-c. maps showing (a) heat flow regime, b) bouguer anomaly and (c) k-horizon depth. a b c 111 resistivity and heterogeneity of earth crust in an active tectonic region, southern tuscany (italy) wide at its top and possibly wider as it approaches the moho, at depths of more than 8 km below ground level (see fig. 4a,b) (batini et al., 1995). a low velocity body was also identified below the amiata area, but resolution here is much lower than at larderello (chiarabba et al., 1995). another clue is provided by a seismic horizon, k, which was identified in the amiata area and given the same name as the seismic horizon first observed at larderello. seismic reflectors having the same seismic signature as the k-horizon were recognised not only in the geothermal fields of larderello and mt. amiata, but also along the deep seismic reflection profiles crop-03 and crop-18 (e.g., cameli et al., 1998). these horizons are often referred to as a single horizon, but this is not strictly true. the k-horizon locally shows «bright spot» features and is located at 3-4 km depth below the central parts of the geothermal fields of larderello and mt. amiata, at 5-6 km depth along the margins of these same fields, and at 10-12 km depth outside the geothermal areas, where it becomes relatively flat. the fig. 4a,b. a) 2d inversion results of the tm-mode mt data along aa′ profile in the larderello area; b) local earthquake and teleseismic inversion results along the same profile (from batini et al., 1995, modified). a b 112 adele manzella form of the k-horizon shows many similarities with the heat flow and gravity maps (fig. 3c), rising sharply above the deep anomalous bodies defined by seismic tomography. the most common interpretation of this horizon is that it represents the brittle-ductile transition of the tuscan crust, where fluid-filled fractures determine the bright-spot appearance in geothermal areas (cameli et al., 1998; and references therein). 3. resistivity data and discussion resistivity data have never achieved a decisive importance in southern tuscany. due to the low resistivity of the shallower formations, the thousands of geoelectrical data acquired in the geothermal areas have never proved to be of real interest since they were able at most to define the resistivity distribution down to the resistive basement, whereas the main geothermal target for the past 30 years has been the deep reservoirs within the metamorphic basement. moreover, due to the geological characteristics of the formations hosting the geothermal reservoirs in tuscany and to their low permeability, geothermal fluid circulation does not produce a widespread alteration zone and the contribution to resistivity of very conductive clayey minerals can be neglected. hence, the decrease in resistivity caused by thermal fluid circulation was not expected to be high, especially in the larderello geothermal area, where the dominant fluid phase is steam. the geoelectrical data acquired in the larderello and mt. amiata areas, together with the results of a few resistivity logs in the larderello area, and laboratory data (losito, 1991), have been used to determine the resistivity values of the shallow geological units and to formulate hypotheses with regard to the deepest units. the shallow neogenic deposits and flysch units are electrically conductive (10-20 ω⋅m). the carbonate formations are only moderately resistive (usually around 100 ω⋅m), whereas the anhydritic formations, interbedded with the carbonates, are occasionally very resistive, reaching values of 1000-1500 ω⋅m. the resistivity of the metamorphic units measured in a few deep wells in the larderello area are quite low (about 100 ω⋅m) compared to those of the anhydrites and to the values measured on laboratory samples. this resistivity is probably representative of the upper, most fractured part of the metamorphic basement and a bulk value of many thousands of ω⋅m is expected on the basis of the laboratory data, even accounting for the very high temperature and pressure conditions (losito, 1991). it is difficult to define the resistivity values in geothermal areas accurately because of the presence of geothermal fluids; by definition warm and salty, these fluids decrease the resistivity of the formations affected by fluid circulation. formations that are usually resistive, such as metamorphic formations, in these conditions can assume values that are not so different from those of carbonatic formations. the presence of geothermal fluids also triggers dispersion phenomena over the frequencydependent mt sounding data (stoyer, 1976; patella, 1987). this is not a problem when many electrical sounding data are available, and comparison of the high-frequency mt data and the electrical data defining the shallower structures gives an idea of the dispersion rate (patella et al., 1991). in our case, many electrical data were available in the larderello and amiata geothermal areas and the dispersion effect was found to be negligible. outside the geothermal area the dispersion effect produced by hydrothermal fluid circulation is also probably negligible, as shown in giammetti et al. (1996). looking at the distribution of sites in fig. 1 it is evident that most of the mt sites are distributed over profiles with a roughly wswene trend. this is because the main regional strike is defined by the apenninic structure and the coast, i.e. ese-wnw, and the profiles were designed orthogonal to the main strike. the profiles and the sparse sites are too widely separated to allow a 3d modelling of the mt data. therefore, 2d interpretation of the data have been attempted. depending on inductive and geological length scales of the target, 2d interpretation may provide useful information of the features of the subsurface, provided that near surface galvanic distortion are removed (ledo et al., 2002). the mt data along the profiles 113 resistivity and heterogeneity of earth crust in an active tectonic region, southern tuscany (italy) shown in this paper have been modelled using a 2d inversion code, which use a regularized inversion algorithm that minimizes the sum of the chi-squared measure of data misfit and the squared norm of the laplacian of the model function (rodi and mackie, 2001). since tm-mode data are less affected by complex effects caused by near 3d structures (wannamaker et al., 1984; and references therein), we refer here to models obtained by inversion of tm-mode data only. ledo et al. (2002) have discussed the applicability of this procedure and found results consistent with previous work, which suggest that the models obtained by inversion of tm-mode data only are more robust than those obtained using both polarization. however, the resistivity of the conductive 3d bodies is overestimated and the dipping structures are not well defined. the mt data span a frequency range of about 300-0.001 hz, with some slight changes due to the different equipment used during fieldwork. the mt data considered in this study were acquired in the region after 1991. before 1991 the data were acquired in the larderello area as single-site or were referenced to a local site at a distance of, at most, 3 km. the strong electromagnetic noise affecting the data for frequencies lower than 1 hz cannot be separated from the mt signal in this case, and only shallow information can be gathered for these data (fiordelisi et al., 1998). the first mt data acquired in remote-reference mode and processed with robust algorithms were from larderello, in 1992 (fiordelisi et al., 1995). two profiles, for a total of 34 sites at an average distance of 2 km, crossed the main geothermal fields and the surrounding areas (aa′ and bb′ in fig. 1). these data were recently reprocessed (manzella et al., 2002) and the results obtained by 2d inversion of tm data on profile aa′ are shown in fig. 4a. the rms misfit of the model is 10.5 with a floor error of 5% and 2% for resistivity and phase, respectively. the modelling shows, on both profiles aa′ and bb′ (the latter is not shown here) that the area is characterized by many low resistivity anomalies. there is a good correlation between the most productive and exploited geothermal fields and the low resistivity anomalies. the main anomalies show a very large vertical extension, and their location is comparable to those of the low seismic velocity anomalies defined by seismic tomography and described in section 2 (fig. 4b). since only tm-mode data were inverted, the resistivity of the anomalies is probably overestimated. unfortunately, the lateral and vertical extension of the anomalies, and hence of the reservoirs and their connection to deeper features, does not emerge in great detail since the distance between sites was quite large and the structure is very complex. the distance between mt sites was found to be of critical importance for defining the extension of these low resistivity areas. as an example, we consider two models obtained inverting the mt data acquired in the mt. amiata water-dominated geothermal area on 1994 (fig. 5a,b), along a profile named cc′ in fig. 1 (manzella et al., 1999). the model on the top was obtained by inverting the tm-mode mt data from just 26 sites at an average distance of 1 km, and the one on the bottom by inverting the tm-mode mt data from 126 sites at an average distance of 200 m on the same profile. the rms misfit of the two models is 1.65 and 1.87, respectively. the two models define very different features of the subsurface, in particular at large depth. the one on the bottom is probably a better and more detailed representation of the subsurface, taking into account, however, the limitation of the 2d interpretation of 3d structures. a spacing of just 1 km between mt sites is unable to resolve the lateral changes in resistivity occurring in an area with a sharp transition to lower resistivity, since this is caused by an abrupt increase in permeability through fractures and faults. the data, for the moment, are unable to predict the exact location and direction of the fractures at such great depths, which would be of considerable interest in geothermal exploration, but they identify the areas that are affected by deep circulation of geothermal fluids. the correspondence between areas of low resistivity inside the resistive basement and geothermal reservoirs is very evident in the mt. amiata waterdominated system, which was investigated by another continuous profiling survey, named dd′ in fig. 1 (volpi et al., 2003). it is notewor114 adele manzella fig. 5a,b. 2d inversion results in the amiata area along profile cc′: a) tm mode inversion of 26 mt sites along a profile; b) tm mode inversion of 126 sites along the same profile. fig. 6. 2d inversion results in the amiata area along profile dd′. a b 115 resistivity and heterogeneity of earth crust in an active tectonic region, southern tuscany (italy) thy that, below the mt. amiata exploited geothermal area, the resistivity is never as low as in larderello at a depth of more than 7-8 km (figs. 5a,b and 6). the main decrease in resistivity, comparable to that of some sites in the larderello area, is found outside the geothermal area, about 15 km sw of mt. amiata. apart from the surveys in geothermal areas, mt data were also acquired within the framework of deep crustal exploration projects. although many of these data are too affected by noise to be of any use, some proved to be good and were considered in this study. the profile ee′ in fig. 1 included many sites where noise was absent or very low (de angelis et al., 1998). the model obtained by 2d forward modelling is shown in fig. 7, and depicts a continuous increase of resistivity with depth. despite the clear limitation of the 2d model where the sites have an average distance of 10 km, it is evident from the figure that the most conductive formations, with resistivity lower than 50 ω⋅m, are confined to depths of less than 2-3 km. the only exceptions to this general trend occur near the eastern border of the model, with an uplift of the tuscan nappe that appears reversed at the surface, and below the outcropping volcanics (site 11 in fig. 7), where the resistive layer rises to a depth of about 8-9 km. this shallow resistive body was interpreted as an intrusion of hot material that rose in the past and cooled completely. other data, acquired on sites located 20 km west of mt. amiata, show a very low, deep resistivity that is comparable only to that of some sites in the larderello area and sw of mt. amiata (manzella and fiordelisi, 1995). in general we observe that mt data can be divided into three main categories, which are named only for representation purposes: 1) normal sites, located outside the geothermal area, whose apparent resistivity at frequencies lower than 10−2 hz (i.e. at greater depths) is higher than 100 ω⋅m; 2) deep, low resistivity sites, characterized by an apparent resistivity lower than 10 ω⋅m at frequencies lower than 10−2 hz; and 3) geothermal sites, located in intensely exploited geothermal areas, whose apparent resistivity at lower frequencies is lower than in normal sites but higher than in low resistivity sites. the geographical distribution of the three types of sites is shown in fig. 8, as well as a sample curve for each of the three categories. comparing fig. 8 with fig. 3a, we observe that the areas where the heat flow values are highest do not always correspond to the areas with the lowest resistivity below 8-10 km depth. the low resistivity data in the larderello area show a good correspondence both with the maximum heat flow values and the location of the low velocity anomaly defined by seismic tomography. this evidence, together with other geophysical and geological information described in section 2, can be interpreted as due to the presence of a still hot and partially molten magmatic body, representing the heat source of the larderello geothermal area. with the actual data it is impossible to say whether there is one, large magmatic body only or many different bodies, possibly interconnected. geochemistry has shown that the helium values at larderello are anomalous, suggesting a mantle origin for part of the geothermal fluids, and hence depicting a direct connection of the magmatic body/-ies with the upper mantle (magro et al., 2003). with the data available at present, however, we are unable to verify this hypothesis. fig. 7. 2d forward modelling results along profile ee′. 116 adele manzella although its reservoir temperatures can be as high as 320°c, the amiata area appears quite different from larderello. both the mt and the seismic data are unable to identify any clear reduction in resistivity and seismic velocity below this area. this difference with the situation at larderello could be explained by assuming that the magmatic body is deeper and maybe smaller than at larderello. nor does the geochemistry show any evidence of a mantle origin in the geothermal fluids, so the difference could also be due to a different connection with deeper crustal and mantle structures. fifteen kilometres sw of amiata we identified an area with deep, low resistivity and no apparent relation to the amiata area itself (manzella et al., 1999). this area also shows an increase in heat flow values, but no seismic tomography data are available. in the other area with low resistivity at depth, located 20 km west of mt. amiata, neither heat flow nor seismic tomography data are available for comparison. fig. 8. distribution of the three main categories of mt sites (see text): dotted ovals indicate areas of very low deep resistivity (few ω·m); slashed ovals correspond to areas of average deep resistivity (100-200 ω·m) and densely lined ovals indicates areas of high deep resistivity (1000-2000 ω·m). an example curve of the three main categories of sites is given on the bottom. 117 resistivity and heterogeneity of earth crust in an active tectonic region, southern tuscany (italy) magmatic bodies could be present below these areas, at depths similar to those at larderello, but have never been identified before because of a lack of surface manifestations. the general behaviour of resistivity in the medium crust is another matter of some interest here. many papers report a decrease of resistivity in the medium-deep crust, which was interpreted as being linked to the transition from brittle, shallow to ductile, deep conditions, and, possibly, the presence of fluids (see, e.g., simpson, 1999; and references therein). a region such as southern tuscany, so richly endowed in geothermal fluids, some of which are of deep origin, should be a perfect example of how deep crustal fluids can affect resistivity. as already mentioned in section 2, the brittle-ductile transition in this region is possibly evidenced by the seismic marker k and it probably follows the same shape of the k-horizon «surface» shown in fig. 3c at a slightly different depth. hence, the transition was definitely explored by the mt data described in this study, since it is quite shallow. indeed, no evidence of a decrease of resistivity can be seen at depths corresponding to the ductile condition of the crust. 4. conclusions the mt data show evidence of an extremely heterogeneous distribution of crustal resistivity in southern tuscany. mt penetration depths of more than 4 km were achieved only where electromagnetic noise was absent, or could be eliminated. in general, the mt data show that below this depth the resistivity reaches values of only a few thousands of ω⋅m, which is far lower than in metamorphic formations in other parts of the world. this is not unusual, however, for a tectonically active area such as southern tuscany, where the high reflectivity and the high heat flow could indicate the presence of fluids and/or melts on a fairly wide scale. besides the resistivity anomalies created by local conditions at depths of 2-5 km, the mt data also reveal that the resistivity distribution is not homogeneous below 8-10 km depth, and that low resistivity subsurface does not show a simple relation to the heat flow distribution. the deep, low resistivity anomalies were found below the geothermal area of larderello and in areas located west and south-west of the amiata area but outside the main geothermal area, whereas deep resistivity in the central amiata area has been observed to increase. the mt data seem to indicate that the intrusions lie at greater depth in the amiata area than in the larderello area, and that intrusions comparable to those at larderello may be present in areas that have still to be explored. the connections at depth between these many magmatic bodies has still to be investigated. a clear transition to low resistivity in the medium and deep crust is absent in southern tuscany, suggesting that the effects of temperature and fluid distribution do not depend on the transition to ductile conditions. more mt data are needed to obtain a clearer picture in the region. future experiments will include the acquisition of data at lower frequency than have been acquired till now, and longer recordings in order to obtain a better quality. acknowledgements the author thanks enel and the cofin2000 project for providing data and for partly funding this research, and marnell dickson for proof-reading the original manuscript. references barelli, a., g. bertini, g. buonasorte, g. cappetti and a. fiordelisi (2000): recent deep exploration results at the margins of the larderello travale geothermal system, in proceedings of the world geothermal congress 2000, kyushu-tohoku, japan, 965-970. batini, f., a. fiordelisi, f. graziano and m.n. toksoz (1995): earthquake tomography in the larderello geothermal area, in proceedings of the world geothermal congress 1995, firenze, italy, 817-820. bernabini, m., g. bertini, g.m. cameli, i. dini and l. orlando (1995): gravity interpretation of mt. amiata geothermal area (central italy), in proceedings of the world geothermal congress 1995, firenze, italy, 859-862. cameli, g.m., i. dini and d. liotta (1998): brittle/ductile boundary from seismic reflection lines of southern tuscany (northern apennines, italy), mem. soc. geol. it., 52, 153-163. chiarabba, c, a. amato and a. fiordelisi (1995): seismicity and velocity images of the roman magmatic 118 adele manzella province, in proceedings of the world geothermal congress 1995, firenze, italy, 827-832. de angelis, m., a. fiordelisi, a. manzella and a. zaja (1998): two-dimensional analysis of a magnetotelluric profile in the crop 03 area in southern tuscany, mem. soc. geol. it., 52, 295-304. fiordelisi, a., r. mackie, t. madden, a. manzella and s. rieven (1995): application of the magnetotelluric method using a remote-remote reference system for characterizing deep geothermal system, in proceedings of the world geothermal congress 1995, firenze, italy, 893-898. fiordelisi, a., r. mackie, a. manzella and a. zaja (1998): electrical features of deep structures in southern tuscany (italy), ann. geofis., 41 (3), 333-341. fiordelisi, a., a. manzella, g. buonasorte, j. larsen and r. mackie (2000): mt methodology in the detection of deep, water-dominated geothermal systems, in proceedings of the world geothermal congress 2000, kyushu-tohoku, japan, 1121-1126. giammetti, s., d. patella, a. siniscalchi and a. tramacere (1996): the siena graben: combined interpretation of des and mt soundings, ann. geofis., xxxix (1), 189-200. gianelli, g., a. manzella and m. puxeddu (1997): crustal models of the geothermal areas of southern tuscany (italy), tectonophysics, 281, 221-239. larsen, j.c., r.l. mackie, a. manzella, a. fiordelisi and s. rieven (1996): robust smooth magnetotelluric transfer functions, geophys. j. int., 124, 801-819. ledo, j., p. queralt, a. martí and g. jones (2002): twodimensional interpretation of three-dimensional magnetotelluric data: an example of limitations and resolution, geophys. j. int., 150, 127-139. losito, g. (1991): proprietà elettriche delle rocce lungo il profilo crop 03, studi geologici camerti, 1, 33-40. magro, g., g. ruggieri, g. gianelli and s. bellani (2003): helium isotopes in paleofluids and present-day fluids of the larderello geothermal field: constraints on the heat source, j. geophys. res., 108 (b1), doi: 10.1029/2001jb001590. manzella, a. and a. fiordelisi (1995): resistivity structure on the western side of the mt. amiata region, in proceedings of the world geothermal congress 1995, firenze, italy, 881-886. manzella, a., r. mackie and a. fiordelisi (1999): mt survey in the amiata volcanic area: a combined methodology for defining shallow and deep structures, phys. chem. earth, a, 24 (9), 837-840. manzella, a., g. volpi, f. corsi and a. fiordelisi (2002): distribution of resistivity in the upper crust of the larderello geothermal area, in studi per l’interpretazione del profilo sismico crop18, ii stato di avanzamento, 16-24. patella, d. (1987): tutorial: interpretation of magnetotelluric measurements over an electrically dispersionale one-dimensional earth, geophys. prospect., 35, 1-11. patella, d., a. tramacere, r. di maio and a. siniscalchi (1991): experimental evidence of resistivity frequency-dispersion in magnetotellurics in newbury (oregon), snake river plain (idaho) and campi flegrei (italy) volcano-geothermal areas, j. volcanol. geotherm. res., 48 (1-2), 61-75. rodi, w. and r.l. mackie (2001): nonlinear conjugate gradients algorithm for 2d magnetotelluric inversion, geophysics, 66, 174-187. simpson, f. (1999): stress and seismicity in the lower continental crust: a challenge to simple ductility and implications for electrical conductivity mechanisms, surv. geophys., 20 (3-4), 2001-227. stoyer, c.h. (1976): consequences of induced polarization in magnetotelluric investigation, pure appl. geophys., 114, 435-449. volpi, g., a. manzella and a. fiordelisi (2003). investigation of geothermal structures by magnetotelluric (mt): an example from the mt. amiata area, italy, geothermics, 32 (2), 131-145. wannamaker, p., g. hohmann and s. ward (1984): magnetotelluric responses of three-dimensional bodies in layered earths, geophysics, 49, 1517-1533. completo_def.qxd 1311 annals of geophysics, vol. 47, n. 4, august 2004 1311 key words ionosphere – f2-layer critical frequency – ionospheric disturbances 1. introduction ionospheric disturbances, being manifestations of extreme space weather, may severely degrade terrestrial and earth-to-satellite technological systems. consequently, long-term predictions and short-term forecasting of such phenomena are essential. recent investigations, mainly by global-scale numerical simulations of first-principle models (schunk, 1996) and by measurement studies (yeh et al., 1994), have improved our understanding of the thermosphere-ionosphere system. however, the agreement between simulations and observations is still rather qualitative, whereas a quantitative modeling for immediate space weather application has not been achieved yet (szuszczewicz et al., 1998; fuller-rowell et al., 2000). one way to establish accurate specification of the temporal and spatial development of a ionospheric storm is to fully understand disturbance mechanisms and insert suitable input to the global simulation models. although research has grown rapidly in this direction (prölss, 1993, 1995; buonsanto, 1999; mikhailov, 2000), cause-effect relationships are hard to establish in many cases. alternatively, ionospheric empirical storm-time models in disturbed magnetic conditions have been developed (e.g., cander and mihajlovic, 1998), improving thus existing ionospheric empirical models (araujo-pradere et al., 2002, 2003). such an advance deals with one major cause of ionospheric storms, hence not with the only one, since ionospheric storms can be regional and are not directly linked to geomagnetic activity (wilkinson, 1995). another potential way of storm modeling is first to produce a detailed climatology and morphology of ionospheric storm independently of the cause-effect mechanisms and then correlate those morphologies presenting important frequency of occurrence in time and space, with certain physical parameters so as to improve long-term predictions. it is in this latter direction that this work contributes, providing radio users and researchers with an analytical climatology mailing address: dr. dimitris n. fotiadis, national telecommunications and post commission (eett), 60 kifisias ave., 15125 marousi, greece; e-mail: dfot@eett.gr climatology of ionospheric f-region disturbances dimitris n. fotiadis (1), stamatis s. kouris (1), vincenzo romano (2) and bruno zolesi (2) (1) electrical and computer engineering department, aristotle university of thessaloniki, greece (2) istituto nazionale di geofisica e vulcanologia, roma, italy abstract after more than 60 years of research, ionospheric disturbances are today a most challenging topic of upper atmospheric physics. although the understanding of the thermosphere-ionosphere system has increased, quantitative predictions of ionospheric perturbations, valid for space weather assessment, are still imprecise. using a long fof2 dataset, an analytical climatology of the f-region storms is presented as a function of appropriate variables. local phenomena are then detected. 1312 dimitris n. fotiadis, stamatis s. kouris, vincenzo romano and bruno zolesi which is expressed with caution to appropriate variables and prominent features of ionospheric storms, also emphasizing regional phenomena. 2. definitions and method of analysis distinct from any geophysical definition of storminess, kouris et al. (1998, 1999) have developed a ionospheric definition of disturbed days and periods for the day-to-day variability of the f2-layer critical frequency (dfof2). this definition of disturbances is based solely on their power, amplitude and duration, being consistent with suggestions of other workers (gulyaeva, 1996). initially, day-to-day variability of fof2 was calculated with the transformation: dfof2=(fof2+ − fof2median)/fof2median, for each hour, day, month, year, station of table i. then, the algorithm for disturbed periods (kouris et al., 1999) was applied. thus, a catalogue of positive and negative f-region disturbances was compiled. in order to sufficiently illustrate ionospheric disturbances’ climatology in time and space and to avoid grouping storms of different mechanisms and morphological aspects, the climatology of disturbances is presented according to: i) their phase, positive or negative, since it is attributed to different disturbance mechanisms (e.g., prölss, 1995). ii) their season, since it is long supported that seasonal variations are prominent features of ionospheric storms and directly linked to their phase. in this work each month has been dealt with separately. table i. list of stations and years of fof2 data used in the analysis. the geomagnetic latitude is calculated by the international geomagnetic reference field for the year 1986 for a 300 km height. dip inclination angle and modip (rawer, 1963) are calculated for 1980. station geographic igrf corrected station code lat. (°) long. (°) geomagnetic lat. (°) dip (°) modip (°) years of data nicosia nic 35.1 33.2 28.5 50.8 44.4 87-96 el arenosillo are 37.1 353.2 30.5 52.2 45.6 82, 83, 93-96 gibilmana gib 37.6 14 30.4 52.8 46.0 76-94 tortosa tor 40.8 0.5 34.8 56.4 48.5 64-87, 91-95 rome rom 41.8 12.5 35.7 57.6 49.4 76-94, 96 grocka gro 44.8 20.5 39.4 61.1 51.7 85-93 poitiers poi 46.6 0.3 42.3 62.3 52.7 64-94 freiburg fre 48.1 7.6 43.6 63.7 53.7 64-74 lannion lan 48.8 356.6 45.4 64.3 54.1 64-94 kiev kie 50.5 30.5 45.8 66.5 55.5 64-92 slough slo 51.5 359.4 48.4 66.5 55.8 67-95 juliusruh jul 54.6 13.4 50.8 68.9 57.7 64-95 moscow mos 55.5 37.3 51 70.4 58.5 64-96 uppsala upp 59.8 17.6 56.3 72.4 60.7 64-96 leningrad len 60 30.7 55.9 72.9 60.9 64-96 arkhangelsk ark 64.4 40.5 60 75.8 63.6 69-93 lycksele lyc 64.7 18.8 61.3 75.2 63.5 64-96 sodankyla sod 67.4 26.6 63.6 76.8 65.2 64-89 kiruna kir 67.8 20.4 64.4 76.8 65.4 64-86, 91-96 loparskaya lop 68 33 63.9 77.3 65.6 64-77, 81-84, 91-96 dakar dak 14.8 341.6 2.9 16.1 16.0 64-89 johannesburg joh – 26.1 28.1 – 35.6 – 62.0 – 48.8 64, 67-91 grahamstown grw – 33.3 26.5 – 41.2 – 64.2 – 50.8 74-76, 78-84, 86-96 syowa base syo – 69 39.6 – 66.4 – 66.7 – 62.8 66, 68-79 1313 climatology of ionospheric f-region disturbances table i (continued). singapore sin 1.3 103.8 – 7.7 – 17.6 – 17.1 64-71 kodaikanal kod 10.2 77.5 1.6 3.7 3.8 64-80, 85, 86 manila man 14.6 121.1 6.2 14.2 14.1 64-94 taipei tai 25 121.5 17.5 34.9 32.6 64-96 okinawa oki 26.3 127.8 18.8 36.6 34.0 64-88 yamagawa yam 31.2 130.6 23.8 43.9 39.7 64-88 kokubunji kok 35.7 139.5 28.2 48.8 43.4 64-90 ashkhabad ash 37.9 58.3 32.6 55.8 47.6 64-86, 89-95 akita aki 39.7 140.1 32.3 53.5 46.8 64-89 wakkanai wak 45.4 141.7 38.1 59.4 51.1 64-88 karaganda kar 49.8 73.1 44.8 68.1 56.0 64-88 irkutsk irk 52.5 104 46.8 71.0 57.8 64-90 tomsk tom 56.5 84.9 51.4 74.1 60.1 64-90, 92-96 magadan mag 60 151 53.2 71.2 60.4 68-96 yakutsk yak 62 129.6 55.5 75.6 62.6 64-91 provideniya bay pro 64.4 186.6 60.3 73.9 63.0 64-70, 79-83 norilsk nri 69.4 88.1 63.8 82.0 67.5 68-88 tiksibay tik 71.6 128.9 65.2 82.0 68.6 64-71, 87 dikson dik 73.5 80.4 67.7 83.3 69.9 82-96 vanimo van – 2.7 141.3 – 11.4 – 22.5 – 21.5 64, 65, 67-93 darwin dar – 12.4 130.9 – 22.2 – 40.8 – 35.8 83-88, 90-94 townsville tow – 19.3 146.7 – 28.7 – 49.1 – 41.4 64-95 brisbane bri – 27.5 152.9 – 36.8 – 58.1 – 47.1 64-86 norfolk nor – 29 168 – 35.9 – 56.8 – 46.7 64-94 mundaring mun – 32 116.2 – 44.5 – 66.7 – 51.7 64-94 canberra can – 35.3 149 – 45.8 – 66.4 – 52.1 64-91, 93, 94 christchurch chr – 43.6 172.8 – 50.3 – 69.0 – 54.8 64-70 kerguellen ker – 49.4 70.3 – 58.4 – 68.6 – 56.0 65-84, 87, 88 campbell island cll – 52.5 169.2 – 60.2 – 76.0 – 59.5 70-85 macquarie island mac – 54.5 159 – 64.4 – 79.2 – 61.1 84-92 casey cas – 66.3 110.5 – 80.7 – 82.3 – 66.2 64-74, 90, 91 mawson maw – 67.6 62.9 – 70.5 – 70.1 – 63.2 64-87, 91-93 davis dav – 68.6 77.9 – 74.8 – 73.6 – 64.8 85-88, 90, 91, 93 scott base sco – 77.9 166.8 – 79.9 – 82.0 – 72.3 70-83 maui mau 20.8 203.5 21.3 38.5 34.8 64-94 point arguello pnt 34.6 239.4 40.5 59.8 49.0 72-74, 77-96 wallops island wal 37.8 284.5 49.5 68.0 53.2 67-87, 91-96 boulder bou 40 254.7 49.1 67.6 53.4 64-96 ottawa ott 45.4 284.1 56.9 73.6 56.9 64-93 st. johns stj 47.6 307.3 55.2 70.3 56.2 66-77 winnipeg win 49.8 265.6 60.7 76.8 59.1 66-76 goosebay goo 53.3 299.2 62.6 75.8 59.7 87-96 churchill chu 58.8 265.8 69.5 82.5 63.4 66-74 college col 64.9 212.2 65 77.1 64.2 64-67, 88, 89, 91, 94 resolute bay res 74.7 265.1 83.8 89.0 71.7 64-91 huancayo hua – 12 284.7 1.4 1.9 1.9 64-87 tahiti tah – 17.7 210.7 – 16.3 – 30.8 – 28.9 71-89 concepcion con – 36.6 287 – 22.5 – 36.4 – 35.3 64-79 port stanley por – 51.7 302.2 – 37.3 – 49.2 – 47.5 64-92 argentine is. ari – 65.2 295.7 – 49.8 – 59.2 – 57.9 64-92 halley bay hal – 75.5 333.4 – 61.5 – 65.4 – 66.3 64-80 1314 dimitris n. fotiadis, stamatis s. kouris, vincenzo romano and bruno zolesi iii) their local time (lt) of commencement (jones, 1971; prölss and von zahn, 1978; danilov and morozova, 1985; prölss, 1993). a disturbance may commence any time during the day; on the other hand, a minimum frequency of occurrence of such phenomena should be ensured, without harming the analytical picture. thus, by calculating the solar zenith angle at 300 km height, we have defined four local-time zones: day (cosχ > 0.20), night (cosχ = 0), while dawn and dusk fall in between. iv) their universal time (ut) of commencement by studying each station separately within its longitude sector and hemisphere (fullerrowell, 1994; hajkowicz, 1998). furthermore, investigating climatology in this way may reveal regional (local) phenomena (cander, 1993). v) their duration. this morphological aspect – rather than the depth – is directly linked to different disturbance mechanisms, especially on positive storm effects (buonsanto, 1999). therefore, before stepping to climatology, the basic morphological features, depth and duration, need to be investigated. 3. results and discussion 3.1. basic morphology table ii shows the percentage of positive and negative storms for which the respective variability level (i.e. the deviation from the monthly median) is not exceeded, giving thus the cumulative depth distribution for 54 ionospheric stations in different longitudes and latitudes. selecting as a boundary 49% of the total number of disturbances, positive disturbances display an important variation of depth with the geomagnetic latitude in different longitude sectors. a stable distribution is observed in mid-latitudes up to about 55°ϕm. in higher latitudes positive storms become deeper, being more intense around 62°ϕm, boundary of auroral oval. then, phenomena seem to be less deep at about 70°ϕm, however they are somewhat deeper in the polar region. furthermore, positive storms always grow deeper approaching the geomagnetic equator. on the contrary, negative storm effects are more shallow and their depth distribution may not table ii. percentage of positive (left) and negative (right) disturbances which does not exceed the respective variability level – ‘depth’ (x-axis). greek ‘σ’ denotes total percentage of positive/negative storms. top to bottom: european, asian, australian and north american stations. ϕm .4 .5 .6 .7 .8 .9 1 σ –.4 –.45 –.5 –.6 –.7 –.8 –.9 –1 σ 64.4 kir 14 31 40 45 49 52 54 57 16 26 33 41 43 63.9 lop 14 32 43 49 52 54 55 56 15 26 33 42 42 43 44 63.6 sod 10 24 35 42 46 49 50 54 13 24 33 43 46 61.3 lyc 9 23 33 40 44 47 49 56 13 23 30 41 44 60 ark 11 25 35 41 45 47 49 56 15 26 33 42 44 56.3 upp 13 30 40 45 48 49 51 54 17 27 33 43 45 46 55.9 len 13 30 41 45 48 50 51 54 17 27 33 42 45 46 51 mos 18 39 48 54 55 55 56 21 32 37 42 44 50.8 jul 18 38 48 52 53 54 21 31 37 44 46 48.4 slo 18 38 48 52 54 55 22 32 38 43 45 45.8 kie 20 38 49 52 54 54 54 55 24 35 40 44 45 45.4 lan 19 37 47 51 53 53 54 25 35 41 45 46 42.3 poi 18 39 50 54 56 57 23 34 39 42 43 35.7 rom 22 46 57 61 63 63 63 64 21 30 33 36 1315 climatology of ionospheric f-region disturbances table ii (continued). ϕm .4 .5 .6 .7 .8 .9 1 σ –.4 –.45 –.5 –.6 –.7 –.8 –.9 –1 σ 34.8 tor 19 44 58 62 66 67 20 28 31 33 30.4 gib 17 39 53 61 65 66 66 67 16 25 30 33 63.8 nri 9 23 33 41 46 48 50 54 14 24 31 43 46 55.5 yak 13 32 42 48 50 52 52 54 18 28 35 43 46 53.2 mag 14 33 45 50 52 53 53 55 19 30 36 42 44 45 51.4 tom 19 40 50 56 58 58 58 59 22 32 35 40 40 41 46.8 irk 19 41 51 55 57 57 58 22 32 37 42 32.3 aki 21 43 58 64 66 67 67 70 18 25 28 30 28.2 kok 20 45 58 64 67 68 69 71 18 25 27 28 29 23.8 yam 17 44 59 67 71 74 75 79 12 18 20 21 18.8 oki 11 31 46 56 62 65 68 72 15 22 26 28 17.5 tai 9 26 40 50 56 60 62 66 15 25 30 34 6.2 man 9 23 34 43 49 52 54 59 14 25 33 40 41 – 7.7 sin 5 20 33 41 45 49 52 55 15 25 34 43 45 – 11.4 van 7 21 31 39 44 48 50 55 14 24 33 42 45 – 22.2 dar 12 32 46 53 58 62 63 67 16 24 28 33 – 28.7 tow 17 39 52 58 61 62 63 64 21 29 33 36 – 35.9 nor 21 45 56 60 61 62 22 30 34 37 38 – 36.8 bri 21 46 57 61 62 63 63 64 22 30 33 36 – 44.5 mun 21 41 50 53 54 55 23 33 39 44 45 – 45.8 can 19 39 47 50 51 27 36 42 48 49 – 54.2 hob 14 34 46 52 55 56 57 58 19 29 34 47 42 – 58.4 ker 11 27 37 44 48 52 54 61 16 24 30 31 39 – 60.2 cll 11 25 35 41 44 46 47 50 18 29 37 46 50 – 64.4 mac 11 29 40 44 47 49 51 53 20 32 38 46 47 – 70.5 maw 12 33 46 54 59 61 62 64 15 24 30 35 36 – 74.8 dav 6 23 39 57 55 59 60 66 13 22 28 33 34 – 79.9 sco 8 23 35 43 47 51 53 57 11 21 28 39 42 43 – 80.7 cas 8 24 36 46 52 54 56 62 13 23 30 37 38 83.8 res 6 21 33 41 47 50 52 57 14 24 32 41 43 69.5 chu 15 32 42 48 51 52 52 54 19 31 38 45 46 65 col 10 25 34 41 44 47 49 59 13 22 29 39 40 41 62.6 goo 9 22 32 39 43 45 46 52 12 21 29 41 46 48 60.7 win 11 25 33 38 43 45 46 56 15 24 29 41 44 56.9 ott 15 33 41 44 46 47 19 30 39 49 52 53 55.2 stj 12 29 39 44 46 47 47 48 15 28 35 47 51 52 49.5 wal 17 35 45 48 50 51 51 52 22 32 38 46 48 49.1 bou 20 41 51 56 58 59 60 22 29 33 38 40 40.5 pnt 18 39 52 59 63 65 66 69 16 22 26 30 31 21.3 mau 9 25 38 47 53 58 60 67 15 25 30 33 1316 dimitris n. fotiadis, stamatis s. kouris, vincenzo romano and bruno zolesi 1316 fig. 1. percentage of positive (left) and negative (right) disturbances for which the corresponding duration is not exceeded (cumulative distribution). top: asian stations; bottom: australian stations. be interpreted using the above method. negative disturbances are deeper around 55°ϕm and 65°ϕm, at the edge of auroral oval boundary which moves under geomagnetically disturbed conditions. on the other hand, negative storms are more shallow (about –50% of the monthly median fof2) in low midlatitudes (22-35°ϕm) and the equatorial anomaly crest (15-22°ϕm), but they are somewhat deeper on the geomagnetic equator. figure 1 shows cumulative distributions of the duration of positive/negative disturbances in asian and australian stations. accordingly, we calculated the duration threshold for which the gradient of the distribution tends to zero, defining thus a maximum duration. we assess that the above criterion may be satisfied with a potential error of 1% between sequential distribution values. figure 2 illustrates the respective thresholds for positive and negative disturbances with the geomagnetic latitude. since much dispersion of values is observed, we have drawn two polynomial regression lines. it is evident from fig. 2 that the greatest duration of positive storms is not more than 15 h and it is observed in higher midlatitudes (5560°ϕm) and the upper boundary of the equatorial anomaly crest (20-25°ϕm). in midlatitudes great dispersion of this maximum duration is observed which may be attributed to a longitudinal dependence (fotiadis et al., 2004) and the regional character of positive disturbances (hajkowicz, 1998). from 60°ϕm polewards maximum observed duration is delimited to 12 h and even more in the polar region (10 h), as also happens on the geomagnetic equator region. on the contrary, negative storm effects may last more than 15 h in midlatitudes (55-60°ϕm). in higher and polar latitudes negative disturbances have a maximum duration of about 9-11 h while they appear to be shortest on the equator (7-8 h). 1317 climatology of ionospheric f-region disturbances synoptically, it may be supported that where great depths are observed the duration of phenomena is delimited and vice versa. definite exceptions are the equatorial anomaly crest and the higher-latitude boundary where the power of positive disturbances maximizes, since these regions constitute the ‘source’ of these phenomena. 3.2. climatology of disturbances in the previous section it is shown that phenomena last for about 16 h, however it should be stressed that a small amount of disturbances (mostly of negative phase) last for more than a day. in the literature, depending on their duration, disturbances are attributed to different mechanisms (e.g., hocke and schlegel, 1996; buonsanto, 1999). similarly, three duration classes are selected here: i) 3-5 h, ii) 6-24 h and iii) more than 24 h. the frequency of occurrence of positive and negative disturbances is presented in fig. 3a-d, and 4a-d respectively, according to the parameters mentioned in the method of analysis section. comments on the results follow categorized by the local time of storm commencement: sunrise – positive storms have mainly short duration and are observed with a frequency of 0.5-1 phenomena per month at 50-65°ϕm latitudes in europe and asia, whereas in australia over 70°ϕm and only in equinox period. however, they occur more frequently in the american zone, at summer period over 60°ϕm and also sporadically at west coast midand low-latitude stations (maui and point arguello). in winter such phenomena are limited only in short latitude strips well above 60°ϕm. positive storms of greater duration mostly affect europe than any other sector. negative disturbances of short duration are more infrequent than the positive ones, occurring mostly in near-summer months at 5065°ϕm. above 65°ϕm they are observed only in the american sector in equinox and winter months (polar region). again, medium duration negative storms affect europe in near-winter months. negative storms commencing at sunrise are totally absent from low and lower midlatitude stations. day – positive storm effects appear to have short and medium duration and are a dominant feature of the equatorial anomaly crest (all year long) and lower midlatitude ionosphere (in summer). such effects are also observed in greater midlatitudes hence at equinox, but never in winter. the above climatology is almost reversed fig. 2. maximum duration of positive and negative disturbances with geomagnetic latitude. 1318 dimitris n. fotiadis, stamatis s. kouris, vincenzo romano and bruno zolesi a si a e u ro p e f ig . 3 a, b . f re qu en cy o f oc cu rr en ce o f po si ti ve st or m s pe r m on th a cc or di ng t o l t of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om le ft to r ig ht : s m al l, m ed iu m , l ar ge d ur at io n) a s a fu nc ti on o f ge om ag ne ti c la ti tu de ( xax is ) in ( a) e ur op e an d (b ) a si a ( li gh t g re y: 0. 51 st or m s/ m on th , gr ey : 12 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). a b 1319 climatology of ionospheric f-region disturbances 1319 a u st ra lia n o rt h a m e ri ca f ig . 3 c, d . f re qu en cy o f oc cu rr en ce o f po si ti ve st or m s pe r m on th a cc or di ng t o l t of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om l ef t to r ig ht : sm al l, m ed iu m , la rg e du ra ti on ) as a f un ct io n of g eo m ag ne ti c la ti tu de ( xax is ) in ( c) a us tr al ia a nd ( d) n or th a m er ic a (l ig ht g re y: 0 .5 -1 s to rm s/ m on th , gr ey : 12 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). c d 1320 dimitris n. fotiadis, stamatis s. kouris, vincenzo romano and bruno zolesi e u ro p e a si a f ig . 4a ,b . f re qu en cy o f oc cu rr en ce o f ne ga ti ve st or m s pe r m on th a cc or di ng t o l t of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om l ef t to r ig ht : sm al l, m ed iu m , l ar ge d ur at io n) a s a fu nc ti on o f ge om ag ne ti c la ti tu de ( xax is ) in ( a) e ur op e an d (b ) a si a (l ig ht g re y: 0. 51 st or m s/ m on th , gr ey : 12 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). a b 1321 climatology of ionospheric f-region disturbances a u st ra lia n o rt h a m e ri ca f ig . 4c ,d . f re qu en cy o f oc cu rr en ce o f ne ga ti ve st or m s pe r m on th a cc or di ng t o l t of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om l ef t to r ig ht : sm al l, m ed iu m , la rg e du ra ti on ) as a f un ct io n of g eo m ag ne ti c la ti tu de ( xax is ) in ( c) a us tr al ia a nd ( d) n or th a m er ic a (l ig ht g re y: 0 .5 -1 s to rm s/ m on th , gr ey : 12 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). c d 1322 dimitris n. fotiadis, stamatis s. kouris, vincenzo romano and bruno zolesi for the australian zone, where positive storms commencing in daytime are observed in summer at much higher latitudes (up to 75°ϕm). furthermore the american zone is affected with greater frequency around 40°ϕm by such phenomena, while the asian sector is practically not at all affected above 30-40°ϕm. positive storms of greater duration are frequent around 20°ϕm and also about 60°ϕm everywhere but in asia. negative short duration disturbances are important all year long at the equatorial crest and mostly at about 60°ϕm in the american zone during equinoctial months. however they are absent at low midlatitudes, except when phenomena of medium duration in american summer are concerned. sunset – positive disturbances of short duration occur more frequently around 60°ϕm all year, but summer, and they ‘penetrate’ to midlatitudes during winter, similarly to traveling ionospheric disturbances’ climatology (hocke and schlegel, 1996). this penetration to midlatitudes is not observed in the american and australian sectors, presenting a ut effect. furthermore, positive disturbances of greater duration (more than 5 h) affect mainly europe. on the contrary, negative storms occurring mostly during winter are observed from 60°ϕm polewards, however, they affect european and australian latitudes of about 55°ϕm in summer. this is not the case in american sector. again, longer duration negative storms occur only in europe. night – equatorial and lower midlatitudes present shortand medium-duration positive storms throughout the year, while at equinoxes and winter they also occur in midand highlatitudes. the only regions which present shortperiod positive storm effects in summer midlatitudes are the sectors including the geomagnetic poles (america and australia). longer duration positive effects are restricted to winter months in america and asia, whereas europe and australia are also affected by such phenomena during equinoxes. negative storms present a similar frequency distribution in time and space with the formerly analysed positive ones. summarizing, the above results seem to confirm prominent features of ionospheric storms such as their local time and seasonal variation (prölss, 1995; mikhailov, 2000). however, here many regional and local phenomena are pointed out and users are provided with analytical maps of storm frequency. 4. conclusions the analysis of basic morphological storm features shows that positive storms present maximum power, i.e. depth and duration, in the equatorial anomaly crest and the auroral oval boundary, presenting durations of the order of 15 and 14 h, respectively. on the contrary, negative storms last longer 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(received january 23, 2004; accepted june 17, 2004) 285_294 de zafra.pdf 285 annals of geophysics, vol. 46, n. 2, april 2003 on the cryogenic removal of noy from the antarctic polar stratosphere robert l. de zafra (1), giovanni muscari (1), and sergei smyshlyaev (2) (1) institute for terrestrial and planetary atmospheres, state university of new york, stony brook, ny, u.s.a. (2) russian state hydrometeorological institute, st. petersburg, russia abstract we review current knowledge about the annual cycle of transport of nitrogen oxides to, and removal from, the polar stratosphere, with particular attention to antarctica where the annual winter denitrifi cation process is both regular in occurrence and severe in effect. evidence for a large downward fl ux of noy from the mesosphere to the stratosphere, fi rst seen briefl y in the limb infrared monitor of the stratosphere (lims) data from the arctic winter of 1978-1979, has been found during the 1990s in both satellite and ground-based observations, though this still seems to be omitted from many atmospheric models. when incorporated in the stony brook st. petersburg two dimensional (2d) transport and chemistry model, more realistic treatment of the noy fl ux, along with sulfate transport from the mesosphere, sulfate aerosol formation where temperature is favorable, and the inclusion of a simple ion-cluster reaction, leads to good agreement with observed hno3 formation in the mid-winter middle to upper stratosphere. to further emphasize the importance of large fl uxes of thermospheric and mesospheric noy into the polar stratosphere, we have used observations, supplemented with model calculations, to defi ne new altitude dependent correlation curves between n2o and noy. these are more suitable than those previously used in the literature to represent conditions within the antarctic vortex region prior to and during denitrifi cation by polar stratospheric cloud (psc) particles. our noy -n2o curves lead to a 40% increase in the average amount of noy removed during the antarctic winter with respect to estimates calculated using noy -n2o curves from the atmospheric trace molecule spectroscopy (atmos)/atlas-3 data set. mailing address: prof. robert l. de zafra, department of physics and astronomy, state university of new york, stony brook, ny 11794, u.s.a.; e-mail: rdezafra@notes. cc.sunysb.edu key words polar stratosphere reactive nitrogen heterogeneous chemistry tracer-tracer correlations and the formation of hno3 in the mid-to-upper stratosphere, was fi rst implied by observations of anomalously large amounts of no2 and hno3 by the limb infrared monitor of the stratosphere (lims) during the arctic winter of 1978-1979 (russell et al., 1984; also see wmo, 1985, vol. ii, for a summary of lims measurements). there was no further direct confi rmation of these observations, however, until the launching of upper atmospheric research satellite (uars) late in 1991. although transport from the winter mesosphere into the stratosphere is now well established over both polar regions, denitrifi cation of the lower stratosphere is quite variable from year to year in the arctic under current climatic conditions, and we concentrate here on the more regular and severe denitrifi cation process of the antarctic. in this paper we review some recent work helping to establish the large fl ux of 1. introduction the production and transport of oxides of nitrogen in the atmosphere and their subsequent cryogenic removal by polar stratospheric clouds (pscs) (denitrification) over polar regions constitutes one of the most fascinating atmospheric cycles traced out in recent years. the infl ux of large quantities of nitrogen oxides from the winter polar mesosphere into the stratosphere, 286 robert l. de zafra, giovanni muscari and sergei smyshlyaev no2 into the antarctic winter stratosphere, the chemical routes by which a signifi cant fraction is transformed into hno3 above 30 km altitude, the condensation of hno3 into pscs at lower altitudes, and their removal from the stratosphere by gravitational fallout. measurements made with the stony brook ground-based millimeter-wave spectrometer (gbms) during the decade of the 1990s, along with measurements by the cryogenic limb array etalon spectrometer (claes) and the microwave limb sounder (mls) onboard the uars, have revealed much more detail about the formation of hno3 in fall and winter over a large vertical range of the southern polar stratosphere, its descent as part of winter vortex dynamics, and its freeze-out in psc particles (kawa et al., 1995; de zafra et al., 1997; santee et al., 1998, 1999; mcdonald et al., 2000). there has been considerable effort to explain how hno3 can form in the mid-to-upper stratosphere in winter (e.g., kawa et al., 1995, and references therein). chemical conversion from no3 + no2 æ n2o5, followed by n2o5 + + h2o æ 2 × hno3 has been generally assumed, but the latter reaction is far too slow in the gasphase, and some sort of heterogeneous (i.e., surface or condensed-state) chemical reaction has been sought. the altitude range where hno3 is formed (30-50 km) in polar winter is much too warm for the formation of polar stratospheric clouds, and sulfate aerosols (which could take the place of pscs) have often been considered non-existent above ~ 30 km (see garcia and solomon, 1994, for an early refutation of this prejudice, and mills et al., 1999, for a more thorough analysis). in section 3 we will discuss recent work (de zafra and smyshlyaev, 2001) in which earlier suggestions for the formation of hno3 have been revisited, with new results which, despite initial doubts, seem to confi rm the viability of these formation mechanisms. regardless of the partitioning of noy in the stratosphere (where noy ª hno3 + no + no2 + 2 × n2o5 + clono2), descent of noy from the mesosphere during polar winters is of great importance for the nitrogen budget in polar stratospheres. since the late 1980’s, correlations between the two long-lived species n2o and noy have been used to estimate denitrifi cation in polar stratospheres, which is of great importance in determining the severity and duration of springtime ozone loss. more recent studies show that seasonally dependent mixing across the vortex boundary can alter such correlations dramatically, so that mid-latitude noy-n2o correlation curves are not representative of the correlation present at high southern latitudes prior to denitrifi cation. transport of large concentrations of noy from the upper atmosphere to the lower stratosphere inside the vortex adds to this discrepancy, increasing the error when computing antarctic winter denitrifi cation using noy-n2o curves obtained outside the southern vortex as a reference. our current work on the noy-n2o correlation is an attempt to remedy this situation through seasonal estimates of noy versus n2o at the southernmost latitudes, and will be discussed in section 4. 2. transport of noy into the polar stratosphere observations with the lims instrument in the arctic winter of 1978-1979 indicated no2 mixing ratios on the order of 100 ppbv in the upper arctic stratosphere and lower mesosphere, as well as anomalously large amounts of hno3, as referenced in section 1. despite these fi ndings, and their confi rmation in the early 1990s by additional ground-based measurements and measurements from instruments onboard uars, many atmospheric chemical and transport models in use through the 1990s have failed to incorporate these fi ndings of a large winter noy fl ux over polar regions: see for instance fi gs. 3.5.1a through 3.5.1c of park et al. (1999), where 11 out of 13 models tested show noy deficiencies by one or two orders of magnitude above ~ 50 km relative to lims observations. noy enters the polar mesosphere both by transport from lower latitudes and by in situ production in the mesosphere and thermosphere in the auroral regions near the magnetic poles (e.g., siskind et al., 1997 and references therein; randall et al., 1998, 2001). callis (2001) has also recently emphasized the inadequacy of most atmospheric models in either realistically parameterizing or dealing directly with creation and downward transport of noy in polar regions. 287 on the cryogenic removal of noy from the antarctic polar stratosphere it is not our intention to deal further with the question of production or transport in the upper polar atmosphere in this paper, but merely to point out that efforts intended to model chemical processes in the polar stratosphere are certain to lead to inaccuracies unless and until realistic parameterizations of downward stratospheric noy fl uxes through the fall and winter seasons are used. 3. the production of hno3 in the winter mid-stratosphere the most complete record of the behavior hno3 in the antarctic stratosphere as a function of both time and altitude comes from the gbms observations taken at the south pole during 1993, 1995, and 1999 (de zafra et al., 1997; mcdonald et al., 2000; de zafra and smyshlyaev, 2001; muscari et al., 2002). figure 1 shows a composite average of these observations: the evolution of gas-phase hno3 follows very nearly the same pattern for each individual year, with minor variations driven mainly by lateral transport. two features are very evident from fi g. 1. first, hno3 increases signifi cantly after sunset in the lower stratosphere, due to the cessation of direct photolysis of hno3 and to the conversion of other nitrogen oxides to nitric acid. second, shortly after day 160 (early to mid-june), signifi cant new hno3 begins to form as high as ~ 50 km, and is carried downward in the general rapid descent of air towards the lower stratosphere, fig. 1. three-year average of hno3 measurements over the south pole (1993, 1995, and 1999). triangles along the upper boundary indicate days when measurements were taken over the three years. the lightest gray contour fi lling indicates mixing ratios between 1 and 3.5 ppbv. contours increase in increments of 2.5 ppbv to a maximum of 21-23.5 ppbv. heavy dashed lines mark sunset and sunrise as a function of altitude. 288 robert l. de zafra, giovanni muscari and sergei smyshlyaev while increasing its mixing ratio along the way. the fi rst of these processes poses no problems in stratospheric chemistry, but the second has been a persistent puzzle since the early lims measurements first suggested anomalously large amounts of hno3 high in the polar winter stratosphere. the most plausible route for formation of hno3 in the upper nighttime stratosphere is by the gas-phase transformation no2 + no3 æ n2o5, followed by reaction with water: n2o5 + h2o æ 2(hno3). it is known from laboratory rate studies that the latter reaction is far to slow in the gas-phase to account for the rapid formation of observed amounts of hno3, and a heterogeneous (surface) reaction seems to be required. the antarctic stratospheric temperature is surprisingly warm at this time and altitude, however, ranging from ~ 225 to 250 k, which is much too warm for the condensation of pscs. the problem, therefore, has been to fi nd a plausible chemical mechanism for rapidly making hno3 under conditions found in the polar winter stratosphere at 35-45 km. in some recent work (de zafra and smyshlyaev, 2001) we have considered earlier suggestions (garcia and solomon, 1994; mills et al., 1999) that extend sulfate aerosol production above the previously accepted cut-off of ~ 30 km over the poles. in common with these earlier studies, microphysical modeling of sulfate aerosol condensation in the stony brook st. petersburg 3d chemical and transport model did not extend the condensation of sulfate aerosols signifi cantly above 36-37 km for the temperature fi eld of the 1993 antarctic stratosphere, even when coupled with enhanced sulfate transport from the mesosphere, as in mills et al. (1999). this mechanism therefore cannot provide heterogeneous conversion of n2o5 to nitric acid in the critical region above ~ 37 km. we also reconsidered earlier investigations involving ion-cluster reactions in place of standard heterogeneous chemistry, to carry the burden of n2o5 conversion above ~ 35-40 km. kawa et al. (1995) had considered the ion-cluster reaction n2o5 + h + (h2o)n æ h + (h2o)n–1 + 2(hno3) fi rst considered by böhringer et al. (1983) in relation to a different problem in stratospheric noy chemistry. similar to standard heterogeneous reactions, ion-cluster reactions are orders of magnitude faster than gas-phase reactions (böhringer et al., 1983). additional, more complex reactions such as h+ (h2o)n + no3 – (hno3)m æ (m+1)hno3 + n(h2o) were considered by aikin (1997), and any of these could serve to create hno3 at altitudes above ~ 35 km, although not all result in the non-clustered gas phase hno3 which is detected by the gbms or claes instruments. in the model studies cited above, it appeared that a large ion-cluster density, such as those associated with unusually large solar flares, would be necessary to rapidly generate enough hno3 in the relatively short time-span required by observations. we believe these studies used models which did not incorporate a suffi ciently large fl ux of noy (mostly no2) coming from the polar winter mesosphere, however, as discussed in section 2. when these combined mechanisms (sulfate aerosol formation, occurring when and where temperatures are cold enough below ~ 37 km, supplemented by ion-cluster reactions extending to ~ 45-50 km), are used along with an increase in the downward noy fl ux to realistic values, we have obtained calculated production of hno3 in quite good qualitative agreement with our gbms observations from the south pole (de zafra and smyshlyaev, 2001). we have gotten this agreement by assuming ion-cluster production only from the relatively constant galactic cosmic ray fl ux, as calculated by beig et al. (1993), without adding input from solar events, or the supplementary ion reactions considered by aikin. we also note that the global calculations of beig et al. (1993) could only be verifi ed against midlatitude measurements, and may be uncertain by a factor of ~ 2 in polar regions (g. brasseur, private communication). given the relatively conservative assumptions we have made about total ion-cluster production, the use of only a single ion-cluster reaction, and the supplemental production by sulfate aerosols at altitudes above ~ 30 km where they can form, we believe the 289 on the cryogenic removal of noy from the antarctic polar stratosphere tained for the spring and summer of 1993 and throughout the year 1995, when the largest overlap of data sets is available from both mls and gbms instruments. the estimation of noy involves the use of several sources of data: hno3 measurements by the gbms (in good agreement with hno3 version 5 retrievals of the microwave limb sounder onboard uars, as shown by muscari et al., 2002); no and no2 by the halogen occultation experiment (haloe) aboard uars (russell et al., 1993), and no2 by the polar ozone and aerosol measurements ii (poam ii) instrument aboard spot-3 (glaccum et al., 1996). calculations of the minor noy constituents n2o5, clono2, and ho2no2, which are not readily available from measurements over the periods in question, are made from a photochemical box model. since gbms and satellite measurements are not co-located, we have used only satellite data that can be connected to gbms measurements by air parcel trajectory tracing, and the box model takes into account any change in species concentrations that may occur along the trajectories. local solar exposure and meteorological conditions along trajectories are taken from ukmo gridded reanalysis data for specifi c dates involved. the noy values obtained, as well as the measured n2o profi les, were binned in 6 pseudoseasons defi ned as: summer (su), fall (f), early winter (ew), late winter (lw), early spring (es), and late spring (ls). results for the year 1995 are shown in fi g. 2a-c, where numbers next to data points indicate various potential temperature levels. in summer and fall (fi g. 2a), before and during the formation of the polar vortex, with temperatures still above the hno3 condensation threshold, the correlation profi le appears similar in shape to those observed in previous studies (e.g., fahey et al., 1989): that is, the correlation shows linearity for n2o values larger than 100 ppbv, with noy reaching its maximum for n2o around 100 ppbv and then decreasing rapidly for smaller n2o values. data points from both seasons were fi tted with a 4th order polynomial, shown as a black solid curve in fi g. 2a. this fi t suggests that the noy mixing ratio at the fall theta levels 585 k (n. 3) and 620 k (n. 4) may be biased high by ~ 10% with respect to the polynomial curve. question of how hno3 is generated throughout the polar winter stratosphere is probably no longer a mystery. no2 can be converted to hno3 throughout most of its journey from the mesosphere through the polar stratosphere in winter. because this process is undoubtedly variable, depending on changes in ion-cluster and sulfate aerosol density as well as on temperature, we believe that care must be exercised when interpreting amounts of no2 that reach the mid-to-lower stratosphere in one year relative to another (e.g., randall et al., 1998 and 2001). these varying amounts may not accurately indicate variable rates of downward transport, or alternatively, variable production rates at high altitudes, until one factors in the role played by conversion to hno3 and the variation of that process over time, altitude, and seasons. 4. annual variations of noy within the antarctic vortex region although descent of noy from the upper atmosphere takes place during fall and winter, its effects on the polar stratosphere are not limited to those seasons. to study such effects, we have recently estimated the variations of the correlation curve between n2o and noy, as a function of time and altitude in the antarctic stratosphere (muscari, 2001). until now, many studies have used measurements of the noy-n2o correlation curve taken outside the antarctic vortex to represent predenitrifi cation conditions (i.e. during fall) inside the vortex region. from this standard or reference relationship, the severity of denitrifi cation within the winter or spring vortex has been inferred. if the vortex region hosts unusually large downward noy fl uxes, however, or is characterized by a small but frequent mixing with regions at the edge of the vortex, the use of correlations from outside the vortex as reference for vortex conditions will lead to false conclusions (see, e.g., plumb et al., 2000); in this case an under-estimation of the amount of noy removed annually from the polar stratosphere. our determinations of noy mixing ratio profi les cover the potential temperature range 465-960 k (~ 18-30 km), and have been ob290 robert l. de zafra, giovanni muscari and sergei smyshlyaev between may 16 (last day of data averaged in the austral fall period) and june 28 (fi rst day included in the early winter) temperatures in the vortex drop below 195 k at most altitudes considered here, and hno3 gas-phase depletion takes place through condensation on pscs. figure 2b shows a correlation clearly affected by noy depletion, and n2o values that have decreased at all levels from fall to early winter, due to descent of n2o-defi cient air from the mid and upper stratosphere. (notice the different x-axis scale between fi g. 2b and fi g. 2a,c). both early winter and late winter data points depict the same relationship between noy and n2o, one sign of a vortex well isolated from the non-denitrifi ed regions outside. the consistency between the noyn2o correlations obtained in early and late winter, together with that observed between summer and fall for n2o mixing ratios larger than 100 ppbv, implies that these estimates have a greater precision than suggested by the formal error estimates indicated by error bars in the fi gure. downward transport inside the vortex is illustrated by the displacement along the winter fig. 2a-c. correlation plots for noy versus no2 at various seasons within the region of the antarctic vortex. a) su = summer, f = fall; b) ew and lw = early and late winter; c) es and ls = early and late spring. the black curves are 4th order polynomial fi ts to all data points in (a) and (b). numbers from 1 to 7 designate increasing altitudes, at potential temperatures of 465, 520, 585, 620, 655, 740, and 960 k, respectively. a b c 291 on the cryogenic removal of noy from the antarctic polar stratosphere correlation that characterizes each theta level. the lw polar stratosphere is characterized by small concentrations of noy at lower theta levels, due to denitrifi cation, and large concentrations of noy at the upper theta levels due to descent of air rich in noy but poor in n2o. a 4th order polynomial fi t to these winter curves is also displayed in fi g. 2b (solid black curve). averaging meteorological data for fall and winter 1995, we calculated approximate noy column densities corresponding to the summer/ fall and winter polynomial fi ts, over the range 10-300 ppbv of n2o (~ 14-33 km for april 1995 over the south pole). we obtained ~ 2.5 ¥ 1026 molecules of noy / km 2 (or 5.8 kg/km2 of nitrogen) for the 1995 summer/fall polynomial fi t, and 1.42 ¥ 1025 molecules of noy / km 2 (or 0.33 kg/ km2 of nitrogen) for the 1995 winter polynomial fi t. if we approximate the mean size of the polar vortex with the size of the antarctic continent, the total loss of nitrogen in the antarctic stratosphere is about 7.6 ¥ 107 kg. this estimate of the winter nitrogen loss is however a lower limit, because downward transport brings noy-enriched air from above 960 k, while noy depletion takes place in the lower stratosphere. thus more nitrogen is removed from the lower stratosphere by the end of the winter/spring period than that which is initially present in the fall. figure 2c shows results for early and late spring, when the antarctic polar vortex starts to weaken, and mixing across its edge leads to renitrification of the lower stratosphere. from lw to es, the lower theta levels show an increase in both noy and n2o. noy increases because outside the vortex the lower stratosphere experienced only a minor nitrogen loss (if any at all), while n2o increases because the air outside the vortex did not experience the strong descent that took place inside the vortex, and n2o concentrations are therefore larger. with the end of winter the downward motion inside the vortex decreases gradually and eventually comes to an end (crewell et al., 1995), cutting off the input of noy from the mesosphere. level 960 k (black circle n. 7) shows the effect of such a cut off, exhibiting a lower noy mixing ratio with respect to winter values, while level 740 k (black circle n. 6) clearly shows the beginning of the effects of denitrifi cation over the noy-n2o correlation curve. the comparison of lw with es results underlines how the nitrogen lost by the stratosphere is not limited to the nitrogen present in the stratosphere at the beginning of the winter, but extends to the reactive nitro gen brought down by winter descent of mesospheric air. the late spring period shows a partial return of the antarctic stratosphere to pre-winter conditions, with an increase of both noy and n2o at most levels. such a recovery is particularly advanced at 740 and 960 k where, in late november, potential vorticity maps show that the vortex has ceased to exist. comparing our results for the austral summer and fall of 1995 with atmospheric trace molecule spectroscopy (atmos)/atlas-3 results obtained during november 1994 at high latitudes outside the southern polar vortex (see fi g. 3), we fi nd that the noy mixing ratio is 18% larger within the vortex at the noy peak. the result is in very good agreement with model estimates of the increase in noy mixing ratio in the antarctic lower stratosphere due to upper atmospheric sources of noy (callis et al., 2001). if the atmos/atlas-3 curve shown in fi g. 3 were used as a reference for vortex conditions prior to the reactive nitrogen loss during winter 1995, instead of our 1995 summer/fall polynomial fi t, the total nitrogen present in the antarctic lower stratosphere before denitrifi cation (and therefore denitrifi cation itself) would be underestimated by about 20%, or a total of 1.63 ¥ 107 kg, if we again approximate the size of the polar vortex by the size of the antarctic continent. a difference between our noy-n2o estimates and the atmos/atlas-3 data from november 1994 is also found when comparing the degree of denitrifi cation reached at the lower levels of the noy-n2o curve (see fi g. 3). the consistency in our results between winter 1995 and early spring 1993 (not shown) suggests that the degree of denitrifi cation in the antarctic lower stratosphere does not have a large variability. (this is also suggested by the large, longlasting volume of the lower stratosphere where temperatures fall well below the psc-formation value of ~ 195 k every winter). early and late winter of 1995 and early spring of 1993 show smaller values of noy with respect to atmos/ 292 robert l. de zafra, giovanni muscari and sergei smyshlyaev atlas-3 data for 1994 from inside the vortex. although we cannot exclude interannual differences between 1993, 1994, and 1995, this discrepancy is more probably due to some mixing of air from the vortex edge with the denitrifi ed air sampled by atmos in november, 1994, when vortex breakdown was well underway. since this discrepancy is in the lower stratosphere, it does have a large impact on the underestimation of winter nitrogen loss discussed above. the effect is to add another 1.60 ¥ 107 kg of nitrogen to the 1.63 ¥ 107 kg computed earlier, making a total underestimate of ~ 40% from use of the atmos/ atlas-3 extra-vortex reference curve. the noy-n2o correlation curves obtained in this recent analysis contribute to improving our knowledge about the transport of noy to the antarctic lower stratosphere, and about winter loss of nitrogen through hno3 condensation on psc particles. both phenomena are closely related to ozone loss and to climate-driving parameters such as temperature and aerosols concentration. 5. summary following a considerable amount of research in polar regions by a large international effort, under the impetus of understanding seasonal polar ozone loss, we now know a great deal more about the annual cycle of oxides of nitrogen in the polar stratosphere. in this paper, we have concentrated on the cycle in antarctica, since it currently results in considerably greater, as well as more regular, denitrification of the lower fig. 3. comparison of noy-n2o correlations determined here, and those from the atmos/atlas-3 campaign of november 1994, obtained both outside (atmos-out) and inside (atmos-in) the antarctic vortex. left panel: the solid curve is the same as in fi g. 2a, while dashed curve is atmos/atlas-3 data measured outside the antarctic vortex boundary. middle panel: solid curve is the same as in fi g. 2b, while the dashed curve is atmos/atlas-3 data observed within the vortex. right panel: the solid curve is the average of summer 1993 and late spring 1995 correlations, while the dashed curve represents atmos-out. 293 on the cryogenic removal of noy from the antarctic polar stratosphere stratosphere during winter. this occurs through the widespread and prolonged formation of polar stratospheric clouds, most likely a tripartite composite of hno3, h2o and h2so4 (e.g., santee et al., 1998, and references therein), and their subsequent gravitational removal from the stratosphere. poleward transport of no2 in the mesosphere and lower thermosphere is supplemented through formation by charged particles penetrating near the magnetic poles. downward transport into the polar stratosphere creates a considerable flux of noy (mainly no2) during the winter. this no2 ‘feedstock’, combined with effi cient conversion to hno3 in the upper and middle stratosphere by ion-cluster reactions and heterogeneous reactions on sulfate aerosols, continually supplements the hno3 available to enter into pscs in the lower stratosphere, where it is delivered by the steady subsidence of stratospheric air throughout the fall and winter. despite the new knowledge gained during the past 10-20 years, it appears that many, if not most, stratospheric chemical models still fail to properly account for the downward fl ux of noy in the polar winter stratosphere, often failing by one or more orders of magnitude (e.g., see park et al., 1999). the larger winter fl ux of noy into the antarctic vortex region, and perhaps other transport anomalies, moreover leads to a different ‘pre-denitrifi cation’ relationship be tween n2o and noy than the ones found experimentally at lower latitudes that have been used to represent this relationship within the vortex before denitrifi cation occurs. this can lead to underestimates of the true removal of noy by as much as 40%. acknowledgements observational data on hno3 over the south pole was collected through the steadfast resourcefulness and dedication of curt trimble during 1993 and 1999, and through that of richard chamberlin in 1995. michael mcdonald contributed to analysis of 1995 hno3 observations. our work has been supported by nasa through grant nag 54071, and through the national science foundation by grant opp-9705667. references aikin, a. 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geophys. res., 104, 8225-8246. siskind, d.e., j.t. bacmeister, m.e. summers and j.m. russell iii (1997): two-dimensional model calculations of nitric oxide transport in the middle atmosphere and comparison with halogen occultation experiment data, j. geophys. res., 102, 3527-3545. wmo (1985): atmospheric ozone – 1985, report n. 16 (world meteorological organization, geneva, switzerland). annals of geophysics, 55 (3), 2012: solarino impact factor, citation index, h-index: are researchers still free to choose where and how to publish their results? stefano solarino istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, sede di genova, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5518 abstract over the last decade, the demand to evaluate the impact of any given research study, the credentials of a researcher, and the influence that any single research unit or agency has on the world of research has constantly grown. many tools have been developed and applied to evaluate the level of innovation, originality and continuity of a single researcher in an objective way. as a consequence, there are comparisons of the performances of different research agencies. some of these tools, which often provide the result as an ‘index’, are briefly described in this study. however, it is clearly evident that the evaluations provided by these instruments do not always correspond to the real impact of the research, nor are they unique. indeed, the same index computed using similar criteria on different databases gives different scores, which can lead to confusion and contradictions. in this contribution, the principal anomalies, problems and failures of these evaluation schemes are described. the most evident of these arise from the nature of the evaluation, which being automated, cannot establish the role of any single researcher in papers of ‘pooled’ research, and cannot recognize similar or duplicated papers by the same researcher(s) in more than one journal. the ‘selecting’ effects that these evaluation indices can have on the research are then discussed. indeed, in an attempt to obtain the highest possible scores in terms of citations, there is a tendency of the single scholar to avoid studies that deal with small areas, or with scientific problems that do not have a broad interest or provide applicative results. in all of these cases, an article describing such studies will in all likelihood appear in a ‘minor journal’ (one with a low impact factor). as a consequence, this will provide a low citation index, will not significantly contribute to the authors’ h-index, and/or will only be published as a report. moreover, a discussion on the role that these evaluation indices can have in the world of research is presented. particular attention is paid to the consequences in the field of the geoethics, where scientific, technological, methodological and socio-cultural aspects need to be considered in a different order to that expected in a pure meritocracy. 1. introduction over the last decade, it has become necessary to objectively evaluate the impact of any research study, and of the ‘credentials’ of a researcher or the influence that any single research unit or agency has on the world of research. to be effective, this evaluation must be rapid, easy to update, based on sound elements, widely available, easy to use, and of course, as objective as possible. once a methodology to evaluate a research study is designed, the applications of such an assay can be very large. indeed, they span the mere impact of the single academic, to the allocation of funds, career advancement, job/position assignment, and awards, even if the very different natures of these applications require additional factors to be taken into account. for example, the amount and quality of the data from which a scientific result is obtained often depend on the availability of instruments, computers and other facilities, rather than simply on the skills and experience of the research unit or the single researcher. given this situation, i discuss here the problems that can arise and are often not taken into account when using these tools, and especially the consequences that the using these indices can have on the research. as known, most of these evaluations are compacted into a number or an index. i will focus on the research field that i know more about, which is geophysics, but i am almost convinced that similar problems affect most research fields, if not all. the debate that i will summarize applies with different relevance across various countries. to cite just a few of the ‘filters’, english-speaking countries have a natural tendency to write papers in the english language (which is one of the requirements for many of the evaluation indices). however, some countries host the most important press companies in any particular science, which are therefore a favourite target for the scientists living in that country. some other countries, and especially those in the far east, have historic roots and origins in science, due to which they prefer to use their own language, and indeed, were not allowed to use other languages in the past, or have advised researchers against publishing in foreign journals. article history received december 20, 2011; accepted march 9, 2012. subject classification: impact factor, bibliometrics, citations, evaluation, ethics. 473 italy is slowly approaching the need for an objective way to evaluate its research and scholars. there are a few elements that render this approach more clumsy: the cultural importance of the italian language; the existence of many locally distributed journals, some of which are very slowly becoming international by opening the editorial board to foreign scientists and by hosting articles in english; and the pioneering nature of geophysics in italy, which was the reason for establishing the xx century relationships, which included bilingual journals, with advanced countries like germany, more than with english-speaking countries. just to mention an example, one of the most important journals in geophysics, the rivista di geofisica pura ed applicata [bossolasco 1939], turned to the current structure and name of pure and applied geophysics in the late 1960’s, although it was still publishing papers in german, italian and english at that time. the first part of this report sketches out the main evaluation tools that are used to score scholars, and describes some of their incongruities. the aim of the discussion propounded here is not to criticize any single index or to label it as good or bad. i do not have enough skill and experience to even dare to attempt this. furthermore, i am firmly convinced that even with the failures that i will briefly describe here, the indices in current use are sensitive and objective enough to grossly describe the credentials of scientists in earth sciences. however, a discussion of the roles that these evaluation indices have in the world of research is well supported. this is especially relevant in the light of the geoethics, where scientific, technological, methodological and socio-cultural aspects have to be considered in an order that is different to what is expected in a pure meritocracy. this aspect will be the argument of the second part of this article, together with a few suggestions on how to render the use of evaluation indices if not more effective, at least more adherent to the real importance of the research. 2. citation metrics over the last few years, many tools to quantify the scientific importance of any research have been proposed, developed and adopted. although some of these are relatively new (e.g. the h-index dates back to 2005 only), they have been rapidly included in the computation schemes and are currently provided by several indexing agencies. these evaluations score the publications or compute the score of the single scholar on the basis of his/her publications. a detailed description of the evaluation criteria or the single number/ index goes far beyond the scope of this article; however, a short summary of the main characteristics might help the reader to get to the point, and so these are described in the next lines. in particular, the discussion here is based on the impact factor (if), the citation index, and the h-index. generally speaking, these numbers are often grouped under the name of ‘citation metrics’, or bibliometrics. the if is a measure that reflects the average number of citations to articles published in science and social science journals. in a given year, the if of a journal is the average number of citations received per paper published in that journal during the two preceding years. for example, if a journal has an impact factor of 5 in 2010, then the papers it published in 2008 and 2009 received 5 citations each on average in 2010. the impact factor of a journal is calculated as follows: a = the number of times articles published in 2008 and 2009 were cited by indexed journals during 2010. b = the total number of ‘citable items’ published by that journal in 2008 and 2009. these citable items are usually articles, reviews, proceedings, or notes, but do not include editorials or letters to the editor. the if for the year 2010 is then a/b, and it is computed only in 2011 because it cannot be calculated until all of the 2010 publications have been processed by the indexing agency. this if was devised by eugene garfield [1998], and it is provided to subscribers by the agency founded by garfield himself, the institute for scientific information (isi; http://apps.webofknowledge.com/), which is now part of thomson reuters. although the if by the isi accounts for more than 11,000 science and social sciences publications, these represent only a part of all of the present-day journals. there are alternative ways to rank the influence of any journals and to compute the if. these include: – the pagerank [brin and page 1998] algorithm of the google search engine; – the eingenfactor (http://www.eigenfactor.org/), in which journals are rated according to the number of incoming citations, with citations from highly ranked journals weighted to provide a greater contribution to the eigenfactor than those from poorly ranked journals; – the scimago journal rank (http:// www.scimago jr.com/), an open-access journal metric that is based on scopus data and that uses an algorithm similar to pagerank. the if is a quick tool to evaluate the breadth of distribution of a journal, but it is not problem free. its value is not merely dependent on the citations (a, in the formula). indeed, if a journal publishes only a few, longer articles, or does not publish any item in a given year, this will increase the if the following year (b will be small); conversely if the number of articles per year is high, the if will be small if the number of citations is not also significantly higher. moreover, when journals are included in the isi list, it takes three years before they are assigned an if, which means that issues published before or during the first two stefano solarino 474 475 years of computation are not assigned an if. finally, the if can change abruptly from year to year, but the authors know the if relative to their publication only at the end of the successive year, making it very hard to forecast the popularity of a journal, and in turn, the benefit of their choice to publish in that journal. on the other hand, it is very clear that the if is a qualitative more than a quantitative tool. in practice, any article published in journals for which an if is computed concur to the credential of a scholar as a single unit, no matter how high the rank of the journal is. a scientist is attributed the number of published articles, and not a sum weighted on the if of the journal where they appear. an exception is with the very high if journals, which are sometimes mentioned separately in a scientist’s resume, because, being top publications, they carry concrete credibility to the authors. nevertheless, there is indeed a sort of if ‘filter’ on the research. indeed, in principle, the higher the if of a journal is, the better known, distributed and read it is. for this reason, the top-ranked journals will not be likely to host studies related to minor topics or small geographical areas, and the editors will strive to publish arguments regarding national to planetary matters, and novel arguments. i will recall this point in the discussion on the roles of these evaluation tools. a second, common way to evaluate research is the citation index, which sums up the number of citations of all articles for a given author. there are several indexing agencies and bibliographic databases that formulate this evaluation. the oldest of these citation indices, the science citation index, was established by garfield in 1960 (http:// thomsonreuters.com/products_services/science/free/ essays/history_of_citation_indexing/), and it is currently accessible to subscribers under the isi web of science webpage or on a cd. then there is the bibliographic database scopus (http://www.scopus.com/ home.url), which is owned and run by elsevier and offers a similar service to subscribers. finally, the google scholar database (http:// scholar.google.com/) can be queried via a program (publish or perish [pop]) [harzing 2010] that is becoming very common among scientists, because it is very easy to use (it self connects to the web and searches for the most updated database), and it is free of charge and open to any user. unfortunately, these three bibliographic databases are very different, and the choice of a reference dataset turns into divergent results with respect to using one over the others. table 1 shows a comparison of a query to evaluate my own research activities using these three databases. the results are very different, with variations of the order of 50% in these citation indices and the h-index, the description of which is below. the most evident problems with the citation indices are: – they do not take into account the number of authors on each paper, so that if an article is written by more scientists, each will get an identical score, no matter how big their contribution; – they do not take into account the if of the journal, meaning that an article published in a very low ranked journal will be rated as being as important as a very highly ranked paper; – they are not ‘time weighted’ or ‘dependent’. the citation indices give the sum of the citations over the career period (or at least over the last 20 years) without any differences between long or short activity periods. very prolific young scholars will have similar performances as older, less productive scientists, and vice versa. finally, the h-index [hirsch 2005] is based on the set of a scientist’s most cited papers and the number of citations that they have received in other publications. the definition of the h index is given as: “a scientist has an index h if h of his/her np papers have at least h citations each, and the other np-h papers have no more than h citations each”. in other words, a scholar with an index of h has published h papers, each of which has been cited in other papers at least h times. thus, the h-index reflects both the number of publications and the number of citations per publication. the h-index is designed to improve upon simpler measures, such as the total number of citations or publications. the h-index works correctly only for comparing scientists working in the same field, as citation conventions differ widely among different fields. the h-index can be manually determined using citation databases or can be determined using automatic tools. subscription-based databases such as scopus and the web of knowledge provide automated calculators. harzing’s pop program (http://www.harzing.com/ pop.htm) calculates the h-index based on the google scholar entries. each database will produce a different h for the same scholar because of the different cover of the scholar’s publications (see table 1): google scholar has more citations than scopus and the web of science, although the smaller citation collections tend to be more accurate the h-index should cover up the main disadvantages of these other bibliometric indicators, such as the total number of papers or the total number of citations. the total number of papers does not account for the quality of the scientific publications, while the total number of citations can be disproportionately affected by participation in a single publication of major influence (for instance, methodological papers proposing successful new techniques, methods or approximations, which can generate a large number of citations), or having many publications with a few citations each. the h-index is intended to simultanebibliometrics and freedom to publish ously measure the quality and quantity of scientific output. there are a number of situations in which the h-index can provide misleading information about a scientist’s output. although most of these are not exclusive to the h-index, they include: – the h-index does not account for the number of authors on a paper. in the original paper, hirsch suggested partitioning citations among co-authors. due to this, the h-index and similar indices tend to favor fields with larger groups, e.g. experimental over theoretical. – the h-index does not account for the typical number of citations in different fields. different fields, or journals, traditionally use different numbers of citations. – the h-index discards the information contained in the author placement in the author list, which in some scientific fields is of significant importance. – the h-index is bound by the total number of publications. this means that scientists with a short career are at an inherent disadvantage, regardless of the importance of their discoveries. this is also a problem for any measure that relies on the number of publications. – the h-index does not consider the context of the citations. for example, citations in a paper are often made simply to ‘flesh out’ an introduction, and can otherwise have no other significance to the article. also, the h-index does not resolve other contextual instances, such as citations made in a negative context, and citations made to fraudulent or retracted work. this is also a problem for regular citation counting. it is of note that various proposals to modify the hindex to emphasize different features have been made, and some of these modifications are currently computed, including the g, hc and hm indices. 3. discussion and conclusions as already stated, in principle it is reasonable to evaluate the impact of a scholar provided that the rating tools apply to the personal merits of that scientist. however, from what i have described above, it is clear that in most cases, a score can be influenced by the environment where the scientist works, including the ‘pool’ he/she belongs to, the availability of money and laboratories, the facilities he/she can profit from, and the importance of the agency where he/she works. this ‘starting’ condition reflects on all of the rest of a career, because the rating of a scholar is often used as a discriminating element for funding, job advancement, and personnel selection. this thus provides academic impact and makes the scholar suitable for more credits in the future, with respect to other colleagues. in addition, within any similar ‘environmental’ situations, a scholar can perform better than others if he/she makes particular choices. in the introduction to pop, it says: “if an academic shows good citation metrics, it is very likely that he or she has made a significant impact on the field. however, the reverse is not necessarily true. if an academic shows weak citation metrics, this may be caused by a lack of impact on the field, but also by one or more of the following: – working in a small field (therefore generating fewer citations in total); – publishing in a language other than english (effectively also restricting the citation field); – publishing mainly (in) books.” it is then clear that the topics, the conditions, and the means of research of a scholar looking for impact are not completely free. indeed, to get a higher score, the results should not be published in a language other than english, should not be printed on books (this last point does not apply to some indexing agencies, like isi, which recently included citations from books in the computation), and should not be devoted to ‘small fields’ of research. however, i would also add that they should not be published as a ‘user’s manual’, nor as a united states geological survey open report file, just to mention two examples. indeed, in the latter case, as these publications are not subjected to peer review, they will not be included in the computation of citations by most of the indexing agencies, although it is well known that anybody using a program code will certainly use and then cite the accompanying manual or report describing its features and use. and what about publication costs? sometimes to publish on a high-impact journal the authors have to cover printing expenses. the amount of money available in research projects depends on many factors, but seldom it is proportional to the target of the research. as an example, theoretical studies are often not adequately funded, as they do not need field or laboratory activities, although these studies can produce results suitable for a widely distributed journal that can only be accessed if the costs for publication can be covered. there are significant implications from all of the arstefano solarino 476 source number of publications number of citations h index web of science 19 199 6 cited reference search 29 235 scopus 20 212 7 google scholar (via pop) 29 247 8 table 1. computation of the author’s personal score as an example of the use of the different databases. comparison of the number of published papers, the citation index and the h-index according to isi (two computations), scopus and google scholar. the scores vary a lot across the different sources, and show variations of up to 50%. the choice of a specific database biases the (absolute) rate of the single scholar, although it probably affects comparisons between scientists only if their scientific production is very dissimilar (many books or conference papers versus journal articles). 477 guments discussed here. i believe that the need to obtain the highest possible score can make a scholar avoid studies that deal with small areas, to give low priority to scientific studies of narrower interest, and to refrain from research activities in applicative fields. in all of these cases, converse to what is required to increase the impact, an article describing the study will probably appear in a ‘minor journal’ (with low impact factor). it will thus have a low citation index, will not significantly contribute to the hindex, and/or will only be published in reports. the search for fame and glory of a scholar and the decisions and choices linked to this can reflect on the society and consequences in the field of geoethics, where scientific, technological, methodological and socio-cultural aspects have to be considered in an order different to what would be expected in a pure meritocracy. the most important scientific discoveries, including those in the earth sciences, have been achieved for ethical reasons, such as to help people, to enlarge perspectives, to investigate causes, and to be prepared to react. sometimes these studies have been rewarded later, although the converse has never happened. no scientist has up to now estimated his/her work in terms of ‘popularity’, but only in terms of benefits to the scientific community, with the related consequences for society. avoiding ‘small’ arguments or lesswidely distributed journals can, in the long term, interfere with the freedom of the single researcher. indeed, even now the ‘recommendation’ to publish in highly cited journals is already a limitation to our freedom. in my opinion, a good compromise between the current rating scheme and a more ethically correct one would be to assign a fraction of the score to the scholar that is proportional to his/her contribution to a study. this can be carried out by simply taking into account the number of authors or by establishing thresholds, like a full score for a single author, a half score for up to three authors, and so on. this feature of dividing the merit according to the number of authors is already included in the computation of some indices, although it has not been steadily accepted. by undertaking this improved assignment, we would avoid the paradox of those very basic studies that sometimes appear in widely distributed journals and are authored by the many scholars , each of which gets a score, participating in a field experiment. although the article is often dealing with the simple description of the field activity and then carries no significant or novel results, it is cited by the hundreds of colleagues using the data collected in the experiment (increasing as a consequence the citation index of each author) and, even worse, with citations that persist for very long times. acknowledgements. i wish to thank the editors, giuseppe di capua and silvia peppoloni, for their will and efforts to publish this volume and an anonymous reviewer for the precious suggestions. references bossolasco, m. (1939). geofisica pura ed applicata, in: pure and applied geophysics, 1, pp. 1-2. brin, s., and l. page (1998). the anatomy of a large-scale hypertextual web search engine, in: seventh international world-wide web conference (www 1998), april 14-18, 1998, brisbane, australia. garfield, g. (1998). the impact factor and using it correctly, der unfallchirurg 101, 413-414. harzing, a.-w. (2010). the publish or perish book. your guide to effective and responsible citation analysis, tarma software research pty ltd, melbourne, australia, 250 pp. hirsch, j.e. (2005). an index to quantify an individual’s scientific research output, proc. natl. acad. sci. usa, 102 (46), 16569-16572; doi: 10.1073/pnas.0507655102. corresponding author: stefano solarino, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, sede di genova, italy; e-mail: stefano.solarino@ingv.it © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. bibliometrics and freedom to publish << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 15hough.qxd 523 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words new madrid earthquakes – intraplate – historic 1. introduction the earthquake sequence that struck the new madrid region of the north american mid-continent in 1811-1812 had remarkably far-reaching effects. by some accounts, they are among the largest – if not the largest – to have ever occurred in a so-called stable continental region (scr) (johnston, 1996). ground motions from the three principal events were felt in places as far away as canada, new england, and at a number of locations along the atlantic coast (mitchill, 1815; bradbury, 1819; fuller, 1912; nuttli, 1973; penick, 1981; street, 1984; johnston, 1996). contemporary accounts document three principal events: approximately 02:15 local time (lt) on 16 december 1811; around 08:00 lt on 23 january 1812, and approximately 03:45 lt on 7 february 1812 (henceforth nm1, nm2, and nm3, respectively; see fig. 1). all three events were felt throughout much of the central and eastern united states. additionally, a large aftershock to nm1 (nm1-a) occurred near dawn on 16 december 1811. substantial aftershock activity following all events was also documented (fuller, 1912; penick, 1981). paleoseismic investigations suggest a repeat time of the order of 400-500 years for the new scientific overview and historical context of the 1811-1812 new madrid earthquake sequence susan e. hough us geological survey, pasadena, california, u.s.a. abstract the central and eastern united states has experienced only 5 historic earthquakes with mw 7.0, four during the new madrid sequence of 1811-1812: three principal mainshocks and the so-called «dawn aftershock» following the first mainshock. much of the historic earthquake research done in the united states has focused on the new madrid seismic zone (nmsz), because the largest new madrid earthquakes may represent the archetype for the most damaging earthquakes to be expected in intraplate regions. published magnitude values ranging from 7.0 to 8.75 have generally been based on macroseismic effects, which provide the most direct constraint on source size for the events. critical to the interpretation of these accounts is an understanding of their historic context. early settlments clustered along waterways, where substantial amplification of seismic waves is expected. analyzing the new madrid intensity values with a consideration of these effects yields preferred values of mw 7.27.3, 7.0, and 7.4-7.5 for the december, january, and february mainshocks, respectively, and of 7.0 for the «dawn aftershock». these values are consistent with other lines of evidence, including scaling relationships. finally, i show that accounts from the new madrid sequence reveal evidence for remotely triggered earthquakes well outside the nmsz. remotely triggered earthquakes represent a potentially important new wrinkle in historic earthquake research, as their ground motions can sometimes be confused with mainshock ground motions. mailing address: dr. susan e. hough, us geological survey, 525 south wilson avenue, pasadena, ca 91106, u.s.a.; e-mail: hough@gps.caltech.edu 524 susan e. hough madrid events; they also suggest that the new madrid seismic zone tends to produce prolonged sequences with multiple, distinct mainshocks, the magnitudes of which are comparable to those of the 1811-1812 events (e.g., tuttle and schweig, 1996; tuttle et al., 2002). thus, the magnitudes of the earthquakes become a critical issue for the quantification of regional hazard in central north america. a repeat of the 1811-1812 sequence would clearly have a tremendous impact. the new madrid seismic zone (nmsz) contributes a nontrival component of seismic hazard in relatively distant midwestern us cities such as st. louis, missouri (frankel et al., 1996). a second impetus to investigate the 18111812 sequence stems from their implications for general issues related to intraplate earthquake processes. the nmsz is among the bestunderstood intraplate source zones in the world, largely because it has been so active throughout the historic and recent prehistoric past. this relative abundance of data affords the opportunity to explore critical unanswered scientific questions regarding large scr earthquakes, most notably the questions of why such events occur in certain regions but (apparently) not in others. because an evaluation of the magnitudes of the 1811-1812 events is so critical for several reasons, tremendous effort has been invested in gleaning quantitative information from the limited available data. available data include i) paleoliquefaction features preserved by the sediments within the mississippi embayment (e.g., tuttle and schweig, 1996); ii) the present-day distribution of seismicity in the nmsz, which is assumed to illuminate the principal fault zones (e.g., gomberg, 1993; johnston, 1996); iii) first-hand reports («felt reports») of the shaking and/or damage caused by the events over the central/eastern united states (e.g., nuttli, 1973; street, 1984). determination of magnitudes for the 18111812 mainshocks hinges exclusively on the felt reports and their interpretation for modified mercalli intensity (mmi) values. nuttli (1973) drew isoseismal contours based on his compilation of approximately 40 felt reports. he determined mb = 7.2, 7.1, and 7.4 for nm1, nm2, and nm3, respectively, based on a relationship between ground motion and intensities from smaller and more recent instrumentally recorded earthquakes in the central united states. with an exhaustive archival search, street (1984) greatly expanded the number of reports (to approximately 100 for nm1) and assigned them intensity values. street (1982, 1984) used these new data and the same method used by nuttli (1973) to obtain mb = 7.1 and 7.3 for nm2 and nm3 and mb 7.0 for the 07:15 lt aftershock of 16 december 1811. street (1982) determined these values by assuming the mb value for nm1 determined by nuttli (1973) and comparing the relative isoseismal areas of the other events. johnston (1996) carried out a comparison between intensity distribution and moment magnitude mw for large earthquakes in stable continental regions worldwide. he compared areas within isoseismals of discrete intensities with instrumentally measured moment magnitudes. on the basis of this calibration, he assigned 8.1 ± 0.31, 7.8 ± 0.33, and 8.0 ± 0.33 for nm1, nm2, and nm3, respectively. in this calculation, johnston (1996) used the only published intensity contours; those determined by nuttli (1973). hough et al. (2000) revisited the magnitude determination for the new madrid mainshocks in two ways. first they reconsidered intensity assignments for the reports compiled by nuttli (1973) and street (1984). this reinterpretation focused on effects that were considered relatively objective, such as descriptions of damage to structures. the reinterpreted mmi values can be used to define new isoseismal contours using subjective as well as systematic approaches, and the isoseismal contours can then be used to obtain mw estimates following the procedure and calibration established by johnston (1996). the results can then be interpreted with a consideration of their historic context, most notably early american settlement patterns. the population of the united states was ≈ 7 000 000 in 1811, with sizable numbers in the states of tennessee, kentucky, and the region including the present-day states of missouri and louisiana. the 1810 census gives the population for several districts for which felt reports are considered, including the dis525 scientific overview and historical context of the 1811-1812 new madrid earthquake sequence fig. 1. map showing location of the new madrid seismic zone as illuminated by microseismicity between 1974 and 1996. locations are from the new madrid catalog (see taylor et al., 1991), which are reported only to two significant figures in decimal degrees. epicenters of the three principal 1811-1812 mainshocks are shown with large open circles (after johnston and schweig, 1996). solid line shows inferred location of reelfoot fault (after odum et al., 1998). rupture scenarios for nm1, nm1-a, and nm3 are also indicated. scenario for nm2, indicated with dashed line, is considered relatively uncertain. 526 susan e. hough trict of st. louis (population 5667), cincinnati (2540), new orleans (24 552), louisville (1357), and new madrid (2103). although present-day missouri was relatively sparsely populated in 1811, available contemporary accounts (e.g., brackenridge, 1817, bradbury, 1819) provide a fairly thorough documentation of demographic and related information. these sources reveal that some towns were more than simple villages by 1811, with solidly constructed houses appearing by the turn of the century. the oldest brick building west of the mississippi was built in the town of sainte genevieve in 1804; this town is along the mississippi river valley north of new madrid. this house and ≈ 50 others that predate the new madrid sequence, are still standing today. this paper summarizes and expands on the results published by hough et al. (2000). the reader is referred to this publication for many of the details regarding the results summarized here. additionally, i summarize evidence that the 1811-1812 sequence included a number of significant, potentially damaging earthquakes, that occurred well outside of the nmsz. 2. intensity reports 2.1. original sources: general considerations hough et al. (2000) concluded that many of the original mmi assignments by nuttli (1973) were too high for two basic reasons: a general bias in the interpretation of reports whose drama is belied by low levels of actual damage reported, and, to a lesser extent, a failure to take site response issues into account. many of the original accounts describe the effects of longperiod shaking; this kind of shaking can be dramatic for large (mw 7 and larger) events at regional distances even when the overall effects of the shaking is low. overall, many of the accounts do not appear to support values as high as those originally assigned. in st. louis, missouri, fuller (1912) describes reports (from the louisiana gazette, 21 december 1811) of people having been wakened by nm1 and furniture and windows having been rattled. he notes that «several chimneys were thrown down», and a few houses «split». to understand such accounts one must be familar with the vernacular of the time; in this case, the word «split» seems to have been used in a number of accounts to mean «cracked» rather than destroyed. consistently, as in the above example, the phrase «thrown down» is used to describe catastrophic damage to chimneys, walls, or houses. the louisiana gazette account goes on to note that «no lives have been lost, nor has the houses sustained much injury». this observation also suggests that the word «split» does not imply substantial damage. on the basis of these reports, a mmi of vi-vii appears to be more appropriate than the value of vii-viii that nuttli (1973) assigns for nm1. at many locations at regional distances (roughly 500-1000 km) event nm1 is generally reported as having been «distinctly» (often the word «sensibly» is used) felt but with no reports of damage. instead, reports describe the rattling of washing stand pitchers, glass, china, and furniture. reports from these locations also generally indicate that «many» were awakened by the event. such descriptions are consistent with an mmi value of iv-v, whereas higher values were assigned in the earlier study. in two instances (arkport, in western new york, and lexington, kentucky) it appears that nuttli (1973) was simply mistaken in either his reading of the original sources, or the mmi assignments. the reinterpretation of hough et al. (2000) thus represents both a revision and a substantial expansion of the original intensity work by nuttli (1973). 2.2. site response in addition to the reassignments discussed above, hough et al. (2000) also assigned – and interpreted – mmi values with a consideration of site response. arguably, the key to understanding the effects of the new madrid earthquakes lies with an appreciation for their historic context. as a first-order observation, the intensity data are very sparse and concentrated along major river valleys and other bodies of water. the latter observation reflects the distri527 scientific overview and historical context of the 1811-1812 new madrid earthquake sequence 1815). however, the potential magnitude of site amplifications at regional distances has perhaps not been fully appreciated until it was so dramatically demonstrated in a number of destructive earthquakes in recent years (e.g., singh et al., 1988). recent dramatic examples of site response have tended to involve lake beds, valleys or basins, and coastal regions such as the san francisco bay area, but significant site amplifications along river valleys have also been documented (e.g., stover and von hake, 1982). a close reading of original sources reveals that the role of site response in controlling ground motions from the new madrid events is documented in several contemporary accounts of the events. for example, fuller (1912) quotes an account by daniel drake of cincinnati, ohio: «(event nm1) was so violent as to agitate the loose furniture of our rooms, open partition doors that were fastened with falling latches, and throw off the tops of a few chimneys in the vicinity of the town». it was this account that apparently prompted nuttli (1973) to assign a mmi value of vi-vii for cincinnati for nm1, yet drake goes on to say that, on the «elevated ridges» in kentucky, less than 20 miles from the river, many people were not awakened by the event. this account (in particular the fact that many people away from the river slept through the event) suggests a mmi value of perhaps iv, certainly not as high as v. considering reported effects from the river valley and those from higher ground, one obtains a mmi range of iv-vi for cincinnati, or an average of v. equivalently, this approach corresponds to separate assignments for river valley and hill sites at cincinnati. of the felt reports from the new madrid sequence, site response is explicitly documented at six different locations. the town of sainte genevieve, which had been moved onto a hard-rock site, provides a key example. no account of the earthquakes from this town was included in the compilation of street (1982). a brief account was discovered by the author following a focused archival search. the account states that the earthquakes were felt in sainte genevieve but caused no damage (rozier, 1890). the pristine, original appearance of brick and other masonry homes in the town also testifies to the absence of dambution of the overall population in the more sparsely populated parts of the central and southeastern united states in the early 1800s. because the new madrid sequence predates the construction of railroad lines into the midcontinent, settlements tended to remain clustered in proximity to waterways. westward expansion followed the major rivers, and virtually all early 1800s settlements in missouri (the extent of the western frontier at that time) were within a few miles of the mississippi river. in addition to the influx of settlers from the east, settlers of french descent also arrived in the area from quebec to the north, primarily along the wabash river. by the early 1820s, early settlers had begun to recognize the pitfalls associated with life on the immediate river banks, which included poor drainage, floods, and disease (missouri historical review, 1911). however, the very earliest settlements of the late 1700s and very early 1800s often were on fluvial sites, immediately adjacent to rivers. new madrid was built so close to the river bank that even before the earthquakes, parts of the town regularly gave way under the continued assault of river currents (penick, 1981). one of the other sizable missouri settlements of the time, sainte genevieve, had been moved to higher ground approximately a mile from the river after a flood in the late 1700s resulted in substantial erosion of the river bank upon which the town had originally been built (brackenridge, 1817). this town, which is 160 km north of the town of new madrid and 75 km south of st. louis, provides a unique hard-rock sample point, as i will discuss later. notwithstanding a handful of exceptions, at the time of the 1811-1812 sequence, the population of the us was clustered in proximity to waterways, especially throughout the sparsely populated mid-continent. intensity data from river bank and other coastal regions will almost certainly reflect a significant site response resulting from the amplification of seismic waves in unconsolidated (and often water-saturated) sediments. the importance of site response in controlling earthquake ground motion has been understood for over a century (milne, 1898), and even correctly inferred by one astute witness to the new madrid sequence (drake, 528 susan e. hough age. this illustrates an important general point about poorly sampled intensity data from historic earthquakes: people are more likely to document their observations (by writing letters, etc) if their experiences were dramatic, than if a felt earthquake had little real impact. it also provides prima facie evidence that the hardrock ground motions from the new madrid earthquakes were not damaging (even to vulnerable structures) at a distance of ≈ 160 km. for those cases where shaking and/or damage is reported to have been worse within a valley or along a riverbank than on adjacent higher ground, one can assign distinct mmi values for riverbank/valley sites and «hard rock» sites away from the waterways. where the reports do not explicitly document relatively higher shaking along shorelines, hough et al. (2000) do not attempt to correct the mmi values for site response. however, in some cases, it appears that high intensity values were assigned based solely on riverbank effects which may have been the result of agitation of the river itself; some of these values were downgraded. clearly, differentiating between the effects of river disturbances along the mississippi and ground shaking is difficult, if not impossible. in the final analysis, some level of bias will inevitably remain in any set of interpreted mmi values. however, in some cases the available data are sufficient to assign a more representative regional mmi level based not on the maximum effects reported at soft-sediment sites but on a full consideration of all available reports. overall, hough et al. (2000) assigned significantly more mmi iv-v values and significantly fewer vi-vii ones compared to the earlier studies, although in a few instances their mmi assessments for a given location were higher than those of street (1982). clearly, however, the reinterpreted values were lower in general than those assigned in the earlier studies, which implies that the differences are due to systematic differences in interpretation rather than random differences in interpretation of ambiguous accounts. a final map of reinterpreted mmi values for event nm1 is shown in fig. 2. these results include mmi values based on data from the following sources: mitchill (1815), fuller (1912), nuttli (1973), and street (1984), as well as a small number of additional sources, including a single point west of the mississippi river, at the location of fort osage. 3. isoseismal areas considering the data shown in fig. 2, it is clear that isoseismal contours are not well-constrained. to obtain magnitude estimates using the equations derived by johnston (1996), however, one must estimate isoseismal areas. hough et al. (2000) employed three different approaches to contour the data from nm1: one subjective, one based on the least-squares minimization schemes presented by seeber and armbruster (1987) (see also armbruster and seeber, 1987), and one in which the mmi values are treated as boolean data. if a data point falls within the appropriate isoseismal area (e.g., a value of iv that falls between the mmi iv and v contours) the residual is zero. if a data point is outside the appropriate contour, the residual is equal to the (whole number) difference between the observed and calculated values. this approach was designed to reproduce the usual conventions applied when intensity data are contoured subjectively. that is, isoseismals are generally drawn to outline areas of equal intensity. in both regressions, the starting model for the falloff of intensity with distance is derived from the empirical equations of johnston (1996). the inversion schemes then allow for iteration away from this model based on the distance decay of the data. using the regression approaches, the treatment of the «not felt» (nf) reports becomes a critical issue. following street (1984), hough et al. (2000) assign a nf value to those locations where a local newspaper is known to have not mentioned an earthquake as having been felt in that location. because nm1 and nm3 occurred at times when people can be assumed to have been asleep, a nf report is taken to indicate a bound of mmi < iv (the shaking level at which «some» people are awakened (stover and coffman, 1993). for nm2, which occurred around 08:00 a.m. lt, a nf report is taken to imply a bound of mmi < iii. hough et al. (2000) do not attempt a subjective contouring of the data for events nm2 and 529 scientific overview and historical context of the 1811-1812 new madrid earthquake sequence fig. 2. mmi values based on a reinterpretation of original felt reports from towns as documented by nuttli (1973) and street (1984). interpolations are done using a standard mathematical algorithm (for details see: hough and martin, 2002); black circles indicate locations where mmi values are available, while outlined gray circles indicate locations where hough et al. (2000) assigned a «not felt» value. < .17 .17-1.4 1.4-3.9 3.9-9.2 9.2-18 18-34 34-65 65-124 >124 < 0.1 0.1-1.1 1.1-3.4 3.4-8.1 8.1-16 16-31 31-60 60-116 >116 none none none very light light moderate moderate/heavy heavy very heavy not felt weak light moderate strong very strong severe violent extreme boolean approach would be preferred given a sufficiently complete set of felt reports, it is yielding overestimates of isoseismal areas for the new madrid events because of the biased sampling of site conditions. the least squares regressions, on the other hand, result in contours that are closer to what one would draw subjectively based on an assessment of site response. the least squares results for events nm2 and nm3 yield the preferred magnitude estimates. for event nm1 the preferred estimate is the one resulting from the subjective contouring. although it is not possible to quantify the uncertainties precisely, the bootstrap results do provide a good general indication of the appropriate error bars. the final, preferred estimates for the three events are mw 7.2, mw ≈ 7.0, and mw 7.4, respectively, with uncertainties of ≈ 0.3 units in each case. hough and martin (2002) estimate mw 7.0 for the dawn aftershock, and conclude that this event most likely occurred on a southeastern segment of the reelfoot thrust fault. 5. remotely triggered earthquakes in his compilation of accounts of the new madrid sequence, street (1982) compiled a list of all events for which there are multiple accounts, identifying a number of «large aftershocks» that were widely felt. at the time of this earlier study the seismological community did not yet generally appreciate the fact that large earthquakes are capable of triggering events at distances far greater than those associated with classic aftershocks. since the 28 june 1992, landers, california, mainshock, however, numerous studies have documented the reality of so-called «remotely triggered earthquakes» (e.g., hill et al., 1992). triggering appears to be associated with dynamic strain associated with the surface wave (gomberg and davis, 1996). although the physical mechanism where by strains cause distant earthquakes remains unclear, remotely triggered earthquakes are generally assumed to be earthquakes that would have happened at some point, but that were «nudged along» by the triggering event. it is possible, however, that remotely triggered earthquakes represent 530 susan e. hough nm3. given the sparsity of the data for these events, both the ellipticity and the shape of the distance decay are fixed to match that determined for nm1. the decision to fix ellipticity is a pragmatic one; allowing another free parameter with the sparse data results in unstable solutions. once the isoseismal contours are determined, mw values can be estimated from each individual isoseismal contour using the equations derived by johnston (1996). johnston (1996) derives western correction factors for extrapolation of isoseismals from the new madrid sequence to the west, using the 1843 marked tree, arkansas earthquake to derive correction factors for nm1 and the 1895 charleston, missouri, earthquake to derive a different set of factors for nm2 and nm3. hough et al. (2000) used the same factors. the method of johnston (1996) yields independent estimates of mw from each isoseismal area (mmi 4-8) from each event. to obtain an average mw for each event, one can estimate seismic moment, mo using the standard formula, log (mo) = 1.5 mw + 16.05, and compute an average mo value that we then translate it to mw. to investigate the uncertainties associated with each regression, we apply a bootstrap analysis in which isoseismals are fit using 50 randomly resampled sets of data points. for each intensity, the five most extreme results are discarded and bounds are estimated from the remaining 45 sets. the uncertainty ranges resulting from the bootstrap analysis are approximately ± 0.1-0.2 units for nm1 and ± 0.2-0.4 for nm2 and nm3. the reader is referred to hough et al. (2000) for a more thorough discussion of these results. 4. interpretation for nm1 the range from both regressions is 0.3 units. for nm2 and nm3, ranges of 0.35-0.7 are inferred. however, considering the range of results from both the boolean and least squares approaches for each event, one obtains uncertainties of approximately a full magnitude unit for all three events. hough et al. (2000) concluded, however, that the magnitudes of each event are constrained to better than ± 0.5 magnitude units. they concluded that while the 531 scientific overview and historical context of the 1811-1812 new madrid earthquake sequence ly strengthening to «tremendous», which is the descriptor brooks reserved for the most severe levels of shaking (mcmurtrie, 1819). according to brooks, the strongest shaking from nm3-b lasted only a few seconds, suggestive of an event in the midwest rather than the new madrid region. daniel drake also described the ground motions from nm3-a and nm3-b as having been qualitatively different from those caused by other events. evaluating the distribution of intensities with the johnston (1996) regressions, one obtains magnitudes of ≈ 4.5 and 5.0-5.5 for nm3-a and nm3b, respectively, and locations well outside the nmsz (fig. 3). event nm2-a, which followed nm2 by 4 days, was apparently smaller than nm3-a and nm3-b; hough (2001) does not determine a magnitude for this event. additionally, a handful of accounts describe the nm2 mainshock as having comprised multiple episodes of shaking within a few minutes. while not definitive, this suggests that remote earthquakes of substantial size could have been triggered in the immediate wake of the s/surface wave arrivals generated by nm2. such immediate triggering is often observed following modern earthquakes (e.g., hough and kanamori, 2002). in addition to the inferred remotely triggered earthquakes discussed above, hough and martin (2002) analyzed accounts from a large events that would not have occurred otherwise. hough (2001) reexamined three of the large «aftershocks» of the new madrid sequence, events occurring at approximately 08:45 a.m. (lt) on 27 january 1812; 08:30 p.m. (lt) on 7 february 1812; and 10:40 p.m. (lt) on 7 february 1812 (hereinafter, nm2-a, nm3-a, and nm3-b, respectively; table i). the first of these events followed nm2 by approximately four days; the second and third events occurred the night following the nm3 mainshock (which occurred at approximately 03:45 a.m., lt). many of the accounts describe the shaking from nm3-b as «severe» or «violent». the nm3a event is generally described as less severe than nm3-b, but still strong. two individuals experienced the new madrid sequence and endeavored to not only document every event they felt, but also to rank the events by severity of shaking: daniel drake of cincinnati, ohio, and jared brooks, of louisville, kentucky (see, mcmurtrie, 1819; fuller, 1912). brooks describes nm3-a as having been, «violent in the first degree, but of too short duration to do much injury». (he presumably means short in relation to the shaking from the new madrid mainshocks, which are typically described as lasting for 2-4 min. in the ohio-kentucky region). brooks describes the shaking from nm3-b as «violent in the second degree», quicktable i. new madrid sequence: mainshocks, principal aftershock, and triggered events. event year month day hr:min long. lat. mw nm1 1811 12 16 02:15 –90.0 36.0 7.2 nm1-a 1811 12 16 07:15 –89.5 36.3 7.0 nm1-b 1811 12 17 noon –89.2 34.6 6.0 nm2 1812 1 23 08:45 –89.7 36.6 7.0 nm2-a 1812 1 27 09:00 –84.0 38.9 ne nm3 1812 2 7 03:45 –89.6 36.4 7.4 nm3-a 1812 2 7 20:30 –84.0 38.9 ≈ 4.5 nm3-b 1812 2 7 22:40 –84.0 38.9 5.0-5.5 event; year, month, day; local time; crudely estimated longitude and latitude in decimal-degrees north and west; preferred moment magnitude estimate, ne: no estimate. 532 susan e. hough fig. 3. map showing inferred locations of principal and remotely triggered earthquakes that occurred during the 1811-1812 «new madrid» sequence. black stars indicate locations given in table i; gray star indicates possible source zone for event nm2 as proposed recently by mueller et al. (2004). event that occurred near noon (lt) on 17 december 1811. they obtain a preferred magnitude estimate of 6.0 and a location well south of the nmsz (table i; fig. 3). 6. discussion and conclusions the magnitude of the principal new madrid mainshocks has been the matter of some debate in recent years. as is often the case with historic earthquakes, macroseismic data provide the most direct constraint on magnitude. as summarized in this paper, the key to interpreting such data in this case is twofold: 1) to assign intensity values based on objective observations such as damage to structures rather than on the apparent drama of anecdotal accounts, and 2) to consider the earthquakes’ effects in light of their historic context. the preferred estimates for the moment magnitudes of the three principal events are mw 7.2, ≈ 7, and 7.4-7.5, respectively, and 7.0 for the dawn aftershock. these values are more consistent with 533 scientific overview and historical context of the 1811-1812 new madrid earthquake sequence other lines of evidence, such as geomorphology, that provide indirect constraint on event size. the geometry and extent of the cottonwood grove fault, which is assumed to have generated nm1, is established primarily from recent microseismicity (e.g., gomberg, 1993; johnston and schweig, 1996). event nm2 is difficult to analyze in any detail because the inferred causative fault, the northern strike-slip limb of the nmsz (johnston, 1996), is the least well-understood part of the zone. also, although the hour of the event provided a better characterization of the low-intensity (mmi iiiiv) field, reporting of the event was likely hampered by a cold spell that had frozen the river and halted boat traffic along the ohio river and mississippi river until 22 january 1812. very recently, mueller et al. (2004) have reexamined this event and questioned whether it occurred within the nmsz at all. at a minimum, this event remains the least well-understood of the three principal mainshocks. however, recent investigations have provided significant constraint of the reelfoot fault, the thrust fault in between the two strike-slip limbs of the nmsz that is inferred to have produced nm3 (e.g., russ, 1982; kelson et al., 1992; johnston, 1996). structure of the reelfoot fault has been elucidated in recent years with seismic reflection profiling. odum et al. (1998) infer an overall fault length of at least 30 km and constrain the dip to be ≈ 31°. more recently, champion et al. (2001) concluded that the reelfoot fault does not exhibit clear geomorphic expression to the southeast of the northern terminus of the cottonwood grove fault. given their inferred spatial extent of the actively deforming reelfoot fault, they conclude the fault could host plausibly a lowto mid-mw 7 earthquake. gomberg (1993) reached similar conclusions based on the fault area estimated from current microseismicity. although no direct measurements of fault scarp height are available for nm3, contemporary accounts from boats on the mississippi describe waterfalls forming on the river. as discussed by odum et al. (1998), these observations correspond to points where the inferred fault rupture crossed the river. the height of these waterfalls is not well constrained, although some information can perhaps be gleaned from available reports. in light of these accounts and the established geometry of the reelfoot fault, one obtains an average slip of 4-5 m. one can account for a mw of 7.5 on the reelfoot fault with plausible rupture parameters: a length l of 40 km, a width w of 30 km (consistent with the maximum depth of microseismicity, see gomberg, 1993), and an average slip d of 5.0 m. these rupture parameters are consistent with established scaling relationships derived from worldwide events (wells and coppersmith, 1994). given a rupture length of 40 km and an average slip of 5 m, one obtains mw 6.9 and mw 7.5 from the scaling relationships for rupture length versus magnitude and slip versus magnitude, respectively. the faulting parameters illustrated in fig. 1 are thus consistent with the lateral and depth extent of the nmsz faults as inferred from microseismicity. the static stress drop values are also consistent with those inferred from finite fault inversions of more recent large earthquakes (e.g., hough, 1996). clearly one cannot prove that the new madrid earthquakes did not have exceptionally high static stress drop values. however, hanks and johnston (1992) showed that highfrequency shaking, and thus isoseismal areas, will depend on stress drop. thus if the new madrid events had higher stress drop values than the average of those used to obtain the regression results of johnston (1996), the magnitudes of the new madrid earthquakes would be lower than the estimates derived from these results. because the regression results of johnston (1996) were calibrated with similarly subjective data, one critical question is the extent to which the hough et al. (2000) assignments are consistent with those on which the regressions were based. to answer this question, one must consider both our general approach to the mmi assignments as well as our treatment of site response issues. in general, there is some precedent for keying an mmi assessment on the most dramatic effects described. however, considering the mmi assignments made for the 1968 mb 5.3 southcentral illinois earthquake (gordon et al., 1970) as an example, it is clear that an mmi of vi is typically assigned when there are multiple instances of the damage usually as534 susan e. hough sociated with this level of intensity: broken windows, cracked plaster, damage to brick chimneys, etc. at some locations the specific report suggesting a high mmi value in the new madrid sequence is one that suggests relatively long-period shaking effects. there is ample precedent for not assigning an mmi value based on such a report when the effects related to higher-frequency shaking (i.e. toppling of small objects and furniture) indicate a much lower value (e.g., armbruster and seeber, 1987). in general, there is a fundamental distinction between the 1811-1812 new madrid events and those used by johnston (1996) to derive the isoseismal area-moment magnitude regressions: the latter events are those for which instrumental magnitudes are available, which means they are from the 1900s (1925 onward). the new madrid sequence is upward of 100 years less recent, and so its collection of felt reports is considerably more sparse than the others. systematic differences in sampling of site conditions can clearly introduce substantial biases. in 1811-1812, logistical constraints induced most of the population to live along river banks (or coasts), which are often characterized by alluvial near-surface geological conditions. later in the nineteenth century, the introduction of round transportation allowed settlement to shift to higher ground, away from potential flooding hazard. sediment-induced amplification is therefore much more likely to affect reports from the early part of the nineteenth century than those from the twentieth century (or even the mid-nineteenth century). although this probably results in a systematic bias in the 1811-1812 intensity data, i do not correct for it systematically in our assignment of mmi values. it would, indeed, not be appropriate to «correct» mmi values for site response and then apply the johnston (1996) regressions because the mmi data used to derive the regressions were not similarly corrected. i have, however, addressed the issue of site response in two ways: i) by revising the mmi assignments where contemporary accounts do document significant site response, which we view as consistent with the usual practice of assigning site-specific mmi values based on sitespecific information, and ii) by using judgement in choosing preferred isoseismal contours. the subjective contouring approach is considered to be the most reasonable proxy for the ideal procedure, which one is unable to do in this case: to allow the mmi values to define the shape of the contours, with clear definition of high-intensity lobes. we conclude that it would clearly be inappropriate to allow the contours to «balloon» out, as was done by nuttli (1973), based on values that surely represent «spokes» of anomalously high ground motions. a systematic site response correction could be done via a careful consideration of intensity distributions from more recent events. hopper et al. (1983) present a map of isoseismals expected from a repeat of a new madrid mainshock in which site response is included implicitly. i note, however, that site corrections for the 1811-1812 data would require a very detailed analysis because settlement patterns changed so drastically in the decades following the new madrid sequence. hough et al. (2000) focused on the moderate intensity contours because their isoseismal areas are the critical inputs to the area-based magnitude determination method of johnston (1996). isoseismal contours for mmi levels ivvii can be constrained by relatively objective reports of damage to structures and the perceptions of individuals who (it can generally be assumed) were asleep at the time of events nm1 and nm3. the felt reports closer to the new madrid seismic zone are relatively incontrovertible in documenting the extent of damage and ground failure. however, interpretation of these reports is greatly complicated by the vast extent of poorly consolidated and largely watersaturated sediments within the mississippi embayment. once again, the natural settlement patterns would have resulted in a strong correlation between population density and proximity to the mississippi river. according to the results of hough et al. (2000), nm1 was also similar in magnitude to the 1886 charleston, south carolina, earthquake, which johnston (1996) estimates to have been mw 7.3. although perhaps at odds with «conventional wisdom» regarding the relative sizes of the two events, we note a striking degree of reciproc535 scientific overview and historical context of the 1811-1812 new madrid earthquake sequence ity between our results and the isoseismal contours from the charleston event determined by bollinger (1977). both events generated values of mmi ≈ v for areas midway between new madrid and charleston, and charleston generated a small swatch of mmi ≈ vi values in the immediate vicinity of new madrid. clearly, magnitude estimates for the new madrid mainshocks will always be plagued by a certain level of uncertainty; a level that is, moreover, difficult to even quantify. we argue that the central issue is not one of precisely determined mathematical uncertainties but rather overall consistency and credibility. magnitudes of 7.2-7.3, 7.0, and 7.4-7.5 for the three principal mainshocks, nm1, nm2, and nm3, respectively, are consistent with both known and plausibly inferred faulting parameters and the shaking distribution as inferred from our reinterpreted mmi values. magnitude values significantly lower than these estimates strain credulity for two reasons: i) comparisons with more recent, better-constrained events, such as the 1886 charleston, south carolina, and 1926 grand banks earthquakes (bent, 1995), and ii) the evidence, discussed above, that nm3 was associated with significant surface faulting. on the other hand, significantly larger magnitude values are problematic for reasons that have been addressed at length in other studies; primarily, the lack of sufficient fault area and slip to generate three separate events with mw close to 8.0. (although one could plausibly argue for a greater depth extent of large earthquake ruptures in the nmsz, even a factor of 2 increase in fault width would increase mw estimates by only 0.3 units). one interesting consequence of our reinterpretation concerns the relative magnitudes of the three principal mainshocks. in the interpretation of johnston (1996), nm1 is larger than nm3, and nm2 is of appreciable size. in contrast, our results reveal nm3 to be substantially larger than the other two. this implies that rather than being a mostly strike-slip system with thrust faulting associated with a compressional stepover, thrust faulting may have been the dominant mechanism associated with the 1811-1812 new madrid sequence. at least, this would be the case if the mechanism of nm3 was predominantly thrust, as has generally been inferred. a predominant thrust mechanism is consistent with the hypothesis that post-glacial rebound provides the driving force for large late holocene earthquakes in the nmsz (e.g., wu and johnston, 2000). although the results of hough et al. (2000) represent a «down-grading» of the magnitudes of the principal new madrid mainshocks, several lines of evidence argue for substantial distributed hazard throughout the north american mid-continent. first, the hazard is a function of the expected ground motions, which, in the case of the new madrid sequence, appear to have been significantly elevated in many cases by site response. an evaluation of site response may therefore be critical for seismic hazard assessment at many locations in the central/eastern united states, particularly those immediately adjacent to major rivers and the atlantic seaboard. secondly, remotely triggered earthquakes potentially represent an additional source of distributed hazard. finally, the recent earthquake history of western india reveals that large earthquakes can occur close together in time, not on the same fault but on neighboring faults (e.g., hough and bilham, 2003). although the 1811-1812 new madrid sequence provides a unique and critically important data set, a more thorough investigation of potential neighboring and regional source zones in the midcontinent appears to be warranted. acknowledgements i thank jim dewey, lucy jones, hiroo kanamori, bill bakun, jim dolan, paul bodin, ruth harris, and jerry hough, for helpful comments and suggestions. i also appreciatively acknowledge the constructive criticism from two anonymous reviewers, as well as substantial effort on the part of the editors to make this volume happen. references armbruster, j. and l. seeber (1987): seismicity 18861889 in the southeastern united states: the aftershock sequence of the charleston, s.c. earthquake, u.s. nu536 susan e. hough cl. regul. comm., washington, dc, rep. nureg/cr4851, pp. 153. bent, a. 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(1982): a contribution to the documentation of the 1811-1812 mississippi valley earthquake sequence, earthquake notes, 53, 39-52. street, r. (1984): the historical seismicity of the central united states: 1811-1928, final report, contract 1408-0001-21251, appendix a, u.s. geol. surv., washington, dc, pp. 316. taylor, k., w. stauder and r. herrmann (1991): a comprehensive unified data set for the new madrid, seismic array, seismol. res. lett., 62, 187. tuttle, m.p. and e.s. schweig (1996): archaeological and pedological evidence for large prehistoric earthquakes in the new madrid seismic zone, central united states, geology, 23, 253-256. tuttle, m.p., e.s. schweig, j.d. sims, r.h. cafferty, l.w. wolf and m.l. haynes (2002): the earthquake potential of the new madrid seismic zone, bull. seismol. soc. am., 92, 2080-2089. wells, d.l. and k.j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seismol. soc. am., 84, 974-1002. wu, p. and p. johnston (2000): can deglaciation trigger earthquakes in north america?, geophys. res. lett., 27, 1323-1326. i n f l u e n z a d e l l e z o n e r e f l e s s o g e n e v a s o s e n s i b i l i s u l l a f u n z i o n e v e s t i b o l a r e n e l c o n i g l i o s o t t o p o s t o ad e l e v a t e a c c e l e r a z i o n i t r a s v e r s a l i (*) c . vacca c . k o c h r i c e v u t o il 19 f e b b r a i o 1903 riassunto. — i n e s p e r i e n z e e s e g u i t e su c o n i g l i d i a m b o i s e s s i e d e l p e s o m e d i o d i k g 2, s o t t o p o s t i a d a c c e l e r a z i o n i t r a s v e r s a l i c o m p r e s e t r a 3 e 9 1 0 g, p r i m a e d o p o l a d e a f f e r e n t a z i o n e d e l l e z o n e v a s o s e n s i b i l i p e r t a g l i o d e i g l o s s i f a r i n g e i , d e l n e r v o d i c y o n e d e i v a g h i b i l a t e r a l m e n t e a l collo, è s t a t o s t u d i a t o il c o m p o r t a m e n t o d e l n i s t a g m o o r i z z o n t a l e p r i m a , d u r a n t e e d o p o le a c c e l e r a z i o n i . d a l l e p r e s e n t i e s p e r i e n z e p i l o t a , si p o s s o n o a v a n z a r e le s e g u e n t i c o n c l u s i o n i : i. n e l c o n i g l i o n o r m a l e , s o t t o p o s t o a d a c c e l e r a z i o n i s u c c e s s i v e di 3 , 0 e 9 1 0 g , il n y p o s t r o t a t o r i o d i m i n u i s c e s e n s i b i l m e n t e p e r d u r a t a , a m p i e z z a e f r e q u e n z a c o n l ' a u m e n t a r e d e l l e a c c e l e r a z i o n i e, t r a 0 e 9 i o g , s c o m p a r e q u a s i t o t a l m e n t e m e n t r e d u r a n t e l ' i n c r e m e n t o d e l l e a c c e l e r a z i o n i si r i l e v a n o s c o s s e d i « a d a t t a m e n t o ». n e l p e r i o d o d i a c c e l e r a z i o n e c o s t a n t e (90" p e r o g n i m a s s i m a l e d i a c c e l e r a z i o n e a 3,0 e 9 1 0 g) n o n si o s s e r v a n o s c o s s e n i s t a g m i c h e a d a c c e z i o n e c h e n e l l a f a s e u l t i m a a l l ' a c c e l e r a z i o n e di 3 g . l ' e c g c o n t e m p o r a n e a m e n t e r e g i s t r a t o , d i m o s t r a v a r i a z i o n i d i f r e q u e n z a a t i p o di a d a t t a m e n t o d u r a n t e l ' i n c r e m e n t o d e l l e a c c e l e r a z i o n i , m e n t r e f o n d a m e n t a l m e n t e la f r e q u e n z a r e s t a c o s t a n t e n e l p e r i o d o (li s t a t o d i e s s a . si r i l e v a n o i n o l t r e le v a r i a z i o n i g i à o s s e r v a t e i n r i c e r c h e d i v a c c a e coli, d u r a n t e lo s t e s s o t i p o di a c c e l e r a z i o n i , n e l r a t t o , e c i o è : r o t a z i o n e d e l l ' a s s e e l e t t r i c o i n s e n s o o r a r i o , o n d a p a t i p o a n o s s i c o e s e g n i di s o f f e r e n z a c o r o (*) n o t a p r e s e n t a t a a l 2° c o n g r e s s o i n t e r n a z i o n a l e t e c n i c o s c i e n t i f i c o d e l l o s p a z i o . r o m a , 1 9 2 3 g i u g n o 1902. (**) i s t i t u t o di f i s i o l o g i a u m a n a d e l l ' u n i v e r s i t à di n a p o l i d i r e t t o r e p r o f . g i n o b e r g a m i . i s t i t u t o m e d i c o l e g a l e p e r l ' a e r o n a u t i c a m i l i t a r e d i n a p o l i dir e t t o r e col. c . s . a . d r . g i u s e p p e p i z z u t i . ( i l o c . v a c c a c. kocii n a v i c a a t i p o i s c l i e m i c o . t a l i m o d i f i c a z i o n i s c o m p a i o n o m o l t o r a p i d a m e n t e s u b i t o d o p o la fine d e l l e a c c e l e r a z i o n i (in m e d i a i n t o r n o a 3' d a l l a fine della r o t a z i o n e ) . 2. n e l c o n i g l i o d e a f f e r e n t a t o , s o t t o p o s t o a l l e s t e s s e a c c e l e r a z i o n i su c i t a t e , n e l l a f a s e c o s t a n t e d i a c c e l e r a z i o n e a 3 g n o n si r i l e v a n o s c o s s e n i s t a g m i c h e clie p e r ò c o m p a i o n o , s e p p u r e a l q u a n t o r i d o t t e d i d u r a t a , a m p i e z z a e f r e q u e n z a , a l l a c e s s a z i o n e d e l l a r o t a z i o n e . a 6 g si n o t a u n ' u l t e r i o r e r i d u z i o n e d e l l a r i s p o s t a n i s t a g m i c a c h e s c o m p a r e a d d i r i t t u r a a 9 1 0 g . p e r q u a n t o r i g u a r d a l ' e c g si n o t a n o p i ù f r e q u e n t i e p i ù i n t e n s e v a r i a z i o n i d e l l a f r e q u e n z a c a r d i a c a sia a v o l t e n e l s e n s o d i a u m e n t a t o d e l l a f r e q u e n z a s t e s s a , s i a n e l s e n s o d e l l a r i d u z i o n e , s p e c i e a (i e 9 1 0 g r i s p e t t o a q u a n t o o s s e r v a t o p r i m a d e l l a d e a f f e r e n t a z i o n e . i s e g n i d i s o f f e r e n z a m i o c a r d i e a c o r o n a r i c a s u e l e n c a t i si a c c e n t u a n o s e n s i b i l m e n t e e d il r i t o r n o a l l a n o r m a , d o p o l a fine d e l l a r o t a z i o n e , è p i ù l e n t o . si è o s s e r v a t o a n c h e u n c a s o di b l o c c o d i b r a n c a s i n i s t r a . q u a l c h e a n i m a l e è a n c h e d e c e d u t o a 9 1 0 g . 3. l e e s p e r i e n z e d i m o s t r a n o c h e s t i m o l i a f f e r e n t i p e r i f e r i c i p a r t e n t i d a l l e z o n e v a s o s e n s i b i l i c a r d i o v a s o r e s p i r a t o r i e , h a n n o i n f l u e n z a s u l l a f u n z i o n e l a b i r i n t i c a e c h e la d e a f f e r e n t a z i o n e r i d u c e a l q u a n t o l a r e s i s t e n z a d e l l ' a n i m a l e a l l e e l e v a t e a c c e l e r a z i o n i t r a s v e r s a l i . s u m m a r y . i n e x p e r i m e n t s c a r r i e d o u t 011 r a b b i t s of b o t h s e x e s , w e i g l i i n g a v e r a g e l y 2 k g , t h e b e h a v i o u r of t h e h o r i z o n t a l n y s t a g m u s b e f o r e , d u r i n g a n d a f t e r t h e o p e r a t i o n s l i a s b e e n s t u d i e d . t h e r a b b i t s w e r e s u b j e c t e d t o t r a n s v e r s a i a c c e l e r a t i o n s b e t w e e n 3, g a n d 9 1 0 e a c h , b e f o r e a n d a f t e r t h e d e a f f e r e n t a t i o n of t h e v a s o s e n s i t i v e z o n e s b y c u t t i n g t h e g l o s s o p h a r y n g e o , t h e c y o n n e r v e a n d t h e v a g i 011 b o t l i s i d e s of t h e n e c k . f r o m s u c h g u i d e e x p e r i m e n t s , t h e f o l l o w i n g c o n c l u s i o n s m a y b e d r a w n : 1) i n a n o r m a l r a b b i t , s u b j e c t e d t o s u c c e s s i v e a c c e l e r a t i o n s of 3.0 a n d 9 1 0 g , t h e p o s t r o t a t o r y n y d e c r e a s e s r e m a r k a b l y in l e n g t h , w i d t l i , a n d f r e q u e n c y a s a c c e l e r a t i o n s inc-rease a n d i t a l m o s t d i s a p p e r a s b e t w e e n 6a n d 9 1 0 , "while d u r i n g t h e i n c r e a s e of a c c e l e r a t i o n s " a d j u s t m e n t " o s c i l l a t i o n s a r e r e g i s t e r e d . ili t h e p e r i o d of c o s t a n t a c c e l e r a t i o n (90 sec. f o r e a c h a c c e l e r a t i c i ! m a x i m u m a t 3,g a n d 9 1 0 g) 110 n y s t a g m i c o s c i l l a t i o n s a r e 0i1s e r v e d , w i t h t l i e e x c e p t i o n of t h e l a s t p h a s e of 3 g a c c e l e r a t i o n s . t h e e c g r e c o r d e r e d a t t l i e s a n i e t i m e s h o w s f r e q u e n c y v a r i a t i o n s b y t h e t y p e of a d j u s t m e n t d u r i n g t h e i n c r e a s e of a c c e l e r a t i o n s , w h i l e f r e q u e n c y r e m a i n s f u n d a m e n t a l l y t h e s a n i e d u r i n g t h e p e r i o d of t h e s t e a d y s t a t e , v a r i a t i o n s a l r e a d y o b s e r v e d i n f o r r n e r w o r k s b y v a c c a a n d coli, d u r i n g t h e s a n i e k i n d of a c c e l e r a t i o n s i n r a t s , t h a t i s : r o t a t i m i f r o m r i g l i t t o l o f t of t h e e l e c t r i c a l a x i s ; a n o x i a t y p e p w a v e , a n d m a r k s of i s c h e m i e t y p e c o r o n a r y s u f f e r i n g . s u c h m o d i f i c a t i o n s d i s a p p e a r v e r y q u i c k l y i m m e d i a t e l y a f t e r t h e e n d of a c c e l e r a t i o n s ( a v e r a g e l y , a b o u t 3 m i n u t e s a f t e r t h e e n d of t h e r o t a t i o n ) . 2. i11 d e a f f e r e n t a t e s r a b b i t s , s u b j e c t e d t o t h e s a n i e a c c e l e r a t i o n s a s a b o v e , 110 n y s t a g m i c o s c i l l a t i o n s a r o n o t i c e d i n t h e c o s t a n t a c c e l e r a t i o n p h a s e a t 3 g . s u c h o s c i l l a t i o n s h o w e v e r , a p p e a r , e v e n if of r e d u e e d l e n g h t , c o n i g l i o s o t t o p o s t o a d e l e v a t e a c c e l e r a z i o n i t r a s v e r s a l i 1 ) 1 7 w i d t h , a n d f r e q u e n c y , a t r o t a t i o n ' s e n d . a t (j g , a f u r t l i e r d e c r e a s e of n y s t a g m i c r e s p o n s e is o b s e r v e d , w l i i c h t o t a l l y d i s a p p e a r s a t 9 1 0 g . i l i t h e e g c , m o r e f r e q u e n t a n d s i z e a b l e v a r i a t i o n s of l i e a r t f r e q u e n c y a r e n o t i c e d , h o t l i a s i n c r e a s e of t h e f r e q u e n c y i t s e l f a n d a s a d e c r e a s e ( e s p e c i a l l y a t (3 a n d 9 1 0 g ) w i t h r e s p e c t t o w h a t o b s e r v e d b e f o r e d e a f f e r e n t a t i o n . t h e a b o v e m e n t i o n e d m a r k s of m y o c a r d i u m c o r o n a r y s u f f e r i n g a r e r e m a r k a b l y i n c r e a s e d a n d t h e r e t u r n t o n o r m a l y a t r o t a t i o n ' s e n d is s l o w e r . a l s o a c a s e of b l o c k of t h e l e f t b r a n d i h a s b e e n o b s e r v e d . s o m e a n i m a l s d e c e a s e d a l s o a t 9 1 0 g . 3. e x p e r i m e n t s s h o w t h a t a f f e r e n t s p e r i p h e r i c a l s t i m u l i , c o m i n g f r o l l i t h e v a s o s e n s i t i v e c a r d i a c r e s p i r a t o r y z o n e s , h a v e a n i n f l u e n c e o n t h e l a b y r i n t h f u n c t i o n a n d t h e i r d e a f f e r e n t a t i o n r e d u c e s i z e a b l y t h e a n i m a l ' s r e s i s t e n c y t o h i g h t r a n s v e r s a i a c c e l e r a t i o n s . i n t r o d u z i o n e e scopo d e l l e r i c e r c h e . è n o t o d a l l a fisiologia classica c h e l'eccessiva s t i m o l a z i o n e l a b i r i n t i c a d e t e r m i n a riflessi n e u r o v e g e t a t i v i sia a carico del s i s t e m a cardio-circolatorio (pallore, s u d o r a z i o n e , a u m e n t o della f r e q u e n z a cardiaca) sia a carico di a l t r i o r g a n i (nausea, v o m i t o , v e r t i g i n e ecc.). n o n ci risulta che ricerche p i ù a p p r o f o n d i t e siano s t a t e c o n d o t t e s u l l ' i m p o r t a n z a che le zone reflessogene vasosensibili p o s s a n o a v e r e , v i c e v e r s a , sulla f u n z i o n e v e s t i b o l a r e . ci è p a r s o p e r t a n t o utile c o n d u r r e ricerche in t a l senso d a t o che la n a v i g a z i o n e spaziale, in a t t o , s o t t o p o n e l ' a s t r o n a u t a a d i n t e n s e accelerazioni nella fase di decollo d a l l a t e r r a ed in quella di r i e n t r o n e l l ' a t m o s f e r a . t e c n i c a e d e s p e r i m e n t i . le ricerche sono s t a t e c o n d o t t e su conigli di a m b o i sessi e del peso m e d i o di 2 k g s o t t o p o s t i ad accelerazioni t r a s v e r s a l i comprese t r a 3,6 e 9-10 g, m a n t e n e n d o l i a t a l i accelerazioni, nella f a s e m a s s i m a l e p e r ciascuna di esse, p e r la d u r a t a di 90". l a f a s e di i n c r e m e n t o delle accelerazioni è s t a t a s e n p r e m o l t o l e n t a e r i s p e t t i v a m e n t e in valori di t e m p o da 2',20" p e r 3 g ; 4',30" p e r 6 g e 7',08" p e r 9-10 g. la f a s e di decelerazione, o v v i a m e n t e m o l t o p i ù r a p i d a , è s t a t a l i m i t a t a a 9" p e r 3 g, 12" p e r 6 g e 17" p e r 9-10 g. p e r t a l e e s p e r i m e n t o è s t a t a i m p i e g a t a u n a c e n t r i f u g a p e r piccoli a n i m a l i già u s a t a in p r e c e d e n t i ricerche da c. vacca e coli. (» « s ). 618 c. v a c c a c . kocii i n via generale si è p r a t i c a t a l'esperienza negli stessi a n i m a l i p r i m a e dopo la d e a f f e r e n t a z i o n e delle zone vasosensibili chemio-e pressocentrici. t a l e i n t e r v e n t o è s t a t o eseguito in a n e s t e s i a locale v y y^y ••--!.;,tlio nomala (2c/5/196?) di doacculerasione da j a o g / v a -j pine d e l l a cuwooaleratioa > ^atoi ) * ì> ~\ ___ _ > » / x . / v / v r ^ y o y \ i c o n i g l i o s o t t o p o s t o a d e l e v a t e a c c e l e r a z i o n i t r a s v e r s a l i 1)17 novocainica e p r e v i a i n t u b a z i o n e t r a c h e a l e p e r t r a c h e o t o m i a , sezion a n d o s u c c e s s i v a m e n t e e b i l a t e r a l m e n t e al collo i n n . glosso-faringei (giorno e seno carotideo), d i cyon ed i v a g h i (zona cardio-aortica t,=6 c s i . / a e c . ?aau d i do»ooal«raziontì c o n i g l i o oy.g.2 n o m a l o da 9 a 0 g (2s/5/1962) w a v v 0 2 0 c. v a c c a c. kocii ed a l t r i r e c e t t o r i e v e n t u a l m e n t e s i t u a t i s u l l ' a r t e r i a p o l m o n a r e ) . gli a n i m a l i v e n i v a n o s o t t o p o s t i alle accelerazioni t r a sei e dodici ore dopo l ' i n t e r v e n t o . t.= 6 ec,/sae. yig. 3 c o n i g l i o kg. 2 c e a f f e r e n t a t o ?uae d i d e a c c e l e r a i o n e da 3 a 0 g (-9/5/••'jsc) pine d e l l a d e a c c e l e r a z i o n e ( s t o p ) i » 1 a ; 1 p i a . 3 c o n i g l i o s o t t o p o s t o ad e l e v a t e a c c e l e r a z i o n i t r a s v e r s a l i 1)17 l a c e n t r i f u g a , a mezzo di c o n t a t t i r o t a n t i , ha [permesso [il rilievo del n i s t a g m o orizzontale, a n c h e d u r a n t e la r o t a z i o n e , a p p l i c a n d o d u e a g h i e l e t t r o d i inseriti s o t t o c u t e all'angolo e s t e r n o della r i m a oculare, t.>ò cz./aoc. ee«s ,a rt /i corsilo f r.s.l d.,ff.«»tom (jw,36=) fase di doaocelora.-ior.e da 9 a 0 g t pina della daacoaltrazlcne («top) — > f i g . 4 m e n t r e l'applicazione di a g h i e l e t t r o d i agli a r t i h a c o n s e n t i t o il rilievo di u n a derivazione p e r i f e r i c a (d2) e l e t t r o c a r d i o g r a f i c a . ci si è s e r v i t i , p e r la registrazione c o n t e m p o r a n e a dei t r a c c i a t i e l e t t r o c a r d i o g r a f i c o ed elett.ronistagmografico, di u n e l e t t r o e n c e f a l o g r a f o t i p o galileo a d 8 canali con c o s t a n t e di t e m p o = 0,3". nelle figure 1, 2, 3 c 4 è i l l u s t r a t o u n esempio completo delle esperienze c o n d o t t e c o m p l e s s i v a m e n t e su dieci a n i m a l i . 022 c. v a c c a c . k o c h r i s u l t a t i s p e r i m e n t a l i . d a l l e p r e s e n t i e s p e r i e n z e p i l o t a (in q u a n t o è n o s t r a i n t e n z i o n e cont i n u a r e r i c e r c h e s u l l ' a r g o m e n t o ) si possono a v a n z a r e le s e g u e n t i osserv a z i o n i : 1) nel coniglio n o r m a l e , s o t t o p o s t o a d a c c e l e r a z i o n i successive di 3,6 e 9-10 g, il x y p o s t r o t a t o r i o d i m i n u i s c e s e n s i b i l m e n t e p e r d u r a t a , a m p i e z z a e f r e q u e n z a con l ' a u m e n t a r e delle a c c e l e r a z i o n i e, t r a g e 9-10 g, s c o m p a r e q u a s i t o t a l m e n t e m e n t r e d u r a n t e l ' i n c r e m e n t o delle a c c e l e r a z i o n i si r i l e v a n o scosse di « a d a t t a m e n t o ». n e l p e r i o d o di acceler a z i o n e c o s t a n t e (90" p e r ogni m a s s i m a l e di a c c e l e r a z i o n e a 3,6 e 9-10 g) non si o s s e r v a n o scosse n i s t a g m i c l i e a d eccezione c h e nella, f a s e u l t i m a n e l l ' a c c e l e r a z i o n e di 3 g. l ' e c g , c o n t e m p o r a n e a m e n t e r e g i s t r a t o , d i m o s t r a v a r i a z i o n i d e l l a f r e q u e n z a c a r d i a c a a t i p o di a d a t t a m e n t o d u r a n t e l ' i n c r e m e n t o delle accelerazioni, m e n t r e f o n d a m e n t a l m e n t e la f r e q u e n z a r e s t a c o s t a n t e nel p e r i o d o di s t a t o di essa. si r i l e v a n o i n o l t r e le v a r i a z i o n i già o s s e r v a t e i n r i c e r c h e di c. v a c c a ed l . v a c c a (l) d u r a n t e lo stesso t i p o di accelerazioni, nel r a t t o , e cioè: r o t a z i o n e d e l l ' a s s e e l e t t r i c o i n senso o r a r i o , o n d a p a t i p o anossico e segni di s o f f e r e n z a c o r o n a r i c a a t i p o i s c h e m i c o . t a l i modificazioni s c o m p a i o n o m o l t o r a p i d a m e n t e s u b i t o d o p o l a f i n e delle accelerazioni (in m e d i a i n t o r n o a 2 ' 3 ' d a l l a fine d e l l a r o t a z i o n e ) . 2) nel coniglio d e a f f e r e n t a t o , s o t t o p o s t o alle stesse a c c e l e r a z i o n i su c i t a t e , n e l l a f a s e e o s t a n t e d i a c c e l e r a z i o n e a 3 g, in g e n e r e , non si r i l e v a n o scosse n i s t a g m i c h e c h e p e r ò c o m p a i o n o , s e p p u r e a l q u a n t o rid o t t e di d u r a t a , a m p i e z z a e f r e q u e n z a , alla cessazione d e l l a r o t a z i o n e . a 6 g si n o t a u n ' u l t e r i o r e r i d u z i o n e della r i s p o s t a n i s t a g m i c a che scomp a r e a d d i r i t t u r a a 9-10 g. p e r q u a n t o r i g u a r d a l ' e c g . si n o t a n o p i ù f r e q u e n t i e p i ù i n t e n s e v a r i a z i o n i della f r e q u e n z a c a r d i a c a sia a v o l t e n e l senso di a u m e n t o della f r e q u e n z a stessa, sia nel senso della r i d u z i o n e , specie a 6 e 9-10 g, r i s p e t t o a q u a n t o o s s e r v a t o p r i m a della d e a f f e r e n t a z i o n e . i segni di s o f f e r e n z a m i o c a r d i c a c o r o n a r i c a su e l e n c a t i si a c c e n t u a n o s e n s i b i l m e n t e ed il r i t o r n o alla n o r m a , dopo la fine d e l l a r o t a z i o n e , è p i ù l e n t o . si è o s s e r v a t o a n c h e a 9 g u n caso di blocco di b r a n c a s i n i s t r o (fig. 4). c o n i g l i o s o t t o p o s t o ad e l e v a t e a c c e l e r a z i o n i t r a s v e r s a l i 1)17 c o n c l u s i o n i . i r i s u l t a t i s p e r i m e n t a l i su d e s c r i t t i d i m o s t r a n o che stimoli a f f e r e n t i periferici p a r t e n t i d a i p r e s s o c e t t o r i v a s a l i senocarotidei e cardio-aortici i n f l u e n z a n o la f u n z i o n e l a b i r i n t i c a e v i d e n z i a n d o p e r t a n t o che connessioni n e r v o s e esistono f r a i c e n t r i c a r d i a c o e quello v a s o m o t o r e , s i t u a t i nella s o s t a n z a r e t i c o l a r e b u l b a r e , ed i c e n t r i viciniori v e s t i b o l a r i . d a t a la nat u r a v e g e t a t i v a dei riflessi in p a r o l a (3) l a connessione t r a essi è verosimilm e n t e e f f e t t u a t a dalla s o s t a n z a r e t i c o l a r e m e s e n c e f a l i c a . le esperienze f a n n o rilevare i n o l t r e che p i ù e l e v a t e sono le accelerazioni m e n o i n t e n s a è la r i s p o s t a v e s t i b o l a r e ; esiste p e r t a n t o u n a soglia o t t i m a l e di stimolazione del l a b i r i n t o . i n o l t r e a p p a r e chiaro che influenz a n o s i c u r a m e n t e la f u n z i o n e l a b i r i n t i c a i m p u l s i p a r t e n t i dalle zone reflessogene v a s a l i , o l t r e che quelli p r o v e n i e n t i dai r i c e t t o r i p r o p r i del l a b i r i n t o ; i n f a t t i la d e a f f e r e n t a z i o n e delle zone vasosensibili r i d u c e not e v o l m e n t e la r i s p o s t a n i s t a g m i c a . i n f i n e la d e a f f e r e n t a z i o n e dei c e n t r i b u l b a r i dalle zone p e r i f e r i c h e vasosensibili d i m i n u i s c e la r e s i s t e n z a dell ' a n i m a l e alle accelerazioni t r a s v e r s a l i t a n t o che in q u a l c h e caso, a d accelerazioni di 9-10 g, si è v e r i f i c a t a la m o r t e d e l l ' a n i m a l e stesso. b i b l i o g r a f i a p ) vacca c. e vacca l . , rivista di medicina aeronautica e spaziale, 2 3 , 34, r o m a 1 9 6 0 . ( 2 ) v a c c a c . , d e f r a n c i s c i s p . e v a c c a l . , rivista di medicina aeronautica e spaziale, 2 4 , 5 0 1 , r o m a 1 9 6 1 . (3) kocii c., minerva otorinolaring., 3 8 0 3 8 1 , t o r i n o i 9 6 0 . 404 not found annals of geophysics, 55 (3), 2012: pievani geoethics and philosophy of earth sciences: the role of geophysical factors in human evolution telmo pievani università di milano-bicocca, dipartimento di scienze umane per la formazione, milano, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5579 abstract this article explores the role of philosophy of the earth sciences in the foundation of the principles of ‘geoethics’. in particular, the focus is on two different examples of philosophical analysis in the field of geosciences: the first is the trial against the italian national commission for forecasting and predicting great risks, which was charged with negligence in communication and prediction on the occasion of the earthquake that almost destroyed the city of l’aquila on the night of april 6, 2009; the second is related to the scientif ic and theoretical consequences of the updated geographical scenario of the human global populating of the earth, based on archeological, paleontological and genetic data. our concept of ‘scientific prediction’ in the case of geophysical phenomena and the new ways to see human evolution that depend on geophysical factors have ethical and philosophical implications that are crucial for the foundations of geoethics. the tentative conclusion is that we need an evolutionary sense of belonging to our planet, and a concept of ‘natural’ phenomena and ‘natural’ disasters that should not be an alibi for the underestimation of our political and ethical responsibilities. 1. introduction to date, philosophers of science have paid insufficient attention to the earth sciences. however, crucial epistemological issues deserve to be tackled through the analysis of these fields of research that are intended as ‘special sciences’. among others, these issues include: the extent to which these disciplines and their principles can be reduced to apparently more fundamental ones, like physics; the role of past events (and so not observable) in their explanations, theories and descriptions that are devoted to the reconstruction of historical and contingent processes through law-like patterns and generalizations that are considered as nonuniversal but context dependent; the need for a plurality of methodologies of explanation, which will be causal and mechanistic in some cases, narrative and historical in other cases, and with various mixes of these two [frodeman 1995, cleland 2001, kleinhans et al. 2005]; and the structures of geological ‘deep time’ (e.g. circular, linear, progressive) that emerge from the earth sciences [gould 1987]. these philosophical lines of analysis, with the results shared at the moment among the community of scholars, are very similar to those discussed in another young field in the philosophy of science: the philosophy of biology, and particularly, the philosophy of evolutionary biology [sterelny 1996]. the influence of local circumstances (under-determined by the data), contingent events, and unpredictable nonlinear processes, together with the search for general regularities, appear to be the main link of resemblance between the two domains [sterelny and griffiths 1999], in a welcome global vision of life sciences and earth sciences. in this perspective, it must be remembered that charles r. darwin’s first training as a naturalist was in geology, a discipline that was intended by him to be a model of methodology (due to its ability to show global patterns of change) and a conceptual framework for evolutionary gradualism [herbert 2005, pievani 2009]. in his youth and his notebooks, we can clearly see that darwin was actually the heir who extended the geological revolution to the life sciences, through charles lyell’s uniformitarianism and actualism. since his first notes during the voyage of the beagle, the geological instability of the earth surface was the culture medium of darwin’s transmutational view of life, as he emerging from a peculiar mix of fossil evidence and biogeographical observations. so, the history of the biosphere and the history of the physical planet have a lot in common, not only in their scientific explanations, but also in their philosophical interpretations and historical roots. 2. predictions in earth sciences: from philosophy of science to ‘geoethics’ a recent italian example illustrates the need for a philosophical and methodological evaluation of the earth sciences and the nature of their explanations. the govarticle history received december 20, 2011; accepted february 21, 2012. subject classification: geoethics, philosophy of earth sciences, scientific prediction, risk, natural disasters, probability. 349 ernmental national commission, composed of six leading earth scientists and experts involved in the public discussion and security policies for the prevention of damage from earthquakes in italy (the national commission for forecasting and predicting great risks), is on trial, with the serious imputation of multiple manslaughter. this is because of the reassuring statements that were disseminated during the seismic swarm of the low-level tremors and the inability to predict the catastrophic earthquake of april 6, 2009, in the city of l’aquila and its surroundings. the position of the scientists is paradoxical, because in the case of a dramatic and generalized warning to the population with every seismic swarm, in the absence of the subsequent earthquake (like in the majority of historically reported cases), they might have been charged with the opposite accusation, of instigating a false alarm and panic. clearly, in this case, we are dealing with the concept of ‘prediction’ in general science, and particularly in earth sciences, and how to communicate it. following the accusations, it is supposed that the scientists should have foreseen the specific occurrence or the exact probability of an earthquake (clearly impossible with our current technologies), or that they would have alerted the population in every similar case (unrealistic for earthquakes like that in l’aquila). probably the style of communication of the commission was truly incomplete and contradictory, mostly with the idea that repeated tremors would mean less probability of a major earthquake. the misleading communication was driven by the political and contingent demand for calming news, and the issue here is the way of communicating scientific evidence based on probability and degrees of risks, and not on certainties, as stephen s. hall pointed out in nature in september 2011 [hall 2011]. nevertheless, it appears that an overestimated interpretation of the concept of ‘scientific prediction’ might become a way to underestimate, at least in public debate, the political, social and ethical responsibility for the heavy human and civil damage produced by a not so infrequent seismic event in the apennine region. however, in terms of the philosophy of science, the case has another interesting side: due to their robust and appreciated tradition of research, italian earth scientists have actually made a lot of specific ‘predictions’, and have even visualized the micro-zones of statistical seismic risk for the whole italian territory, and they have worked for decades on detailed recommendations for institutions and people living in highly seismic regions in italy. furthermore, the occurrence of an earthquake or a prolonged seismic activity in areas with previous low probabilities, like in emiliaromagna region in northern italy since may 2012, shows the evolving and historic features of these maps of risk. the difficulties in the evaluation of the real risk through the analysis of seismic precursory symptoms have to be handled in a probabilistic way. however, probability, which is so crucial in our most advanced scientific models, is one of the less understood concept in public debates. this is exactly the point: these are probabilistic predictions of risks, and not of specific events (like in a prophecy). recommendations and probabilistic risks claim for consequent long-term political actions of preventive measures and for the general education and ethics of prevention. even the most advanced calculations of the fragility of buildings and hazard probabilities are useless, even when frequently updated, without consolidated practices of prevention. what is under discussion here is not only the difficult task of communicating risks and probabilities, but the relationships between human populations and their territories. so, in such cases, a rigorous philosophical and methodological analysis might be extremely useful, and also in public debates and trials, for addressing the concepts and terms we use in the earth sciences, and for the correct foundation of ethical arguments related to our knowledge of the mechanisms and processes of the earth. 3. sons of an unstable earth the same argument, from the philosophy of earth sciences to geoethics, can be outlined for the proposal of more general ethical values related to, and coming from, the geosciences. take the case of the most updated evidence about human evolution. our journey as humans started two million years ago, when the first specimens of the homo genus walked out of the african continent and colonized eurasia. a long time after that, small groups belonging to our species, homo sapiens, once again moved from africa and launched into the exploration of the old and new worlds. the journey is still not over: human beings have crossed or settled in every corner of the land above water. that handful of pioneers has now resulted in a population of seven billion people. until a few thousand years ago, the earth was populated by several human species, of which there were at least five [tattersall 2009, wood and harrison 2011]. now we have ended up with just one. the history of the many humans at the beginning has been shrouded in darkness for a long time, due to a lack of scientific and historical evidence. now, due to the knowledge of paleontologists, archeologists and geneticists, we can retrace the paths of the first explorers, and through them the great journey into human diversity, through genes, peoples and languages [cavalli-sforza et al. 1994]. in this field, we discover that geophysical factors were crucial for our evolution and diversification, which started with the role of the great rift valley for the separation of the first hominin species in the eastern parts of the continent, which were progressively more arid. later, where telmo pievani 350 351 once there were lakes, rivers and green grassland, now we have the expanse of the sahara desert; other humans saw what we can no longer see. continental drift, volcanic eruptions, environmental instability, climate change, and fragmentation of habitats have all radically shaped the evolutionary path for us, as for all species. all of the major events for our genus took place in the midst of the unstable pleistocene climate that started more than two million years ago, and then again one million years ago with the glacial and interglacial periods, the rising and falling of the sea levels, the coming and going of geographical barriers, islands turning into peninsulas, and vice-versa, lands locked in by ice, areas of vegetation shifting latitude, and with them the herbivores and carnivores. the movements of homo in and out of africa were largely due to the alternating dry and wet or damp phases in the sahara and the sahel. these areas were green and fertile expanses with waterways running across them that are now only visible from satellites [osborne et al. 2008]. they attracted hominins from the south and the east, who were then driven in all directions, including north and north-east, when the desertification progressed. oscillations were in turn caused by variations in the intensity of the system of atlantic ocean currents [castañeda et al. 2009]. this was particularly true after the closure of the isthmus of panama. so, who would have thought that something so deep would bond us humans to the sahara, to the continental drift, and to our ability to adapt culturally and biologically to the whims of a living planet. eight thousand generations ago (or about 200 thousand years ago) the first homo sapiens made their appearance in sub-saharan africa, during the dry phase that coincided with the quaternary ice age. it was a small population with some identifying traits, such as a tall and slim build, a large skull and cranial capacity (exceeding 1400 cm3), good stone working skills, and different modulation of gene expression that regulated development and growth. homo sapiens spread throughout africa, and then probably left the continent in several waves, following the shorelines and congenial climate zones, colonizing eurasia and then australia and the americas [cavalli sforza and cavalli sforza 1995]. between 60,000 and 50,000 years ago, the islands of the indonesian archipelago formed an uninterrupted land bridge up to bali, called sunda. however, to reach the australian supercontinent (australia, new guinea and tasmania were all one) a 70 km to 100 km channel had to be forded or sailed to pass timor or sulawesi. at such a distance, it is difficult to see the other shore. some homo sapiens tribes forded the sea stretch and were successful, probably more than 55,000 to 50,000 years ago. elsewhere, in the glacial phases, a land bridge of at least 2,000 km joined also what is now alaska to the chukchi. it was the beringia continent that has now disappeared. this was wild and wind beaten, and populated by woolly mammoths, and it is currently mostly submerged, with the remains of its ancient dwellers. it might have been 25,000 years ago when the siberian hunters crossed beringia and settled there, then moved on in pursuit of the mammoth and caribou herds, and descended into the great plains of north america. in this new geographical scenario of global peopling and diversification of human groups [cavalli sforza and pievani 2011], we discover that on several occasions we were very lucky from a geophysical point of view. molecular data show a drop in homo sapiens numbers around 70,000 to 75,000 years ago, which coincided with a drop in world temperatures, maybe due to the volcanic winter caused by the catastrophic eruption of toba on the isle of sumatra. this was a global environmental disaster, equivalent to many eruptions of krakatoa all together. we had apparently ended up in what evolutionists call an ‘evolutionary bottleneck’: a drastic reduction in population numbers that led us to the brink of extinction. then the few survivors starting up again after the cataclysm, with a reduced internal genetic diversity. it might have been just a coincidence time-wise, but the limited genetic variation of present-day humans suggests that the founding group was rather small, and that later the human population had to overcome a number of bottlenecks due to environmental crises [huff et al. 2010]. other experts believe that the bottleneck (or one of them) took place earlier in africa, during the ice age that lasted from 190,000 to 123,000 years. due to the changes in the winds and rainfall, glaciation led to increased arid zones in africa. the few homo sapiens may have found refuge on the more welcoming coastlines of the cape in south africa, or at the southern tip of the rift valley. whatever happened, it is clear that our evolution, and even our genetic diversity, were shaped by geophysical global phenomena and geographical constraints, which has been underestimated in evolutionary models to date. the weak but intelligible molecular traces of this history are still present. we see maximal genetic diversity in african huntergatherer populations, with a peak in southern africa [henn et al. 2011], and a general decrease in genetic diversity with the distance from africa [ramachandran et al. 2005]. this is a picture of human genetic variation that is coherent with a process of ‘serial founder effects’ from africa [romero et al. 2009, li et al. 2008]. the same role of geophysical factors is valid for the neolithic revolution and for recent times. the domestication of plants and animals began between 12,000 and 7,000 years ago, after the end of the great cold in the last ice age. with agriculture and animal domestication, the ecosystems spontaneously started producing much more than they geoethics and philosophy of earth sciences had done previously. the increase in numbers led to new movement and dispersion of populations, and hybridizations and conflicts. human populations began increasing at an unprecedented rate, with the development of stable settlements and giving rise to the first urban centers in the course of the millennia. so, the interplay between environmental and geophysical determinants, historical contingencies, and cultural diversities is now a keystone for our understanding of the natural history of the human species and human societies [munoz et al. 2010, diamond and robinson 2010]. as in the case of mass-extinctions produced by global geophysical events in long-term evolution, the timing of our young history as a species was punctuated by major geophysical events. we are the sons of the geophysical and ecological instability of our planet, which produced major macro-evolutionary patterns of biological change [eldredge 1999]. 4. geoethics and our ideas of ‘nature’ as causes of change, geophysical factors and geographical constraints are independent from adaptive micro-evolutionary dynamics of change in genetic frequencies, although they can influence these from higher levels of evolution, shaping, for example, the population structure of a species in its territory. this means first that we need an extended and multilevel theory of evolution, with a neo-darwinian core and a surrounding plurality of new patterns of transformation [okasha 2006, pievani 2012]. however, it also means that the macro-evolution of the several human species was the history of their relationships with geophysical phenomena and contingent events. we were not alone in our history of species, and we were not expected as the ‘mainstream’ of human evolution [pievani 2011]. from a philosophical point of view, this discovery means that we need more evolutionary ‘humility’ and a stronger feeling of belonging to our planet. the expressions of violence and unpredictability of natural phenomena that shock our societies so much today are the normal ecological niches where we were born. we would not be here at this moment without them. so, in what sense do we say that a tsunami, an earthquake, a volcanic eruption, or a hurricane are ‘natural’ disasters? the catastrophic effects of these phenomena, as their usual geophysical power or strengthened by anthropically forced climate change, are measured by the disruption of human artefacts and settlements, and by the consequent loss of human lives. so, if we build a little village under a slope subject to landslips, or we regiment torrents between cement barriers, or we erect nonearthquake proof buildings in high risk territories, the disaster itself will be completely human, cultural, political, social and ethical, but certainly not ‘natural’. the role of geoethics here should be not only the awakening of public opinion to environmental problems, hydro-geological instability, and maintenance of territory, but also a more educated idea of what nature is (the only opportunity we have for making a living) and is not (an indefinite repository of available resources). our idealizations of ‘nature’, as a harmonic eden or a wicked nemesis are frequently our preferred alibi for hiding from our responsibilities. in cases like the trial in l’aquila, italy, in the future, we will have to manage problems of risk assessment and public communication about not only known natural disasters like earthquakes and volcanic eruptions, but also unpredictably violent weather events, such as floods and droughts, or hurricanes and tornadoes. earth scientists have to be careful to communicate their noncertainties in a transparent and authoritative way: geoethics will be more and more an ethics of communication and relationships with the wide audience of science. however, a more general philosophical topic cannot be underestimated in these situations: in what sense do we say that these disasters are ‘natural’? from a natural point of view, they are normal geophysical phenomena, given certain circumstances. the responsibility of the disaster is primarily political and social. the negligence in communication, such as that of mother ‘nature’, should not be our alibi. references castañeda, i.s, s. mulitza, e. schefuss, r.a. lopes dos santos, j.p. sinninghe damsté and s. schouten (2009). wet phases in the sahara/sahel region and human migrations patterns in north 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(2009). the fossil trail, oxford, uk/new york, usa, oxford university press. wood, b., and t. harrison (2011). the evolutionary context of the first hominins, nature, 470, 347-352. corresponding author: telmo pievani, università di milano-bicocca, dipartimento di scienze umane per la formazione, milano, italy; email: telmo.pievani@unimib.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geoethics and philosophy of earth sciences << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 62, 5, se571, 2019; doi: 10.4401/ag-7815 “the characteristics of reflection and transmission coefficients of porous medium saturated with an ideal fluid„ dongyong zhou*,1,2,3, xingyao yin4, zhaoyun zong4 (1) college of geophysics, chengdu university of technology, chengdu sichuan, 610059, china (2) post−doctoral research center for geological resources and geological engineering, chengdu university of technology, chengdu sichuan, 610059, china (3) geomathematics key laboratory of sichuan province, chengdu university of technology, chengdu sichuan, 610059, china (4) college of geosciences and technology, china university of petroleum (east china), qingdao shandong 266580, china 1. introduction the underground rock is a porous medium composed of rock skeleton and pore fluids. the properties of rock are not only related to various mineral properties that make up the rock skeleton, but also influenced by many factors, such as pore shape, pore size, pore fluids filling, and so on. when seismic waves propagating in such a porous medium, they show complex reflection and transmission characteristics, which carry a large number of information reflected reservoirs properties of oil and gas bearing. it is of great significance to study the characteristics of reflection and transmission coefficients at an interface of porous media saturated with fluids, which can be used to identify the reservoirs of oil and gas, analyze the properties and distribution of reservoirs, and improve the exploration and development level of oil and gas fields. since biot [1940, 1956, 1957, 1962] proposed the theory of seismic wave propagation in porous medium, article history received june 19, 2018; accepted september 16, 2019. subject classification: porous medium, an ideal fluid, slow p-wave, reflection and transmission coefficients. abstract the underground rock is composed of rock skeleton and pore fluids. when seismic waves propagate in underground medium, it will show complicated change influenced by pore and pore fluids in rocks. it is very important to study the characteristics of reflection and transmission coefficients of seismic waves at the interface and to analyze the properties of the lithology and pore fluids of porous medium, which can reveal the oil and gas bearing in underground medium. based on the relationship among wave functions, displacement and stress in porous medium, the equation of reflection and transmission coefficients at the interface of porous medium saturated with an ideal fluid is derived. a geological model with sandstone porous medium in the top layer and mudstone porous medium in the bottom layer, the rock skeleton parameters of which vary with porosities, is established. based on the equation and the model, the variation of reflection and transmission coefficients with the incident angle at the interface of porous medium is studied under the conditions of different pore fluids filling and different porosities. the study shows that the existence of the pore and pore fluids will impede the reflected and transmitted abilities of seismic waves at the interface of porous medium. combining with the theory of rock physics and well data, the porosities and pore fluids of porous medium can be identified qualitatively by studying the variation of the reflected fast pand svwaves with incident angle. the values of reflection and transmission coefficients of slow p-wave are very small, but the variation of that is relatively large due to the influence of the pore and pore fluids. many geophysical experts have carried out in-depth research. yin et al. [2014, 2015, 2016] and zong and yin [2017] combined this with pre-stack seismic inversion to realize the fluid identification based on the data and model driven. when the seismic wave impinges perpendicularly at the interface of porous medium, some scholars [geertsma and smit, 1961; silin et al, 2004; silin and goloshubin, 2010] analyzed the reflection and transmission problems. they considered that the reflection and transmission coefficients can be expressed as a low frequency asymptotic formula. however, when the seismic wave impinging obliquely, it will bring more abundant information varying with incident angle, which can help us to realize the high accuracy identification of oil and gas reservoirs. lovera [1987] and mu [1996] studied the boundary conditions and the reflected and transmitted problem with a seismic wave inclining at the interface of porous medium. for some complex reservoirs, such as the carbonate reservoirs and the reef-bank reservoirs, there may be two or more kinds of immiscible fluid in porous media. the types of pore fluids, the volume ratio and the interaction among these fluids will affect the propagation characteristics of seismic waves. tuncay and corapcioglu [1996, 1997] and lo and sposito [2005] thought that there are three kinds of p-waves and a class of sv-wave propagation in the porous medium saturated with two kinds of immiscible fluids. some other scholars [vashisth et al., 1991; tomar and arora, 2006; kumar and saini, 2012, 2016; kumar and sharma, 2013] had carried out a series of in-depth studies on the reflected and transmitted characteristics of this type medium. a lot of work had been done on the theoretical research of porous medium. however, the characteristics of reflection and transmission coefficients at the interface of porous media saturated with an ideal fluid still need to be furtherly discussed. the difference of the reflected and transmitted characteristics of seismic waves at the interface has not been considered yet under the situations of different pore fluids filling. the variation of porosity is not only directly responsible for reflection and transmission coefficients, but also indirectly influencing their values by affecting rock skeleton. how to establish a comprehensive relationship between the porosity and the coefficients is still a problem to be solved. in this paper, the equation of the reflection and transmission coefficients at an interface of porous medium saturated with an ideal fluid is derived, whose expression is more concise and geophysical meaning is more clear. based on previous researches, a geological model characterized by the porosity is established with separating a sandstone porous medium half space and dense mudstone porous medium half space. combining coefficient equation with geological model, the variation characteristics of the reflection and transmission coefficients with incident angles are analyzed under different conditions of porosities and pore fluids filling, respectively. some meaningful conclusions are obtained in numerical analysis, which provides theoretical support for the identification of oil, gas and water in reservoirs. 2. formulation of the problem 2.1 reflection and transmission we consider the reflection and transmission problem with a seismic wave impinging obliquely at the interface of porous medium saturated with an ideal fluid. the seismic waves generated at the interface include reflected fast p-wave 𝜑11, reflected slow p-wave 𝜑12, reflected sv-wave 𝜑13, transmitted fast p-wave 𝜑21, transmitted slow p-wave 𝜑22, and transmitted sv-wave 𝜑23 (figure 1). the reflected and transmitted angles of all waves are 𝜃11, 𝜃12, 𝜃13, 𝜃21, 𝜃22, 𝜃23, respectively. 2.2 wave functions assuming that the incident wave 𝜑0 with its angle 𝜃0 is a fast p-wave (figure 1), the functions of the incident, reflected and transmitted waves can be expressed as follows, respectively. zhou et al. 2 figure 1. the reflection and transmission at the interface of porous medium saturated with an ideal fluid. 3 seismic wave analysis of porous media incident seismic wave, φ0=a0 exp�iω�t-k11 x-p11z��. (1) reflected seismic waves, φ1j=a1j exp �iω�t-k1j x+p1jz��, j = 1,2,3. (2) transmitted seismic waves, φ2j=a2j exp �iω�t-k2j x-p2jz��, j = 1,2,3, (3) (4) k= k11 = k12 = k13 = k21 = k22 = k23. (5) where 𝑖, 𝜔, k are the imaginary part, the frequency of incident wave and the horizontal slowness, respectively. 𝐴0, 𝐴𝑙𝑗, 𝑙 = 1,2,𝑗 = 1,2,3 are amplitudes of the corresponding incident, reflected and transmitted waves, respectively. 𝑣𝑝𝑙𝑗, 𝑙 = 1,2,𝑗 = 1,2 represent the velocities of p-wave, and 𝑣𝑝𝑙3, 𝑙 = 1,2 represent the velocities of sv-wave. 𝑙 = 1,2 denote the top and bottom porous media, respectively. 𝑗 = 1,2,3 denote the fast p-, slow pand sv-waves, respectively. 3. reflected and transmitted theory 3.1 displacement and stress in the x-z plane, the wave’s functions of fast p-, slow pand sv-waves are used to describe the displacements of rock skeleton and pore fluids relative to rock skeleton (equations 6, 7 and 8). the total stress forcing on rock skeleton is partly derived from the interaction force between rock skeletons, and the other part comes from the static pressure of pore fluids (equations 9, 10 and 11). (6) (7) (8) where 𝜑1, 𝜑2, ψ are wave functions of fast p-, slow pand sv-waves, respectively. 𝑢𝑠𝑥 and 𝑢𝑠𝑧 represent displacements of rock skeleton along the directions of x and z, respectively. 𝑤𝑓𝑧 is the displacement of pore fluids relative to rock skeleton along the direction of z. 𝛾𝑗,𝑗=1,2, γ1 represent the ratios of seismic wave’s amplitudes of pore fluids relative to rock skeleton corresponding fast p-, slow pand sv-waves, respectively. (9) p = [– αm ∇·us – m ∇·wf ] i, (10) τt = τs – α·p. (11) where, τs is the stress tensor of rock skeleton, p represents the effective pressure of pore fluids, τt denotes the total stress tensor acting on rock skeleton. 𝐾 and 𝜇 are the bulk and shear moduli of rock skeleton, respectively. us = [𝑢𝑠𝑥, 𝑢𝑠𝑧]𝑇 is the displacement tensor of rock skeleton, and wf = [𝑤𝑓𝑥, 𝑢𝑓𝑧]𝑇 is the displacement tensor of pore fluid relative to rock skeleton. 𝛼 and 𝑀 are the biot parameters, and i is an identity tensor. superscript 𝑇 denotes the transposition of a matrix or a vector. 3.2 boundary conditions the boundary conditions at the interface of porous medium are determined by the existing physical situations. the continuity equation for fluid-flow and the energy conservation at the interface are the fundamental requirements [sharma, 2008]. the equation energyconservation can be expressed as follows, u̇1sz · τ1tz + u̇1sx · τ1tx + ẇ1fz · �̄�1z = u̇2sz · τ2tz + + u ̇2sx · τ2tx + ẇ2fz · �̄�2z . (12) assuming that the interface of porous medium is welded contact, and the pores of up and bottom media are fully connected, the appropriate boundary conditions to be satisfied are, u̇1sz= u̇2sz, u̇1sx= u̇2sx , ẇ1fz= ẇ2fz , τ1tz = τ2tz , τ1tx = τ2tx, �̄�1z = �̄�2z . (13) where 𝑢𝑙𝑠𝑥, 𝑢𝑙𝑠𝑧, 𝑙 = 1,2 are displacements of rock skeleton along the directions of x and z, respectively. w𝑙𝑓𝑧, 𝑙 = 1,2 are displacements of pore fluids relative to rock skeleton along the direction of z, and the superscript ‘ . ’ denotes partial time derivative. τ𝑙𝑡𝑥,τ𝑙𝑡𝑧, �̄�lz,𝑙 =1,2 are the total stresses tensor acting on rock skeleton along the directions of x and z, the effective pressure of pore fluids along the direction of z, respectively. 𝑙 =1,2 represent the porous media of the top and bottom layers, respectively. klj = ,plj = l = 1,2,j = 1,2,3, sinθlj vplj cosθlj vplj wfz =� γj + г1 .∂φj ∂z ∂ψ ∂x 2 j=1 τs = k (∇·us) i +μ �∇us+ (∇us)t – (∇·us) i�,2 3 usx =� – .∂φj ∂x ∂ψ ∂z 2 j=1 usz =� + .∂φj ∂z ∂ψ ∂x 2 j=1 3 zhou et al. 4 3.3 coefficient equations taking the functions of incident and reflected waves (equations 1 and 2) into the expressions of displacement and stress (equations 6-11), the boundary conditions for top medium can be expressed as equation 14. similarly, that for bottom medium also can be obtained in the equation 15. t (0) = �u1̇sz u1̇sx ẇ1fz τ1tz τ1tx �̄�1z�t = (iω)2 · {ip a0 + b · a}, (14) t'(0) = �u̇2sz u̇2sx ẇ2fz τ2tz τ2tx �̄�2z�t = (iω)2 · c · a, (15) a = �a11 a12 a13 a21 a22 a23�t. (16) combining equations 13, 14 and 15, the equation of reflection and transmission coefficients represented by the ratio of seismic wave’s amplitudes can be deduced, (b – c)r = – ip, (17) (18) the coefficients solved above are relative to the amplitudes of seismic waves. using the relation between the ratio of displacement amplitudes r and the ratio of seismic wave’s amplitudes r, the equation of reflection and transmission coefficients can furtherly be modified as follows. for incident sv-wave, we only need to replace ip with is in the equation 19. (b c)d –1r = – ip, (19) (20) where, the symbol diag[ ] denotes diagonal matrix, and the specific expression of the parameters b, c, ip, is in equation 19 can be referred to appendix a. 4. porous medium model for furtherly analyze the influence of the porosity, pore fluids and incident anlge on reflection and transmission coefficients, a porous medium model and its equivalent model are established. the porous medium saturated with a fluid is separated by a plane surface with unconsolidated sandstone in the top layer and tight mudstone in the bottom layer. the detailed description is as follows. 4.1 rock skeleton and pore fluid parameters for porous medium, the change of porosities will lead to the variation of the bulk and shear moduli of rock skeleton. the quantitative relationship among the porosity, the bulk moduli of rock skeleton and mineral grain are described by equation 21, and the similar relationship for the shear moduli is shown in equation 22 [walton, 1987; pride and berryman, 2003; zhao et al.,2015]. kd = (1.0 – f ) ks �(1.0 +c · f ) , (21) μd = (1.0 – f ) μs�(1.0 +c · f ) . (22) where, 𝐾𝑠 and 𝜇𝑠 are the bulk and shear moduli of mineral grain [tosaya, 1982; carmichale, 1989; blangy, 1992], as shown in table 1. 𝐾𝑑 and 𝜇𝑑 are the bulk and shear moduli of rock skeleton, and 𝑓 is porosity. the parameters 𝑐 for sandstone and mudstone media are 6 and 4, respectively. the bulk and shear moduli of rock skeleton varying with porosities are as shown in figure 2. the red and green lines represent rock skeletons of porous media for sandstone in the top layer and for mudstone in the bottom layer, respectively. the existence of pore fluids in underground medium will have an important effect on the propagation of seismic waves. in order to analyze the influence of different pore fluids on the characteristics of reflection and transmission coefficients, the bulk modulus and density of gas, oil and brine are given in table 2 [tuncay and corapcioglu, 1996; rubino and holliger, 2012; zhao et al., 2015]. r = � � t . a11 a12 a13 a21 a22 a23 a0 a0 a0 a0 a0 a0 d = diag � �.vp11 vp11 vp11 vp11 vp11 vp11 vp11 vp12 vp13 vp21 vp22 vp23 properties ks (gpa) μs (gpa) ρs (kg / m3) quartz (up) 37 44 2650 clay (bottom) 25 9 2550 table 1. mineral grain parameters. here, ρs denotes the density of mineral grains. pore fluids gas oil brine k𝑓 (gpa) 0.012 2.1 3.0 ρ𝑓 (kg / m3) 78.0 940 1050 table 2. pore fluid parameters. where, k𝑓 and ρ𝑓 represent the bulk modulus and density of pore fluids, respectively. 5 seismic wave analysis of porous media 4.2 equivalent model for verifying the correctness of the equation and comparing the difference of the coefficients between the equivalent and porous media, a solid model which is equivalent to the porous medium model with sandstone (the porosity is 0.3) in top layer and mudstone (the porosity is 0.05) in bottom layer is established. the pore fluids filled in both top and bottom layers are all brine. the parameters are shown in table 3. 5. numerical results and discussion the pore shape, pore size and the properties of pore fluids all affect the propagation of seismic waves in porous medium. studying and identifying pore fluids filling from underground medium is the ultimate aim of oil and gas exploration. basis on this consideration, the characteristics of reflection and transmission coefficients at the interface of porous medium under different pore fluids filling and different porosities are analyzed, respectively. 5.1 different pore filling based on the model established in the fourth part, the porosities are set 0.3 in the top layer, and 0.05 in the bottom layer. the pore fluids filling are gas, oil and brine in the top layer, and brine in the bottom layer, respectively. figure 3 shows the variation of reflection and transmission coefficients with incident angle under the conditions of gas, oil and brine filling, respectively. in figure 3, (a), (c) and (e) are reflection coefficients of fast p-, svand slow p-waves, respectively. (b), (d) and (f) are transmission coefficients of fast p-, svand slow p-waves, respectively. the red, green and blue lines represent reflection or transmission coefficients under the conditions of gas, oil and brine filling, respectively. the black line denotes the coefficients obtained from the equivalent model. from figure 3, we can draw the following conclusions. (1) compared with reflection and transmission coefficients of the pand sv-waves in the equivalent medium (black lines in figures 3a-3d), the values of reflection and transmission coefficients of the fast pand sv-waves in the porous medium (red, green and blue lines in figures 3a-3d) are reduced, that is, the existence of pore fluids hinders the reflected and transmitted ability of seismic waves at the interface. (2) the bulk modulus (and density) of gas is quite different from that of oil or brine (table 2), which leads to different characteristics of reflection and transmission coefficients at the interface of porous medium (figures 3a-3d). this phenomenon can be used to identify gas bearing reservoirs. (3) the values of reflection or transmission coefficients of slow p-wave are very small, and the corresponding coefficients under the conditions of different pore fluids filling also have obvious differences (figures 3e and 3f). 5.2 fferent porosities based on the model established in the fourth part, we also set the brine filling in both top and bottom porous media. the porosities are, respectively, 0.1, 0.2 and 0.3 in the top layer, and 0.05 in the bottom layer. figure 2. the variation of the bulk modulus (a) and shear modulus (b) of rock skeleton with porosities. properties vp (m/s) vs (m/s) ρ (kg / m3) sandstone 5592 3767 2170 mudstone 3390 1625 2523 table 3. equivalent medium model. where, vp, vs and ρ rep− resent equivalent p−wave velocity, s−wave velocity and density, respectively. figure 4 displays the variation of reflection and transmission coefficients with incident angle at the interface of porous medium under the conditions of different porosities. in figure 4, (a)-(f) are the same as in figure 3. the red, green and blue lines represent the coefficients under the porosities of 0.1, 0.2 and 0.3, respectively. the black line denotes the coefficients obtained from the equivalent model. according to figure 4, some meaningful conclusions can be obtained. (1) the volume of pore fluids will expand and the absolute values of reflection and transmission coefficients of fast pand sv-waves will decrease at the interface with the increasing of porosities (incident angles below 30 degree in figure 4a). that is to say, higher porosities can further weaken the reflected and transmitted capability of fast pand svwaves, which again confirms the conclusions obtained in figure 3. (2) within the range of incident angle less than 45 degrees, the reflection coefficient of fast pwave corresponding to the high porosity sandstone (blue line in figure 4a) is basically unchanged, while that corresponding to the low porosity sandstone varies greatly (red line in figure 4a) with incident angle, and the characteristics of the reflected sv-wave also have zhou et al. 6 figure 3. the variation of reflection and transmission coefficients with incident angle under the conditions of different pore fluids filling the similar phenomenon. we can qualitatively identify the porosities of sandstone porous medium using the variation characteristics of reflection and transmission coefficients of fast por sv-waves with incident angle. (3) due to the existence of the pore and pore fluids, the reflected and transmitted slow p-waves are produced at the interface of porous medium. although the values of reflection and transmission coefficients of slow p waves are very small, the influence of porosities on that is relatively large (figures 4e and 4f). conclusion the variation characteristics of reflection and transmission coefficients with incident angle at the interface of porous medium saturated with an ideal fluid are studied. considering the influence of porosities on the parameters of rock skeleton, a porous medium model, the bulk and shear moduli of rock skeleton of which vary with porosities, is established. the numerical analysis shows that the increase of porosity and the existence of pore fluids will reduce the values of reflection and transmission coefficients, and weaken the reflected 7 seismic wave analysis of porous media figure 4. the variation of reflection and transmission coefficients with incident angle under the conditions of different porosities. and transmitted abilities of fast pand sv-waves at porous medium interface. the characteristics of reflection coefficients of the fast pand sv-waves corresponding porous medium saturated with gas or oil are obviously different, which can be used to identify gas and oil (or brine) bearing in the reservoirs. the porosities of porous sandstone medium can be qualitatively distinguished by reflected pand sv-wave. we would like to acknowledge the sponsorship of china postdoctoral science foundation (2019m663452), opening fund of geomathematics key laboratory of sichuan province (scsxdz2019yb08). we also are grateful to the anonymous reviewers for their constructive comments. references biot m.a. 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(1987). the effective elastic moduli of a random pack of spheres, j. mech. phys. solids 35, 213-226. yin x.y., d.p. cao, b.i. wang, z.y. zong (2014). research progress of fluid discrimination with pre-stack seismic inversin, oil geophys. pros. 49(1), 22-34, 46. zhou et al. 8 yin x.y., z,y. zong, g.c. wu (2015). research on seismic fluid identification driven by rock physics, sci. china earth sci. 58,159-171. yin x.y. and x.x. liu (2016). research status and progress of the seismic rock-physics modeling methods. geophys. prospec. petroleum 55(3), 309325. zhao l., x. han, d.h., yao, q.l., r. zhou, f. yan (2015). seismic reflection dispersion due to wave-induced fluid flow in heterogeneous reservoir rocks, geophysics 80(3), d221-d235. zong z.y. and x.y. yin (2017). model parameterization and p-wave ava direct inversion for young’s impedance. pure appl. geophys. 174(5), 1965-1981. *corresponding author: dongyong zhou, college of geophysics, chengdu university of technology, chengdu sichuan, 610059, china, email: zhou_dongyong@163.com © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 9 seismic wave analysis of porous media layout 6 1 annals of geophysics, 62, 1, vo11, 2019; doi: 10.4401/ag-7673 “the value of historical documents for risk reduction: the 1600 huaynaputina eruption (peru)„ maria eugenia petit-breuilh sepúlveda*,1 (1) department of american history, the university of sevilla, sevilla, spain 1. introduction the reduction of natural risks is a social research objective whose interest has significantly increased through time. undoubtedly, to know more details about explosive eruptions that caused severe damage to population and infrastructures is a topic of great interest. this type of research also has relevance when we try to assess the planetary aftermaths of these catastrophic events, such as climatic changes that may altered the daily life of societies also far away from the place where the eruption occurred. this is the case of the 1600 ad eruption of the huaynaputina volcano that is considered the most explosive eruption recorded in the andes, in the last five hundred years of history [thouret et al., 2002]. this research is based exclusively on the analysis of historical documentation and is complementary with the studies based on archaeology, volcanology and historical climatology [de silva and zielinski, 1998; thouret et al., 2002; labazuy et al., 2018]. when analysing the effects of an explosive eruption, it is necessary to take into account the reaction of the population that was affected, since their actions and behaviours will serve as elements to calibrate the criteria to be adopted to interpret the historical descriptions. following this methodology, data will not only validate aspects related to the eruptive phenomenon, but also will provide information on the behaviour and mentality of the affected society. bouysse and bouysse-cassagne [1984] were the first historians to recognise the importance of this eruption, and to analyse the behaviour of spanish and indigenous people. they stated that the huaynaputina activity was interpreted by the hispanics as a punishment for their sins, and by the natives as a powerful act of their ancient gods aimed to destroy the spanish. in a previous study, i referred to this eruption and its regional impact [petit-breuilh, 2004a], whereas details were discussed in a successive work [petit-breuilh, 2016]. lavallé [2011] discussed the reactions of the inhabitants and religious authorities of arequipa at the beginning of the 17th century, including some details article history receveid february 28, 2018; accepted february 27, 2019. subject classification: 1660 ad huaynaputina plinian eruption (peru); historical sources; risk implications. abstract on the 14th of february of 1600, one of the most explosive eruptions occurred in historical times in south america (vei 6) plunged the city of arequipa (peru) and surrounding villages into a severe crisis, with long lasting dramatic economic consequences. the eruption was produced by the huaynaputina dacitic stratovolcano (16º37’s-70º51’w), although many historical sources attribute the activity to the volcano of arequipa or omate, among others. this eruption also produced alterations in the global climate, between 1601 and 1603. the aim of this research is to demonstrate how historical documents and an adequate working methodology, may aid to reduce disasters. of the huaynaputina eruption, but associating their behaviour with previous catastrophic earthquakes experienced in the area. petit-breuilh [2016] suggested that the idea that the spanish and indigenous population of arequipa had about the origin of the disaster produced a series of behaviours on the society. some personages had to assume the organization of their respective groups and to take charge of the material and economic reconstruction of the city in a moment of generalized crisis. on the other hand, the fear felt by the people and how they expressed it leads to suggest that the providentialism of the time is the answer that helps to understand the celebration of various rituals and ceremonies to appease the “wrath of god” [petit-breuilh, 2001]. the indians also had reactions adjusted to their beliefs, because for them as for the spanish, the ruin that caused the huaynaputina eruption was a “divine punishment”, by their ancestral gods such as tice-viracocha or tunupa [bouysse cassagne, 1987]. they believed that all disasters were a punishment because the andean communities had abandoned their ancestral beliefs and no longer fulfilled their cults, as required by the tradition through the shamans, generally called “sorcerers” in the documentation. this information is relevant for the study of extreme disasters since the analysis of the worst possible scenarios is considered from an empirical point of view and not from a theoretical one, as for mathematical models. in our case, the possibility of an explosive eruption is not studied but a real fact is considered, that can be verified through the historical documentation translated into a current scientific language [petitbreuilh, 2013]. the sources used to identify the characteristics of huaynaputina’s 1600 eruptive phases and the behaviour of the society of the time are documents of the archivo general de indias (seville), of the vatican apostolic library, as well as chronicles, reviews and reports of missionaries who lived in the affected area. all these documents allowed to reconstruct the facts and to check the available information. sketches of the time, historical cartography and toponyms of the region were also used, in order to define exactly the gemaria eugenia petit-breuilh sepúlveda 2 figure 1. isopac map of the eruption of the huaynaputina volcano of 1600 (sinpadindeci. gob.pe). ographical details of what was described in the sources. to constrain historical data more carefully, multidisciplinary researches were carried out. volcanologists and regional geologists evaluated this extraordinary natural event to recognize the local and planetary effects of this eruption; architects and art historians evaluated better the characteristics of the damaged infrastructure. data used for this research were obtained from the quoted documentary sources (handwritten and printed). it must be considered that the majority of researchers who have studied this eruption have only used the information obtained from chronicles and printouts that are contemporary with the event, whereas this work presents details of the eruption cycle, the caused damages and the identification of the disaster managers of that time, obtained from the manuscripts of several repositories, especially the general archive of the indies of seville (that preserves the official material of the spanish monarchy in america between the 16th and 19th centuries), the royal academy of the history of madrid and the secret vatican archive. 2. huaynaputina eruption in the sources in the first records, that also include the historical cartography, the region was also identified with the name of omate or volcano of the ubinas, quinistaquillas or chequeputina, that mean volcano of bad luck or agüero [amanqui, 2006]. this is the reason why there is some historical confusion over time to determine exactly the vent of this eruption. the town most affected by the catastrophic event was arequipa together with several indian villages in its surroundings. for this reason and because a volcano with the same name is very close, in some contemporary chronicles the activity was erroneously attributed to the volcano of arequipa or misti and this mistake was transferred in later records (xix and xx centuries). from this urban nucleus, the most shocking stories were written due to the proximity to the active crater and to fact that many survivors of the neighbouring villages moved to arequipa in search of refuge. the 1600 ad plinian eruption generated by a dacitic stratovolcano, lasted for more than one month in its most explosive phase and the worst moments were experienced from february 19 to february 21 (plinian phase), from february 26 to march 4, and ending with a fine ash fall on march 15. during this time, at least three pyroclastic flows were generated, and at least seven indian villages of the surrounding area were buried by them. this eruption is comparable in its general climatic impact to those of laki, iceland in 1783, tambora, indonesia in 1815 and krakatoa, indonesia in 1883, much better known and studied. the ash of the eruption reached europe, causing global climate changes [génova, 2012], and its effects were felt in the low temperatures recorded in 1601 in china and korea [fei et al., 2016]. more than 500,000 casualties in russia are attributed to this eruption [verosub and lippman, 2008]. by analysing these data obtained from historical sources, it is possible to identify at least four main explosive phases during the 1600 ad eruption: 1. february 19-20: which corresponds to the plinian phase with generation of ash fallout and pyroclastic flows. 2. february 21-22: new explosions with generation of pyroclastic flows. 3. february 25-28: explosions with generation of pyroclastic flows. it is specified in the documents that on 25 february the banded pumice emission were identified for the first time during the eruption. possible onset of phreatomagmatic activity [thouret et al., 2002]. 4. march 4-5: new explosions with generation of pyroclastic flows. 3. the photography of the time the only image of the time is a sketch of the plaza mayor of arequipa (figure 2) where most rituals and religious ceremonies were held to placate the “wrath of god” in february and march 1600. it highlights the profile of the old major church of arequipa (1558) destroyed during this eruption and rebuilt in the middle of the xvii century. this sketch tells us how this eruption was perceived in the city of arequipa, as in its caption it is clearly expressed that the darkness was caused by dust in suspension due to the pumice and ash emitted by the plinian explosion and by several pyroclastic flows: “the city of ariquipa: the volcano burst and covered the city and its jurisdiction and districts with ash and sand; in thirty days the sun was not seen, nor the moon, nor the stars” [guamán poma de ayala, 1615]. 3 the value of historical documents for risk reduction maria eugenia petit-breuilh sepúlveda 4 date description feb. 14-15 precursory earthquakes were felt in the villages near arequipa. feb. 18 at 19:00 tremors began. near 21:00 the earthquake magnitude and frequency increased; some walls fell and the inhabitants of the city evacuated their houses. there were loud noises and rumblings during the night. feb. 19 the tremors continued, some of great intensity, with thunderous noises. starting at 5:00 p.m., the city became dark and an abundant fall of pumice with “the size of a mustard” began in arequipa and surrounding areas. the population remained outside their homes. some houses fell by the earthquakes and by the weight of the ashes that accumulated throughout the afternoon-night. pyroclastic flows buried some indian villages. lightning and “falling stars” were observed in the eruptive column. “fireballs” on the city. feb. 20 darkness due to the tephra suspension in air. the inhabitants of arequipa had to quickly discharge ash from the roofs of their houses to prevent roof collapses. at 14:00 the situation worsened: “it was a so dark night that no one could recognize the encountered persons”. at 16:00 sky somewhat clarified, but then pyroclastic fallout began lasting for three hours. no water supply to the city because of the accumulated and still falling ash. the thickness of the pumice was estimated that afternoon to about 7-8 centimetres. feb. 21 because of darkness every think appeared as “closed in a colour between red and pale, which made it look horrible”. ash fell for three hours. feb. 22 pale red dawn and continuous fallout of fine pumice from 9:00 a.m. to 3:00 p.m. there were many tremors. in arequipa, thickness of accumulated ash was estimated to about 21 cm. the roofs of some houses collapsed. feb. 23 some clarity, persistence of fine ash fall and tremors. in the whole city the weight of the fallen ash caused the collapse of some roofs and those still standing had to be unloaded. feb. 24 more clarity, no ash fall; tremors continued. feb. 25 the cloudy day was so dark that they needed light to see. at 20:00 it rained with “fine earth”, probably ash mixed with rainwater. banded pumice was firstly described. part of the structure of the cathedral of arequipa collapsed. feb. 26 dawn was very dark and they had to bring lights to see each other. surely on this day pyroclastic flows were emitted from huaynaputina. a lot of fine ash fell in arequipa and there were tremors and loud noises of volcanic explosions. in some parts of the city the ash reached the people waist. inhabitants had to unload ash from rooftops to avoid collapses. feb. 27 some clarity by the day and finally it was learned that the origin of the disaster was an eruption of huaynaputina volcano. fine ash fallout and tremors continued. at 16:00 explosive activity increased again. feb. 28 some clarity. at 3:00 p.m., the day became darker due to ash suspension in air. at 17:00 the air was clearer but ash fallout continued. there was a strong earthquake and a new explosion. feb. 29 the sun was seen. fine ash fallout continued. mar. 1-3 somewhat clearer days. fine ash fallout continued. mar. 4 clear at dawn, but no visible sun. there was a lot of ash. a new pyroclastic flow buried indian peoples. mar. 5 deep darkness for ash suspension in air. tremors. mar. 6 fallout of fine ash and tremors. mar. 7-15 fine ash fallout. table 1. chronology of the 1600 ad eruption from historical data collected in arequipa (56 km from the volcano). 5 the value of historical documents for risk reduction 4. damages in the regional economy the most important economic activity in the region prior to the 1600 eruption was agriculture, especially vineyard farms and livestock production, with native animals such as llamas and alpacas and those brought from europe such as cows, sheep, horses, mules and donkeys that were also used to carry different goods from the audiencia de charcas to lima or to the coast of arica. the first chronicler of the area, pedro cieza de león (1540-1550) wrote: “in this area an excellent wheat is produced, of which they make very good and tasty bread” [cieza de león, 1984]. martín de murúa also pointed out that at the beginfigure 2. an historical drawings, which allows us to reconstruct an event from the past and clarify what actually happened. ning of the 17th century in this andean region there were “many ranches of castile, sheep and cattle. “it fell in great measure from its prosperity with the ruin of arequipa, which was a business of many rich men that became very poor” [murúa, 1987]. it is evident that one of the main consequences of the explosive activity of the huaynaputina was the damage in the harvests and the death of the cattles; especially considering that the ash is highly harmful to animals because of its abrasiveness and high content of silica. 5. processes associated with volcanic disasters one of the most dangerous phenomena occurred after the eruption were the lahars generated by rain mobilization of loose tephra in the valleys and ravines that, enabled the possibility of short term economic recovery of the region. the chronicler bernabé cobo said: “it highlights the remarkable fury with which these large maria eugenia petit-breuilh sepúlveda 6 figure 2. fragment of the map of peru by nicolas sanson d’abbeville (ca. 1645). of particular note are the cities of lima or ciudad de los reyes, callao and cusco to the northeast, and the port of arica southern in the pacific coast. huaynaputina volcano is located in the mountain area between arequipa and lake titicaca. floods were produced, composed of ash and a very subtle and soft dust. they devastated many productive lands that now are no longer useful” [cobo, 1964]. there were also several reservoirs of stagnant water with volcanic materials, tree trunks and organic matter that with the passage of time broke and surprised the neighbours causing again damages and even the death of some people. cobo [1964] described these processes as follows: “another wave or flood snatched a man, and rammed him with his rapid current into a deep, though narrow, lagoon”. moreover, due to the large number of dead animals a great pestilence was produced and the real threat of an epidemy arose; in fact, many people became ill due to the environmental conditions to which they were exposed after the volcanic eruption. in a letter preserved by the jesuits it is written: “cows have died by the five hundreds. what i fear and what some doctors believe is that it may be the effect of some plague or of the ingestion of ash. we are all like asthmatic. in addition, the large number of dead animals makes us fear worse effects” [egaña and fernández, 1986]. after months and years of misery in arequipa, because of the consequences of the eruption, many neighbours died of hunger; facing this reality some decided to emigrate to other regions losing all faith in recovering their crops, livestock and material goods. most of them were getting sick without any comfort. thus a member of the society of jesus reflected in 1602: “for all these evils many are losing faith and hardening their hearts” [fernández, 1981]. 6. reaction of the indigenous people in the indians community there must have been much concern during the explosive eruption of the huaynaputina, as several of its villages were in the vicinity of the volcano and some were literally buried by pyroclastic flows and by pumice and ash fallout. the disappearance of at least six villages of indians: “omate, lloque, tarata, colaña, chec and quinistaca” had certainly severe mental and behavioural consequences that have been little explained in the sources that are almost all spanish. one of the few information is that of vazquez de espinoza, who points out that “los indios al principio de la tempestad se retiraron a un alto cerro, muchos de los cuales se decían idolatraban en él y le hacían sacrificios en él al demonio, ofreciéndole en tiempos indios que echaban en el volcán para que los tragase” [vázquez de espinoza, 1992]. this information confirms the maintenance of altitude worship and the practice of human sacrifices in the andes, especially in active volcanoes at the end of the 16th century [petitbreuilh, 2006]. according to the sources, it is estimated that more than two hundred people were killed by the pyroclastic flows of the huaynaputina eruption. some survivor indians came to arequipa a few days later and related their experiences [cobo, 1964]. this theme is of high interest in arequipa; on august 30, 2015 it was reported in the local press that “the people of 17 villages of indians were buried by the ashes of huaynaputina in 1600”. the project “huayruro: peruvian pompeii” has just begun and aims to recover the remnants from the villages of: quinistacas, omate, coporaque, quinistaquillas, tassata, escobaya, hanvasi, cupilaque, coalaque, yamana, acambaya, jurama, laji, chica, lloque, colona and checa, “where more than fifteen hundred people died” [labazuy et al., 2018]. 7. conclusions at present it is necessary that interdisciplinary studies on volcanic eruptions become a reality and not just a good wish. especially the volcanology would deserve this dialogue between different disciplines as the results could have a great utility, since they can contribute to save lives and to improve territorial planning in areas prone to volcanic risk. for more than twenty years i have been able to verify that the study of toponymy, historical cartography and the critical analysis of historical sources, allow improving the historical eruptive chronology of active volcanoes and with this the knowledge of the risk of a vulnerable region is broadened. through the historical analysis it has been corroborated that the plinian eruption of the huaynaputina volcano that began in february 1600 has been the most explosive in the last four hundred years (vei 6), at least in south america, and that it would be necessary to use the available information for the territorial planning of the region. at present, the traces of this extreme event are still evident and the data of several scientific disciplines confirm that it had important effects on the global climate between 1601 and 1603. finally, the historical documentation provides rele7 the value of historical documents for risk reduction vant information to understand the volcanic processes and the effects that they produce on the exposed population and infrastructures. history can not be overlooked for the study of this type of natural processes which, as in the case of the huaynaputina eruption in 1600 ad, have a global impact, altering the climate in vast regions of the planet. acknowledgements. this study has received financial assistance from the andalusian research plan of the junta de andalucía granted to the research group “mentality, society and environment in andalusia and ibero-america in the modern age” (hum-785). references alcedo, a. de (1789). diccionario geográfico-histórico de la indias occidentales o américa. 5 tomos, madrid, imprenta de manuel gonzález. adams, n., s. de silva, s. self, g. salas, s. schubring, j. permenter, and k. arberman (2001). the physical volcanology of the 1600 eruption of huaynaputina, southern peru, bull. volcanol., 62, 493-518. doi.org/10.1007/s0044500000105. amanqui, y. (2006). mentalidad ante los terremotos en la arequipa colonial, arequipa, universidad nacional de san agustín. http://www.bvirtualunsa.edu.pe/edicion 7/9-historia-7-amanqui-pdf. bouysse, p. and t. bouysse-cassagne (1984). volcan indien, volcan chrétien á propos de l`éruption du huaynaputina en l`an 1600 (pérou méridional). journal de la société des américanistes, 70, 43-68. bouysse cassagne, t. (1987). de empédocles a tunupa: evangelización, hagiografía y mitos. saberes y memoriasen los andes, paris, édition de l’iheal, 157-212. calancha, a. and b. de torres (1972). crónicas agustinianas del perú (primera mitad del siglo xvii), vol. 1. madrid, consejo superior de investigaciones científicas. cieza de león, p. (1984). la crónica del perú (15401550). crónicas de américa, núm. 4, madrid, historia 16. cobo, b. (1964). historia del nuevo mundo en obras del padre bernabé cobo de la compañía de jesús. biblioteca de autores españoles, vol. 1, madrid. de silva, s. and g. zielinski (1998). global influence of the ad 1600 eruption of huaynaputina, peru. nature, 393, 455-458. doi.org/10.1038/30948. egaña, a. de and e. fernández (1986). monumenta peruana, vol. vii (1600-1602). apud institutum historicum societatis iesu, romae, 1986. signatura: ordini rel. ii, gesniti 6, nº 128. fei, j., d. zhang, and h. lee (2016). 1600 ad huaynaputina eruption (peru), abrupt cooling, and epidemics in china and korea. hindawi publishing corporation, advances in meteorology, article id 3217038, 1-12. doi.org/10.1155/2016/3217038. fernández, e. (1981). monumenta peruana, vol. viii (1603-1604). apud institutum historicum societatis iesu, romae. signature: ordini rel. ii, gesniti 6, nº 120. génova, m. 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(2001). conjuros y exorcismos ante maria eugenia petit-breuilh sepúlveda 8 los desastres naturales en hispanoamérica (siglos xvi-xviii). gonzález cruz, d. (ed.) ritos y ceremonias en el mundo hispano durante la edad moderna. huelva, universidad de huelva, 331351. petit-breuilh, m.e. (2004a). la historia eruptiva de los volcanes hispanoamericanos (siglos xvi al xx). el modelo chileno. serie casa de los volcanes, islas canarias, servicio de publicaciones del exmo. ayuntamiento de lanzarote. petit-breuilh, m.e. (2004b). desastres naturales y ocupación del territorio en hispanoamérica. huelva, servicio de publicaciones de la universidad de huelva. petit-breuilh, m.e. (2006). naturaleza y desastres en hispanoamérica. la visión de los indígenas, madrid, sílex ediciones. petit-breuilh, m.e. (2007). la concepción científica de la dinámica terrestre en los cronistas de indias en orbis incognitvs. avisos y legajos del nuevo mundo, universidad de huelva, 405-420. petit-breuilh, m.e. (2013). la investigación histórica sobre la actividad volcánica de la edad moderna en hispanoamérica en alberola. armando (coord.) clima, naturaleza y desastre. españa e hispanoamérica durante la edad moderna, universidad de valencia, 169-191. petit-breuilh, m.e. (2016). miedo y respuesta social en arequipa: la erupción de 1600 del volcán huaynaputina (perú). obradoiro de historia moderna, 25, 67-94. doi.org/10.15304/ohm25.3154. simkin, t. and l. siebert (1994). volcanoes of the world. geociences press, tucson, smithsonian institution. sinpad-indeci (2016) sistema de información nacional para la respuesta y rehabilitación. lima, instituto nacional de defensa civil. thouret, j.c., e. juvigné, a. gourgaud, p. boivin, and j. dávila (2002). reconstruction of the ad 1600 huaynaputina eruption base on the correlation of geologic evidence with early spanish chonicles. j. volcanol. geotherm. res., 115, 529-570. vázquez de espinoza, a. (1992). compendio y descripción de las indias occidentales (segunda parte). madrid, tomo 68b, historia 16. verosub, k. and j. lippman (2008). global impacts of the 1600 eruption of peru’s huaynaputina volcano. eos, trans. am. geophys. union, 89(15), 141-148. corresponding author: maria eugenia petit-breuilh sepúlveda department of american history, the university of sevilla, sevilla, spain email: mepetit@us.es © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 9 the value of historical documents for risk reduction layout 6 wavelet analysis of the lf radio signals collected by the european vlf/lf network from july 2009 to april 2011 flavia righetti1,*, pier francesco biagi1, tommaso maggipinto1, luigi schiavulli1, teresa ligonzo1, anita ermini2, iren a. moldovan3, adrian s. moldovan4, aydin buyuksarac5, hugo g. silva6, mourad bezzeghoud6, michael e. contadakis7, dimitrios n. arabelos7, thomas d. xenos7 1 università di bari, dipartimento di fisica, bari, italy 2 università di roma "tor vergata", dipartimento di ingegneria meccanica, rome, italy 3 national institute of earth physics, seismological depth., bucharest-magurele, romania 4 azel-designing group s.r.l., bucharest-magurele, romania 5 canakkale onsekiz mart university, department of geophysics, canakkale, turkey 6 universidade de évora, escola de ciências e tecnologia, centro de geofísica de évora e departamento de física, évora, portugal 7 aristotle university of thessaloniki, department of geodesy and surveying, thessaloniki, greece annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5188 abstract in 2008, a radio receiver that works in very low frequency (vlf; 20-60 khz) and lf (150-300 khz) bands was developed by an italian factory. the receiver can monitor 10 frequencies distributed in these bands, with the measurement for each of them of the electric field intensity. since 2009, to date, six of these radio receivers have been installed throughout europe to establish a ‘european vlf/lf network’. at present, two of these are into operation in italy, and the remaining four are located in greece, turkey, portugal and romania. for the present study, the lf radio data collected over about two years were analysed. at first, the day-time data and the night-time data were separated for each radio signal. taking into account that the lf signals are characterized by ground-wave and sky-wave propagation modes, the day-time data are related to the ground wave and the night-time data to the sky wave. in this framework, the effects of solar activity and storm activity were defined in the different trends. then, the earthquakes with m ≥5.0 that occurred over the same period were selected, as those located in a 300-km radius around each receiver/transmitter and within the 5th fresnel zone related to each transmitter-receiver path. where possible, the wavelet analysis was applied on the time series of the radio signal intensity, and some anomalies related to previous earthquakes were revealed. except for some doubt in one case, success appears to have been obtained in all of the cases related to the 300 km circles in for the ground waves and the sky waves. for the fresnel cases, success in two cases and one failure were seen in analysing the sky waves. the failure occurred in august/september, and might be related to the disturbed conditions of the ionosphere in summer. 1. introduction since 1980, studies of the interactions between seismic activity and disturbances in radio broadcasts have been carried out. pre-seismic disturbances in very low frequency (vlf) radio signals that lie in the 20-60 khz frequency band have been investigated mainly in japanese and russian studies [hayakawa and sato 1994, morgounov et al. 1994, hayakawa et al. 1996, 2006, molchanov and hayakawa 1998]. at the same time, pre-seismic disturbances on lf (150-300 khz) radio broadcasts have been proposed mainly in italian studies [biagi et al. 2001a,b, 2005, 2006a, biagi and hayakawa 2002]. generally, the radio data have been collected by receivers located on the ground. recently, some possible seismic disturbances were revealed by the vlf radio signals recorded by the french demeter satellite, as presented by molchanov et al. [2006] and rozhnoi et al. [2007]. all of the previous disturbances are related to variations of some parameters in the ground, the atmosphere and the ionosphere. the ground variations, such as uplift and tilt, gas emissions, underground water level fluctuations, changes in groundwater chemistry, and changes in electrical resistivity of rocks, are clearly related to microfracturing processes that can occur during the preparatory phases of earthquakes [rikitake 1975, 1987, king, 1984/85, roeloffs 1988, wyss and dmowska 1997]. on the other hand, to justify the atmospheric (mainly ionospheric) disturbances, two different models have been proposed. the first model assumes direct special issue: earthquake precursors 171 article history received june 16, 2011; accepted september 22, 2011. subject classification: seismic risk, intruments and techniques, data analysis, radio network, wavelet precursors. effects of ionising radiation from gases (mainly radon) or aerosols, or electromagnetic emissions from the ground [hayakawa and sato 1994, alperovitch 1997, pulinets et al. 1998, biagi et al. 2001b]; the second model assumes indirect effects of the production of gravity waves in the atmosphere/ionosphere [hayakawa et al. 1996, molchanov and hayakawa 1998] as a consequence of pre-seismic processes in the ground. this second model overcomes the problem present in the first model, of the transport up to the ionosphere of particles or electromagnetic waves from the ground. in any case, these results relating to the earthquake precursors obtained through the analysis of radio signals have been very encouraging, and these investigations are now becoming more widespread. in this framework, during 2008, a new radio receiver operating in both the vlf and the lf bands that can monitor the electric field intensities of 10 frequencies was developed by an italian factory (elettronika, palo del colle, bari). since 2009, to date, six of these receivers have been put into operation, two in italy and the remaining four in greece, turkey, portugal and romania, giving rise to the first ‘european vlf/lf network’. a sampling rate of 1 min is used, and the electric field intensity of the signals is expressed in dbm, as dbm = 20 log(vm vpp). for simplicity, in the different figures of this study that show data collected with these receivers, the dbm indication on the y-axis is not reported. the receivers and the network were described in detail by biagi et al. [2011]. figure 1 illustrates this radio network, and some particularities of the lf transmitters are listed in table 1. we present here our first study of the lf data that was collected from july 2009 to april 2011. 2. theoretical features of lf radio signals lf radio signals are characterised by ground-wave and sky-wave propagation modes. the ground waves provide relatively stable signal. in contrast, the sky waves vary greatly from day to night, and in the day-time, from winter to summer. for the ground waves, rotheram [1981a,b] developed a model based on propagation over a smooth, homogeneous, curved earth with an exponentially decreasing refractive index. the meaning of an exponential atmosphere is that it 172 righetti et al. figure 1. map showing the locations of the receivers and the lf transmitters of the european radio network. stars, receivers; circles, lf transmitters, the signals of which were collected by the different receivers. some particularities of these transmitters are reported in table 1. code country power (kw) frequency (khz) rro romania 1200 153 fri france 2000 162 trt turkey 1200 180 eu1 germany 2000 183 ch1 algeria 2000 198 mco france 1200 216 rru russia 2500 261 cze czech republic 500 270 table 1. particularities of the lf transmitters of the european network. 173 represents the average atmospheric conditions more closely than a linear refractive index variation does. the factors that influence the propagation are: a) the refractive index n of the troposphere; b) the scale height h of the troposphere; and c) the permittivity f and the conductivity v of the ground. here, n and v are the most influential factors. practically, the ground-wave propagation is affected by the troposphere and the ground conditions. for the sky wave, the wave hop theory proposed by knight [1973] can be considered. according to this theory, the sky-wave signals received by an antenna can be considered as rays that started from the transmitter and were reflected one or more times (hops) by the lower ionosphere and by the ground. for distances less than 1,000 km, the sky wave is entirely represented by a one hop wave. the main factors influencing the propagation are: a) the ionosphere reflection coefficient r; b) the ionosphere focusing factor d; and c) the sky-wave path length l, which depend on the ionosphere reflection height. practically, sky-wave propagation is affected by the ionosphere conditions mainly in the central part of the path. 3. data analysis 3.1. night-time and day-time data as a first step, we separated the day-time data from the night-time data. to obtain datasets that are always related to the day-time and to the night-time regardless of the season, we selected the range from 08.00 to 13.00 (ut) for the daytime, and the range from 20.00 to 22.00 (ut) for the night-time. this night-time choice is also forced by the occurrence of an interruption of 3-4 h in some of the radio broadcasts, generally after the local 24.00. as an example, in figure 2, the raw mco transmitter signals collected during may 2010 by the it-an receiver with the relevant night-time and day-time content are reported. on the basis of the results presented by biagi et al. [2006b], we can consider that the night-time data represent the sky wave, while the day-time data represent the ground wave. thus, according to the considerations made in the previous section, the sky wave might be affected mainly by the ionosphere conditions, and wavelet analysis of vlf/lf signals figure 2. night-time and day-time mco transmitter radio signals collected by the it-an receiver in may 2010. top panel: intensity of the raw mco (216 khz) transmitter signals. middle panel: night-time (20.0022.00 ut) data extracted from the raw data. bottom panel: day-time (08.00-13.00 ut) data extracted from the raw data. figure 3. day-time and night-time transmitter radio signals and local air temperature trends collected from july 2009 to december 2010. left: intensities of the cze (270 khz) transmitter signals collected by the gr receiver (figure 1). right: intensities of the mco (216 khz) transmitter signals collected by the it-an receiver (figure 1). the local air temperature trends are also reported for both. red dashed lines, long-term smoothed data. the second panels from the top show the 30-day adjacent averaging smoothing of the day-time radio data. the ground wave by the troposphere/ground conditions. to check these influences, we examined the night-time and day-time radio data collected by the different receivers over about two years, together with the data sampled by meteorological stations located in each receiver area. figure 3 shows the night-time and day-time cze transmitter radio signals collected by the gr receiver, and those of the mco transmitter radio signals collected by the it-an receiver, together with the air temperature trends in the two receiver areas over the same period. a very clear correlation appears between the night-time radio signals and long-term temperature trends, demonstrating the influence of solar radiation on the ionosphere, and consequently on the night-time radio signals that represent the sky wave. as the solar radiation disturbs the ionosphere and consequently the reflection of the sky wave, the correlation ratio between these variations is negative. a lag between the temperature and radio-signal variations can be clearly seen in figure 3, which indicates the delay of the heating/cooling on the ground with respect to that in the ionosphere, as would be expected. in figure 3, the long-term trend of the day-time radio (dashed red lines in the plots) appears to be relatively stable, as must be expected for a ground wave. there are only some weak variations that can be seen in the radio data (figure 3, second panel from the top), showing that there is some influence of the ground/troposphere conditions on the radio signal propagation. finally, figure 4 shows the trt transmitter day-time radio signals collected by the gr receiver, and the mco transmitter day-time radio signals collected by the rom receiver, together with the storms detected in the areas of the two receivers over the same period. the disturbances in the radio signals caused by the electricity produced by lightning during the storms are evident. this effect stands out clearly on the day-time radio signals data because the lightning activity occurs mainly during the day, and also because of the lower level of the day-time signal with respect to the night-time signal. 3.2. earthquake selection to uncover possible seismic effects on the radio signals, at first it was necessary to select the relevant earthquakes. the european mediterranean seismological centre (emsc) bulletin from july 2009 to april 2011 was used (emsc website, http://www.emsc-csem.org). the three following criteria were adopted for the choice of the earthquakes: (a) earthquakes with m ≥5.0 located inside the 5th fresnel zone of the different radio paths; (b) earthquakes with m ≥5.0 that occurred inside a circle with a 300-km radius around each receiver; and (c) earthquakes with m ≥5.0 that occurred inside a circle with 300-km radius around each transmitter. requirement (a) takes into account several results that have indicated that the area inside the 5th fresnel zone is the most sensitive for seismic disturbances on radio propagation [molchanov and hayakawa 1998, molchanov et al. 2006, rozhnoi et al. 2004]. requirements (b) and (c) are based on the dimensions of the area affected by possible pre-seismic effects [dobrovolsky et al. 1979, kingsley et al. 2001]. the results of these selections are reported in table 2. the cases denoted with an asterisk in table 2 cannot be investigated due to a lack of radio data or for known local disturbances on the receiver [biagi et al. 2011]. righetti et al. 174 figure 4. day-time transmitter radio signals and storm days collected from july 2009 to december 2010. left: intensities of the trt (180 khz) transmitter signals collected by the gr receiver (figure 1). right: intensities of the mco (216 khz) transmitter signals collected by the rom receiver (figure 1). the storm days are also reported for both. the second panels from the top show the day-time intensities on an enlarged scale to clearly show correspondence with the storms. 175 4. wavelet analysis to reveal any anomalies on the radio data, the wavelet transform [torrence and compo 1998] was applied. in this way, it was possible to highlight the spectral components of the signals using variable-width time windows, by considering that the frequency content of these windows has an inverse relationship to the time widths. thus, the localization of the signals is obtained simultaneously in both time and frequency [daubechies 1992, strang and nguye 1996]. in our analysis, we adopted the ‘morlet function’ as the wavelet [torrence and compo 1998]. in this case the wavelet transform of a time signal is a complex series that can be usefully represented by its square amplitude, i.e. we consider the so-called wavelet power spectrum. the power spectrum is a two-dimensional plot that once correctly normalized with respect to the power of the white noise, gives information on the strength and precise time of occurrence of the various fourier components that are present in the original time series. furthermore, the wavelet power corresponding to the fourier period in which the maximum power is found can be usefully reported; i.e. the wavelet analysis of vlf/lf signals table 2. earthquakes selected according to the requirements indicated in section 4. radio path relevant earthquakes selected coordinates (˚n, ˚e) date (yyyy-mm-dd) magnitude fresnel zones gr/mco 41.86, 19.16 2009-08-21 5.0 gr/mco 41.46, 20.41 2009-09-06 5.6 gr/trt 40.43, 26.30 2010-11-03 5.3 gr/cze 43.74, 20.69 2010-11-03 5.4 it-tc/rro (*) 41.86, 19.16 2009-08-21 5.0 rom/ch1 (*) 41.86, 19.16 2009-08-21 5.0 receiver circles it-tc 41.46, 20.41 2009-09-06 5.6 tur 38.84, 40.00 2010-03-08 6.0 transmitter circles cze 51.45, 16.11 2010-02-06 5.0 figure 5. night-time and day-time spectrograms of the trt (180 khz), mco (216 khz) and cze (270 khz) transmitter radio signals collected by the gr receiver, together with the geomagnetic/solar and the (local) meteorological data, from july to september 2009. top panels: spectrograms of the nighttime (left) and day-time (right) radio signals. continuous black lines, delimitation of the region (coi) of the wavelet spectra in which edge effects become important (torrence and compo, 1998). centre panels: dst and kp geomagnetic indices and solar burst numbers (ss) (left), and air temperature, air pressure and occurrence of rain and storms (right). bottom: map showing the 5th fresnel zone of the gr/mco path, showing the epicentres of the august 21 and september 6, 2009, earthquakes that occurred inside these zones. vertical dashed lines, time of occurrence of these earthquakes. one-dimensional plot that represents a single period with respect to time. for the earthquakes that occurred in the fourier zones, we looked for anomalies in the night-time data (the sky wave); the day-time data (the ground wave) were examined only to identify any anomalies in the night-time data related to some local day/night cause or to meteorological disturbances (in addition to the meteorological data). for the cases within the 300 km circles, both the night-time and daytime data were examined; indeed, in these cases, the seismicity produced local effects and so disturbances in both of these signal should to be detectable. the results of this analysis according to the cases indicated in table 2 are reported in sections 4.1, 4.2 and 4.3. 4.1 the fresnel zones figure 5 shows the analysis of the first two cases reported in table 2. figure 5 reports the night-time and day-time spectrograms of the trt, mco and cze transmitter radio signals collected by the gr receiver, together with geomagnetic/solar and (local) meteorological data. the times of occurrence of the two earthquakes that occurred inside the 5th fresnel zone of the gr/mco path are indicated. these earthquakes occurred close together and not very far apart from each other in time, so a unique perturbation can be expected. as can be seen in figure 5, a clear anomaly appears simultaneously in the different day-time spectrograms at the beginning of july. in our opinion, this anomaly is related to some local diurnal phenomenology that occurred in the receiver zone. for the night-time spectrograms, some anomalies appeared, although no particular correspondence with these earthquakes stands out. at the same time, the geomagnetic/solar conditions also cannot justify the anomalies. therefore, irregular disturbances in the ionosphere that are typical of the summer can in all probability be claimed to justify these anomalies. figure 6 shows the analysis of the next two cases in table 2, with the night-time and day-time spectrograms of the trt, ch1 and cze transmitter radio signals collected by the gr receiver reported together with the geomagnetic/ righetti et al. 176 figure 6. night-time and day-time spectrograms of the trt (180 khz), ch1 (198 khz) and cze (270 khz) transmitter radio signals collected by the gr receiver, together with the geomagnetic/solar and the (local) meteorological data, from october to december 2010. top panels: spectrograms of the nighttime (left) and day-time (right) radio signals. continuous black lines, delimitation of the region (coi) of the wavelet spectra in which edge effects become important [torrence and compo 1998]. centre panels: dst and kp geomagnetic indices and solar burst numbers (ss) (left), and air temperature, air pressure and occurrence of rain and storms (right). bottom panels: wavelet power corresponding to the fourier period in which the maximum power was found for the 180 khz signal (left), and map showing the 5th fresnel zone of the gr/trt and gr/cze paths (right), showing the epicentres of the november 3, 2010, earthquakes that occurred inside these zones. vertical dashed lines, times of occurrence of these earthquakes. 177 solar and the (local) meteorological data. these two earthquakes happened on the same day (november 3, 2010), and their epicenters were inside the 5th fresnel zone of the gr/trt and gr/cze paths, is indicated. from the spectrograms, three anomalies stand out. the most evident occurs in december. this appears in the different radio signals sampled by the receiver, and it is present both in the night-time and day-time spectrograms. in addition, no similar effects were seen in the data collected by the other receivers of the network. this anomaly appears to be due to the relocation of the gr receiver to a more suitable place, which occurred on december 11, 2010. another anomaly is present only on the night-time spectrogram of the trt (180 khz) signal coming from turkey. the anomaly has a period of about 15 days and it can be clearly related to the earthquake (m = 5.3) that occurred along the gr/trt path. the one-dimensional plot corresponding to the maximum power of the 180 khz spectrogram in figure 6 reveals that the anomaly started 10 days before the earthquake occurrence. although weak, the last anomaly appears only in the night-time spectrogram of the cze (270 khz) signal coming from the czech republic. this anomaly is characterized by a period of about 10 days, and it might be related to the earthquake (m = 5.4) that occurred along the gr/cze path. the effects seemed to start 15 days before the earthquake. wavelet analysis of vlf/lf signals figure 7. night-time and day-time spectrograms of some of the radio signals collected by the tur receiver relative to the m = 6.0 earthquake that occurred on march 8, 2010 (left), and by the it-tc receiver relative to the m = 5.6 earthquake that occurred on september 06, 2009 (right). top panels: spectrograms of the night-time data. centre panels: spectrograms of the day-time data. continuous black lines, delimitation of the region (coi) of the wavelet spectra in which edge effects become important [torrence and compo 1998]. bottom panels: maps showing the tur receiver (star) and the march 8, 2010, earthquake epicenter (triangle) (left), and the epicenter of the september 06, 2009, earthquake that occurred inside the 300-km radius circle around the it-tc receiver (right). vertical dashed lines, times of occurrence of these earthquakes. 4.2. receiver circles figure 7 relates to two cases reported in table 2, and shows the night-time and day-time spectrograms of some of the radio signals sampled by the tur and it-tc receivers. in both of these cases, clear anomalies with periods of 2-5 days appear in the spectrograms. from the geomagnetic/solar and meteorological data (for simplicity, not shown in figure 7), no particular activity appears in the anomalous periods. on the contrary, it appears realistic that these anomalies are related to the selected earthquakes. a pre-seismic phase with a duration of a number of days stands out in some of the signals. 4.3. transmitter circles finally, in terms of the ‘transmitter circles’, figure 8 shows the night-time and day-time spectrograms of the cze transmitter radio signal (table 2) recorded by the gr, it-an and it-tc receivers, together with the geomagnetic/solar data and meteorological conditions in the transmitter area. clear anomalies stand out at practically the same time in the night-time and day-time spectrograms. in addition, these do not appear in the spectrograms of the other radio signals recorded by these receivers over the same period. therefore, these anomalies are clearly related to some local effects. the righetti et al. 178 figure 8. night-time and day-time spectrograms of the cze (270 khz) transmitter radio signals collected by the gr, it-an and it-tc receivers, together with the geomagnetic/solar and the (local) meteorological data, from january to march 2010. top panels: spectrograms of the night-time (left) and daytime (right) radio signals. continuous black lines, delimitation of the region (coi) of the wavelet spectra in which edge effects become important [torrence and compo 1998]. centre left panels: wavelet powers corresponding to the fourier period in which the maximum power was found. bottom left panels: dst and kp geomagnetic indices and solar burst numbers (ss). centre right panels: air temperature, air pressure and occurrence of rain and storms in the transmitter area. bottom right panels: map showing the epicentre of the m = 5.0 earthquake that occurred on february 6, 2010, inside the 300-km radius circle around the cze transmitter. vertical dashed lines, time of occurrence of this earthquake. 179 local meteorological conditions in this period are characterized by consistent cooling (down to –20˚c), and this might be the cause of the radio anomalies. at the same time, in this period, an earthquake with m = 5.0 occurred near to the cze transmitter, as indicated in table 2 and shown on the map in figure 8. the one-dimension wavelet power plots at night-time that are shown in figure 8 reveal very good correspondence of the anomalies with the occurrence of the earthquake. therefore, the seismic justification of these anomalies also appears to be consistent. at the moment, we have not found more arguments for accepting one or the other possibility. 5. conclusions the wavelet analysis of the lf data collected by the european radio network has revealed some possible anomalies related to earthquakes. the concurrent analysis of the day-time and night-time signals was useful for the identification of local effects. except for some doubt in one case, success appears to be obtained in all of the cases related to the 300 km circles, for both the ground wave and the sky wave, which reveals that this method of analysis has a good sensitivity for the local disturbances produced by the earthquakes. this is consistent with the occurrence of local perturbations in the so called ‘preparation-zone’. for the fresnel cases, two were successful and one failed in the analysis of the sky wave. in the 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(1981b). ground wave propagation, part 2:theory for medium and long distances and reference propagation curves, iee proc. f, communication, radar and signal process, 128 (5), 285-295, . rozhnoi, a., m.s. solovieva, o.a. molchanov and m. hayakawa (2004). middle latitude lf (40 khz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, phys. chem. earth, 29, 589-598. rozhnoi, a., o. molchanov, m. solovieva, v. gladyshev, o. akentieva, j.j. berthelier, m. parrot, f. lefeuvre, m. hayakawa, l. castellana and p.f. biagi (2007). possible seismo-ionosphere perturbations revealed by vlf signals collected on ground and on a satellite, nat. hazard earth sys., 7, 617-624. strang, g. and t. nguye (1996). wavelets and filter banks, wellesley cambridge press, 490 pp. torrence, c. and g.p. compo (1998). a practical guide to wavelet analysis, b. am. meteor. soc., 79 (1), 61-78. wyss, m. and r. dmowska (eds.)(1997). earthquake prediction state of the art, pure appl. geophys., 149, 272 pp. * corresponding author: flavia righetti, università di bari, dipartimento di fisica, bari, italy; email: rete.fisica@uniba.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. righetti et al. 180 microsoft word 7407-17667-1-ed_abbott.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7407 1 some fundamental issues in geoethics david m. abbott, jr.* aipg ethics committee chairman, denver, colorado, usa dmageol@msn.com abstract ethics and morals concern themselves with distinguishing right from wrong, with actions, and with the volition involved. dictionaries define “ethics” in terms of “morals” and “morals” in terms of “ethics,” so the terms are quite similar and circular. i define “morals” as informal statements and “ethics” as written statements adopted by some group. general morals apply to everyone while professional ethics generally apply only to members of a particular profession. moral and ethical analysis requires careful and consistent discrimination of the relevant facts. it is important to remember that moral/ethical analysis does not resolve all questions because different people rank moral/ethical values differently. we must respectfully agree to disagree. moral/ethical rules apply to all persons at all times. however, exceptions are allowed following careful analysis of the exception. for example, surgeons are allowed to cut people open, most of us are not allowed to do so. moral/ethical aspirations are statements encouraging a particular activity but no demerits follow from choosing to follow one aspiration and not another. continuing professional development (cpd) provides an example. geoscience ethics codes all recommend cpd, which is aspirational. for those societies that require some minimum amount of cpd, an aspiration has become a rule. honesty is the principal geoscience ethical rule. some geoscience societies have disciplinary procedures, most do not; nor should they. disciplinary procedures must allow for due process and appropriate confidentiality until resolved. 1. basics of morality and ethics ecause dictionary definitions of “morals” and “ethics” tend to define one term using the other, a circular argument, i define morals as informal statements about what people are expected to do and ethics as written statements prepared by some group like a geoscience society that describe the conduct expectations for members of the issuing group. discussions of geoscience ethics or geoethics should be grounded in general morality. bernard gert (2004) provided the best summary for the general reader of general morality. rather than formulating a new ethical statement or system, gert examined the informal moral principles on which all societies we know about agree. gert points out that that there are two kinds of moral statements: moral rules and moral aspirations or ideals. moral rules must be obeyed by all people with respect to all other people all the time. gert identifies ten fundamental moral rules that everyone recognizes regardless of whether they had been formally articulated or written down within their group, culture, or society. they are: 1. do not kill. 2. do not cause pain or injury. 3. do not disable. 4. do not deprive of freedom. 5. do not deprive of pleasure/the pursuit of happiness. 6. do not deceive. 7. keep your promises. 8. do not cheat or violate rules of conduct. 9. obey the law includes theft. 10. do your duty. do not deceive is emphasized because i believe that honesty is the basic moral principle underlying all geoscience ethics codes (abbott, 2004). i also added “the pursuit of happiness” to gert’s “do not deprive of pleasure” rule because i feel that pursuit of happiness better captures the concept of gert’s rule. moral aspirations or ideals are admirable activities that some people engage in some of the time but which are not obligatory on everyone all the time. for example, none of us individually can relieve hunger worldwide even if we b annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7407 2 choose to work towards that goal. actions reflecting moral aspirations or ideals include such things as contributing to the cure of some disease, working to ease hunger. within a geoscience setting, actions reflecting moral aspirations or ideals may include volunteering for a professional association, writing papers, giving presentations at professional meetings, and engaging in public outreach about matters that are within our areas of expertise such as natural hazards or the responsible development of natural resources. while promoting a moral aspiration warrants recognition, demerits are not given if one does not work towards one or more moral aspirations. continuing professional development (cpd) provides an example of geoscience activities that are sometimes aspirational and sometimes rules. geoscience codes of ethics or conduct usually contain a statement encouraging continuing education; an aspirational statement. however, increasing numbers of geoscience organizations are requiring at least some members to engage in a specified minimum amount of cpd. for these organizations, cpd has become an ethical rule for the specified membership classes. gert (2004) also recognizes that there are exceptions to the moral rules. for example, selfdefense is a generally recognized exception to the “do not kill” rule. similarly, surgeons are allowed to cut into a patient’s abdomen as part of a surgical procedure in violation of the “do not cause pain or injury” rule. gert provides a checklist for identifying a morally justified violation of a moral rule: • what moral rule is involved? • who is harmed? • what harm is avoided by the violation? • can everyone violate the rule in the same way? • is some sort of emergency involved? • was the violation intentional? • was coercion involved? • are you willing to publicly acknowledge and accept the consequences of the violation? (integrity). “integrity” was parenthetically added to gert’s checklist because publicly acknowledging and accepting the consequences of a violation of a moral rule (or a law) is where integrity enters the discussion. stephen l. carter (1996) notes that “integrity is like the weather: everybody talks about it but nobody knows what to do about it. ...[i]ntegrity is like good weather, because everybody is in favor of it.” carter defines integrity as requiring three steps: (1) discerning what is right and what is wrong; (2) acting on what you have discerned, even at personal cost; and (3) saying openly that you are acting on your understanding of right from wrong. the first criterion captures the idea of integrity as requiring a degree of moral reflectiveness. the second brings in the idea of an integral person as steadfast, which includes the sense of keeping commitments. the third reminds us that a person of integrity is unashamed of doing right. the word [integrity] conveys not so much a single-mindedness as a completeness; not the frenzy of a fanatic who wants to remake all the world in a single mold, but the serenity of a person who is confident in the knowledge that he or she is living rightly. honesty is a basic geoscience ethical principle (abbott, 2002 and 2004). demonstrating the reliability of sampling results through the use of quality assurance/quality control (qa/qc) procedures has become required practice in the mining industry in recent years and should be applied to any sampling program (abbott, 2007 & 2016a). such qa/qc procedures appear to violate the honesty principle by concealing the identity of the standard, duplicate, and blank qa/qc samples in a sample stream from those further down the sample preparation and analysis procedures performance line. but is this apparent violation unethical, or is it actually necessary to assure the validity of scientific results? using gert’s rules for identifying a morally justified exception to a rule for qa/qc samples results in: • rule being violated: do not deceive. • harm done: increased analytical costs. • harm avoided: determining and verifying the reliability of your sampling and analytical processes. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7407 3 • everyone can and should be using qa/qc procedures in data collection. • although the identity of the standards, blanks, and duplicates should be hidden, their existence need not be. gert (2004) makes another very important point about moral or ethical analysis: such analysis will not answer all moral/ethical questions. different people rank moral or ethical principles differently in their analysis of a particular situation and thus can arrive at opposing moral/ethical conclusions each have a valid moral/ethical justification. common examples include abortion and capital punishment. most geoscience ethics codes require geoscientists to maintain the confidentiality of their employer’s or client’s confidential information. however, most geoscience ethics codes also state that protection of the public’s health, safety, and welfare (including financial welfare) is the highest duty or responsibility of members. by stating that this obligation is “the highest duty or responsibility,” these codes place the public’s health, safety, and welfare above employer or client confidentiality. geoscientists should bring confidential evidence of fraud, environmental degradation, or a geohazard situation to the attention of appropriate authorities (when one is faced with a conflict between confidentiality and disclosing a potential public harm, one should proceed very carefully and seek the advice of knowledgeable, independent parties for advice). the efg’s (european federation of geologists) code of ethics (2016) states this principle: … 8. the geologist must avoid any sort of negligence in the practice of his/her profession, especially when this gives rise to risks or of material or moral damage for his/her client or for the environment. 9. the geologist must not alter, or deny the existence of, facts or accepted technical or scientific truths which could thereby favour a client or mislead the public. 10. the geologist must not promise or broadcast specific professional advice that cannot be supported by a genuine, objective possibility, … 10.2 if, after having given his/her advice, a geologist becomes aware that it will not be entirely followed, he/she should, regardless of his/her own position, inform the relevant person of the foreseen risks. … 2. to whom do ethics codes apply? gert (2004) also points out that it is important to determine who or what is covered by a moral or ethical statement. geoscience ethics codes apply specifically to covered member categories of the issuing geoscience society. other codes, such as the 2015 american geoscience institute guidelines for ethical professional conduct (2015 agi guidelines) assert a broader reach. the 2015 agi guidelines begin, “these guidelines address common ethical topics across the geoscience community; the ethics statements of individual societies may expand beyond these guidelines” clearly extending the application of the guidelines to everyone in the geoscience community regardless of their membership in an agi member society. the ethical guidelines for statistical practice issued by the american statistical association (2016) go even further by explicitly stating, “because society depends on informed judgments supported by statistical methods, all practitioners of statistics, regardless of training and occupation or job title, have an obligation to work in a professional, competent, and ethical manner and to discourage any type of professional and scientific misconduct.” at least according to the american statistical association, their ethical guidelines for statistical practice apply to all geoscientists using statistical analysis. this brings up the important point: who decides whether a particular topic, group, location, or cause is covered by moral or ethical principles? i addressed this issue in commenting on a 2015 paper by m. brocx and v. semeniuk, “the development of solar salt ponds along the pilbara coast, western australia a coastline of global heritage significance used for industrial purposes”. the first sentence of their abstract summarizes their view, “the pilbara coast in nw australia stands unique as the most geologically/geomorphically diverse arid coast globally and, as such, it is a coastline of global significance.” i was previously unaware of specifannals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7407 4 ics of the pilbara coast and am unable to determine whether the pilbara coast deserves the protection brocx and semeniuk wish it had. but i strongly suspect that others disagree with brocx’s and semeniuk’s views and with equally good ethical reasons. it is a result of how different people rank different “good” outcomes (abbott, 2016b). 3. moral and ethical analysis analysis of an ethical situation requires careful analysis employing the following: • identifying the ethical principle(s) involved. • discriminating between relevant and irrelevant facts. • being logical. • being objective. • being consistent all similar cases must be treated the same way. discussion of and reflection on ethical case histories demonstrate that small differences in the relevant facts can lead to quite different conclusions. consider the following examples: example 1: a geoscientist has prepared a structure contour map that is part of a project for a client. the geoscientist’s project manager, who is also a geoscientist, reviews the map and concludes that the contours should be changed in a way that has the effect of making the project look better. does the difference between the original and revised version of the structure contour map reflect (a) an effort to please the client or (b) a legitimate difference of professional opinion? what considerations would go into favoring alternative a or b? example 2: a university professor has been retained as a private consultant on a project located several hours by car away from the university. during the same trip, the professor also does a field check of a graduate student’s thesis area, which is located near the consulting project. the professor is permitted to drive a university vehicle to the graduate student’s thesis area. should the professor also drive the university vehicle to the private client’s project site? if yes, should the university be reimbursed for half the whole trip or just for the extra miles required to visit the client’s site? if the university vehicle should not be taken to the client’s project site, how should the professor get to the site? should two separate trips be made or are there other solutions? are there any other facts that would change your view of the situation? the american institute of professional geologists (2017a) has published a large number of articles and columns on professional ethics for over 20 years. 4. does having a code of ethics require discipline? most geoscience professional societies have educational and experience requirements for membership. these qualifications were among the reasons for organizing in the first place. membership was a credential the public could look to for assuring competent work and honest appraisal of natural resource offerings. abbott (2018) reviews the history and application of enforceable codes of geoscience ethics. once a professional organization has decided that it will have an enforceable code of ethics, it must determine which provisions of its code of ethics are ethical rules that cannot be violated and which are aspirational statements. most professional societies have codes of ethics with both types of provisions, while state licensing boards and others will have codes of conduct containing only ethical rules. adoption of an enforceable code of ethics requires that disciplinary procedures be adopted. these disciplinary procedures should describe all of the processes of: 1. review of allegations. 2. initiation of an investigation. 3. filing of formal charges. 4. adjudication process. 5. appellate process. 6. describe the disciplinary sanctions that can be imposed. 7. describe the rights of respondents. 8. avoidance of conflicts of interest within the procedures. 9. confidentiality of the proceedings. annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7407 5 10. reporting of violations to other organizations and regulators. 11. publication of annual summaries of disciplinary activity. abbott (2018) contains detailed descriptions of these 11 activities, and the american institute of professional geologists’ (aipg) disciplinary procedures (2015) provide an excellent example of such procedures. aipg publishes an annually updated summary of its disciplinary activities (aipg, 2017b) the grievance and disciplinary procedures of the european federation of geologists (efg) are contained in regulation c2-ver 5 (2010). references abbott d.m. jr. (2004). are scientific honesty and “best practices” in conflict?: european geologist, no. 18, december 2004, p. 34-38; reprinted in the professional geologist, july-august 2005, p. 46-51. abbott d.m. jr. (2007). assuring the reliability of your sampling results: the professional geologist, november/december 2007, p. 33-38. abbott d.m. jr. (2016a). usgs inorganic geochemical lab manipulated data and proving your data are accurate: the professional geologist, professional ethics & practices column 160, p. 31-32. abbott d.m. jr. (2016b). geoethics: the professional geologist, professional ethics & practices column 157, p. 32-33. abbott d.m. jr. (2018), brief history and application of enforceable professional geoscience ethics codes. in: gunderson l.c., ed., scientific integrity and ethics with applications to the geosciences: special publication american geophysical union, john wiley and sons, inc., pp. (in press). aipg american institute of professional geologists (2015). disciplinary procedures: www.aipg.org/disciplinaryprocedures, accessed march 2017. aipg american institute of professional geologists (2017a). professional ethics and practices topical index: www.aipg.org/codeofethics, accessed march 2017. aipg american institute of professional geologists (2017b). aipg disciplinary procedure actions 1989 through 2016: www.aipg.org/disciplinaryactions, accessed march 2017. agi american geoscience institute (2015). guidelines for ethical professional conduct: www.americangeosciences.org/community /agi-guidelines-ethical-professionalconduct, accessed september 2017. asa american statistical association (2016). ethical guidelines for statistical practice: www.amstat.org/asa/yourcareer/ethical-guidelines-for-statisticalpractice.aspx, accessed september 2017. brocx m. and semeniuk v. (2015). the development of solar salt ponds along the pilbara coast, western australia a coastline of global heritage significance used for industrial purposes. in: peppoloni s. and di capua g., eds., geoethics: the role and responsibility of geoscientists, geological society of london, special publication 419, p. 31-41. carter s.l. (1996). integrity: harper perennial, 288 p. efg european federation of geologists (2010). grievance and disciplinary procedures, regulation c2-ver 5: http://eurogeologists.eu/statutesregulatio ns/c2, accessed march 2017. efg european federation of geologists (2016). code of ethics, regulation c1, ver. 6: http://eurogeologists.eu/wpcontent/uploads/2017/07/reg-c1-ver-6nov-2016.pdf, accessed september 2017. gert b. (2004). common morality deciding what to do: oxford university press, 179 p. * aipg-cpg, fgsl, eurgeol, fausimm 423_434 scarsi.pdf 423 annals of geophysics, vol. 46, n. 2, april 2003 euso: using high energy cosmic rays and neutrinos as messengers from the unknown universe (*) livio scarsi istituto di astrofi sica spaziale e fisica cosmica (iasf), cnr, sezione di palermo, italy abstract exploiting the earth atmosphere as a giant detector for the incoming extraterrestrial fl ux of high energy cosmic rays and cosmic neutrinos, the mission «euso extreme universe space observatory» is devoted to the exploration of the domain of the highest energy processes occurring in the universe up to its accessible boundaries. the observable is provided by the air nitrogen fl uorescence light emitted in the uv band 300-400 nm by the extensive air showers produced by the cascading processes of the primary cosmic radiation particles interacting with the atmosphere. the euso telescope is based on a double fresnel lens optics (diameter 2.5 m) coupled to a highly pixelized focal surface composed multianode pmts; the image at the earth surface is detailed at 1 km2 over a total of several hundred thousand of km2. euso will fl y on the international space station accommodated as external payload of the european space agency columbus module. the mission is scheduled to last 3 years, with the start of operations foreseen for 2007/8. the expectations are of a collection rate of a thousand events/year for cosmic rays at e > 1020 ev together with tens/hundreds of cosmic neutrinos at energy above about 4 ¥ 1019 ev. euso is the result of the collaborative effort of several institutions in europe, japan and usa and it is conceived within the science program sponsored by various space agencies coordinated by esa. mailing address: prof. livio scarsi, istituto di astrofi sica spaziale e fisica cosmica (iasf), cnr, sezione di palermo, via u. la malfa 153, 90146 palermo, italy; e-mail: livio. scarsi@pa.iasf.cnr.it key words fl uorescence cosmic rays neutrinos extensive air showers (*) this text is largely derived from contributions made by the author in 2000/2001 to other workshops and conferences and from documentation submitted by the euso consortium to the european space agency as a part of the euso proposal. 1. introduction cosmic radiation can be considered the «particle channel» complementing the «electromagnetic channel» proper of conventional astronomy. a classic presentation of the cosmic ray energy spectrum is shown in fi g. 1; an unconventional view (which i borrowed from a colleague of karlsruhe, where it was fi rst shown at the ceremonial organized in honor of dr. shatz) is given in fi g. 2 to illustrate in an anthropomorphic perspective the features conventionally called «knee» (around 1015 ev) and «ankle» (above 5 ¥ 1018 ev). the remarkable «feminine leg» in the fi gure is that of the famous german movie star marlene dietrich. today substantial progress has been made in the knowledge of the nature of cosmic rays of the relatively modest energies (up to the «knee» at 1014-1015 ev). on the other hand the cosmic radiation on the higher energy side presents us with the challenge of understanding its origin and its connection with fundamental problems in cosmology and astroparticle physics. 424 livio scarsi fig. 1. the observed cosmic ray spectrum for e > 108 ev showing the principle features. the inset shows the high-energy part with the overall e-3 dependence removed as observed by agasa (takeda et al., 1998), fly’s eye and hires. the dashed line shows the effect of the gzk cut-off assuming a homogenous source population fi lling the universe. the numbers are the actual number of events in each bin. fig. 2. the cosmic ray energy spectrum: anthropomorphic presentation. 425 euso: using high energy cosmic rays and neutrions as messengers from the unknown universe focal points are represented by: i) the change in the spectral index at ~ 5 ¥ ¥ 1018 ev («ankle»). ii) existence of «cosmic rays» with energy e > 1020 ev: (eecr) (fi g. 1). a direct question arising is: what is the maximum cosmic ray energy, if there is any limit? addressing the theoretical issue concerning the production and propagation of 1020 ev primary quanta is problematic and involves processes still little known. 2. the universe and the probing depth of the extreme energy cosmic radiation from the document «report on the accommodation of euso on the columbus exposed payload facility: esa/msm-gu/2000, 462/ap/ rda, december 2000». soon after the discovery of the cosmic microwave background radiation (cmb) by penzias and wilson in 1965, greisen, and, independently, zatsepin and kuzmin pointed out that this radiation would make the universe opaque to cosmic rays of sufficiently high energy. for protons, e.g., this occurs when the pion production threshold is reached (about 5 ¥ 1019 ev, if the lorentz transformations of relativity still hold at g 1011). the reaction p + g æ d+ æ p + p ° or n + p + will lead to an effective attenuation length of 50 mpc for a proton of 1020 ev. this is about the size of the virgo cluster to which our galaxy belongs, and is just a small fraction of the size of the universe. table i summarizes the effects introduced on the primary eecr components, including gamma rays, by the interaction with the cmb. to a much lesser extent, neutrinos decay above 1021-1022 ev by z0-resonance with cosmic neutrino background (cnb). the greisen-zatsepin-kuzmin effect shapes in a complicated way the form of the observed energy spectrum of the eecr as a function of the distribution of the extragalactic sources in the universe (see fi g. 3a,b). 2.1. sources of extreme energy cosmic rays many sources of high-energy particles could exist in the universe at distances beyond the gzk length of 50 mpc. the predicted sharp cut-off of energy spectrum above 5 ¥ 1019 ev may not be present if a possible high abundance of cosmological neutrino events, or others from nearby sources, are dominant. a significant bump should exist at 1019-1020 ev due to the gzk effect for protons, because cosmological protons that were accelerated to the super-gzk energies decay down to the trans-gzk energies, and pile up at sub-gzk energies. the details of the energy spectrum in the trans-gzk energy regime (1019 – 2 ¥ 1020 ev) depend on the model of the evolution of the universe from about 10 billion years ago (or z ~ 5). high event statistics can provide information on the evolution of the highest energy universe. this information on the evolution of universe may be distorted by the probable existence of super-gzk neutrinos. however, using euso the neutrino energy spectrum can be identifi ed and directly measured, and the non-neutrino spectrum can be statistically corrected by subtracting relevant neutrinos and their process cut-off energy mean free path protons p + g 2.7k æ p + x ev 50 mpc nuclei a + g 2.7k æ d ++ + x 5 ¥ 1018 ev/n 100 mpc gamma-rays g + g 2.7k 10 14 ev (at 1020 ev) 10 mpc (at 10 20 ev) neutrinos n + n1.95k æ (w/z0) + x 4 ¥ 10 22 ev 40 gpc table i. extreme energy processes that cut-off the energy spectrum of particles in universe. 426 livio scarsi secondaries. the evolution of energetic sources in the universe will be examined by such statistical means. the current summary of the data from agasa, fly’s eye and hires is shown in the insert of fi g. 1 and suggests that the observed cosmic ray spectrum is not necessarily cut-off following the predicted gzk process for protons and nuclei. it suggests that either the majority of cosmic rays originated within the gzk length (< 50 mpc), or the relativity principle fails at extreme energies, or some other unknown sources exists. concerning the distribution of arrival directions, agasa and worldwide data summary of cosmic rays above about the gzk cut-off energy show a quasi-isotropic distribution in the sky clearly suggesting an extragalactic origin. among them, 6 pairs and 1 triple set of spatially correlated events within 2 years from 58 events were recognized by agasa only, while the world-wide data show 9 pairs and two triples. the chance coincidence probability for these «clusters» of events is less than 0.07%, and therefore, the particles of a «cluster» possibly had the same sources. although the existence of the highest energy cosmic rays is proven, their origin is still an enigma despite the efforts of many theorists and experimentalists. one is led to the conclusion that they have an entirely different origin from the lower energy cosmic rays. the present data raise questions of great importance for astrophysics, cosmology, and fundamental physics. focusing the attention on the primary sources, the general production mechanisms proposed for the eecrs can be classifi ed as: fig. 3a,b. a) energy spectra from a single source of protons with an e –2 spectrum, for various source distances between z = 0:004 and 1 (i.e. between 2 and 5000 mpc). b) the nucleon spectrum at the trans-gzk and supergzk energies predicted from different models of the cosmic evolution, including the hypothetical component at ee up to 1023 ev. curve 1 is for the case of homogeneous evolution for the redshifts 0 £ z £ 2. curve 5 shows non-homogeneous distribution up to z = 4 in which early cosmological era has more active ee sources. curves 2-4 are for intermediate models. cosmological neutrinos and their secondaries are not included in this fi gure. a b euso: extreme universe space observatory 427 euso: using high energy cosmic rays and neutrions as messengers from the unknown universe – bottom-up, with acceleration in rapidly evolving processes occurring in astrophysical objects. the scenario involves astrophysical objects such as, e.g., agns and agn radio lobes. the study of these objects is, besides radio observations, a main goal of x-ray and gammaray astrophysics of the late 90’s. an extreme case in this class is represented by the gamma ray bursts, found to be located at cosmological distances. the observation of «direction of arrival and time» coincidences of grbs and extreme energy neutrinos (e 1019 ev) in the euso mission could provide a crucial test for the identifi cation of the observed grbs as eecr sources in spite of their location at distances well above the gzk limit. – top-down processes. this scenario arises from the cascading of ultrahigh energy particles from the decay of topological defects. cosmic strings would play an essential role for releasing the x-bosons emitting the highest energy quarks and leptons. this process could occur in the nearby universe. the relics of an early infl ationary phase in the history of the universe may survive to the present as a part of dark matter and account for those unidentified eecr sources active within the gzk boundary limit. their decays can give rise to the highest energy cosmic rays, either by emission of hadrons and photons, or through production of ee neutrinos. from the astroparticle physics point of view, the eecrs have energies only a few decades below the grand unifi cation energy (1024-1025 ev), although still far from the plank mass of 1028 ev. 2.2. neutrino induced air showers neutrinos with high enough energy can produce detectable eas observable by euso. this will provide valuable information on their origin together with that of the eecr. not suffering the gzk effect and being immune from magnetic field deflections, or from a delay caused by the quantum relativity effects, neutrinos are ideal for disentangling source-related mechanisms from propagation-related effects. the opening of the neutrino astronomy channel will allow the extreme boundaries of the universe to be probed. however, neutrinos are elusive objects with a low interaction probability, to such an extent that they can be neglected as observable eas initiators for all ground based detectors, present or planned. even for the largest planned ground based cosmic ray detector (the auger project), fig. 4. predicted neutrino fl uxes for various models. the large range of predicted fl uxes should be noted as should the number of models which exceeds the number of events with energies > 1020 ev! (1) stecker and salamon (1996); (2) stecker et al. (1991); (3) sigl et al. (1999), m x = 1016 gev, x > q + q, supersymmetric fragmentation; (4) yoshida et al. (1998), mn = 1 ev, primary fn ~ e –1; (5) waxmann and bachall (1997). fig. 5. shower depth distribution from monte carlo simulations showing how neutrino and proton and nuclei induced events can be distinguished. 428 livio scarsi in the most optimistic case, the expected rate is only a few events per year. euso, with its large sensitive area and accessible mass target of the order of 1013 t of atmosphere, will be sensitive to this class of events. the expected neutrino event rate ranges from a few events per year (gzk processes, agn, grb sources, etc.) to 150 per year according to the effectiveness of the «topological defects» hypothesis (see fi g. 4). observationally, neutrinoinduced eas can be distinguished from background events and from other eecr eas by selecting events with large zenith angles which initiate deep in the atmosphere (fi g. 5). a nearly horizontal t-neutrino event with an energy > 1019 ev can be identifi ed by a «double bang» structure. both the initial shower in the nt æ t interaction, and another, by the t -decay, can be seen because of the long path length (~ 1000 [e/1020 ev] km) for t -decays. 3. observational problems the extremely low value for the eecr fl ux, corresponding to about 1 event per km2 and century at e > 1020 ev, and the extremely low value for the interaction cross section of neutrinos, make these components difficult to observe if not by using a detector with exceptionally high values for the effective area and target mass. the integrated exposure (~ 2 ¥ 103 km2 yr sr) available today for the ground based arrays operational all over the world is suffi cient only to show the «ankle» feature at ~ 5 ¥ 1018 ev in the cosmic ray energy spectrum and the existence of about ten events exceeding 1020 ev; the limited statistics excludes the possibility of observing signifi cant structures in the energy spectrum at higher energies. experiments carried out by means of the new generation ground-based observatories, hires (fl uorescence) and auger (hybrid), will still be limited by practical diffi culties connected to a relatively small collecting area (< 104 km2 sr) and by a modest target mass value for neutrino detection. to overcome these diffi culties, a solution is provided by observing from space (fi g. 6) the atmosphere uv fl uorescence induced by the incoming extraterrestrial radiation, which exploits up to millions km2 sr for the acceptance area and up to 1013 t as target for neutrino interaction. this is the philosophy of the «airwatch programme» and «euso» is a space mission developed in the airwatch framework. the earth atmosphere in fact constitutes the ideal detector for the extreme energy cosmic rays and the companion cosmic neutrinos. the eecr particles, interacting with the air nuclei, give rise to propagating extensive air showers (eas) accompanied by the isotropic emission of ultraviolet fl uorescence (300-400 nm) induced in nitrogen by the secondary charged particles in the eas as a result of a complex relativistic cascade process; an isotropically diffuse opticaluv signal is also emitted following the impact on clouds, land or sea of the cherenkov beam accompanying the eas. a shower corresponding to a primary with e > 1019 ev forms a signifi cant streak of fl uorescence light over 10-100 km along its passage in the atmosphere, depending on the nature of the primary, and on the pitch angle with the vertical. observation of this fl uorescence light with a detector at distance from the shower axis is the best way to control the cascade profi le of the eas. when viewed continuously, the object moves on a straight path with the speed of light. the resulting picture of the event seen by the detector looks like a narrow track in which the recorded amount of light is proportional to the shower size at the various penetration depths in the atmosphere. from a low earth orbit (leo) space platform, the uv fl uorescence induced in atmospheric nitrogen by the incoming radiation can be monitored and studied. other phenomena such as meteors, space debris, lightning, atmospheric fl ashes, can also be observed, while the luminescence coming from the eas produced by the cosmic ray quanta can be disentangled from the general background exploiting its fast timing characteristic feature. euso observes at nadir from an orbital height of about 400 km. it is equipped with a wide angle fresnel optics telescope (60° full fov) and the focal plane segmentation corresponding to about 1 km2 pixel size on the earth surface. the area covered on earth is of about 160 000 km2. exploiting the high speed of the focal plane de429 euso: using high energy cosmic rays and neutrions as messengers from the unknown universe tector (10 ns class), euso is able to reconstruct the inclination of the shower track by the speed of progression of the projected image on the focal surface and to provide the tri-dimensional reconstruction of the eas axis with a precision of a degree (or better) depending on the inclination. by measuring the eas front luminosity with the photoelectrons (pe) detected by the mapts covering the focal surface, euso registers the longitudinal development of the eas. 3.1. euso general requirements and main goals for a signifi cant observation from a space mission the assumed values are: a) geometrical exposure of (5 ¥ 104-105) km2 sr considering a duty cycle of 0.1-0.15. b) eas energy threshold at about 5 ¥ 1019 ev. eecr statistics – about 103 events/year (an order of magnitude above those expected by the presently planned ground based experiments) to allow a quantitative energy spectral defi nition above 1020 ev, together with the evidence of possible anisotropy effects and clustering (if any) for the directions of arrival. neutrino events – the expected event rate ranges from several events/year (agn, grb source) to several events/day according to the effectiveness of the «topological defects» hypothesis. from the observational point of view, the neutrino induced eas can be distinguished from background and from other eecr eas by triggering on horizontal showers initiating deep inside the atmosphere. moreover neutrinos with energy of about 10151016 ev interacting in the solid earth and emerging upward in the atmosphere create showers which can be detected by euso by means of the cherenkov beamed signal induced in the atmosphere, extending the capability of euso to this lower neutrino astronomy energy band. a horizontal t -neutrino event at energies greater than 1019 ev can be identifi ed by a «double bang» structure. both the initial shower at the nt æ t interaction, and fig. 6. observation of eas from space. 430 livio scarsi another, by the t -decay, can be seen because of the long enough path-length (~ 1000 [e/1020 ev] km) for t -decays observable by euso. t -neutrinos above 1015 ev, on the other hand, will be observed and identifi ed as earth-penetrating «upward» showers (by cherenkov). high nt fl ux by the nm æ nt oscillation and the low detection threshold energy for them allow euso to make oscillation experiments in space as well as nt astrophysics of agn above 1015 ev. 3.2. euso schematic outline euso, originally proposed to esa in january 2000 for a free-fl yer leo mission, was approved in march 2000 for an «accommodation study» on the iss international space station (esa/estec, 2000; scarsi et al., 2000). as a result, euso is now undergoing a «bridging phase» to enter «phase a design» carried out by industry under an esa contract (transportation and transfer to the iss/columbus epf (fi g.7)). under the assumption of both a leo (~ 500 km altitude) free-fl yer mission or the iss accommodation (400 km average altitude), the coverage of the observable atmosphere surface at the scale of 1000 km across and the measurement of very fast and faint phenomena like those euso is interested in, requires: – optical system with large collecting area (because of the faint fl uorescence signal) and wide equivalent fi eld of view covering a sizable half opening angle around the local nadir (to reach geometrical factor of the order of 106 km2 sr). – focal plane detector with high segmentation (single photon counting and high pixelization), high resolving time (~ 10 ns), contained values for weight and power. – trigger and read-out electronics prompt, simple, effi cient, modular, capable to handle hundreds of thousands of channels, and comprehensive of a sophisticated on-board image processor acting as a trigger. 3.3. euso payload: the «main telescope» the euso main telescope is presented schematically in the artistic view of fi g. 8. the instrument consists of three main parts: optics, focal surface detector, trigger and electronics system. an effective synergy between the parts constituting the instrument is of fundamental importance for achieving the euso scientifi c objectives. optics, detector elements, system and trigger electronics have to be matched and interfaced coherently to obtain a correct response from the instrument. scientifi c requirements have been of guidance for the conceptual design of the apparatus and in the choice among various possible technical solutions. the observation from space calls for an approach different from that of the conventional ground based fluorescence experiments. for space application the instrument has to be fig. 7. euso at the cof-epf. fig. 8. view of the euso main telescope. 431 euso: using high energy cosmic rays and neutrions as messengers from the unknown universe compact as much as possible, highly effi cient, and with a built-in modularity in its detection and electronics parts. 3.3.1. the optics the optical system required for euso aims at fi nding the best compromise in the optical design, taking into account the suitability for space application in terms of weight, dimensions and resistance to the strains in launch and orbital conditions. the optical system views a circle of radius ~ 220 km on the earth and resolves 0.8 ¥ 0.8 km2 ground pixels: this determines the detector size to be adopted to observe the events. the forgiving resolution requirements of euso suggest the consideration of unconventional solutions, identifi ed in the fresnel lens technology. fresnel lenses provide large-aperture and wide-fi eld with drastically reduced mass and absorption. the use of a broader range of optical materials (including lightweight polymers) is possible for reducing the overall weight. the present fresnel optical camera confi guration study (fov 60°) considers two plastic fresnel lenses with diameter 2.5 m and iris diaphragm 2.0 m diameter. 3.3.2. the focal surface detector due to the large fov and large collecting area of the optics, the focal surface detector is constituted by several hundreds of thousands of active sensors (ª 2 ¥ 105 pixels). the detector requirements of low power consumption, low weight, small dimension, fast response time, high quantum efficiency in uv wavelength (300-400 nm), single photoelectron sensitivity, limit the fi eld of the possible choices to a very few devices. a suitable off-the-shelf device is the multi-anode photomultiplier hamamatsu r5900 series. these commercial photomultipliers meet closely the requirements imposed by the project. pixel size, weight, fast time response and single photoelectron resolution are well adaptable to the euso focal surface detector. the organization in «macrocells» of the focal surface (a macrocell is a bi-dimensional array of n ¥ n pixels) offers many advantages as easy planning and implementation, fl exibility and redundancy. moreover, modularity is ideal for space application. the multi-anode photomultipliers represent, in this contest, a workable solution. 3.3.3. trigger and electronics system special attention has been given to the trigger scheme where the implementation of hardware/ fi rmware special functions is foreseen. the trigger module named oust (on-board unit system trigger) has been studied to provide different levels of triggers such that the physics phenomena in terms of fast, normal and slow in time-scale events can be detected. particular emphasis has been introduced in the possibility of triggering upward showers (emerging from the earth, «neutrino candidate») by means of a dedicated trigger logic. the fire (fluorescence image read-out electronics) system has been designed to obtain an effective reduction of channels and data to read-out, developing a method that reduces the number of the channels without penalizing the performance of the detection system. rows wiredor and columns wired-or routing connections have been adopted inside every single «macrocell» (n ¥ n pixels unit, ª 100 macrocells constitute the focal surface detector) for diminishing the number of channels to read-out. 4. euso duty cycle the euso duty cycle has been estimated taking into account the following factors affecting the level of background: the iss night time; ground locations with signifi cant light output, natural or anthro-pomorphic; lunar cycle; clouds in the fov strongly affecting the detection or interpretation of the eas; iss activities or contingencies that do not allow the operation of euso. the likely euso duty cycle is in the range 0.1-0.15; a more precise evaluation requires a detailed assessment of the various elements, in particular of the cloud related effects: we, 432 livio scarsi in a conservative approach, use a value of 0.1 throughout this report. 5. expected results the slope of the cr energy spectrum in the region of the gzk limit and above is poorly known because of the reduced statistics available (see fi g. 1, insert). the expected counting rates for energies e.g., above 1020 ev, are therefore diffi cult to defi ne and are strongly dependent from the assumed extrapolation for the energy spectrum. as an example, we show fi g. 9 where the integral count rates are given respectively for a spectral index – 2.7 and – 2.3: the counts per year above 1020 ev varies from 500 to 1300, accordingly. in the following, a spectral index of – 2.7 is assumed (in a conservative way). figures 10 and 11 show the predicted number of eecr and neutrino events per year as a function of energy detected by euso in the original free-flyer and iss configurations. both confi gurations give comparable results within a small factor with the lower observational altitude of the iss (380 km) counterbalancing somewhat the sensitivity afforded by the larger optics diameter of the free-fl yer. the integral number of counts above an energy e for the two confi gurations is shown in fi g. 12 assuming the 2 year operational life of the free-fl yer and a requested 3 year lifetime. 5.1. comparison with ground based observations in the following, a spectral index of – 2.7 is assumed whenever absolute values are quoted for the counting rates above 5 ¥ 1019 ev. currently only the agasa and hires instruments are operational. the agasa experiment near tokyo, with coverage of about 100 km2 (about 300 km2 sr), comprises a scintillator array for electromagnetic shower particles and an array of muon detectors. the hires experiment in utah consists of two fl uorescence detectors at a distance of 12.6 km and, in a fi rst stage, just came online. its aperture is energy dependent and rises from 340 km2 sr at 1019 ev to 1000 km2 sr at 1020 ev. the largest fig. 9. (left) this is fi g. 10 from yoshida et al. (1995): «derived primary energy spectra expressed by eqs. (11a) (solid line) and (11b) (dashed line) and the expected values in each bin simulated under the assumptions of these spectra with the energy resolution of the present experiment (open squares and crosses). black dots with error bars are the raw data. the case of a single power up to the highest energy is also shown by a dotted line and shaded circles.» the slope of the continuous line above 1019 ev is 2.3; for the dashed line the slope is 2.7. the superimposed red points are from takeda et al. (1998). (right) euso counting rates under the hypothesis of the two different spectral index assumed (see fi g. 1 left). 433 euso: using high energy cosmic rays and neutrions as messengers from the unknown universe fig. 10. differential eecr counting rate comparison between the iss version of the euso and the original free fl yer. the dashed zone shows the spectral region where structure induced by the gzk cut-off is expected. the lens diameter is the maximum external diameter allowed in each confi guration. fig. 11. the differential fl ux of neutrinos predicted using the topological defects model of sigl et al. (1999) and the gzk model of stecker et al. (1991). fig. 12. the integral count rates above an energy e predicted for the original free fl yer proposal with 2 years of operations and the iss confi guration with 3 years operations. planned ground-base experiment is the pierre auger observatory, currently under construction in argentina. this will consist of an array of 1600 particle detectors covering 3000 km2 and 4 fl uorescence light detectors, similar to the ones used in the hires experiment. the hybrid detector allows cross calibration and a check of the systematic uncertainties inherent in each of the techniques. the construction is expected to be completed in 2004. by then the auger observatory will have an aperture of 7000 km2 sr, leading to about 30 events per year with energies > 1020 ev. though a second auger observatory is planned in the northern hemisphere, it is not clear if, or when, there will be funding for it. auger will produce a comparable number of events to all previously observed above 1020 ev in only 4 months. nevertheless, a rate of 30 events per year is too small to follow the cr spectrum to such energies, or to obtain the detailed form of the spectrum with small statistical errors. at 1021 ev only about 5 events are expected in 10 years of operation. at least one order of magnitude more statistics is desirable. by the year 2006, provided auger is completed on schedule, the world data set will comprise about 100 events above 1020 ev and perhaps one or two events above 1021 ev, if the spectrum continues without a gzk cut-off. the existence or non-existence of the gzk cut-off will most likely be established by then. however, a defi nite answer concerning the origin and possible identifi cation of sources will certainly rely on the precise spectral form and on the arrival direction distribution. in both cases, good statistics are vital to distinguish between different competing scenarios. for euso, an effective geometrical factor of 5 ¥ 105 km2 sr (fi g. 13) and an observing effi ciency of 0.1 gives an expected event rate of ~ 500 per year for eecr with energies 1020 ev (no gzk suppression and spectral index of 2.7), or some hundreds per year with gzk suppression. the 434 livio scarsi fig. 13. a comparison of the euso effective area with ground based facilities. total number of events per year with the lowenergy threshold of > 3 ¥ 1019 ev is > 1700. for eecr neutrinos the expected rates vary from a few per year (gzk processes, agn, grb sources, etc.) to ~ 150 per year if top-down processes dominate. acknowledgements a general acknowledgemt is due to the euso collaboration. i am deeply indebted to m.c. maccarone for her determinant contribution. references esa/estec (2000): euso accommodation study report, esa/msm-gu/2000.462/ap/rda, december 2000. scarsi, l. et al. (2000): euso extreme universe space observatory, proposal for the esa f2/f3 missions, january 2000 (http://www.ifcai.pa.cnr.it/~euso/docs/ eusoproposal.pdf). sigl, g., s. lee, p. bhattacharjee and s. yoshida (1999): probing grand unifi ed theories with cosmicray, gamma-ray, and neutrino astrophysics, phys. rev. d, 59, 043504, pp. 8. stecker, f.w. and m.h. salamon (1996): high-energy neutrinos from quasars, space sci. rev., 75, 341-355. stecker, f.w., c. done, m.h. salamon and p. sommers (1991): high-energy neutrinos from active galactic nuclei, phys. rev. lett., 66 (21), 2697-2700. takeda, m., n. hayashida, k. honda, n. inoue, k. kadota, f. kakimoto, k. kamata, s. kawaguchi, y. kawasaki, n. kawasumi, h. kitamura, e. kusano, y. matsubara, k. murakami, m. nagano, d. nishikawa, h. ohoka, n. sakaki, m. sasaki, k. shinozaki, n. souma, m. teshima, r. torii, i. tsushima, y. uchihori, t. yamamoto, s. yoshida and h. yoshii (1998): extension of the cosmic-ray spectrum beyond the predicted greisen-zatsepin-kuz’min cut-off, phys. rev. lett., 81 (6), 1163-1166. yoshida, s., n. hayashida, k. honda, m. honda, s. imaizumi, n. inoue, k. kadota, f. kakimoto, k. kamata, s. kawaguchi, n. kawasumi, y. matsubara, k. murakami, m. nagano, h. ohoka, m. teshima, i. tsushima and h. yoshii (1995): the cosmic ray energy spectrum above 3 ¥ 1018 ev measured by the akeno giant air shower array, astroparticle phys., 3 (2), 105-123. yoshida, s., g. sigl and s. lee (1998): extremely high energy neutrinos, neutrino hot dark matter, and the highest energy cosmic rays, phys. rev. lett., 81 (25), 5505-5508. waxman, e. and j. bahcall (1997): high energy neutrinos from cosmological gamma-ray burst fi reballs, phys. rev. lett., 78 (12), 2292–2295. santini 725_730 725 annals of geophysics, vol. 46, n. 4, august 2003 a note on northern marche seismicity: new focal mechanisms and seismological evidence stefano santini istituto di fisica dell’università di urbino, italy abstract a geodynamic interpretation of the northern marche region is diffi cult, the zone being characterized by complex structures which cannot be defi ned in the form of a simple, standard model. it is unquestionable that the geodynamic setting, whatever it is, bears a strong infl uence on the seismic hazard assessment of a region, and this is the background reason for the present note. in order to obtain a more detailed picture of seismological evidence in this zone, 11 new fault plane solutions of crustal events with 2.9 < m < 4.3 were calculated, using data recorded by the national seismic network of the istituto nazionale di geofi sica e vulcanologia in the period 1990-2000. the aim is to add local information to the previous studies by frepoli and amato (1997, 2000). a possible result of this new study is the division of the northern marche region into three areas with different focal mechanism categories: the inner area of the apenninic belt, the adriatic on-shore and the adriatic off-shore. this note is intended to be a contribution to update seismological evidence in the northern marche region. mailing address: prof. stefano santini, istituto di fisica dell’università di urbino, via s. chiara 27, 61029 urbino, italy; e-mail: santini@fi s.uniurb.it key words focal mechanisms northern apennines adriatic off-shore seismological evidence 1. introduction for the northern apennines, available data consistently indicate active extension in the axial zone of the chain, where the main active structures consist of border faults to a series of nw-se oriented intramontane basins (lunigiana, garfagnana, mugello, casentino, high tiber valley; e.g., galadini et al., 2001). a general agreement exists on the fact that a roughly ne-sw oriented minimum stress axis (s3) is dominant in this part of the orogen (borghini et al., 2000). in the studied rectangular area (outer northern apennines and related adriatic off-shore; fi g. 1), thrust tectonics has been widely documented by field surveys and by the interpretation of seismic refl ection lines and deep wells (e.g., bally et al., 1986; barchi et al., 1998). short note 726 stefano santini on-shore, available fi eld data show that the most important evidence of active tectonics in the studied area is a well documented and generalised pleistocene uplift (e.g., dramis, 1992; borraccini et al., 2002). the studied area is the locus of a moderate – yet signifi cant – tectonic activity, as also witnessed by historical and instrumental seismicity (imax = ix mcs; mmax = 6; gruppo di lavoro cpti, 1999; frepoli and amato, 2000). the sources of these earthquakes are not well known (galadini et al., 2000) and the possible existence and size of currently silent sources is a matter of study (valensise and pantosti, 2001). 2. seismic data many focal mechanisms of moderate events with magnitudes ranging between 4.0 and 5.8 have been determined for the northern apennines. the available fault-plane solutions of the period 1939-1980, calculated from arrival times and polarities read from seismic bulletins, were reported by gasparini et al. (1985): the best focal mechanisms are shown in fi g. 1 as selected by zoback (1992), which used p and t axes to divide the data into five main stress regime categories and an unknown category when the data provide no information on relative stress magnitudes (see table i). table i. fault plane solutions (fps) categories of zoback (1992) used for classifi cation of events shown in fi g. 4. plunges of axes (pl) fps categories by figure 4 notation legenda p t zoback (1992) (label color) pl ≥ 52° pl £ 35° nf blue labels normal-fault 40° £ pl £ 52° pl £ 20° ns blue labels with small-slip-component pl < 40° pl £ 20° ss red labels strike-slip pl £ 20° pl < 40° ss red labels strike-slip pl £ 35° pl ≥ 52° tf green labels thrust-fault pl £ 20° 40° £ pl £ 52° ts green labels with small-slip-component others others uc black labels unknown category fig. 1. fault-plane solutions from gasparini et al. (1985) (in black), cmt solutions (in light grey), solutions of the four largest events of the porto san giorgio sequence of july 1987 from riguzzi et al. (1989) and of the six largest events of the colfi orito sequence of september-october 1997 from santini et al. (2003) (dark grey), and the studied area (in the box) of the northern marche region. 727 a note on northern marche seismicity: new focal mechanisms and seismological evidence the fi gure displays also two centroid moment tensor (cmt) solutions of the norcia (1979) and perugia (1984) earthquakes, the focal mechanisms of four events of the porto san giorgio seismic sequence of july 1987 (riguzzi et al., 1989) and the focal mechanisms of six events of the colfi orito seismic sequence of septemberoctober 1997 (santini et al., 2003). the two cmt solutions show an evident normal-fault regime along the portion of the apenninic belt, while the strike-slip solution of the ancona (1972) event, shows a compressional axis (p) with ene-wsw orientation (frepoli and amato, 1997). the only events in previous studies suggesting an evident active compression along the outer part of the apenninic belt in the northern marche occurred on january 26, 1990, and december 15, 1991, as recolored in dark grey in fi g. 2 by frepoli and amato (1997). the earthquakes analyzed in this study are located in the northern marche from 43° to 44°n and from 12° to 13.5°e (table ii). arrival times and polarities were accurately re-picked from digital seismic waveforms recorded by the seismic network of the ingv in the period 1990-2000. the duration magnitudes m d of these events range between 3.0 and 4.2. the smaller events are not considered because their mechanism cannot be calculated with good accuracy due to the lack of off-shore seismograph stations: therefore it is advisable to consider magnitudes greater than 3-3.5. the locations of considered table ii. parameters of the seismic events whose epicentres and focal mechanisms are shown in fi g. 4. label year month day lat long depth m d strike dip rake rms f (°n) (°e) (km) (s) 1 1990 august 27 44.02 13.177 5.0 3.9 115° 50° 0° 0.10 0.00 2 1991 november 22 43.842 12.062 5.9 3.0 165° 60° 70° 0.02 0.03 3 1996 june 28 43.769 12.995 27.5 3.4 155° 40° 110° 0.16 0.12 4 2000 february 22 43.79 12.083 8.0 3.0 265° 65° -120° 0.01 0.13 5 2000 may 5 44.014 13.192 5.0 4.1 195° 25° 30° 0.25 0.20 6 2000 june 25 43.886 13.147 5.0 3.5 130° 90° 50° 0.16 0.18 7 2000 june 27 43.883 13.200 5.0 3.4 115° 85° 40° 0.06 0.14 8 2000 august 1 43.929 12.318 5.0 4.2 160° 70° 130° 0.02 0.10 9 2000 december 27 43.678 12.245 5.0 3.2 115° 30° 80° 0.13 0.12 fig. 2. thrust-fault solutions from frepoli and amato (1997): the solutions of seismic events which occurred in the studied area are recolored in dark grey. table iii. velocity model adopted for focal mechanism computation. velocity (km/s) depth (km) 5.7 0-12 6 12-18 6.2 18-26 6.7 26-35 7.5 35728 stefano santini fig. 3. lower hemisphere, equal-area projection of the 11 selected fault plane solution presented in this study. compression and dilatation polarities are indicated with crosses and circles, respectively. the 9 focal mechanisms of fi g. 4 are the fi rst solutions; in the case of double solutions (5 and 7), the second solutions with a worse stability are shown. 1 2 3 4 5 6 7 8 9 729 a note on northern marche seismicity: new focal mechanisms and seismological evidence events were verifi ed with the computer program hypoinverse (klein, 1989) using the velocity structure reported in table iii. good values of rms residuals were obtained for most analyzed earthquakes; in particular the rms values of locations have residuals below 0.25 s; furthermore, a misfi t of polarity quality factor (f ) was below 0.20. the fault plane solutions were calculated with the program fpfit (reasenberg and oppenheimer, 1985) and the 11 fault plane solutions were obtained with more than eight polarities. 3. new focal mechanisms this classification is founded on the assumption that the earthquake focal mechanisms refl ect the state of stress of the northern marche region, implying that the pand t-axes, to a fi rst approximation, correspond to the principal stress axes s1 and s3, respectively. figure 3 shows fault plane solutions for 9 events which occurred in the northern marche from 1990 to 2000. the multiple solutions are due both to the low weight of some polarities and to the low azimuthal coverage of data on the focal sphere. in fi g. 4 the solutions with the smaller uncertainties were chosen, represented by the fi rst solutions of the 9 focal mechanisms of fi g. 3; if one considers the alternative solutions, the tectonic setting does not change much. the 9 fi rst solutions of fi g. 3 are divided as follows: – three solutions (labels 2, 4 and 9) belong to the pure normal-fault category (nf); – two solutions (labels 3 and 5) belong to the pure thrust-fault category (tf); – one solution (label 8) belongs to the thrustfault with small strike-slip category (ts); – three solutions (labels 1, 6 and 7) are included in the unknown-stress-fi eld category (uc). if plunges of the axes of solutions are inside or outside the ranges defi ned in table i with differences of only one degree these solutions were assigned to the appropriate category: in this case, one solution (label 5) belongs to the thrust-fault with small strike-slip category with a difference of 0.2 degrees, as it is possible to see in table ii. the eastern part of the examined area was also affected by a few historical events of magnitude larger than 5.5. the estimated sources of these events are shown in fi g. 4. the northernmost box (west of pesaro) represents the source of the coriano earthquake (december 25, 1786; fig. 4. epicentre location and focal mechanisms of seismic events listed in table ii. fault plane solution (fps) categories refer to zoback (1992; table i). historical boxes after valensise and pantosti (2001). 730 stefano santini 43.98°n; 12.58°e), an event of intensity i0 = viii and equivalent magnitude m e = 5.5 (gruppo di lavoro cpti, 1999). this source is derived by the analysis of the isoseismal map produced by guidoboni and ferrari (1986); it strikes 20º ± 15º and is 5.6 km long and 5.2 km wide. the two other sources, instead, are exclusively derived from intensity data by applying the method proposed by gasperini et al. (1999), as the quality of the obtained solution allowed the representation of the source by an oriented rectangular box (valensise and pantosti, 2001). the box south-west of fano (43.71°n; 12.97°e) corresponds to the september 21, 1897 earthquake (i0 = vii and me = 5.4; gruppo di lavoro cpti, 1999) and strikes 73º ± 48º. the southernmost box represents the seismic source for the cagli event (43.59°n; 12.51°e) which occurred on june 3, 1781 (i0 = ix-x and m e = 6.2; gruppo di lavoro cpti, 1999). this source strikes 118º ± 17º. 4. conclusions the northern apennine chain was described as a tectonic belt characterized by extension (e.g., lavecchia, 1988; decandia et al., 1998). general agreement exists on the fact that a ne-sw oriented minimum stress axis (s3) is dominant in the axial zone of the chain (mariucci et al., 1999; boncio et al., 2000). a possible result of this study is the division of the northern marche region into three areas with different seismological evidences: – the inner area of the apenninic belt, which is caracterized by extension; – the adriatic on-shore, which shows evidence of active compression; – the adriatic off-shore, which shows not very clear evidence of active compression. in the extensional area, the t-axes of normal solutions (2, 4 and 9 in fi g. 4) do not have a homogeneous orientation, while in the compressional area the p-axes of thrust solutions (3 and 8 in fi g. 4) have a dominant nw-se direction (in agreement with compressive stress regime mapped by montone et al. (1999) in northern surrounding zones of the northern marche). finally, the adriatic off-shore zone shows not clear seismological evidence of nevertheless possible active compression; on the other hand no earthquake with magnitude ≥ 5 occurred during historical times in the off-shore zone of the northern marche region (fi g. 5a,b). fig. 5a,b. a) historical seismicity examined by santini (2000), from boschi et al. (1997) and gruppo di lavoro cpti (1999); the montefeltro area (green zone) is characterized by limited values in terms of intensity and frequency (blue ellipse). b) epicentres of seismic events with magnitude ≥ 4 recorded by the national seismic network of ingv from year 1990 to 2003 (blue circles with magnitude < 5 and red circles with magnitude ≥ 5). a b 731 a note on northern marche seismicity: new focal mechanisms and seismological evidence acknowledgements i am grateful to michele dragoni and another anonymous referee for their reviews, which improved the paper. i thank gianluca valensise for his critical and constructive comments on the scientifi c starting point of this study, and alessandro amato for his encouragement with the present note. this research has been financially supported by «cofin 2001» of ministero dell’istruzione, dell’università e della ricerca (miur). references bally, a.w., l. burbi, c. cooper and r. ghelardoni (1986): balanced sections and seismic refl ection profi les across the central apennines, mem. soc. geol. ital., 35, 257-310. barchi, m.r., g. minelli and g. pialli (1998): the crop 03 profi le: a synthesis of results on deep structures of the northern apennines, mem. soc. geol. ital., 52, 383-400. boncio, p., f. brozzetti and g. lavecchia (2000): architecture and seismotectonics of a regional lowangle normal fault zone in central italy, tectonics, 19, 1038-1055. bo rg h i n i, g., m. be r n i n i and g. pa p a n i (2000): individuazione e mappatura di faglie attive e/o strutture sismogenetiche in appennino nord-occidentale, in le ricerche del gndt nel campo della pericolosità sismica (1996-1999), edited by f. galadini, c. meletti and a. rebez, 143-148. borraccini, f., m. de donatis, d. di bucci, s. mazzoli, a. megna, o. nesci, s. santini, d. savelli, m. tramontana and p. triggiani (2002): analisi della tettonica quaternaria nel basso bacino del f. metauro (marche settentrionali) e nell’adiacente offshore adriatico attraverso l’integrazione di dati sismici, geomorfologici, stratigrafi ci e strutturali, studi geologici camerti, 2, 29-43. boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (editors) (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990 (ing, roma sga, bologna), vol. 2, pp. 644. decandia, f.a., a. lazzarotto, d. liotta, l. cernobori and r. nicolich (1998): the crop 03 traverse: insights on post-collisional evolution on the northern apennines, mem. soc. geol. it., 52, 427-439. dramis, f. (1992): il ruolo dei sollevamenti tettonici a largo raggio nella genesi del rilievo appenninico, studi geologici camerti, 1 (special volume), 9-15. frepoli, a. and a. amato (1997): contemporaneous extension and compression in the northern apennines from earthquake fault plane solutions, geophys. j. int., 129, 368-388. frepoli, a. and a. amato (2000): spatial variations in stresses in peninsular italy and sicily from background seismicity, tectonophysics, 317, 109-124. galadini, f., c. meletti and e. vittori (2000): stato delle conoscenze sulle faglie attive in italia: elementi geologici di superfi cie, in le ricerche del gndt nel campo della pericolosità sismica (1996-1999), edited by f. galadini, c. meletti and a. rebez, 107-136. galadini, f., c. meletti and e. vittori (2001): major active faults in italy: available surfi cial data, neth. j. geosci. geol. mijnbouw, 80, 273-296. gasparini, c., g. iannaccone and r. scarpa (1985): fault-plane solutions and seismicity of the italian peninsula, tectonophysics, 117, 59-78. gasperini, p., f. bernardini, g. valensise and e. boschi (1999): defi ning seismogenic sources from historical earthquake felt reports, seismol. soc. am. bull., 89, 94-110. gruppo di lavoro cpti (1999): catalogo parametrico dei terremoti italiani (ing, gndt, sga, ssn), pp. 88. guidoboni, e. and g. ferrari (1986): il terremoto di rimini e della costa romagnola: 25 dicembre 1786. analisi e interpretazione (sga), 127-142. klein, f.w. (1989): hypoinverse, a program for vax computers to solve for earthquake location and magnitude, u.s. geol. surv. open-file rep., 89-314. lave c c h i a, g. (1988): the tyrrhenian-apennine system: structural setting and seismotectogenesis, tectonophysics, 147, 263-296. mariucci, m.t., a. amato and p. montone (1999): recent tectonic evolution and present stress in the northern apennines, tectonics, 18, 108-118. montone, p., a. amato and s. pondrelli (1999): active stress map of italy, j. geophys. res., 104, 25,59525,610. reasenberg, p. and d. oppenheimer (1985): fpfit, fpplot and fppage: fortran computer program for calculating and displaying earthquake fault-plane solutions, u.s. geol. surv. open-file rep., 85/739, pp.109. riguzzi, f., a. tertulliani and c. gasparini (1989): study of the seismic sequence of porto san giorgio: marche 3 july 1987, nuovo cimento c, 12, 453-466. santini, s. (2000): montefeltro seismicity: from serpieri’s seismograph to the rsnc seismograph station, ann. geofi s., 43 (3), 505-517. santini, s., p. baldi, m. dragoni, a. piombo, s. salvi, g. spada and s. stramondo (2003): montecarlo inversion of dinsar data for dislocation modeling: application to 1997 umbria-marche seismic sequence (central italy), pure appl. geophys. (in press). valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger than m 5.5 in italy, ann. geofi s., 44 (suppl. to n. 4), pp.180 (with cd-rom). zoback, m.l. (1992): firstand second-order patterns of stress in the lithosphere: the world stress map project, j. geophys. res., 97, 11,703-11,728. (received april 7, 2003; accepted july 15, 2003) geo 1121 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 11 investigation into the problem of characterization of the hf ionospheric fluctuating channel of propagation: construction of a physically based hf channel simulator vadim e. gherm (1), nikolay n. zernov (1) and hal j. strangeways (2) (1) university of st. petersburg, russia (2) school of electronic and electrical engineering, university of leeds, u.k. a wideband hf simulator has been constructed that is based on a detailed physical model. it can generate an output giving a time realization of the hf wideband channel for any hf carrier frequency and bandwidth and for any given transmitter receiver path, time of day, month and year and for any solar activity/geomagnetic conditions. to accomplish this, a comprehensive solution has been obtained to the problem of hf wave propagation for the most general case of a 3d inhomogeneous ionosphere with time-varying electron density fluctuations. the solution is based on the complex phase method (rytov’s method), which has been extended to the case of an inhomogeneous medium and a point source of the field. results of simulation obtained according to the technique developed have been presented, calculated for a single-hop path 1000 km long oriented to the south from st. petersburg and including a horizontal electron density gradient present in the iri model used as the basis of the ionosphere model. the fluctuations of the ionospheric electron density were characterized by an inverse power law anisotropic spatial spectrum. for this model, the random walk of the phasor at the receiver is determined and shown both for paths reflected in the eand fregions, being significantly larger for the latter. the oblique sounding ionogram is constructed and reveals three propagation modes: the e-mode and low and high angle f-mode paths. the time-varying field due to each of these paths is then summed at the receiving location enabling the calculation of the scattering function and also the time realization of the received signal shown as a function of both fast and slow time. this is performed both with and without the presence of the geomagnetic field; in the former case the splitting of the f2-mode into both eand o-modes is seen. it is also shown how the scattering function can be obtained from the time realization of the channel in a way akin to experimental determination of the scattering function from channel measurements. results from the simulations show the very significant effect of irregularities of even modest magnitude and the comparative effects due to background ionosphere dispersion and the fluctuating irregularities as well as geomagnetic mode splitting. since the simulator is based on a physical model, it should be possible by comparison of experimental results and simulation to identify the correspondence between physical parameters (e.g., the variance and anisotropy of the electron density fluctuations, orientation of the propagation path to the magnetic meridian, bulk ionosphere motions) with observed channel parameters (e.g., doppler spread and shift, time delay spread). 1122 vadim e. gherm, nikolay n. zernov and hal j. strangeways 11.1. introduction to properly characterize the ionospheric reflection hf fluctuating channel of propagation on a physical basis, a comprehensive solution should be devised to solve the problem of high frequency wave propagation in the 3d inhomogeneous (rigorously also anisotropic) ionosphere with local random inhomogeneities embedded. this entails a comprehensive solution to the problem of wave propagation in a random medium for the most general case of a 3d inhomogeneous dispersive medium with fluctuations of the parameters including the case of strong scintillation (strong fluctuation of the field amplitude), or that of the saturated regime of propagation. although a rigorous solution to this problem is not currently available, we consider the best available solution that is based on the complex phase method (in classical terms, rytov’s method), which we have extended to the case of an inhomogeneous medium and a point source of the field. first, zernov (1980) extended this method to the case of a plane-layered background medium and a point source. this permits simultaneous accounting for the ray bending and the scattering by local random inhomogeneities, including diffraction effects on local inhomogeneities. this extension was intensively employed in a series of papers (gherm and zernov, 1995; gherm et al., 1997a,b; gherm and zernov, 1998; gherm et al., 2001a,b, 2002, 2003) to study effects due to fluctuations of the electron density in the plane-stratified ionosphere. gherm and zernov (1995) studied the statistical moments of the phase and log-amplitude of the hf field, in particular, the effect of fresnel filtering. subsequent papers (gherm et al., 1997a,b) were devoted to the effects on propagation of both narrowand wideband pulses in the fluctuating ionosphere. moments of the full field in the hf-band, in particular the scattering function, were studied in a series of papers (gherm and zernov, 1998; gherm et al., 2000, 2001a,b, 2002). a review paper on hf propagation in the ionosphere disturbed by random inhomogeneities was subsequently given at the 27th general assembly of ursi in maastricht, the netherlands by zernov et al. (2002). however, all the investigations described were limited by a plane-stratified isotropic model of the background ionosphere. it was then considered important to extend the complex phase method to the case of a 3d inhomogeneous background medium, which is necessary, in particular, to account for the effects of a horizontally inhomogeneous background ionosphere. this extension was made by gogin et al. (2001) and thus enables the widening of the range of possible conditions of the hf propagation in the ionosphere. finally, the most general theory was developed in the scope of the project «wideband hf and uhf simulators for ionosphericaly reflected transionospheric channels», (under the financial support of the u.k. epsrc visiting fellowships programme, grant gr/r37517/01) where the effects of the anisotropy of the ionosphere (including the background ionosphere) were taken into account. this project was performed in collaboration between the university of st. petersburg, st. petersburg, russia (nnz and veg) and the university of leeds, leeds, u.k. (hjs). this investigation resulted in producing a physically based software simulator for the hf ionospheric reflection fluctuating channel of propagation which overcame limitations of empirically based models such as that developed by mastrangelo et al. (1997) on the basis of the work by vogler and hoffmeyer (1993). this simulator requires a large number of input parameters for each propagation mode which cannot be directly determined from the parameters of the transmission path and model ionosphere. further, it is also based on a limited amount of data for an undisturbed ionosphere and it is rather questionable whether extrapolation to disturbed ionosphere conditions is valid. the basis for the delay profile modelling which is the main difference from the watterson model is also not clear. to overcome these and other limitations, the wideband hf simulator constructed is based on a detailed physical model. it can generate an output giving a time realisation of the hf channel (as well as the statistical moments of a signal, e.g., the scattering functions) for any bandwidth (up to a mhz) and for any given time, path and conditions. to characterise the hf channel of propagation, an analyticnumerical technique has been used, which employs the complex phase method, which, as was just 1123 characterization of the hf ionospheric fluctuating channel of propagation: hf channel simulator mentioned, was recently extended to the case of a 3d inhomogeneous and even anisotropic medium. the main principles of constructing this physically based software simulator will be outlined below. 11.2. main relationships it is assumed that a random realization of a pulsed signal propagated through the fluctuating ionosphere is represented as the following fourier integral in the frequency domain: ( , , ) ( ) ( , ) ( , , )u t t p e r t e dr r rω m m i t m go 0= ~ ~ ~ 3 3 + ~#! . (11.1) here p(ω) is the spectrum of a launched pulse, e go m0 represents the transfer function for a given m-th path in the undisturbed channel, where the field in the undisturbed channel is represented by the sum ( ) ( )e er r m go m0 0 = ! (11.2) of the geometrical optics type fields e go m0 , each propagating from a source to a receiver along its own path of propagation. once the model of the 3d background ionosphere is given, quantities e m go 0 are calculated employing the appropriate ray-tracing code, which also permits calculation of the ray tube divergence. accepting representation (11.2) for the undisturbed field implies a limitation to cases where the observation points are far from any caustic (i.e. far from the skip distance, if the transmitter and receiver are located on the earth’s surface). the random phasor rm(r, ω, t) is introduced in (11.1) to account for the effects of fluctuations of the electron density of the ionosphere. variable t is the flight time of a pulse and t denotes the slow time dependence of fluctuations, which can be treated in the quasi-stationary approximation. the background channel is assumed to be stationary; that is time independent. a corollary of this is the absence of slow time dependence in the transfer functions of the background channel in (11.2). the summation in (11.2) is performed over all paths of propagation from a transmitter to a receiver. according to the complex phase method, random phasors rm(r, ω, t) are represented utilising complex phases as follows: t( ) ( , , ), , expr tr r m m = } ~~ 8 b (11.3) where the first and second order complex phases ψm, in powers of the disturbances of the dielectric permittivity of the medium of propagation, are utilised in the following consideration. complex phases ( , ,, , ,, t is ttr r rm m m= +} | ~ ~~ ) _ _i i (11.4) are random functions with the real part χm representing log-amplitude fluctuations and sm giving the fluctuations of the phase of the field. thus, eqs. (11.1) to (11.4) comprise the formal basis to produce both the random time series (sequences) of the pulsed signal (11.1) and its statistical moments, e.g., the scattering functions. analytical expressions for ψm (r, ω, t) can be found in gogin et al. (2001). the appropriate statistical moments of the log-amplitude and phase, which are necessary to produce both random series of a signal and its scattering function, are constructed and calculated in raycentred variables associated with every path of propagation in the background medium, linking the transmitter and receiver. these paths are constructed by means of the numerical ray-tracing for the background ionosphere. numerical rather than analytic ray-tracing (e.g., using the multi-quasi-parabolic model) is required since the effect of the geomagnetic field is included to determine both oand 1124 vadim e. gherm, nikolay n. zernov and hal j. strangeways e-modes and also to find paths through any 3d ionosphere model including horizontal gradients. the presence of some local deterministic inhomogeneities (e.g., tids) or regular doppler shifts due to bulk motion of the ionosphere can also be accounted for. oblique sounding ionograms are constructed which are then employed to specify the possible paths of propagation between the given transmitter and receiver locations, so that multi-path effects are taken into account. each propagation path for each magneto-ionic mode is determined by employing a homing-in procedure using the neldermead algorithms (strangeways, 2000) together with the 3d ray-tracing program. 11.3. production of random time series to produce time series of a pulsed signal given by the fourier integral (11.1) for a point of observation r, two real random functions χm and sm must be generated in a two dimensional domain (ω, t) for a given value of r. this demands knowledge of the probability density functions for χm and sm, as well as their autoand cross-correlation functions. in the scope of the complex phase method, these functions are given as follows: ( , ; , ) , ,b t t t t1 2 1 2 1 1 1 1 2 2=~ ~ | ~ | ~| _ _i i (11.5) ( , ; , ) , ,b t t s t s ts 1 2 1 2 1 1 1 1 2 2=~ ~ ~ ~_ _i i (11.6) ( , ; , ) , ,b t t t s ts 1 2 1 2 1 1 1 1 2 2=~ ~ | ~ ~| _ _i i . (11.7) according to the complex phase method, quantities χm1 and sm1 are represented by linear integrals over many random inhomogeneities. this guarantees, according to the central limit theorem, a normal distribution for both the random functions χm1 and sm1. when averaging in eqs. ((11.5) to (11.7)), it is also implied that the electron density fluctuations along different paths of propagation are not correlated. this is in reasonable agreement with the requirement that the main fresnel volumes of the neighbour rays do not overlap. the exception is only the case of oand e-modes, where cross-correlation is accounted for, because the magneto-ionic split does not result in wide enough displacement between o-mode and e-mode paths of propagation. all the three functions from eqs. ((11.5) to (11.7)) can be expressed through the two correlation functions of the complex phase ψ1. these correlation functions are of the following form: f, , ( ) ; , , ( ) ( ) ( ) exp exp b t k k s ds d d b s i v t i v t k k i k k d s d s d s 2 0 2 2 n n n n n s n n n n 1 1 2 1 2 0 0 1 2 1 2 2 2 0 $ $ = + + + ~ ~ r f l l l l l l l l l l ∆ ∆-} x x x x x x x x x ##_ _ ^ ^ ^ i i h h h 8 7 b a) 3 (11.8) f, , ( ) ; , , . exp exp b t k k s ds d d b s i v t i v t k k i k k d d d 2 0 2 2 n n n n n s n n n n 2 1 2 1 2 0 0 1 2 1 2 2 2 0 $ = + + + + ~ ~ r f l l l l l l l l l l ∆ ∆-} x x x x x x x x x ##_ _ ^ ^ ^ i i h h h 8 7 b a) 3 (11.9) here, k = ω /c, bε (s; 0, κn, κτ) is a 3d spatial spectrum of the electron density fluctuations with zero value of the spectral variable, fourier-conjugated to the variable s along the path. it is also the function of the variable s along the reference ray. spectral variables κn and κτ are fourier-conjugated to the spatial variables q1 and q2 lying in the plane perpendicular to the reference ray at each point. quantities ∆n and ∆τ are the components of the vector of the distance between the rays corresponding 1125 characterization of the hf ionospheric fluctuating channel of propagation: hf channel simulator to frequencies ω1 and ω1, which also depend on s. additionally, the hypothesis of the «frozen drift» of random inhomogeneities is accepted, so that vn and vτ are the components of the frozen drift velocity, also depending on the point along the reference ray, and t− = t1− t2 is the difference slow time. the central slow time t+ is not involved in eqs. ((11.8) and (11.9)), because of the assumption of the statistical homogeneity of fluctuations. coefficients dn, dτ, and dnτ are the elements of the matrix ( )d b 1 = + -t t , which is the inverse of the matrix b̂ + , introduced in gogin et al. (2001). matrix b̂ + is actually a sum of curvature matrices of the incident field and of the green function. these are obtained by integrating the corresponding differential equations along the reference ray and also depend on the variable s. in the numerical calculations, the model of the ionospheric fluctuations is considered as turbulence, having the anisotropic spatial spectrum with the inverse power law of the form as follows: ( , ) ( ) ( ) .b s c s s k k 1 1n n tg tg tr tr p 2 0 2 2 2 2 2 2 2 = + +l f v l l f f p7 a (11.10) here c n 2 is a known normalization coefficient. k l2tg tg 1 = r , where l tg is the outer scale of the turbulence along the geomagnetic field, and k l2tr tr 1 = r , where l tr is the outer scale of the turbulence across the magnetic field. function ε 0 (s) is the distribution of the dielectric permittivity of the background ionosphere and ( )sn 2v is that of the variance of the relative fluctuations of the electron density of the ionosphere, both along the reference ray in the 3d inhomogeneous ionosphere. as a result, functions ((11.8) and (11.9)) are of a very high degree of the generality, valid for arbitrary three dimensional models of the background ionosphere and fluctuations of the ionospheric electron density. all the above-mentioned results permit us to uniquely produce random time series of functions χm and sm in the domain (ω, t ), if additionally the cross-correlation (11.7) is also properly accounted for when generating the series. in turn, this permits us to generate random values of phasor rm(r, ω, t) in the same domain, and finally to generate random series of a signal propagated through the fluctuating ionosphere employing the appropriate methods of numerical calculation of the integrals in eq. (11.1). below we shall present some results of simulation obtained according to the technique developed. all the results have been calculated for a single-hop path 1000 km long oriented to the south from st. petersburg. the iri model for july at 14:00 lt was chosen for the transmitter site at st. petersburg and for the receiver site 1000 km to the south of st. petersburg. for this path, horizontal gradients of the electron density resulted in a difference of 0.5 mhz in fof2 between the transmitter and receiver. the fluctuations of the ionospheric electron density were characterized by the inverse power law anisotropic spatial spectrum with the spectral index of 3.7, the scale of random inhomogeneities across the geomagnetic field of 3 km and the aspect ratio of 5. the variance of relative fluctuations of the electron density was assumed to be uniform along the path of propagation and equal to 10-6. the hypothesis of frozen drift of random inhomogeneities was utilized with the same horizontal longitude and latitude velocity of 0.5 km/s. the bandwidth of the non-monochromatic transmitted signal was 20 khz. for the first step, the oblique sounding ionogram was constructed for the model of the background ionosphere chosen, which indicated possible high angle and low angle fand e-mode paths of propagation. in fig. 11.1, the random walk is shown for the phasor corresponding to the e-mode propagation, whereas fig. 11.2 demonstrates the same for the phasor of the high angle f-mode. clearly, the spread of possible random values of the phasor on the plot of fig. 11.2 is significantly wider due to the higher density of the background ionosphere at the altitude of the f-layer, leading to the higher value of the absolute fluctuations of electron density. as an example, in figs. 11.1 and 11.2 the random walk is calculated with no geomagnetic field, however, similar plots can be produced for the case when the geomagnetic field is present. 1126 vadim e. gherm, nikolay n. zernov and hal j. strangeways fig. 11.1. random walk of phasor for the e-mode. fig. 11.2. random walk of phasor for the high angle f-mode. the case of stronger fluctuations. fig. 11.3. realization of the received signal plotted in slow time and fast time variables. fig. 11.4. realization of the received signal plotted in slow time and fast time variables calculated for the case of inclusion of the geomagnetic field. 11.3 11.4 in the following two plots (figs. 11.3 and 11.4) random series of a signal at the location of the receiver are demonstrated. to obtain them, random integrals (11.1) were calculated, having the appropriate distributions of the phasors available, as the function of the fast time (time of flight) for different moments of the slow time. the plot in fig. 11.3 shows the case without the geomagnetic field, where the contributions of three paths of propagation (high and low f and e) are well resolved. by contrast with the geomagnetic field present, as in fig. 11.4, contributions due to four paths of propagation are clearly seen. for this case, the high angle f ray is split into both eand o-mode, whereas this splitting cannot be seen for the other modes. 11.4. multi-moded wideband scattering functions the scattering function of a pulsed signal is introduced (proakis, 1983; vogler and hoffmeyer, 1993; mastrangelo et al., 1997; gherm et al., 2001a,b) as the appropriate fourier transform of the 11.1 11.2 1127 characterization of the hf ionospheric fluctuating channel of propagation: hf channel simulator auto-correlation function of the random channel impulse response on the difference slow time variable. utilising (11.1) the auto-correlation function of a pulsed signal on slow time t can be written as follows: , , ( ) ( ) ( ) ( ) ( , ; , ) ( ) ( ) ( ) .exp t t t p p f f t t ik ik i t d d ψ ψu m m rm m m m 1 2 1 2 1 2 1 2 1 2 1 1 2 2 1 2 1 2 $ $ = ~ ~ ~ ~ ~ ~ { ~ { ~ ~ ~ ~ ~ ) )# !_ i 8 b (11.11) here the spatial variable r was suppressed and the following relationships have been introduced ( ) ( ) ( )expe f ik m m go m0 =~ ~ { ~8 a (11.12) , ; , , ,t t r t r tψrm m m1 2 1 2 1 1 2 2=~ ~ ~ ~ ) _ _ _i i i (11.13) equation (11.12) shows explicitly the amplitude and phase of the field e m go 0 , which are calculated for each possible mode of propagation, defined by the model of the background medium. equation (11.13) is the definition of the two-frequency two-time correlation function of the random phasor rm(ω, t). it is convenient to work with the central and difference variables in the frequency and slow time domains ( ), 2 1 1 2 1 2= + = -~ ~ ~ ~ ~ ~+ , (11.14) ( ),t t t t t t 2 1 1 2 1 2= + = -+ . (11.15) utilising new variables in eq. (11.11) and performing fourier transformation on difference slow time t−, the following equation for the scattering function (which is sometimes termed as the wideband scattering function) is obtained , , , ; , ( ) .exp s t t p p f f t t i t t i t d d dtψ 2 1 2 2 2 2 d m m m rm gm d $ $ = + + + ~ r ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ) ) + + + + + + + + + # !_ b b b b _ _ i l l l l i i9 c (11.16) here the summation is performed over all paths of propagation from the source to receiver. the scattering function of the channel s(t, t+, ωd) depends on the doppler variable ωd, (fourier-conjugated to t−), the group-delay spread t and, generally, on the slow time t+. the latter dependence vanishes when the random ionospheric fluctuations are assumed to be statistically stationary (the same as in eq. (11.8)). group delay time tgm(ω+) is given by the equation ~= ( ) ( )t cgm m2 2 =~ ~ ~ { ~+ ~ + ; e (11.17) and is calculated for each mode of propagation. in the framework of the complex phase method, the frequency and time correlation function of the random phasor ψrm, in the integral (11.16), is expressed through the statistical moments of the complex phase (gherm and zernov, 1998; gherm et al., 2001a) as follows: 1128 vadim e. gherm, nikolay n. zernov and hal j. strangeways , ; , , ; ,expt t v v t tψ ψ 2 2 1rm = + -~ ~ ~ ~ ~ ~ ~ ~)+ + + + + + -}_ b b _i l l i9 c' 1 (11.18) ( ) ( ) ( ) .expv 2 1 2 1 2 = +~ } ~ } ~; e (11.19) function ψψ is the auto-correlation function of the complex phase with the main term employing the complex phase method by the relationship , ; , , ,t t t t t tψ 2 2 2 2 1 1= + + -~ ~ } ~ ~ } ~ ~)+ + + + + + -}_ b bi l l . (11.20) functions ((11.18) to (11.20)) have been studied in detail in gherm and zernov (1998) for the case of a horizontally stratified background medium. the scattering function (11.16), also for the stratified background medium, has been studied in gherm et al. (2000, 2001a,b). the extension of the complex phase method, obtained above for the case of a 3d inhomogeneous background medium (gogin et al., 2001), naturally permits the technique of calculation of the scattering functions to be extended to the general case of a 3d background ionosphere. the statistical moments of the complex phases involved in equations ((11.16), (11.18) to (11.20)) are given through the representations ((11.8) and (11.9)). they are all derived analytically and then calculated numerically for a given model of the background ionosphere. figure 11.5 presents the scattering function in the form of a contour plot, calculated according to the outlined technique as the appropriate statistical moment of the signal. the adjacent contours are separated by 5 db and range from 0 to – 30 db. this calculation was performed for the case of no geomagnetic field. there is also another possible method of obtaining the scattering function. in this alternative case, it is determined from the random time series, as represented in the plots shown in figs. 11.3 and 11.4. this is a numerical processing of the simulated time series of a signal analogous to what is really done to real experimental data. in figs. 11.6 and 11.7 this form of the scattering function is presentfig. 11.5. scattering function calculated theoretically as a statistical moment of the field. 1129 characterization of the hf ionospheric fluctuating channel of propagation: hf channel simulator ed for the two cases: with and without the effect of the geomagnetic field. the figures are in the form of contour plots where the adjacent contours are separated by 5 db and range from 0 to –30 db. strictly speaking, these plotted values are not statistical moments, but a sort of realization of the scattering function, obtained after the appropriate partial averaging over a finite series of realizations of the signal. in fig. 11.7, where the effect of the geomagnetic field is accounted for, four distinct regions occur, which present four contributions to the full signal which have propagated along the four paths: e, low angle f and both oand e-modes for high angle f. 11.5. conclusions a wideband hf ionospheric channel simulator has been constructed which is solely based on a physical model of propagation through a time-varying ionosphere. the use of a propagation model including the earth’s field enables the effect of both oand e-modes to be included for all layer reflections. results from this simulation show the very significant effect of irregularities of even modest magnitude and the comparative effects on the variation of received pulse magnitude and pulse width due to background ionosphere dispersion and the fluctuating irregularities. this emphasizes the shortcomings of previous simulation models for which no such stochastic ionosphere model was included. further, precisely because the simulator is based on a physical model, comparison of experimental results and simulation can also be used to identify the correspondence between physical parameters (e.g., the variance and anisotropy of the electron density fluctuations, orientation of the propagation path to the magnetic meridian, bulk ionosphere motions) with observed channel parameters (e.g., doppler spread and shift, time delay spread). this in turn will lead to a better characterisation of the propagation channel and thus a more accurate simulator. such an iteration is not possible for an empirically based simulator. the simulator could also be used to test hf systems and candidate hf modem schemes for various applications, e.g., hf broadcasting (drm) and spread spectrum systems, both direct sequence and frequency hopping. fig. 11.6. scattering function determined from the field realizations shown in fig. 11.3. fig. 11.7. scattering function determined from the field realizations for the case when the earth’s magnetic field is taken into account. the uppermost region corresponds to the extraordinary component of the high angle f-mode. 11.6 11.7 1130 vadim e. gherm, nikolay n. zernov and hal j. strangeways references gherm, v.e. and n.n. zernov (1995): fresnel filtering in hf ionospheric reflection channel, radio sci., 30, 127-134. gherm, v.e. and n.n. zernov (1998): scattering function of the fluctuating ionosphere in the hfband, radio sci., 33, 1019-1033. gherm, v.e., n.n. zernov, b. lundborg and a. vastberg (1997a): the two-frequency coherence function for the fluctuating ionosphere; narrowband pulse propagation, j. atmos. sol.-terr. phys., 59, 1831-1841. gherm, v.e., n.n. zernov and b. lundborg (1997b): the two-frequency, two-time coherence function for the fluctuating ionosphere; wideband pulse propagation, j. atmos. sol.-terr. phys., 59, 1843-1854. gherm, v.e., n.n. zernov, h.j. strangeways and m. darnell (2000): scattering functions for wideband hf channels, in proceedings of the 8th international conference on «hf radio systems and techniques», university of surrey, u.k., 10-13 july, 2000, conference publication no. 474, 341-345. gherm, v.e., n.n. zernov, b. lundborg, m. darnell and h.j. strangeways (2001a): wideband scattering functions for hf ionospheric propagation channels, j. atmos. sol.-terr. phys., 63, 1489-1497. gherm, v.e., n.n. zernov and h.j. strangeways (2001b): scattering functions for multimoded wideband hf channels, in proceedings icap2001, april 2001, exeter, u.k., iee conference publ. no. 480, 393-396. gherm, v.e., n.n. zernov and h.j. strangeways (2002): multipath effects in wideband fluctuating hf channels, acta geod. geophys. hung., 37 (2/3), 253-259. gherm, v.e., n.n. zernov and h.j. strangeways (2003): wideband hf simulator for multipath ionospsherically reflected propagation channels, in proceedings icap2003, april 2003, exeter, u.k., iee conference publ. no. 491, vol. 1, 128-131. gogin, yu.a., v.e. gherm and n.n. zernov (2001): diffraction of the wave field on weak inhomogeneities of the dielectric permittivity in a 3d smoothly inhomogeneous medium, st. petersburg university herald, series 4, 2 (12), (in russian). mastrangelo, j.f., j.j. lemmon, l.e. vogler, j.a. hoffmeyer, l.e. pratt and c.j. behm (1997) a new wideband high frequency channel simulation system, ieee trans. commun., 45, 26-34. proakis, j.g. (1983): digital communications (mcgraw-hill, new york). strangeways, h.j. (2000): effect of horizontal gradients on ionospherically reflected or transionospheric paths using a precise homing-in method, j. atmos. sol.-terr. phys., 62, 1361-1376. vogler, l.e. and j.a. hoffmeyer (1993): a model for wideband hf propagation channels, radio sci., 28, 1131-1142. zernov, n.n. (1980): scattering of waves of the sw-range in oblique propagation in the ionosphere, radiophys. quantum electron., 23, 109-114. zernov, n.n., v.e. gherm and h.j. strangeways (2002): modelling of wideband ionospheric hf channels, in proceedings 27th general assembly of ursi, maastricht, the netherlands, pap. no. 0785, pp. 4. 27stucchi.qxd 659 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words completeness – historical earthquakes – italy – seismic hazard 1. background among investigators working in historical seismology, or making use of this type of data, the assessment of data completeness is a typical problem that seismologists deal with – or, better, create – while historians find hard to understand. of course, historical records are not complete; however, while historians can live in peace with such an issue, seismologists – and, among them, those who make massive use of statistics – need to master it in some way. although obviously incomplete, parametric earthquake catalogues (pec), or historical earthquake datasets, such as those widely described in this volume, supply a picture which represents most of our knowledge of the past seismicity. past seismicity is one of the keys for predicting future seismicity; therefore, the more reliable picture of the past we have, the more successful prediction of the future we expect to perform. this issue invites one to assess how representative the picture we get from the catalogue is with respect to the «true seismicity». this question can be answered – in principle – in several ways. for instance, we are sure that the total energy release described by a mailing address: dr. massimiliano stucchi, istituto nazionale di geofisica e vulcanologia, sezione di milano, via e. bassini 15, 20133 milano, italy; e-mail: stucchi@mi.ingv.it assessing the completeness of italian historical earthquake data massimiliano stucchi (1), paola albini (1), carmen mirto (1) and alessandro rebez (2) (1) istituto nazionale di geofisica e vulcanologia, sezione di milano, italy (2) istituto nazionale di oceanografia e geofisica sperimentale, italy abstract the assessment of the completeness of historical earthquake data (such as, for instance, parametric earthquake catalogues) has usually been approached in seismology – and mainly in probabilistic seismic hazard assessment (psha) – by means of statistical procedures. such procedures look «inside» the data set under investigation and compare it to seismicity models, which often require more or less explicitly that seismicity is stationary. they usually end up determining times (ti), from which on the data set is considered as complete above a given magnitude (mi); the part of the data set before ti is considered as incomplete and, for that reason, not suitable for statistical analysis. as a consequence, significant portions of historical data sets are not used for psha. dealing with historical data sets – which are incomplete by nature, although this does not mean that they are of low value – it seems more appropriate to estimate «how much incomplete» the data sets can be and to use them together with such estimates. in other words, it seems more appropriate to assess the completeness looking «outside» the data sets; that is, investigating the way historical records have been produced, preserved and retrieved. this paper presents the results of investigation carried out in italy, according to historical methods. first, the completeness of eighteen site seismic histories has been investigated; then, from those results, the completeness of areal portions of the catalogue has been assessed and compared with similar results obtained by statistical methods. finally, the impact of these results on psha is described. 660 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez pec over a certain time-interval and above a given threshold magnitude (m ≥ mt) represents a fraction of the «true» energy released in the same conditions; we might then be happy to estimate that this fraction is, say, 1/3 rather than 1/2, or 2/3. for certain purposes, on the other hand, it seems more useful to assess the time-interval for which this fraction approximates the value of 1. in this case we speak in terms of assessment of (time-intervals of) data completeness. this paper mostly deals with the last issue, which is commonly addressed by statistical analysis performed in the perspective of probabilistic seismic hazard assessment (psha). it is to be stressed, however, that the problem of data representativeness is not frequently addressed by other types of analysis, such as the comparison between the strain release assessed by geodetic and seismological data over long time-intervals (centuries). in this case the gap between the two values is often accounted for as due to non-seismic deformation, with little reference to possible seismological data incompleteness. while addressing this issue, some aspects of the general problem will also be discussed. 2. the problem, common solution and some objections the problem we want to solve is the following: how can we assess the time-interval(s) during which seismological data derived from historical records are complete above a given mt? historical-seismological data can be represented on a time scale in many ways. figure 1a-d shows the seismic histories of some localities, that is, the sequences of earthquake effects fig. 1a-d. seismic histories of (or macroseismic intensities reported at): a) san francisco, ca, us, b) montreal, canada (from us earthquake intensity database, http://www.ngdc.noaa.gov/seg/hazard/eqint.html); c) ica, peru (from ceresis, 1985); d) verona, italy (from dom4.1, monachesi and stucchi, 1997). a c b d 661 assessing the completeness of italian historical earthquake data reported for those localities in terms of macroseismic intensity, as derived from the relevant macroseismic databases. figure 2a gives the seismic history of a large area (southern italy), in terms of magnitude; fig. 2b the cumulative energy release for the whole italian pec (working group cpti, 1999). a look to these graphs invites a number of questions, among which – for instance – are the following: a) did san francisco suffer intensity 11 mm only once, and montreal intensity 9 mm two times, since their foundation?; b) is the gap of data of ica (peru) in the 18th century due to seismic quiescence or lack of historical records?; c) how would we feel if we neglect the seismicity around verona before, say, 1600, because we think that the data are not complete?; d) which are the completeness time-intervals in southern italy for m ≥ given thresholds?; e) does the plot of fig. 2b suggest something? actually, it is a common practice to answer such questions looking at the graphs themselves assuming for instance that: i) completeness at ica starts after the gap, because this is likely to be a gap in the historical records; ii) completeness for large events starts in southern italy before 1660 and shortly after fig. 2a,b. a) seismic history of southern italy; b) cumulative strain release computed for events with ms ≥ 5.5 of the italian cpti catalogue. a b fig. 3. seismic history of southern italy and completeness time-intervals used by slejko et al. (1998). 1600 for the whole italy, as from the corresponding jump in the slope of the plot. however performed, completeness assessment usually ends out with time-intervals which go from some ti to present: often these 662 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez time-intervals are made very short in order to ensure completeness. time-intervals can be plotted in the form of a broken line (fig. 3): in the common use the line becomes a true divide, which separates the «complete» world (right of it) from the «incomplete» one (left), usually bound for oblivion. the approaches described above have been made more robust by the use of statistical methods, some of which will be recalled later. they have a common point: they want to infer how complete the data are looking inside the data themselves; and this point shows some inconsistencies, both from the historical and the seismological points of view. factors which contribute to the wealth of historicalseismological data are mainly of external origin and concern how, where and for what purpose historical records have been produced, preserved and investigated; the evaluation of these factors is a typical historical problem which requires historians’ expertise and methods. after all, the problem is similar to the completeness assessment of the data produced by a seismic network; to assess how complete they are above a given mt one would mainly investigate whether and how long some seismic stations were out of order, and how this affected earthquake detectability and parameter determination. one would not normally infer it from the records of the network only. on the other hand, when looking inside the data one has to compare – in a more or less open way – the pattern of one’s data to some model in which one trusts. the above-mentioned approaches make reference – explicitly or not – to a stationary seismicity model, which may be representative over large areas and time-intervals but definitely fails over small areas and time-intervals. this is even truer in areas where seismicity is low or moderate and where faults are slow in comparison with the time-window spanned by the catalogue. in order to avoid shortcomings or inconsistent assumptions, historical approaches should be used. before moving to this, let us examine how the problem is dealt with in some cases. 3. some current solutions following the former considerations, one would expect that most pec or earthquake databases carry some completeness information so to assist users. in the reality this is hardly the case; this fact leaves users helpless in front of the task, and they usually get out of this situation adopting some statistical approach. this also implies that traces of completeness assessment – if any – are to be found in seismic hazard literature more than in catalogues. a complete review of the existing solutions is out of the scope of this paper: in the following, some examples are recalled, going from contributions dealing with the completeness of the parametric catalogues to those choosing a historical approach to evaluate the completeness of historical records for a region or a country. it is a general issue that statistical approach to completeness assessment started with the work by stepp (1972), which had a number of followers. the paper by albarello et al. (2001) clearly describes basic assumptions, pros and cons of the approach and quotes the main literature on the subject, such as kanamori and abe (1979), perez and scholz (1984), mulargia and tinti (1985), rotondi et al. (1994). it also offers the most recent and advanced attempt to assess the completeness with a statistical approach which analyses the seismicity patterns around a given site. it then applies it to one of the italian pec (nt4.1, camassi and stucchi, 1997) and performs a preliminary comparison of the results with some historical background considerations. for the first time results are associated to uncertainty estimates. although not directly suitable for catalogue completeness evaluation it is worth mentioning the analysis method by kijko and sellevoll (1989, 1992) to obtain gutenbergrichter relation parameters and maximum magnitude from an earthquake catalogue that can be partially incomplete. this method is applicable on a set of data composed by a first part of catalogue derived from historical macroseismic observations, a lack of information (no data) in the central part of catalogue and a final part obtained from recent instrumental observations. 663 assessing the completeness of italian historical earthquake data in italy, the completeness of the nt4.1 catalogue was assessed by slejko et al. (1998) with a statistical/pseudo-historical procedure which consisted of dividing the whole italian territory in 4 regions, assumed as homogeneous from the historical side, and then performing stationarity tests in order to identify the periods of completeness (fig. 4a,b). for switzerland, the 1999 version of the mecos catalogue (mecos, 1999) available online, gives estimates of completeness for the whole country (epicentral intensity versus time), without explicitly mentioning the used approach (fig. 5a,b). the mecos (1999) estimates are recalled in the introduction to mecos-02 (swiss seismological service, 2002), the updated version of the macroseismic catalogue. the «completeness» section of mecos-02 contains historical considerations on the availability and retrieving of documents for the swiss territory through time and space. however, these arguments are not clearly linked with the estimates attributed to eight swiss regions, performed for varied time periods and epicentral intensity values. by comparing the regional estimates of mecos-02 with the previous of mecos (1999) it emerges that for some of the eight regions the situation for i0 = 8 is today more conservative. as an example, according to mecos-02 in the luzern region the catalogue is considered complete for i0 = 8 from 1600 onwards (fig. 5a), and for the regions of wallis/valais (fig. 5b) and tessin roughly from 1500, while for mecos (1999) the completeness for i0 = 8 and i0 = 9 for the whole country started on 1300. it is worth noting that for some regions mecos-02 states that completeness for all epicentral intensity values cannot be assessed for some time-intervals and epicentral intensities values, mostly on the basis that no primary sources have been found (an example is shown in fig. 5b for wallis/valais between 1600-1679). the completeness of the catalogue for the u.k. territory is shortly discussed in the introduction to musson (1994); more at length in musson and winter (1996). the completeness is assessed from both statistical and historiographical points of view. from the first type of config. 4a,b. a) homogeneous regions and (b) relevant completeness time-intervals (ms versus time) used by slejko et al. (1998). a b 664 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez siderations it is concluded that the uk catalogue can be considered complete after about 1830 for ml ≥ 4.0, and after about 1720 for ml ≥ 4.5, although with regional variations. the authors also conclude that the statistical results are «generally consistent with what might be expected from a historiographical approach». on the historical side, this aspect is being increasingly addressed, with attempts of the authors of giving an idea of how exhaustive the investigation can be considered or what more one can expect to find, and with growing attention to the investigation of unknown earthquakes. the importance of being aware of the limitations of historical records in assessing «how complete and representative a sample of seismic activity has been recorded» is underlined by ambraseys and melville (1982) in their volume on persian earthquakes. the evaluation of completeness is here supported by describing from which places and which types of historical sources one can expect to find for four unequal time periods, from 7th to 19th century. in his «earthquake history of ethiopia and the horn of africa», gouin (1979) deals with the completeness of the information by dividing the available observations into two periods: 14001874 and 1875-1974. he defines the break at 1875 a merely convenient choice, adding that prior to that year reporting of seismicity in the study area was random and sporadic. his evaluation of the 100 years of the second period are based on the comparison between the observed and computed frequency-magnitude curves. dealing with the seismicity of quebec (gouin, 2001) the author evidences some gaps in the reporting of events between late 17th century and early 19th century. the only reflection on completeness is made by comparing the number of events in the historical period with those reported in the 1990s. in a very interesting and paradigmatic paper, which did not reach the great international circuit, agnew (1991) investigates the completeness of the pre-instrumental record of earthquakes in southern california, analysing the historical sources of the region, comparing the content of varied compilations and proposing some case-histories. after a contribution with a very stimulating title (musson, 2000), musson et al. (2001) published some results of a research of seismicity of the faroe islands, an isolated area with low seismicity and about which no previous studies had been made. after a thorough investigation of historical sources, some considerations are supplied fig. 5a,b. completeness time-intervals (i0 versus time) for mecos-02: a) luzern region; b) wallis/valais. a b 665 assessing the completeness of italian historical earthquake data on the capacity of records and the significance of their absence in seismological terms. in this case, time-intervals could not be assessed in terms of magnitude, but of peak intensity values at a significant inhabited area. some attempts at considering this problem from a historical perspective are present also in some other papers in this same volume, and especially in the contributions by toppozada and branum (2004) on california, by tatevossian (2004) on russia and by downes (2004) on new zealand. summarising, the above recalled examples show that there is no common current solution. in defining the completeness time-intervals for assessing seismic hazard, the statistical approach has been recently used in combination with considerations coming from the historical side. on the other hand, a more properly defined historical approach has been increasingly used by investigators for selected regions and countries. however, the main goal of the latter contributions is still to explore how historical sources survival and their investigation is affecting the «incompleteness» of the existing catalogues, more than immediately using the results for psha purposes. 4. the historical approach in italy the problem we are discussing is well known to italian investigators since many years ago. traces of this thread are found in many published or conference papers which, although not openly addressing it, went around varied aspects, such as the finding of unknown earthquakes, etc. the historical approach to completeness is briefly dealt with in guidoboni and stucchi (1993), a contribution in the frame of the gshap project, in a comprehensive perspective in stucchi (1995) and, concerning the italian town of siena, in castelli and albarello (2000). the search for unknown earthquakes and the results obtained is reported by albini and rodriguez de la torre (1993) for periodical press in 18th century europe, by castelli and camassi (2000) with the support of some case histories, and from a comprehensive point of view by valensise and guidoboni (2000) and mariotti et al. (2000); and surely many more could be mentioned. all these contributions offer useful results; however, they remain on a qualitative side, which often prevents seismologists from trusting them completely. 4.1. investigating the completeness of site seismic histories in the view of overcoming such difficulty and of establishing a semi-quantitative approach, in the last few years a group of historians and seismologists turned their attention to the assessment of the completeness of site seismic histories. the basic idea was: a) to consider a locality, together with its set of historical sources, as a kind of «seismic recorder» and to assess the completeness of the seismic records concerning the locality; b) to infer from it, or from the results on a few localities, some conclusions on the completeness of the data concerning areas. the investigation was performed in 2001 (albini et al., 2001), with the financial support of the national seismic survey of italy, on fifteen localities (fig. 6a) selected according to the following criteria: i) to have suffered a site intensity is ≥ 8 mcs at least once. ii) to be «important» enough, so that a good historical sources coverage could be expected in the considered time-window (1200-1870). iii) to be located in areas with preference to low-to-moderate seismicity. three further localities, treviso, asolo and belluno, were investigated in 2002, in the frame of the research sponsored by the project «damage scenario at vittorio veneto» of gndt, the national group for defense against earthquakes (albini and stucchi, 2002; albini et al., 2003). the main goals of the investigation were: a) to find out new earthquake records leading to is ≥ 9 mcs, unknown with respect to the seismic histories obtained from the available datasets (monachesi and stucchi, 1997; boschi et al., 2000). b) to understand the reasons why such records could have been lost. c) to assess potential information gaps. it must be said that the available resources allowed a good and rigorous, although not exhaustive, investigation. 666 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez goals a) and b) were achieved by investigating historical sources and compilations which: 1) had contributed to the known seismic histories; 2) had not contributed to, so far. the main result was that no unknown earthquake records leading to is ≥ 9 mcs were found for the eighteen localities. in addition, a few cases (locality/events) were investigated, where no records were available for intensities of comparable size predicted by means of attenuation relationships. the results were: i) for a few cases (2) new earthquake records were found, leading to intensity assessments lower than predicted; ii) for another four cases no new records were found, but the predicted intensities were probably overestimated because of a bad fit of the attenuation relationship with the data of the corresponding earthquake; iii) in one case (sulmona, 1315) no records were found; this fact contributed to fix that the completeness should start later than this date. goal c), the main one, to assess completeness and potential information gaps, was achieved by comparing category 1) (historical sources which contributed to the known seismic histories) and category 2) (newly investigated historical sources), and assessing conclusions from both of them. briefly, the investigation was mainly devoted to exploring the time periods where no earthquakes were reported; the main goal was to assess whether in those periods the historical sources reported other events, either natural or political, or did not report any event at all. in the first case, the historian’s expert judgement was able to assess when «no earthquake records» could be interpreted in terms of «no earthquake took place». the investigation was initially foreseen with reference to is ≥ 9 mcs, because it is believed that historical considerations useful for this purpose apply mainly to the range of heavily damaging to destructive effects. actually, a heavily damaging earthquake is not an «instant» event, but something which leaves lastfig. 6a,b. a) the 18 localities where the completeness of the site seismic histories was investigated. b) completeness start times at the investigated localities for site intensity is ≥ 8 mcs (squares), is ≥ 9 mcs (diamonds), is ≥ 10 mcs (circles). a b 667 assessing the completeness of italian historical earthquake data ing traces in the life of the affected territory. it seems reasonable to affirm that in most cases such traces were recorded by the historical sources in the italian region from about 1200 on, with a few exceptions only. therefore, our point was to investigate where and how such sources were preserved and/or earthquake records transmitted to us by the compilers. at the end it was possible to supply results also for is ≥ 10 mcs and is ≥ 8 mcs. the results are given in fig. 6b; they show how varied the completeness time-intervals are, even within short distances. summarising, it can be concluded that this approach appears promising and able to give reliable constraints to the completeness assessment. as for many scientific issues it can be performed according to either an expeditious or an intensive approach; in the first case one can obtain many clues in a shorter time, although with a lower reliability; in the second results are better but larger resources are needed. 4.2. from sites to areas, from is to m, from high to low m at conferences the results presented above were observed with sympathy by seismologists, who soon asked for more localities/datapoints, and when the results in terms of catalogue completeness (time-intervals for m and areas) would be available. more datapoints would cost money and time; therefore, to make our data usable for psha purposes we need to extend them. this requires performing three steps: a – from sites to areas; b – from site intensity to magnitudes; c – from high to low m. step a (extending point data to large areas) requires lot of care, as we are not dealing with measures of physical quantities; on the contrary, we deal with point data which have individual roots and might have only weak relationships one to another, over small areas. having data from eighteen points only, the areal extension was performed by expert judgement, using the fig. 7a,b. a) homogeneous regions and (b) average relevant completeness start times for i0 ≥ 9 mcs or ms ≥ 6.0 (full diamonds), superimposed to the data of fig. 6b. a b 668 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez fig. 8a,b. a) homogeneous regions; b) broken lines representing the final completeness time-intervals (ms versus time) in the five regions. a b wealth of information and expertise acquired by historians during the last years, although not formalisable according to our schemes, yet. the expert judgement was first used to define five regions (fig. 7a) which, according to historical considerations, may be taken as sufficiently homogeneous from the point of view of historical record production and preservation. as an example, during the middle ages and the early modern age, the alpine region in the north and the mountainous calabria region in the south, with sparse settlements and a relatively small amount of local documents survived, can be compared to the regions of the central and eastern po valley, the history of whose communes and lordships is well documented by local and highly reliable chronicles. then, the expert judgement was used to adopt a most reliable areal value for the beginning time of completeness, starting from the value obtained for the localities. it is to be stressed that the choice of the localities was performed for other reasons than this investigation; therefore, their distribution appears rather uneven with respect to the regions (for instance, there was one locality only in sicily and none in the alps). results are presented in fig. 7b and confirm that conservative solutions were adopted. step b (from site intensities to m) is performed as follows: the conclusion that is ≥ 9 is complete at locality x since, say, 1400 means that no earthquake effects of that size at locality x are lost after 1400. such effects can be produced by earthquakes with i0 located at the same locality, or by earthquakes with larger i0 located in a convenient area; therefore, using a i0 / m relationship we can conclude at least that no earthquakes with m corresponding to i0 = 9 (roughly ms = 6.0 using the cpti i0 /ms table), or larger, happened nearby that locality in the period considered as complete. as for step c (from high m to low m), following the issues made above we must conclude that historical considerations cannot be used for assessing the completeness of slightly damaging effects (i0 ≤ 7 mcs and corresponding ms ≤ 5.0). therefore, the values proposed by slejko et al. (1998), assessed according to statistical considerations were adopted with 669 assessing the completeness of italian historical earthquake data small adjustments. figure 8a,b summarises the results in terms of completeness time-intervals for the five regions of fig. 7a. 5. validation as explained, these results are based on good although scanty data points and, therefore, may suffer from some uncertainty in relation with the areal extension and the assessment of the most representative value for each area. there is little hope to assess the reliability of these results with conventional methods; however, we can perform some rough estimates. 5.1. comparison with statistical assessment a first, although preliminary, comparison can be made with the results published by albarello et al. (2001), both using the data for the sites closest to our localities, and assessing a mean, weighted value of their results over the five regions adopted in this paper (fig. 9a,b). results shows that, for i0 = 9 mcs at least, statistical time-intervals are steadily shorter than historical ones, with the exception of two cases: i) crema, because of a significant gap in the surviving local documentation between 1100 and 1450; ii) melfi, which represents an anomaly of unclear origin in the statistical results. 5.2. constraining the estimation of the «true seismicity» as said before, we do not have a method for estimating the fraction f = cs/ts where cs = seismicity in the catalogue and ts = = «true» seismicity, the latter remaining undisclosed. fig. 9a,b. a) sites (stars) investigated by albarello et al. (2001) closest to the 18 investigated localities; b) completeness start times (i0 ≥ 9 mcs) at these sites, adapted from authors’ results (empty triangles), and average start times in the five regions (full triangles), superimposed to the data of fig. 7b. a b 670 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez although ts is not known, as a matter of fact we estimate it in psha from the completeness time-intervals by assessing the seismicity rates (number of events of a given m divided by the time-interval). if we project the seismicity rates over – for instance – 1000 years, which is more or less the time-interval spanned by the italian catalogues, we get the number of events which «should have happened» in 1000 years, under the common assumption that the seismicity rates are representative; in other words, that the seismicity is stationary, that is the current assumption adopted in most psha cases. figure 10 shows: a) the cumulative earthquake distribution of the nt catalogue (camassi and stucchi, 1997) in 1000 years, that is cs; b) the cumulative earthquake distribution in 1000 years which comes projecting over 1000 years the seismicity rates derived by sets of completeness time-intervals of fig. 8 (let us call it «virtual seismicity» vs1); c) the cumulative virtual seismicity vs2 obtained projecting the seismicity rates used by slejko et al. (1998). the gap between vs1, or vs2, and cs, represents at any m the number of earthquakes that should be missing from the catalogue if all the basic assumptions are right; fraction f is given, at any m, by the values of cs divided by the values of vs1 or vs2. although most investigators could live in peace with almost any large gap or small fraction at any m (after all, catalogues are incomplete by definition!), the size of the gap between distribution cs and vs2 attracted the attention of stucchi and rebez (2000) and, later, of stucchi and albini (2000). actually, this gap is about 97-55 = 48 earthquakes with ms ≥ 6.4 (f = 0.56) and 329134 = 195 earthquakes for ms ≥ 5.8 (f = 0.4). in other words, this would mean that the italian catalogue knows only a half or less than the presumed, total number of earthquakes of the same size which should have happened in 1000 years. this rate, very poor for a country where the historical catalogue is widely considered as a very good one, is hard to believe from the historical point of view, too. considering distribution vs1, obtained from the completeness intervals determined in this investigation, versus cs, the figures go down to 19 = 74-55 earthquakes with ms ≥ 6.4 and 53 = 187-134 with ms ≥ 5.8; the corresponfig. 10. comparison of the cumulative nt4.1 catalogue (camassi and stucchi, 1997) (cs, full triangles) over 1000 years with the virtual, cumulative catalogues obtained by the historical completeness time intervals (this paper; vs1, open circles) and the statistical one (slejko et al., 1998; vs2, full squares). 671 assessing the completeness of italian historical earthquake data ding values of f are 0.74 and 0.71, which seem more reasonable although a bit high, still. a more detailed analysis shows that the number of earthquakes which contribute to the gaps described above are not evenly distributed in space but show higher peaks in southern italy. this shows, in our opinion, that the completeness time-intervals determined for that region are very severe and/or that seismicity is there even less stationary than in the other areas. in general, gaps are higher in the areas where strong events took place recently. we do not go into details about this here: this analysis has the scope of exploring the possibility of constraining the completeness time-intervals by means of the mentioned gaps, the size of which can be roughly compared with evidence suggested by historical considerations. 5.3. does it make any difference? we believe that, in the frame of conventional psha, it may. first of all, more rigorous and semiquantitative methods must be welcome as a step of psha which is frequently overlooked (or casually dealt with). next, it is clear that some considerations proposed above suffer from incorrect seismicity assumptions. however, such assumptions are behind psha conventional methods and they drive completeness assumptions and methods, too. we believe that completeness plays a significant role in areas with high seismic activity and moderate to long return periods of destructive earthquakes, such as italy, the iberian peninsula and the balkans. as an example, fig. 11a,b compares the hazard distributions obtained from data which give rise to distribution vs1 and vs2, respectively, keeping all the other input data and using the same code to compute seismic hazard. it should play a minor role in areas with moderate seismicity (in europe north of the alps) or very high one (aegean arc). in the first case this is mainly due to the fact that seismicity is low and long-term fluctuations do not affect conventional psha. for instance, musson (2002) showed that – in a typical moderate seismicity case – changing expert judgement fig. 11a,b. comparison of the psha obtained using distributions (a) vs2 of fig. 10 and (b) vs1, this paper, leaving unchanged all the other input data and using the same code to compute seismic hazard. a b 672 massimiliano stucchi, paola albini, carmen mirto and alessandro rebez about the completeness changed the final seismic hazard value by 10%. in the second case, this is due to the fact that shorter time windows can be sufficient to approach the «true» seismicity pattern. it is clear that we do not believe that distributions vs1 and vs2 really represent the «true» seismicity ts. however, at least we can assess that they overestimate it, may be by far, and that, therefore, using them we are on the safety side. 6. conclusions the problem of how good is the picture that our seismological data of historical origin supply with respect to the truth can be of varied importance, depending on the region and/or the final purpose of our research. we believe that the problem should be mainly addressed with historical methods. this investigation should mainly look outside the earthquake records, in the realm of everyday life history; we are aware, however, that this research can be time-consuming and frustrating, although this is one of the few areas where to find a «credible nothing» may represent a highly valuable result. we have proposed some examples; being aware that they represent a drop in the sea we are ready to use them, in agreement with the principle that little is better than nothing, for making our psha more robust. and we are confident that they will be. acknowledgements this paper summarises the results of a number of ideas, attempts, discussions to which many investigators and friends have contributed, although not always in agreement, over a timeinterval of many years. we want to acknowledge here the contribution of d. albarello, n.n. ambraseys, j. bommer, f. bramerini, r. camassi, v. castelli, g. ferrari, d. giardini, g. grünthal, e. guidoboni, i. leschiutta, d. mayerrosa, c. meletti, p. migliavacca, e. migliorini, d. molin, g. monachesi, v. montaldo, a. moroni, m. mucciarelli, d. perkins, l. peruzza, r.m.w. musson, r. rotondi, f. sabetta, d. slejko, r. tatevossian, g. valensise, j. vogt, g. zonno. without their encouragement this paper would have not come to completeness. references agnew, d.c. 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(1995): short notes from a journey through earthquakes and historical records, in earthquakes in the past. multidisciplinary approaches, edited by e. boschi, r. funiciello, e. guidoboni and a. rovelli, ann. geofis., 38 (5-6), 567-571. swiss seismological service (2002): ecos – earthquake catalog of switzerland, ecos report to pegasos, version 31.3.2002, appendix a: ecos database (sed, zürich), (on line: http://histserver.ethz.ch/). tatevossian, r. (2004): history of earthquake studies in russia, ann. geophysics, 47 (2-3), 811-830 (this volume). toppozada, t. and d. branum (2004): california earthquake history, ann. geophysics, 47 (2-3), 509-522 (this volume). us earthquake intensity database, on line http://www.ngdc.noaa.gov/ seg/hazard/eqint.html valensise, g. and e. guidoboni (2000): towards new research strategies: silent seismogenic areas or silent sources?, ann. geofis., 43 (4), 797-812. working group cpti (1999): catalogo parametrico dei terremoti italiani (ing, gndt, sga, bologna), pp. 92 (on line: http://emidius.mi.ingv.it/cpti/). n a v i g a z i o n e s p a z i a l e ( * ) p . p e r n a z z a ricevuto il 19 f e b b r a i o 1903 r i a s s u n t o . vengono b r e v e m e n t e discussi i principali a s p e t t i della navigazione spaziale, la q u a l e p u ò essere divisa in t r e f a s i : c i r c u m n a v i g a z i o n e della t e r r a (sosta in o r b i t a o p a r t e n z a ; n a v i g a z i o n e m e d i a o i n t e r p l a n e t a r i a ; a v v i c i n a m e n t o e a p p r o d o sul p i a n e t a . l ' a s s i s t e n z a r a d i o dalla t e r r a , p u ò giocare il ruolo p r i n c i p a l e per la prima delle d u e fasi, m e n t r e p e r l ' u l t i m a gli a s t r o n a u t i d e b b o n o f a r e affidam e n t o sulla n a v i g a z i o n e a u t o f r e n a t a , sugli s t r u m e n t i e l e t t r o n i c i e l'osservazione d i r e t t a . l a riserva di energia riduce le generali necessità della navigazione spaziale. s u m m a r y . lì rie fi y are discussed t h e priucipal a s p e c t s of " space n a v i g a t i o n " . w h i c h c a n b e d i v i d e d in t h r e e p h a s e s : c i r c u i n n a v i g a t i o n of t h e e a r t h ( p a r k i n g o r b i t or d e p a r t u r e ) ; m i d d l e or i n t e r p l a n a t e r y n a v i g a t i o n ; a p p r o a c h a n d l a n d i n g 011 t h e pia net. t h e r a d i o a s s i s t e n c e f r o m t h e e a r t h , w i t h t r a n s p o n d e r , m a y play t h e principal iole for t h e first t w o phases, while f o r t h e l a s t p h a s e , t h e a s t r o n a u t a m u s t rely 011 t h e s e l f c o n t a i n e d n a v i g a t i o n : electronics a n d o b s e r v a t i o n . t h e energy reserve reduce» t h e general needs of space n a v i g a t i o n . 1. l a n a v i g a z i o n e s p a z i a l e u m a n a p u ò r i g u a r d a r s i c o m e n a t u r a l e e s t e n s i o n e d e l l a n a v i g a z i o n e a e r e a n o t t u r n a a d a l t i s s i m e q u o t e . t u t t a v i a , l ' a m b i e n t e fisico e o p e r a t i v o n e l l o s p a z i o v u o t o p r e s e n t a a s p e t t i n o t e v o l m e n t e d i v e r s i : — v o l t a c e l e s t e p e r m a n e n t e m e n t e o s c u r a , e q u i n d i gli a s t r i s e m p r e v i s i b i l i ; (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. 4 4 8 p . p e r n a z z a —• sensazione (l'immobilità e d'isolamento; — imponderabilità; — alti valori delle accelerazioni sia alla partenza che al rientro; — radiazioni cosmiche; — calore solare; — imperiose esigenze di condizionamento dell'astronave; ecc. 2. circa gli effetti della « imponderabilità » sull'organismo umano, i pareri dei p r i m i astronauti sembrano discordi. i fisiologi ritengono, t u t t a v i a , che la imponderabilità, prolungata nel tempo, possa determinare i seguenti effetti dannosi: —• perdita di orientamento e di equilibrio, fino a possibile collasso psico-fisiologico dell'individuo; — atrofizzazione del sistema muscolare. a tali inconvenienti si potrà, probabilmente, rimediare creando a bordo una accelerazione artificiale (meglio se propulsiva), e con il lavoro fisico degli astronauti. 3. per la condotta della navigazione, occorrerà assicurare: 1) un sistema di riferimento spaziale; 2) riferimenti per il controllo di assetto e di direzione del veicolo; 3) comandi a reazione (o balistici), con relativa riserva di energia (gas compresso e/o ruote giroscopiche); 4) riserva di energia propulsiva per le manovre e per le correzioni di r o t t a ; 5) più sistemi di navigazione. 4. nella navigazione orbitale circumterrestre, la traiettoria percorsa dal « p u n t o sub-satellite» può riguardarsi come una circonferenza massima della terra. l a navigazione del veicolo sarà ancora riferita alla terra (coordinate geografiche e altitudine); e l'orizzonte fisico terrestre rappresenterà il piano istantaneo di navigazione. l'assenza di peso non p e r m e t t e r à , t u t t a v i a , di poter utilizzare il pendolo e l'usuale orizzonte artificiale giroscopico per la indicazione della verticale locale (o dell'orizzonte). p e r il controllo di assetto e di direzione del veicolo, l ' a s t r o n a u t a dovrà f a r riferimento: o alla visione d i r e t t a dell'orizzonte locale e degli astri; n a v i g a z i o n e s p a z i a l e 4 4 9 —• o agli s t r u m e n t i : orizzonte artificiale a raggi infrarossi (r.i.), indicatore di virata, girodirezionale. dei q u a t t r o sistemi classici di navigazione (osservata, stimata, radio e astronomica), la radioassistenza da t e r r a può giocare il ruolo principale. la distanza dalla terra, o da un pianeta, p o t r à anche determinarsi, con la necessaria precisione, misurandone l'angolo di parallasse sotteso dal suo diametro, a mezzo di sestante telescopico. 5. nella navigazione interplanetaria, il piano dell'orbita percorsa dal veicolo (e definito d a : terra-sole-pianeta di destinazione) rappresenterà il piano di navigazione, il piano per i riferimenti di assetto del veicolo, e per l'orientamento degli astronauti. poiché quasi t u t t i i pianeti (esclusi soltanto plutone e mercurio), giacciono nel piano della nostra eclittica, quest'ultima può assumersi come piano ideale di riferimento. la direzione del polo nord della eclittica rappresenterà, allora, la direzione della verticale fittizia; m e n t r e la congiungente veicolo-centro del disco solare rappresenterà la verticale vera (tuttavia non utilizzabile a bordo a mezzo del pendolo, per assenza di peso). nel campo interplanetario, per le grandi distanze dalla terra, la radio-assistenza non p o t r à , forse, soddisfare t u t t e le esigenze della navigazione. bisognerà, quindi, f a r ricorso agli astri e, in particolare, al rilev a m e n t o dei pianeti. rispetto ad essi, infatti, sono compatibili la velocità e lo spostamento dell'astronave nello spazio. il metodo astronomico per la determinazione della posizione, appare semplice sia nel principio che nella pratica realizzazione. i n f a t t i , una p i a t t a f o r m a inerziale può materializzare, a bordo, il piano della eclittica. la p i a t t a f o r m a sarà orientata e stabilizzata, a lungo termine, a mezzo di giroscopi i quali saranno a loro volta controllati, a breve termine, con il p u n t a m e n t o di telescopi sul sole, su pianeti e/o su stelle opportune. sarà ancora necessario stabilire u n a direzione fondamentale di riferimento (direzione di un nord fittizio) sul piano della eclittica, o prossima a detto piano. a tale scopo, la direzione sole-punto gamma sembrerebbe la soluzione ideale, per quanto la direzione sole-stella (ad esempio sole-regolo) possa risultare una soluzione pratica più conveniente. rispetto a tale direzione, permanentemente fissa nello spazio interplanetario, verranno effettuate, a mezzo di sestanti telescopici di a l t a 4 5 0 p . p e r n a z z a precisione e a p u n t a m e n t o automatico, le misure angolari di p i a n e t i situati in posizione relativa opportuna. d u e o più rilevamenti angolari di pianeti (reiterabili a volontà), sia nel piano dell'orbita che nel piano zenitale, ad essa relativo, saranno sufficienti per poter determinare la posizione spaziale del veicolo rispetto al sistema di riferimento prestabilito. determinata la posizione e v a l u t a t i gli eventuali scarti, sarà opportuno, ai fini del risparmio di energia propulsiva, effettuare p r o n t a m e n t e le necessarie correzioni di r o t t a . 6. avvicinamento e approdo. l'avvicinamento al pianeta-obiettivo può suddividersi in due fasi: —• avvicinamento iniziale — dal limite della « gravisfera » del pianeta all'istante del passaggio al « traverso »; — avvicinamento f inale —• dal « traverso » all'approdo sul pianeta. i n ogni caso, il p u n t o cruciale del problema, se non si dispone di energia propulsiva sovrabbondante, sta nel raggiungere il « traverso » alla distanza giusta e alla velocità prestabilita. gli impulsi di f r e n a m e n t o disponibili e il sistema di navigazione dovranno p e r m e t t e r e di raggiungere le suddette condizioni: — sia che s'intenda approdare, attraverso eventuale corridoio atmosferico di penetrazione; —• sia che s'intenda orbitare intorno al pianeta ad u n a determin a t a distanza; •—• sia, infine, che si voglia semplicemente circumnavigare il pianeta, per f a r subito ritorno verso la terra. 7. il sistema di navigazione per vavvicinamento v a riferito, ovviamente, al pianeta di destinazione perché è rispetto ad esso che vanno, ora, considerate posizione, direzione e velocità relativa dell'astronave. la velocità di arrivo sul pianeta, senza impulsi di f r e n a m e n t o , sarebbe, infatti, iperbolica: uguale, cioè, alla velocità di fuga dal pianeta + velocità relativa residua, posseduta dal veicolo al limite della « gravisfera » del pianeta stesso. la strumentazione nautica, idonea a soddisfare le esigenze dell'avvicinamento, può così configurarsi: —• sestante telescopico, per la misura della distanza e per il m a n t e nimento della linea di m i r a ; n a v i g a z i o n e s p a z i a l e 4 5 1 —radar-doppler, per la misura della distanza e della velocità relativa; — radio-altimetro; — orizzonte artificiale a r.i.; — indicatore di virata, e girodirezionale, per il m a n t e n i m e n t o del piano orbitale e per la misura della velocità angolare del veicolo rispetto al pianeta. l a suddetta strumentazione va considerata, n a t u r a l m e n t e , integrata al sistema generale di navigazione, le cui p a r t i essenziali saranno costituite dalla piattaforma inerziale e dal relativo commutatore automatico, munito di « memoria » e di predispositore di « programma ». layout 6 low frequency signal spectrum analysis for strong earthquakes alexander rozhnoi1,*, maria solovieva1, pier francesco biagi2, konrad schwingenschuh3, masashi hayakawa4 1 institute of physics of the earth, russian academy of sciences, moscow, russia 2 università di bari, dipartimento di fisica, bari, italy 3 space research institute, austrian academy of sciences, graz, austria 4 university of electro-communications, chofu, tokyo, japan annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5076 abstract we examined changes in the spectral composition of the low frequency (lf) subionospheric signals from the nrk transmitter (37.5 khz) in iceland that were received in bari (italy) relative to the earthquake that occurred in l’aquila on april 6, 2009. in our previous studies, we have reported the occurrence of preseismic night-time anomalies using observations from three receivers located in bari, graz (austria) and moscow (russia). the strongest anomalies in the signals were observed in the nrk-bari propagation path during the period 5-6 days before the l’aquila earthquake, as well as during the series of aftershocks. during this period, similar very low frequency (vlf)/lf amplitude anomalies were also observed along several other propagation paths that crossed the l’aquila seismogenic zone. spectral analysis of the lf signals filtered in the frequency range 0.28 mhz to 15 mhz shows differences in the spectra for seismo-disturbed days when compared to those for either quiet or geomagnetically disturbed days. these spectral anomalies, which are only observed in the propagation path between nrk and bari, contain signals with periods of about 10 min to 20 min. these periodic signals are absent both in the spectra of the undisturbed signals for the control paths, and in the spectra of the signals received during geomagnetic storms. the same changes in the spectral composition were observed in the analysis of lf (40 khz) signals from the jjy transmitter in japan that were received in petropavlovsk-kamchatsky (russia) during the occurrence of three strong earthquakes with m ≥7.0. the results of this study support the theoretical prediction that the possible mechanism for energy penetration from the origin of an earthquake through the atmosphere and into the ionosphere is based on the excitation and upward propagation of internal gravity waves. 1. introduction european research into very low frequency (vlf)/low frequency (lf) (20-50 khz) signals associated with earthquakes began in 2002 with the installation of an omnipal receiver at the department of physics, university of bari, in southern italy. a second receiver that operates in the vlf frequency range was installed in graz (austria) at the end of 2007. during 2008, a new type of receiver that operates in both the vlf and lf bands was developed at the factory of the italian company elettronika (palo del colle, bari). at the beginning of 2009, these new receivers were installed in central and southern italy, greece, turkey and romania. in 2010, an elettronika receiver was also installed in portugal. this european network of receivers also includes an ultramsk receiver that operates in moscow. regular signal monitoring by the bari receiver revealed preseismic and postseismic effects in the vlf/lf radio signals when the wave propagation path passed close enough to the epicentres of earthquakes [biagi et al. 2004, 2007, 2008, rozhnoi et al. 2005]. the most significant results were obtained from observation by the russian and two of the european vlf/lf stations, namely those in moscow, bari and graz, for the earthquake that occurred in l’aquila (italy) on april 6, 2009 [rozhnoi et al. 2009]. strong night-time anomalies for long propagation paths, together with a shift in the evening terminator for short paths, were shown to have occurred 5-6 days before this earthquake. direction finding studies have shown excellent coincidence with the actual position of the earthquake epicenter. in this study, we examined the spectral modifications of a lf subionospheric signal from the nrk transmitter (37.5 khz) in iceland hat was received at bari during the period around the time of this earthquake in l’aquila, and also the spectral changes that were observed during three strong earthquakes with m ≥7.0 in the far east. 2. results of the analysis the signals from the nrk transmitter that were recorded in russia and at two european stations (moscow, bari and graz) were compared to investigate any differences special issue: earthquake precursors 181 article history received may 10, 2011; accepted november 10, 2011. subject classification: wave propagation, seismic risk, vlf/lf signal propagation, eartquake precursors. in their spectra that might have originated from seismic effects. the nrk-bari propagation path passes in close proximity to the seismically active region around l’aquila, while the nrk-moscow and nrk-graz signals were used as control paths. strong night-time anomalies that are similar to those that result from strong geomagnetic activity were found up to 5-6 days before the l’aquila earthquake, and also during the series of aftershocks. figure 1 shows the details regarding the reception of these signals at bari, graz and moscow. the top panel in figure 1 shows the dst index. during the period immediately before the earthquake there was no significant geomagnetic activity. the second panel shows when the foreshock, the earthquake, and its aftershocks occurred, together with their magnitudes. the third panel shows the amplitudes of the nrk signals recorded at bari. the peaks represent the day-time amplitudes and the troughs were recorded during the nighttime. it can clearly be seen that in the period leading up to the earthquake, the amplitudes of the day-time maxima decreased, while the night-time amplitudes increased. these changes began around 5-6 days before the main earthquake occurred. finally, the lower panel compares the changes in the amplitudes recorded at bari, graz and moscow. during the period up to the end of march, 2009, the changes in amplitude measured at all three of these stations were very similar. however, at the beginning of april, large changes 182 rozhnoi et al. figure 1. variations in the amplitudes of the lf signals in nrk-bari wave path during the period of the earthquake in l’aquila. upper panels: dst index of geomagnetic activity and magnitudes of the main earthquakes between march 30 and april 9, 2009. middle panel: signal amplitudes (a) of the nrk signals recorded at bari in the period march 7 to april 22, 2009. bottom panel: night-time residual amplitude (da) for the nrk-bari, nrk-moscow and nrk-graz paths. colored region, period in which the residual amplitude exceeded the 2� level (horizontal dotted line) for the nrk-bari path. 183 were observed in the amplitudes measured by the bari station, when compared to those measured in graz and moscow. these anomalies corresponded to the anomalies revealed at the same time in the amplitudes of vlf/lf signals in several other seismic paths of the network that crossed the l’aquila seismogenic zone. figure 2 shows variations in the amplitudes of the nrk signal received at bari during periods of strong geomagnetic activity. the top trace in each panel in figure 2 shows the dst index. periods when the dst index dipped below –400 nt during strong geomagnetic activity can clearly be seen for the periods of october 28-31, 2003 (figure 2a) and november 8-10, 2004 (figure 2b). the lower traces in each panel in figure 2 show the amplitudes of the nrk signals measured at bari. during the main and recovery phases of these strong geomagnetic storms, the lf signals showed large changes in the signal amplitudes in comparison to the regular daily variations observed before and after these events. the signal levels during the night-time increased considerably, while the day-time signal levels decreased. the result of these changes is that the variations in the signal amplitudes are around half of the normal daily peak-to-peak values. these effects in the propagation of a lf signal that were observed during strong geomagnetic activity have been explained through a number of studies [e.g. beloglazov and remenez 1982]. however, the precise mechanisms responsible for these observed variations in the intensities of the radio signals in connection with earthquakes still remain elusive. although the characteristics of the waveform of the nrk signal anomalies appear identical during the l’aquila earthquake and during periods of strong geomagnetic activity, the resulting spectra are different. we applied spectral analysis to signals recorded on quiet days and compared these to similar results for days on which geomagnetic-induced and seismo-induced anomalies were observed. for this analysis, we used the night-time amplitudes of the nrk signals filtered in the frequency band 0.28 mhz to 15 mhz, which corresponds to wave periods in the range t from 1 min to 60 min. in this frequency range, atmosphere gravity waves with periods that would typically be expected to be t >6 min were observed, in agreement with theoretical estimations. figure 3 shows the spectra of these filtered lf signals received at the bari, graz and moscow stations for quiet days long before the earthquake (figure 3, top row), for possible seismo-induced anomalous days in the nrk-bari wave path just before the l’aquila earthquake (figure 3, middle row), and for days after the main aftershocks (figure 3, bottom row). note that for the last period we have data only for the bari station. the spectra lf signal spectrum for earthquakes figure 2. variations in the amplitudes of the lf signals in the nrk-bari wave path during periods of strong magnetic activity. (a) a magnetic storm from october 28-31, 2003. (b) a magnetic storm from november 8-10, 2004. were very similar for all three of these stations during the quiet periods. this contrasts with the differences seen in the spectra of the seismo-disturbed days. only the spectra of the signal propagating along the nrk-bari seismic path exhibited wave activity, with periods of the order of 10 min to 20 min. these periods were absent in the spectra of the undisturbed signals and of the signals used as the control paths, and in the spectra resulting of magnetic-induced anomalous signals (figure 4). this result reinforces previous results in which spectral peaks in the range of t = 10 min to 25 min were observed for several strong earthquakes [rozhnoi et al. 2007]. the same spectral changes were observed in the analysis of lf (40 khz) signals from the jjy transmitter in japan that were recorded in petropavlovsk-kamchatsky (russia) for strong earthquakes (m 6.1-8.3) which occurred in the sensitivity zone of this wave path in november 2004, november 2005 and november 2006. the results of these analyses are shown in figure 5. for comparison, the variations in the amplitude of the jjy signals for the quiet november month in 2002 are also shown. night-time disturbances of the amplitudes of the lf signals (figure 5, yellow-brown color) were conspicuous 2-10 days before the earthquakes and during strong aftershocks (m 5-6.5) activity in november 2006. the spectra of the seismoinduced anomalous days show a main maximum that corresponds to wave periods of t ~30 min, and they also rozhnoi et al. 184 figure 3. normalized spectra of the filtered (0.28-15 mhz) amplitudes of the nrk signals. upper panels: spectra for bari, graz and moscow for quiet days at the end of march, 2009, well before the occurrence of the earthquakes. middle panels: spectra for bari, graz and moscow for days when the seismo-induced anomalies were observed in the nrk-bari propagation path. bottom panels: spectra for bari for the quiet days after the earthquake. figure 4. normalized spectra of the filtered (0.28-15 mhz) amplitudes of the nrk signals registered at the bari station during strong magnetic activity in october 2003 and november 2004. 185 contain waves with shorter periods, in the range of 10 min to 20 min. 3. discussion and conclusions we have presented here a spectral comparison of the amplitudes of lf signals received during periods of strong seismic and geomagnetic activities. although the effects on the lf signals induced by the seismic and geomagnetic activities were very similar, the scale of resulting inhomogeneities in the ionosphere was very different. magnetic storms are global processes, while seismo-induced disturbances in the ionosphere have a local character. this difference can be seen from the spectral compositions of the signals. in the spectra of the seismo-induced lf signals, waves with periods in the range of 8 min to 20 min were clearly observed. these periods were absent in the spectra of geomagnetic-induced anomalous days. these results confirm that the observed effects in the case of the l’aquila earthquake can be produced by the penetration of atmosphere gravity waves into the ionosphere. this penetration leads to perturbations in the plasma in the lower ionosphere [molchanov and hayakawa 2007]. the energy flux of the atmosphere gravity waves originates from the release of gas-water from within the lf signal spectrum for earthquakes figure 5. time-and-frequency analysis of jjy signals received at petropavlovsk-kamchatsky in november 2002, 2004, 2005 and 2006. left panels: contour maps of the amplitudes of the jjy signal variations as a function of time of day (x-axis) and day of month (y-axis). right panels: spectrograms of the filtered amplitudes of the signals as a function of frequency (x-axis) and day of month (y-axis). arrows indicate earthquakes with m >6. earthquake preparatory zone. there are indications that such a release took place before the l’aquila earthquake [grant and halliday 2010]. a secondary effect of this release was the dramatic change in behavior that was observed for the toads in the san ruffino lake area (74 km from the epicenter) 5 days before the earthquake, and some days after the event. the reduced toad activity precisely coincided with the preseismic perturbations in the ionosphere that were detected by vlf/lf radio sounding. changes in the weather or air composition might have affected these toads; however, a detailed analysis of the local weather conditions (e.g. temperatures, humidity, wind and rainfall) at this time did not reveal any perceptible changes. we have every reason to believe that this abnormal behavior of the toads was caused by changes in the air composition due to the release of gases in this area. acknowledgements. this study was supported by the seventh framework programme fp7 under grant agreement n° 262005 semep and rfbr, under grant 11-05-00155-a. data and sharing resources the earthquake catalog used in this study can be found at the site: http://neic.usgs.gov/neis/epic/epic_global.html the dst data were taken from the site: http://wdc.kugi.kyoto-u.ac.jp/dstdir/index.html references beloglazov, m.i. and g.f. remenez (1982). propagation of vlf radio waves in high latitudes, leningrad, nauka, 240 pp. (in russian). biagi, p.f., r. piccolo, l. castellana, t. maggipinto, a. ermini, s. martellucci, c. bellecci, g. perna, v. capozzi, o. molchanov, m. hayakawa and k. ohta (2004). vlflf radio signals collected at bari (south italy): a preliminary analysis on signal anomalies associated with earthquakes, nat. hazards earth sys., 4, 685-689. biagi, p.f., l. castellana, t. maggipinto, g. maggipinto, a. minafra, a. ermini, v. capozzi, g. perna, m. solovieva, a. rozhnoi, o. molchanov and m. hayakawa (2007). decrease in the electric intensity of vlf/lf radio signals and possible connections, nat. hazards earth sys., 7, 423-430. biagi, p.f., l. castellana, t. maggipinto, d. loiacono, v. augelli, l. schiavulli, a. ermini, v. capozzi, m.s. solovieva, a.a. rozhnoi, o.a. molchanov and m. hayakawa (2008). disturbances in a vlf radio signal prior the m=4.7 offshore anzio (central italy) earthquake on 22 august 2005, nat. hazards earth sys., 8, 1041-1048. grant, r.a. and t. halliday (2010). predicting the unpredictable; evidence of pre-seismic anticipatory behaviour in the common toad, j. zool., 281, 263-271. molchanov, o.a. and m. hayakawa (2007). seismo-electromagnetics and related phenomena: history and latest results, terra sci. pub. co., 432 pp. rozhnoi, a.a., m.s. solovieva, o.a. molchanov, p.f. biagi, m. hayakawa and sh. maekawa (2005). disturbances in vlf/lf signals correlated with earthquakes in the europe station and asia network, iwse-2005, abstracts, 428-431. rozhnoi, a., m. solovieva, o. molchanov, p.f. biagi and m. hayakawa (2007). observation evidences of atmospheric gravity waves induced by seismic activity from analysis of subionospheric lf signal spectra, nat. hazard earth sys., 7, 625-628. rozhnoi, a., m. solovieva, o. molchanov, k. schwingenschuh, m. boudjada, p.f. biagi, t. maggipinto, l. castellana, a. ermini and m. hayakawa (2009). anomalies in vlf radio signals prior the abruzzo earthquake (m=6.3) on 6 april 2009, nat. hazards earth sys., 9, 1727-1732. *corresponding author: alexander rozhnoi, institute of physics of the earth, russian academy of sciences, moscow, russia; email: rozhnoi@ifz.ru. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. rozhnoi et al. 186 f u r t h e r r e m a r k s o n e x t r a r o o t s o f r a y l e i g h e q u a t i o n a n d s o m i g l i a n a w a v e s a . l . l e v s h i n ( * ) or. f . p a n za ( * * ) r i c e v u t o il 24 giugno 1970 riassunto. — l e r a d i c i maggiori d e l l ' u n i t à dell'equazione di rayleigh a p p l i c a t e a d u n semispazio elastico c o n t r i b u i s c o n o alla soluzione solo p e r v a l o r i a b b a s t a n z a g r a n d i del coefficiente di poisson (a > 0.309). u n a delle r a d i c i c o r r i s p o n d e ai " l e a k i n g modes " i quali h a n n o velocità di fase m i n o r e della velocità delle o n d e l o n g i t u d i n a l i . u n ' a n a l o g a o n d a con d i v e r s a dispersione p u ò esistere nel caso in cui u n semispazio elastico è c o p e r t o d a u n o s t r a t o c a r a t t e r i z z a t o d a velocità a e fi minori di quelle del semispazio. l o spessore dello s t r a t o n o n d e v e essere t r o p p o piccolo in c o r r i s p o n d e n z a della l u n g h e z z a d ' o n d a . summary. —• t h e e x t r a r o o t s of t h e r a y l e i g h e q u a t i o n for an elastic h a l f s p a c e c o n t r i b u t e t o t h e solution only for large e n o u g h values of t h e poisson coefficient (a > 0.309). one of t h e m corresponds t o leaking m o d e s w i t h t h e p h a s e velocity less t h a n t h e velocity of t h e l o n g i t u d i n a l w a v e . a similar w a v e w i t h d i s t i n c t dispersion m a y exists in t h e case w h e r e a n elastic h a l f s p a c e is covered b y a t h i n l a y e r w i t h lower velocities of elastic w a v e s . t h e t h i c k n e s s of a layer should be n o t too small in c o m p a r i s o n w i t h t h e w a v e l e n g t h . i n t r o d u c t i o n . since the basic paper of rayleigh (l) on surface waves in an elastic halfspace was published, a significant number of papers have considered the physical meaning of extra roots of the rayleigh equation. (*) soviet geophysical c o m m i t t e e , moscow ( u s s r ) . (**) i s t i t u t o di fisica, i s t i t u t o di geodesia, bologna ( i t a l i a ) . 4(> l e v s k i n a. l . p a n z a g . f . recently we h a v e t h e papers of caloi (2), (3), (*), caloi a n d r o m u a l d i (5) which link a t least one of these roots to t h e somigliana waves a n d give some physical a r g u m e n t s a n d e x p e r i m e n t a l d a t a for their existence. iii this review we w a n t to reconsider this problem, basing our a r g u m e n t s on some results of ewing, j a r d e t z k y and press (6). p h i n n e y (7), gilbert a n d l a s t e r (8), gilbert (9). 1. plane wares in a halfspace. let us consider an elastic homogeneous halfspace a n d let a a n d /? be t h e velocities of p a n d s waves y = | j a n d a be t h e poisson's ratio. the f u n d a m e n t a l ravleigh e q u a t i o n for u n k n o w n p h a s e velocity c of surface wave r u n n i n g along a free surface is 2 (0 \2 l/2 (c \2 2 — h — 4 1 — « like rayleigh we assume t h a t t h e signs of radicals are positive, t h e n t h e e q u a t i o n h a s oidy one positive real root ci (0 < c i </?), which is of course t h e rayleigh root. if t h e signs of radicals are n o t specified two additional (extra) roots arise (c2 and c3); if a > a c (etc = 0.263082) b o t h roots are complex a n d c o n j u g a t e d ; moreover if a > a s (as = 0.309) t h e n t h e real p a r t of 02 a n d c3 is less t h a n a, i.e. with u s u a l n o t a t i o n s be 02,3 < a; finally if (jc < a < as t h e n lie 02,3 > a. f o r 0 < (ic b o t h roots are real a n d c3 > c2 > a. these real roots are linked to waves which caloi called somigliana waves. fig. 1 gives t h e roots versus a. i t was shown by f u (10) t h a t t h e p h a s e velocities c2 a n d c3 correspond to such angles of emergence e2 a n d e3 of t h e plane waves a t the free b o u n d a r y for which reflected plane waves of t h e s a m e kind do not exist; this is expressed b y t h e following f o r m u l a e f i g . 2 d e p e n d e n c e of angles a n d i"s2,n on a. f u r t h e r r e m a r k s on e x t r a r o o t s o p r a y l e i g h e q u a t i o n , e t c . 4 9 the dependence of angles of emergence e''2,3 a n d es°,3 on tr < ac is shown in fig. 2. i t is easy to c o m p u t e t h e components of t h e displacement vector a t free surface as a f u n c t i o n of t h e angle of emergence of plane p or s wave of c o n s t a n t a m p l i t u d e . such f u n c t i o n s for a = 0,25 are shown in f i g . 3. tt is seen t h a t no peculiarities exist for angles of emergence e2 a n d e3 in b o t h cases; t h e more complicated behaviour of these f u n c t i o n s for s incident wave is due t o t h e existence of t h e critical angle ec = arcos in this case ( u ) . therefore plane wave t h e o r y gives no clew for u n d e r s t a n d i n g a p a r t i c u l a r role of these e x t r a roots of t h e rayleigli equation. 2. lamb's problem. following gilbert (9) we write t h e solution of n o n s t a t i o n a r y two-dimensional l a m b ' s problem f = re dick jo (kr) | [3] 1 f {a>, k) where r is horizontal distance f r o m t h e source to t h e receiver, z is t h e d e p t h of t h e receiver, k is t h e wave n u m b e r , to = ck is t h e circular f r e q u e n c y ; / (to,k,z) depends on t h e source f r e q u e n c y spectrum, on t h e position a n d model (vertical or horizontal) of the receiver. f (to, k) is t h e e a y l e i g h f u n c t i o n , j0(kr) is t h e bessel function. the e s t i m a t e of t h e inner integral in [3] is usually made by cont o u r integral m e t h o d . d u e t h e presence of t h e radicals 1 ' co2 , 1 „ co2 v = / 1 — — a n d v = / 1 i a 2 k 2 | ' k2 in / a n d f b o t h these f u n c t i o n are multivalued. to eliminate the m u l t i v a l u e n a t u r e of t h e f u n c t i o n b r a n c h c u t s are introduced on t h e p l a n e of t h e complex variable co. usually t h e y are t a k e n along t h e line rev = 0, re v' = 0. the b i e m a n surface is formed b y four sheets identified b y t h e signs of re v a n d re v'. to close the contour t h e sheet with rev > 0, re v' > 0 is used. i t is n o t e d as ( + , + ) sheet. w e are interested only in t h e f o u r t h q u a d r a n t of t h e to plane. f o r real positive k t h e r e is only one f u r t h e r r e m a r k s on e x t r a roots op r a y l e i g h e q u a t i o n , e t c . 5 1 simple pole of t h e i n t e g r a n d r e l a t e d t o t h e bayleigli r o o t of tlie e q u a t ion f(o>,k) = 0 n a m e l y co = lee 1. t h e c o n t r i b u t i o n of t h e b o d y w a v e s is described b y t h e i n t e g r a l a l o n g t h e b r a n c h lines (fig. 4a). imu r ,ko kp reu fig. 4a b r a n c h p o i n t s , b r a n c h c u t s , initial c o n t o u r of i n t e g r a t i o n 011 c o m p l e x p l a n e . this m e t h o d of describing t h e c o n t r i b u t i o n of t h e b o d y w a v e s is of course n o t u n i q u e . if we c u t t h e b r a n c h e s vertically d o w n f r o m b r a n c h p o i n t s co ka, co — kfi a n d we k e e p t h e u p p e r p a r t of t h e p l a n e 011 t h e s a m e sheet as before, i t is possible t o u n c o v e r p r e v i o u s l y inaccessible p a r t s of o t h e r sheets w i t h rev > 0, rev' < 0 ( + j —) a n d rev < 0, rev' < 0 (—, —). t h e n o u r solution m a y b e p r e s e n t e d as a c o n t r i b u t i o n to t h e i n t e g r a l s along t h e n e w b r a n c h c u t s describing m a i n b o d y w a v e s (because of t h e lca a n d kp poles) p l u s t h e c o n t r i b u t i o n of t h e r a y l e i g h pole p l u s t h e c o n t r i b u t i o n of t h e poles f r o m t h e u n c o v e r e d p a r t s of t h e (-{-, —) a n d t h e (—, —) sheets if such poles exist (fig. 4b). a t our case t h e poles a t t h e u n c o v e r e d p a r t of (—, —) sheet are a b s e n t . f o r a < ac t h e poles co = kc2 a n d co = kcs r e l a t e d t o t h e e x t r a r o o t s of [1] are s i t u a t e d t o t h e r i g h t of co = ka on inacessible p a r t of ( + , —) sheet a n d do n o t c o n t r i b u t e t o our solution. t h e s a m e sit u a t i o n exists f o r complex poles rem2,3 ± ilma)2,3 w h e n a < 0,309; only f o r a > 0,309 t h e r e is a complex pole recoil — iimco2,a a t t h e accessible p a r t of ( + , •—) sheet which c o n t r i b u t e s to t h e solution as so called p pulse or pl wave (e.g. p h i n n e y (7), g i l b e r t a n d l a s t e r (8), 5 2 levsilin a . l . p a n z a ( i . f . manuchov, agurtsoff (13). this wave is a weak d i s t u r b a n c e behind p wave. i t becomes more a n d more distinct as a increases. as e x t r a roots do n o t c o n t r i b u t e t o tlie solution for a < 0.309. somigliana waves do n o t exist i n this case. f i g . 4b b r a n c h p o i n t s , b r a n c h c u t s , u s e d c o n t o u r of i n t e g r a t i o n on c o m p l e x p l a n e . 3. layered half space. l e t us consider t h e effect of t h i n elastic layer overlaying t h e same elastic half space with a < ac. such a layer as caloi (') mentioned is necessary t o generate t h e somigliana waves. the thickness of a layer is h a n d its c o n g r e s s i o n a l a n d shear velocities are a 0 a n d fio respectively (/?» < /3). the f o r m u l a of displacement looks like [3] b u t t h e f u n c t i o n / a n d f shoidd be replaced b y t h e new f u n c t i o n s f* (co, h, z) a n d f* (co, 7c) which t e n d t o / a n d f as e — > 0. l e t us fix 1c > 0. then for infinitely small ice we h a v e t h e same situation as before. as ti increases t h e poles of f*(oj,ic) shift f r o m their positions for h = 0. the eayleigh pole keeps on t h e accessible p a r t of ( + , + ) sheet moving f r o m u> = jcci to co = kc0 (here e0 is velocity of eayleigh wave a t t h e halfspace with velocities of body waves a0 a n d fi„ along t h e real axis). t h e behaviour of other poles is much f u r t h e r r e m a r k s on e x t r a r o o t s op r a y l e i g h e q u a t i o n , e t c . 5 3 more complicated. the couple of poles on the real axis to t h e r i g h t of co = iri s i t u a t e d on t h e covered p a r t of ( + , —) sheet unites in t h e c o n j u g a t e d couple a n d moves to t h e left as h increases. the pole which moves a t t h e f o u r t h q u a d r a n t eventually crosses t h e p branch-cut lm u kco k(i0 __tl.kp + , + + , kq\kc 2 ij a+.-) kc 3 ™o— r e u s p fig. 5 m o v e m e n t s of pole p + _ for fixed ic and v a r i a b l e tl. t o a p p e a r on t h e open p a r t of ( + , —) sheet. only now it begins to c o n t r i b u t e to t h e solution as dispersive leaking mode pl (p+ -). as h increases more it crosses t h e uncovered stripe of t h e ( + , —) sheet a n d s b r a n c h c u t , t o t h e covered p a r t of t h e same sheet. h e r e it unites to t h e c o n j u g a t e d pole a t t h e second q u a d r a n t to become real. t h e n one of t h e m still keeps on the same p a r t of ( + , —) sheet, a n d t h e other one overtakes t h e b r a n c h c u t to a p p e a r 011 accessible p a r t of ( + , + ) sheet to t h e left of co = ft/?. now it contributes as t h e first shear mode a n d its evolution is limited b y points co = /.•/?<> and co = 1<!5 (fig. 5). i n a similar way t h e other couple of real poles on t h e covered (—, —) sheet unites a t t h e c o n j u g a t e d couple as h increases. the pole a t t h e f o u r t h q u a d r a n t a f t e r some evolutions appears on t h e uncovered p a r t of ( + , —) sheet to contribute as leaking mode p i n ( p + of gilbert). then if h is large enough it goes to ( + , -¡-) sheet to become t h e t h i r d shear mode. 5 4 l e v s i i i n a . l . p a n z a g . 1'. w e do n o t discuss h e r e t h e b e h a v i o u r of o t h e r poles of f(co, k) which a r e n o t r e l a t e d t o t h e r o o t s of r a y l e i g h e q u a t i o n . t h u s t h e e x i s t e n c e of t h e s u r f a c e l a y e r really " w a k e s u p " t h e e x t r a r o o t s of r a y l e i g h e q u a t i o n p e r m i t t i n g t h e m t o c o n t r i b u t e t o t h e solution. h o w e v e r t h i s h a p p e n s only if t h e l a y e r is t h i c k e n o u g h . f o r t h e c o n t i n e n t a l e a r t h m o d e l , g i l b e r t ' s figure 2 1 t h e pl w a v e gives t h e r e l a t i o n m i n (kh) ^ 0.2: f o r pl>, w a v e we g e t f r o m g i l b e r t ' s figure 23 t h e e s t i m a t e m i n (a:77) «a 1.9. i n f r e q u e n c y d o m a i n it m e a n s t h a t f o r 11 = 35 k m w a v e pl does n o t c a r r y p e r i o d s m o r e t h a n 140 sec a n d pls w a v e m o r e t h a n 14 sec. a k n o w l e d g m e n t s . a. l . l e v s h i n t h a n k s t h e i t a l i a n n . r . c . f o r t h e g r a n t which m a d e possible his visit t o t h e i n s t i t u t e of geodesy. b o t h t h e a u t h o r s t h a n k s p r o f . m. c a p u t o a n d p r o f . v . i . keilis-borok f o r e n c o u r a g e m e n t a n d h e l p f u l discussion. t h i s r e s e a r c h w a s s u p p o r t e d b y t h e i t a l i a n n.r.u. r e f e r e n c e s ( l ) r a y l e i g h l . , on waves propagated along the plane surface of an elastic solid. « p r o o . l o n d o n m a t h . soc. », 17, 4-11, 1885; or «sci. p a p e r s » , 2, 4 4 1 4 4 7 ; « c a m b r i d g e univ. p r e s s » , l o n d o n ( 1 9 0 0 ) . ( ) c a l o i p . , l'equazione di rayleigh e le onde di somigliana. i i . la teoria di somigliana rettifiche e conseguenze. « a t t i acc. naz. lincei, classe scienze fis. mat. e n a t . » , x l i . 5 ( 1 9 6 6 ) . ( 3 ) c a l o i p . , on the upper mantle. « a d v a n c e s in geophysics», 7 9 2 1 1 . « academic press », new york and l o n d o n ( 1 9 6 7 ) . (•*) c a l o i p . , l'equazione di rayleigh e le onde di somigliana. loro importanza per lo studio della crosta terrestre. « a t t i acc. naz. lincei. classe scienze fis. mat. e n a t . , » x l i i i , 6 ( 1 9 6 7 ) . ( 5 ) c a l o i p . and r o m u a l d i & . , more on the somigliana waves. « annali di geofísica» x x i , 3 , 2 3 9 2 6 4 ( 1 9 6 8 ) . (6) e w i n g w . m., j a r d e t z k y w . s. a n d p r e s s f . , elastic waves in layered media. « me graw hill book c o m p a n y , inc. », new york, t o r o n t o a n d l o n d o n ( 1 9 5 7 ) . ( ' ) p i i y n n e y r. a., propagation of leaking interface waves. « bull. seism. s o c . a m . », 5 1 , 9 ( 1 9 6 1 ) . f u r t h e r r e m a r k s on e x t r a r o o t s op r a y l e i g h e q u a t i o n , e t c . 5 5 ( 8 ) g i l b e r t f . a n d l a s t e r , j . l . , exitation and propagation oj pulse on an interface. « b.s.s.a. », 52, 299-319 (1962). ( • ) g i l b e r t f . , propagation oj transient leaking modes in a strati/led elastic wave guide. « r e v . g e o p h y s i c s » , 2, 123-153 (1964). (io) pu c. y., on the irrelevant roots oj the kayleigh wave equation. «science record », 2, n. y. (1949). ( u ) bullen, k . e . , an introduction to the theory oj seismology, p . 106, « c a m b r i d g e u n i v e r s i t y p r e s s » (1963). ( 1 2 ) m a n u c i i o v a . v . , some exact theoretical seismograms for wave propagation in an elastic half space. problems oj dynamical theory of seismic wave propagation, 1 ( e d . g.i. p e t r a s h e n ) , l e n i n g r a d [1957 (in russian)]. ( 1 3 ) o g u r t s o f f k . i . , quantitative investigations oj wave dynamics in an elastic halfspace for different kinds of sources. « p r o c . l e n i n g r a d s k y u n i v . , m a t h e m . seria » 208 (1956). layout 6 1 annals of geophysics, 63, 3, dm329, 2020; doi:10.4401/ag-8197 a staggered-grid lowrank finite-difference method for elastic wave extrapolation qizhen du1,2,3, jing ba*,4, dong han5, pengyuan sun6, jianlei zhang6 (1) key laboratory of deep oil and gas, china university of petroleum (east china), changjiang west road 66th, qingdao, china, 266580 (2) laboratory for marine mineral resources, qingdao national laboratory for marine science and technology, qingdao, china, 266071 (3) key laboratory of geophysical prospecting, cnpc, china university of petroleum (east china) changjiang west road 66th, qingdao, china, 266580 (4) school of earth sciences and engineering, hohai university, nanjing, jiangsu, china, 211100 (5) sinopec geophysical research institute, nanjing, jiangsu, china, 211103 (6) china national petroleum corp bureau of geophysical prospecting inc. zhuozhou, hebei, china, 072750 article history: received may 8, 2019; accepted december 18, 2019 abstract elastic wave extrapolation in the time domain is significant for an elastic wave equation-based processing. to improve the simulation reliability and accuracy of decoupled elastic pand swaves, we propose the staggered-grid lowrank finite-difference method based on the elastic wave decomposition. for elastic wave propagation, a lowrank finite-difference method based on the staggered grid is derived to improve the accuracy. regarding the application of the decoupled elastic wave equation, we derive the finite-difference scheme coefficients which are dependent on velocity. based on the elastic wave decomposition and plane wave theories, we formulate the elastic wave-extrapolation operators, which contain trigonometric adjustment factors. accordingly, by applying the lowrank method to approximating the operators, the finitedifference scheme is designed to discretize the decoupled wave equation. the derivation processing implies the combination of elastic wave-mode decomposition and extrapolation. the proposed method enables elastic pand s-waves to extrapolate in the time-space domain separately and produces accurate p-and s-wave components simultaneously. dispersion analysis suggests that our proposed method is reliable and accurate in a wide range of wavenumber. numerical simulation tests on a simple model and the marmousi2 model validate the accuracy and effectiveness of the method, showing its ability in handling complex structures. although the operators are accurate only when the medium is homogeneous, they are of high accuracy when the velocity gradient is quite small and are applicable when the velocity gradient is large. the subsequent results of reverse time migration for the marmousi2 model also suggest that the proposed method is enough to serve as an extrapolator in elastic reverse time migration. keywords: lowrank finite-difference, lowrank approximation, elastic wavefield extrapolation. 1. introduction over the past few decades, seismic exploration technique has been extended to the elastic wavefield. this remarkable stride benefits from the development of multi-component seismic acquisition technique [granger et al., 2005; robertsson et al., 2008], which provides the accessibility to not only the primary p-wave but also the s-wave. a great deal of information from diverse wave modes exist in multicomponent data, which portray the prominent subsurface structure and discriminate the lithological variation as compared to conventional single-component pwave data [stewart et al., 2003; yan and xie, 2012]. in such case, s-wave becomes an important factor to calibrate the bright-spot and to estimate the fractures [du et al., 2012]. the processing based on the elastic wave equations act as a bridge between multicomponent data and elegant profiles. the subsurface wavefield consists of the pand s-waves. for a physical meaning of pand s-waves, the processing is commonly observed to contain either the wavefield separation [yan and sava, 2008; du et al., 2012, 2014] or the wavefield decomposition [zhang and mcmechan, 2010; wang and mcmechan, 2015]. as for the separation in isotropic media, classic curl and divergence operators are efficient and practical [aki and richards, 2002; yan and sava, 2009; han et al., 2016]. however, sun et al. [2001] showed that they induce amplitude distortion and phase shift. such problems can be fully avoided by using the decomposition operators [zhang and mcmechan, 2010] or the decoupled wave equations [ma and zhu, 2003; li et al., 2007; du et al., 2017]. the decomposition method can fully retain the amplitude and phase of the pand s-waves. the decomposition operators are applied to the subsurface wavefield and are not related to the wavefield extrapolation. therefore, the decoupled wave equations need improved extrapolation methods. the elastic wavefield extrapolation algorithm is the core of processing such as elastic wave simulation, reverse time migration (rtm), and full waveform inversion. those techniques require an efficient and accurate algorithm, in which the large time step is a key to the efficiency. conventional finite-difference methods [virieux, 1984] and pseudo-spectral methods [reshef et al., 1988] are the two popular methods of wave extrapolation. however, conventional finite different methods are constrained to the courant-friedrch-lewy (cfl) condition and require small time steps to guarantee stability [finkelstein and kastner, 2007]. the pseudo-spectral methods, equivalent to infinite order finite-difference, require even smaller time step. a solution to this issue lies in compensating error of approximating the temporal derivatives approximation in spatial derivative computation. for finite-differencebased methods, the optimized coefficients of finite-difference in joint time-space domain could become a good choice to improve the accuracy [tan and huang, 2014; liu and sen,2009, 2011]. for the spectral-based method, incorporation of the temporal compensation factors into the extrapolation operators in the mixed domain is a desirable way to reduce the artifacts caused by the temporal dispersion [soubaras and zhang,2008; zhang and zhang, 2009; stoffa and pestana, 2009; etgen and brandsherg-dahl, 2009; du et al., 2013]. the k-space method is another type of the spectral domain wave extrapolation method. it is essentially an extension of the pesudo-spectral method and reaches high accuracy by using a temporal iteration procedure in combination with the accurate spectral evolution of spatial derivatives [tabei, et al., 2002]. the dispersion errors of the second-order time integration operators are compensated by this procedure [fang et al., 2014]. early attempts of the k-space method include fornberg and whitham [1978]’s application to nonlinear wave equations and bojarski [1982]’s research on the scattering problem. it is then extended by many researches including the application to the elastic scalar wave equation [compani-tabrizi, 1986] and the application to the acoustical wave equation [pourjavid and tretiak, 1992]. the first-order wave equations are capable of handling velocity and density variations and are easier to apply the perfectly matched layer (pml) boundary condition. hence, k-space method on the basis of firstorder wave equations attracts many attentions [mast et al, 2001; tabei et al., 2002; fang et al., 2014]. however, due to its velocity-dependency [etgen and brandsberg-dahl, 2009], solving the k-space operators in the spectral domain involves high computational cost. to reduce computational costs, the flexible-control lowrank methods established a great interest toward further investigation which contains the lowrank decomposition method [fomel et al., 2013; cheng et al., 2016] and the lowrank fd method [song et al., 2013; fang et al., 2014; han et al., 2016]. the lowrank decomposition method generates a sparse separable decomposition of the operator matrix and balance the accuracy and efficiency through the rank. as a branch of the lowrank decomposition method, sun et al. [2015] developed a lowrank one-step method. cheng et al. [2016] proposed a series of approximate mixeddomain integral operators in the anisotropic media for elastic wave extrapolation. the lowrank fd method (lfd) is now widely used to design the fd scheme in relation to the spectral response in the mixed-domain. song et al. qizhen du et al. 2 [2013] proposed the lfd method for the propagation of the scalar wave. it is extended to a coupled first-order system to handle the variable density and velocity by fang et al. [2014]. however, the current literature on the lfd method exerts their efforts to the scalar wave extrapolation based on the acoustic assumption and ignores the elastic nature of the earth. it is essential to develop a new lfd-based method to depict the structures of the subsurface from elasticity and linearity standpoints. in this paper, our prime goal is to design a lowrank fd method for elastic wavefield extrapolation based on decoupled wave equations. the lfd-based elastic wave extrapolation is available to provide accurate pand swavefield vectors. firstly, we begin with the common elastic displacement equations to derive second-order extrapolation operators in the mixed-domain and deduce first-order operators from these second-order operators. then, for heterogeneous media, we formulate a discretization form of the first-order decoupled system. next, the lowrank approximation is applied in the first-order extrapolation operators and the corresponding staggered-grid lfd scheme is designed. finally, to validate the proposed method, we implement the elastic wave simulation on the simple models and on the complex ones, respectively. we also apply our proposed method to elastic rtm on the marmousi2 model. 2. methodology 2.1 second-order elastic wave extrapolation operators in mixed-domain the lfd-based elastic wave extrapolation matters to the construction of the decoupled pand s-wavefield. in consideration of the decoupled wave equations, we derive second-order extrapolation operators to obtain secondorder elastic wave extrapolation in the mixed-domain. according to the elastic wavefield vector decomposition method [zhang and mcmechan, 2010], we could obtain the decoupled elastic wave equations from the common second-order elastic wave equations [aki and richard, 2002]. given that the velocity terms are constant, we employ the analytical solutions to these decoupled elastic wave equations and formulate the second-order extrapolation operators. in a 2d infinite, homogeneous, and isotropic medium, the elastic wave equation is [aki and richards, 2002]: (1) where 𝐮 = (𝑢𝑥,𝑢𝑧)𝑇 denotes a displacement vector, 𝑡 represents the time, 𝑐𝑝 is the p-wave particle velocity, 𝑐� is the swave particle velocity, and 𝐱 = (𝑥,𝑧)𝑇 is the spatial location vector. note that in this paper, the discussions and numerical tests are in 2d cases. given that the velocity terms are constant, we could derive the following equations in the wavenumber domain: (2) where 𝐤 = (𝑘𝑥,𝑘𝑧)𝑇 represents the vector of wavenumber. the vector of 𝐔� represents the displacement vector in the domain of wavenumber, and the forward fourier transform is expressed as (3) equations (1) and (2) are the coupled elastic wave formulas in which the displacement vector contains not only the p-wave but also the s-wave. for a physical interpretation of seismic processing results (e.g., seismic imaging profiles), it is necessary to implement wavefield separation or decomposition. the derivation of decoupled wave equations is based on the condition that pand s-waves propagate independently in homogeneous media. the decomposed pand s-wavefields [zhang and mcmechan, 2010] could be expressed as 𝜕𝑡²𝜕²𝐮 ‒𝑐 𝛻(𝛻 � 𝐮) + 𝑐 𝛻 × (𝛻 × 𝐮) = 0² � ² � 𝜕𝑡²𝜕²𝐔� +𝑐 𝐤 �𝐤 � 𝐔�� ‒ 𝑐 𝐤 × �𝐤 × 𝐔�� = 0² � ² � 𝐔� = ∫𝐮 � 𝑒‒𝑖𝐤�𝐱d𝐱 .1 (2𝜋)² 3 lowrank elastic wave extrapolation qizhen du et al. 4 𝐔�p = 𝐤� (𝐤� � 𝐔�) (4) 𝐔�𝑠 = ‒𝐤� × (𝐤� × 𝐔�) (5) where 𝐤� = is the unit vector of wavenumber, 𝐔�p and 𝐔�𝑠 represent the pand s-wave vectors in the wavenumber domain, respectively. under the linear assumption, the vector wavefield is expressed as the superposition of p-wave (irrotational) part and s-wave (non-divergence) part: 𝐔� = 𝐔�p + 𝐔�𝑠 (6) substituting the vector wavefield expressed by equation (6) into equation (2), and applying the identities in connection with the divergence and the curl, the decoupled wave equations can be given as (7) and (8) equations (7) and (8) characterize the pand s-wave propagations, respectively. the derivation details are given in appendix a. according to the plane wave theory, analytical solutions to equations (7) and (8) are expressed as 𝐔�p = 𝐤� (𝐤� � 𝐔�₀)𝑒±𝑖𝑐�𝐤𝑡 (9) and 𝐔�𝑠 = ‒ 𝐤� × (𝐤� × 𝐔�₀)𝑒±𝑖𝑐�𝐤𝑡 . (10) herein, 𝐔�₀ is the amplitude vectors. the sign ± denotes that the solution stands for the outgoing or the ingoing wavefield. we merge the analytical solutions into the temporal discretization equation of the leapfrog form and obtain the mixed-domain integral solutions for the elastic case as follows: 𝐔�𝑝 (𝑡 +𝛥𝑡)+ 𝐤�𝑝 (𝑡‒𝛥𝑡) ‒ 2𝐔�𝑝 (𝑡)=𝐤 ��𝐤� · 𝐔�(𝑡)� · �2(cos(|𝑘|𝑐𝑝𝛥𝑡)‒1)� (11) 𝐔�𝑠 (𝑡 +𝛥𝑡)+ 𝐤�𝑠 (𝑡‒𝛥𝑡) ‒ 2𝐔�𝑠 (𝑡)=‒ 𝐤 �×�𝐤� × 𝐔��(𝑡)� · �2(cos(|𝑘|𝑐𝑠𝛥𝑡)‒1)� (12) note that the equation related to p-wave only describes the p-wave propagation, which means that if the terms related to s-wave are added to the p-wave equation, the products of the interaction between these terms and the propagation operators are zeros. therefore, under the condition that no effects of the s-wave exist in the p-wave formulas, we can replace the p-wavefield symbol 𝐔�𝑝 with the wavefield symbol 𝐔� at the right-hand side of equation (11). similarly, we can determine the s-wave propagation equation as shown by equation (12). equations (11) and (12) are the exact discretization representations of the second-order time derivatives. when describing equations (11) and (12) at the component level, we could obtain the mixed-domain terms: . (13) indices 𝑗,𝑙 = 1,3 denote xand z-axes of the cartesian coordinate in a 2d medium and the mixed-domain terms 𝐤 |𝐤| 𝜕²𝐔�p 𝜕𝑡² +𝑐 𝐤 �𝐤 � 𝐔�� = 0² � 𝜕²𝐔�𝑠 𝜕𝑡² ‒𝑐 𝐤 ×�𝐤 × 𝐔�𝑠� = 0 .² � � 𝑘�𝑗𝑘�𝑙 �2(cos(|𝑘|𝑐𝑝𝛥𝑡)‒1)�𝑘�𝑗𝑘�𝑙 �2(cos(|𝑘|𝑐𝑠𝛥𝑡)‒1)� are the kernels of the mixed-domain recursive time integration operators [pestana and stoffa, 2010; du et al., 2013] for the elastic case. we use the taylor expansions to approximate these exact mixed-domain operators and find that the pseudo-spectral operators, in the elastic case, are only the second-order taylor polynomial approximations of these operators. in this sense, it is the high-order terms of the mixed-domain operators’ taylor expansion polynomials that compensate the temporal accuracy loss caused by the second-order polynomials. 2.2 first-order elastic wave extrapolation operators in mixed-domain in practice, the wavefield extrapolation methods based on the first-order velocity-stress formulas catch higher ability to handle a variant density situation than the second-order displacement formulas [virieux, 1984 and 1986]. it is easier to combine the pml boundary conditions [bérenger, 1994; yuan et al., 2014] with the first-order system of elastic wave equations [tabei et al., 2002]. therefore, we extend the method demonstrated above to the firstorder system. if we define 𝜈� and 𝜏�� as the particle velocity and stress components, respectively, the velocity-stress formulas [leaney, 1988] follow that: (14) where 𝜆(𝐱) and 𝜇(𝐱) are the lamé coefficients, and 𝜌(𝐱) is density. the subscripts �,�,� represent the components of the cartesian coordinate system, and 𝛿�� is the kronecker delta. for homogeneous media, the formulas in the time-wavenumber domain are determined as (15) where i is the imaginary unit, 𝜆 and 𝜇 are lame’s constants, and 𝜈�𝑗, 𝜏�𝑗𝑙 are the 2d spatial forward fourier transforms of particle velocity and stress components, respectively. to derive the first-order operator in the mixed-domain, we employ the trigonometric identities cos(𝑥) = 1 ‒ 2sin2(𝑥/2) and sin(𝑥) = sinc(𝑥)/𝑥 to reshape operators (13), then we obtain (16) note that, in the equations above, replacing the pseudo spectral operator 𝑖²𝑘�𝑘� with 𝑖²𝑘𝑗𝑘𝑙 𝑐²�,� 𝛥𝑡² sinc² improves the accuracy of the numerical simulation. in a first-order system, the pseudo-spectral operator is changed into the following form at the large time-step without lowering the accuracy (17) however, the velocity and stress components in equation (14) are not only related to p-wave but also to s-wave. we introduce the variables 𝜈�𝑗 = 𝜕𝑢�𝑗 / 𝜕𝑡 and 𝜈�𝑗 = 𝜕𝑢�𝑗 / 𝜕𝑡 as pand s-wave particle velocity components, respectively. ⎧ ⎪ ⎨ ⎪ ⎩ = 1 𝜌(𝐱) �𝜏�� �𝑥��𝜈� �𝑡 �𝜏�� �𝑥� = 𝜆(𝐱)𝛿�� +𝜇(𝐱)� + � ,�𝑣� �𝑥� �𝑣� �𝑥��𝑣� �𝑥� �𝑣� �𝑥� ⎧ ⎪ ⎨ ⎪ ⎩ �𝑣�𝑙 �𝑡 1 𝜌= �𝜏��� �𝑥� = 𝜆𝛿�� 𝑖𝑘𝑚𝜈�𝑚 + 𝜇 (𝑖𝑘𝑗𝜈�𝑙 + 𝑖𝑘𝑙𝜈�𝑗� , 𝑖𝑘𝑗𝜏��� ⎧ ⎨ ⎩ ² �𝑖²𝑘𝑗𝑘𝑙 𝑐 𝛥𝑡 sinc²�|𝐤|𝑐�𝛥𝑡� . ² ² �𝑖²𝑘𝑗𝑘𝑙 𝑐 𝛥𝑡 sinc²�|𝐤|𝑐�𝛥𝑡� ² �|𝐤|𝑐�,� 𝛥𝑡� ₂ 𝑖𝑘𝑙sinc²�|𝐤|𝑐�,�𝛥𝑡� . ² 5 lowrank elastic wave extrapolation these variables follow the linear principle, 𝜈𝑗 = 𝜈�𝑗 + 𝜈�𝑗. the second-order decoupled wave equations are transformed to the first-order ones (li et al., 2007): (18) (19) where 𝜏𝑝 is p-wave stress, 𝜏𝑠�� and 𝜏𝑠�� are s-wave normal stresses and 𝜏𝑠�� is s-wave shear stress. we use the forward 2d spatial fourier transform to derive the representations in the wavenumber domain (20) (21) where 𝜈��𝑗, 𝜈��𝑗, 𝜏�� and 𝜏���� are the fourier transform of , 𝜈�𝑗, 𝜈�𝑗, 𝜏� and 𝜏���. after that, we will design the decoupled first-order system of the elastic wave on staggered grids. figure 1 describes a schematic of the 2d staggered grid scheme. we use 𝛥𝑥 and 𝛥𝑧 to denote the space grid intervals along the xand z-axes, respectively, and i and j to denote the grid cell indices, respectively. 𝜏�, 𝜏��� and 𝜏��� are discretized at space locations (𝑖𝛥𝑥, 𝑗𝛥𝑧), 𝜏��� is discretized at space locations, ((𝑖+0.5)𝛥𝑥, (𝑗+0.5)𝛥𝑧) 𝜈�� and 𝜈�� are discretized at space locations ((𝑖+0.5)𝛥𝑥, 𝑗𝛥𝑧), and 𝜈�� and 𝜈�� are discretized at space locations (𝑖𝛥𝑥,(𝑗+0.5)𝛥𝑧). in the discussion, the staggered-grid schemes are used for theoretical analyses and numerical tests. � � 𝜕𝑧𝜕𝜏�𝜌1=𝜕𝑡𝜕𝜈� 𝜕𝑥𝜕𝜏�𝜌1=𝜕𝑡𝜕𝜈� �𝜕𝑧𝜕𝜈�+𝜕𝑥𝜕𝜈��= (𝜆 +2𝜇)𝜕𝑡𝜕𝜏�⎧ ⎪⎪ ⎨ ⎪⎪ ⎩ 𝜕𝜈�𝜕𝑡 = 1𝜌 � 𝜕𝜏��𝜕𝑧 + 𝜕𝜏��𝜕𝑥 � 𝜕𝜏��𝜕𝑡 = −2𝜇 𝜕𝜈�𝜕𝑧 = 𝑠 𝑠 𝑠 𝜕𝜈�𝜕𝑡 = 1𝜌 � 𝜕𝜏��𝜕𝑧 + 𝜕𝜏��𝜕𝑥 � 𝑠 𝑠 𝑠 𝜕𝜏��𝜕𝑡 = 𝜇 � 𝜕𝜈�𝜕𝑧 + 𝜕𝜈�𝜕𝑥 � , 𝑠 𝑠 𝜕𝜏��𝜕𝑡 −2𝜇 𝜕𝜈�𝜕𝑥 𝑠 ⎧ ⎪⎪⎪⎪⎪ ⎨ ⎪⎪⎪⎪⎪ ⎩ ~𝜈�(𝑡)‒𝜈� (𝑡‒𝛥𝑡)~𝛥𝑡 1𝜌 𝑖sinc(c�|k|𝛥𝑡/2)𝑘�𝜏�~~ ~ = 𝛥𝑡 � � = 1𝜌 𝑖sinc(c�|k|𝛥𝑡/2)𝑘�𝜏�~𝜈�(𝑡)‒𝜈� (𝑡‒𝛥𝑡)� �𝛥𝑡~ ~𝜏�(𝑡+𝛥𝑡/2)‒𝜏�(𝑡‒𝛥𝑡/2) = (𝜆+2𝜇)𝑖sinc(c�|k|𝛥𝑡/2)(𝑘�𝑣�+𝑘�𝑣�)~ ~ ⎧ ⎪⎪⎪ ⎨ ⎪⎪⎪ ⎩ ~��=�� ~𝑖sinc(c�|k|𝛥𝑡/2)�𝑘�𝜏��+𝑘�𝜏���𝜌1𝛥𝑡 ~𝜈�(𝑡)‒𝜈� (𝑡‒𝛥𝑡)~ ~��=�� ~𝑖sinc(c�|k|𝛥𝑡/2)�𝑘�𝜏��+𝑘�𝜏���𝜌1𝛥𝑡 ~𝜈�(𝑡)‒𝜈� (𝑡‒𝛥𝑡)~ ~= ~𝜇𝑖sinc(c�|k|𝛥𝑡/2)�𝑘�𝑣�+𝑘�𝑣�� ,��𝜏��(𝑡+𝛥𝑡/2)‒𝜏��(𝑡‒𝛥𝑡/2) ~~ 𝛥𝑡 = ~‒2𝜇𝑖sinc(c�|k|𝛥𝑡/2)𝑘�𝑣���𝜏��(𝑡+𝛥𝑡/2)‒𝜏��(𝑡‒𝛥𝑡/2) ~~ 𝛥𝑡 = ~‒2𝜇𝑖sinc(c�|k|𝛥𝑡/2)𝑘�𝑣���𝜏�(𝑡+𝛥𝑡/2)‒𝜏�(𝑡‒𝛥𝑡/2) ~~ 𝛥𝑡 ⎧ ⎪⎪⎪⎪⎪⎪ ⎨ ⎪⎪⎪⎪⎪⎪ ⎩ qizhen du et al. 6 in the wavenumber domain, the staggered concepts conveyed by figure 1 are achieved with the shift properties of fourier transform. when the gradient of velocity is small, we can replace constant velocities, 𝑐� and 𝑐�, with the space-dependent velocities, 𝑐�(𝐱) and 𝑐�(𝐱) [fang, et al., 2014]. applying reverse fourier transforms to equations (20) and (21) yields: (22) where (23) these derivative operators are named as the first-order k-space operators [tabei, 2002]. note that equations (23) are actually the approximations of the exact ones which use constant velocities in the sinc operators. its accuracy is still relatively high (fang et al., 2014] in media with small velocity gradient. and it also allows us to testify this method in the complex model with large velocity gradient. for numerical solutions to the spatial derivatives in equation (22), we could introduce the localized fourier transform methods [wards, 2008; etgen and brandsberg-dhl, 2009; zhang and zhang, 2009]. however, these methods have access to all nodes in the mixed-domain and require several multidimensional inverse and forward fourier transforms. let 𝑁𝑥 stand for the total number of the spatial grids, the computational complexity of the localized fourier transform methods amounts to 𝑂(𝑁²𝑥 ). from the mathematical perspective, the mixed-domain operators usually exhibit an oscillatory behavior and approximate lowrank [candes et al., 2007; engquist and ying, 2009]. therefore, one choice for reducing the computational complexity is the application of the lowrank methods [fomel et al., 2013; fang et al., 2014]. in the next section, we will demonstrate the details of the lowrank approximation scheme and the lowrank finite-difference method. �� �� 𝜈𝑧 = 𝜈𝑧 + 𝜈𝑧 𝜈𝑥 = 𝜈𝑥 + 𝜈𝑥� ⎧ ⎪⎪ ⎨ ⎪⎪ ⎩ � ,𝜕𝑧‒ 𝜕�𝑣�+𝜕𝑥‒𝜕�𝑣��= �𝜆(𝐱)+2𝜇(𝐱)�𝜏�(𝑡+𝛥𝑡/2)‒𝜏�(𝑡‒𝛥𝑡/2)𝛥𝑡 𝜕𝑧+𝜕𝜏𝜌= 𝜌(𝐱)1�� 𝛥𝑡𝜈�(𝑡+𝛥𝑡/2)‒𝑣� (𝑡‒𝛥𝑡/2) 𝜕𝑥+𝜕𝜏𝜌=�� 𝜌(𝐱)1𝛥𝑡𝜈�(𝑡+𝛥𝑡/2)‒𝜈� (𝑡‒𝛥𝑡/2) ⎧ ⎪⎪⎪⎪⎪ ⎨ ⎪⎪⎪⎪⎪ ⎩ �� �𝜕𝑧+𝜕�𝜏��+𝜕𝑥+𝜕�𝜏���= 𝜌(𝐱)1�� 𝛥𝑡𝜈�(𝑡+𝛥𝑡/2)‒𝜈� (𝑡‒𝛥𝑡/2)� � 𝜕𝑧‒𝜕�𝑣�= ‒2𝜇(𝐱)𝜏��(𝑡+𝛥𝑡/2)‒𝜏��(𝑡‒𝛥𝑡/2)𝛥𝑡� � 𝜕𝑥‒𝜕�𝑣�= ‒2𝜇(𝐱)𝜏��(𝑡+𝛥𝑡/2)‒𝜏��(𝑡‒𝛥𝑡/2)𝛥𝑡 𝜕𝑥+𝜕𝑧+ �𝜕𝜈�+𝜕�𝑣��� � = 𝜇(𝐱)𝜏��(𝑡+𝛥𝑡/2)‒𝜏��(𝑡‒𝛥𝑡/2)𝛥𝑡 �� �𝜕𝑥‒𝜕𝜏��+𝜕𝑧‒𝜕�𝜏���=�� 𝜌(𝐱)1𝛥𝑡𝜈�(𝑡+𝛥𝑡/2)‒𝜈� (𝑡‒𝛥𝑡/2) ⎧ ⎪⎪⎪⎪ ⎨ ⎪⎪⎪⎪ ⎩ ˖𝜕𝑥�𝜕� = 𝐹‒1�𝑖𝑘�𝑒+𝑖𝑘�𝛥�/2 sinc�c�(𝐱)|k|𝛥𝑡/2�𝐹{·}� 𝜕𝑥�̠𝜕� = 𝐹‒1�𝑖𝑘�𝑒−𝑖𝑘�𝛥�/2 sinc�c�(𝐱)|k|𝛥𝑡/2�𝐹{·}� ˖𝜕𝑥�𝜕� = 𝐹‒1�𝑖𝑘�𝑒+𝑖𝑘�𝛥�/2 sinc�c�(𝐱)|k|𝛥𝑡/2�𝐹{·}� ˗𝜕𝑥�𝜕� = 𝐹‒1�𝑖𝑘�𝑒−𝑖𝑘�𝛥�/2 sinc�c�(𝐱)|k|𝛥𝑡/2�𝐹{·}� 7 lowrank elastic wave extrapolation 2.3 lowrank approximation to first-order operators in the heterogeneous case it is not desirable to extrapolate the elastic wave using the recursive time integration operators because of the high computational cost. in this section, we will derive the decomposition representations of the mixed-domain operators with the lowrank decomposition method [fomel et al., 2013; cheng et al., 2016]. as equation (23) expressed, the mixed-domain operators can be categorized into two groups. one is associated with p-wave and the other is associated with s-wave. for a brief description, we will take as an example to introduce the approximation. let 𝑊𝑗� (𝐱,𝐤) = 𝑘𝑗 sinc(𝑐𝑝(𝐱) ǀ𝐤ǀ 𝛥𝑡 / 2) which is an 𝑁� by 𝑁� matrix, 𝐱 = {𝑥1, ..., 𝑥𝑁�} represent samples in the space domain and 𝐤 = {𝑘1, ..., 𝑘𝑁�} represent samples in the wavenmber domain. the exact operator, 𝑊𝑗� (𝐱,𝐤), could be factorized into the product of three submatrices, which has the form as (24) where 𝑊𝑗� (𝐱,𝐤�) is a submatrix of the original matrix 𝑊𝑗� (𝐱,𝐤) consisting of m columns selected from 𝑊𝑗� (𝐱,𝐤) related to 𝐤�, while 𝑊𝑗� (𝐱�,𝐤) is another submatrix of the original matrix 𝑊𝑗� (𝐱,𝐤) consisting of n rows selected from 𝑊𝑗� (𝐱,𝐤) related to 𝐱�. 𝑎�� represents the middle coefficients which have m-by-n entries, and m and n stand for the rank of the decomposition. the way to construct the decomposition representations follows the method by engquist and ying [2007, 2009]. the computational complexity of the algorithm is 𝑂(𝑁𝑁� log𝑁�), and the computational cost of the fourier transform is 𝑁� log𝑁�. m and n are usually far less than the spatial number, 𝑁�. the ranks are up to the complexity of the model. for homogeneous media, the mixed-domain operators are equivalent to the pseudo spectral operators. there is an actual trade-off in the selection of ranks: larger values lead to a more accurate wave representation but require a longer computational time. in fomel et al. [2013], the authors select the ranks based on an estimate of the approximation accuracy of 10−4. for the isotropic heterogeneous media, ranks between 2 and 4 can lead to satisfying accuracy [fang et al. 2014]. in this paper, we recommend 𝑀, 𝑁 = 4. if we utilize equation (24), the approximate form of could be given by (25) 2.4 lowrank finite-difference schemes on staggered grids for the elastic case in this section, we will further decompose the recursive time integration operators and generate optimal coefficients for elastic wave extrapolation. the submatrix 𝑊𝑗� (𝐱�,𝐤) is only related to the wavenumber and exhibits the behavior of oscillation [engquist and ying, 2009]. thus, we could adopt fourier sine series to represent this 𝜕�𝑣� 𝜕𝑥 ̟� 𝑊𝑗� (𝐱, 𝐤) ≈� � 𝑊𝑗� (𝐱, 𝐤�)𝑎��𝑊𝑗� (𝐱�, 𝐤),� �=1 � �=1 𝜕�𝑣� 𝜕𝑥 ̟� 𝜕�𝑣�𝜕𝑥�̟ ≈� � 𝑊𝑗� (𝑥, 𝑘�)𝑎��𝐹‐1�𝑖𝑒𝑖𝑘�𝛥�/2𝑊𝑗� (𝐱�, 𝐤)𝐹[𝜈�(𝐱,𝑡)]�.� �=1 � �=1 qizhen du et al. 8 figure 1. a schematic of a 2d staggered grid scheme. submatrix and obtain the staggered-grid lowrank finite-difference. we further decompose the third term on the right-hand side of equation (25) into: (26) where 𝐵(𝑙,𝐤) is an 𝐿 by 𝑁� submatrix and determined as sin( 𝑘𝑗𝛥𝑥𝑗), l is the half size of the staggered grid stencil, 𝐶𝑗� (𝐱�,𝑙) is an 𝑁 by 𝐿 submatrix calculated as the product of 𝑊𝑗� (𝐱�,𝐤) and the pseudo-inverse of 𝐵(𝑙,𝐤). we introduce the auxiliary matrix (27) after that, we could overwrite the spatial derivative of 𝜈� as (28) equation (28) is analogous to the one expressed by the standard staggered grid scheme. it is a striking feature of the method that the deduction of the coefficients containing lowrank decomposition and the coefficients depend upon the media. this method could be entitled as an elastic staggered-grid lowrank fd(esglfd) method. the coefficients for the different media are supposed to be stored before the implementation of elastic wave simulation because of the velocity-dependency. through the same processing, the other spatial derivatives of 𝜈�, containing 𝜕�𝜈� / 𝜕𝑥𝑗, 𝜕�𝜈� / 𝜕𝑥𝑗 , 𝜕�𝜈� / 𝜕𝑥𝑗 , could be deduced as (29) (30) (31) herein, we use a 2d simple homogeneous media to analyze the accuracy of the esglfd method. also, we choose the spatial derivative operator, 𝜕�𝑣� / 𝜕𝑥𝑗 as an example. the spatial derivative operator, 𝜕�𝑣� / 𝜕𝑥𝑗 shown in equation (22), is calculated by the k-space method [tabei et al., 2002] and the esglfd method, respectively. the velocity is given by 𝑓(𝑥,𝑧) = 20𝑥 + 25𝑧 + 1200. the velocity value increases to 3900 m/s (figure 2a). figure 2b, 2c and 2d illustrate the exact matrix with the k-space method, the approximate matrix with the esglfd method and the absolute error between these two matrices, respectively. there is almost no difference between figures 2b and 2c at the macroscopic level. by inspection, the absolute error, shown as figure 2d, is almost constrained on the interval (-0.04, 0.04). the error is tolerant in the processing of elastic wave extrapolation. compared to the existing works, the elastic lowrank finite-difference operators are optimal finite-difference operators and constructed in accordance with the spectral response in certain range of frequency. the elastic recursive time integration operator of the proposed method depends on the parameters of the elastic media and the simulation which can efficiently compensate the temporal accuracy loss. it extends the application of lowrank finite-difference schemes from acoustic case to elastic situation, which provides a more accurate approximation in both temporal and spatial domains. 𝜕�𝑣�𝜕𝑥�̟ ≈ � 𝐺𝑗� (𝐱, 𝑙)𝐹‐1[𝑖�𝑒𝑖𝑘�𝛥��/2𝐁(𝑙, 𝐤)�𝐹(𝜈�)]1 2 ≈� 𝐺𝑗� (𝐱, 𝑙)𝐹‐1[𝑖(𝑒𝑖𝑙𝑘�𝛥�� ‒ 𝑒‒𝑖(𝑙‒1)𝑘�𝛥��)𝐹(𝜈�)]. ≈ � 𝐺𝑗�(𝐱, 𝑙)𝐹‐1[𝜈�(𝑥� + 𝑙)‒𝜈�(𝑥� ‒ (𝑙‒1)]1 2 � �=1� �=1 � �=1 𝑊𝑗� (𝐱�, 𝐤) ≈ � 𝐶𝑗� (𝐱�, 𝑙)𝐵 (𝑙, 𝐤),� �=1 2𝑙‒1 ₂ 𝐺𝑗� (𝐱, 𝑙) =� � 𝑊𝑗� (𝐱, 𝐤�)𝑎��𝐶𝑗� (𝐱�, 𝑙).� �=1 � �=1 𝜕�𝑣�𝜕𝑥𝑗 ≈ � 𝐺𝑗�(𝐱, 𝑙)𝐹‐1[𝜈�(𝑥� + 𝑙 ‒ 1)‒𝜈�(𝑥� ‒ 𝑙)]1 2 � �=1 𝜕�𝑣�𝜕𝑥𝑗 ≈ � 𝐺𝑗�(𝐱, 𝑙)𝐹‐1[𝜈�(𝑥� + 𝑙)‒𝜈�(𝑥� ‒ 𝑙 + 1)]1 2 � �=1 𝜕�𝑣�𝜕𝑥𝑗 ≈ � 𝐺𝑗�(𝐱, 𝑙)𝐹‐1[𝜈�(𝑥� + 𝑙 ‒ 1)‒𝜈�(𝑥� ‒ 𝑙)] .1 2 � �=1 9 lowrank elastic wave extrapolation 2.5 dispersion analysis we analyze the accuracy of the esglfd method. according to the plane wave theory, we could acquire a set of ratios containing the ratio of the p-wave numerical velocity to the real p-wave velocity, 𝜉�, and that of the s-wave, 𝜉�. these ratios indicate the errors: (32) and , (33) where, 𝜂�� = 𝐺𝑥� (𝐱, 𝑙)sin((2𝑙‒1)𝑘�𝛥ℎ / 2), 𝜂�� = 𝐺𝑧� (𝐱, 𝑙)sin((2𝑙‒1)𝑘�𝛥ℎ / 2), � � �=1 𝜉𝑝 = 2arcsin (�(𝜂𝑝𝑥 + 𝜂𝑝𝑧)𝜈�𝛥𝑡²/4)𝑘𝛥𝑡𝜈�² ² ² 𝜉𝑠 = 2arcsin (�(𝜂𝑠𝑥 + 𝜂𝑠𝑧)𝜈�𝛥𝑡²/4)𝑘𝛥𝑡𝜈�² ² ² � � �=1 qizhen du et al. 10 figure 2. (a) a linearly increasing p-wave velocity model, (b) an exact matrix for 𝜕� / 𝜕�𝑗 with the k-space method, (c) an approximation matrix for 𝜕� / 𝜕�𝑗 with the esglfd method, (d) the absolute error between the exact matrix and the approximate matrix. a) b) c) d) 𝜂�� = 𝐺𝑥� (𝐱, 𝑙)sin((2𝑙‒1)𝑘�𝛥ℎ / 2), and 𝜂�� = 𝐺𝑧� (𝐱, 𝑙)sin((2𝑙‒1)𝑘�𝛥ℎ / 2). when 𝜉 = 1, no dispersion exists. if 𝜉 keeps a long way off 1, serious dispersion will occur. also, it is clear that the error depends on the propagation angle. suppose that the propagation angle is 𝜃 in a 2d case, let 𝑘� = 𝑘 cos(𝜃) and 𝑘� = 𝑘 sin(𝜃). due to the symmetry of equations (32) and (33), we only investigate the properties of the error when 𝜃∊[0, 𝜋/4]. thereafter, we will compare the difference between the staggered grid finitedifference (sgfd) scheme and the esglfd scheme in terms of difference orders, velocity, time intervals and propagation angles. figure 3 depicts the relation between 𝜉 and 𝑘ℎ for different orders of sgfd and esglfd methods. clearly, when 𝐿 increases, the dispersion error decreases for both sgfd and esglfd methods. however, the proposed esglfd method covers a wider wavenumber range than the sgfd method. the dispersion curves are given in figure 4. note that the errors change significantly with the velocities when employing the sgfd method. by comparison, the errors equal or approximately equal the limit value on the interval [0,2.2] when the esglfd method is used. the esglfd method keeps highly accurate on the wavenumber range mentioned above. from figure 5, we can estimate the impact of time intervals on the dispersion. the dispersion error by the sgfd method becomes larger when the time interval increases. however, as shown in figure 5b, all the dispersion errors are confined in a small neighborhood of 1 with 𝑘ℎ∊[0,2.0]. it proves that the esglfd method is effective for large time-step simulation. as mentioned above, we only investigate the dispersion relation with 𝜃∊[0, 𝜋/4] and select the representative angles including 𝜋/16, 𝜋/8, 𝜋/6 and 𝜋/4 to illustrate the variation of the dispersion errors. from figures 6a and 6b we can see that the trend of the dispersion curves by the two methods for the designed propagation angle is comparable. it is the wider coverage of wavenumber that gives the advantage of the esglfd method over the sgfd method. after the analysis of these figures, we can find that the esglfd method exhibits higher accuracy. especially, the esglfd method is able to yield the promise results when the time intervals become larger. � � �=1 � � �=1 11 lowrank elastic wave extrapolation figure 3. 2d dispersion curves of the fd methods (a) and the esglfd methods (b) for different orders, fourth-order (𝐿 = 2, blue), six-order (𝐿 = 3, red), eight-order (𝐿 = 4, purple), sixteen-order (𝐿 = 8, green), 𝜈 = 3000m/s, d𝑡 = 1ms and 𝛳 = 𝜋/8. a) b) qizhen du et al. 12 figure 4. 2d dispersion curves of the fd methods (a) and the esglfd methods (b) for different velocities, 𝜈 = 2500 m/s (blue), 𝜈 = 3500 m/s (red), 𝜈 = 4500m/s (purple), 𝜈 = 5000m/s (green), 𝐿 = 6, d𝑡 = 1ms and 𝛳 = 𝜋/8. a) b) figure 5. 2d dispersion curves of the fd methods (a) and the esglfd methods (b) for different time intervals, d𝑡 = 0.5ms (red), d𝑡 = 1.0ms (blue), d𝑡 = 1.5ms (purple), d𝑡 = 2.0ms (green), 𝜈 = 3000m/s, 𝐿 = 6 and 𝛳 = 𝜋/8. a) b) figure 6. 2d dispersion curves of the fd methods (a) and the esglfd methods (b) for different propagation angles, 𝛳 = 𝜋/16 (blue),𝛳 = 𝜋/8 (red), 𝛳 = 𝜋/6 (purple), 4 (green), d𝑡 = 1ms, 𝜈 = 3000m/s and 𝐿 = 6. a) b) 3. numerical examples 3.1 two-layers model to investigate the validity of the esglfd method, we implement an experiment on a two-layer model (figure 7). size of the model is defined as , with horizontal and vertical sampling intervals equal 15m. the time sampling interval is 2.0ms. the contrast of the model appears at the depth of 4500m. the density, pand s-wave velocities of the upper layer are set as 2.0g/cm3, 1700m/s and 1000m/s, while the parameters of the lower layer are 2.2g/cm3, 2550m/ s and 1500m/ s, respectively. a ricker wavelet is utilized as an explosive source at the location (4500m, 3975m), with 35 hz the dominant frequency. the x-component snapshots for p-and s-waves at 0.8 s are shown in figure 8. visible and apparent dispersion occurs in figures 8a and 8b. when time and space intervals become larger, it is obvious that the waveform simulated by the sgfd method is distorted. the esglfd method performs well and yields nearly dispersion-free results. figure 9 displays the vertical slices extracted from the x-component snapshots shown in figure 8. notice that there are three waveforms in figures 9a and 9b and two waveforms in figures 9c and 9d. it is in accordance with the fact that the reflected p-wave, the transmission p-wave, and the direct p-wave exist in the snapshots, as is shown in figures 8a and 8b, while the reflected converted s-wave and the transmission converted s-wave exist in the snapshots of figures 8c and 8d. meanwhile, the results also demonstrate that the wave modes, which occur in the elastic media, could be depicted by the proposed esglfd method effectively. the waveforms obtained by the sgfd method do not preserve the wavelet shapes well. vibration curves occur in the neighborhood of the ricker-shaped waveforms. comparatively, the waveforms generated by the esglfd method do comply with the ricker-wavelet forms. the results prove that the esglfd method is accurate and effective for elastic wave simulation when the time interval becomes larger. 3.2 the marmousi2 model to validate our proposed method at the complex model, we select the modified elastic marmousi2 model [martin et al., 2006], and set the grid size of the model as 1700 × 350. the grid interval is redefined as 10 m. we divide the resampled p-wave velocity by �3 to calculate the s-wave velocity. for the elastic wave simulation and the subsequent rtm, we smooth the velocity model. and figure 10a and 10b describe the original and smoothed p-wave velocities, respectively. in figure 10b, subset of the elastic marmousi2 model denoted by the dashed rectangle is used for elastic wave simulation in this experiment. the four corners are all located at the border of the resampled marmousi2 model, the 13 lowrank elastic wave extrapolation figure 7. the geometry and the elastic parameters of a two-layer model. the pentagram denotes the excitation position for elastic wave simulation. qizhen du et al. 14 figure 8. elastic wavefield snapshots at t=0.8s of a two-layer model by using the 4th-order sgfd method and the 4th-order esglfd method: (a) the x-component snapshot of the p-wave by the sgfd method; (b) the x-component snapshot of the p-wave by the esglfd method; (c) the x-component snapshot of the s-wave by the sgfd method; (d) the x-component snapshot of the s-wave by the esglfd method. head waves are highlighted by the red arrows. a) b) c) d) figure 9. vertical slices extracted from the x-component snapshots (a) related to p-wave by the sgfd method, (b) related to p-wave by the esglfd method, (c) related to s-wave by the sgfd method, and (d) related to s-wave by the esglf d method at the distance of 3.0 km. a) b) c) d) exact coordinates of which are (6.0, 0) km, (13.0, 0) km, (6.0, 3.5) km, and (13.0, 3.5) km. both the horizontal and the vertical space increments are 10 m. the partial model is determined with a grid size of 700 × 350, and the ricker wavelet is used as the explosive source term in this case, with the dominant frequency of 25 hz. the time sampling increment is 1 ms. the pml boundary is implemented on the four sides of the model and has a thickness of 300 m. the snapshots corresponding to pand s-wave vector components at 0.8 s (as shown in figure 11) are generated by the sixth-order esglfd method. the distinction between pand s-waves can be clearly delineated with these snapshots. we could also see that the information of the s-wave is a very valuable part to depict the subtle structure of the complex velocity-variation model. figure 12 shows two representative elastic common-source gathers corresponding to horizontal and vertical components, respectively. the direct waves have been removed and we can see that the reflection waves are without any visible numerical dispersion. the experiment verifies that the esglfd method can handle the complex media and provide the accurate wavefield for the subsequent rtm. 3.3 elastic reverse time migration the 2d marmousi2 model, as is shown in figure 10, is used in this section for further rtm, in which the esglfd method serves as the forward and backward wavefield extrapolators. the geometry of the survey is from 15 lowrank elastic wave extrapolation a) b) figure 10. the resampled (a) and smoothed (b) marmousi2 model corresponding to p-wave velocity for elastic wave simulation and reverse time migration, respectively. the subset denoted by the dashed rectangle is used for elastic wave simulation. 0 km to 17.0 km, in which a total number of the split-spread source is 340 with 700 receivers per shot. for each shot, one-half of the receivers are located on either side of the shot. all the sources and receivers are situated on the surface of the model, with the source spacing 50 m and receiver spacing 10 m. similarly, to the elastic wave simulation, the ricker wavelet with the dominant frequency of 25 hz is utilized. total record time is 6 s, with time sample increment of 1 ms. horizontal and vertical sampling intervals are both set at 10 m. additionally, pml with the thickness of 300 m on each side of the model is implemented. with regards to image condition, we utilize the scalar imaging conditions [du et al., 2017], which obtain the scalar images using the normalized cross-correlation qizhen du et al. 16 figure 11. snapshots corresponding to the horizontal component of the p-wave (a), the vertical component of the p-wave (b), the horizontal component of the s-wave (c) and the vertical component of the s-wave (d) at 0.8 s with the 6th-order esglfd method. the model used for elastic wave simulation is the subset, denoted by the dashed rectangle, of the resampled elastic marmousi2, shown in figure 9. a) b) c) d) figure 12. elastic common-source gathers corresponding to the horizontal component (a) and the vertical component (b). a) b) between the source-side wavefields and the receiver-side wavefields. this method eliminates the artifacts resulting from the crosstalk between different types of wave modes and avoids the polarity reversal for ps images, which may lead to destructive interferences in the processing of the adjacent source stack. the final migration images, including the pp image component and ps component, are described in figure 13. it can be observed that not only the stratoid and the flat structure but also the anticline and the faults appear clearly in all of these image profiles. generally, these comparative image results shown in figure 13 verify that the proposed method is sufficient to the function as an extrapolator to reconstruct the forward and the backward wavefields for elastic rtm. 4. conclusions in this work, a method of staggered-grid lowrank finite-difference is proposed to obtain a better representation of the elastic wave extrapolation. numerical experiments on a two-layers model and the complex marmousi2 model validate the precision and efficiency of the method. the following conclusions are achieved. 1. the basic assumption of the method is elasticity and linearity, which is composed of the dynamic and kinetic information of the primary and shear waves. 2. the method is derived on the basis of decomposition and plane wave theory, oriented to decompose the elastic wavefield to the pure-mode wavefield. the wavefield decomposition is the prerequisite of this method. 17 lowrank elastic wave extrapolation a) b) figure 13. the stacked pp image component (a), the stacked ps image component (b). 3. the elastic recursive time integration operator of the proposed method depends on the parameters of the elastic media and the simulation which can efficiently compensate the temporal accuracy loss. 4. the elastic lowrank finite-difference operators are optimal finite-difference operators and constructed in accordance with the spectral response in certain range of frequency. and the numerical tests indicate that the operators are capable of handling media with large velocity gradient. 5. the proposed method still follows the finite-difference scheme. in implementation, its efficiency will be a little lower than that of the regular staggered-grid finite-difference. that is because more memory space is needed to store the space-variant finite-difference coefficient and also more memory access will be introduced. acknowledgements we appreciate the support of the national science foundation of china (41930429, 41774139) and the national key research & development program of china 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(1986). p-sv wave propagation in heterogeneous media: velocity-stress finite-difference method, geophysics 51(4), 889-901. voynov o.y., v.i. golubev, m.s. zhdanov, i.b. petrov (2018). migration of elastic wavefield using adjoint operator and 19 lowrank elastic wave extrapolation born approximation. innovations in wave modelling and decision making, sist series, volume 90, springer switzerland, chapter 8, p. 219-240. wang w., and g.a. mcmechan (2015). vector-based elastic reverse-time migration, geophysics 80, s245-s258. wards b., g. margrave, m. lamoureux (2008). phase-shift time-stepping for reverse-time migration. 78thseg meeting, las vegas, usa, expanded abstracts, 2262-2266. yan j., p. sava (2008). isotropic angle-domain elastic reverse-time migration, geophysics 73(6), s229-s239. yan j., p. sava (2009). elastic wave-mode separation for vti media, geophysics 74(5), wb19-wb32. yan r., x.b. xie (2012). an angle-domain imaging condition for elastic reverse time migration and its application to angle gather extraction, geophysics 77(5), s105-s115. yuan s., s. wang, w. sun, l. miao, z. li (2014). perfectly matched layer on curvilinear grid for the second-order seismic acoustic wave equation, explor. geophys. 45, 94-104. zhang q., g.a. mcmechan (2010). 2d and 3d elastic wavefield vector decomposition in the wavenumber domain for vti median, geophysics 75(3), d13-d26. zhang y., g. zhang (2009). one-step extrapolation method for reverse time migration. geophysics 74(4), a29-a33. *corresponding author: jing ba, school of earth sciences and engineering, hohai university, nanjing, jiangsu, china, 211100, e-mail: jba@hhu.edu.cn © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved qizhen du et al. 20 geo 945 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 2 nowcasting, forecasting and warning for ionospheric propagation: supporting databases richard stamper (1), jean lilensten (2) and norbert jakowski (3) (1) rutherford appleton laboratory, chilton, didcot, oxon, u.k. (2) laboratoire de planétologie de grenoble, bâtiment d de physique, grenoble, france (3) deutsches zentrum für luft und raumfahrt (dlr), institut für kommunikation und navigation (ikn), neustrelitz, germany the use of data is essential in the context of nowcasting, forecasting and warning of ionospheric propagation conditions, with roles to play in the development, evaluation and operation of models and services. descriptions are given of three databases that have been established in the course of the cost 271 action: a database of prompt ionospheric soundings, an extension to a database generated by the eiscat incoherent scatter radars, and a database intended to facilitate evaluation of tec estimation methods. each database includes some background information, a description of the contents and interface, and instructions as to how to gain access to it. 2.1. introduction the memorandum of understanding of the cost 271 action and the terms of reference of working group 1 make clear the importance of data for the work of the action in general and for the specific focus of the work package on nowcasting, forecasting and warning. in particular, there are commitments to collect additional and new ionospheric and plasmaspheric data for now-casting and forecasting purposes, and to establish a space-weather database consisting of both past and new measurements. such commitments are clearly proper when one considers the roles that observational data play in the context of the investigations undertaken during the action into the propagation environment and communication systems. three key roles can be identified: – when developing a model, data provide the framework in which the model is specified and parametrised. – once a model has been specified it needs thorough validation against the full range of conditions it might encounter. – an operational nowcasting or forecasting service will inevitably require data describing the past or current conditions in order to generate its outputs. data have always been used in these ways but the importance of databases is that they simplify access to data for which access would otherwise be difficult. this could be because the data are originally dispersed across multiple sites or because they are in multiple or unfamiliar formats. if anything, these problems have only been multiplied by the great diversity of data sources now accessible on the internet. this paper presents information on three databases developed as part of the cost 271 action: 946 richard stamper, jean lilensten and norbert jakowski – prompt ionospheric soundings at ral, chilton; – eiscat database extension at cdpp, grenoble; – tec evaluation database at dlr, neustrelitz. each of these databases makes access to a wide body of data available via a single source, and presents that data in standardised and readily comprehensible formats. 2.2. prompt ionospheric soundings database 2.2.1. background and motivation the original rationale for compiling the database was the need to understand better how the regional and global ionosphere responds to the impact of space weather events. ionospheric databases have hitherto concentrated on scaled parameters, but to advance our understanding of ionospheric propagation effects more detail is needed. the new database accordingly stores full ionograms and layer traces. this enables the construction of bottom-side ionospheric profiles, and also provides a large of body of data capable of supporting future developments in nowcasting and forecasting of propagation conditions. the database is located at the rutherford appleton laboratory (ral), where it has been developed as an extension of the existing database system of theworld data centre that archives vertical soundings from the ionosondes run by ral at chilton in the u.k. and stanley in the falkland islands. the work to extend this system began in april 2002, funded by a grant from the u.k. radiocommunications agency, and it now accepts prompt ionospheric vertical soundings from across europe. note that although the database is designed to accept and make available data in near-real-time, the accumulated historical data are still valuable. any operational nowcasting or forecasting system will necessarily need to use prompt data as input. it is therefore of little practical use to develop such systems using only high quality, manually scaled ionospheric soundings, since such scientific quality data is rarely available in real-time. it is therefore essential for such tools to use a representative set of historical real-time data when being developed and tested, that is, data that were at one time prompt and have not been adjusted manually since. the ral database provides just such a resource. 2.2.2. database content the database takes data from instruments across europe, currently all dps or dgs sounders manufactured by the university of massachusetts, lowell, center for atmospheric research (umlcar), and known generically as ‘digisondes’. the full list of participating instruments and the institutions that operate them at the time of writing is – chilton, u.k. (ral); – tromsø, norway (university of tromsø, qinetiq); – juliusruh, germany (iap kühlungsborn); – athens, greece (nao); – rome, italy (ingv); – el arenosillo, spain (inta); – pruhonice, czech republic (academy of sciences); – ebro, spain (observatorio del ebro). the standard output files from these sounders are sent to or retrieved by the ral computers within a few minutes of the soundings being carried out. the files are catalogued and archived automatically on arrival. the umlcar digisondes are supplied with a sophisticated software package called artist that carries out automatic scaling of the ionograms. the files delivered to the database therefore contain not 947 nowcasting, forecasting and warning for ionospheric propagation: supporting databases only the ionogram, but also auto-scaled traces and autoscaled parameters. a standard subset of the autoscaled parameters are extracted and stored separately from the raw ionogram files for ease of access. because the initial set of instruments supplying data have all been digisondes, the umlcar sao file format has been adopted as the standard archive format for storing the parameters and traces from each ionogram. the format is sufficiently generic that it should be possible to re-encode the native output from any other type of sounder into the sao format for archiving. 2.2.3. user interface and data products there is public access to the database through the web link http://www.wdc.rl.ac.uk/cgi-bin/digisondes/cost_database.pl/. users are required to register so that they have a defined user area for storing temporary files, but the registration procedure is automatic. the front page is shown in fig. 2.1. it consists of a form which allows the user to select data by station (via a selection list or by clicking on the map of europe) and by arbitrary time intervals; one can choose to download the raw data files and ionograms, or one can display or download a number of other data products. for each of the contributing instruments the database holds some background information. this can be retrieved by choosing the station details option from the web form on the front page. an example of the output of such a request is shown in fig. 2.2. for each instrument there is displayed its ursi code, its physical location, details of the principal contact person and infig. 2.1. the front page of the prompt ionospheric soundings database, showing the options for selecting data by station and date, and for selecting the type and mode of delivery of the data product. fig. 2.2. the details of stations available from ionospheric soundings database. 2.22.1 948 richard stamper, jean lilensten and norbert jakowski fig. 2.3. displaying availability of data files in the prompt ionospheric soundings database. fig. 2.4. listing auto-scaled parameters in the prompt ionospheric soundings database. formation about the institution responsible for running the instrument. wherever possible, links are given to appropriate e-mail addresses and web sites. for example, in fig. 2.2 the section for the sounder at athens has links to the web page for the instrument (the sounder name ‘athens’ is the link), the e-mail address and personal web page of the principal contact, and the web site of the hosting institution. the other selection options allow data to be retrieved in a variety of forms: tables of data availability, a table of auto-scaled parameters, line plots of parameters over time, true-height bottom-side profiles calculated using polan, and zip files of raw data files. for detailed data analysis the file download option enables users to retrieve the raw data files. suitable third-party software such as sao-explorer from umlcar is necessary to process these files. before downloading data it is best for a user to check what files would be retrieved; this is done with the data availability option, for which an example query result is shown in fig. 2.3. to view all the auto-scaled parameters the user should select the parameter data option; this generates a table listing 16 parameters standardly scaled by the artist auto-scaling software on the digisonde sounders. figure 2.4 illustrates the result of such an operation, where the table is sorted by station in this instance; it is equally possible to sort the results by time, where all times are ut. alternatively, one can generate a simple line plot of the fof2, foe and f min auto-scaled parameters (see fig. 2.5), useful for getting a quick view of data availability and quality. the time interval covered by such a plot is set by the user when specifying the query, which makes it possible to focus on the detail of a particular period of interest or, alternatively, to get an overview of a long interval. finally, it is possible to run the polan ionogram inversion algorithm on any auto-scaled traces. for each sounding the result is a list of (frequency, height) pairs, and a short extract of one such profile is shown in fig. 2.6. 2.3 2.4 949 nowcasting, forecasting and warning for ionospheric propagation: supporting databases 2.2.4. future developments there is work continuing to extend the number and geographic spread of contributing instruments. the user interface to the database will also continue to be developed to improve its flexibility and ease of use, with three additions currently planned: 1) some elements of user control will be added to the plotting interface to permit users to select the set of parameters to plot, rather than being given a pre-defined set. 2) access to the wdc holdings of definitive manually checked scaled parameters will be provided through the same interface as the auto-scaled prompt parameters currently available. 3) a facility for setting up automatic transfers of data products will be added. by enabling users to specify a set of data products that should be transferred regularly by e-mail or ftp, this will support the use of the database by operational services that need regular prompt delivery of data inputs. 2.3. eiscat database extension 2.3.1. background and motivation in 2002, the international telecommunication union radiocommunication group (itu-r) issued a recommendation for a proposed data bank for trans-ionospheric model testing. the intention of the recommendation was to make the eiscat radar data available to the wider community in a form that would allow testing of propagation models and, in particular, the tec, hmf2, nmf2, and fof2 parameters had to be easily readable and freely down-loadable. actions to meet this recommendation fig. 2.5. plot of ionospheric parameters over along time interval from the prompt ionospheric soundingsdatabase. fig. 2.6. true-height analysis using polan from the prompt ionospheric soundings database. 2.5 2.6 950 richard stamper, jean lilensten and norbert jakowski have been undertaken by cdpp (centre des données de la physique des plasmas), a data archiving and distribution centre for space plasma physics, implemented by the french national centre for scientific research (cnrs) and the french space agency (cnes), which provides the french eiscat database. 2.3.2. incoherent scatter radars: eiscat incoherent scatter (is) radars are a powerful ground-based tool for the study of the earth’s ionosphere and its interactions with the upper atmosphere, the magnetosphere and the interplanetary medium. the ‘incoherent’ scatter echo is the result of the scattering of electromagnetic energy radiated from the radar by electrons in the ionospheric plasma which are themselves controlled by the much slower and more massive positive ions. most of the returned power is due to scattering from electron density fluctuations caused by the presence of these ions. the frequency spectrum of the received signal therefore provides information about the temperature, composition and velocity of the ion population. the incoherent scatter echo has a characteristic double-peaked ion-line spectrum which also conveys information about the temperature of the electrons themselves. besides the main ‘ion-line’, the incoherent scatter spectrum also contains other, usually weaker, components including two ‘plasma-’ or ‘electronlines’ representing scattering processes where the electrons act as if the ions were absent. the eiscat (european incoherent scatter) scientific association is an international research organisation operating three geophysical research incoherent scatter radar systems, an ionospheric heater and a research ionosonde (known as the dynasonde) in northern scandinavia. two of the radar transmitters are located close to the city of tromsø in norway, while the third, the eiscat svalbard radar (esr), is located near longyearbyen on the svalbard archipelago. the work described here concentrated on data from the auroral region radar over tromsø; this was obtained using one of the standard operating modes of the eiscat radars (rishbeth and van eyken, 1993). the analysis of the scattered signal allows one to deduce the ion and electron temperatures, the ion velocity parallel to the magnetic field, and the electron density. finally, one of the unique features of the eiscat radar is that it has three widely separated receiving stations, thus enabling the observation of the northward and eastward electric field components. 2.3.3. cdpp eiscat database the cdpp hosts the french eiscat database which archives all the ionospheric data from the radars that have been analysed in france since the radars first became operational in 1981, and the database can be accessed at http://www-eiscat.ujf-grenoble.fr/. the data can be retrieved either in graphical format as gif plots or as simple ascii text format tables. the data are processed with either a 5 min or 1 min integration time. the extension to the database has made use of data taken by the cp1 (common programme 1) experiments, in which the tromsø antenna is kept fixed parallel to the magnetic field. the remote sites make continuous measurements at the f-region position (typically at 278.6 km altitude). for the cp1 data the eiscat signal is made up of three parts. the power received on the antenna gives a first estimate of the electron density with a spatial resolution of 3 km up to about 420 km. this measurement does not give the incoherent scatter spectrum and therefore can only give access to the electron density. until the year 2000, eiscat used a single pulse in the f-region, and multipulses or alternating codes in the e-region. the multipulse coding or alternating code have short time and space resolution (about 5 km) and make measure951 nowcasting, forecasting and warning for ionospheric propagation: supporting databases ments of all parameters (electron density ne, electron and ion temperatures te and ti, ion velocity) typically in the eand lower f-regions (altitudes from 90 to 250 km). the single pulse coding is a longer pulse (350 µs) giving a coarser spatial resolution of about 30 km at higher altitudes (from about 140 to 600 km). it also allows retrieval of the four main parameters. 2.3.4. extending the database to act on the itu-r recommendation, the grenoble eiscat database was updated with the integrated total electron content (itec) over the altitude range from 90 km up to about 600 km, and the f2-region parameters (hmf2, nmf2, fof2). since the best electron density resolution is obtained with the power profile, it was necessary to correct its estimate with the electron and ion temperatures measured by the other pulse schemes. this has been done by fitting the multipulse and single pulse temperature data with a spline function. if ne(z) is the true electron density at altitude z, and npp(z) is the power profile estimate, we have / n z t t n z 1 1 2 e d d e i pp 2 2 # = + + +a a ^ ^ ^ ^ h h h h (2.1) where αd is the debye length. finally, in order to get the tec estimate with data measured between 450 and 600 km, we adjusted the single pulse measurement and the corrected power profile with a cubic spline interpolation. this analysis scheme allowed processing of the whole database over more than 1 solar cycle. fig. 2.7. start page of the eiscat database. fig. 2.8. selecting data from the eiscat database. 2.7 2.8 952 richard stamper, jean lilensten and norbert jakowski fig. 2.9. plotting data from the eiscat database. fig. 2.10. listing data from the eiscat database. 2.3.5. user interface and database outputs the data is freely available via the web site http://www-eiscat.ujf-grenoble.fr/. upon entering the database one is presented with the option to select quick-look plots or ascii data listings by year (see fig. 2.7), after which one is taken through to a page (fig. 2.8) containing tables from which individual plots or data listings can be selected. the form of the plot and listing outputs are shown in figs. 2.9 and 2.10 respectively. 2.4. tec evaluation database (teceda) 2.4.1. background and motivation during the cost 271 several projects dealt with estimation of tec using a variety of different techniques. these activities can also be put in the broader context of the interest of the ionosphere working group of the international gps service (igs) in evaluations of global tec maps, with plans to release such evaluations as an official product of the igs in the near future. the cost 271 countries are well placed to contribute to tec validation; they operate a number of ionospheric sounding facilities, are active in ionospheric modelling and have started to develop multi-sensor assimilation techniques for comparative studies. it was therefore decided to establish a database of independent measurements of ionospheric properties that could be used as the basis for systematic comparative studies between different data sets to derive conclusions about the achieved accuracy of tec estimates and the potential of modeling and assimilation work. it was further decided to concentrate on a specific period, chosen to coincide with the hirac high-resolution gps campaign which ran between 23rd and 29th april 2001. 2.9 2.10 953 nowcasting, forecasting and warning for ionospheric propagation: supporting databases 2.4.2. database contents the database is organised to accommodate data of various types and origins in a single scheme, so that data of interest can be easily located and retrieved. the data holdings are presented as a directory hierarchy on an ftp site. the top level directory contains subdirectories named after the location to which the data relates: examples include ‘athens’ and ‘chilton’ referring to the location of ionosondes, ‘u.k.-tomo’ for a u.k. tomographic receiver chain, and ‘iro-champ’ for radio occultation data taken by the champ spacecraft. within each of these location directories are three subdirectories: <station>/tec /pro /rawdata the purpose of these subdirectories is to hold files containing, respectively, derived tec estimates, electron density profiles, and raw data. a file naming scheme applies to the files in the tec and profile subdirectories; each file has a name of the form bbb-ttt+sssss-ooooo_yyyy_ddd where bbb is a 3-character code indicating the base type, namely ‘tec’ or ‘pro’ fig. 2.11. vertical electron density profile from the champ space craft, extracted from the teceda database. fig. 2.12. tec time series from nims satellite transmissions, extracted from the teceda database. 2.122.11 954 richard stamper, jean lilensten and norbert jakowski ttt is a 3-character code indicating the sub-type of the base type to give the derivation of the data, drawn from the set ass assimilation eis eiscat radar data ivs ionospheric vertical sounding mod model output tom tomography sssss is a 5-character code to represent the receiving station ooooo is a 5-character code to represent the organisation or facility which has created the data yyyy is a 4-digit year ddd is a 3-digit day-of-year (january 1st = 001). an example of such a name is pro-mod+at138-dlrnz_2001_101, which would contain ionospheric profiles (pro) produced by a model (mod) for the location of the athens ionosonde (at138), generated by the dlr at neustrelitz (dlrnz) for day 101 of 2001 (11 april 2001). wherever possible these tec and profile files are presented in a straightforward human-readable and self-describing ascii format. the hash character (#) is used to indicate comment lines which are used to create a file header which includes the name of at least one contact person. the header is terminated by a line reading ‘# end’. the data follows this header and is typically arranged in columnar format. two examples of parts of such files are presented in figs. 2.11 and 2.12, showing respectively a vertical electron density profile and a reconstructed tec series. as stated previously, one of the motivations for creating the database was the hirac campaign. consequently, the data is primarily concentrated on the interval 23-29 april 2001, with time series data at 10 min resolution wherever possible. data from other periods can usually be obtained by contacting the data provider, as detailed in the data file headers. where data are held in some non-ascii or non-tabular format, the data providers should have supplemented the data files with a text file giving sufficient supporting information to make the data files usable by the general user. 2.4.3. database access the data are available from an ftp site hosted by the dlr, neustrelitz, and access is restricted by username and password. anyone wishing to have access to the database should contact dr. norbert jakowski (norbert.jakowski@dlr.de) and details of the host computer and account details will be provided. there is also a general invitation to anyone with relevant data, particularly for the interval of the hirac campaign, to deposit data in the database. in particular, for access to more ionospheric radio occultation data obtained on-board the champ spacecraft (jakowski et al., 2002), one should contact the information and science data center (isdc) potsdam (http://isdc.gfc-potsdam.de/champ/welcome.html) or dlr directly (norbert.jakowski@dlr.de). dlr routinely feeds the isdc with ionospheric radio occultation data products. 2.5. conclusions during the course of the cost 271 action significant new data resources have been established, and the three principal examples have been described. the contents, intended user communities, and key features of these three database are different but complementary: – the database of prompt ionospheric soundings at ral is intended to draw together ionospheric vertical sounding data that would otherwise be dispersed across multiple data providers. a key feature of this database is that the data are ingested promptly so that the database can be used as a resource for the development and operation of nowcasting and forecasting tools. 955 nowcasting, forecasting and warning for ionospheric propagation: supporting databases – the eiscat database extension at cdpp is intended to broaden the community of users that can make use of the powerful research tool of the eiscat radars, by presenting the eiscat data in terms of parameters commonly used by the radiocommunications community. – the teceda database is intended to gather together a wide variety of related data for a specific time period. this facilitates detailed study and comparisons of the properties and behaviour of the ionosphere and propagation conditions during this interval. all three databases are resources with many actual and potential users, and represent one of the enduring legacies of the cost 271 action. it is hoped that they will be of use to a global community of users, not just those in the european area, and their wide use is encouraged. acknowledgements eiscat is an international association supported by the research councils of finland (sa), france (cnrs), the federal republic of germany (mpg), japan (nipr), norway (nfr), sweden (vr) and the united kingdom (pparc). the prompt ionospheric database at ral was funded by the radiocommunications agency, now part of ofcom. thanks must go to chantal lathuillère, béatrice pibaret and bénédicte lemieux-dudon for their help in implementing the extensions to the grenoble eiscat database. references jakowski, n., a. wehrenpfennig, s. heise, c. reigber and h. luehr (2002): gps radio occultation measurements of the ionosphere on champ: early results, geophys. res. lett., 29 (10), doi:10/1029/2001gl014364. rishbeth, h. and a.p. van eyken (1993): eiscat: early history and the first ten years of operation, j. atmos. terr. phys., 55, 525-542. layout 6 long time-series of chemical and isotopic compositions of vesuvius fumaroles: evidence for deep and shallow processes stefano caliro1,*, giovanni chiodini1, rosario avino1, carmine minopoli1, berardino bocchino1 1 istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5034 abstract long time-series of chemical and isotopic compositions of vesuvius fumaroles were acquired in the framework of the volcanic surveillance in the 1998-2010 period. these allow the identification of processes that occur at shallow levels in the hydrothermal system, and variations that are induced by deep changes in volcanic activity. partial condensation processes of fumarolic water under near-discharge conditions can explain the annual 18o and deuterium variabilities that are observed at vesuvius fumaroles. significant variations in the chemical compositions of fumaroles occurred over the 1999-2002 period, which accompanied the seismic crisis of autumn 1999, when vesuvius was affected by the most energetic earthquakes of its last quiescence period. a continuous increase in the relative concentrations of co2 and he and a general decrease in the ch4 concentrations are interpreted as the consequence of an increment in the relative amount of magmatic fluids in the hydrothermal system. gas equilibria support this hypothesis, showing a pco2 peak that culminated in 2002, increasing from values of ~40 bar in 1998 to ~55-60 bar in 20012002. we propose that the seismic crisis of 1999 marked the arrival of the magmatic fluids into the hydrothermal system, which caused the observed geochemical variations that started in 1999 and culminated in 2002. 1. introduction the present volcanic–hydrothermal activity at vesuvius volcano is relatively low level. the main evidence of this activity is: (a) widespread fumarolic emissions that are accompanied by diffuse soil co2 degassing in the crater area [chiodini et al. 2001, frondini et al. 2004]; (b) co2-rich groundwaters along the southern flank of vesuvius and in the adjacent plain [caliro et al. 1998, federico et al. 2002, caliro et al. 2005]; and (c) seismic activity with epicenters clustered inside the crater [saccorotti et al. 2002, del pezzo et al. 2004]. the most notable seismic activity was registered on october 1999. this crisis included an earthquake of ml 3.6, which was the highest magnitude recorded for at least 25 years, and possibly since the last eruption of vesuvius in 1944 [zollo et al. 2002, del pezzo et al. 2004]. after the october 1999 seismic crisis, seismic activity decreased to low levels, with shallow hypocenters mainly clustered within the volcanic edifice [del pezzo et al. 2004]. the degassing area of the vesuvius crater is characterized by the presence of fumarolic vents that are sited on the crater rim and at the bottom of the crater (figure 1). fumarolic fluids discharged by these fumaroles on the crater rim are of relatively low temperatures (<75 ̊ c) and are mainly composed of atmospheric components. fumaroles from the crater bottom have a composition that shows h2o and co2 as the major components, followed by h2, h2s, n2, ch4, co and he (in order of decreasing content), and discharge temperature of about 95 ˚c, i.e. the condensation temperature of fumarolic fluids at the crater altitude (patm =0.91 bar). the presence of significant ch4 and nh3 contents provides evidence to the origin of these fluids being from a hydrothermal environment [chiodini et al. 2001]. in particular, on the basis of a comprehensive geochemical study of crater fumarolic fluids collected in the period 1998-1999, chiodini et al. [2001] suggested the presence of a high-temperature hydrothermal system that is located below the vesuvius crater. the aim of this study is to investigate the eventual changes that have affected this hydrothermal system over a relatively long period (1998-2010), on the basis of the data acquired in the framework of volcanic surveillance. these data include chemical compositions of the rim fumaroles, chemical and isotopic compositions of the crater-bottom fumaroles, and soil co2 flux data relative to a continuous monitoring station that is located at the bottom of the vesuvius crater (figure 1). we show modifications of the fumarolic fluids due to the occurrence of seasonal and deep processes. in particular, we focus on the variations that accompanied and followed the seismic crisis of 1999, which caused detectable anomalies in the vesuvius groundwaters [federico et al. 2004, madonia et al. 2008]. article history received december 20, 2010; accepted may 3, 2011. subject classification: vesuvius, hydrothermal system, geochemical monitoring, stable isotopes. 137 1.1. volcanological setting the somma-vesuvius volcanic complex lies in the southern sector of the campanian plain, a plio-quaternary structural depression that extends from the western part of the apennine chain to the eastern coast of the tyrrhenian sea. the latter was generated by an important subsidence of mesozoic carbonate rocks, and was subsequently filled by quaternary sedimentary and volcanic deposits that have reached a thickness of about 2,000 m beneath the volcanic edifice [aprile and ortolani 1979]. somma-vesuvius was formed at the intersection of two regional fault systems with nw–se and ne–sw stress directions [ippolito et al. 1973, pescatore and sgrosso 1973]. the volcanic complex consists of the relatively old strato–volcano, monte somma, that underwent a series of caldera collapses that culminated in 18 ka b.p. [cioni et al. 1999, ventura et al. 1999], and a younger intracaldera cone, vesuvius. the somma-vesuvius activity started between 18 ka and 37 ka [principe et al. 1999], and it was characterized by great variability in eruptive styles: relatively frequent, open-conduit activity, that consisted of effusive eruptions that were sometimes accompanied by strombolian events, and that alternated with less frequent plinian and sub-plinian eruptions that typically followed long periods of quiescence [arnò et al. 1987]. the somma-vesuvius volcanic products generally show alkali-potassic compositions and large ranges of silica contents, from nearly saturated rock (leucite-basalts, leucitetrachybasalts, latites, and trachytes) to highly under-saturated rock (leucititic tephrites, leucititic tephrit-phonolites, and leucititic phonolites) [joron et al. 1987]. effusive and strombolian events discharged small batches (<0.001 km3) of primitive magma, whereas plinian eruptions involved larger batches (0.1 to >1.0 km3) of more evolved magma. these magmas differentiated into reservoirs that are located within a thick sequence of chiefly carbonate rock at depths of 3-5 km b.s.l. [barberi and leoni 1980, barberi et al. 1981, santacroce 1983]. after the last plinian eruption, which took place in 1631 [rosi et al. 1993], vesuvius was marked by open-conduit activity, which ended with the eruption of 1944. since then, the volcano has been characterized by fumarolic activity. 1.2. the conceptual geochemical model of the vesuvius hydrothermal system the crater rim fumaroles are characterized by low temperatures (62-76 ˚c) and they discharge a mixture of air (46%-72%), steam (25%-45%) and co2 (0.2%-2%). the air, which in all of the samples is the main component of the fumaroles, is most probably included in fluids in the upper part of the fumarolic conduits that are located in the very caliro et al. 138 figure 1. sketch map of the vesuvius crater, and location of the sampled fumaroles and the soil co2 flux station (flxov4). inset: location of vesuvius volcano within italy. 139 permeable products of the vesuvius cone. because of this heavy air contamination, these fumaroles are not indicative of the deep system and they have not been considered for the elaboration of the conceptual geochemical model. in contrast, the crater bottom fumaroles discharge fluids hat are representative of the deep systems: h2o and co2 are the major components, followed by h2, h2s, n2, ch4, co and he (in order of decreasing content). excluding minor possible contamination during the sampling, air is absent (n2/ar generally >200). the discharge temperatures, of about 95 ̊ c, are close to the saturation temperature of steam for the fumarolic fluids at the crater altitude. fumarolic gases do not show any detectable so2, hcl, and hf, due to the scrubbing of magmatic gases within the hydrothermal system. in the following we report and discuss the conceptual geochemical model of these fumaroles (figure 2) that was elaborated by chiodini et al. [2001], and that is here assumed as the reference model for the interpretation of the chemical and isotopic variations that have been observed over the last 10 years of monitoring. the origin of the main components, h2o and co2, were investigated on the basis of isotopic data. the interpretation of stable isotopes of water was, however, not sufficiently constrained by the data, possibly because of the occurrence of secondary processes. fumarolic water was thus interpreted as either meteoric water enriched in 18o through water–rock oxygen isotope exchange, or a mixture of meteoric and arc-type magmatic water [chiodini et al. 2001]. the carbon isotopic signature of the fumarolic co2 was compatible, at least partially, with derivation from metamorphic reactions that involve marine carbonates and that take place in the thick carbonate sequence located at depths >2.5 km underneath vesuvius [chiodini et al. 2001]. more recently, this partial derivation of co2 from decarbonation processes was confirmed by petrological considerations regarding the process of carbonate assimilation long time-series of vesuvius fumaroles figure 2. conceptual geochemical model of the volcanic hydrothermal system of vesuvius [after chiodini et al. 2001], refined with the data from the present study (see section 3.3.1). the seismic events registered in the 1995-1999 period [saccorotti et al. 2002] are reported for comparison. by the vesuvius magma [iacono marziano et al. 2009]. to derive indications of the temperature–pressure conditions of the hydrothermal system, chiodini et al. [2001] considered gas equilibria in the co2-ch4-co-h2-h2o gas system in the presence of nacl brines. in particular, fh2o, the fugacity of water, and the vapor–liquid distribution coefficients bi were considered to be controlled by the presence of either 3 m nacl solutions or halite saturated brines. for the fc2 fumarole, i.e. one that is regularly monitored and is considered in the present study, equilibrium temperatures were 445 ±9 ˚c considering 3 m nacl solutions, and 429 ±8 ˚c for the halite-saturated brine. such high equilibration temperatures were confirmed by the individual co/co2, h2/h2o, and ch4/co2 ratios, by the h2-h2s-h2o and h2-n2-nh3-h2o gas equilibria, and by the h2-ar geoindicator [chiodini et al. 2001]. in addition, similar high temperatures (450-500 ˚c) were confirmed on the basis of the c-isotopic exchange between ch4 and co2 [fiebig et al. 2004], leaving little uncertainty of the thermal state of the vesuvius hydrothermal system. due to the uncertainty of the salt content of the hydrothermal liquid, the estimation of total fluid equilibrium pressure (ptot = ph2o + pco2) varies from 260 bar, in the case of halite saturation, to 480 bar, for 3 m nacl solutions [chiodini et al. 2001]. considering a hydrostatic regime, the deepest hydrothermal reservoir might be located at depths of roughly 2.5 km to 5.0 km, within the carbonate sequence, which is present at depths >2.5 km underneath vesuvius [berrino et al. 1998]. it is worth noting that the estimated depths of the deepest hydrothermal reservoir roughly coincide with the major frequency distribution of the hypocenter of earthquakes recorded in the 1995-1999 period [saccorotti et al. 2002], suggesting a possible relationship between these. in particular, saccorotti et al. [2002] proposed that the seismic activity at vesuvius is triggered by diffusion of a pressure front, which is possibly associated with the degassing process of the hydrothermal system. 2. materials and methods this study is based on the compositional data of fumaroles b1 (crater rim) and fc2 (crater bottom) the locations of which are shown in the map in figure 1. although there are other fumarolic vents in the crater area that have been analyzed occasionally, we have limited the discussion here to the fc2 and b1 fumaroles, for which a continuous time-series of chemical and isotopic compositions is available. in particular, we used 88 chemical analyses, and analyzed 72 oxygen isotopic compositions, and 61 hydrogen isotopic compositions of the steam condensate of the fc2 fumarolic vent, and 79 chemical analyses of b1 fumarole, which were collected periodically from 1998 to 2010 in the framework of volcanic surveillance of the campanian volcanoes. gas samples for the determination of the chemical compositions of fc2 fumarole were collected in 200 ml glass vials under-vacuum, which were equipped with a teflon stopcock fixed with a rubber o-ring, which contained a 4 n naoh solution [giggenbach 1975, giggenbach and gouguel 1989]. dry gas samples were collected for determination of co concentrations. condensates of water vapor and noncondensable gases were sampled by flowing the fumarolic gases through a condenser cooled at 20 ˚c to 30 ˚c by water. the chemical compositions of the gases and the isotopic compositions of the co2 were analyzed at the geochemistry laboratory of ingv-osservatorio vesuviano. the gas constituents were analyzed following the methods of cioni and corazza [1981], as modified for the analysis of he, ar, o2, n2, h2 and ch4. the chemical compositions of these nonabsorbed gases that were mainly present in the headspace over the naoh solution were measured by gas chromatography, through a unique injection on two molecular-sieve columns (ms 5å capillary, 30m×0.53mm× 50µm; he and ar were the carrier gases) using tcd detectors. the carbon dioxide and sulfur species absorbed in the alkaline caliro et al. 140 figure 3. chronogram of the co2 contents of the b1 rim fumarole during the 1998-2010 period. 141 solution were analyzed after oxidation via h2o2, and by acidbase titration and ion chromatography, respectively (analytical error, ±3%). because of the reaction in alkaline solution to form cooh− [giggenbach and matsuo 1991], co was analyzed for dry gas samples, using gas-chromatography separation with a ms 5å capillary (30m×0.53mm×50µm); he was used as carrier gas, using a tcd detector. some isotopic analyses (data from 1996 to 2002) of oxygen and hydrogen (water) were performed at the geochemistry laboratory of ingv-palermo, using a finnigan delta plus mass spectrometer. oxygen isotopic compositions were measured after equilibration with co2 at 25 ˚c [epstein and mayeda 1953], h was measured after reduction with zn [coleman et al. 1982], and c after cryogenic purification of co2 (analytical errors: dd ±1‰, d18o ±0.1‰ and d13c ±0.1‰). isotopic data since 2003 were performed at the geochemistry laboratory of ingvosservatorio vesuviano, using a finnigan delta plusxp continuous flow mass spectrometer coupled with a gasbenchii device (analytical errors,: dd ±1‰, d18o ±0.08‰ and d13c ±0.06‰). gas samples for the determination of the chemical compositions of b1 fumarole were collected in 20 ml glass vials that were equipped with a teflon stopcock fixed with a rubber o-ring, by flowing the fumarolic gases through a condenser cooled at 20 ˚c to 30 ˚c with water. the dry gas was analyzed by gas chromatography, through a unique injection on two columns (molecular sieve 5 å capillary 30 m × 0.53 mm × 50 µm, and hysep q packed 90´ × 1/4´) using he as the carrier gas on the columns and tcd detectors. the analytical data are reported in the electronic annex 1. 3. time-series of the chemical and isotopic compositions of the fumaroles during 1998-2010 3.1. time-series of the rim fumarole b1 the b1 fumarole is fed by air that is variably contaminated by co2 of deep origin, as suggested by some of the data on the carbon isotopic composition (d13cco2 = ~−0.7‰) that are close to the values measured in the crater bottom fumaroles (generally from −0.5‰ to 0.7‰) that are not affected by air contamination. the co2 concentration is thus representative of the relative contribution of the deep component. the chronogram of figure 3 shows that over time the co2 content of this fumarole is scattered, probably because the amount of air that enters the fumarolic conduits is controlled by variable meteorological conditions, such as the wind and atmospheric pressure. in spite of this dispersion that is caused by secondary processes, the chronogram shows a general decreasing trend in co2, which can be interpreted as a decrease in the time of the flux of the deep component. 3.2. time-series of the fc2 crater bottom fumarole time-series of selected parameters (d18oh2o, ddh2o, co2/h2o, co2/ch4, he/co2) that were measured at the fc2 fumaroles during 1998-2010 are reported in the chronograms of figure 4. figure 4 highlights the occurrence of compositional variations of different frequencies: shortperiod variations are typical of the stable isotopes of the long time-series of vesuvius fumaroles figure 4. time-series of selected parameters: a) d18oh2o; b) ddh2o; c) co2/h2o; d) co2/ch4; and e) he/co2 measured at fumarole fc2 over the 1998-2010 period. the isotopic compositions of the fumarolic water (4a and 4b) are characterized by an annual cycle, while the long-period variations affect the co2/h2o, co2/ch4 and he/co2 ratios characterized by an unique peak in 2002. condensates, which show an evident annual cycle (figure 4a,b), while the co2/h2o, co2/ch4 and he/co2 ratios are affected by long-period variations that are characterized by a unique peak value, which is positive for he and co2 and negative for ch4, and which occurred in 2002 (figure 4c-e). this annual cycle that characterizes the water isotopic composition testify to the occurrence of shallow processes that are linked in some way to seasonal effects, while the behavior of the gas species might be caused by some deep variations in the system that feeds the fumaroles. 3.2.1. shallow processes the d18o values of steam condensates range from −10‰ to −5‰, while their dd values range from −57‰ to −18‰, with both showing a marked annual cycle (figure 4a,b). this annual cycle is more evident through examination of figure 5, where the values are plotted against the months of sampling. both the oxygen and hydrogen isotopic compositions show sinusoidal trends that are characterized by minimum values from december to february, while the maxima are reached from june to august, indicating strong seasonal control of the isotopic composition of the fumarolic water. a possible process that might cause such seasonal variations is seen in the partial condensation of the fluids within the fumarolic conduits, a process that in winter will be enhanced by the lower ambient temperatures, and/or by the infiltration of cold meteoric water and melted snow. this condensation is a plausible process, because both the relatively low flow rate of the fumarole and its discharge temperature (mean, 94.2 ˚c) are close to the boiling temperature (94.5 ˚c, at this specific altitude). during condensation, the isotopic fractionation causes the formation of a liquid that is enriched in heavy isotopes (18o and deuterium [d]), which leaves a residual vapor of a lighter isotopic composition. this process explains why during the winter season, when condensation is enhanced, the fumarole discharge fluids are characterized by lighter isotopic compositions. these effects of condensation have been modeled using two different approaches. the first approach (model a) considers only the fractionation due to the condensation of the steam at boiling point. in more detail, we computed the dd, d18o compositions of the vapor remaining after condensation (the residual vapor) for different fractions of condensation, in a multi-step process. considering the fractionation factors between liquid and vapor at 94.5 ˚c (1000lnal-v = 30.14 and 5.30 for d and 18o) [horita and wesolowski 1994], the isotopic compositions are computed at each step by: (1) (2) (3) (4) where the subscripts o and r refer to the original and the residual vapors, and y is the fraction of steam condensed at each step. in this exercise, the starting vapor was selected as that of the fumarolic composition characterized by a heavier isotopic composition (sample of 22/08/2007: d18o = −5.58‰, dd= −20.84‰), which is considered as representative of a noncondensed sample. although, this assumption is not entirely constrained by the data, the possible choice of a more positive isotopic composition for the original fluid would be purely speculative. the theoretical compositions of the residual vapor, i.e. of the vapor remaining after condensation, are reported in figure 6, as the "model a" line. this model does not explain the measured compositions that are systematically enriched in 18o, with respect to the condensation line. this systematic deviation is explained by the contemporary occurrence of oxygen exchange between co2 and steam during the condensation. chiodini et al. [2000] demonstrated that in fumarolic gas this process is fast enough to allow rapid isotopic re-equilibration of fumarolic fluids at their discharge temperatures, within a wide temperature range (100–1,000 ˚c). in particular, 52 samples where the oxygen isotopic compositions in both h2o and co2 were measured show d18oco2 -d 18oh2o differences (36‰ ±1‰) that are close to the theoretical fractionation at 95 ˚c (34.1‰). for this reason, the second model (figure 6, "model b") takes into account the occurrence of oxygen exchange caliro et al. 142 figure 5. d18o and dd values of steam condensates versus the months of sampling. more depleted samples (in heavy isotopes) characterize the wintry months, indicating strong seasonal control of the isotopic composition of fumarolic water. rd 1 d d 1000lny yo r l v= + +# #d d d a-^ ^h h d d 1000lnyr o l v= #d d a-^ h r1 1000lny yo o o18 o 18 r 18 l v= + +# #d d d a-^ ^h h 1000lnyo o18 r 18 o l v= #d d a-^ h 143 between co2 and steam during the condensation. practically, this more complex condensation model is based on the solution of the isotopic mass-balance for the oxygen in the h2o + co2 system, which can be written as: (5) (6) where d18otot is the oxygen isotopic composition in the co2 + h2o system before the condensation, |co2, |v and |l are the oxygen atom fractions for co2, vapor and liquid water in the co2 + h2o system, and 1000lnaco2-v = 34.20 is the co2-h2ovapour oxygen fractionation factor computed at 94.5˚c using the equation of richet et al. [1977]. the oxygen atom fractions are given by: (7) (8) (9) the relatively good fit of the measured points with model b suggests that the annual 18o and d variability observed at the vesuvius fumaroles is mainly due to condensation processes, which are more efficient during the cold seasons. the fraction of condensed steam can be roughly evaluated at 0.3–0.4, by comparing the theoretical and measured data. in principle, the same trend of the fumarolic dd-d18o values can be explained by other more complex processes, such as mixing of the deep hydrothermal fluids (either vapor or liquid) with cold groundwaters, formation of mixed parent liquids, and separation of equilibrated vapors [giggenbach and stewart 1982, henley and stewart 1983, chiodini et al. 2001, federico et al. 2010]. these processes possibly occur in peripheral areas of the hydrothermal systems, where meteoric waters can interact with the deep fluids, causing the formation of mixtures at lower temperatures, with respect to the undiluted original fluids. these processes are unlikely to occur at vesuvius, where gas equilibria indicate almost constant and high temperature and pressure values. in contrast, the simplest condensation model that is proposed here for the first time is based on processes that are both directly observable at the fumarole (i.e. condensation) and demonstrated by analytical measurements (i.e. isotopic equilibrium between oxygen in water and in co2; see above). the removal of water from fumarolic fluids by condensation implies a concurrent increase in the relative concentration of the incondensable gas, which should show an annual cycle similar to that of the water isotopes. in this case, however, the occurrence of evident long-term variations, as highlighted in figure 4c-e, can hide the eventual annual variations. for example, the co2 content of the fumaroles shows a clear decreasing trend from 1998 to 2010. we tried to de-trend the co2 chronogram by adding the residual of the linear regression of co2 versus time to the mean co2 value (see figure 4c). the de-trended values, which are plotted against the months of sampling in figure 7, show an annual long time-series of vesuvius fumaroles figure 6. plot of dd vs. d18o of the fumarolic condensates (fumarole fc2). condensation models under near discharge conditions (94.5 ˚c) are also reported, with the parent fluid sample of 22/08/2007. model a refers to simply multistep steam condensation, while model b refers to multistep steam condensation that takes into account the oxygen isotopic exchange between co2 and water vapor during the process. the good fitting of the analytical and theoretical (model b) data support the possible occurrence of this process. figure 7. co2 de-trended contents of the fc2 fluids (see text) versus the months of sampling. a cyclical annual behavior is seen, with the higher values characterizing the cold months (enhanced condensation), which supports the occurrence of condensation processes. 1000ln 1000ln o o o o 18 tot 18 r co2 v co2 v 18 r v 18 r l v l = + + + + # # # # d d a | d | d a | ^ ^ h h 1000ln 1000lno o18 r 18 tot co2 v l v l= # #d d a a |-^ h 2x 2x xco2 co2 co2 h2o= +| ^ h x 2x xyl h2o co2 h2o= +#| ^ h 1 x 2x xyv h2o co2 h2o= +#| -^ ^h h cycle that, in agreement with the isotope variations, has the highest concentrations in the winter season (90,000–110,000 ppm) and the lowest in the summer (70,000–90,000 ppm), which confirms the occurrence of condensation that is driven by seasonal processes. only a few points clearly deviate from the annual trend. these refer to samples collected in 2002, when other parameters showed anomalous behaviors due to the arrival at the surface of a deeply generated geochemical signal. 3.2.2. deep processes in addition to the shallow secondary processes discussed above, other evidence highlights that the vesuvius fumaroles have been affected by compositional changes caused by deep processes. an evident positive peak culminated in 2002 characterizes both the co2/ch4 and the he/co2 ratios (figure 4d,e). in particular, the co2/ch4 ratio increased sharply from a value of ~1,500 in 1998 to ~3,000 in 2002, and then gently decreased to the original values. it is worth noting that ch4 is a gas species that differentiates hydrothermal gases, where it is present at relatively high concentrations, from magmatic fluids, where it is normally absent or present at very low concentrations. due to this large difference between magmatic and hydrothermal fluids, the increase in the co2/ch4 ratio provides a powerful geochemical indicator of the arrival of hot, oxidizing, and possibly magmatic, fluids at the shallow levels of volcanoes characterized by the presence of hydrothermal systems above the magmatic ones [chiodini 2009]. the arrival of deeper fluids, which is possibly characterized by the relatively high he concentrations, also explains the increase in the he/co2 ratio that occurred in 1999-2002, concurrent with the increases in the co2/ch4 ratio (figure 4e). in the case of vesuvius, ch4 is formed in the hottest zones of the hydrothermal system by co2 reduction, as suggested by both chemical and isotopic gas equilibria [chiodini et al. 2001, fiebig et al. 2004]. because of the relatively slow kinetics of the co2 to ch4 reduction, the arrival of a pulse of deeper and hotter fluids that are poor in ch4 would cause a rapid increase in the co2/ch4 ratio, followed by a decrease that reflects the slow re-establishment of the original hydrothermal conditions. on the other hand, caliro et al. 144 figure 8. time variations of the equilibrium pco2 and the soil co2 flux (a), and of the seismic activity reported both as number of events per month and as the strain release [giudicepietro et al. 2010] (b). a pco2 peak starts in concomitance with the seismic crisis of october 1999, passing from values of ~40 bar in 1998 to ~55–60 bar in 2001-2002, in good agreement with the co2 flux data recorded by the automatic flxov4 station that increased by one order of magnitude. 145 the other gas species that are sensitive to redox conditions (i.e. h2 and co) would quickly re-adjust to the new conditions, as these species are characterized by fast kinetics [chiodini et al. 1993, giggenbach 1987]. assuming the presence of halite saturated brines, h2 and co were used as indicators of the pco2 of the hydrothermal system of vesuvius [chiodini et al. 2001]. to better investigate the origin of the 2002 anomaly, we computed the equilibrium pco2 during the period 1998-2005, adopting the same approach as chiodini et al. [2001]. the chronogram reported in figure 8a shows that the 2002 he-ch4-co2 anomaly (figure 4c-e) was also accompanied by an evident increase in the equilibrium pco2, which went from ~40 bar in 1998 to ~55–60 bar in 2001-2002. it is worth noting that these pco2 values are in good agreement with the co2 flux data, an extensive and independent parameter that is recorded by the automatic station located in the bottom of the vesuvius crater, near fumarole fc2 (figure 1, flxov4). during 2002, the co2 flux increased by a factor of ~1 order of magnitude, going from ~1,000 gm−2d−1 in 1998-1999 to ~10,000 gm−2d−1 in 2002 (figure 8a). to sum up, all of the observed variations are in agreement with the arrival of a pulse of deep, possibly magmatic, fluids rich in co2 and he and poor in ch4. to further validate this hypothesis, a simple mass-balance dilution model of fumarolic fluids with a hypothetical magmatic gas is presented in figure 9, where the ch4/co2 and he/co2 values of the 1998-2002 samples are plotted against the pco2 estimated on the basis of the h2 and co contents. in figure 9, the lines represent the values expected for the injection into a box, which represents the hydrothermal system theoretically, for a gas with ch4/co2 = 0 (no ch4) and he/co2 = 9 ×10 −6. practically, we considered that the initial gas composition was that of the lowest pco2 sample (september 1999: pco2 = 36.6 bar). consequently, we computed the initial pch4 and phe on the basis of their composition (pch4 =pco2 ×ch4/co2 =2.35×10 −2bar; phe = pco2 × he/co2 = 1.67 × 10 −4 bar). this simply model can account for the compositional variations of the fumarolic fluids, again supporting the concept that a pulse of deep, possibly magmatic, fluids caused the 2002 anomaly. it is worth noting that the anomaly starts in concomitance with the seismic crisis of autumn 1999 (figure 8b). this crises included the main earthquake recorded at vesuvius after the last eruption of 1944 (the event of october 9, 1999; ml 3.6; depth ~3 km beneath the central cone) [del pezzo et al. 2004]. in our opinion, the earthquakes of 1999 marked the arrival of the magmatic fluids into the hydrothermal system, which caused the above-described variations that culminated in 2002. in agreement with our interpretation, saccorotti et al. [2002] proposed that at least a part of the seismic activity at vesuvius is triggered by diffusion of a pressure front, which is possibly associated with the degassing process of the hydrothermal system. the seismic activity reported in figure 8b (both as number of events per month and as monthly strain release) shows an intense period of activity starting form the october 1999, when there were the highest values, which then remained at considerable levels to the end of 2002. long time-series of vesuvius fumaroles figure 9. ch4/co2 (a) and he/co2 (b) ratios of fumarolic fluids versus equilibrium pco2. lines refer to a dilution model of fumarolic fluids with a hypothetical magmatic gas characterized by the absence of ch4 and by he/co2 =9×10 −6. the good fitting of data supports the concept that a pulse of deep, possibly magmatic, fluids caused the 2002 anomaly. 4. origin of the fumarolic water the large dataset of water isotopic composition that was acquired during the 1998-2010 period of volcanic monitoring can be used to refine the interpretation of the origin of the fumarolic water, with respect to previous interpretations [chiodini et al. 2001]. for this purpose, we computed the oxygen isotopic composition of the h2o + co2 system for the fumarolic fluids under discharge conditions, by means of the following equation: (10) where symbols are the same as for equation (5). the computed d18otot data plot in figure 10a on the right side of the local meteoric water forming a large cluster of points. considering only the samples that were less affected by condensation (figure 10a, black dots), figure 10a suggests a derivation of fumarolic water due to variable mixing of three end-members: local meteoric water, arc-type magmatic water, and seawater. it is worth noting that this isotopic evidence on the involvement of seawater in the vesuvius hydrothermal system is a novel finding, which is consistent with the already recognized presence of saline brines in this hydrothermal system [chiodini et al. 2001]. caliro et al. 146 figure 10. a) dd versus d18o diagram. analytical isotopic compositions of fumarolic condensates and the computed d18otot values of the h2o + co2 system under discharged conditions are reported, together with the local meteoric water line [caliro et al. 1998], isotopic composition of the groundwaters circulating in the volcanic aquifers of vesuvius [caliro et al. 2005] and other possible end-members of mixing (seawater and andesitic waters). the fumarolic data were more or less unaffect by condensation processes (i.e. more positive isotopic values, black dots) and they support an origin of the water due to a mixture of andesitic water, seawater and local meteoric water. b) mixing-condensation model involving seawater, andesitic water and local groundwater. the assumed compositions of the end members are: seawater dd=10‰, d18o=1‰, xco2 =0; andesitic water dd=−20‰, d 18o=10‰, xco2 =0.4; groundwaters: dd=−38‰, d 18o=−6.3‰, xco2 =0. the mixing line a represents the vapor generated by seawater–andesitic water mixtures. the mixing line b represents a dilution of line a by 50% in groundwater. dashed lines represent the condensation patterns of line b vapors. the number of dashed lines refer to the different co2 molar fractions of the pre-condensed line b vapors. o o 1000ln o 1 18 tot 18 v co2 v co2 18 v co2 = + + + # # d d a | d |-^ ^ h h 147 to explain the highly scattered distribution of the samples, in figure 10b the data are compared with the composition expected for a possible mixing–condensation model. in particular, the grid in figure 10b represents the vapor generated by seawater–andesitic water mixtures (figure 10b, line a) which are diluted by 50% in the groundwater (figure 10b, line b). the resulting vapors are subsequently condensed (figure 10b, dashed lines). the effects of the condensation are strongly affected by the co2 concentrations of the pre-condensed mixtures. even if the selected composition for the magmatic component is largely unconstrained, as is the fraction of groundwater, on the whole, the model reproduces well the large variability of the measured isotopic composition. it is worth noting that the samples that would be more rich in the andesitic water component are those of 2002, i.e. the same samples where independent observations highlighted the presence of larger fractions of magmatic fluids (see above). 5. conclusions long time-series of chemical and isotopic compositions of the vesuvius fumaroles have allowed us to identify processes that occur with different frequencies: short-period variations appear to be linked to processes that occur at shallow levels in the hydrothermal system, while relatively long time variations are induced by deep changes in the volcanic activity. the isotopic compositions of the fumarolic waters show evident annual cycles (figure 5), with the more depleted values seen in the autumn-to-winter period. taking into account the oxygen exchange between co2 and steam, condensation processes at near discharge conditions can explain the annual 18o and d variabilities observed at the vesuvius fumaroles. these processes are more efficient during the cold seasons, when the ambient temperatures are lower. the fraction of condensed steam can be roughly evaluated by comparing the theoretical and measured data, as 0.3 to 0.4. taking into account the condensation processes, the analysis of the large dataset of water isotopic composition that was acquired during the 1998-2010 period of volcanic monitoring has allowed the refinement of the interpretation for the source of fumarolic water. considering the isotopic oxygen exchange between co2 and water vapor, the isotopic data suggest a derivation of the water from a mixture of meteoric, arc-type magmatic water, and seawater. the isotopic evidence of seawater is a novel finding that agrees with the presences of high saline brines in the hydrothermal system of vesuvius [chiodini et al. 2001]. superimposed on the seasonal effects, the time series of the chemical composition of fumaroles reveals that important variations in the activity of vesuvius occurred in the 1999-2002 period. a continuous increase in the co2 and he relative concentrations and a general decrease in the ch4 concentrations are interpreted to be the consequences of the increase in the relative amount of magmatic fluids in the hydrothermal system, which are rich in co2 and he, and poor in ch4. gas equilibria support this hypothesis, showing a pco2 peak that culminated in 2002, passing from values of ~40 bar in 1998 to values of ~55–60 bar in 2001-2002 (figure 8a). it is worth noting that the pco2 values are supported by the co2 soil-flux data that were recorded by the automatic station flxov4, which is located close to fumarole fc2 (figure 1); 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(1983). a general model for the behavior of the somma-vesuvius volcanic complex, j. volcanol. geotherm. res., 17, 237-248. ventura, g., g. vilardo and p.p. bruno (1999). the role of flank failure in modifying the shallow plumbing system of volcanoes: an example from somma-vesuvius, italy, geophys. res. lett., 26, 3681-3684. zollo, a., w. marzocchi, p. capuano, a. lomax and g. iannaccone (2002). space and time behavior of seismic activity at mt. vesuvius volcano, southern italy, bull. seismol. soc. am., 92, 625-640. *corresponding author: stefano caliro, istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy; e-mail: stefano.caliro@ov.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. long time-series of vesuvius fumaroles completo_def.qxd 1333 annals of geophysics, vol. 47, n. 4, august 2004 ten years analysis of tropospheric refractivity variations stergios a. isaakidis and thomas d. xenos department of electrical and computer engineering, aristotle university of thessaloniki, greece abstract the refractivity variations of the troposphere are responsible for various effects on radio wave propagation, such as refraction, bending, radio-station interference, etc. in this work, the refractivity variations of the hellenic troposphere are studied using data from helliniko airport of athens/greece. the data were analyzed using various interpolation procedures, i.e. in a day-by-day manner for temperature and relative humidity, to transform the data according to a reference height common for the whole dataset and finally for the refractivity n with respect to time, using piece-wise hermite interpolation polynomials for the low and medium altitudes and linear interpolation factors for the high altitudes. since refractivity varies with time and height, two height independent basic parameters were computed and analyzed: the refractivity at station height n0 and scale height hs. these parameters can be used to calculate the refractivity profile. using statistical tools as the moving average, the β0 parameter and the monthly mean values, together with the corresponding standard deviations, useful results were obtained for the variations of the refractivity with respect to observation hour, height, season, month, day and level. 1980; anderson, 1995), refraction and fading of the electromagnetic waves (flock, 1987), ducting phenomena (patterson et al., 1994), etc. the index of refraction is defined as /c vr =fn = , where εr is the dielectric constant of the troposphere, c is the speed of light and v is the phase velocity of the wave in the medium. because n is slightly greater than unity near the earth’s surface (1.00025-1.00040), in practice we use the scaled index of refraction n which is called refractivity and is defined as (flock, 1987) ( ) . . . n n t p t e t e 1 10 77 6 5 6 3 75 106 2 5 # # = = + (1.1) where p is the total pressure in mbar, e is the water vapor pressure and t is the temperature in kelvin degrees. if the relative humidity (rh %) and temperature are given, the water vapor pressure e can be computed with the following set of equations (lear, 1980): 1333 key words troposphere – refractivity – scale height – β0 parameter 1. introduction the troposphere is the lower region of atmosphere and it extends from the earth’s surface to an altitude of about 10 km at the middle latitudes (6 km at the poles and up to 18 km at the equator). the basic parameters that describe the tropospheric medium are pressure, temperature and relative humidity. propagation in the troposphere is affected by the variations of the refractive index with time and height. these refractivity variations are responsible for various phenomena in the wave propagation, such as range and elevation angle errors in radar acquisition (lear, mailing address: dr. stergios a. isaakidis, department of electrical and computer engineering, aristotle university of thessaloniki, gr-54 124 thessaloniki, greece; email: sisaakid@otenet.gr 1334 stergios a. isaakidis and thomas d. xenos 1334 . . ( . ) x t t 35 85 17 2694 273 15 = (1.2) . ( )( )expe x mbars6 11s #= (1.3) .e e rh 100 s = (1.4) theoretically speaking, the refractivity with respect to height is expressed by the exponential relation expn n h h s0= -_ i (1.5) where n0 is the refractivity at the station’s height, h is the height and hs is the scale height. it is obvious that the parameters n0 and hs are independent of height and since their changes will be reflected in n according to eq. (1.5), they constitute a useful tool for the study of the refractivity variations. the purpose of this work is to study the tropospheric refractivity variations over the hellenic region. these variations describe the tropospheric conditions over the specific region of interest and can be taken into account as an empirical reference for troposheric wave propagation, scale height estimations, bending of rays for terrestrial or satellite paths and so on. 2. data analysis for the calculation of the refractivity variations, data from helliniko airport of athens/ greece (37°53′n, 23°43′e) were used. the data cover the period from 1991 to 1999 and the measured parameters are temperature, relative humidity and height at constant pressure levels. the measured data were provided by the hellenic national meteorological service (hnms) and come from observations with radio balloons launched twice a day (00 and 12 lt). the measured parameter values were classified according to observation hour, day of the year and pressure levels and after a preliminary day-by-day statistical process according to the standard deviation of each pressure level, the data were cleared from any unrealistic values. before calculating the refractivity indices, interpolation procedures were followed to fill in missing values, enrich with data the low altitudes, where the intensity of the tropospheric variations is higher and also to create smoother curves of the initial parameters. the first interpolation procedure was made in a day-by-day manner according to the reference pressure levels and was the most important in the sense that it was applied directly to the initial measured data and provided most of the interpolated values, forming smoothed and enriched curves of temperature, relative humidity and height versus pressure. the next interpolation procedure was also applied each day. its purpose was to transform the data according to a reference height, common for the whole dataset. this procedure also enriched the low altitude values and also detected and corrected days where abnormal fluctuations of the original data were present. after the analysis of the data according to the above procedures, the refractivity index n was calculated using eqs. (1.1) to (1.4). finally, an interpolation procedure for n according to time was followed for each altitude level, in order to fill in missing values that could not be interpolated in the previous methods. for all the interpolation procedures, piecewise-cubic-hermite interpolating polynomials were used, except for high altitudes (higher than 10 km), where linear interpolation functions have been applied. this was due to the fact that at high altitudes, when a large number of data were missing, the cubic functions in many cases returned unrealistic results. spline interpolation functions instead of cubic were also tested, but they resulted in strong oscillations about the measured data, especially in cases where many consecutive values were missing, as is the case for relative humidity. after calculation of refractivity, the scale height hs was determined for each day by applying a least squares nonlinear fitting. the analysis of the data by means of the previously described procedures and the calculation of refractivity were followed by a statistical evaluation in order to assess their variability. 3. results and discussion the statistical treatments applied are the second order 31-days moving averages of n0 1335 ten years analysis of tropospheric refractivity variations 1335 and hs versus days and their monthly mean values, together with the corresponding standard deviations. although calculated n0 and hs with respect to time form a time series with periodic variations, moving averages eliminate strong fluctuations by low pass filtering the data. also, the β 0 parameter was determined for each month for the two observation hours of 00 and 12 lt. this parameter is defined as the percentage of time that the refractive index gradient in the first 100 m above ground exceeds 100 nunits/km and it represents the probability that anomalous propagation may occur. figure 1 shows the second order 31-days moving average of n0 versus all the days of the period 1991-1999 and for both observation hours (00 and 12 lt). in the horizontal axis the serial number of the day is presented with day 1 corresponding to the 1st january 1991 and day 3287 to 31st december 1999. these diagrams constitute a de-noised version of the original calculated refractivity and they show clearly the cycle variations of n0 for all years (separated with vertical grid lines). it can be seen that in general, the refractivity increases from january to june, where it appears to reach a first peak. during the summer period, refractivity levels remain high, but a decrement usually occurs during july, reaching a minimum level in august. then, the refractivity increases till september-october, where it reaches a second peak, higher than the previous one. after october and for the whole winter period, refractivity shows a high rate of attenuation. from the same figure, it can be deduced that only the amplitude of refractivity is changing, being lower at 12 lt, while the refractivity variations with respect to time preserve their initial form. figure 2 shows the second order 31-days moving average of hs versus the days for the two observation hours of 00 and 12 lt. comparing this diagram with fig. 1 it can be seen that the scale height follows the form of n0, but its variations are lower. the results corresponding to 12 lt are similar to those of 00 lt since the scale height does not show time variations, except for its level which seems higher compared to 00 lt. further information on seasonal refractivity variations can be extracted from fig. 3 which shows the monthly mean values of n0 versus months for the whole analyzed period. in this figure, the increment of the refractivity values during the summer period is clear. this diagram also shows comparative variations of no for each year. although all the years do not follow the same level variations, the decrement from june to july is clear, except for 1996, when this phenomenon started on may. except for the seasonal variations of refractivity, diurnal variations are also present (fig. 4). this figure shows the standard deviations of the monthly values of n0 versus months. this diagram shows that the increase in refractivity fig. 2. second order moving average of hs versus days (observation hour 00 and 12 lt). fig. 1. second order moving average of n0 versus days (observation hour 00 and 12 lt). 1336 stergios a. isaakidis and thomas d. xenos levels during the summer is followed by strong deviations within each month. in general, these deviations increase till july where they reach their maximum level and they start to decrease again during the autumn and winter periods. the identification of the monthly variations of the scale height is not as clear as the variations of n0 (fig. 5). this figure shows the monthly mean values of hs versus month for the period 1991-1999. it can be seen that in general hs follows an inverse variation pattern compared to n0. it is decreasing during summer and a local peak appears in july but its deviations are very strong and they do not follow the distribution of the standard deviations of n0 (figs. 4 and 6). the results for the β 0 parameter are shown in figs. 7 and 8, for 00 and 12 lt respectively. by studying fig. 7 it can be seen that the probability of anomalous propagation is very high during the summer months reaching a value of about 90%. during the other periods of the year the β 0 values are much lower but strong variations can be observed in april, september and october where the β 0 values show a fluctuation fig. 3. monthly mean values of n0 versus months (observation hour 00 lt, years 1991-1999). fig. 5. monthly mean values of hs versus months (observation hour 00 lt, years 1991-1999). fig. 4. standard deviations of the monthly values of n0 versus months (observation hour 00 lt, years 1991-1999). fig. 6. standard deviations of the monthly values of hs versus months (observation hour 00 lt, years 1991-1999). 1337 ten years analysis of tropospheric refractivity variations of the order of 50%. for the 12 lt observation hour, it can be seen (fig. 8) that the β 0 values are in general higher during the summer period, but the annual variations are stronger with respect to 00 lt and thus the prediction of anomalous propagation conditions is very difficult. the relation between n0 and hs can be further revealed by comparing their standardized values, no’ and hs’ respectively. the standardization can be made using the equations / ( )nstd= ( )n n nmean0 0 0 0-l 7 a (3.1) / ( )hstd= ( )h h hmeans s s s-l 7 a (3.2) where the moving averages of n0 and hs are used. in the above equations «mean» stands for the global mean value of the moving averages and «std» for their corresponding standard deviation. in fig. 9 the n′0 and – h′s are plotted with respect to days. it can easily be seen that their correlation is very high with respect to time (correlation coefficient ≈ 0.77), whereas n′0 shows weaker variations. references anderson, k.d. (1995): radar detection of low-altitude targets in a maritime environment, ieee trans. antennas propag., 43, 609-613. flock l.w. (1987): propagation effects on satellite systems at frequencies below 10 ghz, nasa reference publication handbook, 1108 (2). lear m.w. (1980): computing atmospheric scale height for refraction corrections, (nasa mission planning and analysis division, lyndon b. johnson space center). patterson, w.l., c.p. hattan, g.e. lindem, r.a. paulus, h.v. hitney, k.d. anderson and a.e. barrios (1994): engineer’s refractive effects prediction systems (ereps) version 3.0, nrad technical document 2648, may. (received february 02, 2004; accepted july 02, 2004) fig. 9. comparison of the n′0 and h′s variations (observation hour 00 lt). fig. 7. β 0 parameter versus months (observation hour = 00). fig. 8. β 0 parameter versus months (observation hour = 12). layout 6 1 preface the strong earthquakes striking italy in this third millennium, in the molise region (2002), in the city of l’aquila (2009), in the emilia romagna region (2012) and just recently devastating the amatrice municipality (august, 2016) and other villages in central italy, once again testify to the very high seismic hazard of the country. such high seismic risk is both due to the geodynamic framework of this part of the mediterranean area and the exceptional vulnerability of its cultural and historical heritage. in this special issue of “annals of geophysics”, the main results of a three-year long project named “seismic risk reduction in eastern sicily” are presented to a wide range of researchers including geologists, geophysicists and environmental, civil, geotechnical earthquake and civil defence engineers. the seismic hazard in eastern sicily is well known, mainly for the long catalogue of dramatic seismic events that struck this sector of italy in the past: messina-reggio calabria in 1908, catania in 1818, val di noto in 1693, siracusa in 1542 and the hyblean area in 1169, are only the more well-known episodes of a long and tragic history of destruction, which has claimed the lives of more than 200,000 people. the drastic increase in building density over recent decades has raised the level of awareness and concern of citizens and authorities about the vulnerability in eastern sicily. an important element in developing strategies for the prevention and reduction of seismic risk is the evaluation of the structural behavior under seismic actions. for existing buildings, the problem mainly arises in the assessment of their vulnerability, a difficult challenge since it involves so many constructions to investigate in a reasonable time. the project, seeking to apply innovative approaches for seismic risk evaluation and reduction, was divided into a number of working teams whose objectives have been defined as follows: wt 2: new protocols for experimental trials of low cost analysis aimed at defining specific dossiers for individual buildings wt 3: installation of geophysical sensors to monitor and acquire seismic data in some selected edifices wt 4: design and production of new low cost sensors for structural monitoring wt 5: geological models and seismic characterization for specific sites wt 6: seismic vulnerability of specific sites based on geotechnical parameters and on the analysis of the soil-structure interaction wt 7: design and implementation of a software (alertsis) for seismic alert warning system and testing the applied methodology wt 8: experimental trials on destroyed buildings wt 9: protocols for the recycling of materials from destroyed buildings the partners of the project were both public institutions and small-medium companies with offices and venues in sicily. this special issue illustrates the results of three years’ research activity by a multidisciplinary research team, with geologists, geophysics, geotechnical, structural and industrial engineers, in the field of analysis and mitigation of seismic risk in a mediterranean area subjected to medium-high earthquake risk. this issue contains the most important results achieved and can be considered a guideline for the multidisciplinary task of assessing the seismic hazard, evaluating the seismic risk, preventing seismic risk of new constructions and for mitigating seismic risk in existing constructions. for task 1: general introduction to seismicity in eastern sicily (part i); on-site investigations for the seismic response analysis of specific sites, based on different and advanced methods (part ii) and innovative technologies for seismic monitoring (part iii). for task 2: static and dynamic geotechnical characterization of soils (part iv); site effects evaluation (part v); vulnerability analysis of buildings including soil-structure interaction of shallow foundations (part vi); developments of early warning systems for the structural monitoring of buildings (part vii) and use of recycled aggregates for construction and concrete mixture design (part viii). part i is devoted to a general introduction on seismicity and to the multiscale interaction structure of several seismic sequences occurred in the eastern sicily in the last decade. (siino et al.). part ii describes outlines/gives many examples of applying standard and innovative procedures to study site response (pino et al., scolaro et al., capilleri et al.(1),capilleri et al., (2) paratore et al.,). part iii is a general introduction to a perspective kind of seismic monitoring in urban areas, with emphasis on the structural monitoring of single edifices (d’alessandro et al., zuccarello et al., massimino et al.). part iv is related to soil characterization and seismic action, which represents the biggest uncertainty in the overall process of designing and retrofitting buildings. in particular, the section discusses the earthquake scenario given by the two destructive earthquakes of 1693, the modelling of the moderate earthquake of december 13, 1990 and the seismic response evaluation from micro-tremors and from numerical analysis. the seismic vulnerability of the city of catania and static and dynamic properties of the soil in catania (castelli et al.(1)) have been widely investigated. a geotechnical dynamic characterization has also been proposed for a test site in the city of messina (castelli et al.(2)). recent improvements for the emergency management have also been achieved (castelli et al.(3)). part v focuses on seismic response analysis and site effects, which is an important tool in earthquake geotechnical engineering studies. site effects evaluation shows great spatial variability in the urban area of catania, due to the geological and geolithological features, as well as the non-linear behavior of the soil. an evaluation of site effects by means of 1-d numerical analysis has been performed (ferraro et al.). part vi concerns the vulnerability analysis of buildings, including soil-structure interaction of shallow foundations. dynamic behavior of coupled soil-structure systems by means of fem analysis for the seismic risk mitigation of a building in catania (italy) has been investigated (abate et al.). part vii reviews the developments of early warning systems for the structural monitoring of buildings (zuccarello et al.). an advanced multi-sensor based early warning system for the structural monitoring of buildings has been proposed (andò et al.). part viii deals with the use of recycled aggregate for construction. use of recycled aggregate for road sub-base construction and concrete mixture design has been proposed by contrafatto et al. the results so far obtained by the research project on seismic risk reduction in eastern sicily, could be considered a pilot project for detailed earthquake scenario analyses and for seismic prevention of damage in many mediterranean cities, characterized, as are the cities of catania and messina, by high seismic risk. mario mattia ingv – osservatorio etneo marco muratore technoside s.r.l., team leader in research project on seismic risk reduction in eastern sicily mattia and muratore 2 annals of geophysics, 61, 2, 2018, se231; doi: 10.4401/ag-7828 © 2018 istituto nazionale di geofisica e vulcanologia. all rights reserved. completo_def.qxd 1471 annals of geophysics, vol. 47, n. 4, august 2004 key words somma-vesuvius – magma chamber – crustal contamination 1. introduction vesuvius is one of the most dangerous volcanoes in the world. at the present, half a million people live in a near-continuous belt of towns and villages around the area possibly exposed to the effects of future eruptions. due to the high volcanic risk, vesuvius is among the mailing address: dr. lucia, pappalardo, osservatorio vesuviano, istituto nazionale di geofisica e vulcanologia, via diocleziano 328, 80124 napoli, italy; e-mail: lucy@ov.ingv.it the 3550 year bp-1944 a.d. magma-plumbing system of somma-vesuvius: constraints on its behaviour and present state through a review of sr-nd isotope data lucia pappalardo, monica piochi and giuseppe mastrolorenzo osservatorio vesuviano, istituto nazionale di geofisica e vulcanologia, napoli, italy abstract vesuvius, dominating the densely-populated neapolitan area, is one of the most dangerous volcanoes in the world. its destructive power derives from energetic subplinian and plinian eruptions, such as the one which occurred in 79 a.d. generally such large-scale events follow a long period of quiescence; a behaviour interpreted as the gradual build-up of magma volumes between periods of major activity. after the 1631 subplinian eruption until the last 1944 a.d. eruption, it experienced an almost continuous and less energetic explosive/effusive activity. the erupted magmas are characterized by undersaturated potassic to ultrapotassic nature, and compositional and sr-isotopic variability. furthermore geobarometric studies indicate two different crystallization depths located at 4 and >11 km, respectively. according to most of the recent literature, the eruptions were triggered by the injection in a shallower magma chamber, of isotopically distinct magma batches derived from heterogeneous mantle source(s) and/or contamination processes occurred within the deep reservoir. in our review of petrochemical data, we consider the period between the 3550 years bp plinian eruption and the 472 a.d. sub-plinian eruption, which includes 79 a.d. event, and the most recent period of activity which started in 1631 a.d. and lasted up to the 1944 a.d. eruption, characterized by a near continuous effusive/explosive activity. for both periods we identify a correlation between sr-isotopical features of magmas and their crystallization depth. in particular, we show that pyroxenes have sr-isotopic ratios lower than 0.7074 and an equilibrium crystallization depth of 22-11 km. moreover feldspars have higher 87sr/86sr values (0.7075-7) and an equilibrium crystallization depth of about 4 km. therefore the most radiogenic magmas did not derive from a deeper reservoir but their higher sr-isotopic ratios have been acquired at a shallower depth likely by crustal contamination during magma evolution. in contrast, the lower sr-isotope compositions characterise the less contaminated magmas coming from deeper crustal levels. on the basis of this evidence, the temporal sr-isotopical variation of magma which erupted in the 1631-1944 a.d. period probably derives from the progressive withdrawal of the shallow magma chamber, which was completely empty before the 1805-1944 a.d. period of volcanism. therefore the effusive and explosive events of the most recent 1805-1944 a.d. period were fed directly by the deep reservoir located at a depth exceding 11 km. 1472 lucia pappalardo, monica piochi and giuseppe mastrolorenzo beststudied and monitored active volcanoes. much research (e.g., santacroce, 1987; civetta et al., 1991; civetta and santacroce, 1992; mastrolorenzo et al., 1993; rolandi et al., 1993, 1998; santacroce et al., 1993; marianelli et al., 1995, 1999; cioni et al., 1995, 1998; arrighi et al., 2001; peccerillo, 2001; somma et al., 2001; andronico and cioni, 2002) has been devoted to reconstructing its volcanic and magmatic history with the aim of understanding the proceeding of geochemical and geophysical precursors and forecasting its future behaviour. in spite of the wealth of studies, many problems related to magma genesis and evolution and to the present state of the vesuvius plumbing system remain open: i) the intraplate versus subduction-related origin of potassic magmatism (e.g., beccaluva et al., 1991; ayuso et al., 1998; peccerillo, 2001); ii) the relative roles of source composition and low-pressure processes in determining the geochemical features of volcanic products (e.g., savelli, 1967, 1968; turi and taylor, 1976; cortini and hermes, 1981; civetta et al., 1991; civetta and santacroce, 1992; santacroce et al., 1993; cioni et al., 1995; fulignati et al., 1995; ayuso et al., 1998; fulignati et al., 1998; del moro et al., 2001; gilg et al., 2001; somma et al., 2001); and iii) the present state of the plumbing system, constituted or not by a shallower magma chamber (e.g., santacroce, 1987; zollo et al., 1996; de natale et al., 2001). in this paper, petrological data are revisited to investigate the role of low-pressure evolutionary processes in magma genesis and their relationships with the style of eruptions (moderate explosive/effusive or highly explosive) during the 3550 years bp-1944 a.d. period of activity for which a complete set of data have been collected. 2. geological and petrological background somma-vesuvius (fig. 1a-c) is a strato-volcano consisting of a recent cone, the vesuvius, which rises within the older somma caldera and build ups on the mesoand cenozoic carbonatic succession cropping out in the nearest apennine. the age of volcanic rocks ranges between not less than 300 ka (brocchini et al., 2001 and references therein) and 1944 a.d. at present, vesuvius is in a quiescent state characterised by fumarolic and low-magnitude seismic activities. somma-vesuvius has been characterized by both highly explosive subplinian and plinian eruptions, as well as intermediate and small scale explosive and explosive/effusive ones. the plinian eruptions, such as the well-known 79 a.d. event, were preceded by long periods of quiescence; a behaviour interpreted as the gradual build-up of the magma reservoir between periods of major activity (e.g., civetta et al., 1991; civetta and santacroce, 1992 and references therein; mastrolorenzo et al., 2002). a ca. 300 years long period of continuous moderately explosive and effusive eruptions followed the 1631 a.d. subplinian event. however a quite moderate explosive activity can also be inferred for the periods between 79 a.d. and 1631 a.d. (medioeval eruptions, rolandi et al., 1998; somma et al., 2001) and between the 79 a.d. and 472 a.d. (ancient historic eruptions, rolandi et al., 1998; somma et al., 2001). a 300 km3 magma volume is inferred to have been erupted throughout the history of somma-vesuvius (civetta and santacroce, 1992). one to a few km3 dre of magmas are generally erupted during sub-plinian and plinian eruptions (e.g., santacroce, 1987; civetta and santacroce, 1992; rolandi et al., 1993), while 0.01 to 0.1 km3 dre of magmas are erupted during intermediate and small scale events characterizing the last 300 years (scandone et al., 1986; mastrolorenzo et al., 1993; arrighi et al., 2001). a near constant averaged output rate of about 0.3 m3/s has been calculated for the whole volcano history (scandone et al., 1986; civetta and santacroce, 1992). the magmatic system of somma-vesuvius produced silica undersaturated potassic (ks) to ultrapotassic (hks) rocks and consisted of multidepth reservoirs located at about 4 and 10 km as deduced by fluid and glass inclusion data (e.g., belkin et al., 1985, 1998; cortini et al., 1985; belkin and de vivo, 1993; cioni et al., 1998; marianelli et al., 1999; cioni, 2000). k-basalts to k-trachytes were produced in the oldest period of volcanism, before 11 500 years bp. between 11.5 ka bp and 79 a.d., the eruptions produced kphonolitic tephrites to k-phonolites. finally, be1473 the 3550 year bp-1944 a.d. magma-plumbing system of somma-vesuvius fig. 1a-c. somma-vesuvio map showing: a) geological sketch map of campanian plain 1 = quaternary and active terrigenous sediments; 2 = somma-vesuvius volcanic rocks; 3 = phlegraean volcanic rocks; 4 = roccamonfina volcanic rocks; 5 = pliocene and miocene terrigenous sediments; 6 = mesozoic carbonatic units; 7 = faults; 8 = nappes. b) reconstructed somma-vesuvius stratigraphic succession, modified after santacroce (1987) on the basis of data from rolandi et al. (1993, 1998) and somma et al. (2001), red = high-energetic eruptions, yellow = moderate/low explosive eruptions, green = effusive eruptions, x-scale indicates the erupted volume. c) geological sketch map of somma-vesuvius, after santacroce (1987). c a b 1474 lucia pappalardo, monica piochi and giuseppe mastrolorenzo tween 472 a.d. and 1944 a.d., leucitic tephrites to leucitic phonolites were erupted. the crystallinity of somma-vesuvius volcanic rocks is strongly variable, from nearly aphiric (mostly in the plinian eruptions) to highly porphiritic (in 472 a.d. eruption and in the products younger than 1631 a.d.) (e.g., villemant et al., 1993). the srisotope data of vesuvius magmas have a large range of values between 0.7069 and 0.7081 (e.g., cortini and hermes, 1981; civetta et al., 1991; civetta and santacroce, 1992; caprarelli et al., 1993; santacroce et al., 1993; cioni et al., 1995; ayuso et al., 1998; somma et al., 2001). the role of evolutionary processes and the nature of source(s) in magma composition is debated: compositional variability of the erupted magma has been attributed to heterogeneous enriched mantle sources, to differentiation and magma mixing (e.g., vollmer, 1976; civetta et al., 1991; civetta and santacroce, 1992; santacroce et al., 1993; cioni et al., 1995; ayuso et al., 1998; peccerillo, 2001; somma et al., 2001; piochi et al., 2004) or to crustal contamination (e.g., rittman, 1933; savelli, 1967, 1968; turi and taylor, 1976). contamination of magmas by assimilation of carbonate material has been also suggested for the genesis of ca-mg-silicate rich ejecta (skarns) on the basis of mineral chemistry, fluid inclusion and isotope data (e.g., savelli, 1968; fulignati et al., 1995, 1998; gilg et al., 1999; 2001; del moro et al., 2001). 3. geochemistry and isotope geochemistry volcanic products of somma-vesuvius have a potassic signature with different degrees of silica undersaturation. the most mafic rocks (sio2 less than 50 wt%) have been produced during the last 300 years of volcanic activity. they commonly have low mg numbers (100 × mg/ /mg + fe = 40 – 20), and low ni (less than 80 ppm) and cr (less than 300 ppm) abundances indicating the non primary nature of these mafic rocks. furthermore whole-rock compositions of these rocks do not always coincide with liquid compositions due to phenocrysts accumulation or depletion (belkin et al., 1993; trigila et al., 1993; villemant et al., 1993). in general, as the fractionation index (cao/al2o3) decreases, sio2, al2o3, k2o, na2o, rb, th, zr and ree (except eu) contents increase, while feo and cao, co, cr, ni, sc and v contents decrease; tio2 and p2o5 contents are roughly constant up to cao/al2o3 of 0.5, then decrease; sr and ba contents slightly increase up to cao/al2o3 of 0.5, then decrease (fig. 2). the strong enrichment in k and sr abundances in the 472 a.d. products could be related to their high porphiricity (e.g., rosi and santacroce, 1983) that generates the nearly vertical trend such as that in fig. 2 and that could be the reason of their ultrapotassic character. these rocks are in fact constituted at 50% in volume of mineral phases (preferentially leucite crystals and subordinately clinopyroxene) in contrast to other vesuvian plinian and sub-plinian rocks that are generally phenocryst poor. chondrite-normalized ree patterns are generally light ree enriched, eu anomaly is present in the most evolved rocks and increases according to the degree of fractionation (fig. 3a). generally the described major and trace element variations range according to fractionation index indicating an origin for most of the evolved rocks by fractional crystallisation in magma reservoirs of the mineral phases observed on the liquidus. constrains on pressure and temperature (p-t) conditions of these reservoirs come from melts and fluid inclusions trapped in phenocrysts which grow in magma chambers. ft-ir analyses on inclusions in phenocrysts (e.g., belkin et al., 1985, 1998; cortini et al., 1985; belkin and de vivo, 1993; cioni et al., 1998; marianelli et al., 1999; cioni, 2000) indicate two different crystallization depths localized at 4 km and > 11 km, and temperatures around 850-900°c and 1200°c, respectively. moreover isotopic variations observed in vesuvius rocks suggest processes other than crystal fractionation and accumulation during magma petrogenesis. in fact 87sr/86sr ratios ranges between 0.7069 and 0.7080; 143nd/144nd ratio vary from 0.51261 to 0.51238 (fig. 3b); although the limited data, a variability is observed also for lead isotope ratios (206pb/204pb = 18.9519.12; 207pb/204pb = 15.62-15.72; 208pb/204pb = = 38.92-39.29) (e.g., civetta et al., 1991; civetta and santacroce, 1992; caprarelli et al., 1993; santacroce et al., 1993; cioni et al., 1995; ayuso et al., 1998; somma et al., 2001). 0.7071 0.7073 0.7075 0.7077 0.7079 0.5124 0.5125 0.5126 143nd/144nd 87sr/86sr 1475 the 3550 year bp-1944 a.d. magma-plumbing system of somma-vesuvius 1 2 3 4 5 6 7 8 9 10 0 0.2 0.4 0.6 0.8 1 1.2 200 400 600 800 1000 1200 1400 1600 0.0 0.5 1.0 1.5 2.0 cao/al2o3 2 3 4 5 6 7 8 9 10 11 0.2 0.4 0.6 0.8 1 100 200 300 400 500 600 700 800 900 0.5 1.0 1.5 2.0 cao/al2o3 na2o (wt%) k2o (wt%) p2o5 (wt%)tio2 (wt%) zr (ppm)sr (ppm) ol + cpx + pl removal c px + f el d+ lc r em ov al m t + b t r em ov al a pa tit e re m ov al leucite accumulation? 1 10 100 1000 la ce pr ndpmsm eu gd tb dy ho ertmyb lu r oc k /c h on d ri te s fig. 2. selected harker diagrams. symbols: open circles = high explosive eruptions of the last 3350 kyrs, full circles = moderate explosive/effusive eruptions erupted in the last 300 years. data from rosi and santacroce (1983); santacroce (1987); civetta et al. (1991); civetta and santacroce (1992); belkin et al. (1993); santacroce et al. (1993); trigila et al. (1993); villemant et al. (1993); cioni et al. (1995); ayuso et al. (1998); del moro et al. (2001) and somma et al. (2001). fig. 3a-b. a) rock/chondrite spiderdiagram (nakamura, 1974) for 1944 a.d. (closed circle) and 1631 a.d. (open circle) eruptions; b) isotope versus isotope diagrams. sr-nd isotope data from cortini and hermes (1981); civetta et al. (1991); civetta and santacroce (1992); santacroce et al. (1993); caprarelli et al. (1993); cioni et al. (1995); ayuso et al. (1998); del moro et al. (2001) and somma et al. (2001). symbols as in fig. 2. a b 1476 lucia pappalardo, monica piochi and giuseppe mastrolorenzo 4. interpretation of petrochemical data and modelling of crustal evolutionary processes figure 4 shows a relationship from the srisotopic ratios of minerals from rocks representative of different eruptions and the crystallization depths. in particular, diopsides derived from magma erupted during the 79 and 1944 a.d. eruptions have sr-isotopic ratios lower than 0.7074 (civetta et al., 1991; civetta and santacroce, 1992; ayuso et al., 1998) and an equilibrium crystallization depth of 1210 km (e.g., belkin et al., 1985; cortini et al., 1985; belkin and de vivo, 1993; belkin et al., 1998; marianelli et al., 1999). in addition, feldspars and salites from magma emitted during the 79 and 1906 a.d. eruptions have higher 87sr/86sr values (0.7075-7) (civetta and santacroce, 1992; santacroce et al., 1993) and an equilibrium crystallization depth of 4 km (cioni et al., 1998; cioni, 2000), moreover leucites have high sr ratios (0.7077) and crystallization depth lower than 1 km (cioni, 2000) (fig. 4). this suggests that the higher sr isotopic ratios are acquired in a late stage of the crystallisation coincident sialic minerals growth at shallow depth. in contrast, the lower sr isotopic ratios shown by diopsides and some whole-rocks could reflect that of the deeper reservoir, probably fed by more primitive magma. the high sr-isotope ratios acquired at shallow depth during crystallisation are consistent with the one inferred for crustal contamination processes, as also suggested by the systematic variations of sr-isotopic ratios with differentiation index (fig. 5a,b). evidence of open-system fig. 5a,b. cao/al2o3 versus 87sr/86sr ratios for high-explosive eruptions of the last 3550 kyr (a) and moderate explosive/effusive eruptions of the last 1631 a.d. (b). sr-isotope data as in fig. 2. dashed lines = mixing trends. a b 0 2 4 6 8 10 12 0.7070 0.7074 0.7078 87sr/86sr d ep th k m fig. 4. sr isotopic ratios of minerals versus equilibrium crystallization depth. symbols: squares = diopside and salite, diamonds = feldspar, triangle = leucite. isotopic data as in fig. 2, pressure data from belkin et al. (1985; 1998), cortini et al. (1985), belkin and de vivo (1993), cioni et al. (1998), marianelli et al. (1999), cioni (2000). 1477 the 3550 year bp-1944 a.d. magma-plumbing system of somma-vesuvius magma processes, such as crustal contamination and/or fluids exchange processes, derives from o-isotope data (turi and taylor, 1976; ayuso et al., 1998) and u-disequilibria (black et al., 1998). plotting cao/al2o3 wt% as fractionation index versus sr isotopic ratios for a complete set of eruptions (fig. 5a-b) shows three main trends. two trends are represented by the evolved rocks (sio2 > 50 wt%) generated during high explosive eruptions: the first shows at decreasing fractionation index against a quite constant sr ratio around 0.7073; the second shows an overall increase in initial sr-isotopic ratios with fractionation from 0.7073 to 0.7077. isotopic disequilibrium between separated minerals and glass is a common feature for these evolved rocks. the third trend is represented by mafic rocks (sio2 < 50 wt%) produced during the last 300 years of moderate explosive/effusive activity and is characterized by an increase in radiogenic sr from 0.7073 to 0.7078 at decreasing fractionation index. isotopic equilibrium has been generally observed between separated minerals and glass. the scattering data observed in fig. 5a,b could be attributed to the existence of isotopic disequilibria, crystal accumulation, magma mixing (dashed lines in figure) different analytical procedures, etc. mesozoic carbonatic succession can be identified as the main source of magma contamination processes. this hypothesis can be inferred on the basis of geological, chemical and geophysical evidence: explosive eruptions which occurred at somma-vesuvius produced volcanic deposits containing carbonatic lithics clearly yielded from the wallrocks in which magma rises and fragments. recently fulignati et al. (1998), del moro et al. (2001) and gilg et al. (2001) recognise that some volcanic magma erupted at vesuvius shows chemico-isotopical evidence of interaction with carbonate country-rocks. furthermore geophysical data identify the mesozoic basement between 2 and not less than 8-11 km of depth (e.g., imbò, 1949; berrino et al., 1998; zollo et al., 1996; 1998), where crystallization of sialic 87sr-rich minerals occurred. 4.1. quantitative modelling of crustal contamination process in order to test the consistence of compositional variations of somma-vesuvius magmas with a crustal contamination model, we adopt the quantitative ec-afc (energy conservation-assimilation fractional crystallisation) approach that accounts for mass and energy conservation (bohrson and spera, 2001; spera and bohrson, 2001). less-evolved samples with highest mgo content and lowest sr-isotopic ratio were selected as magmatic end members (close to the parental magma compositions). the isotopic compositions of these samples are assumed to be unaffected by contamination relative to other samples with low mgo content. carbonatic rocks were also used as end-members (representing the crustal contaminant). the obtained results (fig. 6a,b and table i) show that the isotopic variations are consistent with a crystal fractionation process controlled by the contamination of carbonatic rocks. since melting of limestone is an improbable event at the pt condition of vesuvius magmas, the assimilation was likely a selective process consisting in the inter-exchange between magmas and fluids derived by thermal decomposition (decarbonisation process; tracy and frost, 1991) of the sedimentary wall-rocks. this process has been recently proposed for undersaturated volcanic products from the albani hills on the basis of petrological experiments (freda et al., 2001). in our approach, the values of thermo-chemical parameters (table i) were calculated from the chemistry of rocks and from data on melt inclusions (e.g., belkin et al., 1985; cortini et al., 1985; belkin and de vivo, 1993; belkin et al., 1998; cioni et al., 1998; marianelli et al., 1999; cioni, 2000). calculations based on the above model indicate that the isotopic variations relative to the magmas feeding explosive eruptions (curve b in fig. 6a) are explained by the carbonatic contamination with a ma* (mass of melted wall rock) = 0.10-0.25 and mc (mass of cumulates) = 0.60-0.75. a fractionation mechanism involving sidewall crystallisation and liquid fractionation in the magma chamber (chen and turner, 1980; mc birney et al., 1985) can justify the high mc values despite the crystal content 1478 lucia pappalardo, monica piochi and giuseppe mastrolorenzo fig. 6a,b. ec-afc model following spera and bohrson (2001) and bohrson and spera (2001). lines represent simulations made up using thermal and geochemical parameters listed in table i. symbols and sr-isotopic data as in fig. 2. in fig. 6a, light field indicate the white fallout layers, dark fields indicate the grey fallout layers. observed in most pumice samples. moreover, calculation based on the above model indicates that sr isotopic variations observed in rocks younger than 300 years (curve c in fig. 6b) are consistent with ma* = 0.2-0.4 and mc = 0.60.7. the high mass of cumulates is compatible with the higher crystal content – from 40 to 60% (trigila and de benedetti, 1993; villemant et al., 1993) – observed in lavas. the higher ma*/mo ratio observed in mafic magma erupted in the last 300 years of activity (curve c in fig. 6b) is likely due to their higher t and reduced volume of involved magma that favours the contamination processes. a b 1479 the 3550 year bp-1944 a.d. magma-plumbing system of somma-vesuvius 4.2. evolution of magma plumbing system in the last 300 years and its present state the rocks erupted during the last 300 years of activity show a correlation between sr-isotopic ratios and age (fig. 7). after the 1631 a.d. and until the 1737 a.d. eruption, the sr-isotopic ratio remained constant thus indicating that magma was isotopically and geochemically similar to those erupted during the previous sub-plinian event. from 1737 a.d. to 1805 a.d. more radiogenic (0.708) magma was erupted. then sr-ratio decreases defining two distinct trends, as evidenced by cortini and hermes (1981), below 0.7074 during the 1906 a.d. eruption and below 0.7071 during the 1944 a.d. eruption. this was also evidenced by somma et al. (2001) who reported time-dependent variations for protohistoric, ancient historic and medioeval lavas and tephra also for nd and pb isotopic ratios that table i. ec-afc parameters, somma-vesuvius rocks. thermal parameters compositional parameters (curve b in figure 6a) tlm 1320 °c element sr nd th tm0 1200 °c magma: conc. 1600 35 5 tla 1000 °c bulk d0 2.5 0.25 0.1 ta0 750 °c assimilant: conc. 500 26 10.7 ts 800 °c bulk d0 3 0.25 0.1 cpm 1484 j/kg k isotope 87sr/86sr 143nd/144nd cpa 1370 j/kg k ratio magma 0.7073 0.513 hcry 396 000 j/kg ratio assimilant 0.71 0.5118 hfus 250 000 j/kg (curve c in figure 6b) tlm 1320 °c element sr nd th tm0 1320 °c magma: conc. 1600 35 5 tla 1000 °c bulk d0 0.8 0.25 0.1 ta0 750 °c assimilant: conc. 500 26 10.7 ts 800 °c bulk d0 1 0.25 0.1 cpm 1484 j/kg k isotope 87sr/86sr 143nd/144nd cpa 1370 j/kg k ratio magma 0.7073 0.513 hcry 396 000 j/kg ratio assimilant 0.71 0.5118 hfus 250 000 j/kg terminology from spera and bohrson (2001). tlm = liquidus of magma; tm0 = initial temperature of magma; tla = liquidus of assimilant; ta0 = initial temperature of assimilant; ts = solidus temperature; cpm = magma isobaric specific heat capacity; cpa = assimilant isoba 1480 lucia pappalardo, monica piochi and giuseppe mastrolorenzo show an opposite behaviour with respect to that of 87sr/86sr ratios. consistently with the above interpretation of sr-isotope data, a residual shallower magma chamber was still active beneath the volcano between the 1631 a.d. and 1737 a.d. eruptions. the complete emptying of such a chamber allows mafic magma to ascent directly from the deep reservoir. the increasing isotopic ratios observed in rocks erupted between 1737 and 1805 a.d. likely derives from fractionation and crustal contamination processes at shallow depth in the conduit, consistently with the presence of high sr-isotope ratio leucite (0.7077), crystallized at depth less than 1 km (cioni, 2000). a progressive vanishing of crustal contamination processes occurred between 1805 a.d. and the last 1944 a.d. eruption, as indicated by the continuous decreasing of sr ratios of erupted magmas. this vanishing possibly derives from cooling-induced crystal mush growth along conduit walls or, alternatively, from the rise in the magma ascent rate from the deep reservoir, both reducing magma/rocks exchange. although the two hypotheses are both likely, the latter possibility is consistent with a general increase in eruption explosivity (e.g., arrighi et al., 2001 and references therein) and the emission of poorly crystalline magma (e.g., villemant et al., 1993) between 1805 and 1944 a.d., both characteristics of a degassing in near closed-system conditions (fig. 7). 5. discussion and conclusions the relationship between the depth of magma crystallization and sr-isotopic ratios, interpreted and quantitatively modelled as deriving from crustal contamination processes, indicates that the condition and timing of magma storage in the crust is a major controlling factor of the eruptive activity at somma-vesuvius. less radiogenic (0.70730) mafic crystals have fig. 7. isotopic variations through time after the 1631 a.d. eruption. symbols: open circles = high explosive eruptions of the last 3550 years; full circles = moderate explosive/effusive eruptions of the last 300 years; squares = diopside; triangles = leucite. field defines the compositional range of the 1631 a.d. volcanic products. data as in fig. 2. fig. 8. cartoon showing the supply system for vesuvius reconstructed on the basis of the geochemical and sr-isotopical modeling. 1481 the 3550 year bp-1944 a.d. magma-plumbing system of somma-vesuvius equilibrium crystallization depth exceding 10 km, while more radiogenic (0.7075-0.7077) sialic minerals have shallower equilibrium crystallization depth of about 5 km. this suggests that the magma acquired higher sr-ratios by a process which occurred subsequently to its genesis, when sialic minerals crystallized at shallow depth, in the chamber or in the conduit. magma storage at very shallow depth (likely in the upper conduit) being irrelevant, evidence of high sr-isotope values of leucite at such depths (cioni, 2000) implies that the contamination processes may occur even in a relatively short time. the change in sr-isotopic ratios through the last 300 yr suggests that after the 1631 a.d. sub-plinian eruption and until the last 1944 a.d. eruption no significant shallower reservoir developed in the upper crust (fig. 8). this shallower reservoir probably grew during the long period of quiescence preceding the sub-plinian and plinian eruptions, by the periodical recharge of less contaminated magma rising from deeper storage zones (fig. 8). this shallow reservoir was partially or completely emptied during major explosive events. this magmatological evidence is strongly supported by the results of seismic tomography (e.g., zollo et al., 1996; de natale et al., 2001) which does not reveal seismic evidence of magma storage down to a depth of about 10 km. in contrast, the recurrence of the lower isotopic imprints in the products of most of the somma-vesuvius eruptions suggests that the deep reservoir can be considered a long-lived reservoir acting throughout the volcanic history and furnishing at least 300 km3 (santacroce and civetta, 1992) of magma. consistently with results of the recent seismic tomography (zollo et al., 1996, 1998; de natale et al., 2001), a large (400 km2 wide) reservoir, located at a depth between 8 and 25 km is still present beneath the volcano. acknowledgements authors are grateful to chris kilburn for suggestions that strongly improved the manuscript. references andronico, d. and r. cioni (2002): contrasting styles of mount vesuvius activity in the period between the avellino and pompeii plinian eruptions, and some implications for assessment of future hazards, bull. volcanol., 64, 372-391. arrighi, s., c. principe and m. rosi (2001): violent strombolian and subplinian eruptions at vesuvius during post-1631 activity, bull. volcanol., 63, 126-150. ayuso, r.a., b. de vivo, g. rolandi, r.r. seal ii and a. paone (1998): geochemical and isotopic (nd-pb-sr-o) variations bearing on the genesis of volcanic rocks from vesuvius, italy, j. volcanol. geotherm. res., 82, 53-78. beccaluva, l., p. di girolamo and g. serri (1991): petrogenesis and tectonic setting of the roman volcanic province, italy, lithos, 26, 191-221. belkin, h.e. and b. de vivo (1993): fluid inclusion studies of ejected nodules from plinian eruptions of mt. somma-vesuvius, j. volcanol. geotherm. res., 58, 98-100. belkin, h.e., b. de vivo, e. roedder and m. cortini (1985): fluid inclusion geobarometry from ejected mt. somma-vesuvius nodules, am. mineral., 70, 288-303. belkin, h.e., c.r.j. kilburn, b. de vivo and r. trigila (1993): sampling and analytical chemistry of the recent vesuvius activity (1631-1944), j. volcanol. geotherm. res., 58, 273-290. belkin, h.e., b. de vivo, k. torok and j.d. webster (1998): pre-eruptive volatile content, melt-inclusion chemistry, and microthermometry of interplinian vesuvius lavas (pre-a.d. 1631), j. volcanol. geotherm. res., 82, 79-95. berrino, g., g. corrado and u. riccardi (1998): sea gravity data in the gulf of naples, a contribution to delineating the structural pattern of the vesuvius area, j. volcanol. geotherm. res., 82, 139-150. black, s., r. macdonald, b. de vivo, c.r.j. kilburn and g. rolandi (1998): u-series disequilibria in young (a.d. 1944) vesuvius rocks, preliminary implications for magma residence times and volatile addition, j. volcanol. geotherm. res., 82, 97-111. bohrson, w. and f.j. spera (2001): energy-constrained open-system magmatic processes ii, application of energy-constrained assimilation fractional-crystallization (ec-afc). model to magmatic systems, j. petrol., 42, 1019-1041. brocchini, d., c. principe, d. castradori, m.a. laurenzi and l. gorla (2001): quaternary evolution of the southern sector of the campanian plain and early somma-vesuvius activity, insights from the trecase 1 well, miner. petrol., 73, 67-91. caprarelli, g., s. togashi and b. de vivo (1993): preliminary sr and nd isotopic data for recent lavas from vesuvius volcano, j. volcanol. geotherm. res., 58, 377-381. chen, c.f. and j.s. turner (1980): crystallization in a double-diffusive system, j. geophys. res., 85, 2573-2593. cioni, r. 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(1976): rb-sr and u-th-pb systematics of the alkaline rocks from italy, geochim. cosmochim. acta, 40, 283-295, zollo, a., p. gasparini, j. virieux, h. le meur, g. de natale, g. biella, e. boschi, p. capuano, r. de franco, p. dell’aversana, r. de matteis, i. guerra, g. iannaccone, l. mirabile and g. vilardo (1996): seismic evidence for a low-velocity zone in the upper crust beneath mount vesuvius, science, 274, 592-594. zollo, a., p. gasparini, j. virieux, g. biella, e. boschi, p. capuano, r. de franco, p. dell’aversana, r. de matteis, g. de natale, g. iannaccone, i. guerra, h. le meur and l. mirabile (1998): an image of mt. vesuvius obtained by 2d seismic tomography, j. volcanol. geotherm. res., 82, 161-173. articolo burrato.pdf key words tectonic geomorphology – drainage anomaly – blind thrust – po plain – 1802 oglio valley earthquake 1. introduction alluvial plains are among the most densely populated areas worldwide. this is the result of a combination of morphological, hydrological and geological factors that make them advantageous for human settlements. however, often they are located in active regions, and their flat surface resulting from a high sediment supply from nearby high relief areas may hide blind faults posing a significant seismic hazard. a striking example is the los angeles basin in california, located at the transform pacific margin of the north american plate (wright, 1991). here more than 12 million people live in an area prone to destructive though moderate earthquakes generated by blind or elusive thrusts (e.g., hauksson et al., 1988; dolan et al., 1995; shaw and suppe, 1996). given the hidden nature of these seismogenic faults and related structures, they have been traditionally explored only by means of indirect methods such as interpretation of growth strata (e.g., suppe et al., 1992), geodesy (e.g., lin and stein, 1989; massonet et al., 1993) and morphotectonic studies (e.g., bullard and lettis, 1993). the aim of this work is to present a geomorphological method for the identification of blind active faults associated with subdued topographic expression. we focused on the po plain, a lowrelief sedimentary basin intensely deformed by 865 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. pierfrancesco burrato, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: burrato @ingv.it an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting pierfrancesco burrato, francesca ciucci and gianluca valensise istituto nazionale di geofisica e vulcanologia, roma, italy abstract the po plain is a low-relief area characterised by active shortening accommodated by blind thrust faulting. in this almost flat region depositional rates are similar to tectonic rates and deformation is seldom expressed by noticeable surface anticlines. we adopted a geomorphological approach based on the detailed analysis of the drainage network to identify the location of active thrust faults. a total of 36 anomalies represented by sudden river diversions and shifts in channel pattern were accurately mapped. after comparison with the location of subsurface buried anticlines and of historical seismicity, these anomalies could be related to a tectonic origin and included in a database. their distribution highlights the activity of the buried outer thrust fronts of both the southern alps and the northern apennines. among all the anomalies, we identified one related to the seismogenic source responsible for the 12 may 1802 earthquake (me 5.7), which struck the oglio river valley near soncino (cremona). we propose that this earthquake was generated by an east-west trending, north-dipping, blind thrust fault that roots into the alpine system. if this inference is correct, other faults along the southern alpine margin are potentially seismogenic. buried folds and thrusts, as revealed by extensive oil exploration (fig. 1a,b). in spite of the occurrence of historical earthquakes up to magnitude 6.5, the activity of individual tectonic structures is poorly known. the po plain is one of the most densely populated areas of italy, has a rich patrimony of old historical towns, and represents the most important concentration of industrial facili866 pierfrancesco burrato, francesca ciucci and gianluca valensise fig. 1a,b. a) simplified structural map of the po plain and neighbouring veneto plain, showing the main tectonic elements (from pieri and groppi, 1981, modified), and contours of the base of the plio-quaternary sedimentary sequence (from bigi et al., 1989). the plain is shaded in light grey, while higher topography regions are shown in white. the outer thrust fronts of the southern alps and of the apennines are buried beneath the thick syn-orogenic clastic deposits. the seismogenic sources are from valensise and pantosti (2001b) and are identified in table ii. b) structural cross-section of the central po plain, showing the main buried thrust fronts of the northern apennines and southern alps (from cassano et al., 1986, modified). location of the trace is shown in fig. 1a. ferrarafolds apennines emilia arc romagna folds southern alps milano brescia verona venezia ferrara parma pordenone bologna garda l. idro l. como l. 4000 5000 6000 7000 3000 200 300 400 500 46° 45° 12°1110°8° 30 00 ve ne to pl ain torino v illalvernia-varzi-levanto l. schio-vicenza ad r iatic fo ld s iseo l. 1 2 3 4 5 6 7 8 109 pedealpine homocline main thrust/reverse faults seismogenic sources other faults, mainly reverse base of pliocene surface anticline monferratoarc 0 100km n fig. 4 f ig . 1 b trino vercellese a. s. colombano a. montello a. 0 -5 -10 km po r. adda r. oglio r. southern alps outer fronts apennines outer fronts emilia arc ns a b ties country-wide. in contrast with its high vulnerability, it is traditionally considered a region of low seismic hazard (e.g., albarello et al., 2000). current historical catalogues list very few large earthquakes in the area and show that most of them do not exceed me 5.5 (equivalent magnitude from cpti, working group, 1999). most of the seismicity is concentrated south of the po river along the apennine foothills and on some apennine outer fronts (fig. 2). conversely, the seismicity is more sporadic in time and space north of the po river. no significant historical or instrumental earthquakes are documented west of milan, except for a few events in the monferrato area. since even moderate earthquakes can produce extensive damage due to the high exposure of buildings and facilities, the identification and characterisation of potential seismogenic sources is crucial for mitigating the seismic risk. to locate the areas undergoing tectonic vertical deformation we adopted a geomorphological approach based on the analysis of the pattern and behaviour of the fluvial network. alluvial rivers are very sensitive to modest ground tilting, to which they react with deflections and changes of their sedimentation pattern. we believe this approach is especially promising in areas such as the po plain, where the evolution of the drainage pattern is not affected by morphological obstacles nor controlled by bedrock heterogeneities. only two anticlines are associated with a morphological expression (fig. 1a,b): the trino vercellese anticline (western po plain), which has about 867 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting fig. 2. historical seismicity of the po plain (cpti, working group, 1999). the focal mechanisms of the three largest instrumental earthquakes recorded in the area are consistent with ongoing n-s contraction. in contrast, seismicity recorded in the inner portions of the apennines chain is mainly extensional. only a few earthquakes are reported north of the po river, and seismicity does not follow a regular pattern. 0 100km me=6.8 me=6.0 me=6.5 me=5.5 milano verona ferrara bologna parma alessandria genova firenze venezia adige r. brenta r . po river reno r. oglio r. 9 11.5 45.5 44.5 m in cio ta na ro r . ticino r. adda r. s ec ch ia r . arno r. garda l. iseo l. pavia reggio emilia n po river reggio emilia 1996 parma 1983 parma 1971 1802 valle dell’oglio earthquake 868 pierfrancesco burrato, francesca ciucci and gianluca valensise id r iv er l oc at io n ig m s he et l at n l on g w a no m al y l en gt h a no m al y su bs ur fa ce to po gr ap hi c a ss oc ia te d d is ta nc e c on fi de nc e na m e of a no m al y # ( 1: 10 0 00 0) ty pe of a no m al ou s ra tin g an tic lin e ex pr es si on hi st or ic al fr om of te ct on ic re ac h ea rt hq ua ke (s ) an om al y or ig in (k m ) (m e) (k m ) 1 p o m on ta ro lo ( v c ) 57 45 .1 8 8. 29 c p /d 36 .0 a y y h ig h 2 s es ia p al es tr o (v c ) 57 /5 8 45 .3 0 8. 52 d 22 .4 b m ed iu m 3 a go gn a v es po la te ( n o ) 58 45 .3 3 8. 66 d 13 .9 c y l ow 4 a go gn a o tt ob ia no ( p v ) 58 45 .1 4 8. 77 c p 7. 6 c y l ow 5 l am br o/ s . l am br o s . c ol om ba no ( l o ) 59 /6 0 45 .2 0 9. 48 d /c p 26 .4 a y y h ig h 6 a dd a r iv ol ta d ’a dd a (b g ) 46 45 .4 4 9. 49 c p 13 .3 b y l ow 7 a dd a c or ne gl ia no /c av ia ( l o ) 60 45 .3 1 9. 60 c p /d 48 .7 a y y 17 86 4 7 ( 5. 2) 0. 0 h ig h 8 b re m bi ol o c as al pu st er le ng o (l o ) 60 45 .1 7 9. 65 d 7. 0 c y y m ed iu m 9 s er io b ar ia no ( b g ) 46 45 .3 2 9. 70 c p 8. 7 c y m ed iu m 10 s er io m al pa ga ( b g ) 46 45 .5 4 9. 73 c p 8. 2 c l ow 11 o gl io r om an o di l . ( b g ) 46 45 .5 3 9. 86 c p /d 15 .3 a h ig h 12 m el la p on ca re al e (b s ) 47 45 .4 7 10 .1 2 c p /d 10 .4 b y y m ed iu m 13 m el la b as sa no b re sc ia no ( b s ) 47 /6 1 45 .3 5 10 .1 9 c p /d 13 .0 a y h ig h 14 o gl io s on ci no ( c r ) 60 /7 1 45 .2 0 10 .2 9 d /c p 79 .5 a y y 18 02 5 1 2 (5 .6 ) 9. 8 h ig h 15 m in ci o g oi to ( m n ) 62 45 .2 6 10 .7 0 d 8. 9 b m ed iu m 16 m in ci o m an to va ( m n ) 62 45 .1 7 10 .7 0 d 23 .8 a y m ed iu m 17 a di ge v er on a (v r ) 49 45 .3 4 11 .2 6 d /c p 29 .4 a 11 17 1 3 ( 6. 6) 4. 6 m ed iu m 18 a di ge l eg na go ( v r ) 63 45 .1 6 11 .3 8 c p 14 .9 c l ow 19 p o f ic ar ol o (r o ) 76 44 .9 1 11 .5 9 d 36 .0 b y m ed iu m 20 l am on e a lf on si ne ( r a ) 89 44 .4 4 12 .0 6 d /c p 14 .9 a y 16 88 4 1 1 (5 .7 ) 8. 7 h ig h 21 s en io c ot ig no la ( r a ) 88 44 .3 8 11 .9 5 d /c p 19 .4 a y 16 88 4 1 1 (5 .7 ) 0. 0 h ig h 22 id ic e b ud ri o (b o ) 88 44 .5 3 11 .5 1 d 26 .4 b y 17 96 1 0 22 ( 5. 7) 6. 5 m ed iu m 23 r en o s . a go st in o (b o ) 75 44 .7 7 11 .3 5 d 37 .6 a y h ig h 24 p an àr o b om po rt o (m o ) 75 44 .6 6 10 .9 9 d /c p 30 .0 a y h ig h 25 p an àr o c as te lf ra nc o e m il ia ( m o ) 87 44 .7 9 11 .1 7 c p 32 .6 a y h ig h 26 s ec ch ia m ir an do la ( m o ) 75 44 .7 3 10 .9 9 d /c p 43 .8 a y h ig h 27 p o g ua st al la ( r e ) 74 45 .0 1 10 .6 9 d /c p 28 .6 a y h ig h 28 c ro st ol o r eg gi o e m il ia ( r e ) 74 /8 6 44 .7 0 10 .6 2 d 10 .7 c y m ed iu m 29 e nz a m on ti ce ll i t er m e (p r ) 73 44 .8 1 10 .4 4 c p /d 17 .3 a y h ig h 30 p ar m a p ar m a (p r ) 73 44 .8 6 10 .3 2 c p /d 20 .5 a y m ed iu m 31 a rd a c or te m ag gi or e (p c ) 72 /7 3 45 .0 1 9. 96 d 13 .8 b y h ig h 32 p o c re m on a (c r ) 60 45 .1 4 9. 92 c p /d 36 .4 a y h ig h 33 n ur e s . p ol o (p c ) 60 /7 2 45 .0 0 9. 76 c p 8. 7 c y m ed iu m 34 n ur e s . g io rg io p ia ce nt in o (p c ) 72 44 .9 3 9. 71 c p 5. 0 c y y l ow 35 t re bb ia m an ia co ( p c ) 60 45 .0 6 9. 64 c p 6. 8 c y m ed iu m 36 t id on e p on te ti do ne ( p c ) 60 45 .0 4 9. 53 c p 10 .0 c y m ed iu m 14 38 6 1 1 (5 .5 ) 18 31 9 1 1 (5 .3 ) 18 32 3 1 3 (5 .5 ) 19 71 7 1 5 (5 .6 ) 6. 0 20 .0 t ab le i. l is t of t he d ra in ag e an om al ie s id en ti fi ed i n th e p o p la in a nd p os it iv el y re la te d to a t ec to ni c or ig in ( id n um be rs a s in f ig . 5) . t he m ai n ty pe s of a no m al y th at w er e re co gn is ed a re d ra in ag e d iv er si on s (d ) an d c ha nn el p at te rn s hi ft s (c p ). t he r at in g of th e an om al ie s is b as ed o n th e le ng th o f th e an om al ou s re ac h, on th e ty pe o f an om al y, an d in t he c as e of d iv er si on s it i s al so b as ed o n th e an gl e of d iv er ge nc e be tw ee n th e m ea n fl ow d ir ec ti on a nd t he d ir ec ti on o f th e m ax im um t op og ra ph ic g ra di en t (s ee s ec ti on 4 f or de ta il s) . h ig he r ra ti ng s (a ) w er e as si gn ed t o an om al ie s ch ar ac te ri se d by l on ge r an om al ou s re ac he s an d d iv er si on s (d ) as so ci at ed w it h c ha nn el p at te rn s hi ft s (c p ). h is to ri ca l ea rt hq ua ke s te nt at iv el y as so ci at ed t o ea ch a no m al y ar e sh ow n al on g w it h m in im um e pi ce nt ra l di st an ce f ro m t he a no m al ou s re ac h (m e fr om c p t i, w or ki ng g ro up ,1 99 9) . t he la st c ol um n li st s ho w m uc h w e re ly o n th e te ct on ic o ri gi n of t he a no m al y, ba se d on a ll c on si de ra ti on s. a no m al ie s # . 28 ,2 9 an d 30 o cc ur w it hi n 20 k m o f th e ep ic en tr al l oc at io n of t he f ou r hi st or ic al e ar th qu ak es l is te d in t he n ea rb y ce ll . 40 m of relief and forces the po river to follow an anomalous course (anomaly #.1 in table i); and the san colombano anticline (central po plain), associated with the sharp eastward deflection of the southern lambro river (anomaly #5 in table i). we used basic topographic data (topographic maps and dem) to identify anomalous reaches along several alluvial rivers draining the po plain. in general, anomalies in the drainage pattern may be ascribed to three main families of causes: climate, human activity and tectonic activity. climate changes may induce rivers to cross critical erosional or depositional threshold values controlling major channel pattern changes (schumm and khan, 1972). human activities may induce changes in the fluvial system by direct modifications of the channel (e.g., engineering works and drainage regulation), by indirect alteration of the stream discharge and sediment load transport (knighton, 1984), or by modifying the stream gradient through ground-water withdrawal. tectonic activity produces longitudinal and lateral changes of the stream gradient, that alone may induce channel pattern shifts and diversions. these tectonic modifications, however, may accumulate and become apparent only in conjunction with extreme climatic events altering water discharge and sediment supply. the anomalies we identified consist in river diversions, shifts in channel pattern and longitudinal changes of the channel behaviour. due to their size, they are suggestive of the presence of active buried tectonic structures. their true origin was assessed by spatial comparison between their location and the pattern of buried anticlines reported in the geological maps, and with the historical earthquakes listed in current catalogues. we constructed a database of 36 anomalous reaches along more than 20 rivers. each anomaly is believed to be evidence of the activity of a blind thrust (table i). we could discriminate the seismic versus aseismic behaviour of the underlying fault only for a few cases where the anomaly is unambiguously associated with historical or instrumental seismicity. we present as a case history the application of this approach to the identification of the source of the 12 may 1802 earthquake (me 5.7), that struck the oglio river valley near soncino (cremona). 2. geologic and geomorphic overview of the po plain 2.1. tectonic setting the geologic evolution of the po plain reflects the regional convergence between the african and european plates. this sedimentary basin is located at the northern termination of the adria microplate, a promontory of the african plate. since late cretaceous it represented the foreland of two chains of opposite vergence generated by the collision between the african and european plates (e.g., robertson and grasso, 1995): the north-verging apennines and the south-verging southern alps. thrusting of these two chains loaded and flexed the adria continental crust, giving rise to foredeep basins where a thick syn-orogenic clastic sequence was deposited (doglioni, 1993, and references therein). as suggested by plate motion reconstructions (demets et al., 1994), and confirmed by seismicity (cpti, working group, 1999) relative motion between the two plates is ongoing with a convergence in a nnw-sse direction, that geodesy indicate to have a rate of less than 1 cm/yr (ward, 1994; anzidei et al., 2001). part of this deformation is accommodated seismically within the thrust fronts of the two chains. due to the fast subsidence rates induced in the whole po plain area by the tectonic loading of the two chains, the more external fronts of the two thrust belts are buried beneath the plain. therefore the main tectonic elements were mapped by means of the extensive seismic reflection data used for oil exploration (pieri and groppi, 1981). the map of the base of the plio-quaternary sequence south of the po river (pieri and groppi, 1981) shows the occurrence of three arcs of blind, north-verging thrusts and folds that define the apennine thrust fronts (fig. 1a,b), from west to east: a) the monferrato arc (elter and pertusati, 1973); b) the emilia arc, and c) the ferrara-romagna arc. the latter is further subdivided into three relatively minor structures: the ferrara folds, the 869 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting romagna folds and the adriatic folds. the ferrara folds («dorsale ferrarese») are the most external structures of this arc. north of the po river, the outer fronts of the southern alps thrust belt are arranged in a more simple pattern and deform the south-sloping pedealpine homocline (fig. 1a). the e-w continuity of the thrust front is interrupted by the schio-vicenza line, an important transfer structure that separates the po plain thrust fronts to the west, from those of the veneto plain, to the east. in spite of the diffuse folding and thrusting imaged in the subsurface of the po plain by the geophysical exploration, very few tectonic structures are expressed at the surface. the most noticeable exceptions are represented by the trino vercellese anticline, the san colombano anticline and the montello anticline in the veneto plain (from west to east, fig. 1a). the quaternary activity of these structures is testified by their influence on the drainage pattern and by the presence of young terraced surfaces tilted and uplifted along their flanks (ferrarese et al., 1998; benedetti et al., 2000; castiglioni and pellegrini, 2001). during the tertiary and the quaternary, a considerable thickness of sediments was deposited in the po valley basin, on top of a sequence of mesozoic passive margin carbonatic rocks. as a whole this clastic sequence is a regressive succession. sediment sources included the southern alps and apennines fold and thrust belts. on the southern side of the po plain, the plio-quaternary sediments in the foredeep are 7-8.5 km thick (doglioni, 1993). sediments in the southern alps foredeep, are between 2 and 6 km thick (doglioni, 1993; bertotti et al., 1998). 2.2. seismicity most of the earthquakes listed in current historical catalogues are concentrated south of the po river along portions of the pede-apennines thrust front and on some buried apennines outer fronts. earthquakes are more infrequent north of the po river and west of milan, where only background micro-seismicity is recorded (fig. 2). the most recent large earthquakes of the region occurred in 1971 and 1983 near parma (ms 5.7 and 5.0, respectively), and in 1996 near reggio emilia (ms 5.1). the focal mechanism of the 1971 parma event shows a reverse solution with a large strike-slip component (anderson and jackson, 1987). the centroid moment tensor solutions of the 1983 parma and 1996 reggio emilia events (cmt harvard catalogue, dziewonski et al., 1983) show a thrust solution with a small strike-slip component, and with p-axis oriented n36°w and n24°w, respectively. this compressional tectonic regime is confirmed by the analysis of the reggio emilia main shock and related aftershocks (selvaggi et al., 2001). the focal mechanisms are consistent with a n-s sub-horizontal 1. similar results were reached by frepoli and amato (1997), who obtained n-s oriented compressional p-axes using fault plane solutions, and montone and mariucci (1999) with an analysis of borehole breakouts, that show shmax oriented perpendicular to the trend of the thrust fronts. other recent instrumental earthquakes include the 2000 reggio emilia event (mw 4.5), the 2000 faenza-forlí sequence (mw 3.4-4.4), and the 2000 monferrato event (mw 4.6). 2.3. geomorphology the po plain stretches e-w across northern italy for more than 40.000 km2, and is the widest alluvial basin of the peninsula. it is delimited both south and north by the topographic highs of the apennines and of the alps, respectively, and to the east by the adriatic coastline. the plain is locally up to 100 km wide, and is drained axially by the 652 km long po river. for most of its course the po is an alluvial river characterised by a single meandering channel. its catchment basin is larger than 70.000 km2. the po plain can be separated into several quaternary geomorphological units (murst, 1997a; marchetti, 2002, and references therein). the area located between the southern boundary of the alps and the po river is characterised by the presence of a wide low-gradi870 pierfrancesco burrato, francesca ciucci and gianluca valensise ent, south-sloping depositional surface of fluvio-glacial and fluvial origin. this outwash plain, locally named «plain main level» (livello fondamentale della pianura in petrucci and tagliavini, 1969), was built up during the last glaciation, when rivers flowing from the alps were characterised by water discharge and sediment supply substantially larger than today (marchetti, 1990, 1996). abandoned paleo-channels seen on top of this surface are oversized with respect to the present hydrography, implying a change in discharge occurred at the end of the last glaciation which led to the erosion of the alluvial plain (marchetti, 1996). the present drainage network flows in valleys deeply incised into the aggradation surface. small patches of early to middle pleistocene fluvioglacial deposits («old terrace unit», marchetti 2002), and moraine deposits of pleistoceneholocene age, near the outlets of the main alpine valleys, border the po plain at the alps foothills. south of the po river the drainage network flows on top of a holocene aggradation surface, which merges with the active alluvial plain of the axial po river. continuous sedimentation is suggested by buried archaeological artefacts ranging in age from the neolithic period to the middle ages (marchetti, 2002). close to the apennine margin this surface is covered by coalesced alluvial fans of the same age as the «plain main level», overlain by older fans deposited during previous glacial maximums (marchetti, 2002 and reference therein). in contrast with the rivers flowing from the alps, the po river and its right tributaries flow on top of an active alluvial plain. 3. geomorphic indicators of growing anticlines growing anticlines and tilting over a wide region encompassing an anticline-syncline pair are the surface expressions of slip on an underlying buried thrust or reverse fault (e.g., yeats, 1986; stein and yeats, 1989) (fig. 3). there are numerous geological, geomorphic and landscape features that are sensitive to slip on a buried thrust fault: topography, the thickness and shape of growth strata, the erosional/depositional behaviour of the drainage network, and the geometry of any erosional and/or depositional surfaces. the drainage network is especially sensitive to vertical deformation and thus very useful for analysing low-relief areas, where few or no landforms constrain the location of active faults (holbrook and schumm, 1999; schumm et al., 2000). field studies and numerical models show that the interaction between drainage systems and tectonic sources is governed mainly by the geometry and kinematics of the tectonic source (e.g., champel et al., 2002) and by 871 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting subsidence deposition attraction of drainage tilting uplift erosion antecedence uplift diversion tilting 20° antecedence diversion dranage piracy 0 2 4 6 5 10 15 20km k m a n t ic l in e s y n c l in e diversion growth strata fig. 3. schematic representation in map view (above) and cross-section (below) of the effects that slip on a generic blind, low-angle thrust fault may have on geomorphic/sedimentary processes. the vertical component of the expected deformation is related to the fault’s geometry and kinematics through the elastic dislocation theory. evolution of sedimentation and drainage pattern over a wide area is controlled by the growth of the anticline/syncline, which produces tilting of the ground surface. the relative size of the rivers (e.g., guccione et al., 2002). fault characteristics control the pattern and style of surface deformation, whereas the capacity of the rivers to keep pace with the uplift relies on their size (see for example the longitudinal stream profiles in burnett and schumm, 1983). the fault-induced permanent deformation of the earth surface controls the drainage pattern and the associated geomorphic/sedimentary processes. in particular the uplifted area is subjected to erosion, lack of drainage and antecedence phenomena, while the subsiding area is subjected to attraction of drainage and deposition (fig. 3). river diversions are the most clear indicators that the stream is approaching a rising anticline or entering a syncline. the evolution of sedimentation and topography and of the geometry of geomorphic features is controlled by the growth of the anticline and of the syncline, which may or may not have a perceptible morphologic expression depending on the ratio between tectonic and sedimentation rates. more specifically, if the rate of vertical tectonic displacement is higher than the sedimentation rate, the anticline will have a morphological expression and will be subjected to erosion, while the syncline will be represented by a depressed area that may be filled up with sediments. the evolution of the tectonic topography created by the growth and lateral propagation of the anticline is controlled by linear and areal surface processes that tend to create a smoothed steadystate morphology (champel et al., 2002). when a river enters a zone of active subsidence or uplift it may change its graded longitudinal profile. the river may then cross the zone undergoing deformation, or be deflected by it (holbrook and schumm, 1999). the river incises the area of steepened gradients over the growing structure and aggrades where the slope again decreases downstream. similarly, the increase or decrease in slope caused by an impeding zone of uplift or subsidence may alter the channel pattern of a river (braided, meandering or anastomosing) and its stream power. variations of the stream power have a direct effect on the grain size of the stream bedload. in general, tectonically increased slopes will be characterised by increased bedload grain size, and vice versa (ouchi, 1985). deflection of the river around an area undergoing uplift or subsidence will appear as an abrupt shift in the river course coincident with the deformed zone (holbrook and schumm, 1999). in addition to the deformation seen in a longitudinal profile, tectonic activity may also produce lateral shifting of a river (normal to the topographic gradient of the floodplain). shifting may occur by sudden avulsion or slow migration (combing) of a stream toward the lower down-tilt part of the floodplain depending on the rate of lateral tilt (peakall et al., 2000). schumm and khan (1972) performed a series of experiments in a large flume to determine the effect of changing slope and sediment load on channel patterns. these experiments suggest that sediment loads and slopes are closely related, and that landforms may not always respond progressively to altered conditions. rather, dramatic morphologic changes may occur abruptly when critical erosional and (or) depositional threshold values are exceeded. all of the above effects may be recorded in the alluvial stratigraphic sequence, and their detailed study allows areas of persistent anomalies to be located and characterised. unfortunately, such anomalies may also arise from non-tectonic processes that induce variations of the topographic gradient. for this reason, ruling out other explanations for the observed anomalies is important before they can be confidently interpreted as indicators of tectonic activity (holbrook and schumm, 1999). 4. the case of the po plain: river anomalies versus subsurface data and seismicity very few anticlines in the po plain have a surface expression, probably because sedimentary rates outpace tectonic rates. for example, vittori and ventura (1995) proposed late pleistocene-holocene sedimentation rates of 0.6-2.1 mm/yr for the central part of the basin. these estimates compare with preliminary slip and vertical displacement rates of 1.7 and 0.6 mm/yr, respectively for the fault driving the mirandola anticline, one of the fastest growing 872 pierfrancesco burrato, francesca ciucci and gianluca valensise structures of the entire po plain (ciucci et al., 2002; shown as n. 4 in fig. 1a). the geomorphological map of the po plain (murst, 1997a) shows numerous examples of drainage diversion which in some cases have been dated by radiometric and archaeological methods. this geomorphological record extends back in time to the beginning of the holocene at the earliest, when morphogenic processes triggered by the last glacial maximum formed the «plain main level» on the northern side of the po plain and started continuous fluvial aggradation on its southern side (marchetti, 2002). in this context, several rivers show an evolution of their pattern that is not random, but rather seems to be driven by long-lasting vertical motions that resemble the surface effects induced by the activity of blind thrusts. a striking example is given by the po river just south of mantova, whose course has been shifted northward possibly due to the activity of the ferrara folds, as suggested by the abandoned paleochannels south of its present position. we analysed systematically the response of drainage to the tilt induced by folding. the drainage response varies in trend and strength depending on the location of each given stream section relative to the fault. to reproduce the indications arising from the geomorphological analysis, which constrain the location of the anticline/syncline couple, we use elastic dislocation modelling. our approach assumes permanent deformation of an essentially unfractured uppermost portion of the crust, such that the direction and magnitude of tilting can be derived from the fault parameters via elastic dislocation theory. our starting assumption is that where folds are not expressed at the surface and a thick alluvial bed isolates the drainage network from the fold, rivers should generally flow in the direction of the maximum topographic gradient. regional topography of the po plain is indicated by contour lines that open eastward toward the adriatic coastline, and by a central trough occupied by the po river (see the colour coded topography in fig. 2). this pattern is the compounded result of regional uplift of the northern apennines (along a nw-se axis) and southern alps (along a roughly e-w axis), infilling of the basin and subsequent eastward regression of the coastline. local modifications of the regional slope are highlighted by drainage anomalies of the same wavelength: drainage in subsiding areas is marked by low slope and aggradation, whereas in uplifting zones by increased gradient and incision. we adopted a quantitative method to locate areas undergoing local differential vertical motions, based on the detailed analysis of the topography and of the drainage network. to perform this analysis we used official topographic maps (igmi, scale of 1:25.000), the regional map of relief and vertical movements of the po plain (scale 1:250.000) (murst, 1997b), and digital elevation models with the resolution of 230 m. the methodology used to determine the presence of an anomaly in the flowing direction of the drainage network is shown in fig. 4. it consists in determining the average contour strike of the surface of the plain main level (the depositional surface before the last deglaciation), with tangential segments along each contour line. the tangentials were taken at the down-slope convexities of the contour lines to reproduce with a maximum accuracy the preerosional surface. maximum topographic gradient (red vectors in fig. 4) represents the direction orthogonal to these tangential lines, and we assume that drainage should flow in this direction. we then compared the flow direction of each river between the contour lines (dashed blue vectors in fig. 4), with the topographic gradients. we conventionally declared a drainage anomaly any time we found a divergence 10° between these two vectors. we also considered the length of the reach over which the divergence persisted, adjusted to the river size. this is important to avoid sampling natural irregularities (such as large meanders) as anomalous reaches, and also because we expect that the size of the tectonically driven anomalies is related to the size of the underlying hidden fault. we declared an anomalous reach only if the divergence between the two vectors persisted for more than 5 km. this minimum length was chosen considering the average length of the longer meanders measured in the area. besides being more easily detectable, larger anomalies imply bigger faults that may pose the largest hazard. for some rivers along which we identified such diversions, we searched for 873 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting channel pattern shifts, using topographic maps and air photos interpretation. we then removed all anomalies possibly correlated with human activity (artificial deviations, significant fluid withdrawal, etc.) or for which a tectonic origin can be easily ruled out, using ancient topographic maps and historical accounts. the remaining anomalies are listed in table i, that summarises their main features and possible association with historical earthquakes. geological maps (1:100.000 scale) were used to compare spatially the anomalies with the position of structural axes, buried faults and lithological contacts. this step allowed us to correlate anomalies with buried tectonic structures. a further spatial comparison was made with the historical earthquakes listed in the catalogo parametrico dei terremoti italiani (cpti, working group, 1999). this second step suggested that the activity of at least some of the structures is potentially seismogenic. the observation of seismicity associated with a given anomaly supports the hypothesis of its tectonic origin. some of the areas where the above correlations appear to be most meaningful are shown in fig. 5. areas outlined by black dotted circles exhibit strong correlation between drainage anomalies and buried anticlines (indicated with a and b, respectively). areas indicated with a 874 pierfrancesco burrato, francesca ciucci and gianluca valensise fig. 4. outline of the method used to detect drainage anomalies in the po plain. the figure shows an area located north of the po river, between the adda river and the mella river. straight-line segments at contour line intervals ranging between 5 and 10 m were constructed from the map of relief and vertical movements of the po plain (murst, 1997b). solid red arrows and dashed blue arrows show topographic gradients and actual average drainage directions, respectively. we defined a drainage anomaly as a 10° divergence between the two vectors. anomaly #.14 on the oglio river is analysed in detail. a and b mark the ends of profiles shown in fig. 7. 0 5 10 15 20 km n anomalous drainage sections maximum topographic gradient average direction of drainage contour line (meters) paleochannel of oglio river 80 120 110 100 90 70 60 65 85 75 a b anomaly #12 poncarale (bs) mella river anomaly #13 bassano bresciano (bs) mella river anomaly #6 rivolta d’adda (bg) adda river anomaly #7 cornegliano-caviaga (lo) adda river anomaly #14 soncino (cr) oglio rivero glio river paleo-channel soncino orzinuovi also exhibit some correspondence with historical seismicity. areas shown as red dotted circles do not show any clear correlation and hence require further investigations. this is the case of mantova (mincio river, anomaly # 16), where a clear topographic and drainage anomaly is not related to any known structural element nor to historical earthquakes. in the area between the secchia and panaro rivers, the anomalies (#.24 and #.26, fig. 5) are positively related with known large subsurface structures but there is no report of significant earthquakes. in the case of verona (adige river) the observed anomaly, can not be correlated to any known buried anticline, although it may be associated to slip on the fault responsible of the large 1117 earthquake (me 6.6; cpti, working group, 1999). 5. a case history: anomalies of the oglio river and the 1802 earthquake the anomaly under examination (#.14, table i and fig. 5) is represented by an abrupt southeastward turn of the oglio river south of soncino and orzinuovi, towns located 35 km to nw of cremona (figs. 4 and 5). a paleochannel located south of the anomaly suggests a structural control on the river evolution (fig. 4). this section of the oglio river exhibits anomalies both in terms of trend (sw diversion) and overall behaviour (increase of 875 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting fig. 5. distribution of drainage anomalies in the po plain (marked in yellow). dashed black circles labelled a outline areas where an anomaly, a fold axis, and a historical earthquake coincide. dashed black circles labelled b indicate areas where surface evidence corresponds to buried anticlines, but no historical earthquakes are reported. dashed red circles highlight areas that require further investigations: see text for discussion. drainage anomalies are numbered according to table 1. erosion power and sinuosity). these observations may all be explained with the presence of an area undergoing relative uplift affecting the river bed, and located south of the diversion. to explore the longitudinal behaviour of the oglio river we used a set of three different types of profiles (fig. 6a-c): a longitudinal profile of the river bed, a topographic profile of the river banks, and a differential profile (difference between the previous two profiles). the differential profile was calculated to highlight the possible existence of reaches along which the river is actively eroding its bed, as a consequence of localised uplift. the first two profiles show that the river flows in a valley incised into the plain main level by about 10 m (fig. 6a), whereas the third profile highlights that the differential relief reaches a maximum in the distance range 30-55 km, where relative uplift is inferred to be greatest. most of the valley incision occurred at the end of the last glaciation, and it was the result of changes of water discharge and sediment supply. the two reaches up-stream and down-stream of the diversion are characterised by rather different longitudinal gradients, the higher being along the first reach. instead, the topographic profile along the stream banks shows that there is a change in slope at about down-stream distance 20 km, that may correlate with the toe of the alluvial fan of the oglio river. up-stream of this point the profile is parallel to that of the river bed, indicating uniform erosion along this reach. we interpret the increase of the trend of the differential profile between distance range 30-55 km, as the result of localised uplift due to the growth of a buried anticline (fig. 6c). an additional indication comes from the longitudinal sinuosity profile (fig. 6b), which shows an increment at the beginning of the section characterised by the increase of relief, indicated by the presence of two peaks of high sinuosity. sinuosity was calculated as the ratio between the true (dt) and straightened (ds) distance covered by the river along a given stretch, averaged over a distance of about 1 km sinuosity index = dt /ds. we interpret the two peaks of high sinuosity as a shortening of the wavelength of the meanders due to an obstacle to their down-stream migration. a strong earthquake (me 5.7) occurred on 12 may 1802 in the oglio valley, the flat area 876 pierfrancesco burrato, francesca ciucci and gianluca valensise vertical displacement differential profile longitudinal profile sinuosity -0.1 0 0.1 0.2 0.3 0.4 m 0 10 20 30 40 50 60 70 80 distance along river (km) 0.8 1.2 1.6 2.0 2.4 50 90 130 170 210 m a.s.l. topographic profile 5 9 13 17 21 m fig. 6a-c. profiles of the oglio river from the alpine frontal moraines to the confluence with the mella river. a) the differential profile, obtained by subtracting the elevation of the river bed from the average elevation of the plain main level, shows a dramatic increase of incision in the distance range 30-50 km, suggesting that the river meets an area of localised uplift. b) an increment of sinuosity highlighted at about 30 km from the upper-end of the section is possibly related to a variation of the longitudinal slope of the river bed. c) expected vertical displacement along the river bed generated by slip on a blind, 30°-north dipping thrust fault (full faulting parameters and location of the modelled fault shown in fig. 8 and table ii). this fault explains the geomorphic observations (longitudinal anomalies of the oglio river as well as its se-diversion) and fits subsurface geological data (see upper-left inset of fig. 7). we suggest that this fault is the causative source of the 1802 earthquake. c a b between brescia and cremona near soncino (fig. 7). over twenty towns were severely damaged, including soncino, crema, brescia and cremona, and among those orzinuovi was the most seriously hit. a solution obtained through the automatic elaboration of macroseismic intensity data using the boxer code (gasperini et al., 1999) yielded a hypothetical seismic source (fig. 7). this solution is in good agreement with indications supplied by subsurface geology, particularly as far as the orientation of the seismogenic structure is concerned. e-w buried anticlines belonging to the alpine domain are commonly reported just south of a major drainage anomaly of the oglio river (#.14 in table i and fig. 5), which starts near soncino and continues southward for approximately 25 km. a geological section obtained from geophysical data (cassano et al., 1986; inset in upper left corner of fig. 7) clearly shows the south-verging alpine fronts which, near cremona, face the north-verging thrusts of the apennines domain. assuming that the above geomorphological observations can be explained by the activity of the causative fault of the 1802 earthquake, we calculated the expected displacement for uni877 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting vi-vii v vii vii vii vii vii viivii-viii vii-viii vii-viii v-vivi v vi-vii vi-vii vii-viii vii viii-ix vii vii-viii v-vi 0 10km lodi milano pavia bergamo brescia cremona iseo la ke o g lio r . m e ll a r . l a m b ro r . s o u h e rn l a m b ro r . ticino r . po r. p o r . o glio r. 12 14 32 5 8 7 soncino orzinuovi adda r. from intensity data length: 8.1 km width: 6.4 km azimut: 267°/87° me: 5.7 cpti solution me=5.7; io=viii length: 10 km width: 6 km azimut: 266° dip: 30° depth: 3-6 km (min-max) this work hypothesised causative fault of the 1802 oglio valley earthquake 0 -5 -10 km po r. adda r. oglio r. apennines outer fronts southern alps outer fronts fig. 7. intensity data for the 12 may 1802, me 5.7 earthquake (boschi et al., 2000), the oglio river anomaly (#.14), and buried anticlinal axes from geological maps. two fault models respectively obtained from intensity data (using the boxer code, gasperini et al., 1999) and geomorphological observations (this work) are shown for comparison. the black line is the trace of the geological section obtained from geophysical data shown in the upper left corner (from cassano et al., 1986, modified). it shows that in this area the south-verging alpine front faces the north-verging apennine outermost fronts. we propose that the seismogenic source of the 1802 earthquake is a south-verging blind thrust fault belonging to the alpine system. tary slip on a south-verging, 30°-north dipping, blind thrust fault, extending from 3 to 6 km of depth (table ii). the vertical displacement that is expected to be generated by slip on such fault is shown in fig. 6c and 8. the size of the modelled fault is upper bounded by the magnitude of the 1802 earthquake, while its exact position is constrained by geomorphological observations along the oglio river. we changed progressively the position of the modelled fault until the maximum of relative uplift (anticlinal axis) coincided with the maximum of the differential profile, and could explain the trend of the river itself and the presence of a paleochannel south of the divergence. fault strike was constrained based on the average strike of the buried structures reported in the literature. the contours of the expected displacement of the fault model show an anticlinal uplift south of the diversion, and only slight subsidence north of it (fig. 8). simple scaling relationships suggest that coseismic slip in 1802 was less than 0.5 m, yielding maximum surface vertical displacements of about 0.15 m. our understanding is that, after running into an anticline which slows down its natural course, the oglio river was forced to leave the paleochannel and turn to the ese. this diversion presumably occurred before the incision of the plain main level started, because the reach down-stream the diversion is incised and the trace of the abandoned paleo-channel is on top of this surface. the extent of the diversion represents a balance between the stream power and the uplift rate of the anticline. the seismogenic sources inferred from macroseismic intensity data and geomorphic data do not completely overlap. the macroseismic source may have been influenced by the overestimation of intensity due to the structural weakness of buildings near soncino at the time of the earthquake, as reported in catalogue of strong italian earthquakes, 461 b.c. to 1997 (boschi et al., 2000), or by an incomplete distribution of the data set. 6. discussion and conclusions the morphological analysis carried out in this work revealed the occurrence of several anomalies of the drainage network in the po plain consisting mainly in river diversions. changes in the channel pattern were highlighted in the few areas studied in better detail. to investigate their origin we compared their position and extent with the location of buried anti878 pierfrancesco burrato, francesca ciucci and gianluca valensise table ii. parameters of the seismogenic sources of the po and veneto plains included in the database of potential sources for earthquakes larger than m 5.5 in italy (valensise and pantosti, 2001b). each source is keyed to fig. 1a,b through a code. the orzinuovi seismogenic source (#.1) is related to the 12 may 1802 oglio valley earthquake, and was identified through the geomorphological approach presented in this work. its geometrical and kinematic parameters were used for the dislocation modelling (see fig. 8). # source name length width strike dip rake min depth max depth km km km km 1 orzinuovi 10.0 6.0 266° 30° 90° 3.0 6.0 2 mantova 10.0 6.0 262° 30° 90° 3.0 6.0 3 adige plain 15.0 8.5 255° 30° 90° 3.0 7.3 4 mirandola 12.0 10.0 115° 30° 90° 3.0 8.0 5 bagnacavallo 10.0 6.0 119° 30° 90° 3.0 6.0 6 asolo 27.0 9.0 149° 80° 170° 1.0 9.9 7 montello 15.0 8.0 234° 30° 90° 2.0 6.0 8 alpago 18.0 9.0 206° 55° 50° 1.0 8.4 9 cansiglio 12.0 7.3 230° 50° 64° 1.0 6.6 10 pordenone n. 8.0 5.5 215° 30° 80° 2.0 4.8 clines seen in commercial seismic sections or reported in geological maps. we suggest that the anomalies included in our database represent the surface evidence of growing anticlines driven by blind thrusts. however, only occasionally these anticline do have a discernible morphological expression, probably due to the competition between tectonic rates of vertical displacement and sedimentary rates. we suggest that slip on the blind thrust faults induces slight tilting of the surface that is enough to cause the deflection of the river network, even though it does not produce morphological anticlines. we included in our database 36 drainage anomalies (fig. 5 and table i), most of which occur in the southern part of the plain along tributaries of the po river flowing down from the apennines. all the anomalies except for four (#.1, #.2, #.3, and #.4) occur in the stretch of the plain east of milan. the activity of the buried thrust fronts seems to have controlled also the po river (#.1, #.32, #.27 and #.19), which exhibits shifts in channel pattern (e.g., anomaly 879 an inventory of river anomalies in the po plain, northern italy: evidence for active blind thrust faulting + o glio r . paleo-channel erosional behaviour high sinuosity se ism ic se ctio n apenni nes fro nt southern alps front -0.01 -0.02 -0.03 -0.04 0.0 0.1 0.2 0.3 n surface projection of fault plane intersection of fault plane with surface isolines of expected vertical displacement buried anomalous drainage sections 10 km orzinuovi source length: 10 km width: 6 km min depth: 3 km max depth: 6 km azimut: 266° dip: 30° rake: 90° fig. 8. summary map showing the position of the proposed causative fault of the 12 may 1802 earthquake, contours of the expected vertical displacement and comparison with the geomorphic observations. since this is a qualitative test of fault activity, all displacements are referred to unitary slip. due to the lack of reliable strain and chronological markers, we did not attempt to estimate true cumulative slip and slip rate. the contours express the ratio between slip at depth and surface displacement (e.g., for 1 m slip top uplift is 0.3 m). positions of the buried anticlinal axes and the paleo-channel south of the river diversion are shown. the geometry and location of the modelled fault are constrained using both geomorphological and subsurface geology observations. the abandonment of the paleochannel and the longitudinal anomalies observed along the oglio river are induced by fault-related tectonic uplift south of the diversion. topography is from the map of relief and vertical movements of the po plain (murst, 1997b). #.32) and sudden diversions with abandoned paleo-channels (e.g., anomaly #.27). the correlation of historical seismicity with some of these faults suggests that at least in some cases they slip seismically. the simple consideration that even in the most active areas of peninsular italy the average return time for large earthquakes is longer than 1000 years (e.g., fig. 1 in valensise and pantosti, 2001a) suggests that current historical catalogues, which are considered complete for earthquakes larger than m 5.5 only after the 17th century a.d., may not cover a full seismic cycle for most of the seismogenic sources of the po plain, which could well be the site of very infrequent damaging earthquakes. a comparison between long-term (upper pleistocene) and short-term vertical deformation rates, calculated using geomorphic correlations and repeated geodetic measurements respectively (de martini et al., 1998), indicates that a large fraction of the strain is released aseismically for some of these active thrusts. if extended regionally, this observation would greatly reduce the seismic hazard of the po plain. this inference is supported by the difference between the large number of drainage anomalies that may be explained by the activity of blind thrusts, and the few large earthquakes listed in current catalogues. we applied in detail a morphological analysis to the drainage anomaly shown by the oglio river near soncino (cr) (anomaly #.14). here we found a potential correlation among a river diversion, the longitudinal anomalies of the river bed (fig. 6a), the presence of a buried anticline, and the occurrence of a me 5.7 earthquake in historical times (fig. 7). using simple dislocation model we speculate on the position and extent of the area undergoing differential vertical displacements. according to our hypothesis the geomorphic signal can be explained by the coseismic growth of an anticline located south of the diversion, driven by a south-verging, north-dipping blind thrust (fig. 8) belonging to the southern alps system. this fault model (orzinuovi source) has been included in the database of potential sources for earthquakes larger than m 5.5 in italy (valensise and pantosti, 2001b) as a contribution for the seismic hazard assessment of the po plain. four additional sources identified using the same method exist in the area (fig. 1a,b), but only one of them was correlated to an historical earthquake (bagnacavallo source, #.5 in fig. 1a and table ii, associated with anomaly #.20-21 and 11 april 1688 earthquake in table i). acknowledgements part of this study is the result of the research done by f.c. during her m.s. thesis. andrew meigs, daniela pantosti, bob yeats and an anonymous reviewer are thanked for their thorough revision and for improving the paper. references albarello, d., v. bosi, f. bramerini, a. lucantoni, g. naso, l. peruzza, a. rebez, f. sabetta and d. slejko (2000): carte di pericolosità sismica del territorio nazionale, quad. geofis., 12, 1-7. anderson, h.j. and 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(1986): active faults related to folding, in active tectonics, edited by r.e. wallace (national academy press, washington), 63-79. 882 pierfrancesco burrato, francesca ciucci and gianluca valensise annals 47, 6, 2004def 1827 annals of geophysics, vol. 47, n. 6, december 2004 key words historical earthquakes – british seismicity – scotland – england – border seismicity 1. introduction the purpose of this paper is to examine the early historical seismicity of the scottish borders, that is, southern scotland and northern england. the main intent of the paper is to examine the problems pertaining to the pre-newspaper period (before about 1700), but as some events relevant to the overall discussion take place in the first half of the 18th century, the terminal date for this study is set at the year 1750. the area to be considered is shown in fig. 1. the northern limit is roughly the southern edge of the midland valley of scotland; the southern limit is the southern extent of the english lake district. this area contains four nuclear power plant (npp) sites – thus the historical seismicity is of considerable practical interest. 2. character of the study area the problems of investigating seismicity in border areas are well known (stucchi, 1993). the area under consideration here is notable in that it lost its status as a border between two independent countries. from early times scotland and england were separate countries with separate royal houses. the two countries were frequently at war with one another, with military incursions going both ways across the border. even in times of peace, the border between the two states was a notoriously lawless place where banditry was common and local lords ruled their lands as they pleased by armed force. however, in 1603 the english queen, elizabeth, died without issue. the throne passed to her nearest relative, james stewart, whose great-grandmother was the sister of elizabeth’s father. however, james also happened to be king of scotland (james vi). when james accepted the crown of england, the two countries became united at least as regards head of state, although in most respects they remained two individual countries with their own currencies, parliaments and laws. proper political union didn’t occur until 1707 when the english and scottish parliaments were merged. although, to paraphrase james vi, the borders of the realm became its middle, the characearly seismicity of the scottish borders region roger m.w. musson british geological survey, edinburgh, u.k. abstract this paper considers the seismicity of southern scotland and northern england up to the year 1750. this area was formerly a border area between two states that eventually became politically united. much of the area is uplands, and the seismicity is moderate to low. this makes for some problems in studying historical seismicity, yet the area provides a number of case studies of general interest in the field of historical seismology, including a rare case of being able to track down a «missing» earthquake. mailing address: dr. roger m.w. musson, british geological survey, west mains road, edinburgh eh9 3la, u.k.; e-mail: rmwm@bgs.ac.uk 1828 roger m.w. musson ter of the scottish borders area remained that of a region predominantly of wild upland terrain, much of it sparsely populated. any study of historical seismicity has to take this into account. 3. source materials for the early period in the study area, up to around the 15th century, the predominant (if not the only) source of information on seismicity is provided by monastic chronicles. the presence or absence of information on earthquakes in these documents needs to be treated carefully. one of the first rules of historical research is that any document used as a source should be read with consideration of the purposes for which it was written. this is especially true in the case of historical seismology because most of the material available was written for reasons entirely different to the interests of the seismologist. because the seismologist is himself interested in the reporting of earthquakes, it is easy for him to fall into the trap of attributing the same degree of interest to the writer of a document under consideration (musson, 1998). this is particularly true of the medieval period. the fact that many monastic houses kept annals in which events of note were recorded, and that many of these contain mentions of earthquakes, makes it tempting to imagine a network of monastic chroniclers analogous to a seismograph network – each monk with pen poised ready to record any earthquake as it occurs. or not record it if it was not felt. in fact, this picture is quite false. monastic chroniclers recorded things that they felt were of note, and what these things were, and why fig. 1. the study area for this paper. 1829 early seismicity of the scottish borders region they were of note, varied. in the case of «prodigies» (remarkable occurrences) such as earthquakes, the importance of these to the medieval mind was that they were portents, either of god’s wrath or coming political events. the corollary to this is that if an earthquake is memorable because it may be a portent, it is not all that important exactly where it occurs, or how severe it is. the important thing is the date of its occurrence, and this is probably one reason why many chronicles record the occurrence of earthquakes with absolutely no details beyond the date. consequently, if the chronicle of a particular abbey mentions any earthquake, that does not necessarily mean the earthquake was felt at that abbey, unless this is specifically stated or there is other internal evidence to suggest this. it may be that the earthquake was newsworthy because it was felt in london, or wherever the king was at that time, or for a variety of other reasons. on the other hand, the absence of mention of an earthquake in a chronicle is not necessarily evidence that the earthquake was not felt at that place. it may indicate that the writer himself did not feel the earthquake (for a variety of possible reasons) or that he did not consider it worth recording. a good check can be made on the reliability of «negative evidence» in british monastic chronicles by considering the reporting of severe storms (musson, 1987). these make a good comparison with earthquakes because, from a layman’s point of view, they are far more notable phenomena. in particular, storms in britain are more frequent than earthquakes; they do far more damage; they are observed, and cause damage, over a wider area; they last longer, and therefore are more noticeable – there is less chance that any individual will fail to observe the event through being unfavourably placed; and they cause more deaths. following hardman et al. (1973) one can determine that for the scottish borders one can expect the 50 years storm to have a maximum gust speed of 46 m/s, which is enough to do widespread damage. by contrast, the earthquake with a 50 years return period is about 4.4 ml – probably with a maximum intensity of 5 ems. therefore one would expect any medieval chronicle that concerns itself with local affairs to have more frequent reports of storms than of earthquakes. if a chronicle has few or no reports of storms, particularly where it fails to report storms known from other sources to have occurred, then there is no point in expecting the chronicle to be a reliable reporter of earthquakes. in fact, as will be seen, there are no specific accounts of earthquakes in the study area in the monastic annals, so one needs to interpret this lack of reporting in light of the nature of the documents themselves. the early history of scotland has greatly suffered from the almost complete lack of medieval chronicles comparable to those found in england and on the continent. it is not that no chronicles were written in scottish abbeys; the problem arises largely from the wholesale destruction of ecclesiastical remains in the violence that accompanied the scottish reformation in the mid 1500s (maxwell, 1912). one prominent exception is the melrose abbey chronicle. within the area shown in fig. 1 there are five places where chronicles were compiled: durham, furness abbey, lanercost abbey, hexham priory and melrose abbey. these are shown in fig. 2 and will be considered in turn. not discussed here are some ecclesiastical works of history (the most famous being that by bede) which are of little interest from the point of view of earthquakes as they do not record such information. 3.1. historia regum anglorum et dacorum this is the title given to the chronicle compiled by simeon (or symeon) of durham. it starts in time where bede’s ecclesiastical history of the english nation finishes (731). up to 957 it appears to be a copy of some old durham annals no longer extant. the following section, up to 1119, is mostly copied from florence of worcester (english monastic chronicler d. 1118 – gransden, 1974) with a few interpolations (offler, 1958). then from 1119 to 1129 the narrative is original. five earthquakes are mentioned: 974 («all over england»), 1 may 1048 (worcester, warwick, derby and many other places), 11 august 1089 («throughout england»), 1110 (shrewsbury) and 28 september 1119 («in many places throughout england»). all of these are copied from florence of 1830 roger m.w. musson worcester, except the last, which is probably copied from a continuation of florence of worcester. all of them are either definitely outside the study area or have such poorly-specified locations that they could have occurred anywhere in england – or even in wales or offshore. only for the last ten years is this chronicle an original source, but the one earthquake mentioned (1119) is not clearly an original contribution; no unique information about it is provided. there is a mention of one storm in this final ten years, a gale on 24 december 1122 that threw down buildings. it has been suggested (by stevenson, 1855) that simeon may not actually have been the author of the historia regum at all, as it shows uncharacteristically little interest in the affairs of durham. however, it is still considered to be an important source for events in the north of england and south of scotland (arnold, 1885). 3.2. the furness chronicle the chronicle compiled at furness abbey, near barrow, is a continuation of the historia rerum anglicarum of william of newburgh, and covers the period 1198-1298. three earthquakes are mentioned in it: 20 february 1246 («magnus et horribilis»), 23 december 1247 (no details) and 28 january 1257 («et ventus maximus»). however, it can be shown (howlett, 1884-1889) that these are simply copied from the chronicle written in stanley abbey, wiltshire, and therefore are not necessarily anything to do with furness. the furness chronicle is onfig. 2. sites within the study area where extant monastic chronicles were produced, and other places in the study area mentioned in the discussion. 1831 early seismicity of the scottish borders region ly an original record from 1260-1298, and in this 38-years period no earthquakes are mentioned. a storm on 18 september 1268 is mentioned, as are some eclipses, but no weather phenomena are mentioned after 1272, not even the gale of 25 may 1292 which is known to have affected the nw of england (britton, 1937). 3.3. the lanercost chronicle lanercost abbey is about ten miles ne of carlisle. the origins of its chronicle are mixed. up until 1201 it is simply a copy of roger of howden’s (in yorkshire) chronicle; thereafter it is a copy of two otherwise unknown franciscan chronicles, one 1201-1297, the other 12971346, with some extra material relating to lanercost affairs interpolated (gransden, 1974). the first of these was written by richard of durham, possibly in haddington. the unknown continuator who produced the second chronicle probably lived in carlisle (gransden, 1982). no earthquakes are mentioned in the 12011346 section of the lanercost chronicle at all. whilst the richard of durham section up to 1297 mentions a number of storms and floods affecting the borders, the later section written in carlisle mentions no storms. 3.4. hexham chronicles two chronicles were written in hexham priory (raine, 1864). the first of these was by richard of hexham, and is no more than a short piece entitled «account of king stephen and the battle of the standard». it covers the period 1135-1139. it mentions no earthquakes. the second, and more important work, is a continuation of symeon of durham’s historia regum, by john of hexham. it runs 1130-1153. one earthquake is mentioned, on 4 august 1133, «in pluribus angliae partibus terrae motus factus est magnus». since there is an eyewitness account of this event from william of malmesbury in wiltshire (giles, 1904), it seems unlikely that the event had an epicentre anywhere near hexham. no storms are mentioned, indeed the only mention by john of weather phenomena is subordinate to an account of scottish atrocities (britton, 1937). 3.5. the melrose chronicle the period for which the melrose chronicle is an original and reliable source is 1140-1270 (anderson, 1922). in this period only one earthquake is mentioned: in 1185 «terremotus factus est magnus mense aprili per angliam». this is the earthquake that, on 15 april 1185, caused damage to lincoln cathedral. it is thus not likely to be an event local to melrose – and in any case is only described as being felt in england. mention of storms is irregular. leaving aside those occasions on which natural phenomena are mentioned purely for the bearing they have on political matters, the dates of tempests that are mentioned are august 1165 (in yorkshire), 28 december 1171, 16 august 1173, 24 august 1179 (in «helielandschire»), 1181 (england), 13 and 25 april 1206. this does not suggest that any sort of continuous or careful record was kept of local natural phenomena. thus, consideration of local sources for the medieval period shows two things. firstly, that coverage is limited to the years 1119-1346, although that period is covered more or less continuously by at least one chronicle. secondly, all the local sources are interested in human affairs almost exclusively. the lack of reports of such natural phenomena as damaging storms suggests that damaging earthquakes might equally well have gone unrecorded. it might be speculated that any occasion of really extreme earthquake damage over this period could not have gone unrecorded, but this cannot go beyond speculation. a not exceptionally severe storm can produce levels of damage comparable with an intensity of 7 ems over a wide area; thus one can argue that the highest intensity that might reasonably go unreported is certainly no lower than 7 ems. even if an earthquake did occur producing this intensity value, it would still be necessary for the 7 isoseismal to be reasonably large or located in an area of population. as to the question of a magnitude threshold for the area, above which all events are liable to have been reported, here one is even more in the 1832 roger m.w. musson realms of speculation. the problem is made especially difficult by the paucity of scottish records. if the 1979 carlisle earthquake (4.7 ml, io = 6 ems; musson and henni, 2002) had occurred in 1179, it is quite conceivable it might have gone unpreserved in the annals; its effects in england south of cumbria were limited, the damage was minor and affected only a restricted area, and the earthquake on the whole was felt more widely in scotland than in england. a larger earthquake, felt over a larger area of england, would be more likely to be mentioned by english chronicles, but might only be accompanied with the comment that it was felt «in many parts of england». one would hope that such an earthquake would be described by at least one writer as occurring «in many parts of england and scotland». the lack of earthquakes so described may be indicative of a lack of any such earthquakes occurring, but without comparable scottish records this cannot be taken as certain. 3.6. post-monastic sources the dissolution of the monasteries put an end to monastic chronicles as a source of history, but the tradition had already all but died out, and given way to the writing of histories by secular authors. some of these authors do mention the occurrence of earthquakes (e.g., stow, 1601; baker, 1674), but they are primarily writers of national history rather than local history. to some extent monastic chronicles had a civic sequel in the compilation of mayoral chronicles. just as the first monastic annals grew out of tables giving the dates of easter, these early civic chronicles grew out of lists recording mayors, which were gradually amplified with notes about other memorable events that occurred during the term of office of each mayor. as sources of earthquake data, these chronicles are subject to the same limitation of monastic chronicles respecting completeness. since these documents were in no way intended by the author to be treated as a history of earthquakes, it is not valid to expect every earthquake to have been felt in a town to have been duly noted. this can be demonstrated very clearly by referring to one mayoral chronicle in nw england. this was compiled in kendal. an entry for 1817 reads, «sunday morning, november 9, the shock of an earthquake felt in kendal, this being the fifth or sixth shock here since june 1668» (nicholson, 1861). the number of shocks mentioned previously in the chronicle is one (the 11 august 1786 whitehaven earthquake). it appears that four or five other earthquakes felt in kendal are missing from the record. this will be returned to in discussion of the june 1668 event. on the other hand, it is at least more reasonable to assume that an earthquake mentioned in the town annals without any details was felt at the town in question, since the intention of the document is to record the events of that town. an earthquake occurring elsewhere would not have the same interest to a town chronicler as it might to a medieval monk. it is in this period that miscellaneous sources begin to become useful to a study of historical seismicity – diaries, letters, memoranda, etc. the latter end of the 16th century also saw the first publication of material specifically on earthquakes, including the first british earthquake catalogue (fleming, 1580). in considering documentary sources, firstly, the further back in time one goes, the less written material survives. secondly, the manuscripts most likely to survive are those that have material value, i.e. legal and financial documents. deeds, charters and suchlike documents have no value for earthquake studies in britain. thus, of the amount of manuscript material preserved from past centuries, the amount containing mention of earthquakes is small. in countries like italy where earthquakes can destroy entire communities, financial documents can be useful sources. in cases of heavy damage, one may find repair bills and pleas for remission of taxes. if one finds documents indicative that everyday life was still going on in a community, this may suggest that destruction was not so heavy. in britain one does not expect to find heavy destruction at all, and therefore the only documents that are useful are those that can register the occurrence of an earthquake that was no more than slightly damaging. since the historical end of this study is set at 1750, early newspapers will be used for events after 1700. the use of newspapers in historical 1833 early seismicity of the scottish borders region earthquake studies in britain has been discussed in detail by musson (1986). the only two places in the study area where newspapers were published before 1750 were dumfries and whitehaven. of the drumfries [sic] mercury but a single issue survives (8 may 1721) and of the whitehaven weekly courant (started 1736) there are no surviving copies at all. therefore one relies largely on the edinburgh and london press for this period. 4. chronological list of earthquakes (*) this section of the paper will present descriptions and discussions of the earthquakes in the study area up to 1750. a summary list is provided as table i. assessment of parameters is as discussed in musson (1996). once fake events have been removed (these will be discussed later) only a few events remain, though some of these are rather interesting from a historical perspective. there are no earthquakes in the area at all for the medieval period, as already discussed. thus it is not until the opening of the 16th century that one sees even the possibility of an earthquake in this area. 4.1. 19 september 1508 borders? north sea? rockall? this earthquake presents special problems. it is conceivably one of the largest events to have affected the study area, and has been listed in previous studies as occurring in the study area (e.g., by principia mechanica ltd., 1982; ambraseys and melville, 1983), but very little documentation of the event survives. consequently it has been necessary to examine the little information that does exist with extreme scrutiny. it has not been possible to trace any contemporary account of the earthquake. the nearest sources in time are the scottish historian john leslie, bishop of ross (1527-1596) and the english historian raphael holinshed (?1580). the third source is sir james balfour, writing around 1650. all these sources are summarised in table ii. leslie’s account of the earthquake survives in three forms, all of them brief. in his «history of scotland», a work composed during imprisonment in england ca. 1568-1570, presented to queen mary (herself a captive at this date) in 1571 and eventually published in 1830, it is stated, «in september, ane gret erd quak wes, the xix day thairof in divers placeis, and for the maist part in the kirkis, quhilk wes asueill [as well] in ingland as in scotland.» (leslie, 1830). in 1578 leslie revised and expanded his history and translated it into latin, and it was published in rome under the title «de origine table i. earthquakes in the study area before 1750. parameters in italics are very approximate estimates. day month year hour minute lat. long. depth ml location 1650 (1) 54.98 − 2.78 cumberland 11 04 1650 17 54.98 − 2.78 4.9 cumberland 27 04 1650 55.63 − 2.72 3.5 galashiels 06 1668 55.00 − 2.40 4.4 borders 01 03 1728 04 30 55.58 − 2.81 21 4.2 galashiels 10 1746 55.42 − 3.77 2.0 (2) leadhills (3) 14 02 1749 08 30 55.42 − 3.77 2.7 leadhills (3) (1) or 1649; (2) minimum likely value; (3) mining induced earthquakes. (*) in the text that follows references given in square brackets, mostly newspapers, are given completely in the text and not in the references section. 1834 roger m.w. musson moribus, et rebus gestis scotorum ...» (leslie, 1578). in this the passage reads, «... ingens erat terrae, non solum in scotia, verum etiam tota anglia, motus, quo templa in primis contremuerunt, quod quidam religionis euertendae augurium interpretabantur». this may be translated, «there was a great earthquake, not only in scotland, but also, indeed, even the whole of england, which shook the churches especially, which was interpreted as an omen of the overthrowing of religion». the third version is dalrymple’s 1596 translation of leslie’s latin text. this was published in 1895, and appears to be rather free. the year of the earthquake is given as 1509 instead of 1508, and the text reads, «... terrable erdquakes throuch jngland and scotland war hard, kirkes quaket and trimblet vehementlie, quhilke mony exponet to the over.thraw of religions.» (leslie, 1895). holinshed’s chronicles went through many and various editions, the first being in 1578. his entry on the earthquake comes in the scottish chronicle, and reads, «the 19th of september was a great earthquake in manie places, both of england and scotland, namelie, the same was perceived in churches.» (holinshed, 1578). balfour’s account is a little different. it reads, «1508 the 10 of september, this same zeire, a dreadful earthquack in scotland and england, wich lasted the 10 pairt of ane houre, to the grate terror and astonishment of all the inhabitants.» (balfour, 1825). leslie and holinshed present essentially the same information, and it seems likely they were using a common source, or (very likely) holinshed was using leslie. it appears that the earthquake was felt over a wide, perhaps a very wide, area, and that it was felt both in england and scotland, and caused considerable alarm, and particularly affected churches. balfour leaves out the detail of the churches, but adds an estimated duration of six minutes. he also describes alarm, but mentions no damage. balfour gives a different date; possibly a transcription error. from the fact that the earthquake was felt both in england and scotland, principia mechanica ltd. (1982) conclude the epicentre to have been somewhere in the borders. according to ambraseys and melville (1983) it is an «evident that it [the epicentre] must have been in the border region». the description that the earthquake was felt through england and scotland is almost unique for an early historical earthquake. a consideration of scottish seismicity in the last 300 years shows that activity is largely concentrated north of the central lowlands. thus it almost never happens that a scottish earthquake has been felt table ii. summary of sources for the 1508 earthquake. # source publication approximative date given for earthquake date date of composition earthquake details 1 leslie’s first 1830 1570 19 september 1508 great, in many places in england ms history and scotland, more in the churches 2 leslie’s printed 1578 1578 19 september 1508 great, in scotland and the whole of latin version england, specially shook churches 3 holinshed 1578 1578 19 september 1508 great, in many places in england and scotland, also felt in churches 4 scots translation 1895 1596 19 september 1509 terrible, heard throughout england of (2) by dalrymple and scotland, churches shook vehemently 5 balfour 1825 1650 10 september 1508 dreadful, in scotland and england, lasted six minutes, caused terror and astonishment 1835 early seismicity of the scottish borders region south of the border (the felt area of the 1839 comrie event crossed the border by a few miles only; the 1728 galashiels earthquake will be discussed below). most english earthquakes that have been felt in scotland, even the larger events, have typically only been weakly felt in the southern uplands, effects which would probably go unreported in the early 16th century. the principal exception was the 1979 carlisle earthquake, which was strongly felt as far north as glasgow. to a lesser extent the 1786 whitehaven earthquake also fits the description (musson et al., 1984), although the effects in scotland of the 1786 event were nowhere as marked as those of the 1979 earthquake. thus an earthquake similar to, and possibly larger than, the 1979 carlisle earthquake is a plausible interpretation of the sparse evidence. if, however, such an event was really felt «verum etiam tota anglia», it would have to have been of considerably larger magnitude than the 1979 event, and correspondingly more likely to have caused significant damage in the epicentral area, and to have impinged more strongly upon the local historical record. it is possible that a measure of exaggeration has crept into leslie’s account. holinshed incorporates mention of the earthquake into his scottish chronicle, but not into his english chronicle despite the fact that he mentions that the earthquake was felt in england. this suggests he had no english sources for the event, which in turn suggests that the effects of the earthquake may have been more remarkable in scotland than in england, and that the whole of england was not affected. stow (1601) and baker (1674) are both silent on the event. so are some other scottish sources, though. the first version of leslie’s history covered only the period 1436-1561, and was intended as a continuation of the chronicle of hector boece (batho and husbands, 1941). when leslie prepared the latin version for publication, he took the earlier material mostly from boece. the other main continuator of boece, pitscottie (lived 1532?-1578?), covered a similar period to leslie (1436-1575), wrote at a similar time (the early 1570s), but doesn’t mention the earthquake (mackay, 1899). it would be very helpful if a coeval source for the earthquake could be located. batho and husbands (1941) speculate on the existence of a lost continuation of boece by john bellenden (ballantyne), who translated boece in 1531, and is thus a more contemporary figure than the others mentioned, but such a work has not been found, and its existence is conjectural. it is curious that the main detail to be recorded by leslie is that the earthquake was particularly felt in churches. although churches are described as having shaken, there is no mention of damage. possibly the earthquake occurred during the hours of service, and the effects were comparable with those of the 14 september 1777 manchester earthquake in lancashire churches (burton et al., 1984). however, it should be noted that 19 september 1508 was a wednesday (if balfour’s date were correct, it would have occurred on a sunday, which could be taken as evidence in favour of balfour’s version). the alternative is that churches were shaken more than houses for spectral reasons. if the earthquake were relatively large and distant, churches (with steeples) might be resonant structures, and the shock could be felt alarmingly over a wide area without any particular area of high intensity or damage. it can be mentioned that in the case of the 1580 dover straits earthquake, a church spire was damaged as far away as leicestershire (neilson et al., 1984), so the idea that churches might be particularly susceptible is not far-fetched. an epicentre in the north sea would be likely to be widely felt in both england and scotland without causing significant local damage. it is possible that the 1508 earthquake may be best compared with large events in the viking graben such as the 24 january 1927 north sea earthquake, which was felt over most of scotland and in england as far south as east anglia (musson et al., 1986), without causing any damage. in this case, one would hope for confirmation from norwegian sources, which is lacking. this may be due to inadequacy of the norwegian historical record for this period. alternatively, a large event on the continental slope nw of scotland, perhaps similar to the 1929 grand banks earthquake, is not impossible. one is naturally more inclined to propose an explanation that fits a known local precedent, so it is more credible to suggest a «prequel» of the 5.7 ml 1927 earthquake than an 1836 roger m.w. musson earthquake > 6 ms somewhere near rockall; but this would fit the facts perfectly well. figure 3 shows two possibilities. the broad dashed lines reproduce roughly the isoseismals of the grand banks earthquake (7.2 ms) after smith (1966), moved from the canadian coast to a location on the passive margin indicated by the open star, and with a worst-case orientation. with a smaller magnitude or more favourable orientation of the isoseismals, any area of damage would be confined to the gaelic-speaking nw highlands of scotland, an area with no literary traditions at this period. the rest of scotland and england would only experience intensity 5 or 4 ems, consistent with what is reported. the fine dashed lines show the isoseismals of the 1927 north sea earthquake, after musson et al. (1986); the black star is the instrumental epicentre (neilson, 1980). a large event on the continental slope as in fig. 3 might also be strongly felt in northern ireland. the lack of irish reports is not necessarily significant given the political and social situation in ireland, especially ulster, at the time. there are, for example, no known reports at present of an earthquake felt in antrim at a date of roughly 1600, the existence of which is only known from an anecdote reported in a letter written in 1690, which was subsequently fig. 3. two possible analogues for the 1508 earthquake. broad dashes indicate a worst case scenario for an occurrence of an event equal to the 1929 grand banks (canada) earthquake off the nw of scotland (open star indicates assigned epicentre); variations involving a smaller magnitude or change of isoseismal orientation can easily be imagined. fine dashes are the isoseismals of the 1927 north sea earthquake (black star indicates epicentre). 1837 early seismicity of the scottish borders region published by wilde (1841) – although it is possible that this observation is connected with the large scottish earthquake of 23 july 1597, if the epicentre of that event were further west than previously assumed. an earthquake nearly a century earlier could therefore be unrepresented in the historical record without stretching credulity. in the catalogue of musson (1994) this event is considered to be most likely a viking graben earthquake. the balance of probability is certainly that it was a large distant event somewhere, and not a scottish borders earthquake. 4.2. 11 april 1650 cumberland nothing else is reported from the study area during the rest of the 16th century and the first half of the 17th, so it is, in fact, not until 1650 that the first indisputable borders earthquake occurs. this earthquake appears in a number of catalogues, of which the earliest appears to be turner (1696), from whence it was copied by burton (1734). the earthquake also appears in burton (1737) with the date incorrectly given as 1651. short (1749) gives earthquakes in cumberland and westmoreland in both 1649 and april 1650. davison (1924) uses short as his source and accordingly treats the event with circumspection. the earthquake is known from two contemporary sources. the first is a broadsheet entitled «strange newes from the north ...», printed in london on the basis of a letter received from penrith (anon, 1650). the second is the autobiography of lady anne halkett, written some ten or twenty years after the event, but describing the earthquake at first hand (nichols, 1875). the information from «strange newes from the north ...» is limited, and no specific places are mentioned, although it is reasonable to assume from the phrase «we had a general earthquake» that the earthquake was felt at the writer’s locality, i.e. penrith (or «penreth» in the original). the letter is dated 30 may 1650 and signed with the initials tc. the earthquake description reads as follows: «... upon the 11 of april last about 5 of the clock in the afternoon, in the counties of cumberland and westmerland, we had a general earth-quake; the people were so frightened therewith, that they forsook their houses, and some houses were so shaken that the chimnies fell down ...» (anon, 1650). the following points can be drawn: i) the earthquake was widely felt in cumberland and westmoreland; ii) people ran outdoors in alarm; iii) some chimneys were damaged. an intensity of 6 ems is suggested. the rest of the letter deals with political events related to the civil wars. in the second source, lady anne halkett describes two events observed at naworth castle (about 30 km north of penrith; see fig. 4) of which the second was more severe. the text reads: «one night, being fast asleepe, i was sodainely wakened with the shaking of the bed somewhatt violent, butt of short continuance. in the morning i told sr ch. and my lady that i had heard of earthquakes, butt i was confidentt i had felt one that night, and related how itt was, they laughed at me, and said i had only dreamt of itt. i could nott convince them, nor they mee; butt a litle before dinner came in some gentlemen that lived within 3 or 4 mile, and sr charles asked them what news: they replied the greatest they knew was that there has beene an earthquake that night, and that severall houses were shaken down with itt. then they beleeved what i had told them. another day my lady h. and i was sitting together alone in my chamber, about an ell or more distant from on-another, and sodainely the roome did shake, so that both our heads knockt together. shee looked pale like death, and i beleeve i did the same, and wee were hardly well recovered from our feares when sr c. came in to see how wee were, and told us hee was walking in the gallery with mr n. and that they were so shaken they could skarce hold there feett, and was forced to hold themselves on the sides of the howse. these both hapened in the yeare 1649.» (nichols, 1875). both are given as occurring some time in 1649, but allowing for possible lapses in memory in the time between the events occurring and her account being written it is most probable that the second event was in fact the 11 april 1650 earthquake and the first a foreshock which may have occurred in 1649 or 1650, probably the latter. it may also be recalled that in times past it was general usage for the calendar year to begin on 25 march rather than 1 jan1838 roger m.w. musson uary, so by this reckoning, 11 april 1650 was only seventeen days into the year. further confirmation that the first event described by lady halkett was not the main shock described from penrith is given by the times – the first event occurred during the night, while the second occurred during the day. the options are also restricted by the fact that lady halkett was only at naworth castle for a limited period – between september 1649 and june 1650. since we know that a strong earthquake did occur at penrith on 11 april 1650, during the time lady halkett was at naworth, it would be strange if there were two additional earthquakes in 1649, neither corresponding to the penrith event. lady halkett mentions a propos of the first shock that «several houses were shaken down with it». possibly this is another confusion of memory and this damage, near naworth castle, was occasioned by the second, main shock. the first event described by lady halkett cannot be the shock that caused damage in penrith (i.e. be the main shock) as it was during the night, and the 11 april event was in the afternoon. allowing for an element of exaggeration, lady halkett’s description would suggest that the maximum intensity of the earthquake may have reached 7 ems. the one further piece of information comes from a glasgow clergyman, robert baillie, who gives a contemporary account of an earthquake felt in glasgow on 17 august 1656 (although he did not feel it). he mentions that he did feel an earthquake in glasgow «in the afternoon» five or six years previously (laing, 1841-1842). it would be highly probable that an fig. 4. places mentioned in connection with the 11 april 1650 earthquake. the star indicates the epicentre of the 1979 carlisle earthquake. 1839 early seismicity of the scottish borders region earthquake felt strongly at penrith and naworth would also be perceptible in glasgow; the times match; and the likelihood is that baillie felt the 11 april 1650 earthquake. the fact that the highest recorded intensity occurred near naworth castle, which is itself only about 18 km from the epicentre of the 1979 carlisle earthquake, suggests that the 1650 earthquake may have originated from a similar epicentre. if it was felt at glasgow and throughout cumberland and westmoreland then the felt areas of the two events are similar, and the highest intensities are certainly similar. earthquakes in cumberland reportedly occurring in 1649, as in short (1749), are due to taking the year given by lady halkett at face value. the problems of interpreting these fragmentary accounts are complicated even more when we turn to the next event. 4.3. 27 april 1650 galashiels considering that we have no record of genuine borders earthquakes before 11 april 1650 except a likely foreshock, it seems rather strange that the next independent event should be only sixteen days later. nevertheless, this seems to be the case. the earthquake is known from only a single source, but this source is exactly contemporary. this source is sir james balfour (1600-1657) again. he writes (under the heading of 1650): «the 27 of apryle, this zeire, ther was a grate earthquake, all alonges from the head to the footte of the riuer tueed; and in no place of this kingdome elsquhere felt, bot in these places neir adioyning to the said riuer.» (balfour, 1825). the river tweed runs roughly from west to east, passing the towns of peebles and galashiels, fig. 5. approximate felt area for the 27 april 1650 galashiels earthquake. 1840 roger m.w. musson and reaching the sea at berwick (fig. 5). since to the north and south of the valley are ranges of hills little populated, one can estimate a felt area centred somewhere near galashiels which is not excessively elongated and yet matches the spirit of balfour’s description that the earthquake was felt only in the tweed valley. one can therefore posit an earthquake not dissimilar to the 1728 galashiels earthquake, but a little smaller – somewhere in the region of 3.5 ml. but was it truly a separate event? two possibilities have to be considered: firstly that this was actually the 11 april event felt in the tweed valley (it would be reasonable to suppose that this earthquake was felt in that area if it was felt as far as glasgow); secondly, that this was the earthquake felt in glasgow in 1650 and not the 11 april event. in considering the first possibility, if balfour only had information from tweeddale and not from england, and perhaps knew that the earthquake was not felt in edinburgh, stirling and other distant towns, he could have written as he did that the earthquake was not felt elsewhere. but he was writing around 1651 or 1652, and there is no easy explanation as to why he should get the date wrong by sixteen days. if it were a discrepancy of seven days, one could suppose it was a confusion of weeks. if it were a discrepancy of eleven days, one could suppose it was a confusion of calendars (the julian calendar was still in use in britain at this time). but sixteen days one has to take at face value. equally, the letter from penrith fixes the date of the previous earthquake at 11 april beyond dispute. therefore one can only conclude that this was indeed a separate earthquake. the possibility that baillie felt the earthquake of 27 april rather than 11 april cannot be ruled out, but is inherently less likely. it is unfortunate that balfour gives no time of day for the 27 april event, which would make these comparisons rather easier. lastly, one needs to mention that jeffrey (1857) gives the date of this earthquake as 27 april 1656, which is simply a transcription error or perhaps a misprint, as his source is balfour. this earthquake does not appear in musson (1994) or any other catalogue. it shows that there are still a few historical british earthquakes to be uncovered despite the large amount of research done since 1980. 4.4. ? june 1668 borders when discussing the completeness of earthquake catalogues, one can approach the subject in a statistical way, or one can evaluate the historical sources to try and estimate what sort of earthquake «must have been reported» at any historical period. however, one cannot actually see what sort of earthquakes are missing from the catalogue – because, of course, they are missing. the remarkable thing about the study area is that it provides the opportunity actually to examine a missing earthquake. this is an earthquake lost to the historical record, of which no description survives; yet one can infer that it took place from oblique references. there are two such references to the event. the first is a note written in 1817 to the effect that the 9 november 1817 earthquake was the fifth or sixth shock to be felt in kendal since june 1668 (nicholson, 1861), implying, rather than stating, that a june 1668 earthquake occurred, which formed a sort of backstop to public memory of earthquakes in the town. this would not be conclusive on its own were it not for another reference, from edinburgh in 1728 (weekly journal, 23 march 1728, p. 2). after stating that the 1 march 1728 earthquake (see below) was felt in peebles, kelso, dumfries and berwick, it goes on to say that in some places the shock was as sensible as «that which happened before the year 1668». despite the use of the word «before» in the last quote, the mention of 1668 in both references appears highly significant. there is no other particular reason why two independent writers nearly 100 years apart, one in england and one in scotland, should both pick on the unremarkable year 1668 for remembering previous earthquakes. it is to be considered as highly probable that an earthquake greater than 4 ml occurred in or around june 1668 and was strongly felt in kendal and the scottish borders; this event may have been similar to the 1970 kirkby stephen or the 1979 carlisle earthquakes (musson et al., 1984). figure 6 shows a 1841 early seismicity of the scottish borders region minimum felt area. the epicentre is so uncertain it is doubtful whether it is worth assigning coordinates to it; musson (1994) does not. this demonstrates that, for the 17th century, an earthquake almost certainly greater than 4 in magnitude, could occur and yet leave no direct documentary trace that has been discovered at the present day. part of the problem is the nature of the source materials, as was previously discussed. one cannot read every surviving 17th century letter in the hope that it may mention an earthquake, and one is somewhat dependent on the chance that reports of earthquakes will surface in the course of other research. i spoke some years ago with a local historian in dumfries who was sure that he had once encountered a description of an earthquake in the scottish borders in or around 1637, but he could not recall the reference and i have never been able to find such an event. 4.5. 1 march 1728 galashiels this earthquake is the largest event in the borders area that is definitely on the scottish side. besides the 1650 earthquake already discussed, only a few other events are known in the galashiels area (e.g., 15 march 1844) and they are very small. this earthquake was felt from carlisle in the south to fife in the north, and the magnitude is estimated here as 4.3 ml. the infrequency of earthquakes in this area is actually commented on in some of the original reports. it was also one of only two well-determined earthquakes with epicentres in scotland to be felt in england (the other being the 23 october 1839 comrie earthquake). however, there is no evidence that the intensity was higher than 4 ems anywhere. reports are unanimous that there was no damage, fig. 6. conjectured felt area for the lost june 1668 earthquake. 1842 roger m.w. musson and no effects stronger than rattling objects are reported. this suggests that the focus may not have been shallow. the primary sources for this earthquake are newspapers, chiefly those published in edinburgh and london. at this period all newspapers tended to copy the same few accounts anyway. other sources, such as chambers (1874 – which is used by davison, 1924) all refer back to newspaper accounts. the following two reports give more or less all the data. «edinburgh march 8. we have had some talk these several days bygone of a small shock of an earthquake said to have been felt by some persons in this city early last friday morning; but not damage having thereby happen’d, it was going out of mind, when the two following accounts came to hand from the south country. galashiels, selkirkshire, march 4. [here occurs a long description of the aurora borealis] … the 1st instant …about half an hour after four a shock of an earthquake was felt all over this place, and some miles round; but it pleased god there was no damage sustain’d. selkirk, march 3. i believe you will be surprised at the following relation, which tho’ very extraordinary, is undoubtedly true: last friday morning a little before four o’clock, i was awakened by a noise something like a clap of thunder, which, after it had roared four minutes, dy’d away insensibly. when it ceased, i rose out of my bed, and look’d out at a window, and seeing the air clear ... i concluded it had been the morning-drum by which i had been alarm’d. next morning i was told by every body that there had been an earthquake, and that it shook all the houses in town, but had done no damage: i happen’d yesterday to be at a gentleman’s burial, where were present a great number of gentlemen from all parts of the country, an earthquake being a very extraordinary thing in this part of the world, i took the opportunity to enquire very narrowly of every one of them, if they had felt any shaking of the earth such a night; all agreed they had. some of them were abroad at the time, and both felt the shock and heard the noise which follow’d; others said they were almost rolled out of bed; and others, especially they who live to the southwards, heard no noise, but were dreadfully shaken in their beds.» [london evening post, 14/16 march 1728, p. 2]. «edinburgh march 12. we are informed, that the late earthquake was felt at peebles, kelso, dumfries, and berwick, and in some places as sensibly as that which happened before the year 1668. galashiels, march 4. on friday the 1st of march current, between 4 and 5 in the morning, we felt in this place a very sensible shock of an earthquake. it came from the north east, and went directly south west. it was ... attended with some claps of thunder: it began, as i am informed, as far as at or near lawder, and was felt at blamislie, earliston, stow, melross, selkirk, hawick, longholm, and the length of carlisle; how far to the north i know not. it lasted, according to some, one minute, but most of my acquaintance say, near three minutes ... there have none suffered by it, but there was a strange dancing among the plates, glasses & c. ... there was a very uncommon sound in the air upon the shock of the earth. i hear, it moved at jedburgh too. the above earthquake was felt the same day at falkland in fife.» [weekly journal, 23 march 1728, p. 2]. only at three places (galashiels, selkirk and edinburgh) can intensities be assigned (4, 4 and 3 ems respectively), but the clustering of reports at places around galashiels suggests that the earthquake was generally perceived in this area, and therefore the intensity was 4 ems over this area. on this basis a conjectural isoseismal map has been constructed (fig. 7). the epicentre appears to lie between galashiels and selkirk. some studies (e.g., davison, 1924) give the date erroneously as 1727 – this is due to unfamiliarity with the practice of dating the start of the historical year on lady day (25 march). 4.6. ? october 1746 leadhills this is the earliest of a series of events felt at leadhills and wanlockhead, two small villages in the south of scotland that owed their existence to historical lead mining. all these events have small felt areas, although some 1843 early seismicity of the scottish borders region seem to have been quite strongly felt, especially in the mines. these are characteristics of mining tremors, and there seems little reasonable doubt that that is what these events are. mining activity at leadhills and wanlockhead was quite extensive and of considerable antiquity. a mine on glengonar water, near leadhills, is mentioned as operating in 1239 (porteous, 1876; harkness, 1925), and according to wilson (1921) «there is little doubt that leadmining was in operation long before that time». according to davison (1924) and subsequent authors this earthquake occurred in 1748. this seems to be an error. there are no exactly contemporary reports known, and the earthquake is referred to in the context of the subsequent 1749 event. the original sources for latter event comment that a previous shock was felt «about the time the earthquake happened at lima» (shirreff, 1749). it is clear that the event referred to is the 28 october 1746 lima earthquake, which was a destructive event and made considerable impact on the british press. this fixes the date as late october or early november 1746. the original description of this earthquake at leadhills comes from the mining engineer alexander shireff, and is as follows: «i came here [leadhills] the day after it was felt; severall people told me of it; but it was in the night time, and felt only by a few, which was the reason i took no notice of it, till the acco ts of that earthquake [at lima] came to europe, & found by comparing the time that it was much about the time that the shock was felt here.» (shirreff, 1749). it was a common belief at this time that british earthquakes were the tail end of some fig. 7. intensity and isoseismal map for the 1728 galashiels earthquake, using standard intensity symbols. unfilled circles indicate places where the earthquake was felt, but few or no details are known. 1844 roger m.w. musson large distant earthquake that might have travelled half way round the world. 4.7. 14 february 1749 leadhills a good contemporary account of this earthquake survives in the form of two letters from alexander shirreff, the local mining engineer mentioned above. the text of the two letters is almost identical. one appeared in the glasgow courant [27 february 1749, p. ?] and was copied from thence by other sources; the other survives in the original ms as hopetoun 347 (in private hands). the full text is not given here. the salient details are as follows: at leadhills the shock above ground was accompanied by a sound as of the falling of a house; people ran out in alarm. it was strongly felt underground; in many of the works the men ran out. at one cut-head the shock had the effect of setting a windlass in motion. it was strongly felt by two men outdoors near the meadowhead engine; they thought some piece of machinery had broken. one described the effects of the shock as being as if he had been struck by something. at wanlockhead machinery shook visibly. at penpont «the ground seemed as if going to sink, and the people all left their houses». a man crossing the scar water by a bridge near penpont felt the shock, and imagined the bridge was about to fall. these effects indicate an intensity of 5-6 ems. there are no reports of any damage, which suggests 5 rather than 6 ems. the felt area extends as far south as penpont (fig. 8), suggesting the felt area may have been about 700 km2, though the sparse settlement in the area makes it impossible to estimate the eastern, western and northern extent. fig. 8. places mentioned in connection with the leadhills earthquakes of 1746 and 1749. 1845 early seismicity of the scottish borders region 4.8. fake or mislocated events in the scottish borders a number of spurious events in the study area are found in some previous studies. they need to be listed and explained to prevent future workers rediscovering them. the 1508 earthquake, considered here not to be a borders earthquake, has already been discussed at length. some minor misdatings have also been noted: the 11 april 1650 earthquake placed in 1649; the 27 april 1650 earthquake placed in 1656; the 1 march 1728 earthquake placed in 1727, and the october 1746 earthquake placed in 1748. events not previously noted are the following: 261, 1000 and 1014, all cumberland – these three early earthquakes appear first in short (1749). they are fictitious, and the case against them and a class of further very early earthquakes in britain is discussed in detail in musson (2005). 1658, 20 january 1662, 28 january 1662, 1663, 3 april 1668, 30 january 1728, october 1728, 30 january 1729, all north of england. these appear in various studies, including roper (1889), davison (1924), principia mechanica ltd. (1982) and ambraseys and melville (1983 – they state the events are not confirmed), and all trace back to short (1749), who uses «n. england» as shorthand not for north of england but for new england. these are all early american earthquakes, some with inaccurate dates. 5. conclusions the total number of genuine earthquakes in the study area for the early historical period, up to 1750, is only seven, of which one is a foreshock and two are mining-related. only one of the earthquakes is known to have caused any damage. one might think that such a case history was fairly trivial. however, apart from the fact that the seismicity of this area is quite important in terms of npp safety issues, the study presented here raises a number of issues of interest in general for studying early historical earthquakes in areas of low to moderate seismicity such as western europe or eastern north-america. in particular, one can note how important it is to pay close attention to sources. especially, it is always advisable to treat a source document in its entirety and not rely on little extracts that someone has copied out. one could not properly interpret the medieval chronicles in this area on the basis of just the copied-out entries that mention earthquakes. only by consideration of the entire works can one judge the significance both of the entries on earthquakes and the lack of other entries on earthquakes. in considering whether earthquakes have been duplicated, the date and provenance of sources needs to be considered. balfour (1825) has a different date for the 1508 earthquake than leslie (1830), but because both were writing long after the event it is most likely that a transcription error occurred. in contrast, when balfour disagrees with «tc» on the dates of earthquakes in 1650, a different interpretation has to be made because both sources are closely contemporaneous with the events described. relative dating, where one earthquake is fixed in time with respect to another, allows the correct dating of the 1746 leadhills earthquake, and also throws up the shadow of a significant earthquake in 1668, which is actually missing from the historical record. although this area was historically a border area, the usual seismological problems associated with border areas, of events being either duplicated on either side or falling down the gap between, do not seem to have been particularly prevalent, though one could make a case for the 1668 earthquake being an event that has only been traced when scottish and english sources were compared. after 1750, local newspapers become more widespread and the recording of earthquakes improves. in the english part of the study area quite a number of historical earthquakes are known after 1750, including some as large as 5 ml. historical seismicity in the area north of the border is restricted mostly to small events, none exceeding 3.5 ml, mostly in the area around dumfries. acknowledgements i would like to thank all the various historians i have dealt with over the years for their 1846 roger m.w. musson help and good company. also thanks especially to viviana castelli and an anonymous reviewer who made many helpful suggestions to this paper. parts of this paper, in a less full form, were originally published in musson (1987), a report now out of print, and which was supported by bnfl. this paper is published with the approval of the executive director of the british geological survey (nerc). references ambraseys, n.n. and c. melville (1983): the seismicity of the british isles and the north sea (serc marine technology centre, london). anderson, a.o. (1922): early sources of scottish history (oliver & boyd, edinburgh). anon («tc») (1650): strange newes from the north (clowes, london). arnold, t. (editor) (1885): symeonis monachi opera omnia (longmans, london). baker, r. (1674): a chronicle of the kings of england (london). balfour, j. (1825): the hostorical works of sir james balfour (hurst robinson & co., london). batho, e.c. and h.w. husbands (editors) (1941): the chronicles of scotland, compiled by hector boece, translated into scots by john bellenden 1531 (scottish text soc., edinburgh). britton, c.e. (1937): a meteorological chronology to a.d. 1450 (hmso, london). burton, r. (1734): a general history of earthquakes (london). burton, r. (1737): admirable curiosities, rarities and wonders in great-britain and ireland (london). burton, p.w., r.m.w. musson and g. neilson (1984): macroseismic report on historical british earthquakes iv: lancashire and yorkshire, bgs global seismol., rep. no. 219, 2 vols. chambers, r. (1874): domestic annals of scotland (chambers, edinburgh). davison, c. (1924): a history of british earthquakes (cup, cambridge). fleming, a. (1580): a bright burning beacon (denham, london). giles, j.a. (editor) (1904): chronicle of the kings of england, by william of malmesbury, translated by rev john sharpe (bell, london). gransden, a. (1974): historical writing in england ca. 550 to ca. 1307 (routledge & kegan paul, london). gransden, a. (1982): historical writing in england ii, ca. 1307 to the early sixteenth century (routledge & kegan paul, london). hardman, c.e., n.c. helliwell and j.s. hopkins (1973): extreme winds over the united kingdom for periods ending 1971, meteorol. office climatol. memo. no. 50a. harkness, j.m. (1925): among scotland’s lead mines (courier & herald press, dumfries). holinshed, r. (1578): chronicles (london). holinshed, r. (1806): the scottish chronicle (morison, perth). howlett, r. (editor) (1884-1889): chronicles of the reigns of stephen, henry ii and richard i (rolls series, longmans, london). jeffrey, a. (1857): the history and antiquities of roxburghshire and adjacent districts, from the most remote period to the present time (hope, london). laing, d. (editor) (1841-1842): letters and journals of robert baillie (1637-1662), (bannatyne club, edinburgh), 3 vols. leslie, j. (1578): de origine moribus, et rebus gestis scotorum (roma). leslie, j. (1830): the history of scotland (bannatyne club, edinburgh). leslie, j. (1895): the historie of scotland (scottish text soc., edinburgh). mackay, æ.j.g. (editor) (1899): the historie and cronicles of scotland, written and collected by robert lindesay of pitscottie (scottish text soc., edinburgh). maxwell, h.e. (1912): the early chronicles relating to scotland (maclehose, glasgow). musson, r.m.w. (1986): the use of newspaper data in historical earthquake studies, disasters, 10, 217-223. musson, r.m.w. (1987): seismicity of southwest scotland and northwest england; with a catalogue of earthquakes within 75 km of chapelcross, bgs global seismol., rep. no 316. musson, r.m.w. (1994): a catalogue of british earthquakes, bgs global seismol., rep. no wl/94/04. musson, r.m.w. (1996): determination of parameters for historical british earthquakes, ann. geofis., xxxix (5), 1041-1048. musson, r.m.w. (1998): inference and assumption in historical seismology, surv. geophys., 19, 189-203. musson, r.m.w. (2005): undead earthquakes, j. seismol. (in press). musson, r.m.w. and p.h.o. henni (2002): the felt effects of the carlisle earthquake of 26 december 1979, scott. j. geol., 38, 113-126. musson, r.m.w., g. neilson and p.w. burton (1984): macroseismic reports on historical british earthquakes, i. northwest england and southwest scotland, bgs global seismol., rep. no. 207, 2 vols. neilson, g. (1980): historical seismicity of the north sea, in energy in the balance (westbury house, london,) 120-137. neilson, g., r.m.w. musson and p.w. burton (1984): the «london» earthquake of 1580 april 6, eng. geol., 20, 113-142. nichols, j.g. (editor) (1875): the autobiography of lady jane halkett (camden soc., london). nicholson, c. (1861): the annals of kendal (whitaker, london). offler, h.s. (1958): medieval historians of durham (univ. durham, durham). porteous, j.m. (1876): god’s treasure house in scotland (simpkin marshall, london). principia mechanica ltd. (1982): british earthquakes (pml, cambridge). raine, j. (editor) (1864): the priory of hexham, its chroniclers, endowments and annals (surtees soc., durham). 1847 early seismicity of the scottish borders region roper, w. (1889): a list of the more remarkable earthquakes in great britain and ireland during the christian era (t. bell, lancaster). shirreff, a. (1749): hopetoun ms 347, unpublished letter, (private collection). short, t. (1749): a general chronological history of the air, weather, seasons, meteors, etc. (longman & miller, london). smith, w.e.t. (1966): earthquakes of eastern canada and adjacent areas, 1928-1959, publ. dominion obs., 32, 87-121. stevenson, j. (editor) (1855): church historians of england, part ii. the historical works of simeon of durham (seeley’s, london), vol. iii. stow, j. (1601): the chronicles of england (london). stucchi, m. (1993): through catalogues and historical records: an introduction to the project «review of historical seismicity in europe», in historical investigation of european earthquakes, edited by m. stucchi, and j. vogt (cnr, milano), 1, 3-14. turner, w. (1696): a compleat history of the most remarkable providences, etc. (dunton, london). wilde, w. (1841): gallery of illustrious irishmen no xiii: sir thomas molyneaux, bart., md, frs, dublin university mag., 18, 604-619. wilson, g.v. (1921): the lead zinc copper and nickel ores of scotland, in special reports on the mineral resources of great britain (hmso, edinburgh), vol. 17. (received july 7, 2004; accepted september 23, 2004) s u l l ' i n t e r a z i o n e f r a l e p a r t i c e l l e d e l s o l i d o i d e a l e p . e . v a l l e r i c e v u t o il 18 m a g g i o 19(53 riassunto. — v i e n e m o s t r a t o c h e l ' i n t e r a z i o n e t r a le p a r t i c e l l e d e l s o l i d o i d e a l e p u ò e s s e r e v a l u t a t a s e n z a u s c i r e d a l q u a d r o d e l l a t e o r i a d e l c a l o r e s p e c i f i c o . q u e s t a v a l u t a z i o n e r i c h i e d e il c a l c o l o d i u n ' i n t e g r a l e , d a c u i v i e n e r i p o r t a t a u n a t a b u l a z i o n e . s u m m a k y . t h e i n t e r n a i p r e s s u r e d u e t o t h e i n t e r a c t i o n s b e t w e e n t h e p a r t i c l e s of t h e s i m p l e s t f o r n i of s o l i d is e v a l u a t e d . introduzione. l a teoria del solido ideale conduce a d un'equazione (li s t a t o secondo la quale la pressione i n t e r n a , che equilibra u n a pressione esterna di tipo idrostatico, può essere divisa in d u e p a r t i . u n a p a r t e è costituita dalla pressione di radiazione delle onde elastiche di agitazicne t e r m i c a , m e n t r e l ' a l t r a è d o v u t a all'interazione t r a le particelle del solido. la pressione di radiazione delle onde elastiche di agitazione t e r m i c a è facilmente valutabile, specialmente p e r t e m p e r a t u r e superiori alle temp e r a t u r e caratteristiche, m a il p r o b l e m a della valutazione della pressione d o v u t a all'interazione t r a le particelle del solido p r e s e n t a notevoli difficoltà. si può mostrare p e r a l t r o f1) come sia possibile risolvere questo prob l e m a senza uscire d a l quadro della teoria del solido ideale, la quale, se non altro, h a il merito di aver c o n d o t t o ad u n a valutazione del calore specifico a volume costante conforme ai r i s u l t a t i sperimentali. 0 8 2 p . e . v a l l e t e o r i a . o v e si i n d i c h i con p la pressione t o t a l e , con o l a d e n s i t à di m a s s a , con ui e ut le energie i n t e r n e di agitazione t e r m i c a , a s s o c i a t e r i s p e t t i v a m e n t e alle o n d e l o n g i t u d i n a l i e trasversali, con f(o) la pressione int e r n a d o v u t a a l l ' i n t e r a z i o n e f r a le particelle del solido, l ' e q u a z i o n e di s t a t o del solido ideale, s o t t o p o s t o a pressione di t i p o i d r o s t a t i c o , si scrive v — /(?) + q 2 v'" u" [1] (m l, t, t) . il secondo t e r m i n e a secondo m e m b r o di q u e s t a e q u a z i o n e r a p p r e s e n t a l a pressione d i radiazione delle onde di a g i t a z i o n e t e r m i c a e cont i e n e l ' e n e r g i a ut d u e v o l t e , in relazione ai d u e possibili p i a n i di polarizzazione delle o n d e t r a s v e r s a l i . le f r e q u e n z e m a s s i m e delle onde di a g i t a z i o n e t e r m i c a sono d a t e d a ' 3 i w a , j \ \ 4 7 1 a [2] (to = l, t) nella q u a l e con vt e vt sono s t a t e i n d i c a t e r i s p e t t i v a m e n t e le velocità delle onde l o n g i t u d i n a l i e t r a s v e r s a l i , con n il n u m e r o di a v o g a d r o e con a il peso a t o m i c o . il p a r a m e t r o ym è d a t o poi dalla relazione 3 log (flmg1/3) ì> log p [ 3 ] o r a , i n q u e s t a t e o r i a , le c o s t a n t i elastiche, che d e t e r m i n a n o le f r e quenze delle onde d i agitazione t e r m i c a , d i p e n d o n o s o l t a n t o dalle forze i n t e r a t o m i c h e . p e r t a n t o q u e s t e c o s t a n t i d o v r a n n o essere v a l u t a t e in m o d o d a n o n i m p l i c a r e v a r i a z i o n i delle energie di a g i t a z i o n e t e r m i c a e d o v r a n n o i n o l t r e r i s u l t a r e i n d i p e n d e n t i dalle energie t e r m i c h e stesse. d a t o che l ' e q u a z i o n e di s t a t o [1] è v a l i d a p e r v a r i a z i o n i isotrope ed omogenee di v o l u m e , è c o n v e n i e n t e i n t r o d u r r e il m o d u l o di compressibilità k, il q u a l e è legato alle velocità delle onde elastiche longitudinali e trasversali della n o t a relazione k p 4 = vp — vt" [ 4 ] s u l l ' i n t e r a z i o n e f r a l e p a r t i c e l l e d e l s o l i d o i d e a l e 0 8 3 l a v a l u t a z i o n e d i k d o v r à essere e f f e t t u a t a m e d i a n t e la relazione e d o v r a n n o essere s o d d i s f a t t e le condizioni / 2>2t\ „ / ì>k \ («4 u r ' [6] dall'equazione di s t a t o [1], dalla [5] e dalla [6] si o t t i e n e f a c i l m e n t e df k = do 7] gym = qoyo,, (m = l, t) nelle quali g0 e y„m sono certi valori iniziali di queste grandezze. i n t r o d u c e n d o le [8] nella [31 r i s u l t a vm — vom lpo\u vomii e \ g) (m = l, t) [8] [9] queste espressioni m o s t r a n o che le velocità delle onde di agitazione t e r m i c a longitudinali e t r a s v e r s a l i d i p e n d o n o s o l t a n t o dalla d i s t a n z a m e d i a t r a le particelle del solido. è facile verificare poi che le f r e q u e n z e m a s s i m e r i s u l t a n o d a t e dalla semplice relazione 11 yom 11 — vm — vom e \ gì [10] la pressione i n t e r n a f(g) d o v u t a a l l ' i n t e r a z i o n e t r a le particelle del solido ideale si o t t i e n e ora f a c i l m e n t e . dalla [71 si h a --f = — e q u i n d i t e n u t o conto delle [1] e [9] do o risulta df _ do " \ p) 2u voi e \ o / v-nt e yot 1 — oo [ 1 1 ] 6 8 4 1>. k. valle se si p o n e ìm(g) fm{qo) = v2om i (qoyu d g [ 1 2 ] qo (m = l, t) si a v r a kq) f(eo) = //(e) meo) me) — f'(qo) [ 1 3 ] t e n u t o c o n t o c h e yai e yot sono assai prossimi, è s t a t o m o s t r a t o (') c h e la p r e c e d e n t e p u ò essere s c r i t t a , con b u o n a a p p r o s s i m a z i o n e , nella f o r m a po ' \ o i qo nella quale [14] r = yoi + — (yoi — yot) [15] con a 4 v*0 3 v-0 [10] calcoli n u m e r i c i . l ' u t i l i z z a z i o n e della [13] o della [14] p e r la v a l u t a z i o n e della pressione d o v u t a a l l ' i n t e r a z i o n e t r a le particelle del solido ideale, richiede il calcolo di u n ' i n t e g r a l e del t i p o x f(x) = / 3 e ~ ì dì; [17] o c o m e r i s u l t a d a u n a facile t r a s f o r m a z i o n e della [12] o della [14]. p . e . v a l l e f i g . 1 s u l l ' i n t e r a z i o n e f r a l e p a r t i c e l l e d e l s o l i d o i d e a l e gs7 i l calcolo n u m e r i c o d e l l a f u n z i o n e f(x) è s t a t o e f f e t t u a t o d a l l ' a u t o r e in occasione d i p r e c e d e n t i l a v o r i (*), (2), i q u a l i p e r ò c o n t e n g o n o solt a n t o d i a g r a m m i p a r t i c o l a r i , m e n t r e è c o n v e n i e n t e a v e r e a disposizione u n ' a d e g u a t a serie d i v a l o r i d i q u e s t a f u n z i o n e , p e r l ' u t i l i z z a z i o n e dell ' e q u a z i o n e di s t a t o [1]. si r i p o r t a p e r t a n t o i n q u e s t a n o t a , u n a t a b u l a z i o n e della f(x) eseg u i t a d a l l ' i s t i t u t o p e r le a p p l i c a z i o n i del calcolo, con l a c a l c o l a t r i c e e l e t t r o n i c a f . i . n . a . c . , la q u a l e è d a r i t e n e r e sufficiente p e r m o l t e app l i c a z i o n i . l e f i g g . .1 e 2 i l l u s t r a n o l ' a n d a m e n t o della f u n z i o n e f(.r). b i b l i o g r a f i a p ) p . e . v a l l e , un'equazione di stato per i soliili. « a n n a l i ili g e o f i s i c a », 6. 1 8 3 1 9 7 ( 1 9 5 3 ) . (2) p . e . v a l l e , una stima del punto di fusione del ferro sotto alte pressioni. « a n n a l i d i g e o f i s i c a » . 8 . 1 8 9 2 0 0 , ( 1 9 5 5 ) . 16amponsah.qxd 539 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words ghana – seismicity 1. introduction ghana is located on the southeastern margin of the west africa craton and is far away from the major earthquake zones that mark the present day lithospheric plate boundaries. however, a number of major and minor earthquakes have struck the country in past and present. earth tremors of magnitude ranging from 1.0 to 4.8 on the richter scale have been recorded in recent times (fig. 1). instrumental recording of earthquakes in ghana began in 1914 when the colonial government installed a milne’s single-boom seismograph in the country. it operated until 1933 when it ceased recording (banson, 1970) due to financial constraints. several minor shocks were recorded during the period when the seismograph was in operation. in march 1973, a seismograph observatory equipped with a worldwide standard seismograph network (wwssn) system was established at kukurantumi in eastern ghana. it operated continuously until october 1974 and then intermittently until continuous recording began again in 1977 (quaah, 1980). at present, there is a nine-station radio telemetric network with a central recording station at the head office of the geological survey department in accra. it is an analog recording system. plans are underway to procure a digital recording system. heat pens (hot-stylus) are used for the recordings (amponsah, 2002). 2. geological setting and previous studies about two thirds of the land surface of ghana is covered by birimian rocks that are of paleoproterozoic age. these rocks form the easternmost component of the man shield of west african craton that has remained stable since 1.7 ga. the eastern portion of the country is predominantly underlain by middle to late seismic activity in ghana: past, present and future paulina ekua amponsah geological survey department, accra, ghana abstract though ghana is far away from the major earthquake zones of the world, it is prone to earthquake disaster. ghana has records of damaging earthquakes dating as far back as 1615. the last three major events occurred in 1862, 1906 and 1939. this paper presents the main historical and current instrumental recorded earthquakes of ghana and the steps being taken to mitigate the negative effects of such disastrous occurrences in the country. the discussion is based on historical and current data obtained from the seismological observatories in accra and kukurantumi. historical earthquakes of magnitude greater than 6.0 and current local tremors with magnitudes ranging from 1.0 to 4.8 on the richter scale have been recorded since the establishment of the seismograph stations. mailing address: dr. paulina ekua amponsah, geological survey department, p.o. box m. 80, accra, ghana; e-mail: pekua2@hotmail.com 540 paulina ekua amponsah proterozoic rock units that are differently named as dahomeyan, togo, buem and voltain belts (leube, et al., 1990). work done by bacon and quaah (1981) indicates that most of the earthquakes in ghana occur in the western part of accra at the junction of the two major fault systems namely, the coastal boundary fault and akwapim fault zone. according to them most of the epicentres are located south of weija suggesting that there is little activity north-eastward along the akwapim range and westward along the coastal boundary fault. it is evident from their results that there is a low level of seismic activity scattered along the akwapim fault zone that could be due to normal faulting suggested by burke (1969). in quaah and bacon’s view, the epicentres of the located earthquakes are related to the level of activity of the faults. this they allege to be due to the existence of an old thrust zone, which has been reactivated. burke (1971) associated the seismic activity of ghana with the junction of the chain fracture zone and the african continental margin. fig. 1. seismicity of southeastern ghana. 541 seismic activity in ghana: past, present and future blundell and banson (1975), blundell (1976), suggested that continuous strike – slip movement on the romanche fracture zone adjacent to the west african continental margin may be the cause of the earthquakes. ambraseys and adams (1986) studied the history of earthquakes in west africa and were of the view that accra, the capital of ghana is the most seismically active area in the region. a recent review of geological and instrumental recordings by amponsah (2002) shows that earthquakes have occurred in the past and are still liable to occur within the vicinity of the intersection of the akwapim fault zone and the coastal boundary fault. microseismic studies in southern ghana indicated that the seismicity is associated with active faulting (essel, 1997) between the eastwest trending coastal boundary fault and a northeast-southwest trending akwapim fault zone, defined by a number of active faults. a geophysical study by essel indicated that the seismic activity is related to deep-seated faults. 3. earthquakes in ghana junner (1941) gave a report on historical earthquakes in ghana. ambraseys and adams (1986) studied the seismicity of west africa, which included the history of earthquakes in ghana. their findings on the history of earthquakes in ghana are presented in the paper. the first earthquake in ghana according to ambraseys and adams occurred in 1615. the fortress of sao jorge at elmina was destroyed. this was followed by a few aftershocks a couple of months later. the earliest recorded earthquake in ghana occurred at about 2:00 p.m. on 18 december 1636 in the axim district in southwestern ghana near the border between ghana and the ivory coast. its surface magnitude (ms) was 5.7 and the maximum intensity was ix on the msk scale (ambraseys and adams, 1986). the buildings and underground workings of a gold mine at aboasi, northeast of axim collapsed burying many of the miners (claridge, 1915). the intensities mentioned in the paper are based on the modified mercalli scale of india, unless otherwise stated. it ranges from i to x (junner, 1941). there are no records of earthquakes in ghana between 1636 and 1858 but there is the possibility that there were some minor shocks during the period. in 1858, however, an earthquake was reported to have been felt in accra (ambraseys and adams, 1986). in 1862 a very strong earthquake struck the capital city of accra and caused considerable damage to many important structures. its maximum intensity and magnitude (ml) were estimated to be ix and 6.5 respectively (quaah, 1980). ambraseys and adams (1986) obtained a surface wave magnitude greater than or equal to 6.5 and a maximum intensity of ix (uncertain) on the msk scale for this event. the castle (the seat of government) and some forts were rendered uninhabitable, together with all stone buildings (junner, 1941). three people were killed in accra. the earthquake shock was felt along the coast, east of togo and in benin, which are neighbouring countries to ghana. an earthquake, which was felt in accra in 1863, appears to have been an aftershock of the 1862 earthquake but there is no detailed information on the event to confirm this. two severe shocks rocked accra in 1871 and 1872. the magnitudes of these events were 4.6 and 4.9 respectively and the maximum intensities were vi and vii (ambraseys and adams, 1986). the 1872 event caused damages to buildings in accra. in 1883 there was another minor event, which was reported to have been felt in accra (junner, 1941). two severe shocks were felt in eastern ghana and togo at about 9:00 p.m. and 9:20 p.m. on 20 november 1906. the maximum intensity of the shocks was estimated to be viii at ho in the eastern region of ghana (junner, 1941). the magnitude (ml) estimation was 6.2 (quaah, 1980), however, ambraseys and adams’s estimation of the surface magnitude (ms) was 5.0 and the maximum intensity viii on the msk scale, which they considered uncertain. many buildings were greatly affected by this event, some cracked and others partly destroyed. no casualties were reported for this event, also felt by people in togo and benin. during the same month several minor shocks were felt lasting for three weeks. 542 paulina ekua amponsah on 11 february 1907 a fairly strong shock was felt in accra and lome. an earthquake located out to sea and accompanied by tidal waves destroyed the wharf at lome in togo on 11 may 1911. during the same day at 3:21 p.m., an earthquake was felt in accra but did not cause any damage (junner, 1941). the most destructive earthquake in ghana that caused a lot of damage and loss of life and property occurred on 22 june 1939. the earthquake occurred at about 7:20 p.m. and was felt for about 20 to 30 seconds. the earthquake was assigned a magnitude of 6.5 on the richter scale and a maximum intensity of ix (junner, 1941). ambraseys and adams (1986) estimated the surface wave and body wave magnitudes to be 6.4 and 6.8 respectively and the maximum intensity as ix (uncertain). quaah (1980) also estimated the magnitude (ml) to be 6.4. the intensity of the shock was greatest in james town, a suburb of accra. seventeen people were killed, 133 injured and an estimated one million pounds worth of damage done to buildings (junner, 1941). the earthquake was extensively felt. persons over an area of about 750 000 square kilometres and at places more than 800 kilometers from accra felt it. the epicentre was rather out to sea otherwise the felt area would have been more than the estimated figure. the earthquake was recorded teleseismically at various observatories around the world. the epicentre was located at 5.18° n, 0.13° w (international seismological summary (iss), 1939) with a focal depth of 13 kilometres. the most prominent geological effect of the 1939 earthquake was a line of fissure in the alluvium bordering the akwapim scarp between weija and fete (junner, 1941). in april 1939 two minor shocks believed to be foreshocks of the 1939 earthquake occurred in accra. as many as five aftershocks were felt in a day. this trend continued until july 1939. on 11 march 1964 and 9 february 1969 earth tremors of magnitudes (ml) 4.5 and 4.7 were recorded respectively (quaah, 1980). both events were felt in accra. the 1964 event was located not far from the multi-million dollar hydroelectric dam in akosombo. the latest tremors, which were felt in all the regional capitals, occurred on 8 january 1997, 14 february 1997 and 6 march 1997. these events caused panic among the people in the affected areas (amponsah, 2002). the national earthquake information centre (neic) in the usa located the 6 march event at 5.518n, 0.313w, body magnitude (mb) at 4.4 and at a depth of 10km. the local magnitude was 4.8 on the richter scale. many minor tremors were recorded from 1998 to 2002 with magnitudes ranging from 1.0 to 3.0 on the richter scale. the intensities range from i to iv. the magnitudes of the recorded local earthquakes were obtained from a formula adopted and modified from kanai (1983). it is given by ml = 1.7 log10 ∆ + log10 a/t where ml is the local magnitude, ∆ the epicentral distance in km, a = a/m the ground amplitude in µ, a = 1/2 (peak to peak amplitude), m = magnification and t is the period in seconds. instances where the amplitudes are clipped together, the duration method by bacon and banson (1980) was used. it is given by ml = 2.1 log d – 1.4 where d is the duration time. 4. local seismicity ghana has been struck by damaging earthquakes in the past, and is currently experiencing minor tremors regularly, as indicated by current instrumental records. the latest tremors that occurred on the 14 february and 6 march 1997 and most of the recent microseismic activities were very threatening registering more than 4.0 on the richter scale. accra, the capital city, with a growing number of large industrial activities is located in the earthquake-prone zone. microseismic studies in the country indicate that the seismic activity is associated with active faulting, particularly near the intersection of the two major fault zones, the coastal boundary fault and akwapim fault zone. most of the located earthquakes have their epicen543 seismic activity in ghana: past, present and future tres near the junction of these faults (fig. 1). the epicentres are related to the level of activity of these faults. therefore it is not recommended that any infrastructure be located near them. an awareness programme on earthquakes was began four years ago to sensitise the public about the dangers of earthquakes and what they have to do before, during and after an earthquake. they are also advised to retrofit already built structures and to adhere to building regulations of the country. 5. conclusions this paper has presented the main trends of seismic activity in ghana. the capital city accra has been struck by damaging earthquakes as well as many minor tremors from time to time. instrumental recordings of earthquakes have indicated continuous microseismic activity in southeastern ghana. with the current local events registering more than 4.0 on the richter scale, the geological survey department has embarked on an intensive education on earthquakes to the general public and schools. the outcome of the educational programme is very encouraging. apart from this, engineering infrastructures are being designed to meet the requisite standards and also are properly located to avoid any catastrophe in the near future, if a major earthquake should strike the country. acknowledgements i am grateful to mr. s.k. amponsah and prof. r.k. nkum for their comments and help. my indebtedness to the abdus salam international center for theoretical physics and the swedish international development agency for sponsoring my stay in trieste and making available to me all their facilities. references ambraseys, n.n. and r.d. adams (1986): seismicity of west africa, ann. geophysicae, 4b (6), 679-702. amponsah, p.e. (2002): seismic activity in relation to fault systems in southern ghana, j. african earth sci., 35, 227-234. bacon, b. and a.o. quaah (1981): earthquake activity in southeastern ghana 1977-1980, bull. seismol. soc. am., 71, 771-784. banson, j.k.a. (1970): methods of studying seismicity at short range, m.sc. thesis (university of edinburgh), 5-98. blundell, d.j. (1976): active faults in west africa, earthquakes planet sci. lett., 31, 287-290. blundell, d.j. and j.k.a. banson (1975): interpretation of a seismic reflection survey across the continental shelf south of accra and it’s bearing on earthquakes in the area, ghana geol. surv. rep. no. 75/1, pp. 7. burke k. (1969): the akwapim fault, a recent fault in ghana and related faults of the guinea coast, j. min. geol., nigeria, 4, 29-38. burke, k. (1971): recent faulting near the volta dam, nature, 231, 439-440. claridge, w.w. (1915): a history of gold coast and ashanti (london), 4-90. essel, p.e. (1997): geophysical investigation of the seismicity of the weija area, m.phil. thesis (university of ghana, legon), 61-63. international seismological summary (iss) (1939): annual bulletin, kew observatory, richmond. junner, n.r. (1941): the accra earthquake of june 1939, gold coast geol. surv. bull., 13, 3-41. kanai, k. (1983): engineering seismology (university of japan, tokyo). leube,a., w.hirdes, r.mauer and g.o kesse (1990): the early protorezoic birimian supergroup of ghana and some aspects of its associated gold mineralization, precambrian res., 46, 139-165. quaah, a.o. (1980): microseismicity, past seismic activity and seismic risk in southern ghana, ph.d. thesis (university of london), 10-223. f r o m the s o m i g l i a n a waves to the e v a n e s c e n t waves p . c a l o i ( * ) r e c e i v e d on m a y 1 0 t h , 1972 summary. — t h e r a y l e i g h e q u a t i o n h a s r e a l c o e f f i c i e n t s ; t h e r e f o r e , a l s o t h e c a s e of c o m p l e x c o n j u g a t e d r o o t s m a y b e e x p l a i n e d p h y s i c a l l y . t h e a u t h o r p r o v e s t h a t t h e s o m i g l i a n a w a v e s m a y b e f o r m e d f o r p o i s s o n r a t i o v a l u e s u n t i l 0 . 3 0 5 4 3 ; f o r g r a d u a l l y less r i g i d m e d i a , t h e y a r e m i s s i n g a l t o g e t h e r a n d d e g e n e r a t e i n t o e v a n e s c e n t w a v e s . riassunto. — a c a u s a d e l l ' o m o g e n e i t à d e l l e e q u a z i o n i d e l m o t o e d e s s e n d o l ' e q u a z i o n e d i r a y l e i g h a c o e f f i c i e n t i r e a l i , a n c h e il c a s o d e l l e r a d i c i c o m p l e s s e c o n i u g a t e p u ò e s s e r e fisicamente i n t e r p r e t a t o . v i e n e p r o v a t o c h e le o n d e d i s o m i g l i a n a p o s s o n o i n s o r g e r e p e r v a l o r i d e l r a p p o r t o d i p o i s s o n fino a 0 . 3 0 5 4 3 ; m e n t r e n e l c a m p o di v a r i a b i l i t à 0 . 3 0 5 4 3 < a < 0.5, c i o è i n m e z z i s e m p r e p i ù i n c o m p r e s s i b i l i , le o n d e d i s o m i g l i a n a ( p o s s i b i l i solo p e r i n c i d e n z a t r a s v e r s a l e ) d e g e n e r a n o i n o n d e e v a n e s c e n t i . 1. i n s o m e p r e v i o u s n o t e s 3' 4) i h a v e d e a l t w i t h t h e p h y s i c a l i n t e r p r e t a t i o n of t h e r o o t s of t h e r a y l e i g l i e q u a t i o n w h i c h a r e a b o v e t h e u n i t , f o r t h e v a l u e s of t h e a coefficient of p o i s s o n t o w h i c h corr e s p o n d t h r e e r e a l r o o t s f o r t h e r a y l e i g h e q u a t i o n . a n d i h a v e p r o v e d t h a t t h o s e r o o t s h a v e a n e x a c t p h y s i c a l m e a n i n g : t h e y p e r m i t t h e t h e o r e t i c a l i n t e r p r e t a t i o n of sizable g r o u p s of seismic oscillations w h i c h i n a m e d w a v e s of s o m i g l i a n a . i f o u n d t h e n t h e l i m i t s w i t h i n w h i c h t h e s o m i g l i a n a w a v e s o r i g i n a t e , w i t h i n t h e r e a l r o o t s a b o v e t h e u n i t , a n d i e m p h a s i z e d h o w i n t e r e s t i n g i t w a s t o i n c l u d e t h e s t u d y of such w a v e s i n t o t h e r e s e a r c h of s t r a t i f i c a t i o n s b u i l d i n g u p t h e e a r t h ' s c r u s t . (*) i s t i t u t o n a z i o n a l e di g e o f i s i c a . r o m a . h o w e v e r , as w a s a l r e a d y n o t e d b y s o m i g l i a n a i n t h i s t h i r d cont r i b u t i o n t o t h e p r o p a g a t i o n of seismic w a v e s (6), d u e t o t h e h o m o g e n e i t y of e q u a t i o n s of m o t i o n a n d t o t h e f a c t t h a t t h e e a y l e i g h e q u a t i o n h a s r e a l coefficients, also t h e case of c o m p l e x , c o n j u g a t e d r o o t s m a y b e e x p l a i n e d p h y s i c a l l y w i t h t h e s e p a r a t i o n of t h e r e a l f r o m t h e i m a g i n a r y p a r t of r o o t s . t h i s is w h a t i a m u n d e r t a k i n g as follows. 2. f i r s t of all, l e t u s t r y t o f i n d a n a l y t i c a l l y t h e v a l u e of a separ a t i n g t h e r e a l field f r o m t h e c o m p l e x field f o r r o o t s a b o v e t h e u n i t . this v a l u e h a s a l r e a d y b e e n o b t a i n e d e m p i r i c a l l y in t h e p r e v i o u s n o t e (4). as is k n o w n , t h e e a y l e i g h e q u a t i o n is e x p r e s s e d in i t s m o s t k n o w n f o r m w i t h t h e u s u a l m e a n i n g of s y m b o l s (j): f r o m w h i c h , a f t e r h a v i n g m a d e f1) v32 = x follows vi2 i — 2 a r e m e m b e r i n g t h a t — = , we p u t * 2 ( 1 — a) 1 1 — — 1 6 ~~ 2 (1 — a) ' w h e n c e [1] b e c o m e s x3 — 8 z2 + 8 ( ! + 2 e) 1 — 1 6 6 = 0 • [2] l e t us see h o w t h e r o o t s of t h i s e q u a t i o n v a r y w h e n a v a r i e s b e t w e e n 0 a n d , t h a t is f o r n o w we f r e e [2] f r o m t h e second d e g r e e t e r m i n y. t o t h i s e n d we p u t f r o m t h e s o m i g l i an a w a v e s to t i i e e v a n e s c e n t w a v e s 4 7 3 e q u a t i o n [2] c h a n g e s t h e n i n t o 8 _ c 16 ( 28 \ ¿ 3 + y (6 e 5 ) 6 + — ( 5 e — y ) = 0 . w e m a k e 8 „ 16 / , 28 \ y ( b e 5 ) = p , t h u s o b t a i n i n g d3 + p d + q = 0. if p a n d q a r e real, we k n o w f r o m t h e m a t h e m a t i c a l a n a l y s i s t h a t t h e c o n d i t i o n f o r t h e t h r e e r o o t s of [3] t o b e r e a l is e x p r e s s e d i n t h e r e l a t i o n qp3 + j — < 0 . 4 27 c o n s i d e r i n g t h e v a l u e s of p a n d q, we h a v e in o u r case (45 e — 28)2 + (12 b — 1 0 ) 3 j . f o r t h e r o o t s t o b e real, we m u s t h a v e , = 1 1 4 27 3 6 a(e) = (45 e — 28)2 + (12 £ — 10)3 < 0 . [3] i t is easily f o u n d t h a t t h e v a l u e of e which a n n u l l s zl(£) is £ = 0,6785. a n d since 1 1 a = 1 o £ i t follows a = 0,26308, w h i c h p r a c t i c a l l y coincides w i t h t h e v a l u e p r e v i o u s l y o b t a i n e d (<j = 0,26305). i t is i n c o r r e s p o n d e n c e t o t h i s v a l u e t h a t t h e t w o r o o t s a b o v e t h e u n i t of t h e r a y l e i g h e q u a t i o n coincide; in f a c t o n e o b t a i n s = 3,5754 (4). t h e r e f o r e , i n o r d e r t o h a v e a r e a l r o o t a b o v e t h e u n i t a n d t w o c o m p l e x c o n j u g a t e d r o o t s , i t m u s t b e 4 7 4 p . c a l o i t o w h i c h c o r r e s p o n d s f o r a t h e field of v a r i a b i l i t y : 0,26305 < a < 0,5 . 3. t h e r e r e m a i n e d n o w t o c a l c u l a t e a series of c o m p l e x , c o n j u g a t e d r o o t couples f o r a v a l u e s w i t h i n t h e a b o v e l i m i t s . c a l c u l a t i o n s h a v e b e e n m a d e f o r t h e f o l l o w i n g a v a l u e s : 0,265; 0,27; 0,30; 0,305; 0,35; 0,40; 0,50. t h e e a y l e i g h e q u a t i o n s p e r t i n e n t t o t h e a b o v e a v a l u e s a r e : f o r a = 0,265 3,1277 x3 — 25,022 x2 + 59,065 x — 34,043 = 0, f o r a = 0,27 3,1739 x3 — 25,391 x* + 60,174 x — 31,782 = 0, f o r a = 0,3 3,50 x3 ~ 2 8 > ° x2 + 68;° x ~ 40>° = °> f o r a = 0,305 3,564 x 3 — 28,512 x 2 + 69,536 x — 41,024 = 0, f o r a = 0,35 4,333 x3 ~ 34,66 x2 + 8 8 > ° x ~~ 33>33 = °> f o r or = 0,4 3 x3 ~ 2 4 z2 + 64 x — 40 = 0, f o r a = 0,5 z3 — 8 x2 + 24 x ~ 1 6 = t h e c o r r e s p o n d i n g r o o t s a r e : t a b l e 1 a x i 0 , 2 6 5 0 , 8 4 9 8 3 , 5 7 5 2 ± i 0 , 1 6 2 2 1 0 , 2 7 0 , 8 5 1 2 5 3 , 5 7 4 3 ± i 0 , 3 1 2 0 0 , 3 0 , 8 6 0 0 9 3 , 5 7 1 4 ± i 0 , 7 2 8 4 0 , 3 0 5 0 , 8 6 1 5 4 3 , 5 6 9 0 ± i 0 , 7 8 9 6 0 , 3 5 0 , 8 7 4 0 3 , 5 6 2 5 ± i 1 , 1 7 9 1 0 , 4 0 , 8 8 7 7 3 , 5 5 6 2 ± i 1 , 5 4 0 6 0 , 5 0 , 9 1 2 8 3 , 5 4 3 6 ± i 2 , 2 3 0 2 4. i a s k e d myself w h e t h e r all a v a l u e s i n t h e i n t e r v a l 0,26305 0,5, w e r e l e a d i n g t o r o o t s t o w h i c h c o r r e s p o n d s o m i g l i a n a w a v e s . t h o s e r o o t s a r e c o m p l e x a n d c o n j u g a t e d a n d s o m e of t h e i r v a l u e s h a v e b e e n g i v e n u n d e r 3. t h e y a r e c o m p l e x , h e n c e also t h e v a l u e s of (i. 2), / — vrv3 = v2 v x , t a n g 2 ei = — x — 1 , t a n g 2 e2 = x — 1 . [4] vi~ l e t u s i n d i c a t e a g e n e r a l c o m p l e x r o o t as follows 7.2.3 = r + ic . f r o m t h e s o m i g l i an a w a v e s to t i i e e v a n e s c e n t w a v e s 4 7 5 t h e f o r m u l a s [10j will t h e n b e w r i t t e n v22 . v22 . 1 — t a n g 2 ei = r —— 1 -(-1 c — , t a n g e2 = r — 1 + i c, v3 = v2 v r + i c . vi' vi~ t h e r e l a t i o n r + ic=\jr-\-ic, allows t o o b t a i n r = u j j * + c2 + r y / g c = | v r2 + c2 — r w , a f t e r t h e v a l u e of a, i n c l u d e d in t h e a b o v e i n t e r v a l , h a s b e e n a s s i g n e d , t h e r a y l e i g h e q u a t i o n f u r n i s h e s t h e c o r r e s p o n d i n g c o u p l e of c o m p l e x , c o n j u g a t e d r o o t s . t h u s r a n d c a r e o b t a i n e d a n d t h e n c e t a n g ci, t a n g e2 a n d va p e r t a i n i n g t o t h e c h o s e n a v a l u e . a f t e r s e p a r a t i n g t h e r e a l p a r t f r o m t h e i m a g i n a r y one, we can t h u s a r r i v e a t t h e real v a l u e s of ei (if e x i s t i n g ) , of e2 a n d of v3. l e t u s i n d i c a t e t h e l a t t e r va = vi r while p u t t i n g v3 = t'2 c , w h e r e t h e n e g a t i v e v a l u e f o r c is b e i n g t a k e n . o n t h e b a s i s of t h e r a n d c v a l u e s t a k e n f r o m t h e p r e v i o u s t a b l e 1 we o b t a i n b y v a r y i n g a as follows: t a b l e 2 a v^/vj* t a n g 2 e1 ei t a n g 2 e2 «2 r 0 , 2 6 5 0 , 3 1 9 7 3 0 , 1 4 3 1 0 2 0 ° 4 5 ' , 2 5 2 , 5 7 5 2 5 8 ° 0 4 ' , 2 5 1 , 8 9 1 3 0,27 0 , 3 1 5 0 7 0 , 1 2 6 1 5 19°33' 2 . 5 7 4 3 5 8 ° 0 4 ' 1,8924 0 , 3 0,2857 0 , 0 2 0 3 5 8 ° 0 7 ' 2 , 5 7 1 4 5 8 ° 0 3 ' 1,8995 0 , 3 0 5 0 , 2 8 0 5 0 , 0 0 1 1 0 4 5 1°51' 2 , 5 6 9 0 5 8 ° 0 2 ' 1,9006 0 , 3 5 0 . 2 3 0 7 7 — 0 , 1 7 7 9 — 2 , 5 6 2 5 5 8 ° 0 0 ' 1,9125 0,4 0 , 1 6 6 6 7 — 0 , 4 0 7 3 — 2,5562 5 7 ° 5 8 ' , 5 1,9277 0,5 0 — 1 — 2 , 5 4 3 6 5 7 ° 5 5 ' 1,99605 i t follows f r o m t h e a n a l y s i s of t h e t a b l e 2 t h a t t h e s o m i g l i a n a w a v e s m a y v a r y f o r a v a l u e s u n t i l 0,31; f o r g r a d u a l l y less rigid m e d i u m p . c a l o i t h e y a r e m i s s i n g a l t o g e t h e r a n d d e g e n e r a t e i n t o o r d i n a r y t r a n s v e r s a l w a v e s . t h e v a l u e s of efficient angles, f o r l o n g i t u d i n a l i n c i d e n c e , p r e s u p p o s e i n c i d e n c e s n e a r i n g r a p i d l y t h e r i g h t a n g l e . so f a r as t r a n s v e r s a l i n c i d e n c e is c o n c e r n e d , t h e efficient angles i n c r e a s e s l i g h t l y as t h e r i g i d i t y of t h e m e d i u m d e c r e a s e s a n d r e a c h a l i m i t a n g l e of i n c i d e n c e of a b o u t 32° t o w h i c h c o r r e s p o n d s t h e t o t a l r e f l e c t i o n . c o n s i d e r i n g as well t h e c o m p l e x c o n j u g a t e d r o o t s of t h e r a y l e i g h e q u a t i o n , t h e efficient angles b r i n g i n g a b o u t s o m i g l i a n a w a v e s a r e t h e ones c o r r e s p o n d i n g t o t h e r o o t s f o r t h e f o l l o w i n g field of v a r i a b i l i t y of a : 0 < a < 0,31 , a l t h o u g h , p r a c t i c a l l y (4), t h i s is r e d u c i n g t o 0,25 < a < 0,31 . a n y h o w , t h e use of c o m p l e x c o n j u g a t e d r o o t s i n d i c a t e s a n enl a r g e m e n t a n d t h e l i m i t of t h e field of v a r i a b i l i t y of t h e p o i s s o n coefficient. c o n c e r n i n g t h e p r o p a g a t i o n v e l o c i t y of t h e s o m i g l i a n a w a v e s (if e x i s t i n g ) , it is n o t e d t h a t i t increases as a i n c r e a s e s ( t h a t is as r i g i d i t y decreases) a n d r e a c h e s t h e m a x i m u m v a l u e v'3 = 1,9006 -v2 f o r t h e l i m i t v a l u e of a = 0,305. if in t h e e x p r e s s i o n s of mi, u2, wi, w2 (4) we p u t @ (x — v31) u n d e r p o w e r f o r m (p (e—v31) = fl<p(*—v), a n d o b s e r v i n g t h a t v3 = v'3 + i v3, we will h a v e <t>[x — {v3 + iv„) t] = e<p<*—®'»t) •«?"»*, w h e r e p i n d i c a t e s t h e p u l s a t i o n of t h e oscillation a n d v3 is a n e g a t i v e c o n s t a n t w h i c h m a y b e c o n s i d e r e d as t h e e x t i n c t i o n coefficient of t h e oscillation i n t i m e . f o r a = 0,265, f o r i n s t a n c e , v3 = — 0,043012 v2, a n d f o r a = 0,27, v3 = -— 0,08277-v2-, w h e r e a s f o r a — 0,305, we h a v e v3 = — 0,2076-» 2 . t h e r e f o r e , a t e q u a l f r e q u e n c i e s t h e p r o p a g a t i o n of t h e s o m i g l i a n a w a v e is e x t i n g u i s h e d m o r e q u i c k l y as r i g i d i t y decreases. f r o m t h e s o m i g l i an a w a v e s to t i i e e v a n e s c e n t w a v e s 4 7 7 5. let. u s h a v e a closer look h o w t h e s o m i g l i a n a w a v e s c h a n g e w h e n t h e i r p r o p a g a t i o n is in elastic m e d i a w h o s e a coefficient s h o w s a t r e n d t o w a r d t h e v a l u e of 0,30543. l e t u s consider t h e case of t r a n s v e r s a l i n c i d e n c e , w h e r e w e h a v e t a n g ei = — (y — 1 j 2 , t a n g e2 = [y_ — 1 j ^. if 0 { x — v31) is n o t p e r i o d i c a l , h e r e a p p l i e s t h e r e l a t i o n (••) j t + i / u i f 1 wo / »i2 i ,0 . ai = — a2 (2 — x ) . — + z i r wo \ j x a l w a y s differs f r o m 2. t h e f o l l o w i n g t a b l e 3 is v a l i d : t a b l e 3 a ( ' v 4 ~ t 0 , 2 6 3 0 5 0 , 3 8 6 8 1,6048 0 , 2 6 5 0 , 3 7 8 3 1 , 6 0 4 7 5 0,27 0 , 3 5 5 2 1,60445 0,3 0 , 1 4 2 6 5 1 , 6 0 3 5 5 0 , 3 0 5 0 , 0 3 3 2 3 5 1,6028 0 , 3 0 5 4 3 0,0 1,6028 0, 4 i m a g i n a r y 1,5988 as a t e n d s t o w a r d t h e v a l u e 0,30543, ai is t h u s t e n d i n g t o w a r d t h e i n f i n i t e . n o w , i n t h e e x p r e s s i o n s of u0, w„ [see [1] of (4)], e^, t h e q u a n t i t y c h a r a c t e r i z i n g t h e l o n g i t u d i n a l c o m p o n e n t , a c t s as a d e n o m i n a t o r in t h e r e l a t i v e t e r m s . t h e r e f o r e , t h e s e a n n u l l each o t h e r , n a m e l y t h e c o n t r i b u t i o n of t h e l o n g i t u d i n a l w a v e in t h e f o r m a t i o n of t h e s o m i g l i a n a w a v e falls a w a y , a n d t h i s d e g e n e r a t e s i n t o a t r a s v e r s a l w a v e . h o w e v e r f o r a = 0,30543, t h e efficient a n g l e of i n c i d e n c e of t h e t r a n s v e r s a l w a v e s , is 31°57',4, w h i c h coincides w i t h t h e a n g l e of total reflection of t h e i n c i d e n t t r a n s v e r s a l w a v e . i n f a c t is: 4 7 8 p . calo i so t h a t sin ¿2 = — = 0,52927 , vi w h i c h gives f o r i2 t h e a b o v e v a l u e . t h e r e f o r e , t h e efficient a n g l e f o r t r a n s v e r s a l w a v e s i n c i d e n t a t t h e i n t e r s u r f a c e w i t h t h e o u t e r s t r a t i f i c a t i o n , a p p r o x i m a t e s in less rigid m e d i a t h e a n g l e of i n c i d e n c e t o which c o r r e s p o n d s t h e t o t a l reflection, w h i c h is r e a c h e d , as could b e seen, f o r a = 0,30543. h e n c e , in t h e field w h e r e a v a r i e s f r o m 0,30543 t o 0,5 t h e r e a r e n o s o m i g l i a n a w a v e s , since f o r t h e m t h e t o t a l reflection of t h e i n c i d e n t t r a n s v e r s a l w a v e s t a k e s p l a c e . t h e f o r m a t i o n of s o m i g l i a n a w a v e s r e q u i r e s a p h y s i c a l l y finite m e d i u m b e y o n d t h e s u r f a c e w h i c h is h i t b y i h e w a v e c o m i n g f r o m a n i n d e f i n i t e m e d i u m (3). w h e n t h e l o n g i t u d i n a l r e f l e c t e d w a v e v a n i shes as p r o g r e s s i v e o r d i n a r y w a v e , f o r s a t i s f y i n g t h e c o n d i t i o n s a t t h e i n t e r s u r f a c e it is n e c e s s a r y t o i n t r o d u c e a n e v a n e s c e n t w a v e . if we i n d i c a t e t h e t r a n s v e r s a l r e f l e c t e d w a v e (oscillating, of course, in t h e p r i n c i p a l p l a n e ) b y y> = ekz sin (cot — ax) we will h a v e dm 7)w ... u = — = — a e cos(co t — a x) : w = = ae sin(a>i — a x) , ' 1 t>z " w h e r e u a n d w a r e t h e h o r i z o n t a l a n d v e r t i c a l m o t i o n c o m p o n e n t s . t h e r e s u l t a n t of t h e s e m o v e m e n t s , h o w e v e r , is t h e socalled evanescent wave w h i c h — as t h e r a y l e i g h w a v e s — forces t h e r e a c h e d p a r t i c l e t o d e s c r i b e a n elliptical m o t i o n [(5) p a g e s 300-304], i n t h e v a r i a b i l i t y field ,-i 0,30543 < a < 0,5 , w h i c h m e a n s t h a t in a l w a y s m o r e i n c o m p r e s s i b l e m e d i a t h e somigliana waves (possible o n l y b y t r a n s v e r s a l incidence) degenerate into evanescent waves. i n c o n f o r m i t y t o w h a t h a p p e n s i n t h e p r o p a g a t i o n of t h e l i g h t , t h e v e l o c i t y of e v a n e s c e n t w a v e s i n t h e s e c o n d m e d i u m is t>2/sin¿2, w h e r e ¿2 is t h e a n g l e of i n c i d e n c e . p r a c t i c a l l y i t coincides, t h e r e f o r e , w i t h bv2 w h e r e r is t o b e t a k e n f r o m t h e t a b l e 2. i n case of t h e l i m i t a n g l e of i n c i d e n c e (¿2 = 31°58'), is i n f a c t vífsinu = 1,8894 vo, e q u a l — a t l o w e r t h a n 1/1000 — t o t h e v a l u e of v'3. i f i g . 1 e x a m p l e s of c 0 ,i (t = 6 8 s a b . ) , c i , 2 (t = 3 4 s a b . ) a n d c2,3 (71 = 23 s a b . ) w a v e s b y t h e a u t h o r g e n e r i c a l l y i n d i c a t e a s somigliana waves — r e c o r d a t s o m p l a g o (011 t h e c a v a z z o l a k e ) b y a s e i s m o g r a p h , w i t h f r e e p e r i o d of a b o u t 120s a n d o p t i c a l m a g n i f i c a t i o n , in o c c a s i o n of a l a s k a e a r t h q u a k e of j u l y 30, 1972 ( 5 7 ° , o n 135",9\v: h = 2 1 . 4 5 . 1 1 , 1 g m t : li = 10 k m : m = 7,8) a t a n e p i c e n t r a l d i s t a n c e of a b o u t 8 3 5 0 k i n s . f o r l a r g e e a r t h q u a k e ( a s t h i s a l a s k a e a r t h q u a k e ) c 0 .i w a v e s c a n a f f e c t t h e o u t e r l a y e r of m a n t l e , f r o m t h e t o p of t h e a s t e n o s p h e r e ( l o w v e l o c i t y c h a n n e l ) t o t h e e a r t h ' s s u r f a c e ( t h i c k n e s s of a b o u t 70 k m s ) . 4 8 ( 1 r e f e r e n c e s (') calci p . , 1966. l'equazione di rayleigh e le onde di somigliana. i : la teoria delle onde di rayleigh. " a t t i aoc. n a z . d e i l i n c e i " , classe s c i e n z e fis., m a t . e n a t . , x l i , s e r . v i i i , 1-2 f e r i e . (2) caloi p . , 1966. l'equazione di rayleigh e le onde di somigliana. i i : la teoria di somigliana: rettifiche, conseguenze. " a t t i a c c . n a z . d e i linc e i " , classe s c i e n z e fis., m a t . e n a t . , x l i , s e r . v i l i , 5. (3) caloi p . , 1967. 1/equazione di rayleigh e le onde di somigliana. i l i : le ci,j sono onde di somigliana. loro importanza per lo studio della crosta terrestre. " a t t i a c c . n a z . d e i l i n c e i " , classe s c i e n z e fis., m a t . e n a t . , x l i i i , s e r . v i l i , 6 . (*) caloi p . , 1969. l'equazione di rayleigh e le onde di somigliana. i v : limiti d'insorgenza delle onde di somigliana: loro esclusiva formazione nel piano principale. " a t t i a c c . n a z . dei l i n c e i " , classe s c i e n z e fis., m a t . e n a t . , x l v i , s e r . v i i i , 1. (5) caloi p . , 1955. ci,j. " a n n a l i d i g e o f i s i c a " , v i i i , 3, p p . 2 9 3 3 1 3 . (6) somigliana c., 1918. sulla propagazione delle onde sismiche. nota i i i . " a t t i a c c . n a z . dei l i n c e i " , classe s c i e n z e fis., m a t . e n a t . , x x v i i , s e r . v, 1. layout 6 1 annals of geophysics, 62, 2, vo217, 2019; doi: 10.4401/ag-8048 “understanding basaltic lava flow morphologies and structures for hazard assessment„ sonia calvari istituto nazionale di geofisica e vulcanologia, osservatorio etneo sezione di catania, catania, italy 1. introduction the final morphology of a solidified lava flow is the result of complex interactions between the lava and the environment in which it is emplaced. lava flows display several shapes, sizes and surface morphologies, mainly determined by the physical and chemical properties of the lava, its temperature, rate of effusion, crystal and gas content, but also by the local conditions such as gravitational field strength and topography [peterson and tilling, 1980; kilburn and guest, 1993; harris et al., 2017a, 2017b]. the main parameter influencing the morphology of a lava flow is its non-newtonian rheology, which causes the lava to rest on a slope, although unconfined by topography, as soon as the supply ceases [hulme, 1974]. in turn, lava rheology depends on its composition, temperature, crystal and gas contents [giordano and russell, 2018]. the second most important parameter influencing lava flow morphology is the rate of effusion [walker, 1971], which defines the maximum length that a channelized lava flow can attain [walker, 1973]. morphologies of solidified basaltic lava flows can be broadly divided into two categories: (i) aa or (ii) pahoehoe, whereas the term “block lava” is generally used for thick brecciated lavas that are typically more silicic than basalt [harris et al., 2017a, 2017b], and thus is not considered here. aa is the type of lava that in solidified form is characterized by a rough, jagged, spiny and generally clinkery surface (figure 1a). conversely, pahoearticle history received december 5, 2018; accepted march 26, 2019. subject classification: lava flows; basaltic volcanoes; lava morphology; hazard assessment. abstract lava flow surface morphologies are like pages of a book. if we are able to read the writing of that book, we can understand its content, and learn, act, and react accordingly. in the same way, if we understand lava surface morphology, recognise how it formed and the hazard it poses while flowing, we can adopt actions to protect from lava flow invasion our villages, infrastructures and local population. the surface of lava is a function of intrinsic and extrinsic qualities, and their combination results in different shapes, sizes, and complexities, as well as in different hazards. initial sheet flows spreading at high speed have great potential for devastating land, as happened in hawaii in may-august 2018 [neal et al., 2018]. however, their destructive potential significantly decreases with time and distance from the vent. conversely, lava oozing from the distal exit of lava tubes moves slowly but allows the tubes to expand, increasing gradually and slowly the potential hazard for invasion of more remote lands. in this paper, i present an overview of diverse lava flow surfaces, morphologies and structures in a framework of their generating eruptive parameters, in order to suggest preliminary but prompt hazard evaluations that could be applied during the initial phases of effusive volcanic crises at basaltic volcanoes worldwide. hoe lava displays a smooth, billowy, or ropy surface and normally comprises interconnected multiple flow lobes (figure 1b). harris et al. [2017b] offer a comprehensive review of all terms used to define lava flow morphology, describing also the many different varieties of pahoehoe and aa recognised in volcanology. for the aim of this paper, which is essentially to characterize lava flows in order to reveal their hazard, i consider only the two basic types, aa and pahoehoe. it is here worth noting that aa lava flows generally advance faster than pahoehoe, and therefore pose a greater hazard [kauahikaua and tilling, 2014]. thus, a greater attention will be devoted to the emplacement of the faster and more hazardous aa lava flows. most hawaiian lavas initially erupt as pahoehoe, and may change to aa downstream as they flow away from the vent [peterson and tilling, 1980; lipman and banks, 1987; cashman et al., 1999]. this transition is caused by cooling and by increase in viscosity and yield strength during flowage [kilburn, 1981]. at etna, stromboli and fogo volcanoes instead, aa lavas are the most common surface morphology among early lava flows, with pahoehoe becoming more widespread later in an eruption when effusion rate declines [calvari et al., 1994, 2005, 2018; calvari and pinkerton, 1998; lodato et al., 2007]. although pahoehoe and aa can be part of the same lava flow, they differ for temperature, viscosity, and vesicles shapes, with most active pahoehoe flows being less viscous and erupted at higher temperature than aa flows, and having vesicles normally as regular spheroids [peterson and tilling, 1980]. conversely, aa flows have vesicles that tend to be irregularly shaped, as a result of the deformation caused by movement during the final stages of solidification [peterson and tilling, 1980]. even though aa lava tends to be more viscous, molten lava of approximately the same initial viscosity may form either pahoehoe or aa, with the transition from one to the second determined by a balance between viscosity and motion. thus, in addition to the effect of increasing viscosity, lava tends to change into aa also when subjected to flow turbulence and internal shearing, such as during vigorous fountaining, pouring down steep slopes or prolonged flowage for great distances [peterson and tilling, 1980]. pahoehoe and aa lavas have different modalities of emplacement. pahoehoe starts as small lobes inflating to form wide and thick sheet flows several times greater than initial flow lobes [figure 1b; hon et al., 1994; keszthelyi and denlinger, 1996; harris et al., 2007b]. conversely, aa lavas move as a caterpillar, producing an accumulation of clinkers in the front zone overrun by fluid lava, thus forming a sort of sandwich with clinkers above and below and a fluid compact lava in between (figure 1a). aa lavas move by pulses and display significant variations of the magma flux, with pulses of increased flux in the front region considered as the distal result of more frequent flux changes in the vent region [bailey et al., 2006; james et al., 2007; favalli et al., 2010]. pahoehoe flows also expand intermittently, with alternating phases of lobes inflation and frontal expansion [hon et al., 1994]. as with pahoehoe, aa lava sonia calvari 2 figure 1. (a) expanding aa lava flow front at mount etna (italy), 18 july 2001. the width of the image is about 15 m. photo by ingv. (b) expanding pahoehoe flow lobes at kilauea volcano (hawaii). the width of the image is about 3 m. flows are prone to inflate, forming extensive and complex lava tubes, although this process is more difficult to detect than in pahoehoe flows [calvari and pinkerton, 1998; james et al., 2009]. at etna volcano, pulses during the expansion of aa lava flows [lautze et al., 2004; bailey et al., 2006; james et al., 2007, 2010, 2012; favalli et al., 2010] generate characteristic surface morphologies, influence volume distribution around the lava flow field, and construct the distal, medial and proximal channel segments [favalli et al., 2010]. the largest historic basaltic lava flow field is probably that formed in iceland during the 1783-1784 laki eruption, when 14.7 km3 of lava erupted at initial instantaneous effusion rate (ier) of up to 8.7 × 103 m3 s3 lava flow morphologies and hazard lava type volcano eruption peak tadr (m3s-1) flow front velocity, reported flow front velocity m s-1 reference aa sheet flow o shima (japan) 1951 50 km/h 13.889 mason and foster, 1953 aa sheet flow etna (italy) 1991-93 2 km/day 0.023 calvari et al., 1994 channelized aa lava flows piton de la fournaise, (la réunion) dec 2010 38 7.30 soldati et al., 2018 channelized aa lava flows holuhraun (iceland) 2014-15 350 1.13 km/day 0.131 pedersen et al., 2017 channelized aa flows mauna loa (hawaii) 1984 806 lipman and banks, 1987 channelized aa flows etna (italy) may 2001 0.7 0.29 m/s 0.29 bailey et al., 2006 channelized aa flows etna (italy) 1981 640 1.67 0.56 guest et al., 1987 coltelli et al., 2012 channelized aa flows etna (italy) 1983 50 0.02 guest et al., 1987 aa flow front mauna loa (hawaii) 1950 1,179-1,769 5.8 miles/h 2.59 finch and macdonald, 1953 aa flow front etna (italy) 2006 5 m/h 5.00 favalli et al., 2010 pahoehoe sheet flow kilauea (hawaii) 2014-15 400-500 m/day 0.006 poland et al., 2016; patrick et al., 2017 pahoehoe sheet flow kilauea (hawaii) 23 jan 1988 1.1 150 m/1.25 h 0.029 hon et al., 1994 pahoehoe flow kilauea (hawaii) 1960 58-116 30 km/h 8.333 macdonald 1962 pahoehoe laki (iceland) 1783-84 8,700 15-17 km/day 0.17-0.20 thordarson and self, 1993; guilbaud et al., 2005 table 1. peak values of lava flows emplacement rates (tadr = time-averaged discharge rate) as a function of their surface morphology. sonia calvari 4 1, forming in 8 months ~600 km2 of a mostly pahoehoe lava flow field [thordarson and self, 1993; guilbaud et al., 2005]. lava flow speed during the initial phases of that eruption was up to 15-17 km/day [table 1; thordarson and self, 1993]. the lava flow field initially emplaced as thin pahoehoe lobes that gradually coalesced into larger sheet lobes [guilbaud et al., 2005], an expansion similar to most pahoehoe lava flow fields observed today on kilauea [hon et al., 1994; kauahikaua et al., 1998; patrick et al., 2017]. at basaltic volcanoes like etna and stromboli (italy), kilauea (hawaii), piton de la fournaise (la réunion), fogo (cape verde), or holuhraun (iceland), the general shape of a complex lava flow field is defined by a few arterial lava flows generally displaying aa texture, with its outline modified by secondary lava flows normally having a pahoehoe surface [guest et al., 1987; kilburn and lopes, 1988, 1991; calvari et al., 2005, 2018; rhéty et al., 2017; pedersen et al., 2017]. when lava tubes develop within complex lava flows, their hidden path is revealed by the distribution of skylights, ephemeral vents or breakouts, tumuli, shatter rings, or pressure ridges [guest et al., 1980; mattox et al., 1993; calvari and pinkerton, 1998; kauahikaua et al., 1998, 2003; calvari et al., 1994, 2018]. the formation of lava tubes significantly increases the hazard posed by an expanding lava flow because their insulating effect allows the flow to spread further down slope. in the following sections, i will give a brief introduction of the terms used to describe the effusion rate when calculated on different time spans, and then describe the modalities of emplacement of lava flows and flow fields, their structures and speed of formation considering several examples worldwide, in order to assess their potential hazard and suggest how to organize risk mitigation. 2. tadr, er, ier harris et al. [2007a; 2017a] present a review of effusion rate definitions (figure 2), here summarised to have clear in mind the time scale and size of the phenomena that i am considering. following harris et al. [2007a, 2017a], i use instantaneous effusion rate (ier) for the volume flux of erupted lava that is feeding a flow at any particular point in time; time-averaged discharge rate (tadr) for the volume fluxes over a given time period (e.g., monthly, weekly, daily); eruption rate (er) for the total volume of lava emplaced since the beginning of the eruption divided by the time since the eruption began; and mean output rate (mor) for the final volume of erupted lava divided by the total duration of the eruption, which can be obtained only once that the eruption is over (figure 2). dense rock equivalent (dre) standard is desirable when comparing available data because it is independent from vesicle content, but past data and those collected from ground measurements rather than from satellite often fail to mention if the volume is expressed as bulk (including vesicles) or dre (excluding vesicles). this is the case of most of the data in table 1. at the beginning of many basaltic fissure eruptions, the discharge rate increases rapidly to a maximum value, then gradually drops to lower values as the eruption proceeds and the feeder dike empties [wadge, 1981]. this behaviour has been observed and documented at several basaltic effusive eruptions and on different volcanoes, such as etna and stromboli (italy), fogo (cape verde), holhuraun (iceland), piton de la fournaise (la réunion), kilauea (hawaii), [hon et al., 1994; calvari et al., 2005, 2018; harris et al., 2005a, 2011, 2012; lodato et al., 2007; spampinato et al., 2008a, 2008b; ganci et al., 2012a, 2013, 2018; cappello et al., 2016; coppola et al., 2017; patrick et al., 2017; pedersen et al., 2017]. the general trend of declining discharge rate is overlapped by a local variability giving rise to pulses of the order of hours along the lava flow channel [lautze et al., 2004; bailey et al., 2005; harris et al., 2005b], that refigure 2. scheme modified after harris et al. [2007a], showing a theoretical discharge rate curve and the temporal scale at which each of the measurement definitions apply. ier = instantaneous effusion rate; tadr = time-averaged discharge rate (weekly, daily); er = eruption rate; mor = mean output rate, t0 = eruption start time; tstop = eruption stop time. 5 lava flow morphologies and hazard flect more frequent pulses occurring at the main vent [james et al., 2007]. high effusion rate has been related to a greater crystal content of the lava (up to 70%), whereas it apparently has no effect on lava porosity, which is uniformly decreasing with distance from the vent [soldati et al., 2018]. an important parameter affecting lava flow emplacement speed is the roughness of the topographic surface on which the flow is spreading, with greater roughness increasing flow brecciation and favouring the change from pahoehoe to aa [rumpf et al., 2018]. 3. lava flows and lava flow fields many lava flows are divisible into smaller lava bodies, or flow units, each of which has a top which cooled and solidified before another flow unit superposed on it [walker, 1971]. a lava flow field or compound lava flow is a lava body divisible into flow units (figure 3) produced by a single vent or fissure during an uninterrupted period of effusion [walker, 1971; kilburn, 1996]. superposed flow units in a compound flow are separated by an interval of time ranging from minutes to more than a year [walker, 1971]. effusive volcanic eruptions form either single lava flow units or compound lava flow fields, depending on the eruption duration: short-lived effusive pulses form single lava flows, also defined as volume-limited lava flows [guest et al., 1987], whereas long-lasting eruptions promote the emplacement of complex lava flow fields [kilburn and lopes 1988, 1991; calvari and pinkerton, 1998; calvari et al., 2005, 2010, 2018; patrick et al., 2017; pedersen et al., 2017]. a flow may generate major new lava streams by bifurcation, breaching and overflow of vents, channels, or tubes. bifurcation is controlled by topography and is a random process, breaching and overflows are cooling controlled and more systematic [kilburn and lopes, 1988]. thus, the growth pattern of a lava flow field may follow comparable cooling trends beneath the randomizing effect of bifurcation [kilburn and lopes, 1988]. a simple evolutionary sequence starts with lava flow field widening until an equilibrium width is reached, followed by flow lengthening until inhibited by its crust [kilburn and lopes, 1988]. with further crustal growth, flow thickening starts at the front and migrates upstream, leading to maintained thickening, breaching or overflow [kilburn and lopes, 1988; kilburn and guest, 1993]. the development of lava tubes may significantly modify this general model, promoting longer flows than the chanfigure 3. aerial view of the active complex lava flow field during stromboli’s 2002-03 flank eruption, showing several vents (red) feeding several flow units displaying both pahoehoe and aa surfaces. photo by ingv. nel-fed equivalent [calvari and pinkerton, 1998; kauahikaua et al., 1998]. walker [1971] observed that whether a flow is compound or simple seems to be determined primarily by the rate of extrusion of lava at the surface, with a low rate favouring the formation of compound flows, and a high rate producing simple flows. in addition, the rate of extrusion also influences the morphology of lava flows, with aa surfaces forming in conditions of high ier, and pahoehoe flows forming at low ier [griffiths and fink, 1992a, 1992b; lyman et al., 2005; cashman et al., 2006]. the hazards posed by a single lava flow unit and by a compound lava flow field are extremely different as are their sizes. a compound lava flow field usually has a larger surface area than a single flow unit. in addition, compound lava flow fields often comprise lava tubes, which increase even more the possibility of lava to travel longer distances [calvari and pinkerton, 1998, 1999; kauahikaua et al., 1998]. long lasting eruptions fed by lava tubes develop on the flow field surface several structures like tumuli, pressure ridges, inflation clefts, ephemeral vents, skylights, toothpaste flows, squeeze-outs [walker, 1991]. all these features suggest low emplacement rate, and conditions of a slowly expanding lava flow field with low hazard. 4. sheet flows sheet flows are characterised by flat surfaces and are common during the initial phases of an eruption, when ier is high and the flow spreads laterally from the vent inundating the surrounding topography [figures 4a and b, figure 5; ballard et al., 1979; lipman and banks, 1987; kilburn and guest, 1993; hon et al., 1994; calvari et al., 2005]. at this stage, lava channel development is inhibited by the high ier, that can reach values as high as 180 m3 s-1 [1989 etna eruption; bertagnini et al., 1990], 280 m3 s-1 [2002-03 stromboli eruption; calvari et al., 2005], 350 m3 s-1 [2014-15 holuhraun eruption; pedersen et al., 2017], or even 980 m3 s-1 [20 august 2011 etna paroxysm; behncke et al., 2014]. a flow front consisting of sheet lava is often diagnostic of high advance rate, and it is possible to judge whether a flow was in a fast-advance regime based simply on a quick look at the flow front morphology [patrick et al., 2017]. sheet flow front advance rates of up to 2 km/day (0.023 m s-1; table 1) have been measured during the 1991-1993 eruption at mt etna on aa lava flows [calvari et al., 1994]; up to 500 m/day (0.006 m s-1; table 1) during the 2014-2015 pahoa effusive crisis at kilauea on pahoehoe flows [poland et al., 2016]; and of 50 km/h (13.889 m s-1; table 1) during the initial phases of the 1951 o shima sonia calvari 6 figure 4. (a) example of a sheet flow with aa surface overflowing the rim of the south-east crater during the initial phases of etna’s 1989 eruption. (b) aa sheet flow spreading during the 2002-03 etna’s flank eruption (foreground), while the eruptive fissure was producing lava fountains and ash plume (background). photo by ingv. (c) pahoehoe inflated sheet flow, kilauea, where individual flow lobes can still be distinguished. 7 lava flow morphologies and hazard eruption [japan; mason and foster, 1953]. sheet flows normally do not contain lava channels or lava tubes because they appear to be conditioned by the combination of a flat topography and a high output rate [lipman and banks, 1987; hon et al., 1994]. however, long-lived pahoehoe sheet flows comprising multiple inflated lobes (figure 4c) propagate downslope as a series of interconnected lobes that eventually can develop lava tubes [hon et al., 1994]. sheet flows normally last 1-2 days during the initial phases of an eruption [lipman and banks, 1987], and tend to develop a convex upward upper surface when slowly moving, and a nearly flat upper surface when rapidly advancing [lipman and banks, 1987]. the hazard posed by sheet flow expansion is very high because the spread fast, but it is normally limited to the few first days of an eruption. inflated pahoehoe sheet flows instead advance more slowly, and can be distinguished by their fast counterpart because they display a different top surface, where initial individual flow lobes can still be distinguished (figure 4c). 5. levées when a lava flow spreads along a surface, cooling results in the development of regions of stationary liquid at its margins, called levées [figure 6a; hulme, 1974]. sparks et al. [1976] described four different types of levées: initial, accretionary, rubble and overflow. initial levées are formed in active lava flows by the stagnation and cooling of lava at the margins of an initial flow unit [figure 5; hulme, 1974; sparks et al., 1976], and are characterized by a broad zone of marginal clinker bounding the central flowing plug. effectively these are the rubbly flanks of the aa flow left behind after the initial flow passed through [lipman and banks, 1987]. narrowing of the actively flowing central zone results in inward growth of the initial levées, an effect observed in the morphologies of etna’s proximal channel levées [figure 5; bailey et al., 2006]. initial levées determine channel width, and their sizes depend on slope [sparks et al., 1976]. accretionary levées are slowly built up by calvari figure 5. cinder cone formed along the 2001 eruptive fissure, south flank of mt. etna, in 2001, with initial volume-limited sheet flows (at the two sides of the channel), and in the middle the main lava channel bounded by multiple levées. photo by ingv. sonia calvari 8 smearing of the hot, ductile clinkers onto the sides and tops of the levées, welding together to form a solid levée [sparks et al., 1976]. formation of aa clinker at the flow margins by shearing and milling results in piles of clinker being gradually piled up at the margins of the flow zone [naranjo et al., 1992]. this process is responsible for rubble levées [sparks et al., 1976] that overlie the inner edge of the initial levée. rubble levées are laid down by the initial flow front passage [sparks et al., 1976] and may later become overprinted with overflow leveés [e.g., lipman and banks, 1987; naranjo et al., 1992; bailey et al., 2006]. overflow or accretionary leveés are emplaced during periods of increased effusion rate, when flux exceeds channel capacity, causing the channel to overflow for a short period of time [kilburn and guest, 1993; bailey et al., 2006]. an additional type of levée is called “swollen levées”, and occurs when channel lava laterally intrudes the levée causing swelling, brecciation and local lava extrusion [kilburn and guest, 1993]. similarly, “seeps” of viscous spiny or toothpaste pahoehoe have been observed as extruded from shield flanks or from perched lava channels [patrick and orr, 2012]. active levées become static levées when they become strong enough to withstand pressure from the channelled lava, or when lateral pressure on the margins is removed by draining of the channel, and a static levée may become active if material accumulating in the channel increases the pressure exerted on the levées [guest et al., 1987]. during waning flow, levées become accreted to the inner walls of the channel [naranjo et al., 1992], and nested levées may form [lipman and banks, 1987]. continued narrowing of the central flow zone can leave a series of abandoned rubble levées that attest to sustained inward levée growth. an additional type of levées has been observed at stromboli, and called “excavated debris levées” [figure 6b; calvari et al., 2005] because the lava spreading on the steep sciara del fuoco slope has excavated downward and pushed laterally the cold and loose debris, thus improving channel formation. these levées can be distinguished from the other types by their lower temperature [calvari et al., 2005]. 6. lava channels lava flows are non-newtonian or bingham fluids, and as soon as a lava flow stabilises, it develops a channel zone. thus the channel is a zone of flowing lava contained between static levées [sparks et al., 1976; lipman and banks 1987; kilburn and guest 1993; soldati et al., 2018]. channel formation results from an increase in flow surface velocity in the axial zone of a flow, with the velocity gradient between the rapidly moving central area and the stagnating margins becoming greater with time and eventually abrupt, and is marked by an initial channel generally much wider figure 6. (a) lava channel, about 30 m wide and 20 m deep, formed in aa flow during the 2001 etna’s flank eruption, bounded by high levées. note the person with blue jacket in the middle of the channel for scale. (b) active lava channel during the 2007 stromboli’s flank eruption, with “excavated debris levées” (light brown) around the vent and uppermost channel. the width of the vent is about 6 m. 9 lava flow morphologies and hazard than in the stable channel zone [lipman and banks, 1987]. this process is normally quite fast [at mauna loa in 1984; lipman and banks, 1987], and lava may start spreading within channels just a few hours [at mt. etna in 1991; calvari et al., 1994; calvari and pinkerton 1998] or 24 h after the eruption start [at holuhraun in 2014; pedersen et al., 2017]. lipman and banks [1987] have classically described the architecture of channelfed lava flow systems, recognising four zones down a channel-fed lava flow: (1) a proximal, stable channel zone where levées are well-developed and mostly static (figure 6); (2) a medial, transitional channel zone, where the flow is channelized and bounded by incipient rubble levées still deformable; (3) a distal dispersed flow zone, where there are no levées and the lava is moving across its entire width; and (4) the flow front or flow toe, where the system is still spreading and expanding. the channel propagates downslope along the transitional and dispersed flow zones, where there are no levées present and the flow core is covered by breccia all the way to the flow front [soldati et al., 2018]. this change confined a progressively greater volume of faster moving lava to a smaller channel cross section in an upstream direction, until the surface of the central area became more incandescent and finally consisted dominantly of fluid lava [lipman and banks, 1987]. lava channels develop as a consequence of cooling, which forms as soon as a crust develops, thick enough to prevent lateral flow [hulme, 1974]. following hulme [1974], the time needed to form this crust is proportional to the fourth power of the flow depth, and thus time increases rapidly with increasing depth of flow. pre-existing topography affects the form of the channel network in different ways, depending if we are dealing with aa or pahoehoe flows. at mauna loa, dietterich and cashman [2014] found that steeper slopes correspond to higher braiding indices on pahoehoe flows, whereas soldati et al. [2018] at piton de la fournaise described structures of the aa channel network varying with distance from vent rather than with time, with single channels forming on steeper slopes and braided channels on shallower slopes. soldati et al. [2018] observed that changes in the architecture of the aa flow are reversible because the flow can switch back and forth between single and braided channel configurations multiple times during its emplacement. in addition, soldati et al. [2018] recognise the importance of flow rate on lava flow structures, with lava flows characterized by low effusion rates more prone to braiding compared to high effusion rate flows on any given slope. channel networks govern the distribution of lava supply within a flow, because changes in the channel topography can dramatically alter the effective volumetric flux in any one branch, which affects both flow length and advance rate [dietterich and cashman, 2014]. specifically, branching will slow and shorten flows, while merging can accelerate and lengthen them [dietterich and cashman, 2014], with important effects on lava flow hazard evaluation. consideration of channel networks is thus important for predicting lava flow behaviour and possibly mitigating flow hazards with diversion barriers. 7. lava tubes a lava tube is a roofed conduit through which molten lava travels away from its vent [figure 7; kauahikaua et al., 1998]. lava tubes normally form during long-lasting eruptions in both pahoehoe and aa lava flow fields [kilburn and guest, 1993; calvari and pinkerton, 1998]. at etna, calvari and pinkerton [1998] described a minimum time of 4 days in order to form a tube sector within channelled aa lava flows, and considered a steady magma discharge rate as the main requirement for tube formation. in turn, the discharge rate defines the size of the tube, with longer and wider tubes being formed by higher discharge rates. calvari and pinkerton [1998] describe tube formations at etna within aa lava flows in conditions of discharge rate spanning three orders of magnitude. by contrary, lava tubes within pahoehoe flows described in hawaii apparently form only in a restricted and low range of discharge rate [1 to 5 m3 s-1, peterson et al., 1994]. at nyiragongo during the 2002 flank eruption lava tubes transporting lava within lake kivu apparently formed along the arterial flow in less than a week [komorowski et al., 2003], although it appears from the description that lava surface cooling was favoured by the contact with the lake water. at kilauea, hon et al. [1994] describe the process of lava tube formation within pahoehoe sheet flows in absence of lava channels and as a gradual concentration of the hot flow interior, with the flow margins cooling within hours after emplacement by edge effect, developing preferential pathways along which lava tubes will gradually develop. lava tubes within pahoehoe sheet flows begin to form after 2-4 weeks of their emplacement, but information about this process are scant because normally, being formed on low topographic gradient, they sonia calvari 10 do not drain and therefore are difficult to observe [hon et al., 1994]. lava tubes formed within pahoehoe sheet flows have been observed to develop with time an equidimensional section, whereas immature tubes (less than 1 week old) have widths more than one order of magnitude greater than their height [hon et al., 1994; kauahikaua et al., 1998]. four processes have been recognised by which lava channels evolve into tubes: (i) inward growth of channel crust, rooted to flow margins; (ii) repeated overflow and accretion causing levées to arch and seal over channels; (iii) jamming of crustal fragments on the channel surface; (iv) and wholesale attachment of a complete channel crust to the bounding levées [peterson and swanson 1974; greeley, 1987; kilburn and guest, 1993; calvari and pinkerton 1998; kauahikaua et al., 1998]. broad, flat pahoehoe sheet flows evolve into elongated tumuli with an axial crack as the flanks of the original flow were progressively buried by breakouts [kauahikaua et al., 1998]. sometimes, the tubes began to thermally erode the floor (downcutting), frequently observed through skylights, with rates of 10 cm/day [kauahikaua et al., 1998]. this process increases the insulation of tubes making them increasingly deeper and more difficult to detect. lava streams normally occupy only a small fraction of the tube interior, and the stream has a free surface that is primarily gravity-driven [kauahikaua et al., 1998]. the presence of lava tubes within an active lava flow field is very important for hazard assessment because their insulation can allow lava to flow farther from the vent. in fact, a cooling of just ~1°c/km has been estimated for the lava flowing inside a lava tube [calvari et al., 1994; keszthelyi 1995; kauahikaua et al., 1998; heltz et al., 2003]. tubes allow lava to travel for longer distances than when flowing within channels, threatening areas very distant from the active vent. the effect of lava tubes is thus similar to the downslope displacement of the effusive vent. this is the reason why the most devastating lava flow fields have involved the formation of extensive lava tube networks [calvari and pinkerton, 1998; kauahikaua et al., 1998; crisci et al., 2003; branca et al., 2013; patrick et al., 2017; pedersen et al., 2017; calvari et al., 2018; soldati et al., 2018]. 8. hazard implications one of the more challenging aspects for predicting lava flow dynamics is that the rheological properties of lava flows evolve during eruption and emplacement as a consequence of cooling, degassing and crystallization. this produces strongly heterogeneous flow conditions, with lava textures and morphologies that evolve both in space and time [kolzenburg et al., 2017]. the evolution of the lava’s rheology determines, for instance, whether figure 7 (a) skylight above an aa master lava tube formed during the 2014-15 eruption of fogo volcano, cape verde. note the smooth pahoehoe slabs bounding the topographic surface and inner skylight walls resulting from late tube drainage. (b) skylight above an active lava tube emplaced within pahoehoe inflated sheet flows, kilauea volcano, hawaii. note the gas clouds from skylights that reveal the tube path. 11 lava flow morphologies and hazard lava advances as sheetor channel-like flows, and also dictates its surface morphology, which in turn has great influence on heat loss from the flow [kolzenburg et al., 2017]. in addition to changes in rheology, lava surface morphology changes also as a result of climaxing or declining discharge rate. thus, as an eruption proceeds, we normally observe a first phase displaying the formation of sheet flows when the discharge rate is high and the flow is still free to expand laterally invading the topography that surrounds the effusive vent. this phase normally lasts from one to a few days. this stage corresponds to a great hazard for the areas surrounding the vent, especially because lava can reach high speeds (table 1), but the hazard is limited in time. a second stage corresponds to the formation of levées and one or more well-defined channels, confining the lava within well-established paths and driving it more efficiently to longer distances from the vent. this stage can last from weeks to years, and the greatest hazard is located at the frontal zone, where the flow has not yet established its path and is free to expand laterally. however, sudden changes in the supply rate, or upstream blockages formation and removal, can result in sudden and even large channel overflows that may invade the area around the channel. overflows are normally shortlived, but their damage can be potentially devastating. the hazard posed by the expanding frontal zone, being it increasingly far away from the vent, is also increasingly lower due to its decreasing speed of expansion, a result of cooling and crust formation. this stage normally offers enough time to evacuate buildings or infrastructures avoiding loss of life. with time, and if magma supply is steady enough, lava tubes may develop within the lava flow field [calvari and pinkerton, 1998; kauahikaua et al., 1998]. the insulating effect of lava tubes allow lava to travel longer distances from the main vent, increasing the possibility of inflation or endogenous growth of the lava flow field [hon et al., 1994; calvari and pinkerton, 1998], and resulting in increased hazard for the distal portion of the lava flow field, where breakouts can suddenly open giving rise to fast spreading secondary flows [calvari et al., 1994, 2018]. in addition, an underestimated hazard derives from the reactivation of lava flow fronts due to the overlapping of two or more lava flow units when the lower/earlier has not yet solidified, thus allowing a combination of the molten core of the two overlapped flows [applegarth et al., 2010]. each country with frequently erupting volcanoes has developed proper systems to face volcanic crises, depending on the most common lava flow morphologies, speeds and features. at kilauea volcano, where an effusive eruption is going on since 1983, the scientists of the hawaiian volcano observatory (hvo) in charge of volcano monitoring and hazard assessment produce lava flow maps including potential flow paths based on topographic steepest-descent calculations [e.g., kauahikaua and tilling, 2014; poland et al., 2016; neal et al., 2018]. this is enough to forecast the possible expansion of lava, given that pahoehoe lava flows can be very fast, change rapidly directions following minor topography changes, and cause a reversal topography due to inflation [hon et al., 1994]. by contrast on etna, where frequent effusive eruptions normally occur every few years [harris et al., 2011, 2012; bonaccorso and calvari, 2013], satellite measurements of ier [ganci et al., 2011, 2012b] are crucial during the initial phases of an eruption [bonaccorso et al., 2015]. these are used at first to estimate the maximum distance that a single flow unit can travel [calvari and pinkerton, 1998; wright et al., 2001; bonaccorso et al., 2015], and in the meanwhile the same parameters are used to run lava flow simulations using cellular automata models, that provide a more accurate and reliable forecast of lava flow expansion [crisci et al., 2003; vicari et al., 2011; ganci et al., 2012b; del negro et al., 2013] as well as a probabilistic modelling of future eruptions [cappello et al. 2011, 2012, 2013]. however, the discovery that lava tubes develop and grow even within aa lava flow fields typical of etna [calvari and pinkerton, 1998; 1999] has revealed an important hazard that must be considered when dealing with long-lived effusive eruptions having a steady supply [calvari et al., 1994; calvari and pinkerton, 1998; solana et al., 2017; calvari et al., 2018]. reliable tube growth simulations are not available at the moment, and all we can do when lava tube presence has been detected within growing lava flow fields is to run lava flow simulation moving the effusive vents where new breakouts open at the end of the tube path [cappello et al., 2016; neal et al., 2018]. 9. concluding remarks the analysis of lava flow morphology can help evaluate the possible hazard posed by active lava flows and lava flow fields, because each lava type forms in sonia calvari 12 different phases of an eruption and in different conditions of discharge rate, lava rheology and topography. the initial stages of effusive eruptions normally involve the highest rates of effusion [wadge 1981; harris et al., 2011, 2012], rapidly declining to a lower value and steady state when the feeder dike stabilizes and drain. thus, the initial phases of effusive eruptions at highest flow rate normally produce sheet flows, where a thin sheet of lava spreads laterally at high speed (table 1) around the vent [pedersen et al., 2017]. although very hazardous, they normally last few hours to days, and are followed by aa lava flows spreading at still high discharge rates. these soon form channels, whose levées protect the villages and infrastructures located at the two sides, but at this stage it is still possible that overflows from the main channel may invade properties and land [neal et al., 2018]. when discharge rate declines, earlier aa flows may be covered by slower pahoehoe flows. if we can measure the ier during an effusive eruption, we can apply the simple formula proposed by walker [1971] that relates ier and maximum flow length, in order to estimate a priori the maximum distance that a single flow unit can reach from its vent, as has been done on etna using walker’s formula appropriately modified for etna’s lava [calvari and pinkerton, 1998; wright et al., 2001; bonaccorso et al., 2015; solana et al., 2017]. if instead the first flows forming at high rates are pahoehoe, then we can apply the expertise of hvo and produce a map with lines of maximum steepness of the ground, in a way to forecast the path followed by pahoehoe lobes and inflated pahoehoe sheet flows [kauahikaua and tilling, 2014; poland et al., 2016; neal et al., 2018]. if the eruption proceeds, and a complex lava flow field forms, it is possible to run appropriate models simulating lava flow emplacement [crisci et al., 2003; cappello et al., 2011, 2016; ganci et al., 2011, 2012b; vicari et al., 2011]. however, we must take into account that complex and long-lived lava flow fields may promote the formation of hidden lava tubes, which allow the lava to expand with very little heat loss and spread further downslope than when travelling free on the surface or within channels [calvari and pinkerton, 1998; kauahikaua et al., 1998; calvari et al., 2018]. because no reliable models exist at the moment for simulating the effect of lava tubes, the only possibility to evaluate lava flow invasion in these cases is to apply existing lava flow models to any new major breakouts opening at the edge of the lava flow field [e.g., cappello et al., 2016]. identification of inflated zones of the lava flow field and of potential ephemeral vent sites in its distal regions is critical for hazard assessment because they allow flows to lengthen significantly over their calculated cooling-limited lengths [pinkerton and sparks 1976; calvari and pinkerton, 1998]. in all conditions, the possibility to extract useful lava flow parameters from satellite images may help when ground measurements are impossible or dangerous [ganci et al., 2011, 2012a, 2012b, 2013, 2018; cappello et al., 2016]. the experience gained at well monitored volcanoes can be applied to those cases where a monitoring system is not well developed or is lacking, and the analysis of lava flow morphology can help evaluating the hazard when and where there is no possibility to obtain reliable lava flow mapping and effusion rate measurements from ground or remote sensing techniques. this was the case for example of the 2014-15 eruption at fogo volcano (cape verde), or the 2002 nyiragongo eruption [komorowski et al., 2003; cappello et al., 2016; calvari et al., 2018]. acknowledgements. i would like to thank the editor ciro del negro for handling the manuscript, jim kauahikaua for his precious and valuable revision and suggestions that significantly improved the paper, steve conway for the correction of the english style, and an anonymous reviewer for his/her comments. references applegarth l. j., pinkerton h., james m. r., calvari s. 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(1993) the laki (skaftar fires) and grimsvotn eruptions in 1783-1785. bull. volcanol., 55, 233-263. vicari, a., bilotta, g. bonfiglio, s., cappello, a., ganci, g., hérault, a., rustico, e., gallo, g., del negro, c. (2011) lav@hazard: a web-gis interface for volcanic hazard assessment. annals of geophysics, 54, 662-670, http://doi.org/ 10.4401/ag-5347. wadge g. (1981) the variation of magma discharge during basaltic eruptions. jour. volc. geoth. res., 11: 139-168. walker g.p.l. (1971) compound and simple lava flows and flood basalts. bull volc., 35(2):579-590. walker, g. p. l. (1973), lengths of lava flows, philos. trans. r. soc. london, ser. a, 274, 107-118. walker, g.p.l. (1991) structure, and origin by injection under surface crust, of tumuli, “lava rises,” “lavarise pits,” and “lava inflation clefts” in hawaii. bull. volcanol. 53, 54658. wright, r., flynn, l. p., harris, a. j. l. (2001) evolution of lava flow-fields at mount etna, 27-28 october 1999,observed by landsat 7 etm. bulletin of volcanology, 63, 1 7, http://doi.org/10.1007/s00445 0100124. *corresponding author: sonia calvari, istituto nazionale di geofisica e vulcanologia, osservatorio etneo sezione di catania catania, italy email: sonia.calvari@ingv.it © 2019 the istituto nazionale di geofisica e vulcanologia. 17 lava flow morphologies and hazard articolo clark.pdf key words intraplate seismicity – seismic hazard – australia – paleoseismology 1. introduction on the 27th of december 1989 a magnitude ml 5.6 earthquake near newcastle nsw shattered the commonly held view that damaging earthquakes do not occur in australia. this event resulted in the loss of 13 lives and is, to this time, the most destructive and costly natural disaster to have occurred in australia (e.g., sinadinovski et al., 2002, table 3-1). while being the most costly, the newcastle earthquake was certainly not the first damaging event to affect a populated centre in australia (e.g., 1954 adelaide event, mccue, 1975; 1968 meckering and 1970 calingiri events, gordon and lewis, 1980; 1979 cadoux event, lewis et al., 1981). australian paleoseismology: towards a better basis for seismic hazard estimation dan clark (1) and kevin mccue (2) (1) geoscience australia, canberra, australia (2) australian seismological centre, jamison centre, australia abstract probabilistic seismic hazard analyses in australia rely fundamentally on the assumption that earthquakes recorded in the past are indicative of where earthquakes will occur in the future. no attempt has yet been made to assess the potential contribution that data from active fault sources might make to the modelling process, despite successful incorporation of such data into united states and new zealand hazard maps in recent years. in this paper we review the limited history of paleoseismological investigation in australia and discuss the potential contribution of active fault source data towards improving our understanding of intraplate seismicity. the availability and suitability of australian active fault source data for incorporation into future probabilistic hazard models is assessed, and appropriate methodologies for achieving this proposed. australia experiences an earthquake of sufficient magnitude to cause structural damage (m. .5.5) once every two years on average (mccue, 1990). however due to the very clustered nature of australia’s population, and the continent’s large area, most earthquakes are far from populated centres and infrequently cause more than superficial damage. australia resides within the indo-australian plate, and is classified as a stable continental region (scr) according to the definition of johnston et al. (1994) – the landmass is largely unaffected by currently active plate boundary processes, and has not been influenced by significant orogenic or extensional events for tens of millions of years. scrs worldwide are characterised by very low strain rates, and correspondingly low levels of seismicity relative to active continental regions and plate margin regions. such areas account for less than one percent of global moment release (johnston et al., 1994). consequently, the larger more damaging src earthquake events are rare relative to the period of historic observation. this fact poses problems for seismologists undertaking 1087 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. dan clark, geoscience australia, p.o. box 378, canberra act 2601, australia; e-mail: dan.clark@ga.gov.au probabilistic seismic hazard analyses that rely heavily on the assumption that earthquakes recorded in the past are indicative of where earthquakes will occur in the future – such assumptions underpin the earthquake hazard maps used in the australian building code (as1170.4, 1993). the historic record of 100200 years is far too short to accurately reflect the long-term activity on the faults that generate these events (cf. machette et al., 1993). the problem is compounded when it is considered that the largest, least frequent events dominate the moment budget. strategies that have been employed to overcome this problem fall into two mutually dependant categories: 1) obtaining a better understanding of the patterns and characteristics of contemporary intraplate seismicity, and 2) extending the historic record by consulting the record of seismicity recorded in the landscape (i.e. paleoseismology). the following sections will examine each of these areas in turn for the australian context. after a brief introduction to australian intraplate seismicity and some of its geomorphological ramifications, we present a review of paleoseismological investigation in australia, and offer a discussion on what has been learnt about australian seismicity from these studies. the future of active fault source data in providing a better basis for seismic hazard assessment in australia is also discussed. 1.1. australian intraplate seismicity earthquakes in australia have been monitored with various degrees of completeness since 1888 when the first seismograph was installed. figure 1 presents an epicentre map of earthquakes with magnitude m 4 for the period 1901-2001. the data is from the world earthquake database maintained by geoscience australia with contributions from university, state and private seismological agencies. the database is probably complete for magnitude m 6 since 1901, m 5 since 1963 and m 3.2 since 1995. focal depth control in the database is mostly poor, ostensibly due to a combination of incompletely characterised earth models, and sparse seismograph density. however, some generalisations may be made. with a few notable exceptions (e.g., mccue and michaellieba, 1993), earthquakes initiate in the depth range two to twenty kilometres. within the ancient shield areas of western and south australia, and the northern territory (e.g., the achaean yilgarn, pilbara and gawler cratons, and the pine creek inlier, cf. palfreyman, 1984) most foci are generally at the shallow end of the range, with some events occurring in the top two kilometres. two of the larger australian earthquakes, the 1968 ms 6.8 meckering event and the 1988 tennant creek sequence (see fig. 1 for location) have calculated hypocentral depths of the order of 6 km or less (langston, 1987; bowman and dewey, 1991). hypocentres are more evenly distributed within the range in the predominantly younger rocks (largely phanerozoic) of eastern australia and in eastern south australia. beyond identification of the depth of the brittle-ductile transition, no attempt has been made to explain these apparently shallow hypocentral depths. however, analysis of shallowcrustal stress measurements from across australia (e.g. denham et al., 1979, 1981; denham and windsor, 1991; hillis et al., 1999; hillis and reynolds, 2000), and the propensity for shallow rock failure in mines (max lee, australian mining consultants pty ltd., personal communication), indicates that in the top several kilometres of crust the maximum principal stresses are horizontal, of high magnitude, and are coupled with highly deviatoric stress regimes. such conditions are likely to be conducive to shallow rupture initiation. earthquake fault plane solutions constructed for a selection of larger events are consistent with the stress data, with the majority indicating thrust source mechanisms (hillis and reynolds, 2000; clark and leonard, 2002; leonard et al., 2002). strike-slip mechanisms are locally important in the flinders ranges of south australia, and in northwest western australia. normal faulting mechanisms are relatively rare; there were two in southeastern western australia (1985 ml 5.4 norseman event; 2001 ml 5.2 ravensthorpe event), one in the highlands of victoria (1966 ml 5.6 mount 1088 dan clark and kevin mccue hotham event), and another in central west new south wales (1996 ml 5.1 range event). perhaps the least understood facet of australian earthquakes is their relationship to the geology. hypocentre precision estimates are to tens of kilometres, so earthquake events cannot be confidently assigned to known structures, except where events produce a surface rupture (see section 1.2). australian epicentres were overlain on various geological and geophysical datasets at a continental scale (mccue et al., 1998) to see if any obvious patterns were evident. the study concluded that no consistent relationships were apparent, which is perhaps not surprising given the complexity of australian geology and that the earthquake catalogue contains only forty years of complete data for magnitudes down to five. the possibility that this and other discrepancies are just a function of the short recording period can in part be addressed through paleoseismological investigation. in the next sections we discuss what is known of the geomorphic expression and longevity of features born of earthquake events in australia. the questions we address are: how much paleoseismological information is currently out there and what is its nature? 1089 australian paleoseismology: towards a better basis for seismic hazard estimation sa wa nt qld nsw vic tas 4-4.99 5-5.99 >6 earthquake magnitude tennant creek lort riverhydenmerredin meckering mt narryer lake edgar cadell marryat ck roopena/ ash reef calingiri cadoux ri ve r h uon 0 5 km lake pedder deniliquin echuca rochestervictoria new south wales 0 30 km calcarra 0 400 m calingiri 3 km0 cadoux pipeline 0 6 km meckering 0 5 km roopena ash reef hyden 0 10 km lo rt r iv er 5 km0 2 km0 gosses bore 3 km0 tennant creeklake surprise mt narryer b yro 0 10 km merredin 0 5 km mt lofty/ flinders ranges e as te rn h ig hl an ds fig. 1. seismicity of australia, 1900-2002, and recent fault scarps (modified after mccue, 2001). earthquakes relating to the northern margin of the australian plate have been removed from the image for clarity. earthquake magnitudes are presented as ml for m < 6 and ms for m > 6. wa = western australia; nt = northern territory; qld = queensland; sa = south australia; nsw = new south wales; vic = victoria; tas = tasmania. 1090 dan clark and kevin mccue t ab le i. s um m ar y ta bl e of h is to ri c su rf ac e ru pt ur es a nd s el ec te d pr ehi st or ic t ec to ni c sc ar ps f or w hi ch p al eo se is m ol og ic al i nf or m at io n ha s be en ob ta in ed . n ot e th at t he t en na nt c re ek e ve nt o cc ur re d as a s er ie s of t hr ee m ai n ea rt hq ua ke s in a 1 2 h pe ri od , an d ea ch e ve nt i s th ou gh t to h av e pr odu ce d an in de pe nd en t s ec ti on o f sc ar p (c f. c ro ne e t a l. ,1 99 3) . m ag ni tu de s fo r pr ehi st or ic e ve nt s ha ve b ee n ca lc ul at ed u si ng th e re la ti on sh ip s of w el ls an d c op pe rs m it h (1 99 4) a nd b as ed o n su rf ac e sc ar p le ng th . ea rth qu ak e lo ca tio n d at e of ea rth qu ak e la t lo ng ru pt ur e/ fa ul t m ax sc ar p re ac tiv at io n se le ct ed re fe re nc es la st ev en t m ag ni tu de le ng th (k m ) he ig ht (m ) hi sto ry (i f k no w n) h is to ri c 1 c ut ch ,i nd ia 18 19 m s 7. 25 -8 .2 5 23 .6 69 .6 > 90 79 q ui ttm ey er a nd j ac ob (1 97 9) 2 a cc ra ,g ha na 19 39 m 6. 5 5. 2 – 0. 13 917 0. 5 ya rw oo d an d d os er (1 99 0) 3 c en tra l s ud an ,a fr ic a 19 66 m b 5. 1 12 .6 30 .8 6 0 q ur es hi a nd s ad ig (1 96 7) 4 m ec ke rin g, w a ,a us tra lia 19 68 m s 6. 8 – 31 .7 11 7. 0 37 2. 5 re ac tiv at es a nc ie nt fa ul t g or do n an d le w is (1 98 0) 5 c al in gi ri, w a ,a us tra lia 19 70 m b 5. 7 – 31 .1 11 6. 5 3 < 0. 4 re ac tiv at es a nc ie nt fa ul t le w is e t a l. (1 98 1) 6 c ad ou x, w a ,a us tra lia 19 79 m s 6. 0 – 30 .8 11 7. 2 28 < 1. 4 la ng er e t a l. (1 98 7) 7 g ui ne a, a fr ic a 19 83 m s 6. 2 11 .9 – 13 .5 9 0. 13 m ac he tte e t a l. (1 99 3) 8 m ar ry at c k, sa ,a us tra lia 19 86 m s 5. 8 – 26 .2 13 2. 8 13 0. 9 > 1 ev en t, po ss ib ly re ac tiv at es a nc ie nt fa ul t c ho y an d b ow m an (1 98 9) 9 te nn an t c k, n t, a us tra lia 19 88 m s 6. 3, 6. 4, 6. 7 – 19 .8 13 4. 0 32 1. 8 2 ev en ts ,p en ul tim at e > 50 k a, c ro ne e t a l. (1 99 3) re ac tiv at es a nc ie nt fa ul t 10 u ng av a, q ue be c, c an ad a 19 89 m s 6. 3 60 .1 – 73 .6 10 1. 8 re ac tiv at es a nc ie nt fa ul t a da m s et a l. (1 99 1, 19 92 ) 11 k ill ar i ( la tu r) ,i nd ia 19 93 m s 6. 4 18 .1 76 .6 3 < 0. 5 se eb er e t a l. (1 99 3) pr ehi st or ic 12 m ee rs f au lt, u .s .a . 1. 1 ka < 6. 9 34 .8 98 .5 26 -3 7 3. 6 2 ev en ts < 2 ka ,r ea ct iv at es a nc ie nt fa ul t c ro ne a nd l uz a (1 99 0) 13 c he ra w f au lt, u .s .a . < 7. 7 ka 7. 0 38 .3 10 3. 5 44 4. 0 3 ev en ts < 25 k a c ro ne e t a l. (1 99 7) 14 h yd en s ca rp ,w a ,a us tra lia < 30 k a 6. 9 – 32 .5 11 9. 0 35 2. 5 23 ev en ts < 10 0 ka c la rk e t a l. (2 00 1a ); m ik e m ac he tte (p er s. c om m .) 15 lo rt r iv er s ca rp ,w a ,a us tra lia q ua te rn ar y 7. 0 – 33 .5 12 1. 3 40 2 th om (1 97 2) ; m cc ue (1 97 3) 16 m er re di n sc ar ps ,w a ,a us tra lia q ua te rn ar y 6. 5 – 31 .6 11 8. 3 16 c hi n (1 98 5) 17 m t. n ar ry er s ca rp s, w a ,a us tra lia 18 85 ? 7. 0 – 26 .5 11 6. 4 43 1. 52 re ac tiv at es a nc ie nt fa ul t w ill ia m s (1 97 9) ; w ill ia m s et a l. (1 98 3) 18 c ad el l f au lt, v ic /n sw ,a us tra lia > 20 k a 7. 1 – 36 .0 14 4. 8 50 310 > 1 ev en t < 65 k a ti ck le a nd h um ph re ys (1 98 7) ; b ow le r a nd h ar fo rd (1 96 6) 19 la ke e dg ar f au lt, ta s, a us tra lia < 20 k a 6. 8 – 43 .0 14 6. 4 30 57 2 ev en ts < 20 k a, re ac tiv at es m cc ue e t a l. (2 00 3) an ci en t f au lt 20 g el l r iv er f au lt, ta s, a us tra lia q ua te rn ar y? 6. 2 – 42 .5 14 6. 2 10 m cc ue e t a l. (2 00 3) 21 b ar ro w c re ek ,n t, a us tra lia q ua te rn ar y 6. 1 – 21 .9 13 4. 3 8 12 c ro ne e t a l. (1 99 7) 22 a vo nm or e/ c am pa sp e sc ar p, q ua te rn ar y < 7. 1 – 36 .5 14 4. 8 30 -5 0 2 ti ck le a nd h um ph re ys (1 98 7) v ic ,a us tra lia 23 ta tu ra s ca rp ,v ic ,a us tra lia q ua te rn ar y < 6. 2 – 36 .5 14 5. 3 310 2. 53 ti ck le a nd h um ph re ys (1 98 7) 24 a sh r ee f s ca rp ,s a ,a us tra lia > 14 0 ka 6. 0 – 32 .6 13 7. 3 7 34 h ut to n et a l. (1 99 4) 25 r oo pe na s ca rp ,s a ,a us tra lia < 30 k a 6. 7 – 32 .6 13 7. 3 24 34 3 ev en ts < 14 0 ka h ut to n et a l. (1 99 4) 26 w ilk at an a sc ar p, sa ,a us tra lia < 30 k a > 6. 2 – 32 .1 13 8. 0 > 10 23 w ill ia m s (1 97 3) 1.2. historic fault scarps seven instrumentally recorded earthquakes in australia are known to have produced surface ruptures (table i). curiously, all of these earthquakes have occurred in central and western australia, and in the last forty years – there were no documented surface rupturing earthquakes in the period between european settlement in 1788 and the 1968 meckering earthquake. the magnitude of the earthquakes that produced the seven scarps ranges from ml(i.) 5.6 to ms 6.8 (ml(i.) is a richter magnitude equivalent scale based on the felt area or radius of perceptibility, mccue, 1980). two of these events were smaller than magnitude six, the 1970 ml(i.) 5.6 calingiri wa earthquake and the 1986 ms 5.8 marryat creek sa earthquake. the magnitude of the calingiri earthquake may be an underestimate because in the early seventies there were no wide dynamic range seismographs within 600 km, and all nearby seismographs were saturated. furthermore, the event occurred at 3:15 a.m. western standard time when most people were asleep, so the radius of perceptibility is probably an underestimate. its assigned body wave magnitude (mb.) was 5.7. the surface ruptures relating to these events range from 3.5 km to 37 km in length, and 0.4 m to 2.5 m in height. all the scarps have a dominant component of reverse displacement, and arcuate thrust geometry (cf. everingham et al., 1969). in detail, some of the rupture traces are seen to be a composite of thrust and obliqueslip components. for example, the 1979 cadoux surface rupture comprises northerly oriented thrust segments and easterly oriented strike-slip segments (lewis et al., 1981). similar complexity in the tennant creek surface rupture has been ascribed to complex interactions between subjacent fault sets in the subsurface (crone et al., 1993). the historic and instrumental record suggests that there have been at least 14 m > 6 onshore earthquakes in the last 100 years, yet we know of only seven surface ruptures that can be related to these recorded events. it is suggested that this can to a large degree be related to the sparsely populated nature of australia – it is very likely that there are more ‘historic’ scarps to be discovered. the largest onshore event to be recorded in australia, the 1941 meeberrie wa earthquake of magnitude ms 6.9, is a prime candidate. how recognisable these features may be in the landscape depends upon many factors, including the local rate of degradation of topography. 1.3. degradation of historic scarps all seven historic australian surface ruptures occur in areas with little appreciable relief and low rainfall. the topography in the meckering-calingiri-cadoux area comprises a flat to rolling landscape punctuated by low, bare, rounded granite hills. the average annual precipitation is in the order of 400 mm, which occurs mainly in the wintertime as showers associated with frontal systems (bureau of meteorology, 2001). agriculture is the main land use in the area, with the result that up to 90% of the ground surface has been cleared of its native vegetation and tilled. in general, the soil profile consists of a thin layer of transported sandy soil overlaying a duricrust layer (dominantly ferricrete), which in turn overlays weathered bedrock (gordon and lewis, 1980). without anthropogenic intervention, the scarp preservation potential within this soil profile might be expected to be relatively high, at least on a time-scale of decades. this is certainly the case for the 100 m long section of the 1968 meckering surface rupture that was fenced off as a geological monument shortly after the event. the original scarp height at this location was in the order of 1.5 m (fig. 2a). thirty-four years later some of the fine structural detail has been lost, but the scarp remains a remarkable feature in the landscape (fig. 2b). this good state of preservation is in a large part due to the resistant nature of the ferricrete horizon defining the scarp at this site. elsewhere, farming activities, principally ploughing, have destroyed all evidence for the former existence of the scarp. this is also the case for the calingiri and cadoux scarps. the marryat creek and tennant creek scarps occur in deeply weathered arid lands of 1091 australian paleoseismology: towards a better basis for seismic hazard estimation very low relief. bedrock outcrop is very rare. the average annual precipitation is less than 300 mm, most of which is delivered in the summer months during often violent storms (bureau of meteorology, 2001). in contrast to the meckering area, the soil profile in these regions is characterised by up to tens of metres of alluvium and colluvium overlying deeply weathered bedrock (coats, 1963; connor, 1978; machette et al., 1993). the ground surface is sparsely populated with woody herbs and grasses, typically of the genus’ acacia and spinifex. during dry periods, over-grazing by cattle and rabbits leaves the ground surface very prone to erosion by wind and during storm events. the scarp preservation potential within this soil profile might be expected to be relatively low because of the significant depth to durable bedrock. this suggestion is apparently borne out by repeat topographic data collected from the marryat creek scarp in 1986 and 1990. comprehensive levelling data collected soon after the event in 1986 record a maximum vertical offset of 0.9 m (bowman and barlow, 1991), whereas four years later a maximum offset along the same section scarp of only 0.6 m was measured (machette et al., 1993). despite this rapid degradation, machette et al. (1993) reported that scarps, small ground ruptures and other surficial features were still evident along much of the rupture length after 4 years. however, erosion and the action of cattle had destroyed most evidence for lateral slip and other fine structural features. a similar intensity of scarp degradation was reported from the tennant creek scarp two years after its formation (crone et al., 1993). due in large part to extreme remoteness, neither scarp has been visited recently. the above discussion has important implications for where paleoscarps might be preserved in australia. in general, the arid regions of australia are deeply weathered and bedrock is often covered with a significant veneer of unconsolidated sediment. scarp preservation potential in these areas is low, especially where a significant portion of the local rainfall is delivered by intense storms. the mount narryer scarps in western australia (fig. 1) may be 1092 dan clark and kevin mccue fig. 2a,b. images of a section of the 1968 meckering scarp, looking south. a) captured in 1970, and b) captured in may 2002. scarp preservation has been assisted by the durable ferricrete and weathered bedrock layer exposed in the hangingwall. note the rolling topography of farmland in the background. a b a good example. available evidence suggests that the topographic expression of these scarps has been erased in under one hundred years (see next section for more details; williams, 1979). that scarps do not appear to be preserved in deeply weathered areas also implies very low long-term slip rates on individual structures, else bedrock would be brought to the surface and tectonic relief maintained. in contrast, regions where shallow sediments overly a duricrust and/or bedrock appear to have relatively good scarp preservation potential. the documented occurrence of paleoscarps in southern western australia (e.g., lort river, hyden, merredin), and on the eyre peninsula in south australia (e.g., ash reef, roopena) supports this observation. 2. paleoseismological investigations in australia figure 1 presents a map of quaternary and selected historic fault scarps that have been investigated in more than cursory detail. characteristics of the activity on each feature are summarised in table i. the first trenching investigation of a suspected quaternary fault in australia was in 1973 (mccue, 1973). the objective of the study was to determine whether the then recently identified lort river scarp (thom, 1972) in western australia was a fault scarp, or related to fluvial processes. the investigation arose as part of the considerable interest in determining how common surface ruptures were in australia following the meckering 1968 surface rupture. evidence to suggest that the 40 km long and 2 m high lort river scarp was indeed a fault scarp, and a relatively recent addition to the landscape, was provided by aerial photography over the area. several small streams are clearly truncated, and have ponded against the scarp. the trench log produced by the study clearly shows that the scarp relates to an easterly-dipping reverse fault with about 2 m of vertical displacement (fig. 3; mccue, 1973). the former ground surface, rich in laterite nodules and likely of tertiary age (cf. thom et al., 1977), could be traced back under the overthrust block. no attempt was made by the investigators to take samples for dating. however, the thick accumulation of unconsolidated fine white sand overlying 1093 australian paleoseismology: towards a better basis for seismic hazard estimation 1m0 0 1 2 3 4 5 m fine white sand weathered granite zone of weathered laterite nodules fine brown sand scarp line limit of trench see inset present ground surface thrust block colluvial wedge east west possible fault plane fig. 3. sketch of the ground profile revealed on the southern wall of a trench excavated across the lort river scarp in western australia (modified after mccue, 1973). the scarp suggests an age of late pleistocene or older. several years later, two northeasterly trending fault scarps were identified in the mount narryer region of western australia during routine geological mapping (williams, 1979; williams et al., 1983). the westerly scarp is approximately 30 km long and has little associated relief. the easterly scarp is 13 km long and was reported to have a west-side-up relief of around 1.2 m. both scarps are extremely degraded, and were identified on the basis of the prominent vegetation lineaments associated with them. it was originally though that the scarps might relate to the 1941 meeberrie earthquake as the epicentre for this event occurs less than 50 km to the south west – less than the uncertainty in the epicentre location. however, the results of dendrochronological analysis of acacia sp. trees growing in colluvium related to the scarp were used as evidence to link the scarps with reports of a significant earthquake which was felt in the geraldton area in 1885 (williams, 1979). it has been estimated from felt reports (everingham and tilbury, 1972) that the local magnitude of the 1885 event was in the order of 6.5. because of the sparse population of western australia at that time, the epicentral location of the earthquake is very poorly constrained, so a definitive link cannot be made on seismological grounds. in the early nineties australia became the target for integrated studies of surface-faulting earthquakes in stable continental regions because half of the known historic cases of surface-rupturing earthquakes in such regions of the world have occurred in australia. as mentioned in the previous section, investigations were conducted of the surface ruptures produced by the 1988 tennant creek earthquake sequence (crone et al., 1993, 1997), the 1986 marryat creek event (machette et al., 1993; crone et al., 1997), and the 1968 meckering event (crone et al., 1997). some evidence was revealed at the former two localities for pre-historic surface rupturing earthquakes at the site of the historic events. two trenches excavated across the western arm of the main arcuate tennant creek scarp (crone et al., 1993) revealed the presence of a late quaternary scarp buried by aeolian sand deposits. the vertical relief on this prehistoric scarp is at least equal to that of the contemporary scarp at those sites ( 0.8 m), which formed consequent to the second earthquake event in the sequence (ms 6.4, choy and bowman, 1989). this scarp was not reactivated during the 1988 events, and could not be directly dated. however, thermoluminescence dating of the lower layers of the overlying sand deposits suggests that accumulation of this sediment began ca. 50 ka (crone et al., 1993; hutton et al., 1994). most evidence for prehistoric (pleistocene) movement on the marryat creek scarp has been removed by erosion (machette et al., 1993). however, trenching revealed that the contemporary scarp exploits a 10-15 m wide zone of pervasive shearing and alteration in the granite bedrock. only selected planes in this zone were reactivated in the 1986 event, suggesting that the earthquake reactivated faults that probably have a long history of recurrent movement. a 2-5 m high linear ridge at the southern end of the scarp may relate to a portion of this postulated movement (mccue et al., 1987; crone et al., 1997). the age of the oldest surficial sediments overlying inactive strands of the fault zone, which provide a minimum age for penultimate surface faulting, was estimated visually to be in the order of 100 ka (machette et al., 1993; crone et al., 1997). a cursory investigation of a trench excavated through the meckering scarp as part of a symposium on «recent intraplate seismicity studies» (gregson, 1990) revealed no obvious evidence for earthquake-related features predating the 1968 event (crone et al., 1997). however, gordon and lewis (1980) noted that parts of the 1968 ruptures are marked by abundant quartz fragments, providing circumstantial evidence that the scarps exploit an ancient mineralised fault zone. further evidence that the scarp follows an ancient line of weakness is provided by the course of the mortlock river, which has an arcuate shape which parallels the 1968 scarp for 12 km at a distance of about 1.5 km (gordon and lewis, 1980). this geometry was established prior to the 1968 surface rupturing event. the meckering trench was not logged or interpreted. international interest in australia’s quaternary fault scarps continued in the late nineties with 1094 dan clark and kevin mccue paleoseismological investigations being conducted over the hyden scarp in western australia, and the roopena scarp in south australia (crone et al., 2003). the detailed results of these studies have not yet been published, but preliminary results of optically stimulated luminescence dating suggest at least two surface faulting earthquakes occurred on the hyden scarp, at 30 and 50 ka, and three surface faulting earthquakes occurred on the roopena scarp, at 30, 100, and 150-200 ka (mike machette, pers. com.). estimates of 140 ka were made for the age of basal sediments within small scarp-related lakes occurring at the foot of the roopena scarp (hutton et al., 1994), which is consistent with the age of the oldest event postulated by crone et al. (2003). the total vertical displacement across the 30 km long hyden scarp, distributed amongst the two events, is in the order of 2.5 m. approximately 3.9 m of vertical displacement must be distributed amongst the three postulated roopena scarp events (cf. miles, 1952; dunham, 1992). there is also the suggestion of significant lateral displacement on the 28 km long roopena scarp. the 30 km long and more than 5 m high lake edgar fault scarp in southwest tasmania was identified as a fault scarp in the 1960’s (carey and newstead, 1960; fig. 1). the first trench excavated over the scarp was part of an investigation into the feasibility of the lake edgar dam project (roberts et al., 1975). however, it was not until the nineties that a paleoseismological investigation was conducted (mccue et al., 1996; van dissen et al., 1997; mccue et al., 2003). the paleoseismological study re-occupied the original dam project trench, which was excavated near the centre of the scarp where the vertical displacement is in the order of 2.5 m. the trench excavation provided evidence that the fault had generated at least two surface-faulting earthquakes. geomorphological evidence was used to suggest that these events were of quaternary age, the youngest event having occurred within the last 20.000 years. each event is associated with a vertical displacement in the order of 2.5 m. an attempt to provide better constraint on the time of faulting using the radiocarbon technique proved unsuccessful. despite the apparent age of the lake edgar scarp, and a high local annual rainfall by australian standards ( 2000 mm, bureau of meteorology, 2001), the feature is still a sharp break in the land surface (mccue et al., 2003). this characteristic can in a large part be attributed to the peat-dominated soils displaced by the faulting (cf. williams, 1977, kiernan, 1990). the competence of these materials is demonstrated at the site of the mccue et al. (2003) investigation, where the walls of the 4 m deep trench have remained standing for 30 years, even through the trench is permanently half-filled with water. in mid-2001, a consortium led by mike dentith from the university of western australia retrenched the hyden scarp 4 km to the south of the crone et al. (2003) trench (clark et al., 2001a,b). the soil profile at the site of the trench consists essentially of a mantle of aeolian sand overlying a ferricrete layer that had been disrupted by reverse faulting (fig. 4). the vertical displacement across the various fault strands depicted in fig. 4 is ~ 2.5 m. it is apparent that subsequent to the last faulting event, and prior to the deposition of the sand, the ferricrete surface has undergone significant erosion. stratigraphic evidence that might have revealed the number of surface faulting events that the structures in the trench reflect has therefore been removed. however, preliminary optically stimulated luminescence ages on samples from the base of the sand layer suggest that the last faulting event at this locality occurred prior to 90 ka. it is therefore not clear at this early stage of the study how the structures revealed in this trench relate to those exposed in the crone et al. (2003) trench. there is a wealth of paleoseismological information yet to be recovered from the geomorphological record of australia. many references have been made in australia’s geologic and geomorphic literature to post-tertiary faulting (e.g., victorian scarps, hills, 1960 – cadell scarp, harris, 1938 and bowler and harford, 1964, 1966 – australian scarps in general, twidale, 1968 – wilkatana scarp and other flinders ranges and mount lofty ranges scarps, williams, 1973 and sandiford, 2003 – merredin scarps, chin, 1985 – gell river fault, roberts et al., 1975 and mccue et al., 2003 – tatura scarp, tickell and humphrys, 1987 – palmer fault, bourman and 1095 australian paleoseismology: towards a better basis for seismic hazard estimation lindsay, 1989 – avonmore scarp, cherry and neivandt, 1995 – barrow creek scarp, crone et al., 1997). these accounts generally focus on observations of fault scarps, changes in drainage such as ponding against scarps, and stream diversion, and make no attempt to associate the features with pre-historic earthquakes. 3. discussion: towards a better basis for seismic hazard assessment 3.1. what has been learned about australian seismicity, and surface faulting earthquakes in particular? 3.1.1. magnitude versus scarp length and height relations the largest known earthquake to produce a surface rupture in australia in historic times was the 1968 ms 6.8 meckering event, which produced a scarp 37 km long. similarly, the 1988 ms 6.3-6.7 tennant creek events produced approximately 32 km of surface rupture. there are pre-historic scarps yet to be fully characterised that are significantly longer than those produced by historic surface rupturing earthquakes (up to 50 km, table i), and presumably relate to larger or multiple events. relationships were developed for estimating the magnitude of pre-historic events based upon the lengths and heights of the seven australian historic scarps (mccue, 1990). because of the small sample-space these relationships are not statistically robust, and must therefore be treated with caution. however, the mccue (1990) relationships are comparable to similar relationships derived from reverse faults worldwide (e.g., wells and coppersmith, 1994; fig. 5, table i), and furthermore appear to be indicative of several characteristics that distinguish australian surface ruptures, or at least those from the central and western regions of australia. there is some indication that for a given event magnitude, surface ruptures in western and central australia tend to be longer and involve greater displacements than those in the world data set. this finding is counter-intuitive, as one might expect that rupture planes in intraplate areas would be compact because of the inherent strength of cold, thick crust. the january 2001 mw 7.6 bhuj earthquake is perhaps an extreme example of a compact rupture, being only 40 km long by 36 km deep (arch johnston, personal communication). the reason why this might be so has not been studied in detail and is largely beyond the scope of this contribution to answer. however, it is tempting to speculate that a combination of high shear stress at shallow depth (as suggested in section 1.1) combined with the stress regime becoming less compressive with depth (there is an indication from in situ stress data that this might be the case, hillis and reynolds, 2000; richard hillis personal communication, 2002), might be responsible. in this scenario, hypocentre nucleation would be favoured at shallow depth, and downward propagation 1096 dan clark and kevin mccue fig. 4. grid map of the ground profile revealed on the southern wall of a trench excavated across the hyden scarp in western australia (after clark et al., 2001b). of ruptures along a given structure would be hindered by a progressively less favourable stress regime. alternatively, a barrier to downward propagation of a shallowly nucleated rupture might be provided by the gross geological structure. for example, a seismic reflection profile across the seismogenic zone in south western western australia (dentith et al., 2000) revealed that shallowly dipping near-surface structures favourably oriented for failure in the prevailing east-west stress field sole into a sub-horizontal detachment surface at 8 km depth. because there have been no historic surface ruptures in eastern australia it is not possible to estimate the magnitude of the events which formed pre-historic scarps with any confidence. however, if one were to apply magnitude versus rupture length relationships of the kind presented by mccue (1990) and wells and coppersmith (1994) to the longest known scarps (e.g., cadell, avonmore) events of around magnitude seven, or slightly above, are obtained (table i). there are no indications in the known geomorphic record that events of a magnitude discernibly greater than this have occurred. 3.1.2. reactivation and recurrence several of the scarps discussed in previous sections preserve evidence for multiple quaternary displacement episodes (table i). in addition, there is evidence that many historic and pre-historic surface ruptures exploit ancient fault systems, or zones of weakness. the lake edgar fault is perhaps the best example of both these characteristics. prior to its two quaternary reverse faulting episodes (mccue et al., 2003) the structure is thought to have been active in the paleozoic as a strike-slip fault, with more than 12 km of right lateral slip (carey and newstead, 1960, roberts et al., 1975). similarly, abundant quartz float in the vicinity of the meckering and calingiri surface ruptures was taken as evidence that the ruptures occurred along lines of ancient weakness (gordon and lewis, 1980). direct evidence for reactivation was seen in one of the two trenches excavated across the marryat creek scarp, where multiple fault strands not activated in the 1986 event were recorded within the proterozoic crystalline bedrock underlying quaternary sediments (machette et al., 1993). these older fault strands do not displace the quaternary sediments, and are not associated with significant relief on the bedrock/sediment interface. an attempt to provide a minimum age for the penultimate event by dating the lowermost quaternary sediments (based upon ca++ accumulation in the soil) produced equivocal results, but with the suggestion that the sediments began accumulating in the order of 100 ka. parts of the 1988 tennant creek scarp also show direct evidence for pre-historic surface 1097 australian paleoseismology: towards a better basis for seismic hazard estimation fig. 5. comparison of mccue (1990) and wells and coppersmith (1994) relationships tying rupture length versus magnitude and displacement versus magnitude. note that mccue (1990) uses ml for magnitudes below 6.0, and ms for those above, which approximates mw. a fault dip of 45 degrees was assumed in order to convert displacement in the wells and coppersmith (1994) relationship to scarp height. error bars are 2 . faulting. the 0.8 m high buried scarp exposed in two trenches excavated across the western lake surprise scarp was interpreted to reflect a reactivation of a pre-existing fault zone (crone et al., 1993, 1997). luminescence dating of unfaulted sediments overlying the paleoscarp, suggest that it formed more than 50 kyr before the 1988 scarp (crone et al., 1993). in apparent contradiction to the paleoseismological evidence, there is anecdotal historical evidence to suggest that one or two large (m > 6?) earthquakes occurred in the vicinity of tennant creek as recently as 1883 (mccue, 2001). however, it is not known whether these events activated parts of the future 1988 scarp, or occurred on another as yet undiscovered fault in the region. although the data are still few, there appears to be a pattern emerging where surface rupturing earthquakes in australia occur preferentially on a few pre-existing and often ancient faults. these faults seem to be quiescent for long periods of time between surface rupturing events, in the order of tens of thousands to a hundred thousand years or more (cf. crone et al., 1993, 1997; johnston et al., 1994, figs. 4-8). the preliminary data from the hyden and roopena scarps (crone et al., 2003; mike machette personal communication, 2002) support this interpretation. the data on reactivation of australian surface rupturing faults are too few to meaningfully assess whether they demonstrate the successive occurrence of events of similar displacement, generating ‘characteristic’ magnitude earthquakes (cf. schwartz and coppersmith, 1984). however, paleoand historic scarps of equal height at tennant creek (crone et al., 1993), and evidence that the two quaternary events on the lake edgar scarp both produced displacements of 2.5 m (mccue et al., 2003), suggest that the concept of ‘characteristic event’ may have some merit in the australian context. it is noteworthy that while it appears that long time intervals separate surface rupturing earthquakes australia-wide, the expression of this seismicity in the landscape is very different from east to west. western and central australia is characterised by very low relief (e.g., fig. 2b) and modest fault scarps that relate to one, or only a few events (e.g., crone et al., 1993, 1997; machette et al., 1993; clark et al., 2001a,b). the indication is that after a limited number of events, the fault systems lock-up and seismicity migrates elsewhere. in contrast, the eastern highlands in victoria and new south wales, and the mount lofty and flinders ranges regions of south australia, are characterised by scarps with much greater relief and upon which many events are likely to have occurred (e.g., twidale, 1968; tokarev et al., 1998; sandiford, 2003). this is a fundamental property of australian seismicity which is yet to be satisfactorily explored. the implications for seismic hazard assessment are obvious, and are further discussed in a following section. 3.1.3. segmentation there is no conclusive evidence that australian intracratonic faults rupture in a segmented fashion. however, several lines of circumstantial evidence, including both geometric/geologic and seismological arguments, suggest that segmented behaviour may occur on a limited number of fault systems. for example, crone et al. (1993) postulate that the tennant creek fault system is capable of segmented behaviour. these authors suggest that the topographic low known as lake surprise, which coincides with an area where the 1988 net cumulative offset was small, reflects a segment or rupture boundary in the sub-surface. the discovery of evidence for prior surface rupturing on only one part of the 1988 scarp may reflect the postulated segmental behaviour. it is noteworthy that the 1970 calingiri surface rupture occurred 70 km to the northwest of the 1968 meckering rupture. the dominant structural grain in the area is nw-se (e.g., myers and hocking, 1998; whitaker and bastrakova, 2002). hence, it is conceivable that these localities both occur on a single structural discontinuity, which ruptures in a segmented fashion. a similar pattern is potentially emerging at present from the cadoux area, 90 km to the northeast of meckering. following the 1979 cadoux surface rupture seismicity in the area rapidly decayed to a background rate marginally higher than before 1979. this period of relative quiescence was broken in september 2001 by an ml 5.0 earthquake, located less than 30 km to the nw of the cadoux rupture, again along the regional structural grain (cf. myers and 1098 dan clark and kevin mccue hocking, 1998; whitaker and bastrakova, 2002). since that time, over 18.000 earthquakes have been recorded in the area by geoscience australia (leonard, 2002). four of these events have been of magnitude five or greater, but none have produced a known surface rupture. the epicentres of these events describe a trend to the northwest of the september 2001 epicentre, again parallel to the regional structural grain. the geometric alignment of discrete pre-historic scarps can also be suggestive of segmental rupture behaviour. for example, the gell river fault lies 40 km to the north along the strike of the lake edgar fault. the intervening ground is characterised by an abundance of ultramafic rocks, which mccue et al. (2003) postulate may act as a rupture barrier. the authors suggest that this lithological boundary could force the lake edgar and gell river faults to rupture independently. in the echuca area of northern victoria exist three collinear but separately mapped faults. south to north they are the mount ida./colbinabbin, echuca south and cadell faults (bowler and harford, 1966; cochrane et al., 1995). the latter two are associated with quaternary scarps. the cadell fault has the larger vertical maximum displacement of about 10 m, compared to 3-4 m for the echuca south scarp. very little is known about the rupture history of these structures, including whether they rupture independently or together. the difference in scarp height would suggest that the former possibility is typically the case, but simultaneous rupture cannot be excluded. the above discussion provides an indication that fault segmentation is a phenomenon that should be considered in the australian context. at present the data are too few, and the links too circumstantial, to draw conclusions that may be regarded with any confidence. 3.2. comparison of the australian setting with surface rupturing scr earthquakes elsewhere few scr faults have been studied in detail, and only eleven worldwide are known to have produced historic surface ruptures, the most recent being the 1993 mw 6.2 killari (latur) event in india (table i). discussions of australian surface ruptures in the context of known surface ruptures in other scr regions have been presented by crone et al. (1993, 1997), machette et al. (1993) and johnston et al. (1994). although conclusions are based upon only a few faults, consistent patterns are emerging. quaternary surface ruptures on scr faults often exploit ancient faults or fault zones. paleoseismological investigations of many australian fault scarps (table i), the meers fault in the u.s.a. (crone and luza, 1990), and the ungava fault in canada (adams et al., 1991, 1992), reveal the presence of pre-existing fault strands that were not activated during the latest surface rupturing event. scr faults are also characterised by very low long-term slip rates (e.g. crone et al., 1997) reflecting the low strain rates in scr crust (johnston et al., 1994). evidence from the meers (e.g., crone and luza, 1990) and cheraw faults (crone et al., 1997) in the u.s.a. suggest that activity is also highly episodic, with periods of activity numbering one or two events being separated from prior activity by intervals that may be up to 100 kyr or more. the amazing sequence of three surface rupturing earthquakes in eleven years (1968-1979) in the southwest of western australia suggests that episodic activity might also be the norm in australia. however, more paleoseismological information is required to be confident of this interpretation, as there is some indication, proposed in this review, that faults in central and western australia are abandoned after one or two surface rupturing events. a characteristic that is particularly pertinent for hazard assessment is that scr surface rupturing faults are not typically associated with historic seismicity. the tennant creek, marryat creek, ungava and killari events occurred in areas of low perceived seismic hazard (e.g., gshap report, giardini, 1999), revealing the limitations of relying solely on the record of instrumentally recorded seismicity for hazard assessments. furthermore, because scr events are infrequent, and often not flagged by contemporary seismicity, the populations of scr regions are generally poorly prepared, so even relatively small events can and have been extremely damaging (e.g., the 1989 newcastle 1099 australian paleoseismology: towards a better basis for seismic hazard estimation event and the 1993 killari event). the hazard is compounded by low attenuation of seismic energy in continental interiors (e.g., hanks and johnston, 1992). for example, it has been calculated that an earthquake in the new madrid seismic zone of the eastern u.s.a. may have up to twenty times the damage area than a comparable event in san francisco (hamilton and johnston, 1990). the 2001 mw 7.6 bhuj event in india acts as a warning for those attempting to reconstruct histories of seismicity on scr faults from palaeoseismological information. that the rupture ensuing from this large event did not break the ground surface, nor even approach within 8-9 km of the surface (arch johnston, personal communication) casts doubt on the validity of all estimates of paleoseismicity based upon fault scarp length and height in scr regions (cf. mccue, 1990; wells and coppersmith, 1994). 3.3. the future: improving seismic hazard assessment the recognition that geologic data from active faults can provide an important input to probabilistic seismic hazard analysis is well established. the current hazard maps for the western united states (frankel et al., 1996), and for new zealand (stirling et al., 1998, 2002), incorporate 500 and 350 well-quantified active faults as earthquake source zones respectively. the stirling et al. models in particular show an overwhelming dominance of active fault sources over distributed seismicity sources in controlling the hazard for return periods of 475 and 1000 years. the methodology underpinning the incorporation of geologic data into the hazard modelling process is reasonably robust for plate margin areas where slip rates on the major faults are large and the recurrence intervals for significant or ‘characteristic’ (cf. schwartz and coppersmith, 1984) events are in the order of hundreds of years. however, applying these methodologies to scrs, with very low slip rates and return periods on active structures in the tens of thousands of years, has proved more problematic. several different approaches have been adopted in the central and eastern united states in order to allow for a contribution to the hazard from active intracratonic faults (e.g., epri, 1988; frankel, 1995; frankel et al., 1996). in the new madrid area and the wabash valley in southern indiana and illinois, the geometry of faulting is not well characterised and no surface ruptures are known. the recurrence rates for large events in these areas has been estimated based upon paleoliquefaction studies (e.g., johnston and schweig, 1996; obermeier et al., 1992), while a characteristic magnitude event is derived from historical data (e.g., johnston, 1996a,b). in the hazard modelling process, rupture areas are synthesised using semi-empirical models and the constraints imposed by the geologic data. the combination of 500 year recurrence intervals and characteristic magnitude events in the order of 7.5 to 8.0 results in these features contributing significantly to the hazard for the longer return periods (e.g., 1000 and 2500 years, frankel et al., 1996). where holocene fault scarps are known, for example the meers fault in southwestern oklahoma (cf. crone and luza, 1990) and the cheraw fault in eastern colorado (cf. crone and machette, 1995), maximum magnitude events are derived from the fault length using relations such as those of wells and coppersmith (1994). recurrence intervals are based on geologic measurements of slip rates. the recurrence interval is perhaps the most critical parameter deciding the hazard posed by the active fault source. for example, the meers fault has an estimated return time for its ‘characteristic’ event of 4000 years (crone and luza, 1990), and so makes very little contribution even to the 2500 year return time hazard (frankel et al., 1996). the most recent probabilistic seismic hazard analysis of the whole australian continent was published in 1995 (as1170.4; mccue et al., 1998). this study does not include geologic data from active fault sources, and so relies fundamentally on the assumption that the previously recorded distribution of seismicity is indicative of where earthquakes will occur in the future. in australia, and in other intracratonic regions worldwide, this approach has 1100 dan clark and kevin mccue proved to be unreliable for the larger events. this is mainly due to the order of magnitude difference between the time interval separating m > 6 events in a given region compared to the time interval over which seismicity has been recorded. as the database of paleoseismological information grows in australia, a critical mass will be reached were active faults sources may be used to enhance probabilistic seismic hazard analyses for long return periods. the data provided in this contribution provides important insight into what data is likely to be available in the australian context, and what methodologies should be employed to make best use of it for hazard estimation. the most important implications of what we presently know, or can make an educated guess on, are summarised below. in eastern and southern australia, there are many fault scarps of known length and height for which evidence of quaternary movement has been proposed. perhaps the most easily quantifiable of these are the faults that bound the flinders and mount lofty ranges of south australia (cf. bourman and lindsay, 1989; tokarev et al., 1998; sandiford, 2003). on these faults, which support displacements of up to several hundred metres, the concept of recurrence of a characteristic event (cf. schwartz and coppersmith, 1984) may be applicable. if slip rates were to be quantified for these structures, thus tying down long-term seismicity rates, the faults themselves may be modelled as earthquake source zones, as has been done in new zealand (stirling et al., 2002) and the united states (frankel et al., 1996). a similar strategy could be applied to the lake edgar scarp in tasmania (cf. mccue et al., 2003), the cadell fault in new south wales (cf. bowler and harford, 1964, 1966), and the many reportedly active faults in victoria (cf. hills, 1960). in contrast, the idea of recurrence of a characteristic event may not be applicable to scarps in western and central australia. if the indications prove correct, and activity on these scarps is limited to one or two events before the system/segment is abandoned and seismicity migrates elsewhere, it is not appropriate to assign a characteristic magnitude event and use ruptured quaternary faults themselves as active source zones for hazard assessment. instead, an alternate approach similar to that used for the new madrid seismic zone (e.g., frankel et al., 1996) could be adopted. rather than incorporating individual fault sources into the hazard framework, average parameters derived from faults in a region of concern could be assigned to synthetic source zones whose geometry is constrained by geologic data. data from each quaternary/holocene fault within the region of concern could contribute towards defining parameters such as the maximum likely magnitude event for the region (or possibly characteristic event in some cases), typical source zone geometry, patterns of migration in seismicity, regional slip rates, and recurrence rates for surface rupturing events. these parameters are obtainable through paleoseismological investigation and have the potential to place important constraints on probabilistic hazard models. the immediate value of paleoseismological investigation in central and western australia will be realised in the development of an understanding of the behaviour of the faults in this large portion of australia. 4. concluding remarks the review of paleoseismological investigation presented in this contribution provides important insight into the nature of australian intraplate seismicity, and a guide to how geologic data may be usefully incorporated in future revisions of the australian seismic hazard map. perhaps the most important realisation is that the methodologies employed to include active fault source zones into hazard assessment in new zealand (e.g., stirling et al., 1998, 2002) and the western united states (e.g., frankel et al., 1996) may not be appropriate for the australian context. the limited data available indicate that australian active faults are typified by return periods for surface rupturing events in the order of tens of thousands of years or more (cf. crone et al., 1993, 1997, 2003; machette et al., 1993). if future investigations confirm this indication, each fault, when considered in isolation, is unlikely to contribute significantly to hazard for normal structures (cf. the meers fault, crone and luza, 1101 australian paleoseismology: towards a better basis for seismic hazard estimation 1990). however, if other active faults are present in a region, as is certainly the case in the flinders/.mount lofty ranges and in eastern australia, then the hazard is proportionately larger. we put forward the hypothesis that the concept of recurrence of a ‘characteristic’ magnitude event on a single fault in the western and central parts of australia may not be applicable, as there is no paleoseismological evidence, nor evidence in the landscape, of more than a couple of surface rupturing events on any given fault. in this context, estimates of hazard must be driven by an understanding of the patterns in migration of seismicity, and on an knowledge of the distribution of potentially seismogenic faults. faults active in the quaternary may not themselves contribute to the hazard, but can provide critical information on the long-term character of the seismicity in a region. this information can provide useful constraints on probabilistic hazard assessments. paleoseismology is still in its infancy in australia. many instances of known quaternary faults that could provide valuable information on the character of australian seismicity have not yet been investigated in detail. because of the sparsely inhabited nature of the australian continent it is very likely that more quaternary and younger fault scarps will be found (in spite of high scarp degradation rates over much of arid australia). some of these may even relate to historic events. the future will involve devising methodologies to incorporate this geologic data into our hazard models, as has been done successfully in new zealand and the united states. these methodologies will in part have to be uniquely australian, to match the distinct character of australian seismicity. this is our challenge. acknowledgements tony crone and the mike machette of the usgs and mike dentith of the university of western australia are thanked for providing us access to unpublished data. the authors thank john schneider and clive collins for advice and comments on this manuscript. kelvin berryman and an anonymous reviewer are thanked for their helpful reviews. this contribution is published with the permission of the chief executive officer of geoscience australia. references adams, j., r. wetmiller, h. hasegawa and j. drysdale (1991): the first surface faulting from a historical earthquake in north america, nature, 352, 617-619. adams, j., j. percival, r. wetmiller, j. drysdale and p. robertson (1992): geological controls on the 1989 ungava surface rupture a preliminary interpretation, geol. surv. can. pap., 92-c, 147-155. as1170.4 (1993): minimum design loads on structures. part 4: earthquake loads (homebush nsw), pp. 64. bourman, r.p. and j.m. lindsay (1989): timing, extent and character of late ceinozoic faulting on the eastern margin of the mount lofty ranges, south australia, trans. r. soc. south aust., 113, 63-67. bowler, j.m. and l.b. harford (1964): geomorphic sequence of the riverine plain near echuca, aust. j. sci., 26 (3), p. 88. bowler, j.m. and l.b. harford (1966): quaternary tectonics and the evolution of the riverine plain near echuca, victoria, geol. soc. aust., 13 (2), 339-354. bowman, j.r. and b.c. barlow (1991): surveys of the fault scarp of the 1986 marryat creek, south australia, earthquake, geol. geophys. 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here he tells us of the night of october 9, 1963, when he rushed to the piave valley together with the first-aid teams, to confront the remains of longarone. did the dam collapse? everybody thinks so, but the moon was not shining enough to allow an understanding of what had happened. it was only the first faint light of dawn that revealed that the dam was still standing: it was only edged by the water, from the shaping of the tremendous wave that was raised by the mount toc landslide when it entered the basin at 100 km/h. the dam edge contributed to the projection of the wave onto the opposite slopes, where once the water mass was released from the narrow vaiont canyon, it raised solidly into the air, grazing the casso village and precipitated as a massive block on the longarone area. everybody looked upwards with hate and despair, at the residual water streamlets that were overflowing along the concrete wall of the dam. over the white bare esplanade people appeared extremely small. pitiful people, gathered around the bodies wrenched from the devastated houses, denuded and buried in the mud. 1. introduction as we approach 50 years after the disaster of vaiont, the scientific community has never stopped the thorough study of the combined causes and the final mechanisms of the terrible landslide of october 9, 1963, that brought death and destruction to the villages of longarone, erto and casso, and that ruined a community of over 2,000 inhabitants. the scientists who carried out the most serious studies after that tragedy to provide the most suitable means to prevent such disasters in populated areas could not express emotion when they had to evaluate methods and to design schemes for future public works in front of the elements of nature. by reporting this personal testimony of that night in longarone, the emotion of the author is intended to remind us of the necessary link between the respect for human conditions and scientific knowledge, like a request to keep them improving. 2. the testimony of geologist valerio spagna, who rushed to the disaster site that quiet evening, while waiting for our child to go to sleep, was the beginning of the most dramatic night that i have ever lived [valerio spagna 2010]. suddenly, at a quarter to eleven, the electricity cut out for a few minutes. then the silence was broken by the shrilling of the sirens of the firemen. i got up and followed my instinct towards something i did not know, but that was certainly serious. i got into my car, still dressed in the greyish suit and tie i used to wear for teaching at the school. i greeted leda, who was also wondering about the situation, and worrying for the baby’s sleep. among the few cars on the “agordina” national road, i heard voices talking about a dam that had collapsed. i followed them along the valley of the cordevole river. i then drove through belluno, and arrived at the ponte in the alps, as if attracted by a strong magnet. the road was blocked by the police: nobody could have access to the piave valley. by now it was clear that the vaiont dam had collapsed and had flooded the whole valley. i wanted to reach the dam, which i knew well because i had seen it growing from the foundations to the crest at 262 m above the valley floor. i drove back to agordo, and from there i took the road leading to the passo duran, up to 1,600 m, and down into the zoldo valley. now alone as i was driving down towards the piave valley in front of longarone, i felt surrounded by a tragic atmosphere and my head was aching. i then heard voices coming from the longarone houses set above. along the roadside, men were looking down into the valley. i parked the car and joined them. they were reviewing the familiar houses, but only by remembering their places according to their owners’ names; a painful list. “the julia workshop?” “nija, nija!” answered a choir. “where nani lived?” “nija, nija!” “the postman’s house?” “nija, nija!” the afflicted litany went on. my throat was tightened by those “nothing, nothing!” at one o’clock, a row of miners’ lamps appeared, held article history received december 16, 2011; accepted february 7, 2012. subject classification: environmental risk, vaiont disaster, landslide, flood, first-aid teams. 389 by the alpinists, carrying on their shoulders a coil of rope. without a word they walked past, bound together by a long cord, the noise of their boots on the detritus spread on the path rose up on the background of screams and cries. at that stage, we all looked in the direction of the dam with a mixture of fear and hate. on this mild autumn night, the sky behind the vaiont valley section was brightened by the moonshine, showing a ‘v’ profile so deep that it suggested that the dam was no longer there. when the moon outlined mount toc, we realized that what we thought was a piece of sky was nothing else than the bright reflection of the water, still overflowing the basin. only with the first light of dawn did we see clearly the gray dome of the concrete dam wall, with the residual fall of water. the bottom of the piave valley appeared now like a white desert plain, without any houses, completely bare, as well as the hill where the village of pirago once stood, recognizable now only by the bell tower of the hamlet that had miraculously escaped the wave that had swept everything around. gray bodies, completely undressed by the raging waters, with the hands of compassionate women trying to put together the dislocated limbs. an atmosphere of total defeat, and a sense of uselessness on behalf of the rescuers, who could do nothing but make helpless movements, holding their heads between their hands. everywhere a sea of mud. the stones embedded in the mud made it look like a lava flow. from that loose surface emerged only entire bodies, of both valerio spagna 390 figure 1. satellite image of the vaiont valley, and the tributary torrent of the piave river basin. figure 3. the present rock mass landslide. figure 2. the wall of the arch-dam rising to 262 m from the bottom of the vaiont canyon valley. 391 adults and children, naked, or with their clothes covered in the mud. the large plain through which longarone had expanded appeared like a huge desert, a hallucinatory landscape unknown for the bottom of an alpine valley. the human forms moving without knowing where to go, and the men carrying out the rescue, helpless, and appearing very small. a last distressed look from pirago to the valley. my car was parked by a truck with drop sides that was being loaded with the bodies as they were progressively extracted from the mud. i took the passo duran road, and went straight to the school without changing my clothes. the radio gave the news of the disaster. the principal of the school had gathered the students and teachers in the square in front of the institute. i felt his words inappropriate and out of the place, because he was not conscious of the extent of the catastrophe. i was horrified when he considered the human cost, as happens in the mines, as the usual tribute to the development of the human condition (!). i closed my eyes and saw the human condition in the expanse of mud in longarone that i had just left, and as soon as i opened my eyes again i saw the students of the third class gathered around the reddish hair of dal molin, whose face was convulsed. his eyes were swollen, but they did not cry: his family had disappeared entirely. in the following days, other victims were found in the piave valley, and even further downstream near feltre. on a bend in the river near santa giustina bellunese, the rescuers found a body imprisoned in a tangle of branches two meters above the river bank. this shows how high the wave had been when it was reversed by vaiont lake, and how powerful its destructive capacity had been, with such effects nearly 30 miles downstream. for several weeks the pages of the newspapers were overflowing with the event. in the months before the vaiont landslide, there had been continuous and suspicious movements, but public opinion had not be informed because a very important financial operation was on the way to being settled: the sale of the private hydroelectric power plants to enel, the new agency created to nationalize the electricity throughout italy. the plan of the digoman arch dam on the cordevole river that would have formed a new lake upstream of the alleghe village was abandoned. even today, in front of the village of laste, there are still some concrete blocks on the left shoulder, as a warning to the italian dam builders to stifle their pride. in those years, the kariba dam on the zambesi river was considered an emblem of italian engineering throughout the world. as a countrepoint, the famous writer dino buzzati, a lover of the dolomites, wrote a heartfelt article in the “corriere della sera”, the main national newspaper [buzzati 1963]. he concluded his article about cruel nature with that cry: “the mount that broke and spread death is one of my life mounts, the profile of which is engraved in my mind and will stay there forever. the reason why the fifty years ago: the vaiont night figure 4. the maè torrent at the bottom of the piave valley, south of longarone village. figure 5. the first dawn light on the pirago bell tower, the only building in longarone miraculously left standing. writer has a dry throat and cannot find the right words. the words of disbelief, dismay, anger, crying, mourning, remain inside him, with their cruel weight”. he did not want then to see that it was human cruelty, ignorance, and limits that had let loose this nature! “keeping a diary and writing in his late years one’s memoires should be a duty assigned by the state; the materials stored after three or four generations would have a priceless value: many historical and psychological problems that beset humanity would be solved”. (translation from giuseppe tomasi di lampedusa, racconti, 1961). 3. conclusions the geological and geotechnical model that the enormity of this vaiont event revealed to scientists enlarged the field of scientific knowledge of the dimensions and mode of evolution of slope dynamics, and this is now integrated into the most advanced theories of landslide classification. however, such major environment disasters make us always more aware that knowledge must precede facts, and even in front of the most painful events, human participation, consciousness and professional ethics must be the fundamental attitude of the geologist in providing contributions to basic scientific and technical knowledge in the design of large public works. since that tragic day, now 50 years ago, many other disasters have occurred across our rugged territory. certainly not all of the disasters might have been predicted, but most of them have recurred cyclically and continuously, and not only in relation to large civil constructions. instead, we have found ourselves in circumstances in which even small constructions have challenged “the limits of the safe territory”, because they have been built around sensitive natural elements, such as river banks, slopes, mines, large slopes of roads and railway infrastructure, mostly by following imprudent directives in the planning. very often this was done with the endorsement of valuable professional expertise, but without proper geological-hydraulic or geological-technical evaluation, both in the general framework in which such interventions have fallen, and also in the details of their implementation. the memory brings us back to a ruthless and rhythmic list, from northern to southern italy: 1966: the flood of florence, the flood of the belluno area, the breaking of the murazzi, the walls placed in defense of the lagoon of venice; 1968: the flood of strona valley and biella, piedmont; 1970: the event of genoa and the bisagno stream; 1977: the floods of canavese and val d’ossola, piedmont; 1978: the rail disaster of the train “freccia della laguna” between bologna and florence; 1982: the “barducci” landslide in ancona; 1985: the landslide of the landfill mining of stava, with 290 victims in tesero, in trentino; 1987: the val pola landslide, in valtellina, lombardy; 1994: the flood in val tanaro, piedmont; 1998: the landslides in the sarno valley, campania; 2000: the flood in the tourist village of soverato, calabria; 2009: the landslide of “cancia” in borca di cadore, veneto;… and this list is certainly incomplete. certainly the situation has improved, both with the more effective instruments of control and the quality of prevention studies. as an example: there is now the obligation to draw up a detailed map of the “suitability of the territory” for the planning instruments required, such as in the veneto region for municipal projects. however, disasters continue to follow one after another, relentlessly (2009: the giampilieri event in messina; 2011: the disasters of cinque terre and lunigiana; and always more…). all of the events have also worsened in recent decades, because of the buildings that were constructed along the rivers and streams. this situation imposes a duty to do more; not only a duty in the technical sense for designers and planners, but also a duty for all citizens and administrators, who are often slow to recognize their indispensable role of participation and control. the birth of a new ethical awareness in the practice and research on earth sciences has manifested itself in the successfully geoitalia 2011 conference that was held in turin. for all of us, this is an encouraging sign. references buzzati, d. (1963). natura crudele, in: corriere della sera, 11 ottobre 1963. spagna, v. (2010). il nonno si racconta. edizioni imprimitur, padova (english translation: hélène berta ageron). tomasi di lampedusa, g. (1961). racconti; consulted edition: milano, feltrinelli, 1988. corresponding author: valerio spagna, via bartolomeo cristofori 20, 35137 padova, italy; email: vspagna@inwind.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. valerio spagna 392 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 1, 2004, 01/07def 191 annals of geophysics, vol. 47, n. 1, february 2004 key words ulf – ionosphere – alfven – seismicity 1. introduction while influence of plasma irregularities is important for radio-wave propagation through the ionosphere, the same influence for ulf waves is usually neglected. in the conventional approach to many geophysical problems related to ulf magnetic pulsations (magnetospheric diagnostics, wave-particle interaction inside radiation belts and so on), the ionosphere is approximated by a layer with homogeneous conductivity, which is connected with the regular conductivity profile integrated on height. however there are some indications that such a simple model is not valid even for the ulf frequency range taking into account the existence of ionospheric perturbations in reality. alperovich et al. (2002) using both theoretical computations and laboratory experiments showed that rather small (∼ 10-30%) variations in plasma density can produce a several times increase in «effective» conductivity, included in consideration of ulf wave characteristics. molchanov et al. (2003) reported the results of ulf magnetic field observations (0.003-5.0 hz) at kamchatka region during a long period of seismic activation. they found a remarkable and statistically reliable ulf intensity depression several days before strong seismic shocks. the effect was especially clear at nighttime and for the filter channels 0.01-0.1 hz and it was absent at daytime. they interpreted the effect in asdepression of the ulf geomagnetic pulsation related to ionospheric irregularities valery m. sorokin (1), evgeny n. fedorov (2), alexander yu. schekotov (2), oleg a. molchanov (2) and masashi hayakawa (3) (1) institute of terrestrial magnetism, ionosphere and radiowave propagation (izmiran), russian academy of science, troitsk (moscow region), russia (2) schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia (3) department of electronic engineering, the university of electro-communications, tokyo, japan abstract we consider a depression in intensity of ulf magnetic pulsations, which is observed on the ground surface due to appearance of the irregularities in the ionosphere. it is supposed that oblique alfven waves in the ulf frequency range are downgoing from the magnetosphere and the horizontal irregularities of ionospheric conductivity are created by upgoing atmospheric gravity waves from seismic source. unlike the companion paper by molchanov et al. (2003), we used a simple model of the ionospheric layer but took into consideration the lateral inhomogeneity of the perturbation region in the ionosphere. it is shown that ulf intensity could be essentially decreased for frequencies f = 0.001-0.1 hz at nighttime but the change is negligible at daytime in coincidence with observational results. mailing address: dr. valery m. sorokin, institute of terrestrial magnetism, ionosphere and radiowave propagation (izmiran), russian academy of science, troitsk (moscow region), 142092 russia; e-mail: sova@izmiran.rssi.ru 192 valery m. sorokin, evgeny n. fedorov, alexander yu. schekotov, oleg a. molchanov and masashi hayakawa sumption of atmospheric gravity wave intensification induced by changes in temperature and pressure near the ground due to gas and water release in a course of earthquake preparation. mareev et al. (2002) considered gravity waves intensification in the ionosphere. an appearance of gravity waves in the ionosphere leads to depression of downgoing from magnetosphere ulf waves due to loss of coherency along the wave front (like scattering) and due to a change in effective ionospheric conductivity. here we are going to investigate the latter process theoretically. some results on this subject are obtained in a companion paper by molchanov et al. (2003) in assumption of vertical stratification of the ionosphere. here we consider the same effect in assumption of lateral inhomogeneity but for thin ionospheric layer. 2. electromagnetic ulf field in horizontally inhomogeneous ionosphere we use a simple model, which includes the following: i) source of the magnetic field ulf pulsations is situated in the magnetosphere and it generates downgoing alfven waves. they propagate in a homogeneous magnetospheric medium above ionosphere along z-axis, which is coincident with the direction of the external magnetic field. the frequency of the waves ω << ωci, where ωci is ion cyclotron frequency, therefore they propagate with characteristic alfven wave velocity ca and their vertical wave number kz = = ka = ω / ca. in addition we suppose their horizontal wave number k >> ka, another words we consider oblique alfven waves. ii) ionosphere is a thin layer at z = 0 with integrated pedersen and hall conductivities σp ⋅ ⋅ (x, t), σh (x, t) respectively, which are time-dependent and inhomogeneous along the horizontal x-axis (see fig. 1). for simplicity, we neglect input due to ionosphere thickness ∆h, because of ka ∆h << 1 and present ( , ) ( , )x t x t, , ,p h p h p h0= +σ σ ∆σ . (2.1) iii) atmosphere below ionospheric layer is nonconductive and current-free, but the ground medium is completely conductive and tangential electric field disappears at the ground surface z = h. iv) for simplicity, we assume independence of the all field components on y-coordinate, i.e. ∂ / ∂y = 0, and suppose large conductivity along z-axis for magnetosphere and ionosphere that leads to disappearance of electric field component ez ∼ 0. our model is about the same as was discussed in many other papers (e.g., lyatsky and maltsev, 1983). the only difference is assumption on lateral ionospheric inhomogeneity. alfven wave can reflect from the ionospheric layer and transform in the isotropic mode wave (so-called fast magneto-sonic wave) inside ionosphere. then both waves penetrate into the earth-ionospheric cavity. as usual we consider fourier expansion )i t~( , , ) ( , , )e x z t dk d e k z e 2 2 ( r r ikx = r r ~ ~ 3 3 3 3 # # (2.2) where index r = x, y, and the same for magnetic components br. in our model for alfven wave in the magnetosphere only ex and by exist and 1 2 3 b x z h l fig. 1. the model used to calculate the depression of the ulf geomagnetic pulsations: 1 – conducting ionosphere; 2 – ionospheric inhomogeneities; 3 – earth surface. h is the height of the lower boundary of the ionosphere; l is the spatial scale of the seismic region. 193 depression of the ulf geomagnetic pulsation related to occurrence of the ionospheric irregularities these spectral component are described by following equations: ( , , ) ( , , ) , . z e x z k e k z b i z e k c 0 1 x a x y x a a 2 2 2 2 2 2 2 + = = = ~ ~ ~ ~ (2.3) in opposite only component ey, bx, bz keep in the isotropic wave ( , , ) ( , , ) , , / . z e x z k e k z b i z e b k k e k k k k c 0 1 y i y x y z y i a 2 2 2 0 2 2 2 0 2 2 2 2 + = = = = = ~ ~ ~ ~ (2.4) as concerned the situation below ionosphere (at the atmosphere) we have well-known relationships k b e= = ( , , ) ( , , ) z e x z k e k z ik z e k k ic z e x e k k 0 1 r r x y z y y x z z x 2 2 0 2 2 0 0 0 2 2 2 2 2 2 2 2 + = ~ ~ ~b e= = 0 0 ^ c h m (2.5) where k0 2 << k 2 and kz 2 0 = k0 2 − k 2 ≈ − k 2. then after matching of the magnetospheric and atmospheric fields in the ionosphere layer we obtain integral equation for the fields inside ionosphere (see appendix for details) ( , ) ( , , ) ( ) ( ) ( , , ) k k k k k c dk k k k e e e 0 2 4 0 a 2 $ $ )= ~ ~ r~ ~∆σ 3 3 # l l l " ! (2.6) where , , , e e e k k k k k c e e 0 4, , x y h h p h h p p i p a p h p h 1 2 1 2 2 ) ) = = = r ~ σ ∆σ ∆σ ∆σ ∆σ σ , ( )cothk k k ik kh= = + +∆ k k k k= + = = = = = g g g g " ! , .k c c c4 4a a a a 2 2 = =r ~ rς σ its connection with ulf magnetic field at the ground is also analyzed in appendix, where shown bx (ω, h) >> by (ω, h) and ( , , ) ( , , ).sinhb k h i k kh e k 0x y= -~ ~ ~] g6 @# (2.7) 3. change in the ulf spectrum on the ground induced by ionospheric perturbations using conventional approach of perturbation theory we transform (2.6) as following: ( , , ) ( ) ( , ) . k k k c k k dk k k k k e e e 0 2 4 a a 2 $ $ ) ) = ~ ~ ~∆σ 3 3 1 1 1 # l l l " " ! " (3.1) where inverse tensor k 1-" is easy to find , ( ) ( ) .k k k k k k k k k k 1 h h h 1 2 1 2 2 = = +l l 1= g " after substitution in (2.7) we have ( , , ) ( , ) ( ) ( , ) ( , ) ( ) ( ) ( , ) ( ) ( , ) sinh sinh b k h i k kh ckk k e k i c k kh kk dk k k m k k k m k 2 4 x a h a p h1 2 $ $ = + ~ ~l ~ ~ l ~ ~ ~ ∆σ ∆σ ) ) 3 3 ( , )e k~ # l l l l l l " , (3.2) where ( ) ( ) ( ) ( ) ( ) . m k k k k k k k k k k k 1 1 h h h 1 1 2 2 1 2 2 = + l l ( )m k k= 6 6 @ @ ulf magnetic field at the ground we find as the inverse fourier transform of (3.2) ( , , ) ( , , ) ( , , )b x h b x h b x hx x x0 1= -~ ~ ~ 194 valery m. sorokin, evgeny n. fedorov, alexander yu. schekotov, oleg a. molchanov and masashi hayakawa ( , , ) ( , ) ( ) ( , ) ( , , ) ( , ) ( ) ( ) ( , ) ( ) ( , ) ( , ). sinh sinh b x h ick k dke k kh ke k b x h c ik dke k kh k dk k k m k k k m k e k 2 x a h ikx x a ikx p h 0 1 1 2 # # $ $ = = ~ r~ l ~ ~ ~ r l ~ ~ ~ ~ ∆σ ∆σ ) ) 3 3 3 3 3 3 # # # l l l l l l " , (3.3) in (3.3) the second term bx1 is due to presence of perturbations in the ionosphere and could be compared with bx 0. as example for solitary initial wave e * (x) = e * exp (ikxx) and e* (k) = 2πe*δ (k + − kx). we obtain ( , , ) ( ) ( ) ( , , ) ( ) ( ) ( ) ( ) ( ) ( ) sinh sinh b x h e k k h ick k k e b x h e c ik dke k kh k k k m k k k m k 2 4 x x x a h x ik x x a ikx p x x h x x 0 1 1 2 x # # = = ~ ~l ~ l ∆σ ∆σ ) ) 3 3 # " , (3.4) where kx = 2π / λx and λx is horizontal scale of downgoing alfven wave. let us consider influence a moving density variation in the ionosphere (e.g., gravity waves). alperovich et al. (2002) showed that such a variation leads to mainly perturbation of pedersen conductivity. so in (3.4) we leave only the first term in the integrand and present the perturbation as following: ( ) ( ) cos expx k x l x k l e e e 2 2 4 2 2 1 ( ) ( ) p p g p p k l k k l k k l 2 2 2 g g 2 2 2 2 2 2 $ = = + + { r ∆σ ∆σ ∆σ ∆σ + +{ { $ 0 0 b l< 7 f a & 0 (3.5) where ∆σp0 is averaged amplitude of the perturbation induced by gravity wave, ϕ is random phase of the gravity wave, l is spatial scale of the seismic region. substituting (3.5) in (3.4) we obtain the relative change of ulf magnetic field at the ground )(a kf-1 ( )dkb k r b b b l x x x x x x 0 0 0 1 $= = 3 3 b b # e , ( ) ( ) ( ) ( ) . sinh sinh e e e a k k k k k h b kh k k k k k 2 1( ) ( ) ( ) ( ) i k k x k k l k k k l k k k l a p p x x x h 0 0 1 2 1 2 2 x x g x g x 2 2 2 2 2 2 $ + + + = + = + = + f σ σ ∆σ + + + { { 7 7 a a % . (3.6) finally after averaging we have for the value β (ω) = b bx x0 2 the following resultant relation: .a = + b = exp$ a = =~ , ( , ) , , , re sinh coth sinh coth a d b i a i a i x t kh k h h l h c h c h 1 2 4 x x x x x x x x x x x a a a a a p a p a p a h 2 1 2 2 2 2 2 2 1 0 2 0 2 $= + + + + = = = = = + b f r a w w w w w p a w w w w w w w w w w w w p w w r m ~ ~ r ω ω ω ω ω ω σ σ σ σ σ σ σ σ σ 3 3 2 ,a = =ω 0 0 a 1 2= + #^ ^ ^ ^ ^_ ^ ^_ ^ h h h h hi h hi h 7 a * 4 (3.7) in (3.7) we denote undisturbed integrated pedersen and hall conductivities σp0, σh0 respectively. in a case of the large perturbation zone, α >> 1 after simple calculations we have i= +w w $ , exp re sinh coth sinh coth a i a i 1 4 2 x x x x x x 2 2 0 1 2 0 2 0 0 0 1 2 2 0 2 $= + + + + b p f a p w w w w w w w w w w a p ω ω ω ω ω ` e _ _ _ _ j o i i i i 9 9 c c z [ \ ]] ]] _ ` a bb bb (3.8) in the center of the zone, ξ = 0 relation (3.8) reduced to the following: ( ) f f f f f 1 , , x x x x x x 2 2 3 2 1 2 3 2 1 2 1 2 2 2 3 2 2 = + + b f w w w w ω ω ω ω ω f a= + hω cothf = +h w w _ _ i i (3.9) where η (x ≥ 0) = 1, η (x < 0) = 0. 195 depression of the ulf geomagnetic pulsation related to occurrence of the ionospheric irregularities fig. 2. dependence of the relative magnetic field β at the ground on frequency for night and day hours. thin lines show the results of calculations in the thin film ionospheric model, bold lines correspond to full-wave calculations in the iri-90 model. kx = 0.01 km−1 and other parameters are shown on the picture. fig. 3. spatial distribution of the relative magnetic field β dependence on distance from projection of the perturbation center in the ionosphere, h is height of lower boundary of the ionosphere (here h = 100 km). 196 valery m. sorokin, evgeny n. fedorov, alexander yu. schekotov, oleg a. molchanov and masashi hayakawa appendix let us represent the solution of (2.3) as the sum of downgoing and reflecting waves ( , , ) ( ) ( , ) ( , ) ( ) ( ), e k z k e r k e c c e r k e k dxe x k c e e e x ik z a ik z y a ik z a ik z ikx a a a a a a ) ) ) ) = + = = ~ ~ ~ ~ 3 3 ( , , ) ( )b k z ke=~ # ` j 7 7 a a (a.1) where e * (x, ω) is amplitude distribution on the wave front and ra is reflection coefficient. it is evident that β (ω = 0) = 1 − ε, β (ω → → ∞) = 1 − ε (a1 + 1) / (a2 + 1) and β (ω) decreases on frequency. at night-time ionosphere when σp0 << σa we have ε ≈ ∆σp0 / σa, a1 ≈ 1, and a2 << 1. at day-time ionosphere when σp 0 >> σa we have ε ≈ ∆σp0 / σa, a1 ≈ a2 >> 1. dependence of β on frequency under the center of inhomogeneity is shown in fig. 2 for the night and day hours. the values of integrated pedersen and hall conductivities are given in the figure. for comparison, these dependences calculated for laterally homogeneous ionosphere of the finite thickness are also shown in fig. 2 for the same values of integrated ionospheric conductivities. the values of β (ω) at relatively low frequencies (f < 0.05 hz) are about 0.6-0.7 for nighttime and 0.90.95 at daytime for both models. at higher frequencies (0.2-0.3 hz) β (ω) calculated with the use of iri model grows almost up to unity. several extrema seen at the curve are caused by the ionospheric alfven resonance and ionospheric mhd waveguide. this effect is not obtained in the thin ionosphere model that gives strong monotonous decrease of β with frequency at f > 0.3 hz. at daytime β weakly depends on frequency and is about unity in all the frequency range considered. spatial distribution of β value at the ground found with the relation (3.7) is presented in fig. 3. note that the size of the depression area is larger than the perturbation scale in the ionosphere. 4. discussion and conclusions it seems that theoretical results here coincide with observational data reported by molchanov et al. (2003). recently sorokin et al. (2002, 2003) investigated the possibility of generating seismoinduced geomagnetic pulsations due to preseismic changes in the background electric field. here we try to estimate another possibility in connection with intensification of the atmospheric gravity waves before earthquakes and following the appearance of the ionospheric inhomogeneities. while influence of the horizontal ionospheric irregularities on propagation of the vlf waves is known (e.g., shklyar and nagano, 1998), the same influence on the alfven waves is a rather original problem and we are going to continue this research for a more complicated model taking into consideration both vertical and horizontal stratifications of the ionosphere. acknowledgements this research was partially supported by istc under grant 1121, by commision of the eu (grant no. intas-01-0456) and by rfbr (grant no. 03-05-64553). two authors (o.a.m. and m.h.) are thankful for support from international space science institute (issi) at bern, switzerland within the project «earthquake influence of the ionosphere as evident from satellite densityelectric field data». 197 depression of the ulf geomagnetic pulsation related to occurrence of the ionospheric irregularities the similar relationship for isotropic wave as follows: ( , , ) ( ) ( , ) ( , , ) ( ) ( , ) ( , , ) e k z k r k e b k z k k r k e k e k z k k k e e y i ik z x i i ik z i y i a 2 2 i i ) ) = = = = ~ ~ ~ ~ ~ ~ ~ (a.2) where ri is reflection coefficient for isotropic waves. as concerned solution of (2.5) we need to take into consideration that ex,y (z = h) = 0, and to match with above-mentioned solutions at the lower boundary of the ionosphere, hence ( , , ) ( ) ( , ) , , ( ) ( , ) ( , , ) , , sinh cosh coth e k z k t k k z h b k z i k k t k k z h i k k z h e k z k z h k z k k z h k k z h e e e e x a y a x y i x i y ) ) ) ) = = = ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ sinh( , , ) ( ) ( , ) ( ) ( , ) ( , , )cosh coth e k z k t k b i k t k i e k z = = = ~ ~ ~ ~ ^ _ ^ ^ ^ _ ^ ^ h i h h h i h h 8 8 8 8 8 8 b b b b b b (a.3) where ta is transmission coefficient for alfven wave and t i is coefficient of transformation the alfven wave into isotropic wave in the ionosphere. it is evident that by / bx = (ω / c)2t a/ (k 2t i) << 1, if the transformation is essential. it means that geomagnetic pulsations observed at the ground surface are mainly related to isotropic wave, which is originated from alfven wave inside ionosphere. the electric fields at the upper boundary of the ionosphere are continuous, hence ( ), ( ).sinh sinhr t kh r t kh1 a a i i + = = (a.4) however discontinuity of the magnetic fields equals to surface currents at the boundary ( , , ) ( , , ) ( , ) ( , , ) ( , ) ( , , ) dte dxe b x z t dte dxe b x z t c dte dxe x t e x t dte dxe x t e x t 0 0 4 0 0 i t ikx x i t ikx x i t ikx p y i t ikx h x = + = = + r σ σ 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 ~ ~ ~ ~ = # # # # # # # # r t s ss v x w ww ( , , ) ( , , ) ( , ) ( , , ) ( , ) ( , , ) dte dxe b x z t dte dxe b x z t c dte dxe x t e x t dte dxe x t e x t 0 0 4 0 0 i t ikx y i t ikx y i t ikx p x i t ikx h y = + = = = r σ σ 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 ~ ~ ~ ~ # # # # # # # # r t s ss v x w ww (a.5) suppose now that time variation of the ionospheric conductivity is slow in comparison with geomagnetic pulsations. for example characteristic frequencies of the atmospheric gravity waves are ω 0 ∼ (10−3 − 10−4)c−1 << ω. if so, we can simplify time integration in (a.5). using now (2.1), (a.1a.4) and obvious relation ( , ) ( , , ) ( , ) ( , , )dte dxe x t e x t dxe x t e x0 0, , , , i t ikx p h x y ikx p h x y= =~σ σ 3 3 3 3 3 3 ~# # # 198 valery m. sorokin, evgeny n. fedorov, alexander yu. schekotov, oleg a. molchanov and masashi hayakawa references alperovich, l., i. chaikovsky, yu. gurvich and a. melnikov (2002): laboratory modeling of the disturbed dand e-layers: dc and ac fields, in seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o.a. molchanov (terra scientific publishing co., tokyo), 343-348. lyatsky, w.b. and yu. p. maltsev (1983): the magnetosphere-ionosphere interaction (nauka, moscow), pp. 192. mareev, e.a., d.i. iudin and o.a. molchanov (2002): mosaic source of internal gravity waves associated with seismic activity, in seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o.a. molchanov (terra scientific publishing co., tokyo), 335-342. molchanov, o.a., a.yu. schekotov, e. fedorov, g.g. ( , , ) ( , ) ( , , )e x dk k k t e k0 2 0, , , ,p h x y p h x y0= + -~ r ~σ ∆σ 3 3 # l l l (a.6) we finally find for electric field in the ionosphere ( , ) ( , , ) ( ) ( , ) ( , , ) .k k k k k c dk k k t ke e e0 2 4 2 0a 2)= -~ ~ r~ r ~∆σ 3 3 # l l l " ! (a.7) here time t is parameter and definitions are as follows: , , , ( ), , , . coth e e e k k k k k k k k ik kh k k k k c k c c c e e 0 4 4 4 , , x y h h p h h p p i p a p h p h a a a a 1 2 1 2 2 2 2 )= = = = = + + = + = = = r ~ r ~ r ∆σ ∆σ ∆σ ∆σ ∆σ σ σ σ ) r t s ss r t s ss r t s ss r t s ss v x w ww v x w ww v x w ww v x w ww " ! belyaev, m. solovieva and m. hayakawa (2004): preseismic ulf effect and possible interpretation, ann. geophysics, 47 (1), 119-131 (this volume) shklyar, d.r. and i. nagano (1998): on vlf wave scattering in plasma with density irregularities, j. geophys. res., 103 (a12), 29515-29526. sorokin, v.m., v.m. chmyrev and a.k. yaschenko (2002): ionospheric generation mechanism of seismic related ulf magnetic pulsations observed on the earth surface, in seismo electromagnetics: lithosphere-atmosphere-ionosphere coupling, edited by m. hayakawa and o.a. molchanov (terra scientific publishing co., tokyo), 209-214. sorokin, v.m., v.m. chmyrev and a.k. yaschenko (2003): ionospheric generation mechanism of geomagnetic pulsations observed on the earth surface before earthquake, j. atmos. solar terr. phys., 64, 21-29. p r o g e t t o d i u n o s p e t t r o e l i o g r a f o p e r l y m a n a l f a c o n t e l e s c o p i o p e r s a t e l l i t i (*) p . p o t e n z a r i c e v u t o il 19 f e b b r a i o 1963 r i a s s u n t o . — viene d e s c r i t t o u n s i s t e m a per e f f e t t u a r e la scansione d e l l ' i m m a g i n e solare nel c a m p o di lunghezze d ' o n d a della l y m a n a l f a . esso consiste in u n ' o t t i c a a specchi t i p o cassegrain, u n s i s t e m a di orientazione, u n o s p e t t r o e l i o g r a f o per la dissezione d e l l ' i m m a g i n e solare e u n sis t e m a di codificazione e t r a s m i s s i o n e . vengono d e s c r i t t e a l c u n e p r o v e e controlli d a eseguire p r i m a del lancio. s u m m a r y . — a s a t e l l i t e b o r n e s y s t e m for dissecting t h e solar i m a g e in t h e l y m a n a l f a w a v e l e n g t h r a n g e is o u t l i n e d . i t consist of a cassegrain m i r r o r optics, a n o r i e n t i n g device, a s p e c t r o h e l i o g r a p h dissecting t h e solar i m a g e a n d a coding a n d t r a n s m i t t i n g s y s t e m . some t e s t s to perforili b e f o r e l a u n c h i n g are described. i l sistema rappresentato nella figura è uno spettroeliografo per la lyman alfa con telescopio per raccogliere l'immagine solare e sistema di stabilizzazione. l'immagine solare viene costruita da un sistema cassegrain. esso è il sistema più leggero e più luminoso (anche col sole, a queste lunghezze d'onda la luminosità è importante) che p e r m e t t e agevolmente di ottenere una lunga focale (ad esempio 2 metri), con un ingombro molto limitato. esso è sostanzialmente un teleobbiettivo a specchi. a 1250 a il potere riflettente dell'alluminio e del rodio è basso m a sempre abbastanza elevato (ca. 40-50%) per il sole. questo telescopio è orientato verso il sole mediante la p i a t t a f o r m a stabilizzata della società norden (di cui si vede il profilo in basso a sinistra), essa pesa soltanto 20 libbre ed è costituita da un sistema giroscopico (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e t e c n i c o scientifico dello spazio. r o m a , 19-23 giugno 1962. 4 5 4 f . p o t e n z a il quale p e r m e t t e di riprodurre mediante u n a coppia selsyn l'orientazione che devono possedere i due assi di u n a m o n t a t u r a azimutale del telescopio allo scopo di m a n t e n e r e la sua orientazione nello spazio costante entro pochi secondi d'arco, e indipendentemente dalla orientazione nello spazio della intelaiatura di supporto (incolucro del satellite). tale p i a t t a f o r m a stabilizzata è s t a t a già estensivamente utilizzata su satelliti per scopi analoghi e f o r t u n a t a m e n t e la sua utilizzazione non è più oggi un problema. i l disegno mostra l'uscita dei segnali elettrici per il pilotaggio dei due assi del telescopio (filo rosso) t r a m i t e i selsyn (i cilindri beige sugli assi ortogonali). la p i a t t a f o r m a stabilizzata va orientata in modo che il telescopio punti il sole nel modo desiderato prima del lamcio dell'apprarecchiatura; essa poi m a n t e r r à la sua orientazione indipendentemente dalle vicissitudini impostele dal vettore. l'immagine solare viene p r o i e t t a t a sulla fenditura di u n monocrom a t o r e ad alta dispersione che utilizzerà quindi come mezzo dispersivo, date anche le caratteristiche della radiazione da osservare, un reticolo concavo; evitando quindi l'impiego di altra ottica. sulla f e n d i t u r a di uscita è piazzato il dispositivo di movimento delle fenditure per la scansione dell'immagine solare (esso non è mostrato nel disegno). nel caso in cui si possa effettuare il ricupero del satellite (cosa assai improbabile) l'immagine p o t r à essere costruita su di u n a emulsione fotografica, altrimenti v e r r à inviato a mi fotometro che, m a n mano che l'immagine si sposta di f r o n t e alla fenditura, raccoglie il diagramma delle intensità lungo la f e n d i t u r a e lo invia al sistema di trasmissione dati a terra, indicato nell'angolo inferiore destro del disegno. lo schizzo non m o s t r a i blocchi del sistema di ricezione e codificazione dei d a t i presi dalla f e n d i t u r a ; esso non mostra altresì i sistemi che, i n t e r n a m e n t e allo spettrografo, eliminano l'arrivo, sulla fenditura di uscita, di luce di lunghezza d'onda non desiderata. il fotomoltiplicatore per il rilievo dei dati sulla f e n d i t u r a esige, per il suo funzionamento, che la radiazione della l y m a n alfa sia t r a s f o r m a t a , per esempio mediante un composto fluorescente, in u n a radiazione per cui è t r a s p a r e n t e la chiusura del fotomoltiplicatore dalla p a r t e del fotocatodo (quarzo), oppure, dato che il sistema funzionerà nel vuoto spinto il fotomoltiplicatore p o t r à a d d i r i t t u r a essere privo della finestra di chiusura. un programma minimo potrà consistere nel rilevamento della sola intensità della l y m a n alfa emessa da u n a particolare regione del sole; p r o g e t t o d i u n o s p e t t e o e l i o g r a f o p e r l y m a n a l f a 4 5 5 i dati relativi verranno trasmessi continuamente a terra per essere correlati ad altre manifestazioni dell'attività solare rilevabili anche a t e r r a . il p r o g r a m m a di studio per la messa a p u n t o di siffatta apparecchiat u r a comprende: 1 la scelta delle dimensioni del telescopio e la sua costruzione, messa a p u n t o in laboratorio. si prenderanno in considerazione materiali extraleggeri (alluminio e magnesio per le s t r u t t u r e , berillio per lo specchio, etc.). 2 costruzione, messa a p u n t o e collaudo del sistema di controllo del p u n t a m e n t o ; anche questa costruzione è realizzata con i materiali più leggeri, dovrà venire collaudata in laboratorio, su di una piattaforma che il collaudatore f a r à orientare a capriccio, o vibrare controllando l'efficacia del sistema di p u n t a m e n t o ad annullare questi falsi movimenti t e n e n d o in p u n t a m e n t o l'immagine solare. questa è la chiave per ottenere u n a buona qualità della immagine. 3 costruzione e collaudo del prototipo del sistema di rilevamento e codificazione dell'immagine da teletrasmettere; in particolare si dovrà decidere se rilevare sulla f e n d i t u r a di uscita le intensità con un fotometro di tipo classico o con un contatore di singoli fotoni. costruito il prototipo, si dovrà costruire una prima versione della s t r u t t u r a definitiva, miniaturizzata ed alleggerita da provare per la sua efficacia di f u n zionamento, insieme al sistema 2, in u n a camera a vuoto spinto, sopratt u t t o per controllare la resistenza delle p a r t i elettriche. 4 infine sono opzionali e consigliatali prove generali di funzion a m e n t o del sistema (eventualmente non su l y m a n alfa), per esempio con lanci in pallone con ricupero. prove di questo genere sono estremam e n t e i m p o r t a n t i per la sicurezza del funzionamento finale. layout 6 annals of geophysics istituto nazionale di geofisica e vulcanologia [special issue] earthquake precursor research: ground-satellite observations, laboratory experiments, and theoretical models guest editors pier francesco biagi università di bari, dipartimento di fisica, bari, italy michael e. contadakis aristotle university of thessaloniki, department of geodesy and surveying, thessaloniki, greece masashi hayakawa university of electro-communications, advanced wireless communications research center, chofu, tokyo, japan tommaso maggipinto università di bari, dipartimento di fisica, bari, italy 19 preface annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5691 over the last several years, a lot of evidence has indicated that the earthquake generation process is a critical phenomenon, which culminates in a large event that corresponds to a critical point. a fundamental prediction of this hypothesis is that before the event reaches this critical point, a regional system of faults goes through a period of accelerating seismicity and seismic energy release, which forms the signature of the approach to the critical state. by the end of this stage, the critical point has been attained, and the stress exerted by friction on the shear zone, as well as the strain caused by an avalanche of fusing cracks, lead to irreversible instability and rupture. as a consequence of the processes prior to an event, changes in the physical/chemical state of the earth precede earthquakes. these variations can be called the ground precursors. there are a variety of such precursors, that are evident in their genesis, such as ground uplift and tilt, gas emissions, underground water-level fluctuations, changes in groundwater chemistry, and changes in electrical resistivity of the rock. recently, variations in certain atmospheric parameters have been observed prior to seismic activity. we can call these variations the atmospheric precursors. these atmospheric precursors have mainly been indicated by data collected by satellites. research into the interactions between seismic activity and disturbances in radiobroadcasts has also been carried out recently. these disturbances appear related to variations in certain parameters in the ground, the atmosphere and the ionosphere, and we can call these the radio precursors. there appear to be strict connections between the atmospheric precursors and the radio precursors. indeed, the main disturbances revealed in the atmosphere by satellites generate variations in the propagation medium of the radio signals, and the disturbances in these signals can be detected. two different models have been proposed to justify the atmospheric and mainly ionospheric disturbances. the first assumes direct effects of the ionising radiation from gases (mainly radon)/aerosols, or electromagnetic emissions from the crust during the preparatory phase of an earthquake. the second model assumes indirect effects, such as pre-seismic phenomena in the ground that can produce gravity waves in the atmosphere-ionosphere. this second model overcomes the problems of the transport from the ground up into ionosphere of particles or electromagnetic waves in the first model. in any case, the atmospheric and radio precursors indicate that there is ionosphere-atmosphere-lithosphere coupling. over the last few years, earthquake precursor research has developed greatly. this is seen in particular by the networks for the continuous collecting of data related to different parameters that have been put into operation. in this framework, the pacific vlf radio network and the european vlf/lf radio network stand out. at the same time, a large number of laboratory experiments have clearly suggested that microscopic failure (micro-fracturing) of the rock is associated with the appearance of spontaneous charge production (electrification) and transient electric-magnetic-electromagnetic emission. so, electric, magnetic and electromagnetic emissions from the ground can also be generated during the preparation phase of earthquakes, and these phenomena must be included as ground precursors. thus, both the techniques in the laboratory for measuring electromagnetic radiation emitted during rock fracturing and the analysis of anomalies in the transmission of electromagnetic signals are good candidates for investigating seismic-related phenomena. in this respect, an understanding of the physics of rock cracking and deformation is critical for our understanding of the seismic activity in the earth. this special issue brings together the work of several groups that have been working in these fields, with the presentation of some of the recent results obtained from around the world. these contributions were mainly presented at the european geosciences union general assembly in april 2011, in vienna, austria. guest editors: pier francesco biagi michael e. contadakis masashi hayakawa tommaso maggipinto © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. 435_438 smullin.pdf 435 annals of geophysics, vol. 46, n. 2, april 2003 education for life or life-long education? louis d. smullin massachusetts institute of technology, cambridge, ma, u.s.a. abstract historically, people learned skills that lasted for their lifetime. this was «education for life». in the modern technical and scientifi c world, things change so rapidly that a professional must continuously learn new material. this is «life-long education». technological advances require signifi cant changes the electrical engineering curriculum. studies of vacuum tubes and transformers have given way to studies of control theory, vsli chips, semiconductor technology and lasers. an electrical engineer can not expect to master all aspects of the fi eld within four or even eight years of study. continuing education has become part of an engineering career. mailing address: prof. louis d. smullin, massachusetts institute of technology, 77 massachusetts avenue, cambridge, ma 02139-4307, u.s.a. key words education electrical engineering in the fall of 1961, i received a call from the headquarters of the research laboratory of electronics at mit. they wanted to know if my group could use a physics post-doc, an italian who had just fi nished a job in canada. as it happened, i had recently convinced the laboratory director that it was possible to shine a laser at the moon, and to detect the refl ection. the mit lincoln laboratory had built a 48 refl ecting telescope for nasa that had to be delivered by june, 1961; until then we were free to use it. the raytheon company had agreed to provide us with a 50 j ruby laser. these were the basic elements that we needed; but they had to be integrated into a working system, and i needed help! the help was giorgio fiocco. between then and the following may, there were long hours of work, until we were ready for our fi rst attempt. we worked for several days to assemble all of the equipment at the telescope site. when we fi nally were ready, we opened the roof of the telescope building only to see a sky full of big snow fl akes, in may! we had to wait two weeks until the next half moon reappeared, without snow. we wanted the half moon because it seemed to be the best compromise against earth-shine, that would obscure the refl ected laser light. that was the beginning of a long friendship between my family and his. fiocco told me about the mysterious noctilucent clouds that appeared over the arctic regions, and that was his next challenge – to design and build a portable system that he could take to northern norway. as far as i can tell, he is the same smiling (but now white haired), bouncy guy that we met 40 years ago. i feel privileged to be invited to speak at this event that honors his career. a profession is a career in which a practitioner is expected to have a mastery of certain basic skills and arts. in the old days of guilds and apprenticeships one learned by working for a recognized master of the specifi c craft: medicine, law, masonry, blacksmithing, etc. the apprentice was expected to learn the techniques and practices of his trade, as well as its culture, specifi c technical terms, how to deal with clients, etc. the rates of change of various arts and technologies were slow compared to the working life of an individual, so that it was meaningful to think of «education for life». within the last half-century, or so, things have begun to change 436 louis d. smullin drastically, so that the utility of learned material begins to fade with a half-life of 5 or 10 years, depending on the subject. in 19th century the universities began to expand their educational roles beyond training for the church, and they began to offer educational programs in the law, medicine, the sciences, and engineering. thus were born the academic professions. then governments took on the job of defi ning who was entrusted to practice, by issuing licenses. there are scheduled examinations for medicine, for law, for licensed engineers, etc. today, a licensed electrical engineer would be expected to display a working knowledge of motors, generators, and power distribution systems. but he is not likely to be tested on his knowledge of vlsi chips, or on the fi ne points of optical fi ber systems. today, in the us most engineers are «unlicensed» except for a small number of consulting engineers. most engineers work for an employer, or for a firm, that has the responsibility of weeding out the incompetents. the public is protected by its ability to sue the fi rm for incompetence or shoddy practices, or to not purchase its products. our education system is an extension of that of many other warm-blooded animals: birds, apes, whales, etc. the young animal is kept close to the mother or the group (herd) where it learns the necessary skills for survival (fl ying, hunting, communication). today the fi rst 20 years of a person’s life are spent in learning what to do in the next 40 or 50 years. for physicians or phds we may use up the fi rst 30 years. clearly, we have reached some sort of economic limit to how much time can be devoted to education before one begins to participate in the activities of the community. this is what i call «education for life». but in our technical and scientifi c world, things change so rapidly that a «professional» must continually learn (or study) new material in order to be productive and competitive. this activity we call «life long learning». learning is hard work, and it uses up time and energy that might be more pleasantly spent. we have all observed young boys devoting hours to learning a particular basketball or football maneuver with no supervisor or teacher watching them, they know these are important to learn and they know when their performance is satisfactory. but this sort of focussed learning is less common among adults there are too many other competing demands on one’s time. there is a small number of self educated engineers. the rest attend academic institutions of some sort, and study electrical, mechanical or some sort of engineering in a structured environment. the institution is accredited to award appropriate degrees to students who have completed an accredited curriculum. the curriculum is accredited by examining how much time each student is required to spend on a list of «important» subjects. the facilities (libraries, laboratories, etc.) are examined. the teaching staff is examined for specifi c competencies, and syllabi are examined along with typical homeworks and examinations. the whole process of accreditation is to assure that a student will be able to do the work of an engineer. an electrical engineer will not be expected to know much about reinforced concrete construction; but he may reasonably be expected to know something about electric motor design, linear electric circuits, digital circuitry, design of a web page, communication systems, control systems, programming languages, lasers and optical fi ber optical systems. suddenly it is not reasonable to expect him/her to be skilled in all these areas. as we began to expand the number of computer specialists among our professors, about 25 years ago, our ee department changed its name to eecs (electrical engineering and computer science). since then many other departments have done the same. each having two or even three separate curricula. we are now required to be separately accredited in ee and in cs. a few years ago, we decided that 4 years was too short a time for a fi rst professional degree. we now award a fi rst professional degree in 5 years, the meng (master of engineering). this provides a miniscule 20% improvement for a fi eld that seems to have its own moore’s law for adding new subjects and disciplines under its umbrella. we begin with some constraints: most of our students begin their university study at about age 18. the bachelor’s degree is awarded after about four years. most engineering students expect to fi nish their studies after about 4 years, and then go to work at about age 22. a smaller number stay for an ms degree, and an even smaller number stay for a doctoral degree. 437 education for life or life-long education? there is no reason to believe that the mental powers of students today are any greater than they were, say, 100 years go. i was a student in the 1930s. we thought that we worked fairly hard at our studies, today’s students think the same, «tech is hell» is their description of mit. as time has progressed, so has the domain of ee. as new fi elds emerge, we have had to add new material to the curriculum. the choices have to be made: what old material can be eliminated or covered more lightly? the years after world war ii saw the introduction into our curricula of hollow wave guides and of microwaves, followed by communication theory, control theory, transistors, computers and their languages, vlsi chips with more than a million interconnected elements, semiconductor technology, optical waveguides. lasers are imbedded in the whole curriculum, quantum computing and entangled states are on the near horizon, and we may have to learn how to design and fabricate digital circuits made from organic polymers. study of the detailed design of motor armatures is a thing of the past. vacuum tubes are gone, so is illumination engineering. the detailed study of transformers has vanished but new applications of spin phenomena are growing. what else can we eliminate from the curriculum to make room? i can claim a sort of priority: in 1938 claude shannon tried to explain to me his now-famous master’s thesis on switching theory. later on, in about 1949, charles townes tried to explain his microwave maser. finally, in about 1980 young stephen wiesner tried to explain his ideas about quantum communication systems and entangled states. in none of these cases was my educational background wide enough for me to even grasp what they were trying to tell me. until recently, i felt that there was a culture that defi ned our profession. if i said «matched impedances» to a colleague, i knew the image it would evoke. similarly with «thevenin generator». in 1936, i would have drawn a blank if i were tested with «boolean logic» or «stimulated emission». although i had studied a good course in vacuum tubes, «velocity modulation», or «bunching», or «holes» would have meant nothing. several years ago, i made an informal survey of the research topics of our eecs faculty and their graduate students. the result was that less than 20% required a deep understanding of electricity, as i had known it. they were concerned with control theory, system theory, communications and coding, computer systems and languages, quantum theory, queuing theory, and neuro-physiology. none of these required an explicit knowledge of electricity (my shorthand for this was «ohm’s law») when i was younger, i used to read the german technical literature. they defi ned two domains of electrical engineering: starkstrom, and schwachstrom: strong current (power), and weak current (radio and telephony). today, we must add a third category, keinstrom, no current (everything else). a culture implies a shared set of beliefs and images. in literature i used to think that a quotation from longfellow’s «hiawatha» (by the shores of gitchi goomy, by the shining big sea waters, stood the wigam) would elicit a response from an educated (us) person. my colleague roberto fano used to think that he could use a similar test by rolling off the fi rst few cantos of the inferno. things have changed! the classics of literature have no «practical» value; but the classics of electricity are what hold our society together. the lights, phones, elevators, radio and tv, and air conditioners are absolute necessities. today, there is a movement among conservative educators to restore the classics of literature to their once held central position. whether or not this is a good thing, i have no opinion. i am no expert. there is no similar movement to restore echt electricity to its old position of primacy. it is true that education for electrical systems have to be redesigned and rebuilt. a great deal of the old knowledge has been captured in the literature and in specialized computer programs and handbooks, so that a smart but ignorant electrical engineer can learn what is expected of him. for now, at least, a relatively small number of experts can cope with the situation. very few of our present graduates would be able to recreate our present technical society if they were cast off on a dessert island like robinson crusoe. there is no way that within 4 or even 8 years of study, that an engineer can be educated for life. continuing education has become part of an engineering career. its format will vary. for some, reading the technical journals will 438 louis d. smullin help. for many others a more formal and structured way of learning works better. the mores of the work environment will have to change so that there is time and the opportunity for such study. this can be in the form of a short series of lectures (a few days), or even a longer full course for a semester is better suited. the important thing is knowing how to learn. students are free agents who can choose to study whatever interests or excites them. there is little doubt that the newspaper stories of web billionaires before they are 30 years old is part of the attraction of computer science. for those students, the importance of learning the fi ne points of linear circuit theory corresponds to an english speaking student struggling with the complexities of the ablative case. it is a worldwide phenomenon that computer science is the most attractive of the engineering studies. but it is also true that the lure of new «hot» research fi elds is something that professors fi nd hard to resist, and that few administrators of funding agencies can resist bragging about. what to do? moore’s law speaks about the doubling time of vlsi capabilities. the domain of eecs seems to be subject to the same expansion rate. we can not keep people in school forever. they have to learn some of today’s relevant facts and technologies so that they can begin a job. however, the growth of continuing education seems to be a way of approximating that goal. there was a time when the problem could be handled by focusing on the «basics»: math, physics, chemistry, biology. but even that has become diffi cult to accomplish within 4 or 5 years of formal education. we might balkanize the fi eld even more than it is today. the ieee, today, publishes more than 40 professional journals that cater to various subspecialties. how can a young person decide which of these routes to follow? there was a smug belief that «an electrical engineer can learn anything». to the extent that it was true, it was probably because he (she) had been forced to learn «how to learn». in an old copy of mine of the abraham and becker book on electromagnetic theory, there was an exhortation on the fi rst page to really do the problems, and not to pretend that you «understand» the material. it still seems so today, that problem sets are what fi x ideas in one’s mind. the process of studying a diffi cult fi eld teaches one how to learn, and it makes continuing education possible. maybe the trick is this: given a choice of curricula, a student will choose what excites him/her, and will accept the inclusion of diffi cult material, if it seems «relevant» to the objective of the course. (today, many computer science students are impatient with having to study linear circuit theory, but they will work hard at what they consider relevant.) if later in his career, he suddenly is confronted by switching power supplies, or some other idea or technology, he will be able to read the literature, and to proceed, (we hope!). in closing, let me remind you that the career of giorgio fiocco has been one of life-long learning. when he fi rst told me about noctilucent clouds, the «ozone hole» was not even a concept; and setting up facilities in antarctica instead of in norway was really a crazy idea. but thanks to his work, we now know when to wear a sun hat and what number sunscreen lotion to use. lamb and love wave propagalion in an infinite micropolar elastic plate e. .bosem (>!') reeeive(1 on febrnary 2w1, hli:~ su)i~ialn:. this puper is eolwerne,l with mon(lehl'omatie w;lve propagation in an illfillit.e humogenmluh lllicropolai' elnsti .. plnte ])()unde,l by two purnllcl free l)lunes. two kijl(1s of l)ropagation are ,lis('nsse,l; lamh an,] love waver. \ye fin,l t.hat a (iisplaeement field (n" n" o) uwl n minoi"otution neld (o, 0, 'l'a) le(\(1;; to lnmb';; waye;;, while a ,iisplueellwnt fiel,l (o, 0, n,) an(1 a minol'obtion del(1 (lo!> lo., o) 1ca(18 t.o love'h wnves. ria.~sul>:·i'o. in qnesto lavoro si tratta la l)ropngallionc di on,le monoeromatielw in nn piatto elastieo, mirropolare, omog"elwo e,l illnllito, limitato ,in ,1ue piani pnralleli c liheri. vengono ,liseusri ,lne tipi (ii pl"opaguzione, qndlo ,li lamll e quello ,ii ].ove. :'ii trova ehe il ealllpo di spostamenti (1!'1' n" o) c,l il e:~mllo ,li mierorotullioni (o, 0, 't'a) porta a,l on,le (ii li!mb, mentre nn eampo ,li spostamenti (0, o, u. j ) e,l uli eumpo ,li mkro· l'otllllioni ('p" 'pe, o) port.a n,l owlc ,li loye. i::\t]uldl:ctio:-; oseiljn.tion.~ of an cla~tic plate, ·with stress-f)'{-l(; snrfa.('!'~, lmve been inve~i.igai.c(l by raylcigh (l~ ), j,amò (") n·nll othcrs, aml more recently by presc'ott (12), gogohllllll-l ("), sai.o (1.1) aml bwin)..!.', .fn.rdci.zky ij.ll11 press ("l). in this vlljcr wc want to (leal ·with thl-l ~amc vroblem in the frame·wark of thc thcary oi mierollolal' ejn.si.irity. (~) istituto ,li fisiea, fnivt\1'8itit ,li bolognu. nipartimento ,ii i:)l'ienllc ,iella telti!, lniversitit ,li ,\neona (haly). 3-l-:! l,. boschi 'l'he linear llu'or,\" of miero1jolar ('lahtic'ity has bcpll illtrodlu,(·a by erillgl'n ('), rc~an (1") ha,~ (lcl'ivea the [uihlamenbl equatiolls nsing invariath'e c'01hlitions umlel' sul)('rllosed righi hody motiom, .:\licl'opolal' clastieity ma.y givc intere,~ling re,~llllh khen ullilliea to geollh~-hichi problelll~, \ve have all'eady applicd it 1.0 ilpl'ive an cxplic'it ex1jr(\shioll oi the body iol'ce hnd lhe body coullie eqllivalenls fol' seismie (lihlocatiolls (1,2), itunc eqca1'loxs 'l'hl'oughollt thi,~ 1hqh'1' \h\ em1jloy the uhual in(licial notatiollh, "\11 r<\1,.'1t!arity hnlothcses on thc eollhi(lel'ea funetiol!h \\-ill be omittcll. \\te consiacl' a reela,nglllal' cal'lehiall ft'ame o,t' k (k '--= ',2,3). 'l'he ljh,~ie (\quations in thc lincar thcol'y of honlogenco1lh ami allisotropie ela,~lie ,~olillf; are (la): the kinematic' relaliolls: the eqllatio1lh o[ motion: tll,l +]j'j = e iii -micj + f:ijk t} 1t + jl = iii ti the constitutive law,~: ti) = alllti c~i + bjj~j %kl mi} = bklil ckj + go t i %kl [1.a] il.bj [2.a] [2.bj [3.»j [3.b] iii t-he abovc cqualiolls 'hl havc med thc follo\\'ing llotations: 1t., the ('01111)()]hmts oi tlte (liflphl('pl11ent vectol'; !pl, lite eoi111)ollcnl,~ of the lllierol'otalioll veetol'; ci} aihl %ih thc killematic eharactel'i,~ti(l,~ of lhc ,~train; t}l, the colllj)(mell1:h of t-he stre,~s tenwl'j 1nji, the (l011111onellts oi tlte eouplc htresh tensol'; 1<'j, lhc eomvollents oi the bolly iol'eej _mi, llw comllonents of llw body coulllci (!1 the mass densityi aljiti, 343 b"i·', 0,,1:1, i", t lre chanl{!lawisti c constant.i; {lf t he mllwl"i al ; fjj l:, t!w nlt.l::rml,ting t~~ilsoi"; a· coilullrl (iollot.es partùl,i <1 cl" ivntioll wit.h ros}lcct to ~pn,ce variab lcs, ami a supcrpo~nl dol p a rtial dcrivn.tion with resl) cct to the ti m e t . flu·thel"mol'c , wc hav e: a/ikl = a . i / j 00.' = ol:l/j i" = i j/ , ) [4] jf the bo(l.y is homogf.l! hwl\s , isotropie a ll d l'.cnlros)"mlllct ric, wc hnvo: a l i t i = à ilo (hl + (p ;-; ) b lk bl l + f.l bu b, t bjj~1 = u cij~i = a bj) bkr + r /l/t /lil + {j /lu /ljjr anl1 eql1aliolis {3] are a lteretl to: t'i = à eu /l" + (p. + xl cii + il cii mll = a ~h oli + y i1:h + p xii [5] whel"e l., il, x , a, y a·mi p r\1"e materia l const.an ts satis!,rillg llic foll owing i lleq unlities: 3à+2.u + ;-; ~ o i 21l +r.~ o ;< ~ o 3a + /j + i' ~o [g] \ y ~{j ~y y ;. u whi ch are the necessal".y r ~\lii i; uflicient colh1itiolls lol' tlic internai ciicl"gl' tu be n on-negative. furt·lrermnrc wc cali n lso write: iii =.t (j" [7 ] \\"hcl"t~ i is another m a teri ;11 collstan l e. bohciil ][ cquatiom [5] alhl [7j are flubstitute<l into equationfl [1], we get.; (i. -+/1) l' !.!i + (ft + y.) ui.jj + % [;li 1; rp~.) + e, = ijlli, [.':i.a.] (h + (1) (f!j.h + y rpl.!! + y.[;fj k nk.! -:! % rl' l + j[i = i ij)1 [kh] lpt u~ ilow h}leeiali;.;e oui' hnhlysis to t.ll(' ense in whieh; bquntiom [h] lenl! to: (p + %) v-l' , + (i. ~lt) (',l -1%1f~.2 -ij iii (/1 + r.) 1/-112 i(i. ··i ii) ('t~ % ({:l.l = (}lil ì' p'rp:l + r. (11l.1 101.') -:.! % rp~ = i i"l y v~rpl + (a 1(3) e,l _.:.! % f{'l + %1.:.., = 1 ipl y [7"rp2 -;.. (a + fj) e)2 -:! r. fp2 x u3.l = 1 tè. (p + r.) v : nj + y. (tu 9'1.2) =!! il3 whe!"c we haye intwl!llcci! the [olloll"ln.!! 1l0lntionfl: e = (fl.\ + rp2.2 v 2 = ì)!/ì).rl" + ì)~/ì);r2~ [h.a ] [d.h] the t.wu syhtelllh ui eqllati()n,~ [v.a] njl(! [!i. h] are ilu!epen<lent.. the f!ystem [d.a] i!esel'ibes the dis}llaeemcnt !ipla (1.], il") o) an<l the miel'ul"olation li pii] (o, 0, fr~ ). 'l'he hy.~t.em [h.b] (leflcribefl thc di~]liacellwllt ti eli! (o) 0, nj) nlh! the mic!"o!"otatioll tiel<l (rr'l) (p2) o). om lluqlose ih io show tlmt the system [h.a] le~hlfl to li"aveh o[ lnlllh klll<l) while the syslem [h.h] lcnds io ii"nve~ of l~oye kin<l. i~.uih w..iv.e.<;. 'l'hl' eom})()il{'llls of the ht.rehh and coll]lh~ ~11'('f!h ltmf!ol' <letenniih,a by t.he aispiacmncnl fieli! (lh)u2, o) alli! by the micr()]'olatioll fielil (o, o, rpj) are: ldir a~ll lov,; wave l'rol'agnr!l):\, e're. tll =...c (:3 il + r.) 111,1 + i. e /22 = (2 il + ;.:') !t2.2 + i. e ',,3 = i. c tl~ = il (11:<.1 +11i.~) + % !tu r. 9a i~l -f1 (101,: ...!.. u~ , l ) + y. 111,: -i ~ 'j'3 tla -i:" '-h3 = /32 ,....., u 11113 = y (f".i 11131 = f3 !p3,1 /1/:3 = y r~ .~ '/nj2 =. f3 !p~.2 '/nll = bi·z: = m·a3 = 'l!h2 = '/j'l'21 = o 3.j-5 [ io ) i~pt ns llow assume t.lmt a 1ll00l()('hl'omatie wave propagat c'fi, nloll)! t1le ,t ! llil'edioll, ili ali illfinite llli(']'opolli' pja.~ti(· ]llhte. i~et '2h be t1le thiekness o[ t1lc ('ollsi(lp]'p(l plate. ::\lo]'c'oyc]' wc nsflnme that the following eonditiol1s: /" = hz -= /1113 = u, at ,tl = ± 11 [11] shoul(l be sat.i~!lell. in othe]' w()]'(ls, the sul'faees o[ thc platc are rupllose(l free of stl'esses alhl eouple .~tl'(>ssps. eqnation.~ [u.nj ati' snt·isfip(l by: pl'oviùell lh c-= (j),1 lp,z 112 = (j),z + '1',1 (iì + :3 p + r.) [72 (j~ f! (j) = o (p, + r.) v2 ip f! lp % g;~, = u [y 17 2 '2 r. i ò2 /òp] !f3 + r. 17 ~ !j-' = u [l:3j the fil'~t equnt ion [12j llef\(·l'ibc'.~ thc pl'opag,ltioll of 1ong-itlhtinal (li~­ jlbeement wayes with yt'loeit~·: i,+2 ,u+r. [1:3) the la~t two equations [l:! j ('11ll be rt'(luet'(l to tlll' fol1owillg [orm: {[y [.h _ :! r. i ~ ~/w] [(ii + r.) vz !!ò2/~n -i%! v~} (f1 = u [l·la] { [y v2 '2 % i {l2/w] [(.u --;r.) v~ e ~~/n~j ...!.. r. ~ 17~} !fa = u [l·lb] e. noschi let us assulllc solutiolls of thc fol'lll: w (xi, x" t) = ([j* (x!) ei (k";2 ._--" wt) ':1' (xl, .7:" t) = i.p* (xl ) ef (!ex, wl) rp~ (ah, x" t) = rpa* (xi) ei {kf, wl} the 1irst cqnatioll [12j ancl cquatiolls [hj beeome: where t.he followillg llotntioll~ have ueeu introc1nccc1: b~ = 2 r.fy a2 = u 2 /[y (f' + r.)] c2 2 = /tle c3~ = r./e c4 2 = ì'ii \ ( [15j ) [loj ip*=o [17.a.j le') i ~,' ~o, [17.b] [18j equations [lij (ltlseribe the moc1if'tccl transvcl'flc waves. we seurch solutiolls oi equation8 [16] a!l(l [17j of the fol'm: rp* = a i\illh (1) xl ) + b c08h (1j xl) [19j 1.ji* = c sinh (h xl) + d coflh (7,;1 x'i) + r sinh (k2 xi) + f cosh (7,;. xj) rpa* = g sinh (1'1 xl ) + l cosh (h ,'(1) + l' sillh (1;2 xl) + (j eosh (le. xl) lamu a~d loye \l'ave proragatiq:s", lètc, wher(l 1 ','1 ~~ = k~ + --, . :2 l b2 _ a~_0'~_d2 ± i ( (/.~ + i )i~('l -1. a~ vl~ -b~) . '1 = j,.z_ -( w' ) vi2 347 let ll~ 111m sll(leialize our st.u<ly to the ense or s~-i11illet.rie vibrations, then the eomllonent. 1'~ or the (usplne0ment veeto]', the coillllonents tll aml t~2 or t.he ~tress t-ensor aml the eoillponent "/1113 of the eoullirl stress tensor illust. be symmeb·ic with respeet. t.o the piane xl = o. 'l'his irmds to: a=jj=p=l=(j=o and tile expressions [loj ar(l altere(l t-o: (jj* = 13 eosil (1j xl) ip* =-o o sinh (1:1 xl) + e sillh (k~ xl) rp3* = g sillh (kl:et) + p sillh (k2 xl) [~oj furthermore introdueing thcsc expl'e~sions into nj\wt.ions [12j, "-e get.: g =rl o p =·he whcre thcrcforc rp3* can be writ.t.cn ah: [21j the boundary eomlitions [11j anl express(><l :1.';: 13 [1)~ ( :! p ---ià + 11) à k~j eosh (1) h) ci k kt (~ p + 11) eosh (1.:1 h) e i k k z (2 ft + 11) l'osh (1.2 h) = o, h i k 1) (2 ft ---1 11) ~inh (1) h) + (j 0'1 sinh (kt h) + e o'! sinh (k! h) = o, c kt ti cosh(k. ii) + e k~ tt eosh(k~ h) = 0, [22] i·~. 1i0"ciii wherc li = 1~2. thc e1li1l"acteri~tie eqllation or t.he syf!t.em [~~j is: igli (l/li) t.g·h (k. h) (~/( -+% + al 1)2 !.-2 a (2ft + %}2 1/ k 2 f,-1 ('2 ,tl in the limit. oi t.he usual elastieit.~,) el]lult.ion [~:1j rednper to t.he peri oli wl'mtion ior jj:lmb "'aves (.l .h). tlic llisenrhion oi the traseendenbl eqllation [2:1j presents gl'cat iiiliieultics and) t.herefore, onlr t.he arymlltotic limith ior long and short "'aves are considerell. 1.'or waves 10llg comparpl1 whli tlw t.hiekness '211, t.he pl'olluets 1/ h, h 11, /."2 h are so small titat. tite ityperbolie rnnetiolls are rellhced hy theil' arl:,'11tlwnt.s and cqnation [23j t.akes tite iorm: ('211 + %)~ 1 2 k~ (n n) ... ~ [(2ft + % + à) 1)2 fi2 al . (0"1"t2 112n) [~../.j 1"01" vcrr shoit waves, t.he quantitie~ 1jh, !."ih, /.",h n·re brge alhi tite two ratios or hyperbolic tangent~, whieh ap]wh-j' in eqllatioll l~3j become unit.}', givillg: [~#+.+m~-··m~nh-~t'~ = ('2 ,il -+%)21 fi2 /."1 /."2 ('3 ti) ['25j fn the eln-ssieal lilllit hot.li the equations [2../.j and 1'2;"5j reduce to t.lw col"l"csi)()j\(ling rclat.iom; foull1l in t.hc frame\l'ork of t.lie mmal elasticit.y ("). tn partielùar, equation [~5j can be recogllizcll as tite cham·ctel·ist.ic equation foi' h.n.yleigh wlrves in a micl"o})olar elastie llalf hpacc. so far \l'e have discnsse([ the ease of syllllllet.ric vibl"ation~. t,et us no'\" hpecialize onr st.nll} t.o tite caf\c of allt.isynunetri(~ vihrat.ionf\. the colllponent. u! or the displacement veetor, tite components tn all1i122 of tlm strehf\ tensor anll tlw eolll])(ment 1n13 of the eouple st.ress 'j.i f) temor are no\\' antis,ylilllwtrie ,\"it.h rcsl)cet 10 the ]llnne xl -= (l. tlle8e eon(litioll8 l('aù to: b=c=e_g=]>=o am1 the exih'essions [19] he('ome: w* = a sinh (l/ '1'1) ij'* = j) c'orh (li. xl) +}i' eosh u'2,'l'1) 1f/'" = l e08h (kj .'v i ) + () c'orli (h ;t',) [2lij ~\s in n, pnwiously l'onsi(lere(l l'ase, b~' intro(lul'ing uwsc cx]lmssions into equnt.ions [12], 'hl get.: l =t,d q=n}i' ,dwl'e t i a1\(l n ha"c heen ahea<l,y <lcfmc(l tlim'dol'b (f!~ * can no1\' lw \ltitten as: [27] the houlhlary l'01h1itions [lj are ]un\" ex]lresse(l as: a ['ii" (2 il, + % + l.) -i. l.:~] sin li (l] h) f) i k kt (2 tt + %) sinh(k, il)-ili lo h (2 f.l + %) sinh (lo~ h) ~ 0, .... l i k 1] (2 ft --;%) eosh ('q ii) -+dg, l'orli (l" h) -! p (52 e08h (1.'2 h) = 0, f) ti a'! sin h (a'! ii) + p t2 h f!inll (h h) = 0, [28j l'he ellaraeteristie equntion oi the s'ystem [28j is: (_"':;"c' -;tc'"'''·';:(52 ti kr \ , i ( ii) lgl 11 ' tgh (h h) tgh (k~ ii) l ' (2 tt + %)~ ;" 2 lj~ k l h {tz tj] (2 tt {% + l,) 'i( ". ~ l. [2!jj fn j.jh) dasrieal limit equat.ion [29] h'{luees j.o thc eorrcsl)olhling relation foun(l in the immcwork oi t.hc llsnal elartieity (1), f.. no,h'1i1 for wavcs long comparcll with the thickne108 '2lj, retaining uv to the third tcrnl~ o[ the power expansion of the h.ylwrbolie fullctions, equatioll [29j become8: i alt . u2ti i (3 ' i ll ") !.-i" (3 h" h2) h" (3 _.7.; 12 h2) j = _ (~ f.l + r.)2 (t 2 ti ) ('2 l' + ~ + l,) 1)~ /,;2 j. [30j this is thc pcrioll eqw1,tion [or long llexllml waves. disl)ersion occlll"s for these waves, with l)hase yelocity decreasing t.o .zcro with increasing wayc length. in the classical limit, cqllation [30j rcdllces to the corrcsl)onding relation found in the framcwork of the usual elasticity (8). ]<'or wayc lcngth8 8iwlll comparcd to the thickm1810 'jlj, equation [30] retiuces to equation ['25j. love 'waves 'l'he compollents of t.he 8tre108 ami coup1e stress tensol'8 !letcrmined by the dispbcement lieltl (o, 0, 1ta) alhi b.y the mierorntation fichi (!pl, !p2, o) are: tl3 = (11 + ~) 1t3, 1 + ~ !p2 t:n = i-l u~.l ~ !p2 t' j = (i-l -t %) 1t3.: % ipi tn = f11~3,2 + % !pl mll = (f3 + y) f('1,1 + a e mn = (f3 + y) !p2,2 + a e tn33=ae ma = y f('2.1 + fi f(' 1 .2 'j1h, = ì' 9)1.2 + f3!pu [.31j let us now assmue that a monochromatic wave propagates, alnng thc x~ -ilircct.ioll, in all infinite micropolar clastie plate, whosc thiekness is '2h. liinrcovcr we a810ume that thc sut"faces of theplate lum a:'d love wave 1'1lol'agatio"'. };tc. are free of st.l"esses allll eouple ~tl'ehs(,s. 1'his leadf\ to the followillg boundary eoil(!iti()il~: t.~ = 11111 = "iih~ = 0, at xl ± ll equations [9.b] are sa.tisfied hy: lh'ovided: gh =a,1-f,2 rp~ = a,: + f,l [(a + fi + y) [7~-2% -iò2 j,w]a = o [y [72 -:3 % -i~~j~i~j f-% tt3 = ° [(,u -1%) [7~ f! ò2jòt:j 1t3 -1% (72 r = o [33] the last two equations l33j, aft.er simple manil)ulations, ean be redueed to the following form: {[y [7~ 2 % i i'l~!i'll"] [(/t + %) [7~ -i] i'l~/i'lt2j + % (72} r = o {[y (72 -:3 % -iò2jwj [(/t + %) (72 -,!ò~jw] + % [7:} lt~ = o t>et us a,ssume solutions oi the iorm: a (x" x2, t) = ..1* i ~ ,\ ~'i;' ",,-fi)l) ,_o, y r (xl, x2, t) =r* (xl) ei ( i,; x2 wt) '!i~ (.1.'1, x2, t) = 1~3* (:lh) ei (f~ x, wf) 'l'hen the first. equatioll [33] and equations [34j heeome: , .. 2 , + ":'a l ..1* = o ti!i [3,laj [3u] [35] [36] _ko)!r. ~ o [37.a] 1( "0 ·(do -k2 + a~ -d.l.'l ~ ) d.1.'1 ~ -70"11 , , ti~* = o [37.hj ~:, i\n~('lll where w' \\'e f;eal'eh, as iu the {jn'vious pase, wllll.ious of equalions 13li] a1l(1 [37] or lhe ffjl'lll: whel'e l1'~' =.ii sillh (~,r l ) + n eoslt (~,rl) f* = c sinh (1.-1 xl) + /) eosh (10 1 ;rj) + + p; siuh (b xd + p cosh (h ;:t'l) 1t3 * = u sinh (l'1 xd + l eosh ('" ,td -i· + r f;illh (1.2 ;1'd + q ('osh (h :l',) l~et il,~ uow speeialize om' study to tlw ('.hse or synullet.l'ie vihm·· t.ions. 'j'hen lite pollllloupnt (f2 of t)l(' miel'ol'olalion yw'j:oi', t.he c'om· ponents mll a.jl(1 m~~ or the eollple stl'e~s teusoi' an!! lhe ('omllolll'nl tl~ of lite f;h'es8 tellsor must he symll1etl'ic' with l'espert to the piane xl ~ (i. 'j'hir leadr lo: whel'e: (" ..-1=d=p=l g = el () 'ii=],~. from the a~sllmplion thllt t.he delel'lniuant of lite system or the three homogeneous ewwtions, obtl1.ine!! giying tu! exlllieh expression to the ])ollllllllry tonditiolls f~:.!j, nllv;t. be zero, wc get tlw followiug pel'ifhl equation: l,nlll a:s"d love wave propaga'no:-.-, ~:tc. 3;)3 t~h (l; 11 ) i . ( t ~h (/;~ 11 ) \ t.gh (1.:, h) = (l' + (3) y. kyz kt tg h (/;1 11 ) _ . y l h) + + i la + ~ + y) ,e -" l ' i ( !l,i, -!le i, t":, i;'e 11 1 1) )' i. tg l (" ) , l-l . ; iy + ~i' k' il, i, le id [3dj where: l'n = " j.. r , , i 1 ., ~' 1/ = ,_o for wave iengths hlllau njlnparell to the thieknehh '2il, equatioll [391 rmillceh to: '~+~)~ c lhh-hh)-~h-~n . [(l' -+fi) y. 1.: 2 ((1 -ifl + l') c ~ (12 ""j it ih olrvioilh that we have ilo collntel'llart of eqllationfl l39j ajl(1 [.w] ili eia.~f1ieal rla~ti('ity. iio\l'cver \l'e may eonrlude that ill a micropolar rla,<;tie mediulll wc imve waves of jjove type, while, in uw elahsieal framework, this is possible onlr in a layel'ell mellillin by illlposing some eolhlit.ions on thc materiai eonsta.llts. let us 1i0w sljedalize olll' stluiy to the case of hntisymmet.ric "ibl'ations. 'l'he pollllloncnt rp~ of the mic1'orotatioll veetor, the components 11/.11 and 11122 of the {'oujlle stl'cfl~ tewwr and the componellt 113 of the stress tellsor are alltisymlllctl'ie with respl'eito thc piane;1', = o. these eoihiitionfl icall t o: b=()=}ij=(}=p=o l = ~id ci ,e p all(l by the p1'oeedlll'e, aiready employed in the pretiolls caheh, \l'e get the following ljeriod eqlla.t.ion: l:. boschi tgh (: h) tgll (l'iii) i (y + fil (lo. i, lo, id lo' ( il i (a ~ fi ~i y) c' a lo ' i· i t.gh (lo. il) . fil h y~ ti 19h (k~li) + l i tgh ("'l ii) + . (y ...!.. fi);,:~ h~ f;" , [j" . l (l' li) :: t.g l " 1 )-1 -. lo, y' j ['l] for wave lellgths small ('omp~u·e(l with t.he t.hickness ':!.1i, cqu a hon [h] reaupeh lo equaj.jon [-l-oj. co-'"'cl1.:f-no)oo{ \ye have stwli('(l t.lle i~amb ami i~ove \yavc lll·ollllgation in an infinite homogl'llcolls miel"opolar ela.st.ip plate wit.h hee houlhlary sm·fa.c·es. "'e havc foulla that a dihplacement liel(1 (1/'1, u~, o) alld a miel"ol"otation tiel,1 (0, 0, op,,) leads lo waves which can be coluù,lcrcd of lamb's kin(l, \\"hile a, (lisplaccmcnt. fida (o, o,n3) an(l a mip.j'ol'ot.at.ion fida ((pl, v'e, o) leads to ,,·aves ,,·hich can bc ('(lllsidel'ed of love's kina. 'rhcrcforc ilis possihle to have, in mipropohn mc,lia, i~ovc w,òve;; wit.hout. thc comlit.iolls rcquil·ed in t.he framc'\"ork of elahf<ieal clastieily. t wish to tlmnk l'rof. 1\1. ('aputo for valuable aiscursions, cneollnl.gcmellt;; an(l support in thih wol-k. rel<'erexces (') bù5cili e., 1972. body force /uui body c'o!!plf 1~,/liil'nlfllt~ for sfi,~mie lji.~locnlioll,~ in jlic!"opoln!" jlfdin, to be published. (2) boscili e., h172. body force, body collplr nl/l! 11l'al soil.tec 7~ljlli!.'nlfnl~ for 81'iinnir ])isl()()(1lio! .~ in jjie!"opolar jjcdia, tu he publishcll. (l ) ehi~(;e~ a. c., lfhili. limar 'j ' ltfo!"y 01 jjic!"opoln!" eln~lieily. ",). .\lat.h. aut! .\lej~h .. ', 15, (w9. la~l]l axi' lo\'~: 1\'avj-: 1'110\',\(:,1'1'10>;, etc. (~) ei\"ixg \y. :\1., .l\lwj-:tzky \\'. s., l'rn;~s f., infi,. '''lo",li(~ jran'r in layern! jledill. x('w yorl,. {' i {'i {'i {'i :-:iep, :-:ice, sce, ree, e'. g-. , ('1]. e.g., p(]. e.g.• ci]. p.p:., el]. [(i.121, pa.!!;p 283, [g.151 an,] [(i-17j, [fi.ll], pag-i' 283, [u.211, pag-(' 28fi, or rd. ,. png-c 283, or hd. ,. 01 rel. ,. 01 hd. ,. (') g-ooolaj)ze \'. (;-., in-1-7. j)i~jjcr8ìon 01 nn.yleiqh inn'f~ in a i,ayef. l'ul,1. tnst. :okisill. aead. sci. [t.r.:'..:"., 119, 27, (lo) jll~ax n" i!hhj. ,<,'a/' la 'l'héorif df lo t"'el'll/oaa.~lirill j/iltil/wloiu ('1!!ljilée. "c. bt'ili1. aca,!. :)t:. 1'1iris", 268, :;8. (") la~11\ ii., uliu. on jrlll'es i!l o!l l~iiinlic l'lale. ''l'rot". bo~·. ~()('. (loll11(11)", a, 93, 11-1-. (12) l'rllscol"l" .1., ifl-1-2. t~loslie jral'e,· am! ri/!!"lilif)!ln ili 'j"hin .7:od~. "l'h il. :\iog-."', 33. 703. (1:1) ll\1:ll':wh, 188!). olio iii" f/'e1' j"i/i)"{/iioli~ ili ali lnlinilr' l'lale oi holiioyen"ol!s isolro})il' eh/nlie j[iii/i'i'. ·']'l'o('. 1,(i[1(lon \iatl!. :-<01'.",20, 22". (") ~a't[) l'., wfi1. !slw/y ili !si(./,ia('() jrlln'~ li: j"e/odl!! 01 sii/'focf ]l"lil"fn /')"f)jiaqllll'f/ "!/)fjn hla~lh' l'lale,·. '·bull. earlh. hcs. tnst. (']'ol,:\'())"', 29, 223. layout 6 ionospheric turbulence from ground-based and satellite vlf/lf transmitter signal observations for the simushir earthquake (november 15, 2006) alexander rozhnoi1,*, maria solovieva1, michel parrot2, masashi hayakawa3, pier francesco biagi4, konrad schwingenschuh5 1 institute of the earth physics, russian academy of sciences, moscow, russia 2 laboratoire de physique et chimie de l’environnement et de l’espace, université d’orléans, cnrs, orléans, france 3 university of electro-communications, chofu, tokyo, japan 4 università di bari, dipartimento di fisica, bari, italy 5 space research institute, austrian academy of sciences, graz, austria annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5190 abstract signals from very low frequency (vlf)/ low frequency (lf) transmitters recorded on the ground station at petropavlovsk-kamchatsky and on board the french demeter satellite were analyzed for the simushir earthquake (m 8.3; november 15, 2006). the period of analysis was from october 1, 2006, to january 31, 2007. the ground and satellite data were processed by a method based on the difference between the real signal at night-time and the model signal. the model for the ground observations was the monthly averaged signal amplitudes and phases, as calculated for the quiet days of every month. for the satellite data, a two-dimensional model of the signal distribution over the selected area was constructed. preseismic effects were found several days before the earthquake, in both the ground and satellite observations. 1. introduction seismo-induced electromagnetic effects in the ionosphere were first reported in russia [migulin et al. 1982] from observations by satellite intercosmos-19. a lot of studies were then carried out on satellite recordings of waveplasma disturbances that were possibly associated with individual earthquakes or with several strong earthquakes [for reviews, see parrot et al. 1993, hayakawa 1997, molchanov et al. 2002]. a new opportunity to investigate ionospheric perturbations that might be associated with seismic activity arose with the demeter satellite mission. the first study that showed examples of unusual ionospheric observations made by the demeter satellite over seismic regions at the beginning of the mission was published in 2006 [parrot et al. 2006]. later on, statistical investigations using data from more than 3.5 years confirmed the existence of very small, but statistically significant, decreases in wave intensity a few hours before earthquakes, at a frequency around 1.7 khz [nemec et al. 2009]. these demeter observations also provided the very interesting possibility to analyze ground transmitter signals detected by the satellite above seismic regions. the first results from these analyses were reported by molchanov et al. [2006]. clear drops in the subionospheric transmitter signals around the time of several strong earthquakes were recorded in 2004, including for the catastrophic sumatra earthquake. the method applied estimated the changes in the reception zone of the transmitter signals using the signalto-noise ratio (snr). further analyses of the sumatra earthquakes were made on the data collected over one and half years of observations [solovieva et al. 2009]. evident effects before and during the great sumatra earthquakes were confirmed, with long-term durations of about one month. these results led to the conclusion that the size of the perturbation area in the ionosphere was of the order of several thousand kilometers. after these first publications, the effects of this influence of seismic activity on very low frequency (vlf)/low frequency (lf) signal propagation were further reported from observations of the demeter satellite [boudjada et al. 2008, muto et al. 2008, slominska et al. 2009]. in our earlier studies [rozhnoi et al. 2007, 2010], we presented a correlated analysis of vlf/lf signals radiated by ground transmitters and collected both at ground receivers special issue: earthquake precursors 187 article history received june 19, 2011; accepted october 17, 2011. subject classification: wave propagation, seismic risk, vlf/lf signal propagation, eartquake precursors. and by the demeter satellite. in these analyses, we used the night-time data from the alternative electric field receiver of instrument champ electrique (ice) [berthelier et al. 2006] in lf (20 hz to 20 khz) and high frequency (3 khz to 3.3 mhz) ranges. the discretization of the power spectrum density in the lf range was 19.53 hz, and in the high frequency range it was 3.255 khz. effects in the vlf/lf signal were found in both the ground and satellite observations. in the present study that analyzes the simushir earthquake (m 8.3; november 15, 2006), we used the signal from the australian nwc transmitter (19.8 khz) for the satellite observations, and in addition, for the ground analysis, we used signals from two japanese transmitters: jji (22.2 khz) and jjy (40 khz). the nwc transmitter signal is the most powerful in the vlf range, so that we can analyze the signal in a large area for satellite observations. signals from the japanese transmitters are local and can be used for satellite analysis only if the epicenters of the earthquakes are located in the maximum signal zone. 2. data processing data from the vlf/lf station in petropavlovskkamchatsky and data from the ice receiver collected by the demeter satellite were used for the analysis. the monitoring of the vlf/lf signals was carried out from october 1, 2006, to the end of january, 2007. a very strong earthquake with m 8.3 occurred near simushir island of the central kuril region (russia) on november 15, 2006. following this, a series of strong aftershocks (m 5.0-6.5) was observed over several months. the position of the station at petropavlovsk-kamchatsky and the vlf/lf transmitters, together with the epicenter of this earthquake and its aftershocks, are shown in figure 1. the earthquake epicenter was in the sensitivity zone of wave paths jjy–petropavlovsk-kamchatsky, jji-petropavlovsk-kamchatsky and nwc-petropavlovsk-kamchatsky. for the demeter data, we analyzed the signals in the part of the night-time orbits when the satellite passed above the earthquake area. the zone of analysis had a width of 25˚, which provided one orbit every day. the time averaging of the dynamic spectrum was about 2 s, and the space resolution along the orbit was about 10 km to 15 km. the ground and satellite data were processed by a method based on the difference between the real signal in the night-time and the model signal [rozhnoi et al. 2004, 2007]. the model for the ground observations was the monthly averaged signals of amplitudes and phases calculated for the quiet days of every month. for our analysis we use the residual signals of phase dp or amplitude da: (1) where a and p are the amplitude and phase for the current day, and <a> and <p> are the corresponding averages. for the satellite observations, we calculated the model of the signals for every real orbit based on a two-dimensional model with regular signal distributions over the selected area. the modeling consists of the following procedure: a) computation of a polynomial expression for the surface as a function of the longitude and latitude. we applied the method of local polynomial interpolation, which uses multiple regression methods on data within a localized window to fit a set of trends. the window can be moved around and the surface value at the center of the window, n0(x, y), is estimated at each point, based on a weighted least squares fit to the data points z(xi,yi) as follows: , (2) where n is the number of points within the window, and wi is a weight defined as: wi = exp(–3di0/a), (3) where di0 is the distance of the point from the center of the window, and a is a parameter that controls how fast the weights decay as a function of distance. finally, �n0(xi,yi) is the value of the polynomial. for the first-order polynomial: � (4) d ; da a a p p p1 2 1 2= = ( ( , ) ( , ))w z x y x yi i i i i i n 0 2 1 n= / ( , )x y x yi i i i0 0 1 2n b b b= + + 188 rozhnoi et al. figure 1. map showing the positions of the petropavlovsk-kamchatsky (ptk) station and the jji and jjy vlf/lf transmitters, together with the epicenters of the earthquake on november 15, 2006, and its aftershocks (after the us geological survey catalog). the ellipses show the sensitivity zones for the jji, jjy and nwc transmitters. part of a night-time orbit of the demeter satellite that passes above the earthquake area is indicated. pink rectangle, section where effects are observed. 189 for the second-order polynomial: �, (5) and so on. the minimization occurs for the parameters {bi}. the parameters are re-estimated whenever the center point, and consecutively, the window move [gandian 1963]. b) construction of a regular latitude and longitude grid of 0.32˚. c) computing of a net point model. the models were calculated for the snr defined as the ratio of the signal spectrum density near the transmitter frequency f0 with the minimum value in the frequency band (snr = af0/amin). using this model, we can define the instantaneous variations in the vlf/lf signal intensity in the active region at any time and for any longitude and latitude as the difference between the measured snr of the signal (longitude, latitude, t) and the reference signal (longitude, latitude). 3. results results of ground observations for the wavepaths jjy-petropavlovsk-kamchatsky and nwc-petropavlovskkamchatsky are shown in figure 2. for both wave paths, there were disturbances in the signals (figure 2, yellow and brown colors). these started about two weeks before the earthquake and continued during the aftershock activity, to the middle of december, 2006. the effects were more evident in the 40 khz signal. figure 3a shows the spectra of the vlf signals recorded by the demeter satellite along part of the night-time orbits that passed above the earthquake area on the disturbed day of november 2, 2006. for comparison, we give here the quiet day of october 29, 2006 (figure 3b). the signals from several powerful vlf transmitters (the russian alfa [11.8, 12.6 and 14.88 khz], and the australian nts [18.6 khz] and nwc [19.8 khz] transmiters) are clearly observed in figure 3. a signal at almost 18 khz can be seen in figure 3b, although this was out-of-operation in figure 3a. changes in the nts and nwc signals (decreases in the amplitudes, and spectral broadening of the signals) occurred in the region of 149-152˚e, 45-50˚n, just above the area of the seismic activity (figure 1, pink rectangle along the demeter pass). the same effects were observed in the dynamic spectra of the vlf signals (figure 4). the signals of the same transmitters are easily noted here as the horizontal lines. spectral ( , )x y x y x y x y3i i i i i i i i0 0 1 2 2 4 2 5n b b b b b b= + + + + + vlf/lf disturbances for the simushir eq figure 2. contour map of the diurnal variations of the vlf/lf signal amplitudes at petropavlovsk-kamchatsky from october 1, 2006, to january 31, 2007. left panel: wave path jjy (40 khz)-petropavlovsk-kamchatsky. right panel: wave path nwc (19.8 khz)-petropavlovsk-kamchatsky. broadening of the nwc and nts signals can be seen above the earthquake region. finally, a comparison of the results of the satellite and ground observations is shown in figure 5. here, we used the differences averaged over the night-time for the ground reception, and the differences averaged along the part orbit crossing the seismic area for the satellite data. there is an evident decrease in the amplitude of the vlf/lf signals in both the ground and satellite data that is associated with the seismicity. the amplitude anomalies are always negative for both magnetic storms and seismic activity, because of the loss of the signal in the ionosphere irregularity during the propagation. phase anomalies can be both positive and negative; these depended on the length of the path. in the present case, the anomalies in the phase of the jjy signal were positive. rozhnoi et al. 190 figure 3. vlf signal variations recorded by the demeter satellite along parts of the orbits for the frequency range 11 khz to 20 khz. upper panel: disturbed day of november 2, 2006. bottom panel: quiet day of october 29, 2006. left y axis, longitude; right y axis, latitude, for points of the orbits. figure 4. dynamic spectra of the vlf signal variations for parts of the orbits passing above the earthquake area, for the frequency band 10 khz to 20 khz. the disturbed day of november 2, 2006, is shown. 191 4. discussion and conclusions we have presented here a further comparison of ground and satellite data during periods of seismic activity. these simultaneous analyses provide cross-validation of the results, and they are more reliable for earthquake precursor studies. as a mechanism of the observed effects, we suggest the following: such long-term and large-scale perturbations in the ionosphere cannot be produced by the seismic shock itself (durations of minutes), and so we need to assume a longlasting agent that influences the ionosphere around the date of an earthquake. we believe that this initial agent is an upward energy flux of atmospheric gravity waves that are induced by gas– water release in the earthquake preparatory zone. penetration of atmospheric gravity waves into the ionosphere leads to modifications in the natural (background) ionospheric turbulence, especially over space scales of ~1 km to 3 km and wave numbers kt of ~10–4-10–3 m–1. these weak, but reliable, effects can be revealed by direct satellite observations [e.g. molchanov and hayakawa 2007]. resonant scattering of the vlf signals is possible under the following conditions of frequency–wave number synchronism: ~0 = ~s + ~t, k0 = ks + kt, where ~0 and k0 are for the incident vlf waves, ~t and kt are for the turbulence, and ~s and ks are for the scattered waves. here, the amplitude of the incident wave a0 decreases exponentially during the course of propagation through the perturbed medium: a0 ~exp(–anath), where an is the coefficient of nonlinear interaction, which depends on as and at, and h is the length of the interaction region. in our case with vlf signals: ~t <<~0 ~ ~s, and the interaction is especially efficient because k0 ~ ks ~ kt [molchanov 1985]. therefore, even though the amplitude of the turbulence at is small, the scattering can be significant if the length h is large. acknowledgements. this study was supported by the seventh framework programme fp7, under grant agreement n° 262005 semep. data and sharing resources the earthquake catalog used in this study can be found at the site: http://neic.usgs.gov/neis/epic/epic_global.html the kp data are from the site: http://wdc.kugi.kyotou.ac.jp/dstdir/index.html the demeter data are from the site: http://demeter.cnrsorleans.fr/ references berthelier, j.j., m. godefroy, f. leblanc, m. malingre, m. menvielle, d. lagoutte, j.y. brochot, f. colin, f. elie, c. legendre, p. zamora, d. benoist, y. chapuis and j. artru (2006). ice, the electric field experiment on demeter, planet. space sci. 54, 456-471. boudjada, m.y., k. schwingenschuh, h.k. biernat, j.j. berthelier, j. blecki, m. parrot, m. stachel, o. aydogar, g. stangl and e. weingrill (2008). similar behaviors of navlf/lf disturbances for the simushir eq figure 5. comparison of the ground and satellite observations during october 2006 to january 2007. for the satellite observations averaged along parts of the orbits, the vlf signal differences from the reception of the nwc transmitter signal are shown. for the ground observations averaged through the night-time, the vlf/lf signal differences are shown for the wave paths: jjy-petropavlovsk-kamchatsky (amplitude and phase), jji-petropavlovsk-kamchatsky and nwc-petropavlovsk-kamchatsky. the x axis shows the days beginning from october 1, 2006. two upper panels, earthquake magnitudes, and kp indices of the magnetic activity. tural elf/vlf ionospheric emissions and transmitter signals over seismic adriatic regions, nat. hazard earth sys., 8, 1229-1236. gandian, l.s. (1963). objective analysis of meteorological fields. gidrometizdat. leningrad (translated by the israel programme for scientific translations), jerusalem, 1965. hayakawa, m. (1997). electromagnetic precursors of earthquakes. review of recent activities, rev. radio sci., 1993-1995, oxford univ. press, 807-818. migulin, v.v., v.i. larkina, o.a. molchanov, a.v. nalivaiko, m.b. gokhberg, v.a. pilipenko, v.a. liperovsky, o.a. pokhotelov and s.l. shalimov (1982). detection of earthquake influence on the elf/vlf emissions at the upper ionosphere, preprint of izmiran 25 (2390), moscow, 22 pp. (in russian). molchanov, o.a. (1985). vlf waves and induced emissions in the near-earth plasma, moscow, nauka, 223 pp. (in russian). molchanov, o., m. hayakawa, v.v. afonin, o.a. akentieva, e.a. mareev and v.yu. trakhtengerts (2002). possible influence of seismicity by gravity waves on ionospheric equatorial anomaly from data of ik-24 satellite 2. equatorial anomaly and small-scale ionospheric turbulence. in: seismo-electromagnetics (lithosphere-atmosphereionosphere coupling), edited by m. hayakawa and o. molchanov, terra pub. co., 287-296. molchanov, o., a. rozhnoi, m. solovieva, o. akentieva, j. j. berthelier, m. parrot, f. lefeuvre, p.f. biagi, l. castellana and m. hayakawa (2006). global diagnostics of the ionospheric perturbations related to the seismic activity using the vlf radio signals collected on the demeter satellite, nat. hazard earth sys., 6, 745-753. molchanov, �o. and m. hayakawa (2007). seismo-electromagnetics and related phenomena. history and latest results, terra pub. co., 432 pp. muto, f., m. yoshida, t. horie, m. hayakawa, m. parrot and o.a. molchanov (2008). detection of ionospheric perturbations associated with japanese earthquakes on the basis of reception of lf transmitter signals on the satellite demeter, nat. hazard earth sys., 8, 135-141. nemec, f., o. santolík and m. parrot (2009). decrease of intensity of elf/vlf waves observed in the upper ionosphere close to earthquakes: a statistical study, j. geophys. res., 114, a04303; doi: 10.1029/2008ja01397. parrot, m., j. achache, j.j. berthelier, e. blanc, a. deschamps, f. lefeuvre, m. menvielle, j.l. planet, p. terits and j.p. villain (1993). high-frequency seismo-electromagnetic effects, phys. earth planet. int., 77, 65-83. parrot, m., j. j. berthelier, j. p. lebreton, j. a. sauvaud, o. santolík and j. blecki (2006). examples of unusual ionospheric observations made by the demeter satellite over seismic regions, phys. chem. earth, 31, 486-495, doi:10.1016/j.pce.2006.02.011. rozhnoi, a., m. s. solovieva, o. a. molchanov and m. hayakawa (2004). middle latitude lf (40 khz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, phys. chem. earth, 29, 589-598. rozhnoi, a., o. molchanov, m. solovieva, v. gladyshev, o. akentieva, j. j. berthelier, m. parrot, f. lefeuvre, m. hayakawa, l. castellana and p.f. biagi (2007). possible seismo-ionosphere perturbations revealed by vlf signals collected on ground and on a satellite, nat. hazard earth sys., 7, 617-624. rozhnoi, a., m. solovieva, o. molchanov, p.f. biagi, m. hayakawa, k. schwingenschuh, m. boudjada and m. parrot (2010). variations of vlf/lf signals observed on the ground and satellite during a seismic activity in japan region in may-june, 2008, nat. hazard earth sys., 10, 529-534. slominska, e., j. blecki, m. parrot and j. slominski (2009). satellite study of vlf ground-based transmitter signals during seismic activity in honshu island, phys. chem. earth, 34, 464-473. solovieva, m.s., a.a. rozhnoi and o.a. molchanov (2009). variations in the parameters of vlf signals on the demeter satellite during the periods of seismic activity, geomagn. aeron., 49, 532-541. *corresponding author: alexander rozhnoi, institute of physics of the earth, russian academy of sciences, moscow, russia; email: rozhnoi@ifz.ru. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. rozhnoi et al. 192 layout 6 annals of geophsics, 61, 2, se219, 2018; doi: 10.4401/ag-7704 1 “two-dimensional site seismic response analysis for astrategic building in catania„ piera paola capilleri1,*, maria rossella massimino1, ernesto motta1, maria todaro1 1 department of civil engineering and architecture (dicar), university of catania, catania, italy 1. introduction the town of catania is located in the eastern coast of sicily (italy) at the south of mt. etna. the high level of seismicity that affects the city, and the considerably high density of people living in its urban area, contributes to classify it as one of the town having the highest seismic risk in italy [e.g. biondi and maugeri, 2005]. at the same time it is also high the potential damage to which its historic-architectural patrimony could undergo even in the case of small magnitude events [imposa et al., 2016]. in order to analyse the hazard level of some historical building, a seismic response study of the site in which the “istituto nazionale di geofisica e vulcanologia” (ingv) building is locatedwas carried outby means of 1-d and 2-d numerical analyses, also to clarify the role played by both stratigraphic and topographic effects.the ingv building in catania is located on a slightly sloping area at the south/south-east of the town (figure 1). the area lies in the middle of a terraced alluvial deposit (figure 2), wide around 300 m and develops in the direction nnw-sse for about 700 m. this formation consists of silty-clayey sands or sandy silts and gravels with sandy-loam and clay. the thickness of the most superficial layer has been deduced from results of various geotechnical surveys [capilleri and maugeri, 2008; capilleri et al., 2014]. figure 2 shows the geological map of the catania urban area. this is the result of data and surveys performed by several authors [monaco and tortorici, 1999]. in this study, the seismic hazard of the site where the ingv strategic building is located, has been evaluated and the main results are presented in terms of amplification factors. 2. seismic and geological settings considering recent and past seismic history, the eastern coast of sicily is one of the high seismic risk areas in italy. the seismic history of the sicily is eventful and the earthquakes that involved the area are registered in many seismic catalogues. most earthquakes that damaged or dearticle history receveid march 13, 2017; accepted february 2, 2018. subject classification: fem analysis; seismic response; earthquake; amplification factors; hazard. abstract this study is part of a more extensive research aimed to the seismic risk mitigation in eastern sicily. the earthquakes that occurred in sicily in 1169, 1542, 1693, 1818, 1908 and more recently in 1990, testify the high level of seismic hazard in this region. it is well recognized that local seismic effects can exert a significant influence on the distribution of damage during earthquake. traditionally, these effects are studied by means of simple one–dimensional (1-d) models of seismic wave propagation, which take only the influence of the stratigraphic profile and soil proprieties into to account for the site seismic response. it is known that the seismic response is strongly influenced by stratigraphic and topographic features that can reduce or amplify the earthquake induced ground motion depending on the soil stiffness and on the ground topography. this paper concerns the results of a two-dimensional (2-d) finite element analysis carried out to evaluate the response of the site where the national institute of geophysics and volcanology (ingv) building is located in the town of catania. the analysis, performed using as seismic input the accelerogram recorded in 1990 during the santa lucia earthquake, allowed to make some considerations about the expected accelerations at that the site and some comparison with the peak accelerations prescribed by italian seismic code. capilleri et al. 2 figure 1. view of the istituto nazionale di geofisica e vulcanologia (ingv) building in catania. earthquake date years month day epicentre lat lon magnitude 1125 6 7 siracusa 37,070 15,300 5.8 1169 2 4 eastern sicily 37,333 15,200 7.3 1542 12 10 sortino 37,250 15,067 6.4 1693 1 9 val of noto 37,170 15,070 5.9 1693 1 11 eastern sicily 37,443 15,192 7.0 1727 1 7 noto 36,913 15,045 5.1 1818 2 20 catanese 37,616 15,099 6.2 1846 4 22 catania 37,500 15,083 4.2 1848 1 11 augusta 37,217 15,233 5.5 1903 2 10 noto 36,903 15,014 4.3 1908 12 28 calabro messinese 38,133 15,667 7.3 1990 12 13 southern eastern sicily 37,270 15,070 5.3 table 1. historical earthquakes in the eastern sicily. stroyed the eastern sicily are reported in table 1 [barbano et al., 2000]. the “ingv” site is located in the historical center of catania. the tectonics of eastern sicily is quite complex. available seismic information for south-eastern sicily suggest the existence of two groups of possible sources for the seismicity that affected the town of catania in different times. the sources are located either close to the ionian coast (messina straits and malta-hyblean escarpment), or inland, both in the hyblean foreland and etna areas. the malta-hyblean escarpment, a normal fault system trending nnw-sse, is considered as the possible source of the destructive earthquakes with estimated magnitude m≈7.0 that struck in past centuries the catania area [azzaro and barbano, 2000]. the soil outcrop of the town comes from the combination of three processes linked to volcanic, tectonic and human activities. consequently, the main feature of the area is represented by a complex sedimentary sequence interbedded between a clay basement and an upper volcanic layer made of lava flows and pyroclastic products that sometimes are covered by detritus and ruins due to past earthquakes. the bedrock of the area is composed of a lower-middle pleistocene sequence of marly clays, having thickness up to about 600 m. in the upper part of this succession, sand and sandy clays levels are found frequently. these layers are followed upwards by some tens meters of fluvial-deltaic sandy clay or sand and coarse gravel. geological, geomorphological and topographical conditions are usually very important sources of information for the assessment of the seismic potential hazard. these factors play a fundamental role on earthquake ground motions and distribution of damage. this aspect becomes even more critical in areas with sharp transitions between stiff surface formations and softer soil materials. this condition is typical in a volcanic zone, like catania, which lies at the base of the mt. etna and was affected by many eruptions in historical times. 3. site geotechnical characterization to define the seismic response of “invg” site, two geotechnical investigations were carried out. the first survey in 2010 consisted in four boreholesand laboratory and in situ tests. laboratory tests, included soil classification, direct shear tests and oedometer tests. two multichannel analysis of surface waves tests (masw) were also carried out in that site. additional investigations, conducted in 2014, consisted in three boreholes with standard penetration tests (spt), down-hole tests (dh), cross-hole tests (ch) and seismic dilatometer tests (sdmt). also laboratory tests for soil description and classification, direct shear tests, oedometer tests, resonant column tests and torsional shear tests were carried out on undisturbed samples [castelli et al., this issue]. the main results of the geotechnical properties deduced 3 2d seismic response in catania figure 2. geological map of ingv area (after monaco and tortorici, 1999, modified) and detail with the location of the boreholes, realised in the 2010 and 2014 survey. by laboratory tests in 2014 are shown in table 2. figure 3 shows location of boreholes and of the in situ tests carried out during the geotechnical investigations. two cross sections, named section 1 and section 2, were drawn up from site investigations. section 1 was traced using s1 and s3 soil profiles deducedfrom site investigation in 2010, while section 2 was deduced utilizing s1 and s3 profiles from the site investigation in 2014. results of the shear wave velocities from dh, ch, sdmt tests are shown in figure 4. from the comparison between in situ measurements of shear wave velocities it can be observed that the velocities of s3dh test are significantly greater than the other measurements. for this reasons3dh test was excluded in the determination of shear wave profile adopted in the numerical analysis. to confirm the insitu results, the shear wave profile deduced from spt data and utilizing the otha and goto [1978] expressions are also reported in figura 4. except for s3-dh, both in situ measurements and otha and goto results are in a good agreement. the adopted profile is also shown in figure 5 and it is drawn in red. the relationship between the depth and the shear wave velocities of the adopted profile is given by the following equations: z = 0,1823 · vs 24,537 per 0,0 m < z < 40,0 m (1) z = 0,0006 · v2s 0,1316 · vs + 17,829 per z < 40,0 m (2) capilleri et al. 4 figure 3. plan view of geotechnical cross sections developed from site investigation results [after capilleri et al., 2016, modified]. figure 4. shear wave velocities profile vs depth for different tests. 4 two-dimensional dynamic finite element analysis a 2-d fem analysis was carried out to evaluate the seismic response. to this aim a scenario seismic input was selected. guidelines on procedures for the selection of appropriate acceleration time-series are given by bommer and acevedo [2004]. the accelerogram utilised for the seismic response refers to the main shock of 13 december 1990 [boschi et al., 1997] that was considered significant, having caused severe damage and recently.both the n-s and e-w horizontal components of the sortino records were adopted for the analysis, however in the present paper, only the results for sortino e-w component are presented. the accelerogram is shown in figure 5. for this component of the record, the strong motion duration and the arias intensity are d5-95 = 10.98 s and ia = 5.54 cm/s, respectively; the number of equivalent loading cycles, evaluated according to the procedure proposed by biondi et al. [2012], is neq = 5.4. the analyses were carried out scaling the accelerogram at the peak values prescribed by theitalian code for different limit states that is damage, life and collapse. bommer and acevedo [2004] suggest to not exceed a scaling factor 2.5 while the input for the collapse limit state analysis required a scaling factor about 4, thus the results concerning the collapse limit state must be taken with caution. a dynamic model using the two-dimensional plaxis code [brinkgreve et al., 2002], was implemented. the finite element mesh was modelled to ensure theaccuracy of the dynamic analysis. the mesh was 4800 m in width and 200 m in depth.this depth was considered as the bedrock since, according to the velocity profile of shear waves, at that depth the velocity of shear wave was almost 800 m/s.the lateral and vertical extensions of the mesh were considered enough to minimize wave reflection. in addition, at the lateral sides of the mesh, absorbent boundary conditions were applied to avoid wave reflection. viscous adsorbent boundaries have been introduced, based on the method described by lysmer and kuhlmeyer [1969]. the mesh was modelled with horizontal layers to take the change in shear wave velocity and damping with depth into account. [rizzitano et al., 2015]. the material damping was simulated with the wellknown rayleigh formulation. the damping matrix c was assumed proportional to the mass matrix m and the stiffness matrix k by means of two coefficients, αr and βr according to: (1) the constants αr and βr are obtained by the following expression: (2) where d is the soil damping ratio, ωn and ωm are the control angular frequencies. in particular, ωn is the 5 2d seismic response in catania sample depth [m] γ [kn/m3] c' [kn/m3] φ' [°] cu [kn/m3] s1 c1 4.0-4.5 20.4 11 29 s1 c2 9.0-9.5 20.1 38 16 55 s1 c3 34.0-34.5 20 51 20 s2 c1 5.0-5.4 18.6 14 26 s2 c2 7.2-7.7 19.6 28 22 104 table 2. geotechnical parameters from laboratory tests performed in 2014. figure 5. seismic input of sortino earthquake: a) e-w component accelerogram; b) fourier spectrum. c[ ]=αr m[ ]+ r k[ ]β αr βr ⎧ ⎨ ⎪ ⎩⎪ ⎫ ⎬ ⎪ ⎭⎪ = 2d ωn +ωm n m 1 ⎧ ⎨ ⎪ ⎩⎪ ⎫ ⎬ ⎪ ⎭⎪ ω ω system fundamental frequency and ωm is the later frequency to ωn [sica et al., 2007]. in the analysis a initial damping ratio d = 5% was utilized, while the frequencies ωn and ωm were evaluated by utilizing the spectrum determined by one-dimensional eera code [bardet et al., 2000]. to ensure the numerical accuracy of wave transmission, that is affected by both the frequency content of input wave and the wave velocity characteristics of system, the spatial element size was chosen smaller than 1/10 to 1/8 of the wavelength (l) associated with the highest frequency component of input wave [kuhlemeyer and lysmer, 1973]. according to the above mentioned criteria, the element size δl was defined small enough to allow the seismic wave propagation throughout the analysis: (3) on the basis of the shear wave profile adopted (figure 4) and geological settings, the bedrock was estimated at a depth of about 200 m. however, some analysis performed with bedrock located up to 600 m did not give results significantly different in terms of amplification factors.although two-dimensional fem analysis is widespread used in geotechnical engineering, in this work the two-dimensional numerical analysis has been carried to evaluate the seismic response in terms of both the stratigraphic and the topographic effects [capilleri et al., 2005; biondi et al., 2004]. in order to detect accelerations at the ground surface, five investigation points, named a, b, c, d and e, were selected. the ingv building is located at the point c in figure 6. the mohr-coulomb elasto-perfectly plastic constitutive model with a non-associate flow rule was considered for the soil. computed amplification factors s in the five selected points and for different recapilleri et al. 6 δl = 8 = 1 8 vs fn l state tr [years] pvr [%] ag [g] ss damage 101 63% 0.102 1.50 life 949 10% 0.282 1.29 collapse 1950 5% 0.397 1.13 table 3. seismic parameters according to italian code. [capilleri et al., 2016]. figure 6. geometrical models for fem analysis. turn periods (tr) are shown in figure 8. it should be pointed out that a 2dfem analysis gives both stratigraphic and topographic effects; however on the basis of the low slope of the site it is evident that topographic effect was negligible in this case. details for amplification factors areshown in figure 7 for all the selected points. referring to point c, results give average amplification factors s=1.48 for the damage limit state, s=1,41 for the life limit state and s=1,32 for the collapse limit state. these values are in a fair agreement with numerical and experimental data available in the literature [e.g. massimino and biondi, 2015].for comparison, table 3 shows the acceleration at bedrock (ag) and the amplification factors (ss) for the different limit states prescribed by italian code (ntc08). a comparative study was also performed between 1d and 2-d analysis. the 1-d analysis (caruso et al., this issue) was carried out for the life limit state, using seven accelerograms, concerning the 1693 earthquake (three synthetic records), the 1818 earthquake (three synthetic records) andthe 1990 natural earthquake. considering all the earthquakes, the average value of 1-d amplification factor was 1.56 while for 2-d fem analysis the computed amplification factor for the life limit state was about 1.5 (figure 7). limited to results of the present analysis, the amplification factors given by the analysis and determined for life and collapse limit states, are greater than the amplification factors given by italian code. on the contrary, for the damage limit state, the italian code gives an amplification factor in good agreement with that determined by the fem analysis. 5. conclusions seismic response analysis represents a powerful tool to evaluate the seismic hazard and to reduce the vulnerability of existing and new construction buildings. in this paper a two-dimensional fem analysis has been presented for the evaluation of the seismic response of the site in the centre of catania where the ingv strategic building is located. this study is part of a more extensive activity aimed to “seismic risk reduction in eastern sicily” project, where the seismic hazard was evaluated both in 1-d and 2-d analysis. usually the two-dimensional analysis is to be preferred since it allows to detect both stratigraphic and topographic effects for the amplifications of acceleration at the ground surface. a comparison with the 1-d analysis, that gave similar amplification factors, indicated that in this case topographic effects are negligible. this however was an expected result since the slope of the analyzed site is very low. limited to results of both 1d and 2-d analysis, some difference in theamplification factors with respect to italian code requirements was found for the collapse and the life limit states while a good agreement was found for the damage limit state. references azzaro r., barbano m.s. 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(1969). “finite dynamic model for infinite media”, asce, journal of engineering and mechanical division, vol. 90 no. 4, pp. 859-877. massimino m.r., biondi g. (2015). "some experimetnal evidences on dynamic soil-structure interaction". compdyn 2015 5th eccomas thematic conference on computational methods in structural dynamics and earthquake engineering2015, pages 2761-2774. monaco c., tortorici l. (1999). carta geologica dell'area urbana di catania. s.e.l.c.a., firenze, italy. n.t.c. (2008). norme tecniche per le costruzioni (italian technical regulation for constructions). decreto del ministro delle infrastrutture 14.01.2008, published on gazzetta ufficiale 04.02.2008, roma, 2008. ohta, y. and goto, n. 1978. empirical shear wave velocity equations in terms of characteristic soil indexes. earthquake engineering and structural dynamics, vol.6. rizzitano, s.,cascone, biondi, g. (2015). coupling of topographic and stratigraphic effects on seismic response of slopes through 2d linear and equivalent linear analyses. soil dynamics and earthquake engineering . vol. 67, december 01, 2014, pages 66-84. sica s., melazzo a., santuccio de magistris f. (2007). propagazione e isolamento delle vibrazioni del terreno prodotte dai treni ad alta velocità.rivista italiana di geotecnica,no.4, 149-159. *corresponding author: piera paola capilleri, department of civil engineering and architecture (dicar), university of catania, catania, italy; email: pcapille@dica.unict.it © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. capilleri et al. 8 deidda.pdf 549 annals of geophysics, vol. 46, n. 3, june 2003 inversion of electrical conductivity data with tikhonov regularization approach: some considerations gian piero deidda (1), ernesto bonomi (2) and cristina manzi (2) (1) dipartimento di ingegneria del territorio, università di cagliari, italy (2) centro di ricerca e sviluppo di studi superiori della sardegna, cagliari, italy abstract electromagnetic induction measurements, which are generally used to determine lateral variations of apparent electrical conductivity, can provide quantitative estimates of the subsurface conductivity at different depths. quantitative inference about the earth’s interior from experimental data is, however, an ill-posed problem. using the generalised mcneill’s theory for the em38 ground conductivity meter, we generated synthetic apparent conductivity curves (input data vector) simulating measurements at different heights above the soil surface. the electrical conductivity profile (the earth model) was then estimated solving a least squares problem with tikhonov regularization optimised with a projected conjugate gradient algorithm. although the tikhonov approach improves the conditioning of the resulting linear system, profile reconstruction can be surprisingly far from the desired true one. on the contrary, the projected conjugate gradient provided the best solution without any explicit regularization (a = 0) of the objective function of the least squares problem. also, if the initial guess belongs to the image of the system matrix, im(a), we found that it provides a unique solution in the same subspace im(a). mailing address: dr. gian piero deidda, dipartimento di ingegneria del territorio, sezione di geologia applicata e geofisica applicata, università di cagliari, p.zza d’armi 16, 09123 cagliari, italy; e-mail: gpdeidda@unica.it key words inverse problems – tikhonov regularization – projected conjugate gradient – high-frequency electromagnetics 1. introduction the goal of collecting geophysical data is to gain meaningful information about a given earth property (for example, density, seismic velocity or conductivity of a geologic body). how ever, in many situations the quantities we wish to determine are different from the ones we are able to measure. if the measured data depends, in some way, on the quantities we want, then the data contains at least some information about those quantities. thus, measured data can be used to predict the quantities we really want. unfortunately, the prediction is not straightforward because an inverse problem must be solved. a typical feature of inverse problems is that they are ill-posed. a unique solution may not exist and small errors in the data may cause prohibitively large variations in the estimates of the quantity sought. to overcome these difficulties one has to regularize the original problem, that is the original problem has to be replaced by a nearby well-posed problem in order to obtain a stable solution. one of the bestknown and most used regularization methods is tikhonov regularization. the aim of this work is to illustrate how tikhonov regularization may sometimes be the cause of misleading results, far from the expected solutions. for such a purpose we considered a simple least squares problem for the estimation of the electrical conductivity profile from high-frequency electromagnetic induction 550 gian piero deidda, ernesto bonomi and cristina manzi measurements. high-frequency electromagnetics have enjoyed increased popularity in near surface geophysics and nowadays are routinely used in environmental and hydrogeological studies. shallow applications of electromagnetic methods have been carried out for salinity monitoring in agricultural land (cameron et al., 1981; rhoades et al., 1990; hendrickx et al., 1992), detection of contaminants in soils and shallow aquifers (valentine and kwader, 1985; saunders and cox, 1987; barker, 1990), soil water content measurements (kachanoski et al., 1988, 1990; sheets and hendrickx, 1995), and vadose zone characterisation (cook et al., 1989; scanlon et al., 1999). a popular method employed in these applications is the frequency-domain electromagnetic (fem) induction technique (mcneill, 1980) which uses a ground conductivity meter to measure the apparent electrical conductivity of the subsurface. although this technique is generally used for detection of lateral changes in the apparent electrical conductivity, it can also provide quantitative vertical variations: fem measurements acquired at different heights above the soil surface can be used to predict the electrical conductivity at different depths. adopting the same approach as borchers et al. (1997), we estimated the electrical conductivity profile from synthetic apparent conductivity curves solving a least squares problem with tikhonov regularization optimised with a projected conjugate gradient algorithm. although the tikhonov approach improved the conditioning of the problem, the calculated solution was surprisingly far from the desired true one. in particular, we found that the choice of the point on the l-curve corresponding to a = 0 provided the best solution, that is no smoothing penalty term has to be added to the objective function. the conjugate gradient provided itself sufficient regularization to assure stable solution. 2. em38 electrical conductivity data 2.1. em38 instrument various non-invasive devices can be used to acquire information from the subsurface through electromagnetic induction measurements. the instrument used in this study was the em38, an electromagnetic induction sensor manufactured be geonics limited, ontario, canada. it consists of two coils on a lightweight bar 1 m long, which includes calibration controls and a digital readout of apparent electrical conductivity in ms/m. the em38 instrument operates at a frequency of f = 14.6 khz which corresponds to w = 91.7 ¥ 103 rad/s. the coil spacing s is equal to 1 m. the instrument can be held so that the two coils are either oriented horizontally or vertically with respect to the soil surface, as illustrated in fig. 1. alternate current is sent through the transmitter coil; this generates a magnetic field h p that induces current to flow on the second (receiver) coil, which in turn generates a secondary magnetic field h s . defining the skin depth d as the depth at which the primary magnetic field has been attenuated to 1/e of its original strength, we can introduce the induction number n b , which is the ratio of the intercoil spacing s to the skin depth d. for a soil with uniform conductivity s, it can be shown that (2.1) where µ π0 74 10= ⋅ − henry/m is the magnetic permeability of free space. the em38 measures the quadrature component of the ratio of the two magnetic fields. in general, the secondary magnetic field is a complicated function of the intercoil spacing s, the operating frequency f and the ground conductivity s. it can be shown that, under n s sb = =δ µ ωσ0 2 fig. 1. instrument configurations: horizontal (left) and vertical dipoles (right). s s 551 inversion of electrical conductivity data with tikhonov regularization approach: some considerations the assumption of a homogeneous medium and nb <<1, the secondary magnetic field is a simple function of w, s and s (2.2) where w = 2p f (mcneill, 1980). it is worthwhile mentioning that the field ratio, in the approximation provided by eq. (2.2), is independent of the dipole orientation. since the ratio between the two magnetic fields is linearly proportional to the electrical conductivity of the soil, the conductivity is thus evaluated by measuring this ratio. whatever the structure and composition of the medium under investigation, eq. (2.2) is used to define the apparent conductivity s a , the experimental information required when solving the inverse problem of electromagnetic sounding . (2.3) under the assumption of homogeneity, and normalizing all spatial dimensions with respect to s, mcneill (1980) described the sensitivity f of the instrument to conductivity at depth z, for both vertical and horizontal modes (2.4) . (2.5) figure 2 displays the sensitivity of the em38 for both vertical and horizontal dipole configurations, versus the depth z measured in units of distance between the two coils. 2.2. the forward propagation model borchers et al. (1997) describe and discuss a more general linear model for the instrument response, which can be extrapolated from the model of mcneill (1980) under the following assumption: 1) the subsurface model represents a horizontally stratified medium in which the current flow is entirely horizontal. 2) the current flow at any point of the subsurface is independent of the current flow at any other point, since the magnetic coupling between all current loops is negligible. with the two coils in vertical mode, assuming the instrument at a given height h above the soil, σa v takes the form (2.6) where s (z) is the conductivity at depth z. the sensitivity function fv(z) is described by eq. (2.4). similarly, for the horizontal orientation, the apparent conductivity σa h is written as follows: (2.7) with fh(z) given by eq. (2.5). collecting measurements of σa v and σa h recorded at different elevation, h1, h2, …, hn, above the soil surface, the two integral eqs. (2.6) and (2.7) provide the linear forward model to invert, from which the electrical conductivity profile s (z) can be estimated. h h ss p ≅ ωµ σ0 2 4 σ ωµ a s ps h h =       4 0 2 φv z z z ( )= +( ) 4 4 12 3 2/ φ h z z z ( )= − +( ) 2 4 4 12 1 2/ σ φ σav vh z h z dz( )= +( ) ( ) ∞ ∫0 σ φ σa h hh z h z dz( )= +( ) ( ) ∞ ∫0 fig. 2. instrument sensitivity curves for the two dipole configurations. horizontal mode configuration is more sensitive to contributions from materials at the very near subsurface while the vertical mode configuration better discriminates contributions at lower depth, with a maximum value at about 0.4 times the distance between the coils. 552 gian piero deidda, ernesto bonomi and cristina manzi assuming a stratified medium model (fig. 3) the subsurface is divided into m layers with specified thickness dz j , electrical conductivity s j and magnetic permeability m j equal, in this context, to that of the free space: mj = m0 for j = 1, 2, …, m. let d t(s) = [sv a (h1), s v a (h2), ..., s v a (hn), s ha(h1), s h a(h2), ..., s h a(hn)] t denote the vector gathering data relative to apparent conductivity measurements . (2.8) using the instrument response model described by (2.6) and (2.7), the following system of linear equations establishes a correspondence between the subsurface conductivity profile and the apparent conductivity measurements: (2.9) the system matrix k is constructed as follows: (2.10) where the elements of v and h are, respectively (2.11) and (2.12) for i = 1, 2, …, n and j = 1, 2, …, m. figure 4b shows an example of input data vector d(s) obtained from the model of the electrical conductivity profile s displayed in fig. 4a, for n =11 and m = 30. each value of the apparent conductivity was simulated for different elevations of the instrument above the soil, implementing the forward propagation model (2.9). 3. data inversion 3.1. the least squares problem from the (2n ¥ m)-linear system (2.9) the electrical conductivity profile can now be estifig. 3. stratified subsurface model fig. 4a,b. a) synthetic ground conductivity profile; b) apparent conductivity data computed from the ground conductivity profile in (a). a b d d d σ( )=       v h d kσ σ( )= . k v h =       v z h dzi j v i z z j j , = +( ) − ∫ φ 1 h z h dzi j h i z z j j , = +( ) − ∫ φ 1 553 inversion of electrical conductivity data with tikhonov regularization approach: some considerations 3.2. regularization in the sense of tikhonov a way to enhance the stability of the inverse problem is tikhonov regularization, a method which allows a form of optimal tuning on the sensitivity of the solution to input data errors. this is obtained by the trade-off between the residual norm k dσ − and some desirable property resulting from the action of a discrete differential operator l n on the profile of s. here n denotes the order of the differentiation. a perturbed solution of the inverse problem is computed by solving the least squares problem associated to the new functional (3.5) the norm l sn quantifies the regularity of s = sa, the electrical conductivity profile that minimises (3.5). clearly, for a = 0, ε 0 corresponds to the function ε , eq. (3.1), and thus the optimal conductivity is the solution of the system of eq. (3.2). in the general case of a > 0, the minimum of ε a is reached for the conductivity profile s = sa , solution of the linear system (3.6) where â, a symmetric, positive definite matrix, has the form (3.7) and a is defined, as before, by eq. (3.3). since l n is always a diagonally dominant matrix, the perturbing term in (3.7) becomes absolutely crucial to improve the conditioning of â with respect to that of a (bertsekas and tsitsiklis, 1989). hence, for a > 0, we necessarily have 1≤ <κ κ( ˆ )a a( ). this behaviour is illustrated by the curves displayed in fig. 5. the simplest regularizing operator is l0 = i, where i denotes the identity matrix. another form of control may be obtained through the implementation of l2, the second derivative operator. l2 enforces the smoothness of the conductivity profile while l0 controls its fluctuations. as illustrated in fig. 5, l0 provides a better conditioning to matrix â. mated from the available experimental information d by solving the least squares problem associated with the functional (3.1) at the moment without any requirement, physical or mathematical, on the solution profile. the minimum of the function ε (s) is reached for an electrical conductivity profile s = s, solution of the following system: (3.2) where a = ktk and b = ktd. from eq. (2.10) it follows that: (3.3) where a, an (m ¥ m )-matrix, is symmetric and positive definite. the right-hand side of (3.2), see eq. (2.8), takes the form (3.4) since the instrument response s a depends on the cumulative effect of all sudden changes of the subsurface conductivity profile s, the measured data field is weakly sensitive to the perturbations of the medium conductivity. conversely, this physical property is mathematically translated into a strong sensitivity of the inverse problem solution s with respect to the perturbation of the apparent conductivity s a . the condition number, defined as κ a( )= λ λ= m 1 where lm and l1 are, respectively the largest and smallest eigenvalues of a, plays the prominent role as a measure of the difficulty of computing s = a-1b in face of data uncertainty and roundoff errors. a classical result for non-singular operators (voievodine, 1976) states that, for large values of κ a( ), the system solution s might be highly perturbed even in the case of weak perturbations of both a and b, or one of them. in such a situation, the problem is said to be ill-conditioned. matrix a, defined in (3.3), may be extremely ill-conditioned with the condition number κ a( ) of the order of hundred of thousands. obviously, from the point of view of stability, the condition number should be as close as possible to one. â = a + l lα nt n εε κκs s d( )= − 2 a bσ = a v v h h= +t t b v d h d h= +t v t . εα α αs ks d l s( )= − + ≥ 2 2 0n , . â bσα = 554 gian piero deidda, ernesto bonomi and cristina manzi as already mentioned, the larger the a the more important the diagonal dominance of â is; but increasing a causes the perturbed solution sa to deviate further from the exact solution s. it is then important to achieve an acceptable balance between stability and accuracy of the solution by tuning carefully the regularization parameter a. there are several heuristic ways to proceed in order to select a (wabha, 1990; hansen, 1992; hilgendorf, 1997), but the criterion described below, based on the l-curve construction, is certainly the most used. since the minimum of ε a is a linear combination of two terms, k dσα − and lnσα , the idea behind this criterion is to display one term as a function of the other for different values of the parameter a. the resulting plot is called the l-curve. according to the tikhonov theory, for a going to zero, sa tends to the solution s of the original least squares problem. this implies the sequence of points k d lσ σα α−( ), n moves along a trajectory, denoted as l-curve, presenting a limit point. this point is indicated with a cross in both examples of fig. 6a,b, where the regularization is imposed by l0 (fig. 6a) and l2 (fig. 6b). note that, as expected for large values of a , the norm lnσα decreases while the residual k dσα − increases. in the spirit of the l-curve criterion, the most suitable value of the regularizing parameter a is determined by selecting one intermediate point on the corner of the l-curve (hansen, 1992). such a point, indicated with a circle in fig. 6a,b, is supposed to provide, in terms of accuracy and regularity, the value of the parameter corresponding to the most balanced perturbed solution of the inverse problem. fig. 5. effect of matrix regularization on the condition number: high values of a lower the condition number. fig. 6a,b. l-curve obtained from data d in fig. 4b with regularization imposed by: a) l0, and b) l2 (16-byte arithmetic). b a fig. 7. inverse problem solutions for the input data in fig. 4b. inversions with no regularization and with l0 regularization give the best solutions while inversion with l2 regularization gives a meaningful solution. 555 inversion of electrical conductivity data with tikhonov regularization approach: some considerations figure 7 shows three solutions of the system (3.6). two of them, marked with circles in fig. 6a,b, correspond to a = 104 for l0, and a = 4 . 10-2 for l2. the solution profile obtained with no regularization (fig. 7) gives rise to the limit point framed by a square symbol on the l-curves. note that on the k d lσ σα α−( ), n plane the point corresponding to the true conductivity profile may be far away above the point selected by the l-curve criterion. although this last point represents a compromise between accuracy and regularity of the perturbed solution, the resulting a may lead to a conductivity profile sa which is physically meaningless (fig. 7). 3.3. the projected conjugate gradient algorithm when solving the least square problem (3.5), the solution representing the conductivity profile must satisfy the physical requirement sa 0. under this condition, the conjugate gradient algorithm is no longer applicable because, even if we start inside the feasible set s = œ{ }s r sm 0 , an update may take solutions outside that set (non-physical solutions). the projected conjugate gradient generalized the conjugate gradient algorithm to the case where there are constraints (bertsekas and tsitsiklis, 1989; birgin et al., 1999) assuring the attainment of the physical requirement of the non-negativity of the solution. all the computations run for the analysis illustrated in figs. 6a,b and 7 are performed in 16byte arithmetic. with this level of accuracy, the choice of the point on the l-curve corresponding to a = 0 provides the best solution, in the sense of proximity to the point representing the desired true solution. as a consequence, in this framework the regularization of system (3.2) is simply not necessary. as illustrated in fig. 5, the problem (3.2) is ill-conditioned. however, the computation of the eigenvalues of â, all strictly positive, shows an intriguing result concerning their aggregation close to zero as displayed in fig. 8a. while the eigenvalues relative to the l2 regularization are spread over a large range, in the l0 case and in the case with no regularization, namely â = a, there are only three distinct eigenvalues and the remaining group is practically coincident near the smallest one. this property is of fundamental importance to obtain a fast convergence of the conjugate gradient algorithm. in particular, it can be shown that the method converges faster if most of the eigenvalues of the system matrix â are clustered in a small interval and the remaining eigenvalues lie to the right of the interval (bertsekas and tsitsiklis, 1989). this result is eloquently illustrated by the three convergence curves presented in fig. 8b. it also shows the excellent performance of the case a = 0, in spite of the ill-conditioning of matrix a. it is worthwhile mentioning that a careful construction of the l-curve may present values of a > 0 providing a better solution of the least squares problem, in the sense of proximity to the desired true solution, as shown in the example of fig. 6a where a = 10-12. fig. 8a,b. a) eigenvalues of â. the largest eigenvalue of each matrix, placed around 50, is not displayed. b) convergence history of the conjugate gradient algorithm with different regularizations. 556 gian piero deidda, ernesto bonomi and cristina manzi although these values are the most suitable, the discrepancy between the resulting solutions, with respect to that corresponding to a = 0, is practically unnoticeable. 4. discussion we have seen that clustered eigenvalues allow faster convergence of the conjugate gradient. in fact, the conjugate gradient converges after a number of iterations equal to the number of distinct eigenvalues whose eigenvector is nonorthogonal to the error (hansen, 1998). this signifies that conjugate gradient yields results close to the truncated singular value decomposition solution in which the truncation parameter (an integer at which the singular values are deemed to be negligible) equals the number of iterations for the convergence of the conjugate gradient itself. therefore, the conjugate gradient can also be seen as a regularization tool operating on the original least-squares problem. it is important to note, however, that this is not actually the case of tikhonov regularization since it is only used as a trick to improve the conditioning of the original problem, providing a solution sa, approximating s. another aspect that needs discussion refers to the uniqueness of the solution. the synthetic example here presented belongs to the class of the underdetermined system, the number of unknowns, m, being larger than the number of equations, n. this kind of problem does not admit a unique solution (noble, 1969). however, the strategy implemented with the conjugate gradient algorithm is able to select, in the set of all possible solutions of the leastsquares problem, s n= = ∈ℜ{ }x ax b b, with , a unique solution which is in a suitable subspace of unknowns of dimension n, where n is the rank of the system matrix a. more precisely this subspace is the image of the matrix a, im a a = for all ( )= ∈ℜ{ }y v y v m, orthogonal (see appendix) to the kernel of a defined as ker a a( )= ={ }z z 0 . note that each solution can be decomposed as follows: x = x0+ l z with x0 œs a particular solution, z ∈ ( )ker a and l a real number. hence s represents a translation of the kernel of the system. the conjugate gradient algorithm, starting from an initial guess in the subspace im(a), will necessarily converge to the unique solution x a0 ∈ ( )s i im . as a matter of fact, the iterative procedure of the conjugate gradient performs only transformations by multiplying matrix a to a residual vector. as the gradient, defined by g = ax b , is necessarily in im(a), each iteration of the conjugate gradient will keep the search for the solution within the subspace im(a). obviously, b = ax is in im(a). as schematically depicted in fig. 9, the solution x of the system is reached at the intersection between the im(a) and the set of all solutions s, x = as i im( ). it is the orthogonal projection of all solutions on the image of the matrix of the system ax = b. this solution is unique and it is the best possible within the subspace im(a) being at the intersection with the subset s of all possible solutions of the least-squares problem. this analysis does not contemplate constraints on the solution. the introduction of projections in the conjugate gradient confines the search within a convex set of im(a) and results in highly non-linear iterates converging to the best possible profile x in the sense of proximity to that of the unconstrained problem x. 5. conclusions in this article the simple inversion problem based on mcneill’s linear response model of fig. 9. a schematic view of the evolution of the solution in the admissible subspace im(a). 557 inversion of electrical conductivity data with tikhonov regularization approach: some considerations the em38 ground conductivity meter has been formulated and developed. computer experiments on synthetic data provide credibility to results and conclusions presented in this work. computations have shown how tikhonov regularization and in particular the l-curve criterion can be the cause of misleading conductivity profiles far from the desired true ones. the analysis of the system matrices to invert their structure and eigenvalues shows that, although the original system problem is ill-conditioned, the conjugate gradient algorithm is a robust method for the solution of the electrical conductivity data inversion without any additional regularization of the least squares problem. the projection strategy, implemented together with the conjugate gradient, enforces the positivity of the solution and provides the best possible profile in the sense of proximity to that of the unconstrained problem. in spite of the good performance of the present methodology on synthetic examples here presented, its development certainly needs additional computational experiments adopting more adequate models of two loops em soundings over a stratified half-space. for a more complete validation, it will be necessary to undertake a more detailed analysis on real data acquired in sites where the stratification of the near subsurface and the relative electric properties are well known. appendix. to observe the orthogonality of the subspaces im(a) and ker (a), let us consider the scalar product v . w with νν∈ ( )ker a and w a∈ ( )im . being im(a) spanned by eigenvectors associated to non-zero eigenvalues, we can write the vector w as a linear combination of eigenvectors: w wi=∑α i ; if li are the eigenvalues respectively associated to the eigenvectors w i (that is aw i = l i w i ), and in the hypothesis of symmetric matrix a, for each v and for each w we have this means that im(a) is orthogonal to ker (a). =     = ( )     =∑ ∑νν αα ααννt t i i i t i i i α λ α λ w w 0 νν νν νν νν νν αα⋅ = = ( )=     =     =∑ ∑ ∑w w w w w t t i i t i i i i t i i iα α λ λ α λ references barker, r.d. 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(1976): algèbre linéaire, editions mir, moscou. wabha, g. (1990): spline models for observation data, siam, philadelphia, vol. 59. wang_correctedok:layout 6 annals of geophysics, 56, 3, 2013, r0332; doi:10.4401/ag-5548 r0332 stability analysis of slip of a one-body spring-slider model in the presence of thermal pressurization jeen-hwa wang institute of earth sciences, academia sinica, taipei, taiwan abstract this study is focused on the stability of slip in a one-body spring-slider system, with stiffness l of the slider, in the presence of slip-dependent friction caused by thermal pressurization for two end-member models of thermal pressurization; i.e., the adiabatic-undrained-deformation (aud) model and the slip-on-a-plane (sop) model. analytical studies based on the functions of frictional stress, xf , versus slip, d, of the two models show that lcr=|dxf/dd| at d=0 is the critical stiffness of the system. lcr is a finite positive value for the aud model, and infinity for the sop model. slip is stable when l>lcr and unstable when l<lcr. the analytical stability conditions of slip from the equations of motions of a onebody spring-slider model with the xf – d function of the adu model are also those from |dxf/dd| at d=0. under adu thermal pressurization, the modeled final slip and maximum velocity for l=5 to 31 mpa/m, with lcr=30 mpa/m, both decrease with increasing l. the motions become very weak when l>lcr, thus not resulting in unstable motions. the phase portraits of v versusd for l<30 mpa/m show the existence of a non-zero unstable fixed point. 1. introduction after an earthquake ruptures, the shear stress, x, on the fault plane decreases from a static xo to a dynamic xd, and finally becomes xf. the weakening mechanism is complex [bizzarri 2009] and can be either slip-dependent, or rate-dependent and state-dependent [dieterich 1979, ruina 1983, marone 1998, wang 2002, bizzarri and cocco 2006a]. bizzarri [2011] showed that if two faults that obey different friction laws are energetically equivalent, then it would be virtually impossible to distinguish between them, even on the basis of their frequency content and synthetic ground motions. note that slip, d, and the rate or sliding velocity, v, are both a function of time. for a long time, there has been debate concerning the constitutive laws. some studies are in favor of the slip-dependent law [ohnaka 2004], while other studies prefer the rate-dependent and statedependent law [bizzarri and cocco 2005, wang 2009a, wang 2012]. bizzarri and cocco [2003] showed that the rate-weakening law can also yield slip-weakening behavior of shear-stress evolution. stability analysis of slip of a one-body (one-degreeof-freedom) spring-slider model (see figure 1) is usually carried out to understand the behavior of faulting. for rate-dependent and state-dependent friction, slip stability has been analyzed in numerous studies [rice 1983, ruina 1983, rice and ruina 1983, gu et al. 1984, blanpied and tullis 1986, rice and tse 1986, gu and wong 1991, belardinelli and belardinelli 1996, he et al. 1998, reinen 2000, ryabov and ito 2001]. the equilibrium state of the one-body system subjected to friction exists when the slider moves under a driving velocity, and in the neighborhood of this state it shows that the model parameters control the stability of slip of the system. as mentioned above, stability analyses of this type of system have mainly been focused on the ratedependent and state-dependent friction law. however, this kind of analysis has not yet been performed for the case of slip-dependent friction. to interpret the decrease in the effective frictional stress, several physical mechanisms have been considered, including: hydrodynamic lubrication [brodsky and kanamori 2001], thermal pressurization [sibson 1973, fialko 2004, bizzarri and cocco 2006b, 2006c, rice 2006, wang 2009b], flash faulting [rice 1999, tullis and goldsby 2003a, tullis and goldsby 2003b, prakash 2004, prakash and yuan 2004, hirose and shimamoto 2005, rice 2006], gel formation [goldsby and tullis 2002, di toro et al. 2004], and melting lubrication [sibson 1975, spray 1993, tsutsumi and shimamoto 1997, hirose and shimamoto 2005], among others. a summary of the mechanisms can be found in rice [2006] and bizzarri [2009]. except for flash heating and melting lubrication, heat and the pore fluid pressure both have major roles in reducing the frictional stress. therarticle history received january 6, 2012; accepted may 28, 2013. subject classification: slip stability, one-body spring-slider system, friction, thermal pressurization, aud and sop models. mal pressurization has long been considered to be an important mechanism in controlling fault dynamics. rice [2006] proposed two end-member models for thermal pressurization: the adiabatic-undrained-deformation (aud) model, and the slip-on-a-plane (sop) model. wang [2009b] showed that the aud model is more appropriate than the sop model to interpret the observed xf – d function. to understand slip behavior of a fault under slipdependent friction caused by thermal pressurization, it is significant to analyze the stability of slip in the neighborhood of the equilibrium state on the basis of a onebody spring-slider model subject to slip-dependent shear stress caused by thermal pressurization. unlike rate-dependent and state-dependent friction, to date there is a lack of this a kind of study. as the sop model is defined based on a non-zero constant slip velocity, v, this model cannot work at v=0. hence, in this study, an attempt is made mainly to investigate slip stability of a one-body spring-slider model subject to slip-dependent shear stress caused by thermal pressurization under adiabatic and undrained conditions; i.e., the aud model. for this purpose, three approaches will be used. first, an analytic study of stability conditions based on the xf – d functions will be carried out. for this study, the stability condition for the sop model will also be described. second, the stability of slip at and near v=0 will be analyzed on the basis of a set of equations to represent the motions of the one-body model in the presence of aud-type thermal pressurization. third, numerical simulations will be performed from those equations. 2. one-dimensional thermal pressurization model considering a one-dimensional fault model, the x and y axes denote the directions along and normal to the fault, respectively. under thermal pressurization, the energy and fluid mass conservation equations can be written as [rice 2006]: (1) (2) where t is the time, t is the temperature, c is the shear strain, ct=dc/dt is the shear rate, t is the density of the fault zone, tf is the density of the fluids, c is the heat capacity, at is the thermal diffusivity, ah is the hydraulic diffusivity, and np is the inelastic (or plastic) porosity after deformation. the parameter b is the volumetric pore fluid storage coefficient and it is equal to n(bf+bn), where n is the starting porosity before deformation, bf is the isothermal compressibility of the pore fluid (dtf/tf=bfdp), and bn is the isothermal compressibility of the pore space (dn/n=bfdp). is the undrained pressurization factor that can be written as (mf−mn)/(bf+bn), where mf (dtf/tf=−mffdt) and n (dn/n=mndt) are the isobaric, volumetric thermal expansion coefficients for pore fluids and pore space, respectively. the parameters at and ah are equal to k/tc and k/hfb, respectively, where k, k, and hf are the thermal conductivity, permeability, and fluid viscosity, respectively. rice [2006] proposed two end-member models for thermal pressurization: the aud model and the sop model, detailed explanations of which can be found in rice [2006]. only a brief description is given here. the aud model corresponds to a homogeneous simple shear strain c at a constant normal stress vn on a spatial scale of the sheared layer that is broad enough to effectively preclude heat or fluid transfer. the sop model shows that all sliding with a constant velocity is merely on the fault plane and heat is simultaneously transferred outwards from the fault plane. before slipping, the effective static shear stress x on the fault plane can be represented by xo=f(vn−po), where f is the static friction coefficient and vn and po are the normal stress and pore fluid pressure, respectively, on the sliding plane (y=0). the xf – d functions caused by thermal pressurization [rice 2006] are: (1) for the aud model, (3) in equation (3), dc=tch/fk, where h is the thickness of the slip zone. (2) for the sop model, (4) jeen-hwa wang 2 t/ t / 1/ / yy /c c c tt t2 2 2 2 22vc t t t a=^ ^h h6 @ / t t/ t / t 1/ / y / y ,p /p n ff h p2 2 2 2 2 2 2 2 22b b b t t a k+ = = ^ ^ ^ h h h6 @ figure 1. the functions of shear stress, x, in terms of slip, d: curves for xf versus d: and lines for xe versus d. xo (=ldl) denotes the effective shear strength. / .expfaud o cx d x d d= -^ ^h h /l /l .erfcexpf o / sop 1 2 x d x d d= ) )^ ^ ^h h h 3 to derive equation (4), the sliding velocity, v (=dd/dt), is set to be constant. in equation (4), erfc(z) is the complementary error function, and l*=4(tc/k)2(at 1/2 + + ah 1/2)2/f 2v. the two parameters dc and l * control the xf – d functions of individual end-member models. in equation (3), dc is a characteristic displacement of the xf – d function. for equation (4), there is no characteristic displacement for the xf – d function [rice 2006]. an example to show the xf – d function is given in figure 1a for the aud model, and figure 1b for the sop model. figure 1 thus shows the decrease in xf with increasing d for each of these models. 3. one-body spring-slider model the dynamic one-body spring-slider model (see figure 2) consists of a slider of mass m that is linked by a spring with stiffness l. the slider is also exerted by a normal stress, vn. the bottom area of the slider is a. at time t=0, the slider rests in an equilibrium state. its location is d, measured with respect to its initial equilibrium position, along the horizontal axis. the slider is driven by elastic traction from the spring, which moves with a velocity vp. the slip at the loading point is do=vpt. the elastic traction exerted on the slider by the spring is: (5) the slider is also subjected to slip-dependent frictional stress, xf (d). hence, the equation of motion of the system is: (6) where t is the mass per unit length, with a unit of kg/m. as mentioned above, the sop model is defined on the basis on a nonzero constant slip velocity, v, and thus this model cannot be used for dynamic analysis. in addition, the aud model represents conditions over a sufficiently short time scale to neglect fluid and heat diffusion during the earthquake rupture, while the sop model represents those over a longer time scale, to neglect shear zone thickness. hence, the stability analysis of the aud model is valid for seismic events with short slip durations. on the other hand, the sop model cannot work for such events. hence for the aud model, only equation (3) is taken into account for the dynamic approach. inserting equation (3) into equation (6) leads to: (7) immediately before the slider starts to move, the displacement is d=0, and the total stress exerted on the slider is zero; i.e., t(d2d/dt2)=0. at this moment, do is dl, thus leading to ldl−xo=0 from equation (7). this gives dl=xo/l. when do is slightly greater than dl, the elastic traction from the extended spring can overcome the effective breaking strength, and then it pushes the slider to move. as the rupture duration of an earthquake is several orders shorter than the loading time, dl can be considered to be constant during faulting. after the slider moves, xe decreases with increasing d. the relationship of xe=l(do−d) is shown in figure 1 by the straight line with slope −l. this line crosses the vertical axis for x and the horizontal axis for d at x=xo=ldl and d=dl, respectively. 4. analytical study the xf – d function is shown by the bold curve in figure 1. the derivatives of xf with respective to d for the aud model are: (8) and for the sop model they are: (9) equations (8) and (9) lead to dxf/dd=−xo/dc and dxf/dd→∞ at d=0, respectively. lcr=|dxf/dd| at d=0 is defined as the characteristic stiffness of the system. hence, lcr equals xo/dc for the aud model, and ∞ for the sop model. for the aud model, the xe – d function with l>lcr is shown with a line marked by l>lcr in figure 1a. obviously, the line cannot cross the xf – d function, and xe is always <xf, thus resulting in stable motion. this situation does not exist for the sop model, because l cannot be larger than lcr=∞. when is smaller than lcr=xo/dc for the aud model or lcr=∞ for the sop model, the xe – d function is shown with the line marked by l>lcr in figure 1. this line does thermal pressurization on slip stability d /dt / .expe coo 2 2 d d dt d l d x= --^ ^ ^h h h d /d /l erfc /l / l .exp 1 f o / /1 2 1 2 x d x d d r d = -= ) ) )^ ^ ^h h h6 @ figure 2. a one-body dynamical spring-slider model: m, mass of the slider; vn, normal stress; vp, driving velocity; xf, frictional stress; l, spring constant; d, slip of the slider; and do, slip of the loading point. .e ox d l d d= -^ ^h h d /dt d dtv/ ,e f 2 2t d t x x= = -^ ^h h d /d / /expf o c cx d x d d d=-^ h cross the xf – d function in figure 1a, although not in figure 1b. from d=0 to the displacement related to the intersection point, xe is higher than xf, thus indicating unstable motion. hence, lcr is the critical stiffness (spring constant) for controlling stable or unstable motions of the system. the ratio xo/dc is a function of the thermal and hydromechanical parameters of a fault zone; wang [2009b] indicated that the two microscopic physical parameters xo and dc can be evaluated from macroscopic seismological observations. when l>lcr=xo/dc, the stiffness between the driving force and the slider is higher than the characteristic stiffness of the system, and thus the system shows stable motions. on the other hand, when l<lcr=xo/dc, the stiffness between the driving force and the slider is lower than the characteristic stiffness of the system. this leads to unstable motions of the system. the driving force on a fault system comes mainly from the moving plate. hence, xo/dc is an important factor in controlling the degree of coupling between a moving plate and a fault system: strong coupling when l>xo/dc, and weak coupling when l<xo/dc. the results suggest that large earthquakes can occur more easily in areas with weaker coupling than those with stronger coupling. at the intersection point of the xe – d and xf – d functions, xe equals xf, thus leading to d 2d/dt2=0. in figure 1, the values of dc for the aud model and l * for the sop model are equal. however, under the same xo, the value of d at the intersection point is larger for the aud model than for the sop model. although the total stress is zero at the intersection point, the slider can still move forwards because the velocity of the slider is greater than zero. this could result in overshoot of the system and will be discussed below. 5. stability analysis to study the stability of slip of the one-body system in the presence of aud-type thermal pressurization under adiabatic and undrained conditions, equation (6) is normalized by defining x=d/dc, y=v/vp, and z=xe/xo. the temporal derivatives of x, y and z are xt=dx/dt=(dd/dt)/dc=v/dc, yt=dy/dt=(dv/dt)/vp= =(d2d/dt2)/vp, and zt=dz/dt= (dxe/dt)/xo=[d(vpt− −d)/dt]/xo=[lvp−l(dd/dt)]/xo, respectively. in addition, t1=dc/vp, t2=tvp/xo, and t3=xo/lvp represent three generalized periods of the system. in general, the values of the related parameters are: l=~10 mpa/m and t=~107 kg/m [gu and wong 1991], dc=~1 m [wang 2009b], vn−po=~100 mpa [rice 2006], f=~0.6, and vp=~10 −9 m/s. this leads to t1=~10 9 s, t2= =~10−3 s, and t3=~10 9 s. obviously, t�t3>>t2. the normalized equations of motions from equation (6) are: (10a) (10b) (10c) from taylor’s series, the first-order approximation of e−x is 1−x as |x|<<1; i.e., e−x�1−x. this approximation makes equation (10b) become: (11) differentiating equation (11) leads to: (12) inserting equations (10a) and (10c) into equation (12) results in: (13) the laplace transformation of equation (13) is: (14) where y(s) and 1/s are the laplace transforms of y(t) and 1, respectively. equation (14) gives: (15) as s=0 is the trivial solution, the stability of slip depends only on the solutions of d(s)=s2−[(xo/dc)− −l]/t=0. the slipping state is stable if the equation d(s)=0 has no solutions so with re[so]>0, and unstable if the equation d(s)=0 has some solutions so with re[so]>0. the roots of equation (15) are: (16) if l<lcr, s is a real number. the positive root leads to unstable motions, while the negative root results in stable motions. if l=lcr, s is null. this root represents the stable inflected node in the phase portrait, and slip is marginally stable. hence, the roots represent the saddle point in the phase portrait. if l>lcr, s is an imaginary number. this always results in stable oscillations. hence, the positive and negative imaginary roots denote the center of oscillations in the phase portrait. consequently, the three conditions: l<lcr, l=lcr, and l>lcr represent the unstable (supercritical), marginally stable (critical), jeen-hwa wang 4 y v / z e t ;//z e p o x t x 2t x= -= -^ ^ ^h h h s y s t t s t1/ st 1/ 1/ y /2 3 1 3 2= + -^ ^ ^ ^h h h h6 @ y s t s s t t t s s 1/t / 1/ 1/ / / / / / .o c 2 3 2 1 3 2 2l t x d l t = = = ^ ^ ^ ^ ^ h h h h h 6 6 @ @" , s / / / .o c / cr /1 2 1 2! !x d l t l l t= = -! ^ h6 6@ @ @ @ x y/ /v y/t ;t c p 1d= =^ h z 1 y y/ / v 1 /t .t o p 3x l= = -^ ^ ^h h h y z 1 x /t .t 2= +^ h y z x /t .t t tt 2= +^ h y 1 y y t t/t / / .tt 3 1 2= +^ h6 @ 5 and stable (subcritical) states of the system, respectively. this is consistent with the analytical results shown in the last section. meanwhile, the three conditions are in agreement with the counterparts in the rate-dependent and state-dependent friction law, as shown in the abovementioned numerous articles [gu et al. 1984]. 6. numerical simulations simulations of slip of the one-body system in the presence of aud-type thermal pressurization are performed on the basis of equation (10), using the fourthorder runge-kutta algorithm [press et al. 1986]. as mentioned above, f represents the static friction coefficient between the bottom of the slider and the moving plate. when vn−po and f are set to be 100 mpa and 0.6, respectively, the effective shear stress, i.e., xo=f(vn−po), is 60 mpa. the value of dc is 2 m. hence, the critical stiffness, lcr, is 30 mpa/m. the density is t=10 7 kg/m. first, the slip-dependent variation in x is simulated for an unstable case, with l=10 mpa/m<lcr. the plots of x versus d are shown in figure 3a: a straight line for xe and a curve for xf. at the intersection point of the xe−d and xf−d functions, xe equals xf, thus yielding d2d/dt2=0. the slip at this point is denoted by d1 in figure 3a. here, v reaches the maximum value and thus the slider can still move forward. beyond this point, the frictional stress is higher than the elastic traction, and thus the slider slows down. figure 3a shows that the difference between xe and xf, i.e., xe−xf, becomes negative, and thus forming resistance to decelerate the slider. this makes the slider slow down and finally stop when d=df. the temporal variations in normalized acceleration (a/amax), normalized velocity (v/vmax), and normalized displacement (d/dmax) are plotted in figure 3b. the acceleration and velocity first increase to their respective peaks, and then decrease. the acceleration is zero when the velocity reaches its peak and becomes negative due to a negative value of xe−xf, as mentioned above. this makes the slider slow down, and finally the slider stops when the velocity is zero. the displacement increases monotonously from 0 to its peak. the phase portrait, which is a plot of a physical quantity versus another physical quantity of an object in a dynamical system, can represent the dynamical behavior of the system [thompson and stewart 1986]. the phase portraits of x/xmax (x=xe−xf) versus v/vmax are plotted in figure 3c. when v/vmax increases from 0 to the maximum value, x/xmax is positive. its magnitude increases and then decreases with increasing v/vmax. the dashed line in figure 3c is the bisection line. the intersection point of the bisection line with a phase portrait is called a fixed point [thompson and stewart 1986]. let h be the magnitude of slope of the thermal pressurization on slip stability figure 3. (a) the stress–slip function for the aud model. (b) the temporal variations in a/amax (dashed-dotted line), v/vmax (dashed line), and d/dmax (solid line), and the horizontal dotted line denotes d1/dmax. (c) the phase portrait for x/xmax versus v/vmax and two fixed points: p0=(0,0) and p1=(0.849,0.846). the slope at p1 is —1.78. (d) the phase portrait for v/vmax versus d/dmax and two fixed points: p0=(0,0) and p1=(0.783,0.787). the slope at p1 is —1.81. tangential line at a fixed point. the stability of a fixed point is dependent upon h: it is stable when h<1, marginally stable when h=1, and unstable when h>1. in figure 3c, the bisection line intersects two points. hence, there are two fixed points: one is the zero point (denoted by p0=(0.0, 0.0)), and the other is a nonzero point (denoted by p1). obviously, for the two models, the values of h at p0 are >1 and thus the zero point is a stable fixed point. this is the same as the conclusion from the previous two sections. on the other hand, the value of h at p1 is negative, with a magnitude >1. hence, the nonzero point is an unstable fixed point. as the velocity reaches its maximum, xe−xf becomes negative, thus resulting in resistance to decelerate the slider. then the magnitude of xe−xf reduces with decreasing velocity, and finally becomes zero when the slider stops. in other words, the portrait returns to p0. the driving force continues to pull the slider, thus increasing the value of xe on it. when xe > xf, the slider moves again, and thus the portrait in figure 3c repeats. the phase portraits of v/vmax versus / max are plotted in figure 3d. essentially, v/vmax increases and then decreases with increasing d/dmax. the dashed line in figure 3c represents the bisection line, which intersects the phase portrait at two points: one at the zero point (p0) and the other at a nonzero point (p1). the values of h at p0 are >1. hence, the zero point is an unstable fixed point. this is the same as the conclusion gained in the previous two sections. at p1, h is negative, but its magnitude is >1. hence, this non-zero point is an unstable fixed point. to show the effects of a change of l on slip of the system, numerical simulations are conducted for numerous cases, with l=5, 10, 15, 20, 25, 29, 30, and 31 mpa/m. the first six cases with l<lcr are for unstable slip, the seventh case for marginally stable slip, and the last case for stable slip. figure 4 shows the plots of maximum (final) slip, dmax, and maximum velocity, vmax, versus l. as the maximum acceleration, amax, varies between positive and negative values, it is not plotted in figure 4. it is obvious that dmax and vmax both decrease with increasing l, and the changing rate is higher for dmax than for vmax, especially at small l. obviously, the degree of unstable slip increases with decreasing l. the values of dmax and vmax are very small when l≥lcr. this is because the elastic traction is smaller than the frictional stress, as shown in figure 1, thus resisting slip. numerical results for l=29, 30, and 31 mpa/m are displayed to examine the detailed variations of motions from unstable to stable states when the values of l are close to lcr. results for the xe−d and xf−d functions, the temporal variations in a/amax, v/vmax, and d/dmax, and the phase portraits of v/vmax versus d/dmax are shown in figure 5a for l=29 mpa/m, figure 5b for l=30 mpa/m, and figure 5c for l=31 mpa/m. the values of amax, vmax, and dmax used for normalization for these three cases are actually those from the case with l=29 mpa/m. for the three cases, the values of d1 and df are close to zero, and thus they are not displayed in figure 5a1, b1, and c1. figure 5a2 shows the temporal variations in v/vmax and d/dmax, which are similar to those displayed in figure 3b for l=10 mpa/m. nevertheless, unlike figure 3b, a/amax in figure 5a2 is negative when v/vmax decreases from the maximum value to zero. numerical tests show that when l is increased, amax changes from positive to negative and the magnitude of amax decreases. let amaxn be the largest magnitude of negative accelerations and amaxp be that of positive accelerations. the ratio of amaxn to amaxp increases with l, even though amaxn and amaxp both decrease with l. this can be seen from a comparison between figure 3b and figure 5a2. large values of negative accelerations for large l will resist the motion of the slider. this makes dmax and vmax decrease with increasing l. figure 5b2 and c2 show that the values of a/amax, v/vmax, and d/dmax are very small when l≥lcr=30 mpa/m and the duration time is shorter for l=31 mpa/m than for l=30 mpa/m. like figure 3c, figure 5a3 shows a regular, complete cycle of motions. on the other hand, there are only very small values of v/vmax and d/dmax in figure 5b3 and c3. results again confirm that l is the key parameter in controlling slip stability of the system, and the condition of l≥lcr cannot produce unstable motions. although there have been numerous theoretical studies of modeling and simulation to date, the applications of thermal pressurization models to real faults are rare, especially for the correlation of slip stability to faulting. wang [2009b] applied the thermal pressurization models to explain the slip-dependent stress funcjeen-hwa wang 6 figure 4. plots of dmax (solid line, open squares) and vmax (dashed line, open circles) versus l. the vertical dotted line displays the critical state with l =30 mpa/m. 7 tion on the fault plane during an earthquake, and also to study the slip-dependent radiation efficiency, r, in terms of the parameters of the thermal pressurization model. his results can provide us with something significant and are briefly described here. the radiation efficiency, r, is an important parameter that shows the source property. it is strongly affected by the variation in shear stress with slip. 7. conclusions analytic studies from the functions of frictional stress versus slip show that the value of |dxe/dd| at d=0 represents the critical stiffness (spring constant), lcr, of the system. lcr is f(vn−po)/dc for the aud model and ∞ for the sop model. for the aud model, slip of the system is stable (or subcritical) when l>lcr, and marginally stable (or critical) when l=lcr, and unstable (or supercritical) when l<lcr; for the sop model only unstable slip exists because is always smaller than lcr=∞. analytic analysis of slip stability of a one-body spring-slider model with the xf−d function caused by adu thermal pressurization was carried out. the stability conditions are the same as those from |dxf/dd| at d=0. numerical simulations of slip were made for the one-body system in the presence of aud thermal pressurization. in the phase portrait of x/xmax versus v/vmax for l=10 mpa/m, there are two fixed points: one at the zero point and the other at a nonzero point. the former is a stable fixed point, while the latter is an unstable fixed point. in the phase portrait of v/vmax versus d/dmax for l=10 mpa/m, there are also two fixed points: one at the zero point and the other at a nonzero point. they are both an unstable fixed point. numerical simulations for l=5 to 31 mpa/m (with lcr=30 mpa/m) show that the final slip and maximum velocity both decrease with increasing l and the motion of the slider is rapidly 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(1993). viscosity determinations of some frictionally generated silicate melts: implications for fault zone rheology at high strain rates, j. geophys. res., 98, 8053-8068. thompson, j.m.t., and h.b. stewart (1986). nonlinear dynamics and chaos, john wiley and sons, new york, 376 pp. tsutsumi, a., and t. shimamoto (1997). high velocity frictional properties of gabbro, geophys. res. lett., 24, 699-702. tullis, t.e., and d.l. goldsby (2003a). flash melting of crustal rocks at almost seismic slip rates, eos trans. agu, 84, fall meet. suppl., abstract s51b-5. tullis, t.e., and d.l. goldsby (2003b). laboratory experiments on fault shear resistance relevant to coseismic earthquake slip, scec annu. prog. rep. 2003, south. calif. earthquake cent., los angeles, usa. wang, j.h. (2002). a dynamical study of two one-statevariable, rate-dependent and state-dependent friction laws, bull. seism. soc. am., 92, 687-694. wang, j.h. (2009a) a numerical study of comparison between two one-statevariable, rateand state-dependent friction evolution laws, earthq. sci., 2, 197-204. wang, j.h. (2009b). effect of thermal pressurization on radiation efficiency, bull. seism. soc. am., 99, 22932304. wang, j.h. (2012). some intrinsic properties of the twodimensional dynamical spring-slider model of earthquake faults, bull. seism. soc. am., 102, 822-835. *corresponding author: jeen-hwa wang, institute of earth sciences, academia sinica, taipei, taiwan; email: jhwang@earth.sinica.edu.tw. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. thermal pressurization on slip stability << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice a sludy on lhe displal~emenl compollenls of hayleigh waye (*) k. g. bila'!'!.\. {'" '"l ,r. ('. ])nakol'oltlor ('"'"'") ri{'c'nlto l'l l }'('hhra io 1071 st:)dllltt. 'l'il!' i'.llld)' ha" hecll clllti(>.,1 ont to o])i<c1tc tiie cjl'cct" of l'oi;;~oil mt io on i he di;;pb('cmcllt ('ojll)loilcll ts of hayleigll "\l'an's in clasti(' mc,lia. tll g-encral 1llc prohlem or ~llr!a('e wave;; ljave hel'n slndied hy jiian~' alll!wr". 'l'ijc allthor~ iwve adopled arhitral)' valnes or l'oi""on ndio in onici' lo ;;tlld.v ti,e vllrililions in 1111" ratio of illc ,li~plat'cmcllt (·ojllponl'ld.~ on tlw ;;l11'[a('e and inside thl' ml',lill. thc variat-ioll.~ of 1h(\ iloriwll l al an(1 vcrli('al ajllplitu,]p>< of ~llrfa('c wavc ,li.~pla(·cjllent with depili il' a jiiatler of ,!l;reat intel'e"t l,otll in sei;;jllolog-.i" and l<:nl'lhquake eng-illel'ring. hr ,issu:<ito. (illc><to stwlio (, ~1[1,to c.~ci-.'1tito allo "ropo ,li o><"eryare g-1i efl'ehi ,lei l'apporto ,li poi'~''',n sllll(, "ojllponellti ,leg-li i<poslajllellti delle onde di hayleig"11 in jiie:o::o:i ela~li('i. in g-enerale il prohlejlla delle onde "nperfkiali ì' "talo ><tlhliato ,la molti :mtori. (:ii autol'i llallllo a,lo!tllto yalori arhifrari per il l'apporto ,li pois.~(l1l in jiio,lo ,la i<twlilll'nc lr variaziolli ;;ullc' ('ojiiihiijcllti deg-li ;; postamenti ri s pctto alla i<upel'fkic c nell ' jll!enlo dei jiie:o:zi. le nll'iaziolli (ielle ampie:o:ze ori:o:wntali l' verti('ali (iello i'.]lo sta· mento dell'olllia ~11perfi('iall' ri"pet-lo alla pl'ofoliditit, sono di noj.(\volr inte· rp;;sc' "ia ilei' la :';i"m(,i('g'ia ('he ijf'r l'lng'l'gneria "ismi('a. 1. l:vi'll,olli'('tio-"\. (*) intcruntional in"t ill111' o[ :-\l'i;;jllolog-~' alld l<:al'l!j'illakc eiig-inecrin!!; 'l'ok.vo. ,rapan. (u) :-\('i)(lol or heseardl allol trainillgin l-:'1l'1h(pwke eng-ineel'illg, univer~i!y or hoorkee. [llolia. (***) :-\l'isjllolog'i('al lahorntory, . .\..thcli" l ' niverl<ity. grl'e(·(·,. k. !<. bli.\t!a .1. c. l>jt.·\k()i>o\jr.".~ 11"\1"\, showc(l tlw.t tll(' fl'l'p slil"l":wl' or an cla.4i(· hoillo).!l'iwo\l.~ lu:dl'l·ia.l (·h.il ~lippol't h. (li~tlll"hh.li(·(·. il1\c oh'ill).! (li~phwp1i1(,llt~ iii a yel·ti(·l~l pialle ;~ioll).! tll!' (lil·p(·tioil or pl"opa.).!atioll iil tll!' [01'111 or ho i·pì!·o).!l·a(lp (·llip.~(' alhl ])l·opa).!atp with a· \'(·]o(·ito\" whi("]t \\'a~ about o.d tillh'.~ the ~ih'h,i' wa\'(' ,"(·1()(·ii.\' ill tltat llh'di\llll. ,j('jl'l"l''y.~ an(l stolh'il\~' h.ihllat('i' ewill).!: alu] p]'{',~~ gl'('ah," (\xjh11l(lc(l tlt(· tlt('m',\' to ('0'"('1' (·a .. ~e~ or la~·(']'(·(l ~(,1i1i·illlillitl' llh'(lia., '['lw i"cslilt.~ ilu1kl~te tltat iii h,il hnt tll!' ~i1l1pl(·~t ('lh~{' a,~~\lllw(l h," },01'(1 ha~-l(·ig'h tllp~p wayp~ ~lijt(,l'('(l (li~]){,l"~ion. 'l'hl' ,~\li'fl~("c wan'~ ('01l1])(1,~ill.!! the ita,\'lpi).!h alul l,oyp wa·n·,~ are (li,~tillg'\li~h(\(l fl'ol11 l' alu1 n [)()(l.\· wa.vl·.~ in la·in).! 1i101'{' 01' l('~,~ ('olllilj(\(l to tlw ~\ll"r~we, '['h\l~ tllt'il' alllplit\l(ll' (lt·('i"(%~('." yn," mpi(ll," with (lepth lwlo\\' tlw slil'fa('(· . . , .\.~~1i1l1{' a ,~illlpil\ hh.i·lllolli(" pbilp \nwp j,mill ll'l~,"('l1ing' in tlwa' (lil"edioil ~\ldt tilat 1) '['li/' (li,~t\ll"hl~ih'e i,~ in(lep('lldpllt or .1/ ('oo]'(lillah' 11) it iil('n'h.s{'~ mpidl.\· witlt (li~ì1~i)(,(' z fl"olli uh' fn'e ,~\ll'rlwe. ?{ illn/hm 'l'he [oliowill):{ ,,:vmhol.~ hav!) 11eell 11dopt.ell [ot' ll~e iii thih 11n1l('r; e ela~tidt~" il c'oetli!'lent of rij!idi t y !1 ilensil~o[ liiellia l, lame';; !'onhtajlt <]j ~ealar 1l1jtc'lltial of di;;plac'ellient 'l' ('1'"'1',,,'1'.) yedor potellt ial o[ d j,'plat'ejlic'nl l w:lve \ength u {'olil\h·e;;.;;ional wave ye\o('ity " rhc'ar wave .elo!'it:r c pha.~e vcloc'itr () l'li1)i!'al (lilatat.iojl r:r poi;;soil',; rat.io w all!:{ulat' freqllelu'y k pal'amct.er [or wate ef]lwtiojl i"j (.i":,..l"y,f,) .~ hml.," fon:c~ u,\",iv = dii>plaeelllellt ijl j:,!j an,1 z dircdiojls l'e;;pediycly f" rayleig-h wave \-clol'ity t rate of dencase oi alliplitwlc ') l'atio of ;;.hc'ar \l'a'-e velodly to l'ijjllpl'{'~siollal \l'aye yelodi)" ~ ratio o[ phasc vclo('ity to 8heal' ,un-e ,"c'lol'ity" {'()]ihidel' the [{' . .1/ plalw to be h·~ bonlhlarr 1\o1\(1 t,he ;; nxis llo~iti-ve to\\'h.i'd t.lle intl\rior of thc holi(l. j~et. c l', alhl 11' he tllf' (li~plh'('('nwnt~ in .1', h alld ;; dil'edioll~ re~pc(,ti"\·{'i.'·, 'l'il(' (li~plh'('omp,lit ìllll(·t.joll {}_ t (l', l", 1\') i~ dofill('.(l (3) h.,~ sill('o dihphwement i~ i))(!{'llc)hlcnt. of h wl', lmve r a(/i a'1-'~ \ a;1' a;;; a(f'l alv, . " i " a,1' a([j avi ~ iv " a.r [:: l [,1] whcre j. h,]](l fi h·re t,he lame's (·on~t-a.nt~ a.))(l a·i'(' g-iy!'ll al',' ;l1)(1,1f ~ (1 -1a) (1 --'rr) (l a) o i,~ t,lw (·,nlli(·a.l (lila.t,h,tion and òii i~ ('rolh'(']wr (!{'lta 1.(', =1fori=j 1)/1 " " ofol'i ,z'oj g is the poi~~oll l'l1,t.io all(l e i~ the hl()(llll(!~ 01 f'h~j.i(·i1.y. bqnation 01 illotioll i~ ~ivell :l~ wlwl'o dr i~ the m'\'(md (ll\l'jvm,ivl\ 01' n, with j'('~i)(,(·t t,o time, !l i,~ tllf' (l(,li~it.., h.ii(l !' '-' -' 1"l'om [ij, [~:r, [::)[ alld [ii a (l'u) 'j) j .. af) !} vi (i, + il) --;,ii v" 0, , "' bill o a,ntl w(, ()ht;:~in ti:. (l. 1\ !i .\t 1.\ .1. e. l'j\.\i\ o \·òu j.os '! n = (). + il) v f) -;-ft v~.ii. l' c -c \ il x , r il ii' \ ...j= v. f) = v 2(/> il)" ilzl '! j) = (i. -+.il) v (v · q») -i/' y2 ( d). [h] rllhstit.ut.iug fur 11 lmd flo l' :~ml;i ll~ i"c:~1 :~i\( i i ll lhog i llary part>l wc uht:~i n ,,·he.t·c su l llt.iult u( f'jj whct·c i v~ ,p = l tf '/j i /1ò i. ~ ) i ii~ y'j, v ~ l/il i ,,~ a i ~ i ~ r \1 ' , b' a t~ .. ii .~. :! /i i{= " alltl i " ( '1~1i ho w rjlt c li ;\ .. ..; ) m =_ .·1 '-'xi' [ = q:: -:i (1·,1 /;.1.)1 'p = ij oxp [± .~:: ~ i ((.,1 -'}.-;1")1 1o cxp [ ± .~z -+-i ((.,1 k'f)l a llli ,., a,wl [7] [s] wl [ w .i r i lll'c ioi" ill1l 'fa("{~ w:~\,j!s tl ,o 1il0tioil hl-\(·.qm('i.~ llog1igihlc n.t ; ~ dist.all(,c of a f(\\\' \\':we lcugt.!ts fl"tlm thc f,'oo siil"f:t,(·() i\nlu lioll [9] tak~ i.he funll ) (/) = ai' xp [-ff ...... i (l'lt · -k.r)l iii = ij \lxp [.~;; + -j ("11 k..l· )] v = c (l xp [ . . ilz + i (1"/ 1.-.r)] jll'()yillhi! c < /j < il. [11 1 'rlw ~ii.(-ll ha,~ hecll mi l'i1os(1) so that potcllti:~! he('()mc~ zci'o a,~ ::: telhh to illfillit,\', 'l'o ohtn-i)) i'·ollhtallt~ ..:1, h, c, tlw 1!())!lu!n.i')' ('oluliti01)~ ;~l'e: ali ~tre~sl'~ lh\l~t y:~lli~h at hp(' i\ul'fal'c i,e, ::: = o ::: =0, [1:! j fi'olll [:{i, [11] a!l(l [i:!j ) c " a (:! i'1k) l:i (k~ + s~) -() a [(i, + :! l') q~ },k~j -1n (:! pis) = () [13j b il = u f!'olll thi,~ wl' scl\ that (1)e l'oot of ~ i~ zo!'o lmt this :..dyi>-~ phu,w. yd(wity {'(limi to z('!'o alu! 1)(\1)(,{' i~ of 110t \l~l\. 'flwl'ofm'c .\!~o rl'olll eq. [1-1.1 it is ohyiou~ that tlle eclimtio\! llo\ds ).!"ool! \\"11('1) () <ij <!j < fl. b il = ti lli~plai'clllcllt~ u : ~1ll1 iv are [ alli1 li" à<jj a'p + ax oz 11" ha~ hel'1) !'l'p!u.(,('(l hy vi he·('·:mso othl'l' two i'omihl\lf'-1)t.~ or il' (lo llot ('ome i11 lo l'lay, fi'olll [i ll a.lul [i cii we i!et (i) u = -..:1 l..:1kie-ge -nsc" j cxp [i (ud k.r)j. il-t l" (i, 1l1la'f]a .1. <:. ilrakopoulo;; 'l'akillg olll.\" ma,l jml't a,ii(1 himila.r1\' [w] il' --ai[ tq,~ ,to< cos(u)t-/;.i') l ., k' l 1;2 + i;~ 'l'he mt-l' at whi(']t t.hl' l~mplitnde or tlw dihpla'('('jil('llj, l~lollg t.lw (iil'p('/,ioll or pi'oph,gn.tioll dci'nmms with (iept,h (io]lolldh npoll t 110 iad:lll' [iii . \1:-;0 the mte l~t. whir',h t.he amplitnde 01' thc lllot.iou ill li. (lil'o(,tioll llol'mal lo liw ~nrfh,ce de('l'eahe~ wii:h dcp1h (io]l{)]ul:-; npoll t.lw fl~(·tlli' [181 " ", l<'ig. i coellil'ìcllt hi aera.y of allljllihltie in lite (lirediull or ptupag-alillll u[ hayleigh \\"a\'e ill ela~tie lllc(li;t [or aiitncllt ynlu('.~ of poihrol\'s rali\). ~()\\a nd " -'" 1.: = c = ~i: whe rn j~ i" "he \favo k ng l. h n{ h<~yl(;'igh w.wc. " oo \ " ~ , o l s ~ , o j o o • o" • '" \ ~ o le " . . ~, 00() . -' " " " '\ " ~ o " " " " " " u~-f ig_ :2 cno/lic·-ient. cl f ,ioca)ci i :l lli p litwlc iii t-ile ,li l"ellt·iu ll llf'-rpcli.lit,ulll r lo pn' 111l~!c t i nll {l f ril,\'i~ i gh waye in cla.~ t i e ui (."(lill fn r ,i ilfcre n t. "ailwj; ile l'ui"soil '~ ratio t:ìlllmt.i t.ntin l:-' [3] ill [li] :md [18j wo .l!ol -r" =ox !, i -:!:t "';1 -1}~ , :ju all ~1 [ :! oj (~ee fi~ . 1 a mi :!) . :\g:\in l et (." " au d lv. he th(> di >l p hwelllell t "umpol\cll t>l iii tlw :~lld pcl"p tmdi ("nbl" t o th c dil"(i(·t.ioll of ]ll"op:tgatioll l'e~ptwtiy(\ly at t.lw f]"( .' l~ ~ ll il\f"c z = fi. 'j'ltcu th e ll . 1 ' ';1''''' ';]'' 1 l; ~= ak 1 ~ --=--.;: [i w 'i "2 1[.~ "~9' _ . .. o-o. f,;~ -i~1 ~llbs ti t.util\~ 101" q. if alll i f,; ii-t! ub t.nin [ :!.] j [ :!· ~. l [e3j [:l5 j u \l' ./1 'i = !~ ., -.i;~ , [27j ei/lmti llllil hl, :2n, :w , :':7 , ~g lwe l'lo t tetl fil" diffel't\nt yal uo:; or pojssnn l'm io. 'l'j w plo ul a· .. e o b taill e d no ll diii1bll.<;i ol1a.l. sile fi~. (],:.:, ;~ , 4: ,"» ) . .. " [" ~----------------------==~~ i~. 3 _ r:1 li u fl{ rlllil,jj t mle ill th e d inw-l io u (i[ jlmp:1~a.ti,m al ( ho .. urlncl\ (u~) a ll fl i !\ t,llll l!l(\( tia ( u) [ m' fli ffll nllll yilhlt!~ of pui .'ls( )ji ',~ 1"nti u. k. g. 1i1iatia j. c. dra1\opoui,os ;{. ('o,\t·lduo.'\s. l"ig'. 1 ;~il(l fig'.:! l'ciil'llf;ellt the ('ocftì('ient of dway (lf amplitude ill thc di!"c(,.tioll aliti pci'iiclhliellla.1' to thc (lil'c(·t.ioll of ])j'opag'lt,iù)ji, fig. li indieatcs a eompa,n.tivc ~tlldy of t.hc.~c t.wo ('ocffi('icllt.~ lt,il(l ,. ~ " , " \\\ ,. " \ \ " \ " \\ ~::~:: " " <1. o lo -'::"::::-":: '1 .8 20 ~a . i o a ·o oo o o " " " " l /l _ fig. 4 ratio of alllplitulle in tllc rlired.ion perpendieular to propagation at the ;;urfare (iv,,) and in the tne,lia (w) fol' difiel'ent values of pois~on's ratio. revcals tlmt tlte ('ocftì('icnt ('ol'j'esponding to the (lispla('cmcnt ('ohl])(ii!ellt in the dil'ct'tioll or pi'oii;~g'ation f;how a ('lmng-e in the ph;~he llelh to pi/.!:. ci ratio of amplitwle ill thc (lil'cdjoll (u) all(l pcl'pelhliclllar t.u t.ll(' ,1ij'ediull (w) of pl'o]ja)..!;atioll foj' (lifferellt. valllch of l'oif'.,<.;oil'h ratio, " " " fig. ti coetlidellt of (le{'a~of (lispla{'clloent in lite (1irediull of prop:lgat.ion (~·x) arul in thc ,lirf'.t'tion l'erpejl(lio-nlar to propagat.ion (t,). 100 k. ,;. iliiatia .r. c. llrako!'ovlos the d('pth :: = o :!.fo wlwl"p h.s tlw otiwi' l'ollll'oihmt ilm'ays g-l·allua.]].\" al\(l hav(\ h. t('llilpih'y to hel·oil\p. a,\\"lllptotjl' :òt:: _= 1 .hl alul ol!wl1l"!b. i t is oh,q'ty('ll from fig.;, that \\"ith tlw illl'l"('a~{l il! tlw l'oi~sol! mtio of th(' media. tlw di,~pl:w('ment l'omponent ill the llil"l'diol! o[ tllp jll"opagl~tion l~lso inerea.s{>.s :md tlw nmximllm yl~llle lies \\"lwn ilepth is bet\\"{\{>.n 0.37 l~ll(l ilari tillles the \wiovp leng-th or the hayh>.iidl waye. "\1.~o [01' ali tlw l'ois.~oll l"atio iff tlw lll('(lil1 thi.~ ('ollìpol\(>nt ehange~ phl1.~p at tlw t[ppth jjl'twpell 1l.1;") to il.:!.il tilll(j~ the w;t.ye length. " " " , , o> , • , • , -o> -" -,. " \ " 06 08 ljl _ ... . lo \ 2 vi" , fig. ""i [;/(10, ii"jl!'" iljhi u/ii" vers\l;; t/l. ()n tilp (·olltl"a.l".\" the displa.(·.rnwnt ('ollìponrnt in the (linwtiol! pi'l"i)('ihlil·nla.r t.o pl"opagl~tion or waye does not sho\\" l~ii.\· pha;·w l·ha.ii)!(' . ..\lso uw amplit\lil(\ il!l'n\:t-~ps \\'hrn ilepth jjrs 1wtwren 0.1 to il.:!. tillì('." the wa.v{\ l('ngth l~nll tlwl! it ilep]"(;l1si\s l"apit[l~' an(l hr·i·.o]jl{'s a~~'lllptoj-.i(' at 11 t[(\pth or l.:! tillh's th{' \\"l~\·e [l'llg-th (';l'p fig. j). ,'l'lw g-"alli{\nt or de(·a..'" or.-.. (si'i' fig. :")) ilh·'·el1.q'." with inl'n'l~se " in tlw yl~llle of l'oi~sol! l"lttio and iip to d{>,pth ,il. il i,. 'l'lwl! ilpi'1"p:'.s(\'; ;\ stuo)" 01' t ii e dlsl'lace~ie};'f compo!'le!'its ol,' rayleigii wave lol " with in('rease in poif!soil l'atio. .\180 the ,alue , j lv is illdepejldellt, of t1l8 roi~soil mtio wiwn dept1l is nea l' to 0.5 t.imes t.lte wa,o !f'llgth. u u i\' l<'ig. 'j shows eomparative st.ud.\· of ti'. 1 ::lolld com· ,li. il'. ponellts of tlte (lisplaeement. 4. rmrahk. the study e::m ful'tltel' lw extel!(l('d fol' the y,lol'iation in tlte modulus of ela~titity and the density of tlw media. 'l'iw al1thol's ,we tlmllkfl1! to t,he stall of tomput.el' sedùhl of ill· tel'liatiolla.l illf!tit'l1te of seislllo!oi!y alld hal'thql1ake ]!;ngineering 'rokyo, ja.pan fol' tlwil' tùllcly (·ooljgi'f[·tùhl. reli'erences (i) biiatia k. g., solllr a spccls 01 su1iace ii'nve propa(fntlon in rlaslic alld _j /l.i'iiin/.ic .li edin, .i ji.<sfrl.nlioi' for jl (jstel' rh'gn'cin rm·thqunkc engi1lccring, uni\cl'~ity of rool'l\oo india, (1!j68). (2) hullen il e., ali in/rod/lctiol, lo 7'heory 01 scismology "camb. univo press ", (l !j(j3). (j) ewi!'ig .:'ii., .jarj)el'zky ìy. llllil l''. p.ress, e/astic tval'es in laycnd _uedia :\tccraw·ttill book company, (1!j57). 295_308 deluisi.pdf 295 annals of geophysics, vol. 46, n. 2, april 2003 on the correspondence between surface uv observations and toms determinations of surface uv: a potential method for quality evaluating world surface uv observations john deluisi (1), david theisen (2), john augustine (1), patrick disterhoft (3), kathleen lantz (3), elizabeth weatherhead (3), gary hodges (3), christopher cornwall (3), irina petropavlovskikh (3), amy stevermer (3), dennis wellman (1) and jennifer barnett (4) (1) noaa, boulder, colorado, u.s.a. (2) aerojet corporation, longmont, colorado, u.s.a. (3) cires, university of colorado, boulder, colorado, u.s.a. (4) university of utah, salt lake city, utah, u.s.a. abstract a comparison of erythemally weighted surface uv irradiance observations with similar nasa toms surface uv determinations is described. comparisons are made for two observation periods: the robertson-berger (r-b) meter period from 1974 to the late 1980s and the current period from 1996 to the present when more sophisticated uvb-1 instruments were used. the more primitive r-b meter observations that comprised the fi rst u.s. uv network are seen to drift downward with respect to those of the toms. while the uvb-1 observations did not appear to drift, a substantial bias is noted to exist between the toms and the uvb-1 stations collecting observations; the toms estimations tend to be higher. a portion of the bias may be attributed to errors in calibration, total ozone, and cosine response of the surface instrumentation. unaccounted aerosol effects, although not considered to be large in the toms estimations, present another source of error. comparisons are fi rst done for all sky conditions and then for clear sky conditions. the biases typically agree for all sky conditions within the uncertainties of the surface instruments’ calibrations, liberally defi ned as ± 5%, implying that the toms cloud correction scheme performs reasonably well. snow cover severely impacts the toms observations, giving considerably higher estimations. the biases for clear sky conditions ranged from 15% to 19% with no obvious drifts between the satellite and surface observations. the variation in the biases among stations is within the calibration uncertainties of the instruments, but the absolute bias is unexpectedly large. the standard deviations of the clear sky comparisons among all stations are steady at 4.8% ± 0.7%. a plot of the toms/uvb-1 ratio versus toms cloud refl ectivity observations is noisy, but qualitatively suggestive of a possible slight increase (~ 5% or greater) over the range of clear to overcast skies. the results from these comparisons is believed to be relevant to a wmo goal of uniformly assuring the quality of uv observations made by networks in many countries. the results for clear sky comparisons suggest that a satellite observing system such as toms, which provides global coverage daily, might partially serve as a fi rst-order check to quality assure uv observations being made by networks worldwide. future research should concentrate on determining the causes of the large differences seen between the uvb-1 and toms and the range of uncertainties, using a larger array of stations. mailing address: dr. john deluisi, noaa/arl, 325 broadway, boulder, co80303, u.s.a.; e-mail: john.j.deluisi@noaa.gov key words uv ultraviolet erythermal surface satellite toms 1. introduction during the past three decades the anticipated threat of a serious stratospheric ozone reduction due to human industrial activities has stimulated public and scientifi c concern over 296 john deluisi et al. (http://nrel.colostate.edu/uv-b). broadband sensing instrumentation includes yankee environmental systems’ uvb-1, solar light’s biometer (sl), kipp and zonen’s uv-s-e-x, and eko’s ms-212d. calibrations of these instruments are critical and laborious because their spectral response functions and hemispheric irradiance collectors must be well characterized. moreover, contending with the unpredictable behavior of standard lamps presents a continuous struggle to maintain reliable laboratory irradiance standards. for instance, users of such lamps must contend with their temporal stability, weak uv irradiance output, requirement for highly accurate measurement of electrical power consumption, and susceptibility to changes from slight physical shock or just aging on a shelf. in view of the care that must be given to general instrumentation maintenance, including calibrations, field performance surveillance, data quality control, as well as hazards such as lightning strikes, winds, snow, hail, and curious animals gnawing on electrical cables, it is easily understandable that data quality can be unintentionally compromised. for this reason, it is imperative that whenever possible the quality of long-term observations be evaluated by comparison with other observations containing information that can be related in some manner to a uv observation. in the case of the national oceanographic and atmospheric administration’s (noaa) air resources laboratory (arl) surface radiation monitoring network (surfrad) observations, other types of solar irradiance observations are made. these additional observations are used to check the daily features of the uv observations. for additional information on surfrad see augustine et al. (2000) and the noaa surface radiation research branch home page: www.srrb. noaa.gov. daily uv observations of nasa’s total ozone mapping satellite (toms) described by herman et al. (1996) offer a practical, daily model-dependent observation that can be compared with surfrad’s uv-b network observations. moreover, because the toms is a global observing system, comparisons between it and other networks would be useful for quality assurance checks and to some extent as an environmental impacts. this motivated a strong movement among nations to seek a defi nitive answer on the environmental and human health consequences of increased uv-b radiation (hereafter uv-b) exposure. in response to this concern, the fi rst uv-b network was initiated in 1974 and made use of the robertson-berger (r-b) meter (berger, 1976). later, in the 1990s, several u.s. agencies established uv monitoring programs to acquire scientifi c data that would further shed light on uv-b exposure rates in various regions of the continental u.s. (kaye et al., 1999). other countries, including canada, japan, argentina, new zealand, australia, and several european countries, have also established monitoring programs. the world meteorological organization (wmo) has been actively coordinating the uvb observing activities of participating countries including the world climate research program’s international global atmospheric watch (gaw) environmental monitoring network. a uv monitoring network requires measurements in terms of absolute radiation quantities, i.e. radiance or irradiance. moreover, the wavelength region of the surface uv spectrum of interest is roughly between 295 and 400 nm commonly referred to as the uv-b, 280 nm < < l < 320 nm, and uv-a, 320 nm < l < 400 nm, regions of the solar electromagnetic spectrum. uv levels in this region change by about fi ve orders of magnitude from the shortest to the longest wavelength because of the dramatic increase in ozone absorption with decreasing wavelength in the huggins band. this behavior places severely stringent requirements on the design and construction of rugged observing instrumentation, including accurate wavelength registration, linearity, detector sensitivity, and long-term stability; correspondingly, quality instrumentation costs can soar. the state-of-theart in atmospheric uv measurement technology is still evolving although many advances have been made over the past several years. newer instruments include the brewer double dispersion spectrophotometer, the yankee environmental systems’ uv multi-filter rotating shadowband radiometer (uvmfrsr) (bigelow et al., 1998), and the u-111 spectroradiometer. the latter two are operated by the department of agriculture 297 correspondence between surface uv observations and toms observations independent check to determine how well other observing networks agree among each other. the main intent of the present investigation is to assess the stability of the previous r-b meter network observations, and the current uv-b observations collected by the surfrad network, using toms observations to compare with both. 2. observations the first surface data set that is used to compare with the toms data was produced by the r-b meter network (deluisi and harris, 1983; scotto et al., 1988) and the second data set, collected later by the noaa air resources laboratory’s surfrad network, using the uvb-1. although the fundamental design principles of the two types of instruments are similar, later modifications produced a substantial improvement in fi eld performance characteristics. the fi rst uvb network that was deployed over the u.s. was operated jointly by noaa and temple university (berger, 1976). the network used the r-b meter that was manufactured by temple university in accordance with robertson’s (1972) design. the meter was not thermally stabilized and, therefore, suffered error from temperature fl uctuations (blumthaler and ambach, 1986). network data were archived at the noaa air resources laboratory and the national institute of health. calibration and maintenance records have not been published. it is noted here that the robertson-berger meter network is a separate entity that operated a fi rst-generation instrument and has no connection with the surfrad network and other contemporary networks. the r-b meter network began scaling down in size after 1986 and no longer exists. current networks operate much-improved, updated instruments that are thermally stable and made rugged for long term fi eld operation. the purpose for including the r-b meter data in this comparison is historical and meant to provide a perspective for a possible physical explanation for the negative trend in uv observations reported by scotto et al. (1988). the trend occurred during the years 1974 to 1985 when a slightly positive trend could be expected from an ozone decrease of ~ 5 %/decade reported by bojkov et al. (1990). the surfrad’s uv observations are obtained with the yankee environmental systems (yes) broadband uvb-1 instrument (www.yesinc.com). routine annual instrument checks and calibrations are performed by the central uv calibration facility (cucf) located at the noaa offi ce of oceanic and atmospheric research (oar) facility in boulder, colorado (lantz et al., 1999). the calibration records of the surfrad uv-b instruments indicate that calibration changes are mostly contained within a few percent or less per year, which is within the uncertainty limits of the routine re-calibration process. such stability is highly desirable for maintaining a credible record of long term observations needed for critical scientifi c applications such as environmental impact assessments, trend analysis, and satellite data verifi cation. information on the toms satellite and uv observations can be found in herman et al. (1996). basically, the toms observes total ozone and tropospheric refl ectance each day over the entire globe. the total ozone observations are used to model the incident surface uvb irradiance, and the reflectance is used to estimate the cloud transmittance of uv to the surface, with the assumption that cloud absorption might be negligible, i.e. that conservative scattering applies (schafer et al., 1996; mckenzie et al., 1998). it is yet to be shown that some absorption is possible, especially with polluted clouds. because of their highly variable optical properties, the effects of tropospheric aerosols remains an open question, but have been investigated for example by krotkov et al. (1998) using toms observations. an investigation by wenny et al. (1998) found that tropospheric aerosol absorption in the uv can be quite variable and sometimes strong. the surface uv irradiance measurements and the computed toms estimates of uv-b are slightly different: the spectral response functions (srf) of the various broadband instruments do not strictly emulate the average human skin erythematogenic action spectrum 298 john deluisi et al. while the toms uv retrievals are calculated for the diffey (1991) average human skin erythematogenic action spectrum. therefore, correction schemes are often used to adjust the measurements to more accurately represent the human skin response. nevertheless, because of the unique design of the fi eld instruments, their performance is seen to be remarkably stable with time and, furthermore, display exceptionally good agreement and stability among the array of network instruments’ spectral response functions (deluisi et al., 1992). this trait is highly desirable for continuous long term monitoring observations. 3. procedure five r-b meter monitoring stations were chosen for comparison with toms. the comparison begins in 1980 and ends in 1990. a five station subset, albuquerque (35.1n, 106.4w), el paso (31.5n, 106.3w), minneapolis (44.6n, 93.2w), philadelphia (39.6n, 75.1w), and tallahassee (32.3n, 85.5w), is considered representative of the eight station set used by scotto et al. (1988), because the annually averaged data series closely follows that of the eight stations. these stations were also chosen because of their proximity to dobson ozone observing stations providing total ozone information for modeling uvb irradiance at the r-b meter stations. unfortunately, the overlapping periods of the toms and surface uvb observations are incomplete, but useful information is derived to provide insight into the scotto et al. (1988) analysis showing a negative trend. the r-b meter calibration is in terms of relative units of exposure which are converted into sunburn units. the procedure used to make the rb-meter data equivalent (in units of relative counts per half hour) is to work with normalized observations, viz., departures from the mean of the observation record. this removes any bias, but is useful for determining drifts and climatic variation correspondence between the two. in addition, the procedure removes any seasonal differences such as found by kalliskota et al. (2000). the more updated yes and sl instrumentation data are in terms of absolute irradiance units, w/m2, and furthermore, can be reasonably adjusted for the srf difference by accounting for total ozone and solar zenith angle disparities (lantz et al., 1999) that have been determined during calibration. to reduce solar zenith angle dependence, only data obtained near solar noon are used; the timing happens to be close to the time of the toms overfl ight. the toms is normally in noon orbit, meaning that the station overpass time will be close to noon and large day-to-day variations in solar zenith angle that affect the performance of instruments such as the r-b meter and the uvb-1 can be avoided. corrections for solar zenith angle and total ozone variations are available. tests were performed to quantify errors relating to these factors. the obtained results implied errors on the order of 1% and are not serious. it was decided to calculate a one-hour average of surfrad surface uv-b irradiance around local noontime as a means of averaging cloud effects in the surface instrument’s hemispheric field of view. an additional benefit of the noontime comparison is that the solar zenith angle is not changing rapidly. therefore, the hourly average will only be slightly lower than the exact peak value at solar passage over the local meridian. calculations using 3-min average clear-sky values of observed irradiance indicate that the error between the precise noontime value and the hourly-averaged value around noontime is ~ 1% or less. the report by long et al. (1996) also supports this argument. the surface footprint of the toms is approximately 50-100 km in diameter; comparisons during the presence of clouds are, therefore, expected to be noisy compared to clear sky conditions. on the other hand, an hourly average of clear sky uv-b irradiance values can be expected to usually represent a reasonable portion of clear sky conditions over a large region. the method of long and ackerman (2000) was applied to surfrad observations to obtain the times of clear sky conditions. clear skies over a site can also be determined by the toms refl ectivity channel signal, i.e. refl ectivity < 5%. both kinds of information were used in our analysis with no signifi cant difference noted in the results. 299 correspondence between surface uv observations and toms observations 4. results 4.1. comparison of toms with the r-b meter network figures 1 to 3 are comparisons of toms uv-b with the rb-meter uvb observations. the three surface stations are el paso, albuquerque, and philadelphia, respectively. the plots consist of deseasonalized quarterly averages of the toms and uvb data sets, normalized to the mean value of their respective entire record. the deseasonalizing procedure removes any possible seasonal difference between the two data sets such as found by kalliskota et al. (2000). note that the plots shown in fi gs. 1 to 3 are normalized data, as mentioned previously, because r-b meter units are in terms of counts per half-hour while the toms uv-b data are in terms of w/m2. wintertime toms data will most likely be in error over snow-covered surfaces. this effect occurs because the satellite tropospheric reflection channel senses the snow as cloud cover and falsely estimates the calculated clearsky uv cloud transmission correction factor when the actual cloud cover can be minimal. in these fi gures a downward drift of the r-b meter observations at el paso and albuquerque is seen. the plotted philadelphia data appear to track quite well, within ~ 5%, and show no apparent drift with respect to the toms observations. the observations from this station seem to fl uctuate more than the other two stations shown. it is believed the reason is that greater variation in regional cloud and aerosol climatology exists in the eastern part of the u.s. as compared to the more westerly stations where cloudless skies are more prevalent. the philadelphia r-b meter was used as the temple university’s primary standard to calibrate instruments rotated from the fi eld and to calibrate secondary traveling standard instruments for side-by-side comparison in the fi eld. apparently, the transfer of calibrations to the fi eld instruments by traveling standards was a problem that might not have been adequately resolved. one may notice when reviewing the philadelphia data plot of scotto et al. (1988) that the average annual values were high before fig. 1. time series comparison plots of quarterly averaged deseasonalized toms uv-b and r-b meter uv-b deviations at el paso, tx. 300 john deluisi et al. fig. 2. same as fi g. 1, but for albuquerque, nm. fig. 3. same as fi g. 1, but for philadelphia, pa. 301 correspondence between surface uv observations and toms observations 1980 compared to the data after this time. the minneapolis and tallahassee station comparisons (not shown) were noisier than those shown in fi gs. 1 to 3. next, in fi g. 4 we compare the fi ve station average, starting in 1974, with the averages of toms observations over the same stations. the r-b meter observations in general are high before 1979-1980 and indicate a downward tendency. there is no indication of a downward trend in the toms data. the plotted 1982 and 1983 points of the toms, which were lower than the others, refl ect lower values appearing over the eastern stations at that time. if these points were neglected, one might argue the existence of a slight upward trend for this time period, more consistent with the bojkov et al. (1990) fi ndings. figure 5 compares estimated r-b meter calibration differences between temple university’s actual r-b meter calibrations for the stations under study and the present investigation estimated clear sky calibrations using total ozone observations from dobson and toms instruments and the raw uncalibrated signals from the r-b meters. the ordinate zero line is the reference to the sky calibration. each station symbol is located at the midpoint of a period during which an assigned calibration was in effect. the procedure for calculating surface irradiance that was used to calibrate an r-b instrument if fundamentally similar in principle to that of the toms, which also theoretically computes clear sky uvb surface irradiance using total ozone observations. previous to 1979-1980, most of the differences are positive and contained in the range of 0 to 4%. after 1980, most of the differences are below the zero line. from an average of values before and after the shift, we estimate a total shift of about 4%. the same fi gure shows a calibration difference trend of – 6%/decade (apparently the 4% shift produces a greater trend), which is in the range of the trend reported by scotto et al. (1988). if the total ozone trend reported by bojkov et al. (1990) actually occurred over the fi ve stations during the period of the present analysis, then the temple university calibration drift might be much greater, of magnitude – 8% to – 10% or greater. on the other hand, the present investigation also examined the total ozone over the five stations and no signifi cant trend was indicated. this result leads us to conclude that the 4% shift is mainly responsible for the trend reported by scotto et al. (1988), and that there was very likely no signifi cant trend in the uv, on the average, at least for the fi ve stations examined in the present investigation. around 1979-1980, the temple university r-b meter laboratory calibration standard was changed from a weak power output quartz halogen standard lamp to a more powerful xenon lamp with the intent to improve the radiant power output of their calibration procedure (private communication, dan berger circa 1989). this is a very important point because the spectral emissions of these lamps are quite different. the transfer of a broadband uv calibration to an instrument, having a complex broadband spectral response function calibration such as the r-b meter, with dissimilar irradiance standards is a complicated process. this section concludes with the suggestion that the cause of the negative trend in uv-b reported by scotto et al. (1988) is due to procedural changes in calibration accompanied by fluctuating, moderate uncertainties ~ 5% encountered in transferring calibrations to the fi eld instruments. the results also confi rm the conclusions of the investigation by weatherhead et al. (1997) who performed a statistical analysis on the r-b meter observation record. it is highly unlikely that increased aerosol loading over all stations would be responsible for the negative trend. there is little evidence, if any, that aerosols increased suffi ciently to impact on the uv observations made at different locations. furthermore, if aerosols were responsible, then why was philadelphia not affected? philadelphia is located close to the heart of the eastern seaboard high turbidity region (flowers et al., 1969). the work of wenny et al. (1998) indicated that aerosol absorption can be strong, but their recently obtained aerosol optical depth data also suggested no aerosol trend in the central north carolina region since the time of the flowers et al. (1969) investigation. unfortunately, there are no systematic observations for determining aerosol optical depth trends that are available to unambiguously qualify any statements con302 john deluisi et al. fig. 4. time series plots of deseasonalized average annual observed uv-b deviations from the mean at fi ve stations (see text) in the r-b meter network that were included in the scotto et al. (1988) trend analysis and deseasonalized average annual toms observed uv-b deviations over the same stations. fig. 5. time series plots of temple university calibrations minus reconstructed calibration deviations from the mean showing a period of high values up to 1980 followed by low values thereafter, indicating roughly a shift of 4%. compare this fi gure with fi g. 4. 303 correspondence between surface uv observations and toms observations cerning aerosol affected uv trends at the monitoring stations’ locations. 4.2. comparison of toms uv with srrb broad band uv-b irradiance observations figure 6 shows a plot comparing toms overpass uv-b observations with goodwin creek uv-b observations for all sky conditions (clear and cloudy) from 1996 to 1998. the plotted diamonds represent the percent difference as (toms-uv)/uv. ozone corrections were applied to the surface uv-b measurements. the bias between the two observations is 20.5% with toms being higher. the standard deviation is 21.5% which is not unexpected because of the satellite’s large foot print and the nonhomogeneity of scattered clouds. figure 7 is a plot of goodwin creek comparisons for the same period as in fi g. 6, but for cloud free skies. in this analysis the bias is 18.1%, and the standard deviation is now reduced to 4.1%. the results for the stations at bismarck (46.5n, 100.5w), bondville (40.1n, 88.4w), boulder (40.1n, 105.2w). fort peck (48.3n, 105.1w) and goodwin creek (34.3n, 89.98w) are tabulated in table i. the bismarck station is a noaa solar radiation monitoring station belonging to a different network (hicks et al., 1996) that uses a solar light uv biometer. in the all sky cases, with the exception of fort peck and boulder, toms errors caused by snow cover were removed. the effects of snow cover show up dramatically at fort peck in fi g. 8 and to a lesser extent at boulder (not shown). snow cover at fort peck in the fall and winter of 1997-1998 produced a nearly ~ 100% negative deviation from the average clear sky values. note the distinct boundaries of the snow effect for the winter of 1996-1997, indicating the continuous presence of snow cover in the region, marked by a rapid increase in the fall and a rapid decrease in the spring. also, note the remarkable difference in snow cover for the winters of 1996-1997 and 1997-1998. obviously, any analysis that involves fig. 6. percent deviation of the difference between satellite and surface uv-b observations, toms minus uvb-1, for all sky conditions. the site is goodwin creek, ms. 304 john deluisi et al. toms observations over snow-covered surfaces will be subjected to large uncertainties. this interesting phenomenon may be a way to depict the presence of signifi cant snow cover over a large region as viewed by a satellite, in contrast to point measurements of surface refl ectance at the surface. the presence of clouds can confound location all sky cloud free sky %bias %sd %bias %sd bondville, il 17.0 30.2 19.2 5.5 boulder, co 10.6 27.9 15.6 4.3 fort peck, mt 0.44 51.1 17.1 5.2 goodwin creek, ms 20.5 21.5 18.1 4.1 bismarck, nd* 24.5 29.3 17.9 4.9 * the uv instrument at bismarck is a solar light uv biometer. table i. comparisons of toms uv-b with surfrad uvb-1 observations of surface uv-b for fi ve stations and for all sky and clear sky conditions. bismarck, north dakota results represent a special case. the snow cover error for the all sky case was not removed from the fort peck and boulder data. fig. 7. same as fi g. 6, but for cloud free sky conditions. the actual snow cover surface area (bare versus snow) interpretation of the points intermediate to the maximum and minimum. however, with the tsi observations, or with the use of surfrad cloud cover information, derived by the long and ackerman (2000) method, it seems possible that an interpretation of the intermediate points (i.e., 305 correspondence between surface uv observations and toms observations partial snow cover, versus complete snow cover) can be quantifi ed. from this limited sample of comparisons, the range of consistency among the network instruments’ observational bias for clear skies is ~ 4%. this value is in the range of the year-to-year broadband instrumentation relative calibration uncertainty that existed during the earlier stages of the surfrad network operations. the consistent ~ 5% standard deviation or less is quite surprising and should be further examined to determine whether it is a characteristic feature of clear sky comparisons over a larger sample of stations. also, it is possible that the method for selecting clear sky data may have some infl uence. nevertheless, the present results suggest that clear sky uv-b transmission may not be so erratic to contend with. it would be useful to extend the present investigation to other stations to confi rm the present results. considering the diffi culties of uv-b monitoring, a station-tostation range of comparison uncertainty, even ~ 5%, is forgivable at the present stages of the developing monitoring networks, and especially in light of the diffi culties of maintaining long term irradiance observations. if the small range of differences holds for other stations, then it might be worthwhile for an organization to establish a global uv-b quality assurance program utilizing the toms observations to help uv-b monitoring station managers feel secure that their observations are acceptable and to help certify their long-term measurements for placement in national and international archives. also, it seems feasible that with the use of a satellite, the observations from remote stations that are not directly tied to advanced calibration laboratories could be compared against stations that are tied to the available laboratories, for example, the boulder cucf, and italy’s lampedusa island station operated by italian scientists (di sarra et al., 2001) where systematic spectroradiometric uv observations, with instruments possessing calibrations tied to the cucf, are being made. a small island station such as lampedusa, 2 ¥ 10 km, is ideal for verifying satellite algorithms because of the uniformity of the surrounding sea surface fig. 8. deviation of the ratio toms/uvb-1 compared to no snow observations (lowest values) at fort peck, mt, showing error effects of snow cover on toms uv-b retrievals. 306 john deluisi et al. albedo that is also included in the satellite’s fi eld of view. the large bias between the satellite and surface observations is puzzling and can not be ignored. possible sources contributing to the large disparity are total ozone error, irradiance collector cosine error, aerosol absorption not included in the toms retrievals, and surface instrument calibration factor errors. even so, in light of these possible factors, it is diffi cult to rationalize that the total error would be as large as that being experienced here. at most, the total contribution should possibly amount to not more than a few to several percent. the toms uv-b retrieval algorithm should be further checked because previous comparisons also indicated that toms is giving higher values than surface observations (e.g. see eck et al., 1995). 4.3. examination of the toms cloud correction scheme a simple analysis of the quality of the toms cloud correction scheme was performed by plotting the ratio of surface uv observations to toms as a function of the toms refl ectivity channel value. the result is shown in fi g. 9. the percent difference value for low refl ectivity is close to the value for clear sky conditions (~ 15%-19%), but as the refl ectivity increases the percent difference appears to change. visually, one could perceive a slight increase in the ratio (an increase of ~ 10%) when approaching the highest refl ectivity conditions. the very highest values in the 90%-100% refl ectivity range are believed to be unreliable. if the ratio increase is real, then it could indicate that the toms cloud correction scheme is slightly under-correcting with increasing cloudiness, i.e. the transmission estimation needs to be lower with increasing cloudiness/reflectivity. or, alternatively, the cosine error of the surface uv instruments is responsible, as increasing cloud cover increases the magnitude of the diffuse sky radiation contribution to the hemispheric irradiance fi eld (lantz et al., 1999). on the basis of results from routine cosine error measurements of uvb-1 instruments, as performed by the cucf, a discrepancy of approximately – 5% could be fig. 9. percent deviation of the ratio toms/uvb-1 observations versus toms refl ectivity observations, suggesting a possible slightly non-linear effect in the toms cloud correction scheme. 307 correspondence between surface uv observations and toms observations expected for completely overcast skies. toms cloud transmission is calculated as a linear process and this assumption might not hold exactly for increasing cloud cover. the scatter in the plotted points makes it diffi cult to defi nitively quantify an increasing tendency, with increasing cloud cover, as seen in the plotted points of fi g. 9. accordingly, the result suggests that the cloud correction scheme and the cosine error of the surface instruments are good candidates for closer scientifi c examination. cloud fraction data as well as solar and uv irradiance data for six stations are now available in the surfrad data archive (augustine et al., 2003). the data from this network should be suitable for testing the two hypotheses. 5. conclusions the analysis presented in this report uses nasa toms uv-b observations as a tool for qualifying scientifi c data derived from surface uv-b networks. results of the present analysis of earlier r-b meter network uvb data that were compared with those of the toms imply that the cause of the unexpected negative trend reported by scotto et al. (1988) is most likely due to erratic calibrations of the r-b meter network instruments and a downward shift in their calibrations occurring around 1979-1980. this shift in calibrations is likely to be the most overwhelming factor responsible for the trend that was reported in the scotto paper. from 1978 to 1990 the r-b meter data show a downward trend while the toms data, starting from 1979 show some variations, but tend towards higher values than the r-b meter from 1986 onward. comparisons of toms uv-b observations with observations by upgraded uv-b instruments operating in a contemporary network revealed a substantial clear sky bias in the range 15%19%, with toms higher. all sky conditions biases are on the same order, but the individual day comparisons are much noisier. the standard deviations of the clear sky comparisons is 4.8% ± 0.7% for all stations. at least part of the large bias is believed to be due to errors related to ignoring small ozone corrections, solar zenith angle corrections, and a possible systematic error in the calibration procedure. errors in the toms uv-b retrieval algorithm need to be considered as well. in spite of the bias, a positive aspect of the comparison suggests that it might be feasible to use the toms as a means to check a variety of uv observations made by monitoring stations world wide since the standard deviations of the comparisons for clear skies is ~ 5%. a few of the stations having direct connections with a qualifi ed calibration/research laboratory might serve as regional primary stations to serve as a reference for the satellite and those stations that the satellite overfl ies. however, it needs to be shown that a larger number of stations will behave in a fashion similar to the fi ve stations examined in the present paper. an endeavor such as this could be mandated by a responsible authority such as the wmo. the toms cloud transmission scheme was examined over the range of clear sky to overcast conditions as determined by examination of the toms refl ectance channel. there appears to be a slight increase in the cloud-corrected toms uv-b determinations with increasing cloudiness, but the data are noisy. more work is required to reach a reliable conclusion. it is suggested that the noaa surfrad network observations of uv-b and cloud cover fraction might be suitable for further testing of the toms cloud correction scheme. the atmospheric modeling of radiation experiment (amore) project (see usda web site) that was conducted at boulder, colorado, this past summer will compare theoretical calculations of uv-b with observations made by several uv-b radiometers ranging in technological complexity from high precision double monochromators to single broadband sensors. it is envisioned that this project will help resolve some current issues involving causes for the differences seen between modeled and measured uv-b. acknowledgements support for this investigation was provided by the noaa offi ce of global programs, project # gc97-263, and the national aeronautics and space administration project # s-67815-z. the 308 john deluisi et al. kind assistance received from drs. p.k. bhartia, j. herman, and r. mcpeters of nasa, goddard is deeply appreciated. references augustine, j.a., j.j. deluisi and c.n. long (2000): surfrad – a national surface radiation budget network for atmospheric research, bull. amer. meteorol. soc., 10, 2341-2356. augustine, j.a., c.r. cornwall, g.b. hodges, c.n. long, c.i. medina and j.j. deluisi (2003): an automated method for mfrsr calibration for aerosol optical depth analysis with application to an asian dust outheak over the united states, j. appl. meteorol., 42 (2), 266-278. berger, d.s. 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(1991): solar ultraviolet radiation effects on biological systems, phys. med. biol., 36, 299-328. di sarra, a., t. di iorio, m. cacciani, g. fiocco and d. fuà (2001): saharan dust profi les measured by lidar at lampedusa, j. geophys. res., 106 (10), 10,33510,348. early, e., a. thompson, c. johnson, j.j. deluisi, p. disterhoft, d. wardle, e. wu, w. mou, y. sun, t. lucas, t. mestechkina, l. harrison, j. berndt and d.s. hayes (1998): the 1995 north american interagency intercomparison of ultraviolet monitoring spectroradiometers, j. res. nat. inst. stand. technol., 103 (1), 15-61. eck, t.f., p.k. bhartia and j. kerr (1995): satellite estimation of spectral uv-b irradiance using toms derived total ozone and uv refl ectivity, geophys. res. lett., 22, 611-614. flowers, e.c., r.a. mccormick and k.r. kurfis (1969): atmospheric turbidity over the united states, 19611966, j. appl. meteorol., 8, 955-962. herman, j., p.k. bhartia, j. ziemke, z. ahmad and d. larko (1996): uv-b radiation increases from decreases in total ozone, geophys. res. lett., 23, 2117-2120. hicks, b.b., j.j. deluisi and d.r. matt (1996): the noaa integrated surface irradiance study (isis) – a new surface radiation monitoring network, bull. amer. meteorol. soc., 77, 2857-2864. kalliskota, s., j. kaurola, p. taalas, j.r. herman, e.a. celarier and n.a. krotkov (2000): comparison of daily uv doses estimated from nimbus 7/toms measurements and ground-based spectroradiometric data, j. geophys, res., 105, 5059-5067. kaye, j.a., b.b. hicks, e.c. weatherhead, c.s. long and j. slusser (1999): u.s. uv interagency monitoring program established and operating, eos, trans. amer. geophys. union, 80, 113-116. krotkov, n.a., p.k. bhartia, j. herman, v. fioletov and j. kerr (1998): satellite estimation of spectral surface uv irradiance in the presence of tropospheric aerosols, 1, cloud-free case, j. geophys. res., 103, 8779-8794. lantz, k.o., p. disterhoft, j.j. deluisi, e. early, a. thompson, d. bigelow and j. slusser (1999): methodology for deriving clear-sky erythema calibration factors for uv broadband radiometers of the u.s. central uv calibration facility, j. geophys. res., 16, 1736-1752. long, c.n. and t.p. ackerman (2000): identification of clear skies from broadband pyranometer measurements and calculation of downwelling shortwave cloud effects, j. geophys. res., 105 (d12), 15,609-15,626. long, c.s., a.j. miller, h.-t. lee, j.d. wild, r.c. przywarty and d. hufford (1996): ultraviolet index forecasts issued by the national weather service, bull. amer. meteorol. soc., 77, 729-748. mckenzie, r.l., k.j. paulin, g.e. bodeker and a.p. sturman (1998): cloud cover measured by satellite and from the ground: relationship to uv radiation at the surface, int. j. remote sens., 19, 2969-2985. robertson, d.f. (1972): solar ultraviolet radiation in relation to human sunburn and skin cancer, ph.d. dissertation, dept. physics, university of queensland, pp. 131. schafer, j.s., v.k. saxena, b.n. wenny, w. barnard and j.j. deluisi (1996): observed infl uence of clouds on ultraviolet-b radiation, geophys. res. lett., 23, 2625-2628. scotto, j., g. cotton, f. urbach, d. berger and t. fears (1988): biologically effective ultraviolet radiation: surface measurements in the united states, 1974 to 1985, science, 239,762-764. we a t h e r h e a d, e.c., g.c. ti a o, c. re i n s e l, j.e. frederick, j.j. deluisi, d. choi and w. tam (1997): analysis of long-term behavior of ultraviolet radiation measured by robertson-berger meters at 14 sites in the united states, j. geophys. res., 102, 8737-8754. wenny, b.n., j.s. schafer, j.j. deluisi, v.k. saxena, w.f. barnard, i.v. petropavlovskikh and a.j. vergamini (1998): a study of regional aerosol radiative properties and effects on unltraviolet-b radiation, j. geophys. res., 103, 17,083-17,097. preface 11.pdf annals of geophysics, vol. 46, n. 1, february 2003 v preface: the sintex project the project called «decadal and interdecadal climate variability: scales interactions experiments (sintex)» was conceived in 1997, stemming from a series of discussions and meetings within the euroclivar initiative, a concerted action of the iv framework program of the european union that involved most of the major european laboratories active in the field of climate simulations. the original idea was to address the issue of how different scales of phenomena interact with each other in determining the characteristics of the earth’s climate variability. it was identified early that the interannual time scales act as a pivot, allowing strong interaction towards the longer decadal time scales and at the same time fig. 1. the interactions between atmosphere and oceans in the tropics dominate the variability at interannual scales. wavetrains of anomaly stem from the pacific into the mid-latitudes, as the pacific north american pattern (pna). the tropics are connected through the pacific sst influence on the indian ocean sst and the monsoon, sahel and nordeste precipitation. the monsoon area is also connected to the mediterranean region. also shown is the north atlantic oscillation a major mode of variability in the euro-atlantic sector whose coupled nature is still under investigation. vi antonio navarra intensely interacting with the intraseasonal variability at the other end of the time spectrum. simulating this variability is crucial to several large applications that require an accurate and timely representation of the web of interactions among time scales (fig. 1). seasonal forecasts are an example of how the success of the simulations is influenced by the interactions between intraseasonal variability and the slower interannual variability. full identification of the domain of predictability that we can expect given the strongly turbulent nature of the interactions is a still matter of investigation. climate scenarios represent another case in which the effect of rising concentrations of greenhouse gases will not result in a slow adjustment of the mean, but rather in a remodulation and modification of the space-time probability distributions of the interannual variability statistics. a realistic simulation of the interannual variability can therefore be considered a prerequisite, a prevalidation parameter, for models graduating to the simulation of future climate scenarios. building on the discussions during and after the euroclivar initiative, a group of investigators started to devise a more concrete plan, implementing a program of numerical fig. 2. the partners of sintex in the original slide. imga-cnr, italy; nec supercomputing, germany; lodyc, france; hadley center, u.k.; nansen center, norway; danish meteorological institute, denmark; cineca, italy; knmi, holland; universidad de alcalá, spain; university of reading, u.k.; laboratoire de météorologie dynamique, france; max-planck institute für meteorologie, germany. vii preface: the sintex project experimentations and observational studies. the plan evolved into a proposal and then a project: sintex. the first draft of the project was circulated among the proponents (fig. 2) in 1998 and after much discussion and rewriting was submitted for funding to the iv framework program after a few months. the proposal was successful and the activity started in the spring of 1998. the sintex principles were then set in stone (fig. 3). during the preparatory work for sintex we reversed the method for the design of experiments that has been common in the field of climate numerical simulation for some time. rather than adapt our scientific aims to the available computing resources, we considered the computers as experimental machines to be obtained for doing the kind of science we had in mind. we went through the exercise to assess what level of computation was necessary to perform our scientific plan and then aim for the minimum level of equipment necessary to reach that goal. the main idea of the project was to make available a number of numerical experiments specifically designed to study the intraseasonal, interannual and decadal variability. after much discussion, a consensus was reached that a length of about 200 years was a desirable and sufficient length to allow a comfortable evaluation of the interactions. the project was then made possible thanks to the collaboration of the nec supercomputing company that fig. 3. the original slide of the sintex objectives (1997). viii antonio navarra entered the project as a full partner, providing the required computing capability for the duration of the project and software engineering and optimization expertise. in this way, we achieved a high concentration of computing resources per individual scientist at a very moderate cost. several models were then identified to implement the numerical experiments: the hadley center unified model, the echam4-opyc coupled model, and a new coupled model based on the untried combination of existing atmospheric and oceanic components. in the end, sintex used for the analysis a combination of specifically devised experiments, available experiments and observations. the new coupled model, soon to be known as the sintex model, was based on the atmospheric echam4 model and the ocean model orca, coupled with the oasis coupler. the objective was to aim for a model that was capable of a realistic interannual variability without flux correction. the model was created by a concurrent multi-institutional effort distributed in space, as the groups responsible were divided between italy and france, working on german and french models on a japanese computer located and operated by our danish partner in denmark. amazingly, the logistics worked very well, even if everything was done over the standard science communication network in europe, but obviously our lives would have been a lot easier with faster communication lines. there are a few general points that sintex has contributed to the general scientific community. the first is that sintex has shown that an approach where a scientific goal is first identified and then a particular computing system designed to achive it is possible. after all, this is how von neumann, charney and friends designed their first experiments in princeton in the late 1940s. the approach is not new to meteorology, but it was somewhat overlooked in recent years. second, sintex showed that resolution is also an issue in climate length experiments and significant changes in resolution, in this case horizontal resolution, can lead to different and new physics. we have just started to realize the impact of this point and as the analysis of high resolution experiments progresses it probably will become clearer. it is not a magic wand, problems do not disappear into tiny powdery clouds, but qualitatively different things happen. last, but not least, sintex showed the feasibility of having several research groups working together in a complex multinstitutional, multinational context. the trick here for the success of this model was to concentrate computing resources, and disperse human capital. not everybody may agree, but our experience was that it is easier to network humans than computers. this special issue does not contain all the sintex results. sintex generated more than 20 papers that have been published through the main scientific literature, but we felt that at the end of the project it was a useful idea to have an easy to reach reference that would give a flavor of sintex and at the same time point to the vast wealth of scientific results produced by sintex that could not be contained in a single special issue. the papers published here will give in some cases reference or baseline presentation of sintex activities. hopefully, the issue will be useful to the reader as a guide to the whole of sintex. ix preface: the sintex project sintex has been an exciting and demanding professional and personal experience for several years, and it would not have been possible without the support and friendship of all the partners in the project. it is a pleasure to acknowledge here the contributions of pascale delecluse, julia slingo, leif larsen, mark rodwell, rowan sutton, mojib latif, laurent li, mariá ortiz, claude frankignoul, toine beckers, silvio gualdi, eric guilyardi, susanna corti, and all the students and post-graduates who contributed to sintex. hopefully, most of them learnead something useful, someone even learned to ski. a major part of the science business is to identify the right questions, good directions that let us learn a lot of new things. we can say that in sintex we have attacked several of the most relevant problems of climate dynamics today, made good advances in some cases, smaller steps in others, but we have solved none. this is science and this is how it progresses from one generation of scientists to the next. the individual results are in the sintex papers to be judged. antonio navarra coordinator of sintex istituto nazionale di geofisica e vulcanologia, bologna, italy t h e l a p l a c e t r a n s f o r m s o l u t i o n o f a o n e d i m e n s i o n a l g r o u n d w a t e r r e c h a r g e bv s p r e a d i n g a . p . verma (*) r i c e v u t o il 21 g e n n a i o 19(>9 r i a s s u n t o . — u n ' e s p r e s s i o n e a n a l i t i c a p e r la d i s t r i b u z i o n e d e l c o n t e n u t o di u m i d i t à , in u n p r o b l e m a di carico u n i d i m e n s i o n a l e v e r t i c a l e di a c q u a s o t t e r r a n e a , è s t a t a o t t e n u t a m e d i a n t e il m e t o d o di t r a s f o r m a z i o n e di l a p l a c e . il c o e f f i c i e n t e m e d i o di d i f f u s i b i l i t à , c a l c o l a t o sull'intera fiamm a di v a l o r i di u m i d i t à c o n t e n u t a si c o n s i d e r a c o m e c o s t a n t e , m e n t r e si a s s u m e u n a v a r i a z i o n e lineare di p e r m e a b i l i t à c o n c o n t e n u t o di u m i d i t à . s u m m a r y . — a n a n a l y t i c a l e x p r e s s i o n for t h e m o i s t u r e c o n t e n t distrib u t i o n , in a p r o b l e m of o n e d i m e n s i o n a l v e r t i c a l g r o u n d w a t e r recharge, h a s been o b t a i n e d b y u s i n g t h e l a p l a c e t r a n s f o r m m e t h o d . t h e a v e r a g e diff u s i v i t v c o e f f i c i e n t o v e r t h e w h o l e r a n g e of m o i s t u r e c o n t e n t is regarded as c o n s t a n t , a n d a linear v a r i a t i o n of p e r m e a b i l i t y w i t h m o i s t u r e c o n t e n t is a s s u m e d . 1 . i n t r o d u c t i o n . r e c e n t l y k l u t e i 1 2 ) a n d s a r m a (:1) h a v e d i s c u s s e d t h e n u m e r i c a l m e t h o d s of s o l u t i o n f o r t h e f l o w of w a t e r i n p a r t i a l l y s a t u r a t e d p o r o u s m e d i a . i n t h e p r e s e n t p a p e r w e h a v e o b t a i n e d a n a n a l y t i c a l s o l u t i o n of a o n e d i m e n s i o n a l p r o b l e m of g r o u n d w a t e r r e c h a r g e b y s p r e a d i n g . w e c o n s i d e r h e r e t h a t t h e r e c h a r g e t a k e s p l a c e o v e r a l a r g e b a s i n of s u c h g e o l o g i c a l l o c a t i o n t h a t t h e s i d e s a r e l i m i t e d b y r i g i d b o u n d a r i e s , a n d t h e b o t t o m b y a t h i c k l a y e r of w a t e r t a b l e . u n d e r t h e s e c i r c u m s t a n c e s , w a t e r , f r o m t h e s p r e a d i n g g r o u n d s , w i l l f l o w v e r t i c a l l y (*) d e p a r t m e n t of m a t h e m a t i c s , f a c u l t y of t e c h n o l o g y a n d e n g i n e e r ing, m. s. u n i v e r s i t y of b a r o d a , b a r o d a , i n d i a . :hj a . p . v e r m a d o w n w a r d s t h r o u g h t h e u n s a t u r a t e d p o r o u s m e d i a . i t is a s s u m e d t h a t t h e d i f f u s i v i t y coefficient is e q u i v a l e n t t o its a v e r a g e v a l u e o v e r t h e whole r a n g e of m o i s t u r e c o n t e n t , a n d t h e p e r m e a b i l i t y of t h e m e d i a is a c o n t i n u o u s l i n e a r f u n c t i o n of t h e m o i s t u r e c o n t e n t . t h e n o n l i n e a r p a r t i a l d i f f e r e n t i a l e q u a t i o n f o r m o i s t u r e c o n t e n t h a s b e e n solved b y e m p l o y i n g t h e l a p l a c e t r a n s f o r m t e c h n i q u e , a n d a n a n a l y t i c a l e x p r e s s i o n f o r m o i s t u r e d i s t r i b u t i o n is g i v e n . 2 . f o r m u l a t i o n of t h e b o u n d a r y v a l u e problem. f o l l o w i n g k l u t e (*), we m a y w r i t e t h e f u n d a m e n t a l e q u a t i o n s as below. t h e e q u a t i o n of c o n t i n u i t y f o r a n u n s a t u r a t e d m e d i u m is given b y i m ) = v m , [2.11 w h e r e q, is t h e b u l k d e n s i t y of t h e m e d i u m , 0 is i t s m o i s t u r e c o n t e n t on a d r y w e i g h t basis, a n d m is t h e m a s s f l u x of m o i s t u r e . d a r c y ' s l a w g o v e r n i n g t h e m o t i o n of w a t e r hi a p o r o u s m e d i u m m a y b e w r i t t e n a s : v = —kv<f>, [ 2 . 2 ] w h e r e r e p r e s e n t s t h e g r a d i e n t of t h e t o t a l m o i s t u r e p o t e n t i a l , v t h e v o l u m e flux of m o i s t u r e , a n d k t h e coefficient of a q u e o u s c o n d u c t i v i t y . f r o m e q u a t i o n s [2.1] a n d [2.2], we g e t : * (q.0) = v • (qkv(/>) , [2.3] w h e r e o is t h e fluid d e n s i t y . since, in t h e p r e s e n t p r o b l e m , flow t a k e s p l a c e only i n t h e v e r t i c a l d i r e c t i o n , t h e r e f o r e e q u a t i o n [2.3] r e d u c e s t o : w 3 / \ s m = * r * ) ~ * { e k g ) [ 2 4 ] w h e r e y> is t h e p r e s s u r e (capillary) p o t e n t i a l , g is t h e g r a v i t a t i o n c o n t e n t , a n d (j> — — gz. t h e positive d i r e c t i o n of z axis is t h e s a m e as t h a t of t h e g r a v i t y . t h e l a p l a c e t r a n s f o r m s o l u t i o n o f o n e d i m e n s i o n a l , e t c . 2 7 c o n s i d e r i n g 0 a n d ip t o b e c o n n e c t e d b y a single v a l u e d f u n c t i o n , we m a y w r i t e e q u a t i o n [4] a s : w 3 / w \ 0 j a ' m = ^ r ~dz) + ¿ ; g * z ' [ 2 0 ] w h e r e d — k \ , a n d is called t h e d i f f u s i v i t y coefficient. a» ¡>0 ' j r e p l a c i n g d b y i t s a v e r a g e v a l u e d„, a n d a s s u m i n g k = ko0, ko = ' 2 3 2 (as i n [3j a n d [1]), we h a v e : = l > „ — • k o — = . 2 . ( > i t m 2 qs c o n s i d e r i n g t h e w a t e r t a b l e t o b e s i t u a t e d a t a d e p t h l, a n d p u t t i n g : = | t d " . = t l l2 ' we m a y w r i t e t h e b o u n d a r y v a l u e p r o b l e m a s : j > 0 q k o <>0 i>t p. da ôf ' [2.7] 0 ( 0 , 2 ' ) = 0„, 0(1,t) = 1 , [ 2 . 8 ] 0(1,0) = 0, [2.9] w h e r e t h e m o i s t u r e c o n t e n t t h r o u g h o u t t h e region is zero i n i t i a l l y , a t t h e l a y e r z = 0 i t is 0o, a n d a t t h e w a t e r t a b l e (z = l) i t is a s s u m e d t o r e m a i n 1 0 0 % t h r o u g h o u t t h e p r o c e s s of i n v e s t i g a t i o n . i t m a y b e r e m a r k e d t h a t t h e effect of c a p i l l a r y a c t i o n a t t h e s t a t i o n a r y g r o u n d w a t e r level, b e i n g small, is n e g l e c t e d . 3 . a n a l y t i c a l s o l u t i o n . s e t t i n g e k ° _ » qs d„ p in e q u a t i o n [2.7], we g e t : w **0 ft 00 r11 :hj a . p . v e r m a o n m u l t i p l y i n g e a c h t e r m of e q u a t i o n [3. j j b y e~stdt, i n t e g r a t i n g t h e r e s u l t f r o m z e r o t o i n f i n i t y , a n d u s i n g c o n d i t i o n [2.9], w e o b t a i n d*6 db dp p d£ 86 = 0. [3.2] w h e r e 0(£,s) = i e-sr d(£,t) dt, r e p r e s e n t s t h e l a p l a c e t r a n s f o r m of 0(i,t). tlie l a p l a c e t r a n s f o r m a t i o n of t h e b o u n d a r y c o n d i t i o n s [2.8] y i e l d s 0(0,8) oo 8 6(1,8) = 8 [3.3] s i n c e e q u a t i o n [3.2] is a l i n e a r e q u a t i o n w i t h c o n s t a n t c o e f f i c i e n t , we m a y w r i t e i t s g e n e r a l s o l u t i o n a s : 0(ç,s) = e c o s h (f j//?»/4 + 8) + f s i n h (f j / p * ] i +8) p e2 , [3.4] w h e r e e a n d f a r e c o n s t a n t s of i n t e g r a t i o n . f o r e v a l u a t i n g e a n d f, w e a p p l y c o n d i t i o n s [3.3] t o e q u a t i o n [3.4], so t h a t , a f t e r s o m e s i m p l i f i c a t i o n , w e h a v e : p s io_ 8 c o s h (l /?2m + 8 ) sinh (j//? z /4 + # ) s u b s t i t u t i n g t h e s e v a l u e s i n e q u a t i o n [3.4], w e h a v e : 0 ( f , 0 ) = e •{ ( do 8 sinh (1 — i ) + 8 s i n h j//5 2 /4 + 8 ~ + + e w s i n h f j ' ^ / l + 8 8 s i n h | > / 4 + 8 [3.5] t h e l a p l a c e t r a n s f o r m s o l u t i o n o f o n e d i m e n s i o n a l , e t c . 2 9 t h e i n v e r s e t r a n s f o r m (l~l) of t h e r i g h t h a n d side t e r m s i n e q u a t i o n [3.5] m a y b e d e t e r m i n e d b y r e c a l l i n g a s t a n d a r d r e s u l t [4] viz.. l 1 m r,(8) = x c(<s'n) esvt -o nhs.) [3.6] w h e r e £(<s') a n d r](is') r e p r e s e n t s t w o e n t i r e t r a n s c e n d e n t a l f u n c t i o n s s u c h t h a t d e g r e e of rj{is) is a t l e a s t o n e g r e a t e r in 8 (when e x p r e s s e d £ (8) as p o w e r series) t h a n t h a t of £($), 8 n is a s i m p l e p o l e of b — , a n d v («) i f (sn) d e n o t e s t h e v a l u e of drj(s) d 8 a t s = sn. p u t t i n g : c (8) sinh (f j / > / 4 + 8) s i n h ( i f f > / 4 + £ ) v ('s>) 8 s i n h (yp'li +s) 8 sinh (t j//j»/4 + , [3.7] a n d n o t i n g t h a t t h e r o o t s of e q u a t i o n sinh ( | ' p / i + s ) = 0 , a r e g i v e n b y = — /s2/4 — n* tt2, we m a y w r i t e : *(sn) = sin (n,3t(), c(0) = i sinh ' £ = ^i(o) = i s i n h (/?/2) . [3.8] f r o m e q u a t i o n s [3.6], [3.7] a n d [3.8], we g e t : sinh & l-1 + 271 2 n = l sinh f | /32/4 + <s' « sinh j//s2/4 + s 2 s i n h 0/2 + (— 1)" il sin (« 7t i) (/32/4 + n» jr»)t [3.9] : h j a . p . v e r m a s i m i l a r l y , we h a v e : l 1 p sinh (1 — i) | /?2/4 + s sinh (1 — f ) .v s i n h | /?2/-l +»s' s i n h /3/2 £ » b i n h ( n a g ) ~ (/32/4 + m2 tt2) 2' " „ i £ 2 / 4 + [3.10] t h e i n v e r s e t r a n s f o r m a t i o n of e q u a t i o n [3.5] w i t h t h e h e l p of e q u a t i o n [3.9] a n d [3.10], yields: oo e 2 sinh f f + sinh /î/2 ; (— 1)i » n s i n h (n j r | ] ^ (/92/4 + m2 ti2) t „ _ ! /52/4 + w27t2 0 + e (1 f ) sinh (1 f ) sinh /?/2 f , m sin (wtif) ( / 5 2 / 4 + vzjt2) t 9,7t >i b _ , /? 2/4 + 11*71' [3.11] t h i s is t h e desired a n a l y t i c a l e x p r e s s i o n f o r t h e m o i s t u r e c o n t e n t d i s t r i b u t i o n . i t follows i m m e d i a t e l y f r o m e q u a t i o n [3.11] t h a t t h e g r a p h of t h e m o i s t u r e c o n t e n t v e r s u s d i s t a n c e (for g i v e n v a l u e s of t i m e , s a y t = 1, 4, 16 etc.) m a y b e easily d r a w n (as i n [1]). a n u m e r i c a l i l l u s t r a t i o n is e q u a l l y o b v i o u s . h o w e v e r , t h e s e a r e n o t i n c l u d e d h e r e d u e t o o u r p a r t i c u l a r i n t e r e s t i n o n l y a n a n a l y t i c a l s o l u t i o n . 4. conclusion. a n a n a l y t i c a l s o l u t i o n f o r t h e n o n l i n e a r d i f f e r e n t i a l e q u a t i o n g o v e r n i n g m o i s t u r e c o n t e n t d i s t r i b u t i o n lias b e e n o b t a i n e d , b y u s i n g l a p l a c e t r a n s f o r m m e t h o d , f o r t h o s e cases of g r o u n d w a t e r r e c h a r g e b y s p r e a d i n g w h e r e t h e flow is e s s e n t i a l l y one d i m e n s i o n a l , a n d in t h e v e r t i c a l d i r e c t i o n . t h o u g h no n u m e r i c a l i l l u s t r a t i o n is i n c l u d e d i n t h e p r e s e n t p a p e r (because of our p a r t i c u l a r i n t e r e s t ) y e t t h e c o n v e n i e n t f o r m of t h e m o i s t u r e c o n t e n t e x p r e s s i o n is i m m e d i a t e l y e v i d e n t . t h e l a p l a c e t r a n s f o r m s o l u t i o n o f o n e d i m e n s i o n a l , e t c . 3 1 r e f e r e n c e s ( 1 ) k l u t e a . , a numerical method for solving the flow equation for water in unsaturated materials. " s o i l s c i e n c e , , 73, 2, 105, (1952). ( 2 ) k l u t e a . , some theoretical aspects of the flow of water in unsaturated soils. " proc. soil science s o c i e t y " , 16, 2, 144. (1952). ( 3 ) s a r m a s . v . k . , problem of partially saturated unsteady state of flow through porous media with special reference to groundwater recharge by spreading. " j . sci. e n g g . r e s e a r c h " , i x . 1, 09, (1965). ( 4 ) m i c k l e y h . s . , s h e r w o o d t . k . , r e e d c . e . , applied mathematics in chemical engineering. .me g r a w i l i l l , 29(i, n e w y o r k , 1957. n o m e n c l a t u r e d = d i f f u s i v i t y coefflcient. (cm2 s e c 1 ) i>„ = a v e r a g e v a l u e of t h e d i f f u s i v i t y coefficient over t h e whole r a n g e of m o i s t u r e c o n t e n t (cm2 sec1) k0 = slope of t h e p e r m e a b i l i t y vs m o i s t u r e c o n t e n t plot (cm s e c 1 ) k = p e r m e a b i l i t y coefflcient (cm s e c 1 ) l = d e p t h of p e r m e a b l e s t r a t u m (cm) m = m a s s flux of m o i s t u r e (gm) t = t i m e (sec) t = t i m e (dimensionless) v = v e l o c i t y of flow of w a t e r (cm s e c 1 ) 7j = d e p t h of p e n e t r a t i o n of w a t e r a t a n y i n s t a n t t (cm) ß = a flow p a r a m e t e r (cm2) f = p e n e t r a t i o n d e p t h (dimensionless) ft = t o t a l p r e s s u r e p o t e n t i a l (cm s e c 2 ) ip = c a p i l l a r y p o t e n t i a l (cm s e c 2 ) g = a c c e l e r a t i o n d u e t o g r a v i t y (cm s e c 2 ) o = m a s s d e n s i t y of w a t e r (gm) qs = b u l k d e n s i t y of t h e m e d i u m on d r y w e i g h t basis (gm cm 3) oo = m o i s t u r e c o n t e n t a t z 0 f o r all t i m e ( g m / g m ) 0 — m o i s t u r e c o n t e n t a t a n y d e p t h z (gm g m 1 ) v — v e c t o r o p e r a t o r = i + j + a {), j, k a r e 7>x 7)// 7>z u n i t v e c t o r s ) geo 1131 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 12 behaviour of the f1-region, and es and spread-f phenomena at european middle latitudes, particularly under geomagnetic storm conditions pal bencze (1), dalia burešová (2), jan laštovička (2) and ferenc märcz (1) (1) geodetic and geophysical research institute, hungarian academy of sciences, sopron, hungary (2) institute of atmospheric physics, academy of sciences of the czech republic, prague, czech republic knowledge of the ionospheric electron density distribution and its fluctuations is essential for predicting ionospheric characteristics for radio wave propagation and for other applications such as satellite tracking, navigation, etc. geomagnetic storm is the most important source of the ionisation density perturbations. recent studies of the f1-region electron density distribution revealed systematic seasonal and latitudinal differences in the f1-layer response to geomagnetic storm. at european higher middle latitudes no significant effect has been observed in summer and spring at heights of 160-190 km, whereas well-pronounced depression appears in winter and late autumn at least at 180190 km. a brief interpretation of this finding will be presented. on the other hand, the pattern of the response of the ionosphere at f1-layer heights does not seem to depend on the type of response of f2-layer (fof2) or on solar activity. concerning the main types of ionospheric irregularities sporadic e and spread-f, it has been found that considering sporadic e-layers as thin diffraction screen, it may be modelled for propagation of radio-waves by the determination of the temporal variation of foes representing in ionograms the mean ion density of «patches» of increased ion density embedded in the es-layer. spectrum of these variations indicates the mean period of the variations, which multiplied by the wind velocity gives the mean distance of patches, that is, the mean distance between the screen points. in case of spread-f, it has been found that irregularities causing spread-f are mostly due to plasma instabilities, though the role of travelling ionospheric disturbances may be not entirely neglected. 12.1. introduction the f1-region and its response to geomagnetic storm-induced disturbances has been studied much less than the f2and e-regions and their response to geomagnetic storms, partially due to lower importance of the f1-region for ionospheric propagation of radio waves and also due to substantially weaker geomagnetic storm effects on the f1-region electron density compared with those in the f2-region (above 200 km) and in the lower ionosphere (below about 100 km). for instance, the review papers by buonsanto (1999) and prölss (1995) did not deal with storm effects on the f1-region at all. laštovička (1996) reviewed in brief geomagnetic storm effects on the lower ionosphere, middle atmosphere and troposphere. 1132 pal bencze, dalia burešová, jan laštovička and ferenc märcz however, during geomagnetic storms the f1-region becomes more important for the ionospheric radio wave communications, particularly under the so-called g-conditions, when the substantially reduced f2-region is screened by the f1-region and the latter serves as the radio wave reflecting layer. then the f1-region determines the radio wave propagation conditions. this is the main reason we studied the response of the f1-region to geomagnetic storms in cost 271. concerning the study of ionospheric irregularities and their modeling, the aim was to support understanding of anomalies found in relation to terrestrial and transionospheric propagation of radio waves. the trend of development in telecommunication by radio waves is the reduction of transmitter power and the decrease of the wavelength of radio waves. knowing the characteristics of the propagation medium, i.e. those of the ionosphere and plasmasphere, both efforts led to a decrease of the signal to noise ratio. ionospheric irregularities were not very significant, till the wavelength of radio waves was much larger than the extent of ionospheric irregularities of about 100 m. nowadays radio waves in the wavelength band of 0.1 m and more are used in telecommunication. we may generally distinguish two main types of ionospheric irregularities, sporadic e-layers and the spread-f phenomenon. both types have been known for a long time due to the vertical sounding of the ionosphere. now experiences connected to propagation of vhf and uhf waves necessitate the detailed study of these irregularities. 12.2. geomagnetic storm effects on the f1-region electron density ionisation at the f1-region heights is closely related to neutral composition. changes in the neutral atmosphere ionisation and photochemical processes play important roles in the f1-region electron density distribution due to the shorter lifetime of free electrons compared with the f2-region. rishbeth and garriott (1969) described the photochemical processes in the bottomside ionosphere and placed the transition height between the region dominated by molecular ions (no+ and o2+) and the region where the atomic ions o+ dominate at about 160-200 km. significant progress in the analysis of the f1-region behaviour under geomagnetic storm conditions has been made in the last few years within the cost 271 project. burešová and laštovička (2001), burešová et al. (2002a) and bencze et al. (2002) analysed the effects of a few geomagnetic storms in electron density at f1 heights during daytime, based on data from several european ionosondes. mikhailov and schlegel (2003) attempted to systemize the geomagnetic storm effects on the f1-region electron density and gave a physical interpretation to the revealed electron density profile variations using millstone hill (middle latitude) and eiscat (auroral zone) incoherent scatter (is) daytime measurements. 12.2.1. seasonal and latitudinal dependence of the geomagnetic storm effects an investigation of effects of 35 selected strong geomagnetic storms on the ionosphere at f1 heights over europe showed that, in general, independent of the sign of the geomagnetic storm effect on the f2-layer ( fof2), the effect on electron density at the f1 heights at european high middle latitudes has always been negative, if any at all. another important finding of our studies is the summer/spring versus winter/autumn difference in the effect of strong geomagnetic storms on the daytime f1-region, i.e. an insignificant to moderate effect in summer and spring, respectively, versus an evident decrease of daytime electron density in winter/autumn at higher middle latitudes (burešová and laštovička, 2001; burešová et al., 2002a), as illustrated in fig. 12.1. there is no detectable effect of geomagnetic storm on electron density at f1 heights in the range 160-190 km during summer except for a moderate effect of the super storm of july 2000 at 180-190 km (burešová, 2002; burešová et al., 2002b,c). on the other hand, in winter there is a substantial effect on electron density at 190 1133 f1-region, es and spread-f at european middle latitudes fig. 12.1. the magnitude of the mean electron density decrease at the fixed f1-region heights (the difference between pre-storm quiet days and the storm main phase at 11:00-13:00 ut) for all analysed 35 geomagnetic storms, which occurred during different seasons, for chilton. fig. 12.2. the magnitude of the electron density decrease at the f1-region heights (the difference between prestorm quiet days and the storm main phase at 11:00-13:00 ut) for the storm of february 1998 for warsaw, pruhonice, ebro and arenosillo. 1134 pal bencze, dalia burešová, jan laštovička and ferenc märcz km, there is still a well-detectable effect at 180 km, a weak effect at 170 km and no detectable effect at 160 km except for the super storm of march-april 2001, which caused substantial changes down to 160 km. the winter effect consists in a significant depletion of electron density during the main phase of the storm. winter in this sense means probably autumn and winter, i.e. months with shorter sunlit periods and higher solar zenith angles. observations suggest that the upward motion of the transition height might be an important mechanism responsible for the effects of the geomagnetic storms at the f1-region heights (the electron loss rate is considerably larger below than above the transition height). using the msis-e-90 model burešová and laštovička (2003c) for chilton in 1998 found the transition height to be located above 200 km in mid-summer and for the rest of the year below 200 km, well below 200 km in winter. this qualitatively explains the observed summer-winter difference. the absence of a remarkable effect in summer appears to be due to small compositional change in summer, because even the quiet-time transition boundary is above the f1-region. in summer the regular (solar-induced) and storm-induced meridional winds coincide and the gases with decreased o/n2 ratio are moved from the high latitudes into middle latitudes. the wintertime background circulation does not allow propagation of compositional disturbance from high latitudes well to lower latitudes and thus diminishes or stops the upward motion of the transition boundary. this is probably the reason we sometimes observe substantial geomagnetic storm effects on the f1region at higher middle latitudes but not at ebro and arenosillo (burešová et al., 2002b). figure 12.2 illustrates the decrease in f1-region electron density during february 1998 storm main phase for selected european stations. in general, the f1-region response to geomagnetic storm-induced disturbances at european lower middle latitude is weaker and less regular. 12.2.2. dependence of the effects on solar activity burešová and laštovička (2001) analysed the effect of geomagnetic storms on the f1-region electron density only for events observed under low solar activity conditions. recently several events observed under higher and high solar activity conditions have been examined (burešová et al., 2002a; burešová, 2002; burešová and laštovička 2003a,c). the pattern of the geomagnetic storm effects on the f1-region electron density does not appear to change with solar activity (solar cycle). 12.2.3. f1-region under superstorm conditions according to king (1961), the depth of the storm-induced disturbances penetration into the f1-region depends on the magnitude of the geomagnetic storm, but it also depends on the latitude and season. we found the geomagnetic super storms penetrate deeper into the f1-region than the effects of analysed strong storms, as documented by fig. 12.3 (burešová et al., 2002a). one process, which probably contributes to the deeper penetration of the super storm effects, is the equatorward shift of the auroral zone, which is larger for these super storms. for instance, during the super storm of february 1986, the pruhonice station was for a couple of hours in the auroral zone (boška and pancheva, 1989). 12.2.4. f1-region intra-hour variability 15 min sequences of ionograms recorded during the storms of may and november 1997 and september-october 2002 have been used to investigate the bottomside f-region intra-hour variability. figure 12.4a-c shows the course of the absolute value of the differences between hourly nmf2 and every 15 min nmf2 obtained during each hour september-october 2002 event. for quiet ionospher1135 f1-region, es and spread-f at european middle latitudes fig. 12.3. the magnitude of the electron density decrease at the f1-region heights (the difference between prestorm quiet days and the storm main phase at 11:00-13:00 ut) for the super storm of july 2000 versus the may 1997, june 1998, august 1998 strong storms for pruhonice (top panel) and for the super storm of march/april 2001 versus the strong storms of january 1996, february 1998, march 1998 and november 1998 (middle panel – pruhonice; bottom panel – ebro). full columns – super storms; dashed columns – strong storms. fig. 12.4. storm of september-october 2002: a) course of the hourly dst index; b) course of the differendes between hourly nmf2 and every 15 min nmf2 obtained during each hour, (c) the same for electron density at 190 km. a b c fig. 12.5. the magnitude of the electron density decrease at f-region heights (the difference between pre-storm quiet days and the storm main phase at 11:00-13:00 ut) for the geomagnetic storm of february 1998. 1136 pal bencze, dalia burešová, jan laštovička and ferenc märcz ic conditions these differences are most pronounced near sunrise and sunset, as expected. during ionospheric storms the differences are large and significant also during daytime, particularly for the september-november 2002 event. figure 12.4c presents in the same way the electron density intrahour variability for electron density at 190 km for the same geomagnetic storm. the intra-hour variability during storm maximum days has been found surprisingly small. however, for the septemberoctober 2002 the pre-storm, storm onset day and the recovery phase, the intra-hour variability was approximately twice as high as that for may 1997 (burešová and laštovička, 2001). our f1-region results on intra-hour variability are very similar to those shown in fig. 12.4a-c for all other analysed spring and autumn events. 12.2.5. height of maximum storm effects figure 12.5 displays the height profile of the effect of a strong geomagnetic storm, which has a maximum well below the maximum of the f2-layer (burešová and laštovička 2003b). inspection of a couple of events showed that such behaviour is typical for storms with the positive phase in fof2. broader investigations of this phenomenon are underway. 1137 f1-region, es and spread-f at european middle latitudes 12.3. ionospheric sporadic e-layer investigations carried out in the framework of cost 271 are concentrated to mid-latitudes. it is necessary to emphasize this because the origin of sporadic e(es-) layers differs depending on geomagnetic latitude. considering mid-latitude es-layers, there is a generally accepted theory, the wind-shear theory of mid-latitude sporadic e (whitehead, 1961; axford et al., 1966). according to this theory, it is the vertical shear of the horizontal wind, which changes the vertical distribution of ion (electron) density by the interaction between the wind and the geomagnetic field bu #^ h. this force moves ions upwards from that part of the electron density profile where the wind is in a w-e direction and moves ions downwards, from that part of the profile, where the wind has a e-w direction. thus, ions are concentrated at the height where the zonal wind changes sign. incoherent scatter and coherent scatter radar measurements have shown that patches of enhanced ion density are embedded in the background ion density of the es-layer (e.g., bourdillon et al., 1995; haldoupis et al., 1996). 12.3.1. es parameters the es-layers are characterized by two frequencies, foes and fbes. foes is the maximum frequency of radio waves reflected by the es-layer, however, partially transmitting them, while fbes is the so called blanketing frequency representing that frequency, to which the es-layer is not transparent for sounding radio waves, it screens the overlying ionosphere. fbes corresponds to the background ion density, foes is proportional to the mean ion density of patches. the apparent height h′es may be regarded as the real height of es-layers, since the ion density in an es-layer is much higher than below it. thus, retardation of the sounding radio waves is negligibly small. it has been found there is no change in h′es exceeding the accuracy of ionosonde scaling (5 km) between the solar activity maximum and minimum (bencze and märcz, 2002). the foes-fbes difference may be considered as a quantity proportional to the transparency of eslayers for radio waves (bencze, 1983). it may be shown that transparency of es-layers is related to the wind shear. in fig. 12.6 the seasonal variations of transparency and wind shear are plotted showing a summer maximum in both cases. 12.3.2. modeling of es-layers for radio propagation in case of modeling es-layers to study the propagation of radio waves, it is necessary to take into account the structure of these layers. observations of incoherent scatter radar indicate a structure resembling a diffraction screen from the point of view of radio-wave propagation. this diffraction screen may be identified with a thin diffraction screen, since es-layers represent a thin stratification. characteristic of a diffraction screen is its screening constant, which gives the distance between points of the screen. the screening constant may be determined by the vertical sounding of the ionosphere, if soundings are made frequently enough. three minute ionograms are already enough for determination of the spectrum of the variation in foes. temporal variation of foes indicates the spatial distance between patches, that is, between screen points. spectrum of distances between patches, constructed for a winter day, when ionospheric soundings were carried out every 3 min, is shown in fig. 12.7. the radio waves of wavelength shorter than about half the double distance between screen points in the thin diffraction screen are influenced by the screen. the figure indicates that the predominant separation of sporadic e patches is only a few kilometres. this result is of importance to the oblique propagation of vhf transmissions over long distances by reflection from sporadic e ionisation (edwards et al., 1984). it has been sug1138 pal bencze, dalia burešová, jan laštovička and ferenc märcz gested that the high, possibly unwanted, signal levels sometimes observed for this propagation mechanism arise from the constructive interference of a few modes propagating by reflection from closely-spaced ionisation patches, in accord with the current result. 12.4. the spread-f phenomenon scattered echoes have sometimes been observed in the f-region on ionograms. these scattered echoes are due to ionospheric irregularities and occur as broadening of the trace marking reflections from the ionosphere along the frequency axis (f spread), or along the virtual height axis (h′ spread). development of ionospheric irregularities producing spread-f occurrence has been attributed to travelling ionospheric disturbances (e.g., bowman, 1981, 1990). vhf radar measurements indicated the instability origin of spread-f (fukao et al., 1988). thus, two different mechanisms exist for the development of spread-f causing irregularities. 12.4.1. travelling ionospheric disturbances as source of ionospheric irregularities in the f-region at mid-latitudes travelling ionospheric disturbances (tid) are classified according to their horizontal wavelength. large scale tids are atmospheric gravity waves with a horizontal wavelength exceeding 1000 km-s and vertical wavelength of 400-500 km and a period longer than about 30 min. their source is the auroral region. medium scale tids are probably associated with meteorological phenomena. following bowman here only large scale tids are considered as sources of ionospheric irregularities in the f-region. for the investigation of this possibility the following procedure has been developed (bencze and bakki, 2002). for the indication of a sudden expansion of the auroral zone and – as a consequence – generation of large scale tids, the events have been selected when the ae index is greater than the mean value of ae indices of the five most disturbed days in the corresponding month. the monthly number of these hours has been determined for each hour of the day. figure 12.8 shows fig. 12.6. seasonal variations of transparency of es-layers and wind-shear (bencze and märcz, 2002). fig. 12.7. preliminary spectrum of the distances between patches in sporadic e-layers (indicating the distance between screenpoints in case of a thin diffraction screen). 12.6 12.7 1139 f1-region, es and spread-f at european middle latitudes the daily variations of the simultaneous occurrence of spread-f and enhancements of the ae index for two ionospheric stations, uppsala at high latitudes (59.8°n) and lannion at mid-latitudes (48.45°n). the daily variation of the simultaneous occurrence of spread-f and ae enhancements indicates a maximum by night. number of the simultaneous occurrence attains at the time of the morning a maximum 30% of the total number of spread-f occurrences in uppsala and 23% in lannion. a delay of about 4 h can be observed between the morning maximum of the simultaneous occurrence of spread-f and ae enhancements at the mid-latitude lannion station as compared with the high latitude uppsala station. this delay may be connected with the propagation of travelling ionospheric disturbances from auroral latitudes. 12.4.2. plasma instabilities as sources of ionospheric irregularities in the f-region at mid-latitudes types of plasma instabilities, which may be effective in the development of spread-f producing ionospheric irregularities at mid-latitudes are the rayleigh-taylor, the e × b and the perkins instabilities (kelley, 1989). on the basis of the results obtained in section 12.4.1 related to travelling ionospheric disturbances as a source of ionospheric irregularities in the f-region, instabilities may contribute to the time of the morning maximum by about 70% to the development of irregularities. thus, they are the dominant source of irregularities, but it seems that the role of tids may not be completely neglected. 12.4.3. comparison of the growth rate of plasma instabilities at mid-latitudes connected with study of the spread-f creating irregularities, growth rates of plasma instabilities considered at mid-latitudes are also discussed. rayleigh-taylor instability is not considered here, since it is practically limited to low latitudes. concerning the e b#^ h instability, its growth rate can be expressed by the equation, neglecting the electrical coupling along field lines (maruyama, 1990) cos n dz idn o o $=c o= (12.1) fig. 12.8. daily variation of the simultaneous occurrence of spread-f and ae enhancements (bencze and bakki, 2002). fig. 12.9a,b. latitudinal variation of the instability and the perkins instability. a b 1140 pal bencze, dalia burešová, jan laštovička and ferenc märcz where v⊥ is the drift velocity perpendicular to the geomagnetic field, i is the dip angle and no stands for the background electron density. for computation of the drift velocity perpendicular to the geomagnetic field, the electric field model of richmond et al. (1980) was applied, the electron density and its vertical gradient were obtained using the ionospheric model iri-90. figure 12.9a,b shows that the growth rate increases with growing vertical electron density gradient, but is also enhanced with increasing latitude. conditions for the development e b#^ h of instability are given, if an eastward electric field or southward component of the neutral wind occurs in the presence of a vertical electron density gradient. the growth rate of the perkins instability considering only the dominant term may be written in the form (perkins, 1973; klevans and imel, 1978) / cos sin bh e i 2 n o 2 =c θ^ h (12.2) where eo is the ambient electric field, hn is the neutral pressure scale height, θ stands for the angle between east and the direction of the electric field eo. for calculation of the eastward and northward components of the electric field, for the determination of their resultant and θ, the electric field model of richmond et al. (1980) was used. equation (12.2) shows that the growth rate increases with growing northward electric component by increasing θ, however, the growth rate is reduced with increasing latitude (fig. 12.9a,b). 1141 f1-region, es and spread-f at european middle latitudes using equations of the growth rates of plasma instabilities responsible for ionospheric irregularities at mid-latitudes, growth rates at 300 km and in years of minimum solar activity (1976) and maximum solar activity (1979) have been computed and plotted as a function of latitude (fig. 12.9a,b). the growth rate of the e b#^ h instability is smaller than the growth rate of the perkins instability. 12.5. conclusions an investigation of the effects of selected strong geomagnetic storms and super storms on the daytime ionosphere at f1 heights in europe has been based on electron density profiles inferred from ionograms recorded at the higher middle and lower middle latitude stations juliusruh, pruhonice, chilton, warsaw, ebro, arenosillo and athens. the four main results are as follows: 1) independent of the sign of the geomagnetic storm effect on nmf2, the effect on electron density at the f1 heights at european higher middle latitudes has always been negative, if any at all. at european lower middle latitudes, the effects at f1-region heights are weaker and less regular. 2) there is a substantial summer/winter asymmetry of geomagnetic storm effects on the f1-region electron density at european higher middle latitudes. there is no significant effect of geomagnetic storms on electron density at the f1 heights in the range of 160-190 km during the summer half of the year except for the moderate-to-minor effect of the super storm. on the other hand, in the winter half of the year at higher middle latitudes there is a well-pronounced effect of magnitude increasing with height. the winter effect at higher middle latitude consists in a depletion of electron density during the main phase of the storm. 3) the geomagnetic super storm effects penetrate deeper into the f1-region than the effects of strong storms. 4) the pattern of the geomagnetic storm effects on the f1-region electron density does not appear to change with solar activity (solar cycle). the upward motion of the boundary between the region dominated by molecular ions and the region dominated by atomic ions evidently plays a role in explaining the observed effects. study of sporadic e-layers and spread-f at mid-latitudes resulted in the following main findings: 1) concerning the frequency parameters of sporadic e-layers, foes is considered as a quantity proportional to the mean ion-density of «patches» of increased ion density in the layer. thus, its temporal variation may indicate the mean periodicity of ion density enhancements, which would show the screenpoints of a thin diffraction screen. 2) modelling of es-layers may be based on the mean period of the variation of foes, if it is multiplied by the wind speed at the corresponding time and height. 3) using the daily variation of the simultaneous occurrence of spread-f and sudden enhancements of the auroral electrojet index ae indicating the generation of travelling ionospheric disturbances, it has been found that spread-f producing ionospheric irregularities are mainly due to plasma instabilities, though the effect of travelling ionospheric disturbances may not be excluded. acknowledgements investigations by p.b. and f.m. were supported by the grant tp 121 of the hungarian space office. the work of d.b. and j.l. was supported by grants a3042102 of the grant agency of the academy of sciences of the czech republic and oc.271.10 of the ministry of education, youth and sports of the czech republic. the hungarian authors thank to mr. p.a. bradley and prof. l. kersley for useful discussions. 1142 pal bencze, dalia burešová, jan laštovička and ferenc märcz references axford, w.j., d.m. cunnold and l.j. gleeson (1966): magnetic field variations in temperate zone sporadic e-layers, planet. space sci., 14, 909-919. bencze, p. 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(1989): the earth’s ionosphere (academic press, san diego). king, g.a.m. (1961): analysis of the f1-f2 transition region, j. geophys. res., 66, 2757-2762. klevans, e.h. and g. imel (1978): e-region coupling effects on the perkins spread-f instability, j. geophys. res., 83, 199-202. laštovička, j. (1996): effects of geomagnetic storms in the lower ionosphere, middle atmosphere and troposphere, j. atmos. terr. phys., 58, 831-843. maruyama, t. (1990): instability in the f-region in the lowto mid-latitudes, planet. space sci., 38, 273-285. mikhailov, a.v. and k. schlegel (2003): geomagnetic storm effects at f1-layer heights from incoherent scatter observations, ann. geophysicae, 21, 583-596. perkins, f.w. (1973): spread-f and ionospheric currents, j. geophys. res., 78, 218-226. prölss, g.w. (1995): ionospheric f-region storms, in handbook of atmospheric electrodynamics, edited by h. volland (crc press, boca raton, fl), vol. 2, 195-248. richmond, a.d., m. blanc, b.a. emery, r.h. wand, b.g. fejer, r.f. woodman, s. ganguly, p. amayenc, r.a. behnke, c. calderon and j.v. evans (1980): an empirical model of quiet-day ionospheric fields at middle and low latitudes, j. geophys. res., 85, 4658-4664. rishbeth, h. and o.k. garriott (1969): introduction to ionospheric physics (academic press, new york and london). whitehead, j.d. (1961): the formation of the sporadic e-layer in the temperate zones, j. atmos. terr. phys., 20, 49-58. s0555 annals of geophysics, 60, 5, 2017; s0555; doi: 10.4401/ag-7408 characterization of seismic signals recorded in tethys bay, victoria land (antarctica): data from atmosphere-cryosphere-hydrosphere interaction andrea cannata1,2*, graziano larocca2, paola del carlo3, gaetano giudice4, giovanni giuffrida4, marco liuzzo4, luciano zuccarello2, giuseppe di grazia2, salvatore gambino2, eugenio privitera2, alberto delladio5, paolo grigioni6 1 università degli studi di perugia, dipartimento di fisica e geologia, perugia, italy 2 istituto nazionale di geofisica e vulcanologia, osservatorio etneo-sezione di catania, catania, italy 3 istituto nazionale di geofisica e vulcanologia, sezione di pisa, pisa, italy 4 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy 5 istituto nazionale di geofisica e vulcanologia, sezione di roma, roma, italy 6 enea, laboratory for observation and analyses of earth climate (sspt-proter-oac), roma, italy article history received march 9, 2017; accepted july 2, 2017. subject classification: microseisms, meteorological parameters, icequakes, antarctica. abstract in this paper, we analysed 3-component seismic signals recorded during 27 november 2016 10 january 2017 by two stations installed in tethys bay (victoria land, antarctica), close to mario zucchelli station. due to the low noise levels, it was possible to identify three different kinds of signals: teleseismic earthquakes, microseisms, and icequakes. we focus on the latter two. a statistically significant relationship was found between microseism amplitude and both wind speed and sea swell. thus, we suggest that the recorded microseism data are caused by waves at the shore close to the seismic stations rather than in the deep ocean during storms. in addition, we detected three icequakes, with dominant low frequencies (below 2 hz), located in the david glacier area with local magnitude of 2.4-2.6. these events were likely to have been generated at the rock-ice interface under the glacier. this work shows how seismic signals recorded in antarctica provide insights on the interactions between the atmosphere-cryosphere-hydrosphere. since climate patterns drive these interactions, investigations on antarctic seismic signals could serve as a proxy indicator for estimating climate changes. 1. introduction it is now clear that seismic signals from continuously recording broadband seismic stations contain information beyond tectonic earthquake sources and the earth’s structure. these records include microseism energy and icequakes. the earth is not static but constantly vibrating due to many continuous noise sources such as ocean waves, storms and anthropic activities [e.g. brenguier et al. 2016]. the most continuous and ubiquitous seismic signal on earth is microseisms, mainly composed of surface waves and closely related to ocean wave energy coupling with the earth’s motion [e.g. aster et al. 2008, ferretti et al. 2013, pratt et al. 2017]. two types of microseisms are generally distinguished: primary and secondary. the former is generated by direct ocean waves in the shallow seafloor interacting with the sloping seafloor, by breaking or shoaling [e.g. hasselmann 1963, grob et al. 2011]. this seismic signal has the same period as the generating ocean waves [10-20 s; hasselmann 1963]. secondary microseisms are more energetic and are generated by standing or colliding waves within the ocean wave field near the coast or in the deep ocean, and have shorter periods (3-10 s; grob et al. 2011, and references herein). because of their source, microseisms can be used to make inferences on climate changes [e.g. grevemeyer et al. 2000, anthony et al. 2015]. icequakes are defined as coseismic brittle fracture events within the ice [e.g. podolskiy and walter 2016] or, more generally, are seismic events associated with ice dynamics. many processes can lead to the nucleation of icequakes such as near surface crevassing, cannata et al. 2 iceberg calving, stick-slip motion [e.g. podolskiy and walter 2016]. from the analysis of icequakes, new light has been shed on phenomena such as iceberg calving, glacier and sea ice dynamics [e.g. podolskiy and walter 2016, and references therein]. thanks to the complete lack of anthropic noise, antarctica can be considered a perfect natural laboratory to investigate microseisms and icequakes. in this work, we analyse the seismic signals recorded in tethys bay (victoria land), close to the italian station mario zucchelli, during the xxxii italian expedition in antarctica. 2. seismic data seismic data were acquired by 2 temporary stations (called stn01 and stn02) installed at about 950 m apart nearby tethys bay, close to the italian mario zucchelli station (mzs), from 27 november 2016 to 10 january 2017 (see table 1 for coordinates and figure 1). the stations were equipped with nanometrics trillium 120s compact, that sampled ground velocity at a rate of 100hz. in order to characterise the seismicity of the area, seismograms containing both long (from 1 hour to the whole investigated interval) and short (from a few tens of seconds to 1 hour) periods were visualised and inspected. three different kinds of seismic signals can be recognised (figure 2): (i) long-lasting tremor-like signals, (ii) long-lasting transients, (iii) short duration transients. figure 1. (a) map of victoria land. (b) aerial image of tethys bay from google earth with the locations of seismic stations (triangles), meteorological stations (circles) and mario zucchelli station (mzs, square). (c,d) pictures of installations of the seismic stations stn01 and stn02, respectively (credits graziano larocca; © pnra). table 1. coordinates of the stations installed nearby tethys bay. latitude (°n wgs84) longitude (°e wgs84) stn01 -74.70249 164.05978 stn02 -74.69803 164.08647 seismic data from tethys bay, antarctica 3 signals of type (i) appear to be similar to seismic tremor [volcanic or non-volcanic in origin; e.g. mcnutt 2005, beroza and ide 2011], with emergent onset and tail, amplitude gradually changing within time intervals of several hours, and frequency content below 1.5 hz (figure 2a). signals of type (ii) are seismic events with duration of tens of minutes to hours and most energy lower than 0.5 hz (figure 2b). these are teleseismic events, associated with distant and strong earthquakes. lastly, signals of type (iii) are short transients with duration of 20-30 s, and frequency lower than 2 hz (figure 2c). in this work, we analyse types (i) and (iii) separately. 2.1 long-lasting tremor-like signals the signals of type (i) persisted over a period of several hours to a few days with gradually changing amplitude, and a frequency content mostly below 2 hz. we calculated spectrograms in order to examine the frequency character of the signal. the spectra were calculated from consecutive 163.84-second-long signal windows (containing 214 samples). each segment was hanning-windowed to reduce spectral leakage. all spectra from within a 30-minute period were averaged and are displayed in figure 3. there are two bands of high spectral amplitudes, <0.25 hz and between 0.25 and 1.50 hz, corresponding to classical microseism frequency content [e.g. grob et al. 2011]. we calculated the root mean square (rms) amplitude in both the bands (figure 4). the energy at <0.25 hz is relatively constant, but with isolated periods of high amplitude (29/11/2016 01/12/2016, 06/12/2016 08/12/2016, 15/12/2016 16/12/2016 and 04/01/2017 09/01/2017). the energy in the 0.25-1.50 hz band is more variable, with maxima during 29/11/2016 01/12/2016, 14/12/2016 18/12/2016 and 04/01/2017 09/01/2017. figure 2. examples of long-lasting tremor-like signals (a), long-lasting transients (b), short duration transients (c), recorded by the vertical component of stn01 station. cannata et al. 4 in order to investigate the relationship of these two low frequency bands with meteorological parameters, we compared them with the wind speed, acquired at 1 minute sampling intervals by two stations located in terra nova area, called “eneide” and “maria” (circles in figure 1b; wind speed data and information were obtained from ‘meteo-climatological observatory’ of pnra www.climantartide.it). in addition, we also took into account the data of sea swells in the coast close to mzs, provided by the ufficio meteorologico of mzs; such information, recorded three times per day (at 7:00, 13:00 and 19:00 mzs time, utc+13 hours), is qualitative and starts at the end of december when the sea ice melts at mzs. all the meteorological time series are shown in figure 5. in order to estimate a quantitative relationship between environmental parameters and microseisms, we used two techniques: wavelet transform coherence (wtc) and randomised cross correlation (rcc). to compare seismic rms amplitude and wind speed, both the time series were resampled at 30 minutes intervals. because of the lower sampling rate of sea swells, to compare seismic rms amplitude and sea swells, both the time series were resampled at 3-hour intervals. the seismic rms of stn01 was used because it showed no data gaps. wtc is a method by which the covariance of two signals can be examined in both time and frequency domains. the method provides information on the phase relationship between time series [e.g. torrence and compo 1998, grinsted et al. 2004]. comparisons between time series by wtc have been performed in seismology [e.g. cannata et al. 2010, 2013], as well as in other disciplines such as epidemiology [e.g. yang et al. 2008], meteorology (e.g. jevrejeva et al. 2003] and astrophysics [e.g. donner and thiel 2007]. with respect to the widely used pearson’s correlation coefficient, wtc has the ability to provide information not only in the time but also in the frequency domain at different observation scales. the wavelet transform suffers from edge artefacts, hence it is useful to introduce a cone of influence (coi) in which edge effects cannot be ignored. finally, also the disturbances affecting the considered time series have to be taken into account. geophysical time series are commonly characterized by increasing disturbances figure 3. spectrograms of the vertical component of stn01 (a) and stn02 (b) in the band 0-5 hz. figure 4. rms of the vertical component of the signals recorded by stn01 (red squares) and stn02 (blue diamonds) in the bands <0.25 (a) and 0.25-1.50 hz (b). seismic data from tethys bay, antarctica 5 at lower frequencies and show many distinctive features of red noise (that is, higher noise amplitudes at lower frequencies). following previous studies [e.g. grinsted et al. 2004, jevrejeva et al. 2003], a 5% statistical significance level against red noise is considered in this work. the rcc method is used to determine whether the correlations between two time series (in our case, seismic rms and meteorological time series) is statistically significant or if it may be due to chance. to do so, we calculated the correlation 1,000 times, randomizing at each run the times of the meteorological time series. following saccorotti and del pezzo [2000], martini et al. [2009] and zuccarello et al. [2013], to calculate mean and standard deviation of the randomized correlation, fisher’s z transform was used to convert the correlation values c into z, a new variable following a gaussian distribution: (1) from z we can obtain an unbiased estimation of both mean and standard deviation of c (indicated as and sc, respectively): (2) (3) where and sz are mean and standard deviation of z. the compared time series were considered to be significantly related to each other if the non-randomized correlation value did not fall within the band defined by ± 3 sc. in addition, possible delays between the two time series were tested. the results of these analyses show that the variation over time of rms of both the analysed microseismic frequency bands are related to the meteorological parameters (figures 6, 7). in particular, wtc calculated between seismic rms and wind speed at both the stations (“eneide” and “maria”) shows significant relationships at periods longer than 64 hours. the rcc also shows a positive correlation, confirming that the two series are related to each other. as for the delay, rcc provides clearer information, suggesting that the wind speed time series precedes seismic rms by 5-10 hours. as for the sea swells, the results of wtc do not seem reliable, indeed the 5% significance level areas are mostly in the coi, where the edge effects might distort the picture. this is likely due to the fact that the maximum period, that can be investigated by wtc (about 60 hours), is much shorter than the period range with significant relationships in the case of wind speed (as afore mentioned, longer than 64 hours). such a difference is related to the different duration of the compared windows (~40 days for wind speed and ~10 days for sea swell). on the other hand, rcc analysis exhibits a significant positive relationship between the two time series in both the rms frequency bands. 2.2 short duration transients the signals of type (iii) are made up of transients with duration of 20-30 s, emergent onset, and frequency content below 2 hz. in particular, we noted three events characterised by two phases; the second phase shows a slightly lower frequency and a higher amplitude on the horizontal components (figure 8). on the basis of this, these phases were considered p and s waves. the spectral analysis highlights that these events show dominant low frequencies, indeed most of their energy is in the band 0.3-1.2 hz (figure 8 d,h,l). the similar spectral content and waveform, and the almost equal s-p times of ~16.1±0.6 s observed in all the three events, suggest a common source area. initially, to get an idea of the rough distance of the source area, we made use of the s-p times. we calculated such a distance on the basis of p-wave velocity of 4.5 km/s and s-wave velocity of 2.5 km/s [park et al. 2016, and references therein]. the approximated station-source distance was about 93 km. in order to have an idea about the direction of the source area with respect to the stations, the polarization analysis, based on the eigenvalue decomposition of the covariance matrix of the three components of ground motion z = 1 2 ln 1+c 1−c ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⌢c ⌢c = tanh(z) figure 5. (a) wind speed at maria meteorological station, (b) wind speed at eneide meteorological station, (c) sea swell conditions. wind speed data were provided by ‘meteo-climatological observatory’ of pnra (http://www.climantartide.it), and sea swell information by ufficio meteorologico of mzs. σc = tanh(σz) z ⌢c cannata et al. 6 [jurkevics 1988], was performed. in particular, we calculated source azimuth and rectilinearity coefficient within 2-second-long sliding windows. the p-waves are characterised by high values of rectilinearity (mainly above 0.9) as well as by source azimuth of about 30-35° (figure 9). by merging source azimuth from polarization analysis and source-station distance from s-p differential time, we tried to get an approximated and preliminary source location, which roughly matches the area of the david glacier drygalski ice tongue (figure 10). successively, in order to perform classical location analysis, seismic data of stn01 and stn02 were integrated with signals recorded by sba station (ross island) and vnda station (wright valley), available in the incorporated research institutions for seismology (iris; www.iris.edu) seismic database (see image in figure 11). the sources of the three events were located by hypoinverse-2000 software [klein 2002] by using the prem velocity model [dziewonski and anderson 1981]. in particular, 2 events (29-november and figure 6. (a-d) wtc performed between seismic rms, calculated in two bands, and wind speed measured by maria and eneide meteorological stations. the 5% significance level against red noise is shown as a thick contour. the vectors indicate the phase difference between rms and wind speed (a horizontal arrow pointing from left to right signifies in phase and an arrow pointing vertically upward means the seismic rms lags wind speed time series by 90°). the cone of influence (coi), where the edge effects might distort the picture, is shown with a lighter shade. (e-h) randomised cross correlation analysis performed between seismic rms, calculated in two bands, and wind speed measured by maria and eneide meteorological stations. continuous black lines are the cross correlation functions obtained for the original data testing different time lags; green dots are the cross-correlation estimates derived by randomizing the wind speed time series. dotted black lines mark the 99% (±3s) confidence bounds on the distribution of the correlation values obtained at individual time lags. the lags are negative when seismic rms follows wind speed time series. wind speed data were provided by ‘meteo-climatological observatory’ of pnra (http://www.climantartide.it). seismic data from tethys bay, antarctica 7 6-december) were located by 4 stations, while the third one (23-december) by 3 stations because of the lack of sba signals (see insets in figure 11). the horizontal and vertical errors are in the ranges 17-19 and 3-5 km, respectively. such an analysis confirmed that the event sources were located in the david glacier area, with focal depth ranging between 2 and 8 km b.s.l. on the basis of these locations, these events can likely be considered icequakes. once the hypocentres were defined, we were able to calculate the local magnitude of the three events that ranged between 2.4 and 2.6. finally, following the approach of danesi et al. [2007] and baumbach and bormann [2012], we estimated the source parameters. we calculated the loglog spectra of the displacement p-wave phases for the three events (figure 12), and estimated the amplitude of the flat part (u0), as well as the corner frequency (fc). as also observed by danesi et al. [2007], we found an excess of energy at low frequencies, that was inferred to be due to propagation in the ice layer, acting as a wave guide for surface waves and playing a role as a filter for high frequencies [e.g. sgattoni et al. 2016]. in the light of this, to estimate the amplitude of the flat part of the spectra, we did not take frequencies lower than 2 hz into account. under the assumption of a homogeneous earth model (with density of 2700 kg/m3), constant p and s wave velocity (equal to 4.5 km/s and 2.5 km/s; park et al. 2016, and references therein) and simplified source geometry model, the average value of seismic moment for the three events was ~5.6±0.4·1014 nm. in addition, assuming a circular fault source model, source radius, area and average dislocation were estimated equal to ~163±54 m, 8.9±6·104 m2, and 0.44±0.22 m, respectively. on the other hand, if we assume that the events took place in the ice, seismic velocity and density are different [s-wave velocity 1.95 km/s, and density 917 kg/m3; danesi et al. 2007]. hence, the seismic moment for the three events was ~8.1±0.5·1013 nm, source radius, area and average dislocation were ~122±40 m, 5.0±3.4·104 m2, and 0.60±0.30 m. 3. discussion seismic recordings acquired in tethys bay (victoria land) during 27 november 2016 10 january 2017 allowed recognising a wide variety of signals, such as microseisms, teleseismic earthquakes and icequakes. in this work, we focused on the first and third ones. as for microseisms, two bands of high amplitude signal were identified (<0.25 hz and 0.25-1.50 hz; figure 3) exhibiting similar temporal trends of amplitude variations. it has been shown how microseisms are mostly composed of ocean-generated rayleigh and figure 7. (a-b) wtc performed between seismic rms, calculated in two bands, and sea swell. the 5% significance level against red noise is shown as a thick contour. the vectors indicate the phase difference between rms and sea swell (a horizontal arrow pointing from left to right signifies in phase and an arrow pointing vertically upward means the first series lags the second one by 90°). the cone of influence (coi), where the edge effects might distort the picture, is shown as a lighter shade. (c-d) randomised cross correlation analysis performed between seismic rms, calculated in two bands, and sea swell. continuous black lines are the cross correlation functions obtained for the original data testing different time lags; green dots are the cross-correlation estimates derived by randomizing the sea swell time series. dotted black lines mark the 99% (±3s) confidence bounds on the distribution of the correlation values obtained at individual time lags. sea swell information was provided by ufficio meteorologico of mzs. cannata et al. 8 love waves [e.g. pratt et al. 2017]. in addition, pratt et al. [2017] infer that single-frequency microseisms (13-16 s period) and short-period double frequency microseisms (5-7 s) are generated on the continental shelf and are highly dependent on sea ice concentrations, while long-period double frequency (9-11 s) is excited by storms in the deep ocean. thus, in order to gain insights into the source of the recorded microseisms, the relationship between the rms temporal patterns and the changes over time of wind speed (at two stations installed close to tethys bay) and sea swell in the coast close to mzs, qualitatively evaluated by the ufficio meteorologico of mzs, was investigated (figures 6, 7). a statistically significant relationship was found with a delay of 5-10 hours between wind speed and microseisms; such a delay is in agreement with the values obtained by duennebier et al. [2012], who compared wind speed and acoustic data recorded by a hydrophone installed in oahu (hawaii). these findings suggest that the recorded microseism is caused by waves in shorelines close to the seismic stations, rather than by the generation of seismic energ y in the deep ocean under storms. energetic sources of microseisms were located by lee et al. [2011] and pratt et al. [2017] around the coastline of antarctica, where the continental shelf is ice-free. evidence of microseisms, caused by waves in shorelines, was obtained by bromirsk i and duennebier [2002] by comparing ambient noise data recorded at the near-coastal ocean bottom and inland seismic stations on the oregon coast with buoy data. morefigure 8. vertical (a,e,i), e-w (b,f,j), n-s components (c,g,k) of the seismic signals recorded by stn01, and corresponding spectra (d,h,l) of the vertical components. seismic data from tethys bay, antarctica 9 over, in the time series shown in this work, there is no evidence of amplitude modulations dependent on sea ice concentrations over the continental shelf, as highlighted by many authors [e.g. grob et al. 2011, pratt et al. 2017]. however, the investigated time interval is probably too short to show such seasonal modulations. concerning icequakes, we detected three events (on 29 november, 6 december, 23 december 2016; figure 8), with dominant low frequencies (below 2 hz), located in the area of david glacier, one of the major antarctic glaciers, draining about 4% of east antarctica ice [e.g. rignot 2002, zoet et al. 2012]. in this area, seismic events, associated with the glacier dynamics, were also detected and investigated by gambino and privitera [1994], danesi et al. [2007] and zoet et al. [2012]. in particular, by a dense seismic array operating during 20012003, zoet et al. [2012] identified a huge number of seismic events (20,000 events in 9 months) with epicentres in the same portion of the david glacier as epicentres of the events located in this study (figure 11). this strong seismic activity, which according to our data continued at least up to the end of 2016, is concentrated near the head of david glacier, where the ice thickness is ~1.8 km [frezzotti et al. 2000], and points to the quick dynamics of the glacier, characterised by an advance rate of ~730 m y-1 [wuite et al. 2009]. the low number of events detected in this study during the ~40-day-long recording period can be due to the long distance of the seismic stations from the sources, permitting to detect only the strongest events. in confirmation of this, zoet et al. [2012] showed that the stronger the events, the longer the inter-event spacing, and hence the lower the occurrence rate. the source parameters, calculated on the basis of the spectra features, suggest that not only the source location but also the mechanism of the detected icequakes is similar to the source mechanism of the events studied by danesi et al. [2007] and zoet et al. [2012]. according to figure 9. vertical (a,b,c) component of the seismic signals recorded by stn01, azimuth (d,e,f ) and rectilinearity coefficient (g,h,i). figure 10. approximated source location by using the source azimuth (black line and yellow area) and the source-station distance, calculated by the s-p differential times. yellow triangles represent the two seismic stations stn01 and stn02. cannata et al. 10 them, these events are likely generated at the rock-ice interface under the glacier. unfortunately, the calculated wide depth range, as well as the high vertical errors, do not allow confirming that these events take place at such an interface. in light of the inferred source mechanism, the study of these events can provide information about the glacier dynamics, and thus a continuous seismic monitoring could help detect possible variations in time of such a dynamics. although the seismic monitoring by a local network close to the glacier is desirable, this investigation demonstrates the feasibility of a longrange monitoring of the strongest icequakes produced by the david glacier. this work shows how seismic signals recorded in antarctica provide insights into interactions between atmosphere-cryosphere-hydrosphere. since climate patterns drive these interactions, investigations on antarctic seismic signals could serve as a proxy indicator for estimating climate changes. figure 11. aerial image from google earth, showing the locations of the 4 seismic stations used for the location analysis (yellow triangles), the epicentres of the 3 seismic events analysed in this study (red circles; event 1: 29 november; event 2: 6 december; event 3: 23 december), and the epicentre of the events analysed by zoet et al. (2012) (yellow circle). the rectangular insets show the vertical component of the seismic events recorded by the stations used for the location analysis, with the corresponding p and s wave arrival times (red arrows). figure 12. displacement spectra of the vertical component of the p phases of the three seismic events recorded by stn01 (red line) and stn02 (blue line). u0 and fc indicate the amplitude of the flat part and the corner frequency, respectively. seismic data from tethys bay, antarctica 11 data and sharing resources the seismic data can be requested from andrea cannata (andrea.cannata@unipg.it) or can be downloaded from the project website www.icevolc-project.com. the wind speed data were obtained from ‘meteo-climatological observatory’ of pnra www.climantartide.it. acknowledgements. this work was funded by pnra14_00011 project, called ice-volc (“multiparametric experiment at antarctica volcanoes: data from volcano and cryosphere-ocean-atmosphere dynamics”, www.icevolc-project.com/data). we acknowledge pnra (programma nazionale di ricerche in antartide) for funding the project, enea for providing field logistics, and cnr for scientific support. crosswavelet and wavelet coherence software were kindly provided by a. grinsted. data and information about wind speed were obtained from ‘meteo-climatological observatory’ of pnra www.climantartide.it. we kindly acknowledge the ufficio meteorologico of mzs (francesco sudati and giuliana d’ercole) for providing sea swell data. we are grateful to the helicopter pilots jamie henery and bob mcelhinney and the alpine guide davide de podestà. the facilities of iris data services, and specifically the iris data management center, were used for access to waveforms of sba and vnda seismic stations. iris data services are funded through the seismological facilities for the advancement of geoscience and earthscope (sage) proposal of the national science foundation under cooperative agreement ear-1261681. in 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email: andrea.cannata@unipg.it 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved teisseyre 671_685 671 annals of geophysics, vol. 46, n. 4, august 2003 seismic rotation waves: basic elements of theory and recording roman teisseyre (1), jerzy suchcicki (1), krzysztof p. teisseyre (1), jan wiszniowski (1) and paolo palangio (2) (1) institute of geophysics, polish academy of sciences, warszawa, poland (2) istituto nazionale di geofi sica e vulcanologia, roma, italy abstract returning to the old problem of observed rotation effects, we present the recording system and basic elements of the theory related to the rotation fi eld and its association with seismic waves. there can be many different causes leading to observed/recorded rotation effects; we can group them as follows: generation of micro-displacement motion due to asymmetry of source processes and/or due to interaction between seismic body/surface waves and medium structure; interaction between incident seismic waves and objects situated on the ground surface. new recording techniques and advanced theory of deformation in media with defects and internal (e.g., granular) structure make it possible to focus our attention on the fi rst group, related to microdisplacement motion recording, which includes both rotation and twist motions. surface rotations and twists caused directly by the action of emerging seismic waves on some objects situated on the ground surface are considered here only in the historical aspects of the problem. we present some examples of experimental results related to recording of rotation and twist components at the ojcow observatory, poland, and l’aquila observatory, italy, and we discuss some prospects for further research. mailing address: dr. roman teisseyre, institute of geophysics, polish academy of sciences, ul. ks. janusza 64, 01-452 warszawa, poland; e-mail: rt@igf.edu.pl key words rotation seismograph – asymmetric stresses – defect density – self-rotation nuclei – rotation and twist motions 1. introduction rotation effects at the ground surface associated with earthquakes have been described in several classical monographs, for instance hobbs (1907) and davison (1927); the cited examples concern, among other things, twisting of some obelisks, stone graves and segments of columns. gutenberg (1927) remarked that although such effects are observed, they cannot be explained as being caused by rotation waves, because such waves do not appear in the ideal elastic theory. imamura (1937) explained the rotation effects of some objects on the ground surface by the impact of body/surface waves; due to such impact, an object can be inclined, partly losing contact with the ground surface, and when returning to the vertical some twist occurs with respect to its former position. this explanation relates either to differences in the object inertia properties or to differences in cohesion/friction between some parts of the object and the ground surface. teisseyre (1973, 1974) attributed the appearance of rotation component in seismic waves to coupling between the seismic waves and the micromorphic response of the medium characterized by an internal/granular structure. droste and teisseyre (1976) derived the rotation seismogram from the azimuth recording system (the system used consisted of 6 horizontal seismographs oriented in 6 directions differing by 30 degrees) at a site very close (1-3 km) to mine 672 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio event hypocenters (droste and teisseyre, 1976; teisseyre, 1974; reproduced in teisseyre, 1995, p. 625). in the last few years, seismologists started to record rotational effects using different types of instruments and systems. the ring coil laser interferometer, based on the sagnac effect (gyroscope with zero inertia moment), seems to be the most advanced (takeo and ito, 1997; jaroszewicz et al., 2001; cochard et al., 2003), but some problems arise with application of such gyroscopes to the extremely high sensitivity required in seismology. the rotationseismograph system consisting of two oppositely oriented seismographs has been constructed by moriya and marumo (1998); see also moriya and teisseyre (1999). however, in our present study we refer to the rotation-seismograph system consisting of two oppositely oriented seismographs, having pendulums suspended on a common axis. such a seismograph system can record rotation, twist and stretching effects. twist effects, explained further on, are related to non-uniform rotation, or more exactly to the difference in the rotation effects observed along perpendicular directions (on a plane perpendicular to the axis of rotation). the question of the proper (self or internal) rotation fi eld – different from the rotation of displacements – may be related to the internal structure of the medium, non-symmetric processes of fracturing, and friction stresses. shimbo (1975, 1995) and shimbo and kawaguchi (1976) have considered the asymmetric stress fi eld related to friction processes and rotation of grains; their conclusion for small deformation fi elds is that the antisymmetric part of stresses is proportional to the proper rotation tensor, which is independent of the rotation of displacements (rot u). the theoretical basis for rotation and twist deformations in elastic continua with defects and rotation nuclei is outlined by teisseyre (2002) and by teisseyre and boratyński (2002). for a given shear stress condition, the rotations of grains due to friction are opposite along perpendicular planes (fig. 1a). usually, how ever, the earthquake process proceeds along one slip/fracture plane, the main fault, which produces the asymmetry of rotation and friction stress (fi g. 1b). we will not enter here into the theories of continua with internal structure and of continua containing a dense distribution of internal nuclei (dislocations, disclinations, vacancies, thermal nuclei, or electric nuclei) which may be the objects/sources creating internal/self stresses. some of these internal sources, like dislocations, disclinations and rotation nuclei (rotating grains) cause deviation from stress and strain symmetry; for reference see e.g., anthony et al. (1968), kleman (1980), kossecka and dewitt (1977) and teisseyre (2002). in this paper, we consider theory and experimental results related to micro-displacement motions – rotation and twist – caused by deviations of the real medium from ideal elastic confi guration and state. we shall note in advance that the observed rotation and twist effects as recognized by special recording system bring very small motions in the range of 10 -8 rad/s; it remains an open question whether such motions can cause the macro-rotation effects as experienced by some surface structures after strong earthquakes. a b fig. 1a,b. fracture planes and rotation effects due to friction processes or dislocation motions: a) the symmetric case; b) the fracture along a main fault; the arrows indicate the shear stresses acting along the fracture planes. 673 seismic rotation waves: basic elements of theory and recording 2. rotations as sources of internal stresses when considering an elastic continuum with dense distribution of defects (dislocations, disclinations, vacancies, thermal or piezoelectric nuclei and others), or structured continua (e.g., micromorphic, micropolar continua or other media with internal structures), we should take into account the effects of proper rotations expressed in the records of micro-displacement velocity fi elds. such fi elds can arise due to the motion of some defects or small grains forming the continuum. in earthquake processes, such motions can appear due to relative motions along dislocation planes (medium continuity preserved) or crack surfaces (fracturing process). internal rotation nuclei can thus be related to dislocation motion (resistance stresses causing rotations of grains) or to slip motion (rotations of grains due to friction stresses along the fracture plane). for symmetric stresses, the rotations along perpendicular planes (fi g. 1a) have an opposite sense; the dislocations situated along these perpendicular lines contribute to opposite rotations on those planes. however, earthquake phenomena appear due to motion and fracturing along one main fault (fi g. 1b), hence there arises an asymmetry of self stresses and micro-displacement motion linked to these rotation nuclei. we may expect, as discussed later, that these asymmetry effects can only be recorded in the near fi eld. 3. general approach to real media in ideal elasticity, the deformation rates, for a given boundary or initial conditions, can be described by the displacement motions. when approaching a real medium we should also take into account the internal strains or stresses (stress/strain nuclei) related to defects (vacancies, dislocations, disclinations, etc.), structures (plastic, micromorphic, micropolar, etc.) and interaction of these elements with other fi elds (e.g., thermal, electric, magnetic). to the grain structure, the rotation of grains (rotation nuclei) caused by resistance/friction stresses is added (shimbo, 1975; teisseyre, 2002). considering theories of continua with defects (see e.g., kossecka and dewitt, 1977; kröner, 1981) or those with micro-structures (see e.g., eringen and suhubi, 1964), we shall be aware of the fact that structures of such theories, at least on the level of kinematics, are independent of the length parameter related, e.g., to dimension of grains. similarly, the observations of motions related to micro-displacement fi elds do not bring information on the size of grains or defects typical for geological strata or rocks involved. following the kröner approach (1981), we can assume that the deviations from ideal elasticity – due to the above-mentioned factors – can be taken into account by introducing the self (internal) fi elds. this assures the decomposition of the total fi eld (corresponding to the state of ideal elasticity, expressed by the displacement vector only) into the sum of the self-fi eld and its elastic response. for strain and rotation motions (total, elastic and self parts) we can write (3.1) (3.2) in these expressions, the terms on the left-hand sides represent the total fi eld refl ecting the ideal elastic state, the next terms, the self-fi elds, represent internal sources which together with boundary/initial conditions lead to the elastic responses represented by the last terms. in this approach, we can allow the stress-strain relation to remain that established for ideal elasticity. therefore, similarly to (3.1) we have (3.3) the total fi elds as given by the sum of elastic and self fi elds should fulfi ll the compatibility (integrability) conditions in order to be expressed by displacement derivatives. relations (3.1) and (3.2) can be expressed by the distortion fi elds defi ned as follows: (3.4) e e e e u x u x t s ik t k i i k = + = ∂ ∂ + ∂ ∂     , 1 2 ω ω ω ωt s ik t k i i k u x u x = + = ∂ ∂ − ∂ ∂      , . 1 2 s s st s= + . β ω β β βt t t ik t k i ik ik su e u u u x ( ) = ( ) + ( ) = ∂ ∂ = +, . 674 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio the effects related to deviations from ideal elasticity depend on the real material properties and the thermodynamical state of a body; for rigid brittle rocks under normal conditions such deviations produced by the self/internal distortion fi eld can be very small. however, to study these effects we shall consider the elastic distortion fi eld as it is an observable quantity; from relation (3.4) we obtain (3.4a) in the next sections we will explain how to measure the elastic distortion fi elds. 4. rotation seismograph system the rotation seismograph system consists of two seismograph sensors situated in parallel, one after the other, with the distance l between the suspension axes, and with the pendulums of opposite orientation. in the system designed at the institute of geophysics, polish academy of sciences, the rotation axis of the pendulums is the same (l = 0) and pendulum proper length is l = 0.083 m (fi g. 2a,b). a single pendulum seismograph records the displacement velocity u and the displacement velocity derivative (usually neglected as small in comparison to the displacement) multiplied by the pendulum proper length: l . ∂ ∂u y , and, similarly, the proper rotation lγ (if it exists); in our case l = 0.083 m. with the two opposite seismograph systems we simultaneously record the two fi elds (4.1) it is diffi cult to separate the terms ∂ ∂u y and g ; hence, it is convenient to defi ne the micro-displacement velocity tensor as a sum of the derivative of displacement velocity and the appropriate component of the tensor of real rotation (4.2) thus, the micro-displacement velocity tensor is related to the difference of records between the two opposite sensors (eq. (4.1)) divided by 2l (4.3) it is to be noted that our notion of the microdisplacement velocity tensor is different from that of the micro-displacement tensor used in the micromorphic or micropolar continuum (anthony et al., 1968; teisseyre and nagahama, 1999; nagahama and teisseyre, 2001); our present defi nition is related directly to the observables. relations (4.1) and (4.3) remain valid, however, only when both seismographs forming the system have exactly the same response characteristics. in their system, moriya and marumo (1998) used over-damped sensors; the equalization procedure is, in this case, suffi cient to derive the rotation motion. however, for any rotation seismograph we should consider the infl uence of differences between the response characteristics. we rewrite eqs. (4.1) and (4.3) in the form of the appropriate convolutions (4.4) where we have introduced ground displacement velocity u, the response characteristics of both sensors, r' , r'' , as well as their mean, r, and their difference value, dr, and also the rotation ground motion, f . for the difference, du, we write (4.5) ′ ∂ ∂ +     ′′ = − ∂ ∂ +     u u u l u y = u + l u y γ γ, . φ γik i k ik u x = ∂ ∂ + . φ γik i i k ik u l u x = = ∂ ∂ + ∆ 2 . ′ = ∗ ′ + ∗ ′ ′′ = ∗ ′′ − ∗ ′′u u r l r u u r l r φ φ, ∆ ∆ φu u r l r= ∗ + ∗2 β β β β βik ik t ik s ik k i ik s u x = − = ∂ ∂ −, . fig. 2a. rotation seismograph: scheme. 675 seismic rotation waves: basic elements of theory and recording and for the sum of such records we have (4.6) where we neglect the product of two relatively small quantities, namely the product of rotation motion and difference of response characteristics. to derive the rotation motion, we should minimize the difference between the responses of both sensors, or eliminate such a difference in another way. 5. comparison of rotation seismograph sensors even for very similar sensors, the response characteristics of the sensors are slightly different. in our rotation seismograph system, for the aim of comparing both sensors, we can rotate the position of one seismograph sensor in such a way that both the seismograph pendulums, suspended on the common axis, become oriented in the same direction, one just above the other – this is the test position. the working and test positions for the case of the horizontal seismographs are schematically shown in fi g. 3a,b. the records obtained in the test position fig. 2b. rotation seismograph: photo. a b fig. 3a,b. rotation seismograph: working (a) and calibration (b) positions of the sensor pendulums. σ φ ∆u u r l r u r= ∗ + ∗ ≈ ∗2 2 676 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio could mainly differ due to differences in their response characteristics and (5.1) to minimize these errors, we can use some numerical procedures based on filtering in frequency or in time domains (teisseyre et al., 2003; nowoz . yński and k.p. teisseyre, 2003). the respective fi lters can then be applied to records in the normal working position to reduce the infl uence of response difference dr disturbing the corresponding difference of records in the working position (eq. (4.4)). we should underline that our recorded data of displacement velocities are transformed, usually before any analysis, to ground velocities; in such a situation u ' and u '' are very close to real ground velocity u, and r' and r'' can be treated as corrections to the response characteristics. these characteristics are, thus, very close to unity and dr is much less than unity. however, all these corrections remain unknown; our aim will be to eliminate their infl uence and to estimate the reliability of micro-displacement motion records. in the present paper, we use a new method to discern the rotation motion and to estimate the infl uence of differences in response characteristics of the anti-parallel sensors of our rotation seismograph. considering the difference of the records u ' and u'' in the working position of sensors (eq. (4.5)), ∆ ∆ φu u r l r= ∗ + ∗2 , we should note that it is quite diffi cult to separate these two terms and separately estimate their mean square values. however, when dividing the fft spectra of the difference by the sum of records (eqs. (4.5) and (4.6)) we obtain (5.2) where f(j ) is a complex expression in the frequency domain and j is its counterpart in the time domain. we should note that the fi rst term of expression (5.2) depends only on the sensor characteristics, while the second term on input ground motions. this expression may also lead to many outliers whenever the inequality f u lf( ) << ( )φ occurs. however, we can replace all the outliers with the appropriate mean values plus/minus standard deviations. in our program procedures we divide, at fi rst, the event record into a number of equal segments, that is, into n equal time intervals; then, for each segment i we compute the corresponding complex expressions f(j(i)) and afterwards we take the mean value f j n f j i i n ( ) = ( )( ) = ∑ 1 1 (5.3) where the approximation is justifi ed only for a big enough number of segments n and provided that the outliers are eliminated. the obtained value f j f r f r ( ) ≈ ( ) ( ) ∆ 2 can be easily interpolated to the total data number of the record, and then we can consider the following product and difference (compare eqs. (4.5) and (4.6)) (5.4) from the last expression we can return to the time domain, obtaining the corrected values of micro-displacement motions 2lφ for a given rotation-seismograph. to illustrate the application of this method we refer to the rotation seismograph records at l’aquila observatory, apennines, of june 16, 2002. in fi g. 4 we present the way in which we reach an approximation related to eq. (5.4) (in order to simplify the presentation we have plotted the absolute values of f(j(i)) for selected ′ = ′ +( ) ∗ ′ ′′ = ′′ +( ) ∗ ′′u u l r u u l r φ φ, ∆ φ ∆ ∆u u u u l r u r= ′ − ′′ = +( ) ∗ ≈ ∗ . f j f u f r lf f r f u f r ( ) = ( ) ( ) + ( ) ( ) ( ) ( ) = ∆ φ2 2 f r f r lf f u = ( ) ( ) + ( ) ( ) ∆ φ 2 f j f r f r n lf i f u i f r f ri n ( ) = ( ) ( ) + ( )( ) ( )( ) ≈ ( ) ( )= ∑ ∆ φ ∆ 2 1 21 f j f u f u f r( ) ( ) ≈ ( ) ( )σ ∆ , f u f j f u lf f r∆ σ φ( ) − ( ) ( ) ≈ ( ) ( )2 . 677 seismic rotation waves: basic elements of theory and recording segments – color curves and the absolute value of the mean f j f r f r ( ) ≈ ( ) ( ) ∆ 2 – black curve). from these estimations we can compute for each time moment the values of recorded rotations 2l rφ ∗ and related errors u r∗ ∆ . figure 5 presents the original plot of difference du and its parts 2l rφ ∗ and u r∗ ∆ , proving the successful result of the method applied. then we calculated the ratio of the mean square values of recorded rotations and the mean square values of displacement velocities, mean mean 2 2 2l r uφ∗( ) , and the ratio of the mean square values of respective errors and the mean square values of displacement velocities, mean mean u r u∗( )∆ 2 2 ; for the case under consideration, the fi rst ratio amounts to 0.012, while the second is much smaller at 0.0014. this analysis proves that the recorded microdisplacement motions, as derived from differences of two anti-parallel sensors, exceed the estimated errors due to calibration uncertainties and other system defects. 6. recording the twist and rotation motions: examples to record the components of the microdisplacement velocity f ik (rotation tensor g ik plus fig. 4. examples of the absolute values of the fj(i ) functions in the frequency domain computed for the selected time segments (color lines) and the computed mean values (black line) representing function fj (according to the recordings at l’aquila observatory of 16 june 2002 event in the apennines (22 h 23 min; m = 4.1). 678 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio rotation of displacements: eq. (4.3)), we shall use three rotation seismograph systems: each one with two oppositely oriented pendulums and sensors for each axis of rotation, and another three seismograph systems, each one consisting of the pair of the oppositely oriented coil sensors, to record the velocities of stretching components f11, f22 , f33. for the case of vertical axis, we use a pair of two rotation systems as shown in fi g. 6 (note that the construction of our rotation seismograph is such that the upper sensor coincides with the negative derivative − ∂ ∂u y ; for this reason, the component u should be taken with the «minus» sign to obtain the proper defi nition of the micro-displacement tensor components as given in eq. (4.2)). for this system, we will use the following notations for displacement and micro-displacement velocities (6.1) thus, for these two rotation seismograph systems, we shall use, in addition to eq. (4.1) for the fi rst system, a similar relation for the perpendicularly situated second rotation-seismograph system (6.2) the rotation and twist motions are given by the difference and sum of the micro-displacement velocities, respectively fig. 5. micro-displacement velocity, i.e. the difference between the displacement velocities recorded by the two anti-parallel sensors (black curve) and the corrected micro-displacement fi eld (red curve) and the computed errors due to response difference (blue curve). u u u= = = =1 2 12, , , . υ φ φ ψ φ21 ′ = + ∂ ∂ + ′     ′′ − ∂ ∂ + ′     υ υ υ γ υ υ υ γl x x , .= l 679 seismic rotation waves: basic elements of theory and recording (6.3) (6.4) correspondingly, one term contains rotation/ strain motion related to displacement velocities, while the other contains the proper (self) fi elds of rotation/twist. the theories of continua with structure and defects predict a complex pattern of microdeformations and motions; the rotation tensor becomes asymmetric (not anti-symmetric as for pure rotation), and we also observe twist deformation (fi g. 7). we can now compare the theoretically derived relation for total distortion bt with the defi nition (4.2); comparing these relations we can write (6.5) where we have assumed that the self-rotations (proper rotations and twists) are the unique sources of self-distortions. now, we can write (6.6) identifying the observed fi eld with the distortion fig. 6. scheme of the rotation seismograph system related to one axis of rotation. r u y x xy yx= −[ ] = ∂ ∂ − ∂ ∂       + −[ ]1 2 1 2 1 2 φ ψ υ γ γ t u y x xy yx= +( ) = ∂ ∂ + ∂ ∂     + +( )1 2 1 2 1 2 φ ψ υ γ γ . fig. 7. scheme presenting the micro-displacement components related to the grain surface (left), the related rotation (center) and twist motion (right). ∂ ∂ = + ≈ − = − u x k i ik ik s ik ik ik s ikβ β φ γ β γ, β φik ik= 680 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio fig. 8a,b. example of recording of the displacement velocities by two anti-parallel sensors (ojcow observatory, record of a silesian event at distance about 60 km; 11 july 2001, 18 h 50 min; magnitude about 1.5); parts (a) and (b) are in different time scales. a b 681 seismic rotation waves: basic elements of theory and recording fi eld. the self-rotation is expressed by the proper rotation γ ik[ ] and the twist part γ ik( ) , as in eqs. (6.3) and (6.4). we will now present some records obtained with the use of our system of rotation seismographs at the ojcow observatory, poland, and at the l’aquila observatory, italy. figures 8a,b and 9 present data of a silesian seismic event recorded at ojcow; the distance from the source is about 60 km (11 july 2001, 18 h 50 min; magnitude about 1.5); fi g. 8a,b gives an example of the displacement velocity records u'2 and u''2 in m/s for two anti-parallel sensors of the rotation seismograph: parts (a) and (b) are in different time scales. only very small differences can be seen in these records, contributing to the micro-displacement velocity field; fig. 9 presents an example of recordings for the microdisplacement velocity components, f and y, in rad/s as obtained from the two perpendicularly situated rotation seismographs. according to relations (4.5) and (4.6) these components make it possible to derive the respective rotation and twist motions. figure 10 presents the rotation and twist velocities derived from the records at l’aquila observatory of the 16th june 2002 seismic event (22 h 23 min; m = 4.1; apennines); the black curves show original data, whereas the red ones show the values corrected by the method outlined above (see eqs. (5.2)-(5.4)). 7. discussion and conclusions among many open questions related to seismic rotation waves, the most important concern their generation mechanism and information contained in such waves. we can fig. 9. records of the rotation and twist velocity components (ojcow observatory; as above) 682 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio briefly consider three different, but maybe concurrent, origins of micro-displacement motion. the fi rst relates the micro-displacement fi eld directly to rotation, or generally to space derivatives of displacement. to estimate the order of magnitude of displacement velocity derivatives we can use the plane wave theory and estimation based on direct records where v is the wave velocity near the surface at the station site and we disregard the direction cosine related to the wave approach direction; a possible anisotropy effect would not exceed a factor of 2. the rotation or twist motions can thus be estimated a priori and then compared with the observed data of micro-displacement motions. for the records at the ojcow observatory, we should assume an unrealistically small value of wave velocity, of the order of 50 m/s, to obtain the best fi t of the observed micro-displacement motion and the corresponding fi eld estimated from the plane wave theory (fi g. 11). the interference of different seismic phases related to body shear and surface waves can contribute to somewhat more realistic results. also we should note that such an estimation based on ideal elasticity may differ from estimates based on a more realistic assumption for medium properties. takeo and ito (1997) have estimated, in a very elegant form, using the differential geometry approach, the values of displacement fig. 10. rotation and twist velocities derived from the records at l’aquila observatory of the 16 june 2002 seismic event in the apennines; the black curves show the raw data, whereas the red ones show the corrected values. u a ik x i t u x v u t i i n n i k i= −( ) ∂ ∂ ≈ − ∂ ∂ exp ,ω 1 683 seismic rotation waves: basic elements of theory and recording velocity rotation for a medium with dense defect distribution and found, based on the synthetic rotation seismogram at a distance of about 5 km from the source, that the results of such an estimation can be convincing. the last remark leads us to the second origin mechanism that can attribute the formation of rotation motions due to interaction between the seismic body and surface waves in a medium with internal structure and defects. theories of the micropolar and micromorphic media (eringen and suhubi, 1964) present a continuum approach to granular media and predict the appearance of such micro-displacement motions; the corresponding wave solutions lead, however, to a proportionality relation between the self micro-displacement motion and the derivative of displacement velocity (teisseyre and nagahama, 1999; nagahama and teisseyre, 2001), modifying the estimation considered above to a new form where c is the coupling parameter expressing fig. 11. comparison between the observed micro-displacement velocity fi eld (black curve) and the estimated derivative of displacement velocity (red curve) with the appropriately chosen apparent wave velocity for the best fi t of both curves (here v = 50 m/s); the correlation coeffi cient equals to 0.5 (ojcow observatory; as above). γ ik i k c u x = ∂ ∂ , φik i k i a ic u x c v u t v u t = +( ) ∂ ∂ ≈ − + ∂ ∂ = − ∂ ∂ 1 1 1 684 roman teisseyre, jerzy suchcicki, krzysztof p. teisseyre, jan wiszniowski and paolo palangio interaction of the seismic body and surface waves with the internal structure, defects and any potential rotation nuclei in the medium; this parameter can be in reality considered as a frequency-dependent function appropriate for the medium; v c va = +( )1 can be defi ned as the apparent micromorphic wave velocity. we should note that the micro-displacement motion (4.3) contains two terms, diffi cult to separate in an experiment, unless a special arrangement of seismometers is applied, for example, three pendulum seismometers arranged in one line, or an array of one pendulum and at least two coil sensors. finally, we can consider the seismic source process and its asymmetry pattern as a cause of micro-displacement motions as we mentioned in previous sections of this paper. according to shimbo (1975, 1995), fracture slip motion causes rotations of the grains adjacent to the fracture plane; the anti-symmetric part of selfstresses and self-strains becomes related to the rotation motion where the new constant a depends on friction (for the fracture slip process) or on stress resistance (for dislocation motion) and the grain inertiamoment properties. this friction law indicates the appearance of the anti-symmetric self-stress fi eld and has been generalized by teisseyre (2002) and, teisseyre and boratyński (2002) to become the constitutive law joining the anti-symmetric part of strains and stresses. of course, we should be aware of the fact that the self-rotation motion once generated at a seismic source could be quickly attenuated when propagating through layers with properties not deviating far from those of ideal elasticity; we are close to believing that micro-displacement motion can bear information on source processes only when it is recorded in the near fi eld. concluding, we dare say that classical seismology, based on recording the displacement velocities, remains on the level of an ideal elastic continuum, whereas when introducing recordings of the micro-displacement tensor velocities we approach the level of a real continuum. in further studies, we will encounter many new problems. first of all, we need very sensitive digital and numerically accurate instruments to record all 9 components of the micro-displacement tensor. we will need data at different observation sites and for different earthquakes. we should study many new relations, like that for the mean square ratio between the displacement velocities and rotations and twists as a function of the source-station distance. we should also clarify the infl uence of the local structure and that of the source process. acknowledgements the paper has been partly sponsored by the polish committee for scientifi c research, grant no. 6 p04d 066 20. references anthony, k., u. essmann, a. seeger and h. truble (1968): disclinations and the cosserat-continuum with incompatible rotations, in mechanics of generalized continua, edited by e. kröner (springer verlag, berlin), 355-358. cochard, a., a. flaws, u. schreiber and h. igel (2003): observations and simulations of rotational motions, geophys. res. abstr., 5, 13160. davison, ch. 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(received january 20, 2003; accepted may 27, 2003) giannopoulos_corrected_layout 6 temporal variation of shear-wave splitting parameters before and after the 2008 movri mountain earthquake in northwest peloponnese (greece) dimitrios giannopoulos1, efthimios sokos1,*, konstantinos i. konstantinou2, athanasios lois1, g-akis tselentis1 1 university of patras, seismological laboratory, patras, greece 2 national central university, department of earth sciences, jhongli, taiwan annals of geophysics, 55, 5, 2012; doi: 10.4401/ag-5586 abstract on june 8, 2008, at 12:25 gmt, a mw 6.4 earthquake, the movri mountain earthquake, occurred in the area of northwest peloponnese, western greece. the epicenter was located in the municipality of movri, 35 km southwest of patras. for this study, a crustal anisotropy analysis was performed in the epicentral area of the movri mountain earthquake. specifically, the shear-wave splitting phenomenon and its temporal evolution in relation to the movri mountain earthquake was studied, using the cross-correlation method. the data analysis revealed the presence of shear-wave splitting in the study area. both before and after the movri mountain earthquake, the polarization directions of the fast component of the shear waves followed a general nnw-sse direction. the observed mean fast polarization direction was not consistent with the regional stress field, which showed a general e-w direction of the maximum horizontal compressive stress. the differences between the estimated fast polarization directions and the properties of the regional stress field suggest the presence of a local stress field in the area around the fault. an increase in time delays was observed soon after the movri mountain earthquake. the average value of the delay times before the earthquake was ca. 18 ±2.6 ms, while after the earthquake this was ca. 40 ±4.6 ms. this increase in the time delay indicates changes in the crustal properties, which were possibly caused by variations in the preexisting micro-crack system characteristics related to the movri mountain earthquake, and the possible involvement of over-pressured fluids. 1. introduction shear-wave splitting is a phenomenon in which shearwaves are separated into two components with different polarization directions and propagation velocities. this can occur during s-wave propagation through an anisotropic medium [crampin and chastin 2003, crampin and peacock 2005]. the two splitting parameters that can be measured through shear-wave data processing are the polarization direction { of the fast component of the shear waves, and the time delay dt between the two components. various models have been proposed to interpret the observed seismic anisotropy in the crust. the most widely accepted physical model, which is known as the extensive dilatancy anisotropy model [crampin 1978, 1993, crampin et al. 1984b], explains the principal cause of the shear-wave splitting phenomenon as s-wave propagation through stress-aligned, fluid-saturated micro-cracks with an orientation parallel/ sub-parallel to the direction of maximum horizontal compression. this model has since been modified, and led to the anisotropic poro-elasticity model [crampin and zatsepin 1997, zatsepin and crampin 1997], which explains the way in which such microcracks evolve in response to changing conditions in permeable rock. several studies on local earthquakes worldwide have revealed shear-wave splitting in various geological settings that indicate anisotropic media [kaneshima 1990, crampin and lovell 1991, gao et al. 1998, liu et al. 2008]. changes in the parameters of the shear-wave splitting phenomenon have been observed worldwide in relation to earthquakes that have indicated changes in the anisotropic characteristics of the medium and the stress field [gao and crampin 2004, crampin and peacock 2008, crampin and gao 2012]. these variations in the shear-wave splitting parameters are complicated and have often been the subject of debate. specifically, variations in shear-wave splitting time delays before large earthquakes have given rise to discussion [crampin and peacock 2008]. for instance, liu et al. [2004] analyzed shear-wave splitting of foreshocks and aftershocks of the september 20, 1999, mw 7.6 chi-chi earthquake in taiwan. no evidence was found for temporal changes in time delays over the periods before and after the mainshock. the authors suggested that the behavior in the dataset was affected by spatial variations of seismic anisotropy rather than by temporal variations. on the other hand, in commenting on a previous study, crampin and gao [2005] claimed that stress fluctuates during aftershock sequences article history received february 23, 2012; accepted june 30, 2012. subject classification: earthquake faults: properties and evolution, tomography and anisotropy, waves and wave analysis, stress, tectonics. 1027 research articles and systematic changes in time delays are not expected and have been observed. however, according to crampin and gao [2005], temporal variations of time delays were measured by liu et al. [2004] before the chi-chi earthquake. these variations showed the same characteristic behavior, with comparable magnitude/ time-delay duration similarities as in other studies [e.g., crampin and gao [2005]. liu et al. [2005] maintained their assessment that the results did not show systematic temporal evolution of anisotropy parameters that can be used to forecast impending large earthquakes. in another case, aster et al. [1990] measured time delays on the same seismograms from the anza seismic network that peacock et al. [1988] and crampin et al. [1990] used to show systematic temporal changes before the 1986 m6 north palm springs earthquake in southern california, usa. these authors showed that there were no temporal changes, using automatic techniques to measure time delays. crampin et al. [1991] argued that some of the automatic time-delay measurements of aster et al. [1990] were in error by a factor of up to 3 (200%). they claimed that the inadequacy of all of the automatic measurements invalidated the conclusions of aster et al. [1990], and the estimates of the time delays between the split shear waves were not reliable. other cases in the literature that have provided observations of changes in splitting parameters related to earthquakes have been debated, and those under considerable discussion are seher and main [2004], peng and ben-zion [2005], and munson et al. [1995]. the tectonic and geological setting of western greece is characterized by complex structures. the tectonic framework is dominated by the large dextral strike-slip fault off the coast of kefallonia island, where the change between continent–continent collision in the north and ocean–continent subduction in the south occurs [underhill 1989, sachpazi et al. 2000] (figure 1). during the eocene, northwestern peloponnese was affected by the alpine collision, which led to the formation of the hellenic mountain range [doutsos et al. 1993]. mesozoic and early cenozoic carbonate rock propagated upwards and westwards along n-s-striking and e-dipgiannopoulos et al. 1028 figure 1. geological map of northwest peloponnese showing the active faults, the movri mountain earthquake epicenter (star), the riolos (rls) station (triangle), and the reactivated faults during the 2008 movri mountain earthquake, according to koukouvelas et al. [2009]. figure modified from koukouvelas et al. [2009]. 1029 ping thrust faults [xypolias and doutsos 2000]. from the early pliocene to the present, an extensional stress field has progressively prevailed in the previous compressional tectonic regime in northwestern peloponnese [doutsos and kokkalas 2001]. the movri mountain earthquake occurred on june 8, 2008, at 15:25 local time (12:25 gmt) in the area of northwest peloponnese, western greece. the main shock was accurately located by konstantinou et al. [2009b] at a hypocentral depth of 18 km, using a shrinking grid-search relocation algorithm. several studies have been performed to investigate the aftershock distribution and the rupture process of this event [ganas et al. 2009, gallovič et al. 2009, konstantinou et al. 2009b, papadopoulos et al. 2010, feng et al. 2010]. various organizations have provided moment tensor solutions (e.g., institute of geodynamics of the national observatory of athens, harvard university, united states geological survey, aristotle university of thessaloniki), which have indicated a nearly vertical dextral strike-slip fault striking ne-sw. the event was reported as an earthquake with a mean moment magnitude, mw, of 6.4, which is the largest instrumentally recorded event in this area to date [konstantinou et al. 2009b]. the seismogenic fault of the movri mountain earthquake had no direct surface expression, and none of the major surface-fault ruptures in the area fit with a nne-trending fault at depth [koukouvelas et al. 2009]. konstantinou et al. [2011] suggested that the presence of over-pressured fluids of deep origin might have been responsible for the elevated fluid pressure levels near the hypocenter that led to the reactivation of the unknown fault that generated this event. in this study, we present evidence for shear-wave splitting processes in the epicentral area of the movri mountain earthquake. specifically, we have detected variations in splitting parameters in relation to the movri mountain earthquake. we discuss the measured splitting parameters and their variations in comparison with the regional stress field, and also with the movri mountain earthquake. 2. data for the purpose of this study, three-component recordings from stations in continuous operation during the periods before and after the movri mountain earthquake were needed. the only station in the study area that satisfied this criterion was riolos station (rls). no other seismological station around the epicenter was equipped with a three-component seismometer and was operating continuously in the studied time periods. this station is part of the seismological network of the institute of geodynamics, national observatory of athens (gi-noa) and the hellenic unified seismological network (husn). it is equipped with a threecomponent, broadband sensor and it is located in riolos village, about 13 km from the epicenter of the movri mountain earthquake, in northwestern peloponnese (figure 1). the seismicity studied here consisted of background earthquakes near to the seismogenic fault and aftershocks of the mw 6.4 movri mountain earthquake. the dataset for the period before the movri mountain earthquake was recorded from june 4, 2003, until september 27, 2007, and for the period after the occurrence of the earthquake, from june 10, 2008, until july 13, 2008. the local magnitudes varied between 1.3 and 3.5. we used records from 12 events for the first period and records from 207 events for the second period. for the first period of interest, we used the event locations provided by gi-noa, using recordings from ginoa and the patras seismological laboratory network (http://seismo.geology.upatras.gr/). the location errors in the horizontal and vertical directions associated with event locations by gi-noa did not exceed 2 km. for the second period of interest, we used the event locations determined by konstantinou et al. [2009b]. there were fewer seismic events available before the movri mountain earthquake because the seismicity in the area was low. this reason mostly explains the small number of events available for the first period. an important data-selection criterion in shear-wave splitting analysis is related to the incidence angle of the shear waves. s-waves have such severe interactions with the free surface that almost all of the similarities with the incoming waveform are irretrievably lost when they reach the surface at incidence angles greater than a critical value. the critical value is given by sin−1(vs/vp), where vs and vp are the s-wave and p-wave velocities at the source [booth and crampin 1985]. using the local one-dimensional velocity model by haslinger et al. [1999], the critical angle for our study was estimated at 34.5˚. records of the events within the shear-wave window of the rls station were searched, to obtain clean and impulsive s-phases on the horizontal components. 3. methods for the estimation of the splitting parameters in our dataset, we applied the cross-correlation method [ando et al. 1983], as described by konstantinou et al. [2009a]. before applying this method, we manually picked the p-wave and s-wave arrival times of our waveform dataset. the seismograms were interpolated to 200 samples s−1, integrated to displacement, and then band-pass filtered with corner frequencies of 1 hz and 10 hz. the measurement window for each waveform was defined in the following way, as described by konstantinou et al. [2009a]: the start of the window was fixed as 0.05s before the s-wave arrival, while the endpoint was adjusted each time until the value of the crosscorrelation coefficient c between the fast and slow components reached a maximum. the reasoning behind this choice of window measurement was because the suitable endpoint might actually vary with different waveform characteristics. according to cross-correlation methods, both horizontal seismograms are rotated in the horizontal plane at 1˚ increments s-wave splitting temporal variation of azimuth (a) from −90˚ to 90˚. then, for each azimuth, the cross-correlation coefficient c is calculated between the two orthogonal seismograms, for a range of time delays (x) in a selected time window. when the absolute value of c reaches a maximum, the corresponding values of azimuth (a) and time (x) are chosen as the fast polarization direction and the time delay between the separated shear waves, respectively. the measurement uncertainty was estimated using a t-test at a 95% confidence level on the values of c, as described by kuo et al. [1994]. following liu et al. [2008] and konstantinou et al. [2009a], we accepted the splitting measurements as valid that conformed to the following criteria: (a) the c value was >0.80; (b) the signal-to-noise ratio was >3; (c) the change in the measured dt was <0.02 s when the window size was varied by ±0.02 s; and (d) the change in the measured { was <10˚ when the window size was varied by ±0.02 s. with the application of these criteria, 132 events were selected, and the splitting parameters for both of the periods were computed (before and after the movri mountain earthquake). two examples of the valid splitting measurements are shown in figure 2. 4. results after the analysis of the s-waves, we obtained valid data ({, dt) from 10 seismic events for the period before the movri mountain earthquake (figure 3). the raw delay times varied between 5 ms and 50 ms, with a mean of 18 ±2.8 ms (median, 10 ms), while the polarization directions of the fast components varied between 108˚ and 226˚, with a mean direction of 166˚ ±13˚. the time delays were normalized according to the hypocentral distances, and showed a mean of 4 ±0.6 ms/km. the fast shear-wave polarization directions are presented in figure 5 as rose diagrams. for the period after the movri mountain earthquake, we obtained valid results ({, dt) from 122 seismic events (figure 4). the raw delay times varied between 5 ms and 115 ms, with a mean of 40 ±4.6 ms (median, 43 ms), while the polarization directions of the fast components varied between 106˚ and 231˚, with a mean direction of 161˚ ±8˚. the time delays were again normalized according to the hypocentral distances, and showed a mean of 11 ±1.3 ms/km for this period. the fast shear-wave polarization directions are presented in figure 5 as rose diagrams. in the following paragraphs, we discuss the measured splitting parameters and their variations before and after the movri mountain earthquake. 4.1. polarization directions comparison of the mean polarization directions that were estimated for both periods, and also taking into account the measurement errors, the fast polarization directions followed a general nnw-sse direction during both giannopoulos et al. 1030 figure 2. valid splitting measurements of the s-waves recorded at the rls station for two events that occurred in june 10, 2008, and june 19, 2008. upper panel: contour diagrams of the cross-correlation coefficients in the ({, dt) space. the preferred solutions ({, dt) corresponding to the maximum absolute values (dot) are shown within the 95% confidence region (dotted line). lower panel: superposition of e-w and n-s components (upper traces), and the corrected fast and slow components (lower traces). particle motions are shown to the right of each sub-panel. 1031 s-wave splitting temporal variation figure 3 (top). map of northwestern peloponnese, showing the events from which the valid splitting results were obtained for the period before the movri mountain earthquake. the rls station (large triangle), some of the husn seismic stations (small triangles), the movri mountain earthquake epicenter (star), and the major cities (squares) are shown. the colored circles represent epicenters. the depths of the events are color coded according to the color scale (right). the diameters of the circles are proportional to the magnitudes. figure 4 (bottom). map of northwestern peloponnesus, showing the events from which the valid splitting results were obtained for the period after the movri mountain earthquake. the rls station (triangle), some of the husn seismic stations (small triangles), the movri mountain earthquake epicenter (star), and the major cities (squares) are shown. the colored circles represent epicenters. the depths of the events are color coded according to the color scale (right). the diameters of the circles are proportional to the magnitudes. periods. the movri mountain earthquake had little or no influence on these polarization directions, and this conclusion was confirmed by the statistical test that we applied, relative to the directional data [trauth 2010]. in particular, the test statistic that was used for the testing of the similarity between the two mean directions (before and after the movri mountain earthquake) was a modified f-statistic for the directional data. the application to our data of the specific test with a level of significance of 5% resulted in a test value of f = 0.85, and a critical value of fcr = 1.44. according to theory (see appendix a), as f < fcr, we can conclude that there is no statistical evidence that supports the rejection of the null hypothesis, and therefore these two samples of data have been drawn from populations with the same mean direction. 4.2. delay times delay times are very sensitive to small changes in the geometry of micro-fractures/ micro-cracks in the crust. particularly in cases of earthquakes, there are noticeable variations in time delays that can be ascribed to stress and rock-mass strain changes [crampin and lovell 1991]. the delay times estimated for the period before the movri mountain earthquake had a mean of 18 ±2.8 ms (median, 10 ms), while after the movri mountain earthquake, there was an increase to a mean of 40 ±4.6 ms (median, 43 ms). this change was also validated following a non-parametric hypothesistesting framework. specifically, a two-sample kolmogorovsmirnov (ks) test was applied to our data [gibbons 1971], with a level of significance of 5%. the test value of q and the critical value derived from the kolmogorov distribution, ka, were 0.41 and 0.05, respectively. following theory (see appendix b), as q > ka, we can conclude that the null hypothesis has to be rejected; i.e., the two datasets were not drawn from the same distribution. 5. interpretation and discussion the movri mountain earthquake occurred on a pre-existing strike-slip fault that was probably inherited from past tectonic processes, and it affected the area of northwestern peloponnese. the strike of the fault was 210˚, as determined by seismological means [konstantinou et al. 2009b]. konstantinou et al. [2011] performed a stress inversion of all of the available focal mechanisms in this area, and they estimated that the principal compressive stress axis of the regional stress field has an azimuth of ca. 273˚, which forms an angle of ca. 63˚ with the strike of the fault. the angle between the strike of the fault and the v1 of the regional stress field implies that the fault was severely misoriented compared to the regional prevailing stress field. this means that the movri mountain earthquake occurred within an unfavorable stress regime. according to sibson [1990], for seismogenic fault reactivation to occur, a favorable orientation between the strike of the fault and v1 is needed. konstantinou et al. [2011] suggested that the presence of fluid might have facilitated the reactivation of the fault, to cause the movri mountain event. fluid might have allowed the fault to slip, by rotation of the principal stress axis locally around the fault to more favorable angles. fluids are sourced from the lower crust or upper mantle, and possibly the relatively permeable and fractured fault zone provided a passage [konstantinou et al. 2011]. to confirm that there were fluids in the area, konstantinou et al. [2011] made use of a database of helium isotope measurements around greece and the surrounding areas that was compiled by pik and marty [2009]. one such measurement taken at a location of (37.733˚ n, 21.383˚ e), which was about 20 km from the movri mountain earthquake epicenter, showed a significant percentage (ca. 7%) of mantle helium. furthermore, tomographic imaging of pn velocities performed by al-lazki et al. [2004] indicated abnormally low pn velocities beneath northwest peloponnese. giannopoulos et al. 1032 figure 5. rose diagrams of the fast shear-wave polarization directions before (a) and after (b) the movri mountain earthquake. the arrows represent the mean fast shear-wave polarization directions. 1033 these low pn velocities might represent molten areas in the upper mantle, which can be considered as possible sources of degassing fluids. according to konstantinou et al. [2011], another possible explanation of this reactivation is that the fault gouge consisted of weak material, with friction coefficients much less than 0.6, which would extend the domain of favorable orientation without making necessary the presence of overpressured fluids. however, most fault gouge minerals that show weak behavior at shallow depths have been shown to be significantly stronger at greater depths. therefore, the great depth of the movri mountain earthquake suggests that the misorientation of the fault cannot be explained by the presence of weak materials. s-wave splitting temporal variation date origin time longitude latitude distance (km) hypocentral depth (km) delay time (ms) normalized delay time (ms/km) fast polarization direction (˚) 2006/10/24 18:47:25 37.960 21.450 1.05 16 10 3.02 146 2008/07/11 14:55:31 37.953 21.458 17 55 15.71 153 2005/07/18 01:46:39 38.040 21.570 2.63 18 5 1.01 226 2008/06/27 09:52:24 38.040 21.540 18 60 18.29 175 2005/07/18 01:46:39 38.040 21.570 3.78 18 5 1.01 226 2008/07/12 00:38:13 38.042 21.527 17 30 9.77 175 2007/05/27 20:14:31 38.070 21.570 4.25 19 5 0.80 135 2008/06/27 09:52:24 38.040 21.540 18 60 18.29 175 2005/07/18 01:46:39 38.040 21.570 4.31 18 5 1.01 226 2008/06/14 01:00:22 38.042 21.521 19 30 8.98 210 2007/05/27 20:14:31 38.070 21.570 4.89 19 5 0.80 135 2008/07/12 00:38:13 38.042 21.527 17 30 9.77 175 figure 6. equal area projections of the polarization directions measured at the rls station for the periods before (left) and after (right) the movri mountain earthquake. the radius of the plots is scaled to an incidence angle of 40°. table 1. location and splitting parameters for the selected pairs of events with nearby locations, before and after the movri mountain earthquake ( june 8, 2008). time-delays are normalized to a 1-km path lengths. fast polarization directions are usually parallel or subparallel to the present-day direction of the maximum compressive stress throughout at least the uppermost 20 km of the crust [crampin and lovell 1991]. the mean fast polarization orientation obtained using the complete dataset of the present study was 164˚ ±10˚, and thus we can claim that the direction of the maximum horizontal compressive stress in the study area was almost parallel to this. consequently, this indicates a local stress field in the vicinity of the fault that had different characteristics from those of the prevailing regional stress field. more specifically, the local principal stress axis was measured as approximately orthogonal (ca. 100˚) to the regional compressive stress. this is possibly a case of ca. 90˚-flip above earthquakes in the vicinity of a major strikeslip fault, as has been reported in several studies that have indicated a key role for over-pressured fluids in the splitting parameters and in earthquake source mechanisms [crampin et al. 2002, liu et al. 1997, peacock et al. 1988]. from the comparison of the strike of the fault (210˚) and the mean fast polarization direction (164˚ ±10˚), we infer that the angle between the strike of the fault and the rotated principal local stress axis was 46˚ ±10˚. it appears that the local v1 was rotated towards lower angles to the fault compared to the regional v1. this supports the suggestion of konstantinou et al. [2011], according to which for the movri mountain earthquake to occur, v1 had to form locally smaller and more favorable angles in relation to the strike of the fault. our data characterize a specific area that was the part of the crust within the shear-wave window of the rls station, and not the broader epicentral area. for this reason, we consider our findings as an indication of local rotated fields that acted around the seismogenic fault. in terms of the time-delay variations, we note that the spatial distribution of the events before and after the movri mountain earthquake were different. figure 6 presents the fast polarization directions on equal area projections for the rls station, which shows the different azimuthal distributions of the shear-wave sources of both of the periods. this difference in ray paths might have biased the time-delay observations. to study such an effect, it would be relatively effective if we could compare any doublets or multiplets. as this was not feasible for our dataset, we selected earthquakes from each period that had, as far as possible, nearby locations. the location parameters and the measured time delays (raw and normalized) of the closely located pairs of events are listed in table 1. figure 8 shows the waveforms of the two first pairs from table 1. with this approach, we managed to compare the splitting parameters, and especially the time delays, for similar ray paths. considering that the majority of the events were located at similar depths (see figure 7) and that the geological structure of this studied part of the crust shows the same lithostratigraphic units [kamberis et al. 2005], the previous comparison becomes more credible. this analysis confirms the previously derived results that precluded any ray-path bias. according to our data, we suggest that the shear-wave splitting in the epicentral area of movri mountain earthquake was most probably caused by fluid-saturated microgiannopoulos et al. 1034 figure 7. raw and normalized time delays plotted versus the focal depth of all of the events used in this study, for both periods of interest. no systematic relations are seen between the time delays and the depth. the detected anisotropy is distributed above the depth of 15 km. 1035 cracks that were oriented parallel or sub-parallel to the maximum compressive stress axis of a local stress field in the vicinity of the fault. the earthquake caused a change in the deformation level of the crust and micro-crack system geometry (including the crack density and aspect ratio). the observed mean increase in the time delays on the one hand, and the maintenance of the same mean fast polarization direction after the movri mountain earthquake on the other hand, suggest that the cause of the observed variations in the splitting parameters was the possible migration of over-pressured fluids through the pre-fractured damage zone of the fault. 6. conclusions the shear-wave splitting analysis results presented in this study can be summarized as follows: (a) disagreement between the fast polarization directions and the orientation of the regional stress field; (b) fast polarization directions that did not change after the movri mountain earthquake; (c) time delays that increased soon after the movri mountain earthquake; and (d) these results might further support fluid involvement in the movri mountain earthquake. these results provoke the need for further investigations, to study the disagreement between the polarization directions and the characteristics of the stress field of the broader area in more detail. 7. data and resources the waveform data recorded by rls station were provided by the institute of geodynamics of national observatory of athens (www.gein.noa.gr/services/cat.html). the figures were created using the generic mapping tool software (http://www.soest.hawaii.edu/gmt/) [wessel and smith 1998]. data analysis was performed using the seismic analysis code software that was developed at lawrence livermore national laboratory, ca, usa. acknowledgements. the authors would like to thank the institute of geodynamics of the national observatory of athens for providing the seismic records used in this study. k.i. konstantinou would like to thank the national science council of taiwan for financial support for this study. we are grateful to associate editor francesca bianco, prof. edoardo del pezzo, prof. stuart crampin and an anonymous reviewer for their helpful comments and suggestions. s-wave splitting temporal variation figure 8. three-component waveform plots of two pairs of closely located events, as listed in table 1. the waveforms were band-pass filtered between 1 hz and 10 hz, and the event dates are indicated at the top. references al-lazki, a.i., e. sandvol, d. seber, m. barazangi, n. turkelli and r. mohamad (2004). pn tomographic imaging of mantle lid velocity and anisotropy at the junction of the arabian, eurasian and african plates, geophys. j. int., 158, 1024-1040. ando, m., y. ishikawa and f. yamazaki (1983). shear wave polarization anisotropy in the upper mantle beneath honshu, japan, j. geophys. res., 88, 5850-5864. aster, r., p. shearer and j. berger (1990). quantitative measurements of shear-wave polarizations at the anza seismic network, southern california: implications for shear-wave splitting and earthquake prediction, j. geophys. res., 95, 12449-12473. booth, d.c., and s. crampin (1985). shear-wave polarizations on a curved wavefront at an isotropic free-surface, geophys. j. r. astron. soc., 83, 31-45. crampin, s. 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(1990). rupture nucleation on unfavorably oriented faults, bull. seismol. soc. am., 80, 1580-1604. trauth, s.n. (2010). matlab® recipes for earth sciences, 3rd edition, springer-verlag, berlin/heidelberg. underhill, j. (1989). late cenozoic deformation of the hellenide foreland, western greece, geol. soc. am. bull., 101, 613-634. wessel, p., and w.h.f. smith (1998). new, improved version of the generic mapping tools released, eos trans. agu, 79, 579. xypolias, p., and t. doutsos (2000). kinematics of rock flow in a crustal scale shear zone: implication for the orogenic evolution of the sw hellenides, geol. mag., 137, 81-96. zatsepin, s.v., and s. crampin (1997). modelling the compliance of crustal rock-i. response of shear-wave splitting to differential stress, geophys. j. int., 129, 477-494. *corresponding author: efthimios sokos, university of patras, seismological laboratory, patras, greece; email: esokos@upatras.gr. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. s-wave splitting temporal variation appendix a f statistic test for directional data let us consider: i1, i2,..., in and {1, {2,..., {m as the two sets of azimuth measurements. we wish to statistically test the hypothesis that they belong to the same distribution. before using the appropriate statistic, a specific analysis relative to the directional data has to be followed [trauth 2010]. the characteristics of the directional data are described by measures of central tendency and dispersion, which are similar to the statistical characterization of univariate datasets. initially, we need to calculate the resultant or mean direction for the sets of angular data according to the relationships: the resultant directions of the data are given by: and the mean resultant lengths are: the test statistic that we used for testing the similarity between the two mean directions is the f-statistic, and it is given by the relationship: where k is the concentration parameter, which can be obtained from tables using rall, r1 and r2 are the mean directions (resultants) of the two datasets, respectively, and rall is the resultant of the combined datasets. the calculated f-statistic is compared with the critical values from the standard f tables, and the two mean directions are not significantly different if the measured f is lower than the critical fcr. applying the aforementioned statistical test to our data with a level of significance 5%, we estimate f = 0.85 and fcr = 1.44. as f < fcr, we conclude that there is no statistical evidence that supports the rejection of the null hypothesis, and therefore the two samples of data have been drawn from populations with the same mean direction. appendix b two-sample kolmogorov-smirnov test in the geosciences, there are a lot of occasions where we want to test the hypothesis that two datasets derive from the same statistical distribution. however, the kind and sizes of the samples do not allow any assumption about their distribution (e.g., gaussian distributed). in these cases, we need a nonparametric two-sample statistical test for checking this hypothesis. such a test is the two-sample kolmogorov-smirnov (ks) test [gibbons 1971]. the two-sample ks test is one of the most used and general nonparametric method for comparing two populations, as it is sensitive to differences in both the location and shape of the empirical cumulative distribution functions of the two samples. specifically, let us assume x1, x2, two datasets, of length n and m, respectively, and let f1,n(x), f2,m(x) be their empirical distributions. we need to test under what circumstances the hypothesis f1,n(x) = f2,m(x) (the null hypothesis) is valid. in this case, the ks statistic is: and the null hypothesis is rejected, with a level of significance a if: where ka is a constant threshold value that is derived from the kolmogorov distribution. applying this hypothesis-testing procedure in our phase delay data, with a significance level a = 5%, the values of q and ka are 0.41 and 0.05, respectively. since q > ka, we can conclude that the null hypothesis has to be rejected; i.e., the two datasets have not been drawn from the same distribution. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 [special section] on the use of the internet to collect earthquake information guest editors rémy bossu european mediterranean seismological centre (emsc) paul s. earle u.s. geological survey, denver, co, usa annals of geophysics, 54, 6, 2011 [special section] on the use of the internet to collect earthquake information table of contents on the use of the internet to collect earthquake information rémy bossu and paul s. earle 672 using internet reports for early estimates of the final death toll of earthquake-generated tsunami: the march 11, 2011, tohoku, japan, earthquake xiaolin yang, zhongliang wu, yingchun li 674 crowdsourcing earthquake damage assessment using remote sensing imagery luke barrington, shubharoop ghosh, marjorie greene, shay har-noy, jay berger, stuart gill, albert yu-min lin, charles huyck 680 usgs “did you feel it?” internet-based macroseismic intensity maps david j. wald, vincent quitoriano, bruce worden, margaret hopper, james w. dewey 688 twitter earthquake detection: earthquake monitoring in a social world paul s. earle, daniel c. bowden, michelle guy 708 flash sourcing, or rapid detection and characterization of earthquake effects through website traffic analysis rémy bossu, sébastien gilles, gilles mazet-roux, fréderic roussel, laurent frobert, linus kamb 716 comparison between low-cost and traditional mems accelerometers: a case study from the m7.1 darfield, new zealand, aftershock deployment elizabeth s. cochran, jesse f. lawrence, anna kaiser, bill fry, angela chung, carl christensen 728 community seismic network robert w. clayton, thomas heaton, mani chandy, andreas krause, monica kohler, julian bunn, richard guy, michael olson, mathew faulkner, minghei cheng, leif strand, rishi chandy, daniel obenshain, annie liu, michael aivazis 738 istituto nazionale di geofisica e vulcanologia 672 on the use of the internet to collect earthquake information annals of geophysics, 54, 6, 2011; doi: 10.4401/ag-5549 as geophysicists, we find it difficult to imagine our daily professional and personal lives without the internet. emails have been the mainstay of scientific communication since the 1980’s and websites, since their inception in the 1990’s, have grown to represent the heart of public information dissemination. now, scientific data flow in real-time around the world through custom protocols, portals, and web services, and the advent of social media services such as facebook and twitter offer both a new mechanism to communicate ideas and a potential resource for collecting data. in this issue, authors illustrate ways to augment our traditional methods of collecting data with information obtained through the internet. some of these studies, including the public interpretation of remote sensing data (barrington et al.), are still in their early stages, while others, such as, “did you feel it?” (wald et al.), were in existence before the term “crowd-sourcing” was even coined. the seven papers in this issue can be divided into two different categories: studies that actively engage the public to gather or process data, and those that obtain geophysical insight from internet-based information that is originally intended for other purposes. the first category is related to crowd-sourcing initiatives. crowd-sourcing refers to work being done or data being contributed by an unidentified “crowd” of people focusing on solving a specific task. the geo-can initiative (barrington et al.) uses crowd-sourced analysis of remote sensing imagery to speed the assessment of damage caused by natural disasters. two papers discuss expanding the observational base of seismic ground motions by crowd sourcing citizenoperated seismic networks. cochran et al. show that the quake catcher network (qcn) can be quickly and inexpensively deployed for aftershock surveys and for the collection of strong-motion data. the community seismic network (csn) intends to imbed a dense array of low-cost sensors within a traditional seismic network to rapidly produce maps of ground shaking, potentially before accurate estimates of location and magnitude are obtained (clayton et al.). wald et al. present a retrospective of “did you feel it?”, one of the first internet-based scientific crowd-sourcing initiatives. the second type of study gathers internet-based data not originally intended for geophysical analysis. two studies analyse the online reaction of eyewitnesses in the immediate aftermath of an earthquake’s occurrence to detect felt earthquakes and to characterise their impact within minutes of the earthquake’s occurrence. bossu et al. analyse fluctuations in the use of the european-mediterranean seismological centre’s (emsc) real time earthquake information website and explain how such information can potentially detect damage and be used to improve public information regarding recent events. earle et al. show how twitter, the real-time micro blogging site, can be used to detect felt earthquakes independent of seismic monitoring systems. another internet-based study by yang et al. collect internet reports of the march 11, 2011 tohoku, japan, earthquake to show that they could have provided useful early estimates of the final death toll. these studies are based on citizen engagement and they modify the ways in which we, as a scientific community, interface with society at large. in addition to the data they supply, these internet-based techniques require geophysicists to better understand and respond to public demands and expectations after natural disasters. they will continue to transform the services we provide to society on a daily basis. rémy bossu european mediterranean seismological centre (emsc), earthquake information website (www.emsc-csem.org) paul s. earle u.s. geological survey, denver, co, usa articolo mc cue.pdf key words recent faults – australia – intraplate seismicity – multiple ruptures 1. introduction since 1968, five of australia’s largest earthquakes have ruptured the ground surface (mccue, 1990; crone et al., 1997). these five surface ruptures comprise nearly half of all known historical surface ruptures during earthquakes in stable continental regions throughout the lake edgar fault: an active fault in southwestern tasmania, australia, with repeated displacement in the quaternary kevin mccue (1), russ van dissen (2), gary gibson (3), vagn jensen (4) and bruce boreham (5) (1) australian seismological centre, canberra, australia (2) institute of geological and nuclear sciences, lower hutt, new zealand (3) seismology research centre, richmond, victoria, australia (4) university of tasmania, hobart tasmania, australia (5) department of industry, tourism and resources, canberra, australia abstract the lake edgar fault in western tasmania, australia is marked by a prominent fault scarp and is a recently reactivated fault initially of cambrian age. the scarp has a northerly trend and passes through the western abutment of the edgar dam, a saddle dam on lake pedder. the active fault segment displaces geologically young river and glacial deposits. it is 29 ± 4 km long, and dips to the west. movement on the fault has ruptured the ground surface at least twice within the quaternary and possibly the last ca. 25 000 years; the most recent rupture has occurred since the last glaciation (within the last ca. 10 000 years). this is the only known case of surface faulting in australia with evidence for repeated ruptures in the late pleistocene. along its central portion the two most recent surface-faulting earthquakes have resulted in about 2.5 m of vertical displacement each (western side up). the lake edgar fault is considered capable of generating earthquakes in the order of magnitude 61/2-71/4. the gell river fault is another fault nearby that was apparently also active in the late pleistocene. it has yet to be studied in detail but the scarp appears to be more degraded and therefore older than the most recent movement on the lake edgar fault. the globe (johnston et al., 1994). the fault scarps produced by these earthquakes are quite impressive features in the landscape – extending for some tens of kilometres in length and up to several metres in height. these scarps preserve information related to the earthquake’s location, size and the timing of the past event or sequence of events. there were eight large shallow earthquakes with a magnitude of 6.0 or more in australia between 1901 and 1967 and another eleven between 1968 and 2000. the largest, in 1906, had a magnitude of ms 7.2. none of the pre1968 20th century events are known to have caused surface ruptures. more than a dozen fault scarps of late quaternary age have been discovered to date in 1107 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: mr. kevin mccue, australian seismological centre, po box 324, jamison centre act 2614, australia; e-mail: asc@netspeed.com.au 1108 fig. 1. simplified geological map of tasmania showing the location of the lake edgar and gell river faults (after burrett and martin, 1989). kevin mccue, russ van dissen, gary gibson, vagn jensen and bruce boreham australia (mccue, 2001) so there is little doubt that the past 35 years are not so unusual in relation to the frequency of large earthquakes. the study of these fault scarps and related geology will provide critical information for defining australian earthquake source zones, determining the long-term average recurrence rate of large earthquakes, and assessing earthquake hazard in australia. such study should also contribute to the debate on whether the source regions of the five fault scarps produced since 1968 are still capable of producing further large earthquakes. the purpose of this paper is to present results from our two field studies of the pre-historic lake edgar fault scarps of southwestern tasmania (fig. 1). we describe the features of the fault, and then present evidence for repeated surface-faulting earthquakes on the lake edgar fault within the quaternary (the last ca. 2 ma). we conclude with some speculative comments regarding possible spatial and temporal relations between surface-faulting earthquakes on the lake edgar fault and on a second active fault approximately 40 km to the north, the gell river fault. this part of southwestern tasmania supports temperate rainforest and button grass plains, and experiences ten times the average rainfall of the semi-arid regions of southwest western australia and central australia where the five other scarp-producing events, including the 1968 meckering and 1988 tennant creek earthquakes, occurred. 2. lake edgar fault 2.1. fault geometry the lake edgar fault is located about 70 km inland from tasmania’s western and southern coastlines in the tasmanian wilderness world heritage area and a similar distance west of hobart the capital city (fig. 1). being a world heritage listed area, no trenching is allowed, access is difficult and sampling restricted. the fault was first noted and partially mapped by carey and newstead (1960) who identified it as a recently reactivated fault, initially of cambrian age (ca. 540 ma). it is within a precambrian inlier or region called the jubilee region within which the subsurface continuity of precambrian rocks is thought to be likely (turner, 1989). precambrian rocks underlie about 20% of tasmania. according to turner the inliers are separated by either folded strata of latest precambrian to devonian age or relatively flat lying carboniferous to cainozoic strata. the fault has a northerly trend and is upthrown to the west. carey and newstead also reproduced a copy of an aerial photograph showing two small shallow lakes formed by ponding of streams against the upthrown block (fig. 2). note the fans emanating westward from the ranges to the east. from the photograph we infer that the lakes were once joined, the larger remthe lake edgar fault: an active fault in southwestern tasmania, australia, with repeated displacement in the quaternary fig. 2. airphoto of the lake edgar fault prior to the filling of lake pedder, north up the page. creeks draining from mountains to the east have ponded against the 2 m high scarp, forming a lake which has since partially dried leaving two small lakes, the southern one is lake edgar. 1109 nant was called lake edgar, hence the name of the fault. carey and newstead commented on the youthfulness of the fault scarp based on the observation that the ponds had not yet silted up despite the high average annual rainfall of more than 2500 mm/yr. these ponds were drowned when the gordon, scott’s peak and edgar dams were built in the early 1970s creating lake pedder. in the region of the scott’s peak and edgar dams, the edgar fault separates an older precambrian graphitic phyllite to the east from a precambrian metasiltstone, argillite, quartz sandstone conglomerate to the west. wedges of dolomite and limestone on its western side are truncated against the fault. during investigation of the edgar dam site about 1964 and prior to dam construction, geologists with the then hydro-electric commission dug an 80 m long trench west from the scarp and excavated a number of co-linear pits. their scope was to investigate possible water leakage along the scarp that cuts through the right abutment of edgar dam (roberts et al., 1975). according to the authors «the edgar fault is a sinistral wrench fault trending approximately north-south with an original movement of approximately 12 km. it passes within 76 m of the western end of edgar dam». they note that the «original fault movement dragged a narrow, possibly discontinuous, sliver of dolomite north from the main body a maximum distance of 1200 m.». roberts et al. (1975) also drilled an oblique hole to intersect the fault at about 100 m depth that showed it has a steeply dipping reverse component. starting in 1994 we made two field surveys of the scarp with assistance from hydro tasmania and tasmanian parks and wildlife service (mccue et al., 1996; van dissen et al., 1997). a post-reservoir filling aerial photograph looking north towards the small pond at the southern base of edgar dam is shown in fig. 3. an active fan flowing out of the ranges to the east is clearly dissected by the fault scarp. carey had traced the scarp for nearly 12 km (7 miles) in the 1960s. on our second visit in 1997 we used a helicopter to track the surface rupture and found that it curved around to the southeast from its previously mapped southern end for another 13 km (fig. 4). we subsequently confirmed this using 1960s-vintage black and white aerial photographs. the water-filled trench was in good shape in 1994, 30 years after its completion, the walls were still vertical with only small collapse zones. with continuous water pumping we were able to reoccupy the trench near the centre of the surface fault and on the second field trip in 1997 briefly expose the overthrust glacial gravel layer at the former land surface. the inferred fault plane has a dip of about 60°-70° to the west at this nearsurface location. mckavanagh (in van dissen et al., 1997) made three levelling traverses across the scarp as shown in fig. 5, north up the page. the central 1110 fig. 3. aerial photo of the lake edgar fault, north up the page. a post-last-glaciation fan from the mountains to the east has planed and then been dissected by the last movement on the fault. water ponded against the downstream face of edgar dam (off the top of the page) and the fault can be seen at the top of the photo. the prominent white east-west scar is a former airstrip, the trench and exploration pits are a few hundred metres parallel to and south of the airstrip (see fig. 5). kevin mccue, russ van dissen, gary gibson, vagn jensen and bruce boreham 1111 the lake edgar fault: an active fault in southwestern tasmania, australia, with repeated displacement in the quaternary fig. 4. mapped surface expression of the lake edgar fault, tasmania, west side up. the three arrows show our levelling traverses. the relationship of the fault to two of the dams impounding lake pedder, edgar dam and scotts peak dam is shown. traverse was alongside and parallel to the trench where the scarp is 2.5 m high. the trench and series of exploratory pits can just be seen in this figure on the western side of the scarp as a thin white line and line of white dots. seventy metres north of the trench at the second traverse, the scarp is 4 m high. our last traverse was 500 m south of the trench where the scarp approached its maximum height of 6 m. the unusual variability in the scarp height was a puzzle at first. but air photos such as that in fig. 3 show alluvial fans flowing west out of the ranges to the east and cutting across the central part of the scarp at its lowest point. fan surges have planed off the scarp at least once and the fan has been cut in turn by subsequent fault ruptures. we interpret the 6 m high section to be a measure of the unaltered cumulative fault displacement. our interpretation is shown in cartoon form in fig. 7 and discussed in more detail below. hale and roberts (personal communication) identified another co-linear fault scarp, the gell river fault to the north of the lake edgar fault scarp and about 30 km or a fault dimension away. maximum crustal thickness in tasmania is nearly 28 km with about 5 km of relief on the moho (richardson, 1989). our aerial examination showed that the gell river fault scarp is more dissected and eroded than the lake edgar scarp and apparently older, but clear evidence that there was at least a third large quaternary earthquake in central western tasmania. 2.2. evidence for repeated quaternary displacement geomorphology – deposits and landforms from four distinct glaciations have been identified in the lake edgar region (colhoun and fitz1112 kevin mccue, russ van dissen, gary gibson, vagn jensen and bruce boreham fig. 5. aerial photograph (north up the page) showing edgar dam and the location of surveyed cross sections (three arrows). the trench is the fine white line of exposed glacial gravels opposite the central arrow. cross-section profiles are on the right. simmons, 1990). the glaciations are thought to date from the latest pleistocene (youngest glacial maximum, 14.000 to 25.000 years bp, the glaciation finished by 10.000 years bp), middle pleistocene (middle two glaciations), and early pleistocene or older (oldest glaciation, ca. 2 ma). the lake edgar fault displaces till of the oldest glaciation, the scarp height across these highly weathered deposits is about 6 m. the fault also cuts younger alluvial fans that correlate to the younger glaciations. along the central portion of the fault and across the youngest fan the scarp is about 2.5 m high. across the next oldest fan the scarp is about 5 m high. older fans have higher scarps which indicates that the fault has generated at least two surface-faulting earthquakes within the quaternary, possibly within the last 25.000 years; the youngest being within the last ca. 10.000 years. trench exposure – the 80 m long trench was excavated west from the fault scarp and near its centre where the youngest extensive fan that we consider latest pleistocene in age ( ca. 10.000 years) is draped over the scarp. nearly three decades after the trench was dug, we pumped it dry and cleaned-down its walls and were rewarded with clear stratigraphic evidence of at least two surface-faulting earthquakes. a simplified trench log of the sunlit southern face of the trench is shown in fig. 8. white fan gravels and underlying basement of sands and peats/lignites comprise the main units exposed in the trench (fig. 6). the oldest unit is a dark brown peat/lignite that contains diagenetic quartz laminae. these laminae, which probably formed parallel to the the lake edgar fault: an active fault in southwestern tasmania, australia, with repeated displacement in the quaternary fig. 6. the trench showing the white glacial fan gravels overlying basement. one of the authors (rvand) is pointing out a gravel filled tension crack fig. 7. cartoon showing the sequence of scarp development; scarp formation, scarp planation by glacial fans and subsequent re-faulting. time 4 is current time. 1113 once horizontal bedding in the peat, are now deformed and locally attain dips of 45°-80° (fig. 8). near the fault, fine white sand appears to have intruded into the peat/lignite. we interpret this sand unit as a sand-blow (amick et al., 1990; obermeier, 1994), a liquefaction feature resulting from strong earthquake ground shaking. the fan gravels comprise, in part, the youngest extensive fan, and are warped over the scarp that, at this locality, is about 2.5 m high (we take this to represent the amount of vertical displacement associated with the most recent surface-faulting earthquake). two lines of evidence indicate older faulting: i) the very fine, white, silty sand interpreted as a sand-blow is truncated by, and thus older than, the overlying gravel. the shaking that resulted in the emplacement of the sand-blow must be older than both the gravel and the surface-faulting that later deformed the gravel. ii) quartz laminae in the peat/lignite are more deformed than the overlying gravel, suggesting that the laminae were already deformed by at least one earthquake prior to the deposition, and subsequent deformation, of the younger gravel. several gravel filled tension cracks can be seen in the over-thrust block (fig. 8). 2.3. dating the last event several charcoal samples were collected from pre and post-earthquake soil horizons to bracket the age of faulting (fig. 8), but the c14 dating results were disappointing. the fibrous dark brown peat on the ground surface of the footwall block at the fault was too modern and the samples from the body of the hanging wall block, a fine dark brown silty fine sand were too old (> 39.600 years old). as carey said, the scarp must be recent for the ponds to have survived silting up in this high rainfall environment. the scarp itself is still very clear (figs. 4 and 5), in fact it is more prominent than the 1968 meckering earthquake scarp in western australia which has been severely degraded, thanks in large part to farm management practices. the 1988 tennant creek earthquake scarp in the northern territory has also weathered extensively due to the sandy soil and infrequent but heavy downpours of rain. 1114 kevin mccue, russ van dissen, gary gibson, vagn jensen and bruce boreham fig. 8. simplified log of the trench excavated through the lake edgar fault. the fault trace is at the eastern end of the trench. the disturbed quartz laminae and truncated liquefaction sand lens are evidence of a previous faulting episode while the gravel filled tension cracks in the overthrust block are typical features of reverse faulting. the last large earthquake on the lake edgar fault would have been felt throughout tasmania and in southern victoria. the historic earthquake record of the last nearly 200 years contains no such reports so the earthquake predates the arrival and spread of european immigrants in tasmania nearly 200 years ago. the event then was sometime between 200 years ago and the end of the last ice age ca. 10.000 years ago. we consider that based on the subjective information of c14 dating of sediment deposition on the footwall block, on the non-silting up of lake edgar and on the sharp scarp morphology, the causative earthquake probably occurred nearer 200 than 10.000 years ago. 2.4. earthquake magnitude estimate an approximate magnitude for causative earthquakes can be estimated, based on empirical relations between surface rupture parameters and earthquake magnitude. the mapped length of the lake edgar scarp is 29 ± 4 km, at least 25 km but no more than 33 km depending on its continuation into heavily wooded hilly country at its northern end. vertical offset across the fault ranges from 2.5 m for the last faulting event to an average of 2 m (three events) or 3 m (two events). mccue (1990) plotted magnitude m against rupture length l (km) and maximum vertical displacement u (m) of the five australian earthquakes ranging from m 5.1 to 6.8 that are known to have ruptured the surface. he obtained by least squares fit m = 4.11 (± 0.18) + 1.65 (± 0.15) log.l m = 5.04 (± 0.17) + 0.75 (± 0.10) u. the first equation yields magnitudes of 6.4 to 6.6 for the range of the observed lake edgar fault scarp length while the second yields magnitudes of 6.5 to 7.3 for the imputed uplift. wells and coppersmith (1994) computed the regression of subsurface rupture length l (km) on magnitude m for 167 earthquakes for all slip types. magnitudes ranged from 4.7 to 8.2. m = 4.38 + 1.49 log.l. this equation yields magnitudes of 6.5 to 6.6 for the range of the observed scarp length. we consider that the lake edgar fault is capable of generating earthquakes in the magnitude range 61/2 -71/4. calculations based on seismic moment considerations also yield magnitude estimates in the order of magnitude 7. 3. the gell river fault and possible relations with the lake edgar fault about 30 km north of the lake edgar fault lies another fault with apparent late pleistocene movement, the gell river fault which has a north-northeast trend and is ca. 10 km long (figs. 1 and 9). the active trace of the gell river fault appears to be more subdued than that of the lake edgar fault; this could suggest that the most recent displacement of the gell river fault is older than that on the lake edgar fault. separating the two faults are two large bodies of ultramafic rock, including serpentinite and talc. the ultramafic rocks appear to be bounded by the northern extension of the lake edgar fault (burrett and martin, 1989). the gell river fault is parallel to local geological structure, though based on existing mapping, it is not clear whether it is part of the structure that controls the location of the active trace of the lake edgar fault. we speculate that the recent activity on the lake edgar and gell river faults, and their geographic position relative to each other, is influenced by the occurrence of relatively weak ultramafic rocks along a major pre-existing fault. the more easily deformed talc and serpentinite between the two faults may, in essence, act to concentrate strain in the lake edgar and gell river regions (malcolm somerville, personal communication). this strain may be released by creep in the weak ultramafic rocks. to the north and south, where the rocks are presumably 1115 the lake edgar fault: an active fault in southwestern tasmania, australia, with repeated displacement in the quaternary stronger, strain is released episodically as earthquake rupture along the gell river and lake edgar faults respectively. 4. conclusions the evidence of glacial fans offset by faulting and disproportionate displacement along the fault scarp led us to develop a model of fault growth, planation and subsequent faulting. this model demonstrates that the lake edgar fault has generated at least two reverse surface-faulting earthquakes within the quaternary, the most recent of these being younger than ca. 10.000 years by which time the glaciers had melted. these earthquakes were probably in the order of m 61/2 -71/4 in size, and produced, along the central portion of the fault, at least 2.5 m of vertical displacement. there was at least one other late pleistocene earthquake in central western tasmania on the gell river fault. the spatial relationship between the lake edgar and gell river faults, and temporal relationships between surface-faulting earthquakes on these two faults, are probably influenced by one, or both, of the following: i) the existence of a pre-existing fault which is oriented favourably, both with respect to strike and dip, for movement in the contemporary stress field, and ii) the existence of large bodies of ultramafic rocks, including easily deformed talc and serpentinite, along the pre-existing fault which may concentrate strain in the lake edgar and gell river areas. the lake edgar fault is the only identified australian fault with unequivocal evidence for repeated movement within the late pleistocene. acknowledgements we are indebted to dr. malcolm somerville (deceased), and thank byron mckavanagh, and dr. albert goede who participated in one of the field studies. this study benefited from discussions with numerous people; we would specifically like to acknowledge those with glyn 1116 fig. 9. gell river fault between arrows, oblique aerial photo looking west. kevin mccue, russ van dissen, gary gibson, vagn jensen and bruce boreham roberts (formerly of the hydro-electric corporation), tom laudon (university of wisconsin), and ruth and david wilson (hydro-electric corporation). we also wish to thank hydro tasmania, particularly andrew pattle, the dam safety officer, and the tasmanian parks and wildlife service. this study was sponsored by hydro tasmania as part of an ongoing program of assessing the seismic risk for their dams. some financial support was also provided by the then department of industry, technology and commerce. we thank kelvin berryman as well as des darby, kathleen hodgkinson, and mark rattenbury for their suggestions to improve an earlier draft of this manuscript. references amick, d., g. maurath and r. gelinas (1990): characteristics of seismically induced liquefaction sites and features located in the vicinity of the 1886 charleston, south carolina earthquake, seismol. res. lett., 61, 117-130. burrett, c.f. and e.l. martin (1989): geology and mineral resources of tasmania, geol. soc. australia, spec. publ., 15, pp. 574. carey, s.w. and g. newstead (1960): tasmania university seismic net, publication 84 (geology department, university of tasmania). colhoun, e.a. and s.j. fitzsimmons (1990): late cainozoic glaciation in western tasmania, australia, quat. sci. rev., 9, 199-216. crone, a.j., m.n. machette and j.r. bowman (1997): episodic nature of earthquake activity in stable continental regions revealed by paleoseismicity studies of australian and north american quaternary faults, aust. j. earth sci., 44, 203-214. johnston, a.c., k.j. coppersmith, l.r. kanter and c.a. cornell (1994): the earthquakes of stable continental interiors, epri report tr-102261-v1 us. mccue, k.f. (1990): australia’s large earthquakes and recent fault scarps, j. struct. geol., 12, 761-766. mccue, k.f. (2001) earthquake epicentres in australia 1841-2000 and recent fault scarps, 1:10m map published by geoscience australia. mccue, k.f., b. boreham, r. van dissen, g. gibson, v. jensen and b. mckavanagh (1996): a paleoseismology case study: the lake edgar fault scarp in tasmania, in proceedings of the 13th agc, canberra, 19-23 february 1996, gsa abstracts 41, geoscience for the community. obermeier, s.f. (1994) using liquefaction-induced features for paleoseismic analysis, u.s. geol. surv. openfile rep. 94-663, chapter a, pp. 58. richardson, r.g. (1989) crustal thickness, in geology and mineral resources of tasmania, edited by c.f. burrett and e.l. martin, geol. soc. australia, spec. publ., 15, 465-467. roberts, g.t., b.a. coleb and r.h.w. barnett (1975): engineering geology of scotts peak dam and adjacent reservoir watertightness, aust. geomech. j., 39-45. turner, n.j. (1989): precambrian, in geology and mineral resources of tasmania, edited by c.f. burrett and e.l. martin, geol. soc. australia, spec. publ., 15, 5-46. van dissen, r., k.f. mccue, g. gibson, v. jensen, m. somerville, b. boreham, b. mckavanagh and a. goede (1997): the lake edgar fault: evidence for repeated quaternary displacement on an active fault in southwest tasmania, in proceedings of the seminar «earthquakes in australian cities can we ignore the risks?», australian earthquake engineering society, brisbane 2-3 october 1997. wells, d.l. and k.j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seismol. soc. am., 84, 974-1002. 1117 the lake edgar fault: an active fault in southwestern tasmania, australia, with repeated displacement in the quaternary k k k k k k 17suleiman.qxd 545 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words libya – historical seismicity 1. introduction the complex pattern of deformation between eurasia and africa, the corresponding plate motions and underlying driving forces, lead to strong variations in seismic hazard throughout the region. libya, located at the north central margin of the african continent, has experienced a considerable intraplate tectonism particularly in its northern coastal regions. the information on the seismicity related to the seismotectonic setting of this region mostly relies upon the data from instrumental recording, and the historical earthquakes of the past are scarcely known (fig. 1). although the authors are aware that compiling a uniform catalogue of the earthquakes known to have affected libya in the past centuries is a priority, and especially for seismic hazard assessment purposes, this article is simply an overview of the studies available to date on historical earthquakes in libya, as the basis for establishing an international collaboration in this field. the earthquakes which affected libya up to 1935, though not necessarily located in this country, are presented through a summary of the available information and with a specific reference to the most recent critical interpretation offered by a few studies published in the last twenty years. the most important earthquakes considered by the aforementioned studies are listed according to two time frames, up a short introduction to historical earthquakes in libya abdunnur s. suleiman (1), paola albini (2) and paola migliavacca (2) (1) geophysics department, faculty of science, al-fateh university, tripoli, libya (2) istituto nazionale di geofisica e vulcanologia, sezione di milano, italy abstract as a result of the relative motion of the african and european plates, libya, located at the north central margin of the african continent, has experienced a considerable intraplate tectonism, particularly in its northern coastal regions. if the seismic activity of the last fifty years, at most, is known from instrumental recording, macroseismic effects of those earthquakes which affected libya in the past centuries are still imperfectly known. to try and partly overcome this lack of information, in this contribution we present a short introduction to historical earthquakes in libya, focusing on the period up to 1935. according to the studies published in the last twenty years, the earliest records of earthquakes in libya are documented in the roman period (3rd and 4th century a.d.). there is a gap in information along the middle and modern ages, while the 19th and early 20th century evidence is concentrated on effects in tripoli, in the western part of nowadays libya. the hun graben area (western part of the gulf of sirt) has been identified as the location of many earthquakes affecting libya, and it is in this area that the 19 april 1935 earthquake (mw = 7.1) struck, followed by many aftershocks. further investigations are needed, and some hints are here given at historical sources potentially reporting on earthquake effects in libya. their investigation could result in the needed improvement to lay the foundations of a database and a catalogue of the historical seismicity of libya. mailing address: dr. suleiman abdunnur, faculty of science, al-fateh university, p.o. box 13044, tripoli, libya; e-mail: abdunnur_s@hotmail.com to 1900 (table i) and from 1901 to 1935 (table ii). their location is shown in fig. 1. after short considerations on these studies, the results of sample investigations and some suggestions on potential sources possibly dealing with earthquake effects in libya are proposed. 2. modern studies of past libyan earthquakes only recently the past seismicity of libya attracted the interest of the researchers. according to our review of the studies produced in the last twenty years, the most studied earthquakes are those occurred in the 3rd and 4th centuries (table i) and in the early 20th century (table ii). some contributions dealing with earthquakes affecting libya in the antiquity relied upon historical data, while others used both historical and archaeological data. in particular, the 262 a.d. earthquake, which is considered to have affected cyrene, is described only according to the historical data (guidoboni, 1989; guidoboni et al., 1994; ambraseys, 1994; ambraseys et al., 1994). the archaeological data supplied the most relevant information on an earthquake that caused damage in tripolitania in 306 or 310 (polidori et al., 1999) and two other earthquakes in cyrenaica between 361 and june 363 (guidoboni et al., 1994) and at the end of the 4th century 546 abdunnur s. suleiman, paola albini and paola migliavacca fig. 1. seismicity of libya from the antiquity to 1935. note that the city of brace is today al-maraj and cyrene is today shahat. 547 a short introduction to historical earthquakes in libya table i. earthquake effects in libya up to 1900 according to the studies published in the last twenty years (d = damage; f = felt). date area of origin and libyan affected places level of effects study comments 262 a.d. libya? cyrene d guidoboni (1989) guidoboni et al. (1994) historical data libya? cyrene d ambraseys et al. (1994) ambraseys (1994) historical data 306 to 310 libya? leptis magna, sabratha d polidori et al., 1999 archaeological and historical data 361-june 363 libya? cyrene, balagrae d guidoboni et al. (1994) archaeological and historical data 365 north of creta libya d guidoboni (1989) guidoboni et al. (1994) archaeological and historical data – leptis magna, sabratha d di vita (1995) polidori et al. (1999) archaeological and historical data hellenic arc cyrenaica d ambraseys et al. (1994) historical data hellenic arc apollonia, balagrai, cyrene, hydrax, setif d ambraseys (1994) historical data hellenic arc leptis magna, sabratha, balagrae, cyrene, ptolemais stiros (2001) archaeological and historical data end of 4th century libya? cyrene, hydrax d guidoboni et al. (1994) bacchielli (1995) archaeological and historical data 704 and/or 742 «spurious» – ambraseys (1984) ambraseys et al. (1994) mislocated in libya 1183 «spurious» – ambraseys (1984) tripoli in syria (today in lebanon), damaged by the 1170 earthquake, interpreted as tripoli in libya 1656 libya tripoli d ambraseys (1984) ambraseys et al. (1994) 1803 libya? tripoli f ambraseys et al. (1994) probably a duplication of the 1903 earthquake (see table ii, this study) 1811 egypt libyan desert near siwa f ambraseys et al. (1994) 5 august 1853 libya murzuq (fezzan) f ambraseys et al. (1994) 22 december 1860 libya border with tunisia f ambraseys et al. (1994) 24 june 1870 eastern mediterranean benghazi f ambraseys et al. (1994) august 1883 libya ghadàmes d ambraseys (1984) ambraseys et al. (1994) 548 abdunnur s. suleiman, paola albini and paola migliavacca (guidoboni et al., 1994; bacchielli, 1995). for these two earthquakes, the available studies only mention the places affected in libyan territory, while their epicentral area is considered difficult to assess at this stage. on the other hand, there is a good agreement on the fact that the 365 a.d. earthquake originated in the hellenic arc among the several studies who dealt with this large, mediterranean event. for what libya’s territory is concerned, table i list the main studies describing damage either to both tripolitania and cyrenaica (guidoboni, 1989; guidoboni et al., 1994; di vita, 1999; stiros, 2001) or to tripolitable ii. earthquakes affecting libya between 1901 and 1935 (m≥5.0). date area of origin and libyan affected places study magnitude 22 november 1903 libya tripoli region of tarhuna ambraseys (1984) ambraseys et al. (1994) 5.0 (2) 2 february 1907 – – 5.8 (1) 5.8 (2) 5.8 (3) 18 may 1914 libya al qaddahiyah ambraseys (1984) ambraseys et al. (1994) 5.1 (1) 5.0 (2) 14 october 1918 – – 5.4 (2) 5.4 (3) 26 june 1926 hellenic arc north libya ambraseys et al. (1994) 7.1 (2) 30 august 1926 hellenic arc — ambraseys et al. (1994) 7.4 (2) 19 april 1935 15.23 gmt western gulf of sirt damage at: al qaddahiya bu njaim buayrat al hsun zlitan misratah bani walid zillah sirt tripoli al qaddahiya bu njaim area ambraseys (1984) suleimann and doser (1995) 7.0 (1) 7.0 (2) 7.0 (3) 6.9 (4) 19 april 1935 20.31 gmt al qaddahiya bu njaim area suleimann and doser (1995) 5.7 (1) 5.6 (2) 5.6 (3) 6.4 (4) 20 april 1935 05.10 gmt al qaddahiya bu njaim area suleimann and doser (1995) 6.2 (1) 6.1 (2) 6.1 (3) 6.4 (4) (1) ms from ambraseys (1984), table 1; (2) ms from ambraseys et al. (1994), table 3.1; (3) ms from ambraseys (1994), table 1; (4) mw from suleimann and doser (1995), table 3. 549 a short introduction to historical earthquakes in libya tania only (polidori et al., 1999) or mainly to cyrenaica (ambraseys, 1994; ambraseys et al., 1994). the mentioned studies thoroughly discussed both the provenance and reliability of historical and archaeological data they used, and to them we address the reader for details. here it is only worth mentioning that there are very few reliable written records testifying to earthquake effects in the territory of libya in this early period, namely those related to the 262 and 365 events, and even these are very generic in nature. there is only the study by ambraseys (1984) dealing with the seismicity of tripolitania from antiquity to mid 20th century. between the 5th to mid 17th century, the effects of two earthquakes only were related either to the whole of libya (704 a.d.) or to tripoli (1183) (table i). for both, ambraseys (1984) concluded that they were mislocated or the records misinterpreted. in both cases, the information comes from sieberg (1932a,b). the reliability of sieberg’s lists of earthquakes has been discussed on other occasions for which his list was the only evidence, the main criticism being that he did not supply precise references for his statements, making it impossible to trace back and verify the actual records he used. between mid 17th and late 19th century, the available studies supply information on a 1656 event affecting tripoli, and on six 19th century earthquakes with moderate effects in libya (ambraseys, 1984; ambraseys et al., 1994). few words on the 1656 earthquake, which is considered as having affected tripoli on the basis of the evidence supplied by a later seismological compilation (dresdnissche geleherte anzeigen, 1756), mentioning «die stadt tripoli in der barbaren» (the town of tripoli in the barbary states). two centuries after the earthquake, von hoff (1840) wrongly associated this earthquake with tripoli in syria (today lebanon), and since then this event is uncritically located by earthquake catalogues in the middle east instead of in libya. for the sake of completeness, earthquake effects in libyan territory from the antiquity to the 20th century are included in ambraseys (1994) and ambraseys et al. (1994). since the advent of instrumental recording, a large number of earthquakes have been recorded in libya and its offshore region. figure 1 includes the epicentres of these earthquakes for the period 1901-1935 according to the locations proposed by the studies in table ii. studies on this period concern more the interpretation of instrumental recordings than macroseismic effects (e.g. suleiman and doser, 1995). there is the only exception of ambraseys (1984) and the information he gathered on the 1935 earthquake in the western part of the gulf of sirt. he listed ten damaged places in libya (table ii) and about twenty more places in libya and tunisia where the earthquake was felt; macroseismic effects were found in unpublished sources and reported by «residents in tripolitania at the time of the earthquake». 3. sample investigations for macroseismic evidence the following case histories show the outcome of sample investigations for macroseismic evidence of known and unknown past earthquakes in libya, looked for in italian sources. they are meant to introduce the suggestions on potential historical sources proposed in the following section. 3.1. an unknown 17th century earthquake at tripoli, libya? mention of the 25 may 1685 earthquake appears for the first time in this study. it was retrieved through the bibliography on libya by minutilli (1903) and the reference given by bono (1982). the flysheet (fig. 2) contained an anonymous letter sent from tripoli to a merchant in ancona in which, together with other phenomena and events, there is the following description of an earthquake (anonymous, 1685): «sul fare del giorno [24 may] si cominciò ad oscurar talmente, che pareva si fosse mutato da giorno in notte, e poi venne una gran pioggia accompagnata da terribilissima tempesta, che ruppe tutti li tetti delle case, e uccise più di cinque mila di quei barbaricani, con la 550 abdunnur s. suleiman, paola albini and paola migliavacca morte di tutti li bestiami, che si ritrovavano in campagna. il giorno seguente [25 may] abbaté il terremoto più di 200 case, e frà queste le tre più famose moschee che soliti orare questi maometani ... (on the beginning of the day [24 may] it became so dark, that it seemed that day had changed into night, and then a heavy rainfall came together with hail, and all the roofs were broken, and more than five thousand barbarians died, and the livestock as well, in the country. on the day after [25 may] the earthquake made more than 200 buildings collapse, and among them the three most famous mosques where the muslims used to pray)». the whole letter sounds emphatic, and verging on exaggeration. at this stage, there are no other independent sources available on this earthquake, as well as no substantial elements to doubt of the truthfulness of this record (on this point see bono, 1982). in particular, this record does not coincide with information on any event that affected tripoli in syria (today lebanon) in 1685. anyway, the uniqueness of the available source suggests that further evidence is needed to link without any doubt this record with tripoli in libya. 3.2. an italian source on the 1883 earthquake, ghadàmes concerning the effects of the 1883 event affecting the oasis of ghadàmes and surroundings (south-western part of libya), an anonymous note that appeared in an italian geographical journal has been retrieved (anonymous, 1884). it contained a summary of a report sent to an equivalent french journal (comptes-rendus de la societé géographique) by duveyrier (1883), who is the source mentioned by ambraseys et al. (1994). in his turn, duveyrier had received the information through a letter by «qâsem ben mûsê ben el hâg’ mohamed el ghedâmesi», dated tripoli, 8 october 1883. from the date given according to the libyan use, «end of the month of chawâl» the earthquake could be dated in the gregorian calendar «between 23 august and 2 september 1883» (duveyrier, 1883 as reported by anonymous, 1884). 3.3. looking for macroseismic effects of the 1926 and 1935 earthquakes these two earthquakes differ from each other for having the 1926 origin in the hellenic arc and the 1935 for being the largest 20th century instrumentally recorded earthquake with epicentre located inside libya’s territory (see table ii for magnitude estimates). in that period, libya was under italian domination, and this is the main reason for a supplement of investigation in italian records on these early 20th century earthquakes. a new piece of information on the hellenic arc earthquake of 30 august 1926 was retrieved. two seismological bulletins published at the time in italy (cavasino, 1927; ingrao, fig. 2. first page of the flysheet describing the 25 may 1685 eartquake at tripoli, lybia. 551 a short introduction to historical earthquakes in libya 1930) under the section devoted to «colonie italiane» (italian colonies) report that on 12:40 of the 30 august 1926 at benghazi and derna an earthquake was felt and an intensity of the iv degree according to the mercalli macroseismic scale was assessed. the intensity is in this case higher than that reported at tripoli and benghazi for the 26 june 1926 event (ii-iii for both places) (see table ii). unfortunately, no more details are given. on 19 april 1935, at 15.23 gmt, a large earthquake (mw = 7.1, according to suleiman and doser, 1995; ms = 7.0 according to ambraseys, 1994) hit the hun graben area near the city of al-qaddahiya, in the western part of the gulf of sirt. it was followed by a large number of aftershocks, two with mw 6.4 (suleiman and doser, 1995), which occurred respectively five hours later on 19 april, and in the early morning of 20 april. people all over the country felt this earthquake and the year 1935 was remembered as the «year of the shock». libya being at that time under italian rule, a sample investigation has been carried out in the the italian press, looking for reports on this earthquake. one of the main italian newspaper published at that time, the «corriere della sera», though supplying information on earthquakes in other parts of the world, and accurately describing the visit of umberto, prince of savoy, and his wife to tripoli and ghadàmes at the end of april 1935 (corriere della sera, 1935c), does not mention any earthquake effects in libya. censorship of disasters affecting the «italian territory» was common in that period, and is confirmed by the fact that in the 20 april issue (corriere della sera, 1935a) the generic information of a distant earthquake was in fact reported: «un terremoto catastrofico a 1500 km. registrato dall’osservatorio ximeniano padre guido alfani ... comunica: nel pomeriggio, alle 16.23’ 30”, tutti gli strumenti sismici dell’osservatorio hanno iniziato la registrazione di un terremoto lontano. ... la distanza della località colpita non risulta molto grande perché s’aggira sui 1500 chilometri ... stando tuttavia ad alcuni indizi e con tutte le riserve, sembrerebbe non potersi escludere che si tratti di un terremoto di origine sud-orientale» (a catastrophic earthquake at a distance of 1500 km recorded by the osservatorio ximeniano [in florence] father guido alfani says: in the afternoon, at 16.23′30″, all the seismic instruments of the observatory started the recording of a distant earthquake ... the distance from the affected place is not so large because it is about 1500 km. from some hints and with many doubts, it seems it cannot be excluded that it was an earthquake originated in the south-east. time and distance are in due coherence with an epicentral location in libya, but in the following days the news is disregarded and details on a damaging earthquake in taiwan («formosa») is reported instead (corriere della sera, 1935b). a few pieces of information are given by the coeval seismological bulletins published by the «società sismologica italiana» (agamennone, 1935) and by the «r. ufficio centrale di meteorologia e geofisica» (cavasino, 1937). the latter reported that: «on the 19th of april 1935, at 16h and 25m at tripoli an earthquake was felt. the macroseismic intensity was vi according to the mercalli scale. its epicentre, according to the seismological office in strasbourg, is in the sea, in front of the cape misurata ... the same shock was felt in eastern sicily and more lightly in apulia» (originally in italian, translated by the authors) (cavasino, 1937). 4. potential historical sources on libya’s past earthquakes from the overview supplied in the previous sections and tables, it is clear that our knowledge of earthquakes in libya between the 3rd century a.d. and 1935 relies to date on a meagre set of historical records. the historical and geographical reasons that could account for the silent periods highlighted in our overview have to be found and pointed out as clearly as possible when preparing a feasibility study aiming at collecting new data on past earthquakes in libya. for instance, there is an indisputable concentration of information in the flourishing period (3rd and 4th century) of the main urban settlements of phoenician and greek origins 552 abdunnur s. suleiman, paola albini and paola migliavacca along the coast of tripolitania and cyrenaica. this information derives mostly from studies using archaeological data, and the time-span it covers ends abruptly with the loss of importance of towns like cyrene and leptis magna. there are evident differences in the history of the coastal regions of tripolitania and cyrenaica, which were under the influence and domination of the roman empire, and the vast deserted lands to their south. there are less apparent but still important differences in the respective history of tripolitania and cirenaica. for instance, cyrene and apollonia, its harbour, were at the very border of the eastern roman empire (4th century), while tripoli and the surrounding region were part of the western roman empire. after the arab conquest of both regions in 642, cyrenaica did not regain its prior importance. during the arab domination, tripoli, born from the ashes of the three «poleis» of oea, sabratha and leptis magna, maintained its role as capital, but always as junior of the main capitals of the neighbouring states, e.g., algiers and tunis. the period between the 5th and the 15th century is less known about for the territory of modern libya. historical records on this period have to be looked for mainly and specifically in medieval arab sources. this will require ad hoc expertise and a dedicated feasibility study, which is out of the scope of this paper. the situation changes, for the better in terms of availability of information, during the modern age, when the contacts between the north african countries, and the rest of the european and mediterranean states, increase. what follow are some hints at a possible investigation strategy, oriented towards european depositories, mainly inspired by the exhaustive contribution by bono (1982) on western sources on four centuries of the history of libya (1510-1911). bono’s essay describes the main features of the history and historiography of tripoli in its relationships with the whole of libya, presenting the general 20th century treatises on the whole country, and then detailing less important contributions on its geography or very specific themes. the book develops according to a possible periodisation of the history of libya, subdivided into the short dominations of the spanish and the knights of malta (1510-1551), the first turkish period from 1552 to 1835, with special attention on the age of qarama¯nli¯ (1711-1835), and the second turkish period (1835-1911). unpublished and unexplored sources are thus described in their potential and documentary typology as available in the libyan archives (based on surveys carried out by bono himself in the mid 20th century), but mainly in those pertaining to the governments and trading companies of great and small european countries that, between the early 16th and mid 19th century, had to deal and compromise with the barbary states, as morocco, algeria, tunisia and libya were commonly named at that time. in terms of documentary production and survival to our days, records of these exchanges can be found at the state archives of many italian regional states (e.g., genova, venezia, parma, napoli, modena, palermo, milano, firenze, cagliari), at the french archives nationales (paris and nantes, france), at the arquivo general of the spanish crown (simancas, spain), at the national archives of malta (la valletta, malta), at the secret vatican archives (roma, italy), at the national archives of the united kingdom (kew, u.k.), and at the haus-, hofund staatsarchiv (vienna, austria). the most important typology of documents emerging as potentially including evidence of earthquake effects is the diplomatic and consular correspondence. this material mostly concerns tripolitania and occasionally embraces the entire coastal area of libya. in the period 1835-1911, the documentation increases due to the reports produced by geographical and explorative surveys, including the vast libyan desert. the 20th century westerners’ interest towards libya was in a sense guided further by the italian colonialist attitude, starting with sparse settlements in tripolitania in late 19th century, and developing into an occupation of libya in the period between 1911 and the end of ww ii. the documents on libyan affairs produced in this period are mainly stored in the historical and diplomatic archive of the italian ministry of foreign affairs (roma). 553 a short introduction to historical earthquakes in libya 5. conclusions libya is not usually thought of as a seismically active country; however, a number of earthquakes are reported to have occurred in libya including a major earthquake of m = 7.1 on 19 april 1935. the recorded number of earthquakes in libya is not representative of the actual total number because of the small number of seismological stations in the area and the limitations on instrumental sensitivity before 1950, as well as the lack of seismological stations in libya itself. the data on historical earthquakes are therefore of crucial importance. epicentral locations for the period 19352001 (suleiman et al., 2001) strongly suggest that most of the activity is clustered in two areas: the nnw trending hun graben and the al jabal al akhdar to the northeast of libya. the hun graben is a prominent rift valley feature trending nnw from the city of hun to the city of al qaddahia. the boundary faults of the graben are marked by escarpments and substantial fault displacement can be mapped at the surface. seismic activity seems to be concentrated along the eastern side of the graben. after the earthquake sequence in 1935, it was the site of a number of earthquakes in 1941, 2000, and 2001. the brace (known now as al-maraj) earthquake in 1963 and other tremors recorded in al jabal al akhdar area ne libya suggest that the area is still tectonically active. the review presented above shows that the gaps in the information on historical seismicity of libya before 1900 are so large that there is no confirmation that i) the two areas mentioned above were active in the past centuries as well or, in contrast, that ii) no other areas have to be considered when assessing seismic hazard for the territory of libya. to improve actual knowledge of long-term seismicity of libya, some suggestions on potential sources to be explored were given. we would like the short list proposed to focus the researchers’ attention, not only on their existence, but particularly on that their geographical distribution is a primary reason to implement an international collaboration aimed at improving today’s knowledge on historical earthquakes that affected libya. references agamennone, g. (1935): notizie sismiche, boll. soc. sismol. ital., roma, xxxiii (3-4), 146-151. ambraseys, n.n. (1984): material for the investigation of the seismicity of tripolitania (libya), boll. geof. teor. appl., 26, 143-153. ambraseys, n.n. (1994): material for the investigation of the seismicity of libya, libyan studies, 25, 7-22. ambraseys, n.n., c.p. melville and r.d. adams (1994): the seismicity of egypt, arabia and the red sea. a historical review (cambridge university press, cambridge), pp. 181. anonymous (1685): distinto ragguaglio gionto per lettera particolare ad un mercante nel porto d’ancona, tripoli li 16 maggio 1685, in roma e bologna, pp. 4. anonymous (1884): terremoto nell’oasi di ghadàmes, boll. soc. geogr. ital., a. xviii, v. xxi, s. ii, v. ix, p. 169. bacchielli, l. (1995): a cyrenaica earthquake post 364 a.d.: written sources and archaeological evidences, ann. geofis., xxxviii (5-6), 977-982. bono, s. (1982): storiografia e fonti occidentali sulla libia (1510-1911), quaderni dell’istituto italiano di cultura di tripoli, nuova serie (l’erma di bretschneider, roma), pp. 142. cavasino, a. (1927): macrosismi avvertiti in italia e colonie nell’anno 1926, boll. soc. sismol. it., roma, vol. xxvi 1926, pp. 25-34. cavasino, a. (1937): macrosismi, fasc. ii, bollettino sismico anno 1935, r. ufficio centrale di meteorologia e geofisica, roma, pp. 7-16. corriere della sera (1935a-c): a) 20 aprile; b) 23 aprile; c) 30 aprile, milano. dresdnissche geleherte anzeigen (1756), dresden. di vita, a. (1995): archaeologists and earthquakes: the case of 365 a.d., ann. geofis., xxxviii (5-6), 971976. guidoboni, e. (editor) (1989): i terremoti prima del mille in italia e nell’area mediterranea: storia, archeologia, sismologia (ing, roma-sga, bologna), pp. 765. guidoboni e., a. comastri and g. traina (1994): catalogue of the ancient earthquakes in the mediterranean area up to the 10th century (ing, roma-sga, bologna), pp. 504. hoff, k.e.a. von (1840): chronik der erdbeben und vulkan ausbrüche, i. theil, (justus perthes, gotha), pp. 470. ingrao, g. (1930): italia; colonia, macrosismi, fasc. ii, bollettino sismico anno 1926, r. ufficio centrale di meteorologia e geofisica, roma, 5-18; 19-21. minutilli, f. (1903): bibliografia della libia (bocca, torino), pp. 136. polidori r., a. di vita, g. di vita evrard and l. bacchielli (1999): libya the lost cities of the roman empire (könemann, cologne), pp. 256. sieberg, a. (1932a): die erdbeben, in handbuch der geophysik, edited by b. gutenberg (berlin), vol. iv. sieberg, a. (1932b): untersuchungen über erdbeben und bruchschollenbau im östlichen mittelmeergebeit, denkschr. medizin.-natürwiss. ges., jena, 18 (2), 161-273. stiros, s.c. (2001): the a.d. 365 crete earthquake and possible seismic clustering during the fourth to sixth centuries a.d. in the eastern mediterranean: a review 554 abdunnur s. suleiman, paola albini and paola migliavacca of historical and archaeological data, j. struct. geol., 23 (2-3), 545-562. suleiman, a.s. and d.i. doser (1995): the seismicity, seismotectonics and earthquake hazards of libya, with detailed analysis of the 1935 april 19, m= 7.1 earthquake sequence, geophys. j. int., 120, 312-322. suleiman, a.s., i. daba and e. abdurazag (2001): seismicity of sirt and surrounding areas. deatailed investigations and future planning, in first conference on the natural resources in the sirt area, sirt, libya, pp. 13 (in arabic). annalsft_amatricemedia_revgemformattedlast annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 1! the 2016 amatrice seismic sequence in the media gemma musacchio1* and giovanna l. piangiamore2 1 istituto nazionale di geofisica e vulcanologia, ingv ac, 2 istituto nazionale di geofisica e vulcanologia, ingv, sezione roma 2, sede di portovenere, fezzano di portovenere (sp) *gemma.musacchio@ingv.it abstract media's power in setting the public’s agenda for discussion can affect perception and debate upon disasters. in the frame of a dialogical approach of science communication, here we challenge the paradigm for which those issues that experts consider valuable are not in the media's agenda. we have studied the way newspapers have addressed the amatrice 2016 sequence and discussed the story telling. we have analyzed specific indicators to assess to what extent the scientific coverage, risk reduction and damage issues are discussed. first results show that science is in the media when framing encoding with meaningnews on a natural disaster. the media do think valuable to provide public with an in-depth scientific coverage of an earthquake and refer to authoritative sources. as time goes by and aftershocks magnitude decreases, a more reflexive thinking is triggered; than news stories include more risk reduction indicators than damage. although memory of past earthquakes is always part of the story, only one month after the main shock risk reduction disappears from the media's agenda. we also explored the level of public engagement in risk reduction and found out that, as far as it concern newspapers, media still seem not believe that citizens should be active part of the debate upon their own safety. i. introduction ews stories on natural disasters attract a great audience interest for which the media represent among the most important sources of information (pasquarè and pozzetti, 2007; colombo et al., 2002; fischer, 1994). they perfectly meet media newsworthiness criteria such as human interest, proximity, possible future impact, oddity and timeliness elements. by channeling information in a way that makes some aspects more relevant than others (forsyth, 2003) the media introduce frames of reference (gilliam and bales, 2001) that influence how different individuals or societies perceive disasters. on the other hand the knowledge and understanding of what the media grant be relevant for their public, is nowadays fundamental for scientific institution specifically dealing with natural hazards. here we test whether the paradigm, upon which journalists often avoid n annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 2! providing in-depth scientific coverage on natural causes of extreme geological events and hardly ever highlight the need to prevent and mitigate their consequences, holds, and if yes to what extent. to pursue our target we have studied the way the media have tackled a recent earthquake while the emergency phase was still on going. on august 24th, 2016 (1:36 utc) a mw6.0 earthquake struck an area in the central apennines (italy) between the municipalities of norcia and amatrice. a large aftershock, mw 5.3, occurred about one hour later (2:33 utc) and, in the first month, more than 15 earthquakes with magnitude larger than 4.0 followed the main shock (gruppo di lavoro ingv, 2016). although belonging to a high hazard zone, the quake found population widely unprepared and damage was extensive (gruppo di lavoro ingv, 2016). the earthquake's news has spread in the internet, reaching 200 thousands tweets a few hours after the main shock. we have analyzed the echoes of the seismic sequence in the media, mostly focusing on those issues that if were on the media's agenda they might trigger a change in attitude and in perception of seismic risk. more specifically we have studied to what extent the issues regarding the natural phenomena, those concerning risk and damage were addressed. the rationale behind our approach is that by not properly understanding the difference between hazard and risk laypeople do not have a proper and effective attitude towards disaster reduction. figure. 1. impact of the amatrice sequence in the media: number of printed and on-line news stories talking about the sequence in one month. the distribution of the m>3.5 earthquakes over time is plotted for comparison. ii. method this paper uses a quantitative approach to explore the impact that the 2016 amatrice seismic sequence had on the media. news stories were retrieved from articles published within the first month after the main shock by in print and online newspapers included in the ingv press review (fig. 1). the sample includes news stories from 355 in print and 653 online newspapers and refers to 9 selected days with the on-going seismic sequence. the sampling on days was less tight as the sequence smoothed down. because we are interested on what the press considers be newsworthiness we deliberately have not chosen a software-based automated search of keywords to explore the stories. we rather relied on a in-depth assessment: we annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 3! read thoroughly each story and evaluated each indicator upon consistency. for the online news we explored only titles and assessed just the highlights that journalists think worth of notice. the approach relies on the fact that in print newspapers are addressed to readers who like to take their time to read, while online are for those who might just be caught by the title of news. online newspapers are bound to have sharp and right-to-the-point titles that should highlight in a few words what the journalist thinks worthwhile. we consider newspaper type (specialist, generalist, local, national and international), if the article is signed as a stuff product or with the name of the journalist, whether is on a first page (only for printed). we then set indicators to explore who does the journalist refers to (the source), what is considered to be newsworthiness and who is actually the journalist trying to give voice to. for each specific indicator we measure how many stories were published by the media. in assessing newsworthiness we break down the what to three main categories of indicators, which we think of major interest to understand the image of a natural disaster in the media in the first days of its occurrence. they are: scientific coverage, risk reduction issues and earthquake damage. to assess scientific coverage (fig. 2) we consider how frequently journalists cover topics such as: the geologic setting of the area, the seismic sequence and surface seismic-induced effects (es. landslides, surface rupture), hazard or risk maps, site effects, building code or forecast. risk reduction issues are analyzed distinguishing prevention/preparedness concepts from post earthquake interventions or investigations of responsibilities. we have also counted if risk reduction was not mentioned at all. when considering earthquake damage we assessed whether the article reported on damage, collapse, deaths, and loss of money or whether the damage was related to public buildings (e.g. schools). figure 3. the newsworthiness along time is shown. black contour empty bars are online newspapers. scientific coverage always plays a major role. damage is more discussed than risk reduction in the first days of the sequence. however, later on (i.e. september 2nd), online titles begun to show risk reduction issues more frequently than damage. they follow the decrease in magnitudes of aftershocks. figure!2. the what in the media: assessing the indepth scientific coverage.! annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 4! the voice indicators allow highlight whether the journalist is speaking for citizens, politicians, or scientists. it is the most difficult to assess and its study might be not truly objective. for this reasons we have included the notdefined/journalist all those cases when doubts might arise. iii. results although this is an on-going study that will include the whole world press, newscast and social, some preliminary observation can be retrieved. the earthquake had a strong echo in the media and almost the entire press review was filled with stories on the amatrice sequence (fig. 1). however, as one would expect, the media's attention curve follows the one of aftershock and magnitude distribution in time, and a few days after the main shock the number of news stories dropped significantly. we discuss here the analysis that concerns the what and the voices. scientific coverage result on scientific coverage (fig. 3) is somewhat surprising: more than 60% of the stories included one of the indicators we have chosen to explore (fig. 2). previous study that considered the 2002 molise earthquake have shown that no more than 22% of the stories had mentionany scientific issue (pasquarè and pozzetti, 2007). moreover, in that study no more than 11% had dealt with seismic classification and mapping while here we found a percentage never less than 20%. damage and risk until a few days after the main shock (fig. 3), damage, including causalities, is considered to be more valuable to give attention than risk reduction issues. interestingly, risk reduction becomes a hot topic and titles of articles in cludes more often risk reduction than damage indicators, later on when events decrease in magnitude and number. this can point out that as the emergency cool down a more reflexive thinking shows up. however one month after the main shock and as the aftershocks magnitude drops on-line titles do not have indicators for damage or risk anymore. they are discussed only inside the in-print articles. if we look at specific issues concerning risk over time (fig. 4), we can see that media have well in their agenda the memory of past earthquakes. although not included in the titles, up to 60% percent of the stories refers to past earthquake and recall that the amatrice sequence occurred in an area with high seismicity. another aspect worth to notice is that prevention is actually discussed generally more than post-earthquake and/or emergency related actions. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 5! figure 4. risk reduction and damage indicators versus time. the memory of past earthquakes is always present. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 6! voices we are aware that our analysis is based on a press review from a scientific institution. however giving a first look to international press we have found that news stories concerning the amatrice sequence always refers to a scientific source. if our study holds, than the results concerning the voices indicator, which sets science on a highlight, are in agreement with the fact that in italy only 51% of people acknowledge that we should ask scientist to get answers concerning science (observa 2011). scientists seems not be trivially trustworthy. citizen has a very shy voice that does not come out in the first days after the disaster. it takes several days to have them start speaking up in the newspapers (fig. 5). this is surprising if we consider that they are the first victims of disasters and might be the key to work on for future risk reduction plans. public engagement in science and risk communication and a better involvement of local communities in the discussing on safety can be the way for the future. iv. conclusive remarks media are interested to newsworthiness; politicians want to make sure they can gain votes; scientists might focus in public understanding of science. when it comes to risk communication, citizens live inside this triangle of interests; yet they are main actors in a public's agenda that is set by the media. in this frame the understanding of what the media might acknowledge newsworthiness is fundamental to help scientists to deliver those concepts that can trigger awareness on hazard, risk and safety. risk communication is a social responsibility that can efficiently be accomplished with cooperation between scientists, experts, and journalists (fjæstad, 2007). informed and aware citizen can than take actions towards politics and policy makers. citizens, without the media’s involvement, might not easily have access to hazard-related issues and might not be able to bring them to the attention of decision makers (colombo et al., 2002). the scientific community might play figure 5. who is really speaking? the voices indicators acknowledge scientists or experts as the real speaker in 51% of the stories. citizen are the most unspoken voices annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7263 ! 7! a key role in this process, trying to establish tighter connections with the media and the general public (pasquarè and oppizzi, 2012). here we observe as the relevance of the scientific coverage has improved over time and, from the 2002 saint giuliano earthquake till the amatrice 2016 sequence, the media write 40% more of stories dealing with it. fourteen years later, many more natural disasters, including earthquakes, have occurred, which we should study the impact in the media. one can argue that this study points out that science now meets newsworthiness criteria also thanks to what scientists have done in quiescence time, when reflexive thinking is encouraged. nonetheless if science was in the agenda of general stakeholders risk reduction was not there. memory of past earthquakes can be envisaged as one of those parameters that should trigger actions towards risk reduction. we notice that whereas memory is well in the media, risk reduction issues do not seem to deserve being in their agenda. acknowledgements we thank the journal's unknown reviewer for the interesting comments. we are also grateful to suggestions by francesca noceti (university of ferrara). this work is the thesis for master graduation in scientific journalism and science communication at the university of ferrara. references [björn fjæstad, 2007] fjæstad, b. 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(2001). strategic frame analysis: reframing america's youth. social policy report, 15 (3) http://repositories.cdlib.org/ccc/chidren/005 s [gruppo di lavoro ingv, 2016] gruppo di lavoro ingv sul terremoto di amatrice (2016). secondo rapporto di sintesi sul terremoto di amatrice ml 6.0 del 24 agosto 2016 (italia cantrale), doi: 10.5281/zenodo.154400. [pasquare and pozzetti, 2007] pasquare, f. and pozzetti, m. (2007). geological hazards, disasters and the media: the italian case study. quaternary international 173–174: 166–171. [pasquare and oppizzi, 2012] pasquare, f. and oppizzi p. (2012). how do the media affect public perception of climate change and geohazards? an italian case study. global and planetary change 90–91: 152–157. completo_def.qxd 1409 annals of geophysics, vol. 47, n. 4, august 2004 key words aeolian islands – fluid inclusions – crustal xenoliths – homogenisation temperature – magma plumbing system 1. introduction understanding the role of shallow level evolutionary processes in arc volcanoes has an important bearing on the way the various volcanoes work, which is a necessary preliminary requirement to understand both the petrological (i.e. racf processes) and volcanological evolution of magma. modeling magma ascent rates, however, requires the determination of the residence depths of magma (i.e. location magmatic chambers) based on reliable geobarometers. fluid and melt inclusion studies in magmatic minerals and/or in xenoliths entrained in volcanic rocks represent a valuable technique to reach this objective (e.g., roedder, 1965; clocchiatti et al., 1994; andersen and neumann, 2001). many studies have shown that fluid inclusions may be abundant in lavas and xenoliths, reflecting composition, pressure and temperature conditions of the fluid phases trapped during magma ascent and degassing (see andersen and neumann, 2001). mailing address: dr. rossana bonelli, dipartimento di scienze della terra, università degli studi di siena, via laterina 8, i-53100 siena, italy; e-mail: bonelli5@unisi.it evolution of the volcanic plumbing system of alicudi (aeolian islands – italy): evidence from fluid and melt inclusions in quartz xenoliths rossana bonelli (1), maria luce frezzotti (1,2), vittorio zanon (3) and angelo peccerillo (3) (1) dipartimento di scienze della terra, università degli studi di siena, siena, italy (2) istituto di geologia ambientale e geoingegneria (igag) cnr, roma, italy (3) dipartimento di scienze della terra, università degli studi di perugia, perugia, italy abstract quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from alicudi contain abundant melt and fluid inclusions. two generations of co2-rich fluid inclusions are present in quartz-rich xenolith grains: early (type i) inclusions related to partial melting of the host xenoliths, and late type ii inclusions related to the fluid trapping during xenolith ascent. homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (type i) and 0.47-0.26 g/cm3 (type ii). early type i fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. late type ii fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallization, mixing with source – derived magma, and various degrees of crustal assimilation. the second magma reservoir is located at shallow crustal depths (about 6 km), the site where magma rested for a short time before erupting. 1410 rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo the aeolian arc (southern tyrrenian sea) comprises seven volcanic islands, some of which are active volcanoes (vulcano, stromboli, and, possibly, lipari and panarea) and six principal seamounts (glauco, sisifo, enarete, eolo, lamatini and alcione). volcanic rocks range from mafic to acid and have calc-alkaline (ca), high-k calc-alkaline (hkca), shoshonitic (sho) and potassic alkaline (ks) compositions, which are closely associated in time and space (see de astis et al., 1997, 2000). the alicudi island is a composite volcano made up of dominant lavas and minor pyroclastic rocks of calcalkaline affinity sited at the western margin of the aeolian arc. in spite of the many volcanological and geochemical studies carried out in recent decades, there is little or no information on the internal structure of alicudi volcano and on the way the volcanic plumbing system works. this paper presents fluid and melt inclusions data which contribute to the development of a model for the plumbing system of alicudi, and for its evolution through time. 2. geological and volcanological setting the volcanism of the aeolian archipelago initiated about one million years ago, generated by subduction of the ionian plate under the calabro-peloritani continental margin (barberi et al., 1973; ellam et al., 1988). alicudi is one of the youngest islands in the aeolian arc (90 ka; gillot, 1987) located on the western side of the archipelago. the island represents the summit part of a complex stratovolcano, extending to 675 m above sea level and about 2000 m below (fig. 1). the overall circular pattern of its base (an area of about 5 km2) and the almost perfect conical shape of this stratovolcano suggests a development due to a central activity, without any significant migration of the feeding conduit (villari, 1980). according to several authors (villari, 1980; peccerillo and wu, 1992; peccerillo et al., 1993) alicudi was built during three different subsequent volcanic cycles separated by caldera collapses (fig. 1). the first two cycles (scoglio galera, 90-60 ka and dirittuso, 55 ka) were characterised by emission of calcalkaline basaltic and basalt-andesitic lavas and minor pyroclastic products. high-k andesitic lava flows and domes were emplaced during the third phase (montagnole and filo dell’arpa, 28 ka) from the summit crater on the southern flank of the volcano. the volcanic products exhibit a restricted compositional range (from basalts to high-k andesite) and display the most primitive petrological and geochemical characteristics over the entire aeolian arc (peccerillo and wu, 1992; peccerillo et al., 1993). fig. 1. schematic geological map of alicudi showing the volcanics belonging to the three different volcanic cycles, simplified from manetti et al. (1989). a = first eruptive cycle scoglio galera formation (~ 90-60 ka), b = second eruptive cycle dirittuso formation (~ 60 ka), c = third eruptive stage montagnola and filo dell’arpa formation (~ 28 ka). grey dots indicate sampling localities. 1411 evolution of the volcanic plumbing system of alicudi (aeolian islands – italy) 3. analytical methods sample preparation for fluid inclusion studies was done by making doubly polished sections of 100-150 µm thickness. microthermometry analyses were performed at the university of siena with the linkam th600 heating and cooling stage. synflinc® standard synthetic fluid inclusions were used to calibrate the stage, checking the temperature at the co2 (– 56.6°c) and h2o (0°c) triple points. accuracy at standard points was estimated ± 0.1°c. mac flincor® software package (brown, 1989) with the equation of state by holloway (1981) for co2 were used to calculate the isochores for fluid inclusions. density values of co2 were calculated from angus et al. (1976). density of mixed inclusions of co2 and n2 was derived from kerkhof and thièry (2001). fluid inclusions were further analysed with a confocal labram multichannel spectrometer of jobin-yvon ltd. in the laboratory of the university of siena. the excitation line at 514.5 nm was produced by an ar+ laser. the raman intensity was collected with a poltier-cooled ccd detector. the beam was focused to a spot size of about 1-2 µm using an olympus 100 × lens. the scattered light was analysed using a notch holographic filter with a spectral resolution of 1.5 cm−1 and grating of 1800 grooves/mm. electron – microprobe analyses of mineral phases were performed with a cameca sx 50 (igag-cnr, roma) operated at an acceleration voltage of 15 kv and a probe current of 15 na. mineral analyses were performed with a focused (1 µm) beam. natural and synthetic silicates were used as standards for mineral analyses. melt inclusions microthermometry of a single xenolith was performed at the vrije university of amsterdam using a high temperature (up to 1600°c) heating/quenching stage (sobolev and kostyuk, 1975). temperatures were measured with a pt-pt09rh10 thermocouple, calibrated with gold, silver and synthetic compounds. experiments were performed at 1 atm he, purified by a 700°c ti filter. heating times varied from 1 to 6 hours due to the slow kinetics of high-silica melts. optimal heating rates of 2-5°c/min were used above 700-900°c and much lower rates (530°c/hour) near homogenisation temperature. measurement uncertainties were estimated to be ± 5°c. after quenching, host minerals were mounted on epoxy and polished until melt inclusions were exposed to surface. emp analyses of melt inclusions were performed using a four-wds-spectrometer jeol ltd. jxa 8800m superprobe at the vrije university of amsterdam using an acceleration voltage of 15 kv and a beam current of 25 na. spot sizes were 2 ÷ 10 µm, with single-element counting times of 25 ÷ 50 s on the peak and 10 ÷ 25 s on the bottom. 4. petrography of lavas and xenoliths studied lavas and xenoliths were collected in three distinctive localities in alicudi, which are considered representative of the overall petrological and volcanological evolution of the island (fig. 1): 1st cycle – basaltic lavas at scoglio galera and malopasso (scoglio galera activity: ∼ 9060 ka); 2nd cycle – basaltic-andesitic lavas at bazzina (dirittuso activity: ∼ 55 ka); 3rd cycle – andesitic lavas at the alicudi porto and perciato (montagnole and filo dell’arpa activity: ∼ 28 ka). xenoliths are particularly abundant in the basaltic lavas of the first two cycles, whereas they are scarce or absent in the third stage andesites. 4.1. host lavas the rocks of alicudi are basalts, basaltic andesites and high-k andesites, which define a subalkaline trend showing transitional characters between calcalkaline and high-k calcalkaline according to the classification proposed by peccerillo and taylor (1976). lavas are porphyritic, with plagioclase as the dominant phenocryst phase in all rock types, whereas olivine, although ubiquitous, is abundant (> 30%) only in basalts. clinopyroxene phenocrysts occur in moderate amounts in all rocks. representative mineral compositions of alicudi lavas are reported in table ia,b. 1412 rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo basalts from the first cycle of activity (scoglio galera) display a holocrystalline texture and a phenocryst content ranging from 30 to 50% of the total rock volume. plagioclase makes up more than 50% of the total phenocryst abundance and consists of bytownite-labradorite (an% = 83-73; table ia). olivine is also abundant (≥ 30%) and occurs as phenocrysts (fo79) and microphenocrysts. clinopyroxene occurs both as phenocrysts and in the groundmass and can be classified as augite (wo% = 40-43; en%= = 45-47; fs% = 9-13; table ia). basaltic andesites from the second cycle of activity (dirittuso) are porphyritic in texture and are similar to the basalts described above, but with less olivine (≥ 20%). plagioclase is by far the most abundant phenocryst phase and has compositions similar to those in the basalts, with max an% = 81 (not shown). olivine (fo74) and clinopyroxene (wo% = 40-43; en% = 45-47; fs% = 9-13) have compositions similar to those in the basalt. these two last minerals represent the principal phases in the groundmass as well. high-k andesites from the third cycle of activity (montagnole and filo dell’arpa) show porphyritic textures with dominant plagioclase phenocrysts of labradoritic composition (an%= = 65-58). the olivine (fo78-76) content is less than 20% and clinopyroxene shows a composition similar to that in basalts (wo% = 40; en%= table ia. major (wt %) elements of the studied phenocrysts in basaltic lavas of alicudi. samples sg7 tc5 sg7 sg7 sg7 sg7 sg7 sg7 sg7 mineral ol ol cpx cpx cpx pl pl pl pl note core rim core rim sio2 38.69 36.98 52.31 51.42 52.52 47.53 48.46 47.88 49.76 tio2 – – 0.48 0.41 0.54 – 0.09 0.04 – al2o3 – – 2.48 3.86 2.73 33.06 31.80 32.95 31.17 feo tot 19.07 23.42 8.38 5.91 7.02 0.66 0.59 0.58 0.70 mno 0.28 0.44 0.18 0.07 0.14 – – – – mgo 41.52 38.36 16.26 16.32 16.33 – – – – cao 0.18 0.25 19.94 20.88 21.03 17.06 16.18 16.99 14.91 k2o – – 0.01 – – 0.08 0.11 0.09 0.13 na2o – – 0.29 0.22 0.22 1.83 231 1.96 2.91 totale 99.74 99.44 100.33 99.09 100.53 100.22 99.54 100.48 99.57 fo% 79.50 74.48 – – – – – – – fa% 20.50 25.52 – – – – – – – wo% – – 40.48 43.25 42.61 – – – – en % – – 45.94 47.07 46.05 – – – – fs % – – 13.58 9.68 11.34 – – – – mg* 0.80 0.74 0.77 0.83 0.80 – – – – an% – – – – – 83.34 78.91 82.29 73.34 ab% – – – – – 16.17 20.42 17.19 25.87 or% – – – – – 0.49 0.66 0.52 0.79 ol: olivine, cpx: clinopyroxene, pl: plagioclase; blank entries: not analysed; mg* = mg/(mg+fetot). 1413 evolution of the volcanic plumbing system of alicudi (aeolian islands – italy) = 45; fs% = 13; table ib). the groundmass contains the same phases as the phenocrysts, except olivine, set in glassy matrix. 4.2. quartz xenoliths the lavas of alicudi commonly contain xenoliths of both magmatic and metamorphic origin, similar to observations in the other aeolian islands (e.g., frezzotti et al., 2003). metamorphic xenoliths show variable compositions and textures and can be classified as: i) quartz aggregates, ii) garnet-cordierite and garnet-sillimanite gneisses, iii) vesuvianite-grossular-bearing skarns and iv) metapelites (honnorez and keller, 1968; peccerillo and wu, 1992). magmatic lithologies are represented by gabbros and diorites consisting of plagioclase and clinopyroxene. previous petrological and geochemical investigations suggest that metamorphic xenoliths represent residual material of partially melted gneiss and schists, thus constituting compelling field evidence of interaction between magma and crustal rocks beneath alicudi volcano (peccerillo and wu, 1992). study rocks consist of quartz xenoliths which contain abundant fluid and melt inclusions. quartz xenoliths are composed mainly of quartz (> 95%), with accessory plagioclase, pyroxene, table ib. major (wt %) elements of the studied phenocrysts in andesitic lavas of alicudi. samples arp arp arp arp arp arp arp arp arp mineral ol ol cpx opx opx opx pl pl pl note core rim sio2 37.53 37.31 51.75 52.75 52.38 52.39 52.04 51.87 52.72 tio2 0.01 0.03 0.45 0.22 0.30 0.20 0.06 0.01 0.06 al2o3 0.04 0,03 1,78 0,97 0,80 1,45 30.33 30.47 29.00 feo tot 20.51 21.74 8.53 18.02 21.37 18.15 0.56 0.55 0.73 mno 0.40 0.42 0.36 0.66 0.95 0.83 0.02 0.03 mgo 41.63 39.75 16.10 24.85 21.73 25.57 0.07 0.08 0.09 cao 0.18 0.25 20.37 2.02 1.90 1.32 13.20 13.11 11.83 k2o 0.02 0.01 0.02 0.02 0.10 0.01 0.20 0.19 0.32 na2o – 0.03 0.28 0.06 0.06 0.02 3.84 3.76 4.45 totale 100.32 99.57 99.64 99.57 99.59 99.94 100.32 100.04 99.23 fo% 78.34 76.51 – – – – – – – fa% 21.66 23.49 – – – – – – – wo% 0.24 0.34 40.96 3.94 3.83 2.55 – – – en % 77.82 75.90 45.07 67.54 60.99 68.78 – – – fs % 21.94 23.76 13.97 28.51 35.18 28.67 – – – mg* 0.78 0.76 0.76 0.70 0.63 0.71 – – – an% – – – – – – 64.75 65.09 58.38 ab% – – – – – – 34.09 33.78 39.74 or% – – – – – – 1.17 1.12 1.88 ol: olivine, cpx: clinopyroxene, opx: ortopyroxene, pl: plagioclase; blank entries: not analysed; mg* = = mg/(mg+fetot). rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo apatite, zircon and opaque (table ii). they generally have angular shapes and sizes ranging from a few mm to a few dm. quartz (400-µm – 2mm) shows variable microstructural characteristics and is present both as rounded grains often rimmed by cristobalite (i.e. melting; fig. 2a), and as recrystallised crystal aggregates (i.e. annealing; fig. 2b). significant quantities of internal glass, present both as intergranular veins and as short trails or clusters of silicate-melt inclusions, are often observed within single rounded quartz grains. such a glass does not represent infiltration by host lavas since it has a distinctive high-silica rhyolitic composition. 5. melt inclusions melt inclusions are present only in restitic quartz grains showing rounded morphology (fig. 2a). the dimensions of the inclusions vary according to their textural distribution. their size is commonly about 30 µm, but inclusions of greater diameter were sometimes observed. inclusions are commonly present isolated or in small clusters in most of the quartz grains (fig. 2c). most melt inclusions contain glass and a bubble, ± locally one or more crystals (quartz, feldspars, ilmenite, clinopyroxene, sulphides or oxides) are present in the glass. in some inclutable ii. mineral compositions (wt %) of studied quartz-rich xenoliths. samples tc3 tc3 tc5 tc5 tc5 tc3 tc3 tc3 mineral cpx cpx opx opx opx pl pl pl note core rim core rim sio2 54.73 52.99 52.51 50.54 51.39 54.52 53.40 53.39 tio2 – 0.76 0.11 – 0.02 – – – al2o3 0.40 1.73 0.30 0.74 0.78 27.76 29.01 29.32 feo tot 2.60 4.44 24.23 24.72 21.85 – 0.51 0.38 mno – 0.36 1.72 1.63 1.54 – – – mgo 17.18 16.84 20.84 21.38 22.68 – – – cao 24.61 22.77 0.97 0.22 0.40 11.08 12.42 12.56 na2o 0.17 0.31 0.01 0.03 – 5.19 4.43 4.17 k2o – – – 0.02 0.02 0.37 0.34 0.31 totale 99.69 100.20 100.69 99.28 98.68 98.92 100.12 100.12 wo% 48.68 45.57 1.93 0.43 0.80 – – – en % 47.30 46.92 57.71 58.83 62.81 – – – fs % 4.02 7.51 40.36 40.73 36.39 – – – mg* 0.92 0.86 0.59 0.59 0.63 – – – an% – – – – – 52.96 59.60 61.37 ab% – – – – – 44.93 38.47 36.83 or% – – – – – 2.11 1.93 1.79 cpx: clinopyroxene, opx: ortopyroxene, pl: plagioclase; blank entries: not analysed; mg* = mg/(mg+ + fetot). 1414 1415 evolution of the volcanic plumbing system of alicudi (aeolian islands – italy) fig. 2a-f. photomicrographs of studied quartz xenoliths and fluid inclusions: a) and b) quartz grain textures in a quartz xenolith from a basaltic rock of the 1st cycle of alicudi (scoglio galera): a) this type of quartz is transparent and generally has a rounded contour; b) this type of quartz is milky, highly fractured with cracks filled with trails secondary fluid inclusions; c) trail of texturally early type i fluid inclusions in a quartz grain (scoglio galera). type i fluid inclusions (black arrows) occur associated with silicate melt inclusions (white arrows); d) cluster of early type i inclusions in a quartz grain from dirittuso; e) trail of late type ii fluid inclusions in a quartz grain from dirittuso. these inclusions are clearly secondary origin and contain co2; f) late trails of type ii co2 inclusions distributed along two main directions in a quartz grain (scoglio galera). a c d e f µ µ µµ µ µ b 1416 rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo sions, a large bubble is present, often containing immiscible co2. the presence of co2 in melt inclusions is associated with the co2-rich fluid in host xenolith. homogenisation temperatures of silicate-melt inclusions are between 1060 and 1120 ± 5°c. the glass consists of sio2 (69-78 wt.%), al2o3 (10-11 wt.%), k2o (3-5 wt.%), na2o (12 wt.%) and very little or no feo, mgo (chemical data from four melt inclusions are reported in table iii). 6. fluid inclusions 6.1. composition and distribution fluid inclusions from studied xenoliths melt instantaneously in a narrow t interval (– 56.9 ÷ table iii. results of electron – microprobe analyses of silicate – melt inclusion in a quartz xenolith of alicudi. samples arp arp arp arp sio2 78.10 72.60 69.00 75.10 tio2 0.18 0.11 0.87 0.02 al2o3 10.30 11.60 11.20 11.60 cr2o3 0.01 0.00 0.02 0.02 fe2o3 0.23 0.98 1.78 0.34 feo 0.47 1.97 3.56 0.69 mno 0.03 0.09 0.08 0.05 mgo 0.20 0.72 1.68 0.16 cao 0.48 2.14 3.89 0.69 na2o 1.72 1.67 1.17 1.91 k2o 4.67 3.97 3.33 4.96 p2o5 0.00 0.00 0.29 0.00 so3 0.00 0.03 0.00 0.00 f 0.03 0.04 0.01 0.10 cl 0.23 0.48 0.00 0.28 totale 96.65 96.40 96.88 95.92 7500 2000 2100 2200 raman shift (cm–1) 2300 2400 2500 6500 5500 4500 3500 2500 2142 co 2329 n2 in te n s it y fig. 3. histogram of melting temperatures (tm) for studied fluid inclusions. most co2 melting occurs within a narrow temperature interval between – 56.9 and – 56.3°c (maximum at – 6.6°c). fig. 4. raman spectrum for a fluid inclusion in a quartz xenolith from scoglio galera. the inclusion contains nitrogen (10 mole%) and carbon monoxide (14 mole%) besides co2. ÷ – 56.3; highest frequency tm = – 56.6; fig. 3), suggesting that co2 is the major phase present. raman spectra (28 collected) reveal the presence of other species (table iv). nitrogen (1-10 mole%) n n tm 1417 evolution of the volcanic plumbing system of alicudi (aeolian islands – italy) table iv. chemical composition of fluid inclusions from raman and microthermometric measurements. samples thl thv tm co2 n2 co (°c) (°c) (°c) (mole %) (mole %) (mole %) sg1 d)bis* – 16.5 – 57.9 90 2 8 sg1 d)bis – 17.0 – 58.0 92 1 7 sg1 d)bis – 15.0 – 58.1 82 4 14 sg1 d)bis – 17.6 – 57.9 92 2 6 sg10 d)2 5.0 – – 57.7 95 5 – sg10 d)2 – 24.0 – 57.0 97 3 – sg7 a)1 – 0.5 – 57.0 97 3 – sg7 f) 16.6 – – 57.5 96 4 – 2° cycle of dirittuso (55 ka) tc5a) 22.0 – – 57.2 97 3 – tc5c) – 2.5 – – 58.9 90 10 – tc5e) 8.8 – – 57.3 98 1 1 tc5f) 4.5 – – 58.1 94 6 – tc5f) 4.5 – – 58.5 94 6 – tc5f) 0.8 – – 58.7 93 7 – tc5f) 16.3 – – 57.8 96 4 – tc5f) 5.2 – – 58.9 95 5 – tc5f) 7.0 – – 58.7 95 5 – tc1-1a)1 20.2 – – 57.1 97 3 – tc1-1a)1 20.0 – – 57.1 97 3 – tc1-1a)2 – 23.5 – 57.0 97 3 – tc1-1b) – 25.5 – 57.0 97 3 – tc1-1c) 27.0 – – 57.1 98 2 – tc1-1c) – 29.0 – 57.1 98 2 – tc1-1c) – 23.0 – 57.2 97 3 – tc1-1c) – 23.5 – 57.2 97 3 – * raman spectrum (fig. 4). 1° cycle of scoglio galera (90-60 ka) ± carbon monoxide (1-14 mole%) are always present in fluid inclusions with melting below – 56.9°c and slow melting behaviour (fig. 4; table iv). carbon dioxide is present in two different generations of inclusions: type i early inclusions are present in xenoliths of the 1st and 2nd cycles (fig. 2c,d). they are isolated or in a small clusters with sizes ranging 3 ÷ 10 µm and shapes generally regular. partial decrepitation is visible (haloes of small inclusions around the inclusion cavity) (anderson et al., 1984). early inclusions are often associated with silicate-melt inclusions suggesting concomitant trapping. type ii late inclusions are present in all studied xenoliths and occur in variable shape and size. they are distributed along interand intragranular trails (fig. 2e,f). only a few among late type ii inclusions show evidence of partial decrepitation. 1418 rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo fig. 5. histograms of homogenisation temperatures for fluid inclusions present in xenoliths of the studied volcanic island (alicudi). histograms report homogenisation temperatures to the liquid phase (thl) for early type i fluid inclusions and homogenisation temperatures to the vapour phase (thv) for late type ii fluid inclusions. f re q u en cy f re q u en cy 6.2. homogenisation temperatures and fluid density calculations quartz xenoliths from scoglio galera and dirittuso contain high-density (fig. 2c,d) and low-density (fig. 2e,f) fluid inclusions – type i and type ii inclusions, respectively. the distribution of fluid inclusions from filo dell’arpa is different. here, inclusions are extremely rare and all of type ii. scoglio galera (1st cycle of activity; ∼ 90 ka): type i inclusions always homogenise to liquid (thl) in an interval of temperatures between – 0.5 and 29.5°c (fig. 5), with calculated density values between 0.93 and 0.61 g/cm3. most thl values, clustering around 28°c, represent re-equilibrated fluid inclusions (decrepitation; vityk and bodnar, 1998). type ii inclusions homogenise to vapour (thv) in the range of temperatures between – 6 and 31°c, with corresponding density values between 0.08 and 0.47 g/cm3. the thv histogram shows a few data scattered in a wide range of re-equilibrated inclusions. dirittuso (2nd cycle of activity; ∼ 55 ka): microthermometric data for early type i inclusions show a distribution of temperatures between – 2.5°c and 31.1°c, with resulting density values between 0.93 and 0.47 g/cm3 (the lowest temperatures around – 2.5°c to belong to a co2 + n2 fluid inclusion, with density value of 0.83 g/cm3) (fig. 5). values between 22 and 31°c belong to re-equilibrated inclusions. type ii vapour-rich inclusions show homogenisation temperatures between 0 and 31°c, (d = 0.10 – 0.47 g/cm3). lowest thv values, below 26°c, correspond to re-equilibrated fluid inclusions. th 1419 evolution of the volcanic plumbing system of alicudi (aeolian islands – italy) filo dell’arpa (3rd cycle of activity; ∼ 28 ka): this cycle is represented by two low-density inclusions (thv), with homogenisation temperatures between 6 and 18°c. density values are between 0.12 and 0.18 g/cm3. 7. discussion 7.1. isochore calculation isochoric lines representative for p and t conditions of fluid trapping in xenoliths during ascent of lavas for the different volcanic cycles are reported in fig. 6. the figure shows the presence of two isochoric bands, corresponding to the bimodal distribution of density values for the different inclusion types. the narrowest interval is between 0.47 and 0.26 g/cm3, corresponding to type ii low-density inclusions; the other interval (between 0.93 and 0.68 g/cm3) corresponds to type i high-density fluid inclusions. note that this last isochoric band is much wider due to the presence of density data from reequilibrated inclusions. not all of the fluid inclusion densities observed in xenoliths reflect original densities at trapping conditions: most type i fluid inclusions show textural evidence for decrepitation and fluid density decrease. decrepitation is confirmed by the frequent presence of small haloes of tiny bubbles and/or little fractures around the inclusions, and by the scattered th distribution. reset density data for type i inclusions will not be considered in the following discussion. 7.2. temperature and pressure estimates before any meaningful geological interpretation can be proposed it is necessary: first to fully describe the temperature and pressure conditions under which type i and type ii co2 fluids are trapped, and second to verify the fluid-xenoliths evolution during ascent. the temperature conditions for the fluids at the time of trapping are based on homogenisation temperatures of silicate melt inclusions, since microstructural evidence clearly indicates concomitant trapping of high-silica melt and (type i) co2 fluids in the inclusions. homogenisation temperatures of silicatemelt inclusions show values between 1060 and 1120°c (± 5°c). for this reason, we assume 1100°c as a mean trapping temperature for fluid inclusions present in xenoliths. at 1100°c, pressure estimates from fluid inclusions form two distinct pressure intervals that are interpreted to reflect two different episodes of fluid entrapment. both undisturbed early type i inclusions from the 1st cycle of scoglio galera and from the 2nd cycle of dirittuso show identical maximum pressure estimates at ≈ 6 kbar. such a pressure corresponds to a depth of about 24 km, assuming 2.7 g/cm3 as the average fig. 6. isochoric bands, corresponding to the bimodal distribution of densities for the first two magmatic cycles of activity of the alicudi island: the first at high pressure corresponding to early type i fluid inclusions and the latter at shallower pressures corresponding to late type ii fluid inclusions, see text. t t p p 1420 rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo value of density for the stratigraphic sequence beneath alicudi (zanon, 2001). a second episode of fluid trapping in quartz xenoliths from lavas of both the 1st and 2nd cycles occurred at a later stage, and is represented by low-density type ii inclusions. type ii fluid inclusions indicate similar pressure values and between 1.7 and 0.2 kbar (about 6 km depth). for filo dell’arpa andesitic lavas (3rd cycle), low-density type ii inclusions form a distinct pressure interval, between 0.5 and 0.3 kbar (about 1 km depth). 7.3. polybaric evolution of alicudi’s magmas the data obtained from the study of fluid and melt inclusions hosted in quartz-rich xenoliths allow us to propose a schematic model for the plumbing system of alicudi volcano, and to describe its modification through time, illustrated in fig. 7a-c. fluid inclusion barometry bear evidence that basaltic and basaltic andesitic lavas of the first and second cycles (scoglio galera, ∼ 90 ka and dirittuso, ∼ 55 ka) underwent the same polybaric evolution in the crust. type i fluid inclusion data for the first two volcanic cycles indicate that a deep magma storage level has been present beneath the island, located at lower-crustal depths (about 24 km; fig. 7a,b), since the moho beneath alicudi island was proposed at 21-25 km (falsaperla et al., 1984). such a magma accumulation level probably corresponds to a deep magma chamber located at the crust-mantle boundary, where mantle magmas rested and underwent contamination and very limited crystal fractionation. barometric data from low-density type ii fluid inclusions indicate that a second magma storage level is present located at shallower depths (about 6 km). shallow level magma chambers are fed by the deep magma chambers. the occurrence of quartz xenoliths, which contain both low-density (type i) and high-density (type ii) fluids, the lack of extensive density resetting for type i deep fluids, and the preservation of quartz xenoliths themselves, all suggest that the residence time of deep magmas and related quartz xenoliths in the shallow chambers is very short, possibly a few days. thus, we can conclude that a single volcanic plumbing system was active during the first two cycles, and from 90 to 55 ka, related to the eruption of basaltic and basaltic andesite lavas (fig. 7a,b). the above model implies that the large quantities of mafic magmas which characterise the first two cycles of the activity of alicudi reflect tapping of deep level reservoirs, where dominant mixing processes do not allow magma differentiation toward silicic composition. according to fluid inclusion data, the overall magma ascent evolution shows abrupt variations during the third cycle. for late andesitic lavas of filo dell’arpa, inclusion evidence indicates that a deep magma chamber is conspicuously absent (fig. 7c). a single magma accumulation level is indicated by type ii low density fluid inclusions at about 1 km depth; that is more superficial than those present during the first two cycles (compare fig. 7a,b with fig. 7c). it would be inaccurate, however, to propose a model for andesite crustal evolution based only on fluid inclusion evidence. the absence of type i inclusions and the limited amount of type ii inclusions in quartz xenoliths contained in andesitic lavas might be related to the scarcity of studied samples, since andesites do not contain abundant metamorphic rocks. higher viscosity and lower temperatures of such lavas compared to previous basaltic ones may have prevented melting and incorporation of crustal rocks within the ascending magma. for these reasons, we cannot exclude that andesites formed in the same deep magma chambers where early stages basalts and basaltic andesites rested resulting a similar polybaric evolution. 8. concluding remarks present fluid inclusions data indicate that two principal levels of magma accumulation were present beneath the island during the first two cycles (scoglio galera and dirittuso), indicating a polybaric evolution for alicudìs magmas. a first deep accumulation level is present at depths close to the moho (∼ 24 km). at these stages xenoliths were possibly trapped along with (type i) co2 ± n2 ± co fluids in the basaltic lavas. 1421 evolution of the volcanic plumbing system of alicudi (aeolian islands – italy) fig. 7a-c. schematic section modelling the magma plumbing system beneath alicudi island, as inferred from fluid inclusion study. lithological boundaries are from falsaperla et al., 1984 (see text). a b c p p p h h h 1422 rossana bonelli, maria luce frezzotti, vittorio zanon and angelo peccerillo a second magma storage level at upper crustal depths (about 6 km) has been active throughout the volcanic history of the island. here magmas and xenoliths rested for a time span sufficient for new co2 fluid trapping (type ii) and old type i fluid inclusion density resetting. it is noteworthy that during the 3rd cycle, when magma compositions were more evolved (andesites) there is no evidence from fluid inclusions for a deep accumulation level. acknowledgements the authors are grateful to c. ghezzo for useful suggestions and comments and to g. cavarretta and m. serracino for hosting r.b. and for eds analytical facilities at the igag studio del quaternario e dell’evoluzione ambientale – cnr. v.z. acknowledges w. lustenhower and i. nikogosian for help during his stay at the v.u. in amsterdam. raman facilities were provided by pnra, the italian organisation for scientific research in antarctica. references andersen, t., s.y. o’reilly and w.l. griffin (1984): the trapped fluid phase in upper mantle xenoliths from victoria, australia: implications for mantle metasomatism. contrib. miner. petrol., 88, 72-85. andersen, t. and e.r. neumann (2001): fluid inclusions in mantle xenoliths, in fluid inclusion studies-principles and applications, edited by t. andersen, m.l. frezzotti and e. burke, lithos, 55, 301-320. angus, s., b. armstrong, k.m. de reuck, v.v. altunin, o.g. gadetskii, g.a. chapela and j.s. rowlinson (1976): carbon dioxide, in international tables of the fluid state, vol. 3 (pergamon press, oxford). barberi, f., p. gasparini, f. innocenti and l. villari (1973): volcanism of the southern tyrrhenian sea and its geodinamic implication, j. geophys. res., 78 (23), 5221-5232. brown, p.e. (1989): flincor: a fluid inclusion data reduction and exploration program, in second biennal pan-am conf fluid inclusions program abst., pp. 14. clocchiatti, r., a. del moro, a. gioncada, j.l. joron, m. mosbah, l. pinarelli and a. sbrana (1994): assessment of a shallow magmatic system: the 1888-90 eruption, vulcano island, italy, bull. volcanol., 56, 466-486. de astis, g., l. la volpe, a. peccerillo and l. civetta (1997): volcanological and petrological evolution of vulcano island (aeolian arc, southern tyrrhenian sea), j. geophys. res., 102, 8021-8050. de astis, g., a. peccerillo, p.d. kempton, l. la volpe and t.w. wu (2000): transition from calc-alkaline to potassium-rich magmatism in subduction environments: geochemical and sr, nd, pb isotopic constraints from the island of vulcano (aeolian arc), contrib. mineral. petrol., 139, 684-703. ellam, r.m., m.a. menzies, c.j. hawkesworth, w. leeman, m. rosi and g. serri (1988): the transition from calc-alcaline to potassic orogenic magmatism in the aeolian islands, southern italy, bull. volcanol., 50, 386-398. falsaperla, s., g. neri and s. velardita (1984): struttura della crosta superiore nell’area delle isole eolie, rendiconti dell’osservatorio geofisico reggino, 29, 103-111. frezzotti, m.l., a. peccerillo, v. zanon and r. bonelli (2003): magma ascent rates and depths of magma reservoirs beneath the aeolian volcanic arc (italy): inferences from fluid and melt inclusions in crustal xenoliths, in melt inclusions in volcanic systems, edited by b. bodnar and b. de vivo (elsevier), (185-206). gillot, p.y. (1987): histoire volcanique des iles eoliennes: arc insulaire ou complexe orogenique annulaire?, doc. trav. igal., 11, 35-42. holloway, j.r. (1981): compositions and volumes of supercritical fluids in the earth’s crust, in mac short course in fiuid lnclusions, edited by l.s. rollister and m.l. crawford (mineralogical association of canada), 13-38. honnorez, j. and j. keller (1968): xenolithe in vulkanischen gesteiner der àolischen inseln (sizilien), geologischen rundschau., 57, 719-736. kerkhof, a.m. van den and r. thiery (2001): carbon inclusions, in fluid inclusions: phase relationshipsmethods-applications, edited by t. andersen, m.l. frezzotti and e. burke, lithos, 55, 49-69. manetti p., g. pasquarè, a. tibaldi and a. tsegaye (1989): geologia dell’isola di aliendi (eolie), boll. g.n.v., 2, 903-916. peccerillo, a. and s.r. taylor, (1976): geochemistry of eocene calcalkaline volcanic rocks from kastamonu area, northern turkey, contrib. miner. petrol., 56, 221-246. peccerillo, a. and t.w. wu (1992): evolution of calcalkaline magmas in continental arc volcanoes: evidence from alicudi, aeolian arc (southern tyrrhenian sea, italy), j. petrol., 33 (6), 1295-1315. peccerillo, a., d. kempton, r. harmon, t.w. wu, a. santo, a.j. boyce and a. tripodo (1993): petrological and geochemical. characteristics of the alicudi volcano, aeolian islands, italy: implications for magma genesis and evolution, acta vulcanol., 3, 235-249. roedder, e. (1965): liquid co2 inclusions in olivine-bearing nodules and phenocrysts from basalts, am. mineral., 50, 1746-1782. sobolev, v.s. and v.p. kostyuk (1975): magmatic crystallization based on a study of melt inclusions, fluid inclusion res., 9, 182-253. villari, l. (1980): the island of alicudi, rend. soc. it. miner. petrol., 36 (1), 441-466. vityk, m.o. and r.j. bodnar (1998): statistical microthermometry of synthetic fluid inclusions in quartz during decompression re-equilibration, contrib. mineral. petrol., 132, p. 149-162. zanon, v. (2001): inclusioni fluide e vetrose negli xenoliti delle eolie: implicazioni vulcanologiche e magmatologiche, tesi di dottorato, perugia. 31ambraseys.qxd 733 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words middle east – 12th century – historical earthquakes 1. introduction the purpose of this article is to present the retrieval and uniform processing of information for the study of the 12th century seismicity of the region, which is defined by 33° to 38°n and 35° to 39°e, and includes southeastern turkey, syria and lebanon. this information may be used for the assessment of the location, and size of 12th century earthquakes and their association with regional tectonics. results from historical studies like this will be of value to the earth scientist and engineer alike, only when descriptive effects derived from historical sources can be reliably quantified. the assessment of an historical earthquake requires the documentary information to be reviewed with reference to the environmental conditions and historical factors that have influenced the reporting of the event. in evaluating the evidence for individual earthquakes and the completeness of information as a whole, attention must be given to exaggeration, the role of population distribution, communications and availability of historical documents. the significance of negative evidence and lack of data also need to be considered. 2. environmental conditions tectonic features – the dominant tectonic feature of the levant responsible for destructive earthquakes is the dead sea fault system. it is a zone of left-lateral strike-slip running from the the 12th century seismic paroxysm in the middle east: a historical perspective nicholas n. ambraseys department of civil engineering, imperial college, london, u.k. abstract the dead sea fault and its junction with the southern segment of the east anatolian fault zone, despite their high tectonic activity have been relatively quiescent in the last two centuries. historical evidence, however, shows that in the 12th century these faults ruptured producing the large earthquakes of 1114, 1138, 1157 and 1170. this paroxysm occurred during one of the best-documented periods for which we have both occidental and arab chronicles, and shows that the activity of the 20th century, which is low, is definitely not a reliable guide to the activity over a longer period. the article is written for this workshop proceedings with the archaeoseismologist, and in particular with the seismophile historian in mind. it aims primarily at putting on record what is known about the seismicity of the region in the 12th century, describe the problems associated with the interpretation of macroseismic data, their limitations and misuse, and assess their completeness, rather than answer in detail questions regarding the tectonics and seismic hazard of the region, which will be dealt with elsewhere on a regional basis. mailing address: dr. nicholas n. ambraseys, department of civil engineering, imperial college, london sw7 2az, u.k.; e-mail: n.ambraseys@imperial.ac.uk 734 nicholas n. ambraseys gulf of aqaba in the south to join the east anatolian fault zone near maras in se turkey (fig. 1), accommodating about 8-10 mm/yr of slip between arabia and africa. the east anatolian fault zone is also predominantly left-lateral strike-slip in nature, but the faulting within it is less continuous, less localized and more varied in nature than in the north anatolian fault zone. its junction with the dead sea fault occurs in the region between maras, adana and aleppo forming a poorly understood broad zone of diffused seismicity. seismicity in this region is restricted to the upper 10-20 km of the continental crust while the lower crust is generally aseismic. despite their high tectonic activity the dead sea fault and its junction with southern segment of the east anatolian fault zone have been quiet in the last two centuries, though they have produced many large earthquake in the past such as those discussed in this article. geographic factors – the region consists of large tracts with different geologic site conditions; from predominantly soft rock on the mountains, to loose saturated sands and silts in the basins, areas responsible for foundation and slope failures even under normal conditions. these areas, such as in the adana plains, the amik golu, nahr al ‘asi (orontes) valleys and throughout the mesopotamian plains, always tend to enhance damage and respond strongly to large distant earthquakes. liquefaction and damage to structures due to foundation failure are not indicators of high earthquake intensity. building stock – the vulnerability of the building stock in the region varies enormously. in the 12th century, in towns, and to a lesser extent throughout rural areas, better-built constructions were of stone, laid predominantly in clay mortar, and plastered. however, in most cases, heavy damage to buildings was due to both poor construction and lack of repairs, while for forts and castles built on hilltops or on sloping ground, damage was chiefly due to differential settlements, overturning and incipient sliding of their foundations. in villages rural houses were built chiefly of sun-dried mud and adobe with roofs of tamped earth, which becomes particularly heavy during the rainy season. on the hills and mountains stone was used for more substantial houses, few of them two storey high, built on sloping ground or on hilltops. population density – we have no 12th century figures for the number of people living in fig. 1. the dominant tectonic feature of the levant. dsfs – dead sea fault system; eafs – east anatolian fault zone. open stars show the general location of the epicentral region of the earthquakes investigated. the location of the 1202 earthquake comes from a separate study (ambraseys and melville, 1988). 735 the 12th century seismic paroxysm in the middle east: a historical perspective the region. for the early 16th century barkan (1958) estimates a total population of about 600 000, of which 67000 lived in aleppo and 57 000 in damascus. 3. historical factors the earthquakes occurred in what is today lebanon, syria and southeastern turkey, at a time of continuing conflict during the successive military and religious crusading expeditions organised in western europe against the muslims which intended to take back and defend against islam the holy places of palestine and syria. for a background of the history of the region during that period, see röhricht (1898), runciman (1952) and maalouf (1983). it is important to note, therefore, that continuing warfare and earthquakes before and throughout the 12th century, in both muslim and christian territories, had progressively enhanced the vulnerability of hastily repaired fortified towns, castles, and houses, much of the damage caused by earthquakes reflecting the cumulative effects of both deliberate and earthquake damage. in assessing intensities, this is an important consideration for which allowance must be made. 4. sources of information one of the objectives is to provide a uniform account of seismicity from the assessment of original sources of information. our area has a well-documented history preserved in a variety of sources including mediaeval arabic and occidental chronicles. these are supplemented by syriac, byzantine and jewish sources and also by archaeological studies. in contrast with other parts of the world, for which there is unpublished information awaiting retrieval from archives and repositories, for our region many primary sources are well known, most of them published. thanks to röhricht’s work, and to the work of many others such as berchem and fatio (1913), almost all primary sources of 12th century earthquakes in syria are easy to locate. also a corpus of texts from arabic sources of material on the seismicity of the arab region can be found in taher (1979, 1996). although these work are not without blemishes regarding date conversion and amalgamation of events, their great value is in the unexpurgated data of the arabic texts that they provide. an interesting summary of the effect of earthquake and plague in arabic sources is given by kremer (1880). so, there is no real difficulty in knowing where to look for useful information. the real difficulty is to improve on the factual interpretation of this material by systematically reading all the relevant texts, in their original language if possible, with the aid of reliable commentaries and other pertinent works. historiographic and linguistic problems are addressed only when they have a bearing on the scientific understanding of the earthquake in question. there is little additional evidence to help assess damage or intensities by archaeological reports. archaeological studies on the whole use documentary evidence of earthquakes to support the chronology and identification of repair or reconstruction phases at castles and public buildings, rather than archaeological evidence of rebuilding to indicate the extent of earthquake damage. 5. discussion 5.1. reliability of source material almost all of the 12th century and later documents give an apocalyptic account of earthquake effects and describe the ensuing human suffering in graphic detail, most of this in a standard disaster language. other sources give a muddled picture, but often few do have interesting details. although this is not sufficient to throw these accounts in doubt, it does give different interpretations of the facts, which often lead modern writers to assign grossly inflated intensities, not only to middle eastern earthquakes but also greek and particularly italian. natural exaggeration in historical sources is well recognised and not difficult to detect (berchem and fatio, 1913). the authenticity of 736 nicholas n. ambraseys the source, the style of its narrative, internal evidence in the account and experience gained from processing local information, all combine to permit a realistic assessment of the seriousness of the situation. it is important to keep an open mind about information, which, though beyond one’s experience, is not impossible. descriptions of the 1170 earthquake such as: «… the earthquake razed to the ground a swathe of most ancient and well-fortified cities throughout the whole orient, burying their inhabitants» … «there was no place, undamaged even as far as the ends of the earth, or that it was the greatest and more violent shock than any, which were said to have happened in the memory of men», imply a catastrophic event of an unprecedented magnitude. yet the same authors go on to say that after the earthquake, people were staying outside their houses, that they took refuge in churches and mosques or that it took them only three weeks to repair the damage. allowing exaggeration of damage in early sources to influence the assessment of destructiveness of earthquakes has lead modern writers to assign grossly inflated intensities. with few exceptions all texts are rhetorical and self-consciously literary in their style, and therefore exaggerate the earthquake’s effects, although in terms of content they closely resemble the briefer and plainer texts given by few contemporary writers. the reliability of the sources varies considerably. for instance, runciman describes matthew of edessa as «naïve», and remarks «much of his information about the crusade must have been derived from some ignorant frankish soldier; but about events in his native city and its neighbourhood he was very fully informed». runciman also notes ibn al-athir’s chronological deficiencies and his tendency to transform his sources’ accounts and dates after his own prejudices (runciman, 1951, 1952). it is unusual for sources to note that earthquakes caused disasters in the rural areas as well as the cities. most sources tend to mention the effects of earthquakes only on cities or entire regions. also casualty figures are difficult to check and are anyway not necessary indicative of the intensity of an earthquake. 5.2. location and timing of earthquakes because of the emphasis on effects in major urban centres, reported figures depended to a large extent on population distribution and density. even approximate figures, such as those given for the larger 12th century earthquakes, seem to be a rough popular estimate. duplication of earthquakes also has been a problem, particularly with modern writers. several systems have been used to date earthquake, notably the muslim (a.h.), seleucid (sel.), armenian (ar.), and the julian (old style) calendars. details of these may be found in grumel (1958) and cattenoz (1961). a difficulty can arise from the fact that the muslim and the christian day do not precisely correspond. whereas the christian day is reckoned from midnight to midnight, the muslim day begins at sunset, time being reckoned in twelve-hourly periods from 6 p.m. for this reason muslims likewise reckon the days of the week from sunset to sunset, and what to the muslim in sunday evening is to the christian still saturday evening. in dating earthquakes if these differences are overlooked, or the time of the day is not known, an incorrect date can be put on an event. this is willis’s mistake – see below –, which lay in failing to convert dates in the muslim calendar, with the result that a considerable number of events were dated many years too early. 5.3. assessment of intensity for the purpose of assessing intensity and reducing subjectivity it is important to distinguish between damage caused by vibrational, dynamic or inertia loading, and damage caused by indirect, secondary effects, such as foundation spreading, liquefaction, slides, rock falls and aftershocks. liquefaction will cause damage from excessive differential settlement rather than from ground shaking. this kind of built environment makes it difficult to assess damage and apply damage criteria inherent in modern intensity scales. another factor to be borne in mind is the way in which earthquakes and earthquake damage was perceived and reported in the sources. the 737 the 12th century seismic paroxysm in the middle east: a historical perspective systematic use of terms such as «strong», «frightening», «great», «large» or «not seen before» is merely repeating or adding to what their various sources wrote, rather than applying a uniform reassessment of their own, and one author’s strong is another author’s great. «large» may mean different things in different contexts: a large earthquake in britain is a different matter from a large earthquake in turkey. it is the rule rather than exception of the same shock or degree of damage being given in conflicting ways in the chronicles, and these terms alone cannot be used to assess intensities of earthquakes in the middle ages, not only in the middle east but also in europe. early intensity reports are typically amplified by the high vulnerability building stock and by exaggerated reporting that conspire towards the levels where 20th century scales saturate. mortality caused by earthquakes depends directly on the vulnerability of the built environment and on population density and only indirectly on epicentral distance. major earthquakes may be expected to kill more than 10 percent of the population in the epicentral area. where the housing stock consists primarily of weak, heavywalled structures, the death toll will be much higher. other factors, such as the level of occupancy of dwellings may affect this percentage so that much larger life loss should be expected if the earthquake occurred at night, in the winter. obviously, loss of life is not a criterion, and alone it cannot be used to assess intensity. some of the historical, and modern earthquakes originating from the region are felt at abnormally great distances to the south and to the east than away from the mainland to the north and west. this preferential distribution is due to the enhancing effect the nile delta and nile of alluvial valleys, and also due to the low attenuation of seismic waves across the arabian shield in these directions rather than due to any other less obvious causes (ambraseys, 2001). 5.4. existing earthquake catalogues numerous 20th century catalogues of historical middle eastern earthquakes have appeared and the rate at which they are produced in the 21st century has increased, most of them fatally flawed. the intention towards their authors, particularly seismophile historians, is not vindictive; precisely because they are authoritative as well as recent works, but the problems they contain are the most extensive and pervasive, and therefore most need to be exposed (ambraseys et al., 2004). for instance willis’s remarkable error that was noticed by the author himself (willis, 1928, 1933) but not before erroneous entries from his list had found their way into other catalogues (sieberg, 1932a,b). while willis is therefore rightly treated with caution, the same misgiving is not applied to sieberg and other later authors (sieberg, 1932a,b). sieberg’s work is prominent among those used in the most recent authoritative catalogue of middle east earthquakes by benmenahem (1979). another example is the results of taher’s research, which were presented by poirier and taher (1980). their catalogue unfortunately passes on, and even adds to, the inaccuracies in the original work. this highlights the interesting problem of how to achieve consistency and homogeneity in a catalogue relying on very diverse material, which ideally needs a specialist in each relevant field, and without reading through the full texts from sources written in different languages. 6. conclusions the ultimate purpose underlying the quantitative assessment of earthquake hazard using historical data is to identify active tectonic elements and to obtain an estimate of the regional long-term seismic exposure. the means to obtain this estimate is the identification of sites, which have been associated with large historical events and the assessment of their magnitude (see ambraseys and jackson, 1998). from fig. 1 we notice that the seismic activity in the 12th century occurred in a remarkable series of earthquakes that moved successively from north to south along the east anatolian and dead sea fault systems. the 1114 earthquake and its foreshocks are clearly associated with the east anatolian fault zone, probably of m > 7, the shock occurring 738 nicholas n. ambraseys southwest of the earthquake (m = 7.1) of 2 march 1893 (ambraseys, 1989). the next event of 1138 occurred on the dead sea fault system, south of its junction with the east anatolian fault system, perhaps of m < 7. this was followed in 1151 by an ill-defined activity in the region of bosra and the hauran in se syria, probably associated with the palmyride fold belt. next we have what seems to be a strong foreshock in 13 july 1157 in the region of hama, which was followed on 12 august 1157 by the large hama earthquake (m >> 7) that must be associated with the bent of the fault system. this is common with all major continental strike-slip faults; the dead sea fault system is likely to be segmented, with rupture in individual earthquakes limited in length by structural discontinuities or bends. finally the large earthquake of 29 june 1170, probably of m >> 7, completed the activity in the 12th century, but the paroxysm continued into the next century with the large shock of 1202 (ambraseys and melville, 1988) which extended the activity even further south on the straight yammouneh fault segment of the main left-lateral fault system, which runs nne-ssw for about 150 m and is thus oblique to the overall africaarabia motion. it is likely that in this area the deformation is probably distributed over more than one fault, and may even be separated into its strikeslip and thrust component as is seen elsewhere in regions of oblique convergence. a manifestation of this distributed deformation may be the large earthquake (m > 7) of 1837 in southern lebanon, which appears not to have occurred on the yammouneh fault, and may have been on the roum fault instead (ambraseys, 1997). the size of these events is not different from better-documented large earthquakes in the region (ambraseys and melville, 1988; ambraseys and barazangi, 1989). it is evident from the quiescence of the 20th century compared with the large earthquakes of historical times that the main dead sea fault system itself remains locked between slip events in major earthquakes. oddly enough, despite the relatively large magnitude of these earthquakes and of their clear association with active faults, there is no mention in the sources about ground deformations that could be of tectonic origin. the only allusion we find is in the description of earthquake of 1170 in which it is said that deep fissures opened in the ground in the mountains overlooking baalbek. one of the basic problems with historical earthquakes is the quality of intensities. there are very few catalogues of historical earthquakes which present both, unadulterated original macroseismic data and also the method used to assess intensities. another problem is that there has been more emphasis on the development of methodology to analyse intensity data, than on the intensity data themselves. at this stage of our studies it is not possible to assign intensities to individual locations or magnitudes to different events. the reason for this is that in determining the intensity of an historical earthquake at a place, and in assigning to that place an intensity rating in any scale, allowance has to be made for the vulnerability and rate of aging of buildings at that time, as compared with the vulnerability of man-made structures in those countries for which modern scales were originally drawn up. the building stock in the 12th century in our region and its vulnerability are different from those for which modern intensity scales have been prepared, so that existing intensity scales are not appropriate. our uncertain knowledge of region-specific intensity attenuation scaling laws for that period has yet to be improved (see above assessment of intensity; ambraseys and bilham, 2003). case histories all the earthquakes discussed below, as a matter of principle, are documented as fully as possible. inevitably, however, there are cases where such thoroughness is misplaced when near-contemporary or later historical references, as well as early editions of muslim writers that could have been easily omitted, are included for the sake of completeness, and also because this material had to be read through in the expectation of finding additional information, even when eventually they do not always add very useful or relevant details. 739 the 12th century seismic paroxysm in the middle east: a historical perspective as the aim of this article is to present as clearly as possible all the information on which the assessment is based, modern earthquake catalogues and studies are not cited unless they contain fresh and reliable information such as those of alexandre (1990). 10 august 1114 – the large earthquake of 29 november 1114 – see below – was preceded by two strong shocks: the first of which, on the feast of st laurence, 10 august 1114 (fulcher, rhc hist. occid., 1866), was probably felt in antioch, and allegedly «caused damage to maritime cities and fortified towns with loss of life» which, as these cities are not named, may be pure rhetoric (william of tyre, rhc hist. occid., 1844; walter the chancellor, rhc hist. occid., 1895; robert de torigni, delisle, 18721873). but it is possible that what we have here is an earthquake with an epicentre offshore in the bay of iskenderun (alexandretta). aftershocks continued for two months (gesta, bongars, 1611; see also rhc hist. occid., 1866, though it dates it in 1113). 13 november 1114 – the second shock, for which we have information, occurred on st bricious’ day (william of tyre, rhc hist. occid., 1844) or on the day of the ides of november, 13 november 1114 (sigebertus gemblacensis, mgh ss, 6, 1844; see also alexandre, 1990), and affected the south-eastern part of the plain of adana, in the principality of antioch. the earthquake destroyed a part of maras (mamistra, germanicea) (gesta, bongars, 1611; see also rhc hist. occid., 1866) and allegedly all the towns in the surroundings of the city (walter the chancellor, rhc hist. occid., 1895), causing great loss of life (fulcher, rhc hist. occid., 1866). the neighbourhood of antioch and the city itself suffered less, but in the suburbs of antioch the ground opened up and a number of towers and houses nearby settled into the ground (sigebertus gemblacensis, mgh ss, 6, 1844), probably as a result of liquefaction of the ground. many other towns in caelosyria, isauria and cilicia, the name of which is not given, were also affected (william of tyre, rhc hist. occid., 1844). the large earthquake that followed a few weeks later obscures more details about this earthquake (flores historiarum, luard, 1890; robert de torigni, delisle, 1872-1873; william of tyre, rhc hist. occid., 1844; romoald of salerno, ris 2, 1909-1935 and mgh ss, 19, 1866). 29 november 1114 – the earthquake of 29 november 1114 occurred at night and affected the christian county of edessa and principality of antioch, which lie around the present borders of southern turkey and northern syria (fig. 2). the shock occurred at a time of almost continuous conflict between christian and muslims states. it was strongly felt to the east and southeast in neighbouring muslim territory as well as to the north in armenian and turkish states. the earthquake occurred at night on the sunday of the vigil of st andrew’s day, 29 november 1114 (walter the chancellor, rhc hist. occid., 1895). although there are not many contradictions between the sources, they vary about the date of the event. out of about 25 authors who mention the event, three give the year as 1113, 16 agree on 1114, and 5 put it in 1115 (anselmus gemblacensis, pl, 160, 1854), all of them providing texts the details of which are those of the earthquake of 29 november 1114 (see also alexandre, 1990). a few of the wrong dates are clearly copyist error adopted by later writers, as well as due to the amalgamation of the main shock with its foreand aftershocks, a typical habit of later sources, particularly with syriac writers who drew heavily on earlier material. this would also account for the few references to an earthquake in 1115. in fact, there is strong agreement about the date of the main shock between contemporary and near contemporary occidental sources that give the night of 29 november. the earthquake affected most of all the part occupied by the franks (william of tyre, rhc hist. occid., 1844). it was felt in other parts such as in mesopotamia, syria and other regions (ibn al-athir, rhc hist. or., 1872). in those of the muslims nothing unfortunate occurred (matthew of edessa, rhc da, 1869). maras, a fortified town in the principality of antioch together with its suburbs (william of 740 nicholas n. ambraseys tyre, rhc hist. occid., 1844), was almost totally destroyed with great loss of life (fulcher, rhc hist. occid., 1866). houses and the city walls, which were not in good condition, the fort (robert de torigni, delisle, 1872-1873) and its ramparts (abul faraj, bedjan, 1932) all were completely demolished (flores historiarum, luard, 1890). the church of mar john of kaysun collapsed, along with the church of the forty martyrs (michael the syrian, chabot, 1899-1910) and among those killed in the town were the constable, the bishop, the clergy and many important people (walter the chancellor, rhc hist. occid., 1895). large parts of the villages belonging to maras, which are not named, also collapsed (michael the syrian, chabot, 1899-1910). it is said that maras was a very populous city and that between 24 000 (abul faraj, bedjan, 1932) and 40 000 (matthew of edessa, rhc da, 1869; sembat, rhc da, 1869) persons lost their lives, besides strangers, and more than 100 priests and deacons (chronicon 1234, csco ss, 56, 1937; gesta, bongars, 1611; see also rhc hist. occid., 1866). the casualty figures are naturally suspect as they are comparable if not larger with the population of maras; they sound like a biblical formula for a multitude. the monastery of mashchgavor (mashkur), which must be sought near the north part of the amanus mt. (giaour dagh) (dulaurier, 1861), also fell, killing among others the armenian doctor gregory (matthew of edessa, rhc da, 1869). fig. 2. location map of the earthquake of 1114. 1 – ablastha; 2 – aleppo; 3 – antioch; 4 – atharib; 5 – azaz; 6 – balis; 7 – edessa; 8 – hiesuvank; 9 – harran; 10 – kaysun; 11 – latakiya; 12 – mansur; 13 – maras; 14 – maschegavor; 15 – mopsuestia; 16 – raban; 17 – samosata; 18 – shoughr; 19 – sis; 20 – tell khalid; 21 – zaradna. 741 the 12th century seismic paroxysm in the middle east: a historical perspective the same happened to shoughr, the monastery of the brasilians on the black mountains (lersar), which is between maras and sis (missis, kozan) about 50 km from the former (dulaurier, 1861), was also ruined and its church collapsed killing thirty monks and two officiating priests (matthew of edessa, rhc da, 1869; sembat, rhc da, 1869). a similar incident is reported from the monastery of hiesuvank near maras: it fell crushing all the religious under its ruins (matthew of edessa, rhc da, 1869; dulaurier, 1861). raban was almost totally destroyed (matthew of edessa, rhc da, 1869; sembat, rhc da, 1869) and the same happened to kaysun (matthew of edessa, rhc da, 1869; abul faraj, bedjan, 1932). it seems that damage at mansur (hisn mansur) was serious but not excessive (matthew of edessa, rhc da, 1869; chronicon 1234, csco ss, 56, 1937; sembat, rhc da, 1869). samosata (sumaisat) built on the left bank of the euphrates river, was badly damaged (matthew of edessa, rhc da, 1869). houses in some parts of the town collapsed and elsewhere sunk into their foundations (fulcher, rhc hist. occid., 1866; ibn al-jauzi: ed. hyderabad 1938-1941 and cairo nat. lib., m. 95; ibn al-athir, rhc hist. or., 1872; sembat, rhc da, 1869). according to a chronicler, they disappeared under the ground (al-suyuti, abd al-latif sa’adani, 1971) , killing a number of people among them constantine the lord of gargar (michael the syrian, chabot, 18991910; abul faraj, bedjan, 1932) but not his jailers or other franks. it is possible that much of the destruction was due to ground failures enhanced by the euphrates river overflowing and flooding the town, which happened shortly before or after the earthquake in samosata as well as in other places (sibt ibn al-jauzi, ed. chicago 1907). little we know about elbistan (ablastha, zeitun) where damage should not have been very serious (sembat, rhc da, 1869). we are told that tell khalid (trialeth), a fortified site at the head of sadjour suyu, a tributary of the euphrates river, was also destroyed (fulcher, rhc hist. occid., 1866; flores historiarum, luard, 1890; william of tyre, rhc hist. occid., 1844; robert de torigni, delisle, 1872-1873; gesta, bongars, 1611; see also rhc hist. occid., 1866). the earthquake was strongly felt in the district of aleppo. in the city itself there was no damage to speak of, but near-by azaz, a fort already in ruins, was badly damaged, and its governor fled to aleppo (kemal al-din, rhc hist. or., 1884). damage in aleppo was minimal, but about 25 km southwest of the city, the fortified site of athareb was almost completely ruined (kemal al-din, rhc hist. or., 1884), which is not surprising as two years earlier, the siege engines of the franks had pounded its walls to pieces, leaving little standing to be shaken down by the earthquake (see references in runciman, 1952). zerdana, 10 km south of al athareb had the same fate (kemal al-din, rhc hist. or., 1884). in the principality of antioch the earthquake caused great concern but otherwise only sporadic damage (william of tyre, rhc hist. occid., 1844; benedict of accolti, rhc hist. occ., 1895). in the city itself people fled their homes in panic but since the walls remained intact, no one could escape and many fled to the church of the apostle peter, seeking his protection. from what we know damage was confined to the collapse of the tower of the north gate, of a few houses in a part of the city centre, as well as in the new, upper district (al akaba) of the city where a few people lost their lives (kemal al-din, rhc hist. or., 1884; gesta, bongars, 1611; see also rhc hist. occid., 1866). in the suburbs of antioch, the earth opened up, presumably as a result of incipient sliding or liquefaction of the ground, causing some damage. the patriarch proclaimed three days of fasting, but the authorities did not hurry what was to be done about repairs and the condition of the whole city (walter the chancellor, rhc hist. occid., 1895). they organised repairs asking inhabitants to contribute according to their means (rey, 1896) and toured chief fortresses in the district for repairs (rey, 1901). contrary to what many near-contemporary sources imply, there is no mention in the contemporary sources of extensive damage, let alone that the city collapsed (sembat, rhc da, 1869) – for one thing, churches were left 742 nicholas n. ambraseys standing to which people took refuge (cahen, 1940). repairs carried out after the earthquake suggests that the earthquake was rather strong in latakia. the site of balis (balash) was a former town in syria and a port on the western bank of the euphrates 5 km from modern meskene. almost all writers say that the earthquake ruined 100 houses burying many people in the debris and caused the collapse of half of the citadel while the rest of the town stayed secure (ibn aljauzi, ed. hyderabad 1938-1941 and cairo nat. lib., m. 95; ibn al-athir, rhc hist. or., 1872; abul faraj, bedjan, 1932; al-suyuti, abd allatif sa’adani, 1971; ajami abu darr, sauvaget, 1950). but we are also told that soon before or after the earthquake, which should have been felt at balis, the euphrates river overflowed, ruining 100 houses and sweeping away half of the citadel (sibt ibn al-jauzi, ed. chicago 1907). it is hard to decide between the earthquake and the flood for the loss of 100 houses and citadel. in edessa (ruha, urfa) the earthquake occurred almost immediately after the muslims, who were besieging the city for two months, withdrew. the shock was felt in the edessan countryside where the mountains and hills were shaken violently (matthew of edessa, rhc da, 1869). muslim sources say that 13 towers of the city wall collapsed (ibn al-jauzi, ed. hyderabad 1938-1941 and cairo nat. lib., m. 95; abul faraj, bedjan, 1932; al-suyuti, abd al-latif sa’adani, 1971; ajami abu darr, sauvaget, 1950) with some loss of life (ibn al-athir, rhc hist. or., 1872). oddly, frankish sources, which mention a flood shortly after the earthquake that demolished the near-by dam, do not mention any damage in odessa (sic) caused by the earthquake. the old walls of harran were breached here and there and houses were ruined killing a number of people (ibn al-jauzi, ed. hyderabad 1938-1941 and cairo nat. lib., m. 95; ibn alathir, rhc hist. or., 1872; kemal al-din, rhc hist. or., 1884; abul faraj, bedjan, 1932; al-suyuti, abd al-latif sa’adani, 1971; ajami abu darr, sauvaget, 1950). little we know of what happened in sis except that the town was again damaged and that many villages and monasteries in the plains of its district were destroyed with casualties (matthew of edessa, rhc da, 1869; sembat, rhc da, 1869; cahen, 1940). the earthquake was not felt in damascus where news of the earthquake arrived some days after the event (ibn al-jauzi, ed. hyderabad 1938-1941 and cairo nat. lib., m. 95). it is probable that the shock was perceptible (gesta, bongars, 1611; see also rhc hist. occid., 1866) in jerusalem but that damage extended that far (romoald of salerno, ris 2, 19091935) might be dismissed as gross exaggeration. there is some evidence, however, that this or another earthquake shock about the same caused some concern (enlart, 1925). we could not substantiate the statement that as a result of the earthquake the sea got up (sembat, rhc da, 1869) causing some damage, spurious information, perhaps belonging to the earthquake of 10 august 1114. apparently only the frankish-occupied provinces were badly damaged. recorded repairs of damage in muslim territory was chiefly to the mosques which was caused by the earthquakes or by other causes allotting to the repairs the silver which remained from the treasure of the waqf. 1 january 1120 – a violent earthquake destroyed many places (michael the syrian, chabot, 1899-1910) at the 3rd hour of thursday, 1st of kanun-ii, 1431; the date given corresponds to 1 january 1120, which was a thursday. no location is given, but as michael’s source for many events in this period is basil of edessa (michael the syrian, chabot, 18991910), it is quite probable that the earthquake happened in the locale of that city. february 1127 – an earthquake was felt, probably in edessa. this event is recorded by michael the syrian (michael the syrian, chabot, 1899-1910) and dated to a.s. 1438 in the month of šebat (february 1127). he gives no location. november 1127 – two earthquakes during the day were followed by two more at night, with a long period of aftershocks for «forty 743 the 12th century seismic paroxysm in the middle east: a historical perspective days and forty nights», which can be taken to mean «a long time». the location is not given. this event is recorded by michael the syrian (chabot, 1899-1910) and dated to a.s. 1439 in the month of tesrin (november 1127). a much later source (15th century) implies that this may be the earthquake that sometime in 1128 caused considerable damage in sur (matteo palmieri, ris 2, 1906-1915). 11 october 1138 – a long series of damaging shocks occurred in northern syria continuing for three months between october and december 1138. the main shock occurred on 11 october 1138 and it was followed by many aftershocks (ibn al-qalanisi, amédroz, 1908). alqalanisi, a contemporary chronicler in damascus, places the first shock in a.h. 533, tuesday 4 safar (tuesday 10 october 1138), the next during the night of (after the next) friday, at twilight, which, as the islamic system counts from the night before should be friday 14 safar (thursday 20 october 1138). the next shock is not until 19 safar (wednesday 25 october), followed by another shock during the night of the following wednesday (26 safar = tuesday 31 october-wednesday 1 november), with the last one in the early hours of friday (i.e. 28 safar = = friday 3 november). a later author, kemal al-din, mentions only one shock in a.h. 533, thursday 13 safar (thursday 19-20 october 1138), which does not add up to al-qalanisi’s date. the varied dates given in the sources suggest that it was difficult for witnesses to distinguish the main earthquake from other shocks, particularly if the event was a multiple earthquake. in any case, the sources’ sense of dating and of chronological order are not always their strong points (see also ibn al-’adim, dahhan, 1951). later writers conflate this earthquake with the earthquakes of november 1137 in jazira and the large event of 30 september 1139 in ganjak. for instance the 15th century ibn taghribirdi reports a loss of 230 000 lives in the earthquake, which was worst in aleppo (ibn taghribirdi, shaltut, 1929-1972). in fact these losses were due to the earthquake in ganjak in georgia. following al-qalanisi, the main shock should have been on 10-11 october 1138, a date supported by other sources (kemal al-din, rhc hist. or., 1884; al-’umari, ms., iraq acad., baghdad; ibn al-athir, rhc hist. or., 1872 and cairo 1885), and which we adopt for this event. the earthquake occurred in what was at the time the frontier territory between moslems and christian, in which most sites had already suffered from warfare and previous earthquakes. worst hit was the castle of harim, which was occupied by the franks (fig. 3). the castle was shattered and the church collapsed (michael the syrian, chabot, 1899-1910; ibn taghribirdi, shaltut, 1929-1972). athareb, occupied by the muslims, and already weakened by warfare and the earthquake 24 years earlier, was ruined and its citadel collapsed completely, killing 600 of the guard. the governor with a few survivors fled to mosul. in this respect the earthquake benefited athareb as he had taken possession of the region and raised taxes. the impromptu tax relief benefited reconstruction (kemal al-din, rhc hist. or., 1884; michael the syrian, chabot, 1899-1910; chronicon 1234, csco ss, 56, 1937; ibn taghribirdi, shaltut, 1929-1972). all we know about near-by zaradna is that it was destroyed, which is not surprising as it had already been ruined a few years earlier (kemal al-din, rhc hist. or., 1884; deschamps, 1935). little is known about the small fort at shih, which was totally destroyed (kemal aldin, rhc hist. or., 1884). in aleppo, a large city with a population of a few tens of thousand, the earthquake did considerable damage. the ramparts of the city buckled and the walls of the citadel were shattered (ibn alqalanisi, amédroz, 1908). stones detached themselves from the walls fell into the streets, houses were destroyed, some house walls collapsed, and the two parts of the walls, east and west of the citadel were breached (kemal al-din, rhc hist. or., 1884).warned by foreshocks people evacuated their houses and fled to the country (ajami abu darr, sauvaget, 1950; abu ‘l-fida, rhc hist. or., 1872). throughout the period 4th to 19th safar 533 a.h. (10 to 25 october) (ibn alshihnah, british library, london, ms. or. add. 2, 36), the populace of aleppo lived outside the city (ajami abu darr, sauvaget, 1950). 744 nicholas n. ambraseys many sources, without giving any details regarding loss of life, repairs or tax relief, say briefly that aleppo was destroyed (michael the syrian, chabot, 1899-1910; al-suyuti, abd allatif sa’adani, 1971). however, not all sources agree that the shock reached its maximum at aleppo and in the surrounding towns. kemal’s record shows that the damage to aleppo was not great, the worst being the collapse of the nearby atharib citadel (kemal al-din, rhc hist. or., 1884). in azrab, north of aleppo on the edge of the kuros mountains, the ground opened in the middle of the village and later collapsed totally (michael the syrian, chabot, 1899-1910; ibn taghribirdi, shaltut, 1929-1972) most probably, as a result of a landslide triggered by the earthquake. at bizaah the earthquake brought down towers from the walls (michael the syrian, chabot, 1899-1910). all we know about tell khalid and tell amar, is that they suffered some unspecified damage (kemal al-din, rhc hist. or., 1884). the earthquake and some of its aftershocks were perceptible in damascus but not in jerusalem (chronicon 1234, csco ss, 56, 1937). callinice (nicephorium, raqqa) on the euphrates river is often associated wrongly with this earthquake on the authority of michael the syrian (michael the syrian, chabot, 1899-1910). after recording the damaging earthquake in aleppo in a.s. 1450 tesrin (october 1138), michael moves on to his next item in which he mentions a severe winter from december to february, during which the euphrates river freezes. only after this does he mention the opening of the ground and the swallowing up of 40 men near callinice (raqqa). this incident, which is repeated by later writers in 1140 (abul faraj, bedjan, 1932), he does not connect with the1138 earthquake, nor does he imply that the opening of the ground was the result of an earthquake. there is no evidence that the shock was responsible for the reported effects at raqqa, or that it extended beyond (al-tabbakh, ed. aleppo 1923). 27 september 1151 – an earthquake occurred during the night in the area of bostra and fig. 3. location map of the earthquake of 1138. 1 – aleppo; 2 – atharib; 3 – azrab; 4 – bizza; 5 – harim; 6 – raqqa (120 kn se of tell amar); 7 – shih; 8 – tell amar; 9 – tell khalid; 10 – zaradna. 745 the 12th century seismic paroxysm in the middle east: a historical perspective the hauran in se syria and was felt widely in neighbouring districts. there were three shocks altogether, at least one of which must have been damaging, as a large number of walls were reported to have collapsed in bostra and in other places (ibn al-qalanisi, amédroz, 1908). it is not certain that the shock was felt in damascus. ibn al-qalanisi says that «accounts came to us» (probably orally, in the first instance) about an earthquake during the night of (i.e. preceding) a.h. 546 13th latter jumada (27 september 1151). 27 september 1156 – a strong earthquake caused considerable concern in damascus, «in the 8th hour of the night of thursday 9 shaban 551» (27 september 1156) (ibn al-qalanisi, amédroz, 1908). shock continued until the end of the day. the long series of earthquakes which were felt in damascus after this data, is recorded by this author, from whose work many later chroniclers have drawn, in the process committing a variety of dating errors. 10 october 1156 – there were more shocks felt in damascus but less intense (ibn alqalanisi, amédroz, 1908). according to the same author and to al-suyuti (abd al-latif sa’adani, 1971) in the night of wednesday 22 shaban 551 (10 october 1156) an earthquake happened on saturday night 25 shaban 551 (13 october 1156). gregory the priest, the continuator of matthew of edessa (rhc da, 1869) gives exactly the same description of the event but dates it to 26 october. shocks were felt in damascus on wednesday 29 shabban 551 = 17 october 1156, monday 1 ramadan = 18 october, tuesday ramadan 3 = 20 october, friday night 15 ramadan = 1 november, saturday 16 ramadan = 2 november, at night 17 ramadan = 3 november, friday night 23 ramadan = 9 november, on sunday 2 shawwal = 18 november, thursday 7 shawwal = 22 november, monday 16 shawwal = 2 december, 17 shawwal = 3 december, 22 shawwal = 6 december. 2 april 1157 – another earthquake was strongly felt in damascus after sunrise and the following morning, causing great consternation among the inhabitants, on wednesday 19 safar 552 (2 april 1157) (ibn al-qalanisi, amédroz, 1908). the shock had considerable effects in northern syria (al-suyuti, abd al-latif sa’adani, 1971; abu shama, hilmi, 1956-1962), the exact nature of which is unknown, but probably it was damaging. it was followed by another shock on the same day (ibn al-qalanisi, gibb, 1932) and more shocks the following days (thursday night 20 safar 552 = 3 april 1157; 21 safar = 4 april, 21 jumada = 1 july, thursday 25 jumada = 5 july). 13 july 1157 – this was damaging, probably at midday sunday 4 jumada-ii = 13 july, which was followed by a single aftershock, possibly during the night (ibn al-qalanisi, amédroz, 1908; al-suyuti, abd al-latif sa’adani, 1971). homs (hims emessa), hama (hamath, epiphanea), shaizar, and kafrtab were partly ruined. repairs were affected after the last earthquake had damaged (al-suyuti, abd allatif sa’adani, 1971) or destroyed them (ibn al-qalanisi, amédroz, 1908). it seems that damage in apamea (afamya, qalat al mudiq) was less serious (abu shama, rhc hist. or., 1896). the earthquake did some slight damage in aleppo and it was felt in damascus, and strongly at tayma, south of damascus (ibn alqalanisi, amédroz, 1908; al-suyuti, abd allatif sa’adani, 1971). as a result of the cumulative damage caused by the long series of earthquakes to the defences of the region of aleppo, particularly to the frontier forts of homs, shaizar, kaftab and hama, the frankish armies were preparing to invade the country in the hope of profiting from the earthquakes which had just ruined the forts, citadels and houses of the muslims (abu shama, rhc hist. or., 1896). nureddin set out to repair the defences of hims, shaizar, kafr-tab and hamah on a.h. 552 rajab 3 (11 august 1157), and he makes it clear that this was because he had heard that the franks were preparing to invade the country in the hope of profiting from the earthquakes. this confirms that the cumulative damage caused by all these earthquakes was serious and predates the additional damage caused by the large earthquake of 12 august 1157. 746 nicholas n. ambraseys 12 august 1157 – the culmination of over a year of foreshocks was a violent earthquake in the northern section of the dead sea fault zone (fig. 4). the most part of syria, in the districts where there was no destruction, was damaged (ibn alathir, rhc hist. or., 1872 and cairo 1885; abu shama, rhc hist. or., 1896), and had it not been for prompt response by the muslims to defend the country, the franks would have taken possession of it without siege or combat (abu shama, rhc hist. or., 1896). the situation was the same in the frankish possessions (kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874). the earthquake was so violent that ramparts and castles were overturned by it both sides busying themselves in repairing the damaged fortifications and in making sallies into each others possessions in order to confine the enemy back in their own territory (abu ‘l-fida, rhc hist. or., 1872; ibn wasil, shayyal, 1962). the earthquake occurred during the night (on the 7th hour) of monday, 4 rajab a.h. 552 (12 august 1157) (ibn al-qalanisi, amédroz, 1908 and gibb, 1932; al-suyuti, bibliotèque nationale de france, mss ar. 5929; ibn alathir, rhc hist. or., 1872 and cairo 1885). there is some confusion possible with the earthquake that followed in 1170, particularly in some later writers, which is not difficult to disentangle (ibn kathir, cairo 1932-1939; ibn al-furat, basra 1967; al-aini, bibliotèque nationale de france, ms. arabes 5761; alqalqashandi, ferraj, 1964; ajami abu darr, sauvaget, 1950; al-’umari, ms., iraq acad., baghdad; matthew of edessa, rhc da, 1869; nicetas choniates, bekker, 1835; gregory the priest, rhc da, 1869). the epicentral region of the earthquake may be defined by the localities maraat an-numan. kafertab, apamea, shaizar, hama, masyaf, and barin, a zone about 120 km long running in a north-south direction along the el rhab and the orontes valley. in maarrat an-numan damage was very serious. public buildings collapsed and the minaret of the grand mosque was damaged and repaired (elisseeff, 1951). despite later sources, which say that no one survived, loss of life was not exfig. 4. location map of the earthquake of 1157. 1 – acre; 2 – adad al-arab; 3 – afamiya; 4 – aleppo; 5 – antioch; 6 – arqa; 7 – azaz; 8 – azra; 9 – barin; 10 – beirut; 11 – damascus; 12 – hama; 13 – harran; 14 – hims; 15 – hisn al-akrad; 16 – jebeleh; 17 – jerusalem; 18 – jubail; 19 – kafertab; 20 – latakiya; 21 – maarat al-numan; 22 – masiyaf; 23 – nisfin; 24 – rahba; 25 – saida; 26 – selemiyeh; 27 – shaizar; 28 – shoumaimis; 29 – sur; 30 – tell bashir; 31 – tripolis. jerusalem and rahba are out of the map. cessive (ibn al-jauzi, ed. hyderabad 19381941). the town received 8.2 kg of gold in tax relief (abu shama, rhc hist. or., 1896). kafertab suffered almost total destruction (elisseeff, 1967) and very few of its inhabitants es747 the 12th century seismic paroxysm in the middle east: a historical perspective caped (michael the syrian, chabot, 1899-1910; ibn al-jauzi, ed. hyderabad 1938-1941; ibn alathir, rhc hist. or., 1872 and cairo 1885; chronicon 1234, csco ss, 56, 1937). in apamea, which is built on the edge of a plateau on the right bank of the orontes river at its northward bend, the earthquake caused equally heavy damage and loss of life (michael the syrian, chabot, 1899-1910; ibn al-athir, rhc hist. or., 1872 and cairo 1885; chronicon 1234, csco ss, 56, 1937). its citadel was destroyed (ibn al-jauzi, ed. hyderabad 19381941), most probably by a landslide, which tipped the fortress into a lake, which resulted in many deaths (ibn taghribirdi, shaltut, 19291972). there is no evidence that the castle was rebuilt (berchem and fatio, 1913) but the walls of its citadel were repaired (elisseeff, 1951 and 1967). the ruins of the town still exist, flanked on the west by the later citadel of qalat almadik (nr. apamea). at shaizar (ibn al-jauzi, nat. lib., cairo, ms. 95, gives shiraz for sheizar) on the orontes river, damage was heavy (robert de torigni, delisle, 1872-1873; yaqut al-hamawi, wüstenfeld, 1866-1873; abu shama, rhc hist. or., 1896; chronicon 1234, csco ss, 56, 1937; abu ‘l-fida, rhc hist. or., 1872; ibn taghribirdi, shaltut, 1929-1972). its castle, which was the seat of the local ruling family, fell down upon its governor and his followers, killing all but two. its citadel totally collapsed as well as all the buildings, which it enclosed, and only a small number of people managed to flee and escape (michael the syrian, chabot, 1899-1910; ibn al-jauzi, ed. hyderabad 1938-1941; ibn alathir, rhc hist. or., 1872 and cairo 1885; alsuyuti, bibliotèque nationale de france, mss ar. 5929). we hear that «half of the mountain on which the citadel was built collapsed» suggesting that damage was enhanced by a landslide. the suburb of shalimar escaped, except for what had been destroyed earlier (ibn alqalanisi, gibb, 1932; ibn al-’adim, dahhan, 1951). again here it is difficult to estimate loss of life which is given descriptively as «a countless multitude» (ibn al-athir, rhc hist. or., 1872 and cairo 1885), or 40 000 people (shaizar «was demolished», 40 000 dying there: this must be a gross exaggeration, chronicon 1234, csco ss, 56, 1937). as the earthquake had ruined the castle and its ramparts had been overturned the muslims took possession of the town, rebuilding the houses (ibn al-athir, rhc hist. or., 1872 and cairo 1885) and the citadel (elisseeff, 1951). the fort was repaired but not necessarily rebuilt (berchem and fatio, 1913). the town was probably not totally destroyed (ibn al-qalanisi, gibb, 1932), but as it was of strategic importance and its main defense had been destroyed, it was granted a year’s tax relief of only 2.7 kg of gold (abu shama, rhc hist. or., 1896).the reason for the tax relief at the same time as the muslims took possession of shaizar, is that the towns in question could not afford «to repair the damage done by the franks». our source makes not mention of earthquakes as the reason for the tax relief, but it cannot be insignificant that among these towns are all but those damaged by the earthquake. one should be cautious about inferring the size of the damage from the size of the relief (see also the cases of maarrat an-numan, above, and homs, adad al arab, aleppo, below). ibn al-athir (rhc hist. or., 1872 and cairo 1885) says that shaizar was adjacent to damascus, and that these two places were separated only by half a day’s march, and that shaizar was located on a high and impregnable mountain, and could be reached by only one road. for this mislocation see dussaud (1927). the city of hama built on both banks of the orontes river that at this point winds a great deal, was the worst affected. the town itself and all its large houses that were crowded close to the river (chronicon 1234, csco ss, 56, 1937) collapsed and almost all the inhabitants perished. the citadel of the town built along the bank of the river, and its fortress were demolished (chronicon 1234, csco ss, 56, 1937). there were few survivors (michael the syrian, chabot, 1899-1910; ibn al-qalanisi, gibb, 1932; abu ‘l-fida, rhc hist. or., 1872; alsuyuti, bibliotèque nationale de france, mss ar. 5929). this is attested by the story that when the school collapsed on all of the children, the teacher who survived the earthquake said that none of the parents came to claim them (ibn al-jauzi, ed. hyderabad 1938-1941; 748 nicholas n. ambraseys ibn al-athir, rhc hist. or., 1872 and cairo 1885; abu ‘l-fida, rhc hist. or., 1872; ibn taghribirdi, shaltut, 1929-1972). a contemporary writer (usama, hitti, 1987) describes the deplorable situation in hama in an elegy. it is not known how many people lost their lives. we know that repairs were made, because when hama was damaged again by the earthquake of 30 october, according to one source «the structures which had been rebuilt were destroyed again. the wall of the citadels and the hasanain mosque were rebuild (elisseeff, 1951). an extant inscription on the wall of a small mosque, to the south of the citadel, indicates that this structure was repaired after it had been destroyed by the hama earthquake (al-zalzala alhamawiyya) of a.h. 552 (berchem and fatio, 1913). however hama is not recorded as having received any tax-relief. it is probable that masyaf was also ruined (guyard, 1877) but details are lacking. the castle of barin was seriously damaged (abu shama, rhc hist. or., 1896) and its walls were repaired after the earthquake (elisseeff, 1951). less seriously affected localities were shumaimis, hisn al akrad, arqa, tripolis, homs, adad al arab, salamiya, aleppo and jabealah. at the citadel of shumaimis the earthquake caused numerous deaths (kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874). hisn al-akrad in frankish territory, contrary to what muslim sources say that it was destroyed (michael the syrian, chabot, 1899-1910; ibn al-athir, rhc hist. or., 1872 and cairo 1885; chronicon 1234, csco ss, 56, 1937; abu ‘l-fida, rhc hist. or., 1872), it was badly damaged and some lives were lost (kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874). the damage was quickly repaired and the castle enlarged. arqa (arches), in frankish territory, was probably badly damaged. according to muslim sources the town completely collapsed (michael the syrian, chabot, 1899-1910) and countless multitude died (ibn al-athir, rhc hist. or., 1872 and cairo 1885) for which there is no indication in occidental sources. damage in tripoli should have been less serious than muslim writers describe. they say that most parts of tripolis were destroyed (michael the syrian, chabot, 1899-1910; chronicon 1234, csco ss, 56, 1937; abu ‘lfida, rhc hist. or., 1872; ibn taghribirdi, shaltut, 1929-1972) and a countless multitude died (ibn al-athir, rhc hist. or., 1872 and cairo 1885; kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874), which is very dubious. a contemporary hebrew source (benjamin of tudela, adler, 1907) has two unconnected records of the damage caused by this earthquake: the first is of the collapse of tripoli in an earthquake «in years gone by», with «great destruction in the land of israel» which killed some 20 000 people. his second record is of an earthquake «some time ago»: apparently the 1170 event which in one day it killed some 25 000 people, «and of about 200 jews but 70 escaped». homs is on the eastern bank of the orontes in the centre of a plain. it suffered less serious damage than some authors report (ibn al-jauzi, ed. hyderabad 1938-1941; ibn al-athir, rhc hist. or., 1872 and cairo 1885). the citadel (michael the syrian, chabot, 1899-1910), monasteries (chronicon 1234, csco ss, 56, 1937) and most of its buildings were ruined, but the death toll was low as the population fled to the outskirts (ibn al-qalanisi, gibb, 1932; alsuyuti, bibliotèque nationale de france, mss ar. 5929) during the foreshocks (michael the syrian, chabot, 1899-1910). it received tax relief of 71 kg of gold (abu shama, rhc hist. or., 1896) . nothing is known about the effects of the earthquake on adad al arab the location of which is questionable (dussaud, 1927), except that it received the considerable sum of 27.2 kg of gold as tax relief (abu shama, rhc hist. or., 1896). salamiya was also damaged severely and a great number of people died in the town and in many neighboring villages (ibn al-qalanisi, gibb, 1932; kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874), but is not recorded as having received tax relief. damage in aleppo, a city of about 70 000 inhabitants, was widespread and in certain places serious. some of the towers of the fortifications collapsed together with some houses (michael the syrian, chabot, 1899-1910; ibn 749 the 12th century seismic paroxysm in the middle east: a historical perspective taghribirdi, shaltut, 1929-1972) and the wall between the bab al-jinan and bab qinnarsin was damaged and repaired after the earthquake (elisseeff, 1951 and 1967). most of the inhabitants left the city during the foreshock period (ibn al-qalanisi, gibb, 1932; chronicon 1234, csco ss, 56, 1937) and only about 100 to 500 were killed (ibn al-jauzi, ed. hyderabad 19381941). it is odd that kemal (kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874) adds nothing on the damage to aleppo, as he would presumably have had access to local archives: however, his silence suggests that he accepts ibn al-jauzi’s figure of 100 deaths, as opposed to the 500 given by other sources (michael the syrian, chabot, 18991910). following the earthquake taxes were abolished to the amount of 136 kg of gold (abu shama, rhc hist. or., 1896). jubail (byblus) sustained some unspecified damage (ibn al-qalanisi, gibb, 1932). outside this region damage was widespread but not serious. the walls of nisfin, probably a village lying near aleppo (yaqut al-hamawi, wüstenfeld, 1866-1873) were damaged (ibn taghribirdi, shaltut, 1929-1972); tell nasfin, too, is a hill near aleppo (yaqut al-hamawi, wüstenfeld, 1866-1873). tell bashir, important frontier fort, also suffered some damage, and was given relief of 7.2 kg of gold (abu shama, rhc hist. or., 1896). azaz, in the north, important frontier fort-towns, also suffered some damage, and was given relief of 27.2 kg. (abu shama, rhc hist. or., 1896). at arza there were numerous deaths (kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874). the city of antioch was far less seriously affected than muslim sources imply. there is no evidence that «most parts of antioch were destroyed» (michael the syrian, chabot, 18991910; ibn al-athir, rhc hist. or., 1872 and cairo 1885; chronicon 1234, csco ss, 56, 1937), or that «the majority of the population was annihilated» (kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874), which is very dubious. it is probable that these sources refer to the principality of antioch that included most of the localities destroyed in this earthquake (abu ‘l-fida, rhc hist. or., 1872). damage in latakia should have been serious but not excessive as muslim sources suggest (ibn al-athir, rhc hist. or., 1872 and cairo 1885). the great church remained standing (michael the syrian, chabot, 1899-1910; chronicon 1234, csco ss, 56, 1937), and there is no evidence that the walls of the town were damaged. it is said that as result of the earthquake the ground in the city split open, mud and water filling the cracks, revealing ancient ruins (michael the syrian, chabot, 1899-1910; kemal al-din, rhc hist. or., 1884 and rol, 1896; see also röhricht, 1874; chronicon 1234, csco ss, 56, 1937; ibn taghribirdi, shaltut, 1929-1972), evidence suggesting liquefaction of the ground. the castle at jabalah was damaged but how much, it is not given in the sources (ibn alqalanisi, gibb, 1932). the shock should have been felt at beirut, saida (sidon), sur (tyrus) and acre (akko), but it is rather unlikely that the earthquake ruined these localities, most probably oriental sources (michael the syrian, chabot, 1899-1910; ibn taghribirdi, shaltut, 1929-1972) exaggerating the damage to these frankish coastal strongholds. in damascus the earthquake caused panic. with the first foreshock the people fled their houses (ibn al-qalanisi, amédroz, 1908; ibn taghribirdi, shaltut, 1929-1972). the shocks affected many parts of the city, and caused some damage to the mosque of damascus that shed quantity of mosaics and marble (al-suyuti, bibliotèque nationale de france, mss ar. 5929). we cannot authenticate the assertion made by a near-contemporary writer (al-suyuti, bibliotèque nationale de france, mss ar. 5929) that the mosque itself collapsed (since ibn al-qalanisi, who was an eyewitness, does not refer to damage in damascus and to the collapse of the mosque, it is not improbable that abu shama confused the 12 august event with another earthquake). the city was granted tax relief of 57 km of gold, not so much for repair but as an assistance to the inhabitants whose fortunes had been lost by the attacks of the franks, and also for their inability to meet the annual tax (abu shama, rhc hist. or., 1896). 750 nicholas n. ambraseys repairs to the mosque in baalbek suggest that this old structure had suffered some damage from this earthquake (elisseeff, 1951 and 1967) or from a later shock (berchem and fatio, 1913), information not found in the sources. modern writers extend the destructive effects of the earthquake to harran in jazirah. it is possible that the shock was felt as far as harran but the sources do not say explicitly that the city was destroyed or even damage by the shaking of the ground (ibn al-jauzi, ed. hyderabad 1938-1941; ibn taghribirdi, shaltut, 19291972). what they say is that, as a result of the earthquake the hill, or a part of it, on which the city is built opened up causing the collapse of many houses, and the cleft in the ground revealing tombs and old houses. this fits the description of a landslide in recent deposits overlying earlier urban debris, triggered by the shock. as for the financial aid of 13.6 kg of gold given to harran (abu shama, rhc hist. or., 1896), this could have been used to enlarge the friday mosque, which became necessary since in recent years the number of muslims had greatly increased. ibn jubayr, who passed through harran in 1184, gave a detailed account of the city and new mosque, an inscription on which gives the date of its enlargement as 1174. also for rahba (mayadin) on the euphrates, the sources do not imply that the town suffered any damage or even that the shock was felt there (michael the syrian, chabot, 1899-1910; ibn al-qalanisi, gibb, 1932; chronicon 1234, csco ss, 56, 1937). they say that the earthquake effects spread towards rahba and all the regions of iraq. this is an indicator of the direction in which the shock was felt. the reason for the abolition of taxes of 10 000 dinars (abu shama, rhc hist. or., 1896) was probably not for the repair of earthquake damage but for the repair war damage. as for the statements that the earthquake affected ararat (ibn al-jauzi, ed. hyderabad 1938-1941) and that damage extended to cyprus (elisseeff, 1967), we can find no evidence. the only earthquake felt in cyprus near this time occurred in 1160 (neophytus, delahaye, 1907). aftershocks continued to be felt over a long period of time and are recorded friday night 8 rajab (16 august), saturday night rajab 9 (17 august); saturday, sunday and monday, 9, 10 and 11 rajab (17, 18, 19 august), and monday 29 rajab (6 september) (ibn al-qalanisi, amédroz, 1908 and gibb, 1932). there is no doubt that large earthquakes cause considerable destruction, which is often followed by tax relief and grants awarded by the state for reconstruction. however, this is not always the sole reason. the tax relief after the 1157 earthquake was not for the repair of earthquake damage but because people could not afford to repair the damage done by the franks to a number of towns and forts, which was enhanced by the earthquake. one should be cautious about inferring the degree of damage, even for modern events, from the size of the relief. 30 october 1157 – a violent aftershock caused great panic and more damage in syria, wednesday 24 ramadan 552. in and around hama, there were continual violent shocks for several days, and the buildings that had been rebuilt were reportedly destroyed again (ibn alqalanisi, amédroz, 1908). in aleppo many of the houses and defences were damage and some may have been destroyed (ibn al-qalanisi, amédroz, 1908). the earthquake was strongly felt in damascus, and there may have been damage, though abu shama (rhc hist. or., 1896), does not mention damage in aleppo and damascus. as the shocks went on they became weaker. october 1158-september 1159 – the date of an earthquake, which allegedly destroyed jabalah on the syrian coast and as a result 2000 people were «drowned», cannot be fixed. chronicon 1234 (csco ss, 56, 1937) dates the event in a.s. 1470 (october 1158-september 1159). if the sequence of events in the chronicon 1234 is correct, it is likely that the earthquake happened sometime between those dates. this event is recorded only in one source and it may refer to the earthquake of 12 august (chronicon 1234, csco ss, 56, 1937). aftershocks continued to be felt in syria for a long time: on saturday night 10 shawwal (15 november), saturday night 10 dulqada (14 de751 the 12th century seismic paroxysm in the middle east: a historical perspective cember), sunday night 21 dulqada (25 december), friday night 23 dulqada (27 december), sunday night 25 dulqada (29 december 1157), friday 30 dulqada (3 january 1158), 15 rabi-i (16 april), 25 rabi-i (26 april), 23 rajab (20 august), 24 rajab (21 august), and friday night 2 jumada-ii (21 jun 1159). 29 june 1170 – for this earthquake we have several accounts by eyewitnesses, and virtually unanimity regarding the date from both occidental and oriental sources. preceded by foreshocks, the earthquake occurred early in the morning of 29 june 1170, and it was as destructive as that of 12 august 1157. its epicentral region overlaps that of the earthquake of 1157, making it difficult to define its extent, which includes shaizar, hama, barin, safita, hisn el akrad, homs, qusayr (n. qalat el-zau), hisn alakkar, arqa and baalbek (figs. 5 and 6). in shaizar the earthquake ruined what the war and the earthquake 13 years earlier had spared. it caused considerable destruction to the walls (abul faraj, bedjan, 1932 and salhani, 1890) and citadel (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963) and great parts of the town were destroyed (kemal al-din, 1896 and röhricht, 1874), killing many people (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963). the town did not recover until 1232 when it was finally rebuilt (mallet, 1853). hama, almost totally ruined thirteen years before and partly rebuilt, replacing many of its fig. 5. location map of the earthquake of 1170. 1 – acre; 2 – aleppo; 3 – antioch; 4 – arqa; 5 – ashtera; 6 – ashur; 7 – baalbek; 8 – baghras; 9 – bannyas; 10 – barin; 11 – beirut; 12 – damascus; 13 – edessa; 14 – harim; 16 – harran hims; 17 – hisn akkar; 18 – hisn al-akrad; 19 – hunain; 20 – jebeleh; 21 – jerusalem; 22 – jubail; 23 – latakiya; 24 – mardin; 25 – margat; 26 – mosul; 27 – nazareth; 28 – nisibis; 29 – qusayr; 30 – raqqa; 31 – safita; 32 – samosata; 33 – shaizar; 34 – sinjar; 35 – sur; 36 – tayma; 37 – hama; 38 – tripolis; 39 – baniyas. 752 nicholas n. ambraseys houses with huts (abu shama, al-kauthari, 1947), was again badly damaged (sibt ibn aljauzi, ed. chicago 1907) perhaps totally ruined (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; kemal al-din, rol, 1896 and röhricht, 1874). there was considerable destruction of its hastily rebuilt walls (abul faraj, bedjan, 1932 and salhani, 1890) and citadel and many people lost their lives (ibn alathir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963). it is said that only one house was left intact (abu shama, al-kauthari, 1947). nevertheless, it is likely that what a traveller who passed some years later through the region says, that some 25 000 people perished in hama, and of about 200 jews but 70 escaped, is not but a gross exaggeration (ibn dawadari, almunajjid, 1960). it may be that he refers to the town and rural areas (michael the syrian, chabot, 1899-1910 and rhc da, 1869). again here the walls were quickly repaired (ibn alathir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963). great parts of the border castle of barin were destroyed (kemal al-din, rol, 1896 and röhricht, 1874). of its citadel not a wall remained standing, and the settlement abutting on frankish territory was damaged (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963). the repairs occupied the franks completely for some time (abu shama, rhc hist. or., 1896). also safitha was almost totally destroyed, and not one wall was left standing (abu shama, rhc hist. or., 1896). hisn akkar was probably shattered (richard, 1972) but details are lacking. homs heavily damaged (sibt ibn aljauzi, ed. chicago 1907) and great parts of the town were ruined (kemal al-din, rol, 1896 and röhricht, 1874) causing many victims (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963). its walls and citadel were shattered, but quickly repaired (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963; kemal aldin, rol, 1896 and röhricht, 1874). again here the language used by some occidental authors grossly exaggerates damage (william of tyre, rhc hist. occid., 1844; and babcock fig. 6. location map of the earthquake of 1170. 1 – acre; 2 – aleppo; 3 – antioch; 4 – arqa; 5 – ashtera; 7 – baalbek; 8 – baghras; 9 – bannyas; 10 – barin; 11 – beirut; 12 – damascus; 14 – harim; 16 – harran hims; 17 – hisn akkar; 18 – hisn al-akrad; 19 – hunain; 20 – jebeleh; 22 – jubail; 23 – latakiya; 25 – margat; 27 – nazareth; 29 – qusayr; 31 – safita; 33 – shaizar; 35 – sur; 36 – tayma; 37 – hama; 38 – tripolis; 39 – baniyas. 753 the 12th century seismic paroxysm in the middle east: a historical perspective and krey, 1943; michael the syrian, chabot, 1899-1910 and rhc da, 1869). it is said that he castle of qusayr was also ruined, but details are lacking. hisn al-akrad was seriously damaged, particularly its citadel where, it is said (sibt ibn aljauzi, ed. chicago 1907; abu shama, rhc hist. or., 1896) that its walls were destroyed, which cannot be true (hagenmeyer, 1890). the franks rebuilt part of the damage (abu shama, rhc hist. or., 1896) and the inner enceinte underwent modifications. a massive battered embankment was built up against the wall, which provided resistance against future earthquakes. all we know about arqa from only one source is that the castle collapsed (het’un, hakobyan, 1976 and rhc da, 1869). in baalbek the earthquake caused considerable damage to houses and defenses. some parts of its wall and citadels were ruined (kemal aldin, rol, 1896 and röhricht, 1874; abul faraj, bedjan, 1932 and salhani, 1890; kohl et al., 1925); and a number of people lost their lives (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963). the damage was quickly repaired (kemal al-din, rol, 1896 and röhricht, 1874). in the mountains overlooking the town, deep fissures opened in the ground (ibn dawadari, al-munajjid, 1960). on one of the gates an inscription commemorates repairs in 1168(?) (alouf, 1908). damage was considerable at jabalah, baniyas (valanin), margat (marqab), tripolis, jubail, and damascus. it is said that jabalah collapsed (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; sibt ibn aljauzi, ed. chicago 1907), however, certain churches are reported as having withstood the shock (michael the syrian, chabot, 1899-1910 and rhc da, 1869; abul faraj, bedjan, 1932 and salhani, 1890), and there is no evidence that the earthquake affect this port. the same is said about the castle of baniyas (het’un, hakobyan, 1976 and rhc da, 1869), information not found in another source. margat is reported destroyed (annales 5689: bibliotèque nationale de france, fond latin 5689). it is mentioned by röhricht (1874) among the localities damaged or affected by the earthquake, but the name of this locality does not appear in any of the references that he mentions in support (röhricht, 1898). a large part of the city of tripolis was ruined and its castle (het’un, hakobyan, 1976 and rhc da, 1869) and the great church collapsed (michael the syrian, chabot, 1899-1910 and rhc da, 1869) but the town was far from being destroyed as some chroniclers maintain (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; robert de torigni, delisle, 1872-1873). the syrian church and the harbour survived the shock apparently with no serious damage (michael the syrian, chabot, 1899-1910 and rhc da, 1869). jubail suffered unspecified damage, serious enough for the franks to request immediate repairs. oddly beirut, is not mentioned in the sources. in damascus, one of the large urban centres, the earthquake caused general panic, and in places serious damage to dwellings (michael the syrian, chabot, 1899-1910 and rhc da, 1869; ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963; kemal aldin, rol, 1896 and röhricht, 1874; robert de torigni, delisle, 1872-1873). the inhabitants left the town (sibt ibn al-jauzi, ed. chicago 1907) and made for the plains of ghuta. strangely the only damage to public buildings recorded in the urban area is the knocking down of the crenellations of the great mosque and the collapse of the roof of the rostrum, and only one man died, hit by a stone (sibt ibn aljauzi, ed. chicago 1907). however, damage to the rural district was widespread and serious. further away from the epicentral area in aleppo, the largest urban centre in the region, already shattered by the large earthquake of 1157, damage was widespread and in places serious. the descriptions of damage by christian and muslim authors are somewhat coloured by their religious perspective, considering the earthquake to be of divine justice. they grossly exaggerate damage by saying that the whole city of aleppo collapsed (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; michael the syrian, chabot, 1899-1910; rhc da, 1869; langlois, 1868; sibt ibn al-jauzi, ed. chicago 1907; abul faraj, bedjan, 1932 and salhani, 1890) becoming a hill of ruins (michael the syrian, chabot, 1899-1910 and rhc da, 754 nicholas n. ambraseys 1869). more sober sources point out that damage was not total. a considerable part of the city (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963) or parts of it were damaged so that the people were not able to take refuge in their homes for fear of another shock (kemal al-din, rol, 1896 and röhricht, 1874), an observation suggesting that most dwellings survived the shock. and because the shocks had carried on for several days and despite the fact that the people were apprehensive of camping outside aleppo, lest they found themselves surprised by the franks (ibn alathir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963), they evacuated the city (ibn al-’adim, dahhan, 1951). there is no doubt that many public buildings, dwellings and some parts of the city walls were damaged, some of them seriously. half of the citadel was ruined (sibt ibn al-jauzi, ed. chicago 1907), but the syrian church suffered absolutely no damage and the ulu cami survived with some minor damage to its minaret (ibn al-shihnah, british library, london, ms or. add. 2, 36), the crescent of which was hurled almost 200 m (michael the syrian, chabot, 1899-1910; rhc da, 1869; langlois, 1868; abul faraj, bedjan, 1932 and salhani, 1890; ajami abu darr, sauvaget, 1950). it is said that as a result of the earthquake the ground in the whole city cracked, and was reduced to a series of crevasses and fissures filled with a black fluid (michael the syrian, chabot, 1899-1910; rhc da, 1869; langlois, 1868), probably an allusion to liquefaction of the ground along the banks of the quwayq river. the death count is suspect with estimates ranging from 5000 (ibn al-’adim, dahhan, 1951) to 80 000 (sibt ibn al-jauzi, ed. chicago 1907; al-’umari, ms., iraq acad., baghdad). nevertheless, with aleppo’s jails crowded with several thousand christian prisoners there is no evidence that any of them lost his life. damage was very quickly repaired (ibn alathir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963) with the muslim army camped outside the city supervising the work until they had repaired all of the walls and the principal mosques (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963; kemal al-din, rol, 1896 and röhricht, 1874). it is not certain what happened in harim. all we know from a muslim source about baghras is that its walls «fell down» (abul faraj, bedjan, 1932 and salhani, 1890). antioch on the river orontes, a large urban centre already badly damaged by the earthquake of 1157, suffered some damage. some battlements of the city wall on the riverbank were thrown down (michael the syrian, chabot, 1899-1910 and rhc da, 1869), as well as part of the ramparts (annales vizeliacenses, huigens, 1976) and in places cracks opened in the ground filled with water (annales magdeburgenses, pertz, 1895). the cathedral church of st. peter was shattered and one of the domes fell in rey (1896), on the greek patriarch and his clergy, and the altar of the church of st. cosma caved in (abul faraj, bedjan, 1932 and salhani, 1890). in contrast, the syrian churches st. mary, st. george and bar suma were not damaged (michael the syrian, chabot, 1899-1910 and rhc da, 1869; abul faraj, bedjan, 1932 and salhani, 1890). the death toll was relatively low, about 50; most of whom were in the church of st. peter (michael the syrian, chabot, 1899-1910 and rhc da, 1869; neophytus, delahaye, 1907). the walls of the city and its church were rebuilt (michael the syrian, chabot, 1899-1910; rhc da, 1869; langlois, 1868). internal evidence suggests it is unlikely that antioch was severely damaged; in fact the earthquake seems to have caused more fear than anything else. again here, some chroniclers exaggerates the situation, saying briefly that antioch was completely flattened (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; annales vizeliacenses, huigens, 1976), overthrown (michael the syrian, langlois, 1868), destroyed (sibt ibn al-jauzi, ed. chicago 1907; het’un, hakobyan, 1976 and rhc da, 1869), or swallowed up in the ground (annales magdeburgenses, pertz, 1895). others say that parts of the city collapsed (robert de torigni, delisle, 1872-1873; chronicon universalis senonensis, holder-egger, 1882; chronicon gastinensis coenobii, rec. hist. france, 1781), while better-informed sources restrict damage in diverse 755 the 12th century seismic paroxysm in the middle east: a historical perspective places (michael the syrian, chabot, 1899-1910 and rhc da, 1869). there are no records of «destruction» (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943; sibt ibn al-jauzi, ed. chicago 1907) in the coastal towns of latakia except for its castle that was damaged (het’un, hakobyan, 1976 and rhc da, 1869) but its church was left unscathed (abul faraj, bedjan, 1932 and salhani, 1890). the fortress of hunain may have bee damaged, and there is evidence that the wall towers of sur suffered some unspecified damage (het’un, hakobyan, 1976 and rhc da, 1869), the earthquake causing considerable concern in the port area (william of tyre, rhc hist. occid., 1844 and babcock and krey, 1943). nothing is known in any detail about acre except that allegedly its castle collapsed (het’un, hakobyan, 1976 and rhc da, 1869), which is not substantiated by other sources. the earthquake was felt in the region of nazareth (anonymous, 1846; röhricht, 1898), causing some unspecified damage to the church in nazareth (mayer, 1977), and it was perceptible in jerusalem (chronicon universalis senonensis: holder-egger, 1882). there is no evidence that the shock was noticeable at ashtera (abu shama, rhc hist. or., 1896; berchem, 1902) or cyprus (neophytus, delahaye, 1907). to the east of the epicentral region we know that the walls of samosata had to be repaired but it is not clear whether this was due to the earthquake (michael the syrian, chabot, 1899-1910 and rhc da, 1869). edessa was unscathed, however, the nearby monastery of st. ananias was shaken to the extent that the clergy clung to the altar (michael the syrian, langlois, 1868; abul faraj bedjan, 1932 and salhani, 1890). not only in the monastery but also in the whole country, there had been absolutely no damage (michael the syrian, chabot, 1899-1910 and rhc da, 1869). the earthquake was felt as far as raqqa (sibt ibn al-jauzi, ed. chicago 1907), perhaps causing some unspecified damage (abu shama, rhc hist. or., 1896) and in the region of mardin, sinjar, mosul, and nisibis (sibt ibn al-jauzi, ed. chicago 1907). there is nor evidence that the shock was felt in baghdad, basra, harran, and wasit. these localities are mentioned in the sources merely to indicate the general direction in which the earthquake shaking propagated (sibt ibn aljauzi, ed. chicago 1907). the duration of aftershocks given in the sources vary from a fortnight to four months, but in general there is little hard evidence that aftershocks were either numerous or destructive. in all, an estimated thirty towns and fortified sites were significantly damaged or destroyed, leaving both the franks and muslims open to military attacks from each other. the franks believed that damage was worse in the muslim side (abu shama, rhc hist. or., 1896) and it took them sometime to realise that the earthquake overthrew several cities, as many christian as muslim (annales colonienses maximi, pertz, 1861). thus each side was occupied with repairing the damage, for fear of the other (kemal al-din, rol, 1896 and röhricht, 1874). ironically, this resulted in a period of unofficial truce, as both sides were repairing as quickly as possible the fortifications of border citadels. and as after the earthquake of 1157, repairs were hasty and for many years little proper rebuilding was done. for instance antioch, although not so badly affected, seems to have suffered from a shortage of funds as, according to a source writing about ten years later, the repairs did not reach «even a mediocre standard». it is interesting that it was only after the first large shock of 1157 did wood was used extensively in the rebuilding work, which demonstrates that its earthquake-resistant properties took some time before it was appreciated and the cost was partially offset by tax-relief on wood (abu shama, al-kauthari, 1947). the earthquake was felt in most of the regions of sham, jazirah, as far as the borders of mosul, and in iraq, while the area of maximum intensity was syria (ibn al-athir, cairo 1885; tornberg, 1853; rhc hist. or., 1872; talimat, 1963; kemal al-din, rol, 1896 and röhricht, 1874), misspelled in some occidental sources as styria (annales admontenses, wattenbach, 1851), thus placing a spurious large earthquake in steiermark, in what was then hungary. this 756 nicholas n. ambraseys error passes on unnoticed to modern writers (mallet, 1853; rethly, 1952; hoernes, 1902). undoubtedly the earthquakes of 1157 and 1170 were of sufficient political importance to interest chroniclers throughout europe. 25 february 1181 – an earthquake caused extensive damage to the city of aleppo and villages in the surrounding region; the same event may have precipitated rock falls in the mountains. different manuscripts of al-suyuti date the event either in 1181 or 1182 . acknowledgements i would like to thank jean vogt for his assistance over the years in systematically locating, but not in the most systematic way recording, valuable information from european repositories. charles melville is among those whose help with arabic texts in the early stages of this work contributed more than he realises. great help was provided by dominic white, do, who patiently read through many greek and latin sources during the course of this work. also, i was fortunate to have comments on this article by two anonymous reviewers that proved particularly helpful. this articles draws from my previous but now grossly outdated work, sponsored by unesco thirty five years ago, and later, in 1996, by the ec beecd project. the revision, acquisition of new archival material and publication of these works is aided by the european commission project apame (archaeo-palaeoseismology for the protection of cultural heritage and archaeological sites in the middle east), ica-ct-2002-10024. references journal format restrictions did not allow full pagination of references in the text. mgh ss = monumenta germaniae historica, scriptores, hannoverae, hahn, 1826-1934, vols.1-32. pl = migne j.p., patrologia cursus completus…, series latina, parisiis, 1841-1864, vols. 1-221. rhc da = recueil des historiens des croisades, documents arméniens, académie des inscriptions et bellelettres, paris 1869-1906, vols. 1-2. rhc hist. occid. = recueil des historiens des croisades, historiens occidentaux, académie des inscriptions et belle-lettres, paris 1844-1895, vols.1-5. rhc hist. or. = recueil des historiens des croisades, historiens orientaux, académie des inscriptions et bellelettres, paris 1872-1898, vols. 1-5. ris 2 = rerum italicarum scriptores…., nuova edizione, istituto storico italiano per il medioevo, città di castello, ab anno 1900. occidental sources annales 5689, annales de terre sainte, bibliotèque nationale de france, paris, fond latin, no. 5689, extr. archives de l’orient latin, vol. 2. annales admontenses, edited by w. wattenbach, mgh ss, vol. 9, hannover 1851. annales colonienses maximi, edited by g.h. pertz, mhg ss, vol. 17, hannover 1861. annales magdeburgenses, edited by g.h. pertz, mhg ss, vol. 16, hannover 1895. annales vizeliacenses, edited by r. huigens, corpus christianorum. continuatio mediaevalis, 42, turnholti 1976. anonymous, bibl. de l’ecole d. chartres, vol. iv, ser. 3, 1846. anselmus gemblacensis, sigiberti gemblacensis chronica anselmi continuatio, edited by j.p. migne, pl, vol. 160, parisiis 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(1933): earthquakes in the holy land, a correction, science, 77 (1997), 88-89. annals of geophysics, 55 (3), 2012: fontana et al. risk management: a proposal for communication strategies michele fontana1, mattia de amicis1,*, matteo rossetti2, carolina garcia1 1 università di milano-bicocca, dipartimento di scienze dell’ambiente e del territorio, milano, italy 2 studio architettura rossetti, milano, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5521 abstract disasters related to natural hazards have increased in the last few decades. this increment makes it necessary to develop non-structural risk prevention and mitigation measures to improve people’s safety. an effective non-structural measure that can improve the preparedness of the population is a locally adapted communication campaign that is focused on natural hazards. we have developed a hypothetical communication campaign for a specific area in the north of italy, in which hydro-geological risk is of considerable importance. the content of the campaign is defined by the combination of the requirements of the law with the results of a survey conducted in the study area. the aim of the survey is to evaluate the level of risk perception among the residents, and their attitudes towards prevention activities. the operative procedure of the campaign is modeled on advertising strategies. the campaign is designed to reach each family, and it is aimed at affecting people’s everyday life through a horizontal communication strategy that involves flyers, billboards, umbrellas and a website. the use of umbrellas as a medium for the campaign is the key. people mostly use umbrellas when it rains. rain is linked with hydrogeological risk. as the content of the campaign is printed on the umbrellas, each time people use these umbrellas, they remember the campaign. the campaign is integrated into a broader communication program that includes meetings with stakeholders, activities in schools, and public conferences. the f inal goal is to foster the creation of a shared knowledge about risk in the whole population. 1. introduction natural disasters have increased in europe in the last decade [european environmental agency 2010]. italy is no exception in this trend, and this increment makes it necessary to develop non-structural risk prevention and mitigation measures to improve people’s safety. to this end, in the past few years, the department of environmental and landscape sciences (university of milano-bicocca), together with the italian national research council (cnr), has carried out various studies in a specific area in the north of italy. these studies include a comprehensive geological survey and a detailed hydrological study. furthermore, some activities of information and prevention have already been realized. thus, the present study, which is focused on the communication to the general public, is part of a broader communication program, and it is realized in an area that is well known from a scientific point of view. the central idea that guides our communication choices is that ‘prevention is better than cure’ [bianco and maggi 2000]. to create an extensive control network over the territory, our aim is to involve the whole population in the prevention activities. prevention is a strategy based on simple everyday actions. people can have key roles in risk prevention, if they are aware of the hydrogeological nature of the territory in which they live. if each citizen starts to periodically check her/his neighborhood, she/he will be the first sentry against natural hazards. thus, the goal is to reach as many citizens as possible. 2. territory and population this study is focused on the territory of the ‘comunità montana di valtellina di tirano’ (mountain consortium of municipalities valtellina di tirano; hereafter mcmvt). the mcmvt is made up of twelve municipalities that are scattered over a 451.97 km2 area of the italian alps, in the lombardy region. the altitude goes from 373,000 to 2,902 m a.s.l. (figure 1). starting from the last century, the population of the study area has sharply increased, and there are now 29,000 residents in the whole of the mcmvt. as a consequence of this larger population, over the last 60 years, new settlements have started to appear all along and down the valley. in time, these new villages have become more important and populous than the older ones, which is also due to internal migration from the mountains to the valley. with few people living on the mountainside, the traditional activities linked with life in the forests have disappeared. these activities were important actions in the management of the territory, and among other activities, they included the maintenance of the dry-stone walls article history received december 19, 2011; accepted february 9, 2012. subject classification: risk management, communication campaign, civil protection plan, hydrogeological hazard, risk awareness. 433 and the gathering of the wood. dry-stone walls slow down the velocity of run-off water, and during heavy rains, natural dams formed by water-transported twigs and branches used to form into streams. thus, the consequences of the changes in the demography have resulted in a loss of control over the territory, and a subsequent increase in the risk of flooding. moreover, the hydrogeological regime of the mcmvt is no longer natural, because the rivers are used for hydroelectric purposes (as more people need more energy). in conclusion, today the territory of the mcmvt is characterized by high hydrogeological risk due to its socioeconomic and geographical characteristics [sterlacchini et al. 2008]. 3. local civil protection plans the content of the communication campaign is defined by the combination of the requirements of law with the results of a survey that was conducted in the studied area. the law of the lombardy region1 describes what has to be done when speaking about natural hazard prevention. the law identifies four main subjects: preventive communication, training, emergency drill, and planning. the corner stone of the planning is the civil protection local plan (cplp). each municipality has its own cplp that identifies the risks to which the territory is subjected, as well as the areas that might be affected by a specific risk. also, the cplp locates the safe zones in which people can take refuge during an emergency. thus, the cplp contains the information that the population has to know to be prepared for an emergency: where hazards might strike and where to go to be safe. from this point of view, preventive communication, the first subject identified by the law, can be thought of as the communication of the cplp to the general public. 4. the survey the population of the mcmvt was the subject of a survey carried out in person by one of the authors of this paper [garcia 2011]. the aim of the survey was to evaluate the level of risk perception among the residents, and their attitude towards prevention activities. risk perception is a key factor when planning a communication campaign. if the citizens are aware of the risks that exist on their territory, they will be receptive to a communication campaign focused on these risks. in contrast, if the citizens do not know about the risks, they will be indifferent to the same campaign, because the information will be considered outside of the citizens’ sphere of interest [lombardi 2005]. surprisingly, the results of the survey showed that despite having good knowledge of previous flooding and mass movements,2 the population of the studied area has low levels of perceived risks and preparedness (table 1). however, the population also showed interest in being informed about natural hazards, mitigation activities, risk management and emergency procedures, as 67.3% of the respondents expressed willingness to learn how to be better prepared in the case of a future event. this includes learning about the appropriate mitigation activities that they can directly perform to be less vulnerable. also, the results highlighted that the preferred sources of information are local authorities and not the fontana et al. 434 figure 1. the study area. 435 scientific community.3 this indicates the need to work in collaboration with the local authorities to communicate scientific information, otherwise the same scientific information will not be willingly received by the general public [gallavotti and pistoni 2007, bucchi 2008]. 5. communication strategies the operative procedure of the campaign is modeled on advertising strategies, as the final goal of our campaign is to get close to the goal of a commercial campaign. like an advertiser, we want to suggest to our target to do something [packard 1958]. of course, our aim is not linked to some commercial purpose, but to risk prevention. the center of our campaign is the idea of safety, and the suggestion that through the adoption of specific behaviors, it is possible to make the place in which one lives safer, both in everyday life and during an emergency. it is important to note that we do not think that this campaign is self-sufficient, to create better risk perception, or even that this campaign will be sure to reach everyone. this campaign is only one of all of the initiatives that should be developed to create sensitivity in everyday life in a risk-exposed territory. indeed, extensive work has already been done in the territory. meetings with students and teachers have been arranged to discuss risk prevention in many of the schools of the mcmvt.4 local authorities have met researchers of the department of environmental and landscape sciences and of the national research council, to understand how a cplp is made and how to use it before and during an emergency. this activity has been carried out inside the doctoral research project of one of the authors, and the cost has been borne through the contribution of a european community project, called mountain risk. now, we believe that it is important to try to reach the whole population, to foster the creation of a widespread awareness of risk, and this is the reason that lies behind the design of this hypothetical campaign. the campaign is designed to reach each family and it is aimed at affecting people’s everyday life through a horizontal communication strategy. the focus on everyday life is rooted in the impossibility of defining the exact moment in which an emergency might take place. thus, although the law insists that preventive information has to continuously run all the time, it is clear that repeated excesses in risk communication can easily generate the “cry wolf ” phenomenon, which would cancel out the effects of all of the effort. several studies have shown that if the notice of an imminent disaster is not followed by the disaster itself, further communications will be received with increasing skepticism by the citizens [anderson 1968, baker 1984, atwood and major 1998]. therefore, it is important to calibrate the communication, and to prefer initiatives that seek to change people’s behavior by acting on their everyday lives. communication that establishes artificial situations of emergency should be avoid, as they are destined to fade into nothingness, and they can also erode the general credibility in the source of the communication. combining the requirements of law with the results of the survey, we created four iconic-text symbols (figure 2). on the one hand, these symbols draw attention to some aspect of the cplp (where hazards can strike, and where to go to be safe). on the other hand, these symbols answer the citizens’ desire that was highlighted by the survey: to know which activities they can directly perform to be less vulnerable. these symbols include: – the symbol of “know the territory” (figure 2, top left, “conosci il territorio” in italian). the aim of this symbol is to push the population to know their own territory. if a citizen knows that their house might be flooded by the river, then there are more chances not only that they will be ready to move away in time, but also that they will constantly check the river during (for example) heavy rain. risk management: a proposal for communication strategies figure 2. the four symbols of the campaign. population with knowledge of previous flooding and mass movements 88.3% risk perception level of the population low (2.2/ 5) preparedness level of the population low (2.5/ 5) level of trust of the population in the local authorities medium (3/ 5) table 1. results of the survey on risk perception (adapted from garcia [2011]). – the symbol of “inform yourself ” (figure 2, top right, “informati” in italian). the aim of this symbol is to suggest to people that they should also seek information by themselves. it is unrealistic to think that the mayor of each municipality will go house-to-house to explain how to reach the safe zones from each house. if the majority of the citizens become responsible for their own safety, the authorities can better deal with the extreme situations of an emergency (for example: older people who are not able to move). this symbol is the logical consequence of the previous symbol: to know your territory you have to have the necessary information. – the symbol of “listen” (figure 2, bottom left, “ascolta” in italian). this symbol reminds the population that they will not be abandoned in the case of an emergency. the authorities will coordinate the rescue operations, keeping the population updated on the situation. as different municipalities of the mcmvt can use different methods to communicate with the population,5 this symbol is not focused on a specific media. – the symbol of “protect yourself ” (figure 2, bottom right, “proteggiti” in italian). this symbol represents what should be the first thought of everyone during an emergency: to protect their own personal safety. speaking of hydrogeological risk, this means to move away from exposed zones. to know which are the exposed zones is the goal of the first symbol. thus, if you know the territory, you should be safe. each symbol is made up by text and an image, joined together as a single unit. in this way, the symbols are more understandable because the two parts, the text and the image, clarify one another [calabrese 2007]. the linguistic choice of the text is oriented towards messages that promote citizens’ direct participation, while avoiding negative messages or prohibitions, which are considered as little effective in establishing a dialogue [barszczynska et al. 2006]. from a visual point of view, the shapes of the symbols are very different from shapes usually linked with emergency situations. emergency situations are represented by triangles. the use of the square, with the rounded angles, is designed to support the ideas of solidity and safety [kandinsky 1926], which are central to the campaign. these four symbols are grouped together in a single picture that becomes the logo of the campaign. the campaign is composed of four phases (figure 3). first, a flyer is sent to all of the 10,000 resident families. the flyer presents the logo and explains the meaning of each symbol. the flyer is sent from the mcmvt, an institution that, as the survey highlighted, is trusted by the population. after two weeks, an umbrella is sent to each of the families. the umbrella has the logo printed on the cloth. then billboards are placed in each municipality that explain the specific meaning of each symbol according to the specific territory of that municipality (what kind of hydrogeological risk exists in that municipality; what is the communication strategy that the municipality will use in the case of an emergency; what are the locations of the safe zones in that municipality). parallel to the previous phases, a website is put online, and it will be possibly for this to be hosted within the website of the mcmvt, with the aim of assuring continuity to the campaign. the website contains all of the information that has been spread through the previous phases, and it represents a constant point of reference for the population, supplying both information and contact details to reach the authorities. so, the campaign starts with an institutional communication (mcmvt – flyer), passes through a common object that becomes part of people’s everyday life, and ends with a local institutional communication (municipality – billboards), finally staying on through the website. the choice of the umbrella is not casual. the idea to use an object in the campaign is taken from guerrilla marketing campaigns. the effectiveness of this kind of strategy comes from the capability of the object to enter almost unnoticed into the everyday life of the target of the campaign.6 as hydrogeological hazards are mainly triggered by rain, we choose to use an umbrella. an umbrella is a common object that is used by everyone without regard for gender, age and culture. therefore, an umbrella is an ideal object to reach a wide number of people. moreover, an umbrella presents a wide surface that is available for printing information. in this way, every time a citizen takes this umbrella because it is raining, and opens it, they will see the logo. the logo should remind the person about the hydrogeological risks and the communication campaign. even if some of the umbrellas get lost, we believe that just seeing a few of these umbrellas around on a rainy day will trigger the memory of the campaign. thus, through this dynamic, it should be possible to transform a simple daily gesture into an action of prevention, and to contribute to the creation of a common and shared awareness of the territory and its risks. therefore, the idea is not to generate alarm in the people with sudden announcements of imminent disasters, but to help citizens to recognize that in their particular territory specific risks exist on an everyday basis, and that with risk awareness, that risk can be greatly reduced. the total cost of this campaign is, more or less, 70,000 euros, subdivide in this way: 45,000 euros for the umbrellas (for 10,000 inhabitants), 2,000 euros for the brochure and leaflet, and 3,000 euros for other costs. it is also clear that this campaign can be realized only with the contrifontana et al. 436 437 butions of sponsors. for this reason two of the main banks in the territory, banca popolare di sondrio and credito valtellinese, have been contacted. 6. conclusion this study puts together two issues: (a) the preventive communication that has to be carried out according to law; and (b) the people’s desire to know how to directly reduce the risk in their territory. in doing this, the campaign relies on studies that have been carried out by researchers over the past few years, even if it could be argued that in the campaign the scientific information becomes closer to an advertisement than to a scientific paper. however, there is little value in producing high quality research if the results are not put into practice. with this campaign, we have tried to explore a way to build communication between scientists and residents. scientists know the natural hazards and how to reduce the risks linked with them, but it is the citizens who live in the territory. thus, it is important to create communication between scientists and the population. this communication has to go through the local authorities, as they are more trusted by the population than scientists. this preference should push the scientific community to ask itself what went wrong. why do lay people not trust scientists today? we do not have the answer to this question at present, but we do hope that working together with the authorities and this active contribution to the safety of the population through taking science to the society will help to improve scientists’ credibility in time. the final goal of this campaign is to foster the creation of shared knowledge about the risks in the whole population. hydrogeological risk is caused also by the use that people make of their territory. for reasons that are partially related to industrialization, many inhabitants of the mcmvt have stopped to live in mutual relations with their territory. the traditional practices that were linked to the local culture have been abandoned. today, many residents of the mcmvt live close to the mountains as if they live in a town on the plain. so, the challenge we face is to contribute to the creation (or re-creation) of their sensitivity to the specific environment that characterizes the mcmvt. if we succeed in this, every citizen can become a node of an extended monitoring network. due to their knowledge of the territory and to their behavior during both everyday life and in emergencies, they will surely be able to live in a safer territory. risk management: a proposal for communication strategies figure 3. the phases of the campaign. notes 1. law reference: regione lombardia, direttiva regionale per la pianificazione di emergenza degli enti locali (l.r. 16/2004 art. 7, comma 11, approvata con d.g.r. n.viii/4732 del 16 maggio 2007). 2. flooding and mass movements are the most significant natural hazards in the mcmvt. 3. when asked about which source should provide information on natural hazards, 74.8% of the respondents said ‘the municipality’, while only 8.2% of the respondents said ‘the scientific community’ [garcia 2011]. 4. these activities were coordinated by lisa garbellini (irealp). 5. some of the possible methods are: cell phone sms messaging, the internet, television, and loud-speakers on police cars. of course, one method does not exclude another. it is also possible that in time new methods will arise. 6. more information about this can be found at: http://www.guerrigliamarketing.it/; http://www.advertnews.com/category/ad-agency/. references anderson, j.w. 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(2007). retorica del linguaggio pubblicitario, bologna, archetipolibri, 184 pp. european environment agency (2010). mapping the impacts of natural hazards and technological accidents in europe. an overview of the last decade, publications office of the european union, copenhagen, 186 pp. gallavotti, b., and s. pistoni (2007). la comunicazione istituzionale della scienza: come evitare la deriva autoreferenziale, analysis, 4, 20-23. garcia, c. (2011). mountain risk management: integrated people-centered early-warning system as a risk reduction strategy, northern italy, phd thesis, supervisors: mattia di amicis, simone sterlacchini, department of environmental and landscape sciences, university of milano-bicocca, italy, 150 pp. kandinsky, w. (1926). uber das geistige in der kunst, langen, munchen, bauhaus bucher; consulted edition: punto linea e superficie, milano, adelphi, 1968. lombardi, m. (2005). comunicare nell’emergenza, milano, vita e pensiero, 160 pp. packard, v. (1958). i persuasori occulti, torino, einaudi, 286 pp. sterlacchini, s., m. de amicis, s. frigerio, s. sironi and i. poretti (2008). metodologia per l’analisi della pericolosità da inondazione, cnr – idpa, italy, 135 pp. *corresponding author: mattia de amicis, università di milano-bicocca, dipartimento di scienze dell’ambiente e del territorio, milano, italy; email: mattia.deamicis@unimib.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. fontana et al. 438 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice s c i e i p e r s o n i c h e ( * ) l . d e s o c i o ( * * ) r i c e v u t o il 18 maggio 1963 riassunto. — vengono s c r i t t e le equazioni per le seie ipersoniche ten e n d o c o n t o degli e f f e t t i incrociati t r a i g r a d i e n t i di t e m p e r a t u r a e di concent r a z i o n e e l e t t r o n i c a ed il g r a d i e n t e di p o t e n z i a l e elettrico. l ' a d i m e n s i o n a lizzazione delle equazioni m e t t e in evidenza i p a r a m e t r i f o n d a m e n t a l i c a r a t teristici dei f e n o m e n i connessi con le scie. l ' a n a l i s i di q u e s t i p a r a m e t r i m o s t r a come t a l u n i e f f e t t i incrociati possano avere u n a notevole i m p o r t a n z a in un v a s t o corridoio di volo p r a t i c a m e n t e realizzabile. summary. — the e q u a t i o n s f o r t h e h y p e r s o n i c w a k e s are w r i t t e n , t a k i n g i n t o a c c o u n t t h e cross effects b e t w e e n t h e g r a d i e n t s of t e m p e r a t u r e a n d t h e electronic c o n c e n t r a t i o n a n d t h e g r a d i e n t of electrical p o t e n t i a l . b y t h e dimensional analysis of t h e e q u a t i o n s t h e f u n d a m e n t a l p a r a meters affecting t h e phenomenological b e h a v i o u r of t h e w a k e s are evidenced. the analysis of these p a r a m e t e r s shows t h a t some cross effects h a v e a noticeable i m p o r t a n c e in a large flight corridor of p r a t i c a l i n t e r e s t . i n t r o d u z i o n e . l ' i n t e r e s s e p e r le scie i p e r s o n i c h e si e a c c e n t u a t e u l t i m a m e n t e i n r e l a z i o n e a g l i s t u d i sul r i e n t r o di c a p s u l e s p a z i a l i , sulle t r a i e t t o r i e dei m o b i l i b a l i s t i c i e delle m e t e o r e . i n p a r t i c o l a r e la p r e s e n z a d e l l a scia i p e r s o n i c a c o m p o r t a u n a serie d i p r o b l e m i r e l a t i v i a i s i s t e m i di r i l e v a m e n t o e di r a d i o c o m u n i c a z i o n e p e r i m o b i l i c h e p r o c e d o n o a f o r t i v e l o c i t a n e l l ' a t m o s f e r a . p i n o a d oggi l a l e t t e r a t u r a s e i e n t i f i c a n o n h a t e n u t o (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientiflco dello spazio. r o m a , 19-23 giugno 1962. (**) i s t i t u t o di a e r o n a u t i c a u n i v e r s i t a napoli 8 4 l . d e s o c i o conto, nella determinazione delle caratteristiche delle scie ipersoniche, dei cosiddetti effetti incrociati, cioe della interazione m u t u a t r a gradienti dei p a r a m e t r i di stato, come i gradienti di concentrazione elettronica e di t e m p e r a t u r a , ed il gradiente di potenziale elettrico determinato dai moti relativi t r a particelle cariche di segno diverso. i n effetti, qualora si dimostrasse che tale gradiente di potenziale elettrico ha un peso eospicuo nel bilancio dei f a t t o r i agenti sulle caratteristiche della scia, i metodi di studio finora seguiti risulterebbero inadeguati. fi scopo di questa n o t a dimostrare che, nella valutazione dei proflli di concentrazione elettronica, attraverso la regione diffusiva, bisogna tener conto della presenza del gradiente di potenziale elettrico indotto, a meno che la variazione di t e m p e r a t u r a nella zona dissipativa non sia molto piccola. gli effetti di diffusione termica potranno invece completamente trascurarsi: tale ultima osservazione puo d ' a l t r o canto gia farsi, considerando i risultati ottenuti da chapman et al. (5). le scie ipersoniche andranno quindi, in genere, studiate considerando anche l'equazione di poisson per la distribuzione del potenziale elettrico al tine di rendere eompleto il gruppo di equazioni di solito t r a t t a t o . l ' a u t o r e desidera esprimere il suo ringraziamento al prof. luigi g. napolitano per le discussioni ed i suggerimenti ricevuti nel corso di questo lavoro. equazioni fondamentali. le equazioni per le scie ipersoniche nel caso piano isobarico si scrivono: continuita: (qu)x + (qv)y = 0 diffusione elettroni: ouclx + ovclv = (a10t„ + ancvj + al2cpy)y = (j1)y diffusione ioni: nuc2t. + ovcm = (a2„ty + a2lcvj + a22q}y)y = (j2)y moto: quux + gvuy = {/j,uy)v energia: qu1ix + gvhv = (a00t„ + ancv, + a02(py)y + = (j0)„ dove j 1 ? j2, j 0 sono i flussi di massa del componente 1 (elettroni), 2 (ioni) e di energia (0) rispettivamente. tali flussi possono porsi in funzione dei gradienti vt, vcx e v(p attraverso i coeflicienti fenomenologici an. peraltro la termodinamica dei processi irreversibili dimostra che gli a n non soddisfano il principio di onsager, poiche il sistema dei flussi non e quello d i r e t t a m e n t e coniugato con le forze scelte. s c i e i p e r s o n i c h e 8 5 ii sistema ft, vcu yep comporterebbe i n f a t t i i flussi (x): . m , + . 1 t ' 'z7 ' t dove j j e il flusso di entropia di conduzione, 11011 legato ad alcun trasporto di massa. i e la densita di corrente elettrica, i = e1j1 + e2 j2, e: __ a (fj,( — /j,3) ex a j ^ i — ,m3) 1 dcx e2 3c2 ii sistema di liussi j0, j1 e j2 e invece di r e t t a m e n t e coniugato alle tre forze generalizzate: 17t f0 = — —1 0 m fx = — — p,) — (jsx — <s8) p t — = — — — (s2 — s,) p t — e2vcp attraverso i coefflcienti fenomenologici l*tj (4). ne segue quindi che i coefflcienti ay andranno determinate considerando proprio il sistema congruente e ricavandoli in funzione degli l*n. tralasciando i vari passaggi, si ottiene in tal guisa: 4 — t,* i t ^oo — x ' 00/-*a0j — b2l • 02 a02 = 62l! 02 au = — l * j t an = — bxl*n a12 = — e1l*u a20 = — l*20/t al = — [mai + b2l*22\ a22 = — [e1l*n + e2l*22] t u t t i gli l*ij sono stati determinati in bibl. (') nella ipotesi di plasma lorentziano imperfetto debolmente ionizzato. per procedere alia adimensionabzzazione delle equazioni, facciamo le posizioni: t o u v x t+ = — ; p+ = — ; «+=„-; v+ = — ; x+ = — ; to go 1 o i 0 lo v v i 4r + >/ = v i v — — i = 7 1 — t 5 = —t7 • l„ (p0 (-iii)o gol'o^io mentre le grandezze t0, q0 ecc. hanno il consueto signiflcato di grandezze di riferimento caratteristiche del fenomeno (valori all'inflnito o 87 l . d e socio valori del p u n t o di ristagno del corpo ecc.), la scelta di tp0 e s t a t a compiuta tenendo conto delle considerazioni che seguono. fi s t a t o dimostrato sperimentalmente (") la possibility di ottenere in un plasma u n campo elettrico indotto m e d i a n t e l'applicazione di u n gradiente di t e m p e r a t u r a . si e cioe messa in luce, anche per mezzi gassosi, come gia per i solidi, la presenza di u n effetto seebeck. i n bibl. (') si e ricavato m e d i a n t e la termodinamica dei processi irreversibili ed i risultati della teoria cinetica dei gas la espressione del coefficiente di seebeck per i plasmi lorentziani debolmente ionizzati. in particolare d e t t o coefficiente di seebeck risulta espresso da: sembra quindi opportuno scegliere, quale valore di riferimento per <p, proprio: tp0 = >sa0t , dove. a„t e una o p p o r t u n a differenza di temp e r a t u r a . p e r es. a0t puo essere il valore massimo della differenza di temperat u r a a t t r a v e r s o la scia. con siffatte considerazioni e tenendo conto del sistema che lega gli at) e gli l * u e possibile scrivere le due equazioni della diffusione e l'equazione dell'energia in termini adimensionali come segue (*): 1 5 k ' 8 e n2 dz3 n 3 a 2 quc1x + qvciy = (^•n)o i d'fo^ q0v0l0 ikt0l1w '110 [1] quc2x + qvciy qo^ol0 kt0l2w \ (a201\,) + (a21 ciy\ + h \ )y [2] 1 — v nx 8c re n [ 3 ] (*) l e g r a n d e z z e a d i m e n s i o n a l i vengono d ' o r a in poi i n d i c a t e senza asterisco. scie i p e r s o n i c h e 8 7 i nuineri adimensionali che compaiono nelle precedent! equazioni risultano: re — numero di reynolds = g0v0l0/iu pr = numero di p r a n d t l = x/gp/x e = numero di e c k e r t = v02/gpt0 8c = numero (li schmidt = /u/q0d23 . d i s c u s s i o n i b c o n c l u s i o n i . l'esame delle equazioni su riportate mostra come assuma u n a particolare influenza sul fenomeno della scia la distribuzione del potenziale elettrico provocato dalla diffusione reciproca t r a le particelle cariche. si consideri i n f a t t i la equazione 1. (a ) 1 a ineno del coefficiente , che pu6 ridursi a e che g0l0l0 kc&c rappresenta l'importanza relativa dei processi convettivi e dissipativi, l'influenza del potenziale elettrico sulla diffusione e paragonabile a quello del gradiente di concentrazione elettronica. l'effetto della diffusione termica risulta invece piccolissimo, nel caso da noi studiato: esso puo assumere u n a certa importanza solo in condizioni particolari di altissime temperature, come in alc.uni fenomeni astroflsici (ad es. nella corona solare). l'equazione dell'energia, inflne, mostra come la distribuzione della t e m p e r a t u r a non sia invece p r a t i c a m e n t e controllata dalla distribuzione del potenziale elettrico e dalla concentrazione elettronica. da quanto sopra detto discende la necessita di includere nel calcolo della massima p a r t e delle scie ipersoniche lo studio della distribuzione del campo elettrico. tale determinazione del potenziale elettrico comportera quiudi oltre al sistema di equazioni gia riportate, anche l'equazione di poisson, che sino ad oggi non e s t a t a considerata nella letteratura pertinente, con la conseguenza di una maggiore semplicita ma anche di una scarsa attendibilita, in molti casi. simboli: an = coefflcienti fenomenologici (i, j = 0, 1, 2) ct — concentrazione di massa per la specie i cp — calore specifico a pressione costante 8 8 l . d e s o c i o dt, = coefficiente di diffusione termica della specie i dtl = coelficiente di diffusione molecolare binaria per la specie i ed j e = numero di e c k e r t et = carica elettrica per unita di massa della specie i e = carica elettrica dell'elettrone hd = lunghezza di debye h = entalpia totale per unita di massa i = densita di corrente elettrica jt = flusso di massa della specie i jo = flusso di energia jsc = fluido di entropia di conduzione k = costante di boltzmann l0 = lunghezza di riferimento l„ = coefficienti fenomenologici (i, j = 0, 1, 2) m< = massa molecolare per la specie i n ( = densita di numero per la specie i pi= numero di p r a n d t l re = numero di reynolds si = entropia specifica per la specie i so = numero di schmidt t = t e m p e r a t u r a u, v = componenti di velocita y = rapporto di calori specifici x = coefficiente di conducibilita termica /» = coefficiente di viscosita 0 = densita <p = potenziale elettrico. i pedici 1, 2, 3 si riferiscono agli elettroni, agli ioni ed alle molecole neutre, rispettivamente. i pedici x, y si riferiscono a derivazione rispetto a x, ed a y, rispettivamente. n o t a finanziata dal n a v y d e p t . u . s . a . ( c o n t r a t t o n . o . n . r . 3475 00). b i b l i o g r a f i a ( • ) i . . g . n a p o l i t a n o , l . d e s o c i o , study of electromagnetic properties of non-uniform, plasmas in thermal equilibrium. «i. a. rep.n, (1962). ( 2 ) s . f e l d . m a n , on trails of axisymmetric hypersonie blunt bodies flying through the atmosphere. «j.a.s. », ( j u n e 1961). s c i e i p e r s o n i c h e 89 (3) m. h . b l o o m , thermal and chemical effects in wakes. a g a r d meet, on « high t e m p . a s p e c t s of h y p e r s o n i c plow ». (brussels, april 1962). (•*) j . h i r s c h f e l d e r , c . c u r t i s s , r . b i r d , molecular theory of gases and liquids. wiley & sone, 1954. ( 6 ) s . c h a p m a n , e . t a n d b e r g h a n s s e n , thermal diffusion at high temperatures in ionized gases. « p r o c . conference on e x t r e m e l y high t e m p e r a tures)), (boston 1958). ( • ) s . k l e i n , the direct conversion at thermal energy into electrical energy. icproc. 5tli i n t . conference on i o n i z a t i o n p h e n o m e n a in gases», (munich 1961). a s p e t t i i n t e r n a z i o n a l i d e i p r o b l e m i d e l l e t e l e c o m u n i c a z i o n i s p a z i a l i ( * ) i . r a n z i ( * * ) r i c e v u t o il 19 f e b b r a i o 1963 r i a s s u n t o . — vengono esposti i p r o b l e m i che si p r e s e n t a n o sul p i a n o i n t e r n a z i o n a l e per l'organizzazione dei sistemi di comunicazioni f a c e n t i uso di satelliti artificiali della t e r r a come r i p e t i t o r i , e per lo svolgimento delle ricerche spaziali. i n p a r t i c o l a r e viene t r a t t a t o il p r o b l e m a dell'assegnazione delle f r e q u e n z e ai v a r i servizi spaziali e se ne pongono in rilievo le non lievi difficoltà. s u m m a r y . — t h e p a p e r p r e s e n t s t h e p r o b l e m s t o be t a c k l e d 011 int e r n a t i o n a l level for t h e o r g a n i s a t i o n of c o m m u n i c a t i o n s y s t e m s b y using artificial satellites of t h e e a r t h as r e p e a t e r s a n d for t h e p e r f o r m a n c e of s p a c e r e s e a r c h . p a r t i c u l a r a t t e n t i o n is d e v o t e d to t h e p r o b l e m of t h e allocation of f r e q u e n c i e s t o t h e v a r i o u s space services, e m p h a s i z i n g its considerable difficulties. m i p r o p o n g o di e s p o r r e i n s i n t e s i q u a l i s i a n o i p i ù i m p o r t a n t i p r o b l e m i t e c n i c i d e l l e t e l e c o m u n i c a z i o n i s p a z i a l i i n g e n e r e , i q u a l i e s i g o n o p e r l a l o r o s o l u z i o n e u n a t r a t t a z i o n e sul p i a n o i n t e r n a z i o n a l e . f o r t u n a t a m e n t e e s i s t e g i à l a s e d e a d a t t a p e r t a l e t r a t t a z i o n e , c o s t i t u i t a p r e c i s a m e n t e d a l l ' u n i o n e i n t e r n a z i o n a l e delle t e l e c o m u n i c a z i o n i ( u . i . t . ) , l a c u i f u n z i o n e è fiancheggiata d a i suoi d u e c o m i t a t i c o n s u l t i v i , l ' u n o p e r l a r a d i o c o m u n i c a z i o n e ( c . c . i . r . ) , l ' a l t r o p e r l a t e l e g r a f i a e l a t e l e f o n i a ( c . c . i . t . t . ) . i l g r a v e p r o b l e m a d e l l ' a s s e g n a z i o n e delle v a r i e f r e q u e n z e d e l l o s p e t t r o r a d i o e l e t t r i c o ai d i v e r s i s e r v i z i v i e n e t r a t t a t o e (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. (**) p r o f e s s o r e o r d i n a r i o presso l ' i s t i t u t o superiore delle p o s t e e telecom u n i c a z i o n i r o m a . p r e s i d e n t e della commissione di s t u d i o per le comunicazioni spaziali del c o m i t a t o consultivo i n t e r n a z i o n a l e delle r a d i o c o m u n i cazioni. 4 5 8 i . r a n z i risolto in tale sede, e si può dire che l'importanza e l'efficienza dell'organizzazione in parola si sono ormai validamente affermate. una particolare commissione di studio del c.c.i.r., e precisamente la q u a r t a , è s t a t a creata per l'esame dei problemi delle telecomunicazioni spaziali; essa si è riunita per la p r i m a volta a washington nel marzo .1962, svolgendo i lavori preparatori per l'assemblea plenaria del c.c.i.r. che si t e r r à a ginevra nel gennaio-febbraio 1963. il primo e più i m p o r t a n t e problema che si presenta in questo nuovo e p r o m e t t e n t e campo della ricerca e della tecnica è quello dell'assegnazione delle bande di frequenza ai vari servizi spaziali; ad esso, come vedremo, sono s t r e t t a m e n t e legati in una interdipendenza a volte assai complessa i vari altri problemi tecnici dei sistemi di comunicazione spaziale. 1 . s i s t e m i di t e l e c o m u n i c a z i o n e a mezzo d i s a t e l l i t i . iniziamo il nostro esame dal campo delle telecomunicazioni a grande distanza, facenti uso di satelliti artificiali della terra, quali ripetitori attivi o passivi. le caratteristiche di tali sistemi sono s t a t e così largamente divulgate in questi ultimi tempi, ed anche nel corso di questo convegno esse hanno f o r m a t o oggetto di così approfondito esame, che ritengo del t u t t o inutile ritornare su di esse. le più accreditate previsioni che oggi si fanno circa la larghezza totale delle bande di frequenza che verranno richieste dai sistemi in questione sono di 1000 mhz entro il 1970 e di altri 1000 mhz entro il 1980; è inutile forse precisare che tali cifre si riferiscono ai satelliti ripetitori attivi, gli unici che, allo stato a t t u a l e della tecnica, appaiono a t t i a soddisfare le esigenze di un efficiente sistema per comunicazioni transcontinentali. se si confrontano tali esigenze con i 127 mhz complessivi delle varie bande assegnate per t u t t i i servizi spaziali (comprese quindi le ricerche spaziali) dalla conferenza radio amministrativa di ginevra del .1959, e se si tiene inoltre conto della completa saturazione dello spettro delle microonde specie f r a 1000 e 10.000 mhz, il problema sembra a prima vista di soluzione particolarmente difficile. f o r t u n a t a m e n t e , si è ben presto dimostrata la possibilità di condividere le bande richieste con quelle già assegnate ai ponti radio di piccola potenza in visibilità diretta. sono proprio le frequenze f r a 2000 e 10.000 mhz, cioè quelle che comprendono i ponti radio, che appaiono come le a s p e t t i i n t e r n a z . d e i p r o b l e m i d e l l e t e l e c o m u n i c a z . s p a z i a l i -403 più a d a t t e per le comunicazioni spaziali; infatti, il limite inferiore a 2000 m h z o poco al disotto è imposto dal rumore di origine cosmica, la cui intensità cresce rapidamente al disotto di tali frequenze; verso i 4000 m h z l'intensità di tale rumore si riduce a valori inferiori a quelli relativi al r u m o r e dovuto alla emissione di onde elettromagnetiche per meccanismo termico da p a r t e del vapor acqueo atmosferico; anche la radiazione dei corpi posti alla superficie terrestre o quella dello stesso suolo, d a t a la sua intensità relativamente elevata, può contribuire, intorno e al disopra della suddetta frequenza, al rumore del ricevitore, anche se l ' a n t e n n a non è p u n t a t a all'orizzonte, e ciò per effetto dei cosidetti lobi secondari dell'antenna stessali rumore dovuto al vapor acqueo atmosferico s'accresce poi con la f r e q u e n z a al disopra di quella b a n d a intorno ai 4000 mhz, a cui corrisponde così il minimo del rumore di origine esterna; si progetta pert a n t o di assegnare tale b a n d a (precisamente f r a 3700 e 4200 mhz) alle comunicazioni dirette dal satellite verso terra, tenuto conto della limit a t a potenza dei t r a s m e t t i t o r i posti a bordo dei satelliti, e della necessità quindi dell'impiego di sistemi riceventi a terra di elevata sensibilità. è poi previsto l'uso delle due bande f r a 1750 e 2250 mhz e f r a 5925 e 6425 mhz per le trasmissioni dalle stazioni a terra verso i satelliti; u n ' a l t r a b a n d a f r a 6500 e 7000 mhz potrà essere necessaria per gli ulteriori sviluppi dei sistemi di telecomunicazioni a mezzo di satelliti (verso il 1980). i n linea di massima, t u t t i i paesi occidentali appaiono d'accordo su t a l i assegnazioni; nella riunione di washington, la delegazione dell'unione sovietica ha espresso il parere che sia per ora sufficiente assegnare ai servizi in questione solo t r e bande, u n a di 100, u n a di 400 e u n a di 500 mhz, f r a 3000 e 6000 mhz. la decisione finale spetterà alla conferenza eadio a m m i n i s t r a t i v a prevista per la fine del 1963, a ginevra. la condivisione delle frequenze con i ponti radio presenta vari problemi, i cui aspetti hanno u n a diretta importanza nel campo internazionale. la possibilità di interferenze reciproche fra i due sistemi va attent a m e n t e esaminata. p r a t i c a m e n t e non avverrà m a i che l ' a n t e n n a a terra di un sistema di comunicazione a mezzo di satelliti e quella di un terminale di un ponte radio siano p u n t a t e direttamente l'una verso l'altra; t u t t a v i a l'esistenza dei lobi secondari delle stesse antenne, specie dei primi lobi laterali, può far sì che p a r t e dell'energia emessa da u n ' a n t e n n a entri nell'altra, anche se gli angoli f r a gli assi siano dell'ordine di u n a diecina di gradi o più. nel caso della interferenza p r o d o t t a in un ricevitore spaziale da p a r t e delle emissioni di un ponte radio, è l'elevata sen4 6 0 i . r a n z i sibilità del ricevitore a definire le condizioni di protezione, e nel caso della interferenza inversa, è l'elevata potenza del t r a s m e t t i t o r e del sistema spaziale (dell'ordine di uno o più chilowatt) a definire i termini del problema. è ovvio che il valore limite massimo consentito, per quanto riguarda la potenza del segnale disturbante all'ingresso del ricevitore, dipende in alto grado dai tipi di modulazione del segnale desiderato e del segnale disturbante: nel caso di trasmissioni di telefonia, comprendenti un certo numero di canali (ad esempio, 900 canali a due vie, in u n a b a n d a base di 5 mhz), appare subito la convenienza di disporre nella stessa b a n d a di frequenza, i vari canali dei due sistemi di comunicazione, in modo tale che quelli di un sistema siano intercalati f r a i singoli canali dall'altro sistema. poiché esistono dei precisi piani di canalizzazione raccomandati in sede internazionale dal c.c.i.r. per i ponti radio, sarebbe possibile u n a riduzione delle probabilità di interferenze a d o t t a n d o u n opportuno piano di canalizzazione per i sistemi a saltellite; vi sono, t u t t a v i a , oggi alcune difficoltà per un tale accordo, dato che per alcune bande di frequenza vi sono paesi che, per ragioni tecniche ed anche finanziarie, non sono in grado di modificare i loro impianti di penti radio, così da adeguarsi alle norme del c.c.i.r. e m a n a t e dopo la messa in opera degli impianti stessi. anche quando sia possibile u n a disposizione interlacciata dei canali, la protezione dalle interferenze può essere assicurata solo se viene interposta f r a le stazioni terminali dei due sistemi u n a notevole attenuazione di propagazione; questa attenuazione appare dell'ordine di 160 d b e dovrebbe salire a 180 db circa nel caso in cui l'interlacciamento non sia possibile. attenuazioni di quest'ordine si possono avere nella propagazione troposferica al di là dell'orizzonte, a mezzo del noto meccanismo della diffusione (scatter), oppure nel caso dell'interposizione di rilievi del terreno, quali catene montuose, che producano u n a efficiente azione di schermo; quest'ultimo effetto è oggi assai poco noto nei suoi dettagli e intense ricerche in tale campo sono s t a t e raccomandate dal c.c.i.r. le distanze cui corrispondono le richieste attenuazioni di propagazione vanno dai 40 ai 150 e più chilometri; è ovvio come, anche nella previsione del caso di terminali dei due sistemi posti vicino a f r o n t i e r e f r a due stati, sia necessaria u n a trattazione del problema in sede internazionale. il problema, comunque, va t r a t t a t o tenendo anche presente il f a t t o che le stazioni a terra dei sistemi di comunicazioni spaziali saranno in a s p e t t i i n t e r n a z . d e i p r o b l e m i d e l l e t e l e c o m u n i c a z . s p a z i a l i -403 numero limitato su t u t t a la t e r r a e che, quindi, u n a volta fissati i criteri generali per la soluzione, le soluzioni singole a n d r a n n o studiate caso per caso. soluzioni di carattere generale non possono ora essere date, anche perchè i sistemi di modulazione più convenienti per le comunicazioni a mezzo di satelliti saranno definiti solo in base ai risultati delle esperienze in corso col satellite « telstar » e delle varie altre progettate (satelliti « relay » e « syncom », in particolare). non solo il tipo di modulazione, ma anche la potenza in gioco, t u t t i clementi questi che influiscono sulle possibilità di interferenze con altri sistemi, dipendono dal sistema a satellite che verrà in definitiva adottato. allo stato a t t u a l e delle nostre conoscenze, sembra che i sistemi suscettibili di pratiche applicazioni nel campo delle telecomunicazioni siano i seguenti: a) sistema con numerosi satelliti, distribuiti casualmente, in orbite ellittiche o circolari inclinate sul piano equatoriale, ad altezze da 2000 a 5000 km circa sulla superficie terrestre (tipo « telstar »): b) sistemi con 12 o 20 satelliti in orbita circolare equatoriale ad altezza intermedia (intorno ai 12.000 km), dotati di dispositivi per la correzione dell'orbita e della velocità, così da mantenere l'equidistanza f r a di loro, e d o t a t i inoltre di dispositivi automatici per l'orientamento fisso verso t e r r a così da consentire l'uso di antenne direttive a elevato guadagno; c) sistemi con satelliti sincroni col moto della terra, su orbita equatoriale (alta circa 36.000 km) e quindi stazionari rispetto alla superficie terrestre, d o t a t i ili dispositivi per la correzione dell'orbita e per l'orientamento fisso (3 di tali satelliti sarebbero sufficienti). è evidente che nel caso del sistema b), in cui le orbite dei satelliti sono sempre le stesse per mia determinata stazione e ancor più nel caso del sistema stazionario, in cui le a n t e n n e delle stazioni a t e r r a m a n t e n gono un p u n t a m e n t o fisso, le possibilità di interferenza con ponti radio sono più ridotte. nel caso del sistema con satelliti stazionari vi è, però, da tener presente che forse il tipo di modulazione più conveniente nelle trasmissioni dalla terra verso il satellite potrà essere quello a b a n d a laterale unica; esso, i n f a t t i , consentirebbe u n a maggiore accessibilità al sistema da p a r t e delle stazioni a terra. come già si è detto, solo l'esperienza fornirà gli elementi per la scelta del sistema; non è, però, da escludersi che risulti conveniente l'impiego simultaneo di due diversi «sistemi. 4 6 2 i . r a n z i sempre per ciò che riguarda il problema della condivisione delle frequenze, sono oggetto di studio anche le possibilità di interferenza nei ricevitori dei ponti radio per effetto delle emissioni dai satelliti, e quella di interferenza nei ricevitori a bordo dei satelliti da p a r t e di t r a s m e t t i t o r i di ponti radio. gli studi nel campo della fondi visione delle frequenze mirano ora a stabilire le condizioni generali e i modi di soluzione dei vari problemi, che potranno in seguito delinearsi con maggiore precisione. oltre alle bande di frequenze destinate alle comunicazioni vere e proprie (telefoniche o televisive), altre più r i s t r e t t e bande sono richieste dai satelliti per comunicazioni: una b a n d a per il p u n t a m e n t o e l'inseguimento (tracking) del satellite, e per la trasmissione dei d a t i sul funzionamento degli a p p a r a t i di bordo, sulle radiazioni ambientali, ecc. (telemetria), banda che già è s t a t a assegnata, sia p u r e non in esclusiva, dalla conferenza di ginevra del 1959, f r a 136 e 138 mhz; u n ' a l t r a b a n d a , di pochi k h z , a t t o r n o ai 400 mhz sarà necessaria per la trasmissione di comandi da terra (telecomandi), comandi relativi a l l ' e n t r a t a in funzione dei vari a p p a r a t i o al loro silenziamento, all'eventuale funzionamento di dispositivi per la correzione dell'orientamento o del moto del satellite, ecc. vi sono altri problemi relativi a l l ' e n t r a t a in funzione di u n sistema di comunicazioni intercontinentali a mezzo di satelliti, problemi che esigono u n a discussione in sede internazionale; ci limiteremo qui ad enunciarli: a) diffusione dei d a t i orbitali dei vari satelliti cosi da consentire il p u n t a m e n t o delle a n t e n n e delle stazioni a terra. b) assegnazione dei canali telefonici e televisivi ai vari paesi. c) dislocazione delle stazioni a terra, e interferenze f r a stazioni vicine. d) metodi per la commutazione da u n satellite all'altro, specie nei sistemi a satelliti distribuiti casualmente in orbite relativamente basse. e) metodi per la correzione dell'effetto doppler. /) interconnessioni f r a stazioni a t e r r a dei sistemi a satelliti e sistemi di comunicazioni terrestri. 2 . t e l e c o m u n i c a z i o n i s p a z i a l i i n g e n e r e . anche in questo campo, il problema fondamentale che si p r e s e n t a sul piano internazionale è quello dell'assegnazione delle frequenze. considereremo qui brevemente quali siano jc esigenze nei vari casi. a s p e t t i i n t e r n a z . d e i p r o b l e m i d e l l e t e l e c o m u n i c a z . s p a z i a l i 4 0 3 2.1 sonde spaziali (tipo ((ranger)), « mariner», ecc.).-— p e r q u a n t o riguarda il « tracking » normale p o t r à servire la stessa b a n d a 136-138 mhz già assegnata; il tracking di precisione che tali sonde esigono dovrà farsi su frequenze più alte f r a 1000 e 4000 mhz e con elevate larghezze di b a n d a (da 2 a 4 mhz), dato che si deve determinare con grande esattezza anche la distanza, e si deve f a r quindi uso di emissione e ricezione di impulsi (transponder); tali frequenze potranno essere condivise f r a le varie sonde spaziali se queste sono assai lontane f r a di loro. d a t a , però, la piccola potenza dei segnali ricevuti da sonde lontane, occorrerebbe provvedere all'assegnazione di almeno una b a n d a in uso esclusivo p e r l'esplorazione profonda dello spazio. l a telemetria e il telecomando potrebbero effettuarsi f r a 1000 e 4000 mhz, o, come proposto da taluni, secondo due bande, l'una f r a 300 e 600 mhz, e l'altra f r a 600 e 2000 mhz, per una larghezza totale di 1 mhz. la prima b a n d a appare come la più conveniente, poiché consente l'impiego, a bordo della sonda, di trasmettitori a t r a n s i s t o r , di peso e ingombro assai ridotto. 2.2. satelliti per ricerche. — anche per questi satelliti la banda f r a 136 e 138 mhz viene proposta per u n a definitiva assegnazione alle funzioni di tracking e telemetria. p e r il tracking di precisione, per la trasmissione di d a t i a larga b a n d a e per particolari ricerche su fenomeni di propagazione, si prevede l'impiego di frequenze anche più elevate, sino a 10 ghz. l'unione sovietica tende a impiegare onde h . f . , f r a i 20 e i 90 m h z ; le frequenze più basse offrono il vantaggio di poter essere ricevute, in seguito a riflessioni multiple f r a la ionosfera e la superficie terrestre, anche quando il satellite si trova occultato dalla terra, il che può essere utile quando non si possa disporre di una rete mondiale di stazioni di ascolto; tali frequenze offrono però lo svantaggio di un elevato rumore di origine atmosferica e cosmica e della possibilità di interferenze. 2.3 satelliti di navigazione. •— p e r la telemetria e il telecomando vengono proposte le bande di frequenza già indicate per gli altri satelliti e per le sonde spaziali. le frequenze, su cui verranno effettuati i rilievi che conducono alla determinazione della posizione di chi riceve i segnali, variano a seconda del sistema usato. come è noto, in t u t t i i sistemi si presumono conosciuti t u t t i i p a r a m e t r i dell'orbita del satellite. i n quei sistemi in cui il p u n t o viene f a t t o misurando l'angolo d'arrivo verticale 4 6 4 i . r a n z i (oppure quello orizzontale) dei segnali emessi dal satellite, le frequenze dovranno essere nella g a m m a f r a 8 e 35 ghz. nei sistemi basati invece sul metodo del « transponder » (emissione di impulsi del satellite, su comando da terra, e determinazione del tempo di percorso), la frequenza dovrà rimanere f r a i 300 e i 700 mhz. nei sistemi, infine, basati sulla misura dell'effetto doppler nei segnali ricevuti, le frequenze dovranno preferibilmente cadere f r a 100 e 1000 mhz. 2.4 satelliti meteorologici. — il successo dei vari satelliti meteorologici della serie « tiros » lanciati dagli stati uniti, la preparazione della nuova serie dei satelliti « nimbus », rendono di grande importanza, per lo sviluppo della meteorologia su scala mondiale, l'assegnazione di adeguate bande di frequenza per tali ricerche. mentre per il t r a c k i n g può b a s t a r e la consueta b a n d a f r a 13g e 138 mhz, la trasmissione dei dati (in particolare delle fotografie delle nubi) dovrebbe avvenire nella banda, sino ad oggi usata, f r a 1700 e 1710; questa b a n d a f u già assegnata dalla conferenza di ginevra del 1959, m a non in uso esclusivo. si progetta ora di dotare tali satelliti di piccoli r a d a r per l'osservazione delle nubi (su frequenze intorno ai 35 ghz) e delle precipitazioni (su frequenze vicine a 10 ghz); è prevista anche l'emissione di microonde di frequenze corrispondenti ai massimi di assorbimento da p a r t e del vapor acqueo (23 ghz) e dell'ossigeno atmosferico (60 ghz), ciò al fine di studiare la distribuzione di tali componenti atmosferici. 2.5 veicoli spaziali pilotati. — le comunicazioni fra veicoli spaziali pilotati e le stazioni a t e r r a comprendono, oltre al collegamento telefonico a due vie, la trasmissione dei dati di comando, controllo, guida, navigazione, la trasmissione televisiva, e inoltre uno speciale collegamento per i casi di emergenza. si prevede che la fase sperimentale dei voli pilotati continuerà per almeno dieci anni, e in questa fase, m e n t r e alcune funzioni (come il tracking, i controlli, ecc.) p o t r a n n o essere svolte sui canali già proposti per i veicoli pilotati, occorre disporre (li u n a b a n d a per comunicazioni radiotelefoniche sotto i 25 mhz (così da assicurare la riduzione anche al di là dell'orizzonte visibile dal veicolo), e di u n a b a n d a f r a 100 e 600 mhz, avente u n a larghezza di circa 0,5 mhz, e con una elevatissima protezione dalle interferenze; praticamente, tale b a n d a dovrebbe essere assegnata in uso esclusivo, dato che, in condizioni di emergenza, la vita del pilota dipende soltanto dalla possibilità di trasnettere al pilota istruzioni o comandi diretti da terra al veicolo stesso. a s p e t t i i n t e r n a z . d e i p r o b l e m i d e l l e t e l e c o m u n i c a z . s p a z i a l i -403 specialmente nella fase di rientro le condizioni dei radiocollegamenti si fanno assai diffìcili, a causa della formazione di uno strato ili « plasma » a t t o r n o al veicolo, ossia di uno strato di gas atmosferici fortemente ionizzati a causa del calore che si svolge per l ' a t t r i t o incontrato dal veicolo nel suo rientro nell'atmosfera; soltanto onde di frequenza assai elevata, forse superiore ai 10 giiz, p o t r a n n o sorpassare questo strato senza subire un assorbimento troppo elevato, ed è a p p u n t o in questa g a m m a di frequenze che occorrerà assegnare una delle bande di emergenza per le comunicazioni con i veicoli spaziali pilotati. 2.0 radio diff usione diretta dai satelliti. — allo stato attuale, non si prevede certo prossima la realizzazione di emissioni ili radio diffusione da satelliti con potenza tali che la ricezione a terra possa avvenire con i normali apparecchi ili uso domestico. quando si pensi che occorrerebbero t r a s m e t t i t o r i a bordo dei satelliti con potenze effettive irradiate che nel campo delle y . h . f . dovrebbero superare i 100 kw, e si tiene presente che le potenze massime emesse ora dai satelliti sono dell'ordine di una diecina di w a t t , si vede quali difficoltà si incontrino in tale campo. e ciò senza tener conto delle non meno lievi difficoltà circa la scelta di p r o g r a m m i che devono interessare e risultare gradit i ad assiemi di popolazioni diverse per lingua e s t r u t t u r a politica. 2.7 radio astronomia, —l'impiego di sistemi riceventi di elevatissima sensibilità, dotati in genere di dispositivi di integrazione dei segnali nel tempo, rende assai difficile il problema della scelta delle frequenze per le osservazioni di radio astronomia, sia che si t r a t t i della ricezione delle radioonde emesse da sorgenti cosmiche, sia che si t r a t t i della ricezione delle onde dirette ila trasmettitori terrestri su corpi celesti, e da questi retrodiffuse verso t e r r a (astronomia « r a d a r » ) ; in quest'ultimo caso, l'elevatissima potenza ilei trasmettitori pone il problema della interferenza nei sistemi di comunicazione terrestri. la conferenza di ginevra del 1959 aveva raccomandato la protezione di alcune bande di frequenza, e in particolare di quella relativa alla emissione da p a r t e dell'idrogeno cosmico (1420-1427) mhz; in pratica, t u t t a v i a , l'elevato livello delle interferenze rende in molti paesi assai difficili le osservazioni radioastronomiche. la condivisione delle frequenze con altri servizi, specie nella b a n d a u . h . f . , appare assai difficile, a meno di non installare gli osservatori in località assai lontane dai trasmettitori dei servizi terrestri, e p r o t e t t e da catene montuose. i n alcuni casi si può pensare a u n a condivisione 86 i . r a n z i nel tempo, ossia a una sospensione in certi periodi della giornata delle emissioni dannose per le osservazioni radioastronomiche. c o n c l u s i o n e . il quadro sommario che si è cercato di dare delle esigenze dei vari sistemi di comunicazione connessi con le ricerche e le applicazioni delle nuove tecniche spaziali può dare un'idea della complessità dei problemi che sorgono sul piano internazionale, specie per quanto riguarda l'assegnazione delle frequenze e l'eliminazione delle interferenze. in particolare, i contrasti f r a le esigenze delle ricerche, specie di quelle radio astronomiche, e la pressante richiesta di bande di frequenze per servizi terrestri di estrema importanza sia nel campo civile che in quello militare, danno luogo a problemi che a volte sembrano insolubili. mi sia consentito di formulare il voto che si trovi la possibilità di assicurare lo svolgersi anche di quelle ricerche che, p u r non offrendo ora alcuna possibilità di pratica applicazione, hanno t u t t a v i a il nobile intento di approfondire le nostre conoscenze sull'universo che ci circonda e di elevare il nostro spirito nella contemplazione delle meraviglie che l'universo stesso racchiude. development of nuclear emulsion for muography annals of geophysics, 60, 1, 2017, s0112; doi:10.4401/ag-7387 s0112 development of nuclear emulsion for muography kunihiro morishima1,2,3,*, akira nishio4, masaki moto4, toshiyuki nakano3,4, mitsuhiro nakamura2,3 1 institute for advanced research (iar), nagoya university, nagoya, japan 2 institute of materials and systems for sustainability (imass), nagoya university, nagoya, japan 3 kobayashi-masukawa institute for the origin of particle and the universe (kmi), nagoya university, nagoya, japan 4 nagoya university, department of physics, nagoya, japan abstract muography (cosmic-ray muon radiography) is the non-destructive inspection by using muons produced by interactions between primary cosmic-rays and the earth’s atmosphere. the principle of this technology is the measurement of absorption of incoming cosmic-ray muons, just like x-ray imaging technology. a nuclear emulsion is the high sensitive photographic film used for detection of three-dimensional trajectory of charged particles such as muons with the position resolution of sub-micron. a nuclear emulsion has following properties: high spatial resolution with thin detector structure, lightweight, flexibility of detector size and no requirement of electric power supply. these properties of a nuclear emulsion are suitable for a detector of muography. we are developing the nuclear emulsion at nagoya university. in this paper, the current performances of the nuclear emulsion and the fully self-production techniques developed at nagoya university are described. 1. introduction muography (cosmic-ray muon radiography) is a visualization technique of interior density distribution of large-scale structures such as volcanoes [tanaka et al. 2007a, 2007b, 2007c, 2009, 2010; lesparre et al. 2012, cârloganu et al. 2013, carbone et al. 2014], the pyramid [alvarez et al. 1970], nuclear reactors [morishima et al. 2012, fujii et al. 2013], and so on. muons are produced from collisions of primary cosmic-rays, which consist mainly of proton, with the earth’s atmosphere as shown in figure 1. generated muons in this way, called cosmic-ray muons, come from all direction to the face of the earth consistently and have broad energy distribution. high energetic muon has high penetration power that is able to penetrate through several km of rocks. the relation between muon energy and penetration length of the standard rock thickness [groom et al. 2001] is shown in figure 2. for example, a muon with an energy of 1 tev is able to pass through 1km of rocks. thanks to the broad incoming muon energy disarticle history received july 5, 2016; accepted october 5, 2016. subject classification: solid earth: exploration geophysics, instruments and techniques. figure 1. production process of cosmic-ray muons. electromagnetic components, nuclear components and neutrinos are removed in order to highlight the muon production process in this figure. incoming muon rate on the face of the earth is approximately 1 muon/min/cm2. figure 2. a relation between penetration thickness of standard rock and energy of muon [groom et al. 2001]. tribution, only muons with enough energy are able to penetrate material in the pathway and reach to the face of the earth. the principle of muography, similar to xray imaging technique, is the measurement of penetration rate of cosmic-ray muons pass through a large-scale structure and conversion to two-dimensional integrated density distribution as shown in figure 3. in addition, if we know the three-dimensional shape of the structure, we can convert to average density distribution. in comparison with other various non-destructive inspections, the muography has wide effective detection range and high spatial resolution thanks to properties of cosmic-ray muon. the request performance of muon detector for muography is detection of incoming angle. the required angular resolution to the detector is calculated by dividing required position resolution by the distance from observed object. typically for enough spatial resolution, an angular resolution of a few mrad is needed. in addition, in order to take a high spatial resolution image, enough statistics of detected muons are necessary. thus, a large area detector is required to collect a lot of muons because cosmic-ray muon rate on the face of the earth is constant. from these kinds of reasons, requirements for muon detector are both high resolution to achieve enough spatial resolution and large area to achieve enough statistics. 2. nuclear emulsion a nuclear emulsion is a high sensitive photographic film sensitized to minimum ionizing particles (mips) such as cosmic-ray muons and is able to record three-dimensional particle trajectories with sub-micron spatial resolution as tracks when particle passes through in its sensitive emulsion layer as shown in figure 4. these recorded tracks are detected by using an optical microscope. a nuclear emulsion is very old technology invented in the 1940s [powell et al. 1959]. nowadays, an automated high-speed nuclear emulsion read-out system called track selector (ts) based on an optical microscope with latest computing technologies were developed by our group [niwa et al. 1974, aoki et al. 1990, nakano 1997, morishima and nakano 2010] from 1970s. the read-out system takes sixteen divided tomographic images along the direction of optical axis in the emulsion layer and searches a straight track. after track recognition, track data (position, angle, pulse height) is saved into the database. the read-out speed of emulsion layer was reached to approximately 100 cm2/h by superultra track selector (s-uts) and will be reached to approximately 1 m2/h in near future by hyper track selector (hts) under development. in addition, we put the emulsion gel production machine into operation in 2010 at nagoya university and are able to produce an emulsion gel independently. by using the production machine, we are able to improve and tune performances (sensitivity, long-term stability and so on) of a nuclear emulsion gel for purposes. in addition, we are developing the pouring technique and the mass production method. in this way, we are developing fully self-made emulsion technologies in laboratory level. a nuclear emulsion has the following advantages for muography: high spatial resolution (position resolution is sub-micron, angular resolution is a few mrad) with very thin detector structure, lightweight, flexibility of detector size from 1 cm length to 1 m length and morishima et al. 2 figure 3. a schematic diagram of muography. a photography and a muography in the lower part of the figure are referenced from tanaka et al. [2007a]. 3 no requirement of electric power supply. these properties are ideal for muography in various environments due to portability, scalability and flexibility. 2.1. structure of nuclear emulsion the picture of figure 4a is the nuclear emulsion we produced and figure 4b shows the structure of the nuclear emulsion. an emulsion gel is poured on the 170 µm transparent polystyrene (ps) plastic base, which acts as a supporting material. a huge amount of silver bromide (agbr) crystals, which diameter is 200 nm, are dispersed in 60 µm gelatin layer coated on both sides of ps base. the picture of figure 4c is an electron microscope image of agbr crystals. a charged particle passes through the emulsion layer and some crystals in the line of the trajectory have a latent image as shown in figure 4d. after chemical development process, these latent images become sub-micron silver particles (grains) as shown in figure 4e. the grain size in an optical microscope image is approximately 1 µm and the size is sufficiently smaller than the thickness of the emulsion layer. thanks to the structure, a trajectory of a charged particle (track) can be recognized in three-dimensions with sub-micron accuracy. an angular range of data taking of tracks is depends on the recognition algorithm of the read-out system. the angular range of the ts is 45 degrees in an ordinary condition. 2.2. production of emulsion film we established nuclear emulsion production techniques as shown in figure 5. we are able to produce an emulsion gel for r&d and small amount mass production by using 0.5 l scale machine. in addition, we introduced 1.5 l scale machine for mass production in 2014 at nagoya university. by using the new machine, we are able to produce a nuclear emulsion gel at the speed of approximately 3 l/day. the pouring procedure constructed at nagoya university is shown in figure 5. we pour an emulsion gel inside a frame made of a vinyl tape and spread an emulsion gel with keeping the uniformity of the emulsion layer thickness by using the special glass coating tool. after an emulsion gel becomes solid state (setting), we move emulsion plates to a drying shelf. we process both side of an emulsion plate. the achieved uniformity of the thickness by this method is approximately 10 µm in the area of 10 cm × 10 cm. after that, we perform cutting, humidity conditioning and packing with aluminum-laminated film. an aluminum-laminated film acts as shielding from water, shielding from light and holding water in the nuclear emulsion. by these methods, we can produce nuclear emulsions at the speed of 4 m2/week. the production speed is limited by emulsion setting time and spaces of the shelf. for muography, 12.5 cm × 30 cm size nuclear emulsions, which is shown in figure 4a, are produced and used. 2.3. performance the track recognition efficiency of muons by the read-out system strongly depends on the density of grains per length along the line of a track. the number of grains per 100 microns is called grain density (gd). in order to increase gd, we tuned the volume occupancy of agbr crystals of 55% in the nuclear emulsion development of nuclear emulsion for muography figure 4. (a) a picture of the nuclear emulsion; (b) an illustration of the structure of the nuclear emulsion; (c) an electron microscope image of silver bromide crystals of the nuclear emulsion. the diameter of crystals is approximately 200 nm; (d) and (e) the principle of detection of a charged particle in the nuclear emulsion. black small dots on silver bromide crystals show latent images in (d). layer. in addition, we tuned amount of a few additive chemicals (surfactant, plasticizer and hardener) for achievement of sufficient flexibility in order to prevent an emulsion layer from cracking. the optical microscope image of one of mip track in the nuclear emulsion is shown in figure 6. the gd of the nuclear emulsion is 51±2 by counting an image taken by an image sensor by eye. the nuclear emulsion evaluated was developed with a developer for opera experiment (pd-t) produced by fuji photo film co., ltd. for 25 minutes at 20 degrees celsius. the gd is higher than that of opera film [nakamura et al. 2006], which is produced for the opera experiment and used for muography in the past [tanaka et al. 2007a, 2007b, 2007c; morishima et al. 2012]. the pulse height (ph) distribution of cosmic-ray tracks of its angle|tani|< 0.1 recorded in the emulsion layer taken by the s-uts [morishima and nakano 2010], which is one of the ts, is shown in figure 7a. the angle i is defined by an angle between a track angle and an optical axis of the read-out system. the number of tomographic layers is 16 and the ph is the number of layers, which includes grains of a track. this ph distribution enables us to achieve a track recognition efficiency of more than 97% in a normal radiation background. the angular resolution of cosmic ray tracks of its angle|tani|< 0.1 recorded in the emulsion layer taken morishima et al. 4 figure 7. (a) a histogram of angular resolution of tracks recorded in the emulsion layer; (b) a histogram of the pulse height (ph) distribution of tracks recorded in the emulsion layer. figure 5. an overview of a process of self-made production of the nuclear emulsions. figure 6. a microscope image of a track of minimum ionizing particle in the nuclear emulsion. black dots are silver gains. a chain of silver grains is a trajectory of charged particle. 5 by the s-uts is shown in figure 7b. this resolution is equal to the position resolution of 0.45 µm on the surface of the ps base. this position resolution gives us to achieve the angular resolution of a few mrad by using the track positions on both surfaces of the ps base. the long time stability of the nuclear emulsion is important for muography. especially, the temperature dependency of the latent image fading is very important because the latent image fading is accelerated by high temperature. the fading property of the nuclear emulsion in the combination of temperature (25 and 35 degrees celsius) and relative humidity (20% and 50%) is shown in figure 8. in this evaluation, a time variation rate of fading effect, which is measured gd at each sampling date divided by initial gd (relative sensitivity), is presented. the result shows that high temperature and high relative humidity of the nuclear emulsion accelerate decrease of gd. thus, we need to decide an observation period according to the surrounding temperature with controlling relative humidity of the nuclear emulsion in an aluminum laminated film package in the observation place. 2.4. observation the structure of nuclear emulsion detector is optimized for various observation surroundings. the nuclear emulsion is attached on a flat plate, for example an aluminum plate, as shown in figure 9a. in this case, the nuclear emulsion is fixed in the vertical direction to the ground in order to measure incoming muons in a low elevation angle, for example to measure volcanoes. in order to reflect sunlight and suppress a rise in temperature, the nuclear emulsion is put in the thermal insulation covered with light reflective material on case-by-case basis as shown in figure 9b. the nuclear emulsion is fixed in the parallel direction to the ground in order to measure incoming muons from zenith direction, for example to search for unknown empty space in the upper direction as shown in figure 9c. nuclear emulsions are fixed inside the supporting aluminum material with lead plates to shield from gamma ray emitted from surrounding bedrock and the thickness of the structure is kept in approximately 5 cm in order to install in a narrow space as shown in figure 9d. the main application of nuclear emulsions produced at nagoya university is the measurement of fukushima daiichi nuclear power plant, which was conducted from 2014 to 2015, until now. this is the first experiment using the high sensitive nuclear emulsion produced at nagoya university. in addition, we start to apply nuclear emulsions in various fields such as geoscience, archaeology, civil engineering and so on. 3. conclusion nuclear emulsion has ideal properties for a detector of muography. we developed the nuclear emulsion production technology and improved the muon detection efficiency. the production speed is approximately 4 m2/week by our hands. the sensitivity (gd) of the nuclear emulsion is higher than that of opera film and enables us to achieve the efficiency of more than 97% by the s-uts. the angular resolution of a track is 15 mrad in the emulsion layer. the accuracy is almost the same as that of the opera film. these performances are sufficiently for a detector of muography. the nuclear emulsion we developed is already applied to muography of fukushima daiichi nuclear power plant and are beginning to be widely applied to other fields such as geoscience, archaeology, civil engineering and so on. development of nuclear emulsion for muography figure 8. the fading property of the nuclear emulsion. a relative humidity is written as rh in this figure. in the case of combination between 35 degrees and rh 50%, measurement dates are 1, 3, 5, 14 days. in the other case, measurement dates are 7, 14, 31 days. figure 9. pictures of nuclear emulsion detectors. (a) a picture of a structure of the nuclear emulsion detector fixed in the vertical direction to the ground; (b) a picture of the detector, which is same type as (a), covered by the thermal insulation with light reflective material; (c) a a picture of the nuclear emulsion detector fixed in the parallel direction to the ground; (d) is a picture of the nuclear emulsion detector of a picture (c) placed in a narrow space. references alvarez, l.w., j.a. anderson, f. el bedwei, j. burkhard, a. fakhry, a. girgis, a. goneid, f. hassan, d. iverson, g. lynch, z. miligy, a.h. moussa, mohammedsharkawi and l. yazolino (1970). search for hidden chambers in the pyramids, science, 167, 832-839. aoki, s., k. hoshino, m. nakamura, k. niu, k. niwa and n. torii (1990). fully automated emulsion analysis system, nucl. instum. meth. b, 51, 466-472. carbone, d., d. gibert, j. marteau, m. diament, l. zuccarello and e. galichet (2013). an experiment of muon radiography at mt etna (italy), geophys. j. int., 196 (2), 633-643; doi:10.1093/gji/ggt403. cârloganu, c., et al. 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the inhomogeneous crust with cosmic ray muon radiography: the density structure below the volcanic crater floor of mt. asama, japan, earth planet. sc. lett., 263, 104-113. tanaka, h.k.m., t. nakano, s. takahashi, j. yoshida and k. niwa (2007c). development of an emulsion imaging system for cosmic-ray muon radiography to explore the internal structure of a volcano, mt. asama, nucl. instrum. meth. a, 575, 489-497. tanaka, h.k.m., t. uchida, m. tanaka and h. shinohara (2009). cosmic-ray muon imaging of magma in a conduit: degassing process of satsuma-iwojima volcano, japan, geophys. res. lett., 36, l01304, 1-5. tanaka, h.k.m., h. taira, t. uchida, m. tanaka, m. takeo, t. ohminato, y. aoki, r. nishitama, d. shoji and h. tsuiji (2010). three-dimensional computational axial tomography scan of a volcano with cosmic ray muon radiography, j. geophys. res., 115, b12332. *corresponding author: kunihiro morishima, institute for advanced research (iar), nagoya university, nagoya, japan; email: 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice s u l l ' o m o g e n e i t à e s u l l o s t a t o d i e q u i l i b r i o d e l m a n t e l l o i n t e r n o d e l l a t e r r a p . e. valle introduzione. — i n una nota p r e c e d e n t e ( * ) è stato posto in evidenza che le velocità d e l l e o n d e elastiche di agitazione termica di un solido isotropo, o m o g e n e o , di t i p o m o n o a t o m i c o , sono date dalle rel a z i o n i n e l l e quali p è la densità, y o m un p a r a m e t r o e gli i n d i c i 1 e t si r i f e r i s c o n o alle o n d e l o n g i t u d i n a l i e trasversali r i s p e t t i v a m e n t e . queste velocità sono d e t e r m i n a t e dai m o d u l i valutati ad e n e r g i e t e r m i c h e costanti, ma se la temperatura del s o l i d o è superiore alle t e m p e r a t u r e caratteristiche, c o i n c i d o n o con le velocità isoterme. i n questa nota si studiano alcuni p a r t i c o l a r i d e l l ' a n d a m e n t o di tali velocità in uno strato o m o g e n e o , in e q u i l i b r i o idrostatico, in rel a z i o n e al g r a d i e n t e di temperatura e n e l l ' i p o t e s i che la t e m p e r a t u r a stessa sia s u p e r i o r e alle t e m p e r a t u r e caratteristiche. c o m e a p p l i c a z i o n e v i e n e tentata una v e r i f i c a , per il m a n t e l l o int e r n o della t e r r a , d e l l e ipotesi su cui poggia il calcolo del m o d e l l o a di k . e. b u l l e n . si p r e m e t t o n o alcune c o n s i d e r a z i o n i sulle t r a s f o r m a z i o n i adiabatiche r e v e r s i b i l i . trasformazioni adiabatiche. —• se la temperatura è superiore alle t e m p e r a t u r e caratteristiche, l ' e n t r o p i a di un solido può essere espressa, con sufficiente approssimazione, dalla r e l a z i o n e (m s = s w — (m = l t, l) a t r 0, 'ni [2] n e l l a q u a l e jr è la costante dei gas, a il peso a t o m i c o , 0„, sono l e t e m p e r a t u r e caratteristiche e t la temperatura assoluta. * 3 4 p . e. v a l l e se si indica con l ' i n d i c e z e r o un certo stalo i n i z i a l e si ha dalla [ 2 ] a i 3 g„i 0 „ , — (s — 5„) log —— l3] r e t „ 3 e, e,2 o anche, tenendo presente che le t e m p e r a t u r e caratteristiche sono prop o r z i o n a l i alle f r e q u e n z e massime di v i b r a z i o n e , 4 t s v , v2 , — ( s — s „ ) = log —— r t „ ; ì vi v , 2 ora nella nota già citata ( r ) è stalo d i m o s t r a t o che v 0 „ , ( l v,„ = v m e \ " / e q u i n d i risulta r r . . ^ * ) ^ m nella quale y n è il p a r a m e t r o di griineisen, dato da v , 4 2 v y „ = y < " ^ t < " [ 5 ] e cy è il v a l o r e classico del c a l o r e specifico a v o l u m e costante, ossia ò r r n c v — —— [ 6 ] se si considera una t r a s f o r m a z i o n e adiabatica r e v e r s i b i l e ch e porti il solido dalla densità p„ alla densità p, la [ 4 ] d i v i e n e t = t e''" ( l ~ f ) [ ? ] questa r e l a z i o n e si trova già accennata in p r e c e d e n t i l a v o r i dell ' a u t o r e e può essere scritta anche nella f o r m a t s = costante [ 8 ] p l'i i v s u l l ' o m o g e n e i t à e s u l l o s t a t o di e q u i l i b r i o del m a n t e l l o d e l l a t e r r a 3 5 si può v e d e r e f a c i l m e n t e che se la temperatura i n i z i a l e t„ soddisfa la c o n d i z i o n e 7',, > 0 e y " [ 9 ] una compressione adiabatica mantiene la t e m p e r a t u r a sempre al disopra d e l l e t e m p e r a t u r e caratteristiche. gradienti di temperatura. — si consideri uno strato o m o g e n e o , isotropo, nel q u a l e la v a r i a z i o n e della velocità v,n con la p r o f o n d i t à li è determinata soltanto dalla v a r i a z i o n e della densità p e non dai valori i n i z i a l i dei p a r a m e t r i che c o m p a i o n o nella [ 1 ] . si avrà dvm _ 3 v,„ d p d li 3 p d li [10] l a densità p può essere pensata f u n z i o n e della t e m p e r a t u r a t e della pressione p attraverso l ' e q u a z i o n e di stato, e q u i n d i _ ' i p _ dr\ dh s p \ k t dh dh) nella quale k. è il m o d u l o di compressibilità isoterma e a la dilataz i o n e t e r m i c a . se l ' e q u i l i b r i o è idrostatico la [ 1 1 ] si scrive d vn, _ $ vm p l dt\ p 2 | dh 3 p ky d h d o v e g è il c a m p o g r a v i t a z i o n a l e . c o n v i e n e i n t r o d u r n e il g r a d i e n t e di temperatura t,. per cui la velocità r i m a n e costante al v a r i a r e di li e che v e r r à c h i a m a t o gradiente critico. si avrà n or [ 1 3 ] a kr o anche, r i c o r d a n d o c he a k t = y p cy = y„ p„ i\ , [ m ] y „ c v p „ 3 6 p . e, v a l l e i n d i c a n d o con t il g r a d i e n t e attuale dt/dh, dato che dalla [ i ] risulta 3 a log vm 1 — — = — i r 9 p p p e r c h é y,„ p = y„„, p„ , l ' e q u a z i o n e [ 1 2 ] d i v e n t a l ni "111 d h kt 1 — [15] questa r e l a z i o n e p u ò essere u l t e r i o r m e n t e trasformata i n t r o d u c e n d o il g r a d i e n t e a d i a b a t i c o t s . = l ì * [ 1 6 ] con q u a l c h e passaggio dalla [ 1 3 ] e dalla [ 1 6 ] risulta = a y t tc 1 + a y t [17] dato c he in g e n e r a l e a > 0, tc è sempre m a g g i o r e di t s . ora i n d i c a n d o con k s il m o d u l o di c o m p r e s s i b i l i t à adiabatica si ha k , 1 [18] e q u i n d i la [ 1 5 ] si può scrivere nella f o r m a = v p « l — j l \ — d li ks\ 3 ) ì _ [19] o p p u r e d t'„, _ / 1 ^ dh~s itm "3 1 — 1 — d o v e si è posto — ( « . s ) s — 3 f . 1 [20] [ 2 1 ] s u l l ' o m o g e n e i t à e s u l l o s t a t o di e q u i l i b r i o bel m a n t e l l o d e l l a t e r r a 3 7 si osservi c h e la [ 2 0 ] m o s t r a c h e l e v e l o c i t à i s o t e r m e s o n o s t a z i o n a r i e al c e n t r o d e l l a t e r r a p e r c h é i v i è g = 0. e f f e t t i v a m e n t e la v e l o c i t à d e l l e o n d e s i s m i c h e l o n g i t u d i n a l i , la q u a l e p e r ò è la v e l o c i t à a d i a b a t i c a , presenta un a n d a m e n t o d e l g e n e r e v e r s o i l c e n t r o d e l l a t e r r a . p o s t o o r a = d i / 1 \ dh gly."" t gly."3 / [22] n e l l ' i p o t e s i c h e la densità p si m a n t e n g a s e m p r e m i n o r e d i 3 y o m p„ , ossia n e l l ' i p o t e s i c h e r i s u l t i y m > r i c o r d a n d o c h e s < tc, si h a n n o i casi n o t e v o l i i n d i c a t i n e l l a t a b e l l a i . t a b e l l a i v a l o r e d i rm v a l o r e d i t rm < 0 t > lini — : 0 tt = te rm < 1 t > rm 1 t = rm > 1 t < t s p u ò essere i n t e r e s s a n t e t a l v o l t a c o n s i d e r a r e a n c h e il g r a d i e n t e d i t e m p e r a t u r a „ > n e c e s s a r i o p e r c h é si f o r m i una z o n a d ' o m b r a p e r la p r o p a g a z i o n e d e l l e onde, e l a s t i c h e . c o m ' è n o t o la f o r m a z i o n e d e l l a z o n a d ' o m b r a si ha q u a n d o d v„, dh 1 r„ — li [ 2 3 ] i n cui r„ i n d i c a il r a g g i o d e l l a t e r r a . i n t r o d u c e n d o questa c o n d i z i o n e n e l l a [ 2 0 ] si ha r„ 1 [ 2 4 ] •wibbfc 34 p . e. v a l l e m e d i a n t e la «jiiale si p u ò v a l u t a r e t„. questo g r a d i e n t e , n e l l ' i p o t e s i y m > y^ è superiore al g r a d i e n t e critico. tentativo di verifica delle ipotesi sulle quali è stato costruito il modello a di k. e. bullen per il mantello interno della terra. — i.e p i ù r e c e n t i discussioni sulle p r o p r i e t à elastiche, t e m p e r a t u r a e comp o s i z i o n e d e l m a n t e l l o i n t e r n o della t e r r a , sono p r o b a b i l m e n t e q u e l l e c he si sono sviluppate sulla base dei l a v o r i presentati da f . b i r c h ( 2 ) e j. v e r b o o g e n ( : ì ) alla 34" r i u n i o n e annuale d e l l ' a m e r i c a n g e o p b y sical u n i o n . p e r a l t r o , allo stato d e l l e attuali conoscenze, sembra o p p o r t u n o c o n t r o l l a r e se le note ipotesi sulle q u a l i è stato costruito il m o d e l l o a di k . e. b u l l e n sono v e r i f i c a t e . u n tale c o n t r o l l o p u ò essere tentato m e d i a n t e la r e l a z i o n e [ 2 0 ] del p a r a g r a f o p r e c e d e n t e , p r e n d e n d o in c o n s i d e r a z i o n e le o n d e sismiche di t i p o s, p e r c h é la v e l o c i t à isoterma di queste o n d e c o i n c i d e con la v e l o c i t à adiabatica. l ' a p p l i c a b i l i t à della teoria dei solidi a l l ' i n t e r n o della t e r r a è stata discussa in p r e c e d e n t i l a v o r i ( 4 ' 5 ) . se l o strato d è o m o g e n e o , in e q u i l i b r i o idrostatico e a d i a b a t i c o dovrà risultare 11 p r i m o m e m b r o della p r e c e d e n t e r e l a z i o n e p u ò essere direttam e n t e calcolato dai v a l o r i d e l l e v e l o c i t à d e l l e o n d e sismiche, m e n t r e il secondo m e m b r o può essere valutato soltanto in base ad un m o d e l l o p e r c h é contiene g e y t . n e l l a t a b e l l a i i sono contenute l e v e l o c i t à d e l l e o n d e sismiche c al c ol ate da h . j e f f r e y s e le g r a n d e z z e r e l a t i v e al m o d e l l o a di k . e . b u l l e n | 7 s j. d a t o che si presenta q u a l c h e difficoltà p e r il c a l c o l o di dv,/dh, la velocità i>t è stata approssimata, m e d i a n t e il m e t o d o dei m i n i m i q u a d r a t i , con una curva di t i p o p a r a b o l i c o . rt = 1 [ 2 5 ] ossia [26] s u l l ' o m o g e n e i t à e s u l l o s t a t o iii e q u i l i b r i o del m a n t e l l o d e l l a t e r r a 3 9 t a b e l l a l i h ( ' v s <t (ì g k m k m / s e c k m / s e c g / c m 3 c m / s e c 100(1 11,42 6,36 4,68 995 1200 11,71 6,50 4,80 991 1400 11,99 6,62 4,91 988 1600 12,26 6,73 5,03 986 1800 12,53 6,83 5,13 985 2000 12,79 6,93 5,24 986 2200 13,03 7,02 5,31 990 2400 13,27 7,12 5,44 998 2600 13,50 7,21 5.54 1009 2800 13,61 7.30 5,63 1026 c o n i v a l o r i della t a b e l l a 11 si è ottenuto : 1 = (0,101747 + 0,001862) li' — vt, [ 2 7 ] — (0,011346 + 0,001270) h'-' d o v e li' = io " ( h — 1 0 0 0 ) e eon l ' i n d i c e 1 si è i n d i c a t o il v a l o r e della v e l o c i t à alla sommità d e l l o strato d, ossia alla p r o f o n d i t à di 1000 km. l a derivata risulta q u i n d i = 6,36103 [(0,101747 + 0,001862) — dlt [28] — 0,022692 + 0,0025-10) h ' ] s o n o stati r i t e n u t i esatti i v a l o r i d e l l e velocità (li jeffreys. l ' e r r o r e m e d i o preso in c o n s i d e r a z i o n e è soltanto l ' e r r o r e m e d i o della derivata. in questo m o d o è stato calcolato il p r i m o m e m b r o della rel a z i o n e [ 2 6 ] e i risultati sono contenuti nella seconda colonna della t a b e l l a i i i . t a b e l l a i i i h . du. ' ' db km e in /secc m / s e c 1(10(1 778 + 14 8 3 1 + 4 0 . 9 3 6 ± 0 , 0 1 7 1200 768 + 15 799 + 4 0,961 ± 0 , 0 1 9 1100 760 + 17 771 + 4 0,986 + 0,023 1600 749 + 20 744 + 4 1,007 ± 0 , 0 2 7 1800 738 + 24 722 + 3 1,022 ± 0 , 0 3 3 2000 722 + 29 701 + 3 1,030±0,042 2200 703 + 34 684 + 3 1,028 ± 0 , 0 5 0 2400 678 + 39 671 + 3 1,010 ± 0 , 0 5 8 2600 6 5 2 + 4 4 660 + 3 0,988 + 0,067 2800 610 + 49 655 + 3 0,931 ± 0 , 0 7 5 m e d i a 0 , 9 7 5 ± 0 , 0 1 1 ( p o n d e r a t a ) i n un p r e c e d e n t e l a v o r o ( 5 ) è stato d e d o t t o d a l l e v e l o c i t à di jeffreys e d a l l e densità del m o d e l l o a di b u l l e n , il v a l o r e del param e t r o y, alla sommità d e l l o strato d, cioè alla p r o f o n d i t à di 1000 k m . t a l e v a l o r e , considerato con quattro c i f r e d e c i m a l i , è il s e g u e n t e : y u = 1,1689 + 0,0039 [ 2 9 ] r i c o r d a n d o che y« = y , . — [ 3 0 ] p si possono c a l c o l a r e f a c i l m e n t e i v a l o r i di y, alle v a r i e p r o f o n d i t à , m e d i a n t e i v a l o r i della densità della t a b e l l a i i . l a terza colonna della t a b e l l a i i i c o n t i e n e il p r o d o t t o g ( y t — % i. l ' e r r o r e m e d i o indicato è soltanto q u e l l o che deriva d a l l ' e r r o r e med i o di y t . l ' u l t i m a colonna della t a b e l l a i i i c o n t i e n e i n f i n e il r a p p o r t o r t , il cui e r r o r e m e d i o i n d i c a l o è stato c a l c o l a t o a p p r o s s i m a t i v a m e n t e con gli e r r o r i m e d i a t t r i b u i t i al d i v i d e n d o e al divisore. questo r a p p o r t o è r i p o r t a t o in f u n z i o n e della p r o f o n d i t à nella fig. 1. p r i m a di t r a r r e q u a l c h e conclusione dai risultati n u m e r i c i contenuti nella t a b e l l a i i i , è necessario eseguire una stima del g r a d i e n t e c r i t i c o tv. s u l l ' o m o g e n e i t à e s u l l o stato di e q u i l i b r i o del m a n t e l l o della terra 4 1 se si s u p p o n e , a t i t o l o d i o r i e n t a m e n t o , c h e il p e s o a t o m i c o m e d i o d e i m a t e r i a l i c h e c o s t i t u i s c o n o l o strato v sia u g u a l e a 20, la r e l a z i o n e f v f i a ' p i n e l l a q u a l e l ' i n d i c e 1 si r i f e r i s c e alla s o m m i t à d e l l o strato, c o n s e n t e di c a l c o l a r e t,., i n f u n z i o n e d e l l a p r o f o n d i t à , c o n i d a t i d e l l a t a [ 3 1 ] a • w 7 r . i 7 r/km t = 2 ' / k m velocità di j e f f r e y s • modello a d i b u t t e r i v a l o r e m e d i o t e gradiente critico m e d i o 7 ° / k m 7 = 7 c 1000 1500 2 0 0 0 2 5 0 0 fig. 1 3000 h k m b e l l a 11. a y j si p u ò a t t r i b u i r e il v a l o r e 1,253, o t t e n u t o i n un p r e c e d e n t e l a v o r o (~'l. l a t a b e l l a i v m o s t r a c h e . è c o m p r e s o tra 6,4 g r a d i k m e 7,9 g r a d i k m . i n m e d i a t(. si p u ò assumere p a r i a 7 g r a d i k m . o r a , tras c u r a n d o t s r i s p e t t o o tc, si ha [32] » " « k 34 p. e. valle t a b e l l a i v se si attribuiscono a t i v a l o r i 1; 1,5; 2 « r a d i k m , rt risulta pari a circa 0.86: 0,79; 0,71 r i s p e t t i v a m e n t e . queste c i f r e , pur essendo p u r a m e n te i n d i c a t i v e , d a n n o una idea della dim i n u z i o n e d e l r a p p o r t o rt con l ' a u m e n tare di t. d a l l ' e s a m e della t a b e l l a 111 risulta in p r i m o l u o g o che il g r a d i e n t e attuale t è m o l t o m i n o r e del g r a d i e n t e c r i t i c o tc. a n z i il v a l o r e m e d i o di rt induce a r i t e n e r e clic il m a n t e l l o int e r n o della t e r r a sia, con buona approssimazione, o m o g e n e o , in e q u i l i b r i o idrostatico e a d i a b a t i c o . p e r a l t r o il proc e d i m e n t o di c a l c o l o usato non consente di dare un peso d e t e r m i n a n t e alla precisione dei risultati. l ' i n c r e m e n t o a d i a b a t i c o della t e m p e r a t u r a 1000 e 2800 k m di p r o f o n d i t à è piuttosto p i c c o l o . d a l l a r e l a z i o n e [ 7 ] del secondo para•iragrafo si ha h t e k m g r a d i / k m 1000 6,4 1200 6,5 1400 6,6 1600 6,8 1800 6,9 2000 7,1 2200 7,2 2400 7,4 2600 7.6 28(10 7,9 7 ' [ 3 3 ] nella q u a l e pt e pn i n d i c a n o la densità alla p r o f o n d i t à di 1000 e 2800 k m . r i s p e t t i v a m e n t e . con i dati della t a b e l l a l i . a t t r i b u e n d o a yi i l v a l o r e 1,253, si ottiene j j * 0 0 _ 2 2 3 5 [ 3 4 ] questa c i f r a è assai prossima a q u e l l a d i e si può d e d u r r e dalla t a b e l l a i i i di un l a v o r o ( " ) eseguito q u a l c h e anno fa d a l l ' a u t o r e . da detta t a b e l l a risulta ^ "800 lj62g t„ 1 , 3 1 , = 1,24 135] si può q u i n d i c o n c l u d e r e che se l ' e q u i l i b r i o del m a n t e l l o i n t e r n o della t e r r a è prossimo a l l ' e q u i l i b r i o a d i a b a t i c o , c o m e sembra mostrare sull'omogeneità e sullo stato di equilibrio del mantello della terra 35 la presente ricerca, i l g r a d i e n t e di t e m p e r a t u r a n e l suo i n t e r n o è piccolo, p r o b a b i l m e n t e pari ad alcuni d e c i m i di g r a d o per k m , anche a m m e t t e n d o che la t e m p e r a t u r a a 1000 k m di p r o f o n d i t à sia pari a q u a l c h e m i g l i a i o di gradi. roma — istituto nazionale di geofìsica — marzo 1954. riassunto si studiano alcuni particolari dell'andamento della velocità delle onde elastiche in uno strato solido, omogeneo, in equilibrio idrostatico. che si trovi ad una temperatura superiore alle temperature caratteristiche. come applicazione viene tentata una verifica, per il mantello interno della terra, delle ipotesi in base alle quali è stato costruito il modello a di k e. bull-ai. la iteri fica fornisce, entro certi limiti, un risidtato positivo. summary ari equation is derived for the rate of cliange of the isotermai velocities of elastic ivaves, in a homogeneous gravitating layer ni ili an arbitrary gradient of temperature. an attempi is iliade to test the uniformity and adiabaticity of the layer d of the earth's mimile on the basis of this equation. b i b l i o g r a f i a ( ! ) valle i ' . e., unii equazione ili stalo per i solidi. a n n a l i (li g e o f i s i c a , 6, 183-197 ( 1 9 5 3 ) . ( ' ) b i r c h f „ the earth's mimile elasticitv and constitution. t r a n s . a n i . g e o p h y s . u n i o n , 35, 79-85 ( 1 9 5 4 ) . ( : ! ) verhoocen j., petrological evidence on temperature distribution in the maiale of the earth. t r a n s . a n i . g e o p h y s . u n i o n , 35, 85-92 ( 1 9 5 4 ) . ( 4 ) birch f., elasticity and constitution of earth's interior. j. g e o p h y s . r e s . , 57, 227-286 ( 1 9 5 2 ) . 34 p. e. valle (r>) valle p . e , sulle proprietà elastiche del mantello della terra. a n n a l i di g e o f i s i c a , 6, 373-380 ( 1 9 5 3 ) . ( ° ) jeffreys h , the tinte of p. s and sks and the velocieties of p and s. g e o p h y s . s u p p l . 4, 498-533 ( 1 9 3 9 ) . ( " ) bullen k . e , the probleni of the earth's densitv variations. b u l l . s e i s m . s o c . a m , 32, 19-29 ( 1 9 4 2 ) . ( 8 ) b u l l e n k . e , introduction to the theory of seismology. c a m b r i d g e , u n i v e r s i t y p r e s s ( 1 9 4 7 ) . ( ° ) valle p . e , sul gradiente adiabatico di temperatura nell'interno della terra. a n n a l i di g e o f i s i c a , 5, 41-53 ( 1 9 5 2 ) . studio degli accelerogrammi ottenuti nei recenti eventi sismici dell' italia centrale (on the .tudy 01 acceleragram, obtained during the la.t .e;'mic eventi al centrai ltaly) r. conrole f. peronaci ri('evuto il 23 settembre 19i1 riassunto. si espongono alcune consiùerazioni riguardanti l'analisi delle regifltrazioni ottenibili con gli ar,celerograft m.o. 2. le meùesime consi· ùerazioni vemgono messe in pratica su quattro terremoti effettivamente regi. strati ottenendone alcune grandezze fondamentali e gli spettri di velocith. questi ultimi vengono poi mediati e tradotti in spettri ùi ar,r,elerazione. r risultati dimostrano la validità deu'uso ùel calcolatore elettronico per questo tipo di rtulli e confermano l'importanza dell'installazione di una rete accelerometrica nazionale. sulillary. some considerations about the analysis of recorùs ohtai· nahle hy ~lo. 2 accelerographs are exposed. 'l'he same considerations are put into pract.ice on four eartllquakes reauy recorùeù, obtaining some funda· mental constants and tlle velodty spectra. tlle lattei' are then averageù and translateù in acceleration spectra. the results show the validity of uring an eloc,tronic computer for tllese stllllies and confìrm the importance of a national acr,elerometric network. l in seguito all'installazione di apparecchiature accelerometriche (m.o. 2) in prossimità. delpepicentro di zone colpite da terremoti, si sono ottenute registrazioni di repliche aventi intensità superiori al 498 11. co'lsole f. perokaci fv-v. fn particolarc sono state registrate tre leggere .~eosse a ::\lignano ::\lontelungo (di cui una da parte del c.n.b.~.) cd una più fort-e a valfahbriea. (perugia). nessuna registrazione (~st.at.a ot.tcnuta a 'l'useanih. scbbclie l'i.n.n. vi h.vesse installato uno strumcnt.o sin dal pomcriggio seguente la scossa prindpalc. l. de'l'elt.'liiè'lazioi'i"e delle accelerazio.'il come è noto, gli a{~celerografì ::\1.0. 2 registrano su pellicola fotogra· fiea da 3.') mm non perforata. t,a sensihilità per l',oecelenwione è pari a· circa 1,5 cm/g per le (iue componenti orizzontali, mcntre ho soglia di sca.tto è di circa 0,01 gj pcrtanto i t.erremoti di int.ensit.à, appena sufficiente ad avviare il meceanismo generano una traccia con ondula.zioni di j)()(·hi (iecimi di millimetro; questù si è verificato per le t.re scosse registrat.e a mignano. nome si è già aeeennat() in un precedente articolo, nel tentativo di utilizzare registrazioni di così scarsa. elltità si è proceduto ad inl,,'"randil"e fot.ograficamente (5 volte) il t.rattù di pellicola contenente l'inizio (iella regist.razione per una durata. di ein~a 4 secondi, periodo nel quale sono comprese le oihle di massima ampiezza. dato che non si disponeva di una attrezzatura aut.omatica, su queste riproduzioni fotogranche si sono lette le quote dei singoli punti della t.l"aceia., a(1 intervalli di un millimet.ro, cerelon(lo di apprezzare il decimo di millimetro. i,'intervallo scelto sull'asse dei t.empi corrisponde ilcirca 1/80 (li sccondo, il che ci consente (ii rilevare componenti di frequenza inferiore a. 40 hz. si è const.at.at() che un lettore sufficientemente esercitato può portare a termine l'analisi dei 300 punt.i che compongono l'aecelerogramma in meno di mezz'ora. successivamente i dati sono stati riporta.ti su sehe(le perforate. le quote dci punti sono st.,ot.e rilevate rispetto ad una linea. di zero arhitraria, pertanto occorreva t.radurle in ,oceelerazioni vere, come se queste fossero misurate rispetto alia. posizione di riposo (iella traecia. questo è un prohlema fondament,ole nell'analisi degli aceelerog"l·ammi. si è subito scart.a.t.o il metodo di ricavare (iirettmnente dall'accele· rogramma ho line,o di zero, dato ehe quest() port.a ad errori dello stesso ordine di grandezza delle oscillazioni che si intendono misumre. si ti scelto perciò il met.()(lo analitico dei minimi quadrati. in pl"atiea il metodo consiste nel det.erminare, rispet.to al retieolo usato come riferimento, l'eqmozione (leua retta che rende minima la somma (lei quadrati studio degli accelero grammi ottenuti nei recenti eve:l<ti sismici 49\l (legli scarti tra i punti sperimentali e la, retta cercata. l'intero calcolo vienc rapidamentc svolto daifelaboratore elettronico, e quirhli i valori che interessano vengono (letermimlti per differenza (1 alla retta in questione e moltiplicati per l'opportuno fattorc di scllla. col metodo descritto si compensa automaticamente l'errore che il lettore compie nel valutare il pa,rallelismo tra il reticolo e la linea (li fe(le, errore che, come ri i~ potuto ('onstatare successivamente, i~ dell'or(line di due o tre decimi di millimetro sull'intero accelerogramma. 2. nf.telnrni'i"azione delle grai'i"dezze fondalifei'i"tali det.la scossa. il passo successivo delle nostre elaborazioni consiste nel determinarc l'accelerazione massimll e la potcnzll (lei terremoto. quest'ultima" definita come " p = j a!(t) dt " dove a(t) !ì l'accelerazione istantanea, viene in realtà calcolata come somma dei quadrati delle acccierazioni ottenute punto per punto (vedi paragrafo 1). i valori di queste gran(lezze sono stati riportati nella 'l'abell'l i e messi ll confronto con la magnitudo (determinata, con gli strumenti campione dcll'osscrvatolio sismico di monteporzio oatone) e l'intensità ma.crosismica rileva,ta, sul luogo. tabella i terremutu .l[ 1. c j ! amax p ulllpunen e ((:ill/~ee:) (r:m2/~e('~) --mignanu 21/11/70 3.27 v-vi 8e.nw 16,8 108,7 ne-sw 14,8 77,9 mignanu 26/1l/70 3,15 v se.n'v 9,3 24,3 ne·s\v 10,5 36,5 mignanu o.' 1/71 3,20 v ne-s'" 10,0 31,8 se-nw 11,5 22,7 valfabbrica 14/2/71 4,00 vi·vii ne·sw i 78,9 914,7 se·nw 49,2 361,4 ,j()() r. co"lsoi.e f. pero}o;aci si può notare una maggiore uniformità. tra i valori relativi alle due componenti delle registrazioni efiettuate a mignano rispetto a (juf'lla ellettuata a valfabbrica, dove f.' evidente una polarizzazione delle onde (fig. 1). n' • . ~i\ ,. ~/v . ... i· , valfabbrica 14-2-1871 ____ ,;,;,;,; .......... ;,;";;;.;.;. ___ "",,. , .. "". ,,""" ""li" j •• ~ . f ig. j 3. t'3pettri hl risposta. seguendo la teoria clahsica ùegli spettri di ris}losta, descritta in una }lrecedente memoria, abbiamo elaborato gli spettri per ('iascuna delle otto componenti a nostra disposizione .. allo scopo di risparmiare il tempo impiegato sul calcolatore abbiamo scelto un incremento ùel }wl'ioùo l' non costante, ùiraùando i punti nella zona meno significativa, ed abbiamo ripetuto il calcolo per tre valori ùello flmorzamento. le figure 2·9 si riferiscono agli spettri disegnati ùa un ~ }llotter ~ collegato a.ll'elaboratore elettronico cile ha eseguito i calcoli. le leggere ùifierenze che si possono riscontrare tra i primi quattro ùiagrammi e quelli ripol'tat,i nell'articolo citato sono dovute ad un miglioramento nella determinazione della linea ùi zero. prima di ottenere i valori medi ùello spettro di risposta occorre proceùere alla normalizzazione delle curve, cioè riportarle a valori uguali ~tudio degli accelerogra~nii otn;~utl n}~l heces'l'1 };vkk'l'1 5h!mici 501 }lei' le intensità. a tale si'opo llefìniamo l'intensità spettrale come l'area contenuta dalla curva dello spettro di velocità fra i periodi 0,1 sec e 2,:3 8ec: 2.5 si. r s, {h, ti fft . ./ il.1 applicando tale formula otteniamo i risultati llclla 'l'ahella ii. tahelh~ il terremoto cumpunente i 8 10 8 i o.l sio.~ _ .. -----:\!ignllnu 21i] 1.170 se·~w 4,233 2,753 2,301 ke·::;w 4,240 2,735 2,18u :\!ignano 2el/ii/70 ~e·~w 2,877 1,957 1,61 u ke·::;w 2,874 2,086 1,804 :\!ignano eli 1/71 x}:'~w 2,292 1,532 1,243 he ·nw 2,445 1,668 1,480 valfahhrj('ll14/2/71 ~e · sw 6,994 d,822 5,556 :·m .~\\, 3,237 2,538 2,4{i3 dividenllo le o1'!linate llelle curve relative a ogni componente per la propria intensità spettrale, e moltiplicandole }lei' l'intensità della prima, ne otteniamo i valori normalizz a ti. nell'eseguire i calcoli a bbiamo distinto la media dei soli dati di ?iignano (fig. lo) da quella generale (fig. 11). dallo spettro mellio di valocità, melliante la formula s 2n ~ .. = ....,., uv, " otteniamo quello dell'accelerazione (figg. 12 e 13). j{. coxsou; f. p};ronacj w • 3 z c , -n o z < z o -, < v l/ ~ • , > e in " • • • \ i ~ -. ti c " <:: n -• ~ ~ "', • n c n • c • c c c studio degli accelerogra~nli ottenuti nei recenti eventi sis~iici 503 / \ \ • id w z o , --n z < z o • o " ( -• , > e u> o • • • l"\' ~ / v_ o <p • n -• • , o ~ "'\ o o o • o o o il. cossole f'. p t: ros,l.c i w • ~ z o • -• n o z < z o • ~ • , > e cn u • • • \ ( i ~ ~ ~ i". n o n " o ", n o --~ f ~ n o .; / " ~ .; ( ~ o o • studio degli a ccelerogh a. mmi otte~uti ~ei recenti eventi .'! is~f1ci 1105 -• ~ • • w z o • " o z ~ < z o /' , o ;,\ n " < o ì ~ i) .. < ~ -> e c.,\ <il ·u • • o • ,. o n o • o o h. cosso u : t ' . pt: hos aci ~ , , l • w z -, -o w o o z < z o • l j • , > , <j) '" • • • -• -.... o n " o " < \. ( /, c: 1 o • n o • o < studio degu accelerogr .nnll otte');"uti nei recenti eve');"ti sis~iici 50, . ~ -'" .... • / • z \ , -o • o z in < z " o " -, v i~ 15 k! • , ~ > e "' u • • • " • • " o o .~ .. " 5 (" ,,) • • , r\ h :;o '(\ , j 7o o o., valfab rica " 02·7 1 --j o., lo l"ig. 8 n ·sw l' t ( s) 8 ., .• o " " , stulho 1»,:0 1.1 acc }: u:ltog italuil ottl ;suti sp.i itf.c):s ti evf.sti !!is ~tl c i liod ~ .!) .... " • 3 z " " o • < u -• • < • " < > ~ h'-" l>2 ~ ~ ~ • , • e u> u • • • • • " • " • • 5 10 il . co~ sou: t'. l't:hosa ci , • , • • , • -, , • ~ , > e 1<il ~ • • -• -.... / t n 0\ o ~'3 k--" ~~ n o • n o n o • o o o studlo degl accelerogr .... 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('(j:\ulusio::'\i. ]}allalì.~i (lei primi aecelerognnnmi m/.{istrati dalla rete nazionale ila ('onf!entito di p['oyam .~u dati muli i metodi di elabol":lzione pil'[ adatti per ottenere gli sih'!tri di rispo~ta. l'nso del calcolatore ekttl"ollieo si (', dillllmt.mto idoneo a (inestu s('opo e, nonostante ~i fo.~se partiti d[l,ll'idea di fare (ielle f!emplil"i prove, si è giù pervenuti a dei risllltati eon ('l"pti sullo sijettl"o di l"isposta medio ilella 7.olla di mignano :uontelungo. :-lu ijlm~te hasi possiamo rit.enere che il fnnzionamento della rete aeeelerolllptl"iea del )linistel"o dei 11avori l'ubbliei, in l'orso (ii installazione, forninì risultati di ilotevole importall7.a per il perfezionamento delk ilorlll(' antisismiche. hl h li ()(} ll\ 1>'1 a co~~ole n., l'ero"xm~[ f., so"xmh.ia a., 1-'('"/10"11'"11 i si81/1ici ti jligl/th/o montdungo, "ann. di gcofi~i("u. ", xxiv, i, 1971. noi.is~er g, \v., sprrtt1.!1/1 _ii/(ll!!sis 01 sirollg-]lolioll earlhqlwkl's. 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", 43, ul53. layout 6 1 annals of geophysics, 62, 2, vo231, 2019; doi: 10.4401/ag-8049 “classification of mount etna (italy) volcanic activity by machine learning approaches„ alireza hajian1, flavio cannavò2, filippo greco2, giuseppe nunnari*,3 (1) department of physics, najafabad branch, islamic azad university, najafabad, iran (2) istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, catania, italy (3) dipartimento di ingegneria elettrica elettronica e informatica, università di catania, catania, italy 1. introduction one of the key challenge to modern volcanology is to identify and characterize volcano activity, based on parameters recorded by the monitoring network that might be useful for hazard assessment and risk mitigation. this topic has maximum priority especially for volcanoes located close to densely urbanized areas, such as mount etna volcano, italy. mount etna is nowadays one of the best-monitored volcanoes worldwide. a wide data set ranging from seismic, geodetic, gravimetry, geochemical, video etc., most of them collected in real time, were able to set strict constraints on the timing of the paroxysmal events which occurred in the last decade at etna volcano [i.e. aloisi et al., 2018; bonaccorso et al., 2011; greco et al., 2016]. changes in the state of activity as well as in the eruptive style of the volcano can be recognized in the geophysical and geochemical data collected by this monitoring network. the problem is that any change in these parameters does not especially imply a significant change in volcanic activity. hence, the characterization of volcanic activity based only on one geophysical and/or geochemical parameter, may be lead to ambiguous forecasts. one solution to this problem is through using a combination of different parameters to reduce the level of ambiguity and to enhance the quality of interpretations of the volcano activity. article history receveid december 5, 2018; accepted january 24, 2019. subject classification: decision tree; k-nearest-neighbours; volcano monitoring. abstract assessment of the ongoing activity of volcanoes is one of the key factors to reduce volcanic risks. in this paper, two machine learning (ml) approaches are presented to classify volcanic activity using multivariate geophysical data, namely the decision tree (dt) and k-nearest neighbours (knn). the models were implemented using a data set recorded at mount etna (italy), in the period 01 january 2011 – 31 december 2015, encompassing lava fountain events and intense strombolian activity. here a data set consisting of five geophysical features, namely the root-mean-square of seismic tremor (rms) and its source depth, counts of clustered infrasonic events, radar rms backscattering power and tilt derivative, was considered. model performances were assessed by using a set of statistical indices commonly considered for classification approaches. results show that between the investigated approaches the dt model is the most appropriate for classification of volcano activity and is suitable for early warning systems applications. furthermore, the comparison with a different classifier approach, reported in literature, based on bayesian network (bn), is performed. volcano monitoring aims at the recognition of changes in observable parameters before hazardous activity develops in order to alert the civil defence authorities. however, up to now, the lack of models for automatically recognizing the significance of changes in multiparametric real-time data, makes their integration difficult (or in many cases impossible). using expert tools for real-time volcano monitoring can be helpful in enhancing the quality of decision-making processes in order to reduce the volcano risk [cannavò et al., 2017]. in order to integrate geophysical parameters there are some statistical methods in which a pre-assumption about the model is applied and attempts are made to fit the data to that model; this means that they are modelbased. the other new solution is through using supervised intelligent methods, which can learn from experimental data without any pre-assumption about the model. in the last decades, there has been an increasing tendency to use intelligent methods in geophysical volcanology studies, especially for monitoring the volcano activity. for instance, langer et al. [2009] used support vector machine (svm) and multi-layer perceptron (mlp) as pattern classifiers for volcanic tremor data, which is extremely useful for monitoring volcanic activity at any moment and in whatever condition. they investigated the tremor features and their relationship to regimes of volcanic activity. messina and langer [2011] combined various unsupervised pattern recognition techniques, self-organizing maps, classic k-means cluster analysis and fuzzy c-means clustering for pattern recognition of volcanic tremor data on mount etna. brancato et al. [2016] used different kinds of artificial neural networks (anns) for pattern recognition of etna flank eruption forecasting. malfante et al. [2017] used machine learning for automatic classification of volcano-seismic signatures and presented an automatic classification of volcano-seismic events, based on a comprehensive signal representation with a large feature set. cannavò et al. [2017] proposed a multivariate probabilistic graphical model for real-time volcano monitoring on mount etna, based on an extension of a bayesian network (bn). the resulting probabilistic model was able to encoding the conditional dependencies between measurements and volcanic states. in this paper, we compare two different ml approaches, namely the decision tree (dt) and the k-nearest-neighbour (knn) algorithms, in order to find a classification rule capable of mapping a set of five real-valued geophysical features against observed volcanic activity, represented in categorical form. the data set considered is the one used by cannavò et al. [2017], so that the performances will be easily compared. we focus on the relationships between geophysical parameters patterns and the volcano status and discuss the methodological differences among these classifiers and their results. the paper is organized as follows: in section 2, the problem is stated and the volcanological framework, in which the data set was recorded, is described. in section 3, the methods considered for the problem solution are presented. in section 4, we describe how the data set is organized for training the classifiers and how the model performances will be assessed. in section 5, numerical results are presented and the dt and knn approaches compared. the selected classifier will be finally compared with the bn model proposed by cannavò et al. [2017]. in section 6, the capabilities of the proposed classifier will be illustrated analysing selected episodes occurred on mount etna during 2011-2015. finally, in section 7, the conclusions are drawn. 2. problem statement the main purpose of this paper is to implement a classifier capable of recognizing different phases of the volcanic activity which occurred at mount etna between 01 january 2011 and 31 december 2015, through a set of geophysical parameters (features) recorded in this area by the multidisciplinary monitoring network (figure 1). formally, the input-output representation of the classifier, that we aim to identify, can be expressed as follows: y(t) = f(x(t)). (1) where: y(t) indicates the categorical model output at time t; x(t) is the real-valued vector of features measured at the same time; f is a map representing the unknown classification rule. in a similar manner to what was done by cannavò et al. [2017], the volcano activity is expressed by a set of three different digit classes g = {0,1,2}, where 0 stands for no activity, 1 for strombolian activity and 2 for paroxysmal explosive activity, while the feature vector x consists of geophysical data coming from the hajian et al. 2 monitoring networks. here, with the term strombolian activity we intend burst frequency lower than a fraction of minutes and magma jets usually not exceeding tens to hundreds meters and weak effusive activity. moreover, with the term paroxysmal events, we intend lava fountains and strong strombolian eruptions, characterized by high burst frequency, up to a few per second, and magma jets up to several hundreds of meters, usually accompanied by the emission of lava flows [andronico et al., 2013]. 2.1. volcanological framework and features description after the flank eruption of 2008-2009, lava fountain events and intense strombolian activity, have characterized activity at mount etna. between january 2011 and december 2013, a sequence of 44 lava fountain events, characterized by powerful gas emission, together with ejection of lava fragments to heights ranging from tens to hundreds of meters, occurred from a pit on the east flank of the south-east crater (sec), one of the four summit craters of etna, as well as strombolian activity and intra-crater lava flow emissions at bocca nuova crater (bnc) and voragine (vor). the location of such craters and the area where the measuring stations are installed is shown in figure 1. the main explosive activity built a new summit cone, the new south-east crater (nsec) [behncke et al., 2014]. no major flank eruptions have accompanied this activity. generally, all the 2011-2013 lava fountain episodes from the nsec showed similar main characteristics, with the height of the lava fountain reaching 300-1000 m, ash columns reaching 5-9 km and associated lava flows 4-6 km long, descending the eastern flank of the volcano. the average total density rock equivalent (dre) volume of magma emitted during each of the nsec lava fountains, including both pyroclastic products and lava flows, was about 2.5 × 106m3 per event [de beni et al. 2015]. after the 2011-2013 nsec lava fountains, the eruptive activity switched to moderate lava effusion from the nsec toward ne during january-april and july-august 2014. on 28 december 2014, a further lava fountain episode occurred at the nsec [gambino et al., 2016]. as mentioned in the previous section, for the purpose of the present work, the activity of the etna volcano has been categorized by using three different classifications: “0” to indicate quiet, i.e. no relevant volcanic activity or unknown activity, characterized by low amplitudes of all considered features; “1” to indicate strombolian activity, i.e. mildly explosive activity, characterized by medium amplitude of seismic tremor rms, shallower source of the seismic tremor, clustered infrasonic events, no eruption column but possible ash emission detectable by the doppler radar; “2” to indicate paroxysm, i.e. an energetic activity with lava fountains, characterized by high amplitude of seismic tremor rms, shallower source of the seismic tremor, eruption column formation and ash emission detectable by the doppler radar. for consistency, the observed volcanic activity and the corresponding features were re-sampled with a 10 min sampling rate. the main reasons for choosing the five features listed in table 1 are: (i) they are available in real time at a rate compatible with early warning purposes; (ii) they are usually sensitive to etna volcanic activity; (iii) they roughly represent a summary of all available information from the monitoring networks. in figure 2, the time series of activity at mount etna during 01 january 2011 – 31 december 2015, together with the five features used are shown. we select this period because mount etna exhibited an exceptional variety of explosive activity from strombolian to lava fountain events. follows, a description of the five features used: the feature x1 (weighted normalized median of seismic tremor; rms) is based on seismic rms amplitudes which has been routinely used at istituto nazionale di geofisica e vulcanologia (ingv), osservatorio etneo, 3 classification of volcanic activity at mt etna figure 1. location map of the seismic, acoustic, and tiltmeter stations used in this research. the position of the doppler radar (voldorad 2b) is also shown. the yellow circles indicate the stations destroyed during paroxysmal episodes on 28 february and 11 november 2013 [after cannavò et al., 2017]. hajian et al. 4 for real-time monitoring [cannata et al., 2013]. this feature was calculated on the vertical component of the signal within 10 min long time windows, recorded by 19 stations, belonging to the permanent seismic network (sampling rate of 100 hz). the signal was filtered in the band 0.5 − 5.5 hz, which generally includes the relevant volcanic tremor information. in order to reduce the dimensionality of the model, we averaged the normalized rms time series calculated for each station as performed by [cannavò et al., 2017]; the feature x2 (normalized volcanic tremor source depth) was obtained by a grid search method, based on a spatial seismic amplitude distribution and assuming the propagation in a homogeneous medium, within 30 min long time windows. the use of relatively long time windows is necessary to reduce the effects of transient events (such as long period events, volcano-tectonic earthquakes, and regional earthquakes) on the volcanic tremor locations [cannavò et al., 2017; and references therein]; the feature x3 (normalized number of infrasonic events) was obtained by locating about 160.000 infrasonic events, recorded by the permanent infrasonic network during 2011-2015; the feature x4 (normalized radar rms) was obtained by processing the total radar echo backscattered by particles in the atmosphere crossing the beam of the 1.274 ghz doppler radar, called voldorad 2b and installed in the upper southern flank of the volcano (2600 m a.s.l.). this signal is dominated by reflections from particles, and hence it is sensitive to the formation of eruptive columns; it can be used to detect abundant pyroclastic particles in atmosphere crossing the radar beam [donnadieu et al., 2016]. in particular, periods of intense explosive activity producing abundant ash, are characterized by sharp increases in the considered time series; the feature x5 (normalized tilt derivative) was obtained by using the data recorded by the tiltmeter station referred to a cdb (figure 1), installed at 10 m depth and equipped with a high resolution (< 0.005 µrad) tiltmeter [ferro et al., 2011]. since tilt data is affected by several noise sources (e.g., temperature, earth tides, local earthquakes, and teleseism), we considered its derivative as input to the model. overall, the parameters related to volcanic tremor, infrasonic events, and ground deformation are sensitive to explosive activity, while the radar signal is sensitive to the development of sustained ash-rich eruption columns. since the radar signal and infrasound are not affected by pure effusive activity, and because lava effusion during the considered period was always accompanied by explosive activity with different intensities, we avoided modelling a state of pure effusive activity and included it in the eruption states. table 1 shows the summary of the input/output of the model classifier. 3. methods in this section, a short description of considered classification approaches is provided. 3.1 decision learning tree decision learning trees are popular ml algorithms belonging to the class of supervised learning algorithms. the main motivations to use decision tree (dt) is that they mimic the human level thinking and, differing from svm and nn, are not black box models. another reason for choosing these algorithms is because they are less time consuming with respect to other tested clustering approaches. in order to illustrate how a dt algorithm works, we report in figure 3 an attempt to classify the volcanic activity based on three features, namely the average seismic rms amplitude, the radar signal and the tilt derivative. the classification tree was obtained training the model with a small number of splits (i.e. the number of levels of the tree), in order to obtain a rough, but easrepresentation description x1 weighted normalized median of seismic tremor rms (a.u.) x2 normalized seismic tremor source depth (a.u.) x3 normalized number of distances of infrasonic events (a.u.) x4 normalized radar backscatter power at montagnola station (a.u.) x5 normalized tilt derivative (a.u.) y state of the volcano activity (a.u.) table 1. the machine learner inputs and output variables with related descriptions. au standing for arbitrary unit. 5 classification of volcanic activity at mt etna ily, interpretable classification rule. in this example, the classification algorithm gives a prominent role to the rms feature, which is thus chosen as the root of the tree. visiting the tree start from the root; in each node of the tree a binary condition is indicated by which it is possible to make the tree descent, until a leaf is reached. thus, in this example, the class 2 will be assigned to a set of features which satisfies the condition rms ≥ 1.44219 and depth ≥ 1697.37 and radar ≥ 1.94432. however, it is trivial to understand that the interpretability of the classification rule can be done only for trees with a limited number of levels and features. in this paper, classification trees will be referred to as fine, when the number of splits is of the order of hundreds, medium, when the number of splits is of the order of tens and coarse, when this number is of the order of units. 3.2 k-nearest-neighbours the k-nearest-neighbours (knn) method uses observations in the training set closest to x to form y. specifically, the k-nearest-neighbour fits for y defined as (2) where nk(x) is the neighbourhood of x, defined as the k closest point x1 in the training sample. closeness implies the use of a metric, which in the simplest case can be the traditional euclidean distance. however, several alternatives are possible, such as the chebshev distance or the minkowsky distance just to mention a few. of course, the performance of a k-nearest-neighbours classifier depends on the number k of elements in the nk(x) neighbour. similar to the tree classifiers, knn can be referred to as fine, medium or coarse depending on wheter k is of the order of units, tens or hundreds, respectively. figure 2. from top to bottom: a) time series showing the state of mount etna during 01 january 2011 – 31 december 2015; b) seismic rms amplitudes; c) depth of volcanic tremor source centroid; d) number of distances of spatially clustered infrasonic events; e) signal obtained by the doppler radar voldorad 2b; f) tilt derivative of the signal recorded by the cbd station [after cannavò et al. 2017]. figure 3. example to classify the volcanic activity in terms of three features: average seismic rms amplitude (rms); radar signal (radar); tilt derivative (tilt). 4. training of the classification models and performance assessment in order to identify the unknown learning rule (see equation 1), described in section (2), the available data set, which spans over five years, was organized into two subsets: four years of data was considered for training the classifier while the remaining one year was considered for the test. however, in order to avoid bias of the model error, the testing year was rotated using the leave-one-out approach, described in figure 4. in the figure, it is schematically indicated that the model error e was averaged over five iterations. during each iteration, one year of the overall data set was left out from the training set and considered only for the testing. the performance indices to evaluate the error model e, are described in the next section (4.1). 4.1 classification performance metrics in order to assess the performances of a classifier, different metrics can be considered. a popular metric consists in computing a set of indices referred to as the true positive rate (tpr), true negative rate (tnr), false positive rate (fpr), false negative rate (fnr), accuracy and so on. another example of metrics refers to the evaluation of the precision, the specificity, and the fall-out etc. it is obvious that the choice of metrics influences how the performance of a ml algorithm is measured and compared. in this paper we will refer to the former metrics mentioned above. given a classification experiment, let us indicate as p and n the number of actual positive and actual negative cases for a given class, respectively, and as tp, tn, fp and fn the number of true positive, true negative, false positive and false negative cases, respectively, recognized by the classifier, for the considered class. thus, the following rates are defined: (3) (4) (5) (6) the meaning of the above indices can be expressed as follows: the tpr expresses the proportion of actual positives that are correctly classified by the model as belonging to a given class. best values of tpr approaches to 1, while in the worst case tpr approaches to 0. the tpr is referred in literature also as specificity or recall; the tnr expresses the proportion of actual negatives that are correctly classified as not belonging to a given class. as for the tpr, best values of tnr approaches 1, while worst values approaches 0. the tnr is referred to also as sensitivity or selectivity; the fnr expresses the proportion of false negatives in a given class with respect to all actual positives in the same class. of course, in the best case fnr approaches 0, while in the worst case approaches 1; the fpr expresses the proportion of false positives in a given class with respect to the total number of actual negatives in the same class. similar to the fnr in the best case fnr approaches 0, while in the worst case approaches 1. together with the four indices above, we also consider the index referred as f1, defined as: (7) which represents a harmonic mean of precision (p) and recall (r). it is trivial to understand that f1 approaches 1 in the best case and 0 in the worst case. 5. numerical results as mentioned in the section 2, in this study two kinds of classifiers were considered, namely the dts and the knns. in more detail, as a first stage, for each kind of hajian et al. 6 figure 4. schematic of the leave-one-out cross validation method used in this study. classifier different configurations were considered, meaning that coarse, medium and fine classifiers were trained, in order to select the most appropriate for the application. results obtained considering the tpr and the tnr for each class and for each configuration are reported in tables 2 and 3. looking at these tables, it is possible to infer that the fine tree classifier is the optimum among the dt classifiers, and similarly, the fine knn is the optimum among the tested knns, particularly for the strombolian and paroxysmal events. a direct comparison between the fine dt and fine knn is shown in figure 5 which shows that the fine dt classifier outperforms the fine knn for all the classes and indices (from here, in this paper, the fine dt and the fine knn will be referred to as fdt and fknn, respectively). in particular, the sensitivity of the fdt (figure 5a) alarm is higher than that of fknn especially for paroxysmal activity (class 2). the fpr (fall out) value for strombolian class (figure 5c) in fknn is higher than that of the fdt. when the fall out is high for strombolian class, it means that most of the time when the alarm is ’on’, the volcano state is quiet or in paroxysmal state. the tnr (specificity) of fdt and fknn (figure 5b) are very close to each other for the strombolian and paroxysmal classes, but for the quiet class, the tnr (specificity) of fdt is higher than the fknn. it is necessary to mention that the fnr index was zero for the class 2, for both the fdt and fknn (figure 5d). the evaluation metric we investigated for the competition between fdt and fknn was the f1 score, as defined in (equation 6). the comparison between the fdt and fknn in terms of the f1 index is shown in figure 6. as it can be seen the f1 score, i.e. the harmonic mean of precision and recall, for the fdt is higher than that of fknn for all activity classes. thus, we selected the fdt as the optimum between the two compared approaches. the performance of the selected classifier is summarized in table 4, in terms of a confusion matrix. the confusion matrix for continuous estimation of the 7 classification of volcanic activity at mt etna activity quiet strombolian paroxysm type of dt fine medium coarse fine medium coarse fine medium coarse tpr 0.8703 0.8792 0.8445 0.6471 0.5083 0.3054 0.5878 0.3104 0.1456 tnr 0.6782 0.4456 0.4102 0.8701 0.6587 0.7645 0.9989 0.7915 0.8432 activity quiet strombolian paroxysm type of knn fine medium coarse fine medium coarse fine medium coarse tpr 0.8370 0.8616 0.8551 0.3361 0.2031 0.3309 0.5240 0.2310 0.3985 tnr 0.3569 0.1931 0.2210 0.8372 0.5764 0.66424 0.9984 0.6654 0.7361 table 2. sensitivity (tpr) and specificity (tnr) for different types of tested tree classifiers. table 3. sensitivity (tpr) and specificity (tnr) for different types of tested knn classifiers. figure 5. a direct comparison between fdt and fknn. ongoing volcano state shows in its main diagonal the percentage of events, correctly recognized in each class. the table also shows that a percentage of 12.94% of actual quiet events (class 0), was recognized by the classifier as strombolian activity, while none event of this class was attributed to class 2. a percentage of 25.69% of events belonging to the strombolian class, were attributed to the class 0. finally, a percentage of 1.06% events actually belonging to the strombolian class, were incorrectly attributed to the paroxysm (class 2). the most relevant drawback of the fdt classifier is that a high percentage (74.07%) of events actually belonging to the paroxysm (class 2), was attributed to class 1 (strombolian). referring to the strombolian class, the fdt estimated no activity (quiet state) instead of the correct state for 25.69% of events which is, nevertheless, better than the bn model (about 50%) reported in cannavò et al. [2017]. the main reason that 25.69% of the strombolian class events are confused as quiet state, is due to a few but long periods of weak strombolian activity that is not distinctly reflected in any input variables. 6. evaluation of the fdt classifier for selected episodes to test the performance of the fdt classifier, to track the actual episodes, we compared the output model time series with the true one. as a sample, the fdt result for the episodes between 30 july 2011 and 31 july 2011 is shown in figure 7. this example shows a delay between the predicted state of the model and the actual state of the volcano. this means that the alarm is on with a delay after starting of the strombolian event. conversely, as it can be seen in figure 7, the alarm for the paroxysmal event is on time. in order to calculate the performance of the model for paroxysmal events detection, we changed the model into a binary classifier, which means ’0’ for both quiet and strombolian and ‘1’ for paroxysmal events. the performance indices of this kind of classifier (binary) were calculated for the considered period 2011-2015, based on the leave-one-out cross validation method described in section 4.1. between 19 and 24 february 2013, a series of five compound volcano events (both strombolian and paroxysm; labelled each with a tag number) occurred, as shown in figure 8. for each event, the volcano activity hajian et al. 8 table 4. confusion matrix of the fdt classifier. figure 6. a direct comparison between fdt and fknn in terms of the f1 index. figure 7. afdt classification estimation for the activity of etna between 30 july 2011 and 31 july 2011, note: class 0 as quiet, class 1 as strombolian and class 2 as paroxysm. fdt predicted quiet predicted strombolian predicted paroxysm actual quiet 87.06% 12.94% 0.0% actual strombolian 25.69% 73.25% 1.06% actual paroxysm 0.0% 74.07% 25.93% starts with a strombolian class and then leads to a paroxysmal event. in the events 1, 3 and 5 there is an advanced time (a time measure of the capability of model to predict an event before it happens). the advance time for events 1 and 5 are greater than that of event 3 (figure 8). conversely, the event 4 is predicted by the model with a delay. only two false alarms are detected. this example highlights the good ability of the fdt model to track all the events starting with strombolian activity and leading to paroxysmal event. in order to estimate the fdt classifier capability to detect the beginning of paroxysmal events, we averaged the advance time estimated over the whole data set. the comparison between the performances of fdt and bn proposed by cannavò et al. [2017], both in terms of advance time, tpr and fpr are reported in table 5. it is possible to see that these models performs similarly in terms of tpr and fpr but the fdt outperforms the bn in terms of advance time, which is especially relevant for early-warning alarm in order to alert the civil defence authorities. 7. discussion and conclusions the search for a reliable and quantifiable method of determining the onset of volcanic eruptions is a key challenge for volcanology and has major implications sociologically and political as well as geologically. machine learning methods make no assumptions about mechanisms behind geophysical and volcanological phenomena, but simply attempt to look for relationships between parameters collected in the volcanoes monitoring network to map these onto a set of outcomes, which are classes of the volcano state. in this paper, two categories of machine learning models were investigated for classification of mount etna activities, namely the dts and knns. the global 9 classification of volcanic activity at mt etna figure 8. fdt predicted class activity in a time window between 19 february 2013 and 24 february 2013. index fdt bn model tpr(%) 99.66 99.16 fpr(%) 21.34 23.21 advance time (min) 55.00 32.16 table 5. performance for the fdt working style as a binary classifier to detect paroxysm events calculated by leaveone-out cross validation, and the bayesian network (bn). statistical indices show that the fdt and the fknn are the models with higher performances, compared within their own categories (coarse and medium). furthermore, taking into account the f1 score as a measurement of the total performance of precision and recall, the fdt outperforms the fknn. experimental results show that the fdt model is very sensitive to the paroxysmal activity, being even able to detect in advance the beginning of this kind of activity. we have estimated that the mean advance time is about 55 minutes for the considered set of paroxysmal events. the advance time for events of class 1 (strombolian activity) has not be estimated, because, from early warning perspective, the tested models are not enough sensitive to the beginning of this weakly explosive activity. a comparison between the results obtained in this work is possible only with the ones in cannavò et al. [2017], since (to the best of our knowledge) other studies have not used an extensive set of features as in this work. the comparison with cannavò et al. [2017] showed that the performances among the dt and bn approaches are substantially similar: to a great capacity to detect paroxysms (with a positive rate above 99%) is opposed a not negligible percentage of false alarms (with fpr > 21%, about 1 out of 5 detected paroxsyms was false). this suggests that at this stage of the work the set of features used is more relevant rather than the applied techniques. therefore, in order to improve the classifier performance, we plan to add some new relevant features to the data set. in particular, we expect to consider the gravity signal, so2 emissions and magnetic data. in conclusion, we can say that fdt methods are promising classification algorithms to associate the categorical volcanic activity to a set of features measured in a given area. the use of such tools in real-time volcano surveillance could be helpful for decisionmaking processes aimed at reducing volcano risk. specifically, the tool could be useful both for civil defence authorities, who can evacuate on time the summit area of the volcano, when volcanic activity changes, and the aviation authorities in order to avoid problems with air traffic. acknowledgements. we are indebted to the authors of the paper cannavò et al. [2017] for enabling data used in this manuscript. special thanks are due to the najafabad branch, islamic azad university president and research deputy for supporting the activity of dott. alireza hajian during the sabbatical leave at ingv – osservatorio etneo. prof giuseppe nunnari wish to thank the università degli studi di catania for the financial support under the grant piano della ricerca 2016-2018, linea di intervento 2. we thank the associated editor dr c. del negro and the reviewers prof. c. bonadonna and prof. p. styles for their careful reading of our manuscript and their constructive comments and suggestions. references aloisi m., a. bonaccorso, f. cannavò f. and g.m. currenti (2018). coupled shortand medium-term geophysical signals at etna volcano: using deformation and strain to infer magmatic processes from 2009 to 2017, front. earth sci. 6:109, doi: 10.3389/feart.2018.00109. andronico, d., m.d. lo castro, m. sciotto and l. spina (2013). the 2010 ash emissions at the summit craters of mt etna: relationship with seismo-acoustic signals. j. geophys. res. solid earth, 118, 51–70, doi: 10.1029/2012jb009895 behncke, b., s. branca, r. a. corsaro, e. de beni, l. miraglia, and c. proietti (2014). the 2011–2012 summit activity of mount etna: birth, growth and products of the new se crater, j. volcanol. geotherm. res., 270, 10–21, doi:10.1016/j.jvolgeores.2013.11.012. bonaccorso, a., a. cannata, r. a., corsaro, g., di grazia, s., gambino, f., greco, l. miraglia, a., pistorio, a. 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of a shallow persistent magmatic reservoir from joint inversion of gravity and ground deformation data: the 25 – 26 october 2013 etna lava fountaining event, geophys. res. lett., 43, doi: 10.1002/2016gl068426. langer, h., s. falsaperla, m. masotti, r. campanini, s. spampinato and a. messina (2009). synopsis of supervised and unsupervised pattern classification techniques applied to volcanic tremor data at mt etna. italy, geophys. j. int., 178, 1132-1144. malfante, m., m. dalla mura, j. mars and j. p. metaxian (2017). machine learning for automatic classification of volcano-seismic signatures, 25th european signal processing conference, 2457-2461. messina, a. and h. langer (2011). pattern recognition of volcanic tremor data on mt. etna (italy) with kk analysis—a software program for unsupervised classification, computers & geosciences 37(7):953961, doi: 10.1016/j.cageo.2011.03.015. *corresponding author: giuseppe nunnari, dipartimento di ingegneria elettrica, elettronica e informatica università degli studi di catania catania, italy email: giuseppe.nunnari@dieei.unict.it © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 11 classification of volcanic activity at mt etna 439_449 spinetti.pdf 439 annals of geophysics, vol. 46, n. 2, april 2003 aerosol optical thickness of mt. etna volcanic plume retrieved by means of the airborne multispectral imaging spectrometer (mivis) claudia spinetti, maria fabrizia buongiorno, valerio lombardo and luca merucci istituto nazionale di geofi sica e vulcanologia, roma, italy abstract within the framework of the european mvrrs project (mitigation of volcanic risk by remote sensing techniques), in june 1997 an airborne campaign was organised on mt. etna to study different characteristics of the volcanic plume emitted by the summit craters in quiescent conditions. digital images were collected with the airborne multispectral imaging spectrometer (mivis), together with ground-based measurements. mivis images were used to calculate the aerosol optical thickness of the volcanic plume. for this purpose, an inversion algorithm was developed based on radiative transfer equations and applied to the upwelling radiance data measured by the sensor. this article presents the preliminary results from this inversion method. one image was selected following the criteria of concomitant atmospheric ground-based measurements necessary to model the atmosphere, plume centrality in the scene to analyse the largest plume area and cloudless conditions. the selected image was calibrated in radiance and geometrically corrected. the 6s (second simulation of the satellite signal in the solar spectrum) radiative transfer model was used to invert the radiative transfer equation and derive the aerosol optical thickness. the inversion procedure takes into account both the spectral albedo of the surface under the plume and the topographic effects on the refl ected radiance, due to the surface orientation and elevation. the result of the inversion procedure is the spatial distribution of the plume optical depth. an average value of 0.1 in the wavelength range 454-474 nm was found for the selected measurement day. mailing address: dr. claudia spinetti, istituto nazionale di geofi sica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: spinetti@ingv.it key words aerosol optical thickness volcanic plume mt. etna mivis dem radiative transfer model 6s inversion technique 1. introduction volcanic plumes are often the most visible indicators of volcanic activity. they are a turbulent mixture of solid particles, liquid droplets and gases exsolved from magma, emitted continuously from summit craters, fumarolic fi elds or during eruptive episodes. the direct sampling of volcanic plumes is diffi cult and often hazardous. the use of remote sensing techniques permits a fast, large-scale and above all safe measurement of the volcanic emissions to study the most variable components of volcanic plumes, such as aerosols. volcanic aerosols are defi ned as minute liquid and solid particles suspended in the atmosphere. they consist of solid sulphur-derived particles, aqueous droplets and volcanic ash; typical size is in the range 10–2 mm 10 mm (sparks et al., 1997). volcanic aerosols have important climatic and environmental effects. depending on the particle size distribution and on the number 440 claudia spinetti, maria fabrizia buongiorno, valerio lombardo and luca merucci concentration, sulphate aerosols are able to modify the atmospheric radiative field: they scatter the incoming short-wave solar radiation and absorb the outgoing long-wave radiation (fiocco et al., 1994). different volcanic activities, such as volcanic eruption, volcanic clouds and plumes, inject into the atmosphere volcanic aerosol with different altitudes, latitudes and residence times. explosive volcanoes, such as mt. pinatubo or alaskan volcanoes are able to inject large quantities of sulphate aerosol into the stratosphere with a residence time of several years. as several studies have pointed out (fiocco et al., 1994), large quantities of stratospheric aerosol increase the backscattering of the incoming solar radiation in the short-wave region; the result is the reduction of temperature at the earth’s surface (hansen et al., 1992). conversely, degassing volcanoes produce effusive plumes rich in sulphate aerosol (sparks et al., 1997) and inject them in the troposphere with residence times, variable from hours to weeks, depending on their altitude and geographical location (chin and jacob, 1996). tropospheric sulfate aerosols are destructive for the environment as they may cause acid rains and contaminate local water supplies; moreover they are hazardous for local human and animal populations (neubauer et al., 1996). mt. etna (sicily) is one of the world’s most actively degassing volcanoes (allard et al., 1991). it is a large strato-volcano with a summit height approximately of 3300 m a.s.l. and a circumference of about 200 km at ground level; it produces alkaline and basaltic lava during summit and fl ank eruptions (romano and sturiale, 1982). an important feature of its activity is the continuous and abundant noneruptive gas emissions from the summit craters. this degassing process produces an effusive plume rich in gases (h2o, co2, so2, hci, h2, h2s, hf, co, n2, ch4) (francis et al., 1998; mcclelland et al., 1989), and aerosols (ash and sulfates) (amman et al., 1992). the main gas components and mean fl uxes are shown in table i (jaeschke et al., 1982; symonds et al., 1994). in quiescent conditions, the so2 gases re leased by the magma, rise up in to the atmosphere. during this degassing, aqueous and gas phase oxidation of sulphur species and subsequent nucleation and accumulation of particles in droplets occurs. this process, depending on relative humidity, temperature and environmental conditions (watson and oppenheimer, 2000), results in the formation of liquid droplets composed of a solution of water and sulfuric acid known as sulfate aerosols. mt. etna is an ideal test site, given the copious emissions in the troposphere and the easy access compared to other volcanoes. volcanic plumes modify the signal measured by an airborne sensor since the incident solar radiation is scattered and absorbed by the molecular and particle plume components. the aerosol optical thickness t a is a geophysical parameter which expresses the influence of volcanic aerosol on direct radiation crossing the plume. this parameter is a function of wavelength according to the ångstrom equation. the two ångstrom coeffi cients give a rough idea of the particle size and of the total number concentration of aerosol particles, if their refractive index is known. this work gives a short presentation of the measurements collected during the mvrss «sicily 1997» campaign; then a theoretical overview of the inversion algorithm and the corresponding aod map are presented. 2. data set in the framework of the «mitigation of volcanic risk by remote sensing techniques» table i. main gas components of the etna plume, typical emission rates and the etna average fluxes compared to global volcanism (jaeschke et al., 1982; allard et al., 1991; symonds et al., 1994). gases etna fl uxes (t /d) etna to global volcanism percentages h2o 13 000 000 10% co2 27 000 7-16% so2 5500 11% hcl 140 9% 441 aerosol optical thickness of mt. etna volcanic plume retrieved by means of the airborne multispectral imaging spectrometer (mivis) (mvrss) project, funded by the european community, the «sicily 1997» fi eld measurements campaign was organized to acquire highresolution hyperspectral remote sensed images with the airborne «multispectral infrared and visible imaging spectrometer» (mivis) together with ground-based measurements (buongiorno et al., 1999). the main targets of the «sicily 1997» fi eld measurements campaign were: the analysis of plume gas components; the estimation of the so2 fl ux (important for the evaluation of volcanic activity (caltabiano et al., 1994); the spectral characterisation of geological surfaces; the determination of the thermal emissivity of different geological surfaces to study structural characteristics of volcanoes; the estimation of aerosol burden; the realisation of a plume dispersion model. the mivis instrument represents a second generation imaging spectrometer developed for environmental remote sensing studies (bianchi et al., 1994). it is a daedalus aa5000 electrooptical scanner with 102 spectral channels simultaneously sampled and recorded. the 4 spectrometers collect radiation refl ected by the surface in the visible and infrared spectral range (see table ii). the airborne campaign took place in the week from 11 to 17 june 1997, over etna, vulcano and stromboli. this period was chosen to meet suitable weather conditions since the solar irradiance is near to its yearly maximum, expected relative humidity is low and expected wind direction is n-w prevalently. these factors allow the acquisition of good quality images since the area under the plume is well illuminated and, especially in the early morning, the formation of orographic clouds is limited. indeed, in this period, the time weather conditions were as expected. the sky was cloudless throughout the week, except for 17th june. a temperature mean value of (12.5 ± 1.5)°c, a low relative humidity of 35% ± 5% (except for the 17th june when relative humidity was 42%), a pressure of (723 ± 3) hpa, a wind speed between 11 to 20 m/s and wind direction of about 300 degree north, was measured at torre del filosofo meteorological station (2920 m a.s.l. height) (buongiorno et al., 1999). during the campaign, the 3 observed volcanoes presented different activity levels. mt. etna was in a quiescent period since the 1996 eruption. it showed a major degassing activity in the n-e crater, a minor one in bocca nuova crater and same episodes of lava fountains in the s-e and la voragine craters. stromboli (924 m a.s.l. height) showed small eruptions of ashes and lava with a frequency of 2-3 episodes per hour. volcano (390 m a.s.l. height) presented fumarole fi elds and gas emissions located in the fossa crater. images of the etna plume were taken by 3 fl ight lines having a radial arrangement, one along the plume axis oriented on e-s-e direction and two along each side. most of the groundbased measurement sites were located on the south-eastern fl ank of the volcano, covered also by an additional fl ight line. figure 1 shows the mivis fl ight lines scheme over mt. etna. the platform altitude was about 6500 m a.s.l. and the corresponding ground resolution ranged from 4 to 13 m, depending on orography and aircraft altitude. table ii. mivis spectral bands characteristics (bianchi et al., 1994). optical port spectral range number of channels spectral resolution 1 (vis) 431-833 nm 20 20 nm 2 (nir) 1150-1550 m 8 50 nm 3 (swir) 1985-2479 m 64 9 nm 4 (tir) 8210-12 700 m 10 340-540 nm 442 claudia spinetti, maria fabrizia buongiorno, valerio lombardo and luca merucci different ground-based measurements, derived from atmospheric, radiometric and spectral data, were taken simultaneously to the fl ights, and are reported in table iii (buongiorno et al., 1999). 3. image processing to analyse the optical characteristics of the etna plume, a single digital image has been selected with the following criteria: i) the plume should be well contained in the image. ii) the signal-to-noise ratio should be large. iii) simultaneous ground-based measurements should be available (atmospheric photometric measurements and vertical profi le). iv) cloudless conditions should be present. the image satisfying these criteria was acquired the 16 june. the wind direction was 290 north degree while the aircraft trajectory was about 87 north degree. this imply a difference on same degree between image axis and plume axis. in fi g. 2 the selected image is showed. however, no ground-based photometric measurements across the plume were available for this fi eld measurements campaign. fig. 1. mvrss «sicily 1997» campaign mivis fl ight lines over mt. etna. solid lines: mivis fl ights over the volcanic plume (one along the plume axis and two on each side). dashed line: mivis fl ight over the groundbased measurement sites. table iii. mvrss «sicily 1997» campaign ground based measurements (buongiorno et al., 1999). ground-based measurements type of data use in the mivis data analysis atmospheric vertical profi les with on board balloon radiosonde. pressure, temperature, relative humidi ty, wind direction and speed. modelling of the atmospheric parameters. sun-photometric measurements with 2 sun-photometer located in different places. total atmospheric optical thickness, aerosol optical thickness and ångstrom turbidity parameters. modelling of the atmospheric optical characteristics. radiometric measurements with spectroradiometer. atmospheric radiance modelling of the atmospheric radiative contributes. meteorological station at torre del filosofo. pressure, temperature, relative humidity, solar radiation fl ux and wind direction and speed. modelling of the atmospheric parameters at the ground level. vis-ir spectra and rock sampling for laboratory analyses. spectra of volcanic rock, vegetation and terrain. ground truth for surface refl ectance 443 aerosol optical thickness of mt. etna volcanic plume retrieved by means of the airborne multispectral imaging spectrometer (mivis) 3.1. calibration the first step in image processing is the radiometric calibration of the raw data i.e. the conversion of the digital numbers (dn ) into radiance values, obtained by multiplying pixel by pixel each dn by the calibration factor (bianchi et al., 1994). the calibration equation for visible and infrared channels is the following: (3.1) i = 1,…,92 channels where r i is the radiance value, dni the measured raw data, dnref the reference body digital number, g the operational amplifi er gain (1, 10, 1000 values), a the attenuation factor (values between 0.4 and 1) and f i the calibration factor. the term f*g*a is constant for the entire fl ight line and for each channel (bianchi et al., 1994). the mivis calibration factors were measured with laboratory tests performed before and after the airborne campaign on an optical test bench by using a known radiance source refl ected by a panel of know refl ectance (lechi, 2000). the fi nal result of the radiometric calibration was independently checked using pixels of the calibrated image corresponding to a zone covered by snow situated in valle del bove. the snow albedo of selected pixels was calculated inverting the following radiative transfer equation (spinetti, 2000): (3.2) r* is the radiance measured by the sensor given in apparent radiance units ( ρ π µ ∗ = ⋅ ⋅ l f l s0 , is the radiance at the sensor, f0 is the extraterrestrial solar flux and m s = cosq s is the cosine of the solar zenith angle q s ); rsnow is the snow albedo; t is the atmospheric optical depth measured with sunphotometer; m v = cosq v is the cosine of the sensor view zenith angle q v . the obtained values in the visible range are in agreement with the snow refl ectance available in the bibliography (painter et al., 1996; salvatori et al., 1997). r dn dn f g a i i i i= − ⋅ ⋅ ref fig. 2. mivis image etna june 16th 1997 calibrated in radiance rgb composition. flight altitude 6000 m. pixel resolution ranges to 4 ¥ 4 m2 at summit craters to 11 ¥ 11 m2 at the sea level. ρ ρ τ µ µ∗ − +    = ⋅snow e s v 1 1 444 claudia spinetti, maria fabrizia buongiorno, valerio lombardo and luca merucci (4.1) where the apparent reflectance at the sensor, ρ π µ ∗ = ⋅ ⋅ l f s0 , (l is the radiance at the sensor, f0 is the extraterrestrial solar fl ux and m s = cosq s is the cosine of the solar zenith angle q s ); q v is the viewing angle; f s , f v are the solar and view azimuth angle, respectively; ρ ρ θ θ φ φa a s v s v= ( ), , , is the radiation scattered into the optical path by the atmosphere (atmospheric path refl ectance); t d (q s ) is the downward total transmittance in the path from top of the atmosphere (toa) to the surface (it can be analysed as sum of the direct and the diffuse transmission functions (sifakis and deschamps, 1992; gordon, 1997)); t u (q v ) is the upward total transmittance in the path from the ground to the sensor (sum of the direct and the diffuse transmission functions); r is the surface refl ectance; <r> is the environmental refl ectance; s the atmospheric spherical albedo. the presence of the volcanic plume modifi es eq. (4.1). each term takes into account the effects due to the different path of the radiation traversing or refl ected by the plume (spinetti, 2000) (see fi g. 3). it is important to note that the condition for validity of eq. (4.1) is the low refl ectance of the surface (kaufman et al., 1997). this condition is verifi ed for the selected image. then eq. (4.1) becomes (4.2) where ρ ρ θ θ φ φa a s v s v= ( ), , , is the atmospheric path refl ectance, ρ ρ θ θ φ φa p a p s v s v= ( ), , , is the radiation scattered into the optical path by the plume, , , 3.2. dem coregistration to the image the second step in image processing is the defi nition of the optical path geometry which depends on 3 parameters: target elevation, observer altitude and the viewing angle between the target and the observer (buongiorno et al., 2002). the target elevation was determined by use of a digital elevation model (dem) constructed starting from digital topographic maps at a scale 1:10 000 and using digital photogrammetric techniques. the observer altitude and the viewing angle depend on position and attitude of airborne platform, respectively. for each scan line the position of the sensor is estimated with onboard gps data and the aircraft attitude with onboard gyro and fl ux gate compass inertial references (bianchi et al., 1994). the viewing angle was determined taking into account the scanner geometry and the aircraft attitude. 4. methodology starting from the radiometrically calibrated image of the plume, a map of volcanic aerosol optical thickness was retrieved. the blue channel (454-474 nm wavelength range) was chosen among the mivis channels because it is mostly affected by the aerosol presence. since the aerosol optical thickness is a decreasing function of wavelength (according to the ångstrom equation). to derive the optical thickness, it was necessary to identify and estimate the various contributions that infl uenced the pixel-measured radiance of the digital image. 4.1. radiative transfer the method used to retrieve the aerosol optical thickness is based on the hypothesis of cloudless sky and plane parallel atmosphere. under these assumptions, radiance measured by a remote sensor in the atmosphere is given by the basic radiative transfer equation for monochromatic radiation and no surface emission (kaufman et al., 1997) ρ θ θ φ φ ρ ρ ρ∗ ( ) = + ⋅ − < > ⋅ +s v s v a d ut t s , , , 1 ρ ρ+ ⋅ − < > ⋅ < >d u t t s1 ρ ρ ρ ρ ρ ρ ρ ∗ = + + ⋅ ⋅ − < > ⋅ + < > ⋅ ⋅ − < > ⋅a a p u p d p u p d pt t s t t s ' ' 1 1 ρ ρ ρ ϑ ϑ' = − < > ⋅ ( ) ⋅ ( ) 1 s t td s u v < >= < > − < > ⋅ ( ) ⋅ ( )ρ ρ ρ ϑ ϑ' 1 s t td s u v 445 aerosol optical thickness of mt. etna volcanic plume retrieved by means of the airborne multispectral imaging spectrometer (mivis) fig. 3. scheme of different solar radiation pathways measured by a remote sensing instrument. 446 claudia spinetti, maria fabrizia buongiorno, valerio lombardo and luca merucci td p sθ( )and tup vθ( ) are the downward and the upward plume transmittance, respectively, function of volcanic aerosol optical thickness. in the single-scattering approximation ρa p the plume path refl ectance is given by (4.3) ρ ρ θ θ φ φνm p m p s s v= ( ), , , is the molecular scattering path refl ectance, w 0 single-scattering albedo, p(ys ) phase function and t a the volcanic aerosol optical thickness; m v and m s are cosines of the view and illumination directions, respectively. 4.2. inversion technique in order to invert the eq. (4.2) and extract the aerosol optical thickness t a, it is necessary to estimate the atmospheric terms for each pixel of the mivis image. to this aim, the 6s (second simulation of the satellite signal in the solar spectrum) radiative transfer code (vermonte et al., 1997) was used. the model was calibrated using atmospheric optical thickness measurement simultaneous with the selected image. the digital elevation model (dem) was used to take into account the elevation of each pixel for a correct estimation of atmospheric transmittance in the inversion procedure. moreover, the inversion procedure needed the knowledge of the surface albedo and the ground inclination of each pixel with respect to the sun and sensor relative positions. ground albedo was measured during the campaign (table iii): an average value of 0.05 for spectral basaltic lava in the wavelength range 454-474 nm was retrieved from fi eld measurements (fi g. 4). the ground inclination was obtained from the shadow digital model (shaded relief) using the dem and the geometrical factors of the source, such as solar zenith and azimuth angles. the inclination was interpolated from the elevation of each pixel of the dem and the altitudes of the nearest neighbours pixels (horn et al., 1989). the low refl ectance of basaltic lava permits to neglect the environmental term in the eq. (4.2). before inverting eq. (4.2), a digital mask was applied to the image in order to isolate the pixels belonging to the plume. ρ θ θ φ φ ρ ω ψ µ µνa p s s v m p s s v p , , ,( ) = + ⋅ ( ) +( ) ⋅0 4 τ µ τ µ a s a v exp⋅ − − +             1 fig. 4. volcanic rock refl ectance measured in laboratory and during mvrss «sicily 1997» campaign on mt. etna (1972 lava fl ow). refl ectance value of 0.05 in wavelength range 454-474 nm. 447 aerosol optical thickness of mt. etna volcanic plume retrieved by means of the airborne multispectral imaging spectrometer (mivis) 5. results and conclusions the map in fi g. 5 shows results of the inversion procedure, indicating the spatial distribution of volcanic aerosol optical thickness. the t a values range from a minimum of 0.1 in the far part of the plume, to a maximum of 0.3, close to the summit craters. this result cannot be validated by the direct optical measurements across the plume since they are not available. however, a comparison can be made with the sunphotometric measurements taken in torre del filosofo in october 1997 (oppenheimer, 1998) assuming similar aerosol burden in this two periods. this is a reasonable assumption because mt. etna had similar quiescent conditions in the june and in october 1997 as deduced by the international institute of volcanology observations (coltelli and del carlo, 1997). moreover, also the so2 fl ux shows similar values in these two periods as emerges from the monitoring of weekly measurements performed using cospec correlation spectrometer (caltabiano et al., 1994) on adapted vehicle and from fi xed observation sites (buongiorno et al., 1999). conversely, the time weather conditions for october 26th with respect to june 16th 1997 are different in temperature (about 12 °c lower) and in relative humidity (about 25% higher) while they are similar for the wind speed and direction. recent studies (watson et al., 2001) point out that meteorology play same role in the volcanic aerosol formation but it is not as signifi cant as the volcanic activity. figure 6 reports the retrieved t a range and shows a rather good agreement with sunphotometric measurements (oppenheimer, 1998). this work represents a preliminary and promising study to investigate the aerosol load in volcanic plume with remote sensing techniques. validity of this algorithm is for thin and tropospheric plume, such as the etna plume in quiescent condition (teggi et al., 1999). to investigate explosive and tropospheric volcanic plume, the inversion algorithm needs a different implementation taking into account the multiple scattering on the path radiance term in eq. (4.3). in the near future, it is foreseen to extract from digital images more information on the fig. 5. resulting map of volcanic plume aerosol optical thickness (t a ) in false colours. average value of spatial distribution t a = 0.1. 448 claudia spinetti, maria fabrizia buongiorno, valerio lombardo and luca merucci plume particles. for this purpose, t a will be calculated at different mivis visible channels. all these t a values could be used to derive particle optical characteristics, such as the ångstrom coeffi cients, and an estimation of particle size. however, the development and the validation of a remote sensing technique, as most of the inverse problems, requires input information on atmospheric and surface parameters, such as the meteorological and atmospheric parameters. it is important to measure these parameters simultaneously. the mvrrs project plans to install a ground-based photometer instrument that will record atmospheric and plume data continuously. these data will serve to develop and validate the remote sensing techniques. the future work will regard the development of a suitable methodology, combining remote sensing data, ground-based data and atmospheric modelling to systematically study the volcanic plumes characteristics. acknowledgements the work was developed during a thesis on physics, «la sapienza», university of rome. the work was carried out at ingv, partially funded by gnv and asi. the european community (mvrss project, contract n. env4-ct96-0288) funded the mivis 1997 campaign. we wish to thank particularly prof. g. fiocco for his helpful advice. we thank the reviewer for his thorough and helpful reviews. references allard, p., j. carbonelle, d. dajlevic, j. le bronec, p. morel, m.c. robe, j.m. maurenas, r.f. pierret, d. martins, j.c. sabroux and p. zettwoog (1991): eruptive and diffuse emission of co2 from mount etna, nature, 351, 387-391. amman, m., l. scherrer, w. mueller, h. burtscher and h.c. siegmann (1992): continuous monitoring of ultrafi ne aerosol emissions at mt. etna, geophys. res. lett., 19 (13), 1387-1390. fig. 6. solid lines indicate the t a measured by sun-photometer (october 26th, 1997) and t a modelled using aerosol code (oppenheimer, 1998). the rectangle indicates the t a range obtained from the inversion procedure of mivis data (june 16th, 1997). 449 aerosol optical thickness of mt. etna volcanic plume retrieved by means of the airborne multispectral imaging spectrometer (mivis) bianchi, r., c.m. marino and s. pignatti (1994): airborne hyperspectral remote sensing in italy, in proceedings of the european symposium on satellite remote sensing, rome, italy, september 26-30, 1994, europto series, spie, vol. 2318, 29-37. buongiorno, m.f., m.p. bogliolo, t. caltabiano, v. carrere, s. corradini, l. merucci, s. pugnaghi, s. salvi, a. sterni and s. teggi (1999): mvrrs campaign: mivis mission on sicilian volcanoes and ground measurements, quad. geofi s. 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(2000): caratterizzazione ottica di una piuma vulcanica mediante telerilevamento aereo, ph.thesis, dipartimento di fisica, università «la sapienza», roma. symonds, r.b., w.i. rose, g.j.s. bluth and t.m. gerlach (1994): volcanic-gas studies: methods, results, and applications, in volatiles in magmas, edited by m.r. carroll and j.r. holloway, mineral. soc. am., rev. mineral., 30, 1-66. teggi, s., s. pugnaghi, m.p. bogliolo, m.f. buongiorno and v.j. realmuto (1999): modelling local atmospheric effects by modtran-3 code and vertical profi le in the area of mt. etna volcano, sicily, j. atmos. solar terr. phys., 60, 1059-1070. vermonte, e., d. tanré, j.l. deuzé, m. herman and j.j. morcrette (1997): second simulation of the satellite signal in the solar spectrum (6s): user’s guide, technical report version 2, department of geography, university of maryland, c/o nasa-goddard space flight center – code 923, greenbelt, md 20771, usa, pp. 218. watson, i.m. and c. oppenheimer (2000): particle size distributions of mount etna’s aerosol plume constrained by sun photometr, j. geophys. res., 105 (d8), 9823-9829. watson, i.m. and c. oppenheimer (2001): photometric observations of mt. etna’s different aerosol plumes 2001, atmos. environ., 35 (21), 3561-3572. layout 6 1 annals of geophysics, 63, 6, se663, 2020; doi:104401/ag-8488 crustal seismology: traditions and styles from a historical perspective aitor anduaga basque museum of history of medicine and science, university of the basque country ehu/upv, 48940, leioa, spain ikerbasque, basque foundation for science, bilbao, basque country, spain article history: received may 8, 2020; accepted august 23, 2020 abstract it has often been said that geophysics is an umbrella discipline, and that its various and varied fields remained conceptually autonomous even when configured in the mind of a single scientist. however, to what extent were these fields conceptually autonomous? was there a single accumulation of geophysical knowledge and practices, or rather diverse traditions? furthermore, what happens when there is a confluence of traditions rather than an independent accumulation of knowledge? would it make sense then to talk about any conceptual autonomy and compartmentalized fields? this article examines the historical development of a geophysical specialization developed in multinational settings: crustal seismology. rather than a conglomerate of autonomous fields, the view of geophysics as an intercalated set of inter-disciplinary fields, research schools, programs, and traditions which seem to concur in the same direction, can be applied to a large extent to geophysics of the earth’s crust. the article shows how these elements interacted and were even transferred from one place to another. it concludes with some reflections on the institutional and procedural relations between academic geophysics, physics and geology. keywords: history of science; seismology; exploration geophysics; geophysical methods; earth’s interior. 1. introduction this article examines the historical development of a geophysical specialization cultivated in multinational settings: crustal seismology. analysis covers three countries remarkable for their geophysical advances, england, germany and the united states, in the period from the 1880s to the 1930s. it is important to note that for each of these countries, academic geophysics was institutionalized around contrasting systems of education and research. geophysics has long been regarded as an essentially stable discipline and, hence, formed through regular stages on its way from immature to mature status. this general viewpoint has profoundly influenced the historiography of geophysics, which has often tended to regard its various sub-disciplines as watertight compartments being almost independent of one another. this kind of approach also tends to suggest a cumulative picture of geophysical knowledge that informs us very little about how it was gained, processed, transmitted or disseminated. moreover, the purported firmness and stability of geophysical discipline begs some questions with regard to the autonomy of its sub-disciplines: autonomous to what extent and as to what? issues of this kind have seldom been addressed in historical studies of geophysics, which too frequently tend to overlook crucial questions such as the confluence of traditions or the existence of common threads within the supposedly compartmentalized fields. recent research, however, has started to undermine this once-unquestioned cannon of stability associated to geophysical discipline. historian gregory good has argued that disciplines are ever-changing frameworks within which different activities are organized. he has shown how geophysics was assembled by a process of extrication and recombination of various specializations such as seismology, meteorology, vulcanology, oceanography, geomagnetism, hydrology and geodesy. he defines geophysics as a framework of consensus on conceptual, methodological and socio-institutional questions in which its development is never teleological [good, 2000]. ronald doel, who has studied the formation of the earth sciences in the twentieth century, has shown how military funding for geophysics after 1945 helped to such a formation [doel, 1997]. the existence of autonomous and watertight sub-disciplines in geophysics should be also viewed as doubtful today. for example, during world war i, ludger mintrop suggested using portable seismographs to locate enemy guns [keppner, 1993]. this idea came into fruition in 1917 when the u.s. navy developed sound ranging equipments with the help of the western electric company [owen, 1975]. after the war, john clarence karcher devised a new method, reflection seismology, aimed at determining the location of oil-field structures. interestingly, the bond between electrical engineering and the oil industry would lead to explore the earth’s crust by seismic prospecting techniques [karcher, 1987; green, 1979; anduaga, 2009]. i will argue that crustal geophysics should be viewed as an intercalated set of inter-disciplinary fields, research schools, programs, and traditions, rather than an autonomous branch. only when one examines all these elements does the purported stability and autonomy of geophysical discipline become diluted. i will furthermore show that these elements interacted with each other and were transferred from one place to another. a set of four geophysics cores —or traditions with long pedigree— will be identified in crustal seismology: the cambridge mathematicalphysics, the göttingen mathematical and observational geophysics, the u.s. observatory-based data recording tradition, and the u.s. laboratory-based experimental physics. let me finish this introduction with a disclaimer and a few considerations. this article is not intended to offer an exhaustive description of the historical development of the seismological studies on the earth’s crust; this is too large a planet for a single prospecting. however, it suggests that certain traditions did operate in the maturation of crustal seismology which affected the evolution of this sub-discipline. indeed, if it is possible at all to condense a definite conception of “crustal seismology”, this certainly must be as much concerned with scientific, as with social and institutional issues. i say “if it is possible” for two reasons. first, the blurred limits of what i have called “crustal seismology” or even at times “crustal geophysics” do nothing but reflect the blurred nature of research fields, both in the western world and everywhere. it is well known, as showed by reference works on the history of this sub-discipline [e.g., mooney, 2007; agnew, 2003; doel, 1998], that the earth’s crust was an undefined concept up to the moment croatian geophysicist andrija mohorovičić found the first seismic evidence for it in 1910. some decades later, the use of this term became noticeable, albeit never pervasive, in textbooks and research articles. although it is true that nowadays the term is not used, it is also true that the field of seismology comprises several topics of crustal research such as seismicity, composition, structure, and oil and mineral prospecting, not to mention those related to other fields of geophysics. and second, “crustal seismology” is also a conceptual expression in geographical terms. unquestionably, there were many other research schools, programs, and countries in the pursuit of crustal knowledge. outstanding among them is japan. we have only to recall the contributions to instrumental and theoretical seismology by influential personalities such as fusakichi omori, toshi shida, hiroshi nakano, and kiyoo wadati, who, in different times and contexts, helped expose and solve the problems which studies on the earth’s crust would bring. however, the lack of both clearly defined traditions and connections with the cores identified above makes it their inclusion inadvisable. 2. cambridge mathematical-physics tradition the first core originates in the cambridge mathematical-physics tradition and became visible in what david kushner first called the “british school of geophysics” [kushner, 1990, 1993]. it was shaped by two leaders in victorian physics—william hopkins and william thomson (lord kelvin)— whose concerns focused on the field of aitor anduaga 2 mathematical physical geology and the solution of interdisciplinary problems of the earth sciences [smith, 1985; brush and gillmor, 1995]. although they used a variety of approaches in their investigations, they all showed an important, if not vital, commitment to mathematization, which they applied to three central problems: the earth’s structure and the thickness of its crust, the age of the earth, and causes of glacial eras. an early physical approach toward the history and structure of the earth emerged with the figure who held the plumian chair of astronomy and experimental philosophy at cambridge (1883–1912), george darwin. charles darwin’s son applied to the various series of investigations undertaken throughout his career a scientific methodology that was at once analytical and numerical-computational. through this methodology, his applied mathematics and his persistent search for quantitative results, he contributed to the transformation of the physical geology into modern geophysics. basically, his methodology began with defining the problem and setting its boundary conditions. next he analyzed the bound problem by introducing two simplifications: first, he treated the physical reality with the best possible mathematical model; and then he undertook the first analysis after reducing the number of parameters. he always tried to give results a physical meaning. if they were not satisfactory, he increased the number of parameters. he repeated all this operation as many times as necessary until achieving an approximation of the real-world situation [kushner, 1993]. the simplification of problems as analytically and systematically as possible and the ensuing physical interpretation of results formed the basis of the methodology of his successor, harold jeffreys – the father of twentieth-century british geophysics. historians have unraveled many institutional and personal idiosyncrasies that gave the development of geophysics in britain its peculiar character. the relatively amateur nature of scientific activities in nineteenthcentury britain delayed the consolidation of geophysics into a university discipline. in fact, it was not until 1931 that the first university reader in geophysics (jeffreys) was appointed [cook 1990]. this contrasts markedly with the early institutionalization of geophysics in germany, favored by the university-based tradition of professional science. an important influence was also exerted by the close relationship between geophysics and astronomy, and by the perennial practice of viewing the field of terrestrial physics from the astronomical perspective. the long shadow of newtonian physical astronomy in britain, stronger than in germany, undoubtedly contributed to the late institutional emancipation of geophysics from astronomy. likewise, darwin’s personal style guided the pattern of future methodology in the field, and placed emphasis on analysis and quantity rather than on quality; that is, on the applicability of mathematics and the quest for numerical results rather than on qualitative evidence. as a result, british geophysics advanced in the twentieth century but did so in such a way that it had loose ties with geology and strong institutional and filial links with astronomy [kushner, 1993]. the long-standing dependence of geophysics on astronomy was most frequently viewed as unique to britain in the twentieth century, and there can be no denying that the argument had substance. although it never did become a sub-field of astrophysics, as some advocated at the time, the motives for disciplinary dependence had always been overtly institutional. for example, the first geophysical committee organized to foster meetings and discussions among the branches of this field was the responsibility of the royal astronomical society (ras) in 1919 [jeffreys, 1936]. the first british journal exclusively devoted to geophysics was the influential geophysical supplement (renamed geophysical journal in 1958) of the royal astronomical society. the chair of the leading british authority on geophysics (again, jeffreys) was the plumian professorship in astronomy–geophysics only had the rank of reader. there were enough connections, in all the cases, to understand the inclusion of geophysical studies into disciplines such as planetary science. in fact, issues such as tidal friction or large-scale climate change shared both fields [brush, 1978]. but the effect was clearly and manifestly to provide an approach that gave prevalence to quantitative over qualitative reasoning; whether to promote diffusion and communication (as in the case of the committee and the journal), or to teach fundamentals (as in the case of the chair). the continued readiness of most geophysicists to comply with the quantitative conception of the astronomical methodology cannot conceal the fact that harold jeffreys marked a turning point for the discipline. both the nature and the epistemological status of geophysics, especially crustal seismology, underwent a fundamental change [jeffreys and jeffreys, 1971-1977]. in the 1910s, this cambridge mathematician formulated an entirely new theory of induction that was brilliantly addressed in his work scientific inference in 1931 [jeffreys, 1931]. this book was to reinforce the value of induction and probability as tools for analysis, and to refute the thesis of a hypothetical “geophysical style” based on deductive reasoning and quantitative evidence. the reversal is all the more revealing since it came from someone who was not averse to darwinian analysis. but jeffreys shed new light by formulating an epistemological program based upon inductive inference and probabilistic analysis. as a part of this program, 3 crustal seismology: traditions and styles aitor anduaga 4 he defined “simplicity” as an epistemological category, and formulated what he called the “simplicity postulate”; i.e., “the simpler the law, the greater is its prior probability” [jeffreys and wrinch, 1921]. in the 1920s, jeffreys devised a new method of seismic analysis and applied both this method and his simplicity postulate to the structure of the earth’s crust. as a methodological procedure, he devised crustal models of layerlike simplicity [jeffreys, 1926]. although geological evidence seemed to point in another direction, jeffreys always preferred epistemological criteria to evidential ones. by world war ii, the jeffreysian crustal model achieved a dominant position. it had strong advocates such as cambridge seismologists robert stoneley and keith edward bullen; it had inspired other layered crustal models; and its foundations, in particular the primacy of simplicity and the predilection of seismic analysis over geological evidence, were taught as a guiding core principle in geophysics courses and textbooks. yet, however homogeneous such a dominion may have appeared in the interwar years, it had never been so without the participation of another crustal seismological tradition, as addressed below [anduaga, 2016] 3. göttingen mathematical and observational geophysics the second core of crustal seismological tradition is the other main geophysical focus in europe: göttingen. from its establishment in 1898, the institut für geophysik der universität göttingen housed a good number of graduates who specialized in geophysics under the direction of emil wiechert [wiechert, 1906]. although they were involved in construction of seismographs, their research interests mainly revolved around on observation and the mathematical theory of seismic-wave propagation. they were all uniformly indoctrinated into the practice of seismogram reading and accurate measurement. however, not all were “crustal seismologists” in the sense that such a categorization might indicate. furthermore, seismology was not their unique dedication (e.g., atmospheric electricity and aurora were as well). some devoted themselves to analyze the motion of earthquake waves (wilhelm schlüter, harold d. arnold); others, to the theory of elastic waves (zoeppritz), or travel-time curves from seismograms (herglotz); some also turned to localization of epicenters (geiger), or the structure of the earth’s core (gutenberg); and among those focusing attention on the crust, some studied surface waves (gutenberg, angenheister), while others used artificial sources (mintrop, hans mothes). however, wiechert imbued them all with his unified conception of mathematical and observational geophysics. the königsberg seminar method, based on the handling of instruments and analysis of data and errors, as well as on theory and precision measurement, was the model by which all were inspired [schweitzer 2003; cremer 2001; westermann 2011; kertz, 2002; olesko, 1991]. of several respects in which the göttingen institute differed from other foci such as cambridge or california, two are worth mentioning here. first, by its research projects and organization, göttingen’s geophysics experienced a relatively quick differentiation from astronomy, establishing itself motu propio as an autonomous university discipline. however, in suggesting this more than progressive differentiation, i am making a statement that must be clarified; for even if it is true that 1898 was a turning point, it is also true that the character and objectives of geophysics remained subordinate to astronomy in the various decades up to then. until the end of the nineteenth century, its role had been straightforwardly ancillary: for example, the first pendulums set up at the astronomical observatories of göttingen and potsdam had the mission of measuring earth movements caused by the influence of astronomical bodies [forbes, 1967]; and even as late as 1896, wiechert developed a model of the earth’s interior, based only on density and astronomical observations [brush, 1980]. the idea was not as outlandish as it could seem, as shortly after the first seismological evidence of an earth’s core was given by richard d. oldham [1906] in britain. however, from 1898, when the first geophysical institute in the world was established in göttingen, the context for the geophysical emancipation began to change, at an institutional and conceptual level. all that occurred just at the time in which the plans by göttingen mathematician felix klein for strengthening mathematics, natural sciences, and technology came to fruition through the patronage of several professors and industrialists [neuenschwander and burmann, 1994]. the split of the chair in theoretical astronomy, geodesy, and geomagnetism into two new professorships in 1897 is typical of the way in which the accumulation of professional forces accelerated, rather than slowed, the autonomy of said discipline. a second distinctive feature of the göttingen school was its emphasis on “explosion seismology”. from the very start, göttingen program contained the systematic study of non-seismic sources of ground movements. the case of ludger mintrop shows how innovative this initiative was [schleusener, 1956; kertz, 1991; cremer, 2001]. a professor at the mining school in bochum and a land surveyor, mintrop had conducted research involving the measurement of elastic waves induced by mining explosions. but the idea of deliberately using artificial sources to study seismic-wave propagation was a new challenge by the standards of contemporary seismology. attracted by this singular challenge, mintrop used small explosions to generate elastic waves just after joining göttingen in 1907. he also broke new ground by constructing portable seismographs and patenting novel procedures such as his method for determination of rock structures, a method he patented in 1919 to interpret travel-time curves [lehmann, 1965]. this privilege was not reserved only for him; another of wiechert’s pupils, hans mothes, was the first to measure the thickness of a glacier by applying seismic surveying methods [mothes, 1927]. these endeavors symbolize innovating techniques which contributed to the expansion of explosion seismology and the seismic prospecting method not only in germany, but also in the eastern coast of the u.s. in the 1920s [mintrop, 1947]. it was certainly the perception that göttingen was taking the lead, which helped seismic prospecting take root in germany’s industry in the early 1920s. the pace was set by the commercial company seismos-gmbh, created by mintrop [mintrop, 1930)], though the marks of a competitive struggle can now be seen. as soon as the company was founded, in 1921, richard ambronn, the head of the rival company erda-gmbh, took legal steps to stop mintrop’s patent from being accepted [brinckmeier, 1955]. however, mintrop’s commercial successes in the u.s. tipped the balance in his favor. seismos, which obtained a contract to conduct seismic surveys for marland refining company in 1923, applied its method along the gulf coast of texas. while this first attempt proved fruitless, subsequent surveys conducted for gulf production company led to the first seismic finding of a salt dome, the orchard dome in the texan coast, with significant amounts of oil [lawyer et al., 2001]. consequently, from the mid-1920s a substantial number of geophysical companies in the u.s. began emulating seismos and managed to edge ahead of germany in applying seismic prospecting methods. the third and final characteristic of the göttingen school was its interest in global earthquakes. through teleseismic, or distant, observations, they provided global seismology with the method of studying the earth’s interior. american seismologists, by contrast, maintained their pre-seismographic practice of studying regional or local earthquakes, adopting the pragmatic view of their precursors, geologists, mining engineers, and astronomers. however, global seismology did not originate at göttingen; they further developed the german tradition initiated by astronomer ernst von rebeur-paschwitz in 1886 in setting up horizontal pendulums in remote places to monitor the earth’s seismicity as globally as possible [dewey and byerly, 1969]. although indirectly sought (as his aim was to measure the deformation of the ground by tides), rebeur’s instrument (the clinometer) also proved to be sensitive to earthquakes, and, therefore, was set at different places for seismic purposes. they also retained rebeur’s idea of collaborating with distant stations and using their data as references for teleseismic observations. thus, from 1902, when the apia geophysical observatory was established on the samoan islands in the pacific ocean, such cooperation finally became viable. several wiechert seismographs were set up in samoa, and göttingen pupils, such as angenheister and geiger, spent some time there. in extending their perspectives worldwide and to international cooperation, göttingen seismologists felt the satisfaction of being able to precisely locate large quakes in the pacific area, as well as of having global travel-time curves, important to their investigations on the earth’s core and crust [pyenson, 1985]. ironically, the göttingen institute’s globalist view may well have had significant consequences for american seismology in an episode that began with the arrival of beno gutenberg at caltech university in 1930 [goodstein, 1984]. until that date, this seismologist of jewish origin had meritoriously carved out a name for himself in the world of geophysics [richter, 1962; knopoff, 1999]. his contributions included the estimate of the radius of the earth’s core in 1914, as well as the identification of its elastic properties. in 1926, gutenberg alternated between temporary position of professor of geophysics at the university of frankfurt-on-main and of manager of the soap factory that he inherited from his father. but he failed to occupy the vacant chair at göttingen in geophysics at a time in which the presence of jews in university professorships was becoming an affair of state. the impossibility of realizing his full capacity at home convinced him to accept the invitation given by the head of caltech, robert a. millikan, to join the seismological laboratory [hough, 2007; goodstein, 1984; wood, 1929]. at that time, caltech’s authorities concluded that a “good seismologist” was necessary, if the institute aimed to progress on the study of the earthquakes in california, as the ongoing research lines seemed clearly insufficient. from his new chair, gutenberg made an unprecedented effort to shift the focus of attention from local observations to general dynamical patterns. during his first years at caltech, he examined north american microseisms and analyzed the global distribution of earthquakes. then, in the late 1930s, gutenberg and his caltech colleague charles richter (better 5 crustal seismology: traditions and styles known as the inventor of the scale of earthquakes named after him) revised standard time-distance tables to calculate the velocity of seismic waves [gutenberg and richter, 1934, 1935, 1936, 1939]. as a result, the centre of scientific research and production in earthquake seismology unmistakably shifted from the once fertile ground of göttingen to california. by world war ii, gutenberg’s contribution to crustal seismology was, for all intents and purposes, indisputable. he had stimulated research of outstanding quality, introduced artificial blasting (in the purest style of göttingen) as a tool to study the outer crust, and he had elucidated the structure of the earth’s crust in california with enough precision. yet, however successful such participation may have appeared in the 1930s, the implementation of göttingen’s modus operandi ran into a brick wall in berkeley, northern california. 4. u.s. observatory-based data recording tradition there was an observatory-based data recording tradition in the u.s.—which constitutes our third core—that helped governmental bureaus such as the u.s. geological survey and the u.s. coast and geodetic survey to foster geophysical research [yoder, 2004, pp. 1–3]. this third core originated from the tradition of humboldtian-style terrestrial physics reaching back to the mid-nineteenth century. in most cases, their methods were qualitative rather than quantitative. utilitarian concerns derived from geological and geodetic surveys aroused a growing interest in earthquake research on a scale unknown up to that time. part of this emphasis was placed on researching the relations between faults and earthquakes. but the greatest part of it seems to have been placed on performing geodetic, gravimetric, and magnetic surveying in order to construct charts at a time when geodetic surveys and navigation required accurate observations [good 1991, 1985]. these agencies responded to national needs. civilian-governmental agencies such as the u.s. coast and geodetic survey had in fact been established in most western industrialized countries by the early twentieth century; their aims included magnetic studies and surveys of coastlines which were essential for navigation. likewise, they contributed to the informal training of young geophysicists. however, these were not the only national needs that stimulated the growth of geophysics. the scale and strength of this tradition can be seen in the cases of the university of california observatories, berkeley and lick [louderback, 1942; bolt, 1989]. berkeley, the main seat of the evolution of twentieth-century observational seismology, was a student observatory with a clearly educational orientation. lick was an important professional astronomical observatory situated on mount wilson. while berkeley was financed by the california legislature to provide practical training to civil engineers, lick was the fruit of the bequest from a wealthy man [leuschner, 1904]. however, both shared the same fate in 1887: they were organized under the same plan, equipped with identical instrumentation, and had the honor of being the first seismographic stations with continuous recording set up in north america [bolt,1985]. yet it was astronomers, and not geologists, who were to blame for this initiative. the president of the university, astronomer edward singleton holden, promoted their establishment, not so much by their intrinsically seismological interest, but for the need to keep “a register of all earthquake shocks in order to control the positions of the astronomical instruments” [louderback, 1942]. in any event, the measure had an incentive effect as just one-year latter several instruments were set in california with the objective to have a record of the region’s earthquakes. lick and berkeley’s establishment enabled the organization of a coordinated network of seismographic stations in northern california and nevada; the first network of its kind in america. the records of almost a dozen stations established at local astronomical observatories (such as that at carson city, nevada), and holden’s work, gathered in his catalogue of earthquakes on the pacific coast, 1769-1897 [holden, 1898], all reflected the commitment to astronomy-oriented observational seismology. to a greater or lesser degree, these stations shaped a style of observational practice aimed at finding solutions to the so-called “california problem” of regional earthquakes [goodstein, 1984]. interestingly, the great 1906 san francisco local earthquake galvanized the joint collaboration of geologists and seismologists; up until this point there had been a lack of organization and coordination between sciences in the u.s. [geschwind, 2001]. insofar as the causal link between geological faults and the destructive intensity of earthquakes was searched, collaboration was easy and productive. for instance, three days after the event, the governor of california created the state earthquake investigation commission, half of whose members were aitor anduaga 6 geologists [lawson, 1908–1910]. eight months later, the seismological society of america was established [byerly, 1964; geschwind, 2001]. social and academic demands were high because of the tragedy’s magnitude, and this fuelled the collaboration. as a result, global seismology and the structure of the earth’s interior became secondary objectives; the priorities unmistakably lay in the field of local earthquake seismology and its relation to faulting. hence, at a time when seismology was being mathematized at göttingen, new steps towards its institutionalization were taken at berkeley in ways that even nineteenth-century geologists would have found impressive. among other steps, we can mention the seismological program founded by the carnegie institution in 1921, the first seismology classes for undergraduates in 1911, the first graduate course in seismology in 1923, the program for studying local earthquakes in 1916, or the hiring of key figures in this discipline, all within the university’s department of geology and mineralogy [louderback, 1942, bolt, 1989] the character of the seismological program at caltech was shaped by the established values of the observatories in northern california, namely berkeley and lick. the main function of these observatories was to find a practical solution to the hazards of california earthquakes [wood, 1918]. accordingly, the carnegie institution launched a modest program for the investigation of local earthquakes in 1921 [goodstein, 1984]. as a proof of its commitment to the field approach and to rigorous instrumental standards, the berkeley school hired harry wood, a harvard graduate in geology and mineralogy, as the program’s director, and charles richter, a theoretical physicist very experienced in reading and measuring seismograms, and hugo benioff, a skilful seismic instrument designer, as assistants. research activities were initially conducted at the carnegie institution of washington’s seismological laboratory in pasadena and in a seismographic network. by 1927, this network also included the central station at pasadena and four auxiliary stations in southern california [geschwind, 2001; gutenberg, 1934]. at caltech, a research line in particular—seismic instrumentation—passed from a hesitant infancy to a robust adolescence. caltech housed wood and john a. anderson, who in the 1920s devised a ground-breaking torsion beam seismometer which could photographically record the imperceptible motions of local earthquakes. it was in fact the signals from this seismograph on which richter drew in 1935 to define his magnitude scale for earthquakes [richter, 1935; hough, 2007]. but caltech also housed virtuous benioff, who in the 1930s designed one of the best sensors for decades, of much higher sensitivity than any so far built, the result of the adaptation of a variable-reluctance transducer to a seismograph. benioff’s sensors were in fact used in the world-wide standardized seismic network, which operated from the 1960s to the 1980s. in 1932, he applied his transducer to a strain seismograph, which proved to be particularly useful for recording long-period surface waves from distant earthquakes. to the extent that they provided records of free vibrations at many frequencies, benioff’s strainmeters inaugurated a new branch of instrumental seismology [press, 1978]. it is consistent with my categorization that california represented the confluence of two traditions, which originated in berkeley and göttingen. the former was involved in seeking correlations between seismic phenomena and geological faulting—an issue that had attracted the interest of their precursors, the geologists and mining engineers [wood, 1916]. berkeley’s seismologists devoted little time to interpreting travel-time curves or devising seismological earth models; rather, their concerns were oriented towards the study of local earthquakes, that is, the possibility of predicting them or the problems they posed for engineers and architects. their practical and local approach powerfully contrasted with göttingen seismologists’ globalist view. at berkeley and then at caltech, wood headed the mapping of active faults; also the prediction of large earthquakes (according to him, “weak shocks” were “preliminary to failure in faulting, with resulting strong shocks”) [bolt, 1985]. at berkeley, perry byerly, a graduate in physics and the first person to receive the title professor of seismology at the university of california, who obtained a guggenheim fellowship in 1928 to conduct research at cambridge with jeffreys, developed a graduate research program [verhoogen, 1985; anduaga, 2016]. he instructed his doctoral students (a total of 22 in his long career) how to scrutinize a large earthquake, determining travel-times, its location and depth, and the direction of faulting [bolt, 1979]. for years, wood and byerly’s shadows was very long. however, since 1930, as göttingen-trained seismologist gutenberg initiated a research program of the world’s large earthquakes, the influence exerted by berkeley pupils in american seismology began to wane. their regional seismology had proven socially beneficial, but somewhat scientifically limited [byerly, 1958]. despite the preeminence of these two foci, the variety of institutions and programs involved is one of the most idiosyncratic aspects of the observatory-based data recording tradition. according to the most common list of seismographs in the u.s., compiled by the seismological society of america in 1911, the total number of stations was 44—many of them operated by jesuits (fifteen) and by the coast and geodetic survey (five)—a figure that grew 7 crustal seismology: traditions and styles slightly in the following decades [heck, 1944, p. 224]. as time went by, the share of commitment extended to academia, and it was the jesuit seismological service that set the pace [macelwane, 1926]. founded in 1909 in cleveland by frederick l. odenbach of john carroll university, it constituted the “first seismological network of continental scale with uniform instrumentation” [udías, 2000]. but until 1925, when the service was revitalized by st. louis university, jesuits rarely drew upon stations as a vehicle for research. emphasis was placed on publicity and acclaim rather than analysis and investigation. jesuits (and also the u.s. cgs) were interested not so much in fomenting research as they were in collecting earthquake records to send to the central bureau of the international seismological association in strasbourg within the european seismological program [geschwind, 1998]. with this tradition in mind, it is enlightening to focus attention on the one case that seems, at least in its initial stages, to vie as an exception: the jesuit st. louis university. this case reflects the commercial and industrial dimension that academic seismology held. broadly speaking, the introduction of the seismological program at st. louis in 1925 is comparable to that of caltech in the early 1920s. its promoter was the jesuit father james b. macelwane who first graduated from st. louis and later earned his ph.d. at berkeley in 1923 [macelwane, 1923; byerly and stauder, 1958]. two years later he returned to st. louis as director of the newly created department of geophysics [geschwind, 1998; barth, 2000]. behind this return was his goal—presumably suggested or encouraged by his superiors—to revitalize the jesuit seismological association [udías and stauder, 2002]. like wood at caltech, macelwane designed a seismological program for the department. yet it was a program geared towards graduate studies in earthquake research, and most importantly in applied seismology. training graduates for academic positions was an objective of the program, but macelwane’s educational goal was to train exploration seismologists. basically, his strategy was to build up a network of contacts and acquaintances among the many geophysical consultancies and oil companies in the u.s., some of which macelwane himself had worked for [byerly and stauder, 1958]. then from an academic position of recognized prestige, he would establish the institute of geophysical technology at st. louis in 1944 in which many men who ended up serving in high posts in the geophysical staff of oil companies would be trained [barth, 2000]. some schools of mines and university geology departments—the centers that promoted exploration seismology and applied geophysics in general—form a separate subgroup with limitations and virtualities very similar to those described above. the path was opened by the colorado school of mines, which offered the first official course in geophysical prospecting under the direction of carl a. heiland in 1927 [hollister, 1957; eckley, 2004]. unlike other schools at the time, colorado hired professors with industrial curricula, benefiting from the growing consolidation of applied sciences in mining, particularly in geological engineering [van tuyl, 1949]. at the same time, other universities such as st. louis and houston’s rice institute benefited from the proliferation of posts directly or indirectly related to industrial geophysics. here oil companies and geophysical consulting companies led the demand: by the 1920s, the american oil companies employed a wide variety of geophysical methods, including seismic, magnetic, electrical and gravitational ones [ochs, 1992]. the growth of exploration geophysics was not only parallel to, but also constituent of the expansion of the petroleum industry. 5. u.s. laboratory-based experimental physics the last core has much bearing on the deeply entrenched u.s. tradition of laboratory-based experimental physics. the carnegie institution of washington (ciw) and harvard university were actively involved in the early investigations into the physics of the earth’s crust. the two institutions benefited from philanthropic donations. the ciw was founded in 1902 thanks to a ten-million dollar donation from the magnate andrew carnegie. his fortune was a daughter of industrialization; in the 1880s, carnegie steel was the largest manufacturer of pig iron, steel rails, and coke in the world. the rockefeller foundation, for its part, facilitated with grants harvard’s vigorous entry into geophysics in the 1930s [good, 1991]. such private patronages placed geophysics firmly on the path of experimental physics. the ciw set the pace. it is significant and wholly in character that the most important research centre, the geophysical laboratory (gl), was founded by the ciw early, in 1905 [yoder, 1994]. the gl soon stood out as the authoritative source of data on the physics-chemical properties of rocks and minerals [servos, 1984]. to a certain extent, the gl was the result of a boost in geophysical tradition. in the 1870s and 1880s, a group of geologists from the u.s. geological survey (clarence king, george f. becker, and carl barus) were interested in the inner state and aitor anduaga 8 age of the earth [burchfield, 1975]. but that was primarily due to the growing prominence of young geologists and petrographers (like charles r. van hise and joseph p. iddings), concerned with physical chemistry and the study of the formation of igneous rocks [servos, 1984]. as a result, the gl promoted experimental studies in petrology and geochemistry rather than geophysics. it was not until after world war ii that more attention was given to leading geophysical questions such as the dynamics of crustal deformations or the measurement of physical constants (e.g., the elasticity and thermal conductivity of rocks), which were important for layered crustal models [yoder, 2004]. the ciw’s managers became convinced that the gl’s objective should be focused on physics and seismology rather than on a geochemical tradition that had tipped the balance toward questions relating to petrogenesis and metamorphism [abelson, 1975]. gl director arthur l. day played a major role in this regard [day, 1938]. moreover, he received the favorable response of his physicist colleagues. for example, leason h. adams and erskine d. williamson examined the elastic properties of minerals and rocks by conducting tests of the properties of materials under high pressures. by doing so, they delved into the nature and composition of the earth’s crust [gibson, 1980; hemley, 2006]. others retained their petrologic concerns but centered all their efforts on the issue of propagation of elastic waves through rocks (e.g. [roy w. goranson, 1928]). yet all of them settled on experimentation, making the gl a centre of reference for the increasingly exacting demands of earthquake seismologists. interestingly, the attitude of carnegie authorities had beneficial results for american seismology in the new age that dawned with the development of sensitive motion detectors after world war i. in 1916, wood, in those days a researcher at the hawaiian volcano observatory, wrote a report on the desirability of establishing a network of seismic stations in southern california [wood, 1916]. it was also during world war i that wood designed a device capable of registering vibrations (from cannon fire) that resembled those of local earthquakes. within a few years, wood designed a piezo-electric seismometer, emulating the russian prince boris b. galitzin’s work [wood, 1921]; and, once established in washington, submitted his plan to arthur day who passed it on to john merriam, the ciw’s new president [goodstein, 1984]. merriam, a berkeley geologist and paleontologist who left university teaching in 1920, immediately espoused wood’s plan, forming an advisory committee on seismology composed of five geologists and two physicists under day’s direction at the gl. the committee constituted not only one of the largest cooperative efforts in american science [abelson, 1975], but it was also the seed that led to the agreement between the ciw and caltech, which in 1927 created the seismological observatory in the foothills of the san rafael mountains [anderson, 1974]. there can be no denying that, with the entry of the ciw into seismology, caltech promoted investigations on crustal movements of a scope and magnitude beyond anything previously attempted [goodstein, 1984]. the case of another geophysical committee, harvard’s, is symptomatic of the existence of an ingrained tradition of experimental physics in the u.s. during the period from the 1870s through the 1910s, most of the u.s. physics departments encouraged experimental practice while neglecting mathematical instruction [servos, 1986]. the most renowned physicists were experimentalists; such was the case with robert a. millikan, albert abraham michelson, henry august rowland, to mention but a few. another distinguished experimentalist was percy w. bridgman, also known for his operational doctrines of modern physics, who conducted pioneering investigations on the field of high-pressure physics after joining the jefferson laboratory at the harvard physics department [kemble and birch, 1970]. his efforts were focused not only on the application of high-pressure techniques to conditions like those found in the earth’s crust, but also on the subsequent finding and scrutiny of new phenomena under such high pressures. in the interwar years, he was the only physicist who was able to reach pressures of thousands of kg/cm² (that is, the type of pressures found several kilometers below the earth’s surface) by laboratory techniques [okamoto, 2004]. his contributions proved to be highly promising for his colleagues as they expanded the experimental horizons of crustal seismology. bridgman’s intense personality left a deep imprint on the atmosphere in harvard in the early 1930s, where geophysicists mingled with geologists to promote experimental geophysics in an era when the two disciplines interacted as never before. the committee for experimental geology and geophysics played a major role in this promotion. with the financial support given by the rockefeller foundation [birch, 1960], they helped inaugurate a modern style of geophysics that would be reflected in the fulfillment of a two-fold venture [ahrens, 1966]. on the one hand, a seismological station was established under the direction of don leet, which specialized in the study of seismic velocities by using quarry blats. as the director of the new station, leet published several articles on seismic travel-time curves in cooperation with maurice ewing, then instructor at lehigh university [bullard, 1980]. on the other hand, building upon bridgman’s experience with high pressure physics, a program of physical 9 crustal seismology: traditions and styles measurement was established at harvard. this program soon yielded fruitful results, such as the studies on elastic properties of rocks by william a. zisman, and the works on seismic velocities of rocks by albert f. birch. all these studies aimed at determining the physical properties of geologically relevant materials, and consequently, at harmonizing geophysics with geology [birch, 1979]. 6. some final considerations in a short article it is not possible to introduce highly telling nuances; only the salient features of the historical development of crustal seismology could be raised. four research traditions are present in this text: cambridge mathematical-physics, göttingen mathematical and observational geophysics, u.s. observatory-based data recording, and u.s. laboratory-based experimental physics. there are sound historical grounds for arguing that crustal geophysics should be viewed as an intercalated set of inter-disciplinary fields, research schools, programs, and traditions, rather than a conglomerate of autonomous fields. all these geophysics cores seem to concur in certain directions, or at least, interact with each other by transferring programs and human capital from one place to another. the historical development of crustal seismology also raises some more general considerations. the first consideration concerns the relationship between academic geophysics and physical sciences. in the united states, unlike in england or germany, the socio-political context greatly affected the implantation of academic geophysics, which encountered serious obstacles at the turn of the century. earth sciences enjoyed a preeminent status due to having played a decisive role in the great western surveys of the 1870s. yet, by 1900 these sciences underwent a loss of status, owing to the lack of political support and the increasing weight of industrialization [doel, 1997]. it is no small irony that lies behind these obstacles that led to the primacy of physical sciences over the earth sciences in the u.s. on the one hand, departments of physics from american universities received considerable funds and political backing. moreover, these very departments hosted, with a greater or lesser presence, some physical disciplines related to earth processes—which belonged to geophysics. on the other hand, the disciplines that compose geophysics, such as seismology or meteorology, were only included as research specialities either because they had no political backing, because they were young, or because they had no close links with industrial sectors. this meant that no exclusive institute or centre of geophysics existed in american universities—a strategy that was scientifically disputable and academically inappropriate, if we compare this to the situation of europe. notwithstanding all this, the few programs on interwar seismological research had great impact and academic repercussion. the policy of support received in many departments of physics only increased both. the second consideration concerns the relationship of crustal geophysics with its so-called “borderland science”: geology. in historical terms, this relationship was difficult and complex. although they were bordering sciences, they had differences in methodology, tradition and style. academic geophysicists often deemed their science as archetypal of precision and of mathematical rigor, as the triumph of quantitative methods over what they judged to be the qualitative and unproductive methods of geologists—a “negative” perception that little helped understanding between the actors involved [heiland and wantland, 1940; keys, 1940]. the geologists thus trained in academia, were reluctant to accept the crustal models of academic geophysicists, and until the 1940s and the late post-war years, then within a new sociopolitical context, we cannot speak of the integration as such of geophysicists into the departments of geology (or vice versa) [hubbert, 1940]. in american universities, the departmentalization of the domain of science into conventional fields (such as physics, geology, etc.) did not help understanding between the academic communities of geophysicists and geologists. ironically, many of the problems (or sources of tension) observed in academia were solved by the industrial geophysical community in the interwar years; a variety of factors contributed to this fact: the same objectives, the use of identical prospecting methods, a structure of values and socioeconomic incentives that encouraged scientists to cooperation (not to inbred isolation), and participation in work crews combining geologists’ field experience with physicists’ experimental insight. in the case of the american oil industry, these features reflected not only the strength of industrial geophysics, but also the anomalous functioning of academic geophysics. the industrial scientific community had specialized journals, societies (made up of both geophysicists and geologists), common workplaces, a hierarchy of common values; it formally had all the elements favorable for rapprochement and coexistence. hence, understanding between industrial geophysicists and geologists was not aitor anduaga 10 comparable to that of any homologous academic department. as i showed on another occasion, the industrial geophysical community had a system of values and prospecting methods which were progressively transferred to academia for scientific research. the post-war coexistence between geophysics and geology was impelled from the industrial world to academia—not the other way around [anduaga, 2016]. the last consideration concerns the laboratory-based experimental physics tradition. during the first decades of the twentieth century, the late nineteenth-century trend toward the proliferation of laboratory-based physical research was consolidated in the u.s. however, the laboratories founded in those years were headed by experienced, skillful, and able, physicists trained in the european style. the careers of arthur louis day and beno gutenberg are illustrative in this regard. after working as professor of physics at the universities of yale, day was trained at the physikalische technische reichsanstalt in berlin [abelson, 1975]. it was in this place that day engaged in hightemperature thermometry in 1897 [yoder, 1994]. gutenberg, for his part, was imbued with the göttingen conception of mathematical and observational geophysics being indoctrinated into the procedures of accurate measurement, long before accepting the post of director in pasadena [knopoff, 1999]. day’s and gutenberg’s early impression produced in the us was similar: whereas day was hired by the geological survey to organize a new laboratory because of his experience in high-temperature research, gutenberg displayed his deep knowledge of observational geophysics to caltech’s colleagues and rector. day headed ciw’s geophysical laboratory from 1906 to 1936; 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(1994). development and promotion of the initial scientific program for the geophysical laboratory, in the earth, the heavens and the carnegie institution of washington, gregory a. good (editor), the american geophysical union, washington d.c., 21–28. yoder, h.s. (2004). centennial history of the carnegie institution of washington: the geophysical laboratory. vol. iii. cambridge university press, cambridge. *co r r e s po n d i n g a u t h o r: aitor anduaga, basque museum of history of medicine and science, university of the basque country ehu/upv, 48940, leioa, basque country and ikerbasque, basque foundation for science, bilbao, basque country e-mail: a.anduaga@ikerbasque.org © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 15 crustal seismology: traditions and styles mailto:a.anduaga@ikerbasque.org a noie on the loss of hydrogen and the supply o f hydrogen to the earth as a direct or indirect effect of the solar wind jst. a . b a r r i c e l l i r . m e t c a l f e ( * ) r i c e v u t o il 29 m a r z o 1969 summary. — t h e rate at which h y d r o g e n is supplied t o the e a r t h by the solar w i n d , and the rate at which h y d r o g e n escapes f r o m the e a r t h by diffusion into space are c o m p a r e d 011 the basis of recent estimates. it is f o u n d that the hydrogen supplied by the solar w i n d is roughly comparable to or m a y even be larger than present h y d r o g e n losses. t h e e a r t h m a y h a v e lost most of its hydrogen at an early stage when, according to u r e y and miller (1959), p o x (1960) and others, the e a r t h had a reducing, instead of an o x i d i z i n g atmosphere. h o w e v e r , no sufficient e v i d e n c e is y e t a v a i l a b l e to determine w h e t h e r the e a r t h is still losing more h y d r o g e n than it receives. t h e c o m m o n notion that the e a r t h is still in the process of losing its hydrogen is t h e r e f o r e in question. riassunto. — sulla base di recenti stime, si confronta il tasso di idrogeno f o r n i t o alla t e r r a dai v e n t i solari in r a p p o r t o al tasso di f u g a dell ' i d r o g e n o dalla t e r r a in seguito a diffusione nello spazio. si constata che l ' i d r o g e n o f o r n i t o dai v e n t i solari è pressoché pari, forse anzi superiore a quelle che sono le perdite attuali di idrogeno. e da presumere che la t e r r a abbia perso la m a g g i o r parte del proprio idrogeno in uno stadio primario, quando — secondo u r e y e miller (1959), f o x (1960) ed altri — l ' a t m o s f e r a della t e r r a era del t i p o riducente, i n v e c e che ossidante. comunque, non sono state finora p o r t a t e p r o v e concludenti per determinare se, t u t t o r a , la t e r r a continui a perdere idrogeno in quantità superiore a qnella che r i c e v e . l ' i p o t e s i g e n e r a l m e n t e accettata, secondo cui il processo di riduzione del contenuto di idrogeno della t e r r a sarebbe t u t t o r a in corso, è quindi posta in dubbio. ( * ) u n i v e r s i t y of w a s h i n g t o n , seattle, w a s h i n g t o n . 1 0 6 n. a. b a r r i c e l l i r. m e t c a l f e 1. introduction. after the discovery of the substantial intensity ol' the stream of protons and hydrogen atoms carried by the solar wind, the question lias arisen whether the rate at which hydrogen is supplied to the earth by the solar wind might be comparable to or larger than the rate at which the earth is losing hydrogen by diffusion into space. only very rough estimates of the time rates are possible at present, and the main purpose of this note is to attract attention to the problems involved and their possible implications rather then bring a final solution. 2. e s t i m a t e of h y d r o g e n s u p p l i e d by solar w i n d . only a high estimate of the rate of hydrogen supplied to the earth by the solar wind — obtained by disregarding the affect of the earth's magnetic field on charged particles — seems meaningful on the basis of present information. according to jokipii ('), the flux of the solar wind is approximately 3 x 10* protons/cm2.sec in the vicinity of the earth. other recent estimates (brigde, (2() differ from this by a factor 3 either way. using the first estimate, the amount of hydrogen from the solar wind which would be intercepted by the earth if the earth did not have a magnetic field or if the solar wind contained a comparable amount of uncharged particles would be: / protons\ / 1 \ / gm \ /3.(> x 103 sec\ 3 x 10» 1 w 1 ' 1 7tr2 . cm2 • sec/ \ protons i \ mole of hi \ hr 6.02 x 1023 mole oi i i í876ü 1 u 'u7? 2 == (l.57 x 10 8 k m \ \ vr / \ cm2vr/ hr\ „ „ „ ^ „ „ gm yr (where r is the radius of the earth). because of the earth's magnetic field and the effect of the magnetosphere on charged particles, the actual amount of solar-wind-hydrogen which would reach the earth's atmosphere would be only a fraction of this figure which should therefore be considered as a high estimate. present theories and available information give discrepant estimates of a n o t e on t i i e loss ok h y d r o g e n a n d t h e s u p p l y , e t c . 107 tliis fraction and it has not been decided whether this fraction would be substantial or not. 3. e s t i m a t e of hydrogen loss from e a r t h . according to spitzer (3), (table 5, page 245) the time of escape of free hydrogen from the earth is 3.5 x 104 years, assuming an upper atmosphere temperature of 1000°k. according to farley et al. (4) the approximate temperature from 350 to 500 km is 1000°k; according to evans (5) the ion-temperature at about 400 km is approximately !)00°k. the time of escape is defined as the time in which the amount of hydrogen lost by the earth would be equal to the amount of hydrogen in its atmosphere on the assumption that the hydrogen concentration in the atmosphere were maintained constant by a continuous supply. the amount of hydrogen in the atmosphere per cm2 of the earth's <r]i) surface is 3.5 x 10 5 * (hutchinson, (")). hence the rate of hydrogen loss from the atmosphere is estimated to be: comparing this with the high estimate given in part 2 of the hydrogen accumulation shows that the rate of hydrogen gain could be as large as or even larger than the rate of hydrogen loss. recent work by chamberlain and campbell (7) indicates that the rate of hydrogen loss from the atmosphere may be slightly smaller then the value calculated above. 4. conclusion. in spite of the considerable uncertenity both in the hydrogen loss and the hydrogen supply by the solar wind, the least one can say about the result is that: one cannot take it for granted that the earth's present cm2 • yr/ (where f is the radius of the earth). n . a . b a r r i c e l i . i r. m e t c a l f e hydrogen balance is negative, or in other words that the earth is still losing more hydrogen than is supplied to it by the solar wind. this conclusion does not contradict the explanation commonly given to the fact that the earth as well as the other three terrestrial planets (namely mars, venus and mercury) have a low content of hydrogen compared with the larger outer planets and the cosmic abundance of this element. the explanation that the earth's gravitational field is incapable of retaining a large concentration of hydrogen in it s atmosphere remains valid. however, the main loss of hydrogen by the earth must have occurred at the time when the earth had a reducing instead of an oxydizing atmosphere and probably contained a substantial concentration of hydrogen in its atmosphere (urey and miller (s), fox (!l)). the transformation from reducing to oxydizing atmosphere caused by photosynthesis in living systems may have influenced the earth's hydrogen balance in two different ways: 1 by binding most atmospheric hydrogen into water, thus preventing it from reaching the upper regions of the atmosphere where it could be lost by diffusion into space. 2 by supplying a chemical reagent capable of binding a significant portion of the hydrogen carried to the earth by the solar wind. whether at present the net result of these phenomena is still a reduction or a tight balance or even an increase of the earth's hydrogen is an unresolved question. however the very fact that the two figures are so close to one another that there can be any doubt at all about this point suggests an even balance between hydrogen supplied by the solar wind and hydrogen lost from the earth. otherwise one would have expected to find a difference by several orders of magnitude between the two figures, and the facts presented here would be difficult to explain as pure coincidence. the earth may he losing approximately the same amount of hydrogen which is being supplied to it by the solar wind, and the concentration of hydrogen, and perhaps even the concentration of water vapor in the upper stratosphere may basically depend on this hydrogen balance. acknowledgment. thanks are clue to professor kenneth c. clark for valuable advice and encouragement. a n o t e on t i i e loss of h y d r o g e n a n d t h e s u p p l y , e t c . 10'j r e f e r e n c e s ( ' ) jokipii j. r . , effects of diffusion on the composition of the solar corona and the solar wind. " t h e solar w i n d " , ( r o b e r t j . m a c k i n , jr., marcia n e u g e b a u e r , eds.) 215. j e t p r o p u l s i o n l a b o r a t o r y , california i n s t i t u t e of t e c h n o l o g y , p a s a d e n a , california, (190(3). ( 2 ) bridge u.s.,our knowledge of the solar wind. " t h e solar w i n d " , ( r o b e r t j. m a c k i n , jr., m a r c i a n e u g e b a u e r , eds.), 127. j e t p o r p u l s i o n l a b o r a t o r y , california i n s t i t u t e of t e c h n o l o g y , p a s a d e n a , california, ( i 9 6 0 ) . (:l) spitzer l y m a n jr., the terrestrial atmosphere above 300 km. " t h e a t m o s p h e r e of t h e e a r t h and p l a n e t s " ( g e r a l d p . k u i p e r , ed.), 245, t a b l e 5. t h e u n i v e r s i t y of chicago press, chicago, illinois, (1947). ( j ) p a r l e y i ) . t . , m c c l u r e j . p . , s t e r l i n g i ) . l . , g r e e n j . l . , temperature and composition of the equatorial ionosphere. " journal of geophysical research space p h y s i c s " , 72, 23, 5844-5845, f i g s . 5 (5, (1907). ( s ) evans j. w . , electron temperature and ion composition in the 1<\ region. " j o u r n a l of g e o p h y s i c a l research space p h y s i c s " , 72, 13, 3350. f i g s . 7-10, (1967). (*) hutchinson g. d . , the biochemistry of the terrestrial atmosphere. " t h e e a r t h as a p l a n e t " ( g e r a l d p . k u i p e r , ed.), 372, t a b l e 1, t h e u n i v e r s i t y of chicago press, chicago, illinois, (1954). ( ' ) chamberlain j. w . , campbell f . j., rate of evaporation of a non-maxwellian atmosphere. " a s t r o p h y s i c a l journal " , chicago, 149, 687-705, (1967). ( 8 ) urey h . c., miller e . l . , organic compound synthesis on the primitive earth. " s c i e n c e csss " , 245-251, (1959). (") p o x s. w . , how did life begin? recent experiments suggest an integrate origin oj anabolis, protein and cell boundaries. " science " , c x x x i i , 200-208, (1960). annals of geophysics, 55 (3), 2012: interview with f.ferrarotti social aspects of the earth sciences interview with prof. franco ferrarotti silvia peppoloni annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5632 franco ferrarotti, professor emeritus at ‘la sapienza’ university of rome; since winning the first chair in this discipline in italy in 1961, he has been considered the father of italian sociology. an independent member of italian parliament in the third government (1958-1963), a member of the new york academy of sciences, and a ‘visiting professor’ at many universities in europe, north america, russia, japan and latin america. franco ferrarotti has taught and still teaches in europe and america, and he has received many awards throughout his career. in this interview, he talks about the social aspects and consequences of earth sciences studies. silvia peppoloni (s.p.) good morning prof. ferrarotti. as one of the most important sociologists in the world, you can really help us to reflect on some essential points that must accompany our activities as geologists. as geologists, we study the earth, and the natural phenomena that take place on it, and we try to understand the causes, and to mitigate their negative effects. what are the social consequences of geological practice and research? franco ferrarotti (f.f.) at least for the time being, they are practically irrelevant. as almost all scientists in italy in particular, geologists are not heard. at the present time, italy is still culturally midway between the world of yesterday’s carelessness, the period of the great pre-scientific traditions, and the technologized and rationalized world. scientific evaluation, which should give us the exact coordinates of where we are and suggest what has to be done to reduce the risks, is irrelevant at the moment. science is viewed as entertainment, but it does not involve us, because the prevailing mentality both in policy makers and in ordinary people translates and reduces the political problems into literary perorations, transforms ethical problems into esthetic attitudes. this is the culture of ‘beautiful singing’: geologists say useful things, but unfortunately they do not sing. s.p. what can we do as geologists to change this state of affairs? f.f. geologists can do a lot. it is necessary to have greater moderation in judgments. in schools, there must be more space for scientific subjects. the risk we run is to move from a miracle-working and superstitious mentality to an attitude that believes that science can solve everything. this is wrong: science can have a decisive social impact to the extent that it is aware of its limitations. there are human problems, such as love, justice and death, that cannot be solved by science; however, science can help us. in a country such as italy especially, which is geologically ruined, from sicily to trentino, geology can achieve a lot. this does not happen though, because of the italian mentality, which is still deeply conservative, miracle-working, and devoted to chance and luck. s.p. what role do the media have in this situation? f.f. the media, as television, the internet, youtube, and much more, are important, but they are ethically indifferent: they theatricalize everything; everything becomes entertainment, and so an escape from the real problems. instead, specific problems require the intervention of science. science must make the transition from common opinion to established truths. it is generally believed that problems can be solved by using the media properly. this is a commonly held illusion. this is because there is something intrinsic to the media, which does not help the raarticle history received march 2, 2012; accepted march 10, 2012. subject classification: geoethics, earth sciences, society, scientific information, geological hazard. 347 tional interpretation of the situation. the media crush everything into the present time, and eliminate the memory. moreover, media do not mediate. for geologists, the memory is essential, because geological phenomena must be studied in a temporal dimension. the media, with their foolish ‘presentism’, cancel out the dimension of time. s.p. speaking of ‘memory’, we were all amazed to learn that during the disaster in sumatra, there were some indigenous people who recognized the precursors of the tsunami and saved themselves from the waves. thinking about the italian situation, about our villages inured to living together with the fragility of their territory: how important is it to maintain a collective memory of the hazards, of the disasters, that have occurred in our territory? f.f. collective memory is fundamental, but we are losing it. italy has become industrialized very quickly, in a single generation, and today we live in an industrial country, but we do not have an industrial culture. we have lost the wisdom of our ancestors, but we have not yet assimilated the scientific values. the media work against the memory. also thinking about history, when history is treated by the media, you lose the direct relationship with the ancient existential memory; on the contrary, our ancestors had it. on television, memory looks like an archeological find, which is separated from us: it entertains us, but it does not involve us. and still, without memory, we are nothing; without memory, there is gratuitousness, fatuousness, irresponsibility, and in the end, pure madness. man is only what he remembers he has been. memories are fundamental to understand where we came from, to understand our present situation, and to build a project for our future. as giustino fortunato, an expert in the problems of southern italy, wrote, “italy is a completely ruined territory hanging over the sea”. and then the question can be raised: where are the geologists? they should be in the front rows, they should be worshiped as ‘gurus’. instead, they live in their little corner, without disturbing anyone. they should yell: “we are in danger! the italian territory is one of the most seismic of our entire planet. at least, let us try to identify the real terms of the situation!” in italy, geologists are in an almost worse condition than sociologists. we are in the middle of enormous social problems and solutions are not found; they are not even looked for. we are faced with a territory that is crumbling, with houses that move, with hills that slide down, and geologists in italy are simply not there. their silence is deafening. from my point of view, this is the situation, and surely it is not positive nor adequate to solve the problems we are bound to face in the near future. s.p. thank you, professor, for the strong impulse that you have given us for changing this state of affairs. we will certainly bear your advice in mind. corresponding author: silvia peppoloni, istituto nazionale di geofisica e vulcanologia, rome, italy; email: 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 geochemical monitoring of volcanic lakes. a generalized box model for active crater lakes dmitri rouwet1,*and franco tassi2 1 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy 2 università degli studi di firenze, dipartimento di scienze della terra, firenze, italy annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5035 abstract in the past, variations in the chemical contents (so4 2−, cl−, cations) of crater lake water have not systematically demonstrated any relationships with eruptive activity. intensive parameters (i.e., concentrations, temperature, ph, salinity) should be converted into extensive parameters (i.e., fluxes, changes with time of mass and solutes), taking into account all the internal and external chemical–physical factors that affect the crater lake system. this study presents a generalized box model approach that can be useful for geochemical monitoring of active crater lakes, as highly dynamic natural systems. the mass budget of a lake is based on observations of physical variations over a certain period of time: lake volume (level, surface area), lake water temperature, meteorological precipitation, air humidity, wind velocity, input of spring water, and overflow of the lake. this first approach leads to quantification of the input and output fluxes that contribute to the actual crater lake volume. estimating the input flux of the "volcanic" fluid (qf kg/s) –– an unmeasurable subsurface parameter –– and tracing its variations with time is the major focus during crater lake monitoring. through expanding the mass budget into an isotope and chemical budget of the lake, the box model helps to qualitatively characterize the fluids involved. the (calculated) cl− content and dd ratio of the rising "volcanic" fluid defines its origin. with reference to continuous monitoring of crater lakes, the present study provides tips that allow better calculation of qf in the future. at present, this study offers the most comprehensive and up-to-date literature review on active crater lakes. 1. introduction the crater lakes that top active volcanoes are windows into the deeper parts of the volcano hydrothermal systems. more than 80 post-holocene volcanoes worldwide have a lake in their craters [simkin et al. 1981, delmelle and bernard 2000a]. a permanent crater lake should meet a combination of physical constraints: (i) the bottom of the lake should be protected against water seepage by physical sealing; (ii) the meteorological precipitation should be abundant (snowfall and its melt water, rainfall); (iii) the input of "volcanic" fluid should be sustained; and (iv) the heat input from the active volcano should be limited, to avoid drying out of the lake by evaporation [brown et al. 1989, pasternack and varekamp 1997]. the necessary coincidence of these physical factors stresses the vulnerability of the accumulated water masses inside active volcanic craters. in historical times, only 8% of volcanic eruptions have occurred beneath water bodies (crater lakes or shallow seawater), although these eruptions have produced about 20% of the total fatalities [mastin and witter 2000]. phreatic or phreatomagmatic explosions through crater lakes can eventually expulse water from the crater basin and generate base surges, lahars, tsunamis or seiches [sheridan and wohletz 1983, mastin and witter 2000]. coincidences between eruptive and seismic activities at volcanoes with crater lake systems have been described in previous studies [hurst and mcginty 1999, christenson 2000, martínez et al. 2000, obha et al. 2008]. recent modeling of eruptions from beneath active crater lakes has stated that the eruption style depends on the lake depth, the eruption pressure, and the mass ratio of lake water to superheated vapor [morrissey et al. 2010]. even during non-eruptive periods or quiescent degassing, crater lakes can pose indirect volcanic risks. the acidic crater lake brines can seep into the volcanic edifice, dissolve the wall rock, and decrease the volcano flank stability [rowe and brantley 1993, pasternack and varekamp 1994, rowe et al. 1995, sanford et al. 1995, delmelle and bernard 2000b, kempter and rowe 2000, taran et al. 2008, varekamp et al. 2001, varekamp 2008, varekamp et al. 2009]. these processes can lead to lahars, or flank failure and sector collapses [lópez and williams 1993, kerle and van wijck de vries 2001, reid 2004, procter et al. 2010], which can trigger major eruptions [voight et al. 1983]. the distribution of acidic brines into the hydrological network and plume emission into the article history received february 1, 2011; accepted may 3, 2011. subject classification: geochemical monitoring, active crater lakes, box model, mass budget, isotope and chemical budget. 161 atmosphere can affect vegetation and agricultural activity, leading to human health problems [sriwana et al. 1998, delmelle and bernard 2000b, heikens et al. 2005, löhr et al. 2005, van rotterdam-los et al. 2008]. in dormant volcanic systems, the co2-rich lake bottom waters can lead to catastrophic turnovers (nyos-type events), followed by the dispersion of a toxic co2 cloud in the area around the lake [kling et al. 1987, jvgr 1989, evans et al. 1994, kusakabe et al. 2000a, chiodini et al. 2010]. the presence of a water body (a hydrothermal system or crater lake) along up-rising patterns of volcanic gases causes water vapor and acidic gas species (co2, so2-h2s, hcl, hf) to dissolve, which establishes particular p-t-phredox conditions inside the volcano hydrothermal system. in active crater lakes, the acidic species are abundant, for creation of a ph lower than 3.8 to lose co2 to the atmosphere as a gas phase. the chemical–physical parameters of crater lakes can reflect and preserve signals related to eventual changes in volcanic activity. this "memory" of crater lakes provides an extra tool, as compared to open-conduit degassing volcanoes, where the recording of variations of gas discharge rates related to deep fluid inputs is directly lost to the atmosphere, often without being detected. when using crater lake chemistry and dynamics as monitoring tools, the challenge is to decipher the degassing history of the volcano over a period of observation. over the four decades of crater lake research, some crater lakes that manifest eruptive activity have been extensively studied (ruapehu, new zealand; poás, costa rica; lake yugama, japan, copahue, argentina-chile; santa ana, el salvador; rincón de la vieja, costa rica; raoul, kermadec islands; voui, vanuatu) [giggenbach 1974, brown et al. 1989, oppenheimer and stevenson 1989, takano and watanuki 1990, hurst et al. 1991, rowe et al. 1992a, rowe et al. 1992b, christenson and wood 1993, christenson 1994, ohba et al. 1994, rowe 1994, takano et al. 1994, rowe et al. 1995, christenson 2000, ohba et al. 2000, varekamp et al. 2001, bernard and mazot 2004, tassi et al. 2005, christenson et al. 2007, ohba et al. 2008, bani et al. 2009, christenson et al. 2010a, ohsawa et al. 2010]. the time series of datasets and the interpretations for these events can be considered the first examples of discontinuous volcanic monitoring using crater lake chemistry and dynamics. in the present study, we propose the box model approach for active crater lake systems, to better understand how physical and chemical changes in a crater lake can be related to possible variations in volcanic activity. we give tips on how to improve each parameter that contributes to the box model, to better constrain the relationships between a deep fluid input and its effects on crater lake chemical–physical properties. moreover, this report presents the most complete bibliographical compilation on active crater lakes at present, as a helpful guide for future studies. 2. active crater lakes volcanic lakes are classified into various groups according to their state of activity, based on their physical and chemical characteristics, such as low-activity to peakactivity lakes and no-activity lakes [pasternack and varekamp 1997], and co2-dominated, quiescent and "active" crater lakes [varekamp et al. 2000]. active crater lakes are generally gas dominated, and the solute compounds in crater lake water are thought to depend on the fluid dynamics in the magmatic–hydrothermal systems. variations in the absolute concentrations of chemical compounds in crater lake waters are only partly indicative of variations in the degassing regime. in some cases, the cation (al3+, fe2/3+, ca2+, na+, k+, mg2+, mn2+), sio2, and so4 2− and cl− contents in lake waters act as precursors of eruptive activities [ohba et al. 2008]. however, many nonvolcanic processes (e.g. dilution, evaporation, water mass loss, secondary mineral precipitation) affect the absolute solute contents, which prevents a systematic interpretation of the temporal patterns of compositional parameters. increases in the mg2+/cl− ratio of crater lake water have been interpreted as eruption precursors at ruapehu volcano since the 1970's [giggenbach 1974, christenson 2000, christenson 2010a], and in lake yugama in the 1980-1990's [ohba et al. 2008]. in volcano hydrothermal systems, cl− is generally considered a conserved species [pasternack and varekamp 1994], while mg2+ results from water–rock interactions that involve an unaltered rock (~fresh magma). therefore, mg2+/cl− increases in lake water are believed to indicate the intrusion of a new magma batch at shallow depth [giggenbach 1974]. however, for hyperacidic lakes (ph ≤0; e.g. poás), cl− can escape at the lake surface as a gas phase, while mg2+ remains unaffected, making the mg2+/cl− indicator often useless for the monitoring of set-ups [rowe et al. 1992a]. thiosulfate concentrations in acidic lake water have been correlated to pre-eruption activities of subaqueous fumaroles [takano 1987, takano and watanuki 1990]. thus, thiosulfates can sometimes be used as volcanic precursors, but these species can be detected in only a few lakes, depending on the temperature regime and so2/h2s ratio of the entering volcanic fluid [takano 1987]. the use of these geochemical parameters can help to provide a better understanding of the volcanic activity beneath crater lakes, although they are not systematically applicable for each crater lake. as volcano monitoring aims to quantify variations with time of parameters sensitive to changes in the volcanic status, it is recommended to convert the intensive parameters (solute concentrations and their ratios, ph and temperature) into extensive parameters (mass fluxes of water or chemical compounds). rouwet and tassi 162 163 3. the box-model approach in highly dynamic systems like active crater lakes, the main concept is the residence time of the lake water, which can be defined as [varekamp 2003, taran and rouwet 2008]: (1) where r is the residence time (s) at a certain time of observation, v is the lake volume (or mass) at a certain time of observation (kg), and f is the difference between the input and output water fluxes at a certain time of observation (kg/s). this definition of the residence time assumes steadystate for water (total input = total output, constant lake water volume), which is almost never the case for highly dynamic active crater lakes [varekamp 2003]. on the other hand, the volume of a crater lake arises from the difference between the input and output fluxes of the water mass added to the lake water volume at the previous time of observation (figure 1). if the observation or sampling frequency is higher than the residence time of the lake water, the input and output fluxes can be assumed to be constant for the observation period, and the water mass budget of the lake can be expressed in a differential form: (2) where vl is the resulting crater lake volume (kg), vl0 is the crater lake volume at the previous time of observation (kg), qf is the input flux of "volcanic" fluid from beneath the lake (kg/s), qm is the input flux of meteorological water (kg/s), qsp is the input flux of water from springs located outside the lake (kg/s), qe is the evaporative flux of water from the lake surface (kg/s), qs is the seepage flux of water out of the lake system (kg/s), qo is the overflow flux of water out of the lake basin (kg/s), dt is the period (s) between two times of observation of the crater lake (≈ sampling frequency). in equation (2), all of the fluxes are positive. the "volcanic" fluid can be defined as a fumarolic condensate that enters the lake bottom either as a vapor from the underlying volcano hydrothermal system, or directly, as an often highly saline and acidic liquid brine. to apply the mass balance to an active crater lake, it is thus strictly necessary that the observation period is shorter than the residence time of the crater lake water. if not, possible indicators of the changes in the activity of the volcano hydrothermal system will be "flushed out" of the lake before being detected. for example, at lake yugama (kusatsu-shirane volcano, japan), r is less than one year, so a monthly sampling frequency is adequate to detect possible variations in activity [ohba et al. 1994, 2000, 2008]; while the el chichón crater lake has a very low r of ~2 months, and thus requires a sampling frequency of at least monthly [taran and rouwet 2008]. 3.1. lake depth, surface area and volume the first unknown parameter in equation (2) is the variation in the lake volume (vl vl0). in general, the lake volume is deduced empirically and independently for each crater lake [hurst et al. 1991, ohba et al. 1994, martínez et al. 2000, rouwet et al. 2004, tassi et al. 2009]. the lake surface area can be obtained by global-positioning system (gps) measurements in the field [rouwet et al. 2008], topographic maps, satellite images [zlotnicki et al. 2008], or photographic methods [rouwet et al. 2004, rouwet 2011]. the lake depth can be obtained by direct measurements or by echo-sounding surveys along horizontal profiles from a raft [takano et al. 2004, christenson et al. 2010b]. on actively degassing lakes, bottom profiles from echo-sounding can be disturbed by the rising bubbles [caudron and bernard 2010]. the lake volume can be highly variable due to the presence of sulfur pools at the lake bottom [oppenheimer and stevenson 1989, hurst et al. 1991, christenson and wood 1993, takano et al. 1994], secondary mineral precipitation [christenson et al. 2010a] and changes in the sedimentary regime [rouwet 2011], phreatic eruptions breaching the lake bottom sediments [bani et al. 2009, rouwet et al. 2010], or dome intrusion that results in uplift of the lake bottom [bernard et al. 2010]. to improve the accuracy and frequency of depth measurements, floating buoys with pressure gauges have been recently installed on lakes, to register variations in hydrostatic pressure (y. taran and a. bernard, person. comm.). 3.2. input flux of volcanic fluid the input flux of volcanic fluids (qf), i.e. the only real "volcanic" factor that affects the crater lake mass balance, is an unknown parameter. however, quantifying qf and its variations with time will provide insights into possible variations in volcanic activity, which is within the scope of volcano monitoring programs. solving equation (2) for this "volcanic" parameter (qf) is possible when the physical "nona box model for active crater lakes v v q t q t q t q t q t q t l l0 f m sp e s o = + + +d d d d d dfigure 1. box model for active crater lakes, observed for a period of time t0 to t. r v f= volcanic" factors qm, qsp, qe, and qs are fully understood and quantified. 3.3. input flux of meteorological water the input flux of meteorological water (qm) to a crater lake system depends on the precipitation rate, which commonly refers to a certain period (e.g. daily), and on the surface of the catchment area for meteorological precipitation. to measure the precipitation rate, a rain gauge should be installed as near as possible to the lake. for lakes in tropical or temperate areas, the meteorological precipitation is a real-time parameter, whereas for lakes in cold areas (high altitudes and/or latitudes), meteorological precipitation is often snow, which will be released into the lake with a time delay, after the snow melts in spring and summer. the catchment area of meteorological precipitation is the crater surface area, and considered in this way, it is the total amount of meteorological water that finally contributes to the crater lake water volume by direct entrance, superficial run off or after infiltration. in reality, part of the meteorological water is dissipated by permeation through the volcanic edifice, and by evaporation from the ground of the emerged catchment area before contributing to the lake water volume. such losses strongly depend on the subsurface structure and hydrogeology of the summit system, which is generally a poorly constrained factor. to deduce the "effective catchment area" for meteorological precipitation [ohba et al. 1994], it is best to observe the direct effects of short-term major rain events on the (sudden) lake volume increase. nevertheless, it is not always possible to observe such events. for this reason, taking the crater area as the catchment area for meteorological precipitation is often the best alternative, although this can cause overestimation of the qm value. 3.4. input flux of spring water thermal or cold springs can significantly affect the mass balance of a crater lake [ohba et al. 1994, delmelle et al. 1998, stimac et al. 2004, rouwet et al. 2008]. for example, a geyser-like spring is a major fluid source for the el chichón lake (chiapas, mexico) [taran et al. 1998, rouwet et al. 2004, rouwet et al. 2008, taran and rouwet 2008] (figure 2). the input flux of springs (qsp) located outside the catchment area rouwet and tassi 164 figure 2. discharge of a thermal spring (qsp) towards an active crater lake (el chichón, chiapas, mexico). (a) the white arrow indicates the discharge of thermal spring water towards the lake. the people indicate the scale (photograph: d. rouwet). (b) a detail of the stream flow (photograph: y. taran), and (c) geyser-like thermal spring, with a shoe to indicate scale (photograph: d. rouwet). the white box indicates the location of the geyser-like thermal spring, enlarged in (c). 165 of a lake can be directly measured in the field, by means of a flow-meter device. 3.5. evaporative output flux the most prominent feature that proves that a crater lake acts as a calorimeter and window into the volcano hydrothermal system beneath the lake is the evaporative heat and mass loss from the lake surface (figures 3, 4). the evaporative output (qe) strongly depends on the difference between the temperatures of the lake surface and the ambient air above the lake, and the air humidity at the lake water temperature. the brown et al. [1989] equation, which was applied for the first time at poás laguna caliente (costa rica), is the most widely accepted method to calculate the evaporative heat loss (hl w/m 2) at the lake surface from the latent heat of vaporization of water (l-mj/kg): (3) where e is the average particle flux, which is dependent on temperature and wind speed; u is the friction velocity (m/s), which is dependent on wind speed; qw is the saturation vapor concentration of water in air at the lake temperature (kg/m3); and qa is the vapor concentration in air at the ambient temperature (kg/m3). generally, a low wind speed of 2 m/s is accepted to calculate the evaporative mass losses, considering that a crater lake surface is often screened from the dominant strong winds at high elevations of volcano summits by the crater rim [brown et al. 1989, taran and rouwet 2008] (figure 3). oppenheimer [1997] suggested that the bulk temperature measured a few centimeters beneath the lake surface is tenths to a few degrees higher than the real surface water temperatures (skin effect). often, the measured temperature thus erroneously results in overestimations of calculated evaporative fluxes. however, as broadly discussed in the literature, the wind speed immediately above the lake surface probably dwarfs the dt and humidity effects [brown et al. 1989, adams et al. 1990, hurst et al. 1991, ohba et al. 1994]. hurst et al. [1991] stated that «the calculation of evaporative losses is not straightforward with experimentally based formulae in apparent conflict». with the present instrumental means, it is impossible to measure the water (vapor) lost from a crater lake surface by evaporation. to date, no direct and consistent measurements of wind speed above crater lakes have been produced. it is strongly recommended to install permanent a box model for active crater lakes figure 3. evaporation (qe) from poás crater lake, costa rica (december 2009; photograph: d. rouwet). the lake temperature was ~45 ˚c, while the ambient temperature was ~10 ˚c. h le q q ul w a= -^ h meteorological stations to measure wind speed and direction, air humidity and temperature, and atmospheric pressure near the lake shore, coupled with a fixed temperature probe in the lake, to enhance the confidence of the qe parameter in equation (3). 3.6. seepage output flux the seepage loss (qs) through the crater lake floor is an important subsurface process. a reliable estimation of the seepage flux can be carried out through solving the mass balance for qs during quiescent periods of short duration, assuming a zero qf [rouwet et al. 2004]. although it is often considered as constant afterwards, qs is probably highly variable with time, and depends on: (i) the hydrostatic loading of the lake water mass [terada et al. 2010]; (ii) the lake bottom surface area [taran and rouwet 2008]; (iii) the physical sealing of the lake bottom by clays [rouwet 2011], secondary minerals or molten sulfur pools [oppenheimer and stevenson 1989, hurst et al. 1991, christenson and wood 1993, takano et al. 1994]; and (iv) the opening and sealing of fractures. during periods of peak activity, upward steam pressures probably inhibit direct seepage at the lake bottom, or force the lake water to infiltrate through alternative routes. moreover, phreatic eruptions can breach the lake bottom sediments or sulfur pools, strongly modifying the morphology and permeability of the lake bottom, and inevitably affecting the lake seepage. understanding the local tectonics inside and around craters can help to locate possible seepage routes. structural mapping using co2 flux surveys on and around crater lakes can give qualitative insights into the fluid migration regimes [mazot and taran 2009, mazot et al. 2011]. geophysical surveys (self potential, resistivity, magnetometry, microgravimetry) can increase the knowledge of the seepage regime of crater fluids [fournier et al. 2004, rymer et al. 2009, fournier et al. 2010, jutzeler et al. 2011]. these indirect approaches, however, do not help to quantify the seepage output flux. the results of hydrological and geophysical findings can be subjected to hydrogeochemical modeling to verify if the deduced fluid circulations are physically possible [christenson et al. 2010a]. by trial-and-error, this can allow a semi-quantitative estimation of the permeability that controls the seepage. 3.7. overflow output flux some lakes can reach the crater rim and overflow towards the volcano flank (e.g. lake nyos, cameroon; rouwet and tassi 166 figure 4. evaporation (qe) from paulina lake, newberry caldera, oregon, usa (august 2010; photograph: d. rouwet). the lake temperature was ~18 ˚c, while the ambient temperature was ~5 ˚c. 167 ruapehu, new zealand; boiling lake, dominica) [tuttle et al. 1987, christenson 2000, schaefer et al. 2008, fournier et al. 2009, christenson et al. 2010a]. in some cases, this overflow can have disastrous effects, leading to destructive lahars descending from volcano flanks (chiginagak, alaska; ruapehu, new zealand) [schaefer et al. 2008, procter et al. 2010]. such overflows (qo) can also be induced intentionally by human engineering, as drainage tunnels, to control the lake water volume, to avoid overflows leading to lahars or dissolved gas accumulation in the bottom waters of "nyostype" lakes (e.g. kelud, indonesia; lago albano, italy) [bernard and mazot 2004, carapezza et al. 2008]. as is the case for the input flux of spring waters, the overflow output flux of lake water can be generally measured directly in the field. most crater lakes, however, do not show overflow (qo.=.0 kg/s). 4. isotope and chemical budget analysis so far, the lake water chemistry has not been necessary to calculate the input and output fluxes that affect the mass budget, which is based only on the physical variations of the crater lake (lake level, surface area, volume, temperature, atmospheric parameters, meteorological precipitation). however, when lake water sampling is possible, the boxmodel approach can be expanded to the isotope and chemical budget of the crater lake. this has the great advantage that sources can be better characterized, physical processes can be detailed, and fluxes of chemical compounds can be calculated. considering that active crater lakes do not have a chemocline, due to lake convection [christenson and wood 1993, christenson 1994], a water sample from the surface is generally representative of the vertical water column. sampling and analytical procedures should remain identical during a period of monitoring, to avoid data inconsistencies. when crater walls are nearly vertical, a rappel descent is necessary (e.g. irazú, costa rica), or specific sampling devices have to be lowered into the lake from the crater rim (rincón de la vieja, costa rica and aso, japan) [tassi et al. 2005, miyabuchi and terada 2009]. at erupting volcanoes, sampling can become extremely dangerous. 4.1. isotope budget as all of the terms in equation (2) represent water masses, by multiplying each term with the corresponding values of the water isotopic composition (dd or d18o), the isotope budget of a crater lake is given by (figure 5): (4) where dd is the water isotopic composition, where the l, l0, f, m, sp, e, s, and o subscripts refer to the lake water, the lake water at the previous time of observation, the volcanic fluid entering from beneath the lake, the meteorological water, the spring water discharging into the lake, the vapor coming off the lake surface, the seeping lake water, and the overflowing lake water, respectively. dd is conveniently used because: (i) the d18o is sensitive to o-exchange between oxidized s-species and the water in variable thermal regimes [kusakabe et al. 2000b], which is a common situation for active crater lakes; and (ii) the d18o shows a positive isotope shift due to water–rock interaction processes, which are abundant in volcano hydrothermal systems [craig 1963, giggenbach and stewart 1982]. the isotope balance approach goes beyond a pure mass balance of crater lakes, as the isotopic composition of the contributing fluids provides qualitative insight into the chemical and physical processes that control isotopic fractionation. moreover, with the dd budget, we consider only the water, without considering the chemical compounds entering through the gas phase. the only fluids that are impossible to sample are the entering volcanic fluid (f) and the steam coming off the lake by evaporation (e). therefore, solving equation (4) for ddf or dde is most adequate, although one of the two parameters has to be deduced or calculated separately. if the rising volcanic fluid is assumed to be pure magmatic (andesitic) water, the ddf is expected to range from −30‰ to −10‰ [taran et al. 1989, giggenbach 1992]. however, this assumption is not straightforward, as the condensing volcanic fluid does not necessarily directly originate from the magma, but can be influenced by boiling and steam separation processes, lake recycling, or mixing processes. the isotopic composition of local meteorological water can be determined for rain water samples (ddm). the ddsp is directly obtained from the analysis of the spring water sample. the ddsp of each specific period of observation is often preferred to a long-term average ddsp. the isotopic composition of steam that originates from heated water bodies has been discussed in the literature [giggenbach 1978, giggenbach a box model for active crater lakes figure 5. box model for the isotope budget, observed for a period of time t0 to t. v d q d t q d t q d t+ + +d d d dd d d -= q d t q d t q d td d dd d dddvl l l0 l0 f f m m sp sp e e s s o o and stewart 1982]. the dde of the vapor released at the lake surface is subject to kinetic or equilibrium fractionation, and strongly depends on: (i) the differences between the temperature of the top layer of the lake water and the ambient temperature above the lake [ohba et al. 2000, taran and rouwet 2008]; (ii) the humidity of the boundary layer (the air layer immediately above the lake surface) [varekamp and kreulen 2000, and references herein]; (iii) the ph; and (iv) the salinity of the lake water [horita et al. 1993a, 1993b]. the kinetics and equilibrium isotopic fractionation for dde under a large variety of clearly controllable conditions is quantified in experimental set-ups [horita and wesolowski 1994, cappa et al. 2003]. nevertheless, the natural system, i.e. the evaporating crater lake, is far from an experimentally stable situation, which forces assumptions to be adopted regarding the calculated dde introduced in equation (4). different equations to calculate dde can be applied, although the exact isotopic composition of the evaporative steam from a crater lake at the moment of observation will hardly ever coincide with the calculated dde. the dds and ddo values introduced in equation (4) are simply taken to be the average of ddl for the period of observation ((ddl +ddl0)/2). among the parameters in equation (4), dde is the one with the largest uncertainty. a challenge for the future is to find an adequate technique to sample the evaporative steam. 4.2. chemical budget as already emphasized by the mass balance approach, increased release of volcanic fluids related to changes in volcanic activity are masked by nonvolcanic fluid contributions. the only certainty is that the main anionic species (cl−, so4 2−, f−) and acidity originate from a magmatic-related fluid entering the lake, i.e. volcanic fluid and/or thermal springs. dilution by meteorological and/or spring water, salinity increase by evaporation, and element loss through seepage or lake overflow are inevitable. following the above statements, it is not safe to suggest that an increase in the anionic content of crater lake waters is only due to the enhanced input of the volcanic fluid. chloride is considered be chemically conserved in acidic hydrothermal environments, while sulfur species are subject to precipitation as secondary minerals (anhydrite, alunite, native sulfur). for this reason, past studies have basically used cl− as the guiding species in chemical balances [ohba et al. 1994, rouwet et al. 2004, rouwet et al. 2008, taran and rouwet 2008]. based on equation (2), we can now multiply each term with the corresponding cl− content (figure 6): (5) where cl is the cl− content, and the l, l0, f, m, sp, e, s, and o subscripts are those used in equation (4). equations (2), (4) and (5) can be solved mathematically for the desired unknown parameter. for el chichón crater lake, qsp was calculated [rouwet et al. 2004, 2008], while clf was estimated for yugama lake following a similar approach [ohba et al. 2000]. it is clear that the expression "q cl dt" represents a cl− mass, and equation (5) respects the relative proportions of cl− given by the various q dt. the entering volcanic fluid cannot be sampled; therefore, the most common procedure is to calculate clf using equation (5), when the other parameters are known. unless acidic rain is formed due to the absorption of plume gases into the clouds, the entrance of cl− from meteorological water can be neglected (clm =0mg/l). spring waters can be sampled and clsp should thus be known. a significant hcl loss by evaporation occurs from the surface of hot hyper-acidic lakes (e.g. poás, costa rica; aso, japan). in this case, cle is the unknown term because the cl2-rich vapour is still impossible to sample. a promising tool is to determine the chemical species in evaporation plumes at active crater lakes by direct plume measurements [christenson et al. 2010a, christenson et al. 2010b, shinohara et al. 2010]. for lower temperature, lower acidity lakes, cl2 is not lost from the lake surface (cle =0mg/l). for the cl − lost by seepage, it is most correct to use the average cl− content of the lake water for the period of observation (cls = (cll + cll0)/2). if overflow occurs, the same is true for clo (clo =(cll +cll0)/2). at el chichón, no volcanic cl− is thought to enter the lake system, and the geyser-like springs were found to be the only cl− source for the crater lake during the period 19952007 [rouwet et al. 2004, rouwet et al. 2008, taran and rouwet 2008]. following similar approaches, ohba et al. [1994, 2000] calculated a clf even higher than the cl − contents in high-temperature volcanic gases, which suggested that volcanic fluids entering beneath crater lakes are cl−-enriched due to multi-step steam separation processes, and probably to lake water recirculation. by rouwet and tassi 168 figure 6. box model for the chlorine budget, observed for a period of time t0 to t. v cl v cl q cl t q cl t q cl t q cl t q cl t q cl t l l l0 l0 f f m m sp sp e e s s o o = + + +d d d d d d 169 multiplying clf by qf, cl − fluxes can be calculated. in our opinion, monitoring cl− fluxes of the volcanic fluid is the most direct way to detect changes in volcanic input. 5. conclusions based on a literature review, this study offers a generalized box-model approach that can be applied in continuous or discontinuous geochemical monitoring of active crater lakes. intensive geochemical parameters (solute concentrations, ph) do not systematically relate to variations in eruptive activities at volcanoes with crater lakes. on the contrary, the box model allows the calculation of extensive parameters (mass fluxes), providing a better representation of the chemical–physical variations with time. by quantification of the nonvolcanic factors that affect the physical–chemical state of an active crater lake, the box model allows calculation of the volcanic input flux (qf), which cannot be measured directly. a challenge for the future is to better constrain these nonvolcanic factors, in order to better define qf. as well as giving insights into the changes in volcanic activity (qf), an understanding of the seepage (qs) and overflow (qo) regime of crater lake systems can be useful in volcanic surveillance of indirect risks related to crater lakes (impact of contaminants on the environment, flank failure, lahars). the error of discontinuous monitoring methods is impossible to quantify, as it largely depends on: (i) the error in the single contributing parameters; (ii) the residence time of the lake water specific for each lake; (iii) the related sampling/observation frequency, which depends on access to the crater lake (~safety limits during eruptive cycles, logistics); and (iv) the averaging of the values for observation periods longer than the residence time of the lake. evolution towards continuous monitoring (visual-thermal cameras, and meteorological stations with data transmission systems, p-t-ph-conductivity probes) will increase the frequency, quality and safety of the data gathering. the quantitative and qualitative characterization of temporal variations in the volcanic and nonvolcanic parameters of the box model offers a means to better constrain the physical and chemical processes that occur in crater lakes with volcano hydrothermal systems. beyond the concept of continuous versus discontinuous monitoring, this box model is applicable for low-cost (mass budget), over medium-cost (chemical and isotope budget) to high-cost (continuous) monitoring setups. if volcano monitoring is a too ambitious goal for some crater lakes, within the limits of the available means, the box model proposed here will definitely help to develop basic conceptual models of crater lakes with volcano hydrothermal systems. acknowledgements. the authors wish to thank the editor, s. inguaggiato, for his insightful handling of this study, and reviewer a. caracausi for fruitful discussions that helped us to improve the manuscript. we are indebted to all of the previous workers on active crater lakes for their indirect contributions to this review paper. references adams, e., d.j. cosler and k.r. helfrich (1990). evaporation from heated water bodies: predicting combined forced plus free convection, water resour. res., 26, 425-435. bani, p., c. oppenheimer, j.c. varekamp, t. quinou, m. lardy and s. carn (2009). remarkable geochemical changes and degassing at voui crater lake, ambae volcano, vanuatu, j. volcanol. geotherm. res., 188, 347-357. bernard, a. and a. mazot (2004). geochemical evolution of the young crater lake of kelud volcano in indonesia, water-rock interaction (wri-11), wanty & seal ii eds., a.a. balkema publishers, 87-90. bernard, a., b. barbier, c. caudron, d. syahbana, a. solikhin, s. kunrat and v hallet (2010). the monitoring of volcanic lakes in indonesia, 7th workshop of the commission of volcanic lakes, costa rica. brown, g., h. rymer, j. dowden, p. kapadia, d. stevenson, j. barquero and l.d. morales (1989). energy budget analysis for poás crater lake: implications for predicting volcanic activity, lett. nature, 339, 370-373. cappa, c.d., m.b. hendricks, d.j. depaolo and r.c. cohen (2003). isotopic fractionation of water during evaporation, j. geophys. res., 108, 4525-4534. carapezza, m.l., m. lelli and l. tarchini (2008). geochemistry of the albani and nemi crater lake in the volcanic district of alban hills, j. volcanol. geotherm. res., 178, 297-304. caudron, c. and a. bernard (2010). hydroacoustic quantifications of co2 bubbles in volcanic lakes, 7th workshop of the commission of volcanic lakes, costa rica. chiodini, g., a. costa, d. rouwet and f. tassi (2010). modeling co2 air dispersion from gas driven lake eruptions, abstract v23a-2384, agu fall meeting. christenson, b.w. and c.p. wood (1993). evolution of the vent-hosted hydrothermal system beneath ruapehu crater lake, new zealand, bull .volcanol., 55, 547-565. christenson, b.w. 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di geofisica e vulcanologia. all rights reserved. a box model for active crater lakes completo_def.qxd 1501 annals of geophysics, vol. 47, n. 4, august 2004 key words volcanic gases – volcanic hazard – geochemical monitoring – geophysical monitoring – turrialba volcano – costa rica 1. introduction turrialba (10.03n, 83.77e) is an active stratovolcano belonging to the holocene «cordillera central» volcanic belt and is located at about 35 and 15 km from the towns of san josé and cartago, respectively (fig. 1). this region is part of the central mountain range conservation area and includes seven volcanic edifices. it was colonized by the indians of the oriental huetar kingdom and later by the spaniards. originally this volcano was named «torre alba» (white tower) by the spaniards, due to the presence of a large plume that looked like an enormous white tower. since its last eruption (16 september 1864), turrialba volcano has been characterized by a discontinuous fumarolic activity. a small plume, visible from the immediate surroundings of the volcano, has occasionally been reported. in the last decade, the presence of continuous weak fumarolic discharges has been observed. in the present study, the results of a geochemical monitoring carried out from 1998 to 2002 mailing address: dr. franco tassi, departimento di scienze della terra, università degli studi di firenze, via g. la pira, 4, 50121, firenze, italy; e-mail: francot@steno.geo.unifi.it fluid geochemistry and seismic activity in the period 1998-2002 at turrialba volcano (costa rica) franco tassi (1), orlando vaselli (1), vilma barboza (2), erick fernandez (2) and eliecer duarte (2) (1) dipartimento di scienze della terra, università degli studi di firenze, italy (2) volcanological and seismological observatory, universidad nacional, heredia, costa rica abstract turrialba volcano, located in central-southern costa rica, has been characterized, since the last period of eruptive activity in 1884-1886, by a weak and discontinuous fumarolic activity in the western area of its summit. during the 1998-2002 period, fumaroles discharging from central and west craters were collected for chemical analyses of major and trace gas compounds, 13c/12c in co2 and 18o/16o and d/h (in one fumarolic condensate), isotopic ratios. geophysical measurements (seismic activity and ground deformation), monitored in the same period, were compared to geochemical data to define the status of the volcanic system. chemical and isotopic characteristics of fumaroles of turrialba volcano seem to be related to interaction processes between a magmatic source and a shallower hydrothermal aquifer. since february 1997, seismicity at turrialba volcano gradually increased, while since august 2001 new fumaroles start to discharge from a new fracture system located in the area between central and west craters. since september 2001, strong compositional changes of gas discharges have been recorded at central crater. these occurrences are possibly due to variations in the permeability of the conduit system feeding the fumaroles. heat pulse episodes from a magmatic source have possibly caused the increase of vapour pressure at depth and, consequently, favoured the uprising of the magmatic fluids toward the surface. the observed evolution of chemical and physical parameters suggests that to forecast a possible renewal of the volcanic activity in the near future a full program of both geochemical and geophysical surveillance must be provided at turrialba volcano. 1502 franco tassi, orlando vaselli, vilma barboza, erick fernandez and eliecer duarte on the fumaroles of the summit craters are described and discussed. data from a geophysical monitoring of the seismic activity, carried out by the volcanological and seismological observatory of the national university of costa rica, are also included. thus, the main goals of this work are: i) to investigate the origin of the fumarolic discharges; ii) to evaluate the evolution in time of both chemical parameters and seismic activity and iii) to assess the status of activity of the volcanic system. 2. geological and volcanological background costa rica consists of six main geologic provinces (castillo, 1984): 1) cretaceous-middle tertiary ophiolitic suite; 2) tertiary basins; 3) tertiary volcanic ranges; 4) active quaternary volcanic ranges; 5) intra-arc basins; 6) caribbean coastal plain (fig. 1). the active quaternary volcanic ranges comprise the cordillera de guanacaste to nw and the cordillera central to se (fig. 1). the volcanic activity in costa rica in the last 25 000 years has been characterized mainly by phreatic and phreato-magmatic eruptions, with rare plinian events (alvarado et al., 1992) and is related to the subduction of the cocos plate underneath the caribbean plate (e.g., johnston and thorkelson, 1997 and references therein), with a subduction angle of about 30° (protti et al., 1995). turrialba is a composite stratovolcano (3,325 meters a.s.l.) and belongs to the cordillera central where two more active volcanoes with historical eruptions are present: irazu and poàs. this volcanic range also includes platanar, congo and barva volcanoes. historic eruptive activity of turrialba dates back to 1723, 1847, 1853, 1855, 1861 and 1864-1866 (castillo, 1984). they were probably small phreatic eruptive events, but very little is currently known about these volcanic activities. the last eruption (1864-1866) was characterized by the emission of large volumes of ashes that, during the period of major activity (february 1866), extended up to nicaragua (ovsicori-una, 1998). the west crater on the north-western side of the volcano summit (fig. 2) was formed during this eruptive event. in 1920 and in the period 1950-1960 local newspapers reported that large columns of vapour were observed from the city of cartago, showing that episodes of strong fumarolic emission have occurred several times in the last century. turrialba summit presently hosts three nesw oriented craters: east, central and west fig. 1. schematic geological map of costa rica. the middle america trench, the caribbean plate, the cocos plate, the cordillera de guanacaste volcanic arc, the cordillera central volcanic arc and the location of the turrialba volcano are reported. fig. 2. map of the summit craters of turrialba volcano with the location of principal, central and quemada fumaroles. 1503 fluid geochemistry and seismic activity in the period 1998-2002 at turrialba volcano (costa rica) craters (fig. 2). sulphur deposits of the central crater had been exploited till the end of the 80’s. the age of the craters decreases from east to west and the central and west craters currently have fumarolic emissions. 3. sampling and analytical procedures of gas discharges gas discharge sampling points are shown in fig. 2. fumaroles from both the central and west craters are characterised by a relatively low flux. they seep out, mainly as diffuse emissions, from areas of few tens of square meters and marked by an intense rock alteration. gas sampling of the west crater fumarole started in february 1998, whereas that of the central crater in february 1999, due to the fact that previously the flux of this gas discharge was too low and diffuse. the quemada fumarolic field, formed during summer 2001 but sampled only in april 2002, discharges from a large fracture (about 30 m long and 5 m wide) located between the west and central craters (fig. 2) where burning effects on the nearby vegetation were produced by the hot gas emission. gas sampling was performed with a titanium tube inserted into the gas outlets. gases were convoyed, through pyrex glass dewars, into pre-weighted and pre-evacuated 50 ml thorion-tapped vials to which 20 ml of a 0.15 m cd(oh)2 and 4 m naoh solution was previously added. here, acid gases (co2, so2, h2s, hcl, and hf) dissolve into the alkaline solution, while the unabsorbed gases (n2, o2, co, he, h2, ch4, ar, ne and light hydrocarbons) concentrate in the head-space. sulphur from h2s immediately precipitate as cds (montegrossi et al., 2001). major and trace inert gas compounds were analysed with shimadzu 15a and shimadzu 14a gas-chromatographs equipped with tcd (thermal conductivity detector) and fid (flame ionisation detector), respectively. co was determined, as ch4, after its conversion at 400°c (shimadzu mtn-1 methanizer), using h2 as carrier gas to allow the coch4 fischer-trops reaction (in presence of shimalite-ni acting as catalyser). the complete separation of ar-o2 and he-ne-h2 peaks was performed at low temperature (about 0°c, obtained with a shimadzu crg-15 cryogenic equipment) by using a 9 m length molecular sieve column. co2, trapped in the caustic solution of the sampling vials, was determined by acid titration with 0.5 m hcl, while so2, h2s, s0 and hcl abundances were performed by ionchromatography (dionex dx100 ion-chromatograph), as described in montegrossi et al. (2001). hf contents in the condensates (collected at the «bulbe» temperature by the gentle dropping from glass tubes wrapped with paper tissue) were determined by potentiometry with a specific electrode. analytical precision was < 1% for major components and < 5% for minor and trace compounds. 13c/12c ratios of co2 (expressed as 0/00 pdb) were determined with a finningan delta s mass spectrometer after the standard procedures of extraction and purification of the gas mixtures (evans et al., 1998). the reproducibility of δ-values for c is ± 0.10/00. δ18o and δd values (expressed as 0/00 smow) were determined with a finningan delta s mass spectrometer after equilibrating the waters with co2 and after the reaction of 10 ml of water with 0.3 g of pure zn at 500°c, respectively (vaselli et al., 1997, 1998). 4. gas chemistry and isotopic composition the analytical results for chemical and isotopic composition of the fumarolic discharges from the west crater (10 gases sampled from february 1998 to april 2002), central crater (6 gases sampled from february 1999 to april 2002) and quemada fracture (sampled in april 2002) are shown in tables i and ii. fumarole temperatures ranged between 86 and 92°c, with no remarkable differences among the three sites of gas discharges. gas chemical composition is characterized by the predominance of h2o (up to 980 000 µmol/mol) and co2 (up to 580 000 µmol/mol), with less n2, h2s, hcl, and h2 contents (6700, 2400, 570 and 428 µmol/mol, respectively). s0, hf, co, he, ne, ch4 and light hydrocarbons are present as trace compounds. ar contents are < 11 µmol/mol, indicating a very limited air-contamination on the composition of gas samples. 1504 franco tassi, orlando vaselli, vilma barboza, erick fernandez and eliecer duarte t ab le i. c he m ic al c om po si ti on o f in or ga ni c ga s co m po un ds a nd δ 13 c , δ1 8 o a nd δ d v al ue s of t he t ur ri al ba v ol ca no f um ar ol es . g as c om po un d co nte nt s ar e in µ m ol /m ol . n. a. : no t an al ys ed . t °c c o 2 h c l h f s o 2 h 2s s h 2o n 2 c h 4 a r o 2 n e h 2 h e c o δ 13 c δ 18 o δd w es t 02 -1 99 8 91 17 74 8 40 .5 2. 73 3 < 0. 01 19 2. 2 < 0. 00 1 98 15 51 41 1. 1 0. 00 7 1. 04 1. 0 0. 00 11 51 .9 6 0. 37 7 0. 00 23 n. a. –4 .7 6 –5 5. 1 w es t 02 -1 99 9 90 19 40 4 87 .4 0. 49 4 < 0. 01 83 .8 < 0. 00 1 97 99 85 41 0. 0 0. 04 3 2. 68 1. 1 0. 00 25 24 .6 0 0. 56 7 < 0. 00 05 – 2. 71 n. a. n. a. w es t 02 -2 00 0 91 11 26 5 20 5. 7 0. 24 4 < 0. 01 51 .6 5. 14 4 98 80 26 37 8. 7 0. 01 4 2. 39 1. 3 0. 00 39 63 .6 4 0. 34 4 < 0. 00 05 – 3. 07 n. a. n. a. w es t 02 -2 00 1 92 21 21 9 7. 8 0. 21 4 < 0. 01 44 .4 0. 26 1 97 83 26 29 9. 0 0. 01 3 1. 72 17 .4 0. 00 10 83 .8 0 0. 35 9 < 0. 00 05 n. a. n. a. n. a. w es t 09 -2 00 1 90 41 57 9 4. 1 0. 24 5 < 0. 01 20 .7 0. 50 3 95 80 75 22 3. 7 0. 01 5 0. 35 1. 9 0. 00 02 94 .3 5 0. 25 1 < 0. 00 05 n. a. n. a. n. a. w es t 11 -2 00 1 88 50 84 3 6. 4 0. 37 4 < 0. 01 34 .8 0. 43 9 94 86 65 31 3. 1 0. 01 6 0. 47 3. 3 0. 00 04 13 3. 47 0. 31 5 < 0. 00 05 n. a. n. a. n. a. w es t 01 -2 00 1 90 51 54 9 21 .3 1. 51 2 < 0. 01 14 .5 0. 64 5 94 80 13 20 9. 6 0. 01 9 0. 35 1. 8 0. 00 02 90 .7 8 0. 22 0 < 0. 00 05 n. a. n. a. n. a. w es t 06 -0 220 02 90 55 92 3 91 .9 4. 83 8 < 0. 01 14 .4 0. 49 9 94 33 42 54 0. 3 0. 01 2 0. 99 14 8. 7 0. 00 06 44 .5 4 0. 53 1 < 0. 00 05 n. a. n. a. n. a. w es t 19 -0 220 02 90 46 25 2 11 1. 3 5. 30 1 < 0. 01 17 .6 0. 51 1 95 29 91 45 6. 8 0. 01 8 0. 76 98 .7 0. 00 06 71 .1 2 0. 18 4 0. 00 13 – 2. 7 n. a. n. a. p ri nc ip al 0 420 02 90 63 40 2 2. 8 0. 14 5 < 0. 01 44 .5 0. 30 6 93 63 57 14 2. 1 0. 01 4 0. 28 1. 0 0. 00 03 49 .6 6 0. 20 9 < 0. 00 05 – 2. 83 n. a. n. a. c en tr al 0 219 99 89 20 42 7 17 .6 0. 09 8 < 0. 01 28 9. 3 < 0. 00 1 97 80 14 11 98 .0 0. 05 3 10 .0 7 37 .8 0. 00 97 3. 40 2. 13 5 < 0. 00 05 n. a. n. a. n. a. c en tr al 0 220 01 90 19 40 3 6. 4 < 0 .0 01 < 0. 01 16 .3 0. 21 8 97 95 08 85 7. 1 0. 00 3 8. 05 20 1. 3 0. 00 60 0. 12 0. 03 7 < 0. 00 05 n. a. n. a. n. a. c en tr al 0 920 01 89 18 12 98 31 .1 1. 01 9 < 0. 01 10 5. 0 2. 88 9 81 71 08 10 10 .5 0. 07 0 2. 12 12 .2 0. 00 12 42 8. 11 1. 25 2 < 0. 00 05 n. a. n. a. n. a. c en tr al 1 120 01 86 58 73 67 25 2. 4 8. 26 8 70 03 8. 45 24 04 .6 29 .2 62 33 31 15 67 04 .0 0. 08 9 10 .5 2 61 .9 0. 00 61 13 .7 8 2. 97 1 < 0. 00 05 – 2. 72 n. a. n. a. c en tr al 0 220 02 89 19 22 23 51 0. 2 5. 93 2 15 82 4. 60 34 5. 9 4. 23 3 78 74 61 30 12 .0 0. 03 2 8. 10 59 4. 2 0. 00 65 20 .3 9 0. 62 2 0. 14 46 – 2. 55 n. a. n. a. c en tr al 0 420 02 89 32 40 20 24 5. 5 33 .2 43 34 64 7. 04 13 82 .8 1. 22 9 63 85 91 11 04 .2 0. 02 2 1. 94 2. 0 0. 00 19 2. 84 1. 66 6 0. 06 65 n. a. n. a. n. a. q ue m ad a 04 -2 00 2 90 85 32 0 10 .6 0. 78 4 19 38 .0 2 25 .2 0. 25 2 91 20 58 61 3. 8 0. 09 0 2. 02 7. 0 0. 00 14 25 .3 8 0. 31 8 < 0. 00 05 n. a. n. a. n. a. 1505 fluid geochemistry and seismic activity in the period 1998-2002 at turrialba volcano (costa rica) t ab le ii . c he m ic al c om po si ti on o f or ga ni c ga s co m po un ds o f t ur ri al ba v ol ca no f um ar ol es . g as c om po un d co nt en ts a re µ m ol /m ol . c 2h 6 c 2h 4 c 3h 8 c 3h 6 ic 4h 10 nc 4h 10 1c 4h 8 ic 4h 8 t2c 4h 8 c2c 4h 8 ic 5h 12 nc 5h 12 c 5h 10 nc 6h 12 nc 7h 14 nc 8h 16 c 6h 6 c 7h 8 c 8h 10 m -c 8h 10 w es t 02 -1 99 8 0. 00 05 8 < 0. 00 00 5 0. 00 02 3 0. 00 02 3 0. 00 01 4 0. 00 05 3 0. 00 05 3 0. 00 06 6 0. 00 03 2 < 0. 00 00 5 0. 00 11 4 0. 00 02 3 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 06 0 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 02 -1 99 9 < 0. 00 00 5 < 0. 00 00 5 0. 00 05 1 0. 00 07 9 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 69 1 < 0. 00 00 5 0. 00 21 9 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 39 2 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 02 -2 00 0 0. 00 17 6 0. 00 04 7 0. 00 05 5 0. 00 09 1 0. 00 00 5 0. 00 01 6 0. 00 12 6 0. 00 00 5 0. 00 01 6 0. 00 10 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 02 1 < 0. 00 00 5 < 0. 00 00 5 0. 00 13 4 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 02 -2 00 1 0. 00 09 0 0. 00 16 9 < 0. 00 00 5 0. 00 18 1 < 0. 00 00 5 < 0. 00 00 5 0. 00 45 0 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 04 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 09 -2 00 1 0. 00 18 7 0. 00 03 2 0. 00 10 1 0. 00 06 8 0. 00 03 3 0. 00 03 4 0. 00 01 8 0. 00 07 8 < 0. 00 00 5 < 0. 00 00 5 0. 00 02 4 0. 00 01 1 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 01 1 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 11 -2 00 1 0. 00 20 4 0. 00 03 4 0. 00 11 5 0. 00 06 5 0. 00 04 8 0. 00 03 9 0. 00 01 5 0. 00 09 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 02 5 0. 00 01 0 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 01 0 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 01 -2 00 1 0. 00 28 2 < 0. 00 00 5 0. 00 16 8 0. 00 08 5 0. 00 05 4 0. 00 06 7 < 0. 00 00 5 0. 00 26 6 0. 00 00 7 0. 00 00 9 0. 00 05 3 0. 00 00 9 < 0. 00 00 5 < 0. 00 00 5 0. 00 00 6 0. 00 00 7 0. 00 01 6 0. 00 00 5 0. 00 00 5 0. 00 00 9 w es t 06 -0 220 02 0. 00 14 1 < 0. 00 00 5 0. 00 07 6 0. 00 03 9 0. 00 01 8 0. 00 03 0 < 0. 00 00 5 0. 00 07 0 0. 00 02 0 0. 00 00 6 0. 00 01 8 0. 00 02 0 < 0. 00 00 5 < 0. 00 00 5 0. 00 00 5 0. 00 00 5 0. 00 00 9 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 w es t 19 -0 220 02 0. 00 23 3 < 0. 00 00 5 0. 00 13 7 0. 00 08 0 0. 00 04 3 0. 00 03 6 < 0. 00 00 5 0. 00 15 2 0. 00 00 8 0. 00 01 0 0. 00 02 0 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 00 5 0. 00 00 5 0. 00 01 7 0. 00 00 5 < 0. 00 00 5 0. 00 00 6 w es t 04 -2 00 2 0. 00 14 4 < 0. 00 00 5 0. 00 07 9 0. 00 04 9 0. 00 03 0 0. 00 02 6 < 0. 00 00 5 0. 00 05 5 0. 00 00 5 < 0. 00 00 5 0. 00 02 2 0. 00 01 1 0. 00 00 5 0. 00 02 5 0. 00 00 7 0. 00 00 6 0. 00 04 6 0. 00 00 6 0. 00 00 5 0. 00 00 6 c en tr al 0 219 99 < 0. 00 00 5 0. 00 14 9 0. 00 00 0 < 0. 00 00 5 0. 00 20 3 0. 00 10 8 < 0. 00 00 5 0. 01 16 3 < 0. 00 00 5 0. 00 09 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 73 0 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 c en tr al 0 220 01 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 c en tr al 0 920 01 0. 00 93 9 0. 00 16 2 0. 00 50 3 0. 00 32 7 0. 00 15 6 0. 00 16 5 0. 00 04 9 0. 00 52 4 0. 00 04 4 < 0. 00 00 5 0. 00 13 2 0. 00 10 8 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 08 1 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 c en tr al 1 120 01 0. 00 51 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 47 3 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 c en tr al 0 220 02 0. 00 36 5 < 0. 00 00 5 0. 00 11 6 0. 00 09 4 0. 00 05 4 < 0. 00 00 5 < 0. 00 00 5 0. 00 34 3 < 0. 00 00 5 0. 00 09 0 0. 00 12 3 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 00 5 0. 00 01 2 0. 00 03 5 0. 00 01 2 0. 00 00 8 0. 00 00 5 c en tr al 0 420 02 0. 00 09 0 0. 00 01 6 0. 00 03 6 0. 00 02 3 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 06 2 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 < 0. 00 00 5 0. 00 00 7 0. 00 00 6 0. 00 00 5 0. 00 03 9 0. 00 00 5 0. 00 00 6 0. 00 00 5 q ue m ad a 04 -2 00 2 0. 00 41 8 < 0. 00 00 5 0. 00 08 4 0. 00 23 4 0. 00 04 2 0. 00 03 1 < 0. 00 00 5 0. 00 24 9 0. 00 03 0 0. 00 01 9 0. 00 02 8 0. 00 01 4 0. 00 02 6 0. 00 17 4 0. 00 05 1 0. 00 01 9 0. 00 20 5 0. 00 04 6 0. 00 11 6 0. 00 05 1 1506 franco tassi, orlando vaselli, vilma barboza, erick fernandez and eliecer duarte the wide variability of o2 contents (ranging from 1 to 594 µmol/mol) is possibly related to its partial consumption during fluid circulation, since it is involved in redox reactions with both the other gas compounds and the hosting rocks. so2 in the west gas samples is below the detection limit (< 0.01 µmol/mol). on the contrary, since november 2001, so2 has been detected in the central fumarole (up to 70 000 µmol/mol). quemada fumarole shows an intermediate composition between the west and the central fumaroles (tables i and ii). δ 13c in co2 range between – 3.1 and – 2.7 0/00 pdb, with no remarkable difference among the different gas discharges. δ 18o and d/h ratios, analysed in the condensate of the gas sample collected in 1998 at the west fumarole, are – 4.76 and – 55.1 0/00 smow, respectively. 5. discussion 5.1. origin of gases the chemical composition of turrialba gas discharges is relatively enriched in the less soluble acidic compounds, such as co2 and h2s, although the presence of hcl, so2 and hf (table i) indicates that, at least partly, the magmatic signature still remains. the relative abundances of n2, ar and he are plotted in fig. 3, where basaltic magmatism («mantle»), convergent plate boundaries («andesite») and crust fields («crust») are also reported (giggenbach, 1996). n2/ar ratios of all the samples (up to 640) are higher than that in the air (83), suggesting a non-atmospheric origin for n2, probably partly released from subducted sedimentary material, as commonly observed for gases collected in areas of convergent active tectonics (giggenbach, 1996), although he relative contents seem to differ sensitively from the typical volcanic arc gas compositions (sano and wakita, 1985; sano et al., 1987). as shown in fig. 4, ne/ar ratios are between those of air and air saturated water (asw), suggesting a common atmospheric source for these two compounds. concerning the organic gas fraction, up to 20 different hydrocarbon compounds pertaining to the c2-c8 species (the so-called «light hydrocarbons») were detected. relative fig. 4. ar-n2/100-1000ne ternary diagram for the turrialba fumaroles. air and air saturated water (asw) compositions were reported. symbols as in fig. 3. fig. 3. n2/100-ar-10he ternary diagram for the turrialba fumaroles. basaltic magmatism («mantle»), convergent plate boundaries («andesite») and crust fields («crust») are also reported (giggenbach, 1996). asw: air saturated water open squares: principal crater fumarole; open up triangles: central crater fumarole; open down triangles: quemada fumarole. 1507 fluid geochemistry and seismic activity in the period 1998-2002 at turrialba volcano (costa rica) high contents of hydrocarbons pertaining to both aromatic and alkene groups were systematically observed in fumaroles from volcanic areas and geothermal fields with respect to those measured in gases from quiescent volcanism (capaccioni et al., 1993). the composition of the organic gas fraction can be referred to the occurrence of two chemical processes: i) catalytic reforming reactions and ii) thermal cracking reactions (capaccioni et al., 1995). the catalytic reforming process is typical in hydrothermal environment, while cracking reactions are considered negligible at t < 300°c. at turrialba volcano production of hydrocarbon gas species is probably controlled by catalytic-type oxidation and hydration reactions (seewald, 2001), which may lead to the formation of both alkane and aromatic gas compounds. however, a notable influence of kinetic processes, through the breakage of carboncarbon bonds, produces a general reduction of the molecular weight and possibly leads to the generation of alkene gas compounds. preliminary analytical results of r/ra (where r is the 3he/4he ratio in the sample and ra is the 3he/ 4he ratio in the air: 1.34 × 10–6) values are up to 7.5 and suggest that turrialba gas discharges are strongly influenced by a magmatic component (vaselli et al., 2002). values of δ13c (table i; fig. 6) suggest that co2 derived from a mixing between a possible mantle source and a sedimentary end-member (giggenbach et al., 1993; rollison, 1993; sano et al., 1997). as shown in fig. 7, where the «andesitic water» area (taran et al., 1989; giggenbach, 1992) is also reported, a condensate sample of west fumarole (sampled in 1998) has isotopically heavier δ 18o and d/h ratios than those of the meteoric water (lachniet and patterson, 2002), considering that the altitude of the recharge area is at about 3500 m. this supports the hypothesis that turrialba fumaroles were recharged by significant contribution of magmatic water. from xa = (δg-δm) / (δa-δ m) (giggenbach and corrales, 1992), where δg represent the isotopic ratio of 18o/16o or d/h, δa is the corresponding ratios of the «andesitic waters» and δm is the isotopic composition of the local meteoric recharge, a fraction of about 30% of «andesitic water» was estimated. aromatics alkanes alkanes fig. 6. δ 18o-δd diagram for the condensate of the central crater fumarole at turrialba volcano. the smow field, the «andesitic water» field (taran et al., 1989) and the local meteoric water line (lachniet and patterson, 2002), were reported. symbols as in fig. 3. δ18o δ 2 h mw: δd=7.6 *δ18o+10.5 fig. 5. alkanes-aromatics-alkanes ternary diagram for the turrialba fumaroles. symbols as in fig. 3. 1508 franco tassi, orlando vaselli, vilma barboza, erick fernandez and eliecer duarte fig. 7a-c. temporal variations of 10∗h2s and so2 (a), 20∗hf and hcl (b) and co2 and 2000∗h2 (c) contents for central crater fumarole at turrialba volcano. a b c as a consequence, the chemical composition of the gas discharges at turrialba volcano is likely to be related to the interaction between hot fluids ascending from the deep magmatic system, and a shallow aquifer. the latter is able to lower the temperature of the uprising fluids (temperatures of gas discharges are below 92°c) and dissolve the deep-originated acid species. 5.2. temporal evolution of gas composition during the period the present study was carried out (february 1998-april 2002), major chemical changes in the central fumarole were detected, whilst the west fumarole did not suffer any substantial compositional modification (tables i and ii). the newly formed fracture system (quemada) released gases with a chemical composition approaching that of the central fumarole. in spite of the measured changes in gas composition, no variations of the outlet temperatures of all the fumaroles were observed. on the basis of the data gathered, the chemical evolution of the fumarolic discharges at turrialba volcano can be summarised as follows: 1) between february 1998 and february 2001 gas discharges were characterised by h2o contents > 940 000 µmol/mol, minor contents of co2 and h2s (up to 20 000 and 290 µmol/mol, respectively) and very low concentrations of typical high temperature compounds (up to 205, 5.3, 0.0023 and > 0.01 µmol/mol for hcl, hf, co and so2, respectively). 2) in september 2001 the chemical composition of central fumarole started to change, with a dramatic increase in both co2 and h2 contents (fig. 7c). 3) in november 2001, the same fumarole showed a further increase in co2 (up to 587 000 µmol/mol), appearance of relatively high contents of so2 (up to 70 000 µmol/mol), and an increase in h2s, hcl and hf concentrations of about one order of magnitude (from 105 to 2400, 31 to 252 and 1 to 38 µmol/mol, respectively) (fig. 7a,b). 4) in february and april 2002 significant co contents were measured (up to 0.1146 µmol/mol) (table i). the compositional evolution of central fumarole has to be related to a change of the equilibria between the hot deep source and the cold 1509 fluid geochemistry and seismic activity in the period 1998-2002 at turrialba volcano (costa rica) perature rises up to 1161°c in april 2002), on which, as suggested by chemical and physical evidence, the magmatic influence on fluid chemistry is particularly high. 5.4. seismic activity and ground deformation further information on the state of activity of turrialba has been provided by the geophysical surveillance that started in 1990 with a single station located at the summit of the volcano. since 1999 vdap-usgs assistance to ovsicoriuna has provided 4 seismic telemetric stations, including a three component short-period stashallow liquid buffer, probably due to a «deep hot pulse» which occurred in summer 2001. the sudden increase in h2 contents recorded in september 2001 in central fumarole was possibly produced by a more effective dissociation of h2o related to the described increase of heat flux from depth. as a further consequence of the increase in the vaporization rate of the shallow aquifer, major changes in the permeability of the fracture system that feeds the fumarolic discharges occurred. this allowed the acid species such as so2, hcl and hf, to reach the surface, as confirmed by the composition of central crater fumarole after november 2001. on the contrary, the temperature continued to be buffered by the shallow system (the fumaroles remained below 90°c, the boiling temperature of water at about 3000 m of altitude). 5.3. geothermometry equilibrium temperatures calculated by h2/ar geothermometer (chiodini et al., 2001) are 621 and 495°c for west and central fumaroles, respectively (table iii). it is possible to speculate that such temperatures are consistent with thermodynamic conditions regulating the volcanic system at depth. the difference among the results obtained from the chemical composition of the two fumaroles is likely to be related to air-contamination, which particularly affected ar contents of central gas discharge due to the low gas flux. the relatively high temperature calculated for the sample collected in september 2001 at central fumarole (about 140°c above the mean value; table iii), is related to the previously mentioned sudden increase in h2 (table i). differently, geothermometric calculation based on ch4/co2 redox pair (giggenbach and goguel, 1989) indicates mean equilibrium temperatures of 868 and 963°c for west and central fumaroles, respectively. since these ch4/co2 temperatures are too high to correspond to realistic thermodynamic conditions at depth, they are possibly due to partial methane destruction by thermal degradation, able to modify ch4/co2 ratios established through thermodynamic equilibria. this effect is more evident for the central gas samples collected after february 2001 (equilibrium temtable iii. equilibrium temperatures calculated with h2/ar and ch4/co2, geothermometers for the turrialba volcano gases. temperatures are in °c. ch4/co2 h2/ar west 02-1998 885 325 west 02-1999 702 550 west 02-2000 754 581 west 02-2001 832 598 west 09-2001 894 647 west 11-2001 914 648 west 01-2001 892 646 west 06-02-2002 963 595 west 19-02-2002 884 616 west 04-2002 963 635 mean west 868 621 central 02-1999 687 456 central 02-2001 1020 367 central 09-2001 887 638 central 11-2001 1018 495 central 02-2002 1005 513 central 04-2002 1161 498 mean central 963 495 quemada 04-2002 773 559 1510 franco tassi, orlando vaselli, vilma barboza, erick fernandez and eliecer duarte tion, to improve the geophysical monitoring of the area. in the period 1990-1996, seismic activity was relatively low (0-91 events/month). in may 1996, small magnitude lp events (m < 1.6 in the richter scale) occurred (fernández et al., 1996). since february 1997 seismicity gradually has increased, up to 2000 events/month, and in october 2000 a further increase was recorded. in january 2001, late march 2001, october 2001 and october 2002, four seismic swarms characterized by hybrid (dominant frequency 2.1-3.5 hz) and lp events (dominant frequency < 1.8 hz) with a maximum magnitude of 2.0 were detected. determination of hypocenters located the seismic events at an estimated depth of 4-6 km, beneath a 4 km elongated area comprising the summit craters and the northern flank. in addition to seismic records, dry tilt measurements carried out at the «la silvia» station located 2.5 km w of the volcano summit showed, between october 1996 and january 1999, an accumulated radial inflation of 28 µrad (barboza et al., 2003). the characteristics of the geophysical parameters seem to indicate the occurrence of hydrofracturaction phenomena at depth (barboza et al., 2003), promoted by an increase in pressure due to fluid vaporization possibly triggered by an enhanced heat input, that have also caused a reduction of the efficiency of the aqueous buffer system. eventually, at the top of the volcano, the ground deformation caused the opening of new fractures with radial and concentric direction with respect to the position of the central crater. 6. concluding remarks the evolution of the chemical composition of the fumarolic gas discharges along with the geophysical parameters suggests that a relatively high-temperature system is still active at turrialba volcano and major modifications of the present status of activity are occurring. the described behaviour of the volcanic system is not necessarily related to an incipient resumption of eruptive activity at turrialba volcano, that would eventually be preceded by further changes in gas chemistry, ground deformation and seismic signals. moreover, previous studies on these fumarolic discharges carried out in the earlier eighties detected the presence of so2, h2 and co in the gas phase (cheminée et al., 1982). this may suggest that, before the chemical changes recorded after september 2001, variations in gas chemistry of this volcanic system had already occurred. by coupling these observations with the last volcanic events, mainly phreatic eruptions, at turrialba two different scenarios can be hypothesised: i) the increasing pressure may result in an enhanced fracturing system at the craters, that may allow a release of the accumulated energy without eruptive events; ii) the overpressure may trigger small-to-medium size phreatic eruptions. alternatively, we can speculate that the persistence of high heat-flux from depth may cause the total consumption of the shallow aquifer. in this case, a further rapid evolution of the chemical characteristics of the fluids and a strong increase in the temperatures of the fumaroles should be detected prior to the occurrence of an eruptive event, probably characterised by a significant magmatic contribution. in order to monitor the activity of the turrialba volcanic system, a bi-weekly or at least monthly gas sampling collection, with an improved seismic and ground deformation surveillance, is highly recommended. acknowledgements many thanks are due to f. bergamaschi for his help during the sampling campaign in febraury 2001. this work was supported by the italian space agency (resp. ov). references alvarado, g.e., s. kussmaul, s. chiesa, p.y. gillot, h. appel, g. corner and c. rundle (1992): resumen cronoestratigrafico de las rocas igneas de costa rica basado en dataciones radiometricas, j. south. am. earth sci., 6 (3), 151-168. barboza, w., e. fernandez, e. duarte, w. saenz, m. martinez, n. moreno, t. marino, r. van der laat, e. hernandez, e. malavassi and j. valdes (2003): changes in the 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(1984): geologia de costa rica, editorial de la universidad de costa rica, san josè, costa rica, pp. 188. cheminée, j., m. javoy and h. delorme (1982): temperature and gas data from turrialba, 01/83 (sean 08:01). chiodini, g., l. marini and m. russo (2001): geochemical evidence for the existence of high-temperature hydrothermal brines at vesuvio volcano, italy, geochim. cosmochim. acta, 13, 2129-2147. evans, w.c., l.d. white and j.b. rapp (1998): geochemistry of some gases in hydrothermal fluids from the southern juan de fuca ridge, j. geophys. res., 15, 305-313. fernández, e., e. duarte, v. barboza, r. van der laat, e. hernandez, m. martinez and r. sáenz (1996): number of microseismic events continues to increase, 12/96 (bgvn 21:12). johnston, s.t. and d.j. thorkelson (1997): cocos-nazca slab window beneath central america, earth planet. sci. lett., 146, 465-474. giggenbach, w.f. (1992): isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin, earth planet. sci. lett., 113, 495-510. giggenbach, w.f. (1996): chemical composition of volcanic gases, in monitoring and mitigation of volcanic hazards, edited by m. scarpa, r. tilling (springer, berlin), 221-256. giggenbach, w.f. and r.s. corrales (1992): isotopic and chemical composition of water and steam discharges from volcanic-magmatic-hydrothermal systems of the guanacaste geothermal province, costa rica, appl. geochem., 7, 309-332. giggenbach, w.f. and r.l. goguel (1989): collection and analysis of geothermal and volcanic water and gas discharges, dsir chemistry, rep. 2401. giggenbach, w.f., y. sano and h. wakita (1993): isotopic composition of helium, and co2 and ch4 contents in gases produced along the new zealand part of a convergent plate boundary, geochim. cosmochim. acta, 57, 3427-3455. lachniet, m.s. and w.p. patterson (2002): stable isotope values of costa rican surface waters, j. hydrol., 260, 135-150. montegrossi, g., f. tassi, o. vaselli, a. buccianti and k. garofano (2001): sulphur species in volcanic gases, anal. chem., 73, 3709-3715. ovsicori-una (1998): report turrialba activity, smithsonian institution: global volcanism network bullettin, 23/3. protti, m., g. guendel and k. mcnally (1995): correlation between the age of the subducting cocos plate and the geometry of the wadaty-benioff zone under nicaragua and costa rica, in geologic and tectonic development in the caribbean plate boundary in southern central america, edited by p. mann, geol. soc. am. spec. pap. 295, 309-326. sano, y. and h. wakita (1985): distribution of 3he/4he ratios and its implications for geotectonic structure of the japanese islands, j. geophys. res., 23, 109123. sano, y., h. wakita and w.f. giggenbach (1987): island arc tectonics of new zealand manifested in helium isotope ratios, geochim. cosmochim. acta, 51, 1855-1860. rollison, h. (1993): using geochemical date. longman scientific and technical, pp. 352. sano, y., t. gamo and s.n. williams (1997): secular variations of helium and carbon isotopes at galeras volcano, colombia, in galeras volcano colombia: interdisciplinary study of a decade volcano, edited by j. stix, m.l. calvache and s.n. williams, j. volcanol. geotherm. res., 77, 255-265. seewald, j.s. (2001): aqueous geochemistry of low molecular weight hydrocarbons at elevated temperatures and pressures: constraints from mineral buffered laboratory experiments, geochim. cosmochim. acta, 65, 1641-1664. taran, y.a., b.g. pokrovsky and a.d. esikov (1989): deuterium and oxygen-18 in fumarolic steam and amphiboles from some kamchatka volcanoes: «andesitic waters», dokl. akad. nauk sssr, 304, 440-443. vaselli, o., p. censi, d. abbado, a. minissale, m. paolieri and f. tassi (1997): regolamento operativo per le analisi dei rapporti isotopici 18o/16o e 13c/12c nei carbonati, 13c/12c nella co2 dei gas e 18o/16o nelle acque, internal report cnr-csmga, 1, 1-14. vaselli, o., p. censi, a. minissale, a. casiglia, m. paolieri and f. tassi (1998): regolamento operativo per le analisi dei rapporti isotopici d/h nelle acque, internal report cnr-csmga, 2, 1-12. vaselli, o., f. tassi, e. fernandez, e. duarte, v. barboza, g. magro and a. minissale (2002): fluid geochemistry evolution at turrialba volcano (costa rica). colloque intl., mt. pelèe 1902-2002, explosive volcanism in subduction zones, 12-16 may, 2002, ilede la martinique (france), 90. annals of geophysics, 55 (3), 2012: lambert geoethics: a perspective from australia ian b. lambert secretary general, 34th international geological congress, geoscience australia, canberra, australia annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5556 abstract this short article reflects my experience over many years in provision of the geoethics symposium at geoitalia 2011. i was pleased to be invited to provide a brief address at this meeting because it gave me the opportunity to promote the 34th international geological congress (igc) that will to be held in brisbane, australia, on august 5-10, 2012, and which will have a strong geoethics symposium. my succinct remarks reflect my experience over many years in provision of technical information and advice in support of australian government decisions and policies on mining, energy and groundwater – all of which have core geoscientific elements. further, they reflect the situation in a new world country with a strong economy dominated by mining, which differs in many ways from the countries where the field of geoethics has been nurtured and grown. they also outline a dilemma relating to mining in a developing country. 1. introduction this short article is based on the introductory remarks i made in the geoethics symposium at geoitalia 2011. i was pleased to be invited to provide a brief address at this meeting because it gave me the opportunity to promote the 34th international geological congress (igc) that will to be held – in brisbane, australia, on 5-10 august 5-10, 2012 – and which is towill have a strong geoethics symposium. however, i was also apprehensive, because i had had very limited exposure to “geoethics” until 2010. when considering the scope of the scientific program for the igc, it became obvious that there were some very active researchers and practitioners of geoethics in parts of europe, and russia, and in some developing countries. previously, i was more familiar with terms such as “best practice” and “a social licence to operate”, which i submit encompass the essence of geoethics. this article reflects my experience in provision of technical information and advice in support of australian government decisions and policies on mining, energy and groundwater, – all of which have core geoscientific elements. further, it reflects the situation in a new world democracy with a healthy economy that is dominated by mining. as the flattest, most arid, deeply weathered and least-populated of the inhabited continents, australia differs in many ways from the countries where the field of geoethics has been nurtured and grown, and from others where its practice is particularly important. after outlining some of the factors used in australia to balance economic with environmental and social benefits, it this article considers a dilemma relating to mining in a neighbouring developing country, and makes provides some concluding remarks. 2. the australian situation in australia, the main industries which that have potential for significant environmental and social impacts include mining, manufacturing, chemicals and agriculture. the last few decades have seen a progressively increasing focus on environmental and social responsibilities. the framework for this is provided by the federal ‘environment and biodiversity protection act’, and complementary state and territory legislation. there are also guidelines for good practices for many activities, that have often been developed by governments with constructive industry and community inputs, to increase the probability that they will work in practice. professional societies accredit geoscience professionals and develop and disseminate codes of ethics. many industries and individual companies have also established codes for ethical behaviour and sustainability policies. achieving a social licence to operate is a widely accepted principle. reporting is required for authorised and accidental releases into the environment, and monitoring is undertaken to give early warning of any emerging problems. it is common that governments and industry co-regulate activities, and this generally works well, given the effective checks and balances that are part of the regulatory system. particularly in the case of mining, rehabilitation funds or bonds are commonly required during operations to cover costs of remedial works after closure. the legislative framework and broadly transparent article history received january 9, 2012; accepted january 12, 2012. subject classification: geoethics, mining, sustainability. 377 processes are important in reaching decisions on major projects which that have to balance the various interests and benefits. no system is perfect, but where things are not working out as planned, review processes and public pressure lead to improvements, or closure in extreme cases. the ultimate responsibility to perform satisfactorily is with the companies, and failure to do so can result in heavy fines and even imprisonment of responsible managers. 3. an ethical dilemma in contrast with to australia’s stable setting, the island nations to the northeast, north and northwest are characterised by high rainfall and relief, high seismicity, landslides and volcanic activity, which reflecting their near near-equatorial latitudes and associations with active plate margins. such geological settings are highly prospective, particularly for world-class copper and gold deposits. mining activities in these environments can lead to ethical dilemmas. i will illustrate this for a major multinational company mining a large cu-au deposit in a developing country, the government of which is part owner of the mine. at the outset, a dam was built to contain tailings on the mine lease. given the extreme conditions in the region, it was not surprising that this dam soon collapsed. ever since, tailings have been washing down the river system with government approval. in places, tailings have deposited in overbank areas, fishing has been affected, and some communities have had to be re-located. this attracted a lot of attention from international non-governmental organisations (ngos), which have been very vocal in trying to shut the mine, using electronic and other media to disseminate their fears of major permanent environmental destruction. at this stage, i suspect that many of you will be agreeing with the ngos and thinking of what action you would take to help achieve mine closure. where is the ethical dilemma? government ministers ministers from the developing country have angrily denounced the pressure to close the mining operations. they have questioned the rights of foreigners to protest against the mine, stating that these people are not speaking in the best interests of the developing country, which has received schools, hospitals, jobs and income from the mine. they pointed out that scientists who have studied the river system have concluded that the main impacts will greatly diminish as it is all flushed out by floods after the mining ceases, and that no significant environmental changes have been detected in the marine zones beyond the deposits at the river mouth. i will leave it to geoethical practitioners to consider where the moral high ground is in such a situation, and to ponder other situations where communities are benefitting from better services and lifestyles in the short shortto to-medium term through activities which that need to be conducted ethically to sustain communities in the longer term. 4. concluding remarks many activities that are undertaken to meet societal needs, or simply for financial gain, as well as natural disasters, can have can impact on environments and communities. it is harder to hide and ignore damaging activities and events now than in the past, because the internet and social media are provideing powerful means for exposing these and for exerting pressure on those causing the problems. governments have primary responsibility for putting into place an effective framework of measures to minimise minimize the impacts within their jurisdictions. in most developed countries, there are formal and informal approaches designed to keep impacts within reason. these vary from highly prescriptive – where a fixed set of practices or particular technologies are specified – to outcomes outcomes-based regulation based on underpinning principles. these approaches are generally effective, although there will always be a the need for vigilance, to avoid distortions and cutting corners for financial gain. in those developing and centrally planned countries where bad practices and corruption are widespread, sound foundations for ethical behaviour depends primarily on the appropriate legislation, good governance and a balance of assistance and penalties. geoethicists have particularly important roles to play in promoting the responsible behaviours and practices that are required for sustainable development. a measure of their success will be increasing levels of employment in governments and companies, to help achieve these ends. corresponding author: ian b. lambert, email: ian.lambert@ga.gov.au. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ian b. lambert 378 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice t h e b e h a v i o u r of a large dam at severe frost p . c a l o i ( * ) m . m i g a n i ( * * ) m . 0 . s p a d e a ( * ) r e c e i v e d 011 m a y 5tli, 1972 summary. — s y n t h e s i z i n g t h e p r o b l e m , t h e a c t i o n of t h e t h r u s t s i n t h e b e h a v i o u r of t h e d a m of p i e v e di c a d o r e , m a k e s itself c o n s p i c u o u s e x p e c i a l l y d u r i n g t h r e e p e r i o d s of t h e y e a r : 1. a b o u t t h e e n d of j u n e , t h e a i r t e m p e r a t u r e , 011 t h e a v e r a g e , o v e r c o m e s t h e w a t e r o n e i n t h e w a t e r s h e d u p s t r e a m t h e d a m : t h e b e n d i n g of t h e d a m u p s t r e a m i n c r e a s e s f r o m t h e b o t t o m t o t h e t o p . 2. a b o u t t h e e n d of o c t o b e r , t h e t h e r m a l c o n d i t i o n s c h a n g e ; t h e m e a n a i r t e m p e r a t u r e g r o w s l o w e r t h a n t h e m e a n w a t e r t e m p e r a t u r e ; t h e d a m b e g i n s h e r b e n d i n g d o w s t r e a m . 3. w h e n t h e a i r t e m p e r a t u r e is d i s t i n c t l y b e l o w 0 °c, t h e a c t i o n of t h e t h r u s t s g r o w s m o r e c o m p l e x e ; t h e r o c k y w a t e r l o g g e d s y s t e m d o w n s t r e a m of t h e d a m , w h i l e c o o l i n g , swells a n d p u s h e s t h e b o t t o m of t h e d a m u p s t r e a m ; a t t h e h i g h e r q u o t e , on t h e c o n t r a r y , t h e t h r u s t d o w n s t r e a m c o n t i n u e s . w h e n t h e s t r e n g h t l i m i t of t h e m e d i u m is s u r p a s s e d , a r i s e s a c o n t r a s t b e t w e e n t h e r o c k y s y s t e m a n d t h e c o n c r e t e s t r u c t u r e : t h i s c o n t r a s t c a n origin a t e v e r y s m a l l f r a c t u r e s , r e v e a l e d f r o m s e i s m i c s t a t i o n i n s t a l l e d i n t o t h e c e n t r a l a s h l a r ( x i v ) a t 660 m e t r e s h e i g h t of t h e d a m , u n d e r t h e f o r m of m i c r o s h o c k s w h i c h e n e r g y is of a b o u t 1 0 i 0 1 0 u e r g . 1 . a s i s k n o w n , t h e b e h a v i o u r of a d a m a b o v e 0 ° c m a y b e s y n t h e z i s e d i n t o t w o p r i n c i p a l m o v e m e n t s d u e t o t h e a c t i o n of t e m p e r a t u r e v a r i a t i o n s : a n u p s t r e a m b e n d i n g m o v e m e n t if t e m p e r a t u r e r i s e s a n d a d o w n s t r e a m f l e x u r e if t e m p e r a t u r e d e c r e a s e s f 1 ). t h i s d o e s n o t t a k e i n t o a c c o u n t , of c o u r s e , t h e v a r i a t i o n s d e p e n d i n g o n t h e w a t e r s h e d . a t a b o u t 0 ° c a n d b e l o w t h e b e h a v i o u r of a d a m b e c o m e s m o r e c o m p l e x . u p s t r e a m of t h e b a r r a g e t h e l a k e c o v e r s t h e d a m u p t o a c e r t a i n h e i g h t . w h e n t h e t e m p e r a t u r e f a l l s b e l o w 0 ° c , t h e w a t e r l e v e l a t (*) i s t i t u t o n a z i o n a l e di geo fìsica, r o m a . (**) e . n . e . l . c o m p a r t i m e n t o di v e n e z i a . 5 2 0 i>. o a l o i m . m i g a n i m. c . s p a d k a tlie s u r f a c e f r e e z e s t o a c e r t a i n t h i c k n e s s a c c o r d i n g t o t h e n e g a t i v e t h e r m a l s t a g e . below t h e f r o z e n l a y e r t h e w a t e r m a i n t a i n s a t e m p e r a t u r e a b o v e 0 °c. t h e s i t u a t i o n d o w n s t r e a m d e p e n d s on t h e o u t e r t e m p e r a t u r e w h i c h is m o r e or less, b u t d i s t i n c t l y below 0 ° 0 . t h i s m e a n s t h a t t h e p o r t i o n of t h e d a m w h i c h is c o v e r e d b y t h e w a t e r h a s a h i g h e r t e m p e r a t u r e t h a n t h e p o r t i o n d o w n s t r e a m . t h i s a d d s — a t l e a s t in t h e b e g i n n i n g — t o t h e t h r u s t of t h e d a m d o w n s t r e a m . b u t if t h e t e m p e r a t u r e of t h e air r e m a i n s f o r a l o n g e r t i m e p e r i o d d i s t i n c t l y below 0 °c, new f a c t o r s e n t e r i n t o a c t i o n d u e t o t h e a b n o r m a l b e h a v i o u r of t h e f r o z e n soil. t h e r o c k y s y s t e m on w h i c h t h e d a m f o o t s is o b v i o u s l y w a t e r l o p p e d d o w n t o a c e r t a i n d e p t h . w h e n t h e t e m p e r a t u r e r e m a i n s sizeably below 0 °c f o r s e v e r a l d a y s , t h e f r o s t p e n e t r a t e s m o r e or less d e e p l y i n t o t h e u n d e r g r o u n d d o w n s t r e a m of t h e d a m . w h i l e f r e e z i n g , t h e l i q u i d c o m p o n e n t b r i n g s a b o u t a swell of t h e soil w h i c h is all t h e m o r e p r o n o u n c e d t h e d e e p e r t h e f r o s t p e n e t r a t e s . a t t h e m a r g i n s of t h e p l a i n in c o n t a c t w i t h t h e d a m , a t h r u s t is t h e r e f o r e d e v e l o p i n g t o w a r d t h e b a r r a g e , a n n u l l i n g t h e o p p o s i t e t h r u s t f r o m t h e s u b m e r s e d s u r f a c e a n d c a u s i n g a t l a s t a n u p s t r e a m b e n d i n g of t h e d a m b o t t o m . r e f e r r e d t o h e i g h t , in f a c t , t h e b a r r a g e is s u b j e c t t o t h e swelling a c t i o n of t h e s p r i n g e r r o c k y m e d i u m o n l y i n t h e n a r r o w a r e a c o r r e s p o n d i n g t o t h e d a m t h i c k n e s s . o b v i o u s l y , t h i s a c t i o n is n o t s t r o n g e n o u g h t o c a u s e a sizeable t h r u s t f r o m d o w n s t r e a m t o u p s t r e a m , so t h a t a b o v e t h e level of t h e p l a i n o u t s i d e t h e d a m t h e d o w n s t r e a m t h r u s t m a k e s itself m o r e a n d m o r e felt, as s k e t c h e d in lig. 1. t h a t t h e e f f e c t s a r e s u c h as a b o v e d e s c r i b e d is p r o v e d b y t h e c l i n o g r a p h i c r e c o r d i n g s w h i c h w e r e o b t a i n e d u n d e r t h e said c o n d i t i o n s a t v a r i o u s h e i g h t s of t h e c e n t r a l a s h l a r of t h e p i e v e di c a d o r e d a m , n a m e l y o n t h e levels of t h e d a m b o t t o m ( p i a n delle e r e ) 625 m , 660 m a n d o n t h e d a m t o p (682 m) (fig. 2). 2. so f a r t h e r o c k a n d t h e c o n c r e t e h a v e w i t h s t o o d t h e s t r e s s in a c t i o n w i t h i n t h e l i m i t s of t h e i r p r o p e r s t r e n g t h , b u t w h e n f r o s t p e r sists as f a r a s t i m e , a n d h e n c e , s p a c e is c o n c e r n e d , t h e i n t e n s i t y of t h r u s t of t h e s p r i n g e r r o c k i n c r e a s e s d o w n s t r e a m of t h e d a m , t o w a r d t h e b a r r a g e . t h e r h e o l o g i c a l b e h a v i o u r of t h e f r o z e n soil is still f a r f r o m b e i n g d i s c o v e r e d i n all i t s a s p e c t s . i n a n y case i t is h i g h l y imp o r t a n t i n several g e o m o r p h o l o g i c a l s t r u c t u r e s (7). specific r e s e a r c h w o r k c o n d u c t e d b y t h e d i s a s t e r p r e v e n t i o n r e s e a r c h i n s t i t u t e of t h e u n i v e r s i t y of k y o t o (6) h a v e s h o w n — t h r o u g h t h e b e h a v i o u r o f a l a r g e dam at s e v e r e f r o s t 5 2 1 t e s t s of a x i a l c o m p r e s s i o n on f r o z e n soil s p e c i m e n s e x t r a c t e d ' f r o m v a r i o u s d e p t h s of t h e p l a i n of t o k y o — t h a t t h e h i g h e s t v a l u e s o b t a i n ed f r o m clay s e d i m e n t s w e r e b e t w e e n 10 a n d 20 k g / c m 2 a t — 5 °c a n d a b o v e 20 k g / c m 2 a t — 1 0 °c. t h e d e f o r m a t i o n of s e d i m e n t a r y s a n d s a t t h e s a m e t e m p e r a t u r e s was e v e n s t r o n g e r , a n d f o r r o c k y g r o u n d s t h e v a l u e s m a y rise still higher. i n v e r s e l y , if t h e o t h e r c o n d i t i o n s r e m a i n u n c h a n g e d , t h e l a t e r a l t h r u s t of a h i g h l y f r o z e n m e d i u m t h e r e f o r e increases. besides, if t h e u p s t r e a m t h r u s t of t h e s u r f a c e layers of t h e d a m p l a i n is c o u n t e r a c t e d b y t h e d o w n s t r e a m t h r u s t of t h e d a m b o t t o m p r o p e r l y s p e a k i n g which is u n d e r t h e lake, s u c h as is t h e case w i t h t h e p i e v e d i c a d o r e d a m , a n i n c r e a s i n g c o n t r a s t is b u i l d i n g u p as t h e f r o s t b e c o m e s h a r s h e r u n t i l t h e s t r e n g t h limit of t h e m e d i u m is s u r p a s s e d , t h e e q u i l i b r i u m b r e a k s a n d t h e m e d i u m cracks. 3. t h i s p h e n o m e n o n a s s u m e d s p e c t a c u l a r a s p e c t s d u r i n g t h e first p e r i o d a f t e r t h e d a m b u i l d i n g . t h e s e i s m o g r a p h s i n s t a l l e d in a s m a l l h o u s e on t h e r i g h t a b u t m e n t a t some fifty m e t r e s f r o m t h e d a m r e c o r d e d t h o u s a n d s of small s h o c k s in t h e first w i n t e r s . t h e r e c o r d i n g s were e q u a l l y g i v e n a f t e r w a r d s w h e n t h e t e r n i s h i m o t o s e i s m o g r a p h s h a d b e e n i n s t a l l e d in t h e c o n t r o l cabin a t close b y t h e d a m (2), still on t h e s a m e side ( t h e y w e r e l a t e r r e p l a c e d b y a s t r o n g e r t e r n of girl a n d a s e i s m o g r a p h s ) . m o s t of t h o s e small s h o c k s c a m e f r o m t h e b a r r a g e b o t t o m f1-2). a f t e r a f e w y e a r s t h e seismic s t a t i o n i n s t a l l e d in t h e b a s e m e n t of t h e c o n t r o l c a b i n s t o p p e d r e c o r d i n g m i c r o s h o c k s c o m i n g f r o m t h e b a r r a g e b o t t o m . caloi, w h o h a d followed t h e p h e n o m e n o n since i t s o u t s e t , h a d g o t c o n v i c t i o n , h o w e v e r , t h a t i t could n o t b e a l t o g e t h e r s p e n t . o n l y a seismic s t a t i o n i n s t a l l e d w i t h i n t h e d a m s t r u c t u r e c o u l d solve t h e p r o b l e m . a f t e r p e r m i s s i o n f r o m t h e « e n e l », a « g i r l a n d a » seismic s t a t i o n w a s i n s t a l l e d b y d r . migani in a. s t a p l e p i t of t h e c e n t r a l a s h l a r ( x i v ) a t 660 m e t r e s h e i g h t of t h e d a m (figs 3, 4). w i t h t h e help of t h i s s t a t i o n , w h i c h allowed t o d e t e c t n e w p h e n o m e n a , p a r t l y discussed elsewhere a n d p a r t l y covered l a t e r h u n d r e d s of v e r y small s h o c k s could b e r e v e a l e d d u r i n g d a y s of v e r y s t r o n g f r o s t (see fig. 5) w h i c h t h e s t a t i o n in t h e c e n t r a l i z e d c o n t r o l c a b i n is u n a b l e t o r e c o r d . h e r e arises t h e s p o n t a n e o u s q u e s t i o n w h y a t t h e s t a r t t h e m i c r o s h o c k s w e r e so m u c h s t r o n g e r t h a t t h e y c o u l d b e clearly r e c o r d e d b y t h e t h o u s a n d s e v e n o u t s i d e t h e d a m . t h e r e p l y is ob5 2 2 p . c a l o i m . m i g a n i m . c . s p a d e a v i o u s : a t t h e o u t s e t t h e r e were t w o m e d i a , t h e r o c k y s y s t e m a n d t h e d a m , e a c h w i t h i t s o w n c h a r a c t e r i s t i c s . t h e f o r c e d " s y m b i o s e " b e t w e e n t h e t w o c o u l d n o t b u t l e a d t o i n i t i a l c o n t r a s t s of a c e r t a i n violence, c a p a b l e of r e d u c i n g t h e s t r o n g e s t r e s i s t a n c e of t h e r o c k a g a i n s t t h e m o v e m e n t s of t h e d a m . t h e r e f o r e , t h e s h a t t e r i n g b e c a m e m o r e w i d e s p r e a d a n d h i t t h e e l e m e n t s of m a j o r size, w h i c h e x p l a i n s t h e h i g h e r a m o u n t s of e n e r g y set f r e e b y t h e m i c r o s h o c k s . f u r t h e r m o r e , i t w a s o n l y i n t h i s w a y t h a t t h e s y s t e m could s e t t l e w i t h n e w p o s i t i o n s of e q u i l i b r i u m (it s h o u l d n o t b e o v e r l o o k e d i n t h i s r e g a r d t h a t t h e m o d u l u s of e l a s t i c i t y of t h e r o c k y s y s t e m s u p p o r t i n g t h e p l a i n d o w n s t r e a m of t h e d a m w a s s t r o n g l y r e d u c e d w i t h i n f e w y e a r s , t h e first, f a l l i n g f r o m t h e i n i t i a l 7000 m / s e c (1949) t o 5000 m / s e c (1960) of long i t u d i n a l w a v e v e l o c i t y (4)). a f t e r t h e a c u t e s t a g e of t h e crisis i n t h e c o n t a c t s u r f a c e b e t w e e n t h e u n d e r l y i n g s t r a t a of t h e b a r r a g e a n d t h e c o n c r e t e h a d b e e n o v e r c o m e , t h e m i c r o m o v e m e n t s of t h e l a t t e r could s w i n g o u t i n t h e p l a s t i c field. t h i s c a n n o t h a p p e n in p e r i o d s of s t r o n g f r o s t w h i c h , f o r t h e a b o v e r e a s o n s , give rise t o s h e a r s a n d elastic w a v e s . f i g . 6 g i v e n u m e r o u s e x a m p l e s of m i c r o s h o c k s , r e c o r d e d bet w e e n d e c e m b e r 1970 a n d m a r c h 1971, t h a t is i n p e r i o d s w h e n t e m p e r a t u r e s fell d e c i d e d l y below zero degrees (*). w e h a v e r e p o r t e d o n l y s o m e of t h e m , of course, b u t t h e y give sufficient e v i d e n c e of t h e l a r g e v a r i e t y of a s p e c t s t h e y a r e a s s u m i n g , s o m e t i m e s w i t h c o n s i d e r a b l e u n l i k e n e s s e s b e t w e e n t h e m . u n l i k e n e s s i n d i c a t e s d i f f e r e n t origin a n d d i f f e r e n t p r o p a g a t i o n . r u p t u r e of e q u i l i b r i u m c a n t a k e p l a c e s l i g h t l y d i s t a n t f r o m t h e d a m b o t t o m . d u e t o t h e l i m i t e d a m o u n t of e n e r g y i n v o l v e d , i t is u n a b l e t o c a u s e a s h a k i n g of t h e s t r u c t u r e a n d a c t s m e r e l y as a v e h i c l e of t h e elastic w a v e s , so as t o b r i n g a b o u t , i n t h e s e cases, v e r y f a s t oscillations w h i c h f a d e a w a y i n i n t e r v a l s of o n e or t w o seconds (fig. 6). if t h e c r u s h is closer t o t h e d a m , i t s e n e r g y (*) i t w o u l d n o t b e s u r p r i s i n g if s o m e s u p e r f i c i a l o b s e r v a t i o n a t t r i b u t e d t h e m i c r o s h o c k s t o t h e c r a c k s w h i c h f r o m t i m e t o t i m e a r e p r o d u c e d i n t h e i c e d s t r a t u m of t h e l a k e . w i t h o u t m e n t i o n i n g t h a t c r a c k s a r e n o t f r e q u e n t a n d of a r a t h e r l i m i t e d n u m b e r , i t s h o u l d b e c o n s i d e r e d t h a t d u e t o c o n t i n u o u s v a r i a t i o n s of t h e w a t e r l e v e l i n t h e b a s i n t h e i c e l a y e r a p p e a r s f r a c t u r e d t o a l a r g e e x t e n t , t h a t is t h e s t r i p a l o n g t h e w a t e r s i d e s . t o t h e s e , t h e r e f o r e , i t is n o t a t t a c h e d r i g i d l y , so t h a t t h e e n e r g y s e t f r e e f r o m c r a c k i n g s is d i s p e r s e d a n d r e a c h e s t h e w a t e r s i d e in v e r y l i m i t e d a m o u n t s . t h e b e h a v i o u r o f a l a r g e dam at s e v e r e f r o s t 5 2 3 m a y b e sucli as t o cause s h a k i n g s . since t h e e x c i t a t i o n e n e r g y is i n t o finite m e d i u m , it is similar t o t h e oscillations of a d i a p a s o n : in o t h e r w o r d s , t h e r e will b e a n a n o m a l o u s dispersion, w i t h oscillations of longer a n d longer p e r i o d s which a r e t h e l a t e s t a n d a r e a p p r o a c h i n g t h e u n i n o d a l v a l u e s of t h e a s h l a r s a n d o f t e n of t h e d a m itself (fig. 6). i t is t o b e n o t e d in t h i s r e s p e c t t h a t t h e f r e e oscillations of t h e s t r u c t u r e o r i g i n a t e s o m e t i m e s i m m e d i a t e l y a f t e r t h e b e g i n n i n g of t h e p e r t u r b a t i o n , b u t s o m e t i m e s t h e y a p p e a r a f t e r c e r t a i n t i m e i n t e r v a l s . t h i s m e a n s t h a t t h e d i s t a n c e f r o m t h e o b s e r v a t i o n s t a t i o n (installed, in t h i s specific case, a t t h e c e n t r e of t h e d a m ) t o t h e d i s t u r b i n g a r e a v a r i e s . t h e f a c t s h o u l d bo u n d e r l i n e d , as well, t h a t o f t e n t h e r e c o r d i n g does n o t s h o w t h e p r o p e r f e a t u r e s of a s e p a r a t e m i c r o s h o c k , b u t of s e v e r a l shocks o r i g i n a t i n g a t s h o r t t i m e i n t e r v a l s b e t w e e n each o t h e r , a s t h o u g h t h e r u p t u r e of e q u i l i b r i u m in one p o i n t b r o u g h t a b o u t a n a l o g o u s r u p t u r e s all a b o u t (fig. 6). w e r e s e r v e t o d e a l m o r e specifically w i t h t h e p r i n cipal a s p e c t s of t h e recordings in a following p a p e r . 4. w i t h o u t c o n s i d e r i n g t h e b a r r a g e b o t t o m w h i c h is p a r t i c u l a r l y , if n o t e x c l u s i v e l y , s e n s i t i v e t o t h e v a r i a t i o n s of t h e w a t e r b a s i n , t h e cycle of b e n d i n g s of a large d a m is g o v e r n e d , t h e r e f o r e , b y t h e developm e n t of t h e s e a s o n s — all o t h e r c o n d i t i o n s r e m a i n i n g e q u a l — a n d p a r t i c u l a r l y b y t h e a n o m a l o u s b e h a v i o u r of t h e w a t e r w i t h r e g a r d t o t h e v a r i a t i o n s of t e m p e r a t u r e . t h i s m a y b e s u m m a r i z e d as follows. l e t us consider a d a m (in o u r specific case t h e p i e v e di c a d o r e d a m ) w i t h t h e a d j a c e n t w a t e r s h e d a t a n o r m a l level. a t a c e r t a i n d a t e of t h e first m o n t h s of t h e y e a r (which m a y b e f e b r u a r y or m a r c h , a c c o r d i n g t o t h e p r o c e e d i n g of t h e season), t h e w a t e r a n d t h e air will c o m e t o h a v e a p p r o x i m a t e l y t h e s a m e m e a n t e m p e r a t u r e . a f t e r t h a t d a t e , t o w a r d s p r i n g , t h e air beings t o b e w a r m e r t h a n t h e w a t e r . i t will b e r e m e m b e r e d t h a t w a t e r h a s t h e h i g h e s t specific h e a t of all k n o w n s u b s t a n c e s , w h i c h implies t h a t i t s w a r m i n g u p t a k e s p l a c e v e r y slowly, also as f a r as t h e d a m s u p p o r t i n g r o c k s y s t e m is c o n c e r n e d . t h e i n c r e a s e of t h e g a p of m e a n t e m p e r a t u r e s of air a n d w a t e r is r e s p o n s i b l e f o r a s t r o n g e r h e a t i n g u p of t h e wall d o w n s t r e a m of t h e d a m c o m p a r e d w i t h t h e u p s t r e a m wall w h i c h is u n d e r t h e w a t e r , w i t h t h e c o n s e q u e n c e of a b a r r a g e b e n d i n g u p s t r e a m which increases in t h e f o l l o w i n g m o n t h s , u s u a l l y u n t i l s e p t e m b e r inclusively. besides, t o t h i s slow u p s t r e a m b e n d i n g a d d s t h e d i u r n a l w a v e following t h e s u n r a d i a t i o n s t h r o u g h t h e 24 h o u r s . 5 2 4 p . c a l o i i i . j i i g a n i m . c. s p a d e a i n t h e e o u r s e of t h e s e m o n t h s t h e w a t e r h a s c o n t i n u e d s t o r i n g u p h e a t , w i t h t e m p e r a t u r e s of t h e s u r f a c e l a y e r s g o i n g u p t o m a x i m a of 20 °g. as slowly as h e a t h a s b e e n s t o r e d , as slowly t h e w a t e r c o n v e y e s a r o u n d . so, w h e n t h e o u t e r t e m p e r a t u r e begins t o fall, t h e m e a n air t e m p e r a t u r e soon falls below t h e w a t e r t e m p e r a t u r e of t h e l a k e . this is when the thermal condition of the dam walls is inverted: t h e n o r t h wall is w a r m e r t h a n t h e s o u t h wall. a n d t h e b e n d i n g is i n v e r t e d , t o o , b e i n g d i r e c t e d t o w a r d t h e v a l l e y . as t h e t e m p e r a t u r e falls f u r t h e r t o w a r d s 0 °c, t h e w a t e r of t h e w a t e r s h e d b e g i n s t o b e s u b j e c t e d t o a c o n t i n u o u s m i x i n g process. t h e s u r f a c e w a t e r cooling off, b e c o m e s heav i e r a n d s i n k s a f e w m e t r e s d o w n . i t is r e p l a c e d b y w a r m e r w a t e r f r o m m a j o r d e p t h s . t h e o u t e r c o n d i t i o n s p e r m i t t i n g , t h i s p h e n o m e n o n c a n go on u n t i l a c o m p l e t e b l e n d i n g of t h e l a k e w a t e r is r e a c h e d , w i t h t e m p e r a t u r e s a t t h e l a k e b o t t o m of a b o u t 4 °c w h e r e , as is k n o w n , w a t e r h a s t h e m a x i m u m d e n s i t y a n d , h e n c e , t h e m a x i m u m specific w e i g h t . if t h i s s i t u a t i o n is t o t a k e p l a c e , t h e o u t e r t e m p e r a t u r e m u s t b e c l e a r l y below 0 °c, w h i c h m e a n s t h e s h a p i n g of a n ice l a y e r on t h e l a k e s u r f a c e , t h e c o n t i n o u s b e n d i n g on t h e e n t i r e d a m t o w a r d t h e v a l l e y , as w a s d e s c r i b e d a t t h e b e g i n n i n g , a n d — f r o s t b e i n g s t r o n g — t h e o b s e r v a t i o n of t h e a b o v e i n d i c a t e d p h e n o m e n o n on a p a r t of t h e d a m , t h a t is t h e b o t t o m , w h i c h b e n d s u p s t r e a m , while t h e h i g h e r p o r t i o n is a l w a y s s t r e s s e d i n t h e o p p o s i t e d i r e c t i o n . t h e a b o v e describes t h e p h e n o m e n o n in i t s s i m p l e s t a s p e c t s , of course, w h e r e t h e f a c t o r s i n v o l v e d v a r y u n i f o r m l y in one v e r s u s o n l y . i n f a c t , t h e r e a r e c o m p l i c a t i o n s d e p e n d i n g on r e c u r r e n c e s of s e a s o n s t o w h i c h t h e air t e m p e r a t u r e is p a r t i c u l a r l y sensitive. t h e r e s p o n s e of t h e d a m is i m m e d i a t e : benclings a r e i n v e r t i n g m o r e or less o u t s p o k e n l y , m o r e or less long or r e g u l a r , a c c o r d i n g as t h e cause evolves. n o cons i d e r a t i o n h a s b e e n g i v e n , of course, t o s i m u l t a n e o u s a c t i o n s on t h e valley as a whole, on i t s solid p a r t s . t h e v a l l e y t e n d s t o s h r i n k in t h e w a r m season a n d t o e x p a n d i n t h e cold season. n o t t o m e n t i o n a t all, a t l a s t , t h e c o n t r a s t s f o l l o w i n g t h e m o r e or less m a r k e d v a r i a t i o n s of t h e w a t e r level in t h e r e s e r v o i r . t h e b e h a v i o u r o f a l a r g e dam at s e v e r e f r o s t 5 2 5 r e f e r e n c e s {1 ) caloi p . , 1953. osservazioni sismiche e clinografiche presso grandi dighe di sbarramento. « a n n a l i di g e o f í s i c a », v i . 3. (2) caloi p . , 1962. la geofísica e le grandi dighe. « l ' e n e r g i a e l e t t r i c a », x x x i x , 1. (3) caloi p . , 1962. aspetti della dinamica di rocce, calcestruzzo ed acque, « a n n a l i di g e o f í s i c a », x v , 2-3. ( c o n t i e n e u n ' a m p i a b i b l i o g r a f i a s u p r e c e d e n t i l a v o r i d e l l ' a . s u l l ' a r g o m e n t o ) . (4) caloi p . , spadea m . c., 1966. principali risultati conseguiti durante vosservazione geodinamica, opportunamente estesa nel tempo, di grandi dighe di sbarramento, e loro giustificazioni teoriche. « a n n a l i di geofis i c a », x i x , 3. (5) caloi p . , spadea m . c., 1969. sulla risposta elastica delle dighe e sulla sua variazione nel tempo. « a n n a l i d i g e o f i s i c a » , x x i i , i. (6) pukuo y . , 1966. oli the bheological behavior oj frozen ti o il ( l'art i). « b u l l . d i s a s t e r p r e v e n t i o n r e s . i n s t . », 15, p a r t 3. (') scheidegger e . a., 1970. theoretical geomorpholog;/. s e c o n d a e d i z i o n e r i v e d u t a . s p r i n g e r v e r l a g . b e r l i n , p p . 375-382. annals 47, 1, 2004, 01/07def 199 annals of geophysics, vol. 47, n. 1, february 2004 mailing address: dr. nikolai t. tarasov, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: tarasov@ifz.ru key words earthquake – initiated – seismicity – electromagnetic – pulse 1. introduction the bishkek site is situated in the junction area of north tien shan and chuya basin structures. its southern part consists of paleozoic crystalline rocks and the northern, of mesozoic/ cenozoic sedimentary deposits. the tien shan is a highly seismic region of the earth. the north tien shan earthquake-generating zone, where the bishkek site is situated, contains deep crustal faults. three earthquakes with magnitudes great er then 8.0 occurred there during the past 100 years. deep electrical sounding of the crust was carried out at the bishkek test site of the institute of high temperatures ras in 1983 to 1990. the source of energy was the mhd-generator, and load was electrical dipole of 0.4 ω resistivity, whose electrodes were 4.5 km apart. when the generator was put into operation, the load current was 0.28-2.8 ka, the sounding pulses had durations of 1.7 to 12.1 s, the energy being mostly in the range 1.2-23.1 mj. the dipole was installed within those paleozoic crystalline structures in the north tien shan (42.69°n, 74.68°e), which spatial-temporal structure of seismicity of the north tien shan and its change under effect of high energy electromagnetic pulses nikolai t. tarasov and nadezhda v. tarasova schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia abstract the effect of high-energy electromagnetic pulses emitted by a magnetohydrodynamic generator used as a source for deep electrical sounding of the crust on spatial-temporal structure of seismicity of the north tien shan is explored. five-six years periodicity of changes in spatial distribution of seismicity was revealed. the effect of electromagnetic pulses increases the stability of the spatial distribution of seismicity over time and simultaneously speeds up cycles of its transformations, which develop on stabilization background. increasing of seismic energy release after electromagnetic impacts is observed basically in most active zones. periodic variation of efficiency of earthquakes triggering on the distance to the mhd-generator was detected. it was shown that electromagnetic pulses give rise to an appreciable increase in the rate of local earthquakes, occurring around 2-6 days after the pulses. total earthquakes energy released after start-ups was by 2.03⋅1015 j greater than the energy released before them. at the same time, the total energy transmitted by the mhd-generator was 1.1⋅109 j, i.e. six orders of magnitude smaller. consequently, the electromagnetic pulses initiated the release of the energy that had been stored in the crust due to activity of natural tectonic processes in the form of comparatively small earthquakes, which leads to an additional release of tectonic stresses. 200 nikolai t. tarasov and nadezhda v. tarasova were adjacent to the boundary of mesozoic/ cenozoic sedimentary deposits in the chuya basin. during the time of the mhd operation, 114 start-ups were carried out. a detailed description of the mhd equipment and the sounding procedure can be found in volykhin et al. (1993). the observation area was within coordinates 41.0° to 45.5°n and 74.0° to 81.5°e. by the criterion of geological structure, the area can roughly be divided by latitude 42.9° into a southern part (the paleozoic features of north tien shan) and a northern part (mesozoic/cenozoic sedimentary deposits). analysis of seismicity of the observation area was based on the earthquake catalog for the north tien shan and adjacent areas of n. mikhailova containing earthquake data for the period from 1975 to 1996 (mikhailova, 1990). the catalog contained data on 14 114 earthquakes with magnitudes ml from 0 to 6.2. the recurrence diagram based on all events is linear in the ml ranging from 2.2 to 6.1. the trigged seismicity was identified upon the background of the natural variation by coherent summation of earthquakes in time windows of ± 20 days about the time of each of 114 mhd-generator start-ups (i.e. we superposed all 114 windows in time). for this purpose, selection of earthquakes was made for each of 114 mhd-generator runs in the interval (tj + − 20, tj + 20), where tj is absolute times (in days) of the jth mhd-generator start-up. absolute times ti of all selected earthquakes (more than 4000) were replaced by the times tij = ti + − tj measured from the time of the corresponding start up (here i is number of earthquake in the selection). obtained selection was used to find nt, which is the number of events occurring in the area during each day within time windows, i.e. those with t − 0.5 ≤ tij ≤ t + 0.5 for all integer t between − 20 and 20 days. values nt with t > 0 were analyzed to study the effect of electromagnetic pulses of the mhdgenerator on seismicity over time. the events occurring during 20 days before the start-up were used to estimate the natural seismicity rate immediately before mhd-generator runs. we calculated background level as averaged daily number of events and estimated the 99% confidence interval over all nt with t < 0. for investigation of total seismic energy changes after mhd-generator runs, we calculated the ratio of the sums of the energy of earthquakes with tij > 0 and tij < 0, respectively, using the relationship for earthquake energy estimation presented in rautian et al. (1981). note, this technique allows one to increase the relation of signal/noise n times, where n is the number of the superposed time windows. this brings the relation up to 10.7 when 114 start ups of the mhd-generator are used. for study of spatial variations of total seismic energy changes after the mhd-generator runs, the observation area was divided into 1000 km2 cells of the grid. the ratio of the sums of the energy of earthquakes with tij > 0 and tij < 0, respectively, was calculated for each cell. the map of distribution energy ratios was created using all cells with non-zero earthquake number. a similar technique was used for creation of the map of the flow of total seismic energy over the area, but the total catalog was processed for this purpose instead of the selection made. for a quantitative estimation of variations of a spatial distribution of the earthquakes over time, the spatial correlation of earthquakes density for each two contiguous years rk as a function of the time from 1975 to 1995 was calculated. to do this, the region of observations was divided into square 1000 km2 cells of the grid too. nijk and nijk + 1 are numbers of the earthquakes occurred inside each cell for kth and for k + 1th years, respectively, were calculated for all k varying from 75 to 95 year. the indices i, j denote the numbers of a grid cell for latitude and longitude. they are related with geographical coordinates x, y as ,i x x dx j y y dyint int0 0= = -^ ^h h where dx, dy are the length of sides of the cell on axes x, y in degrees, and x0 = 41°n and y0 = = 74°e are latitude of southern and longitude of western boundaries of the observation area. the function of spatial-temporal correlation of seismicity rk was evaluated for all k = 75, 76, ..., 95 using the expression r s s c , k k k k k 1 1 2 $ = + + 201 spatial-temporal changes of seismicity under effect of high energy electromagnetic pulses where c n n n m n n n n m n n n m n 1 1 1 ,k k ijk ijk j m i n ijk j m ijk j m i n i n k ijk j m i n ijk j m i n k ijk j m i n ijk j m i n 1 1 11 1 1 111 2 11 11 2 1 1 2 11 1 11 2 $ $ $ $ $ $ = -+ + == = + === == == + + == + == s = $ s = !! ! !!! !! !! !! !! d _ d _ d n i n i n n is number of cells along the axis x, and m is number of cells on the axis y. 2. results first, we examined the time variation in seismicity after the mhd-generator runs. figure 1a presents the distribution of daily rates of earthquakes nt for the entire area before and after mhd-generator runs. the values of nt are seen to be appreciably higher one day after the startup than before it. the maximum rate occurred on the second day after start-up with amplitude nearly 1.7 above background level. when only earthquakes in the north part are considered, no significant changes in nt related to mhd-generator runs can be detected (fig. 1b), whereas a well-pronounced maximum can be seen in the south (fig. 1c) occurring on the second day after electromagnetic action, nearly 2.2 above the mean background, to be followed by a gradual falloff in nt during a few days. the triggering effect of electromagnetic pulses of mhd-generator was most active in the upper 5 km layer. figure 1d shows the distribution of the earthquakes occurring within the layer in the south part of the area. it is seen that during the first day after mhd-generator runs, the nt values remain at the background level and then a strong activization of the regime occurs that goes on for about 5 days. during this period, nt exceeds background level 2.7 times. the maximum is observed on the second day after mhd-generator start-up. its amplitude exceeds 99% confidence level. the above results show that high energy electromagnetic pulses emitted by mhd-generator give rise to an appreciable increase in the rate of 50 150 250 50 150 250 -15 -10 -5 0 5 10 15 50 150 150 250 350 n t, days fig. 1a-d. daily number of earthquakes in the area of the northern tien shan and adjacent territories before (t < 0) and after (t > 0) the mhd-generator runs on the bishkek test site plotted against time: a) for the entire area, b) its northern and (c) southern (north tien shan) part, d) events occurring in the upper 5 km layer of the southern part of the region. the vertical axis shows the daily rate of earthquakes; horizontal axis shows the time measured from the mhd start-ups (in days). the dashed lines indicate a mean background level (lower lines) and 99% confidence interval (upper lines). local earthquakes, occurring about 2-6 days after the pulses. it confirmed the initiating effect of action by electromagnetic pulses on the seismicity previously revealed (tarasov, 1997; tarasov et al., 1999) in the garm and bishkek areas. a b c d 202 nikolai t. tarasov and nadezhda v. tarasova 75 76 77 78 79 80 81 42 43 44 45 4 5 6 7 8 9 10 11 12 13 14 n e lg e fig. 2. map of the flow of total seismic energy of earthquakes in northern tien shan and adjacent territories obtained using all earthquakes of the catalog for 1000 km2 cells of grid. the values of energy are reduced to the flow for 1 year. 75 76 77 78 79 80 81 42 43 44 45 -4 -3 -2 -1 0 1 2 3 e n lg ea eb fig. 3. map of total seismic energy ratio of earthquakes in northern tien shan area and adjacent territories for 20 days after and 20 days before mhd-generator runs. 203 spatial-temporal changes of seismicity under effect of high energy electromagnetic pulses on the one hand it can be assumed that this effect results from the extra energy loaded to the earth by electromagnetic pulses, which is radiated in the form of earthquakes. another explanation is that sounding pulses initiated the release of energy stored in the crust due to natural geodynamic processes. to test these assumptions, we estimated the total seismic energy released during 20 days before all mhdgenerator start-ups eb, total seismic energy released during the same period after the start-ups ea, and the post-start-up increase in total seismic energy release ea-eb. these estimates were based on all earthquakes of the area, on events occurring in the north and south, regardless of their focal depths, and the events from the upper 5 km layer. an analysis of the obtained values shows that the total earthquake energy released in the area during 20 days after all start-ups was by 2.03⋅1015 j greater than the energy released for a period of the same duration before them. most of the energy release was due to the earthquakes occurring in the upper 5 km layer in the southern part of the area, which belongs to the crystalline rocks of the north tien shan. at the same time, the total energy transmitted by mhd-generator to the radiating dipole for all start-ups was 1.1⋅109 j, i.e. six orders of magnitude smaller. consequently, the high energy electromagnetic pulses radiated by the mhdgenerator initiated the release of the energy that had been stored in the crust due to other sources. the mean energy of initiated earthquakes per one start-up was 2.2⋅1013 j. tarasov (1997) investigated the initiating effect of mhd-generator pulses on the seismicity of the garm area in tajikistan and concluded that the average increase in total energy release through local seismicity was 1.1⋅1012 j per start-up. however it should be kept in mind that garm site has an area much smaller than our area under study. when the estimates of energy were divided by the values of respective areas, the extra seismic energy release was 7.3⋅107 j/km2 per start-up in the bishkek site and 6.6.107 j/km2 per start-up in the garm site. these similar values indicate approximately similar responses of the earth to electromagnetic pulses in different areas. figure 2 presents the map of the flow of total seismic energy of earthquakes of north tien shan and area of adjacent territories. the density of seismic energy flow is seen to vary widely over the area. there are several zones of high energy flow in the middle and south, while energy flow density in the northwest is significantly lower. figure 3 shows the spatial variations in the changes of total seismic energy after mhdgenerator start-ups. the map demonstrates that seismic energy release increasing takes place within a number relatively small zones located predominantly in southern and central parts of the observation area. one can see some correlation of maps in figs. 2 and 3. however, these maps are not identical. therefore, electromagnetic effects cause the increase in seismic energy release mainly in most active zones. nevertheless a triggering effect is determined not only by a level of seismic activity, but also by other phenomena. using this map, it is difficult to recognize any regular dependence of efficiency of trigger effects of electromagnetic pulses on the distance to location of the mhd-generator (42.69°n, 74.68°e). for detailed analysis, we calculated average ea /eb as function of the distance from location of the mhd-generator to epicenters of earthquakes, irrespective of azimuths. using all mhd-generator runs, the summary seismic energy of earthquakes which occurred 20 days before (eb) and 20 days after (ea) electrical soundings were calculated for ring bands bounded by radii r and r + 50 km with the center in the site of location of the mhd-generator. eb and ea were computed in the distance range from 0 up to 600 km with 50 km step. shown in fig. 4, the dependence of eb on the distance to mhd-generator represents a natural change of the flow of seismic energy in the observational region. similar dependence of ea shows summary effect of natural changes and trigged seismicity. note, relatively low level of eb is observed up to distances of ∼ 200 km. one can see in the figure that the total seismic energy before mhd-generator runs was higher than after them for distances r < 110 km. then, the interval where ea is greater than eb is observed. 204 nikolai t. tarasov and nadezhda v. tarasova -1.5 -0.5 0.5 1.5 2.5 0 100 200 300 400 500 r, km e e b alg further, the position of curves relative to one another changes, and so on. more clearly it is possible to see in fig. 5 where the dependence of lg (ea /eb) being the ratio of the total seismic energy of earthquakes after and before mhd-generator start-ups on the distance is presented. the values lg (ea /eb) change the sign periodically through every 80110 km. hence, bands where the electrical effects increases the flow of seismic energy and bands where the electrical effects diminishes the seismic flow, alternate periodically with increasing the distance to the mhd-generator. it is interesting that lg (ea /eb) < 0 at the short distance (less than 100 km). therefore, the electromagnetic pulses diminish seismic activity inside near the field area of the electrical dipole. at distances of 110-220 km, the seismicifig. 5. the ratio ea /eb of the total seismic energy of earthquakes of the north tien shan and adjacent areas occurring after and before 114 mhd-generator start-ups as a function of the distance to location of the mhdgenerator on bishkek test-site. the values ea /eb are plotted on a logarithmic scale. 10.5 11.5 12.5 13.5 14.5 0 100 200 300 400 500 r, km lg e fig. 4. the total seismic energy of earthquakes of the north tien shan and adjacent areas occurring before eb (dashed line) and after ea (solid line) 114 mhd-generator start-ups as a function of the distance to location of the mhd-generator on bishkek test-site. the values ea and eb are plotted on a logarithmic scale. 205 spatial-temporal changes of seismicity under effect of high energy electromagnetic pulses ty increases after mhd-generator start-ups, further, the curve goes to the negative domain again, etc. as a whole, the sign-variable dependence is observed, whose amplitudes increase up to a distance of 375 km to electrical dipole of the mhd-generator and, then, the amplitudes begin to decrease. it is important to note that the amplitudes of positive half-periods of the dependence are higher than the amplitudes of negative half-periods. this confirms that the effect of the electromagnetic pulses causes substantial growth of seismic energy of the region in the whole. for a quantitative estimation of variations of a spatial distribution of the earthquakes over time, the spatial correlation (rk) of earthquakes density for each two contiguous years as function of the time from 1975 to 1995 year was calculated using the technique discussed in the first part of the paper. this function is presented in fig. 6. first of all, it is interesting that for the period of 19751983 the periodic changes of the function are clearly observed. for this time interval, value rk has rather low values. the correlation regularly varies from 0.1 to 0.45 with a period of 5-6 years. it shows that there are periods of «fast» changes in spatial distribution of seismicity, which are replaced by periods of its relative stability every other 2.5-3 years and vice versa. one can clearly see in fig. 6 that the function abruptly changes the shape since 1983. the 5-6 year periods of variations in correlation are changed by periods of 3 year duration. at the same time, a significant increase in correlation factors rk up to 0.3-0.9 is observed. thus, the spatial distribution of earthquakes becomes more stable after 1983. as the time of change in frequency of periodicity and in increase in values rk coincides with the beginning of experiments with mhd-generator, it is possible to assume that the action of electromagnetic pulses causes an increase in stability of spatial distribution of seismicity over time and simultaneously speeds up cycles of its transformations, which develop on stabilization background. interestingly, the variation of values of fractal dimension shows that the earthquakes temporal distribution becomes more random for time period of experiments with the mhd-generator (from 1983 to 1990) (chelidze et al., 2002). figure 7 shows a similar dependence based on the selection of earthquakes occurring in the upper 5 km layer of the crust. just at the beginning of observations time interval, the rather high values of a correlation function are observed on the curve. further, within 6 years from 1977 to 1983, the values of rk do not exceed 0.23. at the beginning of experiments with 0 0.2 0.4 0.6 0.8 75 80 85 90 t, year r k fig. 6. spatial-temporal correlation of density of earthquakes of the north tien shan and adjacent areas as function of time. the vertical axis indicates the values of a correlation function rk; the horizontal axis indicates time in years. the vertical points are time of beginning and termination of a series of mhd-generator start-ups on the bishkek test-site. 206 nikolai t. tarasov and nadezhda v. tarasova the mhd-generator, the level of spatial-temporal correlation sharply increases. thus, in 1983, rk reaches 0.35, and, further, the values oscillate within the range 0.42-0.97. the 3-4 year periodicity is observed for this time again. these results confirm that the most appreciable changes in spatial-temporal structure of seismicity after the beginning of experiments with a mhd-generator are observed in the upper 5 km layer. previously, we showed that the most intensive increase in total seismic energy of earthquakes after mhd-generator start-ups was observed at the same depths not only in the north tien shan, but also in the garm region of tadjikistan (tarasov, 1997; tarasov et al., 1999). because the initiating action was most effective in the depth range 0-5 km which is the hydrostatic zone of the crust containing free water that fills cracks, pores and cavities, it was concluded that fluids are largely responsible for the initiation of seismicity by electromagnetic pulses. the presence of fluids in rocks gives rise to the seismoelectric and electroseismic effects. to test this assumption, we studied the changes in efficiency of the earthquakes triggering by electromagnetic pulses depending on a level of atmospheric precipitates. first, the values ea /eb obtained on each of 114 start-ups were compared to an average level of precipitates before each of them. to average the precipitate level before the start-ups, the time windows of different duration from 1 to 20 day were applied. however, a significant correlation between the flow of seismic energy and level of precipitates was not found. comparison of long-period changes of ea/eb with average annual level of precipitates was more successful. to do this, the values of average annual level of precipitates were calculated for each year of experiments. for the same period, we computed the ratios ea/eb of total seismic energy of the earthquakes which occurred in layer 0-5 km after and before the mhd-generator start-ups within each year. as the intervals between start-ups varied for different years, the time windows for ea/eb calculation were limited by 5 days. tarasov et al. (1999) have shown that it reduces the noise level which arises because of partial superposition of time windows of different start-ups when they are executed too often. figure 8 represents the variations ea/eb depending on time for 1983-1990 years. the changes in the annual level of atmospheric precipitation in bishkek are shown for the same period in the fig. 9. one can see that these curves are correlated. the correlation factor in this case was equal to 0.845, though this esti0 0.2 0.4 0.6 0.8 75 80 85 90 r k t, year fig. 7. function of a spatial-temporal correlation of density of earthquakes occurringin the upper 5 km layer of crust of the north tien shan and adjacent areas. the vertical axis indicates the values of a correlation function rk; the horizontal axis indicates time in years. the vertical points are the time of beginning and termination of a series of mhd-generator start-ups on the bishkek test-site. 207 spatial-temporal changes of seismicity under effect of high energy electromagnetic pulses mation should be assumed as preliminary, because we have only very small statistics. more clearly it could be seen in fig. 10 where points represents the ratio of total seismic energy after and before all mhd-generator start-ups for each of 8 years as a function of the annual level of precipitates for the related year. the fitted line obtained by the mean square method is also given. it is possible to see the values ea /eb increase depending on the level of precipitates. these results confirm the assumption that fluid is largely responsible for initiation of earthquakes by electromagnetic pulses. the dependence of efficiency of earthquakes triggering by electromagnetic pulses at a hypocenter depth shows that this can depend on pulse duration, as this parameter determines the thickness of the skin layer. figure 11 represents ea /eb as a function of sounding pulse duration. to calculate the diagram, all start-ups of the mhd-generator were divided into a number of subsequences with respect to pulse duration. the total seismic energy of the earthquakes that occurred before and after pulses of some duration was evaluated using all start-ups of mhd-generator included in the relevant subsequence. in the figure we do not observe any regular increase or decrease in efficiency of electromagnetic effects with the pulse duration. one can see that the dependence has sign-variable character. the shortest (less than 1.5 s) and longest (more than 12 s) pulses cause a decrease in total -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 83 84 85 86 87 88 89 90 l g (e a /e b ) t, year 0.7 0.9 1.1 1.3 1.5 1.7 83 84 85 86 87 88 89 90 l , m m /d ay t, year fig. 8. average annual values of ea / eb being the ratio of total seismic energy of earthquakes of the north tien shan occurring 5 days after and 5 days before mhd-generator start-ups in the upper 5 km layer as a function of time. the values ea / eb are plotted on a logarithmic scale. fig. 9. averaged annual level of atmospheric precipitation in bishkek as a function of the time. 208 nikolai t. tarasov and nadezhda v. tarasova seismic energy of earthquakes. the 6.75-9.1 s duration pulses cause a similar effect. the high efficiency is observed for pulses of duration ∼ 2 s, 3.5-6.7 s and 9.4-11.8 s. the sign-variable character of the curve suggest that layers where the electrical effects increases the flow of seismic energy and layers where the electrical impact decreases them alternate with depth. 3. discussion changes in seismicity could be connected with the influence of some external factors. it is shown for the central asian region (nikolaev and vereshagina, 1991a,b; tarasov and tarasova, 1995) that mb > 5 earthquakes and underground nuclear explosions significantly influlg(e a /e b) = 3.708 l – 3.5766 r = 0.845 -1 0 1 2 3 4 0.8 1 1.2 1.4 1.6 1.8 l g( e a /e b ) l, mm/day -4 -3 -2 -1 0 1 2 1 3 5 7 9 11 x, s e e b alg fig. 10. averaged annual values of ea / eb being the ratio of total seismic energy of earthquakes of the north tien shan and adjacent areas occurring 5 days after and 5 days before mhd-generator start-ups in the upper 5 km layer plotted as function of average annual level of atmospheric precipitation l in bishkek site (dots) and fitted line. the values ea / eb are plotted on a logarithmic scale. fig. 11. ratio ea /eb of total seismic energy of earthquakes of the north tien shan and adjacent areas occurring after and before mhd-generator start-ups as a function of the electromagnetic pulses duration. the values ea/ eb are plotted on a logarithmic scale. 209 spatial-temporal changes of seismicity under effect of high energy electromagnetic pulses ence seismicity at a distance up to 1500 km. the influence of geomagnetic storms on seismicity of the observation area is shown in (tarasov and tarasova, 2002; sobolev and ponomarev, 2003). thus, activation of remote earthquakes, nuclear tests or increasing geomagnetic activity before mhd-generator startups could be the reason for the observed increasing in seismicity. in order to check this assumption, the selection of mb > 5 earthquakes, which occurred within the time window ± 30 days about the time of each of 114 mhd-generator runs at distances up to 1500 km from the center the region was made from the world earthquake catalogue of isc. as in a previous study, obtained selection was used to find nt, which is the total daily number of remote earthquakes before and after all 114 mhd-generator start-ups (fig. 12a). estimations of the average background level and 99% confidence interval were made for all earthquakes with mb > 5, which occurred from 1983 to 1990 within the distance range of 1500 km. in the same way we obtained daily numbers of underground nuclear explosions (fig. 12b). for analysis of geomagnetic activity we drew up the curves of changes in average daily values of dst index (fig. 12c) and kp index (fig. 12d), which were averaged for all of 114 time windows. figure 12a-d shows that before mhd-generator start-ups (t < 0) there is no anomalous increase in the number of remote earthquakes or nuclear explosions, as well as anomalous changes of geomagnetic activity. therefore the observed effect is not connected with the influence of these factors. some articles, for example (oike and yamada, 1994), detected a correlation between shallow seismicity and atmospheric thunderstorm activity. unfortunately we have no information on thunderstorms in the region. in order to estimate this factor all mhd-generator runs were divided into two groups according to seasons: from march to august and from september to february. in the observation area, the first period is characterized by high thunderstorm activity, in the second period there are no thunderstorms. however, a significant increase in seismic activity after mhd-generator startups was observed for both groups. it means 0 2 4 6 8 t, day -20 -15 -20 -10 0 10 20 2 3 0 5 10 15 20 n dst kp fig. 12a-d. daily number of mb > 5 remote earthquakes (up to 1500 km) in the area (a) of the northern tien shan and adjacent territories, b) of semipalatinsk underground nuclear explosions, averaged (c) dst and (d) kp geomagnetic indices before (t < 0) and after (t > 0) 114 mhd-generator runs on the bishkek test site plotted against time (in days). the dashed lines indicate a mean background level and 99% confidence interval. that the effect is not connected with thunderstorm activity. one more reason for activation of seismicity could be the mechanical impact of the mhd-generator on the ground surface. unfortunately it was impossible to check this hypothesis in this region. however, such estimaa b d c 210 nikolai t. tarasov and nadezhda v. tarasova tion has been made for analogous experiments in garm region of tadjikistan (tarasov, 1997). along with the deep electrical sounding seismic soundings of the crust were executed by explosions of 400 kg of explosive (gamburtsev et al., 1983) (within the period from 1979 to 1984 there were executed 276 explosions). figure 13a,b show changes in the daily number of earthquakes before and after mhdgenerator start-ups in garm region (from tarasov, 1997), and fig. 13c shows changes of nt before and after explosions in the same area. it is clearly seen that the flow of seismic events grows after the mhd-generator startups. however, unlike nuclear explosions, relatively weak chemical explosions do not cause noticeable changes in seismicity. while their impact on the ground surface is much stronger than the mechanical impact of the mhd-generator, it is possible to conclude that the observed effect is caused by its electromagnetic, but not mechanical impact. it is interesting that in garm region after electromagnetic impacts the most long-standing activation of seismicity was observed (fig. 13a-c), beginnning 6 days after experiments, but not after 2 days as in fig. 1a-d. such delay of activation after the time of impact shows that electromagnetic pulses initiate some process, which causes an increase in seismicity, and the time of its development depends on rock properties. a similar dependence was observed in garm region after remote nuclear explosions (tarasov and tarasova, 1995). it is possible that rock properties determine not only the time of delay, but influence the efficiency of effect. it was shown above that in paleozoic structures of the southern part of the region a strong activization arises after mhdgenerator runs, while in mesozoic and cenozoic structures of its northern part there is practically no increase in seismicity. this fact could have the another explanation. in connection with the geoelectric cross-sections of the observation area presented in volykhin et al. (1993), it is possible to assume the existence of a high conductive layer, which plunges from the mhd-generator site to the south and is a kind of channel which directs most of the pulse energy in that direction. however, the efficiency of the initiating effect of electromagnetic pulses is influenced by other, more complicated factors too. periodic changes in ea/eb with an increased distance from the mhd-generator, shown in fig. 5, could be connected with the block structure of the crust. estimations of prevailing dimensions of the crust blocks for tien shan give 70 and 140 km (sadovsky and pisarenko, 1989), which is close enough to periods of change in ea/eb in fig. 5. the other explanation of such periodicity could be in the long scale anticorrelation of rock properties (sahimi et al., 1993). at this stage, the physical mechanism of the process of initiation of seismicity by electro0 5 10 15 -20 -10 0 10 20 1400 1600 1800 2000 200 300 400 n t, day fig. 13a-c. daily number of earthquakes in the garm region of tadjikistan before (t < 0) and after (t > 0) 34 mhd-generator runs on the garm test site plotted against time for the entire area (a) and for upper 5km layer of the tadjik depression (b), daily number of earthquakes in the same area of tadjikistan before (t < 0) and after (t > 0) 276 local chemical explosions of 400 kg explosive (c). the dashed lines indicate a mean background level (lower lines) and 99% confidence interval (upper lines). a b c 211 spatial-temporal changes of seismicity under effect of high energy electromagnetic pulses magnetic pulses is not clear. now it is difficult to explain with confidence the observed spatialtemporal characteristics of this effect. however, observed phenomena show new properties of triggered seismicity and could be useful for investigation of a process of earthquake initiation. 4. conclusions it has been shown that 2-6 days after effect of high energy electromagnetic pulses emitted by the mhd-generator there is activation of relatively small earthquakes in the region under study. the total seismic energy of initiated earthquakes is six orders higher than the energy transmitted by the mhd-generator to the radiating dipole. efficiency of the initiating effect of electromagnetic pulses depends on the natural level of seismic activity over the region, electromechanical characteristics of rocks, depth and distance to dipole. a 5-6 years periodicity of spatial-temporal correlation of earthquake density was detected indicating the existence of periods of «fast» changes in spatial distribution of seismicity over time, which alternate every 2.5-3 years by periods of relative stabilization. it is shown that the effect of electromagnetic pulses increases the stability of spatial distribution of seismicity over time and simultaneously speeds up cycles of its transformations, which develop on stabilization background. the results of this study show that the effect of high energy electromagnetic pulses radiated by mhd-generators causes substantial spatial-temporal changes in the seismicity of earthquake source zones and accelerates the release of energy stored in the crust due to the activity of natural tectonic processes, thus, serving as a kind a trigger. based on energy balance assumption for the crust, it can be concluded that a man-made increase in part of the seismic energy radiated in the form of flow of relatively small earthquakes leads to an additional release of tectonic stresses, thereby diminishing the likelihood of catastrophic events (or at any rate, reduces the energy of such events). if such assumption is confirmed, the detected effect could be used to develop a technique of reduction of seismic hazard by artificial discharge of tectonic stress by electromagnetic pulses. acknowledgements we are grateful to kostas eftaxias for useful advice and discussion. this work was supported by intas-99-0064 grant. references chelidze, t., v. de rubeis, t. matcharashvili and p. tosi (2002): influence of strong electromagnetic discharges on the dynamics of earthquakes time distribution at the bishkek test area (central asia), in proceedings of esc xxviii general assembly, genova 2002. gamburtsev, a.g., n.g. gamburtseva, s.p. starodubrovskaja and v.n. fridman (1983): recording waves and seismic model of the earthcrust on the dushanbinskiy test site, in seismic sounding of source zones, edited by a.v. nikolaev (nauka, moscow), 109-122. mikhailova, n.n. (1990): catalog of earthquakes in north tien shan and adjacent area (nauka, almaata), pp. 224. nikolaev, a.v. and g.m. vereshagina (1991a): about triggering of earthquakes by earthquakes, dokl. acad. nauk sssr, 318 (2), 320-324. nikolaev, a.v. and g.m. vereshagina (1991b): about triggering of earthquakes by underground nuclear explosions, dokl. acad. nauk sssr, 319 (2), 333-336. oike, k. and t. yamada (1994): relationship between shallow earthquakes and electromagnetic noises in lf and vlf ranges, in electromagnetic phenomena related to earthquake prediction, edited by m. hayakawa and y. fujinawa (terra scientific publishing co., tokyo), 115-130. rautian, t.g., v.i. khalturin, m.s. zakirov, a.g. zemtsova, a.p. proskurin, b.g. pustovitenko, a.n. pustovitenko, l.g. sinelnikova, a.g. filina and i.s. shengelia (1981): experimental exploration of seismic coda (nauka, moscow), pp. 141. sadovsky, m.a. and v.f. pisarenko (1989): discontinuous hierarchical model of the geophysical medium, in multidisciplinary studies in terrestrial physics, edited by s. negmatulaev and v. fremd (nauka, moscow), 9-26. sahimi, m., m. robertson and c. sammis (1993): fractal distribution of earthquake hypocenters and its relation to fault patterns and percolation, phys. rev. lett., 70, 2186-2189. sobolev, g.a. and a.v. ponomarev (2003): physics of earthquakes and precursors (nauka, moscow), pp. 270. tarasov, n.t. (1997): crustal seismicity variation under electric action, dokl. acad. nauk sssr, 353 (4), 542-545. tarasov, n.t. and n.v. tarasova (1995): response of 212 nikolai t. tarasov and nadezhda v. tarasova seismoactive medium to nuclear explosions, in earthquakes induced by underground nuclear explosions, edited by r. console and a. nikolaev, nato asi series 2. environment (sprunger-verlag, berlin heidelberg), 4, 215-223. tarasov, n.t. and n.v. tarasova (2002): the effect of geomagnetic storms on the seismicity, in proceedings of third international workshop on magnetic, electric and electromagnetic methods in seismology and volcanology, moscow, russia, 3-6 september 2002 (institute of physics of the earth, moscow), 206-208. tarasov, n.t., n.v. tarasova, a.a. avagimov and v.a. zeigarnik (1999): the effect of high energy electromagnetic pulses on seismicity in central asia and kazakhstan, volc. seismol., 4-5, 152-160. volykhin, a.m., v.d. bragin, a.p. zubovich, n.l. aitov, j.p. babakov, v.j. batalev, a.g. blokh, e.l. geller, v.a. zeigarnik, e.i. ivanov, j.i. isaev, n.a. koshkin, r.k. kuzmin, t.i. kulevtsova, a.i. matix, n.n. orlenko, a.v. pisakin, a.n. revtov, m.f. retinsky, a.r. rung, a.k. ribin, b.g. tkatchenko, j.a. trapeznikov, p.s. turovsky, v.v. finiakin, e.v. sharonova and a.p. shishkina (1993): geodynamic processes in geophysical fields (nauka, moscow), pp. 158. crocco .pdf 559 annals of geophysics, vol. 46, n. 3, june 2003 gpr prospecting in a layered medium via microwave tomography lorenzo crocco and francesco soldovieri istituto per il rilevamento elettromagnetico dell’ambiente (irea), cnr, napoli, italy abstract the tomographic approach appears to be a promising way to elaborate ground penetrating radar (gpr) data in order to achieve quantitative information on the tested regions. in this paper, we apply a linearized tomographic approach to the reconstruction of dielectric objects embedded in a layered medium. the problem is tackled with reference to a two-dimensional geometry and scalar case when data are collected over a linear domain with finite extent. in particular, in order to increase the amount of independent available data, a multi-frequency/multi-view/ multi-static measurement configuration is considered. with reference to stepped-frequency radar, this means that for each working frequency and for each position of the transmitting antenna (moved along a linear domain), the electric field scattered by the buried targets is measured in several locations along the same linear domain. the proposed inversion approach is based on the born approximation and a regularized solution is introduced by means of the singular value decomposition (svd). the problem of determining the optimal measurement configuration (in terms of number of frequencies and number of transmitting and receiving antennas) is also tackled by a numerical analysis relying on the singular value decomposition (svd). numerical examples are provided to assess the effectiveness and robustness of the proposed approach against noise on data. mailing address: dr. lorenzo crocco, istituto per il rilevamento elettromagnetico dell’ambiente (irea), cnr, via diocleziano 328, 80124 napoli, italy; e-mail: crocco.l.@irea.cnr.it key words gpr – microwave tomography – inverse scattering – ill-posed problems 1. introduction ground penetrating radar (gpr) is widely employed in civil engineering, shallow subsurface prospecting applications and archaeology (daniels, 1996) since it allows non invasive diagnostics of the probed domain in a fast and simple way. gpr works by emitting a modulated electromagnetic pulse into the ground and by recording the strength of the echo produced by the interaction between the impinging waves and the buried objects received at the air-soil interface in a monostatic or bistatic configuration. in the former case, the locations of the transmitting and the receiving antennas are coincident, whereas they are different in the latter case. by moving the antenna along a selected profile above the ground surface, a two-dimensional plot (radargram) is obtained in which the delay time of the recorded echoes (which can be related to the depth of the underground reflectors) is drawn versus the antenna position (daniels, 1996). this fast and simple operating mode is, however, able to survey only the presence and the location of the buried object but not its characteristics, unless additional a priori information is provided. moreover, since it is based on the assumption that the velocity of the investigating wave is constant, it may often give misleading results. finally, the interpretation of the radargrams is itself a difficult task which encompasses the ability and expertise of the user and requires the availability 560 lorenzo crocco and francesco soldovieri of a priori information about the domain under investigation. a tomographic approach represents a possible way to overcome these limitations as it makes it possible to achieve quantitative reconstruction of the domain under investigation, in terms of location, shape and chemical/physical properties of the buried targets (deming and devaney, 1997). in this case, the problem at hand amounts to determining the dielectric and conductive properties of the probed domain starting from the knowledge of the electromagnetic field scattered under the incidence of known impinging fields. to this aim one has to perform the inversion of a pair of coupled integral equations which govern the electromagnetic scattering. such an inversion is not a simple task (colton and kress, 1992) since it amounts to solving a non-linear ill-posed inverse problem. indeed, due to the properties of the kernel of the integral equation relating the unknowns to the scattered field data (bucci and isernia, 1997; pierri and leone, 1999), in the presence of uncertainties on the data only a finite amount of independent information about the unknowns can be achieved. from a mathematical point of view, this means that a proper regularization strategy has to be adopted to restore the well-posedness of the problem (bertero and boccacci, 1998). in addition, the nonlinear relationship between the data and the unknowns makes it very difficult to solve the problem, usually cast as the minimization of a non quadratic cost functional. due to the large number of unknowns one usually has to deal with, stochastic optimization tools cannot be exploited and so deterministic minimization techniques have to be adopted (isernia et al., 1997; kleinman and van den berg, 1993). since convergence of deterministic approaches depends on the starting guess, the minimization can get stuck in a local minimum of the cost functional which actually represents a «false solution» of the problem (isernia et al., 1997; leone et al., 2001). the local minima problem leads to the necessity of making the number of the searched parameters as low as possible, or in an equivalent way to keep as high as possible the ratio between the amount of independent data and the number of the unknowns to be determined (isernia et al., 2001). this requires availability of a priori information about the features of the unknowns and entails constraints on the extent of the probed domain. another class of solution algorithms is instead based on the adoption of linear approximations of the electromagnetic scattering (see, for instance, deming and devaney, 1997; wang and oristaglio, 2000; pierri et al., 2001, 2002a). these linearized inversion algorithms require lower computational cost with respect to the nonlinear ones and no local minima problem arises for the corresponding functionals. the above factors make the linear inversion algorithms suitable in several applications concerning the diagnostics of large probed domain and allow real-time processing. conversely, the adoption of approximate models of the electromagnetic scattering, introduces several limitations on the set of unknown functions which can be dealt with (pierri et al., 2001). in microwave tomography, the linearization is usually achieved by means of the born approximation (ba), which consists in approximating the total electric field inside the investigation domain with the incident one, i.e. the scattering object is a small perturbation with respect to the host medium. this assumption entails some constraints on the extent of the buried objects (which must be small with respect to the investigating wavelength) and on the difference between the dielectric properties of the objects and those of the background medium (which must be as low as possible) (slaney et al., 1984). in addition, also within the validity of the born model, further constraints arise about the unknowns which can be dealt with. these limitations concern their allowable spatial variations and are due to the filtering effect of the relevant linear operator relating the unknowns to the scattered field data (bucci et al., 2001a; pierri et al., 2002a,b). to this end, the singular value decomposition (svd) of the relevant linear compact operator (bertero and boccacci, 1998) provides a powerful tool to examine the amount of independent information carried by data, to discuss the class of retrievable unknowns, to determine the achievable spatial resolution limits 561 gpr prospecting in a layered medium via microwave tomography in the retrieved tomographic images (bucci et al., 2001a; pierri et al., 2002a,b) and to give a stable solution of the problem. in this paper, we apply the ba linear approach to the reconstruction of the spatial map of the dielectric and/or conductive properties of an investigation domain in the case of a layered medium. in particular, we consider a threelayered medium and inquire about the dielectric and conductive characteristics of an investigation domain completely embedded in the second layer. such a scheme is representative of several realistic cases such as for instance masonry diagnostics and prospecting of the shallower layers of soil (crocco et al., 2002). the twodimensional and scalar case is tackled with a filamentary excitation and the data are collected according to a multifrequency/multiview/multistatic strategy. as such, for each working frequency and for each position of the source, the data are measured in several locations along a linear domain of finite extent and very close to the first interface. the only a priori information about the problem regards the location and the extent of the investigation domain within which the objects are supposed to reside. 2. the mathematical model the reference situation is shown in fig. 1. the first region is homogeneous with relative dielectric permittivity e1 and conductivity s1. the second layer has extent equal to d and is homogeneous with relative dielectric permittivity e2 and conductivity s2. the third region is homogenous with relative dielectric permittivity e3 and conductivity s3. the complex equivalent relative permittivity in each layer has an explicit dependency on frequency given by (2.1) e0 being the free space dielectric permittivity and w the pulsation. the magnetic permeability is everywhere equal to that of the free-space, µ0. we suppose that infinitely long cylindrical objects having cross sections invariant with respect to the z-axis are embedded in the second layer and that their cross sections are contained within an a priori fixed square investigation domain d, completely embedded in the second layer. the unknowns of the problem are the spatial distribution of the relative dielectric permittivity and that of the conductivity over the investigation domain d. the source of the incident field is a filamentary z-directed electric current (tm polarization), of non finite extent and constant along the z-axis, which radiates at different frequencies. such a source is located in the first layer and is moved along the line g parallel to the first interface and located very close to it. in order to collect multifrequency/multiview/multistatic data, for each working frequency and at each position of the source the electric field scattered by the buried targets is measured in several locations along g. it is worth noting that such a set-up can be practically realized by using only one pair of antennas. as a matter of fact, exploiting a «synthetic array» strategy, one antenna acts as transmitter and the other one, which acts as receiver, is moved along the measurement line in the desired positions. by repeating this procedure for different locations of the transmitting antenna, one can achieve the desired set of data. under the above assumptions, the electromagnetic scattering phenomenon at hand is described by a two dimensional scalar model and ε ω ε σ ωεi eq i ij( ) = − / 0 fig. 1. geometry of the problem. 562 lorenzo crocco and francesco soldovieri it is governed by the following pair of integral equations (chew, 1995) (2.2a) (2.2b) where the time-factor exp ( jwt) has been omitted, k 2 is the (complex) wavenumber in the second medium, einc, e and es are the incident field in d (i.e. the field in the absence of the object), the total field induced inside the investigation domain d and the scattered field on g, respectively; r, rt, rr denote the generic point in the domain d, the generic location of the source and the generic measurement point, respectively. the quantities g21 and g22 are the sommerfeldgreen’s functions pertaining to the considered geometry and their expressions, as well as that of the incident field are reported in appendix. the contrast function c relates the searched (complex) permittivity (defined in the domain d) to that of the background (medium 2) and is defined as (2.3) where (2.4) such a complex function changes with frequency and this does not allow to consider it as a convenient unknown in the multifrequency case. however, assuming a priori that e i and s i in eq. (2.1) as well as e d and s d in eq. (2.4) are independent of frequency, we can introduce a pair of frequency independent real functions (2.5) wm being the maximum working frequency. these functions are simply related to c (2.6) and in what follows we will assume them as the actual unknowns of our inverse problem. by so doing we are able to deal with frequency independent unknowns, de and ds, without neglecting the dispersive nature of the problem. 3. the linearized tomographic approach in its general formulation, see eqs. (2.2a,b), the problem we are dealing with is non-linear. however, in some cases it is possible to describe the problem by means of an approximated model. the born approximation (slaney et al.,1984) consists in neglecting the mutual interactions between the objects and amounts to assuming the internal field e in d equal to the unperturbed one. this allows a considerable simplification since the electromagnetic scattering is modeled by means of a single linear integral equation. when the targets are embedded in a lossy medium, a large number of cases can be modeled by means of the ba (bucci et al., 2001b), since the presence of losses smoothes multiple scattering effects. considering that in masonry diagnostics as well as in subsurface prospecting involved media are usually lossy, we will consider the ba in the following. accordingly, the mathematical model is given by (3.1) and the problem can be cast as the inversion of the linear operator given by eq. (3.1) and χ ω ε ω ε ω , , r r ( ) = ( ) ( ) −d eq eq 2 1 ε ω ε σ ω εd eq d dj, / .r r r( ) = ( ) − ( ) ( )0 ∆ ∆ε ε ε σ σ σ ω ε = − − d d m 2 2 0 , = χ ω ε ε ω ω σ, r r r( ) = ( ) − ( )   1 2 eq mj∆ ∆ e k e g j s d r r r r r r r t t r' ', , , , , , / ω ω ω ε σ ωε ( ) = ( ) ( ) − ×∫22 21 2 2 0 inc j dmr r r' ' 'ε ω ω σ× ( ) − ( )    ∆ ∆ e e k e d r r r r r r, , , , , ,t t t'ω ω ω( ) − ( ) = ( ) ⋅∫inc 22 e k es d r r r r rr t t' ', , , , ,ω ω χ ω( ) = ( ) ( ) ⋅∫22 g d dr r r r r rt, , , ,' ' 'χ ω ω⋅ ( ) ( ) ∈ ∈22 γ g dr r r r rr r t' ', , ,ω⋅ ( ) ∈21 γ 563 gpr prospecting in a layered medium via microwave tomography defined as ldb: xœx �l 2(d) ¥ l2(d)æe s œy�l2(g ¥ g ¥ f). (3.2) in eq. (3.2) x is a subspace containing all the pairs of functions x = (de, ds) and the data space y is a subspace containing the scattered fields corresponding to all possible measurement positions over g, all possible source positions over g and to the frequency band f. due to the properties of its kernel, ldb is a compact operator, so that its inversion is an ill-posed problem and the necessity of restoring well-posedness arises (bertero and boccacci, 1998). therefore a suitable regularization scheme has to be adopted to set up a stable and reliable solution. a convenient tool to achieve a regularized inversion is provided by the svd of the operator ldb, which defines a triplet σ νn n n nu, ,{ } = ∞ 0 such that ldb(un) = snnn, ldb +(n n ) = s n u n (3.3) wherein ldb + is the adjoint operator of ldb, σ n n{ } = ∞ 0 denotes the sequence of the singular values ordered in non increasing sequence and u n and v n form the basis for the space of the visible objects (that is the objects which can be recovered by the error-free data) and for the closure of the range (that is the space of the noise-free data) of the operator, respectively. exploiting the svd of the relevant operator, the formal solution of integral eq. (3.1) can be cast as (3.4) where <.,.> denotes the scalar product in the data space. the compactness of ldb entails that the sequence of the singular values asymptotically decays to zero (bertero and boccacci, 1998). therefore the ill-posedness of the inverse problem given by eq. (3.1) immediately appears: indeed, in the presence of noise on data, the contributions to the solution related to the singular values closer (*) the model error is the discrepancy between the actual scattered field data and those that would result according to the ba. to zero will be strongly corrupted by noise and the result of the inversion will become unstable. a simple way to tackle such an instability consists in truncating the expansion (3.4) to those terms that are not overwhelmed by noise. therefore we assume as a regularized solution of our problem the one obtained by means of the truncated svd expansion (3.5) by restricting the solution space to that spanned by the first n + 1 singular functions, this regularized solution is stable with respect to errors on data. however, since it is achieved by considering only a reduced number of terms in eq. (3.4), this procedure will lead to an approximate representation of the sought unknown. therefore, the choice of the index n represents a trade-off between the accuracy of the solution and its stability with respect to noise. such an optimal choice, in its turn, is actually related to the estimated level of noise and to the model error (*). as a consequence, the optimal choice may be a chimera, because the model error depends on the unknown scattering object. lacking definite criteria to determine such an optimal choice, we have chosen in our regularized approach to reject the contributions related to singular values 20 db lower than the maximum one. 3.1. the discretization procedure the numerical implementation of the inversion procedure described above is based on an equivalent matrix formulation of the problem in which only real vectors are involved. as a first step, one has to discretize the unknown parameters within the investigation domain d. in order to satisfy the criteria outlined in richmond (1965), the pair of unknown functions is expanded by using as a basis the pulse functions defined within x e u nn n s n n= < > = ∑ 1 0 σ ν, . x e u nn s n n= < > = ∞ ∑ 1 0 σ ν, 564 lorenzo crocco and francesco soldovieri n c ¥ n c square cells whose side is l / 20 long, l being the minimum wavelength, thus rendering the results of the discretized version of the problem very close to those of the continuous problem in eq. (3.1). the vector of the unknowns is now defined as a column vector of dimension 2n c 2, whose first n c 2 rows contain de, and the second n c 2 rows contain ds. a discrete set of measurements n m , incident fields n v and frequencies n f is considered, leading to m = n m ¥ n v ¥ n f complex data. therefore, the data vector is defined as a column vector of dimension 2m, in which the real part of the scattered fields (considered at each frequency, incident field and measurement point) is stored in the first m rows, and the imaginary part is stored in the latter m rows. by following this procedure one achieves the matrix equation: (3.6) wherein a, b, c, d are block matrices of dimension m ¥ n c 2, computed by separating the discretized counterpart of eq. (3.1) in its real and imaginary parts. the discretized problem, as stated in eq. (3.6), can be now solved by means of the numerical svd applied to real matrices. 4. fixing the measurement set-up via svd besides defining the regularized solution of the linear inversion at hand, the svd can be exploited as a tool to fix the number of antennas of the measurement set-up and to investigate the effect of using a different number of frequencies in a given band. this can be performed by examining the effect of the change in the number and locations of the transmitting and receiving antennas on the number of the singular values n retained in the summation (3.5). according to the choice of the above section, we assume n as the number of singular values larger than 0.1 times the maximum singular value. the analysis is performed under the simplifying hypothesis of transmitting and receiving antennas uniformly spaced over the observation domain. in addition, thanks to the arguments in bucci et al. (2001a), it is convenient to assume that the number of transmitter and receiver locations is the same. in this way it results n v = n m , that is for each of the n m positions of the transmitter, n m measurement are collected (multiview/multistatic configuration). one can expect that when the number of transmitters and receivers increases, some independent information about the unknown is added and so the number n of useful singular values to be retained in (3.5) increases. however, since the available information carried by the scattered fields is finite, it is expected that a maximum number of antennas exists beyond which the addition of further antennas does not change in a significant way the number of useful singular values to be employed in the reconstruction. this is a very important point as in any practical application an increase in the complexity of the set-up means increasing costs in terms of measurement time and computational burden. to give an example of how the svd can be usefully exploited to this aim, we have considered a case typical of masonry diagnostics, in which media 1 and 3 are the free space and medium 2 is d = 2 m thick and has a relative dielectric permittivity of 6 and a conductivity of 1 ¥ 10 -3 s/m. the investigated domain is 1 m ¥ 1 m starting from the first interface and the receivers and transmitters are placed on a line g 1 m long placed just above the interface. the following analysis concerns only the relevant linear operator and therefore it does not depend on the targets, but only on the geometrical characteristics of the investigated domain, the background permittivity and the measurement configuration. firstly, let us consider a single frequency case at 300 mhz and determine which is the suitable number of antennas. this is performed by evaluating the svd of the matrix defined in eq. (3.6) when changing the number of transmitters and receivers. figure 2 shows the behavior of the normalized singular values greater than the threshold (assumed equal to – 20 db) in these various situations. as can be seen, the number of a b c d       ⋅       = ( ) ( )       ∆ ∆ ε σ re im e e s s 565 gpr prospecting in a layered medium via microwave tomography fig. 2. behavior of the normalized singular values of ldb at 300 mhz when changing the number of transmitter (and receiver) positions. a suitable choice is n m = 9 (n v = 9). fig. 3. behavior of the singular values of ldb when varying the number of frequency steps in the 100-300 mhz band: n f = 2 (100, 300), dashed-dotted line; n f = 3 (100, 200, 300), dotted line; n f = 4 (100, 150, 225, 300), solid line; n f = 5 (100, 150, 200, 250, 300), dashed line. 566 lorenzo crocco and francesco soldovieri useful singular values remains nearly unchanged when varying n m from 9 to 13 or more, while it is considerably lower when considering n m equal to 5 or less. on the basis of such a simple analysis we can then conclude that a suitable choice for n m (and hence for n v ) is 9. secondly, by assuming such a number of transmitter and receiver locations, we can then investigate the effect of the addition of the working frequencies in the band 100-300 mhz. figure 3 shows behavior of the singular values when increasing the number of frequencies in the 100-300 mhz band. first of all, it appears than increasing such a number has beneficial effects on the number of singular values. however, it also appears that considering more than four frequencies does not bring a definite advantage. in what follows we will assume that the data are collected at 100, 200 and 300 mhz. although a (slightly) larger number of singular values would be achieved considering four frequencies, this choice satisfies the need of keeping the computational burden as low as possible, without dramatically reducing the number of singular values. in conclusion, we have shown that exploiting the svd tool it is possible to simply determine the measurement set-up, both in terms of the number of antennas and in terms of the number of frequencies to consider. 5. numerical examples in this section we will give some examples aimed to show the reconstruction capabilities of the proposed inversion approach. the reference situation is the one depicted in fig. 1. the measurement set-up is designed according to the criteria outlined in the previous section and considers three equispaced frequencies in the band 100-300 mhz. at each frequency, in correspondence of each of the nine positions of the transmitting antenna, the scattered field is measured at nine equispaced positions along the measurement line g, 1 m long and placed very close to the first interface. the investigated domain is 1 m ¥ 1 m wide, starting from the first interface and is embedded in a layer which is d = 2 m thick. the dielectric and conductive properties of the three media are the same as in the previous section. in all the examples, the data have been synthetically generated by means of a forward solver which exactly solves eq. (2.2a,b) and then corrupted by means of an additive white noise to simulate measurement error. the aim of these examples is twofold. first, we want to show that the proposed approach can furnish qualitative information on the investigated region even when the model error is very large. as a matter of fact, unless a very low model error is present, it is not possible to achieve a quantitative reconstruction (even in the noiseless case) due to the discrepancy between the actual (synthetically generated) data and the data that would arise from the ba model. second, we want to show how the proposed regularization is a robust inversion tool when considering noise corrupted data. therefore, we will show for each case the reconstruction achieved in the (ideal) case of clean data and that obtained when data are corrupted by noise. in the first example, we consider the problem of imaging a rectangular void. due to the strong difference between the permittivity of the target and that of the background, the model error is quite large (about 100%), and we do not expect to achieve a quantitative reconstruction. figure 4a shows a comparison between the modulus of the (simulated) scattered data at 200 mhz and the corresponding field that would result according to the ba: as it can be seen the difference between the two fields is very large. figure 4b shows the comparison between the clean data and the data corrupted with an additive gaussian noise with an snr of 10 db, which does not change at the various frequencies. by comparing fig. 4a,b one can observe that in this case the larger error comes from the failure of the ba model. several factors affect the model error. among them the difference between the permittivity of the target and that of the background plays a major role. in table i one can observe how the model error varies when changing the permittivity of the target (the rectangular void we are considering): the void certainly represents a situation in which the ba assumptions are strongly violated. another interesting point is the behavior of the model error as a function of the working 567 gpr prospecting in a layered medium via microwave tomography frequency. in the case of an empty cavity, we can observe that the model error decreases with frequency, being equal to 118% at 300 mhz, 96% at 200 mhz and 52% at 100 mhz. this can be explained observing that when decreasing the working frequency, the corresponding wavelength increases and therefore the electrical dimension of the target decreases too. this entails that, as long as the working frequency decreases, the validity of ba is ensured. however, when decreasing the working frequency the amount of independent information carried by data decreases (pierri et al., 2001, 2002a) and therefore only an approximation of the sought unknown with a lower resolution can be achieved (pierri et al., 2002b). this is the main reason why frequency diversity is very a b fig. 4a,b. case 1: a rectangular void. a) comparison between actual scattered data (solid line) and data that would result according to the ba model (dotted line). b) comparison between clean data (solid line) and data corrupted by noise with an snr = 10 db (dotted line). the plotted quantity is the modulus of the field at 200 mhz for each location of the transmitter and the receiver. table i. behavior of the model error when changing the value of the target permittivity. the larger the relative difference between the background permittivity (e b = 6) and the target one the larger the model error is. target permittivity model error % 1 116 2 115 3 100 4 69 5 33 5.9 3 7 31 8 59 9 80 10 94 568 lorenzo crocco and francesco soldovieri important in microwave tomography approaches. as a matter of fact, exploiting multifrequency data it is possible to jointly consider data affected by a low model error (those pertaining to the lower frequency) and data which carry «more» independent information (those pertaining to the higher frequency). in order to show the effect of the model error on the reconstruction capabilities of our approach, let us first consider the case of clean data. figure 5 plots the modulus of the retrieved contrast (evaluated at 200 mhz placing the retrieved d e and ds in eq. (2.6). as expected, we cannot achieve a quantitative reconstruction, nevertheless the regularized inversion approach locates the target properly (but for a small misplacement). the effect of the model error is also evident as a «ghost» appears in the deeper part of the region. this is related to the mutual interactions between the target and the second interface which are indeed neglected under the ba. in order to prove the stability of our approach against noise on data, we repeated the inversion considering noise corrupted data and the modulus of the reconstructed contrast function is reported in fig. 6. as can be seen, the achieved reconstruction is almost unchanged compared to the clean data case, apart for an obvious (small) enhancement of the artifacts. the inversion procedure is quite fast as it takes only a couple of minutes to run on a 200 mhz pentium ii computer. in the second example, we consider the case of imaging a conductivity anomaly; the target has the same permittivity as the background but a slightly different conductivity (9 ¥ 10-3 s/m). while in the previous example the main (theoretical) contribution to the scattering data was due to d e, in this case the main contribution is due to ds. due to the small difference between the conductivity of the target and that of the background, a lower model error is expected, which indeed is about 13% (and is almost constant with frequency). this entails that the actual data and the data arising from the ba model are very similar; as pointed out by the comparison between the two fields at 200 mhz which is given in fig.7a. on the other hand, when comparing clean and noise corrupted data (fig. 7b), it appears that in this case the effect of measurement error is definitely more relevant than in the previous case. considering the clean data first, the modulus of the reconstructed contrast is plotted in fig. 8. the target can be clearly located and also the quantitative value of the contrast is determined with a good accuracy (the actual value of the modulus of the contrast would be 0.08). also in this case we repeated the test by corrupting the data with an additive white noise (snr = 10 db) and again the achieved result, see fig. 5. case 1: a rectangular void. modulus of the reconstructed contrast when considering clean data. the reference shape is reported as a contour plot. fig. 6. case 1: a rectangular void. modulus of the reconstructed contrast when considering noise-corrupted data (snr = 10 db). 569 gpr prospecting in a layered medium via microwave tomography fig. 9, proves the stability of our approach with respect to noise. in the last example we show the target is an empty pipe whose cross-section has a ring shape. the pipe has a relative dielectric permittivity of 7, a conductivity of 1 ¥ 10-3 s/m and it is 10 cm thick. this case is a quite hard one because both d e and ds contribute to the scattered field, moreover a large model error (70%) is present due to the strong difference between the permittivity of void (inside the pipe) and that of the background, while at the same time a little difference between the pipe and the background is present. in fig. 10 the modulus of the real part of the contrast reconstructed from clean data is plotted. again, due to the large model error, no quantitative reconstruction is achieved, but the presence of the target as well as the «transition» between the a b fig. 7a,b. case 2: a conductivity anomaly. a) comparison between actual scattered data (solid line) and data that would result according to the ba model (dotted line). b) comparison between clean data (solid line) and data corrupted by noise with an snr = 10 db (dotted line).the plotted quantity is the modulus of the field at 200 mhz for each location of the transmitter and the receiver. fig. 8. case 2: a conductivity anomaly. modulus of the reconstructed contrast when considering clean data. the reference shape is reported as a contour plot. 570 lorenzo crocco and francesco soldovieri shell and inner part of the pipe can be inferred from the reconstruction. finally, in fig. 11 the modulus of the real part of the reconstructed contrast when considering noise corrupted data (snr = 10 db) is reported, again proving robustness of the proposed approach against noise. 6. conclusions a linearized approach elaborated within the framework of the ba for the reconstruction of objects embedded in a layered medium has been presented. a suitable formulation of the problem allowed us tackle the dispersive nature of the scattering problem considering a pair of frequency independent unknowns, d e and ds. this exploits multifrequency data at best. as shown by several numerical examples, the proposed approach produces an efficient and reliable solution algorithm. in particular, the regularization strategy proves to be very stable since results obtained with «clean» and noisecorrupted data are almost coincident. moreover, although the assumptions underlying the ba model actually limit the class of retrievable functions (in terms of the permittivity and the extension of the target with respect to that of the background as well as in terms of its spatial variability), the numerical examples show that the approach can provide good (qualitative) results also in cases in which the linear approximation fails. the svd tool has been widely exploited in this paper not only to regularize the inverse problem, but also to determine which is the optimal measurement set-up. as a matter of fact, by means of a simple numerical analysis, we were able to determine the suitable number of measurements to perform and the number of working frequencies to consider. fig. 9. case 2: a conductivity anomaly. modulus of the reconstructed contrast when considering noisecorrupted data (snr = 10 db). fig. 10. case 3: a buried empty pipe. modulus of the real part of the reconstructed contrast when considering clean data. the reference shape is reported as a contour plot. fig. 11. case 3: a buried empty pipe. modulus of the real part of the reconstructed contrast when considering noise-corrupted data (snr = 10 db). 571 gpr prospecting in a layered medium via microwave tomography appendix. the function g21 represents the field generated in the first medium at the location (xp, yp) by a filamentary source located at the point (x',y') in medium 2. by relying on the general expression for a layered medium (chew, 1995) one achieves (a.1) wherein k is the spatial frequency and (a.2) (a.3) accounts for the transmission between media 1 and 2 for a given spatial frequency, (a.4) is the reflection coefficient between media i and j for a given spatial frequency and (a.5) is an «equivalent» reflection coefficient which takes into account the multiple reflections at the two interfaces. the function g 22 represents the field generated in the second medium in the location (x, y) by an elementary source located in the point (x',y') in the same medium. its expression (chew, 1995) is given by (a.6) wherein h0 ( 2) (.) is the zero order second kind hankel function, d x = x x', dy= y y' and dy+ = y + y'. g x y x y j m j y j x x j yp p p p21 21 2 1 2 4 , , , exp exp exp' ' ' '( ) = − ( ) −( )[ ] × −( ) +[ −∞ +∞ ∫π τ β κ β r j d j y d23 2 22exp exp '+ −( ) ( )]β β κ β ω µ ε ε κi i= − 2 0 0 2 . τ β β21 1 2 2 = + rij i j i j = − + β β β β . m r r j d 2 21 23 2 1 1 2 = − −( )exp β g x y x y j h x y22 0 2 2 2 2 4 , , , ( )' '( ) = − +( ) +−β ∆ ∆ j r m r j d y j x d23 2 12 2 2 2 2 2exp cos exp− ( ) ( ) ( ) +− −∞ +∞ ∫π β β β κ κ∆ ∆ j m r j y r j y j d 2 2 2 21 2 23 2 2 4 2exp exp− −( ) + −( ){ + + π β β β β∆ ∆ }} ( ) −∞ +∞ ∫ exp j x dκ κ∆ 572 lorenzo crocco and francesco soldovieri by means of the green-sommerfeld functions it is also possible to evaluate the incident field in the domain d produced by a filamentary current i0 located in the first medium at a distance yt from the interface (a.7) as can be seen, g21, g22 and the incident field can be interpreted as 1-dimensional fourier transforms in the spatial frequency k and therefore can be efficiently computed by means of fast fourier transforms. some remarks on the solution of the forward scattering problem can be found in crocco et al. (2002). e j i m j y j x xt t tinc r r, exp exp( ) = − ( ) −( )[ ] × −∞ +∞ ∫ ωµ π τ β κ0 0 21 2 1 4 ' j y r j d j y dexp exp exp .× −( ) + −( ) ( )[ ]β β β κ2 23 2 22' ' references bertero, m. and p. boccacci (1998): introduction to inverse problems in imaging, institute of physics (bristol and philadelphia, u.k.), chapters 5 and 9. bucci, o.m., l. crocco, t. isernia and v. pascazio (2001a): subsurface inverse scattering problems: quantifying qualifying and achieving the available information, ieee trans. geosci. remote sensing, 39, 2527-2538. bucci. o.m., n. cardace, l. crocco and t. isernia, (2001b): degree of non-linearity and a new solution procedure in scalar 2-d inverse scattering problems, j. opt. soc. am. a, 18, 1832-1845. bucci, o.m. and t. isernia (1997): electromagnetic inverse scattering: retrievable information and measurement strategies, radio sci., 32, 2123-2138. chew, w.c. (1995): waves and fields in inhomogeneous media (ieee press, new jersey), 410-414. colton, d. and r. kress (1992): inverse acoustic and electromagnetic scattering (springer verlag, berlin). crocco, l., r. persico and f. soldovieri (2002): a tomographic approach for imaging targets embedded in a layered medium, in proceedings ninth international conference on ground penetrating radar, gpr 2002, santa barbara, ca, 353-358. daniels, d.j., (1996): subsurface penetrating radar (iee press), chapter 1. deming, r.w. and a.j. devaney (1997): diffraction tomography for multi-monostatic ground penetrating radar imaging, inverse problems, 13, 29-45. isernia, t., v. pascazio and r. pierri (1997): a non-linear estimation method in tomographic imaging, ieee trans. geosci. remote sensing, 35, 910-923. isernia, t., v. pascazio and r. pierri (2001): on the local minima in a tomographic imaging technique, ieee trans. geosci. remote sensing, 39, 1596-1607. kleinman, r.e. and p.m. van den berg (1993): an extended-range modified gradient technique for profile inversion, radio sci., 28, 877-884. leone, g., a. brancaccio and r. pierri (2001): the quadratic distorted approximation for the inverse scattering of dielectric cylinders, j. optical soc. am., pt. a, 18, 600-609. pierri, r. and g. leone (1999): inverse scattering of dielectric cylinders by a second order born approximation, ieee trans. geosci. remote sensing, 37, 374-382. pierri, r., g. leone, r. persico and f. soldovieri (2001): electromagnetic inversion for subsurface applications under the distorted born approximation, nuovo cimento c, 24 (2), 245-261. pierri, r., a. brancaccio, g. leone and f. soldovieri (2002a): electromagnetic prospection via homogeneous and inhomogeneous plane waves: the case of an embedded slab, aeü, int. j. electron. commun., 56 (1), 11-18. pierri, r., a. liseno, r. solimene and f. soldovieri (2002b): resolution limits in the fresnel zone: roles of aperture and frequency, subsurface sensing technologies and applications, 3 (1), 1-18. richmond, j.h. (1965): scattering by a dielectric cylinder of arbitrary cross section shape, ieee trans. antennas propag., 13, 334-341. slaney, m., a.c. kak and l.e. larsen (1984): limitations of imaging with first-order diffraction tomography, ieee trans. microwave theory tech., mtt-32, 860-874. wang, t. and m. oristaglio (2000): gpr imaging using the generalized radon transform, geophysics, 65, 1553-1559. 309_319 girolamo.pdf 309 annals of geophysics, vol. 46, n. 2, april 2003 lidar research activities in potenza, southern italy paolo di girolamo (1), aldo amodeo (2), marco pandolfi (2), gelsomina pappalardo (2) and vincenzo cuomo (1) (2) (1) dipartimento di ingegneria e fisica dell'ambiente (difa), università della basilicata, potenza, italy (2) istituto di metodologie per l’analisi ambientale (imaa), cnr, tito scalo (pz), italy abstract the lidar group in potenza has almost 10 years’ experience in the fi eld of lidar research. important results have been accomplished both in aerosol and water vapour research. the lidar system in potenza has acquired different confi gurations during the years, always preserving the capability to accomplish two wavelength aerosol and water vapour raman measurements. an important up-grade was the introduction of a water vapour dial channel in 1997. the lidar system in potenza has been involved in several aerosol and water vapour measurement campaigns: stratospheric aerosol measurement campaign (1994-1995); lite correlative measurement campaign (september 1994); water vapour intensive observation period (january-february 1997); earlinet project (since february 2000). a second lidar system, primarily dedicated to water vapour raman measurements, is under development and starting january 2002 will go through an intensive observation period dedicated to the validation of sensors on-board envisat (principally gomos, mipas and sciamachy). this paper summarises some of the major results accomplished, as well as expected results from the forthcoming campaigns. mailing address: dr. paolo di girolamo, dipartimento di ingegneria e fisica dell’ambiente (difa), università della basilicata, c.da macchia romana, 85100 potenza, italy; e-mail: digirolamo@unibas.it key words lidar water vapour aerosol 1. introduction atmospheric water vapour and aerosols play a crucial role in the earth climate, being a key element in the global radiation budget, in atmospheric circulation, as well as in the microphysical processes leading to cloud formation and development. the role of atmospheric water vapour and aerosols in the climate system is only partially understood at present, as a result of their highly variable time and space distribution on the global scale. global coverage measurements of aerosol and water vapour with high accuracy, and high spatial and temporal resolution are needed throughout the troposphere up to the lower stratosphere. in spite of the recent improvements in their performances, passive remote sensors from space provide global coverage of aerosol and h2o distribution with accuracy and a vertical resolution still insuffi cient for climate studies, primarily in the middle and upper troposphere. the lidar system based both on the raman and dial techniques can provide accurate measurements of atmospheric water vapour, as well as aerosols, with high space and time resolution. ground-based and airborne lidar systems operational spread around the globe, as well as the shuttle-based experiment lite, have demonstrated the technological maturity of the lidar technique for operational space-borne application. in this paper we review the major results accomplished by one of these systems, namely 310 paolo di girolamo, aldo amodeo, marco pandolfi , gelsomina pappalardo and vincenzo cuomo the lidar system in potenza. we intend to illustrate the performed measurement campaigns in terms of aerosol and water vapour measurements. the earlinet project, mentioned in the abstract, is not included in the present work since a specifi c paper within this special issue is devoted to this effort. 2. instrumentation 2.1. the aerosol / water vapour system the lidar system in potenza is located within the scientifi c laboratories of imaa (40°50' n, 14°10' e, 820 m a.s.l.) and was developed in co-operation with the physics department of the «federico ii» university of naples. the system has been operated throughout the years by personnel from instituto di metodologie per l’analisi ambientale (imaa) consiglio nazionale delle ricerche and university of basilicata. it started to take measurements in july 1993. figure 1 shows the block diagram of the system. the potenza lidar system was originally developed around a nd:yag laser source, including second and third harmonic generation crystals (532 and 355 nm) and providing pulses at a repetition rate up to 20 hz. more recently, a second laser source was added, namely a dye laser, tunable within a spectral region of major absorption for water vapour (690-730 nm) (ambrico et al., 1997; di girolamo et al., 1999). the dye laser source is pumped by a portion of the 532 nm beam. as the laser pulses propagate through the atmosphere, part of their energy is backscattered to the instrument by particles – typically aerosols or hydrometeors – and by the molecules of the gas species comprising the atmospheric mixture. laser beams at 355 nm and 532 nm are used to monitor aerosol and clouds. the 355 nm laser beam is also used to stimulate vibrational raman scattering by water vapour and nitrogen molecules, at 407.5 and 386.6 nm, respectively. the dye laser is exploited for the application of the dial technique to water vapour monitoring. this laser source is able to transmit two laser beams at two different wavelengths (l on and l off), the fi rst one falling on a water vapour absorption line center and the second falling on the line shoulder. for the measurements illustrated in the present paper, l on is set to 723.59, while l off is 723.71 nm. the receiver consists of a vertically pointing cassegrainian telescope (0.5 m diameter primary mirror, 5 m combined focal length). the presence of a series of dichroic mirrors split the collected radiation into four portions (355 nm, 386 nm, 407 nm, 532 nm). interference fi lters or single/double grating monochromator are used as spectral selection devices. the introduction of a fused-silica beamsplitter transmits 50% of the collected radiation into the old section of the receiver, and deflects the remaining 50% into the dial channel. this particular partitioning is considered in order to obtain comparable performances for the raman and dial techniques for the reported water vapour measurements. detection is accomplished by means of photomultipliers. detected signals are sampled by means of both analog-to-digital conversion and photon counting. water vapour dial measurements were supported by a photoacoustic apparatus, used to perform high-resolution measurements of the water vapour absorption spectrum and to check the dye laser tuning on the selected absorption line during the dial measurement. the photoacoustic experiment was developed around a vacuum cell equipped with fused silica windows; distilled water is contained in a small tanker inside the cell. pressure variations inside the cell, resulting from water vapour absorption, are detected by a condenser microphone, whose signals are amplifi ed and processed by an analog integrator and acquired by means of a digital oscilloscope. 2.2. cal / val lidar system a second lidar system, primarily dedicated to water vapour raman measurements, is under development at imaa. this system will mainly be used to validate envisat data products. this system is based on a nd:yag laser equipped with third harmonic generation crystals (repetition rate up to 100 hz). the third 311 lidar research activities in potenza, southern italy harmonic is transmitted into the atmosphere in a coaxial mode. the receiver is developed around a vertically pointing telescope in cassegrain confi guration with a 0.5 m diameter primary mirror and a combined focal length of 5 m. the collected radiation is split into three channels by means of dichroic mirrors. interferential fi lters are used to select the elastic backscattered radiation at 355 nm, the n2 raman shifted signal at 386.6 nm and the water vapour raman shifted signal at 407.5 nm. each wavelength is then split into 2 different channels for both low and high range signals. photomultiplier tubes are used as detectors. low range signals are sampled in analog mode by means of a digital oscilloscope (500 mhz). high range signals are acquired in photon counting mode using a multi channel scaler (mcs) board with a dwell time of 100 ns. lidar measurements will be performed simultaneously with radiosonde launches to calibrate lidar measurements of water vapour. 3. main results 3.1. stratospheric aerosol measurement campaign (1994-1995) lidar measurements of stratospheric aerosol were made in potenza on a routine basis in fig. 1. block diagram of the aerosol / water vapour lidar system. 312 paolo di girolamo, aldo amodeo, marco pandolfi , gelsomina pappalardo and vincenzo cuomo the period 1994-1995 (di girolamo et al., 1995, 1996). this measurement campaign is the continuation of the one started in naples in september 1991. measurements cover the history of the aerosol cloud produced by the eruption of mount pinatubo (june 1991, philippines). the eruption of mount pinatubo in june 1991 determined a large increase in the stratospheric aerosol load on a global scale. this eruption is considered to be the most important of the century in terms of both volcanic material injected into the stratosphere and effects on the global climate (zhao et al., 1995). apart from the pinatubo eruption, the period 1991-1995 was characterized by minor volcanic eruptions (hudson, etc.) which produced negligible effects in the stratosphere on the global scale. this allowed aerosol sensors to observe the restoration of pre-volcanic constant background conditions and thus allowed to clearly distinguish and isolate the effects on a single eruption event. figure 2 shows the evolution with time of the stratospheric aerosol load. data are expressed in terms of aerosol backscattering coeffi cient at 351 fig. 2. vertical profi les of the aerosol backscattering coeffi cient at 351 and 355 nm for 61 selected nights of measurements in the period september 1991 june 1995. data are expressed in units of 10-6 m–1 sr–1. 313 lidar research activities in potenza, southern italy and 355 nm. a large increase in aerosol load is observed starting september 1991, but maximum load is observed in december 1991 and decay started immediately afterwards. integrated backscattering (ib) is obtained by integrating the aerosol backscattering coeffi cient throughout the stratosphere. ib (fi g. 3) reached its maximum value in december 1991, displaying a subsequent decay with an e-folding times of 237 ± 25 days. figure 4 shows the evolution with time of the aerosol optical thickness ta. ta reached its maximum in december 1991, then decaying with an e-folding times of 250 ± 111 days. ta is obtained from the elastic signal assuming an aerosol-free region above an below the stratospheric aerosol layer (di girolamo et al., 1996). while an aerosol free region is always present above the stratospheric aerosol layer, cirrus and upper tropospheric clouds can sometime prevent from locating an aerosol-free region below the stratospheric layer. therefore, the number of data points for ta in fig. 4 is smaller than the number of data points of ib in fi g. 3. furthermore, the considered approach is particularly effi cient for values of ta in excess of 0.3. thus, in the second half of the data record (period 750-1500 days after eruption), when values of ta are often smaller than 0.3, the number of data points in fi g. 4 with respect to fi g. 3 is further reduced. approximately 20 mtons of so2 were injected into the lower stratosphere within the fi rst few days after the eruption (bluth et al., 1992), and by late july 1991 the resulting aerosol cloud was diffused on a global scale (mccormick and veiga,1992). the presence of a massive long-lasting stratospheric aerosol layer leads to an in-crease in the planetary albedo, resulting in a lower energy fl ux at surface level, as well as in an absorption enhancement within the aerosol layer. the latter leads to a temperature increase in the lower stratosphere, together with an enhancement of diffuse sky radiation. effects of stratospheric aerosols of volcanic origin on the global radiation budget are dependent on the amount, height and lifetime of the material injected (pollack et al., 1976). observed values of the aerosol optical thickness (fig. 4) are consistent with a local increase in stratospheric temperature of 4-6 k. figure 5 shows the evolution with time of the aerosol layer center of mass height, together with theoretical descent curves for different particle sizes as obtained from kasten (1968). theoretical data account just for sedimentation of aerosol particles, while vertical eddy diffusion is not considered. the fi gure clearly shows that data in the period 400-1000 days following the eruption follow the 0.3 mm particle size curve, while data in the period 1000-1500 days after the eruption follow the 0.1 mm particle curve. fig. 3. time evolution of integrated backscattering ib for the period 1991-1995. also reported is the exponentially decreasing function fi tted to the data. fig. 4. time evolution of the aerosol optical thickness ta for the period 1991-1995. also reported is the exponentially decreasing function fi tted to the data. 314 paolo di girolamo, aldo amodeo, marco pandolfi , gelsomina pappalardo and vincenzo cuomo 3.2. lite correlative measurement campain (september 1994) lite (lidar in space technology experiment), flying on the space shuttle in the period 10-19 september 1994, represented the fi rst attempt to perform global coverage lidar measurements from space (mccormick et al., 1993). the lidar system in potenza was involved together with another 60 lidar ground stations in the validation of lite measurements. thus, in conjunction with the lite mission an intensive lidar measurement campaign was carried out in potenza. because of the relative distance between lite ground tracks and the validation station for all passes, measurements carried out in potenza were primarily aimed to the validation of lite stratospheric measurements (cuomo et al., 1997,1998; pappalardo et al., 1997). lite accomplished 6 overpasses within 1500 km from potenza. the minimum distance between lite ground track and potenza was 600 km. figure 6a,b shows the comparison between simultaneous lite and potenza lidar measurements at both 355 (fi g. 6a) and 532 nm (fi g. 6b), in coincidence with shuttle orbit 33. data are expressed in terms of the aerosol backscattering ratio ra(z). each fig. 5. time evolution of zc for the period 1991-1995. also reported are the theoretical descent curves for 1 mm, 0.3 mm and 0.1 mm diameter particles. fig. 6. simultaneous lite (thin line) and potenza (solid line) lidar measurements of the aerosol scattering ratio at 355 nm (a) and 532 nm (b) for orbit 33. data area reported with their error bars a b 315 lidar research activities in potenza, southern italy lite profi le is averaged over 300 laser shots, with a vertical resolution of 150 m, while each potenza lidar profi le is averaged over 90 min, with a vertical resolution of 300 m. data are reported with their error bars. assuming that data at different altitudes are uncorrelated for both potenza lidar and lite, it is possible to make a statistical analysis of the two data sets to quantify the degree of agreement between the lite and potenza lidar measurements. potenza lidar measurements of ra,532(z) (r pz a,532(z)) have been plotted as a function of corresponding lite data ( ra ,532 lite ( z) ); that is, for any given z within the stratospheric aerosol layer, points of coordinates ( ra ,532 lite ( z) , r pz a,532(z)) were considered (fi g. 7a-e). since lite data have a vertical resolution of 150 m, while potenza lidar measurements have a resolution of 300 m, the former data have been smoothed down to a resolution of 300 m. this reduces the number of data points within the stratospheric aerosol layer from 70 to 35; however, the number is large enough to guarantee the applicability of the fi tting procedure. points reported in the plot are those corresponding to values of z in the range 16-26 km. values of rpza,532(z) are reported together with their error bars. the points in the plot have been least square fi t by the linear function (3.1) fig. 7a-e. ra ,532 lite ( z) versus r pz a,532(z) for all data points within the stratospheric aerosol layer: a) orbit 33; b) orbit 34; c) orbit 128; d) orbit 129 and e) orbit 145. values of ra ,532 lite ( z) are reported together with their error bars. also reported is the best fi t linear function. fig. 8a-e. r za,355 lite ( ) versus r a pz ,355(z) for all data points within the stratospheric aerosol: a) orbit 33; b) orbit 34; c) orbit 128; d) orbit 129 and e) orbit 145, together with the best fi t linear function. a e b c d a e b c d 7 8 ra ,532 pz z( ) = mra ,532lite z( ) + c. 316 paolo di girolamo, aldo amodeo, marco pandolfi , gelsomina pappalardo and vincenzo cuomo even if both rpza,532(z) and ra ,532 lite ( z) are affected by a non negligible uncertainty, only the error on rp z a,532 (z) was considered in the fi tting procedure. fitting lines are presented in fi g. 7a-e. the correlation coeffi cient of the fi t, r, equal to 1 in case of perfect correlation, quantifi es the degree of correlation between lite and potenza lidar data. if data analysis is properly performed, values of the fi tting parameter c are expected to be equal to 0. the parameter m gets the signifi cance of a normalization coeffi cient to calibrate data: m >1 implies ra ,532 lite ( z) is underestimating rpza,532(z) (assumed as truth for validation), while m < 1 implies ra ,532 lite ( z) is overestimating rpza,532(z). the fi tting procedure was applied to data at both 532 (fi g. 7a-e) and 355 nm (fi g. 8a-e). results can be summerized as follows. at 532 nm, values of r are in the range 0.88-0.93 (average value of 0.90 ± 0.02). such high values of r imply a very good correlation between lite and potenza lidar data in terms of ra,532(z). values of m are in the range 0.96-1.08, with an average value of 1.02 ± 0.07. at 355 nm, values of r are in the range 0.72-0.81 (average value of 0.77 ± 0.04) and values of m are in the range 0.79-1.19 (average value of 0.92 ± 0.19). this latter result implies that lite measurements at 355 nm are slightly underestimating potenza lidar measurements. 3.3. water vapour intensive observation period (january-february 1997) water vapour measurements in the troposphere up to approximately 10 km a.s.l. have been performed by means of a ground-based lidar system through the simultaneous application of the raman and dial techniques. water vapour absorption lines were carefully selected to minimise dial measurement uncertainty. in particular, l on was selected to limit interferences by molecular species different from the investigated one, to minimise the temperature variability of the water vapour absorption line strength and to meet the requirements in terms of optimal optical depth (ambrico et al., 2000). the dependence on temperature and pressure of the water vapour absorption cross-section, as well as laser line alteration due to water vapour absorption and the effects of the limited laser spectral purity, have been carefully accounted for in the estimate of the altitude variability of the effective absorption cross-section, with particular attention to boundary layer measurements. selected values for l on and l off are 723.59 and 723.71 nm, respectively. in early 1997 (20 january-20 february), an intensive observation period was devoted to the simultaneous application of the raman and dial techniques for water vapour measurements. as an example of the performed measurements, fi g. 9 shows the raman measurement of the nh2o(z) vertical profi le carried out on february 18, 1997 (22:20-00:20 gmt). the fi gure also shows the simultaneous humidity profi le measured by the dial technique (22:20-00:20 gmt) and by a radiosonde launched at 22:55 gmt. measurements are expressed in terms of specifi c humidity (g/kg) and are reported with their error bars. raman measurements extend up to approximately 9 km, while dial measurements fig. 9. vertical profi les of nh2o (z) for february 18, 1997: dial measurement (22:20-00:20 gmt, black squares), raman measurement (22:20-00:20 gmt, red circles) and radiosonde profi le (launch at 22:55, green triangles). 317 lidar research activities in potenza, southern italy display large error values above 7 km and have not been reported above this height. the calibration coeffi cient for raman data is obtained on the basis of an extended comparison between raman measurements and simultaneous and co-located radiosonde measurements, the accuracy of raman measurement being strictly related to the reliability of radiosonde humidity measurements used for the calibration. since the dial technique is self-calibrating, water vapour density measurements can be performed in the troposphere by the dial technique with an uncertainty which is primarily limited by the dial signal statistical error. raman, dial and radiosonde measurements appear to be in good agreement up to approximately 8 km agl, with a relative deviation which does not exceed 20% up to 7 km (fi g. 9). in the same period, measurements were also performed to characterize the boundary layer structure (di girolamo et al., 1999). aerosols and moisture tend to be trapped within the pbl and can be used as tracers for the study of the boundary layer vertical structure and time variability (pappalardo et al., 2001). figure 10 shows the comparison between simultaneous lidar and radiosonde data for 11 february; lidar data are expressed in terms of aerosol backscattering coeffi cient at 723 nm, while radiosonde data are expressed in terms of potential temperature and relative humidity. the potential temperature profi le appears to be anti-correlated with both the relative humidity and the aerosol backscattering, the aerosol layer top being associated with a maximum in potential temperature and a minimum in relative humidity. 3.4. forthcoming measurements the lidar system described in section 2.2 will go through an intensive observation period (starting january 2002) dedicated to the validation of the sensors on-board envisat, primarily aatsr, gomos, mipas, meris and sciamachy. aatsr will provide, among others, measurements of cloud top height. the lidar system will be used to validate these measurements, primarily in the vertical interval 0.5-20 km. mipas will provide measurements of several atmospheric constituents in the height region 8-53 km. the lidar system will be used to validate mipas measurements of the water vapour profi le in the vertical region 8-10 km. validation above this height (up to approximately 20 km) will be provided by radiosondes. gomos measurements of stratospheric aerosol will be validated by lidar system up to approximately 20 km. the system will also be used to validate meris and sciamachy measurements of aerosol optical thickness. sciamachy will also provide, among others, measurements of precipitable water to be validated by lidar. fig. 10. vertical profi les of aerosol backscattering coeffi cient at 723 nm for 11 february 1997 (bold line) and simultaneous radiosonde profi les of potential temperature (open up triangles) and relative humidity (solid dots). 318 paolo di girolamo, aldo amodeo, marco pandolfi , gelsomina pappalardo and vincenzo cuomo table i. envisat validation: list of validated and validating sensors. aatsr calval instrument aerosol lidar geophysical parameter cloud top height calval vertical interval 0.5-15 km calval instrument radiosonde system geophysical parameter cloud top temperature and height calval vertical interval 0.5-20 km mipas 8-53 km calval instrument raman lidar geophysical parameter water vapour profi le calval vertical interval 8-10 km calval instrument radiosonde system geophysical parameter water vapour profi le calval vertical interval 8-20 km geophysical parameter temperature profi le calval vertical interval 8-30 km geophysical parameter density profi le calval vertical interval 8-30 km gomos 15-100 km calval instrument aerosol lidar geophysical parameter stratospheric aerosol profi le calval vertical interval 15-20 km calval instrument radiosonde system geophysical parameter water vapour profi le calval vertical interval 15-20 km geophysical parameter temperature profi le calval vertical interval 15-30 km meris calval instrument aerosol lidar geophysical parameter aerosol optical thickness calval vertical interval column calval instrument raman lidar geophysical parameter precipitable water calval vertical interval column calval instrument radiosonde geophysical parameter precipitable water calval vertical interval column sciamachy calval instrument aerosol lidar geophysical parameter aerosol optical thickness calval vertical interval column calval instrument raman lidar geophysical parameter precipitable water calval vertical interval column calval instrument radiosonde geophysical parameter precipitable water calval vertical interval column calval instrument aerosol lidar geophysical parameter stratospheric aerosol profi le calval vertical interval 15-30 km calval instrument aerosol lidar geophysical parameter cloud top height calval vertical interval 0.5-15 km 319 lidar research activities in potenza, southern italy the stratospheric aerosol profi le and cloud top height can be retrieved from sciamachy limb measurements and such measurements can be validated by the lidar system. a complete list of validated and validating sensors is given in table i. calibration and validation measurements (cal/val) will be carried out on a routine basis for the fi rst 6 months after the launch, but will continue afterwards with a reduced schedule for all the duration of the mission. references ambrico, p.f., a. amodeo, s. amoruso, m. armenante, v. berardi, a. boselli, r. bruzzese, r. capobianco, p. di girolamo, l. fiorani, g. pappalardo, n. spinelli and r. velotta (1997): a multiparametric lidar system spanning from uv to the mid ir, laser und optoelektronic, 29, 62-69. ambrico, p.f., a. amodeo, p. di girolamo and n. spinelli (2000): sensitivity analysis of dial measurements in mid ir region, appl. opt., 39 (36), 6847-6865. bluth, g.j.s., s.d. doiron, c.c. schnetzler, a.j. krueger and l.s. walter (1992): global tracking of the so2 clouds from the june 1991 mount pinatubo eruption, geophys. res. lett., 19, 151-154. cuomo, v., p. di girolamo, f. esposito, g. pappalardo, c. serio, n. spinelli, m. armenante, b. bartoli, v. berardi, r. bruzzese, c. bellecci, g.e. caputi, f. de donato, p. gaudio and m. valentini (1997): the lite correlative measurements campaign in southern italy: preliminary results, appl. phys. b, b 64 (5), 553-560. cuomo, v., p. di girolamo, g. pappalardo, n. spinelli, m. armenante, v. berardi and m.p. mccormick (1998): lidar in space technology experiment correlative measurements by lidar in potenza, southern italy, j. geophys. res., 103, 11,455-11,465. di girolamo, p., r.v. gagliardi, g. pappalardo, r. velotta, v. berardi and n. spinelli (1995): twowavelength lidar analysis of stratospheric aerosol size, j. aerosol sci., 26 (6), 988-1001. di girolamo, p., g. pappalardo, v. berardi, v. cuomo, n. spinelli and r. velotta (1996): lidar observations of the stratospheric aerosol layer over southern italy in the period 1991-1995, j. geophys. res., 101, 18,76518,773. di girolamo, p., p.f. ambrico, a. amodeo, a. boselli, g. pappalardo and n. spinelli (1999): aerosol observations by lidar in the nocturnal boundary layer, appl. opt., 38 (21), 4585-4595. kasten, f. (1968): falling speed of aerosol particles, j. appl. meteorol., 7, 944-947. mccormick, p.c. and r. veiga (1992): sage ii measurements of early pinatubo aerosols, j. geophys. res., 19, 155-158. mccormick, m.p., d.m. winker, e.v. browell, j.a. coakley, c.s. gardner, r.m. hoff, g.s. kent, s.h. melfi, r.t. menzies, c.m.r. platt, d.a. randall and j.a. reagan (1993): scientific investigations planned for the lidar in space technology experiment (lite), bull. am. meteorol. soc., 74, 205-214. pappalardo, g., v. cuomo, n. spinelli, p. di girolamo, v. berardi and r. bruzzese (1997): southern italy correlative lidar measurements for lite, in advances in atmospheric remote sensing with lidar, edited by a. ansmann, r. neuber, p. rairoux and u. wandinger (springer-verlag, berlin), pp. 173. pappalardo, g., m. pandolfi, p.f. ambrico, a. amodeo, p. di girolamo and a. boselli (2001): lidar measurements of aerosol extinction and backscatter in the pbl, in advances in laser remote sensing, selected papers presented at the 20th international laser radar conference, edited by a. dabas, c. loth and j. pelon, école polytechnique, 207-210. pollack, j.b., o.b. toon, a. summers, w. van camp and b. baldwin (1976): estimates of the climatic impact of aerosols produced by space shuttles, ssts, and other high fl ying aircraft, j. appl. meteorol., 15, 247-258. zhao, j., r.p. turco and o.b. toon (1995): a model simulation of pinatubo volcanic aerosols in the stratosphere, j. geophys. res., 100, 7315-7328. microsoft word 7410-18027-1-ed_foresta martin-peppoloni_ag-rev daniela.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7410 1 geoethics in communication of science: the relationship between media and geoscientists franco foresta martin laboratorio-museo di scienze della terra, isola di ustica, palermo, italy sidereus@rocketmail.com silvia peppoloni istituto nazionale di geofisica e vulcanologia, roma, italy iapg international association for promoting geoethics silvia.peppoloni@ingv.it abstract the themes addressed by geoethics are becoming central to all scientific debate. a growing number of scientists now consider geoethics as an effective tool to increase, in the scientific community and society as a whole, the awareness of local and global environmental problems that humanity faces. geoethics deals with ethical, social and cultural aspects related to geosciences. geoethics addresses fundamental issues such as the prudent and sustainable use of geo-resources, management of natural hazards, defence of geoheritage as a common value to be protected and enhanced. but above all, geoethics aims to raise, in the community of geoscientists, their awareness of responsibilities in conducting scientific and professional activity. in order to extend this awareness to the whole of civil society and also to foster the recognition of the usefulness of geosciences in daily life, geoscience communication should be recognized as a fundamental activity connected with scientific and professional work, since geoscience knowledge is a tool to contribute to the construction of social knowledge for human communities. but today what is the role played by geosciences in the scientific mass culture? are the geosciences part of a collective “cultural heritage”? do the publishing world and media offer adequate space to geosciences? through the analysis of two italian case studies, the authors highlight the critical features of the relationship between geoscientists and the media and try to suggest some actions that are useful to make the relationship between these two separate fields more functional, with the goal of bringing citizens closer to geosciences and increasing the awareness of the individual and collective responsibility towards the earth. 1. introduction: what is geoethics? he international association for promoting geoethics (www.geoethics.org) has defined geoethics as the “research and reflection on the values which underpin appropriate behaviours and practices, wherever human activities interact with the earth system”, those universal shared values on which to base correct behaviors and practices towards the earth system, considering the different social and cultural contexts and the existing political and economic realities. so, geoethics deals with the ethical, social and cultural implications of earth sciences knowledge, education, research, practice and communication (peppoloni & di capua, 2014; bobrowsky et al., 2017). geoethics analyses ethical and social implications in a wide range of geoscience studies and practices, i.e in exploitation and use of georesources, the management of geo-hazards, climate change adaptation policies, impact of geo-engineering on the natural environments. but above all, geoethics aims to raise in the community of geoscientists their awareness of responsibilities in conducting scientific and professional activities. geoscientists have to consider the indisputable ethical, social and cultural implications of geoscience research and practice and their personal possibility to influence decision-making in regards to land, water and air. t annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7410 2 2. the role of geoscientists in bringing geosciences closer to the public in order to extend this awareness to the whole of civil society and also to foster the recognition of the usefulness of geosciences in daily life, it is important to develop proper communication of geological knowledge, that is capable to contribute substantially to the construction of social knowledge for human communities. scientific information presented in the media is one of the main themes discussed within geoethics and a growing number of geoscientists now consider geoethics as an effective tool to increase in society the awareness of local and global environmental problems that humanity faces. geoscientists have peculiar knowledge and skills. as scholars of the planet earth and experts of their territory, they can highlight the importance of geosciences as a group of disciplines that can contribute to the construction of correct social knowledge, by helping to dispel misconceptions about natural phenomena and to strengthen the link between the identity of human populations and land. 3. geosciences and communication do the publishing world and media in general offer an adequate space to geosciences? how are geoscientists treated and what is their influence in the field of media information? in the case of geosciences the relationship with the media is particularly significant when focused on the issue of natural disasters that strike the earth (i.e. earthquakes, floods, volcanic eruptions). surely there is an interest by the media towards these phenomena greater than other scientific arguments, since their impact on the population is often very strong. consequently, the relationship between journalists and scientists, certainly already delicate in “normal” times, becomes critical after the occurrence of a natural disaster. on the occasion of the 2009 earthquake in l'aquila (central italy) the problem of the relationship between media and science was evident (cocco et al., 2015). did the media play a neutral role in that affair? the communication to the public of the research work carried on in the field of geosciences (stewart & nield, 2012), as well as that relating to other scientific disciplines that are the subject of information and explanation through the general media, is confronted today with two established trends: a) the crisis of print journalism; and b) the globalization and wider spread of scientific news through the internet. these two trends, which were outlined at the end of the last century and which were greatly expanded in the early years of the present century, contain positive and negative potential. between science journalists and the broader field of scientific research, there is currently a subject of debate regarding the reasons for concern or optimism, as noted by a report on the practice and ethics of science journalism in the age of global communication (bauer et al, 2013). on the one hand it could seem positive that information and science communication in the new media has found a fertile ground to multiply, as shown by the most recent surveys that indicate the online newspapers, websites, and even the social networks facebook and twitter, but especially individual blogs, act as the sources of scientific news more so than science journalists (brumfield, 2009). 4. critical points however, it is a fact that while scientific news spreads on the web without the press, its quality is often considerably diminished. two-thirds of a sample of nearly a thousand science journalists from around the world, interviewed on this subject (bauer et al, 2013), admit that the pressures on their productivity, extracted from the publishers of the news media, ultimately damage the content of their work. in anglo-saxon countries we are referring to “web churnalism” (from the verb “to churn out”; https://www.theguardian.com/science/thelay-scientist/2011/apr/25/1) to indicate those journalists who merely compile articles by copying pieces of press release or other articles, without exerting any critical control on sources and content, only with the goal to meet the needs of continuous renewal of web pages annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7410 3 comprising topics of assured emotional grip. the churnalists would be: “journalists who are no longer gathering news, but are reduced instead to passive processors of whatever material comes their way, churning out stories, whether real event or pr artifice, important or trivial, true or false” (davies, 2008). with a definition, perhaps less effective but equally explicit, the authors of the aforementioned report on science journalism do not hesitate to speak, in these cases, of “science news cut and paste journalist” (bauer et al., 2013). with all their limitations and approximations, the major print media , now in serious decline (https://www.theguardian.com/commentisfre e/2013/oct/29/decline-print-mediajournalism-web; bauer et al, 2013), have the merit of having grown several generations of journalists specializing in scientific issues, of having dedicated sections of their pages to scientific news and dissemination of science, of having exercised a sort of review, although unconventional, on texts concerning the subjects of science. in case of disputes regarding content, the authors of the texts and the editorial staff are identifiable and accessible interlocutors, for any required clarifications and corrections. on the web those newspapers that meet the principles of good quality of the information and science dissemination, as well as of a sound professional ethics, are dispersed in an undifferentiated milieu of individual websites, commercial initiatives and groups of opinion that offer few guarantees of fairness and in the midst of which the reader can easily get lost, without distinguishing the true from the false or likely. 5. a warning: the extinction of scientific journalists the decline of the printed newspaper in terms of sales, print runs and advertising revenue, among the many consequences, is leading to the progressive extinction of qualified science journalists who worked full-time for a masthead and are now removed and not replaced with similar professionals. "the securely employed specialist correspondent, writing for print and seriously investigating a story, is an endangered species," concludes the report on science journalism, taking up the data collected by the 2009 survey of nature (bauer et al., 2013). the transition from scientific journalism practiced in the print media to online journalism highlights failures which should be rectified soon, especially considering that disciplines such as geosciences and environmental sciences suffer more heavily the repercussions of such failures, because of their strong impact on social safety, quality of life and environment, and on ethics of development. we must consider that scientific news that is distorted or even totally invented, that is misleading or provides completely erroneous explanations, whether relating to the field of astronomy, particle physics or technology (just to name a few), is a detriment to the mass culture because quite often most readers incorporate these observations uncritically. the same mistakes and incorrectness of information can be related to geosciences, if such indiscretions are made during a seismic event, or a hydrogeological or climate crisis, and can go far beyond the cultural implications and actually endanger human lives and material goods of society 6. two italian case studies in 2014 the italian media dealt with a case that addressed the danger related to a wrong or distorted communication in the field of geosciences. the case became the subject of reflection on popular science magazines by seismologists from the istituto nazionale di geofisica e vulcanologia ingv (italian institute of geophysics and volcanology) (amato, 2014). the case referred to a device, marketed and presented as a potential life-saving instrument, which would have been able to provide an appropriate visual and audible warning of the imminent arrival of a seismic shock above 3.03.2 richter magnitude, taking advantage of the delay between the p and s waves (the latter potentially more destructive). so, in the opinion of the person who favourably presented this product, it would have been possible to have several seconds to evacuate a building or to be sheltered from a possible building collapse. but the seismic history of italy teaches us that most annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7410 4 of the time earthquakes struck in areas very close to towns or villages, so the damages and casualties occur in the vicinity of the epicentre where the time difference between the s and p waves is negligible (fig. 1); and that if this minimum forewarning was instinctively used to leave the house, people would have been exposed to dangers far more serious, for instance the collapse of stairs or cornices (amato, 2014). these considerations didn't prevent some media from speaking of the device as a technological innovation and from promoting it as a kind of "do it yourself" solution to seismic safety. more recently, on the occasion of earthquakes that occurred in central italy starting from august 2016, another example of recurring “fake news” can be illustrated. it refers to the alleged manipulation of seismic data by the ingv, the scientific public body responsible for promptly transmitting the estimate of the magnitude of earthquakes to the italian department of civil protection. for some time now, after the occurrence of earthquakes of medium / high magnitudes, that are unfortunately frequent in the seismogenic areas along the italian peninsula, some newspapers, magazines and social networks started to suggest that ingv underestimates the magnitude purposely, in order to reduce the refunds that the government provides to citizens for the damages suffered (since the magnitude would have been lowering below the refund threshold). this is a typical case of “conspiracy theory” that is refuted repeatedly by the publication of facts and details, but one that is resistant to any rational explanation. this particular type of “conspiracy theory” deliberately ignores two facts: the first concerns the scientific aspect, the second the normative aspect of the alleged plot. as regards the scientific aspect of the issue, it is necessary to clarify that upon the occurrence of an earthquake, the ingv seismologists calculate firstly the value of the richter magnitude ml (also called “local magnitude”), that has the advantage of being obtained in a few tens of seconds, starting from the maximum amplitude of the oscillations registered by a woodanderson seismometer. thus, the value obtained is communicated to the italian department of civil protection within about two minutes, whereas the media receive the value via web after at least 12 minutes. however, this maximum amplitude on the seismometer does not account for all the energy released by the earthquake. to get a more accurate estimate it is necessary to calculate the so-called moment magnitude mw, which requires a longer processing time and is especially important for earthquakes with ml greater than 6: thus for these events the moment magnitude mw can indeed be up to 0.5 higher than the local magnitude ml. obviously, even the mw values are disclosed to the media, but they lag behind the local magnitude (ml), which is communicated immediately after the earthquake. on the occasion of the earthquake that occurred on 30 october 2016 (6h.40m utc) in the province of perugia (lat. 42.83, long. 13.11), the ml magnitude was 6.1, whereas the mw 6.5. this was enough for some bloggers to launch a charge of government conspiracy. as for the normative aspect, the italian law n. 100, dated 12nd july 2012, which regulates the remuneration criteria to the inhabitants of areas struck by earthquakes, makes no reference to the local magnitude, nor to the moment magnitude. the law is based on the mercallicancani-sieberg scale, which classifies earthquakes in relation to their effects on people and objects, and thus refers to the seismic intensity, connected to the extent of the damage. therefore, any differences between the values of ml and mw cannot be invoked to demonstrate the existence of a conspiracy, simply because govfigure 1: delay of just 2 seconds between the p and s waves in instrumental registration of the destructive main shock of the april 6th, 2009 l'aquila earthquake, italy (amato, 2014). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7410 5 ernment refunds are based on seismic intensities, and not on magnitude values. since both of these refutations, scientific and normative, have got wide circulation on the same channels in which “conspiracy theories” are spread, the fact that the idea of a plot persists among the population is a case study to be treated with the mass psychology tools in the internet era, rather than with the instruments of effective information and scientific dissemination. this kind of communication misconduct by the media can have very negative repercussions on the social structure: the loss of trust in the institutions by citizens, the discrediting of science that is not perceived in its important function to serve society. 7. conclusions geosciences have always had a difficult relationship with the general media. newspapers have turned their attention to the geosciences, mostly, on the occasion of crises and natural disasters, rather than follow the most promising developments of the research in a balanced and constant way. this attention centered on the emergency phase after a disaster is probably due to the fact that the same researchers in the field of hazards studies are not committed enough to spread the content of their work in order to make them understandable and exciting for the general public. specialists in other scientific disciplines have been able to manage this undertaking more successfully: disciplines no less “abstruse”, counter-intuitive and complex than geosciences enjoy a more constant and productive relationship with the media. the exploit of scientific communication through the internet is now a great opportunity to reverse this trend because it offers an abundance of space and attention that the print media have not been able to provide, due to their own nature and publishing organization. at the same time the internet has permission to open a direct dialogue between researchers and information professionals, through initiatives such as blogs, forums, social networks, and newsletters that are hosted in the corporate websites of scientific research institutes. an effective way to counter junk science and misinformation that is spreading on the web will require making more useful and attractive the spaces dedicated to the proper communication of geosciences for information operators. some scientists and science journalists show a superficial attitude with respect to science dissemination. they should not underestimate the importance of transmitting reliable scientific information to citizens that should be their fundamental social task. how much are both entities (journalists and geoscientists) aware of the responsibility inherent to this activity? and how is it possible to strengthen this ethical responsibility? the values of geoethics (peppoloni & di capua, 2016), such as integrity, honesty, trustworthiness, accountability, accuracy and impartiality, can orient both scientists and science journalists to find the best way to cooperate, and to ensure the best service to the general population. in fact both groups should share the same ethical values and should aim for the same goal: to make scientific knowledge an integral part of social knowledge. references amato a. (2014). terremoti: prevenire o scappare (earthquakes: to prevent or escape?). scienza in rete, 17 dicembre 2014 (in italian). http:// www.scienzainrete.it/contenuto/articolo /alessandro-amato/terremoti-prevenireo-scappare/dicembre-2014 bauer m.w., howard s., romo ramos y.j., massarani l. and amorim l. (2013), global science, journalism report, working conditions and practices, professional ethos & future expectations. trident printers, uk. bobrowsky p., v.s. cronin, g. di capua, s.w. kieffer, s. peppoloni (2017). the emerging field of geoethics. in: scientific integrity and ethics with applications to the geosciences, edited by l.c. gundersen. special publication american geophysical union, john wiley and sons, inc. brumfield g. (2009). supplanting the old media? nature, vol. 458, pp. 274-277. cocco m., cultrera g., amato a., braun t., cerase a., margheriti l., bonaccorso a., de martini p.m., galadini f., meletti c., annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7410 6 nostro c., pacor f., pantosti d., pondrelli s., quareni f. and todesco m., 2015. the l’aquila trial. in: geoethics: the role and responsibility of geoscientists”. geological society, london, special publications, 419. isbn 9781862397262. davies n. (2008). flat earth news: an award-winning reporter exposes falsehood, distortion and propaganda in the global media. chatto and windus. di capua g. & peppoloni s. (2009), “scientific information: problems and responsibilities”. proceedings of the mining pribram symposium international section on “geoethics” pribram (czech republic), 12-16 october 2009. ingv – istituto nazionale di geofisica e vulcanologia. https://ingvterremoti.wordpress.com/20 16/10/31/la-magnitudo-del-terremotodel-30-ottobre-2016/ matteucci r., gosso g., peppoloni s., piacente s., wasowski j. (2014). the geoethical promise: a proposal. episodes, vol. 37, n. 3, pp. 190-191. peppoloni s. & di capua g. (2016). geoethics: ethical, social, and cultural values in geosciences research, practice, and education. in: wessel g. & greenberg, j. (eds.). geoscience for the public good and global development: toward a sustainable future. geological society of america, special paper 520, pp. 17-21, doi: 10.1130/2016.2520(03). peppoloni s. & di capua g. (2014). the meaning of geoethics. in: peppoloni s. and max wyss (editors): geoethics: ethical challenges and case studies in earth science. elsevier, waltham, massachusetts, 2014. isbn: 9780127999357. stewart i.s. & nield t. (2012). earth stories: context and narrative in the communication of popular geoscience. proceedings of the geologists' association, 124, 4, 699– 712. feddersen.pdf annals of geophysics, vol. 46, n. 1, february 2003 109 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region henrik feddersen climate research division, danish meteorological institute, copenhagen, denmark abstract seasonal mean values of tropical sea surface temperature (sst) and atlantic/european mean sea level pressure (mslp) from a 301-year coupled ocean/atmosphere model run are analysed statistically. relations between the two fields are identified on both interannual and interdecadal timescales. it is shown that tropical sst variability affects atlantic/european mslp in winter. in particular, there appears to be a statistically significant relation between the leading modes of variability, the el niño/southern oscillation (enso) and the north atlantic oscillation (nao). during cold enso (la niña) years the nao tends to be in its positive phase, while the opposite is the case during warm enso (el niño) years, although to a lesser extent. similar analyses that are presented for gridded observational data, confirm this result, although here tropical atlantic sst appears to be stronger related to the nao than tropical pacific sst. the linear predictability of a model simulated nao index is estimated by making statistical predictions that are based on model simulated tropical sst. it is shown that the predictive skill is rather insensitive to the length of the training period. on the other hand, the skill score estimate can vary significantly as a result of interdecadal variability in the climate system. these results are important to bear in mind when making statistical seasonal forecasts that are based on observed sst. 1. introduction interannual variability of the large-scale atmospheric circulation in the atlantic/european region is dominated by variability that is internal to the atmosphere. the internal variability is largely a result of the chaotic dynamics of the atmosphere in the atlantic/european region, and thus the potential would seem limited for seasonal to interannual prediction of european climate (rowell, 1998). however, both observational and model studies suggest that a small fraction of the atlantic/european interannual climate variability is affected by the tropical oscillations through atmospheric teleconnections. several analyses of interannual variability in general circulation models (brankovic and palmer, 1997; davies et al., 1997; may and bengtsson, 1998; feddersen, 2000) as well as analyses of observational data (fraedrich and müller, 1992; pozo-vazquez et al., 2001) have demonstrated a link between the el niño/ southern oscillation (enso) and the winter climate in europe, including the seasonal mean mailing address: dr. henrik feddersen, climate research division, danish meteorological institute, lyngbyvej 100, dk-2100 copenhagen ø, denmark e-mail: hf@dmi.dk key words seasonal predictability − statistical analysis − enso − nao 110 henrik feddersen phase of the north atlantic oscillation (nao). in summary, these papers find that cold enso events are associated with the positive phase of the nao, while warm enso events (to a lesser extent) are associated with the negative phase of the nao. the identified statistical links between enso events and the phase of the nao hold some potential for atlantic/european seasonal to interannual predictability, particularly in years with large tropical sea surface temperature (sst) anomalies, although a study by johansson et al. (1998) found no significant predictability of surface air temperature in northern europe associated with enso. it is possible that in this study no predictability was detected because the region that was considered, is rather small, and the study was based on a relatively short period (39 years) which may not be long enough to identify a teleconnection signal that is hidden in the chaotic internal variability. thus, there is a need for longer time series of data. given the lack of observational data back in time, we focus our analysis in the present study on data time series from a 301-year integration of the echo-g coupled ocean/atmosphere general circulation model. such long time series allow us not only to better identify tropical/ extratropical teleconnection signals, but also to study the robustness over many decades of the teleconnection relations. comparisons with analyses of observational data are made when the observational data series are sufficiently long to do so. teleconnections on interdecadal timescales are also identified between tropical sst and atmospheric variability in the atlantic/european region in the model time series. teleconnections on these longer timescales will show up in interannual climatic time series as apparent trends, and they also contribute (positively or negatively) to the interannual predictability. the paper is organised as follows. the analysed observational data and the 301-year model run are briefly described in section 2. section 3 describes teleconnections between tropical sst and atlantic/european mean sea level pressure (mslp), both for observations (for the period 1903-94) and for the 301-year model run. here and in the following we consider only the january-march (jfm) season. the teleconnection patterns are identified by applying a singular value decomposition (svd) analysis to cross-covariances of tropical sst and atlantic/ european mslp in jfm. in section 4 a similar svd analysis is applied to jfm time series that are averaged over several years in order to study teleconnections that relate to interdecadal variability. section 5 discusses implications for the interannual predictability of atlantic/ european mslp. in particular, data from the model run is used as synthetic data for statistical predictions of a nao index and european surface temperatures in winter. the relatively long model run allows us to discuss aspects of the robustness of statistical predictions which is not possible for the much shorter observational data series. finally, conclusions are presented in section 6. 2. data 2.1. observational data globally gridded observations of tropical sst have been taken from the gisst2.2 dataset which includes monthly mean values for the 1903-1994 period on a 1° × 1° grid (rayner et al., 1996). however, here we include only every third grid box in both directions in order to obtain a resolution comparable to the model’s t30 resolution. the mslp data was taken from a 5° × 10° gridded dataset from the climatic research unit. this dataset covers the northern hemisphere from 15°n-85°n and includes monthly mean values for the 1873-1995 period (jones, 1987; basnett and parker, 1997). 2.2. model data we use seasonally averaged data from a 301year simulation of the present-day climate from the echo-g coupled ocean/atmosphere general circulation model. the model consists of the echam4 atmospheric model (e.g., roeckner et al., 1996; stendel and roeckner, 1998) and the hope-g ocean model, including a dynamic/ 111 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region thermodynamic sea ice model (e.g., legutke and maierreimer, 1999). echam4 has a horizontal resolution of t30 (corresponding to 48 × 96 grid points on the corresponding gaussian grid) and 19 vertical levels (e.g., stendel and roeckner, 1998). in the extratropics hope-g has a horizontal resolution of 2.8125°, corresponding to a triangular truncation of t42. in a band from 30° to 10° off the equator the distance between latitudes decreases to 0.5° in order to better resolve the equatorial dynamics. the vertical resolution of hope-g is 20 layers. 3. interannual covariability 3.1. observational data an svd analysis (bretherton et al., 1992; wallace et al., 1992) was applied to jfm seasonal means of 92 years (1903-94) of gridded observations of tropical sst and atlantic/european mslp. both the sst and mslp fields were standardised prior to the svd analysis so that all grid boxes contribute the same amount to the analysis. the svd analysis can in many respects be regarded as an extension of the well-known principal component (or eof) analysis. in the latter, one or more patterns are derived that maximise the fraction of «explained» variance for a particular field, whereas the svd analysis provides one or more pairs of patterns that maximise the fraction of «explained» covariance between two fields. it is common practice to give the squared covariance fraction (scf) that is associated with a pair of svd patterns, just like it is common practice to always give the fraction of explained variance that is associated with an eof pattern. both the scf and the correlation between the associated svd time series are often found to be very high for the leading svd mode. however, this does not necessarily imply a strong statistical connection between the two fields (wallace et al., 1992; cherry, 1996). therefore, additional tests of statistical significance are required. here we use a monte carlo resampling to test statistical significance (ward and navarra, 1997). the atlantic/european mslp time series are randomly resampled 200 times, and each time an svd analysis is applied to tropical sst and the resampled mslp. an estimate of the significance level (i.e. the probability that an equally strong coupled mode would appear by chance for independent sst and mslp fields) is then given by the number of times that the randomly generated scf exceeds the actual scf divided by the total number of times the svd analysis was applied to the resampled mslp fields and tropical sst. in order for the resampled time series to appear random, the original mslp time series must not be serially correlated. it was checked that serial correlation is not a problem. figure 1a,b shows the leading pair of heterogeneous correlation patterns, i.e. the temporal correlation in each grid box between the standardised time series of one field (mslp in fig. 1a; sst in fig. 1b) and the first svd time series of the opposite field (bretherton et al., 1992). thus, the heterogeneous correlation pattern for mslp shows how well mslp anomalies can be specified from the first svd time series of tropical sst, i.e. the heterogeneous correlation pattern yields an estimate of the skill of svd predictions of atlantic/european mslp if the predictions were based on the statistical link between tropical sst and atlantic/european mslp. the total interannual mslp variance that is «explained» by tropical sst is simply given by the area average of the squared correlations in fig. 1a. the first pair of svd time series for atlantic/european mslp and tropical sst, respectively, are shown in fig. 2a,b. the mslp pattern shows a north/south dipole which is also found in the first eof of jfm mslp (fig. 3, the nao pattern in its negative phase), but in the eof pattern the dipole is much more pronounced over the european continent than what is seen in fig. 1a. likewise, the tropical sst pattern bears some resemblance to the first eof of tropical jfm sst (not shown). but the sst pattern in fig. 1b has relatively more weight in the tropical atlantic, while the first eof looks more like the generic enso pattern of harrison and larkin (1998). the svd patterns are found to be statistically significant at a 3.5% level, using the monte carlo resampling test, so the svd analysis supports the notion that warm (cold) tropical sst is associated with an atmospheric 112 henrik feddersen fig. 1a,b. leading pair of heterogeneous correlation patterns from an svd analysis of 92 years (1903-1994) of gridded observations of jfm seasonal mean tropical sst and jfm seasonal mean atlantic/european mslp. a) mslp correlations (in %); b) sst correlations (in %). squared covariance fraction = 73%. total explained mslp variance fraction in atlantic/european region = 7%. statistical significance level = 3.5%. a b fig. 2a,b. standardised time series for the patterns in fig. 1a,b. a) for atlantic/european mslp; b) for tropical sst. correlation coefficient between time series is 0.44. a b 113 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region large-scale circulation in winter over the atlantic/ european region that more or less corresponds to the negative (positive) phase of the nao. the mslp pattern, however, only explains about 7% of the total variance, so the impact of tropical sst is likely to be felt in the atlantic/european region only in years of extreme tropical sst anomalies. if the svd time series for tropical sst is correlated with the mslp time series in each grid box in the northern hemisphere then the pattern in fig. 4 is obtained. this pattern shows that the atlantic/european mslp pattern is just a weak part of an hemispheric pattern which has a strong centre of action over the tropical atlantic and a secondary centre over the western part of the tropical pacific. 3.2. model data a similar svd analysis was applied to the 301-year of model simulated jfm seasonal mean tropical sst and jfm seasonal mean atlantic/ european mslp. the first pair of heterogeneous correlation patterns are shown in fig. 5a,b. the pacific part of the sst pattern is dominating the pattern and is similar to the model’s enso pattern (calculated as the first eof of tropical pacific sst; not shown). likewise, the mslp pattern in the atlantic/european region bears some resemblance to the first model eof pattern (fig. 6, nao pattern in its negative phase). however, the model’s first eof pattern is located fig. 3. first eof pattern (explaining 43% of the total variance) of gridded observations of jfm seasonal mean atlantic/european mslp. fig. 4. correlations (in %) between the tropical sst time series (fig. 2b) and gridded observations of mslp on the northern hemisphere north of 15°n. 114 henrik feddersen fig. 5a,b. leading pair of heterogeneous correlation patterns from an svd analysis of 301 years of model simulated jfm seasonal mean tropical sst and jfm seasonal mean atlantic/european mslp. a) mslp correlations (in %); b) sst correlations (in %). squared covariance fraction = 84%. total explained mslp variance fraction in atlantic/european region = 7%. statistical significance level < 0.5%. fig. 6. first eof pattern (explaining 38% of the total variance) of model simulated jfm seasonal mean atlantic/ european mslp. a b 115 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region slightly north of the svd pattern, and over the iberian peninsula the eof pattern has a centre which is absent in the svd pattern, while the eof pattern does not show the centre north of scotland. nevertheless, the svd analysis of the model run supports the observational finding that tropical sst modulates the nao so that a negatively phased nao is more likely in winters with warm tropical sst anomalies, whereas a positively phased nao is more likely in winters with cold tropical sst anomalies. the first svd pattern is found to be highly statistically significant (with an estimated significance level < 0.5%), although the atlantic/european mslp pattern explains only 7% of the total interannual variance for the jfm season. if we consider only the years when the tropical sst is most extreme, then the fraction of mslp variance explained by tropical sst is considerably higher. for example, for the years of the 25 most positive values and fig. 7a,b. standardised time series for the patterns in fig. 5a,b. a) for atlantic/european mslp; b) for tropical sst. correlation coefficient between time series is 0.45. a b 116 henrik feddersen the 25 most negative values of the svd time series for sst we find that tropical sst explains about 16% of the corresponding mslp variance. the associated standardised time series are shown in fig. 7a,b. the sst time series has a very strong biennial component which is not seen in the observations (cf. fig. 2a,b). this biennial component reflects the model’s tendency to switch back and forth between el niño and la niña states almost every year. this tendency is not seen in the mslp time series. in fact, the autocorrelation function for the mslp time series is similar to that of white noise (not shown), but although the mslp variations appear as those of a random process, the correlation coefficient between the mslp and the sst time series is 0.45, so the mslp is not statistically independent of the tropical sst variations. the pattern of correlations between the svd time series for tropical sst and the time series for mslp in each grid box in the northern hemisphere is shown in fig. 8. the strongest correlations are found over the tropical pacific, indicating that enso is the dominant source of the variability in the tropical pacific that influences atlantic/european mslp variability. notice that north of the direct influence of the southern oscillation, the pattern has some resemblance with the surface part of the arctic oscillation pattern (thompson and wallace, 1998). it appears that in the model’s climate tropical pacific sst is the most important part of the tropical sst in relation to the atlantic/european mslp variability, and, indeed, if the svd analysis is repeated with tropical pacific sst only, then an almost identical mslp pattern and almost identical svd time series are recovered (not shown). but, interestingly, this is also the case if the svd analysis is repeated with tropical atlantic sst only. so the notion of one dominant tropical oscillation (which itself is dominated by enso) seems reasonable in this coupled ocean/ atmosphere model. the svd analysis gives no indication of cause and effect, but it seems most likely that the tropical sst has a (small) influence on atlantic/european interannual mslp variability and not vice versa. this assumption is confirmed by applying an svd analysis to tropical sst in the ond season and atlantic/european mslp in the following jfm season. the resulting leading pair of heterogeneous correlation patterns (not shown) are very similar to the pair of no-lag patterns in fig. 5a,b, except that the correlations generally are lower in the lagged patterns. 4. interdecadal covariability in order to study teleconnections that relate to interdecadal variability − loosely defined as variations with periods between 10 and 100 years − new time series are constructed by dividing the original long model time series into 5-year chunks, each one of which is represented by an average value. note that we are not using running means; there is no overlap between two 5-year chunks. thus, for the 301-year model simulation the new «decadal» time series can have a length of 60 at most. the period of five years is chosen because it represents the minimal sampling frequency that is needed in order to resolve 10year periods, and also because 5-year averages are long enough to eliminate effects of the strong biennial enso variation. fig. 8. correlations (in %) between the tropical sst time series (fig. 7b) and model simulated mslp on the northern hemisphere. 117 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region an svd analysis similar to the ones described in the previous section was applied to the decadal time series of model simulated tropical sst and model simulated atlantic/ european mslp, but the monte carlo resampling test for statistical significance gave a significance level > 20%, i.e. no statistical significance. however, if sst leads mslp by one year (i.e. fig. 9a,b. leading pair of heterogeneous correlation patterns from an svd analysis of 60 5-year averages of model simulated jfm seasonal mean atlantic sst and jfm seasonal mean atlantic/european mslp. sst leads mslp by one year as described in text. a) mslp correlations (in %); b) sst correlation (in %). squared covariance fraction = 68%. statistical significance level = 1.5%. a b 118 henrik feddersen the decadal sst time series are based on averages of years 1-5, 6-10, 11-15, etc. and the decadal mslp time series are based on averages of years 2-6, 7-11, 12-16, etc.), then the statistical significance level drops to 7.5%. and if the sst domain is extended to also include the extratropical north atlantic, then the statistical significance level drops further to 1.5%. the svd heterogeneous correlation patterns and associated time series are shown in fig. 9a,b and fig. 10a,b, respectively. the sst pattern shows a tripole in the tropical atlantic. this tripole is also seen in the second mode (not shown) of the svd analysis of interannually varying sst and mslp. as the time series and the mslp correlation pattern for the first interdecadal svd mode also appear to correspond to those of the second interannual svd mode (not shown), it seems reasonable to assume that they are in fact the same mode of variability. the interdecadal sst time series appears to have an oscillation period of around 45 years. further analysis, including analysis of additional oceanic datasets, is required in order to attempt an explanation of the driving mechanism for this oscillatory behaviour. however, variability in the thermohaline circulation is a possible candidate (tourre et al., 1999). a similar analysis of interdecadal covariability for the observational data makes little sense as the available 92 years of data is simply not enough to study variability with expected periods of nearly 50 years. 5. interannual predictability seasonal forecasts for surface air temperature and precipitation that are based on statistical relations with tropical sst (e.g., barnston, 1994) fig. 10a,b. standardised time series for the patterns in fig. 9a,b. a) for atlantic/european mslp; b) for atlantic sst. correlation coefficient between time series is 0.73. a b 119 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region are issued routinely around the world (see e.g., the experimental long-lead forecast bulletin at http://grads.iges.org/ellfb/). such statistical forecasts are typically based on records of about 50 years of observational data. in order to estimate forecast skill the statistical predictions are cross-validated, i.e. for each year in the observational record a hindcast is made that is based on a training period that does not include the particular hindcast year (michaelsen, 1987). here the 301-year long model run is used as synthetic data for statistical predictions of an nao index that describes the large-scale circulation over the atlantic/european region. the purpose of this exercise is twofold: 1) to quantify predictability of the nao that can be linearly related to tropical sst; 2) to investigate to what extent the predictive skill depends on fig. 11a-f. leading mslp heterogeneous correlation patterns from svd analyses of six different 50-year periods of model simulated jfm seasonal mean tropical sst and jfm seasonal mean atlantic/european mslp. a) years 1-50; b) years 51-100; c) years 101-150; d) years 151-200; e) years 201-250; f) years 251-300. total explained mslp variance fractions in the atlantic/european region vary between 6.6% and 13.1%. cf. fig. 5a which is based on an svd analysis of all 301 model years. a b c d e f 120 henrik feddersen the length of the statistical training period, and how well the predictive skill is estimated by the cross-validation technique. the answers to the latter questions require long data series, such as those of the present 301year model run, that are not available for the relatively short record of observational data. the leading heterogeneous mslp correlation patterns from svd analyses of model simulated tropical sst and atlantic/european mslp for six non-overlapping 50-year periods show notable differences, although a north/south dipole structure is evident in all six patterns (fig. 11a-f). the fraction of explained interannual mslp variance in jfm varies between 6.6% and 13.1%, i.e. by more than a factor 2. svd analyses of observational data also show differences between the two patterns that are based on the first and the last 46-year period, respectively, of the 19031994 period (fig. 12a,b). these results show that the statistical relationship between tropical sst and atlantic/european mslp varies on interfig. 12a,b. leading mslp heterogeneous correlation patterns from svd analyses of two different 46-year periods of gridded observations of jfm seasonal mean tropical sst and jfm seasonal mean atlantic/european mslp. a) for 1903-1948; b) for 1949-1994. cf. fig. 1a which is based on an svd analysis of all 92 years. a b 121 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region decadal timescales. therefore it is likely that interannual predictability of atlantic/european mslp will also vary on interdecadal timescales. predictions of an nao index are made by statistical specification of the index (calculated as the difference between mslp anomalies in the model grid boxes centred approximately at (39°n, 30°w) and (65°n, 19°w)) from the first three sst time series from an svd analysis between tropical sst in jfm (or ond) and atlantic/european mslp in jfm (that is, for a lead time of zero or one season). the statistical specification is simply made by multiple linear regression of the nao index on the three sst time series. the choice of including three time series is based on the observation that the squared covariances accounted for by subsequent svd modes are small, and generally the scf is tending to zero very slowly for these modes, which would indicate that these modes are close to the «noise level» of the variability (o’lenic and livezey, 1988). moreover, repetition of a few of the predictability calculations described below indicates that including more sst time series in the regression does not lead to an increase of the predictive skill. first, the predictive skill is calculated as a function of training period length. the validation period is the 150-year period from year 151 until year 300. a prediction for year n is always based on a training period that ends in year n-1, i.e. the training period is sliding through the validation period, just as one would like to always use the latest data for a real statistical forecast. as a measure of predictive skill we use the anomaly correlation. figure 13 summarises the dependence on training period length for lead times of zero and one season. for short training periods the predictive skill increases with increasing training period length, but seems to saturate already at training period lengths of about 30 years. this is an encouraging result for statistical seasonal forecasting because it suggests that there is no need for very long training data sets in order to make forecasts for large-scale features such as the nao index. the predicted phase of the nao depends on the phase of enso. figure 14 shows a scatterplot of predicted (at zero lead time and for a 50-year training period) versus actual nao index in the model run for 25 warm and 25 cold enso years. generally, cold enso years lead to a positive nao index, and warm enso years lead to a negative nao index, but the variability of the nao index is much larger in warm enso years than it is in cold enso years. also, a positive nao index is more likely in a cold enso year than a negative nao index is in a warm enso fig. 13. predictive skill as function of length of training period for statistical predictions of model simulated nao index (see text) for jfm, years 151300. top curve: predictions based on tropical sst from the same season (jfm). bottom curve: predictions based on tropical sst from the previous season (ond). fig. 14. actual model nao index versus statistically predicted nao index for 25 warm enso years (circles) and 25 cold enso years (crosses). lead time = 0, length of training period = 50 years. see text for further explanation. 122 henrik feddersen year. for example, the nao index is more than + 1 (indicated in fig. 14) in 84% of the cold enso years, whereas the nao index is less than – 1 (indicated in fig. 14) in only 60% of the warm enso years. thus, it seems that cold enso events affect the nao more than warm enso events, as also found in previous studies (dong et al., 2000; pozo-vazquez et al., 2001). the data from the long model run allows us to compare cross-validated skill scores and skill scores that are estimated from truly independent data. the data is divided into six non-overlapping 50-year periods. using each 50-year period as training period the skill score for prediction of the nao index is estimated as anomaly correlations for: 1) predictions of the nao index in the training period itself (i.e. the predicted nao index is also part of the training data set); 2) crossvalidated predictions in the training period (i.e. for each prediction the predicted nao index is excluded from the training data set), and 3) predictions of the nao index in the remaining five 50-year periods. the estimated skill scores are shown in fig. 15a,b. the skill scores that are estimated from dependent data are higher than the cross-validated skill scores in every case, and they are almost always higher than the «true» skill scores for independent data. overall (i.e. for the average over all cases), the cross-validated skill score matches the «true» skill score very well, but there are individual cases when the cross-validated skill score appears to overestimate the «true» skill score, as well as cases when it appears to under-estimate it. for both lead times, we find in one of the six cases that one skill score for the independent data falls outside a 95% confidence interval for the cross-validated skill score. although the confidence interval is estimated under the assumption that the nao index follows a normal distribution, the result indicates that statistically significant differences between cross-validated and true skill scores can occur. it is likely that these differences are caused by the interdecadal variability that was illustrated in fig. 11a-f. cross-validated and «true» skill scores were also calculated for the observational data. for two non overlapping 46-year periods (and zero lead time) we find skill scores that are comparable to those of the model run as shown fig. 15a. 6. conclusions it has been demonstrated that the large-scale seasonal mean atmospheric circulation in winter (jfm) in the atlantic/european region is statistically related to tropical sst. this has been shown for both 92 years of gridded observations of atlantic/european mslp and for 301 years fig. 15a,b. predictive skill for statistical predictions of model simulated nao index in jfm as estimated from six non-overlapping 50-year training periods for the model run (case 1-6). top curve: validation period identical to training period. bottom curve: as top curve, but using cross-validation (see text). error bars: estimated 95% confidence interval for cross validated skill scores. circles: validation periods not overlapping with training period. a) predictions based on tropical sst from jfm, i.e. lead time = 0. predictive skill scores are also included for two non-overlapping 46year training periods for gridded observations. b) predictions based on tropical sst from ond, i.e. lead time = 1 season. a b 123 impact of tropical sst variations on the linear predictability of the atmospheric circulation in the atlantic/european region of coupled ocean/atmosphere model simulations. although the average effect is small (only about 7% of the total interannual variance of atlantic/ european mslp is explained by the leading mode of interannual variability in tropical sst), it makes sense to take advantage of the existence of this tropical/extratropical teleconnection in seasonal predictions of atlantic/european climate, particularly in years when there is a strong forcing from tropical sst. a statistically significant relation between interdecadal variations of atlantic sst and atlantic/european mslp has been found for the 301 years of model simulations. a corresponding pair of patterns have not been identified for the observational data, as only 92 years of observational data were available, which is insufficient to identify an interdecadal mode with a dominant oscillation period of around 45 years. the statistical relation between tropical sst and atlantic/european mslp in winter is sufficiently strong to allow us to make skilful statistical seasonal forecasts of an nao index, using linear statistical techniques, such as multiple linear regression on the leading svd modes. the training period for the statistical forecasts of the nao index need not be very long in order to obtain good skill scores. increasing the length of the training period beyond 30 years does not seem to add more skill to the predictions. estimation of the expected predictive skill is an important part of seasonal forecasting. skill estimation is typically done by hindcasting, preferably cross-validated, but it has been demonstrated here that interannual predictability is affected by interdecadal variability. thus, skill score estimates obtained by cross-validation of statistical hindcasts can be misleading in the presence of interdecadal variability. acknowledgements this work was supported by the commission of the european union through the «environment and climate programme» under contract env4ct98-0714, «interannual to decadal climate variability: scale interaction experiments». references barnston, a.g. 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(2002), gives a detailed description of the structure and operation of the various constitutive boards. since a detailed analysis is beyond the scope of this paper, only a synthetic description of the ionosonde referring to fi g. 1 (which is a simplifi ed functional diagram of the whole system) will be presented here. 2.2. general description in fi g. 1, the ionosonde has been divided into functional blocks representing the main functions; one block does not necessarily correspond to physical blocks or circuits. the thicker lines refer to digital buses. the blocks are grouped into two sections corresponding approximately to the transmitting and the receiving sections: on the left, there is a power amplifi er (pwa), a frequency synthesiser (syn) and the code generator (code gen); on the right there is the radio receiver (rx), the analog to digital converter (adc) and the digital signal processor (dsp). every block is controlled by a personal computer (pc control and storage), which can store and display data. the antennas (tx ant. and rx ant.) complete the system. in order to increase the signal-to-noise ratio without increasing the transmitted peak power, the pulse compression technique (encoded pulse) is applied. after the echo is received, it is compared to the transmitted code by means of a correlation process. to this purpose a digital biphase complementary code is used. it lets the rf carrier assume only two values for its phase: 0° table i. ais-ingv ionosonde specifi cations. the symbol (*) indicates the maximum range of input received powers, in all possible conditions; it is not required that the system maintains the linearity over it, provided that it allows the recognition of the echo position. parameter requirement height range 90 ∏ 750 km distnace resolution 5 km max. peak tx power (average power) 250 w (5 ~ 10 w) receiver sensitivity ~ 85 dbm for 0 db s/n input dynamic range ~ 80 db (*) frequency range 1 ∏ 20 mhz frequency resolution (step) 25, 50, 100 khz frequency scan duration (max.) 3 min (with 50 khz step) acquisition sampling rate ~ 100 khz acquisition quantization 8 bit storage data rate (max.) 60 kbytes (with 50 khz step) 649 the new ais-ingv digital ionosode and 180° with respect to a reference, following a pattern designed to achieve the cancellation of the correlation «sidelobes». the duration of a single phase of the code (called «sub-pulse») was set to 30 ms after the design considerations based on the specifi cations. 2.3. frequency synthesis and code generation the ais-ingv adopts a receiving heterodyne system based on multiple conversions. this kind of system allows the reduction of internally generated noise due to the «image» interference. in order to make the system effective, it is important to raise the intermediate frequency (if) to move the images far enough from the input passband. the ifs have been chosen at 35.9, 4.1 and 0.1 mhz. in this way, the image related to the fi rst conversion (from 1-20 mhz to 35.9 mhz) lies between 72.8 and 91.8 mhz, well beyond the rf input band. similarly the second conversion (from 35.9 to 4.1 mhz) has its image at 44.1 mhz, the third (from 4100 to 100 khz) at 3.9 mhz; all the images are easily removable by means of a fi lter before the conversion. to accomplish such conversions, three local oscillators (lo) at 36.9-55.9, 40 and 4 mhz were devised (note that only the fi rst has variable frequency). since digital techniques that implement a direct synthesis of a sinusoid with a determined frequency and phase were available, it was decided to use such «dds» devices (direct digital synthesis). information from the received signal is extracted by means of the «quadrature» demodulation. it demands maximum phase coherence between all signals generated in the system. so, though different dds devices are used, their reference is unique: an ox 125 mhz quartz oscillator. all the frequencies and timing signals are derived from this unique reference oscillator. so the whole system is phase locked and a drift (few hertz over some mhz) of the reference oscillator does not affect the coherence of the system. all the dds devices were mounted on a single board (syn): two of them generate the fi rst two local oscillator outputs; the third fig. 1. simplifi ed functional diagram. 650 enrico zuccheretti, giuseppe tutone, umberto sciacca, cesidio bianchi and baskaradas james arokiasamy generates the tx carrier. the third local oscillator output at 4 mhz is derived from the second at 40 mhz by means of frequency division by ten. all the lo outputs are fi ltered to clean the waveforms, to make them as pure as possible. all dds devices can output a specifi ed frequency value by means of proper programming, coming from the pc bus. a supplementary 400 khz square wave is generated from the 4 mhz; it acts as a reference for other sub-systems (e.g., the adc), that must all be phase-locked with respect to the 125 mhz reference. the reference clock (400 khz in fi g. 1) is used to create the codes (30 ms period corresponding to the sub-pulse length). this function was implemented in a different board, the code generator (code gen). the 400 khz clock (2.5 ms period) is also used to generate the quadrature sampling clock. the codes are implemented as digital waveforms, with the 1 s and the 0 s corresponding to the carrier phase rotations. while the proper timing is derived inside the board, the pc has only to send the exact set of 1 s and 0 s. in this way, the system is very fl exible; it is possible to easily change the type and length of the code. ) ) ( p , ) fig. 2. the ionosonde subsystems. interconnections (antennas not included). 651 the new ais-ingv digital ionosode t x o sc il la to r ( ) gg ( ) ( ) ( ) ( ) g f ig . 3. f un ct io na l di ag ra m o f th e m ai n un it o f a is -i n g v i on os on de . 652 enrico zuccheretti, giuseppe tutone, umberto sciacca, cesidio bianchi and baskaradas james arokiasamy a a a a a a a a a a once generated, the codes (code in fi g. 1) are sent to the synthesiser board to modulate the rf carrier. the modulated or «coded» rf carrier (crf in fi g. 1) is then sent to the amplifi er. the modulation of the rf carrier by the code could be suffi cient to avoid the amplifi er energy output out of the transmission phase. anyway, in order to interdict the amplifi er completely, a trigger pulse is generated inside the board (amp trig in fi g. 1). it is obtained starting from the same reference used for the code, considering that a complete tx pulse is a multiple of 16 sub-pulses. table ii. symbols used in the block diagram. 653 the new ais-ingv digital ionosode 2.4. power amplifi cation and antenna system in fi g. 2 the transmitting power amplifi er (pwa) is considered as a separate subsystem. to increase the effi ciency and reliability of the system, an industrial product was bought outside ingv. the peak power in linear conditions was set at 250 w. the linearity specifi cations are not critical, because of their low impact on the useful rf band. on the other hand, the harmonics and spurious signals are to be maintained low in order to increase the power effi ciency and to limit emi. the antenna system employed is a couple of crossed «delta» antennas, to minimise the direct coupling between the tx and rx (fi g. 2). each antenna has a 42 m base and is mounted on a single 24 m mast. such an antenna type is similar to the rhombic antenna (to which it is equivalent considering the image effect due to the ground) and its impedance remains almost constant over a wide range allowing good matching with a gain (1.5-3 db). a couple of baluns are used to match the balanced antennas with the unbalanced coaxial transmission lines. 2.5. receiver and a/d conversion the demodulation is performed by means of a multiple conversion heterodyne system. the three local oscillator outputs are to be used inside the receiver as lo inputs of mixers which actually perform the conversions. before and after the conversion fi lters have to be added to clean the signal from noise. specifi cally, a separate board was thought to be put at the rx antenna input to select a relatively narrow rf band. in detail, a preliminary fi ltering, wide band, is put as the fi rst stage. it has a pass band in the ionosonde operative range (1-20 mhz) and some protection against high voltage bursts. then six band pass fi lters follow; they are switchable, i.e. only one of them is actually working at a time. this combination reduces the number of rf fi lters maintaining the response relatively fl at. the fi lters have different bandwidths, the lower frequency fi lters have a narrower bandwidth with respect to the higher frequency ones (the bandwidth is around the half of the central frequency). switches are used in the receiving chain, in order to interdict the reception during transmission. the output of the rf band selection fi lter is fed to the fi rst mixer, inside the real receiver (the switchable fi lters and the receiver are two boards reported in fi g. 1 as the rx block). in the receiver chain, the three mixers are followed by fi lters and amplifi ers. as previously mentioned, these fi lters limit the noise bandwidth and avoid the image interference. the last fi lter is the narrowest, with its pass band limited by the bandwidth of the code: 66 khz. all hardware fi lters (i.e. excluding fi ltering accomplished by means of digital signal processing) are passive, analog, with lumped elements (capacitors and inductors). inside the receiver, a variable attenuator is added to increase the dynamic range. in the fi rst realisation of the ionosonde, this feature is used only as a way to manually calibrate the system and let it work well, without dynamically changing the attenuation (and the overall gain of the receiver) to match the system to the actual ionospheric attenuation and the actual noises. the attenuator is programmable by means of the pc bus, so a future expansion of the system capability is possible. obviously, many amplifi ers are introduced in the receiving chain, to bring the signal to a good level, suitable to be processed by the following stage. the analog to digital conversion is performed in a different board (adc). the clock is furnished by the previously seen frequency synthesiser. some circuitry is necessary to accomplish the «quadrature», or i-q demodulation. two converters sample the analog receiver output at a 100 khz sampling frequency (10 ms period), but one is delayed by one fourth of a period with respect to the other (2.5 ms). the sampled voltages are stored in two temporary memories, whose address is generated by a digital circuit, driven by the same 100 khz clock used by the adcs. once the acquisition process is completed (about 5.2 ms) data are sent to the dsp using the same address generator. 2.6. pc control and dsp board the personal computer (pc control and storage) supervises the operation of the whole 654 enrico zuccheretti, giuseppe tutone, umberto sciacca, cesidio bianchi and baskaradas james arokiasamy system. a control program, written in a high level language, provides the interface with the operators, letting them decide the operating parameters of the soundings (starting and ending frequency, repetition rate, etc.) and displaying the ionogram while sounding. it also provides the storage of the acquired data on the local hard disk, using the internal timer to mark the time of the sounding. an interface between the pc bus and the external ionosonde bus was introduced. a dedicated bus was designed to treat properly all control and data fl owing from the pc. the control and data lines in fi g. 2 are a «buffered» bus that actually brings the control signals and some data packets (e.g., the code or the words to program the tx frequency) to the main unit. a commercial digital signal processing board (dsp) is employed for on-line analysis. it is programmed at a low level assembler lan guage. the functions to be performed are better explained later. dsp output is a stream of values (an array for each frequency of sounding) representing the time domain signal amplitude. the location of the amplitude peaks is related to the heights of the refl ecting layers. the pc analyses each trace, extracting the position of the peaks and displaying them on the screen. independently of the display, the complete traces are stored on disk to be used in off-line processing (extraction of relevant parameters as critical frequencies, electron density, and so on). 2.7. additional blocks in fi g. 1, some additional blocks were not included because of their minor conceptual relevance. among these, there is a power supply (+ 12 v, 12 v and + 5 v) to furnish the supply voltages to the various blocks, except the pc and the power amplifi er that are self powered. in some cases, a – 5 v is generated locally in boards. in all cases, the incoming supply lines have to be fi ltered to reduce the electromagnetic susceptibility to duct noise. an additional card was designed to calibrate the system during the assembly phase. it is the «synthetic echo board» (syn echo), referring to its capability to output an echo-like signal. its internal constitution is very similar to the code generator; the difference lies in the fact that the rf code is produced after a desired delay (user programmable) from the tx trigger. in this way, the entire system can easily be check without transmitting signals on the air (the use of an external physical delay line would have needed a very long cable, more than 33 km to simulate a 50 km echo!). referring to fi g. 1, in this operating mode the signals are to be connected as follows: the code does not come from «code gen» but from the «syn echo», while the crf does not go to the power amplifi er but directly to the receiver through a proper attenuator. 3. digital signal processing much of the on-line processing is done at software level, inside the dsp board. after reading the data stored in the adc board, a complex fast fourier transform (cfft) is performed. from this point on every calculation is carried out in the frequency domain (texas instruments, 1994; oppenheim et al., 1999). a frequency based fi ltering follows the cfft. this fi ltering reduces the if bandwidth a little more; a better s/n ratio is reached without affecting the code decoding. a second type of fi ltering is then performed to reduce the high amplitude signals with respect to a reference threshold. in fact, overcoming a specifi ed threshold is a sign that a narrow band interfering signal has passed through the last if fi lter. the slight degradation of the spectrum due to the digital fi ltering is compensated by the great improvement of the signal to noise ratio. at this point, the fl ow branches, for the system must perform different correlation in accordance with the transmitted code. as previously mentioned, the code is «complementary» to eliminate the sidelobes of the correlation process. once the type of transmitted pulse is recognised, the correlation is performed. after the code 1 correlation, the system prepares itself for the reception of code 2. the process is repeated n times (where n is the number of integrations) after that the two integrated correlations are summed so that the complementary code becomes effective. 655 the new ais-ingv digital ionosode board parameter values fsy 125 mhz ( fref) reference oscillator stability 15 ppm (0° ∏ 70°) + 1 ppm per year fsy rx oscillator output frequency (lo#1) 36.9 ∏ 55.9 mhz fsy tx oscillator output frequency 1 ∏ 20 mhz fsy dds oscillators frequency resolution 4 ·10– 9 of fref (30 mhz) fsy crf output level – 12 dbm fsy tf#1 center frequency / 3 db bandwidth / min. attenuation 46.4 mhz / 28 mhz / 2 db fsy tf#2 center frequency / 3 db bandwidth / min. attenuation 40 mhz / 5 mhz / 5 db fsy tf#3 center frequency / 3 db bandwidth / min. attenuation 4 mhz / 500 khz / 8 db fsy lo#1∏3 output level + 11, + 8, + 5 dbm ctm pulse compression 16 bit bi-phase complementary codes ctm sub-pulse duration 30 ms ctm code 1 / 0 / tx off voltages + 1 / – 1 / 0 v pwa rf power amplifi er max. gain / 3 db band limits 68 db / 0.5 ∏ 50 mhz pwa rf power amplifi er max. output / max. linear output + 54 dbm / + 53 dbm pwa rf power amplifi er max. spurious level (linear) – 14 dbc @16 mhz, – 27 dbc @2 mhz pwa rf tx residual output power out of transmission swf lbi rff pass band / insertion loss 1 ∏ 25 mhz / 3 db swf rx on/off switches attenuation during rx / tx 3 db / 25 db swf 1p6t insertion loss / min. isolation 1 db / 38 db swf nbs rff central frequency 1.3 2.2 3.3 6.0 10 17 mhz swf nbs rff 3 db bandwidth 0.6 1.1 1.8 3.0 5.0 8.5 mhz swf nbs rff typ. insertion loss 2 db swf overall swf gain / attenuation during rx / tx 8 db max. / 36 db typ. rcv rf#1 center frequency / 3 db bandwidth / attenuation 35.9 mhz / 2.5 mhz / 5 db rcv rf#2 center frequency / 3 db bandwidth / attenuation 4.1 mhz / 200 khz / 4 db rcv rf#3 center frequency / 3 db bandwidth / attenuation 100 khz / 66 khz / 4 db rcv variable gain amplifi er dynamics / resolution 30 db / 2 db rcv overall rcv board gain (0 db attenuation) 52 db adc adc input range 1.55 ∏ 3.26 v adc adc resolution 8 bit = 6.7 mv adc sampling rate 100 khz adc oversampling factor (samples per sub-pulse) 3 adc demodulation system synchronous with quadrature carriers dsp input data stream 290 ns per sample (16 bit) dsp processor speed 40 mips-80 mhz table iii. functional blocks characteristics summary. 656 enrico zuccheretti, giuseppe tutone, umberto sciacca, cesidio bianchi and baskaradas james arokiasamy fig. 4a,b. ionograms recorded at 7 a.m. 2002/5/29: a) ais-ingv ionosonde; b) other digital ionosonde. a b 657 the new ais-ingv digital ionosode this phase coherent integration of the received signal is a powerful technique, because random noises are partially cancelled while the in-phase signals are amplifi ed. the processing gain depends on the number of integrations n and this integration remains effective in accordance with the ionospheric coherence time. lastly an inverse cfft is performed, the amplitude of the signal is computed and the controlling pc is called, so that it can store and display the data. the processing gain, correlation and phase coherent integration, is about 25 db. 4. system performance and conclusions after the manufacturing and assembling of the boards and subsystems, tests were done, determining the performance values reported in table iii. unless differently stated all analog lines are matched to 50 w impedance, all digital levels are ttl compatible. the main unit absorbs less than 30 w of electric power (excluding the power amplifi er and the pc) and the weight of the main unit is 9 kg. total weight and power consumption depend on the particular pc (with monitor) and power amplifi er used to complete the system. at the present stage the total volume is limited approximately: 50 cm width, 50 cm depth, 80 cm height. the ionosonde system has been tested for months, producing ionograms of good quality, comparable with ionograms recorded by other ionosondes. some results will be presented, making comparisons with the two ionosonde working at ingv. in all ionograms on the horizontal axes there are the frequencies in mhz, while the units on the vertical axes are km; they were scaled graphically and placed in the pages to allow an easy comparison among scales; all soundings were performed in the rome ionospheric observatory. figure 4a is a ionogram recorded by the aisingv ionosonde; fi gs. 4b and 4c are ionograms recorded by a digital and an analogue ionosonde respectively. the soundings were performed within a few minutes around 7.00 a.m. (local time). figure 5a,b show another sample of ionograms recorded by the ais-ingv ionosonde and another digital ionosonde. the shown ionograms were chosen as representative of the ionograms recorded by the various ionosondes; the deviations from the «normal» cases (the ones shown) are rare. it is possible to note the good quality of the ionograms recorded by the ais-ingv ionosonde, accompanied by a good background noise level (compared to analog ionosonde). for some conditions, the ais-ingv ionosonde is even better than the digital ionosonde used or comparison, e.g., at the lower frequencies (e layer). these results encourage us to improve the performance of the ionosonde; some areas in which such improvements may occur are: the new functions to recognise the polarization of the echo, doppler analysis, improvement in s/n ratio and digital signal processing using a fl oating point processor and the software adaptation for remote access. fig. 4c. ionogram recorded at 7 a.m. 2002/5/29: other analog ionosonde. 658 enrico zuccheretti, giuseppe tutone, umberto sciacca, cesidio bianchi and baskaradas james arokiasamy fig. 5a,b. ionograms recorded at 5.45 p.m. 2002/5/29: a) ais-ingv ionosonde; b) other digital ionosonde. a b 659 the new ais-ingv digital ionosode references arokiasamy, b.j., c. bianchi, u. sciacca, g. tutone, e. zuccheretti (2002): the new ais-ingv digital ionosonde design report, ingv internal technical note. bibl, k. (1998): evolution of the ionosonde, ann. geofi s., 41 (5/6), 667-680. davies, k. (1990): ionospheric radio (p. peregrinus ltd., london), pp. 580. hunsucker, r.d. (1991): radio technique for probing the terrestrial ionosphere, in physics and chemistry in space, edited by m.c.e. huber, l.j. lanzerotti and d. stoffler (springer-verlag, berlin heidelberg), vol. 22. oppenheim v.a,. w.r. schafer, j.r. buck (1999): discrete-time signal processing (prentice hall, new jersey), 2nd edition, pp. 870. skolnik, m.i. (1980): introduction to radar systems (mc graw-hill, new york), pp. 422. skolnik, m.i. (1990): radar handbook (mc graw-hill, new york), pp. 1200. texas instruments (1994): digital signal processing applications with the tms320 family (prentice-hall). (received january 7, 2003; accepted april 28, 2003) articolo demartini.pdf key words tsunami – liquefaction – gargano – 1627 earthquake 1. introduction the gargano promontory and surrounding sea have been the objects of several recent geological and seismological studies. the promontory is clearly differentiated both geographically and as seismic activity rate from the foreland of the apennines chain that bounds it to the west. historical accounts from the 1627 earthquake (fig. 1), imax = x mcs (mercalli-cancani-sieberg scale; sieberg, 1930), describe extensive destruction with formation of cracks and liquefaction features from the lesina area to san severo and a strong tsunami that inundated the northern gargano coastland, also observed at the city of manfredonia, to the east (fig. 1). despite the fact that the fault responsible for the 1627 event has not yet been identified, this historical depiction suggests that the gargano area is susceptible to tsunami inundation and liquefaction. evidence of these effects 883 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. paolo marco de martini, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; demartini@ingv.it identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication paolo marco de martini (1), pierfrancesco burrato (1), daniela pantosti (1), alessandra maramai (1), laura graziani (1) and hans abramson (2) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) geomatrix consultants, ca, u.s.a. abstract the gargano region (southeastern italy) was hit by a m = 6.8 earthquake and inundated by a subsequent tsunami in 1627. to better define the hazard in the region, we searched for evidence of this and prior earthquakes in the geologic record. we identified potential earthquake-related liquefaction features and tsunami deposits in the stratigraphic sequences of the marsh areas both north and south of the gargano promontory. we recognized clear liquefaction features and possible tsunamigenic sands that can be related to the 1627 seismic event in irrigation ditch exposures and gouge cores along the northern gargano coast. in total, six potential tsunami sand deposits have been recognized in two areas located close to the northern and southern coasts of the gargano promontory. however, ambiguous evidence comes from the paleontological analysis of these sands. although fragments of marine shells have been found in the coarser portion of the sand samples, foraminifera and ostracods assemblages are typical of brackish water condition. radiocarbon dating of three of these deposits from the northern gargano coast, near the town of lesina, suggests an average recurrence interval of 1700 years for tsunami events in this area. assuming that all the paleotsunamis are related to the same seismogenic source responsible for the 1627 earthquake, this average recurrence interval may be typical for that source. radiocarbon dating of three sand layers observed on the southern coast, close to the city of manfredonia, suggests that the average recurrence time for violent sea inundation there is about 1200 years. may be preserved in coastal lowlands around gargano. reliable historical records of large earthquakes (m.>.6) there reach .1000 years into the past, but the geologic record preserved in coastal marshes and lakes may cover much of the late holocene. based of geology, we could provide a longer time-frame to assess the tsunami recurrence and hazard of the study areas, an assessment now based on historical information only. however, a clear limitation of this approach is the intrinsic uncertainty about the earthquake source generating these secondary effects: it is not unusual to have tsunami waves and liquefaction features in italy triggered by seismic events hundreds of kilometers away (galli and meloni, 1993; tinti et al., 2002). we studied the stratigraphy of coastal wetlands to characterize better the recurrence and distribution of tsunamis and liquefaction in the gargano region. these features could likely be the result of the 1627 earthquake and previous holocene events. in doing this, we studied the upper 5 m of stratigraphy in three marshes located on the northern and southeastern coast of the promontory (fig. 2). preliminary micropalaeontological analysis has also been performed to describe the depositional environment of anomalous sand layers. radiocarbon dating of nine samples (listed in table i) helped us reconstruct the inundation history of the studied areas. 884 fig. 1. damage distribution of the june 30, 1627 earthquake. intensities are plotted in roman numbers (boschi et al., 2000). areas inundated by the 1627 tsunami are also shown. the thick dashed line north of lesina represents the best candidate for the source responsible for the 1627 earthquake and tsunami as proposed by tinti and piatanesi (1996). n 0 20 km termoli foggia manfredonia barletta tremiti island lesina lake varano lake adriatic sea apennines fo rto re r ive r bi fer no r ive r mattinata fault system tremiti fault system 1627 flooded areas 1627 flooded area lesinaripalta apricena san paolo di c. torre maggiore san severo sannicandroix ix ix ix-x ix-x ix-x viii-ixviii-ix viii-ix ixx vii-viii vii-viii vii-viii vii-viii viii ix ix x vii-viiivii-viii vii v v vii vii viii viii-ix viii-ix ix ixix x x x vii paolo marco de martini et al. 2. seismotectonic framework 2.1. geological-structural setting the gargano promontory is located on the southern adriatic coast of the italian peninsula and is considered part of the adriatic block (adria): one of the microplates identified in the central mediterranean at the collisional border of the european and african plates (mckenzie, 1972). adria is an elongated continental block which extends from the po plain southward to the adriatic basin. lort (1971) originally identified the margins of this adriatic microplate based on seismicity patterns. many authors have subsequently interpreted this plate as a single, nearly aseismic, rigid block (mantovani et al., 1985, 1993; anderson and jackson, 1987) being overridden (or subducted) along three of its margins: underthrusting the hellenides and the dinarides to the east, the southern alps to the north, and the apennines to the west. some authors (console et al., 1989, 1993; favali et al., 1990, 1992, 1993) suggest complex heterogeneities or lithospheric breaks in this area based on more recent structural data, seismic profiles and seismicity. in particular, they identified two active regional strike-slip discontinuities crossing the adriatic sea at the latitude of the gargano promontory (finetti, 1984; favali et al., 1992; chilovi et al., 2000): the tremiti fault and the mattinata fault systems (fig. 2). the gargano promontory, a structural high area where mesozoic strata rise to about 1000 m above sea level (a.s.l.), is characterized by a positive bouguer gravity anomaly (finetti and morelli, 1973). the presence of several quaternary marine terraces indicates substantial quaternary uplift of the promon885 identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication table i. measured and calibrated ages (according to calib 4.3 by stuiver et al., 1998a,b) of the samples collected in the ditches and cores. sample type measured age 2 cal age probalab adopted interval (*) year bp years bility (**) (corrected for c13/c12) ld207 charcoal 360 ± 50 1447-1638 a.d. 1.000 beta 1450-1640 a.d. le1 charcoal modern beta le2 charcoal 280 ± 120 1438-1713 a.d. 0.663 beta 1440-1890 a.d. 1716-1887 a.d. 0.274 1911-1950 a.d. 0.064 tm5-rc2 charcoal 4750 ± 90 3702-3355 b.c. 1.00 beta 3700-3350 b.c. tm5-rc3 charcoal 4700 ± 40 3631-3577 b.c. 0.199 beta 3630-3370 b.c. 3571-3561 b.c. 0.015 3538-3482 b.c. 0.229 3475-3369 b.c. 0.557 sip-d5-rc3 charcoal 1710 ± 40 244-416 a.d. 1.000 c.a.i.s. 240-420 a.d. sip-d7-rc2 charcoal 1650 ± 40 261-280 a.d. 0.057 c.a.i.s. 320-530 a.d. 293-296 a.d. 0.005 323-473 a.d. 0.820 477-531 a.d. 0.118 si9-a2rc1 charcoal 3250 ± 40 1676-1674 b.c. 0.005 beta 1620-1430 b.c. 1620-1430 b.c. 0.995 si9-a2rc2 charcoal 3630 ± 40 2135-2079 b.c. 0.152 beta 2130-1880 b.c. 2054-1884 b.c. 0.848 (*) the code of the sample reflects the site where it was collected. (**) to simplify the discussion in the text and figures ages are reported adjusted to the nearest decade and with one single range including all the main intervals with probability larger than 0.100. tory (checchia rispoli, 1916; doglioni, 1994). structural and geomorphic analyses (favali et al., 1992; piccardi, 1998; salvini et al., 1999, and references therein) show that the dominant deformation structures are the regional e-w strike slip mattinata fault system together with secondary nw-se strike slip and dip slip faults. in particular, the mattinata fault experienced a reversal from late miocene to early pliocene left-lateral strike slip to right-lateral strike slip movement since late pliocene (chilovi et al., 2000). regional cmt solutions (rcmt, 2002) of the october 31 and november 1 2002 mw = 5.7 earthquakes, occurred 50 km far to the west of the mattinata fault, showed also the same rightlateral sense of movement. 2.2. historical and instrumental seismicity despite its location in the supposedly inactive foreland of the southern apennine fold and thrust belt, the gargano promontory has been affected by strong local earthquakes in the past (fig. 2). historical seismic catalogues (postpischl, 1985; ing, 1991; boschi et al., 2000) include a large number of earthquakes in the last millennium. some earthquakes, in particular those in 1223 (imax ix), 1627 (imax x), 1646 (imax ix-x), 1731 (imax ix), 1875 (imax vii-viii) and 1948 (imax viiviii), were characterized by significant damage. moreover, many of these events were followed by aftershock sequences, some of which lasted over one year (baratta, 1901). 886 paolo marco de martini et al. n 0 20 km termoli foggia manfredonia barletta tremiti island lesina lake varano lake adriatic sea apennines 12231646 1731 1948 1875 1627 fo rt or e r iv er bi fer no r ive r mattinata fault system tremiti fault system rg pe vi is alps dinarides adriatic sea tyrrhenian sea po plain eur afr ss ce adria fortore mouth area lesina lake area siponto area m 6.2 m 6.8 m 6.6 historical eqs instrumental eqs m 4.5 m 4.0 m 3.5 fig. 2. historical (cpti working group, 1999) and instrumental (ingv monthly bulletin, 1985-2000) seismicity of the gargano region. thick rectangles show the location of the areas investigated in the present study. abbreviations: rg rodi garganico; is ischitella; pe peschici; ce cerignola; ss san severo; vi vieste. inset in the upper right shows the location of the gargano region (thick rectangle). the 1223 shock hit the southeast coastline between siponto and vieste causing severe damage to both towns. the 1627 earthquake, the largest seismic event ever in the gargano region, struck a wide area from the lesina lake to san severo, west of the promontory. a precise description of this earthquake is included in the following section. during the 1646 earthquake, the whole gargano suffered severe damage and in particular vieste, rodi garganico, ischitella and peschici, located on the northeastern edge of the promontory, were completely ruined, suggesting a likely offshore location of the epicenter. the 1731 earthquake was probably located south of gargano, as most of the destruction occurred in foggia and cerignola (ix mcs), while in the gargano area only siponto and manfredonia had minor damage. after this shock, however, a sudden sea rise, that nearly wrecked some boats, was reported along the coast between siponto and barletta (south of the gargano promontory), suggesting the 1731 earthquake triggered a tsunami (tinti et al., 2002). the 1875 earthquake has been associated with the western section of the mattinata fault system. (valensise and pantosti, 2001). this interpretation indicates that the mattinata fault can produce large earthquakes and thus its activity is not limited to microseismicity as previously thought. finally, the 1948 earthquake, characterized by three major shocks in three days, was felt in a wide area and produced maximum damage along the eastern section of the mattinata fault system, where it was probably located. the italian seismic network, implemented in the past twenty years, shows seismic activity offshore near gargano localized along e-w trending structures. detailed studies of three seismic sequences in the southern adriatic sea between 1986 and 1990 (console et al., 1993) suggest their association with the tremiti fault. this implies that this offshore structure is seismically active. thus, both the tremiti fault and the mattinata fault system (see previous paragraph), should be considered major candidates for the location of large earthquakes and related tsunamis. 2.3. the july 30, 1627 earthquake on july 30, 1627 around midday a disastrous earthquake (imax = x mcs) hit the gargano promontory, with the strongest shaking concentrated in the northern coastal area between lesina and ripalta (fig. 1). the shock was followed by a strong tsunami and four large aftershocks, the first one occurring about 15 min after the main shock. many contemporary sources described the event, providing a detailed picture of the extensive destruction and many casualties. figure 1 shows the distribution of the macroseismic intensities (boschi et al., 2000), and the location of the flooded areas. the most severely affected villages were apricena, lesina, san paolo di civitate, san severo and torre maggiore, where most buildings were totally destroyed. the earthquake left more than 5000 victims and was felt in a very broad area, as far as the central apennines and sicily. it caused cracks on the ground surface, subsidence, flooding, and gas emissions along the fortore river accompanied by modified active river channels and dried lakes. however no clear evidence for surface faulting was found. the damage distribution (boschi et al., 2000; fig. 1) from which the macroseismic epicenter (fig. 2) is derived does not help resolving the ambiguity of the earthquake source location and leaves open the possibility of an offshore source. a significant tsunami, intensity 5 on the sieberg-ambraseys scale (ambraseys, 1962) which means «very strong tsunami», occurred after the main shock. the tsunami was particularly violent on the northern coast of gargano, between the fortore river and the coastal plane north of sannicandro, around the lesina lake. the lake, approximately 20 km long and 4 km wide, is separated from the sea by a narrow sand spit. few artificial channels through the spit connect the lake to the sea. eyewitness accounts report that lesina lake completely dried for many hours after the shock and many fish were stranded far from water. a coeval drawing by de poardi (1627; fig. 3) depicts this phenomenon. contemporary reports describe an initial sea withdrawal of about three kilometers, followed by a great inundation that extended some length inland. according to ballerani (1627) seawater 887 identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication flooded the village of lesina and the victims were «uncountable». no distinction was made between casualties directly related to the earthquake and those caused by the tsunami. less severe tsunami effects were reported over a very wide area. at the mouth of the saro river (today called foro, about 60 km north of termoli), a sea withdrawal of about 90 m was observed (antinori, 1782). in manfredonia, on the southern coasts of gargano, the waves reached the city walls, about 2.5 m above the mean sea level (cerqua, 1627). accounts of the earthquake are much more abundant and detailed than those concerning the tsunami, precluding assessments of wave height, flow velocity, and run-up limit based solely on historical reports. one potential explanation for this is that most people lived well inland during the 17th century, and the marshy lowlands around lesina lake were almost uninhabited. 3. observations and data collection as already discussed, the 1627 contemporary chronicles clearly state that the july 30 888 paolo marco de martini et al. fig. 3. drawing from de poardi (1627) describing some effects of the 1627. note that fishes are drown as the rows out from the lesina lake. this map can be considered as the ancestor of modern macroseismic maps, since the author represented with various symbols cities which sustained different levels of damage. earthquake produced a tsunami wave that flooded the northern coast of the gargano promontory and the manfredonia harbor to the east, along with liquefaction features over a wide area. field surveys of areas affected by modern tsunamis show that their effects may be preserved in the geologic record. the most prominent of these effects are erosion and transport of beach sand and debris from the beach with subsequent deposition inland where they may be preserved in marsh or lacustrine sediments (bourgeois and reinhart, 1989; dawson et al., 1991a,b; dawson et al., 1995). to investigate the existence of these geologic records in the gargano area we performed a detailed interpretation of aerial photographs to identify coastal wetlands that showed little evidence for anthropogenic modification. if these wetlands show a relatively continuous depositional history over the late holocene with few and short episodes of erosion, they have a high probability of preserving deposits from tsunamis or liquefaction features. we identified marshes at the fortore river mouth in the eastern part of the lesina lake and in the siponto farmlands (fig. 2). these are considered test areas where the features that formed during the 1627 and possibly older earthquakes may have been preserved in the stratigraphic sequence. 3.1. fortore mouth area the fortore river flows, with a nne direction, from the southern apennines and abuts the adriatic sea just west of the gargano promontory. the estuarine zone around its mouth is about 2.5 km wide, and includes several abandoned channels. much of this land is currently used for agriculture purposes (fig. 4). the most impressive description of the 1627 tsunami wave, reported by contemporary chronicles, refers to identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication le sin a l ak efo rt or e r iv er adriatic sea present mouthold mouth n 2 km torre mozza site auger holes le casette 1 site cutbank: water reservoir le casette 2 site ditches pantanello site auger holes masseria longara site auger holes fig. 4. spot image (doi-10m national imagery and mapping agency: contains data (c) cnes/spot image 1992-1994 copyright) of the fortore mouth area with location of the investigated sites. 889 this locality. however, several sites near the fortore river mouth lack significant evidence of tsunami deposits. auger holes from two sites east of the fortore mouth, pantanello and masseria longara sites (fig. 4), revealed mostly sandy and silty fine sand overbank fluvial deposits extending up to the holes end at 2-2.5 m. weakly developed soils cover both sequences. west of the fortore mouth (fig. 4) auger holes and cut-banks show undisturbed stratigraphy at two other sites. at torre mozza approximately 60-80 cm of thick, dark, loose mud overlies well sorted mediumgrained sand extending to a depth of 2-2.5 m. at le casette 1 an undisturbed sequence of alternating fine sand and silty clay is exposed in cutbanks of a water hole. at le casette 2 site distinct sand dikes and ball and pillar structures in a stratigraphy of interbedded sand, silt, and clay signify earthquakeinduced liquefaction (fig. 4). the upper 1.5 m at this site consist primarily of bluish-gray silty clay with planar beds of sand and silt ranging from < 1 cm to over 20 cm thick. these beds are typically planar, horizontal, and show no internal grain size variation. one convoluted layer, however, is exposed at depths from 25 to 40 cm in a 50 m reach of the cut-banks of an abandoned channel now used as irrigation ditch. it consists primarily of very fine to medium grained sand with thin silty layers, and presents a total thickness ranging from 10 to 20 cm. clasts of gray clay, that are sometimes angular and sometimes stretched parallel to the convoluted upper and lower contacts, are entrained in the sand and silt. this layer is graded to finest material both toward its top and bottom. the presence of coarsest material in the middle suggests flow of liquefied sand within it. both the upper and lower contacts of this layer are contorted into «ball and pillar structures», common in liquefied sand (obermeier, 1996; obermeier and pond, 1999). the most convincing evidence for liquefaction, however, are the vertical sand dikes that extrude from the top of this layer, crosscut bedding and rising to within 5-10 cm of the marsh surface where they are overprinted by bioturbation and modern loamy soil (fig. 5). these dikes are very thin (up to 3 cm thick) and planar but locally are paolo marco de martini et al. fig. 5. pictures of the sand dikes observed on the cutbanks of an abandoned channel at le casette 2 site. 890 thicker (5-10 cm) and bulbous. unfortunately no datable material has been found to constrain the age of the liquefaction event. taking into account that the upper termination of the sand dikes reaches the modern soil, we suggest that this liquefaction is related to the 1627 earthquake, the local best candidate for inducing strong shaking and high pick ground acceleration values. more liquefaction features are exposed on the cut-banks of an abandoned estuary channel that parallels the modern beach roughly 300 m inland. the modern beach shows signs of rapid retreat. clayey marsh deposits are being exhumed by wave erosion and retreat of the beach along a 2.5 km reach around the fortore estuary. a house built in the 1970’s 2 km west of the river mouth is crumbling into the water due to wave erosion. finally, there is virtually no natural dune around the fortore river mouth to protect against coastal erosion. for the above reasons, we suspect there may have been a delta associated with the fortore river at the time of the earthquake and that the shoreline was probably much further out. all these considerations on the rapid modification occurring at this site could help explaining the lack of evidence for tsunami deposits. 3.2. lesina lake area contemporary chronicles reported that the 1627 earthquake produced a huge wave that inundated the country side of sannicandro. this city lies at 225 m a.s.l., about 9 km inland on the northern foothills of the gargano promontory, and dominates the eastern lesina lake plain. we selected a 5 km2 marshy area located at the lake’s northeastern edge (fig. 6), where we studied five sites located as far as 1 km inland from the modern shoreline. the marshy eastern end of lesina lake and the low, narrow sand spit that separates it from the sea combine identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication n 2 km lesina lake ma rsh are a adriatic sea low lands limit < 2-3 m tamaricelle site auger holes corso site cutbank: water reservoir triangulation site auger holes black panther site auger holes lesina lake site ditches fig. 6. spot image (doi-10m national imagery and mapping agency: contains data (c) cnes/spot image 1992-1994 copyright) of the lesina lake area with location the investigated sites. the dashed white line encircles the low lands probably inundated in 1627. 891 to create conditions favorable for deposition and preservation of tsunami deposits. irrigation ditches in reclaimed marsh land expose stratigraphy in the northeastern portion of lesina lake between the sea and the regions flooded by the tsunami in 1627. the ditches on the inland edge of the sand spit are oriented perpendicular to the coastline, extending roughly 3-400 m inland. the ground level around these ditches is currently lower than the water level of the lake and they are protected from flooding by an artificial dike. a convoluted layer similar to that found at the fortore river mouth shows evidence of either liquefaction or syn-depositional soft-sediment deformation at this site. this very fine, tan silty sand layer locates from 40 to 60 cm below the modern ground surface (fig. 7). the convoluted layer contains stretched and broken clay clasts in a matrix of fine grained sand with horizontal flow structures around them. the sand is finer on the upper and lower edges of the layer and coarser in the middle, features common in liquefied sand deposits (obermeier, 1996; obermeier and pond, 1999). radiocarbon dating of a piece of charcoal from the top of this layer yields an age of 1450-1640 a.d., providing a limiting maximum age for the disruption (sample ld207 in table i). an additional exposure from the corso site ( 700 m east and 750 m from the shoreline) revealed a 2 cm thick sand layer 1 m below the surface that may correlate to this convoluted layer. to assess stratigraphy in the marsh immediately south of the ditches, we collected 18 gouge cores that penetrated up to 5 m deep. sampling sites spaced along the perimeter of lesina lake (fig. 6) included: 8 cores in the 2000 m2 surrounding the black panther site, 2 at the triangulation site, and 8 in the 2000 m2 surrounding the tamaricelle site. the stratigraphy revealed by these cores is remarkably uniform. figure 8 shows the log of a representative core, bpau2, from the black panther site about 100 m south of the ditches. the general stratigraphy observed at this east end of lesina lake consists primarily of peat and mud with a few thicker clayey and sandy silt intervals. most cores hit refusal below 4 m depth in a dark gray, hard, sticky clay layer. this stratigraphy is punctuated by 3 anomalous sandy or coarse silty layers that indicate high energy deposition and/or marked changes in depositional environment. we think these «disturbance layers» are potentially deposits from the 1627 tsunami and two earlier waves. we refer to the events depositing these disturbance layers as events l1, l2, and l3 where event l1 represents the most recent and event l3 the oldest deposition. the uppermost disturbance layer, found at a depth of 70-90 cm, is 10 cm thick, light brown, medium to fine sand, with shell fragments concentrated at the bottom. the second is 3-5 cm thick, gray, fine sand with abundant shell fragments, lying approximately 270 cm 892 paolo marco de martini et al. fig. 7. lesina lake site: picture of the convoluted layer (above) and simplified sketch of the upper 1 m stratigraphy studied in the ditches (below). the location of one charcoal sample is also plotted with a black circle. deep. the third layer is 3-5 cm thick, sandy silt that is normally graded (coarser at the bottom, where shell fragments have been found) and lies about 340 cm below the surface (fig. 9a). each disturbance stratum has a sharp lower contact, contains shell detritus and sediments coarser than that above and below it. this implies rapid change to high energy deposition, consistent with that from an inundating tsunami wave. one of the three layers accompanies transition from marsh peat (below) to gray silty mud (above), recording also a sudden change in depositional environment. all three layers contain sand: a common element of tsunami deposits (bourgeois and reinhart, 1989; clague et al., 1994; minoura et al., 1996; benson et al., 1997). four radiocarbon dates constrain the time of deposition of two disturbance layers (samples le1, le2, tm5-rc2, tm5-rc3 in table i). dating suggests that the uppermost disturbance layer (l1) is younger than 1440-1890 a.d., with the older age statistically preferred, and the lowermost sand (l3) was deposited in the interval 3630-3350 b.c. other sandy layers occur in the stratigraphy of lesina lake area that are not interpreted as tsunami deposits. they are either too localized (they occur only in one core), lack of sharp contacts, are much thicker than expected for a tsunami deposit so far from the major source of sand (the beach), or do not accom893 fig. 9a,b. a) picture of the event l3 sand found at black panther site; b) picture of event s3 sand collected at mr. paolo’s site. bp au2 5 4 3 2 1 sandy mud sandy silt with shell fragments medium to fine sand with abundant shell fragments at bottom gray to light brown silt with small shells sandy silt with shell fragments sandy shell-rich peat shelly sand blackish brown peat normal graded sand brownish black peat very fine sand blackish brown peat dark gray clay, marine? event l3 -coarse grained -normally graded d e p th ( m ) event l2 -coarse grained -accompanies strong change in deposition event l1 -coarse grained -correlates to convoluted layer in drainage ditch le-1 le-2 tm5rc2 tm5rc3 fig. 8. core bpau2 log from lesina lake area. the location of four charcoal samples is also plotted with a black circle. identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication a b pany a change in depositional environment. these sand layers may be channel deposits, colluvial material that has washed down the neighboring hillside, or simply longer term environmental changes in the lake, such as the opening of the sand spit. 3.3. siponto area the area south of siponto, that was once the seaport of the ancient roman town of siponto, was naturally filled with sediment and is today represented by a low lying area extending inland roughly 1.0-1.3 km (fig. 10). this 0.5 km2 region is currently used for farming and is dissected by a dense network of irrigation ditches draining the abundant spring water away. we investigated stratigraphy of exposures in irrigation ditches throughout the region, but we mostly focused on two locations, mr. paolo and siponto sites. there the water was at least 1.0-1.3 m below the ground surface allowing to study the upper part of the stratigraphy. at mr. paolo site we also collected gouge cores at 8 locations, penetrating approximately 5 m. as exposed in irrigation ditches and in gouge cores, the observed stratigraphy consists almost entirely of brownish black peat. the peat has gastropod fossils scattered throughout and chunks of tan tufa. near the surface several intact plant fossils are preserved in the peat, while lower in the stratigraphy peat is more decomposed and contains muddy and sandy layers. the peat is capped by 0.5-0.7 m of clayey loam. at a depth of roughly 4.5-5 m, peat overpaolo marco de martini et al. lo w la nd s l im it < 23 m n 2 km manfredonia adriatic sea siponto mr. paolo site ditches and auger holes siponto site ditches fig. 10. spot image (doi-10m national imagery and mapping agency: contains data (c) cnes/spot image 1992-1994 copyright) of the siponto area with the sites investigated in the present work. the dashed black line encircles the low lands that could probably be inundated by tsunami waves. 894 lies a hard gray pebbly clay that gouge cores cannot penetrate. the 5 m of fine-grained muddy and peaty deposits, that represent a long period of lowenergy deposition, are interrupted by 3 coarsegrained layers indicative of high energy deposition. similarly to lesina lake, we interpret these disturbance layers as possible evidence of three tsunamis. the two uppermost disturbance layers are exposed at both sites in irrigation ditches (fig. 11). we refer to the events that deposited these 3 disturbance layers as events s1-3, with numbers increasing with age and depth. the upper layer («event s1 layer») occurs at depths of 40-60 cm, typically just at or above the contact between the peat and silt. it consists of mixed pockets of tan fine-grained sand, silt, blackish muddy peat, and broken and ground fragments of yellowish tufa, in varying amounts. it most commonly has a sharp lower and a gradational upper contacts and its thickness ranges from 3-15 cm. in 15 of the 27 ditches examined this layer is clearly distinguishable. radiocarbon dating of a charcoal fragment (sample sip-d5-rc3 in table i) collected just below this sand indicates that it is younger than 240-420 a.d. the next layer («event s2 layer») typically occurs at depths of 70-90 cm, and therefore does not crop out in shallower ditches. it consists of a discontinuous 1-3 cm layer of fineto medium-grained, gray sand in sharp contact with the surrounding black peat. this stratum is visible in 5 of the 9 sections studied that are deep enough to extend down into the black peat that typically surrounds it. two charcoal samples, collected above and below this deposit (samples sip-d5-rc3 and sip-d7-rc2 in table i, see fig. 11) yield radiocarbon dates that bracket its age between 320 and 420 a.d. at the siponto site, ditch 1, two fragments of pottery lie at a depth of 70 cm. local archaeologists from the foggia regional department date them between 400 and 1200 a.d. based on the style of pottery, with the older part of the interval preferred. the stratigraphic relationship between the shards and the middle disturbance layer is unclear, because neither of the upper two disturbance layers was present in the stratigraphy where the pottery was found. however, the depth of the shards and their stratigraphic relationship to the peat and silt imply they were deposited before the last two disturbance events. this archaeological estimate agrees with the radiocarbon ages (samples sipd5-rc3 and sip-d7-rc2 in table i) reported above and further constrains the timing of event s2. the third layer («event s3 layer», figs. 9b and 12) consists of normally graded fineto medium-grain tan sand. sand grains are well rounded and the layer contains no mud in the interstices. it sharply overlies brown peat and 895 0 40 cm 80 silty active soil peaty mud with shells at bottom blackish-brown peat blackish peat orange sand gray sand water orange sand gray sand sipd5rc3 sipd7rc2 shard 240-420 a.d. 320-530 a.d. identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication fig. 11. siponto site: picture of the two tsunami sands from ditch 5 (above) and simplified sketch of the upper 1 m of stratigraphy studied in the ditches (below). the location of two charcoal samples and of the shard is also plotted with a black circle. underlies peaty mud in a gradational contact (figs. 9b and 12). this layer occurs at about 250 cm below the surface and is only visible in gouge cores. over a 900 m2 area it is found in all 8 cores that penetrated to this depth. dating of charcoal fragments (samples si9-a2rc1 and si9-a2rc2 in table i) collected above and below it constrains the age of this layer to the interval 2130-1430 b.c. 4. micropaleontological analysis as discussed above, historical and modern tsunamigenic deposits frequently include sand and/or other material. micropaleontology has been used in several studies to characterize the provenance of anomalous sand layers and their depositional environment (minoura et al., 1994; hemphill-haley, 1996; nelson et al., 1996; hutchinson et al., 1997). this tool may help identify marine surges into freshwater ponds or lakes by detecting marine species in an otherwise freshwater environment. we collected samples of sand from disturbance layers and surrounding material at lesina lake and siponto. selected samples were then washed and sieved into three sizes, 125-250, 250-500 and > 500 µ. material from the 250500 µ fraction was then analyzed under a binocular microscope with foraminifera, ostracods and plant matter being picked, sorted into species and counted. 4.1. lesina lake area we analyzed samples from two sites we deemed to best represent the stratigraphy at lesina lake, black panther and tamaricelle. in this preliminary phase, we limited our analysis to the sand layers deposited by events l1 and l3, as these were the most widespread. microfossil assemblages from the «event l1 sand» in the two cores were nearly identical; likewise, microfossil assemblages from the «event l3 sand» were nearly identical between cores, reinforcing our stratigraphic correlation. event l1 – the samples examined consist of light brown silty sand, containing fragments of rearranged marine shells. the inorganic fraction consists of carbonate concretions and rare crystals of gypsum. microfauna assemblages are abundant and well preserved (more than 90% intact) and a restricted fauna of both foraminifera and ostracods is present. frequent gastropods are also visible (valvata piscinalis). the foraminifera, purely freshwater, are almost completely dominated by ammonia parkinsoniana (parkinsoniana) (about 90%) with ammonia parkinsoniana (tepida) (about 8%) and minor (< 1%) haynesina germanica, elphidium granosum, elphidium sp., aubigyna perlucida also present. 2.3 d e p th ( m ) 1.3 3.3 4.3 5.3 brownish-black peat peaty mud medium sand brown gray peat with shells green gray silty clay dark brown peat whitish yellow sandy silt with many shells brownish black peat gray silty clay with few pebbles whitish silty clay with few pebbles paolo’s au2 event s3 event ? -normally graded -accompanies strong change in deposition si9-a2rc1 si9-a2rc2 fig. 12. core paolo’s au2 log from siponto area. the location of two charcoal samples is also plotted with a black circle. 896 paolo marco de martini et al. the ostracod assemblage displays slightly more variation and includes the brackish species xestoleberis communis, cyprideis torosa, loxoconcha elliptica, leptocythere castanea. these species indicate peripheral marine conditions. cyprideis torosa is especially adaptable to diverse salinity, from almost freshwater to over 60 parts per thousand; loxoconcha elliptica is totally confined to brackish conditions. all shells are well calcified and irregular ones are rare. the characteristics of both foraminifera and ostracods assemblages, certainly autochthonous, refer to a saltmarsh environment. the lack of evidence of ecological stress would suggest an environmental stability. event l3 – the samples consist of gray sand, containing fragments of rearranged marine shells. the inorganic fraction contains frequent carbonate incrustations on plants, some rare crystals of gypsum and mica. in terms of microfauna, the assemblages are abundant and well preserved (more than 90% intact) and a limited fauna of both foraminifera and ostracods is present. frequent mollusks are present (valvata piscinalis, gyraulus laevis, ancylus sp.). the foraminifera, purely freshwater, are almost completely dominated by ammonia parkinsoniana (parkinsoniana) (about 90%) with ammonia parkinsoniana (tepida) (about 8%), aubigyna perlucida (about 3%) and minor (< 1%) haynesina germanica and elphidium sp. also present. the foraminifera assemblages show the presence of frequent deformed specimens and a high intraspecific variability, a sign of ecological stress probably due to environmental instability. the ostracods assemblage contains the brackish species xestoleberis communis, heterocypris salinua, candona sp., herpetocypris chevreuxi. all these species are strictly continental. the presence of many ostracods with closed valves and of foraminifera with very thin shells absolutely intact implies a depositional environment with low energy, in which the conditions were more or less constant. carophitae oogones are frequent, intact and well preserved, their presence indicates a low energy depositional environment, clearly continental. in general, the results of the micropalaeontological analysis, performed on samples from the lesina lake area, suggest that the deposits are typical of a brackish environment (saltmarsh), almost totally cut off from normal marine conditions. this is particularly true for the «event l3» layer. the presence of coarse sand with fragments of bivalve marine shells, typical of an environment with higher energy, sedimented in a quiet environment, indicates a certain degree of rearrangement, possibly due to tsunami waves. 4.2. siponto area we analyzed samples from the «event s1 sand layer» collected from ditch exposures at mr. paolo site, and from the «event s3 sand layer» collected from gouge core at mr. paolo site (fig. 10). micropalaeontological analysis of the «event s1 sand layer» revealed faunal assemblages very similar to those observed in the «event l1» deposit at lesina lake area (see section 4.1.). foraminifera assemblages from this layer are typical of a brackish environment with a minor presence of marine species. some wellpreserved fresh water ostracods were identified and few specimens with spotted decalcified shells were observed. spotted decalcification may be indicator of episodic acidic changes. detailed analysis of the «event s3 sand layer» reveals it is very rich in limonite but sterile, lacking both microfauna and flora. thus we can not make any inference on the possible tsunamigenic origin of this sand layer. 5. discussion the occurrence of several historic earthquakes and evidence for multiple tsunamis in the gargano region, most notably those in 1627, clearly highlight that earthquakes and tsunamis comprise a large portion of the geologic hazard in this area. significant components of the hazard evaluation include tsunamis and liquefaction recurrence, the areas affected by them and the magnitude of their effects. in the following we discuss the interpretation of our findings in terms of: a) distribution of liq897 identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication uefaction features probably related to the 1627 earthquake on the northern gargano coast, and b) recurrence and distribution of anomalous sand layers, interpreted as tsunami deposits, both north and southeast of the promontory. 5.1. liquefaction evidence at the fortore mouth and at the lesina lake the convoluted layer and sand dike in the fortore river estuary is most likely the result of earthquake-induced liquefaction. obermeier (1996) suggests clastic dikes present the most convincing evidence for seismic liquefaction. the association of the convoluted layer with the sand dike suggests the convoluted layer shares the same origin. convolutions and clay clasts probably resulted from water and sand flowing horizontally within the layer during seismic shaking, and ripping clasts from the surrounding material. although no absolute age is available from the ditch wall, the upper termination of the sand dikes (fig. 5), 5 to 10 cm below the surface suggests that the correlation of these features with the earthquake of july 30 1627, the strongest recent earthquake in the region, is reasonable. we have two working hypotheses for the formation of the convoluted layer at lesina lake site. it contains features common in liquefied sand layers (i.e. convolutions), but it lacks definite evidence for liquefaction (i.e. sand dikes). another hypothesis is that when the 1627 earthquake hit, this area was covered with shallow lake water, as suggested by the current position of the ground surface below the lake level. either the tsunami that followed the earthquake or seiches in the lake caused a disturbance that affected this layer. the radiocarbon age of the sample ld207 (table i), collected from the top of the convoluted layer, implies this horizon was deposited between 1450 a.d. and 1640 a.d., allowing either hypothesis. 5.2. tsunami evidence at the lesina lake and siponto area the suspected tsunami deposits (figs. 8, 11 and 12) examined in the lesina lake and at siponto contain coarser grained material, usually normally graded, than the deposits above and below them, have often sharp basal contacts and occasionally include clasts of mud and peat. the presence of sand in all these layers implies deposition from higher energy flow than the surrounding material. normal grading in the sand layers implies deposition from suspension, as expected from a tsunami which entrains sand from the beach as it passes, and carries it in suspension as the flow progresses inland. the sharp contacts imply a sudden inundation rather than a gradual transition to a higher energy environment, consistent with the inundation expected for a tsunami. moreover, the abundance of fragments of marine shells in these layers suggests they were mechanically broken while being transported in a violent flow. all of these traits are common in tsunami deposits, and have been used as criteria for distinguishing a tsunami deposit (clague et al., 1994; atwater et al., 1995; atwater and hemphill-haley, 1997; benson et al., 1997). 5.3. lesina lake area the uppermost sandy layer (fig. 8), «event l1» has to be younger than 1440-1890 a.d. (sample le2) where the lower part of the time interval is statistically preferred (see table i). we also expect the event age to be possibly very close to the age of the sample considering its stratigraphic position and an uninterrupted sedimentation at the site, thus suggesting a probable origin from the 1627 tsunami. the «event l2» sand (fig. 8) accompanies transition from marsh peat (below) to gray silty mud (above). this pattern is common for tsunami deposits found on the cascadia subduction zone, where it is most commonly attributed to the sudden coseismic subsidence of a marsh surface into the intertidal zone (darienzo and peterson, 1990; atwater et al., 1995; atwater and hemphill-haley, 1997). the marsh plants are then killed by exposure to more saline water and buried by intertidal mud. this pattern has also been seen associated with tsunami deposits in coastal freshwater lakes (hutchinson et al., 1997; garrison-laney, 1998) and coastal fresh898 paolo marco de martini et al. water marshes (witter and kelsey, 1996). in these cases the marsh may have been killed by the incursion of saline water from the tsunami, an earthquake-induced change in the morphology of the marsh and beach, or buried by increased sedimentation after the earthquake and tsunami. preliminary micropalaeontological analysis of the «event l1» and «event l3» sand layers does not give unequivocal constraints for interpreting these layers as tsunami deposits. as previously discussed, the microfauna, mainly foraminifera and ostracods, observed in the examined layers is typical of saltmarsh environment and the presence of several intact very thin shells suggests a low energy depositional setting. this intact and fragile microfauna could not have been in place at the time of the deposition of the coarse sands and the broken bivalve shells. the co-existence of sediments indicative of high energy and microfauna indicative of low energy conditions, in our opinion, can only be explained by an imprecise sampling procedure. sand layers observed in the field frequently have sharp basal contacts with high concentrations of broken marine shells. gradational upper contacts make the inadvertent selection of some overlying material sometimes unavoidable. an interesting observation is the total absence of marine microfauna. while this may not have been expected, it is not unique. in fact, micropalaeontological analysis of paleotsunami deposits in fresh water environments show the vast majority of microfossils included in these deposits are fresh to brackish water species (minoura et al., 1994; hemphillhaley, 1996; nelson et al., 1996; hutchinson et al., 1997; garrison-laney, 1998). clearly, further study including careful sampling of sand layers and their surrounding sediments is necessary to answer these questions. the sources responsible for the earthquakes that produced the three tsunami waves remain uncertain. tinti and piatanesi (1996) performed numerical simulations of the 1627 earthquake and tsunami, integrating shallow water equations via a finite elements technique and assuming dip-slip faults placed in different locations and with various strike both on land and offshore. these authors suggest that the possible source responsible for the 1627 tsunami is a fault placed inland along the coastal area enclosing the lesina lake and the mouth of the fortore river, that caused the uplift of the sea floor facing the gargano promontory. we were not able to find evidence of such fault, but we have to consider that this area is dominated by littoral sand dunes and active river channels, that are able to modify the morphology quite rapidly. the age of the lower tsunami deposits («event l2» and «event l3») is not within the historical record and we cannot infer a triggering source for these potential tsunamis. other possible sources include the tremiti fault system, located 25 km offshore to the north, or seismogenic faults on the eastern adriatic sea border. in this frame, it has to be underlined that the coasts of albania and montenegro experienced, in the last two millenniums, a relevant number of tsunamigenic events, particularly in the region of valona. two recent ones, occurred respectively in 1920 at saseno island and in 1979 in kotor bay, have been destructive, causing severe damage and victims along the eastern adriatic coasts. moreover, both these events produced sea perturbations also observed and /or recorded along the italian coasts as in the tide gauge of bari (bedosti and caputo, 1986). on this basis, we suggest an inundation average recurrence interval of about 1700 years. if we could infer that all 3 tsunamis affecting that site were triggered by earthquakes on the same source responsible for the 1627 shock, we could obtain a minimum average recurrence interval of about 1700 years for that fault. 5.4. siponto area although the disturbance layers at siponto have more features common in tsunami deposits than those at lesina lake (all three contain sand, one has rip-up clasts, one has normal grading), the interpretation of tsunami as the depositional mechanism at siponto is less strong because the morphology of the site has changed significantly in the last 2000 years. in fact, the morphology and position of the beach berm 2000 years ago, and thus the approach of the hypothesized tsunami flow is less clear than 899 identification of tsunami deposits and liquefaction features in the gargano area (italy): paleoseismological implication at lesina lake area, where the lake probably retained nearly the same morphology as it has today. the only historically documented tsunami near the site was associated with the 1627 earthquake. during this event, anomalous waves rose as high as 2.5 m on the walls protecting the city of manfredonia. micropalaeontological analysis of samples of the uppermost sand (fig. 11), collected from the ditches at the siponto and mr. paolo sites, reveals the presence of both foraminifera and ostracods typical of brackish environments, together with minor marine species. samples of the «event s3 sand layer» (figs. 9b and 12), unfortunately lack of any microfossils. again, analysis of microfossils does not substantiate the interpretation of a tsunamigenic derivation for these deposits. nonetheless, based on characteristics of the three disturbance layers themselves and the tectonic activity of the region, we suggest a tentative interpretation as tsunami deposits is still valid. we do not have direct evidence for locating the sources of earthquakes that triggered the three tsunami deposits found in the siponto area. in general, the mattinata fault system, and particularly its offshore portion, could be considered a possible local source. similarly at the lesina lake area, the strong seismic activity of the albanian and montenegro coasts in the southern adriatic sea must be considered as probable farfield sources. the uppermost disturbance layer may be associated with the event in 1627, but the distance of this site from the probable epicenter of that earthquake, and the historical reports of very localized waves at manfredonia, with no mention to the southern siponto area, make this association tentative at best. based on these interpretations and considering that no clear historical information exists on local tsunamigenic source, we suggest that the average recurrence time for violent sea inundation at this site is approximately 1200 years. 6. conclusions the recurrence and distribution of tsunami deposits and liquefaction features in the gargano region have been explored through field surveys of irrigation ditch walls and gouge cores. in doing this, we studied the upper 5 m of stratigraphy in three marsh areas on the northern and southeastern coasts of the promontory. we identified clear evidence of liquefaction near the mouth of the fortore river and close to the eastern edge of lesina lake. based on the upper termination of sand dikes rising to 5-10 cm of the surface and the age of a charcoal sample (1450-1640 a.d.; ld 207 in table i) collected from the top of a liquefied sand layer, we claim to have found features related to the 1627 earthquake, for which numerous liquefaction effects have been reported by contemporary chronicles. based on field observations and the constraints given by radiocarbon dating of selected samples, we believe we have identified evidence in the gouge cores for three tsunamis that inundated lesina lake and three that submerged the town of siponto. we must emphasize that this interpretation is preliminary and reflects our current level of uncertainty. in particular, while detritus of marine macrofossils have been found in the coarse sand samples, micropalaeontological analysis reveals assemblages typical of brackish environment and does not provide unequivocal constraints to a tsunamigenic cause. in the lesina lake area, the uppermost disturbance layer identified in gouge cores has a minimum age of 1440-1890 a.d., where the lower part of the time interval is statistically preferred (le2, table i). taking into account the historical reports and this age, we suggest an origin from the 1627 tsunami for this sand layer. two radiocarbon dates (tm5-rc2, tm5-rc3 in table i) constrain the deposit of the oldest sand between 3630-3350 b.c. thus, a conservative interpretation provides a minimum average recurrence interval of about 1700 years for tsunami inundation in this area. if the paleotsunamis are related to the same fault responsible for the 1627 earthquake, this average recurrence interval may be typical for that source. in the siponto area, archeological and radiocarbon (sip-d5-rc3, sip-d7-rc2 in table i) ages indicate that the two upper disturbance layers were deposited in historical time. dating of charcoal fragments (samples si9-a2rc1 and 900 paolo marco de martini et al. si9-a2rc2 in table i) constrains the age of the oldest sand layer to the interval 2130-1430 b.c. accordingly, the average recurrence time for violent sea inundation at this site is approximately 1200 years. at present, age constraints are not enough to discuss possible correlation between inundation events occurred along the northern and southeastern coasts of the gargano promontory. acknowledgements we are grateful to two anonymous reviewers for valuable comments and suggestions that improved the clarity and focus of the manuscript. we wish to thank f. frugoni (ingv), a. piatanesi (ingv) and m. rymer (usgs) for the interesting discussions on tsunami generation and their field identification. we are also indebted with mr. s. persichetti of laboratorio preparazione rocce e sedimentologia (università degli studi roma tre) for his valid support during the preparation of the samples. references ambraseys, n.n. 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(1930): geologie der erdbeben, handbuch der geophysik, 2 (4), 550-555. stuiver, m., p.j. reimer, e. bard, j.w. beck, g.s. burr, k.a. hughen, b. kromer, f.g. mccormac, j.v.d. plicht and m. spurk (1998a): intcal98 radiocarbon age calibration 24.000-0 cal bp, radiocarbon, 40, 10411083. stuiver, m., p.j. reimer and t.f. braziunas (1998b): highprecision radiocarbon age calibration for terrestrial and marine samples, radiocarbon, 40, 1127-1151. tinti, s. and a. piatanesi (1996): numerical simulations of the tsunami induced by the 1627 earthquake affecting gargano, southern italy, j. geodynamics, 21, 141-160. tinti, s., a. maramai and l. graziani (2002): the new catalogue of the italian tsunamis, j. seismol. (submitted). valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger than m 5.5 in italy, ann. geofis., 44 (suppl. to n. 4), pp. 183 (with cdrom). witter, r.c. and h.m. kelsey (1996): repeated abrupt changes in the depositional environment of a freshwater marsh: a record of late holocene paleoseismicity at euchre creek, south coastal oregon, geol. soc. am., abstracts with programs, 28, 125. 902 paolo marco de martini et al. 01ambraseys notes.qxd 335 annals of geophysics, vol. 47, n. 2/3, april/june 2004 the interaction between different disciplines that constitute the basis for mitigation of earthquake risk can best be portrayed by the relation that describes seismic risk, which is the preoccupation of decision makers, planners, engineers and politicians alike. seismic risk can be defined by the following relation: [seismic risk] = [earthquake hazard] ∗ [structural vulnerability] × [value] in which earthquake hazard is the probability of occurrence, within a specific period of time and given area, of a potentially damaging earthquake, which is beyond human control but knowledge of it is possible. vulnerability is the degree of loss resulting from the occurrence of an earthquake of a given magnitude, and it is subject to human control. value may be taken either in the sense of capital value or of production capacity of a vulnerable element. this definition makes a clear distinction between earthquake hazard, which includes tectonics, seismology, strong-ground motions, seismic regionalization and tsunamis, specialties which constitute engineering seismology, and vulnerability, which includes building materials, foundations, structural engineering, and retrofitting, specialities which constitute earthquake engineering. it is the amalgamation of these specialities and their balanced and coordinated support that would lead to the mitigation of seismic risk. to begin with, not all of the existing regional and global parametric earthquake catalogues from which the earth-scientist or engineer may cull the information he needs fulfil the condition of transparency. some of these catalogues are fresh and pertinent, some out-ofdate or at second hand, some misleading. it is not satisfactory, therefore to acquire information from the historian or seismologist and use that information without understanding fully the basic principles on which the information has been obtained and what really means in terms of completeness and uncertainties. the user of catalogues must be aware of the quality of the data, of the uncertainties associated with them and of the completeness of the data. he should prefer data from reliable long-term datasets that give a far fuller understanding of earthquake hazard because they are based on human experience of earthquakes over a much greater segment of the geological time-scale. much of the information in such datasets comes from historical data and their use should aim to be indicative, to expose points for further analytical or field clarification rather than prescriptive, since in fact the prescriptions have to be based on rather arbitrary assumptions. it is not sufficient, therefore, merely to lay hand on a few historical earthquakes and use them to model seismicity. notes on the state-of-the-art and practice of long-term seismicity by nicholas n. ambraseys department of civil engineering, imperial college, london, u.k. mailing address: dr. nicholas n. ambraseys, department of civil engineering, imperial college, london sw7 2az, u.k.; e-mail: n.ambraseys@imperial.ac.uk 336 nicholas n. ambraseys for the early period mislocation of instrumental epicentres is the norm, while for earthquakes before the 1940s macroseismic locations are more reliable but they must be used with caution. for the later period location accuracy is not better than 30 km, but improves to about 5-10 km for more recent events. however, bearing in mind that an earthquake of ms, say between 6.5 and 7.5, will have ruptured faults 30 km to more than 100 km in length, the epicentre indicates nothing more than the general location of the event, and alone, without knowledge of the location of the causative fault, is of little use for design purposes. focal depths are more uncertain. intensity, which was devised two centuries ago, is a convenient means of conveying in a single rating of the scale a measure of the effects of the ground motion on man-made structures and on the ground itself. by definition it is a vague measure, either of the strength of ground shaking or of the weakness of manmade structures or a combination of the two. intensity tells us little about the nature and level of the ground motion. nevertheless the distribution of intensity may be used for the assessment of the magnitude of the associated event, for constraining the dimensions of the seismic source and for the identification of subcrustal events, particularly for earthquakes in the pre-instrumental period. for the purpose of assessing intensity and reducing subjectivity it is important to distinguish between damage caused by dynamic or inertia earthquake loading, and damage caused by secondary, quasi-static after-effects such as foundation spreading, liquefaction, slides, rockfalls and aftershocks. for instance i find that maximum intensity in any destructive historical or modern earthquake in rural areas in south-eastern europe and in the middle east appears to be effectively the same; that is, intensity «saturates» at viiviii msk at which all local type of constructions are destroyed or damaged beyond repair and any town or village would thus appear equally, but no more, devastated at so-called higher intensities. also landslides, spreading and liquefaction of the ground, are too much factors of other conditions besides the ground accelerations, and making appraisals of intensity on the basis of such ground effects, would be subjective and often misleading. in spite of the subtleties, which are involved in its definition and assessment, which requires engineering knowledge, intensity values estimated by seismologists are adopted by the engineer as a means to assess ground accelerations and velocities. modern textbooks and building codes feature tables and formulae for the conversion of intensity into ground acceleration, which is convenient but very unreliable. it is futile to seek a meaningful one-to-one relationship between intensity and any other single quantity which can be used for design purposes, a relationship that can best be described as a 21st century anachronism. the method of contouring intensity data cannot be separated from the method of information collection and intensity allocation. if the information is sufficient then the method of contouring should be allowed to work with modal intensity values. this procedure is particularly important when a few isolated high intensities exist within a background of many sites of much lower intensity, and conversely when isolated low intensities exist in the far-field within a background of «not felt». the use of low intensity radii that include the furthermost location from which the shock was reported, even by single observer, has a considerable bearing on the determination of the radius of perceptibility, which leads to grossly overestimated magnitude. homogeneously-defined isoseismals, may be used, however, to assign magnitudes to their causative earthquakes by calibrating sets of isoseismals against magnitudes, a method that gives stable results, but which must be applied to well-defined tectonic environments rather than to individual countries or global conditions. having assessed the magnitude of the earthquakes in terms of magnitude, one may use an appropriate attenuation model to estimate ground motions. 337 the state-of-the-art and practice of long-term seismicity the derivation of calibration functions between magnitudes and their corresponding sets of isoseismals for a specific tectonic environment needs internally consistent data on both; the isoseismals must be drawn on maps of intensity points assigned from examination of primary historical macroseismic information, and the magnitude data must be re-evaluated uniformly. as for the use of synthetic isoseismal maps for engineering purposes, this reminds me of leo pomerance who once said about the person who was searching under a street lamp for his house key, which he had dropped some distance away, but he searches there because there is more light. reliable magnitudes are essential for the derivation of recurrence relations, particularly at large magnitudes where recurrence curves steepen. they are also important for the scaling of ground motions and for placing constraints on the bounds of rupture zones. a cause for concern is the reliability of magnitudes reported in different parametric catalogues, particularly for events before the advent of the magnitude scale in the 1950s. no formal method can be devised to test the completeness of long-term data other than by testing their implications. formal statistical tests are as valid as the distributional assumptions on which they are based. since these assumptions are rarely likely to be always satisfied, such tests may best be regarded as indicators to the extent to which a particular conclusion would be supported, or not, by the data, if in fact the assumptions were justified, and hence, of the extent to which that conclusion is likely to remain valid despite departure from those assumptions. frequency-magnitude relations calculated with different magnitude scales differ. they also differ when calculated in terms of ms or moment magnitude mw. the reason for this is, that because of scaling, ms is not related linearly to log (m0), and smaller events contribute proportionally more moment than large ones. regarding ground motions, we know how to assess uncertainties in defining them, but we know also that peak ground acceleration is a poor index by which to express the damage potential of a ground motion. at present, and for a broad class of structures, displacement is probably the most widely used parameter to limit damage and also to quantify it in terms of design criteria. for the engineer, analysis of existing strongmotion recordings is the most common method used to estimate future ground shaking in terms of peak or spectral acceleration or displacement. this method must rely on good quality databases, uniformly processed records supported by reliable seismological and soil mechanics information and reliable associated data banks. there is at present a multitude of cds and internet sites that can provide strong motion data. whilst the availability of data via such mass media is extremely valuable, it must be recognised that use of this data cannot be made indiscriminately. and although there is a great need for data storage and dissemination on a european and world level, and that cd-roms or the internet can provide the uninitiated engineer with readily available strong-motion time-histories, the indiscriminate use of some of the data in existing cdroms or active internet-sites is likely to generate misleading results. the derivation of attenuation laws and site-specific design parameters must rely on good quality databases and reliable associated data banks rather than on statistics of many records of questionable quality. to the best of my knowledge there are at present world-wide 125 local or regional attenuation laws for peak ground accelerations, and 85 for response spectral ordinates, derived from the data available at the time, using different definitions for the variables involved and different procedures. uncertainties in the derivation of scaling laws depend on how well dependent variables are known. teleseismic locations are known to have larger uncertainties compared with those 338 nicholas n. ambraseys from local networks and like epicentres, focal depths based on teleseismic arrival times alone lack precision. the use of a unified magnitude scale in attenuation studies is also an important consideration. adoption of ms rather than ml stems from the fact that the former is not only the best estimator of the size of a crustal earthquake, but also because seismicity in europe is generally evaluated in terms of ms. while there can be no objection to modelling and calculating ground motions the best we can, but with so many uncertainties in the input data, whose accuracy for predictive purposes is little known, that there is a degree of precision beyond which refinement becomes pointless. moreover, a too sophisticated model carries with it the danger that its weaknesses and assumptions may not be appreciated. conversely, a too simple model may be discredited just because it exposes the underlying assumptions too clearly. i would prefer a simple model in which the number of variables is justified by the available data. over-parameterisation of the model alone is not recommended. recent studies show that seismic activity is both regional and long-time dependent, which renders particularly problematic the assessment of hazard from short-term observations. the true, long-term nature of the frequency-magnitude distribution is hampered not only because the 20th century record is too short but also because test areas may be too small to disclose the repeat time of large earthquakes as the shape of the frequency-magnitude distribution from short-term observations cannot be defined at large magnitudes. the implication is that large earthquakes in a test area are less frequent, when predicted from the long-term dataset than from the usual 100-year instrumental period, making the notion of recurrence time, in its usual definition questionable, and the characteristic model an artefact of incompleteness of data in space and time. incomplete data and clustered seismicity is the principal reason why statistics from short-term data alone cannot quickly answer the question of seismic hazard evaluation. considering that most major urban and industrial developments are spreading into areas of little-known seismicity, and that time rarely allows for the acquisition of adequate data, the engineer is likely to be forced on occasions to step across that hazy borderline of safety by accepting an element of risk over and above what would otherwise have been considered to be normal. to accept what is an acceptable risk, a certain amount of informed judgement, detailed technical evaluation of the structure and experience is needed, rather than results from a probabilistic treatment of short-term seismicity data. much statistical ingenuity has been spent on devising techniques for tackling this problem, but there are doubts about how useful and how well-founded some of these techniques really are, and the statistician here should take a background role. he can point to features in the data that look anomalous because they depart from some standard model, but whether the anomalies are to be ascribed to peculiarities of the model, or to peculiarities of the process by which the input data were recorded, is not a question the statistician should be asked to answer. it should be referred back to the seismologist, geophysicist or engineer. if an important effect is really present it should not take a statistician to bring it out. finally, to anyone who is really concerned with historical seismicity it is becoming increasingly apparent that the site of a damaging earthquake is a full-scale laboratory from which significant discoveries may be made, by historians, seismologists, geologists, engineers, sociologists or economists, not to mention politicians. as our knowledge of the complexity of earthquakes has increased, we become more and more aware of the limitations which nature has imposed in our capacity to model on purely theoretical bases. it is field observations and measurements that allow the interaction of ideas and the testing of theories. through the field study of earthquake effects on structures and on the ground itself, a unique opportunity exists to develop an understanding of the behaviour of man-made structures, when tested by nature. layout 6 1 annals of geophysics. 61, 5, se560, 2018; doi: 10.4401/ag-7837 “an analytic study of frictional effect on slip pulses ofearthquakes„ jeen-hwa wang1,* (1) institute of earth sciences, academia sinica, nangang, taipei, taiwan 1. introduction heaton [1990] first found the existence of slip pulses of earthquakes. let tr and td be the rise time of displacement at a site on a fault and the duration time of ruptures over the entire fault, respectively. he observed tr<<td and non-uniform tr over the fault plane. this exhibits the complexity of slip over fault plane during an earthquake. several groups of researchers also observed slip pulses from natural earthquakes [wald et al., 1991; wald and heaton, 1994; nakayama and takeo, 1997; chen et al., 2001; huang et al., 2001; nielson and madariaga, 2003; lee et al., 2007; galetzka et al., 2015]. heaton [1990] interpreted the generation of slip pulse-likes using a self-healing assumption based on a crack model in the presence of velocity-weakening (vw) friction. he only considered a small strip which occurs immediately after the rupture front. johnson [1990] discussed short tr by combining forward propagating waves and backward arresting (or healing) waves from the borders. but, day et al. [1998] claimed that it is not necessary to consider self-healing as a factor for earthquake dynamics. the slip pulses have been investigated based on the crack models in the presence of friction [andrews, 1976, 1985; day, 1982; papageorgiou and aki, 1983; boatwright, 1988; andrews and ben-zion, 1996; article history receveid june 26, 2018; accepted november 9, 2018. subject classification: slip pulse; rise time; duration time; 1-d spring-slider model; friction. abstract seismological observations show the existence of slip pulses with tr/td<0.3, where tr and td are, respectively, the rise time at a site on a fault and the duration time of ruptures over the fault. an analytical study of generation of a slip pulse is made based on the continuous form of 1-d spring-slider model, with uniform fault strengths, in the presence of linear, slip-weakening (sw) friction: f=1-u/δ (u=the displacement and δ=the characteristic distance) or linear, velocity-weakening (vw) friction: f=1-v/υ (v=the velocity and υ=the characteristic velocity). let ω0 and tr are, respectively, the predominant angular frequency of the system and the arrival time at a site and define γ=(1-1/δ)1/2 and σ=(1-1/4υ2)1/2. there are complementary solution (cs), and particular solution (ps) of the equation of motion. the cs shows a slip pulse under some ranges of model parameters for sw friction and for vw friction when υ>>1; while the cs shows a cracklike rupture for vw friction when υ is not too large. for sw friction, tr and tr/td decrease when the slip pulse propagates in advance along the fault and when ω0 and γ increase. tr and tr/td also depend on vr (rupture velocity) and increasing l (fault length). for the ps, tp/td is a good indication to show the existence of pulse-like oscillations at a site, because tp (the predominant period of oscillations at a site) is slightly longer than tr. results show the existence of a pulse-like oscillation at a site for the two types of friction. a pulse-like oscillation is generated when δ>1.6 for sw friction and when υ>0.6 for vw friction. tp/td decreases with increasing δ. for the two types of friction, to/td decreases when vr and l increase. beeler and tullis, 1996; cochard and madariaga, 1996; perrin et al., 1995; zheng and rice, 1998; nielsen et al., 2000; lapusta et al., 2000; ben-zion and huang, 2002; nielsen and madariaga, 2003; coker et al., 2005; rice et al., 2005; ampuero and ben-zion, 2008; urata et al., 2008; ando et al., 2010; garagash, 2012]. friction used by those authors includes vw friction, sw friction, velocityand state-dependent friction, and thermal-pressurized friction. results suggest that rupture modes are controlled by several factors, including friction laws, fault strengths, stress conditions on faults, energy and heat generated by faulting, scaling laws of faults, and spatial-temporal complexity of fault slip. in addition, some researchers considered geometrical heterogeneity of slip is a mechanism to stop earthquake rupture. beroza and mikumo [1996] suggested that the short tr could yielded by pre-existing stress with heterogeneous fault strengths. the slip pulses have also been studied by some authors [wu and chen, 1998; chen and wang, 2010; elbanna and heaton, 2012] based on the 1-d spring-slider model (abbreviated as the 1-d bk model hereafter) proposed by burridge and knopoff [1967]. from analytic studies by using sw friction, wu and chen [1998] claimed that sw friction can result in the self-healing slip pulse and the width of a pulse depends on vr and friction strength. from numerical studies by using vw friction chen and wang [2010] found the propagation of slip pulses with tr/td<0.1 along the model. their simulation results are in agreement with heaton’s observations. elbanna and heaton [2012] pointed out the differences between the bk model and crack models. according to linear elastic fracture mechanics, slip pulses are seldom generated in the continuum models because slipping region inside of a fault cannot release applied stress without continuous slip while rupture is extending. on the other hand, in the bk model slip pulses can be produced due to the following reason. each slider can completely release stress exerted by the leaf springs by going back to their equilibrium position even while rupture is extending. in other words, each slider does not transfer stress after their stoppage and information on the length of rupture does not feedback. laboratory experiments also provide significant information on generation of slip pulses. coker et al. [2005] observed the existence of both pulse-like and crack-like ruptures under certain conditions. lykotrafitis et al. [2006] observed the pulse-like shear ruptures with self-healing. lu et al. [2007] found that the rupture modes depend on the level of fault pre-stress and vw friction is important for earthquake dynamics. biegel et al. [2008] found that off-fault damage can affect the slip-pulse velocity. as mentioned previously, in order to generate slip pulses some authors prefer to vw friction, while other favor sw friction. in this study, i will investigate the frictional effects on the generation of slip pulses using the continuous form of the 1-d bk model with linear sw friction or linear vw friction. hence, it is significant to examine which friction (sw or vw friction) is more capable than the other for yielding slip pulses. 2. one-dimensional spring-slider model 2.1 model burridge and knopoff [1967] proposed the 1-d bk model (see figure 1), in which there are n sliders and springs. a slider with mass, m, is connected to its nearest two neighbors by a coil spring of stiffness, kc. of course, the two end sliders are only connected to the respective one nearest slider. a moving plate with a constant velocity, vp, pulls each slider through a leaf spring of stiffness, kl. each slider rests in its equilibrium state at time t=0. the position of i-th slider (i=1, …, n) is denoted by xi, which is measured from its initial equilibrium position, along the horizontal axis represented by the coordinate y. hence, xi is in a function of y and t. each slider is exerted by a frictional force between it and the moving plate. the frictional force is usually a function of displacement, xi, and particle velocity, vi (=dxi/dt), of the slider and shown by the function fi(xi;vi), which has a static frictional force of fsi=fi(xi;0) at rest. the equation of motion is m(∂2xi/∂t 2)=kc(xi+1-2xi+xi-1)-kl(xi-vpt)-fi(xi;vi) (1) in equation (1), there is an implicate parameter ‘a’ which is the space between two sliders in the equilibrium state. the ratio κ=kc/kl has been defined by wang [1995] to be the stiffness ratio of the system. this ratio represents the level of conservation of energy in the system. larger κ is equivalent to stronger coupling bejeen-hwa wang 2 figure 1. an n-degree-of-freedom dynamical spring-slider system. tween two sliders than between a slider and the moving plate, thus leading to a smaller loss of energy through the leaf spring or a higher level of conservation of energy in the system, yet opposite for smaller κ. since the fault system is dynamically coupling with dissipation, κ must be a non-zero finite value. the vp is in the order of ~10-12 m/s. the moving plate pulls a slider and thus gradually increases the elastic force, kcvpt, on it. when kcvpt is slightly higher than static frictional force, fsi, at the i-th slider, the two forces are cancelled out each other and can be ignored during ruptures. after a slider moves, fsi drops to fdi (i.e., the dynamic frictional force). 2.2 friction the frictional force between two contact planes is a very complicated physical process. laboratory experiments have exhibited time-dependent static frictional strength of rocks [dieterich, 1972] and velocity-dependent dynamic friction [dieterich, 1979; shimamoto, 1986]. dieterich [1979] and ruina [1983] proposed empirical, rateand state-dependent friction laws. the detailed description of friction laws and the debates concerning the laws and their application to earthquake dynamics can be found in some articles [e.g., marone, 1998; wang, 2009; bizzarri and cocco, 2006a; bizzari 2011]. several simple friction laws have been taken to theoretically and numerically study earthquake dynamics [see wang, 2016]. the laws are: the velocity-dependent, weakening-hardening friction law [burridge and knopoff, 1967]; the slip-dependent friction law [cao and aki, 1984/85]; the nonlinear vw friction law [carlson and langer, 1989a,b; carlson, 1991; carlson et al., 1991; and beeler et al., 2008]; and the piece-wise, linear velocity-weakening and weakening-hardening friction [wang, 1995, 1996]. purely velocity-dependent friction could yield unphysical phenomena and mathematically ill-posed problems as pointed out by madariaga and cochard [1994]. ohnaka [2003] stressed that the pure velocity-dependent friction law is not a one-valued function of velocity. the problem has been deeply discussed by bizzarri [2011]. nevertheless, for a purpose of comparison the single-valued linear velocity-dependent friction law is still considered below. friction is an important factor in controlling earthquake dynamics. based on the 1-d bk model in the presence of linear vw friction with a decreasing rate, rw, of friction force with velocity, wang [1996] found three types of rupture propagation: (1) subsonic type with rw>2(klm) 1/2; (2) sonic type with rw=2(klm) 1/2; and (3) supersonic type with rw<2(klm) 1/2. supersonic-type ruptures are non-causal, because vr is greater than the sound speed. knopoff et al. [1992] stated that the system is asymptotic to dispersive-free elasticity in the continuum limit when rw=2(klm) 1/2. they also found that large rw is more capable of generating large events than small rw. carlson and langer [1989a,b] used f(v)=1/(v+vc) where vc is the characteristic velocity. the related decreasing rate is 1/vc(1+v/vc) 2 with the values in the range of from 1 to 0 when vc varies from 0 to ∞. hence, their friction law basically exhibits supersonic behavior with rw<2(klm) 1/2, and thus is potentially capable of producing very large events. wang [1997] also stressed the effect of frictional healing on earthquake ruptures. several authors [nur, 1978; carlson and langer, 1989a,b; carlson, 1991; carlson et al., 1991; knopoff et al.,1992; rice, 1993; wang and hwang, 2001] stressed the influence on earthquake ruptures due to heterogeneous fault strengths on the fault. carlson and her co-workers emphasized that de-localized events can be generated when the friction strengths over the fault plane is uniform. in this study, i will analytically study the frictional effects caused by vw friction or sw friction on the generation of slip pulses by using the 1-d bk model. in order to perform analytic manipulation, only the linear laws are taken into account. the sw friction law (see figure 2a) is: f(x)=fo(1x/xc) (2) where x and xc are, respectively, the displacement and the characteristic distance. the vw friction law (see figure 2b) is: f(v)=fo(1v/vc) (3) where v=dx/dt is the velocity and vc is the characteristic velocity. the breaking strengths are uniform over the model. this means that only steady travelling waves are taken into account. 3 an analytic study of slip pulses figure 2. (a) for linear, slip-weakening friction law: f(x)=1x/xc (xc=characteristic displacement) and (b) for linear, velocity-weakening friction law: f(v)=1-v/vc (vc =characteristic velocity). jeen-hwa wang 4 4. analytical manipulation 4.1 equation of motion define xi=xi-vpt. this gives xi=xi+vpt and vi=dxi/dt=dxi/dt+vp=vi+vp. hence, equation (1) becomes m(∂2xi /∂t 2)=kc(xi +1-2xi +xi -1)-klxi -fi(xi +vpt;vi +vp) (4) after a slider moves, vpt and vp can be neglected because of vpt<<xi and vp<<vi during ruptures. this makes equation (4) be m(∂2xi /∂t 2)=kc(xi +1-2xi +xi -1)-klxi -fi(xi; vi) (5) analytic manipulation will be performed based on the continuous form of equation (5). the details how to set up the continuous form can be found in carlson and langer [1989a,b] and wang [2016], and only a brief description is given below. equation (5) is first normalized and then transformed to its continuous form. in order to normalize the equation, some normalization parameters must be defined below. the natural angular frequency of oscillation of a single slider attached to a leaf spring in the absence of friction in equation (1) is denoted by ω0=(kl/m) 1/2, and thus the related natural period is t0=2π/ω0. define τ=ωot as the normalized time. define d0=f0/kl as the characteristic slip distance of a slider exerted by a force f0 through a spring with stiffness of kl. since the ratio d0/vp is the loading time for a leaf spring to stretch enough for overcoming f0, vp/d0ω0 is the ratio of the slipping time ω0 -1(=t0/2π) to the loading time. fi(xi;vi) is represented by f0φ(xi;vi). substituting ui=xi/d0, vi=dui/dt=(dui/dt)/d0, and related normalized (dimensionless) parameters [wang, 2016] into equation (5) and performing some mathematical manipulation lead to ∂2u/∂τ2=h2(∂2u/∂ξ2)-u-φ(u;v) (6) in equation (6), h2, u, and v are, respectively, the limiting values of κa2, ui, and vi when ‘a’ approaches zero. meanwhile, ξ is the normalized coordinate of y-axis which is the horizontal axis coordinate along the model, i.e., ξ=y/d0. wang [2016] discussed in details the trial solution of equation (6), i.e., u=-φ(u;v)≈constant, when ∂2u/∂τ2=0 and ∂2u/∂ξ2=0. 4.2 solutions based on slip-weakening friction the normalized sw friction law from equation (2) is φ(u)=1-u/δ where δ=xc/d0 is the dimensionless characteristic distance. equation (6) becomes ∂2u/∂τ2=h2(∂2u/dξ2)-u-(1-u/δ) (7) since φ(u) is negative and unreasonable when u>δ, equation (7) can work only for u<δ. when the driving force reaches the static strength of the friction whose value is unit in equation (7), stability at the slider is determined by the competition between the rate of friction |∂fstrength/∂u|=1/δ and the rate of stress-relaxation between the slider and the leap spring |∂fstress/∂u|=|∂[h2(∂2u/dξ2)-u]/∂u|=|h2∂(∂2u/dξ2)/∂u-1| at u=0. with the condition 1-1/δ>|∂[h2(∂2u/dξ2)-u]/∂u-1|, we have |∂fstrength/∂u|> |∂fstress/∂u|. this means that stable motions cannot exist, and is in contrast with the known source time function for dynamic ruptures where an initial acceleration phase should exist. this again makes equation (7) work only when u<δ. under sw regime, equation (7) means that max{xp}=d0/γ 2=xc/(δ-1). this means δ>1. to solve equation (7), the laplace transformation (lt, denoted by l), which can be seen in numerous textbooks [e.g., papoulis, 1962], is used to transform it to a different form. the lt of equation (7) is h2∂2u/∂ξ2+(s2+1-δ)u=1/s (8) the solution of u includes the complementary solution, uc, and particular solution, up, that is, u=uc+up. according to the method given in johnson and kiokemeister [1968], the solution of equation (8) is u(ξ,s)=c1e -ψξ/h+c2e ψξ/h-1/sψ2 (9) where ψ=(s2+1-δ-1)1/2. there are two types of waves from equation (9): the first one is the travelling wave represented by the first term along the +ξ direction and the second one along the -ξ direction in its right-handed-side, i.e., uc(ξ,s)=c1e -ψξ/h+c2e ψξ/h. the second one is the oscillation at a site given by the third term, i.e., up(ξ,s)=-1/sψ 2. the second term of the first type with ξ<0 can be rewritten as e-ψ|ξ|/h. the inverse laplace transformation (ilt, denoted by l1) of uc(ξ,s) with |ξ|/h>0 is uc(ξ,τ)=c{1-γ(|ξ|/h)j1[γ(τ 2-(|ξ|/h)2)1/2]/ (10) [τ2-(|ξ|/h)2]1/2}h(τ-|ξ|/h) where c=c1 or c2, γ=(1-δ -1)1/2, j1[…] is the first-order bessel function, and h(τ-|ξ|/h) is the unit step function (h(z)=0 as z<0 and h(z)=1 as z≥0) representing a travelling plane wave. since τ=ω0t and ξ=y/d0 are, respectively, the normalized time and normalized rupture distance, h=vr/d0ω0 is the normalized rupture velocity. when the rupture propagates from 0 to ξl, which is the normalized rupture length and equal to l/d0 (l=the rupture length), the normalized duration time is τd=ξl/h, and thus the duration time is td=τd/ω0=ξl/ω0h, thus giving td=ξl/ω0h=(l/d0)/ω0(vr/d0ω0)=l/vr. let tr be the arrival time of the travelling wave at a site y, that is, tr=|y|/vr. substituting uc(ξ,τ)=xc(y,t)/d0, ξ=y/d0, τ=ω0t, and tr=|y|/vr into equation (10) gives xc(y,t)=cd0{1-(γω0tr)[γω0(t 2-tr 2)1/2]]/ (11) (t2-tr 2)1/2}h[ω0(t-tr)] equation (11) shows a propagating wave which is usually represented by a function of the form g(t-|y|/vr), where t’=t-|y|/vr is known as the retarded time for situations where causality holds [e.g., perrin et al., 1995; nielsen et al., 2000]. the rise time, tr, is measured from t’=0 or t=tr=|y|/v to larger t’=t* when the wave amplitude and the particle velocity reach their respective peak values. the quantities inside {…} multiplied by cd0 of equation (11) show the wave amplitude. in order to further understand the properties of equation (11), we only need to examine the function ss(y,t) =1-(γω0tr)j1[γω0(t 2-tr 2)1/2]/(t2-tr 2)1/2. define θ to be γω0(t 2-tr 2)1/2, and thus ss(y,t) can be represented by 1(γ2ω0tr)j1(θ)/θ. when t=tr, θ=0 and j1(θ)=0. this makes the value of j1(θ)/θ be indefinite. from the l’hospital theorem [see johnson and kiokemeister, 1968], we have lim θ→0[j1(θ)/θ]=limθ→0 [dj1(θ)/dθ]=limθ→0[j0(θ)-j1(θ)/θ]. according to the recurrence relation of bessel functions [abramowitz and stegun, 1972]: j1(θ)/θ=[j0(θ)-j2(θ)]/2, we have lim θ→0[j1(θ)/θ]=1/2 because of j0(θ)=1 and j2θ)=0 when θ=0. hence, xc(y,0)=cd0/2 and c=2xc(y,0)/d0. the value of constant c depends on the initial value of xc(y,0). the waveform xc(y,0) appears before the main rupture and can behaves like the nucleation phase. from the observations, they are very small [beroza and ellsworth, 1996; ellsworth and beroza, 1995,1998; mori and kanamori, 1996; wang, 2017]. this suggests xc(y,0)=cd0/2<<1, and thus c could be small. the firstorder bessel function j1(θ) is positive, but vibrates and decreases with increasing θ, and (t2-tr 2)-1/2 or θ-1 also decays fast with time when t>tr. from tr=t*-tr, t* is the time when ss(y,t) reaches its peak value. this time is commonly calculated from the necessary condition, i.e., dss(y,t)/dt=0. this can be obtained by taking dss(y,t)/dt=[dss(θ)/dθ]dθ/dt=0. since dθ/dt=γω0(t 2-tr 2)-1/2 cannot be zero when t>tr, we only need dss(θ)/dθ=0 or d[j1(θ)/θ]/dθ=j0(θ)-j1(θ)/θ=0. the condition of dss(θ)/dθ=0 at θ*=γω0(t* 2-tr 2)-1/2 leads to the following equality: j0(θ*)-j1(θ*)/θ*=0. actually, numerous values of θ* can make this equality hold. among them, the first value is 1.975 [abramowitz and stegun, 1972], thus giving γω0(t* 2-tr 2)1/2=1.975 and t*=(tr 2+3.901/γω0) 1/2. this makes tr be t*-tr=tr{[1+ 3.901/(γω0tr) 2]1/2-1}. figure 4 demonstrates the plots of tr versus tr for various values of γ, and ω0: (a) for γ=0.1, 0.25, 0.5, and 1.0 (from top to bottom) when ω0=1 hz; and (b) for ω0=0.1, 0.2, 0.5, 1.0, and 2.0 hz (from top to bottom) when γ=1. obviously, tr decreases rapidly with increasing tr, thus meaning that tr decreases when the slip pulse propagates along the fault. it is interesting to investigate the variation in ss(y,t*) with y=vrtr. inserting t* into ss(y,t) leads to ss(y,t*)=1γω0trj 1[γω0(t* 2-tr 2)1/2]/(t*2-tr 2)1/2. hence, ss(y,t*)=1γω0trj 1{γω0[tr 2+3.901(γω0) -2-tr 2]1/2}/[tr 2+3.901(γω0) -2 -tr 2]1/2| due to t*=(tr 2+3.901/γ2ω0 2)1/2 as mentioned above. this gives ss(y,t*)=1-(γω0) 2(|y|/vr) j1(1.975)/1.975, where 3.9011/2=1.975, and thus ss(y,t*)=1-0.294(γω0) 2(|y|/vr) because of j1(1.975)= 0.5782 [see abramowitz and stegun, 1972]. when ss(y,t*)=0, we have 0.293(γω0)2(|y|/vr)=1. this gives |y|=yc=3.413vr/(γω0) 2 and tc=yc/vr=3.413/(γω0) 2, where tc is tr related to yc. clearly, the value of yc (>0) is a function of vr, γ, and ω0. hence, ss(y,t*) changes from positive (when |y|<yc or tr<tc) to negative (when |y|>yc or tr >tc) by passing through zero (when |y|=yc or tr =tc). figure 3a displays an example of a normalized waveform ss(y,t)/ssmax, where ssmax is the value of ss(y,t) at t* as defined before in the figure, of the rupture wave having a propagation velocity of vr=2 km/sec and the predominant angular frequency of ω0=1 hz at a position on the y-axis when the wave propagates with a travelling time tr=3.607 sec under the action of slipweakening friction force with γ=0.9. this plot shows a 5 an analytic study of slip pulses figure 3. figure shows ss(y,t)/ssmax (ssmax=ss(y,t*)) and ss(y,t)/ssvmax (ssvmax=sv(y,t*)) at y=vrtr with vr=2 km/sec and tr=3.607 sec: (a) for ss(yy,t)/ssmax with γ=0.9 and ω0=1.0 hz; and (b) for sv(y,t)/ssvmax with σ=0.9 and ω0=1.0 hz. pulse-like wave with short tr. since the value of tc of this case is 4.216 sec, the related value of t*, which is not displayed in figure 3a, is positive as expected because of tr=3.607 sec<tc. it is necessary to consider a quantitative criterion to confirm the existence of a slip pulse. a simple way is taken based on the value of tr/td, which is tr/td=(vrtr /l){[1+3.901/(γω0tr) 2]1/2-1} (12) where td=l/vr. in equation (12), vrtr (denoted by lr) is the length along which the rupture has propagated on the fault. obviously, tr/td increases with lr/l and decreases with increasing γ, or ω0, or tr. from the measures made by heaton [1990], tr/td is almost between 0.1 and 0.3. chen and wang [2010] gave tr/td<0.1 from numerical simulations. here, i assume that an acceptable quantitative criterion is tr/td<0.3. since, lr/l is always less than 1 during earthquake ruptures, the request of tr/td<0.3 will be mainly dependent on γ, ω0, and tr. nevertheless, it is easier to generate a slip pulse for long l than for short l. the inequality tr/td<0.3 can be represented by [1+3.901/(γω0tr) 2]1/2<1.3. when the rupture proceeds, tr always increases and makes the inequality hold. hence, tr is less important on the inequality especially for tr>1 sec, which appears very soon after an earthquake starts to rupture. since the slip pulses have frequencies in a small range of 0.5 to 2 hz, ω0 is not the main factor. since the value of γ varies in a large range from 0+ to 1, γ must be the main factor in controlling the slip pulse. considering an example of a slip pulse with ω0=1 hz and γ=1, we have (1+3.901/tr 2)1/2<1.3. when tr >2.378 sec, the inequality tr/td<0.3 always holds and thus the slip pulse appears. figure 6 displays the plots of tr versus γ for ω0=0.1, 0.2, 0.5, 1.0, and 2.0 hz (from top to bottom). the area between the curve and vertical line denotes the solution regime. clearly, tr decreases with increasing ω0. the example shown in figure 3a can meet the condition. figure 5 displays the plots of tr/td versus tr with vr=2 km/sec for various values of γ, ω0, and l: (a) for γ=0.1, 0.25, 0.5, and 1.0 (from top to bottom) when l=50 km and ω0=1 hz; (b) for ω0=0.1, 0.2, 0.5, 1.0, and 2.0 hz (from top to bottom) when l=50 km and γ=1; and (c) for l=10, 30, 50, 70, 100 km (from top to bottom) when γ=1 and ω0=1 hz. obviously, tr/td decreases very rapidly with increasing tr, thus meaning that tr decreases when the slip pulse propagates along the fault. meanwhile, tr also decreases when γ, ω0, and l increase. the third term of equation (9), i.e., up(ξ,s)=-1/s(s 2+1δ-1), is not a function of locality and thus only represents the oscillations at all sites on the fault. but, it is still significant to examine if it behaves like a pulse-like oscillation or not. its ilt is up(ξ,τ)=l 1[up(ξ,s)]=[1-cos(γτ)]/2γ. the solution exists when δ>1. for δ>1, we have up(ξ,τ)=[1-cos(γτ)]/2γ=sin 2 (γτ/2)/γ2 (13) the particular solution up(ξ,τ) comes from the ilt of -1/s(s2+1-δ-1). equation (8) leads to h2∂2u/∂ξ2=-(s2+1δ-1)u+1/s, in which -(s2+1-δ-1)u behaves like the lt of an elastic force and 1/s is the lt of 1 (see equation (8)) normalized from the static friction force, f0. when h2∂2u/∂ξ2=0, i.e., the divergence of u along the ξ-axis is zero, we have (s2+1-δ-1)u+1/s. this indicates a baljeen-hwa wang 6 figure 4. the plots of tr versus tr for various values of γ and ω0: (a) for γ=0.1, 0.25, 0.5, 1.0, and 2.0 (from bottom to top) when ω0=1 hz; and (c) for ω0=0.1, 0.2, 0.5, 1.0, and 2.0 hz (from bottom to top) when γ=1. figure 5. the plots of tr/td versus tr for various values of γ, ω0, and l: (a) for γ=0.1, 0.25, 0.5, and 1.0 (from top to bottom) when l=50 km and ω0=1 hz; (b) for ω0=0.1, 0.2, 0.5, 1.0, and 2.0 hz (from top to bottom) when l=50 km and γ=1; and (c) for l=10, 30, 50, 70 km. ance between the elastic force and the static friction force and its ilt shows a vibration on the fault. the two forces behave like pre-existing forces on the fault plane. since the two forces are of localityindependence, the related particular solution is also of locality-independence. inserting equation (13) into equation (7) results in ∂2u/∂τ2=-(1-δ-1)up-1. since the initial value of up is zero, we have ∂ 2u/∂τ2=-1, thus implying a negative initial acceleration at a slider. there are two cases in equation (13): one with 1-δ-1<0 or δ<1 and the other with 1-δ-1>0 or δ>1. the first case is sin[(1-δ-1)1/2τ/2]= i sinh[(δ-1-1)1/2τ/2], and thus sin2[(1-δ-1)1/2τ/2]=-sinh2[(δ-1-1)1/2τ/2]. this solution exhibits a negative hyperbolic sine-type function, which cannot be a common wave, and can be ignored. the second case represents a positive sine-type function and thus can represent a vibration at all sites because it is site-independent from equation (13). replacing up(ξ,τ) by xp(y,t) and τ by ω0τ into equation (13) leads to xp(x,t)=d0sin 2(γω0t/2)/γ 2 (14) define ω*=γω0/2=(1-δ -1)1/2ω0/2 to be the predominant angular frequency of the oscillation, and thus tp=2π/ω* is its predominant period. this gives t0/tp= 2/(1-δ-1)1/2. the plot of t0/tp versus δ is shown in figure 7a. t0/tp increases with δ. the increasing rate of t0/tp with δ is high when δ<2 and becomes low when δ>2. figure 8 shows the typical velocity waveform (in the solid plus dotted curve) and the typical displacement waveform (in the solid curve). since the wave propagates from -y to +y, it stops at the time instant when v=0. in figure 8, tp is the period of velocity waveform. since tr≈tp/2, the ratio of tp over td, i.e., tp/td, is a good indication to show whether a slip pulse can exist or not. the ratio tp/td is 2t0(1-δ -1)-1/2/(l/vr)=2(1δ-1)-1/2vrt0/l, where vrt0 denotes a characteristic distance of wave propagation in the natural period. for real m≥6 earthquakes vr varies from 1.5 to 4 km/s, l from 30 to 300 km, and t0 from 10 -1 to 10 sec. figure 9 shows the plots of tp/td versus δ: (a) for vrt0=0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 km (from bottom to top) when ω0l=80 km/s; and (b) for l=20, 40, 60, 80, 100, 120, 140, and 160 km (from top to bottom) when vrt0=2 km. as mentioned above, the quantitative criterion to confirm the existence of a slip pulse is tr/td<0.3. now, tr is almost equal to tp. hence, a pulse-like oscillation can exist when tp/td<0.3. in figure 9, the upper bound of tp/td is 0.3. meanwhile, the solution 7 an analytic study of slip pulses figure 6. the plots of tr versus γ for ω0=0.1, 0.2, 0.5, 1.0 and 2.0 hz (from top to bottom) based on sw friction. figure 7. (a) the plots of t0/tp versus δ; and (b) the plots of t0/tp versus υ for sw friction. the vertical line shows the value of δ or υ at which the curve becomes flat. figure 8. the solid plus dotted curve represents the velocity waveform and the solid curve denotes the displacement waveform. tp and tr are, respectively, the predominant period of the velocity waveform and the rise time of displacement waveform. exists only when δ>2 (as displayed by a vertical line in the figure). obviously, the major portion of each curve is below the upper bound, thus suggesting the existence of a pulse-like oscillation in the solution regime. 4.3 solutions based on velocity-weakening friction the normalized vw friction law from equation (3) is φ(v)=1-v/υ where υ=vc/d0ω0 is the dimensionless characteristic velocity. equation (6) becomes ∂2u/∂τ2=h2(∂2u/∂ξ2)-u-(1-v/υ) (15) when v>υ, φ(v) is negative and unreasonable. hence, equation (15) works only for v<υ. the lt of equation (15) is s2u(ξ,s)=h2∂2u(ξ,s)/∂ξ2-u(ξ,s)-1/s+su(ξ,s)/υ (16) this gives h2∂2u/∂ξ2-(s2+1)-sυ-1)u=1/s (17) let ζ=(s2+1-sυ-1)1/2. the general solution of equation (17) is u(ξ,s)=c1e -ζξ/h+c2e -ζξ/h-1/sζ2 (18) there are two types of waves from equation (18): the first one is the travelling waves represented by the first term (travelling along the positive direction along the ξaxis) and second term (travelling along the negative direction along the ξ-axis) in its right-handed-side, i.e., uc(ξ,s)=c1e -ζξ/h+c2e -ζξ/h, and the second one is the oscillation at site shown by the third term, i.e., up(ξ,s)= -1/sζ2. the second term of the first type of waves with ξ<0 can be re-written as e-ζ|ξ|/h. the ilt of uc(ξ,s), i.e., l1[uc(ξ,s)] with |ξ|/h>0 is uc(ξ,τ)=c{1-(σ|ξ|/h)eτ /2υj1[σ(τ 2-(|ξ|/h)2)1/2]/ (19) (τ2-(|ξ|/h)2)1/2}h(τ-|ξ|/h) where σ=(1-1/4υ2)1/2. because of σ>0, 1-1/4υ2 must be larger than 0 and thus υ>1/2. equation (19) exhibits a travelling plane wave. the quantities inside {...} before h(τ-|ξ|/h) denote the wave amplitude. unlike equation (9) for sw friction, this equation has an extra term eτ/2υ, which increases exponentially with time. when the wave propagates from 0 to ξl, the normalized duration time is τd=ξl/h and thus the duration time is td=τd/ω0=ξl/ω0h=(l/d0)/ω0h. let tr be the arrival time when the rupture arrives at a site y, that is, tr=|y|/vr. substituting uc(ξ,τ)=xc(y,t)/d0, ξ=y/d0, τ=ω0t, tr=|y|/vr, and ϖ=ω0/2υ into equation (19) leads to xc(x,t)=cdo{1-(σωotr)e ϖtj1[σωo(t 2-tr 2)1/2]/ (20) (t2-tr 2)1/2}h[ωo(t-tr)] when ϖ<<1 hz, eϖt≈1 makes equation (20) be similar to equation (9). the results are similar to those for sw friction as mentioned above. this condition is equivalent to ω0<<1 hz or υ>>1. since the wave with ω0<<1 hz is a very long wave, it cannot be a slip pulse. when ϖ is not a small value, we examine the properties of xc(x,t) by using the function sv(y,t)=1(σω0tr)e ϖtj1[σω0(t 2-tr 2)1/2]/(t2-tr 2)1/2. define θ to be σω0(t 2-tr 2)1/2. sv(y,t) can be represented by 1(σ2ω0 2tr)e ϖtj1(θ)/θ. when t=tr, θ=0 and j1(θ)=0. this leads to an indefinite value of j1(θ)/θ. like the abovementioned mathematical manipulation for sw friction, we have lim θ→0[j1(θ)/θ]=1/2. hence, xc(y,0)=cd0/2 and c=2xc(y,0)/d0. as mentioned above, the value of constant c depends on the initial value of xc(y,0) and thus c must be in general small. the function j1(θ) is positive, but vibrates and decreases with increasing θ, and (t2-tr 2)-1/2 or θ-1 also decays fast with time when t>tr. the value of tr of a slip pulse at a site is measured from the arrival time, tr, at a site to the time, t*, when the amplitude of slip pulse reaches its peak value. this gives tr=t*-tr. to obtain t*, we must calculate the time when sv(y,t) reaches its peak value from the necessary condition: dsv(y,t)/dt=0. this can be obtained by taking dsv(y,t)/dt=[dsv(θ)/dθ]dθ/dt=0. since dθ/dt=t(t 2-tr 2)-1/2 cannot be zero when t>0, we only need to consider if dss(θ)/dθ equals zero. mathematical manipulation leads to dsv(θ)/dθ=-(σ 2ω0 2tr)e ϖt{ϖj1(θ)-[j0(θ)-j1(θ)/θ]}. from jeen-hwa wang 8 figure 9. the plots of tp/td versus δ: (a) for vrt0=0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 km (from bottom to top) when ω0l=80 km/s; and (b) for l=20, 40, 60, 80, 100, 120, 140, and 160 km (from top to bottom) when vrt0=2 km. the rule of bessel functions that not any two bessel functions have the same zero, the function ϖj1(θ)-[j0(θ)j1(θ)/θ] cannot be zero. when j1(θ)=0 at θ=θ*, j0(θ*) is not zero. this results in ϖj1(θ*)-[j0(θ*)-j1(θ*)/θ*]= -j0(θ*)≠0. when j0(θ)=0 at θ=θ*, j1(θ*) is not zero. this results in ϖj1(θ*)-[j0(θ*)-j1(θ*)/θ*]=(ϖ+1/θ*)j1(θ*)≠0. meanwhile, eϖt is equal to 1 at t=0 and larger than 1 when t>0. hence, dsv(θ)/dθ cannot be zero and there is not an extremum for sv(y,t). this cannot yield tr/td<0.3 and thus slip pulses do not exist for vw friction. unlike ss(y,t*), sv(y,t) does change from positive to negative passing through zero. an example of a normalized waveform sv(y,t)/svmax, where svmax is the maximum value of sv in the figure, of the rupture wave having a propagation velocity of vr=2 km/sec and the predominant angular frequency of ω0=1 hz at a position on the y-axis when the wave propagates with a travelling time tr=3.607 sec under the action of velocity-weakening friction force with σ=0.9. this plot which shows an increase in the amplitude with time, thus suggesting a crack-like rupture with long tr. hence, sv friction can yield pulse-like ruptures only when υ>>1 or vc>>1 and in general produces crack-like ruptures. the third term of equation (18), i.e., up(ξ,s)= -1/s(s2+1-sυ-1), is not a function of locality and thus only represents the oscillations at all sites on the fault. but, it is still significant to examine if it behaves like a pulse-like oscillation or not. the function up(ξ,s) can be re-written as -1/s-[1-(1-4υ2)1/2]/2(1-4υ2)1/2(s-b)[1+(1-υ2)1/2]/2(1-υ2)1/2(s-c), where b=(1/2υ)[1+(14υ2)1/2], and c=(1/2υ)[1-(1-υ2)1/2]. the solution does not exist when 1-4υ2=0 or υ=0.5. when 1-4υ2≠0 or υ≠0.5, l-1[up(ξ,s)] is up(ξ,τ)=—{1+(e τ/2v/e)[eτ/2v-eτ/2v+eτ/2v)/2] (21) where e=(1-4υ2)1/2. for uc(ξ,τ),υ must be larger than 0.5 as mentioned above. let q=(4υ2-1)1/2=ie. consider a right triangle with three sides: the longest side with a length of r=(12+q2)1/2=2υ, and the other two with lengths of q and 1, respectively. the angle between the longest side and the side with l=1 unit is set to be θ. hence, we have cos(θ)=1/2υ, sin(θ)=q/2υ, and tan(θ)=q. the tangent term gives θ=tan-1(q). define sin(qτ/2υ)=(eiqτ/2υ-eiqτ/2υ)/2i and cos(qτ/2υ)=(eiqτ/2υ+ e-iqτ/2υ)/2. replacing up(ξ,τ) by xp(y,t)/d0, ξ by y/d0, τ by ω0t, and ϖ=ω0/2υ into equation (21) leads to xp(y,t)=-do[1+e ϖtsin(qϖt-θ)/sin(θ)] (22) obviously, the initial value of xp(y,t) is 0 when t=0. this xp(y,t) shows a sine-function-type oscillation at a site. unlike equation (14), this equation has an extra term eϖt, which increases with time. when t≤0, xp(y,t) is zero and thus equation (22) becomes xp(y,t)h(t). define ω*=qϖ=(1-1/4υ2)1/2ω0 and tp=2π/ω* to be, respectively, the predominant angular frequency and the predominant period of the oscillation. from equation (22), the particle velocity is dxp(y,t)/dt=-doϖe ϖtsin(qϖt)/cos(θ)sin(θ)] and the particle acceleration is d2xp(y,t)/dt 2=-d0ϖ 2eϖt,sin(qϖt,+θ)/ cos2(θ)sin(θ)]. when qϖt+θ=π, we have d2xp(y,t)/dt 2=0. hence, max{dxp(y,t)/dt}=d0ϖe (π-t)/q. this holds before the first zero-crossing of dxp/dt. hence, the condition of d0ϖe (π-t)/q<vc, which gives υ=1.744, is required. the plot of to/tp versus υ is displayed in figure 7b. clearly, increasing rate of to/tp with υ is high when υ<1.744 and becomes low when υ>1.744. although tp is determined by t0 (0.1<t0<10) here, the value of t0 for natural faults is not exactly determined. t0 is defined by the stiffness of a leaf spring and the mass of the slider. as widely known, natural fault systems are usually selfsimilar, so that the unique characteristic period of faulting is td. even if natural faults show pulse-like behaviour, their own tp is not necessarily related to t0. elbanna and heaton [2012] claimed that simultaneous movement of only 10 to 20 sliders in a spring-slider system consisting of 5000 sliders lead to a slip pulse. from figure 8, tp/td=(1-1/4υ 2)-1/2vrt0/l is a good indication of the presence of a slip pulse for vw friction. figure 10 shows the plots of tp/td versus υ: (a) for vrt0=0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 km (from bottom to top) when l=80 km; and (b) for l=20, 40, 60, 80, 100, 120, 160, and 180 km (from top to bottom) when vrt0=2.0 km. as mentioned above, the quantitative criterion to confirm the existence of a slip pulse is tr/td<0.3. now, tr is almost equal to tp. the upper bound of tp/td in the figure is 0.3. meanwhile, the solution exists only when υ>1.744 (as displayed by a vertical line in the figure). obviously, the major portion of each curve is below the upper bound, thus suggesting the existence of a pulse-like oscillation in the solution regime. 5. discussion the theoretical analyses for the conditions of generating slip pulses are made based on the continuous form of the 1-d bk model in the presence of the linear friction laws: f(u)=1-u/δ for sw friction as well as f(v)=1v/υ for vw friction. the parameters δ and υ are, respectively, the characteristic distance and the characteristic velocity of the respective law. first, it is neces9 an analytic study of slip pulses sary to consider physical implications of the results. for sw friction, variable transformation of w=u+γ2 reduces equation (7) to the klein-gordon equation [polyanin, 2002]: ∂2w/∂τ2=h2(∂2w/∂ξ2)-γ2w. this implies that the strength of the leaf spring has a factor of γ2 due to presence of friction, so that γ2 per stored energy within the leaf spring is conserved and 1-γ2 per the energy is dissipated. we can see that 1-γ2=d0/xc is a ratio of actual frictional work dissipated due to sliding from an area under sw line in figure 2a. since the stiffness of a leaf spring multiplied by γ-2 controls energetics, dependence of behavior of the system on k/γ2 should be discussed. for vw friction, the restoring force should always have the same sign for stability of the system. variable transformation of w=u+1 reduces equation (15) to ∂2w/∂τ2=h2(∂2w/∂ξ2)-w+w’/υ, where w’=dw/dτ. without loss of generality, the restoring force can be assumed as negative, i.e., -w+w’/υ≤0. let f[w] be the fourier transformation of w. then the following holds: so that should be required, otherwise the friction is not restoring force but repulsive force in high-frequency content. such a compact supported spectrum in frequency domain is, however, not able to be a compact supported signal in time domain. there are the complementary solution, xc(y,t) and particular solution, xp(y,t), of the equation of motion. for sw friction, the function xc(y,t) shows a slip pulse when 1>γ=(1-δ-1)1/2>0. tr and tr/td both decrease rapidly with increasing tr, thus meaning that tr and tr/td are both reduced when the slip pulse propagates along the fault. this means that the slip pulse is generated when the ruptures are far away from the nucleation point. meanwhile, tr and tr/td also both decrease when γ, ω0, and l increase and when vr is reduced. higher γ associated with a low decreasing rate of friction with slip is easier to produce a slip pulse than lower γ related to a higher decreasing rate. a decreases in tr and tr/td with increasing ωo means that slip pulses should have higher angular frequencies and cannot be long-period rupture waves. the decreases in tr and tr/td with increasing l mean that a slip pulse can be more easily produced in a long fault than in a short one. hence, it would be difficult to detect a slip pulse on a short fault. equation (21) shows that lower vr can more easily result in small tr and tr/td than higher vr. this indicates that the slip pulse has a relatively slow propagation speed. however, vr is less important than other parameters, because the value of vrtr/l in equation (21) is always smaller than 1. for vw friction when υ>>1; while it shows a cracklike rupture for vw friction when υ is not too big. the present results suggest that unlike heaton [1990], sw friction is easier to produce a slip pulse than vw friction. although perrin et al. [1995] assumed that not all friction laws result in steady travelling pulses, the present result exhibits that the two types of friction in use can generate a slip pulse under their respective ranges of model parameters. for the particular solution, the ratio of tp over td of a rupture is a good indication to show the possible existence of a slip pulse, because the tr of slip at a site is shorter than tp. for sw friction with δ>1, the solution clearly shows oscillations with a predominant period of tp. computational results in figure 9 reveal a decrease in tp/td with increasing δ. longer (shorter) δ means a slower (faster) decay of friction with slip. a slower decay of friction with increasing slip is easier to generating a pulse-like oscillation than a faster decay of friction. based on the definition of δ=xc>d0, δ>1 gives xc>d0. this suggests that when the characteristic length is longer than the characteristic distance, the pulse-like oscillation can be generated. figure 9a shows tp/td<0.4 (or tr/td<0.2) when δ>1.2. figure 9b shows that except for l=20 km, tp/td<0.4 (or tr/td<0.2) when δ>1.5. theoretical results suggest that δ>1.5 is a significant condition for generating a pulse-like oscillation at a site under sw friction. for vw friction, when υ>0.5 this solution shows a sine-function oscillation at a site. because of υ=vc/d0ω0, the inequality υ>0.5 leads to vc>d0ω0/2. this means that a pulse-like oscillation at a site can be generated when vc>d0ω0/2. since vc is the characteristic velocity of vw friction law, higher vc indicates a slower decay of friction with velocity. the present result suggests that a slower decay of friction is more capable of generating the pulse-like oscillation than a jeen-hwa wang 10 figure 10. the plots of tp/tp versus υ: (a) for vrt0=0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 km (from bottom to top) when l=80 km; and (b) for l=20, 40, 60, 80, 100, 120, 160, and 180 km (from top to bottom) when vrt0=2.0 km. faster one. unlike the pulse-like oscillation produced by sw friction, the amplitude of pulse-like oscillation generated by vw friction could increase with time due to an extra term eτ/2υ. this suggests that vw friction can cause higher wave energy than sw friction. smaller υ will cause a faster increase in wave amplitude than larger υ. figure 10 reveals a decrease in tp/td with increasing υ. higher (lower) vc means a slower (faster) decay of friction with velocity. clearly, a slower decay of friction with velocity is easier to generating a slip pulse than a faster decay of friction. because of υ=vc/d0ω0, υ>1 means vc>d0ω0/2. when the characteristic velocity of the model is higher than the rate of a slider in the characteristic distance, a pulse-like oscillation can be generated. figure 10 shows tr/td<0.4 (or tr/td<0.2) when υ>0.6. hence, υ>0.6 could be a significant condition for generating a pulse-like oscillation at a site under vw friction. in addition, figures 9a and 10a both show a decrease in tp/td with decreasing vrt0, which is the characteristic rupture distance. lower vr is more capable of generating a pulse-like oscillation than higher vr. meanwhile, shorter t0 is more capable of generating a pulse-like oscillation than longer t0. because of t0= (m/kl) 1/2/2π=(ρa/κl) 1/2/2π, strong coupling (denoted by κl) between the moving plate and a fault system is capable of producing a pulse-like oscillation than weak coupling. on the other hand, lighter or lower-density fault rocks are easier to producing a pulse-like oscillation than heavier or higher-density fault rocks. figures 9b and 10b exhibit a decrease in tp/td with increasing l, that is, longer l is more capable of generating a pulse-like oscillation than shorter l. however, the difference in the values of tp/td between two sequential rupture lengths decreases with increasing l. this means that when l is longer than a certain value, the effect of rupture length on tp/td is reduced. of course, the l-effect also depends on vr and t0. a comparison between figure 9 and figure 10 shows that tp/td is in general smaller for vw friction than for sw friction, thus suggesting that the former is more capable of producing a pulse-like oscillation at site than the latter. in addition, under vw friction a pulse-like oscillation can be produced even though l is short. based on equation (7), wang [2016] obtained that the complementary solution exhibits ω-1 scaling in the whole range of ω for sw friction. but, for the particular solution sw friction results in spectral amplitudes only at three values of ω. based on equation (15), wang [2016] obtained that for vw friction with υ>0.5, the spectral amplitude versus ω exhibits almost ω0 scaling when ω is lower than the corner angular frequency, ωc, which is independent on υ and increases with ω0. when ω>ωc, the spectral amplitude monotonically decreases with ω following a line with a slope value of -1, which is the scaling exponent. this again confirms that crack-like rupture is generated by vw friction. hence, it is easier to yield slip pulses from sw friction than from vw friction. 6. conclusions seismological observations show the existence of slip pulses with tr/td<0.3. for the present model, there are complementary and particular solutions of the equation of motion. for sw friction, the complementary solution shows a slip pulse when γ>1. tr and tr/td both decrease rapidly with increasing tr, thus meaning that tr and tr/td both decrease when the slip pulse propagates along the fault. tr and tr/td also both decrease when γ, ω0, vr, and l increase. for vw friction, a slip pulse is yielded when υ>>1 or vc>>1 and the crack-like ruptures is generated when υ is not too big. when υ>>1 or vc>>1, the results produced by vw friction are essentially similar with those by sw friction. for the two types of friction, a slower decay of friction is more capable of generating slip pulses than a faster one. lower vr is more capable of generating a slip pulse than higher vr. longer l is more capable of generating slip pulses than shorter l. of course, the importance of vr and l are lower than γ and ω0. for the particular solution, the ratio of predominant period (tp) of oscillations at a site is slightly longer than tr, and thus tp/td, is a good indication to show the existence of pulse-like oscillations at a site because of tr<tp. results show the existence of pulse-like oscillations for sw friction when δ>1.6 (or xc>1.6d0) and for vw friction when υ>0.5 (or vc>0.5d0ω0). tp/td decreases with increasing δ for sw friction and with increasing υ for vw friction. for the two types of friction, t0/td and tp/td both decreases when vr and l increase. for the two types of friction, a slower decay of friction is more capable of triggering pulse-like oscillations at a site than a faster one. shorter t0 is more capable of generating pulse-like oscillations than longer t0. in other words, strong coupling between the moving plate and a fault system is capable of producing a slip pulse than weak coupling. lighter or lowerdensity fault rocks are easier to producing pulse-like oscillations than heavier or higher-density fault rocks. longer l is more capable of generating pulse-like oscillations than shorter l. 11 an analytic study of slip pulses acknowledgements. the author would like to thank prof. a. bizzarri (editor of annals of geophysics) and an anonymous reviewer for their valuable comments and suggestions to help him to substantially improve this article. this study was financially supported from academia sinica, the ministry of sciences and technology (grant no.: most-106-2116-m-001-005), and the central weather bureau (grant no.: motc-cwb-107-e-02). references abramowitz, m. and i.a. stegun (eds.) 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(2017). frictional and viscous effects on the nucleation phase of an earthquake nucleation, j. seismol., 21(6), 1517-1539. wang, j.h. and r.d. hwang (2001). one-dimensional dynamical simulations of slip complexity of earthquake faults. earth planets space, 53, 91-100. wu, z.l. and y.t. chen (1998). solitary wave in a burridge-knopoff model with slipdependent friction as a clue to understanding the mechanism of the self-healing slip pulse in an earthquake rupture process. nonl. process geophys., 5, 121-125. zheng. g. and j.r. rice (1998). conditions under which velocity-weakening friction allows a self-healing versus a crack-like mode of rupture. bull. seism. soc. am., 88, 1466-1483. *corresponding author: jeen-hwa wang, institute of earth sciences, academia sinica, nangang, taipei, taiwan email: jhwang@earth.sinica.edu.tw © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. jeen-hwa wang 14 modification of a method o f galanopoulos f o r determining earthquake risk r . m a a z ( * ) r i c e v u t o il 20 m a g g i o 1969 summary. — a m e t h o d f o r the determination of e a r t h q u a k e risk basing on the e n e r g y f r e q u e n c y r e l a t i o n proposed b y galanopoulos is m o d i f i e d . riassunto. — n e l l a nota viene modificato il m e t o d o per la determinazione del rischio legato ad un t e r r e m o t o , m e t o d o già proposto precedentem e n t e da a . g. galanopoulos e basato sulla relazione che intercorre t r a energia e f r e q u e n z a . in the relation lg n = a — bm [1] between earthquake magnitude m and annual number n of earthquakes of the magnitude class with centre m the coefficient a appears to depend mainly on the energy release in the considered region v with area g, whereas b depends on the mean depth of the focus in /'. after reducing [1] to a general standard area g* the accordingly transformed relation [1] informs us about the relative frequency of such earthquakes. supposing all earthquakes have equal depth, then a measure of the earthquake risk would be found by this. to compensate the actual variation of the mean focal depth and the variability of b connected with it galanopoulos (') proceeds as follows. may lg n' = a0 — b0m [2] ( * ) i n s t i t u t p h y s i k der erde, bereich 1 ( g e o d y n a m i k und p h y s i k des e r d i n n e r n ) . jena ( d d r ) . 112 k . ma az substitutes [1] where b0 is a fixed reasonable value, possibly 0.8. the formulas [1] and [2] are connected by postulating that the magnitude m„ corresponds to a certain value n0 in both cases m = m0 * n'(m0) = n(m0) = n0 . [3] therefore from [1] and [2] follows a« = a b° + ( l — m lg n0 . [4] in the next step galanopoulos (') gets to the frequency n'* related to the standard area by thinking the earthquake events being distributed homogenously in f and f* cz f, too. that means n'* _ g* n' ~ g whence lg n'* = a'* — b0m , a'* = a0 + lg — . [5] (r now we consider two regions 71, and f2, the intersection of which need not be empty, with a, ^ at, but. having equal parameters a'*, b a'*, = a'*2 , b, = b2 # b0 . [6] for such regions it holds naturally = gt n2 g2 [6] helps with [5], [4], [7], and [1] to [ 7 ] 0 = therefore b0\atoi n 02 n o t _ n 0 2 g, ~ g2 ~7,0 is a quantity which must be chosen uniquely for all regions r and n0 cannot be independent of f. with this condition we get «'* = ~ (a — lg uog) + lg n0g* as modified measure of earthquake risk in the sense of galanopoulos. m o d i f i c a t i o n of a m e t h o d of g a l a n o p o u l o s 1 1 3 the last term is an arbitrary number independent of a, it and g, suitably defined to zero: >io<t* = 1 • [ 8 ] therefore we finally get: a s = l>0 ( , (r \ the values of b0 and g* must be chosen definitly and according to reality, possibly in conformity with galanopoulos as />„ = 0.8 , g* = 10' km. the arbitrariness in choosing b„, g* and n0g* can be comprehended as a foible of the method. another difficulty is that the investigated region /' cannot be defined without free choice. moreover, there remains the open question mentioned also by galanopoulos to which degree the basing relation [1] fits to reality. furthermore, we consider the difference between a'* and the analogous quantity after galanopoulos who generally postulates no = n0g = 1 , [10] instead of [8] where n0 is defined in [3] and who, therefore, gets b g* a*=}°a + [11] the difference b„ g 1°' b) * g* to [9] depends on the chosen values b0 and g* and of course on b and g. i n tab. i in the paper of galanopoulos, this difference does not exceed the amount of 0.11 in the case of south california where a = 5.25, b 0.86, g = 29.61 g*, and a* = 3.41, a'* = 3.52. finally, we can see at a consequence of [10] that this postulate is unsuitable. let 1\ and i\ be two regions with equal relative quantities a* and b, b different from b0: a*i = <t*2 , bl = b2 # b, geo 985 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 4 a revised method to extract thermospheric parameters from incoherent scatter observations andrei v. mikhailov (1) and jean lilensten (2) (1) institute of terrestrial magnetism, ionosphere and radio wave propagation (izmiran), russian academy of sciences, troitsk (moscow region), russia (2) laboratoire de planétologie de grenoble, bâtiment d de physique, grenoble, france height distribution of ionospheric plasma parameters in the f2-region is closely related to height distribution of the main thermospheric parameters. therefore, they can be extracted from ionospheric observations solving an inverse problem of aeronomy. a self-consistent approach to the ne(h) modeling at the f2-region heights has been applied to solve the problem. using routine incoherent scatter radar observations (ne(h), te(h), ti(h), vi(h) profiles) the method yields a self-consistent set of main aeronomic parameters responsible for the f2-region formation. the list of derived parameters includes: neutral temperature profile tn(h) depending on the exospheric temperature tex, the temperature at 120 km t120 and the shape-parameter s, which determine the temperature profile, concentration of neutral species [o], [o2], [n2], vertical plasma drift w, which may be converted to the meridional thermospheric wind vnx, total solar euv flux and ion composition (o+, o2+, no+, n2+, n+) as a result of ne(h) fitting. therefore, the method gives a complete description of the upper atmosphere condition in the vicinity of incoherent scatter facility for the periods of observation. analysis of all available eiscat (cp-1, cp-2) observations has shown wide deviations from msis-86 model predictions for geomagnetically disturbed conditions while the retrieved parameters are close to the model ones for quiet periods. the approach turns out to be very useful for physical analyses of the f2-layer disturbance mechanisms giving a complete picture of the phenomenon in question. limitations and problems related to method application are discussed. under existing conditions when thermospheric observations are not conducted currently the proposed method may be considered a real tool for thermosphere investigation and monitoring at least for the periods of isr observations. 4.1. introduction the idea of using ionosphere plasma parameter observations to obtain various aeronomic parameters has been implemented and used since the beginning of ionospheric research. ionospheric observations are widely used to obtain thermospheric parameters: winds (mikhailov and ostrovskii, 1978; miller et al., 1986; richards, 1991; titheridge, 1995; igi et al., 1999), vertical plasma drifts in the equatorial f2-layer (hari and murthy, 1995; ramesh and sastri, 1995; batista et al., 1996), neutral composition and winds (richards and wilkinson, 1998; wright and conkright, 2001). using observed tec variations thermospheric winds were found by sethia et al. (1983), förster and jakowski (1988), exospheric temperature by antoniadis (1977) and atomic oxygen by lilensten and blelly (2002). 986 andrei v. mikhailov and jean lilensten incoherent scatter radar (isr) observations turned out to be the most appropriate for such analyses as they provide the most reliable and consistent set of ionospheric plasma parameters in e-, f1-, and f2-regions. much valuable and interesting information on thermospheric winds, temperature and neutral composition has been obtained from isr data analyses (e.g., alcaydé et al., 1974, 1982; babcook and evans, 1979; evans et al., 1979; oliver, 1979, 1990; alcaydé and fontanari, 1986; burnside et al., 1988, 1991; winser et al., 1988; blelly et al., 1992; buonsanto et al., 1992; aruliah et al., 1997; lathuillère et al., 1997; buonsanto and pohlman, 1998; schoendorf and oliver, 1998; witasse et al., 1998; buonsanto and witasse, 1999; litvin et al., 2000). the main approaches are based on the use of the energy equation for o+ ions to find atomic oxygen concentration and neutral temperature at the f2-region heights and on the momentum equation for o+ ions to retrieve meridional thermospheric winds. usually a ionospheric parameter being fitted in model calculations depends on many input aeronomic parameters – some of them are more critical, others are less. it is always possible by varying one of the main aeronomic parameters to fit the ionospheric one, meanwhile the other parameters are taken from outside (e.g., empirical models or observations). therefore, one obtains a partial solution and this is a special problem to estimate its validity. the more ionospheric variables are simultaneously fitted, the more reliable solution (as a set of aeronomic parameters) is obtained. in this case the extracted aeronomic parameters are not arbitrary but constitute a self-consistent set. thus we come to the idea of a self-consistent approach to the ionospheric modeling proposed by stubbe (1973). fitting the whole ne(h) profile opens many possibilities to extract aeronomic parameters as it contains a large amount of information. but this is a complicated problem which requires for its solution an adequate physical model as a tool for analysis, reliable and consistent ionospheric parameter observations and special methods to extract a consistent set of main aeronomic parameters responsible for the observed ne(h) distribution. nevertheless, attempts are being made in this direction and some progress has been achieved. zhang et al. (2001) using isr observations found one to three of five thermospheric parameters by fitting ne(h) distribution. a self-consistent approach to the f-region modeling by mikhailov and schlegel (1997) with later modifications (mikhailov and förster, 1999; mikhailov and schlegel, 2000) also deals with ne(h) fitting to retrieve basic aeronomic parameters responsible for the ne(h) formation in the mid-latitude f2-region. the choice of ne(h) as a fitted parameter is due to the fact that this quantity is the most reliable and easily measured with the is technique parameter while other plasma characteristics are subjected to larger uncertainties. for instance, observed te and much stronger ti (compared to ne) depend on the ion composition model applied in the fit of the theoretical to the measured autocorrelation function during the incoherent data analysis. the uncertainty in the actual ion composition may lead to considerable uncertainties in the derived ti(h) and te(h) profiles especially during disturbed conditions. plasma velocity observation is a complicated technical problem which is not successfully solved at some isr facilities (kharkov, millstone hill, for instance). recent modifications of the self-consistent method improving its numerical stability, an extension of the method to non-stationary (twilight and nighttime) conditions, its limitations as well as problems encountered are discussed in the paper. thermospheric parameters extracted from eiscat (cp1 and cp-2) and millstone hill isr observations both for quiet and disturbed conditions are presented in comparison with the msis-86 thermospheric model. the role of disturbed thermospheric parameters in generating f2-layer disturbances is discussed. 4.2. present day state-of-the-art a widespread method in the f2-region physical modeling to obtain observed nmf2 and hmf2 variations is to fit hmf2 by varying vertical plasma drift or meridional thermospheric wind vnx. while nmf2 is fitted by changing o/n2 ratio (e.g., richards et al., 1994a; mikhailov et al., 1995; pavlov and foster, 2001) or plasmaspheric flux during nighttime hours (e.g., mikhailov and förster, 1999). 987 a revised method to extract thermospheric parameters from incoherent scatter observations this gives some useful information about the thermosphere state especially during disturbed periods when empirical thermospheric models turn out to be inefficient. unfortunately, the obtained thermospheric parameters present only a partial solution as nmf2, hmf2 or tec also depend on other aeronomic parameters which are fixed to default values during this fitting. without an independent control of extracted thermospheric parameters the validity of obtained results remains questionable. as an example recently obtained in this direction let us consider the results by lilensten and blelly (2002), who modeled eiscat observations for june 9, 1994. it was a quiet day (ap = 8) during solar minimum (f10.7 = 83) although some splashes of electric field up to 20 mv/m took place both at the beginning and at the end of the period. the transcar first principle model (blelly et al., 1996) was applied in that analysis. observed ne(h) profiles were used to specify nmf2 and hmf2 variations and ne(h) integration in the 80-425 km height range gave itec. it was shown that [o] varying within ± 20% with respect to the msis model can provide the observed itec and nmf2 variations (fig. 4.1). the latter is the expected result as tec and nmf2 are known to exhibit a pretty good correlation (das gupta and basu, 1973; kane, 1975; jakowski et al., 1991), in particular for this case when only the ionospheric part of tec below 425 km was considered. with the same correction of [o] it was possible to obtain ne, te, ti, and vi height profiles close to eiscat observations (fig. 4.2), [o2] and [n2] were fixedto default msis values. it was also shown that acceptable fig. 4.1. total electric field measured by eiscat (top panel). the nmf2 and itec variations are shown in the third and fourth panels. bold lines are the measurements and the thin lines are the fits with a correction factor for the atomic oxygen density. second panel gives the relative errors for nmf 2 (upper line) and for itec (lower line). the bottom panel shows the values of the time varying correction factor for atomic oxygen. 988 andrei v. mikhailov and jean lilensten results can be obtained varying [o2] and [n2], but a correction factor should be larger (1.5-2) in this case, and this looks unreal for a quiet time period. it is stressed that in reality one can hardly expect solely atomic oxygen or molecules variations and a multi-parameter fit is required. so, in quiet geomagnetic conditions when empirical thermospheric models like msis give reasonable neutral composition and temperature it is possible to describe fairly well f2-region plasma parameter variations and this has been demonstrated repeatedly (e.g., pavlov and buonsanto, 1997; richards and wilkinson, 1998; pavlov et al., 1999, 2000; pavlov and oyama, 2000 and references therein), but the situation is more complicated during disturbed periods when large deviations from the msis model take place (buonsanto et al., 1992; mikhailov and foster, 1997; mikhailov and schlegel, 1998; litvin et al., 2000; pavlov and foster, 2001). however, in the frame of space weather this is a promising and perhaps the only acceptable approach. indeed, space weather requires real-time (or close to) and global control of the ionosphere and this problem cannot be provided with is radars. such service should be based on the use of cheap ionosondes for the f2-layer parameters and the positioning system receivers to estimate the tec. fig. 4.2. the four upper panels are the ionospheric profiles at 6 lt, the middle panel at 12 lt and the bottom panels at 16 lt. from left to right, one gets the electron density, electron temperature, ion temperature, ion velocity parallel to b. the broken lines are the measurements with instantaneous values in dashed lines and the surrounding extreme values in full lines. the modeling outputs are in smoothed lines. the dashed lines stand for no correction factor in the neutral atmosphere. the dotted lines are with a correction factor of 0.7. the full line (in the electron density only) for a time varying correction factor. this factor’s values are 0.915 at 6 lt, 0.99 at 12 lt and 0.65 at 16 lt. 989 a revised method to extract thermospheric parameters from incoherent scatter observations as mentioned earlier, isr observations are widely used to obtain information on the thermosphere. there are two generally used methods based on: i) the use of the energy equation for o+ ions to find atomic oxygen concentration and neutral temperature in the f2-region (e.g., evans et al., 1979; oliver, 1979, 1990; alcaydé et al., 1982; blelly et al., 1992; schoendorf and oliver, 1998; litvin et al., 2000), and ii) the use of the momentum equation for o+ ions to obtain meridional thermospheric wind (e.g., lathuillère et al., 1997; witasse et al., 1998). based on the experience obtained since the first publication of the self-consistent method by mikhailov and schlegel (1997), some crucial points should be mentioned which are to be taken into account extracting thermospheric information from ne(h) profiles. 1) simultaneously measured basic plasma parameters (ne, te, ti, vi profiles) should be internally consistent and demonstrate a sufficient accuracy. for instance, eiscat does provide such observations, while not all millstone hill isr data can be used for such analysis due to rare observations or problems with vi(h) measurements. 2) an optimal set of searched aeronomic parameters should be specified, which is sufficient for describing the observed ne(h) distribution in the chosen geophysical conditions (day or nighttime, high, middle or equatorial latitudes, etc.). only these key parameters can be retrieved with an acceptable accuracy from the observed ne(h) profile as their contributions are the largest. 3) unlike direct f2-layer model calculations when all input parameters can be changed arbitrarily, in case of solving an inverse problem of aeronomy the searched parameters constitute some sort of clusters in which they are self-consistent varying within confined limits. therefore, the main parameters should be found simultaneously within the same algorithm and this strongly complicates the problem. usually this is not taken into account, and searching for an aeronomic parameter by fitting an observed ionospheric one, other important parameters are taken from empirical models – that is the set of aeronomic parameters turns out to be not self-consistent. such approach is acceptable to some extent in quiet geomagnetic conditions when thermospheric empirical models like msis provide reasonable neutral temperature and concentrations, but it can not be justified for disturbed periods. 4.3. method description a self-consistent approach to the f-region modeling using isr observations is described by mikhailov and schlegel (1997, 2000), mikhailov and förster (1999). the method was successfully used to analyze f2-layer storms (mikhailov and foster, 1997; mikhailov and schlegel, 1998), ion composition (mikhailov and kofman, 2001), and f1-layer disturbances (mikhailov and schlegel, 2003). nevertheless the method is still under development and some modifications to improve its numerical stability as well as its extension for non-stationary conditions are considered in this part of the paper. the sketch illustrates the idea of the method. routinely observed and specially prepared (see later) ne(h), te(h), ti(h), and vi(h) profiles are used in the f2-region model to fit calculated ne(h) to the observed one. the variable parameters are: tex, t120, shape parameter s for the tn(h) profile, input routine isr observations ne (h) te (h) ti (h) vi (h) output a self-consistent set of thermosphere parameters tn == ϕϕ (tex, t120, s) [o], [o2], [n2] total euv solar flux w(h) � vnx ion composition f2-region physical model 990 andrei v. mikhailov and jean lilensten fig. 4.3. an example of eiscat observations for a disturbed day of august 05, 1992. initial median (before polynomial smoothing) profiles calculated over 11:00-12:00 ut time interval are shown. standard deviations are not experimental, but show the scatter of profiles within the selected 1 h time interval. factors for msis [o], [o2], [n2] concentrations as well as for the total euv flux. vertical plasma drift w(h), which can be converted to the meridional thermospheric wind vnx, is found from the observed vi(h) (see later). the whole ion composition is available as a result of model calculations. all parameters are found simultaneously constituting a self-consistent set. let us consider the three items shown in the sketch. 4.3.1. input isr data reduction isr observations need a special reduction before being used in calculations. the eiscat cp-1 observations provide range profiles of ne, te, ti and vi every 5 min (cp-2 every 6 min) with the antenna beam directed along the local geomagnetic field line. each particular height profile is not smooth exhibiting fluctuations for some reasons and cannot be used for solving the inverse problem. therefore, median (not average) profiles with standard deviations (sd) at each height step are calculated over 1.0-1.5 h time interval, which is close to the e-fold time of the daytime f2-layer. it gives 13-17 values to find the median. at millstone hill in some experiments the observations are rare (three per hour) and a 3-4 h time interval has to be used to obtain such median profiles. this is not convenient especially during disturbed periods when changes in the f2-region are pretty fast. these median vertical profiles are then smoothed by a polynomial of up to 5th degree before being used in our model calculations. experimental te(h), ti(h) and ne(h) profiles depend on the ion composition used in the fit of the theoretical to the measured auto-correlation function (acf). an uncertainty in ion composition may lead to considerable uncertainties in the derived te(h) and ti(h) profiles and to somewhat smaller uncertainties in ne(h) (e.g., waldteufel, 1971; lathuillère et al., 1983). the effect of varying ion composition is most noticeable during disturbed periods, but an appreciable effect may also take place for quiet periods as well (e.g., mikhailov and schlegel, 2003). the largest uncertainties take place at the f1-region heights where the ion composition changes from molecular to atomic. therefore, a correction of the experimental te(h), ti(h) and ne(h) profiles is required. a simple correction proposed by waldteufel (1971) may be applied when the deviations in ion composition from the model (used 991 a revised method to extract thermospheric parameters from incoherent scatter observations in the isr data analysis) are not large, but in case of strong perturbations this simple correction results in unreal te(h) and ti(h) profiles. in such cases a more sophisticated iterative method considered by mikhailov and schlegel (1997) should be applied, which provides the proper fit to the measured acf. unfortunately, this iterative method requires considerable calculations and cannot be applied routinely. an example of good eiscat observations for a disturbed period of august 05, 1992 is given in fig. 4.3. initial median (before polynomial smoothing) profiles calculated over 11:00-12:00 ut time interval are shown. note small sd in the whole height range telling us about the reliability of observations. 4.3.2. ionospheric model a mid-latitude f2-region model includes transport process for o+(4s) and photo-chemical processes only for o+(2d), o+(2p), o2+(x2′), n+, n2+ and no+ ions in the 150-550 km height range. a two-component model of the solar euv from nusinov (1992) is used to calculate the photoionization rates in 48-wavelength intervals (10-1050 å). to extend the method to twilight and dark hours a nighttime ionization source has been added to the model. nighttime scattered hlyβ (1026 å), hei (584 å), heii (304 å) and radiative recombination emission of o+ ions (910 å) are taken according to the kashirin (1986) model. the photo-ionization and photo-absorption crosssections are obtained from torr et al. (1979), richards and torr (1988), richards et al. (1994b); and from ivanov-kholodny and nikoljsky (1969) for x-ray emission. the list of chemical processes used in the model is given in table 4.i. as long as we work above 150 km the contribution of neutral no can be neglected and corresponding reactions are not in the list. atomic nitrogen being used in some reactions (table 4.i) is taken from the msis-86 model. the most important for the f2-region chemistry is the o+ + n2 reaction. recent flowing afterglow laboratory measurements by hierl et al. (1997) of this reaction rate are used in the model. these measurements were made at tn =ti = tv (where tn – neutral, ti – ion temperatures and tv – vibrational temperature of the excited n2) in a wide temperature range and take into account the effects of vibrationally excited n2*. this may be important for summer high solar activity conditions (e.g., pavlov and buonsanto, 1997; pavlov et al., 1999 and references therein). a comparison of different o+ + n2 reaction rate constants using eiscat observations has shown that the hierl et al. (1997) rate coefficient for this reaction may be recommended for aeronomic calculations (mikhailov and schlegel, 2000). the experimental dependence from hierl et al. (1997) was approximated by a polynomial to be used in the model. in the auroral zone strong horizontal e × b drifts of the ions increase the effective temperature for this reaction. according to schunk et al. (1975) we accept teff = (mitn + mnti)/(mn + mi) + 0.329 e2 where e is in mv/m. concentration of o+(4s) ions is calculated from the continuity equation t n h n v q ni i i i i 2 2 2 2 = + b^ h (4.1) where sin ln ln v m k i t h n t h n k gm h t t wi i ij i i e e i e i2 2 2 2 2 2 2 = + + + + + o! ^ h ) 3 (4.2) vi is measured total vertical velocity of plasma (vi = vl sini at eiscat) and w is vertical plasma drift due to thermospheric winds and electric fields. the first right hand term of eq. (4.2) is the diffusion velocity for o+ ions (≠ ne) from banks and kockarts (1973). observed and smoothed (see earlier) te and ti values are used in calculations. expression (4.2) specifies w, which in principle can be con992 andrei v. mikhailov and jean lilensten verted to the meridional thermospheric wind. this is a standard approach usually applied to find meridional thermospheric winds from isr observations (e.g., buonsanto and wittasse, 1999 and references therein). neglecting vertical neutral winds (possible under small joule heating conditions), the meridional neutral wind vnx can be found from .sin cos cos sin cos cosw v i i d v i i dnx ny= at eiscat due to small magnetic declination (d =1.2°), the contribution of zonal neutral wind vny may be neglected. diffusion collision frequencies νij for o+ are related to momentum transfer collision frequencies ν* by the expression (see eq. 19.13 in banks and kockarts, 1973) m m mij j i j ij ) = +o o_ i , where i applies to o+ ions and j applies to other neutral or ionized gas species. collisions of o+ ions with neutral o, o2, n2 and only with no+,o2+ ions are taken into account. unlike the previous version of the method where all ions were included to νij, here we leave only major ones thereby increasing the numerical stability of calculations. all o+ ion collision frequencies are taken from banks and kockarts (1973). a correction factor 1.2 for ν(o+-o) was applied in accordance with the results of analyses by pesnell et al. (1993), oliver and glotfelty (1996), buonsanto et al. (1997), litvin et al. (2000). the νij, qi, β values in eqs. (4.1) and (4.2) depend on the aeronomic parameters searched for. normally the results are rather insensitive to the choice of the upper boundary height within 400600 km range and any calculation can be used as a solution (mikhailov and kofman, 2001). therefore, calculations are made with the upper boundary at 550 km where [o+] = neobs − ∑ ni is specified. ion concentrations ni are known at each iteration of fitting calculated ne(h) to the experimental one. table 4.i. chemical reactions used in the model. reaction rate coefficient, cm−3 s−1, or rate, s−1 o+(4s) + n2 → no+ + n ∑ci × teff o+(4s) + o2 → o2+ + o 1.0 × 10−9/tn0.7 o+(2d) + o → o+(4s) + o 1.0 × 10−10 o+(2d) + n2 → n2+ + o 1.0 × 10−9 o+(2d) + o2 → o2+(a4′) + o 8.0 × 10−10 o+(2d) + e → o+(4s) + e 6.6 × 10−8(300/te)0.5 o+(2p) + o → o+(4s) + o 1.8 × 10−10 o+(2p) + n2 → n2+ + o 5.0 × 10−11 o+(2p) + n2 → o+(4s) + n2 4.0 × 10−10 o+(2p) + e → o+(2d) + e 1.5 × 10−7(300/te)0.5 o+(2p) + e → o+(4s) + e 3.2 × 10−8(300/te)0.5 o+(2p) → o+(2d) + hν a = 0.173 s−1 o+(2p) → o+(4s) + hν a = 0.048 s−1 o2+(x2′) + n → no+ + o 1.8 × 10−10 o2+(x2′) + n → n+ + o2 4.0 × 10−10 o2+(x2′) + n2 → no+ + no 1.0 × 10−15 o2+(x2′) + e → o + o 1.95 × 10−7(300/te)0.7 n2+ + o → o+(4s) + n2 1.0 × 10−11(300/tn)0.23 n2+ + o → no+ + n 1.4 × 10−10(300/tn)0.44 n2+ + o2 → o2+ + n2 5.0 × 10−11(300/tn) n2+ + n → n+ + n2 1.0 × 10−11 n2+ + e → n + n 3.5 × 10−7(300/te)0.5 n+ + o2 → o+(4s) + no 3.6 × 10−11 n+ + o2 → no+ + o 2.6 × 10−10 n+ + o2 → o2+ + n 3.1 × 10−10 n+ + o → o+(4s) + n 2.2 × 10−12 no+ + e → n + o 4.5 × 10−7(300/te)0.83 where teff, tn, te, ti are effective, neutral, electron and ion temperatures. 993 a revised method to extract thermospheric parameters from incoherent scatter observations usually the low boundary for [o+] is specified at 160 km during daytime, but it may be set at 230260 km during nighttime at eiscat location to avoid particle precipitation effects at lower heights. corpuscular ionization is not included in the model and periods with intensive precipitation cannot be developed with the method (see later). 4.3.3. fitting procedure an important point is the list of aeronomic parameters to be searched for. these should be the main critical parameters responsible for the f-region formation. an analytical solution of the continuity equation for electron concentration in the f2-region (e.g., ivanov-kholodny and mikhailov, 1986) can help select the parameters. in the mid-latitude daytime f2-region they are: neutral temperature tn, atomic oxygen [o], linear loss coefficient β = γ1[n2] + γ2[o2], efficiency of solar ionization, and vertical plasma drift w due to thermospheric winds and electric fields. all aeronomic parameters may be divided into two groups: rate constants and cross-sections of processes, which are supposed to be known from laboratory experiments and those which vary with geophysical conditions. the latter present our main concern, although the former can be also analyzed using ionospheric observations (e.g., ivanov-kholodny et al., 1976; oliver and glotfelty, 1996; buonsanto et al., 1997; mikhailov and schlegel, 2000). atomic oxygen is the main neutral constituent at the f-region heights and it specifies the total production rate as well as the diffusion rate of o+ ions. the recombination rate of o+ ions is specified by linear loss coefficient β, so molecular nitrogen and oxygen should be in the list, moreover they strongly contribute to the total production rate. there is still a pretty large uncertainty with the solar euv fluxes in particular with their day-to-day variations due to varying solar activity. total euv flux rather than spectral distribution of the energy is important for f2-layer ionization (ivanov-kholodny and nikoljski, 1969), therefore this parameter should also be in the list. neutral temperature height profile is a crucial structural parameter for the whole thermosphere. within the msis model formalism (we are working with), the tn(h) is specified by three parameters: tex, t120, and the shape parameter s. although we are not dealing with heights below 160 km it was found that the method works better if the msis t120 value is freed. so, t120 was formally added to the list of searched parameters. but it should be stressed that this is just a technical step and the extension of tn, [o], [o2], [n2] down to 120 km height is just an extrapolation as we do not fit any ne(h) profile below 160 km height. the last very important parameter is vertical plasma drift velocity w due to thermospheric winds and electric fields. unfortunately the success of ne(h) fitting strongly depends not only on the absolute value of w at the f2-layer heights, but also on the height profile w(h). therefore, any attempts to use height-independent w normalized by empirical model values (e.g., hedin et al., 1991) should give unsatisfactory results. the only acceptable approach according to our experience is to use simultaneous isr observations of total vertical plasma velocity subtracting diffusion velocity (see eq. (4.2)). this confines the application of the method to high-quality isr data like the eiscat facility provides. due to technical problems with vi measurements some of millstone hill observations cannot be developed with the method. so we have seven parameters to be found: three parameters (tex, s, t120) specifying tn(h) profile, factors for the msis model [o], [o2], [n2] concentrations as well as for the total solar euv flux with λ < 1050 å. vertical plasma drift w(h) is calculated at each step of fitting using eq. (4.2). the whole ion composition is also available at each step of calculations. using standard multi-regressional methods we fit the calculated ne(h) profile to the observed one and find a self-consistent set of aeronomic parameters listed. it is important to stress that all parameters should be found simultaneously. all the attempts to divide the fitting process by steps gave unsatisfactory results. during quiet and slightly disturbed conditions the initial values of searched parameters are set equal to the model ones with a ± 20% corridor for their variations. usually this is enough to obtain 994 andrei v. mikhailov and jean lilensten an acceptable solution. but in disturbed conditions the thermospheric parameters are beyond the ± 20% corridor and some steps are required to localize the area of possible solution, that is to find approximate values of the searched parameters. after this the standard method is applied to find the final values of the parameters. two criteria are used to select the solution: i) a fit of ne(h) should be good in the whole height range, that is the calculated profile should lie within the experimental ± sd corridor, the goodness of fit being estimated with /log n ne e h h 2 obs bot top cal=∆ ! ^ h8 b . ii) the extracted parameters should be within the ± 20% corridor with respect to the average values. fig. 4.4. observed at eiscat and calculated daytime ne(h) profiles for quiet (top) and disturbed (bottom) conditions. profiles for different levels of solar activity (minimum, june 26, 1997; medium, august 02, 1992; and maximum, july 02, 1990, top panel) and different levels of geomagnetic activity are considered (ap indices for this and previous days are shown next to curves in the bottom panel). note the variability of ne(h) profiles under disturbed conditions. 995 a revised method to extract thermospheric parameters from incoherent scatter observations there are stationary and non-stationary versions of the method. in case of the stationary solution the left hand term of eq. (4.1) is set to zero, while [o+] (≠ ne) height profile from the previous time step should be available to obtain a non-stationary solution. the stationary form of eq. (4.1) is used during sunlit conditions. a comparison of stationary and non-stationary solutions shows close results for solar zenith angles χ ≤ 95°. during twilight and in the night ( χ > 95°) a non-stationary approach gives better results. such behavior is explainable. median profiles of observed characteristics used in calculations (see earlier) are obtained over 1.0-1.5 h time step in case of eiscat observations, but this time interval may be larger for millstone hill observations. therefore, the minimal time step may be 1 h for solving eq. (4.1). this time step ∆t is close to the e-fold time (≈ 1.5 h) of the daytime f2-region. in this case it is better to use a stationary solution (waldman, 1973). during nighttime the e-fold time is much larger than 1.5 h and a non-stationary solution is preferable. unfortunately, a non-stationary solution needs much handy work to prepare median smoothed ne, te, ti, vi profiles for all time steps, and can hardly be applied for routine calculations. an example for one quiet day is given later. the quality of ne(h) fitting is illustrated in fig. 4.4 where eiscat daytime observations for quiet (top) and disturbed (bottom) are compared with calculations. different levels of solar activity (minimum, june 26, 1997; medium, august 02, 1992; and maximum, july 02, 1990) and different levels of geomagnetic activity are considered (ap indices for this and previous days are shown next to curves in the bottom panel). simple quiet time ne(h) profiles as well as much complicated disturbed profiles with well-pronounced f1-layer (august 05, 1992) or profiles without f2-layer maximum at all (april 10, 1990) can be fitted in the whole height range with a good accuracy. 4.4. comparison with msis-86 model the method was applied to eiscat (cp-1 and cp-2) daytime observations (28 magnetically disturbed and 30 quiet days) and the results were compared to the msis-86 thermospheric model predictions. the results for exospheric temperature tex, shape parameter s for tn(h), and total neutral density ρ are given in fig. 4.5, while a comparison on [o], [o2] and [n2] is given in fig. 4.6. total neutral gas density – the msis-86 model overestimates ρ at 300 km height for disturbed days with an average ρcal/ρmsis = 0.52 (the difference is significant at the 99% confidence level). the deviation from msis-86 is insignificant for quiet time conditions. the result may be important for satellite orbital characteristics. exospheric temperature – the msis-86 model underestimates tex by 5-7% on average both for quiet and disturbed conditions. the differences are significant at the 99% level. shape parameter s for tn(h) profile – the msis-86 model strongly overestimates s both for disturbed (average scal/smsis = 0.48) and quiet (scal/smsis = 0.89) days. both deviations are significant at the 99% level. neutral composition at the 300 km height: atomic oxygen – the msis-86 strongly overestimates the [o] concentration both for disturbed (average [o]cal/[o]msis = 0.43) and quiet (average [o]cal/[o]msis = 0.84) conditions. the differences are significant at the 99% level. molecular oxygen – the msis-86 model overestimates the [o2] concentration for disturbed days (average [o2]cal/[o2]msis = 0.59, significant at the 99% level), while [o2] is close to the model values for quiet time conditions (the difference is insignificant). molecular nitrogen – the msis-86 model overestimates [n2] for disturbed conditions (average [n2]cal/[n2]msis = 0.62 significant at the 99% level), but underestimates [n2] for quiet days (average [n2]cal/[n2]msis = 1.17, significant at the 95% level). therefore, there is a problem with msis-86 predictions for disturbed conditions and this has been stressed repeatedly (see earlier), while in quiet conditions it gives the results which in general are close to our estimates. this is not surprising as any empirical model msis-86 presents fairly well only the conditions corresponding to the majority of observations, that is to quiet and slightly disturbed condi996 andrei v. mikhailov and jean lilensten fig. 4.5. a comparison of calculated versus msis-86 exospheric temperature tex, shape parameter s, and total neutral density ρ at 300 km for quiet (left-hand panels) and disturbed (right-hand panels) days. tex is in k, s is in 10-2 km-1 units, and log ρ is in gcm-3. dashed lines indicate a ± 25% band around the model. 997 a revised method to extract thermospheric parameters from incoherent scatter observations fig. 4.6. a comparison of calculated versus msis-86 [o], [o2], and [n2] at 300 km concentrations given in log scale for quiet (left-hand panels) and disturbed (right-hand panels) days. dashed lines indicate a ± 25% band around the model. 998 andrei v. mikhailov and jean lilensten fig. 4.7. diurnal variation of calculated tex, s, o, o2, and n2 to msis-86 value ratios. neutral concentrations are taken at 300 km height. two upper panels give eiscat observations of nmf2 and hmf2 for a quiet period of april 1-2, 1992. solar zenith angle variation is given in the bottom box. 999 a revised method to extract thermospheric parameters from incoherent scatter observations tions which are the most probable, while strong disturbances are relatively rare. although the dependence on 3-hour ap (or daily ap) global indices is formally included in the model, it gives a smoothed averaged pattern of the disturbed thermosphere and cannot describe the peculiarity of an individual disturbance. on the other hand, the closeness of our results to the quiet time msis predictions may tell us about the efficiency of the method developed. but for a convincing conclusion simultaneous isr and satellite probe in-situ observations are required, which are not available at present. 4.5. diurnal variations calculation of diurnal variation of thermospheric parameters is another testing of the method to check whether the found parameters demonstrate consistent variations during the day. all fits are independent in case of the stationary approach normally used during sunlit conditions, while they are slightly related when a non-stationary scheme is applied. eiscat observations for quiet days of april 1-2, 1992 were used for this analysis. the calculations were made for 24 ut moments with median ne, te, ti, vi profiles prepared over 1 h time intervals around the ut moments in question. figure 4.7 gives the observed nmf2 and hmf2 variations as well as ratios of calculated parameters to the msis86 values. solar zenith angle variation for the period in question is shown in the bottom panel. stationary solutions (as mentioned earlier) were used for sunlit ( χ ≤ 95°) period and a non-stationary scheme – for nighttime hours. the results are seen to be consistent for the sunlit hours. average ratios along with sd deviations for the parameters considered are given in table 4.ii for the sunlit hours. the difference from msis for average values is also estimated using the student criterion. the standard deviations with respect the average values are seen not to be large – about 3.5% for tex and s and 15-21% for neutral species. the difference of thermospheric parameters from msis values for this particular quiet day in general agrees with the statistical estimates given earlier. the results for nighttime hours are less satisfactory (fig. 4.7). large ratios for neutral species perhaps reflect large tex and s values, but they look as unreal although the quality of ne(h) fitting is excellent. the calculated tex turn out to be larger than observed ti values and this is impossible during nighttime hours when both temperatures should be close. reasons for such behavior are not clear right now and additional analyses are required. one of the possible factors is the electron precipitation taking place in the auroral nighttime f2-region, but this effect is not included in the model. changes in ne(h) profile resulted from additional corpuscular ionization, the method ascribes to thermospheric parameters which accept unreal values. 4.6. a severe storm (case study) the efficiency of the method can also be illustrated analyzing the periods of severe geomagnetic disturbances when large perturbations of thermospheric parameters are expected. the april 10, 1990 severe storm with ap = 124 presents a good example. both millstone hill isr (american sector, middle latitudes) and table 4.ii. average ratios of calculated thermospheric parameters to msis-86 values over the sunlit hours. the difference from msis values is estimated with the student criterion. parameter r(tex) r(s) r(o) r(o2) r(n2) average ± sd 0.99 ± 0.034 0.91 ± 0.033 0.86 ± 0.126 0.89 ± 0.190 1.03 ± 0.190 diff. from msis insignificant signif. at 99% signif. at 99% signif. at 97% insignificant 1000 andrei v. mikhailov and jean lilensten fig. 4.8. observed and calculated at eiscat (top) and millstone hill (bottom) ne(h) profiles for disturbed april 10, 1990 and quiet reference days. right-hand panels – calculated and standard model o+/ne profiles at the two locations. f2-region is seen to be strongly enriched with molecular ions for the disturbed day, while the calculations are close to the models for quiet days. eiscat (european sector, auroral zone) provided observations for that period, which was considered by mikhailov and foster (1997), mikhailov and schlegel (1998). observed and calculated ne(h) profiles as well as ion composition (o+/ne ratio) are shown in fig. 4.8 for eiscat (top panels) and millstone hill (bottom panels). the severe geomagnetic disturbance resulted in a complete disappearance of f2-layer at usual heights and the ionospheric maximum was presented by f1-layer peak – so called g-conditions. the disturbance picture is seen to be similar at two locations, but the storm effect was larger at millstone hill where electron concentration decreased by a factor of ten at the f2-layer heights. large changes in ion composition (o+/ne ratio) took place during that event. the ionosphere was strongly enriched with molecular ions even at the f2-layer heights, while this ratio was close to the standard model in quiet conditions. large deviations of o+/ne ratio from the standard model being used during the development of isr observations required a correction of the experimental te(h), ti(h) and ne(h) profiles for the disturbed day using the iterative method by mikhailov and schlegel (1997). strong perturbations of the thermospheric parameters were revealed for april 10 relative to the previous quiet days. at eiscat the calculated tex increased by 550 k 1001 a revised method to extract thermospheric parameters from incoherent scatter observations fig. 4.9. calculated and msis tn(h) profiles together with the observed at eiscat ti(h) profile for the disturbed day april 10, 1990. note that msis temperatures below 300 km are larger than ti – the situation is unreal for the conditions in question. compared to 230 k from msis prediction, and o/n2 ratio at 300 km decreased by 4.7 times compare to 2.6 from msis. at millstone hill tex increased by 680 k (173 k from msis) and o/n2 ratio at 300 km decreased by 19.3 times compared to 1.7 according to msis model. therefore, neutral composition and temperature changes are the main reasons for the observed strong decrease of electron concentration at the f2layer heights on april 10, 1990. such changes cannot be described using msis thermospheric parameters. the calculated temperature profile tn(h) also strongly differs from the msis one for that disturbed day. figure 4.9 gives three daytime temperature profiles at the eiscat location on april 10, 1990. a very low value of the shape parameter, s=0.0055 km−−1 instead of 0.016 km−−1 predicted by msis resulted from ne(h) fitting. msis tn(h) values are seen to be larger than observed ion temperatures below 300 km and this is impossible in reality. therefore, the method seems to give reasonable results in strongly disturbed conditions as well. a small value of s (0.011 km−−1 with respect to the msis predicted s = 0.017 km–1) was obtained for the same day, april 10, 1990 from the millstone hill data analysis. therefore, the results of calculations show a pronounced decrease in the neutral temperature in the low thermosphere for this severe geomagnetic storm. it was proposed (mikhailov and schlegel, 1998) that this tn decrease resulted from the enhanced atmospheric cooling through nitric oxide. the increase in no concentration during disturbed periods is well documented (e.g., barth, 1989, 1995; ridley et al., 1999; solomon et al., 1999). 4.7. discussion the proposed method demonstrates high efficiency allowing us to infer basic thermospheric parameters as well as ion composition from routine isr observations both in quiet and disturbed conditions. along with observed plasma parameters this gives a complete picture of the ionosphere and thermosphere state in the vicinity of the isr facility. isr have been operating for many years and this opens an opportunity to analyze thermosphere-ionosphere interaction in various geophysical conditions. among the problems which are the most interesting and challenging both from scientific and practical points of view is the f-region storm effects, and the developed method was widely used for such analyses (see the reference list). 1002 andrei v. mikhailov and jean lilensten our analysis of daytime eiscat observations has shown that msis-86 strongly and systematically overestimates [o] at high latitude both in quiet and especially during disturbed periods (fig. 4.6). similar results were obtained by litvin et al. (2000) analyzing with the energy equation millstone hill observations for a disturbed period of june 5-11, 1991. during the most active phase of the disturbance they found [o] to be lower by a factor of 2 than msis-86 predictions. on the other hand, their tex are lower than msis values by 200 k during daytime. although the authors stress that the shape parameter s strongly affects the results, they used the msis-86 values in their calculations, which are strongly overestimated for disturbed conditions (fig. 4.5). the tex ∗ s product demonstrates a sort of invariant in the inverse problem considered, therefore large msis-86 values of s should result in lower tex values in accordance with their conclusion. optical observations by grossmann et al. (2000) revealed a systematic (by 40% on average below msis-86) difference in atomic oxygen densities in the 130-175 km height range. therefore, independent results on [o] variations obtained with other methods seem to agree with our conclusions. but these are partial comparisons on one-two parameters while the proposed method provides information simultaneously on many thermospheric parameters. only simultaneous isr and satellite (similar to ae-c mission) observations of the whole set of thermospheric characteristics could be used for the overall testing of the method. unfortunately, there is no such opportunity at present. let us consider problems and limitations related to the usage of the method. firstly this concerns the isr observations. the quality of the experimental material is different at different facilities. for instance, due to rare (usually 3 per hour) observations at millstone hill a 3-4 h time interval is needed to calculate median profiles. it is not always possible (especially for disturbed days) to find a 3-4 h period of relative stability in nmf2 and hmf2 variations. this criterion is applied to decrease the scatter in the observations and increase the reliability of median profiles. unlike eiscat observations there is a problem with using routine vi(h) data at millstone hill as they may need an additional correction due to technical reasons. therefore, not all available routine millstone hill observations can be used for our analysis. the same difficulty takes place with kharkov isr facility which does not provide vi(h) observations at all. on the other hand, eiscat observations (each particular experiment) are not normalized by fof2 values as is done at millstone hill. although the declared uncertainty in the measured electron concentration at eiscat is not large − 10 ÷ 12% (farmer et al., 1984; kirkwood et al., 1986) in fact in some cases the difference in nmf2 (compared to nearby ionosonde observations) is much larger as the analysis by mikhailov and schlegel (2003) has revealed. the absence of such normalization also results in a shift between long pulse and multi pulse ne(h) profiles and this shift should be taken into account before the profiles are used for analysis. the problem with dependence of experimental te(h), ti(h) and ne(h) on ion composition was mentioned earlier. but despite the problems encountered the isr data are the most informative and consistent being the only observations which can be used with such a method to retrieve the aeronomic parameters. according to numerous testings of the method acceptable results can be obtained for sunlit ( χ ≤ 95o) conditions only when the f2-region is mainly controlled by photo-chemical processes and the contribution of each parameter is distinct, therefore it can be retrieved. during nighttime hours dynamical processes dominate and the contribution of individual parameters to the ne(h) distribution is not that distinct. this is seen in fig. 4.7 when after 22 ut all parameters are highly correlated and the reliability of each extracted parameter is not very high in this case. additional problems with eiscat nighttime observations are related to horizontal e × b drifts moving plasma via the location as well as with particle precipitation producing fresh ionization. neither effect is included in the model and cannot be taken into account properly. these additional factors are absent at middle latitudes and further analyses of millstone hill nighttime observations are required to clear up the possibility of the method in nighttime conditions. 1003 a revised method to extract thermospheric parameters from incoherent scatter observations the accuracy of calculated aeronomic parameters depends on many input quantities and can hardly be estimated strictly. only direct independent and simultaneous satellite observations of neutral composition and temperature could be used for such a strict comparison. such observations are absent at the moment. but the lowest estimate of expected errors resulting from the uncertainty in measured ionospheric parameters can be made (mikhailov and schlegel, 1997) with the help of expressions relating daytime f2-layer maximum parameters nmf2 and hmf2 with aeronomic parameters [o], β and w (ivanov-kholodny and mikhailov, 1986). from those expressions may be written the expressions for ∆log[o] and ∆logβ . . . .log loghm nm w1 25 10 0 578 1 42 10 log o 7.51 10 0.578 log 1.68 10hm nm w 2 2 3 2 # # # # = + = -b ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ 6 @ the uncertainty in measured electron concentration according to farmer et al. (1984) and kirkwood et al. (1986) gives ∆lognmf2 about ± 0.05. the hmf2 scatter around its median value usually is about ±10 km over the chosen one hour period of observations. the uncertainty in measured field aligned plasma velocity is ± 2 m/s at the height of 300 km (jones et al., 1986). this gives the uncertainty of ≈ 16% in [o] and ≈ 18% in the linear loss coefficient β. as β in the f2-region is mainly determined by molecular nitrogen, this estimate may be applied to [n2]. but it should be kept in mind that the above estimates are the least expected errors for the calculated [o] and [n2]. a comparison with an empirical (i.e. averaged) thermospheric model like msis-86 in quiet geomagnetic conditions can also give an estimate of the accuracy for the retrieved parameters. according to results given in figs. 4.5 and 4.6 the overall uncertainty in modelled tex is less than ± 10% and in neutral species is about ± (20-30)%. this is an acceptable result keeping in mind that the accuracy of many input parameters is of the same order. but the possibility of the method to extract thermospheric parameters for disturbed conditions is the most valuable result. the empirical models like msis are not efficient during storm periods and the proposed approach can provide useful information at least in the vicinity of the isr facility. 4.8. conclusions the results of our analysis may be summarized as follows. 1) further development of the self-consistent approach to the f2-region modeling has been made in three directions: 1.1) an increase in numerical stability has been achieved by taking into account only major ions o2+ and no+ in the expression (4.2) for the o+ diffusion velocity, while a complete list of pertinent chemical reactions is used to calculate ion concentrations. 1.2) an inclusion of nighttime ionization by scattered radiation in 1026, 584, 304, and 910 å lines allowed us to extend the area of possible solutions up to χ ≈ 95-100° at least from the evening side. 1.3) an extension of the method for non-stationary conditions allowed us to consider twilight and nighttime hours when the f2-region is not stationary. 2) application of the method to analysis of eiscat daytime observations (28 magnetically disturbed and 30 quiet days) has shown that msis-86 strongly overestimates atomic oxygen and the shape parameter s in particular for disturbed conditions (average [o]cal/[o]msis = 0.43 and scal /smsis = 0.48), while tex is close (the difference is 5-7% on average) to msis-86 predictions both in quiet and disturbed conditions. with regard to molecular species msis-86 strongly overestimates o2 and n2 concentrations for disturbed periods, while these concentrations are close to msis-86 values for quiet days. the results are statistically significant. a comparison on the total neutral gas density, which is important for practical applications, has shown that msis-86 overestimates ρ at 300 km for disturbed days with an average ρcal/ρmsis = 0.52 (significant at the 99% confidence level), while the deviation from msis-86 is in1004 andrei v. mikhailov and jean lilensten significant for quiet time conditions. but for a convincing testing of the method simultaneous isr and satellite (similar to ae-c mission) observations of the whole set of thermospheric parameters is required. unfortunately, there is no such opportunity at present. 3) the method can be efficiently used in sunlit ( χ ≤ 95-100°) conditions when the f2-region is under essential control of photo-chemical processes. the contribution of various thermospheric parameters to the ne(h) distribution is distinct in this case and they can be retrieved with an acceptable accuracy. during nighttime conditions when dynamical processes dominate the role of individual parameters is not that distinct and found solutions may be unsatisfactory. further analyses in this direction are required. 4) f2-region storm effects which are of great importance both for ionosphere physics and applications can be successfully analyzed with the method. retrieved neutral and ion composition, temperature and meridional wind give a complete picture of the thermosphere state in the vicinity of the isr facility for the conditions in question. keeping in mind that most satellite mass-spectrometric observations were made in the past and are not conducted currently, the method may be considered a real tool for investigation of the thermosphere-ionosphere interaction under various geophysical conditions. acknowledgements the authors thank the millstone hill group of the massachusetts institute of technology, westford; 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dominique lemur (1), louis bertel (1) and alain bourdillon (1) (1) institut d’electronique et télécommunications de rennes, université de rennes 1, france (2) institut d’electronique et télécommunications de rennes, centre de recherches des ecoles de coëtquidan, guer, france the aim of this paper is to present a high data rate transmission system through the ionospheric channel in the hf-band (3-30 mhz). the applications expected in this study are image transmitting and real-time videoconferencing. very high rates are required compared to the standard modems. therefore, an array processing is performed with a set of antennas whose spatial response differs from one another arranged in a circular array or in a collocated sensor. synchronization (zero crossing detector) and source separation (lms algorithm) resort to classical well-tested techniques involving training sequences. experimental results are presented for both antenna configurations. these techniques improve data rate, reaching 20 kbits/s within the 6 khz bandwidth (qam 64) without coding or interleaving. 13.1. introduction hf-waves (3-30 mhz), when propagated through the ionosphere, can achieve very long distance transmissions with a minimal infrastructure compared to satellite links, for example. this multipath multimode channel strongly degrades transmissions (mainly fading and frequency selectivity). for this reason, the data rate of standard scalar hf modems does not exceed 4.8 kbps in 3 khz bandwidth. the originality of this study is to use a heterogeneous array to improve the hf transmission. indeed, it has been shown that such a device could achieve the direction finding regarding the incoming polarization as a decorrelation factor and, consequently, that the diversity of the spatial responses could, to some extent, replace space diversity (erhel et al., 1998). the array processing is performed on a set of four active collocated antennas or a circular array and the corresponding coherent receiving channels. the spatio-temporal equalization resorts to the lms (least mean square) algorithm as the synchronisation is based on a «zero crossing detector». in order to introduce the heterogeneous array and to bring its advantages, it is necessary to start with a short description of ionospheric propagation and the effects on transmission. then it is shown that a heterogeneous array can improve the hf transmission and the real transmission system is described. finally, experimental results are provided to illustrate the theory by practical examples. 1146 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon 13.2. propagation of hf-waves in the ionosphere 13.2.1. multipath and multimode two main phenomena must be considered in ionospheric propagation. the first is that waves can follow several distinct paths by reflecting themselves against different layers of the ionosphere. then the transmitted signal gives birth to as many replicas that are affected in different ways by the channel: their differential time delays can be large, and their doppler shifts and angles of arrival can be spaced out too. the second phenomenon appears because of the earth’s magnetic field that makes the medium anisotropic. within each path, the wave splits into two complementary propagation modes called o (ordinary) and x (extraordinary). both are very close and therefore have similar time delays, doppler shifts and doa’s (direction of arrival). on the other hand, their polarization properties are very different. in the plane orthogonal to the direction of propagation, their electromagnetic fields describe orthogonal ellipses clockwise and counterclockwise (budden, 1985). multipath and multimode propagation are illustrated in fig. 13.1. 13.2.2. effects on transmission all the transmitted signal replicas created by multipath and multimode propagation interfere, degrading transmission quality. again, two cases can be distinguished that do not have the same consequences, although they are of the same nature. if two signals are delayed by an interval smaller than 1/b (where b is their bandwidth), they interfere in a way that the resultant signal fades, making the signal to noise ratio (snr) vary in time (an example is given in fig. 13.2). in the opposite case, the snr remains constant but frequency selectivity occurs. fading is due to multimode propagation. only the oand x-modes of the same path can be close enough to cause it. it is the most important problem for transmitting data since the signal is susceptible to completely disappear for a while. concerning frequency selectivity, it is caused by modes coming from distinct paths. when several modes are present, both phenomena may be observed. the time-frequency analysis of the signal displayed in fig. 13.2 is plotted in fig. 13.3. it represents the mapping of its power in the time-frequency plane, and is obtained directly through the observation of the broadband modulated signal (bisiaux, 2001). two sets of interference patterns are clearly visible, generating both fading and frequency selectivity. they denote the presence of at least 3 modes, among which 2 come from the same path (the line spectrum that appears every 4 s is not a computation aberration, it is due to a periodic pattern inserted into the transmitted signal). fig. 13.1. multipath and multimode propagation in the ionosphere. 1147 a way to increase the bit rate in ionospheric radio links 13.2.3. improvement of hf transmissions resorting to arrays of receiving antennas 13.2.3.1. classical arrays most of the time, arrays are made of identical elements set distant from one another (they are homogeneous arrays). space diversity allows them to steer their main lobe toward the doa of a desired signal, or to create notches in the direction of undesired ones. however, such arrays would not be a very good solution for hf communications applications. the reason for that is illustrated in fig. 13.4: they would be efficient to combat multipath effects, but would not be able to separate the complementary modes of a single path because they usually leave the ionosphere at very close angles. the spatially filtered signal would then still be affected by fading. furthermore, given that the wavelength ranges from 10 to 100 m, such arrays would spread over a large area, making them rather impractical. for these reasons, homogeneous arrays are rarely used for transmission purposes in the hf-band. yet it is possible to suppress fading by separating the oand x-modes, but it requires including some kind of polarization treatment in the whole process, which is not possible with homogeneous arrays. fig. 13.2. example of a signal fading (qam-16, 3 khz bandwidth, f~ 8 mhz). fig. 13.3. time-frequency analysis of an hf signal at intermediate frequency if = 2.25 khz. 13.313.2 13.4 13.5 fig. 13.4. source separation with a classical array. fig. 13.5. source separation with a heterogeneous array. 1148 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon 13.2.3.2. heterogeneous arrays with antenna diversity, it is possible to join the ability of an array to alter its radiation pattern on the one hand, and polarization filtering on the other hand. this leads to the realization of a heterogeneous array, that is to say, made of antennas with various shapes or orientations, as opposed to classic homogeneous arrays. its application to hf transmissions can be illustrated in a simplified way by the drawing in fig. 13.5: the radiation pattern of the array is not the same for all the waves because it depends on their polarization. since oand x-modes coming out of the ionosphere have different polarization properties, two different radiation patterns must be considered instead of one, as is usually the case. one regards o-modes, and is completely unrelated to the other one that concerns x-modes. for this reason it is theoretically possible to separate two complementary modes, even if they come from the same direction, and still being able to create notches in the two radiation patterns. such a principle is ideal for combating both multipath and multimode propagation. as a consequence, the steering vector of a heterogeneous array is expressed as a f e f e f e / ( ) ( ) ( ) ho x j j nc j 1 2 2 2 2 / / / o x o x o x $ $ $ f= { i { i { i r t s s s s v x w w w w where the ϕκ (θ) are the geometrical phases of a classical array and fno/x is the spatial response of antenna n for the two expected polarizations. 13.2.3.3. expressions of the spatial correlation for an array of isotropic sensors the spatial correlation of two incident sources with doa θ1 and θ2 is expressed as 1 nc e a a a a h j nc 1 2 1 2 n n2 1 $ = =t i i i i = { i { i n ! ^ ^ ^ ^ ] ] h h h h g g8 b using a heterogeneous array with the same geometry, the expression of the spatial correlation becomes in a general case (whatever is the incident polarization) a a a a f a f a f a f a h h h h h 1 2 1 2 1 1 2 2 1 1 2 2 $ 7 $ 7 7 7 = =t i i i i i i i i i i i i ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ h h h h h h h h h h h h8 b where f(θ) is the vector of the spatial responses for the doa θ. this quantity can be written under the following form: 1 1 1 e f f f f h n nc n nc n n j nc 1 2 2 2 1 2 n n2 1 $ $ =t i i i i) = = = { i { i n n n ! ! ! ^ ^ ^ ^ ] ] h h h h g g8 b the modulus of the spatial correlation is modified for this type of array: ρh≠ρ. 1149 a way to increase the bit rate in ionospheric radio links 13.2.3.4. improvement due to the heterogeneous array a comparison of the two quantities ρh and ρ does not provide a general result: the heterogeneity of the array does not systematically decrease the spatial correlation. however, it can be mentioned that current situations of reception are characterized by very close angles of arrival though the spatial responses are significantly different for the doa θ1 and θ2: it is the case with diversely polarized electromagnetic waves impinging from the same direction. the geometrical phases are nearly equal in these conditions (ϕn(θ1) ≅ ϕn(θ2) ) and the modulus of the spatial correlation for the homogeneous array is close to 1: ρ ≈1. on the other hand, considering the correlation for the heterogeneous array, the modulus ρh approaches the value 1 1 1 f f f f h n nc n nc n n nc 1 2 2 2 1 2 $ $ ,t i i i i) = = = n n n ! ! ! ^ ^ ^ ^ h h h h . this ratio is less or equal to one, according to the schwartz inequality: the spatial correlation decreases in this case. 13.2.4. numerical simulation in the following simulation, hf radio waves are assumed to be received after propagation through the ionosphere. it involves two uniform circular arrays made up with 8 loop antennas (figs. 13.6 and 13.7). this geometry has a practical advantage since the accuracy of the angular estimation is not dependant on the actual azimuth of arrival in this case. the first array is made up with 8 identical sensors (homogeneous array) and its spatial correlation is ρ. in the second one, the antennas are submitted to a rotation of 30° around a vertical axis every two positions within the array, so fig. 13.6. homogeneous circular array. fig. 13.7. heterogeneous circular array. 13.6 13.7 1150 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon that it becomes heterogeneous with a spatial correlation ρh. this angular value realizes a compromise between a significant diversity of the spatial responses and the existence of redundancy if the rotation exceeds 180°. the spatial responses of the antennas are computed according to the model described in section 13.2. the numerical parameters are: radius r = 0.5 λ, λ denoting the wavelength, and carrier frequency fo =15 mhz. two incident waves are impinging with a common elevation and orthogonal polarizations. the first signal has a fixed azimuth of arrival (az1=180°), and the second azimuth az2 varies in the range of 0° to 360°. the simulation compares the spatial correlation of this couple of incident waves (considered as a function of the variable azimuth az2) for the homogeneous and the heterogeneous array. whatever the method of direction finding, the less is the modulus of the spatial correlation, the easier is the separation of the signals. the critical case corresponds to a modulus close or equal to one for which the separation fails. figure 13.8 plots the spatial correlations for both arrays (modulus of ρ and ρh) as function of the variable azimuth az2. the benefit provided by the heterogeneous array appears in an interval of az2 containing the value of az1=180°. in that area, the modulus ρh is significantly smaller than ρ. in particular, in the critical case of two superimposed incident waves (az2 = az1=180°), the modulus ρh is equal to 0.82 and ρ to 1: the heterogeneous array can separate the incident waves (with orthogonal polarizations) while the homogeneous array cannot. ρh is not less than ρwhatever the azimuth, but when it exceeds this reference value, the corresponding values are significantly smaller than 1, so that the doa estimation works correctly. 13.3. the transmission system the link used for the experiments has been established between poitiers (latitude: 46°35′n, longitude: 0°25′e) and monterfil (latitude: 48°03′n, longitude: 2°00′o) near rennes (both in fig. 13.8. comparison of the spatial correlation of signals. 1151 a way to increase the bit rate in ionospheric radio links france). it is about 300 km long. figure 13.9 gives a schematic view of the link and of the devices employed. this is a short link for an hf transmission that is normally used to cover several thousands of kilometers. 13.3.1. the transmitting system the transmitting system is located in poitiers. in order to provide a good flexibility, it was decided that the whole digital signal processing would be done with a computer: digital modulation and filtering, possibly interleaving and coding, etc. this way it is possible to control very easily the type of modulation used (psk, qam, ofdm, …), the sampling frequency and the signal’s bandwidth. the system is composed of a personal computer, a complex modulator, an hf power amplifier connected to a delta antenna. the complex modulator’s architecture is shown in fig. 13.9. its role is simply to convert digital data sent in real-time by the pc into analog signals, through the means of dacs (digital to analog converters), and then to transpose them into the hf-band. the operation is complex in that both real and imaginary parts of the complex baseband signal (i and q channels respectively) are sent to the modulator, which also generates a complex carrier with a dds (direct digital synthesiser). let x(t) = a(t) + jb(t) be the complex baseband signal. the synthesized signal can be expressed as ( ) ( ) ( ) ( ) ( )cos sins t a t t b t t= -~ ~ . which represent the real part of the equivalent complex signal (called analytical signal) ( ) ( )s t x t e j t 0= ~ # -. the carrier frequency is programmable through the pc, from 0 to 13 mhz by 0.01 hz steps. the phase shift between channels i and q of the dds is adjustable too, which can be used to compensate for phase errors due to electronic components. the sampling frequency can be set between 180 khz and 1 mhz with 20 ns steps in period. it should be set high enough in order to make the rejection of the mirror spectrum easy, while enabling the emission of a wide-band signal. however, it remains limited by the throughput of the pc’s bus, which must send data in real-time. the dacs are followed by low-pass filters with a 100 khz cut-off frequency. thus the maximum bandwidth of the signal is 200 khz. fig. 13.9. schematic of the poitiers-monterfil link. 1152 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon 13.3.2. the receiving system the system used as a receiver is located in monterfil. it was at first designed as a direction finding system and sounder, and then adapted to the current application. therefore it is not as flexible as the transmitter. the array is a set of 8 collocated active antennas, as described by marie et al. (2000) or a heterogeneous circular array, as depicted in fig. 13.7. the collocated array is followed by 8 identical receivers which transpose the incoming hf signals onto a 30 khz intermediate frequency. then they are filtered using a quartz filter with either with a 3 khz or a 12 khz bandwidth. the sampling is performed at a programmable rate and the data is stored on the pc disk. the most restrictive features of the receiver are its bandwidth (12 khz maximum), which might become a concern in future experiments, and the access time of the pc’s hard drive. indeed, a few seconds of data result in files of several megabytes which must be saved in real-time and too long acquisitions often make the acquisition board overflow. 13.3.3. spatio-temporal equalization as ns sources are incident on the array, the signal at the output of one antenna has the following form: 1 ( ) ( ) ( )x t s t n tfi i k k i ns = +i =k ! ^ h . a source separation method tends to estimate a set of weighting factors wik so that the filtered signal ( )s w x tk ik i nc 1 = !t is the closest estimate of sk(t). in the application, the classical least mean square (lms) algorithm is processed. the goal is to minimize the power of the error between the estimated signal ( )s tkt and the reference signal sk (widrow and mccoll, 1976) resorting to a descent along the gradient. it is assumed that a training sequence is transmitted to make the estimation of the weighting factors converge. then, the effective data transmission begins and their real-time computation involves the estimated received data instead of the training data. 13.3.4. synopsis of the signal processing performed at reception in the complete receiving system which has been developed, the array processing is only one among a set of functions including: – matched filtering (to the transmitted wave form); – phase and frequency synchronisation; – time synchronisation (optimal placing of the sampling times); – interpolation. figure 13.10 summarizes all these operations. the input signals xi(t) are the received signals transposed in base band. their phases are corrected by the phase synchronisation loop. the synchronisation techniques are based on an approach of the maximum of likelihood running with the spatially filtered signal. each symbol is over sampled with a ratio of 10: t/ te =10 if t denotes the symbol duration and te the sampling period. 1153 a way to increase the bit rate in ionospheric radio links a good temporal resolution in the placing of the sample times needs an interpolation: a parabolic interpolator with 4 coefficients has been used. the loop yields an estimate kxt of the time delay at the kth iteration; its recursive expression integrates an error signal generated by a «zero crossing detector» 1+ e= +x ck k kxt t with 1 1-/c c kt te 2k k kk 0= + x ) ) st t t t_ bi j( 2 . the quantities with a star denote the conjugates of the estimated complex data and the sample of the filtered signal is picked up with a rate twice faster than the symbol rate (mengali and d’andrea, 1997). 13.4. results 13.4.1. characteristics of the signals received using the 4 collocated antennas the results presented in this section are based on an example that reflects the behavior of the system in the majority of cases. only 4 antennas were used out of the 8 available ones because the other 4 proved to be unnecessary: they did not help to improve the transmission performances due a low level of signals for the tested link and, moreover, they induced a larger computational time. the 4 elements of the array, as pictured in fig. 13.11, are: – a vertical square loop contained in the east-west plane; fig. 13.10. synopsis of the receiver. 1154 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon – a vertical square loop contained in the north-south plane; – a horizontal square loop; – an xyz dipole. the transmitted signal has the following properties: – 16-qam modulation with rolloff factor of 0.2; – 3 khz bandwidth; – 10 kbps (2500 baud); – carrier frequency of 8 mhz. the received signals are plotted in fig. 13.12. they are all affected by fading, which denotes that propagation is mainly provided by 2 complementary modes of a principal path. in such a case, these modes come from very close directions, however the array’s antenna diversity is enough to introduce a significant difference between the 4 observations. this remark is reinforced by the time-frequency representations of the first two signals (figs. 13.13 and 13.14). two interference patterns appear (fig. 13.3); in addition to fading (the most visible one), some frequency selectivity shows that a second fig. 13.13. time-frequency analysis of the second channel at if (vns). fig. 13.14. time-frequency analysis of the first channel at if (vew). 13.13 13.14 13.11 13.12 fig. 13.11. schematic of the collocated array actually used (4 antennas). fig. 13.12. signals received by the array (vew, vns, h, xyz). 1155 a way to increase the bit rate in ionospheric radio links path also exists. but for these two interference patterns, the signal fades at different instants and frequencies from one antenna to the other, what underlines the decorrelation provided by the diversity. 13.4.2. the heterogeneous array: a way to improve hf transmissions the adaptive method based on the lms algorithm has been applied successively to 2, 3 and 4 antennas in order to evaluate the influence of the array’s size. – 2 antennas: the propagation conditions are too constraining during the first seconds of the transmission and the algorithm is unable to keep track. however, it starts to converge again after 6 s and then the ber remains below 10-2 independently of the fading (fig. 13.15). the average ber in this case is 2 × 10-2, not much higher than in the best case with a simple polarization filter. obviously, two antennas are not enough to realize a reliable filter. – 3 antennas: this time the algorithm converges immediately (fig. 13.16) and the ber always remains below 3 × 10-2 with a mean value of 3.7 × 10-3. – 4 antennas: since the previous case led to very good results, the fourth antenna does not bring as much change as the third one (fig. 13.17), but still the ber is improved with an average of 2.8 × 10-3. the snr obtained in the latter case (after adaptive filtering) is reproduced in fig. 13.18, along with the raw snr received on the first antenna (the same pattern can be observed on the 3 others with fig. 13.15. ber obtained using 2 antennas. fig. 13.16. ber obtained using 3 antennas. 13.15 13.16 fig. 13.17. ber obtained using 4 antennas. fig. 13.18. comparison of snr on antenna vew with snr after filtering. 13.17 13.18 1156 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon fading occurring at different times). this figure shows the advantage of antenna diversity: whatever the method employed to demodulate the signal, the quality of the transmission can only be improved when the snr remains almost constant and is not subject to deep fading. 13.4.3. experimental results with the array of 4 collocated antennas the following experiments are obtained with 4 collocated antennas as shown in fig. 13.11. several transmissions have been made at the end of year 2000 with carrier frequencies between 6 and 9 mhz (between poitiers and monterfil). acquisitions were done over 20 s. no coding, interleaving or soft decision was employed. 13.4.3.1. use of a 3 khz bandwidth the waveform is a qam-16 or qam-64 modulation. its envelope has a roll-off factor equal to 0.2 and the symbol duration is equal to 0.4 ms (2500 bauds). 13.4.3.1.1. first example: qam-16 the results presented here have been obtained for a typical case where deep fading appears, due to interferences between two close complementary modes. figure 13.19 shows the set of 4 signals at the array output. their minima and maxima do not occur at the same time, which proves that their association can bring diversity other than space diversity. the complex samples which are collected at the output of the spatial filtering (before the hard decision) are plotted in fig. 13.20. however, the most energetic symbols are not exactly centered at the corners of the map: that indicates that the power stage of the transmitter is not linear for corresponding signals. the corresponding bit error rate (ber) is plotted in fig. 13.21. the mean value is 2.7×10-3 and the data rate is 9700 bps. fig. 13.19. four channel acquisition. fig. 13.20. constellation at the output of the spatial filter. 13.19 13.20 1157 a way to increase the bit rate in ionospheric radio links 13.4.3.1.2. second example: qam-64 in the following experiment, the waveform was a qam-64 modulation and the bit transfer rate equal to 15 kbps. the average signal to noise ratio (before the spatial filtering) equals 20 db. the corresponding bit error rate is plotted in fig. 13.22 and the complex samples collected at the output of the spatial filtering are plotted in fig. 13.23. in this case the mean ber value is 7.2 × 10-3. 13.4.3.2. use of a 6 khz bandwidth the waveform is a qam-16 modulation. its envelope has a roll-off factor equal to 0.2 and the symbol duration is equal to 0.2 ms (5000 bauds) involving a bandwidth of 6 khz. if the channel parameters are still the same, the snr is decreased by 3 db. figure 13.24 shows the set of 4 signals delivered at the array output. the mean snr is 18 db. the variations of the ber is plotted in fig. 13.25 and the mean value is 2.3 × 10-4. the data rate is 19.4 kbps and the corresponding constellation is plotted in fig. 13.26. fig. 13.21. variations of ber. fig. 13.22. temporal variations of ber. 13.21 13.22 fig. 13.23. constellation with a qam-64. fig. 13.24. signals received on the 4 collocated antennas. 13.23 13.24 1158 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon 13.4.4. experimental results: heterogeneous circular array of 4 loop antennas the following experiments involved a heterogeneous circular array of 4 loop antennas and the transmitted data are images. the goal is to design a still image transmission system with a good reception quality in presence of transmissions errors (souhard et al., 2003). that work was developed at the ircom-sic laboratory of poitiers (france) jointly with the ietr in rennes (france). the aim is to find an efficient transmission system, in terms of visual quality for the received image. usual source coding schemes are inappropriate when errors occur during the transmission. many errors correction systems exist but they significantly decrease the compression ratios for a low errors resistance. to solve this problem, a vector quantization (vq) algorithm based on the kohonen topological maps (sofm: self organization feature map) is used. the image used for testing is shown in fig. 13.29. fig. 13.25. temporal variations of the ber. fig. 13.26. constellation at the output of the spatial filter. 13.25 13.26 fig. 13.27. constellation at the output of the spatial filter. fig. 13.28. state constellation diagram (6.7 mhz). fig. 13.29. original transmitted image. 13.27 13.28 13.29 1159 a way to increase the bit rate in ionospheric radio links 13.4.4.1. results several acquisitions were made on 20th february 2003 at frequencies between 6 and 9 mhz. for this experiment, the signal bandwidth was limited to 3 khz. the modulation is a qam-16 and the symbol’s duration is 0.4 ms. the transmitted files duration varies between 4.9 s and 5.8 s. they are sent continuously over 20 s. the number of acquisition files is 72. for many reasons, some of them are unusable: too weak signal due to bad propagation conditions, jammer or important noise pulse, etc. the number of useable files is 57 and table 13.i presents a list of the performances. the results obtained at the first carrier frequency (6.99 mhz) are rather good: the constellation points are clean (fig. 13.27) and the ber is small. when using the second and third carrier frequencies (7.97 mhz and 8.91 mhz), the results are not flawless: the constellation points are not as clean as in the previous example (fig. 13.28) and the ber is larger. the image in the fig. 13.30d contains twice as much errors than others images. however, the coding gain is 7.19 db in relation to spiht (fig. 13.30c), 8.13 db in relation to jpeg2000 (fig. 13.30b) coding and 18.14 db in relation to jpeg (fig. 13.30a). for the same compression rate, the sofm associated with qam modulation fits very well for transmission through the ionospheric channel. table 13.i. number of data files recorded at 3 frequencies and corresponding ber. carrier frequency ber interval number of files 0 ≤ ber ≤ 10-4 21 6.994 mhz 10-4 < ber ≤ 10-3 0 (27 files) 10-3 < ber ≤ 10-2 0 unusable 6 0 ≤ ber ≤ 10-4 2 7.967 mhz 10-4 < ber ≤ 10-3 16 (22 files) 10-3 < ber ≤ 10-2 3 unusable 1 0 ≤ ber ≤ 1.10-4 0 8.917 mhz 10-4 < ber ≤ 10-3 5 (8 files) 10-3 < ber ≤ 10-2 3 unusable 0 fig. 13.30a-d. images received using different number of antennas. a b c d 1160 clency perrine, yvon m. erhel, dominique lemur, louis bertel and alain bourdillon 13.5. conclusions and outlooks this paper presents a way to achieve a significant improvement of the data transfer rate in a hf radio link by the use of a heterogeneous array. the actual rate of the system reaches up to 20 kbits/s (in 6 khz bandwidth) whereas the current standard for scalar modems is 4.8 kbits/s in a 3 khz bandwidth. actually, only one link has been tested with a relatively short range and limited number of paths. moreover, no coding, interleaving or soft decision techniques were employed to reach these performances. the results obtained in terms of ber (about 10-3) are considered to be acceptable for hf modems. with a 4 circular antenna array, a qam modulation and a robust source coding, the sofm method fits very well for transmission through the ionospheric channel. future work will consider longer range links. in this context, the number of received signals will increase in relation to the number of ionospheric paths so that the demodulation processing will have to cope with more severe conditions of reception. in addition, investigations will be performed to adjust the transmitted polarization in order to reduce the number of modes propagating through the ionosphere and this will reinforce the spatial filtering. another development will be the implementation of a blind algorithm for spatio-temporal equalization in the receiving system, replacing the lms algorithm. the benefit will be to avoid the transmission of training sequences and, consequently, to increase the effective data transfer rate. references bisiaux, a. (2001): définition, conception et réalisation d’un modem vectoriel large bande utilisable dans la gamme hautes fréquences, ph.d. thesis (université de rennes 1). bisiaux, a. and l. bertel (2000): antenna diversity as a means of improving hf transmissions, in proceedings of the iee «hf radio systems and techniques», guildford, u.k. budden, k.g. (1985): the magneto-ionic theory and its applications to the ionosphere (cambridge university press). erhel, y.m., l. bertel and f. marie (1998): a method of direction finding operating on an array of collocated antennas, in ieee-ap international symposium, atlanta. marie, f., y.m. erhel, l. bertel and d. lemur (2000): design of a hf compact direction finding system based on collocated antennas, in proceedings of the iee, hf radio systems and techniques, guilford, u.k. mengali, u. and a. d’andrea (1997): synchronization techniques for digital receivers (kluwer academic/plenum publishers). souhard, b., c. chatelier and c. olivier (2003): on the robustness of joint source/channel coding for transmission through an ionospheric channel, in proceedings of the 9th international conference on hf radio systems and techniques, june 2003, bath, u.k. widrow, b. and j.m. mccoll (1976): a comparison of adaptative algorithms based on the methods of steepest descent and random search, ieee trans. ant. prop., 24, 615-637. i n i p o r t a n z a d e l l a l e g g e di h a l l n e i f e n o m e n i i o n o s f e r i c i (*) a . p o z z i g . c0ntub.si (**) r i c e v u t o il 18 maggio 1963 r i a s s u n t o . — i11 questo lavoro si s o t t o l i n e a c o m e p e r s t u d i a r e l'influenza della anisotropia della c o n d u t t i v i t a e l e t t r i c a (fenomeno che d a luogo alia legge di hall) sia necessario risolvere due tipi di p r o b l e m i . ii p r i m o tipo di p r o b l e m i r i g u a r d a la f o r m u l a z i o n e di teorie che cons e n t a n o di esprimere a n a l i t i c a m e n t e la c o n d u t t i v i t a . a t a l i a r g o m e n t i e d e d i c a t a la p r i m a p a r t e di q u e s t a n o t a . nella seconda p a r t e si analizza l ' a l t r a serie di p r o b l e m i , r i g u a r d a n t i lo studio del moto macroscopico di u n a c o r r e n t e fluida il cui coefficiente di c o n d u t t i v i t a sia anisotropo. summary. —• this n o t e deals w i t h t w o p r o b l e m s r e l a t e d t o a n i s o t r o p y of t h e electrical c o n d u c t i v i t y . i n t h e first p a r t of t h e p a p e r t h e possibility t o e v a l u a t e t h e a n a l i t i c a l expression of t h e coefficient of electrical c o n d u c t i v i t y h a s been shown, t a k i n g i n t o a c c o u n t some r e s u l t s s t a t e d b y j a n c e l a n d k a h a n . i n t h e second p a r t a s u i t a b l e simplified analysis e n a b l e d us t o discuss t h e macroscopic motion of a n ionospheric flow. 1 . introdtjzione. numerosi autori hanno segnalato l'importanza del cosiddetto effetto hall per lo studio dei mezzi ionizzati in misura tale quale si riscontra nella ionosfera. (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. (**) i s t i t u t o di a e r o n a u t i c a u n i v e r s i t a di napoli. 146 a . p o z z i g. c o n t u r s i tale studio essenzialmente comporta la impostazione e la risoluzione di due problemi. ii primo di questi eonsiste nella determinazione del coefficiente che definisce l'effetto hall. tale determinazione di u n a proprieta costitutiva del gas, come sara subito illustrato, va correttam e n t e e f f e t t u a t a sulla base di descrizioni statistic-he. al successivo paragrafo 2 tale impostazione verra e l f e t t u a t a valendosi degli accurati risultati stabiliti da j a n c e l e kalian (2) ed utilizzando determinazioni sperimentali della distribuzione elettronica nell'atmosfera o t t e n u t e alcuni anni f a m e d i a n t e satelliti (3). ii secondo jiroblema eonsiste nella descrizione macroscopica delle proprieta di u n mezzo dotato di conduttivita di hall. tale descrizione risulta e s t r e m a m e n t e complessa sta-nte la non linearita che la presenza di campi elettromagnetici aggiunge a quella caratteristica delle equazioni fluidodinamiclie. nel presente lavoro, basandosi su scliematizzazioni semplificatrici, si mettono in evidenza alcune caratteristiclie proprieta derivanti dalla presenza dell'effetto hall. 2 . l a c o n d u t t i v i t ! elettrica n e i gas. u n a delle difficolta coniuni a t u t t e le formulazioni macroscopicbe e quella della determinazione di opportuni coefflcienti che tengano conto di fenomeni che si svolgono su scala microscopica. cosi nel campo della fluidodinamica per tener conto degli sca-mbi di q u a n t i t a di moto su scala microscopica si introduce il coefficiente di viscosita. quanto poi alia possibility di valutarlo si deve ricorrere o all'esperienza o a teorie del t-ipo statistico. analoghi problemi si incontrano nello studio del flusso di cariclie elettriche. la formulazione macroscopica postula u n a relazione del tipo j = cre, dove e e l'intensita del campo elettrico e a e il coefficiente di conduttivita elettrica: vogliamo ora brevemente discutere sulla n a t u r a e sui metodi di calcolo di questo coefficiente. i n condizioni normali, quando cioe il mezzo e isotropo dal p u n t o di vista elettrico, il coefficiente di conduttivita e u n a grandezza scalare; in particolari condizioni invece, come accade nella ionosfera questo coefficiente ha c a r a t t e r e tensoriale. la ionosfera e u n mezzo debolmente ionizzato in cui sono presenti tre specie di particelle: ioni, elettroni e molecole neutre. ii modo pid i m p o r t a n z a d e l l a l e g g e d i i i a l l n e i f e n o m e n i i o n o s f e r i c i 1 4 7 corretto di studiarne le proprieta e quello di ricorrere all'equazioue integi-o-differenziale di b o l t z m a n n (j). alio seopo di effettuare delle valutazioni numeriehe, e il caso di ricordare brevemente la teoria magnetoionica dei gas leggermente ioiiizzati, f o n d a t a sull'equazione di boltzmann, come esposta da j a n c e l e kalian (2). questi considerano u n a miscela di due gas, molecole n e u t r e ed elettroni, sotto l'azione di u n campo elettrico oscillante con legge sinusoidale del tipo e cos w t e di un campo magnetico h0. si trascura il contribute dovuto agli ioni in quanto nei caso di gas debolmente ionizzato questo e trascurabile. altra ipotesi fondamentale e quella di considerare elastici gli u r t i t r a molecole ed elettroni. p e r descrivere le proprieta di questa miscela e allora necessario introdurre due funzioni di distribuzione /l ed /2 rispettivamente per molecole ed elettroni. indicando inolt-re con fx e v2 la veloeita delle due specie e con i<\ l'azione eletromagnetica sugli elettroni del campo e h„ le due equazioni di boltzmann si scrivono: i secondi membri rappresentano i contributi degli u r t i tramolecole ed elettroni. ii gas e considerate lorenziano; valgono cioe le seguenti due ipotesi: a) la massa delle molecole del primo costituente e grande rispetto a quella del secondo costituente. questa ipotesi e senz'altro verificata in quanto il rapporto t r a molecole n e u t r e ed elettroni e molto grande. b) l'influenza degli u r t i t r a le particelle del secondo costituente e trascurabile rispetto a quella degli u r t i tra le molecole del primo costituente. questa ipotesi e accett.abile perche si considera un gas debolm e n t e ionizzato. questa ipotesi e molto i m p o r t a n t e perche consente di concludere che come distribuzione di veloeita relativa alle molecole neutre puo essere assunta quella maxwelliana. si ha cioe: dove ih e il numero di molecole per u n i t a di volume e t e la t e m p e r a t u r a assoluta. 2 [2.1[ [2.2] [ 2 . 3 ] 148 a . p o z z i g. c o n t u r s i ii problema e quindi ridotto a quello della risoluzione dell'equazione [2.2]. essendo la forza f 2 che agisce sull'elettrone d a t a d a : j ^ ^ j j c o s w h («» a h„) [2.4] m2 m2 (dove e2 e la carica dell'elettrone) l'equazione [2.1] si scrive: , „ , [2.5] iu — cos cot + j v2 a ho • g r a d t , 2 f 2 = j j j j^fif*—/i/2j gbdedv^lb dove f \ ed / ' 2 sono i valori delle funzioni /x ed /2 per i valori v\ e v'2, cioe per i valori che le velocita vy e v2 assumono dopo l'urto, g e la velocita relativa delle due particelle [g = | vx — v2 | = | v\— v 2 j e b ed e sono i p a r a m e t r i d ' u r t o (*)]. l a funzione di distribuzione /2 non e maxwelbana per la presenza della forza lorentziana f2 e quindi lo scostamento da questa e t a n t o quanto piu f o r t i sono i campi elettrici e magnetici. j a n c e l e k a h a n (2) prendono in esame campi eletromagnetici di intensita qualsiasi. ii metodo seguito eonsiste nello sviluppo della funzione di distribuzione in termini delle sue armoniche sferiche, ed e spinto fino alia valutazione dell'approssimazione del primo ordine. si pone cioe la funzione f2 sotto la f o r m a : 'e2e \ / ... . . a . / „ . ee2 /2 = /(0)2+ — • v2 a ^ c o s f t r f f / s ^ s e n a r f + \h0 a — th2 j \ r j \ m2 i [2.6] v2ua^2coso)t + r]w2senc»t]j + ho a [ho a — m, v2 cos cot + <5 (1) 2sen cot. i n questa espressione compaiono 7 funzioni incognite: /<•>„ a<v„ ^ > 2 , ywt, . queste vengono determinate sostituendo la [2.6] nell'equazione di boltzmann [2.5] ed eguagbando i due membri. d a questa formulazione appare chiaro come i termini di anisotropia ee2 _ . ee2 _ . /_ ee2 sono: — , h0 a — , h0 a \h„ > —-m2 m2 \ m2 e interessante n o t a r e che nel caso generale, per valori qualsiasi di e e , anche la zero approssimazione /<°> non risulta maxwelliana. si m2 , e2e ^ riduce a questa se e: — < 1 . m . i m p o r t a n z a d e l l a l e g g e d i i i a l l n e i f e n o m e n i i o n o s f e r i c i 149 questa dipende essenzialmente dal libero percorso medio che a sua volta dipende dalla legge di interazione t r a molecole ed elettroni. si ha cioe: x — v212.n wx (v2) e: oo ^12 (v2) = i ( 1 — cos x) gbdb o dove x rappresenta la variazione angolare della veloeita relativa dur a n t e u n a eollisione. d u n q u e per conoscere l'approssimazione di ordine zero e necessario conoscere la legge di interazione t r a molecole ed elettroni. due tipi di schematizzazioni sono i m p o r t a n t i : a) molecole sferiche e rigide con u r t i elastici; a e allora indipendente da v2 b) la forza t r a molecole ed elettroni varia come 1/r5; l a f r e q u e n z a di eollisione v = f2/a c allora indipendente da v2 ed /( 0 ) 2 6 maxwelliana con u n a t e m p e r a t u r a elettronica d a t a d a : r = t + mx (—2)" h(v) 67ctfco2 + ^ ) . \ mi 1 \ 1 u n a volta n o t a la funzione di distribuzione f2 e possibile conoscere la veloeita di diffusione w2 degli elettroni e quindi il flusso di cariche elettriche cioe la corrente i d a t a da: i = n 2 e 2 w 2 . [2.7] tale grandezza h a c a r a t t e r e vettoriale si pu6 scrivere i n f a t t i : [4c7in2e2\ i = \ 3m2 jte + j2 [ho a e + j3ho a [ho a e [2.8] dove jx, j2 e j3 sono grandezze scalari definite in termini delle grandezze introdotte nella [2.6]. come si vede vi e u n contribute parallelo al campo elettrico proporzionale ad jlf uno perpendicolare sia ad e che h0 ed uno perpendicolare ad h0 ed alia normale ad h0 ed e. due casi limiti sono interessanti: h0 parallelo ad e in cui la corrente i e parallela e e non risente del campo magnetieo ed h„ perpendicolare ad e. i n questo caso si manifestano nella massima intensita i fenomeni di aniisotropia del coefficiente di conduttivita elettrica. 150 a . p o z z i g. c o n t u r s i i n particolare in questo caso si h a u n a corrente elettrica perpendicolare al campo elettrico e magnetico. questo fenomeno costituisce la legge di hall. ii coefficiente di hall, definito come rapporto t r a campo elettrico applicato e la corrente i e quindi: e y 3 m2 ho j 2 b = ~ = 4 7i n2 e2 j1 — ho2 js ho2j\ [2.9] per co = 0, per /<°>2 maxwelliana e v\wh < 1, e b = (3tt/8) h0/n2e2 . [2.10] l'interesse per la precedente trattazione risiede nel f a t t o che consent® u n a i m m e d i a t a valutazione della conduttivita di hall, secondo la [2.10], sulla scorta di determinazioni sperimentali russe. pig. l i n (3) e, in particolare, r i p o r t a t a la legge di distribuzione degli elettroni in quota. i n pig. 1 si riporta tale legge di distribuzione. dalla 1 s e n [2.10] si t r a e che la conduttivita a = — = -—~tt 2 e, a p a r t e il f a t t o r e _ k oti id. o o e di scala — 2 , r a p p r e s e n t a t a dal diagramma di pig. 1. i m p o r t a n z a d e l l a l e g g e d i i i a l l n e i f e n o m e n i i o n o s f e r i c i 151 3 . alcuni risultati circa l'influenza sulle caratteristiche del moto. alio scopo di effettuare delle valutazioni quantitative, e il caso di prendere in considerazione qualche caso eleinentare analiticamente t r a t tabile ed atto, quindi, a fornire alcune prime indicazioni. consideriamo, pertanto, il moto bidimensionale di u n a massa d'aria elettricamente conduttrice in presenza di un campo magnetico (*) invposto, normale alia giacitura caratteristica del moto. non esistono campi elettrici imposti: quelli esistenti saranno dunque esclusivamente dovuti a fenomeni di induzione. x sia (fig. 2) x z il piano del moto. d e t t i , al solito i, j, h, i t r e versori della t e r n a prescelta, la forza ponderomotrice per u n i t a di volume assume l'espressione (cfr. formula [2.8]) j a b = fx + f2 = axb\ (ui + w k) + a2 b2„ ( — u h + w i) [3.1] la bidimensionalita del moto u n i t a all'ipotesi di stazionarieta, comporta che le proiezioni dell'equazione di equilibrio si riducano alle seguenti: quux + qwuz = — fx — b-0axu — b'!0(t2w [3.2] quwx + qwivz = — p2 + <j2b"o u — aji-ow . [3.3] (*) t a l e c a m p o s a r a c o n s i d e r a t o c o s t a n t e . cio e q u i v a l e a t r a s c u r a r e gli e f f e t t i dei c a m p i m a g n e t i c i i n d o t t i . cio e lecito i n p r i m a a p p r o s s i m a z i o n e . 152 a . p o z z i g . c o n t u r s i inoltre e: ux + wt = 0 . [3.4] derivando le [3.1] e [3.2] rispettivamente rispetto a z e ad x e sottraendo si h a : gu(uz wx)x + qiv (uz —• wx)t = b2o(ji (wx — uz) [3.5] avendo t e n u t o conto della [3.4]. indicando con r l'unica componente del rotore del campo di velocita della [3.5] nelle ipotesi di moto stazionario si h a : <it? q — = —b*„olr . [3.6] l'ovvia integrazione della [4.6] conduce a b-ooi , r = r0 exp [3.7] ove r0 e il rotore a t = 0. l a [3.7], che gia di p e r se consente di t r a r r e qualche conclusione inerente all'effetto uniformizzatore del flusso esercitato dalla presenza del campo magnetico, da modo di esplicitare il secondo membro dell'equazione cinematica in t e r m i n i della funzione di corrente a*y) = r . [3.8] questa equazione, ove se ne rintracci un integrale particolare, pu6 agevolmente ricondursi all'equazione di laplace di piu agevole studio. se inoltre in qualche p u n t o il rotore e nullo, sara sempre tale. p r e n d i a m o ora in esame le condizioni al contorno. u n a impostazione coerente con lo studio della ionosfera e la seguente. ad x = — oo, u = /(z); sidle superficie esterne della corrente p = cost ( = pressione ambiente). p e r avere delle indicazioni i m m e d i a t e noi sostituiamo a questa geom e t r i a u n a fittizia: supponiamo cioe che il moto a w e n g a t r a p a r e t i rigide. ricaviamo cosi la pressione che agisce su queste p a r e t i ed abbiamo in t a l modo u n a indicazione sulla f o r m a che assumono le superfici delim i t a n t i la massa fluida. prendendo in esame condizioni iniziali uniformi si ha: r = 0 ; px = — b20(xiu ; pz = a2b20u ; u = cost . i m p o r t a n z a d e l l a l e g g e d i iiall n e i f e n o m e n i i o n o s f e r i c i 1 5 3 i n questa schematizzazione la eonduttivita di hall ha influenza solo sul gradiente di pressione trasversale. ne segue che se, come in effetti e, non esistono superflci a t t e a cont r a s t a r e tale pressione si ha un moto trasversale alia veloeita fondamentale funzione del p a r a m e t r o . cr, b20 u l indicando con l lo spessore della vena fluida. ooncltjsioni. i n questo lavoro e s t a t a studiata l'influenza della corrente di hall sui moti ionosferici. nella prima p a r t e e stato m o s t r a t o in che modo e possibile valutare il c.oefficiente di eonduttivita elettrica avvalendosi dei risultati della teoria statistica. in particolare e stato m o s t r a t o quali metodi sono stati suggeriti per risolvere l'equazione di boltzmann e come, con 1'aiuto dei risultati sperimentali o t t e n u t i dai satelliti sovietici, e possibile ottenere il diagramma del coefficiente di e o n d u t t i v i t a in funzione della quota. nella seconda p a r t e e stato m o s t r a t o come schematizzando opportun a m e n t e la geometria del moto sia possibile m e t t e r e in evidenza l'andamento qualitativo del fenomeno. b i b l i o g r a f i a i1) c h a p m a n c o w l i n g , the mathematical theory of non uniform gases. cambridge u n i v e r s i t y press. ( 2 ) j a n c e l k a i i a n , mecanique statistique des plasmas electroniques lorentzians a ses applications a vionosphere. r e p o r t of t h e p h y s i c a l society conference on t h e p h y s i c s of t h e i o n o s p h e r e . p u b l i s h e d b y t h e p h y s i c a l society 1 l o w t h e r g a r d e n s , p r i n c e consort r o a d , l o n d o n s . w . f . ( 3 ) k r a s o u s k y v . , lo studio degli strati superiori della atmosfera mediante il terzo satellite sovietico. memoria p r e s e n t a t a al i x congresso i n t e r n a z i o nale di a s t r o n a u t i c a , a m s t e r d a m 1958. missili, 1959, n. 3. kamland: geo-neutrino measurement in japan annals of geophysics, 60, 1, 2017, s0113; doi:10.4401/ag-7388 s0113 kamland: geo-neutrino measurement in japan itaru shimizu research center for neutrino science, tohoku university, sendai, japan abstract geoneutrinos are antineutrinos produced in radioactive isotope decays inside the earth. those antineutrinos are neutral particle and can be detected by weak interaction, however, due to extremely small cross section, there were no feasible experiments for a long time. the observation of geoneutrinos has been finally made using large-size liquid scintillator detectors, kamland and borexino. in this article, the latest result of kamland and future prospects of geo o−e measurements are reviewed. 1. introduction electron antineutrinos (o−e) emitted by radioactivities within earth’s interior is called geoneutrinos. they are mainly produced by beta-decays of primordial radioactivities, 238u, 232th, and 40k, and have energies below 3.4mev. such series of decays also produce radiogenic heat, which primarily contributes to earth’s energetics. therefore, we can estimate earth’s heat production through the measurement of geoneutrino flux, and make a comparison with the surface heat flow, resulting in an important input for geophysics. geoneutrino detection provides a unique tool to investigate earth’s interior, however, it used to be regarded as almost infeasible for a long time, due to extremely small cross section of the neutrino interaction. in the late 1990s, plans of feasible experiments to detect geo o−e´s, kamland in japan and borexino in italy, are suggested for the first time. in this article, the latest result of kamland and future prospects of geo o−e measurements are reviewed. 2. kamland experiment kamland (kamioka liquid scintillator anti-neutrino detector) is located in kamioka mine, japan, at a depth of 1000 m to suppress the cosmic-ray muon backgrounds. the detector consists of 1000 ton of ultra-pure liquid scintillator (ls) contained in a transparent nylonbased balloon suspended in nonscintillating oil. the containment balloon is a 13-m-diameter spherical balloon made of 135-µm-thick transparent nylon/evoh composite film, and is suspended with kevlar ropes. the neutrino interaction produces energetic charged particle in the ls, and a ls excitation by its energy loss results in a fluorescence emission (scintillation light). this scintillation light is viewed by photomultiplier tubes (pmts), an array of 1879 pmts, mounted on the inner surface of the containment vessel. to recored the signals from pmts, kamland has an electronics system called front-end electronics, based on an analog transient waveform digitizer (atwd). when a pmt channel registers a “hit”, the charge as a function of time is recorded by an atwd chip. each channel has two sets of three atwds, which record waveforms at three available gain levels, allow us to cover signals from wide dynamic range. 3. geo-neutrino results in the kamland geoneutrino measurement, the largest background comes from o−e´s produced in surrounding commercial nuclear reactors in japan. the expected reactor o−e flux is well calculated by neutrino oscillation parameters which are determined from kamland spectral data, as well as the nuclear fission rate based on the reactor operation records. it indicates the geoneutrino contribution can be extracted based on a statistical subtraction of reactor o−e backgrounds. in 2005, kamland showed the first experimental study of geo o−e´s [araki et al. 2005], and the observed flux was consistent with earth model expectations. it was followed by an improved geo o−e measurement owing to statistical increase and radioactive background decrease by a factor of ~20 after the ls purification [gando et al. 2011]. after the fukushima nuclear accident in march 2011, the reactor o−e rate significantly decreased because of the long-term shutdown of japanese reactors in a review of nuclear safety standard. this reactor-off situation provided a good data for the geoneutrino observation. article history received july 5, 2016; accepted october 5, 2016. subject classification: geo-neutrino measurement, anti-neutrino detector, kamland, japan. the latest kamland result [gando et al. 2013] is based on a total livetime of 8.19 years. the data set is divided into three periods: period 1 (march 2002 may 2007); before the ls purification, period 2 (may 2007 august 2011); during and after the ls purification, and period 3 (october 2011 november 2012); after the start of the kamland-zen neutrino-less double betadecay experiment. figure 1 shows the time variation of the observed rate of o−e´s with energies above 3.4mev in agreement with the expectations of reactor o−e´s. the o−e rate in period 3 is remarkably smaller than other periods. to exploit the time variation of the known background rate, o−e candidates are analyzed with an unbinned maximum-likelihood method incorporating the event rate and the prompt energy, including the event time. the kamland o−e analysis constrains both the geo o−e rates and the neutrino oscillation parameters simultaneously. as discussed in gando et al. [2013], the precise measurement of neutrino oscillation parameters was achieved in kamland; tan2i12 = 0.436 +0. −0. 029 025 , dm221 = 7.53 +0. −0. 18 18 × 10 −5 ev2, and sin2i13 = 0.023 +0. −0. 002 002 , incorporating constraints from solar neutrino, accelerator, and short-baseline reactor neutrino experiments. the energy spectrum of the observed events together with the best-fit contributions in the energy region of geo o−e´s is shown in figure 2. the main background contributors are the reactor o−e, 13c(a, n)16o reactions, and accidental coincidence events in descending order. in period 2 and later, the background from 13c(a, n)16o decreased due to the ls purification, and in period 3, the reactor o−e background decreased. it results in the significant low background condition in period 3. we find the clear event excess in the backgroundsubtracted spectrum as shown figure 2 (top panel), and the background-subtracted rate and spectral shape agree with earth model expectations for each data set. the best-fit geo o−e´s are 116 and 8 events from u and th decays, respectively, with confidence level (c.l.) contours as shown in figure 3. assuming the th/u mass ratio of 3.9 based on the geochemical model [mcdonough and sun 1995], the total number of geo o−e events is 116 +28 −27, which corresponds to an oscillated o−e flux of 3.4 +0.8 −0.8 × 10 6 cm−2s−1 at kamland. 4. earth model comparison the result of geo o−e flux measurements in kamland will provide a new constraint on both geophysics shimizu 2 figure 1. time variation of expected and observed rates of o−e´s with energies above 3.4mev in kamland data over 10 years [gando et al. 2013]. the vertical grey bands indicate deadtime during the liquid scintillator purification and the detector modification. the significant reduction of o−e flux from reactors after march 2011 earthquake (red dashed line) is remarkable, and provides a new opportunity of the “reactor on-off ” study for neutrino oscillations and geo o−e´s below 3.4mev. figure 2. energy spectrum of the kamland observed events [gando et al. 2013]. the best-fit contributions are indicated by colored histograms (bottom panel), and the spectrum after subtracting the non-geo-neutrino contributions is compared with the earth model prediction (top panel). 3 and geochemistry. firstly, we calculate the geo o−e flux at kamioka for different bulk silicate earth (bse) compositional estimates based on simple but appropriate assumption, to compare with the kamland geo o−e data. the crustal contribution for u and th can be estimated by compositional data through rock sampling. on the other hand, the chemical composition of the mantle, most massive layer of earth’s interior, is uncertain. so we estimate the total amount of u and th, predicted by the bse models, and assume that the excess above the crustal contribution uniformly distribute in the mantle [enomoto et al. 2007]. the kamland data tested three bse compositional estimates of geo o−e flux and radiogenic heat from 238u and 232th [šrámek et al. 2013], as shown in figure 4. the uncertainties from the abundances and the distributions of u and th in the mantle are indicated by the vertical bands. the upper solid line with a steeper slope corresponds to the estimate in the homogeneous mantle assumption, and the lower solid line the estimate in the sunken-layer which assumes that all of the u and th below the crust are concentrated at the mantle-core boundary. while the statistical treatment of geological uncertainties is not straightforward, assuming gaussian errors for the crustal contribution and for the bse abundances, the kamland data disfavor the geodynamical models with the homogeneous hypothesis at 89% c.l. however, due to the limited statistical power of the data, all bse composition models are still consistent within ~2v c.l. the measured geo o−e flux can be converted into a total radiogenic heat of 11.2+7.9−5.1 tw from u and th (additional ~3.0 tw is expected from other radioactive nuclei). after the publication of the kamland result in gando et al. [2013], new reference models with refined crustal compositions become available in the literature. if the reference model [enomoto et al. 2007] used in the analysis in gando et al. [2013] is replaced with a modern model [huang et al. 2013], the radiogenic heat is estimated to be 14.9+9.8−8.3 tw from u and th. the increase of the radiogenic heat estimation is mainly due to the change in the crustal geo o−e flux contribution, which is the offset of the flux-heat conversion function as shown in figure 4. it indicates, in addition to the determination of the geo o−e flux, a precise estimate of the crustal contribution is of critical importance in proving the radiogenic heat in the mantle. in the future, the estimation needs to be further improved by the revision of the regional crustal compositions around japan, compiling the latest knowledge in geophysics, geochemistry, and local geology. the calculations based on the different earth models support that the total radiogenic heat is smaller than the heat flow of 47 ± 2 tw from earth’s surface [davies and davies 2010], indicating secular cooling of the earth. 5. future prospect currently, the geoneutrino measurements by two large ls detectors are available; kamland in japan and borexino in italy [bellini et al. 2013]. in the future, there are several plans of geoneutrino detectors at different locations; the sno+ detector close to completing the construction in canada [chen 2006], the juno detector in china [han et al. 2016], the lena detector in europe [wurm et al. 2012], and the movable hanohano detector in a deep ocean [learned et al. 2008]. those detectors will have ls mass larger than kamland, so we expect the reduction of the statistical uncertainties on the measured o−e flux. at the kamland location, kamland: geo-neutrino measurement in japan figure 3. confidence level (c.l.) contours for the observed number of geo o−e events in u and th. the small shaded region represents the prediction from the reference model of enomoto et al. [2007]. figure 4. three bse compositional estimates of geo o−e flux and radiogenic heat from 238u and 232th [gando et al. 2013]. the colored shaded regions indicate the different mantle model predictions from cosmochemical, geochemical, and geodynamical estimates, and are compared with the kamland measurement (gray band). the variations due to the radiochemical distributions are represented by the two solid lines with different slopes; the homogeneous and sunkenlayer hypotheses. the variations after adding the uncertainty in the crustal contribution are indicated by the two dashed lines. the geo o−e contribution from the mantle is only about one fourth of the total geo o−e flux, and may be estimated based on the subtraction of the crustal contribution depending on the crustal model. on the other hand, a detector in an oceanic location (e.g. hanohano) has less exposure to geo o−e´s from thick continental crusts, and will be more appropriate to measure the mantle flux. such multi-site data at different geological locations will be useful to construct a detail map of neutrino sources inside the earth. in addition, detectors distant from commercial reactors will reduce the uncertainties on the measured geo o−e flux. alternative approach to mapping neutrino sources is the directional measurement of o−e´s using a detector with a sensitivity of coming o−e direction [shimizu 2007]. such measurements will provide the information on separate contributions from the crust and mantle even at locations near continental crusts, based on the nadir angle distribution. a 6li-loaded detector with relatively good angular resolution can be a candidate, and the possibility of applications for exploring hypothetical magmatic reservoirs was studied in tanaka and watanabe [2014]. they are new experimental techniques imaging earth’s interior using neutrinos. further development of the detector performance and application researches are anticipated in the future. 6. conclusion the kamland data collected between march 2002 and november 2012 provides a most precise measurement of the geoneutrino flux. the observed energy spectra are consistent with the expectation of 238u and 232th decays in the earth. based on an appropriate geophysical assumption, the observed geoneutrino flux is in agreement with the predictions from three bse models within ~2v c.l. currently, due to the limited statistical power, the clear discrimination between models has not been achieved yet. in addition, the crustal contribution needs to be estimated more precisely based on the latest knowledge in geophysics, geochemistry, and local geology. in the future, if multi-site measurements with larger statistics at different geological locations become available, a detail map of neutrino sources inside the earth can be constructed. the directional measurement of o−e can be an alternative experimental technique imaging earth’s interior. references araki, t., et al. 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(2012). the next generation liquidscintillator neutrino observatory lena, astropart. phys., 35, 685-732. corresponding author: itaru shimizu, research center for neutrino science, tohoku university, sendai, japan; email: shimizu@awa.tohoku.ac.jp. © 2017 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. shimizu 4 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 18benouar.qxd 555 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical seismicity – algeria 1. introduction this work discusses some of the most important reported earthquakes that occurred in algeria since 1365, and the issues involved with their study. for the early times in algeria, only one reliable earthquake catalogue for the province of algiers has been published (ambraseys and vogt, 1988), which extends back to the fourteenth century. with regard to regional studies, in tunisia, the earthquake catalogue compiled by rothé (1980) dates from 410 but does not contain enough information about the sources used. for morocco, ramdani et al. (1988) published a list of historical earthquakes. mezcua and martinez-solares (1983) compiled an earthquake catalogue for the ibero-maghreb region for the period 500 b.c.-1980. alexis perrey is without any doubt the pioneer in this domain. perrey published a note on the earthquakes in algeria (perrey, 1848) and he continued his publication by notes on earthquakes in algeria up to 1871 in his annual lists. later m. chesneau published in the annales des mines, a note on the earthquakes during the period 1716-1887 containing some interesting details (chesneau, 1892). at the same time, montessus de ballore (1892) published some comments on the number of earthquakes felt in algeria under the title «la france et l’algérie sismiques». the seismicity of north africa is also addressed in the book «les tremblements de terre, géographie sismologique» published by montessus de ballore (1906). this book includes a map showing the earthquakes reported in algeria. during the last century, several notes were published in the comptes rendus de l’académie des sciences (cras) about seismic events in algeria. a further listing of algerian earthquakes was published in the bulletin du materials for the investigation of historical seismicity in algeria from the records of past earthquakes djillali benouar built environment research laboratory, university of bab ezzouar (usthb), civil engineering department, bab ezzouar, alger, algeria abstract this paper presents some materials relating to the investigation of historical seismicity in algeria. the historical seismicity in algeria is imperfectly known: its coverage is discontinuous and its record is grossly deficient. the seismicity of the twentieth century is relatively well documented; however, the further back in time, the harder it becomes to collect data. even so, important research work in studying the seismicity in algeria has been made in the past by several studies, notably those by alexis perrey, m. chesneau, f. de montessus de ballore, n.n. ambraseys and j. vogt. this paper discusses the problems of historical earthquake investigation in algeria and presents a selection of notable historical earthquakes in the country. mailing address: dr. djillali benouar, built environment research laboratory, university of bab ezzouar (usthb), civil engineering department, bp 32 el-alia, bab ezzouar, alger 16111, algeria; e-mail: dbenouar@yahoo.com 556 djillali benouar service météorologique d’algérie during the period 1889 to 1908. roussel (1973) listed algerian earthquakes during the period 1716 to 1970, to analyse the frequency of the seismicity; he prepared also a map of observed intensities. more recently, ambraseys and vogt (1988) published a study on the historical seismicity in the province of algiers during the period 1365 to 1902, and guidoboni et al. (1994), in her «catalogue of ancient earthquakes in the mediterranean area up to the 10th century», mentions a seismic event, which occurred in setif (algeria), known at that time as sitifis, in 419 a.d. the seismicity of algeria throughout the 20th century is addressed in detail by benouar (1994). 2. the historical period the period of interest for historical studies should be extended as back in time as far as the data allow. the period before 1900 (the pre-instrumental period) has to be studied from macroseismic data. events are typically characterised by a brief description of the macroseismic field, from the data available today. events reported in the older national catalogues cannot be included in the parametric date set to be used in seismic hazard assessment where the data used in these events are not sufficient for a reliable analysis. these events were usually not studied in a systematic manner or following a defined and documented methodology; they were generally collected from various authors from different periods. these catalogues were compiled by their authors for the preoccupation of their times and not with the needs of present studies in mind. 3. general methodology this section presents the methodology used to re-evaluate the seismicity of algeria and adjacent regions. this requires: (i) the retrieval and revision of macroseismic data; (ii) the development of a methodology for the preparation of earthquake catalogues. for historical earthquakes (pre-instrumental events) and even for twentieth century earthquakes for which there are no instrumental data, but for which intensities and radii are available, magnitudes are calculated from macroseismic data (using calibration relationships). 3.1. measurement of time, intensity and magnitudes in algeria, as in other countries, the measurement of the exact time of earthquakes, which occurred in different historical periods, is very important for our preoccupations today. not taking this seriously will affect the catalogue through duplicating events and conflating separated events. through history, different calendars have been used, which makes the measurement of time of seismic events a complex work. destructive earthquakes in algeria and elsewhere have always caused substantial damage and/or total collapse to adobe and other prevailing types of houses. these structures have shown throughout history a high vulnerability and a very low resistance to seismic loads, and even to heavy rain. as a result of their weakness, maximum intensity in any destructive earthquake in algeria seems to saturate; that is, at intensity 9 or less on the msk scale, all adobe houses are destroyed, and thus any settlement or hamlet would be equally, but no more, devastated at higher intensities of the scale. because the building stock in algeria has numerous variable characteristics such as age, building materials and structural systems, an extensive investigation has to be carried out to reveal what type of constructions were exposed and what state they were in during the time of each event. due to certain particularities in the construction materials, the assessment of intensities poses some significant problems. another serious problem that arises is the correct interpretation of written accounts according to the historical context of the period concerned. certainly, exaggerations exist in the sources; usually made to attract more attention to documents, but these are not very difficult to discover, particularly when there is more than one source. the contribution of illustrative photographs to the damage survey, when they exist, is noteworthy. 557 material for the investigation of historical seismicity in algeria from the records of past earthquakes magnitude scales allow the classifying of earthquakes objectively and independently of local ground conditions and environment. for a variety of reasons, many earthquakes in algeria remain without surface-wave magnitudes or simply without any type of instrumental magnitude. to solve this problem, ms is estimated when possible from semi-empirical relationships between ms and mb or ms and ml (benouar, 1993). ms for historical earthquakes may also be estimated from the radius of perceptibility (r3), which is defined as the mean epicentral distance of an area within which the shaking was felt with an intensity equal to or greater than 3 msk, and then using a relationship between ms and r3, or by using a relationship between ms and i0 (benouar, 1993). the location of macroseismic epicentres is also of great value, in terms of understanding tectonics features, particularly during the period before 1900 as well as for the first half of the 20th century. macroseismic information provided by the available source documentary materials usually allows relatively accurate location of the epicentral zone. 4. macroseismic information (literary sources) documentary source materials are essential for a retrospective reconstruction of the macroseismic field data of past earthquakes. such materials are found to be available in libraries and archive centres across algeria and abroad. these archives do not always give full macroseismic information for the historical earthquakes during the period pre-1900. a particular case demonstrating the incomplete macroseismic data of algeria is that of the city of algiers. there are very few literary sources and documentary materials referring to destructive events in algiers. the earthquake damage reported in the few records known to have occurred in the city during the last seven centuries of its history is very infrequent, small and mainly due to relatively large earthquakes originating either on land, away from the site of algiers, or offshore at a considerable epicentral distance from algiers. the earliest known earthquake to have caused loss of lives and considerable damage in algiers occurred in 1365 (ambraseys and vogt, 1988). earlier authors writing on the subject include shaw (1743) who discussed «de la fréquence des tremblements de terre en algérie» in his book «voyages dans plusieurs provinces de barbarie». many factors influence the content and the survival of macroseismic information. these include site accessibility, geographical position of the region, and density of the population, socio-economic conditions, political and military situations, and building stock characteristics. for instance, during the french colonisation period of algeria, the lack of macroseismic information for native settlements was obviously conditioned by the censorship imposed by the french administration. to understand better the importance of an earthquake and the macroseismic data contained in the contemporary source documents, it is imperative that retrieved information be carefully analysed in their whole historical context, to avoid serious miscalculations. for many parts of algeria and other parts of the world with comparable literary production and density of inhabitants, it is well known that the cities, large towns or major settlements constitute the main sources of macroseismic information. in cases when the event occurs within the surroundings of such a town, damage information is more likely to be well reported, while that occurring in remote sites could be left unclear. the scarcity of available macroseismic information obviously makes the catalogue very incomplete. 5. notable historical earthquakes in algeria table i presents a sample of historical earthquakes illustrating the seismic activity in algeria during the period before 1900. both these events and the other known historical earthquakes in algeria still need to be investigated fully in order to be able to include them in the catalogue and use them with an acceptable degree of reliability in seismic hazard analysis. the locations of the principal places affected are shown in fig. 1. 558 djillali benouar table i. selected historical earthquakes of note in algeria (see fig. 1 for location of places). date place notes reference 419 setif mentioned by augustine guidoboni et al. (1994) 3 january 1365 algiers destructive ambraseys and vogt (1988) 10 march 1673 algiers similar to 1716 event ambraseys and vogt (1988) 3 february 1716 algiers destructive; many casualties reported ambraseys and vogt (1988) 29 november 1722 algiers several casualties ambraseys and vogt (1988) 9 october 1790 oran well documented; 2000 dead. lopez marinas and salord (1990) march 1819 mascara extensive damage in mascara; damage also in oran mokrane et al. (1994) 3 march 1825 blida damage at blida and algiers, 7000 dead ambraseys and vogt (1988) 22 november 1851 mascara damage at mascara but no deaths rothé (1950) 22 august 1856 djidjelli destructive; tsunami ambraseys (1982), aucapitaine (1856), senarmont (1856), glautier (1856) 9 march 1858 kherba damage in kherba area ambraseys et al. (1991) 2 january 1867 mitidja destructive cochard (1867), ambraseys and vogt (1988) 16 november 1869 biskra 200 houses damaged and 30 dead rothé (1950), roussel (1973), ollivier (1870) 3 december 1885 m’sila one of the strongest in east algeria: 33 dead rothé (1950), harbi (2000), contemporary press reports 29 november 1887 el kalaâ at least 331 houses collapsed; 20 dead chesneau (1892), contemporary press reports 15 january 1891 gouraya heavy damage; 36 dead pomel (1891), rothé (1950), ambraseys and vogt (1988) fig. 1. location of places mentioned in table i. 559 material for the investigation of historical seismicity in algeria from the records of past earthquakes 6. discussion and conclusions the general conclusion is that long-term historical seismicity of algeria for the period before 1900 is very imperfectly known. the main reasons for this situation are the limited availability and the quality of contemporary documentary records, the prevailing circumstances during the time of the event, the geographical location and problems relating to the intensity. for the period before 1900, the local sources are really the only means of describing the impact of the event for the region, and in the case of nonavailability of such sources, the macroseismic data will remain unknown and thus incomplete. this may explain the apparent low seismic hazard of a number of regions shown on actual modern earthquake hazard maps in algeria that may not be correct; it may illustrate the absence of long-term observations. the types of material and the construction techniques during the period before 1900 are different in many respects from today’s constructions and thus attributing intensities to such destructive events without taking this into account could be misleading. it may be concluded that the historical seismicity in algeria remains incomplete, and unknown events during the period before 1900 cannot be included in the earthquake catalogue to be used for seismic hazard analysis. in algeria, as in other regions where the seismicity is medium to low, the importance of the role of historical data is most evident. the instrumental period represents only a sample of earthquake activity covering less than 100 years, which can rarely reveal the main features of the region’s earthquake activity. for this reason, great and continuous effort should be made to improve the historical data as back in time as possible. references ambraseys, n.n. (1982): the seismicity of north africa: the earthquake of 1856 at jijeli, algeria, boll. geofis. teor. appl., xxiv, 93, 31-37. ambraseys, n.n. and j. vogt (1988): material for the investigation of the seismicity of the region of algiers, eur. earthquake eng., 3, 16-29. ambraseys, n.n., j. vogt and r.d. adams (1991): seismicity of the central cheliff valley in algeria, publ. inst. geogr. nacion., ser. monogr. no. 8, 41-50. aucapitaine, m. (1856): note sur le tremblement de terre ressenti en algérie du 21 au 25 août 1856, c.r. acad. sci., paris, xliii, pp. 765. benouar, d. (1993): the seismicity of algeria and adjacent regions, during the twentieth century, ph.d. thesis (imperial college of science, technology and medicine, civil engineering dpt., london). benouar, d. (1994): materials for the investigations of the seismicity of algeria and adjacent regions during the twentieth century, ann. geofis., xxxvii (4), pp. 416. chesneau, m. (1892): note sur les tremblements de terre en algérie, ann. mines, 9è sér., i, 5-46. cochard, m. (1867): lettre sur le tremblement de terre d’alger, adressée par m. cochard à m. wolf et communiquée par m. le verrier, cras, xliv, pp. 34. glautier de claubry (1856): sur les effets du tremblement de terre des 21 et 22 août 1856 dans certaines parties de l’algérie, cras, xliii, 589-590. guidoboni, e., a. comastri and g. traina (1994): catalogue of the ancient earthquakes in the mediterranean area up to the 10th century (ing, romasga, bologna), pp. 504. harbi, a. (2000): analyse de la séismicité et mise en évidence d’accidents actifs dans le nord-est algérien, thèse de magister (ist-usthb, alger, algérie). lopez marinas, j.m. and r. salord (1990): el periodo sismico oranes de 1790 a la luz de la documentacion de los archivos espanoles, publ. inst. geogr. nacion., ser. monogr. no. 6, pp. 64. mezcua, j. and j.m. martinez-solares (1983): sismicidad del área iberomogrebí, instituto geográfico nacional, madrid, publ. 203, pp. 301. mokrane, a., a. ait messaoud, a. sebai, a. ayadi and m. bezzeghoud (1994): les séismes en algérie de 1365 à 1992, publication du centre de recherche en astronomie, astrophysique et géophysique (département etudes et surveillance sismique, ess, craag, alger, algérie), pp. 277. montessus de ballore, f. (1892): la france et l’algérie sismiques, ann. mines, 9è sér., i. montessus de ballore, f. (1906): les tremblements de terre. géographie sismologique (armand collin, paris). ollivier, m.e. (1870): secousses de tremblements de terre à biskra (algérie du sud), du 16 au 19 novembre inclusivement, cras, lxx, 48-51. perrey, a. (1848): note sur les tremblements de terre en algérie et dans l’afrique septentrionale 1845-1846. mem. acad. sci. arts b. l., dijon, 299-323. pomel, a. (1891): les tremblements de terre du 15 et 16 janvier en algérie, cras, cxii, 643-646. press reports in algeria and france, 1885 and 1887. ramdani, m., b. tadili and t. el mrabet (1988): the present state of knowledge on historical seismicity of morocco, in proceedings of the symposium on calibration of historical earthquakes in europe and recent developments in intensity interpretation, european seismological commission, 23-28 august 1988, sofia. rothé, j.p. (1950): les séismes de kherrata et la séismicité de l’algérie, bull. serv. cart. geol. algérie, 4ème sér., géophysique, n. 3. 560 djillali benouar rothé, j.p. (1980): catalogue des séismes tunisiens (ms.). roussel, j. (1973): les zones actives et la fréquence des séismes en algérie 1716-1970, bull. soc. hist. nat. afrique du nord, 64 (3), 11-227. shaw, m.d. (1743): voyages dans plusieurs provinces de la barbarie et du levant, neaulme. senarmont, m.h. (1856): analyse des documents recueillis sur les tremblements de terre ressentis en algérie du 21 août au 15 octobre 1856, cras, xliv, 586-594. r i s u l t a t i e p r o s p e t t i v e d e l l e r i c e r c h e s p a z i a l i (*) b. r o s s i r i c e v u t o il 19 f e b b r a i o 1963 sarebbe una presunzione da parte degli scienziati il pensare che i grandiosi e costosissimi programmi di attività spaziale in cui, una dopo l'altra, si vanno imbarcando le maggiori nazioni del mondo siano motivati esclusivamente, od anche principalmente, da ragioni scientifiche. vi sono importanti ragioni pratiche, come lo sviluppo di una rete globale di comunicazioni e la raccolta di dati meteorologici che p e r m e t t a sicure predizioni del tempo. vi sono questioni di prestigio nazionale. e vi è infine l'indomabile spirito d'avventura che obbliga l'uomo a tentare t u t t o ciò che è umanamente possibile, a spingersi sempre più lontano nel mondo che lo circonda. e p p u r e è assai probabile che, in ultima analisi, il risultato più importante e più durevole di questa attività saranno appunto quelle scoperte scientifiche che oggi passano quasi inosservate. è probabile che il periodo iniziato quasi cinque anni fa col lancio del primo sputnick passerà alla storia come quello in cui l'umanità ha compiuto uno sbalzo di eccezionale p o r t a t a verso la conoscenza del pianeta su cui vive e dell'universo che circonda questo pianeta. la terra è p a r t e del sistema solare, e l'esplorazione diretta del sistema solare ha già avuto inizio. non v'è dubbio che durante il prossimo decennio strumenti sempre pili complessi e raffinati verranno lanciati a distanze sempre più grandi dal nostro pianeta. e, con ogni probabilità, in questo periodo assisteremo pure ai primi viaggi di astronauti oltre i limiti di quella regione dello spazio che è sotto l'influenza diretta della terra. (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. l ì . r o s s i il sole è u n a delle innumerevoli stelle che formano le galassie, e la nostra galassia non è che una delle innumerevoli galassie che riempiono l'universo. rispetto alle altre stelle e alle altre galassie, la situazione è diversa da quella che riguarda il sistema solare giacché questi oggetti, a causa della loro enorme distanza, non potranno essere raggiunti da veicoli spaziali. p e r quel che riguarda le stelle al di f u o r i del sistema solare e le galassie al di fuori del nostro sistema galattico, non si vede nessuna possibilità di esplorazione diretta a causa della loro enorme distanza. le ricerche spaziali, t u t t a v i a , sono destinate ad a u m e n t a r e grandemente le nostre conoscenze dell'universo galattico e estragalattico. l'atmosfera, come è noto, non lascia passare che u n a piccola p a r t e delle radiazioni elettromagnetiche provenienti dagli oggetti celesti. i raggi infrarossi, i raggi ultravioletti, e i raggi x sono completamente assorbiti dai primi strati dell'atmosfera. i raggi g a m m a di breve lunghezza d'onda penetrano alquanto nell'atmosfera, m a ben presto rimangono nascosti sotto l'intenso flusso dei raggi g a m m a secondari p r o d o t t i dai raggi cosmici. non rimane quindi che u n a s t r e t t a « finestra » nel visibile, ed u n a seconda finestra più ampia, nella regione delle radio-onde. p e r di più le osservazioni astronomiche nel visibile sono seriamente ostacolate dalle distorsioni atmosferiche e dalla luminescenza atmosferica del cielo notturno. p e r contro, da u n satellite che giri a t t o r n o alla t e r r a ad un'altezza di alcune centinaia di kilometri, si può osservare, senza distorsioni e senza lo sfondo di radiazioni secondarie, t u t t o lo spettro delle onde elettromagnetiche, dai raggi y alle onde radio di u n a lunghezza d'onda massima d e t e r m i n a t a dalla densità del plasma. osservazioni astronomiche con s t r u m e n t i a bordo di veicoli spaziali presentano, perciò, grandi promesse; e, per quanto l'astronomia spaziale sia ancora nella sua infanzia, ha già prodotto qualche risultato preliminare di notevole interesse. vorrei ricordare a questo proposito le osservazioni dei raggi gamma di alta energia eseguite per mezzo di explorer x i . tali raggi possono provenire solamente dal decadimento radioattivo dei mesoni ns, i quali a loro volta possono venire prodotti da collisioni nucleari di alta energia o da processi di annichilazione di materia con antimateria. gli apparecchi a bordo di explorer x i hanno registrato un flusso debolissimo di raggi g a m m a , che può interpretarsi come dovuto ad u r t i di raggi cosmici coi nuclei del tenuissimo gas interstellare. questi risult a t i , per quanto t u t t ' a l t r o che definitivi, h a n n o permesso di scartare una delle versioni dell'ipotesi cosmologica della creazione continua. secondo r i s u l t a t i h p r o s p e t t i v e d e l l e r i c e r c h i ? s p a z i a l i 89 questa versione, verrebbero create continuamente nell'universo, ed in particolare nella nostra galassia, coppie di protoni ed antiprotoni, con un ritmo sufficiente a compensare la continua espansione dell'universo, e a mantenere u n a densità costante di m a t e r i a . se così fosse, gli antiprotoni creati nella galassia dovrebbero via via annichilarsi con i protoni, e così facendo dovrebbero produrre raggi g a m m a , ed il flusso di raggi g a m m a predetto da questa teoria è parecchie centinaia di volte più intenso di quello che è stato osservato da explorer x . quali sorprese a t t e n d a n o gli scienziati quando potranno s f r u t t a r e in pieno le possibilità offerte dai veicoli spaziali, nessuno può dire. vorrei menzionare qui solamente il f a t t o che l'assenza della luminescenza di fondo del cielo, l'assenza di distorsioni atmosferiche e la possibilità di estendere le osservazioni nell'infrarosso aumenteranno, a quanto sembra, di un f a t t o r e 10 il raggio di visibilità. ciò permetterebbe di osservare galassie che si trovano vicine al limite estremo dell'universo, che si trovano cioè in quella regione dello spazio dove la velocità di recessione si avvicina alla velocità della luce. non è difficile comprendere la grande importanza cosmologica di tali osservazioni. vorrei ora tornare al sistema solare, su cui si sono già ottenuti important i risultati e su cui si sta rapidamente concretando un organico programma di ricerca, diretto a chiarirne l'origine, l'evoluzione, e la presente s t r u t t u r a . p e r quel che riguarda l'origine del sistema solare, ha particolare importanza lo studio dei corpi solidi in esso contenuti, come la terra, gli altri pianeti interni, e la luna. come è noto, un'ipotesi favorita oggi da molti astronomi è che il sole, i pianeti e la luna siano nati, attraverso locali processi di condensazione, da u n a primordiale e f r e d d a nuvola di pulviscolo a gas. ad un certo p u n t o , raggiunta u n a d a t a densità critica, si sarebbero iniziati nel sole quelle reazioni termonucleari che durano t u t t o r a , e che sono la sorgente dell'energia irradiata. i pianeti avrebbero pure raggiunto, al principio della loro vita, una t e m p e r a t u r a piuttosto elevata, a causa del loro contenuto radioattivo destinato poi a diminuire gradualmente. mentre la luna, dato il minor rapporto f r a massa e area, potrebbe essere rimasta ad u n a t e m p e r a t u r a più modesta attraverso t u t t a la sua storia. le condizioni termiche di un corpo celeste influenzano il grado di plasticità del suo materiale e la rapidità con cui elementi leggeri e pesanti si ridistribuiscono nella sua massa sotto l'influenza del campo gravitazionale. perciò lo studio della costituzione interna dei pianeti e della relativa abbondanza degli elementi presenti alla loro superficie p e r m e t t e r à di confermare le correnti idee sull'origine del sistema solare. 4 7 » l i . r o s s i vi sono ancora molte incognite sulla s t r u t t u r a i n t e r n a della t e r r a , e, strano a dirsi, uno dei primi risultati delle ricerche spaziali è stato un notevole progresso nelle nostre conoscenze del pianeta su cui viviamo. studiando a c c u r a t a m e n t e le orbite dei satelliti artificiali, è stato possibile stabilire che la forma media della t e r r a differisce più di quanto si credeva dalla forma che dovrebbe acquistare u n a massa plastica sotto l'influenza delle forze gravitazionali e centrifughe. ciò significa che la t e r r a , ha una rigidità interna assai maggiore di quella a t t r i b u i t a l e precedentemente. il giorno in cui si m e t t e r a n n o satelliti in orbita a t t o r n o alla luna, a marte o a venere, si potranno usare simili metodi per studiare la forma e conseguentemente la s t r u t t u r a interna di tali oggetti. gli scienziati si attendono risultati particolarmente interessanti per quel che riguarda la luna, che, secondo idee correnti, non avrebbe dovuto passare a t t r a verso la fase di a l t a t e m p e r a t u r a e quindi di a l t a plasticità attraverso cui sarebbero passati la t e r r a e gli altri pianeti di simile massa. le condizioni fisiche e chimiche della superficie lunare sono p u r e di eccezionale interesse. non essendo soggetta a quegli agenti atmosferici, che hanno cosi p r o f o n d a m e n t e alterato la superfìcie terrestre, la superficie lunare è rimasta probabilmente i m m u t a t a per miliardi di anni e quindi p o r t a impressa la storia del sistema solare nel lontano passato. la sua dettagliata esplorazione p o t r à rivelare, per esempio, se è vero, come taluni pensano, che l'iniziale condensazione della nebula solare non abbia dato luogo d i r e t t a m e n t e alla formazione dei pianeti, m a piuttosto ad oggetti di assai più piccole dimensioni, che si sarebbero poi aggregati per formare tali pianeti e i loro satelliti. u n a completa esplorazione della superficie lunare richiederà lo sbarco di complessi strumenti, e probabilmente, di astronauti. ma qualche osservazione preliminare di grande importanza si potrà f a r e p u r e da veicoli spaziali lanciati contro la luna o meglio messi in orbita attorno alla luna. macchine da presa televisive p o t r a n n o mettere in evidenza dettagli della s t r u t t u r a superficiale ancora ignoti. contatori di raggi g a m m a p e r m e t t e r a n n o di misurare l'abbondanza dei vari elementi radio attivi, ed in particolare, del potassio, e di fare così un primo passo nell'analisi chimica della superficie lunare. tornando alla terra, vorrei accennare al f a t t o che la sua atmosfera presenta molti problemi ancora non risolti, i quali sono di grande importanza sia scientifica, sia pratica. già le ricerche spaziali hanno contribuito notevolmente alla soluzione di questi problemi. mi limiterò a ricordare un esempio. l'atmosfera residua nello spazio dove si muovono r i s u l t a t i h p r o s p e t t i v e d e l l e r i c e r c h i ? s p a z i a l i 4 7 1 i satelliti produce u n graduale frenamento, che risulta in un graduale abbassamento dell'apogeo. osservando questo fenomeno, j a c c h i a ha messo in evidenza il f a t t o che la densità dell'aria, ad altezze di qualche centinaio di kilometri, varia periodicamente con un periodo di 27 giorni, che coincide col periodo di rotazione del sole. questo f a t t o si spiega colla circostanza che la radiazione solare varia d u r a n t e la rotazione del sole perché non t u t t e le regioni della superfìcie solare sono ugualmente attive. esso rivela d i r e t t a m e n t e connessione t r a fenomeni solari e fenomeni meteorologici che può avere grande importanza. gli s t r a t i superiori dell'atmosfera sono fortemente ionizzati dai raggi ultravioletti, dai raggi x , e talora dai raggi corpuscolari provenienti dal sole. essi formano la ionosfera la quale, t r a l'altro, per la sua capacità di riflettere onde elettromagnetiche di media lunghezza, perm e t t e le comunicazioni radio a grande distanza, che sarebbero altrimenti impossibili a causa della c u r v a t u r a della terra. p r i m a del lancio dei veicoli spaziali, i soli dati sulla ionosfera erano quelli o t t e n u t i appunto studiando la riflessione di onde elettromagnetiche emesse da stazioni alla superficie della terra. p e r la sua stessa n a t u r a , questo metodo non può dare nessuna informazione sulla densità di ionizzazione al di sopra del massimo, che si trova a circa 200 kilometri. veicoli spaziali hanno già permesso di misurare direttamente la densità di ionizzazione oltre il massimo. prossimamente si inizieranno misure con un altro metodo, che appare assai promettente. esso è basato sull'osservazione di segnali radio emessi da un satellite che circola la terra al di sopra della ionosfera, e riflessi verso il satellite stesso dalla ionosfera sottostante. ben poco sappiamo finora sull'atmosfera degli altri pianeti, come marte e venere, e delle corrispondenti ionosfere. i l loro studio è uno degli scopi principali dei voli interplanetari che avranno luogo nei prossimi anni. un veicolo spaziale che passi nelle vicinanze del pianeta, sia pure senza raggiungerlo, può ottenere importanti dati sulla composizione dell'atmosfera per mezzo dell'analisi spettroscopica della luce solare che, sfiorando il pianeta all'alba o al tramonto, passa attraverso un notevole strato di tale atmosfera. d a t i più precisi si potranno ottenere, in un secondo tempo quando si t e n t e r à di raggiungere la superficie stessa del p i a n e t a per mezzo di s t r u m e n t i contenuti in capsule lanciate fuori dal veicolo nel suo passaggio presso il pianeta. questi s t r u m e n t i perm e t t e r a n n o di ottenere dati più precisi riguardanti sia l'atmosfera come la superficie dei pianeti. 4 7 2 f r a i molti problemi che a t t e n d o n o di essere risolti da esperienze di questo genere vi è quello della t e m p e r a t u r a di venere che, secondo certe misure radiometriche, sembra essere eccezionalmente elevata, raggiungendo forse i 300 °c. se questi risultati sono esatti, essi indicano che il pianeta venere è circondato da u n ' a t m o s f e r a di proprietà eccezionali, la quale lascia passare la luce visibile del sole, m a t r a t t i e n e i raggi infrarossi riemessi dal pianeta impedendone così il raffreddamento. lo studio delle condizioni fìsiche dell'atmosfera e delle superfici dei pianeti è di eccezionale importanza per determinare se f o r m e di vita più o meno simili alla nostra sono possibili su tali pianeti. se lo sono, può darsi benissimo che l'interesse principale delle ricerche spaziali si sposti dalla fisica alla biologia. non occorre essere specializzati in questa scienza per apprezzare l'enorme p o r t a t a scientifica e filosofica che avrebbe la scoperta di forme di vita estraterrene, per comprendere come tale scoperta costituirebbe 1111 grande passo verso la soluzione del mistero che circonda l'origine della vita sul nostro pianeta. a l t r e t t a n t o import a n t e sarebbe la scoperta di forme intermedie di aggregazione della materia fra quelle caratteristiche degli esseri viventi e quelle che si ritrovano nel mondo inanimato. non è impossibile che tali forme possano esistere oggi in un pianeta in cui, per qualche ragione, l'esplosione della vita fosse s t a t a r i t a r d a t a . ritornando u n a volta di più alla terra, sappiamo oggi che, al di là della sua atmosfera e ionosfera, si trova u n a v a s t a regione, nota come magnetosfera. questa regione contiene quelle particelle cariche veloci, t r a t t e n u t e nelle vicinanze della t e r r a dal suo campo magnetico, che formano la grande fascia di radiazione scoperta da van alien. tale scoperta f u il risultato più notevole delle prime ricerche spaziali, e gran p a r t e delle esperienze condotte in questi anni, h a n n o avuto come oggetto, appunto, lo studio della fascia di radiazione. grazie a queste ricerche, sappiamo oggi che la fascia di radiazione è f o r m a t a da elettroni e protoni e possediamo dati piuttosto precisi sulla densità di queste particelle, sul loro spettro d'energia, e sulla loro distribuzione angolare, in funzione dello spazio e del tempo. t u t t a v i a non conosciamo ancora la risposta al fondamentale problema dell'origine della fascia di radiazione e sul modo in cui queste proprietà che caratterizzano il flusso di particelle in un determinato p u n t o variano nel t e m p o e nello spazio. è possibile, ed anzi probabile, che u n a p a r t e di questa radiazione sia d o v u t a ad u n a albedo di neutroni dell'atmosfera sottostante. i raggi cosmici, appena entrano nell'atmosfera, producono interazioni nucleari in cui, f r a l'altro vengono r i s u l t a t i h p r o s p e t t i v e d e l l e r i c e r c h i ? s p a z i a l i 4 7 3 emessi neutroni. i neutroni lanciati verso l'alto viaggiano indisturbati a t t r a v e r s o il campo magnetico terrestre. molti di essi si disintegrano in viaggio producendo elettroni e protoni che, essendo carichi, rimangono imprigionati da questo campo. d ' a l t r a p a r t e non sembra che t u t t a la fascia di radiazione possa spiegarsi in questo modo. altre possibili sorgenti che sono state suggerite comprendono l'iniezione nella magnetosfera di particelle provenienti dal sole, e l'accelerazione locale, per effetto di disturbi idromagnetici, di particelle già presenti nella magnetosfera. se ci allontaniamo sufficientemente dalla terra e dagli altri pianeti, in modo da sottrarci completamente alla loro influenza sia gravitazionale che magnetica, ci troviamo in quello che si suol chiamare lo spazio interplanetario. già prima dell'inizio dei voli spaziali si era a n d a t a affermando l'idea che tale spazio non fosse vuoto, m a contenesse un tenuissimo gas, presumibilmente idrogeno completamente ionizzato. esperimenti con veicoli spaziali sia russi che americani hanno confermato questa idea. particolarmente i m p o r t a n t i furono i risultati ottenuti con explorer x , i quali dimostrarono che questo gas ionizzato, o plasma, ha u n a densità di circa 10 elettroni e protoni per cm3 e si muove con una velocità di circa 300 k m al secondo venendo, sembra, più o meno d i r e t t a m e n t e dal sole. explorer x ha anche dimostrato che la terra è circondata da u n volume in cui il plasma interplanetario non può penetrare, essendone impedito dal campo magnetico terrestre. questo volume, che ha ricevuto il nome di cavità geomagnetica, ha una forma allungata, estendendosi p a r e ano a circa 10 raggi terrestri, nella direzione da cui viene il vento, cioè nella direzione del sole, e fino a parecchie decine di raggi terrestri nella direzione opposta. questi a cui ho accennato non sono che risultati preliminari. lo studio ulteriore del plasma interplanetario è senza dubbio uno dei compiti più urgenti delle ricerche spaziali nei prossimi anni. si t r a t t a di uno studio di grande importanza sia per il geofisico, interessato dei fenomeni a cui dà luogo l'interazione del plasma interplanetario col campo magnetico terrestre, come per l'astrofisico, interessato dei fenomeni solari di cui questo « vento » di plasma è u n a diretta manifestazione. i n realtà il plasma interplanetario può considerarsi come un'estensione dell'atmosfera solare, giacché non vi è nessuna naturale superficie di separazione f r a di esso e la cosidetta corona solare. perciò lo studio del plasma interplanetario è, in un certo senso, uno studio diretto del sole. sarebbe n a t u r a l m e n t e di grande interesse eseguire misure dirette, del tipo di quelle che ho descritto, assai più vicino al sole. è probabile 4 7 4 ti. r o s s i che nel f u t u r o non troppo lontano veicoli spaziali raggiungeranno distanze dell'ordine (li un terzo o u n quarto del raggio dell'orbita terrestre. nel f r a t t e m p o t u t t a v i a possiamo attenderci nuovi dati di notevole importanza per la fìsica solare a t t r a v e r s o l'osservazione dei raggi ultravioletti e dei raggi x , che è divenuta possibile mediante l'uso doi satelliti posti in orbita al di sopra della nostra atmosfera. esperienze preliminari per mezzo di razzi hanno già permesso di misurare accuratamente lo spettro ultravioletto del sole; hanno dimostrato che la corona solare è un'intensa sorgente di raggi x ; ed hanno fornito una p r i m a immagine fotografica del disco solare in raggi x . si attendono ora con grande interesse i risultati (lei primo osservatorio solare in orbita, che h a terminato le sue trasmissioni alcune settimane f a dopo aver funzionato, p a r e soddisfacentemente, per vari mesi. prima (li chiudere vorrei m e t t e r e in chiaro che la possibilità di condurre esperimenti nello spazio non ha diminuito l'importanza delle osservazioni terrestri. quello che sta succedendo è anzi proprio il contrario. vorrei f a r rilevare due cose. la prima è la grande importanza che u n a collaborazione internazionale ha per le ricerche spaziali. si t r a t t a di problemi che interessano t u t t a l ' u m a n i t à e che spesso richiedono osservazioni di carattere globale. la seconda è il f a t t o che vi sono varie ottime ragioni perché ciò sia così. anzitutto le osservazioni terrestri costituituiscono un necessario complemento delle ricerche spaziali. così, ad esempio, le ricerche spaziali dirette a studiare le proprietà della ionosfera al di sopra del massimo avranno assai maggiore significato se saranno accompagnate da quelle osservazioni terrestri (riflessioni ionosferiche a varie frequenze, effetto lussemburgo, etc.) che permettono di studiarne le proprietà al di sotto del massimo. i n secondo luogo le osservazioni spaziali, eseguite contemporaneamente alle osservazioni di certi fenomeni terrestri, permetteranno di interpretare questi ultimi in modo assai più preciso e aumentandone quindi l'interesse. così, ad esempio, si pensa che le fluttuazioni del campo magnetico terrestre e le variazioni di intensità dei raggi cosmici siano provocate da cambiamenti nelle condizioni del plasma interplanetario in cui la terra è immersa. c'è da attendersi che le osservazioni dirette del plasma interplanetario f a t t e per mezzo di veicoli spaziali p e r m e t t e r a n n o di stabilire in modo esatto questa correlazione. quando ciò sia stato f a t t o , magnetometri e monitori di raggi cosmici operanti alla superfìcie della terra diverranno s t r u m e n t i per l'osservazione delle condizioni meteorologiche nello spazio interplanetario. r i s u l t a t i h p r o s p e t t i v e d e l l e r i c e r c h i ? s p a z i a l i 4 7 5 infine le ricerche spaziali, facendo intravedere la possibilità che alcuni f r a i più i m p o r t a n t i problemi della geofìsica, della tìsica solare e dell'astrotìsica possano finalmente venire risolti, hanno intensificato l'interesse in tali problemi e provocato un nuovo stimolo per quelle ricerche terrestri che sono dirette allo studio di questi stessi problemi. possiamo quindi attenderei, nei prossimi anni, di assistere ad un coerente sviluppo di osservazioni terrestri e di ricerche spaziali, che si completeranno a vicenda e che, insieme, rivoluzioneranno le nostre conoscenze del mondo in cui viviamo e della sua evoluzione. li10.pdf annals of geophysics, vol. 46, n. 1, february 2003 39 a 1000-year simulation with the ipsl ocean-atmosphere coupled model zhao x. li and sébastien conil laboratoire de météorologie dynamique, cnrs, université pierre et marie curie, paris, france abstract a 1000-year climate simulation is run with the ocean-atmosphere coupled model developed at the institute pierresimon laplace (ipsl, paris). no flux adjustment is used. the drift of the model is analyzed in terms of the seasurface temperature and deep ocean temperature. when the model’s own equilibrium is reached, it is found that the antarctic bottom water production experiences large-amplitude variation, oscillating between strong and weak episodes. this can yield oceanic temperature variation in the southern hemisphere and for the global mean. 1. introduction ocean-atmosphere coupled general circulation models have become a very useful tool for either climate natural variability studies or future anthropegenic climate change prediction. however, such models have in general important drift when used to simulate current climate, due to mutual amplification of the shortcomings in the oceanic and atmospheric models. an earlier approach to reduce the drift was to use a procedure called flux adjustment by introducing artificially flux compensation at the air-sea interface. recent coupled models, however, try to improve the parametrisation in order to reduce the drift. in the framework of the sintex european project, the ipsl (institut pierre-simon laplace) has built up such an ocean-atmosphere coupled model without any flux adjustment. this paper describes the basic behaviours of the model in a 1000-year long simulation. 2. model and simulation the ipsl coupled model involves the atmospheric gcm (lmdz) developed at lmd (laboratoire de météorologie dynamique, paris) and the oceanic gcm (orca) developed at lodyc (laboratoire d’océanographie dynamique et climat, paris). in this study, the version 3.2 of lmdz is used. it is derived from the standard version described in sadourny and laval (1984). the model is formulated in the finite-difference grid with a horizontal resolution of 5 by 4 degrees in longitude and latitude directions and 19 hybrid vertical layers. a brief description of the physical parameterization and the model’s main performance in simulating interannual climate variability are presented in li (1999). the model has evolved in the following points. the convection scheme is a simple massflux scheme. the closures (both triggering condition and entrainment/detrainment rate) mailing address: dr. zhao x. li, laboratoire de météorologie dynamique, cnrs, université pierre et marie curie, casier 99, 4 place jussieu, 75252 paris cedex 05, france; e-mail: li@lmd.jussieu.fr key words coupled climate model − long-term climate simulation − oceanic overturning circulation − model drift 40 zhao x. li and sébastien conil are based on the surface evaporation and largescale water-vapor convergence. the cloud parameterization is a prognostic scheme, as described in le treut and li (1991). at present, only clouds associated with stratiform precipitation are taken into account. those created by convection are diagnosed following the intensity of convective precipitation. the surface model is a bucket model for which we consider a homogeneous layer of 150 mm. the calculation of the surface temperature is incorporated in the boundary layer and based on the surface energy balance equation. for the surface moisture, a holding capacity is fixed at 150 mm of water, and all the water above this value is lost as runoff. orca is the global version of the opa 8.1 ocean modelling system developed by the lodyc team in paris (madec et al., 1998). a full documentation is available at http://www. lodyc.jussieu.fr/opa). it is a finite difference ogcm formulated on the c-grid. the present configuration uses a rigid lid. the horizontal mesh is orthogonal and curvilinear on the sphere. to overcome the singularity at the north pole, the northern point of convergence has been replaced by two poles located in asia and north america. its space resolution is roughly equivalent to a geographical mesh of 2 by 1.5 degrees (with a meridional resolution of 0.5 degrees near the equator). 31 vertical levels are used with 10 levels in the top 100 metres. vertical eddy diffusivity and viscosity coefficients are computed from a 1.5 turbulent closure scheme. horizontal mixing of momentum is of laplacian type with an eddy viscosity coefficient of 40 000 m2/s, reduced in the tropics to 2000 m2/s at the equator. the lateral mixing of tracers (temperature and salinity) is isopycnal as described in guilyardi et al. (2001). the version used here does not have an interactive sea-ice model and sea-ice cover is relaxed towards observed monthly climatology. lmdz and orca are coupled through oasis (valcke et al., 2000). the coupling procedure is identical to the sintex coupled model (gualdi et al., 2003) involving the same oceanic model, but the echam4 atmospheric model. from levitus (1982), the model is integrated for 2 years in the robust-diagnostic mode. once the oceanic dynamics is spun-up, the integration continues without any adjustment nor restoring (except in regions where sea-ice cover does not agree with climatology). air-sea fluxes and sst are exchanged every 3 h. the simulation was integrated for 1000 years. 3. results 3.1. sea surface temperature global and annual mean of the sea-surface temperature in function of time is plotted in the top panel of fig. 1. other curves of fig. 1 display the sst evolution for respectively the northern hemisphere (30/90°n), the tropics (30°s/30°n) and the southern hemisphere (30/90°s). to facilitate visual inspection, the range for y axis is kept constant (3 degrees) for all the plots. for the global mean, the sst has a cooling of about 0.6 k for the first 50 years. this is the rapid model drift at the initial stage, also visible in the tropics and the southern hemisphere. after this initial drift, the tropics remain relatively stable for the whole simulation and highfrequency variation can be observed. longperiod variation is quite well linked with that of the southern hemisphere. from the simulated year 50 to 300, the southern hemisphere experiences a slow drift to the warm. this is also observed in the tropics and in the global mean. the northern hemisphere has particular behaviour with an initial warming drift during the first 110 years and then a slow cooling that lasts until the year 500. decadal-scale variation can be observed for the northern hemisphere. after the initial quick and slow drift periods, the coupled system reaches its own equilibrium state. a remarkable phenomenon is the largeamplitude sst variation for the southern hemisphere. for example, there is a drop of 1°c from the year 470 to 520 and a similar amplitude warming from the year 720 to 770. the global mean is also influenced by these episodes and the amplitude can reach 0.4 °c. we will show later that the sst variation is correlated to the oceanic convection in the southern ocean and the associated overturning circulation. we examine now the global distribution of the simulated sst. the top panel of fig. 2 shows 41 a 1000-year simulation with the ipsl ocean-atmosphere coupled model the difference between the simulated sst from the year 901 to 1000 and the observed sst deduced from the hadisst 1.1 data set (rayner et al., 1999) for the last 100 years. warming drift is observed in the high latitudes of both the hemispheres: 2 °c for the southern ocean and the north pacific, 5 °c for the north atlantic in the labrador sea. cooling is observed in the mid latitudes: 2 to 3 °c from 20 to 40°n in the north pacific and north atlantic, 3 to 4 °c from 30 to 50 °s in the southern ocean. the tropical indian ocean has near-zero drift and the drift of the warm pool in the western tropical pacific is also small (around – 1°c). the tropical atlantic has a cooling of 1 to 2 °c. many coupled models have too warm sst in the western part of the subtropical oceanic basins, due to insufficient low-level cloud cover. from fig. 2 we can observe that there is almost no drift in the subtropical atlantic near the african coast. this is certainly related to the recent improvements implemented in the boundary-layer parameterization. however the american coast in the subtropical pacific still has a large warming drift (up to 4 °c). the bottom panel of fig. 2 shows the sst difference between two periods of the simulation, the first one is from the year 3 to 10 and the second one is from year 901 to 1000. it is clear that the initial drift (much larger than the interdecadal variations) during the first 10 years takes a large part of the final model drift. for the setup of coupled simulations, it is thus very important to check the initial results before running longer times. 3.2. deep ocean evolution due to the overwhelming information for the three-dimensional ocean temperature, we have to apply a procedure to reduce the amount of information. to do so, we first make a horizontal grid transformation and interpolate the oceanic model output into the grid of the atmospheric model (a regular grid of 5 by 4 degrees in longitude and latitude). then a principal component analysis is applied to the interpolated 3-dimensional oceanic temperature with a normalization by the cosine of the latitude and the depth of each layer. figure 3 shows the obtained leading spatial structure (eof) and the time-varying coefficient (pc). it accounts for 76% of the total variance. for graphic-display purpose, the spatial structure fig. 1. mean sea-surface temperature (°c) in function of simulated year for (from top to bottom) the whole globe, the northern hemisphere (30/90°n), the tropics (30°s/30°n) and the southern hemisphere (30/90°s). 42 zhao x. li and sébastien conil is plotted for different oceanic basins (indian ocean, pacific and atlantic) after making the zonal average. for the three oceanic basins, there is a dipole contrast between the upper layers and the deep ones. taking into account the negative evolution of the pc from the beginning until the year 700, the most important oceanic temperature drift is a deep-ocean warming and a cooling in the surface layer of about 1000 m. figure 4 shows the second eof and corresponding pc, which accounts for 13% of the total variance. we can observe a contrast between the atlantic at one hand and the pacific and indian ocean at the other hand. it is probably related to the variation in the global thermohaline circulation. the pc curve shows a decrease from the beginning until the year 300 and then a recovering until the year 800. 3.3. southern ocean and north atlantic overturning circulation the top panel of fig. 5 shows the global overturning circulation stream function for the last one hundred years of the simulation. it indicates the zonally-averaged mass transport in fig. 2. top: difference of the sea-surface temperature (°c) between the simulation (averaged over 901/1000) and the observation (hadisst1.1, averaged over 1900/1999). bottom: difference of the sea-surface temperature (°c) between two periods of the simulation (901/1000 3/10). 43 a 1000-year simulation with the ipsl ocean-atmosphere coupled model fig. 3. spatial structure and time series (arbitrary units) of the leading empirical orthogonal function (eof) for the 3-dimensional oceanic temperature. the spatial structure is plotted separately for the three basins in the form of zonal average. fig. 4. same as in fig. 3, but for the second eof. 3 4 44 zhao x. li and sébastien conil the meridional plane. the sense of this transport is clockwise arround high-value centers. at the depth of 3500 m and around 40°s, a negative center of – 15 sv represents the antarctic bottom water formation originated mainly in the ross and wedell seas. the bottom panel of fig. 5 plots the time evolution of the maximum overturning mass transport in the southern ocean. after the initial drift stage of about 300 years, largeamplitude oscillations can be observed between 12 and 30 sv. comparison between this curve and that on the bottom of fig. 1 shows clearly that the southern hemisphere sst is very closely related to the overturning circulation. strong (weak) overturning circulation increases (decreases) the heat transport from the tropics to the high latitudes and thus increases (decreases) the sst of the high latitudes. let us now examine two periods with extreme values of overturning circulation. the 20-year averaged period from 770 to 789 is characterized by a strong overturning circulation. however the period from 721 to 740 has a weak overturning circulation. the top panel of fig. 6 shows the difference of overturning stream function befig. 5. top: global overturning circulation mass transport stream function (sv) averaged for the period 901 to 1000. bottom: maximum mass transport (sv) of the antarctic bottom water in function of simulated year. fig. 6. difference for two extreme periods (770/789721/740). top: global overturning stream function (sv). bottom: oceanic temperature (°c). 45 a 1000-year simulation with the ipsl ocean-atmosphere coupled model tween these two periods (strong-weak). the difference value in the center (14 sv) is of the same order of the field itself. it is also closer to the equator, showing an extension of the antarctic bottom water. the bottom panel of fig. 6 shows the difference of the zonally-averaged oceanic temperature between the two periods. for the zone from 30 to 60°s and from the surface to arround 1500 m, warm water (0.8 °c) is observed. however in the south and under the bottom of this zone, cold water (until – 1°c) is observed. this is coherent with the fact that the deep convection is driven by cold and dense water. the north atlantic overturning mass transport stream function is shown in the top panel of fig. 7 for the period averaged from the year 201 to 1000. the maximum value of the stream function is 36 sv at the depth of 1500 m and at 40°n, showing a slightly too intense north atlantic deep water cell. we should note that the mediterranean contribution was taken into account in the calculation. to get the coherent structure for the variation of the atlantic overturning circulation, we performed an eof analysis to the annual mean values for the period from 201 to 1000. the leading eof (accounting for 42% of the total variance) is plotted in the middle panel of fig. 7. it is a monpole structure, quite similar to the field itself, indicating that the north atlantic deep water cell has a dominant stationnary oscillation. the time series (pc, shown in the bottom panel of fig. 7) reveals that important decadal-scale variations exist. lowfrequencies at centennial scale can also be identified through visual inspection. 4. conclusions a 1000-year ocean-atmosphere coupled simulation was presented in this paper. it is the first time that the ipsl coupled model, without any artificial flux adjustment, has been run for such a long period. the results are very encouraging since the model’s drift is relatively small at the end of the simulation and no systematic drift tendancy is observed. for the global sst, there is a warming in the high latitudes of the two hemispheres. however the mid latitudes have a cooling belt. the tropical fig. 7. top: north atlantic overturning stream function (sv) for years 201 to 1000. middle: leading eof structure. bottom: corresponding time series. the middle and bottom panels have arbitrary units. 46 zhao x. li and sébastien conil indian ocean is quite realistic. the warm pool is slightly eroded by the cold tongue and thus a little cold. the tropical atlantic has a more important cooling. for the eastern part of the subtropical oceanic basins, the traditional warming is much reduced in the atlantic, but remains important in the pacific. our results shows that the model’s initial drift during the first ten years is a very important indicator for the longer time-scale behaviour, this is useful for future coupled model development. we have also studied the sst evolution for different regions of the globe, a remarkable phenomenon is the quick sst changes in the southern hemisphere, also visible in the global mean sst. the variation of 0.4°c can be obtained during approximately 50 years for the global mean. this variation is closely related to the deep convection in the southern ocean. the meridional overturning mass transport varies from 12 to 30 sv, making the heat transport from the equator very different. it is possible that the variation of the overturning circulation is due to the interactive coupling between the atmosphere and the ocean for the hydrological cycle. further studies are needed to confirm this point. our results have also implication for the anthropogenic global warming detection. in fact, the quick global mean sst variation, 0.4 °c, due to the southern ocean overturning circulation is almost comparable with the observed global warming (0.6 °c) since the beginning of the industrial era (ipcc, 2001). this implies that the climate natural variability can be large and the global warming detection is indeed a very difficult issue. however, this result should be considered with caution since our model currently does not include the seaice component which can play an important role in the high-latitudes deep oceanic convection. acknowledgements the authors wish to thank the scientific and technique teams at cerfacs, lmd and lodyc who have developed the gcms and the coupler. computer ressources were kindly provided by the idris, the computer center of the cnrs. this work is supported by the european commission through the sintex project. references gualdi, s., a. navarra, e. guilyardi and p. delecluse (2003): assessment of the tropical indo-pacific climate in the sintex cgcm, ann. geophysics, 46 (1), 1-26 (this volume). guilyardi, e., g. madec and l. terray (2001): the role of lateral ocean physics in the upper ocean thermal balance of a coupled ocean-atmosphere gcm, climate dyn., 17, 589-599. ipcc (intergovernmental panel on climate change) (2001): observed climate variability and change, in climate change 2001, the scientific basis, (cambridge university press), 99-181. le treut, h. and z.x. li (1991): sensitivity of an atmospheric general circulation model to prescribed sst changes: feedback effects associated with the simulation of cloud optical properties, climate dyn., 5, 175-187. levitus s. (1982): climatological atlas of the world ocean, noaa professional paper n. 13, pp. 173. li, z.x. (1999): ensemble atmospheric gcm simulation of climate interannual variability from 1979 to 1994, j. climate, 12, 986-1001. madec, g., p. delecluse, m. imbard and c. levy (1998): opa version 8.1 ocean general circulation model reference manual, note ipsl n.11, available at ipsl, paris. rayner, n.a., d.e. parker, e.b. horton, c.k. folland, l.v. alexander and p. frich (2000): the hadisst1 global sea-ice and sea surface temperature dataset, 18711999, hadley centre technical note 17. sadourny, r. and k. laval (1984): january and july performance of the lmd general circulation model in new perspectives in climate modelling, edited by a. berger and c. nicolis (elsevier), 173-198. valcke, s., l. terray and a. piacentini (2000): the oasis coupler user guide version 2.4, technical report tr/cmgc/00-10, available at cerfacs, toulouse, france. annals of geophysics, 55 (3), 2012: borgomeo why i study earth sciences edoardo borgomeo south kensington campus, imperial college london, london, uk annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5635 abstract i present here the reasons why i chose to pursue a university degree in earth sciences. although this topic might at first seem unrelated to geoethics, it is somehow connected to it. i will show how my decision to study earth sciences was influenced by geoethical considerations, even though i did not consider them as such when i made my decision. my choice is explained using four representative words: ‘wonder’, ‘environment’, ‘system’ and ‘anthropocene’. the idea of anthropocene is discussed in detail because of its relevance to geoethics. 1. introduction the reasons that led me to choose a university degree in earth sciences can be briefly explained with four representative words: ‘wonder’, ‘environment’, ‘system’ and ‘anthropocene’. the first word is ‘wonder’. this is perhaps the most common motivation among earth sciences students, and it is linked to the fascination that humans feel for natural phenomena, such as volcanic eruptions, earthquakes, and other representations of the natural world, of mountains or oceans. in my personal experience, the dolomites (figure 1) were the place where this wonder first materialized. the second word is ‘environment’. this is a vague word, often used to mean many different things. initially, i was interested in environmental issues in a very generic way, and i read about climate change, air pollution, and natural hazards. the more i read about the environment, the more i realized that earth sciences are the factor common to all environmental problems. the word environment was fundamental in shaping my decision, because it focused my attention on the earth sciences as a way of understanding environmental problems. the third word is ‘system’, and this mainly refers to the general research method of earth sciences (i.e. from rock to processes). once i became interested in earth sciences and environmental problems, i started to find out more about the activities and methods of geoscientists. above all, the methods fascinated me, because geoscientists always try to group observations together to provide a general, comprehensive explanation of natural phenomena. i started to appreciate the skill that geoscientists have: starting from simple analytical observations (e.g. looking at a rock specimen), they can provide an explanation for the rock formation and its geological history, and they can then integrate various pieces of evidence to construct a general theory to explain the earth as a system. the fourth word, ‘anthropocene’, deserves a slightly longer explanation. 2. the anthropocene the international commission on stratigraphy has recently created a new working group whose task is to solve a rather unusual problem: the stratigraphic significance of the anthropocene [zalasiewicz et al. 2008]. the term ‘anthropocene’ was first proposed in 2002 by the chemist paul crutzen to describe a new geological epoch denoted and characterized by human activity [crutzen 2002]. the possibility of a geological epoch, or even of an era dominated by human activities, had already been hypothesized in the late 19th century by antonio stoppani [1873], an italian geologist, who defined an ‘anthropozoic era’, which began when humans entered the history of the earth for the first time. in stoppani’s time these ideas were criticized by the geological community [zalasiewicz et al. 2010], because it appeared nonsensical to even compare human history to the limitlessness of geological time. in recent years, the reality of climate change and of the major changes that human activities are imposing on our planet has raised new interest in the anthropocene (figure 2). 3. geoethics and the implications of the anthropocene the increasing awareness and interest in the anthropocene in the geological community is only partly due to the stratigraphic debate on where to place the holocene– anthropocene boundary. regardless of the terminology used, the concept of the anthropocene interests geologists, and the public in general, as a metaphor of global enarticle history received december 20, 2011; accepted march 3, 2012. subject classification: anthropocene, geoengineering, student, environmental change. 361 vironmental change. it is a perfect metaphor to represent the present time in which the history of the planet and the history of humankind have become intertwined. natural processes and forces have always influenced human history to an extent, but the reverse had never taken place, until now: human processes can now operate at the same scale as natural processes. as zalasiewicz et al. [2010] remarked, the fate of one depends on the fate of the other. for centuries, geology was a science that was never concerned with human activities; however, now, in the anthropocene, geologists are starting to recognize the importance of humankind as a primary geological factor. it is now known that humans are changing the earth climate system. however, it is not just the climate system that is being affected by human activities: there are many other changes induced by human activities that are happening on a global scale. for instance, the changes in the nitrogen cycle, or the observed increases in the rate of extinction of biological species. geology needs to change to a science that is capable of modeling and describing the interactions between natural forces and human drivers. the anthropocene requires a change in perspective, or what kant would have called a “copernican revolution”: anthropogenic drivers need to be considered as endogenous to the earth system. in the anthropocene, scientists in general, and geologists in particular, will need to study and understand how the earth system functions, and also how human activities fit into this system. in this scenario, geoethics will be an essential tool in the geologist’s toolbox, aiding the interpretation of human activities, and also, and perhaps more importantly, guiding the geologists in the communication of their results to the wider public in an accessible, yet scientific, way. it is the very real existence of the anthropocene problem that makes geoethics relevant nowadays, because it is during the anthropocene that humans have for the first time become able to change geological environments on a planetary scale. bioethical issues really became relevant with the advent of technological advances that made humans capable of consciously changing the basic conditions of biology. in the anthropocene the problem appears to be similar: technology has put humankind in a position where it can consciously determine the fate of its environment and its components. if it is ‘up to us’, then decisions on, for instance, carbon emissions reduction or land use, which can have global effects, need to be weighted up according to geoethical principles; i.e. principles that consider the application of new technologies and policies from a geological point of view. in the anthropocene humankind is ‘in the driver’s seat’, able to intentionally and consciously change the earth climate and environment. some have suggested taking this even further, by developing a new science, geoengineering (figure 3), where the overarching aim would be the engineering of the components of the earth system to reduce the greenhouse effect. this type of science and research cannot overlook ethical considerations. again, to force the comparison with bioethics, it can be said that geoengineering is a cure, a treatment for the sick planet, the earth. as bioethics is edoardo borgomeo 362 figure 1. mount antelao in the dolomites, as seen from the town of s. vito di cadore (from wikipedia). bottom left: figure 2. may 2011 cover of the economist, showing the increasing attention that the public is paying to the anthropocene. the involvement of public opinion provides an important contribution to the debate, although sometimes it can obscure or twist the scientific evidence. bottom right: figure 3. front cover of the 2009 royal society report on ‘geoengineering the climate’. 363 concerned with the morality of the technological innovations applied to medical treatments, so geoethics is concerned with the morality of technologies, and more in general, of human activities that have a planetary impact. 4. conclusions my decision to study earth sciences was influenced by geoethics, although i did not consider my motivations as geoethical at the time. these geoethical considerations did not come from any ‘environmentalist’ ‘save-the-world’ commitment, but rather from a more realistic and selfish view of the world, that takes the management and protection of the earth as a commitment to protect and perpetuate humanity. for any earth scientists who want to think about the ethical implications of earth sciences, it is perhaps a useful and interesting exercise to go back to their early years and reflect on the reasons that led them to choose this area of science. by doing so, many earth scientists will perhaps find that part of their motivation was indeed geoethical. indeed, the discussion on geoethics needs to start from the personal experience of each geologist. references crutzen, p. j. (2002). geology of mankind, nature, 415, 23. stoppani, a. (1873). corso di geologia, vol. ii, g. bernardoni and e.g. brigola (eds.), milan. zalasiewicz, j., m. williams, a. smith, t.l. barry, p.r. bown, p. rawson, p. brenchley, d. cantrill, a.e. coe, a. gale, p.l. gibbard, f. j. gregory, m. hounslow, a. kerr, p. pearson, r. knox, p. powell, c. waters, j. marshall, m. oates, p. rawson and p. stone (2008). are we now living in the anthropocene?, gsa today, 18 (2), 4-8. zalasiewicz, j., m. williams, w. steffen and p. j. crutzen (2010). the new world of the anthropocene, environm. sci. technol., 44, 2228-2231. corresponding author: edoardo borgomeo, south kensington campus, imperial college london, london, uk; email: edoardo.borgomeo08@imperial.ac.uk. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. why i study earth sciences << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 06downes.qxd 399 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words new zealand – seismicity – historical earthquake – earthquake catalogue 1. introduction new zealand’s tectonic setting, astride the obliquely convergent tectonic boundary between the pacific and australian plates, means that it experiences a high rate of small to moderate earthquakes, many large earthquakes and occasionally, great earthquakes (figs. 1 and 2a,b). beneath the north island and northern south island (the hikurangi margin), the pacific plate descends beneath the australian plate, the maximum depth of earthquakes generally increasing towards the north-east, while in the south west of the south island, the australian plate descends beneath the pacific plate, with earthquakes to depths of ca. 140 km. between the two subduction zones is a transition zone, characterised by continental collision and dominated by the ~ 400 km long dextral strike-slip alpine fault. over 300 active faults (surface faults shown in fig. 3), exhibiting a range of rupture styles and characteristics, many capable of producing large to great earthquakes, have been identified throughout the country (see stirling et al., 2002). large earthquakes also occur within the subducting slabs. the subduction interfaces in the north and south of the country are considered capable of large earthquakes, although no large events have been unequivocally identified along the hikurangi margin in the historical record. volcanism occurs at several locations in the north island (fig. 1). procedures and tools used in the investigation of new zealand’s historical earthquakes gaye l. downes institute of geological and nuclear sciences, lower hutt, new zealand abstract new zealand’s tectonic setting, astride an obliquely convergent tectonic boundary, means that it has experienced many large earthquakes in its 200-year written historical records. the task of identifying and studying the largest early instrumental and pre-instrumental earthquakes, as well as identifying the smaller events, is being actively pursued in order to reduce gaps in knowledge and to ensure as complete and comprehensive a catalogue as is possible. the task of quantifying historical earthquake locations and magnitudes is made difficult by several factors. these include the range of possible earthquake focal depths, and the sparse, temporallyand spatially-variable historical population distribution which affects the availability of felt intensity information, and hence, the completeness levels of the catalogue. this paper overviews the procedures and tools used in the analysis, parameterisation, and recording of historical new zealand earthquakes, with examples from recently studied historical events. in particular, the 1855 m 8+ wairarapa earthquake is discussed, as well as its importance for the eminent 19th century british geologist, sir charles lyell, and for future global understanding of the connection between large earthquakes and sudden uplift, tilting and faulting on a regional scale. mailing address: dr. g. l. downes, institute of geological and nuclear sciences, 69 gracefield road, p.o. box 30368, lower hutt, new zealand; e-mail: g.downes@gns.cri.nz 400 gaye l. downes fig. 1. main tectonic features of the plate boundary through new zealand, with north-west dipping subduction of the pacific plate along the hikurangi trench, and south-east dipping subduction of the australian plate along the puysegur trench. the two zones are linked by a zone of continental collision, dominated by the dextral strike-slip transform alpine fault. also shown are the main centres of onshore or near-shore volcanism (m ), and the central volcanic region (cvr), where back-arc spreading is occurring. inset shows 10 years of shallow (depth < 40 km) seismicity. bilistic landslide hazard analysis (g. dellow pers. comm.) and probabilistic tsunami hazard analysis (downes and stirling, 2001). the records of historical earthquakes, landslides, tsunami and volcanic events play important roles in the development of the probabilistic probabilistic seismic hazard analysis (psha) techniques are already in use in new zealand (stirling et al., 2002). new methodologies are being developed to extend psha techniques to probabilistic volcanic hazard analysis (stirling and wilson, 2002), proba401 procedures and tools used in the investigation of new zealand’s historical earthquakes models, and there is ongoing research to ensure that the historical record is as complete and as comprehensive as possible. the recent move to introduce time dependence into probabilistic seismic hazard models means that the record of large shallow earthquakes becomes even more important. since organised european settlement (mostly from the british isles) began in 1840, new zealand has experienced at least 21 m ≥ 7 shallow (depth ≥ 40 km) earthquakes nearshore or on-shore (fig. 2a,b). few m ≥ 7 onshore shallow earthquakes have, however, occurred in what might be considered the modern instrumental era, that is, since the mid-1940s. although two milne seismographs were installed in new zealand in 1900-1901, it was not until the mid-1940s that the new zealand seismic network had developed sufficiently to allow routine calculation of instrumental locations for earthquakes of magnitude 4 and above within the central area of new zealand (38°s42ºs), provided all stations were operating. the record of m ≥ 7 earthquakes (shallow and deep) is considered complete after 1943. the modified mercalli intensity scale was also adopted in 1943, replacing the rossi-forel intensity scale. of the m ≥ 7 shallow earthquakes since 1943 (fig. 2a,b), the 1968 mw 7.1 inangahua earthquake was the only event centred onshore. it occurred in a mountainous, sparsely populated area. in consequence, there is a paucity of data on the effects of strong shaking on the modern built environment, and on «lifelines» (water, gas, electricity distribution systems, road and rail systems, etc.) in major population centres. this makes the development of robust relationships between wellconstrained earthquake parameters (magnitude, location, depth and mechanism), intensity of shaking, and engineering parameters difficult. however, in the 100 years prior to 1943, at least 16 shallow earthquakes exceeded magnitude 7. most were located on shore, most were within the main seismic areas (figs. 1 and 2a), and many occurred within or close to major population centres. not only were large earthquakes more numerous in this era, but also they were generally larger (fig. 2b). multi-disciplinary studies of these events are now providing much needed data for the earthquake engineering community, earthquake geologists, engineering geologists, seismologists and tsunami scientists. for the fig. 2a,b. a) known historical (post-1840) m ≥ 6.0 shallow (depth ≤ 40 km) earthquakes, divided into two eras, pre-1943 and post-1942. symbol (�) shows the approximate location of the 1993 tikiokino and 1958 ashley clinton earthquakes. b) large earthquake timeline, showing the occurrence of large earthquakes (m ≥ 6.5) in time since systematic european settlement in 1840. b a 402 gaye l. downes purposes of the rest of this paper, the descriptor «historical» is used to refer to these pre1943 earthquakes. this paper discusses currently available resources, procedures and tools to analyse these historical earthquakes, with examples given from recent studies. knowledge gaps, completeness levels and parameterisation of the catalogue are discussed. a study of the 1855 m 8+ wairarapa earthquake illustrates the wealth of data available and how well the locations, effects and magnitudes of historical earthquakes can be constrained using multi-disciplinary techniques. the 1855 earthquake is of importance not only as new zealand’s largest historical earthquake, but also, it is of international interest because of its significance for the eminent british geologist of the 19th century, sir charles lyell. 2. historical earthquake record in comparison with european and asian countries, new zealand’s written history is brief, going back just a little more than 160 years, reflecting the arrival of european settlers in new zealand. unlike europe, new zealand’s history is uncomplicated by temporal and spatial variations in language or borders, making it possible for seismologists, geologists and earthquake engineers to unravel the historical earthquake record, largely without the necessity to engage historians, for example, to interpret manuscripts in ancient language forms. the occasional translation and interpretation of maori, german, french, or scandinavian language manuscripts is all that is needed. the last three languages reflect sparse early settlement by non-british immigrants. 2.1. completeness levels and knowledge gaps the completeness levels of the earthquake record depend on four main factors; whether there were people to record events, whether they created records, whether those records are available today in sufficient quantity for analysis of the earthquakes recorded, and lastly, whether those records have been searched out for earthquake information. the first written account of an earthquake in new zealand, on 11 may 1773, is found in the records of captain james cook’s voyages of discovery to the southern oceans (cook, 1777). sealers and whalers arrived from the early 1790s. most ships’ logs and sailors’ journals from this era are relatively inaccessible in overseas repositories, and require extensive, time-consuming searching for perhaps little reward. the result is that few have been searched exclusively by the author for geohazards information or as far as is known by other historical earthquake researchers in new zealand. however, as historians interested in pre-1840 european settlers and sealing and whaling history search out and publish new material, information on large earthquakes and some tsunami events may be uncovered. unfortunately, accounts are almost fig. 3. recognised active surface faults in new zealand (for details see stirling et al., 2002). 403 procedures and tools used in the investigation of new zealand’s historical earthquakes always tantalisingly brief and incomplete, often only record events at sea, and only record effects at one or two isolated sites. christian missionaries arrived in new zealand from about 1814. although excellent chroniclers of day-to-day domestic affairs and clerical duties, they lived for the most part in the northern part of the north island, where seismic activity is low. by the time they extended their missionary activities to more southerly areas in the late 1830s, systematic settlement of these areas by europeans had just about begun. the missionaries were responsible for developing the spoken language of the indigenous people, the maori, into written form. although several oral history records and legends referring to significant pre-european settlement geohazards events have been uncovered and approximately dated, there has as yet been no systematic searching for new events within existing published or unpublished material. hence, it is only from 1840 with the initiation of systematic european settlement, that there is a continuity of written historical material, and it is only from this time that the record of historical earthquakes has the potential to be complete. however, the completeness varies with time as the density and distribution of the population, as well as their predominant activity, changed (fig. 4). the general impression is that town dwellers kept fig. 4. the distribution and predominant activity of the new zealand population in 1860s (adapted from plate 30, mckinnon et al., 1997). more complete personal journals than pastoralists, who tended to record farm activities with very little detail about other activities. the completeness of the earthquake record is also dependent on terrain. even today, some parts of new zealand are still virtually unpopulated because of high, glaciated or densely forested, rugged mountains that run in a spine along the west coast of the south island. the record of earthquakes in fiordland, in the south west of the south island, is almost certainly seriously incomplete and locations very poorly constrained until the 1930s or later. the last factor that affects the completeness levels of the earthquake record, which are summarised in table i, is how intensively historical information sources have been searched for earthquake data. 404 gaye l. downes table i. summary of the completeness levels of the historical earthquake part of the new zealand national earthquake information database. historical (onshore) earthquake catalogue completeness pre-1855 pre-instrumental – extensively researched and events data published (eiby, 1968, 1973). some revision probable with new data. – in-depth studies of major events completed, where data sufficient. – complete to shallow m ≥ 6.5 in central area and north only (estimated). 1855-1900 pre-instrumental – most major/significant earthquakes probably identified through early catalogues, but most await in-depth studies. – extensive research needed to identify all felt earthquakes, to ensure no major earthquakes have been missed, and evaluate known earthquakes. – probably complete to shallow m ≥ 7.0 onshore earthquakes, except in south west, and west coast until 1860s (estimated). 1901-1942 early instrumental – most major/significant and many smaller earthquakes identified – locations, mm intensities, isoseismal maps, and ms for m ≥ 6.5 and other selected earthquakes revised using available material. smaller earthquakes, r-f intensities only. – mw and source parameters obtained for largest events dating from 1917 using body wave modelling. – major multidisciplinary studies on most m ≥ 7.0 earthquakes – many lesser events warrant in-depth studies, possibly resulting in location changes – much available data in files (e.g. felt reports, newspaper clippings, pub. papers). – complete to m ≥ 6.5 over most of the country except fiordland, increasing to m ≥ 4.0 between 38ºs-42ºs by 1942 (estimated). 405 procedures and tools used in the investigation of new zealand’s historical earthquakes table ii. summary of the parameters used in the historical earthquake part of the new zealand national earthquake information database. national earthquake information database (historical section) as at 2003 computer database (format: oracle) earthquake parameters: – times ut, standard time adopted in 1868, pre-1868 times based on the assumption of local solar time; – epicentre – latitude, longitude (the number of significant figures indicating reliability); – magnitude – mw, ms, ml, m, or magnitude range (class a m ≥ 7.5; class b m6-7.4; class c m4.5-.9; class d m < 4.5); – depth or depth class («c», crustal (i.e. there is no reason to suspect a deep focus); «s», shallow or upper crustal; «n», normal or lower crustal); – location code, indicating how earthquake parameters were determined: a number specifies the particular velocity model used in computer solutions; «g» graphical solution; «i» based on a large number of intensity data (i.e. reasonably reliable); «f» based on a small number of intensity data points (i.e. very approximate); «t» based on teleseismic data; «m» a mistaken report; «c» a confused reference (to some other phenomena). – references to key papers only. – no intensity data (intensity data, in form of place name and mm intensity, are included in the electronic database from 1943 only). paper copy only database intensity data: – pre-1943, intensity data as place name and intensity for many earthquakes but not all; – isoseismal maps, 123 available in atlas of isoseismal maps of nz earthquakes (downes, 1995), others in recent publications, others unpublished as yet. future (in planning) computer database (format: oracle) earthquake parameters: – uniformly assigned/determined epicentre and other epicentres, each with depth or depth class, and time as previous; – magnitude: all, as available, in separate fields with references; – mechanism and other rupture parameters, as available with references; – data sources, all relevant references, method of determination and reliability parameters; – metadata; – linked to other databases (e.g., strong motion, active fault, landslide, tsunami, post-earthquake reconnaissance). intensity or effects data: – location defined by latitude, longitude (or nz map grid reference) (+ references); – digitised isoseismal maps (+ references); – brief descriptive account of larger events (+ references); – descriptive summary of effects by place-name; – source material: – intensity data entered using forms processing software; – key-worded, annotated database of source material. 406 gaye l. downes 3. pre-1855 the period prior to 1855 has been studied in detail (eiby 1968, 1973), with every identified felt earthquake being listed, described and catalogued, assigned a magnitude or magnitude class, a location, a depth or depth class and a code to indicate how the earthquake parameters were determined (see table ii for details). all source material used in the compilation of eiby’s catalogues was transcribed and bound in two volumes (grouden, 1966; grouden et al., 1972). the 1848 m 7.5 marlborough earthquake is the only event in this period with sufficient data to warrant an individual scientific paper. due to the priority of completing the later period (1855-1930), searching for new data on earthquakes in this period is not actively pursued at present. however, it is not unusual to find new accounts when searching for data on later period earthquakes. the finding of important new historical evidence on the 1848 marlborough earthquake, the only earthquake in the period recently led grapes et al. (1998) to relocate the earthquake from the wairau fault (eiby, 1980) to the awatere fault (fig. 5). new evidence included accounts of damage, and the length and location of fault rupture – including an 1854 map delineating part of the awatere fault with the statement «opening made by the great earthquake of october 1847 [changed to 1848 on a later version of the map]». based on a new isoseismal map and the historically observed >105 km rupture length and geologically estimated 6 m mean surface displacement, the magnitude was revised from m 7.1 (eiby, 1980) to m 7.4-7.5 (grapes et al., 1998), using dowrick (1992) and smith (1995 a,b) mm intensity attenuation models, as well as wells and coppersmith’s (1994) regression relationships. surface rupture of >105 km is consistent with recent paleoseismological studies of the awatere fault (benson et al., 2001). further, geological studies of the wairau fault indicate that it has not ruptured in historical times (grapes and wellman, 1986; berryman et al., 2000). 4. 1855-1900 the period 1855-1900 has not been studied in detail and it represents a knowledge gap. although the record of m ≤ 7 shallow earthquakes in the period is probably complete for a large part of the country, some may have been missed in remote areas, such as along the west coast of the south island prior to settlement in the 1860s, and in the southwest of the south island for the whole period. further, few felt earthquakes other than the largest events have been identified. there is much searching yet to be done to identify and evaluate the smaller events, to ensure that no significant events have been missed, and to intensively study the largest and/or most significant earthquakes. one example of a significant, but not large, earthquake is the 1869 m 5.0 christchurch earthquake, well constrained by descriptive accounts and the extent of damage to a location within 5-10 km of the centre of christchurch city at shallow depth (≤ 12 km) (yetton and downes, unpub. data). with no local events approaching fig. 5. location map for the wairau and awatere faults in the northern south island. tick marks indicate the ~ 105 km of surface rupture of the awatere fault observed in the 1848 m ~ 7.5 marlborough earthquake. 407 procedures and tools used in the investigation of new zealand’s historical earthquakes this magnitude since 1869, this event is unique in christchurch’s history and hence important for seismic hazard assessment. of the four known magnitude m ≤ 7.0 shallow earthquakes in this period, the 1855 m 8+ wairarapa earthquake and the 1888 m 7-7.3 north canterbury earthquake have been studied in detail (1855 earthquake: eiby, 1990; grapes and downes, 1997; 1888 earthquake: cowan, 1991), while the 1863 m 7-7.5 waipawa earthquake and 1868 m 7-7.5 cape farewell earthquake are undergoing intensive investigation at present (downes, unpub. data). several smaller but significant earthquakes are also in the process of investigation, including three 1876 m 5+ oamaru earthquakes (reyners, pers. comm.), the 1869 m 5.0 christchurch earthquake, the 1870 m 6 banks peninsula earthquake, the 1881 m 6 cass earthquake (yetton and downes, pers. comm.), and the 1897 m 6+ wanganui earthquake (dowrick, pers. comm.). 5. 1900-1942 in the period 1900-1942, the record of m ≤ 7 earthquakes is considered complete, and as the result of recent studies (dowrick, 1994; downes et al., 1999; downes et al., 2001; dowrick, 1998; dowrick, unpub. data) the locations, magnitudes and intensity data of all m ≤ 7 earthquakes, as well as many m 6+ earthquakes, are fairly reliable. table i summarises the knowledge gaps and completeness levels of the catalogue based on expert judgement by eiby (1988) and the author rather than rigorous analysis. 6. analysing historical earthquakes: gathering and interpreting data 6.1. resources collecting data in the form of descriptive accounts, seismograph records, photographs, maps, charts, reports, scientific papers, etc. is but the first step, albeit the most time-consuming step, to analysing historical earthquakes. new zealand is fortunate to have excellent archival resources from which to gather historical source material. although newspapers and government documents are prime sources of descriptive accounts of earthquakes (and other geohazards events), descriptive accounts and comments in diaries and journals kept by the early settlers have proved an excellent supplementary resource. the new settlements seem to have attracted a number of young, literate, scientifically curious people, who wanted to understand their new and foreign environment. some were scientifically trained, some were trained in surveying, and many apparently studied, or at least intelligently observed, botany, geology or geography as leisure pursuits. the general populace seemed to be interested in scientific discovery and discussion also, many recording their thoughts and theories in their diaries, as well as in letters to newspaper editors. some of the most valuable resources are the files and archives of gns. this organisation holds the scientific records of several predecessor organizations that were once part of the nz department of scientific and industrial research (dsir), in particular, the files of the new zealand geological survey and dsir geophysics division. these organisations, in turn, inherited files from their predecessors, back to the 1860s. files relating to historical earthquakes include a large collection of earthquake felt reports (sent in by postmasters, telegraph operators, and later, the general public), post-earthquake geological reconnaissance files, newspaper cuttings, files of earthquake locations, unpublished earthquake catalogues, seismograph records (from 1900), earthquake analysts’ books of phase arrivals (from the 1920s), as well as geological and geophysical reports and publications. gns, a government owned geological and geophysical research organisation, also maintains the earthquake, landslide, volcano and gps (global positioning system) monitoring networks, as well as their associated databases, including the national earthquake information database. other main national repositories of relevant resource material are: – archives of new zealand (several locations), which holds predominantly government papers, including parliament, legislative and 408 gaye l. downes departmental records, photographs, some film archives, maps and charts, dating from the beginnings of plans to settle new zealand in the late 1830s. – national library of new zealand, wellington, a non-lending library with extensive collections including books, including rare books, most new zealand publications, government publications, magazines, newspaper archives, art works, maps, charts, photographs, as well as oral history records – alexander turnbull library, part of the national library of new zealand, wellington, a non-lending library with extensive manuscripts and archives collections, which include private and business papers, journals and diaries, recollections, as well as australian and new zealand joint copying project microfilms (see below for description). – te papa museum of new zealand. te papa’s archives fall into two categories. the first consists of records generated by both the museum and national art gallery from the time they were established in 1865 and 1930 respectively. the second category is made up of archives acquired by both these organisations since they began. in addition, main libraries, museums and universities in each of the main cities, as well as libraries and small specialist museums in many of the smaller centres, each have their own collections, mostly relating to their area or region. genealogical research has become very popular in new zealand, and most libraries endeavour to cater for this audience. interest in genealogy, as well as appeals by national and local archival organisations, have also lead to an appreciation of the historical value of old diaries and journals, many people now depositing the originals or copies in libraries and museums for safe keeping. nevertheless, a large amount of material is still held privately, and stories of the destruction of fifty or more years of diaries dating from the 19th century are not uncommon. microfilms of many private papers and public documents held in british archives relating to australia and new zealand are also available in several libraries. the microfilms were created by the australian and new zealand joint copying project, which operated from 1945-1993. historical newspapers are undoubtedly one of the best and most reliable sources of information. fortunately, many small towns, as well as the provincial centres, published their own newspapers until well into the 20th century. they provide excellent coverage of what was happening in outlying villages and pastoral centres. collections of provincial newspapers are almost complete, and most are available on microfilm at the national library of new zealand. collections of many other newspapers and magazines are also very extensive, with microfilming an ongoing activity of the national library of new zealand with priority given to the most fragile collections. some of the most complete collections are held by the publishers, some protected far more carefully than they were 10 years ago before their historical value was realised. another valuable resource is the listings of earthquakes felt by climatological observers published yearly from 1868 until 1902 in the transactions and proceedings of the new zealand institute, the forerunner organisation to the royal society of new zealand. these listings were replaced in 1904 by the lists of events recorded on the milne seismographs. prompted by a meeting of the australasian association for the advancement of science in christchurch in 1891, hogben (1891) and hector (1891) produced comprehensive lists of earthquakes, which were mainly based on the reports published in the transactions and proceedings of the nz institute mentioned above, but also including some newspaper and other minor reports (eiby, 1988). these are the best early summaries of pre-instrumental shocks, but neither contains any index of severity beyond a few well recognised early shocks. eiby (1988) provides a good summary of other published and unpublished lists of historical earthquakes. sir james hector, director of the geological survey and the colonial museum, initiated an earthquake reporting network in 1868. all telegraph offices throughout new zealand were instructed to calibrate their clocks with wellington daily, and to report the times and effects of the earthquakes that they or the nearby community felt. also, government climatological observers were asked to report earthquakes as part of their duties (eiby, 1988). some of these reports are 409 procedures and tools used in the investigation of new zealand’s historical earthquakes still in the files of the institute of geological and nuclear sciences, wellington, or in the te papa museum of new zealand archives in wellington. this earthquake reporting network worked well until the first few years of the 20th century when it fell into decay as leading seismological personalities became ill and died, and as the result of disruptions caused by the first world war (eiby, 1988). the reporting of earthquakes and the keeping of files of felt reports started again in about 1921, when the first of the new zealand seismological reports was published. reporting of earthquakes seems to have become more comprehensive at about the time of the 16 june 1929 mw 7.8 buller earthquake and the 1931 mw7.8 hawke’s bay earthquake, new zealand’s severest earthquake in terms of casualties, with more than 250 deaths (see downes, 1995). the files of the institute of geological & nuclear sciences contain many felt reports from this period, as well as extensive newspaper cuttings, photographs, geological and engineering reports, particularly on the effects of 1929 buller and 1931 hawke’s bay earthquakes. these two earthquakes were also very well reported in the scientific literature (for references, see downes, 1995; dowrick, 1998), in contrast to the march 1929 mw 7.0 arthur’s pass, 1934 mw 7.4 pahiatua, and 1942 june mw 7.2 and august mw 7.0 wairarapa earthquakes (mw from doser et al., 1999; doser and webb, 2003). in addition to the extensive newspaper and other information on the 1931 mw 7.8 hawke’s bay earthquake, which is summarised and analysed in dowrick (1998), complete documentation on compensation claims for domestic housing damage in the epicentral area, amounting to many hundreds of files, was also found by dowrick, enabling him to calculate the damage ratios for various types of housing as well as quantify the distribution of building damage for rock, and soft and firm soil sites (dowrick et al., 1995). 6.2. limitations once source information on historical earthquakes has been assembled, several factors limit our ability to interpret it. among the problems encountered in new zealand are those caused by changes in place names through time from maori to european names, or successions of european names, the same names applied to multiple locations, multiple names for the same location, and the corruption of maori names as the europeans attempted to write down what they heard. reliable locations cannot be found for many maori landmarks referred to in early manuscripts. in addition, the social and political repercussions following the occurrence of strong shallow earthquakes near wellington in 1848 and 1855 (fig. 2a,b) affected the way earthquake effects were reported in some publications for several decades (see grapes and downes, 1997). the 1848 m 7.5 marlborough earthquake (grapes et al., 1998), which damaged or destroyed almost all brick buildings in wellington and caused three deaths in a large aftershock, was enough to cause many settlers to leave (eiby‚ 1980). the occurrence, eight years later, in 1855, of a second stronger, more damaging and frightening earthquake, the m 8+ wairarapa earthquake meant that more settlers left (grapes and downes‚ 1997). more importantly for the wellington colony’s promoters, the earthquake had the potential to discourage prospective immigrants and to cause the other rival major settlements to consider that wellington was unfit to be the centre of government. as a result, the government of the day, as well as those who had invested heavily in the settlement, attempted to downplay the damage and distress both locally and in official reports and private documents that were sent to britain. analysts of historical earthquakes for some time after these earthquakes need to be aware that some accounts, particularly those from government or immigration authorities, might be distorted. 7. analysing historical earthquakes: determining parameters the two most important parameters usually sought for cataloguing historical earthquakes are magnitude (or magnitude range) and location (with an estimate of its accuracy). our ability to determine these parameters divides new zealand’s large historical earthquakes into two 410 gaye l. downes categories – pre-instrumental period earthquakes, for which there are little or no useful instrumental data, and early instrumental period earthquakes, for which sufficient teleseismic data are available to permit the calculation of surface wave magnitudes (ms), moment magnitudes (mw), mechanisms and depths (from 1917), for shallow and some intermediate depth earthquakes. as will be shown, data on early instrumental period earthquakes, as well as modern earthquakes, have been used to develop the tools for assessing the effects of future earthquakes, but they are also invaluable for analysing pre-instrumental period earthquakes. however, the most important requirement for analysing a region’s historical earthquakes is an extensive knowledge of its tectonics, geology and recent seismicity. to obtain the best understanding and interpretation of historical data, the skills and knowledge of seismologists, earthquake and landslide geologists, geodesists, earthquake engineers, and tsunami scientists often need to be brought together. 7.1. early instrumental period earthquakes (1917-1942) dowrick and rhoades (1998) have determined ms for all shallow earthquakes over magnitude 5.5 since 1900. body wave modelling techniques have yielded source parameters (moment magnitudes, depths and mechanisms) for all m > 6.0 north island, and m ≥ 6.3 south island, shallow and intermediate depth historical earthquakes that have occurred since 1917 (see doser et al., 1999, doser and webb‚ 2003). the tools developed from these recent studies make the task of locating and analysing historical earthquakes of the early instrumental period simpler. in the last ten years, all except one of the m ≥ 7.0 shallow earthquakes from 1929-1942 have been studied in detail (dowrick, 1994, 1998; downes et al. 1999, 2001). these have resulted in well-constrained locations and isoseismal maps, as well as a wealth of data on the distribution of landslides and liquefaction, on microzone effects, on the vulnerability of lifelines (water, gas and sewage systems, road and rail, etc). in addition, much data have been found on the performance of various classes of structures, including reinforced concrete and retrofitted masonry buildings, some of which still exist today. all the studies involved the comprehensive collection and analysis of descriptive, scientific and engineering accounts of effects and damage from many sources. several of the studies were multi-disciplinary or involved multidisciplinary collaboration and sharing of data (for example, downes et al., 1999; 2001, schermer et al., 2003). of the two studies that involved mainshock and aftershock relocations using early local seismograph records, the study of the 1934 mw 7.4 pahiatua earthquake (downes et al., 1999) and its associated paleoseismological study (schermer et al., 2003) were particularly successful. prior to these studies, the location of the 1934 earthquake had an uncertainty of 100 km, the distribution of intensity of shaking data was sparse, and no surface fault rupture was known to be associated with the earthquake. taking into account the shallow depth determined in preliminary (now published) source mechanism results of doser and webb (2003), downes et al. (1999) found new instrumental locations for the mainshock and aftershocks, which correlated well with the newly determined well-constrained intensity distribution (fig. 6a,b). further, the fortuitous location of a wood anderson seismograph almost along the apparent strike of the fault plane allowed an estimation of the approximate length of the fault plane using m > 3.5 aftershock s-p intervals. the s-p intervals also suggested the mainshock epicentre lay at one end of the fault, consistent with the intensity distribution. given the new location and the shallow depth of the earthquake, schermer et al. (2003) investigated active faults in the area, including trenching several of the «freshest» looking fault scarps. the latter resulted in the identification of surface fault rupture in the waipukaka fault zone (fig. 6a,b), which had clearly occurred since european settlement of the area because of the presence of charcoal (the area was heavily forested until after 1874) and historical artefacts (glass objects and the sole of a shoe) em411 procedures and tools used in the investigation of new zealand’s historical earthquakes bedded in the rupture zone. the 1934 earthquake is the most likely earthquake to have caused that rupture. recent research in the early instrumental period has not been confined to the largest earthquakes, or to the shallow earthquakes. to provide the data to develop new attenuation of intensity models, dowrick (see dowrick and rhoades, 1999) and downes have developed new isoseismal maps for another 30 or so wellconstrained pre-1943 smaller events as well as a number of large earthquakes in this period not yet the subject of in-depth studies. previously, for these events, there had been either no isoseismal map, or the existing map had been based on a conversion of rossi-forel to mm intensities without sighting original reports. these data, together with data from the individual large earthquake studies, including some pre-1900 events (those which have a magnitude independent of intensity of shaking distribution, that is, from fault length or dislocation model), and data from modern era earthquakes were used to develop robust mm intensity attenuation relationships involving magnitude, focal depth, source mechanism, fault orientation, and regional characteristics parameters (dowrick and rhoades, 1999). it should be noted here that the sparseness of the population and hence sparseness of intensity points for new zealand typically result in simply contoured elliptical isoseismal patterns. dowrick’s isoseismal map revisions (see dowrick and rhoades, 1999) and the large earthquake studies have also made it possible to refine descriptors in the new zealand version of the modified mercalli intensity scale (dowrick, 1996). one problem encountered when applying the mm scale to early historical earthquakes is the difficulty in determining high intensities in the near absence of masonry buildings in historical rural settings. in historical times, most domestic housing was of wood construction, with brick, reinforced or unreinforced, chimneys depending on the era. the tools, as outlined above, provide the means to analyse pre-instrumental earthquakes, as well as smaller earthquakes not yet studied in the early instrumental period. other useful tools are the relationships between size and extent of fig 6a,b. a) relationship of the instrumental locations of the 1934 mw 7.4 pahiatua earthquake and its aftershocks to the mm intensity isoseismal map and the waipukaka fault zone. b) the distribution of s-p intervals recorded on wellington wood anderson seismograms. note that the five earthquakes with s-p intervals of 19-21 seconds include the porangahau earthquake sequence (ml 5.0, ml 4.6, ml 5.6) five days after the mainshock, as well as the offshore ml 5.2 earthquake just south of 41ºs within 26 h of the mainshock. a b 412 gaye l. downes earthquake induced landsliding and other ground damage, and earthquake magnitude and location, mm intensity, topography and geology, developed by hancox et al. (2002). these relationships were based on detailed study and mapping of landslides and other ground damage caused by 22 well-constrained historical earthquakes from 1848-1995, only two events being pre1900. the relationships are useful for identifying the epicentral region of pre-instrumental earthquakes, most particularly in the absence of intensity data based on building or contents damage, which is the preferred and more reliable basis for assigning intensities. however, the refinement of ground damage descriptors in the mm scale, developed by hancox et al. (2002) also, do help constrain the types and extent of damage observed within in each intensity level. 7.2. pre-instrumental or very early instrumental earthquake period (pre-1917) our ability to locate and determine reliable magnitude, epicentre and depth parameters for earthquakes in this period is clearly dependent on the variable data quantity and quality allowed by the distribution of the population, on whether a surface fault rupture was identified, and on whether the earthquake was shallow or deep. the shallow (depth < 40 km) earthquakes are easier to locate, but still present some difficulties. the best location of a large pre-instrumental earthquake that can be hoped for is the identification of the fault on which it occurred, or which was ruptured to the surface. however, not all m ≥ 7 shallow earthquakes rupture to the surface, at least in parts of new zealand. although four shallow (upper plate) strike-slip mw ≥ 7.0 earthquakes occurred (onshore) in the north island between 1931 and 1942, only two are known to be associated with surface rupture (doser and webb, 2003). in one of these, the 1931 mw 7.8 hawke’s bay earthquake (~ 30 km deep), a surface rupture of only 15 km was observed, although surface deformation occurred in a region 90 km long and 17 km wide (hull, 1990). even in the ideal circumstance of a surface rupture being identified for an earthquake, defining a single parameter as the fig. 7a,b. a) location and isoseismal map of the 1855 m 8+ wairarapa earthquake, showing b) the location of the wairarapa fault, regional deformation, the location of maximum uplift, and tsunami run-up. a b 413 procedures and tools used in the investigation of new zealand’s historical earthquakes earthquake’s location in a parametric catalogue can be a problem. for example, the 1855 m 8+ wairarapa earthquake (see fig. 7a,b) ruptured about 100 km of the onshore part of the wairarapa fault with a possible 30 km more off shore (grapes and wellman, 1988; grapes and downes, 1997). the best fit dislocation modelling of the recorded deformation in this event suggests that the wairarapa fault could curve down towards the west to meet the plate interface some 20 km or so west of the surface expression of the fault at a depth of 25 km (darby and beanland, 1992; grapes and downes, 1997). historical descriptive accounts suggest that the earthquake initiated closer to one end than the other (grapes and downes, 1997), but whether, for catalogue purposes, it is reasonable to use this information or to merely identify the generic centre of the fault as the earthquake’s epicentre is a question that still needs to be addressed. in the absence of a surface fault rupture, unequivocally identifying whether an earthquake is shallow or deep is not without problems. firstly, large earthquakes deep in the subducting slab (> 70 km) can produce isoseismal patterns that resemble moderate magnitude shallow regional earthquakes (fig. 8a,b). this problem will arise mostly when the intensity data are sparse, and the descriptive accounts are too imprecise to identify an s-p interval or aftershocks. fig. 8a,b. similarity of the isoseismal maps of the 24 june 1951 ml 6.3 33 km deep hawke’s bay earthquake (a) and the 5 january 1973 ml 7.3 ~ 170 km deep north island earthquake (b) illustrates that, in the case of sparser intensity data points than shown here, differentiating between moderate magnitude shallow and large magnitude deep historical earthquakes may be difficult. a b 414 gaye l. downes a second problem arises because the presence or absence of aftershocks is not always a clear indicator of shallowness. the occurrence of a large number of aftershocks most often indicates a shallow crustal shock, or an event in the upper seismicity band of a double-banded benioff zone, and the paucity or absence of aftershocks usually indicates a much deeper event or one in the lower seismicity band of a doublebanded benioff zone (robinson, 1994). pacific rim plate interface earthquakes can be aftershock-rich, or aftershock deficient (singh and suárez, 1988). aftershock deficient subduction interface earthquakes have been shown to characterise several subduction zones or parts of subduction zones around the world (singh and suárez, 1988) and it is possible that the hikurangi margin includes one of these zones. for example, the mechanism of the aftershock deficient 1993 ml 5.9 tikokino earthquake at a depth of 25 km (see fig. 2a,b) has been shown to be consistent with the earthquake rupturing the plate interface (reyners et al., 1997). although the data are too poor to constrain their depths, two other low aftershock producing events have occurred within 50 km, the 1958 ml 6.1 ashley clinton earthquake (restricted to a depth of 33 km) (see downes, 1995), and the 1904 ms 6.9 m 7+ cape turnagain earthquake (downes, in prep.) (locations shown in fig. 2a,b). the problems locating the 1904 earthquake exemplify the problems of unequivocally defining depths for historical earthquakes. for the reason that early earthquakes are vulnerable to revision with new data, all reports of felt earthquakes in the pre-instrumental period will be summarised in a descriptive catalogue, similar to that produced by eiby (1968, 1973). transcription and key-wording of source material has been completed for some of the larger earthquakes and is under way for the pre1900 catalogue, to facilitate future revision and to maximise the usefulness of the catalogue for researchers in many fields. 8. catalogue parameterisation the format and extent of the data in the historical earthquake part of the present national earthquake information database are summarised in table ii. there are some inadequacies in this database, and an expansion of it is in the planning stages. the fields under consideration for inclusion in this database are indicated in the second part of table ii. one of the inadequacies of the present database is that none of the felt intensity data on pre1943 earthquakes has been included in the electronic part of the catalogue, that is, the data exist only in written form on paper. another serious inadequacy is that, although epicentres of historical earthquakes were assigned or determined in a uniform and consistent manner when the electronic database was set up in the late 1980s, epicentres have been assigned recently in a variety of ways, which are not always readily apparent in the method of determination parameters. a more serious flaw in the database is that it allows only one entry for some fields where multiple entries might be more appropriate, and some parameters, such as source mechanism, are not included at all. another problem that needs further consideration is that of epicentres of large historical earthquakes generally not being comparable to instrumentally determined epicentres – possibly resolvable by including parameters in the database that better describe source location and rupture area. in developing the new database, such inadequacies and problems will need to be addressed. the database, for both modern and historical earthquakes, should be able to adequately represent all available reliably determined earthquake location, source, magnitude and effects parameters (and their method of determination, reliability parameters, etc.). it should also attempt to satisfy those users who require uniformly determined epicentral or source parameters (for example, for statistical analysis of earthquake occurrence in time and space), as well as those who require the most accurate location, source and rupture propagation parameters derived from special studies, and those who require detailed information on earthquake effects. hence, multiple entries in some fields, as well as new fields, are required, as well as a database interrogation interface that allows researchers to choose the set of parameters most appropriate to their research. 415 procedures and tools used in the investigation of new zealand’s historical earthquakes 9. 1855 january 23 m 8+ wairarapa earthquake the 1855 january 23 m 8+ wairarapa earthquake is new zealand’s largest earthquake since organised european settlement in 1840. this earthquake, and to a lesser extent, the 1848 m 7.5 marlborough earthquake, had a significant social and economic effect on the future nation and building style of new zealand (grapes and downes, 1997). as far as its effects on the environment, the earthquake ruptured to the surface along the wairarapa fault, with what is recognised today as 9-14 m of dextral motion and up to 3 m vertical motion at the fault (grapes and wellman, 1988; grapes and downes, 1997) (fig. 7a,b). regional uplift occurred west of the fault, with up to 3 m uplift along the south coast of the north island coast recorded at the time by surveyors and others, based on changes in the levels reached by the tides along the coast (grapes and downes, 1997). a local maximum has been shown to have occurred near turakirae head, geologically (mcsaveney and hull, 1995; mcsaveney and downes, 2003), and historically, in an 1868 property survey map that delineates the preuplift high tide mark (see grapes and downes, 1997). the earthquake caused damage to the built and natural environment over a very wide area, with up to 9 deaths. a significant tsunami swept through the cook strait region reaching a maximum recorded height of 9-10 m above sea level at the time, and up to 5 m in locations now occupied by the suburbs of wellington city. although this earthquake has been well known as the new zealand’s largest earthquake and its effects generally recognised for some time (for example, see eiby, 1989), the mammoth task of compiling and analysing all the historical data, started independently by seismologist, g. eiby, and geologist, r. grapes, was continued and expanded in a collaborative project by grapes, and downes (a seismologist) after eiby’s death, and only completed in 1997 with a comprehensive analysis of the earthquake and its effects (grapes and downes, 1997). before his death, eiby published a paper showing that a tradition of significant uplift in a small part of the wellington region was not supported by geological evidence (eiby, 1990), exemplifying the problems historical earthquake analysts have in identifying fact and fiction. in the case of the 1855 earthquake, reminiscences in particular frequently confuse the effects with those of the previous large earthquake near wellington in 1848. the analysis of the 1855 earthquake by grapes and downes (1997) is an illustration of the techniques in analysing historical earthquakes in new zealand used in the last few years, following the example set by eiby with his pioneering comprehensive analysis of the 1848 marlborough earthquake (eiby, 1980). using over 200 historical documents as the primary data source for their analysis, but also taking into account the results of more recent geological, geomorphological and seismological investigations of the deformation, grapes and downes (1997) discuss most aspects of the earthquake including: – mainshock magnitude and epicentre, using attenuation models, dislocation models of the earthquake (darby and beanland, 1992), and structural and tectonic models developed from seismicity beneath wellington (robinson, 1989) to constrain the magnitude and likely fault plane. – felt intensity distribution. – descriptive account of the effects of the mainshock on people (including casualties) and man-made structures by location throughout new zealand. a résumé of contemporary building techniques is included. – effects on the environment from strong shaking such as fissuring, liquefaction, sand boils, lateral spreading, settlement and landslides. – effects on the environment from tectonically produced uplift and faulting, including biological (death of seashore animal and plant life) and atmospheric effects: tsunami and seiche; aftershock occurrence; social response and recovery. historical documents included contemporary personal diaries, letters and journals, newspaper reports and articles, memoranda and reports of the wellington provincial government as well as later reminiscences, extracts from published scientific papers, maps and surveys, sketches and art works, books and other articles. 416 gaye l. downes although most of the papers were found in new zealand archives, several were sourced from britain, sometimes through intermediaries. for example, the published accounts of the british geologist, sir charles lyell, who, in 1856, first recognised the importance of the earthquake as causing the greatest deformation and fault rupture then known, are of prime importance as are his private letters (lyell, 1856c), and note books (lyell, 1856d), which were transcribed from the originals and supplied to the authors by leonard g. wilson (u.s.a.). much of the data is presented in grapes and downes’ (1997) paper with extensive quotations from the source material, especially where conflicting accounts on important aspects have been found. all material is analysed with an understanding of geographical, social and political conditions at the time. the reliability of the material is taken account of so that first-hand accounts, that is, those which were recorded no more than several years after the earthquake, and in which there are no obvious inconsistencies or confusion with other earthquakes, are valued most highly. in addition, downes and grapes (1999) have transcribed, key-worded, referenced, cross-referenced, and annotated the source material (over 250 pages), in such a way that it can readily be made into a searchable computer database in the near future, so that extracts relating to any particular aspect or effect of the earthquake and/or from any particular location can be recovered simply and quickly. reliability, events and effects keywords (location keywords are not listed) are given in table iii. many of the extracts are annotated to provide some insight about the author, the reliability of the account, or about important details contained within the extract. compilation of the source material in such a manner provides the means for further detailed studies or reinterpretation, as new evidence – historical, seismological, geotechnical, geomorphological or geological – becomes available. 9.1. the 1855 earthquake, sir charles lyell and global understanding the private and published papers of the eminent 19th century british geologist, sir charles lyell, are the only contemporaneous scientific accounts and evaluations of the 1855 earthquake’s effects on the landscape (for example, lyell, 1856a–d; 1868). as such, they table iii. reliability, events and effects keywords used the compilation of transcriptions of source material for the 1855 m 8+ wairarapa earthquake. reliability event effects primary mainshock artesian well effects primary/reminiscence aftershocks artworks/maps/charts secondary background information atmospheric effects biological effects building damage casualty/injury faulting ground damage response/recovery tsunami/seiche uplift/subsidence volcanic effects notes: primary = contemporary eyewitness account; primary/reminiscence = eyewitness reminiscences; secondary = second or third hand account; artworks = sketches and paintings portraying earthquake effects; background = relevant background information, mainly relating to uplift/subsidence, building construction techniques, or social conditions. other keywords are self-explanatory. 417 procedures and tools used in the investigation of new zealand’s historical earthquakes provide data crucial to understanding of the faulting and regional deformation that accompanied the 1855 m 8+ wairarapa earthquake. the 1855 earthquake was also of great importance for lyell (downes and grapes, 1997), as it provided him with invaluable evidence to support his uniformitarian theories. in 1856, sir charles lyell gave two papers on the geological effects of the 1855 new zealand earthquake, one to the royal institution in london and the other, to the geological society of france. lyell’s evidence was no doubt gathered from reports extracted from new zealand newspapers, the new zealand journal, the reverend richard taylor’s book, te ika a maui (taylor, 1855), all transcribed in downes and grapes (1999), but his understanding of the geological effects of the earthquake were principally derived from interviews with three new zealand eyewitnesses. he met the three men in london on number of occasions in early 1856, as well as maintaining a correspondence. one of the eyewitnesses was the son of the distinguished geologist, gideon mantell, lyell’s friend and colleague. another was a royal engineer who was, at the time of the earthquake, working on a coastal road in the area of maximum uplift. the third eyewitness was a land owner in the south island, whose interests included geology. lyell recognised the 1855 earthquake as «yielding to no other in magnitude of its geological and geographical importance» (lyell, 1856a). in a lecture, given by a colleague on his behalf to the geological society of france, lyell refers to «the formation of a great fault and of an upheaval which is greater in vertical height and horizontal extent than all dislocations of this kind that i [i.e. lyell] am aware of to date» (lyell, 1856b [translated from french by g. lamarche (nz)]). lyell also included data on the 1855 earthquake in vol. 2 of his «principles of geology», stating that «the geologist has rarely enjoyed so good an opportunity... of observing [in the effects of the 1855 new zealand earthquake] one of the steps by which those great displacements of the rocks called «faults» may in the course of ages be brought about. the manner also in which the upward movement increased from north-west to southeast explains the manner in which beds may be made to dip more and more in the given direction by each successive shock» (lyell, 1868). for lyell, the connection between large earthquakes and sudden uplift, tilting and faulting on a regional scale was indisputably demonstrated, as was the principle that geological features of the landscape could be explained as the sum of sudden incremental changes rather than by cataclysmic events. these results were expounded in his books, which were well read by geological students around the world. through these, the 1855 new zealand earthquake was influential in advancing global understanding of earthquakes, faulting, regional deformation and the geological record. 10. conclusions in this paper, the principles and procedures used by the author and others in analysing new zealand’s historical earthquakes have been outlined. it will have been noted that the work of identifying and analysing events is not complete, but is actively being done so that researchers in seismic and other geological hazard events have the best possible data on which to base their assessments of hazard and risk. acknowledgements the author thanks drs martin reyners and warwick smith, of the institute of geological & nuclear sciences, for careful review of this paper. the author also thanks the organisers of the 21st course of the international school of geophysics on «investigating the record of past earthquakes», v. garcia acosta, r.m.w. musson and m. stucchi, for the invitation to participate in the course. the author would also like to acknowledge the support of the foundation for research, science and technology and the new zealand earthquake commission in funding much of the recent (last 10-15 years) research on historical earthquakes. 418 gaye l. downes references benson, a. m., t. a. little, r. j. van dissen, n. hill and d. b. townsend (2001): late quaternary paleoseismic history and surface rupture characteristics of the eastern 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(1856a): on the successive changes of the temple of serapis, in proc. royal institution, 2, 207-214 (given as a lecture at the weekly evening meeting, 7 march 1856). lyell, c. (1856b): sur les tremblement de terre du 23 janvier, 1855, à la nouvelle zélande, bull. soc. géol. france, 2e série, 13, 661-667. lyell, c. (1856c): correspondence from sir charles lyell to his sister, sophia lyell, and to walter mantell, april-may 1856 (originals held at kinnordy house, scotland, unpublished). lyell, c. (1856d): notebooks of sir charles lyell, notebook 213 (originals held at kinnordy house, scotland). lyell, c. 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(1995b): a procedure for modelling near-field earthquake intensities, bull. nz soc. earthquake eng., 28, 218-223. stirling, m.w., g.h. mcverry and k.r. berryman (2002): a new seismic hazard model for new zealand, bull. seismol. soc. am., 92 (5), 1878-1903. stirling, m.w. and c.j.n. wilson (2002): development of a volcanic hazard model for new zealand: first approaches from the methods of probabilistic seismic hazard analysis, bull. nz soc. earthquake eng., 35, 266-277. taylor, r. (1855): te ika a maui, or new zealand and its inhabitants (wertheim and macintosh, london, england). wells, d. l. and k. j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seismol. soc. am., 84, 974-1002. annals 47, 5, 2004 1541 annals of geophysics, vol. 47, n. 5, october 2004 key words fem – ground deformation – dike intrusion – real medium 1. introduction volcanic areas are zones where marked ground deformation often occurs in response to the action of volcanic sources and magma uprising mechanisms. a classic mechanism of magma uprising is related to intrusive processes characterising the emplacement of dikes. a dike can be represented by an opening crack. tensile dislocation theory has been applied with success to model the case of a rectangular shaped source (fig. 1a) with tensile opening in an elastic homogeneous half-space (okada, 1985; yang and davis, 1986). the application of dislocation theory to the case of tensile mechanisms has often been used in the last ten years and applied successfully to model ground deformations by inverting the recorded data to infer source parameters (e.g., yang et al., 1992; bonaccorso, 1999). however, the real characteristics of the medium are not perfectly homogeneous. furthermore, real topography is very different from the flat reference surface assumed in analytical solutions (fig. 1b), whereas an analytical solution in realistic situations with topography and stratifications of the medium is impraticable. a valid alternative is represented by the use of the finite elements method (fem). the study was carried out to investigate the effect of a tensile crack in a volcanic edifice, through fem numeric technique. the use of this analysis has been consolidated in other fields such as engineering, physics, etc., while its application is more recent in geophysics and volcanology (e.g., dieterich and decker, 1975; melosh and raefsky, 1981). in finite element analysis of ground deformation due to dike intrusion with applications to mt. etna volcano rosario occhipinti amato (1), marco elia (1), alessandro bonaccorso (2) and guido la rosa (1) (1) dipartimento di ingegneria industriale e meccanica, facoltà di ingegneria, università degli studi di catania, italy (2) istituto nazionale di geofisica e vulcanologia, sezione di catania, italy abstract a 2d finite elements study was carried out to analyse the effects caused by dike intrusion inside a heterogeneous medium and with a realistic topography of mt. etna volcano. firstly, the method (dimension domain, elements type) was calibrated using plane strain models in elastic half-spaces; the results were compared with those obtained from analytical dislocation models. then the effects caused both by the topographic variations and the presence of multi-layered medium on the surface, were studied. in particular, an application was then considered to mt. etna by taking into account the real topography and the stratification deduced from seismic tomography. in these conditions, the effects expected by the dike, employed to model the 2001 eruption under simple elastic half-space medium conditions, were computed, showing that topography is extremely important, at least in the near field. mailing address: dr. rosario occhipinti amato, dipartimento di ingegneria industriale e meccanica, facoltà di ingegneria, università degli studi di catania, viale a. doria 6, 95125 catania, italy; e-mail: occhsaro@hotmail.com 1542 rosario occhipinti amato, marco elia, alessandro bonaccorso and guido la rosa general, boundary conditions, optimal size of the domain, meshing, characteristics of the medium, and the type of the elements to be used are not known a priori, so it is necessary to calibrate the models. thus, a robust preliminary set-up is needed to obtain realistic results. a finite elements analysis is always preceded by the meshing of the domain. this is a very delicate phase that influences the accuracy of the calculation, since an erroneous choice of the domain size and the type of elements would lead to unreliable results. the present study initially focussed on the analysis of a very simple two-dimensional domain, with the aim of testing the choice of elements. computations are carried out with the fe software marc (marc, 1994). the results were compared with the theoretical ones expected from the dislocation theory in a homogeneous medium (e.g., okada, 1985). then two-dimensional analyses were done by introducing in the model the topography of mt. etna, along an eastwest section, to examine the effect of the topography on the deformation field. in addition to the topography, the effects of the material and stratification of the medium were studied considering an elastic multi-layered medium with the young modulus deduced from recent seismic tomography. the aim was to compare the pattern of deformation obtained analytically from simple half-space medium with that produced by the same source considered inside a realistic layered medium with topography. 2. materials and method 2.1. model without topography in a homogeneous medium as a first step, we considered two 2d fem models reproducing a rectangular tensile dislocation in a homogeneous half-space. the geometry of the intrusion is shown in fig. 1a and in table i. in agreement with other authors (e.g., chevallier and verwoerd, 1988), zero-stress was assumed at the upper surface, while zero displacement values were assumed at bottom and lateral boundaries. material behaviour was assumed elastic with λ = µ, namely poisson ratio ν = 0.25, and young modulus of 40 gpa (paul et al., 1987; russo et al., 1997). after a number of simulations, a domain of 1 0 0 × 21 km2, meshed with triangular elements, proved sufficient to represent the problem under examination (table i). results highlighted that the fig. 1a,b. a) analytic representation: rectangular tensile dislocation in an elastic homogeneous half-space; b) representation with finite elements method (fem): intrusion in a homogeneous medium with topography. table i. parameters of the analysed dike. this geometry is the one of the analytical models obtained by bonaccorso et al. (2002) to model ground deformation changes recorded by permanent geodetic networks during the dike emplacement of the etna 2001 eruption. dislocation width: inclination top crack opening: b → w angle: δ depth: h [m] [m] [degree] [m] 3.5 2300 90° 80 a b 1543 finite element analysis of ground deformation due to dike intrusion with applications to mt. etna volcano fig. 2. comparison between the theoretical deformation field and that found with the fem model for a tensile dislocation in a homogeneous and elastic half-space. dots indicate the fe solution, solid lines the analytical solution (grey colour for horizontal displacement and black colour for vertical displacement). the agreement between the two sets of solution is evident. for the geometrical parameters see table i. fig. 3. map of mt. etna with near e-w section topography and dike position. vertical and horizontal deformation fields practically coincide with the analytical ones (e.g., okada, 1985) (fig. 2). this allowed us to verify the validity of the size domain and element type used in this simulation. 2.2. model with topography in a homogeneous medium we performed numerical modelling with real topography of an east-west section of the etna volcano (fig. 3). this topography permits results from the fem analysis and the analytical model to be compared (bonaccorso et al., 2002) obtained by inverting tilt and gps data recorded during the emplacement and final uprising of the dike of the etna 2001 eruption. the final parameters of this model are reported in table i. the dislocation model assumed the reference plane at 1500 m a.s.l., namely the mean elevation of points belonging to the geodetic permanent networks. after a number of calibrations of the models, a domain of 20 × 60 km2 was considered suffifig. 4. comparison between the deformation fields of a model in a homogeneous half-space medium without topography and a model homogeneous half-space with real topography. the top of the crack is located at 1420 m a.s.l.; the zero of the abscissa corresponds to the crack projection onto the surface. cient (table ii). here too, the medium was assumed homogeneous with λ = µ and young’s module equal to 40 gpa. as in the case without topography, the value of young’s module, in the 1544 rosario occhipinti amato, marco elia, alessandro bonaccorso and guido la rosa table ii. parameters of fem analysis of a dike intrusion for the different medium analysed: elastic homogeneous half-space without topography; elastic homogeneous half-space with topography; medium with topography and seven stratification layers. model width × no. no. mean nodes element crack dislocation no. young depth × depth elements nodes distance in type w × h opening: layers module layer the apex area [km] [m] [m] [m] [mpa] [km] homogeneous 21 ×100 26 850 13804 4 triangular 2300 ×80 3.5 1 40 000 21 000 without topography homogeneous 20 × 60 48 788 24 775 20 triangular 2300 ×80 3.5 1 40 000 60 000 with topography stratification 20 × 60 48 788 24 775 20 triangular 2300 × 80 3.5 7 6800 2000 with 8350 1000 topography 10100 0 12 400 −1000 15 300 −2000 18 700 −3000 22 100 >−4000 condition of λ = µ, does not alter the values of the surface deformations. instead, the results show an evident influence of the topography on the deformation fields (fig. 4). in particular, the fig. 6. comparison between the deformation field of the tensile crack in a homogeneous medium (i.e. without stratifications) with topography and one in a stratified medium with topography. the top of the crack is positioned at 1420 m a.s.l., the geometric parameters are in table i. 1545 finite element analysis of ground deformation due to dike intrusion with applications to mt. etna volcano modulus, we have applied the following equation e=5/6ρv 2p where vp is the seismic p-wave propagation velocity in an elastic medium, with ν = 0.25. results compared to non-stratified medium models (section 2.2) revealed no significant differences (fig. 6). this implies that, as a first estimation in an intrusion model with rectangular source and constant opening, the effects produced in a homogeneous half-space fig. 5. stratifications of the elastic medium and grid used in the fe calculations; zoom of the summit zone is reported. loss of symmetry of the distribution of the vertical and horizontal displacements is recognised, owing to the non-symmetry of the domain. with respect to the half-space model, one can observe a re-distribution of the deformation with higher mean values having a shift of the maximum peaks. in particular, this displacement, also present for the vertical changes, causes a reverse tilt near the zone above the crack, in agreement with other authors (trasatti et al., 2003). furthermore, the maximum amplitude of the displacements undergoes an increase in the part where the topography has a wide depression in the surface profile. 2.3. models with topography in a layered medium starting from the models with topography, another analysis was performed. we added the stratification of the medium by introducing seven layers of elastic linear materials with different young moduli (fig. 5). the thickness and the elastic moduli (table ii) were carried out from a recent seismic tomography of mt. etna (chiarabba et al., 2000). in order to obtain the young 1546 rosario occhipinti amato, marco elia, alessandro bonaccorso and guido la rosa with λ = µ and in a half-space with parallel stratifications with λi = µi, are the same. 3. conclusions finite elements analysis of a geophysical problem, such as an intrusive mechanism, is complex and therefore requires various simplifications. in 2d models the necessary parameters for the successive simulations were tested: material, surrounding conditions and typology of elements. the results showed a good correspondence with the values obtained from the dislocation theory, which allows us to affirm that the finite elements method provides reliable results if the model is suitably calibrated. if we consider the tensile crack with a constant opening, in the condition λ = µ, the effects of the value of young’s module are practically negligible on the behaviour of the surface deformations. we have shown that the effects of a simple model of tensile dislocation with a uniform constant opening in an infinite single-layer with λ = µ coincide with those caused by the same source in a multi-layered stratification with λi = µi. instead, the effect of the topography is important, above all in the near-field of accentuated topographical variations, usually present in summit areas. the deformations in the far-field, for instance on the volcano slopes, where the topography has less than 20° slopes, provide comparable values to those expected also in more realistic situations with stratifications and topography. therefore, the much faster parametric inversions associated with analytical models, which assume half-space conditions, provide reliable preliminary parameters if near-field data are excluded from the inversion. references bonaccorso, a. (1999): the march 1981 mount etna inferred through ground deformation modelling, phys. earth plan. inter., 112, 125-136. bonaccorso, a., m. aloisi and m. mattia (2002): dike emplacement forerunning the etna july 2001 eruption modelled through continuous tilt and gps data, j. geophys. res., 29 (13), doi: 10.1029/2001gl014397. chevallier, l. and w.j. verwoerd (1988): a numerical model for the mechanical behaviour of intraplate volcanoes, j. geophys. res., 93 (b5), 4182-4198. chiarabba, c., a. amato, f. barberi and e. boschi (2000): recent seismicity and tomographic modeling of the mount etna plumbing system, j. geophys. res., 105, 10923-10938. dieterich, j.h. and r.w. decker (1975): finite element modeling of surface deformation associated with volcanism, j. geophys. res., 80, 4094-4102. melosh, h.j. and a. raefsky (1981): simple and efficient method for introducing faults into finite element computations, bull. seismol. soc. am., 71, 1391-1400. marc (1994): analysis research corporation. user’s guides, version 2000 (marc analysis research corporation, palo alto, ca), a-f vols. okada, y. (1985): surface deformations due to shear and tensile faults in a half-space, bull. seismol. soc. am., 75, 1135-1154. paul, a., j.p. gratier and j. bourdon (1997): numerical model for simulating deformation of mount st. helens volcano, j. geophys. res., 92 (b10), 10,299-10,312. russo, g., g. gilberti and g. sartoris (1997): numerical modeling of surface deformation and mechanical stability of vesuvius volcano, italy, j. geophys. res., 102 (b11), 24,785-24,800. trasatti, e., c. giunchi and m. bonafede (2003): effects of topography and rheological layering on ground deformation in volcanic regions, j. volcanol. geotherm. res., 122 (1/2), 89-110. yang, x. and p.m. davis (1986): deformation due to a rectangular tension crack in an elastic half-space, bull. seismol. soc. am., 76, 865-881. yang, x., p.m. davis, p.t. delaney and a.t. okumara (1992): geodetic analysis of dike intrusion and motion of the magma reservoir beneath the summit of kilauea volcano, hawaii, 1970-1985, j. geophys. res., 97, 3305-3324. (received july 4, 2003; accepted december 19, 2003) u n e l t r a n s l o g s c h l u m b e r g e r a . belluigi 1. — c o n s i d e r i a m o il c.e.111. ( c a m p o e l e t t r o m a g n e t i c o ) di una sorgente p u n t i f o r m e d i c o r r e n t e : 0, e l e t t r o d o di d i m e n s i o n i t r a s c u r a b i l i rispetto alla distanza dei p u n t i di misura di t a l e c a m p o , in un m e z z o terrestre o m o g e n e o . si s u p p o n e c o n c i ò c h e i l r i t o r n o della c o r r e n t e a v v e n g a a g r a n d e distanza dalla s o r g e n t e 0, d i m o d o c h é d i p a r t e n d o s i r a d i a l m e n t e l e l i n e e di flusso da essa, l e r e l a t i v e s u p e r f i c i di l i v e l l o si p o t r a n n o c o n s i d e r a r e s f e r i c h e c o n c e n t r i c h e , di c e n t r o 0. cdvo p o s t o l ' e l e t t r o d o 0 ( m o b i l e ) in un f o r o di sonda, un s e c o n d o fisso alla s u p e r f i c i e d e l suolo, le prese d i t e n s i o n e m o b i l i c o l l i n e a r i ad 0 lungo l'asse v e r t i c a l e d e l f o r o c i l i n d r i c o , si realizza il classico d i s p o s i t i v o e l e t t r o d i c o di carotaggio e l e t t r i c o s c h l u m b e r g e r . a l i m e n t a n d o s i f f a t t o c i r c u i t o a c.c. ( c o r b(z^)[ rente c o n t i n u a ) d'intensità /, se (_> è la resistività d e l t e r r e n o , si ha, ( v . f i g . 1), tra i p u n t i a e b la caduta di t e n s i o n e : \ v = i q i a 1 [1] m è il c o e f f i c i e n t e di f o r m a ( o g e o m e t r i c o ) : 4 i 2, l o / l z » — 2 ] ) d e l d i s p o s i t i v o . l a [ 1 ] è v a l i d a , p r e s c i n d e n d o dalla resistività qm dei f a n g h i ( o c o n s i d e r a n d o pnl = p), e dal d i a m e t r o d e l f o r o ( o r i t e n e n d o l o trascur a b i l e r i s p e t t o a z1 ? z2). m e n t r e p e r i r e g i m i s t a z i o n a r i il c a m p o e l e t t r i c o v a l e : e.. = — grad v [ 2 ] per q u e l l i quasi-stazionari ( v . [ 1 ] ) , detta « h » f o n d i t à di p e n e t r a z i o n e della c o r r e n t e nel s u o l o : e„,s. = — e < < + * « • . g r a d v che p e r a)—->-o, i m p o r t a o v v i a m e n t e : eq,s. = ee, p a t f o g 0 fig. 1 ( 2 / a co (vi la pro[ 3 ] 126 a . b e l l v 1 c i 2.— inseriamo ora una step-funetion di correlile nel precedente circuito tellurico, il campo elettrico s'esprimerà c o n : f.(/) = / ( « ) • //4 jict 2 s ; a = q ~ 1 [ 4 ] dove f(t) è una cosidetta « funzione transitoria » (v. [ 2 ] ) , e7,(t) la componente verticale del campo elettrico. introducendo il t e m p o : t = 4 5t az~ : io'9, (che dipende dalla struttura fisica del terreno e dalla distanza dei punti di osservazione, e assume io stesso valore su ciascuna delle superfici sferiche potenziali concentriche intorno ad 0), la [ 4 ] diviene, esplicitando f(t): e, (t) = i • (1 — erj f w /4 * " [ 5 ] è evidente elle: ez(0) = 0, e z ( c o ) = / 4it a s 2 , qualunque siano z e la costante di t e m p o : t(c!,z). la caduta di tensione trans tra i punti sulla verticale, asse del p o z z o : a(zj) e b ( z 2 ) sarà p e r t a n t o : av [ 6 ] dove l'integrale indefinito in [ 6 ] vale, ( p e r l'addendo in « erf » ) : c —z * erj „ 1 / jc a \ , j-n + va/t /gì • e j i i s i ; c = cost. arbitraria, da cui, posto — ] jt a/t : a v ^ 2 (z4 xj ' * , ' ) + *tlerj ?za-zr*erf fa ì ig, e ^ — ig, e i valori estremi di a v(t) risultano: a v\oo) = (zì-3 — zs-*)iqi2iz a f(o) = {z~* — *,-» + z„~< _ s( -i) i p/4 [ 7 ] [ 8 ] u n e l t r a n s l o c s c h l u m b e r c . e r 127 l a derivata prima rispetto al tempo d i a f u l potendosi s c r i v e r e : a v(t) = — / < » lg-,[' ' ) — lgi(e j j /4 jt ty ]j jt [ 9 ] tenendo conto c l i e : — fi > 0, il segno di af'(t) dipenderà dalla differenza dei l o g a r i t m i integrali. posto ora cp(t.) = / g i ( e j , <p (o) = i g , ( 1 ) < ' o ( n e l l ' i n t e r v a l l o : 0, 1, / £ i t < 0 | , ((' (oo)-lgi (o) = o, fp1(;) = 2|"_i e l>0, <p (?) così è crescente. d i s t i n g u i a m o ora 2 casi ( i l che ha un o v v i o significato fisico): z , < z 2 , 2 , > z 2 . s e : z, < z«, ?,<?,, — à v(t) > o a v ( t ) è crescente. i n o l t r e : z,":l — z-_. ' > 0, z2 1 — z f 1 < 0, q u i n d i : /(0) = — — (z, 3 z . -' + s, z , ' ) —^— ( z , = • 4 it a 4 jt a [10] s e : z , > z 2 , zih — zz3 < 0, z/1 — z,1 > 0, e q u i n d i : /(0l > / ( o o ) [11] i n o l t r e essendo: t, > , lg, ( e _ ig, ( « " " j < 0 5 à v (t) < 0 ; a f ( t ) è decrescente. n e l l a fig. 2 è riassunto quanto sopra considerando sia f ( 0 ) > 0, che / ( 0 ) < 0. 3. — occorre, a questo punto, riconoscere i valori di /(0), / ( c o ) , é 0 , in f u n z i o n e della distribuzione t r i p o l a r e verticale. e ciò ai fini della scelta più c o n v e n i e n t e d e l l e g e o m e t r i e polari che si r i p e r c u o t o n o sul m a g g i o r e o m i n o r e evidenziarnento d e l l e caratteristiche t e m p o r a l i dei geoeltrans così attivati. 128 a. b e l l u i g i a ) i n d i c h i a m o : f(0)-4>xa/i = q p ( z i , zo), ed e s a m i n i a m o i l s e g n o ditp a cui sono l e g a t i gli e v e n t i di fig. 2 : ( p ( z ) , z,) = z,-»(h — l)(h* + h + l—za*)i h = z2zl [12] a v ( t ) a ) f a v ( t ) c) d a t o zo > 0 , h > 0, il segno di qp(zi,zo), d i p e n d e r à da q u e l l o d e l p r o d o t t o a 2" m e m b r o d e l l a [ 1 2 ] , e q u e s t o dai segni di (7t — 1) e d e l t r i n o m i o i|> (h) — (.hr f li -j1 — z o 2 ) r i v a r i a r e di li > 0. i l d i s c r i m i n a n t e di t|> (li) = 0 , è : a = 4 z.r — 3 . o r a se a < < 0 , ( 0 < so < | 3 2), il t r i n o m i o t|>(/i) n o n si a n n u l l a p e r v a l o r i r e a l i di h, ed è s e m p r e p o s i t i v o ( p e r li = 0 v a l e : 1 — z o 2 > 0 ) ; i | i ( 7 i ) > 0 s e a = 0, ( z 2 = )'3 2), in q u a n t o si r i d u c e a : ' ' 1 v2)2 e d è positìaro p e r v a l o r i p o s i t i v i di li. se i n v e c e a > 0, ( } ' 3/2 < =0) o c c o r r e d i s t i n g u e r e 2 c a s i : 1 3/2 < z 2 < 1; 1 < zo. n e l i " , 1 — z , 2 > 0, l e r a d i c i h a n n o lo stesso segno, e d essendo la l o r o s o m m a n e g a t i v a : = ( — 1 ) , sono e n t r a m b e neg a t i v e , e p e r v a l o r i p o s i t i v i di li il t r i n o m i o è a n c o r a p o s i t i v o . se inv e c e zo > 1, esiste una r a d i c e p o s i t i v a : h — 0,5 ( — 1 f | 4zo2 — 3 ) , e p e r : 0 < li < li, t ( / i ) è n e g a t i v o , p e r li = h v a l e z e r o , p e r h < h è p o s i t i v o . p r o s p e t t i a m o t a l i e v e n t u a l i t à c o m e segue n e l q u a d r e t t o sottostante : u n e l t r a n s l o g s c h l u m b e r g e r 1 2 9 | 3/2 < 1 ]/3/2 z s = 1/3/2 a < 0 a = 0 a > 0 a < 0 v (h) > 0 i|> (h) > 0 v ( h ) > 0 \|.(/i)<0, h < j i i | . ( f c ) > 0 , n e i p r i m i 3 casi il segno di «(> ( z i , zl>) dipende evidentemente da quello di (li — 11, ed è negativo per li < 1, (cioè se z2 < zi), e positivo per h > 0 (cioè se z 2 > z,). n e l 4" caso, distinguiamo 3 sottocasi: 1") / i < l • a l l o r a se h < h, li 1 < 0 , \ | > ( f t ) < 0 , cp (a,, z „ ) > 0 ; s t i < i i < l , h— i < 0 , , * , ) < 0 ; se 1 < / t . / i — 1 > 0 , c p ( / i ) > 0 , cp ( z t , z 3 ) > 0 . 2°) li = 1 se /i < 7 < 1 , zi — 1 < 0 , h > ( f e ) < 0 , cp(z,, z 2 ) > 0 ; se fc—1>0, i p o ) > 0 , <p ( z t , z 2 ) > 0 . 3") / i > l • se h < 1 < / ( , ft 1 < 0 , h ' ( / h < 0 , <p(z,, z „ ) > 0 ; se h < / i , fc — 1 > 0 , m'(/i)>0, q>(*,, z 2 ) > 0 . riassumiamo quanto sopra nel seguente quadro, che permette di poter stabilire le condizioni, ( v . fig. 2), per c u i : / ( 0 ) ^ 0 . . / . < / i , < r ( z 1 ( z 2 ) > 0 7 t < 1 h<h< 1 , cp (z4 , 2 ) < 0 1 < / i , cp(z t , z „ ) > 0 /t = 1 <p(z,, z2) > 0 fc<i , « p k , * i i > ° h> 1 i<h<h, c p ^ z . x o 130 a. b e l l u i g i b) l a : j ( 0 ) 2 n o / / = c p (m), con m = zjzs = h~l si s c r i v e : < p ( m ) = [ l -m> + z*(m — l)u2z* , <p(±oo)=+'x>, q>(0) = 2 z r d o p o discusso e s a u r i e n l e i n e n t e in a i il segno d e l l a f u n z i o n e iniz i a l e /co), a'ediamo c o m e p u ò v a r i a r e il m o d u l o di / ( 0 ) , con la distrib u z i o n e e l e t t r o d i c a . p o i c h é : cf)'(m) = ( — 3 m2 + zi2)/2 zi3 tale d e r i v a t a si a n n u l l a int a n t o p e r : m = + z\ / ) 3 , risultando positiva se / m i < zi / | 3 , n e g a t i v a se | m | > z j / y 3. se ne d e d u c e c h e : cp ( m ) è decrescente n e l l ' i n t e r v a l l o ( — — z i / )'3), crescente n e l l ' i n t e r v a l l o ( — z / ]' 3, z i / |'3), d e c r e s c e n t e nell ' i n t e r v a l l o ( z , / y 3 , f o o ) , ed è dotata di m i n i m o r e l a t i v o n e l p u n t o — z i / y s j d i m a s s i m o r e l a t i v o nel punto z\/ ] 3. i n o l t r e la ip i m ) si a n n u l l a p e r : m = 1; — l / 2 — ) / z t 2 — % : — v i + v z i 2 — zi > i' 3/2 p e r la realtà d e l l e r a d i c i ) . c i ò posto d i s t i n g u i a m o le seguenti s i t u a z i o n i : 0 < = , < v ' 3 / 2 , j / 3 / 2 < z , < l , l < z , < | 3 . =, > 1°) 0 < z, < \ 3 2. o s s e r v i a m o c h e z, < 1 , zi < y 3, si ha cp ( 0 ) > 0, z i / }' 3 < 1„ z i 2 — % < 0, q u i n d i <p(m) = 0 ha la r a d i c e r e a l e m = 1 e la f u n z i o n e <p ( m ) consegue il m a s s i m o r e l a t i v o p e r un v a l o r e d i m < 1. i l v a l o r e m i n i m o r e l a t i v o ( c o n s e g u i t o per m — — z i / / 3 è n e c e s s a r i a m e n t e p o s i t i v o , non p o t e n d o la f u n z i o n e a n n u l l a r s i f u o r c h é p e r m = 1. 2°) j ' 3 / 2 < z i < 1 si ita: <p ( 0 ) > 0, z , } t < 1, z , 2 — % > 0, o n d e le 3 r a d i c i di cp ( m i = •() sono r e a l i ; anzi esclusa la r a d i c e m = 1, l e a l t r e 2 sono n e g a t i v e . 3») 1 < zi < y ' i . si i l a : <p (0| < 0, z, 1 1 < 1, z , 2 — % > 0, tutte e tre le r a d i c i di c p ( m ) = 0 reali. i l caso d i f f e r i s c e dal p r e c e d e n t e p e r c h é q u i 2 radici reali sono p o s i t i v e e l'altra n e g a t i v a . 4"| zi > 3. si h a : c, ( 0 ) < 0, z, > f3, zr — % > 0 : tre r a d i c i r e a l i , di cui una m a g g i o r e d e l l ' u n i t à . si osservi che, in tutti i casi, m a s s i m o e m i n i m o r e l a t i v i v a l g o n o : cp ( z j y t ) = u ( * , ) _ „ = ( 1 + 2 v / 3 / t z , * ) 2z* [ 1 2 a ] <1 (z j i f j = u (z,) m i „ = (1 _ 2 z , 3 /3 t — z*) /2 z , 3 [121,] u n e l t i i a n s l o g s c h l u m b e r g e r 131 i l luogo (li massimi ( f i g . 3| p e r m e t t e r à la scelta più f a v o r e v o l e delle zi, per zi < < }'3, m e n t r e n e l l ' i n t o r n o (li z, = ] 3 si trovano l e situazioni più s f a v o r e v o l i , p e r v a l o r i (li z\ > > } 3 la i i ( z , | rimane invariata per quanto si aumenti z i : i n f a t t i : [i ( . « ) = 1 3 | 3, u ( 0 ) = o , \i'(z)=(z12— 3 ) / ' 6 z i 4 , che si annulla per z\ = ^3, è positiva se z, > ]' 3, negativa se z i < / 3, o n d e u ( z i ) decresce tra 0 e y 3 , cresce da | 3 a + oc, ha un m i n i m o per zi = )' 3, per c u i : |i(( 31 = 0. c| p o s t o : z i / z ' 2 — r n , f(0) 7 ( o o ) = (l k z,) = 0 , 5 [ 1 — a t * / ( , n ! + m + l ì ] che varia al v a r i a r e di m, ( z i , parametro). g l i estremi v a l g o n o : i|> (0, z, i = 0,5 1 1 — z ] 2 ) ; ( c o , z i ) = 0,5, mentre iti' ( m i = zt{2 ni + 11 2 (ni2 -fni 4l ì 2 , s'annulla se m = — y 2 , è positiva se ni > —1/2, negativa se ni < — '/2, o n d e per ili = — y2, la funzione presenta un minimo r e l a t i v o che è anche assoluto: ma : \ji l m l = m+ 1 + z12)/2(m'+ ni + 11, si annulla altresì per m — 0,5 ( — 1 + ]/4ri2'-—3), radici reali se / zl j | 3/2, ( > , " % > 0 ) , complesse se / zx / < | 3/2, ( z i 2 — % < 0). n e l 1" caso si v e d e subito che si h a : iji ( — z ' 1 < n e l 2"> — x/2> « 0 > 0 : l ' a n d a m e n t o della f u n z i o n e è d e l t i p o : 132 a. bf.lluigi si conclude che quest'« eltranslog s c h l u m b e r g e r » rientra in una categoria già da noi illustrata di geoeltrans ( x ) di « m e d i o r e n d i , m e n t o » agli effetti indicativi del parametro resistività elettrica. a parte la nota comune a tutti gli elmatranslog, di un decorso t e m p o r a l e di misura « tensione trans » , in questo p r o c e d i m e n t o appare particolarmente caratteristico, l ' i n i z i o del decorso t e m p o r a l e , f u n z i o n e 0) del valore di regime, della resistività del m e z z o e della geometria del dispositivo tripolare. ciò trova stretta analogia con ciò che a b b i a m o c h i a m a t o « prima r e l a z i o n e f o n d a m e n t a l e » dei geoeltrans ( 2 ) , e con un t i p o p a r t i c o l a r e di matranslog ( 3 ) , particolarmente da noi illustrato. cagliari — facoltà d'ingegneria — novembre 1953. riassunto l'autore si è proposto il calcolo d'un eltranslog nelle condizioni quadripolari, d'un classico carotaggio resistivo schlumberger. gli elmatranslog finora esaminati sono a• carattere induttivo o galvanico-induttivo. il « rendimento indicativo » di tale eltranslog è ben caratteristico, e s'inserisce nella classifica fatta dell'autore dei « rendimenti medi », dei carotaggi transitori, col vantaggio di una maggior semplicità realizzatrice della corrispondente apparecchiatura. su m mary the author proposes a calculation of an eltranslog under quadripolar conditions of a classica! schlumberger resistence ivell log. the elmatranslogs thus far exarnined are of the induttive or galva-inductive types. the « indicated beliavior » of such an eltranslog is very characteristic and is inserted in the classification given by the autlior of the « meati results » of the transitory measurements, with the advantage of greater simplicity of tlie corresponding apparatila. u n ki,ti)ansi.oc s c i i l i miikhger 133 b i b l i o g r a f i a ( ! ) j. koenigsbergeu, das magnetische feld einer stromquelle in raum i l'iiysik. z c i t . 1927). a . pethowski, b u l l c l i n o f p r a c t i c a l g c o p l i y s i c s , l e n i n g r a d 1928, n. 4. s. s. stefanbscu, eludes lliéoriqiies sur la prospection du sous-sol, 1° p a r t i e . b u c u r e s t i 1929. a . bklluici, lierechnung des magnetisclien feldes einer stórung in elektrisch durch stranitela hoden 1933. e r g i i n z . f . a n g . g e o p h . —, curve universali per il calcolo delle influenze e.m. b o l l . u f f . g e o l . d ' i t a lia 1946. , campi e.m. dei suoli galvanicamente elettrizzali. p u b b l . f a c o l t à d ' i n g e g n e r i a m i n e r a r i a di c a g l i a r i 1946, n . 11. —, l'elettrologia galvanica nei conduttori tridimensionali (suoli). b o l l . a s s o c . m i n e r a r i a s a r d a , 1948. ( 2 ) a . belluigi, fondamenti teorici dei geoeltrans. b o l l , d e l s e r v . g e o l . d italia, 1952, v . l x x i v . ( 3 ) a . belluigi, lineamenti teorici di nuovi m. di carotaggio fisico: matranslog. phaselog. g e o f i s i c a p u r a e a p p i . , v . 25, 1953. active faults in the epicentral and mesoseismal ml 6.0 24, 2016 amatrice earthquake region, central italy. methodological and seismotectonic issues an n als of geoph ysics, 59, fast track 5, 2016; doi: 10.4401/ ag -7266 1 active faults in the epicentral and mesoseismal ml 6.0 24, 2016 amatrice earthquake region, central italy. methodological and seismotectonic issues eman uela falcucci*, stefan o gori*, fabrizio galadin i*, gian domen ico fubelli**, marco moro*, mich ele saroli***, * *istitu to n azion ale d i geofisica e vu lcan ologia em an u ela.falcu cci@in gv.it; via d i vign a mu rata 605, rom a **un iversità d egli stu d i d i torin o ***un iversità d egli stu d i d i cassin o e d el lazio merid ion ale abstract the a ugust 24, 2016 a matrice earthquake (m l 6.0) struck a region of the central a pennines (italy) where several active faults were known since decades, most of which are considered the surface expression of seismogenic sources potentially able to rupture during earthquakes with m of up to 6.5-7. the current debate on which structure/s activated during the mainshock and the possibility that conterminous faults may activate in a near future urged us gathering all the data on surface geological evidence of fault activity we collected over the past 15-20 years in the area. w e then map the main tectonic structures of the 2016 earthquake epicentral and mesoseismal region. our aim is to provide hints on their seismogenic potential, as possible contribution to the national database of individual seismogenic source (diss) and to the database of the active and capable fault italy ha zard from ca pable faults (itha ca ). i. in troduction h e ml 6.0 au gu st 24, 2016 am atrice earth qu ake occu rred on a n w-se tren d in g n or m al ru p tu re, m an ifestation of th e exten sion al tecton ic re gim e on goin g in th e cen tral ap en n in e ch ain sin ce th e late pliocen e. th e active n e-sw tren d in g stress gen erated sets of ch ain -p ar allel n orm al fau lt system s, arr an ged as tw o-to-th ree tecton ic rails, alon g w h ich m ajor h istorical an d in stru m en tal seism icity con cen trates. som e of t mailto:emanuela.falcucci@ingv.it annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7266 2 th e ou ter m ost active n or m al fau lts of th e cen tral ap en n in es are con sid ered as “silent”, th at is seism ic gap s (galad in i an d galli, 2000). th e am atrice earth qu ake occu rred in th e sector betw een tw o of th ese silen t fau lts, th e laga mts. an d mt. vettore n orm al fau lts. th e form er affects th e base of th e w ester n slop e of th e laga mts. its su rface exp ression is rep resen ted by a n w-se ~26 km -lon g fau lt-scarp carved on to clayey -aren aceou s miocen e flysch sequ en ces. th e mt. vettore fau lt, n w-se to n n w-sse tren d in g, is m ad e of d ifferen t sp lays an d segm en ts w h ose scarp s are carved on to meso-cen ozoic lim eston e sequ en ces, exp osed alon g th e sw slop es of mt. vettore, mt. le porch e an d mt. bove. th ese com p ose an abou t ~27 km -lon g (at su rface) tecton ic stru ctu re. seism ological, geod etic, rem ote sen sin g an d geological in vestigation s cu rren tly u n d erw ay seem to in d icate th at p arts of th ese tw o stru ctu res p layed a p rim ar y role in th e seism ogen ic p rocess of th e au gu st 24 m ain sh ock. th e aftersh ock sequ en ce is d istribu ted in th e territory affected by th e laga mts. an d mt. vettore fau lts, an d beyon d th e area of m axim u m coseism ic d eform ation (gru p p o d i lavor o in gv su l terrem oto d i am atrice, 2016). sin ce research es are still on goin g abou t th e cau sative fau lt(s) an d con sid erin g th e in h e ren t v ariation of static stress in th e region s n earby, w e h ere rev iew d ata w e collected over th e p ast 15-20 year s of geological field su rveys in th e ep icen tral an d m esoseism al areas, to d escribe an d m ap th e m ajor active fau lts of th e region , basin g on critically selected geological criter ia for active fau ltin g d efin ition in ter m s of both ch ron ology an d su rface evid en ce. fin ally, w e w ill p r op ose som e h yp oth eses abou t th e ch aracter istics of th e related seism ogen ic sou rces, as p ossible con tribu tion to th e n ation al database of in d ivid u al seism ogen ic sou rces (basili et al., 2008) an d to th e database of th e active an d cap able fau lt italy h azard from cap able fau lts (ith aca) (h ttp :/ / w w w .isp ram bien te.gov.it/ it/ p rogett i/ su olo-e-territorio-1/ ith aca-catalogo-d ellefaglie-cap aci). ii. geological “ration ale” for active fault defin ition an d mappin g th e vast geological liter atu re on th e ap en n in e active exten sion al tecton ics p r ovid es n u m er ou s evid en ce of active n orm al fau ltin g th at allow to d erive a con cep tu al m od el for d efin in g an active an d cap able fau lt as th e p rim ary exp ression at su r face of a seism ogen ic sou rce. as for fau lt cap ability, i.e. a fau lt able to ru p tu re th e su rface, w e ad op t th e 6.0±0.2 th resh old m agn itu d e for su rface fau lt in g, on th e ord er of th at p r op osed by mich etti et al. (2000) for th e ap en n in e fau lts. we th erefore con sid er th e fau lts w e m ap as th e su rface exp ression of seism ogen ic sou rces able to ru p tu re w ith m≥6.0±0.2 earth qu akes. we also m ap oth er n orm al fau lts bu t w ith d ifferen t sym bology to m ark th at, based on several geological observ ation s, w e con sid er th em as n ot (or n o m ore) able to n u cleate seism ic even ts larger than m 6.0±0.2. as for th e tim e in terval to assess active an d cap able fau ltin g, w e ad op t th e criteria p r ovid ed by galad in i et al. (2012), for th e italian exten sion al d om ain : a fau lt sh ou ld be con sid ered as active an d cap able if it d isp lays evid en ce of activation in th e last 0.8 myr, u n less it is sealed by d ep osits or lan d form s n ot you n ger th an th e last glacial maxim u m . th is d efin ition h as been basically ad op ted by seism ic m icr ozon ation regu latory in force in an n als of geoph ysics, 59, fast track 5, 2016; doi: 10.4401/ ag -7266 3 italy (com m ission e tecn ica p er la m icrozo n azion e sism ica, 2015). h en ce, to assess an d m ap an active an d cap able exten sion al fau lt / fau lt system as p rim ar y exp ression of a d eep seism ogen ic sou rce, w e d efin e con cep tu al an d factu al cr iteria based on qu atern ar y geological, geom orp h ological an d stru ctu ral field evid en ce. from th ese cr iteria w e d erive “requ isites” th at a fau lt m u st all h ave: 1) th e fau lt m u st sh ow evid en ce of d isp lacem en t of d ep osits an d / or lan d form s (d erived from field observation s an d p aleoseism olo gical tren ch in g across fau lt traces) in d iver se sectors of its tr ace, for d ifferen t ages (mid d le pleistocen e-h olocen e in terval), w ith offsets th at m u st in crease w ith th e age of d isp laced featu res (fig. 1). th e latter issu e is to av oid to con sid er as p rim ary th ose secon d ary ru p tu res su ch as extr ad os or ben d in g fractu res, n ot p rim arily con n ected w ith th e seism ogen ic fau lt at d ep th , as th ey in h eren tly close at certain d ep th . th is can be on ly u n raveled by con sid erin g th e d isp lacem en t of d ep osits an d / or lan d for m s over lon g tim e sp an s. figure 1. structural scheme of the different displacement features related to primary surface faulting and secondary (extrados) displacement. we p referen tially con sid er th e d isp lacem en t of th e base of a given stratigrap h ic u n it across th e con sid ered fau lt. in d eed , th e top su rface cou ld be affected by ero sion al/ d ep osition al p r ocesses th at m ay alter its actu al sh ap e an d m orp h o-stratigrap h ic sign ifican ce. as best p ractice, geological/ geom orp h ological d ata m u st ascertain th at th e con sid ered d isp laced top su rface is p rim ary (or su b-p rim ary) an d / or ascribable to a d ep osition al/ erosion al sp ecific ord er. th is asp ect is fu n d am en tal to av oid to err on eou sly correlate acr oss a fau lt su rfaces (er osion al/ d ep osition al) p ertain in g to d ifferen t ord ers or h avin g d ifferen t m o rp h ostratigrap h ic sign ifican ce. 2) th e scarp of a m ajor n or m al fau lt h as to be con tin u ou sly d etectable for several kilom etres. th is relates to th e observ ation s th at fau ltin g occu rs an d m ain tain s on th e d iscrete stru ctu r al elem en ts for lon g tim e p eriod s, an d stru ctu r al featu res m u st h en ce be geologically an d geom orp h ologically recogn isable in th e field . min or syn th etic or m in or an tith etic str u ctu res, or secon d ary fau lts m ay p rovid e evid en ce for d etectin g p rim ary a ctive fau ltin g in th e field . bu t as th ese fau lts m ay n ot accom m od ate a large am ou n t of slip an d , th erefore, m ay n ot be rep resen tative of th e m ajor fau lt beh aviou r, w e h ave n ot r ep orted th ese secon d ary fau lts. 3) a several km -lon g n or m al fau lt segm en t, sp lay or br an ch for w h ich evid en ce of d isp lacem en t of d ep osits an d / or lan d form s are n ot sp ecifically av ailable h as to be con sid ered as p rim ar ily con n ected to th e seism ogen ic fau lt if it eith er d isp lays overlap w ith or is p ar allel to oth er stru ctu ral fea tu res for w h ich th ese evid en ce are available, u n less sp acin g exceed s ~4 km (for syn th etic sp lays) in m ap v iew (wesn ou sky, 2006); if so, th e fau lts m ay in d eed h ave d ifferen t kin em atic h istory an d beh av iou r an d cou ld be related to d ifferen t seism ogen ic str u ctu res. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7266 4 at th e sam e tim e, m ajor cau tion s m u st be taken before d efin in g an active fau lt as p r im arily lin ked to a d eep seism ogen ic stru ctu re: a) it m u st be con sid ered if n orm al fau ltin g h yp oth esised on th e lateral con tact of d ifferen t sed im en tary bod ies or of sed im en tar y bod ies w ith th e bed rock, cou ld be in stead d u e to m orp h o-sed im en tary p rocesses, th at cou ld be m islead in gly in terp reted . for in stan ce, sed im en tary u n its em bed d in g, flu vial en tren ch m en t an d terracin g. at th is p u rp ose, d etailed geom orp h ic an d sed im en tological an alyses, ch ar acterisation of th e m ech an ism of tran sp ort an d sed im en tation , an d (p aleo)en v iron m en t assessm en t of d ep osition are cru cial asp ects. b) it m u st be evalu ated t h e p ossible occu rren ce of an y oth er n on -tecton ic m orp h ogen etic p rocess th at cou ld im itate su rface fau ltin g, esp ecially w h en occu rrin g alon g fau lt scarp s. am on g th ese factors, local an d areal lan d slid in g (i.e. of sed im en ts/ r ocks at th e base of a fau lt-scarp ), lar ge scale grav itatio n al m ass m ovem en ts (su p er p osin g on a fau ltscarp ), local sed im en t com p action (resem blin g fau lt d isp lacem en t), local erosion (e.g. of th e d ebris d ep osited at th e base of th e fau lt-scarp ), m orp h o-selection p rocesses an d d ifferen tial er osion (i.e. d u e to d ifferen tial erod ibility of d ep osits/ rocks across a fau ltscarp ), local grou n d su bsid en ce or collap ses (e.g. sin kh oles or kar stic featu res), h u m an activity (qu arryin g, excav ation s, d ebris accu m u lation s) m u st be th orou gh ly ackn ow led ged . a n u m ber of stu d ies d iscu ssed abou t th is (bu cci et al., 2007; fu belli et al., 2009; kastelic et al., 2015). most of th ese stu d ies d efin ed th at active fau ltin g on ly based on th e recogn ition of su p p osed m orp h otecton ic featu res, su ch as th e sole local exp osition of th e fau lt p lan e at few p laces at th e base of a given fau lt-scarp , or th e recogn ition in th e field of geom orp h ic featu res (e.g. trian gu lar facets) su p p osed to be fau lt-r elated , can n ot be con sid ered satisfactory w h en taken alon e. iii. discussion data collected th r ou gh th e p ast 15-20 year s in th e area affected by th e 2016 am atrice seism ic sequ en ce an d su rrou n d in gs allow u s to trace series of n orm al fau lt system s, th at w e con sid er as th e p rim ary exp ression at su rface of seism ogen ic sou r ces. as for th e laga mts. fau lt, evid en ce of late pleistocen e-h olocen e activ ity w as rep orted (galad in i an d galli, 2003). th e fau lt can be sp litted in to tw o segm en ts, th e ~18 km -lon g sou th ern segm en t, bou n d in g th e cam p otosto plateau, an d th e ~8 km -lon g n orth er n segm en t, bou n d in g th e am atrice basin . th e cam p otosto segm en t sh ow s a con tin u ou s su rface fau lt-scarp an d d isp lays geological an d geom orp h ological evid en ce of late pleistocen e-h olocen e m ovem en ts; su rface fau ltin g ep isod es w ere id en tified by p aleoseism ological tren ch in g. con versely, n o ev id en ce at su rface of late qu atern ary fau lt a ctivity w as fou n d alon g th e am atr ice se gm en t. galad in i an d galli (2003), th erefore, con sid ered th is segm en t as in active or th at th e fau lt w ou ld n ot be n o m ore able to gen erate earth qu ake lar ge en ou gh to ru p tu re th e su rface. n otew orth y, th is is con sisten t w ith th e su rface observation s m ad e after th e a u gu st 24 m ain sh ock, as n o evid en ce of su rface ru p tu re occu rred alon g th e am atrice fau lt segm en t (em ergeo workin g grou p , 2016). in terestin gly, th e d eep p ortion of th e cam p otosto segm en t activ ated in its low erm ost p ortion d u rin g th e 2009 seism ic sequ en ce (bigi et al., 2013). as for th e mt. vettore fau lt, galad in i an d galli (2003) d efin ed th at th e 18 km lon g su ran n als of geoph ysics, 59, fast track 5, 2016; doi: 10.4401/ ag -7266 5 face exp ression of th e seism ogen ic stru ctu re is m ad e of d ifferen t segm en ts an d sp lays. evid en ce for th e pleistocen e-h olocen e activ ity of th e stru ctu re w as obtain ed by p aleoseism ological in vestigation s. th e mt. vettor e fau lts th erefore sh ow s geological evid en ce at su rface of recen t activity, d ifferen tly from th e am atrice segm en t. th erefore, th e abou t 5 km lon g su rface ru p tu re alon g th e easter n m ost sp lay of th e mt. vettore fau lt follow in g th e au gu st 24 m ain sh ock (em ergeo workin g grou p , 2016) w ou ld cor robor ates su rface fau ltin g p oten tial of th e mt. vettore fau lt. ou r an alyses revealed th at th e fau lt sh ou ld be len gth en n orth w ard for ~8 km . iv. con clusion s th e ml 6.0 au gu st 24, 2016 am atrice eart h qu ake stru ck a region of cen tral italy w h ere several n orm al active n or m al fau lts w ere alread y kn ow n . debate on th e seism otecto n ics of th e 2016 even t, th e p resen ce of m an y oth er n earby active n orm al fau lts an d th e fact th at som e of th ese u n d erw en t in crease of static stress after th e 2009 l’aqu ila earth qu ake (falcu cci et al., 2011) an d likely after th e 2016 seism ic sequ en ce, u rged u s to gath er an d su m u p th e resu lts of field in vestigation s w e p erform ed in th is r egion an d to m ap in d etail th e active n orm al fau lts w e con sid er as th e p rim ary exp ression at su rface of seism ogen ic sou rces able to ru p tu re d u rin g eart h qu akes w ith m agn itu d e of u p 6.5-7. in th is p er sp ective, as for th e m axim u m d ep th reach ed by th e con sid ered fau lts, th e an alysis of th e seism ological d ata related to th e 2009 l’aqu ila earth qu ake (e.g. valor oso et al., 2013) an d 2016 am atrice seism ic sequ en ce (gru p p o d i lavor o in gv su l terrem oto d i am atrice, 2016) su ggest th at m ajor seism ogen ic fau lts in th e cen tr al ap en n in es can reach ~10 km d ep th . figu re 2 su m s u p all th e active fau lts w e m ap p ed , an d th e related seism ogen ic sou rces, as p ossible con tribu tion to th e italian database of th e active an d cap able fau lts (italy h azar d from cap able fau lts, ith aca) an d to th e italian database of in d ivid u al seism ogen ic sou rces (basili et al., 2008). figure 2. m ap of the active faults of the region (coloured lines). faults: m t.v ettore-bove fault, m v bf; n orcia fault, n f; cascia fault, cf; a matrice fault segment, a fs; campotosto fault segment, cfs; capit ignano fault, caf; upper a terno v alley-paganica fault ststem, ua v -pf. seismogenic sources, white boxes; purple boxes indicate seismogenic sources not able to generate surface faulting earthquakes. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7266 6 referen ces basili, r., valen sise, g., van n oli, p., bu rrato p., fracassi, u., marian o, s., tiberti, m. m., bosch i, e. (2008). th e database of in d ivid u al seism ogen ic sou rces (diss), version 3: su m m arizin g 20 years of r esearch on italy's earth qu ake geology. tecton op h ysics, 453:2043. bigi, s., caser o, p., ch iar abba, c., di bu cci, d. (2013). con tr astin g su rface active fau lts an d d eep seism ogen ic sou rces u n veiled by th e 2009 l’aqu ila earth qu ake sequ en ce (it aly). terra n ov a, 25: 21-29. bu cci, f., d’on ofrio, d., tavarn elli, e., pr o sser, g. (2007). trian gu lar facets or flatiron ? a n ote of cau tion fr om th e lu can ian ap en n in es, italy. com m ission e tecn ica m icr ozon azion e sism ica (2015). lin ee gu id a p er la gestion e d el te rritorio in aree in teressate d a faglie attive e cap aci (fac), ver sion e 1.0 con feren za d elle region i e d elle pr ovin ce au ton om e – dip artim en to d ella p rotezion e civile, rom a . emergeo workin g gr ou p (2016). coseism ic effects of th e 2016 am atr ice seism ic sequ en ce: first geological resu lts; doi 10.4401/ ag-7195.. falcu cci e., gor i s., mor o m., pisan i a.r., melin i d., galad in i f., fred i p. (2011). th e 2009 l’aqu ila earth qu ake (italy): w h at n ext in th e region ? h in ts from stress d iffu sion an alysis an d n orm al fau lt activity. earth plan et. sci. lett., 305:350-358. fu belli, g., gori, s., falcu cci, e., galad in i, f., messin a, p. (2009). geom or p h ic sign atu res of recen t n orm al fau lt activity versu s geological evid en ce of in activity: case stu d ies from th e cen tral ap en n in es (italy). tecton op h ysics, 476:252–268. galad in i f., falcu cci e., galli p., giaccio b., gori s., messin a p., mor o m., sar oli m., scard ia g., sp osato a. (2012). tim e in terv als to assess active an d cap able fau lts for en g in eerin g p ractices in italy. en gin eerin g geology, 139/ 140:50-65. galad in i, f., galli, p. (2000). active tecton ics in th e cen tral ap en n in es (italy)—in p u t d ata for seism ic h azard assessm en t. n at. h azard s 22: 225–270. galad in i, f., galli, p. (2003). paleoseism ology of silen t fau lts in th e cen tr al ap en n in es (it aly): th e mt. vettore an d laga mts. fau lts . an n als of geop h ysics, 46:815-836. galli, p., galad in i, f., pan t osti, d. (2008). tw en ty year s of p aleoseism ology in italy. earth sci. rev. 88: 89–117. gru p p o d i lavor o in gv su l terrem oto d i am atrice (2016). secon d o r ap p orto d i sin tesi su l terrem oto d i am atrice ml 6.0 d el 24 agosto 2016 (italia cen trale), d oi: 10.5281/ zen od o.154400. kastelic, v., bu rrato, p., carafa, m., basili, r. (2015). progressive exp osu re of th e cen tral ap en n in e’s “n astrin i”: a fallin g tecton ics par ad igm ? gn gts 2015.. mich etti, a.m., ferreli, l., esp osito, e., po rfid o, s., blu m etti, a.m., vittori, e., serva, l., roberts, g.p., (2000). grou n d effects d u rin g th e sep tem ber 9, 1998, mw =5.6, lau ria earth qu ake an d th e seism ic p oten tial of th e aseism ic pollin o region in sou th ern italy. seis. res. letts. 71:31–46. valoroso, l., ch iar alu ce, l., piccin in i, d., di stefan o, r., sch aff, d., wald h au ser, f. (2013). rad iogr ap h y of a n or m al fau lt system by 64,000 h igh -p recision earth qu ake location s: th e 2009 l’aqu ila (cen tral italy) case stu d y. j. geop h ys. res. 118:1–21. wesn ou sky, s.g., 2006. pr ed ictin g th e en d p oin ts of earth qu ake ru p tu r es. n atu re 444: 358–360. annals of geophysics, 55 (3), 2012: graziano professional ethics: know-how, deontological code and legality gian vito graziano president, national council of geologists, rome, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5560 abstract the greatest sensitivity and attention to the geoenvironmental problematic underline the urgency to face some matters with new awareness and responsibility, like the management of the territory, the exploitation of the georesources, the energy problem, and the defense against natural risks. the civil community asks for ready and exhaustive answers. the technical-scientific competence of the geologist, linked with the shared deontological practice with respect to the normative in force, can give trust back to the citizens, and support to the political decisoris, the safety and the raising of our professionalism. the italian territory is a great economic, social and cultural resource that must be defended and valued through the contribution of geologists. new strategies, spheres of action, and operational tools are needed. the national council of geologists in italy is working in this direction. 1. introduction for some time, professionals, and also lecturers and researchers, have been asked more and more insistently to take up a social role to serve the community. geologists have been credited with a social responsibility aimed at “encouraging critical analysis of natural resources exploitation, promoting the value and the safeguarding of the geosphere, communicating information regarding natural hazards in a fair way, and engaging society with the idea of a shared geological heritage that can contribute to the social construction of knowledge” [geoethics manifesto 2011]. the social construction of knowledge is an important challenge, and geologists have to make a contribution; however, this requires cultural and social renewal of the earth sciences and of all of its ‘souls’, who are perhaps not always fully aware of the significant social effects of their work. 2. the role of professional associations what is the role of professional associations in the light of this hoped-for ethical renewal? their role is certainly strategic from several aspects: from the spreading of knowledge to all groups of society, to scientific and technical training of the members of the associations, to prepare them for the various requests coming from the community. however, this is totally marginal for matters regarding the often outdated regulations of professional associations, and their institutional roles. it clearly appears that there is the need to reform the professional associations in italy, possibly with modern, incisive, coherent and shared law reform. according to their current structure, professional associations are based on two fundamental pillars: deontology and the social role of the profession. the concept of deontology needs to be the focus of the discussion, because as explained later, it should be extended in the current codes to include the foundational principles of legality, which are often omitted. indeed, even after the italian government reforms (called the bersani reforms), the codes still refer to the noble images of ‘ethics’ and ‘decency’ of the profession as they are intended in the italian civil code, but only in considering their application to professional performance, and not to the conduct of the body’s members. in this context, an active role that has yet to be explicitly indicated will also be played by actions against the spread of the criminal system. the dangers of organized crime have often been, and still are, a cause for concern to professional associations, in terms of the involvement of some of their members. for example, in recent times, cases of complicity between members of professional associations and organized crime have been documented not only in southern italy, where organized crime has a major influence on public life, but also in milan and turin (in northern italy), where the mafia system is expanding dangerously. as every social actor has the duty to take on an active role against this phenomenon, professional associations cannot shirk their responsibility. cultural and anti-racket associations, citizens, and others have started to put pressure on the boards of professional associations, which are not always watchful on these issues, to push for greater attention to be paid to the activities and to the conduct of their members, especially, but not only, when their members are under investigation. article history received january 12, 2012; accepted february 29, 2012. subject classification: deontological code, legality, geoethics, professional association, geologists. 371 it needs to be asked in the first place: “how did the legislator want to make professional associations vouch for the correct and proper work and conduct of their members?” to answer questions on this point, some professional associations have officially stated that “the body’s board cannot take disciplinary measures on a member investigated for a criminal offence; thus even regarding the mafia, independent of the final outcomes of the criminal case, because it is forbidden by legislation and by numerous court of cassation judgments”. it is not quite like this, although it is evident that the issue poses some serious interpretational problems. it is then worthwhile to examine how the activities of the associations are regulated, starting from the institutional law (italian law, n. 112, dt 1963) of the ordine dei geologi (italian order of professional geologists), which, being more recent with respect to other professional associations, borrows some of the principal regulatory aspects from earlier institutional laws. law article 5, entitled prerequisites to become a member, states that to become a member of a professional association there is the need to be an italian citizen with civil rights, to have irreprehensible moral conduct, to be qualified to carry out the profession, and to be resident in italy. the legislator thus emphasizes that irreprehensible moral conduct is an essential pre-requisite to become a member of this professional association. therefore, if the member does not maintain his or her moral conduct in the profession, the professional association is entitled to take disciplinary measures, even resorting to expulsion from the association. this topic raises another issue regarding the link between judgements made by professional associations and official court sentences. it is not clear whether the disciplinary measures taken by a professional association in the case of a severe criminal or administrative offence committed by one of its members are to be considered as a judgement dependent on a possible parallel criminal trial, or as an autonomous judgement regardless of any court ruling. the ‘execution regulations’ for law n. 112 dt 1963 are not helpful in the answering of this question. law article 6 states that the expulsion of a chartered member can occur if the member renounces his or her membership, if there is an issue of incompatibility, or if the member does not meet one of the prerequisites of law article 5 any longer, as either the italian citizenship or residence. moral conduct is effectively excluded from the possible reasons that can be used to justify expulsion from a professional association. the difficulty encountered by professional associations in dealing with this issue is confirmed by law article 14, related to the subsequent integrative regulations regarding the application of law n. 112 dt 1963, where it is specified that “other than the cases of expulsion from a professional association provided for in the criminal code, the expulsion from an association is approved if: (a) an arrest warrant is issued for a member; (b) the member has not paid the professional association dues… the expulsion is legitimate if the chartered member, with the final judgement, has been condemned to incarceration for not less than two years for a nonculpable offence… the expelled member can ask for re-admission if: (a) he or she has been rehabilitated according to the criminal code regulations; (b) two years have elapsed since the date of expulsion”. thus, it is evident that although irreprehensible moral conduct is expressed as a prerequisite for admission, it is not considered in cases of expulsion. piero grasso, director of the direzione nazionale antimafia (national anti-mafia office) of italy, has often reminded professional associations of their duty of carrying out their task as guarantors of the professional conduct of their members. he is effectively upholding the principle of social responsibility as it applies to professionals, a principle that is more relevant than the responsibility of the common citizen. professionals often provide advice to mafiosi (members of the mafia), advising them on money laundering and even on the subsequent management and investing of the laundered money. the problem of professionals who are found to be in collusion with organized crime has not been addressed; indeed, many doctors, lawyers, engineers, architects and geologists who have been put on trial are still chartered members of their respective professional associations; sometimes also even if they have been condemned for a criminal offence. 3. conclusions how can we not respond to piero grasso’s appeal, which is also an appeal by civil society to chartered professionals? given the lack of regulations and of reforms, which would allow professional associations to take measures against their members without the fear of being, paradoxically, charged for not respecting regulations, the only way forward is civil courage. this thus departs from the limited view of institutional regulations, and expands upon the meaning of guarantor by giving professional associations a function that the legislator has not clearly established. by doing so, professional associations can fulfill the social role to which they aspire, and they can also be trusted more by civil society, which often sees professional associations just as corporations. this problem is felt to be a major issue, as shown, for instance, by the subscription of many professional association legality protocols, or by the session dedicated to education and legality by the sicilian geologists at their last congress. the first example of courage that i mentioned eargian vito graziano 372 373 lier comes from sicily, from the institution of engineers, which expelled a chartered member because he acknowledged that he had been paying protection money to a local mafia gang to safeguard his activities. the decision of the institution of engineers was based on the immorality of the member’s conduct, and it set an unfortunately isolated precedent, especially because the decision was taken unanimously by the institution board, even though the final judgement, which condemned the engineer to 15 years of incarceration, had yet to be proclaimed. the national council of engineers re-affirmed that disciplinary measures are autonomous and independent, and confirmed the expulsion decided by the local council because of the serious deontological infringement. if on the one hand civil courage appears to be the only way forward, on the other hand, there still remains a call for reform of chartered professions that will give a guarantor role to professional associations by extending the deontological precepts. in the meantime it will be worth introducing stricter ethical rules in the deontological codes that are going to be revisited due to the new economic measures. this is not an overwhelming task. we could start from the manifesto of the paolo giaccone professional committee. paolo giaccone (figure 1) was a doctor who was killed in 1982 by the mafia just because he was honestly fulfilling his duties, without giving way to requests from the mafia. the manifesto puts together a network of hopeful and honest professionals from all around italy, who pledge “to work bearing in mind society’s interests and the profession’s social role, and not to work against society’s welfare or in a way deleterious to human safety, freedom and dignity” (point 1 of the manifesto). with their attestation and their personal commitment, these professionals want to put forward a basic concept that is the duty of every citizen: to carry out one’s profession with the constant inspiration of achieving that social role that serves the community and that gives one dignity, authoritativeness and civil responsibility. references geoethics manifesto (2011). presented by s. peppoloni and g. di capua at the 8th italian forum of earth sciences, turin. codice civile italiano (italian civil code), istituto poligrafico e zecca dello stato (ipzs). gazzetta italiana n. 79, 4 aprile 1942 (italian gazette n. 79, 4th april 1942); www.ipzs.it. legge n.112 del 1964: “tutela del titolo e della professione di geologo” (italian law n. 112 dt 1964: “protection of the geologist’s title and profession”). websites comitato dei professionisti liberi (italian free professionals committe): www.professionistiliberi.org. consiglio nazionale dei geologi (national council of geologists of italy): www.cngeologi.it. corresponding author: gian vito graziano, consiglio nazionale dei geologi (national council of geologists), rome, italy; email: segreteria@cngeologi.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. professional ethics: know-how, deontological code and legality figure 1. prof. giaccone, a doctor killed in 1982 for having done his duty, without 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 451_457 tampieri.pdf 451 annals of geophysics, vol. 46, n. 2, april 2003 investigations on convective boundary layer turbulence using sodar data francesco tampieri and alberto maurizi istituto di scienze dell’atmosfera e del clima (isac), cnr, bologna, italy abstract acoustic sounding (sodar) data collected in convective conditions were analysed to estimate high order statistics of the vertical velocity in the lower half of the convective boundary layer (cbl). limitations of the instrumentation system were assessed and it turned out that spatial and temporal fi ltering have little effect on skewness and kurtosis, and do not prevent a reliable evaluation of these parameters, provided that a suffi ciently long time period is analysed. vertical profi les of skewness are grouped into two broadly defi ned classes, one which increases almost linearly with height and the other which shows a constant-with-height behaviour. both behaviours are shown to be consistent with different parameterisations used in literature. kurtosis profi les are found to be fairly well described adopting a quadratic relationship between skewness and kurtosis, provided that the correct parameterisation of skewness is used. mailing address: dr. francesco tampieri, istituto di scienze dell’atmosfera e del clima (isac), cnr, via gobetti 101, 40129 bologna, italy; e-mail: f.tampieri@isac.cnr.it key words convective boundary layer acoustic sounding turbulence 1. introduction and review research on turbulence has traditionally been developed in connection with geophysics, as is shown, for instance, in the comprehensive book by monin and yaglom (1971, 1975), which, along with a theoretical presentation, supplies many atmospheric data. studies on boundary layer turbulence structure made important progress from the comparison between atmospheric measurements (very high reynolds number, fullscale size, but coarse resolution) and laboratory experiments (good control, detailed measurements, but low reynolds number, unwanted boundary effects, reduced scale, etc.). two basic shortcomings arise in the analysis and interpretation of atmospheric boundary layer turbulence data. the fi rst refers to the intrinsic non-stationarity and inhomogeneous character of the atmosphere, leading to the impossibility of a full control of the experimental fi eld, which contrasts with the relatively high controllability of the laboratory experiment. this corresponds to diffi culties in interpreting data, but also to new stimula towards understanding the atmosphere. nevertheless, it is possible in general to identify and single out specifi c aspects and study them in spite of the complexities of the atmosphere. a second diffi culty, partly related to the previous one, is more practical but perhaps even more limiting. it arises from the problems in monitoring and measuring the boundary layer as completely as in the laboratory. in reality, data refer often to point measurements, made in few locations. for many years, meteorological towers constituted probably the best approach for investigating the boundary layer (for instance, the boulder atmospheric observatory tower has levels of measurement up to 300 m above the ground). more usually, data refer to the fi rst few meters, 452 francesco tampieri and alberto maurizi or tens of meters, above the ground. moreover towers and masts are always intrusive, and their effects should be accounted for. remote sensing plays an extremely useful role in overcoming part of these difficulties. in this paper, we address turbulence features in the convective boundary layer (cbl) using acoustic remote soundings techniques (sodar), examining the possibility of measuring high order statistics of vertical velocity. turbulence in the atmospheric cbl has specifi c signatures, which are important for both meteorological modelling and dispersion studies. although most of the available experimental information and frequently used models deal with the fi rst two moments of the velocity probability density functions ( pdf ), the importance of skewness s of the vertical component of the velocity in the cbl has long been recognised in relation to dispersion modelling (at fi rst by baerentsen and berkowicz, 1984). starting from the work of thomson (1987), a generation of models has been designed and used, based on an appropriate estimate of the shape of the eulerian probability density function pdf. if non-gaussianity is important, as occurs in the cbl, a knowledge of high order moments is necessary. reliable determinations of the high order moments in atmospheric fl ows are diffi cult to achieve (lenschow et al., 1994), and suitable parameterisations are often appropriate. parameterisations in the atmospheric surface layer, where both shear and buoyancy operate, are based on the similarity theory (monin and yaglom, 1971). vertical profi les of the second and third moments of vertical velocity in the entire cbl have been presented by stull (1988), hunt et al. (1988), weil (1990), sawford and guest (1987) among others. as regard fourth moments, because only very few measurements are available (tampieri et al., 2000), a common choice is to adopt a constant profi le of kurtosis k (see, e.g., du et al., 1994). an empirical relation between skewness and kurtosis emerges quite clearly from sheardominated boundary layer data: shaw and seginer (1987) and tampieri et al. (2000) have proposed a quadratic formula. a similar formula for cbl vertical velocity has been proposed by alberghi et al. (2000). in this paper, we discuss some results concerning skewness and kurtosis of the vertical velocity in the cbl, as derived from investigations carried out using a sodar operated in the rome metropolitan area, and suggest parameterisations useful for applications. advantages and shortcomings connected to the measuring system are briefl y discussed, the main objective being to explore the potential of such instruments in geophysical turbulence research, with a view to stimulate discussion on future work. 2. data description and treatment 2.1. measurement site description the data used for this investigation were obtained at two sites near rome, italy (pratica di mare, hereinafter pdm, and agenzia municipale per l’ambiente, hereinafter ama) during summer 1995, 1996 and 1997, by researchers of the «istituto di fisica dell’atmosfera» (ifa-cnr, rome). instantaneous vertical velocity w was measured in sampling volumes, referred to as «range gates», equally spaced with height with a 6 s sampling period. the radius r of the horizontal section of the sampling volume varies with height z as r = z tana, with a about 9°. the maximum height reached is typically 700-800 m, depending on ambient noise. the signal-to-noise ratio allows quality control on data according to mastrantonio and fiocco (1982) and greenhut and mastrantonio (1989). 2.2. stationarity analysis to obtain reliable statistics, steady time series are necessary. we selected the central part of the day, when the breeze cell due to the neighbouring coast side is well developed. periods with clearly observable (from fac-simile) variations in the cbl thickness, in horizontal wind intensity u and direction f were excluded. ten convective periods were singled out from this fi rst screening. the selected periods were subdivided into half-hour intervals for which mean and w 453 investigations on convective boundary layer turbulence using sodar data variance of vertical velocity were calculated at each height. steadiness of fi rstand second-order statistics was assumed to be verifi ed if the values lay within a band of variability of 20% for each height. table i reports the selected periods after a further analysis of stationarity on the scaling parameters performed in the next section. centred moments are considered, assuming that the presence of vertical advection has a negligible infl uence on the cbl structure. cases with vertical mean velocity signifi cantly less than zero at all heights (in this study: ama site, 199507-30, 1995-08-04, 1995-08-06) were attributed by mastrantonio et al. (1994) to a displacement of the breeze cell centre. 2.3. error estimates an analysis of error sources was performed with the aim of assessing the infl uence of the measuring methodology on the spectrum and momentum estimates. for the reduction of statistical error, the series with more than 15% of missing data were rejected (greenhut and mastrantonio, 1989) to obtain reliable estimates of high-order moments. when a time series relative to a given height was rejected, the time series for greater heights were also rejected. spatial and temporal fi ltering are of some relevance to sodar measurements because of the nature of the technique itself. in order to estimate the effect of both fi ltering effects, power spectra were computed and compared to cutoff scales. errors connected to spatial fi ltering were assessed according to kristensen and gaynor (1986). it turned out that, even in the worst case, i.e., the one that reaches the highest range gate, the correction would be of the order of the accepted error. moreover, temporal filtering proves to be less signifi cant than spatial fi ltering. it is worth pointing out that, both fi ltering scales being in the inertial subrange, it is reasonable to assume that the fi ltered part of the spectrum has less infl uence on the non-gaussian properties, of interest here, than the resolved part. table i. selected stationary periods in convective conditions with scaling parameter and error estimates. the subdivision is relative to the presentation in fi gs. 1a and 2a (upper part) and fi gs. 1b and 2b (lower part). symbols on the last column are used accordingly. reference wind is computed as a time average over the whole period and a vertical average over the selected heights. site date local time ref. wind z i w* symbols yyyy-mm-dd hh:mm (ms-1) (m) (ms-1) ama 1995-08-06 10:30-16:30 3.0 556 ± 22 1.24 ± 0.03 � pdm 1996-08-11 13:00-18:30 4.1 880 ± 180 1.61 ± 0.12 � pdm 1997-07-01 08:30-18:00 1.9 652 ± 56 1.78 ± 0.07 � pdm 1997-08-03 08:30-18:00 3.8 474 ± 25 1.46 ± 0.07 � pdm 1997-08-13 09:45-13:15 2.9 1060 ± 390 1.93 ± 0.26 � pdm 1997-08-13 13:15-16:45 2.9 529 ± 64 1.66 ± 0.09 � ama 1995-07-30 12:00-14:30 4.5 1490 ± 320 1.57 ± 0.13 � ama 1995-08-04 09:30-16:00 4.5 1070 ± 350 1.56 ± 0.18 � ama 1995-08-15 11:30-15:30 4.8 1290 ± 280 1.93 ± 0.15 � ama 1995-09-03 13:30-16:00 3.5 690 ± 180 1.33 ± 0.13 � pdm 1996-07-21 10:00-16:30 5.0 402 ± 27 1.47 ± 0.05 � w2 454 francesco tampieri and alberto maurizi moving on to the analysis of statistical signifi cance, it should be noted that the number of statistically independent data for each time series is proportional to the ratio t/t, where t is the total length of the time-series and t is the eulerian integral time-scale. errors on estimations of moment of order n, due to the fi nite number of independent data, can be evaluated according to lenschow et al. (1994) as σ τn nk t� −( )1 1 2 , where k n is an increasing function of the order n. with typical values of the process under study, a total length t ≥ 2.5 h is required in order to have s2 £ 0.16, s3 £ 0.4, s4 £ 2.3, ss £ 0.15 and s k £ 0.6. these could be taken as the maximum error estimates for the whole data set. furthermore, it is worth noting that the number of independent data required for a good estimate increases with the order of the moment desired. as a last remark, it should be pointed out that relative errors on estimates of non-dimensionalised parameters s and k are smaller than those on thirdand fourthorder velocity moments respectively. 3. some results concerning turbulence structure 3.1. best fi t estimates of w* and zi lacking direct measurements of the heat and momentum fl uxes, the vertical profi le of w2 has been used to estimate w* and zi, leaving indeterminate any estimate of shear (namely, of u*). different formulations are presented in literature, which are essentially composed of two parts: one relies on the similarity in the surface layer, while the other describes the profi les for large z/z i . most of them can be represented by the general expression (3.1) where the parameters a2, b2 and c2 assume different values according to the various authors. the lenschow et al. (1980) profi le (a2 = 1.8, b2 = 0.8 and c2 = 2) was used to compute scaling parameters w* and zi by fi tting eq. (3.1) on data. the same profi le has been used by seibert and langer (1996) and melas et al. (2000) among other authors. this fi tting procedure is commonly adopted (see, e.g., weill et al., 1980; greenhut and mastrantonio, 1989; seibert and langer, 1996): since local equilibrium is better reached in the lower part of the cbl, the estimate of w* is expected to be more robust than that of z i . results of the fi tting procedure on data are reported in table i. stationarity of w* and zi in the selected periods was further investigated by applying the fi tting procedure on w2 data in half-hour sub-periods. in most cases, the scaling parameters were found to be suffi ciently steady (within the error). in one case (1997-08-13), the time interval selected in section 2 was further subdivided into two parts as reported in table i. 3.2. interpretation of the results vertical profi les of s, with height non-dimensionalised by z i , are reported in fi g. 1a,b together with some theoretical profi les from literature (lenschow et al., 1980; sawford and guest, 1987; hunt et al., 1988; weil, 1990; franzese et al., 1999). at fi rst glance, it is clear that skewness data do not collapse into a unique curve, even considering the inaccuracy of the estimation of z i and the errors in the estimation of s. it is interesting to note that s profi les (both theoretical and experimental) can be grouped into two qualitatively different sets: one which shows an increase with height, at least for z/z i < 0.5, and the other which is nearly constant. this suggests that probably two quite distinct mechanisms act in shaping the vertical velocity pdf. the fi rst case corresponds to a picture of convection which organises as the height increases, giving rise to coherent vortices with maximum development near the cbl centre, where the skewness of the vertical velocity is maximum. in this case, a linear fi t s = 1.2(z/z i ) + + 0.1 reasonably describes the present data. on the basis of the relationship k = 2.4(s2 + 1 ) (3.2) (alberghi et al., 2000, 2002) a quadratic dew w a z z b z zi i c2 2 2 2 3 21 2 ∗ =     −     455 investigations on convective boundary layer turbulence using sodar data pendence of kurtosis from height can be established. in fi g. 2a, this relationship is plotted along with the experimental data for the same cases reported in fi g. 1a. the second case corresponds to the idea of strong mixing, leading to a homogeneous structure on the vertical. a value s = 0.5 fi ts the observations. note that in this case kurtosis is a b fig. 1a,b. vertical profi les of s with height normalised with z i , subdivided into two classes (see text). symbols are as reported in table i. lines are: a) thick (linear fi t), solid (lenschow et al., 1980), dashed (weil, 1990); b) thick (linear fi t), solid (franzese et al., 1999), dashed (hunt et al., 1988), dot-dashed (sawford and guest, 1987). z/ z i z/ z i a b fig. 2a,b. vertical profi les of kurtosis. as in fi g. 1a,b, symbols are as reported in table i. lines represent eq. (3.2) with s estimated by linear fi t (see text and fi g. 1a,b). z/ z i z/ z i 456 francesco tampieri and alberto maurizi estimated from eq. (3.2) to be around 3, but in fact presents a large variability (see fi g. 2b). we note in passing that the grouping of profiles evidences that they mostly behave differently in the two sites. the ama site shows a large scatter and a nearly constant with height behaviour, while pdm displays a more coherent increase with height. 4. conclusions in the present paper sodar data were analysed to give a picture of the convective boundary layer turbulence on almost fl at terrain, in synoptic calm conditions. the main objective of the work was to investigate the possibility of obtaining reliable evaluations of some turbulence parameters (moments of 3rd and 4th order, spectra) through remote sensing techniques. in summary, it turns out that the limitations due to the fi nite volume of sampling, and the relatively low time resolution, do not strongly affect the capability of producing robust and consistent statistics. in particular, two different situations were disclosed, refl ecting conditions of vertical homogeneity or not in the cbl. moreover, the existence of a quite universal relationship between skewness and kurtosis (well established in shear dominated boundary layers and poorly documented in the cbl) can account for vertical profi les of kurtosis if a reasonable parameterisation for s is given. the authors would like to close by recalling the pioneering work conducted by giorgio fiocco in the development of remote sensors like sodar, which in fact allowed the present work to be carried out. the senior author was impressed by a paper by fiocco and mastrantonio (1983) where an investigation on the structure of the fl ow around the top of monte rosa was reported, using a sodar located near the mountain top. the paper suggested the possibility of performing studies of atmospheric turbulence statistics in a much easier and more effi cient way with respect to punctual measuring devices (i.e. sonic anemometers). the present work shows an application of such ideas in a relatively simple case; we close with the hope of a detailed documentation of turbulent fl ow in complex terrain, using remote sensors, as an important step along the path initiated at monte rosa some 20 years ago. references alberghi, s., f. tampieri, s. argentini, g. mastrantonio and a. viola (2000): analysis of the pdf of the vertical velocity in the buoyancy-driven atmospheric boundary layer, advances in turbulence viii, proceedings of the eighth european turbulence conference, edited by c. dopazo (cimne, barcelona), 315-318. alberghi, s., a. maurizi and f. tampieri (2002): observations on the skewness-kurtosis relationship of vertical velocity in the convective boundary layer, j. appl. meteorol., 41, 885-889. baerentsen, j.h. and r. berkowicz (1984): monte-carlo simulation of plume diffusion in the convective boundary layer, atmos. environ., 18, 701-712. du, s., j.d. wilson and e. yee (1994): probability density functions for velocity in the convective boundary layer, and implied trajectory models, atmos. environ., 28, 1211-1217. fiocco, g. and g. mastrantonio (1983): characters of the air fl ow inferred from detailed spectral analysis of acoustic sounder echoe, j. acoust. soc. am., 74, 1861-1865. franzese, p., a.k. luhar and m.s. borgas (1999): an effi cient lagrangian stochastic model of vertical dispersion in the convective boundary layer, atmos. environ., 33, 2337-2345. greenhut, g.k. and g. mastrantonio (1989): turbulence kinetic energy budget profi les derived from doppler sodar measurements, j. appl. meteorol., 28, 99-106. hunt, j.c.r., j.c. kaimal and j.e. gaynor (1988): eddy structure in the convective boundary layer new measurements and new concepts, quart. j. r. meteorol. soc., 114, 827-858. kristensen, l. and j.e. gaynor (1986): errors in second moments estimated from mopnostatic doppler sodar winds. part i: theoretical description, j. atmos. ocean. tecnol., 3, 523-528. lenschow, d.h., j.c. wyngaard and w.t. pennell (1980): mean-fi eld and second-moment budgets in a baroclinic, convective boundary layer, j. atmos. sci., 37, 1313-1326. lenschow, d.h., j. mann and l. kristensen (1994): how long is long enough when measuring fl uxes and other turbulence statistics?, j. atmos. oceanic technol., 11, 661-673. mastrantonio, g. and g. fiocco (1982): accuracy of wind velocity determinations with doppler sodar, j. appl. meteorol., 21, 820-830. mastrantonio, g., a. viola, s. argentini, g. fiocco, l. giannini, l. rossini, g. abbate, r. ocone and m. casonato (1994): observations of sea breeze events in rome and the surrounding area by a network of doppler 457 investigations on convective boundary layer turbulence using sodar data sodars, boundary-layer. meteorol., 71(1-2), 67-80. melas, d., g. abbate, d. haralampopoulos and a. kelesidis (2000): estimation of meteorological parameters for air quality management: coupling of sodar data with simple numerical models, j. appl. meteorol., 39, 509-515. monin, a.s. and a.m. yaglom (1971): statistical fluid mechanics, edited by j. lumley (mit press, cambridge), vol. 1, pp. 769. monin, a.s. and a.m. yaglom (1975): statistical fluid mechanics, edited by j. lumley (mit press, cambridge), vol. 2, pp. 874. sawford, b.l. and f.m. guest (1987): lagrangian stochastic analysis of fl ux gradient relationships in the convective boundary layer, j. atmos. sci., 44, 1152-1165. seibert, p. and m. langer (1996): deriving characteristic parameters of the convective boundary layer from sodar measurements of the vertical velocity variance, boundary-layer. meteorol., 81, 11-22. shaw, r.h. and i. seginer (1987): calculation of velocity skewness in real and artifi cial plant canopies, boundarylayer. meteorol., 39, 315-332. stull, r.b. (1988): an introduction to boundary layer meteorology (kluwer academic publishers, dordrecht), pp. 666. tampieri, f., a. maurizi and s. alberghi (2000): lagrangian models of turbulent dispersion in the atmospheric boundary layer, ingegneria del vento in italia 2000, edited by g. solari, l.c. pagnini and g. piccardo (sgeditoriali, padova), 37-50. thomson, d.j. (1987): criteria for the selection of stochastic models of particle trajectories in turbulent fl ows, j. fluid mech., 180, 529-556. weil, j.c. (1990): a diagnosis of the asymmetry in top-down and bottom-up diffusion using a lagrangian stochastic model, j. atmos. sci., 47 (4), 501-515. weill, a., c. klapisz, b. strauss, f. baudin, c. jaupart, p. van gruderbeek and j.p. goutorbe (1980): measuring heat fl ux and structure functions of temperature fl uctuations with an acoustic doppler sodar, j. appl. meteorol., 19, 199-205. m i s u r e d e i r a g g i x s o l a r i p e r m e z z o d e i s a t e l l i t i (*) g . righini r i c e v u t o il 18 maggio 1963 i raggi x solari hanno origine nella corona dove la temperatura del plasma e dell'ordine di 10° h1 0 7 o k . a tali temperature l'energia termica degli elettroni va da 100 a 1000 e y e quando questa energia viene dissipata nel campo dei protoni del plasma si ha emissione di raggi x inolli nella gamma da 10 a 100 a. la prima rivelazione di queste radiazioni f u f a t t a dal gruppo del naval research laboratory di washington ponendo delle piccole camere di ionizzazione nella testa dei razzi aerobee lanciati dal poligono di white sand nel nuovo messico. le misure forzatamente sporadiche mostrarono che l'intensita di questa radiazione solare e estremamente variabile e che in occasione di eventi solari puo essere anche parecchi ordini piu grande del normale. non piu tardi dell'anno scorso altre ricerche eseguite da hinteregger con un monocromatore solare mostrarono anche la presenza di righe di emissione nella gamma dei raggi x molto molli. poiche l'azione ionizzante di questa radiazione e molto forte e quindi di particolare importanza per l'equilibrio di ionizzazione della ionosfera si sono organizzate dal gruppo n r l di washington deue esperienze con satelliti appositamente attrezzati. i n f a t t i nel 1960 f u laneiato il satellite sr i che conteneva oltre alia solita attrezzatura di telemetria e di celle solari, una camera di ionizzazione per raggi x nella banda da 2 a 8 angstrom e due altre camere per la regione ultravioletta lyman alfa nonclie una fotocella per dare vaspetto del satellite, ossia l'orientamento rispetto al sole. altri satelliti della serie seguirono negli anni successivi, l'ultimo chiamato sr i i i f u laneiato in un grappolo contenente un satellite i n j u n e un transit ma non essendosi separato dall'injun assunse un lento moto di precessione per cui i dati rilevati sono di difficile interpretazione. (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientiflco dello spazio, r o m a , 19-23 giugno 1962. 1 5 6 g. r i g h i n i i satelliti di questa serie non sono memorizzati e t r a s m e t t o n o quindi continuamente; per s f r u t t a r e al massimo i dati trasmessi il gruppo clie f a capo al n r l cliiese la collaborazione di altri laboratori e osservatori comunicando il codice per l'interpretazione dei segnali e t u t t i i dati necessari per la predizione dei passaggi. l'osservatorio di arcetri per partecipare a questa ricerca si e pert a n t o attrezzato a d e g u a t a m e n t e installanto u n a stazione di ascolto sulla frequenza di 108.09 mhz. l ' a n t e n n a e u n a cortina di 8 antenne yagi con m o n t a t u r a altazimutale. h a un guadagno di 20 db ed un lobo di 45°; sull'antenna stessa e mon t a t o un preamplificatore. ii segnale o p p o r t u n a m e n t e amplificato e inviato ad un convertitore di frequenza con oscillatore locale stabilizzato a quarzo la cui uscita in media frequenza e di circa 12 mhz. un ricevitore professionale tipo collins r-390 a / u r r provvede all'aniplificazione e rettificazione finale di questo segnale con u n a banda passante di 8 kc/s. ii segnale ricevuto dal satellite ed i segnali di t e m p o s t a n d a r d sono registrati indipendentemente su due piste di u n nastro magnetico dur a n t e il passaggio. successivamente il segnale registrato viene selezionato da u n filtro di b a n d a (uno per ciascun canale di telemetria) e da questo inviato ad u n frequenzimetro che converte le escursioni di frequenza in tensioni. queste sono trascritte sul nastro di carta scorrevole di un registratore a scrittura d i r e t t a «galileo» r 105 a. i segnali di tempo, opportunamente filtrati, eccitano un a t t u a t o r e elettromagnetico clie com a n d a u n a penna marca-tempo; si ottengono cosi su ciascun nastro (uno per ogni canale) delle marche di tempo che, essendo sincronizzate col segnale del satellite, p e n n e t t o n o l'allineamento dei diversi nastri. opportune curve fornite dal n r l permettono di risalire dai segnali registrati al numero di erg emessi per cm2 per sec in un d e t e r m i n a t e intervallo di lungliezze d'onda. lo scopo di questa ricerca era quello di correlare le eventuali fluttuazioni di intensita con i fenomeni osservabili nella cromosfera solare m e d i a n t e lo spettroeliografo e il filtro di lyot. disgraziatamente i d a t i emessi dall'ultimo satellit e solare sr i i i sono di utilita l i m i t a t a in quanto causa il lento moto di rivoluzione dovuto alia m a n c a t a separazione dal gemello i n j u n manca la periodica scansione del sole. ad ogni modo 1'attrezzatura descritta e p r o n t a e funzionante e puo entrare subito in azione non appena sara lanciato il prossimo satellite della serie sr per lo studio dei raggi x solari. numeri caratteristici definitivi dei flocculi di calcio per l'anno geofisico internazionale g . godoli f . mazzucconi b . monsignori f o s s i r i c e v u t o il 15 g e n n a i o 1 9 6 6 riassunto. — si d a n n o i n u m e r i c a r a t t e r i s t i c i definitivi dei flocculi di calcio p e r l ' a n n o geofisico i n t e r n a z i o n a l e , secondo il m a t e r i a l e di osserv a z i o n e r a c c o l t o al w o r l d d a t e center di a r c e t r i grazie al p r o g r a m m a di c o l l a b o r a z i o n e i n t e r n a z i o n a l e . questi n u m e r i c a r a t t e r i s t i c i s o s t i t u i s c o n o quelli p r e l i m i n a r i , già pubb l i c a t i in q u e s t a sede (67 ), i quali v e n n e r o s t i m a t i in b a s e alle sole ossev a z i o n i eseguite alla t o r r e solare di a r c e t r i . summary. — t h e definitive c h a r a c t e r figures of t h e ca b r i g h t flocculi f o r t h e i n t e r n a t i o n a l g e o p h y s i c a l y e a r are g i v e n . t h e o b s e r v a t i o n a l m a t e r i a l used h a s been c o l l e c t e d a t a r c e t r i w o r l d d a t e center for t h e ca b r i g h t flocculi, as a p a r t of t h e i g y i n t e r n a t i o n a l p r o g r a m m e . t h e s e c h a r a c t e r figures t a k e t h e p l a c e of t h o s e a l r e a d y published (67 ) a c c o r d i n g t o t h e only o b s e r v a t i o n s c a r r i e d out a t t h e a r c e t r i solar t o w e r . 1. determinazioni di numeri caratteristici giornalieri dei flocculi solari di calcio sono state pubblicate, per il periodo 1950-1960, in questa sede (!•9) e per gli anni 1961, 1963, 1961 nel bollettino annuale delle osservazioni eseguite all'osservatorio astrofisico di arcetri (10' 12). per queste determinazioni è stato utilizzato il materiale spettroeliografico raccolto alla torre solare di arcetri nella radiazione della componente j i „ l 3 l 2 della riga k del calcio una volta ionizzato. dal 24 ottobre 1961 al 9 ottobre 1963, non sono state eseguite determinazioni di numeri caratteristici dei flocculi di calcio dato che le osservazioni alla torre solare sono state sospese per procedere alla sostituzione della cupola e dei servo-meccanismi di comando. 2. come è noto, sin dall'inizio dell'anno geoflsico internazionale (1° luglio 1957-31 dicembre 1958), l'osservatorio astrofisico di arcetri 9 6 g. godoli f . mazzucconi 15. monsignori fossi è stato centro mondiale (world d a t a center, wdc) per la raccolta e l'elaborazione dei dati relativi ai llocculi solai-i in radiazione k del ca i i . durante l'anno geofìsieo internazionale l'osservatorio di arcetri ha ricevuto regolarmente spettroeliogrammi in radiazione k dagli osservatori di crimea, kodaikanal, me math hulbert. i l materiale di osservazione ricevuto da questi tre osservatori e quello raccolto ad arcetri copre tutti i giorni del periodo 1° luglio 1957-31 dicembre 1958 con le sole lacune relative ai giorni: 1957 novembre 25 dicembre 8, 25, 28, 29, 30, 31 1958 febbraio 5 aprile 20 giugno 20 dicembre 17 allo scopo di colmare queste lacune è stata chiesta la collaborazione di tutti gli istituti impegnati in osservazioni sistematiche della cromosfera solare in radiazione k e cioè degli istituti di freiburg, ikomasan, meudon, mount wilson, tokio, wendelstein. con la collaborazione di questi istituti è stato possibile colmare tutte le lacune con la. sola eccezione del giorno 1957 dicembre 8. le caratteristiche del materiale di osservazione ricevuto sono le seguenti: i s t i t u t o materiale fotografico d i a m e t r o dell'immagine arcetri originali su l a s t r a 63 mm crimea 1 luglio 1957-31 s e t t e m b r e 1957 copia su c a r t a 43 mm 1 o t t o b r e 1957-31 d i c e m b r e 1958 copie su film 43 mm f r a u n h o f e r originali su l a s t r a 50 mm k o d a i k a n a l copie su film 19 mm me math hulbert 1 luglio 1957-31 d i c e m b r e 1957 copie su film o originali 17 mm 1 gennaio 1958-31 d i c e m b r e 1958 originali 17 mm meudon copie su l a s t r a 85 m m mount wilson copie su lastra 52 mm tokio copie su c a r t a 100 mm wendelstein originali su l a s t r a 45 mm n u m e r i caratteristici d e f i n i t i v i d e i flocculi di calcio 9 7 le copie e gli originali inviati dai vari istituti sono stati riportati a negativi su lastra e su carta rispettivamente con una o due inversioni fotografiche. anche gli originali eseguiti ad arcetri sono stati riportati a negativi su carta con due inversioni fotografiche. i l diametro dell'immagine solare è stato sempre riportato alla misura di g3 mm per le lastre ed a 120 mm per le stampe. dal materiale di osservazione così elaborato, sono state costituite due collezioni (una di lastre ed una di stampe) il più omogenee possibile costituite da uno spettroeliogramma al giorno. 3. l a collezione delle lastre era già stata utilizzata per la compilazione delle mappe dei flocculi solari in radiazione k del ca i i per l'anno geofísico internazionale (13> 14> 15> 16). successivamente abbiamo pensato di utilizzare la collezione delle lastre e quella delle stampe anche per la stima dei numeri caratteristici giornalieri dei flocculi di calcio per l'anno geofisico internazionale in quanto le nuove stime, eseguite su lastre e stampe e su materiale che presenta soltanto una lacuna di un giorno, sarebbero state senz'altro più precise e complete di quelle già pubblicate ("• '). abbiamo già notato (7) che, sebbene dal i o gennaio 1957 l'attività dei flocculi di calcio venga seguita all'osservatorio astrofisico di arcetri rilevando area e posizione dei singoli fenomeni in quanto questo metodo di determinare l'attività dei flocculi è più informativo della stima del numero caratteristico e sebbene il numero caratteristico sia mia funzione nota dell'area proiettata, tuttavia è opportuno continuare a stimare i numeri caratteristici dei flocculi di calcio dato che vi sono elementi per ritenere che, durante fasi peculiari del ciclo solare, i due diversi metodi diano informazioni di significato diverso (17). 1. l e stime dei numeri caratteristici sono state sempre eseguite all'osservatorio astrofisico di arcetri attenendosi ai modelli distribuiti dall'osservatorio di mount wilson. i modelli definiscono i punti 0, 1, 2, 3, 1, 5 della scala dei numeri caratteristici. ad arcetri è sempre stato considerato conveniente stimare anche il decimo del numero caratteristico, mentre altri osservatori preferiscono dare solo i cinque decimi. i modelli originali di mount wilson sono costituiti da positive su carta del diametro di 100 mm. recentemente ad arcetri, per migliorare la qualità della stima, si è derivata dai modelli originali di mount wilson mia serie di modelli costituita da negative su lastra del diametro 9 8 g. godoli f . mazzucconi 15. monsignori fossi di 63 mm. dal 1962 i numeri caratteristici sono stati sempre stimati con l'aiuto di questi modelli. 5. i numeri caratteristici giornalieri dei flocculi di calcio per l'anno geofisico internazionale sono stati dapprima stimati sulla collezione dello stampe riferendosi ai modelli originali di mount wilson. l a stima sistematica è stata f a t t a indipendentemente da due osservatori (f. m. e b . m . f . ) ; un terzo osservatore (g. g.) ha saltuariamente controllato le stime. quando lo scarto medio delle singole determinazioni non superava 0 . 3 unità, veniva f a t t a una media delle valutazioni dei due osservatori; nei pochi casi in cui veniva riscontrato imo scarto medio di maggior entità, lo spettroeliogramma veniva riesaminato. 7 6 1 9 5 7 12 1 1se fie. 1 in un secondo tempo i numeri caratteristici sono stati stimati con lo stesso metodo sulla collezione delle lastre riferendosi ai modelli su lastra. dal grafico di fig. 1 in cui sono riportati i rapporti delle medie mensili dei valori rispettivamente determinati sulle lastre e sulle stampe, si nota che l'accordo fra le due serie di stime è assai soddisfacente. per poter confrontare la precisione delle due serie di stime abbiamo applicato il metodo di a. wald (18). indichiamo con (x1, yr), ..., (xr, yr), • • , (xn, y„) le n coppie (nel nostro caso 548) di determinazioni giornaliere del numero caratteristico eseguite rispettivamente sulle lastre e sulle stampe. siano 1) x i ed t i i valori «veri» di xi ed yt ; n u m e r i caratteristici d e f i n i t i v i d e i flocculi di calcio 9 9 2) x = 5>f\n ; 3) y = s w » ; 2 2 4) ax, oy, axy le varianze delle xt, delle yt e le covarianze delle' coppie x,, iji : ox — / s (xi — xy-1 n oy = j £ (yi — yyoxy =' s (xi — x) (2/i — y) . ti definiamo «errori» ài e ai le quantità: di = xi — xt at = yt — tt. supponiamo che: 1) gli errori <5; abbiano distribuzioni identiche e non siano correlati; 2) gli errori ai abbiano distribuzioni identiche e non siano correlati; 3) gli errori <5; e ai non siano correlati; 4) sia yt = a xt + /? ; 5) sia »/2 s< xi — si x, \ i n > k > 0 1 (>>/2+l) con k costante. posto: l»/2 » si xi — si xt i n ; 1 (»/2+1) n/2 ti j s * yi — s i yt i n ; 1 (»/2+i) | a = indicando con e a a le varianze dei òt e zi, si ha, per la legge dei grandi numeri, 1 0 0 g. godoli f . mazzucconi 15. monsignori fossi l ^ i k ^ l = 0 3 1 ' tu / aa 1 indicando infine con r il coefficiente di correlazione fra gli xt e gli 2/i si ha: r = = 0.25 . [3] (txgy ammettendo verificate le nostre ipotesi, si constata che le due serie di stime hanno praticamente la stessa precisione. la nuova serie di stime, eseguita sulla collezione completa delle lastre, è stata anche confrontata, sempre applicando il metodo di a. wald, con la serie di stime già pubblicata (6' 7), eseguita sulle sole lastre raccolte all'osservatorio di arcetri. naturalmente il confronto è stato possibile soltanto per quei giorni in cui si hanno osservazioni eseguite ad arcetri. indicando ora con (xi, ìji),..., (xr, yr),• , (xn, yn) le n coppie (nel nostro caso 245) di determinazioni giornaliere del numero caratteristico eseguite rispettivamente sidla collezione completa delle lastre e sulle sole lastre raccolte all'osservatorio di arcetri, si ha = 0 4 aa = 0 . 5 r = 0 . 2 6 . sempre ammettendo che le nostre ipotesi siano verificate, si constata che la precisione della serie di stime eseguita sulla collezione completa delle lastre è maggiore della precisione della serie di stime eseguite sulle sole lastre raccolte all'osservatorio di arcetri. questo fatto era del resto atteso (cfr. n.ro 3): è stata proprio la convinzione che stime eseguite su materiale che presenta soltanto una lacinia di un giorno sarebbero state più precise, oltreché, naturalmente, più complete, che ci ha suggerito di eseguire questa nuova serie di stime. notiamo che i bassi coefficienti di correlazione ottenuti nei due confronti sono dovuti al fatto che, nel periodo preso in considerazione, le variazioni reali dei numeri caratteristici sono state dello stesso ordine di grandezza degli scarti medi. n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i di calcio 1 0 1 tabella 1 n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i c a l c i o p e r l ' a n n o g e o f í s i c o i n t e r n a z i o n a l e . (periodo 1 luglio 1957-31 dicembre 1957). mese -»• luglio agosto settembre ottobre novembre dicembre giorno | n. c. 0 n c. 0 n.c. 0 n.c. 0 n. c. 0 n.c. 0 1 3 7 a 3 2 a 3 6 c 3 6 c 3 8 c 3 5 mm 2 3 9 a 3 2 a 3 4 mm 3 4 c 3 8 mm 3 4 a 3 4 1 a 3 2 a 3 4 c 3 4 a 3 7 k 3 4 a 4 4 0 a 3 4 mm 3 4 mm 3 4 a 3 6 c 3 4 mm 5 4 0 a 3 2 a 3 6 a 3 4 a 3 6 c 3 4 a 6 3 8 a 3 3 a 3 6 a 3 4 mm 3 5 c 3 4 k 7 3 8 c 3 2 a 3 4 a 3 5 a 3 4 a 3 4 k 8 3 6 a 3 2 a 3 3 mm 3 4 a 3 4 c — 9 7 a 3 4 a 3 4 a 3 4 a 3 4 c 3 2 k 10 3 6 c 3 4 a 3 4 a 3 5 a 3 4 c 3 0 a 11 3 5 c 3 2 c 3 4 c 3 6 a 3 3 c 3 2 c 12 3 2 a 3 4 a 3 6 c 3 6 a 3 2 mm 3 2 k 13 3 2 a 3 4 a 3 5 c 3 6 k 3 4 a 3 3 k 14 3 2 c 3 6 c 3 5 c 3 6 a 3 4 a 3 2 k 15 3 4 c 3 6 c 3 5 c 3 7 a 3 6 c 3 2 k 16 3 4 a 3 6 c 3 4 a 3 6 a 3 5 c 3 2 a 17 3 6 c 3 6 c 3 4 a 3 6 a 3 4 k 3 2 a 18 3 8 a 3 7 mm 3 4 a 3 6 a 3 4 k 3 3 a 19 3 8 mm 3 8 a 3 5 a 3 6 c 3 4 a 3 3 a 20 4 2 c 3 7 c 3 6 a 3 6 c 3 4 c 3 3 a 21 4 0 c 3 6 a 3 6 a 3 6 a 3 4 c 3 4 c 22 4 0 c 3 5 a 3 6 c 3 6 c 3 5 c 3 5 c 23 4 2 c 3 4 mm 3 8 c 3 6 a 3 5 c 3 6 k 24 4 0 a 3 . 6 c 3 7 c 3 6 a 3 5 k 3 7 mm 25 3 8 a 3 5 c 3 7 c 3 6 a 3 6 a 3 7 m 26 3 7 a 3 . 6 a 3 6 mm 3 6 a 3 5 a 3 8 c 27 3 7 a 3 7 a 3 4 a 4 0 c 3 4 a 3 8 c 28 3 6 c 3 6 a 3 5 a 4 0 c 3 4 a 3 6 mw 29 3 6 a 3 6 a 3 6 c 3 7 a 3 6 mw 3 6 mw 30 3 5 a 3 6 c 3 6 a 3 7 a 3 5 a 3 6 mw 31 3 4 a 3 6 a — 3 7 a — 3 6 m w media mensile 3 7 3 5 3 . 5 3 6 3 5 3 4 media semestrale 3 . 6 1 0 2 g. g 0 d o l i f . mazzucconi ß . monsignori f o s s i tabella i i n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i c a l c i o p e r l ' a n n o g e o f í s i c o i n t e r n a z i o n a l e . (periodo 1 gennaio 1958-30 giugno 1958). mese gennaio febbraio marzo aprile maggio giugno giorno ! u.c. 0 u.c. 0 n.c. o il. e. 0 n.c. 0 il c. 0 1 3 4 k 2 9 a 3 4 a 3 8 c 3 6 a 3 7 k 2 3 4 a 3 0 k 3 4 k 3 8 k 3 6 a 3 6 k 3 3 4 c 3 0 k 3 4 a 3 8 c 3 8 a 3 6 a 4 3 4 k 3 1 k 3 5 a 3 9 k 3 8 c 3 6 a 5 3 6 k 3 2 t 3 6 a 3 8 c 3 6 a 3 6 k 6 3 6 c 3 5 a 3 6 a 3 7 c 3 6 c 3 6 c 7 3 5 k 3 6 k 3 6 c 3 8 k 3 6 mm 3 7 a 8 3 4 a 3 6 mm 3 8 k 3 6 a 3 5 a 3 8 k 9 3 4 a 3 9 c 3 8 c 3 7 k 3 6 a 3 8 a 10 3 3 a 4 0 c 3 8 k 3 6 a 3 4 a 3 7 k 11 3 2 c 4 0 c 3 6 k 3 6 k 3 5 c 3 6 c 12 3 4 k 4 0 a 3 6 k 3 6 k 3 4 a 3 6 k 13 3 7 c 3 8 a 3 7 k 3 4 0 3 4 a 3 4 c 14 3 8 k 3 6 c 3 6 c 3 4 a 3 4 a 3 4 c 15 3 7 k 3 5 k 3 6 k 3 2 c 3 2 k 3 4 c 16 3 6 a 3 4 k 3 6 k 3 2 k 3 3 c 3 4 a 17 3 4 a 3 4 k 3 4 k 3 4 k 3 4 c 3 2 mm 18 3 4 a 3 4 k 3 4 c 3 4 k 3 4 c 3 0 c 19 3 6 c 3 4 k 3 4 k 3 4 a 3 2 a 3 1 k 20 3 5 c 3 4 a 3 4 k 3 6 m f f 3 2 a 3 2 m w 21 3 4 c 3 4 k 3 4 k 3 5 a 3 2 a 3 2 c 22 3 4 a 3 3 k 3 2 a 3 6 k 3 2 c 3 2 c 23 3 3 a 3 2 k 3 3 k 3 6 c 3 2 k 3 3 c 24 3 2 a 3 4 k 3 4 c 3 6 a 3 3 a 3 4 c 25 3 2 a 3 4 k 3 4 c 3 6 a 3 3 k 3 5 a 26 3 2 k 3 4 c 3 4 c 3 6 a 3 5 a 3 6 k 27 3 2 a 3 4 a 3 6 k 3 7 c 3 5 a 3 8 c 28 3 2 a 3 2 c 3 7 k 3 6 a 3 . 6 a 3 8 c 29 3 0 a — 3 6 k 3 6 a 3 . 6 a 4 0 c 30 3 0 a — 3 6 c 3 6 a 3 . 6 k 3 7 c 31 3 0 k — 3 7 k — 3 . 6 a — media mensile 3 4 3 4 3 . 5 3 6 3 4 3 5 media semestrale 3 . 5 n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i di calcio 1 0 3 tabella i i i n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i d i calcio p e r l ' a n n o g e o f í s i c o i n t e r n a z i o n a l e . (periodo 1 luglio 1958-31 dicembre 1958). mese luglio agosto settembre ottobre novembre dicembre giorno | n.c. 0 n.c. 0 n.c. 0 n. e. 0 n. c. o n.c. 0 1 3 5 c 3 . 8 a 3 8 m m 3 4 c 3 4 c 3 7 k 2 3 6 a 3 . 8 a 3 6 a 3 6 c 3 5 c 3 6 k 3 3 6 a 3 . 7 mm 3 6 a 3 5 a 3 6 c 3 6 c 4 3 7 a 3 . 6 a 3 6 a 3 6 a 3 6 c 3 6 a 5 3 7 k 3 . 6 a 3 6 a 3 6 c 3 4 a 3 6 a 6 3 8 mm 3 . 6 a 3 6 a 3 6 c 3 5 k 3 6 a 7 3 8 a 3 . 6 mm 3 6 c 3 6 mm 3 4 k 3 6 k s 3 8 a 3 . 6 a 3 4 c 3 5 a 3 4 a 3 6 c 9 3 8 a 3 . 6 a 3 5 a 3 4 a 3 2 c 3 6 mm 10 3 6 a 3 . 5 mm 3 6 c 3 6 c 3 1 c 3 5 c 11 3 6 a 3 . 6 a 3 6 a 3 6 a 3 0 c 3 6 k 12 3 6 a 3 . 6 a 3 8 c 3 6 c 3 2 c 3 6 a 13 3 4 c 3 . 5 c 3 8 a 3 7 c 3 4 c 3 6 mm 14 3 3 a 3 . 6 a 3 8 c 3 8 a 3 4 mm 3 5 mm 15 3 4 a 3 . 5 mm 3 8 a 3 8 a 3 4 a 3 5 mm 16 3 5 a 3 . 5 a 3 8 a 3 8 mm 3 5 k 3 4 k 17 3 6 c 3 . 5 mm 3 8 c 3 8 a 3 4 a 3 4 mw 18 3 6 a 3 . 6 a 3 7 a 3 . 8 a 3 6 a 3 2 c 19 3 6 a 3 . 6 a 3 6 a 3 s c 3 6 a 3 3 k 20 3 6 c 3 . 7 a 3 6 a 3 . 8 c 3 6 a 3 2 c 21 3 7 a 3 . 7 c 3 6 c 3 8 a 3 6 c 3 2 c 22 3 7 c 3 . 7 c 3 4 a 3 . 6 a 3 6 c 3 2 k 23 3 6 c 3 . 8 a 3 4 a 3 5 c 3 4 k 3 3 c 24 3 6 c 3 . 8 k 3 4 a 3 . 4 a 3 4 k 3 4 k 25 3 6 a 3 . 8 a 3 4 a 3 4 a 3 4 k 3 5 c 26 3 8 a 3 . 9 a 3 2 a 3 2 k 3 5 k 3 6 k 27 3 8 c 4 . 0 a 3 3 c 3 2 a 3 4 k 3 6 k 28 3 8 a 3 . 8 a 3 3 c 3 . 3 a 3 4 k 3 8 k 29 3 8 c 3 . 8 a 3 3 a 3 4 a 3 6 a 3 8 k 30 3 8 a 3 . 8 mm 3 4 a 3 4 a 3 6 c 3 7 k 31 3 8 a 3 . 6 c — 3 . 4 c — 3 6 k media mensile 3 6 3 7 3 6 3 6 3 4 3 5 media semestrale 3 . 6 1 0 4 g. godoli f . mazzucconi 15. monsignori fossi in seguito ai confronti che abbiamo eseguito, abbiamo deciso di prendere come numeri caratteristici definitivi per l'anno geofisico internazionale le medie fra la nuova serie di determinazioni sulle lastre e quella sulle stampe. queste medie sono riportate nelle tabelle i i i i . nelle tabelle è indicata anche la fonte dello spettroeliogramma analizzato secondo il codice: a arcetri 0 crimea k kodaikanal mm me math hulbert m meudon m w m t . wilson t tokio. notiamo che lo scarto medio fra la nuova stima e quella già pubblicata rientra generalmente nel limite accettato non superando il valore 0.3. nella seguente tabella sono riportati i giorni per cui lo scarto medio supera il valore 0 . 3 . t a b e l l a i v s c a r t o medio giorni 0.3 < s . m . s< 0.4 1957: v i i , 12, 24, 25, 26, 27, 29, 30, 3 1 ; v i l i , 2, 3, 5, 6, 7, 8, 9, 10, 12, 13, 21. 2 2 ; i x , 16 1 9 5 8 : 1. 27, 2 8 ; i i , 1. 12, 13; i i i , 1: i v , 8; v, 1, 3, 5, 8, 9, 10, 12, 14; v i , 9; v i i i , 18, 19, 2 0 ; x , 3, 4, 9; x i , 5, 8, 17 0.4 < s . m . s i 0.5 1 9 5 7 : v i i , 27, 2 9 ; v i l i , 10, 22 1 9 5 8 : i i , 2 0 ; i i i , 4, 2 2 ; i v , 19, 21, 2 4 ; v, 19, 2 1 ; v i . 16; x , 8, 14, 15, 17, 18, 22, 24, 2 9 ; x i , 15 0.5 < s.m. 0.6 1957: v i l i , 21 1958: i i i , 3, 5, 6; iv, 14; v, 2 0 ; x , 11, 21, 25, 27, 28, 30 0.6 < s . m . 0.7 1 9 5 8 : i i , 27. n u m e r i c a r a t t e r i s t i c i d e f i n i t i v i d e i f l o c c u l i di calcio 1 0 5 b i b l i o g r a f i a ( 4 ) g o d o l i g . , numeri caratteristici dei flocculi d'idrogeno e dì calcio e dei filamenti d'idrogeno per gli anni 1950-51-52, « ann. geof. », 6, 199, (1953). (2) idem per l'anno 1953, 1. c., 7, 135 (1954). (3) idem per l'anno 1954, 1. e., 8, 325, (1955). (4) idem per l'anno 1955, 1. e., 9, 299, (1956). (5) idem per l'anno 1956, 1. e., 10, 133, (1957) (in collaborazione con m. p . strocchi). (6) idem per l'anno 1957, 1. c., 10, 235, (1957). (') idem per l'anno 1958, 1. e., 12. 197, (1959). (8) idem per l'anno 1959, 1. c., 13. 231, (1960). (9) idem per l'anno 1960, 1. c., 14, 319, (1961). (10) osservazioni eseguite all'osservatorio astrofisico di arcetri durante l'anno 1961, (coss. e mem. dell'oss. astrof. di arcetri», 77, (1962). ( " ) idem per l'anno 1963, 1. e., 79, (1964). (12) idem per l'anno 1964, 1. c., 80, (1965). ( 1 3 ) g o d o l i , g . , mappe dei flocculi solari in radiazione k del ca ii per l'anno geofísico internazionale. 1: semestre 1 luglio 1957-31 dicembre 1957, « oss. e meni. dell'oss. astrof. di arcetri», 7 3 . (1961). (1 4) idem per il semestre 1 gennaio 1958-30 giugno 1958, 1. c., 75. (1961). (15) idem per il semestre 1 luglio 1958-31 dicembre 1958, 1. e., 76, (1962). (16) igy solar activity maps dv « annals of the international geopliysical y e a r » , 21, pergamon press, oxford, (1961). ( 1 7 ) g o d o l i g . , attività dei flocculi d'idrogeno e di calcio e dei filamenti di idrogeno per il periodo 1 luglio 1957-30 giugno 1958, « boll, di geod. e scienze affini», 17. 487, (1958). ( l s ) w a l d a., the fitting of straight lines if botti variables are subject to errors, « a n n . math. statistics», 11. 284, (1940). 28albini.qxd 675 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words eastern adriatic – historical earthquakes 1. introduction despite the efforts of many researchers dating back to the unesco project in early seventies (shebalin, 1974; shebalin et al., 1974), the seismicity of the balkan area is still poorly known. this contribution focuses on eastern adriatic from zadar (croatia) in the north to corfu (greece) in the south; it deals mainly with the seismicity of the islands and coastal area of croatia, bosnia and herzegovina, serbia and montenegro, albania and epirus (greece). a two-fold perspective was chosen, aiming at: i) checking the background knowledge supporting the available earthquake catalogue parameters; ii) analysing spatial and temporal seismicity patterns in relation with the actual distribution of historical observations of earthquake effects. since, here as everywhere, geopolitical issues and changes have influenced and shaped the distribution of historical records, the timespan considered is limited to the five hundred years between 14th and early 19th century. selected case histories will allow the author to highlight some sub-regional conditions which have determined both production and transmission of historical earthquake records and which still affect the knowledge of the seismicity of the eastern adriatic region. newly retrieved sources a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) paola albini istituto nazionale di geofisica e vulcanologia, sezione di milano, italy abstract focusing on the eastern adriatic region, from zadar in the north to corfu in the south, the background information supporting our knowledge of the seismicity in the time-span 14th to early 19th century is discussed from the point of view of the historical earthquake records. the late 19th century seismological compilations turn out to be those responsible for the uneven spatial and temporal distribution of seismicity suggested by current parametric earthquake catalogues. this awareness asked for a comprehensive reappraisal of the reliability and completeness of the available historical earthquake records. this task was addressed by retrieving in the original version the information already known, by putting the records in the historical context in which they were produced, and finally by sampling historical sources so far not considered. selected case histories have been presented in some detail also. this material altogether has shown that i) current parameterisation of past earthquakes in the eastern adriatic should be reconsidered in the light of a critically revised interpretation of the available records; ii) collecting new evidence in sources and repositories, not fully exploited so far, is needed. this should aim mostly at overcoming another limitation affecting the evaluation of full sets of earthquake parameters, that is the few observations available for each earthquake. in this perspective, an optimistic assessment of the potential documentation on this area is proposed. mailing address: dr. paola albini, istituto nazionale di geofisica e vulcanologia, sezione di milano, via e. bassini 15, 20133 milano, italy; e-mail: albini@mi.ingv.it 676 paola albini are used also to propose updated studies of key earthquakes in diverse periods and areas. the comprehensive analysis and the new effect distributions here presented turn out to be material useful to highlight tools and paths towards an improvement of the knowledge of the past seismicity in the eastern adriatic. 2. seismicity of the eastern adriatic according to current parametric catalogues the portion of the balkans here considered is shown in fig. 1 by means of a polygonal outline. for this area, from now on referred to as eastern adriatic, data on major seismicity (i0 ≥ 7) up to early 19th century are contained in five parametric catalogues published between 1974 and 1997 (table i). a thorough analysis of these catalogues with respect to earthquake parameters has been performed in the frame of the project «a basic european earthquake catalogue and a database – beecd» (albini and stucchi, 1997; also at http://emidius.mi.ingv.it/beecd), and will not be repeated here. each record included in the 1997 «beecd working file» was chosen as the «best» among those available from the parametric catalogues. it is worth recalling that among the five parametric catalogues only fig. 1. eastern adriatic and its present geopolitical features in the background. the area of interest of this paper is defined by the polygon. parameters for earthquakes between 1000 and 1899 are taken from the 1997 beecd working file and were chosen as the best among those supplied by the main available parametric catalogues for this region (see table i). i0 677 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) papazachos and papazachou (1989 and 1997) supply magnitude values. thus epicentral intensity (i0) is used instead of magnitude both in text and figures, though this value also does not express homogeneously the size of the catalogued earthquakes. table i shows chronological and geographical coverage of the catalogues dealing with the eastern adriatic. the last three columns show the distribution, deriving from the catalogues, of the seismicity of the eastern adriatic according to three time frames. these three time frames have been chosen primarily according to features of historical significance, especially from the point of view of production and conservation of earthquake records on this area; such geopolitical implications will be highlighted in the following sections. this contribution is focused on the period 1300-1806: this means that it considers 55 earthquakes in all, with an epicentral intensity ranging from 7 to 10. from a first crosschecking of data on time and location of earthquakes between the 14th and early 19th century, some features emerge: i) clusters of seismicity in and around zadar between 1300 and 1400 and in the area of dubrovnik and kotor between the early 15th century and 1667, when the large (i0 = 10) dubrovnik earthquake occurred; in contrast, ii) few earthquakes are listed up to the 15th century for the whole eastern adriatic coast (from split to corfu, with the exception of zadar) and from early 17th century to late 18th century from zadar to shkodër (northern albania). as a consequence of this preliminary analysis, focus was put on exploring the background of the information as available to compilers of parametric catalogues, to try and understand how and how much the sources of information they used are those responsible for this uneven spatial and temporal distribution of the seismicity in the eastern adriatic. 3. background to seismic catalogues 3.1. previous catalogues and seismological compilations the earthquake catalogues prepared and used in the frame of the undp-unesco project «survey of the seismicity of the balkan region» (1969-1974) still form the bulk of the information background of current parametric catalogues for the eastern adriatic (fig. 2). apart from trajic (1961), the others go back to the early seventies (cvijanovic, 1971, 1972; basovic, 1973; sulstarova et al., 1971). they catalogue time-span of the whole catalogue area or sub-areas within eastern adriatic records in east. adr. 1000-1529 records in east. adr. 1530-1806 records in east. adr. 1807-1899 shebalin et al. (1974) 2100 b.c.-1970 eastern adriatic 27 19 106 herak, 1995 (unpublished) 373 b.c.-1993 eastern adriatic 28 20 68 sulstarova and kociu (1975) 3rd century b.c.-1970 albania and epirus (gr) 5 5 73 papazachos and papazachou (1989) 550 b.c.-1986 montenegro, albania, epirus and corfu (gr) 1 13 26 papazachos and papazachou (1997) montenegro, albania, epirus and corfu (gr) 1 12 26 the last three columns give the amount of records listed by each catalogue with an epicentre within the area of interest according to three time-frames. table i. time and space coverage of the main catalogues for eastern adriatic. 678 paola albini were purposely prepared for the publication of the balkan catalogue (shebalin et al., 1974) and are described as «manuscripts» in the reference list; in fact they were typewritten lists of earthquake parameters (which could still be seen in v. karnik’s archive at the seismological institute of the academy of sciences, prague, in 1996). as for the rest of europe and the mediterranean, the compilation of the parametric catalogues on the balkan area started from descriptive collections of earthquakes, usually referred to as seismological compilations, spreading since the mid nineteenth century. apart from those covering europe, such as the well-known works by perrey (e.g., 1850) and by mallet (1853), the main interest in this investigation was to find out those specifically devoted to the eastern adriatic. for historical dalmatia, that is for territories today in croatia, bosnia and herzegovina, serbia and montenegro, the catalogues included in shebalin et al. (1974) do not explicitly mention their sources of information (fig. 2, dashed connectors). that they were partly derived from the seismological compilation by kispatic (18911892) had to be inferred by an accurate reading of the text (the original is in croatian). j. mihailovic (1927), morelli (1942), and d.j. mihailovic (1951) are specific seismological compilations related to albania, epirus and corfu (greece) quoted by sulstarova and kociu (1975). concerning mainly corfu, chiotis (1863) and partsch (1887) are the collections of fig. 2. background of the parametric earthquake catalogues for eastern adriatic. shaded boxes highlight the catalogues supporting and explicitly mentioned by shebalin et al. (1974). 679 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) earthquake records with the longest time-span used by papazachos and papazachou (1989 and 1997). 3.2. behind the seismological compilations to achieve a good understanding of the background of the parametric catalogues, and define how information on historical earthquakes has flowed into them, an analysis was performed on the set of historical earthquake records, those derived from both primary and second-hand historical sources, used by the seismological compilations for the eastern adriatic. from now on place names will be given either according to today’s use only or, preferably, together with the toponym of italian origin; this depends on the latter being the one mostly used during the time-span considered and by most of the coeval historical sources investigated (see for instance the documentary appendix). starting from the southernmost part of the study area (corfu, greece), chiotis (1863) collected information from published historical sources, mainly on the ionian islands; partsch (1887), geographer and scientist, concentrated on corfu, and he mostly used sources that are contemporary to the earthquakes; the limitation of his compilation is its dealing only with earthquakes from 1650 onwards. for albania and epirus (greece), j. mihailovic (1927), morelli (1942) and d.j. mihailovic (1951) relied upon seismological compilations of late 19th-early 20th century. though restricted to the albanian area, these works are not original, in that they mostly derive their information from the classical works of perrey (1850) and mallet (1853). for most of the area investigated, corresponding to historical dalmatia, from cattaro/kotor in the south to zara/zadar in the north, the most important collection of earthquake records dates back to the late 19th century and is the work of miso kispatic (1891-1892). a member of the academy of sciences at zagreb, kispatic introduces reasons and means of his work «potresi u hrvatskoj» (earthquakes in croatia) at the end of an introduction on the theories of earthquakes and before listing the sources for his «chronicle». we learn that the material supporting his compilation was collected during fourteen years of investigation and that his main interest was the territory of croatia at the turn of the 19th century. when kispatic published his essay, croatia was an austrian province, separated from the province of dalmatia from the administrative point of view. but the historical links were so strong that earthquakes affecting dalmatia were obviously considered by kispatic. in acknowledging his helpers kispatic mentions their specific contribution in terms of typology of documents and archive or library storing the historical sources. the then most distinguished researchers on the history of croatia did supply him with primary sources, from medieval chronicles to administrative documents for the modern age: worth mentioning are the librarian ivan kostrencic, from the university at zagreb, and the historian sime ljubic. the latter was among the editors of a «corpus» of medieval documents on this area and the balkans in general (ljubic, 1893); he had also worked at the state archive of venice (ljubic, 18761880) and is the one responsible for the set of venetian documents quoted and summarised in kispatic’s seismological compilation. most of the sources concerning zara were given to kispatic by prof. s. brusina, then working at the library of zara. retrieving the most significant sources listed in kispatic’s work has meant going back to the same repositories in which the researchers he acknowledges were working at that time. on the one hand, a direct reading of kispatic’s sources for earthquakes in eastern adriatic, without the intermediation of the transcription or summary made by his suppliers, has clearly indicated that the main value of kispatic’s work is represented by the assembling and ordering of a good set of primary and secondary sources. on the other hand, for a critical use of his data towards a proper and reliable seismological interpretation there is the need to go back to 680 paola albini the original sources, put them in their historical context, but mainly to look for additional, fundamental information which can put the earthquake records in a correct perspective. kispatic did not read directly the sources he quotes, and this indirect and very partial knowledge is the reason for a sort of uncritical and indiscriminate acceptance of all the pieces of information on earthquake effects his collaborators were supplying him with. nor did he perform or ask for a comprehensive investigation on the historical sources for dalmatia. the most evident gaps in terms of repositories not investigated are: i) the archive of ragusa/dubrovnik: it was not considered and no documents produced by the independent republic of ragusa are mentioned; ii) the archive of zara/zadar, ruled by and seat of the venetian governors through the centuries (1208-1358 and 1409-1797) was not investigated; kispatic’s sources on zadar are mostly printed sources (19th century local histories), and some manuscripts, mainly for 18th century earthquakes; iii) the state archive of venice: venetian documents are in fact used by kispatic, but their use is restricted to some major earthquakes and to some documentary sets only, supplied by the historian ljubic. whether and how these gaps affected the completeness and reliability of kispatic’s work will be considered in the following sections. 4. historical background: settlements and documents 4.1. geopolitical features the main reason for considering this area separately from the rest of the balkan peninsula lies in the common geopolitical features characterising the settlements distributed along the eastern shore of the adriatic sea. it is worthwhile mentioning that this separation can be observed, at least for dalmatia, also in the geomorphology of this area. this was described with few but accurate words by a historian at the beginning of the 20th century: «[...] from the quarnero to the bocche di cattaro [boka kotorska], a series of deep inlets and bays, with rocky mountains [dinarides alps] rising up behind [...] dalmatia is always a narrow strip, and [...] southwards it becomes even narrower, the mountain ranges at various points coming right down to the water’s edge» (villari, 1904). important towns were settled by greeks (epidaurus, cavtat, identified today as the place of ragusavecchia, near dubrovnik) and romans (salona, today solin, near to spalato/ split) in this narrow strip, which has been since then a «border-land», owing its importance to its harbours along the most travelled sea-route linking the northern european countries to the eastern world since the antiquity. some of these settlements were established in a favourable position on the coast as the last stop of the land-routes coming from the far and near east (villari, 1904). most of the towns developed from what originally were military posts and then fortresses giving shelter to travellers on their way from constantinople to venice, for instance, or to merchants fearing to be attacked by the ever-present pirate activities. south of kotor, both the morphology and the density of settlements change dramatically: low coasts and bays less suitable to shelter ships and their cargo, after dulcigno/ulcinj (famous for being a pirate nest) the first safe stop was durazzo/dürres; the town of corfu/kerkyra in the eponymous island took the place of the mainland as privileged stop, especially after it became a venetian dominion (late 14th century). this situation is indeed strictly related to and affected by the longest cultural and administrative influence that the eastern adriatic experienced: in the early 10th century the flourishing republic of venice expanded its dominion towards the eastern adriatic when its territorial extension in the north-eastern italian mainland was still limited by the power of neighbouring communes and ruling families. but the prevalence of venice in the adriatic sea depended upon the logistic supports located on the eastern coast, and this lasted, with ups and downs, for quite eight centuries, until the fall of the republic (1797). a far as the southernmost part of the area under investigation is con681 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) cerned, venetian dominion on albania extended at its peak to dürres only up to early 17th century (eickhoff, 1991). in dalmatia, venetian influence was particularly relevant to the language, both spoken and written. still under habsburg dominion (mid 19th century), italian was widely spoken in the main dalmatian towns, zara, spalato, ragusa and cattaro. the official languages of the republic of ragusa (1420-1808) were latin, a residue of roman and byzantine domination, and the italian spoken by the venetian officers ruling in this area with varying luck between the 10th and the 18th century. it is not surprising that nineteenth century historiography on dalmatia has a sound tradition in italian, croatian and german. the renowned geographical and historical description of dalmatia by petter (1834), who spent four years at ragusa and six at spalato since 1823, was published in italian and then in german (petter, 1857). 4.2. observation points and documents as a matter of fact, the places which, in kispatic’s compilation, are the main observation points of the seismicity of this region, are the same from which observations of earthquake effects are available in the long-term period. a compendium of information on three towns, zara/zadar, ragusa/dubrovnik and cattaro/kotor, chosen because they have the longest observation tradition, is given in the following, going from north to south. this is intended to show the main geopolitical changes they have passed through and what historical sources have been investigated so far, with special reference to kispatic’s contribution, and what is the potential set of historical records still to be looked into. from being a town of the kingdom of croatia in the 10th and 11th centuries, zara/zadar (fig. 3) became in 1202 part of the venetian dofig. 3. zadar: seismic and historical background. major earthquakes described by kispatic (1891-1892) are evidenced on the timeline by triangles. earthquake dates are shown on the time interval spanned by the historical sources (shaded boxes) supplying the record to kispatic. other sources are those available for zadar and not systematically investigated so far. 682 paola albini minion in eastern adriatic. the town was a bishop’s see since 1125, decorated by many churches (brunelli, 1913). in 1358, zara was re-conquered by the hungarian and croatian king, became an independent commune between 1382 and 1409, and was again under the republic of venice until 1797. since late 16th century the officer taking care of venetian territory in eastern adriatic was the «provveditore generale in dalmazia e albania» (governor general of dalmatia and albania), seated at zadar. the long history, and in addition the important administrative role, of zadar makes it an important observation point for events affecting the eastern adriatic. for these reasons, documentation of the town life is substantial, and the archive at zadar is still considered one of the main repositories for studying dalmatia and surrounding regions. it should be stressed here that kispatic (1891-1892) (see above and shaded sources in fig. 3) only partially used the potential of information available for zadar. the town of ragusa (today dubrovnik) was established by the survivors of the avars’ raid in dalmatian epidaurus (cavtat) in the seventh century (krecic, 1972) and the importance of the new settlement as a port city on the eastern coast of the adriatic is well attested since the 12th century. becoming independent from venice since 1420 (villari, 1904; carter, 1972; sugar, 1977), the republic of ragusa, though a small city-state, never abandoned its role at the core of dalmatian history. its harbour maintained its importance, and the ragusans specialised in the particular trade of collecting and spreading intelligence among the main actors of the mediterranean in the modern age (ottoman empire, spain and venice) (preto, 1994). since 1526 ragusa paid an annual tribute to the sublime porte, the powerful ottoman neighbour with whom the small republic had to cope for centuries. the heavily damaging 1667 earthquake (guidoboni and margottini, 1988) brought a serious crisis to ragusan finances and trades which took decades to overcome (e.g., carter, 1972; di vittorio, 1983). the city-state lost its independence in 1806, after the dominion of dalmatia passed from france to austria. late medieval chronicles on ragusa are available and published in the collection of monumenta spectantia historiam slavorum meridionalium (annales ragusini, 15th cent; chronica ragusina, 15th cent.). this material has been abundantly used by later authors of histories of dubrovnik (razzi, 1595; luccari, 1605) as well as by modern historians. published separately, but worth mentioning here is the chronicle by filippo de diversis quartigiani (1440), who wrote an account of the buildings and customs of dubrovnik. according to carter (1972), the original manuscript is in the franciscan monastery in dubrovnik. unfortunately, no earthquakes are mentioned in the portion edited by brunelli in 1882. for the modern age, the repository which should be investigated, aiming at an improvement of our knowledge of the long-term seismicity of dubrovnik, is the archive of the city-state, described as one of the richest in the mediterranean area by many historians dealing with its documentation. the typology and amount of available documents are described in details both by carter (1972) and by lume (1977). cattaro/kotor (fig. 4) owes its birth and increasing importance to its strategic location in the «boka kotorska», at the end of a long and narrow bay. historical tradition for this town is scarce; established around the 8th century, it was in the sphere of influence of the byzantine empire until the end of the 11th century. after some time under serbian influence, it became a venetian stronghold in 1420 until the fall of the republic (1797). an officer appointed from venice took care of the administration and of the trade development of cattaro after the 15th century. for this reason the bulk of information on events involving this place has to be looked for in the venetian archives storing the documents produced by the local governor (fig. 4). its surroundings were small and the walls around the town (fig. 5) were for centuries the only physical separation from its powerful ottoman neighbour. 5. major earthquakes in the eastern adriatic (14th-early 19th century): a review the material presented in this paper aims at contributing to a review of the existing knowledge of the seismicity of the eastern adriatic 683 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) fig. 4. kotor: seismic and historical background. major earthquakes described by kispatic (1891-1892) are evidenced on the timeline by triangles. earthquake dates are shown on the time interval spanned by the historical sources (shaded boxes) supplying the record to kispatic. other sources are those available for zadar and not systematically investigated so far. fig. 5. the walls of cattaro/kotor in a 19th century drawing. 684 paola albini from the point of view of historical records, and specifically those made available from seismological compilations and from the result of recent investigations. selected case histories are introduced to highlight the state-of-the-art and the potential of the information, still unexploited, which could seriously increase the amount of intensity data points, for major earthquakes mainly, and thus improve the completeness and reliability of the parameters of current earthquake catalogues. both the overview and the case histories have been addressed in two time frames: 1300 to 1529 and 1530 to 1806. this ad hoc division roughly follows the main geopolitical changes in the area described in the previous chapter and is used to focus the analysis on the most relevant sets of documents available for each period with respect to earthquake observations. 5.1. up to early 16th century seismicity up to 1529 clusters around zadar and dubrovnik (fig. 6): for the rest of the area in this study few earthquakes are catalogued, scattered both in time and space. 5.1.1. zara/zadar the main historical sources dealing with earthquake effects in zadar have been identified fig. 6. seismicity in eastern adriatic between 1000 and 1529 according to parametric earthquake catalogues. earthquakes here considered are those from 1300 to 1529. i0 685 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) through kispatic (fig. 3). the most important one, from which comes the information on four earthquakes affecting zadar at the turn of the 15th century, is the «memoriale pauli de paulo patritii jadrensis», as kispatic quotes it. but what is the original source and where is it? this «memoriale» and other medieval chronicles for zadar and split were published by giovanni lucio from traù/trogir in 1666, as an appendix to his «de regno dalmatiae et croatiae». this outstanding volume is definitely the most important published contribution on the history of the eastern adriatic from 10th to mid 17th century. its main value, at least for our purposes, lies in the large use lucio made of original documents, from the administrative and ecclesiastic archives of both zadar and split, partially or entirely reproduced in his book. the original manuscript and three later copies of paolo de paoli «memoriale» have all been lost, while the copy used by lucio seems to be stored at the vatican library (brunelli, 1911). paolo de paoli lived in zadar and described relevant episodes of everyday life in his town from 1371 to 1408. in fact, one line is devoted to 1371, then he (or lucio who transcribed the chronicle) jumps to 1378; the chronicle widens its horizons and includes both personal and town affairs in some detail from 1380 to 1408. how detailed the record by paolo de paoli is, is not easy to assess, because we have to rely upon lucio’s transcription of his chronicle and because no other chronicles are at hand to make a sensible comparison. table ii reports the information made available by him on earthquakes in zadar at the turn of the 14th century. how have these earthquake records been interpreted, are they included in current parametric catalogues, how have they been processed in terms of location and size? table ii. records on earthquakes at zadar between 1387 and 1407. earthquake date paolo de paolo (15th cent.) 5 march 1387 «anno 1387 die 5 martii, hora tertiarum fuit in jadra terraemotus magnus». [in the year 1387, on march 5, at the third hour, there was a great earthquake at zadar.] [lucio ed., 1666, p. 424] 5 dec 1390 «anno 1390 die 5 decembris circa mediam noctem fuit terrae motus magnus». [in the year 1390, on december 5 around midnight there was a great earthquake.] [lucio ed., 1666, p. 425] 6 nov 1399 «1399 25 iunii [...] eodem anno die 21 oct. hora quasi vesperarum fuit max. terraemotus in iadra bene septies successive, ipsorum primus fuit terribilissimus, et nocte sequenti similiter fuerunt plures, et etiam nocte post diem 24 ejusdem men. circa mediam noctem et circa matutinos fuerunt terrae motus, et per omnes dies inter diem, et noctem usque ad dies 13. subsequentis modo cessavit terrae motus, et deinde die 6 novem. hora quasi vesperarum fuit similiter unus magnus et terribilis terrae motus». [1399, june 25 ... in the same year, october 21. around vespers there was a very strong earthquake in zadar, for seven times, and of them the first was the worst, and the following night there were many too, and also in the night after 24 of the same month around midnight and around matins earthquakes occurred, and for many days, day and night, until the 13th day of the following earthquakes stopped, and then again on november 6 around vespers a great and terrible earthquake happened.] [lucio ed., 1666, p. 430] 25 june 1407 «1407. [die martis iunii 21 consecrata fuit ecclesia s. grisogoni de jadra] die sab 25 iunii in aurora fuit vale terribilis terraemotus». [1407. on tuesday june 21 the church of st. grisogonus from zadar was consacrated and on saturday june 25 at daybreak there was a really terrible earthquake.] [lucio ed., 1666, p. 438] 686 paola albini three are listed and have been located at zadar (fig. 6), with an epicentral intensity equal to 9 msk (shebalin et al., 1974). even if the 1407 earthquake is also reported by kispatic (18911892), it is not included in current catalogues. no new records have been found on these events, zadar being still the only place about which information is available. as for the damaging effects, the usual wording by paolo de paoli «terraemotus magnus fuit» (a large earthquake occurred) is generic and needs to be interpreted through some hints taken from other places in his same chronicle. after each recorded event, nothing changes in zadar life, measured by the customary administrative meetings. but the chronicler is very precise about day and hour of the earthquake, as he is in the case of the burning of a single building («1381 – die 10 mens. sep. ea die de nocte combusta fuit domus») or of an uncommon atmospheric phenomenon («1387 – die 8 jan. in caelo splendor magnus»). these considerations, and a thorough reading of paolo de paoli «memoriale», strongly suggest that the epicentral intensity of the four earthquakes affecting zadar between 1387 and 1407 is overestimated and in need of a reappraisal. also bianchi (1888) made use of paolo de paoli and other later sources, very imprecisely referenced to and only in his introduction. from him kispatic took the information on some earthquakes in the 12th century and in the first half of the 14th century (see fig. 3). a supplement of investigation on this earlier period should cast light on these poor data, but both this information and bianchi’s reliability will not be further considered here. nonetheless it is worth noting that: i) the other chronicles for this area edited by lucio (1666) in the same volume as the «memoriale» cover 13th and 14th century; ii) such chronicles register natural phenomena (eclipse, lightning) and damaging events (fire) but do not mention earthquakes. 5.1.2. ragusa/dubrovnik there are few but reliable sources for medieval ragusa (annales ragusini, 15th cent.; chronica ragusina, 15th cent.; filippo de diversis quartigiani, 1440). it comes consequently as a surprise that no records on earthquakes affecting fig. 7. relationships between seismological and historical compilations and coeval sources on the earthquake of 17 may 1520. 687 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) table iii. records on earthquakes at ragusa/dubrovnik (late 15th to mid 16th century). earthquake date serafino razzi (1595) 14 feb 1481 «dell’istesso anno 1481 alli 14 di febraio, intorno alle cinque hore di notte, venne in raugia un terremuoto grandissimo, il quale fece gran danno, e rouine di case». [p. 64] 5 hours in the night (*) feb 1482 «l’anno 1482 del mese di febraio, vennero di nuovo grandissimi terremuoti à raugia, e per tutto il contorno». [in the year 1482, in february strong earthquakes hit again dubrovnik and surroundings, p. 64] 26 sept 1482 «et alli 26 di settembre di nuovo furono terremuoti». [p. 64] 28 nov 1496 «dell’anno 1496 alli 28 di novembre, à hore 17 essendo congregato il consiglio generale in palazzo, venne un così fatto terremoto, che dubitarono non rouinasse il palazzo, e che non morisse tutta la nobiltà. ma dio grazia non seguì tanto male». [p. 68] (*) 7 dec 1504 «del 1504 alli 7 di dicembre, in martedi, venne un grandissimo terremuoto, il quale guastò molti edificij».tuesday [on december 7th, tuesday, a strong earthquake caused damage to many buildings, p. 71] (*) 17 may 1520 «dell’anno m.d.xx. alli 17 di maggio, à hore 11 nella festa della santissima ascensione, venne in raugia un terremoto tanto grande [...]». [p. 75] (the complete text is in the documentary appendix). 11 hours 3 feb 1534 «dell’anno m.d.xxxiiii alli tre di febraio, festa di san biagio, protettore di raugia, circa le tre hore di notte venne un gran terremuoto in detta città. ma per la dio grazia non fe danno alcuno». [p. 81] 3 hours in the night 13 aug 1543 «alli 13 d’agosto dell’anno istesso alle 7 hore di notte, venne a raugia un grandissimo tremuoto». [p. 101]7 hours in the night 6 march 1544 «l’anno istesso alli 6 di marzo, alle 14 hore, stando tutta la nobiltà alla predica in duomo, venne in raugia un gran tremuoto: ma per la dio grazia non fe danno alcuno nella città. ma rouinò in narenta l’habitazione del venditore del sale per la republica con tutta la salinaria». [p. 102] 14 hours 11 sept 1546 «alli 11 di settembre 1546 alle tre hore di notte venne in raugia un gran terremuoto». [p. 110]3 hours in the night 7 feb 1547 «et alli 7 di febraio dell’anno seguente [1547] à un’hora, e mezzo di notte ne vennero due altri, uno dopo l’altro: ma per grazia del signore non fecero danno alcuno». [p. 110] 1 and a half in the night (*) the earthquake date indicates events listed by parametric catalogues. the town are supplied by kispatic before 1481. on the basis of the catalogue by traijc (1961), current parametric catalogues (shebalin et al., 1974; herak, 1995) locate the first earthquakes at ragusa in 1451 and 1471, while the 1472 event is attested by kispatic through skurla (1871) and is located to the west of the town (fig. 7). as already said, when the only source of current catalogues is another catalogue, one faces a dead-end on the way to the original sources. recent histories of dubrovnik (carter, 1972; krecic, 1972) are not more informative about earthquakes before the one in 1520, with the exception of villari (1904). on the basis of gelcich (1899) he wrote: «the year 1395 proved an unfortunate one for ragusa [because of] heavy im17 hours 688 paola albini positions on ragusan trade [in the period of the wars between hungary and venice for the dominion over dalmatia]. at the same time heavy rains flooded the city and its immediate neighbourhood, destroying all the crops, and on may 19 a severe earthquake, the first great shock felt in dalmatia for many centuries, wrought great havoc». the book in question (gelcich, 1899) was published few years after kispatic’s (1891-1892) and this is probably the reason why it was not taken into account; a previous work by the same author on ragusa (gelcich, 1884) could have been unknown to kispatic’s collaborators. further, as we have already noticed above, against all expectations, few are the local documentary sources about earthquakes in ragusa used by kispatic’s seismological compilation. apart from those on the 1520 earthquake, all records between 1481 and 1547 derive from one unique source: the «storia di raugia» (history of ragusa) by the priest serafino razzi (1595), originally from lucca (tuscany, central italy), who spent many years in ragusa in the 16th century. the information (table iii), as in the case of zadar, is concentrated on ragusa, as one could expect from a local chronicle. the exceptions are the records on the 1520 and 1544 earthquakes. how has this information been processed by parametric earthquake catalogues? as evidenced in table iii by an asterisk before the date, out of eleven records supplied by razzi, and reported by kispatic, only three have been included in current parametric catalogues: 1482, 1504 and 1520. catalogues list also an earthquake in 1516, mentioned by kispatic through adamovic (1884): «on may 6, before midday, at dubrovnik an earthquake was felt and it lasted 18 months. some mountains cracked and the earth opened in some places». since this event is not mentioned by razzi, who is considered a reliable and complete source for this period, even if he was not in ragusa at that time, the record of the 1516 earthquake appears a doubtful one. on 17 may 1520, at 11 hours, an earthquake struck central dalmatia, causing considerable damage and some deaths. this is the only earthquake, before the 1667 one, known in some detail as to its effects in and outside dubrovnik, both in seismological compilations and histories of dubrovnik and dalmatia. but it is also the first earthquake for which information from venetian sources plays a fundamental role in letting us know the effects outside dubrovnik. as shown in fig. 7, kispatic (1891-1892) relied upon skurla (1871) and adamovic (1884), but on coeval sources as well: razzi (1595) and marino sanudo (1496-1533). what the latter reported (see documentary appendix) came from letters sent or read to the senate in venice by the son of simplicio rezo, merchant at constantinople (on 30 may 1520), by marciò nadal, from curzola (may 1520), by piero marzelo from zara, who got a report from ragusa (4 june 1520) and by vicenzo trun, governor at cattaro/kotor (22 may 1520) and daniele contarini, captain of a «galea» (galley) in kotor (20 may 1520). from sanudo and razzi we learn that the earthquake caused the following: i) at ragusa, part of the lord’s palace collapsed, and some houses were seriously damaged; repair costs were estimated in «centomila ducati» (100 000 ducats) for ragusa and 50 000 ducats for the neighbouring places, not mentioned; according to razzi 20 people died, but venetian sources refer to 156 dead and 400 injured; the church of the holy saviour (san salvatore or sv. spas) «was founded, as the inscription on the door states, as a result of a vow made by the government of the time, to commemorate those who suffered in the great earthquake of 1520» (carter, 1972; see also razzi, 1595, documentary appendix). ii) at cattaro, seven houses collapsed and several were damaged; the walls and the castle suffered too, the latter fissured especially in the gunpowder storeroom; no dead; daniele contarini was on his galley and the ship was rolling «as a leaf». iii) at «budua» (budva) and at «dulzigno» (ulcinij) some minor damage. in all, macroseismic effects recorded for this earthquake make it the largest event in the history of dalmatia before 1667, and its maximum intensity could be estimated as 8-9 or 9 in ems98 at ragusa/dubrovnik. 5.1.3. south of dubrovnik if there are relevant gaps in our knowledge of the seismicity of the northern part of 689 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) the eastern adriatic between 1300 and 1529, the situation is even more dramatic south of cattaro/kotor (fig. 6). one earthquake only is listed by parametric catalogues, in 1380, located at elbasan, an albanian settlement far away from the adriatic coast (for a discussion on the date of this earthquake through a historical interpretation of its epigraphic record see evangelatou-notara, 1993). seismological compilations for albania (mihailovic, 1927; morelli, 1942) do not supply earthquake records for this period. moreover, the possibility of easily collecting more records on this region appears to be very low. when dealing with corfu, the situation does not change significantly. the island was under venetian rule since late 14th century, but the documents on the early venetian period up to late 15th century are few, both at the state archive of venice (da mosto, 1937-1940) and locally (gueze, 1970). 5.2. under venetian rule and influence (early 16th to 18th century) patterns of seismicity deriving from current parametric catalogues are different for the second time frame, 1530 to 1806 (fig. 8). in fact, the doubtful 1530/1531 earthquake is the only event located by catalogues at dubrovnik between 1530 and 1667 and after 1667 until the mid 19th century. with the exception of 1667, epicentres migrate from dubrovnik to kotor fig. 8. seismicity in the eastern adriatic between 1530 and 1806 according to parametric earthquake catalogues. i0 690 paola albini and local seismicity appears at corfu only eventually mid-17th century. there are no specific chronicles dealing with kotor, albanian towns and corfu in those centuries. in recent years, a series of investigations have been performed on the potential of venetian documentary sources aimed at understanding the seismicity of the southern part of the eastern adriatic in this time frame. their main results are reported in the following, going from corfu northwards. 5.2.1. corfu seventeenth century corfu was the seat of the venetian governor of the «three islands» (corfu, kefalinia and zakynthos) in the southernmost part of the eastern adriatic, where it merges with the ionian sea. an investigation of three earthquakes located by parametric catalogues in corfu between 1650 and 1674 (fig. 8) was carried out through a systematic reading of dispatches of the local venetian governor (albini, 1999). a synthesis of the results is presented in the following. parametric catalogues (papazachos and papazachou, 1989 and 1997) record an earthquake at corfu in 1650 (no month no day), with i0 = 7. the information is supplied by chiotis (1863), relying in his turn upon a history of corfu by andrea marmora (1672). venetian documents (asve, 1651) do report an earthquake, but on 26 february 1651. the damage described was to a rampart in the walls, but though a survey was conducted the morning after the earthquake, cracks were discovered only eight days later. this makes less reliable the connection between damage and earthquake, and uncertainty on the actual effects due to the event is increased by the silence from places other than corfu. the november 1666 earthquake is listed in three parametric earthquake catalogues (shebalin et al., 1974; sulstarova and kociu, 1975; papazachos and papazachou, 1989 and 1997, with i0 = 7) as a damaging one. the latter catalogue took the record from the seismological compilation on corfu by partsch (1887), who in his turn used a bibliographical source published about one century after the earthquake (dresdnische geleherte anzeigen, 1756). the identification of a dispatch written on 20 november 1666 by the «provveditore e capitano» foscarini (museo civico correr, 1666) casts light on this episode: no earthquake was reported in corfu, or in other places, and the collapse of porta rimonda, one of the entrances to the fortified town, is attributed by foscarini to the continuous and heavy rains since august 1666. the 1674 earthquake occurred on 16 january, about «ten hours of the night». its effects are described in two dispatches by giacomo nani, governor and captain in corfu (asve, 1674a) and andrea valier, governor general «da mar», in charge of the eastern dominions of venice, then visiting corfu (asve, 1674b). both dispatches are dated 21 january and describe the effects at the following places: i) corfu, which suffered no damage; ii) the region of alefchimo, the southern part of the island of corfu, where many houses and some churches were seriously damaged, some people died and many were injured; iii) the village of paxos in the northern part of the island, to the south-east of corfu, suffered some damage as well. from a third dispatch, by the newly appointed governor and captain in corfu, gritti (asve, 1674c) written six month later, we learn that for parga, a fortress in the epirus mainland, facing the island of paxos, «recent earthquakes» are to be considered among the reasons of its bad state of repair. this set of information is particularly valuable because it concerns not only the main town, corfu, but it describes effects at other places, including the mainland. though this earthquake is not included among those considered in this paper, because its epicentral location is between the adriatic and the ionian sea, for the sake of completeness the 20 february 1743 event should be mentioned here, since it seriously damaged places on both sides of the adriatic sea and the ionian islands. it was one of the most damaging in corfu, and a recent reappraisal of this event (boschi et al., 1995), based on contemporary venetian sources, assessed at the town of corfu an intensity of 8 mcs. 691 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) 5.2.2. albania venice never had a strong hold on albania, and she lost her possessions in northern albania, mainly durazzo/dürres, to the ottomans early in the 17th century. in venetian documents the word «provintia» is used by venetian governors as synonymous with «albania» and also to indicate just the northern part of modern albania, roughly as far south as scutari/skhoder only. an investigation carried out on documents by the «provveditore generale in dalmazia e albania» (albini, 1999) has shown that, though such material is available from the early 17th century, the number of documents tends to decrease towards the end of the same century. the research was focused on checking whether what was happening in an area not under direct venetian rule would be reported in the official documentation. though this sample was disappointing in terms of information on earthquake effects, a parallel sample in other venetian archival holdings has shown that a better observatory on albania was corfu’s governor, who was reporting from many places situated inland and thus far from the actual borders with the ottoman empire (albini, 1999). 5.2.3. cattaro/kotor the «seismic history» of cattaro is less known, but perhaps not less damaging than ragusa’s. the earliest earthquake located by the catalogues in kotor is in 1559, and from that moment on and through the 18th century, damaging earthquakes in the eastern adriatic seem to cluster around both dubrovnik (1667) and kotor. on 13 june 1563 around noon, an earthquake in kotor made 168 houses collapse and left the rest seriously damaged. the governor’s palace was hit and the venetian governor of that time, francesco priuli, and his family died under the ruins, together with at least a dozen more. kispatic took this piece of information from two dispatches, written in july and september by the newly elected governor in kotor, andrea duodo. these dispatches should be found at the state archive of venice, but a preliminary investigation in some possible documentary sections storing this material (asve, 1563; asve, 1559-1581) did not succeed in retrieving the original documents. anyway, a confirmation of the damage caused by this earthquake was found in a dispatch that the «provveditore generale in dalmazia e albania» wrote after the april 1667 earthquake (asve, 1667). the same description given by bonito (1691) through giustiniani (1575-1576) is taken over by kispatic, who relied upon the venetian sources above mentioned, and upon an anonymous fly-sheet in german, «newe zeytung» (1564). the latter added a description of a subsidence phenomenon at the place then called «belvedere», not identified so far, ending with a prophecy about the inhabitants abandoning that place in the years to come. the description of effects is compatible with a large earthquake; but the scarcity of information from the surrounding places makes impossible at this stage to assess intensity at places other than kotor, and for it as well it is possible only a (very) tentative evaluation of intensity as 9 ems98. in terms of epicentral intensity, the 25 july event of the 1608-1609 sequence affecting kotor has been estimated equal to 10 msk (shebalin et al., 1974), and is the largest earthquake ever located in kotor by parametric catalogues. apart from kispatic (1891-1892) no previous seismological compilation mentions this event, starting from bonito (1691) to von hoff (1840), perrey (1850) and mallet (1853); all, however, describe the 1563 earthquake. coeval venetian documents (asve, 1608a,d-f,h; asve, 1608b,g,m; asve, 1608c; asve, 1609a,d; asve, 1609b,c), which were known and imprecisely summarised by kispatic (1891-1892), describe the sequence as starting on 25 july 1608, st. james’s day. news of the july earthquake reached the senate in venice and the governor general at zadar by means of a dispatch from tomaso contarini, governor at kotor, dated 5 august 692 paola albini 1608 (asve, 1608b). contarini wrote that the residences of the two high-rank public officers were damaged, one more seriously than the other, that the inhabitants were abandoning the town because the earthquakes were repeating, that the walls over the harbour were also damaged. the army barracks within the citadel were ruined, as well as some buildings by the seaside, where a bridge collapsed too. from «castel novo» (herceg novi), then in ottoman territory, the news spread that damage was even worse than in kotor, with many buildings in ruins and some deaths. the second large earthquake happened on 15 september 1608, at 16:30. and this time it was the turn of the new governor, francesco morosini, to inform both senate and governor general that the situation had worsened (asve, 1608g). he and his family had not been killed by the partial collapse of the palace, already shaken by the previous earthquake, simply because the earthquake had not happened during the night. three houses belonging to merchants, buildings of good quality on the main square of kotor, had the walls collapse. the clock tower was now leaning. also damaged were the citadel and the fortification walls. one old woman died and there were some injured. the venetian governor was also eager to know the consequences of the second earthquake in «castelnuovo» (herceg novi), and thus sent there an informer with an excuse (a visit to an old relative): one third of the buildings had collapsed, as well as a portion of the castle walls, and within it havoc was great. there were neither deaths nor injured because the inhabitants were living in their villas outside the town for fear of a spanish galley attack. smaller earthquakes continued to be felt until mid december 1608 (asve, 1608m). on early january 1609 heavy rains caused the collapse of three buildings which had been weakened by the repeated earthquakes (asve, 1609a). and another earthquake was felt «in the evening of st. paul’s day», that is on 25 january (asve, 1609d). a tentative intensity assessment for kotor and herceg novi for the 25 july 1608 earthquake could result in 8 ems98 at both places. assessing intensity for the 15 september 1608 earthquake would not be free from the previous earthquake’s influence on building stock and vulnerability, and would in reality mean to evaluate the accumulative effects of the two earthquakes. this is a delicate issue, still without agreed procedures, but a tentative estimate for macroseismic effects resulting from both events could be intensity 9 ems98 in kotor and herceg novi. no information was found on an earthquake dated by shebalin et al. (1974) on 14 may, with i0 = 9: letters and dispatches of the governor at kotor are available and have been systematically investigated from the beginning of may 1608. in particular, the dispatch sent on 25 may by the governor in kotor to the «provveditore generale in dalmatia e albania» deals with «things going on in albania» but does not mention any earthquake (asve, 1608a). on the basis of this new evidence, the parametric record on the 14 may earthquake should be considered fake, probably a duplicate of the 25 july earthquake. in this paper no space is given to the already mentioned 6 april 1667 earthquake, which caused serious damage both at dubrovnik and kotor (guidoboni and margottini, 1988). but since we are dealing with venetian documents, it is worth mentioning that venice tried to take advantage of the crisis induced to ragusa/dubrovnik by this earthquake, and put energy and money in strengthening kotor, in open competition with the independent republic of eastern adriatic. parametric catalogues for the eastern adriatic list no earthquakes with i0 ≥ 7 in the area of dubrovnik and kotor between 1667 and 1780. what has been shown about gaps and incompleteness in the sets of historical records supporting kispatic’s earthquake compilation, gives enough evidence to suggest a reasonable explanation of this «silent» period: the two main repositories, venice and dubrovnik, which potentially store the documentation on the history of this area in this time span, were not systematically investigated. this is further supported by the fact that a 693 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) proper description of the 1780 earthquake is also a last-minute addition to kispatic’s compilation, in the appendix to his work. he owes this information to «the work by prof. srecko vulovic, at kotor. i had his personal permission to copy it and here i am reporting his text» (kispatic, 1891-1892). the appendix gives supplementary information on the 1632 earthquake at kotor as well, but this event will not be considered here. in 1780 the venetian «provveditore generale in dalmazia e albania», paolo boldrin, wrote in his 10 december dispatch (asve, 1780) that in the letter of 12 october (not available) the governor in cattaro had informed him of the damaging effects caused in cattaro by an earthquake that occurred on 21 september. damage was sustained by budua and castelnuovo as well. the governor in cattaro made a first request for supplies needed for the restoration of the damaged buildings and a portion of the fortification walls. the governor general appointed the engineer gaetini to make a proper survey of the damage; the survey was made and sent to the senate in venice with the 30 january 1781 dispatch (asve, 1781). unfortunately, the survey is not enclosed in the dispatch and the information on effects remains generic. much more detailed is what kispatic found in vulovic’s work and reported in the appendix. this information, supplemented by reports on coeval french and spanish press (mercurio historico y politico, 1780; gazette de france, 1780) is summarised below. at kotor, some of the poorest buildings laid in ruins and two children died under them; cracks and the fall of chimneys were the only damage suffered by more solid buildings; the cathedral was seriously fissured (kispatic, 1891-1892). at castelnuovo/herceg novi, located at the entrance of boka kotorsks, the lighthouse collapsed and killed an old woman (kispatic, 1891-1892). fissures opened in the ground between cattaro/kotor and perasto/perast, but damage spread inland to scutari/shkodër (albania) (mercurio historico y politico, 1780). within the «bocche di cattaro» (boka kotorska), the sea retreated from the coast and then violently went back (kispatic, 18911892). at ragusa/dubrovnik some buildings were damaged (gazette de france, 1780). the earthquake was felt in the north, as far as the korcula island, midway between dubrovnik and split, and at antivari/bar, 50 km southwards of kotor (kispatic, 1891-1892). 6. discussion and conclusions the information sets supporting current parametric catalogues for earthquakes in the eastern adriatic between 14th and early 19th century have been traced and commented on with respect to their availability, reliability and completeness. it turns out that our knowledge of the seismicity of historical dalmatia, coastal albania and corfu still relies upon those records collected by the seismological compilations produced between mid 19th and mid 20th century. gaps in the information such seismological compilations supplied the compilers of the parametric catalogues with, have been checked versus the actual availability of historical sources on the main observation points in the region, at the same time highlighting the potential of documentation still awaiting to be investigated. in particular kispatic’s work on dalmatia (1891-1892) has been analysed and a «map» of what sources and repositories have contributed to his compilation has been drawn. the review of the major earthquakes focused on retrieving the known historical records in their original version, and on identifying and sampling historical sources so far not considered; this has been done according to two time frames, which made easier accounting for production and survival of local and coeval evidence. concerning the typology of sources supplying information on earthquake effects, town-centred chronicles largely dominate the first time frame, from the 14th to the early 16th century. conversely, the history of this area from the 16th to the 18th century so strongly influenced the document production 694 paola albini and distribution that it made prevalent, in availability, investigation and use, the earthquake records contained in reports by venetian officers, both in their official role and in the occasional ones of historian and reporter. the documentary gap identified corresponds mostly to the documentation produced by the republic of ragusa (15th to early 19th century): documents from the historical archive at dubrovnik were not considered by kispatic, and its investigation, in the author’s opinion, would bring to light outstanding evidence on effects of known earthquakes, as well as a confirmation of the time distribution of the seismicity of the central part of the investigated area. putting the historical records known to seismological compilations in the historical context in which they were produced has shown that their current parameterisation should be reconsidered in the light of a more systematic reappraisal of long-term seismicity of the eastern adriatic. the case histories presented showed that this of course depends on the collection of new evidence, but also on a revised interpretation of the records reading them in a historical and critical perspective. tentative intensity assessments have been given at those places for which details on serious damage effects were available, and this was possible in a few cases only. on the other hand, this is an important limitation in estimating location and size of earthquakes in this area, since descriptions containing data useful to assess macroseismic intensity are mostly restricted to one place only for each earthquake. in practice, with the historical earthquake records now available, the seismic histories of zadar, dubrovnik and kotor could be completed and evaluated much easily than a full set of parameters for the earthquakes of 1407 at zadar, or 1563 at kotor. an improvement of the knowledge of the past seismicity of the eastern adriatic is at hand: historical sources not fully exploited survived and are simply waiting for the historians and seismologists to ask them the right questions to disclose their potential of information on earthquake effects. acknowledgements this paper owes much to the fruitful exchanges through the years with many colleagues. on the occasion of the publication of this volume my prevalent feeling is that i owe a heartful thank to my «masters in historical seismology»: massimiliano stucchi, jean vogt and nick ambraseys. further, i could reckon on roger musson, to whom i am indebted for the writing of this paper. previous and unpublished reports on this area were prepared with the collaboration of carmen mirto and ilaria leschiutta. i would like to thank carlo meletti for the preparation of some figures, valentina montaldo and paola migliavacca for supporting me in translation, transcription and bibliographical matters. documentary appendix – records on 17 may 1520 earthquake serafino razzi (1595) «dell’anno m.d.xx. alli 17 di maggio, à hore 11 nella festa della santissima ascensione, venne in raugia un terremoto tanto grande, che peggiorò dentro alla città per valore di centomila ducati: e fuori per le ville per più di 50.mila. e vi morirono intorno à venti persone, e più assai ne rimasero ferite, e percosse. e narrano come da più persone fuori dalla porta di levante, luogo detto alle plocci: e eziandio da alcuni turchi, che quivi si trovavano, fù veduto il monte vergato, che sopra stà alla città, il quale minacciava di cadere sopra di quella. ma opponendosi la gloriosa vergine con san biagio, non lo lasciarono cadere. e fù in ringraziamento edificata una divota chiesa dell’ascensione di nostro signore, vicino al convento di san francesco. e ci si và ogn’anno, in tal giorno, con pubblica, e solenne processione» (p. 75). 695 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) marin sanudo (1496-1533) [ad annum 1520] «a dì 30. [maggio] la matina, vene in colegio ... fiol di simplicio rezo marchadante di zoie, è a constantinopoli [...] etiam lui averà letere di suo padre, [...] dice come a dì 17 di questo mexe, fo el zorno di la sensa, a hore [...], a ragusi fo un grandissimo terremoto, qual ha ruinato parte dil palazo dei la signoria e altre caxe, morto 12 homeni; siché ha fato danno in ragusi per più di ducati [...] milia» (vol. xxviii, coll. 562-563). «di curzola, di sier marchiò nadal conte di [...] avisa come a dì ..., fo il zorno di la sensa, principiò uno gran terremoto a ragusi, qual durò 9 zorni, over nove volte, ha ruinato parte dil palazo di signori e altre caxe, amazato homeni et fato gran danni; et uno san biagio vardava verso ponente, di marmo, il terremoto l’ha voltato; siché’l varda verso levante» (vol. xxviii, col. 564). «[giugno] di zara, di sier piero marzelo conte et sier zuan nadal salamon capitanio, di ..., manda una relation di un de ragusi, di uno novo terramoto stato de lì, la copia de la qual noterò qui avanti, videlicet a dì 3 di questo è stato sì gran terremoto a ragusi che ha aperto uno monte lì contiguo, e l’aqua qual intrata dentro ruinà caxe etc.; el qual etiam fo a dì 17 mazo el dì de la sensa. item, a constantinopoli è stà uno altro grandissimo teramoto» (vol. xxviii, coll. 621-622). «a dì 18 [giugno][....] di cataro, di sier vicenzo trun retor et provedador, fo letere di 22. di uno terremoto seguito de lì el zorno di la sensa, a dì 17 dil mexe, comenzò a hore una de dì, et à ruinato 7 case, risentito le mura di la terra et dil castelo, sfeso in certo locho, maxime dove stava la polvere. per il che quella terra è in grandissimo spavento; fanno ogni dì processione. scrive è stato etima de lì atorno, et maxime a ragusi grandissimo. di liesna, di sier zaccaria valaresso conte et provedador, di 7, tamen le altre fo di 9.[...] scrive come da ragusi, per molti venuti, dicono quella terra esser in grandissimo spavento per li terremoti che ogni zorno usano dal dì di la sensa in qua, quali hanno fato e fanno ruine grandissime, et ultimate è stà dito ozi l’ha ruinà la caxa di jacomo di zulian fidelissimo nostro, siché la terra è meza abandonata. el soradito vien di constantinopoli, à ditto esser stato a constantinopoli terremoto grandissimo et ruinato gran parte di le mure di quella città. exemplum sumario di una letera di sier daniel contarini soracomito, data in cataro a dì 22 mazo 1520, drizata a sier jacomo corner di sier zorzi el cavalier procurator, ricevuta a dì [...] zugno. scrive, a traù, spalato et liesna non poté trovar homeni da interzarsi, solum 6; siché l’era disperado e desiderava la morte. a dì 13 ditto vene lì a cataro, dove per la dio gratia se ha interzato, dil che è tutto resusitado. a dì 17, fo el zorno di la sensa, a una hora de dì, fo lì a cataro uno gran terremoto, il qual ha fato uno gran dano di caxe, ma non è morto niuno, neanche maculado niuno per la dio gratia. lui si atrovava su la sua galia, la qual tremava come una fogia; mai vide cosa più spaurosa; vete fino le montagne tremar. questo teremoto è stà fino a ragusi, e lì ha fato mazor danno che quì; à mazado 156 persone, magagnadi da 400. è stato etiam a budoa et a dulzigno, ma in questi luogi non ha fato tropo dano. a dì 20, questo provedador ave una letera dil retor di dulzigno, di 19, el qual li avisa come era zonto li uno bregantino armado [...]» (vol. xxviii, coll. 625-627). «a dì 19 [giugno][....] fo scrito eri, per colegio, a sier vicenzo trun retor e provedador di cataro, in risposta di sue di 26 zugno, dil terremoto sequito de lì, et che le mure si ha resentito, sì de la terra come dil castelo, e parte dil turion, con jactura di la polvere e ruinate alcune caxe. pertanto fazi intender a quelli citadini fidelissimi si dolemo, e laudemo siino drizati al signor dio implorando misericordia, et il zeneral presto sarà de lì et provederà a tutto. et per una letera a parte, fo scrito al prefato rector laudandolo aver fornito et interzato la galia soracomito sier daniel contarini» (vol. xxviii, coll. 630-631). 696 paola albini – records on the 1608-1609 sequence asve (1608a) serenissimo prencipe [...] tengo avisi del signor rettor di cattaro de 25 del passato [may] intorno alle cose di albania ... in zara alli 20 giugno 1608, zangiacomo zane asve (1608b) serenissimo prencipe è gionto il capitano alfonso forno con la sua compagnia, che per quanto vegio dalle littere della serenità vostra di 14 del passato da lui resemi sotto li 30 del medesimo et da me riverentemente ricevute vole essere di cento fanti come nel ruolo che mi ha presentato anco si vede, ma io li ho ricevuti con la sua persona, et quatro offitiali in tutto novanta tre, et di quelli che mancherano darò riverente esecutione à detta lettera, si come farò in tutte le altre parti che in esse si contengono. et certo che sono venuti molto à tempo, si per il bisogno che vi era, come perche doppo che il giorno di s. giacomo fece un terramotto in questa città molto grande, et che quasi ogni giorno dal hora in qua se ne sente alcuno (seben piccolo) quasi tutti della città fugono fuori, siche ella era poco meno che abbandonata, et seben io non permettevo a tutti la uscitta con intimationi, et altri modi necessarij, però fugivano ascosamente, et in casi tali in questi luochi massime, conviene passar molto destramente con queste genti per molti respetti. questo terramoto hà fatto della rovina grande in tutta la città, ma in particolare al publico, per più di tre mille ducatti senza il danno delle muraglie buona parte sopra la marina che non è posibile che possino durare, et di fuori dove cadono non si può pontolarle a niun modo, et quando si potesse sarià un far scale da ascendervi sopra, oltre che sono appoggiate al palazzo alla casa del signor governatore che gli va dietro et à quella del signor camerlengo le quali cioè il pallazzo è in molte parti ressentito con qualche pericolo, quella del signor governatore è del tutto rovinata, siche non si può più abitarla chi non la reffa tutta dalle fondamenta ne si può reffarla chi non reffà anco la muraglia, quella del signor camerlengo che continua dietro ad essa muraglia è in grandissimo conquasso et per l’accomodamento di questa è necessario l’istessa fatura come nelle altre, et chi non accomoda dette muraglie si core grandissimo rischio che tosto cadano et che la città resti aperta. molti alogiamenti poi delli soldati in cittadella sono caduti à terra, il luoco dalle monitioni da megli si è scoperto quasi tutto et in molte parti minatia rovina grande. al scoglio degli stradiotti sono cadute tutte le case, et il ponte che è di gran longezza è tutto portato via ma tutte queste cose convengono esser refatte et accomodate, pero mi è parso debito mio darne riverente conto a vostra sublimità perche possi prender quel espediente sopra di ciò che stimerà per sua prudenza necessario. l’istesso terramotto ha ancor fatto notabilissimo danno à castel novo che gli sono cadute quasi tutte le case della città, et buona parte delle muraglie con morte de molti per quanto si hà inteso, si che tutti li altri l’hanno abbandonata. di cattaro li 5 agosto 1608, tomaso contarini, rettore e provveditore asve (1608d) serenissimo prencipe [...] et in questa mia andata a cattaro lascierò una buona banda di gente in quelle parti, affine che non segua alcun inconveniente. grazie. in zara alli 24 agosto 1608, zangiacomo zane asve (1608e) serenissimo prencipe [...] hoggi sarà di ritorno la galea moceniga, et io mi imbarcherò per passar a cattaro conforme al comandamento di vostra serenità. in zara alli 28 agosto 1608, zangiacomo zane 697 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) asve (1608f) serenissimo prencipe ero quasi col piede in galera li di passati per trasferirmi a cattaro, conforme all’ordine di vostra serenità, quando venerdì notte che fu alli 29 del passato [ebbi un] parosismo di febre [...] in zara alli 5 settembre 1608, zangiacomo zane asve (1608g) serenissimo principe luni li 15 del presente alle 16 hore e mezza il terremoto non ancor satio di travagliare questa fortezza e di haver si lungo tempo tirato si fece sentir gagliardissimo con molto spavento e non poco pericolo di tutta la città, havendo rovinate molte case, et conquassate quasi tutte le altre, ma in particolare doi camere di questo palazzo, nelle quali dormivono quattro de miei figli, le ha redotte inhabitabili, et posso bene, e devo anco ringratiare infinitamente il signore iddio, che l’habbi mandato di giorno, si per il mio, come per il comune interesse, poiche se fussi occorso di notte, oltre la certa morte de miei, ne i letti, de quali caderno infinite pietre di non poca grandezza, sarebbero ancor stati ammazzati molti altri; che cosi ha solo causato la morte d’una povera vecchia, la quale (fuggendo per salvarsi) per il cadere d’alcune pietre fu accoppata, et ha ferito solo un soldato et il salinaro, ma di tutti doi si spera bene. et perche sotto di esse camere vi è la sala delle armi la quale è molto pericolosa, ho fatto pero levarle di quel luoco, e riponere in miglior stanza, fino che sarà accomodata. ha gettato a terra molte muraglie, ma spetialmente tre case di mercanti particolari sopra la piazza, et parte della casa, dove soleva habitare l’illustre signor governatore dalla quale si partì per il terremoto che tirò il giorno di san giacomo, il quale fu così gagliardo che quella tutta mosse insieme con il quartiero de soldati posto in cittadella e questo presente molto maggiore per quanto tutti questi signori affermano del primo l’ha del tutto rovinato, ne se li può rimediare, se non con il fabbricarlo di nuovo, il che è necessarissimo. ho però fatto venire molti huomini di queste ville, accio levino li coppi, le pietre, et il legname di esso che potesse esser buono da opera, perche in ogni occasione di fabbricarlo, li vada minor spesa. ancora ha cagionato la rovina della casa del carissimo signor camerlengo, nella quale, se non con evidente pericolo vi si può habitare, essendo tutta aperta, e quasi scoperta. ha fatto molto danno nel loco delli melgi [sic], havendo rovinato parte della muraglia, et il resto sta con assai pericolo, il quale cadendo eccederia à grave danno della serenità vostra, perche rovinerebbe affatto tutti li melgi, li quali non si sanno ove riponere per sicurezza, non vi essendo loco al proposito in questa fortezza ne del publico ne del privato. la muraglia sopra la marina per questo nuovo terremoto ha patito assai, et è ritirata in fuori molto. nel castello poi ha del tutto rovinato il coperto dell’artiglieria, redutto in pericolo il loco della monitione, conquassata la casa del carissimo signor castellano, fatto rovinoso il muro sopra la porta, scoperto tutto il corpo di guardia, et rovinate molte scale che conducono ad esso. qui si vive con molto spavento, e pericolo per il grandissimo danno causato da questi terremoti; et in vero non è cosa al mondo, che possi più atterrire gli animi humani, quanto questi flagelli, poiche in tutte le altre si di guera peste fame et altro, la prudenza, et autorità degli huomini vale non poco, ma in questa nulla giova. non si resta però di far che la città sij ben custodita, poiche e per la quantità dei soldati e per la molta devotione del signor governatore e di questi signori capitani verso la serenità vostra è compitamente guardata. tutto questo popolo ha posto ogni suo pensiero nel volere del onnipotente dio, per il che si fanno continue orationi e processioni per placar la sua ira, e certo che si vede gran conversione universale: onde voglio sperare che la divina bonta sia per riceverle e per l’avvenire per guardarci da questi mali irremediabili. subito, che fu gionto di qui il signor capitano angelo migliorati, mi presentò iuxta l’ordinario un mandato per polvere da distribuire alli suoi soldati per li loro bisogni accio lo sottoscrivessi, et vidi, che la polvere fina viene pagata ragione di soldi quattordici la lira, la quale so io, che molto più costa alla serenità vostra per tanto ho deliberato avvisarne gli illustrissimi signori sopra l’altelleria [...] 698 paola albini subito successo questo terremoto ho spedito un huomo a posta à castelnuovo per intendere il danno, che havesse cagionato in quel loco, il quale con fingere d’andarsene a visitare un suo barba, che habita in quella città procurò secretamente d’intendere ogni particolare, et ritornato mi referì come anco lì era stato gagliardissimo, e che ha conquassato tutta quella citta havendo rovinate delle tre parte una delle case, et gran parte della coltrina sopra il molo senza però offesa d’alcuna persona. il castello drento della città è tutto conquassato, et ha nella muraglia una apertura gagliardissima, per la quale potrebbero doi al paro entrarvi drento. la fortezza di sopra è rimasta intatta, nella quale vi ponno essere da dieci turchi; tutto quel popolo è ritirato alla larga fuori nelle loro ville, ne vi è nella città al presente alcuno, e certo che se da uno segno d’una nave ragusea non le fosse stato detto quattro giorni avanti, che tirasse il primo terremoto, che si dovessero salvare con accennarli che l’armata spagniuola di hora in hora doveva venire a svalligarli, e mettere tutta la città a sacco, che percio da ogn’uno fu abbandonata, sarebbero gran parte di essi stati ammazzati dal terremoto, e queste sono l’istesse parole refertemi dal messo. io sto assai allegramente, sperando nell’aiuto della divina bontà, e molto meno penserei di queste rovine, se di qui mi trovassi solo, e senza famiglia, benche per causa di questi così spaventosi terremoti m’habbi convenuto ritirar con tutti li miei sopra un baluardo non già per pericolo delle camere della mia habitatione, le quali sono sicurissime e cadendo queste si profonderia questa città, ma bensì per schivar la rovina della torre dell’horologio, che pende sopra esse camere, ne è molto sicura, minacciando da ogni parte rovina. starò vedendo qualche giorno quello faranno questi muri e poi procurerò di rimediari al meglio si potrà. ho giudicato però esser bene far del tutto la serenità vostra riverentemente consapevole, acciò con il suo prudentissimo giuditio ordini quello si haverà à fare. non essendo neanco sino a’ hora venuto di qui l’eccellentissimo generale zan, che pur si lasciò intendere di dover venire dal principio del mio reggimento per provvedere a’ quello facessi bisogno; ma trattenuto da alcune sue indisposizioni non si crede perciò che per hora possi venire. significandoli in oltre con l’istessa mia reverenza com’io andrò rimediando a quelle cose, che stimerò esser bisognose, come nell’habitatione de soldati, et altro, nelle quali spese vi potriano andare da cento ducati incirca. restando la serenità vostra sicura che in tutte le cose procurerò ogni avantaggio publico, come è mio debito, e molto più di quello, se fussi mio particolare interesse. stimerei però essere anco bene che vostra serenità desse ordine, che per li bisogni di queste fabbriche fossero mandati di qui, dieci, o dodici mila coppi, e doi, o tremila tavole. rimettendomi però in tutto, e per tutto alla somma sua prudenza, la quale ho voluto con ogni reverenza di ciò avvisare per anticipare il tempo, essendo sicuro che dell’eccellentissimo provveditor generale zan le sarà per l’istesso effetto scritto. grazie. di cattaro alli 19 settembre 1608, francesco morosini rettor e proveditor asve (1608h) serenissimo prencipe [...] intendo per lettere da cattaro che il terremotto fa sempre maggior progresso essendosi sentito di novo molto grande alli 15 del presente, che ha fatto gravissimi danni nella fortezza, et nel castello ancora, onde mi duole in estremo di non haver modo di potermene passare in quella città, come mi comandò già la serenità vostra, ma sa il vostro dio che non ho mai avuto commodo di poterlo fare, essendo prima stato impedito dalla indisposizione dalla quale non sono ancora intieramente ricuperato, et poi dal passaggio di questa principessa che mi ha levate le galere [...] in zara alli 25 settembre 1608, zangiacomo zane asve (1608m) [...] che dire a vostra serenità se non che il terremoto si fa ancora sentire, et mercore li sedeci del presente ne tirorno tre ma non molto gagliardi, niente di meno hanno apportato assai confusione in questa città, con tutto che da quel giorno in qua non se n’habbino più sentiti. [...] di cattaro li 23 decembre 1608, francesco morosini 699 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) asve (1609a) serenissimo prencipe sono da giorni dieci incirca che per gratia del signore si vive da qui alquanto consolati per non sentirsi più terremoti li quali tanto afligevano questa città; ma hora bene si incominciano manifestamente a vedere di quanto danno sijno stati à questo loco, essendo che li giorni passati per il conquassamento da essi cagionato et per le molte pioggie che sono state caderno tre case, ma senza alcun’offesa di persone et cosi questa fortezza per il passato in molte parti desolata viene anco con questi novi danni non poco destruta poiche quelle fabriche una volta cadute ò rovinate non vengono più da questi riedificate. et quando io sperava di poter con qualche consolatione governare questi popoli vengo da molte altre bande di cattivi avisi oppresso, essendo che corre voce che nella villa di corenichi sopra di pisano et lontana da esso loco miglia dieciotto si è scoperto il mal contagioso et che in una settimana sono morti da esso male cinque persone. [...] di cattaro li 6 gennaio 1609, francesco morosini rettor e proveditor asve (1609b) serenissimo prencipe arrivai in questa città con felice navigazione à 13 del presente, [...] all’incontro dentro della citta, ha fatto gran rovine il terremotto, et molte fabriche pubbliche hanno patito estremamente, onde si come ho ordinato ad esso capitano cesare, che prenda nota, et faccio disegno diligente, e distinto di ogni cosa, così prima che io parta che sarà fra doi ò tre giorni, credo di dar ordine che sia principio a risarcire le cose più necessarie, et che non patiscono dilazione, come sono alloggiamenti de soldati, de quali un quartiero è interamente caduto a terra, magazeni de munizioni, casa del governatore et alcune rovine nel castello di sopra, ove andai l’altro giorno, et medesimamente il ponte al scoglio de stradiotti che io vidi, e dove smontai nel passare, che è rovinato del tutto, et che è necessarissimo; di tutto che poi con piu commodo e con maggior chiarezza, e distinzione darò conto a vostra serenità et tra tanto invio con le presenti nota particolare de legnami, ferramenti et copi che fanno bisogno per parte di esse fabriche, li quali le piacerà di mandar quanto prima. [...] in cattaro alli 20 gennaro 1609, zangiacomo zane asve (1609c) serenissimo prencipe sabato passato che fu li 24 del presente io partii da cattaro et havendo prima con esquisita diligenza vedute, et essaminate quelle rovine per occasione del terremotto, fù concertato fra l’illustrissimo proveditor et me, che immediate egli dovesse por mano à ristaurare il quartiero de soldati della cittadella, li magazeni delle munizioni, il ponte al scoglio dei stradioti, et quegli alloggiamenti; et alquanto di muraglia in castello che è risentita, et minaccia di cadere, perche queste sono operazioni grandemente necessarie, et non patiscono dilazione, et havendo io già scritto alla serenità vostra colla fregata ordinaria la quantità de’ legnami, ferramenti et copi, che mi bisognano, mentre che tarderanno à comparire, attenderà s.s. illustrissima à far fare la provisione delle altre materie di calcina, et pietre, et à tirar su i muri dove fanno bisogno, percioche al quartiero de soldati per ritornarlo habitabile si dovverà rifar il coperto, et i pavimenti, et passa 118 incirca di muraglia. a i magazeni si ha da rassettar i coperti, et farli circa 70 passa di muraglia. et al castello medesimamente bisognerà rifar circa 30 passa di muro; et al scoglio de stradiotti sicome medesimamente potrà s.s. illustrissima applicarsi immediate nel risarcimento di quelli alloggiamenti per li quali è bisogno di poco muro, così potrà anco attendere alla refattione del ponte, essendovi li legnami, et la ferramenta et non mancandovi altro che li palli, de quali si potrebbe provvedere parte nel medesimo territorio di cattaro, e parte nell’albania, ma perche questi dell’albania che dovevano esser al numero di 110 80 de quali doveranno essere di longhezza di passa 5173 et 30 di passa 6, per le informazioni che si hanno, potranno costare tre scudi l’uno con la conduttura, et crede l’illustrissimo signor proveditor che quando la serenità vostra gli mandasse [...], si spenderia molto meno, doven700 paola albini domi io sopra ciò riportare a s.s. illustrissima vengo à dire à vostra serenità che esso signor provveditore stava aspettando in questo proposito il comandamento suo. le altre parti, che hanno bisogno di essere accommodate, sono la muraglia della città, il palazzo del rettore, la casa del signor camerlengo e quella del signor governatore et fabrica di altri magazeni, et alloggiamenti circa; che essendo noi conformi di opinione come per avventura le sarà scritto anche da s.s. illustrissima resterà che da vostra serenità sia comandato ciò che le piacerà che si faccia. la muraglia risentita è in quella parte che guarda sopra’l molo, et comincia a piegarsi dodeci passa distante dal fianco della cittadella, et continua per 40 passa di longhezza; i primi 25 passa pendono all’infuori un piede e mezo: doi: e fin doi e mezo: et il restante pende un piede e mezo et uno: et se bene si può quasi dire che essa muraglia non habbia patito se non dalla mettà in su, è nondimeno opinione nostra per maggior sicurtà che si cominci a rifabricarla dal piano naturale per tutto questo corso di 40 passa, alzandola al segno che ella è al presente di piedi 27, ma facendola nel fondo di grossezza de piedi cinque, che venga a poco a poco ristringendosi, cosi che nella sua sommità resti piedi 4 cioè con mezo piede di scarpa di fuori, et mezo di dentro; il che apporterà anco maggior fermezza, et stabilità ad essa muraglia, perche quanto al fabricar sopra i fondamenti vecchi, si come si tiene per fermo da ogn’uno che essi non si siano mossi punto, così si conclude, che essendo sopra palli non è dubbio alcuno, che siano più sicuri, che se si facessero da novo. et perche in questa parte che è appo la porta vi è il volto, ove alloggia il corpo di guardia, il qual volto fa spalla nella muraglia, si giudica che sia meglio non fermarsi qui il volto, ma solo il pavimento, che servirà molto meglio, perche in occasione di altro terremotto non sarà così facile à conquassarsi; et con l’accomodamento di questa muraglia si torna in pristino anco il palazzo del signor rettore, il quale non ha patito altro, se non che essendosi allargata la muraglia, e disgionte da essa le travature de pavimenti, hanno minacciato di cascare, mancatole il suo appoggio. la casa del signor camerlengo che ha sotto di lei alcuni magazeni da sale, è fuor di modo risentita nelle muraglie dal pavimento in su, ma le muraglie inferiori non hanno patito niente, in modo che si è giudicato buon consiglio, provedendo al signor camerlengo di altra casa, abbassar il coperto di quella, che senza altra spesa servirà all’uso di prima per i sali. per l’habitatione poi del signor camerlengo et del signor governatore si è pensato di comprar alcuni fondi de particolari contigui alla muraglia, che sono tutto quel tratto, ch’è tra il palazzo del signor rettor, et la casa del signor camerlengo et oltre di ciò alcuni altri posti all’incontro di spazio di passa 47 quadri, tutti i quali si come non costeranno più di 300 toleri in circa et daranno commodità non solo di fabricarvi le dette case, ma anco de’ magazeni per il pubblico servizio, di che vi è di bisogno, così scosteranno le persone private dalla muraglia, alla quale non è bene che habbino congionte le case. havemo appo considerato che la torre, ove sta riposta la polvere per l’artiglieria, è in sito tale, che si è scoperta da ogni parte, et sottoposta ad ogni ingiuria et machinazione di chi si sia, onde sarebbe a giudicio nostro, molto più sicuro ch’ella fosse riportata in una piazzetta, ch’è vicina alla sentinella, et al quartiero della porta del gordichio [sic], che ivi starebbe senza tanto pericolo, et fuori di prospettiva. queste sono tutte quelle cose che mi sono parse degne di considerazione et di provvisione delle quali non havendo potuto dar conto particolare a vostra serenità con le mie che le scrissi da cattaro sotto li 20 del presente non ho voluto tardare a farla consapevole del tutto, affine che essendo tutte materie di molto momento, ossi ella colla sua somma prudenza deliberar tosto ciò che si doverà fare, poiche da quell’illustrissimo signor rettor che è diligentissimo et volto tutto al publico servizio, sarà con ogni industria, e sollecitudine esseguito quanto le sarà comandato da lei, dovendo egli fra tanto andar preparando tutte quelle circonstanze, che possono esser necessarie a queste operazioni. grazie. curzola à 30 gennaro 1609, zangiacomo zane asve (1609d) [...] il terremoto di qui si fece sentire la sera di s. paulo et da quell’hora in qua si è acquietato, onde voglio credere che haverà preso buona licenza, che il signore ce la concedi. grazie. di cattaro li 14 febbraio 1609, francesco morosini rettor e proveditor 701 a survey of the past earthquakes in the eastern adriatic (14th to early 19th century) – records on 21 september 1780 earthquake asve (1780) serenissimo prencipe [...] quell’illustrissimo estraordinario [di cattaro] con sue lettere 12 ottobre decorso spiega diffusamente il massimo terrore cagionato in que’ sudditi, e li gravissimi danni che derivarono nelle private e in tutte le pubbliche fabbriche, chiese, e conventi da una gagliarda scossa di terremoto, verificatasi nel giorno 21 settembre antecedente, e mi accompagnò con molti fabbisogni le lettere pure de’ nobil huomini rappresentanti di castel nuovo, budua, rettore di cattaro e governator dell’armi, li quali ragguagliano i medesimi mali anche in quelle piazze, non senza grave danno come essi rappresentanti asseriscono nelle case di loro abitazione, ed effetti, supplicando anzi (massime quello di budua che partecipa aver dovuto ritirarsi in altra casa di maggior affitto) che nel generale della sciagura siano tolte in riflesso anche le particolari loro convenienze. accenna inoltre lo stesso estraordinario che attesa l’indispensabile necessità ha dovuto far seguire sul momento provisionali ristauri dei luochi pubblici in cattaro, tra quali il riffacimento d’un pezzo di mura per piedi 18 di antica costruzione che rovesciò dalla parte del castello verso montenero, e che per particolari politici riguardi esiggeva, com’egli scrive, istantaneo riparo. in così dolorosa avventura, e veramente seria secondo i rapporti che mi derivano, ho colto l’opportunità del tenente d’ingegneri gaetini, che attrovavasi a quella parte per altre pubbliche esiggenze, a cui ho rilasciata commissione immediata di riconoscer gli esposti danni e di estender i relativi fabbisogni, che mi riservo di far tutti presenti all’eccellenze vostre tosto che li abbia con quelle avvertenze e previsioni che si convengono. [...] zara li 10 dicembre 1780, paolo boldrin, provveditore generale in dalmazia e albania asve (1781) serenissimo prencipe [...] già ho prevenuto l’eccellentissimo senato con il riverente mio dispaccio n. 10 [asve, 1780] di aver commesso al medesimo gaettini [sic], coll’opportunità, che attrovavasi a quella parte, di dover riconoscere anche i danni cagionati nella pubbliche fabbriche di quelle piazze dalla scossa del terremoto seguita nel giorno 21 passato settembre, sicché in riscontro della prestata obbedienza mi presentò pure tutti quegli altri fabbisogni, che accompagno, li quali riguardano la necessità di ristauri nelle fabbriche stesse, esclusivamente da quelle di budua, ove per combinazioni de’ tempi non si trasferì, e da quelle ancora, che, come rassegnai nel dispaccio medesimo, fece ristaurar tra le più bisognevoli il nobil huomo estraordinario quasi su lo stesso momento. in quel medesimo incontro osservò lo stesso ufficial ingegnere, e assicura con la tutta l’asseveranza nella stessa sua relazione lo stato cadente, e pericoloso della pubblica loggia di cattaro, a tal segno, che si mossero que’ giudici di far pervenire a questa carica con lettere di quel nobil huomo rettor l’annessa supplicazione, che io non so dispensarmi, secondando l’istanza, e in vista delle circostanze, di rassegnare a vostra signoria [...] zara li 30 gennaio 1780 more veneto, paolo boldrin, provveditore generale in dalmazia e albania references adamovic, v. (1884): o tresnjama grada dubrovnika, biblioteca storica della dalmazia, dubrovnik, libro 7, 1-44. albini, p. (1999) venetian documents on earthquakes within and at the western borders of the ottoman empire (17th18th centuries), in natural disasters in the ottoman empire, edited by e. zachariadou, institute for mediterranean studies, halcyon days in crete iii, a symposium held in rethymnon (greece), 10-12 january 1997, 6788. albini, p. and m. stucchi (1997): a basic european earthquake catalogue and a database for the evaluation of long-term seismicity and seismic hazard (beecd), in 702 paola albini seismic risk in the european union, edited by a. ghazi and m. yeroyanni (brussel-luxembourg), vol. 1, 53-77. annales ragusini (15th cent.): annales ragusini anonymi item nicolai de ragnina, in monumenta spectantia historiam slavorum meridionalium, edited by s. nodilo (zagreb, 1883), vol. 14, scriptores, vol. i, 1-328. asve (archivio di stato di venezia) (1559-1581): senato, dispacci, provveditori da terra e da mar e altre cariche, provveditore dell’armata, b. 1196. asve (archivio di stato di venezia) (1563): senato, decreti, mar, 5 march-28 august, f. 28. asve (archivio di stato di venezia) (1608a,d-f,h): senato, dispacci, provveditori da terra e da mar e altre cariche, b. 423, 1608-1610: a) zangiacomo zane, dispatch, zara 20 june; d) zangiacomo zane, dispatch, zara 24 august; e) zangiacomo zane, dispatch, zara 28 august; f) zangiacomo zane, dispatch, zara 5 september; h) zangiacomo zane, dispatch, zara 25 september. asve (archivio di stato di venezia) (1608b,g,m): senato, dispacci, rettori, dalmazia, b. 7, 1607-1609: b) tomaso contarini, dispatch, cattaro 5 august; g) francesco morosini, dispatch, cattaro 19 september; m) francesco morosini, dispatch, cattaro 23 december. asve (archivio di stato di venezia) (1608c): senato, decreti, mar, june-august 1608, deliberation, 12 august, f. 180. asve (archivio di stato di venezia) (1609a,d): senato, dispacci, rettori, dalmazia, 1607-1609, b. 7: a) francesco morosini, dispatch, cattaro 6 january; d) francesco morosini, dispatch, cattaro 14 february. asve (archivio di stato di venezia) (1609b,c): senato, dispacci, provveditori da terra e da mar e altre cariche, b. 423, 1608-1610: b) zangiacomo zane, dispatch, cattaro 20 january; c) zangiacomo zane, dispatch, curzola 30 january. asve (archivio di stato di venezia) (1651): senato, dispacci, rettori, corfù, b. 27, 1649-1651: girolamo contarini dispatch n. 21, corfù 21 february (1650 more veneto) 1651 s.v. asve (archivio di stato di venezia) (1667): senato, dispacci, provveditori da terra e da mar e altre cariche, b. 497: caterino cornaro, dispatch n. 247, zara 24 september. asve (archivio di stato di venezia) (1674a): senato, dispacci, rettori, corfù, b. 32, 1668-1677: giacomo nani, dispatch n. 62, corfù 21 gennaio (1673 more veneto) 1674 s.n. asve (archivio di stato di venezia) (1674b): senato, provveditori da terra e da mar, provveditore generale da mar, andrea valier, 12 ottobre 1671-26 novembre 1675, b. 941; andrea valier, dispatch n. 73, 21 gennaio (1673 more veneto) 1674 s.n. asve (archivio di stato di venezia) (1674c): senato, dispacci, rettori, corfù, b. 32, 1668-1677: gritti, dispatch n. 2, corfù 10 giugno 1674 s.n. asve (archivio di stato di venezia) (1780-1781): senato, provveditori da terra e da mar, provveditore generale in dalmazia e albania, b. 641, september-august 1781, nn. 1-38: paolo boldrin, dispatch n. 10, zara, 30 december 1780; paolo boldrin, dispatch n. 16, 30 january 1781. basovic, r. 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(1904): the republic of ragusa. an episode of the turkish conquest (london), pp. 424. 321_329 ferretti.pdf 321 annals of geophysics, vol. 46, n. 2, april 2003 the role of the high resolution weather forecast in estimating the run-off using a simple hydrological model rossella ferretti (1), tiziana paolucci (2) (*), livio bernardini (2) (*) and guido visconti (1) (1) dipartimento di fisica, cetemps, università dell’aquila, coppito (aq), italy (2) parco scientifi co e tecnologico d’abruzzo, università dell’aquila, coppito (aq), italy abstract recent and repeated episodes of severe weather in italy have stressed the need to have a suffi ciently accurate forecast to give adequate warning to the involved areas. the impact of the precipitation, however, is also a function of the characteristics of the hydrological basin. from this point of view, a rather startling example is the disaster which hit the campania region on 5th may, 1998 in which a moderate precipitation (about 100 mm in 24 h) produced a huge landslide which killed or injured several tens of people and produced serious damage to the area. such localized events require among other things the use of a high resolution weather forecast. in this paper, a forecast of the campania event using a limited area model at 3 km grid resolution is presented. the forecast rainfall at several grid resolution is used to initialize a simple hydrological model to estimate the run-off. the numerical experiments suggest that high resolution may be a key factor in predicting the run-off. mailing address: dr. rossella ferretti, dipartimento di fisica, cetemps, università dell’aquila, via vetoio, 67100 coppito (aq), italy; e-mail: rossella.ferretti@aquila.infn.it key words high resolution forecast heavy precipitation 1. introduction the precipitation forecast over complex topography is still a challenging problem. the mediteranean area is often affected by catastrophic events related to heavy precipitation. in the last 10 years more than one fl ood hit italy producing severe damage. during 2-5 may 1998, cyclogenegesis over the tyrrhenian sea produced rainfall over campania, a region of central italy (fi g. 1); the rainfall caused a severe landslide destroying several villages and killing many people. to prevent these events, both a good forecast of the precipitation and knowledge of the characteristics of the terrain are necessary. most of the italian weather forecasts are made using hydrostatic models reaching resolutions lower than 9 km. a few studies using this resolution for the model simulation showed good skill for the precipitation forecast but at times missing the location of maximum rainfall (paccagnella et al., 1992, 1995). the hydrostatic version of the mm5, from penn state/national center for atmospheric research (psu/ncar), also showed good skill for the precipitation forecast, but the location of the maxima of the rainfall were not correct (ferretti et al., 2000b), whereas the same event analyzed using the non-hydrostatic version of the mm5 showed better results (paolucci et al., 1997). a study of the piedmont fl ood using the non-hydrostatic version of the mm5 showed good skill for both the precipitation amount and the extent of the rainfall (ferretti et al., 2000a). on the other hand, a comparison between the mm5 score and a few other models (richard, (*) present address: cetemps, università dell’aquila, via vetoio, 67100 coppito (aq). 322 rossella ferretti, tiziana paolucci, livio bernardini and guido visconti 1998) revealed a poor skill by the mm5, but the resolution used for that study was 10 km. recently, in the framework of cooperation between university of l’aquila and scientifi c and technology park of abruzzo the mm5 has been made operational, being the only high resolution non-hydrostatic model used for weather forecast in italy. the probability of occurrence of post events (landslides and avalanches), like the one analyzed in this study, is known to be correlated with complex statistical moments of the meteorological variables (in this case precipitation) besides, of course, the structural parameter, like ground conditions. therefore, a high resolution forecast of the meteorological variables may help to forecast these events. an attempt to forecast the hydrological impact of this event is presented: the run-off, related to the heavy precipitation of this case, using an off line scheme for the soil is computed. the aim of this paper is to show how a high resolution weather forecast may help to correctly forecast a few hydrological parameters. for this purpose, sensitivity tests to the grid resolution are carried out using a non-hydrostatic model. the model results are used to initialize a simple hydrological scheme, the biosphere-atmosphere transfer scheme (bats). the meteorological characteristics of this event and the precipitation recorded by a high resolution rain gauge are presented in section 2; the experimental set up for both mm5 and bats is discussed in section 3. the results are presented in section 4 and the conclusions are given in section 5. 2. meteorological characteristics this event was characterized by cyclogenesis over the mediterranean sea a deep low level depression entering this region from the south intensifi ed over the tyrrhenian sea on 3rd may, and reached a minimum of 990 hpa by 4th fig. 1. model domains: domain 1 has grid resolution of 27 km, domain 2 of 9 km and domain 3 of 3 km. 323 the role of the high resolution weather forecast in estimating the run-off using a simple hydrological model fig. 2a,b. ecmwf data analyses for 4th may at 0000utc: a) sea level pressure (c.i. = 2 hpa) and wind (one barb = 10 knots) at 925 hpa; b) 500 hpa-height fi eld (c.i. = 30 m) and wind (one barb = 10 knots). b a 324 rossella ferretti, tiziana paolucci, livio bernardini and guido visconti a 10e 10egm 20e fig. 3a,b. as fi g. 2a,b but for 4th may at 1800utc. b 325 the role of the high resolution weather forecast in estimating the run-off using a simple hydrological model may at 0000 utc (fi g. 2a). the cyclone was associated with a cut-off low (fi g. 2b) which swept over the mediterranean area. during its passage, the axis rotated clockwise producing a westerly fl ow both at upper and lower levels by the late afternoon (fi g. 3a,b). advection of warm and humid air persisted for the rest of the day over most of southern italy. the day before the event, the local rain gauge recorded light precipitation over the disaster area, reaching about 75 mm in 24 h (not shown). during 5th may the rainfall intensifi ed both over this area and south of it (fi g. 6, the rain gauge data are superimposed to the model results). however, the amount of rain was less than half of that recorded during the piedmont fl ood of 1994, which was a major severe weather event. therefore, it makes sense to assume that the damage produced by precipitation is strongly related to the nature and morphology of the terrain for this event, besides the correlation with the meteorological variables as already pointed out. 3. numerical experiment set up 3.1. mesoscale model description the mm5 (psu/ncar) is a non-hydrostatic limited area model fully compressible at the primitive equations (dudhia, 1993; grell et al., 1994). to enhance to forecast over the area of the slide, three domains two-way nested and 24 vertical levels (sigma coordinate) unequally spaced are used. the mrf (troen and marth, 1986; hong and pan, 1996) planetary boundary layer parameterization and the grell (grell, 1993) cumulus convection parameterization associated with an explicit computation of rain water for domains 1 and 2, whereas only the last one for domain 3, are used. the forecast is made using three domains, the fi rst domain (1) has a resolution of 27 km and contains 50 (latitude) ¥ 66 (longitude) grid points; the intermediate domain (2) has 9 km resolution and 58 ¥ 67 points and the high resolution domain (3) has 61 ¥ 58 grid points with a 3 km grid size. to enhance the local forcing by the soil a 1 km resolution land use data (usgs) specifi es the vegetation and the soil characteristics. model domains grid size (km) acronimus mm5 1 27 d11 mm5 2 27 9 d21 d22 mm5 3 27 9 3 d31 d32 d33 table i. the model simulation performed using different nested levels. d11 for model simulation using one domain only. d21,2 for model simulation using two domains. d31,2,3 for model simulation using three domains. a few model simulations are made to verify the sensitivity of the run-off to the grid resolution of the mm5. a simulation using only one domain (d11) at 27 km is performed, and the results are compare with both the observations (obs) and the results of domain 1 (d21) obtained by using two domains two way nested at 27 km and 9 km respectively for domains 1 and 2. a last simulation performed using three domains (d3) respectively at 27, 9 and 3 km for domains 1, 2 and 3 is performed. the d3 results are compared with both the observations and the previous simulations: domain 2 of two domains only (d22) is compared with domain 2 of three domains (d32), and fi nally d33 (domain 3) is compared with the obs only (table i). all the mm5 forecasts are initialized by using the european center for medium-range weather forecast (ecmwf) data analysis and the boundary conditions are upgraded every 6 h by using ecmwf forecast. 3.2. hydrological model description the mm5 output is coupled off line with the biosphere atmosphere transfer scheme (bats) to evaluate the run-off expected from the precipitation. bats can predict a number of soil variables based on the specifi cation of soil characteristic and vegetation cover. the bats (dickinson et al., 1986) estimates the transfer of momentum, heat and moisture 326 rossella ferretti, tiziana paolucci, livio bernardini and guido visconti between the surface and the atmosphere when the boundary meteorological conditions are specifi ed, it also computes soil moisture content and run-off. in the past, this scheme has been coupled to the mm4, the hydrostatic version of the mm5, by giorgi (1989) for climate studies. bats is applied to the landslide area and it is initialized by using both the observations and the mm5 precipitation, temperature and wind forecast for 5th may. the 1 km land-use for the mm5 allow for estimating the vegetation type in the slide area: cropland and woodland mosaic with urban and built-up land in the nearby area is mostly the soil type of this region. 4. results 4.1. mm5 precipitation forecast two operational forecasts covering the whole period were made: one starting at 1200utc on 3rd may and lasting for 48 h; the second one starting at 1200utc on 4th may and ending at 1200utc on 6th may. the results of both of them, for the experiments in table i, are used for initializing the bats. for each run the fi rst 12 h are discarded in order to take into account the spin up time, therefore, the two forecasts have an overlapping period from 0000utc of the 5th to 1200utc of the same day. the obs are available from 0000utc of the 4th to 0000utc of the 6th. the comparison between the model rainfall and the observations is now presented. to ensure a multi-scale analysis all the results of the model on domain 1, at the same resolution but using different nesting levels, are compared with the obs. the same comparison is performed for domain 2. the comparison between the 24 h accumulated precipitation for d11 (fi g. 4a) and the obs (fi g. 6) shows a large underestimation of the rainfall, furthermore the location of the maximum is too far inland. if higher resolution associated with the two way nesting for domains 1 and 2 (d22) is used, a measurable improvement both in the total amount of rain and in the position of the maximum is found (fi g. 4b). indeed, the most intense precipitation has now moved toward the coast. sarno, the town most affected by the fl ood, is located in this area. the differences between d11 and d21 are produced by the feedback process due to the two-way nesting technique; indeed, in this case a feedback from the fi ne mesh to the coarse resolution ensure the smallscale forcing to affect the large scale. the rainfall produced on domain 2 (d22, fi g. 5a), which feeds back to domain 1, is closer to the obs (fi g. 6) than both d11 and d21. this imply a large gain in the precipitation forecast by increasing the resolution only. if three domains two way nested are used, the rainfall on domain 2 (d32) is further improved (fi g. 5b). it can be easily noted that there is a considerable improvement in the rain predicted in both cases d22 and d32 with respect to domain 1. a direct comparison between the high resolution rainfall (d33) and the obs is now presented. figure 6, where the rain gauges data fig. 4a,b. 24 h accumulated precipitation (mm) at ending at 0000utc 5th may predicted on domain 1 by using: a) one domain at 27 km grid resolution; b) 2 domains two-way nested at 27 and 9 km resolution respectively. a b 327 the role of the high resolution weather forecast in estimating the run-off using a simple hydrological model are superimposed to the model prediction, shows that the rainfall in the southern area agrees reasonably well with the d33 results, with peak values around 100 mm in 24 h. on the other hand, the model seems to underpredict the rain in the southernmost region while the northern patch of rain seems an almost complete artifact. the rain produced by the model is strongly infl uenced by the updraft forced by the orography and this may very well be the cause of the northern patch of rain. the 24 h accumulated precipitation recorded (not shown) light precipitation starting from 1st may and decreasing by the end of 3rd may; during 4th may a maximum of the rainfall reaching 84.6 mm in the campania region was recorded, whereas during the day of the slide 103 mm are reached south of the slide area. in the disaster area only 60 mm are reached. d33 shows a similar pattern to the obs, both for the amount and the distribution of the precipitation, reaching values higher than 50 mm. the day before the catastrophe light precipitation occurred reaching 24.4 mm and doubling during the following 24 h. during the fi rst day, the precipitation forecast (not shown) produced 25 mm, and it reached 50 mm the day after (fi g. 6), showing a correct tendency. these results confi rm the good skill of the model in forecasting precipitation even at very high resolution. indeed, the model forecast shows a very good agreement with the high resolution precipitation data on the fi ne mesh; the precipitations are highly localized, and the maximum of the precipitation is close to the hill where the landslide occurred. 4.2. run-off results an attempt to estimate the run-off using bats is now presented. the observed rainfall is used to estimate the run-off that will be used fig. 5a,b. as for fi g. 4a,b but on domain 2 using a two way nesting: a) between domain 1 and 2 at the same resolution of fi g. 4b; b) between domain 2 and 3 at 9 and 3 km resolution, respectively. b a fig. 6. 24 h accumulated precipitation (mm) ending at 0000utc 5th may both predicted on domain 3 using three domains two way nested and observed at a few stations (colored circles). the circles are fi lled with the color corresponding to the scale shown on the right. 328 rossella ferretti, tiziana paolucci, livio bernardini and guido visconti as reference (obs). the run-off produced by bats using the rainfall from d11, d22 and d33 is compared with obs. to ensure a correct response by bats to the precipitation event the simulation started a few day before the disaster to allow the soil to reach the right amount of soil moisture. in fact, the day before the disaster precipitation occurred making the soil wet. the run-off produced by this precipitation is shown on fi g. 7a-d before 3rd may, but it is not real. the comparison between the run-off produced by both the observed and the forecast rainfall shows (fi g. 7a-d) that as the resolution increases the signal get closer to the real one. indeed, the rainfall obtained using one domain only produces a strong underestimation of the run-off (fi g. 7b), only a weak signal during may 5th is obtained. if the resolution is increased, that is the rainfall from d22 is used to estimate, the run-off (fi g. 7c), a slight improvement is obtained. finally, if higher resolution is used, the run-off (fi g. 7d) is in good agreement with that obtained forcing bats with the observations (fi g. 7a). the maximum run-off is still underestimated but the bimodal structure is clearly reproduced by the high resolution rainfall. it is clear from these results that increasing the weather forecast resolution the right amount of run-off is obtained. fig. 7a-d. the run-off in mm/days from 1 to 7 may 1998, obtained when bats is coupled to: a) the observations (obs); b) the rainfall produced by the model simulation performed using one domain only at 27 km; c) the rainfall produced by the model simulation performed using 2 domains with the highest resolution being 9 km; d) the rainfall produced by the model simulation performed using three domains with the highest resolution being 3 km. c b d a 329 the role of the high resolution weather forecast in estimating the run-off using a simple hydrological model 5. conclusions the mm5 forecasts and the run-off produced by bats show that increasing the resolution a large improvement in both the rainfall and the run-off is obtained. at low resolution the rainfall is largely underestimated and the maximum of the precipitation is displaced. increasing the resolution (up to 3 km), both a good precipitation forecast and an estimation of the run-off in very good agreement with the one produced using the observed precipitation are found. the high resolution rainfall forecast is in good agreement with the observations, but a model tendency to overestimate the rainfall, if strong uplifting is present as for the mountain regions, is found. however, d33 is the only one able to separate areas with different rainfall. these results suggest that the high resolution is required to correctly evaluate the hydrological impact. indeed, even using a simple hydrological model the role of the grid resolution in the rainfall forecast is highlighted. acknowledgements this work has been partly supported by parco scientifi co e tecnologico dell’abruzzo and by gndc (gruppo nazionale grandi catastrofi ). servizio idrologico of campania region is also acknowledged for the rain gauge data. ncar is acknowledged for the mm5 model. references dickinson, r.e., a. henderson-sellers, p.j. kennedy and m.f. wilson (1986): biosphere-atmosphere transfer scheme (bats) for the ncar community climate model, ncar/tn-275+str, national center for atmospheric research, boulder, co, pp. 69. dudhia, j. 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(1993): prognostic evaluation of assumption used by cumulus parameterization, mon. weather rev., 121, 764-787. grell, g.a., j. dudhia and d.r. stauffer (1994): a description of the fifth-generation penn state/ncar mesoscale model (mm5), ncar/tn-398+str, national center for atmospheric research, boulder, co, pp. 117. hong, s.-y. and h.-l. pan (1996): non local boundary layer vertical diffusion in a medium-range forecast model, mon. weather rev., 124, 2322-2339. paccagnella, t., s. tibaldi, r. buizza and s. scoccianti (1992): high-resolution numerical modeling of convective precipitation over northern italy, meteorol. atmos. phys., 50, 143-163. paccagnella, t., p. patruno and c. cacciamani (1995): operational quantitative precipitation forecast of the piedmont fl ood event at the regional meteorological service of emilia-romagna, map newsl., 2, 7-11. paolucci, t., r. ferretti and g. visconti (1997): numerical simulations of summer convective precipitation cases in the alpine region using different parameterizations: preliminary results, preprints, seventh psu/ncar mesoscale model user’s workshop, 21-23 july 1997. richard, e., j. charney, s. cosma, r. benoit, s. chamberland, a. buzzi, l. foschini, r. ferretti and j. stein (1998): intercomparison of simulated precipitation fields for some map episodes, map newsl., 9, 12-13. troen, i. and l. mahrt (1986): a simple model of the atmosphere boundary layer: sensitivity to surface evaporation, boundary layer meteorol., 37, 129-148. das skalare, d u r c h einen rechteckigen iinpuls in einem h o m o g e n e n m e d i u m erzeugte potential (erste naherungsrechnuiif») a . b e l l u i g i wir wollen vorerst das problem der erregung des erdbodens mit impuls-stròmen grundlich untersuchen, um die messung der « lueckenpotentiale », (praktische anwendung des elflex), durchfuhren zu konnen. die autoren, die sich in der vergangenheit mit dieser methode beschàftigt haben, weisen zwar auf die anwendung der impuls-stròme hin oline sie jedoch in ihren theorien iiberhaupt zu berucksichtigen. dies bedeutet gleich von anfang an eine schwerwiegende unzulangliclikeit, da dadurch die « periodizitàt der lùcken » ausfallt. es seien: r die ladungsdichte, <z> das skalare potential, /li die magnetische permeabilitat, le die dielektrizitàtskonstante, a die leitfàhigkeit. dies vorausgestellt, erfiillt die ladungsdichte (z1), wie wir bereits gesehen haben, in einem homogenen boden, folgende gleichung: mit der ladungsdichte y fur t = 0; fùr t = t ist f = y/2,718. andererseits genùgt das « skalare potential » folgender gleichung: b i n a t = 0 , ì>t [1] wobei a = ajk , 1/a = r und somit: r = ye-faa(== ye~intlt [2] [3] die infolge gì. [2] und wenn man die beziehung v — i/i7ih (signal1 5 0 a . b e l l u i g i geschiwndigkeit) berucksichtigen will, folgende form (ftìr t > 0) annimmt: 1 w ^ttct -ina! a 0 _ _ = — y e . [3 ] v2 a*2 v2 -òt h y l j setzt man: 0 : rpe~2 7 l a t [4] so verwandelt sicli gì. [3'] in folgende: . 1 52w 4 ti2 ain _ 9 w zj w — h — w — — — y e znax . [5] r "2 m2 v2 y a: l j wenn man annimmt, das (a = ajh) genùgend klein ist, sodass die gròssen (a2)-ter ordnung vernaclilàssigt werden kónnen, so reduziert sich gì. [5] auf nachstehende beziehung: ^ 1 ^v> —9rrni rr-f-, die dieseble form der « wellengleichung »: 2 y ) = — 4jif [6] r v2 ì<2 ' besitzt und aus der man die losung mit hilfe der « verzogerten potentiale » erhàlt. w i r heben sofort hervor, dass es nicht immer gestattet ist, die gròssen (a2)-ter ordnung zu vernachlàssigen. in einer zweiten nàlierungsrechnung wird hiervon eechnung getragen werden. ini falle eines unendlich ausgedehnten homogenen mediums nimmt die losung folgende form (formel von kirehhoff) an: + ce y> = [ [ [ e 1 [/ di dvdc . [7] jj.ì ht-v/e) der weggelassene flàchenintegral-summand ist in identischer weise gleich nuli, was man erkennt, wenn die flàche s vom sender des einzelnen z. b. recheckigen impulses so weit entfernt ist, dass zur zeit (t) noch kein punkt derselben erreicht worden ist; das symbol [ ] bezeichnet « verzogerte werte ». die entfernung: q = [(:x-i)2 + ( y v ) 2 + ( z c ) 2 ] 1 ' 2 ausserdem muss (t) in (/) durch (t = qjv) ersetzt werden. d a s s k a l a r e , d u r c h e i n e n r e c h t e c k i g e n i m p u l s , tjsw. 151 nelimen wir rum eine « punktfòrmige elektrode » im nullpunkt an, die ini zeitpunkt (t = 0) eine momentane ladung (q) liefert. dies bedeutet, dass die ladungsdichte gleich: y = g • s ( i ) ó(v) d(c) [8] gesetzt wird, wobei ó(£) die funktion von dirac bezeichnet, welche bekanntlich liberali gleich nuli und fiir f = 0 gleich unendlich ist, sodass: + co f m ò(o cu = /(o) — co g(x,y,z; £,v,0 ò(0 ò(ri) d(0 d£ drj dc = q (x,y,0; 0 , 0 , 0 ) . da ferner, fùr t < 0, das feld gleich nuli ist, muss man fur (/) in gì. [6] folgenden ausdruck setzen: / = 0 , fiir t < 0 f = -j^ò(oà(ri)ò(oe[9] mit diesem w e r t von (/), infolge der vorherigen deflnition der funktion von <( dirac », liefert gì. [7] sofort: y> = 0 , fùr t < rjv ; r2 (x2 + y2 f z2) \ y> = ® . e x p . ( l — 2 j z a ( t — r/v)] , fiir t>r/v. \ ^ ^ t li ) aus den gì. [10] und [4] erhàlt man unmittelbar fùr die 0: 0 = 0 , fùr t < rjv 0 = exp [—2na(2t — r/vj] , fùr t>rjv. ' ^ v li wenn der impuls liingegen ini zeitpunkt ( r ) anstatt im zeitpunkt (< = 0) erzeugt wird, dann genùgt es in die gì. [11], (t — r ) an stelle von (t) zu setzen und man erhàlt: 0 = 0 , fùr t — r < r/v | 0 = q, exp [ — 2na(2t — 2 r — r/v)] , fùr t — x > rjv . ì [11 ] t li j nelnnen wir an, dass der rechseckige impuls die dauer ( t ) habe und dass somit ini zeitintervall (0 < r < t) v o m stroni j = q/t die ladung (q) gehefert werde. uni dio diesem zustande entsprechenden formeln zu erhalten, genùgt es offensichtlich die wirkungen der ein1 5 2 a . belltjigi zelnen impulse zu integrieren. man gelangt auf diese weise zum folgenden ausdruck fiir das potential (<p): t <p = j 0(t) dr [12] d wobei (&) durch gì. [11'] gegeben ist und an stelle von gì. [9] die beziehung j = q/t eingesetzt wurde. auf grund der doppelten definition von (0) siehe gì. [11'] muss man drei falle unterscheiden: a) t < r/v, d. h. vor ankunft des impulses in der entfernung (r). b) r/v < t < r/v + t d. li. die der ankunft des impuls-beginnes folgende jedoch seinem ende vorstehende augenblicke. c) t > r/v t d. h. augenblicke die der ankunft des gesamten impulses folgen. i m falle a) ist die 0 (t) = 0 im gesamten intervalle (0 < t < t) und somit ist (cp = 0). ini falle b) ist 0 (r) = 0 fuer r > t — r/v und verschieden von nuli fuer (0 < r < t — r/v < t) sodass die (cp) siehe gì. [11'] und [12] aus folgender beziehung berechnet wird: t—r/v cp = i" exp [ — 2na (2t — 2 r — r/v)] dr o i m falle c) ist & (r) 0 im gesamten intervalle (0, t) und wird infolgedessen die integration auf das gesamte intervall ausgedehnt. die (cp) wird somit durch folgende beziehung: t <p = i exp [ — 2jza (2t — 2r — r/v)] dr t k ! 0 berechnet. nach durchfiihrung der angegebenen integrationen, gelangt man fiir die (9?) zu folgenden ausdriicken: <p = 0 , fiir t < r/v = -j—\e-2narlv_e-2na{2t-r/v)} = 2tzar l > | j »' ' , m ì m 9 sin h 2 ti a (t—rlv) e-zjtal , fiir — < < < — + t 2nar v v — sin/i 2na t • e-2na(2t-t-r/v) fùrt > — + t 2:zar v gap potential d a s s k a l a r e , d u r c h e i n e n r e c h t e c k i g e n i m p u l s , u s w . 1 5 3 man beachte, dass fiir einen festen wert von (r) die ( $ ) den wert (0) bis zum zeitpunkt rjv (ankunft des impuls beginnes) annimmt, bis zum zeitpunkt (r/v + t) (ankunft des impuls-endes) bis zum hòchstwert: j rn e — w,n = — sin a, 2na t e~2nat • [141 2 no r anwàchst und nachher rapid bis nuli abnimmt. fiir t = t + rjv nimmt das « gap-potential » folgende form an : <p = <p 7 = 2 nar j r r 2t + 2 t— — v v 2nar j 2nar sinfe 2nate~2jla h2nat e-2na(t-r!v\ oderfur t = t+ — +r [13'] — sin m\h 2nat e~2na t + 2 r v abstract the theory exposed in tliis paper gives an approximative evaluation of the « electric scalar potential » in a low conductivity undefinite homogeneous medium, generated by an electrode ivhich supplies a « unit current pulse », with velocity of propagation v. the case of a « square » pulse of duration t is examined, obviously it is sufjficient to integrate the effects of each pulse. therefore the « direct scalar potential » and the « gap potential » are approximatively evaluated. riassunto la teoria qui data comporta una valutazione approssimativa del « potenziale elettrico scalare » in un mezzo indefinito omogeneo a bassa conduttività, generato da un elettrodo che fornisce un « impulso unitario » di corrente, propagantesi con velocità v. è esaminato anche il caso di un « impulso rettangolare » di corrente di durata t, integrando gli effetti d'ogni singolo impulso unitario. pertanto si calcola approssimativamente sia il potenziale diretto che il gap potential. die a m p l i t u d e n von p p u n d s s w e l l e n in der n a h e von 40° und die struktur des ausseren erdmantels j . y a n e k 1. einfuhrtjng. die anomale zunahme von amplituden der pund s-wellen in den epizentralentfernungen um 20° wurde von melireren autoren beobachtet (13). diese erscheinung, die hòhstwalirscheinlich mit den beobaehteten krummungsànderungen der laufzeitkurven der pund s-wellen in demselben distanzbereicli zusammenliàngt, trat sehr deutlich bei der empirischen ableitung der eichfunktionen e (zi) fùr die bestimmung von magnituden aus raumwellen auf grund prager beobachtungen hervor (4). eine ahnliche zunahme von amplituden der ppund ss-wellen wurde bei dieser gelegenheit in der nahe von 40° beobachtet. von verschiedenen autoren (5>6) wurde die notwendigkeit der systematischen untersuchung von amplituden der pund s-wellen in einem gròsseren distanzbereich fùr die erklàrung der 20° diskontinuitàt mehrmals betont. deshalb wurden in prag die untersuchungen in dieser eiclitung auf grund eines noch zahlreicheren materials vorgenommen. die untersuchung des amplitudenverlaufs der pund s-wellen in dem distanzintervall von 4° bis 30° hat frau ruprechtovà (') dureligefiihrt. sie benutzte dazu einerseits prager beobachtungsmaterial und andererseits beobachtungen europaischer und mittelasiatischer stationen von 5 ausgewahlten erdbeben. bei den amphtuden von p-wellen stellte sie ein klares minimum in der epizentralentfernung von 11,5° und ein ausgepragtes maximum bei 19° fest. bei den s-wellen lag das minimum bei 14° und das maximum bei 21°. in dieser mitteilung wird iiber die untersuchung der amphtuden von ppund ss-wellen in dem distanzintervall von 25° bis 52° berichtet. 240 v j. vanek 2. a v i p l i t u d e n k u r v e n der p p xjnd s s w e l l e n . das beobachtungsmaterial fiir diese untersuchung wurde durch sorgfàltige ausmessung der maximalen amplituden und der entsprechenden perioden der p p und ss-wellen aus den seismogrammen der wiechertschen seismographen von den erdbebenstationen prag und jena gewonnen und zwar aus dem zeitintervall 1927-1956 fiir jena und 1935-1956 fiir prag. um jede verwechslung mit anderen wellentypen zu vermeiden, wurden die ankunftszeiten der betreffenden wellen sorgfaltig kontrolliert. die anzahl der beobachtungen war 90 fiir p p h , 45 fiir p p v und 66 fiir ssh. die epizentralentfernungen der benutzten erdbeben wurden mit einer genauigkeit von ± 0,1° meistens nach den iss parametern berechnet. es handelte sich um asiatische erdbeben mit normaler herdtiefe; einige atlantische erdbeben wurden getrennt untersucht. die tiefenangaben von iss wurden durch vergleich mit den angaben von gutenberg und richter (8) kontrolliert. die erdbeben waren natiirlicli von verschiedener magnitude. als vergleichsgròsse wurde die gròsse a* = log ajt — m eingefiihrt, wobei die magnitude m nur aus raumwellen auf grund der in (4) abgeleiteten eichfunktionen b(a) bestimmt wurde und zwar als mittelwert der zur verfiigung stehenden magnitudenwerte der einzelnen raumwellen ( p h , p v , sh). die abhàngigkeit der a* werte als funktion der epizentralentfernung a fiir ppi-i-wellen kann man aus eig. 1 ersehen. schwarze punkte in fig. 1-4 sind jenaer werte, weisse punkte prager werte; kreuze sind schwerpunkte der natiirlichen zi-intervalle, die mit hilfe der wendepunkte minimaler hàufigkeit der summationskurve bestimmt wurden. die amplitudenkurve ist dann die mittlere durch diese schwerpnnkte hindurchgelegte kurve. der repràsentative charakter dieser kurve wurde durch die ilaufìgkeitskurve der abweichungen à * — a * kontrolliert. fiir pph-wellen zeigt die kurve ein ausgepragtes minimum bei der epizentralentfernung von 34° und ein klares maximum bei der entfernung von 41°. ein ahnliches bild bekommen wir fiir die p p v wellen (fig. 2); das minimum der kurve liegt wieder bei 34° und das maximum bei 40, 5°. die beobachtungen der ssh-wellen haben natiirlich eine gròssere streuung (fig. 3), aber wir sehen eine kurve vom gleichen charakter wie bei den pp-wellen mit einem minimum um 38° und einem maximum um 44°. d i e a m p l i t u d e n v o n p p u n d s s w e l l e n , u s w . 2 4 ! -6,5 -7,0 -7.5 -8,0 • 4 oc • • ( \ » i > • • • • o< 1 d c r 1 • •• •q» o • / 0 oo 7 o o o o r • • <n » o o » u o • o « p • • 25 27 29 31 33 35 37 39 41 43 45 47 49 51 a fig. 1. amplitudenkurve der pph-wellen. e jena, o p r a g ; + schwerpunkte der natiirlichen a-intervalle. « \ » n < » • • + o \ 1 n f, • t 3 • ( ® 9 o • • r-+« o •• • » 25 27 29 31 33 35 37 39 41 43 45 47 49 51 pig. 2. amplitudenkurve der ppy-wellen. 2 4 2 j. v a n e k -6,0 -6,5 -7,0 -7,5 -8,0 • -f n s \ s x n o* o > s. ( c c c ) d cm j • -+n •fo \ c ) » « i • o • o o ' j • •'o§ • • c • • • 25 27 29 31 33 35 37 39 41 43 45 47 49 51 ù fig. 3. amplitudenkurve der ssh-wellen. 6,5 -7,0 -7,5 8,0 \ • \ \ \ / / o / f — % \ \ \ * « t \ . o f • s • x n ( e % t ^ ® • o 25 27 29 31 33 35 37 39 41 43 45 47 a fig. 4. amplitudenkurve der ppi1-wellen fiir atlantische erdbeben. d i e a m p l i t u d e n v o n p p u n d s s w e l l e n , u s w . 24! alle beobachtungen in fig. 1-3 betreffen ausschliesslich die asiatischen erdbeben. fiir beben mit herden im atlantischen ozean liatten wir leider nur wenige beobachtungen zur verfugung; es ist aber interessant, dass die amplitudenkurve der pph-wellen in diesem falle das minimum und maximum bei anderen epizentralentfernungen hat. es scheint, dass das minimum bei 30° und das maximum bei 36° vorkommt (fig. 4). wenn wir jetzt die ergebnisse zusammenfassen und mit den werten fùr pund s-wellen vergleichen, die von frau euprechtovà (7) bestimmt wurden, so bekommen wir folgende interessante tabelle (amia bzw. a 2 — epizentralentfernungen des minimums, bzw. maximums der amplitudenkurve, m-anzahl der beobachtungen): p h p p h p p y p p h a t l . sh ssh ^min 11,5° 34° 34° 30° 14° 38° a2 19 41 40,5 36 21 44 n 125 90 45 12 110 66 das merkwiirdigste in dieser tabelle ist die asimmetrie der werte von zi tur p p und ss in bezug auf die werte fùr p und s. 3. deutungsversuche. uni diese merkwùrdige asymmetrie zu erklàreu, mùssen wir uns unbedingt mit den ursachen des anomalen amplitudenverlaufs, d. h. mit der struktur des àusseren erdmantels emgehender beschàftigen. nehmeii wir zuerst die gutenbergsche interpretation (2) an, nacli welcher diese erscheinungen mit hilfe einer schicht von geringerer geschwindigkeit gedeutet werden. diese schicht ist mit der existenz einer schattenzone verknùpft, in der eine markante abnahme von eaumwellenamphtuden vorkommen soli. wenn wir mit a1 die innere und mit a 2 die aussere grenze dieser schattenzone bezeichnen, so kann man das beobachtete maximum der amplitudenkurve wie zl2 deuten. das minimum der amplitudenkurve ist schwer interpretierbar, weil wir praktisch keine kenntnisse ùber die amphtudeneigenschaften und ùber den charakter der existierenden wellen in der schattenzone haben. die entfernungen ax und a2 kònnen wir fùr verschiedene modelle des erdmantels berechnen. es handelt sicli nur um die bereclinung 2 4 4 j. v a n e k von epizentralentfernungen und deshalb ist die anwendung der normalen strahlentheorie sicher berechtigt. wenn wir fiir die variation der wellengeschwindigkeit mit der tiefe das einfache gesetz vk = va (ujrkfk und fiir die struktur des ausseren erdmantels prinzipiell folgendes einfaclies modell annehmen, »'o h ri b2 r2 b3 »3 b.< dann ist allgemein die innere grenze der schattenzone ax = / (bi, b2, r„ r2) und die aussere grenze a2 = f (blt b2, b3, bt, rlt r2, r3). weil der einfluss der variation von verschiedenen parametern bh, rh allgemein schwer zu iibersehen ist, wurden alle einparametrischen abhàngigkeiten fiir a2 berechnet. als allgemeines ergebnis diesel' untersuchung kann man angeben, dass die variation der tiefe viel gròsseren einfluss auf a2 als die variation der gradienten bk hat. der einfluss der regionalen verseliiedenheiten ini aufbau der erdkruste auf die aussere grenze a2 kann in keineni falle den wert 0,5° iiberschreiten. dasselbe gilt auch fiir den einfluss der herdtiefe, die bei normalen erdbeben gewòhnlich nicht grosser als 50 km ist. uni die asymnietrie der werte von a2 bei p p und ss-wellen erklaren zu kònnen, miissen wir eine regionale variation der schicht mit niedrigerer gesckwindigkeit, d. h. eine regionale variation der struktur des ausseren erdmantels voraussetzen. als wahrscheinliclies modell erdkruste scliicht mit negativem gradienten der gescliwindio'keit d i e a m p l i t u d e n v o n p p u n d s s w e l l e n , u s w . 24! kònnen wir folgende verteilung der geschwindigkeit von p-wellen mit der tiefe h annehmen: h hk (km) bk vk (km/s) 0 0 6,2 26,4 30 7,0 8,0 2,0 2 1 0 0 8,2 4 , 0 3 150 7,95 2,5 fùr dieses modell ist a2 = 19° (europa). fùr /), = 22° (asien) bekommen wir danu bei einer schicht mit konstanter maehtigkeit (h:t — h2 = konst) fùr das minimum der geschwindigkeit die tiefe li3 = 250 km und fùr a2 = 17° (atlantik) die tiefe ìi3 = 120 km. fùr eine schicht von veranderlicher maehtigkeit mit h2 — konst, haben wir dann fùr asien h3 = 200 km und fùr den atlantik li3 = 130 km erhalten. eine grosse unsicherheit ist in der bestimmung der oberen grenze h2 der schicht mit negativem geschwindigkeitsgradienten, da die innere grenze a1 der schattenzone aus dem yerlauf der amplitudcnkurve nicht oline weiteres bestimmt werden kann. diese obere grenze mùsste mit einer anderen methode untersucht werden, z. b. mit der methode von shebahn (9), mit welcher die obere grenze dieser schicht in europa in einer tiefe von 100 km festgestellt wurde. ini falle der jeffreyssclien hypothese (10) ùber die ursache der ei'scheinungen, die mit der 20° diskontinuitàt verknùpft sind, ist die berechnung von a2 wesentlich einfacher. auch in diesem fahe kann man nicht die asymmetrie der a2 werte fùr p p und ss anders als durch die regionale yariation der diskontinuitàt erklaren. wenn wir die jeffreyssche verteilung der geschwindigkeit mit der tiefe ( u ) annehmen, so bekommen wir aus pp-wehen fùr den atlantik die tiefe der diskontinuitàt zu 250 km, fùr europa zu 300 km und fùr asien zu 400 bis 450 km. ein àhnlicher deutungsversuch fùr ss-wellen wurde noch nicht vorgenommen. 2 4 ( 5 j. v a n e k 4. schlussbetrachtung. als allgemeines ergebnis unserer untersuchung folgt, dass die ursache des anomalen verlaufs von amplituden der pund s-wellen in der umgebung von 20° sowohl bei erklàrung nach der hypothese von gutenberg als auch nach der hypothese von jeffreys einen ausgepràgt regionalen charakter hat, d. h. dass der aussere erdmantel eine klare regionale struktur haben muss. es ist wahrscheinlich, dass die yariationen mehr durch regionale verànderungen der tiefe der betreffenden scliieht oder diskontinuitàt als durch regionale verànderungen der geschwindigkeitsgradienten verursacht werden. abstract on the basis of observations of seismological stations jena and prague the amplitude variation of pph-, ppvand ssh-waves with epicentral distance in the range between 25° and 52° was investigated. a clear minimum (at the distance /!,,„„) and maximum (at the distance a2) ivas found for both pp and ss-waves. the results of this investigation together ivith the results of mrs. ruprechtovà (7) for phand sh-ivaves are given in the table on pg. 5. the observed asymmetry in zl2 for pp and ss can be explained by regional variations in the structure of the earth's upper mantle. riassunto è stata studiata la variazione dell'ampiezza delle onde pph, ppv ed ssh con la distanza epicentrale, in un raggio compreso fra 25° e 52°, sulla base delle osservazioni ottenute presso le stazioni sismiche di jena e praga. i risultati di questa ricerca, unitamente a quelli ottenuti da mrs. ruprechtovà c), per le onde ph ed sh, sono riportati nella tabella di pag. 5. l'asimmetria osservata in a2 per le onde pp ed ss può essere spiegata dalle variazioni regionali nella struttura della terra sopra il mantello. d i e a m p l i t u d e n v o n p p u n d s s w e l l e n , u s w . 24! l i t e r a t u r ( x ) g u t e n b e r g b . , zeitschr. f. geophys., 2, 2 4 , ( 1 9 2 6 ) . (2) — bull. seism. soc. am., 3 8 , 121, ( 1 9 4 8 ) . ( 3 ) b a t h m . , annali di geofisica, 9 , 4 1 1 , ( 1 9 5 6 ) . ( 4 ) v a n e k j . , z à t o p e k a . , travaux de vlnst. géophys. de vacad. tchécosl. se., n. 2 6 , geofysikalm sbornik, 9 1 , ( 1 9 5 5 ) . (5) j e f f r e y s h „ 3inbas, «geophys. suppl.», 6 , 348, ( 1 9 5 1 ) . ( 6 ) l e h m a n n i . , trans. amer. geophys. union, 3 4 , 4 7 7 , ( 1 9 5 3 ) . ( 7 ) r u p r e c h t o v à l . , studia geophys. geodaet., 2 , 3 9 7 , ( 1 9 5 8 ) . ( 8 ) g u t e n b e r g b., e i c i i t e r c . f., seismicity of the eartli, princeton univ. press 1949. ( 9 ) s h e b a l i n n . v . , studia geophys. geodaet., 2, 86, ( 1 9 5 8 ) . ( i » ) j e f f r e y s h „ mnbas, « geophys. suppl. », 3 , 4 0 1 , ( 1 9 3 6 ) . (") — 3inbas, « geophys. suppl. », 4 , 4 9 8 , ( 1 9 3 9 ) . m o v e m e n t s o f t h e f a u l t o f t h e l a k e o f cavazzo i n c o n n e c t i o n w i t h t h e l o c a l r a i n f a l l s p . c a l o i * m . m i g a n i * * r e c e i v e d on o c t o b e r 15, 1971 summary. — u n d e n i a b l e w i t n e s s e s a r e g i v e n f o r t h e e x i s t e n c e of m i c r o m o v e m e n t s of t h e f a u l t of t h e l a k e of c a v a z z o ( t o l m e z z o ) i n c o n n e c t i o n w i t h local r a i n f a l l s . t h e y a r e r e c o r d e d a s v e r t i c a l r o t a t i o n s b y t h e c l i n o g r a p h i c s t a t i o n s i n c o r p o r a t e d i n t o e n e l ' s p o w e r s t a t i o n of s o m p l a g o w h i c h lias b e e n b l a s t o u t of t h e r o c k o n t h e w e s t e r n e d g e of t h e l a r g e f a u l t of t h e l a k e . t h e f a u l t in i t s e l f , w h i c h e x t e n d s p r o b a b l y d o w n t o t h e c o n f l u e n c e of t h e j l e l o t o r r e n t w i t h t h e t a g l i a m e n t o r i v e r a t a b o u t 10 k i n s s o u t h of t h e s t a t i o n , lies in t h e n w q u a d r a n t of t h e v a s t a l l u v i u m a r e a of t h e t a g l i a m e n t o e x t e n d i n g b e t w e e n v e n z o n e , g e m o n a a n d o s o p p o . t h e r o t a t i o n of t h e v e r t i c a l t o w a r d s s e m a y b e a t t r i b u t e d t o t h e w e i g h t i n c r e a s e of t h e a l l u v i u m d u e t o r a i n f a l l s . i n f a c t , t h e d e v i a t i o n p r o v e s t o b e p r o p o r t i o n a l t o r a i n f a l l s : t h e h e a v i e r a r e t h e l a t t e r , t h e l a r g e r is t h e f o r m e r . t h i s s u g g e s t s t h e i d e a t h a t b e l o w t h e b e d of t h e t a g l i a m e n t o r i v e r t h e r e m u s t b e a l a r g e u n d e r g r o u n d flood w h i c h is a l i m e n t e d b y l e a k a g e of r a i n w a t e r . i t s h o u l d b e c o n s i d e r e d in t h i s r e s p e c t t h a t d u r i n g t h e s u m m e r w h e n r a i n f a l l s a r e s c a r c e , t h e v e r t i c a l is s u b j e c t e d t o a slow o p p o s i t e m o v e m e n t a t s o m p l a g o , a s if i t w e r e u n d e r t h e i n f l u e n c e of a c o r r e s p o n d i n g slow l i g h t e n i n g of t h e s s e a r e a . t h e m i c r o m o v e m e n t s of t h e t w o l e d g e s of t h e f a u l t m a y e x e r t a " r e l e a s e " e f f e c t on t h e f a u l t a r e a s u c h a s t o p r e v e n t s i z e a b l e a c c u m u l a t i o n s of e l a s t i c t e n s i o n s i n t h e s u r r o u n d i n g m e d i u m . a s a m a t t e r of f a c t , t h e s o m p l a g o a r e a h a s n e v e r b e e n in t h e e p i c e n t r e of d e s t r u c t i v e e a r t h q u a k e s a s f a r a s r e c o r d e d h i s t o r y g o e s b a c k , n o t w i t h s t a n d i n g t h e h i g h s e i s m i c c h a r a c t e r of c a r n i a . * d i r e t t o r e d ' o s s e r v a t o r i o , i s t i t u t o n a z i o n a l e di geofísica, r o m a . ** e . n . e l . c o m p a r t i m e n t o di v e n e z i a . 32 p . c a l o i m . m i g a n i 1. i t was given in account of working — in experimental stage — of two horizontal p e n d u l u m s of exceptional power (x) located in a well of 136 metres hight which had been sank in t h e rock a n d contains t h e e q u i p m e n t of e n e l ' s power station of somplago (tolmezzo). under normal conditions t h e recordings of t h e p e n d u l u m s are those pertaining to t h e e a r t h ' s tides. local p e r t u r b a t i o n s depending on t h e m o v e m e n t s of water in t h e piezometric well of t h e s t a t i o n are n o t missing, b u t t h e y can be readily distinguished. there are often abnormal p e r t u r b a t i o n s of tectonic origin or attrib u t a b l e to o u t w a r d reasons. the l a t t e r are t h e subject of this s t u d y . 2. the recording of variations of t h e vertical as a consequence of passages of more or less a c c e n t u a t e d variations of t h e atmospheric pressure is certainly nothing new. such variations h a v e been recorded m a n y times during t h e last decades. the e x t r e m e sensitivity of t h e clinographie s t a t i o n of somplago to such p h e n o m e n a and to others connected with t h e m appears to be q u i t e new, however. e v e r since the station of horizontal p e n d u l u m s is operating a t t h e somplago power station, the a m o u n t of cases recorded was such as to remove a n y d o u b t : t h e r e is no passage of atmospheric p e r t u r b a t i o n of some intensity which did n o t bring a b o u t t h e more or less rapid shifting of t h e image of one of t h e two components (the s33°e) out of t h e recording-paper, causing s h a r p deviations also of t h e other component a t 90°. we are limiting ourselves to report some of t h e most recent examples: 20-25 f e b r u a r y 1969 (fig. 1), 21-26 april 1969 (fig. 2), 16-20 may 1969 (fig. 3), 3-9 j u n e 1969 (fig. 4), 7-13 j u l y 1969 (fig. 5) and m a n y others. making a first comparison between t h e clinographic curves and t h e atmospheric pressure it m a y lead to t h i n k of an obvious correspondence between t h e deviation (more or less sharp) of the n33°yv-s33°e clinographic component and t h e decrease of atmospheric pressure observed in t h e area, b u t when looking closer one observes two f a c t s which cannot be explained: t h e time gap between t h e two events (variation of the vertical and decrease of atmospheric pressure) and t h e a p p a r e n t independence between t h e amplitudes of the two phenomena, since small pressure variations correspond sometimes to strong vertical deviations, a n d viceversa. m o v e m e n t s o f t h e f a u l t o f t h e l a k e o f c a v a z z o 1 7 here one question had to be p u t : is t h e sudden and ample deviation of « a » t o w a r d s33°e, followed b y a rapid shifting of t h e image out of the recorder, really b o u n d — and exclusively so — to t h e passage of a minimum of pressure above somplago! one f a c t is assured: generally, when t h e atmospheric pressure gives a strong, negative leap, t h e vertical is deviated with t h e image being shifted out of t h e recorder. the contrary, however, is n o t always true, since there are f r e q u e n t and ample shiftings of t h e light-image toward s33°e w i t h o u t t h e atmospheric pressure having suffered variations beyond normal. w e wondered if this could be due to exceptional high tides of t h e venetian lagoons (grado), b u t we got a negative answer. i t could be excluded as well t h a t phenomena of this kind are brought a b o u t by variations in t h e level of t h e near l a k e of cavazzo, of too limited a surface t h a t even considerable variations of its level (daily variations of some decimeters) according to t h e requests of t h e near power s t a t i o n are a p t to cause such large effects, a n d it could be considered v e r y unlikely t h a t such a regular a n d f r e q u e n t p h e n o m e n a should h a v e some tectonic origin, so t h a t another weather f a c t o r h a d to be suspected: the rainfalls. which was actually t h e case. to give one example only, let us consider t h e event of november 19, 1970. as will be seen f r o m fig. 9, t h e variation of atmospheric pressure on t h a t d a y was negligible. this notwithstanding, however, t h e image of component «a» of the horizontal p e n d u l u m to overstep quickly a n d a b u n d a n t l y its limits. the rainfall — a t somplago solely p a s t 90 m m s in 24 hours — on t h a t d a y justifies t h e strong a n d sudden v a r i a t i o n of t h e vertical which happened in the area. and by referring to t h e rainfall, all t h e observed vertical variations, besides those reported by figures 1 and 16, are justified as well. 3. which is t h e reason for such an outspoken sensitivity to phenomena which elsewhere remains almost unobserved? i t should n o t be forgotten t h a t t h e upper suspension of t h e p e n d u l u m extends vertically lip to 130 ms inside t h e mountain. w a s there a differential action between t h e upper and t h e lower connection point a t t h e basis of t h e power station which m a y have enhanced t h e effect of rotation? or did the entire m o u n t a i n rotate, as a rigid means? the a l t e r n a t i v e could be settled only by an horizontal pendulum which was conditioned exclusively by its base support, which was done. dr. migani had two horizontal p e n d u l u m s built and fixed on t h e ground floor of t h e power station, near t h e seismic station and t h e other clinographic station. 1 8 p . c a l o i m . m i g a n i t h e new pendulums, whose sensitivity was much less t h a n t h a t of t h e original couple, offered an a d v a n t a g e over t h e l a t t e r by allowing generally the entire recording of the phenomenon, with t h e exception of particularly a c c e n t u a t e d variations of t h e vertical. t h e recordings of t h e new couple of p e n d u l u m s allowed to prove t h a t as a m a t t e r of f a c t t h e entire m o u n t a i n is uniformly subjected to t h e inclination, even in t h e case of superficial p e r t u r b a t i o n s such as t h e one here examined. figures 14-16 show example recordings of the p h e n o m e n o n b y t h e new couple of horizontal pendulums. 4. the rainfalls in t h e area a r o u n d somplago are, therefore, t h e main reason of t h e vertical deviation recorded b y t h e clinographic stations installed in the power station which had been blast out of t h e rock in t h e extreme n o r t h of the l a k e of cavazzo. how can this high sensitivity of t h e clinographic stations to rainfalls be explained? spontaneously one is led to t h i n k of a p a r t i c u l a r tectonic f e a t u r e of t h e area where t h e above said stations are located. geologists observe t h a t t h e p r e d o m i n a n t tectonic f e a t u r e of t h e area consists of a series of tectonic parallel chips which overslid t o w a r d s s and syv, b y overlapping surfaces falling off t o w a r d n. there are, besides, transverse faults. one of t h e most i m p o r t a n t of t h e m is t h e f a u l t which can be distinguished along t h e w e s t side of t h e l a k e of cavazzo and which was discovered and studied first by e . feruglio (4) (figs. 18,19), a f r a c t u r e caused by a n horizontal shifting crosswise to t h e folds and m a r k e d by their sudden deviation t o w a r d s in correspondence to t h e melò valley. the f r a c t u r i n g plane is accompanied b y a vertical ejection a r o u n d which t h e rock was thinly smashed. the f r a c t u r e m a y be followed only for a limited length as b o t h in the n o r t h and in the south it disappears under t h e lake and t h e alluvium. i t is impossible to check w h e t h e r it proceeds — as e . selli (5) believes — u n t i l t h e confluence of t h e melò in t h e tagliamento river near which, on the w e s t e r n side of t h e melò, a n o t h e r transverse fract u r e comes in sight (fig. 18). this f a u l t belongs to more recent orogenetic periods (post-lielvetian). between the h e l v e t i a n and t h e q u a t e r n a r y occurred the most intense diastrophic period with one-sided t h r u s t s f r o m s o u t h which caused an heavy crush followed by upheaval of t h e m o u n t a i n folds and by m o v e m e n t s o f t h e f a u l t o f t h e l a k e o f c a v a z z o 1 9 their upset or sliding, while a vigorous compression f r o m e a s t twiste d them and caused transverse f r a c t u r e s (*). 5. the clinographic movements recorded a t somplago on the western edge of t h e lake of cavazzo a f t e r heavy rainfalls m a y be in some way connected with the f a u l t ' s activity. a more or less deep f r a c t u r e in the e a r t h ' s crust causes t h e bonds of t h e edges to slacken and permits relative micromovements with a certain degree of freedom. this is w h a t m a y h a p p e n in t h e area of t h e lake of cavazzo during rainfalls. the area west of t h e f a u l t is altogether mountainous. i n t h e e a s t , between gemona, osoppo, and venzone, f u r t h e r there is a v a s t extension of alluvium which is p a r t l y cultivated and a p t to absorb a m o u n t s of water when it rains, with a considerable increase of the " g r o u n d wreight" so as to a t t r a c t t h e w e s t e r n edge of the fault. i t m u s t be emphasized t h a t t h e r e s u l t a n t of t h e observed bendings is always directed toward sse, t h a t is toward the g r a v i t y centre of t h e area considered. i n t h a t case it m u s t be a d m i t t e d , however, t h a t according to selli's hypothesis, t h e f a u l t is n o t limited to t h e western side of the l a k e of cavazzo, b u t extends f u r t h e r towrard south where it links up with the f r a c t u r e which comes to sight between avasinis and t h e confluence point of t h e melò with t h e tagliamento kiver (fig. 18). one f u n d a m e n t a l f e a t u r e of the m o v e m e n t is its taking place with sudden and swift deviations, while a t the end of the rainfalls t h e r e t u r n to the original position of equilibrium is very slow (figs. 15, 16), as if an underground flood, suddenly alimented, loses gradually its flow when the rain has subsided. a p a r t from the way in which the micromovements of the f a u l t of the lake of cavazzo t a k e place in connection with rainfalls, and which could be looked into closer in following studies, there remains t h e outstanding f a c t of a large f r a c t u r e which is a n i m a t e d by an unsuspected outward cause. if the cause is t h a t which has been viewed in the previous section, there is no d o u b t t h a t the two contact surfaces of t h e f a u l t (*) i n t h i s c o n n e c t i o n , it is e m p h a s i z e d t h a t , p r e s c i n d i n g f r o m t h e p h e n o m e n o n o b j e c t of o u r s t u d y , i n t h e r e c o r d i n g s of t h e c l i n o g r a p h i c s t a t i o n of s o m p l a g o , a s l o w d e v i a t i o n n o r t h w a r d is a l w a y s p r e s e n t . t h e p e r m a n e n c e of a m i c r o r o t a t i o n n o r t h w a r d ( g e n e r a l l y n o r t h w e s t w a r d , s o m e t i m e s n o r t h e a s t w a r d ) , m a y b e of t e c t o n i c o r i g i n a s a p e r s i s t e n t p u s h s o u t h e r l y i n t o t h e a r e a . 2 0 p . c a l o i m . m i g a n i carry out relative m o v e m e n t s when t h e cause is in action. t h e frequency of rainfalls in this area is r a t h e r high so t h a t t h e said micromovements recur q u i t e o f t e n and more or less a c c e n t u a t e d according to t h e intensity of t h e p e r t u r b i n g cause. i t m a y therefore well be affirmed t h a t a t least in the outer, surface p a r t of t h e f a u l t hardly great elastic tensions will a c c u m u l a t e d as t h e y are continuously " r e l e a s e d " by t h e above said relative micromovements. i n other words, t h a n k s t o those micromovements no surface e a r t h q u a k e s of high intensity should t a k e place in this area. as a m a t t e r of f a c t , t h e l a k e of oavazzo has never been in t h e epicentre of large e a r t h q u a k e s as f a r as recorded history can tell. as can be seen f r o m t h e s t u d y of marcelli — feliziani (2>3) (fig. 20), t h e area of t h e l a k e of cavazzo appears surrounded b y t h e epicentres of e a r t h q u a k e s , a m o n g which very h e a v y ones, from the year 0 up to 1963. this does n o t give t h e impression of mere coincidence. r e f e r e n c e s (*) c'ai.oi p . , 1909. relazioni fra sismicità e moti lenti nella crosta terrestre. (relations between seismicity and slow movements in the earth's crust). « a n n a l i di g e o f i s i c a », x x i i , 3, p p . 2 0 9 2 2 8 . (2) feliziani p . , marcelli l . , 1965. il terremoto di tolmezzo del 26 aprile 1959. i p a r t e : venti secoli di storia sismica della regione cantica e dell'italia nord-occidentale. « a n n a l i di g e o f i s i c a » , x v i i i , 3, p p . 165-207. (3) feliziani p . , marcelli l . , 1966. ii p a r t e : studio macrosismico-geologico: determinazione analitica delle grandezze ipocentrati e natura della scossa. « a n n a l i di g e o f i s i c a » , x i x , 2, p p . 191-214. (4) feruglio e . , 1925. le prealpi fra l'isonzo e varzino. « e s t r . b o l l . a s s . a g r . f r i u l a n a ». (s) selli r . , 1963. schema geologico delle alpi gamiche e giulie occidentali. « g i o r n a l e di g e o l o g i a », x x x . n u m e r i c a r a t t e r i s t i c i d e i f l o c c u l i d ' i d r o g e n o e d i c a l c i o e d e i f i l a m e n t i d ' i d r o g e n o p e r l ' a n n o 1953 g. godoli s o n o già state sottolineate in questa sede le r a g i o n i che ci hanno spinto a r i p r e n d e r e in considerazione l ' o p p o r t u n i t à di d e t e r m i n a r e l'attività dei f e n o m e n i c r o m o s f e r i c i [ f l o c c u l i d ' i d r o g e n o , flocculi di calcio, filamenti d ' i d r o g e n o ] m e d i a n t e i numeri caratteristici (*). n e l l a nota citata f u r o n o riportati i numeri caratteristici dei fenomeni c r o m o s f e r i c i per gli anni 1950-51-52; qui, nelle seguenti tre tabelle di i m m e d i a t a i n t e r p r e t a z i o n e , sono riportati q u e l l i per l'ani n u m e r i caratteristici sono stati stimati sugli s p e t t r o e l i o g r a m m i eseguiti alla t o r r e solare di a r c e t r i con l ' i m m a g i n e di 64 nini; le caratteristiche strumentali e la tecnica d e l l e osservazioni sono state descritto in altra sede ( ) . sono state esaminate complessivamente 126 lastre in h x e 120 lastre in k>, l a stima v e n i v a eseguita con l'aiuto dei m o d e l l i di m t . w i l s o n . p e r e v i d e n t i ragioni di u n i f o r m i t à è stata stimata p r i m a tutta la serie dei n u m e r i caratteristici per i flocculi d ' i d r o g e n o ; q u i n d i tutta la serie per i flocculi di calcio ed infine tutta la serie per i filamenti d'idrogeno. l o scarto m e d i o d e l l e singolo d e t e r m i n a z i o n i è sempre stato inf e r i o r e a 0.2 unità. i i c o n t r i b u t o d e i s i n g o l i osservatori nella raccolta d e l l e lastre è stato il s e g u e n t e : no 1953. o s s e r v a t o r i k. t o t a l e r i g h i n i . . r i g u t l i . . r o m a g n o l i . a b e t t i b a l l a r l o g o d o l i 4 88 23 •l 6 1 3 86 20 4 6 1 174 43 12 8 c. c o n o u t a b e l l a i c \'iimeri caratteristici dei filamenti d'idrogeno per ranno 1953. g io r n o | f e b b ra io l << a p r il e 'tl te ti exj 3 a g o s to « v « r. o tt o b re 1 n o v e m b r e « u u o 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 21 22 23 24 25 26 27 2-i 29 30 31 0.1 0.1 0.1 0.8 0.2 0.1 0.2 0.2 0.0 0.0 0.8 — 0.9 0.3 0.0 0.0 0.0 0.1 0.2 0.0 0.2 0.1 0.1 0.3 0.2 0.4 0.3 0.0 0.1 0.1 0.0 0.2 0.0 0.2 0.4 0.6 0.3 0.1 0.3 1.1 1.5 0.0 0.'> 0.2 0.6 0.8 1.2 1.1 1.2 1.8 0.4 0.3 0.7 0.6 0.3 0.0 0.1 0.0 0.3 0.2 0.6 0.5 0.4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 21 22 23 24 25 26 27 2-i 29 30 31 0.0 0.9 0.4 o.'l 0.0 0.0 0.0 0.2 0.2 0.1 0.1 0.2 0.0 0.0 0.1 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.1 0.2 0.4 0.2 0.6 1.3 1.0 1.4 0.2 1.6 1.2 1.2 0.9 0.2 0.2 0.1 0.3 0.2 0.1 0.1 0.1 0.1 0.4 0.1 0.0 0.9 0.2 0.0 0.1 0.1 0.2 0.0 0.2 0.1 0.1 0.3 0.2 0.4 0.3 0.0 0.1 0.1 0.0 0.2 0.0 0.2 0.4 0.6 0.2 0.4 0.0 1.2 0.4 0.3 0.7 0.6 0.3 0.0 0.1 0.0 0.3 0.2 0.6 0.5 1.0 1.4 0.1 0.2 0.3 m e d i a 0.2 0.2 0.1 0.2 0.9 1.0 0.2 0.2 0.5 0.8 0.3 0.6 m e d i a a n n u a l e 0.35 numeri caratteristici per l'anno 1953 139 gli osservatori sono stati s e m p r e c o a d i u v a t i dal sig. r o m a g n o l i . la d i s t r i b u z i o n e m e n s i l e d e l l e lastre ridotte è qui sotto i n d i c a t a : 1 1 « 2 1 1 3 t | 5 1 fi i 7 1 8 ' 9 10 11 12 il a 1 1 « 7 16 8 5 6 | 20 j 18 | 10 10 12 fi kc-i 7 6 16 6 6 5 19 18 i 10 10 11 6 firenze — osservatorio astrofisico di arcetri — centro di astrofisica del c.n.r. febbraio 1054. riassunto si danno i numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per l'anno 1053, secondo le osservazioni eseguite alla torre solare di arcetri. summary the character figures of ca flocculi and of h a bright and dark flocculi are given according to the observations carried out at the solar toiver of arcetri. b i b l i o g r a f i a ( ! ) g . godoli, numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per gli unni 1950-51-52. a n n . g e o f . fi, 199, ( 1 9 5 3 ) . ( ) g . r i g h i n i e g . godoli, riduzione del materiale spettroeliografico raccolto alla torre solare di arcetri nel periodo 1932-1949. m e n i . s a i . 21. 333 h 9 5 0 ) . annals of geophysics, 55 (3), 2012: ferrero et al. geodiversity action plans for the enhancement of geoheritage in the piemonte region (north-western italy) elena ferrero*, marco giardino, francesca lozar, enrico giordano, elena belluso, luigi perotti università di torino, dipartimento di scienze della terra, torino, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5527 abstract a geoethical approach to geodiversity allows better understanding of the value of abiotic nature and enhances its conservation and development. our basic assumption is that even during an economical crisis, geoheritage sites can serve both public and private interests. a set of nine strategic geothematic areas were chosen to represent the geodiversity of the piemonte region, north-western italy, each of which is characterized by great potential for scientif ic studies, enhancement of public understanding of science, recreational activities, and economic support to the local communities. specialized research teams individuated critical aspects to advance our knowledge of the geological history of the piemonte region, through climate and environmental changes, natural hazards, soil processes, and georesources. the scientific concepts and techniques were coupled with geodiffusion actions and products: not only geosites, but also museum collections, evidence of mining and quarrying activities, science exhibitions, and nature trails. the preliminary results have allowed action plans to be developed with local partners, to assess the geoheritage management requirements. a series of investigations were carried out to improve the visual representation of the geological processes and the evolutionary scenarios. further outcomes of the project will include didactic tools for educators, schools, and the public in general. 1. introduction as a general view, geoethics is seen as the set of guidelines that pertain to human behavior that can affect larger planetary geophysical systems, including atmospheric, oceanic, geological, and biological systems [cascio 2005]. this approach mainly refers to behavior that can result in long-term, widespread and/or hard-to-reverse changes in global systems; even if transient, local and superficial alterations can be considered through the prism of geoethics. indeed, other studies have considered geoethics as a new discipline (in both earth sciences and applied ethics) [martinez-frias 2008, nemec and nemcova 2011]. whenever ethical dilemmas occur in problems connected with the sustainable use of the earth resources, geoethics helps decision makers to undertake scientifically based political decisions [martinez-frias 2011]. this view links geoethics to ‘geodiversity’, which is considered as “the range of rocks, fossils, minerals, landforms and soils that occur on our planet, along with the natural processes that shape the landscape” [gray 2004]. by developing a geoethical approach to geodiversity, we can understand the value of abiotic nature and devise the correct strategies to conserve a legacy of geological assets. in a few words, we can enhance the ‘geoheritage’ of a region, i.e. the range of sites or areas of geological features with significant scientific, educational, cultural, or esthetic value [o’halloran et al. 1994]. moreover, the geoethical approach helps the potential of geological heritage sites to be identified. these sites might have tourism and economic relevance as local and regional resources [knudsen et al. 2008], or they might be identified as sites of historical and cultural value that are deserving of conservation (e.g., for the piemonte region, the langhian and serravallian historical global stratotype section and point) [mayer-eymar 1858, rio et al. 1997]. knowledge of regional geodiversity is fundamental not only for the successful exploration of natural resources, but also for geohazard assessment and mitigation. moreover, by developing the correct plans for geodiversity identification and management, a range of social, economic and environmental benefits can be delivered. enhancement of geodiversity awareness can provide not only the opportunity to study the evidence of past environmental changes and evolutionary processes, but can also reveal information needed for more accurate prediction of future environmental change and for adequate adaptation plans [giardino and progeo-piemonte research team 2011]. 2. methods and stages of the ‘progeo-piemonte’ research program from a geoethical perspective, the multidisciplinary research project known as ‘progeo-piemonte’ (proactive management of the geological heritage in the article history received december 21, 2011; accepted january 31, 2012. subject classification: geodiversity, geoheritage, geoethics, education, public issues, geohazards. 487 piemonte region) aims to define a new comprehensive conceptual and operational procedure to promote geodiversity knowledge, assess regional geoheritage, and support geoconservation activities in the piemonte region (north-western italy). the scientific background derives from both the geological and geomorphological literature of the area (see below), and from methodological studies relating to the development of techniques for the identification of geodiversity and for the management of geoheritage [serrano and ruiz-flano 1995, ellis et al. 1996, erikstad 1999, wimbledon et al. 1999]. the geoethical management of geoheritage requires a balance to be reached between the protection strategies of georesources and the legitimate needs for fruition by the local population or by visitors. this approach was introduced by the european working group for earth sciences conservation that was created on 1988. then in 1993, this transformed into the progeo association, which is devoted to geodiffusion activities and to establishment of an international network for geosite inventory and conservation [progeo 2011]. following this approach, a ‘progeo-piemonte’ multidisciplinary research group (earth sciences, biology, economics, informatics, social sciences, engineering) was created, which involved 58 participants from eight academic research and administrative institutions in the piemonte region.1 our assumption was that even during economic crisis, geoheritage sites (‘geosites’) can serve both public and private interests. the basic principles of geoconservation have been applied in several projects throughout the world. in italy, we found a gap between the acceptance of this conceptual framework and the application of geodiversity action plans. this is why we consider the application of a balanced mixing of ict (information and communications technology) instruments and geoethical principles innovative in the recognition of geodiversity, for the promotion of geological knowledge, the preparation of functional guidelines for the management of the geoheritage, and the support of geoconservation activities. the preliminary activities started during 2010. then, after a long evaluation process by an international committee, the project received financial support from university and regional bank foundations.2 ferrero et al. 488 figure 1. ‘visual’ approach of the progeo-piemonte project (see text for description). a. the piemonte region. b. a representation of the piemonte regional geodiversity (lithological map by arpa-piemonte: http://webgis.arpa.piemonte.it/geoportale/). c. the ‘disassembled rubik cube’ analogy. d. the operating selections of the rubik cube analogy. e. the ‘re-assembled’ rubik cube of the geological and geographical components of the piemonte region. a b d e c 489 the main stages of the progeo-piemonte research program are: (a) analysis of piemonte geodiversity at a regional scale, through a comparative study of past and present geological and geomorphological environments. (b) identification of ‘symbols’ for the piemonte geoheritage, the outstanding scientific and didactic values of which are coupled with the high potential for attracting tourists. (c) advanced scientific research on selected geosites and related geothematic areas, to provide a basis for dissemination of regional earth sciences knowledge. (d) experimentation on innovative didactic tools and visual representations of geological processes, and evolution of landscapes, for the enhancement of awareness of the natural resources and hazards. (e) promotion of geosites as valuable products, not only as material resources, but also as cultural and geotourism assets that can bring enjoyment to individuals and that are deserving of protection. (f ) elaboration of strategies for promotion of economic support for local communities responsible for geoheritage management and protection. 3. preliminary results on selected geothematic areas the application of these methods by specialized research teams from different scientific backgrounds (structural geology, sedimentary geology, paleontology, petrography, mineralogy, physical geography, geomorphology, lichenology, geoscience education) produced a preliminary report on the piemonte geodiversity at a regional scale. studies of the geodiversity contents are based on scientific knowledge that was either produced by the participants of this project, or was obtained from the literature. ‘hot topics’ for advancing our knowledge on the geological history of piemonte were analyzed, including climate and environmental change, natural hazards, soil processes, and georesources. as a preliminary result, several geosites were located to symbolize these different themes of piemonte geodiversity. a selection then had to be defined, both to enhance the individuality of different provinces of piemonte (figure 1a), and for the recognition of new strategic areas to be studied in the region. we applied a visual/conceptual approach to this problem that was based on geographical and geological criteria. earth scientists used different colors to highlight different geological units. the geothematic map of figure 1b shows the piemonte regional geodiversity, the analog of which can be individuated in the planar view of a ‘disassembled’ rubik cube (figure 1c). here, each solid color represents a different geological unit, in term of distinct processes, materials and structural settings. we considered the analogy between this three-dimensional mechanical puzzle and an idealized crustal block, with superficial, shallow and deep geoenvironments as a key for operating selections (figure 1d) and for the ‘re-assembly’ (figure 1e) of geological and geographical components of the piemonte region. this exercise offered different solutions that we evaluated in terms of geothematic contents and geographical distribution. our choices were addressed to the obtaining of a balanced sample of piemonte territories, and an (almost) complete representation of its geodiversity. as a result, we identified nine geothematic research themes (‘geothemes’; figure 2, 1 to 9) where we considered the ‘symbols’ of the piemonte geoheritage, which was also useful for the enhancement of the cultural awareness of the earth science topics, and for the promotion of geotourism throughout the region. each geotheme represents a distinctive geodiversity feature of the piemonte region that is deserving of advanced research, and that has particular management and protection needs. proactive management of these scientific themes (both as diffusion and protection) is now in progress, according to the following four interdisciplinary thematic actions: (a) geomatic applications for evaluation and management of piemonte geoheritage [ghiraldi et al. 2011]. (b) visual representation of geological environments and processes. (c) geodiversity action plans for dissemination activities. (d) tool-integrated management of a geosite: application of a territorial integrated quality management system (tiqms). our innovative approach to the piemonte geodivergeodiversity action plans in the piemonte region figure 2. the ‘disassembled rubik cube’ analogy for the description of the geothematic areas of the progeo-piemonte project. sity received approval from an international committee, and it is now available to the regional administration for the assessment and management of the geoheritage. the same visual/conceptual approach is useful to spread awareness of geoheritage among the general public. a better understanding of the complexity of regional geodiversity and its interactions with human activities and territories can promote the correct selection of geosites according to geothematic contents and targeted management activities. this can result in a balanced approach, between geoheritage protection issues and the needs of local populations and of visitors. the preliminary findings of the progeo-piemonte research groups for both the study of the regional geothemes and in the interdisciplinary thematic actions are analyzed and commented upon in the following sections. 4. analysis of symbols of the piemonte geoheritage piemonte is very rich in geological symbols that can be used to open people’s hearts and minds to the magnificence of nature and to the need for her conservation for the enjoyment of future generations. here, two examples are presented to provide an understanding of the role of geodiversity in the environmental dynamics of the piemonte region and of its geoheritage as a cultural asset. the first example is the belvedere glacier (north-eastern face of mt. rosa; figure 2, geothematic area 6; figure 3), the view of which offers a spectacular analog of the pleistocene glacial ages, very far from the anthropogenic spread. here, the most extensive piemonte glacier has changed its appearance rapidly in the last few years as a consequence of the temperature increases that are affecting the western alps [haeberli et al. 2002, gianotti et al. 2011]. from an educational point of view, the comparison of the present and past geomorphological landscapes allows us to understand the earth surface processes and these environmental dynamics. the variations, extent and thickness of the glacier, and its landform changes and mass wasting processes, can be used as lessons to show the effects of climate change and natural hazards [haeberli et al. 2002]. these can be used to analyze the possible interactions between human activities and the environment. a better understanding of these processes will favor respectful attitudes toward geological assets. as an example, the putting together of a movie sequence of thousands of images of glaciers that were taken through the temporal window of the last hundred years can provide very convincing evidence of a climatedriven dynamic process for a wider public and will be a useful tool for didactic activities. moreover, it is a possible tool to widen the consensus on environmental problems, and a way to reach broader adherence and participation in certain choices for the protection of our georesources. widespread awareness in the general public leads and influences the choices of managers towards the correct management of our georesources. another example among the piemonte geo-symbols is the mining district of brosso and traversella (figure 2, geothematic theme 7) [costa et al. 2011]. this area has been well known since prehistoric times, when our ancestors began to extract iron, as the ‘resistant and strong’ material that was useful to produce artisan tools for hunting and working the fields. in ancient times, the local people underwent impositions and abuses by foreign conquerors and local potentates, with pressure to increase the amount of extracted minerals. in more modern times, the mining activities allowed the local people to enjoy greater economic wealth, until the closures of the brosso and traversella mines (in 1964 and 1971, respectively). so, the mining trails, the forges spread throughout the area, the extraction tunnels, the mortuary chapels where ‘white deaths’ were cried over, and the yellow to red sulfur-rich veins all remind us of the hard but fruitful work humans can do. also, ferrero et al. 490 figure 3. the belvedere glacier (photo: gnemmi). 491 the remains of this activity and witnessing the world of the old miners teaches our generation, and future generations, of the need to reduce the human environmental impact and to enhance the safety of the workers. 5. from scientific knowledge to diffused geo-awareness geological literacy has a strategic role for the improvement of our awareness of earth science topics that impact on human affairs. the progeo-piemonte project conceived geothematic themes (figure 2) to aid in the diffusion of the knowledge of ‘hot’ earth science topics, such as: – long-term climate change through past geological times (figure 2, geothematic theme 3), to show its effects on the ecosystem (population and habitat) and geosystem (environmental and landscape changes) [dela pierre et al. 2011a, 2011b]; – time scales of geological processes (stratigraphic and tectonic combined in an orogenic succession) (figure 2, geothematic theme 2), to reveal long-lasting impacts on our planet, and to show their possible interactions with human activities and on our future [bertok et al. 2011, d’atri et al. 2011]; – limitation of the available georesources (figure 2, geothematic theme 7) and is influences on the lifestyle of ancient and future generations [costa et al. 2011]. up-to-date scientific research on these themes is essential to address the correct management of georesources, and the adaptation for remedial measures for hazardous geological processes. accurate dating of paleoclimatic and tectonic events can clarify: (a) how long climate change has lasted in comparison to the human and geological time scales; (b) how these events have impacted on life and the geosystem; and (c) if and when the biome recovered after these events. as an example, new data on this debated topic have been realized through recent studies on the messinian succession of the tertiary piemonte basin (figure 2, geothematic theme 3) [dela pierre et al. 2011a]. an understanding of the major environmental changes that occurred around 6 million years ago during a short time span (compared to earth history) is useful, to improve our knowledge of the related modifications to biological communities and the environment. improvements in the interpretation of the regional structural settings, stratigraphic and metamorphic characters, and seismo-tectonic activity of the western alps (figure 2, geothematic themes 1 and 2) [d’atri et al. 2011, rolfo et al. 2011] are legacies to future generations, because they can help in a better assessment of the georesources and geohazards of the piemonte mountains. these are other valuable products of the project: they support the idea that geology is an array of ongoing processes that primarily involve the everyday life of our and future generations. eventually, this newly achieved awareness will strengthen the general idea that continuous earth science education can improve our life style and our place on earth [belluso et al. 2011]. with respect to the educational goals of the project, our strategy is to develop the cognitive aspects of the teaching/learning processes of earth sciences, while enhancing the sensory and emotional aspects that condition the acquisition of this knowledge [orion 2007]. educational materials should create moments of wonder and surprise, including positive feelings related to the moments of discovery. also, sometimes, they should promote a sense of distress, with the realization of the existence of ‘problems’ and the personal inadequacy to deal with them [king 2008, p. 197]. all of these sensations and feelings are part of people’s emotional intelligence, and they become a powerful stimulus for: (a) keeping the focus on not-intuitive issues, which are sometimes abstract and rarely explored by direct experience; (b) motivating personal knowledge improvement, such as looking for visual documents, broadening and deepening the range of interest, repeating the experience in the same place, or in different ones. according to the suggestions of a holistic approach for science education [orion 2007], we believe that learning in nonformal educational situations provides an excellent opportunity for the students [king 2008, p. 201]. it makes it easier to perform participatory actions, to produce positive sensations through active personal involvement, with the association of the enjoyment with recreational activities and with feelings of satisfaction for the conquest of a ‘goal’. 6. geo-resources and geohazards following the international decade for natural disaster reduction (indnr 1990-2000), many projects have been carried out on different scales (national, regional, local) to provide better understanding of natural hazards. then the international year of planet earth (2007-2009) [iype 2008, unesco 2008] promoted a global view of geoheritage and georesources. in this perspective, our project aims to: (i) enhance a geo-ethical approach to these fundamental fields; (ii) increase the competence of local administrators and educators for their understanding of the value of local georesources, and their sustainable exploitation; (iii) mitigate risks related to hazardous earth processes. three of the geothematic themes of the progeopiemonte project (figure 2) offer significant enhancements for the awareness of geohazards and geoheritage: geodiversity action plans in the piemonte region 1) the interactions of geological processes and human activities in the monferrato hills (figure 2, geotheme 4) show that the cultural heritage of this land and the economic value of its worldwide-known vineyard terrain have deep roots on its ancient and recent geological history. earth sciences knowledge of this area is fundamental for the protection of the local resources from geohazards, and progeo-piemonte promotes the correct assessment and mitigation of local accelerated erosion and slope instabilities [van den eeckaut et al. 2010, masciocco et al. 2011]. 2) studies on the quaternary evolution of the landscape and the local climates have been used to offer a comprehensive view of the interactions of major geomorphological systems within geothematic theme 5 (figure 2), where glacial, fluvial and anthropic processes are relevant for local georesources and hazards [ajassa et al. 2011]. 3) as a result of the systematic review of geoscience and natural hazards information, the middle susa valley (figure 2, geothematic theme 8) has become a strategic area for the enhancing of public understanding of natural hazard studies; the achievement of these objectives was made easier by the recreational and didactic activities that are supported by the local communities. visual representations and digital evolutionary models of large slope instabilities (cassas landslide and serre la voute deep-seated gravitational slope deformation) have popularized scientific results of the recent stages of local geological history, the effects of climate and environmental change and natural hazards, and the related risk management practices in the susa valley [borgogno mondino et al. 2009, lombardo et al. 2010, giardino et al. 2011]. 7. geosites as cultural and geotourism assets geothemes 1, 3, 4 and 9 of figure 2 have offered excellent opportunities to illustrate multiple aspects of earth sciences, due to the variety of geological and geomorphological phenomena they represent. each of these themes also differs from the others in term of territorial and environmental context: they contain a variety of locations with respect to settlements, infrastructure and manufacturing activities, and different possible accessibility of geosites in relation to the seasonal conditions and the type of public. a coherent and comprehensive selection of geosites makes the geoheritage a valuable cultural resource that can provide local and regional economic benefits as tourist destinations. as an example, geothematic theme 9 (figure 2) has been selected, studied and exploited for its very complex and interdisciplinary significance: it emphasized the scientific and cultural value of many historical piemonte stones [borghi 2010, borghi et al. 2011] which have been used in the architecture of the city of turin, the first capital of italy (figure 4) [favero longo et al. 2009]. the urban geosites of this geothematic theme are always easily accessible; they offer great potential to promote geotourism, with the integration of historical and artistic aspects of the town with the petrographic nature of the stone used for construction. these geosites are also ideal for educational use by students if they are coupled with visits to the temporary and permanent exhibitions at local museums (e.g. museo regionale di scienze naturali/regional natural sciences museum) and with ‘virtual tours’ that can be offered through the internet (e.g. museotorino) [lucchesi and giardino 2011]. 8. economic support for geoheritage management communities living in a territory with an important geological site should be considered as owners of this property. as a consequence, a program for geoheritage enhancement should have positive local economic impacts. this is a necessary theoretical and practical approach for creating ‘virtuous circles’ that are aimed at maintaining the site and at raising the geological awareness, both of which are fundamental actions to attain the correct management of territorial assets. the economical benefits derived are then available for an improved quality of life of the resident populations. administrators should be made aware of the historical, cultural and economical value of geoheritage, so that they can introduce limitations to the exploitation of the site, as geological assets are finite resources. by following the principle of sustainability, these assets will be long lasting, and open to and usable by future generations. the process that links life quality and sustainability will be analyzed in the progeo-piemonte project by the commodity science department of the university of turin, by using tiqms, a certified system that takes into account environmental protection, occupational safety, protection of the landscape, social responsibility and prodferrero et al. 492 figure 4. ornamental stones of piemonte: the superga basilica, via roma, turin (photo: borghi). 493 uct quality. using international evaluation standards, tiqms analyses every step of geoheritage management and development, starting with the involvement of the local community [beltramo et al. 2011]. preliminary applications of this methodology have suggested successive steps in the economic development and geoconservation of the geothematic themes; for example: – in geothematic theme 1 (figure 2), economic development must take into account the geographical and social contexts. geologically active areas of the western alps are also affected by population loss. here, the correct evaluation, selection and management of geoheritage sites will boost geo-aware tourism growth, leading to social improvement and geoconservation actions; – in geothematic theme 4 (figure 2), the geological and geomorphological assets of the monferrato hills are the main constraints for the economic development of the area. a balanced interplay between territorial development and soil conservation policies is suggested, as excess exploitation would lead to the loss of a key resource (vineyards) for the population. 9. final remarks and future developments by capitalizing on previous successful experiences [giardino and mortara 1999, giardino and mortara 2004, boschis and giardino 2004, boschis 2010], the progeopiemonte project supports the development of the correct plans for geodiversity identification and geoheritage management of nine geothemes of the piemonte region (north-western italy). a new comprehensive conceptual and operational procedure has been set up: 1) fundamental knowledge of regional geodiversity is achieved by up-to-date scientific research that is addressed to key regional issues of the earth sciences; 2) management of geoheritage sites is developed with a geoethical approach, which involves local residents and administrators; 3) the correct plans for geoheritage management, to increase the value of the local geodiversity and its sustainable exploitation, is achieved by applying tiqms. as one of the main focuses of the project has been on the actions for spreading geological culture, different products have been prepared: teaching materials for primary and secondary schools [king 2008, page 209]; training courses for teachers, restorers, and tourism operators; events for the general public (museum exhibitions, geological field trips); geoheritage promotion through digital media (virtual tours, interactive websites). other similar products are in progress to further enhance the cognitive aspects of teaching earth sciences, coupled with sensory and emotional aspects related to the acquisition of knowledge. all of the progeo-piemonte educational materials are conceived to be shared with local organizations, schools, and museums. in addition, the long-term and short-term objectives of the project are integrated into the needs of the local communities, who receive information about the scientific value of the geoheritage of their territory and participate in setting objectives and steps of intervention, to attract visitors and tourism. local communities and administrators provide human resources and funding for the achieving of the objectives of the project and for the enhancing of the dissemination and use of geological knowledge (e.g. museotorino) [scoprialpi 2010, lucchesi and giardino 2011]. the development and dissemination of ‘geological skills’ to children, youths and adults is addressed to a positive impact on the daily life of the local community, both under normal conditions and during geo-hazardous events. moreover, the progeo-piemonte project promotes healthy and respectful interactions with the physical and biological components of the environment, and with other human communities, by favoring better understanding of the value of geodiversity and stronger perception of the natural risks. a geoethical approach to the geodiversity of the piemonte region will reduce local vulnerability to disasters and will support the balanced management of geoheritage through the application of the tiqms, which is suitable for tourism, sustainable development, and geoconservation. notes 1. dipartimento di scienze della terra, dipartimento di scienze della vita e biologia dei sistemi, dipartimento di informatica, dipartimento di scienze merceologiche, università degli studi di torino; dipartimento di ingegneria del territorio, dell’ambiente e delle geotecnologie, politecnico di torino; museo regionale di scienze naturali, torino; comitato glaciologico italiano; cnr – istituto di geoscienze e georisorse, torino. 2. co-financing plan by the compagnia di san paolo bank foundation and the università degli studi di torino (‘progetti di ricerca d’ateneo’, 2011). acknowledgements. the authors wish to thank all the colleagues involved in the progeo-piemonte project and the two anonymous reviewers. thanks also go to silvia peppoloni and giuseppe di capua for inviting us to contribute to this volume and for their accurate editing. references ajassa, r., e. destefanis, m.g. forno, s. fratianni, f. gianotti, e. martinetto, l. masciocco and l. perotti (2011). natural and artificial waters of north-eastern piedmont (progeo-piemonte project, geothematic area 5), epitome, 4, 113, issn 1972-1552. belluso, e., e. ferrero, m. giardino, f. lozar and l. perotti (2011). geodiversity action plans for dissemination acgeodiversity action plans in the piemonte region tivities of progeo-piemonte project (interdiscipliary research area “c”), epitome, 4, 278, issn 1972-1552. beltramo, r., e. vesce, s. duglio and m. giardino (2011). tool-integrated management of a geosite: application of tiq (territorial integrated quality) for the progeopiemonte project (interdiscipliary research area “d”), epitome, 4, 112, issn 1972-1552. bertok, c., l. martire, e. perotti, a. d’atri and f. piana (2011). middle-late jurassic syndepositional tectonics recorded in the ligurian brianconnais succession (marguareis-mongioie area, ligurian alps, nw italy), swiss j. geosci., 104, 237-255. borghi, a. 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(20072009): http://portal.unesco.org/science/en/ev.php-url _id=6318&url_do=do_topic&url_section =201.html. corresponding author: elena ferrero, università di torino, dipartimento di scienze della terra, torino, italy; email: elena.ferrero@unito.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. geodiversity action plans in the piemonte region << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice registrazione di t e r r e m o t i vicini con s i s m o m e t r i a l u n g o p e r i o d o (recording of near earthquakes by long-period seismometers) e . mantovani (*) r i c e v u t o il 10 g i u g n o 1972 riassunto. — v e n g o n o r i p o r t a t e le c a r a t t e r i s t i c h e d e l l a n u o v a r e t e a l u n g o p e r i o d o i n i t a l i a e l a p o s s i b i l i t à di s f r u t t a r e i s i s m o m e t r i d e l l a r e t e p e r l a r e g i s t r a z i o n e d e i s i s m i v i c i n i . summary. — t h e c h a r a c t e r i s t i c s of t h e l o n g p e r i o d s e i s m i c n e t w o r k i n i t a l y a r e d e s c r i b e d a n d a m e t h o d t o u s e t h e l o n g p e r i o d s e i s m o m e t e r f o r t h e r e c o r d i n g of n e a r e a r t h q u a k e s is s u g g e s t e d . l ' i s t i t u t o di geofisica d e l l ' u n i v e r s i t à di b o l o g n a , i n c o l l a b o r a zione con le u n i v e r s i t à di k a r l s r u h e e u c l a di los angeles, s t a svolg e n d o u n a ricerca sulla c r o s t a del m e d i t e r r a n e o con p a r t i c o l a r e rig u a r d o al b a c i n o del t i r r e n o . q u e s t a ricerca è c o n d o t t a col m e t o d o b a s a t o sulla d i s p e r s i o n e delle o n d e superficiali, p e r la r e g i s t r a z i o n e delle q u a l i è s t a t a i n s t a l l a t a u n a r e t e d i s t a z i o n i a l u n g o p e r i o d o (4) (fig. 1, t a b e l l a 1). u n a serie di p r o v e h a p e r m e s s o di m e t t e r e a f u o c o p r i m a e s u p e r a r e p o i i p r o b l e m i connessi con la realizzazione di u n a s t a z i o n e sismica a c o r t o p e r i o d o che u t i l i z z a al p o s t o dei n o r m a l i p e n d o l i a c o r t o i s i s m o m e t r i a l u n g o p e r i o d o della c i t a t a r e t e . (*) i s t i t u t o d i g e o f i s i c a . u n i v e r s i t à di b o l o g n a ( i t a l i a ) . r e g i s t r a z i o n e d i t e r r e m o t i v i c i n i con s i s m o m e t r i a l u n g o p e r i o d o 5 5 3 l a s c e l t a degli s t r u m e n t i di u n a s t a z i o n e sismica a c o r t o p e r i o d o è c o n d i z i o n a t a dalle s e g u e n t i esigenze: a) m a s s i m a sensibilità i n c o r r i s p o n d e n z a delle f r e q u e n z e d o v e è g e n e r a l m e n t e c o n c e n t r a t a l ' e n e r g i a delle o n d e p (0,5 -f1,5 sec). q u e s t o f a v o r i s c e u n a s a l i t a r i p i d a del p r i m o i m p u l s o e di conseg u e n z a precisione nella d e t e r m i n a z i o n e del t e m p o di a r r i v o . b) discesa ripida d e l l ' a m p l i f i c a z i o n e sia p e r i p e r i o d i > 3 sec, sia p e r quelli < 0,6 sec. l a p r i m a , p e r e v i t a r e il d i s t u r b o di t u t t a l ' e n e r g i a sismica a l u n g o p e r i o d o (onde 8 e superficiali), m a s o p r a t t u t t o p e r r i d u r r e al m i n i m o quello microsismico ( 3 + 7 sec) d o v u t o a p e r t u r b a z i o n i a t m o s f e r i c h e . l a s e c o n d a p e r e v i t a r e l ' a l t r o t i p o di a t t i v i t à m i c r o s i s m i c a a p i ù a l t a f r e q u e n z a (0,4 + 0,6 sec) c a u s a t a d a d i s t u r b i di c a r a t t e r e locale (traffico, i n d u s t r i e . . . ) . c) m i n o r ritardo possibile t r a l ' a r r i v o del segnale e la s u a reg i s t r a z i o n e . l e condizioni a) b) c) n o n sono realizzabili c o n t e m p o r a n e a m e n t e . i n f a t t i p e r o t t e n e r e u n r i t a r d o m i n i m o p e r le f r e q u e n z e delle o n d e p s a r e b b e necessario u s a r e g a l v a n o m e t r i e s i s m o m e t r i d i p e r i o d o r i s p e t t i v a m e n t e < 1 sec e > 1 sec, il che c o n t r a s t a con i p u n t i a) e b), che c o m e si p u ò v e d e r e dalle c u r v e di r i s p o s t a delle figg. 2, 3, p o r t a n o a d u n a s c e l t a di t„ e ts c o m p r e s i t r a 0,5 e 1,5 secondi. g e n e r a l m e n t e v i e n e d a t o m a g g i o r peso a i p r i m i d u e p u n t i cerc a n d o di l i m i t a r e il r i t a r d o al m i n i m o possibile. l a scelta del p e r i o d o degli s t r u m e n t i v a f a t t a s t a z i o n e p e r s t a zione in b a s e ai v a l o r i della c o n t i n g e n t e a t t i v i t à m i c r o s i s m i c a . s t a z i o n e a c o r t o p e r i o d o c o n s i s m o m e t r o d i i s = 1 5 s e c e g a l v a n ó m e t r o d i t„ — 1 , 8 s e c . a n a l i z z i a m o o r a l a p o s s i b i l i t à di o t t e n e r e le c a r a t t e r i s t i c h e prec e d e n t i con u n s i s t e m a che al p o s t o di u n p e n d o l o di 0,5 + 1,5 sec, n e i m p i e g h i u n o di 15 secondi. l ' i n c o n v e n i e n t e l e g a t o a l l ' u s o di u n t a l e t i p o di s i s m o m e t r o è lo scarso filtraggio dei p e r i o d i l u n g h i (fig. 2). p e r e l i m i n a r e o p e r lo m e n o a t t e n u a r e la difficoltà b i s o g n a ricorr e r e a d u n filtro e l e t t r o n i c o che p u ò essere sia di t i p o a t t i v o , cioè com5 5 4 e . m a n t o v a n i f i g . 2 c u r v e di r i s p o s t a del s i s m o m e t r o , c r i t i c a m e n t e s m o r z a t o , p e r d i v e r s i v a l o r i d e l p e r i o d o . f i g . 3 c u r v e d i r i s p o s t a n o r m a l i z z a t e d e l g a l v a n ó m e t r o c r i t i c a m e n t e s m o r z a t o . p o s t o (li e l e m e n t i a m p l i f i c a t o r i , sia d i t i p o p a s s i v o , c o m p o s t o d i soli c o n d e n s a t o r i , i n d u t t a n z e , r e s i s t e n z e . col p r i m o t i p o è possibile o t t e n e r e , o l t r e a d u n a a m p l i f i c a z i o n e del segnale, a n c h e u n t a g l i o m o l t o n e t t o alle f r e q u e n z e v o l u t e : esiste r e g i s t r a z i o n e d i t e r r e m o t i v i c i n i con s i s m o m e t r i a l u n g o p e r i o d o 553 t u t t a v i a il p r o b l e m a , n o n s e m p r e risolvibile, d e l l ' e l i m i n a z i o n e del r u m o r e e l e t t r o n i c o . i n a t t e s a di p e r f e z i o n a r e u n filtro a t t i v o a d a t t o allo scopo si è ricorso a d u n semplicissimo filtro p a s s a a l t o di t i p o or (fig. 4). o 1—11 1 1—t 1 i i l — l — — w! 0,05 o.l li 1 2 3 4 5 io t (.«, f i g . 4 a ) s c h e m a del filtro c r e s u a r i s p o s t a in a m p i e z z a b ) e f a s e c ) . i l s i s t e m a i n q u e s t i o n e , che p e r c o m o d i t à è s t a t o c h i a m a t o l c p , v i e n e così a d essere c o m p o s t o d a : 1) s i s m o m e t r o a l u n g o p e r i o d o t i p o s p r e n g n e t h e r s-5007-v m a s s a = 11,2 k g sensibilità = 89 v / m / s e c p e r i o d o = 15 sec. 554 e . m a n t o v a n i 2) g a l v a n ó m e t r o a c o r t o p e r i o d o t i p o c a m b r i d g e 41127/1 s e n s i b i l i t à = 300 m m / j x a p e r i o d o = 1 , 8 sec. 3) f i l t r o p a s s i v o c o s t i t u i t o d a u n a singola cella c e . i n f i g . 5 è r i p o r t a t a la r i s p o s t a i n a m p i e z z a del s i s t e m a i n e s a m e a c c a n t o a quelle d i a l t r i complessi di r e g i s t r a z i o n e sismica. l e c u r v e sono n o r m a l i z z a t e p e r a g e v o l a r e il c o n f r o n t o d e l r a p p o r t o s e g n a l e r u m o r e d a cui d i p e n d e n o t e v o l m e n t e la q u a l i t à d i u n s i s m o g r a f o . d a q u e s t o c o n f r o n t o r i s u l t a che il s i s t e m a l c p p r e s e n t a , r i s p e t t o agli a l t r i di f i g . 5, u n filtraggio l e g g e r m e n t e i n f e r i o r e dei m i c r o s i s m i e la s e n s i b i l i t à m a s s i m a s p o s t a t a v e r s o f r e q u e n z e a p p e n a p i ù a l t e . i l c o n f r o n t o f r a i s i s t e m i u s u a l i e quello q u i p r o p o s t o v a comp l e t a t o con a l c u n e c o n s i d e r a z i o n i sul r i t a r d o del segnale. u n a n o r m a l e s t a z i o n e a c o r t o p e r i o d o , che p e r s e m p l i c i t à consid e r i a m o r a p p r e s e n t a t a d a u n s i s t e m a con t, = l s e c e t„ = l s e c , p r e s e n t a u n r i t a r d o t r a l ' a r r i v o delle o n d e p e la loro r e g i s t r a z i o n e 554 e. mantovani d i circa m e z z o p e r i o d o (tc/2 d o v u t o a l s i s m o m e t r o e tc/2 al g a l v a n o m e t r o ) . il s i s t e m a l o p p u r e s s e n d o c o m p o s t o d a s t r u m e n t i di c a r a t t e r i s t i c h e d i v e r s e ( s i s m o m e t r o e filtro) d a l p r e c e d e n t e p r o v o c a u n r i t a r d o p r e s s o c h é a n a l o g o . p e r q u a n t o r i g u a r d a il r i t a r d o del g a l v a n o m e t r o l ' a n a l o g i a t r a i d u e s i s t e m i è e v i d e n t e . il s i s m o m e t r o a l u n g o p e r i o d o del s i s t e m a l c p , i n v e c e , si differ e n z i a d a quello c p i n q u a n t o la s u a m a g g i o r e i n e r z i a gli p e r m e t t e di r i d u r r e al m i n i m o il r i t a r d o t r a l ' e v e n t o e la sua r e g i s t r a z i o n e . l ' u nico e f f e t t o è il r i b a l t a m e n t o del segnale, che p r e s e n t e r à q u i n d i m a s simi al p o s t o di m i n i m i e v i c e v e r s a . l a q u a s i a s s e n z a di r i t a r d o d o v u t o al s i s m o m e t r o v i e n e p e r ò c o m p e n s a t a d a quello i n t r o d o t t o d a l filtro. i n f a t t i p e r f r e q u e n z e di ~ 1 sec, l ' i m p e d e n z a è q u a s i p u r a m e n t e r e a t t i v a e p r o v o c a u n r i t a r d o t r a t e n s i o n e e c o r r e n t e di circa tt/2 (pig. 4). stazione a corto e lungo periodo l a s e c o n d a p a r t e d e l l ' e s p e r i m e n t o consiste n e l l ' i m p i e g o di u n solo s i s m o m e t r o con p e r i o d o d i 15 sec p e r la r e g i s t r a z i o n e s i m u l t a n e a dei c o r t i e l u n g h i p e r i o d i . 0 p i g . 7 c i r c u i t i d i c o l l e g a m e n t o t r a il s i s m o m e t r o e i d u e g a l v a n o m e t r i c l p e g c r a q u e s t o scopo si sono s f r u t t a t e le d u e b o b i n e di u s c i t a di cui è f o r n i t o il s i s m o m e t r o s p r e n g n e t h e r s-5007-v. r e g i s t r a z i o n e di t e r r e m o t i v i c i n i con s i s m o m e t r i a l u n g o p e r i o d o 553 u n segnale è s t a t o n o r m a l m e n t e i n v i a t o al g a l v a n ó m e t r o l p (tg = 100 sec) e l ' a l t r o al g a l v a n ó m e t r o c p (tg = 1,8 sec). i l p r o b l e m a p r i n c i p a l e è quello d i e v i t a r e q u a l s i a s i t i p o di i n t e r a z i o n e t r a i d u e complessi. l e u s c i t e sono e l e t t r i c a m e n t e i n d i p e n d e n t i ed è perciò d a esclud e r e u n d i s t u r b o d i r e t t o t r a i d u e circuiti. l ' u n i c a possibilità di i n t e r a z i o n e r i m a n e q u i n d i nella r e a z i o n e che ogni g a l v a n ó m e t r o p u ò a v e r e sul p e n d o l o . su q u e s t o a r g o m e n t o , esistono già l a v o r i teorici (') in cui si chiarisce che p e r v a l o r i della m a s s a s u p e r i o r e a 7 k g la r e a z i o n e del g a l v a n ó m e t r o sul p e n d o l o si p u ò c o n s i d e r a r e t r a s c u r a b i l e a t u t t i gli e i ì e t t i p r a t i c i . p e r a g g i u n g e r e alle p r e v i s i o n i t e o r i c h e u n a c o n f e r m a s p e r i m e n t a l e , si è p r o c e d u t o al c o n f r o n t o delle c u r v e di c a l i b r a z i o n e o t t e n u t e con i d u e g a l v a n o m e t r i collegati sia s i n g o l a r m e n t e che c o n t e m p o r a n e a m e n t e al s i s m o m e t r o , col r i s u l t a t o di t r o v a r l e s o v r a p p o n i b i l i e n t r o le fluttuazioni p r o d o t t e da microsismi, e r r o r i di osservazione ecc. c o n f r o n t o t r a l e c u r v e d i c a l i b r a z i o n e l a verifica c h e il complesso a l u n g o p e r i o d o n o n v i e n e i n f l u e n z a t o d a l c o l l e g a m e n t o del g a l v a n ó m e t r o c p al s i s m o m e t r o , è f a t t o m e d i a n t e il c o n f r o n t o t r a le d u e c u r v e di c a l i b r a z i o n e che si o t t e n g o n o nelle condizioni di c o l l e g a m e n t o r a p p r e s e n t a t e in f i g . 8 dagli s c h e m i a) b). l a p r e s e n z a p e r ò d i u n a serie di d i s t u r b i a l e a t o r i c o m e m i c r o s i s m i , d e r i v e t e r m i c h e ecc. r e n d e poco significativo u n c o n f r o n t o d i r e t t o t r a le d u e c u r v e . si è p e n s a t o perciò di d e t e r m i n a r e p e r la condizione a) u n p a r a m e t r o r a p p r e s e n t a t i v o della s i t u a z i o n e m e d i a e r i s p e t t i v o c a m p o di v a r i a b i l i t à . si sono r i c a v a t e e d i g i t a l i z z a t e 10 c u r v e di c a l i b r a z i o n e n e l l a condizione a). si è o t t e n u t a u n a serie di 45 coefficienti di c o r r e l a z i o n e p r o v e n i e n t i d a t u t t e le c o m b i n a z i o n i di c o n f r o n t i t r a le c u r v e digit a l i z z a t e . d a q u e s t a serie si è o t t e n u t o u n v a l o r e m e d i o g = 0,998 e u n o s c a r t o q u a d r a t i c o m e d i o £ = 0,001. si è poi c o r r e l a t a u n a c u r v a o t t e n u t a nella condizione b) con t u t t e le 10 o t t e n u t e nella condizione a) r i c a v a n d o 10 coefficienti che c o i n c i d e v a n o t u t t i , e n t r o lo s c a r t o f , con il v a l o r m e d i o c. 554 e . m a n t o v a n i r i p e t u t o q u e s t o p r o c e d i m e n t o p e r a l t r e q u a t t r o c u r v e di b) e o t t e n u t i , t r a n n e 2 s u 40 coefficienti, gli stessi r i s u l t a t i p r e c e d e n t i si è d e d o t t o c h e il complesso a l u n g o p e r i o d o n o n s e n t e la p r e s e n z a del g a l v a n ó m e t r o c p . nza di smorzamento critico del pendolo f i g . 8 a ) c o m p l e s s o a l u n g o p e r i o d o ; b ) e n t r a m b i i c o m p l e s s i ( l p c p ) i n f u n z i o n e c o n t e m p o r a n e a m e n t e ; c ) c o m p l e s s o a c o r t o p e r i o d o . l o stesso m e t o d o c o n d o t t o con le c u r v e o t t e n u t e nelle condizioni b) e c) h a escluso i n f i n e q u a l s i a s i t r a s f e r i m e n t o a p p r e z z a b i l e di e n e r g i a dal g a l v a n ó m e t r o l p a quello c p . r e g i s t r a z i o n e d i t e r r e m o t i v i c i n i con s i s m o m e t r i a l u n g o p e r i o d o 553 c i r c i ' i t i d i c o l l e g a m e n t o u n c o r r e t t o f u n z i o n a m e n t o degli s t r u m e n t i r i c h i e d e c h e : 1) i d u e gal v a r i o m e t r i e s i s m o m e t r i s i a n o v i c i n i allo s m o r z a m e n t o critico. 2) l ' a t t e n u a z i o n e sia del complesso c p che l p sia t a l e d a t e n e r e il livello dei microsismi, nelle r e g i s t r a z i o n i , i n f e r i o r e a 1-2 nini. i n f i g . 7 sono r i p o r t a t i i c i r c u i t i richiesti dalle condizioni p r e c e d e n t i . l a connessione a t del l u n g o p e r i o d o p r o d u c e lo s m o r z a m e n t o critico sia del p e n d o l o che del g a l v a n o m e t r o l p e la a t t e n u a z i o n e n e c e s s a r i a . i l c i r c u i t o d i a c c o p p i a m e n t o del c o r t o p e r i o d o (leve p r o d u r r e lo s m o r z a m e n t o critico del g a l v a n o m e t r o c p e la g i u s t a a t t e n u a z i o n e del segnale. i n o l t r e d e v e p r e s e n t a r e al s i s m o m e t r o u n ' i m p e d e n z a m o l t o m a g g i o r e della s u a r e s i s t e n z a di s m o r z a m e n t o critico p e r e v i t a r e u n s o v r a s m o r z a m e n t o del p e n d o l o . c o n c l u s i o n i l e c o n s i d e r a z i o n i p r e c e d e n t i p e r m e t t o n o di t r a s f o r m a r e le a t t u a l i s t a z i o n i a l u n g o p e r i o d o i n stazioni a d o p p i a r e g i s t r a z i o n e , col r i s u l t a t o d i c r e a r e u n a r e t e a c o r t o p e r i o d o che p e r m e t t e r e b b e u n n o t e v o l e m i g l i o r a m e n t o n e l l ' a c c u r a t e z z a con cui v e n g o n o d e t e r m i n a t i i p a r a m e t r i sismici dei t e r r e m o t i vicini e, s o p r a t t u t t o , ili (nielli che s f u g g o n o alle b a s s e sensibilità dei v e c c h i sismografi w i e c k e r t . p e r la t r a s f o r m a z i o n e è sufficiente c o m p l e t a r e ogni s t a z i o n e con u n g a l v a n o m e t r o c p di a l t a sensibilità e con u n r e g i s t r a t o r e f o t o grafico con velocità d i r o t a z i o n e del t a m b u r o di 6 c m / m i n u t o . l ' a c c u r a t e z z a del t e m p o è g a r a n t i t a dagli orologi a q u a r z o (sinc r o n i z z a t i v i a r a d i o con u n a s t a z i o n e e m i t t e n t e svizzera) di cui sono o r m a i c o r r e d a t e q u a s i t u t t e le s t a z i o n i della r e t e . s v i l u p p i i r i s u l t a t i c o n s e g u i t i fino a d o r a n o n sono che u n p r i m o p a s s o d e l l ' e s p e r i m e n t o i n corso e r i m a n g o n o q u i n d i s o t t o e s a m e a l c u n i a s p e t t i del n u o v o s i s t e m a c h e p o t r e b b e r o essere passibili di m i g l i o r a m e n t o . 554 e . m a n t o v a n i il p r i n c i p a l e , c o m e già v i s t o , è il filtraggio dei m i c r o s i s m i , il q u a l e p u r e s s e n d o già q u a s i allo stesso livello degli a l t r i s i s t e m i p e r , noi n o n è a n c o r a del t u t t o s o d d i s f a c e n t e . a q u e s t o scopo si s t a n n o s t u d i a n d o a l t r i t i p i d i filtri c a p a c i di f o r n i r e t a g l i p i ù n e t t i s e n z a i n t r o d u r r e sensibili r i t a r d i del segnale. vi è i n o l t r e la p o s s i b i l i t à di collegare al s i s m o m e t r o l p u n a serie di g a l v a n o m e t r i con d i v e r s o p e r i o d o . si o t t e r r e b b e cosi u n i n s i e m e d i c o m p l e s s i sensibili a i n t e r v a l l i d i v e r s i d i f r e q u e n z e , i n t e r e s s a n t i d i v e r s i c a m p i d i s t u d i o c o m e : s i s m i c i t à locale (0,1 1,5 sec), m i c r o s i s m i (2 -f8 sec), o n d e superficiali (oltre 10 sec) ecc. r i n g r a z i a m e n t o e i n g r a z i o il p r o f . m. c a p u t o p e r le f r u t t u o s e discussioni ed i preziosi consigli. b i b l i o g r a f i a (1) chakrabakty s. k . , 1949. response characteristics of electromagnetic seismographs and their dépendance on the instrumental constants. « b u l l . s e i s m . , s o c . a m . », 3 9 , 2 0 5 2 1 8 . (2) eaton j . p . , 1957. theory of electromagnetic seismographs. « b u l l . s e i s m . , s o c . a m . », 4 7 , 3 7 7 5 . ( 3 ) n u t t l i 0 . , m c e v i l l y v . , 1 9 6 1 . the response characteristics of long period seismographs of the saint louis university network. e a r t h q u a k e n o t e s , s e p t . d e c . , 1961. ( 4 ) p r e s s p . , e w i n g m . , l e h n e k p . , 1 9 5 8 . a long period seismograph system. « t r a n s . a m . g e o p h y s . u n i o n », 3 8 , 106-108. (6) wenner p . , 1925. new seismometer equipped for electromagnetic damping and electromagnetic and optical magnification. b u r e a u of s t a n d . , « j o u r n . of r e s . », 2 . 9 6 2 9 9 9 , r e s . p a p e r n ° 66. narvaez 20.pdf annals of geophysics, vol. 46, n. 1, february 2003 125 scale interactions in the tropical atlantic variability simulated with a coupled gcm william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá departamento de física, facultad de ciencias, universidad de alcalá, madrid, spain abstract warm and cold events in the gulf of guinea are an important feature in the interannual variability of the tropical atlantic ocean, and partly a manifestation of the equatorial atlantic system’s intrinsic variability. due to the relatively reduced zonal extension of this ocean, the latter variability is comparatively weak and thus strongly modified by other factors at play, either local or remote, like the seasonal cycle or enso. we present here an analysis of the tropical atlantic variability in a 100-year-long chunk of the output of a coupled gcm. through it, we obtain a better understanding of this variability and of its interactions with the seasonal cycle and with the enso signal. following hints in the observations, we separate warm or cold events of the simulation in a few types, according to their similarities and differences. this classification is carried out as a spatio-temporal cluster analysis of the values, from nine months before up to the peak of the event, of the heat content anomalies. this is an optimal variable to monitor the generation of the events. one of the warm event classes can be explained by enso interactions. one of the cold event types can be explained by this influence as well, while the seasonal interactions might explain the characteristics of another of them. 1. introduction recent years have seen a notorious increase in the interest of the scientific community in the interannual variability of the tropical atlantic. observational analyses (stramma and england, 1999; tourre et al., 1999), modelling of the observed variability (carton et al., 1996), coupled models of the atmosphere and the ocean (simple (xie, 1999), hybrid (chang et al., 2001) or general circulation models (dommenget and latif, 2000)) and predictability studies (penland and matrosova, 1998), all of them have contributed to enhance our knowledge of the phenomena that take place in this ocean and are responsible for its variability at different time scales. meridional displacements of the intertropical convergence zone constitute the main characteristic of the seasonal cycle of the tropical atlantic. as for the interannual variability, its most outstanding feature are the warm and cold events in the gulf of guinea, that suggest an atlantic counterpart for the pacific enso (el niño-southern oscillation). additionally, there is an important decadal variability, associated with a ‘dipole pattern’, that explains roughly a 12% of the variability of the climatology-subtracted sea surface temperature (hereinafter sst) field (servain, 1991). several signs seem to indicate that scale interactions might be more relevant to the variability of the tropical atlantic than to that of mailing address: dr. william d. cabos narváez, departamento de física, facultad de ciencias, universidad de alcalá, alcalá de henares, madrid 28871, spain; e-mail: william.cabos@uah.es key words coupled model – gulf of guinea events − el niño-southern oscillation – timescale interactions – clusters 126 william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá a b fig. 1a,b. a) seasonal distribution of warm sst anomaly peaks in the observed sst anomalies (coads + igoss, 1950-1999). the horizontal line for the 1.5 standard deviation threshold is depicted in order to emphasize the stronger events; b) as in (a), but for the cold sst anomaly peaks. 127 scale interactions in the tropical atlantic variability simulated with a coupled gcm other oceans, particularly the pacific: i) patterns of meridional gradient are linked to the first annual harmonic of the seasonal cycle and to the second empirical orthogonal function of the seasonal anomalies, respectively. ii) the interannual warm and cold events are actually phaselocked to the seasonal cycle. the occurrence of these events is usually monitored by the gulf of guinea (hereinafter gg) index, built as an average of the sst anomalies in the gg region (20°w-10°e, 5°s-5°n). as shown in our fig. 1a,b, both warm and cold events peak during late spring-summer of the northern hemisphere (may-june-july) with few exceptions. iii) the correlation between the unfiltered gg index and niño3 index is barely significant. this last index, that characterizes the enso variability, is computed by averaging the anomalies of sst in the niño3 region (150°w-90°w, 5°s-5°n). if a low-pass filter is applied to these indices, removing variability with periods shorter than one year, the correlation between them notably rises. the gg index typically lags, though not all atlantic warmings are preceded by a corresponding enso episode. in the present work, we try to understand how other scales, either temporal (the seasonal cycle) or spatio-temporal (enso variability), exert their influence, if effectively any, on the interannual variability of the tropical atlantic: can they, in particular, affect the generation of the gg events? to answer this question, we will analyze the development of the latter in 100 years of a coupled gcm simulation. we are aware that such simulations still exhibit some unrealistic features, but on the other hand they offer the advantage of a far more satisfactory sampling than is available for observations as well as the certitude about the consistency of the generated data. the fundamental physical variable for our analysis is the thermal energy stored in the upper layers of the ocean or ‘heat content’, which previous studies (chao and philander, 1993; latif and graham, 1992) have shown to be the optimal parameter to trace the generation of the events. the interannual variability of the equatorial atlantic from 1980 through 1989 was simulated by cabos narváez et al. (1998), who found striking differences in the generation of the two warm events that took place in this decade. while the 1984 event displayed the typical accumulation of warm off-equatorial hc anomalies in the west, in contrast to the cold ones in the gg, during its antecedent stage, the 1988 episode was characterized by the presence of positive hc anomalies in the gg already one year in advance of its peak. simulations of the same period (carton et al., 1996) with other ocean models confirm these differences are not an artifact of a particular ocean gcm or of the forcing used. subsequently, cabos narváez et al. (2002) have investigated similarities and differences in the generation of the warm and cold events in a coupled gcm experiment with echam4(t42)opyc. the spatio-temporal analysis pursued there is applied in the present work to data produced by another coupled gcm, the echam4(t30)-orca. details of the analyzed run will be given in section 2. the classification technique we utilize is described in section 3. the results for the warm events are specified in section 4, while those for the cold events can be found in section 5. in section 6 we discuss how the different kinds of events are related to interactions with the seasonal cycle or with enso. we conclude with a summary and discussion. 2. data and method the data used for this study were basically those extracted from the output of the sintex experiment performed with the echam4 atmospheric model (roeckner et al., 1995) at t30 resolution coupled to the orca (global version of opa8.1 (madec et al., 1998)) oceanic model. although differences between the observed and simulated mean state are still important in some regions (for instance the eastern equatorial pacific), the simulated enso variability seems to be well represented (gualdi et al., 2003). in the tropical atlantic, the occurrence of the events is also quite realistic, though their magnitude is underestimated. the connection between the equatorial pacific and atlantic oceans appears to be correctly modelled, as shown by our fig. 2, where we have represented the lagged correlation between the niño3 and the gg index in the observations and in the sintex run. we have 128 william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá also examined the correlation maps (not shown) at one season lag between the niño3 index and the sst anomaly field in the tropical atlantic, for both observed and simulated data. there is considerable agreement between these spatial patterns, confirming our belief that the impact of enso in the tropical atlantic is adequately captured. no trend in surface or subsurface variables was found in the 100 years chunk of simulated tropical pacific variability selected for our study. this was also true for the surface variables of the tropical atlantic, while a very slight trend was noticeable in the subsurface. this trend was removed for the purposes of the present analysis. one of the basic variables for our analysis is the sst. the other, as stated above, is the thermal energy stored in the upper oceanic layers, defined by the depth h0 taking the value of 300 m. results from cabos narváez et al., (2002), will be mentioned later in this paper. they applied the methodology used here to 200 years of data simulated with the echam4-opyc3 coupled model (roeckner et al., 1995). its atmospheric component has a t42 resolution, while the oceanic component has a comparable zonal resolution but a much coarser vertical discretization (only eleven layers). in a recent intercomparison study of 16 models (achuta rao et al., 2000), this cgcm simulation was rated among the best for its simulated seasonal cycle, the statistical characteristics of several climatic indices, the soi-niño3 teleconnections, the phase locking to the seasonal cycle, and the global atmospheric response to enso events. some deficiencies of the model, common to most of them, are warmer than observed sst anomalies in the tropical pacific off the south american coast, and the overestimation of the biannual component of enso (bacher et al., 1998). observed sst from a global data set covering the period 1950-1999 will also be considered for fig. 2. lagged correlation between the niño3 and the gg indices (niño3 leading at negative lags), for both the sintex simulation (solid line) and the coads + igoss observed sst anomalies (dot-dash line). hc t r c t t r z dz h p hh , , , r r( ) = ( )( )∫ρ 0 0 129 scale interactions in the tropical atlantic variability simulated with a coupled gcm the sake of comparison. the data set is described in sanchez gomez et al. (2001). as for the enso case, events are identified from the gg index as those periods when anomalies exceed the threshold of one standard deviation of the time series for more than three months, with the condition that two events cannot occur within the same year (trenberth, 1997). this criterion identifies correctly the accepted warm events for the equatorial atlantic (zebiak, 1993) from the observed gg index. in the 100 years chunk analyzed, 51 warm events and 46 cold events were identified. the highest peak in the gg sst anomaly during the period of occurrence of an event will be used as reference from now on, as detailed below. once the events have been identified, we proceed to their classification. we apply an agglomerative cluster analysis based on the minimization of the following distance between two events with where yp(l ) and yq(l ) stand for the spatial patterns of the hc anomalies in the atlantic basin within 15°s and 15°n for events p and q respectively, l months before their respective peaks in the sst anomaly gg index. thus defined, the distance d l pq will vary between 0 and 2. the delay l is computed with respect to the peak of each event. that is, in the following, month – 1, for instance, refers to the month previous to the peak, and month + 1 to the one after. the classification is carried out in two stages: at a first stage, when clusters are still undefined, values of for every pair of events, with l ranging between − 9 and 0 months, are computed. an average (over l ) of these distances is used to establish the initial groups, which are formed by a few events (typically 4 or 6) with minimum distance (under a threshold of 0.3) among them. each group is then characterized by a composite at each lag l and on the second stage the distance to these composites is computed for all the events that remain unclassified. each one is aggregated to its nearest group, using as a criterion the distance averaged over the values of l from − 6 to 0, chosen for the sake of a more distinct classification. the latter’s robustness, however, has been examined, with positive results, by using other intervals within the (– 9,0) months range. the number of events present in the sample analyzed gives barely two years as mean recurrence time between events, in good agreement with the observations. also the time interval chosen for the cluster analysis points to a shortening of the generation period, in good accordance with the reduced dimensions of the atlantic, as compared with the pacific. 3. the warm events generation the results of our classification of the warm events are summarized in table i. the 51 events are distributed among five groups, that are characterized by their hc composites at different lags. a greater diversity is found in this analysis for the atlantic than in the one for the pacific (see companion paper), which is also the case for the studies performed on the echam4opyc model, reported in cabos narváez et al. (2002) and álvarez garcía et al. (2002). the statistical details of the clustering are satisfactory, d pq l table i. warm events classification results. n1, d1, s1: number of elements, average distance and standard deviation of the latter within a given group. n2, d2, s2: same as above for events outside a given group. fv: fractional contribution to the variance of the gg index due to all warm events by those in a given group. group n1 d1 s1 n2 d2 s2 fv i 12 0.43 0.11 39 0.98 0.26 26% ii 7 0.35 0.14 44 0.97 0.25 13% iii 8 0.45 0.13 43 1.06 0.20 14% iv 5 0.37 0.13 46 1.08 0.24 11% v 7 0.39 0.12 44 0.93 0.27 12% d s pq l pq l= −1 s y l y l y l y l pq l pj qjj n pjj n qjj n = () () ()( ) ()( ) = = = ∑ ∑ ∑ 1 2 1 2 1 130 william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá with values of the average distance within group plus its standard deviation always smaller than the average distance outside group minus its standard deviation. in the hoevmueller diagrams of fig. 3 we represent (top row) the evolution in time (measured with respect to the peak) of the hc composite at the equator for three out of the five fig. 3. hoevmueller diagrams showing the evolution of (top) heat content (107 jm−2) and (bottom) sst (°c) anomalies in the equatorial atlantic (averaged within the equatorial strip 5°s-5°n) for warm events of type i (left), ii (center) and iv (right). 131 scale interactions in the tropical atlantic variability simulated with a coupled gcm table ii. cold events classification results. n1, d1, s1, n2, d2, s2, fv: same as in table i but for cold events. group n1 d1 s1 n2 d2 s2 fv i 9 0.33 0.13 37 1.06 0.26 25% ii 6 0.26 0.10 40 0.98 0.28 18% iii 12 0.40 0.10 34 1.08 0.30 27% iv 9 0.38 0.06 37 1.15 0.24 16% clusters identified. in the bottom row, the corresponding sst composites for each type are shown. in events of types i and ii, there is an accumulation of anomalies of hc in the western part of the basin, nine months before the peak of the events. but while in events of type i hc anomalies have already propagated into the gg region by month – 6, in events of type ii this propagation does not take place till month – 3. in events of type iv, hc anomalies are negative at the equator east of 20°w from month – 9 up to the peak of the events, but positive sst anomalies are nevertheless produced in the gg. such a situation is likely to correspond to a thin mixed layer lying above colder waters; atmospheric heat fluxes are distributed then over a smaller mass, yielding a stronger effect on the surface. this class of events has a counterpart among the cold ones (that is, characterized by cold sst along with positive anomalies of hc), as will be shown. events of type iii (not shown) belong to a type that we will call ‘static’: warm hc anomalies can be found at the equator, east of 10°w, already in month − 9, while sst anomalies are still cold in the region (but positive off the african coast south of 8°s). they bear some likeness to cold events type iii, whose behaviour is depicted in fig. 5. the evolution of hc anomalies in events of type v (not shown) is roughly similar to that of events type ii, only the pattern of sst anomalies is considerably different now, strong positive sst anomalies being more confined to the eastern equator. 4. the cold events generation using the same criterion as above, 46 cold events are identified from the gg index of the 100 years of simulation analyzed. we apply to them a classification procedure identical to the one followed in the case of the warm events. cold events appear to be generated according to four different ways. the statistical details of this classification are given in table ii, where it can be appreciated how the distance between the centers of each cluster exceeds the value of the average distance within a group plus one standard deviation. figure 4 shows the most characteristic features of the hc (top row) and sst anomalies (bottom row) for cold events type i, ii and iv. the first two of them are similar in the same way warm events of type i and ii are: there is an accumulation of negative hc anomalies in the west, north and south of the equator and there are positive hc anomalies in the gg region by month − 9. as shown in the corresponding hoevmueller diagram of the fig. 4, there is a gradient of hc anomalies along the equator, that is more pronounced in events of type i, likely as fallow-up of a previous warm event. this gradient is maintained up to month – 6 in events type i, but has already disappeared by that time in events of type ii, all the equator being covered by negative hc anomalies. events of type iii are the negative counterpart to the warm events of type iii, as mentioned above: there are negative hc anomalies in the gg region more than six months in advance of the peak; their in situ growth finally leads to the cold sst event, as shown in fig. 5. events of type iv are characterized by the existence of positive hc anomalies in the gg during the cold sst episode, which supports the view that cold sst anomalies appear as a consequence of the subduction of warm surface waters. 5. seasonal and enso inf luence figure 6a represents the seasonal distribution of the simulated warm events. we note in the first place their agreement with the seasonal distribution of the warm events in the observations (fig. 1a), only peaks tend to occur earlier in the model, in spring rather than in late spring. 132 william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá warm events of a given type do not show any preferred seasonal signature. the corresponding seasonal distribution for the simulated cold events appear in fig. 6b. notice how the events that take place in spring belong mostly to type iv, and how events of type iii happening in the first half of the year are stronger than those in the second half. fig. 4. hoevmueller diagrams showing the evolution of (top) heat content (107 jm−2) and (bottom) sst (°c) anomalies in the equatorial atlantic (averaged within the equatorial strip 5°s-5°n) for cold events of type i (left), ii (center) and iv (right). 133 scale interactions in the tropical atlantic variability simulated with a coupled gcm to study the enso interaction, we computed the correlation at different lags between sst anomalies of the niño3 index and the gg index isolating the periods of occurrence of events of distinct types (juxtaposing 19-month-long segments centered at the peak of the events). figure 7a displays the correlations for the different types of warm events. for those of group iv, correlation with enso starts to be significant with the latter leading by 2 months, and peaks at zero months lead. events of type i are also preceded by enso some 4 months in advance, but their correlation is weaker. turning to fig. 7b, cold enso events seem to exert a stronger effect on the cold gg events rather than on the warm ones. events of type i are apparently preceded by cold enso episodes almost one year in advance, while events of type iv begin to show significant correlations when enso is leading them by some 3 months. the maximum value of this correlation is attained at zero months lead. events of type ii follow warm enso events, while the events of type iii (the ‘static’ ones) do not seem to be connected to enso in any way. fig. 5. evolution of hc (left) and sst (right) anomalies for cold events of type iii. time is measured with respect to the peak in the sst (in month 0). units are 107 jm−2 and °c, respectively. 134 william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá a b fig. 6a,b. a) seasonal distribution of warm sst anomaly peaks in the sintex simulation gg index. the horizontal line for the 1.5 standard deviation threshold is depicted in order to emphasize the stronger events; b) as in (a), but for the cold sst anomaly peaks. 135 scale interactions in the tropical atlantic variability simulated with a coupled gcm a b fig. 7a,b. a) lagged correlation between the sintex simulation gg and niño3 indices (niño3 leading at negative lags). different time series have been formed by isolating the periods of occurrence of the distinct types of warm gg events. solid thick lines correspond to correlation values significantly greater than zero at the 5% level; b) as in (a), now for cold gg events. 136 william d. cabos narváez, francisco j. álvarez garcía and maría j. ortiz beviá 6. summary and discussion equatorial atlantic warm (cold) events have been identified from the simulated gg index. for each event, the hc patterns from 9 months before the peak of the event till the peak itself were separated. all of them were considered samples of the warm event generation, and subjected to a spatio-temporal statistical classification procedure. this analysis identifies five (four) types in the generation of warm (cold) events. when the same analysis was applied to the study of the enso variability in the sintex experiment (see companion paper), the number of types was smaller. this increase in the variety of types in the tropical atlantic with respect to the pacific also appears in the analysis, with the same technique, of another coupled simulation experiment (cabos narváez et al., 2002; álvarez garcía et al., 2002). the statistical properties of the classification are satisfactory in all cases, regardless of how many classes are established. all four cold event types seem to possess a counterpart among the five warm event classes. warm events of types i and ii are characterized by a gradient of hc anomalies along the equator, nine months before the peak of the events. cold events of types i and ii are, likewise, characterized by a gradient of hc anomalies along the equator of reverse sign. the difference between these two types of warm (cold) events lies in the pace at which the positive (negative) hc anomalies of the west propagate into the gg. in warm (cold) events of type iii, warm (cold) hc anomalies can be found in the gg more than six months before the event peaks. as they grow in situ, those events are called here ‘static’. lastly, warm (cold) events of type iv are mainly distinguished by the feature that their hc and sst anomalies in the gg are of opposite signs. with a few exceptions, events in the observations are locked to the seasonal cycle, with a manifest preference to peak in late spring. this usually occurs earlier in the sintex run, in spring, an error which might stem from deficiencies in the simulation of the mean state, with modelled temperatures in the gg warmer than observed ones. the fact that this problem is also present in the other simulation analyzed supports this hypothesis. interaction with enso is important for events of type iv (warm or cold). the generation of warm events of type i is also, though more faintly, affected by it, at greater lead times. more generally, enso has a stronger effect on atlantic cold events than on warm ones. the physical mechanism that exerts these influences is being examined. acknowledgements this work was supported by contract env4ct98-0714 (sintex) from the european union environmental and climate program. references achuta rao, k., k.r. sperber, and the cmip modelling group (2000): the el niño-southern oscillation in coupled gcms, pcmdi report series 61, pp. 46. álvarez garcía, f.j., w.d. cabos narváez and m.j. ortiz beviá (2002): different kinds of enso generation in coupled gcm simulations, climate dyn. 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(1993): air-sea interaction in the equatorial atlantic region, j. climate, 6, 1567-1586. 03satake.qxd 369 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical earthquakes – seismic intensity – seismicity – kuril subduction zone 1. introduction japan is one of the most seismically-active regions in the world, surrounded by four tectonic plates: the pacific, philippine sea, eurasia and north america plates (fig. 1). the last two, on which most japanese islands are situated, are also known as the amurian and okhotsk plates, respectively (seno et al., 1996; heki et al., 1999). most of great (m~8) earthquakes around japan occur in subduction zones, including the kuril trench where the pacific plate subducts beneath hokkaido, and the nankai trough where the philippine sea plate subducts beneath southwest japan. japanese historical records document earthquakes as far back as the 7th century (usami, 2002). until around a.d. 1600, most historical records were kept in western japan, particularly around kyoto where the emperor’s capital was located between the 8th and 19th centuries. hence many earthquakes were reported in kyoto and its vicinity. on the basis of such historical documents, recurrence of great earthquakes along the nankai trough with approximately 100 year intervals has been inferred from historical documents (imamura, 1928; ando, 1975). in a.d. 1603, the shugun established a centralized government in edo (present tokyo; fig. 1), and each local government started keeping official records. both the quantity and quality of hisdetection of kuril subduction-zone earthquakes from remote historic records in honshu, japan, between 1656 and 1867 kenji satake active fault research center, geological survey of japan, national institute of advanced industrial science and technology, tsukuba, japan abstract earthquakes before a.d. 1800 along the southern kuril trench, although before the start of written history on nearby islands, probably account for some of the earthquakes noted by local records in honshu, hundreds of kilometers to the southwest. earthquake historians have identified about 4800 felt earthquakes in edo (present tokyo) and about 3000 felt reports in selected local government records in tohoku, northern honshu, for the years a.d. 1656-1867. on the average, 19 earthquakes per year were felt in edo. of the tohoku records, 361 (an average nearly 2 per year) were felt at multiple tohoku locations; 95 of these (0.4 per year) were also felt in edo. since 1926, tokyo has had a yearly average of 15 felt earthquakes with seismic intensity 2 or more on the japan meteorological agency scale (corresponding to iii or more on modified mercalli scale). for tohoku the average annual frequency is about 4. among them, an average of 0.6 events per year also reached intensity 2 in tokyo. about one quarter of these events occurred in the southern kuril trench. if the seismicity is temporally constant, about 80 of the earthquakes recorded in 1656-1867 probably had a kuril origin. mailing address: dr. kenji satake, active fault research center, gsj/aist, national institute of advanced industrial science and technology, site c7 1-1-1 higashi, tsukuba 305-8567, japan; e-mail: kenji.satake@aist.go.jp 370 kenji satake torical records dramatically increased in the edo period (a.d. 1603-1867), and earthquakes were documented throughout japan except for hokkaido (fig. 2). the oldest documented earthquakes along the southern kuril trench occurred in the 19th century. great earthquakes along the southern kuril trench cause damage from ground shaking and tsunamis on nearby hokkaido. most of hokkaido had long been occupied by native people (ainu) and very few written records existed until the 19th century. the oldest record in the eastern hokkaido is «nikkan-ki», the official record of a temple in akkeshi. it started in a.d. 1804, and reports about 70 earthquakes between 1816 and 1861. the earthquake record, however, is not uniform: 32 fig. 1. plate tectonic configuration around the japanese islands. the pacific plate subducts beneath northern honshu, the largest island, and hokkaido, the northernmost island, at the japan and kuril trenches, respectively. the philippine sea plate subducts beneath southwestern japan at the nankai trough. events were recorded between 1816 and 1821, followed by ten years with no reported earthquakes. in this paper i attempt to detect the kuril earthquakes from remote historical data. because of the short written history of hokkaido, it seems difficult to infer the pre-19th century seismicity along the kuril trench. however, great earthquakes along the southern kuril trench are felt in tohoku (northern honshu) and tokyo, and relatively uniform historical records exist in these regions. the seismic intensity distribution is examined from the edoperiod and modern data. then an attempt is made to detect the kuril earthquakes hidden in the historical earthquake records in tohoku and edo. 371 detection of kuril subduction-zone earthquakes from remote historic records in honshu, japan, between 1656 and 1867 2. earthquakes along kuril subduction zone along the kuril trench, where the pacific plate subducts at a rate of about 8 cm/yr beneath hokkaido (seno et al., 1996), great (m ~ 8) earthquakes repeatedly occurred in the 19th and 20th centuries (kanamori, 1977; fukao and furumoro, 1979) (fig. 3). in the 20th century, five great interplate earthquakes occurred between 1952 and 1973. it should be noted that great earthquakes also occur within the subducted slab (e.g., in 1993 and 1994) fig. 2. yearly number of earthquake reports since the 7th century. the top figure shows total records for all japan, the center figure is for kyoto (including nara), and the bottom figure is for tokyo (including kamakura). historical earthquake data exists since around 7th century in the southwestern japan, mostly around ancient capital of kyoto. since a.d. 1603 (in edo period), historical data are available throughout japan with exception for hokkaido. historical records exist only after a.d. 1800 in hokkaido. data are from ueda and usami (1990). and cause ground shaking and tsunami similar to those from interplate earthquakes (satake and tanioka, 1999). great earthquakes also occurred in the 19th century (fig. 3), but their exact location, size and mechanisms are not well known. the kuril earthquakes are associated with unusual intensity distributions. the isoseismals are not concentric around the epicentre, but are elongated along the japanese arc because of the lower attenuation of seismic waves in the subducting pacific plate (utsu, 1971). figure 4 shows an example for the 1952 tokachi-oki earthquake (mw 8.1). seismic intensity 5 (on the japan meteorological agency scale; see fig. 4 for conversion to the modified mercalli scale) was recorded on the pacific coast of hokkaido near the source, and regions of intensity 4 and 3 extend along the western hokkaido and northern honshu. this earthquake was felt as far as at tokyo, where intensity 2 on the jma scale was registered. large kuril earthquakes also cause tsunami damage on the pacific coasts of hokkaido and northern honshu. the tsunami heights from the 1952 earthquake were mostly 2-4 m on the hokkaido coast and 1-2 m on the northern honshu coast (watanabe, 1998). the tsunami caused extensive damage on hokkaido’s pacific coast and lesser yet noticeable damage on tohoku’s sanriku coast. 3. historical records japanese historical documents describing earthquake damage have been systematically collected and published in three different phases: in 1899-1904 by tayama, in 1941-1951 by musha, and in 1980-1994 by usami and his colleagues (usami, 2002). the latest compilation, led by usami, «collection of materials for the history of japanese earthquakes» (21 volumes) was published from earthquake research institute, university of tokyo. the number of earthquakes (damage or felt reports) has dramatically increased in these three stages: approximately 1900, 8400 and 45 000. on the basis of these documents, seismic intensity at each location is estimated from descriptions of ground shaking and damage. the size and epicentre of earth372 kenji satake fig. 3. source regions of great interplate earthquakes along the southern kuril trench. the 1968 and 1856 earthquakes occurred around the corner of kuril and japan trenches. magnitudes are mw (moment magnitude) for the 20th century events and mt (tsunami magnitude; abe, 1999) for the 18-19th century events except for the 1893 event for which m was estimated by utsu (1999). mw mw mw mw mw m fig. 4. seismic intensity distribution (on the japan meteorological agency scale) of the 1952 tokachi-oki earthquake. conversion table for the jma and modified mercalli scales is also shown. mw 373 detection of kuril subduction-zone earthquakes from remote historic records in honshu, japan, between 1656 and 1867 quakes are further estimated from the seismic intensity distribution, by comparing modern earthquake data and using empirical relations. as a result, several tables of damaging earthquakes in japan have been published (usami, 2002). the seismic intensity distribution of some historical earthquakes in tohoku is similar to that of modern kuril earthquakes (fig. 5). an earthquake on 15 april 1674 caused slight damage to the castle at hachinohe, corresponding to seismic intensity 5 on jma scale. the earthquake was strongly felt (intensity 3-4) in tohoku and slightly felt (intensity 2) in edo. on the basis of the intensity distribution, usami (2002) estimated the epicentre off hachinohe with m ~ 6. another earthquake on 10 november 1692 was also felt strongly in hachinohe without causing any damage (intensity 4); it was slightly felt (intensity 2) in edo. without any documented damage, this earthquake has not been included in the tables of damaging earthquakes, and neither the epicentre nor magnitude has been estimated. although the spatial extent is limited, the intensity distributions suggest a possibility of kuril origin of both of these earthquakes. i examined selected historical documents that continuously recorded earthquakes in the edo period. at hirosaki, the official record of local government, «tsugaru-han on’nikki» (about 3300 volumes) is a daily record of various events for 204 years (a.d. 1661-1864). earthquake and tsunami damage in the territory was described in detail in this record. even if no damage was caused, earthquakes were noted in daily records along with weather of the day. such earthquake reports are found in about 1300 days during the 204 years, an average of 6.4 days per year (fig. 6). the daily records sometimes describe multiple earthquakes without specifying the exact number (e.g., «a few» or «several»), hence i decided to count the number of days («earthquake days») rather than number of earthquakes. similar documents report earthquakes on 730 days, or average of 3.6 days per year, at hachinohe, and 950 days in total or average of 6.2 days per year at morioka. in edo, various records report about 4800 earthquakes in the edo period, an average of 19 events per year (ueda and usami, 1990; fig. 6). continuous records were also kept at fig. 5. seismic intensities estimated from historical documents for an earthquake on 15 april 1674 (left) and 10 november 1692 (right). for the 1674 event, epicentre (star) and m estimated by usami (2002) are also shown. 374 kenji satake these, 95 events were also recorded in edo or nikko. among the 361 events recorded at multiple locations, epicentres for 52 events have been previously estimated on the basis of damage and intensity distributions (usami, 2002). additionally, about 20 events have been located off hachinohe (fig. 5), but these could be of kuril origin, as discussed earlier. the earthquakes with known epicentres include great (m ~ 8) earthquakes along the japan trench. along the northern japan fig. 6. annual frequency of days in which earthquakes were recorded in historical documents in three tohoku locations (hirosaki, hachinohe, and morioka) and edo (tokyo). see fig. 5 for the locations. fig. 7. annual frequency of earthquakes recorded at multiple locations. the bottom panel shows those recorded at least two locations in tohoku. the central panel is for those recorded at multiple locations in tohoku and edo (tokyo). the top panel indicates tsunami damage on the sanriku coast. the white bars are trans-pacific tsunamis, black bars are those from northern japan trench and gray bars are from central japan trench. nikko, where guards of the first shogun’s graveyard kept daily records. from the list of earthquakes individually recorded in tohoku and edo, i selected those reported at multiple locations. if earthquake descriptions from different locations or documents differ by up to one unit of time, a «koku», or about two hours, i assume that they represent the same earthquake. of the about 3000 descriptions from three localities in tohoku, more than 2000 were reported at only one location and seem to be of local origin. during the 212 years between 1656 and 1867, there are 361 events recorded at multiple locations in tohoku region (fig. 7). of 375 detection of kuril subduction-zone earthquakes from remote historic records in honshu, japan, between 1656 and 1867 trench, at the source area of the 1968 event (fig. 3), great (m ~ 8) earthquakes and their aftershocks occurred in 1677, 1763, and 1856. to the south along the central japan trench, the miyagioki earthquakes occurred in 1717, 1793, 1835 and 1861 in the edo period, followed by 1897, 1936 and 1978 events (utsu, 1999). during the edo period 16 tsunamis were recorded on sanriku, the pacific coast of northern honshu, but the origin of these tsunami are all known (watanabe, 1998). among these, five tsunamis (1687, 1700, 1730, 1751 and 1837) were trans-pacific tsunamis from north or south america, supported by historical or paleoseismological evidence. four were from great earthquakes in the northern japan trench (including one from the 1763 aftershock) and other four were from the central japan trench. the 1611 tsunami has been considered to be from a tsunami earthquake (watanabe, 1998). a tsunami in 1616, though included in watanabe (1998), has been questioned of its existence (yoshinobu tsuji, manuscript in revision). the last one, in 1843, is considered to be from the penultimate event of the 1952 tokachi-oki earthquake along the southern kuril trench (fig. 2). unknown kuril earthquakes in the edo period, if any, apparently did not generate damaging tsunamis along the sanriku coast, because such tsunami damage would have been documented. 4. modern seismic intensity observations the japan meteorological agency started systematic seismic intensity observations in 1926 at weather stations distributed throughout japan. i used the jma seismic intensity database (ishigaki and takagi, 2000) for the 76 years 1926-2001, and compared the modern and historical felt reports. because there was no weather station in hirosaki, i used nearby aomori data. at aomori, morioka and tokyo, the seismic intensity data have been available since 1926, but the hachinohe data have been available only since 1936. the jma seismic intensity observations detect more earthquakes than the historical reports. the average annual frequency of earthquakes with intensity 1 or more on jma scale is 18 at aomori, 48 at hachinohe, 37 at morioka, and 40 at tokyo (table i; fig. 8). if the threshold is raised to seismic intensity 2 (corresponding to iii on modified mercalli scale), then the average annual frequency becomes 7 at aomori, 18 at hachinohe, 13 at morioka, and 15 at tokyo. these numbers are comparable to annual earthquake days in the historical documents: 6.4, 3.6, 6.2 and 19, respectively, at these locations. it indicates that historical data are complete for modern seismic intensity 2 or higher, if the seismicity is assumed to be temporally constant. at hachinohe and morioka, the modern records show hightable i. felt earthquakes of historical and modern jma data. historical data jma seismic intensity observation annual frequency period 1 or higher 2 or higher 3 or higher period hirosaki 6.4 1661-1864 17.5 7.2 1.9 aomori: 1926-2001 hachinohe 3.6 1665-1869 48.2 17.6 4.1 1936-2001 morioka 6.2 1664-1796 37 13.1 3.4 1926-2001 tokyo (edo) 19 1601-1872 39.9 14.9 4.1 1926-2002 tohoku 1.7 two or more 9.8 3.6 0.9 aomori, hachinohe, morioka tohoku and tokyo 0.4 1.4 0.6 0.1 above three and tokyo 376 kenji satake fig. 8. annual frequency of felt earthquakes reported by japan meteorological agency (1926-2001). shading indicates different intensity values on the jma seismic intensity scale. fig. 9. epicentre distribution of earthquakes reported at three stations in tohoku (open circles) and those reported at tohoku and tokyo stations (solid circles). the symbol size is proportional to earthquake magnitude. triangles indicate the station locations. nearly a quarter of these events occurred along the kuril trench, in the region surround by dashed lines. 377 detection of kuril subduction-zone earthquakes from remote historic records in honshu, japan, between 1656 and 1867 er numbers; hence the historical data may be complete for intensity 3 or higher, for which the average annual frequency is 4 at both locations. however, considering the slight difference in counting methods (events in modern data whereas earthquake days that possibly contain multiple events in historical data) and the incompleteness of historical data, i conclude that historical data detect earthquakes with seismic intensity of 2 or higher on the jma scale. earthquakes recorded at multiple jma stations yield similar conclusions. during the 66 years between 1936 and 2001, the number of felt earthquakes at the three tohoku stations (aomori, hachinohe and morioka) is 276 if the threshold is intensity 2, and 68 for the threshold intensity of 3. the average annual frequencies are 4 and 1, respectively, comparable to that from historical data: 1.7. the number of events recorded at tohoku stations and tokyo is 46 and 11 for intensity thresholds of 2 and 3, respectively. the annual frequencies are 0.6 and 0.1 events, again comparable to that of historical data, 0.4. epicentres of earthquakes felt in tohoku and tokyo are distributed around northeast japan (fig. 9). many earthquakes are located on the pacific coast of hokkaido and honshu, but inland earthquakes, earthquakes along the eastern margin of japan sea, and deep events beneath the russian coast are also included. among them, 59 events (21%) of those felt (with intensity scale 2 or larger) only in tohoku, and 11 events (24%) of those felt in both tohoku and tokyo occurred along the southern kuril trench. 5. edo-period kuril seismicity a comparison of historical and modern earthquakes suggests historical data detect earthquakes with seismic intensity of 2 or more on the jma scale. under an assumption that the historical data are complete for this threshold, and further that the seismicity rate has not changed since the 17th century, the number of kuril earthquakes can be estimated for historical data set. of the 361 earthquakes recorded at multiple tohoku locations between a.d. 1656 and 1867, about 76 events (21%) are estimated to be of kuril origin. of the 95 events recorded at multiple tohoku locations and edo, about 23 events (24%) are also estimated to be of kuril origin. are the above assumptions, data completeness and temporally constant seismicity, valid? regarding the second assumption, earthquakes often occur as clusters and the seismicity rate may change with time. in this study no declustering, including aftershocks, was made for either historical on modern records. intraplate seismicity rate in tohoku is reported to change with association of larger interplate earthquakes (shimazaki, 1978). because the time period is very long (> 200 years for the edo records and 75 years for modern data), effects of clustering and temporal change may not be significant. the present study cannot identify individual kuril earthquakes, but the candidates are listed. further investigation of historical documents could focus on these dates and might reveal more detailed information on individual earthquakes. historical documents are still being found from local archives. while it is unlikely that continuous documents as used here are newly found, historical records for shorter period of time are likely to be found. the candidate tables will be therefore useful to examine such records. acknowledgements the japanese version of this paper with a table of the 361 events was published in historical earthquakes (rekishi jishin). the full text can be downloaded at http://staff.aist.go.jp/ kenji.satake/rekishijisin/05-satake.pdf. kazue ueda provided me with her database for the index to the collections and a summary of felt earthquakes (fig. 2). akimichi takagi provided me with his software to analyze the jma seismic intensity database. yuichi nishimura carefully reviewed the japanese version of this paper. i thank two reviewers whose comments improved the presentations of the paper. i also thank virginia garcía acosta, roger musson, and max stucchi for their efforts to organize the workshop «investigating the records of past earthquakes» and offering me an opportunity to participate. 378 kenji satake references abe, k. (1999): quantification of historical tsunamis by the mt scale, zisin, j. seismol. soc. jpn, 52, 369-377 (in japanese with english abstract). ando, m. (1975): source mechanisms and tectonic significance of historical earthquakes along the nankai trough japan, tectonophysics, 27, 119-140. fukao, y., and m. furumoro (1979): stress drops, wave spectra and recurrence intervals of great earthquakes implications of the etorofu earthqaukes of 1958 november 6, geophys. j. r. astr. soc., 57, 23-40. heki, k., s. miyazaki, h. takahashi, m. kasahara, f. kimata, s. miura, n.f. vasilenko, a. vashchenko and k.d. an (1999): the amurian plate motion and current plate kinematics in eastern asia, j. geophys. res., 104, 29147-29155. imamura, a. (1928): on the seismic activity of central japan, jpn. j. astron. geophys., 6, 119-137. ishigaki, y., and a. takagi (2000): jma seismic intensity data base and several operational utility examples, quart. j. seismol. (kenshin jiho), 63, 75-92 (in japanese). kanamori, h. (1977): seismic and aseismic slip along subduction zones and their tectonic implications, in island arcs, deep sea trenches and back-arc basins, edited by m. talwani and w.c. pitman, iii (american geophysical union, washington d.c.), 163-174. satake, k., and y. tanioka (1999): sources of tsunami and tsunamigenic earthquakes in subduction zones, pure appl. geophys., 154, 467-483. seno, t., t. sakurai, and s. stein (1996): can the okhotsk plate be discriminated from the north american plate?, j. geophys. res., 101, 11305-11315. shimazaki, k. (1978): correlation between interplate seismicity and interplate earthquakes in tohoku, (northeast japan), bull. seismol. soc. am., 68, 181-192. ueda, k., and t. usami (1990): number of earthquakes in historical records, hist. earthquakes (rekishi jishin), 6, 181-187 (in japanese). usami, t. (2002): historical earthquakes in japan, in international handbook of earthquake and engineering seismology, part a, edited by w.h.k. lee, h. kanamori, p.c. jennings, and c. kisslinger (iaspei, academic press, san diego, u.s.a.), 799-802. utsu, t. (1971): seismological evidence for anomalous structure of island arcs with special reference to the japanese region, rev. geophys. space phys., 9, 839-890. utsu, t. (1999): seismicity studies: a comprehensive review (university of tokyo press, tokyo), pp. 876 (in japanese). watanabe, h. (1998): comprehensive list of tsunamis to hit the japanese islands (university of tokyo press, tokyo), pp. 238 (in japanese). geo 1201 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 16 development of algorithms and software for forecasting, nowcasting and variability of tec ersin tulunay (1) (2), erdem t. senalp (1), ljiljana r. cander (3), yurdanur k. tulunay (4), ayse h. bilge (5), eti mizrahi (5), stamatis s. kouris (6) and norbert jakowski (7) (1) department of electrical and electronics engineering, middle east technical university, balgat, ankara, turkey (2) tubitak marmara research center, information technologies research institute, gebze, kocaeli, turkey (3) rutherford appleton laboratory, chilton, didcot, oxon, u.k. (4) faculty of aeronautics and astronautics, istanbul technical university (i̇tü), maslak, istanbul, turkey (5) department of mathematics, istanbul technical university, maslak, istanbul, turkey (6) department of electrical and computer engineering, aristotelian university of thessaloniki, greece (7) deutsches zentrum für luft und raumfahrt (dlr), institut für kommunikation und navigation (ikn), neustrelitz, germany total electron content (tec) is an important characteristic of the ionosphere relevant to communications. unpredictable variability of the ionospheric parameters due to various disturbances limits the efficiencies of communications, radar and navigation systems. therefore forecasting and nowcasting of tec are important in the planning and operation of earth-space and satellite-to-satellite communication systems. near-earth space processes are complex being highly nonlinear and time varying with random variations in parameters where mathematical modeling is extremely difficult if not impossible. therefore data driven models such as neural network (nn) based models are considered and found promising in modeling such processes. in this paper the nn based metu-nn model is introduced to forecast tec values for the intervals ranging from 1 to 24 h in advance. forecast and nowcast of tec values are also considered based on tec database. day-to-day and hourto-hour variability of tec are also estimated using statistical methods. another statistical approach based on the clustering technique is developed and a preprocessing approach is demonstrated for the forecast of ionospheric critical frequency fof2. 16.1. introduction unpredictable variability of ionospheric parameters due to disturbances related to the ionosphereplasmapause system limits the efficiency of hf and other communications, radar and navigation systems by causing serious technological problems including range errors, rapid phase and amplitude fluctuations, in other words, radio scintillations of satellite signals and others. with the future advancement of technology, the above-mentioned risks and financial losses will certainly increase unless swift measures are taken in advance. the ionospheric plasma interacts with the trans-ionospheric radio waves and modifies wave parameters such as amplitude, phase and po1202 ersin tulunay et al. larization from a broad frequency range. the travel time delay of transionospheric navigation signals is in the first-order approximation directly proportional to the tec of the ionosphere and amounts up to 60 m for gps signals. strong gradients in the horizontal tec structure as well as small-scale structures of the ionospheric plasma may seriously complicate or even prevent the resolution of phase ambiguities in geodetic or surveying networks. disturbances, which are of stochastic nature, cause variability in the parameters of electromagnetic wave propagation media such as tec and fof2. therefore monitoring and development of algorithms and software to treat disturbances in earth-space and satellite-to-satellite communications are of crucial importance in the planning and operation of communication systems. therefore nowcasting and forecasting of tec value may prove beneficial for many scientific and technological studies and applications. near-earth space processes are highly complex in nature being nonlinear and time varying with the parameters open to the effects of random variations in near-earth space such as solar activities. it is known that in such cases mathematical modeling based on first physical principles is extremely difficult if not impossible. therefore data driven models such as the nn based models are considered in connection with various near-earth processes such as the ionospheric processes and found promising in modeling such processes (cander et al., 1998; e. tulunay et al., 2000, 2001; y.k. tulunay et al., 2000, 2001, 2004b; senalp et al., 2002; vernon and cander, 2002). the only requirement for the success of data driven models is the availability of reliable data which can represent the characteristics of the process to be modeled. in this paper, the nn based middle east technical university metu-nn model is introduced to forecast the 10 min tec variations during the high solar activity in the current solar cycle for the intervals ranging from 1 to h in advance by running the model in ral using ral data. forecast and nowcast of tec values are also considered based on dlr/ikn tec database and system. day-to-day and hour-to-hour variabilities of tec are also estimated using statistical methods. another statistical approach which is based on the clustering technique is developed and a processing approach is demonstrated for the forecast of fof2. 16.2. forecasting gps-tec values up to 24 h in advance by neural network based models the middle east technical university neural networks (metu-nn) technique to forecast 10 min values of the total electron content (tec) values up to 24 h ahead during high solar activity in the current solar cycle has been examined. the network is designed to forecast tec data evaluated from gps measurements from 2000 to 2001 at chilbolton (51.8 n, 1.26 w) receiving station. an additional validation was performed on an independent validation data set by producing the forecast tec values at hailsham (50.9 n, 0.3 e) receiving station for selected months in 2002. the tec problem and preparation of data are outlined, the artificial neural network models as a databased approach for forecasting ionospheric processes are explained, the results with error tables, cross correlation coefficients and scatter diagrams are given, and the generalized and fast learning and operation of the metu-nn are discussed in the context of the cost 271 action studies and applications. neural network models are designed and trained with significant inputs. in our approach, the basic inputs for the model are the temporal inputs, the present tec value, first difference, second difference and relative difference of tec values. in addition, the models also contain intrinsic information on solar activity. the neural network architecture has one input layer, one hidden layer with the neurons and one output layer. levenberg-marquardt backpropagation algorithm is used in training the neural network based models. then the trained neural network is used to forecast the tec values. 1203 development of algorithms and software for forecasting, nowcasting and variability of tec 16.2.1. preparation of data tec data evaluated from gps measurements from 2000 to 2001 at chilbolton (51.8 n, 1.26 w) receiving station are used for training, test and validation within the development mode of the neural network. an additional validation was performed on an independent validation data set by producing the forecast tec values at hailsham (50.9 n, 0.3 e) receiving station for selected months in 2002. table 16.i summarizes the train, test and validation time intervals selected. the basic criterion in the selection of the train, test and validation years is choosing the years corresponding to similar solar activity. in this work the current high solar activity time periods, i.e. years with current high sun spot number values, are selected. 16.2.2. construction of the neural network based model the construction work of the neural network based model is carried out in the development mode. it is composed of «training phase or learning phase» and «test phase» (y.k. tulunay et al., 2004a). data sets of same month, different year are used for training and validation phases within development as in table 16.i. for fast learning of the process with the huge sized input data, the «levenberg-marquardt backpropagation» algorithm is used within training in the development mode. as the training advances, the training error starts to decrease, and it eventually reaches zero, which corresponds to the memorization. memorization means the loss of the generalization capability of the neural network. to prevent memorization, the training is halted, and independent validation data are used. errors are calculated. the decrease in the validation error is noted. training is restarted, and the training cycle is repeated. when the gradient of the error in the validation phase becomes near zero, a «stop training» signal is produced, and thus the training is terminated. the model is then ready for its actual use in the operation mode for forecasting of the tec. in the operation mode the validation data are used for calculating the errors, point by point, to measure the performance of the model. the value of the tec at the time instant k is designated by f (k). the output is f (k+h). it is the value of the tec to be observed h hour(s) later than the present time, and h is 24 at most. the 8 inputs used for the neural network are as follows: 1) the present value of the tec: f(k); 2) first difference: ∆1(k) = f(k) − f(k-h); 3) second difference: ∆2(k) = ∆1(k) − ∆1 (k-h); 4) relative difference: r∆(k) = ∆1(k) / f(k); table 16.i. selection of the time periods for the input data. year month train 2000 april and may test and validation 2001 april and may within development validation within operation 2002 april and may fig. 16.1. architecture of the neural network model. 1204 ersin tulunay et al. 5) cosine component of the minute, m, of the day: cm = – cos (2.π.m / 1440); 6) sine component of the minute of the day: sm = sin (2.π.m / 1440); 7) cosine component of the day, d, of the year: cd = – cos (2.π.d / 366); 8) sine component of the day of the year: sd = sin (2.π.d / 366). among the various neural network structures the best configuration is found to be the one with one hidden layer. there are 8 inputs, 8 hidden neurons and 1 output in the feed-forward structure (fig. 16.1). the levenberg-marquardt backpropagation algorithm is used in training. 16.2.3. results in the operation mode, forecast of the tec values 1, 3, 6, 12, and 24 h in advance is performed separately for the validation data sets, minute by minute. then the root mean square, normalized and absolute error values are calculated. also the cross correlation coefficients are calculated. the analyses and results of the tec forecast in table 16.ii covers the time interval between april and may 2002 for the hailsham receiving station. figure 16.2 exhibits the tec values versus the order of data points in april and may 2002. superimposed on a solid line are the 1 hour advance forecast values of the tec. table 16.iii gives the daily solar-terrestrial indices for the times of interest, i.e. 5-7 april 2002 and 18-22 april 2002. figures 16.3 and 16.4 are the enlarged portions of some data points of fig. 16.2, i.e. the diurnal variations of the observed, and forecast tec values during 18-22 april 2002 and 5-7 april 2002 respectively. those portions are selected from the data of fig. 16.2. that is, the horizontal axes are expanded. figure 16.5 is the scatter diagram of the forecast and observed tec values for april and may 2002. figure 16.6 exhibits the tec values versus the order of data points in april and may 2002. superimposed in a solid line are the 3 h advance forecast values of the tec. fig. 16.2. observed gps-tec results (dotted), 1 h ahead forecast (solid) tec values for the whole time of validation period: april-may 2002 for halisham. table 16.ii. error table. hour(s) ahead forecasting (h) 1 3 6 12 24 root mean square error (el/m2∗ 1016) 1.8573 4.0863 5.2893 6.3761 6.0922 normalized error 0.0639 0.1512 0.1998 0.2513 0.2357 absolute error (el/m2∗ 1016) 1.1796 2.9256 3.8946 4.8183 4.4129 cross correlation coefficient 0.9855 0.9263 0.8791 0.8108 0.8212 1205 development of algorithms and software for forecasting, nowcasting and variability of tec fig. 16.3. observed gps-tec results for disturbed solar-terrestrial conditions (dotted), and 1 h ahead forecast (solid) tec values for the enlarged portion of the time of validation period: 18-22 april 2002. fig. 16.4. observed gps-tec results for quiet solar-terrestrial conditions (dotted), and 1 h ahead forecast (solid) tec values for the enlarged portion of the time of validation period: 5-7 april 2002. table 16.iii. solar-terrestrial indices. date rc 10cm ak bkg m x 05 apr 2002 213 217 004 c1.3 0 0 06 apr 2002 249 206 004 c1.1 0 0 07 apr 2002 211 208 010 c1.5 0 0 18 apr 2002 155 188 043 c1.3 0 0 19 apr 2002 147 180 045 b8.6 0 0 20 apr 2002 224 177 056 c1.0 0 0 21 apr 2002 142 173 006 c1.3 0 1 22 apr 2002 170 014 c1.0 0 0 rc – sunspot index from catania observatory (italy); 10cm – 10.7 cm radioflux (drao, canada); ak – ak index wingst (germany); bkg – background goes x-ray level (noaa, u.s.a.); m, x – number of x-ray flares in m and x class (noaa, u.s.a.). 16.3 16.4 figure 16.7 and 16.8 are the enlarged portions of some data points of fig. 16.6, i.e. the diurnal variations of the observed, and forecast tec values during 18-22 april 2002 and 5-7 april 2002 respectively. figure 16.9 is the scatter diagram of the forecast and observed tec values for april and may 2002. it is seen in the scatter diagram shown in fig. 16.5 that the deviations from straight line are small. therefore the correlation coefficients are very close to unity. in other words, the neural network model learned the shape of the inherent nonlinearities. this result demonstrates that the model has a high sensitivity. it is also observed that the fitted line has a slope close to 45° passing through the origin. therefore the forecasting errors are small. this fact is an indication of the neural network system reaching the correct operating point and demonstrates that the model has a high accuracy. in other words, the neural network system reaches the global minimum. 1206 ersin tulunay et al. fig. 16.7. observed gps-tec results for disturbed solar-terrestrial conditions (dotted), and 3 h ahead forecast (solid) tec values for the enlarged portion of the time of validation period: 18-22 april 2002. fig. 16.8. observed gps-tec results for quiet solar-terrestrial conditions (dotted), and 3 h ahead forecast (solid) tec values for the enlarged portion of the time of validation period: 5-7 april 2002. fig. 16.5. one hour ahead forecast tec values versus observed gps-tec results (dots) with the linear fit (line). fig. 16.6. observed gps-tec results (dotted), 3 h ahead forecast (solid) tec values for the whole time of validation period: april-may 2002 for halisham. 16.5 16.6 16.7 16.8 fig. 16.9. 3 h ahead forecast tec values versus observed gps-tec results (dots) with the linear fit (line). 1207 development of algorithms and software for forecasting, nowcasting and variability of tec 16.3. tec database, forecast and nowcast as emphasized in the previous sections, it is of crucial importance to monitor the tec. permanent monitoring is performed by dlr for the european and polar regions. nowcasting and forecasting of the tec values are also performed based on the dlr database (stankov et al., 2001; wehrenpfennig et al., 2001; jakowski et al., 2002a,b; klaehn et al., 2003). 16.3.1. tec database since 1995 dlr/ikn has been operating a new system for regularly processing data and producing tec maps over the european region based on gps measurements by the international gps service (igs). the 30s data from the gps stations of the european igs network allow the determination of slant tec values along numerous satellite-receiver links over the european area with high time resolution. the instrumental biases are separated from the observations by assuming a second-order polynomial approximation for tec variations over the observing gps ground station. both tec and the instrumental satellite-receiver biases are estimated simultaneously by a kalman filter run over 24 h. the slant tec data are then mapped to the vertical by applying a mapping function which is based on a single layer approximation at hs p = 400km. finally, the observed tec data are combined with a regional tec model (neustrelitz tec model – ntcm) in a way that the map provides measured values near measuring points and model values at regions without measurements. the advantage of this procedure is that (in case of a low number of measurements) it delivers reasonable ionospheric corrections which can be provided to users to enhance accuracy and integrity of positioning. the existing large database, containing data from all solar/geomagnetic conditions, is an optimal background for the validation of all types of ionospheric correction especially at highly disturbed ionospheric conditions where other measurement techniques (e.g., ionosondes) are limited. 16.3.2. nowcast the computed european tec maps (comparable to waas and estb ionospheric correction maps) cover a region of 32.5°n to 70°n in latitude and –20° to 60° e in longitude. the measurements have a routine time resolution of 10 min. former verification studies by independent data sources (eiscat, ionosondes) have shown that the absolute errors of the estimated tec values are less than about 2-3 tecu. furthermore, dlr/ikn has developed the software modules for deriving ionospheric grid errors in the egnos system testbed (estb) in real-time. 16.3.3. forecast in addition to the nowcast data, forecasts of tec, based on regular and reliable gps measurements, would also be very helpful to improve the surveying practice. autoand cross-correlation procedures have been recently developed for predicting both the critical frequency and the tec, strongly relating the short-term forecast to present and future geomagnetic activity. preliminary results of these methods/procedures have already been tested and reported for the one-dimensional case when forecasting is performed at a given location based on gps-tec measurements, solar and geomagnetic activity indices. if such a prediction is made at several locations in a given region, then instantaneous maps of the forecast can be constructed covering the region of interest. the short-term forecast method is capable of delivering a forecast up to 24 h ahead based on a prediction of the ‘quite-time behaviour’ of tec and a subsequent correction on the relative deviations of the measured tec from its median (quiet-time) values. these deviations, if large enough, are related to the perturbations induced by the eventual geomagnetic storm developing at the same time. this method relies on the long gps-tec time-series data. research and development activities continue according to the envisaged combined nowcast and forecast service as in fig. 16.10. 16.4. tec variability day-to-day and hour-to-hour variability is a permanent feature of the ionosphere. therefore a statistical approach is also necessary for forecasts and predictions (rawer, 1993; rawer et al., 2003). moreover, it is known that the variability from hour-to-hour or from day-to-day of the total electron content (tec) could be estimated from the diurnal variation of the relative deviation of the hourly daily value with respect to the corresponding monthly-median value (kouris and fotiadis, 2002). that is from the expression dt t t t m d m = (16.1) where dt is the relative deviation, td stands for the hourly daily value of tec and tm for the corresponding monthly-median value. using faraday rotation tec data and also gps measurements made in florence and matera (italy), respectively we have calculated quartile and decile levels of variability at each hour/month/year. figures 16.11 and 16.12 report upper and lower quartiles and deciles counted at some selected hours of each month using all available data measured during years of low and high solar activity, respectively. it can be seen that the variability in tec is higher at months/years of high solar activity fig. 16.10. a schematic view on the dlr/ikn nowcasting and forecasting activities. 1208 ersin tulunay et al. 1209 development of algorithms and software for forecasting, nowcasting and variability of tec fig. 16.11. plots of quartiles and deciles of tec variability at randomly selected hours. data measured at years of low solar activity. fig. 16.12. plots of quartiles and deciles of tec variability at randomly selected hours. data measured at years of high solar activity. fig. 16.13. diurnal variation of quartiles and deciles of tec variability at low (left) and high (right) solar activity and different seasons. than during corresponding months/years of low activity (kouris et al., 2004). moreover, the variability in tec is higher after midnight up to before dawn than during the other hours of the day. figure 16.13 reports the diurnal variations of quartiles and deciles of tec relative deviations at low (left) and high (right) solar activity and different seasons. the dependence of variability on season is evident. the variability in tec exhibits higher values during winter and at nighttime, whereas at summer the variability could be assumed practically constant with time of day. it is also evident from the same figure that the variability is higher at high than low solar activity. precisely, the variability in tec is usually close to that in fof2 at low solar activity, whereas at high solar activity it overlaps that of maximum electron density. finally, we may state that the variability in tec for the 90% of the time does not exceed (in absolute value) practically the 40% of the corresponding monthly median value at any time of day and any month except for enhancements in winter, i.e. when very disturbed conditions occur. 16.5. classification of the monthly medians of the ionospheric critical frequnecy fof2 using clustering technique cluster analysis is used to classify monthly medians of the ionospheric critical frequency fof2 data for 13 stations in europe during the period 1958-1998. the algorithm used agglomerates the data consisting of 4801 samples of daily variations into 6 sets of sizes ranging from 1334 to 431 samples, characterized mainly by r12 and seasons. 16.11 16.12 16.13 1210 ersin tulunay et al. monthly medians of the ionospheric critical frequency fof2 for the years 1958 to 1998 for the 13 european ionosonde stations ashkhabad (37.9n-058.3e), novokazalinsk (45.5n-062.1e), poitiers (4.66n-000.3e), lannion (48.5n-356.7e), dourbes (50.1n-004.6e), slough (51.5n-359.4e), kaliningrad (54.7n-020.6e), moscow (55.5n-037.3e), sverdlovsk (56.4n-058.6e), uppsala (59.8n017.6e), arkhangelsk (64.6n-040.5e), lycksele (64.6n-018.8e), kiruna (67.8n-020.4e) are studied. the data from these stations were not completed and we also disregarded data samples with missing values and worked with a total of 4801 samples of daily variations indexed by the month, year and station. based on our previous experience, we eliminated the longitudinal dependency by a local time shift (mizrahi et al., 2002). we overlooked any dependency on the geomagnetic coordinates and identified the dependency on the calendar year by a dependency on r12, hence over looking any possible dependency on atmospheric conditions (mizrahi et al., 2002). the work aimed to be a preprocess to obtain a forecast curve for the ionospheric critical frequency fof2. the clustering algorithm we use starts with a random element from the sample and finds those samples that lie in a certain neighborhood (here 22%) with respect to the l2 norm. we then repeat the procedure after removal of the samples lying in this cluster from the whole data. this procedure gave a total of 27 clusters, 15 of which containing less than 30 elements (less than 0.62% of the data). a qualitative study of these tiny clusters showed that clusters containing less than 10 elements were rather related to bad data and they were overlooked. as a first step we studied the larger 12 clusters, containing 1296 to 34 elements. the representative curves were characterized by 4 parameters, width (w), baseline (b), center (c) and peak amplitude (a). based on these parameters, we merged the 16 clusters (the larger 16 of the 27 clusters), obtaining 6 groups. in order to study the structure of each cluster, we obtained histograms of the distribution of the stations (latitude dependency), months (seasonal dependency) and years (r12 dependency) inside each of the 6 clusters, as shown in figs. 16.14 to 16.22. fig. 16.14. histogram of the stations from low latitude to mid and high latitude (37.9n to 67.8n), for cluster 1 and cluster 2. fig. 16.15. histogram of the stations from low latitude to mid and high latitude (37.9n to 67.8n), for cluster 3 and cluster 4. 16.14 16.15 1211 development of algorithms and software for forecasting, nowcasting and variability of tec from figs. 16.14 to 16.16, (13 stations are ranged from low latitude to mid and high latitude), we can see that cluster 3 is linked to high latitude stations while cluster 4 contains low latitude stations. samples in other clusters seem to have a uniform latitude distribution. in figs. 16.17 to 16.19, (12 months from january to december), we can clearly see that cluster 4 represents equinox conditions, cluster 1 and 2 represents summer, while cluster 3, 5 and 6 contain winter data. fig. 16.18. histogram of months (from january to december), for cluster 3 and cluster 4. fig. 16.19. histogram of months (from january to december), for cluster 5 and cluster 6. 16.18 16.19 fig. 16.16. histogram of the stations from low latitude to mid and high latitude (37.9n to 67.8n), for cluster 5 and cluster 6. fig. 16.17. histogram of months (from january to december), for cluster 1 and cluster 2. 16.16 16.17 fig. 18.22. histogram of r12 values (from 8.6 to 184.8), for cluster 5 and cluster 6. 1212 ersin tulunay et al. fig. 16.20. histogram of r12 values (from 8.6 to 184.8), for cluster 1 and cluster 2. fig. 16.21. histogram of r12 values (from 8.6 to 184.8), for cluster 3 and cluster 4. figures 16.20 to 16.22, (r12, (8.6-184.8), values corresponding to years, 1958-1998), show that low r12 data is in cluster 2 and 3 while high r12 data is clearly in cluster 4. the clusters 1, 6 and 5 contain data corresponding to diminishing r12 level. 16.6. conclusions algorithms and software were developed for forecasting, nowcasting and variability of tec. nn based metu-nn model was introduced to forecast 10 min tec variations during the high solar ac16.20 16.21 1213 development of algorithms and software for forecasting, nowcasting and variability of tec tivity in the current solar cycle for the intervals from 1 to 24 h in advance by running the model in ral by using ral data. it was seen that nn model has a satisfactory sensitivity and accuracy. forecast and nowcast of tec values are also considered based on dlr/ikn tec database and system. day-to-day and hour-to-hour variability of tec are also estimated by using statistical methods. another statistical approach based on clustering technique was developed and a processing approach was demonstrated for the forecast of fof2. as a result of the studies on which this paper is based, data driven and statistical tools were developed for forecasting, nowcasting and investigating the variability of tec. the methods developed can be used for characterizing the electromagnetic wave propagation medium for the purposes of radio system planning and operation. acknowledgements the author, s.s. kouris, is indebted to k.v. polimeris for his assistance in the present work. references cander, lj.r., m.m. milosavlijevic, s.s. stankovic and s. tomasevic (1998): ionospheric forecasting technique by artificial neural network, electronics letters, 34 (16) (online no. 19981113, 6 august 1998), 1573-1574. jakowski, n., s. heise, a. wehrenpfennig, s. schlüter and r. reimer (2002a): gps/glonassbased tec measurements as a contributor for space weather, j. atmos. solar-terr. phys., 64, 729735. jakowski, n., a. wehrenpfennig, s. heise and i. kutiev (2002b): space weather effects on transionospheric radio wave propagation on 6 april 2000, acta geod. geophys. 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(1993): wave propagation in the ionosphere (kluwer publ. co., dordrecht, nl). rawer, k., s.s. kouris and d.n. fotiadis (2003): variability of f2 parameters depending on modip, adv. space res., 31, 537-541. senalp, e.t., e. tulunay and y.k. tulunay (2002): neural network based approach to forecast the total electron content values, on cd-rom of abstracts of the egs 2002 conference, 27th general assembly of the europaen geophysical society, 21-26 april 2002, nice, france, egs02-a-00867. stankov, s.m, i. kutiev, n. jakowski and a. wehrenpfennig (2001): a new method for total electron content forecasting using global positioning system measurements, in proc. estec space weather workshop, 17-19 december 2001, noordwijk, the netherlands. tulunay, e., y.k. tulunay and e.t. senalp (2000): neural network based approach to forecast ionospheric parameters, in special workshop, 8-11 november 2000, lindau, germany. tulunay, e., e.t. senalp and y.k. tulunay (2001): forecasting the total electron content by 1214 ersin tulunay et al. neural networks, in book of abrstracts of the proceedings of the 1st cost 271 workshop, 25-27 september 2001, sopron, hungary, p. 47. tulunay, y.k., e. tulunay, e.t. senalp and c. ozkaptan (2000): neural network modeling of the effect of the imf turning on the variability of hf propagation medium, in ap 2000, millennium conference on antennas & propagation, icap&jina, 9-14 april 2000, davos, switzerland, p. 132. tulunay, y.k., e. tulunay and e.t. senalp (2001): an attempt to model the influence of the trough on hf communication by using neural network, radio sci., 36 (5), 1027-1041. tulunay, y.k., e. tulunay and e.t. senalp (2004a): the neural network technique-1: a general exposition, adv. space res., 33 (6) 983-987. tulunay, y.k., e. tulunay and e.t. senalp (2004b): the neural network technique-2: an ionospheric example illustrating its application, adv. space res., 33 (6) 988-992. vernon, a. and lj.r. cander (2002): regional gps receiver networks for monitoring local mid-latitude total electron content, ann. geophysics, 45 (5), 649-656. wehrenpfennig, a., n. jakowski and j. wickert (2001): a dynamically configurable system for operational processing of space weather data, phys. chem. earth, 26, 371-390. layout 6 1 annals of geophysics, 62, 5, gd561, 2019; doi: 10.4401/ag-8054 “evaluation of different gravimetric methods to moho recovery in iran„ sahar ebadi*,1, riccardo barzaghi2, abdolreza safari1, abbas bahroudi1 (1) school of surveying and geospatial engineering, college of engineering, university of tehran, iran (2) politecnico di milano, department of civil and environmental engineering, milan, italy (3) school of mining engineering, college of engineering , university of tehran, tehran, iran 1. introduction the knowledge of the mohorovičić discontinuity (moho) can provide valuable information to understand some topical issues in solid earth sciences [sampietro, 2016]. its knowledge in the iran block is one of the crucial issues in this region. several unique events like tectonics and orogenic activities in iran led to a complex geological structure of the area. thus, it is important to study in deep the peculiar structure of the iranian moho applying different methods and different observations in order to have a comprehensive definition of its main features. moho interface is commonly estimated by either seismic or gravimetric methods. although seismic moho estimates have a significant accuracy (at the level of 1-2 km), their coverage over entire of the earth is quite poor. thus, in regions where seismic data are sparse or missing, results of gravimetric studies can be profitably used. at global scale, this has been made possible after dedicated gravity-satellite missions, namely article history received december 12, 2018; accepted march 17, 2019. subject classification: moho, gravimetric inverse problem, collocation, isostasy, seismic data. abstract the complexity of geological units in iran because of several unique events like tectonics and orogenic activities in this region led to extensive investigations for moho recovery by seismic methods therein. in this research, three gravimetric methods have been evaluated by some point-wise seismic data. we applied collocation method as an iterative process as well as modified forms of sjöberg and jeffrey’s theory of isostasy for local moho depth recovery. the gravity data has been generated by goco03s model reduced by topography/bathymetry, sediment and consolidated crust effects. although the iteration process in collocation approach only slightly changed the estimated depths, this method led to a better agreement with seismic data rather than others. differences between collocation, jeffrey and sjöberg’s solutions with seismic studies are similar but jeffrey and sjöberg’s methods displayed a systematic bias. the standard deviations of the residuals among seismic data and gravimetric solutions are around 6 km. overall, the evaluation of these approaches indicated that moho from gravimetric approaches reduced only slightly the standard deviation of seismic moho estimates. significant discrepancies with seismic data have been detected in makran subduction zone, oman sea, persian gulf and caspian sea. the explanation of such inconsistency can be partially due to the poor quality of crust1.0 data in these areas, as this model has been used to correct the gravity values that were input in the inversion procedures. the gravity recovery and climate experiment (grace) [tapley et al., 2004a; tapley et al., 2004b] and the gravity field and steady-state ocean circulation explorer (goce) [floberghagen et al., 2011]. these two missions have provided global gravity field with an accuracy and a resolution which is suitable for investigating the moho structure. among successful seismic estimates, the early results dated back to beloussov et al. [1980]. the most widely known crustal model based on seismic refraction is crust5.1 model [mooney et al., 1998]. bassin et al. [2000] further upgraded it and called the new version, crust2.0. the crust1.0 [laske et al., 2013] including ice layers, water, sediments and consolidated crustal layers, is the most recent version compiled with a 1×1 arcdeg spatial resolution. as already pointed out, because of insufficient seismic data coverage over large areas, gravimetric or combined gravimetric/seismic solutions have been utilized. based on some isostasy hypotheses on compensating the earth’s topographic masses, gravity data can be used to determine the moho depth. several basic theories were suggested to explain the mechanism of isostasy like those proposed by pratt-hayford [pratt, 1855; hayford, 1909] and airy-heiskanen [airy, 1855; heiskanen, 1931]. vening meinesz [1931] modified the airy-heiskanen theory by considering a regional instead of a local compensation based on a thin plate lithospheric flexure model [watts, 2001]. vening meinesz theory had been modified by parker [1973] in an iterative approach for moho determination and oldenburg [1974] made an attempt to stabilise this method by applying a low-pass filtering technique. the combination of these two methods was known as a parkeroldenburg method and it has been generalized for the 3-d gravity inversion by gómez-ortiz and agarwal [2005] and shin et al. [2007]. braitenberg et al. [2000] developed a similar method with integration of seismic data; also they estimated variation of the moho under the tibet plateau by an iterative inversion method. moritz [1990] improved the vening meinesz inverse problem for a global compensation by adopting the spherical approximation model of the earth. since there were some theoretical deficiencies in this isostatic method, sjöberg [2009] reformulated moritz’s theory and called it as the vening meinesz moritz (vmm) problem. in this approach he solved a non-linear fredholm’s integral equation of the first kind. bagherbandi and sjöberg [2012] made a comparison between the gravimetric vmm and local airy-heiskanen methods in the determination of moho depth. another approach based on the inversion of gravity data to determining the moho depth is collocation method [krarup, 1969; tscherning, 1985; moritz, 1990]. barzaghi et al. [1992] proposed an approach based on collocation principle, which propagate the covariance structure of the depth interface to the covariance function of the observed gravity field. barzaghi et al. [2015] applied collocation method by combining the global gravity-gradient information of goce and local gravity data to moho recovery. barzaghi and biagi [2014] implemented then a further version of the collocation method by including seismic moho depths as input data. braitenberg and ebbing [2009] studied the structure of the crust by combination of grace and terrestrial gravity data. some other scientists made attempt to estimate moho depth by applying goce gravity gradient data [braitenberg et al., 2010; sampietro, 2011; sampietro et al., 2014]. reguzzoni et al. [2013] combined seismic and goce data to obtain a new global moho model. tenzer et al. [2009, 2011, 2012a, 2012b] proved that applying the crustal density-contrast stripping corrections is appropriate for a gravimetric moho recovery. they showed that gravitational contributions of topography and major known crustal density structures have a large spatial correlation with the moho geometry. in the area under investigation in this paper, several studies based on seismic data have been performed for the determination of the regional moho model. most of these studies concentrated on a specific area, for instance profiles between shiraz-mashhad, tehran-mashhad and mashhad-tabriz [asudeh, 1982], the southern part of the caspian sea [mangino and priestley, 1998], tehran region [hatzfeld et al., 2003], mashhad [doloei and roberts, 2003; javan doloei, 2003], central alborz and the northern iran [sodoudi et al., 2009… radjaee et al., 2010], central zagros [paul et al., 2006, shad manaman et al., 2011], kopeh-dagh [nowrouzi et al., 2007], naein [nasrabadi et al., 2008], the northwest iran [taghizadeh-farahmand et al., 2015], and sanandaj-sirjan zone [sadidkhouy et al., 2012]. furthermore, based on terrestrial gravity data in this area with a quite homogenously coverage, dehghani and makris [1984] combined gravimetric and seismic data to determination of the crustal structure of iran. abbaszadeh et al. [2013] compared the effective elastic thickness of the lithosphere estimated by terrestrial and satellite data in iran. eshagh et al. [2017] reformulated two isostatic methods for moho recovery by considering contributions of mean moho over the whole moho spectrum. in this paper, we adopt collocation method as well as two isostatic approaches to determine the moho depths inverting gravity data over iran. the collocation inversion method for a two-layer model devised by barzaghi and biagi [2014] is applied. in addition, the generalized form of jeffrey and sjöberg’s method proposed by esebadi et al. 2 3 gravimetric moho over iran hagh et al. [2017] have been utilized in this study. these methods have been applied to gravity from goco03s gravitational model [mayer-gürr et al., 2012] reduced for the srtm30_plus topographic/bathymetric data [becker et al., 2009], the sediment and the crystalline data of crust1.0 [laske et al., 2013]. comparisons of these moho depth estimates have been finally performed with the seismic estimates available from literature. 2. the theoretical background in this section, we review three gravimetric inversion methods that can be successfully applied to estimate the moho depth. at first, we describe the procedure based on the collocation principle. subsequently, we also discuss two alternative isostatic approaches, which are reformulations of jeffrey and sjöberg’s theories. 2.1 the collocation solution the collocation method for moho estimation is based on a stochastic approach derived from the wiener filtering and prediction theory that allows estimating the signal correlated component based on the covariance structure of the data [barzaghi and biagi, 2014]. in the years, it has been applied to determine the gravimetric estimate of the moho [barzaghi et al., 1992; barzaghi and biagi; 2014, barzaghi et al., 2015]. in this context, the covariance structure of the moho depth is propagated to the covariance of the observed gravity in a simple two-layer model. in order to apply collocation for estimating the moho depth, we should consider the following linear relationship which holds in planar approximation [barzaghi and biagi, 2014]: (1) δ𝑔 is the bouguer gravity anomaly minus its mean, g is the newton’s gravitational constant, 𝑇₀ is the mean moho depth, 𝜀 is the moho depth undulation with respect ,𝑇₀, δ𝜌 stands for the mean constant density contrast between the two layers and 𝑑�� = √𝑥² + 𝑦². also, the following conditions must be fulfilled [barzaghi and biagi, 2014]: 1) 𝜀 is a weak stationary stochastic process, ergodic in the mean and in the covariance 2) the noises in gravity and depth, n𝑔 and n𝜀 are spatially uncorrelated zero mean signals 3) the cross-correlations between signals and noises are zero to perform the computation, the gravity auto-covariance and the cross-covariances between gravity and depth are needed, i.e.: 𝐶�δ𝑔�, δ𝑔�� = 𝐶δ𝑔δ𝑔��𝑃� – 𝑃��� = 𝐶�δ𝑔�, δ𝑔�� (2) 𝐶�𝜀�, δ𝑔�� = 𝐶𝜀δ𝑔��𝑃� – 𝑃��� = 𝐶�δ𝑔�, 𝜀�� (3) assuming that the observed gravity values contain a noise component, we can write: δ𝑔𝑂𝐵𝑆 = δ𝑔 + n𝑔 (4) the collocation estimate of 𝜀 is [moritz, 1980; barzaghi et al. 1992; barzaghi and biagi, 2014]: (5) with c𝜀δ𝑔�= 𝐶�𝜀�, δ𝑔�� , 1 = �δ𝑔𝑂𝐵𝑆� and c𝑙𝑙 = �𝐶δ𝑔���δ𝑔����. as the first step to use this approach, the empirical covariance function of δ𝑔𝑂𝐵𝑆 should be estimated and modelled with appropriate positive definite model functions [moritz, 1980]. under the hypotheses previously stated, the empirical covariance can be estimated as [barzaghi et al., 1992]: (6) also, auto and cross-covariance models must be defined in order to derive the estimator of 𝜀 for moho determination. one possible model for the auto covariance of δ𝑔 [see barzaghi and biagi, 2014] is: (7) where 𝐽₁(�) is the first order bessel function. if this auto-covariance is considered, one can prove [barzaghi et al., 1992] that: (8) which can be evaluated by numerical integration methods. the parameters a and 𝛼 are determined so that the model (7) fits the empirical estimated covariance values of δ𝑔. after deriving 𝐶𝜀δ𝑔�𝑃, 𝑄� by the expression 𝑅2 δ𝑔�𝑥,𝑦,0� = 𝐺∬𝑑𝑥𝑑𝑦 δ𝜌 ��[𝑇2 +𝑑2 ]3/2 𝜀𝑇₀ ₀ 𝜀̂ = �c𝑇 �c–1l𝑙𝑙𝜀δ𝑔 𝐶�δ𝑔δ𝑔(δ𝑃𝑘) = � �δ𝑔𝑂𝐵𝑆 �𝑄��δ𝑔𝑂𝐵𝑆 �𝑄�� 𝑃𝑘–1 <�𝑄� – 𝑄��<𝑃𝑘 , δ𝑃𝑘 = 𝑃𝑘 – 𝑃𝑘–1 𝑁 1 � 𝑖=1 𝑁� 1 �� 𝑗=1 𝐶δ𝑔δ𝑔(𝑟) = 𝐴𝐽₁�𝛼𝑥� 𝛼𝑥 𝐶𝜀δ𝑔�𝑃, 𝑄� = � 𝛼 𝑑𝑘𝑘𝑒𝑘𝐻�𝐽0�𝑘�𝑃–𝑄��2π𝐺𝜌̄𝛼 𝐴 0 3 ebadi et al. 4 above, we can then obtain by applying formula (5). the final moho estimate is then given as 𝑇 = 𝑇0 + 𝜀. also, in order to refine the estimate, iterations on gravity residuals can be performed so that the final solution is obtained as 𝑇 = 𝑇0 + 𝜀1 + ... + 𝜀𝑛 . usually two or three iterations are computed. 2.2 the jeffrey’s solution jeffrey [1976] has solved the problem of isostasy for moho modelling in a very similar way to the vmm method [sjöberg, 2009]. eshagh and hussain [2016] proposed a different modelling of the isostatic gravity disturbance 𝛿gi by considering the gravitational effects of topography and bathymetry, sediments and consolidated crystalline basement as follows: 𝛿gi = 𝛿g – 𝛿gtb – 𝛿gs – 𝛿gcrys+𝛿gc (9) in this equation, 𝛿g denotes the observed gravity disturbance which is the difference between measured and normal gravity at a computation point on the earth, 𝛿gc is the compensation attraction, 𝛿gtb is the topographic/bathymetric effect on 𝛿g, 𝛿gs and 𝛿gcrys represent the gravitational effects of sediment and consolidated crust, respectively. moritz [1990] and sjöberg [2009] applied this equation as a fundamental condition to solve the vmm problem. in order to define the compensation potential in equation (2-10), eshagh et al. [2017] arranged the formula in jeffrey [1976] in the following way: (10) as stated before, 𝜀 is the variation of the moho depths relative to the mean value indicated by t0. in this equation, r is the radius of the earth, 𝜎 is the unit sphere, 𝑟’ denotes the geocentric distance of the infinitesimal mass element while d𝑟’ and d𝜎 = sin𝜃d𝜃d𝜆 are the radial and the surface integration elements, 𝑃𝑛(cos𝜓) is the legendre polynomial of degree n for the argument of the geocentric angle 𝜓. 𝜃 and 𝜆 represent the spherical co-latitude and longitude of the element, l is the euclidean spatial distance which is a function of the 𝑟’ (see. heiskanen and moritz [1967]). further, eshagh et al. [2017] solved the radial integral and expanded 𝛿gc in a spectral form according to the heiskanen and moritz [1967] scheme: (11) where 𝐾= 1– . also eshagh et al. [2017] approximated the term [𝜀/(𝑅‐𝑇0)]n+3 by a binomial series and after simplification, (2-11) reduces to: (12) they wrote this equation according to laplacian harmonics of δ𝜌𝜀: (13) by considering eq. (2-9) and inserting (2-13) into that, they got: (14) eshagh et al. [2017] took the summation from both sides of eq. (2-14) and extract 𝜀 from the resulting expression and finally arrived at: (15) since 𝜀 is the undulation of moho depths with respect to 𝑇0 , the total moho depth is given as 𝑇 = 𝑇0 + 𝜀. 2.3 the sjöberg’s solution the vening meinesz moritz (vmm) inverse problem of isostasy [sjöberg, 2009] has been developed and applied successfully over different areas of the earth. the main difference between jeffrey and sjöberg’s method is how to write the compensation potential. sjöberg [2009] used the compensation potential vc derived by moritz [1990]: (16) eshagh et al. [2017] solved the integrations by assuming t = t0 and obtained the compensation attraction of the gravity disturbance as follows: 𝑉c = 𝐺�� � d𝑟’d𝜎 = σ 𝑅–𝑇0 –𝜀 𝑅–𝑇0 𝑙 δ𝜌𝑟’2 σ 𝑛=0 𝑅–𝑇0 –𝜀 𝑅–𝑇0 𝑟𝑛+1 δ𝜌∞ = 𝐺��� � 𝑟’𝑛+2d𝑟’𝑃𝑛(cos𝜓)d𝜎 𝑇0 𝑅 𝛿gc = � (𝑛+1) 𝐾𝑛+3 �� (δ𝜌𝜀) 𝑃𝑛 (cos𝜓)d𝜎 σ𝑅–𝑇0 𝐺𝑅 ∞ 𝑛=0 𝛿gc,𝑛 = 4𝜋𝐺 𝐾𝑛+2 (δ𝜌𝜀)𝑛2𝑛+1 𝑛+1 (δ𝜌𝜀)𝑛 = 𝐾–(𝑛+2) �𝛿gn – 𝛿gtb – 𝛿gs – 𝛿gcrys� 4𝜋𝐺 1 𝑛+1 2𝑛+1 n n n 𝜀 =– � 𝐾–(𝑛+2) �𝛿gn – 𝛿gtb – 𝛿gs – 𝛿gcrys� 4𝜋𝐺δ𝜌 1 𝑛+1 2𝑛+1 n n n ∞ 𝑛=0 𝛿gc� 𝑟=𝑅 =𝐺�� δ𝜌� 𝐾𝑛+3 ×�1–�1 – � 𝑛+3 �𝑃𝑛(cos𝜓)d𝜎 𝜀 𝑅–𝑇0 𝑛+3 𝑛+1∞ 𝑛=0 𝑉c =𝐺δ𝜌��� 𝑃𝑛(cos𝜓) � � 𝑟’𝑛+2d𝑟’– � 𝑟’𝑛+2d𝑟’�d𝜎 σ 𝑅–𝑇 𝑅 𝑅–𝑇0 𝑅1 𝑟𝑛+1 ∞ 𝑛=0 5 gravimetric moho over iran (17) furthermore, they considered the spectral form of 𝛿gc and got: (18) where . they proved by inserting eq. (18) into (9) that the moho depth can be obtained as: (19) where . as it can be seen in eq. (19), t0, besides the zero-degree term, affects all frequencies. 3. the iran case study in this section, we present the iran case study and the application of the three methods previously described to the gravimetric estimate of the moho in iran. we divide this section into four parts. in section 3.1 we present a brief geological structure of the area. the used gravity data are described in section 3.2 while in section 3.3 the local gravity moho estimates over the study area by collocation, the jeffrey and sjöberg methods are presented. finally, in section 3.4 comparisons among different gravimetric and seismic derived moho values are shown. 3.1 the study area all methodologies have been applied in iran over an area limited by the parallels 20° and 45° north and the meridians 40° and 65° east (see figure 1). several unique events like tectonics and orogenic activities led to complicated structural units in this area. by considering the geological features of iran, some models and interpretations have been proposed for this region [nabavi, 1976; eftekharnezhad, 1980; alavi-naini, 1993; aghanabati, 2004; ghorbani, 2013]. according to these studies, a geological setting of this area showing a geological classification of various structural zones of iran has been devised and is presented in figure 1, superimposed on the regional topography. topography/bathymetric heights were generated by the srtm30_plus model to degree and order 2160 with a resolution of 5'×5' over the study area [becker et al., 2009]. as seen in figure 1, the ranges of topography vary from -3182 to 4142 m. significant topography is seen over the alborz and zagros mountains which continues from west-north until east-south. most of iran is surrounded by a rough topography except for southern border of caspian sea, central iran, lut block, jazmourian and makran basins. the convergence of the arabia-eurasia plate led to the complex features in the iranian crust and lithospheric mantle. the closure of tethys ocean and collision of arabian-eurasian plates caused the formation of iranian plateau during the mesozoic and cenozoic period [berberian and king, 1981; berberian et al., 1982]. some active and young tectonic structures consist of the collision zones in zagros, alborz, kopeh-dagh and subduction zones in the makran and south caspian basin were formed as a result of the arabia-eurasia convergence [shad manaman et al., 2011]. this convergence led to formation of the two tectonometamorphic and magmatic belts of sanandaj-sirjan zone and the urumieh-dokhtar magmatic assemblage. central iran is a triangle located in the middle and bordered by the alborz mountains in the north, lut block in the east and urumieh-dokhtar in the south. in this zone, there are rocks of all ages, from precambrian to quaternary, and several episodes of orogeny, metamorphism and magmatism. sanandaj-sirjan is located to the south-west of central iran and the northeast of zagros mountains. a remarkable feature of this zone is the presence of immense volumes of magmatic and metamorphic rocks of paleozoic and mesozoic eras. zagros ranges separate the arabian block from the rest of eurasian tectonic plate. in this area, there are no abundant outcrops of paleozoic rocks and the area is without magmatic and metamorphic events. alborz range is located in north of iran, parallel to the southern margin of caspian sea. this mountain is characterized by different sedimentary rocks. the kopeh-dagh mountains and basin consist largely of extrusive igneous rocks belong to paleogene volcanic areas. makran is separated from jazmourian depression by a long range of ophiolites extending from west to east [cf. ghorbani, 2013]. σ 𝛿gc = 𝐺δ𝜌���(𝑛+1) �1 –(𝑛+2) �𝑃𝑛 (cos𝜓)d𝜎 – ∞ 𝑛=0 𝑅 𝑇 2𝑅 𝑇 ∞ 𝑛=0 (𝑛+3) (𝑛+1) – 𝐺δ𝜌��� �1–𝐾𝑛+3�𝑃𝑛(cos𝜓)d𝜎 𝛿gc,𝑛 = 4𝜋𝐺δ𝜌𝛽𝑛 𝑇𝑛– �1–𝐾𝑛+3�𝛿𝑛0 2𝑛+1 𝑛+1 3 4𝜋𝐺𝑅δ𝜌 𝛽𝑛 = 1–(𝑛+2) 2𝑅 𝑇0 𝑇 = 𝐴c0+ � 𝛽 –1�𝛿gtb + 𝛿gs – 𝛿gcrys – 𝛿gn� 4𝜋𝐺δ𝜌 1 𝑛+1 2𝑛+1 nn n n 𝑛=0 ∞ 𝐴c0 = (1–𝐾3)� �𝑅–𝑇0 𝑅 3 𝑅 3.2 the gravity data set in this study, the considered gravity data have been synthetized from the goco03s gravitational model up to degree and order 180 and were computed on a 0.5×0.5 arc-deg surface grid. in order to obtain the topography/bathymetry (tb) corrections, we applied the spherical harmonics expansion of digital elevation model from srtm30_plus to degree and order 180 consistent with the resolution of crust1.0 model. moreover, for computing the gravity corrections due to sediment and consolidated crustal layers, we used the earth’s crustal model crust1.0, which in this area gives mean densities values for the sediments and the consolidated crustal layers that are, respectively, 2060 kg m3 and 2670 kg m-3 (see figures 2b and 2c). by applying all these corrections we obtained the gravity data that were used in the inversion procedures. figure 2d represents the map of the refined bouguer gravity, in unit of mgal, reduced for the gravitational contribution of crustal density heterogeneities over the study area. this illustrates the largest and positive value at oman sea and the negative one over the zagros and sanandajsirjan belts and in the north-east part of iran around its border to azerbaijan and turkey and in the south part in bam. all the mentioned corrections are displayed in figure 2 and the related statistics are given in table. 1. ebadi et al. 6 figure 1. topography heights and geological setting of the study area of iran [km]. max mean min std tb 227.3 -35.0 -371.1 96.7 sediments 183.5 66.0 3.9 34.5 crust 46.5 -199.5 -424.7 84.2 total 486.8 230.6 -195.1 123.2 table 1. statistics of the tb, sediment and consolidated crust corrections to gravity disturbances [mgal]. 3.3 the gravimetric moho estimates the three methods described in section 2 for determination of moho were applied in the study area. the computations were accomplished by setting 𝑇0 = 44 km, which is the mean of depths estimated from seismic studies [mangino and priestley, 1998; paul et al., 2006; taghizadeh-farahmand et al., 2010; radjaee et al., 2010; tatar and nasrabadi, 2013; taghizadeh-farahmand et al., 2015; motaghi et al., 2015; abdollahi et al., 2018]. as stated above, the collocation approach in planar approximation has been applied to goce derived reduced bouguer gravity, i.e. bouguer gravity anomalies reduced for sediment and consolidated crust effects. moho depths have been determined with respect to mean value 𝑇0 while the 𝜀 values have been obtained according to the scheme described in section 2. in this computation, the constant density contrast between crust and mantle has been set to 600 kg m-3. the gravity data, obtained by applying combined topography/bathymetry gravitational effect from srtm30_plus data and corrections for the sediment and consolidated crust data generated from the crust1.0, has been re-gridded on a regular (x, y) grid in the investigation area (see figure 3). the collocation procedure has been then applied iteratively. the behaviour of the empirical covariance function of gravity data and the best-fit model with the related parameters can be seen in figure 4 for the three steps that have been performed to get the final estimate. 7 gravimetric moho over iran figure 2. a) the tb gravitational effect; b) the gravitational effect of sediments; c) the gravitational effect of consolidated crust; d) the reduced bouguer gravity anomalies [mgal]. a) b) c) d) ebadi et al. 8 figure 3. bouguer gravity anomaly reduced by topography/bathymetry, sediment and consolidated crust corrections on a (x,y) grid [mgal]. a) b) in this figure, parameter a is the covariance value in the origin, 𝛼 is the scaling factor for argument of bessel function and 𝜎�̂�2 is the noise variance, i.e. the difference between the empirical value and the model function at the origin. a and 𝛼 are found by letting the first empirical zero coincide with the zero of the model function and assuming the model function coincide with the empirical one at the second point [barzaghi et al., 1992]. in the first step, the model function which best described the empirical values is j1 bessel function divided by its argument, with a=14000 mgal2 and parameter �̂� =0.0055 km-1. a value is sufficiently close to initial one of empirical function, which is 14815 mgal2. the difference between the two values in the origin, i.e. the noise variance, has been fixed to 815 mgal2. in the second step, residuals of gravity from the first inversion step have been used as input data. as can be seen in figure 4 the variance of these residuals, i.e. the value in the origin of the empirical function, decreased drastically to 3490 mgal2 and the signal variance, i.e. the a value, has been set to 3200 mgal2. the �̂� and 𝜎̂𝑛2 quantities have been fixed at 0.0135 km-1 and 290 mgal2, respectively. furthermore, in third step, based on the residual gravity from the second iteration, model covariance parameters were fixed at 𝐴�𝛿𝑔 =900 mgal2, �̂� =0.0317 km-1, 𝜎�̂�2 =192 mgal2. this iterative process has been stopped at the third iteration since the covariance function of the third step residuals has a correlation length (the distance at which the covariance function is half of its value in the origin) that is comparable with the grid step. results of this procedure are shown in figure 5 and related statistics are summarized in table 2. the final estimate (see figure 5c), obtained by adding the values coming from the iterative procedure to the mean depth 𝑇0 =44 km, shows maximum depths under the zagros mountain, the sanandaj-sirjan and the urumieh-dokhtar belts with spread under the alborz mountain and kopeh-dagh, while the minimum depth is under the oman sea and the border of caspian. 9 gravimetric moho over iran figure 4. the empirical covariance function of the gravity data and the best−fit model. a) first step: 𝐴�𝛿𝑔 14000 mgal2, �̂� = 0.0055 km−1, 𝜎�̂�2 = 815 mgal2. b) second step: 𝐴�𝛿𝑔 3200 mgal2, �̂� = 0.0135 km−1, 𝜎�̂�2 = 290 mgal2. c) third step: 𝐴�𝛿𝑔 900 mgal2, �̂� = 0.0317 km−1, 𝜎�̂�2 = 192 mgal2. c) collocation method step max mean min std first step 49.0 44.0 35.5 2.4 second step 51.9 44.0 32.8 3.0 third step 54.6 44.0 31.6 3.4 table 2. statistics of moho depth computed according to the collocation method [km]. in first step it can be observed that the moho depth varies between 35.5 km and 49.0 km and in most areas, depth of moho is below 40 km. in second step, statistics are similar to previous with a slight increase of the standard deviation of the estimated moho depths. as seen in fig. 5c the moho depths coming from the third step are more high frequency (standard deviation increases to 3.4 km) and ranges between 31.6 km and 54.6 km. based on gravity disturbance data, coming from the goco03s, completed to degree and order 180, we then estimated the moho depth over iran with the gravimetric approaches devised by sjöberg and jeffrey. as done in the collocation solution, the mean moho depth has been set to 44 km. also, the constant density contrast between crust and mantle has been set to 600 kg m-3 and corrections including topography/bathymetry, sediment and consolidated crust has been accounted as well. the results are plotted in figure 6 and the summary of statistics is given in table. 3. these solutions show a similar pattern with a large moho depth under zagros and alborz mountains and also along sanandaj-sirjan and urumieh-dokhtar belts and kopeh-dagh. the areas with minimum depth are under the oman sea, some parts of caspian and persian gulf while central iran, tabas and lut block have relatively shallow moho depth. as seen in figure 6, sjöberg method gives a smoother solution than the one based on jeffrey method. minimum moho depths in both these methods are lower than minimum depths estimated using the collocation method. statistics in table. 3 show a minimum moho depth of 17.6 km and 33.9 km for jeffrey and sjöberg methods, respectively. also, the maximum moho depth according to jeffrey solution seems to be overestimated since this method has given a depth ebadi et al. 10 figure 5. map of moho model derived from collocation method in a) first step b) second step c) third step [km]. a) b) c) around 67.4 km near the northwest of iran. on the contrary, by applying the sjöberg method, the maximum moho depth in iran has been estimated to 59.6 km. 3.4 comparing gravimetric and seismic moho estimates to validate the gravimetric moho solutions, we compared them with existing regional seismic studies for iran [mangino and priestley, 1998; paul et al., 2006; taghizadeh-farahmand et al., 2010; radjaee et al., 2010; tatar and nasrabadi, 2013; taghizadeh-farahmand et al., 2015; motaghi et al., 2015; abdollahi et al., 2018]. a compilation of these numerous seismic datasets has been prepared and checks for their consistency were performed. in this way, we defined a selected collection of seismic moho values in the iran area, which consists of 277 points. these models are shown in figure 7 and their statistics are given in table 4. the maximum moho depth is located under the sanandaj-sirjan zone and surrounding mountains. by seismic estimates, the minimum depth of moho is under the oman sea and makran subduction zone. as it can be seen, moho depths derived from seismic studies varies between 18.5 km and 66 km. these seismic values were then compared with the gravity derived estimates. in order to perform the comparison, we interpolated the gridded gravity estimates on the sparse seismic point using linear interpolation. the differences are plotted in figure 8 and the statistics of the differences are summarized in table. 5. as one can see in figure 8a, the differences between the collocation solution and seismic data are between -17 km and 21.9 km. the differences between jeffrey’s and sjöberg’s solutions and seismic data range from -11.1 km to 22.2 km and -10.8 km to 23.4 km, respectively (see figures 8b and 8c). as it can be seen in table 5, the stds of the differences with respect to seismic data are around 6 km, the smaller std values being obtained by the sjöberg estimate. since the std of seismic data 8.2 km (see table 4), we can conclude that the gravimetric moho estimates are not so highly correlated with the seismic estimates. furthermore, the statistics show that jeffrey and sjöberg’s method give estimated moho values that have biases with respect to the seismic values larger than collocation. this reflects in the rmss values that show how collocation gives a moho estimate which is, overall, closer to the seismic values. 11 gravimetric moho over iran max mean min std jeffrey 67.4 44.9 17.6 7.2 sjöberg 59.6 46.6 33.9 3.7 table 3. statistics of moho depth computed according to jeffrey and sjöberg’s method [km]. figure 6. bouguer gravity anomaly reduced by topography/bathymetry, sediment and consolidated crust corrections on a (x,y) grid [mgal]. a) b) this overall analysis can be further specified for different sub-areas in the iran region where existing regional seismic solutions are available. central zagros and specifically sanandaj-sirjan zone are regions where the maximum depth of moho has been revealed therein. according to our computations, collocation solution indicated a 55 km moho depth for this area. sjöberg and jeffrey’s methods also indicated moho depths at the level of 55 km and 60 km for this zone, respectively. dehghani and makris, [1984] by using integrated gravity and seismic data estimated a crustal thickness beneath the central zagros in 55 km, which fully agrees with the results of collocation and sjöberg’s methods. hatzfeld et al. [2003] proposed a depth of 46 km for the crustal thickness at the single station close to the town of ghir in central zagros. another investigation of the lithospheric structure of the iranian plateau has been performed by asudeh [1982] along three profiles connecting mashhad to shiraz, tehran to mashhad and shiraz to tabriz. he reported crustal thickness along these profiles 43, 45 and 46 km, respectively. doloei and roberts [2003] and javan doloei and ghafory-ashtiany [2004] have proposed 52 km for depth of moho in mashhad. by collocation solution, depth of moho in mashhad has been estimated in about 52 km which is in agreement with javan doloei [2003] and javan doloei and ghaforyashtiany [2004] results. also, sjöberg and jeffrey’s method estimated the moho depth of about 52 and 56 km in mashhad, respectively. doloei and roberts [2003] used seismic data and suggested a crustal thickness of 46 km in tehran located southern part of the central alborz. results of collocation, sjöberg and jeffrey’s method gave values of moho depths of 43, 45 and 47 km in tehran, respectively. dehghani and makris [1984] showed that the crustal thickness varies from 35 km beneath alborz mountains to 54 km in central alborz. sobouti and arkani-hamed [1996] identified a 45 km crustal thickness along the alborz mountains. in the analysis of 290 teleseimic events carried out by sodoudi et al. [2009] at 12 shortperiod stations of the tehran telemetric network, an average depth of 44-46 km for moho under central alborz has been estimated. radjaee et al. [2010] reported the crustal thickness 48 km under the northern part of the central iranian plateau. also, they found a variable crustal thickening between 55 and 58 km under central alborz. shad manaman et al. [2011] estimated a thick moho with 55-60 km depth beneath the central alborz. jiménez-munt et al. [2012] and taghizadeh-farahmand et al. [2015] estimated crustal thickness 50 and 54 km in central alborz, respectively. the results of collocation and sjöberg’s method indicated depth of moho in that area around 53 km which is close to the values reebadi et al. 12 figure 7. moho derived from seismic results [km]. max mean min std seismic data 66.0 44.0 18.5 8.2 table 4. statistics of moho depth from seismic estimates [km]. ported by dehghani and makris [1984], radjaee et al. [2010] and shad manaman et al. [2011]. the estimation of jeffrey’s method has given a quite overestimated depth of 60 km in this area. paul et al. [2006] estimated crustal thickness beneath urumieh-dokhtar magmatic area around 42 km. nasrabadi et al. [2008] indicated that moho depth is 40 km beneath maku station in northwest of iran. taghizadeh-farahmand et al. [2010] applied p and s seismic waves to recover the crustal thickness around 48 km in this area. according to taghizadeh-farahmand et al. [2015], the average of moho depth varies from 41 km in the northwest iran to 45-49 km in the northeast. jiménez-munt et al. [2012] estimated the crustal structure from combination of the geoid height and elevation data with thermal analysis. 13 gravimetric moho over iran table 5. statistics of differences between gravimetric moho estimates and seismic results [km]. a) b) c) max mean min std rms collocation method 21.9 2.4 -17.0 6.3 6.7 jeffrey method 22.2 5.7 -11.1 6.1 8.3 sjöberg method 23.4 5.2 -10.8 6.0 7.9 figure 8. differences between seismic data and moho derived from a) collocation method b) jeffrey’s method c) sjöberg’s method [km]. their results showed the crustal thickness of 50 km beneath the alborz and kopeh‐dagh mountains. by our gravimetric solutions depth of moho in kopeh‐dagh has been estimated slightly more than 50 km. mangino and priestley [1998] estimated the moho depth at 30‐33 km beneath the south caspian basin. also, shad manaman et al. [2011] estimated similar val‐ ues for moho depth in this area. our results from gravi‐ metric methods there indicate the moho depth of about 37, 38 and 33 km by collocation, sjöberg and jeffrey’s methods, respectively. shad manaman et al. [2011] investigated the moho depth through the makran subduction zone and reported values around 25-30 km for the oman seafloor and makran. taghizadeh-farahmand et al. [2015] presented a depth of 35 km over this area. abdollahi et al. [2018] reported range of moho from 18 to 28 km in oman sea. the collocation solution gave the moho depth at around 33 km in oman sea. also, results of jeffrey and sjöberg’s methods has led to depth values of 20 and 34 km in this area, respectively. our estimations by collocation and sjöberg’s method are different from the results of seismic studies at the subduction zone and the tectonic border in the oman sea. this could be related to the procedure that we adopted for correcting the gravity values. indeed, eshagh et al. [2017] concluded that the sediment and crystalline corrections computed using the crust1.0 have a low quality in oceanic areas. dehghani and makris [1984] reported that the crustal thickness varies between 45 and 48 km in the eastern iran. nowrouzi et al. [2007] estimated depth of moho around 44-50 km under kopeh-dagh. jiménez-munt et al. [2012] suggested moho depth minima of about 36 km beneath the lut block. according to shad manaman et al. [2011], moho depth varies from 35 km to 40 km in central iran and lut block. nasrabadi et al. [2008] mentioned that the crustal thickness deepens up to 56 km under the naein station in central iran. the results of sadidkhouy et al. [2012] showed that depth of moho in isfahan area is variable between 38.5 and 43 km. our solutions by gravimetric methods give the moho depth ranging from 47 km in central iran to 44 km in lut block, which is in a good agreement with paul et al. [2006] and sadidkhouy et al. [2012]. in the coast of the persian gulf, a moho depth of about 25 km has been suggested by paul et al. [2006] where we have different values (here our estimates are around 35 km). all these comparisons between seismic estimates and regional moho depths obtained from the different gravimetric inversion methods for each of the different sub‐areas have been summarized in table. 6. all in all, these comparisons show that our results are in most cases in the same range with seismic esti‐ mations in literature. by considering the effects of sed‐ iment and consolidated crust we provided satisfactory results for most of the continental crust where our estimates have a relatively good agreement with local seismic studies. larger discrepancies are present between seismic and gravimetric estimates in the offshore area around 22 degrees of latitude. this problem has to be further investigated also following the discussion presented in eshagh et al. [2017]. 4. summary and concluding remarks in this study, we applied collocation method as well as two approaches based on isostasy principle presented by sjöberg and jeffrey for estimating the regional moho in iran using gravity observations. for this purpose, we considered the data of the goco03s satellite only global geopotential model that were subsequently reduced by topography/bathymetry, sediment and crystalline crust data effect by using the srtm30_plus dtm and the crust1.0 model. the three different gravimetric approaches gave coherent estimates. the estimated moho depths obtained using collocation, jeffrey and sjöberg’s approaches proved to be statistically equivalent when compared to seismic derived values. the collocation method has been applied iteratively in three steps and the numerical computations showed that an iterative process in collocation method could not change the results significantly even though the moho estimate based on collocation method in the third step contains more high frequency details. furthermore, the collocation solution proved to be less biased than those based on jeffrey and sjöberg’s methods when considering discrepancies with respect to seismic moho estimates. to evaluate our results, we have compiled a 277 points collection of local seismic estimations in this area. the overall standard deviation of the differences between the results of the collocation, sjöberg and jeffrey’s methods and the seismic estimates is around 6.0 km. the minimum rms of differences is between collocation estimates and point-wise seismic data since, as mentioned, collocation led to less biased discrepancies with the considered seismic values. although the application of sediment and consolidated crust corrections in our solutions provides a reasonable agreement with point-wise seismic data over most of continental areas like central zagros, sananebadi et al. 14 daj-sirjan, kopeh-dagh and alborz mountains, this leads to unrealistic estimates under the makran subduction zone, oman sea, persian gulf and caspian sea. this can be the effect of the poor quality of the crust1.0 data in this region [see eshagh et al., 2017]. the comparisons performed in this paper prove that further analyses are needed to come to a better consistency between gravity and seismic derived moho depths in iran before computing any joint seismic/gravimetric estimate. 15 gravimetric moho over iran region reference moho depth (km) differences between gravimetric solutions and regional seismic moho depths collocation jeffrey sjöberg std rms std rms std rms central zagros dehghani and makris (1984) hatzfeld et al. (2003) motaghi et al. (2015) tatar and nasrabadi (2013) 55 46 49 47 3.2 3.3 5.3 6.2 5.5 5.3 central alborz dehghani and makris (1984) sobouti and arkani-hamed (1996) sodoudi et al. (2009) radjaee et al. (2010) shad manaman et al. (2011) jiménez-munt et al. (2012) taghizadeh-farahmand et al. (2015) 35 45 44-46 55-58 55-60 50 54 3.5 5.6 4.2 4.3 3.2 3.2 northwest of iran nasrabadi et al. (2008) taghizadeh-farahmand et al. (2010) taghizadeh-farahmand et al. (2015) 40 48 41 4.8 5.0 4.7 9.9 4.3 5.9 northeast of iran dehghani and makris (1984) doloei and roberts (2003) javan doloei and ghafory-ashtiany (2004) nowrouzi et al. (2007) jiménez-munt et al. (2012) taghizadeh-farahmand et al. (2015) 45-48 52 52 44-50 50 45-49 1.6 2.3 2.9 6.3 1.8 5.3 lut block shad manaman et al. (2011) jiménez-munt et al. (2012) taghizadeh-farahmand et al. (2015) 40 36 41 3.0 7.4 5.5 13.0 3.5 11.2 yazd block nasrabadi et al. (2008) motaghi et al. (2015) taghizadeh-farahmand et al. (2015) 56 38 42 2.3 8.1 4.3 11.8 2.2 10.8 central iran paul et al. (2006) shad manaman et al. (2011) sadidkhouy et al. (2012) 41 35 38.5-43 1.1 5.3 1.2 8.2 0.5 6.4 south caspian basin mangino and priestley (1998) shad manaman et al. (2011) 30-33 30-33 10.5 7.6 10.4 7.6 13.9 10.9 urumieh-dokhtar and sanandaj-sirjan paul et al. (2006) motaghi et al. (2015) taghizadeh-farahmand et al. (2015) 48 59 48 6.0 6.1 6.1 6.8 6.3 6.5 oman sea floor and makran shad manaman et al. (2011) taghizadeh-farahmand et al. (2015) abdollahi et al. (2018) 25-30 35 18-28 5.0 8.2 6.0 9.5 4.8 9.9 coast of the persian gulf paul et al. 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(2001). isostasy and flexure of the lithosphere, cambridege: cambridge university press. *corresponding author: sahar ebadi, school of surveying and geospatial engineering, college of engineering, university of tehran, iran, email: sahar.ebadi@ut.ac.ir, asafari@ut.ac.ir © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved ebadi et al. 18 527_536.pdf annals of geophysics, vol. 45, n. 3/4, june/august 2002 527 the august 17, 1999 izmit, turkey, earthquake: slip distribution from dislocation modeling of dinsar and surface offset salvatore stramondo (1), francesca r. cinti (1), michele dragoni (2), stefano salvi (1) and stefano santini (3) (1) istituto nazionale di geofisica e vulcanologia, roma, italy (2) dipartimento di fisica, università di bologna, italy (3) istituto di fisica, università di urbino, italy abstract we show the results of application of differential sar interferometry to the m w 7.4, august 17, 1999, izmit earthquake, western turkey. the differential interferogram is obtained using an interferometric ers2 ascending pair with a time interval of 35 days (august 13th september 17th). the fringe pattern clearly defines the coseismic displacement field extended in an area of about 100 km n-s and 120 km e-w. the analysis of the interferogram shows the right-lateral strike-slip movement on the activated section of the north anatolian fault system. the maximum sar-detected displacement ranges between 117.6 cm and 134.4 cm in the proximity of gölcük. we invert sar data for uniform dislocation on a single fault plane using a montecarlo procedure, with the aim of testing a large set of a priori possible asperity distributions on the fault. we then use a forward modeling approach to evaluate the slip variability for the dislocation using additional constraints as surface offsets and seismicity distribution: in this case we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. misfits between sar data and modeled slant range displacements are generally low for all our models (~ 12 cm). our results indicate that slip is concentrated in the central-western part of the fault, in the upper 10-15 km, tapering to the fault tips. for the izmit case, we note that a well constrained fault model can be obtained only integrating dinsar data with additional observations. this is mainly due to an undersampling of the displacement field by dinsar, caused by decorrelation and lack of image data. 1. introduction on august 17, 1999, western turkey was shaken by a destructive, m w 7.4 earthquake. the event struck the highly developed urban and industrialized area surrounding the gulf of izmit (marmara sea) and east of adapazari (fig. 1). the mainshock originated on the northern strand of the north-anatolian fault system, approximately 9 km southeast of the city of izmit, at a depth of 16-18 km. the focal solution shows a right-lateral strike-slip movement on the fault (barka, 1999; toksöz et al., 1999). the izmit event was accompanied by about 120 km of surface rupture. the western limit of the rupture observed on land is in gölcük and proceeds offshore in the marmara sea. eastward, the breakage goes as far as akyazi maintaining an e-w strike, then turns to ~ ene toward düzce mailing address: dr. stefano santini, istituto di fisica, università di urbino, via s. chiara 27, 61029 urbino (ps), italy; e-mail: santini@fis.uniurb.it key words 1999 izmit earthquake differential sar interferometry coseismic displacement field montecarlo method asperity distribution 528 salvatore stramondo, francesca r. cinti, michele dragoni, stefano salvi and stefano santini (fig. 1), the change in direction being marked by a gap in the surface displacement. the surface rupture, although continuous, shows a certain slip variability: maximum offsets of 4-5 m (gölcük, e of sapanka lake), and an average of about 2.5 m (barka, 1999; u.s. geological survey, 1999 and references therein). we analyze the information relative to this seismic event derived from the application of the differential sar interferometry (dinsar) technique and discuss the significance of the fringe pattern with respect to the rupture mechanism. unlike other point based (gps), or line based (levelling) geodetic techniques, dinsar is an image based technique which allows the spatial deformation pattern associated with an earthquake to be mapped at high resolution (see dinsar theory and applications in massonnet and feigl, 1998 and references therein). this characteristic of dinsar data is particularly favourable to develop well constrained dislocation models sensitive even to local slip inhomogeneities (peltzer et al., 1994). we quantify the maximum ers-sar displacement and invert the dinsar data for a uniform slip dislocation using a montecarlo procedure; sar data are used in order to constrain the geometry of the dislocation surface. to this aim, an asperity model is employed, where the fault plane is divided into a large number of square asperity units which can slip by a constant amount or remain locked. we then start from this model epicentre surface rupture north anatolian fault system fig. 1. location and hcmt focal mechanism of the izmit earthquake (17 august, 1999). the north anatolian fault system and the surface rupture (dashed when offshore) (u.s. geological survey, 1999; ipgp web site, 1999) are indicated. 529 the august 17, 1999 izmit, turkey, earthquake: slip distribution from dislocation modeling of dinsar and surface offset (fig. 2). this general setting indicates that large displacement shown by the interferogram occurs in the western part of the fault. this observation seems to be in agreement with the high offsets measured on land that reaches 4.5 m in gölcük before the fault enters the marmara sea. the concentric pattern on the shorelines of the marmara sea suggests the continuation of the rupture offshore for at least 20 km. the satellite-to-ground distance between the two image acquisitions increases north of the surface rupture and decreases to the south (fig. 2). this is in agreement with the right lateral kinematics of the fault. we first estimated the maximum measured range change based on the number of fringes observed at both sides of the fault in the proximity of gölcük (fig. 2). we counted about 27-30 fringes in the northern side and 15-18 fringes in the southern side, each fringe representing a slant range displacement of 2.8 cm. note that for most of the fault extent in the near range of the fault sar data are missing due to de-correlation (marmara sea and sapanka lake) and/or excessive deformation gradient. we manually unwrapped the phase starting from the fringes closer to the fault trace, along the e coast of the izmit gulf, assuming a surface offset of 3.4 m in this area (u.s. geological survey, 1999; ipgp web site, 1999), a pure right lateral slip and e-w direction of the fault to project the slip vector onto the satellite line of sight. we assume that the uncertainty associated with our unwrapping procedure is two fringes. the interferogram is perturbed by atmospheric artifacts that can affect the propagation of the sar signal (massonnet and feigl, 1998; reilinger et al., 2000). for the izmit interferograms, reilinger et al. (2000) attribute differences of 3-4 cm between gps and dinsar displacements to changes in the atmospheric conditions between the two ers image acquisitions. they claim these variations should occur over a spatial wavelength shorter than the distance between the gps stations (~ 10 km). indeed, considerable moisture variations (water vapor is the most important parameter affecting tropospheric path delays in radar images, zebker et al., 1997) can occur even on 1 km scale in case of strong tropospheric turbulence (weckto evaluate the slip distribution on the fault, using additional constraints and a forward modeling procedure. the dinsar data of the august 17, 1999, izmit earthquake have already been presented and discussed in previous works (delouis et al., 2000; reilinger et al., 2000; wright et al., 2001) with the aim of inverting the details of the seismic source. however, these studies suffer from the typical problem of data inversion, notably that they fail to clarify the effective constrains that the data impose on the inverted parameters. in this work, the distribution of asperities (or slip) on the fault plane is found using only the fault geometry, the geodetic data and the seismic moment related to the 1999 izmit earthquake. 2. the dinsar data we selected an ers 2 ascending pair, with an altitude of ambiguity h a (i.e. the altitude difference corresponding to a phase change of 2 ) of 144 m. the two images span 35 days (august 13th september 17th, 1999), the minimum interval between two overpasses of the same ers satellite on a specific area. the topographic phase contribution was subtracted using an interferometric dem (courtesy of e. fielding). differential interferometry is more sensitive to vertical than to horizontal movements (the mean state vectors are 0.38 east, 0.08 north, 0.92 up, for ascending orbits and for turkey region latitude). however, the izmit strike slip fault has the best geometric setting to detect horizontal displacement with dinsar. in fact, the ground projection of the satellite line-of-sight (slant range) being about parallel to the e-w fault, the displacement component on the slant range is maximized (peltzer et al., 1994). the differential interferogram (fig. 2) shows the surface coseismic displacement extending on both sides of the fault and covering a wide area, about 120 km e-w and 100 km n-s. at large scale, fringes are parallel to the fault, showing variable gradient whose maximum is located west of the epicenter and converging towards the fault trace about 15 km west of the city of gölcük 530 salvatore stramondo, francesca r. cinti, michele dragoni, stefano salvi and stefano santini 3. sar data inversion by the montecarlo method let us consider an elastic, homogeneous and isotropic half-space, occupying the region x 3 0 in a cartesian coordinate system, and assume that the fault is a rectangular surface intersecting the earth’s surface with a dip angle . the fault is strike-slip, right-lateral, and is made of 216 square asperity units. we assume that the lamè constants are equal (poisson solid) and calculated according to prem (dziewonski and anderson, 1981): = µ = 4.5 × 1010 pa. in the case of a rectangular dislocation, the displacement field at the earth’s surface can be obtained by available analytical solutions (okada, 1985). werth et al., 1997), with relative humidity variations of several tens %, giving rise to phase shifts of 1-2 cycles over distances of kilometers (hanssen and fejit, 1996). over longer wavelengths though (30-50 km), the changes in relative humidity are usually smaller and the sar phase shifts expected are of the order of a few cycles (hanssen and fejit, 1996). in our interferograms we do not see strong signs of tropospheric turbulence (in fact the interferogram difference shows only long wavelength residuals), therefore, assuming that only minor relative humidity variations have occurred over the scenes, we estimate a maximum atmospheric phase shift over the entire scene of 2 fringes. modelled fault and surface slip (cm) 0 80 160 240 320 400 epicentre fig. 2. ers2 co-seismic differential interferogram. fringes show the displacement field associated with the izmit earthquake. the fault trace and the surface offsets (u.s. geological survey, 1999 and barka, 1999) of the dislocation models in fig. 3b-d are shown. fringe contours (blue lines) retrieved from the interferogram are in the inset. each line indicates a surface movement of 2.8 cm onto the satellite line of sight. 531 the august 17, 1999 izmit, turkey, earthquake: slip distribution from dislocation modeling of dinsar and surface offset we set up a montecarlo method (metropolis and ulam, 1949) with the aim of testing the performances of a large set of a priori random asperity distributions on the fault plane. due to computing time constraints, we assumed a uniform slip on the fault (u = 1.6 m). for the same reason, we decided to approximate the izmit earthquake rupture with a single fault surface whose parameters were defined on the basis of earthquake hcmt focal parameters (www.seismology.harvard.edu/), tectonic studies (barka, 1997, 1999; u.s. geological survey, 1999; ipgp web site, 1999), and seismicity studies (özalaybey et al., 2002). our model fault surface is oriented e-w, dipping 87° to the south, 180 km long, 30 km deep (fig. 3a-d). in our simulations, the depth of the dislocation bottom is variable (20 d 25 km). due to inhomogeneity of friction on faults, dislocation surfaces have usually irregular shapes. in order to take this into account, we assumed that the fault rupture is the consequence of the failure of a large number of small square asperity units (table i; fig. 3a-d). from the solution for a square dislocation, it is easy to obtain the analytical solution for any dislocation with a polygonal contour. as a consequence, it is possible to model the effect of dislocations with any shape by employing suitably small asperity units (e.g., dragoni, 1988; santini et al., 2000). for our inversion, we used asperity units of 5 × 5 km. failure of such asperity would correspond to a magnitude m w = 6 earthquake (the order of magnitude of the largest aftershocks of the izmit sequence) for the unit dislocation, using the constant slip of 1.6 m (kasahara, 1981). assuming a seismic moment m0 = 1.95 × 10 20 n m (reilinger et al., 2000), we can model the dislocation area on the fault with n = 108 unit cells. we generated a set of 25 000 random slip distributions, and we employed the symbol u 3,k p to indicate the vertical displacement predicted w e hersek delta gölcük izmit sapanka lake slip= 0.0 meters (asperity unit) slip= 0.4 meters slip= 1.6 meters slip= 3.2 meters slip= 0.8 meters slip= 2.4 meters slip= 4.0 meters fig. 3a-d. slip distributions for our dislocation models; each asperity unit is 5 × 5 km. a) montecarlo inversion of dinsar data with a uniform slip assumption (u = 1.6 m); b-d) slip distributions from forward modeling of sar and fault surface offsets (see text for explanation). table i. source parameters used and rms values obtained for fault models in fig. 3a-d. for all models: strike = 90°, dip = 87°s, rake = 180°. model no. length width slip rms geodetic moment 3a 120 km 20-25 km 1.6 m 12.8 cm 1.95 × 1020 n m 3b 135 km 20-25 km 0.4-4.0 m 11.9 cm 1.95 × 1020 n m 3c 150 km 20-25 km 0.4-4.0 m 11.8 cm 1.95 × 1020 n m 3d 150 km 20-25 km 0.4-4.0 m 11.9 cm 2.04 × 1020 n m a b c d 532 salvatore stramondo, francesca r. cinti, michele dragoni, stefano salvi and stefano santini by the model at the generic k-th point at the earth’s surface. it is given by a sum over the n square asperity units (3.1) in order to compare the displacement values predicted by the model with the observed values (sar), we considered a square grid covering the earth’s surface in the fault region. the grid side is equal to 120 km and the distance from each point of the grid to one of the surrounding points is equal to 6 km, implying that the total number of points is n = 21 × 21 = 441. we indicate the observed values of vertical displacement at the k-th point of the earth’s surface by the symbol u 3,k o . the 2 value is calculated for each slip distribution according to the formula (3.2) where k o is the standard deviation of the observations. in order to find the best shape of the dislocation surface, we chose the slip distribution having the smallest 2 value. when we considered a variable slip on this surface, in the set of 25 000 distributions generated by montecarlo method, we retained as acceptable the subset of slip distributions, corresponding to 1% of the total number, having the smallest 2 values. for them there is at least a 99% probability of finding a worse solution. u u k p j p j n 3 3 1 , , .= = 2 1 3 3 2 1 = ( ) = n u u k o k p k o k n , , fig. 4. distribution of 2 values obtained from the montecarlo simulations. the histogram shows which is the percentage of iterations in which a given value of 2 is found. 533 the august 17, 1999 izmit, turkey, earthquake: slip distribution from dislocation modeling of dinsar and surface offset we defined 299 the minimum of 2 values associated with 1% of better distributions: in our case 2 99 = 0.25 (fig. 4). in the subset, we retained as the best solution the one for which the slip is a function as smooth as possible of the position on the fault. in this way, we used the dinsar detailed spatial sampling of the slant range displacement to constrain the shape of the dislocation and to minimize the model uncertainties due to the noisy and/or missing dinsar information. to account for the atmospheric phase shifts, uncorrected orbital effects, and unwrapping uncertainty, we assumed a ± 2 fringes uncertainty of the sar observations, so that k o = 11.2 cm. using this inversion procedure our best fit model was obtained with a minimum 2 test value equal to 0.15. the inverted shape of dislocation is shown in fig. 3a and the misfit between observed data and model data is shown in fig. 5. 4. forward modeling of slip distribution we used a forward modeling approach to infer the actual slip distribution over the dislocation area as determined in the previous section for the uniform slip (fig. 3a). the dinsar data set, although yielding a high resolution representation of the displacement field, does not cover the entire extent of the earthquake rupture, due to image decorrelation along the fault trace and in the eastern fault zone. we therefore used all other available constraints to model the variable slip on the fault, in particular the published data on the surface fault displacements (barka, 1999; u.s. geological survey, 1999; ipgp web site, 1999), and the aftershock distribution in the few days following the mainshock (özalaybey et al., 2002). constraints to all our tests were the total geodetic moment equal to the seismic moment, and the shape of the dislocation, maintained as close as possible to fig. 3a. considering the surface displacement data (fig. 3a-d) as additional constraints to the model in fig. 3a, we noted that the montecarlo inversion of dinsar data yields a dislocation length shorter than the rupture length observed in the field of about 15 km (figs. 2 and 3a-d) according to the surface displacement data. an asperity (zero dislocation) is present in the eastern part of the fault (figs. 1 and 3b), where a fault bending exists (from e-w to sw-ne); this feature was not detected by the montecarlo inversion. the cause of these discrepancies is presumably due to the poor coverage of dinsar data in the eastern half of the ruptured area (fig. 2). our next models incorporated the surface offsets (fig. 2). these data constrain exactly the dislocation of the first layer (0-5 km) of our model, while the slip on the deeper patches is imposed to be gradually decreasing with depth. this trend was already observed in other fault models (reilinger et al., 2000; delouis et al., 2000). we also increased the length of the dislocation surface to account for ~ 0.8 m further surface displacement observed in the easternmost 15 km of the fault trace (fig. 3a-d). for the westernmost 25 km of the dislocation surface, the fault trace is hidden under the marmara sea; as an initial attempt to our forward modelling, we imposed here a slip of 1.6 m at the surface, decreasing towards the deepest part of the fault tip to 0.8 m (fig. 3b). the rms of residuals with the dinsar data for the fault model in fig. 3b is 11.9 cm; the highest misfits (up to 41 cm) are fig. 5. misfit ( u = u3,k o u3,k p ) relative to the fault model 3a, produced by the dislocation with a uniform slip equal to 1.6 m. the green line is the modeled fault trace. 534 salvatore stramondo, francesca r. cinti, michele dragoni, stefano salvi and stefano santini located in the near field, within 5 km from the fault trace (fig. 6a). the previous model, while in agreement with the observed surface displacements, still underestimates the fault length. in fact, the model fault ends 10 km west of the hersek peninsula (fig. 1), whereas the distribution of aftershocks which occurred shortly after the mainshock (özalaybey et al., 2002) clearly shows a further e-w continuation of the ruptured zone for about 15-20 km into the marmara sea. we use these data to constrain a new model where the total fault length is 150 km (i.e. further increasing the dislocation surface length of 15 km to the w). we performed some tests to determine the slip distribution in this area and found that, while west of the hersek peninsula the slip tapers to the fault tip, the presence of an l = 20 × w = 15 km patch of at least 1.6 m-slip in the area between gölcük and the hersek peninsula (fig. 3c) is required to obtain a smaller misfit. to maintain the geodetic moment but enlarge the dislocation area, we imposed a more gradual tapering of the slip at depth in the central part of the fault. this model is shown in fig. 3c; its misfits with the dinsar data are shown in fig. 6b. beyond the high values along the fault trace, which are constant in all models, the misfits of model 3c show an improvement with respect to model 3b (its rms is 11.8 cm), except in the e part of the displacement field. our final attempt was to model the slip distribution dropping the fixed moment constraint and introducing a few variations trying to reduce the highest residuals of model 3c. performing a limited number of tests, we were not able to reach a significant improvement of the previous model. a ~ 5% geodetic moment increase in model 3d, corresponds to a slight rms increase with respect to model 3c (11.9 cm). figure 6c shows the model 3d residuals; a general increase in maximum misfits in the ne and w part of the fault is observed. fig. 6a-c. misfits relative to the fault models 3b (a), 3c (b), 3d (c). misfits have been obtained for each 6 × 6 km cell, after masking of dinsar data for decorrelated areas. the white line is the modeled fault trace. a b c 535 the august 17, 1999 izmit, turkey, earthquake: slip distribution from dislocation modeling of dinsar and surface offset 5. discussion and conclusions our modeling procedure allowed us to: i) model the general dislocation shape through montecarlo inversion of sar data only; ii) evaluate the distribution of slip over the rupture plane using additional constraints from geological and seismicity data. assuming first a uniform slip, we obtained a best fit of surface coseismic deformation for a dislocation surface with an irregular shape. the agreement with observations was improved by redistributing the fault slip on the dislocation surface. we note that our uniform slip model yields rather low misfits with sar data (rms = 12.8 cm); this is surprising, considering the strong variability of slip inferred by all studies (reilinger et al., 2000; delois et al., 2000; wright et al., 2001). we attribute this behaviour to undersampling of the displacement field by the sar data (we estimate that sar data are available for only about 60% of the actual displacement field) and possibly to a smoothing effect due to uncorrected orbital errors and long wavelength atmospheric perturbations. the small differences in the average residuals among all our variable slip models (0.1 cm) suggest that the data at our disposal are unable to give full details of the slip distribution. in any case, considering the single fault approximation, we obtain results for the most part in agreement with published studies performed also using different data sets (reilinger et al., 2000; delouis et al., 2000; wright et al., 2001). the depth of dislocation varies between 20 and 25 km, and is maximum in the fault western-central segment; the largest slip values are concentrated in the central part, with two maxima in the gölcük and east sapanka segments. most of the large misfits in fig. 6a-c are concentrated along the fault trace, although these misfit values are biased by the presence, in each of the 6 × 6 km discrete map units, of a large percentage of decorrelation noise, i.e. underestimation of actual displacement. other large misfits (~ 30 cm) are present south of the hersek peninsula in all our models. the absence of surface faulting in this area and our large misfit values can be explained if, at some point between gölcük and hersek, the rupture actually steps north, so that its surface expression is hidden under the marmara sea. on the east side of the fault we also observe high misfits (~ 25 cm); the reason is probably the difference between our approximated model fault (e-w) and the direction of the actual fault segment (swne, fig. 1). notwithstanding the model assumptions and the single fault approximation, for most of the areas showing a well defined fringe pattern we obtain misfits smaller than 10 cm (fig. 6a-c). the application of sar interferometry to the retrieval of the izmit earthquake source parameters has shown once more that for an improved earthquake monitoring strategy in non-arid environments, more effective sar data acquisition policies must be implemented. decorrelation due to rapid surface changes can be reduced using a shorter revisiting time as well as a longer radar wavelength (l band). atmospheric effects should be more accurately estimated, e.g., using independent meteorological data, to assess the data set consistency; their correction could be attempted only if a large number of interferometric data sets are available and/or if simultaneous images from other sensors allow for atmospheric parameters retrieval. acknowledgements we wish to thank the european space agency that delivered the set of ers sar images, making possible the accomplishment of this work. special thanks go to eric fielding (university of oxford, u.k.) for providing the dem. we are also grateful to alessandro amato for his useful comments and to enzo boschi for his encouragement. the diapason software, by cnes (toulouse, france), was used for the ers image 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(received february 22, 2002; accepted july 15, 2002) lorito.pdf 573 annals of geophysics, vol. 46, n. 3, june 2003 wavelet analysis on paleomagnetic (and computer simulated) vgp time series stefano lorito (1) (2), grazia giberti (1), agata siniscalchi (3) and marina iorio (4) (1) dipartimento di scienze fisiche, università «federico ii», napoli, italy (2) istituto nazionale di geofi sica e vulcanologia, roma, italy (3) dipartimento di geologia e geofi sica,università di bari, italy (4) istituto iamc, geomare sud, cnr, napoli, italy abstract we present continuous wavelet transform (cwt) data analysis of virtual geomagnetic pole (vgp) latitude time series. the analyzed time series are sedimentary paleomagnetic and geodynamo simulated data. two mother wavelets (the morlet function and the first derivative of a gaussian function) are used in order to detect features related to the spectral content as well as polarity excursions and reversals. by means of the morlet wavelet, we estimate both the global spectrum and the time evolution of the spectral content of the paleomagnetic data series. some peaks corresponding to the orbital components are revealed by the spectra and the local analysis helped disclose their statistical significance. even if this feature could be an indication of orbital influence on geodynamo, other interpretations are possible. in particular, we note a correspondence of local spectral peaks with the appearance of the excursions in the series. the comparison among the paleomagnetic and simulated spectra shows a similarity in the high frequency region indicating that their degree of regularity is analogous. by means of gaussian first derivative wavelet, reversals and excursions of polarity were sought. the analysis was performed first on the simulated data, to have a guide in understanding the features present in the more complex paleomagnetic data. various excursions and reversals have been identified, despite of the prevalent normality of the series and its inherent noise. the found relative chronology of the paleomagnetic data reversals was compared with a coeval global polarity time scale (channel et al., 1995). the relative lengths of polarity stability intervals are found similar, but a general shift appears between the two scales, that could be due to the datation uncertainties of the hauterivian/barremian boundary. mailing address: dr. stefano lorito, dipartimento di scienze fisiche, università «federico ii», complesso universitario monte sant’angelo, via cintia, 80126 napoli, italy; e-mail: lorito@na.infn.it key words cwt – vgp – geodynamo simulations – excursions and reversals – earth’s orbital parameters 1. introduction the long term phenomenology of the earth’s magnetic field can be inferred from paleomagnetic data as they record the field behaviour on time scales which go from hundreds to millions of years. in particular, sedimentary data can achieve the time resolution to allow the extraction of details of the past geomagnetic field when the estimated sedimentation rate is stable and sufficiently high to affirm that samples preserve the imprinting of their first magnetization (merrill and mcfadden, 1999). a drawback of these data is the complexity of the formation/ magnetization process of sedimentary rocks and of their subsequent history. the measured magnetic signal could be thus have been filtered in a not well understood way (merrill and mcfadden, 1999). the need for accurate and non-standard analyses of the available data with all refined techniques therefore emerges (dormy et al., 2000; mcmillan et al., 574 stefano lorito, grazia giberti, agata siniscalchi and marina iorio 2001). here we utilize the continuous wavelet transform (hereafter cwt) together with the discrete fourier transform (dft) to analyze paleomagnetic sedimentary data. cwt is, in fact, a versatile analysis’ technique (mallat, 1998) which, in principle, could investigate and discriminate among regular/singular and stationary/non-stationary data features. in this work, both paleomagnetic sedimentary (iorio, 1995) and computer simulated (glatzmaier et al., 1999) virtual geomagnetic poles (vgp) latitude time series are analyzed. one aim is to explore how useful the cwt can be in estimating both the power spectrum of the whole data series than the time evolution of the spectral content. similar studies, using cwt, have been performed by some authors on geomagnetic paleointensity data (guyodo et al., 2000; yokoyama and yamazaky, 2000). this tool is recognized as suitable to analyze non-stationary signals (mallat, 1998; torrence and compo, 1998 and references therein), the periodicity over the whole series not being imposed a priori, as in every dft analysis. moreover, local spectral features result better characterized. the cwt uses windows that are shorter in time (and longer in frequency) the higher the frequency analyzed, allowing for a higher joint time-frequency resolution with respect to the windowed dft (mallat, 1998). to assess the reliability of the cwt analysis, the usual dft is performed on all the data. in fact, the accuracy in estimating the decay of the power spectrum at high frequencies by means of cwt (perrier et al., 1995) depends on the kind of fourier spectrum and in turn on the nature of the signal. the cwt is also utilized to look for local regularity versus sharp variations in the latitude time series with the aim of localizing and characterizing reversals and polarity excursions. a similar approach was followed by alexandrescu et al. (1995) in studying geomagnetic jerks of the present geomagnetic field, there defined as singularities in the second derivative of the signal. the definitions of polarity reversals and excursions are a matter of debate (merrill et al., 1996; gubbins, 1999) both with regard to the amplitude and duration/stability of these processes. it is, however, generally accepted that reversals are fast transitions between opposite polarities characterized by a long polarity stability before and after the transition whereas the excursions would present instead a sudden return to the original polarity. we will detail later the definitions we use to quantify local regularities with the appropriate cwt. the efficacy of wavelet analysis depends on the particular mother wavelet employed, so the choice is usually made on the basis of the specific features one wants to investigate and quantify. we utilize two kinds of mother wavelets: i) the morlet (morlet et al., 1982), to analyze local and global spectral content of the series; ii) the first derivative of gaussian (1dog), to localize and study reversals and polarity excursions. this paleomagnetic data-set has already been analyzed by standard spectral and non spectral methods (iorio et al., 1995, 1998, 2001; tarling et al., 1999). in the framework of the timefrequency analysis (mallat, 1998), this cwt study addresses a quantitative verification of some previously found results as: i) the influence of the main milankovitch periods (berger et al., 1992) on the paleomagnetic signal; ii) similitudes between observed and simulated data. 2. data description 2.1. paleomagnetic data series the paleomagnetic data come from boreholes carried out at mt. raggeto (41.1°n, 14.2°e), italian southern apennines. rock samples were extracted from a carbonate sequence formed in the early cretaceous by deposition of shallow water sediments (iorio et al., 1995, 1998). they constitute a composite core, with an overall length of 88 m. the (paleo)magnetic remanence intensity and directions were measured every 2 cm along the cores, so obtaining a sequence that spans an interval of approximately 3.2 myr, when assuming that, on average, 2 cm were deposited in 720 yr (tarling et al., 1999). as the sequence contains the hauterivian-barremian boundary (127 ± 1.6 myr before present, iorio et al., 1998), it was inferred that the sediments analyzed 575 wavelet analysis on paleomagnetic (and computer simulated) vgp time series were deposited in the time interval included in the range 124-128 myr before present. this geological age collocates a few million years before the so-called cretaceous superchron, which is generally considered a very long (about 35 myr) period of normal polarity (merrill et al., 1996). here and in the following, normal polarity is defined as the present polarity of the geomagnetic field. the samples were analyzed from a lithologic, petrological, textural and paleontological point of view (iorio, 1995; d’argenio et al., 1999). after these analyses, some intervals were excluded as corrupted by recent karst phenomena. then, the longest evenly spaced data set is 2.3 myr long. the estimated velocity of sedimentation of 1/360 cm yr-1 renders these data useful to study polarity reversals, if it is accepted that a transition occurs in a time interval ranging from 1 to 10 kyr. the stability of the sedimentation process, together with the relatively fast diagenesis of the samples (d’argenio et al., 1999) and the above mentioned value of the sedimentation rate allow us to infer that the magnetization characteristics can be reliable for this kind of analysis (merrill and mcfadden, 1999). the vgp’s positions with respect to the mean paleomagnetic pole were calculated by means of standard paleomagnetic techniques (tarling et al., 1999). figure 1 presents the latitude of the longest evenly spaced sequence versus relative time. it corresponds to the data segment analyzed here. in this plot, when the latitude value is 90°, the vgp coincides with the mean paleomagnetic pole. the sequence shows a very high rate of variability and a prevailing normal polarity is evident. nevertheless, several reversed chrones and excursions were identified and correlated with those in other pelagic lower cretaceous magnetostratigraphic sequences (iorio et al., 1998). 2.2. simulated data series the simulated data here analyzed (fig. 2) were produced by the geodynamo numerical simulation to which the authors refer as the tomographic case (glatzmaier et al., 1999). this indicates that, as a condition for the heat flow at core mantle boundary (cmb), a pattern was used that includes lateral variations as inferred by the interpretation of seismic tomographic results. the total heat flow through cmb was chosen equal to 7.2 tw. the equations were integrated using 3d spherical geometry and the simulation was carried ahead for a total time of 320 000 «model» years. this total time is estimated by the authors (coe et al., 2000) long enough in order to assert that the field is self-sustained, when it is compared with the free dipole decay time (roughly 20 kyr). the results of the simulation are in the form of schmidt quasi normalized fig. 1. mt. raggeto vgp latitude versus relative time (myr). the instant t = 0 myr is assumed coincident with the beginning of the analyzed data segment. 576 stefano lorito, grazia giberti, agata siniscalchi and marina iorio gauss coefficients, up to degree 21, of the scalar magnetic potential. the obtained magnetic field space spectrum is in good agreement in shape, but with a lower intensity, with respect to the spectrum based on magsat data of the modern earth’s magnetic field (roberts and glatzmaier, 2000). the derived vgp latitudes here used have a time step of 100 «model» years, considered in this paper as real years. it appears that the field reversed twice, the first reversal being faster, whereas in the second reversal the vgp latitude goes up to the normal polarity with a few minor excursions (coe et al., 2000). moreover, before of the first inversion two major excursions occur and, in between the two reversals, some major excursions or «rebounds» appear. 3. wavelet transform the basics of the cwt are described in a number of papers and books (mallat, 1998; torrence and compo, 1998 and references therein). here only the aspects of the cwt that are relevant to the analyses are detailed. the cwt of a function f œ l2 ( ¬ ), is defined as (3.1) where (3.2) is the family obtained by scaling by s and translating by u the argument of the mother wavelet y (t) œ l1 (¬) i l2 (¬).the values wf (u,s) of the cwt for a given pair (u, s) are called the wavelet coeffi cients. the mother wavelet must satisfy the admissibility condition (3.3) where ŷ (w) is the fourier transform of y (t). this condition ensures the signal energy is conserved in the time-scale domain, i.e. that . (3.4) different wavelet functions can be used and the choice is usually made on the basis of the specific features in the data set that one wants to investigate and quantify (mallat, 1998). here two kinds of mother wavelets are used: i) the morlet wavelet, to estimate the local and global frequency content of the series; this choice is classical in harmonic time-frequency wf u s f t t du s, ,( ) = ( ) ( )−∞ +∞ ∗∫ ψ ψ ψu s t s t u s u s s, , ,( ) = −    ∀ ∈ℜ > 1 0 c dψ ψ ω ω ω= ( ) < + ∞ +∞ ∫ ˆ 2 0 f t dt c wf u s du ds s ( ) = ( ) −∞ +∞ −∞ +∞+∞ ∫ ∫∫ 2 2 20 1 ψ , fig. 2. vgp latitude of the numerical geodynamo simulation (glatzmaier et al., 1999) versus «model» time (kyr). 577 wavelet analysis on paleomagnetic (and computer simulated) vgp time series sented by the terms . (3.8) the index i runs on the possible different singularities present at each time t i and a i and b i are real constants. c(t) models a superposition of harmonic components and n(t) a gaussian normal distributed noise. the chosen analyzing wavelet can detect singularities for which a i £ 1 (mallat, 1998), the number of its vanishing moments being equal to 1. the quantities a i (lipschitz exponents) and the t i values (corresponding to translations u i in the cwt) for the singular terms defined by eq. (3.8) are found by identification of the connected lines of local extrema (ridges) of the wavelets coefficients as functions of the scale s. it can be shown (alexandrescu et al., 1995; mallat, 1998) that the instants to which the ridges converge, as the scale decreases, give the localization in time t i of the terms in eq. (3.8). the local degree of regularity a i of these terms is found by analyzing the behaviour of the absolute values of the ridge functions, i.e. the local maxima of |wf (u, s)|, versus the scale s in a neighborhood of the translations u centered around t i . it can be shown, in fact (mallat, 1998) that, at fine scales, log , ~ log .wf u s si( ) +( )α 1 2 as the above technique is applied here to localize and study reversals and polarity excursions in the vgp latitude time series and in view of the above mentioned debate on their definitions, to infer the analytical behaviour of these processes we adopt a working hypothesis. a reversal is considered a transition between the opposite polarities occurring in a few thousands years and characterized by a polarity stability before and after the transition for at least 20-50 kyr. for excursions, we assume a change in the latitude of the pole greater than 40-50 degrees, followed by a return to the original value in a time interval shorter than that occurring between a pair of reversals (guyodo and valet, 1999). as a consequence, we expect that when a reversal occurs the 1dog will localize a ridge function with a lipschitz coefficient ranging from 0 (which characterize a noiseless heaviside function) to 1 (characterizing a noiseless ramp function). when analyses as the morlet is sine-like shaped and limited in time (morlet et al., 1982; daubechies, 1992; torrence and compo, 1998). ii) the first derivative of a gaussian (1dog) to localize regular and singular behaviour in the signal (mallat and hwang, 1992), as could be considered reversals and polarity excursions. the normalized morlet wavelet is defined as (3.5) where w0 is the central frequency. the presence of the correction term is necessary so that the admissibility condition holds (mallat, 1998) and it is not specified in eq. (3.5) as numerically negligible for the choice w0 = 6 (torrence and compo, 1998) here adopted. the morlet wavelet is employed to evaluate: i) the local wavelet power spectrum, i.e. the pattern of the morlet squared coefficients |wf (u, s)|2 in the time-scale plane (u, s). ii) the global wavelet power spectrum, i.e. the sum of the morlet squared coefficients over the translations, that defines a consistent and unbiased estimator of the power spectrum (torrence and compo, 1998; percival, 1995). iii) the time evolution of the squared coefficients over a chosen limited interval of scales. the last item allows us to analyze the energy content versus time of a given period in the signal as a definite factor of proportionality between the scale s and the fourier equivalent period t exists for each wavelet (torrence and compo, 1998 and references therein). in our case t = 1.03 s dt, where dt is the sampling rate of the data series. to estimate the local degree of regularity of a signal, we utilize the normalized 1dog wavelet, i.e. the function (3.6) following alexandrescu et al. (1995), we assume here that the signal can be modeled as (3.7) where the searched singular behaviour is repreψ π ωt e ei t t( ) = +− −1 4 20 2 / correction term ψ π t d dt e t ( ) =     −2 1 2 2 2 . f t t t c t n ti i i i( ) = −( ) + ( ) + ( )+∑ β α t t t t t t t t i i i i i i −( ) = −( ) ≥ <    + α α for for 0 578 stefano lorito, grazia giberti, agata siniscalchi and marina iorio an excursion occurs, the 1dog should localize two nearby (in time) ridges and the ridge function should decrease sharply at high scales because of the possible smaller amplitude of the process and/or shorter duration with respect to reversals. ideally, an excursion in the signal should be seen as a bell function of small width which can possibly collapse into a d -function. 4. morlet results the local wavelet spectrum obtained with the morlet wavelet is shown in fig. 3, plotted as a 2d color image versus the scale s (converted in periods) and the translation u (times in our plot). the series has been padded with the necessary zeroes to limit, in the time-scale plane, the region of influence of the edges (delimited by the bold black line in the plot). this procedure also allows to decrease the computational time of the cwt by using a fast fourier transform algorithm (torrence and compo, 1998). local energy maxima appear at periods corresponding to the main orbital periods estimated for the cretaceous, i.e. 22 (precession), 40 (obliquity), 100 (short eccentricity) and 400 (long eccentricity) kyr (berger et al., 1992). figure 4 shows the obtained global wavelet power spectrum in comparison with the scargle periodogram (scargle, 1982). both are plotted in units of the latitude series variance s 2 = 1442 deg2 (standard deviation s � 38 deg). the scargle periodogram is obtained padding the data series with zeroes as before. the white noise level at 90% for the scargle periodogram is also plotted. the two curves give similar results, the wavelet spectrum being less scattered than its fourier counterpart, due to the smoothing introduced by the gaussian exponential present in the morlet wavelet. it appears that the possible bias at high frequencies that the smoothing by the gaussian exponential could introduce (perrier et al., 1995), does not affect the calculated morlet spectrum. in fig. 5, the red noise theoretical spectrum and the confidence levels at 50% and 90% are superimposed to the morlet global periodogram. the red noise confidence level is calculated assuming as noise-model a univariate lag-1 autoregressive process (torrence and compo, 1998). the first order autocorrelation, after the subtraction of the mean value, is equal to 0.88. with the exception of the peak related to orbital obliquity (� 40 kyr), the periods corresponding to the cretaceous main orbitals emerge only over the 50% confidence level, and so they all are statistically not well resolved against the red noise background. to address the issue of distinguishing the peaks in the power spectrum from the oscillations inherent to a stochastic process, we analyze the time changes of the spectral content in bands around the equivalent periods where peaks fig. 3. local morlet spectrum of mt. raggeto latitude (in units of time series variance) in the time-periods plane (see text for more comments). 579 wavelet analysis on paleomagnetic (and computer simulated) vgp time series fig. 4. global morlet spectrum and scargle periodograms (in units of variance) of the mt. raggeto latitude. the 90% confidence level for the scargle periodogram is represented by a dashed line. fig. 5. global morlet spectrum of the mt. raggeto latitude, along with 90% and 50% red noise confidence levels. spectral peaks corresponding to cretaceous main orbitals are evidenced (see text for more comments). 580 stefano lorito, grazia giberti, agata siniscalchi and marina iorio emerge. to do that we follow torrence and compo (1998), performing the weighted sum of the wavelets coefficients over a limited range of scales (equivalent periods) around three chosen periods (100, 40 and 22 kyr). the results for the three intervals selected are shown in fig. 6a-c. the per-interval red noise 50% and 90% confidence levels are plotted as horizontal dashed lines. for the scale interval including the short eccentricity orbital period (�100 kyr), we observe (fig. 6a) that the spectral energy emerges twice above the 90% confidence level, whereas all the other peaks are well above the 50% level. the long eccentricity orbital period (� 400 kyr) seems to modulate the whole pattern. a different and less regular behaviour appears in the scale intervals of data which refer to smaller orbital periods (fig. 6b,c). in both cases, the highest maxima dominate over the noise level and the spectral energies seem to have the same modulation found in the previous band. the global wavelet spectrum of the simulated data has been calculated, following the same procedure used for paleomagnetic data. this spectrum is plotted in fig. 7 together with that of mt. raggeto data. the two series exhibit comparable energy content. both have been fitted with piece-linear shaped functions, each of which presents two different slopes. a roughly similar slope is found in the high-frequency regions, whereas in the low-frequency region paleomagnetic data exhibit a less steep spectrum. fig. 6a-c. scale averaged local morlet spectrum, versus time (myr). intervals averaged are: a) 120-80 kyr; b) 50-30 kyr, and c) 30-15 kyr. the per-interval 90% and 50% red noise confidence levels are indicated by dashed lines. a b c 581 wavelet analysis on paleomagnetic (and computer simulated) vgp time series this could be expected as a consequence of the low intensity of magnetization of these data, that could result in a higher amount of random noise and as a consequence in a flattening of the spectrum. 5. 1dog results to analyze excursion and reversals, preliminary tests using, beyond the 1dog, the 2dog and 3dog (second and third derivatives of a gaussian, respectively) have been carried out both on simulated and paleomagnetic data sets. the tests were undertaken to check out the possible different features that different wavelets can reveal in the data. in fact, increasing the order of derivatives, the number of zeroes and vanishing moments of the analyzing wavelet increases. as a consequence, it is possible to resolve local features with greater lipschitz coefficients and the multiplicity of the ridges increases (mallat and hwang, 1992; alexandrescu et al., 1995), so that they can be compared among them. the different results we obtain exhibit only a progressive loss of resolution as the order of derivatives increases, both in terms of coefficients images and ridges in them. as the processes we are exploring do not need, in our working hypothesis, more than one fig. 7. comparison between global wavelet spectra of paleomagnetic and simulated data (see text for more details). 582 stefano lorito, grazia giberti, agata siniscalchi and marina iorio vanishing moment, the value of the maximum positive lipschitz exponent we expect being one, we choose to work with the 1dog function. this choice was made also taking into account that a more complex structure of the wavelet, when associated with the inherent noise and high variability of the paleomagnetic data, would induce at any scale correlations which render difficult the localization and interpretation of singular behaviours. we begin reporting the analysis of the simulated data. as they have a simpler structure, some of the results features will guide the analysis of the more complex paleomagnetic data results. the obtained wavelet coefficients are represented in fig. 8 as an image in the time-scale plane. the excursions and reversals, whose localization and characteristics can be easily identified by visual inspection of the data (fig. 2), also show up clearly as extrema (ridges) of the cwt. only some among the found ridges are selected and plotted in fig. 8, i.e. the most energetic (highest values of the cwt modulus) and those connected from finest to coarsest scales. figure 9 shows the corresponding ridges functions versus the scale s in a log-log plot. the reversals (last two plots in fig. 9) are characterized by an almost linear trend up to the biggest scales, whereas the excursions ridge functions die out at relatively smaller scales. all the ridges, with the sole exception of the fourth, have at the finest scales the positive slope which characterizes the transition, in the time series, between the regular behavior and the sudden change we are looking for. the lipschitz coefficients are listed in table i, which also reports the total duration of the processes as estimated from the scale (converted in time) at which the ridge functions die out. this time interval roughly corresponds to the scale at which the full process has been scanned by the wavelet, as far as the excursions regard and coincide for the first two ridges almost exactly with the time interval during which the first fig. 8. image of 1dog wavelet coefficients of the simulated data. red lines indicates the selected ridges (see text for details). blue lines indicate the boundary cone of influence. the coefficients are scaled with respect to their maximum. 583 wavelet analysis on paleomagnetic (and computer simulated) vgp time series excursion fully develops (cfr. fig. 2). as far as the second excursion is concerned, worse results in terms of ridge shape, exponents and possibility of estimating durations, are obtained and attributed to the greater (in comparison with the first excursion) variability of the data around it. as regards the reversals, the difference between their exponents a (0.2 and 0.5, respectively) reflects quite accurately the character expected when looking at the data. the first exponent corresponds to a (slightly) noisy heaviside step and the second to that of a noisy ramp. both ridge functions conserve their almost linear trend within the boundary cone of influence, a limit imposed by the finiteness of the data set. it has to be noted that the estimated duration of 45 kyr of the first reversal (table i) corresponds to the time interval of its polarity stability, suddenly interrupted by a energetic excursion after the reversal. the wavelet coefficients obtained for paleomagnetic data are represented in fig. 10 as an image in the time-scale plane, and the corresponding selected ridge functions in fig. 11. the selected ridges are the most energetic and those connected from finest to coarsest scale, as above. the values of t i and a i and of the total duration of the identified processes are listed in table ii. in the table we indicate which among all the selected ridges we interpret as reversals or excursions. this interpretation needs specific comments. the variability of the data series reflects in a less clear behaviour of the ridges functions. linear trends are interrupted by oscillations, some randomly distributed, some probably due to a positive interference with the orbital periods which envelop the entire data series. this feature does not allow us to decide, only looking at the plotted ridges functions as done with the simulated data, the character of the polarity change. to interpret the ridge characteristics it has been necessary to zoom on segments and look if data trends around ridge roots would confirm the indications coming from ridge functions. in any case, couples of nearby ridges were expected to indicate excursions, whereas a prevalent linear trend up to the coarsest scales along with higher absolute values to indicate reversals. with this in mind, only the ridges numbered 4, 10, 13, 14 and 21 could be identified as reversals. to these, with the exception of ridge 4, are also associated the longest estimated intervals of stability (about 100 l o g 2 ˜w /w m a xá log2s fig. 9. simulated data ridge functions versus the scale in log-log plots. table i. results of the analysis of the ridge functions (fig. 9) of simulated data (see text for details). excursions ti (kyr) a dt (kyr) ridge 1 27.360 0.1 � 10 ridge 2 38.960 0.2 � 12 ridge 3 116.640 0.7 � 15 ridge 4 123.840 0.3 > 8 reversals ti (kyr) a t (kyr) ridge 5 190.800 0.2 > 45 ridge 6 295.920 0.5 > 20 1 2 3 4 5 6 584 stefano lorito, grazia giberti, agata siniscalchi and marina iorio fig. 10. image of 1dog wavelet coefficients of mt. raggeto latitude (see caption of fig. 8 and text for more details). fig. 11. mt. raggeto latitude ridge functions versus the scale in log-log plots. l o g 2 ˜w /w m a x á log2s 585 wavelet analysis on paleomagnetic (and computer simulated) vgp time series kyr). the identification of ridges numbered 3, 7, 8 and 9 as reversal has been made looking at the data, their ridge functions being less definite and resembling those of the remaining excursions. actually, the stability in polarity around the corresponding transitions is found scarce but they do not look, for amplitude and duration, like a sequence of nearby excursions. as no nearby ridges were found to confirm them as excursions, we classify them as unstable reversals and their way to die out is attributed to the correlations at high scales with the neighboring data. the overall not well definite character of the ridge functions for the paleomagnetic data is reflected in the given ranges of the a exponents. their values are on average around zero, with the exception for the couple of ridges numbers 5 and 6, both corresponding to a «d -like» excursion, with a duration of around 2 kyr. finally, the negative exponents of the ridges 1, 2 and 12 indicate a sudden variation, in this being similar to ridges 5-6. the overall estimated duration of all excursions is always much less than 30 kyr, shorter than those estimated for the reversals; table ii. results of the analysis of the ridge functions (fig. 11) of mt. raggeto latitude. legend for the fifth column: rev. and exc. mean, respectively, reversal and excursion. « + æ -» indicates a transition from normal to reversed polarity, « æ +» the opposite transition. n. t i (kyr) a dt (kyr) 1 56.16 0.5 < a < 0 > 20 exc. 2 66.24 a < 0 > 20 exc. 3 306.72 – 0.1 < a < 0.1 > 30 rev. + æ – 4 331.20 – 0.1 < a < 0.6 > 30 rev. – æ + 5 422.64 – 1 < a < – 0.5 ~ 2 exc. 6 424.80 a � – 0.7 ~ 2 exc. 7 686.88 a £ 0 > 30 rev. + æ – 8 715.68 – 0.5 < a < 0.3 ~ 30 rev. – æ + 9 830.16 – 0.5 < a < 0.3 > 30 rev. + æ – 10 966.24 a � 0 > 90 rev. – æ + 11 1141.20 a < 0.2 ? exc. 12 1157.76 a � – 0.6 > 15 exc. 13 1299.60 – 0.5 < a < 0.5 ~ 100 rev. + æ – 14 1530.00 0 < a < 0.5 ~ 100 rev. – æ + 15 1675.44 a < 0.2 ~ 25 exc. 16 1686.96 a � 0.3 ~ 20 exc. 17 1972.80 a � 0.2 ~ 20 exc. 18 1985.04 a � 0.2 ~ 20 exc. 19 2067.12 – 0.3 < a < 0.4 ~ 20 exc. 20 2083.68 – 0.2 < a < 0.4 ~ 20 exc. 21 2126.88 – 0.5 < a < 0.2 > 100 rev. + æ – 586 stefano lorito, grazia giberti, agata siniscalchi and marina iorio ridge 11 constitutes the only exception as it gives an anomalously long timescale. all the mr2-mr12 mt. raggeto polarity changes as detected in iorio et al. (1998) result related to some of the identified reversals, with the exception of mr10 and mr11 that emerge as excursions in this analysis. an indication of the nature of the process can be retrieved also by comparison of the localization of ridge functions with those of the reversals in the coeval interval of the global polarity time scale (channell et al., 1995), shown in fig. 12. if the hauterivianbarremian boundary (h/b) is well located in both the series, no correlation appears but, looking at the tentative correlations among chrone boundaries and taking into account the uncertainties in the h/b datation (± 1.6 myr), a better correspondence would be achieved by shifting the data. 6. conclusions the global morlet cwt spectrum of paleomagnetic data reveals some peaks related to orbital frequencies with a feeble statistical significance. the same periods are again evidenced in the local cwt analysis of three selected ranges (respectively around 22, 40 and 100 kyr), with a better significance. the long eccentricity orbital period (� 400 kyr) seems to modulate the whole pattern. these results also indicate the non-stationary character of the analyzed time series. records of geomagnetic paleointensity by sediments, spanning the time interval 0 -1 myr, have been analyzed by a number of authors (channell et al., 1998; guyodo and valet, 1999; guyodo et al., 2000; yokoyama and yamazaki, 2000) to address the relevance of orbital contributions. controversial results emerge. the debate is mainly about the possibility of climatic contamination that could affect the paleointensity of sedimentary records. more recently, yamazaki and oda (2002) reported evidence of eccentricity signature in the inclination of a sedimentary sequence (0-2.25 myr). here we have tested with the cwt technique previous results (iorio et al., 1995) from the older (124-128 myr) mt. raggeto sequence. these two last results could be indications of orbital influence on the geodynamo, as a mechanism of climatic contamination of paleomagnetic directions seems less obvious than for paleointensity. on the other hand, we noticed that the local energy maxima in the morlet analysis (especially at 22 kyr) occur at roughly the same times as most of the found excursions and some reversals. this could indicate two opposite situations: the polarity transitions are influenced by orbitals or, vice versa, the energy maxima reflect the average durations of the transitions. discussions on the same problem is found also in worm (1997), yokoyama and yamazaki (2000) and kent and carlut (2001). fig. 12. comparison among the positions of found reversals by means of 1dog cwt (gray and white) and the global polarity time scale (channell et al., 1995). white corresponds to reversed polarity while black and gray to normal polarity. 587 wavelet analysis on paleomagnetic (and computer simulated) vgp time series the global wavelet spectra of simulated and paleomagnetic data exhibit roughly similar slopes in the high-frequency region and this can be due to the presence in both series of the same maximum degree of singularity (mallat, 1998). the observed differences could be explained when considering the age of mt. raggeto data and that simulated data are based on the present day cmb seismic tomography. by using 1dog we localized reversals or excursions in the paleomagnetic data, with some difficulties due to the data inherent noise. the analysis of the simulated data was a useful guide in this connection. the analysis of the cwt results could be more robust if applied also to other paleomagnetic data sets with the possible widest geographical and temporal distribution and if also magnetized continental and oceanic lavas with the proper time resolution were available. such a detailed study would serve to confirm the debated features of paleomagnetic data and render them as data on which theoretical models could be validated. this, in particular, as far as the possible role of orbital periods on the geodynamo is concerned (malkus, 1968; buffett, 2000; roberts and glatzmaier, 2000). all the results of this work have been obtained by analysis of latitude data only. the longitude and intensity of remanence were not used at this first stage as they are more difficult to treat with respect to the latitudes. in view of the results obtained so far and the methodological experience acquired, their analysis, along with the analyses of some other paleomagnetic sequences, is now in progress. acknowledgements we wish to acknowledge sergio cavaliere for suggestions about signal analysis and interpretation. as far as cwt is concerned, special thanks are due to umberto amato and the iam-cnr people. we are indebted with gary glatzmaier and lionel hongre for kindly providing access to their computer simulated data. the paper was greatly improved during its revision, thanks to the criticism of maurizio fedi and of an anonymous referee. notes the numerical codes used in the morlet wavelet analysis have been developed adapting the wavelet software provided by c. torrence and g.c. compo, which is available at url: http://paos.colorado.edu/research/wavelets/. the numerical codes for the dog wavelet analysis have been developed starting from those included in the freeware tool wavelab 802, available at url: http://wwwstat.stanford.edu/~wavelab/. references alexandrescu, m., d. gibert, g. hulot, j.-l. le mouël and g. saracco (1995): detection of geomagnetic jerks using wavelet analysis, j. geophys. res., 100 (b7), 12,557-12,572. berger, a., m.f. loutre and j. laskar (1992): stability of the astronomical frequencies over the earth’s history for paleoclimate studies, science, 255, 560-566. buffett, a.b. 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(1997): a link between geomagnetic reversals and events and glaciations, earth planet. sci. lett., 147, 55-67. yamazaki, t. and h. oda (2002): orbital influence on earth’s magnetic field: 100 000-year periodicity in inclination, science, 295, 2435-2438. yokoyama, y. and t. yamazaki (2000): geomagnetic paleointensity variation with a 100 kyr quasi-period, earth planet. sci. lett., 181, 7-14. annals 47, 1, 2004, 01/07def 213 annals of geophysics, vol. 47, n. 1, february 2004 key words geoelectricity – earthquakes prediction – method van 1. introduction the study of ultra low frequency (ulf) geoelectric signals with steep fronts is popular at the moment. recently the author presented the new examples illustrating widespread occurrence of such signals in seismoactive zones (chirkov, 2000, 2002). it is supposed that these signals may be associated with the specific evolution of mode of deformation of rocks before earthquakes. the present work is devoted to an elaboration of the method for monitoring ulf electrotelluric signals with steep fronts. it is further development of an old idea (sobolev and morozov 1970; ponomarev, 1987) concerning the search for electrotelluric precursors of earthquakes (see also sobolev, 1992; varotsos and sarlis, 2002). the application of the electrotelluric method in geodynamic monitoring is attractive because it gives the possibility to provide a distant control of the medium state in real time using point measurements. in addition, it may be sensitive to detect a variation of the medium parameters in the wide range of time intervals – from a fraction of a second up to days and more. in all, the method of monitoring based on the detection of geoelectric signals with steep fronts appears to be particularly importance, because these signals are easily recorded and recognized, and namely such signals are considered the possible precursors of earthquakes. signals with steep fronts were discovered on kamchatka by prof. g.a. sobolev’s group (sobolev and morozov, 1970). signals have not the study of local sources of ulf geoelectric signals with steep fronts yeugeny b. chirkov schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia abstract the method for recognition and monitoring of ulf electrotelluric signals with steep fronts is presented. the method is based on the multicomponent measurements of the geoelectric field and recognition of the clusters of signals using the data of spatial-polarization filtering. this detects the local sources of the signals associated with the variation of mode of deformation of the medium. the method was applied to the study of geoelectric sources associated with aftershocks of the racha-djava earthquake. the field observations show with a fair degree of confidence that these signals are generated by a small number of local zones in the vicinity of the observer (at a range of tens-hundreds meters). the spatial distribution and the dynamic properties of the local sources are described. the analysis of observations leads to the conclusion that the dynamics of the local sources of ulf signals with steep fronts may be considered possible electrotelluric precursors of the earthquakes. consideration is given to the feasibility of the well-known van method for earthquake prediction. the overall methodological aspects of the current studies of the electrotelluric precursors of the earthquakes are discussed. mailing address: dr. yeugeny b. chirkov, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: chirkov@ifz.ru 214 yeugeny b. chirkov been registered synchronously on several stations situated from each other at a distance of some tens of kilometers. on that basis the conclusion on their local origin has been made. as similar signals were not seen before the strong ust-kamchatka earthquake (1971, m = 7), interest in signals in russia subsided. then prof. p.a. varotsos paid attention to these signals. signals received the name ses (seismo electric signals) and for about twenty years have been used by the van group for the prediction of earthquakes in greece. as a result of these researches by the van group, some interesting phenomenological features of signals have been discovered and many successful predictions have been made (varotsos and alexopoulos, 1984a,b, 1987; varotsos et al., 1988, 1993, 1996; varotsos and lazaridou, 1991). however, despite the large number of recorded signals and considerable experience of the practical prediction of the van group, the scientific community has not come to a consensus on the reliability of the prediction on the basis of electrotelluric precursors. this is a difficult problem mainly because the location and origin of sources of the electric precursors are not easy recognized. as a result, «the wind prevailing in the scientific community does not appear to be favourable for earthquake prediction research» (uyeda, 2002). though works of the van group were subject to multilateral criticism most intensive during the years 1996-1998 (e.g., bernard, 1992; geller, 1996a,b, 1997; gruszow et al., 1996; kagan and jackson, 1996; bernard et al., 1997; geller et al., 1997; pham et al., 1998, 1999, 2000; pinettes et al., 1998), the latest works (uyeda, 2002; varotsos and sarlis, 2002) testify that the the question continues to remain open. the sides find newer and newer arguments in support of their points of view, but experimental results preclude a well-grounded conclusion on the correctness of one of the sides and despite perennial efforts, we have no essential advances in understanding the nature of electrotelluric precursors. what is the reason for such a paradoxical situation? in our opinion the general cause of the paradox is mainly the local origin of precursory signals with steep fronts and unsuitability of existing techniques of measurements, data processing and prediction of earthquakes for such kind of signals. in discussion devoted to the earthquake prediction of the van group, there are many unsettled problems which are concentrated around the same main three topics: the localization of the sources, the method of detection of the precursor signals against background noise taking into account the real inhomogeneity of media, and the overall scheme of the earthquake prediction. the following three separate sections of the article will be devoted to these three topics. 2. the local origin of the ulf signals and causal dependence of their sources on the change mode of deformation: experimental evidence in our opinion, the principal cause of mistrust in the results of the van group is connected to the contradiction between the model of source ses in the epicenter of an impending earthquake which is proposed by the van group and their own results, which can be more easily explained on acceptance of a hypothesis about a local origin of source. there are a number of indirect indications testifying the inadequacy of the experimental data to the model of source ses in the epicentre. this is the existence of sensitive zones and maps of selectivity, which is evidence of absence of simultaneous precursory signals on a network of the van group; absence of a large number of signals coincident with the time of the earthquake, when there are most dynamical processes in the epicenter; appearance of a signal not on all lines of a given station and so on. there are also many reasons to think that the sources of observed electrotelluric signals are not arranged within the focal zone of the future earthquake, but are placed in the immediate vicinity of the observer. for example, (gershenzon and bambakidis, 2001) believe that the ulf signals have a local origin (see also dobrovolsky et al., 1989; dobrovolsky, 1991; gershenzon and gokhberg, 1993; gruszow et al., 1996) and presented arguments in favor of the 215 the study of local sources of ulf geoelectric signals with steep fronts hypothesis of regional stress as a prime cause of the abnormal ulf signals observed before earthquakes. ponomarev (1987) noted the remarkable similarity of empirical relation lg (∆u ⋅ ⋅ r) = (0.3 − 0.4)⋅ m + ak established by prof. varotsos (between the amplitude of the signal ∆u, the magnitude m of the earthquake and the epicentral distance r) with other relation lg ⋅ ⋅ (ε1/3 ⋅ r) = 0.43 ⋅ m − 2.73 (between the amplitude of the deformation ε, the magnitude m of the earthquake and the radius of the manifestation of deformation’s precursor r) which was established by dobrovolsky (1983) theoretically. to test the hypothesis about the local origin of signals with steep fronts a special experiment was conducted during an aftershock sequence of racha-djava earthquake (29.04.1991, m = = 6.8). figure 1 shows the map of epicentral zones of the main shock and the strongest aftershock (15.06.1991, m = 6.3). it should be noted that the problem in hand demands a special measurement technique. long lines are unsuitable for monitoring signals since the signal is registered only by one electrode of the line while the signal/noise relation is reduced due to the contribution of ionospheric disturbances, which is proportional to the length of a line. furthermore, a change in potential on the common electrode can result in a false anomaly, which seems recorded by several lines. using the amplitude of anomaly, normalized on length of the line, hides the false character of this anomaly. at best with such technique we receive a signal recorded on one or two lines, that it is not enough for an estimation of parameters of sources. fig. 1. the epicentral zones of the racha-djava earthquake (29.04.1991, m = 6.8) and the strongest aftershock (15.06.1991, m = 6.3). here (1) is the point of observation, (2) indicates the epicenters, (3) is the pleistoseist zone, and (4) indicates the faults. rion i river djo dj ora river oni iri ambrolauri sachere chiatura tschinval djava 216 yeugeny b. chirkov the basic idea is that two horizontal components of the electric field at two closely spaced points are recorded. in other words, the four independent lines are used to control the locality of sources. figure 2 presents the disposition of measuring and generating lines. the distance between points is 70 or 200 m depending on the period of observation. from 23.05.1991 to 19.07.1991 the measurements were carried out at 1 and 2 points, and from 20.07.1991 to 20.08.1991 at 1 and 3 points. the daily measurements of the electrical resistance of the rock were made as well. rock resistance was controlled using the same dipoles used for electrotelluric monitoring. two additional orthogonal dipoles situated near measurements points were used as sources of a current (see fig. 2). the measuring system consists of blocks of preamplifiers and a digital recording system. the former involves four identical preamplifiers with an amplification factor equal to 41. analog filters were equivalent to the 0.033 hz lowpass butterfig. 2. the disposition of measuring and generating lines. the arabic numbers designate the points of measurement. worth filters. 12 bit adc was used, the weight of lsb was 2.5 mkv, sample recording time was 30 or 60 s. high-quality «travers» electrodes were kindly presented by dr. m.m. bogorodsky for this experiment. the special way of installation of electrodes in the ground was applied. measurements were carried out at least every minute. noise level allowed recording of ulf electric signals with steep fronts if their amplitudes were more than 20-30 mkv. the observations have shown an abundance of signals with steep fronts. we do not use the ses designation because we are not taking into account van «rules» of ses detection. in our opinion, ses in the overwhelming majority of cases are a subset of signals selected on a technique offered by us. many signals were recorded at two points with the ratio of amplitudes on measuring lines varying in a wide range. this fact itself may be easily attributed to the presence of at least several local sources. by manual processing some tens signals were detected with similar spatial characteristics. some increase in the number of signals was revealed before the strongest aftershock of 15.06.1991. our opportunity for recording signals from the local sources depends on the intensity of sources, their position relative to the measuring system, and the registration threshold of the system. the lower the level of threshold is, the larger the area we can control. however, manual detection of signals of small amplitude is practically impossible. to solve this problem, a new algorithm for detecting synchronous signals with steep fronts has been developed. the idea consists in the analysis of spatial and polarization characteristics of the signals. the algorithm has two generic parameters. one of them is used for adaptation to the level of noise, and the other defines the level of requirements to quality of the analysis of spatial structure of sources. these two constants need to emerge once on the basis of the analysis of records by duration about one week and a further procedure of detection of signals is completely formalized. several thousand signals were detected using this algorithm. figure 3 gives an example where the significant number of detected signals of different amplitude with the same spatial structure is visible. 217 the study of local sources of ulf geoelectric signals with steep fronts fig. 3. example of the records containing signals with steep fronts, which were registered at 1 and 3 points of measurement on 28.07.1991. the arabic numbers designate the points of measurement. ns and ew specify directions of measuring lines. fig. 4a-c. the distribution of relative amplitudes of signals at the points of measurement produced by the cluster centers of sources for the three independent intervals of monitoring. the roman numbers designate the points of measurement. ns and ew specify directions of measuring lines. a b c 218 yeugeny b. chirkov the data processing is based on the assumption that the ratio of signal components of different measuring lines should remain a constant at a constancy of properties of environment and geometry of source of a signal. in this case, it is possible to take advantage of cluster analysis for determination and classification of sources of signals. so the detected signals were subjected to the standard cluster analysis using the k-means clustering algorithm. the metrics of space-l2, the dimension of space, as clearly seen from fig. 4a-c is 3 or 4. we used for clustering ratio of amplitude of a signal on a line to the sum of modules of amplitudes of signals on all lines. for an estimation of time-stability of the received results the period of observation was broken into three intervals and clustering was carried out formally for each interval independently. results of data processing are given in table i and fig. 4a-c. we can see the distribution of relative amplitudes of signals for the cluster centers table i. the result of cluster analysis of electrotelluric signals with steep fronts (see the text). numbers of points are designated by the roman numbers, ew and ns specify orientation of measuring lines. statistical significance relative amplitudes of signals for the cluster centers 23.05.1991-19.06.1991 point_component f p 1 2 3 4 5_i i_ns 4205 0.000 0.712 0.483 0.110 0.200 0.245 i_ew 4218 0.000 0.171 0.370 0.770 0.178 0.527 ii_ew 2002 0.000 0.117 0.147 0.120 0.623 0.227 number of signals 587 855 519 405 726 20.06.1991-18.07.1991 point_component f p 1 2 3 4 5_ii i_ns 6811 0.000 0.678 0.375 0.123 0.206 0.030 i_ew 7462 0.000 0.177 0.416 0.726 0.201 0.037 ii_ew 13 641 0.000 0.145 0.210 0.151 0.593 0.932 number of signals 663 644 715 866 1570 20.07.1991-20.08.1991 point_component f p 1 2 3 4 5_iii 6_iii i_ns 4889 0.000 0.658 0.395 0.122 0.142 0.091 0.100 i_ew 4350 0.000 0.136 0.364 0.640 0.151 0.466 0.107 iii_ns 4244 0.000 0.118 0.140 0.120 0.108 0.300 0.730 iii_ew 2494 0.000 0.088 0.101 0.117 0.599 0.144 0.063 number of signals 541 875 1024 380 1736 561 of sources on the points of measurement for the three independent intervals of monitoring (fig. 4a,b and c). as is clearly seen, some sources exist throughout the observation (1, 2, 3, 4), some sources arise and disappear (5-i, 5-ii). in table i the clusters designated by simple numbers exist throughout the period of observation. the clusters with numbers containing an interval number exist only in this given interval of observation. the clusters are characterized by f-value, p-level and number of signals for more detailed representation of the clustering. as is seen in fig. 4a-c and table i, the sources of signals are well clustered. for all cases the fvalue (the ratio of the between-groups variance over the error variance) is more than 2000. the distribution of relative amplitudes of signals for the cluster centers of sources clearly demonstrates the local nature of the signal sources. this result appeared unexpected in some respects. if the threshold of selected signals is lowered, we see the greater number of 219 the study of local sources of ulf geoelectric signals with steep fronts signals of close located sources, but not the greater number of sources. detection of the same cluster of sources using formal cluster analysis for different time intervals and different measuring arrays adds strength to the hypothesis that these local sources really exist. to clarify the relations between the cluster analysis results and the parameters of sources of signals, we estimated parameters of a source for the centers of clusters (fig. 5). the results were obtained using the data of points 1 and 3 (20.07.91-20.08.91), all estimations were made on the basis of the assumptions of homogeneous medium and a position of sources near to the earth surface. no sources could not be approximated by a model of a point source, but all sources with a small error were approximated by a model of a dipole. as is clearly seen from fig. 5, all sources are located in immediate proximity to the measuring arrays. it should be noted that sources for clusters 1 and 2 have very similar position and orientation. dynamics of every cluster can be estimated using several characteristics. for the arbitrary interval of time it is possible to obtain estimations of number of signals, dispersions of spatial characteristics of the cluster sources, average amplitude of signals, the maximal amplitude of signals. the corresponding calculations testify that these characteristics behave independently of one another. as an example let us consider the daily occurrence of the signals for the period including the largest aftershock (m = 6.2, 15.06.91). figure 6a-f shows the variations of average daily rock resistivity and number of ulf signals for the five clusters at the period from 23 may up to 19 june 1991. the plot in the top right corner in (fig. 6a) shows time dependence of the u/i, which is proportional to resistivity. on other fragments of (fig. 6b-f) variations of the number of signals for each of five clusters are given. we see that the sources radiate practically continuously and on some days there are anomalies exceeding an average level approximately in four time. the clusters behave rather independently of one another, showing the absence of dependence on the external magnetotelluric field. sometimes there are synchronous anomalies, in our opinion connected with the change in mode of deformation of the rocks. the dynamics of clusters 1 and 2 (which have sources with similar position and orientation) are quite similar. indeed, the moments of all extremes coincide with each other, although the amplitudes of anomalies are dissimilar. dynamics of 3 and 4 clusters (which have sources sharply differ by position and orientation from sources of 1 and 2 clusters) sharply differ from dynamics of 1, 2, 5_i clusters. during observation each day there were many aftershocks of different magnitudes with different distances from point of observation. to compare the dynamics of sources with dynamics mode of deformation near the point of measurement we calculated a posteriori theoretically estimated value of changes in mode of deformation. estimation of a theoretical variation of deformation in point of observation was made using the catalogue aftershocks produced by the group of dr. aref’ev, and theoretical formulae obtained by dobrovolsky (1991). figure 5 shows the results of corresponding calculation. we propose that the deformation from every aftershock is independent from each other, allowing us to make daily estimations of components of deformation by summing up the square fig. 5. the disposition of electric dipoles approximating a field of sources of the recorded signals for the third interval of observation. the disposition of measuring and generating lines are also shown in the figure. 220 yeugeny b. chirkov of amplitudes of components for all aftershocks which occurred this day. to take into account the duration of aftershock preparation times, we used phenomenological estimation for duration of a short-term precursor (dobrovolsky, 1991) as weight at summation. as for the overwhelming majority of events duration of a short-term precursor is less than a day, this procedure seems to be appropriate. however, for the aftershock of june, 15 with m = 6.2 the duration of a short-term precursor is 4.7 days. thus anomaly june, 15 would spread till june, 11. but we have not reflected it in the diagram because of uncertainty of the law of change in amplitude of a short-term precursor in time and for preservation of uniformity of processing. from fig. 7a-c a significant excess of all calculated components at june, 15 is clearly seen. in view of this, it is possible to consider a synchronous local maximum of the number of signals for clusters 1, 2, 5_i at june, 12 as a possible precursor for aftershock june, 15. the level of theoretically estimated value of the changed mode of deformation caused by other aftershocks this day is low (fig. 7a-c). anomaly in occurrence of the signals in the cluster 1 makes about 400 % in comparison with the previous and next days. anomalies of the signals in clusters 1, 2 and 3 fig. 6a-f. variations of the average daily rock electroconductivity and numbers of ulf signals for the five clusters at the period from 23 may up to 19 june 1991. the roman numbers designate the points of measurement. here ns and ew specify the directions of measuring lines, whereas the italic ns and ew specify the directions of generating lines. the arabic numbers specify the clusters. the arrow indicates the time of the aftershock. a b c d e f 221 the study of local sources of ulf geoelectric signals with steep fronts at june, 12 is anticipated and accompanied by a change in resistivity on several measuring lines, that is the additional argument for their connection with changed mode of deformation. the day of the strongest aftershock is marked by a local minimum of quantity of signals for all clusters. since may, 23 in clusters 1, 2, 5_i synchronous fluctuations of gradually increased amplitude fluctuations are noticed, similar data were obtained by meyer and teisseyre (1988). we propose that these facts are indirect evidence of the relation of changed mode of deformation with a process of preparation of aftershock june, 15. 3. reliability of establishing the correspondence between the signal and its source: a practical example some critical remarks on the empirical earthquake prediction developed by the van group are not eliminated by acceptance of a hypothesis of a local origin of the precursor’s sources. these are remarks concerning reliability of the data of monitoring electrotelluric field. opponents assert that the van group cannot identify and eliminate ambient natural and artificial geoelectrical noise sources before claiming to have observed geoelectrical precurfig. 7a-c. average daily variations of the number occurrence and energy of the earthquakes (a), and a posterior theoretical estimates of average daily anomalies in the displacement and inclination (b), and in the linear and volumetric deformations (c) at the observation point. here u, v and w are components of displacement, gx and gy are the anomalies of components of inclination; exx, eyy and ezz are the linear and volumetric deformations before the earthquake (see the text). a b c 222 yeugeny b. chirkov sors of distant earthquakes (chouliaras and rasmussen, 1988; gruszow et al., 1996). most scientists consider that rules for discriminating between alleged earthquake precursors and noise proposed by van are inadequate (chouliaras and rasmussen, 1988; gershenzon and gokhberg, 1993; pham et al., 1998; uyeda et al., 2000). the problem is complex and breaks down into several aspects. for the sake of convenience, electrotelluric monitoring can be provisionally divided into a «technical stage», where relations between the electric field and its source are considered, and a «prediction stage», where relations between the signal, its source and parameters of impending earthquake are studied. let us consider the problems concerning the reliability of detection a signal, discrimination between signal and noise, and an estimation of characteristics of the signal source. the data of electrotelluric monitoring have the additional degree of uncertainty in relation to the data of other methods recording local variations. the area of gathering of the information depends on the intensity of source, and it is practically unlimited. signals with steep fronts can be ionospheric in origin (e.g., ssc), they can also be generated by a number of other sources of industrial noise. in a vicinity of measuring array, as a rule there are some numbers of sources of the natural electric field (oxidation-reduction, membrane, a piezoelectric or electrokinetic nature), which may or may not be connected to the process of preparation of earthquake. recorded signals can be a reflection of changed resistivity of the medium (yamazaki, 1974, 1975) in an external field of ionospheric or terrestrial origin. and last but not least, the electrodes, which are used for measurements can be sources of false anomaly signals due to instability and high sensitivity of potential of a double electrical layer to the numerous external influences. all this together with the non formalized method of isolation of precursor signal causes natural mistrust of the scientific community in the results of monitoring. we shall notice that the methods, which are not using manual isolation of a signal (lukk et al., 1996), based on an analysis record as process, can also appear ineffective in this case. it is connected by that on a measuring line we record the signals determined by various processes, and the common views on the dynamics of such mix hardly have physical sense. spectral estimations used in such approaches are also unsuitable means for detection and an evaluation of parameters of small signals with steep fronts. lacking clear morphological distinctions between the signals, methods of spatial filtration are the only means of separation of sources by spatial localization and scale. as mentioned above, a set of long and short lines having the common electrodes is not effective for detection of signals of local sources. our approach to the detection of signals described in the previous section is appreciably reducing these gaps. allocation of the signal from the local source more reliably because four independent lines record the signal, and for its detection the formalized procedure of a spatial filtration is used. moreover, reliability of the result of monitoring also increases since the significant number of signals with the same spatial structure can be analyzed instead of one signal. thus, we are going from monitoring signals to monitoring characteristics of their sources. there are two questions. how far can we classify and identify different clusters of sources? can we estimate position of a source? for an estimation of to what extent the ratio a component of a field recorded on two orthogonal lines in one point of measurement allows us to estimate a direction on a point source in inhomogeneous medium a special field experiment was carried out. at the point of measurement two orthogonal measuring lines in length of 6 m were located. at the point of measurement the artificial heterogeneity with infinite resistance (l = 1.5 m, w = 0.8 m, d = 0.8 m) was created. on two orthogonal measuring lines we measured signals of the controllable point source being moved by the operator along radial profiles from the center of array. (the second point electrode was grounded at a distance 10 times exceeding the maximal distance between the center of measuring array and the first electrode). this kind of survey may be termed the calibration of measuring array. this array establishes the correspondence between the image of source and its real position for the cho223 the study of local sources of ulf geoelectric signals with steep fronts fig. 8. calibration of the standard measuring array (two-component orthogonal dipoles centered at the point (0, 0) by using point source placed at the point (x, y)). the circular diagram at each given point of the tablet shows the relation between the signal components of measuring array produced by a point source located at this point. fig. 9. the arrangement of electrodes of the proposed «nine-points» array. measured finite-difference approximation for the first and second derivative of the electric potential are also shown in the figure. latin letters designate potentials suitably located electrodes. fig. 10. calibration of the proposed nine-electrode measuring array centered at the point (0, 0) by using point source placed at the point (x, y). the circular diagram at each given point of the tablet shows the relation between the signal components of measuring array produced by a point source located at this point (see text). sen type of a source. figure 8 gives the results of the carried out calibration of measuring array. apparently the influence of heterogeneity does not allow estimation of a direction on a point source, and the ratio of the orthogonal components remains practically constant under changes in an azimuth of a source in a range of 80°. from here it is possible to draw a conclusion on the need for multivariate observations for an estimation of source’s parameters in the inhomogeneous medium. the symmetric measuring array offered for this purpose is shown in (fig. 9). for realization of measurements of a set of the first and second derivatives of potential of electric field specially developed equipment was used. the measuring system consists of blocks of preamplifiers and a digital system of recording (adc 10 bit, amplification factors from 0.25 up to 8, automatic compensation of zero’s shift, sample rate 200 hz). figure 10 presents the results of the calibration of this measuring array at the same point under the same conditions as calibration of standard measuring array. we see one of several gauge maps which shows the relational values of angles between components of a vector field measured by two pairs of two orthogonal measuring lines. it seems clear that opportunities for an estimation of position of a point source here are much greater. the given array uses advanced methods of spatial filtration and gives us more 224 yeugeny b. chirkov opportunities for reliable detection of signals and clustering of its sources on the basis of spatial properties. pairing such arrays together, application of the formalized procedures of detection of signals with steep fronts and clustering of its sources will trace the dynamics of signal sources and with the help of calibration of measuring array estimate the source parameters in the inhomogeneous medium. 4. reliability of establishing the causal relation between the precursor and the impending earthquake: a general approach the model of local origin of signals with steep fronts permits us to explain almost all experimental facts which are seemingly in contradiction with the model of a source in the epicenter of future earthquake (see the section 1). local precursor is less rigidly connected with the preparing earthquake. this also clarifies the gradual changes in terms of a prediction of the future earthquake made by the van group from 6-115 h (varotsos and alexopoulos, 1984a) to one-two month (varotsos et al., 1996). the notion of local origin of signals removes the requirement of one-to-one dependence between signals and earthquakes, what also contradicts experimental data. the model of local sources easily explains the appearance of signal selectively on some isolated lines within the confines of a given station (noto, 1933; yoshimatsu, 1957; varotsos and alexopoulos, 1984a,b, 1987; miyakoshi, 1986; varotsos et al., 1988, 1993, 1996; varotsos and lazaridou, 1991), and paradox with a lack of synchronism of signal occurrences on different lines in some cases (varotsos and alexopoulos, 1984b). it should be noted that the causal relation between precursory signal and preparing earthquake is a very weak point in any method of fig. 11a-c. a) standard and (b) new proposed empirical schemes of the forecast. c) the so-called theoretical scheme is also presented in the figure (see text). a b c 225 the study of local sources of ulf geoelectric signals with steep fronts prediction of the earthquakes, and most notably of predictions based on the control of local characteristics of medium. figure 11a-c illustrates the situation. the top panel (fig. 11a) presents the standard empirical scheme of prediction on the basis of monitoring of local characteristics of the medium. at such an approach an attempt is made to establish some empirical relations between two events that have occurred at different times and in different places. this relation can be proved only statistically because the material objects responsible for this connection are not considered at all. there are the source of the signal, local changed mode of deformation that is cause of signal, and regional changed mode of deformation, determined by process of preparation of earthquake. the theoretical prediction (bottom panel fig. 11c) includes definition of parameters of a source in many points of region, calculation using them of the values of local changed mode of deformation, an estimation of dynamics of regional changed mode of deformation and conclusion on probable parameters of impending earthquake (place, time, magnitude). transition to studying the relation of dynamics of a signal source with changes in the local and regional mode of deformation is the transition from a level of statistical comparisons on a level of research of physical relationships. the theoretical prediction is now impossible due to the uncertainty of the mechanism of source formation and an insufficient degree of development of the theory of preparation of earthquake. however, the standard empirical scheme of the prediction can be improved (see the fig. 11b, middle panel). inclusion in the scheme of predicting the cause of the signal, i.e. its source gives a material object for research. reliability of detection of a precursor is increased due to use of the formalized statistically proved criteria of detection. empirical comparison of dynamics changes the characteristics of a source even in one point of observation with parameters of earthquake seems to be more fruitful, because this is a comparison of characteristics of process with characteristics of the event, which occurred during this process. 5. conclusions the original method for recognition and monitoring of ulf electrotelluric signals with steep fronts has been presented. the method is based on the idea of spatial filtration taking into account the polarization properties of the signals, and recognition of the clusters of the signal sources using the data of spatial-polarization filtering. these procedures allow us to detect the specific anomalies of the ulf geoelectric field against a noise background through the control of intensification and synchronization of the cluster emissions. in addition, a measuring array is proposed which gives the possibility to realize the method by using the multicomponent measurements in each given point. the multicomponent measurements make it possible to detect the source of an anomalous ulf signal under the real condition of heterogeneous media. the method may be used in the search for electrotelluric forerunners of earthquakes. as a first step, the method was applied to the study of geoelectric sources associated with aftershocks of the racha-djava earthquake (29.04.1991, m = = 6.8). the local origin of ulf signals with steep fronts has been shown. namely, these signals are generated by a small number of local zones in the immediate vicinity of the observer (at a range of tens-hundreds meters). the spatial distribution of the sources is rather stable, but the number of pulses per day and their amplitudes may vary several-fold according to the variation of the local stress-strain state of the medium. none of the sources could be approximated by the model of a point source, but all sources with a small error were approximated by the model of a dipole. it is established that overall the sources radiate independent of each other. the remarkable exception is the largest aftershock (15.06.1991, m = 6.2). before this aftershock a synchronous dynamics of the clusters 1, 2 (which have sources with similar position and orientation) was observed, and this event was accompanied by abnormal changes in the electrical resistance of the rock. dynamics of 3 and 4 clusters (which have sources with different position and orientation) was not synchronous. 226 yeugeny b. chirkov also remarkable is a synchronous minimum of the ulf geoelectric activity at the day of aftershock. i believe that the similar abnormal behavior of ulf signals with steep fronts may be considered a possible electrotelluric precursor of earthquakes. in conclusion, the well-known difficulty in the study of the electrotelluric precursors is partly associated with pitfalls in the standard phenomenological method of the earthquake prediction. it is usually based on the search for an empirical relationship between the earthquake and precursor electric signal. however, these two events are separated from each other in space and time. the electric signal may be only causally bound with the future earthquake only via an interface agent, namely, through some current sources, which are generated as a result of evolution of the mode of deformation of the rocks in the course of the earthquake preparation. according to this line of reasoning, attention was focused on the careful detection of the sources responsible for the electrotelluric signals, and also on the study of relations between the dynamics of electric precursors with the dynamics of the local mode of deformation of the earth’s crust. acknowledgements this work was supported by the russian foundation for basic research, projects no. 00-15-98583 and no. 00-05-64770. the author wishes to thank a.v. guglielmi for useful discussion, valuable help and encouragement. references bernard, p. 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(1974): coseismic resistivity steps, tectonophysics, 22, p. 159. yamazaki, y. (1975): precursory and coseismic resistivity changes, pure appl. geophys., 113 (1-2), p. 219. yoshimatsu, t. (1957): the measurements of earth-current potentials and its reliability, mem. kakioka magn. obs. suppl., 1, 1-29. layout 6 1 annals of geophysics, 64, 4, gm440, 2021; doi:10.4401/ag-8507 o p e n acc e s s impact of approximate implementation manner in klobuchar model aghyas aljuneidi*,1 and hala tawfek hasan2 (1) higher institute for applied sciences and technology (hiast), damascus, syria (2) higher institute for earthquake studies and research (hiesr), university of damascus, syria article history: received june 5, 2020; accepted may 11, 2021 abstract this paper focuses on the approximations that john a. klobuchar made in mid 70s in his famous algorithm of ionospheric correction model for single frequency gps receiver. at that time klobuchar used a system of fixed geomagnetic north pole coordinates which are not accurate nowadays according to the international geomagnetic reference field and to the world magnetic model because the geomagnetic poles move slowly. in addition, klobuchar had to do other trigonometry simplifications in his implementation to avoid sophisticated computations. in order to evaluate this approximate implementation in a single frequency gps receiver, ionospheric time and range delay are estimated on the entire day of january 1st 2010, using a different implementation in matlab. the required gps data is obtained from recorded rinex files at udmc near damascus, syria. in this comparative study, we reformulated the standard equations of klobuchar model and examined the influence of its approximations on the ionospheric range delay and found a nonnegligible bias in order of ten centimeters, whereas the influence of the movement of the geomagnetic poles was in order of few centimeters. keywords: gps; geomagnetic north pole; total electron content; ionospheric correction; klobuchar model. 1. introduction global navigation satellite systems (gnsss) began after the launching of sputnik, the first artificial satellite, by the soviet union in 1957, which was very motivating for the us scientists. tracking of sputnik by using the doppler shift inspired the idea to track a receiver’s position on earth by acquiring a certain modulated radio frequency emitted by satellites. starting by the first satellite navigation program, named transit, and because of its numerous weaknesses, the us in mid 70s released its first global positioning system in the world, named gps, which still operate today and is subject to continuous modernizations. four years after the launch of gps in 1978 the soviet union’s counterpart called glonass also started launching first satellites. other systems like the european galileo or the chinese beidou/compass were started decades later. the fundamental principle of satellite positioning is based on the measurements of the signal transit time between the satellite and the receiver, the gps system uses the l band frequencies 1575.42 mhz, 1227.6 mhz and recently 1176.45 mhz for the transmission of data. the observation set for a single channel gps receiver are the pseudorange, the doppler shift, the c/n0, and recently the carrier phase measurements [parkinson, 1996]. the pseudorange is the time difference, when the data was sent by the satellite and when it was received, multiplied by the speed of light. the satellites also transmit data in their navigation message, which includes information like the health status of the satellite, rough clock and orbit correction terms and eight parameters for ionospheric error correction. table 1 presents the most important gps errors for single frequency receivers. table 1. statistical ranging error budget (1𝜎) adopted from [kaplan and hegarty, 2006]. after the end of selective availability (sa) in may 1st 2000, the ionosphere became the highest source of ranging error for gps c/a code receivers, as a result, reducing this error is fundamental [kaplan and hegarty, 2006]. different methods can be adopted to minimize the ionosphere effect in single receiver: • use of multi-frequency technique; • use some kind of augmentation system; • use of ionospheric model. due to the dispersive nature of the ionospheric refractivity, the ionospheric error can be corrected by the ionosphere-free combination with simultaneous measurements at two or more frequencies, [wang, 2016], this method is the most effective, but it cannot be used in a single frequency receiver. instead, single frequency receivers can acquire the ionospheric correction as differential corrections from differential gps (dgps) or from satellite based augmentation system (sbas), by transmitting corrections to the gps receivers either via satellite or terrestrial radio. augmentation systems are not available around the whole globe and involve the use of complex architectures and expensive receivers, while the use of broadcast ionospheric models, such as the klobuchar model, is cheaper and easy to implement in a single frequency gnss receiver. 2. problem statement the estimation and removal of ionospheric delay errors in the real-time applications of the single frequency users continued to remain a potential challenge to be addressed. the klobuchar model was the first ionospheric correction model implemented in near real time. over few decades it has become the standard ionospheric correction used by virtually all single frequency gps receivers in near earth for mid.-latitude. it is still used by a very large class of commercial low-cost single frequency receivers around the world till now. in spite of its numerous advantages, like not requiring additional parameters outside those contained in the aghyas aljuneidi et al. 2 segment source error source gps1σ error space ephemeris data satellite clock 2.1 m 2.1 m user ionospheric delay tropospheric delay multipath receiver noise 4.0 m 0.7 m 1.4 m 0.5 m sum user equivalent error rms 5.3 m gps navigation messages, fastness and ease of implementation, it is considered an outdated model from an accuracy point of view relative to nowadays classifications. however this model can reduce the rms position error about 50% to 60% [klobuchar, 1975]. when it was implemented and because of computational load concerns, john klobuchar, the inventor, used some approximate equations. finally, john klobuchar fixed the geodetic coordinate of the geomagnetic north pole in his algorithm according to some fitting in that time as 69w longitude and 78.3n latitude, which is not accurate nowadays because the poles wander with time. based on the international geomagnetic reference field document (igrf-13) in 2020, the geomagnetic north pole’s 72.7w longitude and 80.7n latitude are slightly different from their counterparts in the world magnetic model (wmm2020) located at 72.68w longitude and 80.65n latitude for the same year. in reality klobuchar adopted the geodetic coordinates of the geomagnetic north pole as 69w longitude and 78.3n latitude, [klobuchar, 1975; davies, 1965], for more details see page 20. 2.1 objectives of study the main objective of this paper is to analytically study some defects in the implementation of the klobuchar model and to do a new implementation with different considerations, then compare the results, in two cases: a) standard implementation of klobuchar equations without any trigonometry approximations; b) set the exact coordinate of geomagnetic north pole. 2.2 importance of study because the gps satellite message has a place for only eight coefficients to describe the worldwide behavior of the earth’s ionosphere and no global coverage of augmentation systems, it is virtually impossible for most of the single frequency gps users around the world to improve the performance of their receivers concerning ionospheric corrections, which is the largest error in gps. 3. ionosphere and geomagnetism the ionosphere is defined as that part of the upper atmosphere where the density of free electrons and ions is high enough to affect the propagation of electromagnetic radio frequency signals [kaplan and hegarty, 2006]. the signals are delayed instead, and this delay is part of the travel time, which represents an error into the computed range. the error is negative for the carrier phases (phase advance; that is, the measured range is smaller than the real range), and positive for the pseudorange (group delay; that is, the measured range is longer than the real range). the phase advance and group delay are equal in value and opposite in sign. for a good first order approximation, the delays that introduced in radio signals by the ionosphere (t����), are correlated with the total electron content (tec) which is defined as the total number of electrons presents in a cross section unit area of 1 𝑚² along the propagation path of the satellite signal to the receiver and inversely proportional to the square of the carrier phase frequency (f ), consequently: (1) thus, all electron density values along the propagation path of the signal are integrated in order to obtain stec, or slant tec, which different from vertical tec or vtec, can be mathematically represented as this integral: 𝑆𝑇𝐸𝐶 = ∫�𝑁�(𝑠). 𝑑𝑠 (2) t����(second) = 40.3 ���� ��² 3 impact of implementation manner in klobuchar model aghyas aljuneidi et al. 4 where, 𝑁�(𝑠) is the electron content per volume unit and 𝑝 is the propagation path between the source and the receiver. stec is measured in tec unit’s i.e. tecu, which equals to 1016 electrons/𝑚². accordingly, estimating and modelling tec is a necessary step to correct the ionospheric effects on electromagnetic waves propagating from satellites to earth, on the other hand, total electron content (tec) measurements provide a rich source of information about the earth’s ionosphere. thus, having gps-derived tec observations in comparison with another expensive method like satellite altimetry, incoherent scatter radar… etc., gps became a promising remote sensing sensor to model the ionosphere and to do many studies about related geophysical phenomenon [shuanggen et al., 2014]. worth to mention, that tec is highly correlated with the critical frequency of the f2 layer (fo f2 or nmf2). the nmf2 is the maximum electron density at the f2 layer of the ionosphere, approximately 300 500 km altitude, which traditionally was measured by ionosonde [kouris et al., 2008]. the solar activity follows a regular periodic variation with eleven years periodicity, commonly known as solar cycle. the solar cycle is the periodic recurrence of sunspots at sun’s surface, therefore tec and nmf2 are affected by solar activity (as measured by solar radio flux f10.7) as a long-term ionospheric variations, on the other, during the solar cycle there occurs periodic change in the solar radiation and the ejection of solar material which induce geomagnetic storms, characterized by the fluctuations of the earth magnetic field under the influence of solar wind and interplanetary magnetic field, which is a short-term ionospheric variations (including tec). particularly, tec changes with time in the forms of diurnal, seasonal, and long-term yearly variations, spatially, it changes with geographic longitude and latitude of the gps receiver (user) location, since the ionospheric properties are based mainly on the interactions between the solar radiation and the earth’s geomagnetic field [davies, 1965], as well as tec is related by the geomagnetic coordinates of the point of ionosphere where its estimation is done. the klobuchar model assumes the earth’s magnetic field to be an earth-centered dipole, so the relationship between the geomagnetic coordinates (𝜙� the latitude and 𝜆� the longitude) and the corresponding geographic coordinates (𝜙₁, 𝜆₁) at a certain point p is given by (3) where 𝜙� and 𝜆� are the geographical latitude and longitude of the north geomagnetic pole, [davies, 1965]. 3.1 geomagnetic poles a certain shift or displacement exists between the geomagnetic poles and the earth’s poles in the two directions of the south and the north, and moves in certain acceleration to the west. its change will affect the sector of navigation, communication and military, and other aspects [e.g., guo et al., 2011]. the earth’s magnetic field is in fact a composite of several magnetic fields generated by variety of sources. these fields are overlaid on each other and interact with each other through inductive processes. the most important of these geomagnetic sources are: a) the earth’s outer core; b) magnetized rocks in earth’s cover; c) fields generated outside the earth by electric currents which are flowing in the ionosphere and magnetosphere; d) electric currents flowing in the earth’s crust (usually induced by varying external magnetic fields); e) ocean current effects. all of these parameters vary with time of measures, ranging from milliseconds to millions of years (i.e. magnetic reversals phenomena) [prö�lss, 2004]. the world magnetic model (wmm) and the international geomagnetic reference field (igrf) are estimated from the most recent data and they are of a comparable quality. the difference between wmm and igrf are within expected model of inaccuracy. the wmm is a predictive only �sin𝜙� = sin𝜙� sin𝜙� + cos𝜙� cos𝜙� cos(𝜆� ‒ 𝜆�) sin𝜆� = cos 𝜙� sin(𝜆�+𝜆�) cos 𝜙� model and the wmm2020 is valid for the current epoch (2020-2025). the igrf is retrospectively updated and the latest update igrf-13 is valid for years 1900-2025. models need to be revised at least every five years because of the varying nature of the magnetic field with both time and location, this change in the geomagnetic field called “the geomagnetic secular variation”, [chulliat et al., 2020; thébault et al., 2015], and each epoch model comprises a set of spherical harmonic coefficients (called gauss coefficients) in a series expansion of the global magnetic field. the geomagnetic poles (dipole poles) are the intersections of the earth’s surface and the axis of a bar magnet theoretically placed at the center the earth by which we approximate the geomagnetic field and can be computed from the first three coefficients from a main field model, on the other hand, the magnetic poles are the points at which magnetic needles become vertical and can be computed from all the gauss coefficients using an iterative method. in table 2, we show predicted geodetic locations of the geomagnetic north pole by “international geomagnetic reference field” (igrf-13) including prediction. 4. ionospheric modelling there have been numerous researches on ionospheric modeling, whether global, regional and local, or near real time and post processing models, these researches include: • empirical models based on ionospheric on extensive world-wide data measurements. usually, the data are collected over an extended period of time and then fitted with simple analytical functions. 5 impact of implementation manner in klobuchar model table 2. locations of geomagnetic north pole in geodetic coordinate based on igrf-13 including prediction adopted from [kyoto university, world data center for geomagnetism, kyoto]. year igrf-13 latitude longitude 1965 78.6n 69.9w 1970 78.7n 70.2w 1975 78.8n 70.5w 1980 78.9n 70.8w 1985 79.0n 70.9w 1990 79.2n 71.1w 1995 79.4n 71.4w 2000 79.6n 71.6w 2005 79.8n 71.8w 2010 80.1n 72.2w 2015 80.4n 72.6w 2016 80.4n 72.6w 2017 80.5n 72.6w 2018 80.5n 72.7w 2019 80.6n 72.7w 2020 80.7n 72.7w 2021 80.7n 72.7w 2022 80.7n 72.7w 2023 80.8n 72.7w 2024 80.8n 72.6w 2025 80.9n 72.6w • global ionospheric map (gim) and regional ionospheric maps are numerical maps, which provide values of ionospheric parameter in a grid. • analytical models are based on orthogonal function fits to the output that obtained from numerical models. • physical models are developed based on typically solution of the continuity equation or the momentum and energy equations for the electrons and ions. 4.1 empirical ionospheric models there are four types of empirical model: • bent model; • international reference ionospheric (iri) model; • nequick model; • klobuchar model. 4.1.1 bent model is an empirical, global model which was created in 1973 using soundings and satellite measurements taken from 1962 to 1969, a period which covered both a solar minimum and a solar maximum. bent model was developed for ground-to-satellite communications by using hundreds of parameters, this global model provides the tec in the altitude range from 150 – 2000 km as a function of position of the observer, time, solar flux and the sunspot number … etc., bent predicts that the model is 75–80% accurate [bent et al., 1973]. 4.1.2 iri model the first version of international reference ionospheric (iri) model was formulated by international union of radio science (ursi) and committee on space research (cospar) in 1978. with the development of techniques and the accumulation of observation data, iri model has been updated frequently. data sources of iri model include ionosonde, radars and several satellites and rockets. [bilitza et al., 2017]. in this model, ionospheric electron density profile can be derived easily after specifying date, time, location, sunspot number and some other parameters, then the tec and ionospheric delay can be calculated, worth mentioning, iri models suffer from lack of knowledge of the upper part of the ionosphere, since it is limited at 2000 km height. 4.1.3 nequick model nequick is a three-dimensional and time dependent ionospheric electron density model, taking advantage of the increasing amount of available data, there are many nequick versions aiming to update better representations of the ionosphere for the whole globe. [nava et al., 2008]. the nequick1 has been used by the european space agency (esa) european geostationary navigation overlay service (egnos) project and has been chosen to single-frequency positioning applications in galileo, it has also been adopted by the international telecommunication union, radio communication sector (itu-r) as an appropriate method for (tec) modeling. 5. klobuchar model basics this is the standard correction used by almost all gps receivers, which operate near earth, with fewer parameters. the klobuchar model is a simple ionospheric model that gives the vertical ionospheric delay at a given time and aghyas aljuneidi et al. 6 location for the gps single-frequency users in near real-time. it was developed in 1975 by john a. klobuchar at the air force geophysics laboratory, u.s. due to quite limited resources of computer microprocessor systems used in navigation receivers at that time, this system had to be extremely undemanding for such resources. klobuchar model is an ionospheric correction algorithm (ica). over many years of operation, this model demonstrated to be efficient. since the tec converted into correction to pseudo-range can vary from a few meters to tens of meters depending on solar activity, time, user location, satellite’s azimuth and elevation relative to user location and many others parameters [klobuchar, 1987]. 5.1 single layer ionospheric assumption ionospheric klobuchar model was designed based on the bent model. it is defined as a single layer ionospheric model (slm-single layer model), because the tec is assumed as concentrated in an infinitesimal layer placed at an average altitude of 350 km from the earth’s surface (i.e. maximum electron density at the f2 peak). in a slm the vtec (vertical value of tec) is calculated at the ionospheric pierce point (ipp) which is a geographic point, obtained by the intersection between the thin layer, (slm) and the line of sight between the satellite at ~22500 km height and the receiver (user) on earth surface (figure 1). where: • 𝜙�, 𝜆�: geodetic location of user. • 𝑍: the zenith angle at the user location. • 𝛽: the zenith angle at the ipp. • 𝜙�, 𝜆�: geodetic location of the ionospheric pierce point (ipp), it is worth to mention that the geomagnetic latitude of the same point is 𝜙�. • 𝐸: is equal to �/₂ ‒ 𝑍, the elevation of the gps satellite relative to user location, at the current moment,, local time (seconds) noticing that the azimuth well be 𝐴𝑧. • re: radius of the earth, 6378137.0m according to wgs84, worth to mention. • hs: altitude of slm, equal to 350 km. • 𝜓pp: earth-centered angle, equal to (𝑍 ‒ 𝛽), then 𝜓pp = 𝑍 ‒ 𝛽 = �/₂ ‒ 𝐸 ‒ 𝛽. (4) with a simple trigonometry 7 impact of implementation manner in klobuchar model figure 1. single layer ionospheric model adopted from [bolaji et al., 2016]. (5) (6) 5.2 ionosphere-mapping function all slant tec measurements are converted to an equivalent vertical value of tec, i.e. vtec by using the thin shell elevation mapping function because mapping to vertical is necessary if the radial electron density structure is not modeled [mannucci et al., 1993], thus for any arbitrary line of sight, stec (the slant angle) of the satellite must be taken into account: (7) as illustrated in figure 2. the gps klobuchar model uses an elevation mapping function, i.e. obliquity factor to convert vertical to slant delay at user level. (8) which leads to: (9) the simplified elevation mapping function [klobuchar, 1975] proposed by klobuchar in function of elevation e (semicircles) is: 𝐹 = 1 + 16. (0.53 + 𝐸)³ (10) 𝑠𝑖𝑛 𝛽 = � 𝑠𝑖𝑛 𝑍� = � 𝑐𝑜𝑠 𝐸��� �� + ���� �� + �� 𝜓pp = 𝑍 ‒ 𝛽 = �/₂ ‒ 𝐸 ‒ 𝑠𝑖𝑛⁻¹ � 𝑐𝑜𝑠 𝐸��� �� + �� 1 ����𝑆𝑇𝐸𝐶 = 𝑉𝑇𝐸𝐶 𝐹 = = 1 �������� ���� 𝑇���� = 40.3 * 𝐹 * ���� ��² aghyas aljuneidi et al. 8 figure 2. stec and vtec in single layer ionospheric model adopted from [janssen, 2012]. bear in mind that stec is equal to tec of equation (1). 9 impact of implementation manner in klobuchar model while according to [mannucci et al., 1993], the elevation mapping function, with a simple trigonometry, became: (11) 5.3 radius of the earth klobuchar articles [klobuchar, 1975; klobuchar,1987] do not state explicitly the length of the earth radius used in the approximated formulas. however, spherical geometry was applied in the mid 70s. we could not use wgs84 formula directly without justification, and the value of radius the earth could be inferred from the numerical equations provided by klobuchar himself. so, by analyzing [klobuchar, 1975] page 10 and comparing these two equations: where • 𝑍 is the zenith angle at the ipp; • 𝑒𝑙 is the elevation angle, • ℎ is the altitude of slm layer, which equal to 350 km, and • 𝑟₀ is the length of earth radius in klobuchar implementation. by solving the equation = 0.948 with taking in account the approximated value 0.948, we found 𝑟₀ = 6380.76 km which is greater than the equatorial radius = 6378.16 km and the polar radius = 6356.16 km and the main radius = 6371.03 [allen, 1976; flock, 1983], where all these previous values when applied to will give the same value, i.e. 0.948, thus, as a decision, we will update as a medium and accepted value of the main radius of the earth, 𝑅� = 6378137.0 m according to wgs84 in all formulas of this article regardless of latitude dependence (considering it as the mean radius of the spherical earth as klobuchar has done in its model of mid 70s). 5.4 model layout klobuchar model assumes that there is a constant delay of 5 ns during night time equal to 9.2 tecu i.e. 1.5 m range error at l1 frequency, and a cosine function (in the very basic variant, expanded in a taylor series in order to save the computing resources) in daytime that is centered at the 14th hour of local time. therefore, the maximum semi period for the cosine function is 20 hours, because the value at the end of the day must be the same at the beginning of the following day (i.e. 5 ns according to the model, figure 3). accordingly, the time delays 𝑇���� (in seconds) at observer’s location [klobuchar, 1975]: (12) where • dc (seconds) is the night-time constant offset term (set to 5 ns corresponding to 1.5 meters ionospheric delay on l1). • a is the amplitude term (seconds). • 𝜙 is the phase of the maximum vertical ionospheric delay which is assumed to be at 14:00hours local mean solar time (50400 seconds). • p is the period (seconds). • t is local time in ipp (seconds), where local time is equal to time zone (related to the longitude) + ut (the 𝑍 = 𝑠𝑖𝑛⁻¹ ( cos (𝑒𝑙)) and 𝑍 = 𝑠𝑖𝑛⁻¹(0.948cos (𝑒𝑙))�₀ �₀ � � 𝐹 = 1��1�� cos 𝐸�²��� �� + �� 𝑇���� = 𝐹�𝐷𝐶 + 𝐴𝑐𝑜𝑠( )��²�⁽�⁻�⁾� � universal time), as there are several versions of ut, [iers, 2018], the most commonly used being coordinated universal time (utc), gps time was zero at 0h 6-jan-1980 and since it is not perturbed by leap second, gps is now ahead of utc by 18 seconds, so according to klobuchar: thus hence 𝑡(𝑠𝑒𝑐𝑜𝑛𝑑) = 12 * 3600 * 𝜆��(semi circles) + 3600 * 𝑈𝑇(ℎ𝑜𝑢𝑟). (13) and • 𝑭 is the elevation mapping function. the a and p terms each have four time and solar-activity dependent coefficients to represent the behavior of the global ionosphere. to analytically represent the amplitude and period terms, a cubic polynomial in geomagnetic latitude is used to represent ionospheric delays which was the best available to model the whole world tec at the time of development. [klobuchar, 1975]. because the bent model did not cover the low latitudes appropriately, klobuchar did not adopt higher than the cubic polynomial, as might be required in the equatorial anomaly region, thus, it is correct to say that the klobuchar model is less accurate in the low-latitude regions that in the middle latitudes. the model is also less accurate in the highly variable polar regions. 5.5 broadcast ionospheric model the klobuchar model is the official ionospheric broadcast model (ibm) for single frequency gps receivers in real time, it is derived from the empirical knowledge of the long-term ionosphere behavior. it is designed for single frequency users in which the required model parameters are broadcast via the navigation message from the satellites, the gps ground control segment updates daily these coefficients according to season and solar activity level. 𝑡(ℎ𝑜𝑢𝑟) = + 𝑈𝑇(ℎ𝑜𝑢𝑟)���₍���₎ ¹⁵ 𝑡(ℎ𝑜𝑢𝑟) = 12 + 𝑈𝑇(ℎ𝑜𝑢𝑟) = 12𝜆��(semi circles) + 𝑈𝑇(ℎ𝑜𝑢𝑟).���₍���₎ ¹⁸⁰ aghyas aljuneidi et al. 10 figure 3. ideal time delay half cosine function in day time adopted from [sanz et al., 2013], simplified from [klobuchar, 1987]. 11 impact of implementation manner in klobuchar model a few broadcast parameters and coefficient are decoded from utc and iono blocks from page 18 sub frame 4 in almanac file, these eight coefficients are determined by a competent processing center in the usa and transmitted to gps satellites, after which they are translated to users as part of gps navigation messages, these parameters are: 𝛼₁, 𝛼₂, 𝛼₃, 𝛼₄, and 𝛽₁, 𝛽₂, 𝛽₃, 𝛽₄. as a rule, the coefficients are updated incomprehensibly once in a few days depending on the ionosphere variability, this makes the model intrinsically insensitive to short-term ionospheric variations (e.g. geomagnetic storms, interplanetary magnetic field effects, etc.). many efforts had been done in the aim to improve the consistency of klobuchar model taking into consideration that only eight coefficients are introduced to describe the global variation of the ionosphere due to the communication limitation in gps navigation message. accordingly, many studies which operate in post processing time attempted to re_estimate these eight parameters [yuan et al., 2008], or to extend the structure of klobuchar model (klobuchar-like model) [orus et al., 2002]. in this study we will take in consideration a set of improved klobuchar-style ionospheric, [igs, 2010], in purpose for comparison with the set of coefficients downloaded from almanac in the same date. while the chinese system, beidou, ionospheric model (bim) is based on the klobuchar model, it has eight parameters although it has some modifications, the difference between them is that bim is situated in terms of geographic coordinates while klobuchar is situated in terms of geomagnetic coordinates, another difference is bim uses the ionosphere height as 375 km, compared with klobuchar model which uses 350 km. [beidou navigation satellite system signal in space interface control document, 2016]. the nequick-g model is a specific implementation of the nequick model for its use in the galileo system for single-frequency ionospheric correction, [european gnss (galileo) open service, 2016], the model coefficients are broadcast in the navigation message since many years, three world-wide coefficients are to be broadcast to the user, these values are computed at the galileo system level, using a set of world-wide distributed monitoring stations to evaluate slant tec needed to determine the coefficients on a current day for use on the following day. the proposed glonass model, [ivanov, 2017], is a semi-empirical model of vertical distribution of the concentration of ionospheric free electrons, the control input parameters are the solar activity index 𝐹₁₀.₇ (the intensity of rf radiation of the sun at the wavelength of 10.7 cm), the geomagnetic activity index 𝐴�, and the correction parameter of electrons concentration at the peak of iono phere region 𝐹₂. the transmission of the first and the third parameters to users in the glonass navigation messages started in 2017-2018. although the index 𝐴� is available on public internet sites. merited to mention, the gemtec model, [gorbachev, 2015] which is another proposed broadcast ionosphere model overcomes the klobuchar model and requires only a single external input parameter 𝐹₁₀.₇. 5.6 the model accuracy the high variability in the ionosphere will prevent any model from being more than 75% accurate without the knowledge of the specific conditions [klobuchar, 1975]. klobuchar model assumes an ideal smooth comportment of the ionosphere, therefore any significant variations from day to day will not be modeled properly. the model can reduce the rms position error due to uncompensated ionospheric delay by about 50% to 60% depending on the solar activity or the region. therefore, its accuracy is often unsatisfactory even for the absolute positioning, especially in the case of the high solar activity [stepniak, 2014]. regardless of our concern in this study about the weakness of implementation of klobuchar model, the model’s accuracy is basically limited due to the following three reasons: • first, the thin layer model currently used in gps has deficiencies resulting from conversion of slant tec to effective vertical tec. the deficiencies come from inappropriate attribution of the thin shell height. this conversion introduces a few errors in the middle latitude where electron density is small. [norsuzila et al., 2008]. • second, a table of predefined values is used to update the eight coefficients in the gps system. these values are predefined for a particular condition in the solar cycle. they are not set to be updated every day. • third, the diurnal variation of the ionosphere is calculated using the eight parameters in the broadcast ephemeris, and setting the delay values at night as a constant, regardless of the latitude, makes the model fail to model the temporal changes, particularly at night. • fourth, the use of a mapping function is clearly an approximation, and can lead to errors of several tecu when applied to regions of large horizontal electron density gradients [klobuchar et al., 1993]. • sixth reason, seems to be an additive approximation, the value of the earth radius was fixed independently of latitude because klobuchar used the spherical model for the earth. 6. klobuchar model algorithm the klobuchar model is a compromise between computational complexity and accuracy, its limited number of inputs makes it an efficient and easy tool of many researches. the tec must be found at the ionospheric pierce point (ipp), since the tec is best ordered in geomagnetic, rather than geodetic coordinates, the assumption is that the tec is only a function of geomagnetic latitude and local time. any two points on the earth having the same geomagnetic latitude are assumed to have the same tec at the same local time. thus, a conversion to geomagnetic latitude is required, [klobuchar, 1975]. 6.1 implementation in this study we considered the coordinate of geomagnetic pole as an input because it is a varying with time. 6.1.1 inputs • 𝜙�₀, 𝜆�₀: geodetic location of geomagnetic north pole. • 𝜙�, 𝜆𝑢: geodetic location of user(rad). • 𝐸, 𝐴𝑧: elevation and azimuth of gps satellite relative to user location(rad), worth to mention that these values could be calculated from the , and the coordinates of satellite ,,. • 𝑡: local time in the ipp (seconds). • re, hs: radius of the earth, 6378137.0 m according to wgs84 and the altitude of slm, equal to 350 km. • 𝛼₁(second), 𝛼₂(second/semicircle), 𝛼₃�( )²�, 𝛼₄�( )³� and 𝛽₁(second), 𝛽₂�( )�, 𝛽₃�( )²�, 𝛽₄�( )²�: the eight parameters broadcast in the navigation message. 6.1.2 outputs • 𝑇���� the slant time delay (seconds), thus we can obtain both the range delay 𝑑����(𝑚) and correspond tec value. • 𝐷���� the range delay (meters). • the correspond tec value. 6.1.3 implementation steps in the days of klobuchar, the calculation complexity was an obstacle for implementation of klobuchar model equations, such several approximations are made in the geometric calculations to further reduce operational user computer storage and running time requirements, hence we cited in detail, with their units, both the approximate equations i.e. indexed by ₋���� and the standard equations without any approximation with no index i.e. which face no problem nowadays looking at the huge progress in the digital era. thus: aghyas aljuneidi et al. 12 13 impact of implementation manner in klobuchar model step1: the earth-centered angle (14) step2: geodetic latitude of ipp (15) as a condition imposed by klobuchar. if 𝜙�� is less than ‒0.416 rad let 𝜙�� = ‒0.416 (semicircles), and if 𝜙�� is more than 0.416 let 𝜙�� = 0.416 (semicircles). take into account that 0.416 = 75/180 which is identical for klobuchar assumption that its algorithm is suitable for only mid. latitude not greater than absolute (75) deg. [klobuchar, 1975], noteworthy for a specific satellite, the geodetic latitude of ipp varies proportionally with the geodetic latitude of user. step3: geodetic longitude of ipp (16) step4: geomagnetic latitude of ipp 𝜙� = 𝑠𝑖𝑛⁻¹�𝑠𝑖𝑛 𝜙�� 𝑠𝑖𝑛 𝜙�₀ + 𝑐𝑜𝑠𝜙�� 𝑐𝑜𝑠𝜙�₀ 𝑐𝑜𝑠(𝜆�� ‒ 𝜆�₀)� (17) with 𝜙�₀ and 𝜆�₀ are the latitude and longitude of geomagnetic north pole respectively, and 𝜙�_���� = 𝜙��_���� + 0.064𝑐𝑜𝑠 �𝜋(𝜆��_���� ‒ 1.617)� (18) notice that constants 0.064 and 1.617 are rounded to geomagnetic north pole coordinates in the original klobuchar algorithm 𝜙� = 78.3 and 𝜆�₀ = 291 , where: 𝑐𝑜𝑠 (𝜙�₀)/𝜋 = 0.0645, and 𝜆�₀ /𝜋 = 1.6167, thus we have: (19) from a mathematical viewpoint on the eq. (17), (19) and based on the igrf-13 secular variation, the slowly drifting value of geomagnetic north pole coordinates towards north and east, knowing that 𝜙�₀= 80.9n for the year 2025 from table 2. we conclude that the geomagnetic latitude of ipp will basically rely slowly with time on geodetic latitude of ipp, thus, for a specific satellite, also on the geodetic latitude of the user, hence in future, geomagnetic latitude of ipp will be greater on high latitude, based on this conclusion and according to sign of 𝛼₁,𝛼2,𝛼3,𝛼4 and 𝛽₁,𝛽2,𝛽3,𝛽4 the slant time delay on high latitude will be highly affected by this deduction. step5: amplitude of half cosine (20) with condition, if a < 0, let a = 0. 𝜓pp (𝑟𝑎𝑑) = �/₂ ‒ 𝐸 ‒ 𝑠𝑖𝑛⁻¹ � cos(𝐸)� 𝜓pp���� (semi circles) = � � ‒ 0.022 �� �� + ��� ₀.₀₁₃₇ ₀.₁₁+ �/� � 𝜙pp (𝑟𝑎𝑑) = 𝑠𝑖𝑛⁻¹ �𝑠𝑖𝑛(𝜙u)𝑐𝑜𝑠(𝜓pp) + 𝑐𝑜𝑠(𝜙u)𝑠𝑖𝑛(𝜓pp)𝑐𝑜𝑠(𝐴𝑧)� 𝜙pp₋���� (semi circles) = + 𝜓pp₋���� 𝑐𝑜𝑠(𝐴𝑧)�� � � 𝜆pp (𝑟𝑎𝑑) = 𝜆u + 𝑠𝑖𝑛⁻¹ �𝑠𝑖𝑛(𝜓pp) 𝑠𝑖𝑛(𝐴𝑧)/𝑐𝑜𝑠(𝜙u)� 𝜆pp₋���� (semi circles) = + 𝜓pp₋���� 𝑠𝑖𝑛(𝐴𝑧)/𝑐𝑜𝑠(𝜙u)�� � 𝜙�_���� = 𝜙��_���� + � � 𝑐𝑜𝑠 (𝜋𝜆������ ‒ 𝜆�₀) 𝑐𝑜𝑠 (𝜙�₀) 𝜋 � 𝐴(𝑠𝑒𝑐𝑜𝑛𝑑) = 𝛼₁ + 𝛼₂ + 𝛼₃ � �² + 𝛼₄ � �³ 𝐴���� (𝑠𝑒𝑐𝑜𝑛𝑑) = 𝛼₁ + 𝛼₂𝜙�_���� + 𝛼₃𝜙�_����² + 𝛼₄𝜙�_����³ �� ��� � �� � � ¹⁸⁰ � ¹⁸⁰ step6: period of half cosine (21) with condition if p < 72000 second, means less than 20h, let p = 72000. step7: the mapping function (22) step8: local time at ipp (23) with condition if t >86400 seconds, means more than 24h, let t = t-86400, and if t <0 let t = 86400. step9: compute the phase klobuchar, in his algorithm, considered that the peak of ionospheric delay occur at 14h local time, where 14h = 50400 second. (24) step10: the slant time delay the slant time delay is converted from vertical time delay when multiplying by the mapping function f. thus, if the phase x <𝜋/2 then it is the day time: (25) otherwise, it is the night time: (26) with 𝐷𝐶 =⁻⁹(second), the nighttime delay constant. step11: the delay range error (27) where 𝑐 = 299792458 𝑚⁄𝑠 𝑃(𝑠𝑒𝑐𝑜𝑛𝑑) = 𝛽₁ + 𝛽₂ + 𝛽₃ � �² + 𝛽₄ � �³ 𝑃���� (𝑠𝑒𝑐𝑜𝑛𝑑) = 𝛽₁ + 𝛽₂𝜙�_���� + 𝛽₃𝜙�_����² + 𝛽₄𝜙�_����³� �� ��� � �� � 𝐹���� = 1 + 16(0.53 + �/� )³� ��1�� 𝑐𝑜𝑠 𝐸�²� 1𝐹 = �� �� + �� 𝑡(𝑠𝑒𝑐𝑜𝑛𝑑) = 12 * 3600 * � � + 3600 * 𝑈𝑇(ℎ𝑜𝑢𝑟) 𝑡����(𝑠𝑒𝑐𝑜𝑛𝑑) = 12 * 3600 * �𝜆������� + 3600 * 𝑈𝑇(ℎ𝑜𝑢𝑟)� ��� � 𝑋(𝑟𝑎𝑑) = 2𝜋 � � 𝑋����(𝑟𝑎𝑑) = 2𝜋 � �� �₋₅₀₄₀₀ � �����₋₅₀₄₀₀ ����� 𝑇����(𝑠𝑒𝑐𝑜𝑛𝑑) = 𝐹�𝐷𝐶 + 𝐴𝑐𝑜𝑠(𝑋)� 𝑇����_����(𝑠𝑒𝑐𝑜𝑛𝑑) = 𝐹�����𝐷𝐶 + 𝐴����𝑐𝑜𝑠(𝑋����)�� 𝑇����(𝑠𝑒𝑐𝑜𝑛𝑑) = 𝐹.𝐷𝐶 𝑇��������(𝑠𝑒𝑐𝑜𝑛𝑑) = 𝐹����.𝐷𝐶� 𝑑����(𝑚) = 𝑐.𝑇���� 𝑑��������(𝑚) = 𝑐.𝑇��������� aghyas aljuneidi et al. 14 15 impact of implementation manner in klobuchar model step12: the tec in the ipp (28) 6.2 data acquisition the data acquisition is done on january 2010 from 0h to 24h (utc) at (udmc: latitude/longitude: 33°30′36′′n / 36°17′06′′e, with leica grx1200 receiver and ashtech antenna) near damascus, syria. the eight ionospheric coefficients downloaded from almanac are: 𝛼₁,₂,₃,₄ = [7.4506e-9, -1.4901e-8, -5.9605e-8, 1.1921e-7]; 𝛽₁,₂,₃,₄ = [9.0112e4, -6.5536e4, -1.3107e5, 4.5875e5]. in purpose of comparison, we downloaded a set of improved klobuchar-style ionospheric coefficients form code’s global ionosphere maps for day 001, 2010, as follows [igs, 2010]: 𝛼��������₁,₂,₃,₄ = [8.8647e-9, -1.4985e-8, -2.5446e-7, -2.9687e-7]; 𝛽��������₁,₂,₃,₄ = [9.1360e4, -6.8880e4, -1.235e6, -5.2002e6]. 6.2.1 satellite visibility the satellite visibility is related to many parameters, one of these parameters is the receiver quality, worth to mention that this study is oriented to such low cost or commercial single frequency gps receiver, with no coverage for augmentation systems, this kind of gps receivers is very widespread for civilian users around the third world. � 𝑆𝑇𝐸𝐶(𝑇𝐸𝐶𝑈) = 𝑆𝑇𝐸𝐶����(𝑇𝐸𝐶𝑈) = 𝑓²𝑑���� ⁴⁰.³𝑓²𝑑����_���� ⁴⁰.³ figure 4. visibility of satellites for a complete day january 1st 2010 at udmc. knowing that the constellation includes nominally a minimum of 24 operational satellites and a certain number of backup satellites are on orbit, making available up to 31 satellites for positioning, in figure 4 gps satellites orbit twice in a sidereal day in inertial space i.e. a gps satellite orbits once every 11 hours and 58 minutes, with the earth rotating once every 24 hours, such a gps satellite pass up over a point above the earth in average once a day, in figure 4. the visibility is very good with eight satellites at least in each moment while all satellites pass twice above udmc except for the satellites 12,13,18,20,21,22,23,24,29,30 and 32 which pass once only and the satellites 1 and 25 never do. 6.2.2 all satellites range error in aim to evaluate the effect of approximation adopted by [klobuchar, 1975], two implementations were written: • approx. implem.: trigonometry approximate implementation with approximate geomagnetic north pole coordinates adopted by klobuchar, i.e. 𝜙�₀ = 78.3n and 𝜆�₀ = 291e using equation (18) to calculate the geomagnetic latitude of ipp. • stand. implem.: standard implementation with geomagnetic north pole coordinates adopted by klobuchar, i.e. 𝜙�₀ = 78.3n and 𝜆�₀ = 291e using equation (17) to calculate the geomagnetic latitude of ipp. while in order to quantify the effect of geomagnetic north poles coordinates shift, two another standard implementation was adopted: • stand. implem_igrf.: standard implementation with geomagnetic north pole coordinates adopted by the international geomagnetic reference field for 2010, 𝜙�₀ = 80.1n deg, 𝜆�₀ = 287.8e deg. (from table 2.). • stand. implem_wmm.: standard implementation with geomagnetic north pole coordinates adopted by the world magnetic model [maus et al., 2010] for the year 2010: 𝜙�₀ = 80.02n deg, 𝜆�₀ = 287.79e deg. aghyas aljuneidi et al. 16 figure 5. ionospheric delay time (left axis in blue) & ionospheric delay range (right axis in green) for all satellites at udmc on 01/01/2010 with standard implementation of klobuchar model with geomagnetic north pole coordinates at 78.3n and 291e. 17 impact of implementation manner in klobuchar model clearly to get these figures (figures 5 and 6) and for a purpose of clarification we had to average the vertical tec values obtained from each of the satellites, while practically for navigation application, the ionospheric correction is applied to the individual pseudorange measurement. the available rinex files for this station near damascus, syria is dated on 01/01/2010, i.e., in mid. winter season which explain why the values of the ionospheric delay range and time are moderate because there no solar activity in that region for that time. from figures 5 and 6, the ionospheric delay range and time reach its peak at ~11.58h ut i.e. 14h local time because udmc is at longitude which means 2.42h in advance, and its minimum values for nighttime. let’s check the ionospheric delay range when using a set of eight improved ionospheric coefficients from [igs, 2010] (figure 7). worth mentioning the remarkable difference between the two ranges, delay range with improved ionospheric coefficients does not act as the ideal range delay half cosine function proposed by [klobuchar, 1975] and there is an approach between day values and night value which is logical because these measurements are done in mid-winter on 01/01/2010. from figure 8 we noticed that there is a small difference in order of centimeters in the values of the ionospheric delay range difference between standard implementation and the approximation. while for the coordinates adopted by igrf (table 2.) for the year 2010: 𝜙�₀ = 80.1n deg, 𝜆�₀ = 287.1e deg. while for the coordinates adopted by world magnetic model [maus et al., 2010] for the year 2010: ‒𝜙�₀ = 80.02n deg, 𝜆�₀ = 287.79e deg. worth to note, figures 9 and 10. that there is also a small difference in order of centimeters in the values of the ionospheric delay range between standard implementation when we adopt the geomagnetic north pole coordinates whether, igrf for 2010 or wwm for the 2010 or the counterpart’s values used by klobuchar in 1975 and the behavior in both case igrf and wmm is very close to each other. merited to mention, the correction for the geomagnetic pole has a geophysical meaning, that might not translate directly into an operational application, because these algorithms are not known in their current implementation. figure 6. fitting curve of 10th degree of ionospheric delay range for all satellites at udmc on 01/01/2010 with standard implementation of klobuchar model with geomagnetic north pole coordinates at 78.3n and 291e. aghyas aljuneidi et al. 18 figure 7. ionospheric delay range (left axis in blue) with ionospheric coefficients downloaded from almanc and with those improved ionospheric coefficients (right axis in green) downloaded from [igs, 2010] for all satellites at udmc on 01/01/2010 with standard implementation of klobuchar model with geomagnetic north pole coordinates at 78.3n and 291e. figure 8. difference of ionospheric delay range for all satellites at udmc on 01/01/2010 between standard and approximation implementation of klobuchar model with geomagnetic north pole coordinates at 78.3n and 291e. 19 impact of implementation manner in klobuchar model figure 9. difference of ionospheric range delay in standard implementation of klobuchar model between klobuchar adopted values and igrf for 2010 of geomagnetic north pole coordinates. figure 10. difference of ionospheric range delay in standard implementation of klobuchar model between klobuchar adopted values and wmm for 2010 of geomagnetic north pole coordinates. 6.2.3 single point range error the inputs were: prn=18. 𝜙𝑝₀ = 78.3n deg, 𝜆𝑝₀ =291e deg. 𝜙� = 33.5102n deg, 𝜆� =36.2852e deg. 𝐸 = 45.032 deg. 𝐴𝑧 = 43.187 deg. 𝑈𝑇 = 11:21:30. 𝑅� = 6378137.0m. ℎ� = 350 km. table 3. standard and approximate implementation of klobuchar model. while for the coordinates adopted by igrf for the year 2010: ‒𝜙𝑝₀ = 80.1n deg, 𝜆𝑝₀ =287.8e deg, and for the coordinates adopted by [maus et al., 2010] for the year 2010: ‒𝜙𝑝₀ = 80.02n deg, 𝜆𝑝₀ =287.79e deg. table 4. standard implementation of klobuchar model using igrf and wmm north pole coordinates for 2010. in table 3 and 4 the results in single point are compatible with previous figures, i.e. there is a difference in order of centimeters between standard implementation and the other implementations. aghyas aljuneidi et al. 20 output unit stand. implem igrf. stand. implem1. wwm 𝜓𝑝𝑝 deg 2.9049 2.9049𝜙𝑝𝑝 deg 35.6044 35.6044𝜆𝑝𝑝 deg 38.7304 38.7304𝜙𝑚 deg 31.5726 31.5348𝐹 1.3470 1.3470𝑇𝑖𝑜𝑛𝑜 sec 11.646e-09 11.653e-09 range delay m 3.4913 3.4934 vtec tecu 21.5 21.58 output unit stand. implem. approx. implem. 𝜓𝑝𝑝 deg 2.9049 2.8866𝜙𝑝𝑝 deg 35.6044 35.6149𝜆𝑝𝑝 deg 38.7304 38.7153𝜙𝑚 deg 31.3240 32.1210𝐹 1.347 1.3506𝑇𝑖𝑜𝑛𝑜 sec 11.692e-09 11.575e-09 range delay m 3.5051 3.4700 vtec tecu 21.58 21.27 21 impact of implementation manner in klobuchar model 6.3 analytical study in this section, we’ll formulate analytically the impact of imperfect implementation of the coordinate of the geomagnetic north pole in the standard equations of klobuchar model. let 𝐼(𝜙�, 𝜆�) the vertical time delay, by using taylor expansion at the coordinate of geomagnetic north pole (𝜙�, 𝜆�) [guo et al., 2011], we get: (29) where 𝐴₁ is constant. define (𝜙�₁, 𝜆�₁) = (𝜙�₀ + 𝛥𝜙�, 𝜆�₀ + 𝛥𝜆�) thus first order taylor series expansion of the function can be obtained as follows: (30) such for the second term of right side of (30), let: (31) thus (31) becomes (32) let’s start by the first term of (32), we have: (33) let 𝑓 = 𝑠𝑖𝑛 (𝜙�(𝜙�₀, 𝜆�₀)), thus (34) if 𝐼������₀ = 𝜙�(𝜙�₀, 𝜆�₀), such we got: (35) = �𝐴(𝜙�₀, 𝜆�₀) · 𝑐𝑜𝑠 � = 𝑐𝑜𝑠 · 𝐴(𝜙�₀, 𝜆�₀) + 𝐴(𝜙�₀, 𝜆�₀) · �𝑐𝑜𝑠 �. 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝜕𝛪(𝜙�₀, 𝜆�₀) 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ let 𝐼�����₀ = 𝐴(𝜙�₀, 𝜆�₀), and 𝐼�������₀ = 𝑐𝑜𝑠 𝜕 𝜕𝜙�₀𝜕 𝜕𝜙�₀ 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝐼�����₀ · 𝑐𝑜𝑠� � + 𝐼�������₀ 𝐴(𝜙�₀, 𝜆�₀).𝑇 𝑃(𝜙�₀, 𝜆�₀)𝜕𝛪(𝜙�₀, 𝜆�₀) 𝜕𝜙�₀ 𝐼�����₀= 𝐴(𝜙�₀, 𝜆�₀) = �𝛼₁ + 𝛼₂ + 𝛼₃ � �² + 𝛼₄ � �³� = 𝜙� (𝜙�₀, 𝜆�₀) + 𝜙� (𝜙�₀, 𝜆�₀) 𝜙� (𝜙�₀, 𝜆�₀) + 𝜙²� (𝜙�₀, 𝜆�₀) 𝜙� (𝜙�₀, 𝜆�₀). 𝜙�(𝜙�₀, 𝜆�₀) 𝜋 𝜙�(𝜙�₀, 𝜆�₀) 𝜋 𝜙�(𝜙�₀, 𝜆�₀) 𝜋𝜕 𝜕𝜙�₀𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀𝜕 𝜕𝜙�₀ 2𝛼₃ 𝜋²𝛼₂ 𝜋3𝛼₄ 𝜋³ 𝜙�(𝜙�₀, 𝜆�₀) = (𝑠𝑖𝑛⁻¹ �𝑠𝑖𝑛𝜙𝐼pp 𝑠𝑖𝑛𝜙�₀ + 𝑐𝑜𝑠𝜙𝐼pp 𝑐𝑜𝑠𝜙�₀𝑐𝑜𝑠�𝜆𝐼pp ‒𝜆�₀��) = · �𝑠𝑖𝑛𝜙𝐼pp 𝑠𝑖𝑛𝜙�₀ + 𝑐𝑜𝑠𝜙𝐼pp 𝑐𝑜𝑠𝜙�₀𝑐𝑜𝑠�𝜆𝐼pp ‒𝜆�₀�� = · �𝑠𝑖𝑛𝜙𝐼pp 𝑐𝑜𝑠𝜙�₀ ‒ 𝑐𝑜𝑠𝜙𝐼pp 𝑠𝑖𝑛𝜙�₀ 𝑐𝑜𝑠�𝜆𝐼pp ‒𝜆�₀��. 𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ 1 √1‒𝑓²1 √1‒𝑓² 𝐼�����₀= 𝐼������₀+ 𝜙� (𝜙�₀, 𝜆�₀)𝐼������₀ + 𝜙²� (𝜙�₀, 𝜆�₀)𝐼������₀.2𝛼₃ 𝜋² 3𝛼₄ 𝜋³𝛼₂ 𝜋 𝐼(𝜙�₁, 𝜆�₁) = 𝐼(𝜙�₀ , 𝜆�₀) + 𝛥𝜙� + 𝛥𝜆�.𝜕𝛪(𝜙�₀, 𝜆�₀) 𝜕𝜙�₀ 𝜕𝛪(𝜙�₀, 𝜆�₀) 𝜕𝜙�₀ 𝐼(𝜙�, 𝜆�) = 𝐴₁ + 𝐴(𝜙�, 𝜆�) 𝑐𝑜𝑠� �𝑇 𝑃(𝜙�, 𝜆�) 𝜕 𝜕𝜙�₀ while for the second term of (32), we have: but where thus we get: (36) the same altitude for the third term of the right side in (30), it can be written as: (37) where (38) and (39) and (40) finally, the equation (30) became (41) as follows: (41) finally, we formulate the deviation in vertical time delay 𝛥𝐼(𝜙�₁, 𝜆�₁) as a function of the shift of geomagnetic north pole coordinates 𝛥𝜙� and 𝛥𝜆�. 𝐼�������₀= 𝑇 � �²· 𝑠𝑖𝑛 � � ·1 𝑃(𝜙�₀, 𝜆�₀) 𝑇 𝑃(𝜙�₀, 𝜆�₀) � 𝐼������₀+ 𝜙� (𝜙�₀, 𝜆�₀)𝐼������₀+ 𝜙²� (𝜙�₀, 𝜆�₀)𝐼������₀.�𝛽₂ 𝜋 2𝛽₂ 𝜋² 3𝛽₄ 𝜋³ = 𝐼�����₀ · 𝑐𝑜𝑠 + 𝐼�������₀ · 𝐴(𝜙�₀, 𝜆�₀)𝜕𝐼(𝜙�₀, 𝜆�₀) 𝜕𝜆�₀ 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝐼�����₀= 𝐼������₀+ 𝜙� (𝜙�₀, 𝜆�₀)𝐼������₀+ 𝜙²� (𝜙�₀, 𝜆�₀)𝐼������₀.2𝛼₃ 𝜋² 3𝛼₄ 𝜋³𝛼₂ 𝜋 𝐼������₀ = · (𝑐𝑜𝑠𝜙𝐼pp 𝑐𝑜𝑠𝜙�₀𝑠𝑖𝑛(𝜆𝐼pp ‒𝜆�₀ ))1 √1‒𝑓² 𝐼�������₀= 𝑇 � �²· 𝑠𝑖𝑛 � � ·1 𝑃(𝜙�₀, 𝜆�₀) 𝑇 𝑃(𝜙�₀, 𝜆�₀) � 𝐼������₀+ 𝜙� (𝜙�₀, 𝜆�₀)𝐼������₀+ 𝜙²� (𝜙�₀, 𝜆�₀)𝐼������₀�𝛽₂ 𝜋 2𝛽₃ 𝜋² 3𝛽₄ 𝜋³ 𝐼�������₀= 𝑐𝑜𝑠� � = 𝑐𝑜𝑠 = 𝑠𝑖𝑛� �. � � 𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝑇 𝑃(𝜙�₀, 𝜆�₀)𝑇 �₁��₂𝜙�(𝜙�₀, 𝜆�₀)��₃𝜙²� (𝜙�₀, 𝜆�₀)��₄𝜙³� (𝜙�₀, 𝜆�₀)𝑇 �₁��₂𝜙�(𝜙�₀, 𝜆�₀)��₃𝜙²� (𝜙�₀, 𝜆�₀)��₄𝜙³� (𝜙�₀, 𝜆�₀)𝜕 𝜕𝜙�₀ � � = ‒ 𝑇 � �² · ��₁��₂𝜙�(𝜙�₀, 𝜆�₀)��₃𝜙²� (𝜙�₀, 𝜆�₀)��₄𝜙³� (𝜙�₀, 𝜆�₀)�. 𝑇 �₁��₂𝜙�(𝜙�₀, 𝜆�₀)��₃𝜙²� (𝜙�₀, 𝜆�₀)��₄𝜙³� (𝜙�₀, 𝜆�₀)1 �₁��₂𝜙�(𝜙�₀, 𝜆�₀)��₃𝜙²� (𝜙�₀, 𝜆�₀)��₄𝜙³� (𝜙�₀, 𝜆�₀) 𝜕 𝜕𝜙�₀ 𝜕 𝜕𝜙�₀ �𝛽₁ + 𝛽₂ + 𝛽₃ � �² + 𝛽₄ � �³� = 𝐼������₀+ 𝜙� (𝜙�₀, 𝜆�₀)𝐼������₀+ 𝜙²� (𝜙�₀, 𝜆�₀)𝐼������₀. 𝜕 𝜕𝜙�₀ 𝜙�(𝜙�₀, 𝜆�₀) � 𝜙�(𝜙�₀, 𝜆�₀) � 𝜙�(𝜙�₀, 𝜆�₀) �𝛽₂ 𝜋 2𝛽₃ 𝜋² 3𝛽₄ 𝜋³ 𝐼(𝜙�₁, 𝜆�₁) ‒ 𝐼(𝜙�₀, 𝜆�₀) = 𝛥𝐼(𝜙�, 𝜆�) = � � � �.𝐼�����₀· 𝑐𝑜𝑠 𝐼�����₀· 𝑐𝑜𝑠 𝐼�������₀ .𝐴 (𝜙�₀, 𝜆�₀) 𝐼�������₀ .𝐴 (𝜙�₀, 𝜆�₀) 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝑇 𝑃(𝜙�₀, 𝜆�₀) 𝛥𝜙� 𝛥𝜆� aghyas aljuneidi et al. 22 23 impact of implementation manner in klobuchar model bear in mind that one can use (41) to estimate the geomagnetic north pole coordinates when having an accurate ionospheric time delay for example with multi-frequency technique as mentioned in [guo et al., 2011]. figure 11. ionospheric delay range in standard implementation of klobuchar model with geomagnetic north pole coordinates at 78.3n and 291e and igrf-2010 with values: 𝛥𝜙� = 1.8° and 𝛥𝜆� = 3.2°. figure 12. ionospheric delay range in standard implementation of klobuchar model with geomagnetic north pole coordinates of igrf-2010 and wmm-2010 with values: 𝛥𝜙� = 0.08° and 𝛥𝜆� = 0.01°. to obtain the slant time delay, 𝛥𝑇𝑖𝑜𝑛 ,we have: 𝛥𝑇𝑖𝑜𝑛(𝜙�₁, 𝜆�₁) = 𝐹.𝛥𝐼(𝜙�₁, 𝜆�₁) (42) and slant range delay became: 𝛥𝐷𝑖𝑜𝑛(𝜙�₁, 𝜆�₁) = 𝑐.𝐹.𝛥𝐼(𝜙�₁, 𝜆�₁) (43) where 𝐹 is the elevation mapping function. in the same way, these results are compatible with the previous ones (figures 11 and 12). 7. future work by force of circumstances, this article suffers from the lack of comparison between ionospheric experimental data and the various implementations of klobuchar model. this would require a statistical study on a larger number of days and possibly using stations at various geomagnetic latitudes, comparing with experimental tec would allow to understand if the proposed corrections are participating to improve the model predictions, or if they are below its intrinsic noise level. 8. results and discussion numerical applications were done in aim to estimate the impact of approximate implementation of klobuchar model, and analytical studies were performed to evaluate the impact of the non-exact values of geomagnetic north pole coordinates, we did many comparisons in these two aspects: • the approximate implementation where the used geomagnetic north pole coordinates are those of the original klobuchar model, i.e., 𝜆�₀ = 291e and 𝜙�₀ = 78.3n. • the uncertainty in the exact geomagnetic north pole coordinates in klobuchar model, where the used geomagnetic north pole coordinates are those of igrf and wwm models for the year 2010. for the first aspect, from table 3 and figure 5 and 6, we can conclude that the difference between the standard and approximate implementation of klobuchar model is in general in the level of centimeters and the most value affected by this implementation is 𝜙� the geomagnetic latitude of ipp because it is directly calculated as a function of geomagnetic north pole coordinates where the biggest approximate was done. generally, an order of few centimeters’ impact is critical for precise point positioning (ppp) systems, which is not the case for the receivers covered by this study, even though we supposed that nowadays only civil users continue to use a single frequency gps receiver with no augmentation systems coverage, although this type of low cost receiver plays an essential role in some countries of the third world for application like civil aviation and generally the transport domain, bear in mind that the other constellations like european galileo or the chinese beidou offer a little improvements over the performance of gps concerning the ionospheric delay corrections. for the second aspect, according to figures 8, 9 and 10, we found immeasurably value when comparing igrf with wmm implementations, while there is a significant difference between them and the standard implementation that uses geomagnetic north pole coordinates adopted by klobuchar in his days. 9. conclusions the approximation of the klobuchar model is justified due to the deficiencies in digital technique in the 70s, as well as the global knowledge in geomagnetism. in this brief study, a level of centimeters difference was estimated to be the impact of such approximation, which affects the precise positioning systems. obviously it isn’t the case of the class of devices concerned by this paper. aghyas aljuneidi et al. 24 25 impact of implementation manner in klobuchar model regardless of the type and end user of gps receivers which relay only on klobuchar model to correct the ionospheric range error, this study indicates and affirms the usefulness of klobuchar model as an efficient method for correcting the ionospheric effects till nowadays for a large group of users around the world. the weak point of this formulation is using very few parameters (eight) that included in the broadcast ephemeris for the ionospheric corrections, which is not enough to describe the worldwide behavior of the earth’s ionosphere. 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(2008). refining the klobuchar ionospheric coefficients based on gps observations, ieee trans. aerosp. electron. syst., 44, 4, 1498_1510. *co r r e s po n d i n g a u t h o r: aghyas aljuneidi, higher institute for applied sciences and technology (hiast), damascus, syria, e-mail: aghyas.aljuneidi@hiast.edu.sy © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international aghyas aljuneidi et al. 26 http://dx.doi.org/10.1016/j.asr.2016.01.010 annals of geophysics, 55 (3), 2012: georgiadis and lagorio the communication of science as an ethical issue: the case of raffaele bendandi teodoro georgiadis1,*, paola pescerelli lagorio2 1 istituto di biometeorologia, consiglio nazionale delle ricerche (cnr-ibimet), bologna, italy 2 museo casa bendandi, istituto culturale la bendandiana, faenza, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5561 abstract in a ever more connected world, whether justified or not, the possibility that scientific information can be used to produce generalized behaviors by populations results in the need to better understand the processes of science communication. consequently, it raises serious questions about the ethical message of the communication itself, and the way in which scientists can interface with people with no scientific training. this article analyses the case of the supposed prediction of the well-known geophysicist raffaele bendandi and the earthquake of rome predicted for may 2011 that never happened. 1. overview about the communication of science the communication of science often involves professional scientists and generally refers to public media, whereby the aim (of the scientist) is to talk about science with nonscientists. science communication is important because some information is directly applicable to the ‘real’ everyday life of populations, such as in the case of the geosciences, which are strongly related to the environment and the territory. these issues can be critical in the face of scientific misinformation, and how much easier it might be to spread false theories and information, with the aim to simply scare populations or to convince the people to do something. thus, communication of science is becoming of paramount importance also on the political agenda, to inform populations about policies and to obtain the general consensus that might be needed to carry out such policies. one of the main problems in the communication of science relates to the concept itself of ‘science’ in the mind of the scientist and in the mind of an average individual. in the former case, ‘science’ is the amount of knowledge in a certain field of expertise, which is always related to the level of uncertainty: what we know and what we do not know are two indissoluble aspects of science. the expectations of a nonscientist about science, as a member of the general public, are mostly completely different. in the modern hedonistic world, it is assumed that science can provide certitude, instead of uncertainty. moreover, science is considered as a way to predict the future. obviously, these different interpretations of the crucial word ‘science’ give rise to two different mentalities, as if they were two different languages. the communication of science is a sensible procedure in which the world seen by the scientist should be transferred to the population, to furnish suitable information so that the population can make responsible decisions. sometimes the ‘cultural mediation’ in the different steps of the communication process can introduce misinterpretation, or even contamination of the original message and knowledge. the content is therefore important not only in terms of the knowledge, the certainty and the uncertainty, but also in the way that this knowledge is communicated, as the scientific community has some habits that cannot be considered to be particularly clear and unequivocal to the ‘common’ people. as an example reported by the weekly scientific journal nature: “at the end of may 2011, six italian seismologists and one government official will be tried for the manslaughter of those who died in the earthquake that struck the city of l’aquila on april 6, 2009. during the hearing, the prosecutor called the committee’s risk assessment ‘superficial and generic’, resulting in ‘incomplete, imprecise and contradictory public information’. the prosecutor acknowledged that the committee members had no way of predicting the earthquake, but he accused them of translating their scientific uncertainty into an overlying optimistic message” [nosengo 2011]. the earth sciences appear in the proposed framework to be a crucial cross between science and the population, where the communication itself represents a ‘policy’, and where the knowledge can dispose or propose article history received january 13, 2012; accepted march 21, 2012. subject classification: bendandi, rome earthquake rumors, communication of science. 409 a direction in which to go, or ‘what to do’ in the presence of a strong disparity of resources (knowledge and information). democracy is not only a political methodology dealing with participation, but also an awareness, to be able to freely take informed decisions. thus, the communication of science, and especially the geosciences, is an issue with a strong ethical content. furthermore, nowadays, the internet allows easy access to information to a huge proportion of the population. the possibility to choose between ‘true’ and ‘false’ theories becomes statistically equivalent, and the danger we have to face is that the ‘most appealing’ explanation can become the most quoted. we should not be over confident of the self-compelling evidence of science, nor of the general scientific consensus of a theory. the personal perception of a threat is variable, and it is regulated by many factors, many of which are emotionally driven. in the case of threats to life, behaviors can lead to certainties being dismissed if they are not properly expressed, in favor of potentially spurious, nondocumented, opinions. this happened recently in italy, where the population of the city of rome underwent generalized panic produced by the prediction that rome was to be hit by a destructive earthquake on may 11, 2011. the forecast was attributed to the studies of raffaele bendandi, who had indicated the date in his famous graphs related to his tidal theory of earthquakes. the internet reports that an important italian consumer organization lodged a complaint with the rome prosecutors concerning the impact of the alarm, while adding that figures showed that 20% of the working population of rome, some 600,000 people, had taken that day off. research showed that along with this 20% who had not gone into work, places outside the city, in the countryside and at the beach, were packed with people. this episode represents a peculiar ‘case’ in which all the ingredients of societal threat perception and spurious information formed a potentially dangerous recipe in the complete absence of any kind of ‘reasonable’ danger. 2. the ‘case’ of raffaele bendandi raffaele bendandi was born in faenza on october 17, 1893, to a modest family of workers. their scarce financial resources did not allow him to pursue his education, and so he only completed primary school. he worked in the workshop of a watchmaker and at an engraver’s. the manual experience that he acquired and an innate artistic sense subsequently helped him to make precision instruments and graphs to support his theories (for a complete biography, see pescerelli lagorio [2011]). after the catastrophic earthquake of messina and reggio calabria that occurred on december 28, 1908, bendandi was so emotionally affected by the death and devastation that he decided to devote his life to the studies of geophysics, to develop a theory of earthquakes with which he would be able to make forecasts, to protect and save human beings. over time, he developed his own theories about the nature of earthquakes, which he called ‘seismogenics’. although unsupported by conventional scientific evidence, he believed that earthquakes are caused by planetary alignment in the solar system: that the moon, the sun, and the other planets have gravitational influences on the movements of the earth’s crust. in 1931, he published un principio fondamentale dell’universo (a fundamental principle of the universe), in which he claimed a relationship between earthquakes and sunspot activity. bendandi became famous, not only in italy, but worldwide, for the huge number of his predictions, which started from the one related to the earthquake of january 13, 1915, that killed 30,000 people in avezzano, italy. in the istituto culturale ‘la bendandiana’ (the bendani cultural institute) archives, it is possible to see how bendandi’s forecasts were reported by a large numbers of journals and newspapers, such as the “chicago tribune”, the “new york sun”, “la prensa”, and “le matin”, over a wide period of time. it must be highlighted that an exhaustive ranking of whether bendandi’s forecasts were true or false has not yet been performed; thus, the authors refer only to reported claims by georgiadis and pescerelli lagorio 410 figure 1. raffaele bendandi at the window of his observatory in faenza (from the casa bendandi archives). 411 newspapers and not to actual occurrence of earthquakes. the bendandi phenomenon ended up by being considered a problem for official science, causing prof. agamennone, one of the most considered personalities of that time in the geophysical sciences and director of the rocca di papa geodynamic observatory, to include bendandi in a sort of damnatio memoriae list, and as reported by the newspapers, to suggest to benito mussolini to officially not trust bendandi, and to forbid him from making further forecasts. bendandi never published a precise account of his forecasting methods, which remain largely unknown today, although he clearly related them to the tidal cycle. this lack of knowledge leaves room for a great deal of speculation and some scientists or ‘amateurs’ claim to be the depositary of the ‘real’ methodology developed by bendandi. some of bendandi’s forecasts corresponded to real occurrences of earthquakes, like for the earthquake of may 6, 1976, in friuli, italy (one of the last forecasts of bendandi), and this evidence reinforced the belief in the accuracy of his entire body of predictions. even if many papers reported a remarkable level of confidence in his forecasts, sometimes using a ‘cherry-picking’ approach, no attempt was made to prepare a catalog that reported complete validation. the only systematic approach to the bendandi methodology and forecasting is being carried out by lagorio and ballabene, who are preparing some new software called cruscotto bendandiano (the bendandi control panel) that will be able to reproduce the astronomical configurations of importance for tidal effects. this follows the theory of bendandi’s book [1931], and will be compared with the historical archives maintained by la bendandiana and with the archives of the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geology and vulcanology). it is not certain whether the bendandi archives are complete, for various reasons, amongst which there is their natural dispersion over long periods of time. the archives were also taken over by the municipality only some time after the death of bendandi, and consequently this does not assure the integrity of the archive itself. as far as the bendandi forecasts are concerned, the study of their correspondence is complicated by the fact that the letters are mostly handwritten, which does not render their content instantly intelligible, and which makes their interpretation a relatively difficult job [pescerelli lagorio 2009]. obviously, along with the rumors around bendandi’s death, which happened on november 1, 1979, in his home, these events have contributed to an increase in the cloud of mystery around bendandi and the findings of his studies. 3. the rome earthquake of may 2011 that never happened in january 2011, leaflets with a false claim were distributed around the ciampino municipality (rome), which stated that bendandi had forecast an earthquake on may 11, 2011, and that residents should leave their homes before that day. rumors continued to spread, in particular through social networking sites, and these specified that the earthquake would affect rome. in may 2011, it was reported that people were leaving rome in readiness for the earthquake that was predicted to occur on may 11. the existence of a prediction for that date was denied by la bendandiana, the custodian of the bendandi archives, and the ingv held an open day at its rome institute on that day, may 11, 2011. there was no earthquake in rome that day. nevertheless, some media reports drew attention to an earthquake that hit the town of lorca in spain on the same day, although it is more than 1,000 km away from rome. the ingv stated, “there is absolutely no link between spain and italy, geologically, or with the prediction of an earthquake in rome”. the “corriere della sera” newspaper splashed bendandi on its front page, naming him as, “the man who predicts earthquakes”. among the writings bendandi left after his death, the urban legend attributes some predictions for the future: these were considered so dangerous that someone attempted to burn them, although fragments remain intact and contain two dates: 2011 and 2012. however, the two dates were not accompanied by any geographical locations for the predicted earthquakes. the strangest thing about the whole of this fear of an earthquake that would strike rome on may 11, 2011, is that there is no documentation that bendandi ever made the communication of science as an ethical issue figure 2. the writings of raffaele bendandi that refer to the year 2011 (from the casa bendandi archives). such a prediction. the only thing that supports a supposed prediction of bendandi for that date was the beautiful alignment of the planets in the sky during that month, with may 10-12, 2011, being of especial note (http://media.sky andtelescope.com/video/planet-animation-may2011.mov). 4. conclusions after all of this, it is possible to draw some conclusions. first, it is clear that not a single document in the bendandi archive reports any earthquake in rome. the communi-cation about the falsity of this supposition was absolutely ineffective, even when given directly to the bulk of the people in the social networks. in addition, as a throwback, some people assumed that the claim was true just because ‘official voices’ stated that the information about the earthquake was false. official scientific institutions approached the claim by devoting the day to an open day, for the population to communicate with real science, and to furnish a clear picture that nothing dangerous was likely to happen. the people participating in this ‘open-house’ responded positively. even reassured by official institutions, the huge number of people who left rome represents a parameter that needs to be taken into account should a real potential threat need to be managed, a possibility that could raise again some ethical questions. references bendandi, r. (1931). un principio fondamentale dell’universo, bagnacavallo (ra), società tipografica editrice, 322 pp. nosengo, n. (2011). scientists on trial over l’aquila deaths, nature, 474, 15. pescerelli lagorio, p. (2009). the epistolary archive of raffaele bendandi, a seismologist standing outside the institutions, annals of geophysics, 52 (6), 651-656. pescerelli lagorio, p. (2011). raffaele bendandi – ombre sul sole, faenza, edit faenza, 203 pp. *corresponding author: teodoro georgiadis, istituto di biometeorologia, consiglio nazionale delle ricerche (cnr-ibimet), bologna, italy; email: t.georgiadis@ibimet.cnr.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. georgiadis and pescerelli lagorio 412 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice articolo hessami.pdf key words active tectonics – paleoseismology – iran tabriz – earthquake 1. introduction nw iran is a region of intense deformation and seismicity situated between two thrust belts of the caucasus to the north and the zagros mountains to the south (fig. 1). earthquake focal mechanisms suggest that the convergence between arabia and eurasia has been accommodated mainly through wnw-trending right-lateral strike-slip faults in this region (fig. 1; jackson, 1992). these strike-slip faults appear to be the southeastern continuation into nw iran of the north anatolian fault and other right-lateral faults in se turkey (westaway, 1990, 1994; jackson, 1992). however, right-lateral faulting in the se turkey-nw iran region is not continuous but consists of several discontinuous fault segments (figs. 1 and 2a). three of these segments ruptured during earthquakes in 1930, 1966 and 1976 (mckenzie, 1972; toksöz et al., 1977; jackson and mckenzie, 1984; westaway, 1990; jackson, 1992). the north tabriz fault segment, however, was seismically inactive during the last two centuries. among the many historical earthquakes that have occurred in the tabriz region (e.g., the 858, 1042, 1273, 1304, 1550, 1641, 1717, 1721, 1780 and 1786 earthquakes), the destructive earthquakes of 1042 903 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. khaled hessami, international institute of earthquake engineering and seismology (iiees), p.o. box 19395/3913, tehran, iran; e-mail: khaled@dena.iiees.ac.ir paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results khaled hessami (1), daniela pantosti (2), hadi tabassi (1) (3), esmael shabanian (1), mohammad r. abbassi (1), khalil feghhi (1) and shahryar solaymani (1) (1) international institute of earthquake engineering and seismology (iiees), tehran, iran (2) istituto nazionale di geofisica e vulcanologia, sezione di roma 1 sismologia e tettonofisica, roma, italy (3) department of geology, islamic azad university, ashtian, tehran, iran abstract the north tabriz fault is a major seismogenic fault in nw iran. the last damaging earthquakes on this fault occurred in 1721, rupturing the southeastern fault segment, and in 1780, rupturing the northwestern one. the understanding of the seismic behavior of this fault is critical for assessing the hazard in tabriz, one of the major cities of iran; the city suffered major damage in both the 1721 and 1780 events. our study area is located on the northwestern fault segment, west of the city of tabriz. we performed geomorphic and trenching investigations, which allowed us to recognize evidence for repeated faulting events since the late pleistocene. from the trenches, we found evidence for at least four events during the past 3.6 ka, the most recent one being the 1780 earthquake. on the basis of different approaches, horizontal slip per event and slip rates are found in the ranges of 4 ± 0.5 m and 3.1-6.4 mm/yr, respectively. we also attempted an estimate of the average recurrence intervals which appears to be in the range 350-1430 years, with a mean recurrence interval of 821 ± 176 years. on the basis of these results, the northwestern segment of the north tabriz fault does not appear to present a major seismic potential for the near future, however, not enough is known about the southeastern segment of the fault to make a comparable conclusion. (ms 7.3), 1721 (ms 7.3) and 1780 (ms 7.4) were accompanied by coseismic surface faulting (ambraseys and melville, 1982; berberian and yeats, 1999). the 1721 and 1780 surface ruptures extended for at least 50 and 60 km long, respectively (fig. 2b; ambraseys and melville, 1982) and occurred 60 years apart in time on adjacent fault sections suggesting that large earthquakes along the north tabriz fault are khaled hessami, daniela pantosti, hadi tabassi, esmael shabanian, mohammad r. abbassi, khalil feghhi and shahryar solaymani arabia turkey black sea eurasia iran 40.00 n 31.00 n 40.00 e 50.00 e c aspian s ea zagros m ts m rf naf ea f ntf 1939.12.26 1966.8.20 1966.3.7 1976.11.24 1977.5.26 1970.3.14 1970.10.25 1963.3.24 2 5 m m /yr 47.00 n fig. 1. location map of the nw iran-eastern turkey, adapted from jackson (1992), with focal mechanisms of some of the large earthquakes (mb > 5.3) in the tabriz-chaldiran seismogenic zone (jackson and mckenzie, 1984). north tabriz fault (ntf), east anatolian fault (eaf), main recent fault (mrf), north anatolian fault (naf). fig. 2a-c. a) small-scale regional map of active faults in nw iran-eastern turkey (after jackson and mckenzie, 1984). the rectangle encloses fig. 2b. b) simplified map of the ntf and location of historical earthquakes, modified after berberian (1997). rectangle encloses fig. 2c. c) nw section of the north tabriz fault (after karakhanian et al., 2001). rectangles enclose figs. 4a,b and 6. 904 c a b clustered in time and are inter-related in space (berberian and yeats, 1999). the 1976 chaldiran earthquake and its 55 km long surface rupture in turkey (fig. 2a,b; toksöz et al., 1977) suggest that there may be a typical surface rupture length for the most recent historical events in the chaldiran-north tabriz fault system. total seismic moment release over 80 years implies that the coseismic deformation rate of about 20 mm/yr (jackson, 1992; fig. 1), contributed by strike-slip faults in the se turkeynw iran region, accounts for almost all the strike-slip component of 22 mm/yr predicted by nuvel 1 (demets et al., 1990). this study describes the main geomorphic features and results of the first trench investigations conducted on a portion of the north tabriz fault that is supposed to have ruptured during the 1780 tabriz earthquake (fig. 2b,c). based on the results of radiometric dating and fault displacements during the late holocene, the age of previous large events is estimated together with the possible slip per event and slip rates. 2. the north tabriz fault the north tabriz fault (ntf) is one of the active faults in nw iran that has a clear surface expression. it has an average strike of nw-se over a length of about 150 km and appears to be generally close to vertical in dip. right-lateral movement along this fault, documented by berberian and arshadi (1976) from study of aerial photographs, can also be seen clearly in the field (karakhanian et al., 2001). reporting right-lateral displacement of gullies for about 8 m during late holocene, karakhanian et al. (2001) suggest that the minimum slip rate along the ntf is about 2 mm/yr. the 1780 earthquake ruptured the northwest part of the ntf, whereas the 1721 event ruptured the southeast part of the fault (fig. 2b; ambraseys and melville, 1982; berberian and yeats, 1999). we investigated the northwest part of the fault within the rupture zone of the 1780 earthquake, with particular attention to the fault in the vicinity of the city of tabriz. paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results b a 11.711.7 +_ 0.5 m0.5 m old c anal recent ca nal qanat shaft fig. 3. lateral offsets of the old canal across the north tabriz fault (looking north). the distance between old canal and its original position (i.e. the qanat shaft) is 11.7 m. the new canal is built where the old canal used to be. 905 khaled hessami, daniela pantosti, hadi tabassi, esmael shabanian, mohammad r. abbassi, khalil feghhi and shahryar solaymani fig. 4. offsets of stream beds along the north tabriz fault. a and b show aerial photographs (1:10.000) of cumulative right-lateral displacements along the nw section of the north tabriz fault. top photographs in a and b represent present-day configuration of rivers and stream beds. lateral offsets of the same geomorphic levels are recognized by reconstructing previous steps of deformation during the late pleistocene (top to bottom). 906 paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results the city is located in a pull-apart basin occurring between two major fault sections of the ntf arranged in an en échelon pattern, stepped right (fig. 2b,c; karakhanian et al., 2001). the fault section to the northwest shows clear geomorphic evidence for repeated surface faulting events (fig. 2c). this, and the fact that the fault crosses late pleistocene deposits, led us to start our investigation in this part of the fault. here stream beds and gullies are systematically offset along the fault trace. the most spectacular feature among them is a line of qanat (underground water tunnels, marked by lines of access shafts) displaced right-laterally (figs. 3 and 6). at this locality, the new canal (a in fig. 3) was dug after the older canal (b in fig. 3) was offset right-laterally for about 11.7 m ± 0.5 (see fig. 6 for location). offsets of several stream beds elsewhere along this section of the fault contain evidence for cumulative displacements by several individual offset events, however, these offsets are difficult to interpret. a maximum 256 m dextral offset can be measured by matching drainage systems (fig. 4). the vertical component of displacement varies from place to place between 2 and 11 m, but in any case, the dip slip component is subsidiary to the main right-lateral strike-slip movement, or it may represent separation rather than slip. in fact, although it is the southern block which is downthrown relative to the northern side, the strike-slip movement along the fault has created scarps facing both north and south by shifting topography laterally in this relatively high-relief terrain. 3. trenches: stratigraphy, structure and paleoearthquakes in order to investigate the paleoearthquakes which occurred on this fault, we excavated two trenches 100 m apart perpendicular to the trace of that part of the fault that is supposed to have ruptured during the 1780 tabriz earthquake. trenches were also excavated parallel to the fault on either side at both locations (see figs. 2c and 6 for location). 3.1. trench 1 location of trench 1 was selected where an upstream reach was dammed by a shutter ridge across the fault zone. at this location, two apparently recent scarplets were also recognized (fig. 5). because of logistical problems, the trench exposed the shorter of these fault scarps as a zone of shearing a few meters wide (figs. 5 and 7a). the lower pleistocene deposits forming the shutter ridge to the south (unit 100) are juxtaposed across this fault zone against the upper pleistocene-holocene deposits (hossein-khan-nazer, 1999) of fluvial and debrisflow origin (units 107 to 118 in fig. 7a,c). the record of repeated dextral offset during multiple faulting events along this fault section is preserved in a sequence of scarp-derived deposits trapped into coseismically-formed depressions, referred to here as colluvial wedges. these colluvial wedges are mainly composed of coarse-grained sand, gravel, and angular pebbles within a sandy clay matrix. individual colluvial wedges interpreted as evidence for individual earthquakes have been discriminated on the basis of their sedimentology and by using structural relations (fig. 7a,c). the bottom unit (101) of this sequence is severely deformed so that pebbles are sheared along fractures and are compacted in their matrix. a few fractures appear to be buried under this unit. the second unit (102) is compacted but is less deformed, with pebbles reoriented only within 10-20 cm of the fault plane. the third colluvial unit (103) is cut by only a few joints, but it clearly buries a fault. unit 104 is mainly alluvium deposited over the youngest wedge of colluvium. all these units are bounded on their northern side by the fault zone which is overlain by the youngest stratigraphic unit (106) of fine-grained alluvial sand and silt. the presence of these alluvial deposits suggests that some erosion may have possibly occurred in the upper part of the section. on the basis of this stratigraphic interpretation of colluvial wedges, we could speculate the occurrence of four events; however, for two of them we have also other independent evidence and thus, to be conservative, we consider only these of high reliability. evidence for the oldest 907 908 khaled hessami, daniela pantosti, hadi tabassi, esmael shabanian, mohammad r. abbassi, khalil feghhi and shahryar solaymani fig. 5. topographic profile across the fault trace near trench sites. this profile indicates a total vertical displacement of 9.8 ± 0.3 m across two fault scarps. trench 1 exposed the shorter of these fault scarps (4 m high) as a zone of shearing a few meters wide. table i. measured and dendrochronologically corrected ages of samples collected in the trenches 1 and 2. measured ages have been corrected for c12/c13 in the laboratory and for c12/c14 by using the calibration program oxcal v3.5 (bronk ramsey, 2000). for each sample a probability density is obtained. we report the 2 interval with the associated probabilities. for our study we took under consideration only the age intervals with probability above 10%. sample # unit measured age bp cal age 2 probability % t2n02 t2-? 965 ± 40 990-1190 a.d. 95.4 t2n03 t2-204 1275 ± 40 660-870 a.d. 95.4 t2n01 t2-? 1020 ± 40 890-830 a.d. 5.2 950-1070 a.d. 74.8 1080-1160 a.d. 15.4 t1n02 t1-102 1915 ± 40 0-220 a.d. 95.4 t1n03low t1-103 1510 ± 40 430-640 a.d. 95.4 t1n03up t1-103 1230 ± 70 660-970 a.d. 95.4 t1n01 t1-101 3240 ± 40 1620-1410 b.c. 95.4 t1n04 t1-104 740 ± 50 1190-1320 a.d. 86.7 1359-1390 a.d. 8.7 event (event t1-d) is marked by the oldest of these colluvial wedges (unit 101) which accumulated in the area that sagged against the fault and overlies some fractures and faults in unit 101. the second event (uncertain t1-m?) could be located at the base of unit 102. the third event is well constrained (event t1-a) as it is marked by the upward termination of the south branch of the main fault at the base of unit 103 and by the deposition of unit 103 itself (fig. 7a,c). unit 104 does not seem to record a distinct event of deformation but it may have occurred in a depression formed by a more recent event (event t1-l?). finally, the main fault branch appears clearly capped by the alluvium of unit 106. this contact is clearly an erosional one, but it may represent the most recent event horizon in this trench (event t1-c). the lack of direct stratigraphic relations prevents us from understanding whether event t1-c coincides with t1l? or whether they are two independent events. further evidence of older events in the trench section includes event t1-b at the top of unit 107. the timing of these paleoearthquakes is poorly constrained by few radiocarbon dates on organic rich deposits as no charcoal was found (see table i and fig. 7a for sample age and location). at least four surface faulting events have occurred since 3.6 ka, the most recent one being younger than 1190 a.d. a summary of the age of the events is in table ii. 3.2. trench 2 trench 2 was excavated across the fault scarp near a pond area on the side of a small erosional gully (figs. 6 and 7b). the trench exposes recent fluvial deposits which are mainly composed of fine-grained sand and silt within a clay matrix (units 200 to 221 in fig. 7b). deposits exposed in trench 2 have been intensely disrupted by northand south-dipping, high-angle faults over a 15 m wide zone within which we distinguished two major zones of deformation, indicated as northern fault zone and southern fault zone (fig. 7b). the northern fault zone in trench 2 can be traced to the fault zone in trench 1 (fig. 6). evidence for some small scarplets at the surface suggests late holocene faulting at this site (fig. 7b,d). however, all these scarplets could not be traced to the trench walls, probably due to bioturbation. truncation of fault splays by sedimentary units at different stratigraphical levels as well as scarp-derived colluvium is interpreted as evidence for the occurrence of at least four individual paleoearthquakes. the event horizons (i.e. the ground surface at the time of the faulting) for these paleoearthquakes are located in the following stratigraphic positions: a) event t2-x, top of unit 204 or middle portion of unit 208 as suggested by some open fissures in-filled with unit 208 and fault splays 909 paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results table ii. ages of paleoearthquakes as constrained in the trenches. trench 1 event # max age min age samples used t1-c 1190 a.d. 1780 a.d. t1n04, historical considerations t1-l? 660 a.d. 1320 a.d. t1n03up, t1n04 t1-a 0 a.d. 640 a.d. t1n03down, t1n02 t1-m? 1620 b.c. 220 a.d. t1n02, t1n01 t1-d not available 1410 b.c. t1n01 t1-b no age constraints trench 2 event # max age min age samples used tr2-x 990 a.d. 1780 a.d. t2n04, historical considerations tr2-w 660 a.d.? 1160 a.d. t2n01, t2n03 assuming 204 was offset also by this event tr2-z not available not available tr2-k not available not available apparently truncated by the upper part of unit 208 (if this latter is not an effect of bioturbation). the vertical displacement of unit 204 could be attributed to horizontal juxtaposition of this unit with irregular thickness along the strike-slip fault, but it can also suggest evidence for further events. b) event t2-w, top of unit 210, where fault splays are truncated by unit 211 (at 5 m). evidence for this event possibly exists also between 0 and 1 m where some of the open fissures, in-filled by unit 206, are not aligned with the most recent ones. this suggests the occurrence of surface faulting just after or during deposition of unit 206. c) event t2-z, base of unit 213, where fault splays terminate and are buried by unit 213 at 1011 m and 15-16 m. this event may coincide with event t2-w, however, the lack of direct stratigraphic relations does not allow us to confirm this hypothesis. d) event(s) t2-k, at the top of unit 200, three channel fills are stacked onto the southern fault zone (at 10-16 m) and cap several fault traces. the timing of only two of these paleoearthquakes is constrained by radiocarbon dates on organic rich deposits as no charcoal was found elsewhere (see table i and fig. 7b for sample age and location). the most recent event is younger than 990 a.d., whereas the penultimate event may be constrained between 660 and 1160 a.d. a summary of the age of the events is in table ii. khaled hessami, daniela pantosti, hadi tabassi, esmael shabanian, mohammad r. abbassi, khalil feghhi and shahryar solaymani fig. 6. large scale topography map (1 m contour line) of the study area and location of trenches (t1 and t2) based on 1075 measured points using total station equipment. displacement of qanat shaft can be seen on either side of the fault trace. a-a shows location of topographic profile in fig. 5. double lines show trench locations. 910 911 paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results south 0 5 north event t2-x 10 15 20 m m t2n03 660-870 a.d. t2n02 990-1190 a.d. t2n01 950-1160 a.d. event t2-z event t2-k event t2-w event t2-w? northern fault zone southern fault zone 200 221 218 219 220 209 213 216 217 215 214 209 213210 211 212 205 207 204 201 203 206 208 209 202 117 113 114 118 108 104 105 103 101 north m m trench 1 7 4 0 south t1n04 1190-1320 a.d. t1n03up 660-970 a.d. t1n03down 430-640 a.d. t1n02 0-220 a.d. t1n01 1620-1410 b.c. event t1-a event t1-m? event t1-l? event t1-c event t1-d event t1-b 107 100 111 112 109 110 106 116 115 102 sedimentary contacts fault fracture c14 sample location unit 211unit 211 unit 209unit 209 unit 212unit 212 fig. 7a-d. trench logs along the north tabriz fault west of tabriz city. a) simplified log of trench 1 based on 1:20 field mapping. trench 1 exposed upper pleistocene-holocene deposits of fluvial (units 107-118) and debris flow-colluvial (units 101-104) origin. these are mainly composed of coarse-grained sand, gravel, and angular pebbles within a sandy clay matrix. b) simplified log of trench 2 based on 1:20 field mapping. trench 2 exposed upper pleistoceneholocene deposits of fluvial origin (all units aside from units 211 and 212) which are mainly composed of fine-grained sand and silt within a clay matrix. units 211 and 212 are mainly of debris flow origin. c) detail of the fault zone of trench 1, east wall, looking south. d) view of the northern fault zone of trench 2. notice the small scarplets at the surface on top of the fault zone. trench 2 trench 1 c d a b 4. coseismic slip, slip rates and recurrence intervals 4.1. slip per event unfortunately no direct information about the horizontal movement during each of the surface faulting events recognized in trenches 1 and 2 could be retrieved from the fault-parallel excavations (fig. 6). from the trenches, only rough estimates of the amount of individual vertical displacements can be obtained. these are based only on the thickness of the deposits interpreted as colluvial wedges, assuming that the vertical slip at the trench site reflects true slip on the fault and not lateral topographic juxtaposition. selecting a crest of an interfluve on either side of the fault (marked aa on fig. 6), we avoided juxtaposition of topography along the topographic profile in fig. 5. even taking into consideration these large uncertainties, we can obtain from trench 1 a thickness of colluvium that varies between 0.3 and 0.9 m that directly translate to minimum vertical coseismic slip. an estimate of the horizontal coseismic slip can be obtained by measuring the right-lateral offset of modern stream channels measured along the strike of the north tabriz fault in the study area. a frequency histogram of lateral offsets over 2.5 km distance along the fault trace is provided in fig. 8 to identify peaks, which could represent the amount of offset related to one or more individual paleoearthquakes. figure 8 shows several peaks indicating that the cumulative slip of offset channels could be associated with individual increments of 3.5-4.5 m. this figure could thus represent the coseismic offset produced by the most recent earthquake and also the typical offset for this section of the north tabriz fault. however, even if we consider it dubious, we cannot exclude the possibility that the 4 ± 0.5 m offset increments may have been produced by more than one earthquake. 912 khaled hessami, daniela pantosti, hadi tabassi, esmael shabanian, mohammad r. abbassi, khalil feghhi and shahryar solaymani 0 20 40 60 80 100 120 140 0 1 2 3 4 n u m b e r o f d is p la ce d f e a tu re s displacement (m) ∼ 4 m ∼ 50 m ∼ 98 m ∼ 136 m fig. 8. frequency histogram of lateral offsets (represented as gaussian probability density functions) over 2.5 km distance along northwestern section of the north tabriz fault. several peaks indicate the cumulative slip of offset channels could be associated with individual increments of 3.5-4.5 m. 4.2. slip rate the only direct estimate of the horizontal slip rate along this section of the north tabriz fault is based on the offset qanats (figs. 3 and 6). the maximum age of the qanat line is 3000 years (forbes, 1964; goblot, 1979; kamiar, 1983; potts, 1990); because it is horizontally offset for 11.7 ± 0.5 m, it implies a minimum horizontal slip rate of 3.7-4.0 mm/yr. the ages of the offset drainages are not known, but by making some assumptions on the maximum age of the incised surfaces, we can compare long-term slip rates to the slip rate obtained for the past 3 ka. channels incised within the upper pleistocene (maximum 125 ka) sediments (hossein-khan-nazer, 1999) are right-laterally offset for about 250 m, indicating a minimum slip rate of about 2 mm/yr. on the other hand, young alluvial fans, incised by drainages that show a 67 ± 10 m horizontal offset (fig. 4) and a 9.8 ± 0.3 m vertical displacement (fig. 5) could be attributed to the last important post-glacial deposition following the last glacial peak (18 to 10 ka; pedrami, 1987). if this assumption is correct, the minimum horizontal slip rate is 3.1-6.4 mm/yr, consistent with that obtained from the qanats. on the same age assumption, the total vertical displacement of the same surface indicates a vertical slip rate of 0.50.8 mm/yr. 4.3. recurrence intervals based on the chronological constraints in each trench for the occurrence of the paleoearthquakes, we attempted a correlation of events between the two trenches (fig. 9). at least four earthquakes occurred during the past 3.6 ka. the most recent event is younger than 990 a.d. in both trenches. since the historical reports indicate that the 1780 earthquake ruptured this section of the fault (ambraseys and melville, 1982; berberian and yeats, 1999), we assume that the most recent event recorded in the trenches is the 1780 earthquake. the ages of the other events fall in very wide ranges because of the limited amount of dateable material found in the trenches. even with these high uncertainties we can infer that the penultimate event occurred 910 ± 250 a.d., the third event back 320 ± 320 a.d., and the oldest one 700 ± 920 b.c. comparing these ages with the historical record, the 1042 earthquake falls in 913 penultimate most recent 0 a.d. 2000 a.d.1000 t1-a t1-c tabriz trenches age of events 1 7 8 0 a .d . t1n03u t1n01 t1n04 t1n02 t1n03d t1n02 t2n02 t2n03 t1n04t1-l? t1-m? t1-d t2-x t2n01 t2-w 1500 b.c. t1n01 fig. 9. attempt of correlation of paleoearthquakes between the two trenches. black bars are the age ranges of the samples used to constrain the age of occurrence of paleoearthquakes which are reported as thick gray bars. sample names and event names are shown too. gray boxes show the correlated age of the most recent and penultimate events. on the basis of historical considerations we assume the most recent earthquake be the 1780 one. paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results the age interval of the penultimate surface faulting event found in the trenches. although information on this event is mostly derived from the city of tabriz, and thus it is hard to define the earthquake source only on the basis of felt reports, surface faulting is reported and certainly support the hypothesis that the 1042 earthquake ruptured at our trench site. no evidence for the other events which produced damage in tabriz is found in the trenches. this may be due to different reasons: a) events were too small to produce surface faulting at the trench site; b) events occurred on different faults but this cannot be verified because of the limited historical information on the damaged area, c) mimicking of the geological evidence for these events occurred by more recent and larger earthquakes. on the basis of these ages an average recurrence interval for surface faulting events of ca. 350 to 1430 years and also a mean recurrence interval of 821 ± 176 years can be estimated. average recurrence interval estimates can also be obtained based on the number of events that contributed to displace a given anthropic/ geomorphic elements and their ages. by using the maximum 3000 years old qanat (forbes, 1964; goblot, 1979; kamiar, 1983; potts, 1990) displaced 11.7 ± 0.5 m and the 4 ± 0.5 m slip per event, we can infer that at least 3 events contributed to its displacement. this would convert to an average recurrence interval of ca. 1000 years. on the other hand, comparing the individual offset of 4 ± 0.5 m with the 67 ± 10 m long term displacements recorded by the 12-18 ka surface, we can infer that 12 to 22 events contributed to the lateral offset of the drainages. this translates to average recurrence intervals ranging between 545 and 1800 years, in good agreement with the observations derived from the qanat and trenches. 5. conclusions geomorphic features and trenches opened west of tabriz city show evidence for repeated surface faulting along this section of the north tabriz fault. at least four surface faulting events have been recognized during the past 3.6 ka. because of the lack of datable materials, ages of events are poorly constrained, and correlation between the two trenches is difficult. however, taking into consideration these intrinsic uncertainties, we defined as much as we could the age of these events as 1780 a.d., 910 ± 250 a.d., 320 ± 320 a.d., and 700 ± 920 b.c. (see table ii and fig. 9). slip per event is estimated as 4 ± 0.5 m horizontal and minimum 0.6 ± 0.3 m vertical although this latter may not be truly representative of slip on the fault. horizontal slip rates of 3.7-4.0 and 3.1-6.4 mm/yr are obtained on the basis of faulted anthropic features and on offset drainages, respectively. on a similar basis, vertical slip rates may be of the order of 0.5-0.8 mm/yr. finally, average recurrence intervals calculated following different reasoning lines are as small as 350 years and as large as 1430 years. our results show that large earthquakes (m > 7) on the nw segment of the north tabriz fault occur each 821 years (mean time) which is consistent with historical data. since this section of the north tabriz fault has not produced any major destructive earthquakes with surface faulting since 1780 (the past 220 years), assuming a constant slip rate accumulation and a characteristic slip per event at least 0.7-1.4 m of strain must have been accumulated along the northwest part of the ntf. this would suggest the potential of this part of the fault is not critical for the next few centuries, however, the possibility of smaller ruptures and clustering with periods of more frequent moderate earthquakes along the fault, should be considered. although this paper contains a preliminary understanding of the seismic behavior of this fault, important issues that have a critical importance for the highly populated city of tabriz remain open such as the segmentation of the north tabriz fault zone, the possibility that the fault ruptures also during earthquakes smaller than the 1780 one, the significance of the historical earthquakes not found in the trenches, and the existence of other sources in the area. acknowledgements we thank dr. c.j. talbot for constructive comments and suggestions for improvements on an early draft of this work. we wish to thank 914 khaled hessami, daniela pantosti, hadi tabassi, esmael shabanian, mohammad r. abbassi, khalil feghhi and shahryar solaymani drs. r.s. yeats and m. berberian for their constructive reviews that substantially improved the original manuscript. we also thank m.j. bolourchi and f. ansari-moghadam for help in logging the trenches. we appreciate field support provided by the international institute of earthquake engineering and seismology (iiees), tehran, iran. this work was supported by the national research council, number 5006. references ambraseys, n.n. and c.p. melville (1982): a history of persian earthquakes (cambridge university press, cambridge), pp. 236. berberian, m. (1997): seismic sources of the transcaucasian historical earthquakes, in historical and prehistorical earthquakes in the caucasus, edited by d. giardini and s. balassanian (kluwer academic press, the netherlands), nato asi series, serie 2: environment, vol. 28, 233-311. berberian, m. and s. arshadi (1976): on the evidence of the youngest activity of the north tabriz fault and the seismicity of tabriz city, geol. surv. iran rep., 39, 397-418. berberian, m. and r.s. yeats (1999): patterns of historical earthquake rupture in the iranian plateau, bull. seismol. soc. am., 89, 120-139. bronk ramsey, c. (2000): oxcal program v3.5, university of oxford radiocarbon, accelerator unit (http:// www.rlaha.ox.ac.uk/oxcal/oxcal.htm). demets, c., r.g. gordon, d.f. argus and s. stein (1990): current plate motions, geophys. j. int., 101, 425-478. forbes, r.j. (1964): studies in ancient technology (leiden, netherlands: e.j. brill), vol. 1, pp. 205. goblot, h. (1979): les qanats: une technique d’acquisition de l’eau (paris, mouton), pp. 236. jackson, j. (1992): partitioning of strike-slip and convergent motion between eurasia and arabia in eastern turkey and the caucasus, j. geophys. res., 97, 12471-12479. jackson, j. and d.p. mckenzie (1984): active tectonics of alpine-himalayan belt between western turkey and pakistan, geophys. j. r. astron. soc., 77, 185-264. hossein-khan-nazer, n. (1999): geomorphological map of sardrud, geol. surv. iran rep., sheet 5266 iii, series k753. kamiar, m. (1983): the qanat system in iran, ekistics, 50, 467-472. karakhanian, a.s., v.g. trifonov, h. philip, a. avagyan, k. hessami, f. jamali, m.s. bayraktutan, h. bagdassarian, s. arakelian and v. davtian (2001): active faulting and natural hazards in armenia, eastern turkey and northwestern iran, tectonophysics (in press). mckenzie, d.p. (1972): active tectonics of the mediterranean region. geophys. j. r. astron. soc., 30, 109-185. pedrami, m. (1987): quaternary stratigraphy of iran, geol. surv. iran rep., serial n. 551.79 (55) pe [in farsi]. potts, d.t. (1990): from prehistory to the fall of the achaemenid empire, in the arabian gulf in antiquity (oxford, clarendon press), vol. 1, pp. 460. toksöz, m.n., e. arpat and f. saroglu (1977): east anatolian earthquake of 24 november 1976, nature, 270, 134-150. westaway, r. (1990): seismicity and tectonic deformation rate in soviet armenia: implications for local earthquake hazard and evolution of adjacent regions, tectonics, 9, 477-503. westaway, r. (1994): present-day kinematics of the middle east and eastern mediterranean, j. geophys. res., 99, 12071-12090. 915 paleoearthquakes and slip rates of the north tabriz fault, nw iran: preliminary results òlahdflòkasdfòklaklòjadf microsoft word articolo annals.doc annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 1 the development of a cloud-gis platform for the management and sharing of geographic data during the central italy seismic sequence. maurizio pignone1, rocco cogliano2, raffaele moschillo1 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti,grottaminarda (av), italy 2 istituto nazionale di geofisica e vulcanologia, sezione roma1, grottaminarda (av), italy maurizio.pignone@ingv.it abstract for several years, the istituto nazionale di geofisica e vulcanologia (ingv, national institute of geophysics and volcanology) uses arcgis online (www.arcgis.com), to share information about the real-time seismicity in italy with other geographical data, geological and seismological, through the development of different webgis applications with various customized functionalities. during the earthquake emergency in central italy in august 2016, webgis applications have been widely used. they derive from a common platform developed a few months earlier by the labgis of ingv-irpinia. this platform has enabled the management and updating of the seismic data of the sequence in real time and their integration with the leading ingv databases seismological and seismotectonic and those of other institutes. moreover, these applications have permission to manage and share information and even data from the activities of the ingv emergency groups , such as the location and characteristics of seismic and accelerometric temporary stations installed by the teams of sismiko and emersito and the geological observations from the emergeo group. i. the cloud-gis he cloud-gis is the combination of running gis software and services on cloud infrastructure and accessing gis capabilities using the web. cloud computing offers an alternative to traditional methods of delivering and consuming gis software and services to users. instead of running on their gis systems and computing operations, with cloud-gis, software and services are hosted on the infrastructure made available through the web technologies. the cloud-gis could be defined as a new generation of on-demand gis technology that uses a virtualized platform or an infrastructure in a scalable dynamic environment. t annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 2 for some time different cloud-gis platforms are available on the web, free and fee: these platforms allow you to create web maps by integrating local spatial datasets with geographic services available, as the base maps or thematic map services. one of the most popular platforms is mangomap (https://mangomap.com/) that lets you transform your data into online web mapping solutions without coding, programmers, servers (cloud-based). with mangomap (free only for 30 days) it is possible to create maps completely customized but does not allow you to load data from online sources (for example google drive), but only via upload. another web maps publishing service is qgis cloud (https://qgiscloud.com/), it is an open source personal spatial data infrastructure (sdi) on the internet which allow to publish qgis desktop projects with maps and data. all maps published on the internet are freely accessible by anyone at zero cost and it’s possible to limit the access to your maps. qgis cloud allows only viewing and not the editing of a web map created by qgis desktop project and does not allow to load the dynamic layers. one of the most widely used cloud platforms by gis users worldwide is arcgis online, developed by esri and available on http://www.arcgis.com, is the cloud-based sharing system "ready to use" that allows a user to build a complete gis, containing maps and applications, with the possibility of access to a large collection of basic geographical data with worldwide coverage (esri basemaps). this cloud-gis allows for creating maps and web applications and sharing with different types of users by creating public or private working groups. arcgis online allows the management of different types of geographic data and geospatial applications, made available directly from esri or from community of esri users around the world. it can be defined as a "social network of geographic information". arcgis online (figure 1) allows you to manage and import a large quantity and diversity of information with geographic component from desktop gis (esri shapefile), other gis software, online resources such as the ogc map-services (wms, wfs), google maps and google earth formats (kml and kmz) and even simple text files and tabular data such as csv and txt. with this data it is possible to create, through a very intuitive interface, the web-maps that represent the starting point for the development of applications webgis choosing from the templates available with different features and functionalities. the webgis, once created and saved in the cloud-gis with its layers and its configurations, can be shared figure 1: the scheme of arcgis online. it is possible to manage and import a large quantity and diversity of information with geographic components. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 3 with a group of users or shared with everyone and published on the web. ingv has its own cloud space on arcgis online (subscription plan) where it can host web maps, geographic data, web applications and can be shared within the institute or outside or with individual working groups. ingv uses esri arcgis software for creating and managing geographic databases with scientific datasets from its monitoring networks that allow the development of cartographic products and for the web. arcgis online is fully integrated with the other components of the arcgis system and this is a huge advantage for the sharing of datasets and creating for web publishing applications. furthermore, all data uploaded and managed in arcgis online are hosted in "esri's secure cloud" that ensures their safety and copyright and allowing you to associate to every single dataset, maps or web applications the metadata and a data policy. ii. the seismic data management in real time the real-time seismic data represent a very useful resource especially for emergency management but also for communication and information for the public. the major worldwide centers of seismic monitoring have invested a lot of resources on the sharing of this information in real time through the development of web applications for general users based on digital maps and geographic services. the usgs allows to follow the seismicity around the world through an interactive map (http://earthquake.usgs.gov/earthquakes/ma p/), but also to download different formats data, including geographic (georss, kml, csv , atom, etc.). similarly, even the csem has developed interactive geographical web application to view the real time seismicity in the mediterranean and around the world (http://www.emsccsem.org/earthquake/significant_earthquake s.php). some of these datasets on seismicity are used in several web portals and smartphones and tablets applications that collect and share information on natural risks and emergencies. for example the esri’s disaster response program (http://www.esri.com/services/disasterresponse/earthquakes), a platform that is a supporting for organizations that are responding to earthquake and tsunami disasters with software, data, imagery, project services, and maps. an interactive earthquake map that show the earthquake locations, shaking intensity, and population as well as social media updates regarding earthquakes that have occurred around the world. the web interface for sharing and distribution of real-time seismic data recorded by the national seismic network (ingv) in italy is iside italian seismic instrumental and parametric data-base (http://iside.rm.ingv.it/), where it is possible to search and download the data, also in geographic formats. in recent years different webgis applications have been developed by the labgis of ingv-irpinia (grottaminarda-av) using arcgis online and the real time seismic data from iside. an example is geosis, a webgis application annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 4 showing recent and real time seismicity, extracted by iside, with other useful geographic data derived from other ingv databases and studies, such as historical seismicity, seismic hazard, seismic classification, the last thirty years seismicity (pignone et al. 2012). the evolution of arcgis online and the development and recent publication of the ingvcentro nazionale terremoti (cnt, national earthquake centre) webservices (http://cnt.rm.ingv.it) allowed the design and implementation of a new application software for the management of the seismic data and those of the monitoring networks in real time (called xpy) constituted by a set of scripts and procedures. the architecture of xpy system is formed by two servers located in the data center of grottaminarda on which run a series of python scripts for finding, processing and publishing the real time seismic data and the real time monitoring networks data. the xpy system runs in the background and performs the following operations (figure 2): ● interrogation of web-services of the national earthquake center (http://webservices.rm.ingv.it/fdsnws/e vent/1/). xpy requires the list of earthquakes located by the national seismic network in the last "x" days; ● the event list obtained by web-services is processed and some csv (commaseparated values) files are generated as needed; for example the last earthquake, the last 24 hours earthquakes or those of last 30 days; ● the generated csv files are processed to classify events based on specific attributes such as: magnitude intervals, time intervals, geographic areas (this allows the creation of different gis layers); ● for the information relating to the positioning and characteristics of the ingv permanent monitoring stations, the query is performed at seisface (the web interface for managing and locating information of ingv seisnet database) and contains information in real time about the monitoring networks; ● synchronization of csv files produced by queries with the sharing webserver and online publication. all the parameters that determine the production of the csv files are managed by the xpy system through a configuration file that determines the scripts settings and can be modified at run-time without the stop and reboot of the system. it is possible to edit the configuration file by changing the parameters or by adding new processing requests. the csv data of real-time seismicity, made and shared on the web through the xpy system, can be easily integrated within arcgis figure 2: the operation of the xpy system and the integration of data in arcgis online. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 5 online as a web csv layer with the possibility of defining an update time interval. these csv files can also be downloaded and used with desktop gis software for the production of thematic maps. during the seismic emergency in central italy, the data source for the thematic maps published on the web portal of the ingv (www.ingv.it) and on the ingvterremoti communication platform (http://ingvterremoti.wordpress.com) have been these csv files. the map (figure 3) has been realized using several layers, classified and displayed according to the magnitude classes (symbols size) and the time (colors) to follow the temporal evolution of the central italy seismic activity. the chosen symbology has been conceived to provide an immediate impact of the ongoing activity, and is analogous to the symbology adopted in the cnt web pages (http://cnt.rm.ingv.it) and in the ingvterremoti app on ios and android. iii. web applications for managing and sharing geographic data during the seismic emergency due to the development of the xpy system and to new functionalities available in the recent versions of arcgis online it was possible to improve the integration and sharing figure 3: thematic map of spatio-temporal distribution of the seismic sequence. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 6 of geographic data for the emergency management activities through the development of a work platform in arcgis online. this platform was tested during the earthquake simulation organized by ingv in november 2015 and used during the seismic emergency in august 2016 (magnitude 6.0 earthquake on august 24th between the provinces of rieti and ascoli piceno). the work platform is constituted by several web-maps and web applications for the management and updating of the seismic data and the monitoring networks in real time and their integration with the principal ingv seismological and seismotectonic databases and those of other institutes. in the maps, it is also possible to directly manage and share information and data from the activities of the ingv emergency groups (https://ingvterremoti.wordpress.com/ilmonitoraggio-sismico/attivita-inemergenza/), for example, the position and characteristics of temporary seismic and accelerometric stations installed by sismiko and emersito teams but also the geological observations from surveys of geologists of emergeo. starting from these web-maps, the web applications have been developed using the arcgis online template or the application builder for arcgis; the web applications have been shared within the ingv crisis unit (the team that coordinates all the ingv emergency activities) and emergency groups in order to coordinate the activities on epicentral area. data from the emergency groups during the early days of the seismic sequence in central italy, to simplify the interaction and sharing of information from the different ingv emergency groups operating in epicentral area, a data-sharing system was used through a google spreadsheet (google docs) platform which is used normally by ingv for the mail service, storage, etc. each group was responsible for implementation and updating the table (a google spreadsheet) created specifically for their activities. the table shared among all the participants of the emergency activities could be populated or changed simultaneously by one or more users in real time. the table generated a csv file that, loaded in the web-map of arcgis online as a csv layer, allowed the visualization of geographical information updated in real time (figure 4). for example, for the sismiko group, the information provided on google document table is relative to sites hosting the installed temporary seismic stations and the characteristics of instrumentation. in the arcgis online web-map, the sismiko layer shows figure 4: the integration of google documents within arcgis online web-maps. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 7 the temporary stations displayed in different colors indicating the teams from rome, grottaminarda, ancona and pisa, participating to the installation of the stations. this has allowed for following day-to-day progress in the implementation of the network of each team. it is possible to query the map by clicking on each station to see the pop-up of the detailed information relating to the individual station. the pop-up allows for displaying of the name, the location, the coordinates, the type of power, the type of connectivity for each station and also for incorporating multimedia elements collected during installation phase (figure 5). in this way, all the sismiko members share the information and documents in real time; proving is useful for planning new installations. similarly, other csv layers have been generated for other networks installed by emergency groups like emersito and for geological field observations detected by the geologists of emergeo group. seismicity and other data in real time the real time seismicity has been organized, in the arcgis online web-maps, in layers in which the earthquakes localized by the national seismic network are represented by a different color for each temporal interval and classified in five classes of magnitude value. this representation type is consistent with that used in the maps of the national earthquake center website (http://cnt.rm.ingv.it), in the maps of the ingvterremoti blog and in the ingvterremoti app for ios and android: ● earthquakes in the last hour (red) ● earthquakes in the last 24 hours (orange) figure 5: the arcgis online web map for management of sismiko activities with the attributes of the temporary stations layer. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 8 ● earthquakes in the last 3 days (yellow) ● earthquakes from january 1st 2016 (blue) for the ingv permanent monitoring networks, the following layers have been produced with the detailed information for each station and its operation level: ● national seismic network stations (rsn) ● ingv accelerometric network stations ● national integrated gps network stations (ring) database and catalogues from ingv and other institutes some datasets have been extracted, in different export formats (text and geographic format), from the ingv and other institutes catalogues and databases and, with some operations, have been made compatible as layers for the arcgis online web-maps. for each layer, appropriate classification and thematization have been created and popups have been configured to display some of the attributes included in the database: ● instrumental seismicity 2006-2016, extracted from the iside database (http://iside.rm.igv.it) with events of magnitude ≥ 2.5 (4 different classes of magnitude); ● instrumental seismicity 1985-2005, extracted from the iside database with magnitude ≥3.0 seismic events (4 different classes of magnitude); ● strong earthquakes of the past extracted from cpti15, the 2015 version of the parametric catalogue of italian earthquakes (http://emidius.mi.ingv.it/cpti15dbmi15/) with events of estimated magnitude (mwdef) ≥ 5.0 (5 different classes of magnitude) ; ● national seismic hazard map (gdl mps, 2004; ordinanza pcm, april 28, 2006, n. 3519, all. 1b); it provides the 10% probability of exceedance in 50 years of the horizontal acceleration at rock sites (i.e., vs30>800 m/s; cat. a, 3.2.1 of the d.m. 14.09.2005, http://zonesismiche.mi.ingv.it); ● database of individual seismogenic sources, diss 3.2.0 (http://diss.rm.ingv.it/diss/), divided into individual seismogenic sources, composite seismogenic sources, debated seismogenic sources. in some maps, several map-services of other institutions dataset have been uploaded in wms format (open geospatial consortium standard): for example, the lithological map of italy and the database of faults ithaca from ispra, the maps of copernicus emergency management service for the central italy earthquake. the arcgis online layers of data from emergency teams and real time seismicity are shared exclusively with users involved in emergency. this choice is justified by two reasons: the first is that many of the collected data are still provisional and, therefore, cannot be shared outside, the second is that sharing of arcgis online layers of real time seismicity during an emergency, there are no certainties on esri of cloud platform performance for a high number of viewing requests. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 9 the development of webgis applications once the data within the work platform in arcgis online has been organized and loaded in the web-maps it has been possible to transform maps into webgis applications with basic and customized functionalities. arcgis online provides different web application templates with the possibility to customize them according to the type of features and preferred layout. another possibility is to use the web application builder for arcgis that provides a foundation for building web applications in arcgis online without writing a single line of code. with web appbuilder, it is possible to create a custom html / javascript web application, to configure the look-and-feel of the app by picking a theme that includes a collection of panels, styles, layouts, and pre-configured widgets, add others widgets and functionality. the web applications run on any device including tablets and smartphones. several custom web applications have been developed for the activities management and the data sharing of the ingv emergency groups. for example, for emergeo a webgis has been developed with a layer of earthquake observations (classified in coseismic rupture, co-seismic fracture, coseismic sliding, no co-seismic effect) and a web interface with these functionalities: legend, layer list, add data, print, basemaps, measure, bookmarks, info (figure 6). among the features included within web interfaces, add data allows to add other shared geographic arcgis online layers (they can be searched by keywords), or to add layers from outside in different formats (esri arcgis server map service, wms ogc map service , kml, georss, csv). this feature allows, therefore, to integrate any geographic data of users with scenarios realized through the emergency data. figure 6: the interface of emergeo webgis with functionalities avaible on top-right. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7268 10 iii. conclusions and future developments the realization of this cloud-gis platform has enabled the management and updating of the seismic data of the central italy sequence in real time and their integration with the main ingv seismological and seismotectonic databases. moreover the platform has allowed for better management of sharing of geographic data produced by different ingv emergency groups, such as the location and characteristics of seismic and accelerometric temporary stations installed by sismiko and emersito teams and the geological observations from the emergeo group. due to web applications it has been possible to create scenarios to manage the emergency activities by integrating different types of geographic data from different sources. the opportunity to manage realtime data and integrate with different databases made dynamic scenarios providing support tools for assessments and decisions of the ingv crisis unit for the activities of emergency management in central italy. platform development is continuous and aims at various targets: displacement of xpy system entirely in the cloud; an improvement in the display performance of geographic data; creating customized functionality in web applications (for example some functions for proximity and spatial analysis); mobile applications for data collection and the direct input in google documents; sharing to all of the real-time seismic data using ogc wms services, and arcgis online layers. references pignone m., moschillo r., (2011). geosis web, atti della 15a conferenza nazionale asita, federazione italiana delle associazioni scientifiche per le informazioni territoriali e ambientali, reggia di colorno, isbn 978-88-903132-6-4, http://www.earthprints.org/handle/2122/7489. pignone m. e moschillo r. (2012). geosis: dall’earthquake report al webgis, quaderni di geofisica ingv n. 94, issn 15902595, http://www.earthprints.org/handle/2122/7077, 2011. pignone m., moschillo r., cogliano r., (2012). geosis, an interactive web-map of earthquakes in italy, rendiconti online soc. geol. it., vol. 19/2012, pp. 20-23, issn 2035-8008. pignone m., moschillo r., cogliano r., (2012). a webgis application for visualization and dissemination of italian seismicity, proceedings 7° european congress on regional geoscientific cartography and informations systems, bologna 2012, volume ii, pp. 644-645, http://www.earthprints.org/handle/2122/8032. r e c e n t i t e c n i c h e s u l l ' i n f r a r o s s o (*) w . r u s p a n t i n i r i c e v u t o il 19 f e b b r a i o 1963 r i a s s u n t o . d o p o b r e v i cenili storici, 1"a. pone a l l ' a t t e n z i o n e del l e t t o r e , i f o n d a m e n t i della fisica dell'infrarosso. vengono q u i n d i descritti le applicazioni scientifiche nel c a m p o delle navigazione spaziale e il t r a s p o r t o di s t r u m e n t i specifici al limite dello spazio, i veicoli necessari per guidarli e per raccoglierne i d a t i fisici. s u m m a r y . — following a brief historical i n t r o d u c t i o n , f u n d a m e n t a l s of i n f r a r e d p h y s i c s are b r o u g h t t o t h e reailer's a t t e n t i o n . scientific a p p l i c a t i o n s i n space n a v i g a t i o n field a n d specific i n s t r u m e n t a t i o n carried 011 b o a r d of space veliicles for t h e i r g u i d a n c e a n d for collecting physical d a t a are described. in questi ultimi anni si sono andati sempre più intensificando gli studi sulle radiazioni infrarosse e veramente notevoli sono stati i progressi f a t t i e i risultati finora raggiunti. ma un ben più ampio sviluppo si apre dinanzi a questa nuova branca della scienza, che lascia prevedere applicazioni sempre più importanti e di piii vasta p o r t a t a . anche nel campo spaziale, la tecnologia infrarossa offre già oggi i suoi preziosi servigi, fornendo mezzi d'indagine per le osservazioni scientifiche e la strumentazione più a d a t t a per rendere sicura la navigazione. l'esistenza della radiazione infrarossa è stata messa in evidenza dall'astronomo william ilerschel nella primavera del 1800, si può dire quasi per caso, mentre egli stava ricercando un sistema di filtraggio della luce del sole, che gli permettesse di ottenere questa in sufficiente quantità, pur eliminando al massimo l'indesiderato effetto calorifico. (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. 4 7 8 x v . r i j s p a n t i n i a t a l lino, dopo avere scomposto la luce solare per mezzo di un prisma, in una camera oscura, egli fece un'indagine accurata della distribuzione della t e m p e r a t u r a sullo spettro solare visibile, usando dei normali termometri a mercurio. l'astronomo not ò che la t e m p e r a t u r a a n d a v a aumentando procedendo dal violetto al rosso, e con sua grande sorpresa, che essa continuava a crescere, anche nella zona non illuminata, dove raggiungeva un massimo. le ricerche furono proseguite da suo figlio e da altri sperimentatori, che passarono dall'uso del termometro, che si era dimostrato s t r u m e n t o troppo grossolano per m e t t e r e in evidenza le radiazioni, all'impiego di rivelatori molto più sensibili. l'uso delle termocoppie, che venivano anche raggruppate in serie, consentì di misurare differenze molto piccolo di t e m p e r a t u r a . un ulteriore passo a v a n t i f u f a t t o con l'invenzione del bolometro. infine, con la scoperta dei fotorivelatori e con il perfezionarsi delle tecniche di fabbricazione è stato possibile ottenere cellule sempre più sensibili, che hanno permesso di indagare molto profondamente la n a t u r a e le proprietà delle radiazioni infrarosse. mentre i bolometri sono rivelatori termici, ossia mettono in evidenza l'energia totale della radiazione che cade su di essi, i fotorivelatori si basano sulla liberazione di u n elettrone, per ogni singolo quanto di radiazione che da essi viene assorbito. la differenza essenziale f r a i fotorivelatori e i detector termici risiede nella circostanza, che m e n t r e i primi sono sensibili solamente al numero effettivo di quanti assorbiti, i secondi rispondono all'energia totale ricevuta. tenendo presente che l'energia di un quanto è d a t a dal p r o d o t t o della sua frequenza di vibrazione per la costante h di planck, si n o t a che i fotorivelatori mettono in evidenza i quanti, che hanno un'energia più grande di un certo valore limite ed ignorano completamente t u t t i gli altri quanti, che hanno un valore più basso. a tale valore limite di energia di attivazione, caratteristico di ogni tipo di detector, corrisponde una frequènza e quindi u n a lunghezza d'onda limite ben d e t e r m i n a t a . per i detector fotoemissivi, cioè quelli in cui avviene l'espulsione degli elettroni, la lunghezza d'onda limite è di circa 1,3^. rivelatori di tale tipo, aventi un fotocatodo costituito da cs-o-ag, hanno trovato impiego per m e t t e r e in evidenza radiazioni situate nell'infrarosso vicino. gli elettroni emessi vengono s f r u t t a t i per la visione n o t t u r n a passiva. essi vengono messi in evidenza sulla lastra fluorescente e forniscono così l'immagine della zona osservata. r e c e n t i t e c n i c h e s u l l / l s i ' r a r o s s o 4 7 9 tali apparecchi sono sensibili per quanto si è detto, solo a frequenze elevate e quindi in lina s t r e t t a g a m m a dell'infrarosso molto vicino. essi possono rilevare oggetti notevolmente caldi, ovvero che siano « illuminati» da una sorgente di radiazioni infrarosse. f o r t u n a t a m e n t e , per superare tale difficoltà, viene in soccorso un altro tipo di fenomeno fotoelettrico, che ha avuto un effetto straordinario nello sviluppo dei rivelatori per infrarossi e che si manifesta in quella particolare categoria di corpi, che per la loro proprietà caratteristica, vengono d e t t i semiconduttori. e l e t t r o n i liberi possono essere prodotti nell'interno di tali sostanze, con l'assorbimento di appropriati quanti di radiazione. inoltre, non essendo più necessario disporre di u n a q u a n t i t à notevole di energia, per provocare l'espulsione dell'elettrone, m a solo una frazione di essa, si è riusciti a mettere in evidenza, con il continuo perfezionarsi di tali rivelatori, radiazioni aventi lunghezze d'onda sempre più lunghe. la p r i m a sostanza ad essere impiegata è s t a t a il solfuro di tallio. esso f u introdotto nel 1920 e messo in uso facendolo depositare in sottile strato, su di u n a lastrina di vetro. vennero poi il solfuro di piombo, il teli uri uro di piombo, il seleniuro (li piombo, l'antimoniuro d'indio ed altri composti, che hanno permesso, impiegando particolari tecniche di fabbricazione, di ottenere materiali molto sensibili. inoltre raffreddando le cellule alle temperature di liquefazione dell'azoto, dell'idrogeno o di altri gas, la sensibilità è s t a t a notevolmente a u m e n t a t a , fino a. consentire la rivelazione di lunghezze d'onda di 9u. un altro grande passo a v a n t i è stato f a t t o con il successivo perfezionarsi della tecnica dei semiconduttori. i cristalli di germanio, a t t i v a t i con impurità di oro, rame, zinco ed antimonio hanno consentito di arrivare alla rivelazione di radiazioni, aventi lunghezze d'onda sempre più lunghe. i m p u r i t à di zinco aggiunte al germanio, p e r m e t t o n o i n f a t t i di rivelare lunghezze d'onda fino a 40//, m e n t r e impurità di antimonio, portano il limite a k'>0/(. all'inizio della seconda guerra mondiale le radiazioni infrarosse, impiegate nella tecnica spettroscopica, trovavano applicazione nell'industria dei petroli e specialmente in quella chimica, per l'analisi delle varie sostanze. inoltre, specie in germania, era s t a t a riconosciuta l'importanza delle possibili applicazioni di tali radiazioni ed u n a serie di studi e di ricerche venivano effettuati nei campi fondamentali riguardanti l'emissione, l'attenuazione atmosferica e la radiazione emessa dal background. 4 8 0 r u s p a n t i x i con lo scoppio dell'ultima guerra le ricerche venivano febbrilmente intensificate, con notevoli risultati. le radiazioni infrarosse venivano applicate in sistemi di comunicazione e di segnalazione, di guida dei missili e per la scoperta ed inseguimento di bersagli aerei e navali. venivano prodotti specialmente rivelatori fotoconduttivi al solfuro di piombo, con i metodi di precipitazione chimica o di evaporazione. venivano inoltre sviluppate le tecniche per a u m e n t a r e la sensibilità di tali cellule e venivano costruiti t u b i convertitori d'immagine, che erano in grado di rendere p e r f e t t a m e n t e visibile un bersaglio riflettente radiazioni infrarosse. grande importanza acquistavano anche le applicazioni della fotografia a raggi infrarossi, lo sviluppo delle emulsioni a d a t t e a tale scopo e del materiale ottico. analogo lavoro di ricerca veniva e f f e t t u a t o dagli alleati, che a loro volta m e t t e v a n o in produzione apparecchiature per la visione n o t t u r n a e per la guida di automezzi nell'oscurità. dopo la guerra venivano sempre più perfezionati gli spettrometri, gli s t r u m e n t i di ricerca, i sistemi di guida per i missili e veniva e f f e t t u a t o un intenso studio sistematico in t u t t i i più svariati campi, che hanno attinenza con la tecnica degli infrarossi. il lavoro di ricerca in questi ultimi anni è divenuto sempre più intenso. schiere di ingegneri, di fisici, di scienziati, sono alla ricerca continua di nuovi metodi e perfezionamenti, nei vari istituti scientifici e nelle società produttrici americane; schiere ugualmente operose lavorano in inghilterra, in francia, nel canadà, in svezia, nei paesi bassi, nel belgio, ecc. in i t a l i a non è stato d a t o finora un impulso adeguato in questo settore ed è auspicabile che anche nel nostro paese vengano al più presto promossi studi e ricerche. occorre riguadagnare il tempo perduto e mettersi al passo con le a l t r e nazioni, per non venire irrimediabilmente distaccati. bisogna incoraggiare l'industria, favorire il perfezionamento dei tecnici e formare u n a m e n t a l i t à dell'infrarosso. p e r lo sviluppo di un tale processo sono necessari n a t u r a l m e n t e mezzi adeguati, m a la spesa, d a t a l'importanza dell'argomento, verrà u n domani f o r t e m e n t e r e m u n e r a t a . l a radiazione infrarossa è generata sia dalla transizione di elettroni t r a due livelli energetici, sia da cambiamenti degli stati vibratori e di rotazione degli atomi e delle molecole che si trovano in ogni sostanza. allorché gli elettroni passano da u n livello di energia ad altri più bassi, r e c e n t i t e c n i c h e s u l l / l s i ' r a r o s s o 4 8 1 si lui un'emissione di radiazioni a p p a r t e n e n t i allo spettro visibile ed all'infrarosso molto vicino. quando ha luogo un cambiamento dello stato vibratorio, la radiazione è situata nella regione infrarossa lino a circa 30/*, m e n t r e se ha luogo u n cambiamento della sola rotazione, lo spettro è situato nell'infrarosso lontano. l a banda di frequenza della radiazione infrarossa è compresa approssimativamente f r a un milione e 400 milioni di megacicli, ossia f r a la regione delle microonde e quella della luce visibile. lo spettro della radiazione infrarossa, in termini di lunghezza d'onda è compreso fra 0,72// e 1000/i, ossia fino a un millimetro. di conseguenza la radiazione infrarossa presenta alcune proprietà caratteristiche della luce, ed altre delle radioonde. i n f a t t i può venire otticamente focalizzata mediante lenti o riflessa per mezzo di specchi, come pure trasmessa similmente alle onde radio o radar, attraverso materiali opachi alla luce visibile. i n relazione ai particolari sistemi di rivelazione usati, lo spettro è stato diviso in infrarosso vicino, medio e lontano. i sistemi ad infrarosso possono inoltre largamente dividersi in due grandi categorie: a t t i v i e passivi. nei sistemi attivi è necessario l'impiego di u n a sorgente ausiliaria di infrarossi, per cui l'oggetto da individuare rinvia per riflessione la radiazione sul rivelatore; nei sistemi passivi la radiazione invece è quella n a t u r a l m e n t e emessa dall'oggetto stesso da individuare. mentre i sistemi a t t i v i vanno diminuendo d'importanza pratica, in quanto la sorgente ausiliaria di raggi infrarossi, in caso di impiego bellico, è anche facilmente individuabile da p a r t e del nemico, quelli passivi incont r a n o sempre più crescente favore, non essendo soggetti a tale limitazione. nei confronti dei sistemi radar, gli infrarossi hanno inoltre notevoli vantaggi. i n f a t t i , dato che la frequenza degli infrarossi è di gran lunga più alta delle onde elettromagnetiche impiegate nei radar, si ha la possibilità di ottenere u n notevole dettaglio degli oggetti rilevati, con il vantaggio che non è necessario disporre della sorgente ausiliaria t r a s m i t t e n t e . p e r illustrare la pronunciata capacità risolutiva del sistema ad infrarossi, nei confronti di quello radar, basterà ricordare come classico esempio, che m e n t r e quest'ultimo è in grado di distinguere s e p a r a t a m e n t e due aerei a 8 chilometri di distanza e che volano a 400 m e t r i uno dall'altro, il sistema ad infrarossi è capace di discernere s e p a r a t a m e n t e addiritt u r a i loro motori. i n paragone al radar, i sistemi ad infrarossi inoltre sono relativamente semplici, di ingombro limitato, leggeri e di basso costo. si è visto i n f a t t i , 4 8 2 w . r u s p a n t i n i che in missili in grado di assolvere missioni simili, il sistema ad infrarossi eosta dieci volte meno di quello radar. di converso è fastidioso per gli infrarossi l'assorbimento da p a r t e dell'atmosfera (nuvole, pioggia, nebbia), che alle basse quote può anche essere di u n a certa importanza. alcuni costituenti atmosferici contengono cariche di legame, con frequenze naturali di vibrazione e rotazione, che dipendono dalla particolare costituzione della molecola. quando queste frequenze caratteristiche si accordano con quelle della radiazione incidente, si manifesta l'assorbimento per risonanza. ne risulta un'ampiezza di vibrazione più grande e l'energia assorbita viene reirradiata in t u t t e le direzioni. in effetti perciò, in una direzione particolare, la radiazione incidente è atten u a t a o assorbita. ma per le applicazioni relative alla navigazione spaziale, il problema della trasmissione atmosferica p r a t i c a m e n t e non esiste, in quanto alle quote elevate il cielo è completamente t r a s p a r e n t e . analogamente ai telescopi ottici, gii apparecchi ad infrarossi sono capaci eli « vedere » a grande distanza e misurare le direzioni e gli angoli con straordinaria precisione. siccome nel campo dell'infrarosso t u t t i gli oggetti sono « luminosi » per sé stessi, non occorre che siano illuminati artificialmente. cosicché la luna, la t e r r a e gli altri pianeti, anche se si trovano in ombra diventano visibili se all'occhio umano viene sostituito l ' a d a t t o strumento. l ' a s t r o n a u t a che naviga nello spazio deve per p r i m a cosa conoscere la sua esatta posizione, il suo orientamento e la propria velocità, onde possa essere esercitata sugli s t r u m e n t i di governo la d o v u t a azione, per il raggiungimento dell'obbiettivo prefisso. se intende m a n o v r a r e il suo veicolo spaziale mediante razzi o getti, egli deve conoscere e s a t t a m e n t e il suo assetto rispetto alla direzione del suo moto. nel caso di veicoli spaziali senza persone a bordo, quanto sopra assume u n ' i m p o r t a n z a decisiva, dato che t u t t e le azioni e le decisioni, debbono essere operate in maniera completamente automatica. la terra, circondata dalla sua fascia atmosferica, è vista da un satellite nello spazio, come un corpo notevolmente caldo, la cui t e m p e r a t u r a assoluta è in n e t t o contrasto con quella dello spazio profondo. il limite dell'orizzonte apparente è situato a u n a decina di chilometri al disopra della superficie terrestre, con u n a incertezza che è dello stesso ordine di quella dovuta alle protuberanze terrestri e alla differenza t r a i raggi ai poli e all'equatore. r e c e n t i t e c n i c h e s u l l / l s i ' r a r o s s o 4 8 3 da un satellite che si trova in un'orbita a 750 k m di altezza, il bordo di tale fascia viene a sottendere un angolo di circa 0,3°. dentro tale angolo veramente piccolo, l'orlo dell'orizzonte appare ad un sistema di rivelazione ad infrarossi, molto ripido, con un salto di t e m p e r a t u r a che è di circa 250° k . approfittando di tale circostanza nei recenti lanci spaziali, la capsula mercury è s t a t a d o t a t a per il suo orientamento, di un perfezionato s t r u m e n t o ad infrarossi, la cui funzione è s t a t a di importanza decisiva all'atto del rientro nell'atmosfera. p e r rilevare con sicurezza la discontinuità dell'orizzonte della terra rispetto allo spazio circostante, lo s t r u m e n t o è stato reso insensibile alla luce solare riflessa, impiegando un filtro di germanio puro, che elimina t u t t e le lunghezze d'onda più corte di 1,8ju, ossia circa il 95°,, dell'energia dovuta all'effetto solare. p e r stabilire la verticale locale, necessaria all'orientamento del veicolo spaziale, il sistema sensorio effettua contemporaneamente due scansioni coniche, ortogonali f r a loro, con un ristretto campo di vista di 2° x 8° ed un angolo di a p e r t u r a di 110°. la scansione viene e f f e t t u a t a a 30 giri al secondo e m e t t e in m a r c a t a evidenza ogni passaggio attraverso l'orizzonte, d a t a la differenza di t e m p e r a t u r a esistente t r a la t e r r a e lo spazio. le uscite dei rivelatori producono segnali ad onda rettangolare, che vengono confrontati con altri segnali generati internamente allo strumento. la differenza di fase f r a i segnali in arrivo e quelli campioni, permettono di determinare con esattezza l'angolo di beccheggio e di rollìo. la cellula impiegata è costituita da. un rivelatore termico a base di ossidi metallici. non è stato usato un elemento fotoconduttivo, per esempio uno dei più recenti tipi di detector a base di germanio a t t i v a t o , d a t o che questo avrebbe richiesto un r a f f r e d d a m e n t o molto spinto e quindi, oltre la inevitabile complicazione, avrebbe rappresentato anche un notevole peso supplementare. un altro apparecchio basato sulla tecnica degli infrarossi, verrà m o n t a t o sul veicolo spaziale mariner, destinato all'esplorazione dei pianeti marte e venere. tale s t r u m e n t o consentirà in un primo tempo di localizzare ad esempio uno di questi due pianeti da 150.000 km di distanza, per mezzo di un sistema di scansione a raggi infrarossi e fornirà successivamente l'orientazione fino a circa 8000 k m di distanza. il rilevamento della posizione del veicolo spaziale in relazione al pianeta, al sole ed alla terra, è fondamentale per la esattezza della raccolta e trasmissione dei dati. i n f a t t i il sistema deve essere in grado di 4 8 4 w . r u s p a n t i n i dirigere verso il sole le b a t t e r i e solari, capaci di produrre u n a potenza di 300 w a t t , e di p u n t a r e le a n t e n n e a l t a m e n t e direzionali sulla terra. il p r o g r a m m a comprenderà sette missioni interplanetarie, t u t t e intorno a venere e a marte. il primo lancio di un prototipo di veicolo spaziale è previsto per il 1963 e tale prova preluderà u n a missione a t t o r n o a marte per il 1964. elevatissima dovrà essere la sensibilità degli strumenti sensori del veicolo spaziale, in quanto le radiazioni giungeranno ad esso in q u a n t i t à molto modesta. a causa delle basse t e m p e r a t u r e delle superfici emittenti, l'energia disponibile per la navigazione sarà quella corrispondente ad onde molto lunghe. venere emette i n f a t t i dalla sommità della sua densa atmosfera che si trova a 225° k , lunghezze d'onda aventi il massimo di emissione a 11,5fi marte, la cui atmosfera è piuttosto r a r e f a t t a , e m e t t e invece dalla sua superficie nella zona equatoriale, che si trova a 280° k , lunghezze d'onda aventi il massimo di emissione a 10a e dalle zone polari, che si trovano a 205° k , lunghezze d'onda aventi il massimo di emissione a 11 11 primo compito dello s t r u m e n t o sensorio è quello d'individuare il pianeta. per f a r ciò, due prismi controrotanti a velocità diversa esplorano mediante u n campo di vista di y2° x u n luogo geometrico a forma di quadrifoglio, che è contenuto in mi cono a v e n t e u n ' a p e r t u r a totale di 70°. il quadrifoglio inoltre viene f a t t o ruotare lentamente, finché il bersaglio viene individuato. in tale i s t a n t e il motore di acquisizione si f e r m a e le radiazioni infrarosse raccolte, vengono t r a s f o r m a t e in tensioni elettriche, per mezzo del detector. misurando la d u r a t a delle tensioni ed i tempi ad essa relativi, il sistema dà luogo ad un segnale di errore, in corrispondenza di ciascuna foglia. allorché mediante opportuna orientazione, questi errori vengono resi tra loro uguali, il pianeta viene a trovarsi in corrispondenza del centro del quadrifoglio. un altro importante s t r u m e n t o basato sulla tecnica degli infrarossi, è stato m o n t a t o sui satelliti tiros, che sono s t a t i lanciati negli usa dalla nasa per osservazioni meteorologiche. si t r a t t a di un particolare radiometro a 5 canali, per la misura di a l t r e t t a n t e bande spettrali, che impiega come riferimento, anziché u n corpo nero campione, la radiazione proveniente dallo spazio e che è prossima allo zero assoluto. nello s t r u m e n t o la faccia di un prisma viene d i r e t t a verso lo spazio, dove la t e m p e r a t u r a è p r a t i c a m e n t e costante e un'altra faccia in direzione della terra. uno specchio r u o t a n t e , che ha m e t à r e c e n t i t e c n i c h e sull/l s i ' r a r o s s o 4 8 5 della sua superfìcie ricoperta di sostanza assorbente gli infrarossi, riceve su u n a m e t à la radiazione da misurare. la rotazione di questo specchio rende a l t e r n a t o l'invio della radiazione alla cellula bolometrica, di guisa che il risultante segnale in corrente a l t e r n a t a , fornisce la misura della differenza t r a la radiazione di riferimento e quella osservata. i 5 canali operano t u t t i in questa maniera. le loro uscite vengono poi amplificate, rettificate e raccolte su u n nastro magnetico, per venire ritrasmesse a comando. ii primo canale raccoglie l'emissione di acqua-vapore intorno ai (iju\ il secondo la luce visibile (0,55-^0,75^); il terzo la radiazione solare (0,2-f5,5/i); il quarto l'emissione totale della terra (7,5-^30/*); il quinto, quale finestra atmosferica, misura la t e m p e r a t u r a della terra o della coltre di nuvole che la ricopre. un altro radiometro a due canali è stato inoltre m o n t a t o sul satellite e p e r m e t t e di determinare il rapporto f r a l'energia infrarossa riflessa dalla terra e quella della radiazione solare totale ricevuta. * radiometri particolarmente sensibili permettono di determinare e misurare dalla terra le radiazioni provenienti dai lontani pianeti e di risalire alla loro t e m p e r a t u r a . è stato così possibile determinare la t e m p e r a t u r a della luna, che quando è completamente illuminata dal sole, raggiunge i 120 °c e scende a 150 °c quando si trova in ombra. similmente sono state determinate le temperature degli altri corpi celesti del sistema solare, che vanno da 400 °c nel caso di mercurio a —150 °c per saturno. il rilievo della t e m p e r a t u r a di corpi che si trovano a distanze così elevate è possibile usando i mezzi della radiometria dell'infrarosso, in quanto la radiazione totale emessa da un corpo è una funzione ben n o t a della sua t e m p e r a t u r a assoluta. i satelliti messi in orbita, possono ugualmente venire localizzati da terra ed inseguiti da perfezionati strumenti, che ne misurano la radiazione emessa. radiometri costruiti secondo le tecniche più recenti sono in grado di effettuare rilevazioni di bersagli piccoli e lontani, in rapido movimento, su u n a varietà di sfondi n o t t u r n i e diurni. sistemi di scansione a reticolo, respingono i segnali uniformi del fondo (filtraggio spaziale), permertendo che misure (li contrasto possano venire e f f e t t u a t e d u r a n t e il giorno, 4 8 0 \v. k u s p a n t i x t allorché la radiazione del bersaglio è mascherata dalla radiazione del cielo. apparecchi di tracking automatici all'infrarosso agganciano e seguono missili e velivoli veloci e forniscono gli errori in azimuth ed in elevazione ad un servo-piedistallo, che p u n t a la sua linea di mira e quella di ogni s t r u m e n t o collegato, a c c u r a r a t a m e n t e sul bersaglio in movimento. nelle installazioni a terra tali sistemi vengono usati anche come sicurezza di poligono per seguire i missili durante le fasi iniziali di volo, in cui il r a d a r è ostacolato dal clutter del terreno. p e r lo studio del rientro dei missili nell'atmosfera terrestre, gli strumenti di cui si è f a t t o cenno, trovano la loro più naturale applicazione. negli usa, allo scopo di investigare le caratteristiche fisiche del rientro dei missili balistici, u n a n a v e è s t a t a t r a s f o r m a t a in osservatorio galleggiante ed a t t r e z z a t a con una grande varietà di strumentazione, comprendente apparecchi radar, inseguitori • ad infrarosso e s t r u m e n t i ottici. shahrisvand_finalonline_layout 6 annals of geophysics, 57, 5, 2014, a0543; doi:10.4401/ag-6612 a0543 detection of gravity changes before powerful earthquakes in grace satellite observations mohsen shahrisvand1, mehdi akhoondzadeh1,*, mohammad ali sharifi2 1 university of tehran, college of engineering, surveying and geomatics engineering dept., remote sensing division, iran 2 university of tehran, college of engineering, surveying and geomatics engineering dept., geodesy division, iran abstract the variations of gravity field have been analyzed in this article, in order to find disturbances in the vicinity of recent great earthquakes epicenters including chile (february 27, 2010), tohoku-oki (march 11, 2011) and indian ocean (april 11, 2012) prior to the events. for this purpose, the 10 years long time series of gravitational gradient components obtained from weekly gravity recovery and climate experiment (grace) solutions have been used. some of gravitational gradient components are independent to grace stripy error and amplify high frequency components of gravity field. therefore that preseismic activity can be better illustrated. the interquartile method has been used to construct the higher and lower bounds in time series to detect outlying solution outside the bounds associated with impending earthquakes. afterwards, nonlinear auto-regressive models with exogenous inputs (narx) neural network has been proposed in the detection process of prominent gravity field anomalies prior the earthquakes. both methods detect considerable anomalous occurrences during 2-5 weeks prior to the earthquakes. our results statistics show that the anomalous deviations before the earthquakes have different signs and amplitudes at different cases. 1. introduction earthquakes are one of the most destructive disasters that affect normal life of millions of people every year. therefore, prediction of earthquake before its occurrence can save the life of so many victims. during an earthquake preparation, a dynamic process involves an energy transfer due to crust displacement and, at the time of the shock a breakdown between the source and the environment occurs. the change prior to the earthquake or along with it, may have different physical and chemical effects on the lithosphere, atmosphere and ionosphere, and accordingly makes it possible to be detected. these variations of lithosphere, atmosphere and ionosphere parameters before the main earthquakes are considered as a hint of impending earthquakes (earthquake precursors) [akhoondzadeh et al. 2010]. earthquakes prediction is mainly based on the observation of precursory phenomena. extensive researches in domain of earthquake prediction have resulted in recognition of many earthquake precursors in the lithosphere, atmosphere and ionosphere. the effects of preseismic activity on the lithosphere can be investigated using gravity change. during the primary period of an earthquake, the gravity field of the seismic region may change due to the crust deformation or medium-property variation. therefore, the effects of the pre-seismic activity on the gravity field can be investigated by measuring the variations of functional of gravity field. recently, several ground based observations have shown the possibility of gravity disturbance generation by earthquake preparation processes [e.g., lan et al. 2011, shen et al. 2011]. there is lack of extensive ground experiments to monitor geophysical parameters in most areas. but due to the vast coverage of the seismic zones, satellite experiments are regarded as a promising mean to study earthquakes. the gravity recovery and climate experiment (grace) has been the first space mission which can measure temporal variations of the earth’s gravity field. therefore, it can be a useful tool for assessing changes in the gravity field related to earthquakes. the grace mission is a space borne gravity mission launched in march 2002 jointly by the national aeronautics and space administration (nasa) and german aerospace center (dlr). grace consists of two identical spacecrafts co-orbiting at low altitude (~450 km), separated by ~220 km and linked with k-band micro-wave ranging (kbr) satellite-to-satellite tracking (sst) system. article history received june 24, 2014; accepted september 4, 2014. subject classification: earthquake, precursor, gravity, grace, ionosphere, gravity anomalies. little changes in the distance between grace satellites (measured by kbr) reflect mass anomalies within the earth system and indicate changes in earth’s gravity field [tapley et al. 2004]. displacement taking place immediately during the earthquake are called coseismic. the dislocated masses disturb the isotactic equilibrium of the crust and mantle, thus including a long term postseismic relaxation process [einarsson et al. 2010]. previous studies demonstrated coseismic and postseismic gravity field changes related to recent great earthquakes. han et al. [2006] for the first time revealed coseismic gravity changes by level-1 grace data (range and range-rate measurements) due to great sumatra-andaman (sa) earthquake (mw = 9.1-9.3). ogawa and heki [2007] processed the grace 36 monthly gravitational field data (level 2 or spherical harmonics) to derive coseismic and postseismic geoid variations due to sumatra earthquake. chen et al. [2007] showed that improved grace monthly gravity fields are able to reveal the coseismic changes with greater spatial resolution. they used variations of equivalent water height with p3m6 and 300 km gaussian filter. panet et al. [2007] showed coseismic and postseismic signal for both sumatra and nias earthquakes using wavelet analysis. han and simons [2008] used slepian’s spatiospectral localizing basis functions, as an alternative to the usual spherical harmonic functions. they estimated the coseismic jump at the time of the sumatra earthquake. han and simons [2008] also use slepian basis functions estimated from in-situ data. they determine a coseismic jump as well as an exponential-like post-seismic signal. de linage et al. [2009] determined the earthquake signal of the sumatra earthquake. they use the regularized cnes solution as well as the monthly csr release four solution. they examine the possible effect of hydrological signals on the estimated earthquake signals by using hydrological models. they are able to detect both coand postseismic signals of the sa earthquake. other studies using grace to detect or constrain coseismic or postseismic deformations of great earthquake include: i) einarsson et al. [2010], broerse et al. [2011], cambiotti et al. [2011], wang et al. [2012a], han et al. [2013] for the 2004 sumatra-andaman and nias earthquake; ii) han et al. [2010], wang et al. [2012b], han et al. [2013] for the 2010 chile maule event; iii) han et al. [2011], matsuo and heki [2011], wang et al. [2012c], han et al. [2013] for the 2011 tohoku-oki earthquake; iv) han et al. [2013] for 2012 indian ocean earthquake. despite all the efforts to detect gravity anomalies before large earthquakes [e.g., seiji and nakamura 2013], previous studies only detected coseismic and postseismic gravity field changes. however, there are no reports associated with detection of preseismic gravity changes using grace measurements. in this study, we analyzed grace observations before recent megathrust earthquake in order to detect gravity anomalies as an earthquake precursor. if it can be confirmed that gravity perturbations are a real and systematic occurrence, then it is possible to consider them as short-term precursors that happen before earthquakes. since any particular earthquake precursor can not be considered as an accurate stand-alone tool for earthquake prediction and it should not be expected that every precursor appear in every earthquake therefore it is required to exploit and integrate different precursors from different experiments to achieve more accurate results. in order to determine whether some gravity changes before earthquake have unique characteristics related to seismic activity, this study examines 10 years of gravitational gradient components obtained from grace satellite data in the seismic region. section 2 will explain the method of gravitational gradient calculation by grace data, applied methods for anomaly detection are discussed in section 3 and then seismo gravitational gradient anomalies observed for three last earthquakes with magnitude greater than 8.0 have been studied in section 4. 2. grace data processing in this study, we used weekly level 2 (l2) global gravity solutions of geoforschungszentrum (gfz, 30 potsdam rl05) which are composed of fully normalized spherical harmonic coefficients complete to degree and order 30. a total 448 gravitational field solution covering from first week of january 2004 to third week of february 2013 are used. the main reason for using weekly data instead of monthly solutions is that weekly data reveals short-time (shorter that one month) variations of gravity field in the seismic region. the earth’s oblateness values (c20) has been replaced by those of the satellite laser ranging (slr) because of its accuracy [cheng and tapley 2004]. then in order to compute variations of gravity field the spherical harmonic coefficients of each individual weekly solutions have been subtracted from mean of solutions between january 2004 and december 2012. for the study period, we computed the full gravitational gradient tensor (second derivative of gravitational potential) in spherical coordinates in 1 degree by 1 degree spherical regular grid. to better demonstrate the local mass anomaly, the local north-east-down frame (ned) at each point with spherical coordinate is introduced: x axis is directed to the north, the y-axis to the east, and the z-axis 1 downward. based on the prinshahrisvand et al. 2 3 cipal of converting between coordinate systems, the full gravitational 2 gradient tensor in this local ned frame is obtained [wang et al. 2012a]. to obtain reasonable estimates of time-varying gravity filed signals, spherical harmonic coefficients should be filtered because high frequency components (high sh degree) are poorly determined [han et al. 2005]. this problem leads to creation of features that elongated in the north-south direction, usually described as striping patterns. several authors have shown that non-isotropic filters [e.g., swenson and wahr 2006] account for the grace stripes more favourably compared with traditional isotropic gaussian [jeckeli 1981]. therefore, we make use of non-isotropic ddk3-filter [kusche 2007, kusche et al. 2009] that is specifically designed for this purpose. among five independent components of gravitational gradient tensor, some components are less sensitive to grace striping errors. as mentioned earlier, grace striping errors distributed in north-south direction, thus these strips generate fluctuations in the east-west direction [swenson and wahr 2006]. the north-south component (x-component) of the gravitational gradient tensor has less sensitivity to grace striping errors and have more signal to noise ratio than other components. as an example, we compute coseismic ∞vxz changes induced by the march 11, 2011, mw 9.0 tohoku-oki earthquake using csr release (rl) 05 grace level 2 monthly gravitational field data products without applying any destriping and spatial smoothing filtering procedure and the achieved results are shown in figure 1. as it can be seen in figure 1, tacking derivative related to x-axis caused dramatically suppress stripes errors. similar results can be obtained for ∞vxx component. due to the less sensitivity behaviour of grace striping errors and more signal to noise ratio rather than other components. by considering this fact, we have used only ∞vxx and ∞vxz components for time series calculation. in addition for geophysical phenomena of regional scale which are dominated by high frequency content (such as earthquake), gravitational gradient changes are more useful. these components enhance high frequency components of the earth gravity field and reveal more details in temporal domain. the changes in grace residues due to hydrological effect can be equal or even greater than the seismic effect of earthquake. these effects are larger in continent area compared to offshore area. in the final solutions models that were used for de-aliasing grace raw data, some errors were introduced at long periods which is the case for ocean tidal models [ray and luthchke 2006]. model errors of the s2 tidal wave create an alias at 161-d period that is clearly visible in the grace signal. another source of variability in grace residues are seasonal and inter-annual changes in ocean circulation. these variations are smaller than other hygravity changes before powerful earthquakes figure 1. grace global ∞vxz changes between of 2011 and 2012 and mean of 2009 and 2010 without applying any destriping and spatial smoothing filtering procedure (in unit of mili-eotvos). drologic effects and their amplitude is often at noise level [de linage et al. 2009]. from these considerations, to separate the abovementioned effects in the grace data, we adopted the following strategy. at each point of a 1*1 grid, we simultaneously fitted to the selected gravitational gradient components time series the following time-function [chen et al. 2007]: (1) where t is time and t0 is time of earthquake occur and model parameters are: 1) a1, {1, a2, {2 are amplitude and phase of the annual and semi-annual waves to model the seasonal and annual variations of hydrology and long period oceanic circulation before earthquake 2) a3, {3 are amplitude and phase of a 161-d sine curve to correct the errors on the s2 tidal wave before earthquake 3) a4, {4, a5, {5 are amplitude and phase of the annual and semi-annual waves to model the seasonal and annual variations of hydrology and long period oceanic circulation after earthquake 4) a6, {6 are amplitude and phase of a 161-d sine curve to correct the errors on the s2 tidal wave after earthquake 5) b and b are linear trends before and after earthquake 6) a and a are mean of signal before and after earthquake. we computed these parameters by nonlinear leastsquares adjustment. then we removed effects of linear trend, annual, semi-annual and s2 tidal wave from each time series of each grid point before and after earthquake. 3. methodology at each case study, using the reported geographic latitude and longitude concerning the earthquake epicenter, time series of gravitational gradient components (∞vxx and ∞vxz) obtained from grace measurements during some years before earthquake has been analyzed. by considering this fact that other preseismic anomalies (e.g. ionosphere precursors) does not occur in the vertical projection of earthquake epicenter, outskirt of each epicenter has been tested in order to detect anomaly. in order to detect anomalies in a time series, a reasonable range of gravitational gradient variations are required to be determined. therefore interquartile range (iqr) and artificial neural network (ann) methods have been implemented to observe the discord patterns in the gravity variations. 3.1. inter-quartile range method the inter-quartile range of data is used as the primary method for anomaly detection. the upper and lower bounds of time series is calculated using the following equations: (2) (3) where x, xhigh, xlow, m, iqr and dx are data value, upper bound, lower bound, median value, inter-quartile range of data and differential of x, respectively. according to this, if the absolute value of the data would be greater than k.iqr (|x|> k.iqr), the behaviour of the relevant parameter (x) is regarded as anomaly. according to equation (4), indicates the percentage of parameter change from the undistributed state. therefore in the anomaly detection from ∞vxx and ∞vxz time series, m and iqr values were computed over total time interval of interest for each case study. 3.2. artificial neural network method a time series is a sequence of vectors that depend on time. in order to forecast time series of gravitational gradient variations before and after occurrence of each earthquake, ann technique has been implemented. the problem of prediction can be formulated as finding a function f to obtain an estimated of the vector y at time t + p (p = 1,2,...), given the values of y up to time t: where y(t) represents output of model as time t, dy is the lag of the system and f a nonlinear function. typically, p=1, measure one-step ahead, but can take any value larger than 1 at multi-step ahead [dorffner 1996]. before 1980s, for time series prediction, methods of linear parametric autoregressive (ar), moving-average (ma) or autoregressive moving-mverage (arma) introduced by box and jenkis [pollock et al. 1999]. these .x m k iqrhigh = + .x m k iqrlow = p k dx k 100!= -c m y t p+t^ h , ...,y t p f y t y t dy+ = -t^ ^ ^^h h hh ;x x x k iqr x m k dx iqr x m low high &1 1 1 1= shahrisvand et al. 4 , cos cos y t t a bt a t a b t a t if t t if t t i i i i i i i i 0 1 3 3 3 1 3 0 0 # # ~ { ~ { = + + + + + + = + + = l l ^ ^ ^ h h h z [ \ ]] ]] / / (4) 5 models are linear and are unable to cope with certain non-stationary signals, and signals whose mathematical model is not linear. artificial neural networks are a type of flexible nonlinear models that can discover patterns adoptively from the data. in many researches, ann has been proposed as a promising approach for time series forecasting. several successful applications of this method demonstrated that ann can be a very effective tool in modeling and forecasting non-linear time series [akhoondzadeh 2013]. among equations which require input/output relationships, anns are trained by sample data to build the input/output vector maps in an implicit way. therefore, anns can solve highly nonlinear problems without the need to define the relationship between inputs and outputs [yang et al. 1997]. a neural network can be classified into static and dynamic categories. the static feed forward network has no feedback elements and contains no delays (figure 2). in these networks output is calculated directly from the input through feed forward connections like back propagation (bp) neural networks. in general, bp is effective only when the network architecture is chosen correctly. too small network cannot learn the problem well, but too large sized network leads to over fitting and poor generalization performance. in dynamic networks, the input depends not only on the current input to the network, but also on the current or previous, outputs or states of the network. in this paper, the architectural approach proposed to predict gravitational gradient changes time series is one based on “nonlinear autoregressive models with exogenous inputs (narx model)”, which are therefore called narx recurrent neural networks [menezes et al. 2006]. this is a powerful class of dynamic neural networks and it has been proven that they are well suited for modeling nonlinear systems and specially time series [diaconescu 2008]. the narx model is commonly used in time series modeling, where the next value of the dependent output signal y(t) is regressed using the previous values of the output signal and previous values of an independent input signal. figure 3 shows the basic architecture of a narx model capable in time series forecasting. the defining equation for the narx model is: in order to determine the best network configuration, the effective parameters, which influence the value of predictive error, including the number of pattern of input; lags of input and output; the number of hidden layers and their number of neurons; the activation functions and the learning algorithm have been obtained via iteration process to assess the minimum predictive error when the training process was implemented. the best results indicate the linear and sigmoid activation functions for output and hidden layers, respectively. in general, in function approximation problems, for networks that contain up to a few hundred weights, the levenberg–marquardt algorithm will have the faster convergences [diaconescu 2008]. therefore the levenberg–marquardt optimization has been selected as training algorithm. however other training algorithms were evaluated and the results were poorer. to implement prediction process, n observations y1,y2,...,yn are selected as training set and the remain ones yn+1,yn+2,...,yn+m are considered as test set. the number of input nodes corresponds to the number of lagged observations to discover the underlying pattern in a time series. in this study a network with six nodes in the input layer, 15 nodes in the first hidden layer, 10 nodes in the second hidden layer and one node in output layer has been proposed. the training patterns in the proposed network are: , , ..., , , , ..., y t f y t y t y t n x t x t x t n 1 2 1 2 y x = ^ ^ ^ ^^ ^ ^ ^ h h h h h h hh gravity changes before powerful earthquakes figure 3. narx neural network for time series prediction.figure 2. a static feed forward neural network. usually, the evaluation of prediction performance is done by finding optimized weights such the prediction error (pe) is minimized. pe equation can be written as: (5) where is the output of network. the testing patterns are, finally, if the difference value between the actual value and predicted values is outside the predefined bounds, the anomaly is detected. 4. implementation in order to ensure reliable detection of gravity field variation using grace data before the earthquake, results for three most recent earthquakes with magnitude greater than 8.0 in maule (chile), tohoku-oki ( japan), and indian ocean have been studied. 4.1. chile maule earthquake (february 27, 2010) the february 27, 2010, maule chilean earthquake (mw = 8.8), which was caused by the subduction of the nazla plate underneath the overlying south america plate, is the sixth largest event in the seismic record (figure 4). data from teleseismic networks, coastal/river markers, tsunami sensors, global positioning system (gps), and interfero-metric synthetic aperture radar (insar) have been used to observe and model the coseismic signature and slip history of this devastating event. in addition, spaceborne gravimetry data from grace have been used to observe the coseismic deformation signal of 2010 maule earthquake [han et al. 2010, wang et al. 2012b, han et al. 2013]. figures 5a and 5b illustrate the variations of ∞vxx and ∞vxz components, obtained by grace measurements from first week of january 2004 to third week of february 2013. a vertical dotted line indicates the earthquake time. the x-axis represents actual time and y-axis represents variations of ∞vxx and ∞vxz components by scale of 1013 (unit is s-2). by visual inspection in both time series, we can see a strange decreasing in both signals two weeks before the earthquake date. it should be noted that there is a horizontal derivative operator shift phase of mass anomaly in the spatial domain. therefore we should not expect that preseismic anomaly for different components of gravitational gradient components occur in the same location. therefore, the locations for time series calculation for ∞vxx and ∞vxz components are different. when implementing the interquartile method, dx is calculated using equation (4) for both components. the ∞vxx and ∞vxz values exceed the lower bounds (m2.6*iqr for δvxx and m-3.1*iqr for δvxz) two weeks before earthquake with values of 29% and 30% of the lower bounds, respectively. figures 6a and 6b represent the observed and predicted ∞vxx and ∞vxz values using narx neural network, respectively, during the time period of 2008 to 2013. figures 6c and 6d represent the differences between observed and predicted values during the testing data. these figures clearly show unusual differences two weeks prior to the earthquake. the ∞vxx and ∞vxz values exceed the lower bounds with values of 14% and 49%. in other words, on these weeks, the narx neural network has not been able to predict the gravitational gradient variation based on model deduced from training data. therefore, the observed values at mentioned week could be considered as earthquake anomaly. thus, as indicated previously, the two applied methods on gravitational gradient time series of chilean pe y t k t ky k n 0 = -t^ ^^ h hh/ yt , , , , , , , , , , , , , , , y f y y y t t t y f y y y t t t y f y y y t t t n n n n n n n n n n n n n n n m n m n m n m n m n m n m 4 1 2 3 1 2 3 5 2 3 4 2 3 4 3 2 1 3 2 1 = = = + + + + + + + + + + + + + + + + + + + + + ^ ^ ^ h h h , , , , , , , , , , , , , , , y f y y y y f y y y t t t y f y y y t t t t t t 2 2n n n n n n n 4 1 2 3 1 2 3 5 2 3 4 2 3 4 3 1 3 1 = = = ^ ^ ^ h h h shahrisvand et al. 6 figure 4. the geographic location of maule earthquake epicenter. a vacant star represents epicenter. (figure taken from: http://earth quake.usgs.gov). 7 earthquake detect a noticeable anomalies two weeks prior to the earthquake. 4.2. tohoku-oki earthquake (march 11, 2011) the tohoku-oki earthquake, mw 9.0, which occurred at 05:46 ut, on march 11, 2011, at the japan trench east of ne japan (figure 7), ruptured the fault as large as 500~200 km [ozawa et al. 2011]. earthquake of this size and magnitude cause mass-dislocation on the scale that is measurable by grace gravity satellite mission. some articles demonstrated coseismic and postseismic gravity changes due to this earthquake [han et al. 2011, matsuo and heki 2011, wang et al. 2012c, han et al. 2013]. there are several publications indicating the preseismic tec anomalies concerning tohoku-oki earthquake [e.g., heki and enomoto 2011, akhoondzadeh 2013]. in addition tsuboi and nakamura in 2013 demonstrate sea surface gravity change gravity changes before powerful earthquakes figure 6. (a) and (b) variations of the observed (green curve) and predicted (blue curve) gravitational gradient obtained from narx method. (c) and (d) variations of the differences between the observed and the predicted values of ∞vxx and ∞vxz. figure 5. weekly time series of ∞vxx and ∞vxz components obtained from grace level 2 data after ddk3 filtering. seasonal (annual and semiannual and s2 tidal wave) signals and linear trend are removed from time series using least squares. the black vertical lines indicate the date of the earthquake. the red horizontal lines indicate the upper and lower bounds. the green horizontal lines indicate median values. figure 7. plate configurations of the japanese islands. the focal mechanism of the tohoku-oki earthquake is taken from the global centroid-moment-tensor project. the red arrows indicate relative motion between the two plates at a plate boundary [ozawa et al. 2011]. three months before this earthquake has been measured by shipboard geophysical observations. but there are no reports showing preseismic gravity changes in grace measurements. by implementing the interquartile method (equation 4), the dx values for ∞vxx and ∞vxz exceed the lower bounds, three weeks before earthquake with the values of 18% and 26% , respectively (figure 8). after the event there are some outlier points of predefined bounds. due to the changes of the mass distribution in the studied region, the same behavior should not be expected from the signal before and after the earthquake in the seismic region. in order to analyze signals of gravity field after the earthquake for anomaly detection, the median and interquartile related to this period should be used. figures 9a and 9b represent the observed and predicted ∞vxx and ∞vxz values using narx neural network, respectively, during the time period of 2008 to 2013. figures 9c and 9d represent the differences between observed and predicted values during the testing data. these figures clearly show unusual differences three weeks prior earthquake. the ∞vxx and ∞vxz values exceed the lower bounds with values of 30% and 38%. in other words, on these weeks, the narx neural network has not been able to predict the gravitational gradient variations based on model deduced from training data. therefore, the observed values at mentioned weeks could be considered as earthquake anomaly. thus, as indicated previously the two applied methods on gravitational gradient time series of tohoku-oki earthquake detect a noticeable anomaly on three weeks prior to earthquake. 4.3. indian ocean earthquake (april 11, 2012) the mw 8.6 earthquake occurred on april 11, 2012, at the depth of 40 km (global cmt), located at nw indian, australian and sunda plates (~500km, west shahrisvand et al. 8 figure 9. same as figure 6 for tohoku-oki earthquake. figure 8. same as figure 5 for tohoku-oki earthquake event. of december 26, 2004, earthquake) (figure 10). it is the largest interplate strike-slip earthquake in the known history. on the same day, with delay of ~2 hours, ~120 km away from the main event, another earthquake of mw 8.1 occurred [shrivastava and reddy 2013]. some authors compute coseismic displacement [shrivastava and reddy 2013] and gravity changes observed by grace measurements [han et al. 2013]. figures 11a and 11b show the time series of ∞vxx and ∞vxz variations during 2005 to 2013 obtained from grace measurements. the sudden jump in earthquake date represents coseismic jump. in this case study, since the december 26, 2004, sumatra–andaman earthquake (mw 9.1-9.3) occurred near the epicentre of this earthquake, therefore we computed time series during 2005 (one month after sumatra earthquake) to 2013. by implementing the interquartile method the dx values for ∞vxx exceed the upper bound five weeks before earthquake with values of 18% of higher bound. gravity changes before powerful earthquakes figure 10. the geographic location of indian earthquake epicenter. a vacant star represents epicenter. (figure taken from: http:// earthquake.usgs.gov). figure 12. same as figure 6 but for indian earthquake. figure 11. same as figure 5 for indian earthquake. on three weeks prior the earthquake, the ∞vxz value passes the upper bound by 24% . figures 12a and 12b represent the observed and predicted ∞vxx and ∞vxz values using narx neural network, respectively, during the time period of 2008 to 2013. figures 12c and 12d represent the differences between observed predicted values during the testing data. these figures clearly show differences five weeks prior earthquake. the δvxx value exceeds the upper bound with values of 56%. there is another anomaly that has been detected by narx neural network with values of 30% three weeks before earthquake that iqr method has not been able to detect it. the δvxz value exceeds the upper bound with value of 68% of higher bound five before earthquake. there is another anomaly has been detected by narx neural network three weeks before earthquake with value of 60% that iqr method has not been able to detect it. in other words, on these weeks, narx neural network was unable to predict the gravitational gradient variations based on the model that was deduced from training data. therefore, the observed values at mentioned weeks could be considered as earthquake anomaly. thus, as indicated previously on gravitational gradient time series of indian ocean earthquake, a noticeable anomaly was detected in three and five weeks prior to the earthquake. 5. conclusion it has been shown in this study that weekly time variable gravity solutions obtained from grace satellite data are able to detect striking anomalies in the gravity field, in the vicinity of recent great earthquake epicenters several weeks before their happenings. traditionally, to examine preseismic gravity field activity in the seismic region, gravitational changes have been analyzed. due to grace stripy errors, analysis of gravitational changes does not show any anomalies before earthquakes. in addition, the efficiency of integrating interquartile and narx neural network methods to detect anomalies in gravitational gradient time series has been shown in this study. it may be worthwhile to mention that ann has been used for the first time in this study to successfully detect anomalies in time series of gravity field variations. the results indicate that highest deviations from normal state that were regarded as anomaly appeared within the time interval 2-5 weeks before earthquakes. it should be pointed out that preseismic gravity anomalies before earthquake can be positive as well as negative. in this study, it has been shown that detected anomalies using applied methods can be related to the three recent great earthquake since variations of gravity field represented by gravitational gradient changes shows the anomalies a few weeks prior these earthquakes during several years data over the region of impending earthquakes. references akhoondzadeh, m., m. parrot and m.r. saradjian (2010). electron and ion density variations before strong earthquakes (m>6.0) using demeter and gps data, natural hazards and earth system sciences, 10, 7-18; doi:10.5194/nhess-10-7-2010. akhoondzadeh, m. 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temperature, canadian journal of soil science, 77, 421-429. *corresponding author: mehdi akhoondzadeh, university of tehran, university college of engineering, surveying and geomatics engineering department, remote sensing division, tehran, iran; email: makhonz@ut.ac.ir. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. shahrisvand et al. 12 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 459_465 brunozzi .pdf 459 annals of geophysics, vol. 46, n. 2, april 2003 a lidar for water vapour measurements in daytime at lampedusa, italy paolo tini brunozzi (*) and franco marenco (**) enea-casaccia, gem-clim, s. maria di galeria (rm), italy abstract enea is planning to develop a lidar system for measurement of the vertical profi le of water vapour mixing ratio in daytime at a remote site, the station for climate observations located in lampedusa, italy. the raman lidar technique has been retained because of its experimental simplicity with respect to dial, and the uv spectral range has been chosen because raman cross-sections and detector effi ciencies are larger. for a wavelength larger than ~ 300 nm the signal is limited in daytime by sky background, but extinction is acceptable, and the aims of the system can be reached with a strong laser source. the 355 nm wavelength of a frequency-tripled nd:yag laser has been retained as this laser source permits to reach a large pulse energy while keeping the system simple to operate. geometrical form factor calculations need to be performed to evaluate the near-range overlap between the laser beam and the fi eld-of-view of the receiver. among several options, a dual-receiver system has been retained to account for the several orders of magnitude expected in the backscattered signal intensity: a smaller receiver, with a primary mirror of 200 mm diameter for the 0.2-1 km range, and a larger 500 mm receiver for the 1-3 km range. mailing address: dr. franco marenco, enea-casaccia, gem-clim, via anguillarese 301, 00060 s. maria di galeria (rm), italy; e-mail: franco.marenco@casaccia.enea.it key words raman lidar water vapour (*) present address: isic, foligno (pg), italy. 1. introduction water vapour plays an important role in several meteorological and climate processes, and it can be considered one of the most important substances present in the atmosphere. water vapour infl uences the radiation fi eld directly by acting as a greenhouse gas (absorption in the infrared part of the spectrum). due to cloud formation, evaporation and precipitation, water in the vapour and condensed states affects the transport of latent heat, with effects on dynamics and meteorology. clouds also infl uence the radiation budget, especially through scattering of visible light and increasing of the terrestrial albedo. water vapour profiles can be measured with active sensors, such as lidar. a lidar is based on the emission of light pulses into the atmosphere and on the measurement of the radiation backscattered by the atmospheric layers under investigation. the original idea of probing the atmosphere with light beams dates back to tyndall (1869), but it is only with the advent of laser technology that the lidar began to be used more systematically (fiocco and smullin, 1963; ligda, 1963). since then, the fi eld has progressed rapidly and new systems have been developed (hinkley, 1976; ansmann et al., 1997; burke et al., 1998; bisson et al., 1999; ellsworth et al., 2000; dabas et al., 2001; wulfmeyer and walther, 2001). the advantages of the lidar technique reside in the high vertical and temporal resolutions, with the possibility to make continuous measurements with a (**) present address: dipartimento di fisica, università di genova, italy. 460 paolo tini brunozzi and franco marenco large repetition rate at a relatively low cost of operation. simulation of system performance is a useful method for determining the important experimental parameters (see, among others, whiteman et al., 2001; wulfmeyer and walther, 2001a,b). the national agency for new technology, energy and environment (enea) runs the station for climate observations located in lampedusa, italy (35.5' n, 12.6' e). several instruments are installed in the station and monitor several important atmospheric parameters such as the concentration of greenhouse gases, total ozone, aerosol load, solar radiation and meteorological parameters (chamard et al., 1999; di sarra et al., 2001; marenco et al., 2002; meloni et al., 2003). enea is planning to install in lampedusa a lidar system for measurement of the vertical profi le of water vapour mixing ratio, based on the raman technique: this technique is based on a shift in received wavelength to identify the scattering molecule. raman lidar measurements of water vapour have been performed by vaughan et al. (1988); whiteman et al. (1992); goldsmith et al. (1998); bisson et al. (1999), and many others. raman lidar measurements in daytime are somewhat critical due to low backscattered signal and large background. however, quite powerful systems have been deployed recently and have shown that it is possible to reach the range of 5-7 km in daytime (goldsmith et al., 1998; bisson et al., 1999). the aim of this paper is to perform a simple numerical simulation of system performance, based on realistic component properties. our sizing considerations are based on the goal to reach the 1 km vertical range, with a signal-to-noise ratio of at least 7, in daytime when averaging over two minutes temporal resolution; to reach 3 km in daytime with a 30 min temporal resolution, and to reach the tropopause at nightime. we are aware, however, that the diffi culty of reaching larger altitudes may constitute a limitation for the study of planetary boundary layer meteorology. 2. lidar system simulation raman lidar allows profi les of mixing ratio w to be determined by taking the ratio of raman backscatter by water vapour to raman backscatter by one of the well mixed gases such as nitrogen (whiteman et al., 1992). the emitted wavelength was chosen based on several parameters: a) water vapour backscattering cross-section; b) atmospheric extinction; c) sky background intensity; d) laser source availability and power, and e) detector effi ciency. for raman scattering, the backscattering cross-section follows a l– 4 law, similarly to rayleigh scattering: therefore, if we want to put all chances on our side, short wavelengths are more interesting and the ultraviolet band is most indicated. choosing to work in the uv also achieves a better detector effi ciency than in the visible and near infrared bands. when working in the uv wavelength range, however, the absorption spectrum of ozone must be considered, since it dominates the atmospheric radiation fi eld. for l < 300 nm we have the «solar blind» case, since ozone absorption prevents solar radiation from reaching the ground; however, the signal is strongly reduced by extinction due to even small concentrations of ozone in the troposphere. for this reason «solar blind» systems are confi ned to a ~ 1-2 km vertical range (renaut et al., 1980; petri et al., 1982). we have therefore confi ned ourselves to l > 300 nm. the weakness of the signal due to the small water vapour backscattering cross-section, and the large sky background put a constraint on laser output power, which must be as large as possible. the 355 nm wavelength was chosen due to the availability of the nd:yag laser with third harmonic generation: this source is in fact simple to use and quite powerful. two important parameters are the telescope area and field-of-view (fov). a large receiver aperture determines a larger signal and a narrow fov implies a smaller sky background: therefore, at a first approach it would seem necessary to have a large telescope with a narrow fov. on the other hand, however, such a receiver may not be suitable for observing the nearby atmospheric layers, due to incomplete overlap between transmitted beam and receiver fi eld-of-view. the choice of telescope area and fov is the result of a balance between the need to reach far atmospheric layers and the need to view the nearby layers: it is therefore important to evaluate the geometrical form factors for 461 a lidar for water vapour measurements in daytime at lampedusa, italy different telescopes. such calculations were made according to halldorsson and langerholc (1978), using a computer code by vincenzo santacesaria (see e.g., santacesaria et al., 1998). a coaxial geometry was considered, and we found that two different telescopes (200 mm and 500 mm in diameter, with different fi elds-of-view) must be considered for entirely viewing the 0.2-3 km interval (see table i). using two telescopes with different apertures is important not only for geometrical form factor considerations, but also for properly detecting signals varying over several decades. figure 1 summarizes the results of the geometrical form factor calculations. what we have labelled «half overlap range» refers to the range where the geometrical form factor is 0.5. normally, for elastic backscattering lidar measurements one would consider valid only data obtained with a geometrical form factor of 1 (full overlap). however, since for raman lidar the ratios between signals measured at the same distance at two wavelengths must be considered, it is admissible to use data obtained at shorter distances. possible differences in the overlap between the channels can be corrected for by experimentally determining the ratio between them as a function of range before each measurement session, by using raman nitrogen fi lters on both channels (whiteman et al., 1992). this permits us to reach atmospheric layers quite close to the telescope. we conventionally chose to consider the «half overlap range» as the fi rst usable range for water vapour raman lidar measurements, but we feel that in practice closer ranges can be reached. the other system constants used in this paper are summarized in table ii. values for pulse energy, pulse repetition frequency and quantum effi ciency for commercially available lasers and photomultipliers were assumed. the optical effi ciency was conventionally assumed to be 5%, and the range resolution to be 150 m. the zenith table i. inputs used for geometrical form factor calculations and results. telescope small large beam divergence (mrad) 0.1 0.1 beam diameter (mm) 50 50 primary diameter (mm) 200 500 secondary diameter (mm) 50 120 focal length (mm) 600 1250 diaphragm diameter (mm) 0.6 0.5 field-of-view (mrad) 1 0.4 half overlap range (m) 150 910 full overlap range (m) 280 1830 table ii. lidar system. constants considered for simulation. emitted wavelenght 354.7 nm received wavelenght 407.5 nm receiver bandwidth 0.4 nm pulse energy 325 mj pulse repetition frequency 30 hz quantum effi ciency 0.25 optical effi ciency 0.05 vertical resolution 150 m zenith sky radiance 0.32 wm-2sr-1nm-1 fig. 1. geometrical form factors for the two receivers: small telescope (solid line); large telescope (dashed line). 462 paolo tini brunozzi and franco marenco sky spectral radiance at the received wavelength was derived from measurements taken by a licor spectrophotometer at lampedusa at noon in the summer. the atmospheric profile of water vapour needed for lidar signal calculation, was derived by assuming a 0.005 volume mixing ratio at ground level and an exponential decrease with altitude with 2.5 km scale height. profi les of the atmospheric extinction coefficient were derived taking into account rayleigh scattering and aerosols. rayleigh scattering profi les were derived from the u.s. standard atmosphere et al. (1976), by using the well-known rayleigh scattering cross-section for the atmosphere (see, e.g., hinkley, 1976). aerosol extinction was conventionally accounted for by assuming an aerosol layer near ground, with an optical depth of 0.6 and a uniform vertical profi le from ground to 3 km altitude. regarding integration time, we assumed 2 min integration for the small telescope, and 30 min integration for the large one. the resulting simulated lidar signals in daytime conditions, for both the large and the small telescope, are plotted in fig. 2 for the water vapour channel (a random statistical noise has been included to simulate photon count statistics). the next step was the computation of the signal-to-noise ratio (snr). since nitrogen concentration is much larger than water vapour concentration, the simulated signal due to n2 is larger by 2-3 orders of magnitude than the signal due to h2o, and the uncertainty of the water vapour signal dominates in the global uncertainty on w. therefore, the snr was estimated by considering the contribution of random noise in the water vapour channel, and neglecting the contribution of the nitrogen channel. we must bear in mind, therefore, than in the fi nal system snr can be larger due to the neglected contribution. moreover, possible systematic errors (i.e. on calibration) have not been taken into account. figure 3 displays the calculated snr for both receivers in daytime conditions; fig. 4 shows them for nighttime conditions. if we assume that the maximum height achievable is marked by a value of 7 of the snr (i.e. a 15% error on mixing ratio), we can see that the goals stated in the introduction can be approached with this hypothetical system, but without full achievement. with the smaller receiver, thanks to its small diameter, a partial overlap can be achieved in the very near layers: the geometrical form factor is equal to 0.1 at 75 m, 0.5 at 150 m, and 1 (full overlap) starting at 300 m. with 2-min integration, however, the signal decreases quite strongly, and snr decreases to 7 at around 1 km range. increasing the integration time with this receiver improves the result, but not enough to reach 3 km as we wish: 1.7 km are reached with an snr of 7 when the integration time is increased to 30 min, and 2 km when it is increased up to 2 h. it is thus necessary to consider a dual receiver system. according to the simulation, the 500 mm telescope with a 0.4 mrad fi eld-of-view allows to reach 2.7 km with an snr of 7, and 3 km with an snr of 4.5, but this telescope is less useful in the very near range due to its slowly increasing overlap function (geometrical form factor equal to zero up to 200 m; 0.1 at 500 m, and 0.5 at 900 m). the plot for nighttime measurements shows that in the absence of sky background much larger ranges can be reached: if the limit is put at snr = 7, almost 4 km can be reached with fig. 2. simulated water vapour raman lidar signal for 2 min integration time with the small telescope (solid line), and for 30 min integration time with the large telescope (dashed line). a random statistical error with poisson distribution was added to simulate photon count statistics. 463 a lidar for water vapour measurements in daytime at lampedusa, italy the small receiver (2 min integration time) and approximately 10 km with the large receiver (30 min integration time). 3. discussion besides raman lidar, another possibility is available for water vapour measurements: differential absorption lidar (dial) in the near infrared (browell et al., 1979; browell, 1983; senff et al., 1994; kiemle et al., 1997; giez et al., 1999; wulfmeyer, 1999; wulfmeyer and walther, 2001a,b). for dial, instrument sizing is less crucial and performance is strongly improved. the problem with dial is related to the narrow width of the water vapour absorption lines (~10 pm). this fact increases the experimental complexity, since even a tiny shift in the laser output wavelength causes a huge variation of the cross-section, which directly enters the dial equation and thus affects the water vapour profi les. the system would therefore need a certain degree of complexity, since the variations in the emitted spectrum would have to be monitored continuosly. this consideration pushed us to discard this option, since the system is going to be installed at a remote site, where it is not possible for the operator to make frequent adjustments. when playing with models and simulations, it must always be born in mind that a real system will always behave differently than expected. however, this exercise is useful before designing a new lidar, to get an idea of the crucial parameters. this is particularly important when designing a water vapour raman lidar for daytime operation, since with a weak signal and a large background we are in adverse conditions. simulation parameters should not be set to unreasonably optimistic values, if one does not want to encounter deception when performing the experiment. an important element of the system is the laser source, which should be as powerful as possible. the 355 nm wavelength is therefore particularly suitable due to the availability of the (frequency tripled) nd:yag laser which permits us to reach a large output while keeping the system simple to operate. lasers with up to ~ 400 mj energy at 355 nm and 30 hz are available on the market, but in the simulations we limited ourselves to 325 mj because we believe that it is not reasonable to operate the laser at full power. similarly, we believe that the quantum effi ciency assumed in table ii is reasonable for the uv spectral range. optical effi ciency is hard to evaluate, and we therefore conservatively chose a low value. zenith sky irradiance in the worst case was used (near noon of the summer solstice). however, despite choosing fig. 3. simulated snr in daytime for 2 min integration time with the small telescope (solid line), and for 30 min integration time with the large telescope (dashed line). the thin dotted line denotes the snr = 7 level. fig. 4. simulated snr in nighttime (no background) for 2 min integration time with the small telescope (solid line), and for 30 min integration time with the large telescope (dashed line). the thin dotted line denotes the snr = 7 level. 464 paolo tini brunozzi and franco marenco not excessively optimistic system constants, we must bear in mind that only experiment will yield the correct signal-to-noise ratio. acknowledgements thanks to vincenzo santacesaria for providing his computer code for geometrical form factor calculations. references ansmann, a., r. neuber, p. rairoux and u. wandinger (editors) (1997): advances in atmospheric remote sensing with lidar, selected papers of the 18th international laser radar conference (ilrc), berlin 22-26 july 1996 (springer-verlag, berlin), pp. 590. bisson, s.e., e. goldsmith and m.g. mitchel (1999): narrow-band, narrow fi eld-of-view raman lidar with combined day and night capability for tropospheric water-vapor profile measurements, appl. opt., 38, 1841-1849. browell, e.v. 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(1999): investigation of turbulent processes in the lower troposphere with water vapor dial and radar-rass, j. atmos. sci., 56, 1055-1076. wu l f m e y e r, v. and c. wa l t h e r (2001a): future performance of ground-based and airborne water-vapor differential absorption lidar: i. overview and theory, appl. opt., 40, 5304-5320. wu l f m e y e r, v. and c. wa l t h e r (2001b): future performance of ground-based and airborne watervapor differential absorption lidar: ii. simulations of the precision of a near-infrared, high-power system, appl. opt., 40, 5321-5336. 19roca.qxd 561 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical seismicity – catalogues – macroseismic data – earthquake database – iberian peninsula 1. introduction the knowledge of the historical evolution of catalogues is extremely important to understand the origin of modern catalogues. a detailed analysis of the ways the information contained in these catalogues is transmitted over time helps to better interpret modern catalogues and to be aware of the real problems they may hold. muñoz and udias (1982) gave a thorough outline of main spain’s works related to historical earthquake data compilation and listed the main catalogues produced until 1982. the most significant works reported in that study are mentioned here together with some of the new publications on particular events and some new earthquake catalogues which have appeared in spain and portugal since then. all these works lead to a quality improvement of the knowledge of the seismicity in our study area, the iberian peninsula, including spanish, portuguese and the mediterranean and the atlantic islands – canaries, balearic and the azores – (fig. 1). 2. overview of historical catalogues the first works on seismicity of iberia go back to mid 18th century as a consequence of the 1755 lisbon earthquake. the great damage produced by this event in an important european town shocked the entire european society. a large amount of literature was written, including not only a number of seismic reports and studies concerning damage at different sites an outline of earthquake catalogues, databases and studies of historical seismicity in the iberian peninsula antoni roca (1), arancha izquierdo (2), carlos sousa-oliveira (3) and jose-manuel martínez-solares (2) (1) geological survey, institut cartogràfic de catalunya, barcelona, spain (2) instituto geográfico nacional, madrid, spain (3) instituto superior técnico, lisboa, portugal abstract the general purpose of the present paper is to summarize the state-of-the-art of historical earthquake knowledge and research in the iberian peninsula, giving an account of the main references, the historical developments and the present situation of earthquake catalogues. the most representative historical works for compiling earthquake data (catalogues) up to 1985 are referred together with those of more recent investigations carried out in spain and portugal for the period 1985-2003. existing databases on historical seismicity are presented, mentioning the most important achievements in relation to quality of information. mailing address: dr. antoni roca, geological survey, institut cartografic de catalunya, parc de montjuic, 08038 barcelona, spain; e-mail: antoni.roca@icc.es 562 antoni roca, arancha izquierdo, carlos sousa-oliveira and jose-manuel martínez-solares but also philosophical and scientific discussions, religious explanations and even poetry (muñoz and udias, 1982; martínez-solares, 2001). this earthquake can be considered the initiator of scientific interest in the compilation of seismic events in the iberian peninsula. according to muñoz and udias (1982), the first lists of earthquakes in the iberian peninsula were compiled by nipho (1755) and roche (1756). also, moreira de mendonça (1758), in his world catalogue from 1815 b.c. to 1755 a.d., containing about 400 well documented events with complete references, includes earthquakes in the iberian peninsula. the work by perrey (1847) is an important reference catalogue in iberia, being the basis of many later studies. for earthquakes in southern spain, the catalogue by taramelli and mercalli (1886), as part of a field study on the 25 december 1884 earthquake, an outstanding example of early seismological work, completed the information contained in perrey’s catalogue adding other sources. for earthquakes in ne spain, teixidor cros (1884) gives a summary of the earthquake history of catalonia, which mainly relies on a booklet from bolós (1841) about the olot volcanic area. mengel (1908; 1909; 1929) deals with seismic data for catalonia (both spain’s and france’s) and pyrenees region. this is a reference work for catalonia although the author is not very critical about historical sources. sánchez navarro-neumann (1917, 1920) published a catalogue for spain including data from different sources, mainly from previous catalogues. one of the most valuable references for seismicity of the iberian peninsula is the catafig. 1. map of the iberian peninsula and insular territories of spain and portugal, with geographical names mentioned in the text. rectangles a and b represent the areas covered by the catalogues of martínez-solares and mezcua (2002) and susagna and goula (1999), respectively. 563 an outline of earthquake catalogues, databases and studies of historical seismicity in the iberian peninsula logue from galbis (1932; 1940). it is a catalogue (1032 b.c.-1933 a.d.) which uses a large amount of historical sources, giving the corresponding references, including the works from contemporary spanish seismologists such as rey pastor, sánchez navarro-neumann, faura i sans and fontseré, among others. munuera (1963) in his catalogue, in table format, presents basically the information from galbis with some modifications, removing some events considered not sufficiently supported by data. fontseré and iglesias (1971) is the essential reference catalogue for catalan speaking areas: both spanish and french catalonia, valencia and the balearic islands. it contains historical data from 1100 to 1906 a.d. based on well-reported references. in portugal, besides the already mentioned pioneer work by moreira de mendonça (1758), the most important studies on earthquake compilations prior to 1980 were produced in the early 20th century, based on data gathered in various types of publications. for example, in relation to the azores archipelago, mention should be made of arquivo dos açores (re-edition 1981-1986) and to frutuoso (re-edition 1998 of the 16th century work). the work of pereira de sousa (1919), dedicated essentially to the 1755 lisbon earthquake, is a piece of documentation of major importance. it also refers to earthquakes in the past. choffat and bensaude (1912) made a notable description of the 23 april 1909 benavente earthquake in the lower tagus valley, lisbon and of the large amount of aftershocks. miranda (1930) made an account of all felt events in portugal in the 1920’s. but the greatest contributions to the historical earthquake compilation in portugal were made during the commemoration of the 2nd centennial of the 1755 earthquake in the «simpósio sobre a acção dos sismos». ferreira (1955) and agostinho (1955) presented collections of important earthquakes in the mainland and in the azores, respectively. in the early 1980’s, some revisions of earthquake data in the iberian peninsula were carried out. among others, mention should be made of the work by rodríguez de la torre (1980) on the seismicity of alicante up to the 18th century, the parametric catalogue by suriñach and roca (1982) for catalonia and pyrenees which includes a collection of isoseismal maps published in different works, the parametric catalogue by monge (1981) on earthquakes in the canary islands; an extensive study by bisbal (1984) for the valencia region, and the study by gentil and justo (1983) for the region of seville with data related to the arab period in south spain together with other works on andalusian earthquakes by these same authors. mezcua (1982) published a catalogue of isoseismal maps for earthquakes in the iberian peninsula and, one year later, a parametric catalogue from the instituto geográfico nacional (ign) for the ibero-magreb region (mezcua and martínez-solares, 1983) was completed. in portugal, various studies on historical seismicity were carried out in the first half of the 1980 decade. a catalogue of earthquakes in mainland portugal was published by moreira (1984) and reviewed later (moreira, 1991). in iberia, macroseismic data are compiled by various institutions and published on a routine basis in monthly and annual bulletins. since 1907 the fabra observatory in barcelona has carried out macroseismic surveys for all events felt in catalonia. from 1984 this task has been undertaken in collaboration with the servei geològic de catalunya – at present as part of the institut cartogràfic de catalunya (icc). starting in 1909, the ign obtained macroseismic data for earthquakes in spain. in portugal, since 1947, the instituto nacional de meteorologia e geofísica (former serviço meteorológico nacional) has been responsible for macroseismic surveys. for the azores, in 1998, they started carrying out these tasks in collaboration with the university of azores (consortium sivisa). 3. recent studies and modern catalogues (1985-2003) in the late 1980’s there was an important development on the studies of historical seismicity by the establishment of teams of historians and seismologists working together in a more systematic way. some ec funded projects as for example rhise (review of historical 564 antoni roca, arancha izquierdo, carlos sousa-oliveira and jose-manuel martínez-solares seismicity in europe – 1988-1991) and beecd (a basic european earthquake catalogue and database for the evaluation of long term seismicity and seismic hazard – 1995-1997) were of special interest for increasing international collaboration. within the framework of these two projects efforts were devoted to comparing methodologies used by european investigators of historical seismology and establishing common principles and criteria for cataloguing seismic events, respectively (stucchi, 1993; albini and moroni, 1994; albini and stucchi, 1997). the major problems encountered when historical earthquakes are reviewed are related to: (i) the reliability and insufficiency of the documentary sources; (ii) errors in the transmission of the information through not-independent sources; (iii) difficulties in assessing the characteristics, typologies and, consequently, the vulnerability classes of damaged buildings; (iv) difficulties in assigning damage descriptions to individual events in earthquake sequences; (v) lack of information on damage to the complete building stock when sources report only on one or a few specific buildings like, for instance, churches or royal buildings; and (vi) problems related to the identification of toponyms and to misinterpretations of the calendar. examples of the concern for reliability and quality of macroseismic data in iberia can be found in olivera et al. (1994; 1999), paula and oliveira (1996), susagna et al. (2001), among others. several studies on particular events, earthquake series or specific time windows have been published in the last decade. examples of these studies are as follows: olivera et al. (1994; 1999) and salicrú (1995) studied historical earthquakes in catalonia; espinar (1994) and espinar et al. (1994) investigated historical sources for earthquakes in south spain and also muñoz and udías (1991) studied a few large events in the same region; cruz et al. (1996) initiated studies on historical seismicity in mainland portugal until the end of 17th century, criticising and analysing the sources of information. most of these recent studies on historical seismicity use the technique of family trees to analyse the traceability of the sources, detecting repetition of sources not contemporary with the event. another topic related to the improvement of knowledge of historical earthquakes from the first half of the 20th century is the processing of early instrumental records using modern digitising techniques. this information can complement the available data obtained from the analysis of contemporary macroseismic surveys (susagna et al., 1993). very good records on paper support have been carefully kept in the archives of early observatories and attempts are being made to digitise and to process available records (batlló et al., 1997; badal et al., 2000). this type of investigation has been important to calibrate, for instance, the magnitude of the benavente earthquake of 23 april 1909 in the lower tagus valley, lisbon (dineva et al., 2002). a catalogue of digital historical seismograms recorded on the wiechert seismograph of the toledo observatory between 1912 and 1962 was published by samardjieva et al. (1997). an ongoing project of the ign consists of digitising all the available records existing in its observatories for earthquakes of magnitude equal to or larger than 4.5 which occurred until 1962. the following sections present, as a matter of illustration, a brief review of recent studies in portugal, and of two spanish catalogues, one for catalonia and the other for the entire iberian peninsula. in addition to these studies, a few other works on revisions of earthquake data have been carried out. among others, we should mention rodríguez de la torre (1990), who studied the seismicity of iberia in the 19th century through an intensive work in newspaper archives and other sources, producing a revised catalogue from 1850 to 1900; muñiz gómez (2001) who, based on the works by rodríguez de la torre (1990), extended the historical research in galicia (nw spain) for the 19th and 20th centuries also through the analysis of newspapers; and various historical studies in andalusia based on the analysis of spanish-arabic documentary sources, as the work by bretón and espinar (1996). 4. recent revisions in portugal in early 1980, a working group to study historical earthquakes and produce a national catalogue was launched by the gabinete de pro565 an outline of earthquake catalogues, databases and studies of historical seismicity in the iberian peninsula tecção e segurança nuclear, secretaria de estado do ambiente e dos recursos naturais, involving the participation of several institutions (faculdade de ciências, universidade de lisboa, universidade nova de lisboa, laboratório nacional de engenharia civil, electricidade de portugal, instituto geológico e mineiro) under the presidency of the instituto de meteorologia. several studies were conducted by the department of history of universidade nova de lisboa which compiled an enormous amount of information on sources of earthquakes. among these works mention can be made of the monographs on revisions of historical seismicity in the 17th and 18th centuries (wagner, 1993; wagner et al., 1993), and in the 16th century (henriques et al., 1988). at the instituto de meteorología efforts were made to compile and analyse the historical seismicity, such as the work by moreira et al. (1993), reviewing the historical seismicity in the gulf of cadiz area before 1 november 1755 earthquake. as mentioned before, moreira (1991) made an overview of seismicity in portugal and cruz et al. (1996) initiated studies on historical seismicity in mainland portugal until the end of 17th century. besides the «anuário sismológico» published since 1947, studies have been performed to compile information from different sources. two examples from late 1980 early 1990 are the catalogues by martins and mendes-victor (1989) and sousa et al. (1992). to quantify earthquake parameters from macroseismic information, oliveira and sousa (1991) made a first contribution to reduce subjectivity in treatment of historical data by introducing the concept of quality factor when analysing the large earthquake events in the lower tagus river. paula and oliveira (1996) revised all data from the «anuário sismológico», transformed the information in the earlier ems-92 scale and applied a quality factor for a better definition of macroseismic intensity. the instituto de meteorologia is preparing a catalogue, to be published possibly in 2004, of all earthquakes between 1980 and 2000, including macroseismic events. for the azores, several recent studies have been produced in relation to macroseismic catalogues. among them, reference should be made to the work of nunes costa (1986) who recently compiled historical events for the archipelago and to silveira (2002) which made a contribution on historical seismicity for the evaluation of the hazard in san miguel island. sousa and martins (2000) made a comparison between the seismic parameters of important events from different catalogues for the azores (portuguese and international) and produced a first version catalogue used for hazard studies in the region. nunes et al. (2003; 2004) have revised many documents and compiled a large amount of information; besides analysing the elements contained in the «anuário sismológico» and in the work of nunes (1986), they mention a large amount of historical references transcribing the original texts. they gathered the documentary sources from the authors who dedicated most of their lives to the history of the azores and also revised the information contained in newspapers. 5. a macroseismic catalogue for catalonia (ne spain) an example of a catalogue compiled according to modern criteria, in particular those established within the above mentioned beecd project (albini and stucchi, 1997; http://emidius.mi. ingv.it/beecd) is the one by susagna and goula (1999) for catalonia. the geographical area covered is a rectangle in ne iberia between longitude 0°20′w to 3°30′e and latitude 40°n to 43°30′n (fig. 1). it covers catalonia and surroundings, in particular the central and eastern pyrenees region, both spanish and french parts. this catalogue is part of a project of the institut cartogràfic de catalunya for assessing earthquake hazard, vulnerability and risk. this catalogue, which is based on macroseismic data, is the result of a long process (susagna et al., 2001) that includes: i) consideration of the existing earthquake compilations, representing the most significant information sources; ii) incorporation of the results from the studies carried out in recent years referring to the historical earthquakes; iii) re-assessment of the macroseismic questionnaires corresponding to the earthquakes during the 20th century, and 566 antoni roca, arancha izquierdo, carlos sousa-oliveira and jose-manuel martínez-solares iv) construction of a database which facilitated the critical comparison between the different sources. one of the results is a homogeneous parametric catalogue, which comprehensively represents all the available seismic information for the region under study. an epicentre quality index and an intensity quality are given for each event together with the common macroseismic parameters; the number of macroseismic observations used for the focal parameters assessment is also reported. the catalogue contains 918 individual records, one for each catalogued event. out of them, 306 records correspond to events before 1900; 612 correspond to events after 1900; and 58 earthquakes, considered in former compilations as real events, have been qualified as false or doubtful. out of the remaining 860 earthquakes, 258 events do not have assigned intensity due to the lack of sufficient information. the most interesting part of the catalogue is the individual information for each earthquake, which shows the value of the parameters chosen from the aforementioned analysis. a map with all intensity data point information is also included for a total of 109 earthquakes. as a synthesis, the catalogue includes several maps of epicentres, in particular one with the known earthquakes that have caused damage throughout history (intensity equal to or above vii msk) and another with all events of intensity equal to or above v which occurred in the 20th century. 6. a new earthquake catalogue for the iberian peninsula the recently published new catalogue by martínez-solares and mezcua (2002) constitutes a revision of the former catalogue from the instituto geográfico nacional (ign) by mezcua and martínez-solares (1983) for the region inside the rectangle a in fig. 1. according to the authors of this catalogue, in the time interval between the publication of these two catalogues – 20 years – problems in several records had been detected, mainly duplications, fake earthquakes, errors in epicentre locations and inaccurate intensity assessments together with lack of available references for some catalogued events. for the revision of the catalogue, an evaluation of each one of the entries has been undertaken through an analysis of all the available documents in the ign archive which contains published studies, internal reports, copies of contemporary sources, etc. each document was referenced with a number, which was included in the earthquake database. all available data for each earthquake were then analysed and the corresponding parameters and quality factors for the epicentral intensity and location assigned. a code explaining the modifications in the different parameters in this catalogue with respect to the former mezcua and martínez-solares (1983) is given. for each catalogued event the number of data points with macroseismic information is indicated together with the bibliographic references. the catalogue includes tables and maps showing intensity data points for the earthquakes with epicentral intensity equal to or greater than viii (ems 98) or for those with at least 15 data points available. the equivalent moment magnitude is computed following the bakun and wentworth (1997) methodology. a comparison of the number of entries between this new catalogue and that of 1983 is shown in fig. 2. on the one hand, there is an increase in the number of events of low intensity, due to the inclusion in the catalogue of detailed studies such as those mentioned for the 19th and 20th century by rodríguez de la torre. on the other hand, some of the largest events decreased the value of io as a result of intensity reevaluations. 7. earthquake databases several attempts to provide the scientific community with data easily accessible through organised databases have been made by various institutions in spain and portugal, but up to now most of these projects have not been completed and databases are essentially working systems mostly for internal use in seismological centres. the instituto geográfico nacional (ign) maintains various computer files organised with 567 an outline of earthquake catalogues, databases and studies of historical seismicity in the iberian peninsula database structures. the historical earthquake data available in digital form are: – earthquakes parametric file; – bibliography, references; – tsunamis on the spain’s coast; – tistorical instruments; – some macroseismic questionnaires. some digital seismic cartography is also contained in the ign digital archives: geo-referenced isoseismal maps from mezcua (1982) and maps with geo-referenced macroseismic information as given in the recent catalogue from martinez solares and mezcua (2002). works to establish a more complete database with all the available macroseismic information are currently being carried out. the instituto andaluz de geofísica y prevención de desastres sísmicos has in operation a database of historical earthquakes in andalucia (south spain) which includes parametric data, macroseismic information and isoseismal maps (feriche and botari, 2002). the institut cartogràfic de catalunya has a database with the intensity data points of all the earthquakes in the area of the catalogue from susagna and goula (1999). a computer file with the catalogue parametric data is available from the icc web site (http://www.icc.es). nunes et al. (2004) prepared a database with the available information on azores earthquakes for the period 1850-1998, and published it on a cd-rom. simões (2004) is now preparing the first internet version of a catalogue of the portuguese earthquakes. 8. final remarks it can be considered that the history of earthquake catalogues in the iberian peninsula starts with the occurrence of the 1755 lisbon earthquake. since then, many authors have been involved in the collection and analysis of past earthquakes. in this paper, the leading works have been presented and the main references given. efforts have been made in the recent decades to review the macroseismic information and to improve earthquake catalogues. however, there is still a need to carry out studies on specific historical earthquakes and to take some of the existing – published and unfig. 2. comparison of the number of events for each intensity class contained in the catalogues of mezcua and martínez-solares (1983) and martínez-solares and mezcua (2002) for the period 1000-1900. 568 antoni roca, arancha izquierdo, carlos sousa-oliveira and jose-manuel martínez-solares published – works, review them under modern acknowledged procedures, that is, through an objective classification and analysis of reliability of sources, and a well-stated and justified methodology for interpreting historical data. there is also a need to establish current policies to incorporate the validated research works into the national catalogues and databases. very often the period of time between the publication of new research works on historical earthquakes and the corresponding changes in the institutional databases – through a necessary validation process – is too long. given the unquestionable importance of observed intensities for attenuation studies which are crucial for earthquake hazard and risk assessment, databases with intensity data points should be constructed and made available to the technical and scientific communities. many earthquakes take place on border areas and thus, for these regions a transnational collaboration is clearly needed. it would be desirable to increase this collaboration at european level with northern africa countries in order to go towards a cooperative euro-mediterranean database. acknowledgements the authors want to thank teresa susagna and jorge fleta for their help in the preparation of this paper and also one anonymous reviewer for his/her comments. references agostinho, j. 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(1993): fontes para o estudo da sismicidade histórica do algarve na primeira metade do século xviii: a investigação no archivo nacional da torre do tombo, lisboa, in historical investigation of european earthquakes, materials of the cec project «review of historical seismicity in europe», edited by m. stucchi (cnr, milano), vol. 1, 207-224. wagner, m.n., l. braga, b. guerra and m.r.t. barata (1993): dados sobre sismos oorridos no estrangeiro e referidos em fontes existentes nos arquivos e bibliotecas portuguesas (séculos xvii e xviii), in historical investigation of european earthquakes, materials of the cec project «review of historical seismicity in europe», edited by m. stucchi (cnr, milano), vol. 1, 243-246. les v e h i c u l e s s p a t i a u x a s o u r c e d ' e n e r g i e s o l a i r e le p r o j e t p h a e t a n (*) m . v e r n e t l o z e t r i c e u t o il 18 maggio 1963 r e s u m e . — i i est p e r m i s , a priori au moins, d ' e n v i s a g e r un assez g r a n d n o m b r e de mdtliodes p o u r t r a n s f o r m e r l'6nergie solaire en energie utilisable p o u r la propulsion d ' u n vehicule s p a t i a l . t o u t e f o i s d a n s la mesure ou 1'on s'impose u n c e r t a i n n o m b r e de c o n d i t i o n s , e n t r e a u t r e s celle de p r o d u i r e u n e poussee qui n e soit p a s p a r f a i t e m e n t negligeable d e v a n t le poids du vehicule, on c o n s t a t e que d a n s l ' e t a t a c t u e l de nos connaissances et de nos possibilitcs technologiques, il n ' e s t g u e r e r a i s o n n a b l e de penser utiliser une a u t r e m e t h o d e que celle, bien classique et sans d o u t e demodee a u x y e u x de c e r t a i n s qui eonsiste a passer p a r l ' i n t e r m e d i a i r e d ' u n e m a c h i n e t h e r m i q u e . ii a p p a r a i t c l a i r e m e n t d ' a i l l e u r s q u e l'originalite d ' u n tel mode de propulsion n e reside p a s d a n s le s y s t e m e de p r o p u l s i o n l u i m e m e qui p o u r r a e t r e n ' i m p o r t e quel t y p e de p r o p u l s e u r electrique, le clioix p o u v a n t e t r e f a i t en f o n c t i o n de la mission que l'on se p r o p o s e d ' e f f e c t u e r et de la c h a r g e utile que l'on desire t r a n s p o r t e r , mais au c o n t r a i r e d a n s la c o n c e p t i o n de l ' e n s e m b l e c o n s t i t u e p a r le miroir, la m a c h i n e t h e r m i q u e et le c o n d e n s e u r . nous nous p r o p o s o n s sur la base d u p r o j e t « p h a e t o n » a c t u e l l e i n e n t en cours d ' e t u d e a la societe d ' e t u d e de la p r o p u l s i o n p a r r e a c t i o n , d ' e x a m i n e r u n peu plus en d e t a i l qu'il n e l ' a ete f a i t j u s q u ' i c i , quels i i n p e r a t i f s regissent le d i m e n s i o n n e m e n t d ' u n e telle m a c h i n e . riassunto. — £ lecito, almeno a priori, e s a m i n a r e u n assai g r a n d e n u m e r o di m e t o d i per t r a s f o r m a r e l'energia solare in energia utilizzabile per la p r o p u l s i o n e di u n veicolo spaziale. t u t t a v i a nella m i s u r a che viene i m p o s t a da u n c e r t o n u m e r o di condizioni, t r a cui quella di p r o d u r r e u n a s p i n t a che non sia p e r f e t t a m e n t e t r a s c u r a b i l e r i s p e t t o al peso del veicolo, si c o n s t a t a che alio s t a t o a t t u a l e delle n o s t r e conoscenze e delle n o s t r e possibilita tecnologiche, n o n e a f f a t t o g i u s t o p e n s a r e di utilizzare u n altro m e t o d o che n o n sia quello, classico e sicuro che eonsiste nel p a s s a r e a t t r a v e r s o u n a m a c c h i n a t e r m i c a . (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifieo dello spazio. r o m a , 19-23 giugno 1962. m. v e r n e t l o z e t a p p a r i r a allora chiaro clie, l'originalit& (li t a l e m e t o d o di p r o p u l s i o u e n o n s t a t a n t o nei s i s t e m a di p r o p u l s i o u e stesso clie p o t r a essere non i m p o r t a di q u a l e t i p o di p r o p u l s o r e elettrico, la scelta p o t r a essere f a t t a in f u n z i o n e della missione clie ci si p r o p o n e di eft'ettuare e del carico utile che ci propon i a m o di t r a s p o r t a r e , m a al c o n t r a r i o nella concezione dell'insierne c o s t i t u i t o dallo specchio, dalla m a c c h i n a t e r m i c a e dal c o n d e n s a t o r e . ci p r o p o n i a m o sulla base del p r o g e t t o « p h a e t o n » a t t u a l m e n t e in corso di s t u d i o alia societe d ' e t u d e de la p r o p u l s i o n p a r r e a c t i o n , di e s a m i n a r e un p o ' piu in d e t t a g l i o t u t t o cio che si 6 f a t t o finora, quali i m p e r a t i v i reggono il d i m e n s i o n a m e n t o di u n a t a l e m a c c h i n a . summary. — on p r o v e t h a t b e t w e e n various m e t h o d s t o t r a n s f o r m e s u n energy i n t o utilizable energy to move a s p a t i a l vehicle, t h e most c o n v e n i e n t is t h a t t o m a k e use of t h e r m i c engine. on e x a m i n e , in all p a r t i c u l a r s 011 t h e g r o u n d of p h a e t o n p r o j e c t , this m e t h o d . rappelons que le b u t du projet « p h a e t o n » est de p e r m e t t r e de effectuer un certain nombre de missions a y a n t elles-memes pour b u t aussi bien la resolution des nombreux problemes qui ne m a n q u e r o n t pas de se poser a propos de la propulsion electrique d ' u n vehicule spatial que l'etude des techniques d'emissions et de reception de radio et de television. les vehicules de ce type out ete etudies pour pouvoir etre mis sur orbite p a r les versions evoluees, d'ores et deja a l'etude, du lanceur d i a m a n t . on peut m o n t r e r que pour le poids t o t a l de l'ordre de 300 kg que ces lanceurs seront capables de m e t t r e sur orbite, lequel constituerait le poids initial de « p h a e t o n » (y compris la charge utile), le compromis optimal en ce qui concerne la puissance electrique a fournir se situe a u x environs de 2 a 3 kw. le projet dont la m a q u e t t e est representee sur la planche 1 a ete calcule pour une puissance electrique de 2,25 kw. ii est essentiellement constitue p a r : — un miroir parabolique depliable de diametre egal a 4,9 m, — d ' u n e chaudiere en forme de cavite, l'ouverture de celle-ci e t a n t placee dans le plan focal du miroir, le diametre de cette ouverture etant, egal a 0,18 m, — d'une machine de transformation laquelle dans le projet actuel est un moteur alternatif, le lluide moteur e t a n t du mercure. cette machine e n t r a i n a n t deux alternateurs, — d'un r a d i a t e u r r a y o n n a n t sur les 2 faces, la surface totale radiante e t a n t egale a 2,85 m2. le miroir est suppose etre pointe de fagon permanente vers le soleil a l'aide d ' u n dispositif approprie, l'erreus moyenne sur le pointage e t a n t supposee etre egale a 3/4 de degre. l e s v e h i c u l e s s p a t i a u x a s o u r c e d ' e n e r g i e s o l a i r e 159 la chaudiere contient une reserve d ' h y d r u r e de lithium f o r m a n t volant thermique afin de pouvoir fonctionner de fa§on p e r m a n e n t e y compris pendant les passages dans les zones non eclairees. le diametre pig. 1 vehicule s p a t i a l e a p r o p u l s i o n electrique « p h a e t o n ». du miroir est prevu de fa eon telle que p e n d a n t les passages dans les zones eclairees l'energie collectee soit suffisante pour alimenter la machine thermique et le stockage d'energie. ij'ensbmble miroir-chaudiere. on sait que pour un miroir p a r f a i t , les rayons lunineux en provenance du soleil definissent apres reflexion une taclie lumineuse situee dans le plan focal a l'interieur de laquelle, pour des valeurs de l'ouverture sufflsamment faibles, est concentree la quasi totalite de l'energie mise en jeu. on sait aussi qu'en etucliant de t.res pres la qualite optique du 1(14 m . v e r n e t l o z e t miroir et la precision de son pointage vers le soleil on p e u t realiser des temperatures tres elevees de l'ordre de 4.000 °k voire superieures. on ne p e u t esperer pour un miroir qui sera necessairement leger e t qui devra etre depliable u n a qualite meme voisine de la qualite optique. d a n s ces conflitions il est illusoire de baser sur un p r o j e t sur les performances qui pourraient etre a t t e n d u e d'un miroir optiquement p a r f a i t , ou t o u t au moins proche de la perfection. la solution la plus sure consiste certes a construire des miroirs de differents types et a determiner experimentalement leurs caracteristiques. moyennant quelques hypotheses on peut toutefois aborder le probleme theoriquement, ce qui est t o u t de meme d ' u n assez grand secours dans 1'etablissement d ' u n avant-projet. si on fait i'hypothese que l'iinperfection de notre miroir est essentiellement due a des imperfections locales, c'est-a-dire si l'on suppose que le miroir reel est constitue de petits elements tous situes sur les paraboloide que serait le miroir p a r f a i t mais non t a n g e n t a eelui-c.i, il est alors possible de ealculer approximativement la repartition statistique de l'energie dans le plan focal en fonction de l'ecart-type des erreurs sur 1'orientation de la normale locale. le calcul a ete fait pour un miroir d'ouverture 60°. la repartition est representee p a r l'expression: a = est l'ecart-type sur la normale. e = est egal a 1,36 kw/m 2 au voisinage de la terre. r = est la distance a l'axe d ' u n point du plan focal. it = est la distance a l'axe du point d'ouverture angulaire maon a represente sur la planche i i la valeur de e' en fonction de r/r pour differentes valeurs de a. on peut voir i m m e d i a t e m e n t que pour un ecart-type relativement faible — de 1/4° p a r exemple — on a encore une concentration d'energie extremement i m p o r t a n t dans la zone centrale et il est encore possible avec une cible de dimensions faibles de recuperer une f r a c t i o n appreciable de l'energie incidente. si on suppose que le rendement de reflexion du miroir est egal a 1, c'est-a-dire que son pouvoir emissif est nul on peut ainsi definir un rendans laquelle: ximale. l e s v ^ h i c u l e s s p a t i a u x a s o u r c e d ' e n e r g i e s o l a i r e 1 6 1 dement d'interception 77 pour une cible dc rayon r placee au centre d'un miroir donne dont l'ecart-type sur la normale est connu. la variation de 77 en fonction de r/b. et de a est donnee sur la planche i i i , f'ig. 2 e c l a i r e m e n t ene:'getique d a n s le p l a n focal en fonction de <7 et de r / r (ecart de pointage). on pent constater que dans la mesure oil l'ecart-type n'excede pas 1/2° a la rigueur 1° 011 p e u t avec une cible de dimension faible recuperet une fraction i m p o r t a n t e de l'energie incidente, p a r contre pour a = 2° p a r exemple il f a u t une cible de diametre egal a 10% de celui du miroir p o u r ne recuperet que 5 0 % de l'energie incidente, valeur qui sera encore abaissee si l'on t i e n t compte: —du fait que le r e n d e m e n t dc reflexion 11'est certainement pas egal a 1, — de l'imprecision sur le pointage du miroir. la q u a n t i t e de chaleur qui, ainsi intercepts, peut etre transmise au cycle est fonction de la t e m p e r a t u r e de la cible, c'est-a-dirc de la tempe1(14 m. v e r n e t l o z e t r a t u r e de source chaude du cycle thermodynamique. la densite d'energie dans la surface delimitee p a r la cible correspond p o u r un corps noir a une certaine t e m p e r a t u r e d'equilibre (lorsque l'on augmente la diametre fig. 3 r e n d e m e n t d ' i n t e r c e p t i o n en f o n c t i o n de la d i m e n sion de la cible et de a ( r e f l e c t i v i t e du miroir = !)• de la cible d'ailleurs en vue d'ameliorer le rendement d'interception on diminue cette t e m p e r a t u r e d'equilibre). si notre cible est un corps-noir p a r f a i t elle v a absorber de l'energie selon la puissance quatrieme de cette t e m p e r a t u r e et en rayonner pour la meme surface selon la puissance quatrieme de la t e m p e r a t u r e de source chaude du cycle. la difference entre ces deux quantites sera effectiveinent transmise a u cycle. on a represents sur la planche i v la variation du rendement de captation global du systeme miroir-cible en fonction de la t e m p e r a t u r e de celle-ci (supposee etre un corps noirparfait) pour une valeur de l'ecartt y p e egale a 1/2°. l e s v ^ h i c u l e s s p a t i a u x a s o u r c e d ' e n e r g i e s o l a i r e 161 on peut constater que, dans le mesure on l'on desire conserver une valeur acceptable du rendement il n'est pas raisonnable d'envisager des temperatures de source chaude au dela de 1.000 ou a la rigueur 1.200 °k. t o u t au moins pour le niveau de puissance que nous nous sommes impose, en raison de l'accrolssement considerable de la table du mrioir. fig. 4 r e n d e m e n t de c a p t a t i o n de l ' e n s e m b l e (mirofi + ca1° 3° vite) a — _ ; e c a r t de p o i n t a g e . la t e m p e r a t u r e de fusion de l'hydrure de lithium utilise comme volant thermique e t a n t egale a 950 °k nous avons adopte cet.te valeur comme t e m p e r a t u r e de la source chaude. p a r ailleurs le corps noir p a r f a i t n'existant pas il a ete prevu que la chaudiere serait une cavite laquelle si elle est bien dessinee peut a^-oir des caracteristiques voisines de celles du corps noir. 1(14 m . v e r n e t l o z e t evacuation par r a y o n n e m e n t d e l ' e n e r g i e non u t i l i s e e dans l e cycle. l a t e m p e r a t u r e de la source froide ne pent pas etre non plus choisie arbitrairement, en fait dans le mesure 011 1'on est capable de se fixer des valeurs pour les poids p a r unite de surface de miroir et de r a d i a t e u r il est possible de m e t t r e en evidence l'existence d'une valour optimale du rapport de t e m p e r a t u r e , c'est-a-dire du r e n d e m e n t de carnot de la transformation pour des valeurs donnees de: — la t e m p e r a t u r e de source chaude (to) — le rendement de captation de l'ensemble miroir-cavite (r/c) — le rendement mecanique de la machine (r/t) — le pouvoir emissif de la surface du radiateur (lequel peut d'ailleurs en premiere approximation etre pris egal a l ) . on obtient pour le poids de l'ensemble miroir + r a d i a t e u r l'cxpression ci-dessuos: 0,1xw 1 (iw l — f i t i l — tjto) v , r , t ( l — tjt.) 2 a r]t ti* (1—tjto) }. et ft, e t a n t respectivement les poids speciflques de miroir et de radiateur. a e t a n t la constate de stefan. en derivant p/w p a r rapport a tjto on peut trouver la valeur optimale de ce r a p p o r t . p o u r les precedes de construction auxquels on peut actuellement penser il est raisonnable de prendre /, = m, dans ces conditions pour des valeurs raisonnables des rendements r)e et r/t et pour la t e m p e r a t u r e de source chaude precisee plus liaut, on peut m o n t r e r que 1 \ doit etre de l'ordre de 500 °k, correspondant a un rendement de carnot de 47%. l e cycle d e transformation. e n t r e ces deux t e m p e r a t u r e s on p e u t envisager un certain noinbre de cycles utilisant differents fluides moteurs, dans la mesure ou l'on desire faire seulement un seul cycle il semble que ee soit le mercure qui soit le mieux adapte. retom8ee du i " e t a o e vehicule en cours de transfert \ • oriite' de transfer! vehicule spatial a propulsion electrioue phaeton _ e x e m p l e d e m i s s i o n possible vehicule dans son o o i v e v e r n e t l o z e t vehicule en utilisation ' sur oriite finale . l e s v ^ h i c u l e s s p a t i a u x a s o u r c e d ' e n e r g i e s o l a i r e 161 nous avons ehosi, au moins dans la premiere version, une machine alternative, essentiellement pour deux raisons: — il nous a semble e x t r e m e m e n t probable qn'en raison du tres faible debit le rendement d ' u n e t u r b i n e serait mauvais (au moins pour u n e t u r b i n e du t y p e conventionnel); — le fonctionnement selon u n cycle de rankinepur, sans surchauffe, qui presente l ' a v a n t a g e de simplifler de fa con assez sensible la machine, impose au cours de la detente une fraction croissante de phase condensee qui a priori n'est guere compatible avec le bon fonctionnement d ' u n e turbine. les etudes en cours m o n t r e r o n t en f a i t si ces hypotheses sont lustifiees p o u r les conditions de fonctionnement qui viennent d'etre enoncees je poids t o t a l de la source d'energie comprenant l'ensemble miroircavite, la machine et le r a d i a t e u r est egal a 84 kg se decomposant comme suit: — miroir 28 kg — chaudiere 5 kg — h y d r u r e de lithium 20 kg —moteur pompe + a l t e r n a t e u r 19 kg — r a d i a t e u r 7 k g —mercure 5 k g le poids d ' h y d r u r e de lithium et la surface du miroir ont ete prevus pour u n e mission consistant en u n t r a n s f e r t continu depuis une ordite basse jusq'ua u n e orbite equatoriale synchrone. nous avons la certitude que pour cette puissance l'usage ou t o u t a u t r e mode de conversion direct ou indirect de l'energie solaire en electricite conduirait a u n poids considerablement plus eleve t o u t au moins sur la base des possibiutes actuelles. le developpement de l'etude m o n t r e r a p e u t etre que les hypothese que nous avons faites sur la qualite du miroir sont pessimistes et qu'il est possible d'obtenir des t e m p e r a t u r e s de sources chaudes plus elevees avec u n rendement de captation acceptable, dans le b u t de preparer u n vehicule p r e t a fonctionner dans les 5 qui annees viennent, il nous a semble toutefois qu'elles etaient raisonnables. layout 6 gas discharges from four remote volcanoes in northern chile (putana, olca, irruputuncu and alitar): a geochemical survey franco tassi1,2,*, felipe aguilera3,4, orlando vaselli1,2, thomas darrah5,6, eduardo medina7 1 università degli studi di firenze, dipartimento di scienze della terra, firenze, italy 2 cnr-igg istituto di geoscienze e georisorse, firenze, italy 3 universidad de atacama, departamento de geología, copiapó, chile 4 universidad católica del norte, programa de doctorado en ciencias mención geología, antofagasta, chile 5 university of rochester, department of earth and environmental sciences, rochester, ny, u.s.a. 6 university of massachusetts boston, department of environmental, earth, and ocean sciences, boston, ma, u.s.a. 7 universidad católica del norte, departamento de ciencias geológicas, antofagasta, chile annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5173 abstract we analyzed gas samples collected from fumaroles and bubbling pools at irruputuncu, putana, olca and alitar volcanoes located in the central andes volcanic zone (northern chile). the irruputuncu and putana fumarolic discharges showed outlet temperatures ranging from 83 ˚c to 240 ˚c and from 82 ˚c to 88 ˚c, respectively. the chemical and isotopic (3he/4he, d13c-co2, d 18o-h2o and dd-h2o) compositions of these discharges were similar to medium-to-high temperature volcanic gases from other active volcanoes in this sector of the andean volcanic chain (e.g. lascar volcano). inorganic and organic gas geothermometers for the h2o-co2-co-h2, co2-ch4 and c2-c3 alkenes-alkanes systems indicated equilibrium temperatures that exceed 500 ˚c at the gas sources. these relatively high temperatures are in agreement with the presence of relevantly high concentrations of magmatic gas emissions, including so2. olca and alitar volcano fluid chemistries indicated lower amounts of magmatic-derived gas species, while both the helium and the water isotopic compositions suggested significant fractions of shallow, crustal/meteoric-originated fluids. these indicate contributions from a hydrothermal environment with temperatures <400 ˚c. the geochemical and isotopic features derived from the present study show that the irruputuncu, putana, olca and alitar volcanoes should be considered as active and thus warrant periodic geochemical monitoring to determine the evolution of these systems and their potential hazards. 1. introduction the central andean volcanic zone (cavz) located in northern chile is a 1,500-km-long volcanic arc (figure 1) that originates from when subduction thrust the oceanic nazca plate beneath the south america plate [de silva and francis 1991, stern 2004], and it includes 44 active or potentially active volcanoes [de silva 1989a, de silva 1989b, de silva and francis 1991, gonzález-ferrán 1995, springer and förster 1998]. lascar volcano is considered the most active volcano in the cavz [francis and rothery 1987, gardeweg et al. 1998], and its largest historical eruption occurred on april 19-20, 1993 [gardeweg and medina 1994]; it is currently characterized by extensive fumarolic activity [tassi et al. 2009, and references therein]. there are recently documented phreatic to phreato-magmatic events at isluga, irruputuncu and san pedro volcanoes [gonzález-ferrán 1995, global volcanism program 1997, céspedes et al. 2004], while there is intense fumarolic activity without recorded historical eruptions at tacora, guallatiri, olca, ollagüe, putana, alítar and lastarria volcanoes [casertano 1963, de silva and francis 1991, naranjo 1992, gonzález-ferrán 1995, trumbull et al. 1999]. little is known about the historical eruptions, activities, and/or volcanic structures at putana, olca, irruputuncu and alitar because of their remote locations and the limited accessibility of these volcanoes. we present the first analytical data of the chemical and isotopic compositions of the gas discharges (fumaroles and bubbling pools) from irruputuncu, olca, putana and alitar volcanoes, which were collected during two sampling campaigns in march and may, 2007. the main aims are: (i) to provide the first geochemical and isotopic dataset for the fluids discharged from these volcanoes; and (ii) to investigate the physical-chemical processes that occur between the magmatic source and the shallower hydrothermal aquifers. we also use these geochemical proxies to define a monitoring strategy for these volcanic systems. article history received november 23, 2010; accepted march 3, 2011. subject classification: fluid geochemistry, subduction zone, volcanic risk, geochemical data, volcano monitoring. 121 2. geological setting and volcanic activity (historical and present) 2.1. irruputuncu volcano irruputuncu (20˚45´ s; 68˚34´ w; 5,165 m a.s.l.) is a composite stratovolcano that is located at the border between chile and bolivia (figure 1) [de silva and francis 1991]. irruputuncu lies within the sw portion of the collapse scarp from a pre-holocene volcano [gonzálezferrán 1995] the products of which cover upper miocene-pleistocene dacitic ignimbrites (ujina and pastillos ignimbrites) [vergara 1978, vergara and thomas 1984]. the irruputuncu cone edifice contains two coalescent summit craters of ~200 m in diameter (figure 2) [de silva and francis 1991, wörner et al. 2000]. the cone is primarily composed of andesitic domes with an age of 0.14 ±0.04 ma, as indicated by k-ar dating, and pyroclastic deposits [wörner et al. 2000]. the most recent historical activity consisted of a series of small phreatic eruptions that produced a 1,000-m-high ash/vapor plume that was observed on november 26, 1995 [global volcanism program 1997]. the current fumarolic activity along the inner sw flank of the southern summit crater produces an ~200-m-high plume. recently, local inhabitants reported that a thermal spring located 13 km west of the volcano [hauser 1997] disappeared after an earthquake (mw 7.9) occurred in this area on june 13, 2005. 2.2. olca volcano olca (20˚57´ s; 68˚30´ w; 5,450 m a.s.l.) is a stratovolcano that forms part of a 20-km-long, ew-oriented volcanic chain that includes paruma and michincha volcanoes [de silva and francis 1991, gonzález-ferrán 1995]; it presents strong evidence of past glacial activity along the southern sector (figure 3). olca lies above upper miocenepliocene dacitic and andesitic volcanic products that form the ujina ignimbrite [vergara and thomas 1984]. andeciticdacitic lava flows extend up to 7 km north from the active olca crater [de silva and francis 1991, gonzález-ferrán 1995, wörner et al. 2000]. unconfirmed historical eruptions are suspected to have occurred in 1865-1867 [gonzález-ferrán 1995], while more recently, anomalous seismic activity hat was most likely related to intense degassing from the volcano summit was recorded in november 1989 and march 1990 [global volcanism program 1990]. the present fumarolic activity of olca began within the last ~60 years [casertano 1963, global volcanism program 1990, clavero et al. 2006]. at present, the main fumarolic field is restricted to the dome within the summit crater (figure 3) [aguilera 2008]. 2.3. putana volcano putana (22˚34´ s, 67˚52´ w; 5,890 m a.s.l.) is a stratovolcano with a summit crater of ~0.5 km in diameter (ø, ~500 m) that consists of two inner craters, one located in its center (ø ~130 m), and the other near the ne flank (ø ~300 m) (figure 4) [gonzález-ferrán 1995, aguilera 2008]. the putana edifice has been built on the andesitic-to-dacitic pliocene-pleistocene tatio and purificar ignimbrites [marinovic and lahsen 1984], and it consists of multiple sequences that include andesitic-basaltic, andesitic and dacitic lavas, and pyroclastic deposits [marinovic and lahsen 1984, de silva and francis 1991, gonzález ferrán 1995]. persistently active degassing from the putana summit has produced a 100mto 500-m-high plume since the 19th century [riso patrón 1924, brüggen 1950, casertano 1963, deruelle 1979, gonzálezferrán 1995]. the present survey included the four main active fumarolic fields, known as f1, f2, f3 and f4. the fumarolic field location are (figure 4): f1, sw sector of the main crater; f2, nw sector of the ne crater; f3, se sector of the ne crater; f4, nw outer flank of the main crater. fumarolic vents at putana volcano range from centimeter fractures to vents of up to 10 m in diameter, in the f1 and f2 fields. tassi et al. 122 figure 1. schematic map of the central volcanic zone of the andes in northern chile, with the locations of the irruputuncu, olca, putana and alitar volcanoes. 123 fluid geochemistry of chilean volcanoes figure 2. aerial photograph (a) and schematic map (b) of the summit crater of irruputuncu volcano, showing the locations of the fumarolic fields and sampling sites. figure 3.aerial photograph (a) and schematic map (b) of the summit crater of olca volcano, showing the locations of the fumarolic fields and sampling sites. tassi et al. 124 figure 4. photograph (a) and schematic map (b) of the summit crater of putana volcano, showing the locations of the fumarolic fields and sampling sites. figure 5.aerial photograph (a) and schematic map (b) of the summit crater of alitar volcano, showing the locations of the fumarolic fields and sampling sites. 125 2.4. alitar volcano alítar (23˚09´ s, 67˚38´ w; 5,346 m a.s.l.) is a stratovolcano that has a 500-m-wide, 50-m-deep maar (broad, low relief) crater at the base of its sw flank [gonzález-ferrán 1995] (figure 5). alítar has no documented historical activity. the basement of the alitar volcanic structure consists of upper miocene rhyolitic la pacana ignimbrite and pliocene dacitic atana ignimbrite [ramirez and gardeweg 1982], and in comparison, the volcanic edifice consists of pliocenepleistocene andesitic-to-dacitic lavas [ramirez and gardeweg 1982]. fumarolic activity has been recognized in the northern sector of the alitar maar and in a small area 400 m nw of the maar (figure 5). its current thermal activity also includes six pools that discharge thermal water and gas along a small, ns-oriented creek that is located ~200 m west of the maar (figure 5) [aguilera 2008]. 3. sampling and analytical methods we collected gas samples using a 1-m-long titanium tube (ø, 2.5 cm) that was inserted into the fumarolic vent and connected through quartz-glass dewar tubes to a preevacuated, 60-ml, glass thorion-tapped flask filled with 20 ml 4 n naoh and a 0.15 m cd(oh)2 suspension [giggenbach and goguel 1989, montegrossi et al. 2001, vaselli et al. 2006]. we collected aliquots of fumarolic vapor condensates in-line with the main sampling apparatus using an ice-cooled glass tube. the condensate was used to determine hf concentrations by ion chromatography (metrohm basic761) and the d18o and dd isotopic ratios. we determined the inorganic gases (n2, o2, h2, he, ar and ne) in the flask headspace using a shimadzu 15a gas chromatograph equipped with a 10-m-long 5a molecularsieve column and a thermal conductivity detector. we analyzed hydrocarbons, including ch4, using a shimadzu 14a gas chromatograph equipped with a 10-m-long stainless steel column (z, 2 mm) packed with chromosorb paw 80/100 mesh coated with 23% sp 1700 and a flame ionization detector. carbon monoxide was analyzed by gas chromatography–flame ionization detection after conversion of co to ch4at 400 ̊ c using a shimadzu mtn-1 methanizer. we separate the alkaline solution from the solid precipitate by centrifugation to determine: 1) co2, as co3 2−, by titration (metrohm basic titrino) with a 0.5 n hcl solution; 2) so2, as so4 2−, after oxidation with h2o2; and 3) hcl, as cl-, by ion chromatography (metrohm compact 761). we determined h2s by first oxidizing cds to so4 2− with h2o2, and then using ion chromatography [montegrossi et al. 2001]. the analytical error was <5%. to determine the 13c/12c isotopic ratio of co2 (hereafter expressed as d13c-co2 ‰ v-pdb), we added ~5 ml anhydrous phosphoric acid to 2 ml of the soda solution under vacuum, and allowed the co2 emanated to equilibrate at 25 ±0.1 ̊ c in a thermal bath overnight. the extracted co2 was then purified using liquid n2 and n2-trichloroethylene cryogenic traps, and analyzed using a finningan delta s mass spectrometer. we used internal (carrara and san vincenzo marbles) and international (nbs18, limestone, and nbs19, carbonate) standards to estimate the external precision. the analytical error and the reproducibility were ±0.05‰ and ±0.1‰, respectively. we determined the 18o/16o and 2h/1h isotopic ratios of the water vapor (steam) condensates (hereafter expressed as d18o-h2o and dd-h2o ‰ v-smow, respectively) using a finningan delta plus xl mass spectrometer at the geokarst engineering laboratory (trieste, italy). oxygen isotopes were analyzed according to the method described by epstein and mayeda [1953]. the hydrogen isotopic measurements were conducted on h2 generated by the reaction of 10 µl water with metallic zinc at 500 ˚c, according to the analytical procedure described by coleman et al. [1982]. we used v-smow and slap as analytical standards and ar-1 as an internal standard. the analytical error was ±0.1‰ and ±0.1‰, for d18o and dd values, respectively. the 3he/4he (hereafter expressed as r/ra ratios, where r is the 3he/4he measured ratio and ra is the 3he/4he ratio of the air: 1.39 ×10−6) [mamyrin and tolstikhin 1984] and 40ar/36ar ratios were determined at the university of rochester rare gas facility using a vg 5400 rare-gas mass spectrometer fitted with a faraday cup (resolution, 200) and a johnston electron multiplier (resolution, 600) for sequential analyses of the 4he (f-cup) and 3he (multiplier) by the method described by poreda and farley [1992]. for noble gas isotope analysis, the gas samples were purified in a highvacuum line constructed of stainless steel and corning-1724 glass to minimize helium diffusion. water vapor and co2 were cryogenically trapped at −90 ˚c and −195 ˚c, respectively. nitrogen and o2 were removed with a zr-al alloy chemical getter (saes-st707), whereas ar and ne were adsorbed onto activated charcoal at −196 ̊ c and at −233 ̊ c, respectively. saes-st-101 getters reduced the hd+ background to ~1,000 ions/s. we corrected the measured 3he/4he ratios for the addition of air (monitoring 4he/22ne), by assuming that the fumarolic ne is low and of atmospheric origin [craig and lupton 1976, sano and wakita 1988]. analytical error for the r/ra determination was ≤0.3%. after completing the he isotopic analysis, we analyzed the 40ar/36ar ratios with a vg 5400 mass spectrometer. sensitivity for the ar concentrations was about 4×10−4 amps/torr on the faraday cup. precision for the 40ar/36ar ratios averaged at 0.2%. 4. results 4.1. chemical composition of gases we present the fumarole characteristics and compositions in tables 1 and 2, which include outlet fluid geochemistry of chilean volcanoes temperatures, the chemical composition of the dry gas fraction (expressed as mmol/mol), and gas fraction (xgas = rdry gases/vapor ×100) for gas discharges from irruputuncu (9 fumaroles; figure 2), olca (2 fumaroles; figure 3), putana (4 fumaroles; figure 4) and alitar (5 fumaroles and 3 bubbling pools; figure 5). irruputuncu volcano has fumarole outlet temperatures between 83 ˚c and 240 ˚c, while the other three volcanoes have temperatures between 82 ˚c and 91 ˚c. all of the emission temperatures are above or similar to the boiling point for water at 5,000 m a.s.l. (~83 ˚c). the temperatures of the alitar bubbling pools (al1, al2 and al3; table 1) are between 54 ˚c and 57 ˚c. the fumarolic gas composition is dominated by water vapor (xgas from 1.45% to 7.24%), while the bubbling gases show xgas values between 91.9% and 96.4%. the dry gas fractions are mainly composed of co2 (from 764 to 983 mmol/mol). at the irruputuncu and putana volcanoes, the fumarolic gas species are dominated by relatively high so2 and h2s concentrations (up to 170 and 52 mmol/mol, respectively), followed in order of abundance by hcl, n2, h2 and hf (up to 14, 14, 4.0 and 1.8 mmol/mol, respectively) and then o2, co, and ch4 (up to 0.41, 0.039, and 0.052 mmol/mol, respectively). the trace concentrations of ar, he and ne are lower than 0.039, 0.007 and 0.000021 mmol/mol, respectively. hydrocarbons are all present at trace levels, with the sum of the seven most-abundant species <0.0002 mmol/mol (c2h6, c2h4, c3h8, c3h6, c4h4o, c6h6 and c4h4s; table 2). the olca and alitar gas compositions vary significantly from the fumaroles of the irruputuncu and putana volcanoes. the main differences include: 1) significantly lower concentrations of h2, co and acidic gases (at least one order of magnitude), with the exception of h2s (up to 26 mmol/mol); 2) significantly higher concentrations of hydrocarbons (including ch4) and atmospheric-related (i.e. o2, ar, ne) compounds (up to one order of magnitude). the n2 and he concentrations are between 1.8 to 6.1 mmol/mol and 0.004 to 0.017 mmol/mol, respectively, and they do not show significant differences from the irruputuncu and putana gases. of note, the co2, ch4, h2, co, he and light hydrocarbon concentrations in the dry gas fraction of the alitar bubbling gases are similar to the other "dry" fumarolic gases at alitar (tables 1 and 2), whereas the acidic gases (i.e. hf and so2) are significantly lower, or even non-detectable. 4.2. isotopic compositions of steam (�d18o and �d) and gases (r/ra, 40ar/36ar and �d13c-co2) each of the four volcanoes can be clearly distinguished according to the stable isotopic composition (d18o and dd) of their condensed steam emissions (table 3). the irruputuncu gas stable isotopic composition is relatively heavy, and it ranges from d18o: +6.8‰ to +10.4‰ and dd: −35.1‰ to −44.1‰ v-smow, respectively. from irruputuncu, the stable isotopic composition of the condensed steam gases are progressively lighter in composition to putana (d18o and dd: 1.7‰ and −57.2‰ vsmow, respectively), alitar (d18o: −1.6‰ to −1.0‰ and dd: −73.4‰ to −67.8‰ v-smow, respectively) and olca (d18o: −8.4‰ and −8.0‰ and dd: −97.5‰ to −95.6‰ v-smow, respectively). conversely, our study found no significant differences in the d13c-co2 values for each of the volcanic tassi et al. 126 ! table 1. outlet temperatures, chemical compositions (dry gas fractions) and xgas (rdry gases/steam in %) of fumaroles from the fumaroles of irruputuncu (ir), olca (ol), putana (pu) and alitar (al) volcanoes. concentrations of gas compounds are in mmol/mol; n.a.: not analyzed. 127 systems, which ranged from −8.54‰ to −6.53‰ v-pdb. fumarolic isotopic gas measurements including: noble gases (i.e. 3he/4he [r/ra] and 40ar/36ar) and co2 (d13c-co2); selected gases are given in table 3. the irruputuncu fumaroles have the highest 3he/4he (r/ra) value (7.27), followed by putana (pu1 and pu4 gases with 3he/4he [r/ra] of 6.15 and 7.14, respectively). the olca (ol1 3he/4he [r/ra] of 6.11) helium isotopic composition is similar to that of pu1, while alitar is characterized by a significantly lower r/ra value (4.80). the argon isotopic composition (40ar/36ar) for the irruputuncu, olca and putana gases range between 312 and 396 and contain a moderately radiogenic component, whereas that of alitar (al5, 297) is similar to that of air (295.5) [ozima and podosek 2002], and reflects significant influence from air or air-saturated water in this system. 5. discussion 5.1. origin of fumarolic fluids volcanic fluids are discharged from volcanic systems along subducting convergent plate boundaries that typically originate from one of the following sources (and may mix to varying extents amongst these sources): 1) mantle or mantle-wedge magmatic degassing; 2) fluid–rock interactions at the various temperatures along the arc; and 3) air or air-saturated water (dissolved in permeating meteoric fluid geochemistry of chilean volcanoes ! table 2. chemical compositions (dry gas fractions) of light hydrocarbons (c2h6, c2h4, c3h8, c3h6, c4h4o, c4h4s and c6h6) of fumaroles from the irruputuncu (ir), olca (ol), putana (pu) and alitar (al) volcanoes. concentrations of gas compounds are in mmol/mol; n.a.: not analyzed; n.d.: not detected; n.c.: not calculated. table 3. d13c-co2 (‰ v-pdb), r/ra, d 18o and dd (both in ‰ v-smow) values and 40ar/36ar, 4he/20ne, co2/ 3he and ch4/ 3he ratios of fumaroles from the irruputuncu (ir), olca (ol), putana (pu) and alitar (al) volcanoes. l, s and m are the limestone, organic sediments and mantle end-members (%), as defined by sano and marty [1995]. ! water or degassed from water in subducting slab sediments) [poreda et al 1988, poorter et al. 1991, giggenbach 1992a, giggenbach 1996, snyder et al. 2001, snyder et al. 2003]. we determined the relative contributions of each of these sources for this region in the cavz. we used the arco2-(so2 +h2s) ternary diagram (figure 6) to demonstrate that the fumarolic gas composition is accounted for by a mixture between hydrothermal and magmatic sources, with minimal atmospheric (air or air-saturated water) contributions. the irruputuncu gases are characterized by c/st ratios <10 (where st is the sum of so2 +h2s), which is typical of magmatic gases from andesitic volcanoes [symonds et al. 1994] and is similar to those of other active volcanoes located along subducting convergent plate boundaries, i.e. avachinsky (kamchatka), galeras cumbal and galeras (colombia) [fischer et al. 1997, taran et al. 1997, lewicki et al. 2000]. interestingly, this low c/st ratio is less than those observed at medium–high-temperature (between 150 ˚c and 385 ˚c) fumaroles from other cavz systems, including lascar (la-ht) [tassi et al. 2009], a frequently eruptive stratovolcano located few dozen kilometers south of putana [gardeweg and medina 1994, aguilera et al. 2003, aguilera et al. 2006, clavero et al. 2006]. compared to irruputuncu, the olca and alitar fumaroles show significantly higher c/st ratios (ranging from 33 to 65), that we suggest is related to a relatively high contribution from hydrothermal fluids, while the gases at putana have intermediate c/st ratios (between 15 and 27). several fumaroles included in this study have anonymously high ar contributions than described above (and cannot be explained by a simple two-component magmatic/hydrothermal mixture) including: 1) the pu1 fumarole, which we attribute to air contamination because of the concomitant increase in o2 concentrations (0.41 mmol/mol; table 1); and 2) the three bubbling gas pools from alitar volcano, which were also characterized by relatively low h2s concentrations (low st), which we attribute to liquid-gas interactions at shallow depths and increased interactions with shallow air-rich groundwater. these types of secondary processes might, at least partially, explain the high c/st ratios of the olca and alitar fumaroles, as the solubility in water of the sulfur gas compounds is significantly higher than that of co2. to better define this system, we constructed the (hcl+h2)-so2-ch4 ternary diagram (figure 7), assuming: (i) gas–water–rock interactions produce h2 and hcl at relatively high temperatures [martini 1993]; (ii) so2 is a direct proxy for magmatic contributions [giggenbach 1980, 1996]; and (iii) ch4 in volcanic environments originates from thermal decomposition of organic material and/or abiogenic processes (i.e. sabatier reaction involving co2 and h2 that occurs under hydrothermal conditions) [schoell 1988, whiticar and suess 1990, giggenbach 1997, darling 1998, horita and berndt 1999, sherwood-lollar et al. 2002, fiebig et al. 2004, fiebig et al. 2007]. our findings here are in agreement with those indicated by the c/st ratios (figure 6), and they suggest that the fumarolic gases from these volcanoes can be defined according to only magmatic and hydrothermal contributions. we find that the irruputuncu fumarolic gases, and to a lesser extent those of putana, have a distinct magmatic signature that is more prominent than the highly active lascar (la-ht) fumaroles. conversely, the alitar and olca gases show a significant enrichment in ch4, and are thus strongly affected by fluid contributions from a hydrothermal source (figure 7). tassi et al. 128 figure 6 (left). ar-co2-(so2+h2s) ternary diagram for the irruputuncu (open circles), olca (open squares), putana (open triangles up) and alitar (open triangles down) fumarole gases. for comparison, the compositions of lascar (la-ht) [tassi et al. 2009], avachinski (kamchatka) [taran et al. 1997], galeras (colombia) [fischer et al. 1997] and cumbal (colombia) [lewicki et al. 2000] gases are also shown. figure 7 (right). (hcl+h2)-so2-ch4 ternary diagram. symbols as in legend to figure 6. for comparison, the composition of the lascar gases (la-ht) [tassi et al. 2009] is also shown. 129 by evaluating the stable isotopic compositions (d18o and dd) of the steam condensates (figure 8), we show that all four of the chilean volcanoes included in this study, and including lascar, plot along a binary mixing line between the northern chile local meteoric water line [aravena 1995, chaffaut et al. 1998, aravena et al. 1999] and the suggested "andesitic water magmatic" field [taran et al. 1989, giggenbach 1992b]. in relative terms, the trend shown in figure 8 supports our findings for the magmatic, hydrothermal, and air-saturated water contributions shown in figures 6 and 7. we find that an andesite water magmatic end-member dominates the irruputuncu gases (figure 8), which agrees with our findings for the gas composition (figures 6 and 7) and the high helium isotopic composition (r/ra, 7.27) at irruputuncu (table 3). helium isotopic compositions between 7-9 ra indicate the presence of a strong contribution from upper mantle or mantle-wedge fluids without the significant addition of crustal fluids [craig and lupton 1976, javoy et al. 1982, poreda et al. 1988, rollinson 1993]. conversely, the d18o and dd isotopic signatures of the putana, alitar and olca fumaroles can be regarded as derived by mixing processes between magmatic fluids and meteoric water in comparable amounts (figure 8). aside from water vapor, co2 is the most prominent gas present in all of the samples included in the present study. we determined the source of co2 using carbon isotopic composition and the relative proportion of co2 to 3he, a conservative, primordial mantle-gas tracer. all of the samples included in this study have d13c-co2 values in the range of mid-ocean ridge basalt (morb) gases (−9‰ to −4‰ v-pdb) [marty and jambon 1987, javoy and pineau 1991, sano and marty 1995], and do not vary systematically in the volcanoes included in the present study. the co2/ 3he ratio is a sensitive tracer that can distinguish the removal ("scrubbing") and/or addition of co2 in fumarole gases, and specifically the addition of co2 to magmatic sources [e.g. tedesco et al. 2010]. we find that the co2/ 3he ratios from all the investigated volcanic systems range between 1.73×1010 and 6.05 × 1010. this range is consistent with typical of fumarole gases from an arc setting and are approximately one order of magnitude higher than those of morb and ocean island basalt gases [des marais and moore 1984, marty and jambon 1987, marty and tostikhin 1998], probably due to the addition of co2 from carbonate-rich sediments released from the subducting slab [giggenbach and poreda 1993, sano and williams 1996, patino et al. 2000, snyder et al. 2001]. in subduction zones, co2 primarily originates from one of three sources: 1) sedimentary organic (s); 2) limestone (l); and 3) morb (a representative for upper mantle gas) (m), with minimal crustal-derived contributions [e.g. hilton et al. 2002]. we evaluated the relative contributions of each source in the cavz using the following equations [sano and marty 1995]: (1) (2) (3) where subscripts obs, morb, lim and sed refer to the sample, morb (a proxy for upper mantle contributions), limestone and sediment, respectively. we assume that the end members have the following compositions: the results for each contribution are presented in table 3. we find that the calculated relative amounts for each of the three potential carbon sources (i.e. m, s and l) result in a maximum contribution of 8.7% magmatic (m) co2 with a ratio of limestone/sedimentary carbon (l/s) ~3 at irruputuncu, olca and putana. these data are consistent with previous findings for lascar volcano [tassi et al. 2009], as well as for other arc volcanoes in salvador and costa rica [shaw et al. 2003, de leeuw et al. 2007]. alitar gas contains fluid geochemistry of chilean volcanoes figure 8. d2h versus d18o binary diagram. the "andesitic water" field [taran et al. 1989, giggenbach 1992b], and the local meteoric water line (lmwl: d2h = 8.15d18o+15.3) [chaffaut et al. 1998] are also shown. symbols as in legend to figure 6. for comparison, the isotopic composition of lascar gases (la-ht) [tassi et al. 2009] is also shown. m s l 1+ + = co co co com l s 13 2 13 2 13 2 13 2 obs morb lim sed = + + c c c c d d d d ^ ^ ^ ^ h h h h 1 c he m c he l c he s c he 12 3 12 3 12 3 12 3 obs morb lim sed = + + ^ ^ ^ ^ h h h h co co co . %, %, 30%, c he 1.5 10 , c he 1 10 , c he 1 10 . 6 5 0 13 2 13 2 13 2 12 3 9 12 3 12 3 13 morb lim sed morb lim sed 13 = = = = = =# # # c c c d d d ^ ^ ^ ^ ^ ^ h h h h h h the lowest magmatic contribution (m: 3.7%), which is more consistent with geothermal systems in northern chile than these cavz volcanic systems [tassi et al. 2010a]. these results suggest that the majority (>90%) of co2 at these volcanoes in the cavz is derived from nonmagmatic (non-mantle) sources. instead, the co2 is primarily derived from the devolatilization of subducting carbonates and to a lesser extent from thermogenic interactions with carbon in rocks and sediments, which will be discussed further below. to further constrain the origins of the gases from potential thermogenic interactions of hot fluids with rocks/sediments and slab devolatilization, we evaluated the relative abundances of n2, ar and he (figure 9). the n2/ar ratio can be used to distinguish the relative contributions to a gas mixture from air, air-saturated fluids, thermogenic interactions with subducting and surficial rocks/sediments, and mantle gases [e.g. snyder et al. 2001, 2003]. the gas composition of the volcanoes in the present study is marked by high n2/ar ratios (up to 1,509) relative to air (82). these high n2/ar ratios are typical of subduction-zone andesitic gas emissions [matsuo et al. 1978, giggenbach 1992b], where n2 is extracted from the subducting slab and slab sediments as the temperatures increase during the subduction process [snyder et al. 2003]. note that in the present study, the n2/ar ratio identifies two distinct trends (irruputuncu and putana vs. olca and alitar) for the source of the thermogenic n2, including: 1) subducting-slab-generated n2; and 2) "crustal-like" n2 that suggests the addition of n2 from crustal (near surface) sources, and most likely from the interactions of heated groundwater with crustal volcanic sequences closer to the surface. the addition of crustal 4he is supported by the decrease in r/rair for olca and alitar (6.11 and 4.80), as compared to irruputuncu (7.27) and putana (6.15-7.14) (table 3). the irruputuncu and putana gases are similar to those of arc-type andesitic volcanism and they contain relatively low he and ar, while the olca and alitar fumaroles show significant he enrichment (specifically 4he enrichment, as inferred from the lower r/ra; table 3), with minimal addition of ar in most samples. note that the alitar bubbling pools and the one putana sample (pu1) suspected of mixing with air, as discussed above, each plot along a mixing line between other gases from the respective volcano and the air composition (figure 9). interestingly, these samples do not appear to mix with air-saturated water, indicating minimal interactions with noble gases dissolved in groundwater for these samples. the origins of light hydrocarbons are commonly investigated on the basis of the relative abundance of the c1c3 alkanes in fumarolic gases [oremland et al. 1987, whiticar and suess 1990, kiyosu et al. 1992, darling 1998]. gases produced by decay of organic matter at temperatures >150 ̊ c are characterized by ch4/(c2h6 + c3h8) ratios <100, whereas higher ratios are expected when processes related to bacteria activity at low temperatures are responsible for hydrocarbon production. based on these assumption, the light hydrocarbons present at irruputuncu, putana, olca and alitar would be characterized as originating from a bacteriarelated source, because the ch4/(c2h6 +c3h8) ratios range between 158 and 603. however, we suggest that a bacteriarelated origin is not likely. we do not anticipate that hydrocarbon production can be attributed to bacterial activity in fluids so strongly related to magmatic degassing, and specifically those with discharge temperatures higher than the temperatures at which bacteria are anticipated to survive (~120 ̊ c). instead, we prefer an alternative process in which the high temperatures and highly oxidizing conditions created by the presence of water-rich, high-temperature, magmatic-related fluids results in a thermal cracking process that preferentially destroys long-chain hydrocarbons, altering the ch4/(c2h6 +c3h8) ratio. this hypothesis can explain: (i) the lack of c3+ species, with the possible exception of benzene, furane and thiophene, which are stable at relatively high temperatures [montegrossi et al. 2003, capaccioni et al. 2005, tassi et al. 2010b]; (ii) the ch4 "excess" with respect to the c2h6 and c3h8 concentrations; and (iii) the relatively low ch4/ 3he ratios (1.55×105 to 1.85×107) that are lower than those observed for gases produced by thermogenic processes in an organic-rich hydrothermal environment (up to 1×1012) [poreda et al. 1988]. relatively high ch4 concentrations in hydrothermal fluids from submarine ultramafic-hosted systems [charlou et al. 2002, proskurowski et al. 2008, konn et al. 2009] and in springs discharging from ophiolites [abrajano et al. 1990] were ascribed to ch4 production through catalytic hydrogenation of carbon oxygenated species, the so-called fischer-tropsch-type reaction. fiebig tassi et al. 130 figure 9. ar-n2-he ternary diagram. asw, air-saturated water. symbols as in legend to figure 6. 131 et al. [2007, 2009] suggested that the occurrence of a fischertropsch-type reaction is responsible of excess methane in continental hydrothermal emissions. isotopic data (d13c and dd) of ch4 and light alkanes, which were not available for the present study, could provide useful information about the origin of these species [sherwood lollar et al. 2006, fu et al. 2007, taran et al. 2007, mccollom et al. 2010, taran et al. 2010]. however, on the basis of the reported chemical evidence, we can argue that the concentrations of ch4 are independent of light alkanes, with the former being regulated by the co2-ch4 chemical equilibrium instead of the decay of organic matter. 5.2. geothermometry we can estimate the thermodynamic conditions of a deep reservoir source for fumarolic fluids by assuming equilibrium conditions for reactions involving specific gas compounds. the h2/h2o log-ratio (rh) is considered to be the most suitable parameter for describing the redox state of most natural fluids [giggenbach 1987]. in hydrothermal fluids, redox conditions are regulated by the feo-feo1.5 pair in rocks, which produces a rh of −2.8 almost independent of temperature. alternatively h2 concentrations and the redox potential in gases related to magmatic degassing are controlled by the so2-h2s gas buffer [giggenbach 1997, and references therein]. equilibrium constraints in the h2o-co2-co-h2 system are provided by the following pressure-independent reaction: (4) reaction (4) is strongly controlled by a temperaturedependent equilibrium constant, according to [giggenbach 1996]: (5) in the log(h2/h2o) versus log(co/co2) diagram (figure 10), the irruputuncu and putana gases correspond to equilibrium temperatures ranging between 420 ˚c and 580 ˚c, whereas the olca and alitar fumaroles appear to equilibrate at 340 ̊ c to 410 ̊ c. it is worth noting that in these systems, the so2-h2s pair is the dominant redox buffer, which is consistent with the presence of significant concentrations of these two gas compounds in the fumarolic gases (table 1). the h2o-co-co2-h2 geothermometer indicates ureliable low temperatures (<100 2c) for the alitar bubbling gases that are significanlty affected by h2o condensation and co dissolution within the bubbling pools. the gas species in the h2o-co2-ch4-h2 system are controlled by the sabatier reaction, as follows: (6) assuming that log fh2o = 4.9 − 1820/t [giggenbach 1987], the temperature dependence of the equilibrium constant of reaction (6) can be expressed by the following equation: (7) according to equations (6) and (7), the thermodynamic conditions of the equilibria among the carbon gas species (i.e. co2, ch4 and co) can be shown by the log(ch4/co2) versus log(co/co2) diagram [giggenbach 1987], where compositions that have equilibria controlled by the feo-feo1.5 and so2-h2s redox pairs are also reported (figure 11). the co2-ch4-co equilibrium temperatures of the irruputuncu and putana gases are 320 ˚c to 390 ˚c, whereas those of the olca and alitar fumaroles are <180 ˚c. the decoupling between the fluid temperatures, calculated by using the h2o-co2-co-h2 and co2-ch4-co geothermometers, is probably caused by the relatively low kinetics of reaction (6) with respect to that of reaction (4). indeed, the gas samples in figure 11 plot far from both the feo-feo1.5 and so2-h2s equilibrium lines. the co2-ch4 pair is quenched at relatively high temperatures (between 370 ˚c and 650 ˚c, considering the feo-feo1.5 redox buffer), whereas h2 and co tend to reequilibrate at lower temperatures as fumarolic fluids rise toward the surface. because of non-equilibrium (quenching) conditions, we additionally investigate the thermodynamic conditions that characterize deep fluid source regions on the basis of the fluid geochemistry of chilean volcanoes figure 10. log(h2/h2o) versus log(co/co2) diagram. solid lines refer to equilibria controlled by the feo-feo1.5 and the so2-h2s redox pairs. dashed lines are the calculated isotherms for the simultaneous equilibrium of the h2-h2o and the co-co2 geothermometers. symbols as in legend to figure 6. co h co h o2 2 2+ +* log co co h h o 2.49 2248 t k2 2 2 =-^ ^ ^h h h ch 2h o co 4h4 2 2 2+ +* log ch co 4log h h o 5181 t k4 2 2 2 =-^ ^ ^h h h chemical features of the c2 and c3 alkenes-alkanes pairs [capaccioni and mangani 2001, taran and giggenbach 2003, tassi et al. 2005a, tassi et al. 2005b]. de-hydrogenation of c2h6 to produce c2h4 is, as follows: (8) the temperature dependence of the equilibrium constant for reaction (8) is [capaccioni et al. 2004]: (9) de-hydrogenation of c3h8 to produce c3h6 is as follows: (10) the temperature dependence of the equilibrium constant for reaction (10) is [capaccioni et al. 2004]: (11) we calculated equilibrium temperatures from equations (9) and (11) (tc2-c2 and tc3-c3, respectively) using the h2 concentrations measured. these calculations were not performed for the alitar bubbling gases, as the h2 concentrations of these samples are strongly affected by liquid-gas interactions at low temperatures (figure 10). as shown in table 2, both of the sets of calculated temperatures indicate that the irruputuncu and putana gases equilibrate at 478 ˚c to 536 ˚c, whereas those of the olca and alitar gases range from 280 ˚c to 400 ˚c, in agreement with the temperature estimations in the h2o-co2-co-h2 system. 6. concluding remarks the chemical and isotopic (r/ra, d 18o-h2o, dd-h2o and d13c-co2) compositions of gases emitted from the irruputuncu, putana, olca and alitar volcanoes suggest a mixture between magmatic, hydrothermal and atmosphericrelated fluids, as typically occurs in volcanic systems along convergent/subduction plate boundaries. we suggest that magmatic degassing is the dominating fluid source at irruputuncu and putana. the olca and alitar gases show high-temperature gas compounds (i.e. presence of so2), and significant contributions from hydrothermal components. the r/ra and the co2/ 3he ratios of the four investigated volcanic systems are consistent with those for the cavz and subduction arc systems worldwide. all four systems emit gases with magmatic carbon components below 10%, while the lower proportion of magmatic carbon in alitar fluids supports our assertions that fluids discharging at this volcano are fed by a well-developed hydrothermal system. gas geothermometry in the h2o-co2-co-h2, co2-ch4and c2-c3 alkenes-alkanes systems, consistently indicates that the irruputuncu and putana fluids are quenched at relatively high temperatures (>500 ˚c), which suggests hot magmaticrelated gases rapidly interact with cooler hydrothermal fluids as they ascend to the surface. however, only a very small amount of interaction between magmatic fluids and shallow aquifers is feasible based on our data. conversely, significantly lower equilibrium temperatures (<400 ˚c) of the fluids from the olca and alitar volcanoes are likely to be attained within hydrothermal systems overlying the magmatic source. on the whole, the irruputuncu, putana, olca and alitar volcanoes are variably affected by mixing between deepseated (magmatic) and shallower (hydrothermal) sources, both which suggest that each of these edifices remain active. consequently, although these volcanoes are located in remote areas of northern chile, we suggest that periodic (e.g. on a yearly basis) sampling of the fumarolic gas discharges is required to monitor the evolution of the most relevant geochemical and isotopic parameters. periodic monitoring will allow researchers to develop an accurate geological hazard assessment for these volcanoes and the gain an understanding of the volcanic hazard potential that can result from their activity. acknowledgements. this study was partially financed by dirección general de investigación y postgrado (ucn-chile) and by d-21050592 conicyt grant (government of chile). a. rizzo and s. inguaggiato are warmly thanked for the comments and suggestions they provided on an earlier version of the manuscript. the authors wish to express their gratitude to jaime llanos (inorganic chemical laboratory-ucn-chile) for facilities in sample preparation, and to josé luis mercado and benigno godoy (ucn-chile) for their help during the sampling campaigns. tassi et al. 132 figure 11. log(ch4/co2) versus log(co/co2) diagram. solid lines refer to equilibria controlled by the feo-feo1.5 and the so2-h2s redox pairs. dashed lines are the calculated isotherms for the simultaneous equilibrium of the h2-h2o and the co-co2 geothermometers. symbols as in legend to figure 6. c h c h h2 6 2 4 2+* 7.43 8, 809 t k log c h c h logh2 4 2 6 2= +^ ^h h c h c h h3 8 3 6 2+* 7. , 0 t k log c h c h logh15 6 6 0 23 6 3 8= +^ ^h h 133 references abrajano, t.a., n.c. sturchio, b.m. kennedy, g.l. lyon, k. muehlenbachs and j.k. bohlke (1990). geochemistry of reduced gas related to serpentinization of the zambales ophiolite, philippines, appl. geochem., 5, 625-630. aguilera, f., c. martínez, f. tassi, j. viramonte, 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(1978). cuadrángulo ujina: región de tarapacá, escala 1:50.000, instituto de investigaciones geológicas, n˚ 33, p. 63. vergara, h. and a. thomas (1984). hoja collacagua: región de tarapacá, escala 1:250.000, servicio nacional de geología y minería, n. 59, p. 79. whiticar, m.j. and e. suess (1990). hydrothermal hydrocarbon gases in the sediments of the king-george basin, bransfield strait, antarctica, appl. geochem., 5, 135-147. wörner, g., k. hammerschimdt, f. henjes-kunst, j. lezaun and h. wilke (2000). geochronology (40ar/39ar, k-ar and he-exposure ages) of cenozoic magmatic rocks from northern chile (18–22˚s): implications for magmatism and tectonic evolution of the central andes, revista geológica de chile, 27, 205-240. *corresponding author: franco tassi, università degli studi di firenze, dipartimento di scienze della terra, firenze, italy; e-mail: franco.tassi@unifi.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. tassi et al. 136 o n t h e a n a l y s i s o f r a y l e i g h w a v e i n e l a s t i c m e d i a i t . g . b h a t i a ( * ) j . c . d r a k o p o u l o s ( * * ) r e c e i v e d o n o c t o b e r 20, 1971 summary. — i n t h i s p a p e r t h e r a y l e i g h w a v e e q u a t i o n h a s b e e n s i m p l i f i e d t o t w o s i m p l e e q u a t i o n s , t h e b i f u r c a t i o n p o i n t b e i n g t h e r a t i o of s h e a r t o l o n g i t u d i n a l w a v e v e l o c i t y . t h e p u r p o s e of t h e s t u d y is t o obt a i n t h e p h a s e v e l o c i t y w i t h o u t b e i n g i n v o l v e d i n t o t h e s o l u t i o n of s i x d e g r e e e q u a t i o n w h e n t h e c o n s t a n t s of t h e m e d i a a r e k n o w n . riassunto. •— i n q u e s t a n o t a l ' e q u a z i o n e delle o n d e di r a y l e i g h è s t a t a r i d o t t a a d u e s e m p l i c i e q u a z i o n i , il cui p u n t o di i n t e r s e z i o n e r a p p r e s e n t a il r a p p o r t o f r a la v e l o c i t à delle o n d e s u p e r f i c i a l i ( t r a s v e r s a l i ) e q u e l l a delle o n d e l o n g i t u d i n a l i . s c o p o dello s t u d i o è di o t t e n e r e l a v e l o c i t à d i f a s e , q u a n d o lo c o s t a n t i dei m e z z i s o n o c o n o s c i u t e , e v i t a n d o cosi di risolv e r e u n ' e q u a z i o n e di s e s t o g r a d o . 1 . i n t r o d u c t i o n i t is shown in treatises on theory of elasticity t h a t two types of wave motion are propagated in an isotropic homogeneous and unbounded elastic medium. these are the longitudinal or t h e compressional (p) waves and t h e transverse or the shear (s) waves. their velocities are respectively given by are lame's constants of the media and o is t h e density. i n any media the p waves travel faster t h a n t h e 8 waves. all the e a r t h q u a k e records show the presence of surface waves in addition to these. 1/ and where a, ¡i (*) school of r e s e a r c h a n d t r a i n i n g i n e a r t h q u a k e e n g i n e e r i n g , u n i v e r s i t y r o o r k e e , i n d i a . (**) s e i s i n o l o g i c a l l a b o r a t o r y , a t h e n s u n i v e r s i t y , g r e e c e . 2 2 k . g . b h a t i a j . c . d r a k o p o u l o s the surface waves comprising t h e rayleigli and love waves are distinguished f r o m t h e bodily p a n d s waves in being more or less confined to t h e surface. thus, their a m p l i t u d e s decrease v e r y r a p i d l y with d e p t h below t h e surface. i t has always been t h e necessity to o b t a i n t h e p h a s e velocities of surface waves in order to analyse t h e behaviour of these waves in a n y media. i n this p a p e r t h e a u t h o r s have paid p a r t i c u l a r a t t e n t i o n on t h e rayleigli t y p e of wave only. 2 . f o r m u l a t i o n o f t h e p r o b l e m a n d c o n c l u s i o n the well k n o w n rayleigli wave equation i s f 1 2 ) : f 2 [ | « — 8 f 4 + | 2 (24 — 16?;2) + 16 { r ¡ ~ 1)] = 0 [1] c b where f = a n d »? = , c is t h e p h a s e velocity of rayleigli wave in b a elastic media and r¡ is t h e ratio of shear wave velocity to compressional w a v e velocity, a a n d b are t h e longitudinal a n d shear wave -• 12 u -}"k -j / velocities in t h e m e d i a given bv a = 1/a n d b = / . f r o m i q i s equation [1] we see t h a t one root of f is zero b u t this gives p h a s e velocity equal t o zero a n d hence is of not use. therefore f r o m [1] we o b t a i n |o _ + p (24 — 1 6 ^ ) + 16 (7f— 1) = 0 [2] we m a y write it as r¡2 (16 — 16£2) + — 814 + 2 4 f 2 — 1 6 = 0 or 16 — 24i2 + 8f 4 — 11 = 16 ( i ! 2 ) [ 3 ] therefore 16 — 2 4 | 2 + 8£4 — £« 16 (1 — i2) w since c is always less t h a n b, f is always less t h a n u n i t y . u n t h e a n a l y s i s o f r a y l e i g h w a v e i n e l a s t i c m e d i a 2 3 f r o m [4] we can obtain a plot of •>] versus we can also solve t h e e q u a t i o n [2] for f for different values of rj. a simple computer program was written for this and t h e results were obtained. m a x i m u m value of i] has been t a k e n as 0.70 because for ?/ higher t h a n this, poisson's ratio becomes negative a n d no m a t e r i a l y e t has been obtained with negative poisson's ratio (3). t a b l e i = 0 . 9 5 5 2 9 1 £ t e s 0.00 0 . 9 5 5 2 9 0.00 0.05 0 . 9 5 5 2 0 — 0 . 0 0 0 0 9 0.10 0 . 9 5 4 6 5 — 0 . 0 0 0 6 4 0.15 0 . 9 5 3 9 3 — 0 . 0 0 1 3 6 0.20 0 . 9 5 2 7 4 — 0 . 0 0 2 5 5 0.25 0.9511 1 — 0 . 0 0 4 1 8 0.30 0 . 9 4 9 0 4 — 0 . 0 0 6 2 5 0.35 0 . 9 4 6 3 4 — 0 . 0 0 8 9 5 0.40 0 . 9 4 2 8 6 — 0 . 0 1 2 4 3 0.45 0 . 9 3 8 3 5 — 0 . 0 1 6 9 4 0.50 0 . 9 3 2 4 9 — 0 . 0 2 2 8 0 0.55 0 . 9 2 4 7 3 — 0 . 0 3 0 5 6 0.00 0 . 9 1 4 1 6 — 0 . 0 4 1 1 3 0.65 0 . 8 9 9 3 6 — 0 . 0 5 5 9 3 0.70 0.87789 — 0 . 0 7 7 4 0 in order to m a k e t h e first value equal to zero, £„ = 0.95529 has been s u b t r a c t e d f r o m each value. a log-log plot is obtained for (£ — £„) vs 7) (see fig. 1). f o r 0 < ?/ ^ 0.40 we get one s t r a i g h t line a n d for values 0.40 < r) < 0.70 we get another s t r a i g h t line. e a c h s t r a i g h t line will give a n equation of t h e t y p e y = h xsec i.e f —1„ = k r]sec. e x a c t results are obtained by a d j u s t i n g t h e values of c o n s t a n t 7c. f r o m graph for 0 < 7] < 0.40 f = 0.95529 — (0.10 + 0.05?;) [5] a n d for 0.40 < < 0.70 £ = 0.95529 — (0.12 + 0.10?;) r f ™ [6] 2 4 k . g . b h a t i a j . c . d r a k 0 p 0 u l 0 s therefore for 0 < rj < 0.4 c = 0.95529 b — (0.10 + 0.5??) btf-™ [7] a n d for 0.4 < rj < 0.7 c = 0.95529 b — (0.12 + 0.10?;) bif -83 [8] f i g . 1 r e d u c e d r a y l e i g h w a v e e q u a t i o n . l i q u a t i o n s [5] a n d [6] or more practically [7] a n d [8] are reduced r a y leigh wave e q u a t i o n a n d these give results correct to t h i r d decimal place. t h e values of h = i / ^ a n d n = i/,-,-:— are calculated f r o m t h e i q i a + 2^ given values a n d hence t h e p h a s e velocity can i n s t a n t l y , be obtained. i t can be n o t e d t h a t the p h a s e velocity v a r i a t i o n in both of t h e zones on ?/ i.e 0 < rj < 0.4 a n d 0.4 < •>] < 0.7 is different. i t is expected t h a t such equations will help i n t h e analysis of rayleigh wave under certain strict b o u n d a r y conditions. also, in order to analyse t h e rayleigh wave i n anelastic media, where t h e dissipation accompanies v i b r a t i o n , equations of these two t y p e will help in obtaining t h e numerical solution of t h e problem. on t h e a n a l y s i s o f r a y l e i g h w a v e i n e l a s t i c m e d i a 25 r e f e r e n c e s ( 1 ) bhatia k . g . , a n d dkakopoulos j . c . , 1 9 7 1 . a study on the displacement components oj bayleigh wave. " a n n a l i di g e o f i s i c a " , x x i v , 1, p p . 89-101. (2) bullen k . e . , 1963. an introduction to theory oj seismology. c a n i b . u n i v . p r e s s . (3) huang y. t . , 1968. effect oj an elastically restrained boundary on sv-wave radiation patterns. " b u l l . seisin. s o c . a m . " , p p . 4 9 7 5 2 0 . g e o m a g n e t i c v a r i a t i o n s a n d d i u r n a l r a n g e o f a t m o s p h e r i c o z o n e s . l . malt r k a r d o b s o n . h a r r i s o n a n d l a w r e n c e ( 1 9 2 7 | f o n n d " t l i e a p p a r e n t c o n nection b e t w e e n t h e a i n o u n t o l o z o n e a n d t h e 'h' r a n f i e is m u c h m o r e m a r k e d 011 d a y s of h i g h m a g n e t i e e h a r a c t e r , t h e d a v s of l o w c h a r a c t e r s h o w i n g l i t t l e d i f f e r e n t i a t i o n h e t w e e n t h e a m o n n t s of o z o n e c o r r e s p o n d i n g t o h i g h a n d l o w "//' r a n g e " . b u t s u b s e q u e n t l y ( 1 9 2 9 | t h e y r e m a r k e d "it w a s t h o u g h t t l i a t a c l o s e r r e l a t i o n m i g h t h e f o n n d b y using t h e m e a n o z o n e v a l u e s f o r n. w . e u r o p e a n d t h e m e a n m a «netic c h a r a c t e r f o r s e v e r a l s t a t i o n s in t h e s a n i e r e g i o n . . . i n s t e a d of the r e l a t i o n b e i n g c l o s e r , as w a s e x p e c t e d , it is e n t i r e l y a b s e n t . t l i i s is a c u r i o u s r e s u l t w h i c h w e c a n n o t e x p l a i n a t present"'. in c o n n e c t i o n w i t h o t b e r s t u d i e s , it w a s t h o u g h t t h a t t h e q u e stion m a y a g a i n h e l o o k e d i n t o . a p a r t f r o m t h e m e a n v a i n e of o z o n e , it is p o s s i b l e t o get t h e d i u r n a l r a n g e a l s o t l i e s e d a y s . t h e v a r i a t i o n s in t h e m e a n v a i n e a n d t b o s e i n t h e r a n g e of o z o n e c a n b e i n d e p e n d a n t . f u r t h e r , in t b e s e y e a r s it is t h e u s u a i r u l e t o get a t m o r e d e t a i l e d p i c t u r e s of m a g n e t i e v a r i a t i o n s in t h e c a n d k i n d i c e s w h i c h r e p r e s e n t t h e m a g n e t i e c h a r a c t e r f o r t h e w h o l e d a y a n d t h e m a g n e t i e fluctuations a b o u t i t s n o r m a l c u r v e at t h r e e h o u r l y i n t e r v a l s of t h e e a r t b ' s b o r i z o n t a l field. t 0 n s b e r g a n d o l s e n ( 1 9 1 li b a v e p u b l i s h e d t h e d a i l y a m o u n t s of o z o n e a n d its d a i l y r a n g e a t t r o m s o . f o r t h e y e a r s 1 9 4 1 a n d 1 9 4 2 , t h e v a l u e s a r e a v a i l a b l e o v e r a l a r g e n u m b e r of d a y s t o c o m p a r e w i t h t h e m e a n w o r l d g e o m a g n e t i c k figures ( i a t m e 1 9 4 8 ) . t h e d i u r n a ! r a n g e of o z o n e f o r 1 9 5 1 at t r o m s 0 is a l s o a v a i l a b l e ( t r o m s 0 , 1 9 5 3 ) . in t h e s a n i e p u b l i e a t i o n t h e k m a g n e t i e figures f o r t r o m s 0 a n d b e a r id. a r e g i v e n . t h e m e a n v a i n e of k i n d i c e s f o r t l i e s e t w o o b s e r v a t o r i e s bas b e e n u s e d f o r 1 9 5 1 . t h e a c t u a l d a i l y figures of o z o n e d i u r n a l v a r i a t i o n in u n i t s of 0 . 0 0 1 c111 a n d t h e d a i l y siuii of t h e 3 l i o u r c h a r a c t e r figures a r e r e p r o d u c e d in t h e g r a p h s . t h e c o n t i n u o u s l i n e s e o n n e c t t h e g e o m a g n e t i c figures a n d t h e d o t t e d l i n e s e o n n e c t t h e v a l u e s of d i u r n a l a m o u n t s of 2 1 0 s . l . m a l u r k a r f i g . 1 c e o m a g n e t i c v a r i a t i o n s and d i u r n a l r a n g e oe a t m o s p h e r i c ozone 2 1 1 1942 o z o n e . t h e g r a p h s s h o w a g o o d e o o r d i n a t i o n . if t h e v a l u e of d i u r n a l r a n g e of a t i n o s p h e r i e o z o n e i n e r e a s e s : at a l i o u t t h e s a n i e e p o c h ( w i t h i n 2 4 h o u r s o n e i t h e r s i d e i , t h e m a g n e t i e c h a r a c t e r suin is a l s o f o u n d t o l i a v e i n c r e a s e d . w h i l e t h e m a g n e t i e c h a r a c t e r , o n c e h a v i n g i n c r e a s e d , t a k e s s o m e d a y s t o d r o p b a c k t o its n o r i n a l o r o r i g i n a i v a l u e . t h e d i u r n a l r a n g e of o z o n e d r o p s b a c k r a p i d l y ; i. e. w h i l e t h e i n c r e a s e of t h e d i u r n a l r a n g e of o z o n e a n d t h e r i s e of t h e m a g n e t i e c h a r a c t e r seem t o h e c o n t e m p o r a n e o u s , t h e d r o p in t h e d i u r n a l r a n g e of o z o n e is n o t n e e e s s a r i l v c o n t e m p o r a n e o u s w i t h t h e f a l l of t h e m a g n e t i e c h a r a c t e r . a m o n g t h e r e m a r k a b l e c o i n c i d e n c e s a r e j u n e . 1 3 1 4 t h . 1 9 ó l a d a y of s o l a r flare c r o c h e t a t a l i b a g f o l l o w e d b y a m a g n e t i e s t o r i l i o n 1 4 t h a n d m a r . i s t . 1 9 4 1 w h e n a m a g n e t i e s t o r n i o c c u r r e d m a y h e n o t i c e d . i — g e 0 m a g n e t 1 c v a r i a t i o n s and d l l ' r n a l range oe a t m o s p h e r i c ozone 2 1 3 i t l i a n k p r o f . d . d. k o s a m l i i f o r c o n f i r n i i n g t h e g r a p h i c a l d e d u e t i o n . t h e a g r i e u l t u r a l m e t e o r o l o g i " s e e t i o n ( p o o n a i a n d m r . m. p a n i l u r a n g a r a o f o r h e l p i n g m e in o h t a i n i n g t h e d i a g r a m s . r e f e r f . n c e s d o b s o n g . m . b . , h a r r i s o n d . i s . a n < ] l a w r e n c e : j . p r o c . r o v . s o c . a 114, 5 2 1 , 1 9 2 7 . d o b s o n g . m . b . , h a r r i s o n i ) . n . a n d l a w r e n c e : j . p r o c . r o y . s o c . a 122, 456, 1 9 2 9 . 1 a t m e : b u l l . n r . 1 2 . i n t e r n a t i o n a l u n i o n of g e o d e s v a n d g e o p l i y s i c s g e o m a g n e t i c i n d i c e s . c a n d k 1 9 4 0 1 9 4 6 . w a s h i n g t o n , 1 9 1 8 . t0nsberg. e i n a r a n d o l . e n . k a a r e i . a n g l o . g e o f y s . p u b i . o s l o , 13, n r . 1 2 , 1 9 4 4 . tromso ( 1 9 5 3 ) . p u b i . n o r s k e i n s t . k o > m . f y s . n r . 3 4 . bergen, t h e a u r o r a l o b s e r v a t o r y al t r o m s ( ) . o b s e r v a t i o n s , 1 9 5 1 . geo 1009 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 5 long-term trends in the ionosphere and upper atmosphere parameters jürgen bremer (1), lucilla alfonsi (2), pal bencze (3), jan laštovička (4), andrei v. mikhailov (5) and neil rogers (6) (1) leibniz-institute of atmospheric physics, kühlungsborn, germany (2) istituto nazionale di geofisica e vulcanologia, roma, italy (3) geodetic and geophysical research institute, hungarian academy of sciences, sopron, hungary (4) institute of atmospheric physics, academy of sciences of czech republic, prague, czech republic (5) institute of terrestrial magnetism, ionosphere and radio wave propagation (izmiran), russian academy of sciences, troitsk (moscow region), russia (6) centre for rf propagation and atmospheric research, qinetiq, malvern, u.k. the first part of the paper is directed to the investigation of the practical importance of possible longterm trends in the f2-layer for ionospheric prediction models. using observations of about 50 different ionosonde stations with more than 30 years data series of fof2 and hmf2, trends have been derived with the solar sunspot number r12 as index of the solar activity. the final result of this trend analysis is that the differences between the trends derived from the data of the individual stations are relatively large, the calculated global mean values of the fof2 and hmf2 trends, however, are relatively small. therefore, these small global trends can be neglected for practical purposes and must not be considered in ionospheric prediction models. this conclusion is in agreement with the results of other investigations analyzing data of globally distributed stations. as shown with the data of the ionosonde station tromsø, however, at individual stations the ionospheric trends may be markedly stronger and lead to essential effects in ionospheric radio propagation. the second part of the paper deals with the reasons for possible trends in the earth’s atmoand ionosphere as investigated by different methods using characteristic parameters of the ionospheric d-, e-, and f-regions. mainly in the f2-region different analyses have been carried out. the derived trends are mainly discussed in connection with an increasing greenhouse effect or by long-term changes in geomagnetic activity. in the f1-layer the derived mean global trend in fof1 is in good agreement with model predictions of an increasing greenhouse effect. in the e-region the derived trends in foe and h´e are compared with model results of an atmospheric greenhouse effect, or explained by geomagnetic effects or other anthropogenic disturbances. the trend results in the d-region derived from ionospheric reflection height and absorption measurements in the lf, mf and hf ranges can at least partly be explained by an increasing atmospheric greenhouse effect. 5.1. introduction trend investigations of different ionospheric and atmospheric parameters became more important during recent years as such trends could be caused by anthropogenic activities. in this paper we will concentrate on two main questions: 1) are there trends in the ionosphere which have to be considered in radio wave prediction models? 2) what are the reasons for possible trends in the earth’s atmosphere and ionosphere? 1010 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers the results related to these two topics are presented in sections 5.2 and 5.3, discussed in section 5.4 and briefly summarized in section 5.5. 5.2. practical aspects of ionospheric trends in this section some practical aspects of ionospheric radio propagation are discussed. we restrict our investigations to trends in the ionospheric f2-region using their height and maximum electron density data from different ionosonde stations. in section 5.2.1 mean global trends are derived, whereas in section 5.2.2 some practical aspects of the ionospheric radio propagation are presented using data from an individual ionosonde station. 5.2.1. derivation of mean global trends as is well-known from many investigations the behavior of the ionosphere is markedly controlled by solar activity. in most of the ionospheric prediction models the solar activity is described by the solar sunspot number r12. therefore, in the analyses to investigate the practical aspects of possible ionospheric trends this parameter has been used. as the ionospheric radio propagation in the hf range is mainly dependent on the behavior of the ionospheric f2-region (height and maximum electron density), in the following the results of trend analyses of hmf2 and fof2 observations all around the world are presented. the analyses have been carried out for data series mainly starting at 1957 with a duration of at least 30 years. altogether the results of 53 stations with fof2 data and 50 stations with hmf2 data have been investigated. the hmf 2 values were derived from m(3000)f2 data using the simple formula of shimazaki (1955). the trend method used starts with calculations of simple regression equations using monthly mean values the ionospheric parameter x (= hmf2 or fof2) at each full hour in dependence on the solar activity assuming a linear dependence x a b r12th $= + (5.1a) or a quadratic equation x a b cr r12 12 2 th $ $= + + . (5.1b) then the differences between the observed xobs and the calculated values xth are estimated x x x thobs= -∆ (5.2a) or relative differences are calculated x x x xth thobs= -∆ ^ h . (5.2b) using mean yearly ∆x values derived by averaging the monthly ∆x values at each full hour (averages of 12 × 24 data series), mean linear tends have been estimated according to x d e year$= +∆ . (5.3) in fig. 5.1a,b two histograms are shown for the trend results of fof2 observations at 53 different ionosonde stations. in the upper part the solar activity has been eliminated by a linear relation according to eq. (5.1a), in the lower part by the quadratic relation according to eq. (5.1b). in both cases differences according to eq. (5.2a) have been used in the trend estimations. the distributions of the 1011 long-term trends in the ionosphere and upper atmosphere parameters individual fof2 trends are relatively broad, the median trends however are relatively near to zero. as to be expected both data sets are well correlated with a correlation coefficient r = 0.92. in fig. 5.2a,b the trend results of hmf2 observations at 50 different stations are shown in a similar way as for fof2 in fig. 5.1a,b. also here relatively broad distributions of the individual trends can be seen, whereas the median values are again very near to zero. the correlation between both trend ensembles is very good with r = 0.995. in table 5.i the mean trends of fof2 and hmf2 (median as well as mean values) have been compiled. these data can partly be found in figs. 5.1a,b and 5.2a,b and from trend results using a linear solar activity influence (eq. (5.1a)) and relative differences ∆x (eq. (5.2b)) not shown before. for the mean trends the error values with a significance level of 95% are also presented (for calculation see taubenheim, 1969). all mean trends are smaller than the corresponding error limits, indicating that the mean trends are not significantly different from zero. nevertheless from all mean and median values in table 5.i the expected changes for ∆fof2 and ∆hmf2 have been estimated for a time period of 100 years. as to be seen these values are very much smaller compared with changes induced by the variable solar activity. therefore, possible long-term changes have not to be introduced in ionospheric prediction models. until now global mean trends in fof2 and hmf2 have no essential practical meaning. fig. 5.1a,b. histograms of 53 individual fof2 trends derived after a linear (a) and quadratic (b) elimination of the solar influence. the corresponding median values are marked by arrows. b a 1012 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers fig. 5.2a,b. histograms of 50 individual hmf2 trends derived after a linear (a) and quadratic (b) elimination of the solar influence. the corresponding median values are marked by arrows. table 5.i. mean trends of fof2 and hmf2 derived from observations at about 50 ionosonde stations (beginning 1957, more than 30 years) by different methods. ∆fof2 and ∆hmf2 are the expected changes of fof2 and hmf2 during 100 years. trend (r12) trend (r12 2 ) trend (r12) rel fof2 mean trend + 0.0003 mhz/yr − 0.0013 mhz/yr − 0.016%/yr 95% error ± 0.0027 mhz/yr ± 0.0023 mhz/yr ± 0.041%/yr ∆∆fof2mean ++ 0.03 mhz −− 0.13 mhz −− 0.13 mhz median trend + 0.0009 mhz/yr − 0.0010 mhz/yr + 0.001%/yr ∆∆fof2med [mhz] ++ 0.09 mhz −− 0.10 mhz ++ 0.008 mhz hmf2 mean trend + 0.058 km/yr + 0.063 km/yr + 0.021%/yr 95% error ± 0.096 km/yr ± 0.092 km/yr ± 0.030%/yr ∆∆hmf2mean ++ 5.8 km ++ 6.3 km ++ 6.3 km median trend + 0.012 km/yr + 0.009 km/yr + 0.004%/yr ∆∆hmf2med [km] ++ 1.2 km ++ 0.9 km ++ 1.2 km a b 1013 long-term trends in the ionosphere and upper atmosphere parameters 5.2.2. trends at an individual station the ionosonde at tromsø in northern norway (69°n, 19°e) (hall and hansen, 2003) is a unique resource providing continuous observations of ionospheric critical frequencies since 1935 and layer heights since 1952. hall and cannon (2001, 2002) have examined long-term trends in f-region parameters at tromsø using a method similar to that developed by bremer (1992) and used in section 5.3.1 below, eq. (5.4). using daily ionosonde measurements taken at or near noon, linear components of the background variations in r12 and geomagnetic index (ap) are subtracted to leave a residual trend. based on a recently extended set of ionograms, the peak height of the f2 layer, hmf2 is found to fall at − 0.106 ± 0.087 km per year and its critical frequency, fof2 falls at − 0.013 ± 0.002 mhz per year, where the errors indicate one standard deviation. ulich et al. (2003) presented an hmf2 data set for a nearby ionosonde at sodankylä, finland for the period 1958-2003 with a trend of − 0.41 ± 0.04 km per year – a significantly steeper trend than that seen in the tromsø data. modeling of ionospheric radio system performance requires a propagation tool that contains a three-dimensional map of ionospheric electron density. this may be used to find the skip zones of high frequency (hf) radio transmissions, or the group delay, phase advance, etc., in transionospheric systems. parameterized «monthly median» models are often employed, and these use worldwide maps of fof2 and hmf2 and similar parameters, often referred to as ccir coefficients (ccir, 1966; jones et al., 1969) or ursi coefficients (rush et al., 1989). the former were based on ionosonde data for the years 1954-1958 and 1964 only and fof2 was described by a set of special mapping functions. the worldwide rms error in fof2 is ∼ 0.5 mhz. the ursi coefficients are derived from a much larger database of 45 000 station-months and include data from 19751979. ursi coefficients showed improved accuracy over the oceans but at the expense of accuracy over land, so that overall the ccir and ursi maps are of similar accuracy (bilitza, 2002). fig. 5.3. impact of 16 km reduction in hmf2 on ground ranges for propagation due south of tromsø at 8 mhz for december, 12 ut, r12 = 50. 1014 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers five decades have elapsed since the data were collected to generate the ccir coefficients, and in that time the f2-region over tromsø has fallen by 5.3 km. further, fof2 has decreased by 0.65 mhz, an amount already bigger than the rms model error of the ccir model. the impact on hf systems has been assessed by ray tracing through two sets of analytical dudeney profiles of electron density (dudeney, 1983). the first set of profiles was fit to the ccir coefficients and the second was modified to reduce the peak height by 16 km. based on the analyses of tromsø ionograms a 16 km drop in hmf2 is appropriate to the conditions in the year 2100 (provided there is no change in the long-term trends). if we had chosen to use the results of ulich et al. (2003) then a 16 km height reduction was appropriate to the year 1990! for a 16 km drop it was found that ground range errors of ∼ 100 km per hop will be common if system designers and operators continue to employ the old ccir maps. a sample of the ray tracing results is presented in fig. 5.3, representing 8 mhz 1-hop propagation south of tromsø near noon on a december day. overall, the ground range errors (lower curve) are 50-150 km for the 1f2 mode of propagation (above 21° elevation), with greater errors at elevations just above that at which the e-layer is penetrated. these results demonstrate that for trends derived at individual stations their influence on ionospheric radio propagation could be much stronger than for global mean trends. this result could be expected from the broad trend histograms shown in figs. 5.1a,b and 5.2a,b. 5.3. scientific aspects of ionospheric trends this section presents the trend results which were derived by different european groups using partly different methods. the results are separately discussed for the different ionospheric regions. 5.3.1. trends in the f2-region using a similar analysis method as described in section 5.2.1 bremer (1998, 2001) investigated data series of more than 100 ionosonde stations. for the elimination of the solar and geomagnetically induced parts, however, the following twofold regression equation was used: x a b r c apth $ $= + + (5.4) with the solar sunspot number r and the geomagnetic activity index ap. also other solar indices have been tested (f10.7, e10.7, r12), but the derived trends are not markedly influenced by this choice (bremer, 2001). in fig. 5.4 the updated histograms with the individual trends of fof2 and hmf2 are presented together with the corresponding median values. as in the investigations discussed in section 5.2.1, the distributions of the individual trends for both parameters are relatively broad, and the median values are very small. as shown in table 5.ii the mean trends of fof2 and hmf2 are not statistically significant different from zero as follow from the corresponding mean errors. in fig. 5.5 the individual hmf2-trends are presented in dependence on longitude and latitude. negative trends are marked by blue, positive trends by red symbols. significant trends with a 95% reliability level are characterised by full dots, non significant trends by circles. there is no clear dependence of the trends on latitude or longitude, but there seem to be some regional differences. whereas in the western part of europe most trends are negative, in central asia the trends are more positive. the causes of these regional differences are not known. similar trend analyses have been made by ulich and turunen (1997a) at first with hmf2 data of sodankylä and later for more than 50 stations (ulich and turunen, 1997b). these authors also found similar regional differences in the hmf2 trends as shown in fig. 5.5. ulich (2000) also investigated 1015 long-term trends in the ionosphere and upper atmosphere parameters fig. 5.4. histograms for fof2 and hmf2 trends derived from ionosonde observation at different stations. number of stations are given in brackets. table 5.ii. mean experimental trends and error limits (95%) derived from trend analyses of different ionosonde parameters at different stations. significant mean trends are bold-faced. parameter n mean trend error (95%) f2-region fof2 106 − 0.0018 mhz/yr ± 0.0025 mhz/yr hmf2 87 − 0.009 km/yr ± 0.076 km/yr f1-region fof1 51 0.0027 mhz/yr ±± 0.0011 mhz/yr e-region foe 72 0.0014 mhz/yr ±± 0.0007 mhz/yr h′e 31 − 0.040 km/yr ± 0.070 km/yr fig. 5.5. trends of hmf2 in dependence on latitude and longitude derived from data sets of 87 different ionosonde stations. negative trends are blue, positive are red, significant trends are marked by full dots, non significant trends by circles. 1016 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers different formulas to derive hmf2 from ionosonde m(3000)f2 data. he found quantitative differences in the derived trends depending on the hmf2 formula used. this finding is caused by the fact that the ionisation below the f2-layer is differently considered by the different formulas. therefore, trends in the underlying ionisation parameters my also cause differences in the hmf2 trends. bencze et al. (1998) derived trends in hmf2 for some asian and american stations using the same method as bremer (2001) and ulich and turunen (1997a,b). he also found similar regional differences with more negative trends at coastal regions and more positive trends inside the continents. alfonsi et al. (2001, 2002a) derived fof2 trends for different stations at high and middle latitudes using monthly mean values as well as hourly values. in their analyses only such experimental data have been introduced which are not accompanied by qualification letters to reduce the uncertainty of the hourly fof2 values. the investigation of the ionospheric long-term changes follows an analytical approach similar to that shown in eq. (5.4). the original contribution comes from the use of ap(τ) and macap(τ), used for investigating the geomagnetic control on the ionospheric trends. ap(τ) and macap(τ) have been introduced for representing the non-linear relationship between geomagnetic activity and ionospheric behaviour (wrenn, 1987; perrone and de franceschi, 1999). in particular, ap(τ) is a time-weighted accumulation of ap index, and macap(τ) is the related catalogue, introduced for characterising four levels of geomagnetic impact on the ionosphere ranging from 0 (quiet magnetic condition) to 3 (severely disturbed magnetic condition) (de franceschi et al., 1999, 2002). extracting from long data series of hourly fof2 data those values recorded under magnetically quiet conditions (macap(τ) = 0), alfonsi et al. (2002a,b) found a long-term decrease of the f2 plasma frequency. all fof2 trends derived from data of four different stations shown in fig. 5.6 are negative with a median value of − 0.0035 mhz/yr. a totally different method for derivation of trends has been developed by mikhailov and marin (2000, 2001) and marin et al. (2001). they use 12-monthly running hourly mean values of fof2 or hmf2 and of the solar sunspot number (r12) and geomagnetic activity (ap12). for fof2 only data during three years near solar minimum and maximum are taken into consideration to avoid the hysteresis effect whereas for hmf2 data of all years have been analysed. data gaps are filled by the mqmf2 model (mikhailov et al., 1996). the regression model used is a modification of eq. (5.1b) according to x a b r c r d r e apth 12 12 2 12 2 12$ $ $ $= + + + + ^ h. (5.5) the analyses have been made with or without the geomagnetic term. relative deviations of the observed data have been used according to eq. (5.2b). the main results can be summarized: 1) the influence of the geomagnetic activity can only partly be removed also if ap12 is introduced in eq. (5.5). 2) periods with increasing and decreasing geomagnetic activity are related to different ionospheric trends: fof2 trends are in anti-phase, hmf2 trends in-phase with ap trends. this behavior can be seen in fig. 5.7 for long-term observations at the station slough. 3) the diurnal and latitudinal variations of the fof2 and hmf2 trends can be explained by the current geomagnetic storm behavior. therefore, this method is called geomagnetic control concept by their authors. danilov (2002, 2003) developed a method to eliminate the long-term changes in geomagnetic activity in fof2 trends. starting with relative ∆fof2 values, calculated in a similar way as by mikhailov and marin (2000) but using data of all available years, he assumes that the derived fof2-trend k(obs) is the result of a linear combination of a non-geomagnetic trend k(tr) and a geomagnetically caused trend. he further assumes that the geomagnetic activity is described by annual mean values of ap and the geomagnetic trend is proportional to the gradient of ap, k(ap) ( ) ( ) ( ).k k a k aptr obs 1$= + (5.6) 1017 long-term trends in the ionosphere and upper atmosphere parameters here al is a scaling coefficient which includes the efficiency of the geomagnetic activity impact on fof2 and may therefore be different for different stations and local times. the details of the method to estimate the al values can be found in danilov (2002, 2003). using fof2 data of 23 stations with observations of more than 35 years a mean non-geomagnetic trend of − 0.012 mhz/yr was found for the period between 1958 and the mid-nineties and − 0.0075 mhz/yr for an earlier interval between 1948-1985 (here only data of 8 stations were available). another attempt to eliminate the geomagnetic influence on the fof2 trends has been tried by mikhailov et al. (2002). they start their analysis with monthly regressions according to eq. (5.1a), estimate relative deviations after eq. (5.2b) and calculate 11-year running mean values from these deviations: δfof2132. the geomagnetic activity effect in δfof2132 is derived from 11-year running mean values ap132 by ( ) ( )fof b b ap t n b ap t n2 th132 0 1 132 2 132 2 $ $= + + + +d (5.7) where n is the time shift in years of ap132 with respect to δfof2132 variations. from the residuals fof fof fof2 2 2obs th132 132= -d d d (5.8) linear trends according to eq. (5.3) have been calculated with the following main results: 1) the derived fof2 trends are nearly independent of latitude and of the phase of long-term changes in geomagnetic activity (basic point of geomagnetic control concept of mikhailov and marin, 2000, 2001). fig. 5.6. long-term trends of fof2 from different stations under magnetically quiet condition (macap(τ) = 0) and after removal of the solar cycle influence. 1018 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers 2) most of the residual trends are negative, often however not significant. in fig. 5.8 a significant negative trend is shown using the long-term data series observed at slough. summarizing it can be stated that in all analyses with data of different stations the derived mean trends in the f2-region are relatively small and not important for practical applications. at individual stations (ulich and turunen, 1997a; rogers et al., 2002), however, the trends can be markedly more pronounced, resulting in broad global histograms as shown in fig. 5.4. fig. 5.7. (top) long-term variation of geomagnetic activity (11-year running mean of ap) together with relative deviations of fof2 (middle) and of hmf2 (bottom) derived from ionosonde observations at slough. 1019 long-term trends in the ionosphere and upper atmosphere parameters fig. 5.8. relative deviations of fof2 after very long-term ionosonde observations at the station slough (full line: linear trend; kr: trend parameter). 5.3.2. trends in the f1-region trends in fof1 data series were derived by bremer (2001) using observations at 51 different ionosonde stations. for the elimination of the solar and geomagnetically induced parts the twofold regression eq. (5.4) was used. in fig. 5.9 the histogram of the updated individual trends is shown together with the global median value (marked by an arrow). the distribution is asymmetric to zero as slightly seen by essentially more positive (grey) than negative trends (white). the estimated mean trend with 0.0027 mhz/yr is significantly different from zero as seen in table 5.ii where the corresponding mean error can also be found. therefore this trend has been marked there by bold letters. 5.3.3. trends in the e-region trends in the ionospheric e-region were estimated by bremer (2001) from h´e and foe observations at different ionosonde stations. for the elimination of the solar and geomagnetically induced parts the twofold regression eq. (5.4) has been used. the results of the trend analyses are shown by histograms together with their global median values (marked by arrows) in fig. 5.10. the estimated mean foe trend is significant with a reliability level of more than 95%, whereas the mean h´e trend is not significant different from zero. this can be seen in table 5.ii where the mean values and their mean errors are presented. mikhailov and de la morena (2003) investigated foe trends from different ionosonde stations with their revised trend method which was developed for fof2-trend investigations (mikhailov et al., 2002). this method is briefly described above. instead of eq. (5.8), they used a slightly modified equation ( )foe b b ap t nth132 0 1 132$= + +d b (5.9) 1020 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers where β is a fitting parameter and n again the time shift in years of ap132 with respect to δfoe132. their main results are: 1) before about 1970 δfoe132 and ap132 are negatively correlated. the authors call it «natural foe variation». 2) after about 1970 positive foe trends were detected which could anthropogenically be caused. in fig. 5.11 an example is shown of the dependence between δfoe132 and ap132 using ionosonde data observed at the station rome. 5.3.4. trends in the d-region in the ionospheric d-region the available data series for trend analyses are limited, therefore we restricted these investigations to some reflection height and ionospheric absorption data observations. fig. 5.9. histogram for fof1 trends derived from ionosonde observations at different stations. number of stations is given in brackets. fig. 5.10. histogram for foe and h´e trends derived from ionosonde observations at different stations. numbers of stations are given in brackets. 1021 long-term trends in the ionosphere and upper atmosphere parameters from the interference field between the ionospherically reflected sky wave and the ground wave of a far distant radio transmitter it is possible to derive ionospheric reflection heights (often called phase heights, for details of the method see bremer and berger, 2002, and references therein). from long-term field strength observations of a measuring path at 162 khz in mid-latitudes trends of the reflection height have been derived for a constant solar zenith angle χ = 78.4°. in fig. 5.12 the yearly trend can be seen. here the solar and geomagnetically induced variations have been removed according to eq. (5.4) using the solar lyman α radiation instead of the solar sunspot number r. a significant negative trend could be derived with about − 0.028 km/yr. such a shrinking of the lower ionosphere could also be detected from observations of reflection heights at ionospheric drift measurements during night-time (kürschner and jacobi, 2002). these measurements were made in the lf range (177 khz) at short distance between transmitter and receiver (170 km). both mean trends of reflection heights near 82 km (lf phase heights) and near 95 km (drift measurements) as well as the trends in the e-layer peak height near 100-110 km (h´e-trends in section 5.3.3) have negative values which may slightly increase with increasing height. the significance level of the phase height trend is high (mostly higher than 99%), for the two other methods this level is however lower due to the shorter data series (drift method) or due to marked differences between individual stations (ionosonde data, see the broad histogram in fig. 5.10 and the derived mean trend and error values in table 5.ii). ionospheric absorption data have been analysed for three different reflection height ranges, below about 80 km, between about 80-90 km, and above about 90 km (bremer and laštovička, 2002; laštovička and bremer, 2004). from long-term observations of the absorption of a very long measuring path at 164 khz (distance between transmitter and receiver: 1720 km) during summer and noon conditions (nestorov et al., 1991, and updated later by laštovička and pancheva, 1999) a clear positive trend could be detected. the reflection height of this measuring path is clearly below 80 km. fig. 5.11. variation of δfoe132 in dependence on ap132 for foe data series observed at the station rome with the marked increase after 1972. 1022 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers using, however, data from lf absorption measurements at short distances (about 180-220 km) the derived trends change their general behavior. in fig. 5.13 the absorption trends are shown for three different frequencies at the same solar zenith angle χ = 90°. whereas the trend at the lowest frequency is negative, the trend increases with increasing frequency and becomes even positive at the highest frequency. that means that with increasing reflection height the trends become more positive. we observe the same behavior if trends have been estimated for the same measuring path but at different solar zenith angles (example can be found in bremer, 2003). with increasing solar zenith angle the trends becomes again more positive. investigating ionospheric absorption measurements in the mf and hf ranges at oblique incidence (distances between transmitter and receiver are about 170 km up to 550 km) only positive trends have fig. 5.12. long-term variation of yearly mean phase-heights at mid-latitudes (162 khz, χ = 78.4°) after elimination of the solar and geomagnetically induced variations. fig. 5.13. trends of ionospheric absorption for three measuring paths with different frequencies at a constant solar zenith angle χ = 90° after elimination of the solar and geomagnetically induced parts. 1023 long-term trends in the ionosphere and upper atmosphere parameters been estimated. some examples can be seen in laštovička (2001, 2002) and bremer and laštovička (2002). the reflection heights of these measuring paths are above 90 km. the tendency detected at the lf absorption data series that with increasing reflection height the trends become positive, is therefore confirmed by the absorption measurements in the mf and hf ranges. 5.4. discussion two main problems will be discussed here concerning a) the practical importance of ionospheric trends and b) the scientific reasons for possible trends in the different regions of the ionosphere. 5.4.1. practical aspects ionospheric hf propagation is mainly influenced by the behaviour of the f2-region expressed by their characteristic parameters fof2 and hmf2. as shown in section 5.2.1 and summarised in table 5.i, possible global trends of both parameters are very small. this finding is in agreement with other investigations presented in section 5.3.1 where the results of more complex trend analyses are shown using observations from all over the world. therefore, in global ionospheric prediction models it is not necessary to include the effect of ionospheric long-term changes. the solar and geomagnetically induced variations are markedly more important. at individual stations, however, the trends can be essentially more pronounced. this can be seen in the relatively broad histograms of the fof2-trends in fig. 5.1a,b and of hmf2-trends in fig. 5.2a,b. the trend values of the stations tromsø and sodankylä used in section 5.2.2 to investigate their impact on ionospheric radio propagation are well inside the corresponding histograms. unfortunately we do not know the reasons for the strong differences of the trends derived at individual stations. one reason for such differences between the trends at (partly neighbouring) stations could be artificial steps in the data series caused by technical changes at the ionosonde equipment and/or changes of the evaluation algorithms. in bremer (1998, 2001) some examples of such disturbances can be found. therefore, the quality of ionosonde data sets have to be carefully controlled before trend analyses are carried out (ulich et al., 2003). as long as we do not have final decisions about the quality of the ionosonde data sets we can only use mean or median values of all reasonable data sets, hoping that some errors of the derived individual trends may compensate each other. 5.4.2. scientific aspects what are the reasons of trends in the different layers of the ionosphere? at least two candidates are often discussed: anthropogenic pollution (e.g., co2, ch4, o3, h2o, ...) and geomagnetic trends. to investigate the influence of an increasing atmospheric greenhouse effect the trends in the ionospheric f2-, f1-, and e-regions can be compared with model calculations of rishbeth (1990) and of rishbeth and roble (1992). their theoretical results have been derived for a doubling of the atmospheric greenhouse gases co2 and ch4. the effective change in greenhouse gases during the last 40 years where trends of the ionosonde data have mainly been investigated is about 20% (brasseur and de rudder, 1987; houghton et al., 2001). assuming a linear dependence between the content of the atmospheric greenhouse gases and the ionospheric effect, the experimental trends of table 5.ii can be extrapolated to a level of doubled greenhouse gases. these values called co2*2 (exp) are compared with the corresponding model values co2*2 (mod) of rishbeth (1990) and rishbeth and roble (1992) in table 5.iii. 1024 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers table 5.iii. mean experimental (exp) trends of different ionospheric parameters (from table 5.ii) and expected changes of these data assuming a doubling of the atmospheric greenhouse gases (co2* 2). the model data (mod) are from rishbeth (1990) and rishbeth and roble (1992). parameter mean exp trend co2* 2 (exp) co2* 2 (mod) f2-region fof2 − 0.0018 mhz/yr − 0.36 mhz − 0.2 ... − 0.5 mhz hmf2 − 0.009 km/yr − 1.8 km − 10 ... − 20 km f1-region fof1 0.0027 mhz/yr 0.54 mhz 0.3 ... 0.5 mhz e-region foe 0.0014 mhz/yr 0.28 mhz 0.05 ... 0.08 mhz h′e − 0.040 km/yr − 8.0 km − 2.5 km 5.4.2.1. f2-region the agreement between the experimental and model trends in fof2 and hmf2 seems to be quite reasonable looking at the data in table 5.iii. however, the differences between the individual trends in fof2 and hmf2 at different stations are very strong, and the derived mean trends are not statistically significant different from zero as can be seen in table 5.ii. therefore, the agreement between model and experimental data is accidental. the reason for the strong variability in the f2-region is not quite clear. after fig. 5.5 there seem to be regional differences which could be caused by dynamical effects in the plasma of the f2-region. such strong regional differences of the hmf2 trends were also detected earlier by other authors using a more limited data volume (ulich and turunen, 1997b; bencze at al., 1998; bremer, 1998). from satellite observations (keating et al., 2000) it is known that the observed long-term neutral density reduction near 350 km altitude is in good agreement with model calculations of an increasing greenhouse effect (akmaev, 2002). that means that the possible greenhouse effect in ionospheric data series is superposed by unknown dynamical processes which are more important for the variability of the ionized component than for the neutral gas at f2-region heights. using differences of hmf2-h´f bencze (2002) derived long-term trends which contain information about the plasma temperature of the lower part of the f2-region. for most of the used 20 european stations a negative trend was detected in qualitative agreement with an expected cooling of the atmosphere due to an increasing atmospheric greenhouse effect. with the method developed by mikhailov and marin (2000, 2001) the estimated trends in fof2 and hmf2 are explained by the remaining influence of geomagnetic activity (geomagnetic control concept). after their investigations their derived trends depending on local time and latitude can be explained by long-term variations of geomagnetic activity. with a revised method mikhailov et al. (2002) tried to remove this geomagnetic influence. the remaining relative trend in fof2 noon values for the station slough with − 2.2 10−4 year−1 can be converted to a mean trend of − 0.0022 mhz/yr if a mean fof2 value of 10 mhz is assumed. this trend value agrees well with the mean global trend for fof2 in table 5.ii and is thus in reasonable agreement with the model value in table 5.iii. therefore, this fof2 trend can be explained by an increasing greenhouse effect. however, an influence of the very long-lasting increase of the geomagnetic activity, as found in long-term variations of the geomagnetic aa index during the last century (clilverd et al., 1998; gulyaeva, 2002), on trends in fof2 should also lead to a negative fof2-trend due to the negative correlation between trends in geomagnetic activity and fof2 after the geomagnetic control concept. alfonsi et al. (2002a,b) tried to remove the influence of geomagnetic activity on fof2-trends by using only geomagnetically quiet hourly data in their analyses. their mean fof2-trend derived from 4 different stations (see fig. 5.6) with − 0.0035 mhz/yr is slightly stronger but not very different from the expectations after the greenhouse effect. 1025 long-term trends in the ionosphere and upper atmosphere parameters danilov (2002, 2003) used quite another method to derive non-geomagnetic trends in fof2. his mean trend, derived from 23 different stations, is with − 0.012 mhz/yr also negative but markedly stronger than the trends in table 5.ii as well as those from mikhailov et al. (2002) and alfonsi et al. (2002a,b). but such differences are not very surprising if we remember the strong variability between the trends detected at different stations and the fact that these authors used different stations in their analyses. 5.4.2.2. f1-region the agreement of the mean experimental trend in the f1-region (described in section 5.3.2) with model trends in table 5.iii is surprisingly good. therefore, the global fof1-trend can be explained by an increasing atmospheric greenhouse effect. 5.4.2.3. e-region as seen from table 5.iii, in the e-region the experimental and theoretical trend values agree qualitatively with a lowering of the height h´e and an increase in foe. however the experimental trends are markedly stronger than the model values. the derived positive foe trend is also in general agreement with rocket mass spectrometer measurements of the ion density ratio [no+]/[o2+] in the e-region (danilov and smirnova, 1997). the observed negative trends of [no+]/[o2+] cause increasing electron densities and therefore increasing foe values as the dissociative recombination coefficient of no+ is markedly larger than that of o2+. for years after 1970 mikhailov and de la morena (2003) also found with their revised trend method increasing foe values, a quantitative comparison with the model trends in table 5.iii is however impossible. nevertheless they propose an increasing chemical pollution due to an increasing number of rocket launchings and/or an increasing atmospheric greenhouse effect to explain the observed foe increase. 5.4.2.4. d-region the shrinking of the lower ionosphere as derived from lf reflection height observations (fig. 5.12) can be explained by a cooling of the stratoand mesosphere as expected from an increasing atmospheric greenhouse effect (bremer and berger, 2002). the estimated amount of the atmospheric cooling depends however also on possible changes in the density of nitric oxide nno and of the effective recombination coefficient αeff near the reflection height of about 82 km. unfortunately there are no experimental data of long-term changes in these two quantities, however there exist model results which predict a decrease of nno (roble and dickinson, 1989; beig, 2000) and an increase in αeff in the mesosphere (chakrabarty, 1997; beig, 2000). as shown by bremer and laštovička (2002) and bremer (2003) a common interpretation of the trends of the lf reflection heights and of the absorption data at different frequencies is only possible if the trends in nno and αeff as predicted from model calculations are taken into consideration. the mesospheric temperature trends derived from the trends in lf reflection heights and ionospheric absorption data are in reasonable agreement with other experimental trends derived from longterm lidar and rocket measurements (bremer, 2003). as shown by model results (bremer and berger, 2002; bremer, 2003) the experimental temperature trends can qualitatively be explained by an increasing atmospheric greenhouse effect. the experimental trends are however stronger than the model results. 1026 jürgen bremer, lucilla alfonsi, pal bencze, jan laštovička, andrei v. mikhailov and neil rogers 5.5. summary and conclusions using data of reflection height observations in the lf range and ionospheric absorption measurements in the lf, mf, and hf ranges for trend analyses in the d-region as well as data of ionosonde observations for trend estimations in the e-, f1-, and f2-regions, the following short summary and conclusions can be given: general aspects – long-term ionospheric data series have to be carefully checked concerning their homogeneity. discontinuities caused by different technical changes can markedly disturb the results of trend analyses. – the solar and geomagnetically induced variations of most ionospheric parameters are essentially stronger than the long-term trends and have carefully to be eliminated in trend analyses. f2-region – the mean global trends are unimportant for practical ionospheric prediction models. in such models the solar and geomagnetic variability are the most important external factors. – due to a wide variability of the individual trends in the f2-region no significant global trends could be derived for fof2 and hmf2. therefore, the relatively reasonable agreement between the mean global experimental and model results could be accidental. the regional differences of the trends hint at an unknown dynamical process which superposes a possible greenhouse effect in the f2-region. – using different hmf2-h´f however a general cooling of the lower part of the f2-region could be derived in qualitative agreement with the greenhouse effect. – the derived trends after the geomagnetic control concept contain geomagnetically induced components causing typical local time and latitudinal trend variations. – different approaches to eliminate geomagnetic activity changes lead to negative fof2-trends as proposed by model results of an atmospheric greenhouse effect. the experimental trends are however slightly stronger than the model predictions. if the geomagnetic influence could not be totally removed, then these negative trends could also be explained by the very long-term increase in geomagnetic activity. f1-region – the mean global trend in the f1-region (increase in fof1) agrees quite well with model results of an increasing greenhouse effect. e-region – the mean global trends in the e-region (lowering of h´e, increase of foe) are in qualitative agreement with other experimental results and with model results of an increasing greenhouse effect. however, the experimental trends are stronger than the model results. – investigations with a revised trend method lead to an foe-increase after about 1970. d-region – the trends in the d-region (lowering of reflection heights and different trends in ionospheric absorption in dependence on frequency of the measuring path) can qualitatively be explained by an increasing atmospheric greenhouse effect. the experimental trends are however stronger than expected from model results. summarizing, the investigation of ionospheric trends is an important task for the understanding of structure and dynamics of the ionosphere in general and for the derivation of anthropogenic influ1027 long-term trends in the ionosphere and upper atmosphere parameters ences in particular. combined experimental trend analyses (including quality control of existing data series as well as introduction of additional data sets) and model activities are necessary for future trend investigations. acknowledgements the investigations presented here were mainly carried out in the cost 271 project «effects of the upper atmosphere on terrestrial and earth-space communications (eacos)». the authors are very grateful to all participants who took actively part in the analysis of trends in the earth’s atmosphere and ionosphere: p.s. cannon, u.k.; 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(1955): world wide daily variations in the height of the maximum electron density in the ionospheric f2-layer, j. radio res. labs., japan, 2, 85-97. taubenheim, j. (1969): statistische auswertung geophysikalischer und meteorologischer daten, akad. verlagsgesellschaft geest und portig k.-g., leipzig. ulich, th. (2000): solar variability and long-term trends in the atmosphere, sodankylä geophysical observatory publications no. 87, oulu. ulich, th. and e. turunen (1997a): evidence for long-term cooling of the upper atmosphere in ionospheric data, geophys. res. lett., 24, 1103-1106. ulich, th. and e. turunen (1997b): long-term behaviour of ionospheric f2-layer peak height on a global scale, paper presented at session 2.18 of the 8th scientific assembly of iaga, uppsala. ulich, th., m.a. clilverd and h. rishbeth (2003): determining the long term change in the ionosphere, eos trans. am. geophys. un., 84 (52), 581-585. wrenn, g.l. (1987): time-weighted accumulations ap(τ) and kp(τ), j. geophys. res., 92, 125-129. o s s e r v a z i o n i su u n a f o r m u l a d e l c a m p o g r a v i t a z i o n a l e d e l s o m i g l i a n a g i o v a n n i boaga ( * ) c. somigliarla dapprima in due note separate e poi nella memoria fondamentale sulla « teoria generale del campo gravitazionale ellissoidieo » (2) ha reso nota la formula: g1 (cos2 <p2 —• cos2 9?3) j/ 1 + i2 cos2 9?! + + g2 (cos2 cp3 — cos2 9^) )/ 1 + i2 cos2 cp„ + [1] + g3 (cos2 cpl — cos2 cp2) j/ 1 + i2 cos2 cp3 = 0 con glf g2, g3 gravità alle latitudini cpx, (p2, <p3 ed i2 quadrato della eccentricità aggiunta (**). su questa formula il chiaro autore osserva che, « se si considera incognita la eccentricità aggiunta i e note le tre gravità (h, g2, g3 e le corrispondenti tre latitudini <px, <p2, <p3 si otterrà una equazione in i che risoluta rispetto i ci permetterà di calcolare lo schiacciamento a del geoide ellissoidico ( * * * ) e quindi la sua forma, per mezzo di tre gravità e senza la preventiva conoscenza della grandezza degli assi dell'ellissoire terrestre ». l'equazione cui fa cenno il somigliana, senza però scrivere in forma esplicita i rispettivi coefficienti, è la seguente: l-i* + m-i2 + n = 0 [2] ottenuta razionalizzando la [1]. (*) direttore dell'istituto di geodesia e topografia della facoltà di ingegneria dell'università di roma. ( * * ) ossia i2 = (a2 — b2): b2 con a, b semidiametri equatoriale e polare dell'ellisoide rotazionale terrestre. ( * * * ) 1 due parametri a ed t2 risultano tra loro legati dalla relazione: (1 + i2) (1 — a)2 = 1. 1 5 6 g. b o a g a fatte le posizioni i a = gt (cos2 (pi — cos2 953) b = g2 (cos2 cp3 — cos2 ipj) [3] ' c— 9* (cos2 9?! — cos2 <p2) i coefficienti l, m e n della [2] risaltano cosi definiti: / l = (a'+b' + c) (—a'+b' + c) (a'—b' + c) (a'+b'—w) ] m = a'2 (—a2+b2 + g2) + b'2 (a2—b2+c2) + c'2 (a2+b2—c2) [5] ' n = (a+b + c) (—a+b+g) (a—b + g) (a+b—c) . a = a • cos cp,, b' = b-cos 9?2 [4] c' = g-cos 9?3 facciamo presente che la [2] fornisce il valore della eccentricità aggiunta i2 proveniente dalle misurazioni geodetiche (triangolazioni) nel solo caso in cui al posto delle tre gravità che figurano nelle posizioni [3], e quindi nei coefficienti l, m, n, siano sostituite gravità ellissoidiche normali ossia gravità teoriche e non gravità osservate, in quanto queste ultime risultano generalmente affette dal disturbo del campo gravitazionale normale, provocato dalla diversa distribuzione delle masse interne terrestri, disturbo che altera talvolta in maniera sensibile le gravità osservate, ridotte come d'uso al livello medio del mare per l'effetto dell'altezza del punto di osservazione sul livello marino e per la topografia circostante il punto di osservazione. queste alterazioni delle gravità prendono il nome di anomalie gravimetriche-, e sono appunto queste anomalie che influiscono sensibilmente sul calcolo di i2; esse d'altronde non si possono separare dalle gravità osservate, se non confrontandole con le gravità normali, provenienti dalla [1] e quindi ammettendo a priori il valore normale del quadrato della eccentricità aggiunta che si vuole determinare. scopo del presente studio è appunto quello di mettere in rilievo l'influenza delle anomalie della gravità sulla determinazione dello schiacciamento terrestre. ciò premesso, se nella [1] — per maggior semplicità di successivi conteggi — si assumono per g2 e g3 i valori (noti) delle gravità equatoriale (gc = 978,049 gal) e polare (gp = 983,221 gal) per cui cos e/)., = 1 e cos <p:ì = 0, rimane una sola gravità arbitraria, la <h, che ora indicheremo semplicemente con g, e intenderemo con la 9? la corrispondente latitudine. o s s e r v a z . su u n a f o r m u l a d e l c a m p o g r a v i t a z i o n a l e d e l s o m i g l i a n a 157 con questa limitazione la formula [1] diviene: g \ 1 + i2 cos2 cp — ge cos2 cp \ 1 + i2 — gp sen2 cp = 0 [6] e le posizioni [3] e [4] assumono le nuove forme: a = g b = — ge cos2 cp [7] c = — gp sen2 cp a' = g cos cp b' — — gc cos2 cp [8] c" = 0 e successivamente ì l = — cos4 cp (g2 — ge2 cos2 cp)2 m = — cos295 \(g2—ge2 cos2<p) (g2—gi2cos4cp)—gp2se]ii<p{g2+g2co&2cp)\ [9] n = 4 ge2 gp2 cos4 cp sen4 cp — (g2 — ge2 cos4 cp — gp sen4 cp)2 assumendo per gravità g quella alla latitudine cp = 45° con che: sen29? = cos2 cp = sen4 cp = cos4 cp = sen8 cp — cos8 cp = 1/16, le [9] assumono le forme: allorché varia il valore della gravità g per effetto di una anomalia ag, variano i coefficienti [10], generando in base alla [2] una variazione zìi2 di i2 legata alle variazioni al, am, an dei coefficienti, dalla: l = : y4 u/ y2 g/r m = y2 \(g2 y2 g2) (g2 % g2) % gp2 (g2 + % [10] n = y4 g/ gp2 (g2 % gè1 ~ 2/4 gp2)2 dove g = <7j5. = 980,629 gal. ai2 = i*-al + i2-am + an ~ 2 i* l + m [11] con al am an [12] 1 5 8 g. b o a g a introducendo per ge, gp i valori dianzi richiamati si giunge con la [11] alla: ai2 = 0,000949 • zl£r<msai) [13] dove ag[msax) indica che la anomalia va espressa in milligal. indicando con m l'inverso dello schiacciamento a (m = 297), la variazione ai1 è legata alla variazione am di m dalla relazione 2m . am r ,„ 2 = { m ^ l j 3 ' = 4366ct [ 1 4 ] eppertanto eliminando dalle [13] e [14] il ai2 si perviene alla: am = 41,3 • [ l o ] atta ad esprimere la variazione dello schiacciamento della terra in funzione della anomalia gravimetrica ag. eppertanto se ag = + 1 milligal la [1] e conseguentemente la [2] fornisce per i2 un valore tale che rispetto a quello attualmente accettato dalla unione geodetica e geofisica internazionale, produce una variazione di 41 unità sull'inverso dello schiacciamento. e poiché non si possono utilizzare gravità osservate prive di anomalie, se non si vogliono effettuare dei semplici controlli numerisi (*), cosi (*) a proposito di controlli numerici osserviamo clie introducendo nella [6] al posto di ge il valore consaputo 978,049 gal ed al posto di qp e 945» rispettivamente i valori 983,221 3143 gal, 980,629 3867 gal e per i2 il valore proveniente dalle misurazioni geodetiche, che fornisce lo schiacciamento a = ' , ossia i2 = 0,006768170197186374 la [6] stessa non risulta identi297 camente soddisfatta, ma presenta il residuo: + 0,00001545 gal che viene annullato lasciando inalterati i valori gravimetrici sopra scritti, se si altera il valore di i2 della quantità 0,00007466205 il che produce una variazione nel denominatore dello schiacciamento dell'importo 0,6 cioè minore di una unità. si vede pertanto che i valori gravimetrici da introdurre nella [1] debbono essere considerati con sette cifre decimali del gal, ed i2 con ben diciotto cifre decimali data la sensibilità della formula considerata, se si vuole condurre il calcolo con cifre esatte fino al centesimo di milligal ossia fino al milionesimo di gal, ordine di grandezza talvolta raggiungibile oggi con i gravimetri per le determinazioni delle gravità relative. il somigliana che ha eseguito tali calcoli con gravità limitate a sole tre cifre decimali del gal ha ottenuto, data la grande sensibilità della formula per la i2 una variazione che ha portato lo schiacciamento da 1/297 a 1/256! o s s e r v a z . su u n a f o r m u l a d e l campo g r a v i t a z i o n a l e d e l s o m i g l i a n a 1 5 9 dobbiamo concludere, dal punto di vista geodetico, e senza diminuire la grande importanza teorica della formula [1] di c. somigliana, che essa non si presta alla determinazione dello schiacciamento, oppure del rapporto dei semi assi equatoriale (a) e polare (b) dell'ellissoide terrestre (*), in funzione delle gravità osservate, causa appunto la presenza in queste delle anomalie gravimetriche, che influiscono, in maniera assai rilevante, sulla determinazione del quadrato della eccentricità aggiunta. riassunto l'autore mette in evidenza l'alto grado di sensibilità numerica della formula del somigliana, che lega i valori di tre gravità, a tre latitudini diverse, al valore della eccentricità terrestre. abstract the autlior puts in evidence the high degree of numerical sensibility of somigliana's formula that unites the vàlues of three gravities, at three different latitudes, to the value of earth eccentricity. b i b l i o g r a f i a ( 1 ) s o m i g l i a n a , c . , sulla determinazione delle costanti del geoide mediante misure di gravità. « atti dell'aco. delle scienze » di torino, voi xvii, rendiconti del « seminario matem. e fisico » di milano, anno i, voi. 1. si veda pure in detto seminario, ecc. la nota di e. b i a n c h i dal titolo: osservazioni sulla comunicazione del prof. somigliana, ecc. stesso volume. (2) « memorie della società astronomica italiana », iv. (*) si osservi che tale rapporto è legato al quadrato della eccentricità aggiunta dalla relazione a : b = ]/ 1 i2. articolo langride.pdf 1. introduction the hope fault is a 230 km long system of dextral-slip faults at the southwestern edge of the marlborough fault zone (mfz), that can be traced from the alpine fault to the northeast coast of south island near kaikoura (freund, 1119 annals of geophysics, vol. 46, n. 5, october 2003 key words hope fault – kaikoura – conway segment – new zealand – paleoseismicity – neotectonics – slip rate – recurrence – nzms 260 sheet 031 mailing address: dr. robert langridge, institute of geological and nuclear sciences, p.o. box 30-368, lower hutt, new zealand; e-mail: r.langridge@gns.cri.nz paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand robert langridge (1), jocelyn campbell (2), nigel hill (1), verne pere (2), james pope (2), jarg pettinga (2), beatriz estrada (2) and kelvin berryman (1) (1) institute of geological and nuclear sciences, lower hutt, new zealand (2) department of geological sciences, university of canterbury, christchurch, new zealand abstract the conway segment of the dextral-slip hope fault is one of the fastest slipping fault segments along new zealand’s plate boundary, but has not ruptured co-seismically in the historic period and little paleoseismic data exist to constrain its large earthquake record. two paleoseismic trenches were opened adjacent to greenburn stream near kaikoura for the 2001 ilp paleoseismology conference. both trenches were excavated into deposits ponded against an uphill-facing shutter scarp. trench 1, dug through a cobbly soil and surface deposit was dominated by a thick fan/fluvial sequence that was radiocarbon dated at 4409 ± 60 c14 years bp (4844-5288 cal years bp) at the base of the trench. this trench exhibited evidence of complex deformation from many paleoseismic events. the most recent earthquakes are difficult to constrain due to a lack of cover stratigraphy on the fan deposits. however, the modern soil appears to be faulted and is covered by cobbles with a weathering rind-derived age of 220 ± 60 years. trench 2, dug 50 m to the west has an expanded sequence of the younger cover deposits. paleoseismic event horizons have been recognised from the combined evidence of upwardterminating faults, offset and mismatched units, a sandblow deposit, and abrupt landscape change shown by the burial of paleosol surfaces that form the event horizons. two paleosols underlying the modern soil are clearly faulted by two separate rupture events. a dome of sand interpreted as a liquefaction sandblow deposit overlies the lower paleosol (event horizon). both paleosols are overlain by metre-thick debris deposits, interpreted as earthquake-induced rock avalanches that cascaded off the hillslope following mw 7 + events. four radiocarbon dates place some constraints on the timing of the three recent surface-rupturing events. the youngest and lowest date is 548 ± 60 c14 years bp (504-656 cal years bp) and occurs below the lower paleosol. it constrains the maximum duration of time in which the last 2 earthquake events occurred to be 545 years (1295-1840 a.d.). this is consistent with the average recurrence interval (ri) of 180-310 years that we determine using two independent paths. the soil record indicates that each event is separated by a significant period of time, comparable to the calculated ri. the most recent event is constrained between ca. 1780 a.d. ± 60 years, taking into account the dates from these trenches, a weathering rind age, and from stratigraphic correlation at the site. event iii probably occurred before 1220 a.d. a maximum dextral slip rate of 23 ± 4 mm/yr is calculated from the minimum fan age and the offset/deflection of a stream channel along the shutter ridge. in concert with the estimate of single event displacement (5-6 m), these results show that the conway segment of the hope fault is fast-slipping and has ruptured regularly as a result of large earthquakes prior to the european colonisation of new zealand. 1971). deformation across the mfz accommodates the oblique convergence of the australianpacific plate margin in northern south island (fig. 1), marking the transition from the westfacing hikurangi subduction margin to the eastfacing alpine fault and puysegur subduction margin (berryman et al., 1992; holt and haines, 1995). the mfz comprises four main strike-slip faults with subsidiary oblique-slip faults (e.g. kekerengu fault) occurring at its northeastern 1120 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman 40 s o 35 s o 170 e o 175 e o 180 e o australian plate pacific plate h ik ur an gi tr ou gh wairau fault awa tere faul twaihop ai f. kakapo f. north island mf z 47 34 39 37 p u ys eg u r tr en ch tasman sea pacific ocean nelson blenheim kaikoura hokitika south island n christchurch clar ence f. 1848 alpin e faul t kelly f. 1929 poulter f. 171 7 ? 1888 0 50km figure 2 al pi ne fa ul t south island hope fau lt kekerengu f. 1 2 3 4 5 hf 42 s o 172 e o fig. 1. location of the hope fault in the marlborough fault zone (mfz) (bold lines), northern south island. the hope fault system is shown boldest and fault segments are numbered in italics (1 kelly fault; 2 hurunui section; 3 hope river segment; 4 conway segment, and 5 seaward segment), their boundaries being marked by open circles. surface fault ruptures, e.g., 1888 north canterbury earthquake, are shown as grey bands. inset: new zealand plate boundary including alpine fault, mfz and hikurangi trough. nuvel-1 directions (arrows) and plate rates relative to australian plate (in mm/yr) after demets et al. (1994). end (van dissen and yeats, 1991). those four faults account for a large proportion of the plate boundary motion at their location, where almost complete locking occurs between the subducting (pacific) plate and overriding (australian) plate. from northwest to southeast, these faults are the wairau, awatere, clarence, and hope faults (lensen, 1968; campbell, 1973; kieckhefer, 1979; yeats and berryman, 1987; van dissen, 1989; mccalpin, 1996). these faults generally have higher slip rates as one progresses southeast across the mfz. the eastern hope fault (conway segment) has the highest holocene slip rate (bull, 1991; mcmorran, 1991; knuepfer, 1992; pope, 1994) and geodetic rate (bibby, 1976; bourne et al., 1998) for faults within the mfz. however, the conway segment has only a modest level of seismicity and no record of surface-rupturing earthquakes within new zealand’s short historic period (anderson et al., 1993; reyners and cowan, 1993; anderson and webb, 1994). indeed, within the mfz since 1840 a.d. only the eastern awatere fault (1848) and central hope fault (hope river segment 1888) have ruptured co-seismically (mckay, 1890; cowan, 1991; grapes et al., 1998). in addition, the prehistoric (paleoearthquake) record of the conway segment of the hope fault is not well understood at this time. the 2001 ilp conference on paleoseismology offered us the unique opportunity to study and present results from our second fastest slipping onland fault in an international forum. data from two new trenches excavated, logged and dated in conjunction with the conference program allow us to estimate: i) the age of the three most recent earthquake events and their bearing on previous studies, and ii) a new dextral slip rate for the conway segment of the hope fault from the greenburn stream site. 2. the hope fault and conway segment the hope fault has been traced from its branching junction with the alpine fault south of hokitika (fig. 1), northeast to the coast north of kaikoura (freund, 1971; van dissen, 1989) and into the offshore shelf region (barnes and audru, 1999). important early onshore studies relate to the reconnaissance of the 1888 north canterbury earthquake, recognition of strikeslip displacement (mckay, 1890), and subsequent mapping of the fault system in its entirety (freund, 1971; hardy and wellman, 1984). the hope fault has been the subject of numerous recent neotectonic and structural studies, many emanating from the university of canterbury (e.g., cowan, 1989, 1990; van dissen, 1989; bull, 1991; mcmorran, 1991; cowan and mcglone, 1991; pope, 1994; wood et al., 1994; simpson, 1995; eusden et al., 2000). based on tectonic geomorphology and the record of the 1888 event, the hope fault can be divided into a number of geometrically-defined segments or sections (fig. 1), namely the kelly fault, and the hurunui, hope river, conway, and seaward segments (see van dissen, 1989; cowan and mcglone, 1991; simpson, 1995; pettinga et al., 1998, 2001; langridge, 2003). this paper focuses on the conway segment of the hope fault, a 70 km section from the kowhai river to the hanmer basin (fig. 2), emphasising results from a trench site adjacent to greenburn stream. east of the kowhai river, the conway segment splays into a number of dextral and transpressive features, including the fyffe fault, kowhai fault and jordan thrust (van dissen and yeats, 1991) (fig. 3). at its western end the conway segment begins from the hanmer basin, an extensional step-over or pull-apart in the hope fault system, which is also the northeastern termination of the 1888 surface faulting event (mckay, 1890; freund, 1971; cowan, 1991; wood et al., 1994). the conway segment is characterised by a relatively continuous linear trace striking about n73°e and has a slip rate of 18-35 mm/yr based on offset alluvial features (freund, 1971; van dissen, 1989; bull, 1991; mcmorran, 1991; knuepfer, 1992; pope, 1994). the conway segment bounds the southern rangefront of the seaward kaikoura, hawk and amuri ranges (figs. 2, 3). oblique compression across the hope fault system is responsible for > 2 km of topography observed in these ranges (fig. 3). where exposed, the fault dips steeply to the nw at 70° (van dissen, 1989; mcmorran, 1121 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand 1122 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman awatere fault hope fault culverden basin clarence fault pacific ocean con way seg men t hop e fau lt cla ren ce fa ult fig ure 3 hanmer basin 0 20 km10 kaikoura n h k fig. 2. digital terrain model of the southeast mfz, illuminated from the southeast. major active faults are shown by arrows and delineated by the rangefronts and valleys. in the west, the hope fault system consists of ene-trending strike-slip sections separated by the hanmer basin extensional stepover, and passes east into oblique-contractional faults near the coast. the conway segment is shown between the hanmer basin and kowhai river (k), marked by two white circles. the greenburn stream site is shown by a white arrow; hossack saddle (h). fig. 3. structure of the eastern hope fault and location of the greenburn stream trench site (modified after campbell, 2001). the conway segment passes east into the seaward segment east of kowhai river, and the mt. fyffe and kowhai faults. the trench site is adjacent to greenburn stream. 0 5 km mt. manakau 2610 m2151 m jo rd an th ru st halfmoon bay pacific ocean u u u sy nc lin e puh ipuh i fau lt seawar d kaiko ura ran ge 60 mt. fyffe 1602 m 70 8485 1873 m hope fault (seaward segment) hawk range kowhai u greenburn stream trench site u u legend 85 1568 m 947 m 1444 m h apuku r iver 48 28 kaikoura k ow hai r iver k ahutara r iver fyf fe f aul t n active thrust teeth toward upthrown side active strike-slip fault u, upthrown side and dip syncline conway downs hope fault cha rwe ll r iver c onw ay river clarence reserve amuri range sawyers creek (conway segment) 1991). although no large earthquakes have ruptured the conway segment historically (last 160 years), lichenometric studies suggest that the conway segment was recently ruptured twice by large seismic events (mw 7 +) in 1768 a.d. and in the 1830’s, the decade preceding the advent of initial european colonisation (bull, 1998; bull and brandon, 1998). paleoseismic studies have been conducted along this segment of the hope fault by mcmorran (1991) and by pope (1994) near greenburn stream. mcmorran was able to demonstrate a holocene slip rate of 18 ± 8 mm/yr but could not determine the age of any paleo-earthquake events. pope (1994) trenched a small sag pond at clarence reserve (fig. 3) and was able to recognise and date three paleoearthquake events. none of these events are believed young enough to represent the most recent surface-rupturing event. therefore, considerable scope still remained to undertake paleoseismic and slip rate studies along the conway segment. 3. the greenburn stream site near greenburn stream the hope fault has a clear geomorphic trace in aerial photographs and on the ground (fig. 4). the fault scarp at greenburn stream forms a linear trace, characterised mainly by an uphill-facing scarp located near the base of the southern, faceted rangefront of the hawk range. several deeplyincised southward draining streams cross the fault trace (fig. 3). two paleoseismic trenches were opened at a site ~ 700 m upstream from the highway 70 bridge over greenburn stream adjacent to a tributary stream, here informally referred to as «urquhart stream» (nzms 260 o31/423678), shown in fig. 5. the trenches were excavated across a ridge or «shutter scarp», that has been dextrally displaced by the fault. the scarp is cored by sheared basement greywacke sandstones (torlesse group) that formerly created a barrier to stream flow. a fan aggraded against this ridge and it is clear that an 1123 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand shutter scarp fault trace debris fan lobe trench 1 «urquhart» stream greenburn valley ponded fan fig. 4. the greenburn stream site with vehicle for scale. view to the east shows trench 1 excavated through the shutter scarp and into the ponded alluvial fan surface. the site is elevated above modern «urquhart stream» and greenburn valley. drainage at the site was from left to right. recent incision by urquhart stream has abandoned this surface. older channel was dextrally deflected around the western end of the ridge. this fan and channel were later abandoned when urquhart stream incised through the bedrock shutter ridge. scattered, angular blocks of greywacke that remain on the top surface of the abandoned fan yield a weathering rind age of 220 years (pope, 1994). trench 1 was dug through the scarp and fan surface, while trench 2 was excavated 50 m further to the west (fig. 5). immediately north of the fault at trench 2 is a small debris fan and scar that had potential as a datable, event-capping and shaking-induced deposit. in detail, the shutter scarp has a sinuous, almost en échelon stepping scarp morphology. the style of the scarp (tectonic geomorphology) is influenced by the transpressive nature of the conway segment (campbell, 2001). the fault dips 60-70° to the north and the ranges have an up-to-the-north sense of topography, as does the uphill-facing shutter scarp. the torlesse bedrock coring the shutter ridge is highly sheared with overthrust relations in exposures along urquhart stream, and a 15 cm wide gouge (pug) zone, corresponding to the hope fault, is also exposed in its channel. a series of dextral displacement estimates were measured from geomorphic features by pope (1994) between greenburn and sawyers creek. these measurements range from 3.4 to 31 m dextral displacement and reflect the likelihood of multiple events accumulating offset on older features. the average of nine single event displacement (sed) measures in this area is 5-6 m (pope, 1994). three displacement measures were made adjacent to the trenches within the modern, incised banks of urquhart stream. these values are 3.4, 4.9 ± 0.2, and 10.4 ± 0.2 m. pope concluded that the higher value was derived from two displacement events, while the lower value was considered to be a minimum due to erosional trimming of the stream bank at the fault. 4. trench 1 trench 1 (t1) was located 10 m from the west bank of urquhart stream, excavated 1124 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman n 0 50 m d~10.4 m d~4.9 m p p* shutter scarp fault trace abandoned outlet debris fan trench 2 trench 1 d~3.4 m «urquhart» stream alluvial fan hillslope in bedrock hillslope in bedrock fig. 5. simplified geomorphic sketch of the trench site (modified after pope, 1994). modern incised urquhart stream crosses the fault but was formerly deflected dextrally and drained through the abandoned outlet. the fault deflection p to p* is marked by stars. inferred measured single event dextral (d) displacement values (in metres) are shown. trench 2 clips deposits of the landslide lobe at its northern end as shown on log. to a length of 17 m and benched (fig. 4). the scarp height there, relative to the fan surface, is 1.6 m and the total exposure from the highest point on the shutter scarp to lowest point in the trench is 5 m. the site was chosen with several hypotheses about the geomorphology and subsurface materials in mind, including: i) the presence of a thick fan aggradation sequence deposited against the shutter ridge; ii) post-fan abandonment deposits against the scarp, including the surficial cobble deposit, that may contain charcoal and evidence for the most recent earthquakes, and iii) a cross-sectional view through the scarp at the location of an en échelon step, that may give insights into the structure of such features. only the east wall of the trench was logged at the time of the ilp conference and is presented in detail for this paper. however, the west wall of t1 was mapped in january 2002. the west wall comprised similar units to the east wall, though the thicknesses were expanded, and therefore the sequence did not extend as far back in time. 4.1. stratigraphy the stratigraphy in trench 1 was consistent with the three surface hypotheses above, made prior to excavation. the stratigraphy is summarised below and in fig. 6. a more detailed description of the stratigraphy can be supplied by the co-authors. the six major packages recognised in t1 are the: scarp; lower, middle and upper south fan; north fan, and channel packages. these packages are overlain at the ground surface by the modern topsoil. the packages have stratigraphic, unconformity, and fault relations separating them. the scarp package is composed mainly of intensely sheared torlesse greywacke sandstone. in places a dark red to grey clay (fault gouge) is interlayered with the sheared bedrock. in addition, two interbedded scarp units composed of weathered and fragmental greywacke-derived clasts are considered sedimentary in origin. in all probability, the scarp package has a complex deformation history with interleaving and overthrust 1125 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand scarp package debris avalanche package channel fill package c14: 710 ± 55 (17d) cfb scarp package modern soil channel package upper south fan package middle south fan package lower south fan / north fan packages trench 1 east trench 2 mostly sheared and brecciated torlesse group sandstone. trench 1 has clay gouge, and some sheared sedimentary units. trench 2 has weathered colluvium (sc) and soil parent material. fn4 /fs3 c14: 4409 ± 60 (fs1) weathering rind age: 220 ± 60 yr s1 s5 fn1 fn3 &5 /fs2,4, &5 fu c3 c1 c2 fault contact two, thick, very poorly sorted angular breccias (da1, da2) separated by a paleosol, p2. underlying da1 is a buried sand volcano (v). da1 and da2 have well-sorted surge gravels at their bases. interbedded fine-grained sand to clay units and channel gravel facies. package capped by paleosol, p1. s sc msms mre ii iii ii iii mre ? mfs1 ufs1 ufs2 ufs3 modern soil debris fan package p1 p2 dark brown-grey, poorly-sorted, bedded gravels and fine interbeds of silt to clay (fn2,4; fs1,3). graben-filling gravel (fu) is faulted by many events, and caps the package brown to brown-grey medium gravel in silty matrix and angular, bladed gravels. matrix-supported channel gravels c14: 645 ± 60 (2d-8) c14: 548 ± 60 (3c-10) c14: 564 ± 40 (2b-10) well-bedded and sorted fine gravels and sand. ponded silt at base of gravels. da1 da2 2 1 3 4 0 ap p ro xi m at e sc al e (m ) df fn2/fs1 & 3 coarse gravel in sandy matrix yellow-brown fine sandy-silt cfa v { { ? fig. 6. generalised stratigraphy and age of deposits from the greenburn trenches. the youngest cover deposits at trench 1 (channel package and modern soil) increase in thickness across the trench to the west and may be laterally contiguous with a thicker, expanded section in trench 2 (dashed lines). radiocarbon and surface ages are attached to the column. horizontal arrows point to earthquake event horizons. 1126 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman n o rt h s o u th 1d 1 1 1 2 1 5 1 7 1 8 1 9 2 2 2 0 2 1 1 6 1 f -1 8 (4 3 0 9 0 ± 9 9 0 ) 1c 1 3 1 4 1c1b r r r r ri p -u p o f p al eo so l r r r r 1e r r r r r r o ck c la st s la rg e e n o u g h t o m a p 3 c -1 0 (5 4 8 ± 6 0 ) 2 b -1 0 (5 6 4 ± 4 0 ) 1c 2 3 4 5 0 1 6 8 7 9 1 0 r r r r r r 2clbu b 2b r r r r r r r r r r r r p 1 0 21 3 4 5 s c 1g r b en ch b e n c h sa n d 01234 a e g r e e n b u r n s t r e a m t r e n c h 1 e a s t w a ll b e n c h n o rt h s o u th m f n 1 f n 2 s 5 co a rs e g ra ve l b ro w n g ra ve l d ef o rm ed g ra ve l d f g f s1 (4 4 0 9 ± 6 0 )c b d a rk b ro w n co a rs e g ra ve l si lt u n so rt ed b ro w n sa n d y g ra ve l fi n e fr ia b le ch ip g ra ve l f n 4 p eb b ly si lt y sa n d co a rs e g ra ve l 1 5 4 3 2 1 0 1 7 1 4 9 8 7 6 1 2 1 1 1 3 1 6 1 5 1 8 sh u tt e r s ca rp h i n o rt h f a n g ra b e n s 1s 2 s 4 s 3 a s 1 s 3 b s 1 s 3 b f s5 f s4 f s3 f s2 f s1 f n 4 f n 3 s 3 a f u f u f u f n 5 f n 3 f n 5 f n 4 f n 5 c 1c 2 c 3 m s m s m s m s u f s3 u f s2 u f s1 m f s1 m m s o u th f a n j t r e n c h 2 w e s t w a ll ( re v e rs e d ) a b d e f g h c 0 8 3 o r r r r r r r r r co ll u v iu m r 2g 2c r r a b 1 4 s s s c f b c f a m s m s p 1 p 2 p 2 d a 1 d a 2 d a 1 1 7 d ( 7 1 0 ± 5 5 ) d f d f r r r a d io ca rb o n s a m p le lo ca tio n s 1 7 d s 4 3f 3d 2d 2d 2 d -8 (6 4 5 ± 6 0 ) r rv r r r r r r r r r r r r ba f ig . 7a ,b . c ol or ed g eo lo gi c lo g of ( a) e as t w al l of t re nc h 1 (t 1) a nd ( b) w es t w al l of t re nc h 2 (t 2re ve rs ed f or c om pa ri so n) w it h br oa d st ra ti gr ap hi c su bd iv is io ns s ho w n, as d et ai le d in f ig . 6. t he s hu tt er s ca rp c on si st s of i nt en se ly s he ar ed t or le ss e be dr oc k, co ll uv ia l de po si ts ( in p ur pl e) a nd s oi l. t he po nd ed a ll uv ia l fa n se qu en ce o f t 1 is f au lt ed b y m ul ti pl e ea rt hq ua ke e ve nt s be tw ee n an d w it hi n pa ck ag es . in t 2, th re e di st in ct s oi ls ( in b ro w n) r ep re se nt in g pr eru pt ur e gr ou nd s ur fa ce s co nv er ge t ow ar d th e sc ar p, w he re p 1 an d p 2 ar e fa ul te d. t he se s oi ls o ve rl ie a ll uv ia l an d de br is a va la nc he de po si ts . r ad io ca rb on s am pl e si te s an d ag es a re s ho w n by g re en s ta rs . 1127 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand relations likely. all other packages onlap or are faulted against the scarp package. the south fan refers to three packages of distinctive deposits between the scarp and intense graben faulting, all being separated by unconformities. the lower south fan package (fs) comprises five units dominated by poorly sorted, bedded gravels with angular to subangular sandstone clasts in a clay matrix. bands of weakly indurated fine sand to clay act as marker horizons separating distinct gravel units. fine-grained charcoal was located in several of these fine units. a single radiocarbon date from a sandy silt bed (fs1) near the base of the trench (fig. 7a) yielded an age of 4409 ± 60 c14 years bp (table i). we use this age below to develop a slip rate and recurrence data from this site. the middle south fan package consists of only one unit (mfs1), an unconformity-bound coarse gravel deposit. the upper south fan package consists of three units. the main body of this package consists of coarse, brown gravel within a silty matrix. the capping unit is a dark brown-grey gravel of angular bladed clasts of up to 10 cm diameter in a sandy matrix. this unit has accumulated ~ 30 cm of apparent vertical displacement across it against the scarp deposits during recent displacement events. the north fan package (fn) comprises six units. the exposed lower half of this package consists of dark brown-grey, poorly sorted greywacke gravel (fn1). above this are interbedded gravel and silt-clay units. these units are capped by and faulted against undifferentiated, deformed (fu) gravels that fill a graben-shaped feature in the middle of this trench. there is a general correspondence between the north fan and lower south fan packages in this trench. however, it is not possible to directly match gravel or silt facies from north to south along the trench wall. the south and north fan packages are consistent with fan aggradation related to the former course of urquhart stream. the uppermost package is named the channel package (c). it consists of three units (c1-c3) of channel gravels that fine upwards overall, i.e. they show evidence for decreasing stream power up-section. the channel deposits appear to correspond to a scarp-parallel drainage system. package c merges upward into the modern topsoil (ms), which has formed on an angular cobble deposit (fig. 6). other angular blocks of greywacke up to 15 cm occur on the surface of ms. these blocks have thin weathering rinds that yield young geomorphic ages ( 220 years) for the clasts and soil surface (pope, 1994). for reasons discussed below, we apply an uncertainty of ± 60 years to this age determination. as shown in fig. 6 and described below, the c package probably corresponds to the channel fill (cf) package of trench 2, while the ms in t1 corresponds to all deposits above cf in trench 2. 4.2. faulting the log of trench 1 is shown in fig. 7a. the bench affects the geometry of units and continuity of faults whose strike is oblique to the table i. summary of radiocarbon dating results from the greenburn trenches, eastern hope fault. location/ sample sample lab 13c./ 12c radiocarbon calendar-calibrated age (years bp) description i.d. type number (per mil) age (years bp) (1 range) (2 range) trench 1 fan silt fs1 ch nza 14707 – 25.1 4409 ± 60 4869-5050 4844-5288 trench 2 below liquefaction sand 3c-10 ch nza 14708 – 25.5 548 ± 60 520-559; 598-629 504-656 at base of df gravels 1f-18 ch nza 14709 – 21.5 43090 ± 990 – – surge gravel of da1 2d-8 ch nza 15137 – 25.13 645 ± 60 556-603; 624-660 543-671 within paleosol, p2 2b-10 ch nza 15136 – 25.91 564 ± 40 533-558; 599-628 517-649 ponded silt above da2 17d ch nza 14925 – 25.29 710 ± 55 651-679 555-603; 623-731 all radiocarbon analyses are ams and were performed by rafter radiocarbon laboratory, igns ltd., lower hutt, nz. locations co-ordinates are from nzms 260 series map o31/423678. sample location abbreviations come from fig. 6. samples all consist of charcoal, ch. ages are calibrated using intcal98-14c of stuiver et al. (1998). geomorphic trace. t1 has a complex deformation history with many highto low-angle faults dipping both to the north and south. in addition to the dominant dextral motion, the fan of faults in trench 1 between metres 6 to 17 gives the appearance of a significant normal component of motion behind the shutter scarp (fig. 7a). this style of young faulting must occur as a response (in covering fill deposits) to the fault dip and dextral-oblique (transpressional) nature of the rangefront and shutter scarp faulting. the scarp package is intensely sheared (cataclastically deformed) and does not yield any useful paleoseismic data. however, the stepped nature of faulting within the scarp is consistent with the observation of an en échelon character to the shutter scarp. if we consider the age of the fill units against the scarp shown by sample fs1 ( 5000 years, table i), nature of the well developed fault zone, and a conservative estimate of the range of earthquake recurrence, it is clear that at least 10 earthquakes are responsible for the deformation visible in trench 1. this implies a considerable amount of strike-slip separation on the sedimentary packages, explaining why it has been difficult to match units of the north and south fan packages across the zone of faulting. the fan of faults in t1 cut units to several levels within the stratigraphy. evidence for faulting during the most recent surface-rupturing earthquake event (mre) comes from faults b, c and d that appear to terminate upward in the modern soil and clearly displace the unit contacts below the soil. we believe that the modern soil and the ground surface are faulted. at this time we have no radiocarbon dates to bracket the timing of this event in t1. the width of the zone of surface faulting in t1 during the mre may be 8 m. we also observe evidence for the 2nd and 3rd events in trench 1. the cobble deposit that forms the parent material of ms was faulted in the mre. additionally, it appears to drape over faults (a and d) which were active during the second event. channel c1 and is possibly dragfaulted on fault b. the occupation of channels c1 to c3 parallel to the scarp, and faults bounding them, imply that faulting has occurred between fan abandonment (incision of urquhart stream) and the present. fault j clearly faults and juxtaposes deposits of c1. this probably occurred during event iii. evidence for previous events comes from several other faults and unconformities in the trench. for example, faults h and i cut the north fan package, but not the paleosol on unit fu or the channel package above it, and therefore, probably represent events that occurred prior to the current incision of urquhart stream. additionally, fault g cuts the lower south fan package only, while fault f cuts up higher, to at least the base of the upper south fan package. none of these events can be temporally-constrained at this time. in summary, due to a lack of useful datable material and complex deformation over a period of at least 5000 years (table i) it is difficult to separate out the number, or individual timing, of events in trench 1. however, this trench has provided useful information concerning the styles of sedimentation and deformation over this period and the uppermost deposits and deformation form a strong partner to the paleoseismic record observed in trench 2. 5. trench 2 trench 2 (t2) was excavated 50 m west of trench 1 across the shutter scarp at a bearing of 002° near the outlet of the abandoned fan drainage (fig. 5). the trench was sited in the lowest part of the basin behind the shutter scarp (fig. 8), where we suspected there would be a ponded finer-grained sequence of units. the benched trench is 22 m long and has up to 4.5 m relief from the top of the scarp to the bottom of the trench (fig. 7b). only the west wall of this trench was logged as the units and deformation were both very similar across the trench. the trench was excavated here in order to intercept: i) the highest point of the bedrock scarp and therefore scarp-derived colluvia; ii) the toe of a landslide expressed in the geomorphology at the north end of the trench (fig. 5), and iii) a hypothesised thicker sequence of young fault zone ponded alluvial deposits that 1128 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman would be useful in the distinction of discreet paleo-events. 5.1. stratigraphy figure 7a,b shows both the stratigraphic and structural comparisons between the two trenches. the stratigraphic sequence of t2 is generally younger, less complex, and less deformed than that of trench 1, with four packages overlain by the modern soil profile (fig. 6). deposition patterns range from gradual, as in the development of soils and ponded fills, to channel-filling alluvial deposition, to punctuated as in the deposition of debris flows or avalanches. the scarp package (s) again consists mostly of highly sheared torlesse sandstone, which is intensely fractured to chip-sized clasts and is cut by numerous minor shears. above the bedrock is a 40-80 cm thick zone of large sandstone blocks in a weathered matrix of gravel to clay-sized fragments (fig. 9a-d). this unit represents an active soil and colluvial pair forming on the fault scarp. the stratigraphically-lowest onlap package is the channel fill package (cf), which consists of two main units. the lower unit (cfa) contains two channel gravels interbedded with fine-grained deposits. this is overlain by channel-free yellow-brown silty clay which grades upward into a wedge-shaped paleosol (paleosol 1) that includes pieces of charcoal. paleosol 1 (p1) thins and pinches out to the north becoming an organic silt, implying a facies change into a subaqueous pond environment (fig. 10). we interpret the change from gravel lenses to fine fill and soil to correspond to the timing of abandonment of the deflected urquhart stream channel. the debris avalanche package is dominated by two, 1 m thick, very poorly sorted, clast-supported angular greywacke sandstone 1129 fault f fault c shutter scarp to abandoned outlet greenburn stream fig. 8. trench 2 at the time of the 2001 ilp meeting. the picture is taken looking southeast from a debris fan above the trench toward greenburn stream. the trench reaches from the apex of the shutter scarp to the toe of the debris fan. two main zones of recent fault rupture (arrows) were identified cutting the buried stratigraphy. paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand 1130 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman fig. 9a-d. sequence and explanation of retro-deformed logs of the west wall of trench 2: a) the trench as logged; b) through d) show progressive unstripping of stratigraphy and removal of deformation. the area of fig. 10, including sandblow deposit (in black), is shown by a dashed box at metre10. 1d 11121516 1314 1c r r r r r 1f-18 1c 568 7910 r r r r r r 2c lb ub 1g 2d 2b r r r rr r r r r 2d p1 2 4 immediately after the mre (event i) no debris fan (df) deposits or modern soil ponded fines on top of avalanche da2 at m-17 faulting occurred on faults a, b, c, f and g da1 surge gravel "infaulted" by fault g at m-7 2f r r r r r da2 r r a c b h 17d 2c r r r r r r 1c r 3h 3i 3d r r 2d 2 4 immediately before mre (event i) no da2 deposit exists faults a, b, c, f and g backslipped hummocky paleosol (p2) surface on da1 p2 merges into scarp soil faults a, b, and g disappear r r 3g 3f 3e r r r r scarp-derived colluvium 1112 1c 568 7910 3e 3i 3d at time of event ii da1 deposit about to arrive from hillslope p2 merges into scarp soil sand volcano erupting onto p1 paleosurface movement on faults c, f, and h? prior to event ii faulting on faults d, e, f, and h fluvial sedimentation of cf package 3c-10 r r 3f r southnorth 1520 1f-18 1c 1c 1b 1a r r r r r r rr r r r r r rock clasts large enough to map 2b-10 1c 5 010 r r r r r r 2c lb ub 2b r r r rr r r r r r r r p1 1 3 5 sc r bench m trench 2 west wall (reversed) a c b d e f g h r r r r r r r r r r 2g 2f 2c 3g r r 4 s s cfb cfa ms ms p1 p2 da1 da2 da1 17d (710 ± 55) df r r radiocarbon sample locations17d 3f3d r r r r r r r r r r r r r rip-up of paleosol r r r r r r 3c-10 sc d e f g h r r r r r r colluvium r s cfb cfa p1 p2 p2 da1 da2 da1 r 3f3d 2d-8 r r r r r r r s s r r 2b-10 (564 ± 40) 2d-8 (645 ± 60) r r 3c-10 sc d e f h r r r s cfb cfa p1 p2 da1 da1 r r rr s r r figure 10 sc bench bench c h h 3c-10 sc d e f h cfb cfa p1 s c r r r r r r r rr rr r r r r r r r r r da1 scarp-derived colluvium v v v 3e r r s a b1 b c d a breccias (da1, da2), interpreted as debris avalanche deposits. incorporated within these deposits are ripped up blocks of the underlying soil horizons and chunky charcoal fragments. in addition, at the base and toward the toe of these deposits are pockets of well-sorted granular gravels, and deformed obliquely-stacked soil units, respectively. the former are interpreted as debris avalanche surge deposits, while the latter are likely to be «bulldozed» toe deposits related to the avalanches stripping off the soil cover. clast range in size from 1 m (soil rip-ups) to 50 cm (rock clasts), to as little as sand size. the da deposits are densely-packed but have little definable matrix material. this list of features is indicative of an en masse style of deposition or emplacement. it is likely that these deposits emanated from the greywacke hillslopes above the trench. unit da1 probably correlates to the cobbly substrate to the modern soil of trench 1, the latter being a thin, distal equivalent of da1. separating da1 and da2 is a grey-brown silt loam with mottles toward its base and a sharp upper contact. we interpret this unit to be a paleosol (paleosol 2, p2) that was buried by the second avalanche. included as part of the debris avalanche package below da1 is a dome-shaped medium sand deposit with remnant vent, considered to be a preserved liquefaction sand volcano (fig. 10). no continuous feeder dike that terminated downward into a source was exposed. however, a source (sand) layer for the sandblow was discovered ca. 1.5 m below it during later deepening of the trench. the debris fan (df) package represents deposits at the north end of the trench above da2, related to the debris fan that extends a lobe south toward the scarp (fig. 5). unlike the debris avalanche units, the debris fan units are generally well-bedded and sorted, clast-supported gravels and are therefore clearly of different origin. the df gravels have a sheeted, non-waterlain fabric and are therefore consid1131 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand 25 cm da1 3c-10 sandblow remnant vent r r fig. 10. photo of trench stratigraphy at metre 10, including sandblow deposit and remnant vent (arrowed), overlying the lateral equivalent unit to paleosol (p1), the top of which forms the event horizon for event ii, and the overlying debris avalanche deposit, da1. ered to be of debris fan origin. at the base of the df package is an unbedded fine-grained clastic unit limited in its deposition to a small «backbasin» on top of debris avalanche da2. two samples (17d and 1f-18), discussed below, were taken from the upper surface of da2 and overlying fine-grained unit. the modern soil (ms) overlies the scarp, debris avalanche, and debris fan packages. in the middle part of the trench it is a weakly-developed soil with a gravely a horizon and minor development of a nutty soil texture. from metres 12 to 7, the three soils (including p1 and p2) converge toward the fault scarp where, albeit faulted, they appear to merge to form one composite soil profile on the scarp (figs. 7a,b and 9a-d). 5.2. faulting the geologic record in trench 2 has been progressively stripped of units and retro-deformed in fig. 9a-d to clearly demonstrate the sequence of events. the dominant mode of displacement (dextral) cannot be quantitatively accounted for in the trench. however, there is evidence of thickness changes in units and juxtaposition of dissimilar units across faults. the zone of faulting in t2 covers a similar width ( 7 m) to that seen in trench 1 (fig. 7a), where many more events have taken place over a longer timeframe. paleoearthquake events in trench 2 are recognised on the basis of both stratigraphic and structural evidence. the most recent event (mre) faulting appears to be completely buried by instantaneous deposition of debris avalanche da2 (fig. 9b). da2 overlies paleosol 2, which is faulted from metres 12-14 by faults a-c. the apparent vertical separation across these three faults is ca. 25 cm. horizontal offset is implied from the difference in thickness of unit da1 and p2 across these faults. da2 and the modern soil have buried and overlie the surface rupture, creating the impression of a flat unfaulted surface in the rift zone (fig. 9b). the rumpled form of the paleosol (p2) seen in the trench wall is mirrored in the shape of the preserved scarp from the most recent event, immediately west of the trench where no debris avalanche material was deposited. at metre-7, at the base of the scarp where the geomorphic expression of faulting is greatest, p2 was also faulted during the mre on fault g with small vertical separation across it (< 5 cm). we infer that the debris avalanche da2 was initiated due to strong shaking and ground motions in the hillslope above the trench site. the deposit itself is very poorlysorted, has a brecciated texture, and shows evidence of surge facies and toe deposits at its base and front. while the model of burial of the event horizon is equivocal, the faulting itself is not, and there does not appear to be better reason for the presence of such a deposit at that stratigraphic level. the best evidence for event ii is faulting on fault f at the main scarp that cuts paleosol 1 and terminates at its top, not displacing the surge or toe deposits of da1. fault f also juxtaposes paleosol p1 against sheared bedrock (fig. 9c). the penultimate event is also implied by an apparent increase in vertical separation on fault c from 25 cm (offset of p2) to 55 cm (offset of cfb.) for two events. however, we recognise that some of the observed vertical must be the result of strike-slip separation on da1. the liquefaction sandblow deposit (fig. 10) and debris avalanche deposit (da1) provide additional evidence for the penultimate event, as punctuated deposits associated with the strong ground motions of a surface-rupturing earthquake event. both of these deposits have a clear, instantaneous relationship with each other and the underlying paleosol (former ground surface). this model requires that the paleosol developed in the interseismic period. during the earthquake a liquefaction fountain ejected sand onto the paleosol surface, sourced from a sand layer discovered below the floor of the trench (fig. 9d). temporally, this deposit was almost immediately overlain by the debris avalanche, da1. further evidence for a third paleoseismic event is recorded at the bottom of the trench between metres 7 to 8, where faults d and e cut the lower channel fill package deposits (cfa), juxtaposing those units against bedrock. the timing of these three events is discussed below with respect to the ages of radiocarbon samples and the record in trench 1. additionally, there are many mapped shears within the bedrock 1132 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman scarp. most of these have no impact upon the recent paleoearthquake story. however, fault h at metre-5 does appear to cut the base of the colluvial scarp deposits and is therefore likely to have ruptured during one of the recent events. 5.3. radiocarbon dates trench 2 contained an abundance of large charcoal fragments that were recovered from several units. five samples were submitted to the rafter radiocarbon lab to determine radiocarbon age dates (table i). they were selected (blind) in order to bracket the time both before and after the two most recent surface-rupturing earthquake events. four of these age dates were young (< 1000 years old), while the remaining date, on sample 1f-18 from the top surface of da2, was anomalously old. on closer examination this sample had the appearance of lignitic carbon. the means of the four young dates fall within 275 radiocarbon years of each other, but show stratigraphic disorder in their ages, i.e. the oldest is highest and lowest is youngest (fig. 6). however, when calendar-calibrated these four dates are indistinguishable in their calibrated age ranges (fig. 11a,b). charcoal is not generally formed in situ and in this case was probably derived from a common source in the forested hillslope environments above the trench site. it seems likely that the charcoal fragments came from mature, burned trees that have been resident near the ground surface for long periods of time. the reworked charcoal is inferred to have been deposited episodically in the small rift zone basin at the trench site. the four dates themselves may reflect the age of any part of any burned tree above the site and we interpret the stratigraphic disorder of charcoal c14 dates obtained accordingly. those associated with the debris avalanche deposits were instantaneously included and transported into place, but do occur in deposits separated by significant time, i.e. separated by a paleosol. however, it is clear that the lowest sample (3c-10) was deposited first and yielded the youngest uncalibrated radiocarbon age result. therefore it provides a 1133 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand 1200 1300 1400 1500 c al en d ar y ea rs a .d . 3c-10 17d2d-8 2b-10 radiocarbon samples 3c-10 is below event ii 2d-8 is above event ii 2b-10 is below mre 17d is above mre complete age range of forest burning 1200 1300 1400 1500 1200 1300 1400 1500 event ii c al en d ar y ea rs a .d . event i (mre) t r e n c h 2 e v e n t a g e d i s t r i b u t i o n s event iii fig. 11a,b. two-sigma calibrated age ranges for the four young radiocarbon dates on charcoal from trench 2 and their implications: a) the complete range of these samples represents wood probably burned in a single fire event; b) samples from above and below both events i and ii indicate that their event age distributions are indistinguishable. despite their overlap, these ranges can be used to define the likely bounds of events ii and iii. ba maximum age for the timing of the two paleoseismic events that followed its deposition. as discussed above, the four radiocarbon dates have overlapping, indistinguishable calibrated age ranges at the 2 limit (fig. 11a,b). together with the abundance of charcoal of the same age in units deposited by both gradual and en masse means, these results suggest a single burn event of an old forest between 1295 and 1450 a.d., that may be the result of polynesian expansion into northern south island in preeuropean times. 6. discussion 6.1. slip rate of the conway segment a new independent slip rate is determined here from the geomorphic offset of the abandoned paleochannel of urquhart stream in concert with the age of associated fan deposits (fig. 5). one end of the offset, defined by piercing point p, comes from projecting the valley axis at the head of the fan where the stream emerges from the mountain front, to the fault. this assumes that if the hope fault were not deflecting it, urquhart stream would flow straight across (perpendicular to) the mountain front. p is not located in the centre of the modern channel. p* is the other piercing point and is related to the outlet of the former stream course as it crosses the fault and cuts through the scarp. p* is the projection back to the fault of the straightened downvalley stretch of the former urquhart stream beyond the scarp. the distance pp* comes from progressive dextral displacement along the hope fault related to the course and deflection of urquhart stream prior to its current incision. pp* has a value of 115 m and an associated error of ± 15 m. the uncertainty comes from the combination of the position of the modern channel and the outlet of the abandoned channel. the age of deposits related to this displacement is based on radiocarbon date fs1 (table i) taken from charcoal in a basal silt layer in trench 1 (fig. 6). this represents the oldest exposed unit containing datable material. because we did not expose the base of the fan sequence, we must consider this date to reflect a minimum age for fan deposition within the fault furrow related to the stream deflection. therefore, the slip rate derived from this calculation is closer to a maximum rate. the combination of displacement and age for the paleochannel of urquhart stream produces a dextral slip rate for the conway segment at this site of 18.9-26.8 mm/yr (rounded to 23 ± 4 mm/yr). we suspect that the actual slip rate should fall inside this maximum slip rate range. this new data can be used to better define the geologic slip rate for the conway segment. our results compare favourably with other slip rate studies on the conway segment, such as the rate of 18 ± 8 mm/yr derived from hossack station at the western end of the conway segment (mcmorran, 1991). dextral slip rates of ca. 28 and 33 ± 1.5 mm/yr have been determined from offset terraces at sawyers creek, 3 km east of this site (van dissen, 1989; bull, 1991). the weathering rind ages associated with this offset were re-analysed by pope (1994) using the calibration criteria of mcsaveney (1992), yielding slip rates in the range 31-55 mm/yr. clearly, the upper end of this range is extreme, as it is greater than the whole plate motion rate for the entire mfz (bourne et al., 1994; de mets et al., 1994). in addition to our new results and using the same method of offset geomorphology and dated deposits in trenches, we recognise a comparable value for a young offset fan at the clarence reserve releasing step-over (pope, 1994; campbell, 2001), 2 km to the east of our site. this site yields a slip rate of ca. 25 mm/yr, implying that locally our new slip rate is a valid result. the greater weight of evidence, particularly from features dated by radiocarbon methods implies that the slip rate range for the conway segment is likely to be somewhat lower estimates than made using weathering rind data, in accordance with our new results. 6.2. slip per event and recurrence interval the average slip per event, or single event displacement (sed) mapped from offset ridges, terrace risers, and channels at 9 localities be1134 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman tween greenburn stream and sawyers creek by pope (1994) is 5-6 m. this value is comparable to the 6 ± 2 m estimated by bull (1991) farther west. at urquhart stream, where the slip rate is calculated, displacements from offset features are measured at 3.4, 4.9 ± 0.2, and 10.4 ± 0.2 m. no new evaluation of sed has been undertaken, therefore we apply the pre-existing dataset of pope (1994) to the greenburn stream site. the recurrence interval (ri) for this site may then be calculated independent of event ages using the slip rate (23 ± 4 mm/yr) and sed range of 5-6 m. this yields an average ri of 185-263 years. this range overlaps, but is generally higher than (longer ri), the recurrence range for the neighbouring hope river segment (81-220 years) (cowan and mcglone, 1991), the range calculated by pope (1994) from data collected locally, and recent estimates for the conway segment (120-300 years) provided by pettinga et al. (1998, 2001). a caveat of our ri range is that it is calculated from a lower sed range and a slip rate tending toward the maximum. however, as shown in the next section, they compare favourably with the stratigraphic and chronologic evidence for large earthquakes at this site. using a variation of the v = d/t equation we can use results from the greenburn stream site to develop an independent test of our ri. the equation we use is ri = t1 = tn./n; where n = dn./d1. the total displacement (dn) or deflection of urquhart stream at the site is 115 ± 15 m. at 5 to 6 m of displacement per event (i.e. d1), between 17-26 large seismic events (n) are required to achieve the displacement, dn. if these events occurred over a period of time (tn) represented by the age of the fan deposits in trench 1 (sample fs1), then the average recurrence for these events (t1) is 179-311 years, consistent with the range we calculated (185-263 years) above using the slip rate (v). combining these two techniques we present a value of 180-310 years for the recurrence interval from the greenburn stream site. given that only one piece (hope river segment) of the hope fault has ruptured during the historic period ( 160 years), we favour our results, which suggest a longer ri for at least the conway segment of the hope fault. 6.3. timing of the last three large seismic events we recognise and can place some temporal constraints on each of the last three paleoearthquakes on the conway segment of the hope fault from the greenburn stream site. constraints come from the four young radiocarbon dates in t2, the weathering rind data (pope, 1994), and stratigraphic correlation to t1. event iii is recognised to have occurred during deposition of packages c and cf in trenches 1 and 2, respectively (fig. 7a,b). this is observed from the faulting that caused thickening or juxtaposition of channel c1 in t1 and faults penetrating to the top of cfa in t2 (fig. 9d). if c2 and cfb were not faulted during event iii then its age is older than that of sample 3c-10 (1295-1405 a.d.). however, if we consider the 2 range of sample 17d then this event may be older (fig. 11a,b). that is, if 17d represents forest material burned at the same time as the other samples (fig. 11a) and is the oldest material above event iii, then that event must have occurred prior to 1220 a.d. no reasonable lower constraint can be placed on this event at this time, though stratigraphically it occurred within the c and cf packages. this record of faulting indicates that three paleoearthquakes have occurred since a fundamental change in the style of sedimentation observed in trench 1, from an outwash fan style with paleodirections perpendicular to the scarp (f, mf, and uf packages), to scarp-parallel drainage in small constricted channels (c and cf packages). this change may reflect the timing of fan abandonment and urquhart stream incision. what can we deduce about the timing and recurrence of the mre (event i) and event ii? if we consider only the radiocarbon age data (the samples above and below each event), then events i and ii are indistinguishable due to their overlapping age ranges (fig. 11b). the two most recent events have been recognised as discreet events in both trenches based on upward termi1135 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand nations of faults and faulted horizons, supported by stratigraphic correlation, and a model of instantaneous event deposition of debris avalanche deposits and a liquefaction sandblow. first, we must clarify the timing of event ii. the maximum age of event ii is best constrained by sample 3c-10 (the youngest and lowest of 4 dates) to be younger than 548 ± 60 c14 years bp. unlike the other three young samples, 3c-10 was deposited in the organic silt adjacent to paleosol p1 and was faulted during event ii. when calendar calibrated to the 2 level, this age becomes 1295-1405 a.d. (504656 years bp). therefore, even though the four dated samples are likely to have a common source we can isolate sample 3c-10 as the one that provides information concerning the age of that event horizon. event ii must be younger than 1295 a.d. the maximum age is also dependent upon the length and timing of the previous interseismic period, i.e. the period between events iii and ii. we recognise that only a few units were deposited after event iii and before the beginning of soil development (paleosol p1) on the cf package. we argue below that such soil development requires a minimum of 160 years. clearly, two paleoseismic events have occurred on the conway segment since 1295 a.d., and prior to the historic period, i.e. 1840 a.d. is it possible that the most recent (i) and penultimate (ii) events are broadly spaced and occurred close to the edges of the recurrence range? or, were they closely spaced (clustered) in time, as suggested by bull (1998)? as discussed, the four samples that we attempted to bracket the events with have indistinguishable age ranges at the 2 level. therefore, they cannot be used to uniquely date the paleoearthquakes themselves, i.e. the event i and ii ranges in fig. 11a,b reflect the age of a burning event rather than two clustered earthquakes. however, evidence for these earthquakes being spaced by a significant interseismic period is gleaned from the history of soil development at the site. three soils (including ms) exist in the middle part of trench 2 (metres 715), which each formerly merged with the soil on the scarp. they (p1 and p2) formed the ground surface prior to large earthquakes and were buried by earthquake-induced debris avalanches. interestingly, all three soils have been the main time units that have developed over most of the last 3 interseismic periods, and therefore themselves, represent the interseismic period. the modern soil has been forming for a minimum of 163 years (i.e. since 1840). it is weakly developed with little fine material included from fluvial or aeolian input, and little clay accumulation. paleosol 2 is a moderatelydeveloped soil with a thick accumulation of fines and mottled b horizon. it formed between the mre and penultimate earthquakes. paleosol 1 is a dark, moderately-developed soil formed on a fine-grained fill deposit against the paleoscarp. it grades away from the scarp into an organic-rich pond deposit (fig. 9a-d). both of these paleosols required significant time to develop before being buried by debris avalanche deposits, possibly a factor of 2 times more than is seen in the modern soil. this amount of time (160-320 years) is equivalent to the recurrence interval calculated previously. furthermore, soil accumulation at trench 1 was probably not interrupted by the deposition of scattered blocks of sandstone across its surface, cf. the complete burial of the soil p2 by debris avalanche da2 in t2 (fig. 9a-d). therefore, in terms of time, the modern soil in t1 probably represents the elapsed time required to form both paleosol 2 and the ms in t2. in addition, the cobbles on which the ms of t1 formed are equivalent in age to da1 in trench 2, and were deposited en masse following event ii. these statements bear on the age bracketing of the mre. the clasts on the surface of the modern soil adjacent to trench 1 are dated using the weathering rind technique at 220 ± 60 years, while the modern soil in t1 appears to be faulted. this evidence points to the age of the mre being equivalent to that of the weathering rind surface age, i.e. 220 ± 60 years, or about 1780 a.d. ± 60 years. given the uncertainties that may exist in the surface age, the stratigraphic correlation between trenches, soil history and the punctuated landscape change model, this is an impressive result. by constraining the interval across which the mre occurred in concert with the soil history, we yield additional knowledge concerning 1136 robert langridge, jocelyn campbell, nigel hill, verne pere, james pope, jarg pettinga, beatriz estrada and kelvin berryman the minimum age of event ii. we argue that paleosol 2 took much more time to form than the modern soil, based on the accumulation of fines and development of mottling (160-320 years). we assert that the spacing of events i and ii (interseismic period) is probably at the upper end of our recurrence range. this pushes the minimum age of event ii back toward the age of the forest burning, or more reasonably to within the period thereafter. 6.4. implications of the timing of the mre the results of this work argue for a young, pre-european surface-rupturing earthquake (1780 a.d. ± 60 years) on the conway segment of the hope fault. we consider that this range gives the best estimate possible of the mre, while giving due credence to the inherent uncertainties. at the upper end of this range (1840 a.d.), the historic period is encountered; thereafter it is known that no surface-rupturing event exists for the conway segment. at the lower end of the range, an equivalent uncertainty is accounted for, representing that which stems from our stratigraphic, dating, or deformation argument. one implication of this result and the recurrence data is that we are beginning to approach, or have already entered, the lower limit of the recurrence range for the conway segment. how does this mre age compare with previous studies? no previous trench (on-fault) studies have produced event timing for the mre (see mcmorran, 1991; pope, 1994). the only other mre information comes from lichenometric studies of earthquake-induced rockfall throughout the kaikoura ranges (bull, 1991, 1998; bull and brandon, 1998). lichen re-colonisation has been used as a proxy for very strong earthquake shaking. however, these results are off-fault or indirect paleoseismic indicators and so are therefore not necessarily associated with large earthquakes. bull (1998) identifies two surface-rupturing events attributed to the conway segment that occur within the uncertainty range of our results, at 1768 a.d. and at 1832 a.d. our evidence supports bull’s assertion that large shaking events have occurred in the kaikoura ranges in the recent past, immediately prior to european settlement of new zealand. however, our trenches show clear evidence for only one event over this time period on the conway segment, with a significant time break (equivalent to the calculated ri) between the mre and penultimate event, shown by the development of soil (paleosol 2) at the site. we cannot separate or confirm the reliability of the 1768 or 1832 a.d. events, though we consider it possible that the mre observed at greenburn stream is one of these events. other fault segments in the region, e.g., kowhai fault (fig. 3), could be the source of a separate young, rockfall-inducing event. in future, more historical data from kaikoura, an early new zealand whaling and sealing settlement may help confirm the age of the most recent events in the southeastern marlborough fault zone. 7. summary i) two trenches were opened near greenburn stream on the conway segment of the hope fault. these trenches were dug through ponded fill deposits against an uphill-facing scarp next to urquhart stream. ii) trench 1 intercepted alluvial fan deposits up to ca. 5000 years old, and showed evidence for multiple earthquake events. data from this trench was used to develop an independent dextral slip rate of 23 ± 4 mm/yr. iii) trench 2 intercepted a fill of younger deposits and showed evidence for the last 3 paleo-earthquakes. the two youngest events are constrained by radiocarbon dates and the historic period to have occurred between 12951840 a.d. event iii probably occurred prior to 1220 a.d. iv) the age of the most recent event could be constrained between 1720-1840 a.d. given that the modern soil of trench 1 is faulted and that the cobbly surface deposit on it is dated at 220 ± 60 years bp. significant time exists between each of these three events as observed in the record of soil development. v) the single event displacement based on offset features near urquhart stream is 5-6 m. 1137 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand in combination with the calculated slip rate, age, and total displacement, the sed produces a recurrence interval of 180-310 years. acknowledgements the authors wish to thank murray urquhart (hawks hill station) for permission to access his farm – you are now immortalised through the naming of this stream. thanks also to the ilp participants for review in the field. a special thanks for assistance from ilp and the ilp conference organisers for including the idea for this project. we thank tim dawson and ramon arrowsmith for thoughtful review comments. we also thank mark stirling and nicola litchfield of gns for internal review comments and discussion that significantly improved this paper. this work was funded by frst postdoctoral fellowship co5903, frst subduction geology 43014300 and south island active faults 41020411, ilp conference support, and university of canterbury research grant u6451. references anderson, h. and t. webb (1994): new zealand seismicity: patterns revealed by the upgraded national seismic network, n. z. j. geol. geophys., 37, 477-493. anderson, h., t. webb and j.a. jackson (1993): focal mechanisms of large earthquakes in the south island of new zealand: implications for the accommodation of the pacific-australia plate motion, geophys. j. int., 115, 1032-1054. barnes, p.m. and j.c. audru (1999): recognition of active strike-slip faulting from high-resolution marine seismic reflection profiles: eastern marlborough fault system, new zealand, geol. soc. am. bull., 111, 538559. berryman, k.r., s. beanland, a.f. cooper, h.n. cutten, r.j. norris and p.r. wood (1992): the alpine fault, new zealand: variation in quaternary structural style and geomorphic expression, ann. tectonicae, 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(1989): late quaternary faulting in the kaikoura region, southeastern marlborough, new zealand, unpublished m.sc. thesis (oregon state university, usa), pp. 72. van dissen, r. and r.s. yeats (1991): hope fault, jordan thrust, and uplift of the seaward kaikoura range, new zealand, geology, 19, 393-396. wood, r.a., j.r. pettinga, s. bannister, g. lamarche and t.j. mcmorran (1994): structure of the hanmer strike-slip basin, hope fault, new zealand, geol. soc. am. bull., 106, 1459-1473. yeats, r. and k.r. berryman (1987): south island, new zealand, and transverse ranges, california: a seismotectonic comparison, tectonics, 6, 363-376. 1139 paleoseismology and slip rate of the conway segment of the hope fault at greenburn stream, south island, new zealand n n n n n n conil.pdf annals of geophysics, vol. 46, n. 1, february 2003 57 influence of the north atlantic on simulated atmospheric variability sébastien conil and zhao x. li laboratoire de météorologie dynamique, cnrs, université pierre et marie curie, paris, france abstract an atmospheric general circulation model is used to investigate the influence of the north atlantic ocean on atmospheric variability. the study covers the period from 1950 to 1994. the observed sea surface temperature and sea ice extension are used to force the atmospheric model. several configurations of the oceanic boundary conditions were made to isolate the role of the north atlantic and to study its non-linear interaction with forcings from other oceanic basins. the multi-realization character of the experiments distinguishes between the internal random part and the external forced part of the total variability. the potential predictability can thus be evaluated. the response of the atmosphere is also studied with a modal approach in terms of hemispheric teleconnection patterns. the north atlantic ocean has a direct influence on both the northern hemisphere annular mode and the pacific-north-america pattern, leading to a weak predictability. however the direct response is largely modulated by forcings from other oceanic basins. the non-linearity of the system compensates the predictable component of the annular mode induced by the north atlantic forcing. furthermore it reduces the forced component of the pacific-north-america pattern, increasing its chaoticity. 1. introduction atmospheric circulation of the northern hemisphere exhibits variability in a wide range of timescales, from days to decades (see feldstein, 2000). this variability is dominated by a small number of large-scale coherent teleconnection patterns (wallace and gutzler, 1981), such as the pna (pacific-north-america pattern) or the nao (north atlantic oscillation). they have major climatic impacts and are involved in many climate phenomena. understanding the mechanisms controlling the teleconnection patterns will contribute to make reliable climate predictions at different time scales. predictability of the climate at seasonal to decadal timescales arises from two distinct sources as described by lorenz (1975). the first source is the initial states (initial-value problem) and the second one is the boundary conditions (potential predictability). they are called the first and second kinds of predictability respectively (collins and allen, 2002). when an atmospheric general circulation model is used to investigate the second kind of predictability, the boundary conditions are the sst and sea-ice variation. a number of past studies looking for the role of the ocean in building atmospheric variability and its associated predictability utilize forced agcm, as in harzallah and sadourny (1995) and zwiers et al. (2000). they note a robust high predictability in the low latitudes whereas the extratropical regions show weak (and inconsistent) response. mailing address: dr. sébastien conil, laboratoire de météorologie dynamique, cnrs, université pierre et marie curie, casier 99, 4 place jussieu,75252 paris cedex 05, france; e-mail: conil@lmd.jussieu.fr key words atmospheric general circulation model − internal/external variability − climate predictability − teleconnections 58 sébastien conil and zhao x. li tropical oceanic forcing has been used with success, in the toga (tropical ocean-global atmosphere) context, to understand ensorelated (el niño southern oscillation) teleconnections and to forecast associated climate fluctuations (trenberth et al., 1998). the role of the mid-latitude ocean is however much more controversial (see kushnir et al., 2002). in particular, the role of the north atlantic for lowfrequency climate variability is very uncertain. results reported in the literature are often incoherent. rodwell et al. (1999) reported a high hindcast skill of the nao, originated from the north atlantic decadalscale variation by using an atmospheric general circulation model forced by the observed sst for the second half of the 20th century. similar experiments were also repeated by other groups, latif et al. (2000), for example, pointed out that a considerable part of this hindcast skill came from the equatorial pacific through global teleconnection mechanisms. the enso influence on the north atlantic and europe atmospheric variability was also depicted in fraedrich (1994) or venzke et al. (1999). lack of a suitable theoretical framework is the main obstacle to understand the role of the mid-latitude ocean in forcing climate variability. in fact, mid-latitude atmospheric circulation is characterized by a strong transient circulation with large-scale wave-like structures. response of such a regime of atmospheric circulation to the boundary conditions is much more complex. strong scale-interactions also increase the difficulties in understanding the role of the midlatitude ocean. for the synoptic time scales, midlatitude ocean has mainly a passive role in integrating the atmospheric variation, such as surface temperature or wind stress. the oceanto-atmosphere signal at low-frequency scale is often masked by the high-frequency atmosphereto-ocean patterns (frankignoul, 1985). in this work we address the problem of the influence of the oceanic boundary conditions on atmospheric variability in the northern hemisphere. this work is a natural extension of li (1999) with emphasis on the role of the north atlantic ocean. we will study the atmospheric response to the oceanic boundary conditions, by separating internal from external variability. our aim is also to identify the teleconnection structures that are sensitive to the oceanic forcing. we will particularly focus on the influence of the north atlantic and its possible interaction with forcing from other oceanic basins. three configurations corresponding to different oceanic boundary conditions are used. the first experiment uses the global observed oceanic surface conditions. to emphasize the north atlantic role, we perform two other experiments, one forced by observed varying oceanic conditions only in the north atlantic and climatological conditions elsewhere, the other with climatological oceanic conditions in the north atlantic and observed varying forcing elsewhere. such a strategy allows us to assess the influence of the north atlantic ocean in two complementary ways. in fact the north atlantic ocean can have a direct impact on the atmospheric variability, but its influence can be non linear and the direct impact can be modulated by other oceanic forcing. this non-linearity can also act on the internal variability. the structure of the paper is as follows. model, experimental design and statistical methods are briefly described in section 2. results on the simulated variability, its internal and external decompositions, and the potential predictability are presented in section 3. in section 4 we focus on the spatial modes of variability and their predictability. discussion and conclusion are given in section 5. 2. model, experiments and analysis method 2.1. model description the model used in this study is lmdz version 3.3, a state-of-the-art climate model derived from the standard version, as described in sadourny and laval (1984). the model is formulated through finite-difference on the arakawa-c grid. its horizontal resolution is 4° in latitude and 5° in longitude, with regularly distributed grid points. the model uses vertical hybrid coordinates with 19 levels, unevenly spaced to allow a better resolution in the boundary layer. the advection scheme is designed to conserve potential enstrophy for divergent barotropic flow (sadourny, 1975). lateral diffu59 influence of the north atlantic on simulated atmospheric variability sion is calculated through an iterated laplacian operator. six minutes are the time step used to resolve the dynamics, but the physical parameterizations are evaluated only every 30 min. convection is parameterized with a simple massflux scheme developed by tiedtke (1989). the cloud parameterization, presented in le treut and li (1991), uses a cloud water budget and a statistical description of the subgrid water distribution. the radiation package is the same as that used in the model of ecmwf. the shortwave radiation code is an updated version of the fouquart and bonnel (1980) scheme. long-wave radiation scheme was designed by morcrette (1991). the planetary boundary layer scheme is based on a second order closure model. the surface model is a bucket model for which we consider a homogeneous layer of 150 mm. calculation of the surface temperature is incorporated in the boundary layer and based on the surface energy balance equation. for the surface moisture, a holding capacity is fixed at 150 mm of water, and all the water above this value is lost as runoff. a complete description of the model is available at «http://www.lmd. jussieu.fr/~lmdz/doc.html». the same version of the gcm was also used in the coupled ocean atmosphere study presented in li and conil (2003). 2.2. realization of experiments our main objective is to study the influence of the north atlantic oceanic conditions on atmospheric variability and predictability. the separation of external and internal components of the total variability and the evaluation of the potential predictability require an ensemble approach. the ensemble size is critical because the signal-to-noise ratio is weak as shown by li (1999) and mehta et al. (2000). we adopt thus the ensemble approach for all the three experiments. an ensemble of 17 simulations was first run with global sst and sea-ice distributions as boundary conditions, covering the period 19501994. the initial condition differed slightly from one simulation to another. the time-varying sst and sea-ice distributions were obtained from the gisst data set (rayner et al., 1996), monthlymean gisst data being interpolated into daily values through a simple spline interpolation scheme. this experiment will be referred to as global. in order to isolate the role of the north atlantic oceanic boundary conditions two additional experiments (referred to as noatl and atl, hereafter) were also performed. noatl consists of 9 integrations with the same sst and sea-ice information for the global ocean, except for the north atlantic (north of 14°n) where seasonal climatological conditions were prescribed. atl is a complementary experiment containing also 9 integrations with observed varying sst and sea ice for the north atlantic ocean (north of 14°n) but prescribed climatological conditions elsewhere. an additional 150-year integration (referred to as clim) was performed with climatological sst and sea ice during the whole integration. 2.3. analysis method in order to identify the relative influence of the boundary conditions and the internal dynamics in the atmospheric variability, we employed the anova (analysis of variance) technique to analyze our ensemble simulations. this approach has been widely used in the past and is described in detail in harzallah and sadourny (1995), rowell (1998) or von storch and zwiers (1999). if x i (t, s) represents a variable (e.g. winter seasonal-mean geopotential height at 500 hpa) at time t (length t = 44 winters) and spatial position s (northern hemisphere), for the simulation i (total number i = 17 or 9), it can be split into two parts x i (t, s) = e(t, s) + r i (t, s) (2.1) where e represents the variations induced by varying boundary conditions and r i the internally-generated variations (the random part). to be able to make this partition, two conditions are necessary (scheffe, 1959). the first is that e and r follow independent gaussian distributions, and the second is that e and r are independent. these two conditions are generally verified for variables such as geopotential height or temperature at 500 hpa, since their persistence time scales are much shorter than the year. 60 sébastien conil and zhao x. li two averages can be defined (2.2) which is the ensemble mean and (2.3) which is the general mean or climatological mean. the ensemble mean is a natural estimator of the boundary-conditions induced variations e, whereas x i – xem is an estimator of the internal variations r i . the dispersion among the ensemble members gives an unbiased estimate of the internal variability (2.4) the variance σ 2 em of the ensemble mean is given by (2.5) following scheffe (1959), the oceanic-forced variability σsst can be deduced from the variance of the ensemble mean after removing the bias due the internal variability (2.6) an unbiased estimate of the total variance σ 2tot is the sum of the internal and external contributions (2.7) in the following, we use the term internal and external variability in reference to σ int and σ sst . the proportion of the total variance due to the boundary forcing measures the potential predictability (2.8) the statistical significance is conducted through a test as in rowell (1998). the null hypothesis σsst 2 = 0 or ρ = 0 is rejected at β significance level when (2.9) where f t–1; t (i–1); β is the f statistic at β significance level with degrees of freedom t – 1 and t (i – 1). 3. analysis of variance and potential predictability in the following sections we apply the abovedescribed anova technique to the three ensemble experiments, global, noatl and atl. we focus on the northern hemisphere winter, december-january-february, when the variability is stronger, the teleconnections more robust and the potential predictability higher. as mentioned before, the role of the north atlantic is our particular interest and it can be assessed through combination of the three different experiments. two different ways are possible. but they are not equivalent since the non-linearity of the model is activated differently. the first combination uses atl and clim. the difference between these two experiments gives a direct evaluation of the impact exerted by the north atlantic. the second combination uses global and noatl. it gives us not only the direct influence of the north atlantic, but also the non-linear response through interaction with other oceanic basins. µ s t x t s t i x t si i i t t t t ( ) = ( ) = ( ) === ∑∑∑ 1 1 1 111 em , , σ int σ int em 2 2 11 1 1 s t i x t s x t si i i t t ( ) = −( ) ( ) − ( )( ) == ∑∑ , , . σ µ em em 2 2 1 1 1 s t i x t s s t t ( ) = −( ) ( ) − ( )( ) = ∑ , . σ σ σ sst em int 2 2 21= − i . σ σ σ σ σ tot sst int em int 2 2 2 2 21= + = + −i i . ρ β > + − −( ) − 1 1 1 1 1 i f t t i; ; ρ σ σ σ σ σ σ σ σ = = − + − = + − sst tot em int em int em int 2 2 2 2 2 2 2 2 1 1 1 1 1 i i i i i( ) . x t s i x t si i i em , ,( ) = ( ) = ∑1 1 61 influence of the north atlantic on simulated atmospheric variability 3.1. total variance the total variability (interannual standard deviation) of the geopotential height at 500 hpa simulated in global, noatl and atl (fig. 1) can be compared with the estimation from the ncep reanalysis (fig. 5b). while the simulated variability is overestimated in the high latitudes, it is, however, too weak at low latitudes. a local maximum over greenland is present in the model as in the observations. but this maximum is too geographically confined for all three experiments, compared to the observed strong one. furthermore, in the three experiments, a strong maximum is located just in the west of england over the north atlantic. two other local maxima in the north of the siberian plain and in the north of japan are well represented in the model. the center over the north pacific, well identified in the observation, is also present in global and noatl, but not in atl. 3.2. internal variance at first glance, the spatial structure, even the magnitude, of the internal variability (fig. 2) is very close in the three experiments. the resemblance to the clim is also evident (see fig. 5a). it is mainly zonally distributed, increasing from low to high latitudes. the maxima located over the north atlantic, greenland and siberia are still present, but not the center over the north pacific. the fact that different estimations of the internal variance give similar results indicates that the internal variance is a robust variable, almost intrinsic to the model’s fundamental behaviour. however careful examination still reveals subtle differences among the experiments, which is the manifestation of the nonlinearity and the interaction between the model’s forced behaviour and its random variation. in fig. 1. total variability (expressed as interannual standard deviation, in meters) for winter (decemberjanuary-february) 500-hpa geopotential height in respectively global, noatl and atl experiments. 62 sébastien conil and zhao x. li fig. 2. same as in fig. 1, but for the internal variability. fig. 3. same as in fig. 1, but for the external forced variability. 63 influence of the north atlantic on simulated atmospheric variability fact, oceanic conditions are able to modulate the mean atmospheric circulation and also its transient properties. 3.3. forced variance the forced variability is displayed in fig. 3 for the three experiments. the forced variance deduced from atl is a direct estimation of the impact exerted by the north atlantic. it is in general very weak. but we can still observe highvalue centers over the arctic and north pacific. these remote influences are probably due to the hemispheric teleconnection mechanism. the local influence over the north atlantic is however small. the structure of the forced variance has a good resemblance between global and noatl. their difference gives another evaluation for the role played by the north atlantic. visual inspection shows that the forced variance decreases, from noatl to global, over the north pacific and north america, but increases in the atlantic sector. this is probably due to the interaction between the north atlantic variability and that of other oceanic basins. the final forced variance can thus be increased or decreased for different geographical locations. a more precise interpretation of this modulation is presented later. 3.4. potential predictability the potential predictability (fig. 4) induced by the north atlantic ocean alone is very weak (< 10%). it is not significant in large parts of the northern hemisphere, except in a marginal region over the eastern pacific where the total variability is very weak and over the arctic. for the experiments global and noatl, the potential predictability (fig. 4) is also weak, fig. 4. potential predictability (% of total variance) for winter (djf) 500-hpa geopotential height in global, noatl and atl. shaded regions indicate non-significant zone according to an f test (see text). 64 sébastien conil and zhao x. li table ii. same as in table i, but for the region nae (north-atlantic-europe) (80°w/20°e and 30/90°n). with values of 10% over europe and 50% over the north pacific, north america and the gulf of mexico. this seems coherent with results reported in kumar and hoerling (1998) and cassou and terray (2001). the general feature for the potential predictability is its almost zonal distribution with, however, larger values over ocean than over land. in addition, at low latitudes the atmospheric internal variability is weaker and the potential predictability is higher. we already know that the north atlantic forcing can modify both internal and external variances through complex nonlinear interaction with other oceanic basins. modifications in the internal and external variances lead finally to changes in the potential predictability. over the north pacific and north america, comparison between noatl and global reveals a weakening of the forced variability and an increase in the internal variability at the same time. the potential predictability over this region is thus largely reduced by incorporation of the north atlantic. the inverse situation is found in the sector of the north atlantic, i.e., the potential predictability is improved by the incorporation b fig. 5a,b. interannual variability (in meters) for winter (djf) 500-hpa geopotential height in the clim experiment (a) and in the ncep reanalysis data set (b). table i. regional-averaged values of the total, internal and forced variability for the three experiments atl, noatl and global. the average is over the pna (pacific-north-america) region (145°e/80°w and 30/80°n). last raw gives the values of potential predictability. the 99% (or 95%) significance level is reached when potential predictability is 6.4% (or 4.3%) for atl and noatl, and 6.2% (or 4.2%) for global experiment atl noatl global total variability (m) 55.7 59.9 63 internal variability (m) 54.8 50.3 55.4 external variability (m) 9.7 32.6 30.1 pot. predictability (%) 2.4 29.4 22.7 experiment atl noatl global total variability (m) 52.2 56.3 57.5 internal variability (m) 50.6 51.9 52.5 external variability (m) 13 21.9 23.3 pot. predictability (%) 6.6 15.2 16.3 a 65 influence of the north atlantic on simulated atmospheric variability of the north atlantic oceanic information since the internal variability is reduced and the external one enhanced. to summarize and quantify our results, we performed a regional average for these two particular regions. tables i and ii give respectively the total, internal and external variances, and the potential predictability for the pna and nae (north-atlantic-europe) regions. the three experiments atl, noatl and global are all included. there is a slight general increase in the total variance from atl to noatl, then to global. however, their decomposition into internal and external variances and their respective evolution make the final potential predictability very different. in the remote pna region, the direct impact of the north atlantic is weaker than over north atlantic/europe. however the non-linear estimation suggests that this direct influence is largely modified by its combination with other ocean forcings. indeed, over the pna region the potential predictability is substantially reduced by the presence of the north atlantic forcing due to decreased external response and increased chaotic variability. in the nae region the modulation of the potential predictability is weak. the external signal is stronger with the north atlantic forcing but the internal variability is also increased. 4. modal analysis from the results presented above, we can notice some regional coherence in the vicinity of the well-known centers of action in the atmosphere. it is thus possible that the atmospheric response to the oceanic boundary forcing is modal and can be studied through the modification of its basic regimes and global teleconnection structures. we will then pursue our investigation, no longer in the physical domain, but rather in the «spectral» domain. the anova technique will be used to analyze the time coefficients of the dominant modes. a natural decomposition of the atmospheric variability is to use the empirical orthogonal function (eof). the calculation is done in the same manner as in barnston and livezey (1987) through a rotated eof analysis for the northern hemisphere 500-hpa geopotential height. we first perform the analysis for each of the three experiments in a separate way. the obtained principal modes of variability are very close among the experiments and comparable to those deduced from observed data, but their relative importance in explaining the total variance is different. in order to have a common comparison base, we performed the eof analysis for the super-ensemble containing all members of the three experiments, one after another. the ncep reanalysis data were also added to this superensemble as an additional member. the superensemble thus contained 36 (= 17 + 9 + 9 +1) members. in this way, we obtained common coherent spatial patterns x k(s) (where k indicates the eof rank), but different time series, α k i (t), associated with each member i of the superensemble. we studed only the first k = 6 eofs which represent near 70% of the total variance of the super-ensemble. the physical field x i (t, s) for each member i (including the ncep reanalysis data) was thus decomposed into two parts (4.1) our objective was not to extract the most skillful or most reproducible modes, as can be done with svd (singular-value decomposition) analysis applied simultaneously to the simulation and observation, but rather to show the influence of oceanic conditions on the natural atmospheric dominant modes. the combined eof technique extracts thus the common dominant spatial patterns x k of the super-ensemble. the time series α i k(t) indicates the temporal of the corresponding spatial structure x k(s) for each simulation i (including the ncep reanalysis data set). the projection (noted as y i k) of a particular member x i (t, s) on its own time series α i k(t) illustrates the manifestation of the common spatial structure x k(s) on this particular member . (4.2) figure 6a-f shows the common eof x k, while fig. 7a-f presents the particular projection map y k for the ncep reanalysis data set. we also x t s t s i i k k k , .( ) = ( ) ( ) = ∑α x k 1 y i k i k i t t s t t x t s( ) = ( ) ( ) = ∑1 1 α , 66 sébastien conil and zhao x. li fig. 6a-f. common spatial structures obtained through the super-ensemble rotated eof analysis for djf-mean 500-hpa geopotential height in the northern hemisphere. the explained variance is respectively 16.2% (a); 13.4% (b); 12.3% (c); 10.7% (d); 9.5% (e), and 7.5% (f ). total variance is 302 m2s–2. units are in meters. a b c d e f 67 influence of the north atlantic on simulated atmospheric variability fig. 7a-f. projection of the common eof structures (shown in fig. 6a-f ) onto the ncep reanalysis data set. units are in meters. a b c d e f 68 sébastien conil and zhao x. li performed eof analysis separately for each member of the super-ensemble. the obtained spatial structures (not shown) are very similar to each other and close to the common structures in fig. 6a-f. figure 6a-f shows slight geographical displacements or distortion of the simulated structures compared to the observed ones (fig. 7a-f). such discrepancies can be related to the systematic error of the background climatological flow (kushnir et al., 2002). they are mainly due to the intrinsic atmospheric processes but they also mark responses to oceanic forcing (see peng and robinson, 2001). except for some systematic errors, the model is, in general, able to simulate correctly the main observed global teleconnection structures (with spatial shifts, in many cases). the dominant mode (accounting for 16.2% of the total variance) is the annular mode with a strong zonally symmetric pattern, looking like the ao pattern (thompson and wallace, 1998). the second mode (accounting for 13.4% of the total variance) has a strong center in the north atlantic, near the european coast. this center is surrounded by a circle with opposite-sign values. a minor center with the same sign as in the north atlantic can be observed in the north pacific. this pattern (see fig. 7a-f) is similar to the observed east atlantic pattern defined by barnston and livezey (1987). the third mode (12.2%) resembles to the well-known pna pattern. the fourth mode (10.7%) has a strong wave-train pattern linking the north pacific and north atlantic through a cross-pole path. it corresponds to the observed west pacific pattern. the fifth mode (9.5%) has a weak relation with the observed pattern. it is mainly present in the noatl experiment and has a large regional difference in global (no pacific center) and is absent from atl. the last mode that we considered accounts for 7.5% of the total variance. it is close to the observed nao pattern (hurrel and van loon, 1997). after this super-ensemble eof analysis, we then applied the anova technique to the reduced data sets, i.e., the time coefficients for the first six dominant modes. results are given in tables iii, iv and v for the three experiments respectively. for the experiment atl, potential predictability was weak for all the modes, since the oceanic forcing itself was very weak. however the first and third modes (annular mode and pna pattern) reached almost 9%. for both the experiments noatl and global, the third mode has the largest potential predictability. however, the first mode (ao) and the sixth mode (nao) have the smallest potential predictability. the influence exerted by the north atlantic ocean can be assessed through the comparison between noatl and global. further comparison with atl can reveal the non-linear interaction with other oceanic basins. let us take the first mode (ao), the experiment atl shows that the incorporation of the north atlantic oceanic variability leads to a potential predictability of 8.2%. for the noatl and global experiments, the potential predictability is almost table iii. results from anova technique (internal variance, external variance and potential predictability) for different dominant atmospheric modes and for the experiment atl. the 99% significance level is reached when potential predictability is 6.34%. mode number 1 2 3 4 5 6 internal variability (m) 110 98 65 92 76 77 external variability (m) 33 10 21 0 15 11 pot. predictability (%) 8.2 1 9.6 0 4 1.8 table iv. same as in table iii, but for the experiment noatl. the 99% significance level is reached when potential predictability is 6.34%. mode number 1 2 3 4 5 6 internal variability (m) 111 100 63 81 83 78 external variability (m) 54 56 104 57 56 28 pot. predictability (%) 19.4 24.1 73.4 33.6 31 11.5 table v. same as in table iii, but for the experiment global. the 99% significance level is reached when potential predictability is 6.16%. mode number 1 2 3 4 5 6 internal variability (m) 113 99 66 86 79 78 external variability (m) 56 61 94 59 59 31 pot. predictability (%) 19.6 27.7 66.7 32.0 36.0 13.8 α i k t( ) 69 influence of the north atlantic on simulated atmospheric variability identical, showing no contribution of the north atlantic oceanic variability. the non-linear effects through the interaction with other oceanic basins totally cancel the direct effect. this destruction effect is even stronger for the third mode (pna) since we observe a significant decrease for the potential predictability from noatl to global. the inverse situation occured for modes 5 and 6 with an increase in the potential predictability from noatl to global, showing a positive contribution of the north atlantic ocean. 5. summary and conclusions by using an atmospheric general circulation model, we studied the atmospheric response to varying oceanic boundary conditions. the simulations cover the period from 1950 to 1994. three ensemble simulations were performed with different configurations of the oceanic forcing to isolate the north atlantic ocean’s role. we used a standard anova technique to separate internal and external variability from total variability. the first ensemble global was forced by the global observed oceanic surface conditions (sst and sea ice). the second ensemble noatl was forced by the observed varying boundary conditions for the global ocean except for the north atlantic where climatological conditions were used. the third experiment atl is a complementary one with observed varying boundary conditions for the north atlantic and climatological ones elsewhere. climate response to the north atlantic forcing was first assessed through the atl experiment. the direct forcing of the north atlantic ocean is weak. it mainly leads to a reduction of the internal variability and thus an increase in the potential predictability over the north atlantic. the dominant annular mode is significantly affected by the north atlantic forcing and is the major cause of this modification. the north atlantic oceanic forcing creates a weak external signal in the north pacific region but the internal variability also increases. the external variability is associated with the pna pattern while the internal variability seems to be associated with the west pacific pattern. the role of the north atlantic in conjunction with other basins is also assessed through the differences between the global and noatl experiments. the potential predictability over the north atlantic sector is slightly enhanced. it is caused by the eastern atlantic pattern and the mode 5 but not by the ao pattern. over the north pacific and north america sector, internal variability is largely increased when the north atlantic ocean forcing is added. moreover the external signal is significantly reduced and this leads to a decrease of the potential predictability. the reduced potential predictability is explained essentially by the modification of the pna and wp modes. the variability created by the north atlantic ocean can be transmitted to the north pacific region. furthermore, it interacts with the tropical pacific forcing, leading to an increase in the random variability. our results suggest that the north atlantic ocean can influence the atmosphere in two different ways. it can drive an external signal which is potentially predictable but only weakly. it can also modify the internal variability. when those two modifications act together (reducing chaotic and increasing forced signal) the resulted potential predictability can become significant. but the influence of the north atlantic is not linear. it interacts with forcings from other oceanic basins. this interaction can lead to a large modulation of the direct signal. in particular, over the north pacific sector, the interaction of the direct north atlantic signal with other oceanic responses (mostly from equatorial pacific) is destructive for the potential predictability, since it increases the chaotic fluctuations while it reduces the predictable forced response. so the inclusion of the north atlantic forcing in conjunction with the tropical pacific reduces the potential predictability in the north pacific. the potential predictability in the north atlantic europe region is, however, slightly increased. acknowledgements computer resources were allocated by the idris, the cnrs’ computer center. this work is supported by the european commission through the sintex project. 70 sébastien conil and zhao x. li references barnston, a.g. and r.e. livezey (1987): classification, seasonality and persistence of low frequency atmospheric circulation patterns, mon. weather. rev., 115, 1083-1026. cassou, c. and l. terray (2001): oceanic forcing of the wintertime low frequency atmospheric variability in the north atlantic europe sector: a study with the arpege model, j. climate, 14, 4266-4291. collins, m. and m.r. allen (2002): assessing the relatives roles of initial and boundary conditions in interannual to decadal climate predictability, j. climate , 15 (21), 3104-3109. feldstein, s.b. 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(1985): sea surface temperature anomalies, planetary waves and air sea feedbacks in the middle latitudes, rev. geophys., 23, 357-390. harzallah, a. and r. sadourny (1995): internal versus sst forced atmospheric variability as simulated by an atmospheric general circulation model, j. climate, 8, 474-495. hurrel, j.w. and h. van loon (1997): decadal variations in climate associated with the north atlantic oscillation, climatic change, 36, 301-326. kumar, a. and m.p. hoerling (1998): annual cycle of the pacific-north american seasonal predictability associated with different phases of enso, j. climate, 11, 3295-3309. kushnir, y., w.a. robinson, i. bladè, n.m.j. halla, s. peng and r. sutton (2002): atmospheric gcm response to extratropical sst anomalies: synthesis and evaluation, j. climate, 15 (16), 2233-2256. latif, m., k. arpe and e. roeckner (2000): oceanic control of decadal north atlantic sea level pressure variability in winter, geophys. res. lett., 27, 727-730. le treut, h. and z.x. li (1991): sensitivity of an atmospheric general circulation model to prescribed sst changes: feedback effects associated with the simulation if cloud properties, climate dynamics, 5, 175-187. li, z.x. 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res., 105, 7295-7315. annals 47, 1, 2004, 01/07def 229 annals of geophysics, vol. 47, n. 1, february 2004 key words electromagnetic emission – temperature – earth surface – impedance – earthquake precursor – 1 hz frequency range new electromagnetic emission termed «thermal geomagnetic emission» has been described elsewhere (gokhberg, 1998). this emission occurs during the daytime and looks like broad band noise in the frequency range 1-5 hz. the noise is supposed to be related to a thermal regime of the near earth surface atmospheric layer. the example of the so-called type 1 emission is shown in fig. 1a. during the local evening hours and night time the emission occurs with rising frequency noise bands, which look like a «fan». this type of emission (that we called as type 2) has been described in several papers (larsen and egeland, 1968; beamish and tzanis, 1986; belyaev et al., 1987) and explained using the theory of the ionospheric alfven resonator (see fig. 1b). diurnal and seasonal variations of intensity for both types are represented in fig. 2. one can see that the diurnal maximum of «thermal emission» (type 1, dark area) take place in between 12-14 ut or 15-16 lt for borok observatory (58.03°n, 39.97°e) and for «fan» (type 2, light area) between 16-18 ut or 19-21 lt. the seasonal distribution for type 1 emission has one maximum during the summer time while for type 2 emission two maxima take place during the spring and autumn. shown in fig. 3a are the thermal emission intensity (black vertical line) for each day during july 1998 and daily temperature variations measured at borok observatory. one can see a good correlation between the sharp gradient of the temperature decrease and deep decrease of the emission intensity starting from july 12 to 13. similar features are seen in fig. 3b with intensity and temperature variations measured at mondy observatory (52.2°n, 104.5°e) during august 1997. for both observatories the general features of such an emission are similar if we take into account the shift of about 5 h in ut relative to their longitudinal position. the analysis of the impedance z = e /h was produced for close to simultaneous records of pc-1 and «thermal emission». the pc-1 impedbackground electromagnetic noise in the vicinity 1 hz as a possible indicator of earthquake-related anomalies mikhail b. gokhberg schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia abstract measurements of space-time distributions of the background emissions in the frequency range 0.5-5 hz and their seasonal and diurnal variations are presented. the analysis of impedance ratio determines the presence of high space harmonics in the emission spectral distribution. the absence of this emission during the winter time correlates with the absence of any precursors before local earthquakes with m ∼ 6. it is shown that the knowledge of the background emission behavior and its origin are very important for precursor phenomena monitoring. mailing address: dr. mikhail b. gokhberg, schmidt united institute of physics of the earth, russian academy of sciences, ul. bolshaya gruzinskaya 10, d-242, gsp-5, 123995 moscow, russia; e-mail: gmb@ifz.ru 230 mikhail b. gokhberg ance can be described by the plane wave impedance z = − iωµ / k, where ω = 2π f, µ = 4π10−7 g/m, i~nvk = , where σ is the conductivity of the earth crust upper layer. in general cases, the «thermal emission» impedance can be described as z * = − iωµ / k* where k2* = k2 + n2 and n is the space harmonic number. if we know σ and ω we can determine n which is related to the source dimension and the distance from it. the 3d fig. 4 shows the dependence of n−1 number on frequency ω and ratio z /z* for σ = = const. one can see that for the 20% ratio z/z*increase the value 1/n (meter) is about several hundred meters if σ ≈ 1 s/m (rokitianski, 1962) which is roughly related to borok observatory conditions. the experimental statistical value for borok observatory is z/z* (1.2-1.4). it means that due to global temperature modulation of the emission intensity, the space distribution is less than one kilometer and is related to the local observation condition and local sources. the same results were obtained by gradient measurement on the distance less than one hundred meters for the local geomagnetic variation network at borok. using this experimental data one can conclude that type 1 emission correlates with the temperature regime and probably relates to the parametric instability of the near earth surface local atmospheric layer. fig. 1a,b. frequency versus time presentation of hy, hx magnetic components during (a) daytime (thermal or type 1 emission) and (b) nighttime (fan or type 2 emission) conditions. a b 231 background electromagnetic noise in the vicinity 1 hz as a possible indicator of earthquake-related anomalies fig. 2. seasonal and diurnal intensity variations for type1 (thermal) and type 2 (fan) emissions. vertical scale shows the product of events number (n) by intensity of thermal emission (i), horizontal scale shows ut (in hours). for type 2 emission the absence of such a structure in electric field component demonstrates still higher differences z * from plane wave. moreover the presence of the structure in vertical z magnetic component (as in the fig. 5) also shows the presence of a source nearby and strongly complicates the explanation from the far zone ionospheric source. 232 mikhail b. gokhberg fig. 3a,b. intensity (i) and temperature (t) variations (a) at borok measured during july, 1998 and (b) at mondy during august, 1997. a b fig. 4. dependence of wave number (n) on impedance ratio and frequency (z is the impedance of plane wave for pc-1 emission and z * is the impedance of thermal emission). 233 background electromagnetic noise in the vicinity 1 hz as a possible indicator of earthquake-related anomalies fig. 5. evidence of occurrence of the essential z component in fan emission that indicates the near source presence. fig. 6. evidence of absence of the thermal emission during winter time (24-25 february, 1999) observed at mondy observatory before the moment of a local earthquake (shown by the vertical solid line). note that the wavelength for borok earth crust conductivity condition is about several kilometers. an increase in the noise electromagnetic emission intensity around 1 hz before local earthquakes have been reported by wolfgang boerner (chicago university) and jack dea (san diego, navy) (private communications). space and time distributions of the natural background noise level in the frequency range 1-5 hz together with the impedance ratio algorithm which are represented here probably help to discriminate the anomaly related to the local earthquake preparation processes. for example, in fig. 6 the absence of this emission during winter time for siberia does not determine such a precursor anomaly before earthquake with m∼ ∼ 6 (52°n, 105°e) on 25.02.1999 18:58 ut. one can see only the pulsations of the magnetospheric origin. another example of the thermal emission during the summer time is presented in fig. 7. the emission cannot be considered a precursor before the local earthquake with m ∼ 5.4 (the time of the earthquake is shown by black solid line) that occurs in the vicinity of the observation point, because it is the usual background level. these two examples show that we have to be very careful before making any conclusions about precursor occurrence. in addition, it is important to investigate the behavior of the background level. we still hope to use type 1 emission for an earthquake preparation monitoring. an increase in the noise electromagnetic emission intensity around 1 hz before local earthquakes detected by w. boerner (chicago university) and j. dea (san diego, navy) in southern california can be related to the local near surface atmospheric layer heating (see gorny et al., 1988). thus the question is whether it is easier to measure anomalies in the temperature gradient directly or to detect the emission. 234 mikhail b. gokhberg fig. 7. typical diurnal distribution of the thermal emission before the moment of a local earthquake (shown by the vertical solid line) possibly not related with earthquake preparation. 235 background electromagnetic noise in the vicinity 1 hz as a possible indicator of earthquake-related anomalies references beamish, d. and a. tzanis (1986): high resolution spectral characteristics of the earth-ionosphere cavity resonances, j. atmos. terr. phys., 48 (2), 187-205. belyaev, p.p., s.v. polyakov, v.o. rapoport and v.i. trachtengerz (1987): estimation of the atmosphere electromagnetic noice in the short frequency range of geomagnetic variation, dokl. akad. nauk sssr, 297 (4), 840-843 (in russian). gokhberg, m.b. (1998): new type of electromagnetic emission in the range of short-period geomagnetic pulsations, dokl. acad. nauk sssr, 359 (4), 543-544 (in russian). gorny, v.i., a.g. salman, a.a. tronin and b.b. shilin (1988): outgoing infrared earth radiation as the indicator of the seismic activity., dokl. akad. nauk sssr, 301 (1), 67-69 (in russian). larsen, t.r. and a. egeland (1968): fine structure of the earth-ionosphere cavity resonances, j. geophys. res., 77 (15), 4986-4989. rokitianski, i.i. (1962): magneto-telluric curve on borok observation data, izv. akad nauk sssr, geophys., 5, 679-680. 331_340 hamill.pdf 331 annals of geophysics, vol. 46, n. 2, april 2003 an empirical approach to the nucleation of sulfuric acid droplets in the atmosphere patrick hamill (1), raffaella d’auria (2) and richard p. turco (2) (1) department of physics, san josé state university, u.s.a. (2) department of atmospheric sciences, university of california, los angeles, u.s.a. abstract we use quantum mechanical evaluations of the gibbs free energy of the hydrates of sulfuric acid, h2so4 . nh2o and (h2so4)2 . nh2o to evaluate an empirical surface tension for sulfuric acid-water clusters containing few molecules. we use this surface tension to evaluate nucleation rates using classical heteromolecular theory. at low temperatures (t � 213 k) the nucleation rates obtained with the empirical surface tensions are signifi cantly greater than those using bulk values of the surface tension. at higher temperatures the difference disappears. mailing address: prof. patrick hamill, physics department, san josé state university, san josé, ca 95192, u.s.a.; e-mail: hamill@wind.sjsu.edu key words nucleation gibbs energy aerosol microphysics 1. introduction the formation rate of sulfate particles in the atmosphere is usually evaluated using the «classical» theory of homogeneous, heteromolecular nucleation. this is based on the theory of the homogeneous nucleation of a single substance (usually water) as developed by volmer and weber (1926), becker and doring (1935), frenkel (1946), and others. an overview of the theory was presented by mcdonald (1962, 1963). the experimental verifi cation of the theory was based on determining the value of the supersaturation when particle formation was observed to occur. for pure water, the theory agrees reasonably well with experiment, and for most substances the predicted values of super-saturation are typically accurate to within 10%, but for other substances the theoretical predictions and experimental results differ signifi cantly. techniques that have been developed to actually count the number of particles formed in a nucleation process indicate that the dependence of nucleation rate on supersaturation is approximately correctly given by the classical theory but the temperature dependence is not (oxtoby, 1992). the predicted nucleation rates are typically too low at low temperatures and too high at high temperatures, disagreeing with experimental values by several orders of magnitude. the classical theory (as described below in section 2) depends on the change in the gibbs free energy of an embryonic droplet as molecules are added to it. the gibbs free energy depends on the surface tension, a quantity that is poorly defi ned for clusters of a few molecules. in this paper in section 2 we give a brief review of the classical theory of heteromolecular nucleation, particularly as applied to the formation of sulfate particles in the atmosphere. as mentioned below, this theory severely underestimates observed environmental nucleation rates. in section 3 we consider the quantum mechanical calculations that have been carried out 332 patrick hamill, raffaella d’auria and richard p. turco by bandy and ianni (1998) and ianni and bandy (2000) which allow one to determine the free energy increase of a hydrate of sulfuric acid as water molecules are added to it. in section 4 we combine the classical theory with the quantum mechanical results by introducing an empirical surface tension term such that the gibbs free energy predicted by the classical theory roughly agrees with the value obtained from the quantum mechanical calculations. we then evaluate nucleation rates for sulfate particles using the empirical (size, composition and temperature dependent) surface tensions to determine the effect on nucleation rates. in section 5 we present our conclusions. 2. the classical theory of homogeneous nucleation 2.1. nucleation of a single substance from the vapor it is convenient to begin by considering the homogeneous nucleation of a pure substance. the classical theory is based on the difference in the gibbs free energy of n free molecules of the condensing substance, and the gibbs free energy of these molecules when they are incorporated into a droplet. when the molecules are in the droplet, they are assumed to be in the liquid phase, meaning that the gibbs free energy has been reduced by a quantity – nln(s) where s = p /p0 is the supersaturation. here p is the partial pressure of the vapor and p0 is the equilibrium vapor pressure of the liquid. the molecules that form a droplet embryo have not only undergone a phase change from vapor to liquid, they have also formed a sphere with a surface of area of a = 4pr2. the energy associated with the construction of this surface is sa where s is the surface tension. thus the change in gibbs free energy when n molecules are transformed from free vapor molecules to molecules in the liquid state in a droplet of area a is dg = – nkt lns + sa (2.1) where k is boltzmann’s constant and t is the temperature. if the density of the liquid is r this expression can be written in terms of the radius of the droplet r as follows: (2.2) where r is the gas constant per gram of vapor. for s > 1 the function dg passes through a maximum dg* = 16ps3 / 3(rrt lns)2 (2.3) at the «critical» embryo radius r * that satisfi es the kelvin equation r * = 2s/rrt lns. (2.4) the surface energy term in eq. (2.2) increases with r 2 while the bulk energy term grows more negative as r 3. in general a plot of dg versus r shows an increase to a maximum dg* at r * and then a decrease to negative values. note that r is not a continuous variable because the particle grows by incorporating integral numbers of molecules. the functional form of eq. (2.2) implies that dg increases to a positive maximum before decreasing to negative values but since n can only take on integer values (see eq. (2.1)), if the supersaturation s is large enough dg may not exhibit a maximum at all. in the classical theory one assumes that the vapor will contain clusters of molecules (embryos) which satisfy a boltzmann relation (2.5) where n g is the number of clusters containing g molecules. the «critical» cluster contains g* molecules. note that for a cluster with g < g* the addition of one more molecule implies an increase in dg, but for a cluster with g > g* the addition of one more molecule leads to a decrease in dg, and the embryo grows spontaneously. the nucleation rate can be determined by evaluating the number of critically sized clusters and the net rate at which molecules are incorporated into them. one usually defi nes a «current» j g which is the net rate at which g-mers n n eg g kt= −1 ∆ / ∆g r rt s r= − + 4 3 43 2π ρ π σln 333 an empirical approach to the nucleation of sulfuric acid droplets in the atmosphere are formed by j g = c g n g – e g+1ng+1 (2.6) where c g is the condensation rate of single molecules from g-mers and e g+1 is the evaporation rate from (g+1)-mers. it can be shown (mcdonald, 1963, for example) that given several reasonable assumptions (including the assumption of a steady state situation so that j g is a constant independent of g), this current or nucleation rate, can be expressed as . (2.7) here n is the number density of molecules in the vapor, related to the pressure by p = nkt. considering the form of dg* given by eq. (2.3), we appreciate the importance of the surface tension in the nucleation rate calculation as j depends on the surface tension cubed in an exponential. 2.2. heteromolecular nucleation by heteromolecular nucleation we mean the nucleation of liquid drops from a vapor containing two or more substances. we shall limit ourselves to the case of binary system nucleation of solution droplets of sulfuric acid and water. it is well known that solution droplets can form under conditions in which neither one of the pure substances could form. this is because the vapor pressure of a component in a binary solution is less than the vapor pressure of the pure substance. heteromolecular nucleation (sometimes referred to as «binary nucleation» since there are usually only two components participating in the process) was studied by doyle (1961), reiss (1950), heist and reiss (1974), kiang and stauffer (1973), jaecker-voirol and mirabel (1989), hamill et al. (1977) and many others. this is a straightforward generalization of the homogeneous «monomolecular» nucleation theory described above. however, it should be mentioned that the theory originally developed by doyle (1961) and others suffers from a thermodynamic inconsistency in the evaluation of the critical value of the gibbs free energy increase. this has led to a «corrected» theory by wilemski (1984) and others. we shall consider nucleation theory as applied to the formation of a solution droplet with two components labelled a (acid) and w (water). in this case the gibbs free energy change is given by dg = – n a kt lns a – n w kt lns w + 4pr 2 s (2.8) where s p pa a a= 0 is the supersaturation of component a, defined as the partial pressure of component a in the environment divided by the equilibrium vapor pressure of component a over a fl at surface having the same composition as the solution, and similarly for s w . the nucleation rate is then given by j = 4pr *2b a n w exp(– dg*/kt) (2.9) where n w is the number density of w molecules in the environment and b a is the rate at which a molecules impinge upon the embryonic droplet of radius r. in the case of a binary system, the gibbs free energy is a function of n a and n b and consequently we must consider a gibbs free energy surface. the critical values (r * and dg*) correspond to a saddle point on this surface. that is, the dg surface can be envisioned as a mountain range with a pass through it. the highest point in the pass is the critical value. however, at high enough values of supersaturation the gibbs free energy surface will have no barrier to nucleation. in fi g. 1 we present the gibbs free energy surface using bulk values of surface tension and density. this fi gure illustrates the dg surface for the number of acid molecules ranging up to fi ve and the number of water molecules ranging up to ten. the critical size is at about 3 acid molecules and 6 water molecules. the calculation was carried out assuming a temperature of 240 k, an environmental pressure of 125 mb, a relative humidity of 50% and a sulfuric acid concentration of 5 ppt, or about 1.9 ¥ 107 molecules/cm3. this concentration, while common in the lower troposphere, represents an extreme value in upper tropospheric conditions (e.g., clarke et al., 1999). for these conditions the nucleation rate predicted j n m e g kt= ( )( ) − ∗2 2ρ σ π ∆ / 334 patrick hamill, raffaella d’auria and richard p. turco by eq. (2.9) is 1013 droplets/cm3 s, which is, of course, unreasonably high since it predicts that the number of droplets formed per second is six orders of magnitude larger than the total number of available sulfuric acid molecules. if, however, we include the effect of hydrates as suggested by heist and reiss (1974) and jaecker-voirol et al. (1987) the nucleation rate for these conditions drops to 4.6 droplets/cm3 s. the predicted saddle point is at 4 sulfuric acid molecules and 13 water molecules. the results of calculations such as this agree relatively well with those presented by laaksonen and kulmala (1991). those investigators present a table of results including the number of water and acid molecules in a critical cluster and in the majority of the cases given the values of n a and n b are less than ten. our nucleation rates differ from theirs by a factor of about 10 due to the fact that our expression for the prefactor to the exponential does not take into account the effect of hydrates. it is clear, then that the theory of heteromolecular nucleation for sulfuric acid water solution droplets involves very small clusters of just a few molecules. using macroscopic concepts of density and surface tension require extrapolations which are very hard to justify. a particularly serious problem with the theory is that it does not predict the formation of new particles under conditions in which new particle formation is observed to occur (o’dowd, 2001; kulmala et al., 1998). however, clarke et al. (1999) found that particle formation in the upper troposphere is often in agreement with predictions of classical binary nucleation theory if the effect of hydrates is ignored. 3. quantum mechanical calculations recently, bandy and ianni (1998) and ianni and bandy (2000) carried out density functional theory quantum mechanical calculations of the 1 2 3 4 5 0 2 4 6 8 10 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 x 10 12 numb er of acid mole culesm number of water olecules d g ( e rg s) fig. 1. the dg surface as a function of the number of sulfuric acid molecules and the number of water molecules in the cluster, assuming classical theory. the environmental conditions assumed in the calculation are a temperature of 240 k, an environmental pressure of 125 mb, a relative humidity of 50% and a sulfuric acid concentration of 5 ppt. the critical size is the saddle point in this «pass» through the dg barrier. it is located at n a = 3 and n w = 6. 335 an empirical approach to the nucleation of sulfuric acid droplets in the atmosphere hydrates of sulfuric acid. they evaluated the gibbs free energy increase in the reactions h2so4 (n – 1)h2o + h2o æ h2so4 nh2o (3.1) and (h2so4)2 (n – 1)h2o + h2o æ (h2so4)2 nh2o . (3.2) bandy and ianni (1998) considered reaction (3.1) using the «gaussian 94» program for their calculations. an interesting result of the study was the fact that although the bulk sulfuric acid-water solution is highly ionized, the clusters of a few molecules did not exhibit ionization until n = 7. the motivation for the bandy and ianni (1998) study was the fact that the hydrates of sulfuric acid are thought to play an important role in the formation of new atmospheric particles. they point out that previous studies have assumed that the properties of small clusters of sulfuric acid and water could be obtained by extrapolating the properties of the bulk phase. they found that for a temperature of 298 k, reaction (3.1) has gibbs free energies that are slightly negative for n = 1-3 and slightly positive for n = 4-7 with a maximum at n = 5. for lower temperatures, the gibbs free energies decrease. they computed equilibrium constants and found that the values estimated by jaecker-voirol et al. (1987) were orders of magnitude larger than predicted by the density functional theory results. bandy and ianni (1998) suggest that hydrates play a much smaller role in homogeneous nucleation theory than previously believed. it has been supposed that the clustering of water molecules around sulfuric acid molecules would greatly reduce the number of free water and sulfuric acid molecules in the environment. therefore, in this paper we have not included hydrates as a constituent of the environmental vapor in any of our nucleation calculations. ianni and bandy (2000) carried out a study of the hydrates of (h2so4)2. they found that all the higher hydrates of (h2so4)2 linearly decrease the free energy of the system. they developed a kinetic model of the growth of the hydrates and concluded that although (h2so4)2 nh2o with n = 0-6 will spontaneously form from h2so4 h2o it seems to be kinetically and thermodynamically prohibited. they suggest that perhaps a third species such as ammonia or the formation of an excited state is required to stabilize the system and allow for larger hydrate chains to grow. in our analysis we have ignored these problems, and perhaps future quantum mechanical studies will answer some of these questions. 4. evaluation of empirical surface tensions and nucleation rates in view of the fact that the surface tension is the most important parameter determining the gibbs free energy increase and hence the nucleation rate, we used the studies of bandy and ianni (1998) to obtain empirical values of surface tension for very small droplets. for reasons mentioned below, we believe that the bulk value of surface tension is valid to very small droplets, but fails for clusters containing less than about seven molecules. this would not be important if it were not for the fact that the critical size of sulfuric acid-water solution droplets is extremely small, as mentioned in section 1. the reason why we believe that the bulk surface tension is an accurate description of the surface tension of very small droplets is based on a study carried out by d’auria (2001) in which the quantum mechanical evaluations of the gibbs free energy change for water molecules clustered about a hydronium ion were compared with evaluations obtained from the classical theory. figure 2 shows this comparison. note that even for clusters as small as 6 or 7 molecules the classical theory agrees very well with the quantum mechanical simulations. if one were to defi ne a surface tension for the cluster which increases with number of molecules in the cluster, and reaching the bulk value for (say) 7 molecules, then one could fi t the classical theory to the quantum mechanical results. this is what we have done for the binary solution, sulfuric acid and water. of course the system is more complicated, being composed of two substances. furthermore, the quantum mechanical studies are limited to the two pro cesses described by eqs. (3.1) and (3.2). in fi g. 3 we show the quantum mechanically evaluated values for dg, taken from bandy and 336 patrick hamill, raffaella d’auria and richard p. turco 0 1 2 3 4 5 6 7 8 9 10 11 0 5 10 15 20 25 30 number of water ligands per cluster -∆ g ( n -1 ,n ), k c a l/m o l fig. 2. a comparison between the gibbs free energy change associated with the hydronium/water clustering reaction as obtained from the classical model and the quantum mechanical simulations. the solid line is for the classical model and the symbols are the quantum mechanical calculated results (from d’auria, 2001). 0 2 4 6 8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 ∆ g ( kc a l/m o l) 0 2 4 6 8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 ∆ g ( kc a l/m o l) hydrate number n (h 2 so 4 .nh 2 o) hydrate number n (h 2 so 4 ) 2 .nh 2 o) fig. 3. the change in gibbs free energy due to adding a water molecule to h2so4 nh2o (left panel ) and to (h2so4)2 nh2o (right panel ). the top curves (with asterisks) are the quantum mechanical results and the bottom curves are the liquid droplet model results. 337 an empirical approach to the nucleation of sulfuric acid droplets in the atmosphere 0 2 4 6 8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 ∆ g ( kc a l/m o l) 0 2 4 6 8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 ∆ g ( kc a l/m o l) hydrate number n (h 2 so 4 .nh 2 o) hydrate number n (h 2 so 4 ) 2 .nh 2 o) fig. 4. the same as fi g. 3 except that the surface tensions used in the classical calculation have been changed to get a closer agreement with the quantum mechanical results. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 0 10 20 30 40 50 60 70 80 number of acid molecules num ber of w ater mole cule s e m p ir ic a l s u rf a c e t e n si o n d yn e /c m fig. 5. the surface tension that was used to generate the classical curves of fi g. 4, presented in terms of the number of water and sulfuric acid molecules in the embryonic droplet. 338 patrick hamill, raffaella d’auria and richard p. turco ianni (1998) and ianni and bandy (2000), and the values obtained from the classical theory, eq. (2.8). the curves show the change in gibbs free energy due to adding a water molecule to h2so4 nh2o (left panel) and to (h2so4)2 nh2o (right panel). the top curves (with asterisks) are the quantum mechanical results and the bottom curves are the liquid droplet model results. it is not entirely clear from the fi gure at what point the two curves will come together, that is, we cannot predict the size of a cluster at which the classical model agrees well with the quantum mechanical calculation. however, guiding our intuition by the plot in fi g. 2 we shall assume that clusters with about 10 water molecules will be reasonably described by the classical model. the classical curves can be made to approximate the quantum mechanical curves by selecting appropriate choices of surface tension. in fi g. 4 we present the curves of fi g. 3 except table i. empirical surface tension of cluster (dynes/cm2). h2o 1 2 3 4 5 6 7 8 9 10 h2so4 1 0 40.0 57.0 65.0 70.0 75.0 76.2 76.1 76.0 75.9 2 16.0 35.0 57.0 67.0 75.0 76.0 77.0 77.0 76.0 76.1 3 50.0 59.6 64.8 68.5 71.0 72.7 73.9 74.7 75.2 75.5 4 59.6 59.6 62.3 65.8 68.5 70.5 72.1 73.1 73.9 74.5 5 59.6 59.6 60.7 63.8 66.4 68.5 70.1 71.4 72.5 73.3 45 52 58 62 66 69 71 73 75 77 temperature (k) s u rf a c e t e n si o n (d yn e /c m ) 2 200 210 220 230 240 250 260 270 280 290 300 70 75 80 85 90 95 100 45 52 58 62 66 69 71 73 75 77 fig. 6. surface tension as a function of temperature for a bulk sulfuric acid water solution. the numbers on each curve represent the weight percentage sulfuric acid. 339 an empirical approach to the nucleation of sulfuric acid droplets in the atmosphere 10 4 10 6 10 8 10 0 10 10 sulfuric acid concentration (molecules/cm3) n u cl e a tio n r a te ( p a rt ic le s/ cm 3 s ) t =223.15k emp. σ bulk σ 10 4 10 6 10 8 10 0 10 10 sulfuric acid concentration (molecules/cm3) n u cl e a tio n r a te ( p a rt ic le s/ cm 3 s ) t =213.15k emp. σ bulk σ 10 4 10 6 10 8 10 0 10 10 sulfuric acid concentration (molecules/cm3) n u cl e a tio n r a te ( p a rt ic le s/ cm 3 s ) t =203.15k emp. σ bulk σ 10 4 10 6 10 8 10 0 10 10 fig. 7. nucleation rate versus sulfuric acid concentration using the empirically obtained surface tension compared to the bulk surface tension for various values of the temperature. in all cases the water concentration was assumed to be 1013 molecules/cm3. that in evaluating the classical gibbs free energy difference we used surface tension values that brought these curves closer to the quantum mechanical values. we could, of course, have fi t the quantum results exactly, but then the surface tension would not have been a smoothly varying function. the surface tension used in fi g. 4 is presented as a function of composition in fi g. 5. the surface tensions used are presented in table i as a function of the number of sulfuric acid molecules and the number of water molecules in the cluster. in fi g. 6 we show how the surface tension of a bulk solution of sulfuric acid and water varies as a function of temperature and composition. the numbers on each curve are the weight percentage of sulfuric acid in the solution. in our calculations of the nucleation rate using the extrapolations of the surface tensions of table i and fi g. 5 we assumed the same temperature dependence as in fi g. 6. we fi nd, as would be expected, that decreasing the surface tension increases the nucleation rate. in fi g. 7 we illustrate the increase in nucleation rates obtained by using the empirical surface tensions rather than the bulk values. the fi gure presents nucleation rates as a function of sulfuric acid concentration using the empirically obtained surface tension and using the tension compared to the bulk surface tension for various values of the temperature. in all cases the water concentration was assumed to be 1013 molecules/cm3. at the highest temperature (223.15 k), the nucleation rates are low and the change in the surface tension makes essentially no difference, except for the highest sulfuric acid concentrations (107 molecules/cm3). however, for lower temperatures the difference in nucleation rates is quite sig340 patrick hamill, raffaella d’auria and richard p. turco nifi cant. for example, at 203 k (a temperature often experienced near the tropical tropopause) and a sulfuric acid concentration of 10 6 sulfuric acid molecules/cm3, the difference in nucleation rates is about fi ve orders of magnitude. 5. conclusions we have shown that quantum mechanical calculations allow one to defi ne an empirical surface tension for sulfuric acid water calculations. we see that these empirical surface tensions lead to greatly enhanced nucleation rates at lower temperatures. our analysis is based on the two studies of the sulfuric acid-water system carried out by bandy and ianni (1998) and ianni and bandy (2000). these studies involved reactions (3.1) and (3.2) and therefore only include clusters whose compositions are h2so4 nh2o and (h2so4)2 nh2o. therefore, we cannot generate empirical surface tensions for all the compositions that are included in the dg surface illustrated in fi g. 1. consequently, our results are incomplete. for this reason we have not attempted to refi ne our method of determining the empirical surface tensions; such work will be carried out when the quantum mechanical results are available. our main conclusion is, then, that cluster surface tensions can be defi ned and can be used to calculated nucleation rates. the nucleation rates obtained are higher than predicted by using the bulk values of surface tension. acknowledgements this work has been funded by the nsf under grant atm-00-70847, and nasa under grant nag1-1899. rd is also supported by nasa earth system science fellowship ess/ 00-0000-0080. references bandy, a.r. and j.c. ianni (1998): study of the hydrates of h2so4 using density functional theory, j. phys. chem. a, 102, 6533-6539. becker, r. and w. doring (1935): kinetische behandlung der keimbildung in übersättigten dampfern, ann. phys., 24, 719-752. clarke, a.d., v.n. kapustin, f.l. eisele, r.j. weber and p.h. mcmurry (1999): particle production near marine clouds: sulfuric acid and predictions from classical binary nucleation, geophys. res. lett., 26, 2425-2428. d’auria, r. (2001): a study of ionic clusters in the middle and lower atmosphere and their role in aerosol formation, a thesis topic proposal (unpublished ms.). doyle, g.j. (1961): self-nucleation in the sulfuric acid-water system, j. chem. phys., 35, 795-799. frenkel, j. (1946): the kinetic theory of liquids (clarendon press, oxford), pp. 488. hamill, p., c.s. kiang and r.d. cadle (1977): the nucleation of h2so4-h2o solution aerosol particles in the stratosphere, j. atmos. sci., 34, 150-162. heist, r.h. and h. reiss (1974): hydrates in supersaturated binary sulfuric acid-water vapor, j. chem. phys., 61, 573-581. ianni, j.c. and a.r. bandy (2000): a theoretical study of the hydrates of (h2so4)2 and its implications for the formation of new atmospheric particles, j. mol. struct. theochem, 497, 19-37. jaecker-voirol, a. and p. mirabel (1989): heteromolecular nucleation in the sulfuric acid water system, atmos. environ., 23, 2053-2057. jaecker-voirol, a., p. mirabel and h. reiss (1987): hydrates in supersaturated binary sulfuric-acid water vapor: a reexamination, j. chem. phys., 87, 4849-4852. kiang, c.s. and d. stauffer (1973 ): chemical nucleation theory for various humidities and pollutants, faraday sym., 7, 26-33. kulmala, m., a. laaksonen and l. pirjola (1998): parameterizations for sulfuric acid/water nucleation rates, j. geophys. res., 103, 8301-8307. laaksonen, a. and m. kulmala (1991): homogeneous heteromolecular nucleation of sulphuric acid and water vapours in stratospheric conditions: a theoretical study of the effect of hydrate interaction, j. aerosol sci., 22, 779-787. mcdonald, j.e. (1962): homogeneous nucleation of vapor condensation. i. thermodynamic aspects, am. j. phys., 30, 870-877. mcdonald, j.e. (1963): homogeneous nucleation of vapor condensation. ii. kinetic aspects, am. j. phys., 31,31-41. o’dowd, c. (2001): advances in elucidating new particle formation in the marine and coastal boundary layers, in european geophysical society 26th general assembly, nice, 2001, geophys. res. abstr., 3, 4794. oxtoby, d.w. (1992): homogeneous nucleation: theory and experiment, j. phys. condensed matter, 4, 7627-7650. reiss, h. (1950): the kinetics of phase transitions in binary systems, j. chem. phys, 18, 840-848. vo l m e r, m. and a. we b e r (1926): keimbildung in übersättigten gebilden, z. phys. chem., 119, 277-301. wilemski, b. (1984): composition of the critical nucleus in multicomponent vapor nucleation, j. chem. phys., 80, 1370-1372. layout 6 1 annals of geophysics, 62, 2, vo218, 2019; doi: 10.4401/ag-7839 “equilibrium viscosity and disequilibrium rheology of a high magnesium basalt from piton de la fournaise volcano, la reunion, indian ocean, france„ stephan kolzenburg*,1,2,3, daniele giordano3,4,5, andrea di muro6, donald bruce dingwell1 (1) ludwig−maximilians−university munich, department of earth and environmental sciences, munich, germany (2) mcgill university, department of earth and planetary sciences, montreal, quebec, canada (3) universita degli studi di torino, dipartimento di scienze della terra, torino, italy (4) istituto nazionale di geofisica e vulcanologia, sezione di pisa, pisa, italy (5) institute of geoscience and earth resources (igg−cnr), italian national research council (cnr), pisa, italy (6) institut de physique du globe de paris, observatoire volcanologique du piton de la fournaise, paris, france 1. introduction 1.1 motivation and scope of this study the flow of lava is dominantly governed by viscos− ity, effusion−rate and the slope and morphological fea− tures of the underlying topography [cashman et al., 2013; dragoni et al., 1986; hallworth et al., 1987; har− ris and rowland, 2001; pinkerton, 1987]. mitigating the hazards posed by lava flows that threaten infras− tructure has only shown moderate success to date [bar− beri et al., 2003]. this is largely due to an incomplete understanding of how lava flows, how and when flows stop and is further hampered by a lack of methods to provide up−to−date topographic surface models [cash− man et al., 2013; farquharson et al., 2015; favalli et al., 2010; james and robson, 2014; kolzenburg et al., 2016a; kolzenburg et al., 2018c]. these shortfalls drastically re− duce our ability to predict lava flow emplacement. one of the most challenging aspects of predicting the flow dynamics of lavas is that the rheological proper− article history receveid june 26, 2018; accepted september 18, 2019. subject classification: rheology; crystallization kinetics; lava flow; piton de la fournaise; la reunion; disequilibrium processes; high mg−basalt. abstract lava flows are a common hazard at basaltic to intermediate volcanoes and have posed a significant threat to la reunion island over the past centuries. in sustained flow units, the efficiency of lava transport away from the vent is dominated by cooling. for basaltic to inter− mediate lavas, it is the ability of the lava to solidify during cooling which exerts a first−order control on spatial extent and flow distance. as a consequence, understanding the sub−liquidus rheology of lavas has become a key focus in lava flow research in the past decade. to date, the development of a systematic understanding of lava rheology during emplacement conditions has been significantly hampered by a lack of experimental data. here we present new data on the rheological evolution of crystallizing high−mg basalt from piton de la fournaise. sub−liquidus experiments were performed at constant cooling rates ranging from 0.5 to 5 k/min. those rates mimic thermal conditions experienced 1) by lava during flow on the surface and 2) by magma during dike and sill emplacement. our data show that the effective viscosity of the crystallizing suspension increases until reaching a specific sub−liquidus temperature, the so−called “rheological cut−off temperature” (tcutoff), at which the lava becomes rheologically immobile and flow ceases. this departure from the pure liquid vis− cosity curve to higher viscosity is a consequence of rapid crystallization and its variability for a given lava is found to be primarily con− trolled by the imposed cooling rate. based on these experimental data, we adapt the failure forecast method (ffm) − commonly used to describe the self−accelerating nature of seismic signals to forecast material failure − to predict the rheological cut−off temperature (tcutoff). the presented data substantially expand the modest experimental database on disequilibrium rheology of lavas and represent a step towards understanding the underlying process dynamics. ties of lava flows evolve during eruption and emplace− ment since they occur under conditions that impose a state of dynamic disequilibrium. the transient nature of the rheological properties is caused by changes in the temperature, composition and phase state of the lava or magma resulting from gas loss, cooling, crystallization and vesiculation. the importance of non−isothermal effects on the crystallization kinetics and textural de− velopment of silicate melts have in principle been rec− ognized for decades [arzilli and carroll, 2013; coish and taylor, 1979; gamble and taylor, 1980; hammer, 2006; lofgren, 1980; long and wood, 1986; vetere et al., 2013; walker et al., 1976]. yet experimental parameter− isation of these effects on magma flow behaviour has lagged behind badly. such a detailed understanding of the rheological evolution of crystallizing magmas or lavas requires direct measurement of the flow proper− ties at emplacement conditions. to date, direct measurements of the rheology of lavas or magmas under natural conditions are few [be− lousov and belousova, 2018; chevrel et al., 2018; einarsson, 1949; gauthier, 1973; panov et al., 1988; pinkerton, 1994; pinkerton and norton, 1995; pinker− ton and sparks, 1978; shaw et al., 1968]. they repre− sent crucial measurements for benchmarking of exper− imental data. in conjunction with systematic laboratory studies on the evolution of lava flow properties as a function of the physico−chemical environmental con− ditions such data enable the production of new input parameters for predictive models of lava rheology and emplacement. in an effort to generate a systematic em− pirical database that will help to elucidate the system− atics of the underlying processes, a range of concentric cylinder viscometry data has been presented. concen− tric cylinder viscometry typically utilises measurements of the torque exerted by the liquid on a spindle inserted into a sample melt and rotated at a constant rate. the available datasets can broadly be separated into mea− surements at constant, isothermal conditions (i.e. con− stant undercooling and at fixed shear rates and oxygen fugacities) e.g. [chevrel et al., 2015; ishibashi and sato, 2007; robert et al., 2014; sato, 2005; sehlke et al., 2014; soldati et al., 2016; vona et al., 2011; vona et al., 2013] and measurements at constant cooling rates that map suspension rheology at varying experimental shear−rates and/or changing oxygen fugacity [giordano et al., 2007; kolzenburg et al., 2018a; kolzenburg et al., 2018b; kolzenburg et al., 2016b; kolzenburg et al., 2017; kouchi et al., 1986; shaw et al., 1968]. the for− mer datasets aim at attaining mechanical and textural equilibrium and permit the sampling of the equilibrated material. these data are used to correlate the viscosity measurements to the sample textures. the latter dataset aims at reproducing natural flow conditions and allows the quantification of the “rheological cut−off tempera− ture” (tcutoff), at which the lava becomes rheologically immobile and flow ceases. however, sampling of the texture or modelling of the change in crystal content throughout the experiment for correlation to the rheo− logical data is not possible during these dynamic, dise− quilibrium experiments, since crystallization under these conditions occurs far from equilibrium and very rapidly. here we present new experimental data on a high− mg basalt from piton de la fournaise that investigate the sub liquidus evolution of melt rheology as a function of cooling rate at atmospheric conditions. while these data can also facilitate the understanding of kinetic flow evolution at thermal conditions (i.e. absolute tem− perature and cooling rates) of partly degassed magma during transport toward the surface in dike−swarms, the decrease in oxygen fugacity occurring at sub−surface conditions will likely require a shift of the presented data to lower temperatures, as shown in kolzenburg et al. [2018a]. nevertheless, the presented dataset serves to expand a growing database on non−isothermal rheol− ogy, which is necessary for the development of empir− ical models of the effective lava viscosity at emplace− ment conditions. such data are essential for robust prediction of lava flow behaviour and flow geometry. 1.2 compositional characteristics of the 2007 eruption of piton de la fournaise piton de la fournaise volcano is located on la re− union island ~800 km east of madagascar. in 2007, it produced one of its largest volume eruptions of the past centuries. a detailed account of the eruption can be found in staudacher et al. [2009]. the 2007 eruption represents an interesting end−member with respect to lava rheology since it erupted a wide range of basalt compositions and crystal contents, spanning almost the entire range of magma compositions typically erupted at piton de la fournaise [villemant et al., 2009]. based on regular sampling of the lava and compositional analyses, staudacher et al. [2009] and di muro et al. [2014] report that the lava was, initially, evolved (mgo ca. 7wt%) and poorly phyric with less than 5 vol% of small olivine/clinopyroxene phenocrysts and plagioclase microlites. throughout the eruption, the volume fraction kolzenburg et al. 2 (and size) and of olivine crystals increased, reaching up to 40 vol% of olivine phenocrysts at the end of the eruption [di muro et al., 2014; staudacher et al., 2009; villemant et al., 2009]. villemant et al. [2009] report that the april 2007 eruption produced some of the most primitive and most mg−rich melts erupted at piton de la fournaise (mgo~9wt%) and that they correspond to the first step of the differentiation trend for the parental magma at piton de la fournaise [albarède et al., 1997; boivin and bachèlery, 2009; villemant et al., 2009]. villemant et al. [2009] and di muro et al. [2014] further note that early erupted lavas and the matrix glass of the 2007 lava flows (bulk mgo content ~25–30%) are less mg−rich (mgo~6−%) than matrix glass of scoria (glass mgo content ~7% and bulk rock mgo content ~18– 20%) or pélé's hair (glass mgo content=7.5–9%) emit− ted at the same vent; see (tables 1a–b) in villemant et al. [2009]. melt inclusions trapped in the most mg−rich olivines (fo86) record mafic melt compositions (mgo 10−11 wt%) representative of the primary melts feeding the central and shallow plumbing system of piton de la fournaise [di muro et al., 2014]. average lava compo− sition at piton de la fournaise record extensive early olivine+/−pyroxene fractionation at depth, while prim− itive magnesian melts (mgo > 14wt%) are never erupted on the la réunion island (e.g. see fretzdorff and haase [2002), di muro et al. [2016] and references therein). in summary, the available data demonstrate that the magma is significantly more mg−rich during subsurface transport and eruption than is preserved in the lava flow matrix glass and thus the formation of olivine ± clinopyroxene ± plagioclase plays a central role in the solidification of magma and lavas and, thereby their rheological evolution at piton de la fournaise volcano. 2. experimental materials and methodology 2.1 experimental material and melt preparation the samples we selected from the 2007 eruption display relatively high crystal contents (~25−40 vol%). for rheological experimentation we created a partial melt of the crystallized matrix and some amount of the crystal cargo. the rock sample was crushed using a jaw crusher and fragments of ~ 1cm diameter were hand− picked for matrix melt separation. these fragments were then partially melted in large thin−walled pt crucibles to separate the olivine phenocrysts from the matrix. melting was performed in a nabertherm® mosi2 box furnace at 1250°c in multiple, small, batches for ~20 minutes per batch. during re−melting, the dense olivine phenocrysts sank to the base and the interstitial melt percolated through the crystal frame− work, forming a lens of melt on top of the crystal cu− mulate. this melt lens was carefully decanted to col− lect the interstitial partial melt. fifteen batches were required to recover sufficient melt volume to perform viscometry. the quenched melt batches were crushed and ground using an agate mortar and thoroughly mixed for homogenization. these powders were then re melted at 1350°c for 12h to ensure chemical homo− geneity. after homogenization the sample was quenched by pouring it onto a steel plate in order to en− sure a cooling rate fast enough to supress crystal growth. visual assessment and raman spectroscopy following the approach of di genova et al. [2018] af− firmed that the glasses remained crystal−free. the glass was crushed and re−melted into pt80rh20 cylindrical crucibles of 51 mm height and 26.6 mm diameter. the crucible was then transferred into a deltech bottom− loading furnace adapted for rheological experimenta− tion, the device is described in successive configurations in [dingwell, 1991; dingwell, 1986; dingwell, 1990; dingwell, 1992]. 3 rheology of piton de la fournaise table 1. epma analysis of major element components as oxides; data is normalized to 100 wt%. oxide wt % oxide sio2 48.56 tio2 2.40 al2o3 12.15 feo 12.19 mno 0.20 mgo 11.61 cao 9.76 na2o 2.33 k2o 0.56 p2o23 0.25 analytical total 99.76 kolzenburg et al. 4 2.2 glass chemistry analysis the bulk chemical composition of the experimental material was determined for the quenched glass using an electron probe micro analyser (epma). the sample was impregnated in a low viscosity epoxy resin, polished and carbon coated. the major element composition was de− termined at the department of earth and environmen− tal sciences at the university of munich, using a cameca sx100 epma with 15 kv acceleration voltage and 5 na beam current. in order to reduce the alkali volatilization the beam diameter was defocused to 10 μm. synthetic wollastonite (ca, si), periclase (mg), hematite (fe), corundum (al), natural orthoclase (k), and albite (na) were used as standards, and matrix correction was per− formed by pap procedure [pouchou and pichoir, 1991]. the accuracy was better than 2.5% for all analysed el− ements. normalized results of the average of 10 epma analyses are presented in table 1; analytical totals av− erage 99.76 %. 2.3 concentric cylinder (cc) viscometry and differential scanning calorimetry (dsc) glass transition temperatures at onset (tgonset) and peak (tgpeak) were obtained from heat capacity curves derived from differential scanning calorimetry on aliquots of the re−melted glass undergoing cooling/heating cycles at rates of 5, 10, 15 and 20 °k/min. reproducibility of both tgonset and tgpeak are better than 0.5 % (i.e. ~3°k); see also gottsmann et al. [2002] and giordano and russell [2017]. these data are used to recover the low temperature viscosity data using the method presented by gottsmann et al. [2002], that correlates tgonset and tgpeak to fixed values of vis− cosity; these data are reported in table 2. high temperature rheological experiments were performed using a brookfield dv−iii+ viscometer head (full range of torque = 0 − 0.7187 mnm). a pt80rh20 spindle is suspended from the viscometer head, im− mersed into the sample and rotated at a constant rate. the torque exerted by the sample at constant rotation rate is proportional to the melt or suspension viscos− ity. the spindle head used in these experiments is 33.2 mm long with 14.4 mm diameter and a 45° conical top and bottom to reduce edge effects, this bob is hung on a 2.4 mm diameter stem; see also: [dingwell, 1986; dingwell and virgo, 1988]. the torque reading for this spindle and crucible were calibrated against dgg1 standard glass, with a known viscosity−temperature relationship [meerlender, 1975]. calibration was per− formed for shear rates and temperatures exceeding those used in this study. the precision of the viscosity determination is ±3% as described in dingwell [1986]. the thermal evolution of the sample at the imposed cooling rates was calibrated with an accuracy of ±1 °c over the entire experimental temperature range using a platinum sheathed type−s thermocouple immersed in dgg1 standard glass. the super liquidus melt viscos− ity was measured at between 1444 and 1321 °c in tem− perature steps of ~25 k; see table 2 (corresponding to furnace set points of 1450 − 1325 °c). each tempera− ture step was maintained for 120 min and a stable torque reading was commonly achieved after ~70 min, indicating thermal and chemical (i.e. redox) homo− geneity. the high temperature viscosity measurements in combination with the low temperature melt viscosity data recovered via application of a shift factor to the dsc data, are used to fit a vft−model [tammann and hesse, 1926] to the experimental data. this model de− scribes the theoretical crystal−free liquid viscosity at sub−liquidus temperatures [giordano et al., 2008a; gottsmann et al., 2002; hess and dingwell, 1996; hess et al., 1995] and is used to calculate the relative viscos− temperature (k) temperature (c) log η (pa s) standard deviation 1717 1444 0.27 0.03 1692 1419 0.34 0.03 1668 1395 0.41 0.04 1643 1370 0.49 0.04 1619 1346 0.58 0.04 1594 1321 0.68 0.04 977 704 10.24 0.17 973 700 10.37 0.17 967 694 10.56 0.15 959 686 10.90 0.14 942 669 11.67 0.26 939 666 11.81 0.25 937 664 11.98 0.24 927 654 12.24 0.24 a -3.77 b 4163.79 c 673.18 table 2. summary of pure liquid viscosity measurements per− formed by 1) concentric cylinder viscometry (data at temperatures of 1594 k and above) and 2) application of a shift factor to calorimetric data (data at temper− atures of 977 k and below). 5 rheology of piton de la fournaise ity of the suspension (see section 2.4 for details). the model parameters are reported together with the liquid viscometry results in table 2. 2.4 non-isothermal concentric cylinder (cc) viscometry non−isothermal experimentation was performed fol− lowing methods similar to the ones described in previ− ous work [giordano et al., 2007; kolzenburg et al., 2018a; kolzenburg et al., 2017]. a series of experi− ments were run at constant cooling rates of 0.5, 1, 3 and 5 °k/min and an initial shear rate of 0.766 sec−1 (cor− responding to 2.5 rpm). experiments at lower initial shear rates were not possible for the given experimen− tal temperatures because at low rotation rates, the sur− face tension of the melt in the meniscus between spin− dle and crucible is sufficiently high to pull the spindle to the sidewalls of the crucible. during experimentation, the measured torque increased as a function of the ap− parent viscosity of the suspension. upon reaching the maximum torque limit of 0.7187 mnm, the shear rate was automatically dropped to 0.368, 0.184, 0.092, 0.061, 0.031 s−1 (1.2, 0.6, 0.3, 0.2, 0.1 rpm, respectively), each successive time the limit was reached. between consecutive dynamic cooling experiments the sample was re−heated to 1396 °c (corresponding to a furnace set point of 1400 °c) for a minimum of two hours and homogenized by stirring at a shear rate of 3.06 s−1 (10 rpm) in order to ensure complete melting of all crystalline phases formed during previous experi− mentation. after the end of the last experimental cycle, the sample was re−melted and quenched to a glass. based on the results of mysen and virgo [1978] and dingwell and virgo [1987] we assume that redox equi− librium was achieved during the super−liquidus melt viscosity determination, which lasted more than 24 hours at atmospheric conditions. since the crystalliza− tion experiments lasted between 0.5 and less than 5.8 hrs (at lower temperatures; i.e. higher viscosities and slower redox equilibration) and we assume that the melt redox state was re−equilibrated to the initial, pre− experimental state within the time spent at high tem− perature under constant stirring during remelting. the assumption of thermal and redox equilibration and re− melting of previous crystal phases was confirmed by the recovery of a steady torque measurement at ±1% of the value of the pure melt over more than an hour. in order to resolve the crystallization induced rhe− ological deviation from the pure melt viscosity at greater detail than in absolute viscosity space we cal− culate the relative viscosity ηr, that is the ratio between the measured shear viscosity of the mixture ηs of the crystallizing suspension and the liquid viscosity ηl, (equation 1): (1) where ηl was calculated by employing a vft−fitting of the low temperature viscosity data reported in table 2 and the super liquidus viscometry data of each exper− iment. in the following sections, the results are presented as the logarithms of both ηs and ηr. 3. results 3.1 liquid viscosities of magmas at piton de la fournaise the melt composition studied here is reported in table 1. it is representative of the parental melt feed− ing the central activity of piton de la fournaise [di muro et al., 2016; di muro et al., 2014; famin et al., 2009; villemant et al., 2009]. the viscosity measure− ments of the crystal free liquid are summarized, together with the vft model fitted to the low and high r= s l η η η 1000 1200 1400 1600 0 2 4 6 8 10 12 temperature (k) lo g 1 0 v is c o s it y ( p a s ) this study vft model this study gba 89 data from villeneuve 2008 vft model gba89 villeneuve 2008 grd model sgba98 villeneuve 2008 grd model o77 villeneuve 2008 figure 1. summary of crystal free liquid viscometry data on piton lavas. blue dots represent high temperature cc data and low temperature dsc measurements con− verted to viscosity, measured for this study. the blue solid line represents the vft−model fitted to these data. magenta diamonds and dashed line represent viscometry data from villeneuve et al. [2008]. green dotted and brown dash−dotted lines represent grd model curved for compositions sgba90 and o77 re− ported in villeneuve et al. [2008], respectively. errors are smaller than symbol size. kolzenburg et al. 6 table 3. summary of absolute viscosity measurements for all experiments. for simplicity we report interpolated values of the ex− perimental data at equal temperature steps with exception of the final datapoint for some experiments; highlighted in grey. temperatures related to those points are reported in the last two rows in kelvin and celsius, respectively. temperature viscometry data in figure 1. for compar− ison we also report viscosity measurements as well as grd model estimates [giordano et al., 2008b] of three other compositions presented in villeneuve et al. [2008]. these span the compositional variety of the piton de la fournaise volcano, where sgba98 is representative of the more evolved and o77 repre− sentative of the more primitive melt compositions erupted at piton de la fournaise [albarède et al., 1997; boivin and bachèlery, 2009; famin et al., 2009; villemant et al., 2009]. 3.2 viscosity evolution of the crystallizing suspension the temperature−dependent viscosity of the piton de la fournaise lava, when subjected to varying cooling rates is plotted as absolute and relative viscosity vs. temper− ature in figures 2 and 3, respectively; the data are re− ported as absolute and relative viscosity in tables 3 and 4, respectively. the rheological evolution of the lava follows the trend of the pure liquid data (logηr=0) up until the point where the influence of crystalline phases (via nucleation and growth of crystals) starts to measurably increase the sam− cooling rate (k/min) 0,5 1 3 5 shear rate (sec-1) 0.77 0.77 0.77 0.77 t (k) t (c) log η (pa s) 1573 1300 0.79 0.79 0.78 0.78 1568 1295 0.81 0.81 0.80 0.81 1563 1290 0.83 0.84 0.81 0.83 1558 1285 0.85 0.86 0.84 0.85 1553 1280 0.89 0.89 0.87 0.87 1548 1275 0.91 0.91 0.89 0.90 1543 1270 0.94 0.94 0.91 0.92 1538 1265 0.96 0.96 0.94 0.95 1533 1260 1.00 0.99 0.97 0.98 1528 1255 1.04 1.02 0.99 1.00 1523 1250 1.10 1.06 1.02 1.03 1518 1245 1.15 1.09 1.05 1.06 1513 1240 1.20 1.14 1.08 1.09 1508 1235 1.26 1.18 1.11 1.11 1503 1230 1.32 1.23 1.14 1.14 1498 1225 1.44 1.28 1.17 1.17 1493 1220 1.66 1.34 1.21 1.21 1488 1215 2.16 1.41 1.25 1.24 1483 1210 2.58 1.53 1.29 1.28 1478 1205 1.78 1.33 1.32 1473 1200 2.60 1.37 1.36 1468 1195 1.43 1.40 1463 1190 1.48 1.46 1458 1185 1.56 1.51 1453 1180 1.67 1.58 1448 1175 1.86 1.67 1443 1170 2.54 1.78 1438 1165 2.65 1.94 1433 1160 2.28 1428 1155 2.62 t final datapoint k c 1485 1475 1443 1431 1212 1202 1170 1158 ple’s apparent viscosity. at this deviation, the apparent vis− cosity of the suspension increases with respect to the the− oretical liquid (logηr>0). this increase in relative viscos− ity accelerates rapidly as temperature decreases. at high− er cooling rates this departure commences at progressively lower temperatures (i.e. higher degrees of undercooling) since crystallisation of the melt is delayed. further, the na− ture of this departure is correlated with the imposed cool− ing rate, where slower imposed cooling rates result in a steeper rheological departure. this is in agreement with 7 rheology of piton de la fournaise cooling rate (k/min) 0,5 1 3 5 shear rate (sec-1) 0.77 0.77 0.77 0.77 t (k) t (c) log η (pa s) 1573 1300 0.01 0.00 0.01 0.02 1568 1295 0.00 -0.01 0.00 0.01 1563 1290 -0.01 0.00 -0.02 0.00 1558 1285 -0.02 -0.01 -0.01 -0.01 1553 1280 -0.01 -0.01 -0.01 -0.02 1548 1275 -0.01 -0.01 -0.03 -0.02 1543 1270 -0.01 -0.01 -0.02 -0.02 1538 1265 -0.01 -0.01 -0.02 -0.02 1533 1260 0.01 0.00 -0.02 -0.03 1528 1255 0.04 0.00 -0.02 -0.04 1523 1250 0.10 0.02 -0.01 -0.03 1518 1245 0.16 0.04 -0.01 -0.03 1513 1240 0.22 0.07 0.00 -0.04 1508 1235 0.28 0.11 0.01 -0.04 1503 1230 0.36 0.15 0.02 -0.04 1498 1225 0.57 0.20 0.04 -0.04 1493 1220 1.00 0.27 0.06 -0.03 1488 1215 2.07 0.37 0.07 -0.02 1483 1210 3.01 0.56 0.10 -0.01 1478 1205 1.09 0.13 0.01 1473 1200 2.92 0.16 0.03 1468 1195 0.22 0.06 1463 1190 0.28 0.10 1458 1185 0.39 0.16 1453 1180 0.57 0.24 1448 1175 0.94 0.37 1443 1170 2.41 0.54 1438 1165 2.68 0.84 1433 1160 1.53 1428 1155 2.27 t final datapoint k c 1485 1475 1443 1431 1212 1202 1170 1158 table 4. summary of the calculated relative viscosity for all experiments. for simplicity we report interpolated values of the ex− perimental data at equal temperature steps with exception of the final datapoint for some experiments; highlighted in grey. temperatures related to those points are reported in the last two rows in kelvin and celsius, respectively. earlier non−isothermal rheological experiments at at− mospheric conditions [giordano et al., 2007; kolzenburg et al., 2018b; kolzenburg et al., 2016b]. the crystallization induced rheological departure ac− celerates continuously until the samples reach the torque limit of the experimental device. this point is close to the “rheological cut−off temperature” (tcutoff) at which the sample becomes rheologically immobile and flow ceases. albeit direct measurements at high viscosities, approaching solidification, are not possible due to me− chanical constraints of the experimental setup, the data show that tcutoff shifts systematically to lower temper− atures with increasing cooling rate. 4. discussion 4.1 implications for magma transport and lava emplacement at piton de la fournaise the data reported in figure 2 show that the viscosity measurements presented here are in good agreement with the down−flow viscosities calculated by rhéty et al. [2017]. the latter study estimated the bulk lava viscosity of the 2007 lava flow using the texture−based approach for multiphase rheology presented in phan−thien and pham [1997] in combination with the temperature−de− pendent melt viscosity model reported in villeneuve et al. [2008]. direct comparison with our measurements is con− sistent with down−flow cooling rates of 3 to 5 k/min. rhéty et al. [2017], reconstruct initial cooling rates of 7 k/km that increase down−flow to 42 k/km. accounting for the average flow channel geometries (i.e. widths be− tween 4 and 30 m and heights between 0.8 and 2 m) re− ported for the flow zones defined in rhéty et al. [2017] and the average effusion rate of 54 m3s−1 reported in staudacher et al. [2009], the distance−dependent cooling rates convert to time−dependent cooling rates of 0.1 to 2 k/min and 0.7 to 12 k/min for the reported values of 7 k/km and 42 k/km, respectively. lava cooling rates increase sharply during late stages of flow once the lava leaves the well−insulated transport system [cashman et al., 1999; harris et al., 2005; harris and rowland, 2009], therefore the lower cooling rates re− ported in the former studies are likely most representa− tive of the dominant flow phase. values as low as 0.1 k/min, however, are lower than the cooling rate imposed in the experiments (see viscometry data plotted in fig− ure 1). we attribute this mismatch to the changes in oxygen fugacity between the experiments, carried out in atmospheric conditions, and the natural lava, being erupted in a more reduced state; see vlastélic et al. [2016], pichavant et al. [2016] and references therein. the pre− eruptive fo2 for pdf magmas is estimated at ~nno−0.5 and shallow level degassing of so2and h2o might further kolzenburg et al. 8 1400 1450 1500 1550 1600 1650 1700 0.5 1 1.5 2 2.5 3 temperature (k) lo g 1 0 v is co si ty (p a s) 0.5 k/min 1 k/min 3 k/min 5 k/min liquid viscosity rhéty et al., 2017 1440 1460 1480 1500 1520 1540 1560 1580 1600 0 0.5 1 1.5 2 2.5 3 temperature (k) lo g 10 r el at iv e v is co si ty 0.5 k/min 1 k/min 3 k/min 5 k/min figure 2. summary of non−isothermal viscometry data on piton lavas presented as absolute viscosity. black stars represent high temperature cc data. green triangles, blue dots, red squares and ma− genta diamonds represent constant cooling vis− cometry data on the evolving suspension at cooling rates of 0.5, 1, 3 and 5 k/min, respec− tively. black crosses represent viscosity esti− mates based on sample textures from [rhéty et al., 2017]. the precision of the viscosity deter− mination is ±3% of the measured value. figure 3. summary of non−isothermal viscometry data on piton lavas presented as relative viscosity. green triangles, blue dots, red squares and ma− genta diamonds represent constant cooling vis− cometry data on the evolving suspension at cooling rates of 0.5, 1, 3 and 5 k/min, respec− tively. the precision of the viscosity determi− nation is ±3% of the measured value. decrease the oxygen fugacity; this is suggested by late stabilization of sulphides at shallow level; see [vlastélic et al., 2016]. according to the data presented in kolzen− burg et al. [2018a] such a shift in fo2 may result in a de− crease of the rheologic departure and tcutoff of ~ 40 to 80 °k. when accounting for such a ~40−80 °k decrease in the rheologic departure due to varying fo2 conditions, the reconstructed cooling rates of 0.1 to 2 k/min match with the experimental measurements, indicating that the erupted lava did not undergo efficient redox equilibration during subaerial flow. 4.2 towards forecasting the rheological cutoff temperature (tcutoff) the decrease in tcutoff with increasing cooling rate is a result of the balance between crystal nucleation and growth rate and the rate of undercooling experienced by the sample [kolzenburg et al., 2018a; kolzenburg et al., 2016b]. there are two dominant mechanical con− straints of the experimental apparatus that do not al− low to continue the rheological measurement beyond the maximum viscosity values presented here: 1) the torque limit of the rheometer head and 2) the fact that the crucible containing the experimental sample may start to slip and rotate in its holder at high torque, ren− dering the measured torque data invalid. here we pre− sent a new, numerical, approach that uses the self−ac− celerating nature of the rheological departure to forecast the tcutoff value (i.e. the temperature at which the sample becomes rheologically immobile and infi− nite viscosity is reached) of the lavas under the imposed experimental conditions. this approach follows the failure forecast method (ffm) adopted in volcano seis− mology and material science to predict failure of a material based on the rate of increase in seismic energy released prior to failure [voight, 1988; voight, 1989]. it is based on the following equation: (2) where d2 /dt2 and d /dt are the acceleration and rate of the phenomenon being monitored, and a and a are em− pirical parameters. for predicting rock failure, this equa− tion is commonly simplified to describe only peaks in signal acceleration rate using a constant a=2, as shown in kilburn [2003] and lavallée et al. [2008] resulting in: (3) where t is time, and (d /dt)0 is the value of (d /dt) when t= t0. the ffm utilizes the production rate of a precur− sory phenomena and correlates its acceleration to the point at which the process reaches an uncontrolled (i.e. runaway) state. this state is reached at an infinite d /dt, that implies an uncontrolled rate of change and, thereby, the point at which the monitored process reaches infi− nite velocity. the forecasting potential of equation 2 lies in its potential for describing the rate at which d /dt ap− proaches uncontrolled conditions. plotting inverse rate against time returns a negative linear trend, where the time at which the inverse rate equals zero corresponds to the uncontrolled condition when dη/dt tends to in− finity [kilburn, 2003; lavallée et al., 2008]. this point can then be obtained by simple linear extrapolation of the measured trend to the time axis. in volcano seis− mology or laboratory experimentation on the brittle ductile transition of magma, the precursory signal of the fracturing and failure phenomena are for example, seis− micity rate, acoustic emission rate or seismic energy re− lease. the forecasting potential for volcanic eruptions has been demonstrated on seismic data from active volcanoes [kilburn, 2003; kilburn and voight, 1998] as well as for experiments on natural lavas at the brittle ductile transition [lavallée et al., 2008]. for further de− tails see also [cornelius and voight, 1995; de la cruz− reyna and reyes−dávila, 2001; tokarev, 1963]. 9 rheology of piton de la fournaise d2ω dt2 =a dω dt ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ a dω dt ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ −1 =  dω dω ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 0 −1 −a(t −t0) 1450 1500 1550 0 100 200 300 400 500 1486 temperature (k) in ve rs e vi sc o si ty r at e (k /p as ) 0.5 k/min 1450 1500 1550 0 100 200 300 400 500 1475 temperature (k) in ve rs e vi sc o si ty r at e (k /p as ) 1 k/min 1450 1500 1550 0 100 200 300 400 500 1443 temperature (k) in ve rs e vi sc o si ty r at e (k /p as ) 3 k/min 1450 1500 1550 0 100 200 300 400 500 1429 temperature (k) in ve rs e vi sc o si ty r at e (k /p as ) 5 k/min figure 4. plot of viscosity acceleration rate vs temperature. data for cooling rates of 0.5, 1, 3 and 5 k/min are plotted in subplots a, b, c and d, respectively. blue open circles represent the experimental data inter− polated at 1 degree intervals. red filled circles rep− resent the linear part of the data used to fit the lin− ear model of viscosity acceleration for extrapolation to tcutoff. black squares represent the predicted tcut− off (i.e. dη/dt = ∞). the precision of the viscosity de− termination is ±3% of the measured value. in the case of crystallizing lava that experiences ever increasing undercooling, the precursory signal to tcutoff (i.e. the lava becomes rheologically immobile re− sulting from an accelerating crystallization rate) is the rate of increase in suspension viscosity. this is measured as a function of temperature instead of time. the for− mulation for forecasting then reads dη/dt (instead of d /dt), where η and t are the effective suspension vis− cosity and temperature, respectively. the temperature at which the inverse rate reaches zero corresponds to the uncontrolled condition when dη/dt tends to infinity. in figure 4 we present plots of the inverse rate of viscos− ity increase against temperature. the data plot randomly at high temperatures, where the acceleration is small (i.e. large numbers of inverse acceleration). once crystallization sets in, the acceler− ation of the measured suspension viscosity increases, forcing the data to converge on a negative linear trend. thus, tcutoff can be obtained by simple linear extrapo− lation of the measured trend to the temperature axis. figure 5 reports the values forecasted using this method as a function of cooling rate. we fit a power law model to the predicted tcutoff values that describes the data with an r−squared value of 0.9881. extrapolation of the data using this model suggests that further increasing cooling rate would result in a smaller effect of cooling rate on this rheological thresh− old, whereas decreasing cooling rates would further in− crease the temperature at which this threshold is reached. this is due to the longer time spent at higher temperatures, allowing for crystal growth approaching near equilibrium conditions. however, since the model describes a crystallization induced rheological thresh− old, it is important to note that the model is only valid at sub−liquidus temperatures and disequilibrium condi− tions (i.e. at constant cooling rates). incorporating tcutoff measurements in lava−flow models will allow for the concept of yield strength to be replaced by a melt specific rheological tcutoff that is de− pendent on composition, shear− and cooling−rate [kolzenburg et al., 2018a; kolzenburg et al., 2018b; kolzenburg et al., 2016b]. the concepts of yield strength or other arbitrary or empirically chosen parameters such as fixed crystal contents, temperature or degrees of un− dercooling have been introduced in numerical simula− tions of lava flows (especially in cellular automata type models; e.g. [miyamoto and sasaki, 1997]) in order to ascribe a halting criterion to a modelled lava parcel. it has also been introduced as a rheologic criterion in planetary sciences for the derivation of rheological pa− rameters from flow morphology, derived from experi− ments using analogue materials that possess yield strength e.g. [fink and griffiths, 1990; hulme, 1974]. in these analogue experiments these materials were cho− sen in order to mimic the development of a crust dur− ing cooling and they do not to represent actual flow rheology as demonstrated in kolzenburg et al. [2018c]. tcutoff data would therefore represent a more realistic de− scription of the lavas rheological evolution and there− with better ability to forecast lava flow emplacement and flow cessation. the new approach to predict flow cessation may eventually be combined with satellite re− mote sensing data of the thermal evolution of lava flows such as presented in coppola et al. [2013] to pro− vide near real time assessment and forecasting of flow evolution. 5. conclusions in conclusion, we find that: 1. cooling rate exerts a first order control on the sub liquidus rheological evolution of the high mg basalt from piton de la fournaise through its influence on the melt crystallization−kinetics. 2. the failure forecasting method (ffm) can be adopted to extrapolate disequilibrium viscometry data beyond the mechanical constraints of the ex− perimental apparatus for prediction of the lavas kolzenburg et al. 10 0 1 2 3 4 5 6 7 1420 1440 1460 1480 1500 1520 1540 cooling rate (k/min) te m p er at u re (k ) t c utoff = 1470.9 * x -0. 017 r 2 = 0.9881 t c utoff model prediction power law fit figure 5. plot of the predicted tcutoff temperatures as a func− tion of cooling rate. blue circles represent the fore− casted values from the data in figure 4 for cooling rates of 0.5, 1, 3 and 5 k/min. the blue solid line rep− resents a power law model fitted to these data de− scribing the tcutoff lava immobilization threshold as a function of cooling rate. errors are smaller than symbol size. tcutoff. 3. modelling of magmatic flow behaviour requires melt−composition specific flow− and crystalliza− tion−models describing tcutoff. 4. developing a systematic process understanding of the disequilibrium rheology of natural silicate melt suspensions requires an expanded database of dy− namic measurements such as those presented here. 5. implementation of data on the dynamic rheolog− ical evolution of magma during ascent in dykes and during the emplacement of lavas in compu− tational models would allow to more accurately constrain the results of physical property based magma and lava transport models. acknowledgements. we would like to thank werner ertel-ingrisch and kai-uwe hess for support in the laboratory and interesting discussions during the experimental campaign. fabio arzilli and an anonymous reviewer are thanked for constructive comments that helped improve the manuscript. sk and dg acknowledge support from fondazione crt, compagnia san paolo, an erasmus traineeship and the university of torino. sk further acknowledges financial support from a h2020 marie skłodowska-curie fellowship dynavolc – no.795044. dbd wishes to acknowledge the support of erc advanced researcher grant evokes – no.247076. adm was supported by the agence national de la recherche through project anr-lava (anr program: ds0902 2016; project: anr-16 ce39-0009). references albarède, f. et al., 1997. the geochemical regimes of piton de la fournaise volcano (réunion) during the last 530 000 years. journal of petrology, 38(2): 171−201. arzilli, f., carroll, m.r., 2013. crystallization kinetics of alkali feldspars in cooling and decompression−in− duced crystallization experiments in trachytic melt. contributions to mineralogy and petrology, 166(4): 1011−1027. barberi, f., brondi, f., carapezza, m.l., cavarra, l., mur− gia, c., 2003. earthen barriers to control lava 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a., romano, c., giordano, d., russell, j.k., 2013. the multiphase rheology of magmas from monte nuovo (campi flegrei, italy). chemical geology, 346(0): 213−227. walker, d., kirkpatrick, r., longhi, j., hays, j., 1976. crystallization history of lunar picritic basalt sam− ple 12002: phase−equilibria and cooling−rate studies. geological society of america bulletin, 87(5): 646−65 *corresponding author: stephan kolzenburg, ludwig-maximilians-university munich, department of earth and environmental sciences muenchen, germany email: skolzenburg@gmail.com © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved kolzenburg et al. 14 s-p c o n v e r s i o n at the base o f the crust m . b a t h e . s t e f a n s s o n r i c e v u t o il 7 m a r z o 1966 sujimaey. — t l i e l o n g p e r i o d s e i s m i c r e c o r d s a t u m e à ( s w e d e n ) of a n i n t e r m e d i a t e d e p t h e a r t h q u a k e i n b u r m a 011 27 f e b r u a r y , 1964, e x h i b i t a n e x c e p t i o n a l l y c l e a r c a s e of 8-p c o n v e r s i o n a t t h e m o h o r o v i c i é d i s c o n t i n u i t y . t h e r e c o r d s a r e e x a m i n e d f r o m v a r i o u s p o i n t s of v i e w ( a r r i v a l t i m e s , a m p l i t u d e r a t i o s , p a r t i c l e m o t i o n s ) a n d c o m p a r i s o n s a r e m a d e f o r e a c h t e s t b e t w e e n o b s e r v a t i o n s a n d t h e o r y . g e n e r a l a s p e c t s of m o d e c o n v e r s i o n s a t t h e b a s e of t h e c r u s t a r e d i s c u s s e d , b o t h t h e i r i m p o r t a n c e a n d w h y t h e y a r e so s e l d o m o b s e r v e d . riassunto. — i l t e r r e m o t o d i b u r m a del 27 f e b b r a i o 1964 ( p r o f o n d i t à i n t e r m e d i a ) , r e g i s t r a t o s u l l u n g o p e r i o d o d e l l a s t a z i o n e d i u m e à ( s v e z i a ) , m o s t r a u n c a s o e c c e z i o n a l m e n t e c h i a r o d i c o n v e r s i o n e d i o n d e s p a l l a d i s c o n t i n u i t à d i m o l i o r o v i c i c . i s i s m o g r a m m i s o n o s t a t i e s a m i n a t i d a v a r i p u n t i d i v i s t a ( t e m p i d i a r r i v o , r a p p o r t i f r a le a m p i e z z e , m o v i m e n t i di p a r t i c e l l e . . .) e p e r o g n u n o d i essi è s t a t o f a t t o il c o n f r o n t o f r a d a t i di oss e r v a z i o n e e t e o r i a . v e n g o n o d i s c u s s i , i n o l t r e , gli a s p e t t i g e n e r a l i d e l m o d o di c o n v e r s i o n e a l l a b a s e d e l l a c r o s t a , i n s i e m e a l l a l o r o i m p o r t a n z a e d al p e r c h é p o s s o n o o s s e r v a r s i t a n t o r a r a m e n t e . i n t r o d u c t i o n . f o r seismic waves which do not penetrate deeper than the m a n t l e (that is, lip to about 103° distance), t h e most pronounced internal discontinuity encountered during the propagation is t h e base of the crust, the mohorovicic discontinuity. a t this surface, not only reflections and refractions occur, b u t also conversions of 8 to p and of p to 8. such phenomena m a y occur both near the hypocenter and near the station as well as in between (for reflected waves). due to t h e relative p r o x i m i t y of the moho to t h e e a r t h ' s surface, waves deriving f r o m moho arrive only within a few seconds f r o m the p a r e n t waves. this naturally often makes it difficult to discover such waves. they m a y be 1 2 0 m. b a t h r. s t e f a n s s o n immersed in already existing motion, as e.g. for p-s converted waves which arrive a few seconds a f t e r p. the s-p converted waves, on the other h a n d , arrive before the t r u e s and m a y therefore be easier to discover. they are also of great significance as t h e y m a y give rise to too early, false readings of s. the relative difficulty to establish reliably the existence of mohoderived waves is the probable explanation why the seismological lit e r a t u r e contains only some scattered information on this point. befloctions against moho f r o m below will give rise to too early arrivals (as sometimes observed for pp). such reflections were studied b y choudhury (1958) and treskov (1961) and were used for estimating erustal thickness a t t h e reflection point. reflections against moho f r o m above have been observed repeatedly in seismic field investigations (see e.g. b a t h and tryggvason, 1962). ps converted waves were reviewed b y cook, algermissen and costain (1962) and sp converted waves were studied b y p a c h a d z h a n o v a (1962). additional references can be found in t h e papers mentioned. also more complicated wave p a t h s , involving multiple passages between moho a n d t h e e a r t h ' s surface, have been reported in t h e l i t e r a t u r e (see e.g. j u n j u , 1965). the present p a p e r explores records of sp converted waves obtained a t umea (sweden) f r o m an intermediate-depth e a r t h q u a k e in b u r m a . of the publications mentioned, it has t h e closest relations to the paper by p a c h a d z h a n o v a (1962). i t will be convenient to h a v e some notation for moho-derived waves and we are suggesting the following scheme, which is constructed in conformity with existing wave notations: 1) ps for ps converted waves, and sp for sp converted waves (conforming to t h e rule t h a t small letters, s and p, are used for waves which p r o p a g a t e only upwards). 2) pmp, pmp and pmp for reflected waves (in conformity with pcp; this n o t a t i o n was suggested b y choudhury, 1958). m can be used to denote reflection against moho f r o m either side. 3) more complicated wave p a t h s can t h e n be easily described in t h e same system. f o r example, sppmp is an incident s against molio f r o m below, which is converted into p , t h e n reflected as p against the surface and a f t e r t h a t against moho, before it finally arrives a t the surface. 4) i n case of more t h a n one layer, indices 1 , 2 , 3 , . . . referring to t h e respective layers, should be added to the symbols. s p c o n v e r s i o n at t h e b a s e of t h e cuust 1 2 1 o b s e r v a t i o n a l d a t a . u.s. coast and geodetic survey reports t h e following d a t a for t h e investigated e a r t h q u a k e : d a t e a n d origin t i m e : 27 f e b r u a r y , 1964, 15h10m48s8 gmt. epicenter coordinates: 21°.7 n, 94°.4 e (burma). focal d e p t h : 102 k m . magnitude (uppsala, iviruna): 6 . 5 . recording s t a t i o n u m e a (sweden): 6 3 ° 4 8 ' . 9 n , 2 0 ° 1 4 ' . 2 e . i n s t r u m e n t s : long-period standardized press-ewing seismographs (t0 = 15 sec, ta = 100 sec). ? r i rr-s /vm^w^v/v '̂avvv '̂̂ v^/'̂ vav \ v ^ y w w w v m v r w y ^ v • v 1 " a . . j . . . 1 wwwvi/v y t / v a a j ^ ^ ^ ^ ^ \lal\iy m h / ì f i g . 1 s w a v e g r o u p r e c o r d e d b y u m e a l o n g p e r i o d s e i s m o g r a p h s b u r m a e a r t h q u a k e 27 f e b r u a r y , 1964. t i m e i n c r e a s e s f r o m l e f t t o a n d t h e r e is o n e m i n u t e b e t w e e n e a c h t i m e m a r k . f r o m r i g h t e p i c e n t r a l distance: 63°.8. epicenter azimuth a t t h e station: 94° (i.e. only 4° south of east). time reading (see records in fig. 1) for fif (actually sp) on e: 15h29m31s. time difference = 6 . 3 sec. 1 2 2 m. b â t h r . s t e f a n s s o n amplitude readings are summarized in table i. resolving t h e motion into $ 7 , sh a n d sp components we find (u = horizontal component, w — vertical component): usv = 1 7 . 8 m m wd (i.e. directed west and down), us a = 2 0 . 7 m m ne (i.e. directed n o r t h and east), usp = 4 . 2 m m ed (i.e. directed east and down). t a b l e i amplitude readings ( m m t r a c e a m p l i t u d e ) c o m p o n e n t sp s e 4 . 2 e 1 6 . 3 w n — 2 1 . 9 n z 2 . 8 d 5 . 6 d d i r e c t i o n of m o t i o n is i n d i c a t e d b y : e = e a s t n = n o r t h w = w e s t d = d o w n . w e t h e n f o r m t h e following ratios: usv : usp = 4 . 2 and wsv : wsp — 2 . 0 . [1] trace amplitudes can be used as the periods of sp and s are practically t h e same (8-10 sec) and we use only ratios of amplitudes in the following. the sp-s doublet is most pronounced on t h e -b-coinponent, as expected, and t h e r e is no trace of sp on the jv-component, the 2-component shows a more complicated motion prior to s, and on z we h a v e measured t h e amplitudes which coincide with e. i t should be added t h a t t h e phase here interpreted as sp is a significant onset and it cannot be explained as due to microseisms. a r r i v a l t i m e s . a) absolute arrival times. the jeffreys-bullen (1940) tables give a t o t a l travel time for s of 18 min 4 8 . 7 sec for t h e given focal d a t a . this means a computed arrival time of s a t 15h29m37s5. this is in good agreement with the s-reading, whereas t h e $p-phase arrives about 6 sec t a b l e i i c r u s t a l m o d e l s u s e 1 ) . m o d e l c h a r a c t e r i s t i c s c r u s t a l v e l o c i t i e s k m / s e c p 8 m a n t l e v e l o c i t i e s k m / s e c p s c r u s t a l t h i c k n e s s k m r e f e r e n c e a s i n g l e l a y e r e d c r u s t 6 . 3 4 3 . 6 6 8 . 2 0 4 . 7 3 35 r i c h t e r (1958) b » » » 6 . 5 7 3 . 7 7 8 . 1 8 4 . 6 2 35 sellevoll (1964) 0 t w o l a y e r e d c r u s t ( 6 . 1 0 j 6 . 6 5 i 3 . 5 0 i 3 . 7 5 8 . 2 0 4 . 6 0 2 0 + 1 3 s a a s t a m o i n e n (1963) d s a m e a s a b u t molio s l o p i n g 3° w e s t w a r d s , e a r t h ' s s u r f a c e h o r i z o n t a l 6 . 3 4 3 . 6 6 8 . 2 0 4 . 7 3 35 i1) e s a m e a s a, b u t a n g l e of incid e n c e is 33° i n s t e a d of 30° 6 . 3 4 3 . 6 6 8 . 2 0 4 . 7 3 35 (!) v e r t i c a l l y b e n e a t h t h e s t a t i o n . o o 2! <! » m go o g > h o h a h o w ci to co 1 2 4 m. batii k. s t e f á n s s o n earlier t h a n this time. there is almost exact agreement between the observed and calculated arrival times of p . b) time difference s-sp. w i t h the notation of fig. 2, we get t h e following formula for the time difference s-sp: a, „ / c o s t ' s cos i'p\ ro, a t = h \ r , r , ) 1 2 1 sv earth's s u r f a c e y — w u moho f i g . 2 s c h c m a t i c a l p i c t u r e s h o w i n g t h e w a v e p r o p a g a t i o n ( i n c i d e n t s a t m o h o f r o m u n d e r n e a t h , sv a n d sp i n t h e c r u s t ) . t h e h e a v y , o p e n a r r o w s i n d i c a t e t h e d i r e c t i o n of first m o t i o n a n d r e l a t i v e a m p l i t u d e s . obviously, we h a v e to make some assumptions about the crustal struct u r e under t h e station in order t o apply equation [2]. the models which were used are given in table i i . model b represents an average of available measurements in fennoscandia for t h e lower crust a n d the upper m a n t l e . model g is based on explosion studies in finland in 1959-1961. i n model d a moderate westward slope of t h e moho has been introduced, such as could be expected f r o m t h e general topography of t h e area around umea. the angles of incidence, calculated b y ritsema's (1958) tables, are subject t o some uncertainties and therefore also a model e was calculated, the same structure as a b u t with an angle of incidence of s from below to moho of 33° instead of 30°. s p c o n v e r s i o n at t h e b a s e of t h e c u u s t 1 2 5 the calculated time differences s-sp are given in table i i i , which also gives t h e crostili thicknesses necessary to give t h e observed time difference of 6 . 3 sec. despite t h e differences between observed and calculated values, there is no doubt t h a t t h e agreement is satisfactory, taking error limits into account. the measured time difference of 6.3 sec has an error of a t least ± 0-4 sec, a n d t h e requested crustal thickness is only slightly above 40 k m . this thickness is verified b y phase velocities of surface waves (tryggvason, 1961, and crampin, 1964). taking also t h e u n c e r t a i n t y in the assumed velocities into account, we can safely say t h a t our observation falls within reasonable limits. a m p l i t u d e r a t i o s . classical theory was applied to calculate t h e transmission across the mohorovicic discontinuity (costain, cook and algermissen, 1963), assuming a density ratio of 0.8 a t t h e b o u n d a r y and a poisson ratio of 0.25. the amplitudes incident a t the e a r t h ' s surface calculated in this way, were converted into ground displacements b y means of gutenberg's (1944) graphs. the calculated amplitude ratios (table i i i ) are then directly comparable with the observed ratios [1]. t a b l e i i i t i m e d i f f e r e n c e s a n d a m p l i t u d e r a t i o s f o r g i v e n m o d e l s . m o d e l t i m e d i f f e r e n c e s-sp sec c r u s t a l t h i c k n e s s ( k m ) f o r t i m e d i f f e r e n c e s-sp = 6 . 3 sec a m p l i t u d e r a t i o s usv'msp wsv-wsp a 4 . 8 46 5 . 3 2 . 8 b 4 . 7 47 5 . 9 3 . 4 g 4 . 7 d 4 . 8 46 4 . 5 2 . 3 e 4 . 9 4 5 4 . 3 2 . 8 we find t h a t models d and e give t h e best agreement. taking into account t h a t t h e error limits of the observed ratios [1] can be estimated as a b o u t ± 0.4, the agreement with models d and e is satisfactory. this m a y even be t a k e n as a suggestion t h a t moho is in f a c t gently sloping westwards in t h e umea area1 2 6 m. b â t i i ii. s t e f â n s s o n [4] p a r t i c l e m o t i o n . a) direction of first motion. with common notations (see b â t h , 1961) we h a v e : iw d<p , dw io, u = — — ^ ; w = — + — l3j ï>a; sz i s 3a; with ip = b exp [ilc (x + ßz — coi)] + b, exp [¿7c (x — ßz — wt)] > y = b' exp [ilc (x + ß'z — cot)] 0 = exp [¿7c (x — az — coi)] <p' — a' exp [ik (x + az — wi)] . this gives the displacement in the incident $ f w a v e : ii = — bilcß exp [¿7c (x + ßz — cot)] w = bile exp [¿7c (x + ßz — cot)], in the t r a n s m i t t e d £ f w a v e : u = — b'ikß' exp [ik (x + ß'z — wt)] w = b'ik exp [ik (x + ß'z — coi)], and in the t r a n s m i t t e d p w a v e (sp): u = a'ik exp [¿7c (x + a'z — cot)] w = a'ika exp [¿7c (x + a'z — cot)] . [5] [6] [7] if we choose b < 0, we have for t h e incident sv t h a t u > 0, iv < 0, and for t h e t r a n s m i t t e d sv t h a t u > (i, w < 0, and for t h e t r a n s m i t t e d p ( s p ) t h a t u < 0, to < 0, because t h e ratios b'\b and a!/b are real and positive (b'\b = + 0.90 and a!\b = + 0.29 according to b a t h , 1961, table 3, for e = 60°). these theoretical displacements are shown in fig. 2 and t h e y agree with the observations (fig. 1). the f a c t t h a t t h e first motion of sp is opposite to t h a t of sv on t h e horizontal (7?) component, certainly facilitates its discovery, as distinct from t h e vertical component where sp and sv have t h e same sign. b) total particle motion. w e have so f a r been considering only the first onsets of t h e respective waves. e x a m i n i n g the entire particle motion we find t h a t it is nearly linear both for sp and for s, and t h a t s p c o n v e r s i o n at t h e b a s e of t h e c u u s t 1 2 7 t h e motions of b o t h waves agree closely with theoretically expected motions (assuming model a). the minor deviations can be explained p a r t l y b y readidg difficulties, especially of sp on the vertical component, p a r t l y b y inaccuracies in t h e angles of incidence. anyway, t h e r e is no doubt t h a t sp is a longitudinal wave related to sv as expected from theory. d i s c u s s i o n . considering b o t h t h e error limits in t h e observations and inevitable uncertainties in t h e theoretical models, we conclude t h a t t h e interpretation of t h e $p-phase is convincing. this is t h e simplest interpretation, and as it has proven successful there is no need for investigation of more complicated wave propagations. moreover, comparisons of observations and theoretical models suggest a crustal thickness of about 40-45 k m in t h e u m e a area, with t h e moliorovicic discontinuity possibly sloping gently westwards. i t is a justified question w h y this phenomenon is observed so seldom. a correct s t a t e m e n t would probably be to say t h a t it exists much more frequently t h a n generally realized, b u t it is only seldom t h a t very clear cases are observed. there are a n u m b e r of factors which contribute to this effect: 1) i t is advantageous for discovery t h a t t h e direction t o t h e epicenter coincides with one of t h e cardinal directions, as otherwise a n y earlier wave wtill show up on b o t h e and n components and will easily escape detection. 2) i t should show u p on the z-component independent of azimuth, b u t z is usually b a d for the /s'-wave group, and s is generally read from t h e horizontal components. 3) certain angles of incidence a t moho f r o m below are more favourable for s-p conversion t h a n others. this means t h a t certain distances in combination with certain slopes of the moho discontinuity are more favourable t h a n others (see b a t h , 1961). 4) the ratios of ground displacements (i.e. corresponding t o recorded displacements) to incident amplitudes v a r y considerably with t h e angles of incidence a t t h e surface (gutenberg, 1944, pig. 3). i n our case we are close to optimum conditions, b u t only smaller changes 1 2 8 m . b a ' i ' i i i t . s t e f a n s s o n in the angles of incidence will entail considerable changes in the recorded amplitudes. 5) a substantial s f-component radiated in t h e direction of the recording station is needed (this means a certain restriction on t h e focal mechanism). 6) the microseismic noise level should not be too high, as otherwist; a small sp m a y be lost or difficult to discover reliably. 7) two sharp onsets (sp a n d s) are required in clear cases, b u t in most cases s-phases are more blunt t h a n p-pliases and even if a multiplicity exists in s this can often not be established with full certainty. a combination of the factors mentioned is probably enough to explain why clear cases are observed so seldom. press-ewing seismographs are also operated a t uppsala, b u t in the case studied here t h e y do not exhibit the sp-phase. this could p a r t l y be due to smaller magnification a t uppsala t h a n a t umea, p a r t l y to unfavourable conditions a t molio. also, t h e phenomenon does n o t show up on the u m e a short-period records, as t h e periods (8-10 sec) are too long for these i n s t r u m e n t s . other cases of sp have been observed a t our stations, though less clear t h a n t h e present one. several of t h e aftershocks of t h e alaska e a r t h q u a k e on 28 march, 1964, gave »sip-phases on t h e ^ c o m p o n e n t s a t uppsala, k i r u n a a n d umea. this was especially the case for the aftershocks on 3 april (22 33 42.2), a n d 4 april (17"46ra08a.6 and 22"16m54*.5), 1964. reliable readings of s are of basic importance for our knowledge about t h e e a r t h ' s interior, in calculations of transverse wave velocities, focal mechanism studies a n d for the interpretation of surface wave observations. n e x t to p and pp it is no doubt t h e most i m p o r t a n t phase in a seismic record. i n a more detailed analysis it is t h e n necessary to t a k e the possible existence of sp into account. sp is more t r o u blesome for accurate ¿'-readings than ps is for p-readings, as ps arrives a f t e r p , b u t m a y disturb pp. it. is f r e q u e n t l y found in routine measurements t h a t the arrival of s differs a few seconds between t h e two horizontal components. i t is then customary to consider the earlier arrival as t h e t r u e s and to report only this onset in bulletins. however, it is obvious f r o m this paper t h a t such a procedure m a y often be incorrect (the earlier arrival m a y be sp). i n order to decide the question in a n y individual case, it is necessary to apply criteria such as those used here. s p c o n v e r s i o n at t h e b a s e of t h e c u u s t 1 2 9 the time differences ps-p and sp-s have sometimes been used for calculation of crustal thicknesses, b u t for higher accuracy this necessarily supposes t h a t crustal velocities are well known. a c k n o w l e d g e m e n t . one of us (r.s.) would like to express his gratitude to the icelandic science f o u n d a t i o n for a scholarship enabling him to do this research a t t h e seismological i n s t i t u t e , uppsala, sweden. seismological i n s t i t u t e , uppsala, sweden. march, 1966. r e f e r e n c e s bâtii m., polarization of transverse seismic waves. « g e o p l i y s . j o u r . r o y . a s t r . soo. », 4 , 106-123, (1961). bath m. a n d tryggvason e . , deep seismic reflection experiments at kiruna. « g e o f i s . p u r a e a p p l . », 5 1 , 79-90, (1962). choudhury m . a . , étude des ondes séismiques dans quelques tremblements de terre profonds de vhindou-kouch. « a n n . g é o p h y s . », 14, 31-75, (1958). c o o k k . l . , a l g e r m i s s e n s . t . a n d c o s t a i n j . k . , the status of ps converted waves in crustal studies. « j o u r . g e o p l i y s . r e s . », 6 7 , 4 7 6 9 4 7 7 8 , (1962). c o s t a i n j . k . , c o o k k . l . a n d a l g e r m i s s e n s . t . , amplitude, energy, and phase angles of plane sv ivaves and their application to earth crustal studies. « b u l l . s e i s m . soe. a m e r . » , 5 3 , 1039-1074, (1963). crampin s., higher modes of seismic surface waves: phase velocities across scandinavia. « j o u r . g e o p h y s . r e s . » , 6 9 , 4 8 0 1 4 8 1 1 , (1964). gutenberg b . , energy ratio of reflected and refracted seismic waves. « b u l l . s e i s m . soo. a m e r . » , 3 4 , 85-102, (1944). jeffreys h . a n d bullen k . e . , seismological tables. « b r i t . a s s o c . a d v . sci. », 4 8 p p . , (1940). pachadzhanova g . n . , study of transformed waves sp type during deep afghan earthquakes (in r u s s i a n ) . a k a d . n a u k . t a d z k i k s s r , d u s h a n b e , t r u d y , 10, 10s-124, (1962). riciiter c. f . , elementary seismology. w . h . f r e e m a n a n d c o . , s a n f r a n c i s c o , 768 p p . , (1958). ritsema a . r . , (i, a)-curves for bodily seismic waves of any focal depth. « l e m b a g a m e t e o r , d a n geofis.», d j a k a r t a , v e r l i a n d e l , 5 4 , 10 p p . , (1958). 1 3 0 m . b a t h r . s t e f a n s s o n saastamoinen p . , travel times for a erustal model in finland. p u b l . i n s t . s e i s m . h e l s i n k i , 5 8 . (1963). sellevoll m. a . , a -publications summary on erustal and upper mantle structure in fennoscandia, iceland and the norwegian sea. y e s i ac r e p . 4 4 1 0 7 5 x , 111-125, (1964). treskov a. a . , on teleseismic determinations of the earth's crust thickness. p u b l . b u r . c e n t r . s e i s m . i n t . , a 2 2 , 33-39, (1961). tryggvason e . , crustal thickness in fennoscandia from phase velocities of bayleigh waves. « a n n a l i d i g-eofisica », 14, 2 6 7 2 9 3 , (1961). yun-yu d . , sp' phases and the crustal thickness of china (in c h i n e s e ) . « a c t a g e o p h y s . s i n i c a » , 14, 168-172, (1965). u n ' o s s e r v a z i o n e s u l l ' a n d a m e n t o a n n u o dei valox-i orari di grandezze geofisiclie a c o n t r o l l o solare p . d o m i n i c i 1. esiste u n a abbastanza v a s t a categoria di grandezze geofìsiclie il cui valore dipende, in maniera più o meno diretta, anche dall'altezza del sole sull'orizzonte (grandezze a controllo solare), e che cioè possono essere rappresentate da u n a funzione del tipo: y — f (..., xi, . . :, x), [1] essendo x l a distanza zenitale del sole e le xi altre variabili, diverse da poiché x è legata all'angolo orario co del sole dalla ben nota relazione: cos x = s e n 7> s e n + c o s 9 c o s <5 c o s m i [2] dove cp è la latitudine del luogo d'osservazione e ó è la declinazione solare, la y può riguardarsi come funzione delle xi, di cp, ò e co: v = 7 * (•••,»<> <5, co). [3] se le rilevazioni di y fossero e f f e t t u a t e sistematicamente con un programma orario riferito a co, cioè al tempo solare vero, l'andamento nel corso dell'anno del valore y relativo al valore particolare co di co risulterebbe descritto da u n a funzione del tipo: v = / * ( • • •••,<p, ò, co), [4] nella quale, posto che il luogo d'osservazione rimanga sempre il medesimo, comparirebbero come variabili indipendenti solo le xi e ò. se invece, come si f a abitualmente, il programma delle rilevazioni di y viene stabilito con riferimento al tempo civile della stazione d'osservazione p, tale andamento assume la forma: y =/*(..., xi, ...,<p,ò,[(o + co12]), [5] 2 7 4 p . dominici nella quale compare una nuova variabile, l'angolo orario oj12 del sole al mezzogiorno civile di p , che misura la differenza t r a il tempo solare vero e il t e m p o civile in p . com'è noto, è co12 = e + a, [6] dove e, che gli astronomi chiamano equazione del tempo, è la differenza, variabile nel corso dell'anno, t r a il tempo solare vero e il tempo solare medio e a è la longitudinale di p rispetto al meridiano centrale del fuso orario al quale p appartiene, positiva o negativa secondo che p si trovi a e s t oppure a ovest di tale meridiano. naturalmente l'andamento nel corso dell'anno di a>12 ripete, a meno della costante addittiva a, quello di e (v. fig. 1). la [5] può essere scritta, in forma puramente simbolica, come segue: f = y f (wn). [7] l ' a n d a m e n t o annuo dei valori orari della y rilevati in p, y, differisce p e r t a n t o da quello, y, peculiare della y medesima per una grandezza, f(oj12), che, dipendendo dalla scelta del riferimento temporale, è di n a t u r a p u r a m e n t e convenzionale. nel presente lavoro ci proponiamo di discutere, in via del t u t t o indicativa, l'entità dell'errore, o, se si preferisce, della « distorsione di forma », introdotto dalla funzione f(a>n), precedentemente definita, nell'andamento annuo dei valori orari di una grandezza del tipo [1]: precisamente, sull'andamento annuo dei valori meridiani di densità elettronica massima in uno strato ionosferico. 2. consideriamo uno strato ionosferico nel quale l'equilibrio elettronico sia governato da un'equazione della forma seguente: ì>n — = q0 cosx—px, [8] dove n è il numero di elettroni liberi per cm3, q0 il numero di elettroni liberi prodotti per cm3 e per secondo nello strato allorché il sole culmina allo zenit (cioè allorquando è co = 0) e /s una quantità, fisicamente interpretabile come coefficiente di attaccamento t r a elettroni liberi e molecole circostanti, che supporremo costante. osserviamo incidentalmente che la [8] può assumersi, almeno in prima approssimazione, a rappresentare le condizioni esistenti nella regione f2 della ionosfera. poniamo: n (t) = n0 v (t) , [9] u n ' o s s e r v a z i o n e s u l l ' a n d a m e n t o a n n u o d e i v a l o r i orari ecc. 2 7 5 dove n0 è il valore di n per x = 0. risulta poi conveniente, in virtù della [2j, passare dalla variabile t, in secondi, alla variabile co, in radianti (i), sicché scriveremo la [8] nella forma: pl = (a + b cos co) — a b v [10] o co jsi0 a = 86'400/2 n = 1,37 io4 a = sen cp sen <5 [10'] b = cos cp cos (5. a norma della [9] la funzione adimensionata v (co) deve quindi soddisfare la condizione: | a+b = 1 . [10"] posto: <) v \ = g , [ 1 1 ] e: ì) co j x = o [12] -"0 e la [10] diviene: — = (a b + g) (a + b cos co) — a b v . [13] o co riguardando la v come funzione della sola variabile co, la soluzione generale della [13] risulta la seguente: \v(m)=(a p+c) a + {a p+c) (sen co+ct,p cos c o ) e x p [—a,p co] ^ = costante . assumiamo, per semplicità, a p = 1 : ciò equivale ad assumere per p il valore 7,3-io"5, valore del resto fisicamente verosimile. con tale posizione, la soluzione particolare della [13] che soddisfa la [10"] è la seguente: v (co) = (1 + g) a + ( 1 + 0 c ' ) b (sen co + cos co) + 2 [15] 1 _ e — (1 + g) a h n e x p [ ^j" 2 7 6 p . dominici l'andamento annuo dei valori di v relativi all'istante del passaggio del sole al meridiano della stazione d'osservazione p , cioè per co = 0, è espresso dalla funzione: v ( . ) = f l + c)(a+b)+1-c ( [16] 2 che è del tipo [4]. l'andamento annuo dei valori di v al mezzogiorno civile di p è invece rappresentato dalla funzione: n i f \ tt v(m12) = (1 + g)a + (sen co12 + cosco12) + [17] l—o—{1 + 0 a r . j + g e x p wl2l ' che è del tipo [5]. come appare dalla fig. 1, e è sempre abbastanza piccolo, non superando, in valore assoluto, 0,07 radianti; a, da parte sua, non supera, radianti gradi g e n . f e h m a r . ap r. m a g . g i ù . l u g . a g 0. s e t . o t t . nov. d i e . minuti •0,07 •0.06 •0,05 • 4 02 0 02 0 1 02 0 02 0 1 02 0 i 02 0 1 02 0 1 02 0 1 02 0 1 02 0 1 32 0 i 02 0 minuti •0,07 •0.06 •0,05 • 3 = — — _ — — — / * * •15 • 0.04 •0,03 •0.02 i •0,01 0 -0.01 -0.02 -0,03 -0.04 0 . 0 5 -0.06 • 2 • 1 0 1 2 3 5 * / t — s \ s / / i w * -+10 + 5 0 5 —10 f i g . 1 a n d a m e n t o nel corso d e l l ' a n n o d e l l ' e q u a z i o n e del t e m p o . in valore assoluto, 7°30', pari a circa 0,13 radianti. complessivamente (ii) cul2 si mantiene, in valore assoluto, t a n t o piccolo da potersi scrivere, con buona approssimazione: ! s e n a>12 + cos col2 ^ 1 + co12 — —•2 2 f 1 8 ] r t &>122 exp [— o)12] ^ 1 — oj12 + — n u n ' o s s e r v a z i o n e s u l l ' a n d a m e n t o a n n u o d e i valori orari ecc. 2 7 7 sicché la [17] può scriversi: / ft)iaa\ ( l + c f ) ( ^ + j ? ) _ l + 0 + w12 ; [19] confrontando le [16] e [19] si ha infine: " k*) -i \ (r. " a tt + c)(a+b)-l + c rom " h t = 1 + 2 j fl + 0 ) ( a + b ) + l 0 • [20] se: i g | « 1 [21] l'errore percentuale av introdotto da w12 nei valori v(o) vale: w12-\ a+b—l " ^ " ^ ì f r f + f 1221 se invece non vale la condizione [21], in luogo della [22] dovremo considerare la seguente relazione: co, av* = a v + 100 co12 — —— g . [22'] nella fig. 2 è riportato l'andamento della q u a n t i t à a v t r a le latitudini 67°30' s e 67°30' n per stazioni al centro del proprio fuso orario [a = 0) e al limite e s t ( a = 7°30') e ovest (a = — 7°30') del fuso stesso, il 15 di ogni mese. indichiamo con / 0 0 la frequenza critica che si misurerebbe in u n a d a t a stazione al mezzogiorno solare, con /0 1 2 quella che si deduce dagli ionogrammi registrati al mezzogiorno civile. ricordando che è: n = 1,24 io8 /02 , [23] si ha dalla [20]: ovvero, per la relativa piccolezza del termine av 100" t ^ = 1 + 4 f • t25} 7o,o 50 poiché si deve ritenere che, in generale, le frequenze critiche ionosferiche siano misurate con u n a precisione non inferiore a ± 1 % , l'effetto qui considerato non può a rigore essere trascurato appena a v superi il va2 7 8 p . d o m i n i c i lore ± 2, il che, come si vede dalla fig. 2, accade frequentemente alle medie e alte latitudini per stazioni in posizione eccentrica nel proprio fuso orario. nella fig. 3 è riportato l'andamento di v (cu12) relativo a due stazioni a latitudine 45° n. geograficamente assai vicine ma appartenenti a due fusi orari diversi: u n a di tali stazioni (curva a in fig. 3) risulta p e r t a n t o 1 5 g e n 1 5 f e b . 1 5 n 1 a r . 1 5 a p r . 1 5 m a g . 1 5 g i ù 1 5 l u 3 j 1 5 4 g 0 . 5 e t j 1 5 o t t . n n o v . 1 5 d i a s u i u n s • n s n s 0 n j s . n s 0 n j s . n j s n b n s o t t . n n n , b ' | j i i i i ! 1 1 1 7 i l i l 1 ! 1 6 1 : m i 1 ' u ' : i t l 1 1 13 [ | ili ! ! 1 ! 1 1 " ' i 1 1 1 1 i hi 1 1 1 : ' t i i 1 i o , ; 1 ; i m i l h li il 1 . h r t t i e r i : fi 1 ! il i 8 . 1 . : h fa ! \ i l 7 m i i ì ; ' il i • 1 | ' l1 6 ì j l i i i h ! 1 5 • ; ! . . ] . \ i 1 ] i 1 i i i . . l . . j i l i il 1 1 i i 3 i 1 ii 1 \ i i 1 1, 1 1 « 1 1 1 1 2 r lì , ] _t \ / / il i l lì \ u . i k / \ \ ' v \ i / \ i f a ( / o r v v ' , \ ì s j . v i t t 1 t i v i \ " / f \ \ 2 'i ' ì -, i \ p 7 ' / ' \ v \ \ | ; ] h,i \ t r s t u j j : h ; i ; 1 » ! t t i l ì i 7 _ i l i i ^ i i « i j . . . l . i l i 9 : i . . . . . . . . . j i i ' 0 . 1 t i l l i . : i . . . . . . t . l . . . . i i 12 1 i ' 3 i i 1 n i i l i t a 11 n = t r 7 ° 3 = 7 ° 3 ! " i a 11 n = t r 7 ° 3 = 7 ° 3 a 11 n = t r 7 ° 3 = 7 ° 3 3 e o ' o ! i r 1 1 1 m 1 1 1 1 1 i n f i n i i i i i l l i m i i l 1 1 i l 1 n h i i m i a 11 n = t r 7 ° 3 = 7 ° 3 3 e o ' o i i f i g . 2 t u t t a spostata a est, l'altra (curva b in fig. 3) t u t t a spostata a ovest nel proprio fuso orario. nella stessa fig. 3 la curva c rappresenta la differenza percentuale a /0,12 delle frequenze critiche /012 misurate nella prima stazione rispetto a quelle misurate nella seconda: osserviamo che tale differenza sarebbe, naturalmente, nulla se le due stazioni operassero con lo stesso programma orario. la curva tratteggiata, intermedia t r a le u n ' o s s e r v a z i o n e s u l l ' a n d a m e n t o a n n u o d e i valori orari ecc. 2 7 9 curve a e b, indica l'andamento della q u a n t i t à v (0) per le due stazioni considerate. 3. le conclusioni alle quali siamo giunti nella discussione testé conclusa su un caso molto particolare portano a ritenere che, in generale, debba porsi attenzione all'effetto qui considerato ogniqualvolta si debbano considerare a n d a m e n t i annui di grandezze geofisiche a controllo f i g . 3 solare i cui valori orari siano riferiti al tempo civile delle stazioni d'osservazione o al tempo universale. spetta n a t u r a l m e n t e allo sperimentatore, sulla base della n a t u r a della grandezza investigata e della precisione intrinseca dei dati d'osservazione, decidere sull'opportunità o meno di introdurre u n ' o p p o r t u n a correzione sui dati rilevati. la questione ci sembra t u t t a v i a che abbia non trascurabile importanza per le grandezze ionosferiche, per le quali il controllo solare è molto stretto; particolarmente sensibili all'effetto considerato dovrebbero essere le frequenze critiche della regione f2, che, com'è noto, presentano variazioni orarie molto cospicue. 2 8 0 p . d o m i n i c i a p p e n d i c e (i) il legame t r a t e co è espresso dalla seguente relazione: t = — r (co + n) + h 86'400 z 71 ( k = intero , ovvero, d e t t o t0 il valore di t alla mezzanotte di un qualunque giorno, dal grafico di fig. 4. operare il cambiamento di variabile indicato significa p e r t a n t o sostituire a u n a variabile non periodica, t, u n a variabile, co, periodica di periodo un giorno; poiché il legame t r a t e co è lineare, si vengono a introdurre solo costanti moltiplicative nelle espressioni differenziali in cui si opera il cambiamento di variabile. ciò posto, ricorderemo che la funzione n (t) che compare nella [8] (e nelle equazioni similari che possono scriversi considerando diversi e più complessi meccanismi di equilibrio elettronico nella ionosfera) è u n a funzione pseudoperiodica, di pseudoperiodo un giorno: la n (t) si ripete infatti, a meno di trascurabili differenze, in giorni successivi ma, n a t u r a l m e n t e non si conserva inalt e r a t a nel corso di un intero anno. d a t o che la variabile t non è periodica, per rispecchiare tale realtà fisica si impone usualmente alla soluzione generale della [8] di essere periodica di periodo un giorno: il che, come ben si comprende, non è rigorosamente esatto. operando il cambiamento di variabile sopra indicato, la pseudoperiodicità della funzione rimane soddisfatta per la n a t u r a stessa della nuova variabile, e l'unica condizione postulabile per la funzione, cioè il soddisfacimento di u n a conveniente condizione iniziale, rimane di n a t u r a p u r a m e n t e analitica. -tc~ f i g . 4 u n ' o s s e r v a z i o n e s u l l ' a n d a m e n t o a n n u o d e i valori orari ecc. 2 8 1 (ii) ricordiamo che, per convenzione (*), uno ionogramma s'intende rilevato nell'istante in cui la frequenza della ionosonda è di 3 mhz. d e t t o r l'intervallo di tempo impiegato dalla ionosonda per passare da 3 mhz a 8 r 15 mhz, valori propri delle più alte f r a le frequenze critiche ionosferiche, la [6] andrebbe a rigore modificata come segue: c 0 1 2 = £ j a + t , nella quale espressione r s'intende n a t u r a l m e n t e misurato nella stessa u n i t à nella quale sono misurati e e a l a q u a n t i t à r può ritenersi del t u t t o trascurabile per le ionosonde « veloci », che impiegano da 7 secondi a 2 minuti per esplorare l'usuale campo di frequenza da 1 a 20 m h z : può invece giungere a valori notevoli nelle ionosonde « lente », nelle quali la d u r a t a di rilevazione dello ionogramma va da 4 a 10 minuti. comunque, dato che le ionosonde a t t u a l m e n t e in uso rilevano gli ionog r a m m i per valori crescenti di frequenza, r, quando non è trascura vile, è u n a q u a n t i t à sempre negativa. riassunto esiste la possibilità che l'andamento dei valori orari di grandezze geofisiche a conti-olio solare (che dipendano cioè anche dalla distanza zenitale x del sole) possa essere alterato dal fatto che i valori di tali grandezze siano rilevati con un programma riferito al tempo civile della stazione d'osservazione anziché al tempo solare vero. a titolo indicativo viene calcolata l'entità di tale « distorsione di forma » in un caso particolare, quello dei valori meridiani di densità elettronica massima n di uno strato iono(*) i g y i n s t r u c t i o n m a n u a l , v o i . i l i , p a r t . 1 ( i o n o s p h e r i c y e r t i c a l s o u n d i n g s ) , p a g . 157. s o u n d i n g s c h e d u l e » 3. timing. t h e r e f e r e n c e t i m e f o r i o n o s p h e r i c s o u n d e r s is t l i a t of t h e z e r o m e r i d i a n ( u t ) , a l t o u g h l o c a i s t a n d a r d i n e r i d i a n t i m e , d i s e r i n g f r o m z e r o m e r i d i a n t i m e b y a n i n t e g r a i n u m b e r of h o u r s , is m o r e c o n v e n i e n t a n d s l i o u l d b e u s e d i n ali r e p o r t s t h e n o m i n a i t i m e of a s o u n d i n g is d e f i n e d a s t h e t i m e w h e n t l i e i o n o s o n d e r e c o r d s t h e s t a n d a r d f r e q u e n c y 3 me. t h e n o m i n a i a n d s c h e d u l e t i m e of s o u n d i n g s h o u l d b e e q u a ! w i t h i n 0 . 5 m i n . 2 8 2 p . d o m i n i c i sferico nel quale l'equilibrio elettronico sia governato da un'equazione del òn tipo: — f (x)—fin. le conclusioni alle quali si perviene in questo "ò t caso particolare portano a ritenere che, in generale, si debba porre attenzione a tale effetto, massimamente per stazioni a media e alta latitudine in posizione eccentrica nel proprio fuso orario. abstract there is a possibility that behaviour of hourly values of geophysical data depending from the zenith distance x of the sun may be altered by the fact that these values are computed with reference to the civil time of the observing station and not to the solar time. for the salce of an example, the amount of such " distortion of forni has been calculated for the particular case of the meridian values of the maximum electron density n of a ionospheric layer in which the electronic balance, is given ~òn by an equation of the folloiving type: — = / (yj —(ìn. the results obli t tained in this particular case allow us to think that this effeete must be, generally, into consideration, expeciàlly for medium and high-latitude stations whose position in their respective time zones is eccentric. 04bautista.qxd 379 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical earthquake catalog – philippine earthquakes – seismic hazard assessment – archival documents – historical accounts 1. introduction reliable seismic hazard studies depend on having a robust earthquake catalog. the longer the extent of the catalog and the more reliable the parameters are, the better it is for those doing seismic hazard analysis. there are two kinds of earthquake catalog: one is the instrumental or recent catalog and the other is the historical catalog. in this study, instrumental or recent catalog refers to the time when seismic monitoring existed while historical refers to the pre-instrumental period. in the philippines, instrumentally-derived parameters are available for events from 1892 onwards. the year 1892 was selected because the first earthquake with instrumentally derived parameters was determined by abe (1994) for an earthquake in 1892. the historical period for the case of the philippines could be considered as covering the period before 1892. unlike other countries with long earthquake histories and despite the the philippine historical earthquake catalog: its development, current state and future directions maria leonila p. bautista and bartolome c. bautista philippine institute of volcanology and seismology (phivolcs), department of science and technology (dost), quezon city, philippines abstract this report will trace the development of the historical earthquake catalog of the philippines, assess its present state and recommend future research directions. the current philippine historical earthquake catalog is culled from various catalogs, both global and local, that were developed since the first philippine catalog by perrey was published in 1860. while early global catalogs gave simple mention of earthquakes in the philippines, more focused earthquake catalogs about the philippines gave more explicit descriptions of earthquake accounts and adopted descriptions by local historians. over the years, various historians and seismologists continued to compile their catalogs whose contents depended on the author’s perspectives and purposes. these works varied from simple listings to others including detailed descriptions. it was only recently that an attempt made to parameterize the magnitudes and epicenters of philippine historical earthquakes using magnitude-felt area relations was done. a more detailed catalog, however, is now underway that will show details of intensity distribution for each significant historical earthquake. by comparing the historical catalog with the recent catalog and assuming that the recent catalog is complete, we find that there are still a substantial amount of historical earthquakes that needs to be reviewed and located. possible sources of new information are local libraries, museums and archives in the philippines, spain and other southeast asian countries to which the country was in contact with during historical times. mailing address: dr. maria leonila p. bautista, philippine institute of volcanology and seismology (phivolcs), department of science and technology (dost), c.p. garcia avenue, up campus, diliman 1100, quezon city, philippines; e-mail: leyo@phyvolcs.dost.gov.ph 380 maria leonila p. bautista and bartolome c. bautista philippines being seismically active, the philippine historical catalog is not that long compared to other countries. the historical accounts only started during the time when the country was conquered by spain in the late 15th century. this study will discuss the development of the historical earthquake catalog in the philippines, assess its present state and recommend future directions. 2. existing catalogs the current philippine historical earthquake catalog is culled from various catalogs, both global and local, that were developed since the first philippine catalog by perrey was published in 1860. early global catalogs made simple mention of earthquakes in the philippines. meanwhile, catalogs that mainly focused on the philippines gave more explicit descriptions of earthquake accounts and adopted descriptions by local historians. over the years, various historians and seismologists kept on building their own catalogs whose contents varied depending on each author’s perspective and purpose. for example, some authors would make simple listings of earthquakes while some would include detailed descriptions. some authors would only list tsunamigenic events. others gave their own intensity estimates as well as drew their own isoseismal maps. a recent work by bautista and oike (2000) has parameterized the magnitudes and epicenters of well-described philippine historical earthquakes using magnitude-felt area relations. 2.1. global earthquake catalogs it was only when colonization of the philippines started in the late 15th century that earthquakes affecting the country started to be noticed and documented. these earthquake citations may be grouped into those cited in global catalogs while the other group belongs to the country-specific catalogs. global earthquake catalogs such as those made by von hoff (1841), mallet (1853, 1854, 1855) and milne (1912) mentioned earthquakes about the philippines. the work of von hoff (1841), written in german, listed three earthquakes and three volcanic eruptions in the philippines. mallet’s works were in english and among his references were the works of de guignes (1808), garnier (1839) and articles from the singapore chronicle (1824, 1829). meanwhile, milne (1912) listed 66 philippine historical earthquakes from 1589 to 1909 in his catalog. his source is mainly the work of maso (1895). more recently, dunbar et al. (1992) listed epicenters, magnitudes and damages attributable to philippine earthquakes from 1599 to 1990. meanwhile, some authors listed tsunamigenic earthquakes only (rudolf, 1887; heck, 1947; iida et al., 1967; berninghausen, 1969; cox, 1970; nakamura, 1978). reviewing global data since 1616, iida et al. (1967) listed 26 tsunamigenic earthquakes in the philippines. they also listed the areas affected and estimated heights based on descriptions. berninghausen (1969), meanwhile, listed tsunamis in southeast asian region which included 29 events in the philippines. his database covered the period 416 to 1965. the various works of abe (1981, 1984, 1994) and abe and noguchi (1983) especially their redetermination of the magnitudes by gutenberg and richter covering the early 20th century period helped to improve the earthquake data in the philippines including those during the 1892 to 1900 period. 2.2. country-focused historical earthquake catalogs: philippines while the earliest earthquake reported for the philippines was the one which happened in 1589, the first known country-focused earthquake catalog for the philippines is the work of perrey (1860) which was in french. this catalog described both earthquakes and volcanic eruptions in the philippines from the 15th to 18th centuries starting with an earthquake in 1601. his sources included various european travellers in asia who wrote books, chronicles or accounts of their travels. despite exaggerations and misquoted names of places, perrey’s work became one of the most referred to earthquake catalog for the philippines. 381 the philippine historical earthquake catalog: its development, current state and future directions the creation of the manila observatory in 1865 paved the way for the systematic collection of earthquake reports in the country. maso, a researcher at the manila observatory, published an earthquake catalog in 1895 written in spanish. his work could be considered the first attempt to document the historical accounts made by spanish priests about earthquakes affecting their localities. his work is more comprehensive than that of perrey, although the main difference is that it dealt exclusively with earthquakes and excluded volcanic activities. his work also included the observations of the manila observatory and its various stations in the provinces. in addition, maso also drew isoseismal maps of selected large earthquakes using the manila observatory intensity scale of i to vi (table i). later, culling much from the work of maso, algue (1900), who was also from the manila observatory, published his own catalog covering the period 1870 to 1897. in 1901, after the spanish conquerors had left the country, the new american government reorganized the manila observatory and created a philippine weather bureau. aside from weather monitoring and forecasting, the bureau also conducted seismic monitoring. maso continued on his compilation and updating of earthquake events and published several catalogs (maso, 1909; 1927, maso and smith, 1919). the 1895 and 1927 maso catalogs have basically the same contents except that the 1927 version only contained the damaging events. in 1928, fr. william repetti joined the manila observatory and the philippine weather bureau. he was placed in charge of all seismological stations in the philippines. he was chief of the section of seismology and terrestrial magnetism of the philippine weather bureau for more than a decade. in 1946, repetti published the most comprehensive historical catalog of the philippines covering the period 1589 to 1899. repetti translated various accounts written in various languages into english. repetti’s work included the results of perrey and maso and at the same time included other accounts culled from history books, bulletins, letters and newspaper articles. after world war ii, the weather bureau was reorganized into the philippine atmospheric, geophysical, astronomical and services administration (pagasa). in 1982, the office of foreign disaster assistance (ofda) of the united states agency for international development (usaid) undertook a project called «seismic data gathering, southeast asia». this project was undertaken under the auspices of the southeast asia association on seismology and earthquake engineering or seasee. among the projects of seasee is an earthquake catalog published in 1985 that compiled repetti’s historical data, instrumental data from the manila observatory and pagasa (1976) as well as reports of investigations of earthquakes from 1950 to 1983 (seasee, 1985). aside from these, the seasee catalog also included earthquake data determined for the philippines by gutenberg and richter (1954), gutenberg (1956), by the international seismological centre (isc) and the u.s. geological survey (usgs). the seasee catalog was divided into eight chapters: i) catalogue of philippine earthquakes 1589-1864; ii) catalogue of philippine earthquakes 1865-1899; iii) catalogue of philippine earthquakes 1901-1942; iv) catalogue of philippine earthquakes 1948-1983; v) catalogue of destructive earthquakes 1589-1983; vi) assessment of seismic intensity of philippine historical earthquakes; vii) seismic source zones of the philippines, and viii) seismotectonics of the philippines. the result is an 843-page compilation of earthquakes from 1589 to 1983. although some of these previous works, including the seasee catalog, estimated epicentable i. manila observatory intensity scale (after seasee, 1985). intensity description i perceptible ii light iii regular iv strong v violent vi destructive 382 maria leonila p. bautista and bartolome c. bautista ters, maximum intensities and even contained isoseismal maps, there was no attempt to make a systematic parameterization of these historical earthquakes to determine their magnitudes and epicenters. the first attempt was made by bautista (1993) who made use of magnitude-intensity at epicenter (io) relationships derived from philippine earthquake data to infer the sizes of philippine historical earthquakes. later on, bautista (1996) revised her empirical equations through the addition of more recent events and developed new empirical equations relating magnitude to felt area for various intensity levels. the same study estimated the epicenters and magnitudes of 74 historical earthquakes. a more extensive work by bautista and oike (2000) reviewed the historical accounts of more than 3000 earthquake events from 1589 to 1895. from these events, they were able to estimate the magnitudes and epicenters of 485 historical earthquakes in the philippines. while there were 3000 earthquake events, most of these were either felt only in one area or had no damage reported. only these 485 events were sufficiently described or could be assessed for parameter determination. meanwhile, as guides to future users of the catalog, their study also gave a quality assessment for each earthquake. this quality assessment is based on the number of places where accounts are available. to have a higher quality, an earthquake should have many accounts for many different places. having numerous accounts but limited to one place would still mean a lower quality of reporting. out of the 485 events, quality «a» was given to earthquakes with 10 or more reporting places, «b» to those with five to nine reporting places and «c» to those with four or less number of reporting places. using these criteria, they found out that only 52 of the total 485 events may be considered very reliable (quality a) and 80 events as quite reliable (quality b). the remaining events were given quality c and their determined parameters are not considered so reliable. 3. issues and concerns inaccuracies, exaggerations and biases sometimes creep into interpretation of historical accounts. thus, it is always better to go back to the primary source of information. the search for new sources of information started from the original list of references by previous authors. the original list has since more than doubled and potential new sources of information are now being considered. the only issue is how to find or locate these references for historical earthquake evaluation. meanwhile, aside from other factors such as change of names and jurisdiction, it is also important to consider knowledge of local history, geography and existing socio-political conditions when these earthquakes occur when reviewing historical earthquakes. samples of such errors were cited in bautista and oike (2000). one example is after an earthquake in 1645, when only the spaniards who perished during the earthquake were counted, while persons of «no account» were disregarded. since the number of casualties are, at times, used to gauge how damaging an earthquake was, the results from this procedure of counting could give quite misleading results. meanwhile, knowledge of local geography is also important. errors in naming places can be discerned if one is familiar with local geography. for example, the name of a place called «cagayan» was changed by perrey (1860) into «camarines» for an earthquake in 1627, when in fact the two places are 400 km apart. to solve such kinds of issues for various cases, it was always important to find out which place was actually affected by getting the information from the original source, if available, or by relating existing accounts with one another. 4. status of the philippine historical catalog after bautista and oike (2000) have determined the epicenters of 485 historical events, they also evaluated the completeness of the resulting historical catalog. they plotted the typical yearly number of earthquakes with m 6.5 and above for the philippines for both the historical and recent periods. they found out by comparing the historical catalog with the recent catalog and assuming that the 383 the philippine historical earthquake catalog: its development, current state and future directions recent catalog is complete, that there are still a substantial number of earthquakes that need to be reviewed and located especially during the period from 1600 to 1850. on the other hand, they also found out that earthquakes that occurred from the period 1850 to 1900 are almost completely reported. instead, what is missing in the catalog for the same time period are events with magnitudes lower than m 6.5 (fig. 1). 5. future directions the continued search for new information to cover the 1600 to 1850 period for all magnitude ranges and from 1850 to 1900 for magnitudes less than 6.5 will be actively pursued. among the possible sources are local libraries, museum and archives, libraries in spain and in southeast asia. spanish-era old churches in various towns are also promising sources as some of them keep records of when they were damaged by natural phenomena while some of them maintain a museum or archive. specific books on town histories are another potential source of earthquake histories. the internet offers information, although it should still be verified, about histories of churches, dates of their constructions, effects of calamities, architectural styles and many other details. the mindanao area in southern philippines where the muslim mosques are found is also a fresh avenue of research considering that previous research concentrated mainly on accounts from a christian perspective. the geoscientific institutions, archives and libraries around the philippines are also another source of possible new information that must be searched on further. the national records and archives office of the philippine national library is another promising source of data. as it is, archival fig. 1. number of earthquakes per 50-year period in the philippine earthquake catalog. by assuming that the recent period from 1900-1997 is complete, the historical period 1850-1900 may be considered well reported and almost complete, too. meanwhile, there are still many earthquakes that need to be discovered during the earlier period from 1600 to 1850 (table after bautista and oike, 2000). earthquake count year magnitude range 384 maria leonila p. bautista and bartolome c. bautista documents on some historical earthquakes and volcanic eruptions which were not seen by previous authors including maso and repetti had already been found. they are all handwritten and in spanish. they were written by governors, military commandants, priests, mayors and other government officials and may be considered as the official damage reports for these earthquakes. however, they are difficult to read and translate because they were handwritten in very complicated hand characteristic of old spanish documents. but their scientific value cannot be overemphasized because they are able to add significant information to these earthquakes. an initial project to have these translated properly is a possible future action. another future activity envisioned is the eventual production of a catalog that will show the detailed intensity distribution of most of these well-described historical earthquake events. aside from the inclusion of the isoseismal maps, this philippine catalog will include the tabulation of the individual earthquake accounts for the period 1589 to 1899. these tabulated accounts will be further categorized for easy usage into structural damage, effects to people, effects to the environment, human and economic loss estimates, references, and their assigned intensities. acknowledgements we would like to thank two anonymous reviewers whose insights and comments greatly improved this manuscipt. we would also like to thank dr. paola albini of the istituto nazionale di geofisica e vulcanologica, sezione di milano, for her thoughtful and constructive comments throughout the various stages of the editing process. references abe, k. (1981): magnitudes of large shallow earthquakes from 1904-1980, phys. earth planet. int., 27, 72-92. abe, k. (1984): complements to «magnitudes of large shallow earthquakes from 1904 to 1980», phys. earth planet. int, 34, 17-23. abe, k. (1994): instrumental magnitudes of historical earthquakes, bull. seismol. soc. am., 84 (2), 415-425. abe, k. and s. noguchi (1983): revision of large shallow earthquakes, 1897-1912, phys. earth planet. int., 22, 1-11. algue, j. (1900): focos sismicos, in el archipelago filipino, chapter x, 268-377. bautista, m.l.p. (1993): historical earthquakes in the philippines, in proc. 1993 joint conference of seismology in east asia, october 29-november 2, 1993, tottori, japan, p. 53. bautista, m.l.p. (1996): estimation of the magnitudes and epicenters of philippine historical earthquakes, unpubl. m.sc. thesis (graduate school of science, kyoto university). bautista, m.l.p. and k. oike (2000): estimation of the epicenters and magnitudes of philippine historical earthquakes, tectonophysics, 317, 137-169. berninghausen, w.h. (1969): tsunamis and seismic seiches of southeast asia, bull. seismol. soc. am., 59, 289-297. cox, d.c. (1970): discussion of «tsunamis and seismic seiches of southeast asia» by william h. berninghausen, bull. seismol. soc. am., 60, 281-287. de guignes, m. (1808): voyages a peking, manille et l’île de france faits dans l’intervalle des annees 1784 a 1801, de l’imprimerie imperiale (paris). dunbar, p.k., p.a. lockridge and l.s. whiteside (1992): catalog of significant earthquakes 2150 b.c.-1991 a.d., incl. quantitative casualties and damage (world center a for solid earth geophysics. national geophysical data center, boulder, colorado, u.s.a.), report se-49, pp. 320. garnier, j.g. (1839): traite de meteorologie du physique du globe (librairie scientifique-industrielle, paris), pp. 480. gutenberg, b. (1956): great earthquakes 1896-1903, trans. am. geophys. un., 37, 608-614. gutenberg, b. and c.f. richter (1954): seismicity of the earth (princeton university press), 2nd edition, pp. 310. heck, n.h. (1947): list of seismic sea waves, bull. seismol. soc. am., 37, 269-279. hoff, k.e.a. von (1841): chronik der erdbeben und vulcan ausbrüche, ii. theil, (justus perthes, gotha), pp. 406. iida, k, d.c. cox and g. pararas-carayannis (1967): preliminary catalog of earthquakes occurring in the pacific ocean, international tsunami information center and office of naval research, report no. 5. mallet, r. (1853): catalogue of recorded earthquakes from 1606 b.c. to a.d. 1850, brit. ass. rep., 1853, 1-176. mallet, r. (1854): catalogue of recorded earthquakes from 1606 b.c. to a.d. 1850, brit. ass. rep., 1854, 118-213. mallet, r. (1855): catalogue of recorded earthquakes from 1606 b.c. to a.d. 1850, brit. ass. rep., 1855, 2-326. maso, s.m. (1895): la sismologia en filipinas (establicimiento tipo letograpico de ranuirez y compania, manila), pp. 121 (+ figures). maso, s.m. (1909): catalogue of violent and destructive earthquakes in the philippines, 1599-1909 (weather 385 the philippine historical earthquake catalog: its development, current state and future directions bureau, manila central observatory). maso, s.m. (1927): philippine earthquakes, weather bureau seismological bulletin for 1926, 67-103. maso, s.m. and w. smith (1919): the relation of seismic disturbances in the philippines to the geologic structure, phil. j. sc., 8, 199-233. milne, j. (1912): a catalogue of destructive earthquakes, a.d. 7 to a.d. 1899, baas (london), pp. 92. nakamura, s. (1978): on statistical tsunami risks of the philippines, south east asian studies, 15, 581-590. pagasa (1976): catalogue of philippine earthquakes, 15851976, in earthquake catalog (philippine atmospheric, geophysical, atmospheric and services administration, quezon city, philippines). perrey, a. (1860): documents sur les tremblements de terre et les eruptions volcaniques dans l’archipel des philippines, mem. ac. sc., arts et belles-lettres de dijon, vii, 85-194. repetti, w.c. (1946): catalogue of philippine earthquakes, 1589-1899, bull. seismol. soc. am., 36, 133-319. rudolf, e. (1887): über submarine erdbeben und eruptionen, gerl. beitr. z. geophysik, stuttgart, i, 360-365. seasee (1985): philippines, edited by e.p. arnold, southeast asian association of seismology and earthquake engineering, series on seismology, iv, pp. 843. singapore chronicle (1824), november 25: article about the october 26, 1824 earthquake in manila. singapore chronicle (1829), january 1: article about the december 7, 1828 earthquake in manila. annals of geophysics, 55 (3), 2012: viale and ferrero spread of endemic disease and global change in an educational project: proposition of relationships developed in a twin partnership michela viale*, elena ferrero università di torino, dipartimento di scienze della terra, torino, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5531 abstract the sudden event of the spread of dengue fever (or break-bone fever) that appeared for the first time in cape verde in 2009 revealed that inappropriate management of waste can be considered a major cause of the spread of this disease. dengue fever is a tropical infectious disease that is caused by the dengue virus. its vector, the mosquito aedes aegypti, found an optimal environment for its life cycle in the context of cape verde, with the persistence of water in abandoned waste objects. the connection between the topics of earth sciences and the spread of this disease is not obvious, but it was explored and illustrated in a school partnership. activities with pupils and students provided an opportunity to investigate how some global phenomena, like climate change (with an increase in local rainfall and higher temperatures), are related to local events, such as the spread of dengue fever. preventive strategies are conditioned by the geomorphology of the territory and by the complex relationships that connect the geosphere and the biosphere. for this reason, it is important to be aware of the risk of breaking these delicate balances, and hence activating unexpected consequences. the roles played by both partners linked knowledge about the natural dynamics of our planet with the development of appropriate behavior, thus contributing to the formation of responsible citizenship, to preserve and protect the environment. the partnership encouraged students to develop sustainable management strategies against dengue fever, and consequently against waste, actively involving them at school, at home, and in their community. in this study, we present a case study of the role of a school partnership in a complex problem, such as the spread of dengue fever and environmental pollution. 1. premise a twinning project between schools of the piedmont region (north-western italy) and the republic of cape verde explored in its first stage the geomorphological aspects of the territory. in the next stage, it developed an investigation of the pupils’ and students’ conceptions about the geological risks in the area. further educational research of the students’ perception of waste and its dangers highlighted the connection between the physical characteristics of the territory, the different ways of managing waste, and the correlation with socio-economic problems [calvo et al. 2009], which revealed many similarities between the two partners. the school twinning and cooperation project was based on some general principles according to which it was necessary to share from the beginning of planning a deep and not only formal agreement, with reference to the objectives, the procedures, and the methods to follow. the essential basic condition was to develop a common course on selected and shared topics in the planning stage, to make sure that the issues were significant for both of the partners, were easily accessible for school and family, and if possible, were an integral part of the daily reality of each. the level of detail could have been calibrated according to age, but the topics were the same for all pairs of the twinning partners. 2. introduction the research included a curriculum for environmental education in a process of education for sustainability and for the training of tomorrow’s citizens, according to the following aims: – to actively participate in the education and training of responsible citizens; – to highlight the importance of earth sciences for the management and protection of the environment; – to propose a didactic trail, for both the young and adults. the objectives were aimed at achieving disciplinary knowledge (related to the curriculum or not), skills, and abilities (linguistic, analytical, and logical thinking): – to detect and exploit the correlation between the emotional and cognitive study of twinning in various disciplines (history, geography, art, science and computer science); – to propose a course of education for sustainability at different levels: cognitive, emotional, between teachers, article history received december 21, 2011; accepted february 7, 2012. subject classification: twin partnership, dengue fever, education to sustainability, global change, cape verde. 439 between students, and between teachers and students, of the twin countries; – to investigate how the students of the two partner countries perceive waste, and to compare the results in their socio-economic context; – to produce a didactic tool to highlight the correlation between local and global aspects (‘bad’ waste management, climate change, and the spread of dengue fever); – to produce didactic tools to be presented to the student and teacher partners to maintain the path of twinning and to directly involve the participants in the exchange of reports and discussions. several years of international cooperation between italy and cape verde republic in the field of environmental education have led to an enriching interchange among the teachers and students, and to interesting results about primitive conceptions and misconceptions of the topics explored [ferrero et al. 2006, 2007]. the main results relating to some of the students’ skills were: a) to improve the individual awareness of local risks, connected to the geological and topographic nature of the territory; b) to allow the students to discover and experience the information and processes directly in the field; c) to stimulate interest in the connection between human actions and natural processes; d) to facilitate the understanding of complex concepts that require a high level of abstract thought; e) to discuss the results inside the classroom, and to exchange and compare the didactic products with the partners of the other country. the experience continued over 7 years and involved hundreds of students (aged 6-15 years) who were residents in small settlements or in the most important centers. some classes had continued for only a few years, although the results of the previous steps of the partnership were always presented to new partners, through synthesis of the products made by the students involved in the project. the protagonists were ideally matched in pairs, to establish a relationship of personal friendship right from the beginning. although the cape verde classes were more numerous than the italian ones, this was not a problem. at the beginning, they observed the elements of the landscape near the school, and gradually enlarged the field of investigation, both in space and time [ferrero et al. 2009a]. the students noted, either directly or with the help of the older members of the family, that the elements of the landscape are not persistent, as seems at the first sight, but that they change over time and are sometimes subject to sudden and catastrophic changes (floods, earthquakes, volcanoes). later, their attention was driven by the effects of the human presence, and in particular on the pervasive presence of waste, and its origin and destiny, regularly comparing phenomena and processes recognized in both of the countries [ferrero et al. 2009b]. 3. materials and methods in the case study described in this report, both the italian students (about 120, from 2 primary school and 4 secondary school classes) and the cape verde students (about 200, from 2 primary school and 4 secondary school classes) were asked to work on two worksheets that were structured in the same way, to highlight their preconceptions about the complex and varied topic of waste. they subsequently followed a course of study, which was often different in the different classes and which produced many reports. the worksheets were analyzed with methods using a quasi-statistical approach and the template approach [viale 2011]. the method suggested to the participating teachers was based on a research model called ‘action research’, in which the teachers become the main players in the practice of education and teaching. the method has been described [ebbutt 1985, cattadori et al. 2011] as a systematic study of attempts to improve educational practice by groups of participants, by means of their own practical actions, and by means of their own reflection upon the effects of these actions. the case study described here was connected with recent episodes of epidemic dengue fever (autumn, 2009 and 2010) that had not been previously present in the cape verde islands, and which generated many victims and great concern. at the same time, this stimulated a great venture of information and actions directed to prevent the diffusion of the vector, the mosquito aedes aegypti. the children in cape verde were involved both in fighting the disease and in its prevention, undertaking a role of control in the family and in the neighborhood of the school. instead, the italian children were very impressed by the event and were interested in being more informed. this was a significant case involving a partnership, where the students became aware of complex problems of global relevance that they could share. 4. the dengue fever outbreak dengue fever and dengue hemorrhagic fever are a serious problem for public health. the primary vector for this disease is a mosquito that is prevalent throughout the tropics, and particularly in urban areas, where the inhabitants are very susceptible to infection. the process of urbanization, which has left many people without water, sewerage and waste recovery systems, has created new sites where the vector can settle, thus speeding up the spread of infection. the virus is transmitted by the bite of various types of mosquitoes, which are active during the day; the principal vector is aedes aegypti. this virus circulates in the blood of the infected people, and the mosquiviale and ferrero 440 441 toes that feed on infected blood remain infected throughout their life. man acts as an amplifier host for the infection. the female mosquitoes can also transmit the infection to subsequent generations (http://www.cesmet. com/planisfero.4/schede/malattie/dengue.htm). for the first time, in 2009, dengue fever appeared in cape verde, with the most severe outbreak ever recorded in africa. the number of cases grew at a phenomenal rate from the early days of november 2009, with a peak of 1,000 new cases every day. indeed, the intense rainfall associated with high temperatures in 2009 provided the mosquito species aedes aegypti with an excellent habitat to complete its life cycle. following the spread of the outbreak, the government distributed informative material on prevention of dengue fever, both direct (use of mosquito nets, repellents) and indirect methods to stop the reproduction cycle of the mosquito. in particular, people were involved in the work of ‘cleansing’, as the elimination of the waste scattered in the streets and the stagnant water where the mosquitoes lay their eggs. beyond the innumerable hardships, the spread of dengue fever thus had a favorable outcome for the citizens of cape verde, who were active in the fight against garbage and stagnant water in urban and rural areas, thus counteracting the outbreak of the disease [fidalgo de barros 2009]. at the same time, the research team produced some original educational material for both the italian and cape verde schools, which included in particular a cartoon-style presentation of the disease, its vector, and its symptoms (figure 1). for a more engaging and challenging proposal, the vectors directly involved in the disease were chosen and individuated: rino, rina and ritinha (a family of cape verde mosquitoes) and ziro (an italian mosquito). they were presented to the pupils according to the model the students used to introduce themselves to the twinning partners [viale 2011]. after viewing the presentation, with the commentary supplied by their teachers, the children produced reports on dengue fever. some of these were limited to the contents of the ‘lesson’, while others proposed their own solutions to decrease the incidence of the disease. the expedient of using humanized mosquitoes as narrators impressed the students a lot, and helped them to analyze the problem of dengue fever from a different point of view. they went beyond the symptoms, and they sought to investigate the causes of the diffusion, and to try to propose their solutions. drawings showed that the mosquitoes were not perceived as terrible enemies, nor the sole responsible entity for the discomforts of the disease. contrary-wise, the mosquitoes become dangerous in certain environmental contexts; namely, where there are favorable conditions for their reproduction. all of the italian teachers found the effectiveness of communication through drawings; indeed, the drawings revealed the great helpfulness of the misconceptions and spread of endemic disease and global changes figure 1. the mosquito aedes aegypti, the vector of the dengue fever virus. illustration of the mosquito life cycle, and the environmental conditions favorable to its development and its diffusion. contradictions in the students’ cognitive processes [hayes et al. 1994, minuti and vescovi 1998]. the older students preferred not to use drawings, and very often they opted for the development of multimedia presentations. however, with the help of the teachers, they were able to capture the emotional and affective components in the drawings of their partners and the added value that drawings brought to the cognitive component of communication and knowledge acquisition. the italian pupils were particularly involved in the topic, and they expressed interest in deepening these issue. they realized that malaria is still present in italy, although it is no longer an endemic disease in italy. they found that the presence of the so-called ‘tiger mosquito’, which had also been in piedmont for some years, was concurrent with an increase in the average temperature, and that as another species of tropical mosquito, it can potentially carry new diseases, including dengue fever. 5. discussion communications with the partners in cape verde, as in other countries in the sahel, were never easy, because of the distance, the travel and expedition costs, and the linguistic and cultural differences. this has always been a critical problem, previously for the choice of the topics to develop, and more recently for the evaluation of the products obtained. nevertheless, both twin partners were involved in this process of reflection, with the support of didactic tools that were valuable aids. the cape verde students presented their preventive strategies, according to the instructions given by the cape verde ministry of health, such us avoiding the leaving of waste around, and using repellents or mosquito nets during the night. on the other hand, the italian students were involved in the development of the emotional and empathetic value of the partnership, both in terms of knowledge and friendship. both the italian and cape verde students argued during the class work, when they produced their reports or when they received what had been elaborated by their partner. in these moments, the teachers encouraged more mature reflections, which provided good integration of cognitive and behavioral aspects of learning; i.e. the creation of effective and durable learning. it was shown that some of the methods used to battle against the diffusion of the mosquito vector might interfere adversely with the primary natural resources (air, viale and ferrero 442 figure 2. reports of students of primary school in praia to explain to their italian partners the best practices to prevent the diffusion of the epidemic spread of the disease. in these drawings, the two students suggest collection of the waste, cleaning of the roads, avoiding leaving water in containers like bottles or cans, not leaving water in plant vases, covering water tanks, avoiding leaving waste around, using repellents, and using mosquito nets during the night. 443 water, soil). it was also discussed whether it was possible to fight this disease in a sustainable way. preventive strategies were conditioned by the geomorphology of the territory, and by the complex relationships that connect the geosphere and the biosphere. in an area consisting mainly of basaltic rock, collecting and burying waste in landfills is not easy [calvo et al. 2009]. so, waste was often burned outdoors, which produced toxic substances. on the other hand, a massive fight with powerful insecticides might contaminate objects and environments with which people are in contact, and at the same time promote resistance in vectors of the disease. for this reason, it is important to be aware of the risk of breaking the delicate balances by activating unexpected consequences. the strategies proposed by the cape verde students (figure 2) demonstrated an awareness of these risks, as they were involved in a clear example of a sustainable campaign against dengue fever. the students proposed to collect the waste, clean the roads, and avoid leaving water in containers like bottles or cans, and also in plant vases. in particular, they were aware of the link between waste and disease, because water stagnates in waste, and there the mosquitoes can lay their eggs. moreover, they knew that cleaning the environment, both in their town and in the countryside, is an effective method to fight against dengue fever. indeed, mosquitoes can have difficulties in the completion of their life cycle without waste. we recall, for example, the justifiable pride of the teacher luisa and her 42 students from praia (the class that has developed the twinning relationship for 6 consecutive years) to report to their partners how they had been involved in fighting the epidemic, checking around their neighborhood, and urging adults to take the precautions they had learnt at school (the same that were suggested by the ministry of health). they felt that they were active citizens in their community; at the same time, they shared this with their italian friends, aware that they had something to teach their partner. the roles played by both of the partners linked the knowledge about the natural dynamics of our planet with the development of appropriate behavior, to contribute to the education and training of responsible citizens and to preserve and protect the environment. the project did not really propose original solutions; on the contrary, it mainly focused on the personal awareness of hazard determined by the diffused presence of waste, and of the possibility of acting personally to prevent and to mitigate the diffusion of the outbreak. 6. conclusions the affective elements involved in the twinning relationship, combined with the surprise and the sense of wonder aroused by the differences that were discovered during the course of the experience, encouraged the development of personal social and cooperative skills. these were to be a powerful stimulus to improve learning and developing the cognitive skills of the students [cavallini 1995, alfieri et al. 2000]. from the perspective of the cognitive viewpoint, the comparison was highly successful for the detection of the different characteristics of the two countries (piedmont, italy, and cape verde). the most obvious difference was due to the geological setting of these two countries: they are very different both in lithology (cape verde rock is mostly igneous rock) and morphology (in cape verde there are many gorges carved into the rock where waste can accumulate and be transported by the wind and the infrequent but heavy rainfall). from the perspective of affective and emotional understanding, the comparison revealed rather surprising similarities in the ways in which the students interacted with family, in school practice, and in relation to the educators. they recognized similarities in the wrong habits and disrespectful behavior towards the environment and its balance. they discovered a non-sustainable use of resources (water in particular), and a lack of reuse and recycling of raw materials. acknowledgements. this study was supported by the italian ministry of universities and research (miur) as 60% funds (to ef), and by funds from the regione piemonte assigned to the cisao for the projects “reflections and proposals of young people in schools of piedmont and the republic of cape verde, to raise awareness of the risks and develop best practices in waste management” (2008) and “from the discovery to the awareness of environmental degradation situations. knowledge of appropriate practices to mitigate the risks involved and selection of appropriate behaviors by school students of piedmont and republic of cape verde” (2009). references alfieri, f., m. arcà and p. guidoni (2000). i modi di fare scienze: come programmare, gestire, verificare, irrsae piemonte; 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(2011) analisi delle concezioni spontanee sul tema dei rifiuti in allievi di scuole del piemonte e di capo verde. da un’esperienza di gemellaggio alla comparazione e alla sintesi della concezione dei rifiuti in ambiente scolastico, unpublished thesis report, corso di laurea in evoluzione e diversità nei sistemi naturali, faculty of sciences m.f.n, torino university. *corresponding author: michela viale, università di torino, dipartimento di scienze della terra, torino, italy; email: michelaviale@msn.com. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. viale and ferrero 444 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 5, 2004 1581 annals of geophysics, vol. 47, n. 5, october 2004 key words lithospheric flexure – differential compaction – triassic 1. introduction the peculiarity of carbonate platforms is that sediments can be produced only in a few meters of water depth. this notwithstanding, in many regions of the world both past and present platforms commonly consist of several hundred meters of continuous successions of shallow-water sediments and these thick accumulations can be explained only if accommodation space is continuously created, that is if subsidence of the underlying ‘substratum’ and/or sea level rise continuously occurs for relatively long time spans. as concerns sea level variations, nowadays we have a widely accepted knowledge of the first-order global fluctuations scale (haq et al., 1987) and therefore, for almost any geological time interval we can grossly quantify the contribution of this phenomenon in producing accommodation space. whatever the case, begrowth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy claudia marella (1), riccardo caputo (2) and alfonso bosellini (3) (1) dipartimento di scienze della terra, università degli studi di camerino, italy (2) disgg, università degli studi della basilicata, potenza, italy (3) dipartimento di scienze della terra, università degli studi ferrara, italy abstract the phenomenon of subsidence induced by the growth of carbonate platforms has been investigated with the aid of numerical modelling. the research aimed to quantify the relative contribution of this process in the creation of the accommodation space required to pile up thick neritic bodies. we analysed two end-member deformation styles, namely the elastic behaviour of the lithosphere when locally loaded and the plastic-like reaction of a sedimentary succession underlying a growing carbonate buildup. the former process, analysed using a modified flexural model, generates a regional subsidence. in contrast, the latter process, simulated by considering the compaction occurring in soft sediments, generates a local subsidence. we attempted to quantify the amount and distribution of subsidence occurring below and surrounding an isolated platform and in the adjacent basin. the major parameters playing a role in the process are discussed in detail. the model is then applied to the late anisian-early ladinian generation of carbonate platforms of the dolomites, northern italy, where they are spectacularly exposed. taking also into account the tertiary shortening that occurred in the area, both local and regional subsidence contributions of major platform bodies have been calculated aimed at a reconstruction of the map of the induced subsidence. a major outcome of this study is that the accommodation space, that allowed the accumulation of very thick shallow-water carbonate successions in the dolomites, was only partially due to lithospheric stretching while the contribution given by the ‘local’ overload is as high as 20-40% of the total subsidence. our results also shed some light on the water-depth problem of the triassic basins as well as on the basin-depth to platform-thickness relationships. mailing address: prof. riccardo caputo, disgg, università degli studi della basilicata, c.da macchia romana, 85100 potenza, italy; e-mail: rcaputo@unibas.it 1582 claudia marella, riccardo caputo and alfonso bosellini cause eustatic curves commonly show some tens of meters of continuous sea-level rise and, just in a few cases, up to 100-150 m, at a first glance this natural phenomenon cannot explain, at least alone, the growth of thick platforms that occurred during many geological periods. as a consequence, subsidence of the underlying substratum must be claimed as the principal mechanism for the generation of accommodation space during the build up of these sedimentary bodies. subsidence of the earth surface can basically occur as a consequence of three major processes. firstly, if the lithosphere is horizontally stretched, it becomes thinner (e.g., mckenzie, 1978), the top and bottom interfaces become closer due to the occurrence of a largescale pure shear and thus the earth surface subsides. this process is mainly referred to as tectonic subsidence and typically occurs during a rifting stage. secondly, if the crust and/or the lithosphere become denser, a new lithostatic equilibrium must be reached. a typical example of this process is the thermal subsidence associated with the cooling of the lithosphere in passive margins (sclater et al., 1971). thirdly, if the crust is locally overloaded, again new lithostatic conditions are required to keep the equilibrium. depending on the rheologic and mechanical behaviour of rocks, this process can induce ‘local’ subsidence at the scale of the applied load as well as regional effects at distances several times the size of the loaded area. this last case commonly occurs in regions where important topographic structures like volcanoes and oceanic islands can be generated on top of the earth surface in relatively short time intervals. typical examples are the fore-deep basins associated with the mountain chains and the post-glacial rebound of scandinavia. in this paper the subsidence induced in the lithosphere and within the uppermost crust by the growth of isolated carbonate platforms is investigated. one of the principal aims of this research is estimating the amount and the distribution of subsidence during the growth of a carbonate platform. to achieve this goal we set up a numerical model compiled with the matlab software. in order to evaluate the role played by the parameters considered, we performed several tests. a second important aim of this study is the application of the model to the carbonate platforms that grew in the dolomites, northern italy, during middle triassic that represent a spectacular and world famous example of carbonate build-ups. as a major outcome, we can tentatively quantify the amount of subsidence induced by the platforms’ load and its distribution in the palaeo-dolomites region. based on the thickness of the neritic successions and considering the calculated pattern of the ‘nearfield’ contribution, the amount of subsidence induced by a ‘far-field’ process can be easily obtained. according to the middle triassic geodynamics of the area, the tectonically induced subsidence can be roughly estimated and consequently we can have some idea on the amount of stretching which occurred during that time interval. 2. the numerical model in order for subsidence to occur, the lithosphere and/or the crust must deform either locally or regionally. the deformation process can occur according to three end-member behaviours of lithospheric rocks: elastic, plastic and viscous. to estimate the subsidence induced by the growth of carbonate platforms two effects have been considered. firstly, the flexure of a thin elastic plate and, secondly, the plastic deformation due to compaction of the underlying stratigraphic successions progressively buried. as a first approach, we did not consider the possible contribution associated with a viscous behaviour because important effects in terms of amount of deformation are negligible in relatively short time spans, say of the order of a few million years (e.g., ranalli, 1987). the elastic behaviour of the lithosphere has already been recognised in several geodynamic conditions. classical examples are the flexure induced by the hawaiian islands chain (turcotte and schubert, 1982), the bending occurring in front of oceanic trenches along subduc1583 growth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy tion zones (karner and watts, 1983; wang, 2001) or the deflection associated with continental sedimentary basins (e.g. walcott, 1970). the elastic deformation is analysed here using a flexural model of bending lithosphere supported by a denser substratum. the permanent volume reduction occurring in the sedimentary succession, which is progressively buried under the increasing load of a growing carbonate platform, has been considered and consequently modelled as a plastic deformation. the two independent deformational processes are then summed up to calculate the induced subsidence. 2.1. flexural model the flexure of the lithosphere can be analysed considering the bending of an isotropic elastic plate of thickness h, much smaller than the two horizontal dimensions, subjected to a surface load located at the top and supported by a denser substratum (fig. 1). in such conditions, three forces are involved: firstly, the downward facing force exerted by the surface load; secondly, the elastic force generated by the plate resisting bending; thirdly, the buoyant force due to the replacement of dense asthenosphere rocks with less dense lithosphere rocks. considering the three forces acting on the plate, the general equation describing the deflection in the xyplane is (e.g., timoshenko and goodier, 1951) d x w x y w y w gw l2 m i4 4 2 2 4 4 4 2 2 2 2 2 2 2 + + + =t t_ i( 2 (2.1) where g is the gravity acceleration, ρm the density of the asthenosphere, ρi the density of the replacing materials (sediments and water), l is the linear surface load and w is the deflection of the plate relative to the centre of load application. d represents the flexural rigidity that is equal to te3e/12(1− ν 2), where te is the elastic thickness, e is the young modulus and ν is the poisson coefficient. this equation can be applied to linear topographic features like mountain ranges or aligned oceanic islands. indeed, in this case, where the bending geometry is the same in any cross-section normal to the long dimension of the loading body the problem is reduced to a one-dimensional elastic bending d dx d w gw lm i4 4 + =t t_ i . (2.2) for a linear load along the y axis at x = 0 the general solution is (turcotte and schubert, 1982) cos sinw w e x x/ x x 0= +a a a a k (2.3) where wx is the deflection at a distance x from the load, w0 is the maximum deflection at x = 0 given by fig. 1. simplified sketch of the numerical model used to estimate the subsidence induced by the growth of a carbonate platform: deflection and compaction processes occurring within the elastic and plastic layers, respectively, are indicated along with the main parameters. e: young modulus; ν: poisson coefficient; te: equivalent elastic thickness; ρw: water density; ρp: platform rock density; a: platform dimensions; ρm: mantle density. 1584 claudia marella, riccardo caputo and alfonso bosellini w0 = lα3/8d (2.4) and α is the flexural parameter α = [4d/(ρm + − ρi ) g]1/4. keeping in mind that the aim of this study is to estimate the flexure of the lithosphere caused by a point load as a carbonate platform (bidimensional case), we also need to consider the shear stress induced in the yz-plane that obviously plays an important role in reducing the maximum subsidence. according to lowrie (1997), because a load with equal horizontal dimensions causes a central depression that is less than a quarter the effect of the linear load a corrective coefficient is introduced in eq. (2.3), which thus becomes . cos sinw w e r r 0 25 /r 0= +a a a a k; e (2.5) where r is the radial distance from the point load. we are well aware that this coefficient is an oversimplification of the mathematical aspects and it does not correspond to a rigorous solution of the differential equation. nevertheless, we prefer to keep the mathematics to a minimum for several reasons. firstly, because the principal aim of the paper is to assess the order of magnitude of this natural phenomenon and not to quantify it exactly. secondly, the values that some of the parameters included in the above equations can assume within their possible ranges induce an even larger variability in the results. accordingly, eq. (2.5) has been used to estimate as a first attempt the amount of lithospheric flexure due to the growth of a carbonate platform. 2.2. compaction model the plastic deformation is estimated by applying a compaction model. the thickness reduction of the sedimentary successions, buried under an increasing load has been calculated. the compaction in sedimentary units commonly occurs due to the leakage of pore fluids and to a general geometric reorganisation of the particles consisting in the partial collapse of the grain framework, plastic deformation of soft fragments, and fracturing and pressure solution phenomena (rieke and chilingarian, 1974). at greater depths even mineralogical transformations can contribute to a progressive volume (i.e. thickness) reduction. all these factors lead to a reduction in porosity. another important factor that influences the porosity reduction is the precipitation of cement material, though this process is more an obstacle to the compaction process. if analysed in detail, diagenetic compaction of sediments is not a simple plastic deformation mechanism, but for the purposes of this research and particularly at the scale we consider, as a first approximation this natural process can be modelled as a plastic deformation (fig. 1). the mechanical compaction of sediments depends on the buried depth (i.e. lithostatic load) and on the differential stresses. if the compaction is the only working process, the thickness of the solid component remains constant and therefore the ratio between solidity (1− φ ) and the thickness reduction is linear (baldwin and butler, 1985). in this study we use a simple porosity-depth function as proposed by athy (1930). ruby and hubert (1960) showed that the porosity should be an exponential function of depth in the form e cz 0=z z (2.6) where φ 0 is the initial porosity and c is the compaction factor. in this study we follow the approach proposed by sclater and christie (1980), which allows estimation of the depositional (i.e. original) thickness of a sedimentary succession from the present-day thickness as directly measured in the field. for example, if we assume a total overburden thickness that caused a certain compaction, inversion of eq. (2.6) allows to ‘de-compact’ the deposits. accordingly, we can calculate the thickness for any depth of burial and from the difference between this calculated value and the present-day thickness we obtain the amount of compaction that occurred during any time interval. 3. parameter definition a key step in any numerical modelling is the choice of the most suitable and correct val1585 growth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy ues for each parameter and variable introduced in the calculations. many of these values can be easily inferred from the abundant literature on the topic (e.g., jaeger and cook, 1979; turcotte and schubert, 1982; and references therein), while in some cases the choice of the numerical value is more difficult because the lack of geological, mechanical and geodynamic information does not allow a tight constraint of the parameter. in these cases, a more or less wide range of values has been tested. 3.1. elastic deformation table i lists all the numerical values and ranges attributed to the parameters used in the program. however, in the following, only the most important parameters will be discussed. as concerns elastic deformation, one of the most important parameters is the elastic thickness, te, which generally does not represent the entire lithosphere but only that part behaving elastically. indeed, one of the major problems in defining this parameter is that the lithosphere is not perfectly elastic as assumed in the theory from which eqs. (2.1) to (2.5) are derived. in fact, the possible occurrence of hotter and/or less competent volumes within the lithosphere, diffuse creeping phenomena and the possible crust-mantle decoupling de facto strongly reduce the real elastic behaviour of the lithosphere. in order to account for these rheological heterogeneities within the lithosphere, a perfectly elastic but thinner lithosphere is commonly considered, that is an ‘equivalent elastic thickness’ is introduced in the calculations. for the oceanic crust the elastic thickness is a function of age and roughly coincides with the isotherm of 600 c° (mcnutt, 1988). in contrast, the continental lithosphere does not entirely depend on temperature and pressure conditions (burov and diament, 1995). the thermal state of the continental lithosphere is one important factor among others that determines the value of the equivalent elastic thickness. other factors are the state of the crust-mantle interface, the thickness and proportions of the mechanically competent crust and mantle and the local curvature of the plate that is directly dependent on the bending stresses (burov and diament, 1995). burov and diament (1995) document that the continental lithosphere elastic thickness has a wide range of values between 5 and 110 km. these authors also emphasise the occurrence of a bimodal distribution characterised by two peaks at 10-30 km and 70-90 km, respectively. table i. list of parameters, their symbols and values used during numerical modelling. the values in the last column are those selected for the model application to the late anisian-early ladinian carbonate platforms of the dolomites, northern italy. the selected values for the initial porosity and the compaction factor refer to the bellerophon formation (bel) and the werfen formation (wer), respectively. parameter symbol unit value or range selected value young modulus e pa 1010-1011 1010 poisson coefficient ν 0.22-0.25 0.25 equivalent elastic thickness te km 10-50 21 mantle density ρm kg/m3 3000-3500 3500 replacement density average of ρw-ρp kg/m3 1100 1100 platform density ρp kg/m3 1800-2200 2200 initial porosity φ 0 0.3-0.7 bel 0.6 wer 0.5 compaction factor c m−1 2 ⋅10− 4-7 ⋅10− 4 bel 5 ⋅10 − 4 wer 2.7 ⋅10− 4 1586 claudia marella, riccardo caputo and alfonso bosellini similarly, mcnutt (1988) have compiled a database of elastic thickness values reported in the literature for the continental lithosphere under major mountain chains. the minimum value they collected is 15 km, the maximum 30 km. based on the gravity anomalies of the krishna-godavari basin in india and the inferred load induced by the sedimentary infill, krishna et al. (1999) suggest that the best fitting value of the equivalent elastic thickness to explain the observed gravity anomalies is about 30 km. similar values were assumed to calculate the tectonic subsidence and the crustal flexure in the chaco basin, bolivia (coudert et al., 1994). accordingly, though the elastic thickness strongly depends on the broader and specific geodynamic conditions to be numerically simulated, as a first attempt for continental lithosphere we chose a range of values between 10 and 50 km. the variability of te has a twofold effect. firstly, by decreasing the equivalent elastic thickness the maximum deflection increases almost proportionally and, secondly, the peripheral bulge moves closer to the central depression and becomes more pronounced (fig. 2a). also the young modulus is inversely proportional to the maximum deflection, though its effects are less important because by varying e of one order of magnitude (from 1010 to 1011 pa), w0 varies of about ± 25% (fig. 2b). another influencing parameter is the magnitude of the point load that is directly proportional to the maximum deflection (eq. (2.4)). on the other hand, this parameter is a function of i) the initial distribution and extension of the neritic conditions (i.e. the growing carbonate platform), ii) the amount of aggradation, iii) the amount of progradation and iv) the selected value for the density of platform rocks. all these factors will be further analysed and discussed in a later section where the numerical model is applied to real examples of carbonate platforms. elastic deformation is also influenced by the density contrast that determines the buoyant force induced by the replacement of the asthenosphere mantle rocks with less dense materials (fig. 1). indeed, while the density of the replacement materials is close to unity, consisting for more than 95% of water, the replaced mantle rocks can range between 3000 and 3500 kg/m3. whatever the case, the final result is not strongly affected. 3.2. plastic deformation as concerns deformation associated with compaction, the most important parameters are the initial porosity and the compaction factor. initial porosity, φ0, can strongly vary from more fig. 2a,b. a) influence of the equivalent elastic thickness (te ) on the deflection. the larger the value, the stronger the layer, the less important the maximum deflection. b) influence of the young modulus on the deflection. note the inverse proportionality between the young modulus and the maximum deflection, the shift and the magnitude of the peripheral bulge. a b 1587 growth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy than 60% to less than 15% (pryor, 1973; campbell et al., 1974). indeed, these extremely variable values of porosity are associated with strong variations in the possible amount of porosity reduction and therefore in compaction and the consequent subsidence. this parameter is particularly influential during the initial burial history of sediments (fig. 3a), though at depths greater than 5 km, all curves tend to converge to a similar value. the second important parameter playing a role during the compaction process is the socalled compaction factor, c, in eq. (2.6) (fig. 3b). in fact, although all c-curves have the common tendency to a progressive reduction in porosity, at great depths, say more than 5 km, small and large values of c differ up to almost 20%. but even more important is the influence of this parameter during the burial evolution. for example, during the first 1000 m of burial, a compaction factor of 8 ⋅10−4 induces about 30% of porosity reduction, while a value of 2⋅10−4 is associated with only a 10% porosity reduction, that is of sedimentary compaction and thus of induced subsidence. this parameter strongly depends on the lithology of the sediments and especially on their texture characteristics. a further influential factor as concerns the amount of induced subsidence is obviously the original thickness of the sedimentary unit undergoing compaction. indeed, it is clear that whatever the compaction factor and the initial porosity are, the larger the original thickness the larger the final amount of induced subsidence (fig. 3c). again with reference to the plastic deformation and considering a simplified 3d truncatedconical shape for carbonate platforms, it is obvious that the amount of burial of the underlying sedimentary units varies with the position relative to the platform limits. indeed, compaction is nil outside the platform toe of the slope and it is maximum inside the ring defining the shallow-water depositional area. therefore, the slope angle directly influences the dimension of the annular gradient for the amount of compaction occurring in the sediments underlying the carbonate build up. carbonate platforms in the dolomites commonly range between 25° and 45°. 4. applications to triassic platforms of the dolomites as mentioned in the introductory notes, the numerical model has been applied to some of the most spectacular and well-known examples of carbonate platforms in the world: the dolomites of northern italy. indeed, during the middle and upper triassic, the whole region fig. 3a-c. reduction of porosity (i.e. compaction) associated to progressive burial: a) with variable initial porosity and constant compaction factor (c = 4⋅10−4); b) with variable compaction factors (c) and constant initial porosity (φ 0 = 0.5). c) subsidence induced by compaction of a ‘plastic’ layer, for different initial thicknesses and material properties, as a function of the overburden. a b c 1588 claudia marella, riccardo caputo and alfonso bosellini was characterised by several episodes of fast growing isolated carbonate buildups. the diverse generations of platforms generally differ in thickness, amount of aggradation and progradation and their relative rates. an extensive literature exists on the topic (see, for example, bosellini et al., 1996 and references therein). in order to test the numerical model, among the several generations of platforms that can be recognised in the triassic succession of the dolomites, we selected the so-called pre-volcanic late anisian to early ladinian platforms which are 400-800 m thick and show high aggradation rates and relatively simple pregrowth stratigraphic conditions. the relatively short time interval (i.e. late anisian-early ladinian) of platform growth, lasting probably less than 2 myr (gianolla et al., 1998), assures that the assumption of a geologically restricted temporal time window is satisfied and therefore the viscous contribution to deformation can be neglected (e.g. walcott, 1970; beaumont, 1978). due to the strong influence of the equivalent elastic thickness in the numerical modelling, we paid special attention to the selection of this parameter. as previously discussed, the elastic thickness of the continental lithosphere depends on numerous factors. burov and diament (1995) showed that the thermal state of the lithosphere, the crust-mantle interface, the thickness and proportions of the mechanically competent crust and mantle and the local curvature of the plate, which is directly related to the bending stress, are all crucial properties in defining the equivalent elastic thickness of a region. nevertheless, following burov and diament (1995), an estimate of the equivalent elastic thickness of the dolomite region during middle triassic can be attempted. based on the available data for the hercynian orogeny that affected the area during the devonian-carboniferous period, the thermal age of the lithosphere at the time of platform growth was about 100 myr (del moro et al., 1980; zanferrari and poli, 1992). based on this age and if we assume a realistic value for the triassic crustal thickness of about 20-25 km as an average, following burov and diament (1995) it is possible to estimate a te between 20 and 22 km. as a mean value, we selected 21 km for our calculations, bearing in mind a ± 5% of variability induced by this parameter. the next step was quantifying the load produced by each carbonate platform and its area of influence. it is therefore necessary to identify and locate the several buildups belonging to this middle triassic generation of platforms and to define the dimensions of each carbonate body. based on literature data (gianolla et al., 1998, and references therein), published geological maps (leonardi et al., 1968; caputo et al., 1999) and available unpublished data (m. stefani and p. gianolla, pers. comm.), we inferred the possible initial dimensions of the different platforms, the amount of aggradation and that of progradation which characterised the growth of the several late anisian-early ladinian carbonate bodies. due to the locally deep erosion and the tectonic deformation that occurred in the dolomites, the reconstruction of the geometric parameters (thickness and lateral extension) has sometimes been inferred. however, due to the relatively fig. 4. the present-day distribution of the late anisian-early ladinian carbonate platforms considered for modelling in the present note. the filled circles tentatively represent the size of the initial nuclei, while the external circles (radial pattern) are proportional to the final size (maximum progradation). the general geometry of the bodies is obviously highly simplified. platform labels indicated in figure correspond to table ii. major thrusts affecting the region are also represented. 1589 growth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy strong influence of these parameters on the calculated subsidence, we attempted a conservative interpretation keeping the inferred dimensions to the minimum values. accordingly, in fig. 4, the nuclei of the considered platforms are highly simplified and represented as filled circles whose dimensions are roughly indicative of the possible initial size. we emphasise that in some cases the considered platforms, whose corresponding values are reported in table ii, merged during their growth due to convergent progradation thus forming a unique larger carbonate body. this is certainly the case, for example, of the civetta-san lucanosan martino platforms (p14-p15-p16 in fig. 5; bosellini et al., 1996) and the gran vernel-marmolada platforms (p8-p9 in fig. 5; stefani and caputo, 1998). however, in terms of produced load and thus of induced subsidence, our separate and simplified carbonate bodies are roughly equivalent to more realistic platform shapes. with respect to the plastic contribution to deformation, an important aspect of numerical modelling is the selection of the parameters characterising the underlying sedimentary units that are supposed to have undergone compaction table ii. list of parameters used for modelling the 16 carbonate platforms. label: platform label used in figs. 6 and 7; a – initial platform radius (m); b – final platform radius (m); c – final platform thickness (m); d – present-day thickness of the bellerophon formation underlying the platform (m); e – present-day thickness of the werfen formation underlying the platform (m); f – present-day thickness of the post-werfen to pre-late anisian succession; g – estimated maximum overburden on top of the werfen formation. label name a b c d e f g p1 croda dei baranci 1800 3250 400 260 420 400 4220 p2 tre scarperi 1500 3100 400 310 420 400 4310 p3 cadini di misurina 1250 2500 400 310 410 400 4240 p4 putia-sass rigai 2600 4000 650 140 200 150 2300 p5 sassolungo 1200 1950 450 215 140 100 1750 p6 sella 900 1600 450 220 80 100 1780 p7 sciliar-catinaccio 3000 5200 500 150 290 130 1780 p8 gran vernel 1250 2500 750 230 170 150 1700 p9 marmolada 1300 2150 750 235 160 150 1880 p10 ombretta 3000 4800 800 220 200 150 1750 p11 latemar 2400 3000 750 180 320 150 1650 p12 viezzena 2000 2950 750 210 310 150 1650 p13 agnello 2100 3400 750 145 340 150 1600 p14 civetta 3200 4500 500 205 220 500 2840 p15 san lucano 2900 4300 500 190 250 500 2540 p16 san martino 3200 5200 600 160 300 450 2330 fig. 5. tentative palinspastic reconstruction of the dolomites and assumed location and distribution of the carbonate platforms during late anisian-early ladinian. platform labels correspond to table ii. the dotted pattern shows the outer limit of the coalesced platforms. 1590 claudia marella, riccardo caputo and alfonso bosellini during the growth of the carbonate buildups. first of all, we have to define which formations could have undergone some compaction. indeed, the pre-late anisian stratigraphy of the dolomites shows that on top of the palaeozoic metamorphic basement the sedimentary units of some interest for our model due to their thickness, lithology and area distribution are the val gardena sandstone (middle permian), the bellerophon formation (late permian) and the werfen formation (schythian). other sedimentary units also exist but they mainly consist of carbonate layers, like for example the contrin formation, or they are coarse-grained like the sesto and ponte gardena conglomerates. in the former case, diagenesis usually occurs soon after deposition thus impeding any important compaction during burial; while in the latter case, the compaction factor is generally extremely low and so not influential in our calculations. moreover, as concerns the val gardena sandstone, the time elapsed between sedimentation and the growth of the platforms investigated in this paper was probably sufficiently long to allow an almost complete diagenesis. therefore their possible contribution to subsidence as induced by compaction is likely to be negligible. according to geological and stratigraphic data (e.g., bosellini, 1968), the present-day thickness of the bellerophon and werfen formations have been inferred in correspondence with the several carbonate platforms (table ii). however, in order to estimate the amount of compaction suffered by these two sedimentary formations during the growth of the late anisianearly ladinian platform generation and the consequent amount of induced subsidence, it is also necessary to know their pre-platform burial history and particularly the thickness of the stratigraphic succession already overlying the bellerophon and werfen units just before the late anisian. therefore, the amount of the postscythian to pre-late anisian overburden existing in correspondence with each considered platform has been also inferred (assereto et al., 1977; masetti and trombetta, 1998; gianolla et al., 1998) and used during numerical modelling (table ii). there is no way to directly measure the initial porosity (φ0) and the compaction factor (c) of the two above-mentioned sedimentary units. however, by considering comparable presentday sediments (e.g., campbell et al., 1974; sclater and christie, 1980) these parameters have been tentatively assumed for the bellerophon and werfen units. the selected values of initial porosity are 0.6 and 0.5, respectively, while those of the compaction factor are 5⋅10− 4 and 2.7⋅10− 4, respectively (table i). furthermore, in order to correctly calculate the amount of subsidence induced by compaction, the total burial overlying the permoscythian formations just before the occurrence of the oligocene-quaternary widespread erosion must be estimated (table ii). accordingly, following the procedure proposed by sclater and christie (1980), we calculated the thickness of the two modelled sedimentary units just before the growth of the late anisian-early ladinian carbonate platforms. these values have been used as reference values for the calculation of the induced compaction (viz. subsidence). 5. subsidence map it is clear that the present-day distribution of the investigated platforms does not correspond to the original pattern. it is well known that mainly during cainozoic times the dolomites suffered considerable deformation (e.g., doglioni and bosellini, 1987). however, in order to account for the large-scale effects of the elastic deformation induced by each single carbonate body, we had to perform a palinspastic restoration to obtain the original distribution. we are well aware of the complex tectonic evolution that affected the region both in terms of intensity, distribution and kinematics of the different deformational events recognised in the area (e.g., caputo, 1996). nevertheless, as a first approximation we can assume that the bulk of shortening which occurred in the dolomites has a mean n-s direction (doglioni, 1987; doglioni and bosellini, 1987; castellarin et al., 1992; caputo, 1997; caputo et al., 1999). accordingly, we firstly identified the major thrusts affecting the region and the amount of shortening they caused and then we reconstructed a simplified palaeogeographic map of middle triassic times (fig. 5) slightly modifying that proposed by bosellini (1996). 1591 growth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy based on this palinspastic restoration and the consequent re-location of the carbonate platforms we thus performed the calculations of the induced elastic deformation for the broader dolomite region. as a first step, we calculated the subsidence induced by a single platform at different stages of the whole process. figure 6a-d shows an example of subsidence evolution in four time steps, relative to the sciliar-catinaccio platform (p7). both local and regional contributions are evident, while the gradient in between the two patterns is an effect of the prograding slope. the same calculations have been performed for all investigated platforms and all contributions have been summed up to generate the total subsidence map represented in fig. 7. this map represents the regional distribution of the amount of subsidence induced by the growth of the late anisian to early ladinian carbonate buildups. also in this case, both local and regional patterns are evident. fig. 6a-d. four time-steps map (a to d) representing the progressive subsidence induced by the growth of a single pre-volcanic platform: the example corresponds to the sciliar-catinaccio (p7 in figs. 4 and 5 and in table ii). the contour lines of the induced subsidence are in meters (see grey scale). fig. 7. map of the calculated subsidence induced by the growth of all late anisian-early ladinian carbonate platforms. both elastic (regional) and plastic (local) contributions are evident and can be easily quantified. the round geometry of the local contributions is due to the simplified shape of the carbonate bodies assumed in the model. the contour lines of induced subsidence are in meters (see grey scale). the maximum regionally induced subsidence is concentrated in the central dolomites and amounts to about 220 m, while local contributions range between 40 and 90 m. 6. concluding remarks due to the uncertainties in the selection of some of the parameters used in the numerical model and according to their more or less important influence in the calculations as discussed in a former section of the paper, it is clear that our final results should be considered as a first order approximation of the investigated natural phenomenon. nevertheless, these results clearly show that the subsidence induced by the growth of the late anisian-early ladinian carbonate platforms in the dolomites gave an important contribution for generating the accommodation space required for these thick successions of shallow-water sediments. a b c d 1592 claudia marella, riccardo caputo and alfonso bosellini based on the distribution and the diverse thickness values of the platforms (table ii), we can tentatively reconstruct the map of the total subsidence occurred in the dolomites during the middle triassic that caused the necessary accommodation space (fig. 8a). according to our calculations, the subsidence induced by the growth of carbonate platforms gives an important contribution as high as 20% to 40% of the total amount of subsidence. consequently, this contribution should not be disregarded especially when dealing with stratigraphic, palaeogeofig. 8a-c. a) the estimated pattern of the total subsidence in the dolomites during middle triassic as inferred from the thickness of the carbonate bodies (table ii). b) map of the subsidence induced by the growth of the platforms (simplified from fig. 7). c) pattern of the tectonic subsidence obtained from the former two maps and likely induced by a far field extensional stress field. the map emphasises the existence of a ne-sw oriented graben affecting the central dolomites. dashed lines represent the two major border faults (or fault systems). contours lines are in metres in all three maps (see corresponding grey scales). a b c fig. 9a,b. regional (a) and local (b) subsidence of a carbonate platform (bosellini, 1984). in the former case, the water depth of the basin roughly corresponds to the thickness of the carbonate platform. in the latter case, with a similar amount of aggradation, the adjacent basin is much shallower. 1: deep-water deposits; 2: platform slope deposits; 3: internal platform deposits. 1593 growth and subsidence of carbonate platforms: numerical modelling and application to the dolomites, italy 400-800 m thick overlying platforms has a magnitude of some tens of meters, while its gradient is distributed within the progradation zone, that is between the nucleus of the platform, where compaction is maximum, and the toe of the slope, where the compaction is nil. it is thus clear that the middle triassic subsidence induced by compaction generated a ‘very local’ effect. therefore, the latter hypothesis for the fig. 10. map of the calculated subsidence occurred during middle triassic and reduced to its present-day distribution after considering the tertiary contraction. graphic and geodynamic reconstructions of the dolomites. moreover, this local overload by carbonate platforms induces a positive feedback because any increased subsidence induces further growth and consequently a larger load onto the lithosphere. the quantification for the first time of this contribution is a major result of this paper. obviously a remote stress field, whose origin is probably associated with a process of lithospheric stretching, induced the remaining 60-80% of subsidence. by combining the map of the total subsidence (fig. 8a) with that of the induced subsidence (fig. 8b) it is possible to obtain a map of the tectonic subsidence (fig. 8c). according to fig. 8c, the central dolomites are characterised by a 20 km wide nesw trending graben where the two border faults (or fault systems) show a vertical displacement of about 200 m. it is well known that immediately after the growth of the late anisian-middle ladinian carbonate platforms, the central dolomites were affected by a short but intense volcanic event. it is noteworthy that the two major magmatic bodies (predazzo and monzoni) documented in the area, partially intruding the platforms and associated with volcanic edifices, are located exactly along the southern border of the graben. it is likely that the rising of the magma and the emplacement of the plutons were facilitated by a crustal scale extensional structure. further geologic and palaeogeographic implications of this study also regard the depth of the surrounding and/or adjacent basins. in fact, if a platform subsides solidly with the adjacent basin, the water-depth of this basin grossly corresponds to the thickness difference between platform carbonates and basin sediments. in contrast, if a platform subsides independently the former equation is not valid (fig. 9a,b). the results obtained from the application of the proposed numerical model to the middle triassic evolution of the dolomites certainly gave a contribution to unravel this problem and to choose between the two hypotheses. indeed, the approach followed allows the differential subsidence between the areas with a direct overload induced by the platform and the adjacent basin to be quantified. according to the results presented in this paper, the differential subsidence induced by the a b 1594 claudia marella, riccardo caputo and alfonso bosellini occurrence of an ‘independent’ subsidence (fig. 9b) is supported by our numerical modelling. however, because the differential subsidence we calculated is distributed along a distance of several hundreds of metres to some kilometres, corresponding to the dimension of a prograding slope, its influence in terms of thickness variations and original geometry of the sedimentary bodies is probably not detectable with geological means. these results can be easily extended to other generations of carbonate platforms of the dolomites as well as elsewhere in the stratigraphic record. a second major outcome of this research is the reconstruction of the present-day distribution of the subsidence induced by the local overload, after re-considering the tertiary shortening that affected the region (fig. 10). with the exception of the local ‘troughs’ directly caused by, and underlying the platforms, lateral variations in the amount of induced subsidence is generally limited, being about 50 m across the whole dolomite region. consequently, also in this case, even specific and dedicated geological investigations will probably not disclose any significant information in the stratigraphic record documenting these small variations relative to the depth of the basin surrounding the platforms or concerning the thickness of the basinal deposits. acknowledgements we thank to m.r. gallipoli, m. mucciarelli, d. albarello for discussions on the numerical modelling and to m. stefani and p. gianolla for supporting unpublished data about the triassic platforms of the dolomites. references assereto, r., c. brusca, m. gaetani and f. jadoul (1977): the pb-zn mineralization in the triassic of the dolomites. geological history and genetic interpretations, l’industria mineraria, xxviii, 1-15. athy, l.f. 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(received october 6, 2003; accepted march 4, 2004) 07giesecke.qxd 421 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical seismicity – south america – ceresis database – intensity data points – earthquake source parameters 1. introduction the andean chain is the result of subduction of the nazca plate beneath the south american plate, which extends for more than 9000 km along the western margin of south america. the activity associated with plate boundaries defines a zone of convergent tectonics, which causes active internal deformation in the andean block, as evidenced by neotectonic activity and a concentration of seismicity along the inter-andean valleys and coastal fault systems. the andean region is one of the most active seismic regions of the world and it accounts for about 15% to 20% of the total seismic energy released by planet earth. several great earthquakes hit the andean countries of south america (argentina, bolivia, chile, colombia, ecuador, peru and venezuela) in the past centuries. it was generally recognised, half a century ago, that in many parts of the world progress in seismology depended on the degree of organised regional co-operation. for this reason, unesco supported the creation of regional centres. a unesco mission, led by prof. v. v. beloussov, visited latin america in the early the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use alberto giesecke (1), augusto antonio gómez capera (2), ilaria leschiutta (2), elena migliorini (2) and leandro rodriguez valverde (1) (1) centro regional de sismologia para américa del sur (ceresis), lima, peru (2) istituto nazionale di geofisica e vulcanologia, sezione di milano, italy abstract the history of earthquakes in south america starts with the coming of the spanish and portuguese «conquistadores» at the beginning of the 16th century. their chronicles, and those of local historians, are the only source of earthquake information for the following 400 years. the creation of the regional centre for seismology for south america (ceresis) was a major factor for homogenous regional progress, in that ceresis promoted the implementation of the first unified earthquake catalogue and database for the whole andean region. this paper reviews basic information about the intensity database and the focal parameter catalogues proposed by ceresis in 1985. further macroseismic data available from the ceresis database (earthquakes with i0 ≥ 8) are used to obtain preliminary results for the earthquake source parameters of selected south american historical events. the case of the great earthquake of the venezuelan andes, 29 april 1894, is presented in some detail. mailing address: dr. alberto giesecke, centro regional de sismología para américa del sur (ceresis), p.o. box 14-0363, lima, peru; e-mail: giescere@inictel.gob.pe 422 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde 1960s to explore the possibility of establishing a seismological centre for south america. the mission recommended that such a centre be located in lima, peru. in 1966, unesco and the government of peru signed a bilateral agreement to create the centre; unesco provided operating funds for a five-year trial period and for a full-time director; peru provided the local facilities. a board of directors (seismologists, geophysicists, earthquake engineers) was appointed by the government of each participating country (including trinidad and tobago), and a representative of the division of earth sciences of unesco and of paigh (commission of geophysics of the pan american institute of geography and history) formulated policy, planned and supervised the centre’s activities. unesco’s financial support ended in 1971. a meeting of high level government representatives of the ceresis (regional centre for seismology for south america) countries was held in lima. they decided to convert ceresis into a permanent, official and autonomous international organisation for the south american region, belonging to the governments of its member states. a multinational agreement was drafted and signed by five countries (bolivia, colombia, peru, uruguay and venezuela) and went into effect on 1st july 1971; the document was deposited at the ministry of foreign relations of peru and it remains open to adhesion of other countries that wish to become member states. argentina, trinidad and tobago, ecuador, chile, spain, brazil and paraguay have also signed the agreement, and most of them have subsequently ratified it. the goal of ceresis is to improve the knowledge of seismology and related disciplines and to apply such knowledge to improve the region’s capacity to cope with earthquake and volcanic hazard. since its beginning, ceresis recognised that a unified catalogue of earthquake parameters for south america was essential for progress in many areas of seismological research such as regional studies of seismicity, seismotectonic and the evaluation of seismic hazard. the decision was thus made to produce a regional catalogue. this was made within the sisra project (earthquake mitigation program in the andean region project) in 1985, which focused on the determination of hypocentral parameters for south american earthquakes. for the pre-instrumental period and/or poorly monitored areas, intensity values, and focal parameters when possible, were determined by investigating and interpreting historical reports and chronicles. in the nineties, in many of the ceresis countries, there was a loss of interest in integrating national catalogues, and intensity data above all, for regional purposes. fortunately, present generations of seismologists and earthquake engineers recognise the importance of upgrading intensity data and keeping the regional catalogue up to date. despite the fact that «intensity» is not a scale which is based on instrumental measurements, it is nonetheless designed to quantify damage caused by earthquakes, an aspect which no other instrumental parameter can measure as satisfactorily. practical applications of intensities are illustrated by the following examples: i) earthquakes are the largest financial risk faced by insurance companies. therefore, these companies must be as highly informed as possible about the risks they might encounter. for example, the well-known skandia group insurance co. uses the ceresis «maximum intensity map of south america» as a tool to evaluate risk in the andean region. ii) in chile, «quality of soil» maps have been prepared as a function of differences of modified mercalli intensity (mmi), observed between different types of soil during the same earthquake. accurate estimates of the possible intensity at a given location have been achieved as a function of magnitude and distance, on the basis of attenuation equations relating magnitude-distance-intensity. such expressions have the virtue of being very reliable due to the fairly large quantity of data available over a very broad range of intensities, distances and magnitudes. ceresis has presented a proposal to the commission of geophysics of the pan american institute of geography and history (paigh) to interpret and catalogue the large amount of intensity data (mostly in mm, modified mercalli scale) accumulated in south america over the past 20 years. we hope this effort will lead to a ceresis training project, with the participation of historians and seismologists, to produce 423 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use in each country a group of professionals, with the required expertise, training and capacity, who are essential for reliable and consistent assessment of intensity. 2. background of the andean region catalogue and intensity database historical earthquakes in south american countries are found in historical documents from spanish colonial times, ancient newspapers, personal notes, chronicles, records of public offices, etc. these were mainly compiled by ecclesiastical authorities, and governors. early compilations of earthquakes were produced by some scientists and scholars. such compilations, including, for instance, boussingault and roulin (1849) in ecuador, and montessus de ballore (1912, 1916) in bolivia and chile, can be considered the first seismic catalogues of south america. during the past century, jesuit seismologists gave an outstanding contribution to the development of modern seismology in south america. fathers pierre descotes, luis fernández and ramon cabré in bolivia, german saa in chile and peru, rafael goberna in cuba and colombia, jesus emilio ramirez in colombia, were part of a generation of jesuits who played an important role in studying earthquakes in different parts of the world. historical research was proposed by sisra project and supported by the us geological survey, financed by the us office of foreign disaster assistance and the ceresis countries. historical compilations of destructive earthquakes of south america from 1530 to 1894 are published in vol. 10 of ceresis (1985), which contains information collected by silgado (1978, 1985) in several public and university libraries, local and central government offices and archives in south america, spain, france and england. data from the spanish colony were found in spain at the archivo general de las indias in sevilla, at the biblioteca nacional de madrid, at the archivo histórico nacional and at the real academia de historia. in london, silgado found data about earthquakes from colonial and republican time of south america at the british library, «sección de manuscritos españoles». he also consulted reports of the english consuls of the 19th century, stored in the public record office at kew, u.k. in france, silgado searched at the bibliothèque nationale de france in paris, where he consulted the «catalogo de los manuscritos españoles de alfred morel fatio» and the manuscripts of montessus de ballore, which are a compilation of south american earthquakes from colonial time to the end of the 19th century. in strasbourg, silgado consulted perrey’s works at the library of the institut de physique du globe. finally, silgado (1985, 1992) derived intensities of historical earthquakes of south america by interpreting the historical descriptive texts. the intensity scales most commonly used in south america are the modified mercalli scale (mm) and the medvedev-sponheuer-karnik scale (msk). however, there is growing interest in the new european macroseismic scale 1998: ems (grünthal, 1998), which derives from the necessity to introduce in the msk scale both new types of buildings (engineered and antiseismic construction) and the criteria of vulnerability. at best, intensity assessment is a complex endeavour that requires expertise, professional skill and remarkable historical background. therefore, it is better evaluated when historians and seismologists work together. national maps of maximum intensities (mm macroseismic scale) at a scale of 1:2 000 000 were adjusted and integrated to produce a regional map of south america at the scale 1:5 000 000 (ceresis, 1985). isoseismal maps of 220 strong earthquakes and values of intensity at selected locations were integrated. juan carlos castano, director of inpres (instituto nacional de prevención sísmica), argentina, was the regional co-ordinator for the map. ceresis provided an unparalleled framework aimed at unifying and upgrading the various existing national catalogues and intensity data (fig. 1a,b). the most important work on the ceresis hypocentre catalogue was done by the national groups of each participating country, who entered the data using common formats agreed by the national representatives. the regional co-ordinator for the catalogue was 424 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde leonidas ocola of the geophysical institute of peru. the regional catalogue, edited by askew and algermissen (ceresis, 1985), covers a time period which begins with the earliest known historical event (1471) and ends in 1985. below, we briefly describe the state-ofthe-art of historical seismology studies for ecuador, peru, bolivia, chile, argentina, brazil, paraguay and uruguay. the situation for colombia, venezuela, and trinidad and tobago is described in espinosa et al. (2004), altez and grases (2004) and vogt (2004). 2.1. ecuador the observatorio astronómico de quito (1959) compiled a descriptive and parametric catalogue of earthquakes that occurred between 1534 and 1958 as a contribution to the international geophysical year. the catalogue and intensity database compiled in the framework of the sisra project and published in vol. 6 of ceresis (1985), are based on information from observatorio astronómico de quito (1959), egred (1968) and data from international seismological agencies. historical archives were consulted as the archivo municipal de quito, the archivo nacional de historia, the national library and the aurelio espinosa polit library in quito. historical sources are quoted as for example «libros de cabildo de la ciudad de quito» (1645, 1675, 1775), documents from spanish colonial times. at present there is interest amongst historians in ecuador to investigate the social, economic and political impact of major earthquakes and volcanic eruptions. special care was taken to interpret the language and avoid confusion between earthquakes and volcanic eruptions, to attenuate the tendency to exaggerate or ignore evidence according to personal criteria. both descriptions of large events by different authors and documents found in sevilla (archivo general de las indias) and in the archives of the convents within ecuador have been compared. another difficulty was that caused by place names, since some localities have changed their names over the years. for some of them it has been impossible to find a reliable location. fig. 1a,b. a) idp distribution from the ceresis database; b) epicentre distribution from the ceresis database. a b 425 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use 2.2. peru primary historical sources in peru span from the spanish conquest (16th century) and spanish colonial times to the 19th century. these sources are anonymous notes, letters, chronicles, records of public offices prepared by ecclesiastical authorities, governors, etc. polo (1904), barriga (1951) and silgado (1968) investigated both peruvian libraries and archives and the archivo general de las indias in sevilla. the results of this research were used for the catalogue of strong earthquakes (espejo, 1974) compiled by the instituto geofísico del perú and for the peruvian parametric catalogue (ocola, 1984) prepared in the framework of the sisan project (sismicidad andina, andean seismicity). at the same time, huaco (1983) performed historical investigations and quantification of historical information and similar studies were published by silgado (1978, 1985). the results obtained by this second wave of research are at the basis of the peruvian catalogue and intensity database published in vol. 7 of ceresis (1985). further studies (e.g., dorbath et al., 1990; silgado, 1992) made possible a revised and updated peruvian ceresis catalogue from 1470 to 1982, which is available since 2000 at http://www.igp.gob.pe/ 2.3. bolivia the jesuit priest pierre m. descotes (18771964) is considered the father of seismology of bolivia. for more than 50 years, he served as director of the san calixto observatory (osc). father descotes collected and studied all available references for historical earthquakes (descotes and cabre, 1973), particularly ballivian (1909) and montessus de ballore (1912). vega (1978) compiled a first systematisation of seismological information through the project sisan where the main bibliographical sources were descotes’s work and publications from the international seismological agencies. the ceresis catalogue and intensity database for bolivia was compiled on the basis of the sisan catalogue (vega, 1978) and a search of more data in historical archives as the archivo de potosi and the archivo nacional de bolivia. some historical sources and ancient newspapers are quoted by ceresis (1985), as for example: anonimous (1650), lara (1860), la industria (1884, newspaper of sucre town) and la estrella de oriente (1890, newspaper of santa cruz town). this catalogue, critically revised and complemented, was published in vol. 3 of ceresis (1985). the compilation of the catalogue continued up to the 1981, using data of both the u.s. geological survey and the international seismological centre (isc). 2.4. chile the catalogue of earthquakes for chile covers a long history. the compilation of events that occurred in colonial times were in general related to studies aimed at compiling texts on the history and geography of chile, rather than for scientific purposes. montessus de ballore (1911, 1912, 1916) produced the most complete catalogue of historical chilean earthquakes. greve (1964) and lomnitz (1970) have added historical information to this catalogue. during the first four decades of the 20th century several international institutions and universities began the preparation of world catalogues with valuable information related to chile. the department of geology and geophysics of the universidad de chile, santiago, runs the chilean seismological service. a catalogue of destructive earthquakes is available at http://ssn.dgf.uchile.cl/home/ sismohisto.html/ the chilean catalogue and database compiled by sisra project, and published in vol. 5 of ceresis (1985), are documented in detail mainly by greve (1964), lomnitz (1970), kausel (1979a, 1979b). some problems were nonetheless encountered in the compilation of intensity database for chile. for example, some localities related to the 1868 and 1877 earthquakes do not have co-ordinates and their names are not known, possibly because they correspond to places in southern peru, western bolivia or chile, which either were abandoned or changed their names. 426 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde 2.5. argentina a working group at inpres (instituto nacional de prevención sísmica) was appointed to prepare the national catalogue of argentina in early 1980’s. their first step was to compile all available data, historical and instrumental, pertinent to earthquakes that affected the argentine territory. when the sisra project started, what was previously achieved was reviewed in order to proceed within the guidelines and formats proposed by sisra. new information on destructive earthquakes was obtained from different historical sources, mainly newspapers and magazines of the time for different places in the country. thus, the first argentine catalogue and intensity database was compiled (ceresis, 1985). this catalogue quotes studies and compilations from lunkenheimer (1930), olsacher (1935), harrington (1944), and volponi (1962). a parametric and descriptive catalogue is today available at http://www.inpres.gov.ar/ 2.6. brazil, uruguay and paraguay although seismic hazard in brazil, uruguay and paraguay is low, it is worth stressing that these three countries voluntarily joined and support ceresis. since they occupy most of the precambrian shields of the continent and are located in the internal continental region of the south american plate, seismic activity is expected to be low, similar to other intraplate regions on the earth. the first seismological station in brazil was established in 1921 in rio de janeiro. the present national network includes several local and regional sub-networks and many first order stations. the study of the seismicity of the brazilian territory has intensified in recent years. this is explained by i) the interest of brazilian seismologists, ceresis and the university of edinburgh in the installation of the south american (seismic) array system, near brasilia, ii) the necessity of carrying out detailed studies on seismicity and seismic risk for the construction of large hydroelectric dams and nuclear reactors, and iii) the rather constant occurrence of small and, generally, non-destructive earthquakes of intensity v mm. seismic activity is observed on the continent near the atlantic coastline, from belem in the north to st. catarina, south of sao paulo. induced earthquakes of magnitude mb 3.5 to 3.7 have occurred twice in parana. the important contribution brazil gave to south american seismology was to improve the locations of several andean epicentres. a working group of seismologists from the instituto astronómico e geofisico, the instituto de geociencias (usp) and the estação sismológica de brasília, at the request of the national commission for nuclear energy, undertook a study of the seismicity of brazil. the resulting catalogue was published by berrocal et al. (1984) and includes historical events dating back to 1560. previous attempts to compile a catalogue began with capanema (1859). in uruguay, the universidad de la república in montevideo is the liaison institution with ceresis. uruguay’s interest in seismology is related to large engineering projects, e.g., the salto grande dam (a bilateral project with argentina) and the 60 km long bridge across the la plata river, from buenos aires to colonia. the ceresis regional seismicity map shows one seismic event located in the middle of the la plata, close to where the bridge will probably be built. the maximum intensity map shows that the area has an intensity of v mm. in paraguay, the universidad nacional de asunción (facultad de ciencias exactas y naturales) is the national liaison institution with ceresis. paraguay operates one of south america’s four gtsn, high gain, stations located about 100 km south of asunción. three other gtsn stations are located near la paz, in bariloche (argentina) and in brasilia. 3. characteristics of the ceresis database and use macroseismic data are frequently used for parameterisation of the seismogenic sources and evaluation of seismic hazard. consequently, several seismological institutions have formalised their historical data into homogeneous macroseismic databases and have developed 427 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use online databases to increase the dissemination of data through the web (see rubbia, 2004). the ceresis database (http://www.ceresis. org) is the main source of intensity data points (idp) for south america, it contains 16 318 idp (fig. 1a) related to 3183 events (fig. 1b) that occurred from 1471 to 1985 in eight south american countries and one caribbean country (table i). the format of the ceresis database includes two types of record as shown in table ii. for each event, the first line provides the earthcountry time window no. of earthquake no. of idp argentina 1692-1985 45 703 bolivia 1650-1981 45 208 brazil 1767-1981 114 431 chile 1730-1977 61 816 colombia 1566-1981 188 2097 ecuador 1541-1980 153 1416 peru 1471-1981 2023 8354 trinidad and tobago 1825-1981 23 78 venezuela 1530-1981 531 2215 total 3183 16318 cou code ye da da ho mi se ilat ilon mlat mlon dph ms i0 ne hyp pe 120 1586 07 10 00:30:00 –12.300 –77.700 60 10m 12 pe lima –12.10 –77.00 100 76 81 8k dh 421 pe callao –12.05 –77.15 32 62 74 8k dh 421 pe callao –12.04 –77.09 20 10 41 319 8k utlo 411 pe callao –12.04 –77.09 20 10 41 319 9k utlo 411 pe caravel –15.77 –73.36 1779 567 568 136 3k utlo 411 pe cuzco –13.52 –71.97 3326 571 573 106 3k utlo 411 pe huanuco –9.92 –76.23 1894 257 261 20 3k utlo 411 pe ica –14.07 –75.72 439 259 262 147 5k utlo 411 pe ica –14.07 –75.72 439 259 262 147 6k utlo 411 pe lima –12.05 –77.05 32 7 41 333 9k utlo 411 pe lima –12.05 –77.05 32 7 41 333 8k utlo 411 pe trujillo –8.10 –79.03 51 495 496 33 3k utlo 411 hyp pe19795 1960 11 20 22:01:56 -6.800 -81.000 55 6.8 ms 6m 2 pe piura –5.20 –80.62 35 54 81 35 6k utlo 411 pe piura –5.20 –80.62 35 54 81 35 5k utlo 411 table i. time-windows, number of earthquakes and related idp in the ceresis database. table ii. example of ceresis database format. examples of duplicate record, as for different «interpreter» (i.e. callao: dh or utlo) or uncertainties in the intensity assignment (i.e. callao i = 8/9), are reported. 428 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde quake parameters (i.e. epicentral co-ordinates, epicentral or maximum intensity, magnitude, depth, number of related idp – although this is not always reported); in the following lines, the idp related to the earthquake are listed. all intensities are given in either mm (modified mercalli) or msk (medvedev-sponheuer-karnik) macroseismic scales. the investigator who assigned the intensity value is identified as «interpreter» («dh» or «utlo» in table ii). a more detailed description of the catalogue format is given in vol. 1 of ceresis (1985), including the list of the «interpreters» and the references they used. the database contains some cases of duplicate records for the same events and/or localities. this is either due to different interpreter/source or to uncertainties in the intensity assessment. for instance, if the «interpreter» assess an i = 7/8 for a given locality, two records are present in the database, one with i = 7 and another with i = 8. the increased availability of historical data interpreted in terms of idp has stimulated seismologists to develop standard and repeatable procedures to determine earthquake source parameters, in order to lower the level of subjectivity they are usually determined upon. the boxer program (gasperini et al., 1999) is an example of these new procedures. the algorithm computes the parameters of the earthquake source from idp; it provides the seismological parameters (latitude and longitude of the epicentre, moment magnitude) and a geometric model of the source through a box that represents the surface projection of the modelled seismogenic source. the original version of the program is designed only for dip-slip faults and it was initially calibrated for apenninic earthquakes (in most cases shallow events). the program represents each source as the surface projection of a fault dipping 45° towards a direction perpendicular to the fault strike (gasperini et al., 1999). the projection width constrains the fault at depth. the program was largely tested in italy (valensise and pantosti, 2001; mirto et al., 2001) and europe, first in the frame of the ec project faust (faults as a seismologist’s tool, http://faust.ingv.it), whose goal was to determine the source parameters (leschiutta and mirto, 2000; stucchi et al., 2000) of some european earthquakes using the european mediterranean intensity database (emid), and then by other european seismologists (kouskouna, 2002). given the widespread use of boxer, we decided to test it using the ceresis database. we are perfectly aware that in south america earthquakes are chiefly deep crustal or due to subduction processes, and the structures are often strike-slip faults. nonetheless, we applied the boxer method to some earthquakes of the south-american region without considering the applicability limit of boxer to pure dip-slip faults and attenuation of earthquake intensity for the italian territory. as such, our results represent a very preliminary attempt of determining source parameters for some large south american earthquakes. 3.1. examples of use: analysing records to determine sources parameters with the aim of determining source parameters using the macroseismic data provided by ceresis database we focused on large events; 201 earthquakes above the threshold of i0 ≥ 8, are considered in this study. figure 2a,b shows the distribution of earthquakes and number of idp for 50-year time windows, for the 201 selected earthquakes. in some of the studied cases, the ceresis database contains for an individual earthquake duplicated intensity values assessed at the same place. since the boxer program accepts one intensity value only for each place, an a priori choice has been made; both in case of different interpreters and of uncertain intensity, the highest intensity value has been adopted. the number of idp in the ceresis database for the 201 events with i0 ≥ 8 is 5515, while those used in this study are 4245 only (table iii). out of the 201 events above the threshold (i0 ≥ 8) a more detailed selection was carried out considering the characteristics of the boxer program (table iv). therefore only events with an appropriate number of idp (no. ≥ 10) have been considered; all the earthquakes with a focal depth ≥ 30 km have been disregarded as well as the offshore and subduction events. the identification of the subduction events was sup429 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use ported by the available literature. dorbath et al. (1990) assessed the size of large and great historical earthquakes in zones along the trench in peru. these zones correspond roughly to segmentation defined by the geometry of the subduction zone. other authors, such as lomnitz (1970), kelleher (1972), pennington (1981), kanamori and mcnally (1982), nishenko (1985), comte and pardo (1991), mongue (1993), tavera and buforn (1998), tavera (2002) and carpio and tavera (2002) listed the major earthquakes along south american subduction zone. they are the 1746, 1868, 1912, 1913, 1970 earthquakes in peru, and the 1730, 1751, 1822, 1868, 1877, 1928, 1966 earthfig. 2a,b. a) number of earthquakes per 50-year time-windows and b) number of idp per 50-year time-windows, for the 201 earthquakes with i0 ≥ 8 from ceresis database. table iii. number of records in the ceresis database for earthquakes with i0 ≥ 8. country ceresis database i0 ≥ 8 this study i0 ≥ 8 time-window no. of earthquakes no. of idp no. of idp used argentina 1692-1985 17 483 480 bolivia 1650-1947 6 30 30 brazil 1955 1 10 10 chile 1730-1976 13 382 330 colombia 1644-1981 43 1287 921 ecuador 1541-1980 29 912 871 peru 1471-1974 63 1775 990 trinidad & tobago 1825 1 14 14 venezuela 1530-1975 28 622 599 total 201 5515 4245 table iv. criteria used for selecting earthquake records from 201 events with i0 ≥ 8. criteria no. of earthquakes eliminated number of idp < 10 89 focal depth > 30 km 45 off shore events 18 subduction events 11 incoherent idp distribution 1 total events eliminated 164 a b 430 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde intensities are essentially similar. table v list also «∆epic.» that is the distance between the ceresis epicentres and the macroseismic ones computed by the boxer program. this comparison between epicentres shows some cases with distance greater than 150 km. the only justifiable difference is the one concerning 9 april 1928, peru earthquake, because in this case ceresis gives the instrumental localisation. for the other two events (26 march 1812 venezuela and 27 october 1894 argentina earthquakes) a more detailed analysis would be necessary (but it is out of the scope of this study). fig. 3. location of 37 intensity-based sources (box) quoted in table v. the adjacent number to each box corresponds to id_box (table first column). quakes in chile. these are the events not considered by this study. adopting the above described criteria only 37 events have been processed with the method proposed by gasperini et al. (1999). in fig. 3, boxes representing the source dimension, size and location obtained from macroseismic data are shown. table v gives the ceresis parameters, the parameters computed by boxer and a comparison between them. in general, the moment magnitude calculated by boxer program is smaller than the magnitude ms reported by ceresis. the epicentral 431 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use t ab le v . p ar am et er s ob ta in ed f ro m m ac ro se is m ic d at a fo r th e 37 i nv es ti ga te d ea rt hq ua ke s in s ou th a m er ic a. ∆ e pi c. i s th e di st an ce b et w ee n ep ic en tr e gi ve n by c e r e s is an d ep ic en tr e co m pu te d by b o xe r pr og ra m . id _b ox c e r e s is (1 98 5) t hi s st ud y p ar am et er s co m pu te d by b o xe r p ro gr am c om pa ri so n of s ou rc e pa ra m et er s ob ta in ed b y b ox er pr og ra m w it h c e r e s is da ta c ou nt ry i 0 s c d ep th ( km ) m s n o. i d p i 0 m w f au lt l en gt h (k m ) f au lt w id th f au lt a zi m ut h i 0 m w – m s ∆ e pi c. 1 16 98 0 6 20 e cu ad or 10 m s k 14 10 6. 3 20 .3 10 .5 00 9 ± 05 3 0. 0 30 .8 2 17 66 1 0 21 v en ez ue la 9 m m 6. 0 3 8 5. 6 7. 7 6. 2 02 7 ± 00 0 – 1. 0 – 0. 4 13 .1 3 17 85 0 7 12 c ol om bi a 8 m m 6. 5 17 8 6. 3 20 .0 10 .5 13 8 ± 04 8 0. 0 – 0. 2 37 .3 4 17 97 0 2 04 e cu ad or 11 m s k 86 11 7. 3 70 .3 20 .7 06 7 ± 00 1 0. 0 7. 3 5 18 12 0 3 26 v en ez ue la 9 m m 6 6. 3 40 9 7. 2 62 .5 19 .4 05 6 ± 00 9 0. 0 0. 9 29 4. 0 6 18 34 0 1 20 c ol om bi a 11 m m 7. 0 11 10 6. 3 20 .3 10 .5 03 4 ± 00 0 – 1. 0 – 0. 7 16 .2 7 18 59 0 3 22 e cu ad or 8 m s k 20 8 6. 2 17 .5 9. 7 01 8 ± 02 9 0. 0 14 .4 8 18 68 0 8 15 c ol om bi a 8 m s k 7. 0 10 10 6. 3 18 .4 10 .0 06 8 ± 01 8 0. 0 – 0. 7 34 .8 9 18 68 0 8 16 e cu ad or 10 m s k 52 10 6. 7 32 .3 13 .5 00 8 ± 01 6 0. 0 7. 4 10 18 75 0 5 18 c ol om bi a 10 m m 20 7. 3 30 10 7. 0 45 .5 16 .3 09 3 ± 01 6 0. 0 – 0. 3 9. 0 11 18 78 0 4 13 v en ez ue la 8 m m 13 5. 9 18 7 5. 3 5. 2 5. 0 09 6 ± 02 4 – 1. 0 – 0. 6 4. 8 12 18 94 0 4 29 v en ez ue la 9 m m 20 7. 1 71 9 6. 8 38 .9 15 .0 06 0 ± 01 6 0. 0 – 0. 3 12 .0 13 18 94 1 0 27 a rg en ti na 9 m m 30 8. 0 44 9 7. 1 53 .8 17 .9 16 7 ± 01 1 0. 0 – 0. 9 21 5. 0 14 19 13 1 1 04 p er u 10 m m 20 23 9 6. 3 19 .7 10 .4 10 6 ± 05 8 – 1. 0 4. 3 15 19 14 1 2 02 p er u 10 m m 15 18 10 6. 3 19 .3 10 .2 07 3 ± 21 7 0. 0 16 .6 16 19 28 0 4 09 p er u 9 m m 30 6. 9 11 7 6. 0 12 .7 8. 2 10 1 ± 00 0 – 2. 0 – 0. 9 13 7. 8 17 19 28 0 5 14 p er u 10 m m 7. 3 16 9 6. 9 45 .6 16 .3 13 6 ± 14 3 – 1. 0 – 0. 4 91 .3 18 19 32 0 3 14 v en ez ue la 9 m m 25 6. 8 58 9 6. 8 35 .8 14 .3 11 4 ± 04 8 0. 0 0. 0 32 .3 19 19 34 0 6 11 a rg en ti na 9 m m 30 6. 0 13 8 6. 1 14 .0 8. 6 – – 1. 0 0. 1 28 .8 20 19 38 0 8 10 e cu ad or 9 m s k 23 9 5. 8 9. 5 7. 0 09 0 ± 01 5 0. 0 1. 2 21 19 42 1 2 26 c ol om bi a 8 m m 6. 5 12 8 6. 5 24 .9 11 .8 05 6 ± 04 2 0. 0 0. 0 86 .2 22 19 44 0 1 15 a rg en ti na 9 m m 30 7. 4 39 9 6. 9 41 .6 15 .5 07 6 ± 16 5 0. 0 – 0. 5 12 .6 23 19 46 1 1 10 p er u 11 m m 7. 3 34 11 7. 3 70 .8 20 .7 13 2 ± 01 6 0. 0 0. 0 21 .1 24 19 47 0 7 14 c ol om bi a 9 m m 10 5. 5 59 9 5. 9 11 .3 7. 7 15 9 ± 11 5 0. 0 0. 4 36 .4 25 19 50 0 8 03 v en ez ue la 8 m m 8 64 55 8 6. 2 15 .7 9. 2 04 3 ± 04 4 0. 0 – 0. 2 34 .6 26 19 52 0 6 11 a rg en ti na 8 m m 30 7. 0 15 8 6. 0 12 .2 8. 0 17 7 ± 01 6 0. 0 – 1. 0 23 .9 27 19 55 0 5 11 e cu ad or 8 m s k 6. 8 22 7 5. 4 5. 3 5. 1 05 1 ± 02 1 – 1. 0 – 1. 4 35 .0 28 19 57 0 4 21 c ol om bi a 8 m m 6. 6 18 8 7. 0 45 .5 16 .3 03 6 ± 03 7 0. 0 0. 4 90 .3 29 19 61 0 4 08 e cu ad or 8 m s k 24 29 8 5. 7 7. 9 6. 3 15 5 ± 02 3 0. 0 20 .2 30 19 66 0 9 04 c ol om bi a 8 m m 8 10 7 5. 2 4. 1 4. 4 12 1 ± 07 6 – 1. 0 19 .7 31 19 69 1 0 01 p er u 9 m m 20 7 5. 3 4. 6 4. 7 03 6 ± 00 9 – 2. 0 9. 7 32 19 70 0 9 26 c ol om bi a 8 m m 8 6. 6 13 7 6. 0 12 .0 7. 9 00 0 ± 00 0 – 1. 0 – 0. 6 21 .6 33 19 74 0 4 18 c ol om bi a 9 m m 24 4. 5 10 8 6. 3 20 .2 10 .5 15 1 ± 15 8 – 1. 0 1. 8 14 .6 34 19 74 0 7 13 c ol om bi a 8 m m 12 7. 3 17 8 7. 0 47 .0 16 .6 00 6 ± 04 2 0. 0 – 0. 3 57 .9 35 19 76 0 4 09 e cu ad or 8 m m 19 6. 7 67 7 5. 8 5. 7 5. 3 01 7 ± 03 2 – 1. 0 – 0. 9 15 .4 36 19 77 1 1 23 a rg en ti na 9 m m 4 7. 4 13 2 9 6. 9 40 .2 15 .3 10 8 ± 03 1 0. 0 – 0. 5 86 .4 37 19 85 0 1 26 a rg en ti na 8 m m 12 22 8 5. 8 9. 5 7. 0 06 4 ± 05 0 0. 0 24 .4 y r m o d a 432 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde 3.2. the earthquake of 29 april 1894 in the venezuelan andes the 1894 event is one of the largest historical earthquakes in venezuela. it is known as the great earthquake of the venezuelan andes and it almost completely ruined the towns of santa cruz de mora, zea, mesa de bolivar, tovar, san juan de lagunillas, chiguará y mérida (audemard, 1998). altuve (1998), rengifo and laffaille (1998) and rivera de uzcategui and torres (1998) investigated and collected historical sources on this event. ceresis (1985) on the basis of the historical work by grases (1979), gives the epicentre as 8.50°n, 71.70°w, at a 20 km depth, magnitude ms = 7.1, and maximum reported intensity ix (mm). also the epicentre given by rengifo and laffaille (1998) is similar: 8.55° ± 0.05°n, 71.69° ± 0.05°w, but with a focal depth estimated in 14 ± 2 km, and a magnitude ranging between 7.1 and 7.4. recently this event has been directly associated to the southern section of the bocono fault through paleoseismological investigations (audemard, 1997, 1998; audemard et al., 1999) and its magnitude has been estimated between ms 7.1 and 7.3. based on the 71 macroseismic data reported by ceresis (fig. 4a,b), the macroseismic epicentre (8.39°n, 71.70°w) and the moment magnitude (mw = 6.8) were obtained through the boxer program. as shown in fig. 4a,b, the boxer program suggests that the 1894 earthquake ruptured a source with a length of 39 km and an azimuth of n60°e. this result can be considered consistent with the regional tectonic trend, and it is further supported by the conclusions drawn by audemard et al. (2000). in fact, they mention this earthquake as an event related to historical movement (sense of movement: dextral-normal) of the south of mérida section of bocono fault system, in venezuela. 4. conclusions between 1950 and 1970, before the ceresis foundation, compilations, studies and catalogues were produced by various seismologists and south american seismological agencies. in 1985, the earthquake catalogue and intensity database for eight south american countries (argentina, bolivia, brazil, colombia, chile, ecuador, peru and venezuela) and one caribbean country (trinidad and tobago) were compiled through the sisra project, which were then assembled in the ceresis earthquake catalogue and intensity database (ceresis, 1985). data came from previous compilations and new interpretations of hisfig. 4a,b. the 29 april 1894 earthquake in venezuela: a) 71 intensity data points reported by ceresis (1985); b) the box represents the source dimension, size and location obtained from intensity data following the method proposed by gasperini et al. (1999). b a 433 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use torical and instrumental records within a time window of about 500 years; earthquakes occurred in pre-hispanic times are not included, apart from some information on the 1471 earthquake in peru. given the huge amount of data and the level of its organisation, the south american intensity database produced by ceresis is a paramount achievement in the framework of macroseismic studies. a recently developed application, which processes macroseismic data (boxer, gasperini et al., 1999) and was originally tested in italy and europe, has been adopted in this study. even though this application should be carefully tested, given the nature of the south american catalogues, which include both shallow and subduction earthquakes, the results we achieved are to some degree conformable. to conclude, we emphasise that more efforts are needed to develop methods that make use of macroseismic intensity data to constrain the essential characteristics of the seismic sources for south america. acknowledgements we are very thankful to massimiliano stucchi for his thoughtful suggestions and continuous encouragement. we also thank paola albini for reviewing the manuscript and making helpful comments. thanks are extended to an anonymous referee whose comments improved an early version of the manuscript. references altez, r. and j. grases (2004): a brief history of a restless vision: seismological research in venezuela, ann. geophysics, 47 (2-3), 451-464 (this volume). altuve, f. 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(1859): quais as tradições ou vestígios geológicos que nos levem a certeza de ter havido terremotos no brasil, rev. inst. hist. geogr. bras., 22, 135-159. carpio, j. and h. tavera (2002): estructura de un catálogo de tsunamis para el perú basado en el catálogo de gusiakov, bol. soc. geol. perú, 94, 45-59. ceresis, centro regional de sismología para américa del sur (1985): earthquake mitigation program in the andean region (project sisra), ceresis, perú, 14 vols. comte, d. and m. pardo (1991): reappraisal of great historical earthquakes in the northern chile and southern peru seismic gaps, nat. hazards, 4, 23-44. descotes, p.m. and r. cabre (1973): historia sísmica de bolivia, rev. geofís. panam., 2 (1), 251-781. dorbath, l., a. cisternas and c. dorbath (1990): assessment of the size of large and great historical earthquakes in peru, bull. seismol. soc. am., 80 (30), 551-576. egred, j. 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(1979): investigación sobre los sismos detructores que hayan afectado el oriente de venezuela, 434 alberto giesecke, augusto antonio gómez capera, ilaria leschiutta, elena migliorini and leandro rodriguez valverde delta del orinoco y regiones adyacentes (proyecto instituto tecnológico venezolano del petroleo), 2 vols. greve, f. (1964): historia de la sismología en chile, publicación instituto de geofísica y sismología, santiago de chile, pp. 94. grunthal, g. (editor) (1998): european macroseismic scale 1998, cah. cent. eur. géodyn. séismol., luxembourg, 15. harrington, h.j. (1944): volcanes y terremotos (editorial pleamar, serie biblioteca conocimiento, buenos aires, argentina). huaco, d. (1983): catálogo sísmico del perú (proyecto sisra, instituto geofísico del perú, lima). kanamori, h. and k.c. mcnally (1982): variable rupture mode on the subduction zone along the ecuador, bull. seismol. soc. am., 72, 1241-1253. kausel, e. (1979a): comportamientos de diferentes suelos durante el terremoto del 16 de agosto de 1906, inf. comisión chilena energía nuclear, pp. 9 and appendix. kausel, e. (1979b): los terremotos de agosto 1868 y mayo 1977, que afectaron al sur del perú y norte de chile (departamento de geofísica, santiago de chile, inédito). kelleher, j. (1972): rupture zones of large south american earthquakes and some predictions, j. geophys. res., 77 (11), 2087-2103. kouskouna, v. (2002): characteristic fault directions and rapture zones obtained from historical and instrumental earthquakes from the gulf of corinth, in xxviii esc general assembly, sca-1, genoa, 1-6 september 2002, p. 137 (abstract). la estrella de oriente (1890), newspaper, santa cruz, bolivia. la industria (1884), newspaper, sucre, bolivia. lara, m.s. (1860): crónicas anónimas, archivos universidad de santa cruz, santa cruz, bolivia. leschiutta, i. and c. mirto (2000): using historical earthquake data for the determination of the source parameters of some european earthquakes, in proceedings of the esc wg historical seismology workshop, 1-5 september, macerata (italy), 54-59. libros de cabildo de la ciudad de quito (1645): manuscritos originales de las actas de cabildo de la ciudad de quito, a partir de su fundación, archivo histórico municipal quito. libros de cabildo de la ciudad de quito (1675): manuscritos originales de las actas de cabildo de la ciudad de quito, a partir de su fundación, archivo histórico municipal quito. libros de cabildo de la ciudad de quito (1775): manuscritos originales de las actas de cabildo de la ciudad de quito, a partir de su fundación, archivo histórico municipal quito. lomnitz, c. (1970): major earthquakes and tsunamis in chile during the period 1575 to 1955, geol. rundsch., bd. 59, heft 3, 938-960. lunkenheimer, f. (1930): el terremoto del sud-mendocino del 30 de mayo de 1929 (observatorio astronómico, universidad de la plata, la plata, argentina). mirto, c., e. ercolani, i. leschiutta, f. mosciatti and g. rubbia rinaldi (2001): i database macrosismici on-line dei terremoti italiani ed europei e il loro utilizzo per la determinazione dei parametri di sorgente dei terremoti, in atti del vi workshop giast, 14-16 settembre 1999, sansepolcro, arezzo, italy, 77-89. mongue, j. (1993): estudios de riesgo de tsunami en costas chilenas, in jornadas chilenas de sismología en ingeniería antisísmica, santiago de chile, 2, 3-22. montessus de ballore, f. (1911): historia sísmica de los andes meridionales (imprenta universitaria, santiago de chile), pp. 345. montessus de ballore, f. (1912): bibliografía general de temblores y terremotos (sociedad chilena de historia y geografía, santiago de chile). montessus de ballore, f. (1916): historia sísmica de los andes meridionales (editorial cervantes, santiago de chile). nishenko, s. (1985): seismic potential for large and great interplate earthquakes along the chilean and southern peruvian margins of south america: a quantitative reappraisal, j. geophys. res., 90 (b5), 3589-3615. observatorio astronomico de quito (1959): breve historia de los principales terremotos en el ecuador, publicación del comité del año geofísico internacional del ecuador, quito, pp. 35. ocola, l. (1984): catálogo sísmico del perú, proyecto sisan, instituto geofísico del perú, lima, pp. 464. olsacher, j. (1935): terremoto de sanpacho (provincia de córdoba), 10 y 11 de junio de 1934, rev. mus. prov. cienc. nat., 1, p. 52. pennington, w.d. 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(1978): historia de los sismos más notables ocurridos en el perú (1513-1974), instituto geología y minería, lima, boletín no. 3, pp. 130. silgado, e. (1985): destructive earthquakes of south america 1530-1894, in earthquake mitigation program in the andean region (project sisra, ceresis, lima), 10, pp. 328. silgado, e. (1992): investigación de sismicidad histórica en la américa del sur en los siglos xvi, xvii, xviii y xix (concytec/ceresis, perú), pp. 108. stucchi, m., i. leschiutta and c. mirto (2000): on the influence of the uncertainties of historical data on the determination of earthquake parameters, in xxvii esc general assembly, wsa-1, 10-17 september 2000, 435 the ceresis earthquake catalogue and database of the andean region: background, characteristics and examples of use lisboa, pp. 115 (abstract). tavera, h. and e. buforn (1998): sismicidad y sismotectónica de perú, in física de la tierra, edited by e. buforn and a. udias (servicio de publicaciones universidad de complutense), 10, 187-219. tavera, h. (2002): historia de un terremoto anunciado para la región sur del perú (centro nacional de datos geofísicos, instituto geofísico del perú), 11-18. valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger than m 5.5 in italy, ann. geofis., 44 (suppl. to no. 4), pp. 180, with cd-rom. vega, b.a. (1978): catálogo sísmico de bolivia (proyecto sisan-oea, observatorio san calixto, la paz). vogt, j. (2004): a glimpse at the historical seismology of the west indies, ann. geophysics, 47 (2-3), 465-476 (this volume). volponi, f. (1962): aspectos sismólogicos del territorio argentino, in actas primeras jornadas argentinas de ingeniería antisísmica, buenos aires. geo 1215 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 17 ionospheric disturbances generated by different natural processes and by human activity in earth plasma environment hanna rothkaehl (1), nataly izohkina (2), igor prutensky (2), sergey pulinets (2) (3), michel parrot (4), georgyi lizunov (5), jan b ⁄lecki (1) and iwona stanis ⁄lawska (1) (1) space research centre, warsaw, poland (2) institute of terrestrial magnetism, ionosphere and radio wave propagatio (izmiran), russian academy of sciences, troitsk (moscow region), russia (3) instituto de geofísica, unam, ciudad uni, delegación de coyoacán, méxico d.f. (4) lpce/cnrs, orleans, france (5) kyiv shevchenko university, ukraine the magnetosphere-ionosphere-thermosphere subsystem is strongly coupled via the electric field, particle precipitation, heat flows and small scale interaction. satellites in situ measurements and ground based complex diagnostics can provide comprehensive coverage of both time and geomagnetic place effects. human activity also can perturb earth’s environment, but few are connected with controlled experiments in the ionosphere and are transient. most of them are related to industrial activity and have increased in recent years. the most important power sources are broadcasting transmitters, power stations, power lines and heavy industry. at ionospheric altitude some disturbances and physical processes are related to seismic activity, thunderstorm activity and some global changes in the earth environment such as ozone holes. various natural and artificial indicators can affect satellite telecommunication quality. the aim of this presentation is to report progress in understanding the physical processes in the ionosphere described above and to assess the application of these considerations to the study of plasma effects on earth-space and satellite-to-satellite communication. 17.1. electromagnetic and electrostatic instabilities driven by changes geomagnetics activity in the nearest earth environment to develop a quantitative model of the interaction of solar energetic particle events with earth’s magnetosphere it is necessary to consider plasma as well as wave diagnostics of space plasma. despite a long history of ionospheric space investigations, the physical properties of ionospheric plasma are still not well understood. the global description of electromagnetic fields during the variable parameters in the sun-earth system is very difficult to do in the upper ionosphere. natural plasma emission has become an object of interest since it was first received on the orbit of spacecraft. it is very hard to judge which physical process dominates during a geomagnetic disturbance, in particular in particular it seems necessary to bind the observations from low and high altitude together for a better understanding of the physical processes in the magnetosphere and ionosphere and to build a useful physical model for space wether service purposes. 1216 hanna rothkaehl et al. in addition to passive information like local plasma density near space vehicles, wave diagnostics located on board spacecraft also give a dynamic view of plasma behaviour and instabilities. the hfwaves activity manifest in the ionosphere, can describe the property of local electron plasma and can also help to understand the property of remote plasma region. this frequency range covers high frequency whistler waves, langmuir and upper hybrid modes of natural plasma, whereas non-linear plasma iteration can appear as a wide band turbulent spectrum. the region of the main ionospheric trough acts like a lens focusing a variety of instabilities and disturbances from a different region of the earth’s environment. the mid-latitude electron density trough observed in the topside ionosphere has been shown to be the near-earth signature of the magnetospheric plasmapause and thus its behaviour can provide useful information on magnetosphere dynamics since its existence is dependent on magnetospherically induced motions (tulunay et al., 2003). the ionospheric trough is an unique region of the ionosphere, where different type of wave and instabilities can be generated. dynamic changes in the post-midnight outer ionosphere during an intense magnetic storm with ssc have been analysed using wave and plasma measurements (rothkaehl et al., 1997). the investigation shows that hf and vlf wide band frequency emissions are good indicators of a turbulent fig. 17.1. the theoretical position of plasmapause lpp, (solid line), and the position of the trough determined from the hf measurements versus sequence of satellite orbits (dashed line) with typical emissions during strong geomagnetic storm close to the plasmapause position, bottom panel the evolution of dst index. 1217 ionospheric disturbances generated by different natural processes and by human activity in earth plasma environment plasma region closely related to the ionospheric trough features. figure 17.1 presents the position of minimum mail ionospheric trough and typical waves activity. the detected vlf emission follows the quarter-gyrofrequency very well while hf radio emission indicates the enhancement of radiation at frequenciesy larger than the local upper hybrid limit. this broad band emission is mainly detected very close to the plasmapause, inside the plasmasphere but also inside the trough up to the auroral oval. this type of emission is mainly correlated to low frequency radiation, which is excite by the wave-particle interaction in the equatorial plasmapause and moves to the ionosphere along the geomagnetic field line. at the ionosphere the subthermal electron can interact with those electrostatic waves and excite the electron acoustic waves or the hf longitudinal plasma waves. fossil broad band emission was also detected inside the plasmosphere close to the plasmopause after strong geomagnetic disturbances. this type of emissions permit us to construct a more realistic model of the transfer energy in the ionosphere-magnetosphere coupling processes and to describe the region of plasma instability in the top side ionosphere. satellite in situ waves and plasma measurements and theoretical considerations yielded a spatial picture of this ionospherice region. it was shown that the peeling of plasma cloud associated with spatially nonuniform plasma from the low latitude trough wall during enhanced geomagnetic activity can be associated with plasma heating by electrostatic instability and atmospheric acoustic gravity waves generated in the auroral zone and damped in the ionosphere (izovkina et al., 1999). the polar cusp-cleft is a very sensitive region of the earth environment for changes in geomagnetic condition and is one of the regions, where the most intense electrostatic and electromagnetic waves are generated. during reconnection at magnetopause in the cusp vicinity plasma particles are accelerated and plasma particle fluxes are generated. wave diagnostics located on board a low altitude orbiting satellite have demonstrated that during strong geomagnetic storms the cusp and cleft region is characterised by the appearance of: whistler mode, upper hybrid, electron plasma waves and lower hybrid resonance, as well as electron acoustic plasma waves. the observed emissions at low frequency and vlf frequency range are localised in space and have burst character and can be correlated with the filamentary structure of the cusp caused by current instabilities. the broad band hf whistler waves are good indicators of cusp positions particularly during strong geomagnetic disturbances. wave particle interactions are proposed as a mechanism for the energy cascade from low frequency to high frequency waves. the discussed process has a self-consistent character and is accompanied by the heating and acceleration of particles in the polar cusp. disturbances in the magnetopause can produce ion beams, which give rise to alfven waves. as a consequence alfven waves accelerate electrons. on the other hand, lower hybrid waves are also sources of energetic electrons. theoretical consideration shows that the high frequency langmuir waves can be excited by the beam of non thermal electrons or are driven by lower hybrid resonance. the most intensive waves in the cusp are related to the turbulent plasma region (b ⁄lecki et al., 2003). the impact of magnetospheric disturbances on the ionospheric daily fof2 frequencies is remarkable and can be closely related to the cusp feature (fig. 17.2). the geographical position of the cusp determined by high-latitude topside measurements seems to be very close to the disturbance seen in the fof2 maps obtained from the ples (stanis ⁄lawska and rothkaehl, 2002) model even with some doubts concerning the mapping accuracy and topside data interpretations. in spite of these limitations, the almost real-time availability of both vi and topside measurements makes it possible to use them for the practical improvement of radio communication maps. now the topside measurements can be treated not only as an indicator of the high altitude local disturbance that statistically will influence the electron concentration below, but as a physical source of redistribution of the electron concentration that will give exact figures when using simpler models. future works will refine the dayside cusp-cleft model for radio communication purposes (rothkaehl et al., 2003). large-scale spatial structures of electrostatic noise distribution observed near the geomagnetic equator can be related to variations in plasma dispersion parameters in the inhomogeneous magnetic field, the decay of electrostatic modes over the magnetic equatorial surface, and the possible formation of 1218 hanna rothkaehl et al. fig. 17.3. global distribution of hf emission in the ionosphere in the frequency range 0.1-2 mhz during day time sector. the spectral intensity was integrated at around 500 km altitude 31.03.1994 during quiet conditions and on 6.04.1994 during strong geomagnetic disturbances, recorded by the sors-1 instrument on board the coronas-i satellite. the well manifested enhancements of hf frequency (whistler frequency range) follow the geomagnetic equator and some enhancements are also remarkable in the region where energetic proton fluxes are detected. fig. 17.2. fof2 instantaneous map (mhz) for the european area for december 17, 1992 14 ut constructed during a geomagnetic storm (upper panel) and wave measurements display of the cusp region over the atlantic ocean seen from on board the freja satellite 17.12.1992 at 18 ut (lower panel). 1219 ionospheric disturbances generated by different natural processes and by human activity in earth plasma environment plasma vortices (with dimensions exceeding the ion larmor radius) on density and temperature gradients of plasma. if a vortex propagates in the region of the inhomogeneous geomagnetic field, it can break up when the direction of the geomagnetic field vector at the equator becomes opposite. the formation of plasma vortices favors conservation of plasma inhomogeneities and consequently their detection. the appearance of additional broadband noise can be the manifestation of quasi-entrapment of electromagnetic waves between plasma walls of an equatorial cavern (izovkina et al., 2002) (fig. 17.3). the enhancement of broad band emissions in the electron whistler frequency band near equatorial equator is a common feature of global distribution natural electromagnetic noises in the ionosphere. this emission is associated with distribution of energetic particles from the radiation belt region. 17.2. electromagnetic and electrostatic instabilities driven by man-made influences the ionosphere is modified unintentionally by various types of man-made noise. as a consequence electromagnetic emissions detected on board low orbiting satellites and propagated signals are affected by artificial influences. perturbations of the nearest environment are due to different types of waves emitted from the ground and release of different gases into the atmosphere. a large variety of observed phenomena connected with human activity have been detected and analysed such as: wave particle interaction, precipitation of radiation belt electrons, parametric coupling of electromagnetic whistler waves, triggering emissions, frequency shifting and wave spectrum broadening. most of these disturbances were observed at lower frequencies and only a few were correlated with the disturbances of hf electromagnetic emissions. some ground-based experiments were performed and consisted in controlled heating of the ionosphere by dense powerful electromagnetic waves. synchronous detection of hf waves on board the coronas-i satellite over sura heating facility exhibited enhancement of background radiation parallel to the appearance of radiation at the 3rd and 5th order of electron cyclotron harmonics (klos et al., 1997). latest experimental results from ionosphere hf pumping experiments in the night-side auroral region above tromsø, show the triggering of local auroral activation. the authors suggest that these changes in the local ionospheric plasma are parallel to the appearance of the alfven turbulence and the formation of the local magnetospheric current system. the property of natural emissions depends very much of the conditions of the sun’s activity, but the influence of artificial disturbances is complex and very often relate to the weave particle and wave weave plasma interaction. we can observe parallel to direct radiation also in the emissions of ionosfig. 17.4. the example of typical single hf spectrum detected on board the coronas-i satellite, the line marks the local electron gyrofrequency at the satellite altitude and the arrow the cut-off of local plasma emission uhr (upper hybrid resonance). the well pronounced emission for frequencies larger than uhr are related to human activity, mainly broadcasting station activity. 1220 hanna rothkaehl et al. fig. 17.5. the global distribution of mean value of electromagnetic emission in the ionosphere in the frequency range 0.1-15 mhz. the spectral intensity was integrated at various times of day and night 30.03.1994 during quiet condition and on 6.04.1994 during strong geomagnetic disturbances, recorded by sors-1 instrument on board the coronas-i satellite. the resolution is 5°× 5°; the units are db/µv. fig. 17.6. the global distribution of the electric component eh and magnetic component bx recorded during very quiet condition from 11 to 13 january 1982 on board the aureol-3 satellite in the wide frequency band from 9to 16 khz. the intensity of detected emissions is in log(mv/m/√(hz)) for the electric component and log(nt/m//√(hz)) for the magnetic component. 1221 ionospheric disturbances generated by different natural processes and by human activity in earth plasma environment pheric plasma triggered by transmitters of electromagnetic waves located around the world. human activity can be seen on the recorded spectra as separated peaks, growing of the whole background of plasma emission or enhanced broadband emissions in the whole observed frequency range (fig. 17.4). the permanently pumped electromagnetic waves to the ionosphere by the system of broadcasting stations can disturb the nearest space environment. thus this activity can create in the topside ionosphere local langmuir turbulence or ion-acoustic turbulence. however theoretical assumptions show that the generation of ion-acoustic turbulence is far more effective. the scattering of super-thermal electrons on ion-acoustic or langmuir turbulence is proposed as a mechanism of generation of broad-band hf emissions. the hf diagnostics, performed on the low orbiting satellite, detected an enhancement of radiation particularly over the euro-asia region. hf emission in the topside ionosphere is more intense over the area correlated with outer radiation belts. it is prominent on the eastern hemisphere i.e. some longitudinal effect is manifested. the observed effect is markedly visible, and is stronger in the night-side ionosphere. strong geomagnetic disturbances can broaden the area of observed intensification of hf noises in comparison with quiet time, but the intensity of the observed enhancement is not affected by geomagnetic disturbances (rothkaehl and klos, 2002) (fig. 17.5). the vlf frequency emissions range is in principal related to the direct radiation of broadcasting station, high voltage power line and thunderstorm activity, but, the complex non-linear physical processes in ionospheric plasma described above should also change the nearest earth environment. the examination of the data collected on the arcad-3 satellite helps us to find the emissions in the vlf frequency range correlated with broadband hf emissions detected over europe and asia region. detailed examination of global distribution vlf emission detected in the topside ionosphere seems to be very complex for analysis due to propagation effects. the vlf natural plasma emissions are made by different types of natural and artificial origin noises. an effect of particle precipitation on natural whistler and hiss emissions as well as enhancement of made man noises triggered by vlf transmitters, power-line harmonic radiation plhr and enhancement the vlf emissions related to thunderstorm activity have been shows (parrot, 1990). they also concluded that maximum vlf hiss over north america is related to enhancements of thunderstorm activity region and plhr emissions. the presented example (fig. 17.6) shows that during very quiet condition we can detected the enhancements of vlf emission over europe contrary to the more disturbed condition where the maximum is detected over regions of north america. it is important to underline that the described increase was detected during night time at altitudes larger than 1000 km during quiet thunderstorm activity. 17.3. earthquake electromagnetic signatures many observational facts suggest the connection of earthquakes with disturbances of electromagnetic fields (lf vlf hf range) and associated phenomena. the statistical investigations of elf/vlf emissions recorded by low orbiting altitude satellite aureol-3 done during seismic events are presented in the fig. 17.7 (parrot, 1994). the correlation between seismic activity and wave enhancement shows that the extension of increase is in latitude but not in longitude (the ∆long < 10°). this indicates that, due to the propagation wave related seismic processes can be observed all along the magnetic meridian passing the epicentre of the earthquake. changes in the nearest earth environment over seismic activity regions observed by the «atmospheric explorer – e» satellite and by the system of ground-based vertical ionosondes network have been investigated recently. densities and temperature of neutral components o, n2; main ion component o+ and densities of minor ionised components no+, o+2 recorded on board the ae-e satellite as well as typical ionospheric parameters of eand f-layers recorded by vi measurements were analysed. the crosscorrelating spectra analysis of wavelength from 100 km to 2000 km displayed the enhancement of atmospheric gravity waves, agw, wavelike ion variations and travelling ionospheric disturbances, tid. 1222 hanna rothkaehl et al. fig. 17.7. the average amplitude of 140 hz recorded by magnetic component during 24 h window on board the aureol-3 satellite, around 325 earthquakes with ms > 5. fig. 17.8. variation of ion concentration o+ along lt, 77.10.07. the satellite pass perturbed region through 5 h after an earthquake, distant from the earthquakes epicentre r (103 km), detected on board the ae-e satellite at altitudes of up to several thousand km from epicenter. regular features of recorded variations appeared to be connected with solar, moon atmospheric tides, and terminator motion. sporadic variations are caused by solar and geomagnetic activity as well as agw penetrating the thermosphere «from below». characteristic parameters of agw/tid features generated by strong earthquakes (m = 7) at satellite altitudes (near 300 km) have horizontal wavelength near 600 km, while the amplitude of neutral density wave (agw) reaches several percents, variations of ion density wave tids are stronger, reaching tens percent (fig. 17.8). observational effects can be explained in terms of agw-ionosphere interaction. the hypothetical chain of events is as follows: earthquake shock can generate wave packet agw waves propagating upward, than due to the filtering of large-scale agw by the atmosphere we can observe heating of the upper atmosphere and the generation of tids. as a consequence we can detect the electric field irregularities. 1223 ionospheric disturbances generated by different natural processes and by human activity in earth plasma environment the physical picture of the relation between electromagnetic disturbances on the ground as well as at the ionospheric altitudes is not clear and more theoretical studies are needed. in the near future (april 2004) the french satellite demeter will be launched into orbit and its main goal will be to search for ionospheric effects such as disturbances of the electromagnetic field, plasma density, electron temperature, ion composition and temperature, alfven, whistler and ion-acoustic waves associated with earthquakes. 17.4. the interaction between the body of satellite and surrounding ionospheric plasma the level of electromagnetic radio noises observed on board satellites strongly depends on the properties of the satellite environment, as well as geophysical conditions. behind the body of spacecraft the region of strong disturbed plasma is well manifested. the study of plasma wakes behind satellites has a long history in the investigation of earth environment. the mesothermal motion of a satellite in the ionosphere creates a disturbed region behind the spacecraft that is depleted unequally of both ions and electrons. this type of irregularity has been mainly investigated by accurate measurements of electron density in the wake region while an enhancement of electron temperature has been observed in the discussed region. it is well known that for a small body the wake depletion varies approximately linearly, while for a large body the variation is exponential. the existence of excitation of ion acoustic waves behind a small satellite has been well examined. also the unexpected broadband electrostatic emission behind large objects like spacelab-2 has been observed and considered. the radiospectrometer observations recorded on board the intecosmos19 and other plasma waves experiments like active and apex show some broad band emission in the hf frequency range. the measurements also show the enhancement of radiation between the local plasma frequency and critical frequency fof2. it has been shown that the origin of this radiation can be correlated with the interaction between oscillation of electrostatically tapped particles in the wake of spacecraft and superthermal electrons (rothkaehl, 1993). the potential of spacecraft body surrounding in ionospheric or magnetospheric plasma can play a crucial role in understanding the electromagnetic stability of payload and antenna systems. a number of experiments have employed the artificial particle beams form ionospheric satellite to study various ionospheric and magnetospheric phenomena. these studies were used to diagnose auroral plasma properties and also to research the disturbances of propagated electromagnetic signal from one satellite to another. theoretical considerations and data analysis of satellite experiments indicated that the charging of the satellite body by a dense electron beam strongly depends on the local electron density and in cases when local plasma is not sufficiently dense to neutralize electron current the overpotential takes place. the interaction of the injected beam with payload periodical potential can create strong wide band hf emission up to 80 mhz (zbyszyński, 2002). 17.5. conclusions electromagnetic radiation in hecto and decametric range in the nearest space environment is a topic of investigations both as a constituent of the geophysical environment, and as an element of physical processes in which particles and waves of ionospheric plasma participate. those investigations are lead using ground-based techniques and space borne instruments. the level of electromagnetic noise detected at ionospheric altitude depends on properties of geophysical condition, properties of the different earth regions, influences from the earth surface and the satellite surrounding plasma interaction. human activity can perturb earth’s environment, but it does not have a direct impact, so the investigations should cover the wide band subject of physical processes in space. 1224 hanna rothkaehl et al. the various natural and artificial indicators can affect satellite telecommunication quality. the natural ionospheric radiation range around characteristic plasma emission in hf is of the order of 10−10 wm−2hz−2, while noises related solar radio emissions are of the order of 10−16 wm−2hz−2. theoretical considerations show that the emitted power related to the human activity is in order of 10−18 wm−2hz−1. the broad band emission detected in the ionosphere enriches the frequency bandwidth by a few mhz. this type of broad band emissions can influence on the global electromagnetic environment. the presented considerations can be used to construct global signal to noises maps used for telecommunication purposes. theoretical considerations of the physical property of ionospheric plasma bring us to a description of the spatial and temporary distribution of instabilities and disturbances generated in the nearest space environments. it has been shown that broad band emissions are related to the natural plasma instabilities driven by geomagnetic activity, some unintentional heating ionosphere by human activity, interaction between the satellite body and surrounding plasma and some catastrophic events on the earth surface. the different types of ionospheric plasma instabilities described above can in consequence produce different types of irregularities both in ionisation and neutral plasma component. the aim of this presentation was to listen to the typical instabilities and regions, where different types of plasma irregularities can be recognized. the process described above can significantly influence propagation properties. signal quality is affected by irregularities and instabilities in the ionosphere which produces phase and amplitude variations in the received signal. an in-depth understanding of these physical processes is helpful for telecommunication purposes. references b ⁄lecki, j., s. savin, n. cornilleau-wehrlin, m. parrot, h.rothkaehl, k. stasiewicz, r. wronowski, o. santolik and j.-a. sauvaud (2003): fine structure of the polar cusp as deducted from the plasma wave and plasma measurements, adv. space res., 32 (3), 315-322. izhovkina, n.i., v.v. afonin, a.t. karpachev, i.s. prutenskii and s.a. pulinets (1999): structure of the ionospheric trough for different geomagnetic disturbance levels and the sources of plasma heating in the upper day side ionosphere, magn. aeron., 39 (4). izhovkina, n.i., i.s. prutensky, s.a. pulinets, z. klos and h. rothkaehl (2002): electrostatic emissions in magnetoplasma irregularity in the region of the south atlantic geomagnetic anomaly, magn. aeron., 42 (5), 660-665. klos, z., h. rothkaehl, z. zbyszyński, n.i budko, i.s. prutensky, s.a. pulinets and v.v. vaskov (1997): the modification of hf emissions in the upper ionosphere during the heating experiments over sura facility, in plasma 97, research and applications of plasmas, edited by m. sadowski and h. rothkaehl, vol. 1, 379-383. parrot, m. (1990): world map of elf/vlf emissions as observed by low-orbiting satellite, ann. geophysicae, 8, 135-146. parrot, m. (1994): statistical study of elf/vlf emissions recorded by a lowaltitude satellite during seismic events, j. geophys. res., 99 (a12), 23339-23347. rothkaehl, h. (1993): the hf noises related to the satellite wake in ionospheric plasma, adv. space res., 13 (10), 309-312. rothkaehl, h. and z. klos (2002): broadband hf emissions as an indicator of global changes within the ionosphere, adv. space res., 31 (5), 1371-1376. rothkaehl, h., f. jiři ček, j. šmilauer and m. förster (1997): dynamic changes in the outer ionosphere in the region of the ionospheric trough during an intense magnetic storm, adv. space res., 20 (3), 409-414. rothkaehl, h., i. stanis ⁄lawska, j. b ⁄lecki and z. zbyszyński (2003): low altitude cusp-cleft region signatures of strong geomagnetic disturbances, cosmic res., 41 (4), 340-344. 1225 ionospheric disturbances generated by different natural processes and by human activity in earth plasma environment stanis ⁄lawska, i. and h. rothkaehl (2002): ples model in the plasmapause diagnostics, adv. space res., 29 (6), 833-838. tulunay, y.k., i. stanis ⁄lawska and h. rothkaehl (2003): revisiting the ariel trough work for hf telecommunication purposes, cosmic res., 41 (4), 319-331. zbyszyński, z. (2002): spacecraft potential neutralization as a result of interaction ambient ionospheric plasma, on cd-proceedings of the 1st cost 271 workshop «ionospheric modelling and variability studies for telecommunication applications», ral, 25-27 september 2001, edited by a. vernon. l ' e l e t t r o n i c a n e l l a t e c n i c a a e r o s p a z i a l e (*) f . s a l z a r i c e v u t o il 19 f e b b r a i o 1963 r i a s s u n t o . — l ' e l e t t r o n i c a reca u n c o n t r i b u t o essenziale alla t e c n i c a a e r o s p a z i a l e in ogni fase di qualsiasi sua realizzazione. i n f a s e di p r o g e t t o è f o n d a m e n t a l e l'impiego del calcolo analogico o digitale, di quello analogico-digitale e delle tecniche di simulazione. nel volo spaziale la g u i d a e la stabilizzazione si b a s a n o sull'uso di sistemi e l e t t r o n i c i che sono a n c h e i m p i e g a t i l a r g a m e n t e nella essenziale fase di p r e p a r a z i o n e al volo. così p u r e le c o m u n i c a z i o n i coi passeggeri e le telemisure sono possibili s o l t a n t o grazie all'impiego delle t e c n i c h e p i ù a v a n z a t e delle telecomunicazioni. si e s a m i n a infine la situazione a t t u a l e per ciò che si riferisce al g r a d o di a f f i d a m e n t o delle a p p a r e c c h i a t u r e e l e t t r o n i c h e in conseguenza della loro c r e s c e n t e c o m p l e s s i t à : l'impiego di e l e m e n t i allo s t a t o solido e di a d e g u a t e tecniche circuitali c o n s e n t e di conservarlo ad un valore a c c e t t a b i l e n o n o s t a n t e la c r e s c e n t e complicazione degli a p p a r a t i in conseguenza delle s e m p r e più g r a v i esigenze. s u m m a r y . tlie role of e l e c t r o n i c s i n each phase of ali s p a c e p r o j e c t s is essential. in t h e design p h a s e a n a l o g u e , digitai, digital-analogue electronic comp u t a t i o n a n d s i m u l a t i o n t e c l i n i q u e s are lieavily relied u p o n . d u r i n g space flight g u i d a n c e a n d s t a b i l i z a t i o n are a c h i e v e d w i t h elect r o n i c s a n d likewise electronics are widely used in ali pre-flight p h a s e s . space c o m m u n i c a t i o n in m a n n e d a n d u n m a n n e d flight a n d t e l e m e t e r ing m u s t use t h e most sopliisticated c o m m u n i c a t i o n s t e c h n i q u e s . e l e c t r o n i c e q u i p m e n t is b e c o m i n g increasingly complicateci: its reliability is briefly discusseci. solid s t a t e electronics a n d a d v a n c e d c i r c u i t design seem t o give s a t i s f a c t o r y reliability even w i t h t h e i n c r e a s e d d e m a n d s p l a c e d u p o n electronic e q u i p m e n t . (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 giugno 1962. 4 8 8 i ' . s a l z a lo sviluppo della tecnica aerospaziale è s t r e t t a m e n t e legato ad un largo impiego della tecnica elettronica. credo che questa affermazione di carattere generale possa essere a c c e t t a t a senz'altro: le realizzazioni già a t t u a t e e, certamente, anche quelle a cui oggi si tende, non solo non avrebbero p o t u t o essere concepite e raggiunte, ma nemmeno essere verificate, senza l'ausilio della tecnica elettronica e, in particolare, della applicazione a cui si pensa i m m e d i a t a m e n t e ogni volta che si nomina l'elettronica: la tecnica delle radiocomunicazioni, che ha ormai raggiunto attraverso alla teoria dell'informazione u n i n q u a d r a m e n t o preciso per ciò che si riferisce alla definizione della capacità limite di trasmissione. p e r illustrare il contributo dell'elettronica alla tecnica aerospaziale pensiamo di seguire la genesi di un veicolo spaziale, dalla fase di concezione attraverso l'intero sviluppo e fino all'impiego, soffermandoci in ciascuna fase per esaminare le possibilità di utilizzazione e le contribuzioni effettive della tecnica elettronica. vedremo così che le une e le altre sono assai numerose e che spesso l'elettronica fornisce l'unica soluzione possibile a molti dei problemi che si presentano nel corso del progetto e della realizzazione. cominciamo dalla fase iniziale di concezione; la definizione dell'obbiettivo da raggiungere nasce c e r t a m e n t e da u n a individuazione di possibilità al di là dello stato presente della tecnica e, se vogliamo veramente dare u n nome alla causa p r i m a dell'impostazione di ogni nuovo progetto, non può essercene che u n o : lo sforzo creativo ed immaginativo che è all'origine di ogni nostro progresso, di ogni nostra conquista. ma, immediatamente, l'idea iniziale deve essere sottoposta all'indagine scientifica e si può dire che, a cominciare dalla fase di analisi preliminare sulla possibilità di attuazione (feasibility) fino alla definizione funzionale completa, in ogni progetto aerospaziale il contributo dell'elettronica, attraverso i procedimenti di calcoli e di simulazione, è estremamente esteso. i calcolatori analogici e numerici rappresentano lo s t r u m e n t o di lavoro più i m p o r t a n t e e più largamente usato in questo settore: converrà quindi soffermarsi brevemente sulle possibilità e sugli orientamenti odierni. sia il calcolo analogo che quello numerico, come è noto, devono le loro odierne estese possibilità all'impiego dell'elettronica. fino a poco tempo f a si è parlato, in generale, di campi diversi di applicazione per i calcolatori dei due tipi. p e r i calcolatori analogici: calcolo con approssimazioni ridotte, ma con facile variabilità dei p a r a m e t r i di base del sistema. contemporaneal ' e l e t t r o n i c a n e l l a t e c n i c a a e r o s p a z i a l e niente difficoltà di impostazione per problemi con coefficienti variabili e nella realizzazione di operatori non lineari; infine una certa possibilità di soluzione in tempo reale, generalmente t a n t o più limitata quanto più nel problema compaiono, appunto, operatori non lineari. p e r i calcolatori numerici, invece, si parla generalmente della possibilità di precisioni assai più elevate, ma con maggiori difficoltà di variazione dei p a r a m e t r i nei problemi di ottimizzazione; difficoltà ancora maggiore di operare in tempo reale. l'evoluzione dei calcolatori dei due tipi ha p o r t a t o oggi sempre più, nella tecnica aerospaziale, al loro impiego promiscuo in complessi analogico-numerici: questa soluzione è s t a t a resa possibile s o p r a t t u t t o dai sistemi rapidi per effettuare nei due sensi la conversione analogieonumerica, dall'accorciamento dei tempi di accesso delle memorie e di esecuzione delle operazioni nella p a r t e numerica. certamente, t r a i numerosi f a t t o r i che hanno condotto ai più recenti progressi nei campo dei calcolatori, il maggior contributo singolo è quello dovuto all'impiego di circuiti elettronici allo stato solido, sia per la maggior facilità di ottenere tempi brevi di commutazione che per la possibilità di a u m e n t a r e il numero di elementi di calcolo ed ausiliari senza raggiungere configurazioni proibitive sia come ingombro che come potenza dissipata. infine l'elettronica dello stato solido ha permesso di raggiungere gradi di affidamento accettabili anche in apparecchiature con numero assai elevato di componenti, riducendo drasticamente i tempi di indisponibilità dei calcolatori. questo breve esame dell'uso del calcolo elettronico nella fase di progetto, per accertare la realizzabilità del sistema, stabilirne la configurazione ed ottimizzarlo, non sarebbe completo senza un cenno alle possibilità offerte dalla tecnica di simulazione, intesa come sostituzione parziale, nel modello del sistema impostato sul calcolatore (analogico, numerico o misto) di componenti effettivi, già disponibili, del sistema in esame, collegati al modello da opportuni organi di trasduzione. l ' a u m e n t o della velocità nel calcolo elettronico accresce la possibilità di impiego dei calcolatori in tempo reale e ciò risulta particolarmente interessante, nel settore aerospaziale, sia per consentire l'impiego della simulazione nella fase di progetto di cui ci stiamo occupando sia, come vedremo in seguito, per a t t u a r e la guida in fase effettiva di impiego. continuando il nostro esame converrà ora accennare all'impiego vastissimo di strumentazioni elettroniche per la realizzazione pratica e la messa a p u n t o degli elementi di sistema. —ut ..i.*—h i —il ; nw• _ i 4 9 0 f. salza ricordiamo che, accanto all'impiego vastissimo dell'elettronica nelle tecniche delle misure e della registrazione, compare oggi quello, più propriamente tecnologico, di macchine operatrici a controllo elettronico per l'esecuzione di processi di fabbricazione, per esempio per la realizzazione di p a r t i con configurazioni complesse, esprimibili in f o r m a analitica o almeno approssimabili con u n a espressione analitica. passando ora alla fase di impiego del sistema ritroviamo ancora molteplici utilizzazioni delle apparecchiature elettroniche. parlando dell'impiego dei veicoli aerospaziali non si può, p r i m a di t u t t o , non esaminare quel complesso di operazioni di verifica che precedono l'impiego effettivo. molte delle apparecchiature del veicolo e degli eventuali elementi di appoggio a t e r r a sono di tipo elettronico. inoltre, non fosse che per il numero spesso estremamente elevato di funzioni da analizzare, è spesso necessario ricorrere a schemi logici di verifica, impostati su calcolatori elettronici, per effettuare con sicurezza e senza errori l'intera sequenza delle operazioni di controllo cui è necessario sottoporre il sistema. esaminando, successivamente, l'impiego effettivo del veicolo aerospaziale, osserviamo che all'elettronica sono sempre affidate le funzioni di stabilizzazione e di guida. circa la stabilizzazione è ben noto che, s o p r a t t u t t o nelle fasi in cui si utilizza un controllo aerodinamico, è r a r a m e n t e possibile conferire a un veicolo di prestazioni elevate u n a configurazione aerodinamica che soddisfi t u t t e le esigenze di stabilità: in questo caso l'elettronica offre il suo ausilio efficace consentendo di modificare nel senso desiderato le proprietà aerodinamiche intrinseche del veicolo attraverso u n a appropriata azione delle superfici di controllo. problemi analoghi trovano la loro soluzione a t t r a v e r s o l'elettronica nel caso della stabilizzazione a t t o r n o al baricentro dei veicoli spaziali, sia in fase propulsa che in fase orbitale o di volo nello spazio libero. la guida del veicolo (moto del suo baricentro) può, in generale, essere a t t u a t a con sistemi inerziali, a p r o g r a m m a oppure a comando da t e r r a . i n ogni caso l'intervento di apparecchiature elettroniche è essenziale. senza l'elettronica non potrebbero realizzarsi p i a t t a f o r m e inerziali, senza di essa non si potrebbe misurare, nello spazio, la posizione del veicolo e valutare l ' e n t i t à dei comandi da impartire per avere la traiettoria desiderata. a questo proposito è i m p o r t a n t e notare quanto sia utile potere effettuare, sul calcolatore di guida, i calcoli in tempo reale, sia per assil ' e l e t t r o n i c a n e l l a t e c n i c a a e r o s p a z i a l e curare la traiettoria prevista col minimo di provvedimenti correttivi, sia per accertare al più presto, in caso di inconvenienti di funzionamento, il manifestarsi di una situazione pericolosa. è pressoché superfluo ricordare che nella funzione di guida si intende anche compreso, generalmente, t u t t o un complesso di operazioni ausiliarie: accensione dei vari stadi, loro distacco, etc. in funzione di tempi prestabiliti o del raggiungimento di determinate condizioni cinematiche e che, anche per queste operazioni, siano esse p r o g r a m m a t e a priori o comandate in funzione dello svolgimento del lancio, il contributo dell'elettronica è essenziale. questa esposizione non si occupa del problema delle comunicazioni con veicoli spaziali; t u t t a v i a faremo qualche considerazione specifica sulle apparecchiature elettroniche usate per tale scopo. considerando sia le prestazioni proprie del veicolo che la sua eventuale utilizzazione a scopi di ricerche deve in ogni caso essere risolto un problema di telemisura. ecco u n a tecnica la cui nascita e il cui sviluppo sono essenzialmente legati all'elettronica: sia dal p u n t o di vista della q u a n t i t à di informazione da t r a s m e t t e r e , sia per ciò che riguarda semplicemente l'esigenza di ottenere questo complesso di dati su un canale di trasmissione radio, ci si trova di f r o n t e a problemi che sono nati, si può dire, con la tecnica aerospaziale e che, risolti per merito dell'elettronica, sulla tecnica aerospaziale h a n n o trovato le loro prime applicazioni, anche se l'efficienza delle realizzazioni ha poi suggerito l'estensione del loro impiego in altri campi. i dati trasmessi possono essere semplici rilevamenti di posizione, oppure si può avere u n a trasmissione automatica di d a t i di ogni genere, in tempo reale o differito, per acquisizione di informazioni sul comport a m e n t o dei veicolo e sull'ambiente in cui esso si trova, oppure ancora la trasmissione è destinata alla utilizzazione specifica del veicolo (appoggio di telecomunicazioni, assistenza alla navigazione) ed alla comunicazione con passeggeri del veicolo stesso. i n ogni caso t u t t o ciò ò possibile unicamente per i progressi compiuti dall'elettronica, s o p r a t t u t t o nell'ultimo ventennio. a questo proposito può essere interessante osservare che la realizzazione delle molteplici funzioni che abbiamo brevemente elencato è essenzialmente legata a 1111 particolare aspetto della evoluzione dell'elettronica: la possibilità di realizzare con sufficiente grado di affidamento delle apparecchiature destinate a funzioni complesse. benché sia difficile esprimere sinteticamente l'evoluzione dell'elettronica, t u t t o r a in a t t o , essa può forse essere indicata con un esempio f . s a l z a di carattere generale: dal 1920 ad oggi il numero dei componenti di un radiotrasmettitore è a u m e n t a t o , in media, secondo 1111 f a t t o r e di circa 10 per decennio. si t r a t t a , ovviamente, di u n a valutazione e s t r e m a m e n t e sommaria: si può t u t t a v i a osservare che forse essa è ancora in difetto, in quanto l'elettronica è sempre più orientata verso apparecchiature a funzioni qualitativamente limitate, anche se q u a n t i t a t i v a m e n t e più estese. questo sviluppo, con applicazioni che u n a volta sarebbero parse impossibili, dà luogo ad altri gravi problemi. t r a questi i più i m p o r t a n t i sono, certamente, da un lato la necessità di conservare e possibilmente migliorare il grado di affidamento delle apparecchiature e, dall'altro, la necessità di contenere il volume degli a p p a r a t i ed il loro fabbisogno di alimentazione elettrica entro limiti ragionevoli, p u r in presenza del numero crescente dei componenti. p e r ciò che si riferisce al volume degli a p p a r a t i ed al fabbisogno di alimentazione crediamo di poter dire che le prospettive appaiono oggi favorevoli; sia per le nuove tecniche di microminiaturizzazione dei componenti; sia per le possibilità di alimentazione con energia immagazzinata o convertita da energia esterna. anche per il grado di affidamento la risposta è rassicurante: nonostante il continuo a u m e n t o del numero di componenti delle apparecchiat u r e elettroniche l'incidenza delle avarie (espressa, per esempio, col numero probabile di ore di funzionamento t r a due avarie successive, per u n dato tipo di apparecchiature e in d e t e r m i n a t e condizioni di impiego) è rimasta finora pressoché i m m u t a t a . è difficile affermare senz'altro che questa situazione potrà essere m a n t e n u t a 0 migliorata in f u t u r o , con il continuo a u m e n t o della complessità degli apparati, che può essere dato per scont a t o . t u t t a v i a le tecniche specifiche (lei componenti (e principalmente le tecniche dell'elettronica allo stato solido), la tecnica di miglioramento delle condizioni ambientali, le tecniche di controllo funzionale rapido, in condizioni eventualmente più critiche di quelle di impiego effettivo, ed infine quelle dei circuiti di tipo fail-safe hanno certamente contribuito e continueranno in larga misura a contribuire alla possibilità, per l'elettronica, di fornire il suo indispensabile contributo al progresso della tecnica aerospaziale. untitled 1 annals of geophysics, vol. 47, n. 4, august 2004 key words melt inclusion – volatile – basaltic explosive eruptions – etna 1. introduction etna, located on the eastern coast of sicily, italy, is an active basaltic volcano characterized by quasi-persistent effusive and explosive activity. explosive activity has usually been considered very subordinate with respect to the lava flow eruptions, but the last decade activity and some recent studies of holocene pyroclastic deposits (coltelli et al., 1998a, 2000) indicate that explosive eruptions are quite frequent and can be very powerful. since 1990, etna has exhibited an extraordinarily high number of violent explosive events, including more than 150 fire fountain episodes characterized by the formation of eruptive columns from 2 to 12 km high, tephra volumes from 104 to 107 m3 and a magnitude which ranges from intense strombolian to subplinian (calvari et al., 1991; coltelli et al., 1998b). holocene pyroclastic succession includes many subplinian eruptions and one plinian basaltic eruption (coltelli et al., 1998a, 2000, 2003). the tephra composition is generally basaltic, ranging from picritic basalt to basic mugearite, similar to the lava flows which erupted during the same period. it is well known that magmatic volatiles play a key role in explosive volcanism. they control vesicularity and fragmentation processes of the magma and consequently the style of the eruptions. for this reason we started to inmailing address: dr. paola del carlo, istituto nazionale di geofisica e vulcanologia, sezione di catania, piazza roma 2, 95123 catania; e-mail: delcarlo@ct.ingv.it the relationship between volatile content and the eruptive style of basaltic magma: the etna case paola del carlo and massimo pompilio istituto nazionale di geofisica e vulcanologia, sezione di catania, italy abstract fourier transform infrared (ft-ir) spectroscopic analyses of melt inclusions from four explosive eruptions of etna (italy) were conducted to determine pre-eruptive dissolved volatile concentrations. the studied eruptions include the 3930 bp subplinian, the 122 b.c. plinian, and the 4 january 1990 and the 23 december 1995 fountain fire eruptions. preliminary results indicate that h2o varies between 3.13 and 1.02 wt% and co2 between 1404 and 200 ppm. the most basic products (3930 bp tephra) contain the highest concentrations of co2 (1404 ppm), whereas fire fountain hawaiitic tephra present the lowest values (< 200 ppm) indicating a continuous degassing process during the differentiation and rising of the magma. generally, similar behavior has been found for water, characterized by a decreasing content during the differentiation that is mainly found in the 3930 bp eruption, 1990 and 1995 fire fountain products. considering the relevance of volatile content and behaviour in determining the eruptive style, we made some inferences on the eruptive mechanisms based on the initial high volatile content and the degassing dynamics inside the plumbing system. these two factors suggest the cause of the high explosive activity in this basaltic volcano. checking copy 2 paola del carlo and massimo pompilio vestigate the amount of dissolved volatiles based on melt inclusion (mi) studies in some etnean tephra. here we report preliminary results of dissolved volatile concentrations determined by the ft-ir (fourier transform infrared) spectroscopic technique and classical microanalytic methods from four explosive eruptions: the 3930 bp subplinian eruption, the 122 b.c. plinian eruption, the 4 january 1990 and the 23 december 1995 fire fountain eruptions. the 3930 bp subplinian eruption and the 122 b.c. plinian eruption represent two limiting cases in the holocene history of etna because the former produced the most basic magma ever erupted at etna (picritic) whereas the latter is one of the few documented basaltic plinian eruptions in the literature. the two fire fountain episodes were typical of others which occurred in the last 13 years, and were very energetic forming eruptive columns several kilometers high. the actual activity of etna represents a favorable case to study since we can integrate direct observations of explosive eruptions, immediate investigation and sampling of the deposits, with quantitative geophysical data (e.g., seismic tremor and ground deformation) recorded by networks monitoring the volcano. volatiles in etna magmas have been previously investigated by various authors (kamenetskiy et al., 1986; metrich and clocchiatti, 1989; metrich et al., 1993; allard et al., 1994). most of these studies focused on chlorine, sulfur and fluorine and use, with the exception of metrich et al. (1993), the «difference method» (100emp analysis total) as an indirect estimation of h2o and co2 content. however, these volatile species, being the most abundant in magma (johnson et al., 1994), have a major control over crystallization and vesiculation, density, rheology and thus eruption style (metrich and rutherford, 1998; ochs and lange; 1999; giordano and dingwell, 2003). fig. 1. bulk rock and mi compositions in total alkali silica diagram (le maitre, 1989). filled triangle: 3930 b.p. scoria bulk rock; filled diamonds: 122 b.c. scoria bulk rock; filled point: 4 january 1990 scoria bulk rock; filled squares: 23 december 1995 scoria bulk rock; triangle: 3930 bp mi; diamonds: 122 b.c. mi; point: 4 january 1990 mi; squares: 23 december 1995 mi. n o n c o r r is p o n d e a l l a f o t o s t a m p a t a d e g l i o r ig in a l i utente non corrisponde utente alla foto stampata utente originali utente degli utente fig. 1. utente prego verificare. grazie. utente prego verificare. grazie. 3 the relationship between volatile content and the eruptive style of basaltic magma: the etna case 2. volcanological and compositional features of the studied eruptions 2.1. the 122 b.c. plinian eruption tephra produced by the 122 b.c. eruption covered the entire southeastern flank of etna, destroying the ancient town of catania. the sequence comprises pyroclastic fall and flow deposits divided into 7 units, whose depositional characteristics, dispersal and physical parameters are reported in coltelli et al. (1998a). the thickest fallout units, that represent a guide-horizon for the late holocene stratigraphy, are c and e (from 2 to 0.3 m) formed by scoria lapilli and lithic clasts (about 20 wt%). juvenile clasts are dense hawaiitic scoria (fig. 1), characterized by abundant, large tabular phenocrysts of plagioclase, minor olivine and rare clinopyroxene (table i); the same minerals form the groundmass with scarce, interstitial brown glass. very rare amphibole (kaersutite) microlites are present in the whole sequence. 2.2. the 3930 bp subplinian eruption the 3930 bp. subplinian eruption produced a scoria fallout with the dispersal axis toward the east (coltelli et al., 2000). the thickest deposit found (110 cm) crops out 14 km from the summit. the deposit shows a peculiar stratification due to the alternation of fine and coarse lapilli beds also marked by the change in the superficial color (brown, gray, red and purple). the deposit presents features of a distal strombolian fall layer but the thickness and dispersal indicate a greater energy of the eruption. the deposit is composed of highly vesicular, olivine-rich scoria lapilli and very rare lava lithics. the phenocryst assemblage consists of scarce (< 8%) olivine (fo90) and minor (< 3%) cr-diopsidic clinopyroxene. olivine, clinopyroxene and skeletal plagioclase form microlites in hyalopilitic-cryptocrystalline groundmass (table i). the bulk rock is a picritic-basalt according to the tas classification (la maitre, 1989; fig. 1). the groundmass is slightly more evolved, but its composition falls on the primitive side of the basalt field. they are the most basic products ever found among etnàs alkaline suite (pompilio et al., 1995). 2.3. the 4 january 1990 and 23 december 1995 fire fountain eruptions the 4 january 1990 fire fountain eruption from south east crater was a very powerful paroxysmal episode. it produced a copious scoria fall on the northwestern flank of the volcano reaching a deposit thickness of 60 cm about 5 km from the vent along the dispersal axis (calvari et al., 1991). scoria are porphyritic hawaiites (fig. 1) with phenocrysts of plagioclase, clinopyroxene, olivine and microphenocrysts of ti-magnetite (table i). the 23 december 1995 fire fountain eruption from north east crater produced a 9.5 km long eruptive column (coltelli et al., 1998b). the good weather conditions allowed for clear observation of the eruptive column expansion. table i. mineral chemistry of the studied tephra. microl.: microlites. 3930 bp rim/microl. 3930 bp core 122 b.c. rim/microl. 122 b.c. core 4 january 1990 4 january 1990 23 dec. 1995 23 dec. 1995 rim/microl. core rim/microl. core plg an63-73 – an24-85 an60-80 an86-49 an71-81 an55-86 an64-87 ol fo84-88 fo88-91 fo70 fo73-76 fo70-71 fo77-81 fo68 – cpx x(di) 0.76-0.79 0.82-0.78 0.22 0.38-0.43 0.41-0.57 – 0.47-0.57 0.53-0.54 usp% – – 30-48 – – – – – 4 paola del carlo and massimo pompilio the column was bent approximately 40° downwind toward the nne and abundant lapilli and ash covered a wide area of the ne flank down to the coast producing damage to fruit plantations, cars and buildings. the deposit thickness along the main dispersal axis was 6-7 cm at 6 km from the vent and 1-2 cm at 20 km close to the ionian coast. lapilli and bombs were highly vesicular and glassy. their composition is hawaiitic (fig. 1) and the mineral assemblage consists of phenocrysts of plagioclase, clinopyroxene, olivine and microphenocrysts of ti-magnetite (table i) in a groundmass which varies from hyalopilitic to intersertal. 3. melt inclusion analyses 3.1. experimental procedures we used fourier transform infrared spectroscopy (ft-ir) to determine h2o and co2 concentrations in the mi’s hosted in olivine phenocrysts of tephra except for the 4 january 1990 fire fountain, which was measured in a clinopyroxene phenocryst. 122 b.c. eruption carbon dioxide and water were analyzed at dst-university of pisa in doublesided, polished mi by transmission ir spectroscopy using a nicolet magna-ir 560, equipped with an everglo source, a laser gekbr beam splitter and a dtgs detector, coupled with an ir nicplan microscope. the number of scans varied between 512 and 2048, and spectral data were collected utilizing omnic software. carbon dioxide and water in 3930 bp, 4 january 1990 and 23 december 1995 eruptions were analyzed at gps-california institute of technology laboratory using double-sided polished mi’s and a nicolet 60sx ir spectrometer equipped with an mct detector, a globar source, a kbr beam splitter, coupled to a nicplan microscope. the number of scans varied between 1024 and 4096. three spectra were collected on each mi, and the arithmetic mean of the resulting species concentration was calculated. absorbance was measured from peak heights, and after subtraction of the background signal were extrapolated with a flexible drawing curve. concentrations (c) were calculated according the beer-lambert law: c = 100 am/(ερe), where a is the molar absorbance, m the molar mass (g/mol), ε the molar absorptivity (l/mol cm), ρ the glass density, and e the thickness in cm. sample thickness was measured using a digital micrometer with a precision of ± 1µm and checked with calibrated video analyses system. thicknesses of each measured mi are listed in table ii and vary between 10 and 82 µm. glass density values of mi’s of 2800 g/l was used for hawaiitic compositions and 2950 g/l for the basaltic one. water is dissolved as molecular water (h2omol.) and hydroxyl group (oh-) in the mi. the concentrations of total water (h2omol. + + oh–) in mi were determined using the broad band at 3535 cm−1 of the total water. the values for molar absorptivity ε3535 used is 63 ± 3 l/mol cm determined for basaltic alkaline glass according to dixon et al. (1995). carbon dissolved in mi is present only as co3−2 which gives the doublet absorption band at 1350-1430 and 14501570 cm–1. the value for molar absorptivity ε1350 used is 375 ± 20 l/mol cm according to fine and stolper (1986). minerals and glass composition were analyzed using an energy dispersive analytical (link exl) spectrometer linked to the cambridge stereoscan 360 sem, and quantitative analyses were obtained by zaf correction using natural standard for the calibration. analytical conditions were 15 kev of acceleration tension, 500 pa of probe current. glass was analyzed using a raster of about 10 × 10 micron area in order to minimize na loss. replicate analyses of internal natural standards (mineral and glasses) assured an analytical precision of less than 1% for sio2 and al2o3, about 2% for feo, mgo and cao, and from 3-5% for remaining elements. due to the lower value (< 0.5%), chlorine and sulfur concentration obtained from sem analyses must be considered only semi-quantitative and was not used for the discussion. table ii lists the major element compositions and dissolved volatile concentrations of the studied mi. 5 the relationship between volatile content and the eruptive style of basaltic magma: the etna case 3.2. mi features abundant mi of variable size were found in olivine crystals from 122 b.c. tephra. those chosen for this study are about 100 µm in maximum diameter. many others were observed but were too small to be prepared and measured by ft-ir spectroscopy. they are generally glassy, although some show limited crystallization of olivine and clinopyroxene around the border. inclusions in olivine crystals of 3930 bp tephra are generally rare and very small. quite often they appear empty as original trapped fluid or vapor and leaked. in a few favorable cases, where glass was trapped, the inclusions show negative crystal shapes and contain shrinkage bubbles. in 23 december 1995 olivine crystal mi’s are more abundant and show quite often negative crystal or a sub-rounded shapes accompanied by shrinkage bubbles of variable sizes. no mineral inclusions are present in these mi. in the sample from the 4 january 1990 tephra, some large «hourglass» inclusions (anderson, 1991) with small trapped crystals, were found in diopside crystals. 3.3. mi composition a mi analyzed from the 3930 bp subplinian eruption was trapped in high forsteritic olivine (fo90) and has basaltic composition (fig. 1). this inclusion contains 2.45 wt% h2o and 1404 ppm co2. partitioning of fe and mg between olivine and mi (kd 0.33) is close to equilibrium value for crystallization at depth (mysen, 1975). the most complete set of measurements was carried out on products of the 122 b.c. table ii. melt inclusion compositions. h2o and co2 from ft-ir analyses. bdl: below detection limit. s detection limit is ±100 ppm, *=mg# in host clinopyroxene crystal. eruption 3930 bp 122 b.c. 4 jan. 1990 23 december 1995 inclusion 294 362a 362b 362ca 362cb 362cc 363da 363a 363b 363c 363d 367a se66 231295b 231295 thickness (micron) 17 37 20 10 10 10 20 13 31 33 20 31 22 65 82 sio2 45.62 49.91 51.26 50.25 50.20 50.07 49.87 49.97 50.50 52.54 49.90 51.15 46.56 47.27 51.41 tio2 1.00 1.76 1.78 1.66 1.64 1.69 1.70 1.75 1.70 1.81 1.72 1.72 2.14 1.31 1.52 al2o3 10.18 15.94 16.52 16.65 16.29 16.08 17.47 16.11 16.94 16.37 16.14 16.57 15.64 18.43 15.39 feotot 7.36 8.89 8.04 7.54 7.84 7.77 8.75 8.06 7.39 7.50 8.44 7.58 9.69 9.25 9.93 mgo 11.20 3.98 3.64 3.71 3.85 3.84 4.08 4.11 3.72 3.52 4.01 3.68 3.69 3.74 2.94 cao 13.28 7.05 6.94 7.77 7.10 7.18 6.97 6.89 6.44 6.41 6.86 6.97 8.46 8.90 6.35 na2o 1.60 4.94 5.40 5.34 5.48 5.37 5.30 6.48 5.86 3.88 5.25 5.62 4.08 4.58 5.34 k2o 0.74 2.39 2.53 2.07 2.04 2.08 2.23 2.39 2.18 2.22 2.27 2.38 2.65 3.01 2.71 p2o5 0.49 1.09 1.18 0.75 0.68 0.79 0.80 0.84 0.95 1.19 0.94 0.94 0.82 0.86 1.04 s 0.40 0.18 0.16 0.20 0.16 0.18 0.19 0.15 0.20 bdl 0.20 0.18 bdl 0.24 bdl cl 0.29 0.35 0.27 0.35 0.34 0.32 0.32 0.28 0.29 0.29 0.30 0.33 0.15 0.35 0.25 h2o 2.45 2.22 1.02 2.25 2.86 3.13 1.26 1.56 2.63 3.04 2.55 1.94 1.93 1.85 1.63 co2 ppm 1404 272 283 503 880 576 289 451 266 207 615 221 3 210 bdl fo (%) 90 73 73 75 74 75 74 75 75 74 74 74 0.73* 75 68 6 paola del carlo and massimo pompilio plinian eruption (del carlo, 2001). mi have mugearite composition (fig. 1). the measured h2o concentrations range between 1.02 and 3.13 wt% and co2 between 207 and 880 ppm. the host olivine has a homogeneous composition (fo72-75). partitioning of fe and mg (kd 0.17-0.25) between olivine and trapped glass indicates that the inclusions suffered only limited post-entrapment crystallization (< 6%). mi studied in 1990 fire fountain tephra is hosted in diopside crystal. the melt has a hawaiitic/basic mugearitic composition (fig. 1) and contains 1.93% h2o and co2 below detection. mi’s measured in 1995 fire fountain tephra was trapped in olivine crystals with fo68-75. one mi has a mugearitic composition (fig. 1) and contains 1.63% h2o. the other has a lower alkali and silica content but a higher water concentration of 1.85 wt%. an appreciable co2 content (210 ppm) was measured only in mi within olivine with fo75. partitioning of fe and mg between olivine and mi (kd 0.27-0.28) indicates, as in 122 b.c. mi, equilibrium between host minerals and trapped liquid and excludes significant post-entrapment crystallization of host olivine. on the whole, for the studied mi the occurrence of extensive post-entrapment crystallization can be ruled out, therefore the measured volatile concentrations represent the volatile content of the magma at the time of entrapment. figure 3 shows the relationships between volatiles and single element or element ratios, that change during the differentiation process. taking into account the whole set of measurefig. 2a-d. photo of a) mi in 3930 bp tephra; b) photo of mi in 122 b.c. tephra; c) photo of mi in 4 january 1990 tephra; d) photo of mi in 23 december 1995 tephra. a b c d 7 the relationship between volatile content and the eruptive style of basaltic magma: the etna case fig. 3. correlations between co2 and h2o versus k2o composition, cao/al2o3 ratio and feo/mgo ratio in mi from the studied tephra. symbols as in fig. 1. stars: data from metrich et al. (1993). 8 paola del carlo and massimo pompilio ments that span in composition from basalt to hawaiite, or the more restricted compositional range observed in the 122 b.c. mi’s, a general inverse correlation exists between co2 and k2o and feo-mgo ratio (fig. 3). k2o is considered to be incompatible in the magma, and is expected to increase during the differentiation. likewise the feo/mgo ratio is expected to increase in the melt due to crystallization of mafic phases, while pyroxene and plagioclase precipitation would diminish the cao/al2o3 ratio in the melt. in the co2 versus k2o and feo/mgo diagrams (fig. 3), the scatter is limited and all the inclusions plot along a single correlation curve. this pattern is compatible with the concomitant occurrence of differentiation and co2 loss from the magma, occurring during the ascent and cooling of a magma saturated in co2. a similar general relationship is observed using h2o as volatile species in these diagrams. on the whole, the sign of the correlation does not change, but the scatter is large and not a single correlation curve can be drawn. in particular, in 122 b.c. mi’s, we observe a fast decrease in water content coupled with a small changes in compositional parameter related to differentiation, indicating that they are poorly dependent of composition. this is compatible with a rapid decompression and water exsolution accompanied by limited cooling and crystallization processes for the magma feeding the 122 b.c. eruption. a different trend, characterized by a decrease of water during the differentiation, is depicted by 3930 bp, 1990 and 1995 mi’s. the high-k mi’s compositions of metrich et al. (1993), that correspond to recent etna products, fall in the same trend. instead, the low-k mi’s of metrich et al. (1993) define another trend with a comparable slope but shifted toward lower water content (fig. 3). noteworthy is that the inclusion with the most primitive composition does not have the higher water content. this implies that the first (probably deep) differentiation stage that produced the less evolved melts between the 122 b.c. mi, from a primitive magma, such as that feeding the 3930 bp eruption, must have developed in conditions of water undersaturation. assuming that the measured volatile content represents the maximum amount dissolved in magma, a minimum pressure of entrapment can be calculated following dixon and stolper (1995) and a h2o and co2 solubility model (fig. 4). in fig. 4, the highest pressure (≈ 260 mpa) is associated with the inclusion in the 3930 bp tephra, which is the most basic magma, while the lowest one (< 50 mpa) corresponds to the 23 december 1995 fire fountain products. inclusions in crystals of 122 b.c. eruption show a large variation in volatile content that corresponds to a trapping pressure ranging from 210 to 50 mpa. 4. discussion volatile contents (h2o and co2) found in etna tephra are on average higher than those found in other intra-plate basaltic magmas known in literature, such as kilauea (hawaii) with 0.1-1 wt% of h2o and comparable co2 content. previous work on dissolved volatiles from etnean magmas reported similar water concentrations. for example, metrich et al. (1993) found for the 1989-90 fire fountain tephra 1.1 wt% of water and in the 1892 eruption 2.3 wt% of water. thus, fig. 4. h2o (wt%) versus co2 (ppm) diagram. saturation curves at different pressures are represented according to dixon and stolper (1995) model for alkaline magmas. symbols as in fig. 1. 9 the relationship between volatile content and the eruptive style of basaltic magma: the etna case the initial high volatile content and the degassing dynamics inside the plumbing system may have a major role in controlling the eruptive style of etna suggesting the cause for the high explosive activity in this basaltic volcano. inferences on the eruptive mechanisms can be proposed on the basis of the inverse correlation between volatiles and incompatible elements (e.g., k2o) or indexes of differentiation (e.g., feo/mgo). these relationships suggest: i) the devolatilisation processes take place while the differentiation is going on; ii) magmas feeding the explosive eruptions are saturated in both (h2o and co2) investigated volatile species. this pattern even with a distinct rate is visible also in the two evolution trends (high-k and low-k) found by metrich et al. (1993) in mi’s analysed in recent etnean volcanics. the trend in the h2o-co2 diagram (fig. 4) suggests a devolatilisation in a closed system more than an open system. in addition, a large pressure range would exclude the entrapment of mi forming in shallow magma chamber (fig. 4), and further indicate that devolatilisation and degassing occurred in non-equilibrium conditions. these effects are more visible in the 122 b.c. eruption, where a large variation of volatile content is not accompanied by significant changes in mi composition, suggesting limited cooling, crystallization and differentiation processes associated with a fast uprising. normally, the rheological properties and the volatile content of basaltic magmas do not allow plinian eruptions, but the sudden decompression of a basaltic magma may cause the nucleation of a large number of bubbles that rapidly increase bulk viscosity and reduce magma density allowing rapid ascent in the conduit. disruption of low density vesicular magma results in a pyroclastic mixture leading to the formation of a plinian eruptive column. this is the case of the 122 b.c. eruption, for which a sudden decompression model of the large magma body was proposed in coltelli et al. (1998a) to form a plinian eruption. the whole 122 b.c. magma was volatile saturated with an incipient vesicularity. in this condition, even a small decompression of the system could have caused the gas to be exsolved, although the pre-eruptive water content was quite low (about 1 wt%). at low magma ascent rates, gas separates from low-viscosity melt and bubbles coalesce during the ascent producing the explosion of large bubbles forming strombolian activity. conversely, rapid ascent rates in basaltic magmas prevent bubble coalescence and can produce fire fountains and, if the volume is greater, subplinian and plinian eruptions. so, different paths of decompression of initially similar magmas may deeply influence the eruptive style. 5. conclusions the explosive activity of etna represents a continuous hazard for lands and villages located on its flanks. the most recent eruption, which started on 27 october 2002, gives an excellent example having produced a continuous ash emission for more than two months and causing damage to the eastern sicily economy mainly related to the prolonged closure of catania airport. so, understanding of the triggering mechanisms of high explosive basaltic eruptions is of fundamental importance in order to assess the probability of future, large explosive events. this study on mi in minerals provided useful information on differentiation and degassing processes that occur during the storage and ascent of magma before the eruption, in particular the relationships between volatile content and composition. this preliminary study outlines that initial volatile content at etna is higher than in other basaltic volcanoes indicating their important role in controlling the eruptive style. we made also some inferences on the eruptive mechanisms based on the initial high volatile content and the degassing dynamics inside the shallow plumbing system, indicating these two factors as the cause of the high explosive activity in this basaltic volcano. acknowledgements authors are grateful to r. cioni and s. newmann for their kind assistance in ft-ir measurements, and to h.e. belkin and an anonymous referee for the review that greatly improved the manuscript. research supported by gruppo nazionale per la vulcanologia (italy). 10 paola del carlo and massimo pompilio references allard, p., j. carbonnelle, n. metrich, h. loyer and p. zettwoog (1994): sulfur output and magma degassing budget of stromboli volcano, nature, 368 (6469), 326330. anderson, a.t. 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(1975): partitioning of iron and magnesium between crystals and partial melts in peridotite upper mantle, contrib. mineral. petrol., 52 (2), 69-76. ochs, f.a. and r.a. lange (1999): the density of hydrous magmatic liquids, science, 283, 1314-1317. pompilio, m., m. coltelli, p. del carlo and l. vezzoli (1995): how do basaltic magmas feeding explosive eruptions rise and differentiate at mt. etna?, per. mineral., 64, 253-254. utente in press). utente se possibile completare. grazie. utente se possibile completare. grazie. intercomparison of metop-a so2 measurements during the 20102011 icelandic eruptions annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6613 intercomparison of metop-a so2 measurements during the 20102011 icelandic eruptions maria elissavet koukouli1*, lieven clarisse2, elisa  carboni3, jeroen van gent4, claudia spinetti5, dimitris  balis1, spyros dimopoulos1, roy grainger3 , nicolas  theys4, lucia tampellini6 and claus zehner7 1 laboratory of atmospheric physics, aristotle university of thessaloniki,  greece, 2 université libre de bruxelles, brussels, belgium, 3 comet, atmo­ spheric, oceanic & planetary physics, oxford university, oxford, uk, 4 bel­ gian institute for space aeronomy, brussels, belgium, 5 istituto nazionale di  geofisica e vulcanologia, roma, italy, 6 compagnia generale per lo spazio,  milano, italy, 7 european space agency, rome, italy.  *mariliza@auth.gr abstract the european space agency project satellite monitoring of ash and sulphur dioxide for the mitigation of avi­ ation hazards, was introduced after the eruption of the icelandic volcano eyjafjallajökull in the spring of 2010   to facilitate the development of an optimal end­to­end system for volcanic ash plume monitoring and predic­ tion. the eyjafjallajökull plume drifted towards europe and caused major disruptions of european air traffic for   several weeks affecting the everyday life of millions of people. the limitations in volcanic plume monitoring and   prediction capabilities gave birth to this observational system which is based on comprehensive satellite­derived   ash plume and sulphur dioxide [so2] level estimates, as well as a widespread validation using supplementary   satellite,   aircraft   and   ground­based   measurements.  inter­comparison   of   the   volcanic   total   so2  column   and   plume height observed by gome­2/metop­a and iasi/metop­a are shown before, during and after the eyjaf­ jallajökull 2010 eruptions as well as for the 2011 grímsvötn eruption. co­located ground­based brewer spectro­ photometer data extracted from the world ozone and ultraviolet radiation data centre for de bilt, the nether­ lands, are also compared to the different satellite estimates. promising agreement is found for the two different   types of instrument for the so2  columns with linear regression coefficients ranging around from 0.64 when   comparing the different instruments and 0.85 when comparing the two different iasi algorithms. the agree ­ ment for the plume height is lower, possibly due to the major differences between the height retrieval part of the   gome2 and iasi algorithms. the comparisons with the brewer ground­based station in de bilt, the nether ­ lands show good qualitative agreement for the peak of the event however stronger eruptive signals are required   for a longer quantitative comparison. doi: 10.4401/ag-6613 1 francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text francescocaprara typewritten text annals of geophysics, fast track 2, 2014 i. introduction he main objective of the european spa­ ce agency [esa] satellite monitoring of   ash   and   sulphur   dioxide,   so2,   for   the   mitigation   of   aviation   hazards,  sacs­ 2/smash, project is the development, impro­ vement   and   validation   of   satelite­derived   ash  &   so2  products.   this   system   is   based   on   im­ proved   and   dedicated   satellite­derived   ash  plume   and   sulphur   dioxide  level  assessments  following the support to aviation control ser­ vice   (sacs,   http://sacs.aeronomie.be),   which  is a free online service initiated by esa for the  near­real­time   satellite   monitoring   of   volcanic  plumes of so2 and ash [brenot et al., 2014]. the  sacs­2/smash  system   has   been   compre­ hensively   validated   using   auxiliary   satellite,  aircraft   and   ground­based   measurements  [koukouli   et   al.,   2014;   spinetti   et   al.,   2014.].  this   paper   provides   inter­comparisons  between three different so2 column and plume  height assessments from two different satellite  instruments  for   the   2011   grímsvötn   eruption.  the   satellite   so2  products   are   further   being  compared to ground­based so2  column obser­ vations for the eyjafjallajökull 2010 eruptions. ii. so2 observations and methodologies i. iasi sulphur dioxide products the   hyper­spectral   infrared   atmospheric  sounding   interferometer   (iasi)   was  launched  in   october   2006   onboard   the   meteorological  operational   satellite­a   (metop­a)   [clerbaux  et   al.,   2009]   for   meteorological   and   scientific  applications.   global   nadir   measurements   are  acquired twice a day at 09:30 and 21:30 mean  local equatorial time with a 12 km radius foot­ print   in   the   nadir.   in   the   following   analysis  two   iasi   so2  products   are   used:   one   devel­ oped by the atmospheric spectroscopy group  of the université libre de bruxelles, hereafter  ulb, and one developed by the earth observa­ tion   data   group   at  the   university   of   oxford,  hereafter  oxford. both products use similar al­ gorithms for the detection [clarisse et al., 2014]  and   retrieval  of   total   columns   [carboni   et  al.,  2012] however  the oxford algorithm retrieves  the so2  plume height simultaneously with the  so2  column amount, which has the advantage  of   yielding   realistic   error   estimates.  the   ulb  algorithm retrieves the height prior to the col­ umn and has the added benefit of not relying  on any a­priori information.  ii. gome2 sulphur dioxide products the  second   global  ozone  monitoring  experi­ ment (gome­2) is a uv/visible nadir­viewing  spectrometer   covering   the   240­790   nm   wave­ length interval with a spectral resolution of 0.2  – 0.5 nm [munro et al., 2006] also on board the  metop­a   satellite.   its   ground   pixel   size   is  nearly constant along the orbit, around 80 x 40  km². the gome­2 so2  products shown in the  analysis   have   been   developed   at   the   belgian  institute of space aeronomy, (hereafter  bira­ iasb) and were derived with an extended ver­ sion of the godfit trace gas column retrieval  algorithm   [lerot   et   al.,   2010].   the   algorithm  derives the so2 total column amount and effec­ tive plume height simultaneously by applying  an iterative direct fitting method with an opti­ mal   estimation   inversion   scheme.  a   priori  in­ formation   is   only   used   for   the   so2  plume  height.  iii. brewer sulphur dioxide columns measuring total so2  column  using brewer in­ struments   is   discussed   in   kerr   et   al.,   [2007],  and   references   therein.   brewer   spectropho­ tometers measure the uv absorption spectrum  t 2 annals of geophysics, fast track 2, 2014 at five wavelengths. once the ozone column is  calculated, the total so2  column can be deter­ mined  using   semi­empirical   equations   based  on the measured light intensity at each  of the  wavelengths,   equivalent   extraterrestrial   light  intensities,   the   air   mass   factor,   the   pre­calcu­ lated column amount of ozone and the absorp­ tion coefficients of the two interfering species.  since   the   signal­to­noise   level   for   the  so2  ab­ sorption is usually  quite low, very well calib­ rated   instruments   are   required   to   monitor  nominal  so2  levels.  to  the  best  of  our  know­ ledge, no comprehensive error assessments of  the   brewer  total   so2  columns   have   been   re­ ported as yet. after imposing a rigorous qual­ ity   control   on   the   stations   reporting   brewer  so2  columns   available   through   the   world  ozone   and   ultraviolet   radiation   data   centre  (woudc), [koukouli et al., 2014], a total of 16  european   stations   were   accepted   as   possibly  being   able   to   show   signs   of   increased  so2  levels   due   to   the   volcanic   eruptions.  in extreme cases, such as volcanic eruptions [fioletov et al., 1998], so2 levels typically rise well above instrumental noise permitting their comprehensive study using the brewer instrument. flentje   et   al.,  [2010],   showed   enhanced   atmospheric   levels  during the eyjafjallajökull eruptive period us­ ing   the   brewer   observations   at   hohenpeis­ senberg, germany, and rix et al., [2012], report  good   comparisons   between   gome­2/metop­ a measurements and ground­based findings at  valentia,   ireland   for   the   may   2010   events.  here, we show comparisons over de bilt, the  netherlands,   one   of   the   few   european   loca­ tions   that   where   substantial   plume   drifted  over. iii. results in figure 1 the so2 column amount of the three  algorithms is compared for may 23rd  2011;  the  so2 column amounts are gridded on a regular  grid of 0.25 x 0.25 deg latitude/longitude to ac­ count for the differences in footprint. note that  this   is   the   first   time   the   different   retrieval  schemes   and   findings   are   compared   as   such.  the   ulb/iasi   results   are   shown   on   the   left,  the oxford/iasi in the middle and the bira­ iasb/gome­2   in   the   right   column.   no   cut­ offs/flags   were   used   as   part   of   a   quality  screening   of   the   data   since   such   a   screening  eliminates pixels with high so2 amounts in the  central denser part of the plume. for aviation  applications, such a strict quality control is not  needed, as long as so2 is reliably detected. the  results   shown   are   very   promising   with   all  three algorithms capturing quite well both the  magnitude of the loading as well as the central  location of the plume to the nw of iceland and  its dispersion over the north atlantic sea. for  the 190 common grids between the oxford and  ulb/iasi   results,   the   mean   so2  loading   esti­ mated by the oxford algorithm is 14.32±25.14  d.u. and by the ulb algorithm at 11.76±16.60  d.u.   with   a   correlation   coefficient   of   0.85   for  the two algorithms. for the 232 common grids  between   the   bira­iasb/gome­2   and   the  ulb/iasi   results,   the   mean   so2  loading   esti­ mated   by   gome­2   is  9.06±14.33   d.u.   and   by  iasi   10.96±15.15   d.u.   with   a   correlation   of  0.64.   for   the   173   common   grids   between   the  bira­iasb/gome­2 and the oxford/iasi re­ sults, the mean gome­2 loading is 10.97±15.68  d.u. and the mean iasi loading is 16.52±28.60  d.u. with a correlation of 0.64. in figure 2 the  so2 plume height estimated for the 23rd of may  2011 from the iasi and gome­2 observations  is shown. all three retrievals show a well­de­ fined plume, setting off in a westerly direction  over   iceland   before   rising   north­   and   east­ 3 annals of geophysics, fast track 2, 2014 wards, resembling an arm over the arctic sea.  since   for   small   amounts   of   so2,   there   is   not  enough information inherent in the measured  spectra in order to retrieve an independent al­ titude   value,   the   resultant   height   values   will  tend   towards   the   a   priori   values.   for  oxford/iasi   the   retrieval   is   performed   in   a  pressure   grid   and   the   plume   height   a   priori  values is set to 400mbars (~6km) and the a pri­ ori   error   to   1000   mbars.   in   the   bira­ iasb/gome­2 algorithm the a priori height er­ ror   was   set   to   5km   and   so   any  a   posteriori   height error value that approaches the  a priori  error   levels   signifies   non­dependable   detec­ tion. no screening was applied to either iasi  data whereas a minimum  amount of four de­ grees of freedom was permitted for the bira­ iasb/gome2results in order to signify mean­ ingful   plume   height   detection.     as   can   be  noted in the figure, significant differences ex­ ist   among   the   iasi   and   the   gome­2   assess­ ments of the plume height. in numbers, for the  173 common grids,  the mean gome­2 plume  height is found at 7.15±0.84 km and the mean  oxford/iasi plume height at 8.82±2.0 km; for  the   232   common   points,   the   mean   gome­2  plume height is at 7.12±0.95 km and the mean  ulb/iasi height at 9.40±1.44 km. for the com­ parison between the two iasi algorithms, the  190 points provide a mean ulb/iasi height of  9.00±2.00 km and the oxford/iasi of 8.17±2.64  km with a good correlation of r­squared 0.62.  the agreement between the two algorithms is  valid for the entire european domain and not  solely on the location of the maximum plume  load and height. in figure 3, the comparisons  between   satellite   and   ground­based   so2  amounts  in   the   days  before,   during   and   after  the  eyjafjallajökull  2010   events   [grey   shaded  areas] are shown for the de bilt brewer spec­ trophotometer   in   the   netherlands.   the   ox­ ford/iasi product is shown in the left plot and  the   ulb/iasi   product   in   the   right.   the  ground­based   daily   mean   brewer   data   are  shown   in   red,   the   instantaneous   daytime  [nighttime]   satellite   observations  in   light   blue  [light   green]   and   the   satellite   daily   mean   in  black. since only four co­locations were found  for   the   bira­iasb/gome­2   within   a   200km  radius   and   no   time   imposition,   only   the   two  iasi comparisons are presented. the compari­ son   is   quite   good   for   the   two   iasi   products  with   the   levels   rising   equivalently   between  satellite and ground data for the highly loaded  days after the eruption, reaching daily maxima  around   2­3   d.u.,   well   above   instrumental  noise.  iv. discussion in this paper, a few representative examples of  the   validation   activities  performed   within   the  sacs­2/smash,   european   space   agency  project  have  been  presented.  so2  column   loa­ ding and plume height extracted   using three  different algorithms analysing metop­a obser­ vations by the iasi and gome­2 instruments  during the 15 to 26 april 2010 and 4 to 20 may  2010,   eyjafjallajökull,   iceland,   eruptions   and  the 23rd of may 2011, grímsvötn, iceland, erup­ tion are shown.  4 annals of geophysics, fast track 2, 2014 figure 1: estimated total atmospheric so2 load over europe on may 23rd 2011 as seen by the ulb/iasi algorithm   [left], the oxford/iasi algorithm [middle] and the bira­iasb/gome­2 algorithm [right]. figure 2: estimated total so2 plume height over europe on may 23rd 2011 as deduced by the ulb/iasi algorithm   [left], the oxford/iasi algorithm [middle] and the bira­iasb/gome­2 algorithm [right]. taking   into   consideration   substantial   differ­ ences   in   instrumentation,   algorithm,  wavelength   range,   spatiotemporal   resolution,  and so on, the three algorithms are providing  consistent   results   depicting   well   the   central  plume   location   as   well   as   its   dispersive   pat­ terns.   a   highly   satisfactory   agreement,   with  correlations ranging between 0.64 and 0.85, is  found for the comparisons of the so2  loading  during   the   chosen   grímsvötn   day;   the   plume  height concordance provides a far greater chal­ lenge   with   the   two   iasi   algorithms   agreeing  adequately   both   in   location   and   in   height,  however   the   gome­2   results   show   a   more  uniform plume  height across the domain  and  also   seem   not   to   be   able   to   depict   the   plume  height   near   the   source.   the   iasi/metopa  comparisons with a ground­based station in de  bilt,   the   netherlands,   are   also   promising   but  would benefit greatly from a far larger statist­ ical   sample   during   a   stronger   eruptive   occa­ sion.  we hence conclude that all three metop­ a so2  products are mature enough to be used  in a satellite volcanic monitoring system with  possible   improvements.   as   a   side   note,   it   is  equally  important  to  assess  whether  so2  may  act   as   proxy   for   volcanic   ash   during   volcanic  eruptions for an early warning system to pre­ vent   planes   from   potential   encounters   with  volcanic   plumes,   sometimes   not   the   case   sev­ eral   hours   after   an   eruption   [thomas   and  prata, 2011.]  references [brenot   et   al.,   2014]   brenot,   h.,   et   al.,   (2014),  support   to   aviation   control   service   (sacs):  an   online   service   for   near­real­time   satellite  5 annals of geophysics, fast track 2, 2014 monitoring   of   volcanic   plumes,   nat.   hazards  earth syst. sci., 14, 1099­1123.  [carboni, et al., 2012] carboni, e., et al., (2012),  a   new   scheme   for   sulphur   dioxide   retrieval  from   iasi   measurements:   application   to   the  eyjafjallajökull eruption of april and may 2010  atmos. chem. phys., 12, 11417­11434. figure 3: total so2 load over de bilt, the netherlands.   details in the text.  [clarisse, et al., 2014] clarisse, l., et al. (2014),  the 2011 nabro eruption, a so2  plume height  analysis   using   iasi   measurements.   atmos.  chem. phys., 14, 3095­3111. [clerbaux,   et   al.,   2009]   clerbaux,   c.,   et   al.  (2009), monitoring of atmospheric composition  using   the   thermal   infrared   iasi/metop  sounder, atmos. chem. phys., 9, 6041­6054. [fioletov,   et   al.,   1998]   fioletov,   v.   e.,   et   al.  (1998), influence of volcanic sulfur dioxide on  spectral uv irradiance as measured by brewer  spectrophotometers,   geophys.   res.   let.,   25,  (10), 1665­1668.  [flentje,   et   al.,   2010]   flentje,   h.,   et   al.   (2010),  the   eyjafjallajökull   eruption   in   april   2010   –  detection of volcanic plume using in­situ mea­ surements,  ozone  sondes and  lidar­ceilometer  profiles, atmos. chem. phys., 10, 10085­10092. [kerr and davis, 2007] kerr, j. b., and davis j.  m. (2007), new methodology applied to deriv­ ing   total   ozone   and   other   atmospheric   vari­ ables   from   global   irradiance   spectra,   j.   geo­ phys. res., 112, d21301. [koukouli. et al., 2014] koukouli, m. e., et al.  (2014),  sacs­2/smash  validation  report  on  the  eyjafjallajökull  &  grímsvötn  eruptions,  http://sacs.aeronomie.be/documentation/in­ dex.php, last accessed: january 23th, 2015. [lerot, et al., 2010] lerot, c., et al. (2010), the  godfit algorithm: a direct fitting approach to  improve the accuracy of total ozone measure­ ments from gome,  int. j. remote sensing, 31,  543­550. [munro,   et   al.,   2006]   munro,   r.,   et   al.  (2006):  gome­2 on metop, in: proc. of the 2006 eu­ metsat   meteorological   satellite   conference,  helsinki, finland, eumetsat, p. 48, isbn 92­ 9110­076­5. [rix, et al., 2012] rix, m., et al. (2012), volcanic  so2, bro and plume  height estimations using  gome­2   satellite   measurements   during   the  eruption of eyjafjallajökull in may 2010, j. geo­ phys. res., 117. [spinetti, et al., 2014] spinetti, c., et al. (2014),  volcanic   so2  by   uv­tir   satellite   retrievals:  validation   by   using   ground­based   network   at  mt. etna, annals of geophysics, this issue. [thomas   and   prata,   2011]  thomas,   h.e.   and  prata,   a.   j.   (2011),   sulphur   dioxide   as   a  volcanic ash proxy during the april–may 2010  eruption   of   eyjafjallajökull   volcano,   atmos.  chem. phys., 11, 6871–6880. 6 annals of geophysics, 55 (3), 2012: manni paleontological museums and geoethics riccardo manni università di roma “la sapienza”, dipartimento di scienze della terra, rome, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5562 abstract paleontological museums should adopt a code of ethics in order to carry out restorations and to set-up exhibits without any falsification. indeed, alterations can often be voluntary because an exhibit needs to be ‘beautiful’, ‘realistic’ or ‘charming’ for the public. therefore, the reconstructed parts are painted and then ‘soiled’ artfully to look more realistic. an incomplete skeleton might be completed by reconstructing the missing bones, or by adding casts of other bones. sometimes skeletons are ‘created’, by assembling together bones from several specimens of the same species. therefore, the museum staff should also inform visitors if a specimen has undergone such tampering, because otherwise each visitor is convinced that they have seen a ‘true’ fossil. so all museum staff should be trained not only in the techniques of museums, but also in the ethics of restoration and installation. 1. introduction science museums are the places that are responsible for the preservation and dissemination of culture. therefore, we expect that what they propose is ‘scientifically true’. unfortunately, this is not always respected. it happens indeed that material on display is actually an artifact, while any notification of such for visitors is missing. an alteration is sometimes ‘desired’ because the exhibit needs to be beautiful and charming for the public. this philosophy is probably inherited from former ‘museums’, in which only weird findings were considered for display. these museums were much more ‘rooms of wonders’, or ‘wunderkammer’ in german, where from the 16th to the 18th century, wealthy collectors displayed their curiosities to surprise their guests. a striking example is the archeological site (archeological sites are to be considered in all respects as openair museums) of knossos (isle of crete, greece). during the systematic excavation of the site from 1900 to 1931, the british archeologist sir arthur j. evans brought to light many of the archeological remains visible today. the monuments were restored in questionable ways. some rooms of the palace were colorfully rebuilt. even the famous paintings of the minotaur and the dolphins were almost completely re-depicted. this method of restoration has been much criticized, and visitors should be told to carefully read a well-written guide to be sure of the truth. unfortunately, it is not unusual to find ‘disciples’ of evans in some modern paleontological museums. 2. the paleontological museums fossils are stored in paleontological museums. by their intrinsic nature, fossils are usually not intact, as they are the remains of organisms that lived thousands or hundreds of millions of years ago. fossils are often broken, fractured, or distorted. at present, we see how much restoring is important and necessary as a practice to preserve fossil material. unfortunately, during the restoration process, fossils can be altered in more or less severe ways, and this falsification is often voluntary. 3. restoration restoration is the practice whereby a damaged object is ‘restored’ to its original aspect. moreover, the recovery operation is not always easy. sometimes, and quite rarely indeed, fossils are found intact, whereas more often fossil material is found damaged, and it requires long and painstaking work of restoration. basically, we can recognize two types of restoration, one addressed to mere conservation, and one targeted at the display of the material. 3.1. conservative restoration this type of restoration is performed when a finding is not going to be displayed. in this case, the intervention is aimed only at maintaining the artifact in its optimal condition. so the restorer acts in order to stick together fragments, and eventually to assemble the battered parts. this is therefore like minimally invasive surgery, because there is no need to reconstruct the missing parts. the adhesives used to glue and to impregnate must be reversible. article history received december 21, 2011; accepted january 19, 2012. subject classification: paleontological museum, restoration, mounting skeletons, ethics. 469 3.2. exhibition restoration in this case, with appropriate mastics, the restorer reconstructs the missing parts of the fossil to reproduce the exhibit as it would be if it was intact. it is good practice not to hold the true parts, and to it is recommended to use adhesives that are reversible. the reconstructed parts can be colored so as to make them easily recognizable. this is an invasive intervention, however, which can alter the original appearance of the exhibit. it is important to have deep knowledge of the fossil taxon when it is treated, so as not to falsify it. 3.3. remarks unfortunately, it is clear that restoration of material can lead to misrepresentation. in some cases, ignoring the concept itself of fossils and science, an exhibit might be restored to look as if it is intact, so as if is ‘new’. indeed, the reconstructed parts can painted and then ‘soiled’ artfully, to look more realistic, such that at first sight they are indistinguishable from the real parts (figure 1). only a careful examination will allow the restoration to be detected. unfortunately, until recently, many restored fossils were altered, and new restorations are now necessary to ‘correct’ such false fossils. for mounted skeletons, the falsification of fossils might also occur in the skeleton assembling and mounting. the mounting of a skeleton can be a hard task. the bones are often fragmented or deformed, or even missing. it is therefore mandatory that the assembling must be well designed. for example, if the bones are deformed, the skeleton might slope to one side. so, when the restorer starts facing skeleton mounting, he must consider in which way the bones might be deformed, to find the right balance. in some cases, inexperienced technicians have filed down the bones to have a perfect skeleton. many museums have skeletons that were installed following the example of the infamous dr. victor von frankenstein. in this way, an incomplete skeleton can be completed by reconstructing the missing bones (and in some cases by inventing new bones), or by adding casts of other bones (sometimes from other species), or with bones belonging to specimens of the same species. for these reasons, although some fossils are ‘falsely real’, they are actually counterfeit or chimeras. the intent of those who counterfeit is commendable on the one hand, because they aim to give the viewer an idea of the ‘real’ animal. at the same time, it is evident that the visitor is pushed into believe that what he is seeing is a true and actual fossil. another problem is with the ‘compiled’ skeletons. these skeletons are ‘created’ by assembling together bones from several specimens. from one point of view, these are true ‘monsters’, worthy of the best of dr. frankenstein [mateus et al. 2008]. this mode of operation is generally adopted when a lot of bones that are supposed to belong to a single species are found together. in this case, the bones are all measured. then, using biometric methods, i.e. using bones that appear dimensionally compatible with each other, several entire skeletons can be assembled. scholars consider this as a reliable method from a scientific point of view. however, it is clear that any skeleton compiled in this way is effectively a ‘puzzle’ made of several individuals (figure 2). the visitors are thus unaware that they are only observing a working hypothesis. another problem is the posture of the skeleton. this can vary greatly depending on whether we imagine the animal as standing, as stationary, or is moving. today, curators really like to show the dynamism of certain animals, so we can see skeletons that stand on one leg only, or that are about to prey on an another skeleton. the intention is noble, but even in these cases, reconstructions of behaviors might be arbitrary, or at least scientifically questionable. 4. falsification and scientific theories sometime museums suffer from obsolete scientific theories. scientific theories can change according to new studies and/or new finds. for instance, in several natural history museums of paleontology, skeletons of bipedal dinosaurs (e.g., theropoda) are mounted in a way similar to the tripodal posture of kangaroos (i.e. two legs plus the tail in contact with the ground). this means that those skeletons were mounted with the pelvis and tail in a subvertical position. today it is thought that bipedal dinosaurs hold the pelvis and tail sub-parallel to the ground. the craziest thing is that to maintain the ‘tripod’ theory, bones have been filed down. of note, in the famous original mounting of an iguanodon at the royal institute of natural sciences in brussels, several caudal vertebrae were modified to obtain a tail that rested on the ground. original shapes have thus been filed down to agree with the old and compelling theory of tripod-standing dinosaurs. riccardo manni 470 figure 1. detail of a deer horn on which sand was glued to increase the realism (paleontological museum of rome university). it is evident that in these cases it is not correct to speak of deception. indeed, these skeletons were mounted when that theory was in vogue and was considered to be correct. nonetheless, filing down bones is certainly a hoax. nowadays, the tripod posture can be considered somehow scenic, and the skeleton is significantly higher than it would be if correctly mounted. so it might be preferred to maintain the skeleton just as it is, in the wrong tripod posture. at the same time, it should also be said that any adjustment to the mounting of such an exhibit has significant cost, and this might be why many museums prefer to avoid such extra costs. again, however, it would be appropriate to specify in the captions that the tripod position is no longer accepted by the scientific community. 5. desire for success in a few disgraceful cases, the insane search for fame, glory and cash has lead some private institutions to display material of uncertain origins, if not totally fake. in july 1997, a farmer from xiasanjiazi (in the liaoning province of china) who was also a fossil hunter and seller, came out with a fossilized skeleton of a bird, preserved on a slab of shale. this skeleton was datable to the early cretaceous, and was identified as a primitive bird. an american museum bought the piece and made it available for study by experts. with its particular characteristics (both reptile and bird), the fossil turned out to be, at least the experts said, the missing link between dinosaurs and birds. it was named as archaeoraptor liaoningensis. the national geographic magazine sensed the importance of this scientific discovery and funded research and published scientific results from the study of this extraordinary fossil. the criticism of the scientific community, which highlighting some inconsistencies, destroyed the article in a few weeks. indeed, all of the unavowed doubts at the time of the discovery eventually became certainties, and archaeoraptor was declared a resounding fake. in december 1998, the counter-mould of the tail of a dromesaurid dinosaur (the cast of which was glued to the bird skeleton by the chinese farmer) was recovered from a fossil-bearing locality in china. in this way, rowe et al. [2001] were able to officially confirm that archaeoraptor was a name given to a false fossil. later, computer tomography analysis revealed further forgery of the ‘original’ archaeoraptor specimen [simons 2000]. 5.1. remarks this case is the one that a scientist would never like to comment on, because it overshadows the scientific community due to the disarming superficiality of the researchers who were charged with the study of the fake chinese fossil. indeed, more than pointing out the superficiality of some of the analyses, and that they did not take sufficient account of the criticism, the most worrying aspect is that the scholars were biased by their conviction of having a missing link in their hands, instead of aseptically investigating the specimen. such cultural bias is one of the worst faults that a scientist can have. cultural bias can lead to deliberately falsifying plain facts, because it starts from an incorrect assumption. it is important here to remember that also racism stems from a cultural bias [gould 1983]. 471 paleontological museums and geoethics figure 2. composite skeleton of a dwarf elephant (elephas falconeri) (paleontological museum of rome university). 6. conclusions what we have seen so far comes mainly from the questionable philosophy that a displayed exhibit must be beautiful, even it means sacrificing some science, forgetting in this way that a fossil, for its inherent nature, is imperfect. among other things, these ‘monsters’ are often accepted passively by the scientific community, because they are considered scientifically reliable. from an ethical point of view, this way of working is questionable, because the viewer is often not informed. if the paleontological museums, and also for the knossos ruins, inform visitors about the process of restoration that any ancient remains have gone through, there would be no objections. the visitor should be informed that the finding has been restored, and the visitor is happy anyway. the best thing would be a conservative restoration, possibly revealed by a distinguishing color of the exposed material. at most, we can propose the separate reconstruction of how the fossil would have been if found intact. the important thing is to always inform the visitor of the type of restoration that has been done. finally, it is necessary to train the staff of museums, and not just the technicians, but also the scientists. this should not only include the techniques of fossil preparation, mounting and display, but also the ethics of restoration and installation. acknowledgements. we would like to thank prof. ruggero matteucci for critical advice, and dr. simone d’orazi porchetti for revising the english. references gould, s.j. (1983). il pollice del panda. riflessioni sulla storia naturale, roma, editori riuniti, 371 pp. mateus, o., m. overbeeke and f. rita (2008). dinosaur frauds, hoaxes and ‘frankensteins’: how to distinguish fake and genuine vertebrate fossils, j. paleontol. techn., 2, 1-5. rowe, t., r.a. ketcham, c. denison, m. colbert, x. xu and p.j. currie (2001). forensic paleontology: the archaeoraptor forgery, nature, 410, 539-540. simons, l.m. (2000). il caso archaeoraptor, national geographic italia, 6, 132-137. corresponding author: riccardo manni, università di roma “la sapienza”, dipartimento di scienze della terra, rome, italy; email: riccardo.manni@uniroma1.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. riccardo manni 472 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 1 different damage observed in the villages of pescara del tronto and vezzano after the m6.0 august 24, 2016 central italy earthquake and site effects analysis angelo masi(1), giuseppe santarsiero(1), leonardo. chiauzzi(1), maria r. gallipoli(2), sabatino piscitelli(2), luigi vignola(3), jessica bellanova(2), giuseppe calamita(2), angela perrone(2) , carmine lizza(3), stefano grimaz(4) (1) school of engineering, university of basilicata, potenza (italy), (2) national research council imaa, tito scalo (italy),(3) national association for public assistance (anpas), marsicovetere (italy), (4) dpia-university of udine, udine (italy). angelo.masi@unibas.it abstract the authors have surveyed many damaged villages located at the epicentre of the ml=6.0 earthquake which occurred on august 24, 2016 in central italy. some unexpected anomalies were discovered such as very different levels of damage in vezzano and pescara del tronto villages (arquata del tronto municipality, ascoli piceno province). the two villages are situated just 1300 meters from each other. pescara del tronto suffered very heavy damage with many masonry building collapses and 48 fatalities, while vezzano suffered only light damage to few buildings. this paper provides a preliminar analysis from an engineering and geophysics perspective. particularly, rapid visual surveys were carried out in the two villages in order to detect possible significant differences in the vulnerability of their building stocks and site geophysical investigations were performed to detect possible local amplification effects. i. introduction n the aftermath of the ml=6.0 earthquake which occurred on august 24, 2016 in central italy, the authors surveyed many damaged villages. among them, some unexpected anomalies were found, particularly regarding the severity of damage in vezzano i annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 2 as opposed to pescara del tronto (arquata del tronto municipality, figure 1). figure 1: territorial framework of studied area (administrative territory of arquata del tronto) with indication of the small villages of pescara del tronto and vezzano. despite the villages being situated just 1300 meters from each other, pescara del tronto suffered very heavy damage with many masonry building collapses and 48 fatalities (on 122 inhabitants, istat 2011), whereas vezzano (12 inhabitants; istat 2011) suffered light damage to only a few buildings. considering the recent seismic history of the villages, it is worth noting that for the mw=5.0 earthquake which occurred on 19th december, 1941 (dbmi2015, http:// emidius.mi.ingv.it/cpti15-dbmi15/), different intensities were reported in the macroseismic catalogue, particularly higher in pescara del tronto (vii mcs) as opposed to vezzano (vi mcs). rapid visual surveys were carried out in order to detect possible significant differences in the vulnerability of the building stocks, together with site geophysical investigations aimed at detecting possible local amplification effects. ii. damage in the villages of pescara del tronto and vezzano the extent of damage in pescara del tronto is clearly visible in figure 2-a) showing the post-earthquake condition of a portion of the village. figure 2-b) displays a partial collapse of a masonry building pointing out the poor quality of masonry walls without any connection between wythes. in some cases, the combined effect of in plane actions and out plane actions further exacerbated by pushing wooden roofs was responsible for the collapse. heavy damage also occurred to buildings with reinforced concrete (rc) structure. an example is reported in figure 3-a) and b), where the heavy damage occurred to a four storey rc building in pescara del tronto is shown. the infill walls of the second storey totally collapsed. they were made up of two 12 cm clay hollow brick layers with an interposed layer of insulating material. figure 2: a) overview of pescara del tronto most damaged area and b) collapse of a typical masonry building. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 3 figure 3: a) reinforced concrete building in pescara del tronto, b) detail of the damage to a beam-column joint. some structural damage was also found in a beam-column joint displayed in figure 3-b). concrete spalling affected the joint panel where no hoops are visible as expected considering design rules given in the building codes then in force (masi et al., 2013; manfredi and masi, 2014; masi et al., 2014). in fact, the presence of deformed reinforcing bars indicates design and construction could date back to the early 1980’s (masi et al., 2015), when arquata del tronto was classified as a seismic zone, forcing engineers to consider seismic activity without particular attention to structural details. a possible interaction with a retaining wall could also be partially responsible for the undesired seismic behaviour of the building. both severity and extent of damage in vezzano were found to be quite different, as well as no fatalities or injuries were reported. the lack of heavy damage was confirmed during the rapid survey carried out by the authors and most of buildings appear to be undamaged (figure 4-a), apart from a few abandoned and dilapidated buildings (figure 4-b). iii. comparison between typological characteristics of building stocks in pescara del tronto and vezzano. building stocks in pescara del tronto and vezzano show typical characteristics of central apennine villages affected by earthquakes (mucciarelli et al., 2003; masi et al., 2014b). the 2011 istat survey (see tab.1) reports that the building stock in both figure 4: a) undamaged buildings in the main square of vezzano village and b) partial collapse of an abandoned building. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 4 villages was made up mostly of low-rise masonry structures in a good state of preservation, however, designed without any reference to seismic criteria (arquata del tronto municipality was classified as seismic in 1983). most buildings in pescara del tronto are 2-3 storey, rarely 4 storeys and have an unreinforced masonry structure. about 60% of surveyed buildings were classified as "mixed" and are mainly masonry built structures on the first floor and the remainder in reinforced concrete. only 4 buildings in pescara have reinforced concrete (rc) structures while in vezzano there are not buildings in rc structure. rapid/initial visual surveys, carried out by authors in pescara del tronto, show that the quality of masonry buildings appear to be very poor with highly inhomogeneous stone elements. irregular limestone block walls were frequently found, sometimes with brick elements or travertine stones (very soft and porous rock used mostly for aesthetic purposes, figure 5-a), mortar quality was also checked and found to be very poor. mortar generally appears to be made up of sand and hydraulic lime providing a very low bond between stones, mainly due to age deterioration, thus contributing to poor seismic performances. several buildings in pescara del tronto had been subject to further construction adding one or two storeys. figures 5-b and 5-c highlight a typical case of a building with a limestone basement and the upper two storeys made up of hollow clay bricks that appear inappropriate for structural purposes. another prominent aspect related to the main structural characteristics of masonry buildings in pescara del tronto concerns the wall corners which are generally of poor construction quality, table 1: data extracted from the 2011 italian population and housing census (istat, 2011) pescara del tronto vezzano inhabitants 122 12 typology of dwelling buildings masonry 37 34 r.c. 4 0 mixed 69 2 age of buildings <1919 88 32 1919-1960 14 3 1961-1980 6 1 >1980 2 0 number of floors ≤2 47 12 3 59 23 ≥4 4 1 state of preservation excellent 14 6 good 65 24 mediocre 21 6 very bad 10 0 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 5 figure 5: a) travertine blocks of a collapsed building, b) building with two added floors made of hollow bricks, c) detail of the hollow resulting in a poor collaboration between orthogonal walls. besides, it appears that strengthening tie rods were absent, although their use cannot be excluded as they are rather difficult to detect in the case of a totally collapsed buildings. floors were usually made of wooden or steel beams and in some cases, shallow arch vaults. in addition to the poor quality of masonry walls and the lack of connection between them, heavy reinforced concrete roofs were sometimes found. such a combination caused the disintegration of the load bearing masonry walls and, in turn, the total collapse of the building. masonry buildings of vezzano have generally better quality with respect to pescara del tronto and wall corners with a more effective connection were frequently found (figure 6-a). few cases of additional storeys and heavy rc roofs were found. strengthening tie rods were substantially absent. masonry walls appear to be made up of limestone elements without travertine elements thus making them stronger, although global resistance is also dependent on the mortar quality which could not be checked. in a few cases, enlargement interventions can be noted as in figure 6-b, although they rarely affect the global behaviour of the building. better quality stone elements and corner connections between orthogonal walls, fewer additional upper storeys and heavy rc floors would suggest less vulnerability of the masonry buildings in vezzano with respect to pescara del tronto. figure 6: a) buildings with good quality corners, b) building with badly constructed enlargement. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 6 the seismic vulnerability of masonry buildings can be judged as generally high, considering vertical and horizontal structural types, and in general a seismic vulnerability class “a” and “a-b can be assigned for pescara and vezzano, respectively, according to the ems 98 classification (grunthal, 1998). iv. geophysical survey of pescara del tronto and vezzano a geophysical survey was carried out with the aim of distinguishing sites where increased local damage was due to lower quality buildings and/or to local geo-structural settings responsible for ground motion amplifications (mucciarelli et al., 2009). electrical resistivity tomography (ert), single and multi-station noise measurements (hvnsr, seismic array) and single station earthquake recordings (hvsr) were carried out in the investigated sites (figure 7 for pescara del tronto and figure 9 for vezzano). ert. an ert survey was performed at pescara del tronto using a syscal pro resistivity meter (iris instruments), coupled with a multielectrode acquisition system (48 electrodes), using a constant spacing of ‘a’ = 5m between the adjacent electrodes. in order to improve data quality and resolution, apparent resistivity data was acquired along with each profile through different array configurations (wenner and wenner–schlumberger), and coupling measurements with different combinations of dipole length (i.e., 1a, 2a, 3a) and ‘n’ numbers of depth levels (overlapping data levels). figure 7: sketch map of the investigated area at pescara del tronto with location of the geophysical surveys. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 7 the 235 m-long ert reached an investigation depth of about 40 m. to obtain twodimensional resistivity images of the subsurface, the apparent resistivity data was inverted using res2dinv software (loke, 2001; galli et al., 2006; giocoli et al., 2011). the best results were obtained by means of the wenner array, which showed a higher signal-to-noise ratio. the root mean-squared (rms) error was 5.1% after 5 iterations (figure 8). overall, the electrical image shows a relatively low resistivity range, from 10 to over 320 ωm, and a resistivity pattern characterised by relatively strong lateral and vertical gradients. the shallow relatively high resistivity layer could be attributed to slope deposits, overlying the substratum constituted by the pre-evaporitic member of the laga formation, according to the geological map of the area (marche region geological maps at http://www.ambiente.marche.it). in particular, the relatively conductive sectors could be related to the pelitic-arenaceous lithofacies (lag1e), with a consistent water content, whereas the relatively resistivity sectors, especially in the center of the ert, could be associated with the arenaceouspelitic lithofacies (lag1b) of the laga formation. single station noise measurements (hvnsr). 14 and 3 single station noise measurements (horizontal-to-vertical spectral ratio, hvnsr, bonnefoy-claudet et al., 2006; evg1-ct-2000-00026 sesame, natosfp980857) with 20 minutes of recordings sampled at 128 hz using tromino equipment (moho srl) were carried out in the whole urban area of pescara del tronto and vezzano, even in the most heavily damaged areas where accessibility with other instruments was problematic (figure 7). figure 8 shows the pescara del tronto hvnsr functions of the measurement points close to the ert profile. as shown in ert prospecting, although some sites lay on slope deposits and others are on the laga lithofacies (lag1e and lag1b), all have amplification in the frequency range 3-7 hz, that is the range of greater interest of the built-up environment (mainly low-rise masonry buildings). figure 8: electrical image (ert 01) with hvnsr functions (on the top) carried out at pescara del tronto. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 8 although the lito-stratigraphic geological condition in vezzano is comparable with pescara del tronto (marche region geological maps at http://www.ambiente. marche.it), the hvnsrs estimated in vezzano shows flat amplitude values (figure 9). seismic array. mechanical characterization of shallow soil for both passive (bidimensional seismic arrays processed with extended spatial auto-correlation, esac; ohori m., 2002; okada 2003; parolai et al., 2006) and active techniques (multistation analysis of surface waves, masw; park et al., 1999.) were applied (figure 7). for brevity, only the esac survey is reported in this paper, it was performed using a geode, a 24 bit seismometer with 144 db dynamic range with 8 msec sampling rate. the set-up of arrays was with 24 geophones with 4.5 hz frequency, l-shaped arrays with variable interdistance and length in the 190 – 50 metres range were used for esac. noise acquisition was carried out with a 125 msec sampling rate for a length in the range 3-40 minutes; figure 10 reports the vs velocity profile estimated by esac analysis: two impendence contrasts are present at about 10m (vs from 300 to 600m/sec), probably due to the discontinuity between slope deposits and the underlying laga formation, and at about 30m (vs from 600 to 1000 m/sec), probably due to the different lithostratigrafic members, from pelitic to arenaceous, in the frame of the laga formation. figure 9: ambient noise hvsr functions estimated in vezzano. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 9 single station earthquake measurements (hvsr). to validate the site response evaluated by single station noise measurements, two etna-kinemetrics accelerometers were installed on september 9, 2016 with different equipment arrangements. the aftershocks in three sites of pescara del tronto (as1, as2, as3) and one in vezzano (as4) were recorded (figure 7 for pescara del tronto e figure 9 for vezzano). accelerometric station 1 (as1) acquired 471 aftershocks from 7 to 10 september with magnitude in the range 1.1-4.3 ml. accelerometric station 2 (as2) recorded 1113 earthquakes from 8 to 27 september with magnitudes in the range of 1.1-3.9 ml, while accelerometric station 3 (as3) was installed in the most damaged area from 28 september and is still acquiring data. because the vezzano hvnsrs have a flat response, an accelerometric station as a reference site was installed in vezzano, recording earthquakes simultaneously with as2 and as3 stations. the preliminary horizontal-tovertical spectral ratio (hvsr) functions were performed using 80 earthquakes with 2.4 ≤ ml ≤ 4.3 in as1, 60 earthquakes with 1.2 ≤ ml ≤ 3.3 in as2, 77 earthquakes with 2.2 ≤ ml ≤ 3.9 in as3 and 88 with 2.5 ≤ ml ≤ 3.9 in the vezzano site (figure 11). the vezzano hvsr estimated with earthquakes again confirms a flat amplification function, whereas pescara del tronto shows a significant resonance peak with a clear amplitude value (figure 11). although the stations are located on the same geological formation (laga formation), they show different amplification functions: the as1 and as3 sites have a clear resonance peak at about 6.5 hz and 4 hz respectively, whereas the as2 site was characterised by a slight amplification in a broad frequency range (at about 2-4 hz and 7-10 hz, this behaviour is more evident in the we component). it is important to emphasize that heavy damage was observed in the area close to the as3 station. figure 10:. dispersion curve and vs profile estimated by esac analysis in pescara del tronto. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 10 figure 11: hvsr functions estimated in three sites of pescara del tronto (as1, as2, as3) and in one of vezzano (as4). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7271 11 v. final remarks preliminary surveys of the main structural characteristics carried out on the building stocks in the pescara del tronto and vezzano point out possible significant differences between the building vulnerability, although they are not so remarkable as to justify the high variability in damage distribution. on the other hand, the results of geophysical investigations show remarkable variable local site effects determining seismic intensity amplifications in pescara del tronto. in particular: 1) the vezzano hvsrs are characterised by a flat amplification function using both ambient noise and earthquake recordings; 2) the pescara del tronto hvsrs have a clear resonance peak at frequency between 4-6 hz, the peak amplitude is highlighted using earthquake data rather than ambient noise recordings. moreover the soil resonance frequency is very close to the main period of masonry buildings (~0.2 sec), this could have increased the damage in pescara del tronto. summarizing, the current status of the work allows to state that the different damage distributions in pescara del tronto and vezzano are mainly due to local site effects determining seismic intensity amplifications in pescara del tronto, while a minor role should be attributed to the difference in terms of seismic vulnerability of buildings. however, further engineering, geological and geophysical studies are required, and currently in progress, to better understand and possibly quantify the role of the different factors (vulnerability, site effects both stratigraphic and topographic, ground settlements) on damage enhancement found in pescara del tronto. references [bonnefoy-claudet et al., 2006] bonnefoyclaudet s., cornou c., bard p.-y., cotton f., moczo p., kristek j., fäh d. 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[parolai et al., 2006] parolai s., richwalski s.m., milkereit c., faeh d.,(2006); s-wave velocity profile for earthquake engineering purposes for the cologne area (germany). bull. earthq. eng., 65-94. [park et al., 2009] park c.b., miller r.d., xia j. (1999); multichannel analysis of surface waves, geophysics, volume 64, issue 3, 1999. [sesame, 2004] sesame project, (2004). guidelines for the implementation of the h/v spectral ratio technique on ambient vibrations. measurements, processing and interpretation, wp12, deliverable n. d23.12, http://sesame-fp5.obs.ujf-renoble.fr/papers /hv_user_guidelines.pdf. the s o l a r f l a r e effect a t a l i b a g on june 13th. 1 9 5 1 s . l . m a l u r k a r t h e m a g n e t i c o b s e r v a t o r y a t a l i b a g ( 1 8 " 3 8 ' / v , 72" 5 2 ' e , 1 8 m i l e ss e of c o l a b a , b o m b a y i is s i t u a t e t i a t a l o w m a g n e t i c l a t i t u d e ( 9'-.5 n). t h e v a r i o m e t e r s a r e u s u a l l y r u n o n a t i m e s c a l e of 1 5 m i n , h r . t h o u g h t h e b r o a d f e a t u r e s of a m a g n e t i c p h e n o m e n o n a r e r e c o r d e d , i h e d e t a i l e d p r o f i l e d u r i n g s u c h p e r i o d s c a n o n l y h e o b t a i n e d w h e n a n o p e n t i m e s c a l e is u s e d . f o r t h e l a s t f e w y e a r s , 011 s p e c i a l o c c a sions, t h e i n s t r u m e n t s b a v e b e e n r u n a t 1 8 0 m m / h r . it i n a y j u s t h e p o s s i b l e t o a n t i c i p a t e a n d p u t t h e q u i c k r u n i n a c t i o n f o r m a g n e t i c s t o r m s a n d h e f o r t u n a t e i n o b t a i n i n g d e t a i l e d r e c o r d s of t h e s a n i e . a m o n g t h e f e w p h e n o m e n a t l i a t c a n h a r d l y h e a n t i c i p a t e t i a t p r e s e n t is t h e s o l a r i l a r e e f f e c t o r t h e ' c r o c h e t s ' . a n y s u d i r e c o r d i n g of t h e s o l a r i l a r e w o u l d h e a c c i d e n t a l . w h i l e t h e m a g n e t i c r e c o r d e r s h a d b e e n o n q u i c k r u n , o n j u n e 1 3 t h . 1 9 5 1 , a r e m a r k a b l e r e c o r d w a s o b t a i n e d . t h e s o l a r flare e f f e c t c o m m e n c e d o n t h e r e c o r d a t a l i b a g a t 0 5 4 6 g m t a n d t h e e f f e c t w a s o v e r a f t e r a b o u t 1 4 0 m i n u t e s . s o l a r flares w e r e v i s u a l l y r e c o r d e d a t k a n z e l h o h e , m e u d o n , z i i r i c h , s a l t s j o b a d e n a n d h e r s t n i o n c e u x a b o u t t h a t t i m e of t h e d a y . t h e p a r t i c u l a r o b s e r v a t i o n s c o r r e s p o n d i n g t o t h e c r o c h e t a r e : table s t a t i o n t i m e g m t i m p o r t a n c e n u m b e r o f a c t i v e r e g i o n of q u a r t e r s a l t s j o b a d e n 0 6 1 2 3 2 6 k a n z e l h o h e 0 6 4 1 0 9 0 5 3 2 6 0 6 4 1 0 7 0 0 1 — 2 4 m e u d o n 0 7 1 0 0 7 2 5 2 2 6 z i i r i c h 0 7 1 3 1 0 0 0 2 2 6 o n d r e j o v 0 7 5 6 0 8 0 1 ? 2 6 0 7 5 9 0 9 4 8 ? 2 3 0 9 0 0 0 9 0 6 1 2 4 h e r s t m o n c e u x 0 9 1 0 0 9 2 5 1 2 6 2 1 6 s . l . m a l u r k a i t c a n b e r r a r e p o r t e d flare, r a d i o f a d e o u t a n d i o n o s p h e r i c d i s t u r b a n c e f r o m 0 5 5 0 g m t f o r 3 1 m i n u t e s o n a l a r g e n m n b e r of b a n d s ( 6 0 , 6 2 , 7 3 , 1 0 0 , 1 2 0 0 a n d 3 7 5 0 mc/s). t o k y o h a d i o n o s p l i e r i c d i s t u r b a n c e f r o m 0 6 3 5 g m t f o r 1 0 0 m i n u t e s . ( a l i t h e d a t a a b o v e a r e f r o m o u a r t . b u l l . s o l a r a c t i v i t y . n. 9 4 . 1 9 5 1 . z u r i c h ) . i n a l e t t e r , t h e a l i i n d i a r a d i o h a s s a i d t b a t a s o b s e r v e d al t o d a p u r ( d e l h i i , t h e r e w a s c o m p l e t e r a d i o f a d e o u t of s h o r t a n d m e d i u m w a v e b a n d s b e t w e e n 0 5 5 0 a n d 0 7 3 0 g m t a n d t b a t c o n d i t i o n s s t a r t e d i m p r o v i n g a t 0 8 4 0 g m t a n d b e c a m e n o r m a l at 0 8 5 0 g m t . f r o m d a t a of c r p l b u l l e t i n s of t h e n a t i o n a l b u r e a u of s t a n d a r d s , w a s h i n g t o n ( u . s . a . ) t h e f o l l o w i n g t a b l e h a s b e e n e o n s t r u c t e d . table r e c e i v i n g s t a t i o n t i m e g m t l o c a t i o n of t r a n s m i t t e r s h o n g k o n g ( c h i n a ) 0 5 5 5 0 7 2 5 c h i n a , e n g l a n d , f o r m o s a , f r e m ii i n d o c h i na, j a p a n , m a l a y s t a t e s , p h i l i p p i n e ids., t h a i l a n d . c o l o m b o ( c e v l o n ) 0 5 5 5 0 8 0 0 c h i n a , e n g l a n d , i n d i a , j a p a n . l i n d a u ( h a r z g e r m a n y ) 0 5 5 0 1 1 0 0 m u n i r l i , l i n d a u i p u l s e t r a e m i t t e r a n d rec e i v e r ) , w i e s b a d e n . b r e n t w o o d ( e n g l a n d ) 0 6 0 0 0 8 0 5 a l g h a n i s t a n , a u s t r i a , b a h r e i n i d s . b e l g i a n c o n g o , b u l g a r i a , e r i t r e a , g r e e c e , i n d i a , i r a n , k e n y a , m a l t a , p a l e s t i n e , p o r t u g a l , s. r h o d e s i a , s p a i n , s w i t z e r l a n d , s y r i a , t h a i l a n d , t r a n s j o r d a n , l i . s . r . , y u g o s l a s o m e r l o n ( e n g l a n d ) 0 6 0 0 0 8 0 0 a d e n , a u s t r a l i a , c e y l o n , c h i n a , c y p r u s , f o r m o s a , i n d i a , u n i o n of s. a f r i c a . t h e s u b s o l a r p o i n t m o v e d j u s t s o u t b of t h e t r o p i e of c a n c e r f r o m t h e l n d o b u r m a b o r d e r t o o m a n . a l i b a g o b s e r v a t o r v w a s f a v o u r a b l y s i t u a t e d f o r r e c o r d i n g t h e c r o c h e t . t h e l o c a l i t i e s of t h e r a d i o f a d e o u t s e e m t o fit a t e x t h o o k e x a m p l e of t h e a r e a t b a t s h o u l d b e a f f e c t e d . on j u n e . 1 4 t h at 1 7 5 0 g m t a m o d e r a t e m a g n e t i e s t o r n i s t a r t e d a t a l i b a d w i t h a s. c. of 2 5 r in h a n d t h e s t o r i l i l a s t e d a b o u t 5 2 h o u r s . t h e s o l a r l-i.are e f f e c t a t a l i b a g on j u n k 1 3 t l l . 1 9 5 1 2 1 7 t l i e p r o f i l e of t h e flare f o r t h e h o r i z o n t a l a n d v e r t i c a l m a g n e t i c c o m p o n e n t s a n d d e c l i n a t i o n is g i v e n a f t e r t a k i n g o u t t h e e f f e c t of d i u r n a l v a r i a t i o n ( t a h l e a n d d i a g r a m ) . table t i m e r e l a t i v e d i s p l a c e m e n t f r o m v a l u e s ( c o r r e c t e d f o r d i u r n a l v a r i at 0 5 4 6 g m t a t i o n ) t i m e h i r ) v ( r ) d ( m i n u t e s l o w a r d s e a s t ) 0 5 4 6 0.0 0.0 0.0 0 5 5 1 4.75 0.0 0 5 5 6 10.55 — 2 . 2 8 — 0 . 3 0 6 0 1 1 8 . 1 0 — 2 . 9 5 0 6 0 6 3 0 . 9 0 — 6 . 4 3 — 0 . 9 0 6 1 1 4 7 . 6 5 — 8 . 9 3 0 6 1 6 5 8 . 4 0 — 1 0 . 5 3 — 1 . 1 0 6 2 1 6 3 . 2 0 — 1 1 . 9 0 0 6 2 6 6 6 . 4 0 — 1 1 . 7 5 — 1 . 6 0 6 3 1 7 1 . 6 0 — 1 3 . 5 5 0 6 3 6 6 9 . 5 5 — 1 2 . 0 5 — 2 . 0 0 6 4 1 6 3 . 2 — 9 . 3 0 0 6 4 6 5 2 . 1 5 — 3 . 6 5 — 1 . 7 0 6 5 1 4 3 . 6 5 1 . 0 7 0 6 5 6 3 6 . 4 0 3 . 4 7 — 1 . 0 0 7 0 1 3 0 . 5 0 7 . 0 1 0 7 0 6 2 6 . 6 0 8 . 1 5 — 0 . 6 0 7 1 1 2 4 . 8 0 8 . 8 5 0 7 1 6 2 2 . 4 0 11.71) — 0 . 3 0 7 2 1 1 9 . 7 5 uio 0 7 2 6 1 6 . 1 0 1 1 . 5 5 0.0 0 7 3 1 1 0 . 9 0 1 2 . 2 5 0 7 3 6 8 . 6 0 1 2 . 3 0 0 . 4 0 7 4 1 4 . 8 0 1 3 . 4 5 0 7 1 6 — 1 4 . 9 5 0 . 7 0 8 0 6 0.00 t h e v a r i a t i o n i n d is v c r v s n i a l l 011 m o s t o c c a s i o n s , at a l i b a g . t h e d e c i m a i p o i n t fignres a r e g i v e n t o h e l p t o d r a w t h e r e s u l t a n t c u r v e . t h e i n a i l i f e a t u r e t o n o t i c e is t h e f a c t t l i a t t h e r i s e t o t h e m a x i m u m h a s b e e n i n t w o s t a g e s , w i t l i a n i n f l e c t i o n b e t w e e n 0 6 2 1 a n d 0 6 2 6 g m t . t h e m a x i m u m w a s a t t a i n e d a t 0 6 3 2 g m t . t h e a c t i v e r e g i o n s of flares t l i a t w e r e o b s e r v e d 1 n o s . 2 3 , 2 4 a n d 2 6 of t h e q u a r t e r ) c r o s s e d t h e c e n t r a i m e r i d i a n of t h e s u n 011 j u n e . 7 . 9 , 1 1 . 8 a n d 1 8 . 3 . a s t h e flare c o r r e s p o n d i n g t o a c t i v e r e g i o n n. 2 3 w a s o b s e r v e d a t o n d r e j o v b e t w e e n 0 7 5 9 a n d 0 9 4 8 g m t , t h e c r o c h e t r e c o r d e d a t a l i b a g is d u e t o flares of t h e a c t i v e r e g i o n s n. 2 4 a n d n. 2 6 a n d t h e t w o s t a g e s 2 1 8 s . l . m a l l ' r k a r 0 6 1 1 0 6 3 6 0 7 0 1 0 7 2 6 0 7 5 1 t i m e in c . m . t . f i g . 1 steji to t h e m a x i m u m , w h i c h is e q u i v a l e n t t o t w o s u p e r p o s e d m a x i m a , is d u e t o t w o d i s t i n c t flares. m o s t of t h e r a d i o f a d e o u t s h a v e b e e n r e p o r t e d 4 m i n u t e s a f t e r t h e c r o c h e t a t a l i b a g c o m m e n c e d . t h e i o n o s p h e r i c d i s t u r b a n c e a t t o k y o h a s b e e n r e p o r t e d a b o u t t h r e e t o f o u r m i n u t e s a f t e r t h e absol u t e m a x i m u m of t h e c r o c h e t w a s r e a c h e d a t a l i b a g . t l i o u g h o n e s h o u l d b e r e l u c t a n t t o u s e t h e v r e c o r d s i n t l i e s e l a t i t u d e s , i t m a y b e p o i n t e d o u t t b a t f r o m 0 5 4 6 t o 0 5 5 1 g m t t h e r e h a s b e e n l i t t l e c h a n g e a n d t l i a t f r o m 0 7 2 0 t o 0 7 3 0 g m t t h e r e h a s b e e n a flattenins i | s o l a r f l a r e . 1 3 " j u n e 1 9 5 1 . ( c 0 r r e c t e d f o r d i u r n a l v a t u a t 1 0 n ) y • ^ 6 0 < 2 0 in t h e s m o o t h e d v c u r v e s . w h e t l i e r t l i i s h a s a n y t h i n g to d o w i t h t h e p e r i o d of c o m p l e t e f a d e o u t of s t a t i o n s as r e c e i v e t l a t d e l h i ( 0 5 5 0 t o 0 7 3 0 g m t ) c a n n o t b e d e f i n i t e l y s a i d . t h e d e t a i l e d c o r r e l a t i o n of t h e p r o f ì l e c a n o n l y b e d o n e if a n y o t h e r i n s t i t u t i o n h a s c o l l e c t e d c o g n a t e d a t a o n a c o m p a r a b l e t i m e s c a l e . f r o m t h e p u b l i c a t i o n s of t h e n o r s k e i n s t i t u t t f o r k o s m i s k f y s i k ( n r . 3 4 . o b s e r v a t i o n s of 1 9 5 1 ) , i t is n o t i c e d t l i a t t h e d a i l y r a n g e of t h f . s o l a r f l a r e e f f e c t at a l i b a g on j u n e 1 3 l l l . 1 9 5 1 2 1 9 o z o n e a t t r o n i s p w e r e m u d i l a r g e r o n t h e 1 3 t h a n d ] 4 t h j u n e . 1 9 5 1 (1.19 i n n i a n d 0 . 3 6 n i n i r e s p e c t i v e l y w h i l e t h e a v e r a g e d i u r n a ] r a n g e f o r l h e i n o n t h w a s o n l v 0 . 0 8 i n n i . t l i e r e c o r d i n g w a s d o n e b y m r . a . s . c h a u b a l a n d m r . d . k . d e s h m u k h a n d h e l p w a s r e n d e r e d i n c a l c u l a t i o n s b y ìvir. m . p a n d u r a n g a r a o a n d m r . k . s . r a j a r a o . colobii observatory. — bombay. 5. jan. 21st. 1953. summary a detailed study oj the quick-run magnetograms oj tlie solar flare oj jane 13, l'j51. alibag tv as favourably situated. corresponding to different flares on tliat day the profili' oj tlie solar flare effect has a tteo-stagi' maximum. the time oj the onset oj the radio fade-out in relation to the fiori• has been pointed out. in the vertical force, effect corresponding to the onset and end of radio fade-out tliere has been a temporary no-variation-ivith-time stage or a sort oj inflexion. the diurnal rang oj ozone at ironisti also ivus abnormally large ivithin the next 24 hours. 29glavcheva.qxd 705 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words macroseismology – bulletins and catalogues – supporting data set – site seismic histories – bulgaria 1. introduction the beginning of bulgarian seismology dates back to 1891. at that time spas watzof, the director of central meteorological station in sofia, organized a network of correspondents for observation of felt earthquakes. to unify the collected reports, watzof prepared and spread circulated throughout country a special «instruction and program on the earthquakes observation and documenting» (watzof, 1902). all the reports coming to the central station were included in seismological compilations (reports on the earthquakes felt in bulgaria). thus, watzof formed the proto-type of a macroseismic bulletin in bulgaria which contained: time of perceived shaking, locality, direction of impact, observed effects, intensity according to the volunteers’ (correspondents’) assessment by rossi-forel scale till 1912 or by forel-mercalli scale since then. the initial material on earthquakes felt in bulgaria in the period 1892-1927 is published in this manner. going through the 17 annual volumes edited by watzof (1902-1923), one can find that even at his time watzof was concerned with some important topics, now considered quite common: «seismic centre» location (now, epicentre determination and source zones), density of observation points and territorial coverage (topics of great importance for both non-instrumental and instrumental data acquisition), accumulation of as many witness reports as possible from a given locality (statistical nature of modern intensity scales), etc. the seismological survey employees coming after watzof’s death (1928) carried on with the macroseismic data collection. as a result of their efforts the scientific world received annual reports on earthquake effects in bulgaria in the course of 74 years. so these annual reports were issued until 1965, though in different formats. state-of-the-art of historical earthquake investigation in bulgaria rumiana glavcheva geophysical institute of the bulgarian academy of sciences, sofia, bulgaria abstract this paper deals with non-instrumental seismology development in bulgaria (the central-eastern part of the balkan peninsula). the first steps and products of this scientific branch are discussed because they have traced the road of present-day historical seismology in this country. the sources of information on long-term seismicity are critically reviewed. some recent studies, which contribute to an improvement of the supporting data sets, are also discussed. a special emphasis is laid on the rules adopted to solve different cases as well as on the aspects, by which our understanding of the seismogenesis throughout the present-day bulgarian lands has been enhanced. mailing address: dr. rumiana glavcheva, geophysical institute, bulgarian academy of sciences, acad. g. boncev-st. bl. 3, 1113 sofia, bulgaria; e-mail: glavchev@geophys.bas.bg 706 rumiana glavcheva the successor of watzof was kirov. the first issue edited by kirov (1931) was, however, the last annual seismological compilation in bulgaria which included eyewitnesses’ descriptions. the presentation of data is organized by kirov in the typical form of a classical macroseismic bulletin. therein the main characteristics of the seismic impact are tabulated together with the effect strength uniformly assessed by a specialist. this style of macroseismic bulletin survived till 1966 when the last annual material on felt earthquakes was published. in 1952 the first isoseismal maps appeared (ileff, 1952). one can find macroseismic pictures of most of the bulgarian xx century earthquakes with a m 4 or higher in the monographic work by grigorova and grigorov (1964) and in the atlases by shebalin (1974) and prohazkova and karnik (1978). isoseismal maps of some weaker or later events are published in a number of papers. unfortunately, the intensity data points are not always shown in these maps and, though rarely, the intensity values assigned to individual localities had not been converted into the scale announced by the map’s legend. the main characteristics of the collections of records and products proceeding from this stage of the non-instrumental seismology in bulgaria are summarized in table i. the non-instrumental seismology in this country covers the earlier times as well (sections 2 and 3 below). the first steps in both the regular systematisation of the observations and searching for past earthquakes are the basis of historical seismology development in bulgaria. 2. first bulgarian investigations on long-term seismicity at the very beginning of bulgarian seismology two bulgarians went back to the earthquake history, and their compilations have supplied information to the catalogues. that is why the reader is to be acquainted with the sources of these compilations and to understand how fully and how far critically they have been exploited. watzof was the first person in bulgaria to pay attention to previous earthquakes. his findings refer to seismic events within the balkans and are published in supplements to the annual reports and in several papers (e.g., watzof, 1908, 1912). these findings proceed from isolated records included in the works by 9th century theophanes (de boor, 1883) and by 11th cent. symeon magister and logothete (bekker, 1838), as well as from some widespread secondary sources like the ones by schmidt (1879), mushketov and orlov (1893), etc. as to bulgaria, three destructive earthquakes which occurred prior to the beginning of the 19th century are roughly outlined in the material supplied by watzof. ey nhe ata ed rthtable i. generalization on the sources of macroseismic information since the bulgarian seismological survey started operating. time span characteristic features sources of macroseismic information 1965-1980 the descriptive initial reports are partly transferred into intensity-locality symbols geophysical institute’s archive and publications 1928-1965 macroseismic bulletins for the entire time-span. the archive of initial reports is complete since the late 50s. geophysical institute’s archive since 1956; macroseismic bulletins published (intensity-locality data plus some isoseismal maps). 1891-1927 all the initial reports of the bulgarian seismological survey correspondents are preserved (compiled accordingto a special instruction of how to observe and what to note during earthquakes) reports on earthquakes felt in bulgaria; published; isoseismal maps in papers published later. 707 state-of-the-art of historical earthquake investigation in bulgaria watzof’s most important contribution to past seismicity knowledge consists in gathering data from newspapers and journals in the national library, sofia. this data concerns central balkan earthquakes occurring during the second half of the 19th century (the first bulgarian newspaper «tzarigradski vestnik» appeared on 3rd january 1848). recently performed thorough investigation through the 19th century periodicals (babachkova and rizhikova, 1993) added not too many new pieces of information: a couple of weak (4 msk) earthquakes in kyustendil (sw bulgaria); two local earthquakes in the vicinity of tutrakan at the danube river; some new information connected with the sofia series in 1858-1859, and with the 1875 ne bulgaria earthquake (imax = 7 msk); finally, several weaker events. thus, the latter study verifies the good quality of watzof’s investigation through the periodicals. staiko staikoff was the second to rummage for unknown earthquakes through ancient and recent literature, catalogues, periodicals, etc. his compilation (1930), based on findings in central european libraries, covers 375 earthquakes or clusters occurring all over the balkans and in asia minor, rarely in italy, during the time period 3th century b.c.-1885. the earthquake records are ordered chronologically. a reference is attached to each entry, unfortunately not always supplying the source title or the year of publication. the extracted description is sometimes accompanied by the author’s comment. it is rather probable that some of the sources cited by staikoff have not been exploited directly but through the 19th century seismological compilations by von hoff (1840), mallet (1853), schmidt (1879), etc., and some of them are known as second-hand sources (like most of perrey’s regional works, 1850). despite being very laconic and presumably considering isolated localities, staikoff’s initial sources are not at a low level. when the outputs proceeding from staikoff’s findings and from the rather richer information in coeval sources, provided by ambraseys and finkel (1991, 1995) and guidoboni et al. (1994) are juxtaposed, it turns out that the bulgarian compilation results in nearly the same epicentral area and dates of the earthquake occurrence shifted by about 0-1 to 10-20 years (glavcheva, 1996). that is why the compilation by staikoff (1930) still serves as a starting point towards the data set improvement for some balkan and east mediterranean earthquakes. 3. bulgarian earthquake catalogues with preference for macroseismic data the annual issues on seismic effects in bulgaria and the seismological compilations considered above became the basis toward earthquake catalogues compilation. the first bulgarian catalogue (kirov et al., 1960) is of a mixed descriptive-parametric type, presenting the earthquake occurrence time, summarized description of maximum effects, epicentral or maximum intensity, epicentral geographical coordinates for earthquakes in bulgaria or the geographical region of most likely epicentre location for foreign events, and the focal depth roughly assessed. it covers 344 events, which have caused effects of minimum 5 msk (5 mcs then) in bulgaria during the time-period 1892-1958. the first parametric catalogue was compiled by grigorova and rizhikova (1966). it refers to the time interval 1961-1964. therein, the instrumental magnitude based on records by the wiechert seismograph appears. later on, the joint balkan earthquake catalogue (shebalin et al., 1974), noted further by sha74, was prepared, and was the first to deal with the longterm seismicity of bulgaria. it covers most of the then known bulgarian events (of a magnitude m ≥ 4) since ancient times till 1970. the primary material on this is extracted from the above considered bulgarian compilations (part 2) and the annual macroseismic reports (part 1) according to grigorova (1973). the last bulgarian catalogue which also gives preference for macroseismic data lists medium-size and strong earthquakes in the 70ies (sokerova et al., 1982). the study of any impressive present-day earthquake could be the reason for preparing a detailed historical earthquake catalogue of a certain portion of the country. aiming at updating the seismic zoning in bulgaria, a revised catalogue was compiled at the end of the 1970s (grigorova et al., 1978), noted 708 rumiana glavcheva further by gra78. as a result of applying the principles and techniques already examined when creating the balkan catalogue, the new catalogue presents more than 1400 entries within the territory enclosed in 40-46°n, 20-30°e. about 930 of them correspond to earthquakes occurring in the present-day bulgarian territory: 86.5% are supported by respective datasets and the rest come from other parametric catalogues. two peculiarities of this catalogue should be noted: it does not fix the lowest earthquake size, and the intensity assessments are not unified. later, christoskov and samardjieva (1980) published the main rules accepted for accuracy assessment in gra78. it was postulated that the degree of cultural development was unambiguously related to the elapsed time. proceeding from this, two ideas were suggested: 1) the earlier historical sources represent only quite strong or catastrophic earthquakes, and 2) going back in time the descriptions become more laconic, incomplete and less reliable. and the maximum uncertainty of the parameters is established as: ± 100 years in occurrence time, ± 200 km in epicentral coordinates, ± 2 degrees in epicentral intensity for the old times. the first suggestion above can lead to a systematic overestimation of the earthquake size, especially when the information is poor. concerning the second point, that the information reliability is directly connected with the period when the earthquake record was produced, this is rather simplified. it has been proved many times that the reporter’s social responsibility, the reporter’s attitude to natural events, the stress s/he puts on the political circumstances in the earthquake epoch, are crucial factors for reliability of the report. actually, the accuracy of earthquake parameters depends to a great extent on the quality of the data set from which the earthquake parameters are evaluated (the so called supporting dataset). the supporting dataset quality in bulgaria has been improved in the course of time, although the intensity data points number has not been changed significantly. for instance, more than 50% of the earthquakes in bulgaria which occurred before 1892 still are supported by a single locality report. the next section is dedicated predominantly to various means of improving in the supporting datasets. 4. historical earthquake investigation in bulgaria since the last 20th century decade at the beginning of the last decade of the 20th century, just a century after the pioneering efforts of spas watzof, a new search for earthquakes in the past was commenced through different kinds of compilations (historical sources, marginal notes, memoirs, inventories of greek, bulgarian, slavonic and other manuscripts, etc.) thanks to serious support by the national library, sofia. this thorough work fulfilled by babachkova and rizhikova (1993) led mainly to first hand sources on unknown weak earthquakes in bulgaria and to additional data on some strong earthquakes in the neighbourhood. at that time, i chanced to find a traveller’s account (galt, 1812) which pointed to an earthquake series realized near plovdiv, south bulgaria, in 18091811; the case was discussed together with other problematic seismic events in the xxiv esc general assembly (glavcheva, 1994). considering the importance of supplying a qualitative input information for solving the seismic hazard and risk connected topics, the historical seismology in bulgaria focussed efforts on the following two tasks: 1) to examine again the circumstances related with some key catalogue events, starting from the original information; 2) to clarify the threat to some important sites often subjected to seismic impacts, i.e. to make the data on experienced effects as complete as possible. figure 1 is an illustration of the cases developed below in connection with these two tasks. 4.1. flexibility of «the rules» supporting data set improvement the improvement of some datasets, connected with key-events, has become the goal of bulgarian seismologists since the 1990s. case a – a transfrontier felt area: overcome disagreement by the very primary information. twelve solutions, independently found, belonged in the early nineties to the 14 october 1892, balkan earthquake. the epicentres were 709 state-of-the-art of historical earthquake investigation in bulgaria spread over more than 200 km. such a disagreement is shown to have been produced mainly by fragmentary use of the initial data (glavcheva and radu, 1994). the seismic intensities at 155 localities (previously being 95) are evaluated only on the basis of first-hand sources applying the same scale. after improving the supporting dataset in such a way, the so-called «watzof’s intensity assessments», actually intensity values assigned by the local reporters, have been avoided for the territory of bulgaria. as the romanian territory is concerned, the second hand sources have been omitted at solving this case. the earthquake main parameters have been derived, as accurately as possible, from the intensity distribution: seismic source location in south dobrodja at a depth of a minimum 35 km, epicentral intensity 8 msk, the corresponding magnitude up to 7. case b – the catalogue solution provides matters of uncertainty due to the careless reading of a record. the following account exists in the balkan earthquake catalogue sha74: «1759, june 26 or 29, the towns thessaloniki and plovdiv suffered great damage, epicenter in the pirin mountains or in struma valley (9?; 41n, 24e); 40 3/4n, 23e, 8; two shocks?, according to staikoff (1930), montandon (1953), galanopoulos (1961)». thus, the catalogue provides a matter of uncertainty. the case needs contemporary reports. and the report has been found. the collection academique (guéneau de montbéliard, 1761), among its descriptions on an earthquake sequence near thessaloniki, wrote: «la ville de phillipolis dans le voisinage de salonique, a beaucoup souffert de ce meme tremblement.» so it became clear that the original phillipolis had been replaced by philippopolis (plovdiv) in the classical compilations by von hoff (1840), perrey (1850), mallet (1853). the erroneous reading was inherited by milne (1911), montandon (1953) and finally was entered in the fig. 1. map of bulgaria and cases of study. the rectangles denote the regions; the crosses mark the strong key events. 710 rumiana glavcheva balkan catalog sha74. after analyzing the information from the earthquake time, the following conclusions are drawn by glavcheva (2000a). i) an earthquake series occurred during june-september 1759 in the thessaloniki surroundings. the activated space was most likely somewhere in the narrowest part (the «neck») of the halkidiki peninsula, where the great 1902 activation would take place. the strongest earthquake (or two earthquakes) of the series might have a magnitude ranging between 6.6 and 7. ii) in the area of phillipoe a strong local shock occurred simultaneously with the thessaloniki series. iii) the bulgarian town of plovdiv was involved in the 1759 earthquake sequence due to a careless reading of the initial record. thus, the suspicious 1759, «9 [msk]?, 41n, 24e» event in the border greece-bulgaria region must be dropped from the earthquake catalogues. case c – an «energy» catalogue evidences seismic precursory features. an impressive seismic activity developed in the transfrontier region of sw bulgaria-macedonia during 1904-1906. the seismic sequence started on 4 april 1904 with two catastrophic earthquakes within 23 min (mlh = 7.1 and 7.8 in sha74, ms = 6.9, 7.2 by ambraseys (2001). recently glavcheva (2000b) compiled an «energy» catalogue of the sequence; an excerpt for the first 100 minutes is plotted in fig. 2b. for this purpose two problems had to be solved when processing the initial data (watzof, 1902-1923): interruption of data flow coming from the damaged area (epicentral distance up to about 50-80 km) and accumulation of damage. to overcome difficulties, reports from localities at epicentral distance more than 50 km, outside the area most damaged by the first severe earthquake, are mainly taken into account. to distinguish separate events, the «relative time differences» [tm(i) – ta(i)] are used, as shown table ii. problems and ways of achieving results in cases c and d. case roots intensity data set creation – influence taken into account dataset processing c watzof (1905-1907) – interruption of recordings from the epicentral area; – damage cumulation occurrence time determination ta(abs) = tm(abs) – [tm(i) – ta(i)] (*) magnitude evaluation (glavcheva, 1997) m = 2.36 logr3 (**) r = 0.90 (45 events; m up to 7.2); m = 2.45 logr4 + 0.49 r = 0.92 (55 events; m up to 7.8). d watzof (1902-1923); kirov (1931); mihailovic (1932); local press – observation network geometry; – observational points specificity; – conditions at the moment of recording interpreted four sorts of quality of the macroseismic field (*) tm(i) and ta(i) are times of the main shock and of a concrete aftershock reported from the same locality «i»; tm(abs) is the actual time of the main earthquake; ta(abs) is the true time of a given event a; [tm(i) – ta(i)] is the «relative time difference». (**) r3 and r4 are the mean isoseismal radii of 3 and 4 msk respectively. 711 state-of-the-art of historical earthquake investigation in bulgaria in table ii. the earthquake magnitudes have been assessed by the distant felt area sizes, applying intensity attenuation relations from sha74. the list of events has been controlled by the continuous earthquake recording supplied by two meteorological stations in rila monastery and borovetz. these uninterrupted records guarantee completeness of the seismic events listing, i.e. of the «energy» catalogue, at a lower threshold m 4.5. according to this catalogue, the seismic source maximal activity turns out to have been in the interval between the two largest earthquakes. this very period, unfortunately the most slightly known one until late 1990’s, presents the most curious stage of the rupture process. indeed, the national catalogue gra78 shows that the energy release pattern is quite monotonous (fig. 2a), while the sample derived from the new catalogue discloses precursory time variations of the energy release mode, more precisely – a seismic activity decrease starting in the middle of the interval between the two largest shocks (fig. 2b). case d – an overall picture of a catalogue revision. the maritza seismic zone in central south bulgaria is well known with the 1928 catastrophic earthquakes (m 6.8 and 7.0 according to the catalogue sha74) because they caused two surface coseismic ruptures, each of them several tens of kilometers in length, and much loss as well. the current dense population and industry concentration require better knowledge of the former seismogenesis in the zone. a thorough review of fig. 2a,b. energy release at the start of the 1904 seismic activation in bulgaria macedonia (case c): according to the last bulgarian earthquake catalogue (a) and after the initial records reassessment at fixed regulations (b). note the significant decrease in the mean energy released per single event before the second catastrophic earthquake (the arrow). a b 712 rumiana glavcheva t ab le ii i. e ar th qu ak es p ri or t o th e s ei sm ol og ic al s ur ve y es ta bl is hm en t. a bb re vi at io ns a s fo ll ow s: d s = d at a so ur ce ( s d = s ta ik of ( 19 30 ), w s 02 = w at zo f (1 90 2) , jm ih = m ih ai lo vi c (1 93 2) , in s = i ni ti al s ou rc e ci te d in d s (n p / n p s = n ew sp ap er , -s ; h id = h id de n, i. e. un kn ow n) ; a x = a re a of m ax im um i m pa ct o r of t he r ep or t’ s si te ; n m o = n um be r of m ac ro se is m ic o bs er va ti on s; i x = m ax im um o bs er ve d in te ns it y (5 /7 = 5 t o 7 de gr ee ); e p = e pi ce nt er . t he v al ue s in b ra ck et s ar e un ce rt ai n. d at e t im e d s in s a x n m o ix e m s c oo rd in at es r em ar ks g ri go ro va et a l. (1 97 8) y r m o d ay h r m in °n °e av ai la bi li ty 17 50 10 s d n p s p lo vd iv m ar it sa 1 + a re a ar ou nd (9 ) 42 .1 24 .8 m or e in fo rm at io n ne ce ssa ry y es 18 11 01 18 /1 9 g al t e ye w it ne ss p lo vd iv 1 + a re a 6 42 .1 5 24 .7 5 tw o ye ar s fo rm er a ct iv it y; d at e by o .s t. n o 18 55 04 03 s d v er ol lo t p lo vd iv 1 5/ 7 42 .1 5 24 .7 5 «s ev er al s tr on g sh oc ks » 2 ev en ts 18 58 09 10 s d p er re y p lo vd iv 1 4 42 .1 5 24 .7 5 y es 18 59 06 15 s d p er re y p lo vd iv 1 + a re a 6 42 .1 5 24 .7 5 y es 18 59 07 25 s d p er re y p lo vd iv 1 7 42 .1 5 24 .7 5 «q ui te a l ot o f da m ag e» y es 18 61 01 09 21 55 s d r it te r p lo vd iv 1 ≥ 5 42 .1 5 24 .7 5 y es 18 62 01 09 21 55 s d r it te r p lo vd iv 1 ≥ 5 42 .1 5 24 .7 5 «t hr ee s ho ck s» 2 ev en ts 18 63 09 s d p er re y u pp er t hr ac ia 1 + a re a 5/ 7 a re a «s tr on g an d fr eq ue nt sh oc ks » 2 ev en ts ea rl y s ep t. r ho do pe s b al ka n 42 .2 25 .0 18 80 12 22 s d f uc hs p lo vd iv 2 + a re a 6/ 7 a re a «t w o m or e sl ig ht er s ho ck s af te r th at » 1 ev en t; r ho do pe s 42 .0 24 .9 di ff er en t e p 18 82 06 10 01 15 w s 02 jm ih n p h id p lo vd iv 12 6 42 .1 5 24 .7 0 is os ei sm al m ap a va il ab le di ff er en t e p 18 84 12 09 13 w s 02 jm ih n p h id p lo vd iv 8 4 42 .2 0 24 .7 5 is os ei sm al m ap a va il ab le di ff er en t e p 18 90 05 10 10 05 w s 02 jm ih n p h id p lo vd iv 11 6 42 .2 5 24 .8 0 is os ei sm al m ap a va il ab le di ff er en t e p 18 91 06 27 04 w s 02 jm ih n ps h id p lo vd iv 10 5 42 .2 5 24 .8 0 is os ei sm al m ap a va il ab le n o 713 state-of-the-art of historical earthquake investigation in bulgaria the supporting datasets for the earthquakes prior to 1928 has been carried out. different sorts of primary records corresponding to the earthquake «epoch» are involved in processing these events. for the time prior to the bulgarian seismological survey, the supporting datasets are predominantly extracted from the works by staikoff and watzof presented above. they correspond mostly to contemporary earthquake reports. the resulting revised catalogue (table iii; fig. 3, plot ii) has the following advantages compared to the latest bulgarian catalogue (fig. 3, plot i). it 1) leads to two more main events (14 in total); 2) shows the site intensity and geographic coordinates of the report’s locality without fabricating any epicentre (the first 9 cases in the table), in a case of initial data deficiency; 3) indicates the availability of accompanying shocks; 4) contains some entries being specified after adding a new source. for the time following till the severe earthquakes in april 1928, primary documentation of a good quality (watzof, 1902-1923; kirov, 1931) has come to us. it has been used as a basic source for the catalogues up to now (fig. 3, plot iii). recently, new pieces of information were found (table ii) and the catalogue revision became necessary (glavcheva et al., 2000). when compiling the supporting intensity data set, three important factors have been considered: the influence of the observation network geometry, the observational point’s specificity (systematic or occasional reporting, complete or partial descriptions of effects, detailed or laconic records), and the specific recording conditions (day-time, season population activity, saturation of people’s sensitivity). finally, depending on the macroseismic field quality, four groups of cases have been solved: 1) many intensity data points (idp) and areas with different degrees of effects; 2) many points of the same intensity in the distant part of the macroseismic field and known felt area size; 3) few idp with a different intensity degree (this case is reduced to case 1, the solution being of lower accuracy); 4) few idp with the same intensity degree and surrounding settlements without any other information except being a regular contribfig. 3. past seismicity of central south bulgaria (case d): prior to the seismological survey establishment according to the catalogue gra78 (plot i) and this study interpretation (plot ii, see also table iii); since 1892 until the 1928 catastrophic earthquakes according to gra78 (plot iii) and results from (glavcheva et al., 2000) (plot iv), the latter plot evidences a good coverage of the region by observation sites. 714 rumiana glavcheva utor of data on felt earthquakes; the territorial distribution of such settlements favours the macroseismic area outlining (analogous with case 2; lower accuracy). the manner of the earthquake main parameters determination is that applied in the balkan catalogue sha74. the new catalogue includes 46% more events than the latest bulgarian catalogue gra78 (fig. 3, plot iv). additionally, a settlement – intensity (msk) database has been developed for each event and the intensity data points plotted on a map. case e – false warnings in the latest bulgarian catalogue. this case shows how the unreasonable combination of two pieces of information: «6 december 1866: strong shock in thessaloniki» (fuchs,1886; staikoff, 1930) and «a heavy shock damaged some old houses and poor walls at sofia» (watzof, 1902) has led to the fabrication of the destructive 1866 earthquake in sw bulgaria listed in three catalogues: of the balkan region, bulgaria and greece. the steps of data processing for this false event were: setting the epicenter in the mid-point between the two reported localities; solving an elementary puzzle by the corresponding intensity attenuation relation and, in result, surprisingly high epicentral intensity – 8 or more msk (!?). the catalogue entries preceded are: «december? a heavy shock damaged some old houses and poor walls at sofia (watzof, 1902); grigorova gives i = viii for the region of the struma valley; incomplete information» in karnik (1971); «6 december 1866? old houses and walls damaged in sofia, epicentre in the struma valley (8°?), strong in thessaloniki, according to fuchs, 1886; karnik, 1971; grigorova, 1973» in shebalin et al. (1974); «18 december 1866 [?], 16h 25min ut, 42.0n, 23.0e, h = 40, mms = 7.1» in grigorova et al. (1978); «6 december 1866 16:25 42.0 23.0 h = normal m = 7.0 bulgaria (sofia 7°)» in comninakis and papazachos (1982). the following facts argue against assigning the 6 december 1866. earthquake to the middle struma valley: 1) such an earthquake has not been mentioned by authoritative catalogues as these by schmidt (1879), milne (1911), sieberg (1932), montandon (1953) or galanopoulos (1960); 2) no record from skopje, tatarpazardjik, plovdiv – important transport and trade centres then, which should have been concerned by a magnitude 7 earthquake in sw bulgaria – can be found; 3) the most significant arguments may be found in the primary documentation of later strong earthquakes that occurred in the same region. the well developed interpretation of the data concerning four later earthquakes in the middle struma valley has been summarised in table iv. twelve localities situated at a distance of no more than 100 km from the above supposed 1866 epicentre are shown in the table (columns 1 and 4). after considering the intensity attenuation according to relations published in sha74, it turns out that in each of the considered localities (dupnitsa is the only exception), the 1866 earthquake should have caused intensity (column 5) of one to two degrees higher than the maximum intensity experienced during the four later earthquakes (column 3). however, the quoted phrases (column 2) extracted from the reports on the later earthquakes in 1894, 1903 and 1904 (watzof, 1902, 1904, 1905) completely reject the possibility of a stronger former event. unfortunately, the town of dupnitsa remains the exception according to the results of the comparative analysis here. several records of shocks occurring around the end of 1866, along and near the struma river, exist in the 19th century documentation. one of them is the excerpt of the fuchs’s catalogue (staikoff, 1930), according to which the occurrence of an earthquake in thessaloniki on 6 december 1866 is irrefutable. watzof’s volume on the 19th century earthquakes in bulgaria (watzof, 1902) also supplies data (newspaper «vremya», issues 14, 16, 17, 21) thanks to which we learn two essential things. first, several not very impressive shocks had occurred: 14 november 1866, [n.st.], a slight shock in samokov, about 4 msk; 26 november 1866, an earthquake in sofia which had caused some damage, 6 msk most likely, the same shock was also felt in dupnitsa, 4 to 5 msk; 1867, first half of january, sofia, 5 to 6 msk. second, the only questionable entry in table iv, dupnitsa, is proven not to have experienced even 7 msk. thus, the available 19th century documentation gives strong evidence that no severe shock occurred in the middle struma valley in the late 1866. 715 state-of-the-art of historical earthquake investigation in bulgaria table iv. data for checking the authenticity of an earthquake in 1866 in the middle struma valley. the reports from the national seismological network (watzof, 1902, 1904, 1905) have been used. calibrating earthquakes the false event 1866 in gra78 6 dec 1866 16:25 42.0n 23.0e h = 40 m = 7.0 n date time coord. h io m root 1 25 nov 1894 22:30 42.2 23.0 15 6-7 4.9 gra78 2 26 nov 1894 01:40 42.2 23.2 15 7 5.3 gra78 3 25 nov 1903 23:16 42.1 23.2 5-10 8 5.5 gra78 4 4 april 1904 10:02 41.8 23.0 15-30 9-10 7.1 gra78 locality data for the calibrating earthquake data for the 1866 event name number (n) of the calibrating event and information about former seismicity in the same region ii observed msk locality/ epicentre distance ii calculated 1 2 3 4 5 kotcherinovo 1-2 pt (**) «very strong, even the oldest people do not remember a similar event» 7 10 ≥ 85 boboshevo 3 snt (**) «old people say they have never felt such an earthquake» tpo (**) «people say that such a strong and long lasting earthquake has not been felt till now» 7 15 ≥ 8 smolitchane 3 pt «very strong, people do not remember another similar event.» 6 25 ≥ 8 dupnitsa 4 tpo «nobody remembers such an earthquake» 8 25 ≥ 8 palatovo 3 pt «old people say they have never observed such a strong earthquake» 6 30 ≥ 75 tcherven breg 3 pt «...such a strong earthquake that old people do not remember to have had happened before...» 6 35 ≥ 75 ovtcharci 3 pt «old people do not remember such a strong earthquake in their life» 6 35 ≥ 75 delyan 3 pt «nobody remembers such a strong earthquake» 5/6 40 ≥ 75 dren 3 mnc «everybody says that such a strong earthquake has not been felt till now» 5/6 40 ≥ 75 rayovo 3 pt «such a strong earthquake has never happened here before» 6 50 ≥ 7 bossilegrad 3 tpo «such a strong earthquake has never happened here before» 5/6 75 ≥ 65 trin 4 rs «such an earthquake has never happened before» 5 100 ≥ 6 (**) pt = people’s teacher; snt = senior teacher; tpo = telegraph-post office; mnc = municipal clerk (secretary); rs = rain-gauge station.; ii = intensity at a site. 716 rumiana glavcheva 4.2. on the site seismic history the time seismic histories of two regions, sofia and se bulgaria, have been studied in order to widen the acquaintance with the longterm seismic influence on them. this has been done in the following succession: i) retrieval of all eyewitnesses earthquake records, or scientific summaries on them, i.e. compiling of the maximum complete set of initial data for seismic effects on concrete places; ii) intensity reassessment of the effects; iii) identifying the causal seismic events; iv) calculating the site intensity in case of description deficiency after normalizing the intensity-distance attenuation to some well compiled macroseismic pictures. the two regions are separately considered because of their specific problems. case f – sofia seismic history. prior to the 19th century sofia was a small town, deeply inside the ottoman empire, and only few reports on earthquakes felt therein could be mentioned. for this reason, the strength of excitation on sofia has mainly been determined by analogy with some recent well solved events. later, most of the seismic effects are documented and the intensity is possible to assess. thus, in the period since the early 19th century to date, seismic influences on sofia which have been identified (fig. 4a-c), are: a) from nearly a hundred local earthquakes, without taking into account the long-lasted sequences in 1818 and 1858, and b) twice more than the previously fig. 4a-c. time seismic history of the sofia city (case f): a) frequency of seismic impacts from both the external zones (44 times excited by 1904 bulgaria-macedonia earthquakes) and the local ones (the rest highest bars); b) strongest impacts per annum corresponding to available records (19th-21th century) or to the most likely intensity value (17th-18th century); c) regions (rounded rectangles, numbering as in table v) from where sofia have experienced seismic impacts in the time period 1892-2002; intensity of the strongest impact and the corresponding earthquake magnitude are marked. a c b 717 state-of-the-art of historical earthquake investigation in bulgaria known effects, caused by distant earthquakes. some details on damaging effects at sofia in this period are given in table v. case g – seismic impact on se bulgaria. part of the significant seismic effects near the bulgarian state borders comes from the neighbouring areas. such effects are not to be neglected, especially when the seismogenic zones are rapidly deformed and, consequently, show essential activity. for example, the se territory of bulgaria is under damaging influence from the marmara sea region more frequently than from local and nearly local origins. the long-term record of seismic effects on the south-eastern territory of table v. distribution of damaging seismic effects (imin = 5 ems) on sofia during the period of regular seismological observations in bulgaria, 1892-2002. the seismogenic regions are shown in fig. 4a-c, plot c (here: numbering in brackets). seismic impact source region relative amount of effects, % maximum intensity in sofia magnitude caused imax [study (*)] event causing maximum effect area of r ≤ 30 km centred at sofia (1) 40.0 7 5.1 [gra78] 18 october 1917 struma valley and rila mt. (5) 32.5 6/7 7.2 [amb01] 4 april 1904 maritsa valley (7) 4.0 6 7.0 [gra78] 18 april 1928 vrancea (12) 4.5 5/6 7.2 [cor79] 4 march 1977 macedonia (4) 5.0 5 6.7 [sha74] 8 march 1931 ne bulgaria (11) 2.5 5 7.0 [gra78] 14 june 1913 turkey (10) 4.5 5 7.3 [sha74] 9 august 1912 unknown epicentre 7.0 5 – 4 april 1904 generalized 100 7 6/7 local source: 5.1 external source: 7.2 1917 1904 (*) sha74 = shebalin et al. (1974); amb01 = ambraseys (2001); cor79 = cornea and radu (1979). table vi. time changes of knowledge in the earthquakes felt in the southeastern bulgarian territory. time period all events affected se bulgaria events in turkey only amount % 5th cent. b.c.-19th cent. before the bulgarian seismological survey establishment 113 63 55.7 5th cent. b.c.-11th cent. 34 29 85.3 12th cent.-1891 79 34 43.0 after the bulgarian seismological survey establishment (since 1892) 19th cent.-21th cent. (incl. aftershocks of strong events in local zones, maritsa valley, sw bulgaria & halkidiki) 177 36 20.3 (1991, 1995) and guidoboni et al. (1994) was used. the strength of excitation in bulgaria is calculated combining these catalogue parameters with the intensity attenuation based on well documented macroseismic patterns. around 290 seismic effect observations on south-eastern bulgaria have been identified for the time span since 5th century b.c. till now. the systematization in table vi clearly shows: a) poor information about the earthquake sequences in bulbulgaria has never been studied in detail. the forthcoming gas transmission via this territory and the importance of this area because of concentration of industrial and tourist sites in it encouraged this investigation. for the time before the beginning of 20th century, catalogues of the other balkan countries have been applied. for effects from turkey, a catalogue sketched on the basis of coeval descriptions derived by ambraseys and finkel 718 rumiana glavcheva fig. 5a-d. time seismic history of the southeastern territory of bulgaria, since the 5th b.c. (case g): frequency of impacts from all earthquake sources (a); maximum strength (intensity) per year in site 1 the strongest impacts come from local origins (b), in site 2 the strongest impacts being from the western marmara sea region (c), and in site 3 either from the western marmara sea or edirne area (d); site 1 to site 3 are marked in fig. 1. a b c d 719 state-of-the-art of historical earthquake investigation in bulgaria garia before the establishment of the national seismological survey, and b) a good knowledge on damaging historical earthquakes in turkey, especially since the 18th century. the effects on three different sites (marked in fig. 1) are shown in fig. 5a-d. intensity not exceeding 7 msk is noticed there. consequently, there is no reason to preserve intensity 9 msk by the bulgarian code in the territory adjacent to turkey. bearing in mind the contemporary growth of the urban and industrial settlements and hence the increased density of population, the value of maximum 8 msk might be the most realistic one. 5. comments and conclusions the development of non-instrumental seismology in bulgaria has been reviewed. tracing the history of previous earthquake catalogues and isoseismal maps, the main sources of initial descriptive information are discussed. it is concluded that these sources are qualitative enough but not uniform through the centuries. for instance, the new pieces of information on domestic events, recently disclosed, cover the time after the bulgarian renaissance started in the second half of the 18th century. they proceed from greek, bulgarian, slavonic sources but not from ottoman ones. concerning the five-century ottoman domination especially, the historian gradeva (1999) concludes: «the ottoman sources of local issuance or keeping are rather a poor source for the reconstruction of seismic activity in the balkans through the centuries». how difficult it is to have successful searching for bulgarian earthquakes in ottoman sources, proceeds from the impressive efforts of ambraseys and finkel (1999): «... of some 500 earthquakes known from non-ottoman sources for the balkans during 1500-1800, only 41 were found in ottoman sources but none in bulgaria or macedonia». the time before the 18th century has been too slightly known by the seismology of bulgaria. another important topic is how fully and how far critically the information sources have been exploited. in this connection, the macroseismic materials have been inspected; it is concluded that: 1) the final macroseismic products – bulletins, catalogues, isoseismal and other intensity distribution maps – have not been prepared under clear requirements, and 2) some of the macroseismic materials have been used directly as supporting datasets. that is why the historical earthquake investigation since the last 20th century’s decade has been directed to the supporting dataset improvement. the cases considered above are developed on reappraisal of first-hand primary information and for long-term dataset completing, the regional intensity attenuation features are seldom applied to the catalogue entries of strong past earthquakes (cases f, g). when interpreting and processing the initial records, they are managed in a specific way from case to case. the lessons we have learnt are: – to avoid mixtures of firstand secondhand information sources, or of different intensity scales; – to read the primary records many times and very carefully; – to distinguish events of a series by the «relative time of occurrence»; – to prefer records from distant sections of the felt area in cases after a strong earthquake occurrence; – to check the authenticity of strong catalogue events, especially of those proceeding from a very poor dataset. generalizing, (1) the main factors, which might contribute to the improvement of the supporting dataset, turn out to have been: i) flexibility of the initial information management; ii) applying techniques recently involved in the historical seismology, or developing of new ones; iii) borrowing experience from the instrumental seismological monitoring, and (2) after supporting dataset improvement, the consecutive upgrading of knowledge can be achieved in two aspects: i) the key events and the overall seismicity of the region (cases c, d); ii) understanding of to what extent a certain territory is risky. acknowledgements this work has been accomplished thanks to the helpful discussions within the framework of the esc working group «historical seismolo720 rumiana glavcheva gy». the author is grateful to the referees for their reasonable suggestions. references ambraseys, n.n. 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(1952): les tremblements de terre en bulgarie 721 state-of-the-art of historical earthquake investigation in bulgaria ressentis pendent les annees 1946-1950, in: annual issue of directorate of geological and mine investigations (sofia) 5, 442-491 (in bulgarian, resume in french). karnik, v. (1971): seismicity of the european area, part ii (d. reidel publ. comp., dordrecht, holland), pp. 218. kirov, k. (1931): tremblements de terre en bulgarie ressentis pendant les annees 1917-1927, sofia (in bulgarian, abstract and tables in french). kirov, k., e. grigorova and n. ileff (1960): contribution to the seismicity of bulgaria. izv. geophys. inst., bulg. acad. sci., 1, 137-183 (in bulgarian). mallet, r. (1853): catalogue of recorded earthquakes from 1606 b.c. to a.d. 1850, brit. ass. rep., 1853, 1-176. mihailovic, j. (1932): data on south bulgarian seismicity 1749-1931, spomenik srb. kr. akad., lxxviii (17), pp. 153. milne, j. (1911): a catalogue of destructive earthquakes a.d. 7 to a.d. 1899, baas (london), pp. 92. montandon, f. (1953): les tremblements de terre destructeurs en europe (genève), pp. 195. mushketov, i.v. and a.p. orlov (1893): catalogue of earthquakes in russian empire, 26, p. 582 (in russian). perrey, a. (1850): memoire sur les tremblements de terre ressentis dans la peninsule turco-hellenique et en syrie, in memoires couronnes et memoires des savants etrangers, acad. r. sci. de belgique, bruxelles, 23 (1), p. 50. prochazkova, d. and v. karnik (editors) (1978): atlas of isoseismal maps for central and eastern europe (geophys. inst. czechosl. acad. sci., prague). schmidt, j. (1879): studien ueber vulkane und erdbeben, ii, leipzig. shebalin, n.v. (editor) (1974): atlas of isoseismal maps, undp-unesco survey of the seismicity of the balkan region, part iii, skopje, 275 maps. shebalin, n.v., v. kárník and d. hadzievski (editors) (1974): catalogue of earthquakes of the balkan region, undp-unesco survey of the seismicity of the balkan region, part i and part ii, skopje, pp. 600. sieberg, a. (1932): die erdbeben, in handbuch der geophysik, edited by b. gutenberg (berlin), vol. iv. sokerova, d., s. rizhikova and r. glavcheva (1982): catalogue of earthquakes in bulgaria during the period 1970-1980, in national report of bulgaria, undp/unesco and undro project rer/79/014 earthquake risk reduction in the balkan region, working group a: seismology, seismotectonics, seismic hazard and earthquake prediction, final report, athens, december 1982, a47-a49. staikoff st. (1930): materiel sur la seismographie de la bulgarie, la thrace et la macedoine, j. bulg. acad. sci., xlii, sofia, pp. 15-45. watzof, s. (1902, 1903,..., 1923): the earthquakes in bulgaria. report on the earthquakes felt in xixc., and during 1901, 1902,..., 1913-1916, centr. meteorol. inst., (sofia), 1-17 (in bulgarian, in french). watzof, s. (1908, 1912): materials about the seismography of bulgaria. information on previous centuries earthquakes, spisan, ban, 127-134, 225-228. a l c u n i r i s u l t a t i d i u n a r i c e r c a s u l l a d e n s i t a d e i c o r p i raeteorici ( * ) f . v b r n i a n i ( * * ) r i c e v u t o il 18 maggio 1963 riassunto. — dopo u n a b r e v e discussione delle cause dell'incertezza a t t u a l m e n t e esistente al r i g u a r d o della d e n s i t a e della s t r u t t u r a dei corpi meteorici, viene p r e s e n t a t o u n i n e t o d o p e r calcolare la d e n s i t a delle m e t e o r e utilizzando d a t i fotograflci. i n f l n e v e n g o n o esposti i r i s u l t a t i o t t e n u t i dall ' a u t o r e a n a l i z z a n d o d a t i p u b b l i c a t i a d h a r v a r d . t a l i r i s u l t a t i m o s t r a n o c h i a r a m e n t e clie quasi t u t t e le m e t e o r e h a n n o u n a d e n s i t a m i n o r e di 1 g c m 3 e q u i n d i u n a s t r u t t u r a p o r o s a in accordo con il modello c o m e t a r i o di w h i p p l e . summary. — a f t e r a brief discussion a b o u t t h e causes of t h e p r e s e n t u n c e r t a i n t y on t h e d e n s i t y a n d s t r u c t u r e of meteoroids, a m e t h o d f o r comp u t i n g t h e m e t e o r d e n s i t y is p r e s e n t e d . e v e n t u a l l y t h e results, o b t a i n e d f r o m t h e analysis of p h o t o g r a p h i c d a t a p u b l i s h e d a t h a r v a r d , are s h o w n . a l m o s t all meteors a p p e a r t o h a v e a d e n s i t y less t h a n 1 g c m 3 a n d a p o r o u s s t r u c t u r e , in a g r e e m e n t w i t h w h i p p l e ' s icy-comet model. i . i n t r o d u z i o n e . n o n o s t a n t e gli e n o r m i p r o g r e s s i r e g i s t r a t i d a l l a fisica e d a l l ' a s t r o n o m i a delle m e t e o r e i n q u e s t i u l t i m i a n n i (***), p e r m a n e u n a f o r t e i n e e r t e z z a sulla s t r u t t u r a , s u l l a e o m p o s i z i o n e e s u l l a d e n s i t a d e l l e p a r t i celle s t e s s e . (*) n o t a p r e s e n t a t a al 2° congresso i n t e r n a z i o n a l e tecnico scientifico dello spazio. r o m a , 19-23 griugno 1962. (**) centro n a z i o n a l e p e r l a fisica d e h ' a t m o s f e r a del c . n . r . r o m a . (***) n . d . a . p e r u n r a p i d o p a n o r a m a s u l l ' a r g o m e n t o si v e d a la relazione p r e s e n t a t a d a h ' a u t o r e alia v i r a s s e g n a elettronic.a nucleare del 1959 (l). 1 6 8 f . v e r n i a n i £ noto ehe fino a pochi anni f a era universalmente r i t e n u t o clie t u t t e le meteore avessero u n a composizione chimica e u n a s t r u t t u r a analoga a quella dei meteoriti; p e r t a n t o , essendo la quasi t o t a l i t a dei meteoriti costituita da silieati, si riteneva che quasi t u t t e le meteore fossero particelle di n a t u r a pietrosa con u n a densita di circa 3.4 gem-3. si pensava poi che la p a r t e rimanente, in corrispondenza alia componente ferrosa dei meteoriti, fosse costituita essenzialmente da ferro contenente u n a piccola percentuale di nichel (densita q & 8 gem-3). ma nel 1950 whipple (2) elaboro un modello riguai'dante la costituzione delle comete che e stato in seguito generahnente accettato: secondo questo modello, il nucleo della cometa deve essere f o r m a t o da piccole particelle minerali, immerse in u n conglomerato di gas (h 2 0, n h a , cii4, c02, c2x2) alio stato solido. quando la cometa penetra in u n a regione dove la radiazione solare comincia ad essere sensibile, le sostanze volatili vaporizzano e liberano le particelle minerali. le osservazioni spettroscopiche di alcune comete mostrano la presenza di cn, c2, co+ e di altre molecole instabili che possono provenire dalle sostanze stabili solidiflcate. poiche diversi sciami di meteore si possono chiaramente collegare con alcune comete a causa della grande somiglianza dell'orbita, e logico supporre che le particelle eostituenti questi sciami provengano dalla disintegrazione delle comete o comunque siano dovute a materiale emesso dalle comete stesse. cos! la densita delle meteore di origine cometaria deve essere nett a m e n t e minore di 1 gem-3 e la s t r u t t u r a delle stesse deve essere porosa, cioe il corpo meteorico va pensato come qualcosa di spugnoso costituito dagli ordinari materiali contenuti nei meteoriti. l'ammissione che gran p a r t e dei corpi meteorici abbia u n a s t r u t t u r a porosa p e r m e t t e anche di spiegare il fenomeno della frammentazione, scoperto da jacchia nel 1955 (3). le idee dei maggiori esperti in m a t e r i a sono pero alquanto discordanti, sia sulla percentuale di meteore porose, sia sulla densita delle stesse. l'incertezza e dovuta anche all'attuale impossibilita di determinare con sicurezza la massa delle meteore. £ noto i n f a t t i che l'intensita luminosa i emessa da u n a meteora in u n a certa regione dello spettro e proporzionale alia energia cinetica p e r d u t a dalla meteora nell'unita di tempo: i = • 1 dm, t v ~— 2 dt [1] r i s u l t a t i d i u n a r i c e r c a s u l l a d e n s i t a d e i c o r p i m e t e o r i c i 1 6 9 essendo v la veloeita della meteora, m la sua massa e t l'efficienza luminosa nella regione spettrale considerata. dalla formula [1] si ha subito il valore m0 della massa della meteora f u o r i dell'atmosfera terrestre: oo » , = 2 i dt. [2] j tv2 — oo p u r t r o p p o il valore del coefflciente r e la sua dipendenza dalla veloeita sono noti in modo p i u t t o s t o malsicuro; si accetta generalmente per r u n a dipendenza lineare da v : r = t0 v e per r0 un valore calcolato nei 1933 da opik (4) in un modo che h a suscitato diverse critiche ( 5 , 6 ). eecentemente cook e whipple (7) hanno elaborato u n nuovo metodo indiretto per determinare le masse delle meteore m a hanno p o t u t o applicarlo in un solo caso. d a quest'unica valutaziohe essi hanno dedotto che la scala classica delle masse (quella corrispondente al valore di r0 calcolato da opik) deve essere moltiplicata per un f a t t o r e dell'ordine di 200 e n a t u r a l m e n t e per lo stesso f a t t o r e deve essere diviso il veccliio valore di r 0 . ma la difficult a di determinare la densita delle meteore e dovuta anche ad altre cause, come la difflcolta di v a l u t a r e il coefficiente di frenamento e l'energia di ablazione £ e s o p r a t u t t o l'impossibilita di conoscere la f o r m a del meteoroide stesso. cosi il f a t t o che le orbite di molte meteore mostrino analogic, con quelle delle comete (8), m e n t r e solo in casi eccezionali si pu6 notare u n carattere tipicamente asteroidale •— u n i t a m e n t e ai fenomeni di f r a m mentazione mostrati dalle meteore fotograficlie meno brillanti — h a p o r t a t o a ritenere che quasi t u t t e le meteore abbiano una n a t u r a porosa con u n a densita anche molto bassa e che soltanto poche (alcune delle piu brillanti) siano corpi solidi nei senso ordinario (9). l a notevole incertezza esistente in questo campo ha spinto l'autore a compiere u n lungo studio sulla densita, dei corpi meteorici, utilizzando t u t t i i dati pubblicati. ne esporremo ora le conclusioni. i i . e s p o s i z i o n e d e i risultati. l ' a u t o r e ha analizzato i dati fotograflci riguardanti 512 meteore, pubblicati in massima p a r t e da whipple (10) e da hawkins e southworth ("• 12): di queste, 137 sono meteore molto brillanti (fireballs) aventi grandezza visuale negativa e le rimanenti 375 sono meteore piu piccole di grandezza positiva. 1 7 0 f. v e r n i a n i i dettagli del procedimento impiegato per il calcolo delle densita, il complesso dei d a t i r i g u a r d a n t i le meteore brillanti e i r i s u l t a t i o t t e n u t i sono in corso di pubblicazione sul nuovo cimento ("); i r i s u l t a t i completi dell'analisi dei corpi piu piecoli saranno anche essi pubblicati t r a b r e v e sulla stessa rivista. r i m a n d a n d o a t a l i lavori il lettore che desiderasse maggiori notizie, ci limiteremo qui a ricordare la seguente equazione della teoria dell'ablazione delle m e t e o r e (14): qjo _ 3 (vq-+12 £) p m sec i yf sg£ l j essendo: q = densita del meteoroide, r0 = raggio equivalente del meteoroide f u o r i dell'atmosfera, y = coefficiente di f r e n a m e n t o medio, f = f a t t o r e di f o r m a (definite da «sezione d ' u r t o del m e t e o r o i d e » = fnr-), v0 — velocita della meteora f u o r i dell'atmosfera, f = energia e f f e t t i v a di ablazione per u n i t a di massa, definita da f = y l/a (i = calore l a t e n t e di ablazione per u n i t a di massa; a = coefficiente di t r a s f e r i m e n t o del calore), pm = pressione atmosferica all'altezza zm del massimo di i n t e n s i t a luminosa, = angolo zenitale della p a r t e osservabile della t r a i e t t o r i a della meteora, g = accelerazione di g r a v i t a . l e grandezze che compaiono nel secondo m e m b r o dell'equazione [3] sono d i r e t t a m e n t e misurabili come v0 e ^ oppure possono essere determin a t e i n d i r e t t a m e n t e , come pm (da zm e da u n modello s t a n d a r d di atmosfera) e come l'energia di ablazione f . cosi poiche la massa viene d e t e r m i n a t a m e d i a n t e l'equazione [2], se si f a l'ipotesi che la media a r i t m e t i c a dei f a t t o r i di f o r m a per u n gruppo a b b a s t a n z a numeroso di meteore sia uguale ad uno, cioe al valore che si avrebbe se t u t t e le meteore fossero p e r f e t t a m e n t e sferiche di p u 6 determ i n a r e la densita delle m e t e o r e stesse. p e r l'energia di ablazione £ sono s t a t i u s a t i i valori f o r n i t i da j a c chia (15) e p r e c i s a m e n t e per la fireballs e s t a t o a c c e t t a t o il valore medio | = 16 kin2s-2 m e n t r e per le meteore piu deboli (grandezza visuale por i s u l t a t i d i u n a r i c e r c a s u l l a d e n s i t a d e i c o r p i m e t e o r i c i 1 7 1 sitiva) e s t a t a a d o t t a t a la funzione £ = £ (v0) illustrata in fig. 1. molto complicate e stato il calcolo del valore medio del coefficente di frenamento e non e qui il caso di entrare in dettagli. (km 2;2) 12 10 8 e 4 2 0 10 2 0 30 40 so 60 70 80 fig. 1 ii v a l o r e medio di f i n f u n z i o n e della veloeita v0. (meteore s u p e r s c l i i m i d t ; j a c c h i a 1958) la tabella i mostra un quadro riassuntivo globale dei risultati o t t e n u t i sia per le meteore brillanti che per quelle deboli. le meteore sono s t a t e divise in t r e grandi classi: corpi compatti, aventi u n a densita maggiore di 1 gem 3 , corpi porosi con u n a densita compresa t r a 0.1 e 0.7 g e m 3 e « dustballs » aventi u n a densita inferiore a 0.1 gem 3 . dalla tabella i risulta chiaramente che la percentuale delle meteore « c o m p a t t e » e bassa (inferiore al 20%) men t r e la maggior p a r t e delle meteore ha u n a densita che e in buon accordo col modello della cometa ghiacciata di whipple, a cui abbiamo accennato. occorre dunque abbandonare definitivamente l'idea che le meteore abbiano u n a s t r u t t u r a fisica analoga a quella dei meteoriti e accettare invece l'idea che prevalentemente esse sono di n a t u r a porosa. i risultati riportati nella tabella i non mostrano differenze significative nelle percentuali dei vari tipi f r a meteore brillanti di sciame e meteore brillanti sporadiche. si puo notare invece qualche differenza nei caso delle meteore deboli: le meteore aventi un corpo compatto «di tipo classico» sono piu numerose t r a le sporadiche che non t r a quelle appartabella i l a d i s t r i b u z i o n e d e l l e m e t e o r e i n t r e g r u p p i f o n d a m e n t a l i s e c o n d o l a l o r o s t r u t t u r a f i s i c a . meteore sporadielie meteore di seiame t o t a l e b r i l l a n t i deboli b r i l l a n t i d e b o l i b r i l l a n t i deboli n° 0/ /o /o /o n° /o n° /o n° 0/ /o corpi e o m p a t t i (e > 1 g e m 3 ) 8 16.67 67 23 51 13 14.61 14 15 56 21 15.33 81 21 60 corpi porosi (0.1 g e m 3 < g< 0.7 g e m 3 ) 22 45.83 205 71 93 44 49.44 70 77 78 66 48.19 275 73 33 corpi e s t r e m a m e n t e porosi (dustballs) (e < 0.1 gem3 ) 18 37.50 13 4 56 32 35.95 6 6 66 50 36.48 19 5 07 v r i s u l t a t i d i u n a r i c e r c a s u l l a d e n s i t a d e i c o r p i m e t e o r i c i 1 7 3 tenenti a qualche sciame, m e n t r e al contrario le porose sono leggermente piu nuinerose t r a le meteore di sciame clie non t r a le sporadiche. piuttosto sorprendente si rivela la notevole differenza esistente t r a meteore brillanti e meteore deboli: contrariamente a quanto ci si potrebbe aspettare, la percentuale dei corpi solidi e piu numerosa t r a le piccole particelle clie danno luogo a meteore deboli che 11011 t r a i corpi piu grossi. inoltre la percentuale di meteore di n a t u r a estremamente porosa (densita minore di 0.1 gcm~3) e piuttosto alta t r a le meteore brillanti (36% circa) m e n t r e e molto bassa (5%) t r a quelle deboli. u n a spiegazione qualitativa di questo f a t t o puo essere ricercata nella probabile f r a m m e n t a z i o n e a cui sono soggetti in maggior misura i corpi piu grossi: questo p u n t o di fondamentale importanza andra studiato con molta cura; u n esame della correlazione t r a massa e grandezza fotograflca (o visuale) da un lato e densita dall'altro dovrebbe fornire notevoli elementi chariflcatori al riguardo. i risultati qui presentati sono s t a t i o t t e n u t i prendendo per buoni i valori delle masse calcolati col veccliio coefflciente di efficienza luminosa; se si introducesse il f a t t o r e 200 proposto da cook e whipple del quale abbiamo precedentemente parlato, otterremmo per la densita delle meteore dei valori estremamente bassi, assolutamente inaccettabili. si puo quindi affermare che il valore di r 0 calcolato da opik da il giusto ordine di grandezza, anche se c e r t a m e n t e la sua precisione e scarsa. u n a tale conclusione e avvalorata da u n lavoro di mc crosky (16), che dall'analisi dei dati r i g u a r d a n t i u n a meteora sicuramente asteroidale h a trovato per r 0 un valore compatibile con quello di opik. la determinazione dell'efficienza luminosa con i metodi moderni della fisica quantistica appare estremamente difficile, dato il gran numero di casi in cui pu6 essere emessa luce nell'interazione di un atomo evaporato dalla meteora con u n a molecola d'aria. un po' meno complicato si presenta invece il problema di determinare teoricamente la probability di ionizzazione che u n atomo, distaccatosi dal corpo meteorico, produca un elettrone libero: da questa, attraverso u n a relazione empirica che lega r con /5 p o t r a essere t r o v a t o un valore di r„ piu attendibile e la giusta dipendenza di r dalla veloeita. i l opportuno concludere questa breve relazione ricordando la necessity di svolgere un calcolo analogo per le meteore piu piccole, osservabili solamente mediante u n r a d a r . i risultati che scaturiranno dallo studio della densita delle radiometeore dovrebbero consentire di determinare, se essa esiste, la massa minima (e quindi la dimensione minima) sotto la quale le particelle meteoriche tornano ad avere la normale consistenza. 1 7 4 f. v e r n i a n i b i b l i o g r a f i a i1) v e r n i a n i f . , atti u f f . del congres'so scientifico sezione elettronica della vi rassegna internaz. elettronica e nucleate, ( 1 9 5 9 ) . ( 2 ) w h i p p l e f . l „ astrophys. j., ill, 3 7 5 ( 1 9 5 0 ) . ( 3 ) j a c c h i a l . g . , astrophys. j. 1 2 1 , 5 2 1 ( 1 9 5 5 ) . ( 4 ) o p i k e . j . , harvard reprint series i no. 1 0 0 ( 1 9 3 3 ) . ( 5 ) t h o m a s r . n . and. w h i p p l e f . l . , astrophys. j., 114, 448 (1951). ( 6 ) w h i p p l e f . l . a n d h a w k i n s g . s . , eandbuch der physih 5 2 ; astrophysik 3 ( 1 9 5 9 ) . ( ' ) w h i p p l e f . l . , harvard reprint no. 4 6 5 ( 1 9 5 7 ) . ( 8 ) j a c c h i a l . g. a n d w h i p p l e f . l . , smithsonian contributions to astrophysics 4, 97 (1961). ( 9 ) m c k i n l e y d . w . r . , « meteor science and engineering)), (new york 1961). ( 1 0 ) w h i p p l e f . l . , astron. j., 5 9 , 2 0 1 ( 1 9 5 4 ) . ( u ) h a w k i n s g . s . a n d s o u t i i w o r t i i r. b., smithsonian contributions to astrophysics, 2, 3 4 9 ( 1 9 5 8 ) . ( 1 2 ) h a w k i n s g-. s. a n d s o u t h w o r t i i r. b., smithsonian contributions to astrophysics 4, 85 (1961). ( 1 3 ) v e r n i a n i f . , nuovo cimento, 2 6 , 2 0 9 , ( 1 9 6 2 ) . ( 1 4 ) v e r n i a n i f . , nuono cimento, 19, 4 1 5 ( 1 9 6 1 ) . ( 1 5 ) j a c c h i a l . g . , smithsonian contributions to astrophisics 2 , 1 8 1 ( 1 9 5 8 ) . (16) mc c r o s k y r. e . , astron. j., 65, 493 (1960). t h e mechanism of the earthquakes in t e r m s of the dislocation t h e o r y s. d r o s t e r . teisseyre da ring the last years there originated quite a number of model theories of seismic foci. in such models the dynamic proces in the focus is substituted by the action of a classic system of equivalent mass forces. among works on this subject the publications of the school of w . i. keylis-borok, as well as of h . honda should be mentioned in the first order. let us consider the simplest model of a focus, that is the case of a simple point force. the expression for displacement ii in such a case is given by the classic love's formulas. the dependence of the force from time is here expressed by an arbitrary function x (t). if this function will be discontinuous, as for example in case of a step function (x = 0 for t < 0, x = 1 for t > 0), than love's formula for u determines the creation of a infìnitesimal dislocation element at time t = 0. this problem is discussed in detail by nabarro (10). in this manner model theories represent the creation of dislocation elements for such cases in whicli the time dependence of active forces is discontinuous. to this category there belong also the publication of a. wwiedenskaja (i 2) on the mechanism of earthquakes, represented as a creation of a finite dislocation of the disc type. formally her work differs from the preceding group, because of direct use of the theory of physical dislocation. the localisation of the dislocation created — should be determined by the stresses occurring in the medium however no such relation is clearly given by the theories discussed above. w e should like to present a new theory of the mechanism of earthquakes. the starting point of this theory is the assumption, that there exists a nonhydrostatic part of stress in the medium, as well as a field of inhomogenities in the largest sense. such inhomogenities, even very 180 s. d r o s t e l l . t e i s s e t r e small in size, may serve as points of attachment. they institute the nucleus of dischargement of the strain energy occurring in the medium and together with stresses they determine the earthquake's area. these obvious assumptions may be considered further in the light of the theory of physical dislocations, which may be interpreted as the theory of inhomogenities in a field of forces. the theory of physical dislocation was developped in the solid state physics. before going further on we will stop for a short while on basic deflnitions and consequences of the physical dislocation theory. the simplest example is represented by linear dislocation. classically is will be defined, as fouows: a cut is executed along a chosen halfplane and afterwards the material on both sides of the cut is displaced relatively by a constant weetor b (what is called burgers' vector). the edge of the dislocated halfplane is called dislocation une. the main part of the selfenergy of dislocation, determined as the energy of the deformation created, is acumulated in the neighbourhood of the dislocation line. the displacement of the dislocation line gives an increase in the dislocation area. i t may be said that the potential energy of physical dislocation expresses the tendency of the whole medium's deformation. this potential energy is liberated by the movement in this moment when the dislocation reaches the medium's boundary. in order to defìne fully dislocation burgers' sing convention which serves to determine the relation between the circulation along the dislocation line and the direction of relative displacement of the medium on both sides of dislocation should be used. the dislocation vector b which is situated in the cut piane (which is called also the slip piane) may be parallel to the dislocation une — in this case the dislocation is of the screw type, or it may be vertical to the dislocation line in which case we cau the dislocation edge dislocation. the external stress field exerts it's influence on the dislocation through interaction with the field of dislocation. this influence may be expressed as the action of force on an element of the dislocation une da f = ( p -b) x da [1] where p being the stress tensor. similary tliere exist an interaction between dislocation. dislocations of opposite signs are attracted to each other, of the same signs are being repulsed. these forces are inversely proportional to distance between them. t i i e m e c h a n i s m of t h e e a r t q u a k e s , e t c . 1 8 1 dislocation which have curved dislocation lines, especially, closed ones, will serve as a generalisation of dislocations discussed above. dislocations having curved closed dislocation lines are called loop dislocations. w e will consider now very small loop dislocations. these liave interesting properties, which deserve more detailed study. w e are considering now dislocations situated in a common slip piane. loop dislocations of the same signs are now attracted to eacli other. this is caused by the fact that the elements of dislocations situated nearest one to each other have opposite signs. a s the result of such atraction two loop dislocations may approach each other and afterwards join into one. a s a consequence of such a synthesis a dislocation comprising a greater area is created. different kinds of dislocations m a y arise by means of a chain of such syntheses. n o w to the basic assumptions of the present work we will add that the value of the nonhydrostatic part of the stress field and density of inhomogenities of the medium are extrenally great in the neighbourliood of a suppossed piane (y = 0). a s we will see this piane and it's neighboui'hood constitute the stage upon which dislocative processes and accompaning earthquakes take place. this piane, together with it's neighbourhood institute the area of seismic activity. i t may be called also the hypocentric piane. l e t us assume that the nonhydrostatic stresses are represented by the shear stress pyz. n o w we can proove tliat the constant fìeld pyz on the piane may be represented by an equal distribution of loop microdislocations characterized by burgers vector b, and by normal vector of the direction — y. if n will represent the number of microdislocations for an unit of surface and o0 will represent their radius, than we will obtain: here we can go to the limit n = oo, bz 0, q„ -> 0, but under the condition lim = const. assuming the existence of inhomogenities on the considered piane, bigger loop dislocations will arise around them under the influence of the field pyz. a s an example let us consider the extreme case of inhomogenities in the forni of small incompressible intrusions. inside the area occupied by such an intrusion no microdislocation will appear because of the incompressibility of the medium constituting this intrudo 182 s. d r o s t e l l . t e i s s e t r e sion. instead there appears a bigger loop dislocation around the intrusion. in this manner inhomogenities quantize, as it were, the field pyz. the loop dislocations created around inhomogenities we will consider as dementar loop dislocations. i t may be said that dementar loop dislocations represent inhomogenities in the field pyz. later the action of a sufficiently great field pyz and the interaction between dislocations causes their deformation by an increase in surface which dislocation comprises. now we must assume that a fluctuation in density of inhomogenities exist on the considered piane. i t could be now said also in other words that there exist a fluctuation in the density of loop dislocations. in the region of the maximal concentration of inhomogenities, there arise a greater dislocation by it's own growth and by syntheses with neighz 1 u a ®òz 4 u a ®òz 4 t t , ^ t x fig. 1. bouring dislocation. the creation of greater dislocation elements may be of violent nature giving source to seismic shocks. i t should be mentioned, however, that the greater part of the work of the field pyz is consumed by an incorease in internai energy of the dislocation. therefore it is to note, the earthquakes caused by violent creation of dislocations or of their movements will belong to the weaker category. the dislocation created in the manner described above may be represented by a pair of edge dislocations limited by sides of screw dislocations (fig. 1). in an extreme case we obtain a pure pair of edge dislocations infinitely long, composed with two dislocation of opposite signs. further under the influence of the field pyz the dislocation 1+ and 1~ go asunder. this may occur under the condition that, the forces of the t i i e m e c h a n i s m of t h e e a r t q u a k e s , e t c . 1 8 3 field pyz must be greater, than the force of attraction, between the pair of dislocations (forces are usually expresses for an unit of length). w e will now pay attention to particular cases of dislocative processes on the discussed piane. firstly we will examine the case when there exists one center of generation of dislocations. l e t us assume that it is concentrated along the line " 0 " (fig. 2). a fig. 2. many pairs of dislocations are created around the line " 0 " as the result of constant action of the field pyz. each of these pairs go asunder, as it was described above. i n consequence a situation as on drawing is created. i n such case the neighbouring dislocation 1+ and 2+ are of the sanie sign and are repulsing each other. this leads, as it were, to " jostling " of the extremely situated dislocation by dislocations situated near centre " 0 " towards external directions. 1 8 4 s. d r o s t e r . t f i s s e t r e when there exists also the second center " 0' " of generation of the dislocation ( 3 + , 3~), then the dislocation 1 + , 3~ will attract and approach to each other. in the area ( " b " ) the dislocations 3~ and 1+ will join and annihilate themselves, with accompanying dischargement of their energy. w e assume that initially the approach is slow, when they reach the distance l a violent movement will occur and the dislocations will join and disapear. their energy at distance l is (koehler (3)) where a being poisson's coefficient, r„ being a radius of the dislocation une (a small value), la the length of the dislocation. the expression [3] presents the selfenergy and interaction energy of the dislocation 3~ and 1 + . in the result of the movement the energy [3] is partly converted into kinetic energy and by the confluance of dislocations is discharged in the whole. the value of energy discharged will be of the same order of magnitude as in formule [3]. the work of the field pyz and of resistance forces was not taken into account. the displacement ù" originating from the annihilation of the dislocations may be expressed, as follows: where us reoresents the static field of a pair of edge dislocations (tabarro (10)): [3] a s c u = u u [4] ut = 0 x z u' (k 1 ~ , + j t i i e m e c h a n i s m of t h e e a r t q u a k e s , etc. 1 8 5 and uc represents the field of creation of a pair of edge dislocations (13): u° — 0 , x ' 1 c2 , _/ y2at , aat 4y2aat\(, r k ~ — — h for — < < y ti a2 \ r*a rl r" ) \ a 1 , _ / y2 et , get iy2cct , 1 et \ /, r \ • £ ^ o + -f v + 1 ( 1 e o r < '1' c l e 2 , ( z2at , aat 4z2aat k = 7.1t -iu ^ f o r < t -n a2 \ r1 a r4 t r a 1 , r ( z2 et gct 4 z2 g et , 1 et \ l r \ ~ ~ ^ + ^ + j wg ( f o r t ^ v ( a = j/a212 — r2 , g = |/c2<2 — r2 ) . the creation (annihilation) takes place at the moment t = 0. the annihilation of the dislocation 1 + and 3~ is accompanied by the confluence of the dislocation areas limited by them into one deformation area. n o w we must pay the attenuation to the region " a " near the earth's surface. the influence of the free surface may be expressed by means of the well known image method. the field of the dislocation in the presence of free earth surface may be expressed as the total of the dislocation field and it's image, the image being a dislocation of opposite sign. such resultant field leads to disparition of stresses on the earth surface. i n such a manner the action of the free earth surface may be represented as the interaction between the dislocation and it's image. this action manifests itself always as an attraction of the dislocation to the earth surface. naturally this will concern in first order the dislocation 3 + and after 2 + . i n consequence of movements of dislocation 3+ the dislocation area approach to the earth's surface. when the dislocation reaches it, then it's stresses will be released. a s in this case the field of stresses equals the total of the dislocation and it's image, the energy of dischargement will be expressed as in the former case but with a coefficient equals to 1/2 * 4 r ^ i c r b r ) z ° v ( 5 ) 1 8 6 s. d r o s t e l l . t e i s s e t r e where l represents now the distance between the earth's surface and the dislocation at this moment, when the rapid movement of the dislocation in the direction of the surface begins. one would say, at this moment when the dislocation is about to leap to the earth surface. i n the area " c " the dislocation 2~ moves deep into the earth accompanied by the deformation of the medium. i n such a manner the slip increases also in this direction. i t may be postulated that in deep strata of the earth there will also occur a dischargement of dislocation stresses. the character of this dischargement, however, may be more graduai and it is difficult to say something precise about it at this moment. i will amplify the above, presenting a f e w data concerning the earthquakes of type " a " that is to say of this type in which the dischargement of energy take place on the earth's surface and the deformations represented by dislocations reach this surface. for example let us consider several great earthquakes for which the length and magnitude of slip of dislocations on the earth surface are known (table 1). the mechanism of earthquakes presented above constitutes at the same time a theory of origin of dislocations. from the above considerations follows that they originate as the result of displacement of physical dislocations along a given piane. the physical dislocations are connected closely with the existance of stresses and inhomogenities of the media. table 1 earlhquake length of slip lo magnitudo ni slip k energy (lormula |5|) magnitudo of earthquake m ennrgy irnm eq. : loge = u+l,6m california 450 km 7 m 2 1524 d erg 8 1/4 1,6 io24 erg 1906 8 1/4 io24 erg nevada 30 km 5 m 1io23 d erg 7 3/4 2,1 io23 erg 1915 7 3/4 io23 erg mino-ovari 65-120km 7 m 3-5 io23 d erg 7 1/2 1,0 io23 erg 1891 d erg 7 1/2 io23 erg assam 20 km 12 m 2,5io23 d erg 8 1/2 4,0 io24 erg 1897 8 1/2 io24 erg d = lo t, 1 d = lo cr — d = lo ° r 0 4(1 o r ) " — • — d being of order 2-4 t i i e m e c h a n i s m of t h e e a r t q u a k e s , e t c . 1 8 7 to the end of this communication we shoud like to add that the field pyz acting in the neighbourhood of the piane y = 0 characterized by great inhomogenities, acts also on neighbouring elements of the surfaces z = const. the processes occurring in these planes will be of course less important and may be considered as only accompaning the basic plienomena along the piane y = 0. riassunto viene esposto un nuovo meccanismo per terremoti, alla luce della teoria delle dislocazioni. i presupposti base sono resistenza di una parte non idrostatica delle tensioni nel mezzo (py.) e una certa distribuzione delle inomogeneità. questa distribuzione determina un piano e il suo intorno, come il piano del processo di dislocazione attiva (piano di slittamento, y = 0). si dimostra che le inomogeneità possono essere rappresentate da dislocazioni a cappio, in presenza del campo pyz. le dislocazioni a cappio possono aumentare sotto l'influsso del campo pyz e sotto l'interazione fra le medesime (sintesi dei cappi). come risultato, conseguono coppie di dislocazioni torcenti e di confine. allora le dislocazioni della stessa coppia sono costrette, dall'intenso campo pyz, a procedere separatamente. l'area di scorrimento, in tal modo, aumenta. la fusione delle due dislocazioni associate alle coppie vicine, deternami il loro annientamento e la liberazione della loro energia, il che provoca un ulteriore .aumento dell'area di scorrimento omogeneo. similmente, quando la dislocazione raggiunge la superficie terrestre, la sua energia verrà liberata (nel qual caso, va preso in considerazione il fattore % nci confronti con il caso precedente). il confronto dell'energia liberata, così determinata, è in buon accordo con i dati di osservazione. abstract a new mechanism of earthqualces is evolved in the light of the physical dislocation theory. the basic assumptions are the existence of a nonhydrostatic part of stresses in the medium (pyz) and a supposed distribution of inhomogenities. this distribution determines a given piane and it's neighbourhood, as the piane of active dislocation processes (slip piane, y = 0). 188 s. d r o s t e r . t e i s s e y r e it is shown that the inhomogenities could be represented by the loop dislocations in the presence of the field pyz. the loop dislocations may increase under the influence of the field pyi and under the interaction between themselves (synthesis of loops). as the result, there arise pairs of the screw as well as edge dislocations. then the dislocations from the same pair are constrained by the strong field pyz to go asunder. in such manner the slip area increases. the blending of the tivo dislocations from the neighbouring pairs give their anniliilation and dischargement of their energy, this is accompanied by further increase in tlie homogeneus slip area. similarly when the dislocation reaches the earth's surface it's energy will be discharged (noiv the factor 1/2 in the comparison with the preceding case must be taken into account). the comparison of the dischargement's energy is in good agreement with the observation data. r e f e r e n c e s ( ! ) b i l b y , b . a . , proc. phys. soc. a., 6 3 , 191 ( 1 9 5 0 ) . ( 2 ) b u r g e r s , i . m . , proc. acad. sci. amst., 4 2 , 2 9 3 ( 1 9 3 9 ) . ( 3 ) e s i i e l b y , i . d „ proc. boy. soc. a., 1 9 7 , 3 9 6 ( 1 9 4 9 ) . (4) — phil. trans. boy. soc. lond. a., 2 4 4 , 8 7 7 , 87 ( 1 9 5 1 ) . ( 6 ) — f r a n k , f . c . , n a b a r r o , f . e . n „ pliil. mag., 4 1 , 3 5 1 ( 1 9 5 1 ) . ( " ) f r a n k , f . c . , proc. phys. soc. a.* 6 2 , 1 3 1 ( 1 9 4 9 ) . ( ' ) h o n d a , h . , sci. bep. tolioku univ., ser. 5, geopliys. 9 , supp. ( 1 9 5 7 ) . ( « ) k e y l i s b o r o k , w . i . , d. a. n. ussb., 7 0 , 6 ( 1 9 5 0 ) . ( " ) k o e i i l e r , i . c . , phys. bev., 6 0 , 3 9 7 ( 1 9 4 ). ( 1 0 ) n a b a r r o , f . r . n . , phil. mag., 1 2 2 , 1 2 2 4 ( 1 9 5 1 ) . ( " ) p e a c h , m . , k o e h l e r , i . c . , phys. bev., 8 0 , 4 3 6 ( 1 9 5 0 ) . ( 1 2 ) w w i e d e n s k a j a , a . w . , izw. a. n. ussb, 3 , 2 7 7 ( 1 9 5 6 ) . (13) teisseyre, r., ada geophysica polonica (in press). v patella.pdf 589 annals of geophysics, vol. 46, n. 3, june 2003 on the role of the j-e constitutive relationship in applied geoelectromagnetism domenico patella dipartimento di scienze fisiche, università «federico ii», napoli, italy abstract in current applications of the induced polarization (ip) method, the debye and cole-cole models are used to study relaxation and dispersion properties of rocks, though it is believed that this type of modelisation is confused and vague, because of the lack of a background physical description. in this paper, we show that the debye model can physically be deduced as a consequence of the electrodynamic behaviour of a mixture of bound and unbound charged particles immersed in an external electric field. we also clarify that the cole-cole model is a synthetic model, which can physically be explained as a continuous distribution of debye terms. mailing address: prof. domenico patella, dipartimento di scienze fisiche, università «federico ii», complesso universitario monte sant’angelo, via cintia, 80126 napoli, italy; e-mail: patella@na.infn.it key words rock electrodynamics – induced polarization – impedivity models 1. introduction electrical relaxation and dispersion in rocks are observed in the time-domain (td) and frequency-domain (fd), respectively, using any standard induced polarization (ip) device. these effects are often investigated in mining and environmental exploration. figure 1a-c shows schematically the ip response of a polarizable medium, prospected, e.g., by a quadrupolar electrode device (fig. 1a). in the td, a voltage transient is detected across the two receiving electrodes, after both the onset and the shutdown of a current step excitation through the two emitting electrodes, as depicted in fig. 1b. a remarkable ip feature is that at the onset of the excitation current, the voltage response suddenly jumps up from zero to a finite value and then gradually rises towards a stable value. accordingly, at the shutdown of the current, the voltage suddenly falls down to a finite non-null value and then gradually vanishes. in the fd, one observes a voltage-to-current ratio which is a complex function of frequency, showing amplitude and phase spectra as in fig. 1c. worth noting is the high-frequency asymptote of the amplitude curve, which is a horizontal straightline generally placed at a finite non-vanishing level, lower than that corresponding to the lowfrequency asymptote. despite such different behaviour, relaxation in td and dispersion in fd represent the same physical phenomenon and the relative responses are connected to each other via fourier transform (ft). thus, ip observations are the evidence of a complex form of conduction of electricity in rocks, which cannot be synthesized by ohm’s law. in fact, ohm’s law states that the current density is linearly related to the electrical field by a factor s, known as the conductivity, which is assumed independent of time t in the td, or frequency w in the fd. 590 domenico patella c rigorously speaking, ohm’s law can be assumed valid in rocks only when a direct current (dc) regime is utilized, as in the dc resistivity method. with variable currents, as in many natural or artificial source td or fd electromagnetic (em) methods, ohm’s law can be adopted provided that relaxation and dispersion effects are negligible, but in general this condition cannot be established beforehand. therefore, in order to deal with the influence of ip in all em methods, a generalized current density-to-electrical field relationship, including ohm’s law as particular case, is needed. in earth materials, many mechanisms have been developed to explain ip effects (marshall and madden, 1959; nilsson, 1971; zonge and wynn, 1975; wong, 1979; klein et al., 1984; olhoeft, 1985; wyller et al., 1992). comprehensive treatments can be found in the review books by wait (1959), bertin and loeb (1976), sumner (1976) and fink et al. (1990). of the many empirical laws proposed so far, the most utilised in geophysics are the debye (1928) td and fd pair (see also wait, 1959; patella and di maio, 1989) (1.1a) (1.1b) where a n 0, tn 0, bn 0, sa n = m with 0 £ m £ 1, b n = a n t n , and the cole and cole (1941) td and fd pair (see also pelton et al., 1978) (1.2a) (1.2b) with 0 £ c £1, t 0. in both pairs of formulas, t and w are the time and frequency, respectively, v(t) is the voltage during the td relaxation phase, v0 is the steady fig. 1a-c. the induced polarization phenomenology observed with a quadrupolar electrode array (a) in the time-domain (b) and in the frequency-domain (c). a b v t v td n n n n ( ) = -( ) = â0 1 a texp / r w r wb wt d n nn n i i ( ) = + ê ëá ˆ ¯̃= â0 1 1 1 v t mv t nc cc n nc n ( ) = -( ) ( ) +( )= • â0 0 1 1 / t g r w r w t w t cc c c m i i ( ) = ( ) + ( ) è î í ˘ ˚ ˙0 1 1 591 on the role of the j-e constitutive relationship in applied geoelectromagnetism voltage at the end of the td charging phase, m is the chargeability (seigel, 1959), r(w) is the electric field-to-current density ratio in the fd, called impedivity (patella, 1993), and r0 is the zero-frequency impedivity, i.e. the dc resistivity. finally, g(.) is euler’s function, i is the imaginary unit, and a n , t n , b n and c, t, m are assumed as heuristic parameters required to adapt the formulas to experimental data. it must be mentioned that the cole-cole model includes, as particular cases, warburg (1899) and madden and cantwell (1967) models, for c = 0.5 and c = 0.25, respectively. eqs. (1.1a) and (1.2a) can be derived from (1.1b) and (1.2b), respectively, using the relationship (1.3) for a background knowledge of the analytical properties of the debye and cole-cole formulas, reference is made to shuey and johnson (1973), patella and ciminale (1979), pelton et al. (1983, 1984) and caputo (1993). however, wait (1982) points out that this characterization of ip is rather confused and vague, because fundamentally nonphysical descriptions are employed. to try to overcome this conceptual drawback, in this paper a mechanical approach is suggested to derive a generalised current density-to-electrical field relationship, allowing the physical properties of debye and cole-cole descriptions to be fully investigated. 2. the j-e constitutive relationship in presence of polarization we consider ampère-maxwell equation in the td written as (stratton, 1941) (2.1) where h is the magnetic field, and d' and j' are the bound charges displacement field and the unbound charges current density, respectively. postulating that j' and e, and d' and e are related to each other by a convolutional form typical of a linear, causal and time-invariant system, eq. (2.1) can be expanded as (2.2) where the causal functions σ̃ ' 't( ) and ε̃' 't( ) represent the electrical conductivity and dielectric permeability impulse responses, respectively. applying the derivation rule of a convolution integral and putting (2.3) ampère-maxwell equation takes the compact form (2.4) the right-hand integral in eq. (2.4) defines the total current density vector j(t), say (2.5) as a consequence of eq. (2.3), the causal function σ̃ t( ) , which is called the admittivity impulse response, includes the electrical conductivity σ̃ ' t( ) and dielectric permeability ε̃' t( ) impulse responses. in the fd, putting with j(w), e(w), s(w), s'(w) and e'(w) the ft of j(t), e(t), σ̃ t( ) , σ̃ ' t( ) and ε̃' t( ), respectively, eq. (2.3) and eq. (2.5) are transformed, respectively, into (2.6) and . (2.7) equation (2.7) can equivalently be put in the form (2.8) v t v e d i i t ( ) = − ( ) −∞ +∞ ∫ 0 0 0 1 2π ρ ρ ω ρ ω ω ω . ∇ × ( ) = ( ) + ∂ ( ) ∂h j dt t t t' ' / ∇ × ( ) = ( ) −( ) + ∂ ∂ ( ) −( ) ∞ ∞ ∫ ∫h e et t t t dt t t t t dt˜ ˜σ ε' ' ' ' ' ' ' ' 0 0 ˜ ˜ ˜ /σ σ εt t t t' ' ' ' ' '( ) = ( ) + ∂ ( ) ∂[ ] ∇ × ( ) = ( ) −( ) ∞ ∫h et t t t dt˜ .σ ' ' ' 0 j ' e ' 't t t t dt( ) = ( ) −( ) ∞ ∫ ˜ .σ 0 σ ω σ ω ω ε ω( ) = ( ) + ( )' 'i j eω σ ω ω( ) = ( ) ( ) e jω ρ ω ω( ) = ( ) ( ) 592 domenico patella where r(w) = 1/s(w). the frequency-dependent functions r(w) and s(w) are called here impedivity and admittivity, respectively, and represent a useful extension of the classical parameters of the resistivity r and conductivity s, respectively. using now the identity (2.9) equation (2.2) can also be written as (2.10) putting (2.11) ampère-maxwell eq. (2.1) finally takes also the compact form (2.12) whose right-hand integral defines now the total displacement field vector d(t), say (2.13) according to eq. (2.11), the causal function ε̃ t( ) , which is called permittivity impulse response, also includes the electrical conductivity and dielectric permeability impulse responses σ̃ ' t( ) and ε̃' t( ). in the fd, using the ft, eq. (2.11) and eq. (2.13) are transformed, respectively, into (2.14) and (2.15) where d(w) is the dirac delta function and d(w) is the ft of d(t). equation (2.15) can equivalently be put in the form (2.16) where h(w) = 1/e(w). the frequency-dependent parameters h(w) and e(w) are here called preventivity and permittivity, respectively, and represent an extension of the classical concept of dielectric permeability e. equation (2.13) is fully equivalent to eq. (2.5): in fact, comparing eq. (2.3) with eq. (2.11), we readily derive the transformation formulas (2.17) (2.18) and finally, comparing eq. (2.4) with eq. (2.12) and eq. (2.5) with eq. (2.13), we readily get (2.19) concluding, we can state that for linear, causal and time-invariant natural systems, electrical current density and displacement field are equivalent descriptive vectors of the electromagnetic properties of matter. the bulk response can indifferently be synthesized either by the admittivity or the permittivity parameters, which are linked together with simple transformation formulas. by this formulation, a distinction between unbound and bound charge carriers, in relation to the long or short-distance capacity of movement under the influence of an external force, becomes unessential. in order to make ampère-maxwell equation applicable in practice we must define the physical model of the admittivity (permittivity) td and fd functions, for which we need to study at first the electrodynamic behaviour of a charged particle. to this aim, it is worth explaining beforehand how we shall combine the bulk electrical properties of a geophysical ˜ ˜σ σ τ τ' ' ' ' ' t t d t ( ) = ∂ ∂ ( ) −∞ ∫ ∇ × ( ) = ∂ ∂ ( ) + ( )       −( ) −∞ ∞ ∫∫h ' ' ' e ' 't t d t t t dt t ˜ ˜ . ' σ τ τ ε 0 ˜ ˜ ˜ε σ τ τ εt d t t ' ' ' ' ' ( ) = ( ) + ( ) −∞ ∫ ∇ × ( ) = ∂ ∂ ( ) −( ) ∞ ∫h et t t t t dtε̃ ' ' ' 0 d et t t t dt( ) = ( ) −( ) ∞ ∫ ˜ .ε ' ' ' 0 ε ω πσ δ ω σ ω ω ε ω( ) = ( ) ( ) + ( ) + ( )' ' '0 i d eω ε ω ω( ) = ( ) ( ) e dω η ω ω( ) = ( ) ( ) ˜ ˜ /σ εt t t( ) = ∂ ( ) ∂ ˜ ˜ε σ τt t d t ( ) = ( ) −∞ ∫ j dt t t( ) = ∂ ( ) ∂/ . 593 on the role of the j-e constitutive relationship in applied geoelectromagnetism readily obtain the solution for r(w) as (3.3) supposing that the system has k > 0 equal charged particles per unit of volume, by definition it is (stratton, 1941; parkhomenko, 1967) (3.4) which, using eq. (3.4), becomes (3.5) comparing eq. (2.7) with eq. (3.5), we get (3.6) though limited to only one species of charge carrier, eq. (3.6) is a useful admittivity starting model. it describes a circuit-like cell with a resistancecapacitance-inductance series combination. in fact, where it is wm2 << m1 and m0 << wm1, eq. (3.6) reduces to (3.7) which describes a purely resistive cell, as assumed in dc geoelectrical methods. where, instead, it is wm1 << m0 and w 2 m2 << m0 eq. (3.6) reduces to (3.8) describing a purely capacitive cell. finally, if structure and the microscopic electrodynamic properties of a compound of charged particles. indeed, in all equations so far developed, we have not indicated the spatial dependence of the constitutive parameters and field vectors, tacitly admitting that the medium is a continuum. this is a useful, practical assumption at the scale of the geophysical field and laboratory experiments. however, as will be discussed later, the physical derivation of the analytical expression of the constitutive parameters requires a zooming at the smallest scale of the charge carriers, for which discontinuous properties must be introduced. thus, the medium will be considered as an aggregate of elementary cells with unitary volume, each cell being sufficiently small to assimilate the medium to a continuum and sufficiently large to incorporate the full diversity of a charged mixture. 3. the electrodynamic equation of a charged particle let us consider a charged particle confined in a medium immersed in an external electric field e(t). the trajectory r(t ) of the particle can be determined by solving the following general mechanical differential equation (3.1) where q and m2 are the charge and mass of the particle, m1 is a friction-like coefficient accounting for dissipative effects due to collisions and interactions with other charged particles, and m0 is an elastic-like coefficient accounting for recall effects on bound charges. obviously, ml 0 for any l. in the fd eq. (3.1) transforms to (3.2) where r(w) is the ft of r (t). from eq. (3.2) we m d t dt q tλ λ λ λ = ∑ ( ) = ( ) 0 2 r e m i qλ λ λω ω ω = ∑ ( ) ( ) = ( ) 0 2 r e r eω ω ω λ λ λ ( ) = ( ) ( ) = ∑ q m i 0 2 . j rω ω ω( ) = ( )kqi j eω ω ω ω λ λ λ ( ) = ( ) ( ) = ∑ i kq m i 2 0 2 . σ ω ω ωλ λ λ ( ) = ( ) = ∑ i kq m i 2 0 2 . σ ω( ) = kq m 2 1 σ ω ω( ) = i kq m 2 0 594 domenico patella m1 << wm2 and m0 << w 2m2 eq. (3.6) reduces to (3.9) which represents the admittivity of a purely inductive cell. from eq. (3.6) more complex cases can also be discussed. if unbound charges (m0 = 0) are considered, the admittivity model results to be (3.10) which fits the behaviour of a circuit-like cell consisting of a resistance and inductance in series. this model is assumed to explain dispersion effects in metals (stratton, 1941). for bound charges (m0 0) with negligible inertia (m2 ª 0) the admittivity reduces to (3.11) which characterises a circuit-like cell made of a resistance and capacitance in series. finally, for bound charges (m0 0) with vanishing friction (m1 ª 0) the admittivity is (3.12) which characterises a circuit-like cell made of an inductance and capacitance in series. 4. the debye impedivity model let us consider now a system with two different species of charge carriers and put with kj, qj and mlj the number per unit of volume, the electrical charge and the passive coefficients, respectively, of the carriers of the j-th species ( j = 1,2). such a pair of ionic species can, e.g., be the result of ionic dissociation of a salt dissolved in pore water. since the total current density in the fd is now defined as (parkhomenko, 1967) (4.1) we readily obtain the following analytical model of admittivity (4.2) for our purpose, the most interesting case to investigate is when one species ( j = 1) consists of unbound charges and the other species ( j = 2) of bound charges, both showing negligible inertia. this simplified physical set can already explain, at least qualitatively, non-resonant ip phenomena in rocks (keller and frischknecht, 1966). in fact, we have a two-branch parallel circuit-like cell, with a branch consisting of a single resistance and the other branch of a resistance-capacitance series combination, i.e. (4.3) putting (4.4) (4.5) (4.6) with a,t, b 0 and t b, from eq. (4.3) we obtain the admittivity function as (4.7) and the impedivity function as (4.8) we have thus got admittivity and impedivity functions qualitatively reflecting the ip frequency spectral evolution from s s w w0 0 = ( ) = æ lim a, or r0 = r w w 0 1= ( ) = æ lim a , to s s w t t w• æ• = ( ) = -( )lim a b a, or r r w t t w• æ• = ( ) = -( ) £lim .b a a1 s w w w ( ) = + i kq m i m 2 0 1 s w w w ( ) = i kq m m 2 0 2 2 j rw w w( ) = ( ) = âi k qj j j j 1 2 s w w w w ( ) = + -= â i k q m i m m j j j j jj 2 0 1 2 21 2 s w w w ( ) = + + k q m i k q m i m 1 1 2 11 2 2 2 02 12 . a k q m = 1 1 2 11 t = +k q m k q m k q m 1 1 2 12 2 2 2 11 1 1 2 02 b k q m k q m = 2 2 2 11 1 1 2 02 s w wt w t ( ) = +( ) + -( ) a i i b 1 1 r w w t wt ( ) = + -( ) +( ) 1 1 i b a i . s w w ( ) = kq i m 2 2 s w w ( ) = + kq m i m 2 1 2 595 on the role of the j-e constitutive relationship in applied geoelectromagnetism generalising, we can now consider within each elementary cell either a parallel or a series combination of n two-branch circuits, or a network of both combinations. for a parallel combination, the admittivity becomes (4.9) whereas for a series combination, the impedivity becomes (4.10) which takes the same form as in eq. (1.1b), putting (4.11) (4.12) 5. the cole-cole impedivity model shuey and johnson (1973) showed that the analysis of the decay spectrum allows the properties of an impedivity model to be more easily deduced and understood. the decay spectrum a(k) is defined as (5.1) where ρ γik m( ) is any given impedivity function, where w is replaced by ik m γ with g an arbitrarily small positive number. using the debye impedivity model given in eq. (1.1b), after some simple steps we get the debye decay spectrum ad(k) as (patella and di maio, 1989) (5.2) the limit in the right-hand side of eq. (5.2) defines the dirac delta function d(k-1/t n ). hence, we obtain at last the very important result (5.3) now, considering the definition of the d-function (papoulis, 1962) applied to a generic decay spectrum a(k) (5.4) it follows that a(k) can always be considered as a continuous distribution of impulses with varying time constants, and thereof approximable to a sum as in eq. (5.3) with any desired accuracy. in fact, discretising eq. (5.4) using the trapezoidal rule for expressing the area under the curve and putting j = 1/t, we obtain the approximation (5.5) which conforms to eq. (5.3) putting an n n= ρ β τ0 2/ . this conclusion allows cole-cole, warburg and madden-cantwell models to be considered as a continuous superposition of debye terms with varying time constants distributed around k = l/t. therefore, t in eq. (1.2b) takes the additional significance of main relaxation time constant, when the cole-cole model is considered as a sum of debye terms (pelton et al., 1983), i.e. as . (5.6) 6. conclusions we have demonstrated that the debye impedivity model for rocks can physically be deduced as a consequence of the electrodynamic behaviour of a mixture of bound and unbound charged particles immersed in an external electrical field. we have also clarified that the cole-cole impedivity model is a synthetic σ ω ωτ ω τ ( ) = + + −( )=∑ a i i b n n n nn n 1 11 ρ ω ω τ ωτ ( ) = + −( ) +( )=∑ 1 11 i b a i n n n nn n ρ ρ ω ω0 0 1 1= ( ) = ( ) → = ∑lim an n n β ρn n nb a= 0 . a k ik ik i ( ) = −( ) − +( ) → lim γ ρ γ ρ γ π0 2 a k k d n nn n n ( ) = −( ) +[ ]= →∑ ρ β τ γ π τ γγ 0 2 1 0 2 21 lim . a k kd n nn n n( ) = −( ) = ∑ρ β τ δ τ0 2 1 1 . a k a k d( ) = ( ) −( ) ∞ ∫ ϕ δ ϕ ϕ 0 a k a kn n n n( ) ≈ −( ) = ∑ 1 1δ τ ρ ω ρ ωβ ωτ cc n nn n i i ( ) ≈ − +    = ∑0 1 1 1 596 domenico patella model, which can physically be explained as a continuous distribution of debye terms. though being the debye and derived colecole impedivity models related to the physical parameters describing the motion of ionic species in rock capillaries under the influence of an external electric field, no possibility exists to distinguish each ionic species in a compound by means of only ip meaurements. indeed, admitting that one may be able to single out the n exponential decay terms in the td voltage transient or the n complex fractions in the fd impedivity spectrum and obtain the set of a n ,t n or b n ,t n ip parameters, by no way the mechanical and electric parameters of bound and unbound charged particles can be determined singly. this conclusion contrasts the current belief that using ip it should be possible to distinguish ionic particles and to estimate their abundances, as, e.g., in environmental applications for monitoring contaminants in sediments. in this application field, only vague information on the possible existence of contaminants seems to be deducible from ip experiments. weller and börner (1996), e.g., have shown a reliable correlation between the imaginary part of ad mittivity and porespace internal surface in sediments, and concluded that the observed high sensitivity of the imaginary component to changes at the internal surface may be used as an indicator for contaminations. acknowledgements study performed in the frame of the intas project 97-2/612. references bertin, j. and j. loeb, (1976): experimental and theoretical aspects of induced polarization (gebrüder-bornträger, berlin), 2 vols. caputo, m. 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(1959): mathematical formulation and type curves for induced polarization, geophysics, 24, 547-565. shuey, r.t. and m. johnson (1973): on the phenomenology of electrical relaxation in rocks, geophysics, 38, 37-48. stratton, j. (1941): electromagnetic theory (mcgrawhill, new york), pp. 508. sumner, j.s. (1976): principles of induced polarization for geophysical exploration (elsevier, amsterdam), pp. 277. 597 on the role of the j-e constitutive relationship in applied geoelectromagnetism wait, j.r. (editor) (1959): overvoltage research and geophysical applications (pergamon press, oxford), pp. 158. wait, j.r. (1982): geo-electromagnetism (academic press, new york), pp. 268. warburg, e. (1899): über das verhalten sogennauter unpolarisierbarer elektroden gegen wechselstrom, z. chem., 67, 493. weller, a. and f.d. börner (1996): measurements of spectral induced polarization for environmental purposes, environ. geol., 27, 329-334. wong, j. (1979): an electrochemical model of the inducedpolarization phenomenon in disseminated sulfide ores, geophysics, 44, 1245-1265. wyller, j., n. wellander, f. larson and d. parasnis (1992): burger ’s equation as a model for the ip phenomenon, geophys. prosp., 40, 325-341. zonge, k.l. and j.c. wynn (1975): recent advances and applications in complex resistivity measurements, geophysics, 40, 851-864. sulla v a r i a z i o n e del p u n t o di f u s i o n e c o n la p r e s s i o n e p . e v a l l e i n t r o d u z i o n e . il processo della fusione è un cambiamento di stato, regolato dall'equazione termodinamica di clapeyron dt^ dps a v 1 x [ 1 ] una semplice interpretazione cinetica del processo della fusione è s t a t a data, parecchio t e m p o fa, da f . a. l i n d m a n n (j). secondo lindmann la fusione di un cristallo avviene quando l'ampiezza delle oscillazioni termiche degli atomi diventa abbastanza grande da far entrare in collisione le « sfere d'azione » degli atomi stessi. in u n a precedente nota (3) è stato osservato che tali collisioni si devono riferire alle oscillazioni termiche trasversali, poiché, nel passaggio dalla fase solida alla fase liquida, si verifica u n a brusca diminuzione della rigidità del cristallo. tenuto conto di queste osservazioni e dell'espressione della frequenza massima delle oscillazioni termiche trasversali (2), la relazione di lindmann può essere scritta nella forma 1' qs l's 1 purché la pressione sia tale da rendere sufficientemente piccola la distanza media t r a gli atomi. il calcolo della t e m p e r a t u r a di fusione ts, in funzione della pressione ps, richiede peraltro la conoscenza dell'equazione di stato. nella nota sopracitata (3), mediante la formula di l i n d m a n n nella forma [2] e un'equazione di stato per i solidi precedentemente stabilita (2), l'autore h a ottenuto, t r a l'altro, la relazione dts dps kt voi, qos 1+2 a0 kut ~ (yots — — va t \ 6s [3] [21 in cui yots è il p a r a m e t r o relativo alle predette oscillazioni, v a l u t a t o alla densità gosla [2] lega la t e m p e r a t u r a di fusione ts, alla densità della fase solida qs al punto di fusione. come è stato già posto in evidenza (3), la t e m p e r a t u r a di fusione presenta un massimo per qs = 3 yats qos. quindi la fusione può avvenire a qualsiasi t e m p e r a t u r a , nella quale k r è l'incompressibilità isoterma al p u n t o di fusione, ed a0 la dilatazione termica in condizioni ordinarie. il confronto della [3] con i risultati sperimentali, a pressione normale, è stato eseguito soltanto per il ferro (3). i n questa nota il confronto viene esteso al litio, sodio e potassio. r i c h i a m i ad a l c u n e r e l a z i o n i d e l l a t e o r i a d e i s o l i d i . è opportuno ricordare alcune relazioni contenute nelle note già citate (2), (3). se si indicano con vnl e v„i, le velocità delle onde longitudinali e trasversali alla densità q„, con yo! e y„. i relativi p a r a m e t r i alla stessa densità, l'incompressibilità isot e r m a kt, per t e m p e r a t u r e superiori alle t e m p e r a t u r e caratteristiche, è d a t a da kt = kot il 2 t ( l qn \ 0 ) 146 p. e. valle nella quale r = y„i + con a2 = 1 a £ k 3 k 2(yal voi) [5] 9 d e t t o inoltre gv il calore specifico a volume costante e a la dilatazione t e r m i c a si ha a kr = yggv nella quale e risulta y = yi •vt [6] [7] [8] 7o • k o r a 3 e pn [9] voi = yot = 3 y „ l + 2 r 2 + 3 b 3 y0 (1 + b) r [ 1 0 ] 2+3 b m cui b = 4 1 + «2 5 dalla seconda delle [8] si ricava infine q y„ts = va p ne la densità della fase solida alla t e m p e r a t u r a di fusione tos a pressione ordinaria, con kot l'incompressibilità i s o t e r m a alla densità q0, con kr l'incompressibilità isoterma alla densità gos, con a„ la dilatazione termica alla densità g„, e con yots il paramet r o delle onde trasversali alla densità gos. l a t e m p e r a t u r a ta è l'ordinaria temper a t u r a di laboratorio. p o s t o nella [3] qs = qos, si h a (it. à vs 'a 2 ^ (y0<s va) n i -[3'] 1 + 2 a0 kot (yots va) yt q = cost. yt q = cost. in condizioni ordinarie, p u r c h é la temp e r a t u r a non sia inferiore alle t e m p e r a t u r e caratteristiche, la [6] fornisce dove a è il peso atomico. combinando poi la [5] e la [7] si ottiene i n v e c e di calcolare gos m e d i a n t e l'equazione di stato, come è s t a t o f a t t o per il ferro nella n o t a p r e c e d e n t e (3), si p u ò scrivere, con sufficiente approssimazione, l+ab (tos ta) [12] conviene inoltre sviluppare kt. dalla [4] si o t t i e n e k-r = kot + k t (va + 2 d (qos-qo) + .... qo ]13] i n t r o d u c e n d o la [12] e la [13] nella [3'], con qualche passaggio si h a (dts\ t0 \dps)0= y o t s ~ ' kot 1 + e — d [3"] 111 c u i [ 1 1 ] e s p r e s s i o n e approssimata d i dts/dps a p r e s s i o n e o r d i n a r i a . a0 (2 r + va) (:tos t0) l+a0 (tos t0) e = 2a0 tos (•yots va) è q u e s t a u n a relazione a p p r o s s i m a t a che consente di calcolare f a c i l m e n t e (dts/dps)0. il confronto della [3] con i risultati sperimentali viene e f f e t t u a t o a pressione ordinaria. d ' o r a in a v a n t i v e r r à p e r t a n t o i n d i c a t a con o„ la densità della fase solida a temper a t u r a t0 e a pressione ordinaria, con qos conpronto con i r i s u l t a t i s p e r i m e n t a l i nella tabella i sono c o n t e n u t i i d a t i sperimentali relativi agli elementi litio, sodio e potassio. le t e m p e r a t u r e di fusione, in funzione sulla variazione del punto di fusione con la p r e s s i o n e 147 della pressione, e le r e l a t i v e variazioni di v o l u m e t r a la fase liquida e la f a s e solida, costituiscono i r i s u l t a t i delle m i s u r e eseguite d a p . w . b r i d g m a n ('), m e n t r e le entropie di fusi one sono s t a t e d e d o t t e dai calori l a t e n t i di fusione c o n t e n u t i nell ' h a n d b o o k of c h e m i s t r y a n d p h v s i c s (5). l a tabella i i contiene i valori delle grandezze che i n t e r v e n g o n o nella [3"]. le g r a n dezze 2 f e k„t sono s t a t e p r e c e d e n t e m e n t e v a l u t a t e dallo scrivente (2), (3). il calcolo dei valori di y„ts è s t a t o invece eff e t t u a t o m e d i a n t e le relazioni [9], [10], [11] e [12], t a b e l l a i e l e m e n t o p r e s s i o n e e l e m e n t o 1 4 0 0 0 8 0 0 0 1 2 . 0 0 0 li tos 1 7 8 ° , 4 1 9 1 " , 8 2 0 1 ° , 6 — na tos av a 8 9 7 ° , (52 0 , 0 2 7 8 7 0 , 3 5 8 1 2 8 ° , 8 0 , 0 2 0 7 2 1 5 5 ° , 1 0 , 0 1 7 1 1 1 7 7 ° , 5 0 , 0 1 3 9 8 k tos av as 6 2 ° , 5 0 , 0 2 0 8 0 0 , 1 9 6 1 1 5 ° , 8 0 , 0 1 g 7 (i 152<>,5 0 , 0 1 0 7 3 1 7 9 ° , 6 0 , 0 0 0 4 2 l e p r e s s i o n i s o n o in k g c m 2 , le t e m p e r a t u r e di f u s i o n e tus in °c, le v a r i a z i o n i di v o l u m e a v i n c m 3 g 2 , le e n t r o p i e di f u s i o n e as in j o u l e g r a d i t a b e l l a i i e l e m e n t o a qo a0 k0r 2 r vots li 6 , 9 4 0 0 , 5 3 4 1 , 8 1 0 4 1 1 , 6 7 1 0 ' 3 , 6 4 5 0 , 9 3 1 na 2 2 , 9 9 7 0 , 9 7 1 2 , 1 6 1 0 ' 6 , 2 6 1 0 ' 4 , 3 4 5 1 , 0 7 2 k 3 9 , 0 9 6 0 , 8 7 2 , 5 i o 4 2 , 9 1 1 0 ' 4 , 5 2 6 1 , 0 7 5 fe 5 5 , 8 5 7 , 8 7 3 , 3 6 i o " 5 1 , 71410 6 3 , 7 8 4 1 , 6 2 5 l e d e n s i t à q0 s o n o in g c u i 3 , le d i l a t a z i o n i t e r m i c h e u0 i n g r a d i 1 , le i n c o m p r e s s i b i l i t à i s o t e r m e k 0 x i n k g c m 2 . 148 1'. e. valle p e r il ferro si è posto tos = 1808° k e si è a s s u n t o t„ = 293° k. i d a t i c o n t e n u t i nelle tabelle i e i i consentono di eseguire u n confronto f r a i d a t i sperimentali e la teoria. i valori teorici ottenuti per il litio, sodio, potassio e ferro, vengono confrontati con i dati sperimentali. l' accordo risulta abbastanza soddisfacente. t a b e l l a i i i e l e m e n t o (d'tsdsp)° in g r a d i / 1 0 3 k g c m ~ e l e m e n t o v a l o r i s p e r i m e n t a l i v a l o r i t e o r i c i e l e m e n t o d a l l a r e l a z i o n e di c l a p e y r o n d a i d a t i d i p w . b r i d g m a n c a l c o l o r i g o r o s o (3) c a l c o l o a p p r o s s i m a t o m e d i a n t e l a [ 3 " ] li, 3 , 8 4 , 7 na 7 , 7 8 , 5 — 8 , 4 k 1 3 , 4 j 6 , 0 — 1 5 , 9 fe 2 , 9 — 2 , 9 2 , 8 nella p r i m a colonna della tabella i i i sono elencati i valori di ( d t s / d p s ) 0 o t t e n u t i p e r interpolazione dai d a t i di p . w . bridgm a n ; la seconda colonna contiene gli a n a logia valori o t t e n u t i dalla f o r m u l a di olapeyron. giova a v v e r t i r e che, m e n t r e p e r il sodio e il potassio sono s t a t e utilizzate le entropie (li fusione e le variazioni di volum e c o n t e n u t e nella tabella i, il d a t o p e r il ferro è s t a t o v a l u t a t o d a f . simon ("). nella t e r z a colonna della, tabella iti è r i p o r t a t o il valore teorico di (dts/dps)0 p e r il ferro o t t e n u t o , nella n o t a p r e c e d e n t e (3), m e d i a n t e il calcolo rigoroso della [3]. l a q u a r t a colonna infine, contiene i valori di (dts/dps)0 calcolati con la f o r m u l a a p p r o s s i m a t a [3"]. l ' a c c o r d o t r a i r i s u l t a t i sperimentali e i valori teorici s e m b r a a b b a s t a n z a soddisfacente. riassunto viene valutata, per via teorica, la variazione del punto di fusione con la pressione, per un solido ideale, a pressione ordinaria. abstract the rate of change of the melting point with pressure is derived on the basis of the classical theory of solids. the values. which have been obtained for litium, sodium, potassium, and iron, are compared with experimental data. the resulting concordance is rather satisfactory. b i b l i o g r a f i a (!) lindmann, f . a . , uber die berevhnuiuj molelculare eigenfrequenzen. « i ' h v s . z s . », 1 1 . 6 0 9 6 1 2 ( 1 9 1 0 ) . (2) valle, p . e . , uri equazione di stato per ì solidi. « a n n a l i di g e o f i s i c a » , 6 , 183197 ( 1 9 5 3 ) . (3) — una stima del punto di fusione del ferro sotto alte pressioni. « a n n a l i di g e o f i l i c a », 8 , 1 8 9 2 0 0 ( 1 9 5 5 ) . (>) bridgman, p . w . , the plnjsies of high pressures. « b e l l a n d s o n s », l o n d o n , 1 9 4 9 . (5) — handbook of chemistry and physics. « c h e m i c a l r u b b e r p u b l i s h i n g co. », c l e v e l a n d , 1957-58. (6) simon, f . e . , the melting of iron at high pressures. « n a t u r e » , 1 7 7 , 7 4 6 ( 1 9 5 3 ) . 20scotti.qxd 571 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words sisfrance database – macroseismic historical seismicity 1. introduction the development of the french nuclear power program in 1974 imposed a better knowledge of seismicity. to comply with nuclear safety requirements, an in-depth research program for the revision of existing catalogues was initiated. this resulted in the creation of sisfrance: a relational database containing parametric information on french historical seismicity covering about a thousand years (vogt, 1979; vogt, 1981; levret, 1987; lambert and levret, 1996; lambert et al., 1997). the purpose of the database was to collect and archive all the information pertaining to earthquakes that have been felt on french metropolitan territory. the sisfrance database proposes a complete description of earthquakes going from the documentary sources all the way through the quantification of the intensity observations. the strength of this parametric database is the attribution of reliability coefficients at all levels of interpretations going from the documentary sources to the final intensity estimate for all events irrespectively of their size. after a brief recollection of the historical context that motivated the creation of sisfrance, the paper gives a synthetic description of the design of this database. a quantitative appreciation of the evolution of our knowledge the french macroseismic database sisfrance: objectives, results and perspectives oona scotti (1), david baumont (1), grégory quenet (2) and agnès levret (3) (1) institut de radioprotection et de sûreté nucléaire, fontenay-aux-roses, france (2) département d’histoire, université de versailles-saint-quentin-en-yvelines, france (3) groupe aps, site de ruscino, château-roussillon, perpignan, france abstract to comply with nuclear safety requirements, an in-depth research program for the revision of existing catalogues was initiated back in 1974. the priority of the partners involved in the sisfrance project was to establish the most exhaustive documentary databank in order to construct an intensity macroseismic database, concerning both epicentral and punctual observations. the architecture of the sisfrance database is presented. the strength of this parametric database is the attribution of reliability coefficients at all levels of interpretations going from the documentary sources to the final intensity estimate. to ensure homogeneity of the database, a general guideline was defined. the resulting macroseismic relational database sisfrance contains 65 000 intensity observations attesting to the existence of 5283 earthquakes (575 with io ≥ vi) that have been felt on the french metropolitan territory over the past one thousand years. thanks to the homogeneous methodology and to the continuous collaboration between brgm, edf and irsn for the past 30 years, sisfrance is today a reference database and a key tool for seismic hazard assessment. mailing address: dr. oona scotti, institut de radioprotection et de sûreté nucléaire, 92262 fontenay-aux-roses cedex 77-83, france; e-mail: oona.scotti@irsn.fr 572 oona scotti, david baumont, grégory quenet and agnès levret since the mid-seventies is presented through a comparison between the initial rothé (1977) earthquake catalogue with the one extracted from the sisfrance database. 2. historical context at the time of the development of the french nuclear power program, the seismicity felt on the french metropolitan territory was known primarily on the basis of the work of j.p. rothé, who compiled a number of different catalogues. for the events of the 20th century, rothé’s catalogue is based on macroseismic inquiries of the bureau central sismologique français (annales and annuaires de l’institut de physique du globe de strasbourg). for the events prior to the 20th century, rothé’s catalogue gathers mainly previous catalogues established by perrey (1845), montessus de ballore (1906) and other regional and local compilers (e.g., villette, 1905; pallassou, 1815) that were unfortunately built with insufficient historical critical analysis and with only a limited number of references for the documentary sources. under the request of edf, j.p. rothé classified the events both geographically (according to their location) and chronologically. the resulting unpublished catalogue provides for each event that mostly occurred in the 20th century: the date of occurrence, epicentral location, estimated epicentral intensity, the punctual intensities and some references to the documentary sources. figure 1 illustrates the state of knowledge of the french historical seismicity at the beginning of the sisfrance project. the events of the instrumental catalogue fig. 1. state of knowledge at the beginning of the sisfrance project. seismicity (m ≥ 4.5 and io ≥ vi) reported in the historical (rothé, 1977) and the instrumental (cea/ldg seismicity: 1962-1973) catalogues. 573 the french macroseismic database sisfrance: objectives, results and perspectives (laboratoire de détection géophysique, bruyères le châtel, france) that occurred between 1962 and 1973 are also superimposed. only a few events with a magnitude greater than 4.5 were reported in the instrumental catalogue. on the other hand, many events with an epicentral intensity greater than or equal to vi were reported by rothé (1977) highlighting some important active structures, mainly in the south east part of france and along the italian, spanish and to a minor extent the german borders. although france appeared as a country of low to moderate seismicity it seemed to be occasionally subjected to historically strong earthquakes. 3. objectives to comply with nuclear safety requirements, it was necessary to revisit the catalogues. three french institutions were concerned, the brgm (french geological and mining research board), the irsn (french institute for radioprotection and nuclear safety, formerly cea atomic energy commission) and edf (french electricity board), joined their efforts. a research program was launched under the direction of j. vogt with a common objective: produce the first comprehensive seismotectonic map of france (vogt, 1981; goguel et al., 1985; vogt and massinon, 1985). one of six work packages that this project entailed, was the construction of a macroseismic database of all earthquakes felt on the french metropolitan territory (the sisfrance database was originally named sirene). seismic hazard for nuclear installations is based on a deterministic approach. this approach relies entirely on the estimation of the strongest intensities that a nuclear site could be submitted to (see rfs-2001 for the most recent text). the priority of the partners involved in the sisfrance project was thus to establish the most exhaustive intensity and documentary database, and to qualify all the information (for more details see godefroy et al., 1990; godefroy and levret, 1992). the raw and interpreted data were clearly separated allowing an easy update of the intensity evaluations (fig. 2). the different modifications of the sisfrance database were discussed annually and validated by the three partners. the consensual nature of this approach, albeit tedious, has paid off in the long run, because it led to a very homogenous evaluation of the intensity observations from the first through the second level of interpretation (fig. 2). 4. methodology to ensure the homogeneity of the database, the sisfrance partners defined a general guideline as follows: i) search for all the sources referenced in previous catalogues, with rothé’s catalogue as a starting point; ii) find new sources for all events that could be found in regional, municipal, occasionally parochial archives. contemporary national or regional press journals were also searched. perform a critical analysis of the documents by taking into account the historical, geographical and political contexts of the time. archives of neighbouring countries were also consulted for the events that were felt in metropolitan france. iii) evaluate all the punctual intensities according to the msk-64 intensity scale (medvedev et al., 1967). iv) attribute a degree of reliability to the documents and the different estimates. v) upgrade the database on an annual basis. the working group had to face several types of difficulties when re-analysing the original catalogues used by j.p. rothé in his compilation. first of all, it was important to distinguish between real and false earthquakes. in the early modern period, the natural phenomena (storm, landslide, earthquake) were all referred to as «terremotus» which was a source of confusion. a classic example of a problematic earthquake is the 1227 event. due to a 16th century compiler’s mistake between the locality of salins (savoy) and «salviens», an antique tribe in provence, southeast france, a landslide that occurred in savoy at the end of 1248 became a major 1227 earthquake in provence. concerning the middle age events and according to alexandre (1990) who collaborat574 oona scotti, david baumont, grégory quenet and agnès levret ed closely with the working group, 75% of the data contained in the catalogues for this period (e.g., rothé, 1977; perrey, 1845) were misinterpreted. confusion was partly due to the use of different calendars in different regions. this problem remains today in some existing catalogues of the mediterranean countries. an example of an event that was eliminated because of calendar mistakes is the 15 february 1154 and 5 february 1156 events, which correspond in fact to the 18 january 1155 burgundy event. confusion came also from name of localities that were wrongly transcribed. in the 17th century, the word «curensi», relative to the city of chur, located in switzerland, was transposed as «turonensi», associated with the town of tours (lambert et al., 1997). it was thus assumed that a destructive earthquake struck the city of tours in 1295 whereas in reality, it occurred in switzerland. 5. quantifying first level of interpretation – the intensity evaluation results from: i) a classification of buildings (typology and degree of damage) and ii) a statistical analysis of the distribution of the degree of damage for each building typology. in the sisfrance database, the intensities were estimated following the statistical methodology proposed by the msk-64 intensity scale divided into 12 discrete degrees. although half-degrees are not often considered, they were introduced in the infig. 2. schematic relational links of the different levels of data interpretation (adapted from stucchi, 1993). based on the documentary sources archived in the databank, the intensities are evaluated at different sites using a given macroseismic intensity scale. the spatial distribution of the intensities is used to define the epicentral location and epicentral intensity. 575 the french macroseismic database sisfrance: objectives, results and perspectives tensity evaluation. the presence of a half-degree implies that either the intensity is uncertain (quality b or c) or the statistical evaluation is clearly midway between two intensity levels (quality a). since information on materials and building techniques are very rarely available, it was necessary to assume a likely typology (based on the historical context of the area). a second difficulty came from the estimation of the intensity based on only a single building (e.g., church, castle, and convent). in such cases, when possible, we rely on complementary information by studying traces of past earthquakes on private houses still existing to this day (quenet and levret, 2000; rideaud and levret, 2000, 2002). second level of interpretation – isoseismal contours are then plotted by hand, ensuring the exclusion of abnormal intensities. the epicentral coordinates are computed using the barycentre of the isoseismals of maximal intensities (imax or imax -1). the epicentral intensity is taken either equal to the largest observed intensities, when well distributed data is available, or is deduced from attenuation laws, mostly for off-shore epicenters. these methodological choices (io, half-degree, single building description, quality factor) can be illustrated through the interpretation of a few sources relating the 6 october 1711, loudunmoncontour earthquake (sisfrance, 2001; baumont and scotti, 2003). this is one of the strongest events known in the poitou region, felt from nantes to the west, to le havre to the north, and limoges to the south. the most detailed evidence of the severity of the shock comes from loudun (vii-viii qobs = b), moncontour (vii-viii qobs = b), and saint-jouin sur marne (vii qobs = b). considering moncontour for example, the text (archives communales, moncontour, 1711) mentions several collapsed houses, including that of the priest, which corresponds to a damage degree reaching 4 or 5. assuming a building class a, this translates into intensity vii or viii. due to the vague terminology used in the text to describe the number of buildings affected, it is not possible to distinguish between these two evaluations. this uncertainty is therefore converted into a half-degree estimation (vii-viii) accompanied by a weaker quality factor (qobs = b). the uncertainty in intensity evaluation is exacerbated when the description of damage concerns only one building. this is the case in saint-jouin sur marne where the only source available relates damage occurring only to the abbey (chronique de henri duplessis de paumard). the description of the damage corresponds to a degree 3: four collapsed chimneys, open fissures in the vault and most of the window frames and doorframes. in this case, the building typology is assumed to be quality factor epicentre iobs isoseismal qi0 qloc qiobs a 204 203 16 440 101 b 942 333 46 814 599 k 11 c 643 224 2471 184 d 1772 e 214 436 i 1172 2315 void 2097 total 5283 5283 65 725 884 table i. the number of data present in the different tables of the sisfrance 2001 database subdivided by quality factors. void refers to events for which no epicentral intensity could be estimated. 576 oona scotti, david baumont, grégory quenet and agnès levret of type b judging from what remains today. the minimal intensity in saint-jouin is thus assumed to be vii with a degraded quality factor b to reflect the single-building evaluation for this locality. finally, the epicentral location is assumed to be at mid-distance between the highest intensity localities (loudun and moncontour), which are separated by 20 km (lambert, 2003). to reflect the uncertainty of about 10 km in this estimation, a quality b is attributed. the epicentral intensity is assumed to be equal to the highest intensity observed but with a quality factor of b (constrained only by a few sparse observation points). third level of interpretation – due to its consensual nature (three partners), it was decided to only provide intensities. nevertheless, source parameters (magnitude, depth) have been determined for 140 events (levret et al., 1994) but are not calculated within the framework of the sisfrance project. 6. the sisfrance database: architecture and content (2001) sisfrance has been managed through a relational database since 1986 (godefroy et al., 1980; godefroy and levret, 1992) using the same architecture and applying the same methodology. the brgm manages the database on a unix server via oracle-8 and a gis interface. the brgm is responsible for the systematic search of documentary sources, their interpretation and the intensity assessments. the results of these studies can be found in brgm internal reports. in depth research, focused on specific events important for the safety of nuclear plants, were also conducted with teams of specialists from several neighbouring countries, in the framework of programs sponsored by the commission of the european communities (cec) or the international atomic energy agency (iaea). the following case studies are examples of such focused research: the 1909 lambesc, provence earthquake (levret et al., 1986), the 1428 catalonia, 1564 and 1644 alps, earthquakes (lambert, 1993), the 1580 strait of dover and the 1382 north sea earthquakes (melville et al., 1996), the 1356, basel, swiss earthquake (mayer rosa and cadiot, 1979), the 1799 bouin, bretagne (levret et al., 1996; scotti and levret, 2000) and 1887 ligurian earthquake (scotti and levret, 2000), the 1708 manosque, provence earthquake (quenet, 2001; quenet et al., 2003), and the 1704 poitou earthquake (quenet and levret, 2000; rideaud and levret, 2002). in all cases, the working group meets yearly to discuss and validate these interpretations before updating the database. the simplified conceptual model of the relational database is presented in fig. 3. the main table is the event table (evtsirene), which exists if at least one observation (obsirene) exists. each observation refers to a locality (localites) and at least one bibliographic reference (biblio, documents). the observations are then plotted on a map and the epicentral characteristics (intensity and location) of the event are deduced (epcsirene). the database contains 5884 events including 458 doubtful and 143 false earthquakes. for the 5283 true earthquakes, 575 have an estimated epicentral intensity equal to or greater than vi msk (280 with qie = a, b, k and 295 with qie = c, e, i) and 2545 have an unknown epicentral intensity. some 65 000 intensity observations describe seismic events dating as far back as one thousand years. the originality of this parametric database is the attribution of reliability coefficients at all levels of interpretations going from the documentary sources to the final intensity estimate. concerning the documentary sources, reliability coefficients have been attributed based on a number of criteria (original source or copy, direct or indirect testimony, level of precision and of exaggeration). as summarized in table i, sisfrance attributes to the estimated epicentral location a quality factor from a (certain and reliable) to i (isolated intensity observation, insufficient, not to be used as epicentre location). it also qualifies the estimation of the epicentral intensity from a (well defined) through e (doubtful evaluation), i (isolated intensity observation) and k (calculated from an attenuation relationship). it is clear that the quality of this information is directly related to the spatial sampling and the reliability of the intensity data points in the epicentral area. the epicentral intensities are therefore to be considered with care, the uncertainty on their value being often of half a degree or 577 the french macroseismic database sisfrance: objectives, results and perspectives more. similar quality factors are used to describe the reliability of the obsirene table, the isoseimal table (see table i) and the biblio table. moreover, in the obsirene table, there are different codes that qualify the physical phenomena that may have affected the locality associated with the intensity observation: effects on water wells, landslide effects, site effects and so on. these codes are meant to alert the users that the evaluated intensity observation may have to be considered with great care. a careful selection of the data on the basis of these codes is necessary before analysing the data for magnitude estimates. in 2000, the historical database became partly available to the public via internet (www.sisfrance.net). only the most reliable observations are available online. as far as possible, the original documents are also available on the internet site, irrespective of their associated quality factors. 7. evolution of knowledge since rothé’s catalogue thanks to the continuity of the project over recent decades, numerous events are now known on the basis of 10 or more intensity observations, reaching on average 60 observations when focused research was conducted. for example, the 21 june 1660 event, which was known before the seventies on the basis of only three observations, is today described by 70 intensity observations (vogt, 1983). furthermore, since rothé’s catalogue, 94 new events with io ≥ vi were discovered on french territory (fig. 4a), 33 with io ≥ vii. among the events in rothé’s catalogue for which no epicentral intensity was evaluated, 70 of them are now reported in sisfrance with an estimated epicentral intensity, although two thirds of them remain badly constrained (23 with qie = c and fig. 3. structure of the relational database sisfrance: from the (1) documentary sources, describing an event (2) in the different localities, the evaluated intensities (3) are plotted on a map, and (4) the epicentral parameters are deduced (from godefroy et al., 1990). 578 oona scotti, david baumont, grégory quenet and agnès levret 11 with qie = e and 11 with qie = i). this illustrates the philosophy of the database: to register all known events even badly constrained and to ascribe a reliability factor to each of them. nevertheless, only the well-constrained events are made available to the public on the internet site. thanks to the improvement of the documentary sources both in terms of quality and quantity and to the homogeneity of the fig. 5. state of knowledge in 2002. historical (extracted from sisfrance (2001) database) and instrumental seismicity recorded in french catalogues (cea/ldg 1962-2002). historical events are represented by the squares when the epicentral intensity evaluation is sufficiently reliable and by crosses when it is considered uncertain. fig. 4a,b. comparison between the original rothé catalogue (rothé, 1977) and the present day catalogue extracted from the sisfrance (2001) database for real earthquakes that occurred in france before 1962 and for which the epicentral intensity, io is greater than or equal to vi: a) events with an estimated io that were added to the database (all qualities are considered). fourteen quakes listed in rothé’s catalogue were identified as fake events; b) changes in the epicentral intensity evaluation for events with qie a and b. lo (sisfrance, 2001) lo (sisfrance, 2001) lo (rothé, 1977)a b 579 the french macroseismic database sisfrance: objectives, results and perspectives methodology applied to evaluate the intensity, the epicentral intensities were re-evaluated for a better consistency. figure 4b shows the evolution of epicentral intensity for the most reliable epicentral estimates (io ≥ vi and qie a and b). the differences of evaluation between the sisfrance database and rothé’s catalogue (1977), remains overall symmetrical and centred on 0. most of the changes are smaller than half-degree, which can be considered as the minimal uncertainty of the epicentral intensity evaluation. however, it can be noticed that 50% of rothé’s io = vii-viii or viii were augmented by half degree, whereas 50% of io = viii or viii-ix were reduced by half degree. figure 5 shows a map of the most significant earthquakes (m ≥ 4.5 or io ≥ vi) reported in the historical and instrumental french catalogues the present day. it is remarkable that the instrumental catalogue in 2002 does not show any moderate magnitude seismicity in central france and in the strait of dover whereas clearly the historical catalogue shows several earthquakes generating intensities greater than vi for the past centuries. the comparison of fig. 1 and 5 shows that in the 1970’s, the main active zones were already well depicted by the historical seismicity. naturally, thanks to the efforts conceded in the past 25 years, the seismic activity of these zones is now better quantified, but most importantly better qualified. indeed, for half of the events (io ≥ vi), the epicentral intensity is still poorly constrained, particularly in central western france. 8. perspectives although the sisfrance database could be further improved for instance by explicitly distinguishing primary and secondary sources in the bibliography, by allowing the user to access the interpretative study (from the sources to the intensity quantification) for at least the most important events, by far, the main challenge is to find new strategies to improve our knowledge concerning the numerous important earthquakes (io ≥ vi) for which only a few observation points are available (fig. 6). there is little hope of improving our knowledge through medieval archives: there are only a few and they have already been well exploited (as a means of comparison, the number of french medieval sources is only half of those available for the single town of venice, italy). the most fruitful period to focus research may be the 16th century to the 19th century where the french national archives provide 118 000 units (before 1789). the press, which gave rise to abundant information, has already been widely explored, however, this period is also rich in manuscripts and administrative archives that are waiting to be thoroughly exploited because they may contain valuable documentary sources. acknowledgements the authors thank two anonymous reviewers for their constructive comments. the sisfrance database is co-produced by brgm/ edf/irsn. references alexandre, p. (1990): les séismes en europe occidentale de 394 à 1259, nouveau catalogue critique, observatoire royale de belgique, série géophysique (bruxelles), p. 266. fig. 6. analysis of the database for real events with epicentral intensities ≥ vi: a great number of events are only known through less than 10 data points. the number of events decreases as a function of time. the change observed between the 17th and 18th centuries marks the appearance of the printed sources. 580 oona scotti, david baumont, grégory quenet and agnès levret archives communales de moncontour (vienne), registre paroissial (1711). baumont, d. and o. scotti (2003): ré-examen des intensités des secousses produites par les séismes de loudun moncontour (1711), de parthenay (1772), et de ste-maure (1657), int. rep. irsn/dpre/sergd no. 03-24. chronique de henri duplessis de paumard, sénéchal de saint-jouin (deux-sèvres), archives privées. godefroy, p., s. thirion, j. lambert and b. cadiot (1980): informatisation du patrimoine de sismicité historique de la france, bull. brgm, 2ème série, section iv (2), 139-145. godefroy, p., j. lambert, a. levret and p. vaskou (1990): the french macroseismic database «sirene», in 22nd general assembly of the european seismological commission, barcelona. godefroy, p. and a. levret (1992): creation and management of a database dedicated to historical and contemporary macroseismicity in france: various applications in earthquake engineering, in recent advances in earthquake engineering and structural dynamics, edited by v. davidovici (ouest editions, press académiques), 89-116. goguel, j., j. vogt and c. weber (1985): la carte sismotectonique de la france, in génie parasismique, edited by v. davidoci (presses de l’école nationale des ponts et chaussées, paris), 187-192. lambert, j. (1993): materials of the cec project: review of historical seismicity: the catalonian (1428) and alpine (1564-1644) earthquakes: review of research in france, in historical investigation of european earthquakes, materials of the cec project «review of historical seismicity in europe» edited by m. stucchi (cnr, milano), 145-161. lambert, j. (2003): un nouveau repère dans l’aire pléistoséiste du séisme poitevin du 6 octobre 1711, in documentation, séismes 1708-1711, brgm arnjl-l-03/172. lambert, j. and a. levret-albaret (1996): mille ans de séismes en france, catalogue d’épicentres. paramètres et références (ouest éditions, nantes), pp. 70. lambert, j., p. bernard, g. czitrom, j.y. dubie, p. godefroy and a. levret-albaret (1997): les tremblements de terre en france: hier, aujourd’hui, demain (éditions du brgm, orléans), p. 196. levret, a. (1987): historical seismicity in france and its role in the assessment of seismic risk on french nuclear sites, in historical seismicity of central eastern mediterrenean region (enea-iaea, romecre, casaccia). levret, a., j.c. backe and m. cushing (1994): atlas of macroseismic maps for french earthquakes with their principal characteristics, natural hazards, 10, 19-46. levret, a., m. cushing and g. peyridieu (1996) etude des caractéristiques des séismes historiques en france, in atlas 140 cartes macrosismiques, edited by institut de protection et sûreté nucléaire. levret, a., c. loup and x. goula (1986): the provence earthquake of june 1909 (france). a new assessment of near field effects, in proceedings of the 8th conference of eaee (lisbon). mayer rosa, d. and b. cadiot (1979): a review of the 1356 basel earthquake, tectonophysics, 53, 325-333. medvedev, s.v., w. sponheuer and v. karnik (1967): seismic intensity scale version 1964, punl. inst. geody. jena, 48. melville, c.p., a. levret, p. alexandre, j. lambert and j.vogt (1996): historical seismicity of the strait of dover-pas de calais, terra motae, 8, 626-647. montessus de ballore, f. (1906): les tremblements de terre, géographie sismologique (paris). pallassou, p.b. (1815): liste de plusieurs tremblements de terre ressentis dans les pyrénées, in i.d., mémoires pour servir à l’histoire naturelle des pyrénées et des pays adjacents, t1 pau, vignancour, 260-284. perrey, a. (1845): mémoire sur les tremblements de terre ressentis en france, belgique et hollande, depuis le quatrième siècle de l’ère chrétienne jusqùà nos jours (1843 inclus) (m. hayez, bruxelles), p. 110. quenet, g. (2001): les tremblements de terre en france xvii-xviii siècles. une histoire sociale du risque, thèse de l’université paris i, sous la dir. de daniel roche. quenet, g. and a. levret (2000): aspects méthodologiques de la recherche en sismicité historique: application au séisme de 1704 en poitou, in archéosismicité et vulnérabilité du bâti ancien, edited by r. marichal, actes des iv° rencontres du groupe aps, perpignan 1999, 87-105. quenet, g., g. poursoulis, a. levret and n. lambert (2003): le séisme de 1708 à manosque, france: une approche pluridisciplinaire pour une estimation quantifiée de ses effets, in archéosismicité et vulnérabilité: environnement, bâti ancien et société, edited by r. marichal, actes des vi° rencontres du groupe aps, perpignan, p. 200 (in press). rfs-2001, règles fondamentales de sûreté relatives aux installations nucléaires de base (http:// www.asn.gouv.fr/ data/information/decision12b.asp). rideaud, a. and a. levret (2000): traces de séismes historiques sur le bâti ancien en moyenne durance: beaumont-de-pertuis, in archéosismicité et vulnérabilité du bâti ancien, edited by r. marichal, actes des iv° rencontres du groupe aps, perpignan, 1999, 39-64. rideaud, a. and a. levret (2002): traces du séisme de 1704 sur le bâti de la région de vendœuvre du poitou, in archéosismicité et sismicité historique: contribution à la connaissance et à la définition du risque, edited by r. marichal, actes des v° rencontres du groupe aps, perpignan, 2000, 165-176. rothé, j.p. (1977): listes chronologiques d'épicentres en liaison avec les évènements de sisfrance, travaux inédits et annales et annuaires de l’institut de physique du globe de strasbourg. scotti, o. and a. levret (2000): how to calculate seismic parameters for large offshore historical earthquakes, in 27th general assembly esc, lisbon, portugal. sisfrance (2001): fichier de sismicité historique de la france, brgm/edf/irsn (on line: http://www. sisfrance.net/). stucchi, m. (1993): historical seismology and seismic 581 the french macroseismic database sisfrance: objectives, results and perspectives hazard, ann. geofis., xxxvi (1), 177-189. villette, j. (1905): les tremblements de terre dans les ardennes et les régions voisines (sedan, e. laroche). vogt, j. (1979): les tremblements de terre en france, orléans, mémoire brgm no. 96, p. 220. vogt, j. (1981): carte sismotectonique de la france à 1:1 000 000. mémoire brgm, 111, orléans. vogt, j. (1983): the pyrenees earthquake of 1660: effects in france, disasters, 7 (3), 191-193. vogt, j. and b. massinon (1985): sismicité historique et sismicité instrumentale, in génie parasismique, edited by v. davidoci (presses des ponts et chaussées, paris), 179-186. garcia 18.pdf annals of geophysics, vol. 46, n. 1, february 2003 157 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá departamento de física, facultad de ciencias, universidad de alcalá, madrid, spain abstract in the first part of this work, the dominant time scales that explain the tropical variability of the first sintex simulation (echam4(t30)-orca) are identified through a spectral analysis. higher order spectral analysis is used to examine the interactions among these time scales. the time series analyzed are an average of sea surface temperature over the niño3 region. the time scales obtained are compared with those identified in another coupled gcm simulation (echam4(t42)-opyc3). the higher importance of the biannual time scale in this last is explained partly by the strength of the coupling between the annual and the biannual time scales. there is no such strong coupling in the sintex simulation. important differences among the generation of the simulated warm (or cold) event suggest the need of a systematic classification to isolate their relevant features. therefore in the second part of this work, we address this problem. a space-time cluster analysis is performed on a data set built by collecting the values of the heat content anomalies in the tropical pacific region, in the fifteen months previous to a peak in the niño3 index that has been identified as a ‘warm’ (or ‘cold’) event. in the case of the warm events, three types of generation schemes are found. in two of them, there are anomalies of heat content in the west, north and south of the equator, more than nine months before the events start. in the third case, the anomalies appear and grow in the central equatorial pacific. only two types are needed to classify the generation of cold events. negative sea level height anomalies appear six months before the niño3 index reaches the (local) minimum. they are located north of the equator in one of the groups, and south of it in the other. some of these characteristic traits also appear in observations of warm and cold events. 1. introduction in the present paper, we use different statistical techniques to study the tropical pacific variability found in the echam4(t30)-orca simulation for sintex. the most usual tool for the monitoring of this region is the niño3 index. built as an average of the anomalies of sea surface temperature (hereinafter sst) within the area (150°w-90°w, 5°s-5°n), it provides a time series from which the warm and cold events characteristic of the interannual variability can be readily identified (trenberth, 1997). an equivalent index is employed here for the sst, rather than for its anomalies, produced by two coupled model experiments, the above mentioned sintex run and another one performed with echam4(t42)-opyc3. the dominant time scales of variability in these series are mailing address: mr. francisco j. álvarez garcía, departamento de física, facultad de ciencias, universidad de alcalá, alcalá de henares, madrid 28871, spain; e-mail: franciscoj.alvarez@uah.es key words coupled model – el niño-southern oscillation – seasonal – interannual – heat content – clusters 158 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá determined by means of a standard (second order) spectral analysis. in the eastern equatorial pacific, atmospheric processes are known to be not yet correctly represented in general circulation models (gcm). therefore, differences in the location and strength of the spectral peaks between the simulation and observations are not unexpected. on the contrary, the presence in the sintex simulation of some realistic spectral features (a peak with a period of four years) that do not appear in the echam4opyc analysis is interesting. it cannot be related to a better performance in the simulation of the mean state, which is closer to the observed one in the case of echam4-opyc. a possible explanation could be the existence of more realistic interactions among the different timescales in the sintex simulation. in order to test this point, we have resorted to the help of higher order spectral analysis. there are similarities in the way in which observed warm events start and evolve, and also often in the stage previous to the onset, allowing the identification of a common generation scheme. this is best captured in terms of sea level height (slh) anomalies. well before the onset, slh anomalies appear in the west pacific, north and south of the equator (wyrtki, 1985), onto which they converge some time afterwards, propagating eastward and setting on the event. but exceptions to this rule are not lacking. for instance, things happened differently in the case of the 1991-1993 warm event. thanks to the better sampling, in space and time, provided by coupled simulations, these differences can be further investigated. for their study of the generation of the warm and cold events in the equatorial atlantic simulated in the coupled echam4-opyc3 run, cabos narváez et al. (2002) have devised a statistical procedure that seems to perform satisfactorily. it has also been applied with a similar purpose to the study of the same simulation’s enso events (álvarez garcía et al., 2002). the same procedure will be used here to study the enso generation in the sintex simulation with echam4(t30)-orca. the warm and cold events in the latter run have been identified from the niño3 index and grouped into distinct classes by means of a spatio-temporal analysis on the values of the anomalies of heat content (hereinafter hc), the coupled gcm proxy for slh anomalies, during the period previous to the peak of each event. 100 years of sintex simulation have been analyzed and three (two) groups found for the warm (cold) events. in section 2 of this paper, we give details about the simulated data under study. the spectral analysis of the simulated niño3 time series is presented in section 3. the methodology followed to sort out the warm enso events is explained in section 4, together with the results obtained. those corresponding to the classification of the cold events are given in section 5. section 6 examines connections with climatic signals external to the tropical pacific. we finish with a section where methods and results are summarized and discussed. 2. data the data used for this study were basically those extracted from the first experiment carried out for sintex: the echam4 atmospheric model (roeckner et al., 1996) at t30 resolution coupled to the orca oceanic model (madec et al., 1998). although differences between the observed and simulated mean state are still important in some regions (for instance the eastern equatorial pacific), the modelled interannual variability seems to be quite realistic (gualdi et al., 2003). no trend either in surface or subsurface variables was detected in the 100 year chunk selected for the analysis, that corresponds to years 101 through 200 of the simulation. the spatial domain selected goes from 110°e to 70°w and from 15°s to 15°n. the grid resolution in the domain is 2 degrees in longitude and varies from 0.8 degrees at 15°n and 15°s to 0.5 degrees at the equator in latitude. anomalies were computed by subtracting this period’s climatology from the monthly values of the fields of interest, which are mainly two: the sst and the hc. the latter is the thermal energy stored in the upper oceanic layers, and will be taken here as the model’s proxy for the sea level height. it is an optimal variable to monitor the event’s generation (chao and philander, 1993). 159 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm let it be defined by the depth h0 taking the value of 300 m. for the sake of the discussion, other simulated variables like the wind stress will also be used. additionally, we will analyze an sst index obtained from data of an echam4-opyc3 coupled model run (roeckner et al., 1995 or bacher et al., 1996). the atmospheric component has a t42 resolution, while the oceanic component has a comparable zonal resolution, but a much coarser vertical discretization (only eleven layers). in a recent intercomparison study of 16 models (achuta rao et al., 2000), this cgcm simulation was rated among the best by its simulated seasonal cycle, the statistical characteristics of several climatic indices and the soi-niño3 teleconnections. some of the deficiencies of the model, common to most of them, are warmer than observed sst anomalies in the tropical pacific off the south american coast and the overestimation of the biannual component of enso (bacher et al., 1998). 3. spectral characteristics of the simulated enso variability a preliminary spectral analysis (not shown) was carried out on the sintex simulation of the niño3 index. in the spectrum obtained, the biennial spectral peak is the dominant feature, and is overestimated with respect to the observations. this is a characteristic that the sintex run shares with other coupled model experiments, and particularly with the echam4-opyc3. another lower frequency peak, with a four years period, is found in the sintex data, that is missing in the echam4-opyc3. as it happens, such a peak is present in the observed spectrum of the niño3 index and stronger than the biennial one there. reasons for the spectral differences between the two simulations can be found in the differences between their respective mean states. but, as mentioned above, the mean state of the echam4-opyc run is more ‘realistic’ than the one of the sintex simulation. to search for other possible clue, we will look for scale interactions in a higher order spectral analysis. because the interactions of the interannual variability with the seasonal cycle could be of relevance for our study, we will retain the latter in the time series to be analyzed here. for this reason, the spectra represented in fig. 1 were obtained from sst (not anomalies!) in the niño3 region. the most important spectral diagnostics as regards the comparison of the modelled and observed seasonal cycle is the overestimation of the semiannual signal in the formers, that could be traced back to the failure in the representation of the atmospheric feedbacks in the eastern pacific. at the interannual timescale, the most outstanding features are the excess of power at the biennial period in both simulations, and the four years peak, noticeable in the sintex spectrum. its lack of significance is a side effect of the inclusion of the climatological cycle in the spectral analysis. to study nonlinear interactions among the spectral peaks, higher order spectral functions have to be computed. the one we use here is the squared bicoherence, given by at the pair of frequencies ( f 1 , f 2 ). b denotes the true bispectrum of the process under study, and f its fourier transform, while e {.} stands for the expected value of the magnitude inside the brackets. the terms in the right hand side of the expression are estimated by averaging previous computations obtained for a number of segments into which the time series is split. bounded between 0 and 1, the final estimate of the squared bicoherence provides a measure of the degree of phase coupling (indicative of a possible nonlinear interaction) between distinct time scales present in the series. given the distribution of the estimates (elgar and sebert, 1989) under the hypothesis of zero bicoherence, i.e. no phase coupling at all, values significantly greater than zero (at a chosen level, here 95%) can be determined. they are depicted in fig. 1, right, to show which interactions might be present in the indices under study. hc t r c t t r z dz h p hh , , , r r( ) = ( )∫ ρ 0 0 b f f b f f e f f f f e f f f f f f x2 1 2 1 2 2 1 2 2 1 2 1 2 , , ( ) = ( ) ( ) ( ){ } +( ) +( ){ }∗ 160 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá in the sintex simulation, we find strong nonlinear interaction only among the annual and the intra-annual peaks (semiannual, four and three months periods). these interactions, albeit weaker, are also present in the echam4opyc3 analysis, which in addition shows strong interactions between the two years peak and the annual and semiannual ones (marked with arrows in fig. 1). we hypothesize that this nonlinear interaction, that can be related to the flux correction applied in this run, would dissipate an energy that otherwise would be moved to the lower frequencies. 4. classification of warm events according to their generation 4.1. methodology in order to identify the warm enso events in the sample, the niño3 index was used to spot out the periods when anomalies exceed the threshold of one standard deviation of the time series, with the condition that two events cannot occur within the same year (trenberth, 1997). this criterion identifies correctly the events in the observed niño3 index, and yields 34 warm fig. 1. power spectrum (left) and bicoherence of the niño3 sst index for: (top) sintex simulation, (bottom) echam4-opyc. spectra were computed using a parzen lag-window of width 600 months for both simulations. 161 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm events in the 100 years chunk analyzed for the sintex run. the highest peak in the niño3 sst anomaly during the period of occurrence of an event, will be used as a reference in what follows, as detailed below. once the events have been identified, we proceed to their classification. for each warm event identified, data covering the pacific basin within 15°s-15°n for the 15 months previous and the month where the niño3 index peaks are selected. an ensemble is formed by collecting the 15-month-long chunks corresponding to the different events. to this ensemble, we apply an agglomerative cluster analysis based on the minimization of the following distance between two events with where yp(l ) and yq(l ) stand for the spatial patterns of the hc anomalies in events p and q respectively, l months before their respective peaks in the sst anomaly niño3 index. thus defined, the distance will vary between 0 and 2. the delay l is computed with respect to the peak of niño3 index in each event. that is, in the following, month – 1, for instance, refers to the month previous to the peak, and month + 1 to the one after. notice that the peak of the niño3 index does not correspond to the ‘peak phase’ of rasmusson and carpenter’s composites (1982), but rather to an intermediate stage near ‘transition phase’, as pointed out by latif et al. (1993). the classification is carried out in two stages: at a first stage, when clusters are still undefined, values of for every pair of events, with l ranging between – 15 and 0 months, are computed. an average (over l ) of these distances is used to establish the initial groups, which are formed by a few events (typically 4 or 6) with minimum distance (under a threshold of 0.3) among them. each group is then characterized by a composite at each lag l and on the second stage, the distance to these composites is computed for all the events that remain unclassified. each one is aggregated to its nearest group, using as a criterion the distance averaged over the values of l from – 10 to – 5, chosen for the sake of a more distinct classification. the latter’s robustness, however, has been examined, with positive results, by using other intervals within the (– 15, 0) months range. 4.2. results the results of our classification of the warm events are summarized in table i. the good performance of this classification is evidenced by the fact that the one standard deviation intervals for distances within and outside a given group do not overlap. most of the 34 events are distributed among three groups, that are characterized by their hc composites at different lags. in the hoevmueller diagrams of the fig. 2, the evolution in time (lag before and after the peak) of the hc composite at the equator for each of the three clusters is represented in the top row. in the bottom row, the corresponding sst composites for each type are shown. as can be seen there, in events of type i and ii there is an accumulation of anomalies of hc west of the dateline, fifteen months before the peak of the events, while in events of type iii, anomalies appear only eight months before the peak. the accumulation of hc anomalies in the west for events of types i and ii has an extra-equatorial origin; the role of the extra-equatorial regions at this stage is much less relevant in the case of events of type iii. although events of type i and type ii are similar in that regard, there are also differences dpq l dpq l table i. warm events classification results. n1, d1, s1: number of elements, average distance and standard deviation of the latter within a given group. n2, d2, s2: same as above for events outside a given group. fv: fractional contribution to the variance of the niño3 index due to all warm events by those in a given group. group n1 d1 s1 n2 d2 s2 fv i 12 0.32 0.10 22 0.75 0.20 32% ii 9 0.20 0.09 25 0.82 0.24 47% iii 5 0.40 0.09 29 0.70 0.16 12% d s pq l pq l= −1 s y l y l y l y l pq l pj qjj n pjj n qjj n = () () ()( ) ()( ) = = = ∑ ∑ ∑ 1 2 1 2 1 162 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá between them: events of type ii present a greater strength in the characteristic west-east hc gradient, and they start before and last longer (almost one year). more differences can be detected in the extra-equatorial area. 4.3. the warm events seasonal phase locking phase locking between enso and the seasonal cycle has received much attention. for the events included in rasmusson and carpenter fig. 2. hoevmueller diagrams showing the evolution of (top) heat content (107 jm−2) and (bottom) sst (°c) anomalies averaged within the equatorial strip 3°s-3°n for warm events of type i (left), ii (center) and iii (right). 163 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm (1982) composites (those from 1950 to 1981), the phase locking was evident, the niño3 index peaking in late summer (july-august-september). in recent events (1982-1983 or 1997-1998) the peak of the niño3 index occurs later in the year (neelin et al., 2000), a feature that has sometimes been connected to climate change. it is interesting to check if our classification can be related to the seasonal phase. in the fig. 3a we have represented the seasonal signature of a b fig. 3a,b. a) seasonal distribution of warm sst anomaly peaks in the sintex simulation. the horizontal line marks the 1.5 standard deviation threshold for the niño3 index; b) as in (a), but for the observed sst anomalies (coads + igoss, 1950-1999) peaks. 164 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá fig. 4. vertical section of temperature anomalies (°c) along the equatorial pacific for warm events of type ii in months – 13 (top), – 11 (middle) and – 9 (bottom). superimposed are the climatological 20 °c isotherm in january (broken line) and march (solid line). 165 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm fig. 5. vertical section of temperature anomalies (°c) along the equatorial pacific for warm events of type ii in months – 6 (top), – 5 (middle) and – 4 (bottom). superimposed are the climatological 20 °c isotherm in july (broken line) and september (solid line). 166 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá the warm events in the sintex run, marked by a dot in the month when the niño3 index peaks, together with a measure of their amplitude as given by their maximum value in the index. in order to highlight the stronger events, a straight horizontal line corresponding to an anomaly 1.5 times the standard deviation of the index has been drawn. we can appreciate a preference for events of type i to peak more frequently and strongly in autumn, while events of type ii, that are among the more intense ones, peak preferentially in winter. such a tendency might be linked to an interaction with the mean seasonal cycle, as fig. 4 suggests. in the upper panel, we display a vertical section of the anomalies of temperature along the equator in month –13 of the type ii events composite; warm subsurface anomalies are evident in the western margin. these anomalies are seen to travel eastward, rising along the shallower thermocline, in the following months, as depicted in the middle (month – 11) and lower (month – 9) panels. for events of type ii with niño3 index peaking in winter (december-january), the transit just described corresponds to december (month – 13) to april (month – 9), and coincides with the climatological rise of the thermocline illustrated in fig. 5 by the change in the 20 °c isotherm from january to march. later on in the evolution of events type ii, during months – 6 to – 4, there is an intensification in the subsurface temperature anomalies in the eastern pacific, displayed in the three panels in fig. 5, in response to central pacific highest positive zonal wind anomalies (not shown). again for those events of type ii with niño3 index peaking in winter, the latter enhancement of the warm temperature anomalies occurs in july (month – 6) september (month – 4), when the climatological thermocline shallows in the east due to a reinforcement of the trades. the coincidence of these two relevant stages in the development of the events, namely, the passage of anomalies to the eastern equator and their growth after the strong westerly wind anomalies, with the seasonal migrations of the thermocline appears to explain the preference for winter that the events of type ii display in fig. 3a. the autumnal signature of events of type i, shown in fig. 3a, can also be related to the seasonal displacements of the thermocline. in this case, the transit of subsurface anomalies to the east occurs in months – 10 to – 6, and the strongest westerly wind anomalies and subsequent enhancement of temperature anomalies in the east in months – 3 to – 1. the peak of niño3 index in autumn can be explained again by an association of these features with the climatological january-march and july-september movements of the thermocline. 5. classification of cold events according to their generation using a criterion equivalent to the one above (now sst anomalies must fall under a negative threshold), 36 cold events are identified from the niño3 index, and a classification procedure identical to the one followed in the case of the warm events applied to them. cold events appear to be generated along two different ways only. the statistical details of this classification are given in table ii. it can be also appreciated there how the distance between the centers of each cluster exceeds the value of the average distance within a group plus one standard deviation. for these cold events, the most important differences occur outside the equatorial strip. figure 6 shows the most characteristic feature: negative hc anomalies 6 months before the peak of the event are in the west pacific, south of the equator for events of type i (top row, left) and north of it for events of type ii (bottom row, left). at this time, sst anomalies in the east pacific are already negative in events of type i, while they are still positive in events of type ii. table ii. cold events classification results. n1, d1, s1, n2, d2, s2, fv: same as in table i but for cold events. group n1 d1 s1 n2 d2 s2 fv i 13 0.32 0.15 23 0.75 0.19 39% ii 12 0.44 0.15 24 0.71 0.22 38% 167 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm 5.1. the cold events phase locking the seasonal signatures for the simulated and observed cold events are presented in fig. 7a and 7b, respectively. there seems to be a tight locking of cold events of type ii to may-june, which appears to be related to the simulation’s climatological shallowing of the thermocline in the eastern pacific during the winter-spring months. the cold hc anomalies accumulated in the western part of the basin are not as strong as in events of type i, but the said seasonal feature would act to amplify them once they reach the eastern equator. locking to the seasonal cycle is less strong in the case of events of type i, which do not seem to be so dependent on a seasonal amplifying mechanism. the presence of a seasonal effect is nevertheless suggested by the preferential occurrence of events in september. this tendency points, as in warm events of types i and ii, to a connection between the eastbound transit of anomalies from the western margin with the seasonal rising of the equatorial thermocline in the central pacific in january-march. 6. interaction with extra-basin signals in our analysis of the tropical atlantic simulated with the echam4-opyc3 (cabos narváez et al., 2002), a research into the connection of the different types of events with the enso signal, represented by the niño3 index, has proven very fruitful. we would like to follow the same line to investigate the influence of other climatic signals outside the tropical pacific on enso variability. possible candidates for these signals must be found in the tropics, to exclude the midlatitudes trend, present in the sintex simulation, from the analysis. fig. 6. left: heat content (107 jm−2) and right: sst (°c) anomalies 6 months in advance of a cold sst anomaly peak for events type i (top) and ii (bottom). 168 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá figure 8a shows the connection between the gulf of guinea (or gg) index, representing the tropical atlantic variability, and the niño3 index. the three curves depicted correspond to the period of occurrence of different types of warm events. juxtaposing chunks of 19 months, centered at the peak of each event, correlations with the gg lndex are computed. maximum a b fig. 7a,b. a) seasonal distribution of cold sst anomaly peaks in the sintex simulation. the horizontal line marks the 1.5 standard deviation threshold for the niño3 index; b) as in (a), but for the observed sst anomalies (coads + igoss, 1950-1999) peaks. 169 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm a b fig. 8a,b. a) lagged correlation between the sintex simulation gg and niño3 indices (gg leading at negative lags). different time series have been formed by isolating the periods of occurrence of the distinct types of warm enso events. solid thick lines correspond to correlation values significantly greater than zero at the 5% level; b) as in (a), now for cold enso events. 170 francisco j. álvarez garcía, william d. cabos narváez and maría j. ortiz beviá significant correlations with the gg index are found for events of type iii at lags between – 3 months (gg leading) and + 3 months (gg lagging). for events of type i (those that begin their generation earlier) the significant values of the correlation appear at greater negative lags, while for the type ii events (the more intense ones) negative significants values are found at shorter negative lags. for these last, there are also significant positive values with the gulf of guinea index lagging. in the case of cold events, the type i only has a significant correlation (negative) when the gg index precedes from 10 to 5 months. the correlation in the case of the type ii events is positive and significant when the gg index precedes from 4 up to 0 lags. 7. summary and conclusions a spectral analysis of the niño3 index of the sintex experiment reveals the presence of a 4 year peak, in addition to the dominant biennial timescale. this realistic feature is absent from the spectra of the other coupled simulation analyzed. a possible explanation for the absence of the four years peak in the echam4-opyc3 might lie in the different interactions between the interannual and seasonal variability. this last hypothesis is tested here by means of a higher order spectral analysis, which detects in the bicoherence of the echam4-opyc a strong interaction between the biennial variability and the annual and semiannual peaks. this interaction, not present in the sintex run could divert the energy from the interannual to the intraannual variability. this feature can be perhaps related to the flux correction applied to the echam4-opyc3, which, on the other hand, allows a satisfactory simulation of the midlatitudes and thereby of the tropical-midlatitudes interactions. the similarities and differences among the warm enso events in the sintex simulation are investigated with help of a spatio-temporal cluster analysis. warm events can be sorted into three classes. a common feature in the generation of two of these classes is the accumulation of hc anomalies in the west pacific, north and south of the equator, well before the start of the events. the majority of the events belongs to these groups. but there is a minority in which the hc anomalies appear in the central equatorial pacific. the generation of the cold events present in the sintex run can be explained basically with only two classes. the most characteristic difference is the position, six months before the peak of the event, of the maxima of the negative hc anomalies. in the first type, it is south of the equator, north of it in the other. the influence of the climatological cycle is evident in both warm and cold events and appears to be linked to the seasonal migrations of the equatorial thermocline. thus, hc anomalies that generate warm events of types i and ii are enhanced if they coincide with those displacements, leading to stronger amplitudes of the events. in a sense, these two classes of events could be regarded as two alternate ways of adjustment of enso to the annual cycle, which connects our results to ideas outlined in previous works with simple and hybrid models (neelin et al., 2000). the same mechanism of phase locking to the climatology seems to intervene in cold events, especially in those of type ii, which show a particularly tight locking to the annual cycle, also related to seasonal changes in the thermocline position in the eastern pacific. lastly, the connection of the pacific variability to other tropical signals has been examined bearing in mind the classification of both, warm and cold events. values of the correlation between the gulf of guinea index and the different types of warm events (fig. 7a) point to events of type iii being preceded by tropical atlantic warmings, while warm events of type ii are sometimes preceded by tropical atlantic cold events. the relationship in the case of the cold event types is not as strong as in the case of the warm events. acknowledgements f. álvarez wishes to acknowledge lodyc’s hospitality during a short stay there. the authors are grateful to the anonymous reviewers for their useful comments. this work was supported by 171 spatial and temporal analysis of the seasonal and interannual variability in the tropical pacific simulated with a coupled gcm contract env4-ct98-0714 (sintex) from the european union environmental and climate program. references achuta rao, k., k.r. sperber and the cmip modelling group (2000): the el niño-southern oscillation in coupled gcms, pcmdi report series, 61, pp.46 álvarez garcía, f.j., w.d. cabos narváez and m.j. ortiz beviá (2002): different kinds of enso generation in a coupled gcm simulation, climate dyn. (in revision). bacher, a., j.m. oberhuber and e. roeckner (l998): enso dynamics and seasonal cycle in the tropical pacific as simulated by the echam4-opyc3 coupled general circulation model, climate dyn., 14, 431-450. cabos narváez, w.d., f.j. álvarez garcía and m.j. ortiz beviá (2002): generation of equatorial atlantic warm and cold events in a coupled gcm simulation, tellus a, 54, 426-438. chao, y. and s.g.h. philander (1993): on the structure of the southern oscillation, j. climate, 6, 450-469. elgar, s. and g. sebert (1989): statistics of bicoherence and biphase, j. geophys. res., 94, 10,993-10,998. gualdi, s., a. navarra, e. guilyardi and p. delecluse (2002): assessment of the tropical indo-pacific climate in the sintex cgcm, ann. geophysics, 46 (1), 1-26 (this volume). latif, m., a. sterl, e. maier-reimer and m. junge (1993): climate variability in a coupled gcm, part i: the tropical pacific, j. climate, 6, 5-21. madec, g., p. delecluse, m. imbard and c. levy (1998): opa version 8.1 ocean general circulation model reference manual, tech. rep., lodyc/ipsl note 1.1. neelin j.d., f.-f. jin and h.-h. syu (2000): variations in enso phase locking, j. climate, 13, 2570-2590. rasmusson, e.m. and t.h. carpenter (1982): variations in tropical sea surface temperature and surface wind fields associated with the southern oscillation/el niño, mon. weather rev., 110, 354-384. roeckner, e., j.m. oberhuber, a. bacher, m. cristoph and i. kirchner (1995): enso variability and atmospheric response in a global coupled oceanatmosphere gcm, max planck institut für meteorologie, report n. 170, hamburg. roeckner, e., k. arpe, l. bengtsson, m. cristoph, l. duemenil, m. esch, m. giorgetta, u. sclese and u. sculzweida (1996): the atmospheric general circulation model echam4: model description and simulation of present day climate, max planck institut für meteorologie, report n. 218, hamburg. trenberth, k.e. (1997): the definition of el niño, bull. amer. meteorol. soc., 78, 2771-2777. wyrtki, k. (1985): water displacements in the pacific and the genesis of el niño cycles, j. geophys. res., 90, 7129-7132. t h e b a b o l k e n a r ( 1 9 7 1 ) e a r t h q u a k e a n d t h e s e i s m o t e c t o n i c s o f t h e c e n t r a l a l b o r z ( i r a n ) j . s . t c h a l e n k o * m . h . i r a n m a n e s h * * a . m o h a j e r a s h j a i * * * r e c e i v e d o n d e c e m b e r 23, 1971 summary. — t h e b a b o l k e n a r e a r t h q u a k e (9tli a u g u s t , 1971) w a s a r e l a t i v e l y m i n o r e v e n t c o m p a r e d t o p r e v i o u s c a t a s t r o p h i c e a r t h q u a k e s in t h e c e n t r a l a l b o r z m o u n t a i n s of n o r t h e r n i r a n . i t ' s s t u d y h o w e v e r , is i m p o r t a n t f o r t h e u n d e r s t a n d i n g of t h e s e i s m i c i t y of a r e g i o n w h i c h h a s a l r e a d y e x p e r i e n c e d t w o l a r g e e a r t h q u a k e s d u r i n g t h i s c e n t u r y a n d w h i c h c o u l d b e e n t e r i n g a p h a s e of r e n e w e d s e i s m i c a c t i v i t y . t h e p a t t e r n d e d u c e d f r o m t h e s e q u e n c e of all r e c o r d e d e a r t h q u a k e s in t h e r e g i o n o u t l i n e s t h e s t r u c t u r a l u n i t s of t h e c e n t r a l a l b o r z a n d s u g g e s t s t h e e x i s t e n c e of s e i s m o t e c t o n i c p r o v i n c e s w h i c h a r e a l t e r n a t i v e l y a c t i v e a n d q u i e s c e n t . riassunto. — il t e r r e m o t o di b a b o l k e n a r (9 a g o s t o 1971) è s t a l o u n e v e n t o r e l a t i v a m e n t e di s c a r s o r i l i e v o s p e c i e s e r a f f r o n t a t o a t e r r e m o t i c a t a s t r o f i c i p r e c e d e n t i a v v e n u t i nelle m o n t a g n e d e l l ' a l b o r z central© ( i r a n s e t t e n t r i o n a l e ) . il s u o s t u d i o , t u t t a v i a , è i m p o r t a n t e p e r c o n o s c e r e la s i s m i c i t à di u n a r e g i o n e c h e ili q u e s t o secolo h a g i à s u b i t o d u e gl'alidi t e r r e m o t i e c h e p o t r e b b e q u i n d i e n t r a r e in u n a f a s e di r i n n o v a t a a t t i v i t à s i s m i c a . il m o d e l l o d e d o t t o d a l l a s e q u e n z a di t u t t i i t e r r e m o t i r e g i s t r a t i n e l l a r e g i o n e , t r a c c i a i c o n t o r n i d e l l e u n i t à s t r u t t u r a l i d e l l ' a l b o r z c e n t r a l e e s u g g e r i s c e l ' e s i s t e n z a di p r o v i n c e s i s m o t e t t o n i c h e c h e s o n o a l t e r n a t i v a m e n t e a t t i v e e t r a n q u i l l e . 1 . i n t r o d u c t i o n the babol-kenar earthquake of magnitude mb = 5.2 occurred on the 9th august 1971 a t 02h 54m 36.7s about 150 km northeast of tehran in the caspian province of mazanderan. the instrumental * e n g i n e e r i n g s e i s m o l o g y , i m p e r i a l college, l o n d o n . ** g e o l o g i c a l s u r v e y of i r a n , t e h r a n . *** g e o p h y s i c s s e c t i o n , i m p e r i a l college, l o n d o n . f i g . 1 b a b o l k e n a r e p i c e n t r a l r e g i o n ( a p p r o x i m a t e s k e t c h ) . d a s h e d l i n e i n d i c a t e s a u t h o r ' s r o u t e . insert: full circle, t o w n s w i t h s l i g h t d a m a g e . open circle, t o w n s w h e r e s h o c k w a s s t r o n g l y f e l t . shaded rectangle, b a b o l k e n a r e p i c e n t r a l r e g i o n . asterisk, i n s t r u m e n t a l e p i c e n t r e . a = a b a i i ; b = b a b o l ; b h = b e h s h a r h ; b s = b a b o l s a r ; d = d a m a v a n d ; f = f i r u z k u h ; g = g o r g a n ; n = n o w s h a h r ; p = p o l s e f i d ; s = s a r i ; s g = s h i r g a h ; s h = s h a h i ; s i = s a n g c l i a l ; t = t e h r a n ; z = z i r a b . 28 j . s. t c h a l e n k o m. h . i r a n m a n e s h a . m o h a j e r a s h j a i epicentre was calculated by noaa to be 30.2 x 52.7 e a n d t h e focal d e p t h 27 km. the geological survey of i r a n sent a t e a m to t h e area to define precisely the macroseismic epicentral region a n d to assess t h e d a m a g e a n d t h e seismotectonic implications of t h e event. t h e region of m a x i m u m d a m a g e was f o u n d to be a t t h e n o r t h e r n m o s t limit of t h e alborz foothills, south of t h e town of babol a n d a t a b o u t 15 k m f r o m t h e i n s t r u m e n t a l epicentre (figure 1). a b o u t 900 houses i n 42 villages suffered d a m a g e of some kind (generally light) a n d one person died and 39 were i n j u r e d . a s u m m a r y of t h e t e a m ' s visit t o t h e region and conclusions reached is given below. 2 . d a m a g e : 2.1. general considerations. the e a r t h q u a k e felt over m o s t of n o r t h c e n t r a l i r a n a n d t h e shock was reported as being strong in t e h r a n , ab-ali, d a m a v a n d , f i r u z k u h , gorgan, behshahr, nowshahr, as well as in t h e towns nearer to t h e epicentre: babol, amol, shalii, babolsar, shirgah, zirab a n d pol selid (figure 1). occasional damage occurred in a fewr places a t a considerable distance f r o m t h e epicentre: a t f i r u z k u h (65 km), t h e amir k a b i r school built of kiln-bricks was slightly d a m a g e d a n d one person was i n j u r e d ; a t n o w s h a h r (120 km), some brick houses developed cracks a n d glass panes were broken; a t ab-ali (100 km), cracks a p p e a r e d in one wall of t h e casino. i n this last instance, no other d a m a g e could be found cither in ab-ali or in a n y of t h e surrounding villages, a n d it m u s t be assumed t h a t i n m a n y of these anomalous cases d a m a g e was due to severe weaknesses or haws in t h e building or in i t s foundations. i n localities nearer to t h e epicentral region, slight d a m a g e was more general. i n t h e suburbs of babol, a m i n a r e t collapsed (in bidabad) a n d some rows of houses were d a m a g e d and glass panes s h a t t e r e d (in bid-abad and raste-bazar). i n babolsar, one old building was destroyed, the roofs of two houses caved in a n d several houses develop ed cracks. i n shirgah, m o s t houses were cracked, a n d in t h e zivla district near t h e p o s t office some houses p a r t l y collapsed. i n sangchal, epicentre of the e a r t h q u a k e of 2nd j u l y 1957 (see n e x t section, seismotectonics), a few houses were cracked by t h e r e a c t i v a t i o n of old landslides. f i g . 1 b a b o l k e n a r e p i c e n t r a l r e g i o n ( a p p r o x i m a t e s k e t c h ) . d a s h e d l i n e i n d i c a t e s a u t h o r ' s r o u t e . insert: full circle, t o w n s w i t h s l i g h t d a m a g e . open circle, t o w n s w h e r e s h o c k w a s s t r o n g l y f e l t . shaded rectangle, b a b o l k e n a r e p i c e n t r a l r e g i o n . asterisk, i n s t r u m e n t a l e p i c e n t r e . a = a b a l i ; b = b a b o l ; b h = b e l i s l i a r h ; b s = b a b o l s a r ; d = d a m a v a n d ; f = f i r u z k u h ; g = g o r g a n ; n = n o w s h a h r ; p = p o l s e f i d ; s = s a r i ; s g = s h i r g a h ; s h = s h a h i ; s i = s a n g c l i a l ; t = t e h r a n ; z = z i r a b . 3 0 j . s . t c i i a l e n k o m . i i . i r a n m a k e s i i a . m o i i a j e r a s h j a i the severest d a m a g e a n d all casualties occurred in the b a b o l k e n a r region a b o u t 25 k m south of b a b o l (figure 1). mere, w i t h i n a radius of a b o u t 7 km, m o s t of t h e houses in all t h e villages were affected to some degree, albeit m a n y only lightly. buildings were also d a m a g e d in b a n d p e v to t h e west of b a b o l k e n a r and along t h e shahi-pol sefid road (see appendix). the d a m a g e in b a b o l k e n a r is described in more detail below. 2.2 epicentral region. b a b o l k e n a r is located a t t h e limit between t h e caspian plain a n d t h e n o r t h e r n m o s t alborz foothills. the villages d a m a g e d by t h e e a r t h q u a k e are s i t u a t e d on either side of t h e r u d e babol river (also k n o w n as n a h r e mianrud). as in other p a r t s of m a z a n d e r a n , t h e villages here h a v e a loosely-knit s t r u c t u r e of disconnected hamlets in t h e midst of rice a n d c o t t o n fields and jungle forests. the t r a d i t i o n a l houses, either built entirely of t i m b e r or of a timber f r a m e w o r k filled with m u d a n d plaster, h a v e o f t e n two storeys a n d nearly always t h a t ched roofs. the i t i n e r a r y followed in babol-kenar is shown in f i g u r e 1. in t h e peripheric villages such as e h d a s h t , d a r u n kola sharglii a n d rcis kola, d a m a g e was s c a t t e r e d a n d slight. f o r example in d a r u n kola sharghi, some walls showed cracks b u t only one house (owner: f s f a n d y a r mahdari) p a r t l y collapsed due to a f o u n d a t i o n weakness. in govan kola, where people r e p o r t e d to h a v e been t h r o w n down by t h e shock (?), t h e local t i m b e r houses resisted b e t t e r t h a n t h e kilnbrick a n d m o r t a r constructions (plate 1). deraz k o l a a n d k e b r i a kola are typical of t h e more severely d a m a g e d villages. i n deraz kola, 40 o u t of t h e 320 houses would require extensive repairs before re-occupation (plate 2). there were however neither complete collapses, nor serious i n j u r i e d due to collapsing elements. people were n o t t h r o w n down. a well built brick a n d m o r t a r residence a b o u t 50 years old (and k n o w n as r e z a s h a h ' s palace) resisted with only v e r y minor d a m a g e . i t s owner confirmed i n f o r m a t i o n g a t h e r e d in the surrounding villages, t h a t t h e last strong e a r t h q u a k e in the region was felt in 1935, a n d t h a t t h e 1957 e a r t h q u a k e of was much weaker (see section on seismotectonics). the d u r a t i o n of severe shaking in 1971 was e s t i m a t e d between 6 a n d 8 seconds f r o m interviews reconstructing t h e m o v e m e n t s of observers d u r i n g t h e e v e n t . t h e b a b o l k e n a r ( 1 9 7 1 ) e a r t h q u a k e , e t c . .3.3 p l a t e 2 l o c a l t y p e h o u s e i n d e r a z k o l a . all i n n e r a n d o u t e r w a l l s will h a v e t o b e r e b u i l t , b u t t h e t i m b e r f r a m e w o r k a n d roof will o n l y n e e d m i n o r r e p a i r s . p l a t e 1 s h o p s i n grovan k o l a . t h e s e k i l n b r i c k a n d m o r t a r c o n s t r u c t i o n s g e n e r a l l y s u f f e r e d m o r e t h a n t h e local t y p e h o u s e s . t h e b a b o l k e n a r ( 1 9 7 1 ) e a r t h q u a k e , e t c . .3.3 tt is of interest to note t h a t t h e two-storey talar (*) s t r u c t u r e showed no sign of damage or strain by this e a r t h q u a k e b u t were reported by locals as swaying violently during t h e e a r t h q u a k e of 1935. tn conclusions, local t i m b e r f r a m e d houses were severely cracked only if they were old or if they h a d weak foundations. no complete collapse was seen a n d partially collapsed houses were the exception. well-built local houses resisted with much less d a m a g e t h a n t h e few small kiln-brick a n d m o r t a r houses. no evidence was f o u n d for g r o u n d f r a c t u r i n g of a n y type. 3 . seismotectonics 3.1 previous catastrophic earthquakes two e a r t h q u a k e s are known to have caused wide-spread destruction during this century in t h e central alborz. the first one (n° 1 in fig. 2) occurred on t h e 11th april, 1935 and was assigned a magnit u d e of 0.75 (4). little is known of the resulting damage, except t h a t 27 villages were destroyed and a b o u t 400 people were killed. the second e a r t h q u a k e , of m a g n i t u d e 7.4 (n° 3 in fig. 2), occurred on t h e 2nd j u l y , 1957 a n d caused a b o u t 1000 deaths. m a x i m u m d a m a g e was in t h e sangchal region (fig. 1) even though extensive destruction accentuated by landslides and rockfalls occurred along t h e ileraz a n d talar valleys. both e a r t h q u a k e s were followed by strong aftershocks: in 1935 these were confined to the eastern p a r t of the range, whereas in .1957 t h e y nearly all took place in the west. a smaller shock a t t h e eastern limit of t h e area studied here (n° 2 in fig. 2) d a m a g e d gorgan on t h e 5th april, 1944, killing 20 people in the town a n d surrounding villages. 3.2 sequence of earthquakes w h e n t h e chronology of e a r t h q u a k e s in the central alborz of m a g n i t u d e greater t h a n 4 is considered, t h r e e distinct regions can be delineated. f r o m 1932 (may) to 1944 (november), all e a r t h q u a k e s (*) c a g e l i k e r o o m s s e t o n w o o d e n p i l e s a n d r a i s e d b y a b o u t 4 m e t r e s f r o m t h e g r o u n d . t h e y a r e u s e d i n s u m m e r a s s l e e p i n g q u a r t e r s . 34 j . s . t c h a l e n k o m. i i . i r a n m a n e s h a. m o u a j e r a s i i j a i occurred on t h e n o r t h e r n (caspian) flank a n d east of t h e talar and h a b l e h r u d valleys. f r o m 1957 (may) to 1962 (august) e a r t h q u a k e s were also located on the n o r t h e r n flank, b u t west of t h e talar-iiableh r u d valleys (*). f r o m 1962 (september) onwards, these two regions were quiescent, almost all events being confined to t h e s o u t h e r n (tehran-semnan) flank of t h e range. the t h r e e seismicity regions, n o r t h e a s t ern, n o r t h w e s t e r n a n d s o u t h e r n , can be deduced f r o m figure 2 which shows all the i n s t r u m e n t a l l y determined epicentres in t h e central alborz. t h e b a b o l k e n a r e a r t h q u a k e of 1971 is t h e first event to diverge appreciably f r o m this p a t t e r n , [t occurred in t h e n o r t h w e s t e r n seismicity region, whereas t h e great m a j o r i t y of shocks f r o m september 1962 u p to august 1971 were s i t u a t e d in t h e s o u t h e r n region. i t s i n s t r u m e n t a l epicentre (n° 4 in fig. 2) is slightly north of the 1957 epicentre, t h e macroseismic. d a t a in 1957 as well ,as in 1971 being in a g r e e m e n t with these positions. e v e n t h o u g h t h e intensity a t t h e epicentre was f a r g r e a t e r in 1957 t h a n in 1971, t h e areas of m i n i m u m d a m a g e were similar, all d i s t a n t locations h a v i n g suffered minor d a m a g e in 1971 being within t h e isoseismal vi (mm) of the e a r t h q u a k e of 1957. 3.3 seismotectonic provinces. the differentiation of a n o r t h e r n a n d s o u t h e r n region is a s t r u c t u r a l characteristic of t h e alborz t h r o u g h o u t its geological history. a n early r a n g e existed already in t h e paleogene, later developing into a barrier s e p a r a t i n g t h e caspian depression in the n o r t h f r o m t h e neogene basins of central i r a n in t h e south (7'8). different regimes of sedim e n t a t i o n a n d tectonic m o v e m e n t s on either side of this barrier are known for various geological periods, a n d are p e r h a p s m o s t striking for t h e eocene-oligocene time. eocene sedimentation a n d volcanism producing t h e " g r e e n b e d s " or " k a r a j f o r m a t i o n ' ' are widespread and up to 400m thick on t h e s o u t h e r n flank of t h e alborz, b u t entirely missing in the n o r t h . the n o r t h e r n limit of these eocene f o r m a t i o n s coincides a p p r o x i m a t e l y with t h e n o r t h e r n limit of t h e s o u t h e r n seismicity region seen in t h e previous section (fig. 2). several m a j o r f a u l t zones r u n parallel to t h e general t r e n d of t h e range, subdividing it into s t r u c t u r a l regions (1.2.3.«); there are some indi(*) f r o m n o v e m b e r 1944 t o m a y 1957, w i t h o n l y 2 e x c e p t i o n s , n o e a r t h q u a k e s w e r e r e c o r d e d f o r t h e r e g i o n u n d e r s t u d y . t h e b a b o l k e n a r ( 1 9 7 1 ) e a r t h q u a k e , e t c . .3.3 cations t h a t t h e s o u t h e r n m u s h a f a s h a n f a u l t zone a n d t h e zone a t t h e n o r t h e r n border of the range m a y be active. w h e t h e r t h e y are f u n d a m e n t a l to t h e seismicity of the central alborz it is not possible to determine with t h e limited d a t a available a t present. p r o b a b l y of greater significance to t h e seismotectonics of t h e region is t h e sudden change in the s t r u c t u r a l directions f r o m n w s e in t h e west to n e s w in t h e east. the m a j o r tectonic event of the q u a t e r n a r y , the creation of the d a m a v a n d volcano, is located near this hinge, and t h e delineation of a n o r t h e a s t e r n and a n o r t h w e s t e r n region on t h e basis of earthquake sequence indicates t h a t a t present seismicity is also controlled by this change in s t r u c t u r a l directions. the correspondence between t h e seismicity seen in t h e previous section and t h e regional tectonics suggests t h a t t h e northwestern, n o r t h e a s t e r n a n d southern regions are seismotectonic provinces which, to a certain e x t e n t , act i n d e p e n d a n t l y one f r o m another. 3.4 conclusions the s t r u c t u r a l subdivision recognized by geologists for the central alborz, i.e. t h e n o r t h e r n and southern flanks, are also seismicity units on t h e basis of the chronological sequence of recorded e a r t h q u a k e s . f u r t h e r m o r e , t h e larger e a r t h q u a k e s are concentrated in t h e region where the s t r u c t u r a l t r e n d changes a b r u p t l y f r o m n w to n e . the three seismotectonic provinces which can t h u s be defined, t h e northeastern the northwestern a n d t h e southern seismotectonic provinces, have been, since a b o u t 1900, alternatively active a n d quiescent. the babol-kenar e a r t h q u a k e of t h e 9th august 1971, occurring in a province which has been quiescent since t h e last aftershocks of t h e e a r t h q u a k e of 1957, m a y signify t h a t the n o r t h e r n flanc of t h e alborz is entering a new phase of seismic activity. a c k n o w l e d g e m e n t s the field work reported here was u n d e r t a k e n as p a r t of a joint project between t h e geological survey of i r a n and t h e engineering seismology section, imperial college, london, t h e l a t t e r being supported for this work by a g r a n t f r o m t h e national e n v i r o n m e n t a l research council, london. 3 6 j . s . t c 1 1 a l e n k o m . h . i r a n m a n e s h a. m oh a j e r a s h j ai r e f e r e n c e s (') allenbach 1'., 1966. geologie und petrographie des damavand und seiner umgebung (zentral-elburz), iran. " m i t t . c e o l . i n s t . e i d g e n . t e c h . h o c h s h u k u n i v . z u r i c h " , n . f . 6 3 , p p . 144. (2) assereto r . , 1966. geological map of upper djajerud and lar valleys (central elburz, iran). i n s t . geol. u n i v . m i l a n o , s e r i e g . , p u h l . , n o 2 3 2 , p p . 86. (3) gansser a. a n d iiuber, h . , 1962. geological observations in the central elburz, iran. " s c h w . m i n . i ' e t r . m i t t . " , 4 2 (2), p p . 5 8 3 6 3 0 . ( 4 ) g u t e n b e r g b . a n d r i c h t e r c . f . , 1 9 5 4 . seismicity of the earth, 2 n d e d . p r i n c e t o n , n e w j e r s e y : p r i n c e t o n u n i v e r s i t y p r e s s . ( 5 ) n a t i o n a l o c e a n i c a n d a t m o s p h e r i c a d m i n i s t r a t i o n ( n o a a ) : preliminary determination of epicentres. (6) steiger r . , 1966. die geologie der west-firuzkuh area (zentral elburz, iran.) " m i t t . geol. i n s t . e i d g e n . t e c h . h o c h s c h u l e u n i v . z u r i c h " , n . f . 6 8 , p p . 145. (7) stocklin j . , 1968. structural history and tectonics of iran. " a m . a s s o c . p e t r o l . g e o l . , bull. " 5 2 (7), 1229-1258. (8) stocklin j . a n d nabavi m., 1968. summary of the geology of the alborz mountains and iranian azarbaijan. " i n t . session o n t e c t . m a p s of e u r o p e , n e a r a n d m i d d l e e a s t , b a k u . 1968 " . d e l e r m i n a t i o n of earthquake m a g n i t u d e and intensity in the ussr e . e . s a v a r e n s k y the determinatimi of earthquake magnitude in the soviet union is based on instrumentai observations using t h e international magnit u d e scale. the estimations of m for distant earthquakes usually involve observations of amplitudes and periods a t the m a x i m u m phase of surface waves. to simplify the work seismic stations use nomograms (fig. 1, n. v. sliebalin and s. l. solovyov f6'1')). in esthnating t h e seismic energy for near and weak earthquakes recorded b y higlily sensitive i n s t r u m e n t s t h e galitzin method (') is used. such estimates for locai earthquakes are made a t t h e i n s t i t u t e of seismology of t h e academy of sciences of tadjikskaya sse (v. i. bune (9'10)). work is proceeding to improve methods of determining m from observations of body waves as well as to find regional corrections of m depending on t h e location of e a r t h q u a k e epicentres (s. l. solovyov (mm'.imi)). devices recording energy flux of earthquakes are now being designed which will m a k e m determinations more rehable. i n determining instrumentai magnitude m in the soviet union the scientists do not confine themselves to t h e instrumentai seismic investigations and to t h e connection of these with tectonic. processes (e. e. savarensky ("•8' u15 •20)) b u t consider t h e practical problem of connecting t h e intensity near the epicentre with t h e magnitude m. this is i m p o r t a n t for earthquake-proof construction especiauy in cases when forni erly non-inhabitant regions of seismic activity are considered. the first stage of this problem deals witli t h e determination of t h e connection between t h e intensity i0, t h e magnitude m and t h e focal depth h . this dependance was found by interpreting a few hundreds of earthquakes and comparing t h e intensity a t t h e epicentre with t h e magnitude m. finally t h e following empirical formula was deduced: i„ = 1.5 m — 3.5 lge + 3 . d e t e r m i n a t i c i of e a r t h q u a k e m a g n i t u d e , etc. 3 7 1 the formula is t r u e for earthquakes with focal depths of no more t h a n 60 k m . a t greater depths the influence of a layer with a lowered velocity (asthenosphere) is being exerted. (n. v. shebalin (6>l9'22)). the determination of i0 by m is seen from the above nomogram (fig. 1). the estimates assume t h a t average properties between t h e focus and surface of the e a r t h are obtained everywhere. i n conclusion mention is made on t h e systematization and cataloguing of earthquakes by magnitude. w h e n compihng seismicity maps of respective zones of t h e ussr (e. e. savarensky (20)) according to their magnitude ali earthquakes are divided into 5 groups: g r o u p m a n d io i n t e n s i t y ( a t n o r m a l f o c a l d e p t h s ) d i s t a n c e of r e g i s t r a t i o n i 7 1/2 9 a n d m o r e r e c o r d e d b y t h e m o s t s t a t i o n s of t h e w o r l d i i 6.5-7.5 8-9 i i i 5 1/4-6.5 ca. 7 60° a n d m o r e i v 4 1/4-6 1/4 — 20-60° y 4 1/4 — u p t o 20° thus, the first three groups are based on practical considerations. to some e x t e n t t h e y include destructive earthquakes considered to be t h e most violent in the usse. the lower groups include weak earthquakes which are of interest for a general study of seismicity. i n some cases m is determined from the distance of registration. the catalogues a t t a c h e d to t h e seismicity maps are compded according to t h e f o r m : d a t e t i m e of e p i c e n t r e f o c a l m a g n i m d i s t a n c e n o t e s e a r t h c o o r d i n a d e p t h t u d e of r e g i s t r a ( i n f o r q u a k e t e s d e p t h g r o u p t i o n a n d m a t i o n h . r n . s . n a m e of t h e on i n t e n m o s t d i s t a n t s i t y s t a t i o n . a c c u r a c y c l a s s : a e p i c e n t r a l e r r o r is less t h a n 25 k m . , b le ss t h a n 50 k m . 3 7 2 e . f . s a v a r e n s k y to m a k e a more accurate determination of i 0 b y m it is necessary to t a k e into account t h e influence of t h e locai geologie s t r u c t u r e in the epicentral region upon t h e a m p h t u d e of oscillations. tliis influence is witnesses b y t h e results of numerous investigations of destruction caused b y earthquakes. the main principal problem is t o determine t h e interval of periods in which we m u s t t a k e into account t h e influence of t h e layer of comparatively sedimentary rocks. l e t us m a k e b u t a simple estimate of t h e effect of direct f r o m any direction se waves and vertical p waves. observations show t h a t such waves are most dangerous. l e t us consider piane waves of u n i t y a m p h t u d e . i t sliould be recalled t h a t a t the surface of a semi-space t h e amplitude of the oscillations considered is equal to 2. if a layer with thickness i i is situated on this surface, then, as eayleigh showed, waves coming f r o m t h e semi-space a n d reflected m a n y times in the layer will interfere. f o r p waves t h e amplitude magniflcation takes place each time when t h e ratio of the layer thickness to t h e wavelengtli is multiple of the odd n u m b e r of quarters. a t these ratios the surface amplitude is m equal to 2 — » where m1 and m„ are the acoustic rigidity of t h e layer and of the semi-space. witli t h e wavelength x considerably exceeding t h e layer thickness j j h , when fi = 2 n is small, t h e a m p h t u d e of oscillations at. t h e a layer surface will be near to 2 and to t h e influence of t h e layer can be neglected. this error will be determined by t h e difference: sin /3 — /s thus, t h e m a x i m u m value of t h e period in the interval is determined. w e sha.ll f u r t h e r t a k e into account t h e absorption of t h e energy oscillations b y t h e layer assuming t h a t t h e absorption is inversly proportional to the wavelength or t o t h e period, which f a c t is conflrmed b y observations. i n this case t h e a m p h t u d e changes with distance z b y t h e formula: —2jcx» —— e a where is t h e damping coefficient and t h e a m p h t u d e decrease with t h e increase of > or with t h e reduction of the wavelength. this damping a lowers resonance m a x i m a a t t h e layer surface. d e t e r m i n a t i c i of e a r t h q u a k e m a g n i t u d e , e t c . 3 7 3 hence it follows t h a t a t sufficiently large values of t h e absorption will be so great t h a t the layer eft'ect will not be i m p o r t a n t . these d a t a help to find the minimum value of the period in t h e interval. f i g . 2 to calculate a m p h t u d e s on a layer surface (dry loess sandy soil, etc.) a 1 k m deep, lying on a granite semispace and where ratio of acoustic 771 rigidities — — = 10 and damping coefficient = 0.1 (e. y. k a r u s (23,)) 7u1 t h e influence of t h e layer should be taken into consideration within t h e interval of periods from 0.5 to 7 seconds fig. 2). abstract the determination of earthquake magnitude in the soviet union is based on instrumentai onservations under the international magnitude scale. the dependence between intensity gradation b, magnitude m and focal depth h was found by interpreting a few hundreds earthquakes: b = 1.5 m — 3.5 ig h + 3. 3 7 4 e . f . s a v a r e n s k y the formula is true for earthquakes with focal depths of no more than 60 km (n. v. shebalin). the estimates assume that medium properties between the focus and surface of the earth are everywhere equal. to make a more accurate determination of b it is necessary to take into account the influence of the locai geological structure of the epicentral region upon the amplitude of oscillations. compiling seismicity maps of respective zones of the ussr according to their magnitude ali earthquakes are divided into five groups: ^ 7 l / 2 ; 6 v.-7 7«i 5 1/4-6 i/2; 4 74-5 v«5 < 4 7«. in the published quarter bulletins of the seismic stations of the ussr m is given to within 1li and at m > 4. riassunto la determinazione della magnitudo dei terremoti nella unione sovietica è basala su osservazioni strumentali in base alla scala di magnitudo internazionale. la relazione tra gradazione b di intensità, magnitudo m e profondità focale e è stata determinata interpretando alcune centinaia di terremoti: b = 1.5 m — 3.5 ig m + 3 . la formula è vera per terremoti con profondità focali non superiori a 60 km (n. v. shebalin). si presume che le caratteristiche del mezzo tra il fuoco e la superficie della terra siano ovunque eguali. per effettuare una più precisa determinazione di b e necessario tener presente v'influenza della struttura geologica locale della regione epicentrica, rispetto all'ampiezza delle oscillazioni. preparando le mappe della sismicità delle rispettive zone dell'u.r.s.s. secondo la loro magnitudo, tutti i terremoti sono stati suddivisi in cinque gruppi: > 7 v.; 6 v2-7 7.5 51/4-6 7 4 7 4 -5 y 4 ; < 4 7 4 . nel bollettino trimestrale pubblicato dalle stazioni sismiche dell' u.s.s.r. m è dato entro l'intervallo compreso fra m — 1/i e m ^ 4. d e t e r m i n a t i c i o f e a r t h q u a k e m a g n i t u d e , e t c . 3 7 5 r e f e r e n c e s ( ! ) rojinqi:,ih b . e . , o 3cm.iemp/icehuu 18 (fieepajm 1911 iod, h 3 b . h m n . a h , 9 9 1 9 9 8 , (1915)( 2 ) kocehko c. h., k eonpocy o eumicnenuu snepeuu 3emaemphcemiù. t p . reotjjhah. 2 1 , 148, 3 1 5 ( 1 9 5 3 ) . ( 3 ) cojiobbeb c . j i . , o k/iaccutfiuxatfuu semaempnceuuù no ecaimuiie ux auepzmi. tp. reoijjhah 30, 157, 3 2 1 , ( 1 9 5 5 ) . ( 4 ) cabapehckhh e . o . , ceùcmuhhocmb cccp, limoni u nepcnenmubu ee tayueiiun. tp. reoij)hah 25, 152, 5 2 4 , ( 1 9 5 4 ) . ( 5 ) beuihhkob h . b . , o koauhecmeehhoù oiiehke cuau 3emaemphceuuh. h 3 b . a h c c c p , c e p . r e o t j ) h 3 . , 3 , 2 0 7 2 1 4 , ( 1 9 5 5 ) . ( 6 ) ihebajihh h . b., o c8h3u medicdy snepeueù, óaaabhocmbw u layóunoù ovaia 3emaempncehuù. ii3b. ah cccp, cep. reo(j)h3. 4, 3 7 7 3 8 0 , ( 1 9 5 5 ) . ( 7 ) cojiobbeb c . ji. h xl>khejiafl3e 3 . a . , oó u3menenuu nomona ceucmuieckoù ouepmu c paccmohhuem. h3b. ah cccp, cep. reocj>h3. 5, 4 6 2 4 6 3 , ( 1 9 5 5 ) . ( 8 ) cabapehckhh e . o . , paboma ceùcmuneckux cmamtuù u 3aòanu coeemckou ceùcmoaozuu. becthhk ah cccp, 5, 4 0 4 3 , ( 1 9 5 5 ) . (9) eyh3 b. h . , o kaacctii/ìukaijuu 3emaempncemiù no shepiuu ynpyiux boah, u3ayuaembix u3 ouaea. ,h,okji. ah t a f l » . ccp, 2 4 , ( 1 9 5 5 ) . (10) — o kaaccui/iukaijuu 3emaempncehuu no ux cuae ha ocnoee uhcmpymeumaabhbix dauhbix. h 3 b . a h cccp, cep. reotj)h3. 1, 4 8 5 5 , ( 1 9 5 6 ) . ( u ) cabapehckhh e . o . h j1>khej7aa3e 3 . a . , o ceucmunnocmu eojibiuozo kaei<a3a. h 3 b . a h c c c p , c e p . r e o t } ) h 3 . 5 , 5 7 7 5 8 3 , ( 1 9 5 6 ) . ( 1 2 ) e y h 3 b. h . , cpaeuumeabho suepeemuneckan xapakmepucmuka ceùcmuuuocmu mpex paùouoe taòoicukucmaea. .hokji. ah tafl>k. cccp, 19, 3 1 2 , ( 1 9 5 6 ) . (13) — 06 ucnoab3oeahuu memoda roauifbina òah npuóauoicehhoù oi\ehku snepeuu6au3kux 3emaemphceuuù. t p . m h c t . cehcmojlonra a h ta,t>k. c c c p 1, 3 2 7 , ( 1 9 5 6 ) . ( 1 4 ) cojiobbeb, c . j i . — o c6h3u suepiuu ti uwnehcuehocmu semaemp/iceiiuii. h 3 b . a h cccp, cep. reoc{>h3. 3, 3 5 7 3 5 9 , ( 1 9 5 6 ) . ( 1 5 ) cabapehckhh e . <t>., os uckaoicenunx e xapmax ceucmunuocmu. h 3 b . a h c c c p , cep. reo(j)h3. 7, 745-754) ,1956). ( 1 6 ) kohflopckah h . b. h n o c t o j i e h k o t. a . , ceùcmuhhocmb cccp 3a 1955. h3b. a h cccp, cep. reo<j?h3., 2, 2 5 5 2 5 7 , ( 1 9 5 7 ) . ( 1 7 ) cojiobbeb c . j i . h iiieeaiihh h . b., onpeòeaeuue unmeiicumiocmu 3emaempaceuuh no cmeufenuio noneu e nocepxhocmhbix eoauax. h3b. a h c c c p , c e p . reo<j)h3. 9 2 6 9 3 0 , ( 1 9 5 7 ) (18) — k eonpocy 0 coomuouiehuu meyicdy 3hepzueu o6"emhbix boah a umnehcuehocmbto 3emaemphcemiù. ekwijieiehb cobeia no ceìicmojiorhh ah cccp, 6 , 1 0 9 1 2 2 , (1957)3 7 6 e . f . s a v a r e n s k y ( 1 9 ) iiiebajihh h . b . , coomnoiuemie meotcdy óajuibnocmbw u unmehcuehocmbw 3emjiempmcehuu e 3aeucum0cmu om i/ty6uiibi oh aia. e i o j i j i e t e h b c o b e t a n o cchcmojioi-hh a h c c c p , 6 , 1 2 2 1 2 7 , ( 1 9 5 7 ) . ( 2 0 ) cabapehckhh e . <t>., 06 iisynenuu ceùcmuhhoemu cccp. eronnetehb c o b e t a n o cehcmojiorhh a h c c c p , 6 , 1 6 1 9 , ( 1 9 5 7 ) ( 2 1 ) c o j i o b b e b c . j i . , hekotnopbie pe3yabmambi npumenenun luna/iu unmencuanocmu 3emaemphcehuù ha ceùcmuheckux cmanifuhx cccp. g r y f l h h reo(j)h3hka h metoflhka ^ e x o c j i o b a i i k o h a h , 2 , 4 0 4 5 , ( 1 9 5 8 ) . ( 2 2 ) iiiebajihh h . b . , hcnoab3oeahue coomitouienun meoicdy unmehcu6n0cmbw u óaaathocttibio 3emaemphcehuù òah oyehku zayòuhbi acmehocifiepbi e paùoue bpanna (kapnambi). c r y f l h a r e o ( j ) h 3 h k a h reoaethka m e x o c j i o b a i j k o ì i a h 2 , 8 6 8 7 , ( 1 9 5 8 ) . ( 2 3 ) k a p y c e . b . , rioiaoufenue ynpyiux boah e eophbix nopoòax npii cmatfuohaphom eo36yncdehuu. h 3 b . a h c c c p , c e p . r e o(j)h3. 4 , ( 1 9 5 8 ) . i n d a g i n e g e o l o g i c a e geofisica su di un affioramento argilloso nei pressi di pratica di mare roma (geological and geophysical research on an outcrop of clay near pratica di mare • rome) a . s o n a g l i a e . v o l p o n i ( * ) r i c e v u t o il 3 0 g i u g n o 1972 riassunto. — n e i p r e s s i d i p r a t i c a di m a r e ( t a v . a r d e a f o g . 158 i v n . o . ) è s t a t a e s e g u i t a u n a c a m p a g n a d i s o n d a g g i e l e t t r i c i i c u i r i s u l t a t i , a s s o c i a t i a i d a t i g e o l o g i c i di r i l e v a m e n t o e a d a t i r e p e r i t i d a l l a b i b l i o g r a f i a g e o l o g i c a d e l l a z o n a ( s o n d a g g i m e c c a n i c i , e l e t t r i c i e g r a v i m e t r i c i ) , h a n n o p e r m e s s o d i p o r r e i n r e l a z i o n e l a p a l e o m o r f o l o g i a e d il s i g n i f i c a t o s t r u t t u r a l e d i u n a f f i o r a m e n t o a r g i l l o s o c u p o l i f o r m e c o n i l i n e a m e n t i t e t t o n i c i p r e v u l c a n i c i d e l l a r e g i o n e . summary. — i n t h e a r e a of p r a t i c a d i m a r e ( t a v . a r d e a f o g . 158 i v n . o . ) a s e r i e s of e l e c t r i c a l s o u n d i n g s h a s b e e n c o n d u c t e d . i t s r e s u l t s , a s s o c i a t e d w i t h d a t a of g e o l o g i c a l s u r v e y s a n d t h o s e g a t h e r e d b y g e o l o g i c a l r e p o r t s o n t h e a r e a ( m e c h a n i c a l , e l e c t r i c a l a n d g r a v i m e t r i c a l s o u n d i n g s ) a l l o w e d t o e s t a b l i s h l i n k s b e t w e e n p a l e o m o r p l i o l o g y a n d t h e s t r u c t u r a l m e a n i n g of a d o m e s h a p e d c l a y o u t c r o p o n t h e o n e h a n d a n d t h e p r e v o l e a n i c t e c t o n i c f e a t u r e s of t h e a r e a o n t h e o t h e r . n e l c o r s o d e l r i l e v a m e n t o g e o l o g i c o t e c n i c o d e l l a t a v . a r d e a ( f o g . 1 5 8 i v n . o . ) i n s c a l a 1 : 2 5 . 0 0 0 , a t t u a l m e n t e i n f a s e d i e s e c u z i o n e , è s t a t o p r e s o i n p a r t i c o l a r e e s a m e u n a f f i o r a m e n t o a r g i l l o s o n e i p r e s s i d i p r a t i c a d i m a r e , r i p o r t a t o d a l l a c a r t a g e o l o g i c a d ' i t a l i a c o m e p l i o c e n e m e d i o s u p e r i o r e . (*) c e n t r o s t u d i p e r l a g e o l o g i a t e c n i c a , c . n . r . , i s t i t u t o d i g e o l o g i a a p p l i c a t a a l l ' i n g e g n e r i a , r o m a . ó 8 0 a . s o d a g l i a e . v o l p o n i l ' a f f i o r a m e n t o , v e n u t o a giorno i n s e g u i t o a l l ' a v a n z a m e n t o del f r o n t e di c a v a e allo s b a n c a m e n t o della c o p e r t u r a , c o s t i t u i t a d a s a b b i e e p r o d o t t i p i r o c l a s t i c i , v i e n e c o l t i v a t o dalla « soc. l a t e r i z i p r a t i c a di m a r e »; il m a t e r i a l e , con o p p o r t u n i p r o c e d i m e n t i di l a v o r a z i o n e , ris u l t a t r a i migliori del g e n e r e p e r la p r o d u z i o n e di l a t e r i z i p e r l'edilizia. l a c a v a è s i t u a t a t r a il p a e s e di p r a t i c a di m a r e ed il c i m i t e r o o m o n i m o e p r e c i s a m e n t e sul l a t o sinistro del f o s s o di p r a t i c a (fig. 1). f i g . 1 p l a n i m e t r i a e d i r e t t r i c i dei p r o f i l i g e o e l e t t r i c i . 13' — — b , d i r e t t r i c e del p r o f i l o g e o e l e t t r i c o • 1, u b i c a z i o n e e n u m e r o dei p r o f i l i g e o e l e t t r i c i l a z o n a i n generale, p r e s e n t a u n a morfologia q u a s i p i a n e g g i a n t e con q u o t e c o m p r e s e t r a i 70 ed i 90 m s.l.m. e v i e n e a t t r a v e r s a t a d a u n a incisione p i u t t o s t o p r o f o n d a nella q u a l e scorre il f o s s o di p r a t i c a ; a d essa confluisce t u t t a u n a serie di vie d i deflusso a l i m e n t a t e e s c l u s i v a m e n t e nella s t a g i o n e delle p r e c i p i t a z i o n i , di cui la p i ù p r o f o n d a r i s u l t a quella p o s t a alla b a s e del paese. l ' a f f i o r a m e n t o argilloso, ogg e t t o della p r e s e n t e n o t a , è u b i c a t o alla confluenza della incisione del f o s s o di p r a t i c a con quella p o s t a alla b a s e del paese. i n d a g i n e g e o l o g i c a e g e o f i s i c a su d i u n a f f i o r a m e n t o e c c . 58!) s u l f r o n t e di c a v a si osserva u n a p a r e t e argillosa di circa 10 m e t r i , c o s t i t u i t a d a u n ' a l t e r n a n z a di s t r a t i argillosi di colore g r i g i o a z z u r r o d i 40-50 c m d i p o t e n z a , i n c l i n a t i d i 22°-25° con i m m e r s i o n e n 30° w , e i n t e r c a l a t i d a sottili livelli limoso-argillosi d a poco c o e r e n t i a sciolti. a t e t t o d e l l ' a f f i o r a m e n t o si p u ò osservare u n a p a l e o s u p e r f i c i e d i e r o s i o n e che t a g l i a o segue l ' a n d a m e n t o stesso della superficie di s t r a t o (fig. 2). o o o so°o copertura argilla alluvioni recenti f i g . 2 s e z i o n e i n d i c a t i v a d e l f r o n t e d i c a v a . l a c o p e r t u r a del b a n c o argilloso è c o s t i t u i t a d a s a b b i e c o n t i n e n t a l i e m a r i n e del t a r d o q u a t e r n a r i o (*), d a t u f i g r a n u l a r i grigi e pozz o l a n e i n f e r i o r i del v u l c a n i s m o laziale e d a s a b b i e rosse eoliche ferr e t t i z z a t e c h e c o p r o n o u n i f o r m e m e n t e i s o t t o s t a n t i m a t e r i a l i . l a giac i t u r a della c o p e r t u r a è s u b o r i z z o n t a l e e q u i n d i i n d i s c o r d a n z a a n g o l a r e con le argille s o t t o s t a n t i ; nell'insieme la c o p e r t u r a h a u n a p o t e n z a c o m p l e s s i v a di circa 25 m e t r i . l e p r i m e osservazioni e f f e t t u a t e sul f r o n t e d i c a v a , d i s p o s t o i n d i r e z i o n e e w , p o s t e i n relazione con l ' a m b i e n t e geologico c i r c o s t a n t e , h a n n o f a t t o r i l e v a r e l ' e s i s t e n z a di d e t e r m i n a t i c a r a t t e r i che p o s s o n o d a r luogo a d a l c u n e i n t e r p r e t a z i o n i nella r i c o s t r u z i o n e degli e v e n t i c h e h a n n o c o n t r i b u i t o a d e t e r m i n a r e l ' a t t u a l e s i t u a z i o n e l i t o s t r a t i g r a f i c a . (*) l e a n a l i s i m i c r o p a l e o n t o l o g i c h e s o n o s t a t e e f f e t t u a t e d a l d o t t . . m . c h i o c c l i i n i . 163 a. s o n a g l i a e . v o l p o n i si r a p p r e s e n t a s c h e m a t i c a m e n t e la sezione del f r o n t e d i c a v a (fig. 2) i n d i c a t o con la d i r e t t r i c e d i profilo g e o e l e t t r i c o a ' a i n fig. 1, in m o d o d a r e n d e r e p i ù c h i a r a l'esposizione dei d a t i r i c a v a t i . c o m e i n d i c a t o i n figura il l a t o d e s t r o del f r o n t e argilloso p r e s e n t a , r i s p e t t o al l a t o sinistro, u n a a l t e z z a m a g g i o r e (50 m s.l.m.) e la s t r a t i f i c a z i o n e del b a n c o argilloso segna u n a i n c l i n a z i o n e r e g o l a r e e c o s t a n t e p e r q u a s i t u t t a l ' e s t e n s i o n e d e l l ' a f f i o r a m e n t o ; a v v i c i n a n dosi a l f o s s o di p r a t i c a le argille c a m b i a n o r a p i d a m e n t e la loro inclin a z i o n e a s s u m e n d o u n a g i a c i t u r a s u b o r i z z o n t a l e . alla b a s e dell'incisione i c , le argille sono c o p e r t e d a a l l u v i o n i rec e n t i m e n t r e a f f i o r a n o n u o v a m e n t e , a l di là della m e d e s i m a incisione, a d u n a q u o t a a l q u a n t o più b a s s a (q. 43 m s.l.m.). s u t u t t a la p a r e t e del f r o n t e argilloso, c o m p r e s o t r a l'incisione io e il f o s s o di p r a t i c a , è possibile e f f e t t u a r e m i s u r e di i n c l i n a z i o n e ed i m m e r s i o n e ; s u l l ' a f f i o r a m e n t o p o s t o a d e s t r a dell'incisione i c , q u e s t o r i s u l t a p r a t i c a m e n t e impossibile, p r e s e n t a n d o s i la f o r m a z i o n e argillosa f r a t t u r a t a e d i s l o c a t a in t u t t e le direzioni. p u ò s o l t a n t o a f f e r m a r s i c h e l o c a l m e n t e , in condizioni p a r t i c o l a r m e n t e f a v o r e v o l i , è s t a t a oss e r v a t a u n a i m m e r s i o n e a e s t c o n t r a r i a q u i n d i a quella m i s u r a t a sul f r o n t e di c a v a . i n b a s e alle osservazioni e f f e t t u a t e sul luogo, si r i t i e n e che l'affior a m e n t o sia il r i s u l t a t o di u n a m o d e s t a e locale t e t t o n i c a che a v r e b b e i n t e r e s s a t o le argille d i base, sollevandole e dislocandole d a l l a loro posizione o r i g i n a r i a , p r i m a della messa i n p o s t o dei m a t e r i a l i costit u e n t i la c o p e r t u r a . l a p r e s e n z a di u n a serie di faglie a piccolo r i g e t t o , o s s e r v a b i l i sid f r o n t e d i c a v a , i n t e r r o t t e al c o n t a t t o t r a le argille ed i m a t e r i a l i s o v r a s t a n t i , e la g i a c i t u r a degli s t r a t i del b a n c o argilloso c o n t r a r i a sui d u e fianchi dell'incisione i c , i n d u c e a r i t e n e r e possibile la p r e s e n z a di u n a faglia, in direzione a n t i a p p e n n i n i c a , i m p o s t a t a l u n g o l'incisione le, il cui p i a n o di s c o r r i m e n t o s e p a r e r e b b e l ' a f f i o r a m e n t o s u cui è i m p o s t a t o il f r o n t e d i c a v a d a quello r i b a s s a t o . s e m p r e i n b a s e a osservazioni e f f e t t u a t e sul luogo è s t a t o possibile r i c a v a r e considerazioni d e d o t t e d a i r a p p o r t i s t r a t i g r a f i c i che leg a n o le argille con la c o p e r t u r a , (fig. 3). i l c o n t a t t o t r a le argille plioceniche ed i d e p o s i t i fluvio-lacustri c o n t i n e n t a l i , è m a r c a t o d a u n a superfìcie di erosione; p e r t a n t o , i r a p p o r t i d i r e t t i t r a le argille e le ghiaie-sabbiose sono d a a t t r i b u i r e a d i n t e n s i f e n o m e n i erosivi che h a n n o p e r m e s s o t a l e c o n t a t t o . gli e s a m i paleontologici, e f f e t t u a t i su diversi c a m p i o n i , h a n n o f i g . 3 s e z i o n o n o r m a l e a l l a d i r e t t r i c e a' argille ghiaia sabb grigia o legg arrossata sabbia grigia ghiaia sabb. mollo a r r o s s a l a sabbia con ghiaia lievemente arrossata sabbia grigio-giallo sabbia arrossata con livelletti argillosi sabbia gialla con macrofossili e poupè sabbia molto arrossata livello torboso materiale grigio tufaceo sabbia gialla con inclusi vulcanici (scorie ì tufo antico pozzolane rosse sabbie rosse a del f r o n t e di c a v a . 5 8 4 a . s o n a g l i a e . v o l p o n i p e r m e s s o d i d a t a r e le argille al p l i o c e n e i n f e r i o r e (zona a g l o b o r o t a l i a p u n t i c u l a t a ) ; l ' a m b i e n t e d i deposizione è t i p i c a m e n t e m a r i n o , n e r i t i c o m e d i o . i d e p o s i t i fluvio-lacustri a c o n t a t t o e in d i s c o r d a n z a c o n le argille sono c o s t i t u i t i d a u n a serie di livelb di g h i a i a e s a b b i a della p o t e n z a c o m p l e s s i v a di m 1,70, a t t r i b u i b i l i p r o b a b i l m e n t e a d u n a f a s e contin e n t a l e c a l d a d u r a n t e la q u a l e si sono resi possibili i f e n o m e n i di ossid a z i o n e che h a n n o c o n f e r i t o ai depositi stessi u n g r a d o di a r r o s s a m e n t o j j ì ù o m e n o i n t e n s o . s e m p r e i n c o n c o r d a n z a s t r a t i g r a f i c a è s t a t o r i l e v a t o u n a l t r o orizz o n t e fossilifero, c o s t i t u i t o d a s a b b i e gialle a macrofossili, r i f e r i b i l e al t a r d o q u a t e r n a r i o . i macrofossili p r e s e n t i , l a s c a r s a q u a n t i t à in microfossili, d i regola i n c a t t i v o s t a t o di c o n s e r v a z i o n e , ed i r e s t i o r g a n i c i p i ù r e c e n t i n o n f o r n i s c o n o p i ù sicure i n d i c a z i o n i c r o n o s t r a t i g r a f i c h e , t r a t t a n d o s i di t e r m i n i d i f f u s i d a l p l i o c e n e s u p e r i o r e al q u a t e r n a r i o . vi sono, t u t t a v i a , mag g io ri p r o b a b i l i t à che t a l i s a b b i e si s i a n o d e p o s t e in a m b i e n t e m a r i n o , litorale, d u r a n t e il t a r d o q u a t e r n a r i o . l a q u a l i t à del r e s i d u o i n o r g a n i c o di origine v u l c a n i c a f a s u p p o r r e u n a c e r t a c o n t e m p o r a n e i t à t r a la deposizione d i q u e s t i m a t e r i a l i e l'inizio d e l l ' a t t i v i t à v u l c a n i c a della regione laziale. al d i s o p r a delle s a b b i e gialle a macrofossili v i è u n livello di s a b b i a m o l t o a r r o s s a t a d i circa 1 m e t r o di p o t e n z a sul q u a l e p o g g i a u n pìccolo livello t o r b o s o che r a p p r e s e n t a l'inizio di u n a facies c o n t i n e n t a l e c h e si s v i l u p p e r à poi con la m e s s a i n p o s t o dei d e p o s i t i v u l c a n i c i s o v r a s t a n t i . l a serie p r o s e g u e con u n complesso di t u f i « a n t i c h i », c o s t i t u i t i , d a l b a s s o v e r s o l ' a l t o , d a u n livello di m a t e r i a l e p o m i c e o di circa 1 m e t r o , d a u n o s t r a t o di 2 m e t r i di s a b b i a gialla con a b b o n d a n t i inclusi v u l c a n i c i (scorie), d a u n o s t r a t o di t u f o g r a n u l a r e l i t o i d e grigio t i p o « p e p e r i n o » di 2 m e t r i di p o t e n z a , e finalmente dalle p o z z o l a n e i n f e riori rosse con u n a p o t e n z a di oltre 4 m e t r i . a c o p e r t u r a dei s o v r a i n d i c a t i m a t e r i a l i vi sono i n f i n e le s a b b i e rosse f e r r e t t i z z a t e , di t r a s p o r t o eolico, che a s s u m o n o p o t e n z a v a r i a bile t r a i 2 ed i 5 m e t r i . d a l l a sezione di f i g . 3, quindi, r i s u l t a e v i d e n t e l ' e s i s t e n z a di u n a l a c u n a nella successione s t r a t i g r a f i c a dei s e d i m e n t i , che si e s t e n d e d a l p l i o c e n e medio s u p e r i o r e al t a r d o q u a t e r n a r i o ; sono a s s e n t i i dep o s i t i del t a r d o pliocene, del c a l a b r i a n o e del siciliano, che i n v e c e si r i s c o n t r a n o a circa 2 k m in linea d ' a r i a a s u d della c a v a di p r a t i c a di m a r e , nella serie s t r a t i g r a f i c a regolare e c o n t i n u a , visibile i n l o c a l i t à « c a m p o selva » a q. 37 m s.l.m. nei pressi di t o r v a j a n i c a . i n d a g i n e g e o l o g i c a e g e o f i s i c a su d i u n a f f i o r a m e n t o e c c . 58!) i n f a t t i i n u n ' i n d a g i n e e s e g u i t a nella l o c a l i t à « c a m p o s e l v a » è s t a t o possibile o s s e r v a r e i s e g u e n t i m a t e r i a l i d e s c r i t t i qui di s e g u i t o d a l l ' a l t o v e r s o il b a s s o : — u n a c o p e r t u r a d i s a b b i a rossa f e r r e t t i z z a t a c o n t e n e n t e limo e g h i a i e t t o siliceo p o t e n z a m a s s i m a 5 m e t r i ; — esteso b a n c o di argilla grigio-azzurra, a d e l e v a t a c o n s i s t e n z a , s t r a t i f i c a t a ed i n g i a c i t u r a s u b o r i z z o n t a l e p o t e n z a m a s s i m a g m e t r i . a p p a r t i e n e a l c a l a b r i a n o i n f e r i o r e (zona a h y a l i n e a b a l t h i c a ) e l ' a m b i e n t e di deposizione è m a r i n o , neritico m e d i o ; — u n d e p o s i t o c o s t i t u i t o d a s a b b i a sciolta e c e m e n t a t a , ricco di r e s t i di m o l l u s c h i , riferibile a l l ' i n t e r v a l l o p l i o c e n e s u p e r i o r e c a l a b r i a n o i n f e r i o r e , s e n z a p o s s i b i l i t à di ulteriori precisazioni m a che i n d i c a u n a m b i e n t e d i depos i zi one m a r i n o , n e r i t i c o m e d i o p o t e n z a m a s s i m a 2 m e t r i ; — i n f i n e u n a f f i o r a m e n t o d i argilla d i colore grigio-azzurro, m o l t o p l a s t i c a e d a f r a z i o n e g r a n u l o m e t r i a m i n u t a ed o m o g e n e a . l ' e t à di q u e s t a argilla è del p l i o c e n e s u p e r i o r e ( p a r t e a l t a ) ; l ' a m b i e n t e d i deposizione è m a r i n o , n e r i t i c o medio. l a l a c u n a s t r a t i g r a f i c a p r e c e d e n t e m e n t e m e n z i o n a t a s e m b r a int e r e s s a r e s o p r a t t u t t o la z o n a che d a p r a t i c a d i m a r e si e s t e n d e v e r s o n o r d . q u e s t o a s p e t t o s t r a t i g r a f i c o si r i t r o v a a n c h e p o n e n d o i n relazione a l c u n i sondaggi, eseguiti n e l l a z o n a (*). n e l l a s t r a t i g r a f i a del pozzo g6, p r o s s i m o alla c a v a d i p r a t i c a di m a r e , m a n c a n o i t e r r e n i del c a l a b r i a n o ; m e n t r e a i pozzi g ì g2 p o m e z i a 1 p o m e z i a 2, si r i s c o n t r a u n a c o n t i n u i t à di s e d i m e n t a z i o n e d a l p l i o c e n e al t i r r e n i a n o . l a d a t a z i o n e sul p l i o c e n e r i p o r t a t a sui s o n d a g g i n o n c o n c o r d a con la s u d d i v i s i o n e d a noi s t a b i l i t a , i n q u a n t o d a p a r t e n o s t r a si è f a t t o r i f e r i m e n t o ai r e c e n t i s t u d i e congressi sul p l i o c e n e ; t u t t a v i a i m e d e s i m i s o n d a g g i ci h a n n o p e r m e s s o di o t t e n e r e mag g io ri r a g g u a g l i r i g u a r d o l ' a n d a m e n t o paleo-morfologico e s t r a t i g r a f i c o del b a s a m e n t o argilloso p l i o c e n i c o c a l a b r i a n o e di stabilire, con sufficiente approssim a z i o n e , il t e r r i t o r i o m a g g i o r m e n t e i n t e r e s s a t o d a l f e n o m e n o erosivo. (*) v e d i ili « i n d a g i n i s u l l e a c q u e s o t t e r r a n e e d e l l ' a g r o r o m a n o e p o n t i n o » p a r t e i a g r o r o m a n o . s o n d a g g i i n d i c a t i c o n g ì g2 g6 p o m e z i a 1 p o m e z i a 2. 5 8 0 a . s o n a g l i a e . v o l p o n i d a l l ' a n d a m e n t o delle i s o b a t e del t e t t o delle argille, r i p o r t a t e nell ' o p e r a s o p r a c i t a t a , si n o t a d ' a l t r a p a r t e che il f e n o m e n o erosivo si è i m p o s t a t o i n u n a z o n a d i m a s s i m o s o l l e v a m e n t o ; n e r i s u l t a p e r t a n t o che l ' a r e a o g g e t t o del p r e s e n t e s t u d i o , è s t a t a i n t e r e s s a t a d a f e n o m e n i t e t t o n i c i c h e d e v o n o a v e r f o r m a t o u n a serie di c u l m i n a z i o n i e d e p r e s sioni n e l l ' a m b i t o dei s e d i m e n t i argillosi, p e r m e t t e n d o q u i n d i all'erosione d i a g i r e con m a g g i o r e i n t e n s i t à sulle c u l m i n a z i o n i p i ù a c c e n t u a t e . d a l l ' i n s i e m e delle c o n s i d e r a z i o n i s o p r a e s p o s t e si p u ò d e d u r r e q u i n d i che a l l ' a s p e t t o p a l e o m o r f o l o g i c o del s u b s t r a t o argilloso h a n n o c o n t r i b u i t o f e n o m e n i di c a r a t t e r e t e t t o n i c o e c o n s e g u e n t i f e n o m e n i d i erosione c o n c e n t r a t a ; q u e s t i f a t t o r i , m a n i f e s t a t i s i i n g r a n d e p e r t u t t o il t e r r i t o r i o , h a n n o d a t o luogo, in p a r t i c o l a r e nella z o n a d e l l a c a v a d i p r a t i c a d i m a r e , a faglie e dislocazioni. allo scopo d i r i c o s t r u i r e i n d e t t a g l i o l ' a n d a m e n t o p a l e o m o r f o logico del t e t t o del s u b s t r a t o argilloso c i r c o s t a n t e l a z o n a della c a v a , si è a t t u a t a u n a c a m p a g n a d i p r o s p e z i o n e g e o e l e t t r i c a . l ' i n d a g i n e geofisica è s t a t a e f f e t t u a t a m e d i a n t e l ' e s e c u z i o n e di u n d i c i s o n d a g g i e l e t t r i c i v e r t i c a l i d i s p o s t i secondo profili o p p o r t u n a m e n t e scelti (vedi f i g . 1) i n m o d o d a risolvere, nelle g r a n d i linee, il p r o b l e m a d e l l ' a n d a m e n t o e della p r o f o n d i t à del t e t t o del s u b s t r a t o argilloso nella z o n a c i r c o s t a n t e la c a v a . l e c u r v e d i r e s i s t i v i t à r e l a t i v e ai s o n d a g g i e l e t t r i c i v e n g o n o rip o r t a t e n e l l a t a v o l a i . i s o n d a g g i sono s t a t i e f f e t t u a t i con u n ' a p p a r e c c h i a t u r a p o t e n z i o m e t r i c a a c o r r e n t e c o n t i n u a ed è s t a t a a d o t t a t a la disposizione elett r o d i c a di s c h l u m b e r g e r s i n e r i z con stese m a s s i m e f r a gli e l e t t r o d i d i c o r r e n t e a b v a r i a b i l i , a secondo delle esigenze, t r a i 40 m 1. ed i 250 in 1.. l ' i n t e r p r e t a z i o n e dei s o n d a g g i è s t a t a e s e g u i t a m e d i a n t e il conf r o n t o f r a le c u r v e d i c a m p a g n a e le c u r v e t e o r i c h e ; q u e s t e u l t i m e sono s t a t e c o s t r u i t e u t i l i z z a n d o gli a b a c h i della c.g.g. di a r e l l a n a m o o n e y e gli a b a c h i di c a g n i a r d . i s o n d a g g i e l e t t r i c i e f f e t t u a t i sui t e r r e n i i n a f f i o r a m e n t o h a n n o c o n s e n t i t o di a t t r i b u i r e agli e l e m e n t i litologici della z o n a i v a l o r i della r e s i s t i v i t à s o t t o i n d i c a t i : — s a b b i e rosse e p r o d o t t i piroclastici 100 o h m •m — s a b b i e c a l c a r e e con n o d u l i d i a r e n a r i a 40 o h m ' •m — livelli argillosi; s a b b i e e g h i a i e con limo 20 o h m ' m — argille (plioceniche?) 15 o h m •m i n d a g i n e g e o l o g i c a e g e o f i s i c a su di u n a f f i o r a m e n t o e c c . 58!) t a v o l a 1 c u r v e di r e s i s t i v i t à . d a l c o n f r o n t o dei v a l o r i di r e s i s t i v i t à s o p r a r i p o r t a t i , r e l a t i v i alle singole f o r m a z i o n i , con quelli o t t e n u t i d a i s o n d a g g i e l e t t r i c i e f f e t t u a t i i n c a m p a g n a , è s t a t o possibile r i c o s t r u i r e l ' a n d a m e n t o dei v a r i m e z z i geologici ed i n p a r t i c o l a r e del t e t t o del s u b s t r a t o argilloso c o n d u t t i v o (vedi f i g g . 4,5). l ' a n a l i s i dei r i s u l t a t i h a p e r m e s s o di f a r e le s e g u e n t i c o n s i d e r a z i o n i . è a c c e r t a t a l ' e s i s t e n z a di u n a c o n t i n u i t à del t e t t o del s u b s t r a t o argilloso ed è c o n f e r m a t o l ' a n d a m e n t o p a l e o m o r f o l o g i c o dell'« a l t o » 5 8 8 a . s o n a g l i a e . v o l p o n i argilloso a d i n c l i n a z i o n e d i f f e r e n z i a t a , già r i c o n o s c i u t a con l ' i n d a g i n e d i c a m p a g n a ; è d i m o s t r a t o c h e v e r s o x n e , l ' i m m e r s i o n e del t e t t o del s u b s t r a t o è m o l t o p i ù m a r c a t a che n o n sul l a t o o v e s t ed e s t , della c a v a . e v i d e n t e m e n t e l ' a t t u a l e f r o n t e di c a v a t a g l i a , nella z o n a d i m a s s i m a c u r v a t u r a , u n a c u l m i n a z i o n e del s u b s t r a t o argilloso, i cui fianchi p r e s e n t a n o u n a i m m e r s i o n e p e r i m e t r a l e a d a n d a m e n t o a s i m m e t r i c o con v a l o r i p a r t i c o l a r m e n t e e l e v a t i di i n c l i n a z i o n e nella p a r t e o r i e n t a l e . a n a l i z z a n d o i d a t i r e l a t i v i alla coltre e c o n f r o n t a n d o i n s i e m e le q u a t t r o sezioni delle f i g g . 4,5 si p u ò o s s e r v a r e che l ' e l e m e n t o a resistiv i t à 40 o h m m , r a p p r e s e n t a t o d a l l a f o r m a z i o n e delle s a b b i e calcaree, p o s t e alla b a s e del p i r o c l a s t i c o , subisce u n a n o t e v o l e r i d u z i o n e di spessore i n d i r e z i o n e del c i m i t e r o d i p r a t i c a di m a r e ; i n t u t t e le a l t r e i n d a g i n e g e o l o g i c a e g e o f i s i c a su d i un a f f i o r a m e n t o e c c . 5 8 ! ) sezioni q u e s t a f o r m a z i o n e r a g g i u n g e spessori n o t e v o l m e n t e m a g g i o r i c o m e al s o n d a g g i o n. 6, o v e r i s u l t a essere di circa 50 m e t r i . e v i d e n t e m e n t e l a deposizione delle s a b b i e calcaree è a v v e n u t a con m a g g i o r e a p p o r t o d i m a t e r i a l e n e l l ' a r e a c i r c o s t a n t e la c u l m i n a zione argillosa già e s i s t e n t e all'inizio «lei f e n o m e n o ; la d i m i n u z i o n e di spessore nella p a r t e o r i e n t a l e , invece, d e v e essere p r i n c i p a l m e n t e a t t r i b u i t a a f e n o m e n i erosivi locali. l a p a r t e p i ù a l t a della c o p e r t u r a , c o m p r e n d e n t e il p i r o c l a s t i c o e le s a b b i e rosse f e r r e t t i z z a t e , con r e s i s t i v i t à s u p e r i o r e a i 100 o h m • m , p r e s e n t a spessori r e l a t i v a m e n t e u n i f o r m i , v a r i a b i l i f r a i 10-20 m e t r i , v a l o r i che si r i p e t o n o i n q u a s i t u t t i gli a f f i o r a m e n t i della z o n a circ o s t a n t e . c o n c l u s i o n i gli a a . della p r e s e n t e n o t a , c o n s i d e r a t i i r i s u l t a t i o t t e n u t i d a p r e c e d e n t i l a v o r i e f f e t t u a t i nel t e r r i t o r i o c i r c o s t a n t e l ' a f f i o r a m e n t o di p r a t i c a d i m a r e , h a n n o r i v o l t o l ' i n d a g i n e v e r s o quegli a s p e t t i che, i n a l c u n i casi, p e r m e t t o n o d i r i c a v a r e , d a c o n s i d e r a z i o n i locali, m a g giori conoscenze sui f e n o m e n i g e n e r a l i a v v e n u t i i n p a r t i c o l a r i p e r i o d i geologici. l ' a f f i o r a m e n t o argilloso d i p r a t i c a d i m a r e r a p p r e s e n t a l ' u n i c a t e s t i m o n i a n z a visibile di quel processo t e t t o n i c o che h a i n t e r e s s a t o il b a s a m e n t o p l i o c e n i c o c a l a b r i a n o p r i m a che, nel q u a t e r n a r i o , si venissero a d e p o s i t a r e i p r o d o t t i s e d i m e n t a r i e v u l c a n i c i . u n a c o n f e r m a i n d i r e t t a della e s i s t e n z a d i u n a t e t t o n i c a preesis t e n t e si p u ò d e s u m e r e o s s e r v a n d o a n c h e la c a r t a del r i l e v a m e n t o g r a v i m e t r i c o eseguito d a l l a sezione geotìsica del servizio geologico d ' i t a l i a p e r t u t t o il t e r r i t o r i o dell'agro r o m a n o . i n f a t t i a n a l i z z a n d o le c a r t e delle i s o a n o m a l e del c a m p o g r a v i m e t r i c o r e g i o n a l e della z o n a c o m p r e s a f r a p o m e z i a e p r a t i c a di m a r e , si o s s e r v a u n a c c o s t a r s i di a n o m a l i e p o s i t i v e e n e g a t i v e con disposizione a n t i a p p e n n i n i c a e a n d a m e n t o d e c i s a m e n t e p a r a l l e l o e q u a s i r e t tilineo che i n d i c a n o l ' e s i s t e n z a di u n a z o n a e s t r e m a m e n t e s o l l e c i t a t a ; t a l e a n d a m e n t o l i m i t a a n o r d la d e p r e s s i o n e di a r d e a . a n c h e le c a r t e delle d e r i v a t e p r i m a e s e c o n d a del c a m p o g r a v i m e t r i c o c o n f e r m a n o la p r e s e n z a di u n a z o n a s o t t o p o s t a a d i n t e n s e sollecitazioni le q u a l i d e v o n o n e c e s s a r i a m e n t e a v e r i n t e r e s s a t o le p a r t i p r o f o n d e del s u b s t r a t o e quelle superficiali. 171 a . s o n a g l i a e. v o l p o n i i n p a r t i c o l a r e , nella z o n a in s t u d i o , la c a r t a delle a n o m a l i e s u p e r ficiali m o s t r a la p r e s e n z a di u n m i n i m o c i r c o n d a t o d a u n a serie di v a l o r i p o s i t i v i c h e i n d i c a l a p r e s e n z a d i u n a z o n a d i f r a t t u r a . si d e d u c e p e r t a n t o , che la fascia c o m p r e s a f r a p o m e z i a e p r a t i c a di m a r e f a p a r t e di q u e l l a i m p o r t a n t e z o n a di f r a t t u r a c h i a m a t a « f a glia di a r d e a ». a n c h e l ' a n d a m e n t o delle isoliate del t e t t o delle argille, r i c o s t r u i t e m e d i a n t e p r o s p e z i o n i e l e t t r i c h e d a l l a c.g.g. m o s t r a che, n e l l a z o n a a n o r d di p r a t i c a di m a r e , c o m p r e s a f r a p o m e z i a , castel r o m a n o e s o l f o r a t a , il b a s a m e n t o p l a s t i c o d e t e r m i n a u n « a l t o p a l e o m o r f o l o gico » (alto di s o l f o r a t a ) che p o r t a le argille q u a s i in a f f i o r a m e n t o . q u e s t o « a l t o », c o s t i t u e n d o il g r a d i n o p i ù e l e v a t o c h e b o r d a a n o r d la d e p r e s s i o n e c o m p r e s a t r a p o m e z i a e a r d e a (depressione p o m e z i a 1), r a g g i u n g e le q u o t e p i ù b a s s e in d i r e z i o n e delle p e n d i c i o c c i d e n t a l i d e l l ' a p p a r a t o v u l c a n i c o laziale. i n b a s e ai d a t i a disposizione è possibile r i c o n o s c e r e che le c a u s e della d e p r e s s i o n e sono d a r i c e r c a r e sia n e l l a t e t t o n i c a c h e n e l l a erosione, e p o i c h é è p r e p o n d e r a n t e l ' a z i o n e del f e n o m e n o t e t t o n i c o su quello erosivo, si r i t i e n e che la z o n a di r a c c o r d o f r a 1'«alto» di s o l f o r a t a e la d e p r e s s i o n e di p o m e z i a 1, d e b b a r a p p r e s e n t a r e l ' a r e a ove p i ù i n t e n s i sono a v v e n u t i sforzi e sollecitazioni; t a l e z o n a , p r o i e t t a t a sul p i a n o o r i z z o n t a l e , p u ò essere c o m p r e s a in u n a f a s c i a di t e r r e n o che h a c o m e asse d i r e z i o n a l e la c o n g i u n g e n t e s a n t a p a l o m b a , p o m e z i a e f o s s o d i p r a t i c a di m a r e . q u a n t o p r e m e s s o t r o v a c o n f e r m a nel d e t t a g l i o e nelle osserv a z i o n i d a noi f a t t e nella z o n a d i p r a t i c a d i m a r e . l ' a f f i o r a m e n t o argilloso della c a v a r a p p r e s e n t a a n o s t r o a v v i s o , u n a p r o p a g i n e dell'« a l t o » di s o l f o r a t a e la s u a q u o t a , i n s i e m e all'ass e t t o p a l e o m o r f o l o g i c o c i r c o s t a n t e , i n d i c a che t r a t t a s i di u n « a l t o » r e l a t i v o . l a d i s p o s i z i o n e a t t u a l e del b a n c o argilloso di p r a t i c a d i m a r e , in r e l a z i o n e con u n a t e t t o n i c a a p i ù v a s t o raggio a p p a r e o r a decisam e n t e p i ù spiegabile i n q u a n t o la f a s c i a di m a s s i m e sollecitazioni c o n s e g u e n t i alle dislocazioni s e m b r a c o m p r e n d e r e d e c i s a m e n t e la z o n a i n s t u d i o . t r o v a così c o n f e r m a la d i r e t t r i c e di f a g l i a s u p p o s t a l u n g o l ' i n c i s i o n e l e . l'« a l t o » d i s o l f o r a t a , i m p o s t a t o s i p r i n c i p a l m e n t e con processi t e t t o n i c i p r e q u a t e r n a r i , che h a n n o i n t e r e s s a t o l ' i n t e r a r e g i o n e form a n d o u n a serie di c u l m i n a z i o n i e d e p r e s s i o n i n e l l ' a m b i t o dei sedim e n t i argillosi, lia s u b i t o f r e q u e n t i e successivi f e n o m e n i erosivi p r i m a i n d a g i n e g e o l o g i c a e g e o f i s i c a su d i un a f f i o r a m e n t o e c c . 58!) d e l l a d e p o s i z i o n e della c o l t r e s e d i m e n t a r i a e p i r o c l a s t i c a del t a r d o q u a t e r n a r i o . i p r o c e s s i erosivi, e s p l e t a t i s i n e c e s s a r i a m e n t e in m a n i e r a n o n unif o r m e , i n c o n s e g u e n z a d i u n a p a l e o m o r f o l o g i a p r e e s i s t e n t e , h a n n o a s s u n t o l o c a l m e n t e c a r a t t e r e d i erosione c o n c e n t r a t a , p e r m e t t e n d o , c o m e a l l ' a f f i o r a m e n t o di p r a t i c a d i m a r e , di a s s u m e r e u n a s p e t t o cup o l i f o r m e d i s s i m m e t r i c o su cui si è s u c c e s s i v a m e n t e d e p o s i t a t a , con g i a c i t u r a s u b o r i z z o n t a l e , u n a litologia q u a t e r n a r i a d i facies c o n t i n e n t a l e . b i b l i o g r a f i a l1) a m a d e i g . , s e g r e a . g . , triboldo g . alcune considerazioni sulla gravimetria e sulla geologia del jg. 158, latina « c e n t r o g e o l o g i c o d ' i t a l i a », s e r v i z i o g e o l o g i c o . ( 8 ) b l a n c a . c . , conforto b . , r i c c o d . , saffa m . , 1 9 6 1 . indagine sulle acque sotterranee dell'agro romano e pontino. r o m a . (3) conforto b . , 1955. contributo alla conoscenza idrologica della fascia costiera ijaziale. « g e o t e c n i c a », 6, r o m a . ( 4 ) f o r n a s e r i m . , s c i i e r i l l o a . , v e n t r i g l i a u . , 1 9 6 3 . la regione vulcanica dei colli albani (vulcano laziale). c e n t r o di m i n e r a l o g i a e p e t r o g r a f i a , u n i v e r s i t à di r o m a , c . n . r . (6) motta s., 1955. appunti preliminari sui rilevamenti geologici effettuati durante il 1954 nel fg. 158, latina. « b o l l . soc. geol. i t a l . », 7 6 , r o m a . (6) segre a., 1957. note sui rilevamenti eseguiti nel fg. 15s, latina, c a r t a g e o l o g i c a d ' i t a l i a . « b o l l . s e r v . geol. », 7 8 . layout 6 1 annals of geophysics, 63, 2, se217, 2020; doi:10.4401/ag-8055 influence of viscosity on the thermal energy produced in seismic fault: one-dimensional modeling guy pascal konga1, fidèle koumetio*,1, david yemele1, guy bertrand tanekou1 (1) unity of research of mechanics and modelling of physical systems, department of physics, faculty of science, university of dschang, po box 67dschang, cameroon article history: received december 12, 2018; accepted september 24, 2019 abstract this study aims to determine the thermal energy accumulation and the temperature distribution around a fault model when a slip weakening friction is imposed. we model the seismic fault sliding, also including viscosity, through a simplified one-body spring-slider model. we established the expression of thermal energy produced by slip-weakening friction in the case that we considered a simplified linearly slip-weakening friction law and a constant viscosity. numerical simulation with runge kutta method is performed for determining the thermal energy in the case of linear slipweakening friction law, thermal-pressurization (tp) friction law, softening-hardening (sh) friction law and a simple slip-weakening (sw) friction law. the effect of the viscosity strongly modifies the results of the evolution of the temperature and the thermal energy. we obtained that the increase of temperature is due to the heat generated per unit volume and per unit time. we have showed that the temperature increases, attains a maximum value before decreasing and then, the system migrates toward a stationary state. in the fault, temperature distribution decrease when going far from the slip zone. this work establishes that the thermal energy is maximum at a moment when the amplitude of the seismic wave is large and that we describe as an instant of critical rupture of the seismic fault. finally, we indicate that the effects of thermal energy on measurable physical quantities can be exploited to prevent the moment of critical rupture of the seismic fault. keywords: earthquake dynamic, slip-weakening friction, viscosity, heat generation, temperature distribution. 1. introduction earthquake is preceded by many natural phenomena, including the temperature anomaly. it is necessary to study the origin of the anomaly of temperature which is observed before a big earthquake. many authors [chester and higgs, 1992; fialko, 2004; bizzarri and cocco, 2006; wang, 2006, 2007, 2009, 2011; bizzarri, 2011a, 2011b; bizzarri and crupi, 2013] show that the thermal properties of the fault-zone rock are considered as the important code of thermal effect during earthquake rupture. friction is one of most important factors in controlling the rupture processes of a seismic fault [nur, 1978; dieterich, 1979; ruina, 1983; cao and aki, 1986; knopoff et al., 1992; rice, 1993; wang, 1996, 1997, 2007, 2012; rubin and ampuero, 2005; ampuero and rubin, 2008; bizzarri, 2011c; bhattachacharya and rubin, 2014; pelap et al., 2014]. friction is also one of most important factors to evaluate the thermal energy produced in seismic fault before big earthquakes [konga et al., 2017]. the calculation of the temperature distribution on the fault surface and its temporal evolution has been performed by many researches [kato, 2001; andrews, 2002; fialko, 2004; bizzarri and cocco, 2006; rice, 2006; nielsen et al., 2008; bizzarri, 2009a; noda et al., 2009; bizzarri 2010a, 2010b; konga et al., 2017]. this temperature is due to frictional heat on a sliding interface. these works used the friction laws also known as constitutive laws, often motivated by laboratory experiments, determined the shear stress on the fault usually dependent on quantities such as the slip, velocity or other dynamic variables quantifying the internal state. the first works based on the fundamental rateandstate dependent friction law which explained a large amount of laboratory data on rock friction were made by dieterich [1979] and ruina [1983]. the relations obtained by these authors constitute the basis of all the subsequent studies of the seismic cycle and have been further improved to incorporate the effect of temperature variations on friction coefficient [chester and higgs, 1992; blanpied et al., 1998; hirose and shimamoto 2005; di toro et al., 2006; beeler et al., 2007]. kato [2001], bizzarri and cocco [2006] used the constitutive law to find the temperature distribution on the fault surface. bizzarri and cocco [2006] used both slip-weakening and rateand state-dependent constitutive laws; in their work they employ the linker and dieterich evolution law for the state variable, and they couple the temporal variation of friction coefficient with those of effective normal stress. de lorenzo and loddo [2009] developed a numerical method to describe the thermo-mechanical evolution of the pre-seismic sliding phase which takes in account both the rate-, stateand temperature-dependent friction law. numerous studies used different constitutive relations to evaluate the temperature change in seismic fault. recently many papers were published on the dynamic of earthquake [wang, 2016; wang, 2017a; wang 2017b; wang 2018] using the slip-weakening friction law. this slip-weakening friction law has not yet been used to evaluate the thermal energy and temperature distribution in seismic fault. konga et al. [2017] established the first expression of thermal energy produced by friction forces in a seismic fault and explained the anomaly of temperature observed before a big earthquake without taking in account the effect of viscosity. they have considered friction forces only depending on velocity and frictional parameters which measures the homogeneousness of the surface between the two sides of the fault. the viscosity plays an important role in the earthquake dynamics; it has been investigated theoretically and numerically by several authors. jeffreys [1942] published the first paper which studies the effect of viscosity on the seismic fault. schubert [1982] reported that the presence of water in fault, the variation of temperature and pressure change the viscosity. rice et al. [2001] showed that the viscous effect is seen like an implicit factor on the friction law. wang [2007] studied the viscous effect in the earthquake rupture. numerous researchers [knopoff et al. 1973, xu and knopoff 1994, knopoff and ni 2001, dragoni and santini, 2015] showed that the viscous effect is a factor in causing radiation to reduce energy during earthquake ruptures. wang [2016, 2017b] and tanekou et al. [2018] studied the influence of viscosity on slip of sliders for one-degree-of-freedom spring-slider model. the influence of viscosity on the thermal energy produced in seismic fault has not yet been modeled. this statement brings us to the following question: what is the effect of this viscosity on temperature distribution? in this work, we aim to study the role of viscosity on the thermal energy produced in the seismic fault by modelling this energy and temperature distribution in the framework of a slip-weakening friction law. this study is focused on slip of one-body dynamical spring-slider model in the presence of slip-weakening friction law initially proposed by wang [2016]. we will start by the presentation of the slip-weakening friction law and displacement of slider according to the model that we are considering; thereafter we determine the thermal energy for a different form of slip-weakening friction law. building on this, temperature distribution will be investigated. 2. description of physical model and force of friction we consider a model of earthquakes of figure 1a proposed by wang [2016], where m is the mass of the slider, u is the displacement of slider, k is the spring constant, 𝑉� is the speed of the driving force, ɳ is the coefficient of viscosity. guy pascal konga et al. 2 in the literature numerous researchers described several simple forms of friction laws. burrigde and knopoff [1967] introduced the first model of friction law used to study the earthquake dynamics. this model describes the seismic system as a collection of blocks resting on a surface with friction and which are connected to a set of springs. byerlee [1978) shows that the friction depends on the time when both sides of the fault come into contact. carlson and langer [1989] present the dynamic of the fault like a chain of blocks connected by springs (the only non-linear element is the force of friction which exists between the blocks). that same year, they have proposed a purely non-linearity velocityweakening friction law which was also used by other authors [carlson 1991; carlson et al. 1991; beeler et al. 2008]. to model the latter, they used the expression initially suggested by burrigde and knopoff [1967] while regarding the speed of the displacement of the blocks as variable. pelap et al. [2014] added in the burrigde and knopoff model one coefficient which measures the homogeneousness of the surface between two sides of fault. wang [1995, 1996, 2012] in this row considered a linearly velocity-dependent weakening-hardening friction, which is simplified from the friction law initially proposed by burrigde and knopoff [1967]. the weakening and hardening rate of dynamic friction with sliding velocity are two main parameters of this friction law. cochard and madariaga [1994] showed that, the velocity-dependent friction models drive at unphysical phenomena. this model of friction law is very unstable at low velocities both during the passage of the rupture front and during the possible slip arrest. others friction laws are known to slip-weakening friction due to the thermal pressurization [rice and cocco, 2006]. more details of deepening description about thermal pressurization can be found in work of sibson (1973], rice [2006], lachenbruch [1980], raleigh and everden [1981], mase and smith [1985, 1987], lee and delaney [1987], andrews [2002]. in our study we use different slip-dependent friction laws described by wang [2016] to model the thermal energy in the slipping zone and evaluate the temperature distribution, taking into account the viscosity. we consider the simplified linear slip-weakening friction law in the form of 𝐹(𝑢)= 𝐹0 ‒ 𝛾𝑢, where 𝛾 is the weakening rate 𝐹0, is the static force and u the slip of slider in the analytical solution. for numerical simulations of the dynamic model, we used three slip-weakening friction laws: the thermal-pressurization (tp) friction law, the softening-hardening (sh) friction law and a simple slip-weakening (sw) friction law. a thermal pressurization friction law is defined by: 𝐹(𝑢) = 𝐹0 exp (‒𝑢 / 𝑢�), where 𝑢� is a characteristic displacement. in addition, the expression of the displacement softening-hardening friction law is: 𝐹(𝑢) = 𝐹0 exp (‒(𝑢2 ‒ 𝑢�2 )/𝑐2), where 𝐹0, 𝑢� and 𝑐 are the constant used by cao and aki (1985, 1986). 𝐹0, 𝑢�,and 𝑐 are respectively the static force, the characteristic displacement and a constant which has a dimensional of length. at the end, the simple slip-weakening friction law [𝐹(𝑢)= 𝐹0 /(1+𝑢 / 𝑢�)] is similar to the velocity-weakening friction law [𝐹(𝑣) = 𝐹0 /(1+𝑣 / 𝑣�)] (where 𝐹0 is the statistic force and 𝑣� is the characteristic velocity and is the velocity of the slider) proposed by carlson and langer [1989]. the characteristic displacement 𝑢� is controlled by some parameters like the fluid density, heat capacity, frictional strength, the thickness and the untrained pressurization factor of the fault zone [see wang, 2018, and cited references therein]. the first three parameters are dependent of temperature. in this paper, we will consider that the characteristic displacement remains constant when the temperature varies because wang [2018] gave the following indications: • the observation data and theoretical analyses about the values of the fluid density during a seismic cycle are v = 𝑑𝑢 𝑑𝑡 3 viscosity and heat in seismic fault figure 1. a) one-body spring-slider model. b) schematic of the model considered in the present study. a) b) guy pascal konga et al. 4 rare; its effect on characteristic displacement is neglected. • heat capacity is constant when the temperature is greater than 300°k (≈ 27°c) while the average ambient temperature of fault zone with depths ranging from 0 to 20km is around 523°k (≈ 250°c); hence, heat capacity cannot influence the characteristic displacement. • a lack of long-term observations of frictional strength during a seismic cycle makes the study of its effect on earthquake recurrence impossible. for this reason, the effect of temperature on friction strength cannot be considered. in this paper, we determine the slip and slip velocity from equation of motion. we used this slip and slip velocity to calculate the thermal energy (calculated as the integral of the product of slip velocity and slip-weakening friction law) and heat generated per unit volume and per unit time (calculated as the product of velocity and slip-weakening divided by thickness of slip zone). at the end we study the variation of temperature caused by the heat generated per unit volume and per unit time. 3. equation of motion in this paper we have used the equation of motion described by wang [2016]. he considered the slip of a onebody dynamical spring-slider model. this equation of motion of slider is given by: (1) where f is a frictional force between the slider and the background (f could be a function of 𝑢 and 𝑣). 𝐹 is controlled by several factors like displacement of slider, the characteristics displacements and other parameters which can be seen in the paper of wang [2016], φ = 𝜂𝑣 is the viscous force between the slider and the background, 𝜂 is the coefficient of viscosity. more details deepening description about viscosity and viscous force can be found in wang [2016]. the viscosity coefficient of rocks is controlled by the temperature. in this work, we considered the simplified expression where 𝜂₀ is a reference value, 𝛽 is a constant, 𝑇� is the steady coldest temperature of the side walls and 𝑇� is the hottest temperature. diniega et al. [2013] assume that 𝑇� = 1050°c and 𝑇� 𝑇� = 100°c [more details are found in diniega et al., 2013 and wang, 2017a]. the slider is pulled by a leaf spring of strength, 𝐾, with a constant velocity, 𝑉�, which represents the speed of a moving plate. when the driving force, 𝐾𝑉�𝑡, is slightly larger than the static frictional force 𝐹0, friction changes from static frictional strength to dynamic one. 4. work of friction force and thermal energy in this part, we shall calculate the work of friction force for several cases. in the first time we use the simplified linearly slip-weakening friction law (considering on the one hand that the viscosity remains constant, which makes it possible to have an analytical solution and on the other hand that this viscosity varies with the temperature), secondly we use tp, sh, and sw friction law (considering that the viscosity varies with the temperature). the thermal energy is generated within the slipping zone (figure 1b). the work done due to frictional forces is defined by the scalar product of this force and displacement of slider. the general expression is given by: (2) we integrated equation (2) by taking t=0 as the period where the accumulation of energy starts. 𝑚 = ‒𝐾(𝑢‒v�𝑡)‒𝐹(𝑢,𝑣) ‒ φ (𝑣)𝑑²𝑢 𝑑𝑡² 𝜂 = 𝜂₀ exp �𝛽 �1 ‒ �� ,𝑇 ‒ 𝑇� 𝑇� ‒ 𝑇� � ₀𝑊� = ∫𝐹(𝑢)𝑑𝑢 =∫𝐹(𝑢) 𝑑𝜏 = ∫𝐹(𝑢)𝑣𝑑𝜏𝑑𝑢 𝑑𝜏 � ₀ 4.1 constant viscosity in this case, we considered only the simplified linear slip-weakening friction law to determine the analytical expression of thermal energy. from the analytical study, the general expression of the displacement of slider, u(t), is given by wang [2016] 𝑢(𝑡) = (𝑣₀/𝜔�)exp(‒𝜂₀𝑡/2𝑚) sin (𝜔�𝑡) (3) where 𝜔� = �𝜔²₀ ‒ �𝜂²₀ ‒ 4 𝑚𝛾��4𝑚2�½, 𝜔₀ = (𝐾/𝑚)½, 𝜔� is composed by a part 𝜔₀ only depending on the spring-slider system features and the other part depending on friction and viscosity of the interface, the ration (𝑣₀/𝜔�) denoted the amplitude of slip function and varies with four model parameters, 𝜂₀ is the coefficient of viscosity, 𝑣₀ is the initial velocity and 𝛾 is the weakening rate. from equation (3) the velocity of slider v is given as: (4) the simplified linear slip-weakening friction law defined as: 𝐹(𝑢)=𝐹₀ ‒ 𝛾𝑢 (5) the work done by this friction is obtained as: (6) where 𝛼, s1, s2, s3, s4, a1, a2, a3, a4 and a5 are constants defined by the following expression: thermal energy due to the friction is defined as the absolute value of the work done by frictional forces and is defined by: 𝑄(𝑡)= ⎸𝑊�⎹ (7) we have performed a matlab simulation to obtain figure 2a which shows the distribution of thermal energy accumulated by friction at the level of the fault in presence of viscosity as well as figure 2b which shows the evolution of slip according to time for simplified linear slip-weakening friction law. 𝑊� = 𝐴₁��𝐴₂ sin(𝜔�𝑡)‐𝐴₃ cos(𝜔�𝑡)�exp(‐𝛼𝑡) +𝐴₅� ‐ 𝑆₁��𝑆₂ sin (2𝜔�𝑡)‐𝑆₃ cos(2𝜔�𝑡)�exp (‐2𝛼𝑡) +𝑆₄�+ (1‐exp(‐2𝛼𝑡)�𝐴₄ 2𝛼 𝛼 = ,𝜂₀ 2𝑚 𝑆1 = ,𝛼 2(𝛼2+𝜔²𝑛 ) 𝑆2 = (𝛼+1), 𝛾𝜔𝑛 𝐴² 2 𝐴 = ,𝑣₀ 𝜔𝑛 𝑆3 = (𝛼+𝜔²𝑛 ),𝛾𝐴 ² 2 𝑆4 = (𝜔²𝑛 ‒𝛼),𝛾𝐴² 2 𝐴₁= , 𝐴₂ 𝑆₁𝜔�‐𝑆₂𝛼, 𝐴₃ = 𝑆₂𝜔�+𝑆₁𝛼, 𝐴₄ = , 𝐴₅ = 𝑆₂𝜔�+𝑆₁𝛼𝛾𝛼𝐴² 2𝐹₀ 𝛼 ² + 𝜔²𝑛 𝑣 = 𝑣₀ �‒ sin(𝜔�𝑡)+v0 cos(𝜔�𝑡)�exp�‒ �𝜂₀𝑡 2𝑚𝜔� 𝜂₀𝑡 2𝑚 5 viscosity and heat in seismic fault 4.2 temperature dependent viscosity knowing that the viscosity varies with the temperature when 𝛽≠0. we considered in this case all the slip-weakening friction law listed above. for the simplified linear slip-weakening friction law, equation (1) can be rewritten as two first-order differential equations and we added in this system the third equation which gives the thermal energy generated by simplified linear slip-weakening friction law and the system is given as: 𝜂 = 𝜂₀ exp �𝛽 �1 ‒ �� ,𝑇 ‒ 𝑇� 𝑇� ‒ 𝑇� guy pascal konga et al. 6 a) b) figure 2. a) evolution of thermal energy according to time due to the simplified linearly slip-weakening friction law in the fault when the viscosity is constant. b) evolution of slip according to time due to the simplified linearly slipweakening friction law in the fault when the viscosity is constant. c) evolution of thermal energy according to time due to the simplified linearly slip-weakening friction law in the fault when the viscosity varies with temperature. d) evolution of slip according to time due to the simplified linearly slip-weakening friction law in the fault when the viscosity varies with temperature. values used: ƞ0 = 10 n/(m2/s), vp = 10-11m/s, γ = 0.8 n/m, k = 10n/m, m = 10kg and f0 = 1. c) d) (8) since the viscosity is temperature dependent, the heat conduction equation (9) is used to derive it distribution. the heat conduction equation is given by: (9) where k is the thermal conductivity, cs the specific heat of rock at the constant pressure, 𝜌 is the density of rock (fialko, 2004) and 𝛾 represents the spatial coordinate normal to the fault. this system is semi-infinity medium at the initial temperature distribution on the fault plane 𝑇₀ . in equation (9) q is the heat generated per unit volume and per unit time ([q]=w/m3) i.e. the power developed by friction. the volume is defined by v=s(2h), we have taken the area s equal at an unit in this work and 2h is the thickness of the slip zone (see figure 1b). bizzarri and cocco (2006), rice (2006) show that the heat source is uniformly generated in slip zone layer: (10) the slip velocity v and slip u in equation (10) are obtained by resolving the equation (1) for each friction law. we used the runge kutta method to solve this system and we have performed a matlab simulation to obtain the curves of figure 2c which shows the distribution of thermal energy as well as figure 2d which shows the evolution of slip according to time considering the simplified linear slip-weakening friction law when the viscosity varies with the temperature. for tp friction law, we have a system (11): (11) we obtained the curves of figure 3a which gives the distribution of thermal energy as well as figure 3b which shows the evolution of slip according to time considering the tp friction law. for the sh friction law, we have a system (12): 𝑑𝑢 𝑑𝑡 = v �(𝐾‒𝛾) 𝑢 + 𝜂₀ exp �𝛽 �1 ‒ �� v +𝐹₀ ‒𝐾𝑉�𝑡�𝑑𝑢 𝑑𝑡 = ‒ 1 𝑚 𝑇 ‒ 𝑇� 𝑇� ‒ 𝑇� 𝑑𝑊� 𝑑𝑡 = 𝑣 (𝐹₀ ‒ 𝛾𝑢)𝑎𝑡 𝑡 = 0, 𝑤𝑒 ℎ𝑎𝑣𝑒 𝑊� = 0 ⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ �𝑇 �𝑡 𝑘 𝜌𝑐 � �2𝑇 �𝑦2 𝑞(𝑡) 𝜌𝑐 �= + 𝑡≥0, ⎸𝑦⎹ ≤ ℎ ⎸𝑦⎹ ≥ ℎ 𝐹(𝑢)𝑣 , 2ℎ 0,𝑞(𝑡)= � 1 𝑚 𝑑𝑢 𝑑𝑡 = v = ‒𝑑𝑢 𝑑𝑡 �𝐾𝑢 + 𝜂₀ exp �𝛽 �1 ‒ �� v +𝐹₀ exp (‒𝑢/𝑢𝑐) ‒𝐾𝑉�𝑡�𝑇 ‒ 𝑇� 𝑇� ‒ 𝑇� 𝑑𝑊��𝑑𝑡 = 𝑣𝐹₀ exp(‒𝑢/𝑢𝑐)𝑎𝑡 𝑡 = 0, 𝑤𝑒 ℎ𝑎𝑣𝑒 𝑊�� = 0 ⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ 7 viscosity and heat in seismic fault (12) we obtained the curves of figure 4a which gives the distribution of thermal energy as well as figure 4b which shows the evolution of slip according to time considering the sh friction law. 1 𝑚 𝑑𝑢 𝑑𝑡 = v = ‒𝑑𝑢 𝑑𝑡 �𝐾𝑢 + 𝜂₀ exp �𝛽 �1 ‒ �� v +𝐹₀ exp �‒ � ‒𝐾𝑉�𝑡�𝑇 ‒ 𝑇� 𝑇� ‒ 𝑇� 𝑑𝑊��𝑑𝑡 = 𝑣𝐹₀ exp�‒ � 𝑎𝑡 𝑡 = 0, 𝑤𝑒 ℎ𝑎𝑣𝑒 𝑊�� = 0 ⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ (𝑢2 ‒ 𝑢2) 𝑐2 �(𝑢2 ‒ 𝑢2) 𝑐2 � guy pascal konga et al. 8 a) b) figure 3. a) evolution of thermal energy according to time due to the tp friction law in the fault. b) evolution of slip according to time due to the tp friction law in the fault. values used: k = 10n/m, m = 10kg, vp = 10-11m/s, ƞ0 = 10 n/(m2/s), uc = 0.5, f0 = 1. a) b) figure 4. a) evolution of thermal energy according to time due to the sh friction law in the fault. b) evolution of slip according to time due to the sh friction law in the fault. values used: k = 10n/m, m = 10kg, vp = 10-11m/s, ƞ0 = 10 n/(m2/s), uc = 0.5, f0 = 1. for the sw friction law, we have a system (13) (13) we obtained the curves of figure 5a which gives the distribution of thermal energy as well as figure 5b which shows the evolution of slip according to time considering the sw friction law. the observation of figure 2a reveals that for the simplified linear slip-weakening friction law and for a constant viscosity, the thermal energy abruptly increases, reaches a maximum value and then abruptly decreases in this case of an analytical solution. it can also be seen that the maximum value of the thermal energy decreases as the viscosity increases, this maximum value is reached almost at the same moment whatever the value of the viscosity. figure 2b shows that for the low value of the viscosity (𝜂 = 10), the direction of movement of the block changes twice (approximately at times 2.9s and 7.5s). this curve shows that the amplitude of the movement decreases with time, from one phase of the movement to another until a zero amplitude. we finally observe that the absolute value of the slip varies exactly in the same way as the thermal energy produced in the seismic fault whatever the value of the viscosity. the observation of figure 2c reveals that for the simplified linear slip-weakening friction law and for a viscosity that varies with the temperature (𝛽 ≠ 0), the thermal energy regularly increases until it reaches an asymptotic value in this case of a purely numerical solution. figure 2c also shows that the slope of the curve decreases when the viscosity values are larger, and the asymptotic value is nearly the same for all curves. figure 2d shows that the absolute value of the slip varies exactly in the same way as the thermal energy produced in the seismic fault. figures 3a, 3b, 4a and 4b show that for tp and sh friction law, the evolutions of the curves are exactly the same as for the simplified linear slip-weakening friction law in the case of a purely numerical solution for thermal energy as well as for slipping according to time. 1 𝑚 𝑑𝑢 𝑑𝑡 = v = ‒𝑑𝑢 𝑑𝑡 �𝐾𝑢 + 𝜂₀ exp �𝛽 �1 ‒ �� v +𝐹₀ � � ‒𝐾𝑉�𝑡�𝑇 ‒ 𝑇� 𝑇� ‒ 𝑇� = 𝑣𝐹₀ 𝑑𝑊��𝑑𝑡𝑎𝑡 𝑡 = 0, 𝑤𝑒 ℎ𝑎𝑣𝑒 𝑊�� = 0 ⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ 1 1+𝑢 / 𝑢� 1 1+𝑢 / 𝑢� 9 viscosity and heat in seismic fault a) b) figure 5. a) evolution of thermal energy according to time due to the sw friction law in the fault. b) evolution of slip according to time due to the sw friction law in the fault. values used: k = 10n/m, m = 10kg, vp = 10-11m/s, ƞ0 = 10 n/(m2/s), uc = 0.5, f0 = 1. figure 5a shows that for sw friction law, the thermal energy increases along a practically vertical slope after a certain amount of time, reaching a maximum value, then slightly decreases along the same slope before keeping a constant value. in this case, the curve of variation of the absolute value of the slip as a function of time (figure 5b) regularly increases until reaching a point of inflection at the moment when the thermal energy is maximum, then it continues to increase along a steeper slope, reaches the maximum value and starts decay. we notice that the variation of the viscosity influences the slip and the thermal energy produced in the seismic fault for the simplified linear slip-weakening, tp, sh and sw friction law. 5. temperature distribution we compute the temperature changes caused by heat generated per unit volume and per unit time due to slipweakening friction law in seismic fault. we considered the heat generated per unit volume and per unit time as a heat-flux in the 1-d heat conduction equation given by (9). this heat-flux takes in to account the effect of viscosity. for each friction law we have evaluated the heat generated per unit volume and per unit time by the following equations: for the linearly slip-weakening friction law (14) for the tp friction law, we have the following equation (15) for the sh and sw friction law, we have respectively the equations (16) and (17) (16) (17) figures 6a-d illustrate the time variation of heat generated per unit volume and by unit time for the simplified linear slip-weakening, tp, sh and sw friction law. using the analytical expression (equation 10) and numerical results of the heat generated per unit volume and per unit time (equations 14, 15, 16 and 17), equation 9 is solved numerically with the help of the finite difference method. we have numerically simulated the solution of 1-d heat conduction equation. results of these calculation are illustrated in figures 7a-f. in figures 7a and 7b, we used only the linearly slip-weakening friction law in the case of constant viscosity; temperature distribution is monitored simultaneously at several spaces and times. in these tests, we vary distance when the time is fixed and then the time when the distance is fixed. figure 7a illustrates a spatiotemporal evolution of temperature distribution (or temperature distribution versus distance y/h for a series of constant time) on the slipping fault surface. it shows that the temperature variation is maximum at the slipping 𝑡≥0, ⎸𝑦⎹ ≤ ℎ ⎸𝑦⎹ ≥ ℎ𝑞(𝑡)= � (𝐹₀ ‒ 𝛾𝑢)𝑣 , 2ℎ 0, 𝑡≥0, ⎸𝑦⎹ ≤ ℎ ⎸𝑦⎹ ≥ ℎ𝑞��(𝑡)= � 𝐹₀ exp(‒ 𝑢 / 𝑢�)𝑣 , 2ℎ 0, ⎧ ⎪ ⎨ ⎪ ⎩ 𝐹₀ exp�‒ �𝑣 , 2ℎ 0, 𝑞��(𝑡)= 𝑡≥0, ⎸𝑦⎹ ≤ ℎ ⎸𝑦⎹ ≥ ℎ (𝑢2 ‒ 𝑢2) 𝑐2 � ⎧ ⎪ ⎨ ⎪ ⎩ 𝐹₀ exp� �𝑣 , 2ℎ 0, 𝑞��(𝑡)= 𝑡≥0, ⎸𝑦⎹ ≤ ℎ ⎸𝑦⎹ ≥ ℎ 1 1+𝑢 / 𝑢� guy pascal konga et al. 10 zone and decreases monotonically when moving away from this zone. the results presented in figure 7b confirm that the maximum value of the temperature variation decreases with the distance from the fault zone (we can see from this figure that by increasing the distance from the slipping zone, the temperature variation progressively decreases). figure 7b also shows that the time required to reach the maximum value of the temperature variation remains the same for all distances. figures 7c-e, illustrate the temperature distribution in semi-infinity medium obtained from the heat generated per unit volume and per unit time produced by the different slip-weakening friction law (linearly slip-weakening, tp, sh and sw friction law) when the viscosity varies with the temperature. figures 7c-e (for linearly slip-weakening, tp and sh friction law respectively) show that the temperature variation increases abruptly to a maximum value then decreases with a slope that gradually decreases until reaching zero; this maximum value decreases with the increasing viscosity. in figure 7f (for sw friction law), the temperature increases after a certain amount of time, reaching a maximum value and then decreases; the time at which this maximum value is attained increases with the increasing viscosity. 11 viscosity and heat in seismic fault a) b) figure 6. a) evolution of heat generated per unit volume and per unit time due to the linearly slip-weakening friction law in the fault when the viscosity varies with temperature. b) evolution of heat generated per unit volume and per unit time due to the tp friction law in the fault. c) evolution of heat generated per unit volume and per unit time due to the sh friction law in the fault. d) evolution of heat generated per unit volume and per unit time due to the sw friction law in the fault. values used: k = 10n/m, m = 10kg, vp = 10-11m/s, ƞ0 = 10 n/(m2/s), uc = 0.5, f0 = 1. c) d) guy pascal konga et al. 12 a) b) c) d) figure 7. a) variation of temperature according to nondimentional distance due to the simplified linearly slip-weakening friction law in the fault when the viscosity is constant. b) times series of temperature distribution in semiinfinity medium for simplified linearly slip-weakening friction law in the fault when the viscosity is constant. c) times series of temperature distribution in semi-infinity medium for simplified linearly slip-weakening friction law in the fault when the viscosity varies with temperature. d) times series of temperature distribution in semiinfinity medium for tp friction law in the fault. e) times series of temperature distribution in semi-infinity medium for sh friction law in the fault. f ) times series of temperature distribution in semi-infinity medium for sw friction law in the fault. values used: k = 10n/m, m = 10kg, vp = 10-11m/s, ƞ0 = 10 n/(m2/s), uc = 0.5, density: � = 2700 kg/m3, heat capacity:cs = 1000j/kg, thermal conductivity: k = 2.7w/m°c, f0 = 1. e) f ) 6. discussion in this paper we have analytically (when the viscosity is constant) and numerically (when the viscosity varies with the temperature) determined the thermal energy due to slip-weakening friction in the presence of viscosity and we have numerically solved the 1-d thermal conduction equation. we indicate that this was done so because of the difficulty of finding an analytical solution to the differential equations obtained when the viscosity varies with the temperature. in the case where the viscosity does not vary with the temperature, the maximum of the absolute value of the slip decreases when the viscosity increases (figure 2b), which corroborates the result obtained by wang [2016]. in the case where the viscosity varies with the temperature, for linearly slipweakening, tp and sh friction law, the curves of variation of the slip with time (figures 2d, 3b, 4b and 5b) have almost the same appearance as those obtained by wang [2017] who studied the slip of a two-degree-of-freedom spring-slider model, consisting of two sliders, in the presence of slip-dependent friction due to thermal pressurization and viscosity. since (1) slip also depends on the frictional force, (2) the heat energy is a function of both the friction force and the slip, it is normal that the curves of variation of thermal energy and the absolute value of the slip according to time have the same appearance. in all cases (figures 7b-f ), the temperature variation abruptly increases to a maximum value then decreases with a slope that gradually decreases until reaching zero, which means that the system converges to a stationary state. these results are comparable to those ones obtained by kato [2001], fialko [2004], bizzarri [2010c] and bizzarri [2014] with regard to the variation of temperature as a function of time. we find that in the context of our study, as the system converges to a stationary state, the thermal energy and the slip remain constants. we think that the stationary state describes the uniform motion of the blocks corresponding to an aseismic creep along the fault. fialko [2004] shows that the effective thickness of the earthquake slip zone controls not only the magnitude of temperature increase, but also maximum temperature in the fault. our model (figures 7a-b) gives the same observation, but the attenuation of the temperature distribution is more pronounced when one moves away from slip zone. bizzarri [2010] added that the more general solution found by bizzarri and cocco [2006] by using both slip weakening and rateand state-dependent constitutive relations can be reconciled with solution of kato [2001] found by using rate-, stateand temperaturedependent friction law when the slipping zone halfthickness (w) is smaller than 10 μm. it appears that in our model where we use the slip-weakening friction law, the magnitude of temperature increase and the maximum temperature are not only controlled by a thickness of slip zone (figures 7a-b), but also by the viscosity (figures 7c-f ). bizzarri and cocco [2006] report that, before the rupture starts the temperature is equal to its initial value and after crack start the temperature increases, depending on the value of the slipping zone thickness. in this case, we can understand that they suppose that there is not slip or displacement before the earthquake. if the temperature remained equal to its initial value, one cannot observe the temperature anomaly before an earthquake, while our study as well as those of the other authors [saraf and choudhury, 2004; tramutoli et al., 2005; rezapour et al, 2008; qin et al., 2012] establishes well this temperature anomaly. figures 2a to 4b show that the thermal energy produced in the seismic fault is maximum at the same time as the absolute value of the slip while figures 5a and 5b show that the thermal energy is maximum at a time tc corresponding to the point of inflection on the slip curve. in this last case, we can think that there are several phases of the movement since the slip continues to increase after the instant tc (figure 5b), which can be understood with the reality of the replicas during the earthquakes. however, the thermal energy remains practically constant after the instant tc (figure 5a), a phenomenon that remains to be explained in the case of sw friction law. focusing on the first phase of the movement, it appears that at the end of this phase, thermal energy as well as the absolute value of the slip has its greatest value for linearly slip-weakening, tp, sh and sw friction law. when the absolute value of the slip is great, the amplitude of the seismic wave is large, that implies a higher risk of enormous damage. we can therefore say that the thermal energy is maximal at a critical moment that we describe as a time of critical rupture of the seismic fault. knowing that thermal energy influences measurable physical quantities such as geothermal flow, electrical resistivity of rocks, the effects of this thermal energy can be exploited to prevent the moment of critical rupture of the seismic fault. 13 viscosity and heat in seismic fault 7. conclusions this paper investigated the effect of viscosity parameter on the thermal energy produced in the seismic fault via a one-body dynamical spring-slider model with slip-weakening friction. the time evolution of temperature is numerically simulated when the viscosity parameter is present. analytic results (when the viscosity is constant) with numerical calculations show that the thermal energy is controlled by the viscosity parameter. numerical simulations (when the viscosity varies according to temperature) conduct to the time evolution of the thermal energy and temperature in the presence of linear slip-weakening friction law, thermal-pressurization (tp) friction law, softening-hardening (sh) friction law and a simple slip-weakening (sw) friction law. results suggest that the viscosity parameter remarkably affects the temperature distribution and thermal energy due to slip-weakening friction. we have showed that the increase of temperature is due to the heat generated per unit volume and per unit time. we obtained that the temperature increase, attains certain values before decreasing and then, the system migrates toward a stationary state. in the fault, temperature distribution decreases when going far from the slip zone. the maximum temperature is obtained in the surface of slipping zone. the temperature distribution reaches the maximum when the heat generated per unit volume and unit time is maximum. this work establishes that the thermal energy is maximum at a moment when the amplitude of the seismic wave is large and that we describe as an instant of critical rupture of the seismic fault. acknowledgements this paper benefits from the fruitful criticism and suggestions by the a.e. and anonymous reviewers, leading to a significant improvement of the work. they are gratefully acknowledged. we finally thank the sector editor gaetano festa. references ampuero j.p. and a. m. rubin (2008). earthquake nucleation on rate and state faults-agind and slip 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(2017b). multistable slip of a one-degree-of-freedom spring-slider model in the presence of thermalpressurized slip-weakening friction and viscosity, nonlin. processes geophys. 24, 467–480, 2017; https://doi.org/10.5194/npg-24-467-2017. wang j.h. (2018). a study of earthquake recurrence based on a one-body spring-slider model in the presence of thermal-pressurized slip-weakening friction and viscosity, nat. hazards earth syst. sci. 18, 1969–1983; https://doi.org/10.5194/nhess-18-1969-2018. *corresponding author: fidèle koumetio. unity of research of mechanics and modelling of physical systems, department of physics, faculty of science, university of dschang, po box 67dschang, cameroon e-mail: koumetiof@yahoo.fr fidele.koumetio@univ-dschang.org © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 17 viscosity and heat in seismic fault https://doi.org/10.5194/npg-24-467-2017. https://doi.org/10.5194/nhess-18-1969-2018 mailto:koumetiof@yahoo.fr mailto:fidele.koumetio@univ-dschang.org << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice microsoft word 8_appendix_falsaperla_ba.docx 1 annals of geophysics, 64, 3, se328, 2021; doi:10.4401/ag-8394 appendix to dissemination: steps towards an effective action of seismic risk reduction for non-structural damage in the knowrisk project susanna falsaperla1, gemma musacchio2, mónica amaral ferreira3, mário lopes4, carlos sousa oliveira5 1 istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo, piazza roma 2, 95125, catania, italy; susanna.falsaperla@ingv.it; www.ct.ingv.it 2 istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; gemma.musacchio@ingv.it; www.ingv.it 3 ist, dept of civil engineering, architecture and geo-resources, ceris, university of lisbon, portugal, monicaf@civil.ist.utl.pt 4 ist, dept of civil engineering, architecture and geo-resources, ceris, university of lisbon, portugal, mariolopes@tecnico.ulisboa.pt 5 ist, dept of civil engineering, architecture and geo-resources, ceris, university of lisbon, portugal, csoliv@civil.ist.utl.pt table 1: national and international dissemination events grouped according to the targeted audience professionals laypersons general public schools egu general assembly 2016 date(s): 17-22 april 2016 location: vienna, austria action: poster presentations http://www.egu2016.eu comunicação de risco. quando lisboa treme – de 1755 à cidade resiliente date(s): 28 january 2016 location: lisbon, portugal action: talks http://www.cm-lisboa.pt/cidade-resiliente scienzaperta date(s): 12-13 may 2016 location: milan, italy action: project presentation + hands-on activity attendees: ~200 students http://scienzaperta.rm.ingv.it/ sísmica 2016 – 10º congresso nacional de sismologia e engenharia sísmica date(s): 20-22 april 2016 location: azores, portugal action: project presentations scienzaperta date(s): 16-21 may 2016 location: catania, italy action: project presentation for public and students attendees: 600 visitors http://scienzaperta.rm.ingv.it/ tutelaspezia date(s): 1-2 october 2016 location: la spezia, italy action: lecture to elementary school students with a scientific story-telling activity attendees: 500 students http://www.speziafiere.it/index.php?op tion=com_content&view=article&id=38 2 http://sismica2016.azore s.gov.pt/ 6:tutelaspezia&catid=22:calendariocompleto-2016&itemid=72 icur2016 – international conference on urban risks date(s): 30 june-2 july 2016 location: lisbon, portugal action: project presentations http://www.cerueuropa.pt/icur2016/ sons do vale_sustentabilis date(s): 11 september 2016 location: lisbon, portugal action: project presentations https://www.facebook.com/sonsdovalesust entabilis/?fref=ts settimana del pianeta terra date(s): 16-23 october 2016 location: varese and la spezia, italy action: t0-focus group, lectures, handson laboratory attendees: 100 students http://www.settimanaterra.org/ 35th general assembly of the european seismological commission (esc) date(s): 4-11 september 2016 location: trieste, italy action: poster presentations http://www.35esc2016.e u/ european researchers night date(s): 30 september 2016 location: milan, italy action: conferences attendees: 100 participants http://www.museoscienza.org/attivita/mee tmetonight/meetmetonight2016.asp) bimbi per bimbi date(s): october-november 2016 location: viareggio, italy action: lecture, hands-on laboratory attendees: 100 students https://fiabeefrane.com/2016/09/24/bi mbi-per-bimbi/ jornadas pela segurança (santa casa da misericórdia de lisboa) date(s): 2-4 november 2016 location: lisbon, portugal action: project presentations http://www.scml.pt/ptpt/destaques/jornadas_p ela_seguranca/ bergamoscienza date(s): 8 october 2016 location: dalmine, italy action: conferences attendees: 80 participants https://www.bergamoscienza.it/eng/home page.aspx security day in schools date(s): 22 november 2016 location: northern italy action: lecture attendees: 50 students http://hubmiur.pubblica.istruzione.it/w eb/ministero/cs201115 knowrisk general meeting date(s): 15-17 december 2016 location: catania, italy action: project presentations http://www.ct.ingv.it/kno wrisk2016/index.php settimana del pianeta terra date(s): 16-23 october 2016 location: ferrara, italy action: role game “playdecide” attendees: 20 citizens http://www.settimanaterra.org/ the week of passions date(s): 31 january 2 february 2017 location: la spezia, italy action: knowrisk eas at “mazzini” school of la spezia attendees: 100 students http://www.istitutocomprensivo6laspez ia.it/pvw/app/spme0045/pvw_sito.php ?sede_codice=spme0045&from=1&page=1979591; http://www.istitutocomprensivo6laspez ia.it/pvw/app/spme0045/pvw_sito.php specialized course fundec, ist date(s): 9-10 march 2017 location: lisbon, portugal action: lecture http://www.fundec.pt/pt -pt/content/reabilitacaoestrutural-de-edificiosfórum fazer política date(s): 2 november 2016 location: lisbon, portugal action: project presentations o risco sísmico em portugal date(s): 12 february 2017 location: lisbon, portugal action: lecture for students from escola camilo castelo branco –oeiras attendees: 50 students 3 avaliacao-do-riscosismico-tecnicas-dereforco-e parliamentary questions date(s): 7 november 2016 action: “earthquake information for european citizens” (answer given by mr. stylianides on behalf of the commission) http://www.europarl.europa.eu/doceo/doc ument/e-8-2016-006821asw_en.html?redirect lectures for teachers date(s): 24 february 2017 location: bolzano, italy (museum of natural history) action: lecture on how to teach earthquakes at school attendees: 50 teachers http://www.provincia.bz.it/intendenzascolastica/sistema-scuola/elencoiniziative-2016-2017.asp?kurs_id=22871 icesd 2017 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teachers and civil protection date(s): 27 february 2018 location: almada, portugal action: lecture attendees: 100 students, teachers and 20 agents from almada, cacilhas and trafaria civil protection 467_479 tomasi.pdf 467 annals of geophysics, vol. 46, n. 2, april 2003 marine and continental aerosol effects on the upwelling solar radiation fl ux in southern portugal during the ace-2 experiment claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé istituto di scienze dell’atmosfera e del clima (isac), cnr, bologna, italy abstract an overall number of 447 spectral series of aerosol optical depth were determined in the 0.4-3.7 mm wavelength range by examining the ir-rad sun-radiometer measurements carried out at sagres (portugal) on six clear-sky days, during the clearcolumn (ace-2) experiment in june and july 1997. these spectral series were then analysed with the king inversion method to defi ne the size-distribution curves of columnar aerosol particle total number and volume, assuming values of both real and imaginary parts of the particulate refractive index obtained on the six days by combining our measurements with simultaneous sky-brightness measurements taken by the leipzig university group. for these results, we then calculated the daily time-patterns of the average single scattering albedo of the columnar aerosols, fi nding instantaneous values ranging between 0.70 and 0.96 on those days, with daily mean values varying from 0.83 to 0.95. furthermore, for each spectral series of aerosol optical depth, we determined the instantaneous change df induced by the columnar aerosols on the upwelling solar radiation fl ux leaving the atmosphere, over oceanic areas presenting low surface albedo. the 24-h average values of df obtained on the six days were found to increase as a function of the daily mean value of aerosol optical depth at the 0.55 mm wavelength, following relationship curves whose positive slope coeffi cients decrease gradually with the single scattering albedo of the columnar aerosols. the said curves can be used to achieve reliable estimates of change df directly from daily ground-based multispectral measurements of aerosol optical depth and skybrightness at different angular distances from the sun. mailing address: dr. claudio tomasi, istituto di scienze dell’atmosfera e del clima (isac), cnr, via gobetti 101, 40128 bologna (italy); e-mail: c.tomasi@isac.cnr.it key words aerosol optical depth – aerosol particle size-distributions – aerosol radiative properties – direct aerosol radiative forcing 1. introduction it is well known that airborne aerosols act on the earth’s climatic system in two ways, directly through scattering and absorption of both solar and terrestrial radiation, and indirectly by modifying the optical properties and lifetime of clouds (charlson et al., 1991). the uncertainties in estimating direct aerosol-induced forcing arise from the limited information on the spatial and temporal distribution of aerosol particles and on their optical properties. in fact, while global three-dimensional distribution data of sulphate aerosols are currently available (langner and rodhe, 1991), the global scale distribution features of the other aerosol components are only scantily known. in addition, the close dependence of the optical properties on the size-distribution characteristics of airborne particles and their chemical composition makes it diffi cult to defi ne simple and realistic relationships between direct 468 claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé aerosol-induced forcing and columnar particulate content parameters. for these reasons, several regional experiments have been carried out over recent years in different areas of the planet to study the direct effects produced by aerosols in scattering and absorbing the incoming solar radiation: among them, the most important in oceanic areas have been astex/mage in the eastern atlantic (huebert et al., 1996), ace-1 in the southern pacific (bates et al., 1998), indoex in the indian ocean (jayaraman et al., 1998), tarfox in the eastern coastal region of u.s.a. (russell et al., 1999a), and ace-2 in the northern atlantic (raes et al., 2000). during the ace-2 experiment, our group participated in the clearcolumn project, along with other groups from the u.s.a., germany, france, portugal, sweden and the netherlands (russell and heintzenberg, 2000). the clearcolumn campaign took place in june/july 1997 in portugal, the canary islands and the surrounding regions of the eastern atlantic ocean. our specifi c role was to perform routine measurements of direct solar irradiance at sagres (37°01' n, 8°57' w; 50 m a.m.s.l.) on the atlantic coast of southern portugal and at the nearby mountain stations of monchique (37°19' n, 8°34' w; 500 m a.m.s.l.) and mt. foia (37°19' n, 8°36' w; 920 m a.m.s.l.), with the aim of obtaining precise measurements of aerosol optical depth (aod) at several visible and infrared wavelengths on numerous days of the campaign. the mountain measurements were performed using portable sun-photometers, providing direct solar radiation output voltages at 11 and 13 window-wavelengths, respectively, in the 0.32-1.05 mm spectral range. the sagres measurements were taken with the ir-rad sun radiometer, designed and manufactured by us at the isac institute, which is equipped with 13 narrow passband interference fi lters centered within the main windows of the solar spectrum from 0.401 to 3.676 mm. we collected large data-sets on 21 completely clear-sky days and other 6 partially cloudy days, for an overall number of more than 2100 spectral series of direct solar irradiance output voltages (vitale et al., 2000a). covering so wide a spectral interval, the evaluations of aod obtained from the ir-rad measurements were found to be particularly suitable for analysis by means of retrieval procedures to determine the sizedistribution curves of the columnar aerosol particle polydispersions and obtain useful information on the refractive index of particulate matter and other radiative properties of airborne aerosols. this paper briefl y presents: i) the procedure followed to determine the aod spectral series at various times on six clearsky days; ii) the inversion procedure adopted to infer the columnar particle number and the volume distribution curves as a function of radius, determining the radiative properties of the columnar aerosol particles in terms of the complex refractive index of particulate matter, and iii) the calculations of the changes in the upwelling solar radiation fl ux at the atmospheric top-level produced over an oceanic region by columnar particle polydispersions of marine, continental and mixed origins, as observed on six selected days of the clearcolumn campaign. 2. the spectral aod measurements the optical and electronic characteristics of the ir-rad sun-radiometer are described in detail by vitale et al. (2000a), along with the procedures followed to attain an accurate calibration of the instrument at the 13 selected window-wavelengths. in particular, the instrument is equipped with a collimator tube containing a set of circular diaphragms defi ning a fi eld of view of 1°16' diameter at all wavelengths. the use of such a limited fi eld of view prevents important errors due to the sky diffuse radiation entering the radiometer and reaching the thermopyle employed as sensor. thus, we assumed that the output voltages were only due to direct solar irradiance on all the measurement days presenting relatively high atmospheric transparency conditions, according to the results found by tomasi et al. (1999) for different atmospheric scattering models. all the optical and electronic components are housed in a thermostated box which is maintained at the internal temperature of 15 °c by a set 469 marine and continental aerosol effects on the upwelling solar radiation fl ux in southern portugal during the ace-2 experiment of four thermoelectric (peltier) heat pumps. this device enables us to minimize the errors caused by temperature drift effects on the fi lter transmission curves and/or thermal instability of the thermopyle, whose signals are regularly checked by means of a thermostated chopper disk providing the zero signals. precise values of the calibration constants of a sun-radiometer are needed in analyzing the sun-radiometer output voltages to determine the spectral values of aod. in fact, the calibration constants are the spectral signals which would be given by the instrument if used outside the atmosphere, when the earth-sun distance assumes the yearly mean value. they were fi rst determined during an intercomparison campaign held at the schneefernerhaus observatory (2665 m a.m.s.l.) on the zugspitze (germany) in october 1996 (tomasi et al., 1998), and subsequently checked at sagres on the final days of the campaign (vitale et al., 2000a), in both cases by applying the langley plot method to the field measurements. all the spectral series of ir-rad signals were then examined in terms of the bouguer-lambert-beer law to determine the values of total atmospheric optical thickness at the 13 window-wavelengths. the values were subsequently corrected for the rayleigh scattering contribution and the terms due to absorption by water vapour, ozone, nitrogen dioxide and other minor gases, thus obtaining the spectral series of aerosol optical depth d(l) at the window-wavelengths (vitale et al., 2000a). three examples of spectral series of d(l) are shown in fi g. 1, for different times on three measurement days. in order to assess the variability of the wavelength dependence features of d(l), we examined all the ir-rad spectral series of d(l) taken within the 0.4011.025 mm wavelength range in terms of the wellfig. 1. spectral series of aerosol optical depth d(l) determined from the ir-rad sun-photometer measurements carried out on june 19, 1997, at 09:52 gmt (circles), july 5, 1997, at 17:47 gmt (triangles), and july 20, 1997, at 11:23 gmt (squares). the corresponding ångström (1964) best-fi t lines determined in the 0.401-1.025 mm spectral range are also drawn (dashed lines), together with the corresponding best-fi t values of atmospheric turbidity parameters a and b. 470 claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé known ångström (1964) formula to determine the best-fi t values of the atmospheric turbidity parameters a and b. the results obtained for the three above examples are also reported in fi g. 1, showing that considerably higher values of d(l) were found on july 20, 1997, compared to those of the two other examples of june 19 and july 5, 1997. the spectral features of d(l) relative to the two examples of july appear to be quite smooth, very closely following the ångström (1964) relationship. the example of june 19, 1997 presents lower values of aod, with a well-defi ned minimum at about 0.7 mm wavelength. such spectral features are very similar to those observed by quenzel (1970) in spectral series of aod derived from direct solar radiation measurements taken in the tropical atlantic ocean, where they were ascribed to the presence of very peaked modes of marine aerosols in the columnar size-distribution curve, appropriately represented in terms of narrow lognormal curves with mean radii ranging between 0.5 and 0.9 mm. the three examples presented above suggest that important variations in the spectral dependence features of aod may take place from one day to another, indicating that marked changes occur in the columnar particle sizedistribution curve, as well as in the refractive index of particulate matter. more generally, all the radiative properties of aerosol particles strongly depend on both size-distribution curve and refractive index and, therefore, are closely related to the origin of the particulate matter. thus, when studying the aerosol radiative effects, it is of crucial importance to establish the particulate origin. this can be more confi dently achieved, for instance, by sampling the particulate matter and defi ning the backward lagrangian trajectories of the air masses passing over the measurement station, as was systematically done during the clearcolumn campaign at sagres, to ascertain the origin of aerosol particles at the various altitudes and the climatic effects produced by airborne aerosols. in fact, the aerosols of marine origin usually scatter very strongly and very weakly absorb the incoming solar radiation, while particles of continental (polluted) origin or mixed composition may cause very intense absorption effects if they contain important fractions of soot. thus, marine aerosol particles suspended over a sea surface characterised by low albedo are expected to induce an increase in the outgoing fl ux of solar radiation at the top of the atmosphere; this implies a corresponding increase in the local albedo of the surfaceatmosphere system and, consequently, a cooling effect in the energy budget. conversely, aerosols containing a relatively high fraction of soot matter are expected to cause weaker cooling effects or effects of opposite sign, giving rise to a decrease in the local albedo (paltridge and platt, 1976). 3. application of the inversion method to the aod spectral series the main purpose of the present work is to evaluate the variations caused by marine and continental aerosols of different composition in the upwelling solar radiation fl ux at the atmospheric top-level, when these particles are suspended over a sea surface. to provide evidence of the large differences between the radiative effects caused by pure marine particle polydispersions and those due to continental aerosols mainly generated by anthropic activities, we decided to examine a limited number of daily data-sets collected during the clearcolumn campaign; they were selected so as to represent a variety of columnar aerosol contents presenting different aerosol composition cases, gradually passing from the extreme case of a nearly entirely marine aerosol population to the opposite case of particle populations mainly consisting of polluted continental aerosols originating from industrial and highly populated areas of europe and, consequently, containing only a negligible fraction of maritime particulate matter. hence, we selected six measurement days to determine the columnar size-distribution features, the radiative properties of particulate matter and the aerosolinduced effects on the outgoing solar radiation. in particular, we examined each spectral series of d(l) to determine the ångström (1964) best-fi t values of parameters a and b: the latter in practice gives the measure of the aerosol optical depth at 1 mm wavelength, while a is determined as the slope coeffi cient of the best-fi t line. parameter a closely depends on the shape-parameters of 471 marine and continental aerosol effects on the upwelling solar radiation fl ux in southern portugal during the ace-2 experiment the aerosol size-distribution curve (especially on those characterising the size range of the large and giant particles, which are known to cause the most important extinction effects) and, to a lesser extent, on the complex refractive index of particulate matter (curcio, 1961; tomasi et al., 1983). therefore, a provides a reliable evaluation of the relative contribution of the large and giant particles to the overall aerosol extinction in the atmospheric column. the six selected days were: 1) june 19, 1997, when the analysis of the backward trajectories drawn during the previous fi ve days showed that the air masses arrived from the northern atlantic with prevailing directions from the ne quadrant at all the tropospheric levels. this indicates that the columnar aerosol particle polydispersion mainly consisted of marine particulate matter. the daily mean values of a and b were found to be equal to 0.63 ± 0.37 and to 0.06 ± 0.04, respectively. therefore, we considered this day as particularly appropriate to represent a totally marine aerosol case. 2) june 23, 1997, when the analysis of the backward trajectories showed that the air masses fl owing at altitudes lower than 3 km arrived from the north after circulating over the british isles, the bay of biscay and portugal, while those at the upper levels came directly from the atlantic ocean. therefore, the columnar particle polydispersions observed on this day at sagres were assumed to consist of mixed aerosols with a prevailing content of marine particles. this assumption fully agrees with the daily mean value of parameter a found equal to 0.15 ± 0.08 and with that of b equal to 0.08 ± 0.03. 3) july 5, 1997, when the backward trajectories drawn at levels below 4 km indicated that the air masses arrived from the northern atlantic but circulated over portugal and the western regions of spain during the previous thirty hours. therefore, the columnar particle polydispersions were presumably composed of mixed aerosols, with prevailing radiative effects due to marine aerosols, the daily mean values of a and b being evaluated as equal to 0.35 ± 0.23 and 0.05 ± 0.02, respectively. 4) july 7, 1997, when the air masses circulated for a long time period within the ground layer of about 3 km depth over the iberian peninsula, the western part of the mediterranean and the atlas mountain region, while only those at levels higher than 5 km came from the atlantic ocean. therefore, the columnar aerosol particle polydispersions were estimated to be mostly composed of continental-anthropic particulate matter, with the presence of a very low fraction of marine aerosols. the daily mean values of a and b were found equal to 0.84 ± 0.11 and 0.09 ± 0.02, respectively. 5) july 18, 1997, when the air masses suspended within the fi rst 4 km of the atmosphere had previously circulated for at least fi ve days over the european regions and mainly over the iberian peninsula and the western part of the mediterranean, while only those at levels higher than 5 km clearly appeared to be of oceanic origin. thus, the aerosol particle loading in the vertical column was evaluated to have a mixed composition, with a prevailing fraction of continental-anthropic particulate matter and a smaller part of marine aerosols. the daily mean values of parameters a and b were found to be equal to 1.04 ± 0.10 and to 0.14 ± 0.03, respectively. 6) july 20, 1997, when the air masses arriving above the sagres station at all levels circulated at length over the bay of biscay and the coastal regions of france before reaching the iberian peninsula. thus, the columnar aerosol particle polydispersions were all considered to be predominantly composed of polluted substances of anthropic origin. this assumption agrees very well with daily mean values of a and b, found to be equal to 0.88 ± 0.05 and 0.15 ± 0.02, respectively. in order to examine the different spectral features presented by the columnar aerosols on the above-selected six days for determining the corresponding particle size-distributions, we analysed the 447 spectral series of d(l) measured on those days using the king (1982) inverse method for inferring the size-distribution curves of the columnar total particle number n within the 0.07-10 mm radius range, according to the procedure followed by vitale et al. (2000b). in order to apply this inversion procedure, we needed to use realistic values of both real part n and imaginary part k of the particulate matter refractive index. for this purpose, we 472 claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé determined the daily mean values of these two optical quantities by taking into account the results obtained by vitale et al. (2000a) from the analysis of the aod spectral series, as well as the daily estimates of both n and k found at the sagres and mt. foia stations on the same days by the leipzig university group (von hoyningen-huene, private communication). the latter estimates were obtained following the procedure described by von hoyningen-huene et al. (1999) and were based on the combined examination of simultaneous measurements of aod spectral series, solar radiation fl uxes and sky-brightness, these last measurements being taken in almucantar for various angular distances from the sun. this analysis provided the following daily mean values of the real part n and imaginary part k of the particulate refractive index: i) n = 1.43 and k = 0.0037, on june 19, 1997, in the presence of pure marine aerosol polydispersions; ii) n = 1.47 and k = 0.0067 on june 23 and july 5, 1997, in the presence of mixed aerosols with predominant contents of maritime particles; iii) n = 1.48 and k = 0.0122 on july 18, 1997, when the predominant content in the atmospheric column was given by continental particles and a smaller fraction of marine aerosols was present at higher altitudes and iv) n = 1.50 and k = 0.0138 on july 7 and 20, 1997, both presenting columnar contents of prevailing continental-anthropic (polluted) aerosols. using these values of n and k, we applied the king (1982) inversion method to all the spectral series of aod measured on the six days selected above, inferring from them the size-distribution curves of the columnar total particle number n in the 0.07-10 mm radius range, from which we also calculated the size-distribution curves of the columnar total particle volume v. figure 2 shows the size-distribution curves of quantities n and v, as found for the three aod spectral series shown in fi g. 1. the three number size-distribution curves exhibit clear bimodal features, characterised by the presence of pronounced modes of large and giant particles in the 0.3-3 mm radius range, which are expected to cause the most important extinction features at the visible and near-infrared june 19,1997 (09:52 gmt) july 5,1997 (17:14 gmt) july 20,1997 (11:23 gmt) d n / d (l o g r) ( c m -2 ) fig. 2. size-distribution curves of the columnar total particle number n (left) and columnar total particle volume v (right), found by analysing the three spectral series shown in fi g. 1 with the king (1982) inverse method. 473 marine and continental aerosol effects on the upwelling solar radiation fl ux in southern portugal during the ace-2 experiment wavelengths. correspondingly, the three volume size-distribution curves show that such modes of large and giant particles provide the predominant contribution to the columnar aerosol volume, since they all exhibit high values throughout the radius range from 0.3 to 10 mm. 4. calculation of the single scattering albedo for marine, mixed and continental aerosols the determination of the radiative properties of a columnar aerosol polydispersion requires a knowledge not only of the size-distribution curve of the columnar total particle number n but also of the dependence curves of the refractive index parts n and k on wavelength. therefore, we determined the spectral functions n(l) and k(l) for each day, representing each columnar aerosol polydispersion as a linear combination of three of the four aerosol basic components defi ned in the 6s computer code, i.e. oceanic, water-soluble and soot components (vermote et al., 1997). this was carried out following the procedure proposed by vitale et al. (2000b), obtaining spectral curves of n(l) and k(l) yielding average values of n and k over the 0.4-3.7 mm wavelength range equal to those defi ned above on the six days and used in applying the retrieval procedure to the aod spectral series. the choice of the linear combination of the three 6s basic aerosol components and the defi nition of the retrieved columnar size-distribution curves of n are both crucial for determining the radiative properties of the particle polydispersions. this is because the radiative parameters of each particle, like the single scattering albedo, the phase function and the asymmetry factor, all closely depend on the spectral features of both real and imaginary parts of particulate refractive index, as predicted by mie theory (deirmendjian, 1969). thus, the monochromatic single scattering albedo w(l) of each aerosol particle polydispersion is given by the integral of the single scattering albedo of a spherical particle of radius r multiplied by the columnar size-distribution curve n(r) = dn/dr over the whole particle radius range. parameter w(l) provides the fraction of the solar radiation undergoing aerosol extinction which is subject to scattering, the residual percentage being lost through aerosol absorption. therefore, we calculated w(l) at 10 signifi cant wavelengths in the 0.4-3.75 mm range for each spectral series of aod, using the spectral curves of n(l) and k(l) determined above for the corresponding day and the retrieved size distribution curve n(r) as weight function. we then calculated for each aod spectral series the weighted average value w of the single scattering albedo of the corresponding columnar aerosol polydispersion, using as weight function the spectral curve of the solar radiation reaching the ground, as calculated with the 6s computer code for a standard midlatitude summer atmosphere model (anderson et al., 1986), where we assumed that the aod spectral series is the one measured at that time and the solar zenith angle is equal to 60°. figure 3a,b shows the time-patterns of aerosol optical depth d(550 nm) obtained from the ir-rad measurements carried out on the six selected days and the daily time-patterns of the average single scattering albedo w. the time-patterns of d(550 nm) appear in general to slowly decrease during the morning, presumably as a result of the gradual decrease in the marine breezes moving from the ocean toward the inland areas, and the simultaneous reduction in the large particle sizes due to the decrease in air relative humidity caused by the solar heating. during the afternoon, a slow increase of d(550 nm) was often observed, generally associated with variations in the local circulation favouring the arrival of continental particulate matter from the more polluted region of lisbon or from other urban areas of the iberian peninsula. with regard to the time-variations in w, it can be noticed in fi g. 3b that on june 19, 1997, characterised by the large presence of marine aerosols, this parameter was found to present appreciably higher values than those obtained for june 23 and july 5, when mixed aerosols were present with predominant fractions of marine particles, and for july 18 in the presence of continental aerosols and a less important fraction of marine particles. in fact, a daily mean value v of the single scattering albedo equal to 0.95 was found on june 19, while values of v equal to 0.87, 0.86 and 0.89 were determined on the three above-mentioned days, respectively. a lower value of v = 0.83 was found on july 7 474 claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé and 20, 1997, both days being characterised by the presence of optically prevailing contents of continental aerosols. these evaluations of w are very important in improving our understanding of the role played by aerosols in modifying the radiative balance of the atmosphere through a greater or lesser intense scattering of the incoming solar radiation. the above evaluations of w were obtained by using the columnar aerosol size-distribution curves retrieved from the ir-rad measurements of aerosol optical depth employing fi xed values of n and k and assuming both refractive index parts not to vary with time throughout the daily period. our assumption of fi xed values of n and k was required since the leipzig university group followed the von hoyningen-huene et al. (1999) procedure, providing us with only mean daily values of the refractive index parts (private communication). in reality, both n and k are expected to vary frequently with time as a consequence of transport and/or physical processes, such as the evaporation of haze droplets, formation of new particles through gas-toparticle conversion and growth of aerosols by coagulation or condensation. together, such processes give rise to appreciable modifications in the shape of the columnar aerosol sizedistribution curve, which generally presents multimodal features like those shown in fi g. 2. thus, the use of fi xed values of n and k in the king (1982) inversion procedure led us to obtain retrieved size-distribution curves appreciably different from those which would be found using the true values of n and k. in order to give a measure of how large could be the variations in the calculations of w arising from the above assumptions, we estimated that the diurnal variations in n and k occurring during the day, for anticyclonic conditions and weak transport of particulate matter, as in the cases of july 5, 7 and 20, 1997, can be reasonably assumed to be limited within ± 0.02 for the real part n, and within ± 50% for the imaginary part k. thus, calculations of w were made for the above three days, fi rst by changing the values of n and using stable values of k, and subsequently for stable values of n and different values of k, determining correspondingly the columnar aerosol sizedistribution curves through the king (1982) retrieval procedure. the results indicate that w can vary by ± 6.0% on july 5 for n changing from 1.47 to 1.47 ± 0.02, and by no more than ± 3.5% on july 7 and july 20 for n varying from 1.50 to 1.50 ± 0.02, the values of k remaining unchanged. conversely, for fi xed values of n on the three days, w was found to vary by no more than ± 10% on july 5 for values of k = 0.0067 ± ± 0.0033, and by about ± 8% on the other two days for values of k = 0.0138 ± 0.0069. these and other more extended calculations indicate that the assumption of fi xed values of n and k during the whole day can imply overall relative errors in the estimates of w of no more than ± 30% for anticyclonic conditions favouring only weak processes of particulate matter transport. 5. calculation of the diurnal changes in the upwelling solar radiation fl ux the radiative effects caused by airborne particles on solar radiation can be properly represented in terms of both intensity and sign of the aerosol-induced change df in the upwelling solar radiation fl ux f leaving the atmosphere. calculations of change df were performed by us for all the aod spectral series determined from the ir-rad measurements taken on the six selected clear-sky days. in these calculations, we followed the procedure proposed by vitale et al. (2000b), which employs the 6s computer code (vermote et al., 1997) to calculate the upwelling fl ux of solar radiation at the top-level of the atmosphere, over the 2p solid angle. this was carried out by calculating the fl ux components at numerous different zenith and azimuth angles and integrating them over the hemisphere. the calculations of f were performed for each ir-rad spectral series of aod assuming that: i) the vertical profile of temperature and relative humidity is that determined at each measurement-time by interpolation in time between the profiles obtained from the radiosounding measurements taken on the same day at the sagres station and san teotonio station (37°33' n, 8°43' w; 119 m a.m.s.l.), 60 km nne of sagres, for an overall number of 4 to 8 radiosonde launches per day (tomasi et al., 2000). 475 marine and continental aerosol effects on the upwelling solar radiation fl ux in southern portugal during the ace-2 experiment ii) the vertical contents of o3, co2, o2 and other minor gases are those assumed in the standard mid-latitude summer atmosphere model defi ned by anderson et al. (1986). iii) the apparent solar zenith angle is that corresponding to the median ir-rad measurement time. iv) the columnar particle size-distribution curve is that retrieved from the ir-rad spectral series of aod. v) the spectral curves of particulate refractive index parts n(l) and k(l) are those determined above on the various days. vi) the spectral refl ectance curve of the surface is that defi ned by the 6s computer code for a clear water surface, which simulates with a good approximation the refl ectance characteristics of the sea surface for calm wind conditions. thereupon, we calculated the ougoing fl ux f0 for the same atmospheric model confi guration, in which the columnar aerosol mass content was instead assumed to be null. the difference df between the value of fl ux f with aerosols and that of f0 without aerosols was considered to be a realistic estimate of the change instantaneously produced by the airborne aerosols at the measurement time (hänel et al., 1999). the time-patterns of df obtained on the six days are shown in fi g. 4a. they indicate that the highest values of df (in general greater than 10 wm-2) were achieved on july 18 and 20, 1997, that is, on days presenting values of aod higher than 0.20 and atmospheric particle loadings predominantly consisting of continental aerosols, although with different values of the columnar aerosol single scattering albedo: it was estimated to assume values ranging mainly between 0.85 and 0.90 on july 18 and between 0.75 and 0.85 on july 20, as shown in fi g. 3b. values of df mainly ranging between 5 and 10 wm-2 were obtained on the three days characterised by the presence of mixed aerosol polydispersions, when values fig. 3a,b. a) time-patterns of aerosol optical depth d(550 nm) measured on the six selected days; b) timepatterns of the average single scattering albedo w of the columnar aerosols, calculated on the same six days for marine aerosols (june 19), mixed aerosol polydispersions (june 23, july 5 and july 18), and continental (polluted) aerosols (july 7 and 20, 1997). a b 476 claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé of w varying mainly between 0.80 and 0.85 were found together with values of aod lower than 0.20. values of df mostly lower than 5 wm-2 were obtained on july 19, 1997, in the prevailing presence of marine aerosols, presumably as a result of the very low values of aod measured on that day, in spite of the values of w all higher than 0.94. because of the above assumptions of fi xed values of n and k during the same day, it is reasonable to bear in mind that the present estimates of df are affected by errors due to the use of both aerosol size-distribution curves and single scattering albedo characteristics different from the real conditions. calculations similar to those perfomed in the previous section for determining parameter w as a function of n and k indicate that df can vary by no more than ± 1.0% on july 5 for n changing from 1.47 to 1.47 ± 0.02, and by no more than ± 2.0% on july 7 and july 20 for n varying from 1.50 to 1.50 ± 0.02. on the other hand, using fi xed values of n on the three days together with values of k modifi ed step-by-step, df turned out to change by about ±15% on july 5 for values of k = 0.0067 ± 0.0033, and by percentages varying between ± 35% and ± 50% on the other two days for values of k = 0.0138 ± ± 0.0069. these results and other supplementary calculations made for the other measurement days allow us to estimate that the use of our procedure produces on average overall relative errors in df ranging between ± 15% for values of d(550 nm) equal to about 0.1, and ± 30% for values of d(550 nm) equal to about 0.2. based on the comparison of the daily timepatterns of df with those of d(550 nm) shown in fig. 3a, the above results clearly indicate that more marked values of df are in general obtained in cases where aod assumes relatively high values at the visible wavelengths. to test this statement, at each measurement time of the six days we evaluated the ratios df /d(550 nm), obtaining realistic estimates of the change df per unit aod in the visible range. the results are given in fi g. 4b, which shows values of this ratio mostly higher than 80 wm-2 in the fig. 4a,b. a) time-patterns of the change df in the upwelling solar radiation fl ux at the top of the atmosphere, determined for the six selected days considered in fi g. 3a,b. b) time-patterns of change df /d(550 nm) per unit variation of aod at the mean visible wavelength, determined for the same six days. a b 477 marine and continental aerosol effects on the upwelling solar radiation fl ux in southern portugal during the ace-2 experiment middle part of june 19, 1997, in the presence of pure marine aerosols, values ranging mainly between 50 and 80 wm-2 on the days with mixed aerosols and between 20 and 60 wm-2 on the two days presenting mainly continental (polluted) particles. these fi ndings indicate that, at parity of aod, marine aerosols cause the most intense changes of f , since they produce very high single scattering albedo features, while continental particles containing particulate matter of anthropic origins are expected to produce less pronounced effects, which are lower by more than 50% in the most polluted cases. this demonstrates that the composition of particulate matter and the related microphysical and radiative properties (represented on the whole by the single scattering albedo of columnar aerosols) are of basic importance in determining the intensity of the aerosol effects on the upwelling solar radiation fl ux over oceanic areas. 6. discussion of the results and conclusions the change df represents the instantaneous radiative effect caused at the top of the atmosphere by aerosols suspended within the atmospheric vertical column of unit cross section. the present analysis provided results which point out that the variations df caused by aerosols in the radiation balance of the surface-atmosphere system can be realistically interpreted in terms of the aerosol optical depth and the single scattering albedo of the columnar particle polydispersion, the surface albedo being the third most important parameter to be taken into account in similar studies. however, we have assumed in the present study a constant surface albedo in all the examined cases, representing it through features typical of a clear water surface. in order to get a measure of the daily mean effect produced by airborne aerosols on the radiative budget of the surfaceatmosphere system, we integrated the daily timepatterns of df over the 24 h period. in doing so, we calculated the local times of sunrise and sunset on the six days and assumed that df is null at the initial and fi nal time of each diurnal period, increasing linearly with time during the early morning and decreasing linearly after the last sun-radiometric measurement taken in the late afternoon. the integrals of df in time were then divided by 24 h to obtain the estimates of the 24-h average change df on the six days. they are plotted in fi g. 5 as a function of the daily mean values of d(550 nm), as calculated from the time-patterns of this quantity shown in fi g. 3a. the results are also compared in fi g. 5 with the average relationship curves obtained from those calculated by russell et al. (1999b) on two days of the tarfox experiment for three different values of the daily mean single scattering albedo v of aerosols, i.e.: i) v = 1 for totally scattering aerosols; ii) v = 0.935 for marine aerosols presenting radiative properties similar to those observed by us on june 19, 1997, and iii) v = 0.86 for mixed aerosol polydispersions containing a prevailing fraction of maritime particulate matter and less important percentages of continental origins, similar to those observed by us on june 23, july 5 and july 18, 1997. the comparison shows that a substantial agreement exists between our evaluations and those of russell et al. (1999b), although the present daily average estimates of df turn out to be overestimated with respect to those of the tarfox measurements, by no more than 15% for values of d(550 nm) > 0.15 and by percentages varying between 8% and 62% in the other four cases. such discrepancies can be in part explained by the different radiative properties of the airborne aerosols observed during the two experiments and in part by the irregular features of the aod time-patterns. however, our results indicate that relationship curves like those proposed by russell et al. (1999b), but with higher slope coeffi cients in the range d(550 nm) < 0.15, could provide reliable evaluations of the 24-h average change df for aerosol radiative characteristics more similar to those of the clearcolumn campaign. the present evaluations of df have been obtained for low surface albedo conditions and a large variety of aod values, columnar aerosol size-distribution curves and particulate radiative properties. the change produced by aerosol particles in the upwelling fl ux f is the result of highly complex radiative transfer processes occurring in the atmosphere, which mainly arise from scattering and absorption 478 claudio tomasi, vito vitale, angelo lupi, alessandra cacciari, stefano marani and ubaldo bonafé fig. 5. estimates of the 24-h average fl ux change df determined on the six selected days as a function of the daily mean aerosol optical depth d(550 nm) and for different daily average values of the columnar aerosol single scattering albedo v. the results are compared with the relationship curves determined by russell et al. (1999b) during the tarfox experiment for three different values of v: (a) v = 1.00; (b) v = 0.935, and (c) v = 0.86. (a) (b) (c) caused by aerosols. the results show that the intensity of these processes over a calm oceanic region substantially depends on the two physical quantities aod and w, which implicitly contain a large information set on the various microphysical and radiative parameters, such as the columnar total particle number, the shapeparameters of the particle size-distribution and the complex refractive index. the diagram in fi g. 5 presenting average relationship curves obtained from the tarfox results (russell et al., 1999b) constitutes a useful tool for deriving reliable estimates of the 24-h average values of df in oceanic regions as a function of aerosol optical depth d(550 nm) and single scattering albedo w, both quantities being determined by combining sun-photometric measurements of direct solar irradiance with sky-brightness measurements taken in the almucantar at different angular distances from the sun. therefore, we can conclude that the solar radiation fl ux changes caused by aerosols in planetary regions presenting low surface albedo features are expected to induce cooling effects, whose entity is proportional to aod with proportionality coeffi cients increasing with the aerosol single scattering albedo. references anderson, g.p., s.a. clough, f.x. kneizys, j.h. chetwynd and e.p. shettle (1986): afgl atmospheric constituent profiles (0-120 km), air force geophysics laboratory (l.g. hanscom field, massachusetts), environ. res. papers, n. 954, afgltr-86-0110, pp. 43. ångström, a. 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dust and its effect on radiative balance, j. aerosol sci., 30, 489-502. dong sutton.pdf annals of geophysics, vol. 46, n. 1, february 2003 87 variability of atlantic ocean heat transport and its effects on the atmosphere buwen dong and rowan t. sutton centre for global atmospheric modelling, department of meteorology, university of reading, u.k. abstract the variability of the atlantic meridional ocean heat transport (oht) has been diagnosed from a simulation of a coupled ocean-atmosphere general circulation model, and the mechanisms responsible for this variability have been elucidated. it has been demonstrated that the interannual variability in atlantic oht is dominated by windstress-driven ekman fluctuations. in contrast, the decadal and multidecadal variability is associated with the fluctuations of the thermohaline circulation (thc), driven by the fluctuations in deep convection over the greenland-iceland-norwegian (gin) sea. the fluctuations of oht induce ocean heat content (ohc), and sea surface temperature (sst) anomalies over the tropical and subtropical north atlantic. the sst anomalies, in turn, have an impact on the atmosphere. the lead-lag relationships between the fluctuations of thc-related oht and those of ohc and sst raise the possibility that a knowledge of oht fluctuations could be used to predict variations in atlantic sea surface temperatures, and perhaps aspects of climate, several years in advance. a comparison of results from a second, independent, coupled model simulation is also presented, and similar conclusions reached. 1. introduction the fact that the ocean and atmosphere transport heat polewards is one of the most fundamental features of the climate system. substantial changes in these heat transports would inevitably provoke major climate change (e.g., manabe and stouffer, 1995, 1999). yet the processes that regulate poleward heat transport, and control its variability, are poorly understood. a major aim of the sintex project was to advance understanding of the processes that determine the variability of heat transport in the atlantic ocean, and its effects on the atmosphere. the atlantic ocean plays a particularly important role in global heat transport. whereas the total ocean heat transport is poleward in both hemispheres, the heat transport in the atlantic is northward even in the southern hemisphere (e.g., peixoto and oort, 1993). this surprising situation is a consequence of the thermohaline circulation, in which deep water formation in the north atlantic plays a key role. an implication of this situation is that major changes in the north atlantic thermohaline circulation could have a substantial global impact. efforts to understand the variability in atlantic ocean heat transport are hindered by a mailing address: dr. rowan sutton, centre for global atmospheric modelling, department of meteorology, university of reading, po box 243, earley gate, reading rg6 6bb, u.k.; email: r.sutton@reading.ac.uk key words climate variability – north atlantic oscillation (nao) – ocean heat transport (oht) – sea surface temperature (sst) – thermohaline circulation (thc) 88 buwen dong and rowan t. sutton lack of sufficient ocean observations. consequently it is necessary to turn to models. in this paper, work with a version of the u.k. hadley centre climate model hadcm3 is reported. this model requires no flux corrections to maintain a stable climate and is thus an excellent tool for studying heat transports and their variability. the structure of the paper is as follows. in section 2 the model and simulation are described. the variability of atlantic ocean heat transport, and its effects on the atmosphere, are discussed in section 3. in section 4 comparisons are made with a second coupled model that was developed as part of the sintex project. summary and conclusions are in section 5. 2. description of model simulation the principal model we use is a version of the united kingdom hadley centre global coupled ocean atmosphere general circulation model known as hadcm3, described in gordon et al. (2000). the atmospheric model component in hadcm3 is a version of the united kingdom meteorological office (ukmo) unified forecast and climate model run with a horizontal grid spacing of 2.5° × 3.75° and 19 vertical levels using a hybrid vertical coordinate. the oceanic component of the model is a 20 level version of the cox (1984) model on a 1.25° × 1.25° latitudelongitude grid with a sea ice model using a simple thermodynamic scheme and containing parameterizations of ice drift and leads (cattle and crossley, 1995). the model does not require flux corrections to maintain a stable climate. the mean climate and its stability in a 1000 year control simulation are discussed in gordon et al. (2000). we modified the model in such a way as to restrict atmosphere-ocean interactions to regions outside the tropical pacific and tropical indian ocean. over the tropical pacific and tropical indian ocean, the ssts were relaxed to the monthly mean hadcm3 climatology with a time scale of 2.5 days. the main effect of this relaxation is to eliminate the el niño-southern oscillation (enso) phenomenon in the simulation. this allows us to study ocean-atmosphere interactions in the atlantic basin without interference from the tropical pacific and tropical indian ocean. we performed a control integration of 100 years duration employing this relaxation. the initial state was taken from a 1000 year control simulation of the globally coupled model. the coupled model reproduces the principle features of the north atlantic ocean circulations in a satisfying manner (dong and sutton, 2002a,b). the dominant convective region occurs over the northeast north atlantic, over the greenland, iceland, and norwegian (gin) sea, where the annual mean mixed layer depth reaches a maximum of about 200 m while the mixed layer depth in the winter is over 500 m. 3. results 3.1. variability of atlantic meridional ocean heat transport the annual mean atlantic oht anomalies are given in fig. 1 for the specially configured hadcm3 simulation. the heat transport anomalies range from – 0.2 pw (1015w) to 0.2 pw. these correspond to about 20% to 30% of the model climatological mean heat transport (1.18 pw at 24.4°n) depending on latitude. there are several interesting aspects of the oht variations. first, the variations in the tropics and subtropics show a high degree of coherence with latitude. secondly, the dominant frequency differs between low latitudes and mid-to-high latitudes. in low latitudes, most of the variability is on multi-annual timescales. in mid-to-high latitudes, lower frequency variations predominate. 3.2. mechanisms of the atlantic ocean heat transport fluctuations in the oht may arise from processes that are internal to the ocean, or in response to atmospheric fluctuations. the latter can arise from windstress-driven variations in ekman transport, from density-driven variations in the thermohaline circulation (thc) and from windstress curl-driven variations in the gyre circulation. to identify the different contributions 89 variability of atlantic ocean heat transport and its effects on the atmosphere to the oht variability, we use a principal component analysis. dong and sutton (2001) showed that the dominant mode (eof1) of the atlantic oht, which explains 50.3% of interannual variance, is associated with wind-driven fluctuafig. 1. latitude-time evolution of the ocean heat transport anomalies (pw: 1015 w) in the atlantic for the hadcm3 simulation. positive anomalies are in full lines and negative ones in dashed lines. tions in ekman transport. in this paper we focus on the lower frequency variability in atlantic oht. figure 2a-e shows eof1 of low pass filtered (5 years running mean) oht and the associated pc1 for the hadcm3 simulation. this mode explains 55.6% of the low frequency variance. it reaches peak amplitude around 30°-40°n (where the mean meridional temperature gradients are greatest), decaying rapidly to the north of this latitude and more gradually to the south. pc1 indicates that the timescale of variability for this mode is decadal-to-multi-decadal. the regression pattern of the mean meridional overturning circulation (moc) to pc1 of low frequency oht is shown in fig. 2c. it is characterized by a single cell extending south from 70°n and reaching well across the equator. the vertical structure is baroclinic. this regression pattern is very similar to eof2 (20.4% of total variance, not shown) of the low-pass filtered moc (pattern correlation is 0.87). figure 2b shows that the associated principal components are also highly correlated. this suggests that low frequency variations of the oht are associated with the atlantic thc variability. eof1 of the low-pass filtered moc, which explains 41.0% of low frequency variance, is associated with a trend in moc. this mode makes only a minor contribution to the oht variability. in fact, pc1 of moc is highly correlated with pc3 of oht with oht eof3 accounting for 8.3% of low frequency oht variance. we, therefore, focus on the pc2 of moc and pc1 of oht, and their associated climate variability. figure 2d shows the regression of upper ocean density onto pc2 of the moc. it is clear that density variations over the greenland-iceland-norwegian (gin) sea region are correlated with fluctuations in the moc, such that a positive density anomaly is associated with a maximum in the moc. the positive density anomalies extend from the surface to about 1000 m with weak negative density anomalies from about 1000 m to 3000 m (not shown), thereby reducing the vertical stability of the water column, enhancing the deep convection rate, and driving a stronger moc. the relationship between the moc and density anomalies over the gin sea is -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 -0.025 0 0.025 0.05 0.1 0.15 0.2 0.25 0.3 0.35 ye ar 20s eq 20n 40n 60n 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 latitude -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 0.05 -0.05 -0.05 -0.05 -0.0 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 0.05 -0.05 -0.05 -0.05 -0.05 -0.05 0.0505 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.050.05 0.2 90 buwen dong and rowan t. sutton 20s eq 20n 40n 60n 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 250 0 -0.1 0 05 0.05 0.05 0.1 0.1 0.2 0.3 0.30.3 0.4 moc 60n 10 20 30 40 50 60 70 80 90 100 -3 3 moc pc2 and density index year 10 20 30 40 50 60 70 80 90 100 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 10 20 30 40 50 60 70 80 90 100 eof1 (55.6%) 20s eq 20n 40n 60n -0.04 -0.02 0.0 0.02 0.04 0.06 0.08 pc1(oht) and pc2(moc) 10 20 30 40 50 60 70 80 90 100 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 10 20 30 40 50 60 70 80 90 100 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 10 20 30 40 50 60 70 80 90 100 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 0 0.19 0.2 0.25 0.3 0.4 0.5 0.6 1 90w 60w 30w 0 20s eq 20n 40n 60n 80n -4 -2 -2 -2 -1 -0.25 -0.25 -0.25 0.25 0.5 0.5 1 1 1 1 2 2 2 2 4 4 4 4 4 upper 500 m density 90w 60w 30w 0 20s eq 20n 40n 60n 80n fig. 2a-e. a) eof1 (pw: 1015w), and (b) pc1 (solid line) of low frequency oht for the hadcm3 simulation. panel (b) also shows (dashed line) pc2 of low frequency moc. c) regression pattern of the moc at lag 0 on pc1 of low frequency ocean heat transport (unit: sv per unit of pc1 of oht). d) regression coefficients of the upper ocean (500 m) density (10−5 g cm−3) to pc2 of the moc at lag 0. e) normalized time series of pc2 of the moc and the density index defined over the deep convection region. shading scale in (c) and (d) indicates explained local variance. a b c d e 91 variability of atlantic ocean heat transport and its effects on the atmosphere further illustrated by fig. 2e, which shows the normalized time series of the pc2 of the moc and density index, which is defined as averaged density over the gin sea (60°n-80°n, 20°w15°e). in general, the two time series fluctuate in phase except in first 25 years. this may be due to the spin-up process of the model. the two time series are highly correlated with a coefficient 0.66. the correlation increases to 0.83 if the last 75-years data are used. 3.3. relationships between variability in oht, ocean heat content and sst in the previous section, we have shown that in addition to the interannual fluctuations, the atlantic oht in the model simulation also exhibits decadal variability. it has long been suspected that the changes in the oceanic thc and associated oht may have been an important factor in the climate fluctuations. in this section and the next, we investigate how the ocean and atmosphere respond to fluctuations in the thc and associated oht. to show the oceanic response to the thc fluctuations, the lagged linear regressions of the annual mean low frequency ocean heat content (ohc) and sst to the pc2 of the low frequency moc have been calculated. shown in fig. 3 are the regression maps at different lags. similar regressions against pc1 of low frequency oht have been performed and the results are very similar to those shown in fig. 3. at lag 0, the phase of maximum thc and oht, there are positive ohc anomalies over the gulf stream region, with negative anomalies to the north and weak negative anomalies in the tropical and subtropical atlantic. as time advances the positive anomalies in the gulf stream region and the negative anomalies in the northern subtropical atlantic grow in strength. the positive anomalies are then seen to propagate northeastward, approximately along the path of the north atlantic current. by lag 8 a welldeveloped dipolar pattern of ohc anomalies is seen. the major features of the ohc evolution could be anticipated from fig. 2a. the structure of the eof suggests that convergence of the northward heat transport in the north atlantic (north of about 40°n) should be associated with divergence of the heat transport in the subtropical atlantic (and vice versa). thus, following a maximum in the thc there is a tendency to build up heat in the north atlantic, and to evacuate it from the subtropical atlantic − which leads to the dipolar pattern in ohc. the regression patterns for sst (fig. 3, right column) show that the ohc anomalies are also manifest at the surface, suggesting the possibility of impacts on the atmosphere. note finally that the northeastward propagation of ohc and sst anomalies along the path of the north atlantic current is in line with the sst features identified in the observational study of sutton and allen (1997). the characteristics of the propagation in the model are further analysed by dong and sutton (2002a). 3.4. the impact of atlantic oht variability on the atmosphere in section 3.3, it was seen that the low frequency fluctuations in atlantic oht induce significant sst anomalies. do these sst anomalies have significant effects on the atmosphere? to investigate this question lagged regressions of atmospheric variables to the pc2 of the moc were performed. figure 4a-c shows the regression maps of sst, mean sea level pressure (mslp), and surface wind 4 years after the maximum of thc and associated oht. the regression patterns are similar to fig. 4a-c when the pc2 of the moc leads by 2-6 years, with regressions reaching a maximum at lag 4. figure 4a indicates that the sst pattern is characterized by a tripole with positive anomalies over the gulf stream region and negative anomalies to the north and south. the mslp and surface wind regression patterns (fig. 4b,c) have a positive projection on the well-known north atlantic oscillation (nao, hurrell, 1995). the height field (not shown) indicates that the tropical response is baroclinic while extra-tropical response is equivalent barotropic. in the tropics, the cross-equator sst gradient induces a southward shift of itcz as indicated by the precipitation regression pattern (not shown). 92 buwen dong and rowan t. sutton 90w 60w 30w 0 20s eq 20n 40n 60n -0.1 -0.025 -0.025 0.1 lag 0 90w 60w 30w 0 20s eq 20n 40n 60n 90w 60w 30w 0 20s eq 20n 40n 60n -0.2 -0.025 -0.025 lag 4 90w 60w 30w 0 20s eq 20n 40n 60n 90w 60w 30w 0 20s eq 20n 40n 60n -0.1 -0.1 -0.025 0.025 0.05 0.10.2 lag 8 90w 60w 30w 0 20s eq 20n 40n 60n 0 0.19 0.2 0.25 0.3 0.4 0.5 0.6 1 90w 60w 30w 0 20s eq 20n 40n 60n -0.05 0.1 0.4 lag 12 20s eq 20n 40n 60n 90w 60w 30w 0 20s eq 20n 40n 60n -0.4 -0.4 -0.2 -0.1 -0.1 -0.05 -0.025 0.025 0.025 lag 0 90w 60w 30w 0 90w 60w 30w 0 20s eq 20n 40n 60n -0.2 -0.2 -0.1 -0.1 -0.1 -0.025 -0.025 0.05 0.1 lag 4 90w 60w 30w 0 90w 60w 30w 0 20s eq 20n 40n 60n -0.1 -0.1 -0.05 -0.025 0.025 0.05 0.1 lag 8 90w 60w 30w 0 0 0.19 0.2 0.25 0.3 0.4 0.5 0.6 1 90w 60w 30w 0 20s eq 20n 40n 60n -0.05 -0.05 -0.025 -0.025 -0.025 0.025 0.05 lag 12 fig. 3. regression coefficients of the annual mean ocean heat content (left) and sea surface temperature (right) to the pc2 of the moc at various lags for the hadcm3 simulation. positive lags mean the moc leading. units are in oc per unit pc2 of the moc. shading scale indicates explained local variance. 93 variability of atlantic ocean heat transport and its effects on the atmosphere 90w 60w 30w 0 20s eq 20n 40n 60n -0.2 -0.2 -0.1 -0.1 -0.1 -0.025 -0.025 0.05 0.1 sst 20s eq 20n 40n 60n 90w 60w 30w 0 20s eq 20n 40n 60n -0.025 0.025 0.025 0.1 0.1 0.2 0.2 0.2 mslp 20s eq 20n 40n 60n 0 0.19 0.2 0.25 0.3 0.4 0.5 0.6 1 90w 60w 30w 0 20s eq 20n 40n 60n 0.2 surface wind fig. 4a-c. the regression patterns of low frequency annual sea surface temperature (a), mean sea level pressure (b), and surface wind (c) to the pc2 of the moc when moc leading by 4 years for the hadcm3 simulation. units are in oc, hpa, and m s–1 per unit pc2 of the moc. shading scale indicates explained local variance. a b c sutton et al. (2001) showed, using idealised experiments with the atmospheric component of hadcm3, that an atmospheric pattern similar to that shown in fig. 4b,c can be induced in response to an sst tripole pattern similar to that shown in fig. 4a. they showed further that changes in tropical convection, over south america and in the itcz, play a key role. this anomalous convection results in anomalous heating and triggers the extratropical equivalent barotropic response through rossby waves (sardeshmukh and hoskins, 1988). further experiments showed that forcing of this response is primarily by the tropical sst anomalies, although the midlatitude anomalies also make a small contribution. putting together the results of sutton et al. (2001) with the results from this coupled model study clearly suggests that thc-related fluctuations in the atlantic ocean heat transport can impact the atmosphere, in particular by modulating sst in the northern tropical atlantic. more detailed study of the role of ocean dynamics in north atlantic decadal climate variability in our model simulation will be reported elsewhere (dong and sutton, 2002b). 4. comparison with the echam4t30/opa simulation in this section, we compare the results from our hadcm3 simulation with results from another sintex model: the echam4t30/opa coupled gcm. we analyse a 200-year simulation of this model. a detailed description of the model, and an account of the enso variability in this simulation, is given by guilyardi et al. (2001). because the simulation is subject to some climate drift, a 20-year running mean was removed prior to analysis to eliminate very low frequency variability. the annual mean atlantic oht anomalies from the echam4t30/opa simulation are shown in fig. 5. the heat transport anomalies range from – 0.15 pw to 0.15 pw, or about 20% of the mean heat transport (0.85 pw at 20.4°n). as in the hadcm3 simulation (fig. 1), the fluctuations of oht show a high degree of coherence across the equator and in the subtropics. as for the hadcm3 simulation, a principle component 94 buwen dong and rowan t. sutton fig. 5. latitude-time evolution of the ocean heat transport anomalies (pw: 1015w) in atlantic for the echam4t30/ opa simulation. positive anomalies are in full lines and negative ones in dashed lines. ye ar 20s eq 20n 40n 60n 0 20 40 60 80 100 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 -0.025 0 0.025 0.05 0.1 0.15 0.2 0.25 0.3 0.35 latitude 20s eq 20n 40n 60n 100 120 140 160 180 200 -0.1 -0.05 -0.05 -0.05 0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 05 0.05 latitude -0.1 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 0.05 -0.05 -0.050.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 -0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.1 95 variability of atlantic ocean heat transport and its effects on the atmosphere analysis of the oht was performed. the dominant mode (not shown) of interannual variability, which explains 49.5% of interannual variance, is − as in hadcm3 − associated with windstress-driven fluctuations in ekman transport. the dominant mode of low frequency atlantic oht, which explains 45.0% of the total low frequency variance, is shown in fig. 6a,b. the mode is characterized by anomalous heat transport convergence at about 30°n. this mode is mainly associated with surface buoyancy flux eof1 (45.0%) 30s 10s 10n 30n 50n 70n -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 n pc1 20 40 60 80 100 120 140 160 180 200 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 20 40 60 80 100 120 140 160 180 200 fig. 6a,b. a) eof1 (pw: 1015w), and (b) pc1 of low frequency oht for the echam4t30/opa simulation. a b 90w 60w 30w 0 20s eq 20n 40n 60n -0.05 -0.025 -0.025 0.025 0.025 0.025 0.025 0.1 0.1 sst 90w 60w 30w 0 20s eq 20n 40n 60n 90w 60w 30w 0 20s eq 20n 40n 60n -0.2 -0.1 -0.025 0.025 0.025 0.05 0.1 mslp 90w 60w 30w 0 20s eq 20n 40n 60n 0 0.132 0.2 0.25 0.3 0.4 0.5 0.6 1 90w 60w 30w 0 20s eq 20n 40n 60n surface wind 0.2 90w 60w 30w 0 20s eq 20n 40n 60n fig. 7a-c. the regression patterns of low frequency annual sea surface temperature (a), mean sea level pressure (b), and surface wind (c) to the pc1 of the oht when oht leading by 2 years for the echam4t30/opa simulation. units are in oc, hpa, and m s–1 per unit pc1 of the oht. shading scale indicates ex-plained local variance. a b c 96 buwen dong and rowan t. sutton variability over northeast north atlantic, but windstress driven ekman transport fluctuations also play a role (not shown). this implies that the low frequency oht fluctuations are mainly associated with thc variability. detailed comparison of the mechanisms responsible for low frequency oht variability in the two models is beyond the scope of this paper. the impact of oht variability on atmosphere in the echam4t30/opa simulation is illustrated by fig. 7a-c, which shows the regression maps of sst, mslp, and surface wind onto the pc1 of low frequency oht when the oht is leading by two years. the sst pattern is characterized by a tripole with positive anomalies over the midlatitudes of north atlantic and negative anomalies to the north and south. this pattern is consistent with anomalous oht convergence in midlatitudes and divergence to the south and north (fig. 6a). the mslp regression patterns resemble a positive phase of the north atlantic oscillation (nao). the results here suggest similar behaviour regarding the impact of oht variability on the atmosphere to that found in the hadcm3 simulation. 5. summary and conclusions the dominant mechanisms responsible for fluctuations in atlantic oht in simulations with two of the sintex models have been analysed. we have shown that, in both the simulations, atlantic oht exhibits both interannual and decadal variability. the interannual variability of atlantic oht is dominated by ekman transport fluctuations forced by windstress variability. the lower frequency variability of atlantic oht is associated with fluctuations in the thermohaline circulation which, in turn, are driven by fluctuations in density and deep convection in the gin seas region (hadcm3 simulation). further analysis of the mechanisms responsible for the decadal variability of the thc and its effect on climate in the hadcm3 simulation will be reported elsewhere (dong and sutton, 2002b). the changes in the low frequency oht result in ocean heat content and surface sea surface temperature variations not only over the highlatitude north atlantic, but also over the tropical and subtropical north atlantic. anomalous negative sst anomalies over the tropical north atlantic and positive sst anomalies over the midlatitudes of the north atlantic are associated, respectively, with anomalous oht divergence and convergence, with the sst fluctuation lagging the oht fluctuation by a few years. the results suggest that the variations in atlantic oht are playing an important role in the decadal variability of the upper atlantic ocean. negative (positive) sst anomalies in the tropical north atlantic tend to be associated with the positive (negative) phase of the nao. both model simulations suggest a similar response of the north atlantic climate to the oht fluctuations with a time lag of a few years. the relationship between the oht and sst fluctuations raises the possibility that knowledge of oht fluctuations could be used to predict variations in ocean heat content, and therefore sea surface temperatures, several years in advance. then, the possibility of climate predictions with multi-year lead times is conceivable. acknowledgements this work was supported by the ec sintex project under contract env4-ct98-0714. the computing time was provided by the ugamp, which is funded by the u.k. natural environment research council. we like to thank eric guilyardi for providing us the echam4t30/ opa simulation. comments from an anonymous reviewer are appreciated. references cattle, h. and j. crossley (1995): modelling arctic climate change, phil. trans. r. soc. london, a352, 201-213. cox, m.d. (1984): a primitive equation, 3 dimensional model of the ocean, gfdl ocean group technical report n. 1, princeton nj, usa, pp. 143. dong, b.-w. and r.t. sutton (2001): the dominant mechanisms of variability in atlantic ocean heat transport in a coupled ocean-atmosphere gcm, geophys. res. lett., 28, 2445-2448. dong, b.-w. and r.t. sutton (2002a): variability in north atlantic heat content and heat transport in a coupled ocean-atmosphere gcm, climate dyn., 19, 485-497. 97 variability of atlantic ocean heat transport and its effects on the atmosphere dong, b.-w. and r.t. sutton (2002b): mechanism of thermohaline circulation variability and north atlantic decadal climate variability in a coupled ocean-atmosphere gcm (draft manuscript available from authors). gordon, c., c. cooper, c.a. senior, h. banks, j.m. gregory, t.c. johns, j.f.b. mitchell and r.a. wood (2000): the simulation of sst, sea ice extents and ocean heat transports in a version of the hadley centre coupled model without flux adjustments, climate dyn., 16, 147-168. guilyardi, e., p. delecluse, s. gualdi and a. navarra (2002): the role of meridional recharging in enso phase change, j. climate (in press). hurrell, j.w. (1995): decadal trends in the north atlantic oscillation: regional temperatures and precipitation, science, 269, 676-679. manabe, s. and r.j. stouffer (1995): simulation of abrupt climate change induced by freshwater input to the north atlantic, nature, 378, 165-167. manabe, s. and r.j. stouffer (1999): the role of thermohaline circulation in climate, tellus, 51, 91-109. peixoto, j.p. and a.h. oort (1993): physics of climate, american institute of physics, new york, pp. 520. sardeshmukh, p.d. and b.j. hoskins (1988): the generation of global rotational flow by steady idealized tropical divergence, j. atmos. sci., 45, 1228-1251. sutton, r.t. and m.r. allen (1997): decadal predictability of north atlantic sea surface temperature and climate, nature, 388, 563-567. sutton, r.t., w. a. norton and s.p. jewson (2001): the north atlantic oscillation – what role for the ocean?, atmos. sci. lett., asle.2000.0018. layout 6 recent advances in ground-based ultraviolet remote sensing of volcanic so2 fluxes giancarlo tamburello1, andrew j. s. mcgonigle2,3,*, euripides p. kantzas2, alessandro aiuppa1,3 1 università degli studi di palermo, dipartimento distem, palermo, italy 2 university of sheffield, department of geography, sheffield, uk 3 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5179 abstract measurements of volcanic so2 emission rates have been the mainstay of remote-sensing volcanic gas geochemistry for almost four decades, and they have contributed significantly to our understanding of volcanic systems and their impact upon the atmosphere. the last ten years have brought stepchange improvements in the instrumentation applied to these observations, which began with the application of miniature ultraviolet spectrometers that were deployed in scanning and traverse configurations, with differential optical absorption spectroscopy evaluation routines. this study catalogs the more recent empirical developments, including: ultraviolet cameras; wideangle field-of-view differential optical absorption spectroscopy systems; advances in scanning operations, including tomography; and improved understanding of errors, in particular concerning radiative transfer. furthermore, the outcomes of field deployments of sensors during the last decade are documented, with respect to improving our understanding of volcanic dynamics and degassing into the atmosphere. 1. introduction volcanic gases have very important roles in driving manifested activities, whether these involve passive release of these volatiles from summit craters, or pressurization to generate violent paroxysms. furthermore, measurements of these gases provide important information on underground conditions, enabling the definition of, for instance, the masses of underground magmatic bodies, defining whether these are rising or falling, or if a system is sealed, or whether hydrothermal interactions are involved. such data have therefore been implicit in the activities of volcano monitoring, with data concerning emitted gas ratios and emission rates contributing significantly to hazard assessment endeavours at a number of targets worldwide [e.g., aiuppa et al. 2007, olmos et al. 2007, arellano et al. 2008]. these volatiles are also highly significant with respect to their impact upon the atmosphere, both chemically and radiatively. released so2 reacts to form sulfate aerosol droplets, which back-scatter incident solar radiation, so increasing the albedo of the earth. better constraints upon volcanic loading of the atmosphere, both temporally and spatially, are therefore highly important in terms of improving climate models. furthermore, when entrained at ground level, acidic gas species can have deleterious impacts upon ecosystems, a prominent example of which was the 1783-1874 eruption of the laki fissure in iceland, in which pasturelands and fields were severely fumigated, with catastrophic consequences [thordarson and self 2003]. traditionally, volcanic gas studies have been conducted by collecting samples manually from active vents and craters; for instance, by bubbling the volatiles through alkaline solutions, then transporting these to the laboratory, where the analyses are carried out [e.g., symonds et al. 1994]. while this provides very detailed geochemical information, the requirement of access does pose risks and precludes observations during elevated eruptive periods. furthermore, characterization of individual fumaroles might not be representative of the degassing characteristics of a volcano as a whole. in response, scientists have increasingly adopted remote sensing during recent decades, which involves measurements of the bulk plume and which can provide high temporal resolution and near real-time processed data. these are of great value during activity crises. finally, these techniques are also applicable many kilometers from a source, thus allowing for far greater safety, even during eruptions. volcanic gas remote sensing is mainly predicated upon the absorption of radiation by the volatile species, from which gas emission rates and chemical compositions are inferred [mcgonigle 2005]. the most ubiquitously used spectral window for these observations has been the ultraviolet (uv) [oppenheimer 2010]. this has largely concerned down-scattered solar radiation that is sampled article history received october 8, 2010; accepted march 3, 2011. subject classification: volcanology, volcanic gases, volcano monitoring, volcanic effects upon atmosphere, atmospheric instruments and techniques. 199 from the ground to assess so2 emission rates. following on from 30 years of valued service in this application area, the previously used correlation spectrometers [moffat and millán 1971, stoiber et al. 1983] were upgraded some 10 years ago to miniature uv usb-powered grating spectrometers, and in particular the ocean optics inc. usb2000 [e.g., mcgonigle et al. 2002, galle et al. 2003, horton et al. 2006, mcgonigle 2007, kantzas et al. 2009]. in an earlier study [kantzas and mcgonigle 2008], we summarised how these usb2000 units are used to assess so2 emission rates in standard scanning and traversing configurations, covering the associated differential optical absorption spectroscopy (doas) evaluation routines [e.g., platt et al. 1979, platt and stutz 2008]. in the present study, we go further, through reviewing the outcomes of field installations of these units, with the aim to improve our understanding of volcanic dynamics and degassing into the atmosphere. in addition, more recent empirical developments in the remote sensing of uv volcanic plumes are discussed, including: ultraviolet cameras; wide-angle field-of-view doas systems; advances in scanning operations, including tomography; and improved understanding of errors, and in particular those concerning radiative transfer. it should also be noted that a considerable and increasing volume of work that is not covered here has also been performed in the infrared, using fourier transform infrared spectroscopy, to derive plume-gas ratios, from which the source depths of the degassing have been inferred. this has led to substantial advances in the understanding of volcanic systems [e.g., edmonds 2008]. however, the uv sensors are cheaper and involve simpler retrievals, and hence they have been more widely used. in addition to these ground-based configurations, uv techniques have also been applied from satellite platforms, such as total ozone mapping spectrometry and ozone monitoring instruments, which have been largely focused on the need to define volatile inputs to the atmosphere, with uses including the quantification of global climatic and chemical impacts and for aviation-hazard mitigation [e.g., krotkov et al. 2006]. whereas in the past only the release of explosive gases was resolvable, due to improvements in sensor technology, passive degassing can now also be measured under favorable conditions. notwithstanding this, high time resolution, high spatial resolution and low detection limits are more achievable from the ground, such that in the uv, this arrangement has been the focus of efforts that are aimed at studying volcanic dynamics. 2. field operations – investigating volcanic dynamics miniature uv spectrometers have been widely deployed on volcanoes, both internationally in the context of field campaigns (e.g., as shown in figure 1), and for routine operations by volcano observatories. this has provided insights into the dynamics at a number of target sites of varying magmatic compositions and eruptive styles. for instance, arellano et al. [2008] reported elevated gas fluxes during the explosive activity phases at tungurahua volcano, ecuador, and more gas released than can feasibly be contained in the erupted magmas; they thus postulated that the additional volatiles might arise from exsolution from magma batches that overturn convectively within the conduit. olmos et al. [2007] detailed enhanced so2 release in line with increased seismicity in advance of the eruptions at santa ana volcano, el salvador, as well as modulation of outgassing, which is also potentially related to convection or to tidal processes. periodic fluctuations in gas release over timescales as short as ≈10 min have also been investigated for erebus volcano in antarctica, with respect to open-system degassing dynamics through the lava lake [sweeney et al. tamburello et al. 200 figure 1. left – so2 flux measurements performed via motorcycle traverses on roads downwind of la fossa crater, vulcano. inset – instrumentation used: laptop-computer-controlled miniature (usb2000) uv spectrometer, coupled to a vertically pointing telescope, and a gps unit for geo-referencing. right – schematic drawing of traverse route beneath the volcanic plume. gas fluxes are obtained by integrating the recorded so2 column amounts over the plume cross-section, then multiplying by the plume transport speed. 201 2008]. furthermore, magma–crater-lake interactions at ambae volcano, vanuatu, were studied by bani et al. [2009b] using miniature spectrometer data, for recent surtseyan activity and changes in lake coloration. burton et al. [2009] reported elevated fluxes from stromboli volcano, italy, during the 2007 eruption, which were proposed to arise from the conduit containing a higher ratio of ascending-degassing to descending-degassed magmas than is usually the case. a decrease in the flux immediately prior to the march 15, 2007, paroxysm was proposed to have arisen from a collapse in the permeable gas escape network. this left volatile rich magma pockets at depth that ultimately coalesced and rose, and culminated in the explosion. finally, a transition in the activity of soufrière hills volcano, montserrat, was studied, whereby, following on from five vulcanian events, at the anticipated time of the sixth, a strong degassing event was observed instead [edmonds and herd 2007]. this was proposed to arise from crystallization and degassing in the upper conduit that caused radial gradients in viscosity and magma flow rates, and lead to brittle failure at the walls that formed a permeable shear zone through which gas could escape effusively rather than being explosively pressurized. 3. field operations – constraining atmospheric impacts miniature spectrometer field deployments have also led to many data points that have enhanced the definition of the spatial and temporal volcanic volatile loading into the atmosphere, e.g., from japan [mori et al. 2006a], the marianas [hilton et al. 2007], hawaii [elias et al. 2006], central america [mather et al. 2006, barrancos et al. 2008], papua new guinea [mcgonigle et al. 2004a], italy [aiuppa et al. 2005a, aiuppa et al. 2005b, bagnato et al. 2007, aiuppa et al. 2008], ethiopia [oppenheimer et al. 2004, sawyer et al. 2008a], chile [mather et al. 2004], the democratic republic of congo [sawyer et al. 2008b], mexico [grutter et al. 2008], antarctica [oppenheimer et al. 2005, sweeney et al. 2008] and vanuatu [bani et al. 2009a]. these reports do not only focus on so2, however, with many of them using collated so2 flux data in parallel with contemporaneously obtained gas-composition ratios (e.g., x/so2). these have been derived from various additional techniques, allowing the emission rates of other volcanic species to be inferred (e.g., x as h2s, hg or h2o). in particular, an unmanned aerial vehicle has been used in this way to remotely determine co2 emission rates, which allow for deeper geochemical investigations of magmatic systems than the so2 data [mcgonigle et al. 2008]. the spectral ranges of the miniature spectrometers also overlap the wavelengths at which the differential absorption cross-sections of additional volcanogenic species are sufficiently strong to allow detection, as has been achieved using either diffuse skylight or various active sources, e.g., via a deuterium lamp to measure volcanic h2s/so2 ratios [o'dwyer et al. 2003]. bro [bobrowski et al. 2003, oppenheimer et al. 2006, bobrowski and platt 2007, kern et al. 2009], clo [lee et al. 2005] and oclo [bobrowski et al. 2007] have been detected in addition. these halogen oxide observations have been particularly exciting, as the presence of these species implies halogen-catalyzed ozone depletion, which was subsequently reported empirically for tropospheric plumes by vance et al. [2010]. hence volcanic plumes are now believed to be far more than passive sources of volatile outgassing; they can also host a suite of novel and fascinating atmospheric chemical processes [e.g., mather 2008]. 4. developments in scanning operation one of the first capabilities to arise from the volcanological implementation of miniature spectrometers was the scanning configuration that enabled flux measurements in a matter of minutes from fixed positions below a plume, e.g., as shown in figure 2, thus vastly improving on time resolution relative ultraviolet volcanic remote sensing figure 2. left – computer-controlled scanning unit situated beneath the plume of stromboli volcano during so2 flux measurements. right – schematic representation of the scanning configuration, showing the scan plane inclined nearly perpendicular to the wind direction. to the conventional traversing approach [e.g., edmonds et al. 2003, mcgonigle et al. 2003]. a number of volcanoes are now monitored using networks of these systems that have been installed to capture the plume under all wind directions, and are automated to provide near real-time fluxes throughout the daylight hours [e.g., salerno et al. 2009a]. much of the data discussed above have been captured in this way. of particular note here has been the activity of the european community network for observation of volcanic and atmospheric change project, which has led to widespread dissemination of these systems to 18 volcanoes worldwide [galle et al. 2010]. in certain cases, novel spectral retrievals have been implemented in which the reference spectrum is modeled, to account for scenarios where the entire scanning track of an instrument intersects the monitored plume, such that it is not possible to empirically characterize the background illumination [salerno et al. 2009b]. scanner network data have also been used to tomographically define two-dimensional profiles of the cross-sectional so2 concentration distributions of plumes [wright et al. 2008, johansson et al. 2009a]. this has also been realized using a number of miniature spectrometers during under plume flights, each with a different viewing orientation [kazahaya et al. 2008]. a variety of reconstruction algorithms and scanner–plume orientations have been considered, and reasonable agreement has been found between flux estimates from the conventionally processed raw scanner data and via summation of tomographically derived concentrations over a plume. the ability of tomography to define the limits of a plume can be used to militate against volcanic gas-related hazards, in particular concerning aviation. finally, twodimensional imaging of volcanic plumes from a transverse vantage point has been achieved by scanning horizontally with a vertical array of spectrometers across the field of view of interest, via the so-called imaging doas approach [bobrowski et al. 2006]. louban et al. [2009] used this technique to investigate halogen chemistry in volcanic plumes, via the retrieval of the so2 and bro abundances. 5. wide-angle field-of-view doas observations very recently, novel uv approaches have been developed that can acquire gas fluxes with far higher time resolution (of the order of 1 hz) than is possible even with scanning systems. one such technique involves two miniature spectrometers, each of which is coupled to a cylindrical lens telescope with quasi-rectangular fields of view that encompass the width of a rising or horizontally advecting plume. their fields of view are separated by a small vertical angle in the former case (rising), and a small horizontal angle in the latter (horizontally advecting). each spectrum from these devices therefore captures a proxy for the average concentrations across the field of view at a rate of about once per second. by multiplying these data by the width of the field of view, the so-called integrated column amounts (icas) can be obtained through a more rapid alternative to traversing or scanning, where icas are computed by summing the concentrations from the individual spectra over the cross-section of the plume. the plume speed is derived via the cross-correlation method, as detailed previously [e.g., mcgonigle et al. 2005a, mcgonigle et al. 2005b, williams-jones et al. 2006; johansson et al. 2009b]. as usual, the fluxes are found by multiplying together the plume speed and the ica data. this approach has been deployed on stromboli volcano, italy [mcgonigle et al. 2009], e.g., as shown in figure 3, and erebus in antarctica [boichu et al. 2010]; in the latter case, to study the gas phase and magmatic dynamics of the lava lake. for stromboli volcano, individual strombolian explosive events were resolved and their eruptive masses were defined. the gas flux data were also supported by the 1 hz geophysical records in a non-aliased sense, showing clear correlations between the gas flux, and the thermal and seismic traces. such an achievement is potentially very important for observational volcanology, in the sense that tamburello et al. 202 figure 3. configuration of the wide-angle field-of-view doas approach to determine high-time-resolution volcanic so2 fluxes, as applied to the rising plume from stromboli volcano. the orientation of the telescopes, and their quasi-rectangular fields of view are shown, in addition to the fiber connections to the miniature uv spectrometers and the controlling laptops. note that the spectrometers themselves are concealed within the shade of the backpack. 203 the previously derived scanned or traversed gas fluxes had relatively lower time resolution, which limited the possibility of understanding rapid volcanic events in the most holistic combined geophysical and geochemical manner. notwithstanding the possibilities of this approach, there are errors, as detailed by mcgonigle et al. [2009] and boichu et al. [2010]. these are associated with high gas concentrations, and where the gas plume only fills a relatively small portion of the width of the field of view, although these do need to be considered operationally. 6. uv cameras ultraviolet camera technology has also been applied to the capture of data for so2 fluxes on the order of the 1 hz time resolution (e.g., figure 4) [mori and burton 2006, bluth et al. 2007]. these units typically capture images of the plume through narrow-band transmission filters that are centered at around 310 nm and 330 nm. the filters can either be mounted, one apiece, in front of the fore-optics of two cameras, or on a single camera in combination with a filter wheel. in the 310-nm spectral region, the plume so2 absorbs radiation, in that of 330 nm, it does not. hence, by comparing the pixel intensities of the 310 nm and 330 nm plume images a measure is obtained of the optical density of the so2 across the field of view. these data are converted to gas column density via a calibration relationship, which is normally determined by measuring the absorbance observed for a series of quartz cells containing known so2 column amounts, and plotting these known column densities against the measured optical densities. the fluxes are obtained from the cameras by summing the concentrations across a line of pixels to generate the ica. the plume speeds are defined by observing the rate at which the plume transects the field of view in a series of images. the high temporal resolution of the cameras has allowed investigations of a number of transient volcanic phenomena, ranging from quantifying gas masses released during strombolian explosions [mori and burton 2009, dalton et al. 2010], to probing the dynamics of vulcanian plumes [yamamoto et al. 2008]. furthermore, the imaging ability has enabled the characterization of emissions across heterogeneous fumarole field sources at the la fossa crater, vulcano island [tamburello et al. 2011]. by virtue of their high time resolution, ease of alignment and operation, and relatively low cost (starting from around $8,000), the use of these cameras represents a highly promising technique in volcanology. this being said, as the approach is spectrally broadband, great care is required concerning the operating protocols, as without finer spectral information, a series of error sources can go uncorrected, as investigated by dalton et al. [2009], kantzas et al. [2010] and kern et al. [2010a]. these reports considered factors such as the calibration relationship varying as a function of the solar zenith angle, the total o3 column, the filter illumination angle, and the volcanic plume so2 concentration, as well as the scattering of radiation within the plume, or between the plume and the observer. a number of recommendations have been posited as a result, including: (i) that it is important to use two filters rather than just one at around 310 nm, to characterize and correct for attenuation related to the aerosol phase with the ≈330 nm filter; (ii) that co-aligned doas miniature spectroscopic observations can be used to correct for radiative transfer effects in the so2 concentrations determined by the camera; (iii) that appropriately chosen fore-optics will minimize the effects of the calibration that varies as a function of the illumination angle, or that calibration coefficients can be characterized for each pixel across the field of view of the camera; and (iv) that frequent empirical calibration in field or modeling efforts can be used to allow for changes in the calibration relationship due to the environmental factors detailed above. 7. studies into errors recent years have also seen a number of studies concerning the sources of errors associated with so2 flux measurements, which have been aimed at improving ultraviolet volcanic remote sensing figure 4. left – ultraviolet camera pair with bandpass filters at 310 nm and 330 nm, tripod-mounted during a field campaign on mt. etna. right – schematic configuration of a typical camera-viewing inclination, such that the field of view captures the plume from the side as it moves horizontally, downwind of the volcano. measurement accuracy. in particular, the assumption implicit in the flux computation that radiation is scattered downwards from above the plume while retaining straight photon paths until captured by the telescope has been reassessed. indeed, to varying degrees, this situation is rather more complicated, with scatter occurring both within the plume and between the plume and the instrument, which can lead to significant overestimation or underestimation in the measurements, potentially by several hundred percent and >70%, respectively, according to kern et al. [2010b]. with the correct implementation of correlation methods to reduce plume-speed-related errors [e.g., mcgonigle et al. 2005a, mcgonigle et al. 2005b, williams-jones et al. 2006], these radiative transfer issues become the dominant source of uncertainty in the flux measurements. these optical-path-related effects depend upon factors such as the absorbing gas concentration within the plume, the plume and the ambient atmosphere aerosol conditions, and the plume-measurement distance. mori et al. [2006b] and kern et al [2010b] have studied these, and as a consequence, they reported that the apparent retrieved column amounts vary with wavelength, as a consequence of the spectrally dependent nature of the magnitudes of the so2 absorption and scattering effects. these dependencies need to be accommodated into the measurement protocol, therefore, either using only the longest section of the uv so2 absorption band in the fitting, where scatter between the plume and detector is the weakest, or using comparisons of field-observed absorbances with model results. a further proposed solution is to use a relatively longer wavelength band (360–390 nm) for retrievals than is normally applied [bobrowski et al. 2010]. future avenues of research will involve using spectral information to define microphysical information on aerosol conditions, to further limit this problem. another error source that has been investigated recently with these miniature spectrometers relates to the possibility that during the transit from the source, and prior to the remotely sensed measurement, the released so2 can oxidize to form sulfate aerosols. this can lead to underestimation of the true source strength. to this end, an empirical link has been sought between the measured gas fluxes and the plume age in a number of studies [e.g., mcgonigle et al. 2004b, rodriguez et al. 2008]. with mcgonigle et al. [2004b], for the masaya volcano, nicaragua, this effect was shown to be negligible, while with rodriguez et al. [2008], for the soufrière hills volcano, montserrat, a relatively more pronounced temporal depletion of the gas flux was reported. additional investigations may further expedite our understanding here; however, as emphasized by nadeau et al. [2009], it is imperative that instrumental and measurement factors that can cause apparent losses are eliminated. 8. conclusions the last decade has seen extraordinary advances in our ability to remotely monitor volcanic gas emissions in the uv, with respect to the adaptation of miniature usb spectrometers, and uv cameras in this application area. it is now possible to take measurements faster, more accurately, covering more chemical species, and at far lower prices than previously possible, which has resulted in the rapid proliferation of these units at volcanoes worldwide. this has led to significant advances with respect to our understanding of volcanic dynamics and the impact of volcanism upon the atmosphere, with a great prospect for further discoveries in the decades to come. with permanently installed scanning units, for instance, coupled with the stalwart efforts of volcano observatories, entirely unprecedented long-term degassing datasets are now emerging. this information will prove invaluable for monitoring and forecasting purposes. future areas of study on the metrological side should focus on further reductions of the associated errors, with respect to the scattering issues. regarding volcanic dynamics, a very promising prospect is seen by the emerging possibility of supporting high-time-resolution, 1-hz, uv-camera gas fluxes with contemporaneous geophysical datasets, which should furnish us with far broader perspectives on a range of rapid degassing-driven or -related processes than was possible previously. acknowledgements. amcg gratefully acknowledges research councils uk for an academic fellowship, the istituto nazionale di geofisica e vulcanologia (ingv) sezione di palermo, and the royal society (grant rg072241) for supporting this work. epk thanks the axa research fund for a post-doctoral fellowship. we thank salvo inguaggiato and an anonymous reviewer for their extremely careful and helpful comments, which improved the quality of the manuscript enormously. references aiuppa, a., c. federico, a. franco, g. giudice, s. gurrieri, s. inguaggiato, m. liuzzo, a.j.s. mcgonigle and m. valenza (2005a). emission of bromine and iodine from mount etna volcano, geochem. geophys. geosyst., 6, q08008; 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doi: 10.1029/2007gl032008. *corresponding author: andrew mcgonigle, university of sheffield, department of geography, sheffield, uk; email: a.mcgonigle@shef.ac.uk. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. tamburello et al. 208 341_352 jorgensen.pdf 341 annals of geophysics, vol. 46, n. 2, april 2003 on particles in the arctic stratosphere torben s. jørgensen danish meteorological institute, copenhagen, denmark abstract soon after the discovery of the antarctic ozone hole it became clear that particles in the polar stratosphere had an infl uence on the destruction of the ozone layer. two major types of particles, sulphate aerosols and polar stratospheric clouds (pscs), provide the surfaces where fast heterogeneous chemical reactions convert inactive halogen reservoir species into potentially ozone-destroying radicals. lidar measurements have been used to classify the pscs. following the mt. pinatubo eruption in june 1991 it was found that the arctic stratosphere was loaded with aerosols, and that aerosols observed with lidar and ozone observed with ozone sondes displayed a layered structure, and that the aerosol and ozone contents in the layers frequently appeared to be negatively correlated. the layered structure was probably due to modulation induced by the dynamics at the edge of the polar vortex. lidar observations of the mt. pinatubo aerosols were in several cases accompanied by balloon-borne backscatter soundings, whereby backscatter measurements in three different wavelengths made it possible to obtain information about the particle sizes. an investigation of the infl uence of synoptic temperature histories on the physical properties of psc particles has shown that most of the liquid type 1b particles were observed in the process of an ongoing, relatively fast, and continuous cooling from temperatures clearly above the nitric acid trihydrate condensation temperature (tnat). on the other hand, it appeared that a relatively long period, with a duration of at least 1-2 days, at temperatures below tnat provide the conditions which may lead to the production of solid type 1a pscs. mailing address: dr. torben s. jørgensen, danish meteorological institute, lyngbyvej 100, dk-2100 copenhagen, denmark; e-mail: torben.s.j@mail.tele.dk key words arctic stratosphere ozone layer polar stratospheric clouds sulphate aerosols 1. introduction the occurrence of particles in the stratosphere has been known for more than a century. stanford and davis (1974) presented results from a careful and systematic search for reports of polar stratospheric clouds (pscs) over the period 1870-1972 and found for the high-latitude northern hemisphere a total of 156 dates on which so-called mother-of-pearl clouds were reported. considering the sparsity of antarctic observers in this period it appears that stratospheric clouds occur much more frequently in the high-latitude southern hemisphere than in the corresponding region of the northern hemisphere. this is of course now known to be a result of the much colder antarctic stratosphere. a peculiar fact about the cloud observations compiled by stanford and davis (1974) is that whereas clouds were observed regularly and most of the years during the periods 1870-1895 and 1926-1972, observations of stratospheric clouds from 1896 to 1925 were not reported. the reason for the lack of clouds in this period is not known, but it does not seem to result from lack of observations. thus the very competent and diligent norwegian observer størmer for example reported: «after 1892 i did not see them at all in spite of a very careful watch. it was not until december 27, 1926, that i saw them again» (størmer, 1929). a reason for the 30 year long period without observations of stratospheric clouds might be that the stratospheric temperature was too high for pscs to be formed due to 342 torben s. jørgensen the unusual high volcanic activity at that time. thus krakatau (1883), santa maria (1902) and novarupta also known as katmai (1912) are listed with the highest volcanic explosivity indices since tambora (1815) (briffa et al., 1998). stanford and davis (1974) reported only published observations of stratospheric clouds and found as mentioned no events between 1895 and 1926. they did not know that a danish painter, harald moltke, in the winter 1900-1901 participated in an expedition to utsjoki in northern finland with the purpose of studying the aurora, and that he accidentally one day saw and painted a psc. moltke’s job was to produce pictures of the aurora, since colour fi lm was not yet invented. he should not make artist’s impressions but observations. he made several paintings in oil, and on the backside of each canvas he wrote date and time in hours and minutes of the event. some of these paintings are reproduced in books (e.g., eather, 1980). at the danish meteorological institute, where all moltke’s auroral paintings are kept, a few years ago we were wondering why one of the paintings did not show any aurora but just a rather boring landscape with the sun a few degrees below the horizon. suddenly it became obvious that the painting was made because of a cloud appearing in the upper left corner, and that this cloud was a psc. date, year, and time was january 5, 1901, at 1215 p.m. (fi g. 1). it is not known if the participants in the auroral expedition a hundred years ago had any knowledge about clouds in the stratosphere, but they found the cloud that day interesting enough to document in very likely the fi rst picture of a psc ever made. sidney chapman, who often had the right notions about processes in the earth’s atmosphere once wrote: «the physical implications of the presence of these clouds at their considerable height in the stratosphere are, and for some time may remain, obscure; but possibly they will play a signifi cant part, along with such other remarkable facts as those of the ozone relations ...» (chapman, 1932). however, it was not until more than half a century later, after the discovery of the antarctic ozone hole (farman et al., 1985) that it became clear that particles in the polar stratosphere have an important role in the process that results in destruction of the ozone layer (solomon, 1986). two major types of particles are observed in the lower stratosphere, namely the sulphate aerosol and pscs. the stratospheric aerosol and cloud particles provide the surfaces where fast heterogeneous chemical reactions convert inactive halogen reservoir species into potentially ozone-destroying radicals. the stratospheric sulphate aerosol layer consists of sub-micron sized liquid particles, composed of sulfuric acid and water (turco et al., 1982). the particles are found globally between the tropopause and up to an altitude of about 30 km. for a few years following major explosive volcanic eruptions the abundance of sulphate aerosol particles may increase dramatically, as it happened after the eruptions of el chicon in 1982 and mt. pinatubo in 1991 (fi g. 2). explosive volcanic fig. 1. polar stratospheric cloud, january 5, 1901, 1215 p.m., utsjoki, finland. oil painting by harald moltke. 343 on particles in the arctic stratosphere eruptions are the major source of sulphur in the stratosphere. gaseous sulfuric acid in the stratosphere is generated from the oxidation of predominantly carbonyl sulphide resulting from biological processes and biomass and fossil fuel burning (weiss et al., 1995) and sulphur dioxide injected directly into the stratosphere by volcanic eruptions. fig. 2. time variation of the stratospheric aerosol layer at northern hemisphere midlatitudes. the plot shows the integrated aerosol backscatter at a wavelength of 0.6943 mm from lidar measurements at garmisch-partenkirchen, nasa langley, and mauna loa for the period 1974-1998. also shown for the period 1991-1997 is integrated aerosol backscatter calculated from size distributions fi tted to in situ data above laramie. the calculations assume particles with an index of refraction of 1.45. also shown are column integrals of surface area from the 1991-1997 in situ data. the column integrals are from the tropopause to near 33 km for all the data. arrows indicate prominent volcanic eruptions (world meteorological organization, 1999). sz ( fr 4pr -2 n(r)z dr) dz 344 torben s. jørgensen the pscs are composed of nitric acid and water. gravitational sedimentation of the particles may lead to a vertical redistribution of these species with irreversible denitrifi cation and dehydration in the altitude range where the particles form, prolonging chemical ozone destruction by reactive chlorine. the arctic interannual temperature variability will also introduce a large variability in psc properties. only occasionally has denitrification been observed in the arctic stratosphere in contrast to antarctica where a nearly complete and regular denitrifi cation takes place each year in the lower stratosphere inside the polar vortex. 2. observations stratospheric particles are observed by a variety of measurement techniques. instruments to measure the stratospheric sulfate aerosol can be broadly divided into two categories, those that provide ensemble measurements and those that provide individual particle measurements. ensemble measurements detect the signature of a population of particles at one instant and include active scattering measurements, such as lidar (fiocco and grams, 1964). satellite measurements have provided a climatology of the stratospheric aerosol (hitchman et al., 1994). measurements by sage ii (trepte et al., 1993) allowed the evolution and dispersion of the pinatubo volcanic aerosol plume to be studies on a global scale. lidar backscatter and depolarisation measurements have been used to to classify the pscs: two types of pscs forming respectively above (type 1) and below (type 2) the ice frost point temperature (tice) were originally identifi ed. the former type was later subdivided into lidar depolarising (presumably solid) type 1a pscs and non-depolarising (liquid) type 1b pscs (fi g. 3). whereas type 2 pscs are composed of water ice particles, it was originally suggested that type 1 pscs were composed of nitric acid trihydrate (nat) which is stable at the stratospheric conditions. however, some type 1 pscs were observed in the arctic at high supersaturations with respect to nat. airborne particle volume measurements later showed an excellent agreement with models, assuming the particles to be composed of liquid supercooled ternary solutions (sts; hno3/h2so4/h2o) (carslaw et al., 1994; tabazadeh et al., 1994). various lidar observations and airborne and balloon-borne measurements are consistent with the description of type 1b pscs as composed of sts particles. the composition and, in particular, the formation of solid type pscs and their fig. 3. scatter plot of 532 nm backscatter ratio versus volume depolarisation from lidar measurements at ny ålesund, spitsbergen (upper panel) and antarctic measurements at mcmurdo (lower panel), illustrating the grouping of different particle types observed in the polar winter stratosphere (adapted from world meteorological organization, 1999, courtesy r. neuber and a. adriani). 345 on particles in the arctic stratosphere northern hemisphere implications are much more uncertain. airborne lidar measurements in mountain waves over northern scandinavia have provided short-term quasi-lagrangian pictures of psc formation and evolution (e.g., carslaw et al., 1998). basically, these measurements are made by fl ying parallel to the wind at the level of the cloud under mountain wave-induced pscs that are assumed to be stationary. such measurements of particles in mountain wave clouds have shown that essentially all liquid particles freeze directly into ice when the air is cooled adiabatically to temperatures at least 3-4 k below tice. in situ balloon-borne experiments have measured the chemical composition of psc particles for the fi rst time. particle compositions from the fi rst fl ight in 1998, downwind of the scandinavian mountains, are consistent with sts droplets (schreiner et al., 1999; larsen et al., 2000). more comprehensive analysis of two later fl ights identifi ed for the fi rst time psc particles composed of nat at temperatures up to or even slightly above tnat, with layers of sts particles at much lower temperatures close to tice (voigt et al., 2000). these fl ights showed a clear correspondance between chemical composition, size distributions, and optical properties of the particles. direct measurements of particle size distributions have been obtained from balloon-borne optical counters and airborne particle spectrometers and impactors. as indicated lidars are important instruments for investigation of particles in the stratosphere. this was also realized in 1991, when the network for detection of stratospheric change (ndsc) was established, and aerosol lidars were recommended as standard instruments at all ndsc primary stations, and they were quickly installed at the four arctic sites eureka in canada, ny ålesund at svalbard, and thule and søndre strømfjord both in greenland. lidars are also installed at the ndsc complementary sites at andøya in norway, kiruna in sweden, and sodankylä in finland. in the following some of the arctic lidar observations will be discussed. aerosol backscattering profi les were measured at thule with the university of rome lidar, which is equipped with a 0.8 m diameter telescope and a neodymium: yttrium/aluminum/ fig. 4. sequence of vertical profi les of aerosol backscattering (thin line) and ozone number density (heavy line), observed with lidar at thule from december 3, 1991 to march 2, 1992 (di sarra et al., 1992). 346 torben s. jørgensen garnet (nd:yag) laser emitting pulses of approximately 100 mj at 532 nm. a more detailed description of the instrument and of the data analysis procedure is given by di girolamo et al. (1994). in the winter 1991-1992 intensive series of aerosol measurements were carried out. in the same period ecc ozone sondes were launched also from thule by the danish meteorological institute. it was found (di sarra et al., 1992), that the arctic stratosphere was loaded with aerosols originating from the mt. pinatubo eruption in june 1991, and that both the aerosol and the ozone profi les displayed a layered structure. the aerosol and ozone contents in the layers frequently appeared to be negatively correlated, particularly in the height region around 16 km (fi g. 4). as in previous observations (e.g., grams and fiocco, 1967) the most straightforward interpretation of the anticorrelation between aerosols and ozone is to relate it to different properties of the air coming from different regions and the history of the air parcels. layered structures in the ozone profi le throughout the lower stratosphere are common at high latitudes; it has been suggested that transport processes could be responsible for such laminations (e.g., reid and vaughan, 1991). throughout the period of observation, thule was close to the edge of the vortex; it appears that air coming from inside the vortex was richer in ozone and poorer in aerosol than air coming from outside. moreover, the trajectory analysis indicates that the air parcels reaching thule at the same time at different height levels often come from different latitudes. the relation between ozone and aerosol at thule was also investigated in the two following winters 1992-1993 and 1993-1994 with corroborative results (di sarra et al., 1995). the average correlation coeffi cient between the aerosol and ozone deviations as a function of height for the three winters is shown in fi g. 5. the correlation coeffi cient is consistently negative in the three winters throughout the region 13 to 19 km, and the negatively correlated region seems to progressively descend, probably following the aerosol displacement. the presence of the negatively correlated behavior between aerosol and ozone may be explained by the motion of air layers with different characteristics across the vortex boundary, occurring mainly in the stratospheric region below 18 km. the presence of this dynamically induced modulation masks possible ozone-aerosol interactions originated through chemical or radiative processes. however, although most of the negatively correlated behavior of ozone and aerosol may be explained through transport phenomena, evidence of chemical processing inside the vortex (enhanced clo concentrations) at thule during february and march 1993 have been reported (shindell et al., 1994). the increase in clo may be attributed to reactions taking place either on pscs or on the aerosol surface. an increase of clo and a reduction of ozone was observed by the uars satellite in the same period (manney et al., 1994), and larsen et al. (1994a) calculated, using ozone soundings carried out in 1993 at thule, that an ozone reduction of 1.9%/day occurred at 400 k during february and march in the arctic. di girolamo et al. (1994) detected the pinatubo aerosols in the stratosphere at thule from late september 1991, and observed that the aerosol fig. 5. average correlation coeffi cient between the aerosol and ozone deviations as a function of height for the winters 1991-1992, 1992-1993 and 1993-1994 (di sarra et al., 1995). 347 on particles in the arctic stratosphere load slowly increased during autum and early winter. above approximately 18 km’s altitude the aerosol load depended on the location of the station relative to the polar vortex. a temporary increase in the aerosol concentration was detected up to about 26 km at the beginning of february, when thule was located for a few days outside the vortex (fi g. 6). a fi nal increase of the aerosol load was related with the fi nal breakup of the vortex. there was no evidence of large psc events during this winter over thule consistent with the relatively high stratospheric temperatures in that period and in that region of the arctic. the time series of lidar backscatter ratio (defi ned as the ratio of the total backscatter intensity from both molecules and particles to the backscattered intensity from molecules alone) are shown in fi g. 7 (neuber et al., 1994). at midlatitudes the volcanic cloud was observed in early july 1991 less than a month after the eruption. the aerosol increase at high latitudes through meridional dispersion then proceeded and was completed about 10 months after the eruption (chazette et al., 1995). at polar latitudes the volcanic particle distribution was clearly related to the polar vortex location.volcanic aerosol were removed from the lower stratosphere through sedimentation followed by stratospheretroposphere exchange (jäger et al., 1995). mt. pinatubo aerosols were also observed during 6 balloon-borne backscatter soundings in january and february 1992 under different polar vortex conditions from thule (larsen et al., 1994b). it was found that the vortex boundary seemed to retard stratospheric aerosol mixing into the inner parts of the vortex from lower latitudes; however, when thule was outside the vortex large aerosol loadings were measured. the backscatter sonde, developed by the university of wyoming (rosen and kjome, 1991) was equipped with a xenon fl ashlamp, giving a collimated horizontal light pulse about once every 7 s. the backscattered light from the aerosol was detected in two wavelengths bands around 480 nm (blue) and 940 nm (red). the payload also included an ecc ozone sonde and a temperature and pressure sensor. the vertical resolution was about 30 m. the aerosols were simultaneously observed with the university of rome lidar, whereby aerosol backscatter measurements in three different wavelengths made it possible to obtain information about the particle sizes. parameters of lognormal distributions were derived, using a least square approach between observed and calculated backscatter coeffi cients from mie theory. the aerosol surface area density was increased by factors 10-50, compared to the background levels at pre-volcanic conditions. two vertical profi les of backscatter ratios (i.e. the ratio between the particulate and molecular (rayleigh) volume backscattering coeffi cients to the molecular backscattering) in the 3 wavelengths are shown in fi g. 8a,b together with the observed ozone partial pressures. in the period covering the fi rst four fl ights from january 12 fig. 6. backscatter ratio profi les measured with lidar at thule (di girolamo et al., 1994). 348 torben s. jørgensen fig. 7. temporal development of the backscatter ratios at six lidar stations in the period december 1991 through march 1992 during easoe (european arctic stratospheric ozone experiment). the bars above the plots indicate times of the measurements. isolines are set to values of 1.2, 1.6, 2.8, and 5.4 in order to reveal greater details for the lower values. adapted from neuber et al. (1994) (copyright by the american geophysical union). to 23, 1992 thule was inside the polar vortex. the profi les from this period are rather similar (cf. fig. 8a) with increasing particle loading from the tropopause up to about 17 km, where a sharp cut-off occurs, and with clear layered structures between 13 and 17 km, where the backscattering is strongly enhanced, compared to pre-volcanic levels. layering can also be seen in the ozone profi les in the same altitude range, and are probably caused by horizontal transport 349 on particles in the arctic stratosphere of particles and ozone of different origin into the layers. above 18 km (above the upper particle layers) the ozone profi les are more smooth. in the period of the last two fl ights (february, 2 and 5) thule was outside the vortex; the profi les (cf. fi g. 8b) show large enhancements in the backscatter ratios, and also increased values of the ozone concentrations. temperatures (not shown) were too high for psc formation at thule throughout the winter. aerosol profi les in the winter north polar vortex were also measured at thule and alert with balloon-borne backscatter sondes in the winter 1992-1993 (rosen et al., 1994). stratospheric temperatures were suffi ciently low to expect the formation of pscs, but the positive identifi cation of typical pscs was made diffi cult by the presence of considerable volcanic aerosol background. an unusual scattering layer that can probably be identifi ed as some type of psc fig. 8a,b. a) vertical profi les of aerosol backscatter ratio in three wavelengths (solid lines) and ozone partial pressure (dashed) from 23 january 1992, when thule was inside the polar vortex. b) observations on 5 february, when thule was outside the polar vortex. the lidar backscatter profi le is intermediate between the 940 nm (large values) and 480 nm sonde profi les. the arrows indicate layers where parameters of size distributions have been derived. the tropopause is located at 8 km where the ozone pressure increases sharply. the pressure scale is approximate (larsen et al., 1994b). a b 350 torben s. jørgensen was not associated with as much backscatter as observed in previous non-volcanic years with similar stratospheric temperatures. the arctic volcanic aerosol layer was found to produce a small but signifi cant depolarized backscattered signal. this preliminary result could be interpreted as indicating the presence of some nonspherical particles containing inclusions of irregular shapes with differing refractive index. an investigation of the infl uence of synoptic temperature histories on the physical properties of psc particles was carried out by larsen et al. (1997). a large set of psc observations, obtained throughout four winters between 1989 and 1996 by balloon-borne backscatter sondes from seven locations in the arctic at low stratospheric temperatures, were analyzed by means of isentropic airparcel trajectories, providing the synoptic temperatures of the observed particles. it appeared from the observations that liquid type 1b psc particles can be cooled to very low temperatures, approaching the ice frost point, without causing them to freeze. most of the liquid type 1b particles were observed in the process of an ongoing, relatively fast, and continuous cooling from temperatures clearly above the nat condensation temperature. the particles are mostly seen at the edge of a cloud shortly after they enter the cold area. on the other hand, it appears that a relatively long period, with a duration of at least 1-2 days, at temperatures below tnat, provides the conditions which may lead to the production of solid type 1a pscs. solid psc particles are therefore expected to be observed in aged clouds. figure 9 shows a histogram of the time that psc particles have spent at temperatures below tnat before being observed. it is seen that most of the type 1a pscs have existed at temperatures below tnat for more than 1-2 days, while the type 1b pscs typically have spent less than 24 h at low temperatures. 3. conclusions/outstanding issues the problem of quantifying heterogeneous processing rates in the stratosphere is compounded by the problem of identifying the various particle types that are likely to be present. so, much of the effort in psc research has been directed towards understanding the processes that control psc type; thus, directly addressing the question of why pscs are sometimes liquid and sometimes solid. there are two reasons for wanting to know the answer to this question, which are: 1) the composition and phase of the psc particles determines the rate of heterogeneous reactions, which in turn infl uences the abundance of active chlorine and hence the ozone loss rate. 2) the phase of the pscs controls the process of denitrifi cation, which occurs only when a few particles solidify as nitric acid hydrates and grow large enough to sediment. the signifi cance of denitrifi cation for ozone depletion in the arctic has been demonstrated by waibel et al. (1999), who were able to reproduce the observed denitrifi cation in the winter 19941995. investigations show that the arctic denitrification is highly sensitive to temperature changes. thus a reduction of the arctic vortex temperature by 5 degrees, predicted to occur in fig. 9. histogram showing the time that solid type 1a (fi lled bars) and liquid type 1b psc particles (open bars) have spent at temperatures below tnat before being observed. 351 on particles in the arctic stratosphere a doubled co2 climate, leads to removal of more than 80% of the available noy. the simulations show that reducing the inorganic chlorine concentration to prescribed year-2070-conditions (comparable to 1980) will result in nearly the same ozone depletion as in the winter1994-1995, indicating that enhanced denitrifi cation will delay the recovery of the arctic ozone layer until late in the century. better understanding regarding composition and formation of type 1a pscs is needed, because solid particles play a signifi cant role in denitrifi cation. increasing greenhouse-gas concentrations are expected to result in a more stable arctic polar vortex with colder temperatures in the lower stratosphere and increased ozone depletion (shindell et al., 1998), and despite the limits on cfc production uncertainties remain about the exact timing of recovery of the ozone layer. the results of shindell et al. 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p.s., j.e. johnson, r.h. gammon and t.s. bates (1995): reevaluation of the open ocean source of carbonyl sulfi de to the atmosphere, j. geophys. res., 100, 23,083-23,092. world meteorological organization (wmo) (1999): scientifi c assessment of ozone depletion: 1998, rep. 44, global ozone research and monitoring project, geneva, switzerland. han impaginato.pdf key words wellington region – changes on coulomb failure stress – earthquake hazard 1. introduction there are now many international examples that demonstrate that static stress changes, generated by large earthquakes, influence the timing and locations of subsequent earthquakes (harris, 1998; stein, 1999). the changes in stress can promote or trigger earthquake occurrence and cause the clustering of large events in space and time (harris and simpson, 1992; king et al., possible reduction of earthquake hazard on the wellington fault, new zealand, after the nearby 1855, m 8.2 wairarapa earthquake and implication for interpreting paleoearthquake intervals zhujun han institute of geology, china seismological bureau, beijing, china abstract based on the indicative modelling, the changes in coulomb failure function ( cfs) suggest that the w-hv segment and the t-p segment could be stable in at least the future 300 years and 190 years respectively, for these periods should be needed to accumulate the stress released by the m 8.2 wairarapa earthquake, assuming that there is no influence from other sources, the earthquake did not alter the failure threshold, and that failure is a fairly deterministic process. the results also show that the influence on the w-hv segment and t-p segment of the wellington fault caused by the 1855, m 8.2 wairarapa earthquake is significant considering that the average fault rupture recurrence interval on the wellington fault is about 500-770 years. with our present understanding of the wellington and wairarapa faults, it can be concluded that the 1855 wairarapa earthquake retarded earthquake occurrence on the w-hv segment and the t-p segment of the wellington fault. thus the seismic hazard in the wellington region may be over-estimated. 1994; stein et al., 1994; robinson and benites, 1996; deng and sykes, 1997; mcginty and darby, 2001). paleoseismological studies also demonstrate that the clustering of large events has occurred widely (sieh et al., 1989; mccalpin and nishenko, 1996). on the other hand, earthquakes can also be delayed or prevented by static stress changes. the great 1906 san francisco earthquake ruptured the san andreas fault in central and northern california, and generated a stress shadow for all faults of similar orientation (strike) and slip direction (rake) (harris and simpson, 1998). the stress shadow has dominated the san francisco bay area for decades (simpson and reasenberg, 1994). the record of only four m 6 earthquakes in the 78 years after 1906 is in marked contrast to the record of 18 m 6 earthquakes in the 71 years leading to the great san francisco earth1141 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. zhujun han, institute of geology, china seismological bureau, beijing 100029, china; e-mail: zhujunh@btamail.net.cn quake (ellsworth, 1990; jaume and sykes, 1996). a large earthquake with magnitude 8.2 occurred on the wairarapa fault in 1855 in the wellington region, new zealand. horizontal displacement during the 1855 wairarapa earthquake was dominant (wellman, 1955). this suggests that the 1855 wairarapa earthquake may have created a stress shadow on subparallel strike-slip faults with similar rakes in the wellington region. as a consequence of oblique subduction of the pacific plate beneath the australian plate at 40 mm/yr at an azimuth of 265° at the latitude of wellington, several active right-lateral strike-slip faults have developed in the wellington region (fig. 1a,b). they have a strike of approximately 225°. this tectonic setting implies that earthquakes pose a serious natural hazard in the wellington region (van dissen and berryman, 1996), especially considering that most population centers in the wellington region, encompassing about 400.000 people (~ 10% of the country’s population), are within 10 km of one of the active strike-slip faults. by applying synthetic seismicity models to the wel1142 zhujun han fig. 1a,b. a) oblique subduction of the pacific plate beneath the australian plate at 40 mm/yr at an azimuth of 265° at the latitude of wellington (w). b) the main active faults in the wellington region. 0 10 20 km nn w ai ra ra pa f au lt fa u lt lower hutto h a r iu w el lin gt on fau lt w el li n g to n fa u lt mastertonmasterton palmerston north levin kaitoke martinborough ta ra ru a r an ge r im ut ak a r an ge paraparaumu no rt he rn oh ar iu fa u lt wellington turakirae head palliser bay cook strait pla tepa cif ic au str ali an pla te north island south island 40 mm/yrw kapiti island putara masterton a b lington region and generating catalogs of 200.000 years duration each, robinson and benites (1996) concluded that mutual inhibition mostly occurs between the subparallel strike-slip faults. the occurrence of the 1855 wairarapa earthquake should have had great influence on its adjacent faults, subparallel in strike. it is important to estimate this influence in order to evaluate the seismic hazards in the wellington region. this paper will try to address the following questions: 1) the effect of the 1855 wairarapa earthquake on the wellington fault in terms of the changes in coulomb failure function ( cfs) in both amplitude and sign; 2) what kinds of indications, retardation or advancement, do the modelling results provide to events on the wellington fault? this indication is helpful for seismic hazard evaluation on the wellington fault. 2. the wairarapa fault and the wellington fault 2.1. the wairarapa fault darby and beanland (1992) attempted to model the (limited) data on surface deformation due to the 1855 wairarapa m 8.2 earthquake and suggested that a listric wairarapa fault was possible, the dip (to the nw) becoming progressively less steep at depth (fig. 2a,b). the listric wairarapa fault model for computational simplicity is approximated by a sequence of five planar segments of increasing dip towards the surface, all with the same length of 130 km (table i, figs. 2a,b and 3). the deepest segment is taken to extend tangentially from the subduction interface at a depth of 25 km. in this model, dextral slip of 12 m and reverse slip of 3 m are assigned. the listric wairarapa fault model provides a very good fit to the regional data in terms of uplift and tilting northwest of the fault and almost no deformation to the southeast. grapes and downes (1997) plotted the dextral and vertical displacement data of the 1855 earthquake from turakirae head in the sw to mauriceville in the ne, a length of 100 km. the results show that from pigeon bush to turakirae head, the horizontal displacement decreases southward and from waiohine river and kaipaitangata stm to mauriceville, the horizontal displacement decreases northward. generally, the horizontal displacement has the feature of decreasing towards the rupture ends. based on this feature of horizontal displacement along the 1855 earthquake rupture, each segment 1143 possible reduction of earthquake hazard on the wellington fault, new zealand w -h v se gm en t, w ell ing to n fa ult w ai ra ra pa f au lt distance (km) e le va tio n (k m ) 40 20 0 -20 -40 -60 -50 0 50 100 distance (km) e le va tio n (k m ) 40 20 0 -20 -40 -60 -50 0 50 100 tp se gm en t, w ell ing to n fa ult t w ai ra ra pa f au lt nw se nw se fig. 2a,b. a listric wairarapa fault model with a nw dip (modified from darby and beanland, 1992). the deepest segment is taken to extend tangentially from the modeled subduction interface at a depth of 25 km. a vertical dip for the w-h segment (a) and a 70° dip to the northwest for the t-p segment (b) are assumed (russ j. van dissen, personal communication). depth to bottom of the wellington fault or seismogenic thickness is assigned as 20 km. ba zhujun han table i. model parameters for the 1855 wairarapa earthquake (modified from darby and beanland, 1992). fault segment length width bottom dip/dip central location strike dip (km) km depth strike direction rake (°) slip.(m) slip (km) (°) latitude (°) longitude (°) (m) listric wairarapa fault upper segment 130 4 4.0 225 90 – 41.206 175.205 166 12.0 3.0 segment 2 130 3.2 7.0 225 70/315 – 41.2 175.2 166 12.0 3.0 segment 3 130 7.8 13.0 225 50/315 – 41.18 175.173 166 12.0 3.0 segment 4 130 17.0 21.5 225 30/315 – 41.12 175.09 166 12.0 3.0 segment 5 (deepest) 130 10.2 25.0 225 20/315 – 41.043 174.988 166 12.0 3.0 tapered displacement wairarapa fault upper segment, 13 patches along the strike 10 (km) × 13 4 4.0 225 90 – 41.206 175.205 166 see see direction table ii table ii segment 2, 13 patches along the strike 10 (km) × 13 3.2 7.0 225 70/315 – 41.2 175.2 166 see see direction table ii table ii segment 3, 13 patches along the strike 10 (km) × 13 7.8 13.0 225 50/315 – 41.18 175.173 166 see see direction table ii table ii segment 4, 13 patches along the strike 10 (km) × 13 17.0 21.5 225 30/315 – 41.12 175.09 166 see see direction table ii table ii segment 5, 13 patches along the strike 10 (km) × 13 10.2 25.0 225 20/315 – 41.043 174.988 166 see see direction table ii table ii flexed wairarapa fault southern segment. 21 21.9 19 225 60/315 – 41.462 174.866 175 12.0 1.0 segment 2 3 10.9 9.5 220 61/310 – 41.403 174.987 166 11.7 2.9 segment 3 2 21.5 19 212 62/302 – 41.375 174.978 162 11.5 3.7 segment 4 2 10.7 9.5 203 64/293 – 41.375 175.012 149 10.4 6.2 segment 5 3 20.5 19 192 68/282 – 41.352 175.0 148 10.3 6.3 segment 6 2 20.1 19 203 71/293 – 41.324 175.019 154 10.9 5.2 segment 7 3 19.8 19 212 74/302 – 41.303 175.046 165 11.6 3.2 segment 8 2 19.7 19 220 75/310 – 41.282 175.066 173 12.0 1.4 northern segment. 81 19.7 19 225 75/315 – 41.016 175.413 175 12.0 1.0 1144 1145 possible reduction of earthquake hazard on the wellington fault, new zealand of the wairarapa fault can be subdivided into 13 equal-sized, rectangular patches along the strike direction and the slip is linearly tapered from a maximum of 12 m horizontal displacement and 3 m vertical displacement at the center to 1.71 m and 0.43 at the ends (fig. 4, table i and ii). other parameters about the segments are kept the same as the listric wairarapa fault model. however, the vertical displacement increases rapidly to 4-6 m near turakirae head (grapes and downes, 1997). at the central section of the 1855 earthquake rupture, the vertical displacement is between 1 to 2 m. north of kaipaitangata stream, it is less than 1 m. this feature may be related to the variation of fault structure along strike. the offshore extension of the wairarapa fault appears to be offset about 8 km from its onshore position. within the overlap region, an anticlinal fold was mapped by grapes and wellman (1988). such features are consistent with a left step in a right-lateral strike-slip fault. darby and beanland (1992) modeled the fault as having effectively a straight bottom edge along the subduction inter 4 2 u pp er s eg m en t w -h v1 w-h v3 whv 2 t-p 1 tp2 tp 3 tp 4 3 5 w ai ra ra pa f au lt w el lin gt on w el lin gt on w el lin gt on fa ul t location of fig. 2a location of fig. 2b 0 10 20 km lower hutt mastertonmastertonmasterton levin martinborough otaki palliser baypalliser bay cook straitcook strait waikanae n fig. 3. the surface projections of the wairarapa fault, and the w-hv segment and t-p segment of the wellington fault. the listric wairarapa fault for computational simplicity is approximated by a sequence of five planar segments of increasing dip upward, all with the same length of 130 km (darby and beanland, 1992). face with a variable dip so that the top edge follows the surface trace interpolated between the mapped offset segments. for simplicity, this flexed model is approximated by a series of planes. the dips of the planes decrease from 75° at the northeastern end to 60° at the southwestern end (fig. 5; table i). as the flexed wairarapa fault model can create huge vertical 1146 sw ne southern patch patch 2 patch 3 patch 4 patch 5 patch 6 patch 7 patch 8 patch 9 patch 10 patch 11 patch 12 northern patch fig. 4. the tapered-displacement wairarapa fault still has the listric shape and consists of five segment in the dip direction as shown in fig. 3, but each segment of the wairarapa fault is subdivided into 13 equal-sized, rectangular patches along the strike direction and the slip is linearly tapered from a maximum of 12 m horizontal displacement and 3 m vertical displacement at the center to 1.71 m and 0.43 m at the ends (table iii). the arrow represents the rake direction. zhujun han w -h v1 w-h v3 whv 2 t-p 1 tp2 tp 3 tp 4 w ai ra ra pa f au lt w el lin gt on fau lt 0 10 20 km lower hutt waikanae masterton levin martinborough palliser baypalliser bay cook straitcook strait n southern segm ent 2 3 4 5 67 8 n orthern segm ent fig. 5. the surface projection of the wairarapa fault, and the w-hv and t-p segments of the wellington fault. the flexed wairarapa fault model is approximated by nine segments, whose dips decrease from 75° at the northern end to 60° at the southern end (darby and beanland, 1992). although the northern segment has the same depth as the southern segment, its surface projection is narrower. displacement at the turakirae head, it is also a reasonable model for consideration. in conclusion, three wairarapa fault models are assumed. they are the listric fault, the tapered-displacement fault and the flexed fault (table i). as there is no other geophysical evidence on the wairarapa fault available at present, its deep tectonics used in the above models may be far from the actual situations. the three models only reflect our present understanding about the wairarapa fault. 2.2. the wellington fault the wellington fault can be divided into three rupture segments (berryman and van dissen, 2001). the wellington-hutt valley (whv) segment is the southern-most 75 km long part of the fault, from offshore in cook strait to the ca. 2 km wide right side-step at kaitoke at the southern end of the tararua ranges. the next part of the fault has been called the tararua-putara (t-p) segment with a 62-65 km length. the distance from the northern boundary of t-p rupture segment to woodville at the northern end of the pahiatua section of the fault is only about 30-35 km, so the woodville (w) segment is expected to extend perhaps an equal distance further north. as the northeastern end of the w segment is not well defined, only the w-hv segment and the t-p segment are considered in this study. the w-hv segment and the t-p segment are divided into three and four sub-sections, based on the difference of their strikes (table iii and fig. 3). we know little about the deep tectonics of the wellington fault. a vertical dip for the w-h segment and a 70° dip to the northwest for the t-p segment are assumed (russ j. van dissen, personal communication) (fig. 2a,b). robinson (1986) estimated that the range for seismogenic crustal thickness is between 15 to 30 km in the wellington region. the cross section of seismicity indicates that the cluster of events nw of the kapiti island extends to a depth of about 20 km, but it is difficult to characterise the seismicity se of kapiti island as being on the subduction interface, or on the overlying faults (robinson and benites, 1996). the seismogenic depths are taken as 12.5 km both in california, 1147 possible reduction of earthquake hazard on the wellington fault, new zealand table ii. horizontal and vertical displacements on the 13 patches along the strike direction in the tapereddisplacement wairarapa fault model. displacement southern patch patch patch patch patch central patch patch patch patch patch northern patch 2 3 4 5 6 patch 8 9 10 11 12 patch horizontal (m) 1.71 3.43 5.14 6.86 8.57 10.29 12 10.29 8.57 6.86 5.14 3.43 1.71 vertical (m) 0.43 0.86 1.29 1.71 2.14 2.57 3 2.57 2.14 1.71 1.29 0.86 0.43 table iii. parameters about w-hv segment and t-p segment of the wellington fault for calculating cfs (modified from berryman and van dissen, 2001, and personal communication with russ j. van dissen). fault section length width strike dip fault central location segment no. (km) (km) (°) (°) rake latitude (°) longitude (°) w-hv1 32 20 40 90 180 (*) – 41.377 174.663 w-hv w-hv2 27 20 52 90 180 – 41.194 174.913 w-hv3 16 20 65 90 180 – 41.092 175.121 t-p1 15 21.5 50 70 180 – 41.01 175.25 t-p t-p2 14 21.5 37 70 180 – 40.92 175.359 t-p3 22 21.5 20 70 180 – 40.776 175.443 t-p4 12 21.5 32 70 180 – 40.633 175.523 (*) 180° rake means right-lateral. u.s.a. and north anatonia, turkey (jaume and sykes, 1996; stein et al., 1997), but toda et al. (1998) used 20 km as the depth of faulting in the osaka-kobe region, japan. it is appropriate to assume 20 km as the depth to bottom of the wellington fault or the thickness of seismogenic layer, as the deep tectonics of the wellington region is more similar to that of the osaka-kobe region, japan, located above a plate subduction. in this case, the downdip width is 20 km for the w-h segment and 21.5 km for the t-p segment (table iii). compared to the downdip width range of 20 25km for all the faults above the plate interface in the wellington region (robinson and benites, 1996), our assumption on the fault bottom depth can be accepted. the downdip extent of the faulting can affect the results of both the cfs and tectonic loading rates on the concerned fault, so a seismogenic depth of 15 km is also considered in the 4th part (conclusion and discussion) of the paper to assess the sensitivity of our model to seismogenic thickness. 3. results we use the program gnstress1_5, available from ftp.gns.cri.nz/pub/robinson/gnstress1_5/ gnstress1_5.exe (r. robinson, personal communication, 2001) to calculate the cfs (using an average rigidity of 2.68 × 1010 n m–2) which are rotated to any required orientation, and tectonic stress loading rate. the induced stress changes may be resolved onto some specified fault plane and rake to indicate whether events are retarded or promoted. the program gnstress1_5 has been used to study earthquake triggering in hawke’s bay, nz (mcginty and darby, 2001). the induced coulomb failure stress (cfs) is calculated using cfs = s + µ ( n + p) where s is the induced changes in shear stress, µ is the dry coefficient of friction, n is the induced change in normal stress, and p is the change in pore pressure given by p= – ./3 ( ii) where is skempton’s coefficient. a value of 0.75 for the coefficient of friction is normal, as observed in the german continental deep drilling project (ktb) ultra deep borehole (0.7) and deduced from induced seismicity in oil fields (0.8) (raleigh et al., 1972; brudy et al., 1997). p is calculated with = 0.5. reasonable variations in µ or have little effect on the values of induced cfs. the cfss due to the occurrence of the 1855 wairarapa earthquake were calculated on rectangular surfaces representing the different sections of the w-hv segment and t-p segment of the wellington fault of our interest, with centres at mid-seismogenic depths of 10 km. since we lack hypocentral depths for strong earthquakes in the wellington region, we sample stress in the central part of seismogenic layer in the same way that harris and simpson (1992), simpson and reasenberg (1994) and stein et al. (1997) have done. if cfs > 0 (positive cfs), it means that the 1855 wairarapa earthquake brought a wellington fault event on the w-hv segment or t-p segment closer to failure. if cfs < 0 (negative cfs), the 1855 wairarapa earthquake sent a wellington fault event on the w-hv segment or t-p segment farther away from failure and into a stress shadow. the listric wairarapa fault – negative cfs is dominant on both the w-hv and t-p segments (fig. 6a,b), except for the offshore part in cook strait of w-hv1 section and t-p3 section. the results also demonstrate that the cfs is highly dependent on the orientation of the fault. as a wellington fault event is assumed to usually rupture either the entire whv segment or the entire t-p segment (berryman and van dissen, 2001), it is reasonable to give an averaged cfs for the segment by considering all the sections in the segment. the cfss are resolved to the w-hv segment and t-p segment at the mid-seismogenic depths of 10 km and in their rakes (fig. 6b). the average cfss at the depth of 10 km are about – 20.4 bar for the w-hv segment and – 8.5 bar for the t-p segment. the values show that the occurrence of the m 8.2 1855 wairarapa earthquake caused a stress shadow on 1148 zhujun han the two segments and potentially reduced their seismic hazard. the tectonic loading rate for the w-hv and t-p segments can be approximated from fault long-term slip rates (simpson and reasenberg, 1994; deng and sykes, 1997). for the w-hv and t-p segments that have been relaxed by a stress shadow, estimates of the tectonic loading rate allow us to calculate how long it will take the fault segments to return to its state of loading before the 1855 wairarapa earthquake. during the recovery period, we would not anticipate large earthquakes on the fault segments, assuming that there is no influence from other sources, the earthquake did not alter the failure threshold and failure is a fairly deterministic process. there are two approaches to calculate the tectonic loading rate. simpson and reasenberg (1994) proposed a deep slip model, in which it is assumed that only the seismogenic layer is moving in stick-slip fashion. the deeper layers are moving at steady rates approximated by the long-term slip rates and transfer stress to the seismogenic portion of the fault. the long-term slip rates are assigned to dislocations extending from 20to 100-km depth under the fault segments. then, the cfs calculation technique is used again to look at the stress changes at the seismogenic layer. deng and sykes (1997) used 1149 sw ne section distance (km) c f f (b a r) 0 30 20 10 -10 -20 -30 -40 -50 -60 0 10 403020 6050 t-p1 t-p2 t-p3 t-p4 30 sw new-hv1 w-hv2 w-hv3 section distance (km) c f f (b ar ) 0 40 20 10 -10 -20 -30 -40 -50 -60 0 1010 403020 6050 70 sw ne t-p3 t-p4 section t-p2t-p1 sw ne w-hv1 w-hv2 w-hv3 section -50 bar50 w id th (k m ) length (km) 0 10 0 5 -25 0 25 -60 bar60 w id th (k m ) length (km) 0 5 0 5 0 30-30 b a fig. 6a,b. the distribution of cfss on the wellington fault for the listric wairarapa fault model. a) the calculated stress changes were resolved onto the rectangular surfaces, representing the fault planes of the w-hv segment and the t-p segment of our interest. b) the cfss is resolved to the w-hv segment and t-p segment at the mid-seismogenic depths of 10 km. since we lack hypocentral depths for the strong earthquakes in the wellington region, we sample stress in the central part of the seismogenic layer in a way as harris and simpson (1992), simpson and reasenberg (1994) and stein et al. (1997) have ever done. possible reduction of earthquake hazard on the wellington fault, new zealand a virtual dislocation model to approximate the tectonic loading rates, which only assume the seismogenic portion of the fault to move. the long-term slip rates are assigned to dislocations extending from 0to 20-km depth. both approaches reveal almost the same results (r. simpson, personal communication). although in reality the w-hv and t-p segments probably intersect the underlying subduction interface and the upper crustal faults may only conceptually slip at great depths, it does not affect calculation of the tectonic loading rates. the average dextral slip rate estimates for the w-hv segment and the w segment, north of the t-p segment, are 6-7.6 mm/yr and 4.67.2 mm/yr, respectively (van dissen and berryman, 1996; berryman and van dissen, 2001). as the t-p segment is mainly located in the tararua range, it is difficult to collect slip-rate data on the segment. the fault slip rate is assigned as 4.6-7.6 mm/yr from the minimum to the maximum in both w-hv segment in its south and w segment in its north. the tectonic stress loading rates per decade for the whv and t-p segments are 0.34-0.43 bar and 0.26-0.43 bar. by applying the coulomb-failure model (e.g., simpson and reasenberg, 1994), the average retarded years can be approximated as 470-600 years for the w-hv segment and 200-330 years for the t-p segment (table iv). although rate-and-state (r & s) friction model was also used to calculate the retardation or advancement years, both the coulomb and r & s models create similar results when the time to zhujun han table iv. the average cfss and their influence on the seismic risk. fault segments and the average cfs (bars), slip rate bars per sections on taking the lengths as weights (mm/yr) decade retarded years wellington fault section segment listric wairarapa fault w-hv1 – 8.1 w-hv w-hv2 – 31.4 – 20.4 6.0-7.6 0.34-0.43 470-600 w-hv3 – 30.2 t-p1 – 24.3 t-p t-p2 – 10 – 8.5 4.6-7.6 0.26-0.43 200-330 t-p3 20.1 t-p4 – 37.1 the tapered-displacement wairarapa fault w-hv1 – 1.9 w-hv w-hv2 – 11.7 – 12.9 6.0-7.6 0.34-0.43 300-380 w-hv3 – 37.0 t-p1 – 31.6 t-p t-p2 – 8.4 – 8.2 4.6-7.6 0.26-0.43 190-320 t-p3 8.1 t-p4 – 8.8 the flexed wairarapa fault w-hv1 8.2 w-hv w-hv2 – 34.8 – 19.0 6.0-7.6 0.34-0.43 440-560 w-hv3 – 46.8 t-p1 – 42.1 t-p t-p2 – 28.1 – 19.7 4.6-7.6 0.26-0.43 460-760 t-p3 0.6 t-p4 – 19.3 1150 failure is long enough. besides, the r & s assumptions are based on laboratory experiments and the parameters in the model have a large uncertainty (dieterich, 1994; harris and simpson, 1998; stein, 1999). the tapered-displacement wairarapa fault – the distribution of the cfs for this fault model is also characterised by negative values on the w-hv and t-p segments (fig. 7a,b), except for the offshore part in cook strait of the w-hv1 section and the northern part of the t-p3 section. as each of the five planar segments of the listric wairarapa fault is divided into 13 equal-sized, rectangular patches along strike, the boundary effect of each patch is partly reflected on the fig. 7a,b. the average cfss on the w-hv segment and t-p segment at the mid-seismogenic depth of 10 km are –12.9 bar and –.8.2 bar (table iv and fig. 7b). the values are smaller than those for the listric wairarapa fault model, but still suggest that the occurrence of the m 8.2 1855 wairarapa earthquake caused a stress shadow on the two segments and potentially reduced their seismic hazard. based on the tectonic stress loadings per decade and cfss on the w-hv and t-p segments, the average retarded years are approximated as 300-380 years for the w-hv segment and 190-320 years for the t-p segment by applying coulomb-failure model (table iv). the flexed wairarapa fault – the distribution of cfss on the w-hv and t-p segments is shown on fig. 8a,b. the difference from the above two fault models is that most of the whv1 section has positive cfs. if the w-hv1 section ruptured independently, the occurrence 1151 possible reduction of earthquake hazard on the wellington fault, new zealand 30 sw new-hv1 w-hv2 w-hv3 section distance (km) c f f (b ar ) 0 40 20 10 -10 -20 -30 -40 -50 -60 0 1010 403020 6050 70 sw ne section distance (km) c f f (b ar ) 0 30 20 10 -10 -20 -30 -40 -50 -60 0 10 403020 6050 t-p1 t-p2 t-p3 t-p4 -30 bar30 w id th ( km ) length (km) 0 10 0 5 sw ne w-hv1 w-hv2 w-hv3 section w id th ( km ) length (km) 0 5 0 5 sw ne t-p3 t-p4 section t-p2t-p1 0 1515 fig. 7a,b. the distribution of cfss for the tapered-displacement wairarapa fault. a) the distribution of cfss on the fault planes of the w-hv and t-p segments. b) the distribution of cfss at the mid-seismogenic depth of 10 km. b a of the 1855 wairarapa earthquake might advance a rupture event on the w-hv1 section. the average cfss on the w-hv segment and t-p segment are – 19.0 bar and – 19.7 bar (table iv and fig. 8b). the average retarded years are approximated as 440-560 years for the w-hv segment and 460-760 years for the t-p segment. 4. discussion and conclusions considering uncertainties on our knowledge of the wairarapa fault, listric, tapered-displacement and flexed wairarapa fault models are assumed to estimate the cfss on the w-hv and t-p segments caused by the 1855 wairarapa earthquake. the results suggest that the w-hv segment and the t-p segment could be stable in at least the future 300 years and 190 years respectively, for these periods should be needed to accumulate the stress released by the m 8.2 wairarapa earthquake, assuming that there is no influence from other sources, the earthquake did not alter the failure threshold and failure is a fairly deterministic process. the influence on the w-hv and t-p segments of the wellington fault from the 1855, m 8.2 wairarapa earthquake is significant, especially considering that the average fault rupture recurrence interval on the wellington fault is about 500-770 years (van dissen and berryman, 1996). fault interaction demonstrates that the event intervals can be variable. four events in the w segment have been identified (berryman and van dissen, 2001). the most recent faulting 1152 sw ne section distance (km) c f f (b a r) 0 30 20 10 -10 -20 -30 -40 -50 -60 0 10 403020 6050 t-p1 t-p2 t-p3 t-p4 30 sw new-hv1 w-hv2 w-hv3 section distance (km) c f f (b a r) 0 40 20 10 -10 -20 -30 -40 -50 -60 0 1010 403020 6050 70 w id th ( km ) length (km) 0 100 5 sw ne w-hv1 w-hv2 w-hv3 section w id th ( km ) length (km) 0 5 0 5 sw ne t-p3 t-p4 section t-p2t-p1 -30 bar300 1515 -40 bar400 2020 b a fig. 8a,b. the distribution of cfss for the flexed wairarapa fault. a) the distribution of cfss on the fault planes of the w-hv and t-p segments. b) the distribution of cfss at the mid-seismogenic depths of 10 km. zhujun han event (mfe), the penultimate faulting event (pfe), the third event and the fourth event in the w segment are 1860-1670 a.d., 1160-890 a.d., 150-400 b.c. and 1960-2130 b.c. respectively. the intervals between mfe and pfe, and between the 3rd and 4th events are 740 ± 230 years and 1975 ± 375 years respectively. thus the variation of event intervals can be as much as 1235 years. compared to this value, the retardation in years for events on the w-hv segment and t-p segment are acceptable. although our modelling suggests that the 1855 wairarapa earthquake may retard events on the wellington fault, robinson and benites (1996) pointed out that mutual enhancement occurs between the two segments of the wellington fault that almost join end to end. if an event, for example, happens in the w segment of wellington fault, it will advance the occurrence of earthquake in the t-p segment. it, in turn, will advance earthquakes on the w-hv segment. width, strike, dip, and rake of fault have effects on the cfs. for example, if 15 km is used as the depth to the bottom of the wellington fault and the listric wairarapa fault model is applied, the average cfss on the whv and t-p segments at 7.5 km depth become – 25.8 bar and –.10.8 bar. the tectonic loading rates per decade on the w-hv and t-p segments are respectively 0.45-0.57 bar and 0.320.54 at the central part of seismogenic layer. the retarded years can be approximated as 450570 years on the w-hv segment and 200-340 years on the t-p segment. compared to the retarded years of 470-600 years on the w-hv segment and 200-330 years on the t-p segment when 20 km is assumed as the seismogenic thickness (table iv), it can be concluded that when the seismogenic thickness of the wellington fault has a reasonable difference, it should not change the basic indications of the cfs. the situation when the w-hv segment has the same fault dip of 70° as the t-p segment is also considered. the average cfs on the whv segment becomes –.33.1 bar, when the listric wairarapa fault model is assumed, and the tectonic loading rate per decade is 0.31-0.40 bar. the retarded years can be approximated as 830-1070 years. besides, to have a component of dip-slip is reasonable in some sections of both w-hv segment and t-p segment. we use the geological slip-rate of faults to calculate the loading rate, but the actual strain or stress accumulation rate on the wellington fault is changeable. although these uncertainties in the model assumptions may not change the basic indication as to the possible effects of the 1855 wairarapa earthquake on the wellington fault, the retarded years highly depend on these assumptions. it means that our modelling results can be used only as an order-of-magnitude indication. they are not constrained enough to be used as a prediction. with our present understanding of the wellington and wairarapa faults, it can be concluded that the 1855, m 8.2 wairarapa earthquake has retarded earthquake occurrence on the w-hv segment and the t-p segment of the wellington fault. thus the seismic hazard in the wellington region may be over-estimated (stirling et al., 1998). acknowledgements the work was done by using gnstress1_5, provided by dr. russell robinson, when i was a visitor at gns (institute of geological and nuclear sciences), new zealand, and supported by the china scholarship council and national natural science foundation of china (40274008). figure 1a,b, showing the tectonic framework of the wellington region, is provided by dr. kelvin r. berryman. my thanks to dick beetham, russell robinson, rafael benites, russ. j. van dissen, colin mazengarb (gns), ruth a. harris and robert w. simpson (usgs). i also thank russ van dissen and an anonymous reviewer for their constructive comments. references berryman, k. and r.j. van dissen (2001): recent rupture of the tararua section of the wellington fault and relationships to other fault sections and rupture segments, eqc res. rep. 97/248. brudy, m., m.d. zoback, k. fuchs, f. rummel and j. baumgartner (1997): estimation of the complete stress 1153 possible reduction of earthquake hazard on the wellington fault, new zealand tensor to 8 km 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(1955): new zealand quaternary tectonics, geol. rundsch, 43, 248-257. 1154 zhujun han articolo malik.pdf key words active faults – northwestern himalayan front – paleoearthquake – thrust and right lateral strike-slip faults – slip-partitioning 1. introduction active faults are considered to be the source for earthquakes in the seismically-active zones of the world. their identification bears significant importance towards recognizing the seismic hazard of these zones. identification of active faults is of prime importance in the himalayan region, one of the most seismically active intercontinental regions of the world. ongoing crustal deformation is well reflected by the occurrence of small-, mediumand large-magnitude earthquakes along the himalayan arc. the most prominent largemagnitude events that struck the foothill zone of himalaya in the span of the last 100-120 years are the 1897 shillong (m 8.7), 1905 kangra (m 8.6), 1934 bihar (m 8.4) and 1950 upper assam (m 8.5) earthquakes (seeber and armbruster, 1981; yeats et al., 1997) (fig. 1a). along with these 917 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. javed n. malik, department of civil engineering, indian institute of technology, kanpur 208 016, india; e-mail: javed@iitk.ac.in active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india javed n. malik (1) and takashi nakata (2) (1) department of civil engineering, indian institute of technology, kanpur, india (2) department of geography, hiroshima university, higashi hiroshima, japan abstract numerous newly-identified traces of active faults in the himalayan foothill zone along the hff around chandigarh, in pinjore dun, along the piedmont zone of the lower siwalik hill front and within the lower tertiary hill range reveal the pattern of thrust and strike-slip faulting, striking parallel to the principal structural trend (nnw-sse) of the orogenic belt. the active chandigarh fault, pinjore garden fault and barsar thrust have vertically dislocated, warped and backtilted fluvial and alluvial-fan surfaces made up of late pleistocene-holocene sediments. westand southwest-facing fault scarplets with heights ranging from 12 to 50 m along these faults suggest continued tectonic movement through late pleistocene to recent times. gentle warping and backtilting of the terraces on the hanging wall sides of the faults indicate fault-bend folding. these active faults are the manifestation of north-dipping imbricated thrust faults branching out from the major fault systems like the main boundary fault (mbf) and himalayan frontal fault (hff), probably merging down northward into a décollement. the taksal fault, striking nnw-sse, shows prominent right-lateral movement marked by lateral offset of streams and younger quaternary terraces and occupies a narrow deep linear valley along the fault trace. right stepping along this fault has resulted in formation of a small pull-apart basin. fault scarplets facing ene and wsw are the manifestation of dip-slip movement. this fault is an example of slip-partitioning between the strike-slip and thrust faults, suggesting ongoing oblique convergence of the indian plate and northward migration of a tectonic sliver. slip rate along the taksal fault has been calculated as 2.8 mm/yr. preliminary trench investigation at the base of the chandigarh fault scarp has revealed total displacement of 3.5 m along a low angle thrust fault with variable dip of 20° to 46° due northeast, possibly the result of one large magnitude (mw 7) prehistoric earthquake. taking into consideration the height of the pinjore surface (20 to 25 m), tentative age (8.9 ± 1.9 ka), displacement during one event and average angle of fault dip (25°) gives slip rate of about 6.3 ± 2 mm/yr, a rate of horizontal shortening of 5.8 ± 1.8 mm/yr and recurrence of faulting of 555 ± 118 years along the himalayan frontal fault. events, few moderate earthquakes with m > 6 < 7 have also been recorded in recent years, viz., 1988 bihar-nepal (m 6.6); 1991 uttarkashi (m 6.6) and 1999 chamoli (m 6.5) events. it is believed that although there are few large and moderate quakes along the arc, energy is accumulating in the areas confined between large magnitude events, designated as seismic gaps. the area between the 1905 918 javed n. malik and takashi nakata x x x x 75° 80° 85° 90° 95° 30° 25° 35° 30° srinagar lahore delhi kathmandu shillong t i b e t a n p l a t e a uz o n e o f i n t e r m e d i a t e s e i s m i c i t y dehra dun i n d o g a n g e t i c p l a i n s h i m a l aya n f ro n t 1885 a.d. 25 1905 1934 1897 1950 1947 1833 1803 v v v v v v v v v chandigarh study area fig. 1a,b. a) map showing meizoseismal zones of four great earthquakes along himalayan front and other large magnitude events (after yeats and thakur, 1998). box marks the study area around chandigarh. b) generalized tectonic framework of himalaya and its surrounding area. itsz indus tsangpo suture zone; mct main central thrust; mbf main boundary fault; hff himalayan frontal fault; srff salt range front fault; ksff kirthar sulaiman front fault; sub-h sub himalaya. 5000 1000 km a b kangra and 1934 bihar-nepal earthquakes (fig. 1a) has been categorized as the central seismic gap by khattri and tayagi (1983) and has a high probability for one or more m > 8 himalayan earthquakes in this century (bilham et al., 1998). investigations carried out on active tectonism for the past several decades have provided vital information on the ongoing crustal deformation and on recurrence of earthquakes in the himalayas (gansser, 1964; nakata, 1972, 1975, 1989; valdiya, 1984, 1989, 1992; valdiya et al., 1984, 1992; nakata et al., 1990; yeats and lillie, 1991; yeats et al., 1992; powers et al., 1998; yeats and thakur, 1998; sukhija et al., 1999a,b; wesnousky et al., 1999; lave and avouac, 2000; kumar et al., 2001; oatney et al., 2001). the ideal place to investigate active tectonics is along the front of the himalaya, demarcating the present active tectonic boundary between the indian and eurasian plates (nakata, 1972). bearing this in mind, emphasis was given toward identification of active faults in the area around chandigarh and pinjore dun (dun = valley) located southeast of kangra, which falls within the meisoseismal area of the 1905 earthquake (fig. 1a). numerous new traces of active faults were identified for the first time using corona declassified satellite photos in the himalayan foothill zone along the hff around chandigarh, in pinjore dun, along the piedmont zone of the lower siwalik hill front and within the lower tertiary hill range. the geometric pattern and distribution of the faults in this area reveal the pattern of thrust and strike-slip faulting, striking parallel to the principal structural orientation (nnw-sse) of the orogenic belt. for detailed paleoseismic studies, a preliminary trench investigation was carried out near chandigarh along a branching fault of the hff system. here we summarize our preliminary findings from the trench study. 2. general tectonic outline the mighty himalaya stretching east to west over a length of 2500 km is a result of continental convergence and collision between india and eurasia. ongoing convergence is estimated as about 2000-3000 km since late cretaceous-early tertiary (molnar and tapponnier, 1977), at rates ranging between 44 mm and 60 mm/year (minster and jordan, 1978; armijo et al., 1989). recent studies based on gps measurements show that india and southern tibet converge at 20 ± 3 mm/year (bilham et al., 2001). this can be compared with the rates of 21 ± 3 mm/yr based on geological findings of deformed terraces in southern nepal (lave and avouac, 2000), 14 ± 4 mm/yr in western himalaya near dehra dun (wesnousky et al., 1999); 11 ± 5 mm/yr and 14 ± 2 mm/yr across the himalayan front in dehra dun and northwest of dehra dun (powers et al., 1998), and 15 mm/yr in central himalaya (yeats and thakur, 1998). it is believed that a convergence of about 300-400 km has been accounted for along the himalayan mountain belt (gansser, 1977), which involves the imbricate stacking of the frontal portion of the underthrusting indian plate (lefort, 1975). this is one fourth of the total (i.e. 44 to 60 mm/yr) convergence between the indian and eurasian plates (minster and jordan, 1978), with the rest of the convergence accommodated along the major strike-slip faults of central asia in tibet and nepal (molnar, 1990; avouac and tapponnier, 1993). geological investigations suggest that the himalaya was built-up by the sliver of the indian continent that was overthrust to the south (gansser, 1964; lefort, 1975). from the beginning of convergence in late cretaceous time, followed by the collision in the middle eocene (50 ma) along the indus-tsangpo suture zone, the successive zones of deformation have progressively advanced or jumped southward, resulting in faulting and folding along the prominent structural features of the himalayan orogenic belt (gansser, 1964; seeber and armbruster, 1981; lyon-caen and molnar, 1983). these major tectonic features (indus-tsangpo suture zone itsz; main central thrust mct; main boundary fault mbf; and himalayan frontal fault hff) have played a pivotal role in the structural and topographic architecture of the himalaya (fig. 1b). the mct and mbt are considered to be the sites of cenozoic shortening along the entire length of the himalaya (gansser, 1964; 919 active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india valdiya, 1984), however, the present tectonically active boundary between the indian and eurasian plates is marked by the hff (nakata, 1972, 1975, 1989), where active anticlines and synclines are the surface manifestations of displacement on a buried décollement fault (yeats et al., 1992, 1997) the present study area is characterized by numerous nnw-sse trending faults, part of the mbf and hff systems (fig. 2). in the northeastern part, subsidiary thrusts of the mbf system, namely the palampur./.nahan thrust, barsar./.gambhar thrust and jawalamukhi thrust, cut through the tertiary succession (valdiya, 1980). the barsar thrust marks the boundary between the lower siwalik hill range and the pinjore dun, whereas the jawalamukhi thrust cuts the middle and lower siwalik succession. the southernmost edge of the study area is marked by the hff that delineates the boundary between the detached complex folded-faulted upper siwalik hills comprising molassic sediments of early pliocene-early pleistocene age and the undeformed succession of the indogangetic plains. 3. geomorphology the terrain around the study area can be divided into four major geomorphic zones, from north to south: 1) the uplands comprising 920 31° 76° 77° sutlej r. g ha gg ar r ive r lesser himalayan rocks subathus nummulitic limestone m e so zo ic a n d o ld e r te rt ia ry eocene murrees middle and lower siwaliks upper siwaliks alluvial plains (igp) recent river terraces oligo-miocene q ua te rn ar y indo-g angetic plains bilaspur nahan simla b arsar t. (g am bhar t.) jaw a la m u kh i t. palam pur t./nahan t. h im a laya n p lio -p le is to ce n e pin jo r e d u n study area fault 20 km n chandigarh fro ntal fig. 2. generalized geological map of northwestern himalayan front around chandigarh and pinjore (after gansser, 1964 and valdiya, 1984). javed n. malik and takashi nakata lower siwalik./.lower tertiary rocks of the dagshai, kasauli and subathu formations; 2) a longitudinal alluvial fill-depression, pinjore dun; 3) the isolated upper siwalik frontal range, and 4) indo-gangetic plains (figs. 3 and 4). pinjore dun is a typical «piggyback» basin. it has been postulated that such basins or depressions were formed as thrust fronts propagated southward into the foreland, causing numerous structural and depositional changes and leading to subsequent development of new «piggyback» basins behind the thrusts (ori and friend, 1984; burbank and raynolds, 1988). the duns are the most common and conspicuous feature of the foothill zone and are well documented from the kumaun, panjab and nepal himalaya. the sinuous nature of the mbf has controlled the development and distribution of these intermontane-basins (powers et al., 1998). the pinjore dun is 8 to 10 km wide extending parallel to lower siwalik./.lower tertiary hills with altitude ranging from 600 to 1600 m in the northeast and hogback ridges of upper siwalik hills with altitude of 400 to 600 m in the southwest (figs. 3 and 4). the dun is about 45 km in length, merging farther north with the soan dun. it is made up of thick alluvial fill of late pleistocene to holocene age overlying the middle and lower siwalik succession. these sediments represent debris deposits of coalesced alluvial-fans that were brought down by the himalayan rivers, along with finer-grained fluvial and lacustrine deposits. due to recent crustal movements, these sediments have been dislocated along several tectonic lines parallel to the nahan thrust resulting in development of different levels of surfaces (nakata, 1972). major antecedent streams emerging from the lower siwalik./.lower tertiary hills in the east flow perpendicular to the pinjore dun and join the longitudinally-flowing major tributaries of the sutlej and the ghaggar rivers. sirsa river, a major tributary of the sutlej river, flowing northwest parallel to the dun, is characterized by a larger area of drainage as compared to the southeast-flowing jhajra river, a tributary of the ghaggar river. the divide of the sirsa and ghaggar drainage systems is located near basdevpura at the southeastern end of the dun, an unusual geomorphic feature (figs. 3 and 4). it is difficult to understand the cause for the drainage diversion near village basdevpura. however, we assume that differential crustal uplift, along with the minor climatic fluctuations in pinjore dun during the late quaternary, was one of the causes for the diversion. the morphology of the various surfaces and the sediment succession comprising mainly alluvial-fan debris made of cobble-boulder size sandstone clasts ranging in size from 25 to 100 cm, along with fine-grained fluvial and lacustrine sediments, suggests that initially the pinjore valley was aggraded under an alluvialfan environment, and continued until the stream flows were unconfined. later, the channelized flows might have caused incision of the alluvial-fan surfaces and contemporaneous aggradation within the confined riverbanks, resulting in formation of different terrace levels all along their course. distribution of terraces varies along the different river valleys. the terraces in the southeastern part of pinjore dun and along the ghaggar gorge, where the ghaggar river crosses the upper siwalik hills, have been classified and described in detail by nakata (1972, 1989). five terrace levels were observed in the pinjore dun and along the ghaggar river. these are named the ghaggar surface, kalak surface, pinjore surface, koshallia surface and younger alluvial surface (nakata, 1972, 1989) (fig. 3). the degree of dissection and the distribution of these surfaces suggest that the ghaggar surface is the oldest, followed by the kalak, pinjore and koshallia surfaces. along the ghaggar gorge, on the right bank of the ghaggar river, five terraces were recognized. scattered occurrences of terraces were also observed along these streams in the upper reaches within the lower siwalik./.lower tertiary hills, and are classified as lower and higher upland terraces. they are tentatively correlated with the pinjore and koshallia surfaces of the dun. the pinjore surface is one of the most widespread surfaces. according to nakata (1989), this surface must have developed during the last glacial. however, a radiocarbon date of the oldest paleosol from the middle gangetic plain along the 921 active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india 922 javed n. malik and takashi nakata 4 km 76° 50 76° 55 30° 50 30° 55 chandigarh ghaggar surface kalka surface pinjore surface koshallia surface younger lower alluvial surface m1 m2 fault scarplets (comb on the down thrown side) active fault trace (concealed) strike slip fault inferred active fault trace active folding active flexure direction of tilting u pper siw alik h ills low er siw alik/ low er tertiary h ills upper siwalik hills h ff c f trench site hff p g f p g f p g f b t b t b t b t tf hff chewag mandhala pinjore kalka taksal besdevpura gariran s irsa r . raru dora ko sh al lia r . jhajra r . bhanetjamli gunai kot beja koti satiwala makhnu majra kiratpu r r. ram naga r r. nanakpur r. ba la d r. ratta r. baddi g h a g g a r r . 30° 45 fig. 5 ghatiwala fig. 7 fig. 10 fig. 12 fig. 3. map showing distribution of terraces and active fault traces along northwestern himalayan foot hill zone around chandigarh and pinjore dun. cf chandigarh fault; hff himalayan frontal fault; pgf pinjore garden fault; bt barsar thrust and tf taksal fault. combs on the active faults indicate down thrown side. 923 active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india 40 0 n 0 4 km 76° 50´ 76° 55´ 30° 40´ 30° 45´ 30° 50' 30° 55´ chandigarh g ha gg ar r . pinjore kalka taksal mandhala 600 600 500 600 5 0 0 500 600 110 0 1000 800 10 00 14 00 16 00 1200 1100 1 0 0 0 8 0 0 900 gunai chewag 1 0 0 0 8 0 0 90 0 900 1100 800 70 0 8 0 0 700 6 0 0 700 1100 10 00 1 0 0 0 600 50 0 400 40 0 4 0 0 50 0 400 400 50 0 50 0 50 0 40 0 50 0 400 500 40 0 600 400 6 0 0 7 0 0 500 500 600 400 4 0 0 400 500 70 0 4 0 0 barotiwala kona satiwala besdevpura ghatiwala isharnagar mansa devi nadah belwali saketri sukhna lake makhnu majra baddi gariran fig. 4. detailed contour map of the area of fig. 3. contour interval is 20 m, dashed lines show outline of major as well as minor streams. ghaghra and the gandak rivers (tributaries of the ganga river) gives an age of 10 ka, which developed during a dry period (mohindra, 1995), and a terrace near mohand town along the nw himalayan front gives an age of 3663 ± 215 bp (wesnousky et al., 1998). also the osl dates of the alluvial-fan sediments from dehra dun valley located 150 km southeast of the study area suggests that the fan sedimentation in dehra dun valley started at around 50 ka and lasted up to 10 ka. after 10 ka, a thin prograding sequence was deposited due to uplift of the fans (singh et al., 2001). it is also suggested that a major change in climate from a cold, dry climate with strong seasonal variations prevailing since 50 ka to warm and humid climate at about 10 ka resulted in a change in depositional processes from sediment gravity-flows to braided streams (singh et al., 2001). singh et al. (2001) have dated 4 levels of terraces exposed along the left bank of the yamuna river: terraces yt4, 10.7 ± 2.2; yt3, 9.5 ± 2.3; yt2, 8.9 ± 1.9 and yt1, 6.1 ± 1.2 ka. the geology, geomorphology and tectonic setting of dehra dun and pinjore dun are fairly similar. hence with the presumptions: 1) that the climatic conditions might have remained the same during upper pleistocene and holocene in both the areas; 2) alluvial fan sedimentation took place in similar fashion in both the duns by the rivers flowing down the thrust front with a similar provenance of lower siwalik hills, and 3) also the tectonic scenario along the himalayan front was similar, it can be envisaged that the fan aggradation in the pinjore dun might have initiated at around 50 ka and lasted up to 10 ka. we have correlated the 4 levels of the yamuna river terraces with the 4-level of terraces exposed along the ghaggar river-ghaggar, kalka, pinjore and koshallia terraces. this gives a tentative age of 8.9 ± 1.9 ka for the pinjore surface. however, dating of the individual surface material will help to confirm this chronology. 4. geometry and pattern of active faulting many prominent geomorphic features such as warping and backtilting of fluvial and alluvialfan surfaces and southwest-facing fault scarplets in the quaternary succession were observed along the active fault traces showing thrust movement. along the faults showing strike-slip movement, lateral offsets of streams, offset of quaternary terraces, linear valleys running along the fault and narrow deep gorges were the common features observed (figs. 3 and 4). 4.1. active faulting along the himalayan frontal zone the surface expression of the active fault traces along the hff is discontinuous (fig. 3). however, it is presumed that the fault connects in the subsurface but has not ruptured up to the surface at all places along its length. the length of individual active faults ranges between 2 and 10 km, with fault strikes between n25°w and n-s. active faulting at places has vertically displaced and warped the late pleistocene to holocene alluvial-fan succession, developing sharp vertical scarplets facing southwest towards the gangetic plains (figs. 3 and 5). the most prominent deformation along the hff was observed on the right bank of the ghaggar river, about 5 km southeast of chandigarh (nakata, 1989), named the chandigarh fault. two parallel fault traces were identified between panchkula and mansadevi by satellite photo interpretation as well as in the field (figs. 5 and 6). these faults striking n25°w to n-s have uplifted the ghaggar, kalka, pinjore and koshallia terraces, resulting in fault scarps 15 to 38 m high transverse to the ghaggar river channel. the distance between the faults varies from 0.25 to 0.3 km. the faults are downthrown to the southwest, superficially resembling a pattern of normal faulting. however, the topographic expression observed in the field and on satellite photos shows gentle warping of the surfaces near the fault traces (fig. 6). to the north, these faults follow the range front around saketri village. here the upper siwalik hill range is marked by a straight dissected front with well-developed triangular facets. these active fault traces represent the younger branching faults of the hff system. these probably merge downward and northward into the 924 javed n. malik and takashi nakata décollement; their geometry typically resembles the imbricate faulting pattern seen in fold and thrust belts (burbank et al., 1992). evidence of the displacement of different terrace levels along the branching faults, as well as the maximum height of the scarp (38 m), suggests continued tectonic movement through late pleistocene to holocene times. due to lack of age constraints on the respective surfaces, it is difficult to estimate the timing of events. however, if we consider that the pinjore surface was formed at around 8.9 ± 1.9 ka, then the koshallia and other younger surfaces might have developed during the late holocene. later, these surfaces were subjected to periodic uplift and deformation. the gentle warping of the terraces near the fault line on the hanging wall side suggests that the sediment succession has been warped or folded in response to riding over a bend in a fault with a steeper fault plane near the surface. this type of geometry of deformation is commonly associated with the thrusts that normally step up in the direction of slip to a higher décollement or a ramp in a décollement resulting in fault-bend folding as defined by suppe (1983). the straight faceted range front is suggestive of ongoing tectonic activity. 4.2. active faulting in pinjore dun the active fault traces extending across the pinjore dun have vertically displaced and backtilted various surfaces along them (figs. 3, 4 and 7). the fault traces marked by the discontinuous pattern belong to one single fault system that extends laterally for more than 45 km nnw-sse, named here the pinjore garden fault. it is well exposed between the pinjore and baddi town, along the pinjore-nalagarh state highway. the pinjore garden fault has displaced the older kalka surface and pinjore surface along 925 active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india fig. 5. stereo-pair of satellite photos showing active fault traces (marked by arrows) along the himalayan front southeast of chandigarh on the right bank of ghaggar river (refer fig. 3 for location of active fault traces). two active fault traces (chandigarh fault) running parallel to one another between panchkula and mansadevi with strikes n25°w to n15°e have vertically displaced and warped the ghaggar, kalka, pinjore and koshallia terraces comprising an alluvial-fan sediment succession of late pleistocene to holocene age. warping is clearly seen near the fault line. vertical uplift along the faults has resulted in 15 m to 38 m high southwest facing fault scarplets. these scarps are transverse to the course of the ghaggar river channel, representing the younger branching out faults of the hff system. javed n. malik and takashi nakata fig. 6. view of chandigarh fault scarp near mansadevi. two southwest-facing fault scarps, marked by arrows show vertically displaced flat alluvial-fan surfaces (ghaggar and kalka surface). the height of the lower scarp is about 15-16 m, and the higher one is about 38 m from the foreground in the indo-gangetic plains. the alluvial-fan surface extends northeast for about 2-3 km. trench site is marked on the left side of the photo. (photo looking towards northeast). fig. 7. stereo-pair of satellite photos showing active fault traces cutting across the pinjore dun and along the lower siwalik range front (refer to fig. 3 for location). the active faults are marked by arrows. the lower portion of the photo shows vertically dislocated pinjore and kalka surfaces along pinjore garden fault trending n25°w-n35°w. this fault passes through the mughal garden. the kalka surface near the fault line is warped and highly dissected compare to the pinjore surface. in the central portion of the photo, an active fault near ghatiwala village has vertically displaced and back-tilted the pinjore surface. prominent westand southwest-facing scarp seen in the upper left of the photo represents the southeastern extension of barsar thrust and taksal fault, respectively. 926 with the younger surfaces giving rise to wswand sw-facing fault escarpments. average height of the fault scarps ranges from 2 to 12 m, although at some localities, the scarps are as high as 15 to 16 m. investigation around pinjore reveals that an active fault striking n35°w passes through the mughal garden located between the koshallia and jhajra rivers, where the 12 m high «big step» marks the fault scarp (fig. 8). this garden was constructed around 1600 a.d., using the fault scarp as the «main step». the height of the scarp in the south is about 8 to 10 m and extends further south up to isharnagar village on the left bank of koshallia river. towards the north, the height of the scarp is only 1.5 m to 2 m, and still farther north, the scarp has a height of about 8 to 12 m near village ratpur. here, along with the alluvial fan deposits, the older siwalik bouldery deposits (upper siwaliks?) are also displaced. farther north, the fault passes through the village basdevpura uplifting and warping the sediments of kalka surface located between the jhajra and sirsa rivers, which is presently marked by intensive gully erosion (nakata, 1972, 1989). prominent backtilting of the pinjore surface was noticed near gariran village on the left bank of the kiratpur river, at kona village between the surajpur and the nanakpur rivers and at makhumajra village on the left bank of ratta river. the general tilt ranges from 6° to 8°.ne. local warping or folding was recognized on the satellite photos near marhanwala, where the pinjore surface is tilted sw and ne. long-term displacement along this fault is well documented on the left bank of the balad river, where three levels of terraces are displaced. two active faults were identified east of pinjore town, striking n-s and nnw-sse near ghatiwala and ramsar villages. these faults, between the koshallia and jhajra rivers (figs. 3 and 7), have displaced the pinjore surface, resulting in a west-facing tectonic escarpment. the active fault near ghatiwala extends laterally for about 2 km. maximum uplift of about 31 m along this fault can be seen in the central portion (nakata, 1989), uplifting the older tertiary succession along with the overlying 3 to 5 m thick quaternary deposits and backtilting the surface 8° to 10° east (fig. 9). this fault dies out active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india fig. 8. active fault scarp used as main step (big step) at mughal garden in pinjore town. the height of the scarp is about 12 m and reduces to about 8-10 m in the south and 1.5-2 m in the north. this garden was constructed across this scarp around 400 years ago in 17th century. arrow shows trace of active fault scarp. 927 northward into the floodplain of jhajra river, whereas to the south, it follows the range front. the pattern of displacement along the pinjore garden fault system revealed by the active fault scarps, developed on older as well as younger surfaces, suggests that this fault system has remained active. backtilting and warping of the surfaces near the fault line are suggestive of movement along a low-angle thrust fault dipping northeast, similar to the pattern of deformation observed in the himalayan frontal portion along the chandigarh fault. looking to the intact structure of the mughal garden at pinjore, it is noteworthy that since the construction of this garden, no major earthquake has struck this region, i.e. no movement has been recorded along the pinjore garden fault in the last 400 years. thus, it is presumed that strain is accumulating along this fault, indicating high seismic risk and potential for a large magnitude earthquake in the future. the dislocation of the pinjore surface by two locally-developed parallel faults east of pinjore town according to nakata (1989) is the geomorphic expression of imbricated thrusts branching from the nahan thrust. however, the present study reveals that these faults are the manifestation of a branching fault of the boundary fault or barsar thrust. 4.3. active faulting along the lower siwalik range front the lower siwalik hill range trending nnw-sse marks the northeastern boundary of the pinjore dun (figs. 3 and 4). to the south, this range is flanked by the boundary fault, probably the barsar thrust of valdiya (1980), a branching fault of the mbf system. at some localities along this thrust, there is little evidence for active faults displacing the proximal portion of the late pleistocene-holocene alluvial fan sediment succession. the rest of the portion along the hill range is marked by a sharp contact with the plain showing a faceted range front. 928 javed n. malik and takashi nakata fig. 9. back-tilting of pinjore surface along with the underlying older tertiary succession along active fault (marked by arrow) near ghatiwala village. tilt is about 8°-10° east (view looking southwest). discontinuous geomorphic expression of the active fault traces with strike n-s and nnw-sse suggests dislocation of the alluvial-fan surface around villages like kalka, raru, dhora, and between mandhala and satiwala (figs. 3 and 10). at kalka the active fault trace striking n-s has displaced the fan surface (pinjore surface), giving rise to a 50 m high scarp. the scarp is located on the right bank of the jhajra river and extends laterally for about 0.5 km. three sub-parallel active fault traces showing en échelon pattern were identified around raru and dhora villages, on the right bank of ramnagar river (figs. 3 and 10). these faults striking nnw-sse are characterized by a sw-facing tectonic escarpments developed by displacement of the fan surface. the average length of these faults ranges between 0.25 and 0.6 km; the faults are about 0.3 km apart. the height of the first scarp (on the way to dhora from raru) is about 18 to 20 m; the second scarp is 9 to 12 m high and the third more than 25 m high. farther north, the tectonic activity follows the boundary of the range front marked by welldeveloped triangular facets around koti village. active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india fig. 10. stereo-pair of satellite photos showing three sub-parallel active fault traces marked by typically en échelon pattern around raru and dhora villages and prominent backtilting of alluvial fan surface near mandhala village (refer to fig. 3 for location). the height of the active fault scarp at mandhala is about 20 m. these active faults represent the barsar thrust following the range front. between raru and mandhala, the range front is marked by straight dissected front with well-developed triangular facets. 929 the active faults exhibit a consistent left-stepping pattern. however, no distinct lateral offset was observed on the ground as well as on satellite photos. at mandhala village, the active fault extending n-s for about 1.5 km has vertically uplifted the fan surface, giving rise to a west-facing fault scarp 20 m high and backtilting the surface towards the east (figs. 3 and 11). the slope of the surface is about 10° e. north of mandhala, the discontinuous traces of active faults displacing fan sediments with west-facing fault scarplets are seen as far as satiwala village; no prominent backtilting was noticed. the above-mentioned evidence suggests that the active fault traces along the barsar thrust are the manifestation of reactivated tectonic activity along the mbf that propagated southward along these imbricated thrust faults. the vertical uplift and associated backtilting suggest a similar pattern of deformation observed along the chandigarh fault and pinjore garden fault. the prominent left-stepping pattern of the active faults probably suggests some component of strike-slip movement associated with the thrust faulting, but due to lack of evidence, it is difficult to pinpoint the lateral offset on the surface. 4.4. active faulting within the lower tertiary hills in addition to the fault traces showing thrust movement in the southwest portion of pinjore dun, an active fault trace showing right-lateral displacement of the koshallia river channel by 250 m along the nahan thrust near taksal town was observed by nakata (1989). in the present study, we were able to identify the extension of this fault on satellite photographs as well as in the field. the right-lateral strike-slip fault striking nnw-sse (n15°w) and extending for a distance of about 20 km between taksal and chewag, is here named the taksal fault (figs. 3, 4 and 7). this fault has right laterally displaced the lower tertiary succession as well as younger quaternary fluvial terrace sediments. lateral offset of streams as well as fluvial terraces and occurrence of narrow deep straight valleys are common tectonic features observed along the entire fault trace. fault scarplets of 12 to 30 m height facing ene and wsw are the manifestation of the dip-slip movement. a maximum vertical displacement of about 50 to 56 m was observed near taksal on the left bank of koshallia river. the offsets of the streams are not consistent all along the length of the fault trace. right-lateral offset of the minor as well as the major streams varies from 250 m to 1350 m. minimum displacement of 250 m and maximum of 1350 m has been observed around taksal town and around kot beja village, respectively (figs. 3 and 12). no distinct branching pattern was observed along the fault, except at chewag village (figs. 3 and 12). the rightstepping pattern near chewag has resulted in the formation of a small, elongated lenticularshaped pull-apart basin due to normal faulting along the releasing bend of the faults. this 930 javed n. malik and takashi nakata w e fig. 11. faulted and back-tilted surface representing proximal part of alluvial fan (pinjore surface) along the range front fault (barsar thrust) at mandhala. height of the fault scarp is about 20 m. arrow indicating fault scarp. (view looking north). small basin is about 230 m long in nnw-sse direction along the strike of the fault, and 30 m wide in its center, narrowing to 11 m at the northern end and 10 m at the southern end. the sediment succession within the pull-apart basin shows a typically horizontally stratified succession marked by alternating layers of fineand coarse-grained silty sand. displacement of the lower tertiary succession, younger quaternary fluvial terraces and variable stream offsets suggests continued tectonic movement from late tertiary to recent times. the small pull-apart at chewag is indicative of a right stepover resulting in formation of a dilatational secondary structure. the occurrence of this right lateral strike-slip fault parallel to the principal structural trend (nnw-sse) of the himalaya as well as the thrust fault to the west (i.e. barsar thrust, pinjore garden fault and chandigarh fault) is a clear evidence of slip-partitioning taking place between the reverse faults and the strike-slip fault. this right-lateral strike-slip fault marks a part of the mbf system and has been due to the influence of ongoing oblique convergence between india and eurasia. our observations are consistent with the observations made by mc caffrey and nabelek (1998) and oatney et al., (2001). 5. paleoseismic investigation along himalayan front at the base of a 16-20 m high fault scarp striking n-s along chandigarh fault, a 14 m long; 2 to 4 m deep and 4.5 m wide e-w trench was dug to confirm the amount of displacement and the pattern of faulting of the hff system (figs. 3, 5 and 13). the exposed sediment succession in the trench predominantly shows two sedimentary units comprising poorly-sorted pebble-cobble clasts with occasional occurrence of boulders and a mediumto coarse-grained massive sandy unit. exposed sediments in the trench were subdivided into five facies (fig. 13). unit ocd represents older channel fill deposits composed of loose matrix-supported pebble-cobble-boulders 931 fig. 12. stereo-pair of satellite photos showing right-lateral stream offsets along nnw-sse trending taksal fault between banet and chewag villages (refer to fig. 3 for location). the fault has displaced along it tertiary strata as well as quaternary sediments. along with the streams, younger terraces are seen displaced right laterally along the fault. right stepping along the fault has resulted in formation of small pull-apart near chewag village. active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india of sandstone of dagshai, kasauli and subathu formations: the matrix is mainly fine gravel, coarse sand and silt. units s1 and s2 are the massive medium to coarse sand facies, unit cd is a channel fill deposit made of pebbles and cobbles with a sandy matrix, and unit cb is a channel bar made of comparatively finer gravel clasts seen in ocd and cd facies, with matrix of medium-fine sand, silt and clay. only one fault strand (f) was identified, which has displaced ocd, s1 and cd exposed in the eastern portion of the trench. total displacement of about 3.5 m was inferred along a low-angle thrust fault with dip ranging from 20° to 46° east at a depth of 4 m below the present surface. the fault is marked by an angle of dip of about 46° near the surface in the western portion of the trench and a lower angle of dip of 20° towards the east. the movement from east to west along this fault has resulted in slight deformation within the channel deposit, revealed by the pattern of clast fabric marked by reoriented gravels, resembling folding near the tip of the fault plane. due to lack of numerical age constraints, it is difficult to pinpoint the time of this event. however, looking to the displacement along fault (f.) cutting the unit cd, the youngest unit, and an insignificant cover of well developed soil, it is suggested that this event is the latest event in this area along the chandigarh fault. the net displacement along the fault (f.) of about 3.5 m indicates one single large paleoearthquake. if we consider that the 3.5 m is the maximum displacement during one event, then the empirical relationship of wells and coppersmith (1994), suggests mw 7 for this event. taking the average angle of the fault (25°) gives a vertical displacement of about 1.5 m and a horizontal shortening of about 3.2 m for one large magnitude event. 6. discussion and conclusions the present study along the northwestern himalayan frontal zone around chandigarh and pinjore dun has helped in understanding 932 javed n. malik and takashi nakata s2 s1 cd ocd f cb s1 f f w e fig. 13. north wall of e-w trending trench at the base of chandigarh fault scarp near mansa devi (refer to figs. 3 and 5 for location). low-angle thrust fault with dip ranging from 20° to 46° east has displaced poorlysorted gravelly unit representing old channel trough and massive sand units by 3.5 m. in the lower portion of the trench, the fault plane is distinctly marked by 10 to 12 cm thick crushed zone or brecciated zone comprising angular clasts that were crushed and dragged along the fault during the movement from the underlying unit. aligned discoidal pebbles along their elongated axes mark the fault passing through the channel deposit, which are oriented parallel to the fault plane contact. string on the wall shows grid of 1 m. cd channel deposit; s1 and s2 massive sand deposit; cb channel bar; ocd older channel deposit. the ongoing crustal deformation and its relationship with the plate motion along the indoeurasian plate boundary. the sense of movement along these fault traces identified on the basis of geomorphic as well as field investigations indicates thrust and strike-slip faulting. overall pattern and geometry of active fault traces identified along the himalayan front, within pinjore dun and along the lower siwalik range front suggest that the ongoing tectonic deformation within the himalaya has propagated southward along the various branching faults that belong to the major fault systems like the mbf and hff. these branching faults are the manifestation of imbricated thrust faults in this region. the displacement of various terraces along these branching faults marked by vertical scarplets with maximum height ranging between 12 and 56 m suggests continued tectonic movement through late pleistocene to holocene times and cumulative slip along the chandigarh fault, pinjore garden fault and barsar thrust. gentle warping of the terraces near the fault line on the hanging wall and backtilting suggests faultbend folding. the «big step» of pinjore mughal garden representing the active fault scarp suggests that no movement has been recorded along the pinjore garden fault since the construction of this garden 400 years ago. thus, this fault has a high seismic hazard for a large magnitude earthquake in the future. the occurrence of the right-lateral strike-slip taksal fault striking nnw-sse in the northeastern side of the thrust faults is a clear evidence of slip-partitioning between reverse faults and a strike-slip fault. seven streams showing prominent lateral offset along the fault were taken into account to calculate offset ratio (a = d/l), where d is the amount of stream offset along fault and l is upstream length of the displaced stream (matsuda, 1966) (fig. 14). this gives an offset ratio of about 0.28. a relationship between long-term slip rate (s) along lateral slip of fault and offset ratio (a = d/l) has been rough933 active faults and related late quaternary deformation along the northwestern himalayan frontal zone, india fig. 14. relationship between amounts of stream offset along the fault and upstream length from the fault trace of the taksal fault between banet and chewag. ly calculated as s (m/1000 years), i.e. 10 a (matsuda, 1975). this relationship gives a slip rate along taksal fault of about 2.8 mm/yr. the strike of taksal fault almost parallel to the principal structural trend (nnw-sse) of the arcuate himalayan belt as well as the thrust faults is indicative of change in the direction of maximum principal compressive stress axis from n-s in the eastern himalaya to ne-sw in the western himalayan frontal zone (nakata et al., 1990). this also explains the phenomenon of ongoing oblique convergence of the indian plate (nakata et al., 1990). however, the model on the basis of small-circle approximation with radius 1696 ± 55 km of himalayan arc suggests that the regional convergence vector and stress fields are radial to the arc (bilham et al., 1998; bendick and bilham 2001). mccaffrey and nabelek (1998) suggested that active tectonics of the himalaya and the southern half of tibet show a strong resemblance to the subduction style at the curved oceanic trenches such as sumatra, with oblique convergence and occurrence of trench-parallel strike-slip and extension. they also suggested that radial vergence in earthquake slip vector along the front, east-west extension in southern tibet, and right-lateral slip on the karakoram fault are on account of the basal shear caused by the oblique sliding of indian plate along the arcuate plate boundary. also recent studies around dehra dun valley suggest that strikeslip movement on the trans-yamuna active fault system may be related to arc-parallel lateral translation of the karakoram fault block and east-west extension of the southern tibet block, in response to the oblique convergence between the indian and eurasian plates (oatney et al. 2001). our observations are consistent with the model suggested by mccaffrey and nabelek (1998) and the observation made by oatney et al. (2001). this also explains the westward reduction of convergence rates from nepal to india (yeats and thakur, 1998) and phenomenon of slip partitioning between thrust and strike-slip faults. a preliminary trench investigation at the base of the chandigarh fault scarp revealed total displacement of 3.5 m along a low-angle thrust fault with variable dip of 20° to 46° northeast, suggesting a large magnitude (mw 7) prehistoric earthquake. presuming an average dip of the fault of 25°, the vertical component of displacement is about 1.5 m, and horizontal crustal shortening is about 3.2 m for one event. it is quite logical that the chandigarh fault has displaced various surfaces along it such as the ghaggar, kalka, pinjore and koshallia surfaces. it is envisaged that the fan aggradation within the pinjore dun initiated at around 50 ka and lasted up to 10 ka, and the pinjore surface gives an age of 8.9 ± 1.9 ka. taking into account the age and the height of the pinjore surface along the chandigarh fault ranging between 20 and 25 m, the displacement of 3.5 m per event with dip of the fault 25° gives a slip rate of about 6.3 ± 2 mm/yr, a rate of horizontal shortening about 5.8 ± 1.8 mm/yr and recurrence of faulting 555 ± 118 years along the himalayan frontal fault. a recent paleoseismic investigation along the black mango tear fault in the same region has also revealed evidence of two large surface rupture earthquakes during the past 650 years, subsequent to 1294 a.d. and 1423 a.d. and another at about 260 a.d. (kumar et al., 2001) also paleoseismic investigations along the sirmurital fault in dehra dun valley on the basis of two colluvial wedges suggest that two earthquakes have struck this region in the last 1000 years (oatney et al., 2001). the direct estimates of the convergence rate along the hff based on geological findings suggest shortening rate of 11 ± 5 mm/yr and 14 ± 2 mm/yr across dehra dun and northwest of dehra dun respectively (powers et al., 1998); horizontal shortening of 11.9 ± 3.1 mm/yr, and a slip rate along fault of 13.8 ± 3.16 mm/yr near dehra dun (wesnousky et al., 1999). the offset of late pleistocene and holocene terrace deposits indicates a convergence rate of about 21 ± 3 mm/yr in nepal (lave and avouac, 2000), and the investigation along the black mango tear fault suggests fault slip and crustal shortening of 9.62.+7–3.5 mm/yr and 8.42.+7–3.6 mm/yr (kumar et al., 2001). these rates are consistent with gps measurements that give a convergence rate of 20 ± 3 mm/yr. this suggests that our results are consistence with the results of the above 934 javed n. malik and takashi nakata mentioned workers, because the slip rate of about 6.3 ± 2 mm/yr which we got along the chandigarh fault shows the minimum slip rate. trench investigations along the other active faults, viz., the pinjore garden fault, barsar thrust and taksal fault will be able to give a complete summary of the slip along these faults in this region. our investigation has thrown considerable light on the occurrence of active faults and evidence of large magnitude pre-historic earthquake in this area. the concentration of active faults and their geographic location clearly show that these faults pass through or are near populated areas such as chandigarh, through pinjore dun where lie many small towns and villages. similar is the case along the piedmont zone and in the hill range where traces of active faults pass through small villages. it is envisaged that these areas fall under a high seismic hazard zone and the active faults are the biggest threat if large magnitude earthquake struck this region in future. detail paleoseismic studies are very much important to build up a long-term earthquake database of large magnitude prehistoric events. our finding will play a key role and path for future detailed studies and towards evaluating seismic hazard of this region. acknowledgements we are thankful to both the referees for giving constructive and valuable comments which helped us in improving our text and discussion part. jnm gives his due respect and gratitude to prof. robert yeats for giving invaluable suggestions and encouragement during this study. the financial support provided by jsps (japan society for promotion of science), tokyo, japan is gratefully acknowledged. jnm is grateful to jsps for providing him a post-doctoral fellowship for the year 1999-2001 (id no. p99074), to undertake this work in japan at department of geography, hiroshima university under the guidance of prof. takashi nakata. funding provided to jnm by dst (vide project no. sr/ftp/es-46/2001) is duly acknowledged. references armijo, a.p., p. tapponnier and h. tonglin (1989): late cenozoic right-lateral strike-slip faulting in southern tibet, j. geophys. res., 94, 2787-2838. avouac, j.p. and p. tapponnier (1993): kinematic model of active deformation in central asia, geophys. res. lett., 20, 895-898. bendick, r. and r. bilham (2001): how perfect is the himalayan arc?, geology, 29 (9), 791-794. bilham, r., f. blume, r. bendick and v.k. gaur (1998): geodetic constraints on the translation and deformation of india, implications for future great himalayan earthquakes, curr. sci., 74, 213-229. bilham, r., v.k. gaur and p. molnar (2001): himalayan seismic hazard, science, 293, 1442-1444. burbank, d.w. and r.g.h. raynolds (1988): stratigraphic keys to the timing of thrusting in terrestrial foreland basins: applications to the northwestern himalaya, in new prospective in basin analysis, edited by k.l. kleinspehn and c. paola (springer-verlag, new york), 331-351. burbank, d.w., j. verges, j.a. munoz and p. bentham (1992): coeval hindwardand forward-imbricating thrusting in the south-central pyrenees, spain: timing and rates of shortening and deposition, geol. soc. am. bull., 104, 3-17. gansser, a. 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m. fort, m.a. mirza, m.r. pandey and r.s. stein (1992): the himalayan frontal fault system, ann. tectonicae, 6 (suppl.), 85-98. yeats, r.s., k. sieh and c.r. allen (1997): geology of earthquakes (oxford university press), pp. 568. 936 javed n. malik and takashi nakata annals of geophysics, 55 (3), 2012: lucchesi and giardino the role of geoscientists in human progress stefania lucchesi, marco giardino* università degli studi di torino, dipartimento di scienze della terra, torino, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5535 abstract like any science, geology has a key role in the development and progress of human culture and society. in this context, scientists, professionals and practitioners of earth sciences must inevitably confront themselves with the purposes, methods and results of their studies, concerning relationships between man and his environment, which thus deals with ethical questions. an essential base for any geo-environmental action should be respect for the natural ecosystem. this can be achieved by encouraging an ‘affectionate attitude’ towards nature or ecology. some ancient cultures had a great awareness of the close relationships between humankind and the earth. the recent web-based museo torino (turin museum) multimedia product shows the ‘history of a city’ (turin, italy) in a dimension of unity and as a continuum of space-time-life between the history of the earth and of humans. geoscientists are not limited to merely having a pragmatic vision of the earth, but should pursue harmonious collaboration between man and nature. within complex earth systems, scientists and professionals rediscover their geoethical roles by responsibly evaluating and managing georesources, for progress aimed at improving conditions of life and human dignity. geoscientists can also promote respect for ‘human rights’ through appropriate educational and training actions, for the balanced exploitation of our georesources. an example from mendoza (argentina) is presented here, to encourage opportunities for meetings on environmental issues among people of different backgrounds and cultures. last, but not least, there is the ethical role of geoscientists in ‘the service of the truth’. a misleading view of earth systems by geoscientists can lead to fatalism or myths that often affect people from the psychological and sociological points of view. 1. introduction geoethics is a new discipline that involves scientific, technological, methodological and social-cultural aspects of earth sciences and ethics. it represents the meeting of humanistic disciplines, such as philosophy, epistemology, sociology and ethics, and scientific disiplines, such as geology, natural sciences and ecology. geoethics can be also considered as part of environmental ethics, as it originates from man’s inevitable question about the place he occupies in nature, about the thoughts and motivations that animate him in his everyday life when dealing with the environment where he lives. after its beginning as philosophical reflection by several geoscientists worldwide, such as felice ippolito in italy in the late 1960s, and its later developments by vaclav nemec in 1991, geoethics has now been widely accepted by both earth and social scientists because of the need for appropriate protocols, scientific integrity issues, and a code of good practice regarding the study of the abiotic world. we can consider geoethics as a largely articulated discipline, dealing, for example, with environmental conservation, georesources prospecting and exploitation, ecosystem protection, geo-research and geosciences education, risk management and mitigation of geohazards, and promotion of geodiversity. in this report, we want to underline how geoethics is the pulsing heart that provides nutrients to several branches of earth sciences and their related applications, and we analyze some of the relevant roles that geoscientists can have on the life, culture and society of man, with their studies and professional occupation. we also want to investigate if and how geoethics, in its transversal and multidisciplinary meaning, can have important effects also at a practical level: should geoethics define reference criteria for human behavior both on personal and socio-political grounds? our preferred scenario is that geoethics has a fundamental role in defining the ‘balanced’ environmental policies of the future, including sustainable development and adaptation to climate change. 2. the role of geoscientists in human progress nowadays, the general meaning of ‘ethical scientific behavior’ appears to be confined to the observance of some international laws and decisions. in the case of earth scientists and professionals, they are required to respect certain principles deemed as fundamentals by the scientific community and by international policy, such as the principles of subsidiarity [treaty of maastricht, european community 1992], of precaution [united nations conference on environment and development, united nations 1992] and of responsibility [jonas 1990]. article history received december 21, 2011; accepted march 27, 2012 subject classification: geoethics, environment, sustainability, scientific communication. 355 in this respect, we emphasize the need to consider as fundamental steps in the path of geoethical behavior those that stress the welfare of society. the starting point and the point of arrival of this path should be man: by carefully considering human identity and needs, our society can reach a new awareness of problems that are related to physical and biological environments. if there is no such centrality of the respect for human life, environmental ethics and geoethics are at risk of being manipulated by ideological and political issues, rather than offering an authentic proclamation of truth. this ‘anthropocentric’ analytical perspective is in agreement also with the italian law that recognizes the centrality and importance of man in defining society’s quality of life within a given environment: “the environment is protected as a determinative factor in the quality of life. its protection is not pursued for theoretical or naturalistic aesthetic purposes, but it expresses the present need for a habitat where man lives and acts, and it is necessary for... citizens” [constitutional court of the italian republic 1987]. it follows that there is a close connection between freedom and responsibility. also in the case of earth sciences and geo-environmental applications, responsible management of freedom means making the commitment to work according to general and common rules. this has to be accomplished by all of the people defining, guiding, formalizing and practicing a certain way of acting in the geo-environment. 3. basic principles for geoethics applications at the base of each project of studying, safeguarding or exploiting the geological environment, we consider it is essential to follow some attitudes or basic principles to give a geoethical meaning to any operational choice in this context. these include the following: a) it is necessary to stimulate an ‘affectionate attitude’ towards nature, and hence to enhance an attitude of taking care of nature. we believe that it is fundamental to educate, especially the younger generations, to pay better attention to earth sciences, and not only through the enhancement of their cognitive and practical content, but also by stimulating their sensational and emotional dimensions. these objectives can be taken into account by using practical and interactive methodologies in educational programs, either involving school children or the general public. several recent applications of such a new educational approach confirm that the first steps for inspiring interest in a scientific topic are to rediscover the ability to be amazed by its contents. b) another essential base for geoethics approaches to environmental actions is the enhancement of the order, beauty and harmony inherent in the earth systems, and in general in nature. as earth scientists experience in their everyday research activities (e.g. analyzing crystals, glacial landscapes, sedimentary structures), the earth and the universe follow ‘laws’, and their forms are an expression of the orderly system of the kosmos (= order), even if elements can show different levels of complexity. in this context, the role of geoscientists is fundamental, because their studies can allow people to discover and understand this dimension of the earth systems, thus considering the possibility of geoheritage evaluation and of a sustainable use of georesources. c) it should not be forgotten that even if only addressed by some modern tendencies to dominate and exploit nature, the relationships between man and the kosmos were originally (and still are) characterized by a sort of dependency of man on the earth. an aid in the understanding of this relationship can be found in some cultures, as generally those native to less economically developed countries than ours. these populations have a greater awareness of the close relationships between humankind and the earth (e.g. pachamama-mother earth in latin america, the aborigines of australia). although all of these native peoples remind us of ancient and ‘subservient’ societies, they can also help us to rediscover our original contact point with mother nature. a careful consideration of nature can provide a more balanced approach to georesources, by avoiding over-exploitation of nature in a wasteful manner, which is typical of an unsustainable but highly luxurious life. observing the principles of geoethics can lead us to a sober and modest, and sustainable, way of life. d) at the same time, it is necessary to increase our knowledge, with the ‘modern’ awareness that we belong to a complex system that is characterized by constant, mutual and inevitable interactions between the earth and human beings. there are many examples of interactions or symbiotic relationships between biotic and non-biotic elements, from the coral reefs to the human body. as well as interconnections at the physical-chemical-biological level, there are strong interconnections due both to global ecological issues, which include those related to earth science, and to the present phenomenon of globalization. it is more and more clear that the quality of the earth system and the management of its resources determine the quality of human life and all living beings as part of one global ecosystem [praturlon 2004]. the strong interconnections between the major earth systems (lithosphere, hydrosphere, atmosphere, biosphere) were the starting point for the development during the 1970’s of the gaia hypothesis by lovelock [1979] and his school, who saw the earth systems as being part of a single ‘living organism’. beyond this hypothesis, the aim of our geoethical approach sees human beings as interconnected with geo-environments, together with the community of all of the living beings [hugget 1995]. the planet earth is our home ground: we have the planet as a common heritage and our vocation should be the balanced management of lucchesi and giardino 356 357 the earth resources, rather than their unhealthy exploitation and manipulation. this responsibility should be extended in space and time. indeed, it has to be underlined that every action has consequences in both space and time; this needs to be considered to ensure inter-generation and intra-generation equal opportunities [united nation 1987]. following these principles, we carried out our geological research and dissemination experience for the recent museo torino project, a web-based and multimedia product of the turin municipality for the 150th anniversary of the unification of italy. on the website and in the introductory part of the didactic film “turin, history of a city”, we outlined the different stages of the history of its geographical location from 5 million years ago to the near future. through the representation of what we have called “the city before the city”, we have tried to communicate that there is a dimension of unity and a continuum of space-time-life in the geological and human histories. 4. different possible roles for geoscientists based on the above-mentioned principles, geoscientists are not limited to conform to a merely pragmatic vision of the earth with the aim being to only provide technical instruments, to elaborate on scientific advances, or to perform services related to geohazard mitigation, with noble and high social objectives (e.g. in the contributions of geoscientists to civil protection activities). rather, geoscientists can have several value-added aims that have different ethical implications, as follows: 1) mankind is the only living species that can give rational sense to its actions, to consciously modify the earth system. man can plan and realize plans to govern the geoenvironment, to interact with natural processes, and to change or introduce new equilibria. for these reasons, geoscientists have an important role in the rediscovering of the human role and dignity. indeed, human beings can safeguard and responsibly manage the assets of their home, the earth, to improve the conditions of life and to enhance human progress. a historical perspective reveals that the term geologia appeared for the first time in the early 17th century in the posthumous work of ulisse aldrovandi [1648], and the rise of a systematic geological thought developed only since the second half of the 17th century with the danish geoscientist niels steensen [1669]. nevertheless, due to their intelligence and knowledge of the earth processes, human beings have always made determinate, geologically oriented choices in ‘building their habitat’. for example, they had to choose where to build cities, where and how to find raw materials, and how to avoid involvement in natural disasters [panizza 2005]. nowadays, these fundamental choices should also consider global change, which is a problem that encompass the full range of global issues and interactions concerning natural and human-induced changes to the earth environment. in a global geoethical perspective, we should use both adaptation measures and proactive responses to global change in order not to alter the ability of the earth to sustain life [munn 1996]. 2) every geological action should (ideally) be seen as part of a harmonious collaboration between man and nature, in a relationship of inter-dependence between the biotic and abiotic components, typical of a complex system like our planet earth. in this view, any human projects and actions involving the earth systems should not be considered ‘for’ or ‘towards’ the environment but more and more in harmony ‘with’ the environment! for example, by dealing with the effects of accelerated erosion in different regions through time, man has developed different ‘synergic’ management techniques, such as hillslope terracing, drainage systems, and replanting. these solutions can protect soils from erosion and landscapes from degradation; at the same time, soil conservation is useful for the development of agriculture. on the other hand, some other agricultural techniques, such as excavation of settling ponds, and seeding and mulching of bare surfaces, can be used themselves to reduce sediment removal [reed 1980, hooke 1994]. morever, landscape ‘recovery’ is possible from impacts due to exploitation of georesources, as in the case of postmining restoration of geoenvironments. by following a geoethical approach, rather than simple restoration by waste infilling, naturalization processes should be suggested, to return the system to a condition that can be sustained by natural processes [goudie 2000]. 3) geoscientists are also called on to promote respect for human rights, by providing their knowledge and studies for educational and training actions which affirm the earth and its resources as a common heritage, to be shared with the community of living beings. as an example, the case of an international political debate related to the exploitation of a gold mine in san carlos, mendoza (argentina) is analyzed. here, the use of arsenic for gold extraction compromised the quality of the water resources over a vast urban area. comprehensive information of a large sector of the population and increased awareness of local administrators, with support from earth scientists and experts, were fundamental to thwart this dangerous exploitation approach and to favor land conservation together with respect for the human rights in the area. 4) regarding human rights, in a globalized society, where poor distribution of resources is often the cause of conflict between nations, geoscientists can indirectly become promoters of justice and peace, through correct management and equal distribution of the benefits derived from the georesources. indeed, these georesources should the role of geoscientists in human progress be considered as shared heritage, to serve and benefit all of humanity, including the poorest people. in terms of oil or other mineral resources having a crucial role from an economic standpoint, it is evident that they often cause international conflict. it can be seen, for example, that near the oil fields and pipelines in the middle east there is increased concentration of conflict and political vulnerability. geopolitics in this case can be correctly guided by geoethical principles: by careful analysis and planning of the natural placement of resources and their distribution lines, geoscientists can favor balanced economic and political choices that are as peaceful as possible [biancotti and biancotti 2004]. 5) furthermore, as a safeguard for the earth, the home of all peoples, there is a common target of different cultures: geoscientists can have a significant role in multi-ethnic and multi-cultural dialog, to encourage opportunities to meet on environmental issues with people of different backgrounds and cultures; e.g. concerning the risk management and mitigation of geohazards. the recognition of shared values can become an occasion for encounters between different cultures. this was the case for the adoption of the hyogo framework for action by the world conference on disaster reduction of the united nations general assembly hold in kobe-hyogo, japan, in 2005, and its subsequent endorsement by 168 countries at the general assembly of the united nations. from the shared recognition of the crucial role of human actions in reducing the vulnerability of societies to natural hazards and disasters, a series of strategic goals and related activities were initiated worldwide to promote both substantial reductions in loss of life during disasters, and the social, economic and environmental assets of communities and countries [keller et al. 2008]. 6) last but not least, and also very timely, there is also the role that the geoscientist can have in the service of truth. indeed, more or less consciously, geoscientists can convey different ‘visions of the earth system’ through their scientific communication and/or popular publications. different proposals can lead to different ideologies (e.g. catastrophism, fatalism, mythologizing) or simply help to develop a ‘common feeling’, which can have important effects on public opinion and therefore on human thoughts and behaviors, and on our models for the future. each prediction and future scenario for climatic change or natural hazards has to deal with conceptual models. some of these that relate to global warming were synthesized by maslin [2004]. today, we are witnessing increasing public attention on issues related to the environment and the changing earth, although often this does not necessarily correspond to better information. indeed, misinformation is often more prevalent than correct information. in particular, there are several types of such misinformation that can arise from different causes: ignorance or superstition and fear of the unknown, uncritical acceptance of some unfounded ‘pseudo-scientific’ claims, incorrect information from groups of people who are interested in giving guidelines to determine specific commercial or political choices. for example, in the early 1990’s, an incorrect prediction of a potential earthquake in new madrid, missouri, usa, resulted in a state of agitation and anxiety in the local and surrounding population. all the area suffered huge economic damage, fomented also by the national media coverage of the information delivered by climatologist iben browning. he predicted in the new madrid seismic zone that there was a 50% probability of a magnitude 6.5 to 7.5 earthquake in the new madrid area sometime between december 1 and december 5, 1990. despite the lack of scientific support, browning’s prediction was widely reported in the international media, which caused public alarm. this event has become a case-study that revealed how important it is to define some important guidelines for communication between scientist and the media: – scientists must promptly evaluate any prediction that attracts media coverage. – the media need to verify their sources of information. – there should be a coordinated system with an authoritative evaluation of any predictions [spence et al. 1993]. a correct and honest way of disseminating such information about the earth system to the public, including both its resources and its hazards, it is very important to gain popular credibility and trust in science. this is the starting point for an educational action that can lead people to follow the correct behavior and lifestyle, and to inspire policy choices for action and prevention. however, society should also be told that despite all of the scientific knowledge and instrumentation nowadays, it is still not possible to predict the exact timing and strength of a potential earthquake, or of volcanic activity, or of a tsunami. a better approach will be to increase public knowledge through education of the following aspects: – the preparedness of society for dealing with geohazards. – the efficiency of the warning systems. – the coordination of post-disaster handling of emergency situations by government agencies and nongovernmental organizations. 5. conclusions from the consciousness of the complexity of the earth system and of the continuous interactions between the various elements of which it is composed (living beings, or not), it is essential to define the role and responsibility that geoscientists have in this context. it is fundamental to underline that every thought, effort, choice and action within the geoenvironment needs to remain firmly attached to the issue of respect for human life. for this reason the activities lucchesi and giardino 358 359 of earth scientists cannot ignore the values that are generically known as ‘human rights’. in this context, the work of geoscientists is not merely pragmatic, to perform a service, but rather it should be aimed at promoting sustainable living conditions for each person. geoscientists have many responsibilities in policy making and in taking care of civil activities. through education and training activities, geoscientists can develop renewed attention in society towards nature and the earth sciences. to enhance the prudent use and management of the earth resources, geoscientists can contribute to the fostering of north-south inter-governmental cooperation, with particular attention to people in low-income countries. to summarize these concepts, here we would like to borrow a comment from an important personality of our time, pope john paul ii, who although not a scientist, believed strongly in the value of science. in his sermon given in rome in 1983 for the 350th anniversary of the publication of the works of galileo [1632], he said: “ladies and gentlemen, you who have cultivated the sciences have considerable power and responsibility that might become crucial in the guiding of the world of tomorrow… your job is noble and huge. the world is watching and expecting from you a service that is worthy of your intellect and your ethical responsibilities”. references aldrovandi, u. 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(1669). de solido intra solidum naturaliter contento dissertationis prodromus, firenze, ex typographia sub signo stellae. united nation (1987). report of the world commission on environment and development: our common future, brudtland commission, 20 march 1987, oslo, norway. united nation (1992). united nations conference on environment and development, 3-14 june, 1992, rio de janeiro, brazil. *corresponding author: marco giardino, università degli studi di torino, dipartimento di scienze della terra, torino, italy; e-mail: marco.giardino@unito.it © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the role of geoscientists in human progress << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals 47, 5, 2004 1597 annals of geophysics, vol. 47, n. 5, october 2004 key words seismic attenuation – southern italy – q 1. introduction the main goal of this study is to find empirical attenuation functions that describe the spectral amplitude decay with distance using local and regional events located in southern italy. southern italy is a complex region, located at the boundary between the apulia micro plate and the extensional tyrrhenian sea basin (see e.g., mantovani et al., 1996). important volcanic activity has been related to the formation of the tyrrhenian-apennine system (mele et al., 1997). southern italy is also a region with high seismic activity. intermediate and deep seismicity has been associated to the subducting apulia slab dipping toward the tyrrhenian basin (mckenzie, 1972; gasparini et al., 1982). shallow crustal seismicity is mainly concentrated along the apenninic chain, where normal faults trending nw-se are separated by transfer zone with strike-slip seismicity (valensise and pantosti, 2001). important historical events have also occurred, for instance the 1857 val d’agri (m = 7.0), 1980 irpinia (m = 6.9) earthquakes. the tyrrhenian region is characan attenuation study in southern italy using local and regional earthquakes recorded by seismic network of basilicata raúl ramón castro (1), maria rosaria gallipoli (2) (3) and marco mucciarelli (3) (1) cicese, división ciencias de la tierra, departamento de sismología, ensenada, baja california, méxico (2) istituto di metodologie per l’analisi ambientale (imaa) – cnr, tito scalo (pz), italy (3) di.s.g.g., università degli studi della basilicata, potenza, italy abstract we determined a set of empirical functions that describe the spectral amplitude decay of s-waves with distance in southern italy. we analyzed 32 earthquakes with magnitudes ml 2.0-5.4 and hypocentral distances ranging between 12 and 216 km. we obtained attenuation functions for 14 frequencies (1.0 < f < 20.0 hz). we compared these functions with average non-parametric attenuation functions reported by castro et al. (1999) for different regions of italy, and we observe that at low frequencies ( f < 5.0 hz) the spectral amplitudes from earthquakes in southern italy decay faster than the average. however, at high frequencies ( f > 5.0 hz), the spectral amplitudes are above the average. at higher frequencies ( f > 10 hz), the attenuation functions obtained for southern italy are slightly above the standard deviation of the average attenuation functions. it is possible that in this frequency range (10-20 hz) site effects may influence the amplitude decay. in order to quantify the attenuation of the s-waves, we estimated the quality factor q modeling the empirical attenuation functions using the following parametric form: a( f , r)=10/r b⋅ e−π f r/qβ; where 1.6 ≤ f ≤ 10.0 hz is the frequency band with minimum effect of instrument and site response, r ≤ 120 km is the distance range where the rate of decay of the spectral amplitudes is approximately constant, r=(r −10) and β = 3.2 km/s. we found that the exponent b =1.0 ± 0.2 in the frequency band analyzed and q shows a frequency dependence that can be approximated by the function q = 32.1 f 1.7. mailing address: dr. raúl ramón castro, cicese, división ciencias de la tierra, departamento de sismología, km 107 carratera tijuana-ensenada, ensenada, baja california, 22860 méxico; e-mail: raul@cicese.mx 1598 raúl ramón castro, maria rosaria gallipoli and marco mucciarelli terized by high s-wave attenuation in the crust and mantle and early p-wave arrivals (caputo et al., 1972; del pezzo et al., 1979). because reliable characterization of seismic attenuation is important for seismic hazards and risk assessment, we study the characteristics of the spectral amplitude decay of s-waves with distance using small to moderate earthquakes recorded at stations of the basilicata seismic network of southern italy. in a previous study, castro et al. (1999) obtained average nonparametric attenuation functions (hereafter referred to as naf) using seismic records from other regions of italy. in this paper we will use those naf as a reference frame to compare the attenuation characteristics in southern italy. 2. data we analyzed horizontal component records from 32 earthquakes with magnitudes ml = 2 .05.4 recorded by four stations of the basilicata table i. earthquake coordinates and hypocentral distances. no. date origin lat. long. ml depth ven mat tit vil d.m.yr hr.min.s deg. deg. km km km km km 2 14.09.01 08.01.49 40.664 15.741 2.0 10 73.00 12.00 34.00 5 20.11.01 18.29.03 41.468 15.676 3.4 11 58.00 120.00 97.00 123.00 6 21.11.01 06.10.40 40.507 15.799 2.8 20 55.00 72.00 23.00 25.00 7 21.11.01 06.20.54 40.520 15.809 2.7 11 69.00 16.00 20.00 8 30.11.01 03.17.37 41.751 15.860 3.4 12 88.00 138.00 129.00 153.00 9 09.12.01 12.15.21 40.786 15.348 2.8 9 41.00 103.00 36.00 12 02.01.02 02.17.08 40.753 15.429 2.8 11 39.00 30.00 52.00 13 15.01.02 00.07.07 40.737 15.801 2.6 7 27.00 69.00 18.00 14 15.01.02 01.29.16 40.734 15.796 2.4 9 27.00 69.00 18.00 19 06.04.02 06.35.23 39.224 16.544 2.9 5 201.00 159.00 139.00 20 05.04.02 14.54.07 39.144 16.804 3.8 10 216.00 168.00 156.00 22 13.04.02 17.04.12 40.569 16.394 3.3 6 62.00 26.00 51.00 50.00 23 18.04.02 20.56.47 40.612 15.594 4.1 5 43.00 84.00 11.00 31.00 24 18.04.02 21.00.34 40.583 15.516 3.2 9 49.00 18.00 33.00 25 18.04.02 21.36.52 40.569 15.524 3.1 5 50.00 16.00 31.00 26 18.04.02 22.58.31 40.577 15.547 3.0 7 48.00 87.00 18.00 31.00 27 18.04.02 23.19.06 40.572 15.524 3.1 8 50.00 15.00 32.00 29 21.04.02 23.39.19 40.569 15.580 3.5 10 85.00 15.00 28.00 36 31.10.02 11.34.42 41.760 14.940 5.4 10 110.00 181.00 142.00 167.00 37 31.10.02 11.56.22 41.710 14.825 3.5 10 112.00 184.00 167.00 38 31.10.02 13.04.11 41.670 14.890 3.7 10 105.00 177.00 134.00 160.00 41 01.11.02 15.09.15 41.690 14.830 5.3 10 109.00 184.00 139.00 164.00 42 01.11.02 15.20.04 41.740 14.850 4.1 10 113.00 143.00 168.00 43 01.11.02 15.42.23 41.720 14.840 3.4 10 111.00 184.00 166.00 45 01.11.02 17.21.15 41.710 14.810 4.3 10 112.00 141.00 166.00 46 01.11.02 22.30.26 41.720 14.850 3.1 10 111.00 183.00 166.00 47 01.11.02 22.44.13 41.690 14.800 3.8 10 111.00 184.00 140.00 165.00 48 02.11.02 02.36.27 41.720 14.810 3.7 10 113.00 186.00 167.00 49 02.11.02 06.22.06 41.690 14.810 3.8 10 110.00 183.00 164.00 50 03.02.03 11.24.24 40.760 15.538 3.1 10 33.00 89.00 25.00 48.00 51 03.02.03 12.18.39 40.760 15.538 2.7 10 33.00 89.00 25.00 48.00 52 04.02.03 08.31.33 40.740 16.240 2.5 10 41.00 38.00 43.00 54.00 1599 an attenuation study in southern italy using local and regional earthquakes recorded by the seismic network of basilicata seismic network. the network was set up by the institute for methodology of environmental analysis of the national research council and the department of structure, soil mechanics and engineering geology of the university of basilicata in the framework of a multiparametric geophysical observatory (balasco et al., 2001). all the stations are equipped with 1 hz tridirectional seismometers (lennartz 3d lite at ven, vil, mat and mark l4c at tit) connected to 24 bit a/d converter (praxs) with an instrument response approximately constant between 1 and 25 hz. table i lists the epicentral coordinates of the events used and the hypocentral distances to the recording stations. the distribution of the epicenters and the location of the seismic stations are also displayed in fig. 1. the source-station paths cover mainly the southern end of the apennines and northeast of calabria, italy. the fig. 1. location of earthquakes and seismic stations used. the circles with identification number are the epicenters (see also table i) and the triangles are the location of the stations. we also draw the source-station paths. fig. 2. hypocentral distance versus magnitude distribution of velocity records used. 1600 raúl ramón castro, maria rosaria gallipoli and marco mucciarelli fig. 3. examples of acceleration spectra and the frequency band used. the frequency band used for further analysis is shown with brackets. tion spectra were instrument corrected and smoothed around 14 pre-selected frequencies (1.0 < f < 20 hz). we visually selected, for each spectral record, the frequency band with the highest signal to noise ratio. the frequency at which the spectra became flat (in a semi-logarithmetic plot) was chosen as the end of the frequency band. the same procedure was used to estimate naf in other regions of italy (castro et al., 1999). figure 3 shows four examples of the frequency window selected for further analysis. for most of the records the spectral amplitude flattens beyond 25 hz. figure 4 displays velocity records from an earthquake m = 3.0 used in the analyses. this figure shows the time windows selected to calculate the acceleration spectra of pre-event noise and s-wave signal as well. in general, the signal to noise ratio is at least a factor of 10 between 1-10 hz. at higher frequencies the ratio remains high for stations vil and mat, and for stations tit and ven the spectral amplitude of the noise approaches that of the s-wave at f > 18 hypocentral coordinates were taken from the seismic bulletin of the italian national institute of geophysics and volcanology (ingv). events 36 to 49 in table i are events of the molise seismic sequence of 2002 that caused damage in san giuliano (mucciarelli et al., 2003a). in particular, events 36 and 41 are the main events of the sequence that generated intensity up to viii-ix mcs in san giuliano. according to the information reported by the ingv bulletin the focal depth of these events and events 50-52 was fixed at 10 km. the distancemagnitude distribution of the records, plotted in fig. 2, shows that most of the data is in the ranges of m < 4.0 and r < 180 km. the records were base line corrected subtracting the average of all the points of the record. then we calculated the acceleration spectra using a time window containing the first s-wave arrivals. the beginning of the window was chosen at the first s arrival and the end when 80 % of the total energy was reached, the window was also cosine tapered. the accelera1601 an attenuation study in southern italy using local and regional earthquakes recorded by the seismic network of basilicata hz. we can infer from the spectral inspection of the spectral records that the quality of the data set is very good up to 10 hz for all the stations used and up to 25 hz for vil and mat. 3. method we model the spectral amplitude decay with hypocentral distance r for each sampled frequency using a nonparametric approach. we assumed that the observed spectral amplitude ui(r, f ) at frequency f depends on the size of the earthquake that can be represented by a scalar si, and the attenuation due to geometrical spreading and anelasticity of the medium. thus, we can write ui(r, f ) = si ⋅ a(r, f ). (3.1) in eq. (3.1) the attenuation function a(r, f ) is the same for all source-station paths and is not limited to have a particular functional form. the site effects are eliminated using a smoothing constraint and by making a(0, f ) = 1.0, since at r = 0 only the source term controls the value of the spectral amplitude. we calculated independent inversions for 14 different frequencies between 1.0 and 20 hz. the distance range fig. 4. the frames on the left show the velocity records from event 26 as recorded for the four stations used. the time series are order from minimum distance (top frame) to maximum hypocentral distance. it is also shown the window use to calculate the fourier transform. the frames on the right are the corresponding acceleration spectra of the s-wave signal (solid line) and the pre-event noise (dashed lines). 1602 raúl ramón castro, maria rosaria gallipoli and marco mucciarelli from 5 km to 200 km was discretized into 40 bins, 5 km wide. anderson and quaas (1988) and castro et al. (1990, 1999) used this nonparametric model before to describe the amplitude decay in other regions. 4. results figure 5 shows a sample of 6 attenuation functions obtained between 1 and 20 hz. for comparison we also plotted with different symbols the observed amplitudes, circles for event 5 (m = 3.4) and asterisks for event 8 (m = 3.4). the curves shown are scaled accordingly with the corresponding value of si. the functions obtained show a change in the rate of amplitude decay at r > 100 km probably due to the effect of geometrical spreading, which for that distance range is less severe. it is also noticeable that at f > 10 hz the scatter of the data tends to increase, perhaps as a result of site effects. for some of the sites used, mucciarelli et al. (2003b) observed that site effect is a remarkable and stable feature. at high frequencies ( f > 10 hz) the near-site attenuation (κ 0) can also explain the observed scatter. in order to evaluate how the naf of southern italy compare with those for other regions of italy, we plot them in fig. 6 (with circles) together with the average attenuation functions obtained by castro et al. (1999) (solid lines) ± 1 standard deviation (dashed lines). the naf reported for castro et al. (1999) were obtained averaging attenuation functions from the regions of lombardy, piedmont, eastern sicily, friuli and marche. at 1 hz the naf of southern italy (circles) are slightly below the standard deviation of the average, but in general at low frequencies ( f < 5hz) the attenuation is stronger than the average. at high frequencies (f > 5 hz) attenuation in southern italy is minor than the average, and at higher frequencies (f > 10 hz) the naf are slightly above the standard deviation of the average. the frequency dependence of the attenuation can be appreciated in fig. 7, where we compared naf between 2 and 10 hz. it can be seen in this figure that for a given distance r the swave attenuation factor tends to decrease with frequency. the naf obtained can be also quantified in terms of the quality factor q and the geometrical spreading. for this purpose, we can paramefig. 5. nonparametric attenuation functions obtained (solid lines) and observed spectral amplitudes from event 5 (circles) and from event 8 (asterisks). 1603 an attenuation study in southern italy using local and regional earthquakes recorded by the seismic network of basilicata fig. 6. average naf (solid lines) and ± 1 standard deviation (dashed lines) obtained by castro et al. (1999) from different regions of italy and the attenuation functions obtained for southern italy (this study). terize the attenuation functions as ( ) .a r r e 10 / b fr q $= -r b ,f (4.1) where g(r) = 10/r b accounts for the geometrical spreading, r = (r −10), q is the quality factor of the s-waves and β = 3 . 2 km/s. note that eq. (4.1) is normalized at 10 km, because we do not have information at closer hypocentral distances (see fig. 2). we only considered the naf at frequencies where the instrument response is constant ( f ≥ 1.6 hz) and the specific near-surface attenuation at each site is less severe ( f ≤ 10 hz). we also used only the distance 1604 raúl ramón castro, maria rosaria gallipoli and marco mucciarelli where a (r) = log a( f, r) – log g(r) and m = π f⋅ ⋅ log e/qβ. for each frequency f, we searched for values of b and m that give the best leastsquares solution of eq. (4.2) (e.g., castro et al., 2003). figure 8 and table ii show the resulting values of the exponent b and q for the frequency band analyzed (1.6 ≤ f ≤ 10.0 hz). the values of q (triangles in fig. 8) show stronger frequency dependence than the exponent b of the geometrical spreading function. for comparison, we also plotted in fig. 8 (asterisks) recent estimates of coda q (qc) obtained by bianco et al. (2002) for the southern apennine zone. between 3 hz and 5 hz the values of qc are similar to those obtained using s-waves (triangles); at higher frequencies ( f >5 hz) qc has lower values (5659 % lower). b is approximately 1.0 ± 0.2, so that g(r) ≈ 1/r, as is expected for body waves. the following relation can approximate the frequency dependence of q: q = 32.1 f 1.7. (4.3) because of the trade-off between q and the geometrical spreading function, it is more meaningful to compare the combined effect of q and g(r) together. in fig. 9 we compare the nonparametric attenuation functions (open circles) with the spectral amplitude decay predicted by eq. (4.1) using the values listed in table ii range where the effect of site response and reflected waves is minimum (r ≤ 120 km). we linearized eq. (4.1) by taking logarithms, thus we can write, a(r) = − mr. (4.2) fig. 7. a sample of naf obtained for frequencies between 2 and 10 hz. fig. 8. left frame shows the values of the exponent b obtained for the geometrical spreading function assumed (see eq. (4.1)). the right frame shows the estimates of the quality factor qof the s-waves (triangles) as function of frequency. the asterisks are the values of coda q reported by bianco et al. (2002) for the southern apennine zone. 1605 an attenuation study in southern italy using local and regional earthquakes recorded by the seismic network of basilicata fig. 9. parametric attenuation functions (paf) reported in different regions of italy. the nonparametric functions obtained in this study are plotted with open circles. the solid lines are the attenuation functions calculated with eq. (4.1) using the values listed in table iii. the dashed lines are the paf calculated with the q and g(r) obtained by malagnini et al. (2000) (see table iii). the dotted curves are the paf reported by rovelli et al. (1988) (eq. (4.4)).the asterisks are the paf of castro et al. (2003b). the squares and the triangles are the pga and pgv attenuation functions, respectively, of sabetta and pugliese (1987) (eqs. (4.5) and (4.6)). table ii. resulting parameters of eq. (4.1). f (hz) b q 1.6 1.2 80 2.0 1.2 118 2.5 1.1 121 3.2 1.0 188 4.0 1.0 302 5.0 1.0 413 6.3 0.9 643 7.9 0.9 1112 10.0 0.9 1608 (solid line in fig. 9). in this figure we also compare other parametric relations reported in the literature and that describe the amplitude decay with distance of s-waves (see also table iii). in table iii we did not list estimates of coda q, as those reported by del pezzo and zollo (1984), del pezzo and scarcella (1986), bindi et al. (2001) and others, but those are compared and discussed by castro et al. (2002). the dotted lines in fig. 9 correspond to the amplitude decay calculated using the attenuation model of rovelli et al. (1988) , .exp expa f r r f q r1 0 0 $ $= -rl b r ^ ^ eh h o (4.4) where κ 0 = 0.07, β = 3.2 km/s, and q0 = 100. the dashed lines correspond to the attenuation curves proposed by malagnini et al. (2000) for the entire apennines (see table iii). this curves are characterized for having significant discontinuities at 30 and 80 km that are possibly related to the complex crustal structure of the apennines. it is interesting to note that at 3.2 hz and r > 80 km the attenuation curves of malagnini et al. (2000) are the same as those calculated 1606 raúl ramón castro, maria rosaria gallipoli and marco mucciarelli with eq. (4.1). the asterisks in fig. 9 are the attenuation functions calculated using the relation obtained by castro et al. (2004) for central italy, namely q = 31.2 f 1.2 in the frequency band of 0.3-9.5 hz and g(r) = 1/r. these curves are similar to those calculated with the model of rovelli et al. (1988) at short distances (r < 40 km) in the frequency band of 1.6-3.2 hz. it is important to keep in mind, when comparing these attenuation functions, that the data used to obtain them sampled not only different frequency bands but also different crustal volumes. for instance, while the data set used by malagnini et al. (2000) sampled the entire apennines, our results are only valid for southern italy. because these attenuation functions may have an important impact in the estimation of peak ground acceleration (pga) and peak ground velocity (pgv), we also compare the empirical relations obtained by sabetta and pugliese (1987) to estimate pga and pgv in italy from magnitude (m), site type (s = 0 for rock and s=1 for soil) and the closest distance to surface projection of the fault rupture (r) logpga= − 1.562 − log(r2+5.82)1/2+ 0.169 s + + 0.306 m (4.5) logpgv= − 0.710 − log(r2+3.62)1/2+ 0.133 s + + 0.455 m. (4.6) we plotted in fig. 9 the first 3 terms of eqs. (4.5) and (4.6), with triangles and squares respectively, using both s = 1 and s = 0. the pga relation (triangles) gives higher attenuation factors at low frequencies relative to the other curves. however, at 10 hz and r > 80 km the pga curve approaches the moel of rovelli et al. (1988). on the other hand, the pgv curve approaches the model of rovelli et al. (1988) between 3.2 and 5.0 hz. it is important to point out that sabetta and pugliese (1987) and rovelli et al. (1988) used similar data sets, namely, records from the earthquakes of valerina, 1979; irpinia, 1980; umbria, 1984 and lazio-abruzzo, 1984. sabetta and pugliese (1987) also included events from the regions of friuli and sicily. the result of comparing the parametric attenuation functions reported (fig. 9 and table iii) indicates that a significant spatial variability of the attenuation parameters q and g(r) must exist in italy. this variability must be the result of a heterogeneous complex crustal structure. spatial variability of q can be significant, particularly near seismogenic zones (castro et al., 2000). li et al. (1997), for instance, found a reduction of s-wave velocity (30-50%) and q within tens of meters of the fault trace at parkfield, california. changes in the geologic characteristics of the rocks are also responsible for the spatial variability of q. thus, we can expect different behavior of q and g(r) for different s-wave propagation crustal volumes. table iii. s-wave attenuation relations reported for italy. region distance frequency q geometrical reference range (km) range (hz) spreading central and r ≤ 100 0.1≤ f ≤ 20. q =100 r 1 rovelli et al. (1988) central-southern and κ 0 = 0.07 apennines entire apennines r ≤ 400 0.24≤ f ≤ 5.0 q = 130 f 0.1 r − 0.9, r ≤ 30 km malagnini et al. (2000) r 0, 30 ≤ r ≤ 80 km r − 0.5, r ≥ 80 km umbria-marche r ≤ 100 0.3≤ f ≤ 9.5 q = 31.2 f 1.2 r 1 castro et al. (2003a) southern italy r ≤ 120 1.6≤ f ≤ 10 q = 32.1 f 1.7 r 10 b , b = 1.0 ± 0.2 this study 1607 an attenuation study in southern italy using local and regional earthquakes recorded by the seismic network of basilicata 5. conclusions we determined nonparametric attenuation functions for 14 frequencies (1.0 < f < 20.0 hz). at low frequencies (f < 5.0 hz) these functions show a faster decay than those obtained by castro et al. (1999) averaging naf from different regions of italy. however, at intermediate frequencies (f > 5.0 hz) the spectral amplitudes are above the average, and at higher frequencies (f > 10 hz) the attenuation functions obtained for southern italy are slightly above the standard deviation of the average functions. the data set used in this frequency band (f > 10 hz) may be affected by stronger site effects than expected, in particular near-surface attenuation may have a strong effect on the spectral amplitudes at high frequencies. we also estimated the quality factor q of the s-waves using a parametric approach and we found that q shows a frequency dependence that can be approximated by the function q = 32.1 f 1.7, (1.6 ≤ f ≤ 10.0 hz) and g(r) =10/r b, (b = 1.0 ± 0.2). this parametric model of s-wave attenuation shows significant differences with respect to attenuation models reported for neighboring regions (sabetta and pugliese, 1987; rovelli et al., 1988; malagnini et al., 2000). this indicates that a significant spatial variability of the attenuation parameters q and g(r) exists in italy. further developments of this work will include separation of ray-paths crossing mainly the apennines and those crossing the apulian plate. when enough data are available, it will be interesting to verify whether the apulian plate is less attenuating than the apennines, as geodynamic models predict. acknowledgements the basilicata network was set up and maintained thanks to grants of the italian ministry for university and research. many thanks to dr. enzo lapenna from the imaa-cnr for his constant help. we also acknowledge luca malagnini and the anonymous reviewer for their comments and suggestions. luis inzunza help us preparing the figures. references anderson, j.g. and r. quaas (1988): the mexico earthquake of september 19, 1985: effect of magnitude on the character of strong ground motion: an example from the guerrero, mexico strong motion network, earthquake spectra, 4, 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(received october 13, 2003; accepted february 12, 2004) e s p r e s s i o n e f o r m a l e p e r f u n z i o n i a p i c c o silvio d e f r a n c e s c o c i p r o p o n i a m o di m o s t r a r e c o m e s i a p o s s i b i l e g i u n g e r e a d u n a f o r m a m a t e m a t i c a c h e e s p r i m a u n a f u n z i o n e di d u r a t a i s t a n t a n e a e di v a l o r e finito o m e n o , ( p i a l e l i m i t e di u n a f u n z i o n e r e t t a n g o l a r e d i d u r a t a s , q u a n d o s t e n d a a 0. fi il c a s o d e l l e f u n z i o n i a p i c c o ( d i t e n s i o n e , di c o r r e n t e , chiam a t e d a g l i a n g l o a m e r i c a n i « t r i g g e r » e d u s a l e l a r g a m e n t e nei r a d a r , n e l l a t e l e v i s i o n e e n e i c a l c o l a t o r i e l e t t r o n i c i ) . s i a : ( 1 per i i n t e r n o a (t, t + z) r ( f ; t j t + s) = 1 i 0 » » e s t e r n o » » e c o n c e p i a m o r f o r m a t a d a l l a s o m m a d e l l e d u e f u n z i o n i a g r a d i n o ( s t e p f u n c t i o n ) : ( 1 per t > t ( — 1 per t>~ + s g(t~ t) = j ' — — g{t — t — £ = 6 ( 0 >. / < t i 0 » ' < + £ r ( f ; t . t + e ) = g(t — t ) — g(t — t — e ) [ 1 ] p e r r a g i o n i c h e d i v e r r a n n o e v i d e n t i in s e g u i t o , r i c o r r i a m o a l l o a r t i f i c i o di p r e n d e r e p e r f u n z i o n i a g r a d i n o le s e g u e n t i : , _ p e r ' < t h ( lini = 1 [ìer 1 > t , o s s i a ; ( 0 per t — glt — t — £ = 1 i _ l i m e — r ( ' — 0 = _ 1 per l ] c o n y r e a l e e p o s i t i v o ( c f r . g u i l l e m i n , loc. cit., p a g . 531ì. p r e n d i a m o o r a la t r a s f o r m a t a d i f o u r i e r d e l l a [ 1 ] : f)r\ = f ) g ( t z ) \ f ) g ( t t z ) \ [ 2 ] 41(1 s i l v i o l ) k f r a n c e s c o e r i c o r d a n d o c h e : = um — i y — > o 2 7t j f ) g ( t — t ) | = uni— / e v ( t x ) e j o > t ( u i g v* 1 y — > o ~ 7c t l i m — i e ( r + i « > ) ' d l = limf \ g ( t — ; — * ) [ = lim y + 7 w c y ( t + e ) e ( r + j « o ) ( x + s ) 2 — t + 7 t , j e q u i n d i a n t i t r a s f o r m a n d o la [ 2 ] o t t e n i a m o : ( + s ) r _ ( y + ] „ ) ( , + , ) f ' . f ) r \ = r ( f , t , t + s) = ztm — — i \ —v-1) £ tu ' y + 7 <'-> e " 0 1 d c o + x + / / , „ i gjwt d w m a y—>o 2 re _ ' y + 7 c o ( e r < " + s l — y : — j w i — f a — j m s + j t o t i k m 11 d j = y + 7 t » ) i r l l i f > h i ; = « u n -, / : (/ coi y + 7 y — > o / (0 e d a n a l o g a m e n t e : a a / i m i i . — c ' " , { d co [ = k m ! / co ) < " i , — i r r j o v . , j ( p j " » . 1 ^ i m . — t j m t d co : = / i m _ .„ ( 2 j v + ) , (2 j y + 7 1 d a cui : a a [ | j ' p j < o ( t — t — s ) j p j c j ( x ) j r ( / ; t , + s ) = l i m — / 7 d u + — / r — d <o | " 3 ] r » o f 2 ~ j y + 7 c o 2 ~ . ' y t 7 6 j ) a e s p r e s s i o n e f o r m a l e p e r f u n z i o n i a p i c c o 411 c o n s i d e r i a m o q u i n d i : ( 1 r ) a = = lini i — i — d co w e p o n i a m o : 2 ti j y + j w ( l / " p i u f t — • r ) ) p = k m — dco [ 5 ] a, = / — t — e , a „ = f — t . p e r £ i n t e r n o a l l ' i n t e r v a l l o (t, t + s ) s a r à a„ > 0 , o s s i a = — d t , a„ = / ) „ , c o n d ] , do q u a n t i t à p o s i t i v e . a v r e m o a l l o r a : — i d , a d [ a = z t / » i — — : — / d w d , • r _ j 2 j y dt + j oj l>4 j r ^ 0 | 2 7 r j y d l + j < * d l a s o o + l d i — a d , ( 1 / • " > d e ) 3 = = zini ' -— / — 7 d w p ,im -, — i —— : —o w / ' „ r ad., d a l l e q i i a l i : a d , ' a d , a = b j j f c o s « d < d c p ~ l ! s i ™ d < d c ó d ) y > " > ( 2 n j y d l \ j u > d l 2 j y d l -\-j co d , ) u->co — a d i — a d , [6] a l ) . . a l ) 2 p = / i m | j f c o s " d ° d o > d ì + ì i s ì n i ù d ° d c o d . j y ^ o ( 2 7 t j y d „ + 7 w d „ " 2tt 7 y d s + j ) a ^ x — a d , — a l ) o [ 7 ] l a s o m m a a + (3 è u g u a l e a l l ' u n i t à , m a a n c h e zim (x + [3) = 1. i n f a t t i p e r s—>-o , d ] t e n d e a do e q u i n d i i s e c o n d i t e r m i n i d e i sec o n d i m e m b r i di [ 6 ] e [ 7 ] si e l i d o n o , m e n t r e i d u e p r i m i v a l e n d o c i a s c u n o 1 / 2 , d a n n o l ' u n i t à ( * ) . b e n c h é o v v i o i n r e l a z i o n e a l t i p o di f u n z i o n e a s s u n t a fin d a l l ' i n i z i o , si p u ò v e r i f i c a r e a g e v o l m e n t e c h e p e r t e s t e r n o a ( t , t + e) e d .. 41(1 s i l v i o l)k f r a n c e s c o a s i n i s t r a d i t , l a s o m m a x + d e l l e [ 4 ] e [ 5 ] è n u l l a c o m e p u r e lini ( « + p ) = 0 . £—>0 i n t a l c a s o i n f a t t i a t = — d^ , a j = — d^ p e r c u i : a i ) , a l ) ) ( 1 f coslùd j f s i n o / ) j a = l i n i — / — d c c d , — 1 5 — d w d , «, ( 2 — / + j w d , 2 t u . / y l > l + j t * o l l \ a—>x — a i ) i — a l > £ [8] a l ) » a l ) 2 i r , j f ì m d o d..+ ìf s ì n " d > d c o p j r -->-» ( 2 tu j y /)„ + j co />., " 2 j y />., + y co /">., " } a v v a l ) » ' " " — à i ) . , [ 9 ] p e r z finito, i s e c o n d i t e r m i n i d e i s e c o n d i m e m b r i «li [ 8 ] e [ 9 ] s o n o e n t r a m b i e g u a l i i n v a l o r e a s s o l u t o a d i 2 e si e l i d o n o e lo s t e s s o a v v i e n e p e r i d u e p r i m i . p e r e—>• o , dt t e n d e a d-i c h e t e n d e a z e r o ; i s e c o n d i t e r m i n i d e i s e c o n d i m e m b r i si a n n u l l a n o , m e n t r e i d u e p r i m i si e l i d o n o c o s i c c h é lini ( a + p) -..0 . s—v— o i n m o d o a n a l o g o p e r t e s t e r n o a ( t , t + s) e d a d e s t r a di t + z, a + p = 0 , lini ( a + p ) = 0 . s — > o s i c o n c l u d e c h e ; lini ( 1 f e j « > ( ' ) _ , x ' 0 l ^ 1 p e r i = t r r : m j ~ — r + j » ) = ? 0 1 > e r / ^ t s i v e d e p u r e c h e : j j . ( t — ~) \ x. p e r / t 1 — a (j — t) j1 0 jier t =f= t ( * ) i n f a t t i : a l ) ( 1 r cos co d , ) ( 1 / ' c o s co , / i r a — — d co d [ = / , ' , „ . — c / c o y—>-o ( 2 n j y d + j o > d j y + y c o 1 o ^ d . v l v u — a l ) l ) > o a e s p r e s s i o n e f o r m a l e p e r f u n z i o n i a p i c c o 4 1 3 è v e r o clic se y = 0 l ' i n t e g r a l e d i v e n t a i m p r o p r i o p e r c h é l'integ r a n d o d i v i e n e i n f i n i t o por co = 0. t u t t a v i a se y = 0, l ' i n t e g r a n d o è una f u n z i o n e d i s p a r i di co: e p o i c h é i l i m i t i di i n t e g r a z i o n e s o n o simm e t r i c i , il v a l o r e d e l l ' i n t e g r a l e , e c c e t t u a n d o il c o n t r i b u t o c r i t i c o dell ' i n t o r n o di oj = 0, d o v r e b b e e s s e r e n u l l o ; q u i n d i il v a l o r e d e l l ' i n t e g r a l o ( q u a l u n q u e esso s i a | è p e r ù c e r t a m e n t e d e t e r m i n a t o dal contrib u t o ili t a l e i n t o r n o . d e n o t a n d o q u e s t o i n t o r n o con ( — p , p | si p u ò p o r r e cos co d = 1 p e r c h é p e r piccoli i n c r e m e n t i d a co = 0, cos oj d d i f f e r i s c e d a l l ' u n i t à p e r u n a p i c c o l a q u a n t i t à d i s e c o n d o o r d i n e ( a m a g g i o r r a g i o n e è cos oj d = 1 se d = 0 | e q u i n d i l ' i n t e g r a l e a l sec o n d o m e m b r o d i v i e n e : p l i n i -— i —— = l i m •(—>• o 2 — ' y + / co 2 7 j i n (y + j co) 1 \ l n ' ( + j p 1 " 2 7 z ] . y — j p . 2 l ' a m b i g u i t à r e l a t i v a a ln(—1) = j ( 2 / i + 1 ) —, s p a r i s c e se si t i e n e c o n t o del c a m m i n o d e l l ' i n t e g r a l e o b b l i g a t o d a y — j p a y + j ? • ' i n t e g r a l e [ a m e n o di i n f a t t i p o s t o 1 2 j p r e n d e l a = y t j o ) , r + i p / d z l o r m a : / , o s s i a e una p u r a q u a n t i t à i m m a g i n a r i a p a r i in r — ì p g r a n d e z z a a l l ' a n g o l o l'ormato d a l l e c o n g i u n g e n t i l ' o r i g i n e del p i a n o z con i p u n t i y + y p , y — j p , c h e t e n d e a — q u a n d o y t e n d e a z e r o . p e r ipiesta a n a l i s i c f r . g u i l l e m i n . t h e m a t h e m a t i c s of c i r c u i t a n a l v s i s . .). w i l e y . n . y . e d . 1919, p a g . 535-36. v clic, u n i n t e g r a l e d e l t i p o i < o s % d y e g u a g l i y ( 2 « + l)— si p u ò . y — v ( ( a n c h e m o s t r a r e r i c o r r e n d o a d u n a n o t a f o r m u l a r e l a t i v a a l c o s e n o i n t e g r a l e c h e r i p r e n d i a m o da j a h n k e e n d e d o v e r e d . o v v e r o l a : f cos t , , i" 1 — cof i , ci (.v) = — i — d i = i n (x y) — j d i d o v e y è la c o s t a n t e di e u l e r m a s c h e r o n i . 41(1 s i l v i o l)k f r a n c e s c o i n f a t t i : y „ b > o b b ' cos y /' cos y f cos y dy f cos ydy y v. v > w /' cos y d y , /' c o s y d y , , , f i — cos y , = — -—+ = >n(y „ y ) — = ^ d y + / v ' y j y y „ — y -in _ j o y ) + j j—z£^ll.dy=ln ( — 1) — d y = j l 2 n + l ) * , j y j y i n q u a n t o l ' i n t e g r a n d o è u n a f u n z i o n e d i s p a r i c l i c s ' a n n u l l a p e r y = 0. t a l e m e t o d o è s t a t o s u g g e r i t o d a l d r . d . c . b o r g h i . riassunto viene ricercata tin espressione formale per le funzioni di durata istantanea (picco), considerate quali limite di una funzione rettangolare. la validità della formula poggia su quella di espressioni date da e. a. guillemin (loco cit. nel testo). su m mary an expression for trigger functions is given as a limiting forni of a rectangular function. the vididity of the formula lies on that of an expression given someuhere bv e. a. guillemin (see reference in te thext). s u l l a n a t u r a fisica d e l l e o n d e i n t e r n e d e l l a g o di b r a c e i a n o (*) p . c a l o i m . m i g a n i r i c e v u t o il 15 g i u g n o 1963 riassunto. — e u o t o d i e , alia p r o f o n d i t a m e d i a di 15 m c i r c a , d u r a n t e i m e s i e s t i v i a u t u n n a l i , n e l l a g o d i b r a c c i a n o si f o r m a n o c o s p i e u e o n d e i n t e r n e , d a r i t e n e r s i — c o m e h a p r o v a t o caloi — o s c i l l a z i o n i l i b e r e d e l l a s u p e r f i c i e d e l s a l t o t e r i n i c o , c h e si f o r m a n e l l a g o d u r a n t e i m e s i s u d d e t t i . s u l l a b a s e d i u n a l u n g a serie di r e g i s t r a z i o n i , o t t e n u t e i n d u e s t a z i o n i l i m n o g r a f l c h e , f u n z i o n a n t i a g l i e s t r e m i d i u n d i a m e t r o d e l l a g o , e s t a t o p o s s i b i l e p r o v a r e , c o n f o r m e m e n t e a l i a t e o r i a , clie le o n d e i n t e r n e del l a g o di b r a c c i a n o s o n o del t i p o a s i m m e t r i c o . summary. — i t is a well k n o w f a c t t h a t , a t a n a v e r a g e d e p t h of 15 m , d u r i n g t h e s u m m e r a n d t h e f a l l , r e m a r k a b l e i n t e r n a l w a v e s a p p e a r i n t h e l a k e of b r a c c i a n o — a s p r o f . caloi h a s s h o w n — s u c h w a v e s m u s t b e e x p l a n e d b y f r e e o s c i l l a t i o n s of t h e s u r f a c e of t h e r m i c j u m p , w h i c h t a k e s p l a c e i n t h e l a k e i n t h o s e m o n t h s . o n t h e b a s i s of a l o n g s e r i e s of r e c o r d i n g s , t a k e n i n t w o l y m n o g r a p h i c s t a t i o n s o p e r a t i n g a t t h e e s t r e m e s of a d i a m e t e r of t h e l a k e , i t w a s p o s s i b l e t o p r o v e t h a t t h e b r a c c i a n o l a k e i n t e r n a l w a v e s a r e of a s y m m e t r i c t y p e , a c c o r d i n g t o t h e t h e o r y . 1 . £ s t a t o p r o v a t o , i n p r e c e d e n t i l a v o r i , c l i e l e o n d e i n t e r n e d e l l a g o d i b r a c c i a n o , a s s o c i a t e a l i a f o r m a z i o n e d e l l o s t r a t o d e l s a l t o t e r i n i c o a p r o f o n d i t a v a r i a b i l i i n t o r n o a 1 5 m c i r c a , s o n o d e l t i p o a s i m m e t r i c o (»), (2). m a n c a v a n o p e r o o s s e r v a z i o n i d i r e t t e , c o m p i u t e c o n s t r u m e n t i e s c l u s i v a m e n t e d e s t i n a t i a q n e s t o s c o p o . s i e p e n s a t o d i o w i a r e a q u e s t a l a c u n a p r e d i s p o n e n d o u n a s e r i e d i o s s e r v a z i o n i , i n d u e s t a z i o n i l i m n o g r a f i e l i e , s i s t e m a t e a g l i e s t r e m i d i u n (*) q u e s t o l a v o r o c s t a t o c o n d o t t o c o n c o n t r i b u t i d e l consiglio n a z i o n a l e delle r i c e r c h e . s u l l a n a t u r a f i s 1 c a d e l l e o n d e i n t e r n e d e l l a g o d i b r a c c i a n o 2 1 5 2 1 0 p . c a l o i i i . m i g a n i s u l l a n a t u r a f i s i c a d e l l e o n d e i n t e r n e d e l l a g o d i b r a c c i a n o 2 1 9 t l i a m e t r o (lei lago; nel caso speciflco, a v i g n a d i valle e a t r e v i g n a n o . i l i m n o g r a f i e r a n o del t i p o gia p r e c e d e n t e m e n t e d e s e r i t t o (1), in p a r t e flltrati p e r le oscillazioni di p i n b r e v e p e r i o d o . 2. le osservazioni si sono seguite, pressoche in m o d o c o n t i n n o d a l s e t t e m b r e a l n o v e m b r e 1962. e q u e s t o il p e r i o d o piii i n d i c a t o p e r la r e g i s t r a z i o n e delle o n d e i n t e r n e , q u a n d o si pon.si che la loro esistenza e l e g a t a a quelle p a r t i c o l a r i condizioni t e r m i c h e , alle quali si d e v e la form a z i o n e dello s t r a t o del s a l t o t e r m i c o , che p e r i laghi della zona t e m p e r a t a si f o r m a e p e r m a n e a p p u n t o nei mesi che v a n n o d a l l ' a g o s t o al nov e m b r e . le registrazioni o t t e n u t e h a n n o p i e n a m e n t e c o n f e r m a t o l ' a s p e t t a t i v a : o l t r e a d oil'rire u n a n u o v a , n o t e v o l e t e s t i m o n i a n z a sulla r e a l e esistenza di cospicue onde i n t e r n e nel lago di b r a c c i a n o , h a n n o a l t r e s i c o n s e n t i t o di c o n f e r m a r e la loro n a t u r a di o n d e s t a z i o n a r i e a s i m m e t r i c h e , a v c n t i cioe le superflcie n o d a l i l u n g o sezioni t r a s v e r s e del lago c n o n l u n g o superflcie cilindriche c o n c e n t r i c h e . n e l caso di oscillazioni a s i m m e t r i c h e u n i n o d a l i , a-ll'estremita di u n d i a m e t r o d o v e v a n o veriflcarsi, c o n t e m p o r a n e a m e n t e , s p o s t a m e n t i di senso c o n t r a r i o : a d u n s o l l e v a m e n t o a t r e v i g n a n o , p . es., d o v e v a corrispondere u n c o n t e m p o r a n e o a b b a s s a m e n t o a y i g n a di valle; e v i c e v e r s a . 15 cio che g e n e r a l m e n t e h a n n o t e s t i m o n i a t o le registrazioni, con e s e m p i p a r t i c o l a r m e n t e c h i a r i ed i n o p p u g n a b i l i (v. f i g g . 1 , 2 , 3 , 4 ) . si e d e t t o « g e n e r a l m e n t e » p e r c h e p u o c a p i t a r e — ed e c a p i t a t o (l) — che, i n d i p e n d e n t e m e n t e d a l dissincronismo dei r e g i s t r a t o r i dei d u e limnogi'afi, le oscillazioni u n i n o d a l i n o n a p p a i a n o r i g o r o s a m e n t e s f a s a t e di 180° o — a d d i r i t t u r a — s e m b r i n o r i s u l t a r e i n f a s e . cio e conseguenza del f a t t o che, t a l v o l t a , il p i a n o d'oscillazione r u o t a i n t o r n o a l c e n t r o del lago, con cons e g u e n t e a l t e r a z i o n e della d i n a m i c a delle oscillazioni. a d ogni m o d o , c o n f o r m e m e n t e alia t e o r i a (1), p o s s i a m o c o n c l u d e r e che le oscillazioni di 6 ore circa, che si r e g i s t r a n o sulle r i v e del lago s a b a t.ino d u r a n t e i m e s i della t a r d a e s t a t e e del p r i m o a u t u n n o , sono onde i n t e r n e u n i n o d a l i a s i m m e t r i c h e . 3. a n c h e q u e s t a n u o v a c a m p a g n a l i m n o g r a f i c a , e s e g u i t a sul lago di b r a c c i a n o , h a m o s t r a t o le notevoli a l t e r a z i o n i (li livello cui il lago viene s o t t o p o s t o , in c o r r i s p o n d e n z a d i p e r t u r b a z i o n i a t m o s f e r i c h e . di esse si e gia discusso in n o t e p r e c e d e n t ! (-), con p a r t i c o l a r e r i g u a r d o all ' i n t e r a z i o n e i n t e r c e d e n t e f r a v a r i a z i o n i della pressione a t m o s f e r i c a e v a r i a z i o n i del livello delle a c q u e s o t t o s t a n t i . a n c h e in q u e s t a serie di registrazioni non sono m a n c a t e le conseguenze di t a l e i n t e r a z i o n e . n o n 2 2 0 p . c a l o i m . mtgamt s e m p r e i i n o v i m e n t i della s u p e r f i c i e d i d i s c o n t i n u i t a t e r m i c a d i v e n g o n o d i f a c i l e i n t e r p r e t a z i o n e , e s s e n d o s o v e n t e o b b l i g a t a a m o t i f o r z a t i , d i c a r a t t e r e c a o t i c o ( f i g . 4). l a d i s t r i b u z i o n e t e r m i c a d e l lago n o n s e m b r a p e r o s u b i r e a l t e r a z i o n i p r o f o n d e , t a l i d a d i s t r u g g e r e la z o n a d e l s a l t o t e r m i c o . b a s t a i n f a t t i clie d u r a n t e i p e r i o d i di b u r r a s c a , c o l l e g a t i a f o r t i a g i t a z i o n i a t m o s f e r i c h e e d e l l a p r e s s i o n e b a r o m e t r i c a , q u e s t ' u l t i m a a c c e n n i a s t a bilizzarsi i n t o r n o a d u n v a l o r m e d i o , p e r c h e le o n d e i n t e r n e r i p r e n d a n o in m o d o a m p i o e r e g o l a r e (pig. 4), s e g n o e v i d e n t e d e l l a c o s t a n t e p r e s e n z a d e l l a s t r a t i f i c a z i o n e e s t i v o a u t u n n a l e . 4. circa l ' o r i g i n e di q u e s t e o n d e , n n o d i n o i (x) h a g i a p r o y a t o c.liel a loro a m p i e z z a , il loro r i p e t e r s i d u r a n t e s e t t i m a n e e s e t t i m a n e , p u o s p i e g a r s i solo con la r i s o n a n z a , p r o v o c a t a d a u n c o n t e m p o r a n e o f e n o m e n o c o p e r i o d a l e . t a l e p u 6 e s s e r e il f o r m a r s i di m a r e e solide q u a i ' t o d i u r n e , cons e g u e n t i alle m a r e e dello s t e s s o p e r i o d o del v i c i n o t i r r e n o (come si e r i t e n u t o in u n p r i m o t e m p o (3)), o p p u r e — q u a s i c e r t a m e n t e — d e l l ' a r i n o n i c a s u p e r i o r e dell'oscillazione s e m i d i u r n a d e l l a p r e s s i o n e , s e m p r e ])resente n e l l ' a t m o s f e r a (4) (figg. 2 e 5). b i b l i o g r a f i a p . c a l o i , termoloqia del lago di bracciano: onde interne. « a n n a l i di geoflsica >., x i i , 1 (1959). ( 2 ) p . c a l o i , m . m i g a n i , g-. p a n n o c c h i a , ancora sulle onde interne del lago di bracciano e swi fenomeni ad esse collegati. « annali di geoflsica », x i v , 3 (1961). ( 3 ) p . c a l o i , sulle oiule interne del lago di bracciano e sulle loro probabili cause. « rend. acc. naz. dei lincei, classe sc. fis. m a t . e n a t . », serie v i i i , x x x , 5 (19fjl). ( 4 ) p . c a l o i , interazioni tra atmosfera ed idrosfera. « a n n a l i di geoflsica », x v i , 1, (1963). 05mccue.qxd 387 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words australia – historical earthquakes 1. introduction aboriginal occupants of australia for the last 60 000 years (horton, 1994) left no written records; history was passed from generation to generation orally as dreamtime stories, and rock art depicted many of their rituals. unfortunately there are no dates or times for stories or art that may be interpreted to relate to earthquakes or volcanic eruptions. an example of such a story about the creation of port phillip bay was quoted by mctaggart (1932): «aboriginal belief (is) that in the past there was no passage between points nepean and lonsdale, and an earthquake broke the shoreline to make it what it is today». another quote is in a personal letter from fisher to la trobe (1853) as follows: «1844 – geelong district earthquake, native informed author that he has felt the same a long time ago». aboriginal middens have been discovered buried under volcanic ash, the charcoal enabling age estimates of the eruptions that are described in some of the dreamtime stories. the youngest eruptions in western victoria and southeast south australia are dated at between 1000 and 5000 years ago (barbetti and sheard, 1981). portuguese and dutch seafarers began visiting the australian continent at least as early as 1606 but the first written record of an earthquake relates to one on sunday 22 june 1788, felt in sydney just five months after the first british settlers arrived in southeastern australia. subsequent british settlements spread south, north and west from sydney across the continent in the next 50 years (hobart tasmania, 1803; brisbane queensland, 1824; perth western australia, 1829; adelaide south australia, 1836; and melbourne victoria, 1837); and newspapers followed soon after in all cities and most large towns. surprisingly for the early european settlers, earthquakes were felt in each of these future state capitals within months of their establishment. australia achieved independence from britain in 1901, becoming a commonwealth of six states and two territories with separate state australia: historical earthquake studies kevin mccue australian seismological centre, canberra, australia abstract historical studies of earthquakes in australia using information dating back to 1788 have been comprehensive, if not exhaustive. newspapers have been the main source of historical earthquake studies. a brief review is given here with an introduction to the pre-european aboriginal dreamtime information. some of the anecdotal information of the last two centuries has been compiled as isoseismal maps. relationships between isoseismal radii and magnitude have been established using post-instrumental data allowing magnitudes to be assigned to the pre-instrumental data, which can then be incorporated into the national earthquake database. the studies have contributed to hazard analyses for the building codes and stimulated research into microzonation and paleo-seismology. mailing address: dr. kevin mccue, australian geological survey organisation, australian seismological centre, jamison centre, po box 324, act 2614, canberra, australia; e-mail: asc@netspeed.com.au. 388 kevin mccue governments and state constitutions. commonwealth and some state agencies monitor earthquakes in their jurisdictions but arguments about mandated responsibility, and therefore who should pay, have caused network problems over the years. the first seismograph in australia, a graymilne instrument, was installed at melbourne in 1888 but records only remain from the second phase of installations when milne seismographs were established in perth (1901), melbourne (1902), sydney (1906) and adelaide (1909). between 1788 and the early 1900s, newspaper articles were the main source of information about earthquakes and continued to be important until the 1960s. 2. the pre-instrumental period mention of earthquakes in australia was made in several early texts including the series historical records of australia published by the library committee of the commonwealth parliament in 1914. these were a reproduction of the original reports of the founding governor phillip to lord sydney, the home secretary in england, which started in 1788. phillip sent several handwritten versions of the same report on different ships to try to ensure that at least one arrived. the reports were not always identical. a 1789 publication reproduced by the library of australian history in 1978 mentions that three earthquakes were felt during the first six months of abode there without further details. one of these would have been the event on 22 june 1788 which is reported to have occurred in the evening in one account by phillip (historical records of australia, 1914), or at 11:00 a.m. (howe, 1952). in another account, phillip does not mention a time of day (historical records of australia, 1914). the first publication on australian earthquakes was jevons’s (1859) description of earthquakes in new south wales between 1788 and 1842. the reverend w.b. clarke, known as the father of australian geology (mozley, 1965), collected anecdotal information on 160 earthquakes in australia and new zealand (clarke, 1869) but his list of earthquakes was destroyed in a fire in 1882. heaton (1879) listed a number of earthquakes in most australian states except western australia from 1788 to 1877. towards the end of the 19th century, geologists, surveyors, astronomers and meteorologists all took an active interest in earthquakes in australia and new zealand and formed the seismological committee of the australasian association for the advancement of science (aaas). this committee was responsible for the introduction of the milne pendulum seismometers and for the compilation of lists of reported earthquakes with intensities (rossi-forel scale) for the period 1892 to 1913. the committee included a. biggs, r. ellery, h. russell, c. todd and g. hogben from both sides of the tasman sea. their lists have been the nucleus for subsequent historical studies of individual events and seismicity studies. government geologist edgeworth david prepared a map of earthquakes in se australia for publication in the sydney paper the daily telegraph in 1902, reproduced in fig. 1. another early scribe (dodwell, 1910) provided descriptions of earthquakes in south australia for the four years prior to the installation of the seismograph there, in 1908 according to dodwell, though other references suggest the milne was not in operation until 1909. prior to its shipment to adelaide this instrument was under test in london at the time of the 1906 san francisco earthquake and that seismogram was on display at the science museum, london until at least the 1970s. several seismologists including the author investigated early earthquakes in south australia but the most significant study was undertaken by malpas (1993) who produced five volumes of isoseismal maps covering the period 1837 to 1964, including copies of the source material. thirty five of the isoseismal maps were drawn for the pre-instrumental period. she also made special studies of the most important events, the larger earthquakes and those near the capital adelaide. burke-gaffney sj (1952), whilst based at riverview college observatory that was founded in 1909, used the aaas lists and press reports to compile a list of 82 australian earthquakes from 1883 up to 1949. a quality indicator was assigned to each location. he apparent389 australia: historical earthquake studies ly compiled isoseismal maps for the pre-instrumental earthquakes though these have not been found. he did not assign magnitudes to events prior to 1912 in new south wales, 1922 in victoria and tasmania and 1937 in other states. the observatory, which houses the longest running seismograph station in australia, kept a fairly complete newspaper clippings book which provided much useful information for subsequent studies of early earthquakes throughout australia and in papua new guinea. doyle et al. (1968) summarised the history of earthquakes and seismological research in australia up to 1966 and prepared a comprehensive reference list of historical earthquake sources. they tabulated 161 australian earthquakes and their lists formed the core of the preliminary computerized australian earthquake catalogue (denham et al., 1975). everingham and tilbury (1972) compiled a list of earthquakes in western australia from newspapers, meteorological reports and a review of the perth observatory seismograms for the period 1849 to 1960. the same year underwood (1972) published a catalogue of victorian earthquakes with intensities, dating back to 1841. the first and only historian not also an earthscientist, to investigate and publish information on historical earthquakes in australia was cynthia hunter. her self-published book on earthquakes in the hunter region of new south wales 1788-1989 (hunter, 1991) was ready for publication when the 1989 newcastle earthquake occurred. a very remarkable sequence of earthquakes occurred northwest of tasmania in the period 1883 to 1892 (carey and newstead, 1960; ripper, 1963). in the first three years, 2540 earthquakes were felt in nw tasmania, some of them throughout the island and up to 600 km away in kiama new south wales, 50 km south of sydney. these events are shown on mallet’s (1858) map. michael-leiba (1989) drew isoseismal maps for the larger tasmanian earthquakes of the series and the later coincident 1946 event. rynn (in rynn and others, 1987) searched newspapers and other sources to make a comprehensive list of earthquakes in queensland, and prepared isoseismal maps for the larger earthquakes dating back to 1841. 3. some damaging earthquakes earthquakes are not widely considered a significant threat in australia but as the historical and recent record demonstrates, earthquake risk is increasing with time as the population expands both numerically and spatially as discussed below. people seem to have a short memory about earthquakes except for those that cause great loss of life or property. no deaths or significant structural damage were observed in the 19th and late 18th centuries fig. 1. a 1902 daily telegraph newspaper map of earthquakes in eastern australia compiled by edgeworth david. 390 kevin mccue in australia. since then fifteen deaths resulted from two earthquakes in australia; in 1902 a magnitude 6.0 earthquake caused two deaths by heart attack in adelaide, and in 1989 twelve people died in damaged buildings and a thirteenth of a heart attack in newcastle. the 1989 magnitude 5.6 newcastle earthquake was the most damaging natural disaster in australia’s modern history to date, causing more than $a 1.2 billion damage. most of this damage was to houses, either unreinforced masonry or timber framed houses with unreinforced masonry footings and chimneys. fourteen schools, two hospitals and the ambulance and fire stations also suffered extensive damage. the pattern of damage in newcastle in 1989 mirrored that in the previous most damaging australian earthquake, in adelaide south australia in 1954. this caused about $a120m damage, most of it to unreinforced masonry buildings. instrumental data led the international seismological summary (iss) to locate the epicenter 400 km south of adelaide. bolt (1956), whilst a phd student of bullen at sydney university, studied this earthquake and was able to relocate it close to the eden-burnside fault south of the city where the damage and intensity was greatest. a major hospital and a university now stand at the site of the epicentre. most australian houses have earthquake insurance cover because it is offered free with fire insurance though the deductible amount varies between companies. the problem with insurfig. 2. seismicity of australia 1788 to 2000. the dot size is related to magnitude, the smallest magnitude 4, the largest magnitude 7.2. 391 australia: historical earthquake studies ance is that it will only cover the cost of restoring the building to its pre-earthquake state, not strengthened or made more resilient to prepare for the next damaging earthquake. a map of known earthquakes of magnitude 4 or more between 1788 and 2000 is shown in fig. 2. the historical events form a subset of the pattern of recent events. the occurrence of recent large earthquakes, including five of the world’s ten known intraplate surface-rupturing events in the 20th century (johnston and kanter, 1990), should dispel the notion that earthquake risk is negligible in australia. the very short instrumental record makes it difficult to be confident when selecting source zones, recurrence rates and maximum magnitudes for either a probabilistic or deterministic seismic hazard analysis, but several attempts have been made making use of the historical data. the australian earthquake code is into a third edition since its introduction following the 1968 meckering earthquake (bubb, 1999). the goal of the code is life safety but in only a small percentage of australian buildings have engineers used earthquake engineering principles in their design and construction. consequently, the next m 5+ earthquake near an urban area will cause a repeat of the damage at adelaide and newcastle. nothing will have been learned from history. the australian earthquake engineering society (aees) was established following the 1989 newcastle earthquake to educate the public and engineers about earthquakes in australia, to affiliate with the international association of earthquake engineering and to promote links with the new zealand society for earthquake engineering. it became a professional society of the institution of engineers australia and many of its members are on the earthquake loading code committee. 4. the frequency of earthquakes in australia the australian continent is wholly intraplate but suffers a relatively high level of seismicity for such an environment. based on the number and magnitude of recorded events since 1900, the seismicity can be approximately described by a recurrence relation of the form log nc = 5.3 − m where nc is the cumulative number of earthquakes per year of magnitude m or more (richter or local scale (ml) for magnitude 6.0 or less and surface-wave scale (ms) above magnitude 6.0). in the last 100 years in australia the largest known earthquake had a magnitude of ms 7.2 and there were about 20 earthquakes that were at least magnitude ms 6.0. from the above equation, the expected once-per-year earthquake has a magnitude (richter scale) of 5.3, the ten year event magnitude m 6.3. earthquakes are not randomly distributed across australia, and nor is the population, most australians live in cities in a narrow coastal belt in southeastern australia, so understanding the causes of intraplate earthquakes becomes a critical goal of the hazard computation process. compare the australian record with the earthquake record of the last 100 years in italy where the largest known earthquake had a magnitude of ms 7.5. there were about seven shallow independent earthquakes of magnitude ms 6.0 or more, and the once-per-year (shallow) earthquake has a magnitude of about 5.1. in new zealand, the rate of shallow seismicity (≤ 40 km) is also similar to that in australia. the land area of australia is about 25 times that of italy and 15 times that of new zealand which accounts for the higher relative hazard rating of italy and new zealand. 5. australia’s current earthquake catalogues only three independent seismic monitoring organisations now exist in australia and they maintain their own catalogues. they are the commonwealth government funded geoscience australia (ga), the privately funded seismology research centre (src) of environmental systems and services and the state government operated south australian department of primary industries and resources (pirsa). 392 kevin mccue 5.1. history of the catalogues canberra is the hub of a modern telemetered broadband national network operated by ga, some 30 stations, supplemented by networks of triaxial digital mainly triggered recorders operated by src in eastern australia and pirsa in south australia. altogether there are more than 200 seismographs currently installed in australia. ga’s national network has an average station separation of about 500 km, the src and pirsa networks a station separation of about 50 km. the national detection capability is about ml 3.2 whilst src and pirsa claim ml 1.5 within their networks. in the mid-1980s state and university agencies agreed to exchange basic epicentral and arrival time information and to store the epicentre data in a datafile held by the commonwealth government’s bureau of mineral resources (renamed australian geological survey organisation and now geoscience australia). this datafile was migrated to an oracle relational database in 1991/92 (lenz et al., 1992). geoscience australia and src operate independent earthquake alarm systems. 5.2. description of the national catalogue geoscience australia’s database quakes is a world earthquake database built on the iss, isc and noaa historical catalogues. all known solutions are included for each event with a flag marking the preferred solution. a link has been established to the ga tsunami database (mccue and lenz, 1996) so that the preferred earthquake solution can be linked to observations in the tsunami database. most of the tsunamis along the australian coast originated with major earthquakes around the circum-pacific belt or banda sea arc. the largest tsunamis along the southeast coast were from south america in 1868, 1877 and 1960, and along the northwest coast from the 1883 krakatoa eruption and 1977 sunda earthquake. agencies solicit felt reports for significant or widely felt earthquakes, by questionnaire mailouts, phone and recently by email. there were ten maps compiled each year on average in the 1990s compared with a total of some 70 maps for the historical pre-1900 period. a table has been set up in quakes for intensities which includes the event identification, place name, state, latitude, longitude and intensity with a link to the australian place names gazetteer to allow selection of the place name and automatic download of the latitude, longitude and state. isoseismal maps have been drawn up for some 450 earthquakes. 6. isoseismal maps many observers and researchers have published isoseismal maps in australia, using first the rossi-forel, then the modified mercalli scale (eiby, 1966). ian everingham initiated a project to collect all known maps and reproduce them in a standard format and on the modified mercalli scale. these were subsequently published in three isoseismal atlases (everingham et al., 1982; rynn et al., 1987; mccue, 1996). the magnitude range represented by the maps is from ml 1.6 to ms 7.2 and the period range 1841 to 2000. reports of the australasian association for the advancement of science covering the period 1892 to 1913 provided a valuable reference source for intensities. news articles and letters to the editor in local and regional newspapers were the main source of information. other useful sources were lighthouse keepers’ logs and weather reports compiled by a wide network of meteorological observers. these valuable records have been archived. the occasion of the centenary and then bi-centenary of australia triggered the publication of a number of regional historical studies that include anecdotal information on important local earthquakes. with two exceptions historical earthquake reports have all been written in english. the german naturalist leichhardt happened to be in newcastle (new south wales) when the october 1842 earthquake occurred there and his letter to a friend in england (translated by s. lenz, personal communication) regarding the felt effects was reproduced by mccue (1996). sieberg (1932), another german naturalist and explorer, published a list of earthquakes in aus393 australia: historical earthquake studies tralia and papua new guinea that was translated by g. hoch (personal communication). the many technical terms in these reports, especially the latter, were a challenge for non-geologist german speakers. perrey (1844-1873) too was consulted whilst the author was a student in london but the reports often consisted of «australian meridian sans details». 7. magnitudes from isoseismal maps the location of historical earthquakes is usually taken as the centre of maximum damage or the centre of the felt area. focal depth is difficult to estimate when the reporting population is sparse. the one missing parameter from all these early catalogues which prevents their use for hazard analyses and addition to databases is the earthquake magnitude. several attempts have been made to use the australian intensity information to estimate magnitude as shown in fig. 3. there is a linear relationship between magnitude and felt area, but there may be some unexplained change in the way magnitudes were computed about the mid 1970s. 7.1. radius of perceptibility mccue (1980) derived a relationship between magnitude m (richter scale equivalent) and rp, the radius of a circle equivalent in area to that enclosed by the mm iii isoseismal for australian earthquakes with measured magnitude ranging from 3.6 to 7.0 m = 1.03 ln rp + 0.13. (7.1) this proved to be a simple if approximate method to assign magnitudes for pre-instrumental earthquakes or recent earthquakes that were widely felt but too close or too far from the nearest seismograph to measure the magnitude. the aim of this study was to determine the magnitudes of important historical earthquakes for which isoseismal maps had been drawn. 7.2. other intensities greenhalgh et al. (1988) extended this study using ten times more earthquakes in a similar magnitude range to derive relationships between magnitude and i0, iiii, iiv ml = 0.35 (0 ± .12) (logriii)2 + 0.63 (± 0.41) ⋅ ⋅ (logriii) + 1.87 (± 0.36) (7.2) ml = 0.38 (0 ± .19) (logriv)2 + 0.49 (± 0.66) ⋅ ⋅ (logriv) + 2.38 (± 0.57) (7.3) ml = 1.35 (± 0.34) + 0.57 (± 0.06) i0. (7.4) where i0, riii, riv are the epicentral intensity and radius of mm iii and mm iv isoseismals respectively. they found that the best estimate of magnitude with the lowest rms deviation was obtained using riii (equivalent to rp and iiii). 7.3. the completeness interval it is supposed that in australia all the m 6 or greater earthquakes during the 20th century have been documented, possibly as far back as 1890. the record is not so good for smaller events as indicated in table i. as the networks fig. 3. plot of magnitude versus radius of perceptibility rp for pre 1977 (diamond) and post 1977 (squares) earthquakes. 394 kevin mccue improved so the detection and location capability improved. the record is better for small areas in the vicinity of local networks established after the international geophysical year in southeastern australia, tasmania and south australia in 1957/1958. 8. the last 20 years records of historical earthquakes are not complete, but it would be surprising if any large earthquakes m ≥ 6 had been missed since about 1890 when most parts of the continent were populated by europeans, and a national telegraph network was in place. unfortunately the historical record can not extend back past 1788 so the resulting catalogue is still a short sample for hazard assessments. this has motivated australian seismologists to get involved in paleoseismological studies. recent more detailed examination of intensity observations has encouraged study of the relationship between intensity and soil foundations, their depth and shear-wave velocity. 8.1. microzonation isoseismal maps show that the maximum intensity is not always at the epicentre. this was the case at newcastle in 1989 where the maximum damage was 15 km from the epicentre. this observation led to an examination of foundation soils throughout the mesoseismal area and the realisation that shallow sediments correlated closely with damage in one and two storey un-reinforced masonry buildings (institution of engineers, 1990). this in turn led seismologists to undertake a nakamura-type study of the area and collate all known borehole log records (somerville and others, 1993). resonance observed in ground motion spectra generally coincided with the computed quarter wavelength. 8.2. paleoseismology if the 20th century earthquake history of australia is typical, and if all australian earthquakes are shallow, if most large earthquakes have reverse mechanisms and most of them cause surface ruptures then there should be at least 100 recent fault scarps detectable. surface faults in australia since 1968 show that such scarps are about 30 to 40 km long and have vertical surface displacements of about 2m. such features should be observable which would give us better confidence that the historical record was indeed a representative sample of large earthquakes. a comprehensive paleoseismology study would cast light on the concept of whether a maximum magnitude exists for continental australia. the current view of seismologists is that m 7.5 represents a reasonable upper bound. fewer than twelve pre-historic recent fault scarps have been noted during routine geological mapping (mccue, 1990; 2001). four of these have been investigated and mapped by paleoseismologists. satellite photographs of the prominent surface ruptures of the 20th century have been examined but the resolution is still not sufficient to detect such features unless the photo interpreter knew they were there. air photographs have better resolution but some of the mapped scarps are still difficult to see because of erosion and lack of geographical features. table i. magnitude completeness intervals in australia. magnitude ≥ 6.0 ≥ 5.0 ≥ 4.0 ≥ 3.0 period 1901196319751995reason first seismic network and telegraph line world-wide seismograph network arrays set up in central australia improved and expanded networks 395 australia: historical earthquake studies notwithstanding the problems, a systematic search of these airphotos should be made to locate possible scarps followed by ground reconnaissance to map and trench each scarp, to date them and confirm their cause. 9. case histories 9.1. the 1989 newcastle earthquake a shallow magnitude ml 5.6 earthquake struck the regional city of newcastle new south wales at 10:27 a.m. on 28 december 1989. thirteen people were killed and 120 seriously injured, nine died in the partial failure of one building and three pedestrians were crushed under shop awnings which collapsed into the street. at the time, schools and colleges were closed and most workers were on christmas holidays so injuries and deaths were relatively few. the first question asked of seismologists by the local council charged with response and recovery was: what is the chance of a damaging aftershock in the next 12 hours? rescue workers were still trying to find injured people in the partially collapsed building and were at great risk of being killed themselves if a damaging aftershock occurred. the lack of knowledge of past events meant that a considered answer could not be given. two years later this information was available (hunter, 1991): past earthquakes of about magnitude 5 near newcastle in 1837, 1841, 1842, 1868 and 1925 were not followed by aftershocks, at least not large enough to be commented on in contemporary newspapers. these earthquakes ruptured palaeozoic rock under mesozoic sediments of the northern sydney basin. on the other hand, earthquakes of similar size under the southern sydney basin in 1961 and 1973 did have extensive aftershock sequences and on that basis the rescuers were ordered from the building until it could be made safe. 9.2. tennant creek northern territory on 22 january 1988 between local noon and midnight, three large earthquakes shook tennant creek in the northern territory. the earthquakes were rated mw 6.3, 6.4 and 6.7 and caused a 35 km long, 2 m high fault scarp. most buildings in the town including the local hospital suffered considerable non-structural damage, but there was remarkably little real damage given that the epicentres and surface scarps of the three events were only 40 km from the town. a paleoseismological study of the fault scarps was made in 1992 (crone et al., 1997). they dug two 25 m long trenches through the western lake surprise fault and could not conclusively demonstrate that an ancient fault scarp existed at the first trench site but did find our best evidence of prehistoric surface rupturing in the second trench. similar short trenches through the eastern lake surprise and kunayungku scarps showed no evidence of previous faulting. this study led to speculation that large earthquakes in australia do not recur at the same place, at least not within the last 10 000 to 100 000 years. some engineers have concluded from this that the site of an historic large event may be a safe place to build a high hazard facility. this is not the philosophy of the working group who compiled the current australian building code hazard map. so what does the historical record tell us? the australasian association for the advancement of science (aaas) has provided the foltable ii. earthquake felt reports northern territory 27 august 1883, ~10 am local time. daly waters explosion like blasting and vibration alice springs two distinct explosions sheep camp (9 miles w of alice springs) ditto undoolya ditto 396 kevin mccue lowing few reports (table ii) of a previous earthquake in the northern territory in 1883. eminent authors of this report of committee no. 1 of aaas assigned intensity rf3 (equivalent to mm3) to the felt reports. given the time of day this assigned intensity may be on the low side. daly waters and alice springs are about 800 km apart and almost equidistant from tennant creek. if the epicentre was about midway between them then this felt area would correspond to an earthquake with a magnitude of at least ms 6.2 from eq. (7.1). the further the epicentre is east or west of tennant creek, the larger the earthquake magnitude would have to have been. the aaas authors mention that daly waters was struck by an earlier earthquake at about midnight the previous day, sufficiently intense to wake sleepers. data are sparse but we conclude that the 1988 large earthquake sequence near tennant creek was preceded by one, possibly two large earthquakes 105 years earlier, not 10 000 years ago. as noted above, the sequence of large events off ne tasmania between 1883 and 1892 was followed by a large event in 1946. therefore the code committee premise is not unreasonable. we should be wary of accepting the alternative suggestion that sites of large recent earthquakes are relatively safe from future large earthquakes. 10. discussion the current database of historical and instrumental earthquakes is very sparse despite all the historical studies undertaken to date. that has serious implications for hazard analyses. if australian earthquakes are randomly distributed across the continent (as argued by some engineers), the hazard at any point is reduced to below the level considered necessary to provide for earthquake-resistant design. most australian seismologists do not agree with this hypothesis. in particular, large earthquakes (m ≥ 6) in the last 100 years are distributed in the approximate ratio of 3:2:1 from archaean western to proterozoic central and mesozoic eastern australia. the australian population is clustered along the southeast coastline so associated vulnerable structures, infrastructure and lifelines are sparsely distributed. conversely, oil and gas pipelines, highways and railways are extensive and have high exposure. the most sensitive structures apart from schools and hospitals (which have not been designed to resist earthquakes) are an animal health laboratory near geelong (note the earthquake reference by aborigines in the introduction) and a new experimental nuclear reactor near sydney. these two structures have been designed using earthquake engineering principles. a hazardous chemical waste and a radioactive waste material storage facility are planned but final sites have not yet been selected. useful information has been gleaned from the historical record on: i) the frequency of large and moderate earthquakes; ii) the spatial extent of possible source zones; iii) the contribution to damage caused by magnification of ground shaking within the foundation soils and; iv) the likelihood of potentially damaging aftershocks in different regions. the historical information was used in the hazard map of australia compiled for the current loading code and for the global seismic hazard assessment program (gshap). intensity remains the strong motion parameter of choice in australia because of the lack of accelerographs. a few accelerographs have been installed, two in each city with a population exceeding 50 000 residents. half of the instruments are on rock, the other half on typical foundations for that city. additional accelerographs are on important structures, mainly large dams. valuable data has already been recorded on most instruments from small and moderate regional earthquakes and large distant interplate earthquakes. earthquakes near m 8 in indonesia shake buildings in darwin and tall buildings in perth and adelaide (more than 2000 km away), so strongly that buildings have been evacuated. to date no building has suffered observable damage but the number and height of tall buildings is growing rapidly. to investigate the long-term frequency of large shallow earthquakes and source zone models, and to tackle the uncertainty in estimates of maximum magnitude, a systematic australia-wide paleoseismological study needs to be undertaken to extend the historical record. 397 australia: historical earthquake studies acknowledgements i would like to thank sonja lenz and the two anonymous referees for their valuable comments on the first draft of this paper. references barbetti, m. and m.j. sheard (1981): paleomagnetic results from mounts gambier and schank, south australia, j. geol. soc. aust., 28, 385-394. bolt, b. 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(1972): studies of victorian seismicity, proc. r. soc. victorian, 85, 27-48. s u l l a p o s s i b i l i t à ' d i r e a z i o n i a c a t e n a n e l l a c r o s t a t e r r e s t r e ( * ) c . f e s t a f . i p p o l i t o 1. -—• l a s c o p e r t a d e l l a f i s s i o n e d e l l ' u r a n i o e d e l l e r e a z i o n i a cat e n a ( j ) i n d u s s e a l c u n i g e o l o g i a d a t t r i b u i r e , sia p u r e in m a n i e r a dub i t a t i v a , a q u e s t i p r o c e s s i l ' o r i g i n e di f e n o m e n i c o l l e g a t i con il prob l e m a del b i l a n c i o t e r m i c o t e r r e s t r e e cioè la f o r m a z i o n e dei g r a n i t i (-) e il v u l c a n i s m o ( : ! ). a l l ' e p o c a d e l l a s c o p e r t a d e l l a f i s s i o n e , il f e n o m e n o d e l l a i n t e r a z i o n e dei n e u t r o n i con la m a t e r i a n o n e r a c o m p l e t a m e n t e c o n o s c i u t o e d ' a l t r a p a r t e m a n c a v a q u a l s i a s i i n f o r m a z i o n e r e l a t i v a a d u n a e v e n t u a l e p r e s e n z a di n e u t r o n i n e l l ' i n t e r n o d e l l a t e r r a . s i c c h é q u a n t u n q u e u n o d e g l i a u t o r i ( 4 | a v e s s e s p i n t o a b b a s t a n z a a v a n t i la t r a t t a z i o n e t e o r i c a del p r o b l e m a , non si p o t è g i u n g e r e ad a l c u n a conc l u s i o n e p o s i t i v a p e r m a n c a n z a di d a t i s p e r i m e n t a l i . p o i c h é sotto q u e sto a s p e t t o la s i t u a z i o n e è o g g i c o m p l e t a m e n t e d i f f e r e n t e , v a l e la p e n a di r i p r e n d e r e l a q u e s t i o n e p e r m o s t r a r e c o m e i n u m e r o s i d a t i sper i m e n t a l i , a c q u i s i t i d a l 1945 in p o i , p r o v a n o c h e l e r e a z i o n i a c a t e n a 11011 p o s s o n o s u s s i s t e r e n e l l a c r o s t a t e r r e s t r e . c o m i n c i a m o col c o n s i d e r a r e le c o n d i z i o n i fisiche e c h i m i c h e n e l l e (piali si v e r i f i c a n o i p r o c e s s i c o n s i d e r a t i . è o r m a i c o m u n e m e n t e acc e t t a t o il m o d e l l o di u n a t e r r a c o m p o s t a da u n c e r t o n u m e r o di s t r a t i s f e r i c i c o n c e n t r i c i . q u e s t a s t r u t t u r a c o n c e n t r i c a si d e s u m e , c o m ' è n o t o , d a v a r i d a t i s p e r i m e n t a l i r i g u a r d a n t i la d e n s i t à e la s u a v a r i a z i o n e con la p r o f o n d i t à , la v e l o c i t à d e l l e o n d e s i s m i c h e , la r i g i d i t à ecc. l'issi i n d i c a n o u n a b r u s c a v a r i a z i o n e d e l l e p r o p r i e t à fisiche del c o r p o t e r r e s t r e p e r d e t e r m i n a t i v a l o r i d e l l a p r o f o n d i t à . l a q u e s t i o n e s e a d o g n i v a r i a z i o n e d e l l e p r o p r i e t à fisiche c o r r i s p o n d a u n a v a r i a z i o n e d e l l o s t a t o di a g g r e g a z i o n e o d e l l a c o m p o s i z i o n e c h i m i c a è a n c o r a in dis c u s s i o n e . r e s t a c o m u n q u e s t a b i l i t a la p a r t i z i o n e d e l l a t e r r a n e l l e t r e p a r t i p r i n c i p a l i : n u c l e o , m a n t e l l o e c r o s t a . f i n o a p o c h i a n n i f a si a t t r i b u i v a s e n z a d i s c u s s i o n e al n u c l e o u n a c o m p o s i z i o n e s i m i l e a q u e l l a d e l l e m e t e o r i t i f e r r o s e . n e l 1941 k u h n e r i t t m a n n (r'i h a n n o a v a n z a t o l ' i p o t e s i c h e il n u c l e o n o n s i a costi( * l c o m u n i c a z i o n e p r e s e n t a l a a l c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i l i c a i t a l i a n a , t e m i l o a r o m a il 28-29 m a g g i o 1 9 5 4 . ! 18 c . f k s t a f . i p p o l i t o t u i t o (la f e r r o n i c h e l m e t a l l i c o , m a d a m a t e r i a s o l a r e i n d i f f e r e n z i a t a ; a l d i s o p r a di q u e s t a p a r t e c e n t r a l e v i s a r e b b e u n o s t r a t o m o l t o r i c c o d i f e r r o e di a l t r i e l e m e n t i p e s a n t i . s e c o n d o q u e s t i a u t o r i le discont i n u i t à s i s m i c h e n o n s o n o u n i n d i c e d i v a r i a z i o n e d e l l a c o m p o s i z i o n e c h i m i c a ai l i v e l l i c o r r i s p o n d e n t i b e n s ì di u n b r u s c o c a m b i a m e n t o d e l l e p r o p r i e t à f ì s i c h e . a d u n a r e p e n t i n a v a r i a z i o n e d e l l e p r o p r i e t à fisiche d e l l a m a t e r i a a t t r i b u i s c e l ' o r i g i n e d e l l e d i s c o n t i n u i t à a n c h e w . h. r a m s e y (®ì, c h e c o n s i d e r a il m a n t e l l o ed il n u c l e o c o m e f a s i t e r m o d i n a m i c h e d i f f e r e n t i di u n a i d e n t i c a s o s t a n z a . è i n v e c e g e n e r a l m e n t e accett a t a l ' i p o t e s i s e c o n d o la q u a l e a il m a n t e l l o » si c o m p o n e d i s i l i c a t i b a s i c i a c c o m p a g n a t i da u n a f a s e m e t a l l i c a , d i e va d e c r e s c e n d o al d i m i n u i r e d e l l a p r o f o n d i t à . p e r l ' e s a m e d e i f e n o m e n i , di c u i v o g l i a m o q u i o c c u p a r c i , d o b b i a m o p e r a l t r o c o n s i d e r a r e s o l t a n t o l a c r o s t a t e r r e s t r e e la z o n a m a g m a t i c a a d essa i m m e d i a t a m e n t e s o t t o s t a n t e , c o m e q u e l l e c h e p a r t e c i p a n o ai f e n o m e n i g e o l o g i c i di cui a b b i a m o f a t t o c e n n o s o p r a . s e c o n d o le p i ù r e c e n t i i n t e r p r e t a z i o n i dei «lati s i s m o l o g i c i ("l, c h e in l i n e a di m a s s i m a c o i n c i d o n o con le d e d u z i o n i di c a r a t t e r e geolog i c o e p e t r o g r a f i c o ( 8 ) ( ° ), la c r o s t a t e r r e s t r e p u ò c o n s i d e r a r s i c o s t i t u i t a d a t r e g u s c i s o v r a p p o s t i , d a l l ' a l t o in b a s s o : s i a l i, s i a l l i , s i m a . 11 s i a l i è il g u s c i o s i a l i c o s u p e r i o r e , c o s t i t u i t o p r e v a l e n t e m e n t e «la r o c c e d i t i p o g r a n i t i g n e i s s con v e l o c i t à «li p r o p a g a z i o n e «ielle o n d e s i s m i c h e l o n g i t u d i n a l i , v oo 5,6 k m s e c : «piesto g u s c i o non è c o n t i n u o e d h a u n o s p e s s o r e c h e v a r i a da 40 k m ed o l t r e , n e l l e z o n e o r o g e n i c h e , fino a d a n n u l l a r s i in t a l u n e z o n e o c e a n i c h e ( a d e s e m p i o : o c e a n o p a c i f i c o ) . il s i a l 11 è un g u s c i o i n t e r m e d i o ( o v e v oo 6,3 k m / s e c ì , di c o s t i t u z i o n e a n c o r a a l q u a n t o c o n t r o v e r s a : s e c o n d o rittm a n n ( 1 0 ) ( u ) si t r a t t e r e b b e di m a t e r i a l e o r i g i n a r i a m e n t e g r a n i t i c o , d e p a u p e r a t o d e l l ' e u t e t t i c o q u a r z o f e l d s p a t o e q u i n d i di t i p o d i o r i t i c o l a m p r o f i r i c o . e s s o h a u n o s p e s s o r e v a r i a b i l e d a u n a v e n t i n a di chilom e t r i ( z o n e c o n t i n e n t a l i ) a z e r o ( a r e e d i b a s s o c r a t o n e ) . i l s i m a è a l l ' i n c o n t r o u n g u s c i o c o n t i n u o , o v e v oo 7,9 k m sec, c o s t i t u i t o d a m a g m a di t i p o b a s a l t i c o — m a g m a «« o r i g i n a r i o » sec. rittm a n n —• s o l i t l i f i c a t o . l o s p e s s o r e del s i m a v a r i e r e b b e tra i 2 5 e i 30 k m . l a c r o s t a t e r r e s t r e a v r e b b e , in t o t a l e , s p e s s o r i v a r i a b i l i d a u n a t r e n t i n a d i k m di s o l o s i m a ( n e l l e z o n e o c e a n i c h e ) a d u n m a s s i m o d i u n a s e t t a n t i n a d i k m di s i a l i, s i a l l i e s i m a ( n e l l e z o n e c o n t i n e n t a l i di a l t o e r a t o n e l ( v e d i fig. 11. i m m e d i a t a m e n t e al d i s o t t o d e l l a c r o s t a t e r r e s t r e — e le m a g g i o r i p r o f o n d i t à e s u l a n o , c o m e a b b i a s u l l a p o s s i b i l i t à iii r e a z i o n i a c a t e n a n e l l a c r o s t a t e r r e s t r e 4 l i i ino a c c e n n a t o , d a q u e s t a t r a t t a z i o n e — v i è il s i m a l'uso: m a g m a b a s a l t i c o r i c c o di g a s s e c o n d o la m a g g i o r a n z a d e i g e o l o g i e dei p e t r o g r a f i . è a n c o r a d a t e n e r p r e s e n t e c h e s u l s i a l i p o g g i a , in t a l u n e a r e e c o n t i n e n t a l i , un e s i l e s p e s s o r e di s e d i m e n t i p i ù o m e n o m e t a m o r f o s a t i . si t r a t t a o r a di e s a m i n a r e se a l l ' i n t e r n o d e l l a c r o s t a c o m e s o p r a o c e a n o p a c i f i c o o r o g e n e c o n t i n e n t e f i g . 1 • s e z i o n e s c h e m a t i c a a t t r a v e r s o la c r o s t a t e r r e s t r e c o n t i n e n t a l e e s o t t o o c e a n i c a i d i t i p o p a c i f i c o ! c o n i n d i c a z i o n e d e l l e g e o i s o t e r m e i d a r i t t m a n n ) . d e f i n i t a : n l s o n o p r e s e n t i e l e m e n t i fissionabili: b i s i t r o v a n o n e u t r o n i in g r a d o d i p r o d u r r e l a f i s s i o n e : ci s i v e r i f i c a n o l e c o n d i z i o n i n e c e s s a r i e p e r l ' i n n e s c o di l i n a r e a z i o n e a c a t e n a . 2. — g l i e l e m e n t i fissionabili n a t u r a l i s o n o c o m ' è n o t o l ' u r a n i o e il t o r i o . s e a b o r g e c o l i . i 1 ! h a n n o r i v e l a t o la p r e s e n z a in n a t u r a di pi,-»', e l e m e n t o t r a n s u r a n i c o fissionabile u s a t o n e i r e a t t o r i n u c l e a r i : m a le c o n c e n t r a z i o n i m i s u r a t e d a s e a b o r g s o n o t a l m e n t e b a s s e c h e q u e s t o e l e m e n t o p u ò e s s e r e s e n z ' a l t r o s c a r t a t o n e l l e n o s t r e c o n s i d e r a z i o n i . l a fissione di e l e m e n t i di n u m e r o a t o m i c o i n f e r i o r e a 9 0 è stala o t t e n u t a s o l o d a p o c o m e d i a n t e n e u t r o n i d i e n e r g i a e l e v a t i s s i m a 184 \ i e v i | , : , | . v e d r e m o in s e g u i t o c h e s u l l a t e r r a n o n si h a n n o a d i s p o s i z i o n e n e u t r o n i c o n e n e r g i a t a n t o e l e v a t a . p e r c i ò n e l l a d i s c u s s i o n e r e l a t i v a a l l a p o s s i b i l i t à di r e a z i o n i a cai s l n o n vi è a c c o r d o s o s t a n z i a l e s u l c h i m i s m o d i q u e s t o m a g m a , s e c o n d o a l c u n i d i t i p o llioleiilico ( a d e s . n i g g l i l , s e c o n d o a l t r i d i t i p o oliviniro ( a d e , r i t t m a n n i . m a c i ò n o n ha i m p o r t a n z a ai n o s t r i fini. ! 18 c . f k s t a f . i p p o l i t o t e n a s p o n t a n e e c o n v i e n e c o n c e n t r a r e l ' a t t e n z i o n e su l ' u r a n i o e il t o r i o , q u a l i e v e n t u a l i c o m b u s t i b i l i d e l l a r e a z i o n e . è b e n n o t o c l i c il c o n t e n u t o d i u r a n i o e d i t o r i o n e l l e r o c c e d i m i n u i s c e c o n l a a c i d i t à di q u e s t e . q u e s t o f a t t o p r o v a c h e g l i e l e m e n t i r a d i o a t t i v i s o n o p r e v a l e n t e m e n t e c o n c e n t r a t i n e l l a c r o s t a . d a l l e a n a l i s i d e l c o n t e n u t o di u r a n i o e t o r i o n e l l e r o c c e e d a l l e a b b o n d a n z e i s o t o p i c h e d e i s i n g o l i n u c l i d i c h e li c o m p o n g o n o , si p o s s o n o d e d u r r e i v a l o r i m e d i d e l l a c o n c e n t r a z i o n e di l / 2 3 8 , l / 2 3 5 e th-:i2 nei v a r i s t r a t i d e l l a c r o s t a t e r r e s t r e . q u e s t i v a l o r i s o n o r i p o r t a t i n e l l a t a b e l l a 1 d a l l a q u a l e r i s u l t a e v i d e n t e la d i m i n u z i o n e n e l c o n t e n u t o t o t a l e .in j7~3i>, f / ; i s e t/t 2 : 1 2 nel p a s s a g g i o d a l s i a l i ( g r a n i t i c o ) a l l o s t r a t o di s i a l ii e d a q u e s t o al s i m a . t a b e l l a 1 s t r a t o jj 1*s 77r'-'ì2 s t r a t o x x 1 0 1 8 g x 1 0 1 8 g s e d i m e n t i i 0 , 0 6 8 s i a l i ) ( 126 0.9 4 4 4 s i a l i i j 126 0.9 s i m a 31 0.2 131 i f e n o m e n i d e i q u a l i ci i n t e r e s s i a m o h a n n o s e d e n e l l a c r o s t a e s p e c i e in q u e l l e z o n e o v e s o n o p r e s e n t i s i a l e s i m a . o r a l ' u r a n i o e il t o r i o p o s s o n o c o n s i d e r a r s i c o m e e l e m e n t i « d i s p e r s i » n e l s e n s o c h e a q u e s t a e s p r e s s i o n e è s t a t o a t t r i b u i t o d a y e r n a d s k y e si c o m p o r t a n o , n e i p r o c e s s i g e o c h i m i c i , c o m e e l e m e n t i litofili, nel s e n s o di g o l d s c h u i i d l ( l u ) o a n c h e c o m e jmi'iimatofili n e l s e n s o di r i t t m a n n i " ' i . s e c o n d o q u e s t o u l t i m o i n o l t r e la p o v e r t à in li e tli d e l l a z o n i d e l s i a l i i è c a u s a t a p r o p r i o d a q u e s t o c a r a t t e r e g e o c h i m i c o di t a l i elem e n t i . 11 s i a l l i , s e c o n d o l ' i n t e r p r e t a z i o n e di v a r i a u t o r i , s a r e b b e c o m e g i à a c c e n n a t o , c o s t i t u i t o d a r o c c e g r a n i t i c h e d e p a u p e r a t e s i a d e l l ' e u t e t t i c o q u a r z o f e l d s p a t o s i a di q u e g l i e l e m e n t i , e l l e , c o n t a l e e u t e t t i c o , tend o n o a c o n s o l i d a r e n e l l e s o l u z i o n i m a g m a t i c h e r e s i d u e , t r a i (piali a p p u n t o l ' u r a n i o e s u b o r d i n a t a m e n t e il t o r i o . t a l e d e p a u p e r a m e n t o sar e b b e a v v e n u t o s p e c i a l m e n t e p e r e f f e t t o d e l « r a s t r e l l a m e n t o » o p e r a t o d a l p a s s a g g i o di « f l u i d i » p r o v e n i e n t i d a l l e p r o f o n d i t à s u b c r u s t a l i l ' i. i * ) s a r e b b e a p p u n t o d o v u t o a f e n o m e n i ili q u e s t o t i p o il f a t t o c o n s t a t a t o c h e i g r a n i t i g e o l o g i c a m e n t e p i ù r e c e n t i , e l l e h a n n o c i o è s u b i t o p i ù c i c l i p e t r o g e n e t i c i , a r r i c c h e n d o s i s e m p r e p i ù d i e l e m e n t i l i t o f i l i , s o n o p i ù r i c c h i d i u e iii d i q u e l l i p i ù a n t i c h i . v e d i r i t t m a n n i 1 t >, l a b . a p a g g . 2 4 8 2 4 9 . sulla p o s s i b i l i t à iii r e a z i o n i a c a t e n a n e l l a c r o s t a t e r r e s t r e 4 lii è i n o l t r e d a t e n e r e b e n p r e s e n t e c h e l ' u r a n i o n o n è c o n t e n u t o n e l l e r o c c e i g n e e di t i p o g r a n i t i c o s e n o n i n s o s t i t u z i o n e i s o n i o r f o g e n a di tli o di a l t r i e l e m e n t i di p r o s s i m o r a g g i o i o n i c o (ti, zr, e c c . ì in m i n e r a l i a c c e s s o r i di p r i m i s s i m a c o n s o l i d a z i o n e e d è p e r t a n t o e s t r e m a m e n t e « d i s p e r s o » . v a p o i a c o n c e n t r a r s i , p e r i b e n n o t i p r o c e s s i m a g m a t i c i , c o m e o s s i d o c r i s t a l l i z z a t o o n o n ( u r a n i n i t e e p e c h b l e n d a l in f o r m a z i o n i p e g m a t i t i c h c , p n e u m a t o l i t i c h e o i d r o t e r m a l i , c h e n o n h a n n o a l c u n a i m p o r t a n z a , a c a u s a d e l l e l o r o d i m e n s i o n i g e o l o g i c h e , p e r i p r o b l e m i di c u i t r a t t i a m o , s e b b e n e c o s t i t u i s c a n o i « g i a c i m e n t i » , c u i si a p p r o v v i g i o n a l ' u o m o . e c o m u n q u e si t r a t t a di f o r m a z i o n i p r e s e n t i p e r la l o r o g e n e s i g e o l o g i c a , n e l l a p a r t e più e s t e r n a d e l l a l i t o s f e r a ( 1 7 l . q u e s t e c o n s i d e r a z i o n i , d e r i v a n t i d a l l e c a r a t t e r i s t i c h e g e o c h i m i c h e d i u e th, i n d u c o n o p e r t a n t o a r i t e n e r e c o m e a l t a m e n t e i m p r o b a b i l e l ' e s i s t e n z a d i c o n c e n t r a z i o n i r i l e v a n t i di q u e s t i e l e m e n t i , c h e c o s t i t u i s c o n o il « c o m b u s t i b i l e » d e l l e r e a z i o n i a c a t e n a . c i ò p e r q u a n t o c o n c e r n e l ' o r i g i n e d e l l e r o c c e g r a n i t i c h e . p e r i f e n o m e n i vulcanici o c c o r r e a g g i u n g e r e a l l e c o n s i d e r a z i o n i d i c u i sop r a ( [ n e l l e i n f o r m a z i o n i c h e c i v e n g o n o d a l l e p i ù r e c e n t i e d a m p i e t r a t t a z i o n i in p r o p o s i t o ( , s ) ( s | ("i. 11 v u l c a n i s m o —i n t e s o c o m e m a n i f e s t a z i o n e e s t e r n a di p r o c e s s i m a g m a t i c i — si r i e o n n c t t e ai f e n o m e n i di p l u t o n i s i n o , di c u i a b b i a m o f a t t o c e n n o (li s o p r a , o v v e r o p i ù di i r e q u e n t e t r a e o r i g i n e pili p r o f o n d a — e d è q u e s t o il vulcanismo tipico c i o è d a l l e z o n e m a g m a t i c h e s o t t o c r o s t a l i ili t i p o s i m a t i c o . l a c o n c e n t r a z i o n e di l e th in q u e s t e z o n e d e l l a c r o s t a t e r r e s t r e o d e l l a z o n a m a g m a t i c a s u b c r u s t a l e , c o m e si v e d e d a l l a t a b e l l a i. e c o m e si d e d u c e d a t u t t e l e r i c e r c h e d i r e t t e s u v u l c a n i t i i n t e r m e d i e e basic h e , è n o t e v o l m e n t e p i ù b a s s a di q u e l l a d e l m a t e r i a l e s i a l i c o . n o n è q u i n d i il c a s o di i n s i s t e r e u l t e r i o r m e n t e s u q u e s t o a s p e t t o d e l p r o b l e m a . — n o n p o s s e d i a m o a l c u n d a t o s p e r i m e n t a l e r i g u a r d o a l l a pres e n z a d i n e u t r o n i n e l l a p a r t e p i ù i n t e r n a d e l l a c r o s t a t e r r e s t r e . e s s a è s t a t a p e r a l t r o p r o v a t a n e l l e r o c c e d e l l a s u p e r f i c i e t e r r e s t r e . i n e u t r o n i ivi o s s e r v a t i p r o v e n g o n o d a : il la r a d i a z i o n e c o s m i c a ; 2) l a fiss i o n e s p o n t a n e a d e l u ~ 3 s : 3l i p r o c e s s i ( a , n i p r o d o t t i d a l l e p a r t i c e l l e e m e s s e d a g l i e l e m e n t i r a d i o a t t i v i n a t u r a l i . s e a b o r g ( 1 2 ) v a l u t a a c a . 30 m i n il n u m e r o d e i n e u t r o n i g e n e r a t i in 100 g di p e c h b l e n d a cont e n e n t i c a . il 13' r di u r a n i o . a l l ' e p o c a i n c u i s e a b o r g h a f a t t o l a s u a v a l u t a z i o n e , il v a l o r e d e l l a c o s t a n t e di d e c a d i m e n t o p e r fissione s p o n t a n e a d e l f : ! s , e il n u m e r o d e i n e u t r o n i e m e s s i n e l l a fissione s p o n t a n e a e r a n o c o n o s c i u t i c o n q u a l c h e i n c e r t e z z a . f a c e n d o u s o d e i va! 18 c . f k s t a f . i p p o l i t o l o r i o t t e n u t i r e c e n t e m e n t e c o n m i s u r e |>iù p r e c i s e ( 1 ! , | si t r o v a c h e nel c a m p i o n e e s a m i n a t o d a s e a b o r g s o n o p r e s e n t i 12 n e u t r o n i m i n . v e d i a m o o r a s e d a q u e s t i d a t i r e l a t i v i ai n e u t r o n i p r e s e n t i s u l l a s u p e r f i c i e t e r r e s t r e si p u ò d e d u r r e q u a l c h e i n f o r m a z i o n e s u l l a event u a l e e s i s t e n z a di n e u t r o n i a g r a n d i p r o f o n d i t à . l a m i s u r a d e l l a intensità d e l l a r a d i a z i o n e c o s m i c a t o t a l e è s t a t a s p i n t a fino a 600 m e t r i ea. di p r o f o n d i t à ( 2 0 l . i r i s u l t a t i di q u e s t e m i s u r e i n d u c o n o a d e s c l u d e r e la p o s s i b i l i t à d i u n a p e n e t r a z i o n e d e l l e p a r t i c e l l e c o s m i c h e n e l l a p a r t e p i ù i n t e r n a d e l l a c r o s t a t e r r e s t r e . l e s o l e s o r g e n t i di n e u t r o n i effic i e n t i in q u e l l e z o n e s o n o d u n q u e la fissione s p o n t a n e a d e l l j ì s e i p r o c e s s i (a, n). p o i c h é a b b i a m o v i s t o c h e il c o n t e n u t o di e l e m e n t i r a d i o a t t i v i d i m i n u i s c e con la p r o f o n d i t à p o s s i a m o c o n c l u d e r e c h e il n u m e r o d i n e u t r o n i p r e s e n t i n e l l a p a r t e i n t e r n a d e l l a c r o s t a t e r r e s t r e è i n f e r i o r e a l n u m e r o di n e u t r o n i p r e s e n t i in s u p e r f i c i e . d u n q u e o l t r e gli e l e m e n t i r a d i o a t t i v i , clic c o s t i t u i s c o n o il c o m b u s t i b i l e d e l l a reaz i o n e , si r i d u c e con la p r o f o n d i t à a n c h e il n u m e r o dei n e u t r o n i , c i o è d e g l i a g e n t i p r i n c i p a l i d e l l a r e a z i o n e a c a t e n a . r i m a n e o r a d a s t a b i l i r e q u a l e p r o b a b i l i t à h a un n e u t r o n e , e m e s s o p e r fissione s p o n t a n e a d e l l 2 : ! s o p p u r e in un p r o c e s s o ( a . /il di innes c a r e u n a r e a z i o n e a c a t e n a . s u p p o n i a m o clic, a t t r a v e r s o uno dei d u e p r o c e s s i s u d d e t t i in u n a r o c c i a r a d i o a t t i v a v e n g a e m e s s o un c e r t o numer o di n e u t r o n i . l a l o r o e n e r g i a m e d i a è d e l l ' o r d i n e di io" el . l a p r o b a b i l i t à c h e u n n e u t r o n e di q u e s t a e n e r g i a v e n g a c a t t u r a l o «la u n n u c l e o di f,r-:ìs d a n d o l u o g o a l l a fissione del n u c l e o s t e s s o è m o l t o p i c c o l a . 1 n e u t r o n i c o n t i n u e r a n n o a m u o v e r s i e n t r o la r o c c i a s u b e n d o u n a s e r i e «li u r t i c o n t r o i n u c l e i d e g l i a l t r i e l e m e n t i c h e acc o m p a g n a n o l ' u r a n i o n e l l a r o c c i a m e d e s i m a . d u r a n t e q u e s t i urti un c e r t o n u m e r o di n e u t r o n i v e r r à a s s o r b i t o p e r c a t t u r a «la p a r t e «lei nuclei stessi. i n c o n s e g u e n z a d e g l i u r t i la e n e r g i a dei n e u t r o n i non catt u r a t i d i m i n u i s c e r a p i d a m e n t e . s e e s s a p o t e s s e r i d u r s i fino al v a l o r e t e r m i c o i n e u t r o n i c o m i n c e r e b b e r o a d i f f o n d e r e fino a q u a n d o s a r e b b e r o a s s o r b i t i d a un n u c l e o di p e r c h é la p r o b a b i l i t à «li c a t t u r a dei n e u t r o n i con e n e r g i a t e r m i c a «la p a r t e del l -:lr' è p a r t i c o l a r m e n t e e l e v a t a ( c>r= 500 b a r a s i . q u e s t o p r o c e s s o di c a t t u r a c h e dà l u o g o a l l a f i s s i o n e d e l n u c l e o d e l l: 'c o s t i t u i s c e c o m ' è n o t o il p r i m o s t a d i o d e l l a r e a z i o n e a c a t e n a . in r e a l t à i n e u t r o n i non r a g g i u n g o n o le e n e r g i e term i c h e p e r c h é d u r a n t e il p r o c e s s o di r a l l e n t a m e n t o e p r e c i s a m e n t e « p i a n d o la loro e n e r g i a si a g g i r a i n t o r n o a 7 r i la p r o b a b i l i t à «li cattura da p a r t e «lei i / 2 : ! s d i v i e n e g r a n d i s s i m a icr, = 1800 b a r a s i . si t r a t t a s u l l a p o s s i b i l i t à iii r e a z i o n i a c a t e n a n e l l a c r o s t a t e r r e s t r e 4 lii <li un p r o c e s s o l /i, y l nel q u a l e v i e n e g e n e r a t o pu-39 s e c o n d o l a reaz i o n e : uq u e s t o p r o c e s s o di c a t t u r a d a p a r t e d e l £/-: ! s è i n c o n c o r r e n z a c o n il p r o c e s s o di fissione i n d o t t a d e l v-3 r' sul q u a l e s i f o n d a n o le r e a z i o n i a c a t e n a , in q u a n t o a s s o r b e i n e u t r o n i p r i m a c h e e s s i a b b i a n o r a g 1000 £ r> • s 100 10 0 025 «v i al m a x di 1800 b a r n s 6.75 « v 1.11 _1_l 0.01 0,1 1 i o e n e r v a n e u t r o n i c a ( e v ) f i g . 2 s e z i o n e d ' u r t o t o t a l e d e l l ' u r a n i o n a t u r a l e i n f u n z i o n e d e l l ' e n e r g i a n e u t r o n i c a ( ' ) . g i u n t a la e n e r g i a n e c e s s a r i a p e r la fissione i n d o t t a del ( 3 " . s e si t i e n e c o n t o d e l f a t t o c l i c l ' a b b o n d a n z a i s o t o p i c a d e l u'2:is è d e l 99.39'< si c o m p r e n d e p e r c h e n e l l ' u r a n i o n a t u r a l e n o n [ t o s s o n o p r o d u r s i r e a z i o n i a c a t e n a , se n o n r i c o r r e n d o a p a r t i c o l a r i a r t i f i c i ( 1 ) c h e r i d u c o n o l a p r o b a b i l i t à d i c a t t u r a d a p a r t e d e l l : 2 3 s . d a ( p i e s t e c o n s i d e r a z i o n i di c a r a t t e r e n u c l e a r e si d e s u m e p e r c i ò la i m p o s s i b i l i t à d i a t t u a z i o n e d i r e a z i o n i n u c l e a r i n e l l e r o c c e t e r r e s t r i . p o i c h é a b b i a m o v i s t o c h e p e r q u a n t o r i g u a r d a l ' a b b o n d a n z a d e i c o m b u s t i b i l i n u c l e a r i e d e g l i a g e n t i d e l l e r e a z i o n i a c a t e n a l a s i t u a ! 1 8 c . f k s t a f . i p p o l i t o z i o n e (1 i v i e n e s e m p r e p i ù s f a v o r e v o l e a l l a a t t u a z i o n e d e l l e r e a z i o n i med e s i m e , m a n m a n o c h e si p r o c e d e n e l l ' i n t e r n o d e l l a c r o s t a , p o s s i a m o c o n c l u d e r e e s c l u d e n d o l a p o s s i b i l i t à d i r e a z i o n i a c a t e n a n e l l a c r o s t a t e r r e s t r e . e s s e p o t r e b b e r o a v e r l u o g o s o l o n e l c a s o c h e il r a p p o r t o isotop i c o [/ / j / 2 3 " = 139 f o s s e n o t e v o l m e n t e a l t e r a t o in c o n s e g u e n z a di u n p r o c e s s o d i a r r i c c h i m e n t o d i u~s~'. m a ( i n o r a v a r i a z i o n i c o s ì r i l e v a n t i n o n s o n o s t a t e o s s e r v a t e n é n e l l e r o c c e n é n e l l e m e t e o r i t i , c h e c o s t i t u i s c o n o l a s o l a f o n t e d i i n f o r m a z i o n e s u l l e a b b o n d a n z e d e i n u c l e i a l l ' i n t e r n o d e l l a t e r r a , n é si c o n o s c o n o p r o c e s s i g e o c h i m i c i c h e pot r e b b e r o c o m u n q u e p o r t a r e a t a l i a r r i c c h i m e n t i . e m e r g e q u i n d i c h i a r a m e n t e d a l l e b r e v i c o n s i d e r a z i o n i s o p r a e s p o s t e c o m e s i a p r a t i c a m e n t e i m p o s s i b i l e , a l l o s t a t o a t t u a l e d e l l e n o s t r e c o n o s c e n z e , p e n s a r e a d u n a g e n e s i d e l l e r o c c e g r a n i t i c h e o d e l l a a t t i v i t à m a g m a t i c a i n g e n e r e d a f e n o m e n i di c a r a t t e r e n u c l e a r e . p e r il m o m e n t o c i ò d e v e b a s t a r e , s e v o g l i a m o , s e c o n d o l ' a n t i c o i n s e g n a m e n t o d i h u t t o n , p e r s p i e g a r e i f e n o m e n i g e o l o g i c i « n o n u t i l i z z a r e f o r z e c h e n o n s i a n o c o n n a t u r a l i al g l o b o , n o n a m m e t t e r e r e a z i o n i a d e c c e z i o n e d i q u e l l e di c u i ci è n o t o il p r i n c i p i o , n é i n v o c a r e a c c a d i m e n t i e c c e z i o n a l i p e r s p i e g a r e f e n o m e n i c o m u n i » . roma — istituto nazionali' ili gi'ofisica. napoli istituto ili geologia applicata — giugno 1954. riassunto alcuni autori hanno ricercalo nella presenza (li reazioni a catena all'interno della crosta terrestre la soluzione di alcuni problemi geologici e geofisici. nel presente lavoro si discute la possibilità (li tali reazioni e dal punto di vista geologico e dal punto di vista nucleare. come risultato di i/uestc discussioni si è indotti ad escludere la presenza di reazioni a catena nell'interno della crosta terrestre. si \1 mary the existence of nuclear chain-reactions inside the eartli's crust has been supposed hy some autliors in order to explain some geological and geopliysical problema. the possibili tv of sitcli processes is diseussed s u l l a p o s s i b i l i t à di r e a z i o n i a c a t e n a n e l l a c r o s t a t e r r e s t r e 4 4 9 frani the geological and nuclear point of vieic. .4s a result of tliis discussion the existence of nuclear chain-reactions in the earth's crust seenis to be excluded. b i b l i o g r a f i a l ' i t i r n e r ! . . a . , rev. mod. phys., 12, 1 5 0 , 1 1 9 4 0 ) . < ) a n d r e a t t a c „ rend. soc. min. it., 1, 4 3 ( 1 9 4 6 ) . ( : l ) n o e t z l i n j „ c . r. ac. sci., 2 0 8 , 1 6 6 2 , ( 1 9 3 9 ) . l ' i n o e t z l i n j . , ] . de p h y s . et riul., ti, 9 0 ( 1 9 4 0 ) . rilans énergetiques en géophysique. h e r m a n n , p a r i s 1948. i " ' l k u h n w . u n d r i t t m a n n a . , geol. rundschau, 32, 4 6 , ( 1 9 4 2 ) . c i ) r a m s e y w . h „ m.n.r.a.s., imi, 4 0 6 , ( 1 9 4 8 ) . i " ) gutenberg b . , the internai conslitution oj the earth, n e w y o r k ( 1 9 5 1 ) . (,») dalv r . a . , igneus rocks and the depth of the earth, n e w y o r k ( 1 9 3 3 ) . i ! , l nicoli p „ gesteine und minerallagerstatten, b a s e l ( 1 9 4 8 ) . i wi rittmann a . , rend. acc. sci. fis. e mat. della soc. ii. di napoli; s . 4 , v o i . 13, ( 1 9 4 2 4 5 ) . i " ) ippolito f . , meni, e note ist. geol. appi. univ. di napoli, 1, 3 9 , ( 1 9 4 8 ) . ( ' ) l e v i n e c . a . a n d s e a b o r g g . t . , j . ani. chem. soc., 73, 3 2 7 8 , ( 1 9 5 1 ) . ( 1 3 ) p o l l a r d e . a n d d a v i d s o n \ v . l . , applied nuclear p h y s i c s , n . y o r k ( 1 9 5 1 ) . i 1 4 > v e r n a r s k v w . j . , la géochimie. p a r i s ( 1 9 2 4 ) . < 1 3 ) g o l d s c h m i d t w . m . , j . chem. soc., 6 5 5 , ( 1 9 3 7 ) . i i n ) r i t t m a n n a . , e x p e r i e n t i n , 3 , 8 , ( 1 9 4 7 ) . i ' " ) rittmann a . , vulcani, attiviti) e genesi, n a p o l i ( 1 9 4 4 ) . c o n f r . a n c h e archives des sciences. 4, 5, g e n è v e , 1951. ( l s ) c l o o s h . , e i n j i i h r u n g in die g e o l o g i e , b e r l i n ( 1 9 3 6 ) . 1 1 » ) s e g r è e . , phys rev^ kb, 2 1 , ( 1 9 5 2 ) . i ( l ) j a n o s s v l „ cosmic rays, o x f o r d ( 1 9 4 9 ) . ( ' ) m u r r a y r . l „ introduction lo nuclear engineering, n e w \ o r k ( 1 9 5 4 ) . layout 6 1 annals of geophysics, 62, 6, oc672, 2019; doi: 10.4401/ag-7854 “multiplicative error model on northern hemispheric volcanic sulfate and european temperature anomalies„ nazario tartaglione* norce norwegian research centre, bjerknes centre for climate research, bergen, norway 1. introduction volcanic eruptions are one of the main natural forc‐ ings of the earth’s climatic system. eruptions can inject large or small amounts of sulfur in the atmosphere [hofmann, 1987; mccormick et al., 1995], in the form of so2 and occasionally of h2s [rampino and self, 1984]. sulfate in the stratosphere, obtained by the chemical transformation of sulfur injected by volcanoes into h2so4 aerosols, has been recognized as one of more im‐ portant factors influencing volcano‐climate interac‐ tions [toon and pollack, 1980; robock and mao, 1992, 1995; camuffo and enzi, 1995; briffa et al., 1998; robock, 2000; dunn, 2004; langmann, 2014]. although volcanic sulfate represents only a fraction of the total sulfate present in the atmosphere, the strength of vol‐ canic explosions and the height of volcanic eruption clouds contribute to the injection of sulfate into upper levels of the troposphere and the stratosphere where it can persist for more than one year [robock and mao, article history received july 7, 2018; accepted march 15, 2019. subject classification: multiplicative and additive errors; linear models; volcanic sulfate; surface air temperature; climate. abstract it is generally accepted that volcanic eruptions may have a climatic impact involving overall cooling of the troposphere, as the sulfur injected into the stratosphere is transformed into sulfate. however, it is intrinsically difficulty to distinguish this impact because of natural climate variability. in this paper, we look for a relationship between sulfate in the northern hemisphere (nh) and european temperature during the last 500 years using two independent data sets: stratospheric sulfate retrieved from ice cores and european seasonally and annually averaged temperature. relationships between temperature and sulfate are obtained as a function of the season (summer and winter) and location (tropics vs extra‐tropics) one and two years after the sulfate record year. after one year, we always find a negative correlation in the winter for both tropical and extra‐tropical eruptions. a correlation value of ‐0.56 is obtained for the relationship between sulfate and annually averaged temperature anomalies after one year, and a linear regression predicts a cooling of 1 °c for an eruption of 100 tg of sul‐ fate, an amount similar to that estimated to have been emitted by the laki eruption in 1783. the variability of the cooling effect after one year is evaluated by introducing a multiplicative error model for sulfate that accounts for sys‐ tematic as well as random errors in retrieved sulfate. to evaluate the impact of sulfate uncertainties on the regression slope, a resampling approach with 104 simulations is applied. results indicate that when uncertainties in sulfate are introduced, the variability of cooling is in the order of several 10‐3 °c/tg. temperature anomaly uncertainties impact slope uncertainty but have little influence on slope variability. 1995; sear et al., 1987; crowley and kim, 1999; hegerl et al., 2003; jones et al., 2004; fischer et al., 2007; esper et al., 2013]. the injection of large amounts of sulfur can have an important impact on temperature and, hence, on human activities. while it seems undoubtable that large volcanic eruptions can lead to a decrease in surface temperature [sadler and grattan, 1999], the estimation of how many degrees the atmosphere cools owing to a given amount of volcanic sulfate is problematic. there is uncertainty with the total mass of sulfur emitted [ar‐ feuille et al. 2014; gautier et al., 2016], especially in the stratosphere, and how and where the majority of the sulfur transforms into sulfate [baroni et al., 2008]. the size of volcanic particles also plays a fundamental role in radiative forcing, as well as atmospheric residence times, and, consequently, it affects surface temperature [timmreck et al, 2009; timmreck, 2012]. for single eruptions, the magnitude of the climatic signal can be masked by climate noise unless the eruption is large enough that the impact on climate can be unambigu‐ ously distinguished [swingedouw, 2017]. hence, the magnitude of climatic response to volcanic eruptions is still debated. real data are sparse, and uncertainties with sulfate estimates can affect the model response, as volcano forcing is based on observations [ammann et al., 2003]. although an intrinsic difficulty to depict the temper‐ ature response to volcanic eruptions exists [bradley, 1988; mass and portman, 1989], there have been at‐ tempts in the past to estimate the cooling effect in quan‐ titative terms. devine et al., [1984] found that the mean surface cooling associated with five eruptions appeared to vary linearly with the cube root of the estimated min‐ imum mass of sulfur released by the eruptions. mass and portman [1989], analyzed nine years of eruptions, and, among those considered, five were classified as in‐ tense. most of the eruptions analyzed were located in the nh or in tropical regions. the eruptions they used were relatively recent, occurring from 1883 onwards, and thus large eruptions like tambora and laki were not included. they found a mean cooling effect of ap‐ proximately 0.3 °c and claimed that eruptions do not play an important role in the earth’s climate. reanalyzing the data from devine et al. [1984] and mass and portman [1989], using a regressive model, we find a relationship between stratospheric sulfate mass, as estimated from ice cores, and surface temper‐ ature anomalies. we even investigate the role of the season and of the eruption type. we propose applying a bootstrap to evaluate variability and uncertainty for the slope that relates sulfate and temperature. we de‐ fine variability as the result of having different mean values of the slope, while uncertainty is the error bar associated with each average slope value. we present a multiplicative model for the (un‐ known) sulfate error and, with these errors, we re‐cal‐ culate the slope under different sets of parameters that define the error model. some hypotheses on tempera‐ ture uncertainty are discussed, and we demonstrate that sulfate uncertainty contributes to the majority of slope variability, while temperature uncertainty has lit‐ tle impact. 2. description of data the amount of volcanic sulfate is a difficult quantity to measure, and there is debate on how much sulfate is obtained after each eruption. if nowadays (since ~1978), we can measure sulfur emissions with satel‐ lites, we cannot say the same for past eruptions when satellite data were not available, and the amount of sul‐ fur injected during past eruptions is, therefore, neces‐ sarily associated with larger uncertainties [scaillet et al., 2003; textor et al., 2005]. 2.1 sulfate data set this data set consists of the mass of sulfate released by volcanic eruptions from both hemispheres from the year 500 to the year 2000. however, for our study, only data from the nh and the period from 1500–2000 were considered because the temperature dataset covers that period (figure 1). the original data set was built by an‐ alyzing 54 polar ice cores, and the sulfate released was obtained with a calibration factor that relates sulfate tartaglione 2 figure 1. the sulfate (in tg) as retrieved by gao et al. [2008] for the northern hemisphere. 3 volcanic sulfate errors and temperature 3 table 1. a selection of volcanic events over the last 500 years using data from gao et al. [2008]. the first column is the year of the eruption, as estimated by ice core dataset. the second column shows the volcano associated with the eruption (figures close to the name are other estimates of the eruption year found in the literature). the third column shows the latitude of the volcano (approximated to the first decimal digit). the forth column shows the sulfate retrieved (in tg) by the ice cores for that year [after gao et al., 2008]. in the last column lists the authors that reported the eruption. in particular, the works of ammann and naveau [2003], bradley and jones [1992] and briffa et al. [1998], identified in the table as am03, bj92, and br98, respectively, were used. year volcano latitude estimate of sulfate injected in northern hemisphere (tg) reference 1503 st. helen 1500 atitlan, guatemala 1504 46.2 n 14.6 n 1.72 bj92 am03 1512 hekla 1510 sangeang api gunungapi wetar 64.0 n 8.2 s 6.6 s 4.24 bj92 am03 am03 1526 arenal 1525 10.5 n 3.54 bj92 1584 billy mitchell 1580 kelut, java 1586 6.1 s 8.0 s 24.23 br98 bj92 br98 am03 1600 huaynaputina quilotoa 16.6 s 0.9 n 46.08 br98 am03 bj92 1641 komaga 1640 parker 42.1 n 6.1 s 33.81 br98 br98 am03 1673 san salvador 1671 gamkanora 13.7 n 1.4 n 6.34 bj92 bj92 br98 am03 1719 wudalianchi 48.8 n 31.48 feng and whitford-stark, 1986 1729 krafla 65.7 n 12.02 brandsdottir et al., 1997 1755 katla 63.0 n 7.96 bj92 1761 mount vesuvius 44.6 n 8.41 rosi et al., 1993 1783 laki asama 64.0 n 36.4 n 92.96 bj92 bj92 1796 pogrommi 1795 amak bogoslov 54.6 n 55.4 n 53.9 n 6.69 bj92 plummer, 1898 plummer, 1898 1809 unknown 27.56 1815 mayon 1814 tambora 11.3 n 8.2 s 58.69 bj92 bj92 br98 am03 1831 unimak 1830 mount st. helens babuyan claro 54.8 n 46.2 n 19.5 n 16.97 plummer, 1898 plummer, 1898 am03 1835 cosiguima 13.0 n 26.36 bj92 br98 am03 1883 krakatau augustine nasu 1881 6.1 s 59.4 n 34.1 11.16 bj92 br98 bj92 bj92 1912 katmai novarupta lolobau 1911 taal 1911 58.0 n 58.0 n 4.9 s 14.0 n 11.04 bj92 br98 bj92 am03 am03 1925 raikoke 1924 santorini 48.3 n 3 6.5 n 11.15 bj92 washington, 1926 1943 parìcutin 19.5 n 6.61 pioli et al., 2008 1963 agung 6 s 7.61 bj92 am03 1976 augustine fuego 1974 59.4 n 14.5 n 4.72 bj92 bj92 1982 el chicòn 17.4 n 11.20 am03 1991 pinatubo 15.1 n 15.05 br98 am03 tartaglione 4 depositions measured in the ice cores to the total mass of stratospheric aerosol [gao et al., 2008]. hemispheric calibration factors were determined to allow for com‐ parisons between polar regions for ice core sulphate depositions (in kg km−2) and the total mass of stratos‐ pheric aerosol formed after each eruption (in kg). as we limited our analysis to a period of 500 years in the nh, only 22 out of the 25 events of figure 1 were considered, with the first three events excluded. not all the eruptions considered in the present work are from nh volcanoes. as can be seen in table 1, some of the volcanic eruptions that led to the injection of sulfate in the nh occurred in tropical regions. most of the events reported in table 1 were taken from bradley and jones [1992], briffa et al. [1998], ammann and naveau [2003] and others (see references in table 1). the event associated with the highest value of sulfate oc‐ curred in 1783, corresponding to the eruption of the laki volcano, in iceland [highwood and stevenson, 2003]. it should be noted that the eruption of the asama volcano, located in japan, was also reported in 1783 [zielinski et al., 1994]. the problem with these kinds of data is the uncer‐ tainties associated with them. as observed by arfeuille et al. [2014] there are uncertainties both with calibra‐ tions and with the partitioning of volcanic sulfate be‐ tween the northern and southern hemispheres in the case of eruptions of tropical volcanoes. arfeuille at al. [2014] discusses a few cases where the calibration led to discrepancies between sulfate computed by ice cores and other observations. possible errors may be pres‐ ent in dating as suggested by baillie and mcaneney [2015], who found discrepancies between tree‐ring and ice chronologies. we note that a problem might be related to the proximity of volcanoes to the polar re‐ gions. certainly, this might be the case with laki, for example. gao et al. [2008] argued that the total uncer‐ tainty of their data is about 50% caused by different volcanic signal extraction criteria, sulfate deposition owing to the transport simulations, and the choice of calibration factor. 2.2 temperature data set the temperature dataset used in this article is that presented in luterbacher et al. [2004]. these data are obtained by multiproxy reconstructions of monthly and seasonal surface temperature fields for europe back to 1500 by using the instruments described by mitchell and jones [2005] and hansen et al. [2001]. this data set was obtained with multiproxy recon‐ structions of monthly and seasonal surface tempera‐ ture fields for europe back to 1500 using the instruments described in xoplaki et al. [2005] to eval‐ uate variability, trends, uncertainties, and extreme changes of reconstructed and observed european spring and autumn temperature back to 1500. while uncertainties cannot be estimated with pre‐ cision, we know that they decrease with time, and, for the sake of simplicity, we suppose a random uncer‐ tainty following a normal distribution having a stan‐ dard deviation of 1 °c. to know how temperature uncertainty affects the slope value, we performed a monte carlo simulation by varying the temperature uncertainty and keeping almost as a constant the sul‐ fate uncertainty (a small uncertainty was still associ‐ ated with it). the temperature used is shown in figure 2. all the computations are made using the temperature anom‐ aly with respect to the 30‐year moving average (the red line in figure 2). figure 2. temperature averaged on europe (after luterbacher et al. [2004]) and 30 years moving average (red line). 5 volcanic sulfate errors and temperature 3. statistical analysis and role of eruption type and season we analyzed the relationship between european temperature anomalies and stratospheric sulfate by means of a simple linear regression that accounted for the annually averaged temperature and all the erup‐ tions from the data set (subsection 3.1), and also for season and eruption type (tropical or extra‐tropical; subsection 3.2). the role of the eruption magnitude on the linear relationship is also investigated in subsec‐ tion 3.3. 3.1 linear regression analysis with temperature anomalies we start our statistical analysis with a linear re‐ gression between northern hemispheric sulfate emis‐ sions and european mean temperatures. as it is known that aerosols injected by volcanoes have a short time influence on climate, we analyzed the correlation between sulfate emissions and the tem‐ perature from one year after the eruption. one year after the eruption event, the correlation between sul‐ fate and temperature is moderately strong, with a pear‐ son correlation between temperature and sulfate mass of 0.56. however, there are a few outliers correspon‐ ding to emissions larger than 40 tg (figure 3). the linear model predicted a cooling of 10x10‐3 °c per tg of sulfate injected into the atmosphere; thus large or multiple eruptions that inject 100 tg of sulfate would cause an average cooling of 1 °c in europe. we verified all the conditions for applying a linear model; namely linearity, nearly normal residuals, and constant variability of data or residuals. within the framework of linear regression, we used the student’s t‐test to evaluate the strength of evidence that the slope that re‐ lates sulfate mass with temperature cooling is signifi‐ cantly different from zero at a 5% significance level. moreover, we performed a test of significance by using a nonparametric bootstrap technique. the null hypothesis (h0) is that the values of correlation and slope found in the linear model occurred by chance. if h0 is false, then the alternative hypothesis is true, and the correlation and the slope values should fall in the tail of the distributions of correlations and slopes ob‐ tained by resampling the temperature data. we ran‐ domly selected 22 values of temperature from the temperature data set and randomly assigned them to the northern hemispheric sulfate observed between the years 1500 and 2000. for each sample, we com‐ puted the parameters of the linear model. this proce‐ dure was repeated 2000 times, and statistics of the results were analyzed in light of the values of the cor‐ relation and the slope found previously. figure 4 demonstrates how the observed value of the slope found is in the tail of the distribution. the horizontal line is the fifth percentile, and the vertical line is the slope found with the original data. the slope of ‐10x10‐3 °c/tg was therefore found to be significant, as there is a less than 5% probability of finding this result by chance. the probability of finding a correlation of ‐0.56 by chance is also less than 5%. therefore, we found that the relationship between stratospheric sulfate and temperature was significant. figure 3. the relationship between sulfate mass (in tg) and temperature anomaly (in °c) obtained using the “lm” function from the r statistical package [r develop− ment core team, 2008]. figure 4. cumulative frequency of the slopes obtained by a bootstrap of data. the vertical line represents the slope of the original data (see fig. 3). the horizontal line indicates the 5th percentile. as eruption effects are supposed to last for up to two years, we looked for a relationship between sul‐ fate mass and temperature two years after the erup‐ tions. this correlation after two years was found to be lower than that obtained after one year, with a pear‐ son correlation of only ‐0.28. the linear regression, after two years, gives a slope of about ‐5x10‐3 °c/tg, suggesting a cooling effect on the average temperature value between the first and second year. after three years, the correlation was completely lost. thus, from these data, there is evidence that the impact of an erup‐ tion is important primarily during the first year after the eruption. observations and model simulations gen‐ erally demonstrate a significant cooling only in the first two or three years after the eruption; afterwards, a marginally significant temperature increase reflects the relaxation to an equilibrium climate state without vol‐ canic forcing [hegler et al., 2003; merlis et al., 2014]. 3.2 accounting for seasons and locations the analysis described above was replicated on extra‐tropical and tropical eruptions. among the 22 eruptions selected, we classified 11 as tropical and 11 as extra‐tropical. the analysis was applied for two dif‐ ferent seasons, namely summer and winter, and for one year and two years after the eruption year. table 2 shows the results obtained. when we used all the eruptions, we obtained after one year since the erup‐ tion, a strong cooling during the winter and a slight increase of the temperature anomaly. no statistical significance was found during the summer. after two years, there is not enough statistical significance to claim that eruptions have an influence on climate. looking at the type of eruption, we noticed, even after two years, that extra‐tropical eruptions caused a cooling in europe during the wintertime, although the cooling was statistically significant for only one year after the eruption. for the summer, the relation‐ ship found was not statistically significant whether we considered all the eruptions, or we limited our analysis to the two eruption sub‐groups. while the tropical eruptions are equal in number to the extra‐ tropical eruptions, they produced temperature anom‐ alies that could barely be classified as statistically significant. however, when we lowered the limit of the statistical significance to 90%, the warming ob‐ served in europe after two years from the tropical eruptions became statistically significant. in general, after one year a cooling effect was observed. however we want to underline how statistical significance is important to discriminate whether a correlation ac‐ tually exists. in general, the impact of volcanic eruptions on the earth’s climate is controversial. it is quite well known that tropical eruptions may induce winter warming and a cooling effect on summer at higher latitudes [fischer et al., 2007] although the mechanism is not perfectly clear. it has been suggested that atmos‐ pheric aerosols induce and strengthen arctic oscilla‐ tion or north atlantic oscillation [shindell et al., 2004; fischer et al. 2007]; [wunderlich and mitchell, 2017]. for example, even the observations and simulations of tropical eruptions having the same magnitude as the pinatubo eruption exhibit a possible warming ef‐ fect at higher latitudes [robock and mao, 1992; merlis et al. 2014]. there is also evidence of a winter warm‐ ing at the higher latitudes of the northern hemisphere associated with tropical eruptions [robock and mao, 1992; luterbacher et al., 2004; fischer et al., 2007] this relationship is difficult to measure, as different eruptions can occur in the same year in both the trop‐ ical and extra‐tropical regions. 3.3 the role of eruption magnitude there are three events with estimated emissions of more than 40 tg of sulfate in the data set. in 1783, the eruptions of laki and asama were recorded, but the stratospheric mass loading of sulfate for the asana eruption was estimated to be 3.5 tg [zielinski et al., 1994], a fraction of that injected by laki. thus, the major impact on european climate should be because of the laki eruption [zielinski et al., 1994; witham and oppenheimer, 2004]. the tambora and huaynaputina eruptions were tropical eruptions, and they are two out of the three eruptions with the highest sulfate levels. one year after these three eruptions, average temper‐ atures were lower than the mean, both in summer and winter. a winter warming effect did occur for the two major tropical eruptions, but only two years after the eruption. to test whether the slope we found is still valid even without outliers, we removed the three eruptions with stratospheric sulfate larger than 40 tg, and we found that the slope of the linear regression is 8x10‐3 °c/tg. however, this result is not significant when applying a t‐test and the bootstrap technique adopted previously, so the most intense eruptions, in reality, give us more information than all other simulations together. although these three sets of data are important in de‐ termining a significant slope for the linear regression that describes the relationship between stratospheric sulfate and temperature, we can say that the linear model is robust as the three eruptions are real and not spurious data. moreover, the value of 8x10‐3 °c/tg is tartaglione 6 still a high value. as for a large eruption of 1000 tg, the model, without outliers, would predict a cooling of 8 °c. a technique to estimate the slope uncertainty is the jackknife procedure [wilks, 2006] for which, given a sample of n independent observations, the slope is re‐ computed n times, removing a different pair of the data (sulfate yield and temperature) each time leaving only the other 21 couples. once we compute n slopes we can have a statistic for the slope and the correlations. with 22 slopes computed with the jackknife method, the mean and median values are surprising close to that found with 21 couples of values, the mean value of the slope is still ‐10x10‐3 °c/tg. the jackknife methods give also an indication of possible uncertainty to associate with the mean value. the standard deviation is 1x10‐3 °c/tg, so taking twice the standard deviation as meas‐ ure of the uncertainty associated with the determina‐ tion of the slope we can estimate a cooling, by means the jackknife procedure, of (10 ± 2)x10‐3 °c/tg. 4. dealing with sulfate and temperature uncertainties in this section, we introduce a multiplicative error model that has been generally concluded to be more appropriate for quantifying errors for positive value variables, such as precipitation [tartaglione, 2010]. the estimation of the true value of sulfate is mod‐ eled with the following equation 𝑆𝑡 = m(μ,σ) 𝑆𝑜𝑒 𝑟 (1) in which so is the sulfate estimated from ice cores, st is the supposed true sulfate value, and m is related to the multiplicative error that we can associate with sys‐ tematic errors in determining the true sulfate actually present in the atmosphere. this error has an average factor μ, and a standard deviation σ; r is related to the random error, and it represents the standard deviation of the gaussian process associated with these random errors whose mean is supposed to be zero. although not frequently used in geoscience, eq. 1 can be found in error precipitation estimation [tartaglione, 2010; tian et al., 2013] and it is often used in other disciplines such as biostatistics [baskerville, 1972] and econometry [brownlees et al., 2011]. despite technological improvements, sources of error in so2 flux measurements are still present even when direct measurements are performed [tambu‐ rello et al., 2011]. the first approach is to suppose that there is not a multiplicative error, but only a random gaussian error, with mean value equal o zero, whose standard devia‐ tion is set from 0.01 to 0.5. in such a case equation 1 becomes 𝑆𝑡 = 𝑆𝑜𝑒 𝑟 (2) the second scenario is that with a multiplicative error with a varying mean value μ from 0.5 to 1.5 to provide an uncertainty of 50%. however, a small ran‐ dom error with a small standard deviation of 𝑟 = 0.1 is maintained. by setting the standard deviation to σ = 0.1, we suppose that the same systematic uncer‐ tainty in the term m(μ,σ) still has random variability. in both these error models, temperature is assumed to have a random gaussian error with a standard devia‐ tion of 1 °c. however, when we consider the past, tem‐ perature estimates may have large errors. to evaluate the impact of these temperature errors on slope un‐ certainty, we performed a sensitivity analysis keeping almost constant the sulfate error, which had m = 1 and 𝑟 = 0.1, but the standard deviation for temperature var‐ ied from 0.5 °c to 3 °c. for each error model scenario we hypothesized, 10000 simulations were performed to obtain the average value and associated error bar of the slope that relates sulfate with temperature. figure 5 shows the result of the linear slope that re‐ lates sulfate and the temperature anomaly obtained by changing the value of the standard deviation r of ran‐ dom errors describe in equation 1. when the r value is close to 0, the expected cooling is still about 10‐3 °c/tg, but when it increases to 0.5, the expected cooling is only 7.5 x 10‐3 °c/tg. the error bar associated with each slope is about 0.5 x 10‐3 °c. such variability demonstrates that even a small error in evaluating the amount of the sulfate mass can have an impact on the estimates of cooling. we noticed that the mean value 7 volcanic sulfate errors and temperature table 1. slopes (°c/tg)x10-3 found by distinguishing extra-tropical and tropical events after one and two years after the eruptions. the figures in bold are significant at 95% based on a student’s t-test. extra-tropical tropical all +1 +2 +1 +2 +1 +2 winter -20.9 -5.0 -16.1 +22.7 -20.0 +4.7 summer +1.0 -9.3 -10.2 +9.4 -2.8 -3.5 annual -8.3 -10.3 -13.1 +7.9 -10.1 -4.4 of the slope does not depend on the mean sulfate error that is supposed to be zero, but only on the uncertainty of that error. sulfate measurement uncertainty can be related to an underestimation or overestimation of the method of estimation of sulfate from the ice cores. in such a case, the hypothesis is that there is a stochastic multi‐ plicative term whose average goes from 0.5, where the real sulfate would be half of that retrieved, to 2, where the real sulfate would be underestimated by 50%. the standard deviation value of this stochastic error is as‐ sumed to be small (σ = 0.1) compared to the mean value. the variability of the slope obtained in this man‐ ner demonstrates that if the sulfate emitted in past eruptions had been more than that retrieved by gao et al. [2008] the cooling in terms of °c/tg would be re‐ duced by up to 0.5 °c for 100 tg of sulfate if the error is 100% of the sulfate retrieved. on the other hand, a 100% overestimation of the measured sulfate would lead to cooler temperatures in europe by up to 2 °c per 100 tg of sulfate. 4.1 temperature uncertainties one could object that the temperature anomaly error may also have an influence on the slope, changing the responses shown earlier. we first consider only an additive stochastic error on the temperature anomaly having a mean equal to zero with a specific standard deviation up to 3 °c, which is a large value, as the ab‐ solute anomalies with respect to the moving average are less than 1 °c. when we make a linear regression with only a stochastic temperature error, we obtain an error on the slope of 6 x 10‐4 °c/tg (figure 7). even when we consider stochastic errors having a mean value different from zero, the mean and standard deviation of the slopes found by assuming a multi‐ plicative error on sulfate and an additive error on tem‐ perature do not depend on the temperature anomaly error (see figure 8). in short, we can say that the sul‐ fate error has a larger impact with respect to the tem‐ perature anomaly error. 5. discussion and conclusions defining a relationship between stratospheric sul‐ fate and changes to the surface temperature is a chal‐ lenging task. considerable uncertainty exists with measurements of aerosols injected into the atmosphere during volcanic eruptions. this is especially true with those from the pre‐satellite era (before 1978). more‐ tartaglione 8 figure 5. relationship between sulfate and temperature as a function of exponent r of eq. 1, with μ=1. figure 6. relationship between sulfate and temperature as a function of the mean value μ of eq. 1, keeping constant r. figure 7. relationship between sulfate and temperature as a function of an additive temperature error. over, the natural variability of climate can mask the im‐ pact of eruptions. as we have shown in our analysis, this is particularly true when the stratospheric sulfate emis‐ sions are limited to a few tg. we propose a simple model using nh stratospheric sulfate aerosol loading to calculate the european temperature change following 22 large eruptions events using deposited sulfate meas‐ urements from ice cores and average annual tempera‐ ture estimates for the european region. this empirical model is based on historical observations and takes the form of a linear regression. large volcano eruptions are quite rare, and the data set we used contains only three events that were associated with sulfate mass larger than 40 tg. these three events determined the signifi‐ cance of the slope of the linear regression we found. this slope of ‐10x10‐3 °c/tg was found with two dif‐ ferent techniques: a simple regression with all the 22 events included and with a jackknife procedure, which also provides an estimate of possible errors. the slope is significant at 95% following a student’s t‐test and a non‐parametric test. the correlation value, equal to ‐ 0.56, between sulfate and average northern hemi‐ spheric temperature is also significant at 95%. these results are in agreement with those found by oliver [1976], who analyzed the response of temperature to volcanic dust rather than sulfate. earlier studies such as sigurdsson [1990] and devine et al. [1984], for instance, have found a power law relationship, but they considered only a few vol‐ canic eruptions that led to negative variations of tem‐ perature. we questioned these studies, as the selection of only a few eruptions may have introduced selection and small sample bias. with the power law found by sigurdsson [1990] and the linear relationship that we found, we try to make a few predictions of temperature as a function of stratos‐ pheric sulfate mass, and we can see that the power law is a poor predictor for small eruptions and even for super‐volcano eruptions. in fact, the change of tem‐ perature with an eruption injecting a small amount of sulfate mass (e.g., 10 tg) is about 0.5 °c cooler follow‐ ing a 1/3 power law versus a change in the range of 0.1 °c given by the linear model that appears more appro‐ priate. only when the sulfate mass is in the range of 100 tg are the two models in agreement. however, studies performed by using climate models suggest that a super‐eruption with 2000 tg of sulfate mass in‐ jected into the atmosphere, an unlikely but possible event, would cool the earth’s surface by more than 10 °c. our linear model, with all the cautionary notes related to data extrapolation, gives a prediction of ‐18 °c for the european region. this result falls between 9 volcanic sulfate errors and temperature figure 8. relationship between sulfate and temperature as a function both of the error term μ and the temperature error. the ‐10 °c of jones et al. [2005] and the ‐25°c of harris [2008]. however, we should remember that such cool‐ ing effects could be less marked in some regions de‐ pending on where the eruption occurs. in this paper, we tried to quantify the variability of the relationship obtained by a linear regression be‐ tween european temperature anomalies and sulfate as retrieved from ice cores by adopting a monte carlo ap‐ proach. as sulfate is a positive quantity, a multiplica‐ tive stochastic error model composed of two terms was used to define the sulfate. one term accounted for er‐ rors in the underestimation or overestimation of the sulfate, while the other, an exponential term, accounted for random errors. the former, as expected, had a large impact on slope variability, while the latter had a lim‐ ited impact on the slope, but in general there was a re‐ duction of the cooling estimate of about 3 x10‐3 °c/tg. this result comes as a surprise, as the average value of the random error r is 0, whereas only the standard de‐ viation changes in our experiments. the error on the temperature anomalies, instead, was expected to be ad‐ ditive with the anomalies both positive and negative. we found that the temperature error has no impact on the slope, suggesting that reducing errors on the sul‐ fate will have an improvement on the relationship that relates volcanic sulfate to temperature. our results, al‐ though limited to observation data, are consistent with the extremes and variability found in other studies with numerical models [harris and highwood, 2011; meronen et al., 2012; timmreck et al., 2012; pausata et al., 2015]. analysis reported in this paper was performed by using the language r. we wish to thank you the data provider igbp pages/world data center for paleoclimatology noaa/ncdc paleoclimatology program, boulder co, usa for the european seasonal temperature reconstructions and 1500 year ice core-based stratospheric volcanic sulfate data. references ammann, c.m. 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(1926). santorini eruptions of 1925, bull. geol. soc. am., 37, 349–384. witham, c.s., c. oppenheimer (2004). mortality in eng‐ land during the 1783‐4 laki craters eruption, bull. volcanol., 67, 15‐26. wunderlich, f., d.m. mitchell (2017). revisiting the ob‐ served surface climate response to large volcanic eruptions, atmos. chem. phys., 17, 485‐499, doi: 10.5194/acp‐17‐485‐2017. xoplaki e., j. luterbacher, h. paeth, d. dietrich, n. steiner, m. grosjean, h. wanner (2005). european spring and autumn temperature variability and change of extremes over the last half millennium, geophys. res. lett., 32: l15713. zielinski g.a., r.j. fiacco, p.a. mayewski, l.d. meeker, s. whitlow, m.s. twickler, m.s. germani k. endo, m. yasui (1994). climatic impact of the a.d. 1783 asama (japan) eruption was minimal: evidence from the gisp2 ice core, geophys. res. lett., 21, 2365‐2368. *corresponding author: nazario tartaglione, norce norwegian research centre, bjerknes centre for climate research, bergen, norway; email: nazario.tartaglione@norceresearch.no © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved tartaglione 12 silva_final:layout 6 a simple statistical procedure for the analysis of radon anomalies associated with seismic activity hugo gonçalves silva1,*, mourad bezzeghoud1, maria manuela oliveira2, antónio heitor reis1, rui namorado rosa1 1 university of évora, school of sciences and technology (ect), geophysics centre of évora and physics department, évora, portugal 2 university of évora, school of sciences and technology (ect), mathematics department, évora, portugal annals of geophysics, 56, 1, 2013, r0106; doi:10.4401/ag-5570 abstract this study presents an analysis of data from various radon anomalies that were compiled by toutain and baubron [1999], to investigate their relationships with the earthquake parameters of magnitude and distance from epicenter. the simple methodology applied here reveals significant and positive correlation between the duration of the radon anomalies and the ratio between the earthquake preparation radius and the distance between the sensor and the event epicenter. this shows an important relationship between seismic activity and duration of radon anomalies at a local scale. the consequences and implications of this relationship are discussed. 1. introduction the study of earthquake precursors has a long history that goes back to the late 19th century [e.g., kagan and knopoff 1987, geller 1991, wyss 1991; and references therein]. in the last few years, the study of these phenomena has been gaining new widespread interest. the reason for this is clearly due to the valuable tools that are being developed in this field to complement existing hazard assessment facilities. in the literature, it is possible to find a number of different effects with many occurrences [e.g., biagi et al. 2006, ouzounov et al. 2007, chauhan et al. 2009, silva et al. 2011]. the interested reader is referred to a recent review by cicerone et al. [2009]. many studies have attributed these effects to radon emanations that can be caused by variations in the bedrock strain and the soil permeability during the preparatory phase of earthquakes, and particularly by severe changes near to the end of this phase [see, e.g., pulinets and ouzounov 2011]. for example, according to harrison et al. [2010], the radon released in this process can ionize the lower atmosphere, to produce a considerable reduction in the atmospheric electrical field. due to its relevance, comprehensive analyses of radon emanations are crucial, and many studies have dealt with this issue. in particular, toutain and baubron [1999] carried out a very extensive review of various degassing processes in relation to seismic activity for different gases (especially radon), with specific attention to the multiple factors that can influence these processes (mainly weather conditions). as a discussion of these aspects is not within the scope of the present study, the reader is referred to toutain and baubron [1999], where many further details can be found. in the present study, we applied a simple statistical procedure that was recently developed by silva et al. [2012] to the radon anomalies associated with seismic activity that were compiled by toutain and baubron [1999]. we focused, in particular, on a study of the relationship between the duration of the anomalies and the magnitude of the earthquake and distance from its epicenter. these anomalies are defined based on deviations in the radon levels from the soil or groundwater. here, after presenting the database used and describing our methodology in section 2, we apply this to the occurrences reported by toutain and baubron [1999], including together both the soil and groundwater radon anomalies, as they are linked by the same physical processes discussed in section 3. our analysis indicates that the duration of the radon anomalies is linearly related to the earthquake preparation radius (which is exponentially dependent on its magnitude), and is inversely related to the distance from the earthquake epicenter to the radon-measuring site. 2. data in this study, we have analyzed the various radon anomalies that were compiled by toutain and baubron [1999]. their review includes nearly 150 anomalies of different geochemical indicators, including: rn, h2, n2, ar, he, ch4, h2s and hg (gas). nearly 72% of these anomalies are related to radon, which provided a significant number of cases to which we can apply our methodology. these are defined as sizearticle history received february 8, 2012; accepted november 8, 2012. subject classification: atmosphere: composition and structure, geochemical exploration, radon emissions, seismic precursors, seismic activity, seismic risk. r0106 research articles able deviations from the mean background levels of radon, , which is a typically criterion used in various studies that considers a given measurement as an anomaly when the radon levels rise above + 2v, with v as the standard deviation. this is a well-established criterion in the literature that provides good support for our analysis, although, unfortunately, the values of and v are not presented in the review by toutain and baubron [1999], because normally they are not mentioned in the original papers. most of the anomalies (83%) correspond to groundwater measurements, with 10% from soil measurements, and 7% measured using both groundwater and soil observations. these values should imply a dominance of the groundwater processes in our results; however, interestingly, virk and baljinder [1994] showed that groundwater and soil radon measurements show similar behaviors as earthquake precursors. thus our dataset should be representative of the general radon emanation dynamics in connection with earthquake occurrences. the measurements were made for intermediate to longterm periods (more than 1 year of observations), and different techniques were used. the soil radon anomalies were measured using the track-etch method [virk and baljinder 1994], which is usually a low-frequency technique (integrated over one or two weeks) that does not allow the correct estimation of the anomaly duration and its starting time. on the other hand, groundwater measurements typically consider daily sampling from natural springs, and a subsequent closedcircuit system enables the collected gases from these samples to pass through a detector; e.g., a zns(ag) cell. this detector measures the number of decays recorded for a given period after radon and its progeny reach equilibrium [virk and baljinder 1994]. this technique was used for 90% of our dataset, and it allows better control of the anomaly duration and starting time. it is important to note that 63% of the anomalies considered in the present study corresponded to m ≥5 earthquakes. indeed, different precursor behaviors would be expected for small and large earthquakes, and a careful analysis regarding this will be carried out in the future. for each radon anomaly, toutain and baubron [1999] presented (for the information that is available): the magnitude, m, and depth of the earthquake, z (km); the distance from the earthquake epicenter to the measurement site, d (km); the relative amplitude of the radon measurement, da (%); the duration of the anomaly, d (days); and the time interval between the beginning of the anomaly and the occurrence of the earthquake, dt (days). we selected 109 anomalies where that the earthquake parameters m and d are available (we did not considered z in the analysis, and thus this parameter was ignored), along with at least one of the three parameters of the radon anomalies, da, d, and dt. the ranges of theses parameters are: m = [1.9, 8.1]; d = [1, 1000] (km); da = [−80, 1200] (%); d = [1, 1370] (days); dt = [1, 180] (days). 3. methodology we will now focus on a description of the methodology applied. first, we should note that when analyzing the possible effects of an earthquake on anomalous radon emanations, two parameters must be considered together: the event magnitude, m, and the distance from the epicenter to the measuring site, d. this can be clarified when two earthquakes of the same magnitude are considered, where they occur at different distances from a measuring site: it is expected that the nearer event will influence the radon emanations more significantly than the farther event, such that we cannot compare them directly. this is because, in principle, both of the earthquakes would be expected to generate similar deformation fields, while the deformation caused by the nearer one to the measurement site is expected to be stronger, thereby causing a substantial increase in the local soil permeability, and enabling greater radon emanation, as compared to the farther event. thus, it is important to develop a parameter that is suitable to simultaneously account for both the effects of m and d, as these two parameters by themselves are insufficient to describe the significance of an earthquake with respect to the radon levels at a measuring site. on this basis, we considered the concept of the earthquake preparation radius, r, that depends on m [see dobrovolsky et al. 1979]. it is assumed that there should be an approximately circular region around the epicenter of an earthquake that undergoes elastic crustal deformation prior to seismic events, with this radius estimated as r ≈ 100.43m (km). bearing in mind these observations, we can assume that for relevant events, the radon measurement site will fall within a circle of radius r that is centered at the seismic epicenter, such that r ≥ d. indeed, we have represented the 109 occurrences of radon anomalies that are relevant to the present study in the r-d plane in figure 1. from figure 1, it is clear that more than 84% of the events obey the r ≥ d consilva et al. 2 figure 1. representation of all radon data points from toutain and baubron [1999] in the r-d plane, where r is the earthquake preparation radius, and d the distance (km) from the epicenter to the measuring site. x x x 3 dition. in the remaining 16%, 10 anomalies almost coincide as r = d (black line), while seven of the anomalies correspond to microseismicity data from the same study, virk [1995], as cited by toutain and baubron [1999]. in line with the above arguments, silva et al. [2012] introduced a dimensionless parameter, s, that was defined as follows: (1) where s should be positive, or very small if negative, for occurrences considered to be relevant to the present study; i.e., those that obey the r ≥ d condition. in the present analysis, we used the restrictive criterion of s ≥0 for the relevant cases. figure 2 shows the parameters that characterized the observed radon anomalies; namely, da (figure 2a), d (figure 2b), and dt (figure 2c), as a function of s for these cases. in the s-d plot (figure 2b), it is possible to identify a clear outlier point for s ≅ 16.38 and d = 1370 days, which corresponds to an earthquake with m = 7.8 at 130 km from the sensor, and a radon anomaly with da = −40% and no information about dt, as reported by hauksson [1981]. indeed, toutain and baubron [1999] noted that in areas with frequent highmagnitude earthquakes, more than one event might cause an apparently single anomaly. in the particular case of this outlier, where dt was not identified, it might have been related to either the haicheng (february 4, 1975, m 7.3) or the tangshan ( july 28, 1976, m 7.8) earthquake [toutain and baubron 1999], and for that reason this point was not included in the analysis. finally, we performed a pearson correlation analysis of s with da, d and dt using the statistical computing program "r". 4. results and discussion the pearson correlations of da (%), d (days) and dt (days) with s are shown in figure 3. a p <0.01 level of significance was considered (figure 3, blue line). even though no correlation was found between da or dt with s, stimulatingly, there was a significant positive correlation between d and s, with a coefficient of 0.503 with a p of approximately zero. in addition, we carried out linear regression, which fit the data reasonably and showed a reasonable coefficient of r2 ~0.25, as can be seen in figure 4. although both of these parameters are not high, they are statistically significant. thus we consider this as an important result, as it shows a possible relationship between the radon anomaly duration, d, and both the magnitude of the impending earthquake and the distance from the earthquake epicenter to the measurement site. in other words, this reveals that for two seismic events with the same magnitude, the nearer one to the measurement site will show a longer anomaly than the farther one. this tends to confirm the physical hypotheses presented at the beginning, as it is expected that an event that occurs near to a measurement site should result in a stronger local deformation field, together with a larger increase in the soil permeability (while also implying a slower recovery to the equilibrium state), and this should induce a longer radon emanation episode, as compared with the farther event. likewise, a stronger earthquake that has a larger preparation radius will tend to generate a radon source of larger superfiradon anomalies related with earthquakes figure 2. the three parameters of interest presented as functions of s (defined in equation 1). (a) the time interval between the beginning of the radon measurement anomalies and the occurrences of the earthquakes, dt (days). (b) the duration of the anomalies, d (days). (c) the relative amplitudes of the anomalies, da (%). the data shown were extracted from toutain and baubron [1999]. figure 3. results obtained for pearson correlation tests for da, d and dt with s correlations. ,s d r 1= cial area, together with anomalous radon levels (in both the soil and the groundwater) that last longer. it therefore appears that parameter d is related more directly to the overall process of deformation and the permeability state than the other parameters. indeed, the amplitude of the anomalies, da, depends on the amount of radon released, which is unavoidably related to the specific lithology at the measuring site, such that comparisons of data from different sites has relatively little meaning. likewise, the interval between the beginning of the anomaly and the occurrence of the earthquake, dt (days), is strictly reliant on the intricate process of the radon source, the emanation and flow of the radon through the fissure network of the rocks, and its diffusion through the overlying soil at the observation site, such that, once again, the results from different sites are blurred by several uncontrolled site-related factors. at this point, it is important to note that recorded radon anomalies are very much dependent on the local weather conditions; namely, the precipitation and water run-off, and also the temperature. in contrast, there is no relevant influence from the wind [chen and thomas 1994]. actually, precipitation affects the soil permeability, therefore reducing the radon release rate, and this might be responsible for the short duration of radon anomalies like the points on the s axis (d ~0) in figure 2b. thus special care must be taken with meteorological conditions. indeed, a possibility to minimize any meteorological influences on these radon anomalies is to consider only the anomalies with s > sc (a critical value to study only the events with the earthquake epicenter close enough to the measuring site; e.g., sc = 20). however, due to the restricted size of the database considered in the present study, this would significantly reduce the number of selected occurrences and thereby preclude the realization of any statistical analyses. unfortunately, as mentioned by toutain and baubron [1999], there is almost no information in the literature of very important parameters like the focal mechanics of these earthquakes, and the meteorological conditions during the anomalies, among others. thus, the results shown here are preliminary and deeper investigations will be required to provide more robust results. in particular, the effect of the focal mechanism on the radon anomalies is definitely important, and although it requires a significant monitoring effort, it could complement the present analysis in the future. nevertheless, stimulating perspectives for earthquake hazard assessment are found here, and they are presented at the end of this report (section 4). on the other hand, from a comparison between the results from an analysis performed by cicerone et al. [2009] and the present study, it is interesting to note that a clear relationship between the radon anomalies and the earthquake parameters is found here, while no particular relationships were found by cicerone et al. [2009]. from our point of view, this is because they did not take into account the balance between the distance of the earthquake epicenter to the measuring site and its magnitude; thus, it is not physically appropriate to compare anomalies from different magnitudes at different distances (see discussion in section 2). for this reason, we plan to apply our methodology to the very relevant data that was compiled by cicerone et al. [2009], and to compare their data with the present data. indeed, new studies using larger databases with careful attention to meteorological conditions, and particularly using more advanced statically methods (like the kolmogorov-smirnov test), need to be carried out in the near future. 5. final remarks the present study constitutes a first step to a more profound analysis that must include a larger and more complete database of radon anomalies, with particular attention being paid to the meteorological conditions, and especially to precipitation (this is a crucial aspect for future studies). nevertheless, by using a simple approach, this supports significant, although not decisive, evidence of a correlation between the seismic activity and the duration of the radon anomalies reported by toutain and baubron [1999]. however, the present viewpoint of many studies, e.g., pulinets and ouzounov [2011], tends to emphasize the analysis of satellite data, e.g., sarkar et al. [2011], as a crucial approach to the study of earthquake precursory phenomena. we believe that similar to the case of meteorology, local stations are determining in the better constraining of models, and for that reason, much effort is still needed in multiple-parameter-based research of earthquake precursory phenomena at the local level. to this end, we suggest the combination of satellite insar analysis, which provides an accurate calculation of the deformation area (especially if compared to the rough estimation by the formula of dobrovolsky et al. [1979]), with local radon measurements in seismically active regions. in this way, it will be possible for future studies to fully exsilva et al. 4 figure 4. linear regression of d as a function of s. 5 plore the effects of deformation area on radon emissions, to provide more assertive results. acknowledgements. hgs acknowledges the support of two portuguese institutions: the science and technology foundation for the grant sfrh/bpd/63880/2009, and the calouste gulbenkian foundation for the grant estímulo à criatividade e à qualidade na actividade de investigação in the science program of 2010. important discussions with prof r. giles harrison, dr cláudia serrano, and marta melgão are strongly acknowledged. we are also thankful for the support of the annals in geophysics editor, prof salvatore barba, and the reviewers of our manuscript, dr valerio curcio and dr anna riggio; the final form of our manuscript was greatly enriched by their suggestions. references biagi, p. f., l. castellana, t. maggipinto, r. piccolo, a. minafra, a. ermini, s. martellucci, c. bellecci, g. perna, v. capozzi, o.a. molchanov and m. hayakawa (2006). lf radio anomalies revealed in italy by the wavelet analysis: possible preseismic effects during 1997-1998, phys. chem. earth, 31, 403-408. chauhan, v., o.p. singh, v. kushwah, v. singh and b. singh (2009). ultra-low-frequency (ulf) and total electron content (tec) anomalies observed at agra and their association with regional earthquakes, j. geodyn., 48, 68-74. chen, c., and d.m. thomas (1994). soil processes and chemical transport, j. environ. qual., 23, 173-179. cicerone, r.d., j. e. ebel and j. britton (2009). a systematic compilation of earthquake precursors, tectonophysics, 476, 371-396. dobrovolsky, i.p., s.i. zubkov and v.i. miachkin, (1979). estimation of the size of earthquake preparation zones, pure appl. geophys., 117, 1025-1044. geller, r.j. 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(1995). radon monitoring of microseismicity in the kangra and chamba valleys of himachal pradesh, india, nucl. geophys., 9, 141-146. wyss, m., ed. (1991). evaluation of proposed earthquake precursors, 94 pp., agu, washington, d.c.; doi:10.1029/ sp032. * corresponding author: hugo manuel gonçalves da silva, university of évora, school of sciences and technology (ect), geophysical centre of évora and physics department, évora, portugal; email: hgsilva@uevora.pt. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. radon anomalies related with earthquakes << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice geo 1249 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 1 impact of variability of space environment on communications: working group 1 overview anna belehaki (1) and iwona stanis ⁄lawska (2) (1) national observatory of athens, institute for space applications and remote sensing, p. penteli, greece (2) space research centre, warsaw, poland the paper presents an overview of the scientific research carried out in the frames of working group 1 activities of the cost action 271. the basic objective of this working group was the study of the impact of ionospheric variability on communications. the main achievements and an overall evaluation are reported. 1.1. introduction the well-recognized impact of the space environment variability on communications affects many modern life activities and various sectors of economy. therefore, the systematic development of both scientific research and applications in this field is a high priority. the wg 1 of cost action 271 was established to serve this priority. the first working group (wg 1) is focused on the impact of variability of space environment on communications, having two main objectives. the first is scientific oriented and concerns on the one hand the collection of historical and new ionospheric and plasmaspheric data for nowcasting and forecasting purposes and on the other hand the development of methods and algorithms to monitor, predict and minimize the effects of ionospheric perturbations and variations on communications. the second concerns the stimulation of cooperation, at european level, in the domain of ionospheric and plasmaspheric prediction and forecasting for terrestrial and earth-space communications including interactive repercussions on the corresponding standards in this field, taking into account users present and future needs. the need for cooperation in the field of scientific and technical research on this topic in europe is well established considering the situation worldwide in providing data and services for ionospheric and plasmaspheric nowcasting and forecasting. several non-european organizations especially in the u.s.a. and australia provide ionospheric data and products significant for scientific and operational applications, mainly based on the global network of ionospheric stations and gps receivers. the center for atmospheric research (university of massachusetts, lowell) provides access to ionospheric observations from the worldwide network of digisondes (http://ulcar.uml.edu/stationmap.html) and access to the digisonde database (didb) through the sao explorer software (reinisch et al., 2003). ips radio services, australian forecast center (http://www.ips.gov.au/) provides hf information for europe, north america, australasia, and asia, ionospheric maps of fof2 and tec, estimates for 1250 anna belehaki and iwona stanis ⁄lawska the t index, hap charts and hf-conditions. northwest research associates (http://www.nwraaz.com/) provides space weather and ionospheric scintillation services. ises (http://www.isesspaceweather.org/) encourages and facilitates near-real-time international monitoring and prediction of the space environment by the rapid exchange of data and its application to the proper services that can reduce the impact of space weather. on the other hand, the collection and distribution of ionospheric data and products in europe for scientific and commercial use is currently possible mainly through the following research centers with open access to the public: rutherford appleton laboratory in united kingdom (http://www.wdc.rl.ac.uk/cgi-bin/digisondes/cost_database.pl), and space research centre in poland (rwc warsaw http://www.cbk.waw.pl/rwc). another important database with limited historical ionospheric observations was developed at ictpas, in italy, under the previous cost action 251 (http://cost251.ictp.trieste.it/). a major advantage of europe with respect to other regions is the satisfactory coverage by ionospheric observatories that provide data for radio-communication purposes. some of the european observatories provide access to their observations on the internet. a list of the european ionospheric observatories and their internet addresses is presented in table 1.i. nevertheless, the operations of european observatories are mainly independent creating several barriers for transforming the available information into usable usable data, products and services for nowcasting and forecasting the impact of ionospheric variability on communications. it is therefore obvious that the european research community should exploit the major advantage of the dense network of observations, and the very significant research expertise on this field, aiming to develop products and services which are similar or even more sophisticated from the corresponding tools developed by other non-european organizations to serve the needs of terrestrial and earthspace communication. 1.2. our scientific results – overview the challenges that cost 271 faced were wide and ambitious: monitoring of earth environment, assimilation of disparate data, development of end-user information, demonstration of its application to the experimental and operational service systems. moreover, studies to influence the technical development and the implementation of new communication services, particularly for gnss, as well as regional radio service systems were provided. the progress reached in the frames of wg 1 could be grouped as follows: 1) state-of-the-art study; 2) feasibility study; 3) databases and validation of prompt ionospheric parameters; 4) operational services; 5) modeling techniques leading to new operational services; 6) studies that improve our understanding of the upper attable 1.i. european ionospheric observatories. rome (italy) http://dps-roma.ingrm.it/ chilton (u.k.) http://www.wdc.rl.ac.uk/ionosondes/view_latest.html juliusruh (germany) http://www.ionosonde.iap-kborn.de/ionogram.htm tromsø (norway) http://www.eiscat.rl.ac.uk/~ian/ionos_now.html norilsk, irkutsk, yakutsk, zhigansk (russia) http://ds.iszf.irk.ru/ dourbes (belgium) http://digisonde.oma.be/ kiruna, uppsala, lycksele (sweden) http://www.irf.se/~ionogram/ roquetes/tortosa (spain) http://www.obsebre.es/w3/ionosfer.php athens (greece) http://www.iono.noa.gr el arenosillo (spain) http://www.inta.es/iono/ pruhonice (czech republic) http://147.231.47.3 tromsø (norway) http://www.eiscat.no/updating_dynasonde.html 1251 impact of variability of space environment on communications: working group 1 overview mosphere structure and dynamics; and 7) advances in mathematical and physical tools development. the flowchart of wg 1 activities is given in fig. 1.1. in the following sections we will highlight the most important achievements. 1.2.1. state-of-the-art study the main objective of this cost action is to study the effects of the upper atmosphere on terrestrial and earth-space communications. it was therefore, normally, the first step of our activities, to list present and future anticipated terrestrial and earth-space radio systems that their operation can be affected by ionospheric space weather disturbances (tulunay and bradley, 2004). in this respect the radio systems have been taken as including all those associated with communications, navigation and surveillance. this distinction is particularly important because some phenomena, such as for example those concerned with varying signal group delays, influence the performance of navigation and surveillance systems, but are of lesser significance to systems concerned only with the transmission of intelligence. to assess the reliability and compatibility of a radio system, i.e. whether it will achieve its intended purpose for the necessary time in the presence of the natural noise background, and whether it is compatible with co-channel and adjacent channel transmissions, appropriate propagation models are needed. these in turn depend on the relevant ionospheric models, which may be formulated in terms of either long-term predictions for system planning, or near-real-time forecasts for operational assessments. 1.2.2. feasibility study a feasibility study regarding the development of new models to nowcast and forecast the impact of variability of space environment on communications was performed by tulunay and bradley (2004) and bremer et al. (2004). the first target was to identify phenomena that can lead to impairments of radio systems and need to be modeled. a summary of individual impairments that can arise under some circumstances is given. a radio wave is specified in terms of five parameters: its amplitude, phase, direction of propagation, polarization and frequency. the principal effig. 1.1. flow chart of working group 1 activities. 1252 anna belehaki and iwona stanis ⁄lawska fects of the ionosphere in modifying these parameters are considered. as a second task, the space weather key parameters creating impact adversely and significantly on propagation conditions have been identified. different phenomena that arise under separate conditions with different time scales affect the ionosphere. such phenomena are the solar flares, x-ray enhancements, coronal mass ejections and magnetic effects, and solar radio burst emissions. a detailed examination of the impact of these phenomena on radio propagation is given by tulunay and bradley (2004). a third task of the feasibility study focused on the definition of the potential effect of the longterm trends on prediction models (bremer et al., 2004). using observations of about 50 different ionosonde stations with more than 30 years data series of fof2 and hmf2, trends have been derived with the solar sunspot number r12 as index of the solar activity. the final result of this trend analysis is that the differences between the trends derived from the data of the individual stations are relatively large, the calculated global mean values of the fof2 and hmf2 trends, however, are relatively small. therefore these global trends can be neglected for practical purposes and could not be taken into consideration in ionospheric models. as shown with the data of the ionospheric station tromsø, however, at individual stations the ionospheric tends may be markedly stronger and lead to essential effects in ionospheric radio propagation (rogers et al., 2002). 1.2.3. databases and validation of prompt ionospheric parameters 1.2.3.1. databases two very important databases of public access have been developed in the frames of working group 1 activities. the ionospheric database developed at rutherford appleton laboratory (http://www.wdc.rl.ac.uk/ cgi-bin/digisondes/cost_database.pl) provides real-time and historical data from 11 european observatories: athens, chilton, juliusruh, rome, tortosa, el arenonsillo, dourbes, lerwick, pruhonice, stanley and tromsø. the database provides all possible ionospheric parameters derived from the ionograms, and electron density profile data in plot and ascii format (stamper et al., 2004). this database is continuously updated with real-time data that are transferred automatically after each sounding. the eiscat database developed in the university of grenoble (http://www-eiscat.ujf-grenoble.fr) stores ionospheric data. this data stems from measurements performed by the uhf eiscat radars from 1981 to 1999. the database provides the classical incoherent scatter parameters (ne, te, ti, vi), the ionospheric electric field and the following reduced ionospheric parameters: the integrated total electron content (itec), the f2-layer maximum altitude and density with the corresponding plasma frequency (hmf2, nmf2 and fof2). 1.2.3.2. direct validation of prompt ionospheric parameters an attempt has been made to validate the ionospheric total electron content (itec) based on statistical results from the analysis of long time serried of data. the itec is a relatively new parameter (reinisch and huang, 2001) computed routinely from the european real-time digisondes and its validation was important to be able to use this parameter for operational applications. systematic comparison between itec and gps derived tec values, estimated for the same geographic location (athens) using 12 consecutive months of measurements, demonstrated that their residual differences have the same qualitative characteristics as the plasmaspheric electron content, as deduced from the diurnal and seasonal behavior and the variation during geomagnetic storms (belehaki et al., 2003). it therefore resulted that the itec parameter is a realistic measure of the electron content up to 1000 km. 1253 impact of variability of space environment on communications: working group 1 overview as a next step, a statistical investigation of the relationship between the itec and the total electron content, both calculated from the same set of electron density profiles that had been obtained from 4000 malvern incoherent scatter radar (isr) profiles (belehaki and kersley, 2003), showed that especially during nighttime intervals the correlation between the two quantities is very high. this work is in progress to disclose to the derivation of the quantitatively relation of the two parameters. 1.2.3.3. indirect validation of prompt ionospheric parameters a significant advantage of ionospheric sounding techniques during the last decade is the possibility to derive in real-time from the ionograms all important ionospheric parameters. the software algorithms built in the real-time ionospheric stations have become more and more intelligent and as a result the automatic derivation of prompt ionospheric parameters is very reliable. the development of nowcasting and forecasting models of the earth’s upper atmosphere able to provide results that can be used for operational applications is based to a great extent on the availability of such real-time data. in the frames of wg 1 activities two efforts have been made aiming to demonstrate the improvement in the models’ performance when updated with real-time parameters and therefore demonstrating indirectly the validity of these autoscaled ionospheric parameters. this work is focused on two areas: improving standard ionospheric maps of the fof2 parameter (zolesi et al., 2004) and improving the iri-2000 electron density profiles (burešová et al., 2003). from another point of view, a comparison was made between the eiscat isr and ionosonde data (lilensten et al., 2003) in an attempt to intercalibrate all instruments able to provide realtime data for space weather applications. a comparison of ionospheric measurements (fof2 parameter) between eiscat and two ionosondes located at the same place showed that the calibration of the eiscat incoherent scatter radar through ionosonde data is not a simple task and can lead to erroneous values when compared to gps. therefore a useful approach could be to calibrate is radars with the plasma line and to inter-calibrate ionosondes with the radars over a large set of cases. 1.2.4. operational nowcasting and forecasting services the main category of deliverables provided by wg 1 concerns the development of operational nowcasting and forecasting services. a very important system has been set-up to operate on-line as a result of the collaboration between the rutherford appleton laboratory and the national observatory of athens (http://ionosphere.rcru.rl.ac.uk/). real-time measurements of the critical frequency of f2-layer, fof2, and the propagation factor for a 3000 km range, m(3000)f2 from four european digisondes operating in athens, rome, chilton and juliusruh and the bz-component of the interplanetary magnetic field, bz-imf, from nasa advanced composition explorer (ace) spacecraft mission are combined for the development of a real-time dynamic system, oriented to monitor the ionospheric propagation conditions over europe (cander et al., 2004). the system enables extreme conditions to be quantified so that particularly for telecommunications planning likely variability bounds can be defined. the presented facility is available for both operational purposes and scientific studies and it will be upgraded to accept data from real-time digisondes operated in other areas of the planet according to the users’ requirements. our community can also significantly benefit from the ionospheric nowcasting and forecasting information provided by the regional warning center of warsaw in poland (stanis ⁄lawska and zbyszyński, 2001) and the space weather web operated by rutherford appleton laboratory (cander and ciraolo, 2002; cander, 2003; cander et al., 2003). the services provided by the two research centers are listed in table 1.ii. 1254 anna belehaki and iwona stanis ⁄lawska 1.2.5. modeling techniques leading to new operational services various natural and artificial indicators can perturb earth’s environment and affect the radio-communication quality. therefore, a deeper understanding of the complex, nonlinear physical processes would be very helpful for communication purposes. for instance, the identification of the mutual relations between the quantified ionospheric variability and communication circumstances is expected to enable the construction of relevant models for prediction and forecasting. gathering recent findings and providing follow on studies on the coupling of different atmospheric layers, aiming to improve our understanding, is an important target of the wg 1 activities. the changes in the ionosphere and plasmasphere during space weather events, such as geomagnetic and ionospheric storms are very intense. therefore a sophisticated approach taking into account the global solar wind – magnetosphere – ionosphere interaction is adapted. the investigations undertaken in the frames of cost 271 action covered a wide spectrum of studies that improve and enlarge our knowledge of the ionospheric behavior, which is a requirement for the reliable operation of telecommunication systems. the development and evolution of new techniques and models into operational tools was one of the most important challenges of wg 1 activities. we could distinguish the modeling techniques into two main groups: a) regional ionospheric mapping (burešová et al., 2003; pietrella et al., 2003; stanis ⁄lawska et al., 2004; zolesi et al., 2004). b) modeling and forecasting of ionospheric characteristics (y.k. tulunay et al., 2001; de franceschi et al., 2002; e. tulunay et al., 2002; kutiev and muhtarov, 2003; stankov et al., 2003; antonov et al., 2004). the main characteristics of these models are summarized in table 1.iii. 1.2.6. studies that improve our understanding of the upper atmosphere structure and dynamics two very important studies have been carried out in the frames of the working group 1 activities aiming to study in depth the physics of the upper atmosphere structure and dynamics under the influence of solar and geomagnetic disturbances as well as under anthropogenic activities. table 1.ii. operational services for nowcasting and forecasting the state of the ionosphere over europe. research center operational services regional warning center of warsaw – up to 24 h fof2 forecasts at selected locations. (http://www.cbk.waw.pl/rwc) – up to 24 h regional maps of the ionosphere. space weather web operated by rutherford – real-time plots of fof2 and muf(3000)f2 appleton laboratory at selected locations. (http://ionosphere.rcru.rl.ac.uk) – archive of fof2 and muf(3000)f2. – european tec maps for the previous day. – daily station tec profiles for the previous day. – european tec maps archive. – archive of daily profiles of tec for all european stations. – ionospheric forecast maps of fof2, muf(3000)f2 and nequick derived tec provided up to 24 h ahead using stif. – archive of fof2, muf and nequick derived tec forecasts. 1255 impact of variability of space environment on communications: working group 1 overview t ab le 1. ii i. s um m ar y of t he m od el ’s c ha ra ct er is ti cs . m od el in pu t t ec hn iq ue o ut pu t r ef er en ce t ul un ay e t a l. (2 00 1) . f or ec as ts o f fo f 2 in m id la ti tu de t ro ug h. d at a dr iv en n eu ra l ne tw or k m e t u -n n m od el a pp li ed i n fo f 2 pr ed ic ti on s. k p in de x, lo ca l ti m e, ge og ra ph ic a nd i nv ar ia nt la ti tu de s an d fo f 2 va lu es . f or ec as ti ng f o f 2 in t he m id la ti tu de t ro ug h. s ta nk ov e t a l. (2 00 3) . e le ct ro n de ns it y pr of il es a t m id dl e la ti tu de s. c on st ru ct io n of t he b ot to m si de an d to ps id e el ec tr on p ro fi le , as su m in g e xp on en ti al ,e ps te in or c ha pm an t yp e of v er ti ca l de ns it y di st ri bu ti on . g ro un dba se d m ea su re m en ts o f th e to ta l el ec tr on c on te nt , io no sp he ri c ve rt ic al s ou nd in gs , an d em pi ri ca l va lu es o f th e up pe r tr an si ti on l ev el . r ea lti m e co ns tr uc ti on o f el ec tr on d en si ty p ro fi le s. t ul un ay e t a l. (2 00 2) . r ad ia l di st an ce o f th e e ar th ’s m ag ne to pa us e. n eu ra l ne tw or k sy st em pr op er ly c on st ru ct ed a nd tr ai ne d to l ea rn t he s ha pe o f th e m ag ne to pa us e. s ol ar w in d pa ra m et er s (v el oc it y, de ns it y, pr es su re ) an d in te rp la ne ta ry m ag ne ti c fi el d b z. m ag ne to pa us e cr os si ng . d e f ra nc es ch i et a l. (2 00 2) . f or ec as te d k p in de x. t im e w ei gh te d ac cu m ul at io n of t he 3 h k p in de x. k p in de x fo r pr ec ed in g ho ur s. f or ec as ti ng g eo m ag ne ti c ac ti vi ty . k ut ie v an d m uh ta ro v (2 00 3) . r el at iv e de vi at io n of t he f o f 2 pa ra m et er a s a fu nc ti on o f lo ca l ti m e an d k p in de x. d et er m in at io n of m od el p ar am et er s by f it ti ng t he m od el e xpr es si on t o th e da ta i n ea ch b in . d at a fr om 5 5 io no so nd es , 11 ye ar s of o bs er va ti on s. g lo ba l em pi ri ca l m od el o f fo f 2 pa ra m et er . a nt on ov e t a l. (2 00 4) . f or ec as te d k p in de x. e st im at io n of a m od if ie d b z fu nc ti on w hi ch e xh ib it s a de la ye d re ac ti on t o b z ch an ge s. in te rp la ne ta ry m ag ne ti c fi el d b z, so la r w in d sp ee d an d pr es su re . k p in de x. s ta ni s⁄ l aw sk a et a l. (2 00 4) . t hr ee d im en si on al e le ct ro n co nc en tr at io n pr of il e in st an ta ne ou s m ap s. c on st ru ct io n of m ap s fr om sp at ia ll y di st ri bu te d m ea su re d da ta u si ng k ri gi ng a nd f it ti ng in te rp ol at io n te ch ni qu es . n eq ui t u r m od el , el ec tr on de ns it y pr of il e m ea su re m en ts . u pd at in g th e n eq ui t u r m od el to p ro du ce e le ct ro n co nc en tr at io n m ap s. b ur eš ov á et a l. (2 00 3) . d ef ic ie nc ie s of i r i20 00 w it h an d w it ho ut s to rm o pt io n. c om bi na ti on o f ir i20 00 n (h ) pr of il es w it h re al -t im e n (h ) pr of il es f ro m c o s t 2 71 s pa ce w ea th er d at ab as e. ir i20 00 , re al -t im e m ea su re m en ts o f io no so nd es . r ea lti m e up da ti ng o f ir i el ec tr on d en si ty p ro fi le s. p ie tr el la e t a l. (2 00 3) . r eg io na l m ap s of f o f 2. s pa ce w ei gh te d co rr ec ti on o f th e ba si c gr id v al ue s. t he s pa ce w ei gh te d io no sp he ri c l oc al m od el (s w il m ), pr om pt f o f 2 m ea su re m en ts . t he i s w il m m od el f or in st an ta ne ou s m ap pi ng o f th e cr it ic al f re qu en cy o f th e f 2la ye r in t he e ur op ea n re gi on . z ol es i et a l. (2 00 4) . r eg io na l io no sp he ri c m ap pi ng . e st im at io n of t he e ff ec ti ve su ns po t nu m be r. s ir m , au to m at ic s ca le d fo f 2 va lu es f ro m e ur op ea n di gi so nd es . r ea lti m e up da ti ng o f th e s im pl if ie d io no sp he ri c r eg io na l m od el ( s ir m ) fo r op er at io na l ap pl ic at io ns . 1256 anna belehaki and iwona stanis ⁄lawska a real tool for investigation of the thermospheric-ionospheric interaction under various geophysical conditions has been proposed by michailov and lilensten (2004). ionospheric observations were widely used in the past to obtain thermospheric parameters such as winds, vertical plasma drifts in the equatorial f2-layer, and neutral composition winds. incoherent scatter radar observations turned out to be the most appropriate for such analysis as they provide the most reliable and consistent set of ionospheric plasma parameters in e-, f1and f2-regions. much valuable and interesting information on thermospheric winds, temperature and neutral composition has been obtained from isr data analysis. the main approaches are based on the use of energy equation for o+ ions to find atomic oxygen concentration and neutral temperature at the f2-region heights and on the momentum equation for o+ ions to retrieve meridional winds. in the frames of the work carried out as part of wg 1 activities, at a preliminary stage, worldwide ionospheric reference data on nmf2 and hmf2 were used to establish a system of aeronomic input parameters. at the next stage, this system of parameters was substituted to the ionospheric model to find nmf2 and hmf2, and the results were compared with another set of experimental data, thus confirming the validity of the model, which has been developed. using routine isr observations (ne(h), te(h), ti(h), vi(h) profiles) the method allows to obtain a self consistent set of the main aeronomic parameters responsible for the f2-region formation. the list of derived parameters includes: neutral temperature profile tn(h) depending on the exospheric temperature tex, the temperature at 120 km and the shape parameter which determines the temperature profile, concentration of neutral species, vertical plasma drift, which may be converted to the meridional thermospheric winds, total solar euv flux and ion composition as a result of ne(h) fitting. therefore the method gives a complete description of the upper atmosphere condition in the vicinity of isr facility for the periods of observation. the approach turns out to be very useful for physical analysis of the f2-layer disturbance mechanisms giving a complete picture of the phenomenon in question. a second topic investigated to improve our understanding on the physics of upper atmosphere is the reasons for possible trends in the earth’s atmosphere and ionosphere. trend investigations of different ionospheric and atmospheric parameters have become more and more important during recent years as such trends could be caused by anthropogenic activities (bremer et al., 2004). mainly in the f2-region different analyses have been carried out. the derived trends are mainly discussed in connection with an increased greenhouse effect or by long-term changes of the geomagnetic activity. in the f1-layer the derived mean global trend in fof1 is in good agreement with model prediction of an increasing greenhouse effect. in the e-region the derived trends in foe and h′e are compared with model results of an atmospheric greenhouse effect, or explained by geomagnetic effects or other anthropogenic disturbances. the trend results derived from different ionospheric reflection height and absorption measurements in the lf, mf and hf ranges can at least partly be explained by an increased atmospheric greenhouse effect. 1.2.7. advances in mathematical and physical tools development development of the new tools, techniques, and algorithms can decrease the unreliability of the proposed approach, or, at least, compensate some not fully satisfied effects. lifetime of the action prevents exploiting new techniques and finalizing them to the end-user’s figure, but the demonstration of their necessity creates well-defined further development for diagnostic and application purposes. such direction was represented by the development of the updating of the global or limited-area models, as updating the iri model to produce maps of n(h) over europe, where the findings on deficiencies of iri-2000 model with and without the fof2 storm option were shown (burešová et al., 2003), or updating the swilm model to produce local nowcast maps (pietrella et al., 2003). another very interesting technique for short-term prediction of ionospheric parameters was presented by muhratov et al. (2002). the model consists of one part representing the weighted past time variation of the ionospheric parameter that is to be predicted (φ = fof2 − fof2med ⁄ fof2med) 1257 impact of variability of space environment on communications: working group 1 overview and a second part representing geomagnetic activity as described by the index ap. each part is constructed according to the autocorrelation function of the relative quantity while the interdependence between the two parts is represented by the cross correlation function. the model is termed geomagnetically correlated autoregression model (gcam). testing the model with data from 2 years of high solar activity and 2 years of low solar activity showed a large decrease in the mean squared error by gcam in the first 8-10 h of prediction relative to median based prediction. finally in order to present a better comparison between the gcam and median based prediction errors during disturbed ionospheric conditions, a new error estimate called «prediction efficiency» is used. 1.3. overall evaluation of the progress achieved working group 1 has two main objectives, first the development of tools and models for nowcasting and forecasting ionospheric variability and secondly the stimulation of cooperation at european level. from the review presented it is clear that all objectives have been fulfilled. work of excellent quality published in scientific journals with high impact factors is considered a great scientific achievement of wg 1. operational and experimental services based on the newly developed and/or improved models and algorithms provide substantial improvement in the telecommunication performance. as a result of international collaboration between different institutes in the frames of wg 1 activities (appendix), our colleagues raised funds for specific projects from international organizations. an important output in this direction was our contribution to demonstrate the necessity for action on space weather, since this field is not covered by cost 271, but our community needs this kind of input. the final result was the approval for a new cost action on «developing of scientific basis for monitoring, modeling and prediction space weather» (lilensten et al., 2004). also several proposals and projects were submitted and funded within eu calls based on consortia formed by cost 271 participating institutes. finally, the number of bilateral agreements between cost 271 participating partners has sufficiently increased for the implementation of the cost 271 research programs as well as new subjects. overall the activities in the frames of working group 1 contributed significantly to ec policies. with respect to research and innovation, wg 1 activities contributed a) to the cooperation of several entities at different levels; b) to an increase in the mobility of individuals and ideas between different research organizations; and c) to the reinforcement of european competitiveness mainly with the us and australia. our activities have successfully contributed to maximize european synergy and added value in research cooperation and they have been proved to be a step for further european integration, in particular concerning central and eastern european countries. more advanced work towards the practical implementation of the obtained knowledge is further needed. for future development the major orientations would be in three directions: – the development of models able to operate on line to satisfy the requirements of the scientific community and of the industry oriented users community. – the linkage with other non-european organization in our effort to develop a global system for monitoring and predicting ionospheric and plasmaspheric conditions. – further increase in scientific and technological capacity of the european community by integrating cost 271 results. acknowledgements a.b. would like to thank ioanna tsagouri for her substantial contribution in compiling this report. thanks are also due to ljiljana cander for proof reading and helpful comments. 1258 anna belehaki and iwona stanis ⁄lawska appendix. collaborations in the frames of wg 1 activities – izmiran (russia)-laboratoire de planetologie de grenoble (france). – faculty of aeronautics and astronautics, instabul technical university (turkey)-izmiran (russia)-department of electrical and electronics engineering, middle east technical university (turkey)-information technologies research institute (turkey). – ingv (italy)-aristotle university of thessaloniki (greece). – institute of atmospheric physics (czech republic)-ral (u.k.)-ingv (italy). – laboratoire de planetologie de grenoble (france)-ral (u.k.)-eiscat association (norway)qinetiq (norway). – laboratoire de planetologie de grenoble (france)-ral (u.k.). – src (poland)-observatorio de l’ebre (spain). – noa (greece)-university of wales (u.k.). – noa (greece)-ral (u.k.). – ral (u.k.)-cnr (italy). – noa (greece)-dlr (germany). – bulgarian academy of sciences (bulgaria)-dlr (germany). – ingv (italy)-noa (greece)-ral (u.k.). – dlr (germany)-royal observatory of belgium (belgium). – itu, metu (turkey)-src (poland). – src (poland)-institute of atmospheric physics (czech republic). – institute of atmospheric physics (czech republic)-src (poland)-observatory de l’ ebre (spain)inta (spain)-geophysical observatory (finland). – leibnizinstitute of atmospheric physics (germany)-institute of atmospheric physics (czech republic)-qinetiq (u.k.)-university of tromsø (norway). – ingv (italy)-izmiran (russia)-src (poland). – department of electrical and electronics engineering, middle east technical university (turkey)-goddard space flight center, nasa (u.s.a.)-faculty of aeronautics and astronautics, instabul technical university (turkey)-marmara research center (turkey). references antonov, b., p. muhtarov and i. kutiev (2004): analogue model, relating kp index to solar wind parameter, j. atmos. sol.-terr. phys. (in press). belehaki, a. and l. kersley (2003): statistical validation of the itec parameter, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece (on line: http://www.cost271.rl.ac.uk/). belehaki, a., n. jakowski and b.w. reinisch (2003): comparison of ionospheric ionization measurements over athens using ground ionosonde and gps derived tec values, radio sci., 38 (6), 1105, doi:10.1029/2003rs002868. bremer, j., l. alfonsi, p. bencze, j. laštovička, a.v. mikhailov and n. rogers (2004): longterm trends in the ionosphere and upper atmosphere parameters, ann. geophysics, 47 (suppl. to no. 2/3), 1009-1029 (this volume). burešová, d., lj.r. cander, a. vernon and b. zolesi (2003): results of real-time ionospheric n(h) profile updating over europe using iri-2000 model, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece (on line: http://www.cost271.rl.ac.uk). cander, lj.r. 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(in press). stankov, s.m., r. warnant and j.c. jodogne (2003): improved operational model for real-time reconstruction of the electron density profile at a single location, on cd-proceedings of the 2nd cost 271workshop, 2-4 october 2002, faro, portugal. tulunay, e., e.t. senalp, lj.r. cander, y.k. tulunay and l. ciraolo (2002): forecasting gpstec during high solar activity by neural network technique, on cd-proceedings of the 2nd cost 271 workshop, 2-4 october 2002, faro, portugal. tulunay, y.k. and p.a. bradley (2004): the impact of space weather on communication, ann. geophysics, 47 (suppl. to no. 2), 929-944 (this volume). 1260 anna belehaki and iwona stanis ⁄lawska tulunay, y.k., e. tulunay and e.t. senalp (2001): an attempt to model the influence of the trough on hf communication using neural network, radio sci., 36 (5), 1027-1041. zolesi, b., a. belehaki, i. tsagouri and lj.r. cander (2004): real-time updating of the simplified ionospheric regional model for operational applications, radio sci., 39 (2), rs2011, 10.1029/ /2003rs002936. ag_56.05.13_oladipo_finalonline:layout 6 annals of geophysics, 56, 5, 2013, a0565; doi:10.4401/ag-6247 a0565 magnetic storm effect on the occurrence of ionospheric irregularities at an equatorial station in the african sector olushola abel oladipo1,2,*, torben schüler3 1 university of ilorin, physics department, ilorin, nigeria 2 alexander von humboldt postdoctoral research fellow, institute of space technology and space application, university faf munich, neubiberg, germany 3 institute of space technology and space application, university faf munich, neubiberg, germany abstract large-scale ionospheric irregularities usually measured by gps tec fluctuation indices are regular occurrence at the equatorial region shortly after sunset around solar maximum. magnetic storm can trigger or inhibit the generation of these irregularities depending on the local time the main phase of a particular storm occurs. we studied the effect of nine (9) distinct storms on the occurrence of ionospheric irregularities at fraceville in gabon (lat = −1.63˚, long = 13.55˚, dip lat. = −15.94˚), an equatorial station in the african sector. these storms occurred between november 2001 and september 2002. we used tec fluctuation indices (i.e. roti and rotiave) estimated from 30 s interval rinex data and also we used the storm indices (i.e. dst, ddst/dt, and imf bz) to predict the likely effect of each storm on the irregularities occurrence at this station. the results obtained showed that most of the storms studied inhibited ionospheric irregularities. only one out of all the storms studied (i.e. september 4, 2002 storms with the main phase on the night of september 7-8) triggered post-midnight ionospheric irregularities. there are two of the storms during which ionospheric irregularities were observed. however, these may not be solely attributed to the storms event because the level of irregularities observed during these two storms is comparable to that observed during previous days before the storms. for this station and for the storms investigated, it seems like a little modification to the use of aarons categories in terms of the local time the maximum negative dst occurs could lead to a better prediction. however, it would require investigating many storms during different level of solar activities and at different latitudes to generalize this modification. 1. introduction the ionosphere over the magnetic latitude of about ± 20º centered on the earth’s magnetic equator is referred to as equatorial ionosphere. post sunset equatorial ionosphere shows electron density irregularities of scale sizes varying from centimeters to kilometers and these are generated by plasma instability processes [rishbeth 1981, kelley 1985, 1989, fejer 1996, abdu 2001]. shortly after local sunset e x b vertical drift causes upward lifting of the f-region to an altitude where rayleigh-taylor instability processes are favourable for the generation of ionospheric irregularities. morphology of the equatorial ionospheric irregularities has been studied [e.g., sobral et al. 2002, chu et al. 2005, kintner et al. 2007, oladipo and schüler 2012]. irregularities occurrence has been found to depend on season, geographic location, solar activity, and geomagnetic activities. ionospheric irregularities usually manifest in amplitude and phase fluctuation of trans-ionospheric radio waves passing through the ionosphere which contains these irregularities. these could cause fading and loss of lock on gps l1 and l2 with significant impact on gps positioning accuracy [e.g., knight et al. 1999, skone et al. 2001, doherty et al. 2004, seo et al. 2011]. it is well known that 400-m smallscale ionospheric irregularities are responsible for amplitude scintillation of the trans-ionospheric radio waves like gnss signals. these 400-m small-scale irregularities coexist with large-scale ionospheric irregularities [basu et al. 1999]. large-scale ionospheric irregularities are the focus of the current study. the condition described above for the generation of ionospheric irregularities could change drastically during magnetic storm activity. a magnetic storm could enhance the vertical drift of ionization and this could create the right condition for the generation of ionospheric irregularities even during off-season period. studies on the impact of magnetic storm on the occurrence of ionospheric irregularities have been article history received october 30, 2012; accepted june 11, 2013. subject classification: equatorial ionosphere, ionospheric irregularities, tec (total electron content), rot (rate of change of tec), magnetic storm. done mainly in the america and asian sectors of the equatorial region [e.g., aarons and dasgupta 1984, dabas et al. 1989, aarons 1991, basu et al. 2001, biktash 2004, li et al. 2006, campos de rezende et al. 2007, li et al. 2008]. depending on the local time the main phase of a storm occurs, the storm could trigger or inhibit ionospheric irregularities occurrence. among the indices that are being used to predict the effect of a storm on the occurrence of ionospheric irregularities are disturbance – storm time (dst), rate of change of dst (ddst/dt), and bz component of the interplanetary magnetic field (imf bz). in the current study, we examine nine (9) storms and their consequent effect on large-scale irregularities occurrence at franceville in gabon, a station in the equatorial region of the african sector. as mentioned above, previous studies were from the american and asian sectors. the current study is an attempt to see the likely effect of storms on irregularities occurrence, using gnss data, at a station in the african sector of the equatorial ionosphere. 2. data and analyses we have used rinex data from the igs network for franceville in gabon (lat = −163˚, long = 13.55˚, dip lat. = −15.94˚) during 9 distinct storms that occurred between november 2001 and september 2002. dst values available online at http://wdc.kugi.kyoto-u.ac.jp/ dstdir/ and bz component of the interplanetary magnetic field (imf bz) from high resolution omni (hro) available at ftp://nssdcftp.gsfc.nasa.gov/spacecraft_da ta/omni/high_res_omni/ for the period of these storms were also used. to obtain relative tec, we have used the leveled carrier phase measurement of tec, i.e. the so called carrier phase to code leveling. because phase measurements are sometimes affected by or subjected to cycle slips, we used blewitt [1990] algorithm for detection and correction of cycle slips in the phase measurements. the algorithm describes the procedure on how to detect and correct cycle slip in phase measurements using both the wide lane combination and the ionospheric combination methods. the algorithm employs the use of undifferenced, dual frequency carrier phase measurements and a smoothly varying ionospheric electron content. full description of this algorithm can be found in blewitt [1990]. butterworth filter of order 4 was used to remove the trends from the data. butterworth filter of order 4 is a filter developed by butterworth [1930]. the full description of the butterworth filter can be found in butterworth [1930]. two tec fluctuation indices (i.e. rot derived indices) in equations 1 and 2 are used for this study. (1) (2) where n is the satellite number, 0.5hr is half an hour (0, 0.5, 1, ... 23.5, 24 ut), i is the 5 minutes section within half an hour (i=1,2,3,4,5,6), nsat(0.5hr) is the number of satellites observed within half an hour and k is the number of roti values available within half an hour for a particular satellite. roti (after pi et al. [1997]) values are for a single satellite while rotiave (after oladipo and schüler [2013]) gives 30 minutes average values for all satellites and the values are valid over a station. using gnss data to derive tec fluctuation indices, the choice of elevation cut-off mask is very important so as not to mix multipath effect with ionospheric irregularities. to screen out multipath effects elevation cutoladipo and schüler 2 (a) (b) (c) figure 1. largescale irregularities as manifested in (a) relative stec (uncalibrated) in tecu, (b) rot in tecu/min, and (c) roti index after pi et al., 1997 in tecu/min for prn 9 at fraceville on may 22, 2002 (1 tecu=1016 electrons m-2). elevation cut-off mark of 25 degree was used. the plots indicate depletion of ionization (shown in a) due to large scale ionospheric irregularities and these irregularities are captured by rot and roti indices. ,roti rot rot rot t tec2 2 d d = = . . , . , roti hr nsat hr k roti n hr i 0 5 0 5 1 0 5 ave n nsat i k =^ ^ ^ h h h/ / 3 off mask of 25 degrees was used in the current study. figure 1 shows a typical example of large-scale ionospheric irregularities as captured by tec depletion (figure 1a) and the corresponding rot (figure 1b) and the phase fluctuation index roti (figure 1c). the irregularities as shown in this figure is observed for satellite with prn 9 around 03.00 lt at fraceville and the two indices (i.e. rot and roti) really captured the level of depletion as observed in tec values. 3. results and discussions we have studied the effect of nine (9) distinct storms that occurred between november 2001 and september 2002 on the occurrence of ionospheric irregularities at fraceville. some of these storms have more than one main phase i.e. storm(s) that occurred during the recovery phase of another storm. however, three selected storms out of all the storms studied are presented in detail in sections 3.1 to 3.3 while the summary of all the storms studied and discussions are presented in section 3.4. 3.1. magnetic storm of november 5-9, 2001 figure 2 shows the panel plots for the storm of november 5-9, 2001 at fraceville for (a) dst in nt, (b) rate of change of dst (ddst/dt in nt/h), (c) imf bz in nt, (d) rotiave in tec/min, and (e) roti in tec/min. the vertical dashed line indicates the time in ut when minimum ddst/dt occurred. it is important to mention here that figure 2e is the plot of roti values obtained for all the observed satellites for the day. as shown in figure 2a, the storm started with sudden commencement at 12:00 ut on november 5 and was followed by an initial phase which lasted for about 7 hours (i.e. from 12:0019:00 ut, november 5). the main phase which lasted for about 13 hours was from ~20:00 ut, november 5 to 09:00 ut, nov. 6, while the recovery phase lasted for about 3 days (i.e. november 7-9). the maximum negative dst value was −292nt and it occurred at 07:00 ut (08:00 lt). there was a data gap in imf bz on november 6, 2001, the day the main phase of the storm occurred. for this storm, maximum negative ddst/dt of -168nt/h was recorded at around 04:00 ut (05:00 lt). storm and ionospheric irregularities (a) (b) (c) (d) (e) figure 2. effect of november 5-9, 2001 storm on ionospheric irregularities occurrence at franceville, gabon. the plots are for (a) dst in nt (b) rate of change of dst (ddst/dt in nt/h), (c) imfbz in nt, (d) rotiave in tec/min, and (e) roti in tec/min. the vertical dashed line indicates the time in ut when maximum negative ddst/dt was observed. as indicated in figure 2d,e this storm inhibited irregularities occurrence during the night of november 6-7. 3.2. magnetic storm of april 17-22, 2002 figure 3 shows the panel plots for the april 17-22, 2002 storm. the plot is similar to figure 2. the vertical dashed lines indicate the time in ut when maxima negative values of ddst/dt were observed. as shown in 3 (a), magnetic storm of april 17-22, 2002 has no sudden commencement. the storm has about four main phases which covers about four days (i.e. april 17-20) and has its maximum on april 20 at 09:00 ut (10:00 lt at franceville) with dst value of −149 nt. each of these four main phases has its maximum at 18:00 ut (19:00 lt) on april 17 with dst value of −98nt, at 12:00 ut (13:00 lt) on april 18 with dst value of −124nt, at 19:00 ut (20:00 lt) on april 19 with dst value of −126nt, and at 09:00 ut (10:00 lt) on april 20 with dst value of −149nt. the ddst/dt value (figure 3b) followed similar pattern as dst during the storm with more than four minima values that nearly coincided with that of dst values. imf bz (figure 3c) turned southward more than five times during the main phases of this storm. for each of these southward, imf bz values remain negative for a fairly long period of time. for the four days that the main phases of this storm occurred, ionospheric irregularities were inhibited as shown in figure 3d,e. 3.3. magnetic storm of september 3-9, 2002 similar to figures 2 and 3, figure 4 shows the panel plots for the september 3-9, 2002 storm at franceville. the storm of september 3-9, 2002, with sudden commencement at ~05 ut on september 3, has an initial phase which lasted for about 20 hours (i.e. from ~06:00 ut on september 3 to ~ 02:00 ut on september 4) and it has two main phases. the two main phases occurred on september 4 and on september 7-8 and their negative maximum dst values are −109nt at 06:00 ut (07:00 lt) and −181nt at 01:00 ut on september 8 (02:00 lt) respectively. the vertical dashed lines indicate the time in ut when maxima negative ddst/dt were observed. for this first storm, imf bz (figure 4c) turned southward oladipo and schüler 4 (a) (b) (c) (d) (e) figure 3. effect of april 17-22, 2002 storm on ionospheric irregularities occurrence at franceville, gabon. the plots are for (a) dst in nt (b) rate of change of dst (ddst/dt in nt/h), (c) imf bz in nt, (d) rotiave in tec/min, and (e) roti in tec/min. the vertical dashed lines indicate the time in ut when maxima negative ddst/dt were observed. 5 twice on september 4 the day the first main phase occurred. however, the maximum negative ddst/dt is only observed once at around 05:00 ut (06:00 lt) and the value is −24nt/hr. the effect of this storm is an inhibition of ionospheric irregularities on the night of september 4-5. similarly for the second storm, imf bz turned southward twice on september 7 the day the second main phase occurred. these were also observed in ddst/dt (figure 4b) with maximum negative values at around 19:00 ut (20:00 lt) and 00:00 ut (01:00 lt) and the values are −59.0nt/hr and −28.0nt/hr respectively. this particular storm triggered post-midnight ionospheric irregularities on the night of september 78 as shown in figure 4d,e. 3.4. summary of results and discussions table 1 shows the summary of the results obtained for all the storms studied. as indicated in the table, most of the storms studied inhibited ionospheric irregularities occurrence at fraceville. only one out of all the storms studied (i.e. september 4, 2002 storms with the main phase on september 7-8) triggered post-midnight ionospheric irregularities. there are two of the storms (i.e. storms of february 6 and august 21, 2002) during which ionospheric irregularities were observed. however, these may not be solely attributed to the storms event because the level of irregularities observed during these two storms is comparable to that observed during previous days before the storms. previous results have shown that the effect of a magnetic storm on the occurrence of ionospheric irregularities at a particular station depends on the local time of the day at which the main phase on the storm occurs. the general consensus is that a magnetic storm can trigger (or inhibit) ionospheric irregularity occurrence when the storm enhances (or reduces) the e × b vertical drift responsible for the upward lifting of the fregion to an altitude where rayleigh-taylor instability processes are favourable for the generation of ionospheric irregularities. different researchers have put forward different explanations, using different indices, for the likely effect of storm on the occurrence of ionospheric irregularities. for example, the disturbance – storm time (dst) variations have been considered to be the best criteria to determine the development of the equatorial irregularities during a storm [dabas et al. storm and ionospheric irregularities (a) (b) (c) (d) (e) figure 4. effect of september 3-9, 2002 storm on ionospheric irregularities occurrence at franceville, gabon. the plots are for (a) dst in nt (b) rate of change of dst (ddst/dt in nt/h), (c) imfbz in nt, (d) rotiave in tec/min, and (e) roti in tec/min. the vertical dashed lines indicate the time in ut when maxima negative ddst/dt were observed. 1989, aarons 1991, aarons et al. 1997]. as pointed out by aarons [1991], if the maximum ring current energy, as indicated by maximum negative value, occurs during the midnight to post-midnight time period irregularities would be generated. if the maximum negative dst, the period before recovery set in, occurs in the early afternoon, irregularities would be inhibited. if the maximum occurs around sunset or shortly after sunset, then there would be no effect on the generation of irregularities that night. these have led to three categories based on the local time that maximum negative dst value occurs; 00-06 lt (generation of irregularities), 10-16 lt (inhibition of irregularities) and 18-22 lt (no effect). when these categories by aarons [1991] based on the local time the maximum negative of dst occurred were used in the current study, we observed that only 5 out of 15 main phases studied could be predicted correctly using these categories. it is very important to note that when we have complex shapes in dst variations, the dst category conception cease to have its conventional meaning. we observed complex shapes in dst variation for some of the storm studied but there are some without complex variation in dst that are still predicted wrongly. also it is important to note that these categories by aarons [1991] left out some hours of the day around sunrise, sunset and before midnight. as observed in this study, some of the main phases had their maximum negative dst recorded during these hours. we then applied the categories by dabas et al. [1989], i.e. 06-18 lt (inhibition of irregularities), 18-00 lt (no effect) and 0006 lt (generation of irregularities). nine (9) out of the 15 main phases could be predicted correctly, three fell close to the boundary, while the remaining four could not be explained based on these categories. li et al. [2008], instead of using dst, gave three categories which are similar to those of aarons [1991] based on the local time the maximum negative value of ddst/dt occurred i.e. 00-06 lt (inhibition of irregularities), 10-16 lt (no effect) and 18-22 lt (generation of irregularities). we observed that only 5 out of 15 main phases studied fit in into these categories. another index being used is imf bz. the southward turning of imf bz (i.e. when imf bz is negative) usually signals the transfer of energy, mass, and momentum from the solar wind flow to the magnetosphere. the negative imf bz is believed to enhance e x b vertical drift while positive imf bz does the opposite. a good example of this is the storm of september 2002. during the second main oladipo and schüler 6 nov. 05, 2001 feb. 02, 2002 feb. 06, 2002 mar. 23, 2002 apr. 17, 2002 may 11, 2002 may 23, 2002 aug. 21, 2002 sept. 03, 2002 12:00 ut (309) 00:00 ut (033) 12:00 ut (082) 11:00 ut (131) 10:00 ut (143) 20:00 ut (230) 05:00 ut (246) -292 (07:00), 310 -86 (10:00), 033 -82 (21:00), 036 -100 (10:00), 083 -94 (17:00), 083 -98 (18:00), 107 -124 (19:00), 108 -126 (09:00), 109 -149 (09:00), 110 -110 (20:00), 131 -109 (18:00), 143 -47 (02:00), 231 -71 (00:00), 232 -104 (03:00), 233 -109 (06:00), 247 -181 (01:00), 251 -168 (04:00), 310 -25 (05:00), 033 -28 (21:00), 036-17 (09:00), 083 -9 (14:00), 083 -27 (17:00), 107 -14 (05:00), 108 -24 (14:00), 109 -38 (08:00), 110 -41 (17:00), 131 -83 (12:00), 232 -24 (00:00), 131 -12 (00:00), 232 -21 (00:00), 233 -48 (05:00), 247 -59 (19:00), 250 -28 (00:00), 251 inhibit inhibit no effect inhibit inhibit inhibit inhibit inhibit inhibit inhibit no effect no effect no effect inhibit trigger post mind-night storm time of ssc, (doy) min. dst (time in ut), doy (lt = ut+1) min. (time in ut), doy irregularities occurrence table 1. summary of the results obtained on the effect of magnetic storms on ionospheric irregularities occurrence at fraceville 7 phase which occurred on the night of september 7-8, imf bz turned negative and post-midnight irregularities was observed. 4. conclusions we have studied the effect of nine (9) different storms on the occurrence of ionospheric irregularities at fraceville in gabon, an equatorial station in the african sector. these storms occurred between november 2001 and september 2002 which was a period around the last solar maximum. we used tec fluctuation indices (i.e. roti and rotiave) estimated from 30 s interval rinex data and these give the level of large scale ionospheric irregularities. these irregularities of course are known to coexist with the 400-m small-scale irregularities responsible for the amplitude scintillation of gnss signals. we also used three storm indices (dst, ddst/dt, and imf bz) for this study. out of all the storms studies only one triggered post-midnight ionospheric irregularities, two distinct storms with four main phases had no effect, while the remaining storms (or main phases) inhibited ionospheric irregularities at franceville. for this station and for the storms investigated, it seems more plausible to use a modified aarons [1991] categories (i.e. 06-18 lt (inhibition of irregularities), 18-00 lt (no effect) and 00-06 lt (generation of irregularities) based on the local time the maximum negative dst occurs to predict the likely effect of a storm on the occurrence of ionospheric irregularities. however, a comprehensive study, involving many storms during different level of solar activities and at different latitudes, would be required to generalize this modification. acknowledgements. the authors would like to thank the igs community (international gnss service) for granting access to the dual-frequency gps data on the igs network used in this study. also we acknowledgement the space physics data facility (spdf) and national space science data center (nssdc) for making high resolution omni (hro) data publicly available for scientific use. o.a. oladipo would like to appreciate alexander von humboldt foundation for supporting his postdoctoral research fellowship in germany during which the work presented in this paper was carried out. references aarons, j. and a. dasgupta (1984). equatorial scintillations during the major magnetic storm of april 1981, radio sci., 19, 731-739. aarons, j. 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(1981). the f-region dynamo, j. atmos. terr. phys., 43, 387. seo j., t. walter and p. enge (2011). availability impact on gps aviation due to strong ionospheric scintillation, ieee transactions on aerospace and electronic systems, 47 (3), 1963-1973. skone s., k. knudsen and m. de jong (2001). limitations in gps receiver tracking performance under ionospheric scintillation conditions, phys. chem. earth (a), 26 (6-8), 613-621. sobral, j. h. a., m. a. abdu, h. takahashi, m. j. taylor, e. r. de paula, c. j. zamlutti, m. g. de aquino and g. l. borba (2002). ionospheric plasma bubble climatology over brazil based on 22 years (1977-1998) of 630 nm airglow observations, j. atmos. sol. terr. phys., 64, 1517-1524. *corresponding author: olushola abel oladipo, university of ilorin, physics department, ilorin, nigeria; institute of space technology and space application, university faf munich, neubiberg, germany; email: ooladipo@unilorin.edu.ng; olushola.oladipo@unibw.de. © 2013 by the istituto 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice sulla d i s p e r s i o n e a n o m a l a d e l l e o n d e m a g n e t o i d r o d i n a m i c h e p . e . v a l l e i n t r o d u z i o n e . e noto clie le onde magneto-idrodinamiche hanno una notevole importanza in questioni di fisica cosmica in generale e di geofisica in particolare. la possibile esistenza di tali onde f u posta in evidenza da ii. alfvén nel 1942. in seguito vari autori hanno condotto ricerche sulla propagazione delle onde magneto-idrodinamiche, anche nel tentativo di rendere meno restrittive le ipotesi fondamentali su cui si basa la teoria di alfvén. non sembra peraltro che sia stata p r e s t a t a u n a sufficiente attenzione al f a t t o che le onde magneto-idrodinamiche presentano il fenomeno della dispersione anomala. questa nota ha lo scopo di mettere in evidenza l'andamento di questo fenomeno, il quale potrebbe avere, in qualche caso, u n certo interesse. nessun cambiamento è stato a p p o r t a t o alle ipotesi fondamentali della teoria di alfvén. è s t a t a soltanto riveduta la trattazione formale per renderla più semplice e rigorosa. % * l e o n d e m a g n e t o i d r o d i n a m i c h e . i n seno ad u n fluido indefinito, omogeneo ed isotropo, esista un campo magnetico primario costante h0. si supponga che il fluido sia incompressibile, non viscoso, non ferromagnetico e scarico ed abbia u n a conducibilità elettrica a. se il fluido viene p e r t u r b a t o , il suo movimento e il campo elettromagnetico, di cui diverrà sede, si condizioneranno reciprocamente. ove si prendano in considerazione movimenti sufficientemente lenti da rendere trascurabile la corrente di spostamento nei confronti 3 7 8 p . e . v a l l e della corrente di conduzione, il sistema delle equazioni del campo elettromagnetico e del moto del fluido, è, con evidente significato dei simboli ed in u n i t à giorgi, [1] [2] rot h = j à h rot e — — u j = a (e i / ì v a h ) q j ^ = ^ j a h g r a d (p+gu) a cui devono essere aggiunte le condizioni div h = 0 div e = 0 div j = 0 div v = 0 posto h = h0 + h [3] tenendo conto della identità dv ì>v ,1 7l t = v a r o t v + grad — v2 e ponendo rot h al posto di j nella q u a r t a equazione del sistema [1], si lia rot h = j i h rot e = — u ~ì)t j = ( t [ e + / w v a ( h 0 + h)] [1'] a v q — — g v a rot v = — m h a rot h — u h„ a rot h — 3 t • grad [p + g v + y va le onde magneto-idrodinamiche di alfvén si ottengono dalle [1'] imponendo che le forze di massa e di pressione siano equilibrate dalle forze magnetiche, in modo che il moto del fluido sia determinato dal solo campo primario h 0 . si dovrà p e r t a n t o porre grad (p + e u + — = 0 [4] v i " h [5] s u l l a d i s p e r s i o n e anomala d e l l e o n d e m a g n e t o i d r o d i n a m i c h e 3 7 9 ossia grad (p + q u + ^ p l f i j = 0 . [4'] dalla posizione [5] risulta che le linee di forza di h coincidono con le linee di flusso della velocità del fluido. il campo magnetico h è, per così dire, congelato nel fluido. tenendo conto delle [4] e [5] il sistema [1'] diviene rot h = j 3 h rot e = — li ^ ìt j = a ( e + / i v a h 0 ) 1 j 5 v q — = — h 0 a rot h . u z si esegua ora la divergenza di ambo i membri della terza equazione del sistema [1"]. tenuto conto delle condizioni [2], si ottiene div v a h 0 = 0 ossia h 0 x rot v = 0 . [6] questa relazione mostra, in v i r t ù della [5], che la densità di corrente j è ortogonale al campo primario h0. se poi si moltiplicano scalarmente per h0 ambo i membri della q u a r t a equazione del sistema [1"], risulta ± ^ ( v x h . ) = 0 [7] e quindi, supposta nulla la componente stazionaria della velocità in direzione del campo primario, anche v ed h sono ortogonali al campo primario h0. p e r ottenere un'equazione contenente la sola incognita h, si esegua la rotazione di ambo i membri della terza equazione del sistema [1"], esprimendo j ed e in funzione di h mediante la prima e la seconda equazione. si ha / ~òh \ rot rot h = /u a ( r o t v a h„ — j . [8] poiché div h = 0, l'identità rot rot h = — ah + grad d i v h consente di scrivere la [8] nella forma 1 ò h ah + rot v a h 0 = — • (j[x ot 3 8 0 p . e . v a l l e p e r eliminare v, si derivi la precedente parzialmente rispetto al tempo e vi si introduca la q u a r t a equazione del sistema [1"]. risulta 1 ?» — a h — rot g[l t> « h„ a rot h a h , s2h a<2 [9] d a t o che (h0a r o t h ) a h , = — h„ (h0 x r o t h ) + h0 2 r o t h t e n u t o conto della [5] e della [6], la [9] diviene o/li s t e [10] si deve quindi cercare u n a soluzione della [10] soddisfacente aue condizioni div h = 0 h0 x rot h = 0 h0 x h = 0 . [11] l a velocità, la pressione, la densità di corrente e il campo elettrico, saranno d a t e dalle relazioni v = po— qu h* j = rot h 1 [12] e = r o t h — ll 1 / — h a h0 . a \ p i n vista dello scopo che si propone di raggiungere questa nota, è sufficiente studiare la propagazione di u n ' o n d a elementare piana. assunto u n riferimento cartesiano col campo primario h0 in direzione dell'asse z, le [11] si scrivono 3 hx ~òhv — + — ~òx iy = 0 c> llv ~òhz ìix ~òy k = 0 . = 0 [11'] sulla d i s p e r s i o n e anomala d e l l e o n d e m a g n e t o i d r o d i n a m i c h e 3 8 1 se ci si limita a considerare un'onda polarizzata nel piano xz, preso l'asse y coincidente con la direzione h, risulta hx = 0 . le [11'] mostrano poi che hy non dipende dalle coordinate x ed y, pert a n t o la [10] diviene ofl mìz* + q 0 • l10j un'integrale della precedente equazione ha la forma hy = kyen<»t-iz) [ 1 3 ] con ì i ^ s ^ i — ] = a>>. [14] \ e a" daue [12] risulta inoltre vy = e j { u t f z ) p = po — qtj—^kye / i l ! „ 2 i ( a > t / s ) 2 ìf .. 1 / v jt \ 7, j{<ot — fz) ex = i fi | / ^-h0jh0ye [12'] le relazioni [13] e [12'] mostrano che le onde magneto-idrodinamiche sono onde trasversali, le quah si propagano in direzione del campo primario h 0 . l a d i s p e r s i o n e a n o m a l a d e l l e o n d e m a g n e t o i d r o d i n a m i c h e . la velocità di fase e il coefficiente di assorbimento delle onde magneto-idrodinamiche, si ottengono dalla [14]. indicando con a la lunghezza d'onda e con le il coefficiente di assorbimento, la [14] diviene 3 8 2 p . e . v a l l e è conveniente i n t r o d u r r e la frequenza ridotta dove q = w [16] «o ft)0 = — [17] q e la velocità delle onde magneto-idrodinamiche nel caso che la conducibilità sia infinita, cioè f . ^ f . l'equazione [15] risolta rispetto a 2jr/a ed a a; fornisce 2n w0q ìt/t+f + 1 x v01/2~ | / l + q2 [18] q [19] voi/2 |/l+92 se si indica con v la velocità di fase e si pone a-o = v ° [21] to0 i valori ridotti della velocità di fase e del coefficiente di assorbimento risultano o anche vo _ 1/2 f / l + g2 _ 7 +1 7 = 1 3 ] / i + ~ t y , 7 i t 7 1 ' o q le q / j / l + ? 2 1 | / 2 f / l + ^ [22] [22'] [23] si osservi che se q tende a zero, la velocità di fase t e n d e a v0 e il coefficiente di assorbimento tende a zero, m e n t r e se q cresce, crescono la velocità di fase e il coefficiente di assorbimento. s u l l a d i s p e r s i o n e anomala d e l l e o n d e m a g n e t o i d r o d i n a m i c h e 3 8 3 p e r t a n t o , ferme restando le altre grandezze, la velocità di fase delle onde magneto-idrodinamiche cresce con la frequenza. come è noto tale fenomeno prende il nome di dispersione anomala. 4 . 2 1 | 4 . 2 / / / / / / / 2 . 6 / 2 . 6 / 2 . 2 / 2 . 2 1 8 1 . 6 1 . 4 1 8 1 . 6 1 . 4 / 1 8 1 . 6 1 . 4 / 1 8 1 . 6 1 . 4 v a l o r i r i d o t t i d e l l a v e l o c i t à v a l o r i r i d o t t i d e l l a v e l o c i t à 1 i d r o d i n a m i c h e m f u n z i o n e d e i v a l o r i r i d o t t i d e l l a f r e q u e n z a i d r o d i n a m i c h e m f u n z i o n e d e i v a l o r i r i d o t t i d e l l a f r e q u e n z a i d r o d i n a m i c h e m f u n z i o n e d e i v a l o r i r i d o t t i d e l l a f r e q u e n z a 7 0 1 2 3 4 5 6 7 8 9 1 0 i 1 i 1 i . i 1 fig. l. d a t o che la dispersione non interviene se g < 1, m e n t r e è sensibile per q » 1, conviene .sviluppare in serie l'espressione della velocità di fase e del coefficiente di assorbimento nei due casi. a) q < 1, ossia co < aju f 0 2 . l a [22] e la [23] forniscono f ~ f0 [22"] * — y > 3 [23'] 2 a /x f03 3 8 4 p . e . v a l l e queste relazioni mostrano che se la conducibilità del mezzo è grande, le onde magneto-idrodinamiche si propagano come previsto da alfvén. fig. 2. b) q » 1, ossia co > o/u v02. la [22] e la [23], in questo caso, divengono / 2 a l ( 1 _ l a j ^ l ] [ 2 2 n a [x \ 2 co l a prima dehe precedenti relazioni mostra che, in questo caso, la velocità di fase tende a divenire indipendente dalla densità di massa del mezzo e del campo primario al crescere della frequenza. s u l l a d i s p e r s i o n e a n o m a l a d e l l e o n d e m a g n e t o i d r o d i n a m i c h e 3 8 5 la possibilità che onde siffatte abbiano un basso coefficiente di assorbimento dipende dada conducibihtà del mezzo, poiché al diminuire della conducibihtà la velocità di fase cresce e il coefficiente di assorbimento decresce. si può quindi concludere che per opportuni valori deha frequenza, delle costanti che caratterizzano il mezzo conduttore e del campo primario, la velocità di fase delle onde magneto-idrodinamiche potrà risultare notevolmente superiore a quella prevista da alfvén, senza che ciò comporti necessariamente un elevato valore del coefficiente di assorbimento. tale velocità risulterà inoltre praticamente indipendente dal campo primario e dada densità di massa del mezzo. la fig. 1 e la fig. 2 mostrano rispettivamente l'andamento dei valori ridotti deda velocità di fase e del coefficiente di assorbimento, in funzione deha frequenza ridotta. questi valori sono contenuti nella tabella 1. tabella 1. v k v le 9 vo k 9 v* h 0,01 1,000 0,00005 2,50 1,982 0,854 0,05 1,001 0,00125 2,75 2,088 0,922 0,10 1,004 0,00497 3,00 2,192 0,986 0,25 1,023 0,0301 3,50 2,390 1,105 0,50 1,086 0,109 4,00 2,576 1,212 0,75 1,178 0,212 4,50 2,752 1,311 1,00 1,287 0,322 5,00 2,920 1,404 1,25 1,404 0,428 6,00 3,232 1,572 1,50 1,523 0,527 8,00 3,787 1,865 1,75 1,641 0,619 10,00 4,276 2,117 2,00 1,758 0,703 20,00 6,21 3,08 2,25 1,871 0,781 100,00 14,2 7,04 3 8 6 p . e . v a l l e l ' a n d a m e n t o della velocità ridotta in funzione del coefficiente di assorbimento ridotto, è illustrato dalla fig. 3. v/ 4 , 5 v/ 4 , 5 / v/ 3 . 5 v/ 3 . 5 / v/ v/ v a l o r i r i d o t t i d e l l a v e l o c i t i d i ' d i r d e l l e o n d e m a g n e r ò i d r o d i n a s m i c h e in r u n z i o n e d e i v a l o r i r i a o t = t i d e l c o e f f i c i e n t e di a s s o r b i m e n t o v/ v a l o r i r i d o t t i d e l l a v e l o c i t i d i ' d i r d e l l e o n d e m a g n e r ò i d r o d i n a s m i c h e in r u n z i o n e d e i v a l o r i r i a o t = t i d e l c o e f f i c i e n t e di a s s o r b i m e n t o v/ v/ 0 , 1 0 0 , 2 5 0 , 5 0 0 , 7 5 1 1 . 2 5 1 , 5 0 1 , 7 5 2 2 , 2 5 v fig. 3. se q » 1, il coefficiente di assorbimento è approssimativamente proporzionale alla velocità di fase, come risulta dalla [22"'] e dalla [23"]. riassunto viene valutato l'effetto della dispersione anomala sulla propagazione delle onde magneto-idrodinamiche. si mostra che, per opportuni valori della frequenza, delle costanti che caratterizzano fisicamente il mezzo conduttore e del campo primario, la velocità di fase delle onde magneto-idrodis u l l a d i s p e r s i o n e anomala d e l l e o n d e m a g n e t o i d r o d i n a m i c h e 387 namiche può risultare notevolmente superiore a quella prevista da alfvén, senza che ciò comporti necessariamente un elevato valore del coefficiente di assorbimento. abstract the effect of an anomalous dispersioni on the propagation of magnetohydrodynamic waves is discussed in the present article. it is demonstrated that the phase velocity of tlie magneto-hydrodynamic ivaves may be considerably higher than the one envisaged by alfvén. layout 6 1 annals of geophysics, 62, 2, vo223, 2019; doi: 10.4401/ag-7782 “far-field boundary conditions in channeled lava flow with viscous dissipation„ marilena filippucci*,1, andrea tallarico1, michele dragoni2 (1) dipartimento di scienze della terra e geoambientali, università di bari, bari, italy (2) dipartimento di fisica e astronomia, università di bologna alma mater studiorum, bologna, italy 1. introduction laboratory studies have extensively demonstrated that lava rheology under certain conditions including vesicularity [stein and spera, 1992; badgassarov and pinkerton, 2004], crystalline concentration [pinkerton and stevenson, 1992; smith, 2000; sonder et al., 2006; champallier et al., 2008] and in a certain temperature range [shaw et al., 1968] assumes non-newtonian pseudoplastic properties. when the problem of lava flowing under the effect of gravity is resolved, the power law rheology introduces a non-linearity in the diffusion term of the momentum equation and consequently an analytical solution of the differential equations governing the motion does not exist. furthermore, if the viscosity function also includes temperature dependence, the thermal equation is coupled to the dynamic equation. hence the need to solve numerically the thermo-dynamic equations describing the flows of a fluid as complex as lava. in the thermal modeling of lava flows, the greatest difficulties are due to the different thermal exchanges both external (surface thermal radiation, forced convection, conduction at the base) and internal (axial advection, viscous dissipation, latent heat, internal conduction) to be taken into account. despite the great progress made in numerical modeling, it is necessary to assume some simplifying hypotheses. the need for simplifying assumptions in numerical models of lava flows is described in review articles [costa and macedonio, 2005b; cordonnier et al., 2015; dietterich et al., 2017]. the authors examine the most relevant works on the numerical modeling of lava flows and what emerges is that, due to the high complexity of transport equations, the numerical solution of the complete threedimensional problem for real lava flows is often inarticle history receveid may 30, 2018; accepted october 23, 2018. subject classification: rheology; magmas; thermodynamics; lava flow; viscous dissipation; solid boundary. abstract cooling and dynamics of lava flowing in a rectangular channel driven by the gravity force is numerically modeled. the purpose is to evaluate the thermal process as a function of time involving the liquid lava in contact with the solid boundary that flanks lava. lava rheology is dependent on temperature and strain rate according to a power law function. the model couples dynamics and thermodynamics inside the lava channel and describes the thermal evolution of the solid boundary enclosing the channel. numerical tests indicate that the solution of the thermo-dynamical problem is independent of the mesh. the boundary condition at the ground and at the levees is treated assuming a solid boundary around the lava flow across which lava can exchange heat by conduction. a far field thermal boundary condition allows to overcome the assumption of constant temperature or constant heat flow as boundary conditions, providing more realistic results. the effect of viscous heating is evaluated and discussed. tractable. this has led to concentrate the main efforts on the development of software able to quickly describe the evolution of a lava flow, based on simplified theoretical models, for the purpose of volcanic hazard assessment. parallel to the development of general numerical models closely linked to volcanic hazard, it is necessary to study the details of what happens when, during an effusive process, a fluid like lava flows in a channel. filippucci et al. [2017] discussed the problem of viscous dissipation in channeled lava flows and found that the effect of viscous heating is irrelevant for the most of the channel domain apart from the boundaries where some temperature differences where noticed. these temperature differences brought the authors to hypothesize that the fluid motion can lose the laminarity at the boundary (since locally the reynolds number exceeds the laminar/turbulent threshold). the same effect was found by costa and macedonio [2005] who found that the fluid can develop secondary flows at the boundaries as an effect of viscous heating. in dealing with problems involving the heat equation, in this case coupled to the momentum equation by the rheological function, the boundary condition for a channeled flow is classically chosen between constant temperature and constant heat flow [patankar, 1980; ferziger and peric, 2002]. from the observation that the effect of viscous heating is observed only at the boundaries, in this paper we considered a third possibility, that is a conductive solid boundary with which the fluid can exchange heat. in particular, in this paper we focus on the thermal process involving the liquid lava in contact with the solid edges of the channel, assuming that the fluid can cool down by losing heat by conduction in the hosting rocks and by radiation in the atmosphere and can heat up by the effect of heat advection and viscous dissipation. some authors [costa et al., 2007; filippucci, 2018] used far-field boundary conditions to model the thermal interaction between the hot fluid flow and the host rock with numerical methods. similarly, in this work, the solid boundary condition is treated realistically considering that the rock that hosts the lava flow can exchange heat by conduction. 2. mathematical problem the constitutive equation for a power law fluid is (1) where σij is the stress tensor, eij is the strain rate tensor, k is the fluid consistency, n is the power law exponent which is a measure of nonlinearity, and (2) where i2 is the second invariant of the strain rate tensor. the apparent viscosity ηa of the fluid is (3) if n is lower than 1, the fluid is pseudoplastic and it thins with an increase in stress. if n is equal to 1, the fluid is newtonian. if n is greater than 1, the fluid is dilatant and it thickens with an increase in stress [white, 2005]. we assume a viscous fluid flowing in the x direction in a rectangular conduit inclined at an angle α, perpendicularly to the section of the conduit in the yz plane. the channel is surrounded by edges of solid material of thickness as with which it can exchange heat by conduction. the channel width is 2al, thickness hl and length l. the sketch of the model with the coordinate system is shown in figure 1 and the values of the parameters are listed in table 1. the flow is laminar and subjected to the gravity force. pressure changes are negligible with respect to body forces. the velocity is approximately axial and varies with the lateral coordinates vx(y,z), vy =0, vz=0. the fluid is isotropic and incompressible with constant density ρ, thermal conductivity k, specific heat capacity cp. the equation of motion in the transient state for a gravity driven flow down an inclined rectangular channel is [filippucci et al., 2013a]: (4) where vx is the x component of velocity, g is the gravity acceleration and α is the slope angle. the apparent viscosity is [filippucci et al., 2005]: (5) where vx is the x component of velocity and both the fluid consistency k and the power law exponent n depend on temperature. the temperature dependence of k and n is given by hobiger et al. (2011): filippucci et al. 2 (6) (7) where k0, p1, p2, p3 and p4 are constant parameters listed in table 1. the numerical problem of the dynamics of lava flows was already detailed, by studying the sensitivity of the solution to the variation of the power law exponent n and to the fluid consistency k [filippucci et al., 2010; 2011; 2013b], by using a temperature-dependent power-law rheology and analyzing the thermal effects due to heat advection [filippucci et al., 2013a] and to viscous dissipation [filippucci et al., 2017]. the problem of the solid boundaries has been considered by filippucci [2018]. we assume that the fluid flow is transient, laminar and subjected to the gravity force. downflow pressure changes are negligible with respect to the body force. since the channel cross section is rectangular and the reynolds number is low almost everywhere in the domain [filippucci et al., 2017], the assumption of laminar flow implies that the velocity is approximately axial, but it may depend on all the coordinates: vx = vx(x,y,z). 3 far-field boundary conditions in lava flow -a/2 a/2 y z -h as =3al -al -hl al t =t w t =t w t=tw t=tw t=tw h s =3 h l solid edge liquid lava a) b) sy m m et ry ax is figure 1. coordinate system and geometrical parameters. boundary conditions are also indicated. parameter description value al half channel width 1.5 m a total width 12 m hl channel thickness 1.5 m h total thickness 6 m l channel length 100 m g acceleration of gravity 9.8 m s−2 cp specific heat capacity 837 j kg −1k−1 k thermal conductivity 3 w k−1m−1 te effusion temperature 1100 °c ts solidus temperature 900 °c tw wall temperature 30 °c k0 rheological parameter 1 pa s n pl rheological parameter 18.71 p2 rheological parameter 33.4 10 3 k p3 rheological parameter 1.35 p4 rheological parameter 0.85 10 −3k−1 α channel slope 20° εc thermal emissivity 1 ρ density 2800 kg m−3 σ stefan constant 5.668108 w m−2k4 χ thermal diffusivity 1.28 10−6m2s−1 table 1. values of the fixed model parameters filippucci et al. 4 differently to the approach of filippucci [2018], we include the effect of viscous dissipation as an internal heat source. we neglect the effect of the latent heat of crystallization/fusion. the boundary conditions are the no-slip at the walls and the zero-stress at the top of the flow. since the solution is computed in a half domain of width a/2, thickness h and length l, we consider as boundary condition the symmetry of the problem with respect to the xz plane. so, the boundary conditions are the following: (8) (9) (10) t h e initial condition for velocity is the steady state numerical solution at the initial effusion temperature vx (x,y,z, t=0) = vx (te) . the time dependent heat equation inside the channel takes into account the effect of thermal exchange by heat advection, conduction and viscous dissipation: (11) we can neglect the effect of thermal conduction in the flow direction as it is of secondary importance with respect to thermal advection in the flow direction [filippucci et al., 2013a]. the time dependent heat equation outside the channel, in the solid boundary, is purely conductive: (12) the thermal boundary conditions are the assumptions of a radiative heat flux qr at the upper flow surface, a constant temperature tw at the outer solid walls (such a far field condition is imposed at a distance equal to three times the half-width of the channel), a constant effusion temperature te at the vent and the symmetry of the problem with respect to the xz plane: (13) (14) (15) (16) where qr = σεtu 4 and σ is the stefan-boltzmann constant, ε is the surface emissivity of lava and tu is the temperature of the upper surface at z = 0 (the atmospheric temperature is assumed negligible with respect to tu). at time t = 0, the liquid lava has a uniform temperature te, the velocity is the stationary solution of the dynamic equation with t = te and the outer solid boundary has uniform temperature tw. the choice of the initial condition was made in order to compare this solution with that of filippucci et al. (2017). moreover, the assumption of the extrusion temperature of the fluid as starting condition in numerical studies is widely used and accepted in finite element/volume modeling [costa and macedonio, 2003; patrick, 2004; bernabeu et al., 2016; among others]. at time t > 0, the radiative heat flux qr, the far field constant temperature tw and the constant effusive temperature te are imposed. since x = l is the outflow boundary and both temperature and velocity need to be computed there, no boundary condition at x = l is necessary. the dynamic and the thermal equation are coupled by the temperature dependence of viscosity in the dynamic equation and by the viscous dissipation term in the heat equation. the algorithm is written by the authors in fortran language. the space discretization is obtained by the control volume integration method [patankar, 1980] using a static mesh approach and power law interpolation functions between the nearest grid points. the radiative condition at the boundary z = 0 depends on the fourth power of temperature and needs to be linearized in order to be treated as a source term of the heat equation. for the discretization of the transient term, the integration over the time interval t is made by using a fully implicit scheme. the iterative solutor of the discretized equation is the gauss-seidel one [filippucci et al., 2013a, for details of the flow chart procedure]. the solution is tested to verify the independence of the mesh and the test is shown in figure 2. the computational problem was solved by considering three grids of different sizes (52×52, 102×102, 202×202) to 5 far-field boundary conditions in lava flow discretize the (y, z) section, transversal to the fluid direction. the space discretization along the x direction was fixed to 101 control volumes. the time solution is stopped at t = 106 s since for long times the temperature approaches the steady-state solution. the channel geometry for the numerical test is described by the parameters in table 1. the temperature profile along the z coordinate at the outflow boundary slightly varies with varying the mesh size, indicating that the problem is independent of the control volume size. as expected, the finest mesh (201×201) needs a very large computational cost to achieve the convergence, which means a long time for calculation. in the following, for the problem with geometrical parameters listed in table 1, the mesh y × z × x = 102 × 102 × 101 was used as the best compromise between accuracy and computation time. in particular, in this case, the computational time is approximately 2 days of computation for the problem with the viscous dissipation term, and approximately 1 day for the solution without the viscous dissipation term. 3. results and discussion the problem is illustrated in figure 1 with the physical and geometrical parameters given in table 1. we have evaluated the temperature distribution in an inclined lava channel flanked by solid levees, with which the flowing lava interacts thermally: while lava cools down, the levees are heated in turn. results are obtained considering that the lava cools by thermal radiation at the free surface and by thermal conduction at the solid boundaries. moreover, the lava can be heated by the effect of viscous dissipation. the solid surrounding rocks, in turn, heat up by thermal conduction. the problem is transient, and the first 106 s (approximately 1 day) are modeled, assuming that at t = 0 the whole channel has a temperature t equal to the effusion temperature te and the levees have uniform wall temperature tw. the choice of the distance of the constant temperature boundary condition is arbitrary. costa et al. [2007] used a far-field temperature at an arbitrary distance of 10 times the radius of the magma conduit. our choice is dictated as a compromise between far-field and computational time costs. this choice can be considered adequate since, after t = 106 s of cooling, the boundary is still not heated, as it can be seen in figure 3a. in a previous paper, filippucci [2018] has numerically solved the same problem, but neglecting the viscous dissipation term in the heat equation. in figure 3b, we plotted the difference with the solution of filippucci (2018) in terms of temperature t (y, z) map in the vertical cross section at the outflow boundary (x = l) at t = 106 s. it can be observed that the viscous dissipation term has an effect in the parts of the lava channel in contact with the boundaries and the ground. if we consider the whole lava channel, the effect of viscous heating can be considered of secondary importance with respect to the advective term. if we consider the behavior of the fluid in contact with the solid edges, the heat addition due to viscous dissipation can bring the fluid flow to change the motion from laminar to locally turbulent [filippucci et al., 2017] and to develop secondary flows [costa and macedonio, 2003, 2005a] in figure 4, we plotted the temperature variation with time at 8 monitoring points (p1, ..., p8) selected inside the lava channel and corresponding to the black points in figure 4a. if we observe the temperature evolution with time at some points of the channel section in the flow outlet area (figure 4), we realize that there are fluid regions that are almost at constant temperatures equal to those of te and areas with temperature that oscillates without differences between dissipative and non-dissipative case with the exception of point p5 in contact with the ground at the center of the channel. at points p1, p6 and p8, temperature remains constant and equal to that of effusion t(t) = te during the prescribed time. at these points, the temperature difference between the dissipative and non-dissipative case are in the order of figure 2. temperature profile along z coordinate at x=l, y=0, t=105s, for different mesh sizes, as indicated in the figure legend. tenths of a centigrade degree. at point p2, temperature decreases monotonically during the first 103 s approximately and then remains constant for the following time. the effect of viscous dissipation is to dampen the cooling process and to allow the fluid to be at a constant higher temperature in less time. at points p3, p4 and p7, the temperature has an oscillating behavior in time: at first it decreases and then returns to increase without significant differences between the dissipative and nondissipative case and with a greater oscillation amplitude at point p4 than in the other points. at point p5, the temperature behavior with time is similar to the one just described, but in this case the difference between dissipative and non-dissipative case is sharper. the main difference with the work of filippucci et al. [2017] and then between boundary conditions with constant temperature gradient and far field boundary conditions, where temperature gradient is free to vary as a function of the thermal conduction rate, is just shown in figure 4 (points p3, p4, p5 and p7). in fact, at the same points, in the case of boundary condition with constant heat flux, temperature increases monotonically in the presence of viscous dissipation, while it decreases monotonically in the absence of dissipation [figure 7 in filippucci et al., 2017]. considering far field boundary conditions, the temperature initially decreases and after a certain time internal it starts to increase and this oscillation is independent from the viscous dissipation term in the heat equation. this oscillation can be interpreted considering that, as the lava flow is emplaced, the difference in temperature between the host rocks and the hosted lava is so high that it causes a very intense conductive heat flow. this flow cools the lava in contact with rocks, as shown by the descending part of the curves in figure 4 (points p3, p4, p5 and p7). over time, the host rocks heat up as a result of heat conduction and the heat flow begins to decrease and so does the cooling rate of the lava in contact with rocks, leading to the minimum of the curves in figure 4 (points p3, p4, p5 and p7). continuing with filippucci et al. 6 figure 4. temperature vs time at different monitoring points (p1,..,p8) of the lava channel, as indicated in (a). the label on each plot corresponds to the point in (a). solid line: solution of the heat equation considering the viscous dissipation term; dashed line: solution of the heat equation neglecting the viscous dissipation term [from filippucci, 2018]. time, the conductive heat flow becomes no longer the dominant mechanism, since advective heating begins to prevail due to the isothermal core at the center of the channel, that flows with temperature equal to that of effusion. this change in heat transfer mechanism leads the hosted lava in contact with the host rocks to heat up, as shown by the ascending part of the curves in figure 4 (points p3, p4, p5 and p7). the effect of viscous dissipation during the described thermal process is that of reducing the oscillation amplitude figure 4 (points p5). 4. conclusions the purpose of this work is to investigate how the choice of boundary conditions may affect the results of a model of a channeled lava flow, in particular by evaluating the importance of the viscous dissipation term. to do this, we developed a flow model with far-field constant temperature boundary conditions, allowing the liquid lava to exchange heat with the host solid rock. the results were compared with those obtained in a previous work [filippucci et al., 2018] in which the cooling at the levees and at the ground was modeled by imposing a constant conductive heat flow. with the model presented in this paper, we go beyond the classical boundary conditions at the channel walls, which assume constant temperature or constant heat flow, usually adopted in works dealing with lava flow simulation, as reviewed by costa and macedonio [2005b]. far field boundary conditions are more realistic since it is not necessary to impose a constant temperature nor an arbitrary constant temperature gradient at the channel levees and at the ground. we solved numerically the dynamic and heat equations of a lava flowing by gravity inside a channel with rectangular cross section, flanked by a thick solid levee, by using the finite volume method [patankar, 1980]. the dynamic equation is solved only in the liquid domain, while the heat equation is solved both in the liquid and in the solid domain. the viscous dissipation term is considered in the heat equation. the solution was tested in order to verify that the convergence of the numerical problem is independent of the mesh size. the results indicate that the solid edge interacts with the liquid lava, both at the levees and the ground, causing an initial cooling due to heat conduction and a subsequent heating due to heat advection. this thermal process is not affected by viscous dissipation, which only acts by decreasing the temperature variation interval between the cooling and the heating process. on the contrary, when the boundary condition at the levees and at the ground is a constant temperature gradient [filippucci et al., 2017] or a constant temperature [costa et al., 2007], the effect of viscous dissipation at the boundaries appears to be of great importance, causing an increase of the reynolds number from laminar to turbulent values [filippucci et al., 2017] and triggering secondary flows [costa et al., 2007]. filippucci [2018], using the same far field boundary conditions but neglecting viscous heating, found that the channel levees can melt, since they can heat over the solidus temperature. in this work, including viscous dissipation, we observe the same thermal behavior of the edge (figure 3a), but the effect of thermal erosion cannot be evaluated quantitatively. in order to analyze melting of the solid edges and changes in the channel morphology, we should adopt a moving boundary, so as to account that portions of the host rocks may pass from solid to liquid behavior and, vice versa, portions of lava in the channel may pass from liquid to solid behavior. as reviewed by costa and macedonio [2005b], works dealing with lava flows impose that the channel boundaries are taken at constant temperature or at constant temperature gradient. differently, far field thermal boundary condition allows to overcome this simplification and makes the physical treatment of the problem more realistic. acknowledgements. we thank two anonymous reviewers for their suggestions. during this research, m. filippucci was supported by the intervention co-financed by the fund of development and cohesion 2007-2013 apq research puglia region regional program to support intelligent 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(2005). viscous fluid flow. new york: mcgraw-hill. *corresponding author: marilena filippucci, dipartimento di scienze della terra e geoambientali, università di bari bari, italy email: marilena.filippucci@uniba.it © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 9 far-field boundary conditions in lava flow p r i m e m i s u r e s u l l a r a d i o a t t i v i t à d e l l e p r e c i p i t a z i o n i a t m o s f e r i c h e ( * ) l . santomauro a . cigna apparecchiatura strumentale. — i r i l e v a m e n t i della r a d i o a t t i v i t à f u r o n o c o m p i u t i con un c o n t a t o r e di g e i g e r m i i l l e r s e n s i b i l e alle rad i a z i o n i b e t a e g a m m a . l ' a p p a r e c c h i o c o n s i s t e in un a l i m e n t a t o r e ad at e in un amp l i f i c a t o r e a bf con a l i m e n t a z i o n e s e p a r a t a . p e r l ' a l i m e n t a z i o n e ad at è stata a d o p e r a l a u n a r a d d r i z z a t r i c e a due s e m i o n d e , la 5 t 4 , c o l l e g a t a con l e p l a c c h e in p a r a l l e l o . p e r il l i v e l l a m e n t o della c o r r e n t e r a d d r i z z a t a si sono i m p i e g a t i due c o n d e n s a t o r i a c a r t a da 0.25 |.if-1500 v l c o l l e g a t i in serie con due resistenze da 5 m q c i a s c u n a , in p a r a l l e l o , p e r a s s i c u r a r e una unif o r m e r i p a r t i z i o n e del c a r i c o ai t e r m i n a l i dei c o n d e n s a t o r i di filtro. p e r a v e r e la r e g o l a z i o n e della t e n s i o n e n e g a t i v a d'uscita u n a delle due r e s i s t e n z e è stata s d o p p i a l a i n un p o t e n z i a m e t r o da 2 m lì e in u n a fissa da 3 m q . n e l l ' a m p l i f i c a t o r e a bf sono stati i m p i e g a t i due p i c c o l i p e n t o d i rv12 p 2 0 0 0 a g r a n d e a m p l i f i c a z i o n e . l ' a p p a r e c c h i o è m u n i t o di un a l t o p a r l a n t e , di u n a presa per cuffia e di u n a uscita p e r n u m e r a t o r e o o s c i l l o g r a f o . u n diodo vr 92 provvede al r a d d r i z z a m e n t o della c o r r e n t e per l ' a l i m e n t a z i o n e d e l l ' a m p l i f i c a t o r e a bf. l ' a u t o t r a s f o r m a t o r e dell'alim e n t a t o r e è f o r n i t o di due s e c o n d a r i i s o l a t i t r a l o r o p e r l ' a c c e n s i o n e delle valvole. 11 g r u p p o di filtro è c o s t i t u i t o c o m e di c o n s u e t o da due c o n d e n s a t o r i e l e t t r o l i t i c i e da u n a p i c c o l a i m p e d e n z a . i l c i l i n d r o m e t a l l i c o del c o n t a t o r e g m è stato c o l l e g a t o a l l ' i t n e g a t i v a , l ' e l e t t r o d o i n t e r n o a l l ' i n g r e s s o del c i r c u i t o a m p l i f i c a t o r e med i a n t e un cavo s c h e r m a t o e un p i c c o l o c o n d e n s a t o r e . p a r t i c o l a r e c u r a è stata posta n e l l o s c h e r m a r e l ' a l i m e n t a t o r e ad at. metodo delle misure. — l e m i s u r e v e n n e r o c o m p i u t e dal f e b b r a i o 1951 fino al n o v e m b r e 1952 con i n t e r r u z i o n e d u r a n t e i m e s i estivi. s o n o state f a t t e due serie di m i s u r e : la p r i m a (m") con il contatore a v u o t o e la seconda ( m ' ) in presenza d e l l ' a c q u a piovana. ( * ) comunicazione presentata al convegno dell'associazione geofisica italiana, tenuto a r o m a il 17-18 giugno 1953. 3 i ! 2 l . s a n t o m a u r o a . c i g n a come valore indicativo della radioattività delle precipitazioni è stata assunta la differenza r delle medie degli impulsi delle due serie di misure. queste u l t i m e sono state eseguite su una quantità di acqua pari a 250 cc raccolta in una b a c i n e l l a di vetro. nel corso delle r i c e r c h e , la distanza bacinella-contatore è stata sempre la mer • o o 0 a • • • • | g e n n a i o | febbraio m a r r o | a p r i l e | m a g g i o [ g i u g n o 1 | ottobre | novembre | dicembre 1 1952 • la.ì vegaj a eniwefok m h montebello m a r z o a p r i l e ] m o g g i o i g i u g n o l u g l i o agojto i settembre ) ottobr e. [ novembre • fig. 1 desima. allo scopo di ridurre l ' e n t i t à delle radiazioni estranee, il sistema contatore-vaschetta è stato schermato m e d i a n t e un r i p a r o in p i o m b o dello spessore di alcuni m i l l i m e t r i . p e r ciascuna esperienza è stato tenuto conto della pressione atmosferica, della tensione del c o n t a t o r e di g-m n o n c h é del tipo di precipitazione (pioggia a c a r a t t e r e temporalesco, pioggia n o r m a l e , pioggia leggera, neve). l e misure furono eseguite per l a maggior parte a milano, a l t r e a casalborgone ( t o r i n o ) e a s. cesarea t e r m e (lecce). la diversità nel n u m e r o degli impulsi m" riscontrata n e l corso delle esperienze sono dovute a variazioni a m b i e n t a l i e di tensione di rete nelle f u t u r e r i c e r c h e questo inconveniente sarà e l i m i n a t o . p e r quanto riguarda l e misurazioni dei giorni 31 ottobre, 2 e 4 n o v e m b r e 1952, eseguite a s. cesarea t e r m e , sulla costa del m a r j o n i o , è da n o t a r e la particolare attendibilità dei valori ricavati in p r i m e m i s u r e s u l l a r a d i o a t t i v i ! à d e l l e p r e c i p i t a z i o n i a t m o s f e r i c h e 3 8 3 n. data località m " m' r durata raccolta tipo della precipitazione 1 4-2-51 milano 70 105 35 0 h 3 0 m 3 h 0 0 m neve 2 10-2 » 81 100 19 » pioggia 3 11-2 » 79 105 26 » pioggia 4 12-2 » 83 111 28 » pioggia 5 15-2 » 80 115 35 » pioggia 6 24-2 » 79 91 12 » neve 7 26-2 » 80 90 10 » pioggia forte 8 3-3 » 79 90 11 » pioggia leggera 9 7-3 » 81 89 8 » pioggia 10 13-3 » 82 91 9 » pioggia 11 30-3 » 88 95 7 » pioggia 12 3-5 » 89 99 10 » temporale 13 10-5 » 89 96 7 » temporale 14 15-5 » 82 89 " » temporale 15 16-5 » 87 103 16 » pioggia forte 16 17-5 » 88 104 16 » pioggia 17 27-5 » 88 113 25 » pioggia 18 5-6 » 90 122 32 temporale 19 8-6 » 83 92 9 » temporale 20 ' 23-6 » 93 107 14 a pioggia forte 21 24-6 » 93 107 14 » pioggia 22 22-10 » 105 117 12 )) pioggia 23 3-11 » 77 94 17 » pioggia leggera 24 7-11 » 82 100 18 » pioggia 25 8-11 » 96 112 16 )) pioggia 26 11-11 » 98 118 20 » pioggia leggera 27 18-11 » 90 116 26 pioggia leggera 28 6-12 » 113 121 8 )) pioggia leggera 29 30-3-52 » 93 102 9 3 h 30"' pioggia 30 2 4 » 103 114 11 6 h 00»' pioggia 31 2 1 4 » 116 128 12 0'' 0 5 m temporale 32 2 2 4 » 121 123 2 5 h 15'" pioggia leggera 33 2 3 4 » 118 123 5 141' 30"' t e m p . e pioggia 1. 34 2 4 4 » 122 129 7 13" 30"' pioggia 35 4-5 » 111 130 19 8'1 00"' pioggia 36 5-5 » 135 163 28 21' 00"' pioggia 37 6-5 » 146 168 22 4» 45"' pioggia 38 19-6 » 123 133 10 2'' io"' temporale 39 5-9 casalborg. 72 79 7 oh 44"' temporale 40 12-9 » 66 70 4 oli 22"' temporale 41 27-9 milano 111 116 5 oh ] l r a temporale 42 29-9 » 97 106 9 oli 1 7 " i pioggia 43 20-10 » 105 136 31 li' 00"' pioggia legg. 4 4 22-10 » 120 135 15 01' 30"' pioggia 45 31-10 s. cesarea 83 87 4 0 i i 07"' pioggia 46 2-11 » 95 98 3 01' 14"' temporale 47 4-11 » 98 101 3 oh j 3 m pioggia forte 48 18-11 milano 120 127 7 3 " 49"' pioggia leggera l . s a n t o m a u r o a. c i g n a quanto risulta p r a t i c a m e n t e assente ogni possibile i n q u i n a m e n t o dell'atmosfera dovuti a f u m o o a pulviscolo. 11 n u m e r o r , c b e nella t a b e l l a risulta dalla differenza ( m ' m " ) si r i f e r i s c e a una scala a r b i t r a r i a ed è il n u m e r o di impulsi al minuto registrati dal contatore. t a l e n u m e r o indica, in media, u n a attività pari a quella di acque t e r m a l i non molto radioattive. 1 valori m', m", r n o n c h é il tempo di raccolta delle precipitazioni stesse sono riportati nella tabella c b e precede. risultati. — s u l l ' o r d i n a t a di un sistema di coordinate normali si sono r i p o r t a t i i valori r m e n t r e sull'ascissa sono i n d i c a t e le date delle esperienze (fig. l ì . a causa del c a r a t t e r e discontinuo del f e n o m e n o considerato (precipitazioni a t m o s f e r i c h e ) è stala data la slessa rappresentazione usata da a b r i b a t , p i n o i r , p o u r a d i e r e venct (1952). si è constatato che si sono verificati degli aumenti temporanei della attività delle precipitazioni nei periodi di tempo suc1 e ' cessivi ad esplosioni a t o m i c h e . l e esplosioni ato—g 1 m i c h e prese in considerazione ai fini della presente r i c e r c a e l e cui date sono state desunte dai c o m u n i c a t i stampa, sono le seguent i : 24, 27 e 2 8 gennaio, 1 e 6 f e b b r a i o 1951 a k x i i -25 | i las vegas (u.s.a.). il a p r i l e 1952 , . . , . — . — 14 maggio (data ìncerfig. 2 ta) a e n i w e t o k (oc. pacifico). i l 22, 29, 30 ottobre, il 5 e 19 n o v e m b r e 1951, il 1°, 15 e 22 aprile, il 1°, 7 e 25 maggio, il 1° e il 7 giugno 1952 a las vegas (u.s.a.). i l 3 o t t o b r e 1952 p r i m e m i s u r e s u l l a r a d i o a t t i v i ! à d e l l e p r e c i p i t a z i o n i a t m o s f e r i c h e 385 alle is. m o n t e b e l l o (australia) e il 1" n o v e m b r e 1952 a e n i w e t o k (oc. pacifico). gli aumenti constatali in r si sono verificati dopo circa 8 giorni dalle esplosioni di las vegas, dopo due settimane da quelle di eniwetok e dopo c i r c a tre settimane da quella delle isole montebello. l a correlazione tra la variazione del regime t e r m i c o e della pressione atmosferica con l'aumento di r lia dato risultato negativo. così pure la maggiore attività nelle precipitazioni a c a r a t t e r e nevoso o temporalesco rispetto alle precipitazioni n o r m a l i (a. stefanizzi, 19501 a volle è in netto contrasto con gli aumenti di r osservati. non è superfluo r a m m e n t a r e che la radioattività naturale della pioggia è dovuta agli atomi radioattivi n a t u r a l i ancorati all'aerosol atmosferico e n o n è i m p r o b a b i l e che un aumento di particelle nell'atmosfera possa maggiorare il numero dei nuclei di condensazione. l a m i s u r a della radioattività dell'alta e media atmosfera per mezzo delle precipitazioni presenta il grave inconveniente di essere discontinua. p e r contro, la misura della radioattività delle precipitazioni necessita di u n a tecnica più semplicistica di quella usata per l'attività dell'aria (aliverti, 1933) e quelli sul valore della radioattività delle p r e c i p i t a z i o n i (a. stefanizzi, 1950), si h a : 220 x 10 1 8 curie/cc (in media per l'aria l da 10 x a 51 x io"13 curie /ce per le precipitazioni. l a radioattività media, quindi, di queste u l t i m e è dell'ordine di io4 volte l'attività dell'aria. i n o l t r e il grande volume d'aria scopato dalle precipitazioni stesse assicura un carattere più generale alla misura in quanto essa non si riferisce ad una zona l i m i t a t a dell'atmosfera. l ' e r r o r e nelle misure dovuto alla radioattività delle precipitazioni, generalmente non costante e legata invece al c a r a t t e r e della precipitazione stessa, viene m a n t e n u t o entro l i m i t i abbastanza ristretti, poiché essendo l'attività naturale rappresentata quasi esclusivamente da radiazioni alfa (a. stefanizzi, 19501, essa non viene registrata dal nostro contatore, sensibile solo, come a b b i a m o già accennato, a radiazioni più penetranti. v a notato, infine, che i l ripetersi per cinque volte del fenomeno dell'aumento t e m p o r a n e o dell'attività delle precipitazioni q u a l c h e tempo dopo le esplosioni atomiche rende oltremodo i m p r o b a b i l e la eventualità di una coincidenza casuale di fatti indipendenti tra loro e 3 8 6 l . s a n t o m a u r o a. c i c n a coincide, del resto, con i risultati conseguiti da alcuni r i c e r c a t o r i francesi ( h . garrigue, 1949, 1950, 1951 e 1 9 5 2 ; a b r i b a t e coli., 1952). p u ò ora essere interessante considerare in un caso particolare (fig. 2) l'andamento della radioattività al verificarsi delle precipitazioni. si sono scelti per questo i giorni 21, 22, 23 e 2 4 a p r i l e 1952 durante i quali a m i l a n o piovve quasi senza interruzione e per i quali si hanno rispettivamente quattro misure. l a radioattività misurata il giorno 21, all'inizio della pioggia, diede un valore i? = 12 ; il giorno seguente, subito dopo un violento acquazzone, la misura riferita alla susseguente pioggia leggera diede jr = 2 ; nei due giorni c h e seguirono, 23 e 2 4 aprile, la radioattività tornò ad a u m e n t a r e gradatamente per riprendere il suo valore solito. l a repentina diminuzione a 2 del valore di r la si può a t t r i b u i r e al precedente acquazzone c h e avrebbe operato una intensa azione di scopatura dell'atmosfera riducendo così al m i n i m o il valore della radioattività di quest'ultima. t r a le 4 8 misure eseguite, ne sono state scelte 22, c h e per la loro data c per i valori presentati non sono state influenzate da esplosioni a t o m i c h e e si è potuto d e t e r m i n a r e il valore medio della radioattività n o r m a l e delle precipitazioni a t t r i b u e n d o così un valore r = 9 + 1. conclusione. — è da prospettare p e r t a n t o l'ipotesi che gli anormali aumenti dell'attività p o t r e b b e r o essere dovuti appunto alle grandi quantità di prodotti radioattivi e s t r e m a m e n t e suddivisi c h e vengono sparsi per l'atmosfera in conseguenza delle esplosioni atomiche. da quanto sopra emerge l'importanza di codeste osservazioni e la necessità di c o n t i n u a r l e con mezzi più completi e in m a n i e r a più sistematica. milano — osservatorio astronomico di brera — giugno 1953. riassunto vengono esposti i risultati ottenuti da misure di radioattività relativa delle precipitazioni atmosferiche in alcune località italiane (casalborgone, milano e s. cesarea terme), in seguito alle esplosioni atomiche. tali misure sono state eseguite con un contatore di g-m, sensibile alle radiazioni beta e gamma. è stata trovata una maggiore attività nelle precipitazioni in seguito alle esplosioni atomiche. p r i m e m i s u r e s u l l a r a d i o a t t i v i ! à delle p r e c i p i t a z i o n i a t m o s f e r i c h e 3 8 7 su m mary during last years, from februar 1951 to november 1952, the radioactivily of atmospheric precipitates ivas tested ivitìi a g-m counter. this activity ivas no costanti indeed it became greater after atomic experiments at las vegas, eniwetok and montebello is. therefore these facts are probably associated. the activity increased about 8 days after las vegas experiments, two iveeh after eniivetok experiments and about three iveeks after montebello is. explosions. gamma and beta rays of the precipitates have been measured ivi!h g-m counter and the background of cosmic radiation ivas considered in withdraiving the diagrams. b i b l i o g r a f i a ( ! ) a b r i b a t m . , p i n o i r r . , pouradif.r j . , v e n e t a . m . ( 1 9 5 2 ) , c o m p t e s r e n . n. 2 3 4 , n. 1 1 . ( ) aliverti g. (1931), l'elettricista, anno x x x x , n. 8, r o m a . ( : ! ) — (1931) nuovo cimento, v i l i , n. 7. (•») — (1932), zeit. f. geopliysik, j . 9 ; h . 1 / 2 . (r>) — (1948), rivista geomineraria, anno ix, n. 1. («) — e lovera g. (1949), annali di geofisica, voi. i i , n. 1, 1949. ( 7 ) garricue h . (1949), comptes rendus n. 228. ( s ) — (1950), comptes rendus n. 230, n. 26. (») — (1951), comptes rendus n. 232, n. 10. ( 1 0 ) — (1951), comptes rendus n. 233, n. 16. ( » ) — (1951), comptes rendus n. 233, n. 23. ( ! ) — (1952), comptes rendus n. 235, n. 23. < 1 3 ) j a u f m a n n j . ( 1 9 0 5 ) , m e t . z e i t . , 2 2 , 1 0 2 . ( " ) priebsch j . a . (1932), b e i t r . geophysik, 36, n. 2 / 3 . (io) stefanizzi a . (1950), j o u r n . of geof. res„ voi. 55, n. 4. ( ] l i ) webb (1949), phys. rev., voi. 76, n. 3. gli e l a b o r a t o r i e l e t t r o n i c i n e l c o n t r o l l o d e i v o l i s p a z i a l i (*) e . p e n n a c c h i r i c e v u t o il 19 f e b b r a i o 1 9 6 3 riassunto. — v i e n e e s p o s t a l ' o r g a n i z z a z i o n e d e i c e n t r i d i c a l c o l o d i g i t a l e u t i l i z z a t i p e r il c o n t r o l l o d e i v o l i o r b i t a l i , i n p a r t i c o l a r e il g o d d a r d s p a c e c e n t e r e d il b e r m u d a c e n t e r . è a n c h e i n d i c a t o lo s c h e m a di c a l c o l o p e r l a d e t e r m i n a z i o n e d e l p i a n o d i v o l o e d e i p a r a m e t r i o r b i t a l i . summary. — t h e a u t h o r e x p o s e s t h e o r g a n i z a t i o n of t h e d i g i t a i c a l c u l a t i o n c e n t r e s u t i l i z e d f o r t h e c o n t r o l of o r b i t a i flights, w i t l i s p e c i a l r e g a r d t o t h e g o d d a r d s p a c e c e n t e r a n d t h e b e r m u d a c e n t e r . t h e c a l c u l a t i o n s c h e m e , f o r t h e d e t e r m i n a t i o n of t h e f l i g h t p i a n a n d of t h e o r b i t a i p a r a m e t e r s , is a l s o s h o w n . 1 . p r e m e s s a . i l p r o b l e m a della realizzazione dei voli spaziali è, forse, la b a t t a g l i a p i ù estesa nella quale la scienza si sia m a i i m p e g n a t a , nella sua c o n t i n u a sfida a l l ' u n i v e r s o . si p u ò dire che t u t t i i c a m p i della conoscenza v i p a r t e c i p i n o con p a r i i m p o r t a n z a : dalla fìsica all'ingegneria, dalla biologia a l l ' a s t r o n o m i a , dalla logica alle scienze d i organizzazione i n d u s t r i a l e , alla c i b e r n e t i c a , alle m a t e m a t i c h e p i ù e l e v a t e . e d a l c e n t r o d i questo i m m e n s o p a n o r a m a s t a l ' u o m o , con la sua f r a g i l i t à fìsica, m a s o r r e t t o dalle f o r z e f o r m i d a b i l i del suo ingegno e della sua v o l o n t à . ma p r o p r i o p e r la v a s t i t à dei p r o b l e m i che i n t e r v e n g o n o nella realizzazione dei voi i spaziali, m a i si sarebbe p o t u t i p a s s a r e d a l l a f a s e degli (*) n o t a p r e s e n t a t a a l 2° c o n g r e s s o i n t e r n a z i o n a l e t e c n i c o s c i e n t i f i c o d e l l o s p a z i o . r o m a , 1 9 2 3 g i u g n o 1 9 6 2 . 564 ]{. p e n n a c c h i s t u d i tecnici a d u n a e f f e t t i v a a t t u a z i o n e se l ' u o m o non avesse a v u t o a l suo servizio n o n s o l t a n t o i mezzi più p r o g r e d i t i p e r l'esecuzione della p a r t e più m a t e r i a l e dei lavori occorrenti, m a a n c h e quegli esecutori veloci ed infallibili dei lavori più c o n c e t t u a l i , che sono i sistemi elettronici p e r la elaborazione e lo s m i s t a m e n t o dei d a t i . n o n è nostra i n t e n z i o n e t r a t t a r e qui, in f o r m a teorica, q u a l i siano le t e c n i c h e i m p i e g a t e ed i p u n t i nei quali i n t e r v i e n e l ' e l a b o r a t o r e elettronico in u n a organizzazione p e r lo s t u d i o , la p r o g e t t a z i o n e ed il lancio d i un veicolo spaziale; v o r r e m m o invece descrivere alcuni esempi di applicazioni concrete che, in q u e s t o c a m p o , sono s t a t e d e t e r m i n a n t i nella realizzazione di i m p r e s e che h a n n o s t u p i t o il m o n d o , quali il p r o g e t t o m e r c u r v il p r o g e t t o e c h o , il p r o g e t t o t i t a n , etc. il c o n c e t t o generale di e l a b o r a t o r e elettronico è o r m a i t r o p p o f a m i liare p e r c h é sia necessario r i c o r d a r e qui, quali siano i suoi f o n d a m e n t i logici, le sue u n i t à c o m p o n e n t i , il suo m o d o di operare. m a p e r q u e s t a specifica applicazione, i mezzi che v e n g o n o richiesti alla tecnologia sono di n a t u r a a f f a t t o p a r t i c o l a r e : in q u e s t o c a m p o quelle soluzioni che, nei sistemi elettronici d e s t i n a t i alle n o r m a l i applicazioni commerciali o scientifiche, possono a p p a r i r e come t r o p p o complesse o a d d i r i t t u r a a v v e n i r i s t i c h e , risultano a t t u a l i ed indispensabili. i n q u e s t o c a m p o , si richiedono sistemi capaci di svolgere le loro f u n zioni non p i ù l i m i t a n d o le loro elaborazioni a d a t i p r o v e n i e n t i d a rilevazioni ed esperienze p a s s a t e , m a devono essere i n g r a d o di o p e r a r e su u n f e n o m e n o fisico che è in a t t o , e di cui il sistema deve e f f e t t u a r e u n c o s t a n t e controllo, a n t i c i p a n d o e p r e v e d e n d o n e gli sviluppi f u t u r i ed o f f r e n d o la possibilità di i n t e r v e n i r e nel f e n o m e n o stesso p r i m a ancora che esso sia g i u n t o alla sua conclusione. è q u e s t a la situazione tipica che si p r e s e n t a nel controllo di u n veicolo o r b i t a n t e i n t o r n o alla t e r r a : compito questo t a n t o più d r a m m a t i c o q u a n d o il veicolo t r a s p o r t i esseri u m a n i . 2 . l e f a s i ih u n a m i s s i o n e s p a z i a l e . p e r meglio chiarire quali siano i c o m p i t i svolti d a u n sistema elettronico p e r l'elaborazione dei d a t i , nel controllo di u n volo spaziale, riepilog h i a m o qui b r e v e m e n t e le diverse fasi, nel t e m p o , di u n t a l genere di missione, p r e n d e n d o a modello q u a n t o è s t a t o r e a l i z z a t o negli s t a t i u n i t i p e r il p r o g e t t o m e r c u r v : g l i e l a b o r a t o r i e l e t t r o n i c i n e l c o n t r o l l o d e i v o l i s p a z i a l i 5<)7 a) pre-lancio. — c o m p r e n d e u n periodo di circa 4 s e t t i m a n e prec e d e n t i la p a r t e n z a del veicolo, ed è d e d i c a t o a t u t t e le operazioni d i controllo. d u r a n t e q u e s t a f a s e , v e n g o n o eseguite t u t t e le p r o v e necessarie a d o t t e n e r e la c e r t e z z a i n t o r n o al p e r f e t t o f u n z i o n a m e n t o di t u t t i gli e l e m e n t i e di t u t t i i c o m p o n e n t i della b a s e (li lancio, del veicolo e, princip a l m e n t e , delle a p p a r e c c h i a t u r e di c o m u n i c a z i o n e e di calcolo c h e costituiscono il s i s t e m a nervoso della i n t e r a i m p r e s a . b) lancio. •— v a d a l l ' i s t a n t e di accensione dei r e a t t o r i lino al m o m e n t o in cui si p u ò stabilire se il veicolo si sia inserito o no nella o r b i t a p r e f i s s a t a . n e l caso del p r o g e t t o m e r c u r y , dopo il periodo di g e t t o a t t i v o , u n s i s t e m a elettronico i b m t i p o 7090 era in g r a d o di f o r n i r e al c e n t r o di controllo m e r c u r y di cape c a n a v e r a l u n a risposta circa la a l t e r n a t i v a go no (jo. la decisione di cape c a n a v e r a l di c o n t i n u a r e o i n t e r r o m p e r e la m i s s i o n e d o v e v a segnare la fine della f a s e di lancio. c) interruzione. — i n t e r m i n i di possibilità, u n a i n t e r r u z i o n e della fase di lancio inizia con l a segnalazione d a p a r t e del s i s t e m a e l a b o r a n t e , di u n a q u a l c h e a n o m a l i a di c a r a t t e r e c i n e m a t i c o o d i n a m i c o e si conclude con u n t e m p e s t i v o a z i o n a m e n t o dei r e t r o r a z z i . n a t u r a l m e n t e , possono essere prese decisioni di i n t e r r u z i o n e a v a r i livelli: d u r a n t e l'inizio o alla fine del lancio, secondo il f u n z i o n a m e n t o dei v a r i s t a d i . i n q u e s t a fase, il c o m p i t o pr i nci pale del calcolatore elettronico è quello di p r e v e d e r e il t e m p o ed il p u n t o di i m p a t t o in b a s e alle n u o v e condizioni che si sono v e r i f i c a t e . d) orbita. — i n i z i a d a l l ' a c c e r t a m e n t o che il veicolo è e n t r a t o in u n ' o r b i t a a c c e t t a b i l e e d u r a finché n o n v e n g o n o messi in azione i r e t r o razzi. i n q u e s t a f a s e il calcolatore d e t e r m i n a le v a r i a b i l i c i n e m a t i c h e del veicolo, le c o n f r o n t a con le a n a l o g h e variabili pre-calcolate, d e t e r m i n a il p r o b a b i l e p u n t o e t e m p o di i m p a t t o . e) rientro. —c o m p r e n d e il t e m p o che v a d a l l ' i s t a n t e di a t t i v a zione dei r e t r o r a z z i di f r e n a t a fino a l l ' a t t e r r a g g i o (o a m m a r a g g i o ) della c a b i n a spaziale. d u r a n t e t u t t a la f a s e il calcolatore definisce la posizione a t t u a l e del veicolo e calcola, a p p r o s s i m a n d o l o c o n t i n u a m e n t e , il p u n t o di i m p a t t o . / ) recupero. — q u e s t a f a s e si e s t e n d e d a l l ' a t t e r r a g g i o del veicolo fino a che l ' a s t r o n a u t a (o, in generale, il carico t r a s p o r t a t o ) non sono s t a t i r e c u p e r a t i . 564 ]{. p e n n a c c h i 3 . | t j ' e l a b o r a z i o n e d e i d a t i n e l p r o g e t t o m e r c u r y . nel q u a d r o generale dell'organizzazione del p r o g e t t o m e r c u r y , culm i n a t o , c o m ' è n o t o , con il volo o r b i t a l e del maggiore glenn, f u r o n o affid a t i alla i b m i compiti i n e r e n t i l' elab o razio n e dei d a t i , c o m p i t i che si p r e c i s a v a n o nei p u n t i s e g u e n t i : — sviluppo di un m e t o d o di simulazione p e r le p r o v e d e l l ' i n t e r o s i s t e m a e p e r l ' a d d e s t r a m e n t o del p e r s o n a l e ; — sviluppo di un s i s t e m a a u t o m a t i c o p e r il controllo di efficienza d u r a n t e il « c o u n t down » nella fase di pre-lancio; —• sviluppo e costruzione di speciali a p p a r e c c h i a t u r e da calcolo per la r a c c o l t a , l'organizzazione, la t r a s m i s s i o n e e l'esposizione in t e m p o reale dei d a t i di posizione r i l e v a t i dagli a p p a r a t i sensibili (radars, teodoliti) c o s t i t u e n t i la c i n t u r a di a v v i s t a m e n t o ; — impiego di u n s i s t e m a i n t e g r a t o p e r la elaborazione in t e m p o reale dei d a t i di r i l e v a m e n t o del satellite d u r a n t e le fasi di o r b i t a e di r i e n t r o ; — trasmissione dei d a t i ai c e n t r i di r a c c o l t a p e r il calcolo delle o r b i t e i n t o r n o alla t e r r a , p e r il loro a g g i o r n a m e n t o i s t a n t a n e o e per la r a p p r e s e n t a z i o n e in t e m p o reale. l ' i n t e r o s i s t e m a era costituito da una c i n t u r a di a v v i s t a m e n t o comp r e n d e n t e r a d a r s e teodoliti i n s t a l l a t i in v a r i e b a s i d i s t r i b u i t e i n t o r n o alla t e r r a , d a u n s i s t e m a di t r a s m i s s i o n e digitale a d a l t a velocità p e r convogliare i d a t i r i l e v a t i ai c e n t r i di elaborazione, da 1111 s i s t e m a di elaborazione c o s t i t u i t o da 7090 i b m e 709 i b m i n s t a l l a t i presso t r e c e n t r i : — m e r c u r y control center in cape c a n a v e r a l — g o d d a r d center in beltsville —b e r m u d e center c o m p l e t a v a il s i s t e m a u n insieme di a p p a r e c c h i a t u r e p e r la r a p p r e sentazione in t e m p o reale, d e s t i n a t o a esporre con c o n t i n u i t à , in f o r m a grafica e n u m e r i c a , le r i s u l t a n z e dell'intera missione agli ufficiali della n a s a , agli scienziati ed ai tecnici. d u r a n t e t u t t a la d u r a t a della missione, i c e n t r i di rilevazione, i c e n t r i di controllo ed i c e n t r i di elaborazione e calcolo, s c a m b i a v a n o f r a loro i n f o r m a z i o n i , d a t i e r i s u l t a t i di elaborazioni a t t r a v e r s o q u a t t r o linee di trasmissione ad a l t a velocità (1000 bits al secondo) con messaggi s t a n d a r d di 108 bits, alla cadenza di ogni mezzo secondo d u r a n t e il lancio, di g l i e l a b o r a t o r i e l e t t r o n i c i n e l c o n t r o l l o d e i v o l i s p a z i a l i 5<)7 ogni (i s e c o n d i d u r a n t e la f a s e o r b i t a l e e di ogni 3 secondi d u r a n t e il r i e n t r o . d u e s i s t e m i t i p o 7090 i b m , i n s t a l l a t i presso il g o d d a r d center, c o s t i t u i v a n o il c e n t r o p r i n c i p a l e di calcolo e di s m i s t a m e n t o dei d a t i . q u e s t a e n o r m e m a s s a di i n f o r m a z i o n i p o t e v a essere e l a b o r a t a in t e m p o r e a l e dai d u e sistemi, grazie alla loro elevatissima v e l o c i t à operat i v a , di cui p u ò d a r s i u n ' o r d i n e di g r a n d e z z a c o n s i d e r a n d o che il t e m p o i m p i e g a t o p e r e f f e t t u a r e u n a s o m m a (con a d d e n d i di 10 cifre) è di 218 centesimi di milionesimo d i secondo. -1. r i l e v a z i o n e d e l s a t e l l i t e e d e l l a sua o r b i t a . a) impostazione generale del calcolo. si s u p p o n e e s i s t e n t e u n s i s t e m a d i stazioni di rilevazione. di q u este, però, si c o n s i d e r e r a n n o solo quella che si t r o v a alla m i n i m a d i s t a n z a dal p u n t o p i r i l e v a t o i n i z i a l m e n t e . i l calcolo c o m p l e t o v e r r à i m p o s t a t o a t t r a v e r s o f a s i successive, e cioè: 1. d e t e r m i n a z i o n e del p i a n o d e l l ' o r b i t a ; 2. d e t e r m i n a z i o n e dei p a r a m e t r i orbitali. i n q u e s t o calcolo p r e l i m i n a r e si s u p p o n i che il p u n t o 1° sia m o l t o prossimo a l p i a n o tt,, c o n t e n e n t e l ' o r b i t a del satellite. b) determinazione del piano dell'orbita. r i f e r i r e m o t u t t i i p u n t i c o n s i d e r a t i ad u n s i s t e m a di c o o r d i n a t e c a r t e s i a n e ortogonali a v e n t i l'origine nel c e n t r o della t e r r a (che si indic h e r à con m) e l'asse z p a s s a n t e p e r la stazione di rilevazione 8 consid e r a t a . siano (0, 0, £) le c o o r d i n a t e di s r i s p e t t o a l s i s t e m a di r i f e r i m e n t o prescelto. p e r ciascuno dei p u n t i l ' i r i l e v a t i , la stazione 8 m i s u r e r à : = la d i s t a n z a 8pi = di = l'angolo y)i f o r m a t o f r a la v e r t i c a l e in 8 e la r e t t a 8pi = l ' a n g o l o (pi che il p i a n o 8mpi f o r m a col p i a n o x 0. i n d i c a t e con (xi, t/,-, zì) le c o o r d i n a t e —• p e r ora i n c o g n i t e — del p u n t o p i r i s p e t t o a l r i f e r i m e n t o fissato, si aarrà: di = v a ? + y) + ( a — 0 « . [1j 564 ]{. p e n n a c c h i i n o l t r e , poiché l a v e r t i c a l e i n s h a l'equazione ^ x = 0 i y = 0 e la r e t t a s'pi h a l ' e q u a z i o n e xt (z — zi) + xi y — (z — zi) + yt ih zi — q si ricava, d a u n a n o t a f o r m u l a di g e o m e t r i a a n a l i t i c a , 1 cos f i = j l ) + ri-+1 ove è xj zi-c vi zi — c ' r i c o r d a n d o la [1], la [4] si scrive h = hi cos y>i = di da cui si r i c a v a subito zi = f + di cos y) i . i l p i a n o s m p i h a l ' e q u a z i o n e x y z 0 0 c xt yi zt = 0 ossia x y i — y xi = 0 e q u i n d i l'angolo f r a esso ed il p i a n o x = 0 è d a t o d a cos (fi ih d a cui ossia v y) + x°cos2 9?i (y] + »'•) = y] cos2 q>t [ d j — (zi — c)2] = y\ . g l i e l a b o r a t o r i e l e t t r o n i c i n e l c o n t r o l l o d e i v o l i s p a z i a l i 5<)7 si h a n n o così le t r e e q u a z i o n i + + (zi — = si = c + di cos y>i [14] y2 + (z« — f ) 2 cos2 (pi=w,i cos2 (fi d a cui si r i c a v a zi = f + di cos ipi i / i = di cos (pi sen yn [15] xi — di sen q>i sen %pi . r i l e v a n d o s u c c e s s i v a m e n t e due p u n t i i \ e p 2 , è allora d e t e r m i n a b i l e il p i a n o no p a s s a n t e p e r p , , p 2 , ed m. t a l e p i a n o , che c o n t i e n e l ' o r b i t a ellittica del missile, a v r à l ' e q u a z i o n e x y z i x, vl zx | = 0 [16] x2 y2 z2 | con x1, yj, zt e x2, y2, z2 f o m i t i dalle [15] p e r i — 1,2, ossia ax + fi y + yz = q [17] con a = ih z2 — y2 z1 p = x 2 y 1 — x l y 2 [18] y = «i v2 — «2 ?/i • <?) determinazione dell'orbita del satellite. i n d i v i d u a t o il p i a n o no c o n t e n e n t e l ' o r b i t a del satellite, è possibile, r i l e v a n d o un t e r z o p u n t o p 3 , d e t e r m i n a r e l ' o r b i t a stessa. consideriamo, i n f a t t i , u n secondo s i s t e m a di r i f e r i m e n t o m (x', y', z ) a v e n t e come p i a n o z' = 0 [19] il p i a n o rio, come asse y' (x' = z' = 0) la r e t t a p a s s a n t e p e r il c e n t r o della t e r r a m e p e r il p u n t o i di i n t e r c e t t a z i o n e (che si s u p p o n e c o n t e n u t o in tio ed a v e n t e le c o o r d i n a t e xit y,, zj e come asse x' (y' —z' — 0) la norm a l e in il/ alla r e t t a mi (e c o n t e n u t a in ti0). 5 6 4 ]{. p e n n a c c h i se le t r e r e t t e x', y', z' h a n n o in m (x, y, z) le equazioni p e r l'asse x' [20] x y h m i x y z /2 in,, ti, x y z i-, rtu n. p e r l'asse y' [2 li p e r l'asse z' [22] dalle condizioni sopra i n d i c a t e si r i c a v a : .1) p e r l'asse y' u — /,-2 xj = cos y'x m2 k2 y, cos y'y [23] «,2 = 7r2 zj = cos y'z che i m p o n g o n o il passaggio della [21] p e r i : 2) p e r l'asse z' l3 = k3 a = cos z'x m , = t> = cos [24] '"a = y = cos z'z che i m p o n g o n o la n o r m a l i t à al p i a n o tz„; 3) p e r l'asse a;' ' l + l ?«2 + " 1 »2 0 ?! z3 + m l m 3 + "1 ": ì 0 che i m p o n g o n o la n o r m a l i t à alle due r e t t e y' e z'. d a l l e [25] si ricava h — ''"1 (m2 n3 — m3 nt) = cos x'x ml = /,•] (la « j — i j -»s) — cos afy [26] » ! = /,-j (/»3/2 — m2 i3) = cos ,r's . ciò p o s t o è facile d e t e r m i n a r e le f o r m u l e di t r a s f o r m a z i o n e che p e r m e t t o n o il passaggio d a l sistema m (x, y, z) al sistema m (x',y',z'). i n f a t t i è x = x' ll + y' l2 + z' l3 y = x ni, + ?/' m 2 + z w , [ 2 7 ] a = »! -j(/'«,, + z' il 3 [ 2 5 ] g l i e l a b o r a t o r i e l e t t r o n i c i n e l c o n t r o l l o d e i v o l i s p a z i a l i 5<)7 e, i n v e r s a m e n t e x lyx + »!y + «! z y ?2 x + »u n + li 2 z [281 z — l3x + m3y + n3 z . è o v v i o , come si r i c a v a d a l l e [24], che se un p u n t o considerato giace sul p i a n o tto, p e r esso s a r à z' = 0. consideriamo ora, nel p i a n o n0, il s i s t e m a di r i f e r i m e n t o m (x', y'). i n esso i p u n t i 1\ e p 2 a v r a n n o le c o o r d i n a t e (xu y\) e (aj'2, y'2) d a t e dalle [28]. consideriamo a n c h e u n t e r z o p u n t o r i l e v a t o , a v e n t e coord i n a t e (x3, y3, z3) rispetto a l s i s t e m a m (x, y, z) e m (x'3, y'3, 0) r i s p e t t o a l s i s t e m a m (x', y', e'). i t r e p u n t i p 2 , p 3 , d o v e n d o a p p a r t e n e r e ad u n a ellisse con f u o c o in m, d e t e r m i n a n o l'ellisse stessa che a v r à , r i s p e t t o a l r i f e r i m e n t o m (x', y', z') le equazioni i coefficienti a, b, c possono r i c a v a r s i i m p o n e n d o le t r e condizioni di passaggio p e r p „ p 2 , p3. allora l a q u o t a di passaggio del missile sulla v e r t i c a l e d i i s a r à d a t a dall'intersezione d e l l a ellisse [29] con l'asse ?/', ossia d a v x'z + y' z' -0 . 4 x + b y' + g [29] ± y' = b y' + e z 0 x' = 0 [30] ossia dai due p u n t i s c a r t a n d o il secondo ed i n d i c a n d o con li il raggio della t e r r a (o meglio la d i s t a n z a i m ) , la q u o t a cercata sarà 564 ]{. p e n n a c c h i 4 . l ' e l a b o r a z i o n e d e i d a t i i n t e m p o r e a l e . i n t u t t e le fasi della missione m e r c u r y , e r a di vitale i m p o r t a n z a che i d i f f e r e n t i e n u m e r o s i tipi di calcoli richiesti fossero eseguiti con precisione assoluta e resi disponibili i s t a n t a n e a m e n t e , e questa r i c h i e s t a si i m p o n e v a come necessità f o n d a m e n t a l e p e r la sicurezza d e l l ' a s t r o n a u t a ed il successo d e l l ' i n t e r o p r o g e t t o . p e r d a r e u n ' i d e a del v o l u m e di calcoli d a sviluppare, si pensi, a d esempio, che nei p r i m i s s i m i i s t a n t i d e l l ' i n s e r i m e n t o del veicolo in o r b i t a e r a necessario calcolare i p a r a m e t r i d e l l ' i n t e r a o r b i t a , allo scopo di decidere se la missione p o t e v a proseguire o p p u r e era necessario i n t e r romperla . considerando che la r o t a z i o n e di 180 g r a d i del veicolo, necessaria p e r l ' a z i o n a m e n t o del r e t r o r a z z i di f r e n o , richiedeva u n c e r t o t e m p o e che il p u n t o di i m p a t t o d ' e m e r g e n z a d o v e v a essere sul m a r e , si v e d e facilm e n t e q u a n t o b r e v e era il t e m p o a disposizione p e r il calcolo dei p a r a m e t r i o r b i t a l i . p e r c i ò fin dalla f a s e di lancio, i sistemi 7090 i b m del g o d d a r d cent e r e f f e t t u a r o n o i calcoli necessari a decidere, in ogni i s t a n t e , l ' a l t e r n a t i v a yo no go, m e d i a n t e la d e t e r m i n a z i o n e dei p a r a m e t r i critici della t r a i e t toria e, n e l l ' e v e n t u a l i t à di u n a i n t e r r u z i o n e , il t e m p o di azione dei retrorazzi necessario p e r c h é il veicolo potesse ricadere in u n o dei p u n t i p r e s t a b i l i t i come zone d i r e c u p e r o . è n o t o quali siano le difficoltà di u n simile calcolo, s o p r a t u t t o t e n e n d o conto, che in q u e s t e ipotesi, le equazioni del m o t o devono necessariam e n t e t e n e r conto della v a r i a z i o n e della g r a v i t à con la q u o t a , della azione di forze a e r e o d i n a m i c h e che si s v i l u p p a n o in condizioni supersoniche e con d e n s i t à dell'aria variabile, dell'azione del r i s c a l d a m e n t o p e r a t t r i t o sui coefficienti d i r e s i s t e n z a e di p o r t a n z a del veicolo. si p u ò dire che il calcolo d e l l ' i s t a n t e di a z i o n a m e n t o dei r e t r o r a z z i e della r e l a t i v a d u r a t a o n d e o t t e n e r e che il p u n t o d ' i m p a t t o cadesse in u n a delle zone p r e s t a b i l i t e , f u il m a g g i o r lavoro che f u svolto dai calcol a t o r i 7090 i b m del g o d d a r d center, t a n t o p i ù che t u t t o il calcolo d o v e v a essere eseguito n u o v a m e n t e via via che v e n i v a s t a b i l i t a u n a n u o v a d e t e r m i n a z i o n e dei p a r a m e t r i orbitali. u n a l t r o calcolo che, s i m u l t a n e a m e n t e , d o v e v a essere e f f e t t u a t o era la d e t e r m i n a z i o n e s e m p r e più a p p r o s s i m a t a delle variabili c i n e m a t i c h e , allo scopo di p o t e r m i g l i o r a r e l ' e s a t t e z z a delle previsioni p e r le acquisizioni f u t u r e . g l i e l a b o r a t o r i e l e t t r o n i c i n e l c o n t r o l l o d e i voli s p a z i a l i 5<)7 i calcolo svolti a q u e s t o scopo, e r a n o b a s a t i sulla risoluzione n u m e rica di equazioni d e r i v a t e dalle relazioni f o n d a m e n t a l i di n e w t o n . i m e t o d i n u m e r i c i i m p i e g a t i consistevano nella i n t e g r a z i o n e n u m e r i c a di cowell p e r la e s t r a p o l a z i o n e e la correzione dei p a r a m e t r i o r b i t a l i e in correzioni differenziali p e r l ' e f f e t t o delle p e r t u r b a z i o n i m i n o r i . allo scopo d i r i d u r r e gli e f f e t t i degli e r r o r i d o v u t i a i r a d a r s e p e r t e n e r conto delle m u t e v o l i c a r a t t e r i s t i c h e a t m o s f e r i c h e , f u r o n o a p p l i c a t i pesi s t a t i s t i c i ai d a t i p r o v e n i e n t i dalle stazioni di rilevazione. i n t a l m o d o , la posizione del veicolo spaziale, r e l a t i v a m e n t e a 18 s e t t o r i t e r r e s t r i , f u c o n t r o l l a t a c o s t a n t e m e n t e e nel m o d o più c o m p l e t o . 5 . i l concetto d i programma m o n i t o r . ma, oltre alla d i r e t t a esecuzione dei calcoli un a l t r o e forse p i ù imp o r t a n t e compito d o v e v a essere svolto d a i sistemi e l e t t r o n i c i p e r l'elab o r a z i o n e dei d a t i , ossia il controllo ed il c o o r d i n a m e n t o di t u t t i i calcoli che v e n i v a n o via v i a eseguiti. i n f a t t i , in ogni i s t a n t e , u n d a t o calcolo d o v e v a essere eseguito senza con ciò, sacrificare q u a l c h e a l t r o calcolo che a v r e b b e p o t u t o essere di i m p o r t a n z a d e t e r m i n a n t e . p o t e v a , p e r esempio, a c c a d e r e che u n i m p r o v viso g u a s t o di u n ' u n i t à di u s c i t a , c o m p r o m e t t e s s e t u t t o il sistema di calcolo in t e m p o reale. p e r r e n d e r e i n t e r a m e n t e c o o r d i n a t o , in ogni i s t a n t e ed in ogni event u a l i t à , l ' i n t e r o s i s t e m a di elaborazione v e n n e s t u d i a t o e messo a p u n t o u n apposito p r o g r a m m a : il m o n i t o r . i l p r o g r a m m a m o n i t o r , c a r i c a t o in m e m o r i a assieme ai p r o g r a m m i di calcolo, svolgeva u n a f u n z i o n e di supervisione dello s v o l g i m e n t o dei singoli calcoli, c o o r d i n a n d o n e lo svolgimento secondo l ' a r r i v o dei ciati grezzi e la possibilità di emissione dei r i s u l t a t i , e s t a b i l e n d o s i t u a z i o n i di p r i o r i t à in b a s e alle condizioni di e n t r a t a , di elaborazione e di u s c i t a dei d a t i disponibili. come a c c a d e s o v e n t e , a n c h e in questo caso la risoluzione di u n p a r t i c o l a r e p r o b l e m a tecnico, h a d a t o l ' a v v i o a d u n a m e t o d o l o g i a n u o v a la cui i m p o r t a n z a t r a s c e n d e l'applicazione p a r t i c o l a r e clic l ' h a p r o m o s s a . è s t a t a , in q u e s t a occasione, a p p l i c a t a p e r la p r i m a v o l t a u n a gerarchia c o n c e t t u a l e di p r o g r a m m i di m a c c h i n a : il p r o g r a m m a m o n i t o r che c o o r d i n a il l a v o r o d a eseguire, i p r o g r a m m i non-monitor che eseguono il lavoro. ji. p e n n a c c h i un così d i n a m i c o c o n c e t t o di p r o g r a m m a z i o n e c o n d u c e a concepire un sistema p e r l 'el ab o razio n e dei d a t i c o m e u n ' o r g a n i z z a z i o n e in sé chiusa ed in g r a d o di a u t o d e t e r m i n a r s i . i p r o g r a m m i non-monitor o p e r a n o i n d i p e n d e n t e m e n t e , ricevendo di volta in volta ordini dal m o n i t o r in m e r i t o a l l ' a v v i o dei calcoli, all'indicazione di dove sono disponibili i d a t i da e l a b o r a r e e di d o v e devono essere indirizzati i risul t ati o t t e n u t i in modo t a l e che l'elaborazione, p u ò svolgersi in f o r m a del t u t t o i n d i p e n d e n t e dalla configurazione del s i s t e m a e l a b o r a n t e , e p u ò c o n t i n u a r e n o n o s t a n t e i m p r o v v i s e e casuali variazioni di q u e s t a . 6 . c o n s i d e r a z i o n i g e n e r a l i . l ' i m p o r t a n z a ed i p r i n c i p a l i a s p e t t i della elaborazione e l e t t r o n i c a dei d a t i nei p r o b l e m i del volo spaziale sono s t a t i qui s u c c i n t a m e n t e esposti nella esemplificazione offerta dal p r o g e t t o m e r c u r y . ma è n o t o che n o n è questo l ' u n i c o esempio, né il p i ù complesso. b e n altri p r o b l e m i già p r e s e n t a n o la necessità di u n a soluzione a l t r e t t a n t o efficiente negli s t u d i e nei p r o g e t t i , la cui m e t a è quella di p a s s a r e dalla fase dei voli o r b i t a l i a quella, ben p i ù ambiziosa, dei voli t e r r a l u n a . qui la necessità di calcolo in t e m p o reale, di raccolta e di s m i s t a m e n t o veloce delle i n f o r m a z i o n i acquisite si p r o p o n g o n o s o t t o u n a f o r m a assai più a m p i a e complessa, sì da richiedere t e c n i c h e e tecnologie, a n c o r a più r a f f i n a t e , a n c o r a più a v a n z a t e . in q u e s t i p r o g e t t i è necessario inserire u n o p p o r t u n o c e n t r o di calcolo nel cuore del veicolo spaziale stesso, e f a r sì che a n c h e q u e s t o sia in collegamento con gli a l t r i c e n t r i di calcolo (lei s i s t e m a , così che u n a inint e r r o t t a rete di messaggi a u t o m a t i z z a t i c o m p r e n d a in u n unico c a m p o la t e r r a , la l u n a ed il veicolo spaziale. già sono s t a t i realizzati modelli di e l a b o r a t o r i che u s u f r u i s c o n o delle t e c n i c h e di m i c r o m i n i a t u r i z z a z i o n e . calcolatori elettronici, come il sistema i b m tipo t i t a n i i , del peso complessivo di 90 libbre e capaci di compiere circa 150.000 operazioni al secondo, s p e r i m e n t a t i s o t t o accelerazioni c o n t i n u e di 20 g. e u r t i di 100 g., in g r a d o di svolgere il loro lavoro nelle condizioni p i ù d i s p a r a t e . su q u e s t i calcolatori, i n t e g r a t i da u n i t à di t e l e c o m u n i c a z i o n e a d a l t a velocità, i p r o b l e m i del volo spaziale t r o v a n o la loro soluzione, cosi che gli a s t r o n a u t i di oggi, ed a n c o r p i ù quelli che seguiranno, possano disporre di quegli e l e m e n t i che sono f o n d a m e n t a l i p e r la sicurezza della p r o p r i a v i t a e p e r il successo della p r o p r i a missione. t e o r i a p e r l a d e t e r m i n a z i o n e d e l c o n t e n u t o r a d i o a t t i v o d e i m i n e r a l i i donato p a l u m b o introduzione. — m o l t i s p e r i m e n t a t o r i , ( 1 ) , ( 2 ), ( 3 ), ( 4 ), ( 5 ), si sono r e c e n t e m e n t e o c c u p a t i d e l l a d e t e r m i n a z i o n e del c o n t e n u t o di m a t e r i a l i r a d i o a t t i v i i n m i n e r a l i p e r m e z z o d e l l a a u t o r a d i o g r a f i a , o t t e n u t a m e d i a n t e l ' u s o di e m u l s i o n i n u c l e a r i . a l c u n i d i essi ( 3 ), ( 5 ), (°) h a n n o a n c h e c e r c a t o di d e t e r m i n a r e , o l t r e l ' a t t i v i t à t o t a l e d e l m a t e r i a l e , i r a p p o r t i t r a l e q u a n t i t à d i e l e m e n t i r a d i o a t t i v i a p p a r t e n e n t i a l l e v a r i e f a m i g l i e , s u p p o s t e o 110 in e q u i l i b r i o s e c o l a r e . d ' a l t r a p a r t e , l a d e t e r m i n a z i o n e d e l l ' e s i s t e n z a o m e n o d i t a l e e q u i l i b r i o r a p p r e s e n t a u n n o t e v o l e p r o b l e m a , l a cui s o l u z i o n e i n t e r e s s a l a g e o l o g i a e l a v u l c a n o l o g i a . o r a , m e n t r e l ' a t t i v i t à t o t a l e p u ò e s s e r e d e d o t t a d a l s e m p l i c e c o n t e g g i o d e l l e t r a c c e di p a r t i c e l l e n n e l l ' e m u l s i o n e , gli a l t r i d u e p r o b l e m i r i c h i e d o n o l a d e t e r m i n a z i o n e d e l l a dis t r i b u z i o n e d e l l e t r a c c e in f u n z i o n e d e l l a l u n g h e z z a di esse n e l l ' e m u l s i o n e , o di g r a n d e z z e a n a l o g h e ( p r o i e z i o n i o r i z z o n t a l e o v e r t i c a l e d e l p e r c o r s o ) . d a l p u n t o di v i s t a p r a t i c o ci s e m b r a p i ù c o m o d a l a r a p p r e s e n t a z i o n e di q u e l l o c h e p o t r e b b e c h i a m a r s i l o s p e t t r o i n t e g r a l e d e l l e t r a c c e , c i o è l a d e t e r m i n a z i o n e d e l n u m e r o di t r a c c e a v e n t i c a m m i n o t o t a l e n e l l ' e m u l s i o n e ( o p r o i e z i o n i o r i z z o n t a l e o v e r t i c a l e ) , m a g g i o r e di u n a q u a n t i t à p r e f i s s a t a . d e l r e s t o , a n a l i t i c a m e n t e , si passa s u b i t o , p e r s e m p l i c e d e r i v a z i o n e r i s p e t t o ad r, d a l l o s p e t t r o i n t e g r a l e a q u e l l o d i f f e r e n z i a l e ( n u m e r o di t r a c c e a v e n t i c a m m i n o t o t a l e , o p r o i e z i o n i , o r i z z o n t a l i o v e r t i c a l i , c o m p r e s o t r a r ed r f dr). si n o t i c h e , p e r d e t e r m i n a r e l a p r o i e z i o n e v e r t i c a l e v e r a , e q u i n d i il p e r c o r s o t o t a l e d e l l e p a r t i c e l l e a n e l l ' e m u l s i o n e , ( e l e l o r o e q u i v a l e n t i i n c m . di aria), o c c o r r e c o n o s c e r e i l m o d o i n cui l ' e m u l s i o n e si è d e f o r m a t a d u r a n t e l o s v i l u p p o . d i s o l i t o si t i e n e c o n t o d i t a l e d e f o r m a z i o n e m o l t i p l i c a n d o l a p r o i e z i o n e v e r t i c a l e , m i s u r a t a al m i c r o s c o p i o , p e r u n o p p o r t u n o c o e f f i c i e n t e ; a m m e t t e n d o cioè c h e l a c o n t r a z i o n e sia s t a t a u n i f o r m e i n t u t t o l o spessore d e l l ' e m u l s i o n e . 2 3 0 d o n a t o p a l u m b o d e l l ' a s p e t t o t e o r i c o dei p r o b l e m i s o p r a a c c e n n a t i , si sono o c c u p a t i d i v e r s i a u t o r i ( 6 ), ( 7 ), ( s ), (°j, ( 1 0 ) . s e b b e n e quasi t u t t e l e f o r m u l e c h e r i c a v e r e m o in questa p r i m a n o t a s i a n o s t a t e t r o v a t e , n e l caso p a r t i c o l a r e in cui sia v a l i d a l a [ 1 ' ] , da q u a l c u n o degli a u t o r i citati, ci s e m b r a n o n i n u t i l e d e d u r l e con u n m e t o d o u n i t a r i o d i e , c o m e sarà m o s t r a t o i n seguito, si p r e s t a ad essere f a c i l m e n t e g e n e r a l i z z a t o . ci p r o p o n i a m o q u i n d i di d e d u r r e , i n m o d o m o l t o s e m p l i c e , l e f o r m u l e elle d a n n o il n u m e r o di t r a c c e p e r c m 2 di e m u l s i o n e esposta e p e r s. d'esposzione, in f u n z i o n e , sia d e l l a l u n g h e z z a t o t a l e , c h e d e l l e p r o i e z i o n i o r i z z o n t a l e e v e r t i c a l e , e di m o s t r a r e c o m e d a l l a m i s u r a di q u e s t e g r a n d e z z e , su u n n u m e r o c o n v e n i e n t e m e n t e g r a n d e di tracce, si possa r i s a l i r e a l l a s o l u z i o n e dei p r o b l e m i a c c e n n a t i . caso di un solo range. — c o n s i d e r i a m o u n m i n e r a l e l i m i t a t o i n f e r i o r m e n t e d a l p i a n o di t r a c c i a ab, posto a c o n t a t t o con e m u l s i o n e n u c l e a r e , l i m i t a t a s u p e r i o r m e n t e d a l l o stesso p i a n o . s u p p o n i a m o c h e il m i n e r a l e c o n t e n g a n u c l e i r a d i o a t t i v i di d i v e r s a s p e c i e ; sia nt il num e r o di t a l i n u c l i d i p e r c m 3 ; la r e l a t i v a costante di d i s i n t e g r a z i o n e , e q u i n d i n\ il n u m e r o di p a r t i c e l l e a emesse in 1 s. da t u t t i i n u c l i d i d e l l a specie i m a c o n t e n u t i in 1 c m 3 di m i n e r a l e (ni"i.'l è i n d i c a t o da c h. v . b u t t l a r ed f. g. h o u t e r m a n s ( s ) con ni). s i a n o r n , r»i i r a n g e s d e l l e p a r t i c e l l e i n q u e s t i o n e n e l m i n e r a l e e n e l l ' e m u l s i o n e . si h a e v i d e n t e m e n t e (v. fig. 1 ) : — + — = 1 [ l ' i in questa n o t a s u p p o r r e m o v a l i d a , c o m e v i e n f a t t o a b i t u a l m e n t e , (-1), ( 3 ), (7), ( s ) l'ipotesi di b r a g g k l e e m a n n , c h e ci p e r m e t t e di p o r r e ru = 7?| ; r2i = (i2 ri ; essendo r-, il r a n g e in c m d ' a r i a uj, l̂o d u e coefficienti c a r a t t e r i s t i c i del m i n e r a l e e d e l l ' e m u l s i o n e , m a i n d i p e n d e n t i d a l l ' e n e r gia d e l l e p a r t i c e l l e a , l a cui d e t e r m i n a z i o n e a p p r o s s i m a ta p u ò f a r s i , m e d i a n t e l e r e gole di b r a g g k l e e m a n n , con o s c e n d o l a c o m p o s i z i o n e c h i m i c a del m e z z o ( 1 1 ) . fig1 i n d i c a n d o con s la distana \ b b t e o r i a p e r l a d e t e r m i n a z i o n e d e l c o n t e n u t o r a d i o a t t i v o d e i m i n e r a l i 2 3 1 za d e l g e n e r i c o n u e l i d e e m i t t e n t e d a l p i a n o ab, e con 0 l ' a n g o l o f o r m a t o d a l l a t r a i e t t o r i a d e l l a p a r t i c e l l a emessa con l a n o r m a l e ad ab, e p o n e n d o s = [ i j z, b = | j r ' , cos 1) = v, la [].'] si s c r i v e : r / — ri — [1] v z ^ 0 ; 0 ^ v < 1 [2] 11 n u m e r o fi di t r a c c e , s o d d i s f a c e n t i a t a l u n e c o n d i z i o n i geomet r i c h e , c h e s a r a n n o s p e c i f i c a t e sotto, p r o d o t t e p e r c m 2 di e m u l s i o n e e p e r s., sarà d a t o d a : fl= — i n, ha ds . d q = j _ / n a;« dz . dv [ 3 ' ] 4 « j 2 j s essendo l ' u l t i m o i n t e g r a l e esteso ad u n d o m i n i o s del p i a n o (v, z), dom i n i o d e f i n i t o , u n i t a m e n t e a l l e [ 2 ] , d a l l e a l t r e l i m i t a z i o n i i m p o s t e a v, z d a l l e c o n d i z i o n i g e o m e t r i c h e v o l u t e . n e l l ' i p o t e s i c h e ili '/ '1 e i< a n o n d i p e n d a n o da s, cioè c h e il m i n e r a l e , a l m e n o n e l l e p o c h e d i e c i n e di m i c r o n utili (*l, sia o m o g e n e o r i s p e t t o ad s, la [ 3 ' ] d i v e n t a : f ; = — li. 71; a" i dz .dv— — u, n,).-" a [ 3 ] 2 j s 2 ' essendo o l ' a r e a del d o m i n i o s. il n u m e r o di t r a c c e /i(r), p r o d o t t e p e r c m 2 di e m u l s i o n e e s., a v e n t i n e l l ' e m u l s i o n e c a m m i n o r', ( m i s u r a t o in cm d'aria), t a l e c h e sia ri 5=r ' s5r, sarà dato d a l l a [ 3 ] , in cui, p e r l a [ 1 ] , il d o m i n i o s è d e f i n i t o d a l l e [ 2 ] , e d a l l a r i ^ r , — da epiesta: v (ri — r)t>r~z^?0. q u i n d i s c o i n c i d e col t r i a n g o l o oab, (fig. 2 ) ; o = j/2 ( « i — r)ne s e g u e : h (r) = (r, r), p e r 0 ^ r ^ r> ; f , (r) = 0, p e r r > 4? [ 4 ] r i s u l t a t o b e n n o t o . c a l c o l i a m o o r a il n u m e r o di tracce, i|»i(r), a v e n t i p r o i e z i o n i v e r t i c a l i y', (in cm d'aria), t a l i c h e s i a : r^y'^y (*) s e |i, rij n o n è c o s l a n t e su tutta la s u p e r f i c i e , i n d i c h e r e m o con t a l e p r o d o t t o il v a l o r e m e d i o . 230 d o n a t o p a l u m b o m o l t i p l i c a n d o la [ 1 ] p e r v — cos t), si l i a : y' — vrt — z s i a v r à q u i n d i , p e r il d o m i n i o s, l a l i m i t a z i o n e ri > v r i z ^ y ^ c i o è vri-y^-z^r, (v—1) c h e , u n i t a m e n t e a l l e [ 2 ] , dà, p e r s, il t r i a n g o l o cad (fig. 2). q u i n d i : _ ( r i r ) 2 2 r, ne segue, p e r l a [ 3 ] : t|'.(y)=htra,x" { r \ / y , p e r o ^ j r ^ t f , ; ^ ( y ) = 0 , p e r y>r,. 4/?; [ 5 ] c a l c o l i a m o i n f i n e il n u m e r o , rpil*), di tracce, p r o d o t t e p e r ciudi e m u l s i o n e e p e r s., a v e n t i p r o i e z i o n i o r i z z o n t a l i x', ( i n cm. d'aria), t a l i c h e sia r ^ x'^ x. l a [ 1 ] d à : x! = sen il>(/?; — c o s t ì = ( ! » » ) i i ì ( r ì ~ i l d o m i n i o s, i n q u e s t o caso è d e f i n i t o , u n i t a m e n t e a l l e [ 2 ] , d a l l a l i m i t a z i o n e : (ri— — t e o r i a p e r l a d e t e r m i n a z i o n e d e l c o n t e n u t o r a d i o a t t i v o d e i m i n e r a l i 231 c i o è : r,—x(l—v , r v . v r -, 1 ( 1 — q u i n d i s (fig. 2) è d e l i m i t a t o dall'asse v e d a l l ' a r c o , con z > 0 , d e l l a c u r v a di e q u a z i o n e : z = v ri — x(l-v"-) l a q u a l e taglia l'asse v n e l p u n t o e di ascissa v° =—(h2—x2) ' . ri s i lia q u i n d i : ri v — xv (1 — t r ) dv— (ri-x)2 hi d a cui, p e r l a [ 3 ] : (r—x)cpi(x)=\ilnilia ' ^ , p e r ; cpi(*) = 0 , p e r x>r{. 4 r, [6] la d e t e r m i n a z i o n e esatta di questa f o r m u l a , a p a r t i r e d a l l e ipotesi s o p r a a m m e s s e , è stata t e n t a t a da b u t t l a r e h o u t e r m a n s ( 8 ) ; questi a a . , p e r ò , f o r s e p e r u n a p o c o f e l i c e scelta d e l l a v a r i a b i l e i n d i p e n d e n t e , n o n p e r v e n g o n o a l l a [ 6 ] , la f o r m u l a è stata i n v e c e trov a t a s u c c e s s i v a m e n t e da v a n s t i j v e n d a e l ( 1 2 ) ed i n d i p e n d e n t e m e n t e d a i m b ò e c a s e r t a n o ( 1 0 ) . è n o t e v o l e la c o i n c i d e n z a d e l l a [ 6 ] con la [ 5 ] : p e n s i a m o e b e t a l e c o i n c i d e n z a possa s f r u t t a r s i p e r u n c o n t r o l l o d e l l a l e g i t t i m i t à d e l l e ipotesi f a t t e . caso generale. — s u p p o n i a m o o r a di a v e r e n e l n o s t r o m i n e r a l e , c o n d i s t r i b u z i o n e o m o g e n e a r i s p e t t o a z, p s p e c i e d i v e r s e di n u c l i d i a t t i v i (<. p e n s i a m o questi n u c l i d i o r d i n a t i s e c o n d o i r a n g e s c r e s c e n t i d e l l e n da l o r o emesse, ed i n d i c h i a m o con n, l e l o r o c o n c e n t r a z i o n i m e d i e p e r c m ' , con à'.1 l e r i s p e t t i v e c o s t a n t i di d i s i n t e g r a z i o n e a , con r, il p e r c o r s o in a r i a d e l l e « emesse (r[ ^ rj^j), (i = 1 , ... p). a m m e t t i a m o p e r s e m p l i c i t à clic si possa s u p p o r r e c h e ciascun n u c l i d e e m e t t a p a r t i c e l l e <t m o n o e n e r g e t i c h e . ( i n caso c o n t r a r i o bisog n e r à c o n t a r e ciascun n u c l i d e (ante v o l t e q u a n t i sono i g r u p p i m o n o e n e r g e t i c i di p a r t i c e l l e a da esso emesse), e c h e li, sia costante. 230 donato p a l u m b o a) i n d i c a n d o con / ( r ) il n u m e r o di t r a c c e p e r c m 2 d i g e l a t i n a e s., a v e n t i n e l l ' e m u l s i o n e c a m m i n o t o t a l e r ( i n cm di a r i a ) , r' <> r si h a : f(r)= ì k j k ( r ) , e p e r l a [ 4 ] : 1 j(r)= ^ vk nk \ka (rk _ r ) ^ i i l v k „ t x t a rk — ìknkhar] (r^r^rk) 4 ì+ì 4 i+i 4 i+i [7] 4 c h e r a p p r e s e n t a u n s e g m e n t o di r e t t a . p o r r e m o p o i : u(r)= —/(r). hi l a /(r), cioè l o s p e t t r o i n t e g r a l e d e l l e t r a c c e , e l a g r a n d e z z a p r o p o r z i o n a l e zi(r), s a r a n n o q u i n d i r a p p r e s e n t a t e da p o l i g o n a l i , i cui l a t i a v r a n n o r a p p o r t i d i r e t t i v i , i n v a l o r e a s s o l u t o , d e c r e s c e n t i . l e ascisse d e i v e r t i c i d e l l a s p e z z a t a zt(r), s o n o i v a l o r i d e i r a n g e s . i n d i c h e r e m o c o n i/0, « i . . . l e c o r r i s p o n d e n t i o r d i n a t e . l a d i f f e r e n z a t r a i r a p p o r t i d i r e t t i v i mi, n»i4-t, di d u e s u o i l a t i c o n s e c u t i v i dà e v i d e n t e m e n t e : | m i | | b . h . 1 | = n 1 x , a [ 7 ' ] si n o t i c h e i l l a t o imo t a g l i a l'asse d e l l e o r d i n a t e u n e l p u n t o (0, u0i) d a t o d a : u,° = 2 k r k n k x k a q u i n d i 7/0 ,,o. f t i = ' [ 7 " ] l a [ 7 ' ] p u ò s c r i v e r s i n e l l a f o r m a , f o r s e p i ù c o m o d a : x « * n | = = » ' i ' " [ 8 ] r—/?i_t r,+l-ri si p u ò q u i n d i d e t e r m i n a r e g r a f i c a m e n t e , o con s e m p l i c i s s i m i calc o l i , l a c o n c e n t r a z i o n e d e i v a r i n u c l i d i , senza f a r e a l c u n a i p o t e s i sull ' e q u i l i b r i o d e l l e f a m i g l i e r a d i o a t t i v e a l l e q u a l i a p p a r t e n g o n o , c o n o s c e n d o s o l o i v a l o r i d i / ( r ) p e r i soli v a l o r i di r c o i n c i d e n t i c o n i r a n g e s d e i d i v e r s i n u c l i d i a t t i v i . e v i d e n t e m e n t e è p i ù s e m p l i c e i l c a l c o l o r e l a t i v o a l n u c l i d e d i r a n g e m a s s i m o , c h é i n t a l caso, l a [ 7 ] a s s u m e l a f o r m a m p _ 1 = ( r p k p _ 1 ) nf xpa, d a c u i : t e o r i a p e r l a d e t e r m i n a z i o n e d e l c o n t e n u t o r a d i o a t t i v o dei m i n e r a l i 231 s e i v a r i n u c l i d i c o s t i t u i s c o n o u n a f a m i g l i a r a d i o a t t i v a in e q u i l i b r i o s e c o l a r e , n e l l ' i p o t e s i c b e n o n v i s i a n o b i f o r c a z i o n i , ( i l c b e p u ò , con o t t i m a a p p r o s s i m a z i o n e a s s u m e r s i n e l caso d e l l a f a m i g l i a d e l l ' u 2 3 8 , m a n o n p e r q u e l l a del th 232), si a v r à : ili ai" = cost. q u i n d i , p e r l a [ 7 " ] , i p r o l u n g a m e n t i dei l a t i d e l l a p o l i g o n a l e c h e r a p p r e s e n t a l o s p e t t r o i n t e g r a l e d e l l a f a m i g l i a i n e q u i l i b r i o , incont r e r a n n o l'asse d e l l e u i n p u n t i a v e n t i , tra l o r o , d i s t a n z e p r o p o r z i o n a l i a i ranges. s e m p r e n e l caso d e l l ' e q u i l i b r i o , se si d i s p o n e di u n a statistica sufficientemente ricca, l a [ 8 ' ] p e r m e t t e di r i c o s t r u i r e l o s p e t t r o i n t e g r a l e d e l l a f a m i g l i a , p a r t e n d o d a l l a sola conoscenza d e l n u m e r o di t r a c c e a v e n t i l u n g h e z z a s u p e r i o r e al p e n u l t i m o r a n g e . nel caso d e l l a f a m i glia del t o r i o , bisogna t e n e r e o v v i a m e n t e conto d e l l a b i f o r c a z i o n e del tlic. questa f a m i g l i a , a causa d e l l a b r e v i t à d e l l e v i t e m e d i e dei suoi c o s t i t u e n t i , p u ò s e m p r e r i t e n e r s i i n e q u i l i b r i o . in t a l e f a m i g l i a , i l thc' e m e t t e p a r t i c e l l e a a v e n t i r a n g e m a g g i o r e , n o n solo di q u e l l e degli e l e m e n t i d e l l a stessa f a m i g l i a , m a a n c h e di t u t t e q u e l l e d e l l e f a m i g l i e del l'i! 2 3 8 e d e l l ' u 2 3 5 . q u i n d i , d i s p o n e n d o d e l l o s p e t t r o i n t e g r a l e p r o d o t t o da u n m i n e r a l e c h e c o n t e n g a u n m i s c u g l i o di f a m i g l i e r a d i o a t t i v e , a n z i c h é usar e l e [ 7 ' ] , [ 7 " ] , [ 8 ] , sarà p i ù c o m o d o d e t r a r r e , p e r m e z z o d e l l a [ 8 ' ] , l o s p e t t r o i n t e g r a l e d e l l a f a m i g l i a del th i n e q u i l i b r i o . lo s p e t t r o r i m a n e n t e a n d r à i n t e r p r e t a t o p e r m e z z o d e l l e f o r m u l e g e n e r a l i . ò l p r e n d e n d o c o m e v a r i a b i l e l a p r o i e z i o n e o r i z z o n t a l e (o v e r ticale), d e l l e t r a c c e n e l l ' e m u l s i o n e , p e r il n u m e r o cp (.r i, o p p u r e i|> ( y ì , di t r a c c e a v e n t i p r o i e z i o n e o r i z z o n t a l e x', (o v e r t i c a l e y'), x' ^ x, ( o p p u r e y ' > • y ) , si a v r à , p e r [ 5 ] , [ 6 ] : i' u, i' ( r k — x ) 2 cp(*) = nkak cpk ( x ) = -1— n k a" • — = i 4 i + i l \ k u. p ti, p ti. p " k a k a = ~l^nk ak rk ( v k „ k ak ) x+ ( 2 k ) *s (ri<x^rì+l) 4 i + i 2 i + i 4 i + i rk [ 9 ] la cp(a) r i s u l t a c o n t i n u a con l e sue d e r i v a t e p r i m e , i n t u t t o l ' i n t e r v a l l o 0 <; x r p. lo s p e t t r o i n t e g r a l e sarà q u i n d i r a p p i ' e s e n t a t o da 2 3 6 d o n a t o p a l u m b o 4 l i n a c a t e n a di a r c h i di p a r a b o l e . p o s t o , c o m e s o p r a u(x) = — <p(.r), e d i n d i c a n d o c o n u h i ì ì + j , mì+i|2 r i s p e t t i v a m e n t e l e o r d i n a t e d e i p u n t i d i ascisse ri, ri-^,, + jrì-j_t) si t r o v a f a c i l m e n t e : np » k h a _ " i + » i + i ~ 2 " i + ' l ^ i+j rk ~ 2(ri+l-rf" i ai" _ h i _ t + » i — 2 llj—1|2 _ l l j + l l i + l — 2 «i-l-l|2 2 r i ~~ ( r i r i , ) 5 ( r i + 1 r i ) ' ! p e r l a d e t e r m i n a z i o n e di fi,, si h a l a f o r m u l a , a n a l o g a a l l a [ 8 ' ] : n f à „ 0 = [ 9 " ] ( r p k p , ) 2 a n a l o g a m e n t e a q u a n t o d e t t o s o p r a , n e l caso d e l l a d i s t r i b u z i o n e d e i c a m m i n i t o t a l i , q u e s t ' u l t i m a p e r m e t t e , di d e t r a r r e d a l l o s p e t t r o i n t e g r a l e o t t e n u t o s p e r i m e n t a l m e n t e , q u e l l o d e l l a f a m i g l i a del t o r i o . n e l l ' i p o t e s i c h e l e s o l e f a m i g l i e p r e s e n t i n e l m i n e r a l e s i a n o q u e l l e d e l th, e d u 2 3 8 , l ' e q u i l i b r i o o m e n o di q u e s t ' u l t i m a f a m i g l i a p u ò e s s e r e s t u d i a t o p i ù c o m o d a m e n t e , a n z i c h é s u l l o s p e t t r o o t t e n u t o sper i m e n t a l m e n t e , su q u e l l o r i c a v a t o d o p o a v e r d e t r a t t o da q u e s t o l o s p e t t r o i n t e g r a l e d e l l a f a m i g l i a d e l t o r i o . l ' i n t e r p r e t a z i o n e d e l l o s p e t t r o d e l l a f a m i g l i a d e l l ' u r a n i o , così o t t e n u t o , p o t r à e s s e r e f a t t a p e r m e z z o d e l l a [ 9 ] , 0 , p i ù s e m p l i c e m e n t e , ( e q u e s t a o s s e r v a z i o n e v a l e a n c h e p e r i l caso «), si p o t r a n n o d e t r a r r e da q u e s t ' u l t i m o s p e t t r o , n o t a n d o c h e i l r a n g e p i ù l u n g o d e l l a f a m i g l i a a p p a r t i e n e al r a d i o c/, l e t r a c c e d o v u t e agli e l e m e n t i c o m p r e s i t r a i l r a d i o e d i l p o l o n i o , f a c e n d o u s o d e l l a [ 9 " ] . i n f a t t i q u e s t i e l e m e n t i s o n o t u t t i di v i t a m o l t o b r e v e , e q u i n d i , c e r t a m e n t e , i n e q u i l i b r i o t r a l o r o . c) g l i s p e t t r i d i f f e r e n z i a l i s o n o d a t i d a l l e d e r i v a t e , ( c a m b i a l e di segno), d e l l e [ 7 ] , [ 9 ] . il p r i m o è c o s t i t u i t o da u n a g r a d i n a t a di s e g m e n t i r e t t i l i n e i , par a l l e l i a l l ' a s s e r ; l a d i s t a n z a t r a d u e g r a d i n i c o n s e c u t i v i è p r o p o r z i o n a l e ad n. il s e c o n d o si r i d u c e , a l l a = [10] 2 i-h \ rk c h e r a p p r e s e n t a u n a s p e z z a t a di s e g m e n t i di r e t t a . t e o r i a p e r l a d e t e r m i n a z i o n e d e l c o n t e n u t o r a d i o a t t i v o dei m i n e r a l i 231 ringraziamo il prof. m. santangelo, per averci proposto questo calcolo, e per i numerosi consigli datici. palermo — istituto di fisica — febbraio 1953. riassunto considerando una lamina piana di spessore grande rispetto al percorso delle jxirticelle <», conlenente atomi radioattivi < , sovrapposta ad una emulsione nucleare, viene risolto il problema della determinazione del numero di tracce prodotte nell'emulsione dalle particelle « emesse, in funzione della loro lunghezza, della loro proiezione orizzontale e della loro proiezione verticale, nell'ipotesi, generalmente ammessa, che, per le particelle ti, i percorsi nella materia, siano proporzionali a quelli nell'aria. indicando con f ( x ì , ip (x), ip (xi il numero di tracce prodotte per air di emulsione e secondo, aventi rispettivamente, nell'emulsione, lunghezza totale, proiezioni orizzontale o verticale, maggiori di x, (essendo x misurato in cm d'aria equivalenti), si ha: 4 i-j-i /?; < x < ri+l cp (.t)=h< (x)= v k ^ (rk x)* 4 i+i rk in queste formule 111 è il rapporto tra il percorso nel minerale e quello in aria; r i ? . . . r p i ranges delle particelle a emesse (lai vari nuclidi, misurali in cm d'aria e disposti in ordine crescente, n k , à k " il numero di nuclidi della specie k"m per cm3 e la corrispondente costante di disintegrazione a. queste formule sono indipendenti da ogni ipotesi sull'equilibrio delle famiglie radioattive presenti nel minerale. summary considering a piane lamina containing alpha-radioactive atoms, u liose tliickness is great ivith respect to the range of the alpha particles, and exposed to nuclear emulsion ; the problem of the determination of the number of tracks produced in the emulsion by the alpha parti230 donato p a l u m b o cles is resolved as a function of their lengtlis, and of their liorizontal and vertical projections. il e liave used the hypothesis, gencrally admitted, that for alpha particles, the range in materials is proportional to that in air. indicating icitli f (x), qp (xì, i|'(x) the number of tracks produced per cm2 of emulsion and second, iihose total length, liorizontal and vertical projection in emulsion is greater tlian x , (x in cm of air equivalent), ice obtain: 4 i 4 i qp ( * ) = h' ( x ) = 7 ^ k — — ( i h — 4 i + i k k in these formulae iti is the ratio of the range in the minerai to that in air: rj. . . rp the ranges of the alpha particles emitted by the various nuclides, in cm airequivalent and arranged in increasing order, nk and xka are respectively, the number of nuclides of the specie k n m per cm3, and the constant of alpha disintegration. these formulae are independent of any hypotheses on the state of equilibrium of the radioactive families present in the minerai. b i b l i o g r a f i a f1) h. yagoda' radioactive measurements ivith nucìear emulsiona ( n . v . 1 9 4 9 ) . gap. v i i . ( 2 ) f . b a c h e l e t , e . m . f e r r e t t i s f o r z i n i , f . i p p o l i t o : n . c i n i . , v i l i , 8 5 1 ( 1 9 5 1 ) ( 3 ) s . milone-tamburino, a . stella: n . c i n i . , i x , 2 5 3 ( 1 9 5 2 ) . ( l ) e. e. picciotto: b u l l . p l i y s . n u c l . b r u x e l l e s , n. 33 ( 1 9 5 2 ) . ( 5 ) h. v . buttlar, n. isaac: b u l l . p l i v s . n n c l . b r u x e l l e s , n. 36 ( 1 9 5 2 ) . ( « ) j . w . b r e m n e r : p i o c . p h y s . s o c „ a 6 1 , 2 5 ( 1 9 5 1 ) . c ) i . c u r i e : j o u r . p l i y s . e t r a d . , 7 , 3 1 3 ( 1 9 1 6 ) . ( s ) h. v . buttlar, f. g . houtermans: g e o c . et c o s m o c . a c t a , 2, 43 ( 1 9 5 1 ) . (0) r . d. evans: p l i y s . r e v . , 4 5 , 2 9 ( 1 9 3 4 ) ; i b . 65, 2 1 6 0 944). ( 1 0 ) g . i m b ò l . c a s e r t a n o : a n n a l i d i g e o f i s i c a , i i i , 4 ( 1 9 5 0 ) , p a g . 4 7 5 . : a n n a l i di g e o f i s i c a , v , 1 ( 1 9 5 2 ) , pag. 1. ( n ) h . y a g o d a : o . e . , p a g . 8 3 . ( 1 2 ) van stijvendael : b u l l . p l i y s . n u c l . b r u x e l l e s , n. 2 8 ( 1 9 5 1 ) . p r i n c i p a l i risultati c o n s e g u i t i d u r a n t e l ' o s s e r v a z i o n e geod i n a m i c a , o p p o r t u n a m e n t e estesa n e l t e m p o , di g r a n d i d i g h e di s b a r r a m e n t o , e l o r o giustificazioni t e o r i c h e p . c a l o i m . c . s p a d e a ricevuto il 20 f e b b r a i o 1966 r i a s s u n t o . — vengono e s a m i n a t i , nelle loro giustificazioni teoriche, i r i s u l t a t i di sistematiche osservazioni geodinamiche (eseguite con l'ausilio di v i b r o m e t r i , sismografi e clinografi). c o m p i u t e presso g r a n d i dighe dell ' e . n . e l . anche da q u e s t ' e s a m e , scaturisce la g r a n d e u t i l i t à — che spesso confina con la necessità — di tali osservazioni. p e r q u a n t o si riferisce all'uso dei clinografi, un quindicennio di registrazioni o t t e n u t e presso la diga di val gallina, p e r m e t t e di f a r r i e n t r a r e il tragico evento del v a j o n t in un f e n o m e n o di p o r t a t a secolare specialmente a t t i v o nell'ultima era geologica nell'orogenesi a l p i n a ; e. che, in condizioni particolari, ])uò condurre (come ha c o n d o t t o ) ad a n o m a l e situazioni locali. s u m m a r y . — are here e x a m i n e d — in their theoretical e x p l a n a t i o n s — t h e results of s y s t e m a t i c geodynamic observations (carried out b y vibrometers, seismographs and clinographs), achieved on e . n . e l . big d a m s . this investigation also, shows the great usefulness — being, indeed, a, necessity — of such observations. a period of fifteen years of clinographic records on t h e val gallina's d a m , include t h e v a j o n t ' s tragic event into a secular range p h e n o m e n o n , p a r t i c u l a r l y active — during t h e last geologic e r a — in t h e alpine orogenesis who, in special conditions, m a y be t h e cause (just like it h a p p e n e d ) of anomalous local situations. 1. d i v e r s e d i g h e del v e n e t o , e le r e l a t i v e rocce d ' i m p o s t a , a pai-tire d a l 1949, f u r o n o s o t t o p o s t e a c o n t r o l l o g e o d i n a m i c o . p a r t i c o l a r m e n t e s e g u i t e , f u r o n o le d i g h e d i p i e v e d i c a d o r e , d e l l u m i e i e del v a j o n t . p e r a l c u n e d i esse t a l e c o n t r o l l o è t u t t o r a i n a t t o . i l c o n t r o l l o g e o d i n a m i c o si a p p l i c a , e s s e n z i a l m e n t e , n e l r i l e v a m e n t o delle c a r a t t e r i s t i c h e f o n d a m e n t a l i del c a m p o e l a s t i c o n e l l a z o n a int e r e s s a t a d a l l a d i g a , e delle loro e v e n t u a l i v a r i a z i o n i n e l t e m p o . 2 6 2 p . c a l o i m . c . s p a d e a . i principali elementi del controllo sono: a) eilevamento del modulo elastico della roccia e del calcestruzzo (modulo di young) e sue variazioni nel tempo. b) e i l e v a m e n t o della verticale a p p a r e n t e e delle sue variazioni: p e r cause periodiche (alternarsi delle stagioni, insolazione e sua variazione nell'anno, ecc.); secolari (di origine tettonica o comunque geologica); accidentali, legate a deformazioni locali del mezzo, come conseguenza della variazione delle caratteristiche elastiche, ecc. c) eilevamento delle brusche r o t t u r e d'equilibrio del campo elastico, per microsismicità naturale o provocata. i rilievi, di cui al p u n t o a ) , possono essere eseguiti con vibrometri, completati dall'ausilio di geofoni; quelli di cui al p u n t o b) con fotoclinografi, m e n t r e le indagini contemplate nel p u n t o o) sono riservate a speciali stazioni sismiche. di questi strumenti è stato detto, a più riprese, in precedenti pubblicazioni (1-3). 2. la determinazione del modulo d'elasticità è s t a t a f a t t a (e ciò avveniva per la prima volta in europa) rilevando contemporaneamente le velocità di propagazione delle onde longitudinali e trasversali del mezzo. talvolta ci si è serviti delle sole onde longitudinali. anche su questo argomento si è a lungo discusso in precedenti lavori (4) e qui non intendiamo ripeterci. dobbiamo solo aggiungere che u n a delle obiezioni, recentemente mosse a questo metodo da geotecnici italiani (che, finalmente, dopo circa venti anni, si sono accorti della sua esistenza), consiste nel rilevare che le formule relative avrebbero valore solo per u n mezzo omogeneo, isotropo. a rigore, ciò risponde al vero. va però i m m e d i a t a m e n t e sottolineato che t u t t i i t e n t a t i v i f a t t i per tener conto dell'anisotropia del mezzo, oltre a comportare rilevanti complicazioni analitiche, hanno condotto a risultati, l'ordine di grandezza dei quali non si discosta sensibilmente da quello relativo al caso dell'isotropia, le differenze rientrando largamente entro i limiti di errore. del resto, chi lia qualche cognizione di sismologia, sa che i valori dei tempi di t r a g i t t o delle onde longitudinali e trasversali, o t t e n u t i su percorsi di migliaia e migliaia di chilometri nell'interno della terra, con le formule valevoli per mezzi isotropi, coincidono con i valori osservati fino al decimo di secondo. p a t t a questa precisazione, ricapitoliamo rapidamente ciò che è stato largamente esposto in relazioni, a suo tempo inviate alla sade, e in pubblicazioni specifiche (4). r i s u l t a t i c o n s e g u i t i d u r a n t e l'i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 6 3 i l p r i m o f a t t o messo in luce f u la n e t t a diminuzione della velocità delle onde elastiche, r i l e v a t a dopo lo s b a n c a m e n t o , nei c o n f r o n t i dei valori o t t e n u t i p r i m a dell'inizio dei lavori di scavo. questo, in t u t t i i casi s p e r i m e n t a t i . poiché il f a t t o si t r a d u c e v a in u n a sensibile diminuzione del modulo elastico, i n d i c a v a m o il fenomeno col t e r m i n e decadimento (*) del modulo elastico (5). i l f a t t o f u a t t r i b u i t o , nella sua p a r t e p r e p o n d e r a n t e , alla decompressione conseguente al distacco di grosse b a n c a t e di roccia. i l decad i m e n t o però non si a r r a s t a v a al lavorio d e t e r m i n a t o dallo s b a n c a m e n t o . u n a v o l t a c o s t r u i t a la diga, specie nede zone d ' i m p o s t a , esso r i p r e n d e v a e c o n t i n u a v a p e r anni, sia p u r e g r a d u a l m e n t e a t t e n u a n d o s i , t e n d e n d o a s i n t o t i c a m e n t e verso valori c o s t a n t i (figg. 1-4 di p i e v e di 0.). g 1 1 1 1 1 1 1 1 1 1 1 53 5 4 5 5 5 6 57 5 6 59 bo 61 62 6 ì anni fig. 1 diga di pieve di cadore. d e c a d i m e n t o del modulo di elasticità per u n t r a g i t t o in spalla d e s t r a . ciò f u spiegato (6) come conseguenza di una m i n u t a microsismicità, p r o v o c a t a nel sistema roccioso a cui la diga è a n c o r a t a , dai c o n t i n u i m o v i m e n t i cui lo s b a r r a m e n t o viene1 sottoposto d a variazioni di carico nel bacino idrico a m o n t e e d a variazioni della t e m p e r a t u r a e s t e r n a , per l'insolazione d i u r n a e l ' a l t e r n a r s i delle stagioni. l a microsismicità p r o v o c a t a f u i n f a t t i messa in risalto dalla stazione sismica, f u n z i o n a n t e presso la diga di p i e v e di cadore, a p a r t i r e d a l 1950 (4-8). solo u n a stazione sismica, i n f a t t i , p u ò rivelare l'esistenza di u n a microsismicità locale, p r o v o c a t a o no. p e r q u a n t o concerne p i e v e di cadore, p . es., b a s t e r à osservare che delle parecchie diecine di migliaia (*) il t e r m i n e riusci ostico a geologi e geotecnici, p o r t a t i a d d i r i t t u r a a negare il f e n o m e n o . r i t e n i a m o che, a t t u a l m e n t e , si siano r i c r e d u t i (almeno lo speriamo). 2 6 4 p . c a l o ! >[. c . s p a d e a di microscosse, registrate dalla stazione sismica nel primo quinquennio di funzionamento, solo un'infima p a r t e — forse u n a diecina in t u t t o — f u a v v e r t i t a dal personale addetto alla diga. 1 » ' r 1 1 1 1——, , 5 4 5 5 5 6 5 7 5 8 5 9 6 0 61 62 a n n i fig. 2 diga di p i e v e di cadore. d e c a d i m e n t o del m o d u l o di e l a s t i c i t à in spalla sinistra p e r t r a g i t t i t r a c c i a t i t r a s v e r s a l m e n t e alla valle. 2 «•> o x in 3 54 5 5 56 57 5 8 5 9 6 0 61 62 a n n i fig-. 3 d i g a di p i e v e di cadore. d e c a d i m e n t o del m o d u l o di e l a s t i c i t à in spalla s i n i s t r a . il deterioramento del campo elastico locale, si t r a d u c e v a in una conseguente diminuzione della velocità delle onde elastiche, e quindi, nel decadimento del modulo elastico. r i s u l t a t i c o n s e g u i t i d u r a n t e l'i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 6 5 veniva spontaneo di attribuire tale deterioramento a l l ' a u m e n t a t a porosità della roccia, come conseguenza del gran numero di piccole f r a t t u r e , associate alle microscosse verificatesi nel mezzo (6). fig. 4 diga di p i e v e di cadore. valori m e d i del m o d u l o di elasticità nel 1962 p e r le v a r i e zone della s t r e t t a del p i a v e (in u n i t à io6 kg/cm 2 ). 3. accurate ricerche di laboratorio furono compiute sull'argomento della porosità e dei problemi connessi. l a porosità è indubbiamente uno dei più seri ostacoli nello studio dell'elasticità delle rocce. una volta eliminato l'effetto della porosità con l'applicazione di pressioni limite, molte caratteristiche delle rocce 2 6 6 i ' . c a l o i m . c . s p a d e a possono essere agevolmente studiate. l'esistenza di vuoti f r a cristallo e cristallo costituisce p e r t a n t o un grosso ingombro nello studio delle proprietà fìsiche delle rocce a pressione ordinaria. applicando elevate pressioni idrostatiche (più di 2000 atmosfere), l'effetto della porosità sulla velocità delle onde elastiche diviene piccolissimo, t a n t o da rendere superfluo il tenerne conto. ad un iniziale a u m e n t o della pressione, la velocità cresce molto rapidamente, e solo per pressioni molto elevate l'aumento relativo comincia a decrescere. questo anormale aumento della velocità alle basse pressioni, viene attribuito all'esistenza di vuoti fra i grani della roccia. se la roccia è compatta, l'iniziale aumento della velocità è più limitato. esiste, manifestamente, una relazione fra porosità e densità per rocce della stessa n a t u r a , nel senso che l'una cresce quando l'altra diminuisce. furono f a t t o esperienze intese a dedurre la velocità in funzione della densità di volume (« bulk density ») — e quindi della porosità — in rocce dello stesso tipo. la densità f u misurata per ogni esemplare (campione di roccia), pesandolo e misurando l'immersione nel mercurio. le esperienze hanno dimostrato che, a p a r i t à di altre condizioni, all'aumentare della porosità non solo diminuisce la velocità ma anche l'ampiezza del movimento, a testimoniare che la porosità provoca anche un aumento di dissipazione. l a pig. 5 dà la velocità delle onde longitudinali in funzione della densità di volume per arenarie. p e r la determinazione della porosità rispetto alla densità di volume, esiste una formula empirica ottenuta da d. h . davis (7) da 601 campioni di arenarie ed altre rocce: nella relazione scritta, 2.65 rappresenta la media densità dei grani di minerale componenti: arenarie, argille, ecc. l a porosità viene dedotta dalla densità di volume, sulla base della formula [1]. la relazione f r a la velocità delle onde longitudinali e la porosità è r i p o r t a t a nella pig. 6. l a velocità decresce esponenzialmente con l a porosità, e la sua relazione è approssimativamente espressa dalla formula <p = (1 — qb/2 .65) x 100 [ 1 ] dove 0 = porosità, ob = densità di volume. vp = vpo e~y0 , vpo = 5750 m/sec , y = 0.036 , [2] r i s u l t a t i c o n s e g u i t i d u r a n t e l'i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 6 7 dove vpg esprime la velocità delle onde longitudinali nell'esemplare di roccia, per il quale la porosità è nulla. dalla combinazione delle [1], [2] si trae, per la velocità delle onde longitudinali in « arenarie » di n o t a densità di volume («bulk density »): vp (m/sec) = 5750 exp (1,36 qb — 3.6) . [3] f i g . 5 velocità delle o n d e l o n g i t u d i n a l i i n f u n z i o n e della d e n s i t à p e r a r e n a r i e (secondo d . sliimozuru). esistono altre relazioni sull'andamento della velocità con la porosità, specie in relazione alla profondità del materiale roccioso investigato (10). l a teoria più rigorosa svolta su questo argomento resta, in ogni modo, quella sviluppata da y . sato (»). 2 6 8 i'. c a l o i m . c . s p a d e a p e r la velocità delle onde elastiche, che si p r o p a g a n o in m a t e r i a l e elastico c o n t e n e n t e c a v i t à sferiche v u o t e , egli h a d e d o t t o le seguenti f o r m u l e : vp = vpo ve = v. so v e h é 1 2 <r„ 4 — 5 oo i — (i—e) 7 — 5 a 0 a 0 • 5 a . [4] km/svc 5.5 -• 4.5"' °o°„o 3.5 2 . 5 " 1 . 5 12 16 20 % pig. 6 velocità delle onde longitudinali in f u n z i o n e della porosità per arenarie (secondo d. h. davis). d o v e vp0, vso r a p p r e s e n t a n o le velocità delle onde longitudinali e t r a sversali nel m a t e r i a l e privo di c a v i t à , oi l a d e n s i t à relativa del m a t e r i a l e all'esame, oo, il r a p p o r t o di poisson p e r il m a t e r i a l e reale (privo di c a v i t à ) , 1 — e, la porosità. p e r oo = , = — 0.768 (1 — e ) ] , vs =vso [1 — 0.478 (1 — 9 ) ] . [5] r i s u l t a t i c o n s e g u i t i d u r a n t e l'i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 6 9 l a f i g . 7 si riferisce alla tabulazione dei valori della velocità delle onde longitudinali in relazione con la porosità, calcolata d a s a t o . poiché si conoscono le velocità delle onde longitudinali (e trasversali) o t t e n u t e p r i m a della costruzione delle dighe (di p i e v e di cadore, p . es., v a j o n t , ambiesta, val gallina, ecc.) l'uso delle f o r m u l e [1] [2] [3], opp u r e [4] e [5] p e r m e t t e di calcolare — in via a p p r o s s i m a t i v a , n a t u r a l m e n t e — l ' a u m e n t o della porosità, nelle zone d ' i m p o s t a , conseguente allo s b a n c a m e n t o ed ai m o v i m e n t i della diga. 10 2 0 5 0 % porosi fa" ( f p ) fig. 7 a n d a m e n t o della velocità in funzione della porosità (secondo y. sato). 4. u n a l t r o f a t t o , messo ben p r e s t o in evidenza d u r a n t e le indagini sul campo elastico nei pressi di u n a diga, f u l a constatazione di velocità di propagazione delle onde elastiche, nella p a r t e della roccia a c o n t a t t o col bacino idrico, più o meno sensibilmente inferiori a quelle r i l e v a t e a valle dello s b a r r a m e n t o . ciò si rese p a r t i c o l a r m e n t e m a n i f e s t o nel bacino del lumiei (lago di sauris). veniva spontaneo a t t r i b u i r e il f e n o m e n o alla penetrazione dell'acq u a nei pori della roccia. e t a l e f u a p p u n t o la spiegazione d a noi d a t a (5). anche su questo a r g o m e n t o sono s t a t e f a t t e , nel f r a t t e m p o , i n d a gini q u a n t i t a t i v e . 2 7 0 i'. c a l o i m . c . s p a d e a giova premettere alcune precisazioni circa il significato delle parole: contenuto d'acqua, porosità, rapporto di vuoto e r a p p o r t o di saturazione. si definisce come contenuto d'acqua w, il rapporto f r a il suo peso ww e quello del materiale reale ws: w = x 100 . • v s si definisce porosità 0 il rapporto del volume del vuoto vn a quello di t u t t o il sistema v: 0 = y x 100 = — x 100 , dove pb, g sono la densità di volume e la densità sostanziale del materiale effettivo, rispettivamente. diremo rapporto di vuoto e, il rapporto f r a il volume di vuoto vn e quello del materiale vs: = z ? = 0 e ve 100 — 0' 0 = x 100 . infine diremo aliquota (grado) di saturazione s, il rapporto del volume di acqua ww a quello dell'intero vuoto vn: s = ~ x 100 . • n furono f a t t e indagini sulla variazione delle velocità delle onde elastiche con la saturazione di acqua. è risultato che, in effetti, la velocità delle onde elastiche viene fort e m e n t e influenzata dal grado di saturazione di acqua negli interstizi vuoti. la teoria più rigorosa svolta a questo riguardo è sempre quella di y. sato (8), che conduce alle seguenti espressioni, per quanto concerne le velocità di propagazione: vp = vpo 1 — (1 • (1 — fc) ( 1 + do) ve = v, 'so (1 — e) 2(11 15 -2oo) + l c ( l + a „ ) 1 — oo 1 0 ^ c l d ) 7—5cr0 7 o (so (1 —d) [6] r i s u l t a t i c o n s e g u i t i d u r a n t e i . ' o s s e r v a z i o x e g e o d i n a m i c a . e t c . 2 7 1 dove i simboli h a n n o il significato già esposto; i n o l t r e : k è il r a p p o r t o del « b u l k m o d u l u s » del liquido a quello del materiale reale k0 {le — j^j ; i ) è il r a p p o r t o della densità del liquido a quella del m a t e r i a l e reale l a f i g . 8 m o s t r a la variazione di velocità delle onde longitudinali d o v u t a alla porosità per a r e n a r i a secca e s a t u r a t a , calcolata con le porosità'' ( 1-p j fis;. 8 velocità delle onde longitudinali in l'unzione della porosità per a r e n a r i a secca e s a t u r a t a (secondo y. sato). f o r m u l e [4] e [6]. l a velocità per arenaria s a t u r a t a d ' a c q u a è minore di quella p e r arenaria a s c i u t t a . la teoria, b a s a t a sul sistema d i c a v i t à chiuse (cioè non i n t e r c o m u n i c a n t i f r a loro), non predice la velocità in funzione del grado di saturazione. i n ogni modo, per esemp l a r i n o n s a t u r a t i , la velocità p u ò essere d e d o t t a nella regione compresa f r a i d u e casi estremi, come in figura. le esperienze eseguite, sono g e n e r a l m e n t e a f a v o r e della teoria di s a t o . f r a z e r ( u ) t r o v a che l ' a c q u a interstiziale a u m e n t a l ' e f f e t t o di angolarità. p e r es., la porosità delle particelle di quarzo f r a n t u m a t o cresce 2 7 2 i'. c a l o i m. c . s p a d e a dal 4 8 % in un insieme asciutto, al 5 4 % in un insieme u m i d o . perciò, a p p a r e n t e m e n t e l ' a c q u a a u m e n t a la porosità. i n ciò deve vedersi la. causa della diminuzione di velocità al crescere della saturazione. i n ogni caso, sembra che l ' e f f e t t o sia decisamente in questo senso nei pressi della saturazione. f u r o n o f a t t e esperienze anche sulla variazione della velocità in mezzi solidi con c o n t e n u t o d ' a c q u a (mescolanza di m a t e r i a l e solido e acqua), come sedimenti di oceano profondo, f o n d o di laghi, ecc. fig. 9 velocità delle onde longitudinali e trasversali in funzione del contenuto d'acqua (secondo k. iida). è n o t o che l'osservazione sismologica m o s t r a q u a n t o sia piccola la velocità delle onde sismiche in t e r r e n o soffice, nei c o n f r o n t i di quella delle rocce. tale velocità è f o r t e m e n t e a f f e t t a dal c o n t e n u t o d ' a c q u a . ricerche di laboratorio di k . i i d a , c o n d o t t e su t e r r e n i di diverso cont e n u t o d ' a c q u a , p r o v a n o che la velocità delle onde longitudinali decresce al crescere del c o n t e n u t o d ' a c q u a . ciò che v a sottolineato è che la velocità ài un miscuglio di due componenti, p. es. particelle solide ed acqua, varia con il rapporto di composizione ed è sempre considerevolmente al di sotto delle velocità di entrambi i componenti. poiché la velocità delle onde longitudinali p e r solidi a cui le particelle a p p a r t e n g o n o p u ò variare d a 4 a 6 km/sec, e quella d e l l ' a c q u a è 1.5 km/sec, la differenza f r a le velocità m i s u r a t e (dell'ordine delle centinaia di m/secvedi r i s u l t a t i c o n s e g u i t i d u r a n t e l ' i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 7 3 fig. 9) e quello dei componenti è molto grande. la velocità sembra raggiungere un minimo con un certo contenuto d ' a c q u a , e tende a raggiungere i 1500 m/sec quando il contenuto d'acqua tende al 100%, e il sistema risulta composto esclusivamente d'acqua. 5. d a quanto precede, risulta chiaro come, a p a r i t à di porosità, possano essere registrate velocità di propagazione sensibilmente diverse, in rocce della stessa n a t u r a , a valle e a monte di una diga (come è stato verificato per le dighe in gamia e in cadore). ison solo, m a come possa spiegarsi perchè nello stesso massiccio roccioso —• senza che si sia verificata variazione appi'ezzabile nella sua porosità — si possano ottenere, nel tempo, valori della velocità sopra o sotto la media: ciò può dipendere dal diverso grado di saturazione in cui la roccia è v e n u t a a trovarsi in epoche diverse; come è stato effettivamente constatato nella zona d'imposta della diga di pieve di cadore, in sponda destra e in sponda sinistra (figg. 1-3). un altro f a t t o è degno di menzione: finché l'ossatura solida di un mezzo conserva carattere di continuità, la velocità delle onde elastiche condotte dal mezzo è sempre elevata, quantunque la sua porosità — in senso lato — possa provocarne un'apprezzabile diminuzione. non appena il tessuto solido comincia ad incrinarsi, la velocità delle onde elastiche diminuisce bruscamente, per ridursi a valori bassissimi in caso di f r a t turazione multipla, che renda il mezzo una semplice sovrapposizione di elementi, singolarmente compatti, ma senza reciproco legame. i n questo senso, l'osservazione continua del campo elastico nelle zone d'imposta di u n a diga e delle sponde del relativo bacino, riveste un eccezionale interesse, come sensibilissima misura di controllo. una testimonianza preziosa, a questo riguardo, è stata data dal comportamento, in campo elastico, dell'ammasso roccioso sovrastante in sinistra la diga d i p o n t e s e i : nell'intervallo di pochi anni (dal 1952 al 1958), la velocità delle onde longitudinali del complesso roccioso or detto, scese da 3500 m/sec ca a 1000 m/sec ca. ìjel 1958, i n f a t t i , esso aveva già raggiunto l'inconsistenza del materiale di f r a n a ; e solo un tempestivo intervento di alleggerimento della zona valse a scongiurare l'effettivo movimento franoso. u n ' a l t r a ben più drammatica prova a conferma di queste conclusioni è s t a t a offerta dallo sperone roccioso in sponda sinistra a monte della diga del y a j o n t , risultato ancora compatto nel dicembre del 1959 e che un anno dopo — nel dicembre del 1960 — si era rivelato elasticamente debellato; conseguenza dell'azione di demolizione, determinata nel f r a t t e m p o , dalle centinaia di microscosse verificatesi nella zona (17). 2 7 4 i». c a l c i m. c . s p a i ) e a 6. anisotropia elastica delle rocce. un altro problema di complessa t r a t t a z i o n e è quello legato all'anisotropia delle rocce. è noto che rocce dello stesso tipo possono p r e s e n t a r e considerevoli differenze di velocità, i n d i p e n d e n t e m e n t e dal loro s t a t o di porosità. p e r es., le velocità delle onde longitudinali nel m a r m o possono v a r i a r e d a 2.8 k m / s e c a 7.3 km/sec. f r a le cause di tali variazioni, u n a delle p i ù efficenti p u ò essere l'anisotropia, o, comunque, le deviazioni dalla perf e t t a isotropia delle rocce. i n n a t u r a l'anisotropia elastica delle rocce ignee e m e t a m o r f i c h e è diversa d a quella delle rocce sedimentarie. rocce ignee ordinarie, quali il granito, l'andesite, la diorite, a p p a r e n t e m e n t e appaiono isotropiche. i n eff e t t i , anche in questi tipi di rocce, il m e t o d o ad ultrasuoni p u ò rivelare variazioni di velocità delle onde longitudinali e trasversali a seconda deliadirezione. i n ogni caso, si t r a t t a di anisotropia poco a c c e n t u a t a . e s s a viene a t t r i b u i t a alla n a t u r a eterogenea delle rocce, legata sopra t u t t o alla orientazione dei cristalli componenti. se essi sono d i s t r i b u i t i a caso, in t u t t e le direzioni possibili, allora gli aggregati policristallini r i s u l t a n o e l a s t i c a m e n t e isotropici; in t a l caso, v e r r a n n o a m a n c a r e variazioni direzionali di velocità. nelle rocce sedimentarie, tali variazioni direzionali di velocità possono essere a c c e n t u a t e . ciò deriva dal meccanismo di sedimentazione o dal m o d o di associazione dei grani c o m p o n e n t i . f u r o n o f a t t e ricerche sui due tipi di anisotropia. noi accenneremo qui s o l t a n t o al secondo tipo, quello che più ci interessa. i l f e n o m e n o dell'anisotropia di velocità in rocce s t r a t i f i c a t e o sedim e n t a r i e è s t a t o r i p e t u t a m e n t e osservato in ricerche di prospezione sismica. il grado di anisotropia di velocità si suole indicare con il così d e t t o f a t t o r e di anisotropia a , definito come il r a p p o r t o della velocità lungo gli s t r a t i alla velocità considerata p e r p e n d i c o l a r m e n t e agli s t r a t i . p . es., p e r sedimenti di c a r b o n a t o , giacenti f r a m 230 e in 262 sotto la superfìcie, u h r i g (12) t r o v a u n f a t t o r e a variabile f r a 1,10 e 1,19. ma il divario può essere anche maggiore. così nasu ed o m o t e (13) s p e r i m e n t a n d o sopra u n a r u p e di a r e n a r i a t r o v a r o n o un f a t t o r e di anisotropia p a r i a 1,83 (fig. 10). l ' a n i s o t r o p i a di velocità per le rocce sedimentarie d e r i v a da d u e diversi meccanismi. u n o è legato alla loro s t r u t t u r a l a m i n a t a o periodica, l ' a l t r o alla s t r u t t u r a dell'associazione (« p a c k i n g ») dei grani componenti. i l p r i m o è quello dell'argilla (stratificata), il secondo quello d e l l ' a r e n a r i a . f u r o n o f a t t e misure di l a b o r a t o r i o con il m e t o d o degli u l t r a s u o n i . un campione di a r e n a r i a , p r e l e v a t a ad u n a p r o f o n d i t à di 70 m e sagor i s u l t a t i c o n s e g u i t i d u r a n t i ' . l ' o s s e l i v a z i ! > n e i; e d i ) in a m i c a . e t c . 2 7 m a t o , f u esaminato secondo due direzioni ortogonali ed una intermedia: come mostra la figura e la t a v . di fig. 11. l a velocità massima lungo u n a direzione (x) risulta minima nella direzione ortogonale (z) f i g . 10 a n i s o t r o p i a della v e l o c i t à in u n blocco di a r e n a r i a (secondo n. n a s u e g. oniote). anche la trasmissione dell'energia è diversa, a seconda delle direzioni: essa i n f a t t i viene fortemente a t t e n u a t a nella direzione perpendicolare (x) all'asse verticale (z), dove si osserva il minimo di velocità. i n questo caso il f a t t o r e di anisotropia risultò di 1.7. f i g . 11 a n i s o t r o p i a della velocità in u n e s e m p l a r e di a r e n a r i a ( t r a t t o d a u n a p r o f o n d i t à di 70 iti) di non c h i a r a a n i s o t r o p i a s t r u t t u r a l e (secondo d. s h i m o z u r u ) . 2 7 6 i ' . c a l o i m . c . s p a d e a in n a t u r a , con particolare riguardo alle rocce d ' i m p o s t a di u n a diga, lungo la sezione di una valle, le stratificazioni appaiono g e n e r a l m e n t e più o m e n o inclinate, lungo giaciture spesso contorte, c o m u n q u e f o r t e m e n t e piegate. le determinazioni della velocità conducono p e r t a n t o — nella generalità dei casi — a valori i n t e r m e d i . p e r avere nozioni sulle variazioni di velocità — e quindi del m o d u l o elastico — nel t e m p o , si s p e r i m e n t a o v v i a m e n t e su direzioni fisse, prestabilite. 7. p e s t e r e b b e da accennare all'elasticità delle rocce sotto pressioni limiti; m a di essa a v e m m o modo di occuparci a più riprese (4), e qui riteniamo superfluo ripeterci. 8. sull'uso dei clinografi presso i grandi sbarramenti. — sull'utilità dei clinografi nel r i l e v a m e n t o delle condizioni di s t a b i l i t à locale (esistenza o meno di blocchi confinanti, a n i m a t i da m o t i r e l a t i v i diversi, ecc.), sul rilevamento delle variazioni della verticale a p p a r e n t e in senso lato, sull'insostituibile funzione di controllo continuo, sensibile, preciso, i m m e d i a t o , è s t a t o da noi a m p i a m e n t e riferito in n u m e r o s e relazioni e pubblicazioni. qui ci preme soffermarci su alcuni aspetti delle osservazioni clinografiche, che rendono il clinografo un prezioso complemento del sismografo e del vibrometro, nei rilevamenti delle variazioni del campo elastico. p u ò i n f a t t i rivelare variazioni p e r m a n e n t i della verticale a p p a r e n t e , in relazione con variazioni p e r m a n e n t i delle caratteristiche elastiche della roccia, come è s t a t o messo in chiara evidenza dalle registrazioni clinografiche nella roccia d ' i m p o s t a , in spalla destra, della diga di pieve di cadore (6). il suo uso c o n t i n u a t o nell'interno di unsi diga, a cominciare dall'ultimazione dello s b a r r a m e n t o , oltre a rivelare i m o t i lenti o rapidi, periodici e n o n periodici, sistematici e accidentali del m a n u f a t t o , fornisce d a t i e v i d e n t i sulla variazione nel t e m p o della risposta elastica del calcestruzzo. si n o t a , i n n a n z i t u t t o , u n graduale irrigidimento del calcestruzzo nei p r i m i a n n i della costruzione della diga (rivelato, f r a l'altro, d a l l ' a t t e n u a zione dell'onda d i u r n a alla base della diga — pieve di cadore, a m b i e s t a , val gallina, . . . — dal lento migrare in direzione spalla-spalla della r i s u l t a n t e dei m o t i del pulvino o t a m p o n e , legati a d a s i m m e t r i a d'insolazione, inizialmente con sensibile componente anche verso valle-monteambiesta, . . . —). 1 ' . c a i . 0 1 m . c . s p a d f . a l ' i n t e r p r e t a z i o n e d i q u e s t e r e g i s t r a z i o n i . fig. 13 c o n f r o n t o d a t i clinografici con d a t i t e m p e r a t u r a i n v a s o , o t t e n u t i nel n o v e m b r e 1963 presso le p o s t a z i o n i clinograflche site al c o r o n a m e n t o , a q u o t a 437 e, a u n m e t r o s o t t o nel t a m p o n e della diga d e l l ' a m b i e s t a . 2 7 8 i ' . c a l o i m . c . s p a d e a l'osservazione clinogralica prova poi elle, costruita per sopportare in modo uniforme le pressioni provenienti dall'invaso, generalmente una diga si comporta come rigida nei confronti delle variazioni dell'invaso .stesso (salvo casi di una sorta di effetto di « raddrizzamento » intermedio, come per la diga dell'ambiesta) (14), mentre, per ovvii motivi, l'effetto dell'insolazione (a p a r i t à di altre condizioni) cresce sensibilmente dalla base al coronamento (15). di particolare interesse risulta — nelle rivelazioni dell'osservazione clinografica — il comportamento del pulvino presso alcune dighe nei confronti delle variazioni della t e m p e r a t u r a esterna dell'aria, e, in particolare, dell'insolazione. nel caso di asimmetria di insolazione (abbastanza comune nel complesso delle dighe) il pulvino integra, per così dire, gli effetti dell'azione dinamica legata all'insolazione stessa, nel senso che subisce l'ineguale distribuzione delle tensioni verticali nei singoli conci, come conseguenza del diverso riscaldamento degli stessi. avviene così che, mentre alla base del concio centrale l'onda diurna non influisce in modo apprezzabile sulla variazione della verticale apparente (specialmente in direzione spalla-spalla), un metro sotto, il clinografo associato al pulvino, registra ampie olirle diurne, quasi esclusivamente nella direzione spalla-spalla. tale effetto è particolarmente n e t t o nella diga dell'ambiesta ("); un po' meno accentuato presso la diga di pontesei. u n ' a l t r a caratteristica dinamica che distingue il pulvino dai conci sovrastanti è la seguente. i n t u t t e le dighe le variazioni dell'invaso si trasferiscono in nette variazioni della verticale apparente fino alla base dei cónci. è naturale che, sui conci centrali, le su dette variazioni abbiano luogo, quasi esclusivamente, in direzione valle-monte. nelle dighe, come in quella dell'ambiesta, dove alla base — separate da u n solo metro di quota — funzionano sulla stessa verticale due identiche coppie di elinografi una ancorata al concio centrale, l'altra al sottostante pulvino o tampone, le registrazioni delle variazioni di pressione da p a r t e dell'invaso vengono registrate identiche nella forma (anche nei minimi particolari), però lievemente diverse nell'ampiezza; una leggera diminuzione osservandosi in quelle ottenute sul pulvino (tampone). il che sta a significare che la superficie di discontinuità dinamica esistente f r a il pulvino o t a m p o n e e il concio sovrastante, è, sufficiente a provocare u n a leggera attenuazione dei movimenti (analoga alla dissipazione che subiscono le onde elastiche nell'attraversamento di una superfìcie di discontinuità). tale fenomeno appare chiaramente registrato presso la diga dell'ambiesta (pigg. 12, 13). i f a t t i accennati non devono stupire: essi rientrano nell'ambito dei micromovimenti. diga di val gallina anno 1963. p . c a l o i m . c . s p a d e a f i g . 15 c o n f r o n t o f r a le osservazioni clinograflclie e i d a t i di t e m p e r a t u r a ed i n v a s o o t t e n u t i presso la p o s t a z i o n e elinograflca sita alla b a s e della diga di val gallina n e l l ' a n n o 1963. d i g a di v a l g a l l i n a a n n o 1964. p . caloi m. c. spadea vvv'vx/ 660 650 640 630 620 610 600 680 6 7 0 660 6 4 0 / quoh. invaso 620 bacino complaltimui/l (60z1 (602) si io 15 zo 25 51 5 10 15 ! 0 ! 5 ! j 5 10 15 20 25 51 5 io 15 20 25 50 5 10 15 20 25 31 5 10 15 20 v) 30 5 10 15 20 25 31 5 io 15 20 25 31 5 10 «uro fiw x so— . .uh i j i l l i l , .hi iii ufi« i l l p ì l i f l a l 11 temperature (il 1 mnlìm tl\énl p ^ f ww'^ui \f 1 v i . j i f o f ^ . n / ^ i i i i n i v v v i v i i v i i ! 2 0 ' 10" 0* -10" 2 0 * pig. ](> ... .registrazioni in val gallina, dopo l'evento del v a j o n t . nel periodo a b a r i n o v u o t o si è p o t u t o osservare clic l'azione dell'insolazione diurna, alla base del concio centrale, risulta di lieve e n t i t à . r i s u l t a t i c o n s e g u i t i d u r a n t i : 1 , ' o s s e r v a z i o n e g e o d i n a m i c a . e t c . 2 7 9 ma anche altri f a t t i possono essere rivelati dai fotoclinografi dopo un sufficientemente prolungato funzionamento presso una diga: alcuni sistematici, altri legati all'ambiente geologico, in particolare tettonico. f r a i primi, merita particolare menzione il lento, progressivo flettere verso valle del corpo della diga, specie nella sua parte eentrale (fenomeni» espresso, f r a i tecnici, con la brutta locuzione di «effetto pancia»). a questo riguardo, possiamo citare il chiaro esempio fornito dalla diga di val gallina, alla base del cui concio centrale funziona, dal 1951, un fotoclinografo a due componenti, una orientata secondo la direzione vallemonte, l'altra nella direzione a 90° (spalla destra-spalla sinistra). come vedremo più partitamente fra poco, presso questa diga le variazioni normali di carico — derivanti dalle variazioni di livello delle acque nel bacino idrico —• si traducono in variazioni della verticale apparente lungo una linea formante con l'asse della diga un angolo di 12°-15°; nel senso che una spinta verso valle provoca un moto angolare risultante nel quadrante valle-destra, mentre un alleggerimento del carico determina una rotazione nel quadrante monte-sinistra. ebbene, da quando è in funzione il fotoclinografo, è stata registrata una costante « deriva » della verticale apparente verso valle, a cui si sovrappongono naturalmente molti altri movimenti, legati a diverse cause. tale deriva, nella sua quasi totalità, è senz'altro da attribuire all'effetto, sopra accennato, di flessione verso valle dello sbarramento. nel periodo 1951-1965, la « caduta » verso valle della parte centrale della diga è stata di ca 281 sec d'arco (fig. 11). accenniamo ora agli altri movimenti. sotto l'azione dell'invaso, come si è detto, le variazioni angolari si verificano lungo una linea che forma con l'asse della diga un angolo di 13°-14° circa (figg. 15, 16). l'onda diurna non è apprezzabile alla base della diga. ivi infatti il clinografo registrafedelmente le minime variazioni dell'invaso, che risultano in rapporto di perfetta proporzionalità con le variazioni della verticale apparente (2> 4). la poca rilevanza dell'onda diurna alla base della diga è confermata dai periodi di registrazione a bacino vuoto: essa è effettiva nei mesi autunno-inverno (fig. 16), mentre appare appena accenn a t a nei mesi più caldi (fig. 17). la variazione della temperatura dell'aria, come presso le altre dighe, fa ruotare la diga a monte per temperat u r a crescente, a valle per temperatura decrescente. ciò si rende più manifesto a bacino vuoto. questo, in condizioni normali. i n certi periodi però, si verificano movimenti che non possono essere spiegati con nessuna delle cause sopra citate. vedasi la fig. 14, che riassume, mediandole, le osservazioni compiute dal 1951 al 1964. 2 8 0 i'. cai. 01 m. c. ii'adea solo cause naturali, ambientali, intese in senso lato, possono essere invocate per dar ragione di simili movimenti; in particolare, cause legate alla tettonica della zona. come già è stato provato altrove (16), queste possono essere chiara-mente rivelate dai clinografi. nel caso specifico, quella v a l grluhft agosto 19b/(. la « •12. f i g . 17 u n esempio di c o n f r o n t o f r a d a t i clinograflci e d a t i di t e m p e r a t u r a ed i n v a s o o t t e n u t i presso la p o s t a z i o n e di val gallina, a b a c i n o v u o t o . 5 m m = 1". specie di grande cappio che la verticale apparente è stata obbligata a seguire dal 15-vii-1952 al 26-iv-1953 (v. fig. 14), risulta probabilmente dal contrasto di movimenti cui sono ancora soggette le stratificazioni esterne della terra, specialmente nelle zone dove l'orogenesi ha potentemente agito nelle ultime ere geologiche (i7). il comporsi di moti, pure se generalmente unidirezionali ma localmente di diversa ampiezza, o addirittura di ampiezza variabile anche nello stesso luogo, non possono non tradursi in r i s u l t a t i c o n s e g u i t i d u r a n t e l'i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 8 1 u n a sorta di m o t o r i s u l t a n t e torsionale, di cui il cappio citato p u ò essere p o r t a t o a testimonianza. i n certi periodi, il contrasto f r a i massicci che f o r m a n o la valle può essere tale da costringere la diga decisamente in u n a rotazione in direzione f o r z a t a , soverchiatile i suoi normali movim e n t i . ciò che si è palesemente verificato nel periodo dal 26-iy-1953 al 26-v-1954, in cui lo s b a r r a m e n t o e f f e t t u ò una. sensibile, esclusiva pressione monte-valle, come costrettovi da un'azione di «schiacciamento» f r a le due sponde. ma dove la deviazione dai m o v i m e n t i legati alla temperatura, e all'invaso si è m a n i f e s t a t a , nel periodo accennato, più a c c e n t u a t a è nella quasi c o s t a n t e «. deriva » verso la sponda destra della valle. a d intervalli di arresto ne sono seguiti altri in cui il fenomeno a p p a r e n o t e v o l m e n t e accelerato. vedasi ancora la fig. 14. l ' u l t i m a fase del f e n o m e n o riveste, al riguardo, particolare interesse: quella che p r e n d e le mosse dal maggio 1960. d a quell'epoca, con r a l l e n t a m e n t i e accelerazioni, la deriva verso destra ha c a r a t t e r e pressoché continuativo. a questo p u n t o , v a ricordato che la val gallina, si trova i m m e d i a t a m e n t e a sud della valle del v a j o n t (ent r a mbi i t o r r e n t i sono t r i b u t a r i del piave), dalla quale a p p a r e divisa dal massiccio del toc. non sembra azzardato collegare il fenomeno in questione a q u a n t o s t a v a m a t u r a n d o sulla sinistra del v a j o n t , alle pendici del toc. risale i n f a t t i al 22 maggio 1960 l'inizio della microsismicità nella s t r e t t a del v a j o n t , a m o n t e della diga, ir. sponda sinis t r a ("), microsismicità che, a l t e r n a n d o periodi di intensificata frequenza, ad altri di r e l a t i v a calma, doveva in poco più di t r e anni demolire l'edificio roccioso che reggeva la s o v r a s t a n t e zona di f r a n a . non sembra, r i p e t i a m o , a z z a r d a t o associare la sistematica flessione della diga di val gallina verso n o r d , specialmente nel periodo sopra accennato, ad u n c o n t r a s t o tettonico interessante l'intera zona compresa f r a le due valli contigue, dal crep n u d o al toc; contrasto tettonico che a v e v a nella forra del vajont, il suo p u n t o più critico, e dove doveva sfociare n e l l ' i m m a n e scoscendimento del 9 o t t o b r e 1963. i l f a t t o che la flessione verso la spalla destra della diga di val gallina sia c o n t i n u a t a (fig. 18) anche dopo il tragico evento del v a j o n t , sembra testimoniare a favore di questa ipotesi. quell'evento, e il perd u r a r e di questo fenomeno, sarebbero due aspetti diversi di uno stesso grandioso movimento tettonico, interessante la nona nella sua totalità e •non soltanto in superficie. negli ultimi anni, è un pullulare all'estero di studi sui moti recenti della crosta t e r r e s t r e e sulle loro possibili cause. non è qui, n a t u r a l m e n t e , il luogo per soffermarci a lungo su questi studi. r i t e n i a m o però interes2 8 2 p . c a l d e u . c . s p a d e a sante osservare che, tali movimenti, vengono a t t r i b u i t i a dislocazioni sub-crostali o a residui di moti convettivi, ritenuti liberamente agenti in epoche geologiche passate, ed ai quali oggi si è p o r t a t i ad attribuire i massimi corrugamenti della superficie terrestre (18-21). f r a t u t t i , merita 18-xii f i g . 18 r i s u l t a n t e m o v i m e n t i a n g o l a r i s u b i t o dopo l ' e v e n t o del v a j o n t . particolare menzione un lavoro di wagenbreth (22). ad un certo p u n t o del suo lavoro, accettando le conclusioni di pflug (23), egli ritiene che nell'europa centrale le spinte (tensioni), ancora in atto, nella crosta t e r r e s t r e siano come da fig. 19. . l'esistenza di faglie o di discontinuità o comunque di inomogeneità attraverso la crosta terrestre (comunissime in regioni montuose), può r i s u l t a t i c o n s e g u i t i d u r a n t e l'i i s s e r v a z i o n e g e o d i n a m i c a , e t c . 2 8 3 generare situazioni locali del tipo di quella rappresentata nella fig. 20. colpisce l'analogia con quanto si è andato osservando attraverso gli anni nella zona yal gallina-valle del v a j o n t . tenuto conto della direzione della spinta subcrostale (fig. 19), la deriva verso la spalla destra in val gallina e i fenomeni sismici osservati nella s t r e t t a del v a j o n t appaiono legati ad un'unica causa. nelle nostre relazioni (secondo h. d. pflug). alla sade del 1962-1963, sottolineavamo l'esistenza di un contrasto t e t t o nico in sponda sinistra del vajont, a monte della diga (17), contrasto clie si manifestava sotto gli aspetti di uno sprofondamento immediatamente a sinistra del torrente e di un innalzamento a ventaglio lungo le pendici del toc. quest'ultimo fenomeno era da noi attribuito ad un residuo di orogenesi (17). col legando quelle nostre conclusioni al meccanismo suggerito dalla trattazione di wagenbreth, si resta sorpresi nel constatare come tale meccanismo possa in pieno spiegare quanto è avvenuto nella s t r e t t a ' del v a j o n t : si nòti infatti che le dilatazioni vengono a corrispondere alla piccola freccia che indica moto verso il basso (fig. 20), mentre le com2 8 4 p . o a i . o i m . c . s p a d e a pressioni osservate dalla stazione sismica hanno la loro esatta rispondenza nella piccola freccia associata al sollevamento. i n tal modo, verrebbe pure confermato il modello ad origine singola delle microscosse le registrazioni clinografìche ottenute presso la diga di val gallina testimonierebbero a favore di questo meccanismo. ciò consentirebbe di concludere che l'evento del vajont era legato ad un contrasto tettonico di vaste proporzioni, probabilmente non ancora conchiuso. va i n f a t t i grosse f r e c c e : direzioni della c o r r e n t e subcrostale (secondo w a g e n b r e t l i ) . osservato, a questo riguardo, che — sulla base delle registrazioni clinografìche di val gallina — la deriva verso destra ebbe una fase di arresto nei mesi che precedettero il tragico 9 ottobre 1963. sembra quasi che, in questa fase, la spinta verso nord della p a r t e esterna della crosta avesse t r o v a t o un intoppo al conseguente slittamento della p a r t e in lentissimo moto. dopo l'enorme scoscendimento del 9 ottobre, la der i s u l t a t i c o n s e g u i t i d u r a n t e l ' o s s e r v azion e g e o d i n a m i c a . e t c . 2 8 5 riva verso destra denunciata dai clinografi di val gallina è lentamente ripresa, come se in quella tremenda notte del 9 ottobre l'intoppo su detto avesse ceduto. b i b l i o g r a f i a (*) caloi p . , il pendolo orizzontale come clinometro. « a n n . geofis. », i i i (1950). ( 2 ) c a i , o i p . , osservazioni sismiche e clinografiche presso grandi dighe di sbarramento, « a n n . geofis.», v i (1953). (3) caloi p . , diamanti m., il fotoclinografo a pendolo orizsontale nella teoria e nell'applicazione. « l ' e n e r g i a e l e t t r i c a » , x x x i i i (1956). (4) caloi p . , aspetti della dinamica di rocce, calcestruzzo ed acque. « a n n . geofis. », x v (1962). (contiene, fra l ' a l t r o , u n ' a m p i a bibliografìa sui l a v o r i p u b b l i c a t i s u l l ' a r g o m e n t o d a l l ' a u t o r e , a p a r t i r e dal 1950). (5) caloi p . , spade a m. c., decadimento del modulo elastico in roccia a contatto con bacini idrici artificiali. « a n n . geofis.», v i (1954). (•) caloi p . , la geodinamica al servizio delle grandi dighe. « a n n . geofis. », x v i (1963). (') davis d. ii., « j o u r . geol. », 62, (1954). (e) sato y., velocity oj elastic waves propagated in media with small holes. « bull. e a r t l i q res. i n s t . », t o k y o , x x x (1952) e x x x i (1953). ( 9 ) s a t o y . , i. c., 1 8 5 , ( 1 9 5 2 ) . (10) siiimozuru d., elastici/i/ oj iiocks and some related geophysical problems. « j a p . j o u r n a l geopliys. », 2, 3, (i960). (>l) frazer h . yx., « j o u r . g e o l . » , 4 3 , 9 1 0 , ( 1 9 3 5 ) . (12) uniiig c. f . , « geophysics », 20, 774, (1955). (13) jstasu n . , omote s., « z i s l i i n », 13, 4, 91. ( 1 9 4 1 ) . (14) caloi p . , aspetti geodinamici della diga dell' ambi-està, « a n n . geofis.», x v i i , (1964). (15) caloi p . , la geo/isica e le grandi dighe: « l ' e n e r g i a e l e t t r i c a », x x x i x , (1962). (16) caloi p . , spadea m. c., relazioni fra lente variazioni d'inclinazione e moti sismici in zona ad elevata sismicità. « r e n d . acc. n a z . lincei », x v i i i , 3 (1955). («) caloi p . , l'evento del vajont nei suoi aspetti geodinamici. « a n n . geofis. », x i x , (1966). (ls) sciieidegger a. e . , statistical analysis of recent fault plane solutions of earthquakes. « b u l l . seism. soc. a m e r . », 49 (1959). 286 p . caloi m. c. s p a d e a (19) scheidegger a. e . , orosstektonische bedeutung von erdbebenherdmechanismen. « z e i t s c h r i f t f ü r geophysik », 31, h e f t 6, (1965). (20) l u . paul i i . a n d sgheideggekb a. k., an intensive local application of lensen's isallo stress theory to the sturgeon lake south area of alberta. « bull, of c a n a d i a n p e t r o l e u m geology », 13, 3, (1965). (21) de boer jelle, paleoniagnetic indications oj megatectonic movements in the tethys. « j o u r n . of geophys. r e s e a r c h » , 70, 4, (1965). (22) wagenbbeth 0 . , zur theoretischen und regionalen deutung des elbtalgrabens und der lausitzer uberschiebung. « geologie », j a h r g . 15, h e f t 1, (1966). (23) pflug h . d . , versuch einer teklonischen deutung der tertiären sedimentationsräume nw-deutschlands. « f r e i b e r g e r f o r s c h i i . , c. 80» (1960). 08espinosa.qxd 437 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words earthquakes – historical seismology – catalogue – intensity – colombia 1. introduction the colombian territory located in the northwestern corner of south america, is the place of convergence of three lithospheric plates: nazca, caribbean and south american. the movement of these plates produces large deformation zones characterized by the presence of different types of geologic faults (paris et al., 2000). the region surrounding these faults has produced several destructive earthquakes in the past centuries. the most significant earthquake in casualties and financial loss occurred on 20 january 1999 in the coffee growing region (armenia earthquake), with magnitude 6.2 and imax = x mm 56; it killed 1230 people and destroyed more of 5600 homes, the economic impact of this earthquake symbolizes a direct economic loss of approximately us$ 1.8 billion (cardona, 1999). most of the studies carried out for colombian strong earthquakes give information only on epicentral coordinates and magnitude. moreover, incomplete reports of building damage in the main affected cities conclude sometimes in a layout of incomplete isoseismal maps. also, it cannot be assumed that all the earthquakes with intensity (i0 ≥ vii) that have happened in colombia are included in the existent seismic catalogues. the first scientific effort to map the earthquakes with epicentres in colombian territory was carried out by the geographer francisco javier vergara y velasco (vergara y velasco, 1906a; vergara y velasco, 1906b), who published the first seismological map of the country and wrote some notes on its historical seismicity (vergara y velasco, 1898). another important contribution to the seismic history of the country are the studies carried out by the jesuit priest simón sarasola (1924, 1937). many investigators consider him the father of seisstate-of-the-art of the historical seismology in colombia armando espinosa baquero (1), augusto antonio gómez capera (2) and elkin de jesús salcedo hurtado (3) (1) facultad de ingeniería, universidad del quindío, armenia, colombia (2) istituto nazionale di geofisica e vulcanologia, sezione di milano, italy (3) departamento de geografía, universidad del valle, cali, colombia abstract in colombia are available a discreet number of historical seismology investigations, dating back 50 years. this paper reviews basic information about earthquakes studies in colombia, such as primary sources, compilation of descriptive catalogues and parametric catalogues. father jesús emilio ramírez made the main systematic study before 1975. during the last 20 years, great earthquakes hit colombia and, as consequence, historical seismology investigation was developed in the frame of seismic hazard projects. mailing address: dr. armando espinosa baquero, facultad de ingenieria, universidad del quindío, avenida bolívar calle 12 norte edificio de ingeniería, quindío, armenia, colombia; e-mail: armandoespinosa@uniquindio.edu.co 438 armando espinosa baquero, augusto antonio gómez capera and elkin de jesús salcedo hurtado mology in colombia, because in 1923 he installed the first three seismographs in colombia, and in 1941 participated in the creation of the geophysical institute of the andes, which included also a meteorological observatory. the first systematic work on historical seismicity in colombia from the 16th century, is history of the earthquakes in colombia by the jesuit priest jesús emilio ramírez, published for the first time in 1969 and reissued in the year 1975. the main seismic catalogues for the colombian territory are those by ramírez (1975a), ceresis (1985), itec (1988), and ingeominas (1993, 2002). most of these catalogues are based on ramirez’s work. recent contributions to the knowledge of the seismic history of colombia have been made by espinosa who has also partially revised the seismic catalogues. new seismic events, previously ignored (espinosa, 1994b-d), have been included in the catalogues and at the same time some doubtful seismic events were discarded, after being recognized as other type of natural events (espinosa, 1994e-f). 2. historical background 2.1. early compilations during the first century of spanish colonial times, only special chroniclers and curious persons dedicated their time to narrate what occurred in the towns seriously affected by earthquakes. early compilations of colombian historical earthquakes are found in personal notes and autobiographies, containing information on earthquake records in different time periods, made by some foreign scientists and a few nationals, with sufficient education and interest in science and history, who dedicated themselves to recording and describing the earthquakes felt in their own localities. such compilations, which have a local character and are only partial, can nevertheless be considered as the first seismic catalogues of colombia, among which are the documents of luis vargas jurado (edited by caballero in 1902), the list of don santiago pérez de valencia (edited by acosta in 1849 as quoted by espinosa, 1997) for popayán, since 1800 through 1841, that of don josé maría caballero (1902), from 1813 to 1819. in the middle of the century there were historical sources as boussingault (1829, 1833, 1835), acosta (1828) and international scientists such as montessus de ballore (1884) and others, who collected and catalogued seismic data. these authors are mentioned by ramírez (1975a), ceresis (1985) and espinosa (1997, 2004). luis vargas jurado was an employee of the «nueva granada» viceroyalty, who wrote a diary in santa fe de bogota, which is the oldest document with information on natural phenomena and their consequences in colombia from 1703 to 1780. this work was published in 1902 by josé maría caballero. the main earthquake quoted by vargas jurado occurred on 18 october 1743 and was felt in bogota, surrounding areas and the eastern colombian region (vargas jurado, 1902). santiago pérez de valencia was a dignified gentleman and member of the parliament, who made a compilation of earthquakes felt in popayan city from 1785 to 1841. this work is the oldest site catalogue in colombia. the year, month, hour and sometimes the effects and duration of the earthquakes are provided in chronological order. the catalogue by santiago pérez was published by joaquín acosta in paris in 1849 (espinosa, 1997). josé maría caballero was an infantryman of santa fe de bogota in the independence wars of colombia. in the military campaign, caballero wrote in his daily personal notes from 1813 to 1819, within which are found the descriptions of the effects caused by great and small earthquakes in santa fe de bogota. when the war finished in 1819, caballero continued his diary adding information on previous events in santa fe de bogota among which are the 1743 and 1785 earthquakes that were very destructive. his compilation includes 1600-1819. earthquakes outside the city of santa fe de bogota are also described, for instance the honda earthquake in 1805, located in the region of the middle magdalena valley in colombia. this document was published in 1902 (caballero, 1902). the first synthesis of colombian historical seismology was written in 1898 (fig. 1) by francisco javier vergara y velasco in the journal anales de ingeniería de bogotá (vergara y velasco, 1898). vergara y velasco was a high-grade of439 state-of-the-art of the historical seismology in colombia ficer, an academic and distinguished professor, who compiled records from secondary sources such as classical books of national history of colombia, venezuela and ecuador and regional history of colombia. as a result of this study, a chronological compilation of textual descriptions of earthquakes and volcanic eruptions from 1530 to the end of the 19th century, was obtained. he published the first seismological map of colombia, which includes the epicentres from his catalogue (vergara y velasco, 1906a). this document is the base for the study by ramírez (1975a). 2.2. the first investigations between 1930 to 1960 research on regional historical seismology were made by different fig. 1. family tree of the contribution to the seismic history and specific earthquakes in colombia before 1990. 440 armando espinosa baquero, augusto antonio gómez capera and elkin de jesús salcedo hurtado authors from pasto city, popayan city and tunja city among which are the historians arcesio aragón, ramón correa, the lawyer ignacio rodríguez guerrero and the father jesús emilio ramírez. aragón published works about history of popayan and compilations about data of purace volcano (aragón, 1926). correa published a paper about damage caused by great earthquakes in boyaca region, among which are the 1827, 1884, 1917, 1928, 1961 and 1962 earthquakes (correa, 1962). rodríguez guerrero was the author of historical, geographical and literary books. rodríguez guerrero published a compilation about volcanic eruptions in galeras and doña juana volcano and earthquakes occurred in colombian southern region from the epoch of the spanish conquest to his days (rodríguez guerrero, 1959, 1961). it is worthwhile to highlight that earthquakes and volcanic eruptions were studied on the same level, in this manner other compilations were made by muñoz (1925), friedlaender (1927) and forero duran (1933). ramírez (1975a) and espinosa (1997, 2004) quote these authors. 2.3. studies by ramírez the investigation of colombian historical seismicity started with the studies of jesús emilio ramírez, a geophysicist and jesuit priest, born in 1904 in the small town of yolombo not far from medellin, in northern colombia and dead in 1981 in bogota. he was a co-founder in 1941 of the «instituto geofísico de los andes colombianos» (today «instituto geofísico universidad javeriana»). for 38 years, he served as director of this institution and he is considered one of the pioneers of seismology in colombia. he produced studies about some individual earthquakes from 1933 to 1981 (fig. 1). ramírez published compilations about historical seismicity in colombia (ramírez, 1933, 1937, 1947, 1958a, 1968, 1969). the whole study was finished and published in 1975 with the name historia de los terremotos en colombia (ramírez, 1975a). this document is divided into three parts: 1) introduction to seismology, description of colombian volcanoes and history of volcanic eruptions in colombia; 2) colombian historical seismicity; 3) description of the seismicity and seismotectonic map of colombia and compilation of the first colombian seismicity catalogue. the systematic compilation of historical sources made by ramírez supplied records for 250 earthquakes with textual description from 16th to 20th century. the first colombian event described is the 1566 earthquake in the popayan and cali region; the historical compilations which quote this event are arroyo (1907) and aragón (1941). the last earthquake described is 12 july 1974 in the juradó region. the historical references investigated by ramírez are contemporary sources of information about the earthquake like chronicles, letters, historical books, daily newspapers, scientific articles, historical compilations of catalogues and individual earthquakes, personal communications, records of public offices, local seismological records and international table i. parametric catalogues in colombia. author period number of earthquakes included/century 16th 17th 18th 19th 20th total ramírez (1975a) 1566-1974 2 8 23 187 1036 1256 ceresis (1985) 1566-1995 3 6 31 220 10 828 11 088 ingeominas (1998) mb ≥ 4.0 1541-1998 5 22 53 240 4807 5127 441 state-of-the-art of the historical seismology in colombia seismological summary. some of these sources give a relatively good description of damage, others give a highly accurate timing but only report that the earthquake was felt in distant places with no information of damage. among the historical sources are acosta (1828), boussingault (1829, 1833, 1835), anonymous sources as for example «letras anuas de la compañia de jesús de la provincia del nuevo reino de granada» from 1638 to 1643, historical records of public offices found in the national archive in bogota and regional archives as for example the archivo central del cauca in popayan. historical compilations are quoted as for example caballero (1902) and febres cordero (1931). in conclusion, the study by ramírez (1975a) is well documented, it supplies good quality information from primary and secondary historical sources to be interpreted in seismological terms. 3. earthquake parametric catalogues 3.1. ramírez catalogue (1566-1974) the first historical and instrumental earthquake catalogue of colombia appeared in ramírez (1975a). this catalogue includes a total of 1256 events (table i). for each event the earthquake parameters are provided: date, epicentral coordinates and locality, earthquake intensity and depth. for the parameters of location of historical earthquakes, the method followed by ramírez consisted in locating the epicentre of an earthquake in the places where the effects and damage were the greatest. in the case of damage and effect reports approximately equal in two different localities, the epicentre was located mid-way between them (see for example ramírez, 1954; ramírez, 1975b). if the damage and effects were greater in one locality than in the other the epicentre was located at a distance inversely proportional to such effects. for the most recent events the epicentre and depth were defined by instrumental methods (see for example ramírez, 1967a; ramírez, 1971). epicentres of earthquakes that occurred in bordering countries (ecuador, panama, peru and venezuela), whose effects were felt in colombia, are included. for historical events the depths are not given, however in some cases the depth of earthquakes was assumed empirically using the isoseismal map (ramírez, 1975b). the intensity scale used by ramírez ranges from i to iii and has an approximate correlation with the modified mercalli scale of 1956 (mm56): i) i corresponds to iii, iv and v; ii) ii corresponds to vi, vii and viii; iii) iii corresponds to ix, x, xi and xii. 3.2. ceresis catalogue (1566-1995) the catalogue of earthquake parameters for colombia was compiled by ceresis (centro regional de sismología para américa del sur) during the project «the program for the mitigation table ii. number of earthquakes and idp included in the colombian intensity database of ceresis (1566-1981). century number of earthquakes included number of idp/century 16th 2 5 17th 5 16 18th 10 61 19th 26 210 20th 145 1805 total 188 2097 442 armando espinosa baquero, augusto antonio gómez capera and elkin de jesús salcedo hurtado of earthquake effects in the andean region» (known as project sisra, ceresis, 1985, available through http://www.ceresis.org). the catalogue has been compiled using macroseismic and instrumental data related to earthquakes from 1566 to 19 january 1995. the ceresis catalogue contains events without a lower limit of magnitude (table i). the main focal parameters are listed (ceresis, 2003). the compilation of the catalogue was divided in two parts. one was the historical catalogue, where the main bibliographical source was ramírez (1975a). the second was the instrumental one beginning after 1900 (goberna et al., 1985). for the parameters of location of historical earthquakes, the method used is that described in the ramirez catalogue (ramírez, 1975a). the evaluation of epicentral intensities used the modified mercalli scale of 1956. for historical events the depths are not given. the magnitude, for historical earthquakes, is based on the formula of gutenberg and richter for shallow earthquakes of california: m = 1 + 2/3 i0 or on the tables of equivalence which have been calculated from this formula (richter, 1958; bath, 1967). the instrumental catalogue was compiled using data from international sources and from local instrumental networks. 3.3. the colombian earthquake catalogue (1566-1998) the earthquake catalogue for colombia is a parametric hypocentre catalogue which has been compiled, updated and homogenised in magnitude ms by ingeominas (colombian geological survey) (ingeominas, 1993, 1995a, 1995b, 1997, 1998, 1999a and 2002) using historical and instrumental data, available through http://bases4d.ingeominas.gov.co:10084/. the colombia earthquake catalogue covers a time period from 1566 to june of 1998 (table i). for each event there is a line providing the main focal parameters. quality factors for time, epicentre, depth, and magnitude have been assigned for most of the events. the bibliographic sources are given. the lower magnitude cutoff has been set to 4.0 mb (ms ≥ 2.4). the catalogue includes aftershocks. a description of the hypocentre catalogue format can be found in ingeominas (1995b). the historical sources compiled are secondary, i.e. catalogues from ramírez (1975a), ceresis (1985), itec-isa (1988), osso (1996), isc and neic, and catalogue of colombian national seismological network (ingeominas, 2002). 3.4. intensity database: ceresis (1566-1981) the ceresis intensity database for colombia (ceresis, 1985, available through http://www.ceresis.org) contains 2097 intensity data points (idp) related to 188 events from 1566 to 1981 (table ii) (gómez et al., 2002). the proposed epicentre and macroseismic parameters come either from the critical analysis of the different historical sources or from spefig. 2. example of ceresis database format (ceresis, 1985). hyp co00005 1566.00.00 24:00:00 3.000 −76.500 5.0 ms 7 m 2 co cali 3.45 −76.53 995 7 m tprg 801 co popayan 2.45 −76.62 1738 7 m tprg hyp co00010 1595.03.12 16:00:00 4.500 −75.400 4.0 ms 7 m 3 co volcan ruiz 4.55 −75.46 5300 7 m tprg 801 co mariquita 5.20 −74.90 495 4 m tprg co honda 5.20 −74.93 225 4 m tprg 443 state-of-the-art of the historical seismology in colombia cific studies. the main bibliographical sources are ramírez (1975a) and the archive of the instituto geofísico de los andes (goberna et al., 1985). the format of the ceresis intensity database (fig. 2) has two types of record. for each event there is a first line providing the parameters (epicentral coordinates, epicentral or maximum intensity, magnitude, depth, number of related idp – although not always reported) of an earthquake; in the following lines, the idp related to the earthquakes are listed. all intensities are given using either the mm 56 or msk (medvedev-sponheuer-karnik) macroseismic scale. the investigator who assigned the intensity value is identified as «interpreter» (tprg in fig. 2). the references are given with a code (801 in fig. 2). references given in the ceresis database do not allow one to distinguish which event and/or data point is supported by which source. a description of the catalogue format is given in goberna (1985), including the list of the «interpreters» and the references they have used. figures 1 and 3 summarize the background of the historical seismic investigations in colombia, including catalogues, in the periods before and after 1990 respectively. fig. 3. family-tree of the seismic history and macroseismic analysis of a specific earthquake as contribution to the seismic hazard and microzonation studies in colombia after 1990. 444 armando espinosa baquero, augusto antonio gómez capera and elkin de jesús salcedo hurtado 4. most recent investigations 4.1. studies by espinosa during the last 20 years, cities as popayan, pereira and armenia have been affected by great, shallow earthquakes. regional seismic hazard studies began after of the popayan earthquake, which occurred on 31 march 1983, and the first national building code was developed and adopted in 1984. in both of these national projects, the role played by histable iii. main seismic hazard projects in colombia with historical seismicity studies. type project, date and place historical seismicity study remarks regional 1985-1992 valle del cauca espinosa (1992b, 1996b) intensive and extensive research of regional historical sources. analysis of historical sources. regional catalogue. analysis of seismogenic sources. intensity map for main events. example of historical earthquakes studied: 1766. microzonation 1988-1992 popayán espinosa (1992a, 1994f) intensive and extensive research of local historical sources. study of 1983 popayan earthquake. analysis of historical sources. analysis of regional seismicity. local effects. intensity map. analysis of local seismogenic sources. example of historical earthquakes studied: 1736, 1751, 1817, 1878, 1885, 1906, 1936. regional and volcanic hazard 1989 manizales jaramillo and arango (1994) intensive research of regional historical sources. analysis of sources. regional 1994 medellín arango and velasquez (1993) intensive research of local historical sources. descriptive site compilation. microzonation 1993-1995 bogotá espinosa (1994a, 1994b, 1994c, 1994d, 1994e) intensive and extensive research of local historical sources. analysis of historical sources. local effects. isoseismal intensity maps for main events. example of historical earthquakes studied: 1805 with imax = 9; 1785, 1827, 1917 with i0 = 8; and 1743, 1826, 1923 and 1967 with i0 = 7. new events: 1616, 1644. false quake as: 1687 in santa fe de bogotá. regional 1996 pereira espinosa (1996a) intensive and extensive research of regional historical sources. analysis of historical sources. isoseismal intensity maps for main events. historical and recent earthquakes as: 1938, 1961, 1962, 1979, 1991, 1995. microzonation 1997 bucaramanga salcedo (2002) intensive research of local historical sources. analysis of historical sources. descriptive and parametric site catalogue. 445 state-of-the-art of the historical seismology in colombia torical seismology turned out to be very important. espinosa, using primary and secondary historical sources, studied historical regional seismicity in the frame of seismic hazard projects (fig. 3 and table iii). these historical sources have been consulted in national, regional and local archives and libraries of historical cities as popayan, pasto, cali, buga and bogota, including the archivo general de la nación. the studies by espinosa, which include historical and seismological analysis of local and regional seismicity, are considered intensive and extensive (espinosa, 2004) and will be available through http://www.accefyn.org.co/ 4.2. main problems in historical research as in many other countries, the main problems encountered in historical studies in colombia regard: manuscript reading due to the physical deterioration documents or to the grammatical and calligraphic language evolution, change of place-name, etc. moreover, in colombia, the completeness and reliability of historical earthquake documents depend on many elements, such as population density and socio-cultural background in the different regions in the country. as a consequence, the distribution of historical earthquake documents is not balanced. most of them are concentrated in the main historical cities, such as bogota, popayan, pasto and tunja. another problem is the presence of false quakes included in primary historical sources. for example, a flood of the san cristobal river is proposed to explain the terrible and mysterious roar which terrified the town of santa fe de bogota during the night of sunday 9 march 1687 which was reported as an earthquake in some texts and catalogues (espinosa, 1994e). 4.3. isoseismal maps the «atlas macrosísmico de colombia» by salcedo hurtado and gómez capera (2000) contains a compilation of 35 isoseismal maps of about the same number earthquakes and covers a time period from 1812 to 1999. the isoseismals maps are presented in chronological order. every map contains at least three isoseismals. in this compilation the macroseismic field were selected from the following sources: ramírez (1948, 1953, 1975a, 1975b), vall (1950), fiedler (1961), woodward-clyde consultants (1981), sarria (1985), meyer et al. (1986), coral and salcedo (1992), romero et al. (1994), salcedo (1994), espinosa (1994d, 1996a), pulido and tapias (1995), gómez capera et al. (1999). in this atlas, macroseismic parameters from isoseismals are proposed for every earthquake in a database format: macroseismic epicentre, macroseismic focal depth, epicentral distance, epicentral elongation distance, transverse of epicentral elongation distance, direction of elongation, isoseismal area and attenuation coefficient. the macroseismic epicentre was determined as the barycentre of the macroseismic field. the epicentral distances were determined using «equivalent circle» method, by setting them equal to the average radii of equivalent concentric circles having areas equal to that encompassed by the corresponding isoseismals. the attenuation coefficient was determined by the kövesligethy formula. different intensity scales were used for the construction of the isoseismal maps: rossi-forel scale (rf scale), the modified mercalli scale of 1956 (mm56), the medvedev, sponheuer and karnik scale (msk), the mercalli-cancani-sieberg scale (mcs) and the european macroseismic scale-92 (ems92). this atlas is available through http://www.ingeominas.gov.co/ 4.4. site catalogues and others the site catalogue of medellin (arango and velasquez, 1993) is a seismic-historical catalogue of medellin and antioquia region in colombia with commentary, it was made in the framework of medellin seismic hazard study (table iii). a chronological relation of effects of 49 earthquakes in the region studied is listed. the historical sources are dated in the time period from 1730 to 1979. the historical sources consulted are: ramírez (1975a), ceresis (1985), woodward-clyde consultants reports (e.g., 446 armando espinosa baquero, augusto antonio gómez capera and elkin de jesús salcedo hurtado 1981), local chronicles, national and reports from local newspapers, bulletins, magazines, university theses, regional and local historical books. the catalogue of the effects of recent earthquakes (salcedo hurtado and gómez capera, 1998) is a compilation of historical records of 63 earthquakes that occurred in the 19th and 20th century in colombia, in chronological order. this study is in database format with information on historical source, date, location and descriptions of earthquake effects at single localities. additionally, there is a parametric catalogue (date, coordinates, site and bibliographic source) on 63 earthquakes studied. the total number of bibliographic sources is 41 among which historical books, magazines, technical reports, studies, reports from national and local newspapers and others available at the archives and libraries in bogota. the catalogue is available through http://www.ingeominas.gov.co ingeominas (1999c) compiled a wall map of epicentres of ms ≥ 5.5 and imax ≥ vii earthquakes from 1566 to 1999. this historical map lists the date, coordinates, locality, magnitude and maximum intensity of the earthquakes. some inset maps show the isoseismals of great earthquakes that hit colombia between 1875 and 1999 (salcedo hurtado and gómez capera, 2000). the site catalogue of bucaramanga (salcedo hurtado, 2002) is a seismic-historical catalogue with commentary and also a site catalogue of the effects in bucaramanga city and metropolitan area from 1644 to 1997. the bucaramanga site catalogue has hypocentral parameters of 69 earthquakes (date, epicentral coordinates, focal depth, magnitude, epicentral intensity and quality). this study was made in the framework of the bucaramanga seismic microzonation project (table iii). the main historical sources consulted are ramírez (1969, 1975a,b) and reports from local newspapers available at the archives and libraries in bucaramanga. relationships between intensity, focal depth and epicentral distance for earthquakes in colombia have been recently investigated. the isoseismal maps for 34 earthquakes in different parts of colombia were analysed to study the focal depth (gómez capera and salcedo hurtado, 2000) and the attenuation of intensities with distance (gómez capera and salcedo hurtado, 2002). these estimates are based upon the blake-shebalin and köveslighety models. 5. conclusions the historical seismicity of colombia is not documented before the spanish arrival in the 16th century. descriptive compilations and parametric catalogues are available covering the time period between 16th and 20th century. the main studies on historical seismicity of colombia can be grouped in two categories. the first comprises the work by ramirez and all subsequent studies based on this compilation. ramirez published a descriptive compilation and parametric catalogue of the seismicity occurred between 1566 and 1974, which identifies the most important destructive events through primary and secondary sources found in national and regional archives and libraries of colombia. this compilation and catalogue is a very important guide for the knowledge of the seismic history of colombia, since it has been intensively used by ceresis and ingeominas to compile the most recent parametric catalogue of colombia. in the second group are included the studies by espinosa, in the framework of seismic hazard projects. espinosa found new historical sources and earthquake records that he was able to locate in time and space. despite the important contributions of espinosa’s work, which partially supplement ramírez, the seismic catalogues in colombia cannot be considered complete. according to ambraseys et al. (1983), in a general way the data contained in seismological catalogues need a meticulous revision and reinterpretation. for the colombian catalogues, this has not been done for the entire period corresponding to the historical seismicity. the studies of historical seismicity have been carried out in an inhomogeneous way, benefiting the needs of the main cities or of the more developed regions, which required seismic hazard analysis for microzonation studies, in this framework, compilations, site catalogues and isoseismal maps have been developed by different authors but more efforts are still needed. 447 state-of-the-art of the historical seismology in colombia the different historical seismic catalogues done for the colombian territory have not shown meticulous efforts to calculate seismic parameters such as epicentre and focal depth that might be used reliably for the solution of the problem of the seismic hazard. this is a great task for future research. acknowledgements special thanks to massimiliano stucchi for inviting us to write this paper. thanks are extended to valentina montaldo for help in the english translation and mario locati for figures editing. references acosta, j. (1828): lettre sur le tremblement de terre de la colombie, adressée a m. alex barbie du bocage, bull. soc. géogr. (paris), 12 (60-61), 200-203 (reprinted in arago’s «oeuvres complètes», paris 1859). ambraseys, n., e. banda, j. irving, d. mallard, c. melville, t. morse, r. muir-wood, d. muñoz, l. serva, d. shilston, e. surinach and j. vogt (1983): notes on historical seismicity, bull. seismol. soc. am. 73 (6), 1917-1920. aragón, a. (1926): el puracé y sus leyendas. mundo al día (julio, bogota), 14-26. aragón, a. 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localisation of em discontinuities in the subsurface with high resolution. this paper describes an application of gpr to evaluate the state of maintenance of some travertine panels attached to the inside walls of a building housing the bank of naples in campobasso and in danger of falling because of the numerous voids present between the wall and the panels. the aim of the survey was to assess whether the gpr technique could be used to detect the voids behind the travertine panels. the study was made to assist the design of the restoration works of the travertine covering. because of the very narrow thickness of both the travertine plates and the voids, special care was needed in the acquisition and processing steps. the measurements were performed on two panels: one purposely put on to the laboratory wall with a known position of the voids; the other one selected from among the panels to be restored. although pushed to the limit of the resolution achievable by the available antenna, the study has given quite good results. mailing address: dr. giovanni leucci, osservatorio di chimica, fisica e geologia ambientali, dipartimento di scienza dei materiali, università di lecce, via per arnesano, 73100 lecce, italy; e-mail: gianni.leucci@unile.it key words gpr investigation – gpr resolution – travertine panel – time slices 1. introduction the gpr method can detect superficial bodies with relative efficiency depending on the field context, the dielectric properties of the host material and the nature and size of the bodies (davis and annan, 1989). the scale of resolvable features can range from a few centimeters to ten or more meters, depending on the frequency band of the operating antennas. therefore, a careful selection among the available antennas is needed with regard to the aims of the survey (annan and cosway, 1994). numerous studies describe efficient gpr techniques in civil engineering (mellet, 1995; bae et al., 1996; carcione, 1996; davidson and chase, 1998; godio et al., 1999; leucci et al., 2002). however, since each technique is generally considered individually in a specific context, it is difficult to compare the results because of different field conditions. the objective of this work is to verify the capabilities of gpr to estimate the state of maintenance of some travertine panels attached to the inside wall of the bank of naples in campobasso. these panels are affected by instability mainly caused by numerous voids, that denote an absence of mortar. the main target of these pre482 giovanni leucci, sergio negri and maria teresa carrozzo liminary measures is therefore to verify the reliability of the gpr methodology to detect voids between the wall and the travertine panels. the measures were carried out on two panels: – one in the laboratory with a known position and thickness of the voids; – the second panel was placed inside the bank of naples in campobasso, with the position and thickness of the voids completely unknown. because of the very narrow thickness of the travertine (less than 4 cm), special care in the acquisition of the data on the voids (parameters of acquisition, geometry of acquisition) was required. although the only available choice of antenna with the higher centre frequency band was 1 ghz and therefore with well-known limits of resolution, the results seem to be satisfactory. 2. outline of the problem the bank of naples in campobasso is located inside a building of ancient construction. the inside of the building is entirely covered with travertine panels 70 cm wide, 90 cm high and 3 cm thick. last year the static condition of one of these panels presented some problems due to the presence of voids and therefore to the lack of mortar between the wall and the panel itself. figure 1 represents a hypothetical stratigraphical section that shows the problem. the problem (given the narrow thickness to investigate) is to establish whether gpr is able to map the void present between the wall and the travertine panel. the void detection is mainly based on the different em properties between the travertine, the mortar, the wall and the air filling the voids. the size and the geometric shape of the structures play an important role. in the range of the radar frequencies the em wave velocity v and the em wave attenuation a in the medium are used to characterise the medium itself. in materials of low magnetic susceptibility, velocity v and attenuation a are related to the relative dielectric permettivity (rdp) k and loss tangent, tgd, by the expressions (annan and davis, 1988) (2.1) (2.2) where w = 2pf is the angular frequency, f is the frequency and c is the velocity of the em wave in the empty space (30 cm/ns). the attenuation decreases as the frequency decreases (davis and annan, 1989). on this basis, the attenuation and, therefore the frequency play an important role in the penetration depth and both vertical and horizontal resolution for the gpr system. vertical resolution is a measure of the ability to differentiate between two signals adjacent to each other in time. simplistically, vertical resolution is a function of frequency. each radar antenna is designed to operate over a range of frequencies (bandwidth) where the peak power occurs at the centre frequency of the antenna. the centre frequency is also inversely proportional to the pulse period (in nanoseconds). the equivalent length (in meters) of the pulse is the product of the pulse period and the em wave velocity in the medium. resolution can be taken as one-quarter of the wavelength (l) of em wave (burger, 1992) l = v/f (2.3) where v is the em wave velocity in the medium and f is its frequency. the horizontal resolution is related to the em wave which travels from the transmitter antenna in a cone of radiation with a finite-sized footprint (fig. 2), (conyers and goodman, 1997). the first fresnel zone describes the minimum area in which features with smaller dimensions will not ν = c k α ω δ= ⋅ ⋅ ⋅ k c2 tg fig. 1. hypothetical stratigraphical section with the panel of travertine 3 cm thick, the mortar with layer ranging from about 1 cm to some cm, and the wall. 483 ground penetrating radar (gpr): an application for evaluating the state of maintenance of the building coating be imaged. the finite size of this footprint affects both the vertical resolution (when interfaces are steeply dipping or have high amplitude surface roughness relative to the wavelength of the incident em waves), and the horizontal resolution. the larger the first fresnel zone, the lower the horizontal resolution in discriminating between adjacent targets will be. the radius of footprint is (conyers and goodman, 1997) (2.4) where l is the center frequency wavelength of em wave, d is the depth from the ground surface to the reflection surface, k is the relative dielectric permettivity of material from the ground to the surface depth (d). horizontal resolution is inversely proportional to (a)1/2, (burger, 1992). consequently, the horizontal resolution is actually better over a high-loss material than over a low-loss medium. the analysis directly performed on the radar sections (see next section) acquired in a controlled situation and using the relationships (2.1), (2.3) and (2.4), allowed table i to be realized. table i summarises the problem well. another problem is the near-field effect. an em wave radiated from a surface antenna generates an em field around the antenna within a radius of about 1.5 wavelengths of the center frequency (balanis, 1989; sheriff, 1984). for the 1 ghz antenna this effect is approximately 30 cm if the em wave radiated in air (conyers and goodman, 1997), while it is approximately 17 cm if the em wave radiated in travertine. within this zone, the em wave is coupled with the ground, generating an advancing wave front in the standard conical transmission pattern outside the radius. it can be said that the ground within about 1.5 wavelengths of a standard dipole antenna is part of the antenna where no radiation is occurring within this zone, and therefore technically no propagation. in the gpr sections, this zone is visible as an area of few or no reflections. to obviate the near-field effect zone a travertine layer (6 cm thick) between the antenna and the panel was used in the survey. a d k = + + λ 4 1 table i. the travertine, mortar and empty space (air) em properties with vertical resolution l/4 and footprint radius a. 1 ghz antenna, d = 10 cm material velocity (cm/ns) k l/4 (cm) a (cm) travetine 11.4 6.9 2.85 6.4 mortar 9.2 10.7 2.3 5.2 travertine + air 12.8 5.5 3.2 7.1 travertine + mortar 11 6 2.75 6.5 air 30 1 7.5 14.6 fig. 2. the elliptical cone of gpr into the ground (from conyers and goodman, 1997). 484 giovanni leucci, sergio negri and maria teresa carrozzo 3. field instrumentation, data acquisition and analysis the gpr survey was carried out with a georadar system-2 (sir 2) and a 1 ghz (centre frequency) antenna manufactured by geophysical survey systems inc. (gssi). the 1 ghz antenna selection was due to the resolution required by the survey objectives. the following acquisition parameters were selected: – data word length: 8 bit; – samples per scan: 512; – recording time window: 10 ns; – gain function: manual (5 gain points (12, 12, 22, 35, 47) db); – reference marks distance: 10 cm. to obviate the near-field effect zone, a travertine layer (6 cm thick) between the antenna and the panel was used in the survey. 3.1. measures in the test site (travertine panel hung in the laboratory) a travertine panel (100 cm wide, 80 cm high and 3 cm thick) was hung in the laboratory (fig. 3a,b). a layer of mortar was hung between the wall and the panel (20 cm wide, 80 cm high and 2 cm thick) every 20 cm on the whole length of the panel. such 20 cm of mortar and 20 cm of void were alternated along the whole panel (fig. 3a). a reconnaissance survey was made in continuous mode, in a rectangular area of 100 cm by 80 cm, along 5 cm spaced parallel profiles. the problem with the data acquisition is the near field. the quality of the original data did not require advanced processing techniques. in fact, only horizontal scaling normalisation (2 scan/cm) was performed for an easier interpretation. figure 4 shows the reflection profile, labelled a2 in fig. 3a. an analysis of fig. 4 points out: – the clear enough interface that represents the contact between the antenna and the interposed panel. – the series of anomalies with the first arrivals between 1.7 and 2 ns that represent, given their position along the profile, alternatively the wallmortar interface (2 · ns) and wall-empty interface (1.7 ns), the latter with clearly greater amplitude than those related to the wall-mortar interfaces. as already stated, we could not resolve the top and bottom of the mortar and voids. table i used both the first arrivals related to wall-mortar interface (2 ns) and wall-empty interface (1.7 ns) and the relationship v = (2 s)/t where v is the em wave fig. 3a,b. photo of the travertine panel hung in the laboratory: a) before the assemblage on the wall with the position of mortar and void; b) hung on the wall with location of radar profile labelled a2. a b 485 ground penetrating radar (gpr): an application for evaluating the state of maintenance of the building coating fig. 4. radar section related to the profile labelled a2 in fig. 3b. fig. 5. time slice for the 1 ns 3 ns time window with the energy amplitude values. several events are visible as high amplitude anomalies (dark region) that coincide with the shape and position of the voids between the travertine panel and the wall. the weak amplitude anomalies (clear region) coincide with the shape and position of the mortar between the travertine panel and the wall. 486 giovanni leucci, sergio negri and maria teresa carrozzo velocity (unknown) in the studied medium, s is the measured depth to the voids and mortar (known) and t is the two-way travel time from the surface to the voids and mortar and back to the surface. one way to obtain visually useful maps to understand the plan distribution of reflection amplitudes within specific time intervals is the creation of horizontal time slices (conyers and goodman, 1997). this data representation plays an important role in gpr investigations as it allows an easier correlation of the most important anomalies found in the area at the same depth, thus facilitating the interpretation (basile et al., 2000). in reality, because of possible velocity changes across the area and with depth, horizontal time slices must be considered only as approximate depth slices. however, this is generally sufficient for most common applications. in the present work, the time slice technique was used to display the amplitude variations within the 1-3 ns time window, where the majority anomalies were observed. the selected two-way time interval corresponds to a soil layer, approximately 12 cm thick, located between 6 and 18 cm in depth, where the voids were lodged. in the slice shown in fig. 5, several events are visible as high amplitude anomalies (dark region) that coincide with the shape and position of the voids between the travertine panel and the wall. the weak amplitude anomalies (clear region) coincide with the shape and position of the mortar between the travertine panel and the wall. 3.2. measures in situ (travertine panel inside the bank of naples in campobasso) a travertine panel set into the lower part of the wall (fig. 6), inside the bank of naples in campobasso, was considered. the data were acquired using the same acquisition parameters and the same shrewdness fixed in the preceding case of the measure on the test site (section 3.1). fig. 6. photo of the travertine panel inside to the bank of naples in campobasso with the location of b2 radar profile. b2 487 ground penetrating radar (gpr): an application for evaluating the state of maintenance of the building coating figure 7a shows the reflection profile (labelled b2) acquired in the upper part of the panel after the horizontal scaling normalisation (2 scan/cm) processing. a general characteristic of all the profiles acquired on the panel is the near-field effect in the 0-3.4 ns zone (fig. 7a) corresponding to a depth from 0 to about 19 cm if the mean velocity value of 11.4 cm/ns is used (see table i). most of the expected anomalies are confined in this zone. the traditional background removal filter (conyers and goodman, 1997), applied to all data, does not allow the reflection events present in the near field zone to be emphasized. usually, most of the em waves that are reflected on a subsurface interface between two materials are transmitted directly back to the surface and recorded at the receiving antenna. some of these reflected em waves, however, can be re-reflected back into the subsurface at the ground surface-air interface, and then re-reflected back again on to the surface from the same buried interface prior to recording (fig. 7b). this multiple reflection effect creates a double echo of the subsurface reflector, with a recorded time about twice that measured for a wave reflected only once. the multiple phenomenon was also observed in the data acquired on the test site (fig. 4). in order to overcome the near field zone the multiple phenomenon was considered. figure 7a shows some interesting reflection events at about 4.2 and 4.0 ns (multiple reflections) that represent respectively the wall-mortar interface and wallempty interface. if the average value of the em wave in the travertine + air is 12.8 cm/ns (see table i) the void thickness is about 1 cm. a b fig. 7a,b. a) model of radar transmission and reflection with multiple reflections off of subsurface interfaces prior to the wave’s being recorded at the surface antenna; b) radar section related to the profile b2 shown in fig. 6. the underlined multiple reflections are clearly related to the bottom of the interface air-wall and mortar-wall. 488 giovanni leucci, sergio negri and maria teresa carrozzo a time slice map is also made in this case. the time interval from 3.8 to 5 ns was considered. the result is shown in fig. 8. the slice shows the presence of strong amplitude anomalies (labelled m) that, because of their shape and position, could be correlated to metal objects placed to fix the panel. the anomalies of weaker amplitude, in comparison with those suitable with m (labelled void) could be the voids. where the anomalies are more intense, the void thickness could be about 1 cm. the clear zones in the slice (labelled mortar) are probably related to the presence of the layer of mortar. 4. conclusions the gpr investigation was divided into two phases: – the first one (testing phase) was carried out on a travertine panel hung in the laboratory with position and thickness of the voids known; – the second one, considering the principal objective of the survey, was carried out on the panels of the travertine placed inside the bank of naples in campobasso, with the position and thickness of the voids completely unknown. the testing phases allowed a semiquantitative evaluation of the em characteristic of the materials considered (table i) and disclosed more problems linked both to the methodology and geometry of the target. the encouraging results obtained in the first phase of measures were used to understand and interpret the results obtained in the second phase of measures better. using table i, and the hypothesis of similar electromagnetic conditions in the two phases, we have given a probable maximum thickness of void (1 cm) present between the travertine panel and the wall. even if they are still to be verified (to make a core boring on a panel the object of the investigation), the results obtained to date seem to be encouraging. fig. 8. time slice for the 3.8 ns 5 ns time window with energy amplitude values. note the presence of strong amplitude anomalies (labelled m ) that, because of the shape and the position, could be correlated to metal objects placed to fix the panel. the anomalies of weaker amplitude, in comparison with those suitable with m (labelled void), could be the voids. 489 ground penetrating radar (gpr): an application for evaluating the state of maintenance of the building coating acknowledgements the authors wish to thank the technician serafino corriero for his collaboration during data acquisition. the authors thank dr. tiziana di biaso and dr. wendy hudson for revising the english form of the manuscript.the authors are also grateful to the reviewers and to the editor for their valuable comments. references annan, a.p. and j.l. davis (1988): radar sounding in potash mines, saskatchewan, canada, geophysics, 53 (12), 1556-1564; annan, a.p. and s.w. cosway (1994): gpr frequency selection, in proceedings of the fifth international conference on ground penetrating radar, waterloo centre for groundwater research, waterloo, canada, 747-760. bae, s.h., h.s. kim and w.s. yoon (1996): cases studies on the application of ground penetrating radar technology in detection of underground utilities an structure safety diagnosis, in proceedings of the sixth international conference on ground penetrating radar, september 30-october 3, sendai, japan, 467-472. balanis, c.a. (1989): advanced engineering electromagnetics (john wiley and sons, new york), pp. 981. basile, v., m.t. carrozzo, s. negri, l. nuzzo, t. quarta and a.v. villani (2000): a ground-penetrating radar survey for archaeological investigations in urban area (lecce, italy), j. appl. geophys., 44, 15-32. burger, h.r. (1992): exploration geophysics of the shallow subsurface; englewood cliffs, new jersey, print hall, pp. 489. carcione, j.m. (1996): ground-radar numerical modelling applied to engineering problems, eur. j. environ. eng. geophys., 1 (1), 65-82. conyers, l.b. and d. goodman (1997): ground-penetrating radar: an introduction for archaeologist, altamira press, walnut creek, london and new delhi, pp. 232. davidson, n.c. and s.b. chase (1998): radar systems for tomographic bridge deck inspection, in proceedings of the 7th international conference on ground penetrating radar, may 27-30, laurence ks, vol. 2, 749-751. davis, j.l. and a.p. annan (1989): ground penetrating radar for high-resolution mapping of soil and rock stratigraphy, geophysics, 37, 531-551. godio, a., l. sambuelli and v. socco (1999): misure georadar per lo studio della volta della scarsella nel battistero del duomo di firenze, in proceedings 18th convegno gngts, roma 9-11 novembre 1999, cdrom. leucci, g., s. negri, m.t. carrozzo and l. nuzzo (2002): use of ground penetrating radar to map subsurface moisture variations in an urban area, j. environ. eng. geophys., 7 (2), 69-77. mellet, j.s. (1995): ground penetrating radar applications in engineering environmental management and geology, j. appl. geophys., 33, 157-166. sh e r i ff, r.e. (1984): encyclopedic dictionary of exploration geophysics, second edition, society of exploration geophysics, tulsa, oklahoma, pp. 323. t h e r e c i p r o c i t y t h e o r e m f o r p o r o u s a n i s o t r o p i c m e d i a e . b o s c h i ( * ) received oil .july 17tli, 1972 summary. •— in this p a p e r we give a reciprocity t h e o r e m for anisotropic porous media in t h e q u a s i s t a t i o n a r y case. t h e d i s t r i b u t i o n of t h e pores is assumed s t a t i s t i c a l l y h o m o g e n e o u s . riassunto. viene s t a b i l i t o un t e o r e m a di r e c i p r o c i t à per mezzi porosi a n i s o t r o p i nel caso quasi-stazionario. l a distribuzione dei pori è a s s u n t a s t a t i s t i c a m e n t e omogenea. 1 . i n t r o d u c t i o n . the theory of deformation of porous materials containing a fluid has been developed by biot f1-2), for t h e case of an elastic solid. i n the last years various generalizations a n d particular applications have been considered. a porous solid is represented as an elastic skeleton, having compressibility a n d shear rigidity, with a statistical distribution of interconnected pores containing a compressible fluid. i t is understood t h a t t h e term " p o r o s i t y " refers to the effective porosity, namely, t h a t encompassing only the intercommunicating void spaces as opposed to those pores which are sealed off. i n the following, t h e word " p o r e " will refer to t h e effective pores while t h e sealed pores will be considered as a p a r t of t h e solid. (*) i s t i t u t o di fisica, u n i v e r s i t à di bologna, i t a l i a . ( ¡ 0 8 e . b o s c h i this s y s t e m of fluid and solid is a general elastic system with conservation properties. the deformation of a u n i t cube is assumed to be completely reversible. by d e f o r m a t i o n is m e a n t , here, t h a t determined by both strain tensors in t h e solid and in the fluid phases. i e f a n ("8) lias established a m e t h o d by means of which he was able to give reciprocity theorems in t h e d y n a m i c t h e o r y of continua, w i t h o u t using laplace t r a n s f o r m s in the case of nonhomogeneous initial conditions. moreover, by this m e t h o d , he obtained reciprocity relations which involve only t h e displacement vector a n d t h e known functions. boschi (;i) has obtained a reciprocity theorem for isotropic porous homogeneous m e d i a in the quasi-stationary case. h e (4) has also been able to give a v a r i a t i o n a l theorem in t h e linear t h e o r y of porous media, a n d in t h e t h e o r y of viscoelastic porous m e d i a (5). this kind of speculations is of great interest because t h e y f o r m t h e basis for t h e solution of problems arising in m a n y diversified fields such as seepage in soil mechanics, ground w a t e r hydrology, petroleum engineering, w a t e r purifications, acoustic engineering a n d so on. 2. basic equations. t h r o u g h o u t this p a p e r we employ a rectangular coordinate system, oxk (k = 1, 2, 3), and t h e usual indicial n o t a t i o n s . l e t v be a regular (in the sense of kellog) region of space occupied by a n anisotropic porous solid, whose b o u n d a r y is j7. moreover v is t h e interior of v, ni are t h e components of t h e u n i t o u t w a r d n o r m a l to z. f o r convenience a n d clarity in presentation, all regularity hypotheses on considered f u n c t i o n s will be omitted. on these basis t h e field equations for anisotropic porous solids, in t h e qnasi-static case, are: — t h e constitutive equations: da = gijki eici + eia e [ l . a ] [ l . b ] a = an en + fie — t h e equations of equilibrium: (<fu + aòi,),i +qfi = 0 [2.a] t h e r e c i p r o c i t y t h e o r e m for p o r o u s a n i s o t r o p i c m e d i a till the d a i r y ' s law: a,i + oi f( = ht, ( u] — ùj). [2.b] the equation [2.b] is t h e d a r c y ' s law in a generalized form for an anisotropic medium. — t h e strain-displacement relations: in these equations we h a v e used the following notations: ui, t h e components of t h e displacement vector for t h e solid phase; ut, t h e components of t h e displacement vector for the fluid inside the pores; oij, en, the components of the strain a n d of t h e stress tensors for t h e solid phase; a, the h y d r o s t a t i c s t a t e of stress of t h e liquid filling t h e pores, i.e., if we consider a cube of u n i t size of the bulk material, a represents the t o t a l normal tension force applied to t h e fluid p a r t of t h e faces of such a cube; e<j, t h e components of the strain tensor for t h e fluid phase; e — en, dilatation of t h e fluid; q, qi densities of t h e solid and of t h e fluid, respectively; cini, bn, characteristics of t h e solid phase; a n , (i, characteristics of t h e fluid phase (2). l e t us also remember t h a t a comma denotes p a r t i a l derivation with respect to the space variables, xk, a n d a dot denotes p a r t i a l derivation with respect to t h e t i m e /. f u r t h e r m o r e t h e following s y m m e t r y relations: are satisfied. to t h e system of t h e field equations we m u s t a d j o i n t h e initial conditions: 13. a] [3.b] ctjici = c)iki — cxni an = an bu = bu [la] [ l b l ui (x, 0) = at u, (x, 0) ='vli [5.a] [5.b] a n d t h e b o u n d a r y conditions: {an + <rôn) >'i = vh on i [«j where at, ai, pi are prescribed functions. (¡10 e . boschi 3 . p r e l i m i n a r i e s . l e t /, g a n d h be f u n c t i o n on v x [0, oo), continuous on [0, oo) with respect t o t h e time t for each x e v. w e denote b y / * g t h e convolution of / a n d </: t [/ * 9] 0 = | / (®, t — s) g (.;•, s) (h. o w e will h a v e occasion to use t h e following well-known properties of the convolution (9): / * 9 = 9 * / / * (g *h) = (/ * g) *h — / * g * h f * (9 + m = f * 9 + f * hh e n c e f o r t h we will denote by i t h e f u n c t i o n defined on [0, oo) b y : l(t) = 1. i t is easy t o show t h a t : l*(uj—11 j) = uj — u j — (a} — a,) [7] i * {a,, + ei ft) = i * + qil* fi. [8] then from the equations [2.b], [7] and [8], we get: l*tt,i + q,=btj(ui — u,) [9] where: q, = qi i * ft -f blf (a] — ai). [10] t h e equations [9] are equivalent to t h e equations [2.b] a n d to t h e initial conditions [5.a], [5.b], 4 . t h e r e c i p r o c i t y t h e o r e m . l e t us now consider t h e body subjected to two different systems of elastic loadings: l^ = {f(,a\p({a),a,ia),a,ia),} a = 1 , 2 . [11] t h e r e c i p r o c i t y t h e o r e m f o r p o r o u s a n i s o t r o p i c m e d i a t i l l t h e t w o corresponding configurations a r e : /-i(a) — / 77 (a) „, (a) „ (a) .(a) _ (a) _.(a) \ „ 10 n on o =\ut , ht ,eit ,e ,(ju ,a , ja — l, j. | l-!j from the equations [l.a], [l.b], we get: _ (a) „ _(a) rt „ (a) j tij —tti) £ = oiiklski i > a = 1 2 jo — at, e (a) = p £ (a) j i t is t r u e t h a t (crij*1 > — cn, e<d) * e£i<2) = gijki e*i<d * (o-fj«2» — au e(2>) * c<j (1) = ciik, * a d d i n g these t w o relations, we h a v e : (at,'1' — a,j e'1») * eti121 = (,at,<2> — a « e<2>) * e t , »>. [13] i t is also t r u e t h a t : (ffi1) — an ey(d) * £'2> = e<i> * e<2> (ffi2) — an eti&>) * «(1) = /? e<2) * £(1) a n d , b y a d d i t i o n , we o b t a i n : (cr<n — an <?<j(1)) * «(2) = (fff2) — a»j e«(2>) * e«1'. [14] a d d i n g t h e relations [13] a n d [14] we g e t : a n a ) * e t / 2 > + o a > * e < 2 > = a n i s ) * e < i a ) + ff'21 * e ( 1 ) [ 1 5 ] if we i n t r o d u c e t h e n o t a t i o n : = i i * ( f f « ( a ) * + ff(a) * e^) dv-, a, — 1, 2 [16] v f r o m e q u a t i o n [15] we h a v e : ii2 = /21 [17] <>12 f.. bosch i ltsing t h e relations [3], [9], [10] a n d the divergence theorem, we g e t : iafi = | i * v r ] * ut& dz + | / * a'a) * i!,'?' n< dz + v v + i i * o f,(a) * m'p' dv — | / * ah1) * dv — v v j b„(ui,a) — u,,a)) * ut'p' (iv + j q,m) * u,^ (iv, [181 v v a,p = 1 , 2 finally f r o m equations [17] a n d [18] we can s t a t e the reciprocity theorem for anisotropic porous solids: if a n anisotropic porous solid is s u b j e c t e d to two different systems of elastic loadings [11], t h e n between t h e t w o corresponding configurations [12] there is tlie following reciprocity relation: j i * />,<>> * w(<2> dz + | i * c' 1 ' fii * r/v2> dz + + | i g * f,id * mi<2) (iv — j z * cr") * e<2> rff — f f [6(/ (f.^'1» — m/«1») + d] * r/f(2) f / r = i l * p,<*> * u,<'i dz + | i * ff'2> ill * {/,»> d r + + j i * q ft<2> * mi») dv — j z * a<2> * ell) f f j (?7/(2) — wi'2») + r/i(21] * e > rff. [19] w e should w a n t to p u t in evidence t h a t , t h r o u g h o u t this p a p e r , we did not use a n y restrictive assumption. the theorem [19] is valid under very general conditions. t h e r e c i p r o c i t y t h e o r e m f o r p o r o u s a n i s o t r o p i c m e d i a till i n t h e special case of homogeneous b o u n d a r y conditions, of great interest in t h e applications, t h e relation [19] reduces t o : i * (q jv1) *mt<a' — <rll> *e<2>)— [bt)(ui^ — uj'1') + * ut,2> dv = j 11* (q — ff«) *e^) — [btl(u]^ — uii2)) + ?i, 2 )] * e7i(1) | dv. tlie reciprocity theorem derived in this p a p e r can be used to obtain variational theorems in t h e t h e o r y of porous anisotropic materials. these theorems will be developed in a n o t h e r p a p e r . 0 ) b i o t m. a., 1955. " j . a p p i . p h y s . " , 26, 182. ( 2 ) b i o t m. a., 1956. " j . a p p i . p l i y s . " , 27, 210. ( 3 ) b o s c h i e . , 1 9 7 2 . " l e t t . n u o v o c i m e n t o " , 4 , 9 7 3 . ( 4 ) b o s c h i e . , 1972. to be p u b l i s h e d . ( 6 ) b o s c h i e . , 1972. to be p u b l i s h e d . (6) ieçan d., 1967. " r e n d . a c a d . sc.", p a r i s , 265, 271. (7) iesan d., 1969. " i n t . j . e n g n . sci.", 7, 1213. ( 8 ) i e § a n d., 1970. " r e v . r o u m . m a t h . p u r e s et a p p i . " , 15, 1181. ( " ) m i k u s i n s k i j . , 1959. operational calculus, p e r g a m o n press, new y o r k . r e f e r e n c e s 23mantyniemi.qxd 611 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical earthquakes – fennoscandia and baltic republics 1. introduction the region addressed here covers fennoscandia, denmark, the baltic countries, kola and nw russia (fig. 1a,b). it comprises the adjacent sea areas of norway out to the eurasian and north american plate boundaries along the midoceanic ridges in the norwegian and greenland seas. this region is situated in the nw eurasian intraplate domain, which is nearly devoid of earthquake activity in global terms. however, earthquakes are felt occasionally and areas with enhanced seismic activity can be found, particularly in the coastal areas of norway. assessing macroseismic observations in this region can be complicated, because offstate-of-the-art of historical earthquake research in fennoscandia and the baltic republics päivi mäntyniemi (1), eystein s. husebye (2), tarek r.m. kebeasy (2), andrei a. nikonov (3), valery nikulin (4) and andrius pacesa (5) (1) institute of seismology, university of helsinki, finland (2) department of earth science, university of bergen, norway (3) schmidt united institute of physics of the earth, russian academy of sciences, moscow, russia (4) state geological survey of latvia, riga, latvia (5) geological survey of lithuania, vilnius, lithuania abstract we review historical earthquake research in northern europe. ‘historical’ is defined as being identical with seismic events occurring in the pre-instrumental and early instrumental periods between 1073 and the mid-1960s. the first seismographs in this region were installed in uppsala, sweden and bergen, norway in 1904-1905, but these mechanical pendulum instruments were broad band and amplification factors were modest at around 500. until the 1960s few modern short period electromagnetic seismographs were deployed. scientific earthquake studies in this region began during the first decades of the 1800s, while the systematic use of macroseismic questionnaires commenced at the end of that century. basic research efforts have vigorously been pursued from the 1970s onwards because of the mandatory seismic risk studies for commissioning nuclear power plants in sweden, finland, nw russia, kola and installations of huge oil platforms in the north sea. the most comprehensive earthquake database currently available for northern europe is the fencat catalogue covering about six centuries and representing the accumulation of work conducted by many scientists during the last 200 years. this catalogue is given in parametric form, while original macroseismic observations and intensity maps for the largest earthquakes can be found in various national publications, often in local languages. no database giving intensity data points exists in computerized form for the region. the fencat catalogue still contains some spurious events of various kinds but more serious are some recent claims that some of the presumed largest historical earthquakes have been assigned too large magnitude values, which would have implications for earthquake hazard levels implemented in national building codes. we discuss future cooperative measures such as establishing macroseismic data archives as a means for promoting further research on historical earthquakes in northern europe. mailing address: dr. päivi mäntyniemi, institute of seismology, university of helsinki, p.o. box 26, fin00014 helsinki, finland; e-mail: paivi@seismo.helsinki.fi 612 päivi mäntyniemi, eystein s. husebye, tarek r.m. kebeasy, andrei a. nikonov, valery nikulin and andrius pacesa shore events may be poorly sampled and sturdy timber houses were the predominant type of dwelling in the past. also, macroseismic evidence for significant amounts of damage is very rare. when stone churches and some masonry buildings in sweden were inspected for traces of permanent earthquake damage as part of a programme of seismic risk assessments for nuclear power plants, the outcome was negative. many intensity estimates for the historical data have been given using the modified mercalli (mm) scale, which differs somewhat from the medvedev-sponheuer-kárník scale (msk 64) and the revised european macroseismic scale (ems). this is not considered a severe drawback, since most events have an intensity value of v and below for which range these scales are similar. this would not constitute a serious problem even for the largest earthquakes with intensities up to vii, because magnitude estimates are tied to the perceptibility area as defined by intensity i = ii-iii observations. historical times are defined here as those beginning with the earliest written account in 1073 until the use of short period instruments. the latter is not clearly defined because short period fig. 1a. region addressed in the present study. letters denote present-day denmark, estonia, finland, latvia (lv), lithuania (lt), norway, russia and sweden. squares show epicentres for earthquakes of magnitude 4 and above occurring between 1375 and 1964 as given in the fencat catalogue (ahjos and uski, 1992) and its updated version (http://www.seismo.helsinki.fi). the reported event magnitudes are of the ml type but stem from a variety of sources. some events are located in offshore areas reflecting mechanical seismograph installations up to the mid-1960s. 613 state-of-the-art of historical earthquake research in fennoscandia and the baltic republics 2. a brief history of macroseismic studies in northern europe many studies have addressed the problem of compiling a homogeneous and comprehensive earthquake catalogue for northern europe. these activities date back to about the 1750s, coinciding with a growing interest in science and natural phenomena. learned societies were established in the nordic countries for the advancement of scientific thinking and debate. earthquakes were well established as natural phenomena, but their origin was vigorously deseismograph station installations commenced in most countries during the 1960s, while in the russian part of fennoscandia the network remained very sparse until the 1980s and in norway the national network remained somewhat incomplete until 1990. here we define historical records from the earliest available report up to the 1960s. the following section presents some background on the earthquake catalogues available for the region. the third section describes historical earthquake research in various parts of the region during the past 20 years, and the fourth section discusses today’s situation and future plans. fig. 1b. region addressed in the present study. letters denote present-day denmark, estonia, finland, latvia (lv), lithuania (lt), norway, russia and sweden. circles show epicentres for earthquakes occurring between 1965 and 2002. the smaller symbols refer to magnitudes in the range of 3.5-3.9 and the larger symbols to magnitudes 4 or above as given in the fencat catalogue (ahjos and uski, 1992) and its updated version (http://www.seismo.helsinki.fi). 614 päivi mäntyniemi, eystein s. husebye, tarek r.m. kebeasy, andrei a. nikonov, valery nikulin and andrius pacesa bated in theological, philosophical and natural science contexts. although a proper physical understanding of earthquakes developed slowly, descriptions of local events were written down. the oldest documentation was related to a danish earthquake in 1073 (lehmann, 1956), while the first attempt on a swedish catalogue is credited to hjärne (1706). the kattegat, denmark earthquake in 1759 is a unique example of the early use of macroseismic questionnaires. bishop c. horrebow wrote to all the priests in his zealand diocese, asking how the earthquake was felt locally. the original material was rediscovered recently and has now been published together with newspaper clippings from adjacent norway, sweden and northern germany (bondesen and wohlert, 1997). the flow of scientific earthquake studies began in the 1800s. the swedish academy of sciences promoted interest in earthquakes, and these efforts resulted in many annual and biannual reports (ehrenheim, 1824; berzelius, 1823, 1824, 1826, 1827). earthquakes were frequently reported in norway as well, and the first compilation there was due to keilhau (1836). the finnish society of sciences and letters collected information on natural phenomena, which led moberg (1855) to give the first earthquake report in finland. these activities inspired further investigations. a comprehensive listing of all known norwegian earthquakes up to 1912, including the corresponding publications, was later published by kolderup (1913). svedmark (e.g., 1886, 1902) published several papers on seismic events felt in sweden, and renqvist (1930) compiled the first descriptive earthquake catalogue in finland. systematic interest in earthquakes was also shown in neighbouring areas and countries. mushketov and orlov (1893) compiled the first comprehensive earthquake catalogue for the russian empire and adjacent areas, comprising more than 2500 entries. some of these events occurred in russian lapland and the kola regions, which are parts of nw russia. only the narva earthquake in 1881 was included for the se baltic area. more updated earthquake catalogues for nw russia and kola were later published by gorshkov (1947), panasenko (1969) and kondorskaya and shebalin (1977). doss (e.g., 1898, 1909, 1910, 1911) began historical and contemporary earthquake research in what is today latvia and estonia. as usual, the main sources of information were ancient church chronicles, newspapers and eyewitness reports. he compiled an earthquake list that comprised 18 events occurring in latvia and estonia between 1616 and 1896 (doss, 1909). no historical earthquake records are known for lithuania, which is not surprising in light of the present-day very low seismic activity. some large regional events were reportedly felt within lithuanian territory (gudelis, 1958). an important milestone in studies of historical earthquakes was the systematic use of macroseismic questionnaires, which commenced in fennoscandia in the 1880s. initially the rossi-forel intensity scale ranging from i to x was used. later the mercalli, mm and msk64 scales and, most recently, the ems (grünthal, 1993, 1998) came into use. these scales are similar up to intensity i = v or even i = vi and thus comparable for the large majority of reported historical earthquakes. basically, the use of questionnaires has continued without interruption until now, although wars naturally hampered such activities. with the deployment of several modern seismograph stations in the 1960s and afterwards, the interest in macroseismic studies waned and returned questionnaires were often simply archived without much further analysis. this practice changed in the 1970s when seismic hazard studies became mandatory for nuclear power plant operations and later for commissioning oil production platforms in the north sea. since it is not scientifically sound to restrict seismicity studies to events occurring only within national borders, it naturally followed that fennoscandian catalogues began to be compiled. an important work was that of båth (1956), whose catalogue comprised earthquakes occurring in fennoscandia (without its russian part) between 1891 and 1950. ahjos and korhonen (1984) extended båth’s work by compiling a fennoscandian earthquake catalogue covering about 500 years. it can be seen as one step towards the fencat catalogue, which is the present regional catalogue available in parametric form (ahjos and uski, 1992). it is constantly updated 615 state-of-the-art of historical earthquake research in fennoscandia and the baltic republics of particular interest are large recent earthquakes, for which both macroseismic and instrumental data are available. they are helpful in establishing formulas relating intensity and magnitude, thus enabling magnitude estimates for historical earthquakes as well. initially, maximum intensity was used, as in the case of the 1904 earthquake near oslo, but this proved far too unreliable for several reasons, not the least because many epicentres lie outside populated areas. more representative are log-linear relations comprising the entire area characterized by i = iii observations such as ms = 0.69 ⋅ log(aiii) + 0.0006 ⋅ aiii 1/2 + 0.95 (3.1) ml = 0.86 ∗ log(aiii) + 0.21 (3.2) where the ms formula is given by muir wood and woo (1987) and the ml formula by almhjell et al. (2001). many local magnitude formulas have been published (see bungum et al., 1998), but our preference is that of eq. (3.2) because it is calibrated against ml magnitudes stemming from modern instrument recordings. the above relationships should in principle be simple enough to apply but are not so in actual practice due to the presence of so-called outliers. an illustrative example here is the earthquake that occurred in lurøy, northern norway in 1819, which has generally been taken to be the largest in nw europe in historical times. muir wood (1989) assigned it a magnitude value of ms = 5.8, which was entirely due to his acceptance of reports from stockholm more than 800 km away as genuine i = iii observations, while husebye and kebeasy (2003) rated these observations as outliers. when the stockholm observations are classified as belonging to category i = ii, the radius of perceptibility reduces to only 350 km and the corresponding magnitude to only ml = = 4.8 or, equivalently, ms = 5.1. another interesting example is the kattegat, denmark earthquake of 23 december 1759. the peculiar feature here is that this event was felt as far (500 km) as schleswig-holstein (northern germany) to the south but at a far less distance to the north (norway) and northand available over the internet: http://www.seismo.helsinki.fi/ 3. historical earthquake studies during the past 20 years since the fennoscandian shield is characterized by the relatively infrequent occurrence of small rather than devastating earthquakes, the attention focused on these during the years of serious study was largely determined by the level of academic interest. increased interest in fennoscandian seismicity followed from the mandatory seismic risk analysis for nuclear power plants and plans for radioactive waste disposal in the bedrock of sweden, finland and russia and large offshore oil installations in norway commissioned over 20 years ago. exploration of the rich north sea oil and gas resources motivated seismic hazard assessment in that area and greatly increased the need for a homogeneous seismic record compiled on a multinational basis. a comprehensive multinational programme of historical earthquake information retrieval from libraries and record offices in norway, denmark, sweden and britain was launched. it is rated very successful in terms of many new publications on this subject (e.g., ambraseys, 1985; bungum et al., 1986; muir wood et al., 1988). during the search new information on several known events was found and event duplications, meteor-induced tremors and previously unknown events were identified. the new database was used to quantify the earthquakes in terms of msk intensity and to prepare a collection of isoseismal maps following a standard method. a noteworthy outcome of these studies was also that magnitude estimates were given to most of the historical earthquakes. among the largest events was the earthquake in the vicinity of oslo on 23 october 1904 which was initially given a ml magnitude of 6.4 mainly based on i = vii reporting near its epicentre. however, this appeared a bit excessive since no significant damage was reported, and later readjustments gave a ml value of 5.4 using the magnitude formula tied to felt area for i = iii as given below. 616 päivi mäntyniemi, eystein s. husebye, tarek r.m. kebeasy, andrei a. nikonov, valery nikulin and andrius pacesa east (sweden). a rational explanation is the strong wavefield amplification on the northern flanks of both the danish and north german basins, causing persistent biases in intensity observations. kebeasy and husebye (2003) used 2d finite-difference waveform modelling to evaluate amplifications due to subsurface basin structures and reassessed intensity on the ems, using original questionnaire data from bishop horrebow mentioned above. when correcting for the amplifications the shape of the perceptibility area became symmetric, but most significantly the revised kattegat earthquake magnitude became ml = 4.9 (ms = 5.1) compared with ms = 5.6 given by muir wood (1989). musson et al. (2001) conducted a study on the historical seismicity of the faroe islands, which today are a self-governing community within the danish realm. no previous search for historical earthquakes was ever performed there. the study involved a search for earthquake records, especially in the national archives and the national library of the faroe islands. the main conclusion was that the absence of reports of historical earthquakes was evidence that no events occurred there. no historical earthquakes are known for greenland. wahlström (1990) presented a historical earthquake catalogue for sweden covering the period between 1375 and 1890. the purpose was to investigate and quantify known historical earthquakes rather than perform a search for new data, although a few previously unknown events were found. the historical data were quantified on the macroseismic magnitude scale designed by wahlström and ahjos (1984), which has been calibrated against the instrumental local magnitude scale. furthermore, wahlström and grünthal (1994) compiled and systemized earthquake data in the southern baltic sea area, which covered southern sweden, denmark and parts of northern germany and poland. existing catalogues and special studies were made use of, but new evaluations of macroseismic parameters were also carried out. intensities were given on the mm and msk scales. wahlström and grünthal reinvestigated the macroseismic data of the 1930 earthquake, which is the largest known in the southern baltic sea. it was felt in parts of denmark, southern sweden and northern germany. one conference to increase interest in historical earthquake catalogues was the 1988 meeting of former ussr scientists in minsk, where seismotectonic zoning was planned for the western ussr, including estonia, latvia, lithuania, belarus, ukraine and moldova. avotina et al. (1988) published an earthquake catalogue for belarus and the baltic countries that covered the period between 1616 and 1987. it included the epicentral coordinates for 54 shocks, their intensities given on the msk-64 scale and brief explanations of the macroseismic effects reported to accompany the tremors. garetsky et al. (1989) and later boborykin et al. (1995) provided improvements and additions to this catalogue, such as the determination of macroseismic magnitude and focal depth. the seismicity of the se baltic became particularly interesting after the unexpected osmussaar earthquake in the gulf of finland in 1976 (klaamann, 1977; slunga, 1979; ananjin et al., 1980; kondorskaya et al., 1988). nikonov and sildvee (1991) investigated historical earthquakes in estonia and published a comprehensive earthquake catalogue including presumed seismotectonic settings for many events occurring from 1670 to 1976. the parametric catalogue was later enlarged and updated from 19 to 27 events occurring between 1602 and 1987, and a map of maximum observed shaking and seismogenic zones of the area was presented (nikonov, 1992a). another parametric catalogue for estonia was published by sildvee and vaher (1995). a reinvestigation of the 16 november 1931 earthquake in central finland was carried out by mäntyniemi (2004). statistical methods such as correspondence analysis were used to reassess intensity on the ems. a more problematic data set on the earthquake of 18 august 1926 is currently being analyzed by mäntyniemi and nikonov. the felt observations were distributed between finland and nw russia in a very sparsely populated area. studies on historical earthquakes in the russian part of fennoscandia were conducted by nikonov (1991, 1992b) and assinovskaya and nikonov (1998a,b). the most comprehensive of 617 state-of-the-art of historical earthquake research in fennoscandia and the baltic republics these was devoted to the felt effects of 37 earthquakes occurring in the 1900s and included known and previously unknown primary data, data analysis and intensity assessment as well as sources of information (nikonov, 1991, 1992b). later, also magnitude estimation was carried out (assinovskaya and nikonov, 2003). when studying seismicity in nw russia, a severe problem is the scarce information on historical events (bungum and lindholm, 1996). however, large industrial installations in kola and nw russia have been subjected to extensive seismic hazard studies as part of safety managements (kuzmin, 1993). assinovskaya and nikonov (2003) have studied in detail certain large earthquakes there, one occurring in the white sea area in 1627 and the others in the lake ladoga basin in 1861 and 1921. the epicentral intensity of the white sea event may have been up to viii degree, while the epicentral intensities of the others have been assessed at vii and vi, respectively. an updated version of the fencat catalogue given by ahjos and uski (1992) served as the main input data for hazard computations in fennoscandia during the global seismic hazard asssessment programme (gshap) in the 1990s (giardini and basham, 1993; grünthal et al., 1999). this database is also frequently used for seismic zoning and national building codes in the respective fennoscandian countries. 4. today’s situation and future plans all historical events of magnitude 4 and above have been plotted in fig. 1a while, for comparison, the largest instrumental events have been plotted in fig. 1b. the seismicity features displayed in the two maps are similar except for offshore areas (gregersen et al., 1991). differences can be noted especially in southern-central norway and sweden, offshore northern and western norway and in the west coast of denmark. some offshore events shown in fig. 1a have obviously been located with the help of instrumental recordings and have been assigned magnitudes above 4.5. compilation of the historical part of the regional fencat catalogue (ahjos and uski, 1992) was based on existing literature and priority was given to the most recent solutions for macroseismic parameters. as ambraseys (1985) stated, it is often problematic to reconcile earthquake listings given in various catalogues and therefore advocated the use of original sources. in other words, much work remains before we may have a uniform northern europe historical earthquake catalogue based on common analysis procedures. recent studies show that large historical events often warrant reinvestigation (e.g., husebye and kebeasy, 2003; kebeasy and husebye, 2003), as depicted in the previous section. another problem is that historical fennoscandian earthquake data were more frequent during winter than summer, which has been attributed to frost shocks having been accepted as earthquakes (renqvist, 1930; wahlström, 1990). it was noted recently that the historical norwegian events were more abundant in winter than in summer only in the 1880s, which was a very cold decade in norway (boulaenko and husebye, personal communication). winter events in the latvian earthquake data are also more numerous than those of other seasons; nikonov (1995) pointed out that this may be due to sharp changes in temperature. 4.1. future plans seismic hazard analysis is critically dependent on the knowledge of historical seismicity. for real progress here we advocate cooperation in northern europe for realising research goals as tentatively listed below: i) refining the fencat catalogue: – procedures for processing macroseismic data; – reanalysis of the largest earthquakes; – removal of spurious events such as explosions, etc. ii) establishing a macroseismic data base, comprising: – an intensity map data base; – intensity observation files for all major events; – archives of written earthquake descriptions. iii) cooperative measures for establishing: – a uniform magnitude-intensity scale for northern europe; framsteg, afgifne af kongl, vetenskaps-academiens embetsmän, physik och oorganisk chemie, stockholm, 7-288 (in swedish). berzelius, j. (1826): årsberättelser om vetenskapernas framsteg, afgifne af kongl, vetenskaps-academiens embetsmän, årsberätterse om framstegen i physik och chemie, stockholm, 1-318 (in swedish). berzelius, j. (1827): årsberättelser om vetenskapernas framsteg, afgifne af kongl, vetenskaps-academiens embetsmän, årsberätterse om framstegen i physik och chemie, stockholm, 1-364 (in swedish). boborykin, a.m., v.g. vladimirov, r.g. garetsky, v.a. hermann, a.p. emelyanov, v.i. zui, v. ilganyte, m.v. matveenko, g.i. reisner, a.s. savitsky, o.n. safronov and z.p. hotko (1995): seismotectonic studies of the western part of the east european platform (belarus-baltic region), seismol. res., minsk, 2, 5-54 (in russian). bondesen, e. and i. wohlert (1997): præsteindberetninger om jordskælvet den 21. 22. december 1759, roskilde museums forlag, roskilde, pp. 315 (in danish). bungum, h. and c. lindholm (1996): seismoand neotectonics in finnmark, kola and the southern barents sea, part 2: seismological analysis and seismotectonics, tectonophysics, 270, 15-28. bungum, h., p.h. swearingen and g. woo (1986): earthquake hazard assessment in the north sea, phys. earth planet. inter., 44, 201-210. bungum, h., c.d. lindholm, a. dahle, e. hicks, s. høgden, f. nadim, j. holme and c. harbitz (1998): development of a seismic zonation for norway, final report (submitted to the norwegian council for building standardization, nbr). doss, b. 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(1824): om climaternas rörlighet; tal hållet vid praesidii nedläggande uti kongl, vetenskaps academien, stockholm, pp. 208 (in swedish). garetsky, r., a. boborykin, a. emelyanov and h. sildvee (1989): historical earthquakes in the territory of belarus and the baltic republics, calibration of historical earthquakes in europe, madrid, 217-230. giardini, d. and p. basham (1993): the global seismic hazard assessment program (gshap), ann. geofis., xxxvi (3-4), 3-13. gorshkov, g.p. (1947): on seismicity in the eastern baltic shield, proceedings of the seismological institute, 199, 86-96 (in russian). gregersen, s., h. korhonen and e.s. husebye (1991): fennoscandian dynamics: present-day earthquake activity, tectonophysics, 189, 333-344. 618 päivi mäntyniemi, eystein s. husebye, tarek r.m. kebeasy, andrei a. nikonov, valery nikulin and andrius pacesa – intensity and acoustic (sound) decays with distance; – a strategy for searching unknown earthquakes. iv) coordinating data centre bulletins in northern europe: – clearly marked genuine earthquakes; – report macroseismic material as well; – coordinate efforts on earthquake location procedures. in short, macroseismic observations may still be considered a research challenge and remain important for properly assessing past, present and future earthquake activity in northern europe. references ahjos, t. and h. korhonen (1984): on a catalogue of historical earthquakes in fennoscandian area, report s-10 (inst. seismology, univ. helsinki, finland), pp. 24. ahjos, t. and m. uski (1992): earthquakes in northern europe in 1375-1989, tectonophysics, 207, 1-23. almhjell, t., g.e. ekerold and t. vindenes (2001): makroseismisk studie av sotra jordskjelvet 8 des. 2000, technical report (inst. solid earth physics, univ. bergen, norway), pp. 9 (in norwegian). ambraseys, n.n. 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(2004): pre-instrumental earthquakes in a lowseismicity region: a reinvestigation of the macroseismic data for the 16 november 1931 events in central finland using statistical analysis, j. seismol., 8 (1), 71-90. moberg, a. (1855): om i finland inträffade jordskalf och varseblifna eldkulor åren 1842-1850, öfversigt af finska vetenskaps-societetens förhandlingar, 2, 48-52 (in swedish). muir wood, r. (1989): the scandinavian earthquakes of 22 december 1759 and 31 august 1819, disasters, 12, 223-236. muir wood, r. and g. woo (1987): the historical seismicity of the norwegian continental shelf, elocs earthquake loading on the norwegian continental annals 47, 5, 2004 1609 annals of geophysics, vol. 47, n. 5, october 2004 key words regional main field modelling – spherical cap harmonic analysis – magnetic repeat stations 1. introduction the importance of a magnetic repeat station network, where the three-component magnetic field can be measured at intervals of some years, is enhanced in those situations in which no magnetic observatories are operating in the near neighbourhood. the values of the magnetic components, and especially the secular variation field deduced from these measurements, can be used to define regional models to produce magnetic charts or reduce magnetic surveys to a given epoch. the balkan peninsula is one of the european regions with the poorest coverage of magnetic observatories, so the need for a regular monitoring of the magnetic field at some given points is fundamental. a scalar survey (measurements of total intensity f only) was undertaken in epoch 1990.0 by the ex-geophysical enterprise of tirana that covered all albanian territory. the normal field model derived from those data was presented by duka and bushati (1991). in the framework of a joint project between the center of geochemistry and geophysics of tirana, the physics department of tirana university, and the national institute for geophysics and volcanology of italy, a vector magnetic survey (f as well as declination d and inclination i were measured: chiappini et al., 1997) covering the albanian territory was carried out during september 1994 (epoch 1994.75). more recently, in august 2003 (epoch 2003.6) a restricted scalar a geomagnetic reference model for albania, southern italy and the ionian sea from 1990 to 2005 bejo duka (1), luis r. gaya-piqué (2) (3), angelo de santis (2) , salvatore bushati (4) , massimo chiappini (2) and guido dominici (2) (1) department of physics, faculty of natural sciences, university of tirana, albania (2) istituto nazionale di geofisica e vulcanologia, roma, italy (3) observatori de l’ebre, roquetes, spain (4) academy of sciences of albania, tirana, albania abstract taking advantage of the measurements undertaken during the albanian and italian magnetic repeat station networks since 1990, as well as of a selected set of ørsted satellite total field measurements, a magnetic reference model for the region comprising the albanian territory, the southern part of the italian peninsula, and the ionian sea is presented. the model, designed to model the components of the main geomagnetic field for epochs between 1990 and 2005, has been developed by means of spherical cap harmonic analysis applied to a cap of semiangle 8°, larger than that investigated to take into account the appropriate spatial wavelength content of the main geomagnetic field over the region. the goodness of the fit to the real data suggests that the model can be used as a reference model to reduce magnetic surveys developed in the area during the time of validity of the model. mailing address: dr. luis r. gaya-piqué, istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143 roma, italy; e-mail: pique@ingv.it 1610 bejo duka, luis r. gaya-piqué, angelo de santis, salvatore bushati, massimo chiappini and guido dominici survey (6 points at repeat stations in the albanian geomagnetic network and two new points) was carried out during the short visit of one of the authors (ads) to albania, funded by the italian foreign ministry. in this paper we take advantage of all these datasets, together with contemporary measurements from the italian repeat station network and with a selected total intensity data set from the ørsted satellite mission to develop a regional reference model for the considered area, i.e. albania, the southeastern part of the italian peninsula, and the nearby ionian sea. the model, developed by means of spherical cap harmonic analysis (scha) with polynomial time dependency, represents the values of the magnetic field better (with smaller scale) than global models such as the international geomagnetic reference field (igrf), and is, therefore, of utility in reducing magnetic surveys carried out in the same region. 2. data used scalar values at 1990.0 and 2003.6 from 31 and 8 albanian stations, respectively, together with vector and scalar measurements reduced to epoch 1995.0 from a total of ten albanian refig. 1a,b. a) location of the albanian and italian repeat stations from which vector and scalar (stars) and only scalar (squares) magnetic field data were used to develop the model; b) location of the satellite data used. peat stations were used to develop the reference model. details on most of these stations can be found in chiappini et al. (1997). in order to define a model not only for the albanian territory but also for the ionian sea and the southeastern part of italy, measurements at seven locations from the italian magnetic repeat station network (coticchia et al., 2001) at epochs 1990.0, 1995.0, and 2000.0 were also taken into account. a comparison of the real data with the values predicted by the igrf showed that two repeat stations, one italian (madonna di servigliano) and one albanian (rubik), had such high bias values with respect to the reference model, that we finally decided not to include them in the inversion to produce the model. figure 1a shows the spatial distribution of the italian and albanian stations used. to obtain proper temporal behaviour of the model and improve the stability of the inversion, considering the lack of vector data surveys at different epochs in the albanian territory, we decided to synthesize x, y, and z components at 1990.0 and 2000.0 for the same 9 albanian stations reducing the real vector measurements of 1995.0 to those two epochs, using the secular variation predicted by the italian geomagnetic reference model (itgrf; de santis et al., 2003), a b 1611 a geomagnetic reference model for albania, southern italy and the ionian sea from 1990 to 2005 a model that has been demonstrated to predict the temporal change of the internal magnetic field better than the igrf. a selected subset of total intensity magnetic field measurements from the ørsted satellite was also included to develop the model. the inclusion contributes to a homogeneous coverage of the studied region, especially in sea areas. moreover, the satellite data act as a boundary to develop a three-dimensional model, valid not only at sea level but also at any distance between the surface and the satellite height. a total of 30 scalar values measured between 1999.5 and 2002.5 were selected according to restricted criteria to reduce the presence of external magnetic fields in the data. in this way, only nightside data were selected (in order to reduce the effect of ionospheric fields) for periods in which the 3-hourly index kp was lower than 1+. the absolute value of the dst index was also limited to a maximum of 10 nt. the data chosen are distributed in a height range between 650 and 850 km above the earth’s surface. the spatial distribution of these satellite data is shown in fig. 1b. the ground and satellite data were weighted differently according to the reciprocal of the variance σ 2. this variance represents the contributions to the measurements of the crustal field σc and the measurement error σm. the ground data (vector and scalar) were assumed to have σc = 50 nt and σm = 10 nt, whereas no crustal contribution was assigned to the satellite data, and σm was set equal to that of ground data. 3. model the reference model was obtained applying spherical cap harmonic analysis (scha; haines, 1985). this choice represents an improvement with respect to the previously presented polynomial models for the region (chiappini et al., 1997), due to the fact that a scha model allows for the computation of the field component values through expressions that satisfy laplace’s equation. moreover, the radial variation of the magnetic field is implicitly described in the model, without need to assume a dipolar continuation of the field like as with polynomial models. although a similar approach was used by chiappini et al. (1999), the field was analysed for a fixed epoch only, without secular variation modelling. the solution of laplace’s equation for the magnetic potential due to internal sources over a spherical cap in spherical coordinates (r, θ, φ) can be written as 00 0 ( ) ( ) cos cos sin v a r a p g m h m t ( ) ( ) , , k n m n m m k k q m k q m q q 1k k $ $ $ = + i z z = + = = k q m !! ! a ]k g # (3.1) where the polynomial time dependency is included. the spherical cap harmonic coefficients g mk,q and h m k,q are those which determine the model. the number of coefficients depends on the maximum spatial and temporal indices of the expansion, k and q respectively. the associated legendre functions p mnk(m)(cosθ) that satisfy the boundary conditions (a zero of the potential or its derivative with respect to colatitude at the border of the cap; haines, 1985) have integer order m but a generally non-integer degree nk(m), where k is the index used to order the different roots for a given order m. legendre functions were computed using a procedure proposed by olver and smith (1983), since it seems to provide more reliable values than the original approach suggested by haines (1985) when the cap is rather small (thébault et al., 2002). the magnetic components are obtained as the derivatives of eq. (3.1) in spherical coordinates, since the potential is non-observable. the fact that vector measurements are combined with total field measurements introduces a nonlinearity in the equations involved to obtain the coefficients of the model. to avoid this problem, a first order taylor expansion of the total magnetic field intensity, as a square root function of the x, y, and z components, was used (haines and newitt, 1997). after many tests, the parameters that defined the best model in terms of fit to the input data and spatial and temporal behaviour corresponded to k = 2 and q = 1, covering the period between 1990.0 and 2005.0. the coefficients were obtained through a least squares regression procedure. haines and torta (1994) pro1612 bejo duka, luis r. gaya-piqué, angelo de santis, salvatore bushati, massimo chiappini and guido dominici table i. coefficients of the albanian geomagnetic reference model developed by scha. k m nk(m) gmk,0 h m k,0 g m k,1 h m k,1 0 0 .0000 45.735 −219.959 1 0 16.7209 6.221 7.429 1 1 12.7139 −.564 16.519 5.745 −40.357 2 0 26.9471 −2.824 −3.997 2 1 26.9471 −1.448 −2.800 −1.972 8.893 2 2 21.4163 −5.127 −4.654 −.315 .257 posed a statistical estimation to include coefficients in the regression or remove them according to an f level. in our case this f level was fixed to 0 due to the small number of coefficients involved in the regression, so no coefficients were rejected. the cap over which the model was originally defined had a semiangle of 3°. nevertheless some problems arose when such small cap was considered, as typically happens when scha is applied to small caps. figure 2 shows the behaviour of the magnetic field components at 1990.0 over the studied region. it is clear the presence of fictitious oscillations in the charts, especially for the y component. this is due to the lack of presence of lower harmonics in the spherical cap harmonic expansion (3.1): the smaller the dimensions of the cap, the higher the degree of the legendre functions (for a 3° cap, the minimum degree nk(m) involved was roughly 45). to avoid these probfig. 2. maps (in nt) for x (top left), y (top right), z (bottom left), and f (bottom right) elements for epoch 1990.0 at sea level obtained from a scha model developed on a 3° semiangle cap. clear large oscillations appear as symptom of lack of the proper spatial spectral content. 1613 a geomagnetic reference model for albania, southern italy and the ionian sea from 1990 to 2005 table ii. rms fit to input data of scha and igrf models (in nt). ground ørsted model rmsx rmsy rmsz rmsf rmsf igrf 50.0 73.6 49.2 57.3 9.9 scha 26.8 53.5 32.7 36.4 4.3 lems, we decided to enlarge the cap over which to impose the boundary conditions up to 8° semiangle, in order to cover the most significant harmonics (the minimum degree is approximately equal to 12 for this cap for a maximum spatial index k = 2). the final model has a total of 18 coefficients (table i). the igrf2000 model computed at 2000.0 was previously removed from the input data to avoid mathematical problems related to the combination of small and large values during the inversion procedure when determining the coefficients, and to be used as the first value in the iterative process to obtain the model. the value of the igrf2000 computed for a given location at 2000.0 has therefore to be added to the values provided by the coefficients in table i to obtain the final field value. 4. results and conclusions table ii shows the goodness of the scha model in terms of fit to the input data. the spherical cap model improves the fit to the observatory x component and to the satellite total field with respect to igrf by about 50%, whereas the improvement is around 30% for the y, z, and f ground elements. figure 3 shows the difference fig. 3. difference (in nt) for x (top left), y (top right), z (bottom left), and f (bottom right) elements between scha and igrf models at sea level for epoch 2000.0. fig. 4. maps (in nt) for x (top left), y (top right), z (bottom left), and f (bottom right) elements for epoch 2000.0 at sea level obtained from a scha model developed on an 8° semiangle cap. 1614 bejo duka, luis r. gaya-piqué, angelo de santis, salvatore bushati, massimo chiappini and guido dominici fig. 5. maps (in nt) for y component for epochs 1990.0 (top left), 1995.0 (top right), 2000.0 (bottom left), and 2005.0 (bottom right) at sea level obtained from the scha model developed on an 8° semiangle cap. fig. 6. maps for the secular variation of x (top left), y (top right), z (bottom left), and f (bottom right) magnetic elements for epoch 1995.0 at sea level obtained from the scha model. contour lines are shown at 1 nt/year interval. between the scha and the igrf models for epoch 2000.0. this difference is between 20 and − 60 nt for all the components in the area from which data were available, being a bit higher as we move towards areas without data. once the validity of the model has been demonstrated, we can recommend its use as reference model over the investigated region, with particular application to the production of regional charts as those shown in fig. 4 for epoch 2000.0 at sea level, to reduce magnetic surveys developed in the area, or to study phenomena related to the earth’s internal magnetic field. for example, the temporal evolution of the isolines in the magnetic charts for the y component shows a clear westward drift of the east component of the magnetic field (fig. 5). the value of the drift deduced from these charts is about 0.6° per year, which is comparable to the values observed elsewhere in the atlantic hemisphere (barraclough and malin, 1999). other kinds of studies can be developed through the inspection of the secular variation of the magnetic field. the secular variation for all the magnetic elements for epoch 1995.0 (fig. 6), obtained from the scha model as the field differences between epochs 1995.5 and 1994.5, confirms that the region under study presents low values for the temporal variation of the geomagnetic field for this period (see for example gubbins, 1990). all these results will be confirmed when new real vector data are available for the region. it is planned to make a vector survey over the italian region as well as in albania during the period 2004-2005, and the model presented in this paper will be a great help to know a priori the values one can expect to find during the surveys. 1615 a geomagnetic reference model for albania, southern italy and the ionian sea from 1990 to 2005 acknowledgements part of this work was funded by the italian foreign ministry. all the figures contained in this paper were obtained using the generic mapping tools subroutines (wessel and smith, 1991). the satellite data selection was done with the collaboration of nils olsen, from danish space research institute. we are grateful for the support of the ørsted project office and the ørsted science data centre at the danish meteorological institute. references barraclough, d.r. and s.r.c. malin (1999): a fast moving feature of westward drift, ann. geofis., 42 (1), 21-26. chiappini, m., o. battelli, s. bushati, g. dominici, b. duka and a. meloni (1997): the albanian geomagnetic repeat station network at 1994.75, j. geomagn. geoelectr., 49, 701-708. chiappini, m., a. de santis, g. dominici and j.m. torta (1999): a normal reference field for the ionian sea area, phys. chem. earth a, 24 (5), 433-438. coticchia, a., a. de santis, a. di ponzio, g. dominici, a. meloni, m. pierozzi and m. sperti (2001): italian magnetic network and geomagnetic field maps of italy at year 2000.0, boll. geod. sci. affini, 4, 261-291. de santis, a., l.r. gaya-piqué, g. dominici, a. meloni, j.m. torta and r. tozzi (2003): italian geomagnetic reference field (itgrf): update for 2000 and secular variation model up to 2005 by autoregressive forecasting, ann. geophysics, 46 (3), 491-500. duka, b. and s. bushati (1991): the normal geomagnetic field and the igrf over albania, boll. geofis. teor. appl., xxxiii (130/131), 129-134. gubbins, d. (1990): geomagnetism: the next millennium, palaeogeogr. palaeoclimatol. palaeoecol., global planet. change sec., 89, 255-262. haines, g.v. (1985): spherical cap harmonic analysis, j. geophys. res., 90 (b3), 2583-2591. haines, g.v. and l.r. newitt (1997): the canadian geomagnetic reference field 1995, j. geomagn. geoelectr., 49, 317-336. haines, g.v. and j.m. torta (1994): determination of equivalent currents sources from spherical cap harmonic models of geomagnetic field variations, geophys. j. int., 118, 499-514. olver, f.w.j. and j.m. smith (1983): associated legendre functions on the cut, j. computat. phys., 51, 502-518. thébault, e., j.j. schott and m. mandea (2002): geomagnetic field modelling on small spherical caps, in proceedings of the egs xxvii general assembly, nice, france, egs02-a-04673, se6.07-1we2a-003. wessel, p. and w.h.f. smith (1991): free software helps map and display data, eos, trans. am. geophys. un., 72 (41), 441, 445-446. (received october 27, 2003; accepted march 30, 2004) latif 17.pdf annals of geophysics, vol. 46, n. 1, february 2003 99 tropical pacific influences on the north atlantic thermohaline circulation mojib latif max-planck-institut für meteorologie, hamburg, germany abstract most global climate models simulate a weakening of the north atlantic thermohaline circulation (thc) in response to enhanced greenhouse warming. both surface warming and freshening in high latitudes, the so-called sinking region, contribute to the weakening of the thc. some models simulate even a complete breakdown of the thc at sufficiently strong forcing. here results from a state-of-the-art global climate model are presented that does not simulate a weakening of the thc in response to greenhouse warming. large-scale air-sea interactions in the tropics, similar to those operating during present-day el niños, lead to anomalously high salinities in the tropical atlantic. these are advected into the sinking region, thereby increasing the surface density and compensating the effects of the local warming and freshening. the results of the model study are corroborated by the analysis of observations. 1. introduction the atlantic thermohaline circulation (thc) is an important component of the global climate system (broecker, 1991). it transports about 1 pw of heat poleward in the north atlantic, thereby warming western europe. the thc is forced partly by convection at high latitudes, which causes dense surface waters to sink to deeper ocean layers, forming the so-called north atlantic deep water (nadw). strong and rapid changes in the intensity of the nadw formation have been reported from paleoclimatic records (broecker et al., 1985), and it is well established that such changes exert a strong impact on the climate over large land areas (e.g., manabe and stouffer, 1995, 1999; schiller et al., 1997). several papers have suggested that the thermohaline circulation may weaken in response to greenhouse warming (e.g., mikolajewicz et al., 1990; manabe et al., 1991; stocker and wright, 1991; cubasch et al., 1992; manabe and stouffer, 1994; rahmstorf, 1997, 1999; wood et al., 1999). here, we investigate the sensitivity of the thc to greenhouse warming using a global climate model that has been applied in various climate variability and response studies (roeckner et al., 1996; bacher et al., 1997; oberhuber et al., 1998; timmermann et al., 1999; roeckner et al., 1999; christoph et al., 1998; ulbrich and christoph, 1999). it is shown that the tropical feedbacks associated with an increased el niño frequency can stabilise the thc (latif et al., 2000). 2. model the model (echam4/opyc) is flux-corrected (using annual mean corrections) with respect to heat and freshwater. the application of flux correction reflects serious errors in the individual model components. the effects of flux correction on the response characteristics of a model are largely unknown, but it has been mailing address: dr. mojib latif, max-planck-institut für meteorologie, bundesstr. 55, d-20146 hamburg, germany; email: latif@dkrz.de key words thermohaline circulation – air-sea interactions – climate variability 100 mojib latif shown that flux correction may cause misleading results (e.g., neelin and dijkstra, 1995). it should be noted, however, that many studies addressing the stability of the thc have been conducted with flux-corrected models (e.g., manabe and stouffer, 1994). our model employs a horizontal resolution over most of the globe of 2.8° × 2.8°. a special feature of the model is the higher meridional resolution of 0.5° used in the tropical oceans, which enables a realistic simulation of the el niño/southern oscillation (enso) phenomenon (roeckner et al., 1996; bacher et al., 1997; oberhuber et al., 1998), the strongest natural interannual climate fluctuation. we performed two integrations. the first experiment is a 240year long control integration with fixed presentday concentrations of greenhouse gases. in the second experiment the model was forced by increased greenhouse gas concentrations, giving approximately the historical increase in radiative forcing from 1860 to 1990 (roeckner et al., 1999), and subsequently (up to 2100) by increases according to ipcc scenario is92a (ipcc, 1992). the effects of anthropogenic sulfate emissions are not included in the simulation. 3. results the control run simulates the thc in the north atlantic reasonably well (zhang et al., 1998), with a maximum overturning of about 23 sv at a depth of about 2000 m and an nadw outflow at 30°s of about 15 sv (1 sv = 106m3/s) at a depth of about 1500 m. however, the model fails to simulate the inflow of the very dense antarctic bottom water (aabw) into the north atlantic. the thc is relatively stable during the control integration, but exhibits some superimposed interdecadal variability (fig. 1). the range of this internal thc variability is consistent with that simulated by other global climate models (delworth et al., 1993; timmermann et al., 1998). in the transient greenhouse warming simulation the thc remains also stable, with a slight decrease during the first half and a slight increase during the second half. thus, our model predicts a stable thc rather than a weakening thc as simulated by most other global models (rahmstorf, 1999). the physics responsible for the stabilisation of the thc in our model is related to tropical air-sea interactions. the regional distribution of the simulated sea surface temperature (sst) trend is characterised by warming in most ocean areas, with some cooling tendency in certain regions around antarctica (not shown, see also roeckner et al., 1999). of particular importance is the el niño-like warming trend in the eastern equatorial pacific. as has been described in timmermann et al. (1999), this warming results from air-sea interactions similar to those producing present-day el niños and has strong impacts on the entire tropical climate system. the model simulates strongly enhanced precipitation over the central equatorial pacific (fig. 2), a feature observed during present-day el niños (philander, 1990; ropelewski et al., 1987). this leads to adiabatic warming and drying through anomalous subsidence over northeast brazil and the tropical atlantic, thereby reducing precipitation and river runoff from the amazon catchment area and increasing evaporation from the tropical atlantic. the model simulates an additional freshwater export from the atlantic to the pacific of about 0.015 sv/decade, in comparison to only 0.005 sv/decade in another maxfig. 1. time series of the maximum overturning (sv) in the north atlantic for the control integration with present-day greenhouse gas concentrations (black line) and the transient greenhouse warming simulation (red line). 101 tropical pacific influences on the north atlantic thermohaline circulation planck-institute coupled model (echam3/ lsg) simulating a weakening of the thc (mikolajewicz and voss, 2000) as discussed by rahmstorf (1999: his fig. 2). such an enhanced freshwater export is also observed during present-day el niños (schmittner et al., 2000). the reduced freshwater flux induces anomalously high salinities in the tropical atlantic ocean (fig. 3b). we averaged the freshwater fluxes over two regions (30°s-45°n and 45°n90°n) and display their time evolutions in fig. 4a. the freshwater input to the atlantic towards the end of the integration is strongly reduced by about 0.3 sv in the tropical region, while it is enhanced by about 0.1 sv in the middle and high latitudes. thus, the changes in the freshwater flux will tend to enhance the salinity in the tropics and to reduce the salinity in the middle and higher latitudes, with changes in the tropics being considerably stronger in the tropics. we timeintegrated the anomalous freshwater flux forcing into the atlantic and arctic poleward of 30°s. this quantity has been compared to the anomalous salt content of the upper 375 m at the end of the integration. the actual salt gain of the upper atlantic amounts to about two third of the total implied freshwater induced salt gain. we do not expect a perfect match, since the integration domain is not closed. however, this computation shows that the salinity anomalies can easily be explained by the changes in atmospheric moisture transport. the poleward transport of the salt anomaly and the mixing within the subtropical gyre result in increased surface salinity over the entire north atlantic ocean. the poleward propagation of the anomalously high salinities is visualised by a hovmoeller diagram showing the temporal evolution of the salinity anomalies averaged over the upper 375 m in a meridional section along 45°w (fig. 4b). the salinity transport outweighs the counteracting local freshwater influx through enhanced precipitation (figs. 2 and 4a) and warming in the high latitudes of the north atlantic fig. 2. centennial linear trend in the freshwater flux (p – e + r, precipitation minus evaporation plus river runoff ) [mm/(d*decade)] obtained from the transient greenhouse warming simulation and computed over the period 2000-2100. note the freshening in the equatorial pacific associated with the increased el niño frequency and the strongly reduced freshwater influx over the tropical atlantic. 102 mojib latif (fig. 3a), which are the principal feedbacks in models predicting a reduced thc. we computed separately the salt and temperature contributions in the change of the surface density, which is the crucial quantity for deep water formation. the linear expansion coefficients for temperature and salinity for a temperature of 5 °c and salinity of 35 psu, which are characteristic values for the sinking region, amount to – 0.12 (kg /m3)/k and 0.79 (kg/m3)/psu, respectively. given a sst change of about 0.3°c/ decade and a salinity change of about 0.05 psu (fig. 3a,b) yields comparable density changes of about 0.4 kg/(m3* decade). a calculation using the complete equation of state yields a similar result, with almost no density change in the regions of strongest deep water formation (fig. 3c). thus, in our transient greenhouse warming simulation the poleward salt transport is an important feedback process, in addition to the freshening through increased precipitation and warming in the high latitudes, and the net outcome is a stable rather than a weakening thc. are the changes in the hydrological cycle large enough to have a significant effect on the thc? typical threshold values for a complete collapse of the present-day thc are in the range of 0.1-0.4 sv for anomalous freshwater input into the north atlantic (e.g., stocker and wright, 1991; rahmstorf, 1995, schiller et al., 1997). given a change of the order of – 0.3 sv in our simulation, it is not surprising that this effect is large enough to balance the locally induced weakening of the thc. a b c fig. 3a-c. centennial linear trends of selected quantities in the north atlantic obtained from the transient greenhouse warming simulation and computed over the period 2000-2100. a) trend in sst (°c/decade); b) trend in sea surface salinity (psu/ decade); c) trend in surface density [kg/ (m3*decade)]. the depth of the convection (m) simulated for the month of february in the control integration is shown as contours (contour interval is 250 m). the two main convection regions in the labrador and gin (greenland-iceland-norwegian) seas are simulated realistically by the model. no significant change in the density is simulated in these regions. 103 tropical pacific influences on the north atlantic thermohaline circulation fig. 4a,b. a) time series of anomalous freshwater input (sv) into the atlantic in the region 30°s-45°n (black curve) and into the mid and high latitudes (45°n-90°n, red curve). b) temporal evolution of the salinity anomalies ( psu) averaged over the upper 375 m along 45°w. the northward propagation of the salinity anomalies from the subtropics to higher latitudes can be clearly seen by the tilt of the contours. the anomalies are calculated by subtracting the initial values. a b 104 mojib latif 4. further modeling and observational evidence next, simulations with an atmospheric general circulation model (agcm) and some observations are analysed (latif, 2001). unfortunately, long time series of the freshwater flux over the atlantic are not available from observations. instead agcm simulations with observed ssts prescribed globally for several decades were used to study further the connections between the tropical pacific and tropical atlantic oceans. the tropical atmosphere is highly predictable (e.g., lau, 1985), and it is believed that such simulations provide useful insights about the nature of the response of the tropical atmosphere to tropical pacific sst anomalies associated with enso-type multidecadal variability. the agcm used here is echam4 (roeckner et al., 1996) with t42 resolution (2.8° × 2.8°), a model that has been used in many climate applications (see, e.g., roeckner et al., 1999). in particular, echam4 served as the atmospheric component of the coupled model echam4/opyc described above. an ensemble of three agcm integrations was performed for the period 1903-1994, and the ensemble mean is shown here. the ssts used to drive the model are the gisst ssts (parker et al., 1995). the kaplan (1997) data set is used in the investigation of the ssts themselves shown below, since it is the longer data set. however, since the response characteristics of the model do not depend on the forcing ssts, the use of the different sst data sets in this study is acceptable. the atmosphere model simulates an out-ofphase relationship between decadal fluctuations in eastern tropical pacific (niño-3) sst and the freshwater flux over the tropical atlantic (10°s30°n) as shown in fig. 5, confirming the observational results of schmittner et al. (2000). the correlation between the two time series amounts to – 0.75. furthermore, both the tropical pacific sst and the simulated freshwater flux exhibit rather strong trends during the last 50 years: while the tropical pacific sst is slowly increasing by about 0.5 °c, the freshwater flux over the tropical atlantic is slowly decreasing by about 0.06 sv (106m3/s) (fig. 5). this behaviour is consistent with that found in the greenhouse warming simulation described above and supports the picture that there exists a potential «atmospheric bridge», by which the tropical pacific and atlantic oceans can interact with each other. further evidence for the existence of the atmospheric bridge comes from fig. 5. time series of the niño-3 sst anomaly and the anomalous tropical atlantic freshwater flux as simulated by the echam4 model averaged over the region 10°s-30°n, which is the region with the most consistent response over the atlantic. both time series vary out of phase and exhibit strong trends during the last 50 years. both time series were filtered with a 11year running mean. fig. 6. time series of the salinity observed at bermuda (33°n, 65°w). please note the strong increase of the salinity during the most recent decades. 105 tropical pacific influences on the north atlantic thermohaline circulation fig. 7a,b. a) spatial distribution of correlation coefficients between the niño-3 sst anomaly time series and the global sst anomalies at zero lag. b) spatial distribution of correlation coefficients between the niño-3 sst anomaly time series and the global sst anomalies at lag 30 years. the anomaly structure in (b) is reminiscent of variations in the thc, indicating that variations in the thc follow variations in tropical pacific sst with a time lag of 30 years. the data were low-pass filtered with a 11-year running mean prior to the correlation analyses. a b 106 mojib latif schiller et al., 1997). anomalously warm temperatures are found in the northern part and anomalously cold temperatures in the southern part of the atlantic ocean. thus, the results indicate that interdecadal changes in tropical pacific sst are followed by basin-wide changes in atlantic sst, and that periods of high (low) ssts in the eastern tropical pacific are followed by a strong (weak) thc in the atlantic. the statistical significance of the results was tested using a t -test and assuming 10 degrees of freedom, and correlations above about 0.5 are significant at the 95% level. more important than the level of the significance, however, is the fact that physically motivated sst anomaly patterns result from the two correlation analyses. 5. concluding remarks the tropical air-sea interactions that stabilise the thermohaline circulation in our model simulation are not adequately represented in most global climate models applied hitherto to the problem of anthropogenic climate change, mainly because of their too coarse resolution in the tropics. this feedback needs to be studied more carefully in relation to the destabilising feedbacks considered in previous investigations. at this stage of analysis we can conclude only that the response of the thc to enhanced greenhouse warming is still an open question. the uncertainty arises from model shortcomings in the representation of key physical processes, such as the hydrological cycle. our results may be important also in view of multi-decadal and paleoclimatic variations, because they indicate that variations in the tropics may synchronise changes globally. the hypothesis of the tropical influence on the thc is very difficult to verify by observations. there is, however, some observational evidence that multi-decadal variations in the tropical pacific sst feed back on the north atlantic thc. acknowledgements i would like to thank u. mikolajewicz, j. oberhuber and e. roeckner. this work was supported by the european union’s sintex project. salinitiy observations taken at bermuda (33°n, 65°w). a strong increase in the surface salinity is observed during the last few decades (fig. 6), which is expected given the strong increase in tropical pacific sst and the decrease of tropical atlantic (model) freshwater flux during the recent decades. these trends may continue in response to greenhouse warming as hypothesised above. in the next step, it is investigated whether the thc in the atlantic responds to the variations in the freshwater flux. this can be done only indirectly by using ssts, since the required direct observations of the freshwater flux, surface salinity, and the thc do not exist. ocean model and coupled model studies have shown that variations in the thc are associated with changes in the poleward ocean heat transport and an interhemispheric sst dipole (see, e.g., manabe and stouffer, 1999 and references therein). thus, this characteristic sst anomaly pattern can be used as a «fingerprint» to identify variations in the real thc. a correlation analysis was conducted using the kaplan sst data set. first, the zero-lag correlation of the low-pass filtered tropical pacific (niño-3) sst anomaly time series with the global sst anomaly field has been computed. the resulting correlation pattern (fig. 7a) shows the well known picture in the tropical pacific, with an el niño-like positive signal in the eastern and central equatorial pacific and negative anomalies in the mid-latitudes of both hemispheres. a teleconnection to the indian ocean can be seen also, a feature known from present-day el niño events. strong negative correlations are found in the north atlantic near greenland, which is consistent with the recent increase of the north atlantic oscillation. based on the arguments described above, the time it will take to develop sufficiently strong salinity anomalies and to transport those poleward into the so called «sinking region» in the high latitudes is of the order of a few decades. therefore, a time lag of 30 years has been introduced in the correlation 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(2) universidade federal de santa catarina, departamento de geociências, computageo – laboratório de geofísica computacional, campus universitário − trindade, florianópolis, sc, brasil. (3) universidade de são paulo, instituto de astronomia geofísica e ciências atmosféricas, departamento de geofísica, cidade universitária, são paulo, brasil. 1. introduction large amounts of data in the form of irregularly sampled time series have emerged from several different areas, such as astronomy, meteorology, biology, oceanography and cyclostratigraphy [baldysz et al., 2016; bowdalo et al., 2016; dawidowicz and krzan, 2016; jalón-rojas et al., 2016; péron et al., 2016; article history received september 11, 2018; accepted april 2, 2019. subject classification: paleoceanography and paleoclimatology, paleoclimate; inverse methods; statistical analysis. abstract irregularly sampled time series are common in several different areas, such as astronomy, meteorology, biology, oceanography, cyclostratigraphy, and others. the periodogram is a primary tool to extract meaningful information from irregularly spaced and noisy time series. it is an element of decision theory, meaning the periodogram usually transforms the data, and its ordinates are subsequently submitted to a statistical test compared to a population originating from a known stochastic model (white gaussian noise). if some ordinate f0 (usually a local maximum, a peak) fails in this test, we declare that it is a ‘periodicity’ at a frequency f0. besides its full usage, this method until now suffer from numerous theoretical difficulties in adapting to real case situations and shows lack of usefulness for very poorly sampled and high noise cases. all of it implies low usefulness for applying in most sedimentary sequences at our disposal nowadays. the lstperiod is an application, written in matlab, conceived to perform spectral analysis of multiple irregularly sampled time series. it combines information from lomb-scargle periodogram estimates over different time series sampling the same phenomenon, enabling the recovering of signals from very poorly sampled and noisy time series. the software comprises a set of four graphical user interfaces (guis) that allow the user to: 1) have broad choices of the frequency-domain range and density for spectral estimation; 2) select possible spectral features (i.e., pick “t”) for testing as a model [𝐴∗sin( 𝑡–𝜃)] through the visualization of several goodnessof-fit statistics; 3) visualize the fitting residuals in the time domain, for each time series, for the chosen sinusoidal model. these tools help the user to identify and analyze any suspected feature in the estimated spectra through its related linear system responses. all estimated parameter can be saved on worksheets and the visualizations in several different figure formats. we illustrate the use of the software with a set of ocean drilling program (odp) data series that show long-period milankovitch-related spectral features and demonstrate its performance using synthetic time series. 2𝜋 � mortier and cameron, 2017]. in all these fields, the time series are very often the product of irregular sampling. for example, in astronomy, the time series are regularly sampled but usually present inevitable gaps due to interruptions on observations, due to daylight time or bad weather conditions. additionally, in cyclostratigraphy, it is not possible to control the sampling times at all, leaving us with a set of averaged data points defined by the deposition rate. these issues pose a significant challenge to spectral analysis since it prevents the building of an equivalent set of independent points in the frequency domain similar to that obtained from applying a discrete fourier transform (dft) to a regularly spaced time series. therefore, over the last years, many efforts have been devoted to understanding the advantages, as well as limitations, of the methods dedicated to extracting meaningful spectral information from irregularly sampled time series, and vast literature on the subject has been produced [e.g., schwarzenberg-czerny, 1989; babu & stoica, 2010; baluev, 2013; vio et al., 2013; munteanu et al., 2016; jalón-rojas et al., 2016; vanderplas, 2018]. deep-sea sedimentary columns have revealed valuable records of climatic fluctuations, and they are currently abundant due to global ocean drilling projects such as the dsdp, odp, and iodp. a great deal of literature has been devoted to quaternary cyclostratigraphy [see hinnov, 2013 for a review], and its recording of the climate response to the external forcing (i.e., the milankovitch theory of orbital forcing) has become a leading issue [e.g., berger, 2013]. global analyses and comparisons of sedimentary systems at different latitudes and in different environments aim to evaluate the sensitivities of sedimentary records (climatic proxies) to orbital forcing over time [e.g., lisiecki and raymo, 2007]; however, as stressed by berger [2013], there is not a simple link between orbital forcing and climate response. also, climatic components, as expressed in sedimentary records, are not usually stationary, showing regional modulated expressiveness, and correlation with the sampling process itself (sedimentary deposition rate). there is currently a multitude of different methods available to analyze time series – both in time and frequency domains. among them, fourier methods accomplish a decomposition of the time series in an orthogonal basis (very frequently sines and cosines). especially the so-called discrete fourier transform (dft) could be highlighted mainly for being numerically fast (through fast fourier transform fft algorithm), physically insightful (simple physical interpretation for its decomposition terms), and have straightforward extensions to non-equally sampled data (as in the periodogram, classical or the least squares). considering the inevitably irregularly sampling of deep-sea records (as for records from many other sources), the usual methods of spectral decomposition based on the dft, as the fast fourier transform (fft) algorithm, are not directly applicable. therefore, the least squares periodogram, also called the lomb-scargle (ls) periodogram [lomb, 1976; scargle, 1982; vanderplas, 2018] appears as a natural extension of fft methods. the ls periodogram and its variants, which add statistical significance tests for spectral peaks [stoica et al., 2009; vio et al., 2010; baluev et al., 2013; vio et al., 2013; mortier et al., 2015], have been the tool of choice for these cases. the lack of orthogonality requires several strong assumptions for the time series (i.e., regarding the high total length compared to signal period, high average sampling rate, and noise frequency domain characterization) to conduct these peak significance tests. also, the recovery of spectral information from time series data also introduces artifacts such as aliasing (power leakage to distant frequencies), spectral leakage (power leakage to nearby frequencies), mixing with red noise, mixing with sampling noise, and other distortions of the original spectral content. additional issues such as the lack of a sufficient frequency resolution and the presence of statistical biases remain unsolved [e.g., zechmeister and kürster, 2009; mortier et al., 2015]. moreover, as stressed by hernandez [1999], some procedures, including de-trending, filtering or pre-whitening, routinely applied to data subject to spectral analysis may modify the signal power and displace the original spectral peaks. besides that, real-world dynamical systems do not always exhibit sharp lines since they generally evolve. therefore, stationary frequency components for all times are not very common on well-sampled natural time series, especially in cyclostratigraphy. therefore, new methods of analysis are required; particularly those devoted to properly analyzing and displaying these signals, as well as to access uncertainties on the estimated spectral content from irregularly sampled time series. caminha-maciel and ernesto [2013] presented a method to address these issues through a bayesian combination of independent experimental information derived from multiple time series as a stacking procedure applied within the frequency domain aimed to smooth the periodogram and to enhance the signal. we developed this method to study weak signals in caminha-maciel et al. 2 3 lstperiod software short and arbitrarily sampled time series with spectral distortions caused by noise and sampling deficiencies. we tested the method numerically for synthetic gaussian noisy and poorly sampled time series. this estimation process is based on the ls periodogram, but instead of decision theory method (statistical significance test), it incorporates the inverse problem approach envisioned by tarantola and valette [1982], tarantola and mosegaard [2000] and tarantola [2006] for a combination of experimental and theoretical information. for this reason, we will designate it as the lomb-scargle-tarantola (lst) periodogram a process not based on optimized quantities such as averages or standard deviations (i.e., confidence limits) but in smoothing the periodogram estimates through a combination of information. indeed, in this approach, we assume that we can express the information for the estimated parameter using freely normalizable probability functions called "state of information" functions, defined over a finite set of domain points. then, the solution of the inverse problem can be stated as the result of the logical operations or and and applied among these independent state of information functions. the results of the or and and operators can be shortly explained in the following way: the or operator represents the generalized creation of histograms and is related to the arithmetic average of the curves; while the and operator represents the generalized concept of conditional probabilities and suggests an initial null probability distribution conditioned to several independent experimental probability distributions. the and curve enhances the common features between each of the individual spectra primarily and represents the set of 'surviving models', and it is assumed to be the main result of the inversion process. here we summarise the main steps of the algorithm: 1) for each time series, define the minimal time interval between two consecutive points as 𝑡(𝑖)𝑚𝑖𝑛, and define 𝑡(𝑖)𝑚𝑎𝑥 as the length of each time series. 2) the nyquist frequency is set as 𝑓𝑁𝑦𝑞𝑢𝑖𝑠𝑡 = ½*max𝑖{𝑡 (𝑖) 𝑚𝑖𝑛}, which defines the minimum periodicity 𝑇𝑚𝑖𝑛=1/𝑓 ������� (or the highest frequency) tested in the analysis. the maximum permitted periodicity (or lowest frequency) is set as 𝑇𝑚𝑎𝑥 = 1.5 ∗ min{𝑡(𝑖)𝑚𝑎𝑥}. 3) the program calculates the lomb-scargle periodogram for each series between 𝑇𝑚𝑖𝑛 and 𝑇𝑚𝑎𝑥 using a pre-selected first grid density n0 over the interval. we set the initial n0 as 100, but the user can later change this number to an adequate number. 4) each periodogram is normalized by its total power. the resulting state of information functions have total areas equal to one and retain the estimated signal/noise ratio (s/n) for each time series, i.e., it maintain 𝑃�𝑓1�/𝑃�𝑓2� for any grid points 𝑓1 and 𝑓2, where 𝑃�𝑓� stands for the power of 𝑓. 5) combine these state of information functions with the or and and operators [see tarantola and mosegaard, 2000] and then conduct the normalization step again for a total area (i.e., power) equals one. in this paper, we present the software lstperiod for spectral analysis according to the caminha-maciel and ernesto [2013] method. the software also provides complementary information to help during the interpretation process including a set of four graphical user interfaces (guis) allowing for broad frequencydomain visualization of periodogram estimates and selection of spectral features regardless of any interpretation of its dynamical origin (i.e., signal or noise). we also illustrate the use of the program with an application to a set of odp data series and some synthetic data series. the remainder of this paper is organized as follows: section 2 contains the essential background to introduce the lst periodogram; section 3 contains a description of the lstperiod software with its main features e mode of operation; section 4 shows the application to a real set of cyclostratigraphic series with very well known results, and its performance when applied to synthetic time series; and section 5 presents the final considerations. 2. theoretical background this section contains a brief review of the main mathematical background needed to introduce the lstperiod, briefly discuss the lomb-scargle periodogram and the tarantola’s combination of information approach for inverse problems. we also present the new idea of periodogram analysis: linear fitting on multiple time series of a known (pre-chosen) physical model followed by statistical diagnosis metrics (goodness-of-fit statistics). the fourier transform (e.g., dft) is a function that gives a sequence of n0/2 (different) complex coefficients from a time series x(t), with n0 equally time spaced points. in this case, the modulus of those coefficients makes a function known as the “spectrum” (its estimator is the periodogram). this function gives information about the splitting of energy among frequencies (or 3 caminha-maciel et al. 4 periods) and has central physical meaning in the study of dynamical systems. these frequencies are spaced by an increment of 1/t, where t stands for the total time covered by the series (this is called “resolution” of the spectrum). moreover, another important parameter is the nyquist frequency, which is the highest frequency about what there is, theoretically, any recoverable information on the time series. the nyquist frequency is defined as 1/(2*δt), where δt stands for the sampling interval. one important result in this area, the so-called shannon-nyquist theorem states that if the time series is equally spaced, and it does not contain information above the nyquist frequency, then all the information in the time series is in its dft. when the series are unevenly-spaced, this theorem no longer applies. 2.1 periodogram analysis since a very long time periodograms have been used to make estimates of times series spectral distribution and to search for hidden periodicities in experimental data. at first, a formulation similar to the squared absolute values of dft was used, the so-called “classical periodogram”. this periodogram shows very noisy results even for a slightly noisy time series. one of the reasons for this is the lack of orthogonality on fourier basis for irregular spaced data. after the lomb and scargle’s works [lomb, 1976; scargle, 1982; vio et al., 2013], a new form of periodogram takes place, the now-called “lomb-scargle periodogram”. scargle introduced a shift parameter τ, calculated for each frequency, that plays the role of minimizing the inter-cross terms in the fourier basis, ∑𝑗𝑠𝑖𝑛(𝜔𝑡𝑗)𝑐𝑜𝑠(𝜔𝑡𝑗), improving overall separation between the basis. this periodogram, which was proved equivalent to a least square fitting of a sinusoidal signal, shows much smoother results: (1) where the parameter τ is given by: (2) scargle also introduced a statistical test for a periodicity on the ordinates of a periodogram (a decision theory). this theory suggests testing the maximum ordinate (also called fisher criteria) of the periodogram against the hypothesis of an entire stochastic generated population of ordinates. what means to test if a white gaussian noise process, which implies an exponential distribution of the ordinates, could generate this maximum ordinate. hence the probability α that at least one of p(ω) would exceed a certain level is given by: (3) throughout this equation, it is possible to determine a “false alarm level” (or detection threshold) given by: 𝐿𝑡ℎ= – 𝜎𝑛2 𝑙𝑛 �1 – (1– α)1/𝑁* � (4) a critical aspect of this statistical test is that it can be applied only over independent ordinates, what bring limitations to poorly sampled time series (with a small number of points): a) the maximum number of independent points ideally allowable over the frequency interval would be n/2 – what may not coincide with the interest signals frequencies; b) since there is no orthogonality on the basis, there is no way to re-establish independence between periodogram ordinates. a large part of the subsequent literature on the subject is dedicated to finding conditions to establish independence between the periodogram ordinates. most of this literature indicates conditions of the statistical regularity of the time series – a long time length of the series compared to periodicities of interest, uniformly random sampling, stationary periodic components and stochastic regularity of the noise (a closed model for it). however, rarely, in real data cases, we can verify these premises beforehand. 2.2 state of information functions inverse problems is a set of techniques developed in geophysics to deal with broad classes of statistical problems related to incomplete information – incomplete theoretical formulation or incomplete data (ill-posed problems). albert tarantola’s theory of inverse problems also called “combination of information” has some advantages such as: a) simplicity being physically intuitive, b) it allows incorporating in a natural way multimodal (or weakly defined) distributions for data as well as for the solution. this technique has, however, the drawback of being computationally costly, although not so acute for lowdimensional problems. also, this computational issue is continuously waived as modern computers improve. τ (𝜔)= 𝑎𝑟𝑐 𝑡𝑎𝑛 � � 1 2𝜔 ∑𝑗𝑠𝑖𝑛2𝜔𝑡𝑗� ∑𝑗𝑐𝑜𝑠2𝜔𝑡𝑗 𝛼 = 1 – �1 – 𝑒 � 𝑁*_ 𝑃𝑘� 𝜎𝑛2 𝑃(𝜔)= � + � [∑�𝑋�𝑐𝑜𝑠𝜔(𝑡� –τ)]² ∑�𝑐𝑜𝑠²𝜔(𝑡� –τ) [∑�𝑋�𝑠𝑖𝑛𝜔(𝑡� –τ)]² ∑�𝑠𝑖𝑛²𝜔(𝑡� –τ) 1 2 5 lstperiod software here we summarise this approach (tarantola and valette, 1982; tarantola and mosegaard, 2007; tarantola, 2009): a) replace analytical theories with optimized quantities like as means and variances, for operations between probability distributions over the parameter space. these probability distributions could be freely normalizable and multi-modal, and should express our actual knowledge on the variable; 2) operate these probability distributions (also called “state of information functions”) with the two logical operands “and” and “or”. the operator or can be seen as a generalization of doing histograms and is defined as an arithmetic average of the individual distributions ∑𝑖𝑃𝑖(𝑓). the operator and represents the generalization of the idea of conditional probabilities and is defined as , where μ is the null information function – depending on the geometry of the problem. the null information function 𝜇 is usually assumed as constant over the whole interval. in some cases, for example in physical problems involving the variable frequency, it has been shown that it is better if it takes another functional form as 1/𝑓. in our case however, these frequencies (or periodicities) are only labels for a wide class of eigen-functions (sines and cosines) and we can use a constant function as null information distribution. 2.3 lst periodogram due to shannon’s theorem, we already know that an irregularly sampled time series may not contain complete information on the frequency components of the original series. any attempt to recover these components should apply some additional a priori information – explicitly or implicitly. here we consider all the information that could potentially be resolved by each time series. then we choose searching bandwidths that start with a period which is one and a half times the length of the shortest time series, up to the shortest period (or maximum frequency, the nyquist frequency) taken as twice the shortest distance between two consecutive points, considering all series. the basic idea of lstperiod is, for each frequency, to define for each time series a linear factor proportional to the estimated power in that frequency. to do this, we need to find invariant statistics that capture the s/n (signal to noise ratio) without being much sensitive to each series total variance – which depends on noise variance, length of the series, sampling pattern, among other aspects. the solution found was to normalize each periodogram by the total power (area under the periodogram), i.e., all periodograms exhibits total variance equals one. this procedure is equivalent to stretching the x variable in the time domain (i.e., x(t) versus t) and does not alter p(fi)/p(fj), with i ≠ j. after this normalization, these distributions are operated with the or and and logical operators described above, and so normalized again. these two distributions represent the stacking of all experimental data periodograms and incorporate within all available information on s/n. the density of calculation is initially set as 100 points on the interval, but it is allowable to change depending on the user’s needs and computational hardware. 2.3.1 linear systems and goodness-of-fit statistics any point in these curves (including the original periodograms) can be set as a possible candidate for a sinusoidal model fitting. several visualizations are possible to the complete set of parameters originated from this fitting – powers, amplitudes and phases. another window also displays some residuals visualizations and statistics (commonly used goodnessof-fit metrics). in this way, the results (amplitudes and phases), as well as the quality of fitting (residuals metrics), can be compared among all-time series. this method allows us to study the coherent signals present in times series as well as the sampling process itself. moreover, it could be more enlightening, from the physical standpoint, than merely to determine some confidence level for a periodicity, regardless of critical analysis over the time series and the sampling process underlined. 3. the lstperiod software the lstperiod software was developed within the matlab platform, and is freely available (http://www.iag.usp.br/paleo/sites/default/files/lst period‐files.zip) as a matlab (r2012b or newer) m-code (with some graphical “.fig” files and “.dat” test files) to run within the matlab environment, or as an executable “.exe” windows 10 version (which does not requires the matlab previously installed). the software comprises four guis: lstgui, guifitfreq, guiresiduals, and lst_table. with the software, we also deliver a brief manual and a set of synthetic data to help users to ∏𝑖𝑃𝑖(𝑓) 𝜇 format their time series and have an initial experience. to run lstperiod, the program (executable file, or source code plus “fig” files) and all of the data files must be in the same folder. data files should be low-level text files saved with “.dat” extension and should contain only two columns of numbers separated by space: time (t) and the measured variable x(t). for the files, we highly recommend adopting short names starting with a letter; otherwise, lstperiod may display chunks in some visualization labels. as the software starts, it will read all data files in the same folder and exhibit the periodograms. however, three blank panels will appear, because the frequency range must be selected. after that, the user can select the wanted files and run again. the user may run as many files as wanted; the only restriction is the processing time, which will increase for a large number of files running simultaneously. it is widely known that the periodogram (as well as the dft) is a slow algorithm, and the processing time will quickly rise as the number of points used in the calculation of the spectra increases. for this reason, lstperiod comes with two specific tools to manage processing time: an adjustable grid density slider and a small wait-bar window to display the evolution of any actual calculation going on (figure 1). at the first run, lstperiod opens three panels – one (bottom of the window) containing all the spectra for all series, the or spectrum (middle), and the and spectrum (top) (figure 1). however, at this point, the panels will be blank as an adjustment of the frequency range is necessary, and this is done by sliding the bars. the next step is to set the analysis bandwidth: the user can type a numerical value into the ‘central bw’ (central bandwidth) editable box, which corresponds to the central value of the desired frequency range. the number of points (density) in the calculation can be set using the bottom (larger) slider. after the adjustments, the program must be rerun by pressing the run button. this operation can be repeated as many times as necessary until the features of interest are all highlighted. if wanted, one or more data files may be removed from the batch of data files by pressing the file icon (‘open file’) at the top left of the window. the software will list all data files, and the desired files must be simultaneously marked (i.e., selected using the mouse and pressing the ctrl button on the keyboard) for a new run. 3.1 guifitfreq: linear system – parameter analysis a thorough investigation of any peak found in the combined and or or periodograms or any of the individual periodogram may be performed by marking the peak with the ‘datatip’ tool at the top of the window and then pressing the fit(t) button. this action will caminha-maciel et al. 6 figure 1. first lstperiod window showing the periodogram calculated for each data file separately (bottom), and the combined results for the or (middle) and and (top) spectra. the sliding bars at the bottom of the window set the analyzed range of frequencies and the number of points in the calculation. the fit(t) push button opens a second window once the pe− riod has been selected with the selection (period selector − datatip) tool. open a new window showing a bar plot displaying the amplitudes of the selected period among each of the series. the pop-up menu in the bottom right (figure 2) will display several commonly related parameters, including the phase, mean squared error, the coefficient of correlation, and coefficient of determination. all of the results for each file can be automatically sent to a table by pressing the ‘table’ button. the software will open a new gui called lst_table where the numeric results can be seen, selected and copied (using the ctrl+c and ctrl+v keys), and they can be sent to a worksheet file (“.xls” – what only works with microsoft excel installed). for each analyzed period the corresponding parameters will be saved separately in a different sheet of a spreadsheet so that the data will be available for further analysis according to the user. this spreadsheet will be saved with the temporary file name ‘temp_lst_table.xls’. 3.2 guiresiduals: linear system – residual analysis we can see the deviations (or errors) in the fit for a selected period for each data file just by pressing the residuals button (figure 2). a new window will open displaying the graphics for the selected data file and a goodness-of-fit parameter. in the residuals pop-up menu, we can select several options from a list of possible visualizations, including the standard error, error against the model, error against the data, gaussian fit for the errors, and quantile-quantile statistics. the definitions of these characteristic parameters are available in statistical books and texts [e.g., walpole et al., 2007; rodgers and nicewander, 1988]. each window can be printed and saved as a matlab figure (“.fig” files) or in any of several other formats, such as “.jpeg”, “.tiff”, “.eps” or “.pdf”. 4. examples 4.1 real data to illustrate the performance of lstperiod, we used a set of series showing the variation of benthic �18o along sedimentary cores drilled by the ocean drilling program (odp) and reported by lisiecki and raymo [2005]. the time series were already submitted to spectral analyses [lisiecki and raymo, 2007] showing climatically modulated periods of approximately 19, 23 and 41 kyr, the last one being relevant only before 1.4 myr. for this reason, we limited the maximum length of the time series to 1 myr, although some of the time series are longer than 5 myr. the data were organized merely into two-column (i.e., time t and data values x(t)) text files (“.dat” extension) and were processed without any pre-treatment (e.g., filtering or detrending). the selected bandwidth was 14.5 45 kyr, although longer periodicities could also be investigated. the results are displayed in figure 1. the several individual spectra (bottom of the figure) are noisy, and the real periodicity may be challenging to identify. the and spectrum (top of the figure) is far smoother than 7 lstperiod software figure 2. second lstperiod window (guifitfreq) for the selected parameter analysis, which is chosen using the error analysis menu box. the estimated parameters can be exported and saved to a “.xls” table. the individual spectra, and it expresses features that are common to all series. mainly, we are interested in better evaluating the ~19, ~23 and ~41 kyr periods. the period of 41 kyr seems to be the most prominent feature in the spectrum as stressed by previous work [lisiecki and raymo, 2007]. the 23 kyr period is also detected, whereas the 19 kyr period, which is thought to be persistent throughout the last 5 ma, does not appear clearly within our and results. however, the 19 kyr period is present in the or spectrum and is clearly defined in many of the individual spectra. it is worth mentioning that these data are derived from distant sites in the atlantic and pacific oceans and that they might not show the same components of the forcing system. furthermore, lisiecki and raymo [2007] mentioned a low signal-to-noise ratio at this frequency. other peaks at approximately 27 and 31 kyr are also present in the and spectrum and seem unrelated to orbital forcing. however, it is beyond the scope of this paper to discuss these results further. 4.2 synthetic data here, we illustrate the performance of the method and software by analyzing a set of synthetic data series with known harmonic content (figure 4). the synthetic time caminha-maciel et al. 8 figure 3. lstperiod window for one of the options in the guiresiduals. the error analysis refers to the modeling of the selected file displayed in the bottom box. each error type displayed in the menu has a graphical output for visual inspection. figure 4. synthetic series produced as described in the text. the green line (left) represents the original series corresponding to a superposition of sinusoidal components; blue dots and line (left) are one of the inhomogenous sampling series; the other four are plotted on the right diagram. series is the result of ten cycles of the superposition of two components with periods of 90 (with an amplitude of 6 and a phase of zero) and 150 (with an amplitude of 4 and a phase of π/4) time units. the resulting time series was then randomly sampled (30 to 50 points) over the entire length of the original series, thereby producing five different short time series named s1 through s5. to each point of the five series, we added a non-constant gaussian noise level of 5% of its amplitude. considering the poor quality of the final series (figure 4), the recovery of the periodicities was excellent. figure 5a showed periods of ~90.78 and ~147.52; a very smooth and spectrum was produced compared to the noisy individual spectra. regarding amplitudes and phases (figure 5d and 5e), it was possible to recover amplitudes of ~4 for the 147 period and phases of approximately π/4 for at least two series. however, it is not surprising that the amplitude and phase were not rigorously recovered considering the noisy and poorly sampled time series. the other diagrams in figure 5 show the comparison of the original and modeled data along with the error bars (5b), the error histogram (5c), and the standardized errors for the series. 9 lstperiod software figure 5. spectral results for the synthetic time series with periods of 90 and 150 arbitrary time units. small boxes show some examples of graphical outputs corresponding to analyses of the spectral peaks selected: a) original generated series (green line) and the sampling points (blue dots and line); b) calculated model plus error and data; c) error histogram; d) peak amplitude for each time series; e) rose diagram for the phase; and f) standardized errors for a particular time series. 5. final remarks the ls periodogram is a well-known algorithm for detecting and characterizing signals in unevenly sampled data. however, the statistical significance of the detected periods may be difficult to ascertain, especially if the series has no characteristic sampling interval, and is deficient in accurately recording the periodic or quasi-periodic event. for this reason, many records of this type that carry some valuable information are often discarded as considered inappropriate for spectral analysis. lstperiod is a software specially designed to overcome problems in the calculation of the periodograms (e.g., poor data window) of short, noisy and irregularly sampled time series through the selective choice of the frequency range and grid density. the combination of various poor data sets that represent the sampling of the same phenomenon allows a reduction of the noise through smoothing and signal-noise gain, and consequently the extraction of relevant information with a low s/n ratio. lstperiod runs on matlab (r2012b or newer) environment and provides ample possibilities to investigate the reliabilities of the periodicities in time series data. the examples presented in section 4 show the power of lstperiod in comparing variances of different records and statistically analyzing each suspected spectral feature. we demonstrated with synthetic examples that periodicities could be reliably identified in a set of poorly sampled time series. we illustrated the use of the lstperiod method with cyclostratigraphic data because this is one prevalent issue in geosciences, but the software can be applied to any data set that can benefits from the combination of information as proposed here. time series with periodic gaps, as often found in astronomy, are also troublesome because they can generate unpredictably common patterns (like leakage and aliasing) within the spectral windows. in this case, the stacking procedure will enhance not only the common signals but also the common spectral anomalies. another significant advantage of lstperiod is that it offers a statistical diagnosis for models derived from any suspected spectral feature, and thus, we can test as many frequency points as desired. on the other hand, as periodograms are highly time-consuming [deeming, 1975; hernandez, 1999; townsend, 2010], lstperiod may become slow for long time series (i.e., exceeding 10,000 readings), which may be a disadvantage. this software does not intend to be a new tool for implementing old statistical methods. indeed, there is much room for improvement in the user’s interface and graphical capabilities (we count on the community’s feedback to improve future versions). instead, it is a proof-of-concept tool, to demonstrate de very idea that spectral analysis of irregularly sampled natural time series is a process of extracting incomplete information. as so, there is always a need to input some a priori information. nowadays, in the literature, this has been performed through the assumption of obscure statistical hypotheses (as white noise), and the so-called “time series carpentry” – meaning the cutting and manipulation of the time series to obtain desired results. as a consequence, most of the data obtained as time series (especially in cyclostratigraphy) is considered non-suitable to spectral analysis. here we made a clear choice for the input of a priori information: starting for a broadest possible spectral bandwidth, we show graphics of invariant estimates and 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lomb-scargle periodogram. astron. computing, 1, 5-16. walpole, r.e., myers, r.h., myers, s.l., ye, k., (2007). probability & statistics for engineers & scientists. pearson prentice hall, nj, usa. zechmeister, m. and kürster, m., (2009). the generalised lomb-scargle periodogram. a new formalism for the floating-mean and keplerian periodograms. astron. astrophys., arxiv:0901.2573v1 [astro-ph.im]. *corresponding author: george caminha-maciel, university of hawai’i at manoa, soest, hawaii institute of geophysics and planetology (higp), petrofabrics and paleomagnetics laboratory, honolulu, hawaii 96822, usa; email: caminha.maciel@ufsc.br © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved caminha-maciel et al. 12 articolo wei.pdf key words northeastern margin of tibet plateau – paleoseismology – slip-rate 1. introduction the tibetan plateau is a fast deforming zone characterized also by important seismicity. the northeastern margin of the tibetan plateau is characterized by several main systems of active faults from southwest to northeast. they are (fig. 1c): the haiyuan fault (hyf), the zhongwei-tongxin fault (ztf), the yantongshan fault (ytf), niushoushan (nsf)-luoshan (lsf)yunwushan fault (ywf), huanghe fault (hhf), the helanshan fault (hlf), zhengyiguan fault (zgf) and the sanguankou fault (sgf). the niushoushan (nsf)-luoshan (lsf)yunwushan fault (ywf) zone separates the characteristics of the active luoshan fault since late pleistocene, north central china min wei (1), jiao decheng (2), chai chizhang (2), zhang peizhen (1) and mao fengying (1) (1) institute of geology, china seismological bureau, beijing, china (2) seismological bureau of ningxia province, yinchuan, china abstract the luoshan fault located at the northeastern margin of tibet plateau strikes roughly n-s, and is composed of six left-stepping sections with a total length of 60 km. much evidence suggests that the luoshan fault is a reverse right-lateral strike-slip fault. the largest right-lateral strike-slip displacement and the most abundant dextral offset phenomena are located along the central section. based on the right-lateral strike-slip offsets of the oldest alluvial fan, and of a gully and on the average displacement of the same order of gullies, the minimum slip-rate has been 2.15 ± 0.2 mm/yr since late pleistocene. many surface rupture phenomena, such as fault scarps with fresh free-face, ground fissures, displacements of very young gullies, imply that a recent earthquake occurred along this fault. combining the historical catalogue and our results, we believe that the 1561 a.d. earthquake was produced by the luoshan fault. three paleoearthquakes were determined by means of paleoseismic studies along the luoshan fault: they occurred after 8200 ± 600 years bp, between 3130 ± 240 years bp and 4150 ± ± 120 years c.bp, and before 2230 ± 170 years bp, respectively. yinchuan graben and the ordos block. since 1980s, many advances in earthquake geology have been made and many important results have been obtained in the region (qidong et al., 1984; deng et al., 1986; zhang et al., 1988, 1990, 1991; deng and zhang,1990; burchfiel et al., 1991). strong earthquakes frequently occur in this area. according to the institute of geology, seismological bureau of china (deng and zhang, 1990), the 240 km long haiyuan fault (hyf) was responsible for the1920 m 8.5 earthquake (gu et al., 1989), being characterized by a 6-10 mm/yr left-lateral strike slip rate during the holocene. wang et al. (1990) concluded that the 200 km long zhongwei-tongxin fault (ztf) was responsible for m 7.5 of the 1709 earthquake (gu et al., 1989), being characterized by a 3.58 mm/yr leftlateral strike slip rate during the holocene. evidence for holocene activity has hardly been found along the yantongshan (ytf) and niushoushan faults (nsf). the nne trending helanshan fault (hlf) along the western margin of yinchuan graben is a normal and right-lateral strike-slip fault. it cuts late pleistocene to holo997 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. min wei, institute of geology, china seismological bureau, beijing 100029, china; e-mail: minww@263.net.cn cene alluvial fans, forming a se facing scarp, and offsets right laterally for 1.45 m and vertically for 0.95 m the great wall of the ming dynasty (about 400 years bp). the offset of the great wall is probably a result of the m 8 earthquake of 1739 (zhang et al., 1986) and four holocene seismic events have been identified by deng (1996) on the basis of paleoseismological trenching. the huanghe fault (hhf) forms the eastern boundary of yinchuan graben. liao et al. (2000) and chai et al. (2001) studied its southern part and estimated a normal slip rate is 0.23-0.25 mm/yr and recognized two holocene earthquakes that ruptured the fault. according to the research group on «active fault system around ordos massif» (1988), the luoshan fault (lsf) is a left-lateral strike slip fault. moreover, in 1561, another strong earthquake m 7.3 (gu et al., 1989) struck an area close to the luoshan mountain. in order to cast light on the seismotectonics of the northeastern margin of the tibetan plateau, we performed detailed mapping along the luoshan fault, finding geological and geomorphic evidence that this fault is a right-lateral strike-slip fault which could be related to strong earthquakes, and particularly to the 1561 a.d. m 7.3 earthquake. at two investigated sites, the slip rate of the fault was calculated, while three holocene paleoearthquakes have been recognized through paleoseismological analyses. 2. seismotectonic framework of the tibet region at least 1500 km of north-south crustal shortening have occurred because of the collision of india and eurasia which led to eventual construction of the cenozoic tibetan plateau since the late cretaceous (molnar and tapponnier, 1975). under steady ne directional compressional stress from india plate, three main types of structures were formed in the plateau (fig. 1b): thrust fault belts associated with indenting basins; strike-slip fault belts associated with the lateral extrusion and rotation of blocks; normal fault belts associated with extensional graben-type basins. the compressive convergent structures are located around the tibet plateau. the himalaya compressive convergent structure zones are located on the southern margin of the plateau, involving different structures with a convergence rate of 20 ± 10mm/yr (avouac and tapponier, 1993). western kunlun thrust belt is located to the northwest of the plateau and defines the western termination of the altyn tagh fault system. the qilianshan thrust belt is located on the northeast boundary of the plateau, the rate of horizontal convergence on some main faults is estimated as 4-5 mm/yr, while that for the entire convergence zone is up to 15 mm/yr (avouac and tapponier, 1993). the liupanshan thrust belt is located on the northeast margin of the plateau and consists of cenozoic fold and thrust faults, and the amount of convergence since quaternary has been deduced to be 11.4-15.4 km (zhang et al., 1991). longmenshan thrust belt is located on the middle segment of the eastern margin of the plateau. avouac and tapponier (1993) deduced a horizontal shorting rate of about 10 mm/yr for this zone. the rather rigid tarim massif might be a large obstacle to the northward transfer of deformation. the main part of tibet was compressed and contracted, while the blocks within tibet were moved laterally along some major boundary faults. the 998 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying fig. 1a-c. a) sketch map of china active faults. insert a shows the area of fig. 1b. b) active tectonic map of the tibet plateau. earthquakes which only occurred in china are labeled on the map (from gu et al., 1989 and china seismostation report compiled by the institute of geophysics, csb, unpublished). inset b shows the location of the study area. c) active faults map of the eastern margin of the tibet plateau. inset b in fig. 1b shows the location of the study area. faults: zgf zhengyiguan fault; hlf helanshan fault; sgf sanguankou fault; hhf huanghe fault; nsf niushoushan fault; ztf zhongwei-tongxin fault; ytf yantongshan fault; lsf luoshan fault; hyf haiyuan fault; ywf yunwushan fault; lpf liupanshan fault. mountains: hlm helanshan mountain; nsm niushanshan mountain; xtm xiangshan and tianjinshan mountain; lsm luoshan mountain; ywm yunwushan mountain; lpm liupanshan mountain. magnitude and timing of five m 7 historical earthquakes are labeled on the map (gu et al., 1989). 999 characteristics of the active luoshan fault since late pleistocene, north central china ca b altyn tagh fault is one of the largest intraplate strike slip faults with a strike of nee and a total length of 1600 km, constituting the boundary between the tibet plateau and tarim massif (fig. 1b). there is still noticeable disagreement in the estimates of the left-lateral slip rate along the fault, ranging from 6.4 mm/yr (research group on altyn active fault, 1992) to 30 mm/yr (avouac and tapponnier, 1993). the karakorumjiali fault zone is considered to be the southern limit of the eastward extrusion of the plateau, and displays right-lateral strike-slip, in contrast to the altyn tagh fault. a right lateral strike slip rate of 20-30 mm/yr has been estimated for this fault zone (avouac and tapponnier, 1993). the kunlun shan fault, ganzi-yushu-xianshuihe and red river fault are also very famous strike-slip faults within tibet. the 1997 manyi m = 7.9 and the 2001 kunlunshan m = 8.1 earthquakes occurred along the kunlun shan fault. the surface rupture length of 2001 kunlunshan earthquake is between 350 and 426 km (xu et al., 2002). sn-trending extensional graben systems have been recognized in central and southern tibet (tapponnier and molnar, 1977; molnar and tapponnier, 1978). the strike of the graben system varies from northsouth to northeast and then to northwest. the ne-trending boundary faults of the extensional graben show left-lateral strike slip component whereas the nw-trending boundary faults have a right-lateral strike slip component. the luoshan fault locates the easternmost segment of the qilianshan-haiyuan fault zone, many historical strong earthquakes occurred in this area. 3. geologic setting and geometry of the luoshan fault the niushoushan-luoshan-yunwushan faults are in the northeasternmost margin of the tibet plateau (fig. 1c). in this region, the structures are dominated by normal faulting bounding the east and west sides of yinchuan graben to the north, whereas structures are dominated by strike slip and thrust faulting to the south. the luoshan fault belongs to this southern domain. many strong earthquakes have been recorded in this region (fig. 1c), five m 7 earthquakes occurred in the past hundreds of years. they are: 1561 north of luoshan, m 7.3 earthquake; 1622 north of guyuan, m 7.0 earthquake; 1709 zhongwei, m 7.5 earthquake; 1739 yinchuan, m 8.0 earthquake, and 1920 haiyuan, m 8.5 earthquake (gu et al., 1989). the luoshan mountain range, with a 2624 m peak altitude, trends nearly sn and is composed of ordovician rocks. the 60 km long luoshan fault is located along the eastern piedmont of the luoshan mountain (fig. 2), it strikes 340°-360° and is composed of six sections. the displacements of gullies and streams imply that luoshan fault is characterized by right-lateral strike-slip, and the largest observed horizontal offset is located in the middle part of the fault (sf4). the fault sections show reverse components of oblique right-lateral strike-slip along the northern part of the fault (sf1 and sf2) and normal components of slip along the southern part of the fault (sf5 and sf6). therefore, they display pivotal movement. the northernmost section sf1 (fig. 2): strikes 350°, dips westwards and is 14 km long. the average altitude of luoshan mountain is in general over 2200 m, while it suddenly descends to 1700 m north of tanzhuang along sf1. the fault is located between the mountain and alluvial fans. fault scarps with heights of 2-4 m can be seen on the quaternary alluvial deposits, and some fault scarps show fresh freeface. the fault is characterized by thrusting along this section, and a fault section can be seen in tangzhuang where ordovician rocks thrusted on quaternary deposits (fig. 3). north of xiquan, the fault disappeared in the cenozoic basin. the segment sf2 (fig. 2): strikes 310°-330°, dips sw and is only 5 km long. the trend of this segment is not consistent with the trend of the others, and this segment is characterized by important thrusting. south of miaogou, ordovician rocks thrusted onto quaternary loess and alluvial deposits, and the fault broke the surface (fig. 4). 1-2 m high fault scarp is observed along this section. the section sf3 (fig. 2): strikes 345°-350°, dips east and is only 9 km long. starting from this segment, the active fault gradually locates on the eastern alluvial fan of the luoshan mountain, not 1000 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying 1001 characteristics of the active luoshan fault since late pleistocene, north central china fig. 2. detailed map of the luoshan fault. 1002 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying quaternary alluvial gravel with sand 260° 0 10 20 m ordovician sandstone thrust fault 350°/ne/32° f1 f3f2 quaternary alluvial deposits ordovician rocks ew loess fault trace fig. 3. sketch of fault section in tanzhuang. fig. 4. reverse splay of the fault south of miaogou. the ordovician rocks overlaid by alluvial gravels thrusted onto loess and the reverse fault broke the surface. view to the northeast. along the foot of the mountain. the horizontal displacements affect the quaternary alluvial fans. northward from hongmiaogou, three gullies craved on the alluvial fan were dextrally offset by 26-22 and 16 m, respectively. many other gullies and streams were also dextrally offset. in hongmiaogou, the fault cut a 9 m high river terrace, which is displaced by a 5.2 m high fault scarp. west of mawangmiao, a trench across a fault scarp on a 2 m high terrace was excavated. figure characteristics of the active luoshan fault since late pleistocene, north central china 0 1m e fig. 5. schematic log of machanggou trench. 1 redeposited loess with gravel, gravel increases in the lower portion. 2 filled wedge that consisting of redeposited and gravel. 3 mixed gravel and sand. 4 sandy soil. 5 gravel bed of the alluvial terrace, consisting of gravel and sand. the thick lines indicate faults. 14 0 500 m n shiyaodong 23 8.4 10 12 1591012171 m ac ha ng go u 171 a b fault trace sf4 machanggou site 1821 12 13 10 shiyaodong trench 10 18 21 uplift fault trace sf5 sf4 shiyaodong site luoshan mountains fig. 6. aerial view of the fault section sf4 cutting all the alluvial fans on the eastern piedmont of the luoshan mountain and forming a remarkable fault scarp. the fault right laterally offsets most gullies. thin arrows show offset gullies (displacement in meters); thick arrows show fault traces; the filled box indicates the trench area. the strike-slip rate is studied through two sites along this segment and empty boxes indicate the locations of the two sites, the triangle indicates sample location. a pressure ridge was formed in the overlapping area between sf4 and sf5. 1003 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying ns ew fault scarp luoshan mountains fault trace fault sag f1 f2 ordovician rocks loess alluvial deposits alluvial deposits 1 1 1 1surface graben scarp ew gully gully w e fault scarp 18m right-lateral displacement fault section illustrates in fig.8b terrace i terrace ii terrace i fig. 7a-d. evidence of activity of the luoshan fault. a) a gully was dextrally offset by 18 ± 1 m in shiyaodong. thick line indicates the trace of the fault; the large thick arrows show the movement direction of the fault. dashed lines with arrows indicate the gully. two terraces developed on the upthrown block and one terrace on the downthrown block. the topographic map of the gully is illustrated in fig. 10. b) fault exposure with nearly horizontal striae near shiyaodong. c) fault sag in wangdengjuan. thick line with arrow indicate fault. d) cross section of a graben in wangdengjuan. the dashed line indicates buried fault. thick lines indicates faults. a c b d 5 shows the log of north wall of the trench exposing quaternary deposits displaced by several fault splays burried by a thin layer of fine sand with pebbles. the shiyaodong section sf4 (strikes ns, dips east and is only 4 km long) is a continuous straight fault scarp affecting the alluvial fan developed at the foothill of the luoshan mountain (fig. 6). most gullies or streams show a right slip when they cross the fault. this observance and a gully offset by 18 m (fig. 7a) and a fault plane exposed along it showing nearly horizontal fault striae (fig. 7b), imply a right-lateral strike-slip movement along the luoshan fault. the wangdengjuan section sf5 (strike 340°, length of 11 km) shows an eastward facing fault scarp north of wangdengjuan while south of wangdengjuan, the scarp faces westward, thus forming a 60 m wide trough formed from south of wangdengjuan along the foot of the luoshan mountain. the west-dipping fault scarp dammed the normal deposition from upstream or hillside, causing many fault sags (fig. 7c). some trenches were excavated in these sags. in a terrace in wangdengjuan, two normal faults f1 and f2 depict a graben (fig. 7d). f2 cut all deposits and reached the surface, f1 is buried by loess. 1004 1005 characteristics of the active luoshan fault since late pleistocene, north central china the zhangjiashanpo section sf6 (fig. 2): strikes 340°, being 20 km long. the fault signs the boundary between the quaternary deposits and ordovician rocks, and is enhanced by an east-dipping normal fault. many remarkable fault scarps comprising fresh free-faces are observed (fig. 8a). because free-faces will be eroded away within hundreds of years (wallace, 1977), the latest rupture event possibly occurred within one thousand years. several meters west of the fault scarps, another normal fault section outcrops in many gully walls (fig. 8b), however, there are no traces on the surface along this fault splay. this suggests that the fault ruptured the surface many times. south of miaoshan, the fault disappeared in the loess plateau. because of left-stepping of secondary shear faults along the right-lateral strike slip fault zone, pressure structures occur (deng and zhang, 1984; chen and deng, 1985). the thrust fault sf2 formed between sf1 and sf3, and pressure ridges formed between the other overlapping segments (fig. 6). 4. historical earthquakes rupture zone in the luoshan fault as mentioned before, a historical earthquake m 7.3 occurred in the area close to luoshan fault in 1561(gu et al., 1989). although we know nothing about the earthquake fault, the we f1 f2 fault scarp fault trace fault trace ns fault scarp ew eew fig. 8a-d. surface rupture features; some of them may be related to the most recent slip on the fault. a) view of a fault section and scarp with free face at zhangjiashanpo. b) normal fault section near zhangjiashanpo. thick lines with arrows indicate normal faults. 1 quaternary loess; 2 ordovician rocks. c) small graben feature in shiyaodong segment. thick lines with arrows indicate normal faults. the graben is formed in a younger alluvial fan. the deposits between faults f1 and f2 consist of gravel mixed with redeposited loess, and alluvial gravel bed exposures on the other portion. d) small gully dextrally offset by 3.6 m during the last rupture event at shiyaodong. a c b d surface ruptures found along the luoshan fault could indicate that the luoshan fault is responsible for the 1561 earthquake. the fault traces always offset the quaternary deposits, and frequently offset holocene deposits and geomorphic units, with the last rupture buried by recent alluvial material (figs. 3, 4, 5 and 8c). moreover, the displacement of «v» shape small gullies, only tens of meters long and one meter deep, could suggest that they were formed recently. the headwater erosion velocity is estimated to be 8.46 cm/yr in this region (cheng et al., 1984), while these gullies should reasonably be formed within one thousand years. sixteen of these gullies (order iv in table i) show dextral offsets of 1.5 m-5.4 m (fig. 8d), and could account for the most recent event which occurred along the luoshan fault. finally, further evidence for recent slip of the fault can be decided by the study of the free-face (wallace, 1977). for example, a free-face in loess deposits in zhangjiashanpo is 0.7 m high with 71° (fig. 9). here the original upper slope is 6°, a transition portion with 16°-20° formed between free-face and original upper slope, and a 26° debris slope developed near free-face. the dip of the fault is 55° that can be seen in the north wall of a gully (fig. 8a). thus, the slope angle of the free-face is larger than that of the fault, indicating that the fault scarp is still under the evolution of gravity, thus very young. 5. horizontal slip rate horizontal offsets of streams are the most ubiquitous and prominent phenomena along the luoshan fault (fig. 6), and they can be used both to evaluate the kinematics of the fault and to calculate the horizontal slip rate. 1006 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying fig. 9. topographic profile across zhangjiashanpo fault scarp surveyed by total station. free face debris slope 71 26 16 20 13 20 8 6 8 1 m 1 m 2 m 2 m 3 m 3 m w table i. tl sample data (dated by tl laboratory of institute of geology, china seismological bureau, beijing). sample location depositional temperature equivalent dose rate tl age (yr) number environment range (°c) dose (grays) (grays/ka) st terrace ii in syd fine alluvial sand 310-350 44.80 ± 3.18 4.57 ± 0.36 9800 ± 750 ma alluvial fan in mcg fine alluvial sand 310-350 319.24 ± 24.58 4.62 ± 0.37 69 000 ± 5400 sd1 syd trench eolian loess 310-350 19.74 ± 1.18 3.97 ± 0.31 4970 ± 390 sd2 syd trench eolian loess 310-350 14.56 ± 1.08 4.65 ± 0.36 3130 ± 240 sd3 syd trench eolian loess 310-350 10.08 ± 0.87 4.52 ± 0.35 2230 ± 170 sm3 sm trench fine alluvial sand 310-350 31.81 ± 2.14 3.95 ± 0.34 8200 ± 600 syd.-.shiyaodong, mcg.-.machanggou, sm.-.shuangmiao. we studied the right-lateral strike-slip rate of the luoshan fault in the shiyaodong and machanggou sites (fig. 6, two inset boxes). 5.1. shiyaodong site (.fig. 6) here, streams and gullies are dextrally displaced by different amounts. the topographic map of a 18 m ± 1 m-displacement gully was measured by total station (fig. 10), and this gully is illustrated in fig. 8a. two depositional terraces were formed in the upthrown block and one depositional terrace, being eroded away close to the fault, was formed in the downthrown block. terrace i is about 1-1.5 m high and the top of the terrace is composed of 0.5 m gray black soil. the other parts of terrace i were composed of gravel and sand layers. terrace ii is 5-6 m high. its top and bottom are composed of light yellowish sand with a height of 2 m and lower than 1 m, respectively. the middle part of terrace ii is composed of gravel. the 18 ± 1 m-offset is the displacement of the central line of the gully between the two sides of the fault. the incision age postdates the deposition age, so the age of the oldest depositional terrace represents the maximum age of the displacement. a rough indication of the terrace age is indicated by a tl (numbering st) dating of 9800 ± 750 years bp (table i) collected below the top surface of the terrace ii. therefore, the minimum right-lateral strike-slip rate could have been about 1.8 ± 0.2 mm/yr during the holocene. 5.2. machanggou site (.fig. 6) machanggou alluvial fan is the oldest in the eastern piedmont of the luoshan mountain, and it is clearly cut and dextrally displaced by the fault. determining the offset of the fan is difficult, because we cannot find suitable indications to measure it. as soon as an alluvial fan forms, streams and gullies will carve on it, the largest streams may be regarded as coeval with the alluvial fan, and the displacement of the largest stream could approximate the displacement of the fan. the machanggou gully is the largest gully that incised into the highest and oldest machanggou alluvial fan, and the displacement of the southern gully wall between the two points a and b is 171 ± 10 m measured by total station, it is the largest offset value along the luoshan fault. from the south wall of the machanggou gully, a sample (numbering ma) of alluvial fine sand was collected 3 m below the surface of the oldest alluvial fan (black triangle, fig. 6), its tl dating being 69.000 ± 5400 years bp (table i). according to the displacement of the alluvial fan, the right-lateral strike-slip rate is 2.5 ± 0.3 mm/yr since late pleistocene. the formation age of the alluvial is older than the formation age of the gully, so the 2.5 ± 0.3 mm/yr slip-rate should be a minimum value. it is worth noting that the longer the streams and gullies developed, the larger they have been displaced by the fault motions. by grouping these streams and gullies according to their scales, it is possible to use the average displacement of characteristics of the active luoshan fault since late pleistocene, north central china 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 111 1 1 1 11 111111111 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 1 1111111 11 111111 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 100 200 m n t1 t1 t1 t2 t2 gully t1 t2 1 1 1 1 1 terrace i terrace ii cliff fault trace tl sample location contour interval = 1 m 18 1 m fig. 10. topographic map of fault landform near shiyaodong surveyed by total station (shiyaodong site in fig. 6). the gully has been right-laterally offset 18 ± 1 m and illustrated in fig. 7a. 1007 each group to compute an average slip rate. according to our observation, the gullies along the luoshan fault can be divided into four groups characterized by similar width-length and terrace orders. in group i, the gullies eroded upstream the ordovician rocks but did not erode upstream to the watershed, and three alluvial terraces are developed along the gullies. the gullies of group ii are located only within the alluvial fan surface, and did not erode upstream to the ordovician units, and two terraces developed. the gullies of group iii are tens-of-meterslong and 1-2 m deep, and are also craved only in the alluvial fan. finally, the gullies of group iv only several meters long and less than 1 m deep are the smallest gullies along the luoshan fault. 87 offset values were measured along the fault (table ii) and arranged into related groups. most measurements are made with a tape measure and some of them with total station. considering that the average displacement of order ii is 21.7 ± 1 m, and that the age of group ii in shiyaodong is 9800 ± ± 750 years bp, the rough average slip rate related to this period is 2.2 ± 0.2 mm/yr. altogether, the minimum slip-rate, considering the three values above, is 2.15 ± 0.2 mm/yr (mean value of 1.8 ± 0.2 mm/yr and 2.5 ± 0.3 mm/yr). 6. paleoseismic analyses although six trenches were excavated along the luoshan fault (fig. 2), evidence for individual paleoearthquake can only be found in the shiayaodong and shuangmiao trenches. the other trenches were excavated on the youngest surfaces, and where exposing only the most recent faulting event, one log of these trenches was shown in fig. 5. 6.1. shiyaodong trench this trench was excavated across the sf4, and particularly across a straight 2.2 m high fault scarp cutting the alluvial fan near the farm of shiyaodong. the stratigraphical succession observed in the south wall of the trench is composed mainly of eolian silt (loess) and alluvial gravel (fig. 11a,b). two faults f1 and f2 were exposed on the wall. the sediments west of the f2 are composed of alluvial gravel with horizontal beddings. the sediments between f1 and f2 are also composed of alluvial gravels, but layers have been titled eastward by the movement of faults. the deposits related to the faulting events are found on the eastern side of f1. the faulting history could be tentatively suggested by the presence of lenses of gravels in the hangingwall that could be interpret1008 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying table ii. statistics of horizontal displacements of gullies along the luoshan fault. order of displacement values the range of averaging gullies (m) displacement values value i 59, 52, 60, 50, 60 50-60 80 ii 16, 22, 26, 18, 15, 23, 19, 21, 15-31 21.7 22, 18, 18, 18, 20, 31, 25, 28.3, 15.3, 24, 25, 28, 23, 22 iii 8, 7.5, 8.5, 11, 8, 9, 10, 12.5, 12, 14, 8.4, 7-15 10.02 8, 10, 13, 12, 7, 14, 10, 14, 15, 10, 9, 10, 7, 15, 8.3, 8.3, 9, 7, 7.5, 7, 13, 7, 7, 9, 8, 12, 15, 10, 13, 8, 10.7, 8 iv 4, 4.7, 2, 1.5, 4, 2.7, 3.6, 5.4, 1.5-5.4 3.8 3, 3, 5.4, 4, 5, 3, 5, 5 1009 characteristics of the active luoshan fault since late pleistocene, north central china 0 1 m f1 f2 f1 f2 sd1 sd2 sd3 w e 350°/ne/85° 2230 ± 170 3130 ± 240 4970 ± 390 0 0.25 0.5 m fig. 11a,b. south wall of shiyaodong trench. a) photo of the south wall. arrows indicate faults. b) log of the south wall. the numbers represent stratigraphic units: 1 recent alluvial loess mixed with small gravel; 2 redeposited loess, whereas a layer of gravel exists in the upper part of the bed; 3 colluvial wedge consisting of gravel and redeposited loess. the thickness of this unit is about 0.4 m near the fault and extends 1.5 m away from the fault; 4 redeposited loess, and they are brought by sheet flow on the upthrown block of the fault and eolian deflation; 5 colluvial wedge which consists of gravel mixed with redeposited loess. this unit has a thickness of 0.2 m in the vicinity of the fault and thins away from the fault. it extends 2 m and then lenses out; 6 redeposited loess; 7 colluvial wedge (not well exposed) composed of coarse gravel and loess; 8 fracture zone of about 0.5 m width consisting of gravel from alluvial fan, in which the beddings of gravel dip eastwards; 9 gravel bed of alluvial fan, consisting of coarse gravel bedded with sand. the large box shows the area of photo. stars represent tl sample locations. sample names and ages are also labeled on the log. a b 1010 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying fig. 12a,b. south wall of shuangmiao trench. a) photo of the south wall. arrows indicate faults. b) log of the south wall. the numbers represent stratigraphic units: 1 recent hillslope deposits; 2 light black soil containing a little peat; the materials of this unit came from hillslope and deposited in the fault sag; 3 redeposited loess interbedded with three layers of gravel; 4 gravel interbeded with sand; 5 fine alluvial sand. the big box shows the area of the photo. stars represent tl sample locations. sample names and ages are also labeled on the log. a b ed as colluvial wedges that formed during the degradation of unit 9 after each surface faulting. the colluvial wedges consist primarily of coarse cobbles mixed with fine silts without beddings. it is thought that the right lateral strike slip luoshan fault is characterized by pivotal movement, a minor normal element was accompanied by right lateral strike slip along the shiyaodong segment, the fault scarps have shown that normal faulting existed all the time, hence, colluvial wedges are small along the luoshan fault. two paleoearthquakes are determined from this trench. units 3 and 5 can be interpreted as two colluvial wedges and each of them probably represents one paleoevent. unit 7 is perhaps another colluvial wedge, but is not well exposed. the most recent event occurred along fault f2 which cut layer 3 and is covered by layer 1, there was no colluvial wedge related to this paleoearthquake probably because of the geometry of f1 (fig. 11a,b). three tl age samples of sd1, sd2 and sd3 were collected from the bottom of debris slope over every possible colluvial wedge (fig. 11a,b). their ages are 4970 ± 390 years bp, 3130 ± 240 years bp and 2230 ± 170 years bp (table i), so the two paleoearthquakes (de1 and de2) occurred before 3130 ± 240 years bp but after 4970 ± 390 years bp, before 2230 ± 170 years bp and after 3130 ± 240 years bp. the most recent event occurred after 2230 ± 170 years bp and was responsible for the 1561 a.d. earthquake. 6.2. shuangmiao trench this trench is excavated across a west-facing scarp. here, the fault is shown by a 2 m high reverse fault scarp formed on the alluvial fan (fig. 7c). the sediments observed in the south wall of the trench are mainly alluvial gravels, sand and soil (fig. 12a,b). five high-angle reverse faults can be recognized. the sediments eastward of f1 are alluvial gravels, which could be reasonably tilted by the thrusting motion. the growing of the east-dipping-reverse-fault trapped the deposits at the foothill. firstly, the right lateral strike slip is dominant and the thrusting motion is minor, secondly, the hillslope deposits (unit 2) accumulated quickly along the fault scarp, so the colluvial fabric was undeveloped. at least three paleoearthquakes can be recognized at this site. the first paleoevent occurred during the deposition of unit 3. f4 cuts unit 4 and the lower part of unit 3, and then was buried by the upper part of this unit. the second paleoevent occurred during the deposition of unit 2 as showed by faults f2, f3 and f5. these faults cut units 3, 4 and 5, particularly, f5 cut the lower part of unit 2, and were buried by the upper part of unit 2. the last paleoevent occurred along fault f1, it cut unit 2 and was buried by unit 1. in order to determine the timing of these paleoevents, three samples were collected and dated. these are: sample sm1 composed of fine sand with tl age of 8200 ± 600 years bp (table i); sample sm2 consisted of peat deposits and its 14c age is 3900 ± 70 years bp, dendrochronologically corrected age is 4150 ± 120 c.bp; sample sm3 was taken from the peat deposits of unit 2, its 14c age is 2550 ± 100 years bp, dendrochronologically corrected age is 2550 ± 190 c.bp 14c samples were dated by 14c laboratory of institute of geology, china seismological bureau. according to these datings, the first paleoevent (ge1) occurred after 8200 ± 600 years bp; the second paleoevent (ge2) occurred after 4150 ± 120 years c.bp; the third paleoevent (ge3) occurred after 2550 ± 190 years c.bp and should correspond to 1561 a.d. earthquake. 7. sequence of faulting events and recurrence intervals figure 13 summarizes the paleoseismic events identified in the two trenches. four paleoevents can be determined since holocene along the luoshan fault. the oldest paleoevent (e1) is recognized only in shuangmiao trench, and it occurred after 8200 ± 600 years bp. the second paleoevent (e2) was recognized both in shiyaodong and shuangmiao trenches; it occurred between 3130 ± 240 years bp and 4150 ± 120 years c.bp. the third (e3) paleoevent can only be recognized in shiyaodong trench, where it occurred before 2230 ± 170 years bp. the most recent event was recognized in both trenches, and it occurred after 2230 ± 170 years bp in shiyaodong trench and after 2550 ± 190 years bp in shuangmiao 1011 characteristics of the active luoshan fault since late pleistocene, north central china trench. combining with historical data, the last event could be the earthquake of 1561 a.d. 8. conclusions on the basis of field and aerial photo mapping and paleoseismological trenching, we have achieved the following results: 1) the luoshan fault is composed of six sections, and trending 350° with a total length of 60 km. the fault striae, and the right-lateral displacement of gullies and streams indicate that, unlike other faults in the northeastern margin of tibetan plateau, the luoshan fault is a right-lateral strikeslip fault. according to the displacement of terraces and alluvial fans and their dating ages, the minimum right-lateral strike-slip rate was determined to be 2.15 ± 0.2 mm/years since late pleistocene. 2) a m 7.3 earthquake occurred in 1561 a.d. in the area close to luoshan mountain. through detailed fieldwork in this area, we related the luoshan fault to this earthquake. the whole fault-rupture is about 60 km, while the largest horizontal displacement evaluated for this earthquake is 5.4 m, the average being 3.8 m. 3) by means of paleoseismological analyses carried out within 2 trenches, besides the 1561 earthquake, we find evidence for three paleoearthquakes along the luoshan fault, and they occurred after 8200 ± 600 years bp, between 3130 ± ± 240 years bp and 4150 ± 120 years c.bp, and before 2230 ± 170 years bp, respectively. acknowledgements this research was supported by china 973 project (g1998040701) and china seismic science fund (102086-95087421). the authors thank their colleagues yang pin and wang ping for their help. we are grateful to dr. paolo galli for carefully review of the manuscript that greatly improved this paper. references avouac, j.p. and p. tapponnier (1993): kinematic model of active deformation in central asia, geophys. res. lett., 20 (10), 895-898. burchfiel, b.c., p. zhang, y. wang, w. zhang, d. jiao, f. song, q. deng, p. molnar and l. royden (1991): 1012 min wei, jiao decheng, chai chizhang, zhang peizhen and mao fengying shiyaodong trench shuangmiao trench 1 3 4 5 7 8 9 ka 2 6present e 1 e 2 e 3 1 5 6 1 ge1ge2ge3 de1 de12de2 fig. 13. comparison of paleoearthquakes and recurrence interval for large earthquakes documented in trenches along luoshan fault. thick bars show the age range of tl from samples. arrows indicate whether the earthquake occurred prior to or after that period. dashed dark line indicates probable historical earthquake in 1561 a.d. e1-e3 indicate paleoevents and sequence, the time estimate corresponds to the shaded area. geology of the haiyuan fault zone, ningxia hui autonomous region, china and its relation to the evolution of the northeastern margin of the tibetan plateau, tectonics, 10 (6), 1091-1110. chai, c., y. liao, w. zhang and w. xu (2001): late quaternary paleoearthquakes and their rupture features along the lingwu fault, seismol. geol., 23 (1), 15-23. cheng, s. and q. deng (1985): the 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(1977): profiles and ages of young fault scarps, north-central nevada, geol. soc. am. bull., 88, 1276-1280. xu, x., w. chen, w. ma, g. yu and g. chen (2002): surface rupture of the kunlunshan earthquake (ms 8.1), northern tibetan plateau, china, seismol. res. lett., 73 (6), 884-892. zhang, b., y. liao, s. guo, r.e. wallace, r.c. bucknam and t.c. hanks (1986): fault scarps related to 1739 earthquake and seismicity of the yinchuan graben, ningxia huizu zizhiqu, china, bull. seismol. soc. am., 76,1253-1287. zhang, p., p. molnar, b.c. burchfiel, l. royden, w. zhang, d. jiao, q. deng, y. wang and f. song (1988): bounds on the holocene slip rate along the haiyuan fault, north central china, quat. res., 30, 151-164. zhang, p., b.c. burchfiel, p. molnar, w. zhang, d. jiao, q. deng, y. wang, l. royden and f. song (1990): late cenozoic tectonic evolution of the ningxia-hui autonomous region, china, geol. soc. am. bull., 102, 1484-1498. zhang, p., b.c. burchfiel, p. molnar, w. zhang, d. jiao, q. deng, y. wang, l. royden and f. song (1991): amount and style of late cenozoic deformation in the liupan shan area, ningxia autonomous region, china, tectonics, 10 (6), 1111-1129. 1013 characteristics of the active luoshan fault since late pleistocene, north central china j j j j j j j j j j layout 6 dissolved co2 in natural waters: development of an automated monitoring system and first application to stromboli volcano (italy) salvatore inguaggiato1,*, lorenzo calderone1, claudio inguaggiato2, sabina morici1, fabio vita1 1 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy 2 università degli studi di palermo, facoltà di scienze matematiche, fisiche e naturali, palermo, italy annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5180 abstract the study of geochemical parameters applied to natural systems has provided improved knowledge of geochemical mechanisms of gas/rock dissolution in natural waters that are linked to gas–water and/or water– rock interaction processes. here we present the results of our studies focused on the development of an automated monitoring system for measuring the amount of dissolved co2 in natural waters. the system is based on the principle of a dynamic equilibrium between water and the air as the host gas. the pco2 measurements were carried out every four hours, and the equilibration time was around 20 minutes. moreover, application to the thermal aquifer of stromboli volcano during the 2009-2010 period is shown and analyzed. the data highlight a clear correlation between the changes in the pco2 in the thermal aquifer and the changes in volcanic activity. introduction over the last 15 years, fluid geochemistry as applied to active volcanic areas has markedly improved our knowledge of the physico-chemical relationships between magmatic and crustal fluids. this improvement has allowed the formulation of geochemical models that are more and more refined, and that are suitable for an understanding of the geochemical variations linked to changes in volcanic activity [bonfanti et al. 1996a, bonfanti et al. 1996b, capasso et. al 2000, inguaggiato et al. 2000, inguaggiato et al. 2005, de gregorio et al. 2007, inguaggiato et al. 2010, inguaggiato et al. 2011]. within this framework, many studies have addressed the monitoring of geochemical parameters that can help in the collection of information relating to the levels of volcanic activity. geochemical surveillance of an active volcano is aimed at recognizing possible signals that are related to changes in the volcanic activity. indeed, as a consequence of magma depressurization inside the "plumbing system" and/or of the refilling of the magma reservoirs from new batches of gas-rich melt, the volatiles dissolved in the magma are progressively released according to their relative solubilities in the basaltic melt. as they approach the surface, the fluids discharged during magma degassing can interact with shallow aquifers and/or can be released along the main volcano-tectonic structures. high-frequency acquisition of geochemical parameters is a target of many studies that are aimed at improving our knowledge of a volcanic system and at highlighting shortterm variations in volcanic activity. the first step is to select the "sensitive" parameters to be monitored, and to select an adequate frequency for the data acquisition. the next step is to identify the "sensitive" sites where the monitoring instrumentation needs to be installed. continuous monitoring of geochemical parameters often needs complex equipment. furthermore, on active volcanoes, the best monitoring sites are often located in areas that are difficult to reach and are sometimes dangerous, like active crater areas. for these reasons, in many cases, it is better to search for suitable sites for monitoring systems in peripheral areas of a volcanic system. the aim of this study was both to develop an automated and remote monitoring system for measurement of pco2 in equilibrium with the natural ground-waters, and to test such in the field by measuring cold and thermal waters located in active volcanic areas (i.e., etna and stromboli volcanoes). finally, considering that stromboli volcano is characterized by high frequency of strombolian activity, we carry out a long-term test of the prototype of this automated system in a stromboli thermal aquifer, to investigate possible relationships between pco2 and summit volcanic activity. geochemical approach to monitoring volcanic fluids shallow magma located beneath active volcanoes releases volatiles both during eruptive activity (active degassing) and during inter-eruptive periods (passive degassing). fumarolic fluids in active volcanic areas arise directly from the magma, and in order of abundance, their article history received march 11, 2011; accepted may 25, 2011. subject classification: gas–water interaction, dissolved co2 , fluid geochemistry, geochemical monitoring. 209 composition is characterized mainly by h2o, co2, so2, h2s, hf and hcl (condensable gases), and by some noncondensable gases (he, h2, n2, co). however, during their migration towards the surface, steam and gas can condense and/or dissolve in shallow fluids, such as ground-water, giving origin to hydrothermal aquifers. in terms of mass balance, the relative contributions of each degassing process are mainly dependent on the type of volcanism and on the intensity of the volcanic activity. considering their high mobility relative to magma, volatiles can provide useful indications about the state of the volcanic activity. there are always gas emissions in a volcanic system during paroxysmal activity, and these are often the only visible manifestations during the inter-eruptive periods (solphataric activity). gas–water interaction processes modify the pristine physico-chemical characteristics of shallow fluids, thus providing useful information regarding both the origin of the fluids and the degree of this interaction at depth. in many cases, significant geochemical changes have been observed in the thermal states of some shallow groundwaters located near active volcanic systems [capasso and inguaggiato 1998, capasso et al. 2000, inguaggiato et al. 2000, inguaggiato et al. 2005, inguaggiato et al. 2010]. this suggests that the relevant input of magmatic gas into the shallow aquifer occurs before changes in the level of volcanic activity; i.e., before eruptions and explosions, and before increases in the number of fumarolic vents and the total volatiles flux [inguaggiato et al. 2011, capasso et al. 2005, carapezza et. al 2004]. the thermal waters, therefore, look promising as targets for geochemical monitoring of active volcanoes. they might represent probable connections with the deep-gas sources, such as a volcano magma chamber and/or deep-seated geothermal reservoirs. to provide better insight into the importance of hydrothermal systems associated with active volcanoes, the geochemical behaviors of several chemical parameters of naturally discharged fluids have been reported. within an overall geochemical model, these parameters can provide important information regarding possible changes in volcanic activity. among these, there are: • ph and pco2: ph measurements provide indications about the presence of acid fluids that reach the surface, and that can interact with ground-waters and change their normal physico-chemical characteristics. in particular, acid fluids can be represented by acid condensates that are rich in hf, hcl and sulfur species, or by pure gas phases, like co2, which once dissolved in an aquifer, results in a decrease in ph. on the basis of the chemical balance between carbon species in solution and ph, it is possible to estimate the partial pressure of the co2 in equilibrium with the aquifer (pco2). therefore, both ph and pco2 provide good indications of the degree of the gas–water interaction processes [inguaggiato et. al. 2000, carapezza et. al. 2004, inguaggiato et. al. 2005, inguaggiato et al. 2011]. • temperature: measurements of the well/spring temperatures, coupled with their water discharge, can provide information about the energy transfer from both the hot and the deep fluids towards the surface. this energy inguaggiato et al. 210 !!!!!!!!!!!! figure 1. stability fields of the carbon species (as indicated) in pure water on the basis of the water ph. the phs of 6.3 and 10.3 (vertical dotted lines) indicate the inflection points, where the ratios of the carbon species are equal to 1.0 (for the h2co3-hco3 and hco3-co3 pairs). 211 transfer can result in significant variations in the average temperatures of shallow aquifers, with an increase in the water temperature that can overlap, and exceed, the normal seasonal oscillations of an aquifer [bonfanti et al. 1996a, bonfanti et al. 1996b]. • conductivity: measurements of the electrical conductivity of natural waters can provide information about their salinity. changes in salinity can indicate changes in the end-member mixing (meteoric and sea waters) and/or the input of deep and saline fluids. • phreatic level: close to volcanic areas, the groundwater levels can be influenced by interactions between volcanic and shallow fluids. indeed, under particular conditions, the gas accumulation in the soils caused by high gas efflux along active tectonic structures can produce temporary decreases in both the permeability and the flow of the ground-water, thus resulting in increases in the local phreatic level [capasso et al. 1999]. dissolution of co2 in natural waters: theoretical principles the co2 released from a magma batch during magma degassing processes produces a series of physico-chemical reactions as it interacts with shallow fluids during its rise towards the surface. during its dissolution in water, co2 reacts with h2o, forming carbonic acid (1) carbonic acid is a weak acid that is ionized in natural waters through the following reactions: (2) (3) the equilibria of these reactions are ruled by the constants k1 and k2, with values of 10−6.4 in equation (2) and 10−10.3 in equation (3), in pure water at 25 ˚c. consequently, the ratio among the carbon species of h2co3, hco3 − and co3 2− in solution is strongly dependent on the ph, and through simple calculations it is possible to obtain the theoretical percentages of the single species as functions of ph (figure 1). dissolution of co2 in water produces a decrease in ph. therefore, measurement of ph can provide indications on the amounts of dissolved co2 in water, and allow calculation of the pco2 in equilibrium with an aquifer: (4) (5) (6) unfortunately, continuous measurements of ph are affected by many technical problems that do not allow the use of ph for long periods of monitoring (weeks to months). this is mainly when the natural waters have high temperatures and/or high salinities. for this reason, we focused our efforts on providing automated equipment to continuously measure the content of the dissolved co2 in these natural waters. the system is based on the dynamic equilibrium between gases dissolved in a liquid phase and a host-gas phase (in this case, air). henry's law describes this physico-chemical principle: at chemical equilibrium and at constant temperature, a known volume of liquid in contact with a given gas dissolves a quantity of gas that is proportional to the gas pressure on the liquid (henry's law). this is expressed by the following relation: dissolved co2 in the stromboli aquifer figure 2. schematic representation of the automated monitoring system for measuring pco2. there are two main circuits: an open water system, driven by a peristaltic pump that pumps the water from the well, and a closed gas system that is driven by a small gas pump that circulates the co2 between the head-space of the equilibration chamber and the ir spectrometer. co h o h co2(g) 2 2 3+ ) h co h hco2 3 3+) + hco h co3 3 2+) +co h o hco2(g) 2 3+ ) logk log hco log h log co3 2= + +-^ ^ ^h h h ph log hco co pk3 2= + -^ ^h h (7) where pi is the partial pressure of the ith gas in the vapor phase (in atmospheres) and xi is its molar fraction in the liquid phase. henry's constant (kh) does not depend on the total gas pressure, and it varies according to both temperature and salinity [capasso and inguaggiato 1998]. technical features of the automated system for monitoring pco2 the automated system that was designed during the present study is represented by two main circuits: an opensystem circuit, where water flows through, and a closed-system circuit, where gases exsolved from and reequilibrated with the water circulate and are analyzed using an infrared (ir) spectrometer (figure 2). a peristaltic pump sends the water to a plexiglas re-equilibration chamber that has a total volume of about 500 ml, with the proportions of 450 ml and 50 ml between the water and the air, respectively. the water level inside the chamber is kept constant by a u-shaped system that discharges any excess water (figure 3). the water that is circulating inside the open system starts re-equilibrating its dissolved co2 with the host gas (the 50 ml of air inside the chamber), according to henry's law. the system reaches equilibrium in about 20 min. at the same time, the gas pump allows the circulation of the gas inside the closed-circuit system that is between the re-equilibration chamber and the ir spectrometer. the amount of co2 in the circulating gas phase is measured by the ir spectrometer, and it increases with time until it reaches the value corresponding to that at complete equilibrium with co2 in water: pco2(gas) )pco2(water) (figure 4). the concentration of co2 is automatically measured in the air inside the chamber both before and after each reequilibration cycle. the values of co2 reached at the end of the equilibration time (about 20 min) represent the partial pressures of the co2in the natural system being studied at the measured water temperature (figure 4). the data acquired are recorded by a data logger, and they are then sent by gsm to the geochemical acquisition centre of the ingv in palermo. the automated system to measure the dissolved co2 comprises the following main items: • re-equilibration chamber (developed at ingv, palermo); • peristaltic pump (rietschle thomas, germany); • data logger (developed at ingv, palermo); • ir spectrometer (edinburgh sensors, uk); • gas pump (knf, germany); • phreatic level sensor (sts, switzerland); • temperature sensor inside the chamber (analogical devices, usa); • temperature probe inside the well (analogical devices, usa); • conductivity sensor (b&c, italy). laboratory tests and calibration system calibration tests of the ir spectrometer were carried out in the laboratory using standard gas mixtures with different concentrations of co2 (range, 0% to 98% vol. co2). inguaggiato et al. 212 !!!!!!!!!!!!!!!! figure 3. photograph of the prototype of the continuous monitoring station for measuring dissolved co2 in natural waters. k pi xi,h = 213 dissolved co2 in the stromboli aquifer !!!!!!!!!!!! !!!!!!!!!!!! figure 4. example of measurement cycles of the dissolved co2 concentration (mv). these cycles reach equilibrium in 15–20 min. the air co2 content is measured both before and after every cycle. six measurement cycles were performed each day (one every 4 h). figure 5. calibration of the ir spectrometer used in the automated monitoring system for pco2, performed with standards of mixed gases with known co2 concentrations in air (from 0.034% to 100% vol. co2). figure 6.comparisons between co2 partial pressures and phs obtained with the laboratory tests. the three curves represent the best fits of the pco2 values both measured (using the automated system and gas chromatography analysis) and computed on the basis of the ph and the hco3 content. the different values of pco2 and ph were obtained by continually bubbling pure co2 through natural water. the data highlight the good correlation between the different ways used for estimating the pco2 and the reliability of the automated prototype system. the good linear response obtained with the ir spectrometer is shown in figure 5. operation system tests were performed using six aqueous solutions at different phs (range, 5.90 to 7.25) and different amounts of dissolved co2. (range, 0.01 to 0.3 atm pco2). to perform this experiment, a 20 l tank of water was used. the aqueous solutions at different phs were prepared by bubbling pure co2 through the water. the phs and the bicarbonate contents of the solutions were measured during the experiments, to estimate the corresponding pco2 values. samples of the dissolved gases (co2) were also collected at different phs for gas-chromatography analysis. figure 6 shows the pco2values both as computed and as measured with the different methods (gas chromatography and automated ir spectrometer) plotted against the corresponding phs. the plot in figure 6 highlights the good correlation between the pco2 analyzed using gas chromatography and the pco2 analyzed using this new automated ir spectrometer system. moreover, the computed pco2 values estimated on the basis of the phs and the hco3 − contents, also provide good data that are very close to the measured values. only in the range of ph close to 6.4 (figure 1, inflection point) are there small differences with the measured values. this is probably due to the strong logarithmic relationship between ph and pco2 at this value, where small differences in ph can result in very large differences in computed pco2. short-term experimental test on natural systems after verifying the good operation of the prototype system in the laboratory, we tested it in the field by measuring natural waters with different physico-chemical characteristics. therefore, we installed two experimental monitoring systems, one in a cold spring (ciapparazzo, nw flank of mt. etna volcano), and the other in a thermal well (ossidiana, stromboli island). ciapparazzo spring is located inside a drainage gallery, with a drainage flow of about 700 l/s. this spring is characterized by low temperature and low salinity (13 ˚c and 700 µs/cm, respectively). ossidiana well is located on the north-east side of stromboli island (an active volcano that is characterized by persistent strombolian activity), and it is characterized by temperatures of around 40 ˚c and high salinity (about 30 ms/cm). the equipment installed in these different natural waters worked successfully, and figure 7 shows the results of the daily averages of pco2 values acquired over the 2–3 months of observation. long-term variations in dissolved co2 on the basis of the promising results obtained from these first two prototype stations, the geochemical application of this methodology was carried out on stromboli island, by installing a revised and improved second version of the prototype monitoring system in a thermal well that had been drilled for scientific purposes (saibbo well, on the north-east side of stromboli island). we analyzed the complete pco2 dataset acquired at the saibbo well station during 2009-2010, which represented the first continuous dataset of dissolved co2 recorded in a well located in an active volcanic system. to improve our knowledge of the behavior of the pco2, and to link its variations to the different degassing regimes, we processed these pco2 data using a cumulated probability graph [sinclair 1974]. this graph is shown in figure 8, where the pco2 is plotted against the cumulated probability for the entire dataset (2009-2010). on the basis of the thresholds identified, we divided these data into three different subsets (b, h and p). group b includes values below 0.12 atm, which represents about 60% of the total acquired dataset, and it refers to a low degree of gas–water interaction. group h includes about 40% of the total acquired dataset, with higher values that range between 0.12 atm and 0.32 atm, and which are interpreted as a high degree of gas/water interaction. the third group (p) includes very few data (only 1% of the total acquired dataset), with values up to 0.6 atm. the histogram of the relative frequencies within the full inguaggiato et al. 214 !!!!!!!!!!!! !!!!!!!!!!!! figure 7. field measurements of pco2 for the ossidiana well (a) and the ciapparazzo spring (b). ossidiana well (temperature, 40 ˚c; salinity, 30 ms/cm) is located in the village of stromboli (stromboli island), and ciapparazzo spring is located on the flank of mt. etna (temperature, 13 ̊ c; salinity, 700 µs/cm). 215 acquired dataset (figure 9) shows an asymmetric unimodal distribution of pco2, which is centered at around 0.1 atm, with a residual "tail" of values up to 0.6 atm. the average value of pco2 (0.1 atm) computed from the data in group b represents the background value during "low" gas–water interactions in the saibbo well. the right residual tail values (group p) range from 0.4 atm to 0.6 atm and warrant separate discussion and elaboration. indeed, even though group p includes only about 1% of the pco2 dataset –– which is insufficient for statistical elaboration –– it represents the highest pco2 values and it would identify the transient variations due to the very high degree of gas–water interaction. the daily averages of pco2 normalized at standard temperature and pressure (stp) conditions are plotted against time in figure 10, together with the main explosive events that occurred at the summit craters of stromboli volcano. two important behaviors can be seen: an increase in pco2 with time from january 2009 to may 2010, with values starting from 0.07 atm that reach 0.60 atm. this behavior suggests a slow and persistent increase in the dissolved gases in the thermal aquifer; moreover, a series of transient increases in pco2 were recorded at different times during the 2009-2010 period, thus suggesting sharp and significant increases in the gas–water interactions that are likely to be due to new input of volatiles from deep magmatic sources. it can also be seen that many of these sharp increases in pco2 were followed by increases in the summit volcanic activity. in particular, a slow increase in pco2 was recorded from january 2009 (0.07 atm) to april 25, 2009 (0.32 atm). the pco2 then quickly returned to background values (0.10 atm), and one week later a paroxysmal event (major explosion) occurred at the summit crater of stromboli volcano. the pco2 of saibbo well then remained almost constant until early october 2009, when a new series of sharp increases in pco2 were recorded, until november 1, 2009. then, two major explosions occurred, on november 8 and 24, 2009. a further sharp increase in pco2 with a significant peak (up to 0.32 atm) occurred on december 14, 2009, followed by two strong explosions on january 4 and 10, 2010. the first semester of 2010 was characterized by a general increase in pco2 driven by two large increments that occurred on april 14, 2010 (0.35 atm), and may 27, 2010 (0.61 atm), and that culminated in a pco2 of 0.61 atm. after this large increase in pco2, the explosive activity of stromboli volcano increased significantly, leading to a major explosive event that occurred on june 30, 2010. dissolved co2 in the stromboli aquifer figure 8.pco2 values measured for the saibbo well plotted against cumulated probability of pco2 values for the entire dataset (2009-2010). boxes with letters p, h and b indicate the three subsets defined in this analysis (see main text). figure 9. histogram showing the distribution frequency (%) of the pco2 values from saibbo well. the plot shows unimodal distribution of the pco2 values centered around 0.1 atm, with residual ‘tail’ values up to 0.6 atm. !!!!!!!!!!! discussion and conclusions high-frequency acquisition of geochemical parameters is one of the main targets of geochemical volcano monitoring. the acquisition of these parameters at different sites and for different types of shallow fluid manifestations represents a good way to obtain a complete picture of a geochemical model of a volcanic system under study. co2 is one of the main species that needs to be monitored in a volcanic system. the prototype of the monitoring station described in this study provides us with the opportunity to significantly improve the data acquisition frequency of pco2, and the possibility for the better modeling of hydrothermal systems in active volcanoes. moreover, the improved amounts of data allow investigations into the relationships among hydrothermal systems, peripheral soil degassing, and summit volcanic activity. inguaggiato et al. 216 !!!!!!!!!!!! figure 10. long-term variations of pco2 (stp, atm) for saibbo well, as acquired from continuous recordings. vertical blue dashed lines, major explosions from the summit craters of stromboli volcano from january 2009 to june 2010. figure 11.long-term variations in the pco2 (stp, atm) for the coa well, as acquired with low-frequency sampling. this graph highlights the positive trend of pco2 with time (arrow) from the end of the last major eruption of stromboli volcano (april 2007) to the present. the 2007 eruption (red column) and the major explosions that occurred in the period from january 2009 to june 2010 (red transparent area, vertical blue dashed lines) are shown. 217 we also investigated the relationship between the continuous pco2 data acquired for saibbo well and a discontinuous dataset of pco2 measured for another thermal well of stromboli volcano (coa) that was also drilled for scientific purposes (figure 11). this well is located in stromboli village, and its water is characterized by high salinity (around 20 ms/cm) and a temperature of around 40 ˚c. considering the very different sampling frequencies between these two wells, the two pco2 datasets are not comparable for short-term variations. however, it is possible to observe good similarity between the two wells as the longterm trends, as for the long-standing increase recorded for both of these wells (figures 10, 11). in particular, after the last eruption that occurred in february-april 2007, when the coa well showed its lowest pco2 values [inguaggiato et al. 2011], there was a constant increasing trend in the dissolved co2 in the aquifer (from 0.10 atm to 0.40 atm pco2). stromboli volcano is normally characterized by periodic mild-to-moderate explosive activity (known as strombolian activity) that is continuously fed by small batches of volatile rich magma. to explain this behavior of the co2 dissolved in the aquifers, we need to consider the geochemical model of stromboli volcano that was recently formulated [inguaggiato et al. 2011]. the "normal" strombolian activity is the result of a "delicate" dynamic balance between the continuous refilling of deep volatiles exsolved from each magma batch, and the superficial degassing [inguaggiato et al. 2011]. the exsolved magma volatiles rise towards the surface and interact with the shallow fluids (i.e., aquifers) and dissolve in the waters, thus increasing the partial pressure of the dissolved gases. during "normal" strombolian activity, furthermore, slowly, but continuously, the volatiles exsolved from the magma increase the total amounts of dissolved gases inside the peripheral hydrothermal systems (figure 12a), thus producing long-term variations that are characterized by a positive increasing trend in pco2 (figure 10). however, sudden and strong impulses of volatiles arriving from deeper zones of the magma chamber can result in sharp increases in pco2 inside the hydrothermal systems (figure 12b), and can dissolved co2 in the stromboli aquifer figure 12. schematical model of interactions between magmatic and shallow fluids in stromboli volcano under the different conditions of the magma/pressure dynamics inside the "plumbing system" of the volcano. a) input of exsolved gases from a magma body causes a progressive increase in the partial pressure of the dissolved gases in the peripheral thermal aquifers. b) input of strong volatiles exsolved from the magma produce a sharp increase in the pco2 inside the hydrothermal system. c) strongly increased degassing from the shallowest portion of the plumbing system (due to paroxysmal events) causes a decrease in the partial pressure of the dissolved gases in the peripheral thermal aquifers. produce transient variations that are characterized by marked pco2 peaks (figure 10). when the magma/pressure levels inside the feeding conduits of a volcano are such that huge amounts of magma rise suddenly towards the surface, the summit strombolian activity increases a lot, which produces increases in both the energy and the frequency of explosions and/or effusive activity. this type of activity usually interrupts the normal strombolian activity (figure 12c), and a large amount of magma and volatiles are expelled from the volcano, thus decreasing the total pressure inside the ‘plumbing system’ of stromboli volcano (fig. 10). after the 2007 effusive eruption, typical strombolian activity was again recorded at stromboli volcano, which then allowed a slow, but continuous, increase in the pco2 of the hydrothermal system, which reached its highest values in 2010. in conclusion, the setting-up of our automated system for measuring pco2 in these natural waters has allowed us to acquire much larger amounts of data that have helped us to better define the geochemical model of stromboli volcano, which will help us further in future volcanic monitoring programs. acknowledgements. we thank franco tassi and salvatore giammanco for their extremely well considered and helpful comments, which improved the quality of our manuscript. references bonfanti, p., w. d'alessandro, g. dongarrà, f. parello and m. valenza (1996a). medium-term anomalies in groundwater temperature before 1991-93 mt. etna eruption, j. volcanol. geotherm. res., 73, 303-308. bonfanti, p., w. d'alessandro, g. dongarrà, f. parello and m. valenza (1996b). mt. etna eruption 1991-93: geochemical anomalies in groundwaters, acta vulcanol., 8, 107-109. capasso, g. and s. inguaggiato (1998). a simple method for the determination of dissolved gases in natural waters. an application to thermal waters from vulcano island, appl. geochem., 13, 631-642. capasso, g., r. favara, s. francofonte and s. inguaggiato (1999). chemical and isotopic variations in fumarolic discharge and thermal waters at vulcano island (aeolian island, italy) during 1996: evidence of a new increase of volcanic activity, j. volcanol. geotherm. res., 88, 167-175. capasso, g., r. favara and s. inguaggiato (2000). interaction between fumarolic gases and thermal groundwaters at vulcano island (italy): evidence from chemical composition of dissolved gases in waters, j. volcanol. geoth. res., 102, 309-318. capasso, g., m.l. carapezza, c. federico, s. inguaggiato, and a. rizzo (2005). geochemical monitoring of the 2002-2003 eruption at stromboli volcano (italy): precursory changes in the carbon and helium isotopic composition of fumarole gases and thermal waters, b. volcanol., 68, 118-134. carapezza, m.l., s. inguaggiato, l. brusca and m. longo (2004). geochemical precursors of the activity of an open-conduit volcano: the stromboli 2002-2003 eruptive events, geophys. res. letters, 31, l07620. de gregorio, s., p. madonia, s. gurrieri, g. giudice,and s. inguaggiato (2007). contemporary total dissolved gas pressure and soil temperature anomalies recorded at stromboli volcano (italy), geophys. res. letters, 34, l08301; doi: 10.1029/2007gl029578. inguaggiato, s., g. pecoraino and f. d'amore (2000). chemical and isotopical characterization of fluid manifestations of ischia island (italy), j.volcanol.geoth.res., 99, 151-178. inguaggiato, s., a.l. martin-del pozzo, a. aguayo, g. capasso and r. favara (2005). isotopic, chemical and dissolved gas constraints on spring water from popocatepetl (mexico): evidence of gas-water interaction magmatic component and shallow fluids, j.volcanol.geoth.res., 141, 91-108. inguaggiato, s., s. hidalgo, b. beate and j. bourquin (2010). preliminary geochemical characterization of volcanic and geothermal fluids discharged from the ecuadorian volcanic arc, geofluids; doi: 10.1111/j.1468-8123.2010.00315. inguaggiato, s., f. vita, d. rouwet, n. bobrowski, s. morici and a. sollami (2011). geochemical evidence of the renewal of volcanic activity inferred from co2 soil and so2 plume fluxes: the 2007 stromboli eruption (italy), b. volcanol.; doi: 10.1007/s00445-010-0442-z. sinclair, a.j. (1974). selection of threshold values in geochemical data using probability graphs, j. geochem. explor., 3,129-149. *corresponding author: salvatore inguaggiato, istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy; email: s.inguaggiato@pa.ingv.it. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. inguaggiato et al. 218 suggestion f o r a new classification o f earthquakes according to the focal depth p . h e d e r v a r i ( * ) r i c e v u t o il 25 marzo 196-1 summary. — i n this paper the a u t h o r suggests to regard the lower limit of the category of the so-called shallow earthquakes identical with the level of the regional isostatic compensation and the upper limit of the so-called deep shocks identical with the byerly-discontinuity. riassunto. — in questa nota l ' a . suggerisce di identificare il limite inferiore della categoria dei cosi detti terremoti a profondita normale con il livello della superflcie di compensazione isostatica e il limite superiore dei cosi detti terremoti profondi con la discontinuity di b y e r l y . i n t h e g e o p h y s i c a l p r a c t i c e it is c u s t o m a r y t o c l a s s i f y t h e e a r t h q u a k e s i n t o three d i f f e r e n t g r o u p s a c c o r d i n g t o t h e d e p t l i of l i y p o c e n t e r . i n a c c o r d a n c e w i t h this c l a s s i f y i n g w e distinguish shallow, i n t e r m e d i a t e a n d d e e p shocks r e s p e c t i v e l y . a c c o r d i n g t o t h e classical d i v i s i o n : t a b l e i . denomination l i m i t s of focal depth, km shallow intermediate deep 0 70 (sometimes 0-00) 70-300 300-750 ( * ) f . r . a . s . vice-president of international lunar society. 3 7 0 p . i i e d e r v a i i i some authors, e. g. ritsema (2) and honda (3) used a somewhat different classifying than that of in table i . on the 6th figure of ritsema's paper we find the following division of shocks: table i i . gro up limits of focal depth, km i . 0-40 i i . 40-130 i i i . > 1 3 0 in his quoted paper honda wrote the followings: " the earthquakes are classed here as shallow when the focal depth does not exceed 100 km, intermediate when it is from 100 km to 250 km, and deep when it exceeds 250 km " . there is no particular geophysical reason to maintain further the classical division of earthquakes (see in table i ) . it seems to be more real if we connect the limits of focal depth with certain special geophysical levels in the earth. such levels may be the different discontinuities-, the mohorovicic-surface, the byerlyand repetti-ones. however here appears a rather great difficulty. as it is wellknown, the depth of the mohorovicic-discontinuity is varying from place to place in accordance with the regional relief of the surface and the rule of isostasy. although there are many data of the earth's crustal thickness, due to gravitational and seismical explorations, nevertheless the number of useful values is as the present too small for a theoretical world-wide investigation. on the other hand the crustal thickness under oceanic territories is only 5-15 km or somewhat larger. if we connect the lower limit of the shallow shocks' category with the mohorovicic-discontinuity, the greatest part of earthquakes over the oceanic territories (earlier pertaining to the shallow category), — would now belong into the group of the intermediate quakes. hereby much of the regularities discovered concerning the shallow shocks all over the world would disappear. naturally this were not desirable. therefore ice suggest to regard the lower limit of the shallow shocks' category identical with the level of the regional isostatic n e w c l a s s i f i c a t i o n of e a r t h q u a k e s a c c o r d i n g . 3 7 1 compensation. according to the calculations of gutenberg (4) the pressure between the surface and the depth of 50 km is as follows (see on table he): table i i i . depth, pressure of column in bars pressure difference in bars km a b c a-b a-c b-c 0 770 0 0 770 770 0 6 2340 1550 590 790 1750 900 11 3650 2800 2000 790 1050 860 15 4700 3910 3300 790 1400 610 20 6010 5220 4920 790 1090 300 25 7320 0680 0530 640 790 150 30 8780 8140 8150 640 630 — 10 40 11700 11380 11390 320 310 — 10 50 14630 14620 14630 10 0 — 10 a) l>) c) pressure under a 3 km high mountain area; pressure under continental lowlands; pressure under a 6 km deep ocean. i t is to be seen that the difference of pressure in a depth of 50 km is almost zero. therefore we may regard the depth of 50 km as the level of regional isostatic compensation. in accordance with these considerations the suggested new classification of earthquakes regarding their focal depth is as follows: • table i v . category limits of focal depth km s r i c surface, level of regional isostatic compensation 0-50 r i c b level of regional isostatic compensation, byerly-discontinuity 51-413 b r /?i/e)zy-discontinuity, bepetti-discontinuity > 414 table v . date l a t . long, (or approximate location) m t y p e focal depth km earlier class 1896 jan. 19 central honsh u 8,1 s r i c normal shallow 1896 march 4 •! •>. 8,3 1 s r i c ? normal ? shallow ? 1896 may 5 ? * 8,3 « s r i c ? normal ? shallow 1 1896 j une 15 n e honshu 8,7 s r i c normal shallow 1896 june 15 n e honshu 8,5 s r i c normal shallow 1896 june 15 n e honshu 8,5 s r i c normal shallow 1896 june 17 central honshu 8,5 1 s r i c normal shallow 1896 aug. 31 n e honshu 8,5 s r i c normal shallow 1897 febr. 7 40 n 140 e 8,3 s r i c normal shallow 1897 febr. 19 38 n 142 e 8,3 s r i c normal shallow 1897 febr. 19 38 n 142 e 8,3 s r i c normal shallow 1897 may 28 e mediterran 8,0 r i c b ? 1 intermediate '! 1897 j une 12 26 n 91 e 8,7 s r i c 25 shallow 1897 aug. 5 38 n 143 e 8,7 s r i c normal shallow 1897 sept. 20 6 n 122 e 8,6 s r i c normal shallow 1897 sept. 21 6 n 122 e 8,7 s r i c normal shallow 1897 oct. 18 12 n 126 e 8,1 s r i c normal shallow 1898 apr. 22 39 n 142 e 8,3 s r i c normal shallow 1898 june 29 °l 8.3 1 s r i c normal ? shallow ? 1899 jan. 24 17 n 98 w 8,4 s r i c normal shallow 1899 sept. 4 60 n 142 w 8,3 s r i c normal shallow 1899 sept. 10 60 n 142 w 8,6 s r i c normal shallow 1900 jan. 20 20 n 105 w 8,3 s r i c normal shallow 1900 july 29 10 s 165 e 8,1 s r i c normal shallow 1900 oct. 9 60 n 142 w 8,3 s r i c normal shallow > > > > © > > > > > > > > fe 2 > > > > > > > © jo 1 -= 1 a g a s _o s t — j — ; £ w co co co "al co co 'co h i h o co co & co & h i co 57 'r t © o 'cs o ci c3 cc "3 c z g lo § § £ r ' co £ r ^ £ £ g r £ g g uo § o o £ 2 io 2 § lo "" c lo o c c § o 2 _ — o c2 o o o rj o 5 6 6 o «« o s s rj l cc.d̂ ccph m s z fl 2h p? co « o o p5 u lv lt fl x fl t> co co i> ^ co co cc cc co co cc co 00 go 00 go 00 oc* x 00 00 gc x x 00 oo" cc cc cc 00 oo" c c oc 00 00 co bb — g o _ l> r 1 p k*" i— ci o _ o go (m x co i(m 00 _ cq ac o z d co cs ^ (m co co c5 i© co lo t> >0 (m cs x cs tcc (n r-h co co co co <n co cs co co r-h •-h ĥ 1— i 1— 1 1— 1 r-h r-h — ' r-h ĥ c3 m z r^ a iz n 02 25 25 t/2 525 02 m 25 25 25 s5 25 cc co 0 2 £ •sm vtt -r. •w rh r-1 *h co — " « r" rt r-î c3 co o o o <n o c [ cj2 co -t os <m o cc x co x co u lo co co — co cc <m —" cc (m ^ (n h— 1 xo -t o o i i -rf th (n io (n (m co cq csi 1—1 <n — <n <m s i c i im c ^ s3 d © > > > bb ti > o ^ ti: d t'c tl o o © o £1. <d c5 c . ^ £ o p_ qj o s o © £ h-5 << p ) is < i q ^ p i x x c5 c5 c5 0 , <n (m <n (m co co co co 5c cc cs 0 o 0 0 co co co co co -t -+ -+ -+ -+ -t -t -t -t o o o c5 c l o o o c5 c5 cs o cs c5 c-. c5 cr. cr. c i os a cs cs cs o c5 c5 c5 ( continuation) date 1952 march 4 1952 n o v . 4 1953 n o v . 25 1955 febr. 27 1957 march 9 1957 april 14 1957 dec. 4 1958 july 10 1958 n o v . 6 1959 may 4 1959 sept. 14 1960 may 21 june 20 1960 may 21 june 20 1960 may 22 i 963 oct. 13 l a t . long, (or approximate location) 421/, n 143 e 523/4 n 159 % e 34 n 141% e 28 s 175% w 51% n 176% w 15% s 173 w 45% n 99% e 58 % n 136 w 44% n 1483/4 e 52% n 159% e 28% s 177 w 37 % s 73% w 38% s 733/ >w 38% s 74% w 45% s 73v4 w 373/„ s 393/4 s 443/. n 73 w 74i/2 w 149% e m 8,0 8.4 8,3 8,0 8,25 8,0 8,3 8 , 0 8,7 8,25 8,0 8.5 8,5 8,3 8,0 8,25 8,9 8,0 t y p e s-ric s-ric ric-b s-ric s-ric s r i c s-ric s-ric ric-b ric-b s-ric s-ric s-ric s-ric s-ric s-ric s-ric ric-b focal deptli km normal normal 60 normal normal normal normal normal 75 60 normal normal normal normal normal normal normal 60 earlier class shallow shallow shallow shallow shallow shallow shallow shallow intermediate shallow shallow shallow shallow shallow shallow shallow shallow ( * * ) shallow ( * ) great chilean earthquake-sequence. ( * * ) according to rotlie. 3 7 8 p . i i e d e r v a r i l a s t l y , w e show the n e w classification in t h e case of t h e greatest e a r t h q u a k e s ( m > 8,0) b e t w e e n j a n u a r y 1896 a n d o c t o b e r 1963. t h e m a g n i t u d e d a t a w e r e d e t e r m i n e d b y g u t e n b e r g (5), r i c h t e r ( 6 ), galanopoulos ( 7 ), e a t o n ( 8 ), tlie bulletin of the seismological s o c i e t y of a m e rica ( 9 ) and u s coast a n d g e o d e t i c s u r v e y (1 0) r e s p e c t i v e l y . r e f e r e n c e s (*) bulletin of the seismological society of america, 44, 4, (1954). (2) ritsema a . r., the seismicity of the sunda arc in space and time. « indonesian journal for natural science », 1-3. january-june, (1954). (3) honda ii., the mechanism of the earthquakes. publications of the dominion observatory, ottawa, x x . 2, (1957). ( ' ) gutenberg b., geophysical data implied in isostatic calculations. division of the geological sciences, california institute of technology, 675. contribution. (5) gutenberg b., great earthquakes 1896-1903. transactions of american geophysical union, 37. 5, (1956). — major and great earthquakes of 19o7 and 19os. « bulletin of the seismological society of america », 49, 50, (1959, 1960). — seismicity of the earth. second edition. princeton, 1954. (together with richter, c.f). ( 6 ) richter c. p . , elementary seismology. san francisco, 1958. ( ' ) galanopoulos a . g., a catalogue of shocks with to ji vi or .1/ js 5 for the years 1s01-1958. athens, 1960. (8) eaton j. p . , takasaki k . j., seismological interpretation. . . « bulletin of the seism. soc. a m . i, 49. (1959). (9) bulletin of the seismological society of america, 43-52. (1953-1962). (10) preliminary determinations of epicentres, 1962 and 1963. u.s. dept. of commerce coast and geodetic survey. shen_correctedok:layout 6 annals of geophysics, 56, 3, 2013, r0325; doi:10.4401/ag-5749 r0325 a new formulation of stokes’ approach in determining the global gravimetric geoid wen-bin shen1,2 1 wuhan university, key laboratory of geospace environment and geodesy of ministry of education, school of geodesy and geomatics, wuhan, china 2 state key laboratory of information engineering in surveying, mapping and remote sensing, china abstract according to stokes’ approach, given the gravity anomaly on the geoid as the boundary, a disturbing potential function satisfying some boundary conditions should be solved. the basic requirement is that the disturbing potential function should be harmonic in the region outside the geoid. however, since the normal gravity potential is not defined inside the reference ellipsoid (taking the wgs84 ellipsoid as an example), when the geoid is below the ellipsoidal surface, the disturbing potential function is not harmonic in the whole region outside the geoid, and is not defined on the whole geoid itself. these are theoretical difficulties in stokes’ approach. to remove these difficulties from stokes’ approach, this study provides a new formulation of stokes’ approach. an inner ellipsoid with four fundamental parameters is chosen, two of which, the geocentric gravitational constant and the rotational angular velocity, coincide with the corresponding parameters of the wgs84 ellipsoid. the other two parameters, the semi-major axis and flattening, are different from the corresponding ones of the ellipsoid. then, the normal gravity potential generated by the inner ellipsoid is determined, by requiring that it holds a constant on the surface of the inner ellipsoid or on the surface of the ellipsoid. with this new formulation, the disturbing potential function is harmonic in the whole region outside the geoid, and the difficulties in stokes’ approach disappear. the new formulation proposed in this study is also adequate for analogous geodetic boundary-value problems. 1. introduction in physical geodesy, to determine the global gravimetric geoid, one of the most frequently used methods is stokes’ approach [stokes 1845; see also, e.g., heiskanen and moritz 1967, wong and gore 1969, vaníček and kleusberg 1987, torge 1989, torge 1991, vaníček and martinec 1994, vaníček et al. 1996, martinec 1998, sjöberg 2000, sjöberg 2001, sjöberg 2003, vaníček et al. 2004, hofmann-wellenhof and moritz 2005, ellmann and vaníček 2007]. the key to this approach is to solve the following boundary-value problem: given the gravity anomaly dg = gp − cq on the geoid g (as the boundary), where gp and cq are the gravity at point p on the geoid and the normal gravity at the corresponding point qp on the surface e of a reference ellipsoid (hereafter, taking the wgs84 ellipsoid as the reference ellipsoid, simply named as ellipsoid for convenience), respectively, a disturbing potential function t(p) = w(p) − u(p) should be found that satisfies the following conditions (see figure 1): (i) t(p) is defined in the region xg outside the geoid and on the geoid g: (1a) (ii) harmonic in xg: (1b) (iii) satisfying the fundamental equation of physical geodesy on the boundary g: (1c) (iv) regular at infinity: (1d) here, w(p) and u(p) are the geopotential and the normal gravity potential, respectively, n is the external (outward) normal on the geoid g, c is the normal gravity at point qp on the ellipsoidal surface, and article history received april 21, 2012; accepted february 15, 2013. subject classification: stokes’ approach, reference ellipsoid and normal gravity potential, disturbing potential, global gravimetric geoid determination, new formulation. , ;t p w p u p p ggd , /x x= g^ ^ ^h h h 0 , ;t p p g 2 dd x=^ h , ;t n t p n g p g1 d2 2 2 2c c d+ =^ h .lim t p 0 r = "3 p ^ h x =g is the sum of xg and g. as is well known, the external normal gravity potential field, or somiglianapizzetti normal gravity potential field, of a level ellipsoid [pizzetti 1913, somigliana 1929] is uniquely determined by only four fundamental parameters [see e.g., heiskanen and moritz 1967, moritz 1980b, torge 1989, 1991, martinec and grafarend 1997, martinec 1998, ardestani and martinec 2000, ardalan and grafarend 2001, yurkina and pick 2002, vaníček et al. 2004, hofmann-wellenhof and moritz 2005]: the semi-major axis of the ellipsoid, a; the geocentric gravitational constant, gm; the dynamical form factor of the earth, j2 (or equivalently replaced by the flattening, f ); and the earth rotational angular velocity, ~. then, a solution can then be found, which is given by stokes’ formula [see e.g., heiskanen and moritz 1967, vaníček et al. 2004, hofmann-wellenhof and moritz 2005], and further, based on bruns’ formula, the geoidal undulation can be determined as n = tg (p)/ce (qp), p ∈ g, qp ∈ e, where t g (p) is the disturbing potential at point p on the geoid g, and ce (qp) is the normal gravity at the corresponding point qp on the ellipsoidal surface e. in stokes’ approach, there is an a-priori requirement: the masses outside the geoid should be taken away or removed into the inside of the geoid [e.g., heiskanen and moritz 1967, rapp 1997, hofmannwellenhof and moritz 2005], so that it is guaranteed that the disturbing potential function is harmonic in the region xg. to satisfy this requirement, one of the most frequently used methods is helmert’s first or second condensation [see, e.g., heiskanen and moritz 1967, moritz 1968, vaníček and kleusberg 1987, wang and rapp 1990, martinec et al. 1993, vaníček and martinec 1994, martinec 1998, sjöberg 2000, heck 2003, vaníček et al. 2004, ellmann and vaníček 2007], which means that the masses outside the geoid are condensed as a surface layer either below the geoid by 21 km, or on the geoid, respectively. hence, hereafter, we will assume that the masses outside the geoid have been removed into the inside of the geoid (e.g., by helmert’s first condensation method) or on the geoid (e.g., by helmert’s second condensation method). in this case however, both the earth external gravity field and the geoid will be changed, and further corrections are needed [e.g., heiskanen and moritz 1967, vaní ek et al. 2004]. this is the well-known disadvantage in the conventional stokes’ approach [see e.g., heiskanen and moritz 1967, hofmann-wenlenhof and moritz 2005]. as well as this disadvantage, in the present study we indicate that there are still theoretical difficulties (which are different from the ‘well-known disadvantage’) in the conventional stokes’ approach for determining the global gravimetric geoid (see section 2 for details). in section 2, we briefly discuss the difficulties in the conventional stokes’ approach. then, in section 3, a new formulation of stokes’ approach is provided, and further, a modification of this new formulation is suggested in section 4. finally, section 5 provides a summary of the present study and discussions related to this topic. 2. comments on stokes’ approach in stokes’ theory (or the stokes boundary-value problem), although the earth (external) geopotential w(p) is defined in the region , the normal gravity potential (or pizzetti normal gravity potential) u(p) is defined only in the region , which denotes the sum of the ellipsoidal surface e and the region xe outside the ellipsoid (see figure 2), because u(p) is not defined inside the ellipsoid. this conclusion can be referred to e.g., heiskanen and moritz [1967], pick [1978], moritz [1990], hofmann-wenlenhof and moritz [2005], and other various relevant publications. for instance, heiskanen and moritz [1967, p. 64] and hofmann-wellenhof and moritz [2005, p. 65] indicated that the density distribution in detail inside the rotational ellipsoid (e.g., the wgs84 ellipsoid), which produces the normal gravity potential u(p), is not interesting and need not be known at all. indeed, there is no ‘reasonable’ mass distribution inside the rotational ellipsoid [moritz 1990]. this clearly demonstrates that the normal potential field generated by the rotational ellipsoid has no definition inside it, because different distributions provide different inner potential fields. pick [1978] also indicated that the pizzetti normal gravity potential and its derivatives have no definitions inside the rotational ellipsoid, and consequently, pick [1978] proposed an approach to define the inner field. the idea is stated as follows [pick 1978, 1990]: assuming that the density wen-bin shen 2 figure 1. the closed elliptic thin curve denotes the surface e of the wgs84 ellipsoid; the closed irregular bold curve denotes the geoid g. the region filled with slanted lines is the region bounded by the ellipsoidal surface and the geoid where the geoid is above the ellipsoidal surface, indicated by i. the region filled by points is the region bounded by the geoid and the ellipsoidal surface where the geoid is below the ellipsoidal surface, indicated by ii. gg ,x= t p p d xg^ ^h h gg g ,x x= eg g ,x x= 3 profile of the actual earth is known from its surface to a certain depth d, then it is possible to define (or compute) the normal gravity potential and its derivatives inside the ellipsoid from its surface until the depth of d. this can be realized, of course, by combing the newtonian integral formula with a ‘remove–restore’ technique. indeed, given the density distribution inside the ellipsoid or the earth, it is possible to define and determine the normal gravity potential, or the geopotential, in the whole (outer and inner) space based on the newtonian integral. however, as “the density profile of the actual earth up to a certain depth d” is not known, or not precisely known, the inner pizzetti normal gravity potential field is not defined or cannot be precisely determined. although vaníček et al. [2004] declared that “one can certainly compute the value of normal gravity everywhere on, above and even a little below the surface of the reference ellipsoid”, they did not specify how to compute the normal gravity in the region of “a little below the surface of the reference ellipsoid”. indeed, even if the normal gravitational potential field (which is u(p), taking away the centrifugal potential) is defined inside the ellipsoid (where some kind of density distribution is given), it is still not harmonic inside the ellipsoid, unless it is assumed that u(p) is generated by a smaller ellipsoid. hence, in the conventional stokes’ approach, the disturbing potential t(p) = w(p) − u(p) is defined only in the cross-region , and har monic only in the cross-region . the cross-regions and are respectively smaller than the regions and xg, referring to figures 1 and 2. therefore, conditions (i) and (ii) listed in section 1 are not satisfied. as the reference ellipsoid that generates the normal gravity potential field is chosen in such a way that it can best fit the geoid, it is clear that some parts of the geoid are above the ellipsoidal surface and other parts are below the ellipsoidal surface (see figure 1). this conclusion can be confirmed by examining a global gravimetric geoid, e.g., the global geoid associated with the egm96 [lemoine et al. 1998] or egm2008 [pavlis et al. 2008, 2012]. hence, the disturbing potential t(p) is not defined in the whole boundary g, as the normal gravity potential u(p) has no definition inside the ellipsoid. hence, condition (iii) listed in section 1 is not satisfied either. it can be argued that the normal gravitational part of the potential u can indeed be harmonically continued inside the ellipsoid down to a surface of an inner ellipsoid. therefore the definition of the disturbing potential t = w − u should be kept in the domain outside the geoid, with u (without the centrifugal potential) specified by a harmonic function defined at least in the domain above the first layer that is below the geoid. however, the key problem is in which way the normal potential u(p) is defined, but not artificially interpreted. a standard definition of u(p) is provided in e.g. heiskanen and moritz [1967], torge [1991], and hofmannwenlenhof and moritz [2005], where an external normal potential field u(p) is uniquely determined using only four parameters based on stokes’ theorem [heiskanen and moritz 1967, p. 17] or dirichlet’s principle [hofmann-wellenhof and moritz 2005, p. 27] (or see any general text book), taking the ellipsoidal surface as the boundary. then, the normal potential field must be defined only in the external region of the ellipsoid. heiskanen and moritz [1967, p. 64] and hofmannwellenhof and moritz [2005, p. 65] wrote: “the normal potential function u(x,y,z) is completely determined by: 1. the shape of the ellipsoid of revolution, i.e., its semiaxes a and b, 2. the total mass m, and 3. the angular velocity ~.” clearly, inside the ellipsoid, u(x,y,z) has no definition. if u(x,y,z) is continued inside the ellipsoid down to an inner-ellipsoidal surface, it is in fact generated by an inner ellipsoid, not the conventional ellipsoid. hence, the three conditions, (i), (ii) and (iii), listed in section 1 are not satisfied, and there are theoretical difficulties in the conventional stokes’ approach. similar comments are suitable for the boundary-value problems that are similar to the stokes’ boundary-value problem. in the following section, we will propose a new formulation of stokes’ approach, and then the theoretical difficulties in the conventional stokes’ approach mentioned here will disappear. 3. a new formulation to remove the difficulties as listed in section 2, we choose an inner reference ellipsoid (hereafter, simply the inner ellipsoid) that is completely enclosed by the geoid, a new formulation of stokes’ approach figure 2. the closed irregular bold curve denotes the boundary cthat consists of the parts of the geoid that are above the ellipsoidal surface, the parts of the ellipsoidal surface that are above the geoid, and the parts where the geoid coincides with the ellipsoidal surface. the region outside c is the cross-region xg ∩xe (not denoted in the figure). e+x xg e+x xgg e+x x g e+x x xg with its center coinciding with that of the ellipsoid (see figure 3). the condition can be set on the surface ei of the inner ellipsoid, so that the normal gravity potential field ui(p) generated by the inner ellipsoid is equal to the constant w0 = u0 on the ellipsoidal surface with a high accuracy level of 0.3 mm, which is much higher than the present accuracy requirement (e.g., about a 1-cm-level geoid determination), where w0 is the geopotential constant on the geoid, and u0 is the normal gravity potential constant on the ellipsoidal surface, as conventionally given (i.e., u0 = w0). the accuracy level of 0.3 mm and the boundary value on the surface of the inner ellipsoid were simultaneously determined by the following iterative procedures: (1) setting the initial (first) constant value (= w0 or a little larger than w0, as e.g., w0+ 1000.0 m 2s-2) on the inner-ellipsoidal surface, and solving the external boundary-value problem, to obtain the first solution ; (2) calculating the difference between the value u0(= w0) and the average value of over the ellipsoidal surface, denoted as , to obtain the first difference , which is a constant; (3) taking the sum of the first value and the difference as a new boundary value on the inner-ellipsoidal surface, repeating procedure (1), to obtain the second solution . repeating the above procedures, the value was determined. if a higher accuracy level (<0.3 mm) is required, then the boundary value on the inner-ellipsoidal surface should be more precisely determined. however, since at present the accuracy of the value w0 determined in practice is no better than 2 cm [see, e.g., burša et al. 2007a, burša et al. 2007b, sanchez 2007, sanchez 2009, luzum et al. 2011, dayoub et al. 2012], it is accurate enough to put the constant on the inner-ellipsoidal surface, so that the potential field ui(p) generated by the inner ellipsoid coincides with the normal potential field u(p) outside the ellipsoid. then, the disturbing potential ti(p) = w(p) − ui(p) based on the normal gravity potential generated by the inner ellipsoid is harmonic in the region xg. for convenience in later use, the normal gravity potential ui(p), normal gravity ci(p), etc., generated by the inner ellipsoid are simply referred to as the new normal gravity potential, new normal gravity, etc., respectively, and the corresponding disturbing potential ti(p), gravity anomaly dgi(p), geoidal undulation ni, etc., are respectively referred to as new ones. by examining the egm2008 geoid [pavlis et al. 2008, 2012], it can be seen that the maximum absolute value of the geoidal undulation over the globe will not exceed 120 m. hence, for convenience in practical use, the semi-major axis of the inner ellipsoid can be chosen as ai = a − 150 m, so that it is guaranteed that the inner ellipsoid lies completely inside the geoid, where a is the average equatorial radius of the earth, one of the four fundamental parameters of the ellipsoid (see table 1). for the other three fundamental parameters, fi, gmi and ~i (which denote the flattening, geocentric gravitational constant, and rotational angular velocity, respectively), two of these, gmi and ~i, should hold the same as given by the ellipsoid; namely, gmi = gm, ~i = ~; and fi is chosen in such a way that on the whole surface of the ellipsoid, the average value of the new normal gravity potential field generated by the inner ellipsoid equals the geopotential constant w0, as conventionally given within the range of accuracy required. this can be realized by iterative numerical calculations, as mentioned above. based on the iterative numerical calculations, the four fundamental parameters of the inner ellipsoid were determined, as listed in table 1. in this way, the new normal gravity potential equals the constant u0= w0 on the ellipsoidal surface. based on the four fundamental parameters of the ellipsoid (see table 1), the derived parameters, the semiminor axis b of the ellipsoid and the normal gravity potential generated by the ellipsoid on the surface of the ellipsoid, were determined as b = 6356752.31424 m and u0 = w0 = 62636851.7146 m 2s-2, respectively, which are listed in table 1. based on the four fundamental parameters of the inner ellipsoid (see table 1), those corresponding to the inner ellipsoid were determined as bi = 6356603.10537 m and , respectively, which are also listed in table 1. it can be seen that is greater than w0 by 1467.0868 m 2s−2 ; namely, . u0 is related to gm according to the following rewen-bin shen 4 figure 3. new formulation of stokes’ approach. the dashed elliptic curve denotes the surface e of the wgs84 ellipsoid, the closed irregular bold curve denotes the geoid g, the closed solid elliptic curve denotes the surface ei of the inner ellipsoid. the inner ellipsoid (which is drawn exaggeratedly in the figure) is completely enclosed by the geoid. 1467.0868 m su w0i 0 2 2= + 1467.0868 m su w0i 0 2 2= + 1467.0868 m su w0i 0 2 2= + 1467.0868 m su w0i 0 2 2= + u w0i 0= + 1467.0868 m s2 2+ u (1)0i u (1)0i u (1)i u (1)i e;u (1) i eu ( ) i 1 ;d = eu ( ) i 1 ;d e;u u ( ) i0 1= u ( )i 2 m s.u 62638318 8014i 0 2 2= ui 0 5 lationship [e.g., heiskanen and moritz 1967]: (2) where is the linear eccentricity. the theoretical foundation is stated as follows. it is required that the new normal gravity potential generated by the inner ellipsoid holds constant on the surface of the inner ellipsoid, and then the conventional way can be used to express the new normal gravity potential ui(p) generated by the inner ellipsoid. the constant value on the inner-ellipsoidal surface was determined by an iterative procedure, so that the value of ui(p) on the ellipsoidal surface is equal to w0. in this way, the conventional stokes’ approach can be applied. in the region outside the inner ellipsoid, the new normal gravity potential field ui(p) is uniquely determined by the following formula [see, e.g., heiskanen and moritz 1967]: (3) where is the linear eccentricity, ai, bi, ~, gm are the four fundamental parameters of the inner ellipsoid, u is the first component of the ellipsoidal coordinates (u,i,m), b is the reduced latitude, and and qi are defined as follows: (4a) (4b) on the surface ei of the inner ellipsoid, ui (u,b) takes the constant value (5) based on equations (3) to (5), by numerical calculations, bi (or fi ) and were determined, as listed in table 1. the new normal gravity potential ui(p) is equal to the constant w0 at some points, and different from this constant at other points on the ellipsoidal surface. the numerical comparisons and their statistical analysis between the new normal gravity potential field ui and the constant w0 on the ellipsoidal surface are shown in figure 4 and given in table 2. we see that on the ellipsoidal surface, between ui|e (which denotes the value of ui on the surface of the ellipsoid) and w0, the maximum difference, the mean, and the rms are, respectively, 3.3 × 10-3 m2s-2 , 0.21 × 10-3 m2s-2 and 2.2 × 10-3 m2s-2 (equivalent to 0.3 mm, 0.02 mm, and 0.2 mm in height, respectively). hence, for the present accuracy requirement, this demonstrates that the new normal gravity potential ui holds a constant w0 on the ellipsoidal surface. then, the conventional concepts (e.g., geoidal undulation, etc.) can be completely applied without any more difficulties. the new disturbing potential ti(p) is defined by equation (6): (6) which is harmonic in the region xg: (7a) following heiskanen and moritz [1967] or hofmann-wenlenhof and moritz [2005], the fundamental equation of physical geodesy can be immediately arrived at: (7b) a new formulation of stokes’ approach fundamental parameters a (m) f gm (m3s-2) ×108 ~ (rad/s) ×1011 derived parameters b (m) u0 (m 2s-2) wgs84 ellipsoid 6378137 1/298.257223563 3986004.41800000000 7292115 6356752.31424 62636851.7146 inner ellipsoid 6377987 1/ 298.261243404 3986004.41800000000 7292115 6356603.10537 62638318.8014 difference 150 4.518778778×10-8 0 0 149.20887 1467.0868 table 1. fundamental parameters and derived parameters of the wgs84 ellipsoid and the inner ellipsoid. ui 0 ui 0 ui 0 ,tanu gm b a3 1 0 1 2 2l ~= +l a b2 2l= ~ , , tan sin cos u u gm a q q u2 3 1 2 i i i i i i 1 2 2 0 2 2 2 2 2 b ~ b l b = + + + + l ^ ` ^ h j h a bi i i 2 2l = qi 0 3 ,tanq b b b2 1 1 3i i i i i i i0 2 2 1 l l l= + -c m; e 3 .tanq uu u2 1 1 3i i i i 2 2 1 l l l= + -c m; e .tanu gm b a3 1 i i i 0 1 2 2~= +l i i l , ;t p w p u p pi i d x= g^ ^ ^h h h ,,n t p n t p p gg p 1i i e i e i i d2 2 2 2 c c d+ = ^ ^ ^ h h h , .t p p0i g 2 dd x=^ h where, the new gravity anomaly dgi (p) is the difference between the gravity g(p) at the point p of interest on the geoid and the new normal gravity at the corresponding point qp on the ellipsoidal surface, n is the external new normal gravity line (i.e., the line coinciding with the new normal gravity vector ), and the new disturbing potential ti(p) satisfies the regular condition at infinity: (7c) now, based on equation (7), following the conventional way [e.g., heiskanen and moritz 1967], ti(p) can be determined based on the new gravity anomaly dgi (p) given on the boundary g (the geoid). then, the similar bruns’ formula (the new bruns’ formula) can be immediately arrived at: (8) where and are the new disturbing potential value on the geoid and the new normal gravity value on the surface of the ellipsoid, respectively, ni is the new geoid undulation, which is the distance from the corresponding point qp on the surface of the ellipsoid to the point p of interest on the geoid along the external new normal gravity line, and which is now related to the new normal gravity potential field ui. the formula of equation (8) is quite similar to the conventional bruns’ formula [see e.g., heiskanen and moritz 1967]. the only difference is that in the conventional bruns’ formula, the relevant quantities (tg and ce) are related to the normal gravity potential field u(p) generated by the ellipsoid, while in equation (8), the relevant quantities ( and ) are related to the new normal gravity potential field ui(p) generated by the inner ellipsoid. as on the surface of the ellipsoid the maximum absolute difference between ui and u is 0.3 mm and the rms is 0.2 mm (see table 2), it can be concluded that the maximum absolute difference between and tg = wg − ug on the geoid (where u has a definition on the geoid) is ≤0.3 mm and the rms is ≤0.2 mm. hence, where the geoid is above the ellipsoidal surface, the geoid undulation determined by the new formulation coincides with that determined by the conventional approach (i.e., by stokes’ approach) at the accuracy level of 0.2 mm. however, where the geoid lies inside the ellipsoid, the conventional approach (i.e., stokes’ approach) has difficulties, as indicated in section 2 (in this case, tg has no definition), while the new formulation presented in the present study can be applied. now, if u is continued downwards until the inner-ellipsoidal surface, it is replaced by the field ui generated by the inner ellipsoid (see section 5). hence, in practice, the numerical calculations have no difference between the conventional stokes’ approach and the new formulation. the new formulation has at least two advantages: (a) the ellipsoidal surface can still be used as a conventional reference surface, and consequently the conventionally used quantities (such as the three-dimensional coordinates in the wgs84 system, the geoidal undulation, the geodetic height [namely, the height above the surface of the ellipsoid], etc.) hold the same meaning; (b) the new normal gravity potential ui(p) and the new normal gravity ci(p) have definitions in the whole domain outside the inner ellipsoid, and as the geoid lies completely inside this domain, the difficulties that were in the conventional stokes’ theory (see section 2) have disappeared. 4. a modification of the new formulation it can be argued that the new formulation proposed in section 3 has the drawback that the new normal gravity potential ui(p) generated by the inner ellipsoid does not hold a constant w0 on the surface of the ellipsoid. hence, in this section, we introduce a modification of the new formulation provided in section 3. for the new formulation of stokes’ approach as provided in section 3, we can modify it a little, as stated as follows. we still choose the inner ellipsoid. however, wen-bin shen 6 maximum minimum mean standard deviation 3.27 -2.86 0.207 2.16 figure 4. the distribution of the differences between the values (ui|e) of the new normal gravity potential ui on the surface of the wgs84 ellipsoid, and the constant u0 = w0. table 2. statistical analysis of the differences between ui|e (values of ui on the surface of the wgs84 ellipsoid) and u0 (units: 10 -3 m2s-2). i ec ic .lim t p 0 r i = "3p ^ h ,n t i i e i g c = ti g ti g i ec i ec t w ui g g i g= 7 we do not constrain the condition that the new normal gravity potential ui(p) equals the constant (which is larger than w0 by 1467.0868 m 2s-2) on the surface of the inner ellipsoid, but the condition that the new normal gravity potential ui(p) equals the constant u0 = w0 on the surface of the ellipsoid. in this case, a modified new normal gravity potential field ui(p) generated by the inner ellipsoid can be uniquely determined, which has definition in the region outside the inner ellipsoid and coincides with the normal gravity potential field generated by the conventionally used ellipsoid in the region outside the ellipsoid. then, all the concepts used in physical geodesy can still be applied. to distinguish the new normal gravity potential field ui(p) from the modified one, we use to express the potential field generated by the inner ellipsoid with the requirement that it holds a constant w0 on the surface of the ellipsoid. indeed, is almost equal to ui(p), the confirmation of which is provided in the following. suppose that on the surface ei of the inner ellipsoid there is some kind of continuous distribution , then based on the boundary value , the modified new normal gravity potential field generated by the inner ellipsoid can be uniquely determined [see, e.g., kellog 1929, heiskanen and moritz 1967]. the field is required to coincide with the normal gravity potential field u(u,b) generated by the ellipsoid in the region (see figure 5). this requirement can be satisfied if the following condition is satisfied: . then, following heiskanen and moritz [1967], the field can be determined as follows: (9) where , a, b, ~, u0 are the four fundamental parameters of the ellipsoid (see table 1), and q0 and q are defined as follows: (10a) (10b) on the boundary ei, takes the value: (11) where: (12) and bi = b − 149.209 m, as listed in table 1. referring to figure 5, the modified new normal gravity potential is defined by the inner ellipsoid with requirement that is equal to the constant w0 = u0 on the surface of the ellipsoid, and the modified new disturbing potential is harmonic in the region xg. equation (11) provides the distribution of on the boundary ei. with this distribution, the modified new normal gravity potential field is determined using equation (9). now the modified new normal gravity potential function is defined in the region outside the inner ellipsoid and has the constant on the surface of the ellipsoid. indeed, the distribution of on the boundary ei almost holds the constant . on the surface of the inner ellipsoid, the maximum fluctuation of from the constant is about 0.3 mm, as shown in figure 6, with the relevant statistical analysis data listed in table 3. comparing table 3 with table 2, we find that at the present accuracy requirement, in the region outside the inner ellipsoid, the new normal gravity potential field ui generated by the inner ellipsoid with the requirement that ui holds a constant on the surface of the inner ellipsoid can be replaced by the modified new normal gravity potential field generated by the inner ellipsoid with the requirement that holds a constant w0 on the surface of the ellipsoid, and vice versa. this implies that either ui or can be used to replace the normal field u, and the numerical difference between the conventional stokes’ approach and the new formulation or its modification can a new formulation of stokes’ approach figure 5. a modified new formulation of stokes’ approach. the dashed elliptic curve denotes the surface e of the wgs84 ellipsoid, the closed irregular bold curve denotes the geoid g, the solid thin elliptic curve denotes the surface ei of the inner ellipsoid (drawn exaggeratedly in the figure), which is completely enclosed by the geoid. ui 0 u pi )^ h u pi )^ h u pi )^ h u pi )^ h ,u ui ib ) e^ h ,u ui ib ) e^ h ,u ui ib ) e^ h ,u ui ib ) e^ h ,u ui ib ) e^ h ,u ui b )^ h ,u ui b )^ h ,u ui b )^ h ,u ui b )^ h ,u ui b )^ h ,u ui b )^ h xe ,u u u wi 0 0ib = = ) e^ h , , tan sin cos u u gm u a q q u2 3 1 2 i 1 2 2 0 2 2 2 2 2 b l l ~ b ~ l b = + + + + ) -^ ` ^ h j h a b2 2l= ,tanq b b b 2 1 1 3 30 2 2 1 l l l= + -` j8 b .tanq u u u 2 1 1 3 32 2 1 l l l= + -` j8 b , , tan sin tan cos u b b a q q p u a b b3 3 2 i i i i i 1 2 2 0 2 0 2 2 1 2 2 2 2 b ~ l l ~ b ~ l b+ = + + + ) ^ ` ^ ^ h j h h tanq b b b 2 1 1 3 3i i i i 2 2 1 l l l= + -c m; e t p w p u pi= ) )^ ^ ^h h h u w ui 0 0 0= = ) 1467.0868 m su w0i 0 2 2= + 1467.0868 m su w0i 0 2 2= + ui 0 ui ) ui ) ui ) be neglected at the present accuracy requirement. although the new normal gravity potential field ui or the modified new normal gravity potential field has a similar or the same expression as given by the conventional stokes’ approach [e.g., heiskanen and moritz 1967], the conceptions are quite different. by the conventional stokes’ approach, the normal gravity potential field u is defined only in the region outside the ellipsoid, while by the new formulation as provided in section 3, or the modified one as provided in this section, the new normal gravity potential field ui or the modified one is defined in the region outside the inner ellipsoid. hence, the difficulties in the conventional stokes’ approach mentioned in section 2 are removed. 5. discussion and conclusions for the purpose of determining the global gravimetric geoid or external gravity field, dealing with the disturbing potential field t = w − u is much easier than dealing directly with the geopotential field w. hence, in physical geodesy, geoscientists focus on determining the disturbing potential field t, which is required to be harmonic in the region outside the boundary, based on different kinds of boundary-value conditions. the most representative conventional approaches might be stokes’ approach [stokes 1849] and molodensky’s approach [molodensky et al. 1962]. by stokes’ approach, the boundary is the geoid, and the disturbing potential field t is determined based on the given values (gravity anomalies) on the geoid. then, difficulties arise as the disturbing potential field t is not defined in the region between the geoid and the surface of the conventional ellipsoid, where the geoid lies inside the ellipsoid, because in this case, since t is not defined, its harmonicity is not guaranteed, which is, however, a basic requirement. to remove the difficulties in stokes’ approach, a new formulation and a modified one were proposed in sections 3 and 4, respectively, and by both the new formulation and the modified one, a new normal gravity potential field ui or a modified one generated by the inner ellipsoid, has been defined in the whole region outside the inner ellipsoid. hence, the new disturbing potential field ti = w − ui or the modified one is defined and harmonic in the region outside the geoid, as the geoid encloses the inner ellipsoid completely. then, the difficulties in the conventional stokes’ approach disappear. similar comments and proposals are suitable for modifying molodensky’s approach, by which the boundary is the earth surface, and the disturbing potential field t is determined based on the given values (gravity anomalies) on the earth surface. in molodensky’s approach, the gravity anomaly is the difference between the actual gravity measured at point p on the ground and the normal gravity at the corresponding point qp on the telluroid [see, e.g., heiskanen and moritz 1967, moritz 1980a]. then, the difficulties arise because the disturbing potential field t is not defined in the region between the earth surface and the surface of the ellipsoid, where the earth surface is below the surface of the ellipsoid. to remove these difficulties, analogous to the new formulation (as provided in section 3) or the modified one (as provided in section 4) of stokes’ approach, we can establish a new normal gravity potential field or a modified one generated by an inner ellipsoid that is completely enclosed not only by the ellipsoidal surface, but also by the earth surface. by checking the high space-resolution digital elevation model gtopo30 (available at http://eros. usgs.gov/#/find_data/products_and_data_available/ gtopo30_info), which was established based on the global digital data derived from a variety of sources, through a collaborative effort led by the personnel at the u.s. geological survey center for earth resources observation and science (eros), the semi-major axis of the inner ellipsoid can be chosen as a − 400 m (to guarantee that the inner ellipsoid lies completely inside not only in the ellipsoid, but also the earth), and its semiminor axis can be determined by requiring that the normal gravity potential field generated by the chosen wen-bin shen 8 figure 6. distribution of the differences between the constant and the values of on the surface ei of the inner ellipsoid. maximum minimum mean standard deviation 3.27 -2.86 0.208 2.16 table 3. statistical analysis of the differences between (values of on the surface of the inner ellipsoid) and (units: 10-3 m2s-2). ui ) ui ) ui ) t w ui i= ) ) ui m ui m) ui i ) e ui i ) e ui ) ui ) ui 0 ui 0 = 1467.0868 m sw0 2 2= + 9 inner ellipsoid holds a constant on the surface of the inner ellipsoid (note that should be larger than w0) or holds the constant w0 on the surface of the ellipsoid. in this way, using a new formulation or a modified one of molodensky’s approach (with similar techniques as provided in sections 3 or 4), the difficulties in molodensky’s approach [see, e.g., molodensky et al. 1962] will disappear. finally, we note why the conventional stokes’ approach (as well as other similar approaches) can still be effectively applied in practice. conventionally, the normal gravity potential field u generated by the ellipsoid can also be expressed in the form of equation (9). then, we can imagine that this field is generated by an inner ellipsoid; e.g., as defined in sections 3 or 4. in this case, the normal gravitational part of the potential field u is harmonic in the whole region outside the inner ellipsoid. this means using the simple continuation of the usual normal potential u can give the same results in terms of the estimate of t. comparisons of the details between the standard and the new approaches were provided in sections 3 and 4: in the accuracy level of 0.3 mm, there is no difference between t and ti. however, in theory and conceptually, a normal field generated by an inner ellipsoid should be used; namely, the field u should be replaced by ui, as defined in section 3, or replaced by , as defined in section 4. the reason is that the conventional normal gravity potential field u has no definition inside the ellipsoid. extending the definition domain of the potential field u downward until the surface of an inner-ellipsoidal surface implies that it has been replaced by a new potential field generated by an inner ellipsoid. fortunately, at the present accuracy requirement, if the conventional formula [e.g., heiskannen and moritz 1967] is used to calculate the value of the potential u in the region between the surfaces of the inner ellipsoid and the wgs84 ellipsoid (note that the formula has the same form as given by equation 9), it will be seen that it has the same value as that (see equation 3) given by the potential field generated by the inner ellipsoid. this is really an occasional coincidence! this might be also the reason why geodesists did not pay attention to or find the difficulties in the conventional stokes’ theory (and other similar theories; e.g., molodensky’s theory). although the numerical calculations hold the same, the conception should be changed. acknowledgements. the author expresses his sincere thanks to dr. jiancheng han, who prepared the figures in this manuscript, and dr. jin li for providing the numerical calculations relating to the new formulation presented in this study. the author also thanks the reviewers and the associated editor of the journal for their valuable comments and suggestions, which have greatly improved the manuscript. this study is supported by nsfc (grant no.40974015), national 973 project china (grant no. 2013cb733305) and nsfc (grant no.41174011; no.41021061; no. 41128003; no. 41210006). references ardalan, a.a., and e.w. grafarend (2001). somiglianapizzetti gravity: the international gravity formula accurate to the sub-nanogal level, j. geodesy, 75, 424-437. ardestani, v.e., and z. martinec (2000). geoid determination through ellipsoidal stokes boundary-value problem, stud. geophys. geod., 44, 353-363. burša, m., s. kenyon, j. kouba, z. šíma, v. vatrt, v. vítek and m. vojtíšková (2007a). the geopotential value w0 for specifying the relativistic atomic time scale and a global vertical reference system, j geod., 81, 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author: wen-bin shen, wuhan university, school of geodesy and geomatics, department of geophysics, wuhan, china; email: wbshen@sgg.whu.edu.cn. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. wen-bin shen 10 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 55 (3), 2012: crescimbene et al the science of rumors massimo crescimbene*, federica la longa, tiziana lanza istituto nazionale di geofisica e vulcanologia, rome, italy annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5538 “every rumor has its audience” gordon allport e leo postman the psychology of rumor abstract this study takes a soft scientific cut to talks about rumors, hoaxes and urban legends. social psychology, more elegantly, uses the latin word rumor (rumour in british english), which means sound, voice, or gossip. in social, economical, political, cultural and scientific communication, rumors indicate news that is presumed true, that circulates without being confirmed or made evident. the scientific history of rumors is briefly described starting from the period of ancient rome, throughout the second world war and the internet era, up to today. we will try to answer some questions that can be useful to scientists today. what are rumors? how are they born? how do they spread? by which laws are they regulated? how do we need to fight them? a final question regards the collocation of rumors into modern science. science today is divided into ‘hard’ and ‘soft’ science (the latter of which generally lacks a basic mathematical structure); these terms, respectively, indicate the natural sciences, which investigate nature, and the social/human sciences, which investigate man in all his facets. maybe rumors can be thought of as a bridge suspended between two banks: those of ‘scientific truth’ and ‘human truth’. 1. introduction the history of rumors is as old as human history. even in remote antiquity, rumors, gossip, and hoax were always in circulation – in good or bad faith – to influence human affairs. the ancient romans had a god devoted to rumors: the goddess fame.1 this goddess was represented as a woman always in motion, constantly shouting and spreading good and bad news. young and impetuous, she was often pictured with wings covered with eyes, mouths and tongues, in the act of playing a trumpet or two, one for truth, the other for lies. this winged monster allegorically represented rumors that arise, spread, gain credibility, do not distinguish between truth and falsity, and amplify and distort the facts at will.2 let us now give an example of what a rumor is and what effects it can produce. on october 30, 1938, the radio drama the war of the worlds was broadcast from the columbia broadcasting system (cbs) studios in the u.s.a., starring by orson welles and based on the science fiction novel by herbert george wells.3 this program became famous, as it triggered panic by describing an alien invasion. despite notices sent out before and after the program, many radio listeners did not realizing that it was fiction, and they believed that the landing of hostile aliens in u.s.a. territory was really happening. indeed, the adaptation of the novel simulated a special newscast that was at times inserted above the rest of the scheduled radio programs to provide updates on the landing of the bellicose martian spacecraft in the town of grovers mill, new jersey. with these words, orson welles described the uproar that his interpretation had caused: “the size of the reaction was incredible. six minutes after we aired, the houses were emptied and the churches were filled, from nashville to minneapolis people raised cry and tore their clothes on the street. we began to realize, while we were destroying new jersey, we had underestimated the extent of the vein of madness of our america”. today with the development of mass media, the rise of the internet and social networks, rumors are ubiquitous. “did we really ever go to the moon?”. higgs boson: “what has god got to do with it?”. “harold camping’s prophecy about the end of the world today, october 21, 2011, did not come true. now we have to wait until december 21, 2012, to see if the prophecy of the maya will come true”. then to the rumors and prophecies that relate more closely to the earth sciences and seismology: “on may 11, 2011, there will be a devastating earthquake in rome. the prediction of raffaele bendandi…”. article history received december 21, 2011; accepted march 20, 2012. subject classification: rumor, communication, social psychology, urban legends. 421 2. epistemological digression talking about rumors puts us ahead of some philosophical questions regarding epistemology. epistemology is derived from epistème (certain knowledge, science) and logos (discourse). in a narrower sense, epistemology is identified with the philosophy of science, which deals with the fundamentals of the various scientific disciplines. in ancient philosophy, the term dòxa (opinion) was used to indicate an uncertain knowledge. plato, in the fifth book of the republic, reports the words of socrates: “science and opinion are not tangible things, but forms of dynamis, that is, faculty or potential”. the problem, therefore, is a problem of demarcation between what is science and what is not, or the difficulty to distinguish science from pseudo-science and religion. if we try to exemplify these brief epistemological notes, we are faced with some questions. is truth closer to science or to what people believe? how can we approach scientific truth? can scientific truth be established by the scientific community on the basis of a majority? and then, how should minorities and voices of dissent be considered?4 while rumors express common opinion (doxa), what should science do to combat against rumors? 3. scientific studies on rumors in 1940, in america, rumors about world war ii began to spread. the u.s. government was worried about these rumors, as there was a risk that they would affect the social unit of the united states before they entered the war (the u.s.a. entered world war ii on december 8, 1941). to fight rumors that were deemed dangerous to civil morality and national security, the u.s. government decided to set up institutional committees; at the same time, social scientists began to study the phenomenon of rumors. rumor clinics were born in america (in chicago, in 1942) as operating units of a larger project, known as the rumor project, that involved several government agencies and american universities. using data collected by the boston clinics (1941-1942), every sunday, the boston herald published the most famous rumors of the week, where the newspaper dedicated a column of the first page to dismantling the rumors according to the evidence from the facts. after their studies on rumors during the second war, in 1947, allport and postman publish the psychology of rumor. this study is considered by many as a milestone of social psychology, because this science came from the universities to become a science that deals with reality. allport and postman [1947] defined rumors as propositions of faith on specific (or current) topics that pass from person to person, usually by word of mouth, without any evidence of their truth. although rumors are usually communicated from person to person by word of mouth, the media have a key role in their spread.5 for allport and postman [1947] the basic characteristics of rumors are: – they are transmitted by word of mouth; – they provide information about people, events and conditions; – they express and meet the emotional needs of the community. a crucial aspect in the definition of rumors is the emphasis given to their characteristics: – disclosure (word of mouth, which is amplified by newspapers and media); – content (news that is in the public domain, to be distinguished from private and trivial matters); – listening (rumor spreads to the community because it responds to the emotional needs). according to psychoanalysis, allport and postman [1947] supported the idea that a rumor can respond to a state of uncertainty through the production of a response, and that this can have a cathartic effect.6 robert knapp, a student of allport, collected and categorized rumors, and divided them in three categories based on their content: – rumors of impossible dreams (‘pipe-dream rumors’), which reflect public wishes and expected results; – ghost rumors and rumors based on fear (‘bogieman or fear rumors’), which reflected popular fears and anxieties; crescimbene et al. 422 423 – rumors that lead to disagreement (‘wedge-driving rumors’), which are intended to undermine alliances and relationships. knapp concluded that negative rumors expand more easily than positive rumors. jung [1959] proposed a classification of rumors based primarily on their temporal duration. he distinguished the rumors as ‘ordinary’ and ‘visionary’. for jung [1959], visionary rumors have a universal symbolic value and they live a long time. this kind of rumor that expresses chronic anxiety is detectable in the archetypal images and the collective fears about the situation of the world, or in the universal desire for a supernatural power to redeem. other authors, such as festinger [1957] and prasad [1935], argued instead that anxiety and fear are supported and supplied by rumors (e.g. an earthquake in india in 1934). in this interpretation, the function of rumors is precautionary and to alert against disaster, and they also have an adaptive function in relation to catastrophic events. in 2004, prashant bordia and nicholas di fonzo published an article entitled problem solving in social interactions on the internet: rumor as social cognition, where they stated that transmission of rumors is probably indicative of a “collective process of interpretation” (sense making). they reported also that in this process of collective problem solving, every rumor followed four stages of development: – introduction of the rumor; – spontaneous generation of opinions, from where discussions are created; – resolution of the problem; – loss of interest. in their book psychology of rumor, allport and postman [1947] also formulated the basic law of rumor, in which the strength of the rumor (r) is linked to the importance (i) and to the degree of ambiguity (a) of the topic, such that: r ≈ i × a (1) finally, according to recent studies, rumor strength is now considered a complex function of anxiety and uncertainty that can occur from internal states of individuals (trait anxiety) or from the outside, as state anxiety, or from their interactions. 4. how to fight rumors fighting rumors is not a simple task. generally there are only a few simple pieces of advice that can be issued to general public, such as: – don’t believe information coming from traditional media and new media; – check the sources from where the news came – do not consider any source as a-priori authoritative; – increase your own basic scientific knowledge; – always maintain a genuine scepticism and develop critical thinking. to face up to rumors, a number of tools have been proposed that cover two areas: legislative and communicative-educational. in italy, from a legislative point of view, reference is made to article 21 of the constitution, which protects the freedom of the press and free expression of thought, through any medium of communication. as an european union member state, italy is obliged to respect the freedom of the press, as enshrined in the charter of fundamental rights of the european union (art. ii-71). recently, in many countries, controversy has been stirred up by some legislative measures that have been enacted to defend users from information inaccuracy. a specific type of these interventions provides what is called a ‘chilling effect’: some rules that make it possible to ‘freeze’ or delete a range of information that is deemed to be incorrect or inaccurate. in italy, for example, much controversy has been aroused by the introduction of the communications authority7 (agcom) resolution no. 668/2010 which provides for an ‘erase system’ for websites suspected of violating copyright rules. further legislative examples concern false alarms. in italy, as in other countries, this offence is punishable by a fine.8 a false alarm is considered as an act that triggers emergency procedures without there being the presence of a real danger, from cases of small importance, to events of greater social impact. in terms of seismic risk and announced earthquakes, it is worth mentioning the case of garfagnana (tuscany, italy). in 1985, based on predictions made by the italian scientific community, giuseppe zamberletti, head of the italian civil protection, evacuated about 100,000 residents in an area where an occurrence of an earthquake was said to be possible. the predicted earthquake did not happen, and giuseppe zamberletti came under inquiry for procured alarm. in opposition to the case of garfagnana, we can consider the events in relation to the earthquake in l’aquila (abruzzo, italy) on april 6, 2009. based on the analysis of the concentrations of radon gas in the area, giampaolo giuliani, a technician at the national institute of nuclear physics, spread the news around about an earthquake that would occur on march 29, 2009, in sulmona (a town situated about 80 km from l'aquila). the earthquake at sulmona did not occur, and giampaolo giuliani received a notification of procured alarm after the head of civil protection, guido bertolaso, had requested for him to receive “exemplary punishment”.9 a few days later, the devastating earthquake hit l’aquila. up to today, the trial remains in progress and the department of civil protection will have to answer for its choices. the members of the department of civil protection, major risks committee are accused of multiple manslaughter and serious personal injury, as they provided reassuring messages to the science of rumors the public six days before the earthquake occurred. beyond the idea that the courts do not seem to be the most appropriate place to discuss problems of such relevant scientific and social impact, this theme of prediction versus unjustified alarm has generated vibrant scientific debate in italy and abroad, which is still going on [see, for example, hall 2011]. as regards the area of communication-education, the fight against rumors involves: communication, popularizing science and science education. scientific communication is defined as the kind of information published by journals dedicated to the scientific community (a famous example is nature), while using the expression of the ‘popularizing of science’ when what is meant is the spread of scientific knowledge to the general public. this is carried out in italy by magazines such as the sciences, television programs such as quark, and internet sites like molecularlab. an article to be published in a scientific journal must pass a series of steps to assess its reliability, the validity of the research, and the expertise of the authors, among other aspects. so how can it be that prestigious scientific journals can publish errors such as the discovery of cold fusion.10 for reasons still not entirely clear, fleischmann and pons held a press conference on march 23, 1989, before their research was to be published by a scientific journal. the publication was released on 10 april, 1989, as a short article written for the journal of electroanalytical chemistry [fleischmann et al. 1989]. the article, in the opinion of many scientists, was written in haste and was incomplete, and it contained some substantial errors in the measurements of the emission of gamma rays. therefore, it would seem that rumors cannot be effectively fought by the journals that deal specifically in the science, and this is also true for magazines, newspapers and the internet, which all contribute to the public understanding of science. generally, in italy, and it seems true for others european countries too,11 journalists who deal with science do not have full scientific training, and this often leads to inaccurate scientific information that is offered to the public in a superficial and inappropriate way, and which is sometimes totally incorrect. 5. conclusions in our opinion, to combat rumors and to disseminate accurate scientific information, we should go in two directions: on the one hand, there must be the promotion of quality science journalism, by trained journalists in different fields; on the other hand, scientists must be trained to engage in communication and dissemination of scientific information. a further consideration relates to science education. we note that the assumptions themselves of a science education are often misleading and incorrect. science is often taught at schools as a dogmatic ‘truth’. school programs do not provide epistemological training to develop observation, phenomenological approaches, and critical thinking. an example of a phenomenological scientific approach12 has been developed by the edurisk project13 (istituto nazionale di geofisica e vulcanologia; ingv, italy) over 10 years of activity. it is an educational project to reduce seismic and volcanic risk in italy, and it is proposed to italian teachers as a multidisciplinary method, and with a strong phenomenological approach. the themes of seismic and volcanic risk can be explored through a variety of disciplines, ranging from history to literature, from religion to mathematics and engineering, and from psychology to natural sciences, geology and physics. this approach initially disorients teachers that expect truths and certainties by the ingv researchers. instead, our approach proposes a path to knowledge that is built together, teachers, students and researchers.14 in conclusion we emphasize that the most effective response to combat rumors is to encourage a process of science education, ever more wider and open to the public, accepting the challenge of bringing reliability, accuracy and depth of science to the popularization of science. in this direction, we are supported by the thoughts of italian philosopher giulio giorello: “we believe that scientific truth is nothing but a statement that somehow we can control and that can also be discarded and replaced by another, which allows us to better understand the experiences we have, the observations that are recorded. in this sense, what we care about is not so much the possession of something, but the tension, the effort we make. what i have checked, you can also check, because as rightly said by galileo, descartes, pascal, and all the great founders of modern science any person who is able to understand and desire, and who will apply this, is able to make and control the experience. science is public and verifiable by everyone. if it is controllable and public, it is also teachable”.15 in our opinion, giorello’s words emphasize four main points that relate to science and how science is studied: (1) the guarantee of scientific data objectivity, that to maintain such, it must always be clearly identified, separated from the processing and interpretation of what was done, so it can be taken and checked by other researchers, and therefore there is the importance of repeatability of scientific experience; (2) the importance of calling into question acquired scientific truths, so that science really aims at arriving at the ‘truth’; (3) the importance of knowing how to teach science, not as dogmatic truth, but as a cognitive process that starts from experience, observes a phenomenon, questions its crescimbene et al. 424 425 mechanisms, its meaning, and the relationships between its parts, and verifies that its learnings are useful to explain observed phenomena; (4) the human dimension of science, both in an individual sense as science made by man and therefore fallacious and characterized by human experience, and as a collective process of knowledge that in order to be such must be of all. notes 1. fame (derived from the latin fari, which means to speak) was a deity as the allegorical personification of rumor in roman mythology. virgilio speaks of her personification created by the earth after ceo and encelado. literally translated, the latin phrase fama volat means that fame [the news] flies. (virgilio, eneide, iii, 121). 2. ovidio gives a broad description in the xii book of the metamorphoses. 3. the war of the worlds, published in london in 1898, is considered one of the first novels of science fiction. 4. critics of the thoughts of popper say that science would not have progressed without the minorities and ‘dissidents’. 5. for example, the great cabbage hoax, [hall 1965, 1977]. 6. allport and postman [1947] refer to freud’s psychoanalytic model of emotions. this is a hydraulic model, emotions (internal) find a way (acceptable) to be expelled, and act on the internal tension. 7. the communications regulatory authority is an independent authority, established by law n. 249 of july 31, 1997. independence and autonomy are elements that characterize its activities and deliberations. 8. according to article 658 of the penal code. 9. on december 22, 2009, the court of sulmona acquitted giuliani of the accusation of procured alarm in connection with the march 29, 2009 events. 10. cold fusion was suddenly in the spotlight on march 23, 1989, when chemists martin fleischmann of southampton university in england, and stanley pons of the university of utah in the usa, announced to the press that they had achieved it. 11. see, for example, verhoeven 2010. 12. for a phenomenological perspective of the educational event, see for example pezzella 2007. 13. the edurisk project was proposed in 1999 at the national group for earthquakes defense (gndt) of cnr, merged in 2001 in the ingv. the project was conducted from 2002 to 2006 and then from 2008 to 2009. for more information see: sintesi dei lavori del workshop edurisk 2002-2011: 10 anni di progetti di educazione al rischio – roma, 30 novembre 2011, edited by vera pessina and romano camassi, miscellanea ingv, 13, 2012; available at: http:// istituto.ingv.it/l-ingv/produzione-scientifica/miscellanea-ingv/. 14. for educational programs fulfilled in the project see the website: www.edurisk.it. 15. il grillo (11/12/1997): giulio giorello, la verità scientifica, in: enciclopedia multimediale delle scienze filosofiche, rai educational, http://www.emsf.rai.it/grillo/archivio.asp. references allport, g., and l. postman (1947). the psychology of rumor, new york, henry holt. bordia, p., and n. di fonzo (2004). problem solving in social interactions on the internet: rumor as social cognition, soc. psychol. quart., 67 (1), 33-49. festinger, l. (1957). a theory of cognitive dissonance, evanston, il, row, peterson. fleischmann, m., s. pons and m. hawkins (1989). electrochemically induced nuclear fusion of deuterium, j. electroanal. chem., vol. 261, p. 301, and errata in vol. 263 hall, s.s. (2011) scientists on trial: at fault?, nature, 477, 264-269. jung, c.g. (1959). a visionary rumour, journal of analytical psychology, 4 (1), 5-19; available online, doi: 10.1111/j.1465-5922.1959.00005.x. pezzella, a.m. (2007). the philosophy of education, lateran university press. prasad, j. (1935). the psychology of rumour: a study relating to the great indian earthquake of 1934, brit. j. psychol., 26, 1-15. verhoeven, p. (2010). sound-bite science: on the brevity of science and scientific experts in western european television news, sci. commun., 32, 330-355. *corresponding author: massimo crescimbene, istituto nazionale di geofisica e vulcanologia, rome, italy; email: massimo.crescimbene@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the science of rumors << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice le sesse d e l l a g o di c o m o p a r t e i i (*) oscillazioni interessanti l'intero lago p . caloi m . c . s p a d e a ricevuto il 30 settembre 1959 i stima approssimativa d e i periodi d i onde stazionarie i n u n sistema d i tre canali chiusi. il lago di como ha una forma caratteristica, che si può approssimare a quella di un canale (parte settentrionale), che si biforca all'altezza di bellagio in due canali, aventi gli estremi l'uno (occidentale) a como e l'altro a lecco (di dove esce l'adda). indichiamo con blt b2, b3 e hu h2 h3 la larghezza media e la profondità media dei tre rispettivi canad presi in considerazione. l'approssimazione che così si ottiene è tanto più attendibde quanto più le larghezze bl b2 b3 sono piccole rispetto ada lunghezza dei canali; condizione questa pienamente soddisfatta nel lago di como. in questa ipotesi, l'innalzamento deda superfìcie, sotto l'azione di un'onda lunga passante per d punto di confluenza dei tre rami, può considerarsi identica in tre sezioni (appartenente ognuna ad uno dei tre rami) il più vicino al punto di confluenza. per la trattazione matematica, le sezioni considerate possono ritenersi attraversare l'origine delle coordinate di superfìcie di ogni canale; in tal modo, la sezione 1 può passare attraverso ^ = 0 e la lunghezza ded'intero primo canale viene contata da questa sezione ad'estremità del canale stesso. il moto di onde lunghe in un canale a sezione uniforme è rappresentato dade due equazioni differenziali: 9 (*) le parti i, ii di questo lavoro sono state condotte con contributi del consiglio nazionale delle ricerche. 4 9 0 p . c a l o i m . c . s p a d e a dove f rappresenta lo spostamento orizzontale delle particelle d'acqua, rj lo spostamento verticale della superfìcie nello stesso punto e h la profondità media della sezione trasversale del canale considerato. 277 nel caso di movimento armonico semplice di periodo t = a possiamo porre, ( g co (j (vx a cos + b sin —— j , con c = |' g h [1] ha , ( t . ax „ ax v = cos at [a sin — b cos c \ c c espressioni che soddisfano le equazioni del moto. evidentemente c rappresenta la velocità di propagazione di un'onda libera lungo il canale di profondità li. le costanti a, b, h, c e la coordinata x si intendono affette degli indici 1, 2, 3, a seconda che si riferiscano al pruno canale o alle due ramificazioni, nell'ordine. per il punto di confluenza, dove = = x3 = 0, si ottiene b1 — = b2 -— = b3 — . [2] ci c2 la condizione di incompressibilità richiede che la quantità d'acqua fluente, in un certo tempo, attraverso la sezione 1 verso la confluenza, uguagli la somma delle quantità attraversanti le sezioni 2, 3, oltre la confluenza. ciò porta come conseguenza che per x1 = x2 = x3 = 0, si ha: v a i b, \ = 0 [3] ì all'estremità dei canali chiusi, avremo poi ti = 0 per set = li. (i = 1, 2, 3) dalla [1] consegue allora a = _ t g . (i = 1, 2, 3) [4] bi ci dalla [1], ricordando la [4], otteniamo t a b e l l a i l e c c o b e l l a g i o l2 = 1 9 . 0 0 0 m como b e l l a g i o ^ = 2 3 . 3 7 5 m sez. 8 (x) b (x) h sez. 8 (a) b (x) h io2 m 2 io2 m m io2 m 2 io2 m m 0 a 0 0 0 0 0 0 0 la 5 0 0 8 . 0 0 2 9 . 2 5 1 3 8 2 . 5 0 7 . 0 0 2 3 . 5 6 2a 9 8 5 1 3 . 7 5 5 7 . 6 2 2 9 7 2 . 7 5 1 1 . 2 5 5 9 . 9 2 3 a 1 3 5 5 1 2 . 7 5 7 9 . 2 7 3 1 3 8 3 . 2 5 1 6 . 0 0 8 5 . 2 1 4a 1 3 2 5 1 1 . 7 5 7 7 . 5 1 4 1 5 6 0 . 7 5 1 3 . 2 5 9 6 . 1 4 5 a 1 8 0 5 1 4 . 2 5 1 0 5 . 5 9 5 1 8 7 5 . 0 0 1 6 . 0 0 1 1 5 . 5 0 6 a 1 4 4 0 1 3 . 5 0 8 4 . 2 4 6 1 2 3 0 . 5 0 8 . 2 5 7 5 . 8 0 7 a 1 2 7 5 1 2 . 0 0 7 4 . 5 9 7 1 5 4 9 . 7 5 1 0 . 7 5 9 5 . 4 6 8 a 1 8 7 0 1 6 . 5 0 1 0 9 . 3 9 8 3 7 7 0 . 7 5 1 8 . 0 0 2 3 2 . 2 8 9 a 1 4 2 0 1 6 . 5 0 8 3 . 0 7 9 3 2 4 8 . 7 5 1 7 . 5 0 2 0 0 . 1 2 10a 1 0 6 5 1 4 . 7 5 6 2 . 3 0 1 0 2 6 4 7 . 2 5 1 3 . 2 5 1 6 3 . 0 7 1 la 1 9 4 5 1 8 . 2 5 1 1 3 . 7 8 11 2 1 6 8 . 7 5 9 . 7 5 1 3 3 . 5 9 12a 2 5 8 5 2 2 . 2 5 1 5 1 . 2 2 12 2 1 8 7 . 0 0 8 . 0 0 1 3 4 . 7 2 13a 2 7 9 0 1 9 . 7 5 1 6 3 . 2 1 1 3 4 3 6 2 . 5 0 1 5 . 0 0 2 6 8 . 7 3 14a 2 8 1 0 2 0 . 0 0 1 6 9 . 3 8 1 4 4 8 9 2 . 5 0 1 7 . 2 5 3 0 1 . 3 8 15a 2 9 2 5 2 3 . 7 5 1 7 1 . 1 1 1 5 4 7 7 0 . 5 0 1 4 . 7 5 2 9 3 . 8 6 16a 4 1 7 0 2 6 . 0 0 2 4 3 . 9 4 16 4 9 8 9 . 5 0 1 7 . 0 0 3 0 7 . 3 5 17a 5 7 1 0 2 4 . 5 0 2 9 8 . 9 3 17 5 6 6 3 . 5 0 2 0 . 5 0 3 4 8 . 8 7 18a 4 1 0 0 1 9 . 5 0 2 3 9 . 8 5 1 8 5 9 9 2 . 7 5 2 0 . 2 5 3 6 9 . 1 5 "6 = m 1 7 1 0 . 0 0 19 5 5 1 0 . 2 5 2 0 . 7 5 3 3 9 . 4 3 "6 = m 1 7 1 0 . 0 0 h = m 1 2 1 . 8 0 2 0 4 9 0 0 . 0 0 2 3 . 5 0 3 0 1 . 8 4 2 1 6 3 0 0 . 0 0 2 3 . 5 0 3 8 8 . 1 0 2 2 4 8 5 0 . 0 0 1 9 . 0 0 2 9 8 . 7 6 2 3 4 8 6 9 . 7 5 2 4 . 5 0 2 9 9 . 9 8 2 4 5 3 5 3 . 5 0 2 5 . 7 5 3 2 9 . 7 8 2 5 2 5 1 1 . 5 0 1 6 . 7 5 1 5 4 . 7 1 2 6 2 3 8 3 . 7 5 1 6 . 0 0 1 4 6 . 8 4 2 7 1 2 6 7 . 7 5 1 8 . 0 9 7 8 . 0 9 ~b = m 1 6 2 3 . 0 0 a = m 2 0 1 . 5 1 r a m o u n i c o l3 = 2 8 . 0 0 0 m sez. s (x) b (x) h i o 2 m 2 io2 m m 2 8 4 6 3 7 4 0 . 5 0 1 5 5 . 7 6 2 9 6 1 0 7 4 0 . 0 0 2 0 9 . 3 9 3 0 7 8 1 8 3 7 . 0 0 2 7 4 . 7 0 3 1 8 1 7 0 3 3 . 5 0 2 8 9 . 2 2 3 2 9 5 0 5 3 7 . 5 0 3 3 6 . 4 8 3 3 9 3 3 0 3 4 . 2 5 3 3 0 . 2 8 3 4 9 0 1 0 3 7 . 0 0 3 1 8 . 9 5 3 5 7 4 9 0 3 4 . 5 0 2 6 5 . 1 5 3 6 7 5 3 0 3 3 . 5 0 2 6 6 . 5 6 3 7 2 6 0 5 2 0 . 2 5 9 2 . 2 2 3 8 2 8 0 0 1 8 . 0 0 9 9 . 1 2 3 9 4 3 4 0 2 6 . 5 0 1 5 3 . 6 4 4 0 5 0 7 0 2 8 . 5 0 1 7 9 . 4 8 4 1 5 6 3 5 3 1 . 7 5 1 9 9 . 4 8 4 2 4 7 3 0 2 7 . 5 0 1 6 7 . 4 4 4 3 3 9 9 0 2 2 . 0 0 1 9 1 . 2 5 4 4 3 3 2 5 3 3 . 2 5 1 1 7 . 7 0 4 5 5 3 2 5 4 2 . 5 0 1 8 8 . 5 0 4 6 2 5 9 5 2 0 . 7 5 9 1 . 8 6 4 7 2 6 8 0 2 6 . 2 5 9 4 . 8 7 4 8 2 9 6 5 3 0 . 7 5 1 0 4 . 9 6 4 9 1 5 2 5 2 5 . 5 5 5 3 . 9 8 5 0 2 3 5 1 1 . 0 0 8 . 3 2 5 1 0 0 0 ~b = m 2 8 2 2 . 0 0 h = h 1 1 7 4 . 5 5 t a b el 1 a i i l e c c o b e l l a g i o l2 = 19.000 m. c o m o b e l l a g i o k = 23.375 m . sez. aax ? 2 lo 2 a rjo 2 rjo sez. 1 2 l 0 2 a vo 2 vo i o 1 io9 io3 c m c m c m io9 io3 c m cm cm oa 7.907 0 0 0 100 0 0 0 0 100 la » 825 — 1,65 — 1,30 98,70 1 1111,25 — 2,91 — 2,30 97,70 2 a » 2034,07 — 2,06 — 1,63 97,07 2 1931,93 — 1,99 — 1,57 96,13 3 a » 3344,51 — 2,47 — 1,95 95,12 3 3104,72 — 2,24 — 1,77 94,36 4 a » 4533,51 — 3,42 — 2,70 92,42 4 4116,73 — 2,64 — 2,09 92,27 5 a 5804,28 — 3,22 — 2,55 89,87 5 5279,33 — 2,82 — 2,23 90,04 6 a » 7287,13 — 5,06 — 4 , 0 0 85,87 6 6359,81 — 5,17 — 4,09 85,95 7 a » 8424,91 — 6,61 — 5 , 2 3 80,64 7 7172,04 — 4 , 6 3 — 3,66 82,29 8 a » 9715,15 — 5,20 — 4,11 76,53 8 8519,54 — 2,26 — 1,79 80,50 9 a » 11188,35 — 7,88 — 6 , 2 3 70,30 9 10103,38 — 3,11 — 2,46 78,04 10a » 12401,02 —11,64 — 9,20 61,10 10 11422,26 — 4,31 — 3,41 74,63 l l a » 13592,47 — 6,99 — 5 , 5 3 55,57 1 1 12340,21 — 5,69 — 4 , 5 0 70,13 12a » 14828,90 — 5 , 7 4 4 4 , 5 51,03 12 12859,17 — 5,88 4 , 6 5 65,48 13a » 15977,07 — 5 , 7 3 4,58 46,50 13 13612,19 — 3,12 — 2,47 63,01 14a » 16965,19 — 6,04 — 4 , 7 3 41,72 14 14710,14 — 3,01 — 2,38 60,63 15a » 17914,32 — 6,12 — 4 , 8 4 36,88 15 15655,97 — 3,28 — 2,59 58,04 16a » 18928,52 — 4 , 5 4 — 3,59 33,29 16 16518,59 — 3,31 — 2,62 55,42 17a » 19744,12 — 3,46 — 2,74 30,55 17 17633,92 — 3,11 — 2,46 52,96 18a » 20500,23 — 5,00 — 3,95 26,60 18 18746,08 — 3 , 1 3 — 2,47 50,49 19a » 21072,13 — 4,79 — 3,79 22,81 19 19806,37 — 3,59 — 2,84 47,65 20 20832,04 — 4 , 2 5 — 3,36 44,29 21 21971,40 — 3,49 — 2,76 41,53 22 22926,59 — 4 , 7 3 — 3,74 37,79 23 23662,55 — 4,86 — 3,84 33,95 24 24490,08 — 4,57 — 3,61 30,34 25 25142,39 —10,01 — 7,91 22,43 26 25533,23 —10,71 — 8,47 13,96 27 25799,52 —20,35 —16,09 — 2,13 r a m o u n i c o l3 = 28.000 m sez. 2 2 l„ 2 a vo 2 no io9 io3 cm cm c m 23772,00 0 0 2,13 28 23687,55 — 5,11 4,04 — 6,17 29 23466,23 — 3,84 3,04 — 9,21 30 23067,44 — 2,95 2 , 3 3 — 11,54 31 22609,65 — 2,77 2,19 — 13,73 32 22046,72 — 2,32 — 1,83 — 15,56 33 21397,09 — 2,29 — 1,81 — 17,37 34 20789,14 — 2,31 — 1 ,83 — 19,20 35 20073,94 — 2,68 — 2,12 — 21,32 36 19274,44 — 2,56 — 2,02 — 23,34 37 18550,90 — 7,12 — 5 , 6 3 — 28,97 38 18029,44 — 6,44 — 5,09 — 34,06 39 17177,94 — 3,96 — 3,13 — 37,19 40 16193,11 — 3,19 — 2,52 — 39,71 41 14882,68 — 2,64 — 2,09 — 41,80 42 13388,33 — 2 , 8 3 — 2,24 — 44,04 43 11890,97 — 2,98 — 2,36 — 46,40 44 10487,37 — 3,15 — 2,49 — 48,99 45 8409,55 — 1,58 — 1,25 — 50,14 46 6529,30 — 2,52 — 1,99 — 52,13 47 5239,08 — 1,95 — 1,54 — 53,67 48 3374,05 — 1,14 — 0,90 — 54,57 49 1586,88 — 1,04 — 0,82 — 55,39 50 396,00 — 1,69 — 1,34 — 56,73 51 112,35 494 p . caloi m. c. s p a d e a mentre dalla [3] consegue, nel caso dei tre canad chiusi, ì , bl bx \ tang — = 0 oppure (v. [2]) £ b, et tang ^ l = o . 1 ci [5] è questa l'equazione di zeilon ('). ci siamo proposti di applicare, neda determinazione del periodo, approssimativo ded'oscidazione libera uninodale del lago di como, la formula [5]. le lunghezze dei tre rami, dei quali questo lago consiste, sono rispettivamente ll = 23.375 m. (como-bedagio), l2 — 19.000 m. (leccobellagio), l3 — 28.000 m. (bedaio-riva) ; le rispettive larghezze medie sono bi = 2822 m., b2 = 1710 m., b3 — 1623 m., come risulta dada tabella i. la stessa tabeda fornisce anche le profondità medie, le quah permettono di calcolare i valori dede c(: si è ottenuto c± — 41,36 m/sec., c2 = 34,75 m/sec., c3 = 44,44 m/sec. con questi dati morfometrici, risolvendo la [5] con d metodo delle approssimazioni successive, si è ottenuto come periodo dell'uninodale dell'intero lago. si tratta, naturalmente di un valore approssimativo, che — come vedremo — non ddì'erisce molto da quello ottenuto con altro metodo. i i il metodo di defant è stato già esposto in lavori precedenti (2). l'applicazione di tale metodo ad un lago ramificato, comporta alcuni accorgimenti da tener presenti per la zona del lago dove avviene la saldatura dei tre rami. siano, per esempio, % il numero dede sezioni del ramo più corto (lecco), n2 il numero dede sezioni dell'altro ramo (como). siano inoltre a e j s i valori di 2 r;0, corrispondenti alle sezioni di % ed n2 rispettivamente e siano, inoltre a e b i relativi valori di q. per determinare il valore di q corrispondente ada sezione nt, quale sarebbe se 2 rj0 fosse uguale a fi, a va moltipdcato per /3/a. pertanto d calcolo relativo al ramo nord inizierà con la sezione + «2, dove q avrà d valore espresso da r + a • fifa e 2 rj0 avrà d valore fi. per le successive sezioni, d calcolo procederà nel modo abituale. t = 40m, 6 [6] l e s e s s e d e l lago d i como 4 9 5 applicato il metodo di defant con gli avvertimenti sopra detti, dopo alcuni tentativi si è pervenuti ai valori riportati nella tabella ii, corrispondenti ad un periodo t = 37m, 6 [7] per l'uninodale dell'intero lago. i l i u periodo dell'oscillazione uninodale interessante l'intero lago di como, dovrebbe cadere f r a i 38m e i 40m. è interessante osservare che l'applicazione del metodo di hidaka, f a t t a al lago prescindendo dal ramo di como-bellagio ha condotto, come si è visto nel lavoro precedente (3), ad un valore di 39m circa. data la lieve differenza che passa fra il valore dell'uninodale interessante l'intero lago e quello della stessa oscillazione da gera e lecco, dal punto di vista idrodinamico possono realizzarsi entrambe le oscillazioni; fatto che non si sarebbe potuto verificare qualora il lago incompleto avesse avuto un periodo dell'uninodale decisamente maggiore di quello dell'intero lago. soltanto le osservazioni potranno confermare o meno questi valori ottenuti per via teorica. riassunto applicando all'intero lago due metodi diversi abbiamo ottenuto per l'uninodale valori varianti fra 38m e 40»i. poiché l'oscillazione da gera a lecco (vedi parte i) ha un valore di 39m ca., dal punto di vista idrodinamico possono valorizzarsi entrambe le oscillazioni, cioè sia quella interessante l'intero lago che quella limitata al ramo di lecco e al suo prolungamento. abstract applying to the whole lajce two different methods, we have obtained for the uninodal, figures ranging from 38»i to 40»». as the oscillation from gera to lecco (see part i) has reached about 39m, speamng from hydrodynamic point of view, both oscillations can take place, i.e. the one that concerns the whole lake and the other that is limited to the lecco branch and its extension. 4 9 6 p . c a l o i m . c. s p a d e a b i b l i o g r a f i a ( 1 ) z e i l o n n . , on the seiches of the gullmar fjord. g o t e b o r g ( 1 9 1 3 ) . (2) caloi p . , le sesse del lago di garda; p a r t e i i , « a n n a l i di geofisica » (1948). (3) caloi p.-spadea m. c., le sesse del lago di como, p a r t e i, « a n n a l i di geofisica», (1958). articolo bull.pdf key words earthquakes – lichens – paleoseismology – southern alps 1. introduction stratigraphic studies made in trenches excavated across fault scarps continue to be important for paleoseismology. advantages of the trench-and-date approach include: 1) identification of the fault responsible for a specific surface-rupture event. 2) confirmation of seismic slip as revealed by liquefaction disturbance and offset strata. 3) constraint of times of prehistorical earthquakes, usually by radiocarbon dating. 4) estimates of offset of strata and buried stream channels that can be used to estimate slip rates. trenching along the plate-bounding alpine fault of new zealand has provided age estimates lichenometry dating of coseismic changes to a new zealand landslide complex william b. bull geosciences department, university of arizona, tucson, az, u.s.a. abstract lichenometry is a surface-exposure-dating procedure that complements traditional trench-and-date stratigraphic studies of earthquakes. lichens on the surficial blocks of a slump in the seaward kaikoura range, south island, new zealand provide precise, accurate (± 2 years) dating of 20 post-landslide rockfall events. the coseismic character of these rockfall events is apparent when ages of lichen-size peaks are compared with dates of historical earthquakes. most local prehistoric lichen-size peaks are synchronous with peaks at other lichenometry sites in a 20 000 km2 region. lichenometry may be the best paleoseismic tool for describing the extent and intensity of seismic shaking caused by prehistoric earthquakes, and for dating earthquakes generated by concealed thrust faults and subduction fault zones. for several prehistoric (before 1840 a.d. in new zealand) earthquakes (cooper and norris, 1990; yetton, 1998; wells et al., 1999). trench-and-date paleoseismology has several drawbacks. not all faults have accessible scarps including thrust faults in the cores of folds and submarine faults, and faults beneath large cities. earthquakes can not be radiocarbon dated directly, because stratigraphic samples of organic material were created and deposited either before or after a specific earthquake. earthquakes may occur during times of nondeposition at a trench site. trench studies are expensive, time consuming, and provide little information about the extent and intensity of seismic shaking. lichen dating of regional coseismic landslide events (bull, 1996a; bull and brandon, 1998) complements stratigraphic dating of earthquakes. advantages of this new approach to paleoseismology include: 1) estimation of the time of the earthquake itself. 2) precision and accuracy of dating that is ± 10 years or better. 3) ability to make maps depicting patterns of seismic shaking for prehistorical earthquakes 1155 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. william b. bull, geosciences department, university of arizona, tucson, az, 85721, usa; e-mail: bill@activetectonics.com that are as good as mercalli intensity maps for historical earthquakes. deficiencies of earthquake-lichenometry studies include inability to measure amounts of coseismic slip, possible lack of suitable slow growing crustose lichens in a study area, and possible lack of lichens older than 300 years at some sites. lichens older than 1000 years are rare. however, unstable and rapidly changing hillslopes of tectonically active mountains favor detection of regional rockfall events generated by distant earthquakes. 1.1. purpose and scope this article summarizes a surface-exposuredating approach to paleoseismology that avoids the problems associated with radiocarbon dating of organic matter that is younger than 300 years. the purpose is to illustrate the quality and diversity of paleoseismologic information that can be obtained from a lichenometry study of one landslide. each new site in the new zealand study region uses the existing calibration of lichen growth, and is tied into the regional database. lichen-sizes at this landslide document more than 20 historic and prehistoric earthquakes; all done at very low cost and in a time span of only 2 days of field and computer time. this efficiency was possible because of a database of 35.000 lichen-size measurements at 95 sites in the southern alps. the scope of this article also includes a summary of reconnaissance methods and data-collection procedures. 1.2. the seaward kaikoura range the landscape of the seaward kaikoura range (figs. 1 and 2) is dynamic because of unusually fast uplift rates, easily eroded rocks, and large rainfall events. mean annual precipitation is about 1.5 to 2 m. daily rainfall amounts can 1156 william b. bull fig. 2 location 40 mm/yr 168° e 172° e 42° s 44° s n alp ine fa ult hope fault kaikoura christchurch 0 100 200 milford sound mt. cook seaward kaikoura rangeaw ate re fa ult fig. 1. tectonic setting of landslide study area in the south island of new zealand. diagonal zone of faulting is along the transpressional boundary between the pacific and australian plates. plate convergence rate inside the arrow is from de mets et al. (1994). location of fig. 2 image is shown by the rectangle on the inset map showing the north island and south island. be large. cyclone alison dropped 2003 mm of rain on kaikoura in 24 h in march 1974. steep hillsides rise to a 2600 m high range crest above fairly narrow valley floors, and are underlain by highly fractured greywacke sandstone with some argillite and tuffaceous sandstone. uplift of the seaward kaikoura range, and displacements along the fault zones of the marlborough-alpine fault system, is the result of oblique convergence between the pacific and australian crustal plates. the marlborough fault system is composed of steeply-dipping, sub-parallel, right-lateral, strike-slip faults. the hope fault, one of the most active faults in new zealand, bounds the southeast side of the seaward kaikoura range, and major splays of the fault split the range into blocks (van dissen, 1989; van dissen and yeats, 1991). the high angle reverse slip component of faulting decreases to the southwest. late holocene horizontal slip rates are about 30 m ± 3 m/ky, and vertical rates are 2 to 6 m/.ky (van dissen, 1989; bull, 1991; knuepfer, 1992). the southwestern end of the hikurangi trench lies just offshore, and ocean depths exceed 1500 m within 10 km of kaikoura peninsula. progressively greater tilt of older marine terraces on kaikoura peninsula (ota et al., 1996) shows that offshore thrust faults continue to be active. the potential for valley floor erosion in this humid-climate setting is such that surface uplift of ridgecrests may exceed mean mountainrange rock uplift (england and molnar, 1990). harold wellman recognized this elementary dependence of terrain altitude on uplift rate. he published a credible map of uplift rates for the entire south island based on little more than this altitude-uplift-erosion relation (wellman, 1157 lichenometry dating of coseismic changes to a new zealand landslide complex fig. 2. the hope fault stands out on this digital image because of its rapid slip rate. the seaward kaikoura range rises 1500 to 2600 m above the pacific ocean. location of the barretts landslide is shown by the white square. image provided courtesy of scott miller and manaaki whenua landcare research, new zealand. 1979). bull (1984, 1985) used marine-terrace remnants to infer maximum uplift rates of about 6 m/.ky, estimates that are similar to wellman’s. erosional increase of relief causes these rising mountains to crumble instead of forming high, massive cliffs such as one might expect to form in massive plutonic rocks. watersheds yield sediment in excess of 5000 tons/.km2/yr (o’loughlin and pearce, 1982; mackay, 1984), which is equal to a denudation rate of approximately 1.8 m/.ky. this is about 260 times greater than undisturbed forested basins of tectonically inactive watersheds (costa, 1994), but only 17% of the sediment yield measured in wetter parts of the southern alps (griffiths and mcsaveney, 1983) that presently receive five times the mean annual precipitation of the seaward kaikoura range. rockfalls are the most common type of mass movement and debris slides are widespread in the seaward kaikoura range. progressive unraveling (consecutive collapse events) can shift a headscarp a kilometer upslope in only 20 to 50 years. lichen dating of blocks on these active talus slopes provides a record of only recent historical earthquakes. so, i searched for deeper-seated landslides such as slumps, which are rare in the seaward kaikoura range. study of aerial photos revealed only one obvious slump, a large landslide complex in the upper reaches of the hapuku river at an altitude of 750 m; its’ location is 42°14 50 s; 173°40 10 e. the informal name ‘barretts landslide’ is in accord with the name of nearby barretts hut. 2. summary of the lichenometry procedure dating of coseismic landslides with lichens was developed and tested as a paleoseismology tool in the southern alps of new zealand (bull, 1996b; bull and brandon, 1998). the method was then exported to california (bull et al., 1994; bull, 1996a, 2003). many rockfall events in the sierra nevada dated to times of magnitude mw 7.5 to 8 san andreas fault earthquakes, some with epicenters more than 400 km away. the bull-brandon procedure also works well in aseismic regions (bull et al., 1995). lichenometric studies of earthquakes begin with consecutive phases of work. the first is to determine if useful data can be obtained from the lichens that have colonized freshly exposed rock surfaces growing at a site. new rock-fall blocks tumble down hills during rainstorms and avalanches as well as during earthquakes. crustose lichens are preferred. they grow as a tightly attached symbiotic mixture of algae and fungi on the newly exposed rock surfaces of rock-fall blocks. slow growing yellow-green rhizocarpons (fig. 3), rhizocarpon subgenus rhizocarpon, commonly are favored for dating purposes because each millimeter of growth requires 2 to 30 years, varying between geo1158 william b. bull fig. 3. lichens growing on a rockfall block at the barretts landslide site. a wide range of sizes for rhizocarpon subgenus rhizocarpon is shown. these lichens have the typical salt-and-pepper texture of most yellow rhizocarpons. three of the largest lichens at locations a are nearly the same size; 33.46, 34.09, and 34.32 mm. the white lines indicative of the longest axis maximum diameter include the black algal rim, which is part of the lichen. the top lichen is a composite of several yellow rhizocarpons, but the isolated nature of most lichens suggests that this is a first-generation community of lichens. the lichen at b is a fast-growing crustose lichen that would require a separate growth calibration. graphic regions. most new zealand earthquakes are widely spaced in time relative to the rate of growth of rhizocarpon subgenus rhizocarpon, which increases in diameter 1 mm every 6 years. only exposed lichens are measured so as to minimize the effects of a slightly faster growth rate in shady micro-climatic settings. size measurements are made with digital calipers in order to increase precision and reduce bias. the long axis of the largest lichen is measured (fig. 3) on each rock-fall block that has accumulated on a talus slope, hillside bench, or stream terrace. unstable landforms such as fragile cliffs, landslides, and glacial moraines also can provide sensitive records of earthquakes. long axes of elliptical thalli record optimal lichen growth and it is assumed that the largest lichen was the first to colonize a new rock surface. the presence of a second lichen that is nearly as large as the lichen that was measured increases ones confidence that the lichen-size measurement is indicative of the time of exposure of the rock surface. the largest lichen is quickly spotted on blocks smaller than 1 m. work is facilitated where more than two-thirds of the blocks have isolated instead of merged lichens and the blocks are adjacent to one another. it is important to carefully examine the thallus of the largest lichen to make sure that it does not consist of several lichens that have grown together (fig. 3), and that the margins are sufficiently sharp that replicate measurements are within ± 0.1 mm. such replicate measurements are precise (± 0.6 year of growth), so measuring lichen sizes of is a trivial source of error. large numbers of lichen-size measurements, 100 to 1000 lichens, helps define individual lichen-size peaks and allows separation of events only 4 to 6 years apart. graphical portrayal of the size distribution of the largest lichen maximum diameters typically reveals a sequence of prominent histogram peaks. determination of the ages of these lichen-size peaks requires calibration of the rate of growth, which fortunately tends to be constant over a wide range of altitudes within a particular geographic region. preferred calibration sites are those where the date of exposure of the rock substrate is known to the day or year. examples include tree-ring dated landslides, rockfill dam embankments, highway, railroad, and trail cuts, landslides created by historical earthquakes, bouldery deposits of specific floods, abandoned quarries, and known times of diversions of streamflow from bouldery channels and waterfalls. use of radiocarbon dated landforms and geomorphic processes extends a calibration data set back in time, but has much larger uncertainties for individual age estimates. lichen growth begins rapidly, goes through an exponentially decreasing growth phase for several decades, and then begins a prolonged uniform-growth phase. calibration data should document each phase of lichen growth. age estimates of lichen-size peaks at sites in the south island of new zealand date the time of exposure, so are the sum of colonization time, great-growth phase, and uniform phase growth. calibration of the growth rate for rhizocarpon subgenus rhizocarpon in the south island of new zealand is based on calibration sites such as landslides and flood deposits tables 3 and 5 of bull and brandon (1998) describe most of the calibration sites used in d = 315.31 – 0.1552.t (2.1) where d is the mean size of a lichen-size peak (larger than the maximum great-growth peak size of 9.4 mm) and t is the substrate-exposure age in years. lichen sizes were first normalized to the year 1991 a.d. to facilitate comparison with the large regional data set. in order to use eq. (2.1) each lichen size for the barretts landslide site was reduced by 1.55 mm to account for the decade of post-calibration equation growth. plots of lichen sizes at landslide complexes rise to a broad peak and then decline. distinct peaks that rise above this general trend record times of above-normal influx of rock-fall blocks. not all lichen-size peaks record times of seismic shaking. identification of specific lichen-size peaks in probability density plots can be done with simple histograms or with the more robust decomposition of gaussian kernel probability density plots described by bull and brandon (1998). these plots are constructed by converting each meas1159 lichenometry dating of coseismic changes to a new zealand landslide complex urement into a unit gaussian function and averaging the densities of lichen sizes in a probability density plot created by overlapping gaussians. the result is analogous to a histogram, but a unit gaussian width instead of an increment of a histogram bar represents each observation. histogram class-interval size is replaced by a gaussian kernel size. lichen-size peak values are about the same for the two methods, but deconvolution of gaussian probability density plots identifies peaks that might be hidden in standard histograms. standard histograms are used here to illustrate their use and to test their precision and accuracy. lichenometric age estimates are obtained by inserting means of normally distributed lichen-size peaks into the regional equation for lichen growth. precision of dating is estimated by comparing ages of lichen-size peaks for a specific regional rockfall event at many sites in a region. accuracy of dating is determined by comparing lichenometric ages of regional rockfall events with dates of historical earthquakes. the choice of an appropriate histogram class interval involves a trade-off between precision and resolution. numerous, meaningless, small peaks are obvious noise if a class interval is too small. so one is tempted to use a large class interval. resolution is not as good but gives the impression that the few large histogram peaks are real. this can be misleading. merger of real peaks can create false peaks with meaningless mean lichen sizes (see fig. 10 of bull and brandon, 1998). reliability of identification of real peaks (as compared to unrealistic noise) in probability density plots increases with density of lichensize measurements. one or two measurements are worthless, but 10 to 100 measurements tightly clustered in a normal distribution supports the hypothesis of a lichen-size peak. so, density of lichen-size measurements affects choice of histogram class intervals. sizes of lichens from a repository of rockfall blocks typically record many rockfall events with the central part of the probability density plot rising far above low tails at very small or large lichen sizes. the usual approach is to use a class interval that introduces minimal noise into the central part (high-density) part of the display of lichen-size peaks. however the tradeoff is possible spurious lichen-size peaks in the tails of the distribution where density of lichen-size measurements may be borderline. fortunately, the bull-brandon approach to paleoseismology has several ways to confirm the presence of real peaks. real peaks also occur at many lichenometry sites in the study region because the shaking that caused the coseismic rockfalls is regional in extent. it can also be assumed that peaks are real at an individual site when they match the ages of historical earthquakes. the barretts landslide analyses illustrate both ways to confirm real peaks. significant peaks can also be defined as those that rise more than three standard errors above a uniform distribution of lichen sizes defined as lichens growing at a constant rate on blocks in a deposit fed by continuous rockfall events (bull and brandon, 1998, pp. 67-68). 3. application of the lichenometry procedure to the barretts landslide complex a reconnaissance was made of the barretts landslide complex to find old lichens with circular, distinct margins. the initial mass movement created a very unstable head scarp. it still has cliffs of fine-grained, strongly jointed sandstone, with joints parallel the cliff face. this cliff continues to shed rockfall blocks, especially during times of seismic shaking. some large blocks bounce and roll to the distal parts of the barretts landslide, smashing a few blocks along the way to create younger fresh surfaces for colonization by lichens. blocks on unstable parts of the landslide also tip over to expose additional fresh surfaces. most blocks on the landslide surface are in a fairly narrow size range of 0.5 to 2 m. initial measurements recorded a wide range of lichen sizes, which suggested a fairly complex history for the barretts landslide site. this ability to record post-landslide-formation disturbances also applies to other surface-exposure dating methods, such as weathering rinds and cosmogenic isotopes. stratigraphic dating methods, such as dating trees buried by a landslide, focus more on the time of the initial tectonic disturbance. 1160 william b. bull vegetation is sparse on this fairly young landslide that is close to treeline. a few clumps of bushes small trees are present, with patches of ferns and sphagnum moss. large areas have few plants to shade lichens or create a fire hazard, the macro and microclimate favors rhizocarpon subgenus rhizocarpon, and persistent snow cover does not kill lichens. lichens observed along the reconnaissance route shown in fig. 4 were encouraging because the thalli were well defined and isolated (fig. 3). the head scarp has shed so much detritus that talus cones bury the cliff base. i avoided areas of young detritus, abundant trees, and sphagnum moss. the reconnaissance did not find lichens larger than 45 mm. a distal area of surficial blocks with smooth, planar joint faces and isolated nearly circular lichens was selected to get the best possible lichen-size measurements. one could spend several days here if more measurements were needed. if this landslide were three times as old it would take a several days to get the sufficient density of measurements needed to define lichen-size peaks over a much broader range of lichen sizes. the boundaries of small measurement areas (fig. 4) were marked and each block was examined to see if its largest lichen was of suitable 1161 lichenometry dating of coseismic changes to a new zealand landslide complex fig. 4. view of barretts landslide complex from mount stace. unstable headscarp is partially buried with talus cones. the hapuku river has removed portions of the toe of the landslide, thus creating younger landslides. line shows reconnaissance route. most lichen-size measurements were made in the enclosed areas. barretts landslide is 18 km north of the town of kaikoura in the seaward kaikoura range (figs. 1 and 2). quality to be measured. slight variations in color and structure of the lichens indicated that more than one section of rhizocarpon subgenus rhizocarpon is present here. however, that does not present a problem because we already know that all yellow rhizocarpons, with exception for section alpicola (not common at the lower altitudes in the southern alps), grow at the same rate over a wide altitude range and on a diversity of lithologies (bull and brandon, 1998). the reconnaissance indicated that the barretts landslide could provide several hundred good-quality lichen-size measurements. 3.1. data analysis the sample from the barretts landslide consists of 426 lichen-size measurements. examination of these data with a standard histogram using a large class interval (fig. 5) reveals a fairly typical distribution of lichen sizes that rises to a maximum at about 30 mm and then declines. the peak of this overall distribution is the result of several factors. distal parts of the landslide were used for measurements because of a preference for older lichen sizes. work in proximal parts near the active talus cones would have emphasized smaller lichens. the possibility of finding usable lichens decreases with increasing rockfall-block age. old blocks may be broken or buried by incoming blocks, or they may be shaded by growth of bushes and trees. some blocks have large lichens that are no longer isolated. i did not measure the largest lichen if growth of its longest axis was constrained by adjacent lichens. the fig. 5 histogram has 11 peaks, but they rise only a short distance above coalesced background measurements. the distribution declines dramatically above a size of 45 mm. this 1-mm class interval plot appears reasonable, but a smaller class interval might reveal more peaks. the tails of the probability density plot are weak, but the density of measurements between 19 and 46 mm is respectable, averaging 15 measurements per millimeter. a 1162 william b. bull 0 5 10 15 20 25 30 35 n u m b e r o f lic h e n s p e r cl a ss in te rv a l 0 10 20 30 40 50 60 largest lichen maximum diameter (mm) barretts landslide 1.0 mm class interval n = 426 regional earthquake of 1742 a.d. ± 10 years caused this landslide fig. 5. histogram of largest lichen maximum diameters for 426 rockfall blocks. 1163 0 2 4 6 8 10 12 14 16 n u m b e r o f lic h e n s p e r cl a ss in te rv a l n u m b e r o f lic h e n s p e r cl a ss in te rv a l 10 1515 20 25 30 35 40 45 50 barretts hut landslide 0.4 mm class interval n = 424 1770 a.d. 1770 a.d. ± 1010 hope fault hope fault 1834 a.d. 1834 a.d. ± 10. 10. hope fault hope fault 1840 a.d.1840 a.d.± 10 central marlborough earthquake10 central marlborough earthquake on the awatere? fault on the awatere? fault 1742 a.d. 1742 a.d. ± 10 large 10 large earthquake that caused earthquake that caused the barretts landslidethe barretts landslide murchison, 1929, murchison, 1929, mw = 7.8= 7.8 120 km, 1929.8, (0.4), 120 km, 1929.8, (0.4), motunau, 1922, motunau, 1922, mw = 6.4= 6.4 90 km, 1922.1, (0.9)90 km, 1922.1, (0.9) cheviot, 1901, cheviot, 1901, mw = 7= 7 70 km, 1896.3, (5.6)70 km, 1896.3, (5.6) n. canterbury, 1888, n. canterbury, 1888, mw = 7.1= 7.1 100 km, 1891.2, (2.5)100 km, 1891.2, (2.5) hurunui, 1881, hurunui, 1881, mw = 7= 7 110 km, 1878.3, (3.6)110 km, 1878.3, (3.6) bealey, 1866, bealey, 1866, mw = 7= 7 170 km, 1866.7, (0.6)170 km, 1866.7, (0.6) west wairapapa, 1855, west wairapapa, 1855, mw = 8.2= 8.2 200 km, 1856.4, (1.3)200 km, 1856.4, (1.3) marlborough, 1848, marlborough, 1848, mw = 7.4= 7.4 60 km, 1845.4, (3.4)60 km, 1845.4, (3.4) 32.8 39.2 43.4 three rockfalls may not be regional events seaward kaikoura,seaward kaikoura, 1955, mw = 5.1= 5.1 20 km, 1955.6, (0.2)20 km, 1955.6, (0.2) prehistoric regional rockfall events prehistoric regional rockfall events name, year, andname, year, and mw earthquake magnitude estimateearthquake magnitude estimate distance to epicenter, lichenometry date, (accuracy of date in years)distance to epicenter, lichenometry date, (accuracy of date in years) key for historical earthquake informationkey for historical earthquake information largest lichen maximum diameter (mm) fig. 6. histogram of the largest lichen maximum diameters for 424 rockfall blocks. lichen-size peaks with age estimates that coincide with times of historical earthquakes are shown in blue. prehistoric earthquakes are shown in green. the time of creation of the barretts landslide is assigned to the age of the oldest lichensize peak. lichenometry dating of coseismic changes to a new zealand landslide complex class interval of less than 0.2 mm was not be used because it might have introduced spurious lichen-size peaks. a class interval of 0.4 mm is used in fig. 6, resulting in twice as many peaks than when a class interval of 1.0 mm was used (fig. 5). the peaks appear more isolated and stand higher above the background. peak means have a resolution of ± 0.2 mm, or ± 1 year. but, by attempting to use a class interval of 0.4 mm are we introducing spurious peaks? we need to evaluate peak ages. the first step was to use eq. (2.1) to compute ages of the lichen-size peaks. nine of the rockfall events represented by the fig. 6 lichen-size peaks occurred since 1840 a.d., the time span of historical records. comparison of age estimates for lichen-size peaks with historical earthquake dates allows assessment of the accuracy of lichenometric dating. departures of rockfall event lichenometry ages from historical earthquake dates range from 0.1 to 5.6 years and average 2.0 years. accuracy of ± 2 years is the same as for other studies in california and new zealand. the 1855 rockfall event coincides with the time of a north island earthquake whose epicenter was approximately 200 km away. this is not unusu1164 william b. bull 0 20 40 60 80 100 120 n u m b e r o f lic h e n s p e r cl a ss in te rv a l 0 10 20 30 40 50 60 largest lichen maximum diameter (mm) northern south island 0.4 mm class interval n = 6008 fig. 7. histogram of the largest lichen maximum diameters for 6008 rockfall blocks. measurements were combined from 20 lichenometry sites in 20.000 km2 of the northern part of the south island of new zealand underlain by quartzitic greywacke sandstone, syenite, and argillaceous greywacke sandstone. site altitudes range from 380 to 1620 m. altitude and macroclimate not affect lichen growth rates as shown by the sharply defined lichensize peaks. al. earthquakes with mw magnitudes greater than 7 can trigger rockfalls at distances of up to 400 km from their epicenters (wieczorek et al., 1992; keefer, 1994; bull, 1996a; wieczorek and jäger, 1996). these results support a model that most of the lichen-size peaks larger than 29 mm also record coseismic rockfall events. each mountain range has its own mix of climatic and tectonic landslides. the combination of fractured greywacke sandstone in the tectonically active southern alps appears to favor earthquake-generated rock avalanches (whitehouse, 1983) and rockfalls. many new zealand sites have not received an increment of new rock-fall blocks (bull and brandon, fig. 4) since the inangahua mw 7.1 earthquake of 1968 (downs, 1995). rainstorm induced earthflows and debris flows are important in the north island (crozier, 1997) and elsewhere (nielson and brabb, 1977; wieczorek, 1987). in a detailed landslide inventory in yosemite national park, in the sierra nevada of california wieczorek et al. (1992) concluded that earthquakes created only slightly more landslide volume than climatic causes. keefer’s (1994) calculations for peru indicate that 99% of the volume of coseismic landslides is caused by earthquakes larger than mw 6, and 92% larger than mw 7. comparison of lichen-size peaks at the barretts landslide (fig. 6) site with peaks at 20 other sites (fig. 7) underscores the regionally synchronous nature of most rockfall events. peak size varies from site to site and not every hillslope responds to each earthquake, but peak ages are constant. the historical (post 1840 a.d.) events record times of earthquakes, so it is likely that most prehistorical peaks also were coseismic. however, the prominent 32.8, 39.2, and 43.4 mm lichen-size peaks shown in fig. 6 do not coincide with the fig. 7 peaks, in a region that is southeast of the alpine fault and north of christchurch (fig. 1). the 43 mm peak is prominent southwest of christchurch (bull, 1996b), so its presence at the barretts landslide site might record a distant earthquake about 12 years after creation of the landslide headscarp. given the young age of barretts landslide and the instability of its head scarp, one should not be surprised by nonseismic collapses at 12, 42, and 78 years after the slump created the head scarp. these events could be storm related, or perhaps just random events. lichen-size measurements define an isolated lichen-size peak at 45 mm on fig. 6. i assume that it records the time of creation of the barretts landslide, and the prominent peak at 45 mm on fig. 7 supports the impression of that barretts slump was initially caused by an earthquake. the 5 isolated lichen sizes scattered between 45 and 60 mm (fig. 5) are presumed to be inherited blocks-lichens that were already growing and survived the landslide process. 4. potential of lichenometry for paleoseismology the work expended in calibrating lichen growth rates and collecting regional data sets in new zealand (bull and brandon, 1998), california (bull et al., 1994; bull, 1996a), and sweden (bull et al., 1995) allows geomorphologists and paleoseismologists to visit new sites and efficiently assess ages of surficial deposits. paleoseismologists can precisely and accurately date coseismic landslides and rockfalls at sites that are sensitive to seismic shaking. application of the bull-brandon method to other tectonically active study areas will further complement stratigraphic paleoseismic studies. spatial variations in coseismic rockfall abundance can be used to assess patterns of prehistoric seismic shaking. peak-size maps describe patterns of seismic shaking associated with earthquakes. systematic regional trends depicted by peak-size maps for historical earthquakes provide the same level of information as modified mercalli intensity maps (cowan, 1991; downs, 1995). the simple pattern of peak size associated with the 29 mm regional rockfall event (fig. 8) clearly suggests seismic shaking associated with a single large earthquake at about 1840 a.d. in the central part of the study region. the eastern segment of the awatere fault, adjacent and northeast of the maximum shaking illustrated here, ruptured in the marlborough earthquake of 1848 (grapes et al., 1998; benson et al., 2001). peak sizes for the 1840 a.d. event are anomalously low near the conway segment of the hope fault (fig. 1), 1165 lichenometry dating of coseismic changes to a new zealand landslide complex probably because a hope fault earthquake in about 1834 a.d. had already dislodged most of the unstable blocks on steep hills. acknowledgements tim and sally blunt helped explore the hapuku river and measure lichen sizes. alastair wright, environment canterbury, provided essential aerial photographs for the reconnaissance. suggestions by daniela pantosti and two anonymous reviewers greatly improved the organization, content, and readability of this article. references benson, a.m., t.a. little, r.j. van dissen, n. hill and d.b. townsend (2001): late quaternary paleoseismic history and surface rupture characteristics of the eastern awatere strike-slip fault, n. z. geol. soc. am. bull., 113, 1071-1091. bull, w.b. (1984): tectonic geomorphology, j. geol. educ., 32, 310-324. bull, w.b. (1985): correlation of flights of global marine terraces, in tectonic geomorphology: proceedings of the 15th annual geomorphology symposium, state university of new york at binghamton, hemelhempstead, england, edited by m. morisawa and j. hack (george allen and unwin), 129-152. bull, w.b. (1991): geomorphic responses to climatic change (oxford university press, london), pp. 326. bull, w.b. (1996a): dating san andreas fault earthquakes with lichenometry, geology, 24, 111-114. bull, w.b. (1996b): prehistorical earthquakes on the alpine fault, new zealand, j. geophys. res., 101 (b3), 6037-6050. bull, w.b. (2003): guide to sierra nevada lichenometry, in tectonics, climate change, and landscape evolution in the southern sierra nevada, california, edited by g. stock, 2003 pacific cell frieds of the pleistocene field trip guidebook, appendix 8, 100-121. bull, w.b. and m.t. brandon (1998): lichen dating of earthquake-generated regional rockfall events, southern alps, new zealand, geol. soc. am. bull., 110, 60-84. bull, w.b., j. king, f. kong, t. moutoux and w.m phillips (1994): lichen dating of coseismic landslide hazards in alpine mountains, geomorphology, 10, 253-264. bull, w.b., p. schlyter and s. brogaard (1995): lichenometric analysis of the kärkerieppe slushavalanche fan, kärkevagge, sweden, geografiska annaler, 77a, 231-240. cooper, a.f. and r.j. norris (1990): estimates for the timing of the last coseismic displacement on the alpine fault, northern fiordland, new zealand, n. z. j. geol. geophys., 33, 303-307. costa, j.e. (1994): water fact sheet: evolution of sediment yield from mount st. helens, washington, 1980-1993, u. s. geol. surv. open-file rep. 94-313, pp. 2. cowan, h.a. (1991): the north canterbury earthquake of september 1, 1888, j. r. soc. n. z., 21, 1-12. crozier, m. (1997): the climate-landslide couple: a southern hemisphere perspective, in rapid mass movement as a source of climatic evidence for the holocene, edited by j.a. matthwes, d. brunsden, b. frenzel, b. glaser and m.m. weib (gustav fischer verlag), 333-353. de mets, c., r.c. gordon, d.f. argus and s. stein (1994): effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions, geophys. res. lett., 21, 2191-2194. downs, g.l. (1995): atlas of isoseismal maps of new zealand earthquakes, n. z. inst. geol. nucl. sci. monogr., 11, pp. 304. england, p. and p. molnar (1990): surface uplift, uplift of rocks, and exhumation of rock, geology, 18, 1173-1177. grapes, r., t. little and g. downes (1998): rupturing of the awatere fault during the 1848 october 16 marlborough earthquake, new zealand: historical and present day evidence, n. z. j. geol. geophys., 41, 387-399. griffiths, g.a. and m.j. mcsaveney (1983): hydrology of a basin with extreme rainfalls, cropp river, new zealand, n. z. j. sci., 26, 293-306. keefer, d.k. (1994): the importance of earthquakeinduced landslides to long term slope erosion and slope-failure hazards in seismically active regions, geomorphology, 10, 265-284. 1166 william b. bull > 50% 40 50% 30 40% 20 30% < 20% 0 1 0 200 km 42 °s 44° s 168° e 172° e relative size of rockfall event fig. 8. peak-size map for the 29 mm regional rockfall event shows one area of seismic shaking in the center of the study region. contours show the percentage of 29 mm modeled peak size relative to all lichen-size measurements between 27 and 33 mm. from fig. 23b of bull and brandon (1998). knuepfer, p.l.k. (1992): temporal variations in latest quaternary slip across the australian-pacific plate boundary northeastern south island, new zealand, tectonics, 11, 449-464. mackay, d.a. (1984): kowhai management area detritus survey, new department of lands and survey, marlborough catchment board report, pp. 20 + appendices. nielsen, t.h. and e.e. brabb (1977): slope stability studies in the san francisco bay region, california, geol. soc. am. rev. eng. geol., 3, 235-243. o’loughlin, c.l. and a.j. pearce (1982): erosion processes in the mountains, in landforms of new zealand, edited by j.m. soons and m.j. selby (longman paul, auckland), 67-79. ota, y., b. pillans, k. berryman, t. fujimori, t. miyauchi, g. berger, a.g. beu and f.m. climo (1996): pleistocene coastal terraces of kaikoura peninsula and the marlborough coast, south island, new zealand, n. z. j. geol. geophys., 39, 51-73. van dissen, r.j. (1989): late quaternary faulting in the kaikoura region, southeastern marlborough, new zealand, oregon state university m.s. thesis, pp. 72. van dissen, r.j. and r.s. yeats (1991): hope fault, jordan thrust and uplift of the seaward kaikoura range, geology, 19, 393-396. wellman, h.w. (1979): an uplift map for the south island of new zealand, and a model for uplift of the southern alps, in the origin of the southern alps, edited by r.i. walcott and m.m. cresswell, bull. r. soc. n. z., 18, 13-20. wells, a., m. yetton, r.p. duncan and g.h. stewart (1999): prehistoric dates of the most recent alpine fault earthquakes, new zealand, geology, 27, 995-998. whitehouse, i.e. (1983): distribution of large rock avalanche deposits in the central southern alps, new zealand, n. z. j. geol. geophys., 26, 271-279. wieczorek, g.f. (1987): effect of rainfall intensity and duration on debris flows in central santa cruz mountains, california, geol. soc. am. rev. eng. geol., 3, 93-104. wieczorek, g.f. and s. jäger (1996): triggering mechanisms and depositional rates of postglacial slope-movement processes in the yosemite valley, california, geomorphology, 15, 17-31. wieczorek, g.f., j.b. snyder, c.s. alger and k.a. issaacson (1992): rock falls in yosemite valley, u. s. geol. surv. open-file rep. 92-387, pp. 136. yetton, m.d. (1998): progress in understanding the paleoseismicity of the central and northern alpine fault, westland, new zealand, n. z. j. geol. geophys., 41, 475-483. 1167 lichenometry dating of coseismic changes to a new zealand landslide complex j j j j j j j sagnotti.qxp_layout 6 annals of geophysics, 61,3, da341, 2018; doi: 10.4401/ag-7619 1 “stratfit: an excel workbook for correlation of multiple stratigraphic trends„ leonardo sagnotti1,* and chiara caricchi1 (1) istituto nazionale di geofisica e vulcanologia, rome, italy 1. introduction the correlation of sedimentary stratigraphic trends from different basins and/or sections is a critical issue for all paleoenvironmental and paleoclimatic reconstructions, as well as for basin characterization in exploration and characterization of georesources. in particular, the reconstruction of changes in climate and/or environmental conditions at a regional or global scale in the geological past is generally accomplished by detailed studies of stratigraphic variations of suitable proxies in sedimentary sequences, whose tuning results in the set up of reference stack curves [e.g. global stack of oxygen isotope trends in marine sediments; imbrie et al., 1984; karner et al., 2002; lisiecki and raymo, 2005]. stratigraphic correlation between distinct sedimentary sections and/or reference stack curves is usually based upon signal matching between two or multiple records of appropriate proxies. this signal matching is an often difficult and tricky process, which requires multiple efforts to converge toward the most reliable solutions. various attempts have been made in paleoclimate research to make correlation of stratigraphic trends easier. the different adopted approaches span from simple correlation by eye matching of stratigraphic features, which is often time-consuming, to automated correlation [martinson et al., 1982; waterman and raymond, 1987] or dynamic programming techniques [lisiecki and lisiecki, 2002]. these latter techniques require the use of specific software and dedicated algorithms, which may not always be accessible or under the direct control of general users. in this manuscript, we propose a new user-friendly tool, the stratfit program consisting of a single microsoft excel workbook organized in worksheets, which allows an easy correlation of different stratigraphic curves and immediate visualization of the results. the stratfit program was conceived to address the article history receveid january 21, 2018; accepted april 10, 2018. subject classification: stratigraphy; tie-points; core correlation; microsoft excel workbook; forecast function. abstract correlation of distinct stratigraphic sequences is often critical to characterize evolution of sedimentary basins, as well as for exploration of georesources and for paleoenvironmental and paleoclimatic reconstructions. stratigraphic correlation is usually based on signal matching between two or multiple proxies’ records. commonly, stratigraphers have to approach the correlation issue with time-consuming methods or specific software that may not be easily accessible or may not allow a full and direct control of the adopted procedure. in this work, we propose a new simple tool, which consists of a single microsoft excel workbook (that we named stratfit) organized in worksheets and allows an easy correlation of different stratigraphic curves and immediate visualization of the results. the correlation process is based on the forecast function and linear regression between subsequent couples of selected tie-points. the program is open source, user-friendly and allows a full control of the correlation process since all the computations are accessible for user’s inspection and improvement. the stratfit workbook and the user manual are freely distributed and can be downloaded as supplementary material. needs related to the environmental magnetism analyses, as a tool to correlate two distinct stratigraphic sections by taking into account multiple trends in rock magnetic and paleomagnetic parameters [sagnotti et al., 2011a,2011b]. due to the extensive use and distribution of excel, the operability of this software is guaranteed over the longer term for different operating systems (e.g., mac os x, microsoft windows). moreover, the program is open source, user friendly and all the computations are accessible for user’s inspection and improvement. during the correlation process the user can immediately check every change on the graphs and most importantly can take into proper account all useful available information (e.g., 14c ages, lithological factors among others). thus, the power of stratfit stems on the fact that this program allows an aware and interactive correlation of stratigraphic trends between two cores and not only an automatic passive match among peaks and troughs. lastly, the program has the following features, which may be valuable for various users: i) compatibility with nearly all computers; ii) easy input of data by “copy and paste”, immediate visualization of the results and easy export of plots; iii) modular structure in distinct worksheets for each selected stratigraphic trend; iv) easy and prompt visualization of the effects of the adopted matching on multiple stratigraphic trends (up to 11 distinct stratigraphic trends in the released version, but easily expandable at wish); v) complete control of the whole workflow and possibility of implementation of the workbook by users. in the following sections, we describe the software and its main features and applicability. 2. software description the released version of stratfit workbook is organized in 12 worksheets (figure 1). the green one is the “data” worksheet, where the user introduces data for the two cores (one of which considered as “master” and the other as the correlative core) and makes the choice of the tie-points for the correlation. the yellow sheets show the plots of the stratigraphic trends for each parameter, where the user can visualize and check the correlations. the correlation process is based on the forecast function which implies a linear regression between subsequent couples of selected tie-points. this process results in the estimate of the equivalent depth of the correlated curve in the depth scale or age of the “master” curve. in detail the excel forecast function predicts a future point on a linear trend line fitted to a supplied set of xand yvalues. the syntax of the function is: forecast( x, known_y’s, known_x’s ) where the arguments are: i) x, a numeric value for which you want to forecast a new y-value; known_y’s, an array of known y-values; known_x’s, an array of known x-values. the known_y’s and known_x’s arrays correspond to pairs of coordinates defining consecutive tie-points linking the master and the correlative curves (i.e., depth yb of correlative core b linked to a depth xa of master core a). 2.1 data input: green worksheet in the data worksheet (marked in green) the user enters the stratigraphic data of selected parameters (up to 11 distinct parameters) for both the master (core “a”) and the correlative (core “b”) cores and select pairs of tie-points for correlation. the correlation process consists in matching the stratigraphic trends of up to 11 parameters from core b (listed in columns a to l, with headings marked in blue) with corresponding parameters from core a (listed columns t to ae, with headings marked in orange). this correlation is achieved by means of the tie-points, where a depth of b is associated with a depth of a. in detail, the user should copy the data of reference stratigraphic trends of the “master core” in columns t to ae (figure 2a). columns a to l must be filled with sagnotti and caricchi 2 42 43 44 data plot 01 plot 02 plot 03 plot 04 plot 05 plot 06 plot 07 plot 09plot 08 plot 10 plot 11 + normal visualization sum= 0 figure 1. the stratfit excel workbook is organized in 12 worksheets. the green one is the “data” worksheet, where the user introduces data for the two cores and makes the choice of tie-point pairs for correlation. the yellow sheets show the plots of the stratigraphic trends for up to 11 distinct parameters, where the user can visualize and check the correlations resulting from the adopted tie-points. data of the correlative core (figure 2b). stratigraphic coordinates (depths, or ages) are listed in column t for the master core and in column a for the correlative core (figure 2). the columns p and q are dedicated to the input of selected pairs of tie-points chosen by the user to correlate stratigraphic trends finding the best common depth (or age) (figure 3a). the tie-points could be chosen either considering significant and coincident peaks and troughs of the curves or taking into account lithostratigraphic correlations and/or the information coming from other available stratigraphic constraints (e.g., 14c ages). the common depth is computed in the column n according to the forecast function, resulting in a common depth by correlating each depth of core b to a corresponding depth of core a (i.e., in the stratigraphic framework of the master core). the stratigraphic framework (i.e., data in columns a and t) can also consist of ages rather than depths, of course. columns ax, ay, az and ba include formulas (based on the vlookup, index and match functions) used to find the tie-point pairs on which apply the forecast function, for each x value of correlative core b. these columns are colored in red and should not be edited during the use of the software (figure 3b). moreover, the data worksheet includes a graph “depth a vs depth b”, to the right of column ae. this graph allows to visualize the relative variation in the sedimentation rate resulting from the chosen tie-point pairs (figure 3c). 3 stratfit: a stratigraphic correlation tool <==> . home layout tables charts smartart formula data revision strafit.xlsx paste font alignment number format cells themesedit wrap text merge general conditional formatting styles actions themes arial g c s 10 q40 a abc ,0 ,00 ,00 ,0 % 000 aa aa x a b c d e f g h i j k l curve b = xx curve b = xx data in correlative core data in correlative core + 1 2 3 1 2 3 4 5 6 7 8 9 10 <==> . home layout tables charts smartart formula data revision strafit_general form.xlsx paste font alignment number format cells themesedit wrap text merge general conditional formatting styles actions themes arial g c s 10 q40 a abc ,0 ,00 ,00 ,0 % 000 aa aa x t u v w y z aa ab ac adx ae depth a (cm) depth a(cm) curve a = xy data in master core data in master core + 1 2 3 1 2 3 4 5 6 7 8 9 10 parameter 11_aparameter 10_aparameter 09_aparameter 08_aparameter 07_aparameter 06_aparameter 05_aparameter 04_aparameter 03_aparameter 02_aparameter 01_a parameter 11_aparameter 10_aparameter 09_aparameter 08_aparameter 07_aparameter 06_aparameter 05_aparameter 04_aparameter 03_aparameter 02_aparameter 01_a depth b (cm) depth b(cm) parameter 11_aparameter 10_aparameter 09_aparameter 08_aparameter 07_aparameter 06_aparameter 05_aparameter 04_aparameter 03_aparameter 02_aparameter 01_a parameter 11_aparameter 10_aparameter 09_aparameter 08_aparameter 07_aparameter 06_aparameter 05_aparameter 04_aparameter 03_aparameter 01_a parameter 02_a curve a = xy figure 2. “data” worksheet. a) columns t to ae, where the user should list the data of reference stratigraphic trends of the “master core”. b) columns a to l, where the user should list data of the correlative core. a) b) sagnotti and caricchi 4 2.2 graphs output: yellow worksheet each worksheet marked in yellow refers to a specific parameter and shows plots of the stratigraphic trends. on each sheet four graphs are reported. the graphs show i) the stratigraphic variation of the selected parameter in the stratigraphic coordinates (depth or age) of correlative core b (figure 4a) ii) the stratigraphic variation of the selected parameter in the stratigraphic coordinates (depth or age) of master core a (figure 4b); iii) the stratigraphic trend of the selected parameter in a common stratigraphic framework, after correlation of core b to core a, both as horizontal (figure 4c) and vertical (figure 4d) plots. observing the trends in the common stratigraphic framework, the effects of the selected tie-point pairs on the stratigraphic matching can be promptly and easily visualized. 3. working example the correlation process starts entering a first selection of a few tie-points, derived from lithological evidence or available radiometric data, if available, or by a starting assumption about sedimentation rates. . home layout tables charts smartart strafit.xlsx paste font alignment numberedit generalarial g c s 10 a % 000 x l m n o r zp depth b + 1 2 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 q s depth a tie-pointstie-points tie-points depth b depth a depth a (cm) depth a (cm) >> dgrm/dnrm int parameter 11_b common depth depth (cm) 17601-3 correlated to 17603-3 data output common depth data in tie-points data output common depth data in tie-points 35 393 45 409 63 435 85 471 111 542 119 584 134 668 common depth depth (cm) 17601-3 correlated to 17603-3 172 722 184 746 204 235 832 880 3.65e-01 2.76e-01 2.07e-01 1.54e-01 9.23e-02 4.04e-02 4.52e-02 8.69e-02 1.21e-01 1.58e-01 2.18e-01 7 8 9 10 11 12 13 14 15 16 17 393.00 394.60 396.20 397.80 399.40 401.00 402.60 404.20 405.80 407.40 409.00 y = 0.3816x 105.74 0 50 100 150 200 250 0 200 400 600 800 1000 d ep th b depth a . home layout tables charts smartart strafit.xlsx paste font alignment numberedit generalarial g c s 10 a % 000 x axawavau ay ba bbaz + 1 2 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 >> these colums include formulas do not change these colums include formulas do not change min depth b ref min depth b ref max depth b ref max depth b ref min depth a ref min depth a ref max depth a ref max depth a ref 393 409 393 409 393 409 393 409 393 409 393 409 393 409 393 409 393 409 393 409 409 435 35 45 35 45 35 45 35 45 35 45 35 45 35 45 35 45 35 45 45 35 45 63 figure 3. “data” worksheet. a) columns p and q where the user should insert selected pairs of tie-points. the common depth is computed in column n according to the forecast function. b) columns ax, ay, az and ba include formulas used to find the tie-point pairs that should be used to apply the forecast function, for each x value of correlative core b. these columns are colored in red and should not be edited during the use of the software. c) graph of depth a versus depth b. this plot helps to visualise the effects of the selected tie-points on the relative changes in sedimentation rates between the two stratigraphic sequences. a) b) c) 5 stratfit: a stratigraphic correlation tool this process results in an initial visualization of the correlation effects on all the selected stratigraphic trends. afterwards, the match between the distinct stratigraphic trends can be revised and improved by changing and adding tie-points that link directly the main peaks and troughs of distinct stratigraphic trends. this method is very fast and direct and it allows the user to check immediately and simultaneously the effects on the overall correlations among different parameters. in this way the user may identify the main tie-points for a core correlation that satisfy multiple parameters and stratigraphic trends. practically, first of all, the user has to paste the data in the specific columns, and then he has to insert some initial tie-points in columns p-q. for the correct mode of operation of the worksheet the last tie-point of depth b has to be higher than the last value of the depth b. the user then can start an iterative process of improving correlation by changing or adding tiepoint pairs. the selection of tie-points correlating core b to core a has an effect on the relative changes in sedimentation rates. this effect can be visualized in the graph “depth a vs depth b” in the data worksheet. 0.00 0.01 0.01 0.02 0.02 0.03 0.03 0 100 200 300 n r m ( a /m ) depth b (cm) nrm curve b= core 17601-3 curve b= core 17601-3 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0 100 200 300 400 500 600 700 800 900 1000 n r m ( a /m ) depth a (cm) nrm curve a = core 17603-3 curve a = core 17603-3 curve b = core 17601-3 curve a = core 17603-3 0.00 0.005 0.010 0.015 0.020 0.025 0.030 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0 100 200 300 400 500 600 700 800 900 1000 n r m ( a /m ) n r m ( a /m ) common depth (cm) nrm 0.00 0.005 0.010 0.015 0.020 0.025 0.030 0 100 200 300 400 500 600 700 800 900 1000 0.00 0.02 0.04 0.06 0.08 0.10 0.12 c om m on d ep th ( cm ) nrm (a/m) nrm (a/m) figure 4. a) stratigraphic variation of the natural remanent magnetization (nrm) in the stratigraphic coordinates (depth or age) of core b. b) stratigraphic variation of the nrm in the stratigraphic coordinates (depth or age) of the core a. c, d) stratigraphic trends of the selected parameter in a common stratigraphic framework, after correlation of core b to core a, both as horizontal (c) and vertical (d) plots. a) b) c) d) sagnotti and caricchi 6 1 2 3 4 5 6 7 8 9 9.74 m b sf ox 1 3 4 5 m b sf 5.37 2 16°00´e 0 1000 1500 500 m bear island m 00 4 m 00 3 m 00 01 m 00 2 m 0 02 m 001 17601 17603 m 00 01 m 0 05 1 0 10 20 km m 00 1 m 00 51 m 0 03 m 00 2 spitsbergen greenland sea barents sea arctic ocean figure 5. bathymetric map of the kveithola glacigenic trough with location of the cores here discussed. 1 2 3 4 5 6 7 8 9 9.74 m b sf 0 0.015 δgrm/δnrm 0.03 0.045 30 70 110 k (10-5 si) 0.06 0.10 0.14 0.18 0.22 0.26 arm (a/m) 26 mdfnrm(mt) 36 30 90 50 34 28 32 0 20 40 k (10-5 si) 0.01 0.07 arm (a/m) 60 20 40 mdfnrm(mt) 0 0.5 1 1.5 δgrm/δnrm 1 3 4 5 m b sf 5.37 2 0.050.03 0.05 0.1 nrmintensity (a/m) 30 60 90 incl (°) -260 -140 -20 120 220 decl (°) 2 4 mad (°) 00 6 m id d le s lo p e 17603-3 17601-3 nrm/k nrm20mt/arm20mt rpinorm max 0. 0 0. 2 0. 4 0. 6 0. 8 1. 0 o u t e r s h e lf decl (°) 30 60 90 incl (°) mad (°) -180 40 120 -40 nrmintensity (a/m) 6 2 400.02 0 0.04 rpinorm max 0. 2 0. 4 0. 6 0. 8 1. 0 nrm/k nrm20mt/arm20mt 0. 1 ox legend interlaminated faciesheavy bioturbated facies crudely layered faciescoarse sand with shell debris ird-rich facies oxydized layer figure 6. stratigraphic trends of the rock magnetic and paleomagnetic parameters [caricchi et al., 2018]. 7 stratfit: a stratigraphic correlation tool 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 100 200 300 400 500 600 700 800 900 1000 a r m ( a /m ) common depth (cm) arm curve a = core 17603-3 curve b = core 17601-3 668 668 x x 0.000 0.005 0.010 0.015 0.025 0.030 0.035 0.040 0.045 0.050 a r m ( a /m ) 0.000 0.005 0.010 0.015 0.025 0.030 0.035 0.040 0.045 0.050 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 100 200 300 400 500 600 700 800 900 1000 a r m ( a /m ) a r m ( a /m ) common depth (cm) arm curve a = core 17603-3 curve b = core 17601-3 644 668 x x 130 131 132 133 134 135 136 137 138 33.52 32.19 33.54 33.55 34.90 33.58 33.59 33.60 33.61 2.28e-02 2.35e-02 2.41e-02 2.46e-02 2.48e-02 2.47e-02 2.42e-02 2.34e-02 2.24e-02 3.60e-02 3.73e-02 3.87e-02 3.94e-02 3.89e-02 3.79e-02 3.69e-02 3.60e-02 3.52e-02 82.6 82.3 81.9 81.3 80.4 79.6 78.9 78.8 79.1 13.6 13.3 12.9 12.3 11.4 10.6 9.9 9.8 10.1 1.0 1.1 1.5 1.7 1.7 1.5 1.2 1.1 1.1 44.33 44.57 44.79 44.92 44.90 44.75 44.51 44.25 43.91 34.62 35.35 36.45 37.03 36.61 35.70 34.93 34.53 34.30 0.96 0.96 0.95 0.95 0.97 0.99 1.00 0.99 0.97 8.72e-03 1.14e-01 2.36e-01 2.78e-01 2.35e-01 1.48e-01 6.43e-02 6.77e-03 0.00e+00 0.55 0.59 0.59 0.60 0.58 0.60 0.59 0.57 0.54 628.71 632.77 636.84 640.90 644.97 649.03 653.10 657.18 661.23 curve b = core 17601-3 depth b (cm) k (10e-05si) nrm (a/m) arm (a/m) inc chrm (°) mad mdf (nrm) mdf (arm) nrm20mt /arm20mt (norm-max) nrm20mt/k (norm-max) δgrm /δnrm common depth decl chrm (°) a b c d e f g h i l mj k n o tie-points depth b depth a 35 339 45 409 63 435 85 471 111 542 119 548 172 722 184 746 204 832 235 880 tie-points depth b depth a 35 339 45 409 63 435 85 471 111 542 119 548 172 722 184 746 204 832 235 880 150 710 134 668 figure 7. example of the tie-points choice and input. a) b) c) d) 3.1 case study the example that we discuss refers to a study by caricchi et al. [2018], carried out on sediment cores collected in the continental margin of the north-western barents sea, south of svalbard (figure 5). the work aimed to correlate the sedimentary sequences across the continental slope and shelf of the kveithola glacigenic system, through paleomagnetic and rock magnetic analyses. the stratigraphic trends of the measured rock magnetic and paleomagnetic parameters of core 17601-3 (correlative core “b”, from the continental outer shelf) and core 17603-3 (master core “a”, from the continental slope) are shown in figure 6. the initial tie-point pairs for core correlation were deduced from 14c ages and lithostratigraphic constraints. afterwards, the correlation has been refined through an iterative process aimed to improve the matching between peaks and troughs of paleomagnetic and rock magnetic parameters. to combine different peaks (or troughs) we adjusted the initial tie-points and added some additional tie-point pairs as in the example reported the figure 7. initial correlation implied, for example, the hypothesis that stratigraphic depth of 150 cm in the core b correlates to 710 cm in master core a. this assumption resulted in the plot shown in figure 7a, referring to the stratigraphic trend of the arm parameter, that is the intensity of the anhysteretic remanent magnetization, an artificial remanence produced by the simultaneous application of an alternating af and a constant dc magnetic field, which depends on the concentration of ferrimagnetic minerals in the sediment. from this plot, it is clear that the arm peak of core b resulting at a common depth of 644 cm following the correlation under the initial constraints, matches closely the peak measured at 668 cm of depth in master core a (figure 7a). in this case the initial pair of tie-points was adjusted to improve the correlation and impose matching of the arm peak observed in the two stratigraphic trends. to do this adjustment, the proper data were found in the “data” worksheet, searching in column n (common depth) the value closest to 644 cm and finding the corresponding depth in the column a (depth b) (figure 7b). in this case the value is 134 cm, thus the new tie-points pair was defined as 134 688, which implies that depth 134 cm in core b was now linked to depth 668 cm in master core a. typing these values in the tie-point columns sagnotti and caricchi 8 1 2 3 4 5 6 7 8 9 17603-3 m b sf 1 2 3 4 5 m b sf 17601-3 interlaminated facies ird-rich facies heavy bioturbated facies crudely layered facies coarse sand with shell debris legend oxydized layer 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 0.02 0.04 0.06 0.08 0.01 0.12 0 100 200 300 400 500 600 700 800 900 1000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 0.05 0.10 0.15 0.20 0.20 0 100 200 300 400 500 600 700 800 900 1000 0 0.2 0.4 0.6 0.8 1.0 1.2 common depth (cm) n r m ( a /m ) 17 60 3 a r m ( a /m ) 17 60 3 n r m ( 10 -2 a /m ) 17 60 1) a r m ( 10 -2 a /m ) 17 60 1 r p i ( n r m 20 m t /a r m 20 m t ) 17603-3 depth (cm)17601-3 correlated to 17603-3 common depth (cm) common depth (cm) 0 100 200 300 400 500 600 700 800 900 1000 0 5 10 15 20 25 30 35 40 0 10 20 30 40 50 60 70 80 0 100 200 300 400 500 600 700 800 900 1000 k (1 0e -0 5 s i) k (1 0e -0 5 s i) common depth (cm) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 100 200 300 400 500 600 700 800 900 1 000 n r m 2 0m t /k _n or m -m ax common depth 0 10 20 30 40 50 60 70 80 90 100 0 100 200 300 400 500 600 700 800 900 1000 in cl c hr m ( °) common depth (cm) figure 8. resulting correlations between selected rock magnetic parameters of the stratigraphic sequences from the kveithola continental slope and outer shelf. (figure 7c) it is possible to visualize directly the effects of the change in the arm stratigraphic trends from the two cores (figure 7d), as well as on all the other considered parameters, and then proceed with further analogous adjustments until the overall match is satisfactory. this procedure allowed us to define a close correlation between slope and outer shelf sedimentary successions (figure 8). 4. conclusion the stratfit workbook is designed to deal with the correlation between two distinct stratigraphic sections by taking into account multiple stratigraphic trends. the released software improves and simplifies the correlation process. stratfit may be valuable for various users for the following attributes: compatibility with nearly all operating system; easy input of data; immediate visualization of the results; easy export of plots; modular structure in distinct worksheets for each selected stratigraphic trend; easy and prompt visualization of the effects of the adopted matching on multiple stratigraphic trends; full control of the whole workflow and possibility of implementation of the workbook by users. the stratfit workbook and the user manual may be download as supplementary material. we thank alessandro vona and aldo winkler for testing the software. the two anonymous reviewers are kindly acknowledged for their suggestions that contributed to improve the paper. we also thank fabio caratori tontini for the editorial handling. references caricchi, c., r.g. lucchi, l. sagnotti, p. macrì, c. morigi, r. melis, m. caffau, m. rebesco, t. j. j. hanebuth (2018). paleomagnetism and rock magnetism from sediments along a continental shelf-to-slope transect in the nw barents sea: implications for geomagnetic and depositional changes during the past 15 thousand years, global and planetary change, 160, 10-27. karner, d. b., j. levine, b. p., medeiros and r. a. muller (2002). constructing a stacked benthic delta o-18 record, paleoceanography, 17, 3, 2-1 to 2-17. imbrie, j., j. d. hays, d. g. martinson, a. mcintyre, a. c. mix, j. j. morley, n. g. pisias, w. l. prell, n. j. shackleton (1984). the orbital theory of pleistocene climate: support from a revised chronology of the marine d18o record, in milankovitch and climate, part 1, edited by a. berger, pp. 269 305, d. reidel, norwell, mass. lisiecki, l. e., p.a. lisiecki (2002). application of dynamic programming to the correlation of paleoclimate records, paleoceanography, 17(4), 1049, doi:10.1029/2001pa000733. lisiecki, l. e., m. e. raymo (2005). a pliocene-pleistocene stack of 57 globally distributed benthic d18o records, paleoceanography, 20, pa1003, doi:10.1029/2004pa001071. martinson, d. g., w. menke, p stoffa (1982). an inverse approach to signal correlation, j. geophys. res., 87, 4807 4818. sagnotti, l., p. macrì, r. lucchi, m. rebesco, and a. camerlenghi (2011a), a holocene paleosecular variation record from the northwestern barents sea continental margin, geochem. geophys. geosyst., 12, q11z33, doi:10.1029/2011gc003810. sagnotti l., smedile a., de martini p.m., pantosti d., speranza f., winkler a., del carlo p., bellucci l.g. and gasperini l. (2011b), a continuous palaeosecular variation record of the last 4 millennia from the augusta bay (sicily, italy), geophysical journal international, 184, 191-202. waterman, w. s., r. jr. raymond (1987). the match game: new stratigraphic correlation algorithms, math. geol., 19, 109 127. *corresponding author: leonardo sagnotti, istituto nazionale di geofisica e vulcanologia, email: leonardo.sagnotti@ingv.itr © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. 9 stratfit: a stratigraphic correlation tool << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 63, 2, pa211, 2020;doi:10.4401/ag-8066 characterization of ionospheric delay and forecasting using gps-tec over equatorial region nouf abd elmunim*,1,2, mardina abdullah2,3, siti aminah bahari3 (1) department of electrical engineering, princess nourah bint abdul rahman university, riyadh, saudi arabia (2) department of electrical, electronic and systems engineering, universiti kebangsaan malaysia, bangi, selangor, malaysia (3) space science centre, universiti kebangsaan malaysia, bangi, selangor, malaysia article history: received december 28, 2018; accepted july 4, 2019 abstract this paper investigates the characteristic of the ionospheric delay and forecasting using modified statistical holt-winter method over malaysia. this was carried out by using dual frequency gps ionospheric scintillation and tec monitor (gistm) receivers installed at the national university of malaysia (ukm) (geographic coordinates 2.55°n, 101.46°e and geomagnetic coordinates 7.10°s, 174.05°e), and langkawi national observatory (langkawi) (geographical coordinates 6.19°n, 99.51°e and geomagnetic coordinates 3.39°s, 172.42°e), during 2011. in this work, the holt-winter forecasting method is modified to provide a better forecasting result over malaysia. the diurnal, monthly, and seasonal variations of actual ionospheric delay have been analyzed and compared with forecasted ionospheric delay. based on the results, the diurnal variability of ionospheric delay was maximal mostly between 12:00– 17:00 lt, and minimal nearly at 05:00 lt. the maximum monthly ionospheric delays occurred in october, which is attributed to the geomagnetic storm disturbance that caused increase in ionospheric delay. the lowest seasonal ionospheric delay occurred in the summer and the highest was during the equinox. the maximum ionospheric delay on a quiet day was around 41.9 tecu, while it reached up to 86.6 tecu during the geomagnetic storm. the holt-winter method exhibited the highest difference between the measurements and the forecasts in march, with an error of 6%. hence, the modified holt-winter method forecast ionospheric delay effectively during quiet and disturbed periods over malaysia.. keywords: gps, tec, ionospheric delay, forecasting, holt-winter. 1. introduction the earth’s ionosphere is the ionized region of the upper atmosphere consisting of different ionized layers (d, e, and f). the electron concentrations reach their highest values at f layer, which is largely affects the radio waves propagation. the total electron content (tec) is an important ionospheric parameter measuring the total number of electrons in a column of one square meter cross section along the path from satellite to receiver [langley, 2000; abdullah et al., 2009]. the ionospheric tec is a notable source of error that disrupts the accuracy of the global positioning system (gps) signals passing from the satellite to the ground by changing the velocity of the propagated signals causing ionospheric delay. the ionospheric delay changes with the consideration of factors including the user location, the elevation angle, the time of day, the seasons of the year, and the solar cycle. during high solar activity, the delay can generate vertical errors in the range of 5 and 15 m, and it can reach up to 100 m in particular conditions [klobuchar, 1991]. in the low latitude and equatorial regions, the variations of the tec are recognised to be high when compared with other regions such as mid-latitude and high-latitude. the ionospheric delay in equatorial regions is subject of day-to-day variability and is a challenging problem for ionospheric modelers [kumar et al., 2016]. therefore, investigation the ionospheric delay and identification of the precise and accurate forecasting model of transionospheric propagation errors are essential for precise measurements and further contributes valuable information to satellite and space probe navigation, space geodesy, radio astronomy and other applications. a number of empirical models have been developed to effectively forecast ionospheric delay variability at various regions worldwide, such as the bent model [bent et al., 1975], global ionospheric maps (gims) [mannucci et al., 1998], and the international reference ionosphere (iri) model [bilitza, 2001], which is widely used in different scientific areas and continuously developed and updating. generally, the ionospheric delay error correction for single frequency gps receivers is done using the broadcast model, with the accuracy ranging from 50% to at most 60% [klobuchar, 1996]. further techniques have been developed for ionospheric prediction purposes such as the autoregressive moving average (arma) method, and the autocovariance that is used in forecasting the tec timeseries in various stations over europe, demonstrating a precision forecast in the mid latitudes [krankowski et al., 2005; stanislawaka and zbyszynski, 2002]. furthermore, the autocovariance forecasting procedure was used to forecast ionospheric delay in disturbed conditions with acceptable precision [stanislawska and zbyszynsk, 2001; stanislawska and zbyszynsk, 2002]. a number of researchers have used self-consistent and neural network models for forecasting the tec over the equatorial region [williscroft and pool, 1996; oyeyemi et al., 2006; habarulema et al., 2009, klimenko et al., 2011]. later, suwantragul and other researchers demonstrated the application of the holtwinter method in forecasting the ionospheric delay over chiang mai in thailand using the gps-tec measurements [suwantragul et al., 2003]. five-day forecasts were generated by the holt-winter method and compared with the observed data. the method was determined to be capable of predicting ionospheric delays and thus enhancing positional accuracy up to 50%. since the study of the ionospheric delay and forecasting is a new subject in the equatorial region, particularly in malaysia, a precious model has become essential. therefore, the study of ionospheric error and forecasting is very important and could help in representing the model error of the data assimilation, which is useful for improving ionospheric models, it is also important to identify and select a suitable model in order to provide gps data correction to gps users and to improve the accuracy performance of gps positioning as well as to assist the future development of satellite-based augmentation system (sbas) in malaysia. in this research, the holt-winter method is modified and implemented to forecast ionospheric delay accurately. the variations in the diurnal, monthly and seasonal ionospheric delay were estimated from the gps ionospheric scintillation and tec monitor (gistm) receiver and compared with the ionospheric delay forecast using the statistical holt-winter method over the ukm and langkawi stations. besides the differences in ionospheric delay observed during geomagnetic storm disturbances were investigated. 2. data and method of analysis the gps-tec data utilized in this work were inferred from the dual-frequency gistm receivers installed at ukm station (geographic coordinates 2.55°n, 101.46°e and geomagnetic coordinates 2.55°n, 101.46°e), and langkawi station (geographic coordinates 6.19°n, 99.51°e and geomagnetic coordinates 3.39°s, 172.42°e) in 2011. the gistm receiver is capable of tracking up to 11 gps signals at both l1 (1575.42 mhz) and l2 (1227.6 mhz) frequencies. a 50 hz measurement was used at the amplitude and phase, and a 1 hz measurement was used for the divergence of satellite’s code/carrier. the gps data was analysed using an elevation angle greater than 20° because the gps signal suffers from significant fluctuations due to the multipath effect [da silva and camargo, 2010], this multipath effect appears with a small elevation angle. the lock time shows the amount of time the receiver was restricted to the carrier phase on l1 and l2 (l1, l2 lock time), the measurement data for l1 and l2 are restricted for over 240 seconds nouf abd elmunim et al. 2 (4 minutes). this time is required by the gistm receiver for the detrending high pass filter (hpf) to reinitialise when the lock to the carrier phase is lost. the application of a dual-frequency pseudorange and carrier phase allows us the slant tec (stec) through the line of sight. the gistm receiver provided the stec data every 60 seconds, the stec was determined from the receiver independent exchange (rinex) format observation files obtained from the gistm. the gps rinex files were processed using the analysis software for gps-tec developed by gopi seemala at boston college’s institute for scientific research [seemala and valladares, 2011]. the software is available online at http://seemala.blogspot.com. the gps-tec analysis software uses the phase and code measurements for both l1 and l2 frequencies in order to calculate the relative stec values. then the absolute values of stec are obtained by including the differential satellite biases published by the university of bern and the receiver bias that is calculated by minimizing the tec variability between 02:00 and 06:00 lt [doherty et al., 2004; habarulema et al., 2013]. the bias free tec is given in ascii files together with the time, elevation angle, azimuth angle, ipp-longitudes and ipplatitudes parameters. the vtec is computed by averaging the tec for individual satellites in view. the converted vtec in meter unit is termed as vertical ionospheric delays [abe et al., 2017]. additionally, the ionospheric delay was approximated using eq. (1) as shown below (1) where 𝐼�� , is the ionospheric delay in metres, with the carrier frequencies f1 = l1 and f2 = l2. the holt-winter method is a statistical short-term method that utilises mathematically recursive functions to forecast ionospheric delay trend behaviour [makridakis et al., 1982]. it uses a time-series with repeated trend and a seasonal pattern to make the forecasting on the assumption that the future will follow a similar pattern. the exponential smoothing function method is used to minimise the data fluctuations of the time-series to offer a clearer point of view of the time-series. moreover, it can forecast values of future data in the time series by giving the best way to derive accurate readings. three smoothing constants or weights, including 𝛼, 𝛽 and 𝛾 are suitable for a certain period in updating the components. basically, 𝛼, 𝛽 and 𝛾 are used to be 0.2 in the basic holt-winter equations. the holt-winter method includes the multiplicative and additive seasonal models. the multiplicative model is the smoother and more reliable model for forecasting in the equatorial region over malaysia when compared with the additive model [elmunim et al., 2015]. therefore, the multiplicative model was used in this research. in order to improve the accuracy of the method, the holt-winter method equations were modified by identifing some certain smooth coefficients values and added them to the equations. the smooth coefficients founded were 0.9, 0.1, and 0.1 for 𝛼, 𝛽 and 𝛾, respectively. the method was modified and tested in ukm and langkawi stations, resulting in more accurate forecasts than the basic equation results. the holt-winter (multiplicative) method equations are as follows: (2) 𝑏𝑡 = 𝛽 (𝐿𝑡. − 𝐿𝑡‒1) + (1+ 𝛽)𝑏𝑡‒1 (3) (4) 𝐹𝑡 = (𝐿𝑡‒1+ 𝑏𝑡‒1)𝑆𝑡‒1 (5) 𝐹𝑡+𝑚 = (𝐿𝑡 + 𝑏𝑡𝑚)𝑆𝑡‒𝑠+𝑚 (6) where 𝐿𝑡 is the level; 𝑏𝑡 is the trend; 𝑆𝑡 is seasonal; 𝑌𝑡 is the vtec, while t is the time period for the 𝐿𝑡, 𝑏𝑡, 𝑆𝑡 and 𝑌𝑡 components; 𝐹𝑡 is the forecasting value of a period ahead; 𝐹𝑡+𝑚, is the forecasting period. the smoothing coefficients 𝛼(0.9), 𝛽(0.1), and 𝛾(0.1) are the level, trend and seasonal, respectively; m is the forecast period and s is the seasonal duration. the seasonal initial value and the levels of the seasonal duration are determined using eqs. (7) and (8) below. 𝐼�� = 40.3𝑉𝑇𝐸𝐶 � 1�²² − 1�¹² � �� ��𝑆𝑡 = 𝛾 +(1− 𝛾)𝑆𝑡‒𝑠 𝐿𝑡 = 𝛼 (𝑌𝑡 − 𝑆𝑡‒1) + (1− 𝛼)(𝐿𝑡‒1 + 𝑏𝑡‒1)�� ��⁻¹ 3 characterization of ionospheric delay and forecasting http://seemala.blogspot.com nouf abd elmunim et al. 4 (7) (8) where s1 is the initial value of the seasonal component; y1 and l1, are the initial values of the vtec and the level, respectively; ls is the level of seasonal duration s. in order to make the forecast, the data of three days were used to forecast the following day. the mean absolute percentage error (mape) was utilised to define the error between the forecast results and the actual data in order to investigate the accuracy and validate the effectiveness of the method using the formula in eq. (9) below: (9) (10) where n is the total observed value; pet the percentage error; ft the forecast value for period t; yt represents the actual value. considering that mape values less than 10% indicate that the method gives a highly accurate forecast, a range between 10–20% indicates that the method gives a good forecast, a range between 20–50% indicates a reasonable forecast, and lastly a range greater than 50% indicates that the method gives an inaccurate forecast [lewis, 1982]. 3. results and discussion in the equatorial regions, the diurnal variations on quiet days normally vary according to photoionization production and the recombination losses associated with local solar radiation. the diurnal variation of the ionospheric delay at ukm and langkawi stations during a typical quiet day on 12 may 2011, where the kp ≤ 1, has been plotted to illustrate the comparison between actual delay (gps-tec) and forecast (holt-winter) at ukm and langkawi stations and is shown in figure 1. the ionospheric delay is displayed on the vertical axis in tecu unit, and local time (lt) is displayed on the horizontal axis in hours. malaysia’s local time is eight hours ahead of universal time (ut). the diurnal pattern of the ionospheric delay observes an obvious single peak. it exhibits a steady increase from morning to a post-noon maximum between 12:00-17:00 lt and then drops to reach a minimum during sunrise at 05:00 lt. adewale et al. [2011] investigated the diurnal variation of the tec over lagos stations in nigeria. they observed peaks from 15:00 to 17:00 lt, and the minimum at 06:00 lt. the diurnal variation at bangkok, thailand, reported by arunpold et al. [2014], was observed with the trending peak occurring in the post-noon time period between 14:00 and 18:00 lt during 2011. therefore, this indicates that the peaks of ionospheric delay are associated with the smaller chemical losses at the higher altitudes and the production of solar radiation during the post-noon time [kelly et al., 1996]. the diurnal behaviour of the actual and forecast ionospheric delays at ukm and langkawi stations generally demonstrate a good agreement. the holt-winter method underestimates the agreement between the actual ionospheric delay during the early morning time. the ionospheric delay at ukm station was slightly higher than langkawi station. the maximum ionospheric delay at ukm station reached up to 41.9 tecu while at langkawi station it reached 40.9 tecu. the lower value of the ionospheric delay was 4.7 tecu at both ukm and langkawi stations. 𝑆1= , 𝑆2 = , ... , 𝑆𝑠 =�¹ �� �² �� �� �� 𝐿𝑠 = (𝑌1+ 𝑌2 + ��� + 𝑌𝑠)¹ � mape = � ⃒ 𝑃𝐸𝑡 ⃒ ¹ � � �=1 𝑃𝐸𝑡 =� � × 100𝑌� ‒ 𝐹� 𝑌� 3.1 monthly variation the hourly monthly mean of ionospheric delay variations at ukm and langkawi stations are illustrated in figure 2 and figure 3, respectively. in general, it can be observed that the overall actual and forecast ionospheric delays at both stations are comparable in monthly patterns during the year. the ionospheric delays begin to steadily increase from february to reach a maximum during march and april, and then slightly decrease until august before increasing again in september, october, and november. the holt-winter method shows a good agreement in the monthly variations with the actual gps-tec. the forecast trends were nearly comparable to the gps-tec at both the langkawi and ukm stations, except during the peak hours in the post-noon time, which showed a slight underestimation in march, september, october, and november at ukm station and in march, july, august, and october at langkawi station. compared to the langkawi station, the ukm station recorded higher ionospheric delays during march, august, september, and october. considering the results obtained, it can be observed that the highest ionospheric delays occurred in the equinoctial months, which are march, april, september, october, and november, whereas the lowest ionospheric delays occurred in the solstice months of january, june, and july. similar findings over india, thailand, kenya, uganda, and ethiopia were stated by various researchers [bhuyan and borah, 2007; kenpankho et al., 2011; olwendo et al., 2012; arunpold et al., 2014; tariku, 2015a; tariku, 2015b; panda et al., 2015; kumar et al., 2016]. they reported that the maximum monthly variation was in march and the minimum was during june and july. the highest monthly diurnal peak at both langkawi and ukm stations was 69.5 tecu, which was likely influenced by geomagnetic disturbances in october, whereas the comparable peak during quiet periods was 32.3 tecu. 5 characterization of ionospheric delay and forecasting figure 1. figure 1. the hourly diurnal variation of the actual (gps-tec) and forecast (holt-winter) ionospheric delay during a quiet day at ukm and langkawi stations. nouf abd elmunim et al. 6 figure 2. the monthly variation of the actual (gps-tec) and forecast (holt-winter) ionospheric delay at ukm station. figure 3. the monthly variation of the actual (gps-tec) and forecast (holt-winter) ionospheric delay in langkawi station. in the year 2011, a number of moderate geomagnetic storms occurred. august and september were characterised by a number of medium storms. sub-major geomagnetic storms occurred on 6 august and 26 september, with minimum values of dst around -113 nt and -103 nt, respectively. these moderate events did not significantly affect the diurnal gps-tec and therefore the forecast trends. however, the geomagnetic storm that occurred on 25 to 26 october had a major impact, the details of which will be described in later sections. in order to validate the forecasting method, the mape is used to measure the forecasting errors and determine the forecasting method performance. the diurnal hourly average of mape variations, as well as the monthly mape averages at ukm and langkawi stations are shown in figure 4 and figure 5, respectively. the maximum mape for all the months of 2011 was observed during morning time between 00:00 lt and 07:00 lt, for both stations where the mape value was around 8%. in figure 4 (b) it can be seen that in ukm station the mape’s maximum monthly average was observed in january, march, and september with a value of about 5.8%, while the minimum was in april and december at around 2.2%. at langkawi station, the mape’s maximum monthly average was observed in january, march, and july with a value of about 6%, while the minimum was observed in april and december with a value of about 1.7% as shown in figure 5 (b). 7 characterization of ionospheric delay and forecasting figure 4. the diurnal hourly average of mape variation (a) and the monthly mape average (b) at ukm station. 3.2 seasonal variation to investigate the seasonal variation of ionospheric delay, the diurnal variation of seasonal mean values were calculated for all days of the season, where the seasons are described as summer (may, june, july, august), winter (january, february, november, december), and equinox (march, april, september, october). the seasonal actual and forecast ionospheric delay variations at ukm and langkawi stations are given in figure 6. it can be depicted that the maximum value of ionospheric delay was observed during the equinox, where this is due to the sun shines directly over the equatorial region during the equinoctial months causing a strongest ionization over the ionosphere. the minimum ionospheric delay value was observed during the summer, and that is could be due to unequal heating of the two hemispheres. it is produced by the transport of the neutral constituents from summer to winter hemisphere (hot to cold), which reduces the recombination rate in winter compared to the summer and leads to a relatively increase of electron concentration in the winter than the summer. moreover, the change of the direction of the neutral wind is another possible cause for this seasonal phenomenon. a meridional component of neutral wind apparently blows from the summer to the winter hemisphere that can reduce the ionization crest value during the summer solstice where it blows in an opposite direction to the plasma diffusion process originating from the nouf abd elmunim et al. 8 figure 5. the diurnal hourly average of mape variation (a) and the monthly mape average (b) at langkawi station. magnetic equator. consequently, during the equinox meridional wind blows from the equator to polar regions causing a high ionization crest value. other researchers like galav et al. [2010], asmare et al. [2014], and kumar et al. [2016] studied the seasonal variation in the indian region, and they found that the maximum seasonal variation occurred during the equinox while the minimum value was in summer. in this research, the maximum seasonal ionospheric delay was observed in equinox at approximately 54.2 tecu at ukm station and 47.6 tecu at the langkawi station and dropped down to a minimum of 4.7 tecu at both stations. in summer, the maximum ionospheric delay reached up to 35.2 tecu at ukm station and 33.3 tecu at langkawi station, while it reached a minimum of 1.9 tecu at both stations. generally, the holt-winter method exhibited good agreement with actual gps-tec for all seasons and in both stations, except for a slight underestimation around 1.9 tecu which was observed in the peak hours of the summer season at langkawi station. 9 characterization of ionospheric delay and forecasting figure 6. the seasonal variation of the actual (gps-tec) and forecast (holt-winter) ionospheric delay at ukm and langkawi stations. the mape of the hourly mean seasonal variation at ukm and langkawi stations are shown in figure 7 (a) and (b), respectively. the highest value of mape observed in the summer, with values reaching up to 5.7% and 5.9% at ukm and langkawi stations, respectively. the maximum value of mape observed in 04:00 lt at ukm station and in 08:00 lt at langkawi station. the averages of the seasonal mape values at ukm and langkawi stations are shown in figure 7 (c). during the summer, we found that mape was 3% at ukm station and 4.3% at langkawi station, while in equinox it was 1.9% and 2.4% at ukm and langkawi stations, respectively. generally, langkawi station had a comparatively higher value of mape than ukm station. nouf abd elmunim et al. 10 figure 7. the seasonal variation of mape value at ukm station (a) and langkawi station (b) and the seasonal mape average at ukm and langkawi stations (c). 3.3 disturbed period the geomagnetic storm occurred between 25 and 26 october 2011 (24 to 25 october in ut) was classified as an intense or major storm, were the dst value reveals a sharply decrease to -136 nt and the kp index reachs its maximum to 7 approximately. the diurnal variation of the actual ionospheric delay during the disturbed days from 23 to 28 october 2011 at ukm and langkawi stations are shown in figure 8. the geomagnetic storm commenced on 25 october and significantly increased the ionospheric delay from 15:00 lt until the next day at 04:00 lt. the peak in diurnal variation of the actual gps-tec was greatly enhanced on 25 october, when it reached up to 86.6 tecu at ukm station and 80.9 tecu at langkawi station. comparing the diurnal ionospheric delay during the disturbed periods, even before and after the geomagnetic storm, with the quiet day of 12 may as discussed earlier in figure 1, it was found that the ionospheric delay was less during the quiet day than the disturbed periods. during the disturbance of the geomagnetic storm, there is an input of energy into the polar ionosphere, which changes several thermospheric parameters, such as the composition, temperature, and circulation. when the composition changes it directly affects the electron concentration in the f2 region, and then the tec, while the circulation spreads the heated gas to lower latitudes. basu et al. [2007] reported that during major geomagnetic storms the 11 characterization of ionospheric delay and forecasting figure 8. the daily variation of actual ionospheric delay (gps-tec) during 23–28 october 2011. currents associated with inner magnetospheric electric field directed in the dusk-to-dawn direction can no longer shield the mid-latitude and equatorial latitudes from the high-latitude electric fields, and this leads to instantaneous penetration of electric fields from high latitude to the middle and the equatorial ionosphere. consequently, the particle transport and the prompt penetration of the high-latitude electric field to lower latitude which travel equatorward with high velocities during the storm can cause an increase in the tec value and the ionospheric delay. the diurnal variation of the actual ionospheric delay (gps-tec) during the geomagnetic storm disturbance is shown in figure 9. the top panel corresponds to the ionospheric delay hourly variations at the ukm and langkawi stations; the second panel corresponds to the southward component of the interplanetary magnetic field (imf) bz. the next two bottom panels correspond to different geomagnetic perturbation indices: the kp and dst indices, respectively. the dst and kp indices were obtained from the world data centre (wdc) kyoto, japan. http://wdc.kugi.kyoto-u.ac.jp/wdc/sec3.html and the imf bz data was obtained from the omni database http://omniweb.gsfc.nasa.gov/form/dx1.html. during the days of geomagnetic storms, a significant increase in the diurnal variations of the ionospheric delay was observed at both stations. during the geomagnetic storm’s main phase on 25 october, the ionospheric delay reached up to 86.6 tecu. the geomagnetic storm, preceded by a sudden commencement event, occurred at 01:00 lt on 25 october (17:00 ut, 24 october). the imf bz turned sharply southward at 03:00 lt on 25 october (19:00 ut, 24 october), and attained a minimum value of -13 nt at 07:00 lt on 25 october (23:00 ut, 24 october), before gradually rising to reach its maximum value of 22 nt at 00:00 lt on 26 october (16:00 lt, 25 october). the kp index started to increase before the imf bz declined. moreover, it continued to increase even when the imf bz attained its maximum positive value [davis et al., 1997]. the dst started to decrease from 05:00 lt on 25 october (21:00 ut, 24 october) and dropped to a minimum value of -137 nt at 09:00 lt (01:00 ut) on 25 october. after the commencement of the geomagnetic storm, the dst trend did not recover to its initial value until three days later. at high latitudes, the electric field penetration leads to the formation of ionospheric electron density irregularities in the equatorial region. therefore, the greatest decrease in the dst value was observed when the sudden intensification of the ring current occurred [basu et al., 2007]. the positive value of the dst mainly occurs due to the compression of the dayside magnetosphere during the initial phase of the geomagnetic storm, and the negative value is due to magnetic reconnection and the formation of the ring current during the main phase of the geomagnetic storm. several studies have discussed the perturbations during a geomagnetic storm in diurnal variations [rama rao et al., 2009; kumar et al., 2014]. zou et al. [2013] used a gpstec to observe the same storm on 25 october 2011, and they reported that the ionospheric delay increased at a higher rate compared to that seen during the quiet time. this effect was more clearly observed at lower latitudes than at higher latitudes. other researchers studied the effects of the same geomagnetic storm on 24–25 october 2011 on the ionospheric tec for two east african stations; they found that for both stations, the diurnal variations showed a decrease during the storm period [d’ujanga et al., 2013]. figure 10 (a) shows the variations of ionospheric delay forecasting during the disturbed period from 23 to 28 october 2011 for both the ukm and langkawi stations. the vertical axis shows the ionospheric delay in tecu while the horizontal axis shows the hourly mean of the disturbed period days. the forecasting variations in disturbed period ranged from 3.8 – 80.9 tecu at the ukm station and from 2.8 – 80 tecu at the langkawi station. in both stations, the maximum value for ionospheric delay forecasting was observed during the geomagnetic storm day, 25 october. the percentage of the mape in figure 10 (b) shows that the ukm station had a comparatively lower forecasting error than the langkawi station. the maximum forecasting error for the ukm station was 7.3%, while it was 8% at the langkawi station. the mape value was slightly higher during the disturbed period than the quiet period due to the increase of ionospheric delay during the intense geomagnetic storm that occurred on 25–26 october. this increase affected the forecasting and led to a slightly higher forecasting error. however, based on the result of the ionospheric delay variability during the quiet and disturbed periods, it can be observed that there were no significant changes in the trends during these two periods. the same finding was presented by habarulema et al. [2013], for their study of the african equatorial region; they observed two severe geomagnetic storm events which occurred in november 2004. nonetheless, even a slight increase must be taken into consideration, as this has the effect of comparatively increasing the forecasting error compared with the results obtained during the quiet periods. thus, this will lead to a slight decrease in the accuracy of the forecasting method during the disturbed periods. nouf abd elmunim et al. 12 http://wdc.kugi.kyoto-u.ac.jp/wdc/sec3.html http://omniweb.gsfc.nasa.gov/form/dx1.html 13 characterization of ionospheric delay and forecasting figure 9. the diurnal variation of ionospheric delay at ukm and langkawi stations and the parameters of the imf bz component of the interplanetary magnetic field; the kp index; and the dst index of the geomagnetic activity. 4. conclusion this research presents the variability of ionospheric delay and forecast using the statistical holt-winter method in malaysia over ukm and langkawi stations for the year 2011. the results show that the diurnal variability of ionospheric delay has minimum value during sunrise at 05:00 lt and maximum during the post-noon time mostly between 12:00–17:00 lt. the holt-winter method forecast displayed a good agreement, with a slight underestimation compared to the actual gps-tec observed in both ukm and langkawi stations for the entire year. the maximum and minimum monthly ionospheric delay were observed on march and july, respectively. however, the ionospheric delay had a significant increase in october due to the geomagnetic storm effect. ukm station showed a higher ionospheric delay than langkawi station. the ionospheric delay was lower during the summer season and higher during the equinox season. the maximum ionospheric delay forecast was observed during the geomagnetic storm day on 25 october in both stations. comparing the mape values during quiet and disturbed periods, it can be observed that the mape in disturbed periods is slightly higher than the mape during quiet periods. however, since it is still less than 10% it is considered to be an accurate forecasting result. langkawi station had a relatively higher mape than ukm station. the obtained results showed the effectiveness of the holt-winter method in forecasting ionospheric delay, providing good results in the diurnal, monthly and seasonal variations, and also during the disturbed period. these results could be useful in representing the model error using data assimilation. consequently, this could be also useful for developing the regional ionospheric delay model for better understanding and forecasting over malaysia. besides it might help to improve the accuracy and the performance of gps positioning in equatorial regions, and particularly in malaysia. nouf abd elmunim et al. 14 figure 10 (a) the variation for ionospheric delay forecast during the disturbed days (b) the mape’s variances during the disturbed days for both the ukm and langkawi stations. acknowledgments. the authors would like to express their gratitude to the malaysian government, the ministry of education, and the national university of malaysia (universiti kebangsaan malaysia, ukm) for partially funding this research through the frgs/2/2013/tk03/ukm/02/1 and gup-2013-047 research grants. moreover, this research was funded by the deanship of scientific research at princess nourah bint abdulrahman university through the fast-track research funding program. the authors would also like to extend their gratitude to the national observatory langkawi for providing the gistm data from langkawi station. additionally, the authors acknowledge the word data centre for geomagnetism, kyoto, http://wdc.kugi.kyoto-u.ac.jp/dstdir/index.html for providing dst and kp indices data and http://omniweb.gsfc.nasa.gov/form/dx1.html for providing interplanetary magnetic field, imf bz data. references abdullah, m., h .j strangeways, and d. m. walsh (2009). improving 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2017; 2016; icd, 2011]. navic/irnss signals that propagated through the low latitude ionosphere are affected by the inhomogeneous, anisotropic, and dispersive nature of the ionosphere. this characteristic can introduce additionally unknown delay in the signals, which in turns cause a degradation of the positional accuracy of the navic/irnss system [desai et al., 2018; 2015, ruparelia et al., 2015]. these degradations depend on the total electron content (tec), signal frequency and elevation angle of navic/irnss satellites above the horizon. ionospheric tec is the total number of electrons that are integrated between two points along a tube of one square meter cross-section; tec variation depends entirely on geographical locations and geophysical events such as solar flares and geomagnetic storms [chakraborty et al., 2014]. geomagnetic storm occurs when the earth's magnetic field is affected by solar disturbance or solar flare. the severity of a geomagnetic storm is usually classified on the basis of disturbance storm time index dst (i.e., the average value of the horizontal component of the earth’s magnetic field measured in nano tesla (nt)) article history receveid july 12, 2018; accepted september 24, 2018. subject classification: atmosphere; ionosphere; wave propagation; magnetic storm. abstract the total electron content (tec) of navigation with the indian constellation (navic)/ indian regional navigation satellite system (irnss) was examined under the influence of an intense geomagnetic storm occurred on 8 september, 2017, in the low latitudes of the indian region. one week (3 september, 2017 to 9 september, 2017) data from five stations located in the equatorial region and in the equatorial ionization anomaly (eia) area in india are collected from accord navic/irnss dual-frequency (l5 and s-band) receivers for the investigation. the diurnal tec comparison between iri-2007 empirical model and navic/irnss dual frequency model is done. through a comparative study, of tec at the five locations, we clearly observed geomagnetic storms using dual-frequency navic/irnss receivers, while the diurnal tec behavior of the iri-2017 model was the same on all observation days. on the intense stormy day, we observed an increase of about 19 tecu for the area near the equator and decrease of about 20 tecu in the eia region compared to other observed quiet days. as a result, positive correlation between tec and storm occurrence were found in the equatorial region, while a negative one in eia region. in order to support dramatic change in tec during intense geomagnetic storm, geomagnetic indices and solar wind/imf parameters maps are added. the results have been further validated using tec map from igs data and thermosphere o/n2 ratio map from global uv imager (guvi). and a weighted average of k-indices (kp, planetary k indices) from a network of geomagnetic observatory. the k index quantifies the perturbation of the horizontal component of the earth's magnetic field, where the ap index provides the daily average of geomagnetic activity. according to the dst and kp indices geomagnetic storm is classified as super (dst < -25° nt, kp=9), intense (-10° nt > dst > -25° nt, kp=8), or medium (dst <-10° nt, kp=7) [gonzalez et al., 1994, chakraborty et al., 2014; 2015]. the geomagnetic storms can be also identified by various other parameters such as auroral electrojet (ae), the symmetric disturbance the magnetic field h (sym-h), the interplanetary magnetic field (imf), the interplanetary electric field (ief) and the solar radio flux at a wavelength of 10.7 cm (2800 mhz) called the f10.7 index. geomagnetic storm effects on global positioning system (gps) based navigation data were observed by rao et al., in 2009, kumar et al., in 2010, rao et al., in 2013, astafyeva et al., in 2014, de siqueira negreti et al., in 2017. rao et al., [2009], found that in the equatorial ionization anomaly (eia) crest region during the geomagnetic storms on november 9 and 10, 2004, the tec value raised up to 90 tec units, which is equivalent to a ionospheric delay around 15 meters. rao et al. 2013, observed a significant increase of the tec values during the three major geomagnetic storms in 2011; this was obtained from gnss system by using the international reference ionosphere (iri) 2007 empirical model at a low-latitude bangalore (77.510e, 13.030n) station in india.. iri model was also applied by samed et al., [2016] and abreu et al., [2017]; they compared gps tec over the western black sea and brazilian sector respectively, during the unusual solar minimum activity in 2009. astafyeva et al., (2014), showed that the density of loss of lock (lol) increased up to a maximum of 3% during super storm and 1% when an intense storm occurred. during the 2013 high intensity, long duration, continuous auroral electrojet activity event, tec increases from 25 to 80 % compared to quiet time were observed by de siqueira negreti et al., (2017). desai et al., (2018) analyzed the impact of give model for improving the positional accuracy of navic/irnss system under the quiet and disturbed days. however, the study about the effect of intense geomagnetic storms on navic/irnss system still remains open. in this paper, we investigated the impacts of intense (minimum dst = -124, kp = 8, ap = 106) geomagnetic storms beginning from september 08 2017 on newly emerging regional navic/ irnss systems. the effect of the storm is verified by analyzing the diurnal tec behavior of one week data collected by dual frequency navic/irnss receivers at five different locations; one is located near the indian equator (iist trivandrum) and four eia are (svnit surat, iit mumbai, cbit hyderabad, iit gandhinagar). a tec map from the online iri model was also added and the tec changes for all observational days were found to be identical. the dual-frequency navic/irnss tec statistical analysis shows that on a stormy day (8 september, 2017), the tec increases of the navic/irnss satellite near the indian equator is about ~20 tecu (tec units) and while the tec in the eia region decreased of ~14 tecu compared to what observed in quiet days. therefore, the impact of the storm occurred as a positive increment in areas close to the equatorial region, while in the eia area was negative. the geomagnetic indices such as dst(nt), ae(nt), sym-h(nt), imf-bz(nt) and interplanetary parameters (kp, ap) were observed to verify the existence of intense storms on september 8, 2017. the observation is further validated by plotting a world tec map after collecting the data in ionex file format from international gnss service (igs) and thermosphere o/n2 ratio map from online thermosphere ionosphere mesosphere energetic and dynamics (timed) based global ultraviolet imager (guvi) site. 2. geomagnetic parameter analyses and data collection the dst(nt), ae(nt) and sym-h(nt) indices from world data center (wdc) for geomagnetism, kyoto (http:// wdc.kugi.kyoto-u.ac.jp), the kp, ap and f10.7 indexes from omniweb data explorer (https://omniweb.gsfc.nasa.gov) and imf bz (nt) from advanced composition explorer (ace) level 2 data server (http://www.srl.caltech.edu /ace /asc /level2 /lvl2data_mag.html) are obtained and observed for the storm verification. the appearance of the geomagnetic storm that began on 8 september 2017 is verified by plotting kp (see figure 1) dst, ap, ae, sym-h, f10.7, imf bz, and ief ey indexes (see figure 2) from 3 september to 9 september 2017. from the changes in the indices, it can be seen that the storm began on 7 september, 2017 at approximately 22:00 utc (16:30 lt) and ended at 04:00 utc on 9 september, 2017. the minimum values of the dst index (128nt) and sym-h (-146nt) were found at 02:00 utc on 8 september, 2017. similarly, the ae and ap indices are observed with maximum value of 236 and 2677 (nt), respectively. after 8 september, 2017, the ae index indesai et al. 2 creased significantly, indicating that the heat generated during geomagnetic storms in high latitudes may lead to the generation of highly perturbed electric fields. also, note that when the imf bz component downward direction, the imf ey upward and vice versa. the kp value increase to 8 (see figure 1), reaffirming the intense storm 3 impacts of geomagnetic storm on navic/irnss figure 1. graphical represenatation of kp indices. figure 2. graphical represenatation of various geomagnetic parameters. desai et al. 4 on 8 september, 2017. the geographical location of data collection navic/irnss receiver station are as follows; svnit surat (21.16° n, 72.78° e), iit mumbai (19.03° n, 72.91° e), cbit hyderabad (17.39° n, 78.31° e), iit gandhinagar (22.52° n, 72.92° e) and iist trivandrum (8.62° n, 77.03° e); their locations are depicted in figure 3. the iist trivandrum is located nearer to extended equatorial region, whereas the other four stations are located in eia region. the tec for the navic/irnss system can be estimated proportional to the rang difference using the dual frequency model given by equation (1) [icd, 2011]. (1) where, p1 and p2 are the pseudo-ranges of the navic/irnss signal at f1 (l5 band) and f2 (s band) frequencies, respectively. after correcting the satellite and receiver deviations, the slant tec (stec) is converted to a vertical tec (vtec) using an elevation (el) based mapping function. using thin shell model by assuming a height of h=350 km above the earth surface and earth radius re = 6378.137 km at ionospheric pierce points (ipp), the elevation dependant mapping function is given by (2) the navic/irnss satellites data is collected by accord navic/irnss+gps receiver provided by space application centre (sac), isro ahmedabad. the data for the sardar vallbhbhai national institute of technology (svnit), surat (21.16° n, 72.78° e) is acquired by the accord navic/irnss+gps receiver, which is located at the communication research laboratory of electronics engineering department. the navic/irnss satellites data for the location of iit mumbai, cbit hyderabad, iit gandhinagar, iist trivandrum is provided by sac, isro ahmedabad through ftp link. from 3 september, 2017 (time of week count (towc) =0) to 9 september, 2017 (towc=648000), the raw data files of the dual-frequency navic/irnss receivers for the above five locations were collected. at present, navic/irnss 1a is almost unusable due to the failure of the airborne atomic clock, and the navic/irnss 1i, which was navigated on april 12, 2014, replaced navic/irnss 1a [isro, 2018]. therefore, the data of navic/irnss 1a was not considered for result analysis. the received data was distinguished day-wise for each of the navic/irnss satellites (labeled 1b to 1g). initially, the distance between the navic/irnss satellites (1b to 1g) and the user receiver is calculated by extracting the time to travel information from the raw data. after calculating the ranges or distances for the single satellite, l5 and s bands tec is measured using the dual-frequency approach presented in equation 1. data analysis has been carried out using the matlab 2014 software tool. for the above five geographic locations, the effects of geomagnetic storms can also be observed by collecting tec using the online iri model (https://omniweb.gsfc.nasa.gov). the iri is the online standard empirical model developed by the space research council (cospar) and the international radio science union (ursu). the storm effect has been further verified after collecting and plotting data in ionex file format from international gnss service (igs) site (ftp://cddis.gsfc.nasa.gov/gnss/products /ionex) and thermospheric o/n2 data mapped with spatial resolution of 1.75°x1.75° collected and mapped using guvi (http://guvitimed.jhuapl.edu/guvi-galleryl3on2) during the storm event. detailed results analysis are provided in the next section. 3. discussion of detail result analysis figure 4 depicts the diurnal variation of tec comparison of six navic/irnss (1b, 1c, 1d, 1e, 1f and 1g) satellites visible at the svnit surat, eia location. time scales and dates are universal time (ut) or unifigure 3. geographical location of navic / irnss receiver stations. stec = 1 40.3 × f 1 2∗f 2 2 (f 1 2−f 2 2) (p1− p2),(electrons/ m 2 ), vtec = stec*f, f = 1− re *cos(el) re +h ⎧ ⎨ ⎪ ⎩⎪ ⎫ ⎬ ⎪ ⎭⎪ 2 5 impacts of geomagnetic storm on navic/irnss figure 4. tec comparison between iri-model and dual frequency navic/irnss receiver for svnit surat (21.16° n latitude, 72.78° e longitude). figure 5. tec comparison between iri-model and dual frequency navic/irnss receiver for iit mumbai (19.03° n latitude, 72.91° e longitude). figure 6. tec comparison between iri-model and dual frequency navic/irnss receiver for cbit hyderabad (17.39° n latitude, 78.31° e longitude) versal time coordinates (utc). the corresponding local time (lt) or indian standard time (ist) is 5.30 hours ahead to utc. for the available data starting from towc = 47290 (3 september, 2017, utc= 13:08:10, lt = utc+ 5:30 = 18:38:10) to towc = 584267 (9 september, 2017, utc = 18:17:47, lt = utc + 5:30 = 23:47:47) navic/ irnss satellites tec is plotted. the tec values estimated using the online iri-2012 model at the svnit surat location are also added for comparison. in order to observe tec changes during geomagnetic storm events, the dst index is also plotted as a reference. iri-2012 tec changes are almost the same for all the quiet days, with only 1 to 2 tecu increasing on storm days (89 september, 2017). drastic changes in the tec are observed for all navic/irnss satellite on 8 september, 2017 with respect to dst index, confirming that the storm effects on this day. for all observation days, the lowest tec for all navic/irnss satellites to be around 5 to 8 tecus and iri models 0.7 to 0.9 tecus is found. on all observation days, the tecs of the navic/irnss 1b satellite is the lowest with respect to the tecs of all other navic/ rnss satellites. similarly, figures 5 to 8 depicts the tec comparison at the eia locations iit mumbai, cbit hyderabad, iit gandhinagar and near the equatorial location iist trivandrum, respectively for the visible six navic/irnss (1b, 1c, 1d, 1e, 1f and 1g) satellites. it is also observed that average maximum tec for lowest latitude station (located nearer to equatorial region) iist trivendrum (8.62° n, 77.03° e) is around 35 to 40 tecu, while for the other station we have: cbit hyderabad (17.39° n, 78.31° e) is about 45 to 50 tecu, iit mumbai (19.03° n, 72.91° e) is around desai et al. 6 figure 7. tec comparison between iri-model and dual frequency navic/irnss receiver for iit gandhinagar (22.52° n latitude, 72.92° e longitude). figure 8. tec comparison between iri-model and dual frequency navic/irnss receiver for iist trivandrum (8.62° n latitude, 77.03° e longitude). 50 to 60 tecu, svnit surat (21.16° n, 72.78° e) is nearer 60 to 65 tecu and iit gandhinagar (22.52° n, 72.92° e) is around 65 to 85 tecu. similarly, for the average minimum tec, iist trivendrum is around 3 to 4 tecu, cbit hyderabad approximately 3 to 5 tecu, iit mumbai around 6 to 7 tecu, svnit surat nearer 5 to 8 tecu and iit gandhinagar approximately 10 to 15 tecu. therefore, it is found that tec also increased from low latitude to high latitude. similar observations are also found for the tec estimation using the iri-2012 model. sudden, changes in tec are also observed at all navic/irnss receiver locations on 8 september, 2017. however, the changes in the iri-2017 model are the same for all locations for all days. hence, the iri-2012 model fails to detect the event, while the dual-frequency navic/irnss receiver effectively identifies the existence of a geomagnetic storm. details on the maximum and median tec values are presented in table 1. according to the statistical analysis of equatorial iist trivandrum, it can be inferred that ~20 tecus increment of maximum value is observed for 1b to 1g navic/irnss satellites (maximum tecu values 50.83, 48.09, 48.57, 46.05, and 50.55) during the geomagnetic storm event (8 september, 2017) compared to all the others normal days. at the same location, tec median value is in a range between ~17 tecus and ~23 tecus for all navic/irnss satellites. similarly, for cbit hyderabad an increase of the maximum values of ~4 tecus is observed during the geomagnetic storm event. moreover, at iit mumbai the tec values decrease up to 2 to 6 tecus, at svnit surat the decrease was ~10 tecus and iit ghandhinagar reduction of maximum values of ~14 tecus is observed for navic/irnss satellites during stormy days compared to quiet days. the difference between the average value of the quiet days tecu (3 september, 2017 to 7 september, 2017, before the storm day) and the average of all satellite tecus for the equatorial region (iist trivandrum), cbit hyderabad, iit mumbai, svnit surat and iit gandhinagar are shown in figures 9 to 13, respectively. from figure 9, an ~16 tecus increase from the average of quiet days is observed for iist trivandrum (area near the equator) at 10:00 utc, on the stormy day (8 september, 2017). similarly, cbit hyderabad and iit mumbai reduced of ~16 tecus at 12:00 utc, while svnit surat reduced of ~18 tecus at 13:00 utc and iit gandhinagar of ~20 tecu. as a result, the positive change of the tec related to the storm was found in areas close to the equator region (iist trivandrum and cbit hyderabad) and negative one for a eia region (iit mumbai, svnit surat, and iit gandhinagar). to confirm this large diurnal tec variation, we also examined the thermospheric layer o/n2 ratio and tec map constructed from igs station data. in order to verify the neutral component change during intense storm period, the o/n2 ratio retrieved by the guvi instrument is shown in figure 14. based on the o/n2 data, these rates are observed to be higher than other normal days during geomagnetic storm events (8 september, 2017). the higher the o/n2 ratio is related to the increased ionization of the ionosphere, so the tec increases and vice versa. therefore, a correlation between the o/n2 ratio and the perturbed tec was found. the global tec map data for the intense stormy day (8 september, 2017, corresponding to day 251) and day after the storm (9 september, 2017, corresponding to day 252) are downloaded from gnss igs server (ftp://cddis.gsfc.nasa.gov/ gnss/products/ionex/2017) in global ionex file format [eg. codg2510.17i]. the global tec mapping is done using matlab. figure 15 shows the global tec map for the 8-9 september 2017 with a time resolution of 2 hours. as it can be seen from figure 15, an increment of about 10 to 20 tecu is observed on the intense stormy day (8 september 2017) compared to the quiet day (9 september, 2017). the tec map of the igs stations also shows that tec values in the low latitude india region are high during the period of 14:00-20:00 utc are, while in the other hours the tec map show the normal behavior. therefore, both the igs tec map and the guvi-derived thermospheric layer o/n2 ratio show the same significant differences, as we observed in the navic/irnss tec data at the iist trivandrum location. regarding the different behavior of iit mumbai, iit gandhinagar, and svnit surat india, it can be noted that on 8 september, 2017, there was a significant west-facing electric field during the recovery phase of the storm, which seems to inhibit the formation of an equatorial anomaly that usually occurs on a quiet day. this may be the reason for the positive trend of tec response in iist trivandrum, while in iit mumbai, svnit surat, iit gandhinagar is negative. therefore, our results also confirm the observations of buonsanto et al., 1990 and pedatell et al., 2009 that positive and negative storm effects depend on the stage of the storm, the local time and latitude of the location. 4. conclusion the paper shows the impact of intense geomagnetic storm (8 september, 2017) on navic/irnss system for the low latitudes of indian region. the investigation is carried out based on tec, for the location of iist trivandrum (equatorial region), svnit surat, iit 7 impacts of geomagnetic storm on navic/irnss desai et al. 8 tecu (maximum) tecu (median) date (dd/mm/yy) irnss1b irnss1c irnss1d irnss1e irnss1f irnss1g irnss1b irnss1c irnss1d irnss1e irnss1f irnss1g iist trivandrum (8.62 n latitude, 77.03 e longitude) 3/9/17 31.28 34.22 31.36 35.53 35.04 33.17 17.78 17.36 17.47 17.57 18.16 17.71 4/9/17 30.54 32.50 29.69 34.50 33.46 31.83 21.90 22.81 23.54 22.10 21.68 22.38 5/9/17 35.08 37.80 36.01 39.31 37.94 37.48 18.35 18.58 18.68 18.84 18.24 19.23 6/9/17 31.03 33.27 31.75 35.03 34.64 32.92 21.12 21.20 20.88 20.26 21.62 18.41 7/9/17 32.66 35.72 33.16 37.07 36.63 34.77 23.59 23.65 23.84 23.18 23.60 23.43 8/9/17 50.83 48.09 48.57 46.04 50.55 46.53 17.87 17.70 17.58 18.81 18.11 18.21 9/9/17 36.67 39.48 36.75 41.96 39.24 39.38 20.71 21.41 22.92 19.26 21.30 21.49 cbit hyderabad (17.39 n latitude, 78.31 e longitude) 3/9/17 42.28 48.28 46.67 47.85 49.08 47.93 12.27 14.69 14.60 13.74 14.02 15.72 4/9/17 36.75 44.58 40.62 45.66 44.28 43.58 21.60 24.60 25.43 24.21 21.53 25.26 5/9/17 41.03 49.12 45.17 51.24 47.85 46.49 16.65 19.72 20.44 19.84 17.65 19.98 6/9/17 37.47 45.85 42.02 48.69 45.47 51.88 16.55 17.99 19.90 18.07 18.08 17.49 7/9/17 40.39 50.17 45.37 52.58 49.96 47.20 23.16 25.54 27.22 25.64 26.71 25.44 8/9/17 47.43 50.90 49.18 45.69 52.21 49.58 24.01 22.31 22.38 22.98 22.13 19.85 9/9/17 44.45 53.28 49.62 53.61 51.79 52.51 33.42 36.28 36.99 35.02 35.96 35.27 iit mumbai (19.03 n latitude, 72.91 e longitude) 3/9/17 45.48 46.83 46.69 47.14 45.10 46.07 16.00 15.46 15.54 15.58 16.07 13.69 4/9/17 45.85 49.97 44.56 53.39 49.93 48.27 22.58 23.01 24.14 23.84 21.70 24.34 5/9/17 46.36 49.05 47.98 51.45 46.10 49.14 19.01 18.98 19.34 19.52 18.23 19.03 6/9/17 46.21 50.39 46.74 53.40 48.96 50.19 19.73 19.49 19.51 19.35 19.52 17.01 7/9/17 50.15 55.83 51.63 57.80 55.71 53.95 25.24 25.48 25.98 26.03 24.62 25.79 8/9/17 47.96 47.18 46.65 47.04 46.89 45.93 24.38 24.40 24.05 23.67 23.68 23.27 9/9/17 48.74 51.17 50.78 52.08 50.37 50.82 36.11 38.67 41.61 39.29 33.35 43.09 svnit surat (21.16 n latitude, 72.78 e longitude) 3/9/17 -------- -------- -- ---- -- 4/9/17 47.73 58.45 48.83 59.03 55.61 51.59 21.62 25.27 24.55 24.86 22.38 22.81 5/9/17 52.72 61.55 53.05 56.19 56.84 58.90 18.35 21.84 21.24 21.45 19.41 19.10 6/9/17 54.34 65.10 57.82 69.36 64.17 60.51 18.98 20.74 20.81 21.02 21.13 18.26 7/9/17 55.09 63.82 56.31 71.55 67.75 65.41 24.48 28.13 28.07 27.36 28.28 26.25 8/9/17 57.10 58.10 51.19 64.96 57.02 62.80 28.69 27.18 25.91 26.89 29.15 22.25 9/9/17 55.40 61.86 59.28 53.73 59.77 56.85 43.07 42.82 42.62 34.03 40.12 39.18 iit gandhinagar (22.52 n latitude, 72.92 e longitude) 3/9/17 64.11 73.33 65.96 74.41 65.92 62.89 34.79 35.58 31.93 32.01 28.06 25.75 4/9/17 63.56 79.31 62.08 75.53 71.06 64.88 38.70 41.44 37.67 39.07 35.25 30.01 5/9/17 70.84 80.93 64.45 68.82 69.51 75.58 37.40 40.83 35.40 34.47 30.85 29.33 6/9/17 63.85 82.22 66.27 76.35 74.45 70.32 35.73 36.87 34.43 36.31 31.90 26.99 7/9/17 67.84 86.75 71.57 83.35 77.80 77.12 41.38 45.96 42.70 42.14 39.29 36.40 8/9/17 70.50 68.30 62.81 60.24 61.53 55.97 42.10 44.92 40.22 37.57 38.61 31.50 9/9/17 68.41 77.20 69.61 62.28 71.99 65.53 46.31 48.48 44.21 44.57 41.18 38.83 table 1. tec comparisons for dual frequency navic / irnss receiver and iri-2012 online model. 9 impacts of geomagnetic storm on navic/irnss figure 9. dtecu for dual frequency navic/irnss receiver at iist trivandrum (8.62° n latitude, 77.03° e longitude) for september 03, 2017 to september 08, 2017. figure 10. dtecu for dual frequency navic/irnss receiver at cbit hyderabad (17.39° n latitude, 78.31° e longitude) for september 03, 2017 to september 08, 2017. figure 11. dtecu for dual frequency navic/irnss receiver at iit mumbai (19.03° n latitude, 72.91° e longitude) for september 03, 2017 to september 08, 2017. mumbai, cbit hyderabad and iit gandhinagar (eia region). the one week navic / irnss dual-frequency (l5 and s-band) receivers data of above five locations are collected for the investigation. the diurnal tec comparison between iri-2012 empirical model and navic/irnss dual frequency model has been done. from the statistical analysis of navic/irnss dual frequency tec, on the stormy day, iist trivandrum (near the equator) increased of about 10 tecus at 10:00 utc, cbit hyderabad and iit mumbai decreased of about 16 tecus at 12:00 utc, svnit surat decreased of about 18 tecus at 13:00 utc, and iit gandhinagar decreased of about 20 tecu. therefore, the effect of intense geomagnetic storms is easily identified from the observations of tec data analysis using navic/irnss dual-frequency receivers at various location. the diurnal tec behavior of the iri-2012 model is to be found the same for all observation days. in order to support drastic change in tec during intense geomagnetic storm (8 september, 2017), planetary kp indices, dst(nt), ap index, f10.7, ae(nt) index, sym-h(nt) index, imf bz(nt), and ief ey(nt) parameter maps are added. results are further validated using tec map from igs data, thermosphere o/n2 ratio map from guvi. a positive desai et al. 10 figure 12. dtecu for dual frequency navic/irnss receiver at svnit surat (21.16° n latitude, 72.78° e longitude) for september 04, 2017 to september 08, 2017 figure 13. dtecu for dual frequency navic/irnss receiver at iit gandhinagar (22.52° n latitude, 72.92° e longitude) for september 03, 2017 to september 08, 2017 correlation between the storm occurrence and the tec increment is found in the equatorial region, while a negative one in the eia. the delay of the navic/irnss signal caused by the ionosphere is directly related to the tec. therefore, changes in tec during geomagnetic storm events strongly affect the signal propagating through the ionosphere, which leads to a decrease in the positional accuracy of the system. hence, to improve the positional accuracy of the navic/irnss system the estimation and mitigation of ionospheric effects is essential. data and sharing resources •http://wdc.kugi.kyoto-u.ac.jp , world, data center (wdc) for geomagnetism, kyoto for the dst(nt), ae(nt) and sym-h(nt) indexes. •https://omniweb.gsfc.nasa.gov , omniweb data explorer for the kp, ap and f10.7 indexes. •http://www.srl.caltech.edu/ace/asc/level2/lvl2dat a_mag.html, advanced composition explorer (ace) level 2 data server for imf bz (nt). •http://guvitimed.jhuapl.edu/guvi-galleryl3on2, 11 impacts of geomagnetic storm on navic/irnss figure 14. global thermospheric o/n2 mapping using guvi data server. guvi for the investigation of thermospheric o/n2 data. •ftp://cddis.gsfc.nasa.gov/gnss/products/ionex , international gnss service (igs) server, ionex file for world tec data. •ftp://210.212.130.77, sac, isro ftp link for navic/irnss data sharing. acknowledgements. this work is part of the sponsorship project “announcement of opportunity for irnss /gagan navigation data utilization program”, supported by sac, isro ahmedabad. the authors would like to thank iist trivandrum, cbit hyderabad, iit mumbai and iit gandhinagar for their useful navic /irnss data. we would also like to express our heartfelt gratitude to the of the shri atul shukla, director group of nag / ssaa, sac, isro ahmedabad and its team for providing the necessary guidance. references astafyeva, e., yu yasyukevich, a. maksikov, and i. zhivetiev (2014), geomagnetic storms, super storms, and their impacts on gps based navigation systems, space weather, 12(7),508-525, doi: 10.1002/2014sw001072. buonsanto, m. j. (1999), ionospheric storms—a review, space science reviews, 88(3-4), 563-601. desai et al. 12 figure 15. (a) global tec mapping (two hours resolution) comparison (8 september 2017 and 9 september 2017) using global ionex file from gnss igs data server ( 00-06 hours). (a) chakraborty, monti, sanjay kumar, barin kumar de, and anirban guha (2015), effects of geomagnetic storm on low latitude ionospheric total electron content: a case study from indian sector, journal of earth system science 124(5), 1115-1126, doi:10.1007/s12040-015-0588-3. chakraborty, monti, sanjay kumar, barin kumar de, and anirban guha (2014), latitudinal characteristics of gps derived ionospheric tec: a comparative study with iri 2012 model, annals of geophysics, 57(4) 113, https://doi.org/10.4401/ag-6438. de abreu, alessandro josé, inacio martin, rodolfo de jesus, mauricio bolzan, kavutarapu venkatesh, paulo fagundes, mauro alves, mauricio gende, and sanjay kumar (2017), comparison of gps-tec measurements with iri2012-tec predictions in the brazilian sector during the unusual solar minimum 2009, annals of geophysics, 60(3), doi:10.4401/ag7300, 1-6. desai, mehul v., darshna jagiwala, and shweta n. shah (2016), impact of dilution of precision for position computation in indian regional navigation satellite system, proc. ieee international conference on advances in computing, communications and informatics (icacci), jaipur, india, 980-986, doi:10.1109/icacci.2016.7732172. desai, mehul v., and shweta n. shah (2017), analysis of ionospheric correction approach for irnss/navic 13 impacts of geomagnetic storm on navic/irnss figure 15. (b) global tec mapping (two hours resolution) comparison (8 september 2017 and 9 september 2017) using global ionex file from gnss igs data server ( 08-14 hours). (b) desai et al. 14 figure 15. (c) global tec mapping (two hours resolution) comparison (8 september 2017 and 9 september 2017) using global ionex file from gnss igs data server ( 16-22hours). (c) figure 15. (d) global tec mapping (two hours resolution) comparison (8 september 2017 and 9 september 2017) using global ionex file from gnss igs data server ( 24 hours). system based on iot platform, in international conference on future internet technologies and trends, springer, cham, 174-183, doi:10.1007/9783-319-73712-6_18. desai, mehul v., and shweta n. shah (2015), ionodelay models for satellite based navigation system, african journal of computing and ict, 8 (2) 1-6. desai, mehul v., and shweta n. shah (2018), the give ionospheric delay correction approach to improve positional accuracy of navic/irnss single-frequency receiver, current science (00113891) 114(8) 1665-1676 , doi: 10.18520/cs/v114/i08/1665-1676. de siqueira negreti, patricia mara, eurico rodrigues de paula, and claudia maria nicoli candido (2017), total electron content responses to hildcaas and geomagnetic storms over south america, annales geophysicae, 35(6), 1309-1326, doi:10.5194/angeo35-1309-2017. gonzalez, w. d., j. a. joselyn, y. kamide, h. w. kroehl, g. rostoker, b. t. tsurutani, and v. m. vasyliunas (1994), what is a geomagnetic storm?, journal of geophysical research, 99(a4), 5771–5792, doi:10.1029/93ja02867. indian space research organization, isro (2018), available at; https://www.isro.gov.in/spacecraft/list-ofnavigation-satellites last accessed/12 july 2018.. inyurt, samed, omer yildirim, and cetin mekik (2017), comparison between iri-2012 and gps-tec observations over the western black sea, annals of geophysics, 35(4), 817-824, doi:10.5194/angeo-35-8172017, 2017. irnss signal in space icd (2011), isro-isac v 1.1, available at; http://www.isro.gov.in/irnss-programme/, last accessed/12 july 2018. kumar, sanjay, and a. k. singh (2010), the effect of geomagnetic storm on gps derived total electron content (tec) at varanasi, india, in journal of physics: conference series, 208(1), 1-6, doi:10.1088/17426596/208/1/012062. pedatella n m, lei j, larson k m and forbes j m (2009), observation of the ionospheric response to the 15th december 2006 geomagnetic storm: long duration positive storm effect, journal of geophysical research, 114 a12313, doi: 10.1029/2009ja014568. rao, n. venkateswara, g. madhuri, b. venkatesh, and ch dhanunjai (2013) influence of major geomagnetic storms occurred in the year 2011 on tec over bangalore station in india, international journal of electronics and communication engineering , 6(1),105-110. rao, pvs rama, s. gopi krishna, j. vara prasad, s. n. v. s. prasad, d. s. v. v. d. prasad, and k. niranjan (2009) geomagnetic storm effects on gps based navigation, annals of geophysics, 27(5) , 21012110, 10.5194/angeo-27-2101-2009. *corresponding author: mehul v. desai, electronics engineering department, svnit, surat, gujarat, india email: mvd.svnit@gmail.com © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. 15 impacts of geomagnetic storm on navic/irnss geo 1261 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 2 assessment of space plasma effects for satellite applications: working group 2 overview reinhart leitinger (1) and norbert jakowski (2) (1) institute of physics, institute for geophysics, astrophysics and meteorology (igam), university of graz, austria (2) deutsches zentrum für luft und raumfahrt (dlr), institut für kommunikation und navigation (ikn), neustrelitz, germany an important part of the tasks of working group 2 of the cost action 271 «assessment of space plasma effect for satellites applications» is the assessment of novel data sources for information about the state of ionisation of the ionosphere. this report deals with those aspects which are not represented adequately in the scientific papers in this issue. here emphasis is given to the product aspect (data and model collections, descriptions of methods and algorithms, availability of products, expected future developments) and the links between the past cost actions 238 and 251 with the present action 271 and with possible future cooperations. working group 2 was leading in the transionospheric propagation aspects of possible products for the international telecommunication union’s radiocommunication (itu-r) study group 3. this report gives a short overview emphasizing future developments. 2.1. introduction from the beginning, space plasma research (essentially meaning ionosphere and plasmasphere research) had both scientific and applicational aspects. the scientific aspects are multi-disciplinary (from geophysics and plasma physics to wave propagation research) and the application aspects have multiplied with the availability of artificial satellites transmitting radio signals. the transionospheric propagation of radio waves (satellite to ground and satellite-to-satellite) is increasingly important, has a huge user community, and an enormous economic value. all application systems are influenced by plasma propagation effects. recently the applicational interests have moved from communication satellites to global navigation satellite systems (gnss) with satellites having orbital periods of 12 sidereal hours and transmitting l-band signals. their radio signals are not only used for satellite based navigation but also for positioning with various users from advanced geodesy to surveying. transionospheric propagation expertise is needed to assess system capabilities and limitations, to advise on propagation errors and to calculate corrections. on the other hand, gnss signals provide ionospheric information on a global basis. we are moving quickly from the «electron content» point of view to the remote sounding point of view. the latter needs careful assessment, validation and verification of inversion and imaging methods and finally the development of data assimilation systems. 1262 reinhart leitinger and norbert jakowski we have some hope that we will continue to have low earth orbit (leo) satellites with vhf/uhf beacons (frequencies around 150 and 400 mhz) which provide the data for high resolution ionosphere tomography. the novel data sources will not (and should not) replace the classical ones. the main problem with gnss is the coupling of spatial with temporal changes. we continue to need an adequately dense network of ionosondes which provide independent information at well defined locations. 2.2. the main tasks of working group 2 working group 2 has three work packages: – plasma effects on gnss applications. – assessment of plasma propagation errors in navigation systems and merits and shortcomings of novel data sources. – investigation of extremes of ionization. the papers of working group 2 (jakowski et al., 2004c,d; kersley et al., 2004; leitinger et al., 2004) report on most of the important aspects of work packages 1 and 3. work package 2 is divided into several assessment tasks. a part can be considered to be finished and the most important results are found in the wg 2 papers, another part will finish by the end of the action, the foundation for the remainder is laid, but work will have to be finished outside cost action 271. these are: – to assess ionospheric effects in non-ionospheric applications of gnss signals a) ionospheric influences in the use of gnss occultation for stratosphere / troposphere applications. b) the effect of higher order ionospheric propagation errors in advanced ground based applications, like water vapour retrieval. – to assess gnss related systems to gain ionization data by a) 3d and 4d imaging of the ionosphere; b) adaptive modeling; c) data ingestion into ionospheric models / data assimilation systems. detailed reports are available through the working group 2 web site: http://www.uni-graz.at/ igamwww/cost271/. two groups of tools have been developed for and within working group 2: – models for assessment studies; – data collections. these tools have been tested by participants in the cost 271 action and are now available for the ionospheric research community (and strictly for non commercial applications only). relevant information is available through the working group 2 web site. one effort within work package 3 is the collection of «extremes and peculiar cases», meaning unusually high or low electron content values, gradients, fluctuations from: – magnetic storm effects; – travelling ionospheric disturbances (tids); – peculiar (and partially unexplained) effects not related to storms or tids. this collection too is available through the working group 2 web site. 2.3. models and other tools for assessment studies the foundation for all assessment studies are suitable electron density models. a strong effort was made to bind over monthly median modelling efforts and results from actions 238 and 251 to this cost action and to apply modifications and improvements to gain realistic models for actual conditions. 1263 assessment of space plasma effects for satellite applications: working group 2 overview two important developments were made which should be seen in the context of «profiler» type electron density models. these use «anchor points» for the height profile of electron density, in most cases the peaks of the e-, f1and f2-layers. sub-models («maps») are used for peak heights and peak densities. the models developed under the auspices of the cost actions 238 and 271 (a modification of the di giovanni-radicella (dgr)-model for cost 238 (costprof for cost 271) are profiler models. «the model family» developed at trieste and graz (nequick, costprof, neuogplas) and the international reference ionosphere are profilers. the two modifications are: – replace monthly median maps by «data grids» for the f2-layer peak (leitinger et al., 2002a). – add smaller scale static and dynamic 3d structures by means of «model modulations» (e.g., leitinger and rieger, 2004). the «data grids» are files with f2 peak electron density and peak height (and other values) over equidistant grids in geographic coordinates. grid spacing is 2.5° in latitude and 5° in longitude. in general, it is sufficient to produce one set of data grids per hour and to use fourier interpolation to densify temporal spacing (e.g., to one data grid per quarter hour or per 10 min). the data grids are used with spline interpolation in universal time and with third order polynomial interpolation in latitude and longitude (leitinger and radicella, 2002). «model modulation» means multiplication of the large scale background model with three dimensional and time dependent models for ionospheric structures like the main trough of the f-layer (feichter and leitinger, 2002a,b), large scale tids, various other troughs and ridges. the «modulations» provide values larger zero only, a value of one meaning «background model not changed». the general assessment model construction scheme is now as follows: 1) select the background «profiler» model. 2) prepare the data grids. 3) decide on the modulations to be used (e.g., on tid properties). the specialised assessment model is used to calculate satellite to ground or satellite to satellite electron content («forward modelling») to which the evaluation method in question is applied. the evaluation results are compared «with the truth», namely electron contents or electron densities calculated from the specialised assessment model. example: an assessment study is based on «uncalibrated» satellite to ground slant electron contents, the evaluation method produces vertical electron contents for the location of receiving stations which are compared with vertical electron contents calculated from the assessment model. «open to everyone» data allow the comparison of different evaluation methods. «blind» assessments are also possible: the research group that wants to test an evaluation algorithm defines the wanted link geometries (satellite to ground and/or satellite to satellite links), time structure and satellite orbits and gets artificial «observations». the type of model used is not revealed and the comparison between «model truth» and «evaluation results» is done by the producers of the assessment data. 2.4. overview on assessment studies carried out since the german satellite champ happened to be operational during most of the lifetime of the cost action 271 it was considered very important to give relevant assessment studies the highest priority (cf. jakowski et al., 2004a,c,d). the following list is not complete but gives a good impression on champ related activities: – validation of champ ionospheric radio occultation (iro) results in general (jakowski et al., 2002, 2004b). – comparison of ionospheric electron density profiles from champ with the nequick model (cueto et al., 2003) and the iri model (jakowski and tsybulya, 2003). 1264 reinhart leitinger and norbert jakowski – comparison of radio occultation and ionosphere tomography (spalla et al., 2002; malan and jakowski, 2003). the foundations for the assessment of other novel data sources, especially the inter-comparison of different methods to retrieve vertical electron content from gps observations, are now complete (see section 2.3), the assessment and inter-comparison studies themselves will be continued in follow-up studies. another group of assessment studies was done for space based navigation (jakowski et al., 2004c,d). other assessment, validation and verification tasks among others comprise space based radars (cannon and chan, 2003). 2.5. the hirac campaign and regional data collections the high rate measuring campaign (hirac/solarmax) was an enterprise of the international gps service (igs) with the aim to extract ionospheric data with sampling rates between one and three seconds from gps and glonass signals (feltens et al., 2001). the campaign was carried out from 23 through 29 april, 2001. around 100 receiving stations participated and delivered data into a dedicated archive. the data are very well suited to investigate smaller scale structures, especially dynamic ones as tids, on a nearly global basis. hirac was primarily aimed at high latitudes and at the crest regions of the equatorial anomaly, but because of the distribution of igs and other gps receiving stations data coverage in northern mid latitudes was considerably better than in other regions. for cost 271 participation see, e.g., šauli et al. (2003) and stolle et al. (2003). hirac related data with special interest for cost 271 are found in the teceda data archive (see below). in many countries dense networks of dual frequency gps receivers exist which could provide an excellent database for dedicated studies of smaller scale phenomena, e.g., of traveling ionospheric disturbances (tids). in connection with cost 271 the exploitation of such a network was tested at graz (rieger, 2002; rieger and leitinger, 2002). 2.6. data banks, data collections, products of cost actions 238 and 251 there are two european electron content data sets for longterm studies, namely the data derived by means of the faraday effect on the vhf signals of geostationary satellites received at firenze, graz and neustrelitz («faraday-tec») and data derived by means of the differential doppler effect on the 150/400 mhz signals of the (former) navy navigation satellites (nnss-tec) received at graz/austria and katlenburg-lindau/germany. the contact for faraday-tec is paolo spalla (p.spalla@ifac.cnr.it), for nnss-tec reinhart leitinger (reinhart.leitinger@uni-graz.at). the contact for the gps-based regional maps produced at neustrelitz is norbert jakowski (norbert.jakowski@dlr.de; see also the internet address http://www.kn.nz.dlr.de). for the teceda data bank and other data products of interest for studies related to working group 2 of cost 271 see stamper et al. (2004). electron content related products of cost 238 (prime) and cost 251 are found in the working group 2 web page http://www.uni-graz.at/igamwww/cost271/. this is also the place for the electron density assessment models and relevant data and for the collection of extremes of ionization and of peculiar cases (see above). the working group 2 web site will survive the end of cost action 271 at least until relevant decisions are made in the frame of succeeding european projects. for access conditions outside the cost 271 action contact reinhart leitinger (reinhart.leitinger@unigraz.at). 1265 assessment of space plasma effects for satellite applications: working group 2 overview 2.7. products for itu-r so far one product was generated under the auspices of cost action 271 and delivered to itur, namely the monthly median electron density model nequick(itu-r) (leitinger et al., 2002b). another product is the scintillation model gism. cost 271 has declared an interest in the model but is not directly involved in it. for transionospheric propagation of radio waves working group 2 is able to suggest answers or partial answers to several open «questions» of itu-r study group 3 and to provide material to improve itu-r study group 3 «recommendations». material produced are cumulative statistics of solar activity, geomagnetic disturbances and ionospheric conditions. working group 2 has gained a lot of experience with the itu-r (formerly ccir) maps, mainly in connection with its electron density models. this experience is used to give advice for future itur mapping efforts. working group 2 has its own mapping efforts and has produced f2-layer peak density and peak height maps which are offered to itu-r. for transionospheric propagation purposes these new maps are much better suited than the original fof2 and m(3000)f2 maps which are tuned to hf (terrestrial) propagation. with the itu-r (ccir) products it is necessary to combine three maps to produce f2-layer peak height which is a process leading to artificial smaller scale structures in peak height: this is avoided in the cost 271 maps. 2.8. open questions and future aspects not all tasks defined originally could be finished. this is especially true for the assessment studies to assess gnss related systems to gain ionization data by data ingestion into ionospheric models/data assimilation systems. the main reason for the delay is that data ingestion into ionospheric models (one development step from «adaptive modeling») is emerging only now. full data assimilation systems (meaning operational systems which use data from different sources to guide theoretical or hybrid models in near-real-time) do not exist yet. research work is done by several groups but it seems that this work will need considerably more time. relevant assessment studies will be one of the important topics of follow-on projects. references cannon, p.s. and a.h.y. chan (2003): some ionospheric effects on space based radars, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece. cueto, m., n. jakowski, k. tsybulya, s.m. radicella and m.herraiz (2003): comparison of ionospheric electron density profiles from champ satellite with nequick model, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece. feichter, e. and r. leitinger (2002a): properties of the main trough of the f-region derived from dynamic explorer 2 data, ann. geophysics, 45 (1), 117-124. feichter, e. and r. leitinger (2002b): the trough model, acta geod. geophys. hung., 37. feltens, j., n. jakowski and c. noll (2001): high-rate solarmax igs/gps campaign «hirac/ solarmax», cddis bulletin, 16 (3), 2001. jakowski, n. and k. tsybulya (2003): comparison of ionospheric radio occultation champ data with iri-2001, iri newsl., 10 (1,2,4-7). jakowski, n., a. wehrenpfennig, s. heise, ch. reigber, h. lühr, l. grunwaldt and t. meehan (2002): gps radio occultation measurements of the ionosphere from champ: early results, geophys. res. lett., 29 (10), 10.1029/2001gl014364. 1266 reinhart leitinger and norbert jakowski jakowski, n., k. tsybulya, s.m. stankov and a. wehrenpfennig (2004a): about the potential of gps radio occultation measurements for exploring the ionosphere, in proceedings of the second champ science meeting, 1-4 september 2003, gfz potsdam (accepted). jakowski, n., k. tsybulya, j. mielich, a. belehaki, d. altadill, j.c. jodogne and b. zolesi (2004b): validation of gps radio occultation measurements on champ by vertical sounding observations in europe, in proceedings of the second champ science meeting, 1-4 september 2003, gfz potsdam (accepted). jakowski, n., r. leitinger and m. angling (2004c): radio occultation techniques for probing the ionosphere, ann. geophysics, 47 (suppl. to no. 2/3), 1049-1066 (this volume). jakowski, n., r. leitinger and l. ciraolo (2004d): behaviour of large scale structures of the electron content as a key parameter for range errors in gnss applications, ann. geophysics, 47 (suppl. to no. 2/3), 1031-1047 (this volume). kersley, l., d. malan, s.e. pryse and lj.r. cander (2003): further investigations on the verification of tec measurements by different techniques, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece. kersley, l., d. malan, s.e. pryse, lj.r. cander, r.a. bamford, a. belehaki, r. leitinger, s.m. radicella, c.n. mitchell and p.s.j. spencer (2004): total electron content – a key parameter in propagation: measurement and use in ionospheric imaging, ann. geophysics, 47 (suppl. to no. 2/3), 1067-1091 (this volume). leitinger, r. and s.m. radicella (2002): electron density models for assessment studies a progress report, in proceedings of the 2nd cost 271 workshop, 2-4 october 2002, faro, portugal. leitinger, r. and m. rieger (2004): the tid model for modulation of large scale electron density models, ann. geophysics (accepted). leitinger, r., s.m. radicella, e. feichter and b. nava (2001): electron density models suitable for assessment studies which involve satellite to ground and satellite to satellite electron content, in proceedings of the international beacon satellite symposium, boston, u.s.a., edited by p. doherty. leitinger, r., s.m. radicella and e. feichter (2002a): the data grid method to actualize empirical electron density models, in proceedings 10th international ionospheric effects symposium, edited by j. goodman, 139-146. leitinger, r., s.m. radicella and b. nava (2002b): nequick for itu-r: the product delivered and plans for improvements, in proceedings of the 2nd cost 271 workshop, 2-4 october 2002, faro, portugal. leitinger, r., l. ciraolo, l. kersley, s.s. kouris and p. spalla (2004): relations between electron content and peak density: regular and extreme behaviour, ann. geophysics, 47 (suppl. to no. 2/3), 1093-1107 (this volume). malan, d. and n. jakowski (2003): comparison of radio occultation and ionospheric tomography observations over western europe, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece. rieger, m. (2002): smaller scale electron content differences deduced from regional gps data, in proceedings of the 2nd cost 271 workshop, 2-4 october 2002, faro, portugal. rieger, m. and r. leitinger (2002): assessment of tid activity from gps phase data collected in a dense network of gps receivers, acta geod. geophys. hung., 37, 327-341. šauli, p., j. bochníček, j. boška, l. ciraolo, p. hejda and s.m. radicella (2003): short-term variations of the ionospheric f-region measured during campaign hirac 2001 over pr °uhonice observatory. spalla, p., n. jakowski, a. wehrenpfennig, p.s.j. spencer and c.n. mitchell (2002): verification of champ radio – occultation observations in the onosphere using midas, poster presented at the 1st champ science meeting, 22-25 january 2002, gfz potsdam. 1267 assessment of space plasma effects for satellite applications: working group 2 overview stamper, r., j.l. lilensten and n. jakowski (2004): nowcasting, forecasting and warning for ionospheric propagation: supporting databases, ann. geophysics, 47 (suppl. to no. 2/3), 945955 (this volume). stolle, c., s. schlüter, n. jakowski and ch. jacobi (2003): electron density reconstructions using gps measurements during hirac campaign in april 2001, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece. 55.06.2012_frontespizio&prefazione_final:layout 6 annals of geophysics istituto nazionale di geofisica e vulcanologia [55.06.2012_special issue] 5th european repeat station (magnete) workshop rome, italy 2011 editors paola de michelis istituto nazionale di geofisica e vulcanologia, roma, italy gerald duma central institute for meteorology and geodynamics, vienna, austria 1051 preface annals of geophysics, 55, 6, 2012; doi: 10.4401/ag-6313 repeat station measurements are designed specifically to represent and model the geomagnetic field and its secular variations on a regional scale with a level of detail that would not be achievable using only data from permanent geomagnetic observatories. although geomagnetic repeat station surveys have a long tradition in many european countries, they were carried out independently in the individual countries. for this reason, it has been difficult for a long time to use the data efficiently for studies of geomagnetic secular variations of the whole european continent. attempting to solve this problem at a european level, a working group was set up in 2003 at the first workshop on european geomagnetic repeat stations, which was organized by the geoforschungszentrum (gfz, germany) at the niemegk observatory. in the same year, this initiative was also acknowledged by the international association of geomagnetism and aeronomy (iaga) and it is supported by iaga resolution #4, division v. later on, the working group was named magnete (magnetic network of europe). the group aims to coordinate the national efforts to produce more homogeneous, high-quality geomagnetic vector data over the european continent. three consecutive workshops were held in 2005, 2007 and 2009, in warsaw, bucharest and helsinki, respectively, to ensure ongoing cooperation between the participants of more than 20 countries. due to this common effort, it is now possible to use the repeat station data to study the main field, the secular variation, and to some extent, the lithospheric field at a continental scale in europe. the stimulus that led to the conception of this special issue was the 5th magnete workshop that was held in rome, italy, in may 2011, where more than 40 scientists from 20 countries presented the results of recent field surveys and discussed plans and strategies for future data acquisition and geomagnetic modelling efforts. the 12 papers published in this special issue represent only a subset of the scientific results presented at the 2011 symposium. these 12 contributions provide specific information on the numerous magnetic repeat station measurements that have been performed in europe, and on the possible solutions to improve the quality of the repeat data. this special issue also shows one of the main results of the magnete working group: the european declination chart 2006. it is based on 382 magnetic repeat station measurements that were performed over the period of 2005-2007, and on the annual mean values of 42 observatories that are operated in 21 european countries. we gratefully acknowledge all of the reviewers of these papers for their valuable criticism, comments and suggestions that substantially improved the quality of the manuscripts, and we acknowledge the istituto nazionale di geofisica e vulcanologia (ingv) for hosting and financially supporting the 5th magnete workshop. paola de michelis gerald duma istituto nazionale di geofisica central institute for meteorology e vulcanologia, roma, italy and geodynamics, vienna, austria << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 363_372 levizzani .pdf 363 annals of geophysics, vol. 46, n. 2, april 2003 satellite rainfall estimates: new perspectives for meteorology and climate from the eurainsat project vincenzo levizzani istituto di scienze dell’atmosfera e del clima (isac), cnr, bologna, italy abstract satellite meteorology is facing a crucial period of its history since recent missions have revealed instrumental for quantitative rainfall measurements from space and newly conceived missions are at hand. international partnership is rapidly developing and research projects keep the community focused on rapidly developing research and operational issues. a perspective is given through the structure of eurainsat, a project of the 5th framework programme of the european commission. its key objective is the development of algorithms for rapidly-updated satellite rainfall estimations at the geostationary scale. the project is fostering international research on satellite rainfall estimations building a bridge between europe and the u.s. for present and future missions. mailing address: dr. vincenzo levizzani, istituto di scienze dell’atmosfera e del clima (isac), cnr, via gobetti 101, 40129 bologna, italy; e-mail: v.levizzani@ isac. cnr.it key words satellite meteorology precipitation climate nowcasting 1. introduction the latest generation sensors on board the geostationary operational environmental satellites (goes) (menzel and purdom, 1994) and the upcoming meteosat second generation (msg) spinning enhanced visible and infrared imager (seviri) (schmetz et al., 1998) significantly enhance the ability of sensing cloud microstructure and precipitation forming processes (levizzani et al., 2000, 2001) from a geostationary platform. a potential exists for improved instantaneous rainfall measurements from space by combining infrared (ir) and visible (vis) with passive microwave (mw) observations with a more global perspective. ir and vis satellite rainfall estimates have long since been available and suffered from the diffi culty in associating cloud top features to precipitation at ground level. ir methods were used for climate purposes or combined with radar measurements for nowcasting (recent examples are vicente et al., 1998; porcù et al., 1999; amorati et al., 2000) and multispectral approaches start to become operational (e.g., ba and gruber, 2001). physically-based passive mw methods were developed mainly using data from the special sensor microwave/imager (ssm/i) and are based on several different physical principles (see for example wilheit et al., 1994; smith et al., 1998). limitations of mw algorithms include the relatively large footprint and the low earth orbits not suitable for most of the operational strategies. the combined use of mw and ir data for rainfall estimations was already recognized some time ago (vicente and anderson, 1993). adler et al. (1993) used ssm/i data for monthly average rainfall estimations over wide areas and global products such as those of the global precipitation climatology project (gpcp) were 364 vincenzo levizzani conceived (for recent advances see huffman et al., 2001). however, the need for hourly and instantaneous combined estimations was clearly recognized (e.g., vicente, 1994; vicente and anderson, 1994; levizzani et al., 1996). several methods exist (turk et al., 1999; sorooshian et al., 2000; todd et al., 2001) that make use of ir and mw at various degrees of complexity and targeting different rainfall regimes. some of these are running operationally, granting that their validation requires additional work in the years to come. in particular, global rapidupdate estimates with near real-time adjustment of the thermal ir co-localized with mw-based rainrates are operationally very promising (turk et al., 1999). the algorithms for the tropical rainfall measuring mission (trmm) precipitation radar (pr) require a special mention given the novelty and potential of the active instruments for future missions: an example is the trmm algorithm 2a-25 (iguchi et al., 2000). combined mw and rain radar algorithms are relatively new and were developed for the trmm microwave imager (tmi) and the pr (e.g., trmm algorithm 2b-31, haddad et al., 1997). finally, rainfall and humidity assimilation, and microphysical parameterizations for numerical weather prediction (nwp) models, above all limited area models (lams), and general circulation models (gcms) open up the road to very effective operational meteorological applications that incorporate the verifi cation of model output (turk et al., 1997). 2. eurainsat: the project eurainsat is a project partially funded by the european commission with the aim of developing new satellite rainfall estimation methods at the geostationary scale for an operational use in short and very short range weather monitoring and forecasting. it will exploit the new channels in the vis, nir and ir of the msg seviri (see fi g. 1) that will be launched in mid 2002. the project started in january 2001 and will last until december 2003. the seviri channels in the vis and ir portion of the spectrum will gain better insights into the microphysical and dynamic structure of precipitating clouds allowing for a more precise identifi cation of precipitation levels. the method(s) will work as follows: 1) microphysical characterization of precipitating clouds with vis/ir sensors; 2) creation of microphysical and radiative databases on cloud systems using cloud model outputs and aircraft penetrations; 3) tuning of mw algorithms on the different cloud systems (convective, stratiform, ...); 4) combination of data from the different algorithms and application to a rapid update cycle that makes use of the different sensors at the geostationary scale. the consortium has two objectives in mind: 1) contribute to improving the knowledge of clouds and precipitation formation processes using meteorological satellite sensors, and 2) make available new precipitation products for weather analysis and forecasting. seviri will in fact provide better multispectral measurements for the identifi cation of the physical processes of cloud formation and evolution. the 15 min image repetition time is also more compatible with the time responses of cloud systems. the following key geographic areas and major meteorological events are considered: fig. 1. artist’s impression of the meteosat second generation (msg) spacecraft (courtesy of eumetsat). 365 satellite rainfall estimates: new perspectives for meteorology and climate from the eurainsat project – flood-producing episodes (e.g., northwestern and southern italy). – several cases involving the presence/ absence of ice, polluted air masses and maritime conditions. – influence of orography, e.g., the alps during the mesoscale alpine programme (map) special observing period (sop) (bougeault et al., 2001). – sustained light rain and «insignifi cant» rain cases (very diffi cult to detect from satellite) in u.k. and northern europe. – tropical and sub-tropical cases over africa, where the niger catchment was selected given the relatively regular ground raingage network. – a climatological window over europe (30-60 n, 15 w-20 e). the project has gathered together a substantial part of the satellite rainfall community. the team actively participates into the development of scenarios and concept for the future global precipitation measurement (gpm) mission and the international precipitation working group of the coordination group for meteorological satellites (cgms). more information on eurainsat and its findings can be gained at the web site http://www.isac.cnr.it/~eurainsat. 3. microphysical characterization of cloud processes a method was developed by rosenfeld and lensky (1998) to infer precipitation-forming processes in clouds based on multispectral satellite data. the method was originally based on the advanced very high resolution radiometer (avhrr) imagery on polar orbiting satellites (lensky and rosenfeld, 1997). the forthcoming msg seviri is expected to enhance the capabilities of extracting cloud physical properties (watts et al., 1998) more relevant for cloud genesis and evolution and not anymore limited by the insuffi cient number of passages. the effective radius (r e ) of the particles and cloud optical thickness are extracted and used for radiative transfer calculations that defi ne the cloud type and improve its characterization. precipitation forming processes are inferred also using data from the avhrr, the trmm vis and ir sensor (virs) and the moderateresolution imaging spectroradiometer (modis) on board nasa’s terra spacecraft. in particular, microphysical and radiative parameters from satellite sensors are instrumental for defi ning the characteristics of precipitating clouds. an example of global cloud effective radius values derived from modis and ready to be used for eurainsat is given in fig. 2. «microphysically-maritime» clouds grow in relatively clean air with small cloud condensation nuclei (ccn) and low droplet concentrations, which produce very effi cient coalescence and warm rain processes. «continental» clouds normally grow, on the contrary, in polluted air masses having large ccn and high droplet concentrations, i.e. the coalescence is relatively ineffi cient. the better knowledge of cloud microphysical structure and precipitation forming processes will facilitate the development of a new generation of improved passive mw rainfall algorithms. one more promising line of action is the potential use of lightning detection for discriminating between convective and stratiform regimes while estimating precipitation. data from ground-based lightning detection networks and satellite sensors like the trmm lightning imaging sensor (lis) are applied to ir rainfall estimations (grecu et al., 2000) and show considerable potential for rapid update applications. above all, lightning detection represents a fast-response fundamental parameter for discriminating active convection and quantitative relationships have been found between lightning discharges and other measurables of rainfall (petersen and rutledge, 1998). dietrich et al. (2001) have recently shown that the use of concurrent data from trmm pr and lis give unique information about the link between electrification and convection for the discrimination of convective and stratiform regimes. the authors hint at applications for rainfall retrieval using multispectral and mw techniques. the work is based upon the fi ndings of solomon and baker (1998) who examined thunderstorm development and lightning fl ash rates in tropical maritime, subtropical continental, and midlatitude continental storms. the 366 vincenzo levizzani dependence of lightning occurrence and fl ash rate on cloud condensation nucleus concentration, primary and secondary glaciation mechanism, liquid water flux, and updraft velocity is of fundamental importance. 4. rainfall estimation methods 4.1. ir methods ir-based rainfall estimation methods were the fi rst to be applied to a wide variety of scales and phenomena. with the advent of mw sensors they were more and more confi ned to large scale and climate applications. at the instantaneous time scale, methods based on thermal ir data are being integrated by ancillary information, such as data from radar, other vis, ir and nir channels, lightning detection, model output and other meteorological parameters. for example, porcù et al. (1999) have conceived a simple method to ensure a more physical relationship between cloud structure and precipitation rates as derived from the ir thresholding negri-adler-wetzel (naw) technique (negri et al., 1984) by using ssm/i observations. a low-rainrate event with orographic forcing and small scale precipitation in piedmont (nw italy) on 13-16 october 2000 was considered over a 25 000 km2 area to document the impact of the calibration on low precipitation naw areas; nine ssm/i overpasses were available over the target basin (porcù et al, 2000). the impact on the basin-averaged rainfall is rather low, resulting in a slight decrease of satellite overestimation for higher rainrates. the effect of calibration is more evident over the directly calibrated locations, given the very low occurrence of high precipitation areas during the event. the calibration helps for the higher rainrate peaks, while there is a marked underestimation of lower rainrates that is not affected by the calibration. the scattering of the points also increases showing an overall overestimation even for the highest rainrates. the calibration of low precipitation areas does not affect the performance of the technique as fig. 2. combined cloud effective radius retrievals derived for water-phase, ice-phase, and undetermined-phase clouds (10 october, 2001). image courtesy of modis atmosphere discipline group (http://modis-atmos. gsfc.nasa.gov/). 367 satellite rainfall estimates: new perspectives for meteorology and climate from the eurainsat project to the peak rainrates and does not reduce the underestimation for light rain, since it has no effects on the rain/no rain threshold. 4.2. mw passive and active methods many methods have been proposed for measuring rainfall from mw satellite sensors. simple methods using polarization-corrected brightness temperatures (pct) (e.g., kidd, 1998) have been proposed together with more physical approaches that rely upon microphysical characterization by: – stratifying clouds into different microphysical types and examining how much of the variability in the bias of mw rainfall estimation is explained by the microphysical characterization; – developing a library of passive mw signatures from different cloud types, and – using a microphysical cloud classifi cation for improving cloud radiative transfer modeling based on statistical multivariate generators of cloud genera. figure 3 shows an example of mw rainfall retrieval over the mediterranean and western europe using the algorithm by turk et al. (1999) and data from ssm/i, tmi and the advanced microwave sounding unit-b (amsu-b). more details can be found in berg et al. (1998). the scheme of mugnai et al. (1993) and smith et al. (1992) is a good example of such methods, especially in the very complex environment of severe storm microphysics. cloud modeling and mw radiative transfer has been recently applied to stratiform rainfall by bauer et al. (2000). panegrossi et al. (1998) have fig. 3. 3-h average rainrate using a combined ssm/i, tmi and amsu-b algorithm (29 september, 2001). note the storm system stretching from the tyrrhenian sea to tunisia. the black triangle indicates lack of data coverage after the mosaic of the satellite overpasses. image courtesy of j.f. turk (http://www.nrlmry.navy.mil/ sat_products.html). 368 vincenzo levizzani shown the importance of testing the physical initialization and the consistency between model and measurement manifolds. research trends concentrate on improving the interpretation of active and passive mw measurements through better modeling of cloud processes such as the melting layer (bauer, 2001a; olson et al., 2001a,b). the trmm satellite has set the path to new algorithms that still mostly work over oceans (e.g., bauer, 2001b), but new developments are at hand over land (grecu and anagnostou, 2001). polarization and texture information from passive radiometers together with pr data complete the scenario of new methods (olson et al., 2001c). ferreira et al. (2001) have recently contributed to the improvement of the trmm 2a-25 algorithm (iguchi et al., 2000) proposing the use of g drop size distributions and r-k relations instead of r-z (where r is rainrate, k the attenuation coeffi cient, and z the refl ectivity). the importance of separating convective and stratiform precipitation and stratiform and transition regions in the a priori cloud model database of mw algorithms is demonstrated by kummerow et al. (2001). the authors document the latest improvements to the goddard profiling algorithm (gprof) as applied to the trmm data. the new algorithm also uses the emission and scattering indexes instead of individual brightness temperatures. these improvements, together with the elimination of some classifi cation ambiguities over land, are general and apply to other algorithms as well. fundamental ancillary data are finally provided by active and passive mw radiometry for cloud liquid water (lwc) profi ling: measurements from ground based mw radiometers combine with z profi les from cloud radars and cloud model statistics to lower the errors in lwc measurements by as much as 10-20 % (löhnert et al., 2001). 4.3. combined multispectral and mw methods cloud microphysical information, when combined with mw measurements, can lead to improvements in satellite-based rainfall measurements, especially from clouds in the extra tropics and over land (e.g., bauer et al., 1998). eurainsat concentrates on exploiting seviri data in the vis, nir and water vapor (wv) for cloud characterization and screening within a rapid cycle of rainfall estimation based on ssm/i, tmi and geostationary ir data. data from modis simulate msg seviri data during the pre-launch phase. moreover, the project shares in the cloud-related work from the modis team (king et al., 1997). there are two main research lines: develop new msg-mw rainfall algorithms incorporating the observed cloud microstructure and precipitation forming processes. state of the art cloud and mesoscale models (khain et al., 2000; tripoli et al., 2001) and radiative transfer models will be instrumental to detailed cloud and rainfall type discrimination. the need to use such frontier cloud model was recently shown by khain et al. (2001) who demonstrated the weakness of existing cloud parameterizations while trying to give reason of highly supercooled water in convective clouds (1.8 g m– 3 at – 37.5 c) as found by rosenfeld and woodley (2000). introduce such methods into rapid update rainfall cycles for near real time rainfall estimations over oceans and land with the widest possible area coverage. mid-latitude europe, the mediterranean basin, north africa, the middle east and equatorial and tropical african regions are the main targets for operational and climatological applications. applications to the mediterranean have been reported by meneguzzo et al. (1998). 5. applications applications embrace, among others, water availability, global change studies, nowcasting, hydrogeological disaster management, agriculture, famine reduction, and monitoring of remote areas. 5.1. rainfall assimilation for nwp data assimilation procedures that improve cloud and humidity characterization in current analysis schemes for lams are at hand. most important are the sensitivity to orography 369 satellite rainfall estimates: new perspectives for meteorology and climate from the eurainsat project and modeling of moist processes. the model bolam (buzzi et al., 1998) is used in the project to conduct rainfall assimilation experiments that quantify the impact of satellite data onto the forecasting chain. the nudging technique of falcovich et al. (2000) is adopted. motivations for using nudging techniques are that nudging to over-saturation is more gradual, and the reference profi le is useful in tropical areas. the model is now applied to autumn rainfall episodes over the mediterranean. the model rams (pielke et al., 1992) is also used and runs operationally in tuscany with two grids, that are two-way nested and with the highest horizontal spatial resolution of 4 km around tuscany and the arno river basin. the complex cloud microphysics scheme is fully activated (walko et al., 1995), while the kuotype convection parameterization (molinari, 1985) is activated only over the 20 km outer grid and explicit (resolved) convection is allowed over the inner grid. a higher spatial horizontal resolution of 2 km will be reached to ensure fully consistent explicit representation of the convection. quantitative precipitation forecasts (qpfs) are produced hourly over the inner grid. diabatic initialization and rainfall assimilation will be conducted in the operational chain before the end of the project. 5.2. rainfall and climate change the importance of multispectral cloud characterization methods has recently been demonstrated by observing and documenting the inhibiting effects of forest fi re (rosenfeld, 1999), urban pollution (rosenfeld, 2000) and desert dust aerosols (rosenfeld et al., 2001) on precipitation formation processes. it is very likely that rainfall processes have been substantially underestimated against, for example, greenhouse gases in evaluating possible causes of climate changes. a more quantitative assessment is, however, necessary. in particular, ramanathan et al. (2001) argue that manmade aerosols produce brighter clouds with reduced precipitation efficiency and rainfall suppression. this can lead to a weaker hydrological cycle, which connects directly to water availability and quality, a major environmental issue of the 21st century. satellite rainfall data are used also to assess the impact of particular weather systems on the geographical, seasonal and interannual distribution of total rainfall. rodgers et al. (2000) have concentrated on the impact of tropical cyclones on the north pacifi c climatological rainfall. the same authors have repeated the study for the north atlantic (rodgers et al., 2001). such studies are crucial for the quantifi cation of climatic effects and their relationships with indicators such as the el niño southern oscillation (enso) and the north atlantic oscillation (nao). databases of global products such as those of the gpcp (huffman et al., 2001) will be very useful for supporting the defi nition of new local indexes that better describe regional variations, observed over smaller basins like the mediterranean. 5.3. attenuation of satellite communications due to rainfall over the last few years, there has been an increasing demand of large-bandwidth information services coupled with high availability and low-fade margin communication systems (watson and hu, 1994). this scenario has prompted exploring channel frequencies at ka band and above, and developing sophisticated countermeasure techniques to mitigate outage periods (jones and watson, 1993). the implementation of most advanced adaptive countermeasure techniques is related to the possibility of monitoring in quasi real-time the beacon attenuation in a given region and period. spatial and frequency diversity methods, power link control, data rate and error correction on the downlink/uplink can be effectively adopted only if the propagation conditions are known in real time. exploitation of remote sensors and their products represents a natural way to optimize the performances of a satellite communication system with low power margins, specifi cally while applying fade mitigation techniques. mw signatures of precipitation, as given by a spaceborne multi-frequency radiometer, have been shown to be the base for estimating the path 370 vincenzo levizzani attenuation in k-band satellite communications (marzano et al., 2000). a general approach should attempt to estimate rainfall intensity and attenuation by polar-orbiting mw radiometers and temporally track the rain system by means of geostationary ir radiometers. a statistical approach can be used to derive a prediction model of path attenuation from mw brightness temperature and surface rainrate (crone et al., 1996). acknowledgements the work was partially funded by the project eurainsat, a shared-cost project (contract evg1-2000-00030) co-funded by the research dg of the european commission within the rtd activities of a generic nature of the environment and sustainable development sub-programme (5th framework programme). partial support is acknowledged from the italian space agency (asi) within the project studio del ciclo idrologico da piattaforme satellitari: nubi e precipitazioni, and from the italian group for prevention from hydrogeological disasters (gndci). references adler, r.f., a.j. negri, p.r. keehn and i.m. hakkarinen (1993): estimation of monthly rainfall over japan and surrounding waters from a combination of low-orbit microwave and geosynchronous ir data, j. appl. meteorol., 32, 335-356. amorati, r., p.p. alberoni, v. levizzani and s. nanni (2000): ir-based satellite and radar rainfall estimates of convective storms over northern italy, meteorol. appl., 7, 1-18. ba, m.b. and a. gruber (2001): goes multispectral rainfall algorithm (gmsra), j. appl. meteorol., 40, 1500-1514. bauer, p. 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doi: 10.4401/ag-5181 abstract we report here on vulcamera, a stand-alone program for the determination of volcanic so2 fluxes using ultraviolet cameras. the code enables field image acquisition and all the required post-processing operations. remote spectroscopic observations of volcanic so2 fluxes are a mainstay of observational volcanology. such data have greatly strengthened our understanding of volcanic dynamics and the impacts of volcanic degassing to the atmosphere [tamburello et al. 2011a]. a recent development of note has been the imaging of volcanic plumes using ultraviolet (uv) cameras [mori and burton 2006, bluth et al. 2007]. uv cameras can provide numerous benefits, such as: high time resolution, which enables the capture of transient explosive events [yamamoto et al. 2008, mori and burton 2009]; the possibility to spatially resolve heterogeneous operations, e.g., fumarole field sources [tamburello et al. 2011b], and single-point operations. furthermore, the camera images can be used to directly measure the plume transport velocity, potentially a major source of uncertainty in these measurements [e.g., mcgonigle et al. 2005, williams-jones et al. 2006]. here we present vulcamera, a stand-alone, user-friendly code for measuring volcanic so2 fluxes using uv cameras. the code consists of two elements: vulcamera_aq and vulcamera_post, which manage the image acquisition and all of the elements of post-processing, respectively. vulcamera is downloadable from https://sites.google.com/ site/giancarlotamburello/, and it includes detailed instructions. vulcamera will work with the apogee instruments u260 and e6 units; however, we recommend the u260, given its higher signal-to-noise ratio and faster data transfer. vulcamera is designed to operate with two cameras, simultaneously, with bandpass filters centered on 310 nm and 330 nm. it is imperative to use two filters in these observations, to compensate for aerosol attenuation/backscattering, and this approach minimizes temporal mismatches associated with filter changes on a single camera [kantzas et al. 2010]. in particular, the functions of the code include: characterization of vignetting via the collection of clear sky images to compensate for this angular dependency on pixel illumination; determination of the calibration relationships between absorbance and so2 cell concentrations, to enable conversion of the measured field images into ppm m concentration maps; use of simultaneously acquired spectroscopic so2 flux data to calibrate the images; and, finally, feeding back of all of these operations to the main page of vulcamera_post, which leads to the computations of the so2 flux time series and gas masses associated with explosions. vulcamera has been extensively field tested with southern italian volcanoes, and it is hoped that others will find this useful to realize the significant volcanological potential of uv camera technology. article history received march 29, 2011; accepted may 16, 2011. subject classification: gases, volcano monitoring, instruments and techniques, general or miscellaneous. 219 tassi et al. 220 221 references bluth, g.j.s., j.m. shannon, i.m. watson, a.j. prata and v.j. realmuto (2007). development of an ultra-violet digital camera for volcanic so2 imaging, j. volcanol. geotherm. res., 161, 47-56; doi: 10.1016/j.jvolgeores.2006.11.004. kantzas, e.p., a.j.s. mcgonigle, g. tamburello, a. aiuppa and r.g. bryant (2010). protocols for uv camera volcanic so2 measurements, j. volcanol. geotherm. res., 94, 5560; doi: 10.1016/j.jvolgeores.2010.05.003. mcgonigle, a.j.s., d.r. hilton, t.p. fischer and c. oppenheimer (2005). plume velocity determination for volcanic so2 flux measurements, geophys. res. lett., 32, l11302; doi: 10.1029/2005gl022470. mori, t. and m. burton (2006). the so2 camera: a simple, fast and cheap method for ground-based imaging of so2 in volcanic plumes, geophys. res. lett., 33, l24804; doi: 10.1029/2006gl027916. mori, t. and m. burton (2009). quantification of the gas mass emitted during single explosions on stromboli with the so2 imaging camera, j. volcanol. geotherm. res., 188, 395-400; doi: 10.1016/j.jvolgeores.2009.10.005. tamburello, g., a.j.s. mcgonigle, e.p. kantzas and a. aiuppa (2011a). recent advances in ground-based ultraviolet remote sensing of volcanic so2 fluxes, annals of geophysics, this volume. tamburello, g., e.p. kantzas, a.j.s. mcgonigle, a. aiuppa and g. guidice (2011b). uv camera measurements of fumarole field degassing (la fossa crater, vulcano island), j. volcanol. geotherm. res., 199, 47-52; doi: 10.1016/j.jvolgeores.2010.10.004. williams-jones, g., k. horton, t. elias, h. garbeil, p.j. mouginis-mark, a.j. sutton and a.j.l. harris (2006). accurately measuring volcanic plume velocity with multiple uv spectrometers, bull. volcanol., 68, 328-332; doi: 10.1007/s00445-005-0013-x. yamamoto, h., i.m. watson, j.c. phillips and g.j. bluth (2008). rise dynamics and relative ash distribution in vulcanian eruption plumes at santiaguito volcano, guatemala, revealed using an ultraviolet imaging camera, geophys. res. lett., 35, l08314; doi: 10.1029/2007gl032008. *corresponding author: giancarlo tamburello, università degli studi di palermo, dipartimento distem, palermo, italy; email: giancarlotamburello@gmail.com. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. vulcamera: a new code for so2 imaging figure 1 (previous page). screen shot of the main screen of vulcamera_post when in operation during the post-processing of volcanic degassing images from stromboli volcano (italy). graph a and graph b show the raw image files taken from the two cameras, with filters at 310 nm and 330 nm, respectively, showing the gas attenuation in graph a. these images can be flipped as appropriate and are vignette corrected on the basis of operations that take place on the vignetting page. a profile across the rising gas plume is identified in graph a using the l and r cursors, the intensities across which are presented in the profiles in the section graph. the calibration data, as obtained from the calibration page, are entered into the appropriate fields to the centre-right of the screen, to generate both the pseudocolor graph of the concentrations across the images shown in the top right, and the concentrations across the l-r profiles in the section ppm m/absorbance field. all of the above are then used to generate the integrated column amount time series of the explosive gas masses via the bottom-right window. s0551 annals of geophysics, 60, 5, 2017; s0551; doi: 10.4401/ag-7391 italian vs worldwide history of largest pga and pgv akiko suzuki, iunio iervolino dipartimento di strutture per l’ingegneria e l’architettura, università degli studi di napoli federico ii, naples, italy article history received february 21, 2017; accepted may 20, 2017. subject classification: ground motion, seismic hazard, earthquake intensity, engineering seismology. abstract earthquake engineering and engineering seismology greatly rely on the information gathered from observed data. this is especially true when it comes to ground motion intensity measures, which are related to the damaging potential of earthquakes. observed maxima of peak ground acceleration (pga) and peak ground velocity (pgv) can provide reference values for hazard assessment, which often needs to extrapolate towards large return periods. this simple study attempts to reconstruct the history of the largest values of pga and pgv observed in italy since 1972. results reveal that the current maxima of pga and pgv were recorded during the 2016 central italy earthquake sequence. for comparison, up-to-date global trends of pga and pgv maxima are also compiled, following relevant literature on the topic and investigating a number of international databases. it is found that these trends, which are roughly linear in logarithmic scale, correspond to the increase of ground motion records worldwide. 1. introduction according to the italian accelerometric archive (itaca), the history of italian strong motion earthquake recordings dates back to 1972. since then, infrastructures for seismic strong motion recording have been continuously developing, responding to the needs of earthquake-related research and seismic hazard evaluations. the recordings are mainly acquired by the italian accelerometric network (rete accelerometrica nazionale, ran) operated by the italian civil protection department (dpc) and the national seismic network operated by istituto nazionale di geofisica e vulcanologia (ingv). because of evolution of these monitoring networks and the shift from analog to digital recordings, availability of high-quality earthquake data rapidly evolved in the last decades. itaca, started in 2006, archives those ground motion recordings acquired by different institutions in a unified data format [luzi et al. 2016a]. the data-set of itaca contains more than thirty-thousand three-component waveforms from earthquakes with maximum moment magnitude (mw) equal to 6.9. as data increase, it is expected (and actually observed) that the largest recorded value of a ground motion intensity measure (im) tends also to increase over time. this shall occur even if it is postulated that the seismicity shows some stationarity (as it is typically assumed in hazard studies). in fact, the expected value of the maximum in a sample of size n (i.e., obtained from n independent and identically distributed random variables, rvs) increases as n increases. if the rv is the im in a generic earthquake, then the growth of strong motion databases and/or data from longer observation periods are analogous to enlarge the sample size. the larger the sample the more likely the tail of the distribution is sampled. the trend of the maximum recorded values of ground motion ims has been studied somewhat extensively. bommer and martínez-pereira [2000] compiled the history of maximum peak ground acceleration (pga) and peak ground velocity (pgv), in terms of horizontal and vertical components, recorded worldwide until 1999. that study was extended and updated later by strasser and bommer [2009a,b], s&b hereafter, with a particular interest in causative physical process of large-amplitude ground motions. çelebi et al. [2000] also showed that the maximum pga had been increasing with the increase of the recorded strongground-motions, using north american records. yamada et al. [2009] provided an update until 2009, including japanese records the largest of which is the 2008 iwate-miyagi ( japan) earthquake. following this relevant literature, this short study attempts to reconstruct the italian history of the maximum recorded ims and compares it with the worldwide trend, which was updated to 2016, investigating the itaca database and several reputed regional or international seismic suzuki and iervolino 2 data repositories. the remainder of the paper is structured as follows: the next section describes the investigated databases and the methodology to identify national and worldwide strong motion records with largest seismic intensities in the year of recording. subsequently, the obtained histories are presented and discussed for pga and pgv, in terms of both horizontal and vertical components, also linking with data increase per year. final remarks close the study. 2. methodology to compile the italian histories of maximum recorded pga and pgv, a flat-file of the itaca database was obtained (dr. lucia luzi, personal communication). all the processed (both automatically and manually) records in the flat-file, for which pga and pgv were available, were sorted in terms of recording year. then, the maximum im, for each specific year, was determined for horizontal (the largest value between the two as-recorded components was considered) and vertical ground motions. data are those collected until november 2016. for comparison, also the worldwide histories of the largest pga and pgv were updated after the latest relevant studies. in fact, for consistency, herein the worldwide histories were entirely recompiled. to this aim, a number of databases, defined as global or regional depending on the geographic origin of data, was considered. among global databases those investigated are: the peer ground motion database nga west 2 [peer, ancheta et al. 2014], the cosmos virtual data center [archuleta et al. 2005], the engineering strong-motion database [esm, luzi et al. 2016b],1 the selected input motion for displacement-based assessment and design [simbad, smerzini and paolucci 2013, smerzini et al. 2014], and the earthquake strong motion data catalog provided by the national oceanic and atmospheric administration (noaa). among regional databases: the k-net&kik-net seismic networks of the japanese national research institute for earth science and disaster resilience [nied, aoi et al. 2004], as wells as that of the japan meteorological agency ( jma) and of the port and airport research institute (pari), the new zealand strong-motion database [geonet, van houtte et al. 2017, kaiser et al. 2017], the chile strong ground motion flatfile [csgmf, bastías and montalva 2016], the mexican strong motion database [msmd, quaas et al. 1996, alcántara et al. 2000, alcántara and pérez 2004], released as a cd-rom (prof. eden bojorquéz, personal communication), and the database from red acelerográfica del iingen de la unam, still from mexico.2 these data-sources were investigated in one of the following ways depending on the available information. (1) as for itaca, the databases directly providing the flat-files with metadata (i.e., peer, simbad, geonet, and csgmf) were sorted in terms of recording year, then the annual maxima, in terms of the considered ims, were taken for horizontal and vertical components separately. note that in peer, the intensity values are listed in the horizontal orientation-independent scale, rotd50 [boore et al. 2010]. in this case, the values of rotd50 were ranked first. then, only for those top-ranking, the two horizontal im values were retrieved from the corrected waveforms. to complement potential omissions, the horizontal im values of some record-breaking ground motions specified in other databases were also individually examined. with regard to the vertical components, the peer database was only used to check top-ranking im values from other databases. (2) for the databases providing search engines either online or via cd-rom (i.e., cosmos, esm, noaa, k-net&kik-net, pari, msmd, and unam), the maximum intensity histories were obtained identifying the recordings with the peak im value year-by-year, with the aid of parameter-entry search functions. in the case of search engines filtering the records by the vector-sum or the largest value im among the three components, a three-step search was performed: (i) the records were ranked in terms of the im used by the search engine; (ii) only for some top-ranking, the peak intensity values of 1 the latest updates in esm were also tracked to account for the strong motion records from the 2016 central italy earthquake sequence. 2 a number of databases was also considered and then disregarded because of overlapping with the investigated repositories. this is the case of: the european strong-motion database (esmd; ambraseys et al. 2004), center for engineering strong motion data (cesmd; haddadi et al. 2008), iran strong motion network (ismn; iran, alavijeh et al. 2003), national strong-motion network of turkey (turkey, http://kyhdata.deprem.gov.tr/), and swiss seismological service (sed, switzerland; fäh et al. 2011). for other databases, data retrieval or format issues also prevented further investigation. they are: incorporated research institutions for seismology database (iris; https://www.iris.edu/), canadian national seismograph network (cnsn, canada; north et al. 1994), norwegian seismic array (norsar, norway; bungum et al. 1971), and central weather bureau (cwb, taiwan; http://www.cwb.gov.tw/). history of largest pga and pgv 3 each component (two horizontal and vertical) were retrieved using the original waveforms; (iii) the component values were used, in turn, to search again the database for recordings with larger im, when the latter is the one considered by the search engine. this procedure was carried out for the horizontal and vertical components separately. (3) the jma database provides only records from some large japanese earthquakes since the last decades. in this case, all were accessed as well as relevant related literature. the largest intensity value was considered in case of overlapping records among different databases. it is to note that some of the search procedures employed in this study, which has a modest scope with respect to previous literature (i.e., only comparison with italian trend), are simplified and may lead to miss some peak ims. moreover, while s&b [2009a,b] generally considered uncorrected records, herein waveforms are mostly corrected ( japanese and mexican records are an exception). also, the strong motion data sources covered in this study are not exactly the same as s&b [2009a,b]. thus, the generality of the results may be limited. 3. italian trend table 1 and table 2 chronologically show the ground motion records with the largest ims retrieved as discussed in the previous section, and provide the basic ground motion parameters (the soil classification in the list is based on eurocode 8 or ec8, cen 2003). as it regards pga (table 1), the first available recorded values are from 1972. the maximum pga for that year is from the ancona earthquake (local magnitude, ml, 4.7), 0.56g horizontal and 0.28g vertical, at the ancona rocca station. four years later, these values were exceeded by an aftershock of the friuli earthquake on september 15 1976 (mw 5.9), with 0.64g horizontal and 0.49g vertical, at the gemona station. the horizontal maximum was surpassed more than thirty years later by an aftershock of l’aquila earthquake on april 7 2009 (mw 5.5) at the sant’eusanio forconese station: 0.66g. it was further updated to 0.87g by the amatrice earthquake (mw 6.0), that is the event initiating the 2016 central italy seismic sequence.3 interestingly, it seems that, after the 1976 friuli earthquake, the vertical maximum was exceeded by a small-magnitude event (mw 4.3) in the 1997 umbria/marche sequence at the casermette station, with 0.73g. the 2012 emilia earthquake (mw 6.0) increased the vertical maximum pga to 0.86g, followed by the norcia earthquake on october 30 2016 (mw 6.5) with 0.89g at the savelli pg station. from this analysis, it is concluded that the maximum pga values in italy increased by approximately 54% and 220% in horizontal and vertical components, respectively, in the last forty-five years. regarding the italian largest pgv history, the lineup in table 2 is similar to pga, except that no records from the 2009 l’aquila or the 1997 umbria/marche sequences appear; in turn, the 1980 irpinia earthquake (mw 6.9) scores for both horizontal and vertical pgvs at the sturno station. again, the 2016 central italy seismic sequence, the event on october 30 (mw 6.5) in particular, holds the current maxima for horizontal and vertical components, 0.8m/s at the amatrice (t1201, domo ri) station and 0.7m/s at the castelluccio di norcia station, respectively. 4. worldwide trend 4.1 pga figure 1a,c shows the global trend of the maximum recorded pga history for horizontal and vertical components, up to the end of 2016. as already shown in literature, the retrieved maximum pga history starts with the 1933 long beach earthquake (at public utilities building, california, 0.2g and 0.3g for horizontal and vertical, respectively) and terminates with the 2008 iwate-miyagi earthquake, japan (2.5g at the higashinaruse akth04 station, for horizontal; 3.9g at the ichinoseki iwth25 station, for vertical). the present study revealed that, since then, the recordings from the 2011 tohoku ( japan) earthquake led to update the previous maximum pga for the horizontal component to 2.8g (at the tsukidate myg004 station). it is worth to note, however, that the peculiarity of the tsukidate record has been discussed by a number of researchers [e.g., de luca et al. 2011, goto and morikawa 2012, nagashima et al. 2012 and motosaka 2012]. the compiled history up to 2008 is mostly consistent with s&b [2009a,b]. nevertheless, there are a few exceptions. some earthquakes from the aforementioned studies did not show up in this investigation and vice-versa (figure 1a). it is believed that this is mainly due to processing of waveforms. in particular, for the horizontal component, the maximum pga value from the 1984 morgan hills earthquake (corrected: 3 at the time of writing the sequence is still ongoing. moreover, some near-source data show larger recorded pgas (i.e., around 1g), but at this time are not considered fully reliable by the ran managers. therefore, they are not included herein; see reluis-ingv workgroup (2016) for information. suzuki and iervolino 4 1.2g at coyote lake dam, san martin, in noaa, 1.16g in cosmos; uncorrected: 1.3g, shakal et al. 1984) does not appear in this study because of the larger value from the 1971 san fernando earthquake (1.24g at pacoima dam in peer, 1.17g in cosmos). conversely, the 1994 northridge earthquake (corrected: 1.78g at the tarzana table 1. italian ground motion records with largest recorded pga. (†ec8 classification [cen 2003], *not based on a direct shear-wave velocity in the upper 30m measurement, *1epicentral distance, [i] = itaca, [e] = esm). year event (event id) station (station code, soil class†) focal mechanism magnitude mw (ml) distance*1 [km] pga [g] source horizontal 1972 ancona (it-1972-0005) ancona, rocca (anr, b) unknown (4.7) 7.7 0.56 [i] 1976 friuli aftershock (it-1976-0027) gemona (gmn, b) thrust 5.9 6.2 0.64 [i] 2009 l’aquila aftershock (it-2009-0102) sant’eusanio forconese (mi05, a*) normal 5.5 3.6 0.66 [i] 2016 central italy aug. (emsc-2016082 4_0000006) amatrice (amt, b*) normal 6.0 8.5 0.87 [i] vertical 1972 ancona (it-1972-0005) ancona, rocca (anr, b) unknown (4.7) 7.7 0.28 [i] 1976 friuli aftershock (it-1976-0027) gemona (gmn, b) thrust 5.9 6.2 0.49 [i] 1997 umbria/marche (it-1997-0147) colfiorito casermette (clc, c*) strike-slip 4.3 1.3 0.73 [i] 2012 emilia (it-2012-0011) mirandola (mrn, c) thrust 6.0 4.1 0.86 [i] 2016 central italy oct. (emsc-20161030_ 0000029) savelli pg (t1213, a*) normal 6.5 12.0 0.89 [e] table 2. italian ground motion records with largest recorded pgv. (†ec8 classification [cen 2003], *not based on a direct shear-wave velocity in the upper 30m measurement, *1epicentral distance, [i] = itaca, [e] = esm). year event (event id) station (station code, soil class†) focal mechanism magnitude mw (ml) distance*1 [km] pgv [cm/s] source horizontal 1972 ancona (it-1972-0007) ancona, rocca (anr, b) unknown (4.2) 9.3 12.1 [i] 1976 friuli aftershock (it-1976-0027) gemona (gmn, b) thrust 5.9 6.2 68.4 [i] 1980 irpinia (it-1980-0012) struno (str, b) normal 6.9 33.3 70.3 [i] 2016 central italy oct. (emsc20161030_0000029) amatrice, domo (t1201, b*) normal 6.5 23.3 83.0 [e] vertical 1972 ancona (it-1972-0005) ancona, rocca (anr, b) unknown (4.7) 7.7 3.8 [i] 1976 friuli aftershock (it-1976-0027) gemona (gmn, b) thrust 5.9 6.2 20.6 [i] 1980 irpinia (it-1980-0012) struno (str, b) normal 6.9 33.3 23.3 [i] 2012 emilia (it-2012-0011) mirandola (mrn, c) thrust 6.0 4.1 26.4 [i] 2016 central italy oct. (emsc-20161030_ 0000029) castelluccio di norcia (clo, a*) normal 6.5 7.8 68.6 [e] history of largest pga and pgv 5 station, california, from peer) is not in the envelope of the horizontal pga values in the cited studies because of the larger uncorrected value from the 1992 cape mendocino earthquake (corrected: 1.50g at cape mendocino, california, in cosmos; uncorrected: 1.88g, darragh et al. 1992). s&b [2009a,b] also did not include the record from the 1968 ehime earthquake, japan (0.84g at itashima bridge from noaa). for the vertical component (figure 1c), the trends are similar between two studies, there are a few discrepancies in the pga values though. it is also worth to mention that this study verified via literature some large pga values of japanese earthquakes. this is the case of the jma kushiro record from the 1962 hokkaido (hiroo), for which information in dan (1995) was considered more reliable than that from noaa. moreover, the largest pga values from the japanese earthquakes during 2003-2004 were acquired by local seismic networks, and related information is restricted [japan society of civil engineering and japanese geotechnical society, jsce and jgs 2003 and jma 2004]. in fact, the waveform at the naruse town office in the 2003 northern miyagi foreshock (fs) was lost during the aftershock sequence (as well as other records at different stations of the network, jsce and jgs, 2003), while that at the kawaguchi station from the 2004 niigata aftershock (as), was not made available to the authors by jma. it is also to note that the envelope of s&b [2009a,b] excludes these records. for more detailed information of the discussed ground motions, see table 3 (the soil classification in the list is based on nehrp soil classification – bssc, 1994 mostly extracted from the peer flat-file). 4.2 pgv the global maximum pgv histories of horizontal and vertical components were also examined and the results are presented in figure 2a,c. it is to note, however, that it was less straightforward to reconstruct the history for maximum pgv rather than for maximum pga. this is because pgv values for the same records were found to be different among databases, likely because pgv is more sensitive to processing. it is also to mention that pgvs are not provided by all examined databases. for the former case, the discrepancies were solved considering the largest pgv values among those available. the latter case corresponds to the japanese and mexican repositories for which pgv is not provided. therefore, the investigation was limited to some accelero1940 1960 1980 2000 year 1 2 3 4 5 p g a [g ] 1940 1960 1980 2000 year 1 2 3 4 5 p g a [g ] (b) (d) 1930 1940 1950 1960 1970 1980 1990 2000 2010 year 0 1 2 3 4 5 p g a [g ] l o n g b e a c h e l c e n tr o h o k k a id o e h im e s a n f e rn a n d o n a h a n n i c e rr o p e o ri t c a p e m e n d o c in o n o rt h ri d g e m iy a g i f s n ii g a ta a s iw a te -m iy a g i t o h o k u italy worldwide (this study) s&b (2009) 1930 1940 1950 1960 1970 1980 1990 2000 2010 year 0 1 2 3 4 5 p g a [g ] l o n g b e a c h s a n f e rn a n d o g a z li im p e ri a l v a ll e y n a h a n n i iw a te -m iy a g i (a) (c) figure 1. largest recorded values of pga, (a) horizontal; (b) worldwide horizontal in logarithmic scale; (c) vertical; (d) worldwide vertical in logarithmic scale. suzuki and iervolino 6 grams from the seismic events with the largest pgas and some famous large earthquakes. for those records, the pgv values were computed from the corrected accelerograms (linear base correction and forth-order butterworth filter between 0.10 hz and 25hz). the reconstructed horizontal history is composed by the following records: the 1933 long beach, california (0.29m/s at public utilities building), the 1940 el centro, california (0.37m/s at array #9), the 1966 parkfield, california (0.78m/s at cholame 2wa), the 1971 san fernando, california (1.14m/s at pacoima dam), the 1978 tabas (1.23m/s, at tabas, iran), the 1992 cape mendocino (1.26m/s at cape mendocino, california), the 1994 northridge, california (1.48m/s at the rinaldi receiving station), and the 1999 chi-chi earthquakes, taiwan (2.6m/s at taichung, tcu068). s&b [2009a,b] created a single envelope for pgv, while this study accounted for the horizontal and vertical components separately. nevertheless, the updated horizontal pgv history has some agreement with prior studies. exceptions are pgv values from rotated waveforms (e.g., for the 1979 imperial valley, california, the 1987 superstition hill, california, and the 1994 northridge, california, earthquakes) and processing (e.g., the 1992 landers, california, iwan and chen [1995], the 1999 chi-chi, taiwan, earthquake, wang et. al. [2002]). compared to the horizontal envelope, relatively smaller increase in the vertical pgvs was observed over the years. the record-breaking events in vertical component are: the 1933 long beach, california (0.31m/s at public utilities building), the 1971 san fernando, table 3. records determining the compiled pga history worldwide. † nehrp soil classification [bssc 1994], (-) denotes unknown soil conditions. ‡1jma magnitude after the revision in 2004, http://www.jma.go.jp/, ‡2 surface-wave magnitude, ms. *1 closest distance *2 epicentral distance, *3 hypocentral distance, *4 unknown, *5 munguia et al. [1988]. [c] = cosmos, [n] = noaa, [p] = peer nga, [m] = msmd. [j1] = jsce and jgs [2003], [j2] = jma [2004]. year event station (soil class†) focal mechanism magnitude mw (ml) distance*1 [km] pga [g] source horizontal 1933 long beach public utilities bldg., ca (-) strike-slip 6.4 0.8 0.20 [c] 1940 el centro el centro, array #9, ca (d) strike-slip 6.9 12.2*3 0.35 [c] 1962 hokkaido (hiroo) jma kushiro, japan (-) n/a 7.1‡1 92*2 0.53 [n] 1968 ehime itashima bridge, japan (-) n/a 6.6‡1 19*2 0.84 [n] 1971 san fernando pacoima dam, ca (a) thrust 6.6 1.8 1.24 [p] 1985 nahanni nwt -station#1, canada (c) thrust 6.9‡2 *4 1.34 [c] 1987 cerro prieto cerro prieto volcano, mexico (-, rock) transform (5.4) 5.6*2*5 1.45 [m] 1992 cape mendocino cape mendocino, ca (c) thrust 7.0 15.5*3 1.50 [c] 1994 northridge tarzana, cedar hill nursery a, ca (d) thrust 6.7 15.6 1.78 [p] 2003 northern miyagi fs naruse town office, japan (-) thrust 5.6‡1 3.5*3 2.04 [j1] 2004 niigata as kawaguchi town office, japan (-) thrust 6.5‡1 7.3*3 2.08 [j2] 2008 iwate-miyagi higashinaruse, japan (c) thrust 6.9 22.6*3 2.50 [c] 2011 tohoku tsukidate, japan (c) megathrust 9.0 131.0*3 2.75 [c] vertical 1933 long beach public utilities bldg., ca (-) strike-slip 6.4 0.8 0.29 [c] 1971 san fernando pacoima dam, ca (a) thrust 6.6 3.5 0.71 [c] 1976 gazli karakyr, uzbekistan (d) thrust 6.8 5.5 1.70 [p] 1979 imperial valley el centro ca array #6, ca (d) strike-slip 6.5 1.4 1.89 [p] 1985 nahanni nwt -station#1, canada (c) thrust 6.9‡2 *4 2.37 [c] 2008 iwate-miyagi ichinoseki west, japan (c) megathrust 6.9 8.3*3 3.94 [c] history of largest pga and pgv 7 california (0.59m/s at pacoima dam), the 1976 gazli, uzbekistan (0.64m/s, at karakyr), the 1979 imperial valley, california (0.74m/s at el centro differential array), the 1985 algarrobo, chile (1.03m/s at valparaíso), and the 1999 chi-chi earthquake, taiwan (2.13m/s at taichung, tcu068). overall, the study showed no increase of maximum values in the current decade. see table 4 for detailed ground motion information. 5. discussion 5.1 data increase it is noted that maximum pga and pgv values recorded so far qualitatively align if plotted in logarithmic scale versus time. this is shown in figure 1b,d and figure 2b,d, where the recorded maxima are shown in logarithmic scale. these trends can be associated to the rapid increase of the archived strong motion records in the databases, which is illustrated in figure 3, where the cumulative number of three-component accelerograms is plotted for each of the examined databases.4 it can be seen that the amount of data has been rapidly increasing especially in the last decades. 5.2. italy vs world the global and italian trends of maximum recorded ims are compared in figure 1a,c and 2a,c. the italian values, as expected because of the features of the regional seismicity and the period for which instrumental data are available, are significantly smaller and have increased at a slower pace. 6. final remarks the presented study attempted to examine the maximum values of peak ground motion parameters (i.e., pga and pgv) ever recorded in italy. for comparison, starting from relevant literature on the topic, the study also tried to recompile and update the worldwide trend via the analysis of a number of national and international strong motion databases. the notable remarks are listed as follows. 1. the largest recorded intensity values in italy were figure 2. largest recorded values of pgv, (a) horizontal; (b) worldwide horizontal in logarithmic scale; (c) vertical; (d) worldwide vertical in logarithmic scale. 4 the plots were obtained examining the flat-files or counting the waveforms returned by search engines of the web-portals of the databases. note that, because cosmos and noaa databases return individual components, the annual growth was approximately computed dividing by three the number of all archived waveforms. suzuki and iervolino 8 recently updated, by the 2016 central italy earthquakes, to pga values of about 0.9g (both horizontal and vertical) and to pgv values of about 0.8m/s and 0.7m/s (horizontal and vertical, respectively). 2. the records from the recent earthquakes in japan, the 2008 iwate-miyagi and the 2011 tohoku earthquakes, currently hold the worldwide largest pga values, about 4g (vertical) and 3g (horizontal), respectively (although there is discussion about some of these values); the pgv current maxima are larger than 2.0m/s (horizontal and vertical), both from the 1999 chi-chi earthquake. 3. the global trend of peak ims corresponds to a rapid growth of strong motion data available. moreover, the maximum values of pga and pgv recorded worldwide so far qualitatively align when plotted in logarithmic scale versus time. the obtained trend can provide reference values for further observations or for hazard assessment studies, which often need to extrapolate towards large return periods and intensities. however, it is to remark that the generality of the results may be limited by the data available to the authors and the simplified search in some databases. 7. data and sharing resources the following strong-motion databases were considered as data sources for this study (all last accessed on feb. 2017): cosmos virtual data center [http://www.strongmotioncenter.org/vdc]; peer ground motion database nga west 2 [http:// ngawest2.berkeley.edu]; esm [http://esm.mi.ingv.it]; simbad (reluis), noaa [https://www.ngdc.noaa. gov/]; k-net&kik-net (nied, japan) [http://www. kyoshin.bosai.go.jp]; jma ( japan) [http://www.jma. go.jp]; harbor area strong motion network (pari, japan) [http://www.eq.pari.go.jp/kyosin/]; geonet (earthquake commission and gns science, new zealand) [http://www.geonet.org.nz]; csgmf (chile) [https://nees.org]; itaca (ingv, italy) [http://itaca. mi.ingv.it/]; msmd (mexico) [cd-rom] (instituto de ingeniería, unam; comisión federal de electricidad; centro de investigacíon científica y de educación superior de ensenada; fundación ica; centro de instrumentación y registro sísmico; centro nacional year event station (soil class†) focal mechanism magnitude mw (ml) distance*1 [km] pgv [cm/s] source horizontal 1933 long beach public utilities bldg., ca (-) strike-slip 6.4 0.8 29.4 [c] 1940 el centro el centro, array #9, ca (d) strike-slip 6.9 12.2*3 36.9 [c] 1966 parkfield cholame 2wa, ca (e) strike-slip 6.1 3.5 78.1 [c] 1971 san fernando pacoima dam, ca (a) thrust 6.6 1.8 114.4 [p] 1978 tabas tabas, iran (b) thrust 7.4 2.1 123.3 [p] 1992 cape mendocino cape mendocino, ca (c) thrust 7.0 15.5*3 126.1 [c] 1994 northridge rinaldi receiving sta., ca, (d) thrust 6.7 6.5 147.9 [p] 1999 chi-chi taichung, taiwan (c) thrust oblique 7.6 0.3 264.0 [p] vertical 1933 long beach public utilities bldg., ca (-) strike-slip ‡1 0*2 30.8 [n] 1971 san fernando pacoima dam, ca, (a) thrust 6.6 1.8 59.2 [p] 1976 gazli karakyr, uzbekistan (d) thrust 6.7 12.8*2 64.3 [s] 1979 imperial valley differential array, ca (d) strike-slip 6.5 5.6 74.4 [c] 1985 algarrobo valparaíso, varp08r, chile (d) megathrust 7.9 46.8*2 102.8 [cs] 1999 chi-chi taichung, taiwan (c) thrust oblique 7.6 0.3 213.0 [p] table 4. records determining the compiled pgv history worldwide. † nehrp soil classification [bssc 1994], (-) denotes unknown soil conditions. ‡1 unknown magnitude, *1 closest distance, *2 epicentral distance, *3 hypocentral distance. [c] = cosmos, [p] = peer nga, [n] = noaa, [s] = simbad, [cs] = csgmf. history of largest pga and pgv 9 de prevención de desastres; universidad autónoma metropolitana, campus azcapotzalco; instituto de geofísica; and la sociedad mexicana de ingeniería sísmica, mexico); and red acelerográfica del iingen de la unam [http://aplicaciones.iingen.unam. mx/acelerogramasrsm/] (instituto de ingeniería, unam, mexico). acknowledgements. the study presented in this paper was developed within the activities of reluis (rete dei laboratori universitari di ingegneria sismica) for the project reluis-dpc 20142018. data from dr. lucia luzi (istituto nazionale di geofisica e vulcanologia, italy) and prof. eden bojorquéz (universidad autonoma de sinaloa, mexico) are gratefully acknowledged. references alavijeh h. m. and e. farzanegan (2003). iran strong motion network. asian j. civ. eng. 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features of short-period and long-period near-source ground motions. bull. seismol. soc. am., 99(6): 3264-3274. *corresponding author: iunio iervolino, dipartimento di strutture per l’ingegneria e l’architettura, università degli studi di napoli federico ii, naples, italy; email: iunio.iervolino@unina.it. 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved 197_203 balestri.pdf 197 annals of geophysics, vol. 46, n. 2, april 2003 detection of meteorological inconsistencies by gps lorenzo balestri, mauro boccolari, slobodan fazlagiç, sergio pugnaghi and renato santangelo dipartimento ingegneria dei materiali e dell’ambiente (osservatorio geofi sico), università degli studi di modena e reggio emilia, modena, italy abstract gps observations, distances from satellites to receivers and meteorological conditions in neutral atmosphere are known to obey a constraint, which provides a residual or in other words a quality index. a method is discussed which provides a residual epoch by epoch in near real time. in general, distribution of residuals during several consecutive epochs belonging to the same satellites, allows estimates of a mean and a standard deviation of mean. under normal meteorological conditions distribution of residuals appears to be consistent with zero mean as expected. however, consecutive residuals sometimes appear to have a mean different from zero by more than three standard deviations of mean. such signifi cant consecutive epochs provide a warning of existing inconsistencies among gps observations, distances from satellites to receivers as obtained by orbital information, meteorological conditions above receivers (as obtained by ground measurements or by extrapolation of meteorological analysis). a procedure has been set up which warns about these inconsistencies in near real time. mailing address: dr. renato santangelo, dipartimento ingegneria dei materiali e dell’ambiente (osservatorio geofisico), università degli studi di modena e reggio emilia, via vignolese 905, 41100 modena; e-mail: santangelo.renato@unimo.it key words gps – double difference – zenith total delay – residuals 1. introduction soon after its establishment twenty years ago, the global positioning system (gps), became a new and powerful reference frame for active tracking of atmospheric phenomena. microwave radio signals transmitted by 24 continuously operating gps satellites are delayed by the atmospheric refraction as they propagate to earth-based gps receivers. gps satellites transmit two l-band radio signals (19 and 22 cm wavelengths). usually, the receiver’s sampling rate is 30 s (observation epochs). there is a dispersive effect of the ionosphere and a non-dispersive effect of the (neutral atmosphere) troposphere. ionospheric effects could be largely removed by a linear combination of dual frequency data. when the ionosphere disturbance on gps electromagnetic waves is resolved in a satisfactory manner, the main task remains to estimate better the neutral (basically tropospheric) atmosphere impact. the signal delay becomes a function of temperature, pressure and water content. the wet component of the tropospheric delay in gps signals is nearly proportional to the quantity of water vapor integrated along the signal path. water vapor, as the only atmospheric constituent that possesses a permanent dipole moment (asymmetric distribution of charge in the water molecule) is a highly variable atmospheric constituent and also fundamental for the transfer of energy, producing major diffi culties for the parameterization in numerical 198 lorenzo balestri, mauro boccolari, slobodan fazlagiç, sergio pugnaghi and renato santangelo weather prediction (nwp) modeling (businger et al., 1996). with the rapid growth in network of groundbased gps receivers, such increased spatial and temporal resolution of the observing data enabled the birth of the new ground-based gps meteorology which has expanded rapidly in recent years. the present state-of-the-art consists of currently advanced studies and certain operational services for monitoring of the atmospheric water vapor, climatology and weather analysis and prediction (baker et al., 2001). the assimilated gps observation data are used for the atmospheric modeling, primarily in parameter estimation. other sources of signal errors, such as ionospheric refraction, satellite orbit accuracy, antenna phase center modeling, signal multipath and scattering near the receiver, etc. have been signifi cantly reduced during recent years (bock and doerfl inger, 2001). especially when the rather accurate near real time satellite orbits (with only 2 h time delay) became available recently, the space for the validation of numerical weather modeling was opened more widely. gps meteorological estimates are now produced with accuracy comparable with that of existing meteorological measurement techniques (radiosondes, water vapor radiometers). different efforts to use the elaboration of gps specifi c tropospheric refraction as a meteorological profi les data source are being made today. this paper aims to assimilate integrated meteorological measures obtained by gps receivers with conventional meteorological measures in upper air like those provided by a meteorological database, e.g., the database of deutscher wetterdienst (dwd) currently received in real time by osservatorio geofi sico in modena (italy). meteorological fi elds, from the database, compute the corresponding integrated measure performed by gps. a comparison shows possible inconsistencies, in near real time. the amount of detected inconsistencies (measured mean divided by standard deviation of the mean) represents a quality warning about the products (e.g., forecasts) emitted by such database. obviously this quality warning concerns only possible errors of meteorological fi elds used in forecast models, without demanding that this quality warning also concern mechanisms used in the model. 2. summary of gps activity within eec project magic (during map-sop) the work uses gps data observed in padua and medicina (near bologna) during the mesoscale alpine program-special observing period (map-sop – since september 1, 1999 up to november 30, 1999) (bougeault et al., 2001). two gps receivers have been considered since they allow using a double difference technique (better explained later) which increases precision of gps analysis. the receiver in padua is used since it was close to the chosen map site in verona, where about 100 balloons were launched and tracked on command of an map forecaster team in innsbruck (austria). the second receiver chosen was the one in medicina (near bologna), since balloons are also currently launched in capofi ume (near bologna too) every day. upper air observations above capofi ume are currently received and assimilated by meteorological centers (as e.g., dwd). therefore their disseminated meteorological fi elds should represent fairly well meteorological conditions above capofi ume (i.e. above 1 or 2 gps receivers) so that possible inconsistencies should be accounted to a lack of knowledge of meteorological conditions above the other gps receivers (that in padua). 3. «ionofree» combination of the two gps signals emitted by a gps satellite each gps satellite emits two signals, each having its own frequency. a suitable combination of such signals is called «ionofree» signal of that satellite. with a good approximation, the «ionofree» signal does not need any further correction during its travel across ionosphere. however, it undergoes refraction when it travels across the neutral atmosphere. refraction depends on meteorological conditions along the traveled trajectory but does not depend on frequency (no dispersion) (thayer, 1974). 199 detection of meteorological inconsistencies by gps obviously an in void «ionofree» signal would travel with the speed of light. 4. gps observable due to a sophisticated procedure starting time t i of a gps signal from a gps satellite i, and arrival time t a (> t i ) of the same gps signal down to gps receiver a, are appropriately recognized as belonging to the same signal. travel time lag is measured by phase ranging: therefore, such time observable measured by phase ranging has an intrinsic phase ambiguity. moreover, time lag or observable (4.1) has further errors due to unknown constants in both satellite and receiver clocks. in conclusion, observable o should be written as (4.2) where k a and k i are unknown constants. unknown constants disappear if a double difference of observed phases is used i.e. (4.3) 5. constraint if gps signal travels in void an in void gps signal would move with the speed of light (= c), so that the following constraint exists (5.1) where ddo means double difference of observed phases (neglecting any problem concerning phase ambiguity) and ddl means double difference of travel paths from satellites: i, j to receivers a, b. at any epoch distances between receiver and satellite are fairly well known, due to rather well known orbits of satellites, under the assumption that the receiver does not move (no ocean loading). with selfexplanatory symbols (5.2) 6. gps signal traveling across a neutral atmosphere when a gps signal travels across a neutral atmosphere, i.e. a refractive atmosphere, it undergoes an extra-delay t', which exceeds the delay of a signal having the speed of light, due to a velocity lower than that of light. it is useful to convert extra-delay t' into a «tropospheric length» t (6.1) wave front of gps signal which propagates towards earth vertically along a line element ds v in neutral atmosphere, undergoes an extra-delay dt v which depends on meteorological parameters existing along that line element. extra-delay (as length) from i to corresponding vertical point a v on earth is ta i ν . extra delay from a slant gps satellite (say i ) to a receiver (say a) is equal to extra delay from i to point a v along vertical on the same wave front containing a, multiplied by a suitable mapping function, f (6.2) if zenith angle z (complementary to elevation angle e) gps satellite i is less than 75° (duan et al., 1996), it can be assumed to be equal to (6.3) lack of knowledge of the exact expression of mapping function implies that an evaluation of t is rather uncertain under slant conditions. for completeness, expression of ddt is shown (6.4) 7. constraint in neutral atmosphere therefore previous constraint (in double difference technique) must be suitably modifi ed into the following: (7.1) o t ta i= −( ) o t k t ka a i i= +( ) − +( )[ ] ddo = −( ) − −( )o o o oai bi aj bj . c ⋅ =ddo ddl ddl = −( ) − −( )l l l lai bi aj bj . t ct= ′. t t fa i a i= ⋅ ν . f z= ( )1 / cos . ddt = −( ) − −( )t t t tai bi aj bj . c ⋅ = +ddo ddl ddt. 200 lorenzo balestri, mauro boccolari, slobodan fazlagiç, sergio pugnaghi and renato santangelo ddo is obtained by phase ranging; ddl is obtained by known orbits of satellites. ddt can be obtained as follows: 1) by meteorological database disseminated by meteorological centers (e.g., by dwd) and received in real time; 2) by ground meteorological measurements in real time or by climatological data, suitably extrapolated towards upper air, e.g., by a formula suggested by saastamoinen (1972). these two possible choices for evaluation of extra-delay ddt are foreseen in the «bernese» software (to be discussed in next section). the two choices provide, for most epochs, close results. 8. the «bernese gps software» application for a quality index epoch by epoch previous three terms in eq. (7.1) are adjusted by «bernese» software (hugentobler et al., 2001) by a suitable fi t which takes into account phase ambiguity also, and provides fi nally an unbalance or a double difference residual (ddr). if the fi t were perfect ddr would be zero. upper air meteorology, analyzed by the «bernese» software, during map-sop, was provided by: 1) measurements at ground and suitably extrapolated to upper air; 2) meteorological database disseminated by dwd. fig. 1. double differences residuals versus time for doy: 250, 251, 252 and 254 of the year 1999 for the gps stations padua and medicina (italy). lines: cyan mean ddr; blue standard deviation; red anomalous period. 201 detection of meteorological inconsistencies by gps it seems preferable to use meteorological fields provided by dwd database since they are expected to represent better upper air conditions. total number of 13 days, belonging to the map time series of observations have been analyzed so that their ddr is available versus time. 9. pairs of satellites observing a pair of receivers at each epoch: zenith cut-off at any epoch there are several satellites which observe a pair of receivers in sites a and b. therefore there are several pairs of satellites, which provide different values of ddr. in order to avoid management of uncertain ddrs, due to uncertain mapping functions, it would be preferable to choose near vertically positioned satellites or small zenith angles. however, with such choice, only very few satellites would be accepted. therefore a compromise is necessary. zenith angles smaller than 30°, for each pair of satellites, observing the two receivers in a and b, seem a reasonable compromise. certainly this choice of a cut-off angle implies a signifi cant reduction of pairs of satellites observing simultaneously receivers a, b at that epoch. with such choice, at any day, not more than two pairs of satellites are simultaneously present. fig. 2. double differences residuals versus time for doy: 258, 270, 273 and 293 of the year 1999 for the gps stations padua and medicina (italy). lines: cyan mean ddr; blue standard deviation; red anomalous period. 202 lorenzo balestri, mauro boccolari, slobodan fazlagiç, sergio pugnaghi and renato santangelo fig. 3. double differences residuals versus time for doy: 296, 297, 298 and 299 of the year 1999 for the gps stations padua and medicina (italy). lines: cyan mean ddr; blue standard deviation; red anomalous period. 10. evolution of ddr versus epoch; periods of consecutive epochs since ddr fi tted by the procedure previously outlined in eq. (6.1) fl uctuates, it is better to consider an average over several consecutive epochs during which the satellite constellation remain unchanged. in fact, reception of a same pair of satellites lasts several epochs (more than 10 epochs, are chosen, i.e. > 5 min). during that period, every ddr should vanish, if the fi t was ideal. therefore it is preferable to consider the mean value of ddr: ddr, and the standard deviation of the mean: σ ddr , for that period. for all 13 days previously mentioned, gps was observed continuously, and meteorological data from dwd database were obtained. in fi gs. 1 to 4, evolution of ddrand σ ddr is shown for 13 days with suitable colors (doy represents day of year, e.g. doy 250 is september 07th that year) for every period of consecutive epochs with the same satellites observing. in the same plot, anomalous periods are suitably labeled in red. to determine an anomalous period, a hypothesis test was done. taking a signifi cance level of 99.7%, anomalous periods of ddr happen when ddr differs from its expected value (zero) by more than 3 standard deviations of the mean i.e.: − ≤ ≤3 3σ σ ddr ddr ddr . 203 detection of meteorological inconsistencies by gps 11. anomalous periods anomalous periods indicate that a certain inconsistency exists between measured data (ddo from gps), orbital data (ddl), and meteorological fields (ddt). the latter are obtained by ground measurements aside receiver or by extrapolation of dwd analysis. 12. conclusions a procedure has been set up which, in near real time, warns about consecutive epochs with the same satellites, which show inconsistencies among all available information. however, the procedure needs, simultaneous observations of two satellites and two receivers. these inconsistencies may account for a lack fig. 4. double differences residuals versus time for doy: 295 of the year 1999 for the gps stations padua and medicina (italy). lines: cyan mean ddr; blue standard deviation; red anomalous period. of precise knowledge of meteorological information above at least one of the two receivers. the ambiguous assignment of the imprecise meteorological condition is the main drawback of procedure. however, suitable additional meteorological measurements close to one of the two receivers may help in defi ning which receiver site is the imprecise one. references baker, h.c., a.h. dodson, n.t. penna, m. higgins and d. offiler (2001): ground-based gps water vapour estimation: potential for meteorological forecasting, j. atmos. solar-terr. phys., 63, 1305-1314. bock, o. and e. doerflinger (2001): atmospheric modeling in gps data analysis for high accuracy positioning, phys. chem. earth (a), 26 (6-8), 373-383. bougeault, p., p. binder, a. buzzi, r. dirks, r. houze, j. kuettner, r.b. smith, r. steinacker and h. volkert (2001): the map special observing period, bull. am. meteorol. soc., 82, 637-650. businger, s., s.r. chiswell, m. bevis, j. duan, r.a. anthes, c. rocken, r.h. ware, m. exner, t. vanhove and f.s. solheim (1996): the promise of gps in atmospheric monitoring, bull. am. meteorol. soc., 77, 5-18. duan, j., m. bevis, p. fang, y. bock, s. chiswell, s. businger, c. rocken, f. solheim, t. van hoove, r. ware, s. mcclusky, t.a. herring and r.w. king (1996): gps meteorology: direct estimation of the absolute value of precipitable water, j. appl. meteorol., 35, 830-838. hugentobler, u., s. schaer and p. fridez (editors) (2001): bernese gps software, version 4.2, astronomical institute, university of berne. saastamoinen, j. (1972): atmospheric correction for the troposphere and stratosphere in radio ranging of satellites, in the use of artifi cial satellites for geodesy, geophys. monogr. ser., edited by s.w. henriksen, a. mancini and b.h. chovitz (agu, washington d.c.), 15, 245-251. thayer, d. (1974): an improved equation for the radio refractive index of air, radio sci., 9, 803-807 r i s u l t a t i di p r o v e s u m o d e l l i . p r o f i l i d i r e s i s t i v i t à a p p a r e n t e p e r c o r p i p a r a l l e l e p i p e d i r e s i s t i v i i m m e r s i i n u n m e z z o o m o g e n e o i s o t r o p o c o n d u t t i v o (results obtained through model tests. apparent resistivity profiles for resistive parallelepipeda in a homogeneous isotrope and conductive medium hurried). e . b r i z z o l a r i ( * ) r i c e v u t o il 22 luglio 1972 riassunto. — sono s t a t e realizzate p r o v e su modelli p e r d e t e r m i n a r e profili di r e s i s t i v i t à a p p a r e n t e per corpi parallelepipedi resistivi immersi in u n mezzo omogeneo isotropo c o n d u t t i v o . sono s t a t e f a t t e v a r i a r e le prof o n d i t à e le dimensioni del parallelepipedo (spessore, altezza, larghezza) a s s u m e n d o come u n i t a r i a la d i s t a n z a e l e t t r o d i c a . e s t a t o così possibile d e t e r m i n a r e l ' i n f l u e n z a delle dimensioni del p a r a l l e l e p i p e d o e della p r o f o n d i t à s u l l ' a n d a m e n t o e sui valori massimi delle a n o m a l i e . summary. — t h r o u g h model t e s t s , a p p a r e n t r e s i s t i v i t y profiles for resistive p a r a l l e l e p i p e d a h u r r i e d in a h o m o g e n e o u s isotrope m e d i u m are d e t e r m i n e d . d e p t h a n d d i m e n t i o n s of t h e p a r a l l e l e p i p e d u m (thickness, h e i g h t a n d w i d t h ) h a v e been c h a n g e d a s s u m i n g as u n i t t h e electrode spacing. i n f l u e n c e of d e p t h a n d d i m e n t i o n s of t h e p a r a l l e l e p i p e d u m on t h e a n o m a l i e s t r e n d a n d highest values are a s c e r t a i n e d . p r e m e s s e si è a v u t a occasione di effettuare una ricerca geoelettrica, con il metodo dei profili di resistività, di masse resistive in forma presumibilmente parallelepipeda, immerse in u n mezzo conduttivo. (*) assistente ordinario. i s t i t u t o di geofisica mineraria. f a c o l t à di i n g e g n e r i a di r o m a . fill) e . b r i z z o l a hi per la migliore impostazione dell'indagine occorreva scegliere le dimensioni del dispositivo da impiegare nei profili di resistività previsti per la ricerca. occorreva inoltre stabilire la distanza t r a le misure di resistività contigue. dovendo tale scelta essere e v i d e n t e m e n t e f a t t a sulla base di anomalie teoriche, è s t a t a e f f e t t u a t a u n a ricerca di eventuali s t u d i sull'argomento. è s t a t o però possibile reperire soltanto alcune informazioni di c a r a t t e r e q u a l i t a t i v o e, solo per alcuni casi particolari, anche valutazioni q u a n t i t a t i v e delle anomalie (3). si è quindi iniziato u n o studio dell'influenza delle dimensioni e della posizione del corpo rispetto al dispositivo elettrodico sulle anomalie. poiché lo studio è risultato di interesse generale vengono r i p o r t a t i nella presente n o t a i r i s u l t a t i cui si è giunti. è noto che non è s t a t a ancora t r o v a t a u n a soluzione rigorosa del problema del potenziale d o v u t o ad u n elettrodo p u n t i f o r m e in presenza di un corpo parallelepipedico di resistività differente del mezzo circostante. p e r tale ragione il problema è s t a t o a f f r o n t a t o m e d i a n t e l'uso di modelli. p e r semplicità è s t a t o considerato u n corpo a resistività molto maggiore del mezzo circostante (k = 1), di f o r m a parallelepipeda posto v e r t i c a l m e n t e al di sotto di un suolo orizzontale e con la faccia superiore parallela al suolo stesso. è s t a t o scelto il dispositivo elettrodico tipo schlumberger. si sono p e r t a n t o r i c a v a t e le anomalie in funzione dei diversi par a m e t r i che p o t e v a n o essere f a t t i variare. questi p a r a m e t r i sono (fig. .1): la p r o f o n d i t à p della superficie superiore del corpo dalla superficie del terreno, lo spessore s, l'altezza li e la larghezza 1 del corpo. l a dis t a n z a elettrodica ab è s t a t a a s s u n t a come u n i t a r i a . sono p e r t a n t o -1 i p a r a m e t r i che possono v a r i a r e i n d i p e n d e n t e m e n t e l ' u n o dall'altro per oo4 combinazioni possibili. le conseguenze di tali variazioni sulle anomalie non sono in molti casi valutabili i n t u i t i v a m e n t e . ad esempio un a u m e n t o della d i s t a n z a elettrodica ali provoca un a u m e n t o della p r o f o n d i t à di investigazione del dispositivo e quindi, c o s t a n t e la p r o f o n d i t à del corpo, si dovrebbe ottenere u n a esaltazione della anomalia. d ' a l t r a p a r t e però, lo stesso a u m e n t o della distanza elettrodica p o t r e b b e i n c r e m e n t a r e il n u m e r o delle linee di corrente che si chiudono al di sotto e l a t e r a l m e n t e al corpo resistivo se la sua larghezza e altezza non sono tali da impedirlo. un a u m e n t o dello spessore, in riferimento alla distanza elettrodica, provoca un a u m e n t o di valori massimi dell'anomalia e u n aliarr i s u l t a t i d i p r o v e su m o d e l l i . p r o f i l i d i r e s i s t i v i t à e c c . 6 1 7 fig. 1 p i a n t a , sezione e a s s o n o m e t r i a . g a m e n t o della zona in cui è rilevabile l'anomalia. p e r t a n t o , diminuendo il g r a d i e n t e dell'anomalia, nel caso che i terreni superficiali non siano omogenei nel senso della traslazione del dispositivo, si p o t r e b b e r o confondere le variazioni laterali di resistività con l'effetto della presenza di u n a massa a n o m a l a o viceversa. g e n e r a l i t à le prove su modelli sono s t a t e e f f e t t u a t e con l ' a p p a r e c c h i a t u r a sperim e n t a l e d e l l ' i s t i t u t o di geofisica mineraria già descritta da g r a n d i n e t t i (2). è s t a t o impiegato u n dispositivo di tipo schlumberger con elett r o d i di corrente di r a m e ed elettrodi di potenziale di platino. la distanza elettrodica ab è s t a t a fissata in 12 cm. l ' a c q u a che f u n g e v a da mezzo c o n d u t t i v o era s t a t a resa tale con l'aggiunta di nacl in q u a n t i t à sufficiente a p o r t a r e la resistività della soluzione a t t o r n o ai 3 o h m m . il corpo anomalo ad a l t a resistività è s t a t o realizzato m e d i a n t e elementi parallelepipedi di plexiglass tali da p e r m e t t e r e , m e d i a n t e loro combinazione, la realizzazione di corpi di dimensioni diverse. e . brizzolati p r i m a di iniziare le prove con corpo anomalo è s t a t o verificato se, m e d i a n t e la s c h e r m a t u r a della vasca, fosse s t a t o minimizzato l'effetto delle p a r e t i ; si è p o t u t o c o n s t a t a r e che il campo non veniva influenzato dalla presenza del bordo della vasca se non per d i s t a n z a del centro del dispositivo dalla p a r e t e inferiore ad ab. g e o m e t r i a nella fig. 1 è r a p p r e s e n t a t a s c h e m a t i c a m e n t e in sezione, p i a n t a ed assonometria la geometria del corpo e del dispositivo. il corpo viene i n d i v i d u a t o m e d i a n t e il suo spessore s (dimensione nel senso della traslazione del dispositivo); la larghezza l (dimensione ortogonale alla traslazione); l'altezza h. i n o l t r e ciascuna p r o v a viene c a r a t t e r i z z a t a dalla p r o f o n d i t à p della superfìcie superiore del parallelepipedo dal pelo dell'acqua. l a posizione relativa del dispositivo e del corpo resistivo è indiv i d u a t a m e d i a n t e la distanza x dei centri del corpo e del dispositivo. il parallelepipedo resistivo è s t a t o m a n t e n u t o , in t u t t e le prove, ortogonale alla direzione della traslazione e con la superficie superiore parallela alla superficie del liquido. le resistività a p p a r e n t i na m i s u r a t e sono s t a t e riferite alla resistività qi della soluzione per evitare errori connessi a possibile variazione della resistività della soluzione stessa. p r o v e e r i s u l t a t i u n a p r i m a serie di q u a t t r o prove è s t a t a realizzata fissando le dimensioni del corpo (s = 0 , 5 0 0 4 5 ; 7« = 0,833abz = 1 , 6 6 6 4 5 ) e facendo v a r i a r e la p r o f o n d i t à p della superficie superiore del parallelepipedo dal pelo dell'acqua. sono s t a t e realizzate p r o v e p e r p p a r i a 0,083 ab, 0,125 ab, 0,250 ab, 0,375 ab. i r i s u l t a t i di queste p r o v e sono r i p o r t a t i nella fig. 2 nella quale in ascisse compare la distanza xiab del centro del dispositivo dal centro del disturbo e in ordinate il r a p p o r t o qalqi. successivamente m a n t e n e n d o costante la p r o f o n d i t à (p = 0 , 1 2 5 a b e p = 0 , 2 5 0 4 5 ) , l'altezza {li = 0,833.45) e la lunghezza (z = 1 , 6 6 6 4 5 ) è s t a t o v a r i a t o lo spessore (s p a r i a 0 , 0 1 6 4 5 , 0 , 1 6 6 4 5 , 0 , 3 3 3 4 5 , 0 , 5 0 0 4 5 , 1 , 0 0 0 a b , 2 , 0 0 0 a b ) . si sono così o t t e n u t e r i s p e t t i v a m e n t e le curve di fig. 3 e 4 per p 0,125 ab e 0,250 ab. r i s u l t a t i d i p r o v e su m o d e l l i . p r o f i l i d i r e s i s t i v i t à e c c . 6 1 9 è s t a t a f a t t a poi v a r i a r e l'altezza li del parallelepipedo (0,106.-15, 0 , 2 7 7 a b , 0,555.47?, 0 , 8 3 3 a b , 1,666.4.6, 3,000.4.5) m a n t e n e n d o fissa la p r o f o n d i t à (p = 0,125 ab), lo spessore e la larghezza (s = 0,500.47? e 1 — 1,666.4/?). i profili di resistività così ricavati sono s t a t i rappres e n t a t i nella fig. 5. j j j | profilo tlelirico 9 | — i f1— * — i p^. ̂ p / ? 2 » / ^^1.666 a 9 / v d 0,833 ab 0.500ab ^ p / ? 2 » / ^^1.666 a 9 / (7) profondila • q) • p ® • p © • p =0.083 ab = 0.125 ab = 0.250 ab = 0.375 ab © \ f-^, • ! • — . i — i — , — , — . — , — 0.5 7" r— r̂. 5 x « ab f i g . 2 profili di r e s i s t i v i t à . p r o f o n d i t à p variabile. spessore, altezza, larghezza c o s t a n t i . i n ascisse la d i s t a n z a del c e n t r o del dispositivo dal cent r o del parallelepipedo resitivo (xiab), in o r d i n a t a il r a p p o r t o f r a la resistività a p p a r e n t e p„ e la r e s i s t i v i t à ^ della soluzione c o n d u t t i v a oa/qi. infine m a n t e n e n d o costanti gli altri p a r a m e t r i (p — 0,125.a /?, s — 0,500a7?,7i = 0,833 ab) sono s t a t i f a t t i assumere alla lunghezza l seguenti valori: 0,277.42?, 0,555.47?, 0 , 8 3 3 a b , 1 , 6 6 6 4 £ , 3,333.47?. i ris u l t a t i così o t t e n u t i compaiono nella fig. 6. si esaminano ora i profili di resistività r i c a v a t i allo scopo di individuare l'influenza dei p a r a m e t r i p, s, li, l sui valori massimi e sull ' a n d a m e n t o dei profili di resistività. l a lìg. 2 evidenzia la grande influenza della p r o f o n d i t à del corpo sui valori massimi dell'anomalia che esso produce. p e r meglio indiv i d u a r e questa dipendenza nella fig. 7 sono s t a t i r i p o r t a t i in o r d i n a t a i valori massimi dell'anomalia e in ascissa la p r o f o n d i t à ovviamente riferita alla distanza elettrodica ab. è possibile così n o t a r e come i valori massimi a u m e n t i n o con l ' a n d a m e n t o iperbolico verso le prof o n d i t à più piccole per diminuire a l t r e t t a n t o r a p i d a m e n t e verso le p r o f o n d i t à maggiori. p e r p r o f o n d i t à maggiori di 0,4-0,5 ab l'effetto del parallelepipedo resistivo risulta p r a t i c a m e n t e trascurabile. («c ab ® sptisori j 0.016 ab i 0.166 ab i 0.333 ab j 0.500 ab p r o f i l i d i r e s i s t i v i t à . s p e s s o r e s v a r i a b i l e . p r o f o n d i t à ( 0 , 1 2 5 4 5 ) , a l t e z z a , l a r g h e z z a c o s t a n t i . i n ascisse l a d i s t a n z a del c e n t r o del d i s p o s i t i v o d a l c e n t r o del p a r a l l e l e p i p e d o r e s i s t i v o x/ab, in o r d i n a t a il r a p p o r t o « , , / ^ . p r o f o n d i t a p = 0 , 1 2 5 a b f i g . 4 profili di r e s i s t i v i t à . spessore s variabile. p r o f o n d i t à (0,250.1 li), altezza, larghezza c o s t a n t i . in ascisse il r a p p o r t o xiab, in o r d i n a t a il r a p p o r t o qa/ev a b fig. 5 profili di r e s i s t i v i t à . altezza h variabile. p r o f o n d i t à , spessore, larghezza c o s t a n t i . in ascisse il r a p p o r t o x/ab-, in o r d i n a t a il r a p p o r t o g„/ql. f i g . (i profili di r e s i s t i v i t à . l a r g h e z z a l variabile. p r o f o n d i t à , spessore, a l t e z z a c o s t a n t i . i n ascisse il r a p p o r t o x/ab, in o r d i n a t a il r a p p o r t o (?„/(? k. b r i z z o l a r i p e r contro la. larghezza dell'anomalia non sembra dipendere in modo d e t e r m i n a n t e dalla p r o f o n d i t à p. molto influente a p p a r e essere anche lo spessore s del parallelepipedo resistivo sui valori massimi e sulla f o r m a dell'anomalia. l a fig. 8 1 spettare t =0.500 ab alitila h =0,033 ab 0 0 0 0 : profondità/ab fig. 7 valori m a s s i m i del r a p p o r t o oa/£>i in f u n z i o n e della p r o f o n d i t à del corpo a n o m a l o ( p r o f o n d i t à r i f e r i t a alla d i s t a n z a i n t e r e l e t t r o d i c a ab). spessore, altezza, larghezza c o s t a n t i . f i g . 8 valori massimi del r a p p o r t o qalqi in f u n z i o n e dello spessore del corpo a n o m a l o (spessore riferito alla d i s t a n z a i n t e r e l e t t r o d i c a ab) per p r o f o n d i t à di 0 , 2 5 0 a b e 0,125.4b. altezza e larghezza c o s t a n t i . nella quale i valori massimi dell'anomalia sono r i p o r t a t i in funzione dello spessore sia per p = 0 , 1 2 5 4 5 sia per p = 0 , 2 5 0 4 5 , dimostra come l'anomalia sia n o t e v o l m e n t e influenzata dallo spessore soprat u t t o p e r valori compresi t r a 0,300 e 2,000 4 5 . l a massima variazione sembra aversi p e r valori compresi t r a 0,5 e 1 , 5 4 5 . r i s u l t a t i d i p r o v e su m o d e l l i . p r o f i l i d i r e s i s t i v i t à e c c . 6 2 3 inoltre, a differenza di quanto avveniva per la profondità, lo spessore influenza grandemente la distanza a cui si risente la presenza del corpo resistivo, come appare dalle figure 3 e 4. ad esempio nel caso p = 0,250.45 e s = 2,000.47? l'anomalia assume valori inferiori agli errori sperimentali solo per le distanze superiori a 2.4 b. fig. 9 valori massimi del r a p p o r t o qa/ei in f u n z i o n e dell'altezza del corpo anomalo (altezza riferita alla d i s t a n z a i n t e r e l e t t r o d i c a ab). profondità, spessore, larghezza costanti. ________— profondità p = 0.125 ab spessore s = 0.500 ab altena h = 0. 833 ab fig. 10 valori massimi del r a p p o r t o qa/(>i in funzione della larghezza del corpo anomalo (larghezza riferita alla distanza interelettrodica ab). p r o f o n d i t à , spessore, altezza costanti. la fig. 9, nella quale sono riportati i valori massimi delle anomalie in funzione dell'altezza li, m e t t e in evidenza che modeste variazioni dei valori anomali si hanno per valori di li compresi grosso modo t r a 0,3 e 0 , 8 a b . 11 valore dell'anomalia risulta p r a t i c a m e n t e indipendente dall'altezza per valori di li maggiori di 0,8.4 b mentre per valori inferiori a 0,3.47? l'anomalia decresce rapidamente. come risulta poi dalla fig. 5 l'altezza non modifica praticamente la larghezza dell'anomalia. e . r r i z z o l a r i p e r valori di l inferiori a 0 , 5 . 4 5 l'anomalia a u m e n t a r a p i d a m e n t e con l-, tale a u m e n t o risulta m e n o notevole per valori di l compresi t r a 0,5 e 1,5, per divenire molto modesto per valori di i superiori a 1,5 (fig. 10). anche in questo caso, come per l'altezza, l ' a u m e n t o della larghezza (fig. fi) amplia molto poco l'area nella quale è possibile misurare l'anomalia. ( *0 i n c l u s i o n i le prove realizzate p e r m e t t o n o di v a l u t a r e l'influenza dei singoli p a r a m e t r i s u l l ' a n d a m e n t o delle anomalie e sui valori massimi. un a u m e n t o della distanza interelettrodica ab provoca, come previsto, un a u m e n t o dell'anomalia. i n f a t t i in fig. 7 è possibile n o t a r e come a l l ' a u m e n t a r e di ab diminuisca il r a p p o r t o p/ab con conseg u e n t e a u m e n t o dei valori massimi dell'anomalia. d ' a l t r a p a r t e però dalla fig. 8 si p u ò rilevare come u n a u m e n t o della distanza ab provochi a p a r i t à di spessore u n a diminuzione della anomalia. se però il r a p p o r t o s/ab resta superiore a 0,5 (per p — 0,250.42?) ovvero ad 1 (per p = 0,125.1 b) la diminuzione dell'anomalia è r e l a t i v a m e n t e c o n t e n u t a (fig. 8). occorre inoltre considerare che l ' a u m e n t o della distanza elettrodica dà luogo ad u n a u m e n t o della larghezza delle anomalie: i n f a t t i tale larghezza (escludendo le p a r t i marginali spesso non rilevabili) p u ò assumersi in p r i m a approssimazione p a r i allo spessore del corpo più la distanza elettrodica. un a u m e n t o eccessivo di q u e s t ' u l t i m a può allargare oltre il necessario l'anomalia con il rischio di non riuscire a distinguere gli effetti singoli di più masse vicine e v e n t u a l m e n t e presenti. è necessario anche considerare cosa avviene dei valori massimi dell'anomalia, quando, a u m e n t a n d o la distanza elettrodica ab, diminuiscono i r a p p o r t i hi ab (fig. 9) e l/ab (fig. 10). i n e n t r a m b i i casi l ' a n o m a l i a decresce al diminuire del r a p p o r t o : per l'altezza è però sufficiente m a n t e n e r e valori superiori o uguali a 0,3 per non provocare u n a brusca diminuzione dell'anomalia; per la larghezza, per contro, il r a p p o r t o non deve scendere al di sotto di 0,6. l'influenza dell'altezza e della larghezza diviene trascurabile q u a n d o si superino rispettivam e n t e i limiti di 0 , 7 . 4 5 e 1,5.45. si deve quindi concludere che nella scelta della distanza interelettrodica bisogna realizzare il migliore compromesso t r a le influenze c o n t r a s t a n t i della, p r o f o n d i t à da u n a p a r t e e dello spessore altezza e larghezza d a l l ' a l t r a (con maggior riguardo per lo spessore). r i s u l t a t i d i p r o v e su m o d e l l i . p r o f i l i d i r e s i s t i v i t à e c c . 6 2 5 b i b l i o g r a f i a ( 1 ) g o u d s w a r d \ \ \ , 1 9 5 7 . on the effect of the tank wall material in geoelectrical model experiments. « geopli. p r o s p e c t i n g », v, 272. ( 2 ) g r a n d i n e t t i m . , 1 9 6 8 . risultati di prove su modelli relative ad una nuova disposizione elettrodica per la ricerca di corpi di limitate dimensioni. « boll. geof. teor. a p p i . », x , 39. (3) iieiland c. a., 1932. advances in technique and application of resistivity and potential drop ratio methods in oil prospecting. « geophysics », iii, 90. r i s t a m p a t o nel 1947 in: « e a r l y geophysical p a p e r s of t h e society of e x p l o r a t i o n geophysicists », 420. ( 4 ) s a r t o r i r., 1955. la vasca elettrolitica. a t t i convegno i n t e r n o . « i modelli nella tecnica », venezia, acc. n a z . lincei. u l t e r i o r e s t u d i o s u l l e s e s s e d e l l a g o d i b o l s e n a d . di filippo richiami teorici. — c h r y s t a l , p e r l o sviluppo della sua t e o r i a sulle o s c i l l a z i o n i l i b e r e , c o n s i d e r a i l l a g o c o m e u n c a n a l e finito di s e z i o n e v a r i a b i l e con l a p r o f o n d i t à , m a n o n i n m a n i e r a b r u s c a . e g l i p o n e l ' a s s e x s u l l a s u p e r f i c i e del l a g o , l u n g o l a l i n e a di m a s s i m a prof o n d i t à e l ' o r i g i n e ad u n a e s t r e m i t à del l a g o stesso, l'asse y o r i z z o n t a l e e l ' a s s e z v e r t i c a l e , ed i n o l t r e i n d i c a con s ( : r ) l ' a r e a di u n a g e n e r i c a sezione n o r m a l e all'asse x, e con b(x) l a c o r r i s p o n d e n t e a m p i e z z a alla s u p e r f i c i e l i b e r a . c o n s i d e r a i n f i n e t r a s c u r a b i l i , dopo u n t e m p o t, gli s p o s t a m e n t i di u n a p a r t i c e l l a , soggetta ad u n a c e r t a azione, l u n g o l'asse y e c h i a m a con e e £ r i s p e t t i v a m e n t e gli s p o s t a m e n t i orizzontali s e c o n d o l'asse x e q u e l l i v e r t i c a l i secondo z. s c r i t t a l ' e q u a z i o n e della c o n t i n u i t à e t e n u t o c o n t o d e l l ' e q u a z i o n e f o n d a m e n t a l e d e l l ' i d r o d i n a m i c a di e u l e r o , con la c o n d i z i o n e c h e gli s p o s t a m e n t i v e r t i c a l i r i s u l t i n o t r a s c u r a b i l i r i s p e t t o a l l a p r o f o n d i t à del l a g o , p e r v i e n e a l l a s e g u e n t e e q u a z i o n e del m o v i m e n t o ( c h e p o r t a il suo n o m e ) : 8 tò v" dove u=s(x) | è i l v o l u m e del l i q u i d o c h e a t t r a v e r s a s(x) a causa dello s p o s t a m e n t o o r i z z o n t a l e , e v= j b(x)dx è l ' a r e a della supeii ficie del l a g o c o m p r e s a tra l ' o r i g i n e e l a s e z i o n e c o n s i d e r a t a . i n o l t r e egli c o n s i d e r a l a f u n z i o n e a {v) —s(x)b(x) e r i p o r t a t i i v a l o r i di v(x) sulla ascissa e i c o r r i s p o n d e n t i a (v) s u l l ' o r d i n a t a , t r a c c i a l a « c u r v a n o r m a l e » o c u r v a di c h r y s t a l c h e r a p p r e s e n t a l a sezione del l a g o r i d o t t o . p o i c h é u n a sessa è u n a o s c i l l a z i o n e s t a z i o n a r i a , l a « u » è u n a f u n z i o n e p e r i o d i c a e si può m e t t e r e n e l l a f o r m a u — p sen n ( t — r) dove p è u n a f u n z i o n e di v. 4 0 6 i). di f i l i p p o boi sera lago d i b o l s e n a s c a l a 0 1 2 3 u l t e r i o r e s t u d i o s u l l e s e s s e del lago di holsena 4 0 7 nel caso che si tratti (li un lago di ampiezza « b » costante, di sezione rettangolare ma di profondità variabile, tale c h e s(x) = bh(x), supponendo che si abbia a che fare con oscillazioni stazionarie, la [ 1 ] diviene 6 g h (x) 0 . [ 2 ] se poi si considera il caso particolare di un lago parabolico completo, simmetrico e concavo, e l'origine degli assi si sposta al c e n t r o del lago e si indica con h la massima profondità e con a la semilunghezza del lago ridotto i fig. 2), allora a a 0 5 a' fig. 2 li ix) = h • 1 [ 3 ] p o n e n d o ir == — e c a ph la [ 2 ] assume la f o r m a (v p (1 — iv-) fc p = 0 6 u" e questa, introducendo le funzioni delle stesse, ci dà per « u » l'espressione ; h (1 — iv-) = u = ) ac (r, w) + bs(c, w) | sen n (t — t) [ 4 ] nella (piale a c b sono costanti a r b i t r a r i e e c{c, tv) e s(c, w) sono date dalle c ( r , w) = l • s (c, w) = u" c .. c ( c — 1 2 ) 4 u i u . 2 1 • 2 x 3 • 4 [5] 2 3 k 3 + r ( c 2 3 ) i 2 3 x 4 5 4 0 8 i). di f i l i p p o g l i spostamenti v e r t i c a l i r i s u l t a n o di conseguenza l = — — = — — — = — — j a c'{c,w) + b s'(c, w) | sera n ( t — n 8 x a 5 w a ' dove c'(c,w) e s'(c,w) sono l e derivate delle f u n z i o n i delle sesse rispetto a tv. d a l l a [ 4 ] risulta c h e u. = 0 p e r tv = + 1 e dato c h e sussistono le r e l a z i o n i c ( c , — 1) = c(c, 1) e s(c, — 1) — —s(c, 1), d e b b o n o essere soddisfatte l e seguenti condizioni l i m i t e a c ( c , l ) + b s(c, 11 = 0 a c ( c , l ) — b s ( c , 1 ) = 0 t e n e n d o conto della p r o p r i e t à delle soluzioni di una e q u a z i o n e l i n e a r e del secondo ordine, in questo caso si può scrivere c(c,w) s'(c,w) — c ' ( c , tv) s(c,w) — 1 ; r i s u l t a quindi c h e c ( c , l ) e s ( c , 1) n o n possono essere n u l l e contempor a n e a m e n t e e p e r t a n t o o b = 0 , c ( c , l ) = 0 , o p p u r e a — 0 , s(c, 1) = 0 , quando c assume uno dei valori non f r a z i o n a r i 1 * 2 ; 2 3 ; 3 4 ; ... v.(v -j1), a l t e r n a t i v a m e n t e le c ( c , i^) e s(c,w) si a n n u l l a n o o si riduc o n o a f u n z i o n i i n t e r e razionali di w. i n queste condizioni è p e r t a n t o possibile c a l c o l a r e i valori delle ampiezze degli spostamenti v e r t i c a l i £ c h e s a r a n n o dati dalle relazioni t = c ( c 2 s i ; w ) «era ra2s_1 (t — t) a [6] g ^ = s' (c2s ; w) sen n„s (t — t) a r i s p e t t i v a m e n t e per le ampiezze delle sesse dispari e pari. u l t e r i o r e s t u d i o s u l l e s e s s e del lago di b o l s e n a 4 0 9 i periodi relativi si possono ottenere f a c i l m e n t e dalla 7 e ricordando c h e si h a t = , f o 7 ì h con cy — v (v + 1) dove v = 1, 2, 3 . . . applicazione al lago di bolseiui. — i n un precedente lavoro sono stati determinati gli elementi delle c a r a t t e r i s t i c h e i d r o d i n a m i c h e del lago di b o l s e n a con i metodi di h i d a k a e di defant. p o i c h é la curva di chrystal è m o l t o prossima ad una parabola (vedi fig. 3) e la f o r m a del lago assimilabile ad un rettangolo, è stato possibile applic a r e per la prima volta il metodo proposto da chrystal, nel caso di un lago p a r a b o l i c o completo. nella t a b e l l a i sono r i p o r t a t i i dati adoperati n e l primo lavoro e c h e sono stati rilevati dalla carta del lago di b o l s e n a dell'atlante dei 4 1 0 i). di f i l i p p o l a g h i i t a l i a n i di d e a g o s t i n i , ove s o n o state t r a c c i a t e 2 5 se z i on i , dis t a n t i l ' u n a d a l l ' a l t r a 5 0 0 m e n o r m a l i a l l a l i n e a a l l a superficie, assunta c o m e asse d e l l e x. tabella i sez. d t( * ) a: o (v) n. km k m 2 kn : k m 3 0 0,0 0,00000 —56,81275 0,000000 1 0,5 2,21950 —54,59325 0,203290 2 1,0 5,47150 5 1 , 3 4 1 2 5 0,992445 3 1,5 8,87825 —47,93450 1,564414 4 2,0 12,38825 — 4 4 , 4 2 4 5 0 2,303440 5 2,5 16,37375 —40,43900 4,526775 6 3,0 20,75475 —36,05800 6,301200 7 3,5 25,19375 — 3 1 , 6 1 9 0 0 7,493048 8 4,0 29,36450 —27,44825 8,862042 9 4,5 34,37125 —22,44150 8,701275 10 5,0 39,53300 —17,27975 8,900899 11 5,5 44,74500 —12,06775 9,592736 12 6,0 50,19700 — 6,61575 11,226815 13 6,5 55,64250 — 1,17025 11,534864 14 7,0 60,01850 + 3,20575 10,961528 15 7,5 66,00475 + 9,19200 10,490775 16 8,0 71,38075 + 14,56800 10,094545 17 8,5 76,62775 + 19,81500 10,329237 18 9,0 81,77150 + 2 4 , 9 5 8 7 5 9,271762 19 9,5 86,63125 + 2 9 , 8 1 8 5 0 8,122936 20 10,0 91,33350 + 3 4 , 5 2 0 7 5 7,598414 21 10,5 95,77900 + 38,96625 6,754010 22 11,0 100,41600 + 4 3 , 6 0 3 2 5 5,958452 23 11,5 104,33900 + 4 7 , 5 2 6 2 5 4.870289 24 12,0 108,08525 + 5 1 , 2 7 2 5 0 3,468000 25 12,5 111,47475 + 5 4 , 6 6 2 0 0 1,258687 26 13,0 113,62550 + 5 6 , 8 1 2 7 5 0,000000 s e l ' o r i g i n e delle c o o r d i n a t e si p o n e i n s u p e r f i c i e i n c o r r i s p o n denza del c e n t r o della c u r v a n o r m a l e (fig. 2), l a s e m i l u n g h e z z a del l a g o r i d o t t o è a = 5 6 , 8 1 2 7 5 k m 2 e se si c o n s i d e r a n o l e v(x) r e l a t i v e ad ogni sezione e le c o r r i s p o n d e n t i o(v), si h a n n o , a p p l i c a n d o la [ 3 ] , 27 r e l a z i o n i d a l l e q u a l i , con i l m e t o d o dei m i n i m i q u a d r a t i , si p e r v i e n e alla r e l a z i o n e h{x) = 10,972 — 0 , 0 0 3 4 * 2 da cui r i s u l t a p e r la m a s s i m a p r o f o n d i t à del l a g o r i d o t t o i l v a l o r e h — 10,972 u l t e r i o r e s t u d i o s u l l e s e s s e del lago di b o l s e n a 4 1 1 nella fig. 3 la parabola trovata è posta in c o n f r o n t o con la « curva n o r m a l e » ed è messa in evidenza la buona approssimazione prevista. periodi. calcolato il valore di h e noto «, prendendo per l'accelerazione di gravità il valore g = 0,009803 km/sec-', tenendo conto del valore di c per la sessa c b e si considera, dalla relazione t = 2 ,-t i <\. g h con c v = v (v —f— 1 ) e v = 1, 2, 3 ... è f a c i l e d e t e r m i n a r e i vari periodi. i n f a t t i , nell'espressione del periodo, posto i valori detti di a, h, g e con cj = 1 -2, si ottiene per il periodo della uninodale il valore t i = 12m,83 m e n t r e per e;> — 2 • 3 il periodo della b i n o d a l e risulta to = 7r a,41 ponendo invece c:ì = 3 4 per la trinodale t3 = 5 m ,24 e infine per la quadrinodale, con f4 = 4 ' 5 , si perviene al valore t 4 = 4'»,06 come si può vedere dallo specchietto seguente, i p r i m i tre valori trovati vanno d'accordo, anche se leggermente i n f e r i o r i , con quelli ottenuti con i due metodi precedentemente applicati e altresì con i valori medi rilevati dalle registrazioni di palazzo e riportati n e l lavoro citato. unidodale 12'",83 13 ,05 13 ,15 13 ,23 p e r i o d i binodale trinodale quadrinodale 7'", 41 7 ,90 8 ,20 7 ,92 5m,24 5 ,40 5 ,35 5 ,40 4'", 06 3 ,80 metodo di chrystal „ „ hidaka „ " defant medi osservati registrazioni palazzo. 4 1 2 i). di f i l i p p o l ' u l t i m o valore ottenuto è m o l t o prossimo al periodo 3"',8 rilevato c o m e m e d i a di due valori dalla registrazione di palazzo a b o l s e n a , valore elle n e l l ' a l t r a nota si dava appunto c o m e p r o b a b i l e periodo della quadrinodale. spostamenti verticali e nodi. g l i spostamenti v e r t i c a l i lungo la sezione del lago sono stati ottenuti applicando l e f o r m u l e [ 6 ] e ponendo in esse i v a l o r i di c relativi ad ogni sessa. p e r l ' u n i n o d a l e i n f a t t i jl t, = — c' (c, , ir) sen n, (t — t) a dove per ei = l " 2 c ( c i , tv) = 1 — tv2 e r i c o r d a n d o c h e tv = — , si h a in definitiva sen (t — t ) a 2 a x posto come al solito £ = 1 all'estremo del lago si sono ottenuti, p e r l e varie sezioni, i valori delle ampiezze riportati n e l l a t a b e l l a it. l ' u n i n o d o cade n e l centro del lago ridotto in corrispondenza di una distanza di 6,634 k m da marta. p e r la b i n o d a l e gli spostamenti verticali sono dati dalla relazione g 'q = s' (c„ , w) sen n., (t — t) a dove per c 2 = 2 3 si h a dalla seconda delle [ 6 ] s (c2 , tv) = tv — it 3 ; derivando e sostituendo nella espressione di £ , risulta b ! ar — 3 x° , q = i sen n„ (t — t ) i binodi si h a n n o se £ = 0 e precisamente per or — 3 * 2 = 0 da cui r i s u l t a n o <z n ( = 3,367 km e d " 2 = 9,817 k m , sempre contati a partire da marta. u l t e h i o k e s t u d i o s u l l e s e s s e del lago di b o l s e n a 4 1 3 tabella i i " ' unidodale binodale trinodale quadrinodale in. 0 -1-1 + 1 + 1 + 1 1 (1,961 0,885 0,777 0,645 2 0,904 0,725 0,489 + 0 , 2 3 0 3 0,844 0,568 0,236 —0,077 4 0,782 0,417 + 0,022 0,282 5 0,712 0,260 —0,166 0,402 6 0,635 + 0 , 1 0 4 0,313 0,426 7 0,557 —0,035 0,404 0,367 8 0,483 0,150 0,443 0,262 9 0,395 0,266 0,438 —0,104 10 0,304 0,361 0,386 + 0 , 0 6 6 11 0,212 0,432 0,295 0,215 12 0,116 0,480 0,171 0,325 13 + 0 , 0 2 1 0,499 —0,031 0,373 14 —0,056 0,495 + 0 , 0 8 4 0,363 15 0,162 0,461 0,232 0,280 16 0,256 0,401 0,342 + 0 , 1 4 7 17 0,349 0,318 0,417 —0,016 18 0,439 0,211 0,447 0,186 19 0,525 —0.087 0,426 0,326 20 0,608 + 0 , 0 5 4 0,349 0,413 21 0,686 0,206 0,222 0,421 22 0.767 0.384 + 0 , 0 2 1 0,316 23 0,837 0,550 —0,209 —0,106 24 0,902 0,722 0,484 + 0 , 2 2 3 25 0,962 0,889 0,783 0,653 26 1,000 1,000 1,000 1.000 s e si sostituisce c:ì = 3 4 n e l l a p r i m a delle [ 6 ] , si h a c ( c : ì , iv) = 1 6 iv2 -{5 tv3 e q u i n d i , p e r la t r i n o d a l e , l ' a n d a m e n t o degli s p o s t a m e n t i v e r t i c a l i risulta d a l l a £ — — ( — 12 a" x + 2 0 * 3 ) scn n., (t — t ) a* i tre p u n t i n o d a l i si a v r a n n o al solito risolvendo l a 2 0 x3 — 12 a 2 * = 0. i due t r i n o d i e s t r e m i r i s u l t a n o così d i s t a n t i da m a r t a rispettivan i e n t r e c l t m = 2 , 0 5 2 k m e d . , m = l l , 0 5 1 k m , m e n t r e q u e l l o c e n t r a l e coinc i d e con q u e l l o della u n i n o d a l e . 4m d. di f i l i p p o p e r la quadrinodale, dalla seconda delle [ 6 ] si ottiene l ' a n d a m e n t o delle ampiezze verticali, considerando c\ — 4 5 : allora tv s (c4 , tv) = — (1 iv*) (3 7 „ * ) , o s' ( c 4 , w) = — (3 — 3 0 w— 3 5 tv*) , 3 perciò l = _ (3 a 4 — 3 0 cr x-f3 5 x1) sen nt (f —t) . 3 a 5 i nodi si h a n n o risolvendo la b i q u a d r a t i c a f r a parentesi e risultano così u b i c a t i : = 1 , 3 5 5 k m d 3 i v = 4 , 8 6 3 k m <i;,iv = 8 , 4 5 2 k m d 4 i v = 1 1 , 6 8 6 k m , dall'estremo sud del lago. nella fig. 4 sono rappresentati gli andamenti delle p r i m e q u a t t r o sesse del lago di b o l s e n a e risulta evidente c h e verso l'estremità del f i g . 4 lago si accorcia la l o r o lunghezza d'onda. i n o l t r e può essere notato c h e le ampiezze verticali delle sesse sono maggiori all'estremità c h e non al c e n t r o del lago o in vicinanza di esso. a parità di ampiezza agli estremi, n e l l a parte c e n t r a l e del lago h a ampiezza m i n o r e la sessa di lunghezza d'onda maggiore. u l t e r i o r e s t u d i o s u l l e s e s s e del lago di bolsena 4 1 5 l a p a r t i c o l a r i t à della f o r m a di questo lago spiega in parte la frequenza e l'ampiezza rilevante delle sesse, tanto da essere osservate a n c h e ad o c c h i o ; sicché, diventa sempre più interessante eseguire una nuova serie di registrazioni, con vari limnografi opportunamente distribuiti intorno al lago. roma — istituto nazionale di geofisica —• maggio 1953 riassunto si applica per la prima volta, dato vandamento della curva normale del lago di bolsena, il metodo proposto da chrystal per la determinazione degli elementi delle caratteristiche idrodinamiche di un lago jmtrabolico completo. si determinano gli andamenti degli spostamenti verticali, i. nodi e i periodi delle prime quattro sesse. i risultati concordano con quelli ottenuti, in un precedente lavoro, con i metodi di hidaka e di defant e con i dati rilevati dalle registrazioni ottenute da palazzo. summary for the first time, given the behavior of the normal curve of the lake of bolsena, the metliod proposed by crystal for the determination of the characteristic hydrodynamic elements of a complete parabolic lake has been applied. the behavior of the vertical displacements, the nodes and the periods of the first four seiches have been determined ivith this method. the results agree with those obtained in a preceding tvork employing the methods of hidaka. and defant, and ivith the data evaluated from the registrations obtained by palazzo. b i b l i o g r a f i a chrystal g . : on the hydrodinamical theorv oj seiches. transactions of the r o y a l society of edinburgh x l i (1905), x l v (1906), x l v i (1907-8). palazzo l. : studi limnologici sul lago di bolsena. atti del v congresso geografico italiano. voi. ii, sez. 1 (1904). caloi p . : le sesse del lago di garda. annali di geofisica. voi. i (1948). di f i u r r o d . : le sesse del lago di bolsena. annali di geofisica. voi. i v (1951). deduzione delle concentrazioni di uranio e torio nelle sostanze radioattive con il metodo fotografico g. imbò ~ l. casertano l nella nota preccflente (1) si è ,lata la ,listribuzionc statistica delle particelle u raccolte su una lastra foto!!rafica, in hmzione della proiezione sulla lastra del percorso della particella ncll'cmlll~ione; tale ,listribuzione è stata stllfliata sia per le particelle a emesse {la una sola specie di nucleo radioattivo, sia per quelle provenienti da tutti !!ii clementi ili o!!ni singola fami~lia in equili!trio rullioattivo. ora prima (ii dare come si era fletto le distribuzioni pcr le particelle emesse flall'insieme ,ielle tre famiglie, c in funzione anche dci rappol10 in pe~o della concentrazione flell'uranio (c,) rispetto a quella (id torio (c"hl in mollo da poter ricavare que~to rapporto dalle distrihuzioni ebe si ottengono in pratica, si è voluto riesaminare il problema trattato per un completamento flal punto di vista analitico, tenenflo pre~cnte, a tale seopo, i lavori sueee~~ivamente pubblicati sul· l'ar!!omento. quasi contemporaneamente al nostro lavoro, difatti, buttiar ( .. ) c hautenllans e) hanno trattato lo ste~so problema presso a poco nell'identico modo ~e!!uito da noi: unica differenza sostanziale è che es~i per evitare la soluzione analitica in {f fiella !!iù riferita equazione: , x -=rsen{\--tgh [i] (flove r è la proiezione (lei percorso nell'emul~ione di una parti. cella u emessa sotto l'an!!olo {} fla un volume elementare posto a distanza x; r il percorso nell'aria; !j.l il coefficiente di frenamento della sostanza radioattiva e i-l.~ quello (iell'emul~ione) e quinfli flell'integrale ehe permette di ricavare il totale flelle particelle (( rac(.) si è vivament e grati al doti. bt:nur percllé ci 1m inviato copia ciclostilat;> della "ua ic5i. .. 2 c . i m b ò l . c a s e r t a n o colte con proiezione maggiore di r, ricorrono alle soluzioni grafiche; mentre noi preferimmo — come esposto — trovare la distribuzione statistica delle particelle a emesse da uno strato di spessore x = u i r come limite delle distribuzioni di un numero sempre crescente di strati infinitamente sottili posti a distanze comprese fra 0 e ui r. i risultati ottenuti nei due modi non potevano non concordare: in particolare i valori dati precedentemente nella tabella i i i per le tre famiglie radioattive in equilibrio confrontati con quelli delle loro tabelle i l a , i l b , ile presentano al massimo un scostamento intorno all'1%. più recentemente si è occupato ancora dell'argomento il van stijvendael ( 3 ) che, impostando il problema in modo solo formalmente diverso, è arrivato a una formula analoga a quella data dall'evans ( 4 ) per le lunghezze effettive dei percorsi nell'emulsione anziché delle proiezioni. tale formula però può ovviamente essere ricavata anche partendo dall'impostazione data da noi e da buttlar e hautermans. 2. — tenendo conto di quanto detto nella precedente nota si può facilmente vedere come il numero dn di particelle <i emesse dal volume elementare dsdx posto alla distanza x dall'emulsione, raccolte sulla lastra e aventi proiezione maggiore di r, essendo -—-— l'angolo solido corrispondente a tì, è dato d a : dove cos fh e cos sono radici della [ 1 ] per i considerati valori di x e r. poiché la [1] può anche porsi sotto la forma: dn— —• (cos —cos tì,) dsdx n [ 2 ] h, costì= u1 li cos t ì — r — [ao sen si può ottenere, per r costante: [ 3 ] con facili considerazioni (v. fig. 1) si ricava che il numero n di particelle a emesse da uno strato di spessore x, raccolte sulla lastra d e d u z i o n e d e l l e concentrazioni di uranio e torio e c c . 3 10° 20° 30° 1 0 ° 50° 6 0 ° 70° 80° 9 0 ° . 0 f i g . 1 v a r i a z i o n e di f ( 0 ) (ordinate) in funzione di 0 (ascisse) per valori discreti di r/ftsr. per unità di superficie e aventi proiezioni del percorso nell'emulsione maggiore di r è dato da: sen i) | e le coppie dove si è posto /({)•)= (1 — costì')) sen2tt \)'0) e (i^j {>2) sono radici della [ 1 ] in corrispondenza del posto valore di r per le distanze dei due piani che limitano lo strato considerato. in particolare poiché si suppone la sostanza a contatto con l'emulsione si avrà sempre: ft0 = arcsen i r * c . i m 8 0 l . c a s e r t a n o quando si considera , r = j i i r si avrà: «o n iv 2 [ 5 ] questo è da considerarsi un caso limite. infatti per x > it, /ma [ 5 ] risulta sempre valida, perché la parte della sostanza a distanza maggiore di hir non dà alcun contributo in quanto i raggi non penetrano nell'emulsione. invece per a;<li,ì? la [ 5 ] è valida soltanto se si considerano proiezioni r maggiori della proiezione massima corrispondente alla superficie superiore dello strato di spessore x. in generale per spessori le distribuzioni si debbono ricavare dalla [ 4 ] giovandosi dei valori tì, e {}„ deducibili con facilità dal grafico riportato in fig. 2. h r 1.0 0.8 0,6 0,4 0,2 / > / l / • / 01 . — 15 ^ x \ /ds^. — £ ^ a . 5 x \\ 0 0.2 0a 0.6 0 . 8 1.0 f i g . 2 v a r i a z i o n e di c o s 0 (ascisse) in funzione di x/[ìlr ( o r d i n a t e ) per valori discreti di r/(i2r. d e d u z i o n e d e i . l e c o n c e n t r a z i o n i di u r a n i o e torio e c c . 5 nella tabella i sono date alcune distribuzioni. in tale tabella, tra parentesi, su ogni colonna è riportato il totale di particelle a raccolte sulla lastra per i vari spessori (cioè il numero di particelle a con proiezione maggiore di zero): si è posto uguale a 100 quello relativo allo spessore x = jt, r. tabella i \ x \ i<7« 0.0 0.2 0.4 0.6 0.8 1.0 r \ (0.0) (36) (64) (84) (96) (100) uor \ 0.0 0.0 100.0 100.0 100.0 100.0 100.0 0.2 0.0 92.8 96.3 76.2 66.7 64.0 0.4 0.0 68.6 56.3 42.9 37.5 36.0 0.6 0.0 44.4 25.0 19.0 16.7 16.0 0.8 0.0 11.1 6.3 4.8 4.2 4.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 distribuzioni statistiche delle particelle r», in funzione della proiezione (r/ | j r ) sulla lastra del percorso nell'emulsione per diversi valori dello spessore (x/itir) dello strato della sostanza e m i t t e n t e . qualche apparente anomalia della variazione della percentuale delle particelle a con proiezione maggiore di un determinato valore, in funzione dello spessore, trova la piena spiegazione nel comportamento — accennato sopra — dipendente dalla validità o meno della [ 5 ] , per .r < ( j j / { , a seconda che r è maggiore o minore della r massima, caratteristica dello spessore considerato. altro spessore limite può essere considerato quello infinitamente sottile (invece di quello con x = 0) e cioè quello per il quale si ritiene che tutti i nuclei emittenti siano a contatto con l'emulsione. di questo facilmente se ne può trovare la distribuzione. se n' è il numero di particelle a emesse per secondo e per grammo di sostanza, la frazione ( //), rispetto al totale di particelle emesse, con proiezione minore di r è data da : 1 — cos il> p=n' 1 o n' essendo il numero totale delle particelle « raccolte per grammo di sostanza, il la densità della sostanza, e cos ì) = 1 / 1 — ( ] , il r \ u j r ) n « ' 6 c . i m b ò l . c a s e r t a n o mero n di particelle a raccolte per cm2 di superficie e con proiezione maggiore di r sarà : a 2 z 1 (tufi [6] u v in pratica sono prevalentemente considerati solamente i due spessori limiti, e cioè quello infinitamente sottile e l'altro per cui si ha x — \xlr: in quanto al primo vi si ricorre per particolari esigenze e al secondo per la facilità con la quale esso può essere ottenuto (basta che si abbia x > ( j j r ) . si osserva che mentre sul numero totale di tracce si ha nel primo caso un valore inferiore che nel secondo (al quale corrisponde il massimo); sulle distribuzioni invece si ha per 10 spessore infinitamente sottile un netto predominio della percentuale di percorsi a più grandi proiezioni. p e r queste ragioni per l'insieme delle famiglie radioattive vengono prese in considerazione solo distribuzioni che si hanno per questi due spessori. mentre però per l'infinitamente sottile il campo di variazione delle proiezioni sarà esteso da r = 0 a r = l-^r, per a" = 11k sarà limitato da 0,60 < r < [ i j r , in quanto si sono volute, in questo secondo caso, trascurare le più piccole proiezioni. 3. — per le famiglie in equilibrio radioattivo, indicando con n* 11 numero di particelle a emesse al secondo e per gr di sostanza da ognuno degli elementi in equilibrio, e con d la densità della sostanza, le [5] e [6] diventano rispettivamente: n= — i x . d z . f l — ' [ 5 ' ] 4 \ ^ r j -^•ì'hùì con le sommatorie estese a tutti quegli elementi della famiglia il cui percorso nell'aria, r h è maggiore dell'equivalente in aria, — , della proiezione r. f's essendo i capostipiti delle famiglie dell'uranio e dell'attimo due isotopi dello stesso elemento chimico (rispettivamente t/238 e u235) l'insieme delle due famiglie sarà considerato come una famiglia unica (uranio-attinio) con due sostanze madri, cui corrispondono evidentemente due valori diversi di n*. in particolare per i due valori di n* si sa che un grammo di uranio (ui -)uh -)ac-u) è conid e d u z i o n e d e i l e c o n c e n t r a z i o n i di u r a n i o e t o r i o e c c . 7 posto da 0,9928 gr di vi (c/238) e di 0,0072 gr di ac-u (us35), essendo trascurabile il contributo dell'i/// (u234). perciò in base alle costanti di disintegrazione può ritenersi che 1 gr di uranio, in condizione di equilibrio, emette al secondo 24-986 particelle u, delle quali 12210 provenienti da ognuno dei due isotopi un3s e u231, e 566 provenienti dal terzo c / 0 3 5 . p e r la famiglia del torio invece è da considerare che dal rapporto di ramificazione del thc si ricava che di 100 particelle u provenienti dalla disintegrazione del thc, 35 sono emesse dal thc e 65 dal thc'. p e r il numero di particelle « emesse al secondo e per gr di sostanza per ogni elemento in equilibrio della famiglia del torio si ricava il valore di 4100. tabella i i x r x . 0 x-=hi r u r a n i o a t t i n i o t o r i o u r a n i o a t t i n i o t o r i o r (7t< yv — e p-g a di u 7c iv p " g di t h % r m n — p -u[ a ' g di v % n m 8 p.gditli % n.o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 50821 49308 44458 34196 16126 7837 4425 3146 385 0 0 100.0 97.0 87.5 67.3 31.7 15.4 8.7 6.2 0.8 0.0 0.0 12300 12023 11129 8844 7122 3967 1933 951 799 580 c 100.0 97.7 90.5 71.9 57.9 32.3 15.7 7.7 6.5 4.7 0.0 0.60 0.64 0.68 0.72 0.76 0.80 0.84 0.88 0.92 0.96 1.00 1428 966 526 243 70 1 0 0 0 0 0 100.0 67.7 36.9 17.1 4.9 0.9 0.0 0.0 0.0 0.0 0.0 959 744 585 448 329 228 146 82 37 9 0 100.0 77.6 61.0 46.7 34.3 23.8 15.2 8.6 3.8 1.0 0.0 d i s t r i b u z i o n i statistiche per le famiglie radioattive in e q u i l i b r i o . sia gli spessori <x) della sostanza che le p r o i e z i o n i ( r ) del percorso n e l l ' e m u l s i o n e sono state r i f e r i t e al persorso n e l l ' a r i a del t h c ' ( r = c m 8.57) che è il massimo per le partic e l l e (i. nella tabella i i si riportano le distribuzioni ricavate: i valori di ——, per i successivi confronti, sono stati sempre riferiti al massimo ms percorso per le particelle a (uguale a cm 8,57 per thc'). 5. — dalle colonne 2 e 4 per e da quelle 7 e 9 per x = r si possono ricavare le distribuzioni per qualsiasi determinato rap8 c . i m b ò l . c a s e r t a n o porto c„/cth delle concentrazioni in gr per gr dell'uranio (c„) rispetto a quelle, sempre in gr per gr, del torio (cti.). infatti, per x cs: 0, la colonna 2 dà senz'altro la distribuzione per c„ /cth = 0 0 e la 4 quella per c n / c , h = 0 . mentre dalla somma delle due colonne si può ottenere l'altra per cu/cth — 1; e così via. e analogamente può dirsi per lo spessore x=ulr. nelle tabelle i i i e i v sono riportati i valori ottenuti. tabella i i i cu r ct h 0.00 0.20 0.25 0.50 1.00 2.00 4.00 5.00 oo u.2r 0.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 0.1 97.7 97.4 97.4 97.3 97.2 97.1 97.1 97.1 97.0 0.2 90.5 89.1 89.0 88.5 88.1 87.8 87.7 87.6 87.5 0.3 71.9 69.8 69.6 68.8 68.2 67.8 67.6 67.5 67.3 0.4 57.9 46.1 44.6 40.3 36.8 34.6 33.2 32.9 31.7 0.5 32.3 24.6 23.7 20.9 18.7 17.2 16.4 16.2 15.4 0.6 15.7 12.5 12.2 11.0 10.1 9.5 9.1 9.0 8.7 0.7 7.7 7.0 6.9 6.7 6.5 6.4 6.3 6.3 62 0.8 6.5 3.9 3.6 2.6 1.9 1.4 1.1 1.0 0.8 0.9 4.7 2.6 2.3 1.5 0.9 0.5 0.3 0.2 0.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 distribuzioni statistiche per diversi valori del rapporto cu /cth per lo spessore infinitamente sottile della sostanza. i l valore di r / ( i = r è riferito al percorso nell'aria del t h c ' ( r = c m 8,57). tabella i v \ cu r cth (tor \ 0.00 0.20 0.25 0.50 1.00 2.00 4.00 5.00 r o 0.60 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 0.64 77.6 75.3 74.9 73.3 71.6 70.1 69.1 68.8 67.7 0.68 61.0 55.5 54.4 50.7 46.5 42.9 40.3 39.7 36.9 0.72 46.7 39.9 38.7 34.0 28.9 24.5 21.3 20.5 17.1 0.76 34.3 27.6 26.3 21.8 16.7 12.3 9.1 8.4 4.9 0.80 23.8 18.3 17.4 13.7 9.6 6.0 3.5 2.9 0.9 0.84 15.2 11.7 11.1 8.7 6.1 3.8 2.2 1.8 0.0 0.88 8.6 6.6 6.2 4.9 3.4 2.1 1.2 1.0 0.0 0.92 3.8 3.0 2.8 2.2 1.6 1.0 0.6 0.5 0.0 0.96 1.0 0.7 0.7 0.5 0.4 0.2 0.1 0.1 0.0 1.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 distribuzioni statistiche per diversi valori del rapporto c.u /c th per lo spessore x = ) i i r della sostanza. i l valore di r/li2r è riferito al percorso nell'aria del thc'|i(r = cm 8,57). d e d u z i o n e d e i l e c o n c e n t r a z i o n i di u r a n i o e t o r i o e c c . 16 per poter calcolare il rapporto c0/cth ; nei grafici delle figg. 3 e 4 sono riportati — rispettivamente per e per * =: r — gli andamenti, per valori discreti di —— , delle variazioni delle percentuali (ascisse) con il variare del rapporto in esame (ordinate). riportando su ogni curva il valore della percentuale ottenuta sperimentalmente può calcolarsi facilmente il valore del rapporto, ricorrendo eventual5 4 5 2 1 0 f i g . 3 variazioni delle percentuali di particelle a raccolte con proiezioni maggiori di un determinalo valore r / ^ r in funzione del rapporto delle concentrazioni uranio-torio per lo strato infinitamente sottile. le curve a tratto continuo danno l e variazioni delle percentuali i n funzione del rapporto cv/crìi (ascisse crescenti verso destra); mentre quelle a tratteggio danno le variazioni in funzione del rapporto cxh/cu (ascisse crescenti verso sinistra). mente anche a una media ponderata dei valori ricavati per diverse proiezioni. per rendere illimitato il campo di variazione del rapporto c,/cth nei grafici sono riportati anche gli andamenti del rapporto inverso cth/cd ; in tal modo si può scegliere tra i due rapporti quello che più conviene prendere in considerazione. 10 c . i m l t ò l . c a s e r t a n o f i g . 4 variazioni delle percentuali di particelle u raccolte con proiezioni maggiori di un determinato valore r/n^r in funzione del rapporto delle c o n c e n t r a z i o n i uranio-torio per lo strato di spessore x = |air. l e curve a tratto c o n t i n u o danno le variazioni delle percentuali in funzione del rapporto cc/'crh (ascisse crescenti verso destra); mentre quelle a tratteggio danno le variazioni in funzione del rapporto cth/cc (ascisse crescenti verso sinistra). i grafici delle figg. 5 e 6 permettono di ricavare, in funzione del rapporto calcolato distintamente, il numero di particelle <( raccolte per secondo e per unità di superficie relativamente allo strato infinitamente sottile nonché il numero di particelle la cui proiezione equivalente in aria risulta maggiore di cm 5,142 (per lo strato di spessore x = ui ri. quindi il numero effettivamente raccolto, per unità di superficie e per secondo, di particelle u con proiezione analoga a quella considerala per ognuno dei due strati, permette di ricavare anche le singole concentrazioni degli elementi uranio e torio. infatti i calcoli sono stati condotti ammettendo : c„ (gr. per gr) -fc l h (gr. per grj = 1 (gr. per gr) [ 7 ] d e d u z i o n e d e i l e c o n c e n t r a z i o n i di u r a n i o e t o r i o e c c . 1 1 <53.500 o | l 5 a ^ 5 j 5 2 | o f i g . 5 v a r i a z i o n e di n/d ( n num e r o t o t a l e di p a r t i c e l l e « r a c c o l t e p e r s e c o n d o e per unità di superfic i e , d densità della sostanza) in funz i o n e del r a p p o r t o delle c o n c e n t r a z i o n i u r a n i o t o r i o p e r lo strato infin i t a m e n t e sottile. la curva a tratto c o n t i n u o dà la variazione di n/d in f u n z i o n e del rapporto c r / c i li (ascisse crescenti verso d e s t r a ) ; m e n t r e q u e l l a a tratteggio dà la v a r i a z i o n e in f u n z i o n e del r a p p o r t o c ' h / c u (ascisse c r e s c e n t i verso sinistra). 5 « 5 2 1 0 f i g . 6 v a r i a z i o n e di n/ti,d ( n num e r o di p a r t i c e l l e a r a c c o l t e per sec o n d o e p e r unità di superficie c o n p r o i e z i o n e r / u = r 5 ^ 0 , 6 0 ; tii coeffic i e n t e di f r e n a m e n t o e d densità della sostanza) in f u n z i o n e del rapporto d e l l e c o n c e n t r a z i o n i u r a n o t o r i o per lo strato di spessore x = uir. la curva a tratto c o n t i n u o dà la variaz i o n e di n/mid in funzione del rapporto c ti /c th (ascisse crescenti verso destra) ; m e n t r e q u e l l a a tratteggio dà la v a r i a z i o n e in f u n z i o n e del r a p p o r t o ctii/cu (ascisse crescenti verso sinistra). ora se è stato trovalo cv/cth — p e si pone uguale a q il rapporto tra il totale ili particelle « raccolte e quello che si sarebbe ricavato se fosse effettivamente valida la [ 7 ] si ottiene facilmente: c p<ì e c c< 1+p 1+p basandosi pertanto sull'ipotesi dell'esistenza dell'equilibrio radioattivo con questa nota si è voluto indicare il metodo che si può seguire per conoscere, dalle distribuzioni statistiche delle parlicelle «, in funzione della proiezione sulla lastra del percorso nell'emulsione, il valore non solo del rapporto ma anche delle singole concentrazioni dell'uranio e del torio relative alle sostanze esaminate. napoli — osservatorio vesuviano — dicembre 1951. 1 2 c . i m b ò l . casertano riassunto facondo seguito a una nota precedente si danno le formule per le distribuzioni statistiche di particelle a raccolte, su una lastra fotografica, in funzione della proiezione sulla lastra del percorso nell'emulsione delle particelle, per spessori di sostanza compresi fra 0 e quello limite rappresentato dal range delle particelle nella sostanza in esame. in base a queste formule si danno le stesse distribuzioni sia per l'insieme delle famiglie dell'uranio e dell'attimo, sia per quella del torio. le distribuzioni vengono anche date per fissati valori del rapporto delle concentrazioni in j>eso dell'uranio (c l ) rispetto a quelle del torio (cth^ infine viene indicato il metodo per calcolare non solo il rapporto c „ / c t b ma anche le singole concentrazioni cv e c , i , partendo dalle distribuzioni statistiche, ottenute sperimentalmente, in funzione delle proiezioni sulla lastra dei percorsi nell'emulsione delle particelle a emesse dalla sostanza esaminata. summary in pursuance of a previous note, formulae are here given to obtain the statistical distribution of a particles as gathered 011 photographic plates, in function of the projection on the piate of the particle trajectory inside of the emulsion, for thickness values varying from zero to the lim.it thickness represented by the particle range in the substance under examination. starting from these formulae, distributions have been indicated both for the tvhole group of uranium and actinium, and for the thorium group. distributions have been also indicated for fixed ratio values of uranium weight concentra ti ons ( c c ) in respect of thorium weight concentrations ( c t h ) . further has been described the method used for the calculation not only of rations c d / c t i , , but also of the single concentrations c „ arni c t h , as experimentally deduced from the projections on the piate oj the trajectories ili the emulsion of the u particles ivich are emitted by the substance under examination. b i b l i o g r a f i a ( ! ) i m b ò e c a s e r t a n o : a n n . g e o f . v o i . i l i , 4 7 5 , 1 9 5 0 . ( 2 ) b u t t l a r : tesi di laureti; buttlar e hautermans: geoc. et cosm. ac. v o i . i i , 43, 1951. (3) van stijvendael: b u i . c e n . p h y s . n u c l . n. 28, 1951. ( 4 ) evans: p h y s . r e v . , 45, 29, 1934. jt^c. mum^l_ ti'. .. layout 6 annals of geophysics, 56, 6, 2013, s0680; doi:10.4401/ag-6252 s0680 geomorphic evidence of recent activity along the vodice thrust fault in the ljubljana basin (slovenia) – a preliminary study petra jamšek rupnik1,*, lucilla benedetti2, frank preusser3, miloš bavec1, marko vrabec4 1 geological survey of slovenia, ljubljana, slovenia 2 aix marseille université, centre européen de recherche et d’enseignement des géosciences de l’environnement, aix-en-provence, france 3 stockholm university, department of physical geography and quaternary geology, stockholm, sweden 4 university of ljubljana, faculty of natural sciences and engineering, department of geology, ljubljana, slovenia abstract we investigated two prominent, ~e-w trending scarps in quaternary sediments, located close to the town of vodice in the ljubljana basin (central slovenia). by using detailed geomorphological analysis of the scarps, field surveying, and structural observations of deformed quaternary sediments, we conclude that the scarps are the surface expression of a n-dipping thrust fault that has been active during the quaternary. from optically stimulated luminescence and infrared stimulated luminescence dating of deformed quaternary sediments we estimate a slip rate of 0.1 to 0.3 mm a-1 in the last 133 ka. using the published empirical fault-scaling relationships, we estimate that an earthquake of magnitude 5.9 to 6.5 may be expected on the vodice thrust fault. the fault may, therefore, present a major seismic hazard for the densely populated and urbanised region of central slovenia. 1. introduction the ljubljana basin (figure 1) is the most densely populated and a highly urbanized region of slovenia. among the natural hazards affecting the basin and its surroundings, such as flooding, landsliding and earthquakes [e.g. komac and ribičič2006, jemec and komac 2011, komac and zorn 2011, bavec et al. 2012], seismic hazard is probably the least investigated, despite the ongoing seismic activity in the area with earthquake magnitudes frequently reaching 3-4 (based on the earthquake catalogue of živčič [2009] there were on the average 2.6 earthquakes of magnitude 3-4 per year over the last 100 years). the seismic hazard is further increased by the unconsolidated quaternary sedimentary infill of the basin, reaching up to 280 m of thickness, which may significantly enhance site effects during earthquakes [gosar et al. 2010]. the largest recorded event in the basin is the destructive 1895 ljubljana earthquake with macroseismic magnitude (mm) 6.1 [ribarič 1982] and maximum intensities of viii-ix ems-98 [cecić 1998]. despite the recognized hazard, the active faults capable of producing strong magnitude earthquakes are still poorly investigated. for example, the fault responsible for the 1895 earthquake is still unknown. the ljubljana basin is interpreted to have originated as a pull-apart basin in a releasing overstep between two nw-se-striking dextral faults, the sava fault to the north and the žužemberk fault to the south [e.g. vrabec and fodor 2006]. gps measurements suggest that the sava fault is currently active with the slip rate of ~1 mm a-1 [vrabec et al. 2006]. smaller ~e-w oriented reverse faults that displace quaternary sediments in the basin [verbič2006] may indicate a recent change in the deformational regime from transtensional subsidence to transpression. geological observations, earthquake focal mechanisms, and geodetic measurements suggest that the nw-se-striking dextral faults and ~e-w-striking reverse faults may be active in the current regional stress regime with ~n-s oriented axis of maximum horizontal compression [poljak et al. 2000, 2010, vrabec and fodor 2006, verbič2006, weber et al. 2010]. one such presumably quaternary active reverse fault is the vodice fault, located 15 km north of ljubljana. two prominent fault scarps are clearly visible and were previously interpreted as terrace risers of the sava river [žlebnik 1971], later as possible normal fault scarps [vrabec 2001], and more recently as reverse fault scarps [verbič 2006]. the aim of this study was to investigate whether the vodice fault could be currently active and to estimate its possible seismogenic potential. article history received october 30, 2012; accepted april 15, 2013. subject classification: geomorphology, active tectonics, earthquake geology, seismic hazard, ljubljana basin, vodice fault. special issue: earthquake geology 1.1. seismotectonic setting the studied area is located in the northeasternmost tip of the adria-europe convergent margin, in the contact between the eastern southern alps and the dinarides. axis of maximum compression is oriented in approx. n-s direction. e-w-striking thrust faults in ne italy and nw-se-striking dextral faults across slovenia, such as the idrija fault, are most probably seismically active and reflect the present tectonic regime of that region (figures 1a, 1b) [ribarič 1982, poljak et al. 2000, benedetti et al. 2000, galadini et al. 2005, kastelic et al. 2008, burrato et al. 2008, bavec et al. 2012, basili et al. 2013]. this is also supported by the slip-vectors derived from earthquake focal mechanisms evidencing dextral slip on nw-se fault-planes and thrust movements on e-w fault-planes [bajc et al. 2001, zupančičet al. 2001, kastelic et al. 2006, 2008, ložar stopar and živčić2007, 2008, 2011, 2012, čarman et al. 2010]. only few events were attributed to a specific fault, among which the 1998 ms 5.7 [bajc et al. 2001], and 2004 mw 5.2 earthquakes [kastelic et al. 2006] that both occurred on the nw-se right-lateral strike-slip ravne fault (figure 1a, focal mechanism solutions 1 for 1998 and 2 for 2004 earthquake). the strongest recorded historical earthquake in our study area was the idrija 1511 earthquake with an estimated mm 6.8 [ribarič1982] (figure 1b). its exact source and mechanism remain debated, but it most probably occurred along the nw-se striking idrija fault in western slovenia [fitzko et al. 2005, camassi et al. 2011]. 2. geomorphic and structural observations surface expression of the vodice fault was investigated using topographical maps at scales 1 : 5,000 and 1:10,000, the 5 m resolution digital elevation model (dem 5 m), aerial imagery, and 2.5 m resolution spot satellite images in stereo pairs. we performed geomorphological mapping of the alluvial surfaces in the area of the vodice fault and analysed the characteristics of the drainage network. a series of topographic profiles parallel to and across the scarps was extracted from dem 5 m. additional field observations were carried out to investigate deformations of alluvial surfaces.in the vodice area, the general southward dip of the alluvial surface of the ljubljana quaternary basin is perturbed by linear features oriented in ene-wsw direction, which are perpendicular to the general trend of fluvial terrace risers in the area, specifically those bordering the modern n-s flowing sava river (figure 2). two nearly parallel scarps are visible, with the southern one being more prominent and here referred to as the southern vodice (sv) scarp. eastward, the northern vodice (nv) scarp joins with the southern one. the height of the sv scarp varies from 25 to 5 m, whereas the nv scarp varies in height from 18 to 3 m. both scarps are the highest in the central part with heights decreasing towards their eastern and western tips (figure 3). at the top of both scarps, the alluvial surface that was originally sloping southwards is today sub-horizontal or even dipping gently to the north. the top of alluvial surface is strongly incised by recent rivers and creeks. several dry valleys and perched valleys are objamšek rupnik et al. 2 figure 1. (a) instrumental seismicity of the se alps and nw part of the dinarides (anss catalogue, 1976-2012) and earthquake focal mechanisms [bajc et al. 2001, zupančič� et al. 2001, kastelic et al. 2006, 2008, ložar stopar and živčič� 2007, 2008, 2011, 2012, �čarman et al. 2010]. 1 focal mechnism of the 1998 ms 5.7 [bajc et al. 2001], and 2 – focal mechanism of the 2004 mw 5.2 earthquake [kastelic et al. 2006]. square indicates the location of figure 1b. topography is srtm 90 m, available from the global land cover facility (http://glcf.umd.edu/data/). (b) main active faults of the ljubljana basin [modified after buser 2009]. historical earthquake epicentres with magnitude above 3.9 are from živčič� [2009]. earthquake magnitudes are obtained from macroseismic data. locations of historical earthquake epicentres are not well constrained. isoseismal contours of the 1895 ljubljana earthquake according to lapajne [1989]. shaded relief obtained from processing of dem 5 m (public information of slovenia, the surveying and mapping authority of the republic of slovenia, dem 5, 2006). square indicates the location of figure 2a. 3 served near the scarps (figure 3). folding of quaternary sediments in the lokarje clay pit, located in between the two sv and nv scarps, 400 m north of the town of vodice, was reported by drobne et al. [1960] and šifrer [1961] (see figure 4). they describe an asymmetric anticline with the southern limb dipping 40° towards sse and the northern limb dipping around 10° towards nnw. the enewsw oriented anticline axis is parallel to the strike of the sv scarp. at the western tip of the sv scarp, where it crosses the sava river, we observed a fault cutting through quaternary conglomerates (figure 5). the fault plane dips 35° to the north. drag folds in conglomerate layers indicate reverse offset along this fault plane. our geomorphic and structural observations suggest that the sv scarp is the surface expression of an emergent fault. the position, vergence, and orientation of the lokarje anticline are consistent with thrust/reverse faulting along the sv fault scarp. since the distance between the nv and the sv scarps is less than 2 km and because both scarps merge east of vodice, we interpret the nv scarp as also the geomorphic expression of the same thrust fault that splays in two branches at the surface. we constrain the near-surface dip of the fault plane to about 35°n, consistent with our observations along the sava bank and with the geometry of the anticline in lokarje. documented deformations of quaternary sediments and of alluvial surfaces clearly demonstrate that the vodice fault has been active since at least the deposition of quaternary conglomerates. 3. dating in order to estimate the age of the offset surface north of the sv fault scarp, one sample was taken at 3 m depth in the fine-grained sediments from 5 m deep excavation. the sampling site is located in the immediate vicinity of a lokarje clay pit, north of the sv fault scarp, figure 2. (a) surface expression of the vodice fault as seen on the dem 5 m (public information of slovenia, the surveying and mapping authority of the republic of slovenia, dem 5, 2006). locations of figure 2b and figure 5 are indicated as well as the location of the dating sampling site. (b) field picture of the southern vodice scarp (view from the top of the scarp towards southwest). see figure 2a for location of the picture. figure 3. series of topographic profiles extracted from dem 5 m (public information of slovenia, the surveying and mapping authority of the republic of slovenia, dem 5, 2006) across (profiles 14, 2.1 and 2.2), and parallel to (profiles i-iv) the vodice scarps. profiles 2.1 and 2.2 show a close-up on both scarps, where northward sloping is indicated with purple arrows on top of the scarps. locations of profiles 2.1 and 2.2 are indicated in the profile 2. profiles i and iii present the scarps crest, and profiles ii and iv present the scarps base. red arrows in the profiles i and iii indicate the positions of perched valleys, and black crosses indicate anthropogenic excavations due to highway construction. vodice thrust fault activity at the w bank of alluvial breakthrough e of vodice (see figure 2 for location). layers of clay, sand and conglomerate dipping around 10° towards nnw are most probably part of the northern limb of the anticline described in the former clay pit. layers of clays were deposited in lacustrine environment, intermediate conglomerate layers were deposited by fluvial processes. dating was carried out using both optically stimulated luminescence (osl) of quartz and infrared stimulated luminescence (irsl) of k-feldspar. standard chemical pretreatments (hcl, h2o2, na-oxalate) were applied to sand-sized grains (100-150 μm) and heavy liquid separation was used to isolate the quartz and kfeldspar fraction. quartz was additionally etched in 40 % hf for 60 min. determination of the equivalent dose was carried out by the single-aliquot regenerative dose (sar) protocol (quartz: preheat 230°c for 10 s, hoya u340 detection filter; feldspar: preheat 290°c for 10 s, l.o.t. oriel 410 nm interference filter; 2 mm aliquots with ca. 100 grains). the central age model was applied to calculate the mean of repeated measurements. concentration of dose rate relevant elements was determined using high-resolution low-level gamma spectrometry. all relevant data is summarised in table 1. those samples yielded an osl age of 134 ± 15 ka and an irsl age of 132 ± 12 ka for the upper alluvial surface. the ages are very consistent using two different dating approaches. the sediments were deposited by fluvial and lacustrine processes and then consecutively uplifted due to repeated fault movements. because fluvial terraces e-w oriented are found nowhere else along the sava river and because the river was in its most aggradational part, the preservation of those alluvial surfaces is probably of tectonic origin. thus, their emplacement should slightly predate their offset. since the osl and irsl methods allow determining the age of emplacement, those ages yield a maximum value for the offset. moreover, since those samples were taken several meters from the surface, to further constrain the uplift rate, we are currently in the process of determining the surface exposure ages of the deformed surfaces using cosmogenic nuclides. 4. slip rate and seismogenic potential the presence of the presumably seismogenic vodice thrust fault in the ljubljana basin, located only 15 km north of the capital of slovenia, constitutes a significant seismic hazard for the population and a threat for both human lives and the economy of the country. to better assess this seismic hazard, we evaluated the fault slip rate and estimated its possible seismogenic potential. we estimate the maximum cumulative displacement (d) of the fault by assuming it matches the height of the scarp (h) (taken from topographical maps at 1 : 5,000 scale) and the average fault dip (α�) as observed in the field: d = h / sin ( α�). the h on the southern fault segment is 15.5 ± 5 m and the observed dip, α, is 35° n. thus the yielded d is 27.0 ± 8.7 m. note that this d is probably overestijamšek rupnik et al. 4 figure 5.the fault plane of the southern vodice fault exposed on the sava river bank. (a) picture of exposure. (b) fault plane is indicated with red line and fault drag folds are indicated with black lines. the fault plane dips 35° towards n. see figure 2a for location. figure 4. picture and profile of an asymmetric anticline in quaternary sediments in former lokarje clay pit [šifrer 1961]. see figure 2 for location. the orientations of picture and the profile are inferred from the clay pit map in šercelj [1961]. an approximate vertical scale is inferred from the description of the thicknesses of sediment layers in the profile. 5 mated since we have use simple trigonometric relations and a dip for the fault that might be higher at depth. on the other hand, considering scarp diffusional processes [e.g. avouac and peltzer 1993], the offsets might have been higher previously, however to remain conservative we assume this is a maximum value for the d. using osl and irsl ages of sediments (a) in lokarje clay pit as the maximum ages of deformed surface we can estimate the preliminary late quaternary slip rate (sr) of the fault: sr = d / a. this would yield a first estimate for the vodice fault sr of 0.1 to 0.3 mm a-1 over the last 133 ka. the average magnitude (am) expected on that fault was estimated as follows, using the empirical scaling relationships [wells and coppersmith, 1994], with the surface rupture length (srl in km) assumed at this stage to be equal to the length of the sv fault scarp, i.e. 10 km: am = a + b * log (srl) where a and b are coefficients calculated for the reverse faults (table 2). average displacement (ad) per event was calculated using the empirical relationships [wells and coppersmith, 1994] as follows: log (ad) = a + b * log (srl) taken the coefficients a and b for reverse faults as given in table 3. furthermore, a possible recurrence time of large earthquakes (rt) was calculated using our estimated sr and an ad as follows: rt = ad / sr. according to these empirically derived equations [wells and coppersmith 1994], with a srl of 10 km, the vodice fault could trigger an earthquake of m 5.9 to 6.5 with a coseismic ad of 0.1 to 0.9 m. assuming a constant sr, we estimate a rt from 300 to 9,000 yrs. note that the longer the rt interval is, the stronger the associated expected earthquake could be. but even if assuming a coseismic ad of 0.1 m and the shortest rt interval, the expected earthquake magnitude could theoretically still reach 6.1. 5. discussion and conclusions preliminary results of our investigation confirm that the two scarps near vodice are the surface expression of an emergent active thrust fault, striking ene-wsw and dipping about 35° towards n. assuming that the osl and irsl ages yielded from the deposits of the upper deformed surface represent the maximum age for this surface, we estimate that this surface has been offset for vodice thrust fault activity k th u d-q d-f ed-q ed-f age-q age-f sample n (%) (ppm) (ppm) (gy ka-1) (gy ka-1) (gy) (gy) (ka) (ka) lok1 32/6 0.52±0.01 1.45±0.05 2.90±0.13 1.26±0.01 1.74±0.01 174.7±13.5 237.6±13.1 134±15 132±12 table 2. regressions of surface rupture length (srl in km) and moment magnitude (m) [wells and coppersmith 1994]. table 3. regressions of surface rupture length (srl in km) and average displacement (ad in m) [wells and coppersmith 1994]. table 1. summary data of osl and irsl dating giving the number of replicate measurements (n), the concentration of dose rate relevant elements (k, th, u), dose rate (d), equivalent dose (ed), and resulting ages for both quartz (q) and feldspar (f). equation slip type coefficients standard deviation m = a + b * log (srl) reverse 0.28 a b 5.00 1.22 equation slip type coefficients standard deviation m = a + b * log (srl) reverse 0.28 a b -0.60 0.31 about 15 m over the last 133 ka. at this stage, disregarding surface denudation that might be higher in the uplifted hangingwall compartment, and possible syntectonic aggradation in the footwall compartment of the sv fault, the height of the scarp may present the minimum uplift value. assuming that all the surface offsets of 15.5 ± 5 m determined from the topographic maps is due to an uplift along a thrust fault and a fault dip of 35° n, the cumulative displacement of 27.0 ± 8.7 m is a maximum value. from maximum age of the surface, minimum uplift and maximum displacement the estimated vodice slip rate of 0.1 to 0.3 mm a-1 over the last 133 ka probably presents the minimum value. this estimated slip rate is within the range of similar values documented in the southeastern part of the alpine orogenic system, e.g. the broni-stradella thrust fault in northwestern apennines with an estimated uplift-rate of 0.3 mm a-1 since late pleistocene [benedetti et al. 2003] and thrusts in the eastern southern alps, e.g. thiene-udine thrust system in ne italy with estimated vertical sr of 0.1 to 1 mm a-1 [castaldini and panizza 1999, benedetti et al. 2000, galadini et al. 2005, burrato et al. 2008]. moreover, estimated slip rates of the right-lateral strike-slip faults in the vicinity of the investigated area are also within that range, idrija and ravne faults at 0.03 to 2 mm a-1 [burrato et al. 2008, kastelic and carafa 2012] and the sava fault at 0.5 to 1.5 mm a-1 [vrabec et al. 2006]. we estimate that the vodice fault could possibly trigger earthquakes with magnitudes from 5.9 to 6.5. the vodice fault should therefore be monitored considering its location in the densely populated area and its proximity to slovenia’s capital ljubljana. the surface expression of the fault, sharp and with a strong slope, suggests abrupt displacements associated with seismic events [e.g. benedetti et al. 2003, mccalpin 2009]. our preliminary theoretical estimates suggest that possible return time interval for destructive m 5.9 to 6.5 earthquakes would range from 300 to 9,000 yrs. the only known historical destructive seismic event in the area is the 1895 ljubljana mm 6.1 earthquake [ribarič 1982]. the macroseismic intensity dataset [cecić1998] of this earthquake shows that the maximum intensities of viii – ix ems-98 were recorded south of the fault, between vodice and ljubljana. since the vodice fault is a reverse fault, maximum intensities would be expected in the hangingwall of the fault, north of vodice. therefore, unless the maximum intensities distribution is biased by the population, the vodice fault was probably not the source of the 1895 earthquake, but since the time interval between two strong seismic events on the fault may be longer than the span of historical record in the region (about 1000 years), paleoseismological investigations will be required to unravel its possible seismic history. in the future, we will investigate the fault using geophysical survey in order to better understand the geometry of the fault at depth, and further detailed analysis of the surface deformations and their chronology will be done to better constrain the fault displacement and its slip rate. acknowledgements. this work was funded by the slovenian research agency, through the project l1–2383 seismotectonic model of the ljubljana basin (cofounded by the slovenian environment agency), the project j6–4016 the adaptation patterns of human activities to the environmental changes after last glacial maximum in slovenia, the programme p1 – 0011 regional geology and the young researcher grant (contract 1000-09-310068). we also acknowledge spot image and the isis program for the satellite images. we are grateful to a. moulin, r. finkel, i. mrak, j. atanackov, b. milaničand j. horvat for help in the field. we thank to slovenian academy of sciences and arts, the publisher of monograph where the photo and profile of the lokarje anticline were taken from (šifrer, 1961), for their permission to publish this material. we are grateful to vanja kastelic, bruno tomljenovićand the associate editor (christoph grützner) for their reviews and comments that improved the manuscript. references avouac, j. and g. peltzer (1993). active tectonics 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(1971). pleistocene deposits of the kranj, sora and ljubljana fields. geologija, 14, 5-51. *corresponding author:petra jamšek rupnik, geological survey of slovenia, ljubljana, slovenia; email: petra.jamsek@geo-zs.si. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. jamšek rupnik et al. 8 i l t e r r e m o t o s a r d o d e l 1 3 n o v e m b r e 1 9 4 8 f r a n c e s c o peronaci i l 1 3 n o v e m b r e 1 9 4 8 a l l e 9 h 5 2 m c a . t . m . g . f u a v v e r t i t a i n t u t t a l a s a r d e g n a s e t t e n t r i o n a l e u n a scossa di t e r r e m o t o c b e p r o d u s s e d a n n i a g l i e d i f i c i s i t u a l i n e i p a e s i c i r c o s t a n t i l a z o n a d i t e m p i o p a u s a n i a ; p a r t i c o l a r m e n t e c o l p i t i r i s u l t a r o n o a g g i u s , t r i n i t à , b a d e s i . d a l 1 3 n o v e m b r e i n p o i v e n n e r o a v v e r t i t e u n a v e n t i n a d i r e p l i c h e c o n i n t e n s i t à v i a v i a d e c r e s c e n t e , e t r a e s s e l e p i ù i n t e n s e f u r o n o q u e l l e d e l 1 6 a l l e 2 2 " 55'", d e l 2 1 a l l e 22", d e l l ' 8 d i c e m b r e a l l e 4" 35'" e a l l e 1 3 " 15'" t u t t e v a l u t a t e s u l 4"-5" d e l l a s c a l a m e r c a l l i . il p e r i o d o s i s m i c o p r o d u s s e g r a n d e a l l a r m e s u l l a p o p o l a z i o n e l o c a l e , t a n t o p i ù c h e l a s a r d e g n a , p u r e s s e n d o v a s t e z o n e di essa o c c u p a t e d a r o c c e t e r z i a r i e di c a r a t t e r e e r u t t i v o , è da r i t e n e r s i t e r r a p o c h i s s i m o s o g g e t t a a f e n o m e n i s i s m i c i . l a s t o r i a d e l l a r e g i o n e i n f a t t i , c i c o n s e n t e di r i l e v a r e u n c e n t r o s i s m i c o n e l l a p a r t e m e r i d i o n a l e d e l l ' i s o l a c h e d e t t e o r i g i n e a s c u o t i m e n t i n e g l i a n n i 1 6 1 5 , 1 7 7 1 , 1 8 3 5 , 1 8 5 5 , m e n t r e n e l l a s e t t e n t r i o n a l e si e b b e u n s o l o t e r r e m o t o n e l g i u g n o d e l 1 8 7 0 l a c u i i n t e n s i t à è da v a l u t a r s i s u l 5° d e l l a s c a l a m e r c a l l i e i l c u i e p i c e n t r o m a c r o s i s m i c o è d a p o r s i n e l l a r e g i o n e di i t t i r e d d u . n a t u r a l m e n t e q u e s t i d a t i v e n g o n o r i p o r t a t i in b a s e a l l e n o t i z i e d i c a r a t t e r e m a c r o s i s m i c o in q u a n t o , s o l o di r e c e n t e , p e r i n i z i a t i v a d e l l ' i . n . g . è s t a t a i n s t a l l a t a a c u g l i e r i u n a s t a z i o n e s i s m i c a , c b e c o n s e n t i r à d i s e g u i r e d a v i c i n o e c o n r i g o r e s c i e n t i f i c o l a s t o r i a d e l l ' i s o l a . l e r e g i s t r a z i o n i d e l l a r e t e i t a l i a n a e d e g l i o s s e r v a t o r i d e l c e n t r o e u r o p a si r i f e r i s c o n o s o l o a l l a p r i m a s c o s s a : n e s e g u e c b e i l n o s t r o stud i o s a r à l i m i t a t o s o l a m e n t e ad essa. l a r a c c o l t a d e i d a t i m a c r o s i s m i c i è s t a t a e f f e t t u a t a u t i l i z z a n d o o l t r e c h e d e l s e r v i z i o m a c r o s i s m i c o c h e f a c a p o a l l ' l n . g . , a n c h e d e l l e n o t i z i e g e n t i l m e n t e f o r n i t e p e r l a c o r s i c a d a l b u r e a u c e n t r a l s é i s m o l o g i q u e f r a n g a i s n o n c h é da o s s e r v a z i o n i d i r e t t a m e n t e e f f e t t u a t e s u l p o s t o d a l l o s c r i v e n t e d u r a n t e i l p e r i o d o s i s m i c o i n o g g e t t o . l ' e s a m e d e l l e i s o s i s t e t r a c c i a t e i n fig. 1 m o s t r a u n a n d a m e n t o p i u t t o s t o r e g o l a r e n e l l a p r o p a g a z i o n e d e l l ' e n e r g i a , s a l v o n e l l a z o n a di g i a v e e di i t t i r e d d u o v e l ' i r r e g o l a r i t à p u ò g i u s t i f i c a r s i c o l f a t t o c h e d e t t a 5 7 0 f r a n c e s c o p e r o n a c i r e g i o n e t r o v a s i n e l l a z o n a di s a l d a t u r a t r a t e r r e n i s e d i m e n t a r i e g r a n i t i c i . q u e s t a c o n s i d e r a z i o n e s p i e g a c o m e l a scossa s i a s t a t a s e n t i t a d i 4" a b o n o , m e n t r e a f l o r i a n a s e a s a s s a r i , n o t e v o l m e n t e p i ù v i c i n e all ' e p i c e n t r o , essa n o n a b b i a s u p e r a t o i l 3 ° ; q u e s t e u l t i m e i n f a t t i si t r o v a n o su t e r r e n i d i n a t u r a s e d i m e n t a r i a i q u a l i , p u r e s a l t a n d o l e s o l l e c i t a z i o n i a l l ' i n g r e s s o d e l l a s t r a t i f i c a z i o n e , d e t e r m i n a n o s u c c e s s i v a m e n t e u n n o t e v o l e a s s o r b i m e n t o di e n e r g i a p e r t r a s f o r m a z i o n e i n l a v o r o d i a t t r i t o . a n a l o g a m e n t e f a c c i a m o r i l e v a r e c b e m o n t e g g i a n i , c b e d i s t a p o c h i c h i l o m e t r i d a b a d e s i n o n h a s u b i t o d a n n i a p p r e z z a b i l i , essa i n f a t t i è s i t u a t a i m m e d i a t a m e n t e o l t r e l a f r a t t u r a g e o l o g i c a s u d d e t t a . r e s t a n o q u i n d i a n c o r a u n a v o l t a c o n f e r m a t e l e c o n s i d e r a z i o n i f a t t e d a s i e b e r g e a l t r i s i s m o l o g h i , sec o n d o l e q u a l i l a cos t i t u z i o n e d e l s o t t o s u o l o i n f l u i s c e i n m o d o e s s e n z i a l e n e l l a d e t e r m i n a z i o n e d e i d a n n i p r o v o c a t i d a u n a s c o s s a , e c o m e n e l l a c o n s e g u e n t e v a l u t a z i o n e d e l l e n o t i z i e m a c r o s i s m i c h e n o n si p o s s a p r e s c i n d e r e d a l l a c o n o s c e n z a g e o l o g i c a d e l l a r e g i o n e . i n b a s e a l l e r e g i s t r a z i o n i d e l l a s t a z i o n e di r o m a a b b i a m o o t t e n u t o p e r l a m a g n i t u d o 5 ; q u e s t o v a l o r e c o m p o r t a u n ' e n e r g i a l i b e r a t a a l l ' e p i c e n t r o d e l l ' o r d i n e di io2 0 e r g , e c o n f r o n t a n d o q u e s t o d a t o c o n l a s c a l a m e r c a l l i si o t t i e n e c h e a l l ' e p i c e n t r o l a scossa è di 7°. t e n e n d o c o n t o c h e , c o m e v e d r e m o , l ' e p i c e n t r o è i n m a r e i n p r o s s i m i t à d e l l a c o s t a , si h a u n a c o n f e r m a d e l l e n o t i z i e c h e p o n g o n o l a m a s s i m a i n t e n s i t à a l 6°. u n a i n d a g i n e e s e g u i t a s u i d a n n i a g l i e d i f i c i ci h a c o n s e n t i t o d i i l terremoto sardo del 1 3 n o v e m b r e 1 9 4 8 5 7 1 s t a b i l i r e c h e essi p r e s e n t a n o i n g e n e r e l e stesse c a r a t t e r i s t i c h e : d i s t a c c o d i v o l t e d a m u r i d i s o s t e g n o e g r a v i l e s i o n i n e g l i s p i g o l i ; d a t a l a m o d e s t a i n t e n s i t à d e l l a scossa, r i t e n i a m o c b e essi s i a n o i m p u t a b i l i p i ù c h e a l t r o a l l e n o n n e c o s t r u t t i v e d e g l i e d i f i c i stessi. a b b i a m o i n f a t t i n o t a t o c h e n e l l e c a s e d a n n e g g i a t e , t u t t e d i a n t i c a c o s t r u z i o n e , e r a n o s t a t e i m p i e g a t e m a l t e p o c h i s s i m o c o e r e n t i , e c h e s p e s s o i m u r i m a e s t r i e r a n o s u g l i s p i g o l i s o l a m e n t e a c c o s t a t i . e d i f i c i m o d e r n i di u n a c e r t a i m p o r t a n z a e c o s t r u z i o n i i n m a t t o n i n o n h a n n o s u b i t o d a n n i a p p r e z z a b i l i . a b b i a m o v o l u t o i n s i s t e r e s u q u e s t o f a t t o p e r r i b a d i r e q u a n t o è o r m a i a c c e t t a t o da m o l t i e c i o è c h e i l c r i t e r i o d i v a l u t a z i o n e d e l l ' e n t i t à d i u n a scossa i n b a s e a i d a n n i s u b i t i d a g l i e d i f i c i , è q u a n t o m a i a l e a t o r i o , se n o n i n t e r v e n g o n o a s u f f r a g a r l o d a t i s t r u m e n t a l i o b i e t t i v i c o m e l a m a g n i t u d o , l a f r e q u e n z a e l ' a m p i e z z a d e g l i s p o s t a m e n t i d e l s u o l o , o a l m e n o u n ' a c c u r a t a i n d a g i n e s u l l e n o r m e c o s t r u t t i v e c h e si s o n o s e g u i t e . d a t a l a s c a r s i t à d e l m a t e r i a l e a d i s p o s i z i o n e n o n a b b i a m o p o t u t o e s e g u i r e a l c u n a i n d a g i n e s u l l a n a t u r a fisica d e l l a scossa a l l o i p o c e n t r o , a n c h e p e r c h é n e l l e r e g i s t r a z i o n i a d i s p o s i z i o n e , gli i n i z i s o n o q u a n t o m a i d e b o l i se si e s c l u d o n o l e s t a z i o n i d i r o m a e di p u y d e d ò m e . l o s t u d i o m i c r o s i s m i c o d e l t e r r e m o t o è s t a t o e s e g u i t o u t i l i z z a n d o i s i s m o g r a m m i d i 1 4 s t a z i o n i c o m p l e s s i v a m e n t e , c i o è l e u n i c h e c h e a b b i a n o o t t e n u t o d e l l e r e g i s t r a z i o n i s u f f i c i e n t e m e n t e i n t e n s e ; d i q u e s t e s o l o i n 8 si riscontra u n i n i z i o c h i a r o e d i n e q u i v o c a b i l e . d a t a l a e s i g u i t à d e l m a t e r i a l e d i s p o n i b i l e , n e s e g u e c h e i r i s u l t a t i o t t e n u t i d o v r a n n o a s s u m e r s i c o n c a u t e l a , e q u i n d i u t i l i z z a b i l i c o m e e v e n t u a l e p u n t o d i p a r t e n z a p e r s u c c e s s i v i s t u d i n e c e s s a r i p e r l ' a c c r e s c i m e n t o d e l l a c o n o s c e n z a d e l l a s t r u t t u r a d e l t i r r e n o c e n t r o s e t t e n t r i o n a l e , z o n a finora p o c o n o t a s i s m i c a m e n t e . determinazione dell'epicentro. — i n b a s e a i d a t i m a c r o s i s m i c i e a l l ' a n d a m e n t o d e l l e i s o s i s t e , a b b i a m o a s s u n t o c o m e e p i c e n t r o p r o v v i s o r i o q u e l l o i n d i c a t o da s t r a s b u r g o e p r e c i s a m e n t e q p 0 = 4 1 u , l n ; )a = 8 ° , 9 e a s s e g n a n d o p r o v v i s o r i a m e n t e u n a p r o f o n d i t à i p o c e n t r a l e di 5 k m . a b b i a m o q u i n d i e s e g u i t o v a r i t e n t a t i v i p e r l a d e t e r m i n a z i o n e d e l t e m p o o r i g i n e p r o v v i s o r i o u t i l i z z a n d o s u c c e s s i v a m e n t e l e d r o m o c r o n e c a l c o l a t e d a c a l o i ( c a n s i g l i o e a p p e n n i n o ) , m a r c e l l i d i f i l i p p o ( g r a n s a s so), m o h o r o v i c i c , j e f f r e y s ; i n t u t t i i casi si è r i s c o n t r a t a u n a n o t e v o l e d i s c o r d a n z a t r a i l t e m p o o r i g i n e c a l c o l a t o c o n l e s t a z i o n i d e l c e n t r o 132 f r a n c e s c o peronaci e u r o p a e q u e l l o o t t e n u t o d a l l e s t a z i o n i i t a l i a n e . i n p a r t i c o l a r e p e r r o m a , c h e s u l l a c o m p o n e n t e v e r t i c a l e w i l i p p r e s e n t a u n « i m p e t u s » si h a u n a d i s c o r d a n z a d i 4 s . d e t t o s c a r t o , c h e si m a n t i e n e a n c h e p e r l a s t a z i o n e d i p r a t o , ( l e a l t r e s t a z i o n i d e l l a r e t e i t a l i a n a h a n n o i n i z i p i u t t o s t o i n c e r t i ) n o n p o t e n d o e s s e r e i m p u t a t o a d u n a e r r o n e a v a l u t a z i o n e d e l t e m p o a s s o l u t o , p u ò s o l o e s s e r e g i u s t i f i c a t o o d a u n a v e l o c i t à a p p a r e n t e d i v e r s a d e l l e o n d e s i s m i c h e n e l l e d u e d i r e z i o n i s a r d e g n a e u r o p a c e n t r a l e e s a r d e g n a i t a l i a , o d a u n a e r r o n e a v a l u t a z i o n e d e l l a p o s i z i o n e d e l l ' e p i c e n t r o p r o v v i s o r i o . o n d e s c a r t a r e u n a d e l l e d u e i p o t e s i , a b b i a m o c a l c o l a t o l a p o s i z i o n e d e l l ' e p i c e n t r o u t i l i z z a n d o d e i d a t i di t u t t e l e 8 s t a z i o n i a d i s p o s i z i o n e , s e g u e n d o i l m e t o d o c h e si v a l e d e l l e d i f f e r e n z e d e i t e m p i d i r e g i s t r a z i o n e d e l l e o n d e t r a s v e r s a l i e l o n g i t u d i n a l i già a p p l i c a l o c o n s u c c e s s o da c a l o i e s u c c e s s i v a m e n t e d a a l t r i s p e r i m e n t a t o r i . c o m e è n o t o , t a l e m e t o d o p r e s c i n d e d a l l a c o n o s c e n z a d e l l a v e l o c i t à di p r o p a g a z i o n e e f o r n i s c e o l t r e a l l e c o o r d i n a t e d e l l ' e p i c e n t r o , l a p r o f o n d i t à i p o c e n t r a l e e i l r a p p o r t o k= vt — v, i c a l c o l i e s e g u i t i p e r 3 a p p r o s s i m a z i o n i s u c c e s s i v e h a n n o f o r n i t o : f p o = 4 p 3 6 ' , 8 + 5 ' , 9 ; = 8" 2 4 ' . 4 + 0 ' , 5 h„ = 1 3 + 0 , 4 k m ; k = 9 , 2 + 0 , 4 l ' e p i c e n t r o c a d r e b b e q u i n d i a l l a r g o d e l g o l f o di v a l l i n c o ( c o r s i c a ) , i n e v i d e n t e c o n t r a s t o c o n l ' a n d a m e n t o d e l l e i s o s i s t e e q u i n d i c o n l a p o s i z i o n e d e l l ' e p i c e n t r o m a c r o s i s m i c o . r e s t a q u i n d i c o n f e r m a t a l ' i p o t e s i c h e l a v e l o c i t à di p r o p a g a z i o n e d e l l e o n d e è d i v e r s a n e l l e d u e d i r e z i o n i c o n s i d e r a t e ; si è d e c i s o q u i n d i di s c a r t a r e l e s t a z i o n i i t a l i a n e ed e s e g u i r e l a d e t e r m i n a z i o n e d e l l ' e p i c e n t r o u t i l i z z a n d o d e l l e 6 s t a z i o n i d e l l ' e u r o p a c e n t r a l e e p r e c i s a m e n t e z u r i g o , s t o c c a r d a , s t r a s b u r g o , b a s i l e a , n e u c h à l e l , p u y d e d ò m e . u t i l i z z a n d o d e l m e t o d o di c a l o i p e r o n a c i c h e si v a l e d e l t e m p o di r e g i s t r a z i o n e d e l l a f a s e i n i z i a l e e d e i t e m p i d i t r a g i t t o d e l l a m e d e s i m a , e u s a n d o d e l l e d r o m o c r o n e di m o h o r o v i c i c si è o t t e n u t o c o n 2 a p p r o s s i m a z i o n i s u c c e s s i v e cp„ = 4 1 ° 2 9 ' , 4 + 3 1 ' , 8 l a = 8° 2 4 ' , 2 + 1 0 ' , 0 h0= 2 8 + 1 3 k m ha = 5 2 " ' 2 0 s , 8 + 1 \ 5 . i l t e r r e m o t o sardo del 1 3 n o v e m b r e 1 9 4 8 5 7 3 a d a n a l o g o r i s u l t a t o si è g i u n t i a n c h e i m p i e g a n d o l e d r o m o c r o n e c a l c o l a t e d a m a r c e l l i d i f i l i p p o p e r i l t e r r e m o t o d e l g r a n s a s s o . rp„ = 4 1 ° 2 9 ' , 2 + 3 0 ' a„ = 8 " 2 8 ' , 2 + 8 ' h0—20 + 10 km h„= 5 2 " ' 1 7 s , 8 ± 1%2. i n e n t r a m b i i casi i l r i s u l t a t o è s e m p r e in n o t e v o l e c o n t r a s t o c o n 1 d a t i m a c r o s i s m i c i , p e r c u i a b b i a m o r i t e n u t o c h e l e d r o m o c r o n e u s a t e n o n r i s p e c c h i a s s e r o l ' e f f e t t i v o a n d a m e n t o d e l l a v e l o c i t à d e l l e o n d e sis m i c h e n e l t r a g i t t o s a r d e g n a e u r o p a c e n t r a l e . u s a n d o a l l o r a d e i d a t i r e l a t i v i a l l ' e p i c e n t r o m a c r o s i s m i c o a b b i a m o p r o v v e d u t o a c a l c o l a r e u n a d r o m o c r o n a p r o v v i s o r i a d e l l e p,„ u t i l i z z a n d o d e l l e 6 s t a z i o n i s u d d e t t e , si è o t t e n u t o d a l l ' a p p l i c a z i o n e d e l m e t o d o dei m i n i m i q u a d r a t i l ' e q u a z i o n e : t : 8 , 7 4 + 0 , 7 7 , 9 4 ± 0 , 0 7 l a v e l o c i t à d i p r o p a g a z i o n e d e l l e pn si a g g i r a q u i n d i p e r i l t r a g i t t o in q u e s t i o n e sugli 8 k m / s e c , i n f e r i o r e a q u e l l o t r o v a t o p e r il t e r r e m o t o d e l g r a n s a s s o c b e è di 8 , 1 9 k m / s e c . a b b i a m o q u i n d i a p p l i c a t o i l m e t o d o s u a c c e n n a t o u t i l i z z a n d o d e l l e 2 d r o m o c r o n e c a l c o l a t e da c a l o i p e r i l t e r r e m o t o d e l c a n s i g l i o (/io = 1 7 k m ) e p e r q u e l l o d e l l ' a p p e n n i n o t o s c o r o m a g n o l o (h0 — 5 k m ca.) c h e f o r n i s c o n o p e r l e pn l a v e l o c i t à d i 8 , 0 e 7 , 8 6 k m / s e c r i s p e t t i v a m e n t e (si t e n g a p r e s e n t e c h e l a d r o m o c r o n a p e r i l 2° t e r r e m o t o f u c a l c o l a t a u t i l i z z a n d o e s c l u s i v a m e n t e s t a z i o n i d e l l ' e u r o p a c e n t r a l e ) . i l r i s u l t a t o d e i c a l c o l i s p i n t o a l l a t e r z a a p p r o s s i m a z i o n e h a f o r n i t o : rpo = 4 1 ° 3 ' , 6 + 5 ' , 9 ào = 8" 4 1 ' , 4 ± 0 ' , 5 h0 = 1 3 + 5 k m i/o = 52"' 12%5 + 0 s , 7 i n o t t i m o a c c o r d o c o n l ' e p i c e n t r o m a c r o s i s m i c o . n a t u r a l m e n t e s a r e b b e s t a t o d e l m a s s i m o i n t e r e s s e r i p r o c e d e r e a l l a d e t e r m i n a z i o n e c o n i d a t i d e l l e s t a z i o n i i t a l i a n e , m a p e r l e r a g i o n i su e s p o s t e c i ò n o n è p o s s i b i l e . f ** . . . . . . < . 5 7 4 f r a n c e s c o peronaci dromocrone e velocità apparenti. — a b b i a m o c a l c o l a t e l e d r o m o c r o n e r e l a t i v e a l l e v a r i e f a s i ; l o s p o g l i o d e i s i s m o g r a m m i è r i a s s u n t o n e l l a s e g u e n t e t a b e l l a : s t a z i o n i d i s t a n z a p* s „ s* m r o m a 3 3 1 k m 5 2 m 5 7 s , 4 5 3 " ' 3 1 s , 0 5 4 m 0 4 8 f i r e n z e a r e . 3 6 4 3 7 , 8 1 6 ,7 p r a t o 3 7 0 4 1 . 0 1 7 b o l o g n a 4 3 7 5 5 , 0 3 9 8 p a v i a 4 5 9 5 4 0 0 , 3 4 9 5 s a l ò 5 2 6 1 6 , 2 5 5 0 8 n e u c h à t c l 6 7 3 5 3 4 4 , 5 5 4 » 0 1 b , 7 5 3 , 0 5 5 m 0 4 s , 0 55»>17* , 0 5 6 0 7 ,9 p u y d e d ò m e 6 8 9 4 6 , 9 0 4 5 6 ,5 0 6 , 5 2 0 ,0 1 5 z u r i g o 7 0 0 4 8 , 7 0 6 55 0 0 , 6 1 0 , 9 2 4 .6 2 0 b a s i l e a 7 2 5 5 1 , 3 0 6 , 2 1 6 , 2 3 0 ,5 2 8 s t r a s b u r g o 8 3 8 5 4 0 6 ,0 3 1 , 3 4 4 , 0 3 9 ,5 5 7 0 9 5 s t o c c a r d a 8 5 6 0 8 . 0 2 9 ,5 3 5 . 0 5 0 , 2 5 6 0 6 , 7 1 4 p e r q u a n t o r i g u a r d a l e p„ si è p o t u t o c a l c o l a r e l a d r o m o c r o n a s o l o p e r l e s t a z i o n i d e l c e n t r o e u r o p a e si è o t t e n u t a l ' e q u a z i o n e t = [6 , 5 + 0 , 7 7 , 8 4 + 0 , 1 1 l a v e l o c i t à d e l l e p„ c o i n c i d e c o n il v a l o r e r i c a v a t o d a c a l o i p e r l ' a p p e n n i n o t o s c o r o m a g n o l o ; p e r q u a n t o r i g u a r d a l a d i r e z i o n e s a r d e g n a i t a l i a f a c c i a m o s o l o n o t a r e c b e n e l l a s t a z i o n e di r o m a si ris c o n t r a n e l t e m p o di r e g i s t r a z i o n e u n a d i f f e r e n z a di — 3 s , 5 . l a f a s e sn è r i s u l t a l a c h i a r a i n l u t t i i s i s m o g r a m m i : n e s e g u e c h e a b b i a m o p o t u t o c a l c o l a r e l e d r o m o c r o n e n e l l e d u e d i r e z i o n i s u d d e t t e o t t e n e n d o p e r l e s t a z i o n i o l t r e a l p e t = b 7 , 4 4 + 1 , 0 4 , 3 8 + 0 , 0 7 e p e r l e i t a l i a n e a t = + 1 . 8 1 + 1 . 5 4 , 3 2 + 0 , 0 9 t e n u t o c o n t o d e g l i e r r o r i , p o s s i a m o q u i n d i c o n c l u d e r e c h e p u r e s s e n d o l a v e l o c i t à d e l l e t r a s v e r s a l i u g u a l e n e l l e d u e d i r e z i o n i , p e r l e s t a z i o n i i t a l i a n e si r i s c o n t r a u n a n t i c i p o di r e g i s t r a z i o n e d e l l ' o r d i n e d i 5 sec. il n:iiremoto s.~rdo du, 13 "iovembmt; 1948 515 tale anli(~ipo lronl il corri~pondenle per la lon!"itudinale nelb slazione di homa rhe , come ~i è vislo, riceve 3,5 ,c(~ prima (lei lempo calcolalo. lna spiegazione (li queslo flltlo potrehhe ('..!'sere ricercata nel divet·so ~pesso. re dello 8tralo del ~ranilo nelle (lue direzioni in e8ame; m· fatli l'l accerlalo dle tale 81ral0 "l1hisce un ~radl1alc assotli~lia­ menlo quando ~i pa~. " . " "i dall'europa cen,lrale all'italia. nalu· ralmente a i~au~a (lei. lo scar"o maleriale a di"po"izione ~iamo coslrelli a limilarci a quesla considerazio_ " ne (li i"arallere quali_ lali\-o c rimandare ,ll(l allra ol"(~a~ione iii po"sihililù di un'ilhla. ~ine di nlliura ql1anlilllliya ~ulla 51rl1llura della i"ro"la nella zona del tirreno cen_ lrale. " " '" '. " " 'o fi~. 2 le s ~ sono ~tale re~i~trale ('on gran dlillltzza ~olo nelle si azioni del cenlro europa f' i (~alr~oli hanno i~ondotlo all'efjll<lzione 1 --=-_ 4,2 o., 3,74 -i o,m come ~i o~"eryll nella lahella la lon!"iludinale corrispomlenle è rilevahile ~olo in -1 81azioni. la veloeilù delle s* coinci(]e con il valore lrovato da glitenher~ per la germania jferirlionll]e e (la jefireys come me(lia (li vllri lerremoti. :nello ~poglio dei sismogrammi si è ri81~onlrllla la pre~enza (li una 5 7 6 f r a n c e s c o peronaci f a s e p a r t i c o l a r m e n t e e v i d e n t e c o n p e r i o d o p i ù a m p i o d e l l e s„: l a d r o i n o c r o n a c a l c o l a t a c o n i t e m p i di 6 s t a z i o n i è f = h 2 , 8 + 0 , 7 4 , 0 0 + 0 , 0 8 i n b a s e a l v a l o r e d e l l a v e l o c i t à a p p a r e n t e essa q u i n d i r i s u l t a v i c i n a a q u e l l e o n d e già p o s t e i n e v i d e n z a d a c a l o i n e l t e r r e m o t o d e l l ' a p p e n n i n o t o s c o r o m a g n o l o ( 4 , 0 1 ) e d a m a r c e l l i d i f i l i p p o i n q u e l l o d e l g r a n s a s s o ( 4 , 0 4 ) . p a s s a n d o a l l e o n d e d e l l a f a s e m a s s i m a (m) i c a l c o l i ci h a n n o p o r t a t o p e r l e s t a z i o n i d e l c e n t r o e u r o p a a a . . t = d , 6 d 2 , 7 8 m e n t r e p e r l e i t a l i a n e si è o t t e n u t o t = — + 2,8 3 , 0 n a t u r a l m e n t e t r a t t a s i p e r l e r a g i o n i s u d d e t t e , di v a l o r i p u r a m e n t e i n d i c a t i v i ; c i ò n o n t o g l i e p e r ò c b e si n o t i u n a u m e n t o d e l l a v e l o c i t à d i p r o p a g a z i o n e n e i t r a g i t t i i n t e r e s s a n t i il t i r r e n o c e n t r a l e . c o n c l u d e n d o a b b i a m o n o t a t o u n a n t i c i p o s u i t e m p i di r e g i s t r a z i o n e d e l l e l o n g i t u d i n a l i e t r a s v e r s a l i n e i t r a g i t t i i n t e r e s s a n t i i l t i r r e n o c e n t r a l e : t a l e a u m e n t o r i s u l t e r e b b e p i ù o m e n o c o s t a n t e p e r t u t t e l e d i s t a n z e e si a g g i r e r e b b e s u i 5 s c i r c a p e r l e t r a s v e r s a l i ; r i t e n i a m o d i c o n s e g u e n z a c h e esso p o s s a i m p u t a r s i ad u n a d i m i n u z i o n e d e l l o s p e s s o r e d e l l o s t r a t o d e l g r a n i t o n e l l a d i r e z i o n e i n q u e s t i o n e ; t a l e i p o t e s i r e s t e r e b b e s u f f r a g a t a d a l v a l o r e p i ù e l e v a t o d e l l a v e l o c i t à d i p r o p a g a z i o n e d e l l e o n d e d e l l a f a s e m n e l l a stessa d i r e z i o n e , se si t i e n e p r e s e n t e c h e l ' i n f l u e n z a d e l l e s t r a t i f i c a z i o n i s u p e r f i c i a l i r i s u l t e r e b b e n o t e v o l m e n t e d i m i n u i t a . roma —istituto nazionale di geofisica — novembre 1953. riassunto viene studiato il terremoto sardo del 13 novembre 1948. si pone in evidenza un diverso comportamento nella propagazione delle onde elastiche nella direzione sardegna-italia rispetto ai tragitti interessanti le stazioni dell'europa centrale. * jtìiito m i l t e r r e m o t o s a r d o d e l 1 3 n o v e m b r e 1 9 4 8 5 7 7 summary the sardinian eartliquake of novem1>er 13, 1948 lias been studied. lt is shown that the elastic nave in the direction sardinia-i taly lias a different hehavior ivith respect to the tracings oj interest to the stations of central europe. b i b l i o g r a f i a caloi 1 \ , determinazione <ielle coordinale epicentrali di un terremoto ad origine vicina con i tempi delle onde longitudinali e trasversali dirette.. « la r i c e r c a s c i e n t i f i c a » , 1 9 1 1 , p a g . 4 3 1 . c a l o i p e r o n a c i , il terremoto del turkestan. « a n n a l i d i g e o f i s i c a » , v o i . i , n . 2 ( 1 9 4 8 ) . caloi p . , tempi di tragitto per terremoti ad origine vicina. « l a r i c e r c a s c i e n t i f i c a » , 1 9 3 9 , p a g . 3 8 8 . caloi 1'., caratteristiche sismiche dell'appennino tosco-romagnolo. « l a r i c e r c a s c i e n t i f i c a » , 1 9 4 0 , p a g . 2 1 8 . di filippo marcelli, dromocrone per terremoti vicini e velocità delle onde nell'italia centrale. « a n n a l i di g e o f ì s i c a » , v o i . v , n. 2 ( 1 9 5 2 ) . caloi p . , struttura geologico-sismica dell'europa centro-meridionale ecc... « a n n a l i di g e o f i s i c a » , v o i . v , n . 4 , 1 1 9 5 2 ) . caloi p . , attività sismica in italia nel decennio 1930-1')39. l e m o n n i e r , f i r e n z e , ( 1 9 4 2 ) . s u l l a c o n s i s t e n z a d e l l a d e t e r m i n a z i o n e d e i n u m e r i caratteristici d e i f l o c c u l i di c a l c i o g . g o d o l i r i c e v u t o il 3 n o v e m b r e 1959 1. è noto come nel congresso dell'unione astronomica internazionale tenutosi a leida nel 1928 venisse raccomandato di stimare l'attività globale dei doccud e dei filamenti solari con i numeri caratteristici che, in una scala da 0 a 5, dovevano rappresentare sinteticamente sia l'area che l'intensità dei fenomeni (i-2). abbiamo già parlato in questa sede dede vicissitudini che la pubblicazione di questi indici ha subito (1). recentemente, in seguito a ricerche eseguite ad'osservatorio di arcetri ( l i 3 ' 4 ), è stata riconosciuta l'utilità di continuare la pubbdcazione di questi indici, anche per quei fenomeni per cui si effettuano misure più dettagdate. si hanno infatti indizi che stime dell'attività globale con i numeri caratteristici e misure di area offrano informazioni diverse, almeno per quanto riguarda l'attività dei flocculi di calcio ("). l'osservatorio di arcetri ha ripreso la pubbdcazione dei numeri caratteristici dal 1950. annualmente vengono pubbdcati in questa sede i valori giornalieri e le medie mensili ed annuad dei numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno determinati ad arcetri (6). per il periodo 1942-1949 sono state pubbdcate, sempre in questa sede ('), solo le medie mensfli dei numeri caratteristici dei tre fenomeni determinati ad arcetri. precedentemente, i valori giornaderi e le medie mensili, semestrali ed annuali dei numeri caratteristici dei tre fenomeni determinati ad arcetri sono stati pubbdcati, insieme alle determinazioni eseguite negd altri osservatori, nel bollettino « character figures of solar phenomena » che dal 1939 ha preso il nome di « quarterly bulletin on solar activity ». dal 1948 anche l'osservatorio di tokio ha pubbdcato fra l'altro nel suo « budetin of solar phenomena » i valori giornaderi e le medie mensdi dei numeri caratteristici dei flocculi di calcio (6). 5 1 6 g . g o d o l i recentemente anche l'osservatorio di ikomasan, dell'università di kyoto, ha pubblicato un volume di osservazioni solari nel quale sono riportati fra l'altro i valori giornalieri e le medie mensili dei numeri caratteristici dei flocculi di calcio per il periodo settembre 1946-dicembre 1954 ('). avendo a disposizione questo materiale abbiamo creduto opportuno eseguire un confronto fra le determinazioni di numeri caratteristici dei flocculi di calcio effettuate ad arcetri, ad ikomasan e tokio sopratt u t t o allo scopo di valutarne la consistenza. 2. nellavtabella i sono riportate le medie mensili, semestrali ed annuali dei numeri caratteristici determinati ad arcetri, ikomasan, tokio. le medie sono state calcolate per i giorni in cui la determinazione era stata effettuata sia ad arcetri che ad ikomasan oppure sia ad arcetri che a tokio. i numeri in corsivo sotto le medie indicano il numero di giorni di osservazione. la tabella è illustrata dai grafici di figg. 1-9. nei grafici di figg. 1 e 2 sono riportati i confronti fra le medie mensili di arcetri e quelle di ikomasan e tokio. nel grafico di fig. 3 sono riportati l'andamento dei rapporti fra le medie mensili di ikomasan e tokio e quelle di arcetri. i grafici di figg. 4-6 e 7-9 hanno analogo significato per le medie semestrali e quelle annuali. 3. l'esame della tabella e dei grafici permette-di trarre le seguenti conclusioni: a) date le incertezze e le limitazioni inerenti al metodo di misura, la consistenza delle stime dei numeri caratteristici appare assai soddisfacente; b) i numeri caratteristici determinati ad arcetri hanno la tendenza ad essere maggiori di quelli determinati ad ikomasan e tokio. dalle figg. 3, 3 bis, 6, 9 risulta che questa tendenza è più accentuata nei periodi di minor attività solare. c) la notevole discrepanza del 1954 è tale solo percentualmente. in effetti si tratta, come risulta dalla tabella 1, di divergenze non superiori a 0.2. t a b e l l a i m e d i e m e n s i l i , s e m e s t r a l i e d a n n u a l i d e i n u m e r i c a r a t t e r i s t i c i determinati ad arcetri, ikomasan, tokio. (i n u m e r i i n c o r s i v o s o t t o l e m e d i e i n d i c a n o il n u m e r o di giorni di osservazione). o m e s 0 th © ìsemestre © fh © a n n o ce <j pìj r— rq 3 © © a n n o a n n o > ii <d od m c3 s © h © o n (h cs © pfco tao c3 a sao 6p 3 o 05 o te g © © co o b © > o rq 3 © © i i i a n n o o c5 b y <) tts h-ì <1 g © © co o £ q ) a 2 9 3 0 2 . 6 2 8 2 6 2 2 2 3 2 3 1 7 1 9 1 8 2 57 2 13 2.32 ì i 2 7 2 8 2.2 2 3 2 4 2 6 2 3 2 3 2 0 2 1 1 5 2 47 2 19 2.31 1 9 5 0 2 3 5 2 « 6 9 3 5 2 26 33 59 ) a 2 7 2 9 2 . 6 2 8 2 5 2 4 2 3 2 3 1. 5 i 8 2 1 2 58 2 14 2.34 / t 1 9 3 0 2.2 2 5 2 3 1 8 2 1 2 0 1. 5 1 7 1 8 2 21 1 93 2.06 4 2 5 2 9 4 17 5 3 3 2 26 30 56 ( a 1 8 1 6 1.3 2 0 2 4 2 5 1 3 2 0 2. 0 1 5 1 9 1. 7 2 04 1 80 1.92 \ i 1 7 1. 6 1.3 1 4 2 4 2 0 1 4 1 5 1 9 1 2 1 2 1 2 1 82 1 44 1.63 1 9 5 1 3 3 4 4 7 7 4 24 4 4 2 2 25 30 58 ) a 1 6 2 . 2 0 . 9 2 1 2 3 2 6 1 8 1 9 1. 8 1. 6 2. 0 1. 8 2 17 1 87 2.02 1 t 1 2 2 . 0 0 . 5 2 3 2 5 2 2 1 8 1 8 1 5 0 . 5 1. 5 0. 5 2 00 1 57 1.80 5 1 1 2 7 7 5 9 1 2 3 2 23 22 44 ) a ( i 1 5 1. 0 0.6 0 8 0 9 1 3 0 7 1 5 1 7 0 . 5 1. 0 1. 3 1 00 1 15 1.08 ) a ( i 1 7 0 8 0.7 0 5 0 6 0 8 0 7 1 0 1 6 0 . 5 0 6 1. 3 0 87 1 00 0.94 1 9 5 2 4 5 4 3 6 4 9 22 6 4 2 2 26 33 59 1 a 1 5 0. 8 0.4 0 8 0 9 0 6 1 0 1 4 1 5 0. 5 i 0 ] 6 0.92 1 10 1.01 ) t 1 1 0 . 4 0 . 5 0 . 5 0 5 0.4 0 9 1 6 1. 5 0 6 ] 0 2. 0 0 64 1 17 0.90 7 4 3 4 3 4 8 5 4 4 2 2 25 23 48 s a 1 0 0.4 0.2 0 4 0 7 1 4 0 5 0 . 8 1 2 0 7 0 3 0 0 0 55 0 61 0.58 ) t 0 7 0 5 0.1 0 3 0.4 0 . 4 0 1 0.4 0 5 0 3 0 1 0 1 0 29 0 27 0.28 1 9 5 3 3 1 9 5 5 2 7 11 3 4 3 4 25 32 57 s a 1 2 0 1 0 . 3 0 . 5 0 9 0 6 1 4 1 4 1 1 0 2 0 0 0 55 0 64 0.59 ( t 1 i 0 2 0 . 1 0 5 0.4 0 3 j 1 0 5 0 5 0 2 0 0 0.40 0.43 0.41 5 3 10 4 4 4 4 2 2 7 2 26 20 46 > a 0 0 0 . 0 0.1 0 3 0 0 0 0 0 2 0 3 0 2 0 3 0 3 0 3 0 06 0 26 0.19 ( i 0 0 0 . 0 0.1 0 . 1 0 0 0 0 0 1 0 1 0 1 0 1 0 2 0 1 0.03 0 12 0.08 1 9 5 4 3 6 4 4 2 2 7 10 4 3 5 5 22 34 55 i a 0 0 0. 0 0.2 0.4 0 . 0 0 0 0 2 0 . 3 0 2 0 3 0 3 0 . 3 0 08 0 25 0.19 ì t 0 3 0. 0 0 . 3 0 5 0 . 0 0 0 0 3 0 3 0 3 0 5 0 . 5 0 . 3 0 18 il 34 0.29 2 5 5 2 2 2 6' 9 4 4 4 5 27 32 49 1 9 5 5 ( a 0 7 0 2 0 . 6 0 8 0 9 1 7 1 2 2 0 1 2 2 2 3 0 3 2 0.86 1 90 1.47 1 9 5 5 ) t 0 6 0 3 0 . 5 0 5 0 6 0 8 0 . 5 0 9 0 6 1 0 1 8 1 9 0.54 0 99 0.80 4 2 2 10 « 13 8 5 3 6 4 27 39 66 1 9 5 6 i a 2 8 2. 7 3 . 3 3 7 4 0 4 0 3 5 4 . 0 3 5 2 8 3 8 3 7 3 43 3 56 3.49 1 9 5 6 \ t 2 3 3 1 ' 7 4 2 . 2 5 2 5 6 2 7 6 2 4 8 3 6 0 3 6 1 3 6 2 2 4 6 3 2 0 3 0 2 32 27 3 32 00 2.74 63 1 9 5 7 ! a 3 7 2 8 3.6 3 9 3 6 3 9 3 4 2 9 3 5 3 7 3 5 3 1 3 64 3 36 3.48 1 9 5 7 ì t 2 9 2 0 3 . 0 3 3 3 4 3 . 5 3 4 3 0 3 4 3 9 3 5 3 4 3 11 3 46 3.31 10 2 11 2 8 7 7 22 6 24 6 7 40 52 91 fig. 1 c o n f r o n t o f r a le medie mensili di a r c e t r i e quelle di i k o m a s a n . fig. 2 c o n f r o n t o f r a le medie mensili di a r c e t r i e quelle di tokio. fig. 3 a n d a m e n t o dei r a p p o r t i f r a le medie mensili di i k o m a s a n e tokio e quelle di a r c e t r i (1950-1953). 1 \[ { — i / a t/a \ vv v 19s< 1955 7956 (957 fig. 3 bis a n d a m e n t o dei r a p p o r t i f r a le medie mensili di i k o m a s a n e tokio e quelle di a r c e t r i (1954-1957). s u l l a c o n s i s t e n z a d e l l a d e t e r m i n a z i o n e d e i n u m e r i , e c c . 5 1 9 fig. 4 c o n f r o n t o f r a le medie semestrali fig. 5 c o n f r o n t o f r a le medie semestrali di arcetri e quelle di i k o m a s a n . di arcetri e quelle di tokio. — i / a t/a — 195 0 1951 1952 1953 1951 i95s '956 ' 957 fig. 6 a n d a m e n t o dei r a p p o r t i f r a le medie semestrali di i k o m a s a n e tokio e quelle di arcetri. 5 2 0 g. g o d o l i [fig. 7 c o n f r o n t o f r a le medie a n n u a l i di arcetri e quelle di i k o m a s a n . fig. 8 c o n f r o n t o f r a le medie a n n u a l i di arcetri e quelle di tokio. '/a t/a —t— >95 0 19s1 195! 19s3 19 5< >955 / 956 (957 fig. 9 a n d a m e n t o dei r a p p o r t i f r a le m e d i e a n n u a l i di i k o m a s a n e tokio e quelle di arcetri. s u l l a c o n s i s t e n z a d e l l a d e t e r m i n a z i o n e d e i n u m e r i , e c c . 5 2 1 riassunto bai confronto delle medie mensili, semestrali ed annuali dei numeri caratteristici dei flocculi di calcio determinati ad arcetri, ikomasan, tokio, si deducono informazioni sulla consistenza delle determinazioni dei numeri caratteristici dei flocculi di calcio. abstract comparing monthly, half yearly, yearly means of tlie character figures of ca brighi flocculi determined at arcetri, ikomasan, tokio, informations are deduced on the accuracy of the determinations of the character fìgures of ca brighi flocculi. b i b l i o g r a f i a (!) righini, g.-godoli, g., sui numeri caratteristici dell'attività solare, « a n n . di g e o ! », 3, 501, (1950). (2) godoli, g., la torre solare dì arcetri e lo studio del sole in luce monocromatica, «boll, di geod. e scienze affini», 14, (1955). (3) righini, g.-godoli, g., the physical meaning of the character fìgures of solar phenomena, « j . of geoph. r e s . » , 55, 415, (1950). (4) godoli, g., attività dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per il periodo 1° luglio 1957-30 giugno 1958, « boll, di geod. e scienze affini», 17, 487, (1958). (6) godoli, g., numeri caratteristici dei flocculi d'idrogeno e di calcio e dei filamenti d'idrogeno per gli anni 1950-51-52, « a n n . di geof. », 6. 199, (1953). i d e m p e r l ' a n n o 1953, l . c . 7, 135, (1954). i d e m p e r l ' a n n o 1954, l . c . 8, 325, (1955). i d e m p e r l ' a n n o 1955, l . c . 9, 299, (1956). i d e m p e r l ' a n n o 1956, l . c . 10, 133, (1957). i d e m p e r l ' a n n o 1957, l . c . 10, 235, (1957). i d e m p e r l ' a n n o 1958, l . c . 11, 197, (1959). (6) tokio a s t r . obs. b u l l e t i n of solar p h e n o m e n a . (') i k o m a s a n solar o b s e r v a t o r y . p u b i . 1 (1956). layout 6 1 annals of geophysics, 63, 6, gd656, 2020; doi:10.4401/ag-8403 point wise determination of vertical deformation at southern coastal areas of caspian sea by combination of satellite altimetry and tide gauge observations mahmood pirooznia1, mehdi raoofian naeeni*,1, taghi yousefzadeh2 (1) department of geodesy, faculty of geodesy and geomatics engineering, k. n. toosi university of technology (2) department of geodesy, school of surveying and geospatial engineering, college of engineering, university of tehran article history: received december 7, 2019; accepted april 27, 2020 abstract in this paper, a combination of satellite altimetry (sa) and tide-gauge (tg) observations is used to determine the pattern of vertical deformation at southern coasts of caspian sea. satellite altimetry measures the variations of sea water surface with a respect to a reference ellipsoid, while tide gauge data gives the sea level relative to a particular ground station; consequently, the difference between these two observations will give the vertical motion at the ground station. however, altimeter satellites still have problems at shallow waters or coastal areas which leads to inaccurate and corrupted sea surface height at tide gauge stations. to minimize this imprecision, two strategies are applied for the selection of the suitable sa points near the tg stations. the first one is to choose those close passes to the tg stations, provided that the sa data is not corrupted (range observation is available). the second strategy is to extrapolate the sa points at tg stations using spatiotemporal kriging method. in both point of views, use is made of re-tracking methods to improve the ranges of sa observations near the coast. amplitudes of tidal components are removed from both sa and tg observations to derive the residual tide free signals. afterward, a linear trend is fitted to the residual signals in the sense of least square; the difference between these two, supplies the vertical deformation of the ground station. the results illustrate that the average vertical crustal motion at anzali, noshahr and neka are -1.387, -1.903 and -3.14 mm/year, respectively. keywords: vertical crustal motion (vcm); satellite altimetry; coastal tgs; spatio-temporal kriging; caspian sea. 1. introduction caspian sea is encircled by five countries, namely islamic republic of iran, russia, republic of azerbaijan, turkmenistan, and kazakhstan. it is the largest extant part of the old tethys sea; an ancient sea which had extended from north pole to indian ocean in the first three eras of geology. disconnection of tethys occurred in the third period due to geological folding and resulted in the formation of famous caucasus and minor asia mountain ranges; consequently, an uplift happened in european continent and iran plain. this in turn, eventuated in emergence of caspian sea. as shown in figureure1, the caspian sea is situated north of the equator, from 36°33’ to 47°57’ north latitude and 46°43’ to 54°53’ east longitude. this geographical range covers an approximate length and width of 1030 and 240 km respectively [arpe et al., 2000]. the structural tectonic features in the south caspian sea region are particularly important for geodynamical studies. these are mainly due to the active tectonics and structure of the south caspian sea and its role in the collision between arabia and eurasia [jackson et al., 2002; brunet et al., 2003]. the south caspian depression is an autonomous tectonic unit that consists of: the southern part of the caspian sea; the west turkmen depression adjacent to the latter; the northern lowland periphery of the alborz system; the lower kura trough; and the apsheron–kobystan trough [mammadov, 2008]. this interaction between tectonic plates makes the region subject to frequent and strong earthquake. the south caspian basin is surrounded by active earthquake belts on all sides. the southern caspian sea stands out as an aseismic block surrounded by belts of intense earthquake activity. these phenomena cause deformations in the crust of the earth [jackson et al., 2002]. a large number of geodynamic studies based on crustal deformations are done by using observations that typically derived from absolute gravimetry [pagiatakis and salib, 2003], global positioning system (gps) [larson and van dam, 2000; johansson et al., 2002; caccamise et al., 2005; beavan et al., 2010], satellite laser ranging (slr) [noomen et al. 1996], doppler orbitography and radiopositioning integrated by satellite (doris) [soudarin et al., 1999], interferometric synthetic aperture radar – insar [dalla et al., 2012], and very long baseline interferometry (vlbi) [tesmer et al., 2009]. most of the crustal deformation studies are based on global navigation satellite system (gnss) observations due to high levels of accuracy, moderate associated costs, and broad spatial coverage in a relation to slr, vlbi and other techniques [blewitt, 2007]. however, along the south of caspian sea, particularly coastal region, a dense, long term time series and homogenous gps geodynamic network do not currently exist, and gps geodynamic stations are located far from the coastal region. consequently, studying geodynamic phenomena in south coastal area of caspian sea presents difficulties. therefore, other alternatives are necessary. sa and tg records provide alternative and independent methods for estimating vertical crustal motion [cazenave et al., 1999; lin, 2000; nerem and mitchum, 2002; fenoglio-marc et al., 2004]. due to strategic and economic importance of coastal areas and their impact on local people’s lives, dynamical changes of these areas and their sources have captured the interests of scientific communities in the recent years. multitude of factors such as meteorological effects, oceanic influences, tide, climate change and vertical crustal motion control the dynamics of these regions [garcía et al., 2007; pirouznia et al., 2017]. among these factors, the considerable effect that vertical crustal motion (vcm) has on tg measurements, makes it a subject of interest in geophysical studies. for this reason, hitherto, so much effort has been made to estimate this motion [see clark et al., 2002; nerem and mitchum, 2002]. tide gauges measure the relative sea level changes, which is the variation of the mean sea surface with a respect to the solid earth. to obtain such measurements, tg’s must be referenced to a benchmark on the crust of the earth. however, in the most parts of the world, locations of these benchmarks are subjected to the earth’s deformation processes such as earthquakes and their deformation cycles, tectonic plate motion, basin evolution, glacial isostatic adjustment (gia), and anthropogenic effects such as local subsidence due to water extraction, mining, oil or natural gas drilling. therefore, the signals of tg records contain both sea level changes and vcms [kuo et al., 2008]. observations of sa consist of (i) the precise satellite location in a relation to a reference ellipsoid, and (ii) the height of satellite over the surface measured using laser or radar pulses which are sent by satellite and bounced back from the target surface. subtracting (ii) from (i) gives the sea surface height (ssh) relative to the reference ellipsoid at the given location and time. on the other hand, the tg measurement gives the sea level height relative to the solid ground on which the tg is fixed. hence any vertical ground motion, regardless of its source, will be interpreted as an illusory change in sea level height observed by tg. as the land under the benchmark moves up and down, an apparent fall or rise will be seen in sea level which is due to the tectonic movement of sea floor. in other words, tg measures both pure sea level fluctuations which are variations in the water content and sea floor deformation which is due to tectonic activity. it is clear that the vertical ground motion can be obtained by subtracting these two data types, namely, altimetry and tg [nerem and mitchum, 2002; garcia et al., 2007; braitenberg et al., 2011]. shum et al. [2002], used historic tg records and a decade of topex/poseidon (t/p) and geosat altimeter data in a stochastic adjustment to compute the estimation of vertical motion on the great lakes regions. in other paper, mahmood pirooznia et al. 2 fenoglio-marc et al. [2004] calculated the rate of vcm in mediterranean sea, combining t/p data and a set of tg records based on sea level height changes. another research was performed by kuo [2004] in which he introduced a new method, namely gauss-markov stochastic adjustment model, and used it to estimate the vertical crustal motions (vcms) in areas near to open seas, semi-enclosed seas and lakes. kuo et al. [2008] combined geocentric sea level measurements from high-precision t/p satellite altimetry observations and tg records at 25 stations distributed in coastal regions of the baltic sea and around the great lakes. ostanciaux et al. [2012] computed vcm rates by combining tg records and local sa and then compared this database to previous studies that use geodetic techniques and tg records in order to evaluate the consistency of their results and previous ones. it should be pointed out that the quality of sa observations in coastal regions depends on the range between satellite orbit and a water surface, which is derived by measuring the time elapses between sending and receiving radar pulses from the satellite board and the surface of the earth (the altimeter waveforms) [fu and cazenave, 2001]. however, near the coasts, altimeter waveforms may be corrupted due to variety of reasons, and the range observations are infected by some biases [mathers et al., 2004]. to improve the precision of those altimeter observations with complex returning waveforms, a waveform re-tracking method is implemented to reprocess the original returning pulse. “re-tracking” refers to the analyses of the waveforms, received in the satellite antenna as pulses to improve the range [davis, 1995; gomez-enri et al., 2009]. three common re-tracking algorithms such as offset center of gravity (ocog), �-parameter and threshold can be used for this purpose [see rapley et al., 1987; martin et al., 1983; davis, 1997; deng, 2003; lee, 2008; zhang, 2009]. based on the author’s knowledge, the re-tracking correction has not been investigated in the studies of vcm using tg and sa observations and most of the papers ignored this correction. however, this effect may be very significant as it is later shown in this paper. in this study, missions of t/p, jason-1 and jason-2 altimeters are used for vcm’s determination at coastal parts of caspian sea (see figure 1). these missions have 10-day repeat cycles. since vcm is determined by difference between sa and tg observations, those passes of altimeter which cross near the tg station (adjacent to the coast) should be used. due to the problems of sa in the shallow water of coastal regions, choosing a suitable sa points near the tg stations are very important. so in this study, two strategies are implemented (i) selecting the closest sa point to the tg stations on the condition that the sa data is not corrupted. (ii) extrapolating sa points by spatiotemporal kriging method to evaluate the altimeter observation at tg stations. in both cases, however, the re-tracking algorithms are used to improve the range observations near the coast. tide effect should also be removed from sa and tg observations. in this study, to eliminate tide phenomena, it is modeled by considering significant tidal frequencies (empirical tide modeling), and we do not use global tide model such as fes2004 for tide correction. 3 vertical crustal motion at caspian sea figure 1. satellites crossovers above the caspian sea (tp, jason1 and jason2). mahmood pirooznia et al. 4 due to sample rate of sa observations (approximately 10 days for t/p, jason-1 and jason-2), we encounter with aliasing phenomenon for tide modeling (see section 2.4 for more explanation about aliasing phenomenon). so to deal with it, aliasing frequencies are used for tide modeling. eventually the rate of tide free sea level height time series of sa and tg are determined which indicate the absolute and relative changes in sea levels, respectively. difference of the above mentioned two rates gives the vcms. these vcms in turn, have been compared with vcms determined by continuous gps stations. 2. data source and pre-analysis of observations in this section we concisely describe our data sources and some preprocessing strategies, including re-tracking of sa observations, selection of suitable sa point and tidal analysis, being used to prepare the two data sources for determination of vcm at caspian sea. 2.1 data sources tables 1 and 2 show the features of sa and tg data which are used in this study. sensor geophysical data record (sgdr) of sa is used in this study due to containing along-track waveform information that may be used for applying re-tracking corrections for the range observations. tide gauge data in this study is provided from ports and maritime organization (pmo) of iran which were recorded by floating pressure tide gauges. this data is not available in international databases and no processing for quality control of which were applied. table 1. sa data used in this study. mission cycles period source topex/poseidon 001 365 1992/09/23 2002/08/11 (nasa) jason 1 001 259 2002/01/15 2009/01/16 (nasa,aviso) jason 2 001 226 2008/07/04 2014/08/21 (aviso) table 2. location and time duration of tgs (coastal areas of caspian sea). location latitude longitude duration (time) source anzali 37.478° 49.4623° 21/3/2005-20/3/2014 ports and maritime organization (pmo) of iran noshahr 36.6584° 51.5047° 21/3/2006-21/3/2014 ports and maritime organization (pmo) of iran neka 36.8502° 53.3656° 1/1/2000-31/8/2012 ports and maritime organization (pmo) of iran 2.2 re-tracking of sa observations the observable quantity in satellite altimetry is the range observation which is known as satellite range or satellite height above sea level. the range observation is contaminated by some source of errors which must be corrected via global or local modeling and can be summarized as follows: wet troposphere correction, dry troposphere correction, ionosphere correction, electromagnetic bias correction, ocean tide correction, inverse barometer correction, pole tide correction and center of gravity movement. they are better known as “geophysical corrections” [benada, 1997; pirooznia et al., 2016a; soltanpour et al. 2017]. also the re-tracking correction is used to correct range observation. the three common re-tracking methods, namely ocog, �-parameter, and threshold are implemented. these corrections should be applied on satellite range for each repeat cycle to create the time series of sea height [khajeh et al., 2014; khaki et al., 2014]. thus, corrected sshs are derived from the following equations. (1) where c is the speed of light (299,792,458 ), 𝜏 the pulse duration (3.125 ns for t/p), g0 the nominal tracking gate (for t/p, jason-1 and jason-2, it is 32.5, 31 and 31, respectively ), g the computed tracking gate (middle point of the leading edge which is computed from re-tracking algorithms), 𝛥𝑟 the re-tracking correction, h the altitude of the satellite from the reference ellipsoid, r the observed range, 𝛥𝑔 the geophysical corrections, and ssh is corrected sea surface height. as can be seen in computational analysis, the mean computed biases may be greater than 35 cm. table (5) shows the correlation coefficient between tg and sa time series, for some re-tracking methods. it indicates that this correction can improve sa observations in coastal regions. 2.3 selection of suitable sa points due to the well-known problems of sa technique near the coastal regions and shallow waters, choosing a suitable sa point to compare with tg station is of crucial importance. for this purpose, two strategies are considered in this study. for the first strategy, we select those sa ground tracks that pass very close to the tg station provided that the range observation is available on them (see table 3). it should be mentioned that the closer the satellite ground tracks get to the tg stations, the more accurate would be the results. in 2002, nerem and mitchum [2002] showed that as the satellite tracks get closer to the coastal tg station, their time series get more similar. furthermore, increasing the number of cycles will result in the reduction of total error of modeling. thus if there are more than one pass near the tg station, we consider all of them. for example, according to table 3, for anzali tg, three points are suitable points for comparison. this enables more reliable estimation of vcm. choosing the pass of sa close to the tg station as it is done in the first approach, means that the selected sa points are still some distances apart from tg stations. moreover it means that no rigorous criteria is applied to determine the true distance. how much the two passes can be distance apart? moreover, there is no sa pass that exactly crosses the location of tg station. to deal with this issue, an extrapolation method is used to predict the value of sa observation at the location of tg station in order to further improve the accuracy of vcm. in this approach three point-wise sa sea level time series are created. then, the spatio-temporal ordinary kriging is used for predicting point-wise sea level time series at tg stations. ordinary kriging is a geo-statistical interpolation method, which was originally developed for spatial data by krige (1952). in this study, sa measurements along the caspian sea, which are scattered in space and time, are used to predict the sea level at any time and location along the caspian sea. kriging creates a statistically unbiased estimator that is optimal with a respect to the mean squared prediction error. the predictor is a weighted average of the observed values, in which the weights depend on spatial or spatio-temporal dependence of observed and predicted locations. the dependency is expressed by the covariance c ((x1; t1); (x2; t2)) between the sea levels at two space-time points (x1; t1) and (x2; t2). the required covariance models reflect that the sea level, are estimated based on empirical covariance values between sa observations at 𝛥𝑟 = 𝑐 2 × 𝜏 × (𝐺 − 𝐺0) 𝑆𝑆𝐻= ℎ − (𝑅 + 𝛥𝑟 + 𝛥𝑔) 5 vertical crustal motion at caspian sea various locations. it is obvious that the construction of an appropriate covariance function c ((x1; t1); (x2; t2)) to predict the sea level is of particular importance for sa data along the caspian sea. for more details about spatiotemporal kriging and empirical covariance function see boergens, et al. [2016]. figure 2 shows a typical example of an empirical and modeled covariance functions for cycle 10 of t/p satellite pass 16. the modeled covariance function is used for prediction. mahmood pirooznia et al. 6 considerations duration position name of the point distance km a point close to anzali tg from pass 16 1992-2014 lat:38.31° lon:50.18° anzali-16 112.7 a point close to anzali tg from pass 209 1992-2014 lat:38.70° lon:49.26° anzali-209 137.8 a point close to anzali tg from cross over passes 16 and 209 1992-2014 lat:39.20° lon:49.59° anzali-crossover 192.7 a point close to noshahr tg from pass 16 1992-2014 lat:36.95° lon:51.12° noshahr-16 48.7 a point close to noshahr tg from cross over passes 16 and 31 1992-2014 lat:37.09° lon:51.02° noshahr-crossover 64.9 a point close to neka tg from passes 107 1992-2014 lat:37.01° lon:53.8° neka-107 42.5 a point close to neka tg from cross over passes of 107and 92 1992-2014 lat:37.09° lon:53.85° neka-crossover 50.6 table 3. specification of closest points to tg stations derived from satellite ground track by the first strategy. figure 2. empirical and fitted (exponential) temporal (a) and spatial covariance functions (b). a) b) in table 4 the extrapolated sa points near each tide gauge station is shown. figure 3 shows the position of tg stations, sa points, extrapolated sa points and available gps stations in the southern coast of caspian sea. the gps stations will be later used to determine the absolute vertical deformation at this region and in order to make a comparison with the deformation obtained by combination of sa and tg observations. 2.4 tidal analysis finally, the tidal effects should be removed from both data sources. since, global tide models are limited in the number of tidal components (frequencies), and for comparison of these two data sources, it is necessary that remove the similar tidal components from both observational sources; instead of using global tide model, local tide model 7 vertical crustal motion at caspian sea table 4. extrapolated sa points at the location of tg stations using spatio-temporal kriging. considerations duration position name of the points a point close to anzali tg 1992-2014 lat:37.48 lon:49.45 anzali-01 a point close to noshahr tg 1992-2014 lat:36.63 lon:51.50 noshahr-01 a point close to neka tg 1992-2014 lat:36.84 lon:53.35 neka-01 figure 3. distribution of sa, tg, sa kriging and gps stations (kriging stations are extrapolated from observed3 stations on the ground track of satellite altimetry). (the apparent distance between extrapolated sa point and tg 4 station is for better graphical representation. they are actually at the same location). should be conducted in order to calculate the tidal effects precisely. in this case, tide modeling is done by the following equation [hou and vaníček, 1994; okwuashi and ndehedehe, 2017]: ℎ(𝜙,𝜆,𝑡) = 𝑍₀(𝜑,𝜆) + 𝑎 × 𝑡 + ∑�� =₁ 𝑐�cos𝜔�𝑡 + 𝑠�sin𝜔�𝑡 (2) in which 𝑍₀(𝜑,𝜆) stands for mean sea level of caspian sea, 𝜔� = 2𝜋𝑓� , (𝑖 = 1,2,..., 𝑛) is the angular frequency of tidal components, 𝑡 and ℎ(𝜙,𝜆,𝑡) denote the time of observation and sea level height respectively. we have two kinds of tide model, one for sa observation and the other for tg observation each done for all available time series of sa and tg. for the first model, ℎ(𝜙,𝜆,𝑡) stands for the sea level derived for sa measurement while for the second model, ℎ(𝜙,𝜆,𝑡) stands for the sea level derived for tg measurement. 𝑎 is the rate of sea level height and the parameters 𝑐� and 𝑠� denote fourier coefficients which are the unknowns of the problem and must be estimated through least square optimization. after implementing this analysis, the phases φ� and amplitudes 𝐴� of essential tidal components can be derived as (pirooznia et al, 2016b). 𝐴� = �(𝑐²� + 𝑠²� ) 𝛷� = tan‒1(𝑐�⁄𝑠�) (3) to determine the optimum number of tidal frequencies, a spectral analysis is conducted which reveals that tide gauge observations in our case include 40 significant frequencies. it should be mentioned that the analysis of sa time series, suffers from aliasing phenomena. according to shannon sampling theorem (william, 1977; shannon, 1984), if 𝜐 is the highest present frequency in a signal, a sampling rate of at least 𝑓� = 2𝜐 (sample-second) would be adequate to reconstruct the whole signal in which, 𝑓� is the well-known nyquist frequency. in other words, a signal can be fully reconstructed or recovered by sampling it at series of points which are 1/2𝜐 seconds apart. if there is any frequency content in the signal that is higher than this sampling frequency, it will be aliased to the lower frequency. in the case of sea tide, periods of sa observations (10 days) are longer than period of tidal components and thus the determination of tidal effects using this data is subjected to aliasing problem. to deal with this, aliasing period is defined by the following equation [lindsley and long, 2011]. (4) in which, 𝑓 is the frequencies of tidal constituent, 𝛥𝑡 is sampling period in days (orbit repeat period for satellite jason2 is 𝛥𝑡 = 9.9156 𝑑𝑎𝑦𝑠) and [•] is used for a fix function1 which rounds decimal numbers toward zero [lindsley and long, 2011]. it should be emphasized that the sea level time series from tg observations may be contaminated with outliers and it may suffer from offset and gap in the time series [see pirooznia et al., 2019]. these problems have been resolved with a proper strategies and then the time series have been constructed. the main problem with caspian sea tide gauge stations is the improper collecting and compiling of them. in this study, the quality control of data is done according to steps of quality control which previously discussed in pirooznia et al. [2019]. a quality control of sea level observations is important task in a detecting and removing the possible errors, such as outlier, offset that may contaminate the tide gauge data time series [woodworth et al. 2012; ioc 1992]. these errors can greatly affect the results of time series analysis (see http://www.sonel.org/_quality-control-of-the.html). the existence of outliers in the time series of observations can introduce some biases in the estimation of model parameters. outliers may increase the confidence intervals for the model parameters, and thus, they may strongly influence modeling and predictions [chen and liu 1993a, 1993b]. offset in the time series of tide gauge may occur during data acquisition process, due to some phenomena such as, boat hitting, tide gauge damage, earthquake motions and wrong recalibration of the instrument. occurring of offsets in the observations can change the statistical parameter of signal and may affect on the trend computation [gazeaux et al. 2013; montillet et al. 2015; lopez bernal et al. 2016]. the first step in a dealing with these types of errors is modeling sea level time series. in this study, the functional model of the tide (eq.) is used 1 the difference between int and fix is in negative numbers; for a number like -9.2 int returns -10, whereas fix returns -9. 2𝜋𝛥𝑡 2𝜋(𝑓𝛥𝑡-[𝑓𝛥𝑡 + 0.5]) 𝑇𝑎 = mahmood pirooznia et al. 8 http://www.sonel.org/_quality-control-of-the.html to model sea level. before that modeling, the low pass filtering (savitzky–golay filter) is used to reduce the noises in sea level time series [shekhar, 2016]. then, the time series is constructed (modeled) using significant frequencies and the 3-sigma threshold method is used to eliminate the possible outliers in the observations. subsequently, for offset detection, noise assessment algorithm (least square variance component estimation) is used to find the effective noise component in the time series, and using the cofactor matrix of the noise as the weight matrix of observations, the observations are adjusted and the signal is processed to find the possible times of offsets in order to remove their corresponding observations from the time series. for more details see pirooznia, et al. [2019]. for modeling sea level time series, the significant frequencies should be found. for this purpose, spectral analysis is performed on the sea level time series. to this end, angular frequencies are created using the period of observations (𝜔 = 2𝜋/𝑇). assume that �𝜔₁,𝜔₂,...,𝜔�� is the set of real and positive angular frequencies of the observation vector that may be equal to tidal angular frequencies, or they can be constructed by period of observations. in this study, in order to find effective frequencies, the angular frequencies are determined based on the period of observations. to check the effective frequencies, the significant test is employed [amiri-simkooei and asgari, 2011] and the effective frequencies which pass this test are selected. the result of spectral analysis on sa and tg of caspian sea shows that the annual and semiannual components have the highest effect on time series (see figures 4 and 5). as mentioned previously, spectral analysis of sa time series due to sampling rate of observations faces with the aliasing phenomenon which creates the aliasing frequency. as an example, for the component of half daily s2, the main frequency is 2 cycles per day, whose aliased frequency is 0.01702 cycles per day corresponds to period of 58.74 days (see figure 4). figure 4 shows a typical power spectrum for sa time series of anzali-crossover point, alias frequency of s2 component, annual (sa) and semi-annual (ssa) frequencies. the results of tide analysis indicate that the annual (sa) and semiannual solar (ssa) components on all of the sa and tg stations have the highest amplitude in comparison with the other components. the amplitude of annual components in three tg stations (azali, noshahr and neka) are 16, 18 and 15 cm respectively, and for semiannual components are 2.8, 5.4 and 3.7 cm, respectively (figure 5). 9 vertical crustal motion at caspian sea figure 5. tide analysis of three tide gauge stations (anzali, noshahr and neka). figure 4. power spectrum of anzali-crossover point sa time series, alias frequency of s2 component, annual (sa) and semi-annual (ssa) frequencies. the tables which include amplitudes and phases of tidal components (both sa and tg data stations) are added in the appendix a. figures 6-8 show the observed time series (sa and tg), tidal effect (sa tide and tg tide) and the corrected time series (show by residual signal in the figures) in anzali, noshahr and neka stations. in each figure we show, (i) time series of tg observation (ii) time series of sa observations at selected sa points using two adopted approaches (referring to table 3, 4 and figure 3 for more explanations). (iii) corrected time series after removal of tide model. the composed tidal model appropriately estimates the general variations of sea level height time series. judging from sa and tg time series ordinates (see figures 6-8), it is evident that the caspian sea level has been decreasing during the last decades. according to arpe et al. [2012], the volga river as the main source of caspian sea water supply, has brought less water to the sea. mahmood pirooznia et al. 10 figure 7. left: observed sea level time series (purple line), tide model (blue line), and the mean sea level (red dashed line) at points noshahr-16 sa, noshahr-crossover sa, noshahr-01 sa and noshahr tg station. right: residual (green line) at points noshahr-16 sa, noshahr-crossover sa, noshahr-01 sa and noshahr tg station. figure 6. left: observed sea level time series (purple line), tide model (blue line), and the mean sea level (red dashed line) at points anzali-16 sa, anzali-209 sa, anzali-crossover sa, anzali-01 sa and anzali tg station. right: residual (green line) at points anzali-16 sa, anzali-209 sa, anzali-crossover sa, anzali-01 sa and anzali tg station. 3. numerical results and discussion of vcm in this section, we present the numerical results of vcm at the coastal area of caspian sea. to do so, the sa and tg observations that are processed based on the steps of previous section are subtracted from each other. as mentioned previously, to improve the accuracy of sa time series, the re-tracking correction is implemented to range observation. table 5 shows the correlation coefficient between the sea level that is resulted by anzali, noshahr and neka tide gauge observations and sa observations (anzali-16 point, noshahr-16 point and neka-107 point), without applying any re-tracking and while the re-tracking is performed. in this region the threshold re-tracking method, by 35% as threshold value, presents the highest correlation. that means, by using this re-tracking method the most accurate measurement of sea level will be achieved. it should be emphasized that the vertical datum of sa and tg observations are different; thus, to compute the correlation between sa and tg time series, after normalizing the time series of observations, the correlations are computed at the times when sa and tg observations overlap. also due to the well-known problems of sa technique near the coastal regions and shallow waters, choosing a suitable sa point to compare with tg station is of crucial importance. for this purpose, two strategies are considered in this study. 11 vertical crustal motion at caspian sea figure 8. left: observed sea level time series (purple line), tide model (blue line), and the mean sea level (red dashed line) at points neka-107 sa, neka-crossover sa, neka-01 sa and neka tg station. right: residual (green line) at points neka-107 sa, neka-crossover sa, neka-01 sa and neka tg station. table 5. correlation coefficient between tg and sa time series, for some re-tracking methods. method correlation coefficient between anzali tg and anzali-16 sa point correlation coefficient between noshahr tg and noshahr-16 sa point correlation coefficient between neka tg and neka-107 sa point not re-tracked 0.55 0.46 0.53 ocog 0.77 0.68 0.62 �-parameter 0.83 0.74 0.69 threshold (15%) 0.72 0.68 0.73 threshold (25%) 0.82 0.76 0.76 threshold (35%) 0.85 0.82 0.81 threshold (40%) 0.79 0.72 0.77 threshold (45%) 0.78 0.70 0.71 threshold (50%) 0.73 0.64 0.75 sa and tg respectively indicate the absolute and relative changes in sea levels. difference of the abovementioned two rates gives the vcms. table. 6 lists the trends of both sa and tgs, their uncertainties and differences. the average vertical crustal motion estimated from sa-tg at anzali, noshahr and neka are -1.387, -1.903 and -3.14 mm/year, respectively. figure 9 shows mean value of vcm at tg stations. analyzing gps time series at geodynamic stations is an alternative approach to obtain vcm’s. this analysis supplies the auxiliary source to be compared with the method of this study. in this study, four geodynamic stations are used, namely, mabd, nkad, rsht and tkbn. table 7 shows the approximate positions of gps stations, time span of their observations and their distances to closest tg stations. these gps time series were provided by national cartography center (ncc) of iran. all processing steps were done in ncc. the processing of gps raw data was done by gamit software, also it was integrated using globk software [herring et al., 2015]. according to recent report in ncc (www.ncc.org.ir), the first step in processing this data, is controlling the quality of rinex data. this operation is done with teqc software. then data processing on a daily basis is done using double differences method. finally, the precise orbital data (sp3 files from international gnss service (igs)) with a delay of two weeks, mapping function equations for troposphere modeling, ocean and solid tide modeling and solar radiation model are applied. moreover, the igs stations are referenced with a respect to itrf 2014 reference frame to minimize the calculation errors and increase the accuracy. figure 10 shows vertical gps time series at geodynamic stations. mahmood pirooznia et al. 12 figure 9. mean vcm estimated from sa-tg. table 6. the rate of sea level changes via sa and tg’s and vcm estimation using sa-alt. tg stations rate (mm/year) sa points rate (mm/year) vcm=sa-tg (mm/year) anzali -26.40±0.65 anzali-16 -27.78±1.12 -1.38±1.29 anzali-209 -27.24±0.73 -0.84±0.97 anzali-crossover -27.96± 0.91 -1.56±1.11 anzali-01 -28.17± 0.83 -1.77±1.05 noshahr -30.22± 0.81 noshahr-16 -32.85± 0.55 -2.63±0.97 noshahr-crossover -31.50± 0.76 -1.28±1.11 noshahr-01 -32.02± 0.63 -1.8±1.02 neka -21.10±0.48 neka-107 -23.41± 1.03 -2.31±1.13 neka-cross over -26.11± 0.63 -5.01±0.79 neka-01 -23.2± 0.85 -2.1±0.97 http://www.ncc.org.ir in order to compare vcms from gps with those from combination of sa and tg, it is necessary that gps stations are located near to tg stations. even though these gps stations are not located sufficiently close to our tg stations (see table 7), both of them are positioned on the same faults (see figure 11). 13 vertical crustal motion at caspian sea table 7. positions of geodynamic gps stations and observation time span. name of station approximate position duration distance to the nearest tg stations mabd lat:36.58 lon:52.28 6/8/2005-18/7/2013 101.322 km to neka tg station nkad lat:36.68 lon:51.3 18/10/2005-08/9/2012 18.148 km to noshahr tg station rsht lat:37.31 lon:49.61 1/11/2005-18/7/2013 22.184 km to anzali tg station tkbn lat:36.78 lon:50.91 16/8/2005-08/9/2012 54.540 km to noshahr tg station figure 10. vertical gps time series at mabd, nkad, rsht and tkbn stations. figure 11. the location of gps and tg stations in the dispersion of faults. the listed locations for tg and geodynamic stations shows that there is a notable distance between them. due to long distances (few kilometers) between geodynamic (gps) and tg stations, comparing their results may look unsound. however, from tectonic point of view, these stations are situated on a common fault and their comparison would be meaningful. the resulted vcm’s from previous section are compared with the crustal motions deduced from gps time series at geodynamic stations. the comparison shows an acceptable consistency (table.8). table 8 indicates that there is a good consistency between (sa-tg) and gps vertical rates. the mean value of the trend differences between the closest gps and the sa-tg vertical rates is 0.82, 0.21 and 2.71 mm/year for anzali, noshahr and neka tg stations, respectively. at all stations, the rates of vcm are negative which indicates the subsidence phenomenon. various factors may be responsible to this subsidence: tectonic changes due to existing faults, irregular groundwater extraction and etc. [pacheco-martínez et al., 2015; carreón-freyre et al., 2016]. according to the studies conducted by the iranian ministry of energy (http://www.moe.gov.ir) using the data of piezometric wells, the rate of groundwater extraction has significantly increased in the recent years. this conclusion can be used as a confirmation to the negative rates of vcm in these areas. for instance, the highest rate of vertical change at mabd station might have originated from the extravagant amount of groundwater extraction which caused the crust to subside. for more investigation, the correlations between ground water level data of three wells near anzali, noshahr and neka tg stations with gps vertical time series of rsht, nkad and mabd stations are computed. these piezometric wells data were recorded from 23-jul-2002 until 15-dec-2014, from 28-jan-2003 until 21-dec-2013 and from 16-aug-2004 until 29-dec-2013 with sampling interval of approximately one month near anzali, noshahr and neka tg stations, respectively. these wells are located at 37.36° north latitude and 49.86° east longitude, 36.63° north latitude and 51.50° east longitude, 36.72° north latitude and 53.26° east longitude, respectively. to compute the correlations between piezometric wells data and gps time series, after normalizing the time series, the correlations are computed at the times when piezometric wells data and gps observations overlap. table 9 shows the computed correlations. mahmood pirooznia et al. 14 table 8. the rate of vcm via sa and tg’s along with gps vertical rate at geodynamic stations. station sa tg gps points time span rate (mm/year) station time span rate (mm/year) anzali anzali-16 2005-2013 -0.82±0.72 rsht 2005-2013 -0.05±0.46 anzali-209 2005-2013 -1.06±0.84 anzali-crossover 2005-2013 -0.71±0.68 anzali-01 2005-2013 -0.91±0.76 noshahr noshahr-16 2005-2013 -2.32±1.09 tkbn 2005-2013 -2.10±0.60 noshahr-crossover 2005-2013 -2.16±0.76 nkad 2005-2013 -1.51±0.43 noshahr-01 2005-2013 -2.45±1.35 neka neka-107 2005-2012 -5.12±1.07 mabd 2005-2012 -7.92±0.61neka-cross over 2005-2012 -5.64±0.86 neka-01 2005-2012 -4.85±1.01 http://www.moe.gov.ir/ figure 12 shows the groundwater level time series near anzali, noshahr and neka tg stations. the rates of groundwater level time series are negative which may be the sign of groundwater extraction 4. conclusion and recommendation in this paper, the pattern of vertical deformation in coastal parts of caspian sea is determined using a combination of satellite altimetry observations and tide gauge measurements. satellite altimetry measures the absolute sea level, while tide gauge data computes the relative sea level. thus the difference between these two observations can be used to determine the vertical motion at ground station. the result show that the sea level of caspian sea from 1992 to 2014 has had a decreasing trend. moreover, the vcms at the desired stations are negative that may indicate land subsidence. typically, the numerical results illustrate that by considering sa crossover points, the vertical crustal motion at anzali, noshahr and neka are -1.56, -1.28 and -5.01 mm/year, respectively. the innovations of this study are as follows: using re-tracking correction for sa data neat coastal tg stations that increases the correlation between sa and tg sea level time series, application of spatio-temporal kriging method by creating the empirical covariance functions to interpolate sa signal at the tg stations and implementing local and more accurate tide modeling instead of using global tide model such as fes2004. the future issues that may significantly improve the quality of the results are reported below the precision of orbit determination for satellite altimetry is 1-2cm. this leads to a precision of about 4-5cm in sea surface height. an increase in precision of orbit determination will result in a more accurate sea surface height data. 15 vertical crustal motion at caspian sea ground water level data near tg stations gps stations correlation coefficient anzali rsht 0.62 noshahr nkad 0.47 neka mabd 0.43 table 9. the correlation coefficient between ground water level data of three wells near tg stations and gps. figure 12. the underground water level time series near anzali, noshahr and neka tg stations. tg stations should gather data in a unified vertical datum. this unification is of fundamental importance and simplifies the calculation process. in some researches the mean sea level (msl) which is determined from sa and tg data is used as an alternative for estimation of vcm, in this case there is no need to calculate the rate of change and vcm is calculated by subtraction of msl from sa and tg data. this needs all tg stations to be geo-referenced. tg stations should be calibrated precisely. furthermore, tg observations at nearby stations, should be given appropriate weightings and combined with each other in order to increase the accuracy. to integrate the elevations of different tg’s, stations must be fixed to a local benchmark; this benchmark then must be transferred to a national and finally an international datum. this procedure requires a modern geodetic technique. various tectonic factors involved in vertical deformation and land subsidence due to groundwater extraction may be considered as causes of vcm at coastal regions of caspian sea. one of the drawback of this study is the lack of tg data for a long time. 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(2009). satellite radar altimetry for inland hydrologic studies, ohio state univ., columbus, oh, usa, rep. no. 491. *co r r e s po n d i n g a u t h o r: mehdi rao o f i a n n a e e n i, department of geodesy, faculty of geodesy and geomatics engineering, k. n. toosi university of technology, e-mail: mraoofian@kntu.ac.ir © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 19 vertical crustal motion at caspian sea articolo mccalpin.pdf key words surface rupture – kachchh earthquake – thrust fault 1. introduction the 2001 bhuj-kachchh earthquake was one of the largest events in the stable continental regions (scr) of the world. comparable events are the new madrid earthquakes of 1811-1812, the charleston earthquake of 1886, and the 1819 kachchh (allah bund) earthquake (fig. 1). therefore, the 2001 bhuj-kachchh earthquake (mw 7.7) is an important event for the comparative study of two big earthquakes in the same tectonic province. kachchh has a long history of earthquakes and has experienced two major events and tens of smaller events within the past 200 years. the vast wastelands and salty playas are probably the result of a few major earthquakes in the past 3000-4000 years. the nature and the dates of all those earthquakes, except the 1819 event and its penultimate event (rajendran et al., 1998; rajendran and rajendran, 1999, 2001; rajendran, 2000), are yet to be known. in the present paper we attempt to relate the nature of the 2001 bhuj earthquake with the regional structures, especially with the 937 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. james p. mccalpin, geo-haz consulting inc., p.o. box 837, crestone, co 81131 u.s.a.; e-mail: mccalpin@geohaz.com 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india james p. mccalpin (1) and mahesh g. thakkar (2) (1) geo-haz consulting inc., crestone, co, u.s.a. (2) department of geology, shri r.r. lalan college, bhuj, kachchh (gujarat), india abstract primary and secondary surface deformation related to the 2001 bhuj-kachchh earthquake suggests that thrusting movement took place along an e-w fault near the western extension of the south wagad fault, a synthetic fault of the kachchh mainland fault (kmf). despite early reconnaissance reports that concluded there was no primary surface faulting, we describe an 830 m long, 15-35 cm high, east-west-trending thrust fault scarp near where the seismogenic fault plane would project to the surface, near bharodiya village (between 23°34.912 n, 70°23.942 e and 23°34.304 n, 70°24.884 e). along most of the scarp jurassic bedrock is thrust over quaternary deposits, but the fault scarp also displaces holocene alluvium and an earth dam, with dips of 13° to 36° south. secondary co-seismic features, mainly liquefaction and lateral spreading, dominate the area south of the thrust. transverse right-lateral movement along the «manfara fault» and a parallel fault near bharodiya suggests segmentation of the e-w master faults. primary (thrust) surface rupture had a length of 0.8 km, maximum displacement of about 35 cm, and average displacement of about 15 cm. secondary (strike-slip) faulting was more extensive, with a total end-to-end length of 15 km, maximum displacement of 35 cm, and average displacement of about 20 cm. quaternary morphotectonic features along the major e-w and transverse fault systems. a unique model is also proposed herein to show segmented movement on the e-w oriented active faults, with ne-sw, nw-se, nnessw or nnw-sse transverse faults being the limit of the individual blocks, during the 2001 earthquake. this paper is the result of 2 weeks of field mapping in may 2001, during which time the authors traced surface ruptures on the ground and mapped them with gps. in addition we excavated one exposure of the thrust fault in a streambank and logged another trench excavated by a previous french reconnaissance team. 1.1. geomorphology the geomorphology of kachchh is influenced by its regional tectonic setting and is dominated by the basin bounding master faults and main axis of the basin which runs wsw (biswas, 1981). each physiographic division of kachchh is flanked by a major structural feature. there are four physiographic divisions identified, the ranns, the low-lying banni plain, the hilly regions and the southern coastal plain (fig. 2a). the ranns are vast flat regions covered with a fairly hard salt encrustation. these are the areas of large continental, recent sedimentary basins rising not more than 4 m above msl. the little rann is an extension of 938 james p. mccalpin and mahesh g. thakkar pakistan kachchh hyderabad 1819 (7.5 mw) bharodiya budharmora 2001 (7.7 mw) 1956 (6.0 mw) bhuj anjar manfara ahmedabad broach sa ba rm at i r . m ah i r . porbundar arabian sea gulf of kachc hh gulf of cambay km n 0 100 24 68 70 72 e 22 ° ° ° ° ° ° 26 n india gujarat india gujarat india thar desert s ab ar m at i r . m ah i r . nar mad a r . tapti r. mumbai delhi fig. 1. location map showing epicenters of the 26 january 2001 and 1819 allah bund earthquakes, and villages mentioned in the text. the gulf of kachchh when the sea level was high during the holocene transgression. the great rann of kutch is bounded by the nagar parker fault on the north and the island belt fault and the plain of banni (a low alluvial tableland between the mainland of kachchh and the great rann) on the south. the banni is a smooth and nearly flat saline grassland. the uplifted hilly region of kachchh consists of three uplifts: 1) 4 separate anticlines that compose the island belt fault zone; 2) the kachchh mainland uplift, and 3) the wagad uplift. the island belt fault zone comprises four «islands», viz. pachchham, khadir, bela and chorar, controlled and separated by the right-lateral transverse and strike-slip faults. the northern boundaries of these «islands» are controlled mainly by the island belt fault, where steep scarps and colluvial fans are common. the table-like region with low dip due south and southwest of khadir and bela islands is known as the wagad highland. the kachchh mainland uplift is composed of rocky terrain having two sub-parallel e-w trending hill ranges; the katrol hill range in the middle and the dome-dominated northern hill range. the mainland also comprises a strip of coastal plain in the south. 1.2. regional structure and tectonics rifting at the western margin of the indian plate was controlled by the ne-sw trend of the delhi (pre-cambrian) fold belt that swings to e-w in kachchh. the e-w striking master faults are resurgent basement faults (biswas, 1987) that controlled the structural style of the basin. in the post rifting stage, kachchh was under a compressional regime when the foot2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india bhuj bhachau anjar mundramandvi suthri naliya banni plains sa ur as ht ra little rann pu legend streams pu, ku pachham, khadir bu, cu bela, chorar uplift great rann of kachchh 23° 00 70° 71°69° 00´ 00´ 00´ gul f of kac hch h arabian sea n 0 20 km gandhidham lakhpat koteshwar bhujpur cu ranns mudflats 023 30` buku island belt fault northern hill range k m frocky plain katrol hill range k h f coastal alluvial plain kmf kachchh mainland fault khf katrol hill fault swf south wagad fault gdf gora dungar fault khavda rapar nagar parker fault wagad higland (uplift) f f f f swf f f f f ff gedi fault nor th k ath iyaw ad fau lt kachchh mainland (uplift) g df fig. 2a. map showing major geomorphic subdivisions, mountain fronts, and late cenozoic faults on the mainland of kachchh. kmf = kachchh mainland fault. 939 wall blocks uplifted along the e-w master faults and formed the highlands, and intervening half-grabens formed the plainlands. it is during this stage the blocks tilted towards south, and therefore the nature of the master faults also changed from normal to reverse and strike slip. the uplifts are oriented e-w along five parallel faults: 1) nagar parkar fault; 2) island belt fault; 3) south wagad fault (swf); 4) kachchh mainland fault (kmf); 5) north kathiawar fault (nkf) (fig. 2a). in addition to the e-w striking master faults, the associated uplifts are extensively affected by oblique secondary faults of later age. the secondary faults trend ne-sw or nw-se and offset the primary faults, so are termed transverse strike slip faults. 1.3. neotectonics systematic study of quaternary tectonic activity in kachchh was begun only in the past decade, so there is no sizable amount of work available. however, thakkar et al. (1999) and maurya et al. (2002, 2003) constructed a quaternary stratigraphy and tectonic evolution of mainland kachchh, respectively. quaternary tectonic uplift took place in two major phases. the early quaternary tectonic activity took place along the e-w trending faults while the late pleistocene phase took place along the nne-ssw to nnw-sse trending transverse faults. the e-w trending faults were more active during early quaternary, as evidenced by miliolites overlapping the colluvial deposits along the katrol hill fault (thakkar et al., 1999). this geometry suggests that the present configuration of the landscape came into being during the early quaternary due to differential uplift along e-w faults, viz. the kmf and khf (fig. 2a). the early quaternary physiographic setting has been modified by the late pleistocene-holocene tectonic activity along transverse faults (thakkar et al., 2001). the kachchh region has experienced large and moderate earthquakes in historic times. the 1819 kachchh earthquake (mw 7.5) is the largest event to have occurred in the rann of kachchh near the india-pakistan border during historic time, and the 1956 anjar earthquake (mw 6.0) is the largest during the instrumental period. the anjar event was located nearly 40 km west of the epicenter of the 2001 event. it originated at a focal depth of 15 km and is believed to have occurred on the kmf (one of the southern boundary of the kachchh rift), whereas the 1819 earthquake occurred on the northern boundary of the kachchh rift about 140 km nw of the 2001 event. morphological evidence of maximum uplift of about 6 m in 1819 showing a northerly slope suggests that it was associated with thrust motion on a north-dipping fault (oldham, 1926; bilham, 1998; rajendran and rajendran, 1999). apart from the major events, minor earthquakes are also recorded in the area and appear to cluster along the active fault lines. neotectonic landforms of the area suggest that most large holocene earthquakes have occurred along transverse faults. malik et al. (1999) concluded that the seismicity of mainland kachchh was confined to the kmf and khf, but a large number of epicentral plots of earthquakes of historic period compiled by them around bhuj show distinct clustering around transverse faults, indicating that the latest phase of tectonic activity was more along the transverse faults. thakkar et al. (1999) suggested that the compressive stresses accumulating on the e-w latitudinal faults of mainland kachchh, due to locking up of the indian plate in the ne, are transmitted to the nne-ssw and nnw-sse transverse faults. the change in tectonic environment from extensional to compressional is also responsible for the activity of transverse strike-slip faults in kachchh. the focal mechanism of the 2001 bhuj and 1956 anjar earthquakes suggested by usgs also indicates tectonic reversal in the region. the stress field oriented in the n-s to nne-ssw direction is considered to be responsible for this reversal of movement and the ongoing deformation (rajendran et al., 2001). 2. the 26 january, 2001 bhuj earthquake the 26th january 2001 bhuj earthquake is the most disastrous earthquake in the post-instrumental era in india. the 2001 bhuj earthquake originated at 08:46:41 ist with the focal depth of 22 km and a magnitude of 7.7 mw, with the epicenter located at 23.36°n, 70.34°e (bendick 940 james p. mccalpin and mahesh g. thakkar et al., 2001; our fig. 1). the number of lives lost during 2001 bhuj earthquake exceeded 25.000 and nearly 200.000 people were injured. about 400.000 houses suffered severe to complete damage and twice as many were partially damaged. according to the preliminary analysis of part of the aftershock data collected by the middle america earthquake center, the coseismic fault plane dips 40°-50° south. projection of this plane to the surface predicts an intersection 7-8 km north of bharodiya village. kayal et al. (2002) concluded that the best located aftershocks define two major fracture directions, one trending ne and the other nw, and the ruptures meet at a zone 10 km south of main shock epicenter. further, kayal et al. (2002) obtained two well constrained solutions for two clusters of aftershocks at the mid-crustal depth. the nw cluster of events shows pure reverse faulting with sw dipping, nw-se-striking preferred fault plane, and a ne cluster shows reverse faulting with a large strike-slip component on se dipping, ne-sw-striking fault plane. this earthquake generated huge liquefaction and related deformational features in the epicentral area around bhachau (jain and lettis, 2001; singh et al., 2001; tuttle et al., 2002). the ranns and the plains of banni, being made up of unconsolidated holocene sediments, served as vast grounds for liquefaction. the liquefaction was mainly concentrated along the concealed major faults of the region. it was accompanied by lateral spreads, sand blows, and water spouts, mainly between the khadir uplift and the eastern part of the mainland uplift. complete documentation of all liquefaction and related features in the ranns was very difficult, because the area is too marshy with the quick sands, which under liquefaction become dangerous to travel across. however, rajendran et al. (2001) documented many of the features and provide the best postearthquake study. 3. primary surface faulting rajendran et al. (2001) concluded that primary surface deformation possibly existed 2 km north of bharodiya village, in what we refer to herein as the northern dextral fault zone (fig. 2b). we began our reconnaissance at a nwtrending «monocline» in this fault zone, first observed by arch johnston and paul bodin of ceri in february 2001, and later visited on february 18-19, 2001 by c.p. and k. rajendran, m. thakkar, and tish tuttle (rajendran et al., 2001). from the «monocline» we continued northwest and southeast, following a n3040w-trending, 0.9 km long northern dextral fault zone (described later under secondary surface deformation). the right-lateral faulting continued to about 23°34.697 n, 70°24.572 e, at which point the rupture zone abruptly turned west and became a 830 m long, east-west-striking, north verging thrust fault (fig. 3). because this scarp has the same orientation and sense of slip as the mainshock, and exists close to where the mainshock plane would project to the surface, we treat it as primary surface faulting. the eastern 320 m of the thrust lies at the base of a low e-w bedrock ridge underlain by jurassic sandstone (wagar formation), and the remainder crosses an alluviated plain. at the eastern end of the thrust, faults are short (10-30 m long) and en échelon, and expose bedrock on both the hanging wall and footwall. small tent structures and articulated monoclines (fig. 3) are common, as if the 2001 surface displacement was insufficient for the south-dipping thrust plane to break cleanly to the surface. in places a small north-dipping backthrust existed at the crest of the monocline. most of the thrust scarp at the base of the bedrock ridge was developed in thin sandy colluvium or slopewash, and ranged from a sinuous trace to a linear trace (fig. 4). scarp height varied from 15 to 25 cm. the most common scarp morphology was a hanging wall collapse scarp, where the overhanging tip of colluvium failed after (or during) thrusting (see mccalpin, 1996, p. 193). the thrust scarp continues another 135 m west of the bedrock ridge, and then is truncated by a n20e-striking, 30 m long right stepover zone of dextral faults and tension cracks. average displacement in the stepover is 20 cm right-lateral (fig. 3). west of the stepover the thrust crosses a plain underlain by thicker alluvial deposits, and the height and sinuousity of the scarp increase 941 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india james p. mccalpin and mahesh g. thakkar fig. 2b. detailed map showing location of 2001 surface faulting and lateral spreading (thicker gray lines with associated bold labels). kmf = kachchh mainland fault. 942 943 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india fig. 3. reconnaissance gps map of surface faulting north of bharodiya village. the northwest-trending fault zones on either end display dextral slip, whereas the east-west-trending, 1.2 km long scarp between them displays northward thrusting with a small dextral component. we classify the thrust faulting as primary faulting and the strike slip faulting as secondary (tear) faulting, following seeber et al. (2001). total length of surface faulting is 2.6 km (measured between the ends) or 3.0 km (measured along the fault). (fig. 5). the most common scarp morphologies here are: 1) sinuous hanging wall collapse scarp; 2) tent structures, and 3) less sharp moletracks. the upthrown (south) side of the scarp is characterized by horizontal jurassic bedrock at the surface, whereas north of the scarp bedrock is buried everywhere by quaternary deposits of unknown thickness. these observations indicate that the total vertical separation of bedrock across the fault is significantly greater than the 15-25 cm of throw in 2001, and presumably resulted from prehistoric fault movements, which are as yet unstudied. 3.1. exposure of the 2001 thrust in a streambank about 275 m west of the bedrock ridge (at n.23°34.70 , e.70°24.21 ), the 2001 thrust scarp crosses a 1-1.5 m deep stream channel and the fault plane is exposed in the western streambank (fig. 6). the fault cuts late pleistocene and holocene (?) fluvial deposits and creates a 22 to 25-cm high mole track on the surface with tent structures. the thrust dips 13° south and displaces a small channel margin (late pleistocene/.holocene contact?) with a net 944 james p. mccalpin and mahesh g. thakkar fig. 5. sinuous thrust scarp west of the bedrock ridge; white dashed line follows the scarp crest. note cow for scale in far distance to left of dashed line. faulted material here is a thicker quaternary sandy alluvium that contains a hardpan soil profile about 30 cm thick. the tent structure in the foreground (at 23°34.699 n, 70°24.282 e) was formed by horizontal compression and breakage of the hardpan layer. scarp height ranges from 20 to 30 cm. view to the west. fig. 4. straight section of thrust scarp (between white arrows) near eastern end of the rupture, at 23°34.695 n, 70°24.413 e view to southwest. scarp is developed in thin sandy alluvium overlying jurassic sandstone. slip of 22 cm (fig. 7). this fortuitous exposure permitted us to observe how tent structures were created by low-angle thrusting and how their dimensions are related geometrically to fault slip (see next section). from the streambank exposure the sinuous thrust scarp continues west, characterized by moletracks and hanging wall collapse scarps 15-30 cm high. about 120 m west of the streambank the scarp continues up and over a northtrending, 2.3 m high earth dam (fig. 8) and forms a degraded scarp at its crest 20 cm high. the fault trace migrates 3.25 m northward as it climbs the 2.3 m high embankment, resulting in an apparent dip of about 36° south. this angle is considerably steeper than that observed in the streambank. from the dam the thrust scarp continues west as a sinuous hanging wall collapse scarp or moletrack (fig. 9). individual thrust scarp lobes have a wavelength of 5-10 m and an amplitude of 2-3 m in plan view. in this area thrust scarps reach their greatest height (27-35 cm, at 23°34.701 n, 70°24.073 e). farther west the thrust scarp/moletrack dies out and is replaced by a n20w-trending zone of small and widely spaced dextral cracks. these dextral cracks average 1-2 cm of net slip and are about 50 m long, after which they step left and form a small north-facing thrust scarp or moletrack of similar length. northwest of about 23°34.812 n, 70°24.019 e even the dextral cracks vanish, and we were unable to find further surface faulting, despite a lengthy search. 3.2. displacement estimates from tent structures it was difficult to measure displacement on the 2001 thrust fault because vertical exposures were rare. our first estimates assumed that a fault plane dipping south at 15° to 35° had created a simple thrust scarp 15-35 cm high, which implied a net slip vector ranging from 26 cm (35° fault dip and 15 cm high scarp) to 135 cm 945 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india fig. 6. exposure of the thrust fault in a streambank at 23°34.700 n, 70°24.210 e. the 2001 thrust fault (between thick white arrows) dips 13° south and the sense of movement is shown by the thin white arrow. at the bottom of the hardpan layer the fault steepens to subvertical, probably following a subvertical crack caused by ground shaking, and creates a tent structure. 946 fig. 7. log of the streambank exposure. (15° fault dip and 35 cm high scarp). this wide discrepancy shows the difficulty in making an accurate net slip estimates solely from scarp height. however, the streambank exposure showed that the low-angle thrust fault did not continue propagating at 13° once it hit the base of the hardpan layer (carbonate-cemented sand on fig. 7), but instead steepened to subvertical, probably following a subvertical crack created during earthquake shaking of the hardpan. this refraction of the fault plane created head-on compression of the broken hardpan layer, resulting in upward arching of hardpan slabs and the resultant tent structure. based on the streambank exposure, we propose a simple geometric model that relates horizontal tectonic shortening to the surface dimensions of the tent structure (fig. 10). we observed that the ground surface in the tent structure is uplifted and tilted but not horizontally compressed, and inelastic deformation appears limited to thickening of the hardpan layer in the tent structure. therefore, the amount of horizontal shortening (d) can be related to the tent structure as follows: d = 2(l (lcos )) (3.1) where d = horizontal shortening; l = length of one limb of the tent structure; = dip angle of the limb. the height of the tent structure is given by z = lsin (3.2) where z = height of the tent structure. for example, 5 m west of the streambank exposure (where net slip was 22 cm on a 13° dipping fault) is a symmetrical tent structure with z = 25 cm and l = 40 cm. according to eq. (3.2), the dip of the limbs is 39°. entering that value into eq. (3.1), we can compute a horizontal shortening of 18 cm. this compares with 21 cm of horizontal shortening observed in the streambank exposure. the value computed from the tent structure is much smaller than the net slip required for an unrefracted 13° dipping fault to create a scarp 25 cm high. in contrast, the computed 18 cm of horizontal shortening is only 15% smaller than the value measured in a vertical cut, which suggests that a small amount of inelastic shortening (crushing?) did occur in hardpan layer in the tent structure. farther east, the tent structure shown in fig. 5 is asymmetrical, with a right (north) limb of l = 90 cm and james p. mccalpin and mahesh g. thakkar 947 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india fig. 9. photograph of the sinuous trace of the thrust scarp west of the dam shown in fig. 11a,b. view is to the west from the crest of the earth dam. dashed line follows the crest of the scarp. thrust crosses a faint road road in upper center at 23°34.701 n, 70°24.111 e. fig. 8. thrust scarp (between dashed lines) crossing the axis of a north-south-trending, 2.3 m-high earth dam (dike), at 23°34.705 n, 70°24.140 e. the scarp reaches a maximum height of 20 cm at the dam crest, and is slightly higher on the western side of the crest than on the eastern side, implying a small component of dextral slip in addition to north-directed thrusting. the apparent dip of the fault as it ascends the east side of the earth dam is 36° south. 948 james p. mccalpin and mahesh g. thakkar = 25°. applying eq. (3.1) there predicts horizontal shortening of 17 cm. 3.3. summary of primary surface faulting the 830 m long thrust scarp described previously clearly resulted from tectonic faulting, because it involves bedrock, crosses natural and man-made topography indiscriminately, and is not associated with any gravitational failures such as lateral spreads. scarp morphology (hanging wall collapse scarps, moletracks, tent structures) is similar to that of primary surface ruptures during other thrust earthquakes (philip et al., 1992; mccalpin, 1996). however, at this time we cannot prove that this thrust scarp is the surface expression of the 26 january mainshock plane, or even that the thrust was produced during the mainshock, rather than during an aftershock between 26 january and 11 may 2001, when we began our reconnaissance. therefore, the scarp has several possible origins: a) is the surface expression of the 26 january mainshock plane; b) represents shaking-triggered slip on a preexisting fault updip from the 26 january mainshock plane (i.e. induced displacement); c) is the surface expression of an aftershock plane; d) represents shaking-triggered slip on a preexisting fault updip from an aftershock plane (i.e. induced displacement). based on our limited reconnaissance we favour origin (a), but cannot rule out origins (b) through (d) without further detailed study. 4. secondary surface faulting several post-earthquake reconnaissance teams observed nw-trending dextral surface faults east of the mainshock epicenter. their orientation (nw) and sense of slip (right lateral) do not match the focal mechanism of the 26 january mainshock (northward thrusting on an east-west plane), but they were apparently formed wholly or partly during the mainshock. seeber et al. (2001) concluded that the longest of these faults, the 8 km long manfara fault (next section), was a secondary tear fault that bounded the eastern margin of the north-thrusted block. accordingly, we describe two zones of dextral faulting below. the first zone (the northern dextral fault zone) was discovered and mapped by us from may 11-14, 2001, 2 km north of bharodiya village, and the second is the manfara fault described by seeber et al. (2001). 4.1. the northern dextral fault zone near bharodiya village north of bharodiya village, rajendran et al. (2001) first described an elevated zone of unknown origin 80 m long, 10 m wide and 80 40 40 40 40 40 40 40 40 30 30 30 x d l z (a) before faulting (b) after faulting fig. 10. schematic diagrams showing the geometry of an idealized tent structure. (a) hardpan layer is 30 cm thick and is arbitrarily divided into four 40 cm long sections. (b) hardpan layer is subjected to horizontal shortening (d), which creates a tent structure with height (z.), half-length (l), and limb dip angle ( ). after shortening, field observations show that the length of each 40 cm long section remains constant at ground level. thus, the tented sections must thicken as shown, in order to conserve volume (this also minimizes the space problem below the center of the tent structure). the horizontal distance x beneath each half of the tent structure is now less than 40 cm, by half the amount of horizontal shortening (d/2). in this example, dimensions are taken from the streamcut locality on the thrust fault northwest of bharodia village (fig. 6), where w = 40 cm and z = 25 cm, resulting in a computed slab dip angle ( ) of 39° and computed horizontal shortening of 18 cm. equations in the text describe relationships between d, w, x, z, and . cm high, gently dipping towards the northeast. we traced this zone to its northwest and southeast terminations from may 11-14, and found that it connects with the eastern end of the thrust scarp described previously. this «northern dextral fault zone» trends n.30°-40°w (fig. 3) and is composed of: 1) a 325 m long northern section of east facing linear mounds, that intersects the eastern end of the primary thrust scarp; 2) a 310 m long central dextral fault zone, and 3) a 275 m long southern section of west-facing, linear mounds. both linear mounds exist on the edges of cultivated fields and their surface geometry (described below) may have been influenced by the presence of the fields. the northern linear mound section trends n35°w and parallels the western edge of a cultivated field (fig. 11a,b). the mound is up to 0.6 m high and 6-12 m wide and is asymmetrical, bounded by a slightly sinuous thrust scarp on its steeper eastern side and by a more gentle, monoclinal fold on its western side. large tension cracks up to 5 cm wide and 1.1 m deep exist on the mound crest and trend 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india tension cracks dextral thrust wp45 s60w vertical exaggeration= 5x 1 m 0.4 0 0 2 4 6 8 10 12 14 16 18 m fig. 11a,b. southern part of the northern linear mound zone. a) photograph of the mound looking northwest. white dashed line follows the crest of the thrust scarp that bounds the mound on the east. white arrows show axial tension cracks. small black pack to right of scarp in center, and to left of standing figure, is at 23°34.575 n, 70°24.657 e. b) topographic profile across the mound at the site of the pack, with subsurface structures inferred. note vertical exaggeration of 5x. total height of the mound ranges from 40 to 60 cm. 949 a b n20w, or about 15°-20° more westerly than the mound itself. the sinuousity of the thrust scarp lobes varies from south (wavelength of 4.5-5.5 m, amplitude of 1.5-2 m in plan view) to north (wavelength 8-12 m, amplitude 2-3 m in plan view). the 310 m long central dextral fault zone coincides with uncultivated, native scrubland and is dominated by dextral faults, but also contains small, parallel thrust scarps and small linear mounds. at the road crossing in its northern part, the zone is composed of a 22 cm high thrust scarp on the west, and 30 m to the east, a parallel zone of dextral cracks with 7-10 cm displacement. this is the only place where the dextral-reverse slip is partitioned between two discrete, parallel faults. directly south of the faulted road (fig. 3), the dextral fault zone trends n-s and is composed of en échelon, leftstepping, n20°e-striking dextral cracks joined by small thrust faults (fig. 12). total dextral offset across this zone is 10-15 cm. the southern part of the dextral rupture zone is characterized by a low (15-33 cm high), broad (6-12 m wide) linear mound (fig. 13a,b). the mound is bounded on the west by a sinuous, low, degraded thrust scarp (7-20 cm high) and on the east by a weak monocline. fresh tension cracks with apertures 3-6 cm wide run down the axis of the mound. the tension cracks were not observed on february 18-19, 2001 by rajendran et al. (2001), and cut through slopewash deposits from a rainstorm of april 12-15. thus, although the mound and its degraded thrust scarp were probably formed during the 26 january 2001 mainshock, the axial tension cracks must have resulted from some aftershocks between april 15 and may 11, when we first observed them. the 275 m long southern linear mound section trends n35w along the eastern edge of a cultivated field, and is composed of two linear mounds that trend n45w and step right. the northern part of this section comprises the james p. mccalpin and mahesh g. thakkar fig. 12. strike-slip fault zone (between dashed lines) south of the road shown in fig. 3; view is to south. the surface rupture is composed of a system of left-stepping dextral cracks that trend oblique to the strike of the fault zone. at each left step a small north-verging thrust fault is formed (thick black line, teeth on upthrown block). largest thrust at center is at 23°34.495 n, 70°24.682 e. at lower right the rupture becomes a thrust scarp which continues to road shown in fig. 15. 950 «monocline» described by rajendran et al. (2001) and is about 100 m long. this asymmetrical linear mound is 0.6 m high and 12 m wide, bounded by a slightly sinuous thrust scarp on its steeper western side and by a more gentle, monoclinal fold on its eastern side. in other words, it faces the opposite direction to the northern linear mound, and faces the cultivated field. in the center of this section is a stepover zone 30 m long that contains no visible deformation. south of the stepover is another linear mound 170 m long, 75 cm high at its northern end, and 62 cm high at the southern boundary of the field, which slowly dies out southeast of the field. this mound has the same asymmetry as the «monocline» mound, but unlike the that mound, also contains axial tension cracks 10-15 m long, some of which had vented liquefied sand. the linear mounds represent a new style of surface deformation previously unreported in scr earthquakes, but due to lack of exposures we are unsure exactly what type of structures underlie them. because the rupture that crosses unmodified terrain shows 15-20 cm of dextralreverse offset, and because other nw-trending faults such as the manfara fault show similar 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india 951 tension cracks dextral thrust wp15 vertical exaggeration=7x n60e 0.4 m 0.2 0 0 2 4 6 8 10 12 14 16 m fig. 13a,b. the welt north of the southern plowed field, at 23°34.465 n, 70°24.737 e. a) photograph of the n30w-trending welt, looking east. the welt is 16 cm high and 6.3 m wide here, bounded on the west by a sinuous, 7.5 cm, high thrust scarp (dashed line, teeth on upthrown block) and on the east by a smoother monocline (dashed line). arrows show axial tension cracks, 3-5 cm wide. b) topographic profile across the welt, with subsurface structures inferred. note the large vertical exaggeration required to show the welt’s shape. a b james p. mccalpin and mahesh g. thakkar fig. 15. close-up of a small gully displaced by the manfara fault at 23°27.867 n, 70°22.846 e; view is to northeast. the gully axis is displaced 21 cm dextrally and 14 cm vertically, for a net slip of 25 cm (white arrow). fig. 14. the southern part of the manfara fault, south of the highway that runs northeast from kharoi; view to the southeast. the dextral displacement here is 32 cm accompanied by 19 cm of vertical component, east side up. the rupture trace trends n15w and is composed of alternating longer dextral scarp segments trending n35e (white lines) slip and shorter extensional segments trending n20w (black lines). 952 953 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india sense of slip, we presume that the mounds are also underlain by a decimeter-scale dextral reverse fault zone. their peculiar morphology may result from the presence of a wet, topographically lower field adjacent to the rupture. we hypothesize that the subvertical dextral rupture plane propagated nearly to the surface, but then «flopped over» toward the field, forming the sinuous thrust scarps that bound the field side of each linear mound. the cross-sections of figs. 11b and 13b show this inferred geometry, as well as that of the axial tension cracks. the deepest tension cracks were 1.0-1.1 m deep, so any flattening of the fault must have occurred slightly below that depth. tensional forces in the hanging wall above the fault bend then created the tension cracks. confirming this subsurface geometry would require trenching, which was beyond the scope of this reconnaissance study. 4.2. manfara fault close to manfara village, at 23.46°n 70.38°e, the manfara fault ruptured during the earthquake and was first reported by seeber et al. (2001). the fault strikes nw and runs for 8 km with a maximum 32 cm of dextral displacement and 19 cm of vertical displacement, typically east side up (figs. 14 and 15). therefore, the fault was interpreted as a «tear fault» in the hanging wall of the master thrust (rajendran et al., 2001). the surface trace is composed of meter-scale linear segments of different strike that give the rupture a zig-zag appearance (fig. 14). this shape may be inherited from preexisting fractures in the bedrock that underlies the thin alluvium along the fault trace. sand blows were also reported at many places along this fault. for more details, see seeber et al., (2001) and jain and lettis (2001). 4.3. summary of secondary surface faulting the secondary surface faulting does not match the orientation of the mainshock focal plane, so it presumably represents some type of tear faulting above the eastern margin of the thrusted block. however, when measuring the dimensions of the 2001 surface faulting we should probably include both the primary and secondary faulting, because in databases such as wells and coppersmith (1994) no distinction is made between the two. table i shows the dimensional parameters of surface faulting, based on our mapping and that of seeber et al. (2001). 5. landslide north of budharmora village the area around the epicenter was intensely affected by lateral spreads in soft alluvial deposits (rajendran et al., 2002). we studied one particular lateral spread 1 km north of budharmora village where the ground slopes about 1° towards the north. the farmer that owns the field closest to the reservoir shown in fig. 16 reported that he was standing in the field with his family when the 26 january 2001 mainshock struck. he said that members of his family «were pushed up into the air and thrown 3 m to the northwest» by the mainshock. water also sloshed out of the reservoir and spilled out of its northwest corner. farm fields here were cracked at several places and extensional cracks were developed with an e-w orientation, while 150-200 m north of the extensional cracks, there was a 600 m long zone of e-w compressional ridges (fig. 16). in farm fields, irrigation pipe lines initially emplaced 1.5 m below the surface appeared severely broken into pieces and were protruding from the surface, overriding each other (rajendran et al., 2001). our reconnaissance table i. dimensions of surface faulting in the 2001 bhuj earthquake. dimension primary faulting secondary faulting surface rupture length (km) end-to-end, 0.83 15 along strike, 0.95 maximum displacement (cm) 35 35 average displacement (cm) 15 20 gps mapping of the deformed area (fig. 16) supports the interpretation of seeber et al. (2001), jain and lettis (2001), and tuttle et al. (2002) that the feature is a lateral spread landslide. however, the large ground accelerations here may also indicate that the lateral spread overlies a fault (probably the kmf) that moved during the 2001 mainshock, as well as in prehistoric times, as described next. a french reconnaissance team dug two backhoe trenches across the toe thrust of the lateral spread prior to may 2001 (west trench and east trench in fig. 16), perhaps because shortly after the earthquake, this ridge was thought to represent primary tectonic surface rupture of the kmf. we deepened and relogged the western wall of the west trench to see if fault geometry supported a tectonic or a landslide origin. the log of the trench (fig. 17) shows that the fault planes beneath the compressional ridge flatten about 1.5 m beneath the ridge crest, and continue south at a dip of about 10°, parallel to bedding in the holocene sandy alluvium. in reality, the 2001 fault planes were extremely difficult to see once they paralleled bedding, but we infer that the failure plane lay at the base of a gravelly sand unit in the trench wall. because the fault does not displace obvious stratigraphic markers and deformation beneath the pressure ridge was partly ductile, we could not measure net displacement. a relatively weak soil profile is developed on the sandy alluvium, consisting of ack1 and ack2 horizons (weakly organic horizons with stage i calcium carbonate, after the classification of gile et al. (1966)) and a cox horizon (oxidized parent material). this degree of soil development suggests a mid-holocene age for the alluvial deposit. the surprising feature in the trench is the strong soil profile at the northern end of the trench, which is bent down to the south and then truncated by an older, north-dipping fault that was not reactivated in 2001 (fig. 17). this soil is developed on a hard gray clay parent material that is also truncated by the older james p. mccalpin and mahesh g. thakkar 12 3 4 5 6 7891011 1213141516 17 18 2 582 200 m 2 582 100 m 2 582 000 m 2 581 900 m 6 2 1 ,7 0 0 m 6 2 1 ,8 0 0 m 6 2 1 9 0 0 m 6 2 2 0 0 0 m 6 2 2 1 0 0 m 6 2 2 2 0 0 m 6 2 2 3 0 0 m 6 2 2 4 0 0 m n 100 m road road reservoir pipeline faulted pipeline west trench east trench field boundaries 5 m wide, flat bottomed ditch budharmora village, 1 km scarp ends abruptly scarp dies out 2 sharper scarps 4 low anticlinal ridges scarp dies out zone of extensional c racks headscarp of lateral spread fig. 16. reconnaissance gps map of the lateral spread north of budharmora village. coordinates are utm meters, zone 42, wgs84 datum. thick black lines show the margins of the spread, with hachures indicating extensional scarps and teeth indicating compressional scarps or anticlinal ridges. the log of the west trench across the toe thrust is shown in fig. 17. both extensional and compressional scarps die out to the east. 954 fault. the lower half of the clay is riddled with subvertical, rootlike bodies of sand that look like dikes of liquefied sand injected into the clay. however, these sand dikes are also overprinted by soil calcium carbonate, so they were formed in the middle holocene or earlier, not in the 2001 earthquake. the combination of an older fault and older liquefaction features indicates that prehistoric deformation occurred here, but it is unclear exactly when that deformation occurred or whether it was associated with a lateral spread (as occurred in 2001) or with tectonic surface faulting of the kmf. those questions can only be answered by future studies. it does appear that the 2001 lateral spread failure plane was forced to the surface at this location by the buttressing effect of this hard clay deposit. 6. discussion to understand the nature of past and present seismicity in a stable continental region such as kachchh, quaternary tectonic features are the best indicators. thrusting in kachchh is mostly along the pre-tertiary faults (such as the kmf and khf) developed in an older extensional regime. furthermore, the kmf and khf show geomorphic indicators of segmentation, e.g., sharp changes in orientation of the fault, intersection with branch (transverse) faults, abrupt changes in dip, and changes in net-slip. knuepfer (1989) concluded such features are very common structural indicators of possible segment boundaries. the 2001 thrust scarp and the mole tracks we mapped north of bharodiya village are parallel to the major plane of rupture of the main shock, and are linked to two zones of dextral faulting (the northern dextral fault zone and the manfara fault). if these dextral fault zones define the boundary of the north-thrusted block, it follows that the 2001 seismogenic fault which underlies the thrusted block must project to the surface somewhat north of the dextral faults. this constraint appears to rule out the kmf as the seismogenic fault, as does the fact that the 2001 epicenter lies slightly north of the surface trace of the kmf. in addition, the dextral slip on these faults rules out the south wagad fault as a candidate. 955 fig. 17. log of the western wall of the west trench. the 2001 failure plane is inferred to parallel bedding in the southern part of the trench, and then to rise to the surface in a steplike fashion beneath the toe thrust pressure ridge. an older fault zone of unknown origin and sense of slip underlies the steep section of the 2001 thrust, but was not reactivated in 2001. the 2001 thrust may have been forced to the surface at this location by the buttressing effect of the hard gray clay and overprinted carbonate soil at the northern end of the trench. 2001 bhuj-kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, gujarat, western india instead, the 2001 mainshock plane must have been a nearly blind thrust parallel to but 30 km north of the kmf. we say nearly blind because the 0.8 km long thrust scarp was created, and may represent the only location where primary displacement propagated all the way to the surface. however, based on our reconnaissance data we cannot rule out the possibility that the mw 7.7 event was completely blind, and that the mapped thrust scarp was secondary (passive) thrusting induced on a preexisting fault by strong, channeled ground motion extending updip from the seismogenic plane. acknowledgements the post-earthquake studies, especially the study of the primary surface ruptures of the 2001 bhuj earthquake, were supported by nsf funding to boise state university, idaho, u.s.a. we gratefully acknowledge this support and the cooperative efforts of jim zollweg at boise state. references bendick, r., r. bilham, e. fielding, v.k. gaur, s.e. hough, g. kier, m. n. kulkarni, s. martin, k. mueller and m. mukul (2001): the 26 january 2001 «republic day» earthquake, india, seismol. res. lett., 72, 328-335. bilham, r. (1998): coastal tectonics, geol. soc. london, spec. publ., 146, 295-319. biswas, s.k. (1981): basin framework, paleo-environment and depositional history of the mesozoic sediments of kutch basin, western india, q. j. geol. min. metall. soc. india, 53 (1-2), 56-85. biswas, s.k. (1987): regional tectonic framework, structure and evolution of the western marginal basins of india, tectonophysics, 135, 302-327. gile, l.h., f.f. peterson and r.b. grossman (1966): morphological and genetic sequences of carbonate accumulation in desert soils, soil sci., 101, 347-360. jain, s.k. and w.r. lettis (2001): preliminary observations on the origin and effects of the january 26, 2001 bhuj (gujarat, india) earthquake, earthquake eng. res. inst. spec. earthquake rep., eeri newsletter, 1-16. kayal, j.r., r. de, s. ram, b.v. srirama and s.g. gaonkar (2002): aftershocks of the 26 january bhuj earthquake in western india and its seismotectonic implications, j. geol. soc. india, 59 (5), 395-417. kneuepfer, p.l.k. (1989): implications of the characteristics of end-points of historical surface fault ruptures for the nature of fault segmentation, in fault segmentation and controls of rupture initiation and termination, edited by d.p. schwartz and r.h. sibson, u.s. geol. sur., open file rep., 89-315. malik, j.n, p.s. sohoni, r.v. karanth and s.s. merh (1999): modern and historic seismicity of kachchh peninsula, western india, j. geol. soc. india, 54, 545-550. maurya, d.m, m.g. thakkar and l.s. chamyal (2002): quaternary geology of the arid zone of kachchh: terra incognita, in proc. ind. nat. sci. acad. (communicated). maurya, d.m., s. bhandari, m.g thakkar and l.s. chamyal (2003): late quaternary fluvial sequences of southern mainland kachchh, western india: lithostratigraphy and neotectonics implications, curr. sci., 84 (8), 1056-1064. mccalpin, j.p. (1996): paleoseismology (academic press, new york), pp. 588. oldham, r.d. (1926): the cutch (kachh) earthquake of 16th june 1819 with a revision of the great earthquake of 12th june 1897, mem. geol. surv. india, 46, 71-147 philip, h., e. rogozhin, a. cisternas, j.c. bousquet, b. borisov and a. karakhanian (1992): the american earthquake of 1988 december 7; faulting and folding, neotectonics and paleoseismicity, geophys. j. int., 110, 141-158. rajendran, c.p. (2000): using geological methods for earthquake studies: a perspective from peninsular india, curr. sci., 79 (9), 1251-1258. rajendran, k and c.p. rajendran (1999): seismogenesis in the stable continental interiors: an appraisal based on two examples from india, tectonophysics, 305, 355-370. rajendran, c.p. and k. rajendran (2001): characteristics of deformation and past seismicity associated with the 1819 kutch earthquake, northwestern india, bull. seismol. soc. am., 91, 407-426. rajendran, c.p., k. rajendran and b. john (1998): surface deformation related to the 1819 kachchh earthquake: evidence for recurrent activity, curr. sci., 75 (6) 623-626. rajendran, k., c.p. rajendran, m.g. thakkar and m.p. tuttle (2001): 2001 kachchh (bhuj) earthquake: coseismic surface features and their significance, curr. sci., 80 (11), 1397-1405. rajendran, k., c.p. rajendran, m.g. thakkar and r. gartia (2002): the sand blows from the 2001 bhuj earthquake reveal clues on past seismicity, curr. sci., 83 (5), 603-610. seeber, n., d. ragona, t. rockwell, s. babu, r. briggs and s.g. wesnousky (2001): field observations bearing on the genesis of the january 26, 2001 republic day earthquake of india resulting from a field survey of the epicentral region, web document, version of february 28, 2001. singh, r.p., s. bhoi, a.k. sahoo, u. raj and s. ravindran (2001): surface manifestations after the gujarat earthquake, curr. sci., 81 (2), 164-166. thakkar, m.g., d.m maurya, r. rachna and l.s. chamyal (1999): quaternary tectonic history and terrain evolution of the area around bhuj, mainland kachchh, western india, j. geol. soc. india, 53 (5), 601-610. thakkar, m.g., d.m. maurya, r. rachna and l.s. chamyal (2001): morphotectonic analysis of khari river basin of mainland kachchh: evidence for neotectonic activity along transverse fault, bull. indian geol. assoc., 34, 205-220. tuttle, m.p., j. hengesh, k.b. tucker, w. lettis, s.l. deaton and j.d. frost (2002): observations and comparisons of liquefaction features and related effects induced by the bhuj earthquake, earthquake spectra, 18 (suppl. a), 79-100. wells, d.l. and k.j. coppersmith (1994): empirical relationships among magnitude, rupture length, rupture area, and surface displacement, bull. seismol. soc. am., 84, 974-1002. 956 james p. mccalpin and mahesh g. thakkar layout 6 1 annals of geophysics, 62, 4, gm449, 2019; doi: 10.4401/ag-7789 “features of magnetic field fluctuations in the ionosphere at swarm altitude„ paola de michelis*,1, giuseppe consolini2, roberta tozzi1, fabio giannattasio1, virgilio quattrociocchi2 and igino coco1 (1) istituto nazionale di geofisica e vulcanologia, rome, italy (2) inaf−istituto di astrofisica e planetologia spaziali, rome, italy 1. introduction there is a growing body of literature that recognises the importance of the turbulence and intermittent phe− nomena in the ionospheric environment. fluctuations of plasma density, electrostatic potential and magnetic and electric fields have confirmed the existence of a turbulent state in the ionosphere. this state, which can be detected at different spatial scales, is mainly the re− sult of the various phenomena characterising the dif− ferent magnetospheric regions coupled to the iono− sphere via the geomagnetic field lines. it has been shown that the turbulent character of the ionospheric plasma density plays an important role in the formation and dynamics of ionospheric inho− mogenities, which can have a great impact, for exam− ple, on the propagation of electromagnetic waves. these inhomogenities, indeed, may affect the global naviga− tion satellite system (gnss) performance, being re− sponsible for different problems: from a simple signal article history receveid may 31, 2018; accepted september 21, 2018. subject classification: magnetic field, turbulence, ionosphere abstract the magnetosphere−ionosphere system is recognized as a complex and active element affected by space weather and as a region where important scientific questions related to space weather impacts need to be answered. in this framework, there is a high priority on the un− derstanding of how local, regional, and global−scale phenomena couple to produce observed responses across various scales. turbulence provides one pathway by which energy cascades across scales from large to small ones where energy can be dissipated in the form of heat− ing. the swarm mission, that is a true multi−point and multi−purpose constellation, represents a unique opportunity to address some of these scientific questions. in detail, it gives us a chance of investigating the nature and the scaling features of magnetic field fluctuations for different geomagnetic activity levels, and unveiling the role played by turbulence of ionospheric plasma medium on the magnetic field fluctuations. recently, using swarm magnetic field data at high−latitude in the northern hemisphere, the local scaling indices of the 1st and 2nd order structure functions of the magnetic field fluctuations have been evaluated, showing their capability both to give new in− sights about the ionosphere−magnetosphere coupling and to provide information on the ionospheric turbulence. here, we improve and ex− tend the analysis by investigating the scaling features of the geomagnetic field fluctuations of external origin, recorded by swarm a satellite during a period of 2 years (april 2014 – march 2016). maps of the local hurst exponent values, which allow us to study scaling properties of the geomagnetic field’s spatial fluctuations are shown, both at high−latitudes (in the northern and southern hemisphere) and at low− and mid−latitudes (±60°) according to two different geomagnetic activity conditions. the aim is to capture the essential features of the spatial fluctuations of the geomagnetic field and understand their origins. delay to its total loss in case of scintillation events up to the production of misleading information due to strong ionospheric gradients. that is the reason why it has been recognized that plasma density plays a key role in the ionospheric processes. actually, other physical quantities, often underesti− mated, can play a crucial role in this framework and provide interesting information on the features of tur− bulence in the ionospheric environment. since the work of kinter and seyler [1985], the turbulent properties of electric and magnetic fluctuations observed by rockets and spacecraft at different altitudes have been ad− dressed in several papers. great attention was especially paid to the analysis of turbulent properties of the high− latitude electric fluctuations trying to understand the nature of the phenomenon and to clarify its driving mechanisms. many interesting features of electric field fluctuations were obtained using both data from low− altitude polar−orbiting dynamic explorer 2 (de2) spacecraft during its several crossings of the auroral zone and polar cap under different interplanetary mag− netic field orientations [golovchanskaya et al., 2006; golovchanskaya and kozelov, 2010] and data based on ground−based stations and rockets measurements. these works suggested that small−scale electric and magnetic fields in the auroral zone are due to intermit− tent ionospheric turbulence, which develops in the re− gions of large−scale field−aligned currents [see, e.g., kintner, 1976; weimer et al., 1985; basu et al., 1988; tam et al., 2005; golovchanskaya et al., 2006; kozelov and golovchanskaya, 2006; kozelov et al., 2008]. some theories associated the observed ionospheric turbulence with structures of magnetospheric origin [e.g., golovchanskaya et al., 2006] driven, for example, by shear flow instabilities. other studies related the iono− spheric turbulence directly to turbulence in the solar wind [e.g., parkinson, 2006; abel et al., 2009]. iono− spheric turbulence in the form of spatial and temporal fluctuations of plasma density has often been observed also at mid and low latitudes [le et al., 2003; molchanov et al., 2004]. the irregularities, resulting from a variety of plasma instabilities associated with perturbations in the complex arrangement of electric and magnetic field, were observed by ground−based coherent and incoherent scatter radars, sounding rock− ets and satellites. based on plasma density data from the cosmos−900 satellite molchanov et al. [2004] anal− ysed the spatial distribution of the ionospheric turbu− lence (in the spatial range between 15 and 300 km) at the satellite height h=450–500 km and found a power density spectrum in the case of both the plasma and the electric field in the analysed regions. bhattacharyya [1990] studied the chaotic behaviour of ionospheric density fluctuations, using amplitude and phase scintil− lation data, and found the existence of low−dimen− sional chaos. the same chaotic behaviour of ionospheric turbulence was also investigated by wernik and yeh [1994] using scintillation data and numerical modelling of scintillation at high−latitude. a unified picture of plasma irregularities in equatorial spread f was devel− oped from the analysis of satellite, sounding rocket, and coherent scatter radar observations by hysell [2000] in his interesting overview of equatorial ionospheric tur− bulent conditions. it is important to notice that the spatial features of plasma density irregularities have a wide range of scale sizes, spanning from centimeters to hundreds or per− haps thousands of kilometers. this observed broad range of irregularity scale sizes, which is indicative of nonlinear mode−coupling, exhibits a power−law scal− ing, with variable spectral indices. during recent years, large variations of the spectral index, used to describe the ionospheric turbulence, were registered to be de− pendent on the different geomagnetic conditions of ob− servations. the european space agency minisatellite constella− tion mission, called swarm, can be extremely useful in this context. indeed, thanks to both the inclination of the orbits and the precession of their planes, swarm satellites ensure a global coverage in a few months [friis−christensen et al., 2006]. data recorded by these satellites can be consequently used to statistically anal− yse the features of magnetic field fluctuations across the globe. in this framework, we characterise the features of magnetic field fluctuations in the f−region of the iono− sphere using measurements recorded by the swarm a satellite during a period of two years (april 2014 – march 2016). the fluctuations are described under the assumption that, according to previous findings, they obey a power−law distribution. under this assumption, we consider the horizontal magnetic field intensity of magnetospheric and ionospheric origin and analyse the changes in the scaling properties of its spatial/tempo− ral fluctuations, which are valid for a certain range of scales. to do this, we evaluate the local hurst exponent, that characterises the time fluctuations observed in a signal defining their degree of persistence or anti−per− sistence, and discuss the changes in the hurst exponent values in relation to the most important ionospheric and magnetospheric current systems, which could reveal the nature of the magnetic field fluctuations. the paper is organised as follows. at first, the data sources are discussed, and then, a brief summary of the de michelis et al. 2 3 magnetic fluctuations at swarm altitude method of analysis is presented. following this, the analysis is applied on the selected data in three differ− ent latitudinal regions: northern high latitudes, middle and low latitudes and southern high latitudes. finally, the implications of findings are discussed. 2. data this study is based on in situ observations of the ge− omagnetic field recorded on swarm a, one of the three satellites of swarm constellation, in the f−region of the ionosphere. the minisatellite swarm constellation con− sists of three identical satellites, which fly in two dif− ferent orbital planes at two different altitudes. two satellites (swarm a and swarm c) fly side−by−side at a mean altitude of approximately 460 km in a plane of 87.4° inclination, while the third satellite (swarm b) or− bits 50 km above in a plane of 88° inclination [friis− christensen et al., 2006]. in this study we consider low−resolution (1 hz) geomagnetic field observations from swarm a in the time interval from 1 april 2014 to 31 march 2016. we use sw_oper_maga_lr_1b data file types, with file counter equal to 0408 according to esa nomenclature. they are available at ftp://swarm− diss.eo.esa.int upon registration. such data provided us with the utc time, the altitude of the satellite (in km), the satellite position (in local geographic latitude and longitude) and the vector magnetic field in the north− east−center (nec) frame of reference (in nt). we use the chaos−6 geomagnetic field model [fin− lay et al., 2016] to remove from the observed magnetic data the main field and its secular variation. chaos−6 is a high−resolution geomagnetic model, which spans a period between 1997 and 2016, and it is obtained from satellite magnetic data (swarm, champ, and ørsted) along with ground observatory data [finlay et al., 2015]. thus, from the original data we remove the contribution coming from the internal field modelled by chaos−6 to obtain the external magnetic field of magnetospheric and ionospheric origin along the three magnetic field components. from the bx (northward) and by (east− ward) components of the geomagnetic field of external origin in the nec (north−east−centre) frame of refer− ence we evaluate the intensity of the horizontal mag− netic field component bh = (b 2 x + b 2 y) ½, which is analysed in three different latitudinal regions: northern high latitudes (magnetic latitude > 50° n), mid− and low−latitudes (within ± 60° magnetic latitude) and southern high latitudes (> 50° s) and according to two different geomagnetic activity levels. the auroral elec− trojet index (ae) and sym−h index are used to dis− criminate between quiet and active periods. the ae index is used to monitor the level of geomagnetic dis− turbance resulting from the auroral electrojets, being a proxy of the total currents in the auroral zone, while the sym−h index gives information about the strength of the ring current around the earth, which increases during disturbed periods. to select geomagnetically quiet periods we consider the following simultaneous conditions: ae<60 nt and −5 nt <sym−h<5 nt. in this way the chosen time periods are supposed to be devoid of the magnetic perturbations introduced by the occur− rence of storm and substorm events. on the other hand, to select active periods we consider the following si− multaneous conditions: ae>100 nt and sym−h<−30 nt. in this case, the chosen time periods can be reasonably considered to capture the external perturbations in− duced by magnetic storm events. the geomagnetic storms are primarily responsible for geomagnetic vari− ations at low and mid−latitudes, while the geomagnetic substorms, which usually influence the geomagnetic field at high latitudes, can also contaminate the mag− netic observations at lower latitudes due to the equa− torward expansion of the auroral ovals during intense events. these choices of geomagnetic activity levels, which differ from previous works [e.g. de michelis et al, 2015], are chosen to ensure a manageable data set since the simultaneous use of constrains on ae and sym−h strongly reduces the amount of available data for each level. the ae and sym−h data with one minute cadence can be downloaded from the omni website (www.cdaweb.gsfc.nasa.gov/istp−public/). data are presented in terms of a quasi−dipole (qd) coordinate system, which is an apex system, introduced by richmond [1995], and organized with respect to the sun position introducing the magnetic local time (mlt), which is evaluated using the common definition given by baker and wing (1989). it has been shown, indeed, that this coordinate representation of the data in the qd coordinate system is most appropriate when analysing data in terms of ionospheric currents, which are fixed with respect to the sun [laundal and gjerloev, 2014]. 3. method of analysis to investigate the magnetic spatial fluctuation fea− tures we adopt the same method used in our previous studies [de michelis et al., 2015, 2016, 2017] that is based on structure function analysis, a methodology that is widely used in different fields where turbulence phenomena and scale−invariant features play a funda− mental role [frisch, 1995]. in detail, we evaluate the 3 de michelis et al. 4 first−order structure function s1 defined according to the following equation: (1) where bh(t) is the time series to be analysed, < > denotes an ensemble average of the increments of the variable taken over all pairs of points separated by a scale t that quantifies the scale of interest, and t is the time inter− val over which the average is calculated. in the case of a scale−invariant signal, the first−order structure func− tion, s 1 (τ) depends on the scale t by a power−law, s 1 (τ) ∼ t γ (1), where the scaling exponent, γ (1), permits us to characterise the spatiotemporal structure of the analysed signal by describing the rescaling of the sig− nal variance at different scales of observation. in first approximation, it can be considered equivalent to the hurst exponent (h), which is commonly utilized to de− scribe stochastic processes [hurst, 1956]. the hurst ex− ponent, whose values range in the interval [0, 1], permits us to analyse the features of the magnetic field fluctuations, indicating the presence of fluctuations, that tend to cluster along a direction as a consequence of their persistent character (h>0.5) or fluctuations that tend to induce stability in the system due to their anti− persistent character (h<0.5) and regress strongly to the mean. different methods are available to extract the scaling exponents of the structure functions. for ex− ample, wavelet based methodologies, hilbert−based method, or the scaling analysis of probability density function of increments, to name a few. each method has its own advantages and shortcomings. in this paper, a detrended structure−function (dsf) method is proposed to extract scaling exponents [de michelis et al., 2015, 2016, 2017]. this is accomplished by removing a poly− nomial trend within a window size t before calculating the increment. this procedure is designated as detrend− ing analysis. by doing so, scales larger than t are ex− pected to be removed or constrained and the method acts as a high−pass filter in temporal domain. in detail, we consider a moving window of t=400 s, and in each time window we construct a detrended time series, bh_d(t), obtained from subtracting to the original time series, which is in our case the intensity of the hor− izontal component of the geomagnetic field of external origin, its average long−term trend. the latter is ob− tained using a polynomial fit so to eliminate all possi− ble spurious effects. on each time window, the detrended time series bh_d(t) is used to evaluate the first−order structure function considering as scales of interest those in the interval t ∈ [1 – 40] s. the maxi− mum scale investigated (40 s) is 10 times smaller than the size of moving window (400 s) used; this ensures the reliable estimation of the 40 s fluctuation statistics. figure 1 reports an example of the behaviour of the first−order structure function s1(t). a clear power law dependence is recovered at the scales investigated here, thus assessing the scale invariance of the analysed sig− nal in the considered range of timescales. under the taylor’s hypothesis or the “frozen turbu− lence approximation” [taylor, 1938] we can relate the temporal scale to the spatial one. a condition for the validity of this assumption is that the temporal varia− tions of the spatial structures occur on time scales longer than the transit time of the satellite in these structures. this assumption was checked in our previ− ous work [de michelis et al., 2017] using magnetic mea− surements recorded simultaneously by swarm a and c, the two satellites of the swarm constellation which orbit side by side at a distance of about 160 km near the equator and transit in the same region with a time delay of about 10 s. thus, under this assumption and in first approximation, the analysis of the time scales of the geomagnetic field fluctuations in the interval [1–40] s corresponds to the analysis of spatial scales in the in− terval between 8 km and ~300 km, being the velocity of the swarm satellites about 7.6 km/s. 4. results and discussions figure 2 reports the local hurst exponent values during both quiet (top panel) and active (bottom panel) s 1 (τ) = <|b h (t + τ) b h (t)|γ (1) >t figure 1. an example of the behaviour of the first−order struc− ture function s1(𝜏) during an high−latitude crossing in the northern hemisphere. power−law behaviour is observed in the range analysed (between the two vertical dashed lines) with a scaling exponent h = (0.45±0.01). 5 magnetic fluctuations at swarm altitude geomagnetic conditions obtained by analysing the fluctuations of the horizontal intensity of the external geomagnetic field at low and middle magnetic latitudes (±60°). figure 2 allows us to distinguish regions where the geomagnetic field fluctuations have a persistent character (in blue) from those characterised by an anti− persistent one (in red). at low and mid−latitude (±40°), before and after noon (from 06:00 mlt to 18:00 mlt), the geomagnetic field fluctuations have always a strong persistent character (h>0.7) independently from the geomagnetic activity levels. in the other regions, the geomagnetic field fluctuations generally exhibit a small persistent character (h∼0.6). variations from a quasi anti−persistent character to a persistent one ap− pear at higher latitudes (>|50|°) before and after mid− night (from 18:00 mlt to 06:00 mlt) during geomagnetically active periods. furthermore, during active periods we observe an increase of the persistent character also at mid− and low−latitudes in the dusk− to−dawn sector (from 18:00 mlt to 06:00 mlt). these latitudinal structures observed in the spatial patterns of the hurst exponent are expected to result from different physical processes in terms of currents and convection processes, which can cause the differ− ent features of the magnetic field fluctuations. in de− tail, the possible sources of the fluctuations observed on the external component of the geomagnetic field here considered have to be searched among the time− varying electric currents flowing in the ionosphere re− sponsible for signals whose superposition produce the recorded earth’s magnetic field of external origin. at low and mid latitudes these latitudinal structures are responsible for macroscopic variations observed also during geomagnetically quiet conditions, namely for both the regular daily variation of the geomagnetic field, which is referred to as solar quiet variation, and the so−called equatorial electrojet current. the equatorial electrojet (eej) is an intense east− ward electric current flowing on the day−side, in a narrow band of a few hundred kilometers along the dip equator in the ionospheric e−region (90 to 150 km altitude range). it is most intense over a time period that extends from 10:00 to 14:00 lt and reaches its maximum intensity between 11:00 and 12:00 lt [on− wumechili, 1997]. outside of this lt sector, the eej decreases significantly and usually diminishes at night−time. this strong horizontal current causes a magnetic field disturbance, which weakens the hori− zontal component of the terrestrial magnetic field at the earth’s surface over a distance of about 600 km across the equator producing an effect similar to that due to the ring current. the solar quiet daily variation (sq) mainly arises from the ionospheric current system flowing in the so− called dynamo region. this current system, that can be quite well approximated by a 2−d current flowing in the ionospheric e−region between 90 and 130 km [chapman, 1929; richmond et al., 1976], is driven by different processes. it is associated with expansion and contraction of the atmosphere as the sun rises and sets daily, with the global scale horizontal upper−atmo− sphere winds, with the lunar tidal forces on the re− gion, and with variations of sun’s electromagnetic emissions responsible for extra photoionization of the region. as a result, sq variation is a function of lati− tude, local time, season and solar activity level [camp− bell, 2003]. the basic pattern of the equivalent sq current system consists in a near−two−dimensional current circuit at ~110 km altitude fixed with respect to the earth−sun line, and flowing in counter−clock− wise direction in the northern hemisphere and clock− wise direction in the southern hemisphere. the centres of the current vortices lie around 40° north or south magnetic latitude. the sq variation is responsible of an enhancement in the value of the horizontal com− ponent shortly before 06:00 lt with the rising of the sun, which reaches its maximum at about 11:00 lt and then comes back to the night time value at 16:00 lt, remaining constant throughout most of the night. of course, the sq variation of the horizontal component of the geomagnetic field, which occurs in both hemi− spheres, is superimposed onto the variation produced by the eej at low latitudes near the geomagnetic equator. thus, the persistent character of the magnetic field fluctuations seems to correspond with those regions where the intensity of the horizontal component of the magnetic field changes as a consequence of different ionospheric current systems. these current systems develop over spatial scale larger than 300 km and consequently they may be responsible, at the lower spatial scales that we are examining (8−300 km), for the persistent character of the fluctuations that will tend to cluster in a direction. when the geomagnetic activity level increases passing from a quiet period to an active one, the spa− tial distribution of the h values remains essentially the same at low and mid latitudes around noon. there is an increase of the persistent character in the dusk−to− dawn sector, which can be associated with the increase of equatorial currents flowing in the magnetosphere that intensify during active periods. the most impor− tant variations of the h values occur indeed in the higher latitude portion of the considered map, and in the nightside sector from the 18:00 to 06:00 mlt in both hemispheres. these variations could be related to the auroral electrojets, which are known to shift equa− torward drastically during disturbed periods. indeed, it has been found that during the main phase of in− tense storms, the westward electrojet can cover the latitudes from 50° to 80° on the nightside while the eastward electrojet, which flows in the dusk sector, can extend to latitudes even lower than those of the west− ward electrojet. it is interesting to notice that during disturbed pe− riods the global ionospheric electric field and currents increase in magnitude and exhibit rapid fluctuations. the disturbed magnetic perturbations are associated only partly with the ionospheric currents, since a sub− stantial portion comes from more distant magneto− spheric currents like the ring current, the magnetotail current and the field aligned currents. the increase of the persistent character of fluctuations observed both at high latitudes and in the dusk−to−dawn sector of the equatorial region during active periods is an effect of the increase of both the westward auroral electro− ject and the ring current. this result agrees with the increase of the persistent character of ae and sym−h fluctuations during disturbed times [see e.g. uritsky and pudovkin, 1998; balasis et al., 2006]. it is also im− portant to notice that in the dip equatorial region the magnetic field is nearly parallel to the earth’s surface and the horizontal component describes completely the earth’s magnetic field. this provides a constraint on the motion of the charged particles that comprise the ionosphere and produces distinctive behaviour in de michelis et al. 6 figure 2. local hurst exponent values during both quiet (top panel) and active (bottom panel) geomagnetic conditions at low and middle qd magnetic latitudes (±60°) as function of mlt. the equatorial ionosphere. the so−called fountain ef− fect of the ionospheric plasma creates a condition that enhances the growth of the rayleigh−taylor (rt) in− stability. the density depletion referred to as an equa− torial plasma bubble (epb) may rise to the topside of the equatorial f region and also develop smaller scale structure. in recent years, magnetic field fluctuations associated with epbs have also been observed. they are significant only in the components transverse to the main geomagnetic field, which are directed along the z−axis in the equatorial region. this explains the persistent behaviour of the magnetic field fluctuations that are clustered in a direction inhibited by the strong magnetic field, especially around the dip equatorial region. to better visualize the changes of the h values at high latitudes, in figure 3 we show polar maps of the large−scale spatial distribution of the h values both in the northern high latitudes (50° n – 90° n) and in the southern high latitudes (50° s – 90° s). in detail, the h values in the northern hemisphere are shown on the left for quiet (upper left panel) and active (lower left panel) periods, while the h values relative to the southern high latitudes are shown on the right panels of the same figure for the two different activity con− ditions: quiet (upper right panel) and active (lower right panel). the features of the magnetic field fluctuations are similar in the two hemispheres during both quiet and active conditions. during quiet geomagnetic condi− tions, the h values show a strong asymmetry in the daily character of the magnetic field fluctuations. they 7 magnetic fluctuations at swarm altitude figure 3. the local hurst exponent values in the northern high latitudes (top) and southern high latitudes (bottom) in a po− lar representation of magnetic local time (mlt) and magnetic latitude. the maps refer to (left) quiet (ae<60 nt and −5 nt <sym−h<5 nt) and (right) disturbed (ae>100 nt and sym−h<−30 nt) conditions. dashed circles are drawn at qd magnetic latitudes of 50°, 60°, 70°, and 80°. exhibit an anti−persistent character in the sunlit hemisphere (05:00 < mlt < 18:00) and a mostly per− sistent one in the dark sector in a band of magnetic latitude between ∼50° and 75° in both hemispheres. these maps are slightly different from previous results reported by de michelis et al. [2015] where the fluc− tuations of the horizontal component of the earth’s magnetic field from 1 january 2014 to 30 june 2014 were analysed and the hurst exponent evaluated. the difference in the obtained results, which are localized essentially in the polar cap regions, can be due to dif− ferent datasets utilized in the two works. here, we have removed from the original data (2 years instead of 6 months) the contribution coming from the internal field produced by the chaos−6 model to obtain a dataset that describes only the magnetic field which is caused by magnetospheric and ionospheric currents and used different conditions for defining the quiet and disturbed geomagnetic activity levels. during active periods, the region with an anti−per− sistent character becomes smaller. it is localized mainly in a narrow band between 50° and 60° in the sunlight sector. conversely, the persistent region shows an increasing equatorward extent in the dark sector. the only anomaly, with respect to our previous results [de michelis et al. 2015], is represented by the polar cap region, where the h values are always less than 0.5 describing a constant anti−persistent char− acter of the geomagnetic field fluctuations regardless of the geomagnetic activity level. also in this case, the observed difference in the character of magnetic field fluctuations reflects the morphology and dynamics of that part of the iono− sphere, which is crossed by the satellite. at high−lat− itude the ionosphere differs significantly from its mid−latitude counterpart. the high−latitude iono− sphere is characterised by a strong coupling between the magnetosphere and ionosphere through electric fields and currents as well as particle flows, which make the description of this ionospheric region diffi− cult. it is known that this region is more directly mod− ified by magnetospheric processes that are largely controlled by the interaction between the solar wind and the earth’s magnetosphere. indeed, at these lati− tudes the ionospheric currents are joined with currents flowing along geomagnetic field lines into the mag− netosphere and their electrodynamics is dominated by the influences of magnetospheric processes. at high−latitude and low altitude there are two major constituents of magnetic field fluctuations: (1) electric currents in the ionospheric e−layer (at an al− titude of about 110 km) flowing on a horizontal plane that practically perpendicular to the ambient magnetic field and can be decomposed into hall and pedersen currents; and (2) currents flowing along the magnetic field lines connecting the ionosphere and the magne− tosphere (facs). in both cases, due to the significant ionisation caused by particles precipitating into the auroral oval, the high−latitude current flow is con− centrated inside the auroral oval, where it forms the auroral electrojets, which are the most prominent cur− rents at auroral latitudes. they consist of eastward and westward components that are primarily hall currents originated around noon, and are fed by downward facs connecting the ionosphere in the nightside to the magnetospheric current wedge. during disturbed pe− riods, the ionospheric current flow is strongly en− hanced, there is the formation of a substorm electrojet with strongly enhanced westward current flow in the midnight sector and all the current structure expands to lower latitudes. the distribution and the intensity of these currents depend indeed on the magnetic activ− ity and the interplanetary magnetic field orientation. thus, the regions characterised by a persistent be− haviour of the magnetic field fluctuations seem to cor− respond closely with those characterised by intense ionospheric current systems and plasma convection processes. one feature of a self−affine time series, which means of a time series whose rescaled version of a small part has the same statistical distribution as the larger part, is that its power−spectral density is de− fined as a power−law function of the frequency, whose exponent, β, is directly linked to the hurst exponent through the following relationship: β = 2h +1. this means that the ionospheric regions characterised by different h values have different behaviours of the spectral density. in our case the geomagnetic field’s spectral density is therefore characterised by β>2 (β~2.5−2.6) in those regions where the hurst exponent assumes values greater than 0.5 (for instance, in the auroral oval and at mid and low latitudes in the day− side), while the spectral exponent is β<2 (β~1.6 1.8) in those regions with h values smaller than 0.5 (in the polar cap, in the dayside sector at high latitude dur− ing quiet periods and in the nightside sector at mid and low latitudes regardless of geomagnetic activity level). similar values of spectral indices have been found analysing the ionospheric electric field fluctuation spectra. values of spectral index greater than 2.5 have been associated with regions characterised by stronger shear flows and enhanced field intensity [kinter and seyler, 1985]. we can extend this interpretation also to de michelis et al. 8 the case of magnetic field fluctuations. however, it is difficult to identify the shear in the flow as the pri− mary driving mechanism of the observed spectra, as other mechanisms such as drift waves or a current− convective process could be responsible of them. for example, ossakow and chaturvedi [1979] and chaturvedi and ossakow [1981] discussed the current− convective instability in the context of irregularities in the auroral ionosphere, suggesting the important role played by this process in the ionospheric regions. this hypothesis was confirmed by basu et al. [1984], who found that in the spatial region of strong shear flow, the density fluctuation spectrum was relatively flat, while outside the shear flow regions, but staying within the electron precipitation region, the density fluctuation spectrum steepened to an index between −2.0 and −3.0 which was consistent with a current− convective process. these previous results seem to confirm our hypothesis according to which the regions characterised by a persistent behaviour of the mag− netic field fluctuations correspond closely with those characterized by intense ionospheric current systems and plasma convection processes. consequently, mechanisms such as the shear flow or the current− convective process may be the main responsible of the magnetic field fluctuations features. 5. summary and conclusions data series generated by complex systems exhibits fluc− tuations on a wide range of time scales and/or broad distributions of the values. the natural fluctuations are often found to follow a scaling relation over several or− ders of magnitude. when this happens the dynamics of the system, described by the analysed time series, is usually denoted as fractal or multifractal, depending on the question if it is characterised by one scaling expo− nent or by a multitude of scaling exponents. it has been shown that the self−similar (scaling) and fractal properties may characterise, for example, the structure of pedersen and hall conductivities in the au− roral zone and the magnetic and electric field fluctua− tions at small−scale. the aim of this work was to extend the analysis of the scaling features of magnetic field fluctuations, which had been mainly confined to auroral regions in the pre− vious work, to the whole globe. for this reason, the time series of the horizontal intensity of the external geo− magnetic field, measured by swarm a satellite over 2 years (from 1 april 2014 to 31 march 2016), were sta− tistically analysed in terms of their scaling features. the first−order structure function scaling exponent (hurst exponent) of the horizontal intensity was evaluated during quiet and disturbed geomagnetic activity peri− ods at different latitudes. this study was performed in the temporal domain using the continuous 1s measure− ments taken by swarm a along its orbit. from these time series, the spatial structures were inferred under taylor’s hypothesis. the values of the hurst exponent were analysed according to three different latitudinal regions (northern high latitudes, mid and low latitudes, southern high latitudes) and two geomagnetic activity conditions (quiet and disturbed). what is interesting about the hurst exponent values reported in figures 2 and 3 is that they seem to localise different latitudinal structures. there are regions characterised by a persis− tent behaviour of the magnetic field fluctuations and regions where the fluctuations of the geomagnetic field exhibit an anti−persistent character. the regions char− acterised by magnetic field fluctuations with a larger value of the h exponent, which describe magnetic field fluctuations with a larger spatio/temporal coherence than the regions with a smaller h values, seem to cor− respond closely with regions characterized by iono− spheric current system and plasma convection processes. in our previous paper [de michelis et al., 2017], by analysing the high−latitude geomagnetic field fluctua− tions in the northern hemisphere, we found that these fluctuations were characterised by a high coherence in− side convective ionospheric structures and by a lower degree of coherence along the boundaries of the con− vective structures. these results suggested that plasma dynamics play an important role in the coherence range of geomagnetic fluctuations. the conclusions derived from the northern high latitude analysis seem to be supported by the analysis reported in this work, where the southern high latitudes and the mid and low lati− tude regions have also been considered. ionospheric and magnetospheric current systems and plasma convection processes in the equatorial region, at mid latitudes, and at high latitudes determine the scaling features (persis− tence or anti−persistence) of magnetic fluctuations at small spatial scales (8−300 km). the primary mecha− nisms that seem to control these scaling features are probably the shear motions in the plasma flow or the current−convective processes. the results presented in this paper rely on data collected by one of the three satellites of the swarm constellation. we thank the european space agency (esa) that supports the swarm mission. swarm data can be ac9 magnetic fluctuations at swarm altitude cessed online at http://earth.esa.int/swarm. the authors kindly acknowledge v. papitashvili and j. king at the national space science data center of the goddard space flight center for the use permission of 1 min omni data and the nasa cdaweb team for making these data available. giuseppe consolini and 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice e f f e t t i d e l l ' a t t r i t o i n t e r n o s u l l a v e l o c i t a ' d e l l e o n d e s i s m i c h e s u p e r f i c i a l i a b r e v i s s i m o p e r i o d o (*) • pietro caloi in una delle sedute deh'a.s.i. tenute ad oslo nell'agosto 1948 ebbi occasione di presentare un lavoro ( 1 ) che riassumeva i risultati di ricerche^ da me condotte negli anni, precedenti, sulla propagazione delle onde di rayleigh in un mezzo elastico, che ammette attrito interno. r i c h i a m e r ò qui alcuni di detti risultati. la firmo-viscosità determina una sensibile riduzione della componente verticale del moto, riduzione che è tanto maggiore, quanto più piccoli sono il rapporto fi/|i' e il periodo proprio dell'onda: per onde originanti con grossi periodi, l'azione dell'attrito interno sul rapporto delle ampiezze è pressoché nulla e il mezzo si comporta come puramente elastico. la firmo-viscosità può f a r variare i l rapporto delle ampiezze z/h da un valore prossimo all'unità (1,05) a 1,47, valore che la teoria di rayleigh assegna a quel rapporto per la ricerca ha consentito di chiarire un altro aspetto della propagazione delle onde superficiali, finora rimasto insoluto. nella classica teoria di rayleigh, le onde che portano i l suo nome sono considerate come libere, persistenti, non soggette ad assorbimento da parte del mezzo. l'osservazione prova invece che l'assorbimento è fortissimo per i periodi più piccoli e tende a decrescere rapidamente, per raggiungere il valore di circa e~°'0 0 0 3 per periodi dell'ordine di 20s. la nuova teoria prova che in un mezzo firmo-viscoso (con un rapporto n/u' = 50.sec — 1 , conforme alla media dei valori osservati) ciò è pienamente dimostrato: da un assorbimento elevatissimo per piccoli periodi, passando a periodi dell'ordine di 20" e per \i/u'— 50, il coefficiente d'assorbimento diviene dell'ordine di 0.0003, che è appunto quello generalmente osservato. * riassunto di una comunicazione presentata alla i x assemblea generale dell'ug.g.i., tenutasi a bruxelles dal 19 al 31 agosto 1951. 4 7 0 p i e t r o c a l o i di recente, proseguendo nell'investigazione delle caratteri* tiche delle onde superficiali in un mezzo firmo-elastico, ho provato che un altro degli effetti dell'attrito interno riguarda la forma della traiet• toria percorsa da una particella sollecitata da queste onde ("). a n c h e in un mezzo firmo-viscoso, l e onde di tipo rayleigh costringono le particelle raggiunte su traiettorie e l l i t t i c h e ; però, a differenza di quanto avviene in un mezzo puramente elastico, tali traiettorie non sono riferite ai propri assi. solo per periodi infinitamente piccoli o infinitamente grandi, le traiettorie e l l i t t i c h e tendono a quelle riferite ai propri assi, proprie di un mezzo puramente elastico. l o scopo del .mio intervento non si esaurisce però in quanto ho finora detto. è mio proposito richiamare l'attenzione dei colleghi su un altro effetto dell'attrito interno, effetto che ritengo opportuno sia messo nella sua giusta luce. f i n o dal 1944 avevo provato che, in un mezzo elastico, che ammette attrito interno, la velocità di fase delle onde superficiali, a brevissimo periodo, era fortemente influenzata dall'azione dell'attrito, nel senso che la velocità di propagazione delle onde di rayleigh in un mezzo firmoelastico, da un valore infinito per periodi nulli, tende rapidamente, per periodi crescenti, al valore che le compete in mezzi puramente elastici, tanto più rapidamente quanto maggiore è il valore del rapporto fi/u' (u, costante di l a m é ; fi', coefficiente d'attrito interno equivoluminale), come da fig. 1. i n un primo momento, questo risultato mi aveva lasciato alquanto perplesso; una dispersione anomala, relativa alle onde di elevata frequenza, mi riusciva del tutto nuova, anche se essa risultava effettiva per le velocità di gruppo nelle verghe e nelle lastre di ghiaccio (come è stato recentemente provato per queste ultime). f x za 1 . 5 1.0 9 1 3 4 veloci/à' onde di qagleigh 1 • in un mezzo ekiticv(jj.''o.(j~02s) z-yc,-in mezzi firmo -e/astici (2:/j/fs-too) (l.tf/s50) 0-5 0.2 05 f i g . 1 uo rfjce) e f f e t t i d e l l ' a t t r i t o i n t e r n o s u l l a v e l o c i t à d e l l e o n d e s i s m i c h e 4 7 1 restava comunque da provare sperimentalmente l'attendibilità di questo effetto, teoricamente legato agli altri effetti della firinoelasticità, che mi avevano consentito di spiegare le fondamentali caratteristiche delle onde di rayleigh. le cose erano a questo punto quando, qualche mese fa, ho avuto modo di leggere un lavoro di due geofisici giapponesi sulla natura dei microsismi (3). fui colpito sopra tutto dal fatto che lo studio sperimentale sulla propagazione dei microsismi conduceva i due giapponesi a questa conclusione: la velocità di propagazione delle onde superficiali microsismiche cresce al diminuire del periodo. da note a pie pagina risultava che allo stesso risultato era pervenuto il sismologo giapponese k . sassa nello studio della propagazione delle onde provocate da esplosioni ; questo però con relazione alla velocità di grupo 12 qj e 1 0 cu o 8 3 \ <o o , o o po, mentre le conclusioni sui microsismi, raggiunte dai due giapponesi, si riferiscono alla velocità di "°ojso fase. si aveva cosi alleile la conferma sperimentale della validità del principio da me ottenuto per via teorica: l'attrito interno y ( , , , , cioè agisce sulla velo? 3 4 5 6 7 scc cità delle onde superficiali, in modo da determinare dispersione anomala, in modo cioè da far tendere la velocità di fase all'infinito col tenere del corrispondente periodo a zero. tale conferma è stata reiterata dai due giapponesi su nominati in una loro recentissima pubblicazione (4), dalla quale riproduco la fig. 2. dal confronto delle due figure, potrà sembrare che l'effetto dell'attrito interno risulti più notevole nelle conclusioni conseguite dai due giapponesi. va osservato però che i microsismi da essi osservati interessano strati alluvionali. in queste stratificazioni, il rapporto u/n' è molto più piccolo di quello relativo alle rocce ( 5 ) : esso è dell'ordine di 5-10. 4 5 6 periodo f i g . 2 x l 4 ^ àk . x n v < i a 4 7 2 p i e t r o c a l o i pertanto, per tali strati, anche teoricamente l'azione dell'attrito interno riesce sensibilmente più grande. osservo ancora che l'assorbimento del mezzo si mostra crescente con la frequenza dell'onda: tende all'infinito col tendere del periodo a zero. ciò fa sì che le onde legate ai periodi più piccoli (dell'ordine del centesimo di secondo e meno) vengono rapidissimamente assorbiti dal mezzo. ne deriva che l'effetto dell'attrito interno è apprezzabile solo per le piccole distanze. le velocità che in questo caso vengono osservate non sono velocità del mezzo considerato come puramente elastico^ in quanto su esse influisce, più o meno notevolmente, l'attrito interno, tanto più sensibilmente quanto più piccolo è il periodo legato alla fase osservata. concludendo penso che, per una valutazione esatta delle stratificazioni con i metodi della prospezione sismica non sia consentito, quando si registra a piccole distanze, prescindere dall'azione dell'attrito interno sulla velocità di fase delle onde sismiche provocate da esplosioni. roma — istituto nazionale di geo fisica — luglio 1951. riassunto tra i vari effetti dell' attrito interno sulla propagazione delle onde superficiali di rayleigh, va notato quello sulla velocità: tale grandezza è fortemente influenzata dall'azione dell'attrito interno, nel senso che la velocità di propagazione delle onde di rayleigh, in un mezzo che ammette attrito interno, da un valore infinito per periodi nulli, tende rapidamente al valore che le compete in un mezzo puramente elastico. d'altronde, poiché anche l'azione assorbente del mezzo tende all'infinito col tendere del periodo a zero, se ne conclude che l'effetto dell'attrito interno sulla velocità di propagazione può essere constatato solo per piccole distanze. recentemente, una conferma sperimentale di questo effetto è stata ottenuta dai giapponesi ikegami e kishinouye. b i b l i o g r a f i a ( ! ) caloi p . : comportement des ondes de ravleigh dans un milieu firmo-élaslique indéfini. b u r . centr. séism. int., t r a v . scientif., 17 (mómoires presentés à l'ass. d'oslo, 1948). e f f e t t i dell'attrito interno s u l l a velocità d e l l e onde s i s m i c h e 4 7 3 — sulle onde di rayleigh in un mezzo elastico, fìrmo-viscoso indefinito. pontifìcia academia scientiarum, acta, x, 1946. ( 2 ) caloi p . : teoria delle onde di rayleigh in mezzi elastici e firmo-elastici, esposta con le omografie vettoriali. archiv. f. meteorol., geophys. und bioklimat., serie a, met. und geophys., iv, 1951. ( 3 ) ikecami r., kishinouye f . : a sludy on the propagation of microseismic ìtaves. p a r t i i . bull. e a r t h q . res. institute, x x v i i i , parts 1-2, 1950. ( 4 ) ikecami r-, kishinouye f . : a study on the propagation of microseismic waves. p a r t i v . b u l l . earthq. res. institute, x x i x , p a r t 2, 1951. (5) gutenberg b . : physics of the earth vii • internai constitution of the earth. 1939, pag. 367. annals of geophysics, 55 (3), 2012: interview with g.giorello ethical and cultural value of the earth sciences interview with prof. giulio giorello silvia peppoloni annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5755 prof. giulio giorello is amongst the most prominent philosophers of science in italy and in the world. he is currently professor of the philosophy of science at the university of milan, director of the series ‘science and ideas’ (raffaele cortina books editor), and literary journalist of the cultural pages of the “corriere della sera”, one of the most important of the italian newspapers. in this keynote presentation, in interview form, he talks about the value that the earth sciences have had through history, framing this group of disciplines in ethical and epistemological terms, and highlighting some important elements that have to be considered in geological activities. silvia peppoloni (s.p.) for some years, geoscientists have been talking about geoethics, pondering upon all the ethical, social and cultural aspects that are implicit in geological research and practice, because of the close link between earth sciences and human life and activities. in this historical moment, the scientific community needs reference points to develop constructive debate and to frame geoethics in the correct way from an epistemological point of view. starting from the cultural value of science and techno-scientific enterprise, ‘science is culture’, as is maintained. jules henri poincaré argues that “in virtue of science and art, civilizations have a value”. geology is a science, and as such it is culture, and therefore it can transfer cultural values to society, as first of all the value of its own territory. in your opinion, what is the civil value of the geologist’s activities, and the geologist’s social and individual responsibility? giulio giorello (g.g.) i repeat not only that science is culture, but i also add that jules henri poincaré was right in saying that science is one of the characteristics that distinguish a civilization. earth sciences are an example of authoritative sciences, which had and still have immense importance, especially from a cultural point of view, as well as from a technical point of view. the great geology of the 1800’s and 1900’s, the geology of james hutton and charles lyell, two great scottish geologists, opened the way for the cultural development of charles darwin’s theory. indeed, they introduced the fundamental ‘deep time’ concept: the bible claims that the earth is a few thousand years old, while james hutton and charles lyell claimed that it was much older. this was the first step to a great intellectual revolution, which allowed charles darwin’s theory to take off. geology, by full right, is part of the great science of the enlightenment. geology is a fundamental discipline of scientific enterprise. it has been the cause of great conflicts and controversies, and this is a good thing: when there is an intellectual clash, then there is the pleasure of discovery. conflicts occurred also in the study of the sky with copernicus, galileo, bruno, and kepler, and with newton, in the study of physics and chemistry. it was so also for geology. in the u.s., the deep time concept is still opposed by creationists, who reject the whole cultural change that geology has produced, from the enlightenment to today. so, geology is a fundamental part of scientific enterprise. it all started in ancient times, with the watching of the sky, and also the observing of the earth. the ancients tried to correlate events in the sky (such as eclipses) with great events on earth. today this may seem to us naïve, but it shows that the great civilizations of the past had already realized the importance of studying the sky and the earth. we find this attitude in mesopotamia, and also in india and china, and within some pre-columbian civilizations. if we look at the past, we see that civilization has gone through geology, to try to understand the heavens above us, and also the earth beneath us. s.p. many years ago, giuseppe grandori, an italian professor emeritus who recently passed away, and one of the most distinguished scientists in the field of earthquakes, said: “defending oneself against earthquakes means to reduce the consequences of article history received december 21, 2011; accepted february 21, 2012. subject classification: geoethics, earth sciences, geological culture, precautionary principle, sustainability. 343 earthquakes (victims and material damages) below a limit that society deems acceptable, given the costs that would result by a further decrease of this limit”. grandori said this when nobody was talking about civil protection and emergencies yet. what can you say about the precautionary principle in relation to earth sciences? g.g. this statement of grandori expresses much wisdom, as the best of our good sense. scientific enterprise is nothing more than an extension of good sense, which is not intended as conformism, but as the ability to assess the costs of a technology, together with its benefits. a technology can appear expensive now, but everything must be assessed by looking at the probable outcomes of an investment strategy. if there not had been the great geology of the past, and if there were no the current efforts of geologists, today we would be far behind in terms of civil protection and rational planning of the environment. this quote from grandori illustrates well the way to proceed in all of the applied sciences. the same can be said for medicine, when we take precautions against the risk of infections, or for engineering, navigation techniques, and others. in every discipline, there are always elements of risk. john locke, an english philosopher and physician, wrote that we take our decisions under conditions of uncertainty, and therefore of risk. we cannot think of a perfectly safe technology, because ‘absolute’ is not a condition of our world. if we only act when we are sure that there is no risk, we would not even go out of our house, because we would not have the certainty about the risk of a tile hitting us on the head. in all sciences, there is always uncertainty, and under conditions of uncertainty, we have to take our decisions. it is impossible to act with absolute certainty. the precautionary principle in an absolute sense cannot operate. demand for a technology to be put into operation without risk is a metaphysics call. we have to take risks, but risks must be balanced with a calculation; this is what grandori highlighted. i am against the absolutist use of the precautionary principle to stop forms of scientific experimentation, as used by some fundamentalists or by reactionary philosophers. i think that the fear of technology in itself is a dull and reactionary attitude. working in fear is not worthy of a civilized human being. we have to face fear, and the fear should not be denied; it should always be used to understand how to move rationally. for the assessment of risks, we have to use the calculus of probability. we have this tool to assess risks and to see which policies are more or less rational. this is what pascal teaches us, and also the jansenists, who claimed that the calculus of probability is essential to determine our conduct. for example: if a man is afraid of lightning, but he is economically motivated and knows in addition that studies of probability indicate that only one man in a million is hit by lightning, then he wins over his fears, and he leaves the house even if it is raining. moreover, we see that some risks of scientific research are much lower than others. it is much more dangerous going out with the car and driving on the highway during the weekend. we must accustom ourselves to living in an uncertain world. as john locke said, “we take our decisions in the twilight of probability, and not into the clear light of certainties”. however, using the calculus of probability, we can do the best we can, as grandori said. the eminent italian mathematician bruno de finetti, theorist of subjective probabilities, always cites geology as an example of a discipline in which the problem of uncertainty is always present: to know how to handle this can give us great satisfaction. s.p. based on these considerations, the concept of sustainability, which contains in itself the concept of time, in terms of continuity and long-term use of a resource, how has this to be understood? g.g. the concept of sustainability is implicit in the sentence of grandori. there is no sustainability in an absolute sense. the balance between cost and benefit is fundamental, and it can change over time, as well as our intuitions about what we want to achieve or not to achieve. it is important to be clear. a few decades ago, bioethics was born, with the aim to assess the advantages and disadvantages of research on human life and to understand how this research can be directed at improving the quality of life. today bioethics has become the cause of a clash between different stances of ideological, political and religious positions. this is beyond what the founders had in mind. i hope that geoethics does not go in this same direction, and instead remains a sober discipline, that looks primarily at the ethical significance of the correct knowledge of the world we live in. philosophy, born as cosmology, is not a mere intellectual exercise, but a way of life. i believe that the earth sciences can better explain to us how our common home works, indicating the ways to deal with natural disasters, which are often triggered by humans, and therefore refer to human responsibility. the lisbon earthquake of 1755, which was an event that shook the conscience of the enlightenment, including intellectuals like voltaire and kant, destroyed entire neighborhoods and left others intact, for example, the arab quarter. this highlighted the differences in the expertise of those who had built at different times. building safely is an ethical responsibility. interview with prof. giulio giorello 344 345 s.p. plato opposed episteme, the science based on knowledge, to doxa, the opinion of the people. i ask you for a reflection on the l’aquila earthquake. on that occasion, maybe there was a mixing of many elements: disagreement across scientific knowledge, deemed more or less authoritative, and media sensationalism, and the pain and dismay of the citizens of l’aquila, and inside us, as earthquake researchers, the feelings of impotence and uselessness. how can we combine the rigor and the determinism of scientific research with wise and operational good sense, that can reassure the population in case of alarm? perhaps this should be a responsibility that scientists have to assume; or maybe not? g.g. today, we have little faith in scientific certainty, that is plato’s episteme; we are accustomed to work more on our opinions. but opinions should not be despised. “the opinions are our consciousness that is being formed”, as the english poet john milton wrote during the civil war in england in the 17th century. opinions must be evaluated through comparison and intellectual exchange if we want them to be valid and not to be shocked by emotion. the challenges of nature are continuous. i believe that in the tragedy that occurred in l’aquila, it is necessary to start from the legitimate discomfort of citizens, from their pain and suffering, which cannot be removed. the media, subservient to political power, have striven to show the politicians as ‘saviors of the country’. politicians should be servants of the state and citizens. in our country, [italy,] we are discovering acts of awareness of whole categories. if the politicians do not serve the citizens, they must be driven out. if the citizens say “no!” to those who govern us, this will also help scientists: science is allied with democracy. if politicians do not take into account these aspects, they become dictators, and against dictators there are very specific methods that can be used, as the legitimate fight against the iniquity of a power that does not take into account first of all the interests of the health and safety of the citizens. s.p. referring to the relationship between science and politics in the italian context, sometimes we might think that our politicians appear far from an understanding of the importance of scientific research as a tool to serve the citizens, considering it only a mere speculative exercise. a recent example in italy is the referendum on nuclear energy. beyond the ideas that somebody might have about this, do you think that a clear debate on the limits and potentials of this technology has been completely missed? g.g. with regard to nuclear power, it is an interesting case of shilly-shallying and indecision. i think there have been many missed opportunities in the past; now the situation is so intricate that any choice might be wrong. a debate on nuclear power had to be carried out 40 years ago, avoiding being drawn into a strong emotional impact. at the time of the chernobyl disaster, it had to be said that the tragedy was due to the age of the structures. personally, i am not a supporter of nuclear power, but i think that information for the citizens should be increased in italy. we have a cultural lag in science, whereas in other cultural sectors, we are ahead. we must take advantage of the excellence of the many italian scientists who have reached very high levels of skill and are always available to provide their expertise in a frank, honest and democratic debate. my idea of democratic debate is very far from the miserable calculations made by political parties in italy, both of the right and left political parties, as there is not much difference, with a very low level of demagoguery. s.p. one last reflection, on the women who dedicate themselves to science. without going deeply into the theme of discrimination, which still today in the 21st century appears difficult to overcome in our society also in the sector of scientific research: perhaps the intelligence of women, the intuitive capacity and the good sense that often distinguishes them, is an unexploited potential. what do you think about this? g.g. when we talk about the role of women in scientific research, my thoughts go to lise meitner, the nuclear physicist who lived in the 1900’s, a collaborator of max planck. she received no recognition, despite her contribution to the development of the theories about nuclear power. she was discriminated against not only because she was a woman, but also because she was jewish, and for her political beliefs. my thoughts go to rosalind franklin, who contributed to the studies about dna; only after her death was the nobel prize given to the other members of the team, the males, for the discovery of the dna double helix. somebody said she had a bad temper. we are still under the silliness of judging a woman who works in research by her character, and not by the greatness of her scientific discoveries! so, in the same way, we should erase newton from history, who had a notorious bad temper. we have to be careful. the difficult integration of women is not a biological or psychological problem, as some argue, but it is a question of discrimination and social oppression. women should be able to do more than what they do, even in science, as well as in the political field, to gain proper recognition. when there is discrimination, the shame is expanded also to those who are not a part of this discrimination. “every wrong done against my brother or my sister is done against me”: this is the principle which interview with prof. giulio giorello should lead us to remove any obstacle, and to use the injection of energy that comes from the presence of women in science and technology. s.p. to conclude: karl popper says that it is possible to talk even among people who do not share common values; the debate and the language can be built along the way. but then, is it possible to identify a common value, an ethical criterion on which to base the study of our planet? what principle should guide us in geological research and practice, and what, as geologists, can we do to increase the motivation and sense of responsibility that our work implies? g.g. i agree with this wonderful statement of popper. if there had been no tower of babel, it would have been necessary to invent one. the differences in points of view, in languages, in the conceptual apparatus, are resources and not defects in scientific research. contrasts are necessary, especially in the field of scientific research. if we clash over a problem, as galileo said, science takes advantage from this. he was right, even if he paid for his autonomy of thought, for his intellectual freedom, with the shameful processes that silenced him. it is dangerous for scientific enterprise to silence the dissent. false consensus is unmasked in the long run, and scientific truth comes out in the end. one of the most beautiful and tragic histories inside geology regards the wonderful figure of alfred wegener, a scholar who defined the theory of continental drift. he was mocked by his contemporaries, despite the deep and refined articulation of his theory, and this caused legitimate resentment in him. he died as a lone hero on an expedition to the pole. but decades after, his theory was reconceptualized, and has become one of the great revolutions within the earth sciences, which then led to the theory of plate tectonics. this story shows that those who dare to disagree, in the end are right. intellectual tolerance and openess to the proliferation of multiple points of view is a boon to science. there were similar vicissitudes in quantum physics: the clashes between einstein and bohr were useful to refine the theory, and from these discrepancies were born beautiful works. geology has its epic stories too, full of ethical implications. ethics means an attitude that aims to reduce the suffering of the world in a reasonable way, and to intervene with the environment in a controlled and responsible way. in the book of bill mcguire, who teaches in one of the most prestigious british universities, entitled ‘a guide to the end of the world’, there is no history of apocalyptic prophecies, for which we care very little. instead, there is an analysis of our responsibilities to avoid the disasters that can have serious consequences on the global economy. there is a chapter on earthquakes, which shows how the wrong choices of governments and their disregard of geological problems can have heavy repercussions. this shows all the stupidity of the political class. the issues are complex, and geology can help in this complexity. geology is like a red thread that allows us to move ahead: let us make geology as strong as possible! it is important to talk about geology in schools, to make young people aware of the importance of knowing the earth. moreover, we have to tell the heroic stories of geoscientists, like james hutton, charles lyell, john tuzo wilson, and many more, who are worthy enough to stand alongside the best-known figures, such as einstein, copernicus and galileo. s.p. thank you, professor, for the recognition of value that you have given to the earth sciences and for having framed this group of disciplines in ethical and epistemological terms. corresponding author: silvia peppoloni, istituto nazionale di geofisica e vulcanologia, rome, italy; email: silvia.peppoloni@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. interview with prof. giulio giorello 346 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 10vogt.qxd 465 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words west indies – historical seismology 1. introduction this contribution presents a short glimpse at the historical seismology of the west indies (fig. 1). some personal experience of the area leads to a tentative appraisal of the state-of-theart. while methodological and source problems are stressed, mostly from the author’s own experience, no effort has been made to provide an up-to-date inventory of catalogues of different types or specific papers, the discussion being limited to a more or less representative sample of both. indeed available information, often new, could fill a book. actually, the historical seismology of the west indies is not a field of over-research as some parts of the world are sometimes thought to be. while parts of the caribbean area participated in the renaissance of historical seismology, mostly venezuela and central america, parts of it escaped it, aside from sweeping catalogues, often inspired by former ones, and considering only publications founded on genuine research with a renewal of knowledge and/or interpretation. indeed the history of west indian earthquakes forms a patchwork not easily mastered. it should not be forgotten, in a general way, that research often must be undertaken in not always easily accessible depositories, archives and libraries, in europe and the united states. 2. catalogues and studies of west indies seismicity, and associated problems leaving aside classical seismological compilations, such as perrey (1846), poey (1857), scherer’s papers on the seismicity of haiti a glimpse at the historical seismology of the west indies jean vogt rue du docteur woehrlin 1, strasbourg, france abstract this contribution describes the state-of-the-art of historical seismology in the west indies. an overview of the current parametric earthquake catalogues shows that historical earthquakes in the greater and lesser antilles are less well known compared to those which affected the venezuelan and central american regions. problems are still numerous and hard to solve for most former earthquakes, known from the mid 16th century on. the analysis of the catalogues shows that they are incomplete, and that their reliability also should be reconsidered. in recent years, the author has investigated locally stored primary historical sources as well as those in foreign archives and libraries, for instance in france and the united states. the paper deals with some case histories, the 1692, 1751, 1767, 1770, 1830, 1842, 1852, 1860, 1887, 1892 earthquakes in the greater antilles, the 1690, 1727, 1839, 1843, 1867 and 1897 in the lesser antilles. the examples highlight that there is still a need for further improving our knowledge on the historical seismicity of the west indies, and that the investigation of the sources and repositories located outside this region is an open way to fruitfully approach this task. mailing address: dr. jean vogt, rue du docteur woehrlin 1, 67000 strasbourg-robertsau, france. 466 jean vogt (scherer, 1911; scherer, 1912), half a century later, emphasizing major events of the 18th century and mid 19th century (1751, 1770, 1842), should be hailed as a landmark. he encompasses in a modern way, not always attained nowadays, the many facets of historical seismology even if seismotectonic interpretation is still in its infancy. at the same time taber and others investigate specific past and contemporaneous events with the same state of mind (e.g., reid and taber, 1920). however a slump followed, with different time spans from one island or cluster of islands to another. while seismotectonic discussion went on, more or less, often in a straightforward way, historical seismology was neglected to a large extent, sometimes in a paradoxical way. while taber skilfully uses arguments from well-mastered historical seismology for an overall view of «the seismic-belt of the greater antilles» (taber, 1922), the pages devoted to the west indies by sieberg’s erdbebengeographie in gutenberg’s famous handbuch are a disaster, misleading as much by the choice of events as, it seems, by the short information given for them. major events escape it, e.g., the well known 1842 earthquake in northern haiti or the major cuban events of 1766 and 1852, while less important ones are nevertheless considered as wichtigere erdbeben (strong earthquakes). in several cases sweeping information on damage should be carefully checked. for instance, it would be interesting to know if there is really any substance to the küstenabbruch (coastal cliff fall) reported for the 1751 earthquake in haiti, an event i know fairly well. later, a paradoxical message is delivered by rutten and raadshooven (1940). quoting several older catalogues, perrey (1846), poey (1857), salterain y legarra (1884), they state that «they are [...] of no great use to modern researches as the combination of macroseismic and seismographic data, necessary for a good fig. 1. greater and lesser antilles: places mentioned in the text. 467 a glimpse at the historical seismology of the west indies ones have since been produced for the greatest part of the west indies, also with shortcomings. it seems that the trend begins with the early catalogue devoted by robson (1964) for the eastern caribbean. it is mostly based on information easily available in trinidad and neighbouring islands, using, besides mallet (1853-1855) and, partly, perrey (1846), information given by newspapers. he himself writes that «no attempt has been made to examine the source material from which these catalogues (mallet, perrey) [but not poey] were compiled, a costly undertaking..., sources whose reliability it has not been possible to assess» (robson, 1964). on the whole, assessments of intensities, some times clearly conventional, are not reliable. in most cases, macroseismic areas and epicentres are not discussed. even trinidad is not mastered, at least for events of the 18th century. nevertheless this catalogue is useful for collecting a wealth of local, hardly interpreted information that can be used for a wider regional discussion. data banks uncritically fed by robson’s (1964) catalogue would be misleading, with the exception of the use of such basic material. personally i plan to exploit my files to provide a more balanced view of trinidad’s historical seismicity, mostly linked to earthquakes at some distance, which nevertheless are sometimes destructive. jamaica, a larger island, with better possibilities of comparison with other islands at similar scales, and with a larger background of historical seismology, is better mastered by tomblin and robson’s (1977) catalogue, once more fed to a large extent by a systematic search in west indian, chiefly jamaican newspapers. while no thorough catalogue seems to have been recently devoted to haiti, several regional or global catalogues, including of course the haitian seismic activity, have been prepared for the dominican republic in a more or less repetitive way. martinez barrios’ catalogue (1946) in the mid 20th century is followed in 1978 by utrera (1978), adding information to scherer’s fundamental work (covering also the eastern part of the island) and by a succession of catalogues by acosta segura (1987; n.d.). sometimes these do not always meet expected standards, for example, with regard to sources. understanding of the phenomena, is only possible since about 30 years». casually they also mention scherer and some others. while shortcomings of catalogues, old as well as recent, are numerous, the authors clearly underestimate, as a kind of postulate, the value of historical seismology on the whole. limiting their efforts to the instrumental period, from 1913 to 1938, they however have to admit many shortcomings of seismographical data. they are disturbed by the «remarkable fact that in the west indies during the past 27 years quite a number of earthquakes with rather large intensity and spreading have been felt which are not registered in seismograms» (haiti, dominican republic, jamaica, puerto rico). also «as far back as 1914 the strong jamaica earthquakes have not been recorded at other observatories than st. martial» (haiti). besides, a lot of strong events «have not found a place in the international seismological summary». despising historical macroseismology in a general way, they nevertheless have to stress the importance of contemporaneous macroseismology evolving, of course, into the former. later, look, for instance, at the caribbean sketch of a paper devoted in 1973 to prediction criteria of major plate boundaries (kelleher et al., 1973). in a symptomatic way you will find a lot of question marks, not all easily identified and understood, for events of lesser antilles (1831, 1834) and haiti (1751, 1770, 1860, 1887, 1911). clearly the authors are not at ease with events of the past. we read: «estimated rupture zones of caribbean shocks since 1800; three great shocks of the 18th century are also included». what about other events with similar intensities, seemingly also important for such discussion? so the performance of historical seismology, a quarter of a century ago, seems poor at first glance. while question marks are still present for a lot of 19th century events, major events of the 18th century were rather well known at that time, with, of course, a steady improvement since. of course problems are still numerous and hard to solve for most former earthquakes, known from mid 16th century onwards. after a lot of more or less specific catalogues, with more or less shortcomings, new 468 jean vogt most interesting are the maps devoted by nuevo atlas nacional de cuba (1989) to the seismicity of the caribbean and the country. while magnitudes are stressed from 1904 onwards (alvarez gomez, 1989), a map of epicentres is proposed from 1551 to 1983 (chuy rodriguez, 1989). without discussing its conception from other points of view, even locations raise questions. strikingly, a set of important towns appears, trinidad, bayamo, baracoa, etc. once more a frequent urban fixation appears. furthermore, some isoseismal maps are provided, the oldest being for the western cuban 1880 event. isoseismals of major earthquakes of the area can be found elsewhere, notably for the 1842 haitian event, encroaching on cubas’s oriente, used for the preparation of an «isoseismal model» by alvarez and chuy rodriguez (1985). the same authors (chuy rodriguez and alvarez, 1988) also supply data on the 1751 and 1770 earthquakes in the española island. satisfactory standards are attained by feuillard’s (1988) catalogue of the french islands, using not only former catalogues, like poey (1857), famous french authors for several timespans and specific events (like the many books and papers by moreau de jonnès, e.g. 1822), and newspapers (to a limited extent), but also a range of original sources, at least the more easily available ones in french archives. while most assessments of intensities seem reliable, there are once more some shortcomings in appraisals of macroseismic areas, mostly for smaller events, not only from the end of the 17th and the beginning of the 18th century but also from the 19th century. it should be stressed that this effort has been made by a specialist of instrumental seismology, convinced of the need of a better knowledge and a better use of historical information. it is one of the reasons why this pioneering work won wide approval, first, of course, in the french islands. personally i made intensive research over the years, a costly undertaking indeed, in archives and libraries not only in santiago de cuba, port-au-prince, santo domingo, saintecroix, san juan, martinique, barbados and curaçao, with some results, although mostly limited to events of the second part of the 19th and the first half of the 20th centuries. most of the research was made in european and north american depositories storing a wealth of not easily accessible or forgotten sources for former events. most information (some of it could be called «discoveries») was gathered in france and the united states, while a lot of work has still to be done in great britain, copenhagen, etc. besides other harvests, major events have been revised for haiti and the dominican republic (1751, 1770, 1842) and the lesser islands (1690, 1727-1928, 1839, 1843, 1851, 1870, 1897) partly published (vogt, 1994), for the major part unpublished. besides, many forgotten smaller events have been identified, possibly filling «gaps». in a more general way i tried to discuss shortcomings of catalogues for some years, chosen at random, of the mid 19th century (vogt, 2001a), a complex work going on, although slowly, avoiding, of course, poor and/or misleading data banks using to a large extent uncontrolled secondary information. it is too early to assess the degree of craftmanship and originality of a recently announced french data bank for the west indies and the caribbean sea (sisfrance-antilles, 2002), described as containing 1600 events with their epicentral intensity, 4900 observations and 400 «références documentaires», the latter being a rather small number compared with the figures given for the data bank for france, respectively 5000, 9000 and 9800. although the fields are not at all comparable, the gap between the figures for «références documentaires» is nevertheless striking, considering the wealth of material available for the west indies. one can ask, to what extent are these catalogues, learned papers and the like leading to reinterpretations of events, or to the discovery of forgotten ones? we all should admire people able to propose epicentral intensities of so many events, many of which are reported from only one town or even by one ship’s captain. it should really be an outstanding achievement earning worldwide fame. here and there it seems that specialists proceeded straight from their stock of catalogues, monographs, newspapers, etc., to rather summary data banks, with469 a glimpse at the historical seismology of the west indies major pieces of information. first they should systematically give appraisals of reliability, for the location of epicentres, epicentral intensities, etc., from a judgement founded on an intimate knowledge of sources and their whereabouts. in other words it should be the unavoidable task of those responsible for research and not left to somebody else, with talents for computer work. furthermore, sources should always be identified in a proper way, to enable checking and possibly reinterpretation by others. nowadays i could still echo the following words written about the absence of sources in the mid 19th century by poey dealing at that time with cuban earthquakes and storms: «malheureusement cet abandon a trop souvent lieu parmi les auteurs. on dirait qu’ils craignent qu’on puisse y recourir» (unfortunately, researchers too often abandon sources. it seems that they fear to find new information) (poey, 1855). too often data banks escape revision like, in many cases, geological maps. money spent, work done, epicentres located, intensities assessed, magnitudes calculated, etc. new information brought by historical seismology should not only be incorporated in a rough way, but should sometimes lead to drastic reinterpretation, with, possibly, consequences for seismotectonic discussion and appraisal of seismic risk. besides, let’s insist on the correct sense of two words, among others, that are widely used in a haphazard and misleading way. the word «source» should apply to original records or their faithful copies, translations, summaries and to contemporaneous or later echoes using disappeared or inaccessible primary sources, to be exploited, of course, in a most critical way. too often unreliable second-hand records and even catalogues are listed under this heading, in a most deceiving way. similarly the word «archives» too often designates no more than some corpus of documentary material of any kind, including once again, catalogues, while it should be used in a strict sense to indicate genuine archive material with its specific problems of research. in a paradoxical way it is sometimes used by people without the least idea of what a depository is. in these matters, seismologists need to learn from historians. out bothering with any intermediate stage, at least any accessible one. actually the main interest is often for major quakes, for instance from intensity vii on, as also in some cases in other parts of the world. a recent discussion of the seismic potential of a part of cuba takes into account only eight events from 1692 to 1913, a sharp contrast with a wealth of instrumental information for the period 1979-1989 (alvarez and bune, 1978; chuy et al., 1988; cotilla et al., 1997). in a large part of the west indies, as elsewhere, it seems that less important historical earthquakes, even with question marks, could bring suggestions for a better informed seismotectonic discussion and for a better appraisal of the seismic potential at scales different from those of major earthquakes. besides, events now considered as minor, e.g., less than degree vii from available information, could in fact turn out, from further research, to be of higher intensity and a kind of damocles’ sword. such a remark could apply to events known only from some town and not from the countryside, with possibly, in fact probably, a higher epicentral intensity. in a paradoxical way, even the most important events are not always fully exploited from a proper seismological and a seismotectonic point of view. most catalogues and data banks consider only the main shock and possibly one or another important aftershock. it is easily forgotten that the wealth of sources for several earthquakes is such that whole sequences can be followed, sometimes for months, in a rather precise way. fine examples are, for instance, the famous earthquakes of 1751, 1770 (haiti), 1852 (cuban oriente) and 1860 (haiti). such sequences deserve attention not only for a proper assessment of damage and intensity, but also for a tentative discussion of the migration of epicentres in paleoseismic areas and for the knowledge of types of sequences possibly by comparison with instrumentally known sequences in the same areas. such remarks apply mostly to large islands. it is not always easy to distinguish in the lesser antilles, with their «island-hopping» problems, shocks belonging to such sequences from specific nearby events. whatsoever, catalogues and data banks should provide, besides other basic requirements, two 470 jean vogt 3. sample case histories luckily we know more and more examples of well-balanced convergent efforts by historical seismology and seismotectonic discussion with, in between, instrumental seismology. only two examples from the french islands are given here. once more we should mention feuillard’s (1988) «catalogue» with its broad perspective. later a harmonious collaboration of bernard and lambert (1988) led to sizable results, although mostly classical sources, like sainte-claire deville (who produced a lot of publications, e.g. 1861), have been used, with of course unavoidable minor disagreements, here and there, with other sources. but we should avoid general optimism. after a first «renaissance», at the beginning of the 20th century, a long slump, over half a century or even a century, has been described, followed by another «renaissance». some people think that historical seismology is coming to an end, that most work has been done, with wonderful data banks, while some strange elderly people are going on in a stubborn way. the trend is to jump from micro-seismology to bulldozer work relying, between these extremes, on often hasty, uncritical listings, with sometimes a paradoxical preference for important events. in some parts of the world such workmanship would satisfy the needs of seismotectonic overall interpretation. actually, despite a lot of achievements, the west indies are not ready for such a state of mind. while historical seismology question marks have diminished since the seventies they are still numerous, requiring intensive work to gather down-to-earth information and to (re)interpret numerous events and their setting. historical seismology is often used in a narrow, subordinate way, with a tendency to quantitative abstraction illustrated by several data banks, while qualitative informations should be of foremost interest, first of all for mastering the complex field of seismo-geological features. in some cases, in regions with large intervals between major events, only centuriesold earthquakes provide arguments for the assessment of this essential part of the discussion of seismic risk, the indirect one. actually, personal work has been devoted to the identification of cases of liquefaction, rockfalls, etc. in the different geological and geomorphological settings of the greater and lesser antilles, with a wealth of information, still to be located precisely by fieldwork. many examples were given at the workshop devoted to historical seismology at perpignan in 2002 (vogt, 2001b; vogt, 2001c). besides, specific papers have been devoted to such features for two major earthquakes of the lesser antilles (vogt, 1993a; vogt, 1995). so only a few examples are given here, with some references. widespread fissuring and liquefaction occurred during the major jamaican events, foremost in 1692, with fine, easily interpreted information. so we read in a classical description: «... the earth opened in my own parish in multitudes of places and through these chasms spew’d out water to a considerable heigth above ground, in such quantities in some places that it made our gullies run on a suddain, tho’ before exceeding dry... these gaping mouths being no less than 12, 20 or more foot deep under the earth and above two miles up the country, these, especially nigh the river, in the purest mould which had no clay or other consolidating matter beneath to oppose the force of the fountyains of the deep breaking up for where that was we do not find any cracks of the earth at all...» (anonymous, 1693). widespread cracking and liquefaction occurred in 1751 and 1770 in the wide culde-sac plain of southern haiti while such features are described in 1842 in the plain round le cap in northern haiti. under different geological conditions such processes occur only locally, nevertheless in an impressive way. so we hear in 1842 from the mountains of northern haiti, near chatard, that «[...] the earth opened and a clean stream of water spouted forth to the elevation of four feet like an artesian well» (jamaica standard, 1842). they are described similarly in 1843 from nevis, st. christopher, etc. sometimes the lesson has been heard. so a «liquefaction potential map» has been prepared for puerto rico (windeler et al., 1995). rockfalls are widespread in many geological settings, once more in southern haiti, in the 18th century, also in the cuban oriente, but mostly in the lesser antilles, for instance in redonda in 1690, monserrat in 1843, etc. 471 a glimpse at the historical seismology of the west indies further we have a fine knowledge of disastrous tsunamis, e.g., in the virgin islands in 1867, with a wealth of information, and in northern haiti in 1842 with precise information from towns, mostly from le cap. besides this, more modest movements of the sea are often reported, for instance on the same coast in 1897. from saint-louis-du-nord we hear that «la mer s’est retirée très loin laissant beaucoup de poissons à sec, elle n’est revenue que bien doucement et est entrée jusqu’au milieu du bourg» (les antilles, 1887). many sources discuss the behaviour of different types of buildings, the most interesting being the case of mamposteria buildings, a technique using a mixture of materials including irregular fragments of stone, brick or clay tile with soil and sand. while stone buildings, easily destroyed, are forbidden here and there, timber-built towns are easily burnt during earthquakes. to my knowledge, a specific paper has not yet been devoted to such discussions and events, although historians have published background information (luopiès, 1986; geggus, 1990). as an example let’s quote instructions given in 1767 for a saint-domingue (nowadays haiti) sugar plantation: «vous êtes instruit... des malheurs causés dans presque toutes les antilles par les tremblements de terre. ce fléau doit vous engager donner à tout ce que vous avez en pierre la plus grande solidité possible. si les bois étaient plus communs je préférerais cette construction à toute autre, par cette raison qui me fait croire qùelle plus durable et plus propre à y résister...» and, further: «je vous réitère la prière que je vous ai déja faite de ne rien épargner pour la sûreté et la solidité...» (debien, n.d.). of course psychological and social problems, most important in the caribbean, arise in earthquake-prone areas. in 1770 prophetic words uttered by a monk, soon expelled, led to some unrest among slaves at port-au-prince (debien, 1979). in 1842 people from the country-side rushed into the destroyed town of le cap (haiti) for systematic robbing, not easily stopped. after such a prediction made in 1906 in santiago de cuba a slight shock, seemingly confirming it, and possibly announcing a major earthquake, produced a panic. even the helpful intervention of the united states marines in jamaica in 1907 led to a political problem. ultimately these numerous facets of historical seismology could be important contributions for enlarged discussions of earthquake risk and earthquake potential including socioeconomic perspectives (marques et al., 1983). however, earthquakes, whatsoever their cost, whatsoever their potential, are not always the major problem of the caribbean area. they should be considered in a wider frame including, most of all, hurricanes. on the other hand sweeping appraisals of natural risks on the whole do not always master major earthquake problems (cross, 1992). 4. source materials for future studies finally let’s have a glimpse at some types of records and depositories, already hinted at here and there, with some examples. they have been discussed in a wider range west indies, central america and some south american countries at the trujillo (venezuela) workshop in 1997 (vogt, 2002). we will proceed here in a most summary, down-to-earth way, with random examples, among a surprising wealth of hitherto partly unexploited sources of all possible kinds. in the west indies themselves, archive sources, manuscript or in later editions, are mostly not available for the 18th century. as an example the records of san juan’s cabildo give here and there hints of earthquakes, e.g., in 1786. most work has to be undertaken in far-off depositories, first of all in britain and france. so french archives, mainly archives nationales, keep a wealth of reports on major 18th century earthquakes of saint-domingue (haiti) and the lesser french islands. some sources have been printed, officially, for the major jamaican event of 1692 (fortescue, 1901) or otherwise, for the 1727 martinique earthquake (bruneau-latouche, 1991), or the 1751 española event, from audiencia de santo domingo files (archivo nacional de cuba, havana, 18th century). for the 19th century, west indian depositories are sometimes rewarding. as an example not precisely known (anonymous, 1830). similarly the sequence of april-may 1860, in the south-western part of haiti, is best known from such a source, at les cayes: «... im ganzen hatten wir 68 deutliche stösse, ungerechnet die schwankungen...» (anonymous, 1860). even later testimonies are useful. years after the 1842 event in northern haiti, a traveller still describes rents, «larges crevasses» in the vallières mountains (maris, 1860). after many years, souvenirs are recalled. so a former resident of martinique, describing the 1776 oléron (france) earthquake, makes a comparison with his west indian experience: «... j’ai fait ma résidence pendant vingt ans à la martinique; j’en ai ressenti quelque fois jusqu’à deux ou trois chaque année, mais je me suis jamais aperçu que les mouvements aient été aussi violents et aussi vifs...» (journal historique et politique, 1776). clearly he stayed there during a «gap». similarly a corsican, a former resident of haiti, conveys in 1887, the year of the famous ligurian earthquake, felt in northern corsica, these west indian reminiscences: «j’ai habité longtemps port-au-prince... où les tremblements de terre sont fréquents et les habitants sont prévenus de ces évènements lorsque les cloches, mises en branle, sonnent d’une certaine façon» (petit bastiais, 1887). the most informative and precise travellers are often ship’s captains describing shocks felt either in some west indian harbour or at sea, bringing news to european or north american harbours. a number of such reports are printed by the newspapers of liverpool, bordeaux, le havre, bremen, charleston, portland, etc., for instance in 1842 and 1843. first-hand reports, often reprinted elsewhere in extenso, abound in local newspapers with, in the lesser antilles, a steady problem of comparison between them, by «island-hopping». unfortunately in the west indies themselves, such newspapers, affiches américaines (saint-domingue, now haiti), le patriote, sainte-croix avis, saint-christopher advertiser, dominican, palladium (st. lucia), antigua weekly register, les antilles, etc., in many cases have either disappeared or are not easily consultable. it is often preferable and less consuming of time and money to look for (of course, 472 jean vogt the archives of the netherlands west indies in curaçao have valuable reports on the effects of the 1843 earthquake, for instance on st. eustatius. of course, official reports are numerous in major british and french depositories. once more, some of them are printed, for example by the british parliamentary papers, for 1843. most important are consular reports, which, for example, are enlightening for the 1842 haiti event and the 1867 earthquake and tsunami in saint-thomas. archives of families and traders easily escape attention and investigation by specialists. luckily debien, a historian, undertook for years a systematic and fruitful search in the archives of french planters families of former saint-domingue, bringing precise and useful information on a number of events (debien, 1941). the diary of a french family from martinique is a useful source for the 1839 earthquake (fremont and elisabeth, 1984). surprising finds are possible here and there. so we had the luck to find in a philadelphia depository a fine report by a leogane notary on damage produced there by the major 1770 saint-domingue earthquake. of course, many such sources have disappeared. while gaps are sometimes filled in complex, indirect ways other sources are clearly lost. as an example let’s quote a seismological manuscript written by courgerolles at le cap (now haiti): «... un ouvrage fort étendu, perdu dans le premier incendie du cap» gathering his numerous observations «tant par mes voyages que par mes recherches» (courgerolles, 1803). doubtless he had a fine firsthand knowledge of saint-domingue’s seismic activity at the end of the 18th century. reports by travellers should be exploited in a systematic way. several describe saintdomingue events from the end of the 18th century. one of them states, at le cap: «... se firent sentir graduellement les secousses de l’est à l’ouest pendant 39 secondes; plusieurs murs tombèrent et quelques maisons furent lèzardées, des sources furent arrêtées, d’autres parurent; l’église paroissiale fut lègèrement endommagée...» (wimpffen, 1797). from a sailor we have a fine knowledge, from port-au-prince, of the sequence of march-april 1830, otherwise 473 a glimpse at the historical seismology of the west indies maica earthquake (cadbury, 1971). several newspapers, church missionary gleaner, missionsblatt, calwer missionsblatt, etc., have been exploited. informing even on mid-18th century events, the moravian brothers’ publications are most rewarding. unfortunately herrnhut’s german headquarters did not allow access to their archives, which may be a rich resource. even for well documented events of the beginning of the 20th century, religious echoes are of some interest. salesians and jesuits provided papers on the major 1907 jamaican earthquake. reporting on the 1918 puerto rico event die katholische mission stresses the fact that only the catholic church was destroyed in bayamo while the protestant ones were intact, with comments on the wealth and rivalry of the protestants. finally, more or less learned papers, contemporaneous or later, are of foremost interest. reports were immediately compiled on the major earthquakes of haiti 1842 and cuban oriente 1852 by local authors, the first being reedited a century later (márquez, 1842). besides it should not be forgotten that several papers in scientific publications are mainly copies, more or less commented, of information from newspapers, consular reports, etc., e.g., in comptes-rendus de l’académie des sciences. century-old seismological monographs now belong partly to the field of historical seismology. a fine example is of course reid and taber’s (1919) pioneering paper on the 1918 puerto-rican earthquake, a chef d’oeuvre thanks to their broad and modern views. while many reliable sources often give isolated information on minor events, felt for instance at sea, thick files are filled by material for major earthquakes. comparison of information of different origin and kinds allows a balanced appraisal of different facets of an event, discarding utter catastrophist and misleading views, and allowing modern interpretations. a fine example of such a «portfolio» is the 1842 northern haiti earthquake, mentioned several times above, with a wealth of reliable sources. these include: testimonies by witnesses, for instance by a haitian general writoften fragmentary) collections in british, french and north american libraries and/or archives. luckily the barbados press has been exploited, partly, by an earthquake researcher, with a fine file left in the island’s archives. of course historical publications, mainly those of local scope, should be sifted for information given here and there, mostly incidentally, on earthquakes, sometimes from unknown or not easily accessible sources, often filling gaps of knowledge, their authors being not always aware of the worth of their findings. as an example, chronicles of santiago de cuba proved useful despite shortcomings (baccardi y moreau, 1924), among others, for instance, from puerto rico and the french islands. for a long time proper historical research, in many cases, mentions earthquakes only casually, despite the fact that they act as a kind of catalyst for the knowledge of the wide field of economic and social history. typically indexes sometimes skip the very word «earthquake». even the history of art should be considered. so a classical work on the monuments of santo domingo is helpful, not only from the proper architectural view of real or possible earthquake effects, but also for tracing sources (palm, 1955). more or less imaginative pictures sometimes illustrate leaflets and learned papers, for instance for the major 1692 jamaican event (cundall, 1936). later newspapers provide illustrations, for instance for the 1867 saint-thomas earthquake and tsunami (illustrated london news, 1867). works of a more or less encyclopaedic kind also forward valuable information. for saintdomingue (now haiti) the famous moreau de saint méry’s printed or unpublished life-work is a treasure box for hunters of 18th century earthquakes although the information is widely dispersed. whatsoever the kind of material, archives, newspapers, reminiscences, monographs, historical sketches, reports from missionaries and clergy are of particular interest. while psychological and specific religious reactions are of course dealt with at length, numerous valuable and sometimes unique earthquake data are forwarded, mostly from smaller islands. a fine example are quakers’ letters on the 1692 ja474 jean vogt ing down reminiscences from his youth (delorme, n.d.), the dramatic experience of a british missionary writing an informative «earthquake poem» (bird, 1870; eldin, 1878; monthly chronicle, 1842), local newspapers like le patriote, le télégraphe, le temps, not easily located, consular reports, foreign newspapers, for instance portland adviser and hamburgischer correspondent printing ship’s captains reports, etc. even for small islands like the turks we are informed in a satisfactory way. such «portfolios» have been gathered for 1751, 1770, 1843, etc. (grases, 1990; vogt, 1997) and partly exploited. 5. conclusions while specific problems arise in the west indies (far-off sources, «island hopping», generations of research, different conceptions of catalogues and data banks, too often unsatisfactorily) their historical seismology illustrates in a sometimes extreme way more general issues. a cultural problem is often present (vogt, 1993b; vogt, 1996a; vogt, 1996b). authors with a narrow scientific or rather technical mind are not always able to master such a mysterious field, sometimes using unreliable information with an incredible credulity, not even always using the best, most critical, data banks in a proper way. for instance, one finds epicentral locations mapped indiscriminately from information with different degrees of reliability; barycentres are plotted irregardless that they are prone to drastic relocation from a newly discovered source or a reinterpretation. further it should not be forgotten that here and there data banks are elaborated by people unable to master either genuine historical seismology, with cases of drastic misinterpretation, or advanced seismology. besides the weight of shortcomings, not all of the numerous valuable arguments and suggestions given by historical seismology are properly implemented for the needs of seismology as a whole, for instance the analysis of earthquake sequences. consequences could be disastrous in the case of the west indies, with their major events, often at large intervals, easily forgotten, and seismic gaps. despite the work done so far, over-research is not yet a problem for the historical seismology of the west indies. references archivo nacional de cuba, havana (18th century): audiencia real de santo domingo, documents reproduced in a publication edited by sociedad dominicana de sismología e ingeniería sísmica, s.d., santo domingo. acosta segura, 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(1995): seismic zonation in puerto rico, in proceedings of the 5th international conference on seismic zonation, nice. s u l l a f i s s i o n e s p o n t a n e a d e l l ' u r a n i o t e r r e s t r e (*) c . f e s t a m . santangelo 1. — f i n rial 1939, e p o c a d e l i a s c o p e r t a d e l l a fissione (*), w . f . lil)l>y (-) m i s e i n r i l i e v o l a i m p o r t a n z a c h e l a e v e n t u a l e e s i s t e n z a di u n p r o c e s s o d i fissione s p o n t a n e a p u ò a s s u m e r e n e i c o n f r o n t i d e l l a geofisica, o l t r e c h e d e l l a fisica n u c l e a r e . i l t e n t a t i v o c o m p i u t o d a q u e s t o a u t o r e d i r i v e l a r e i n e u t r o n i e m e s s i n e l l a fissione s p o n t a n e a m e d i a n t e u n c o n t a t o r e a p a r e t i d i b o r o n o n e b h e s u c c e s s o : p u r t u t t a v i a c o n s i d e r a z i o n i d i v a r i o g e n e r e l o i n d u s s e r o a s t a b i l i r e u n l i m i t e i n f e r i o r e d i ca. io14 a n p e r l a v i t a m e d i a di q u e s t o p r o c e s s o . l a possib i l i t à t e o r i c a d e l l a fissione s p o n t a n e a d e i p i ù p e s a n t i f r a i n u c l e i f u discussa in u n f o n d a m e n t a l e l a v o r o d a b o h r e w h e e l e r ( 3 ), i q u a l i t e n t a r o n o p u r e u n a t r a t t a z i o n e q u a n t i t a t i v a d e l p r o b l e m a . n e l 1940 f l e r o v e p e t r z h a k ( 4 ) ( 5 ) r i v e l a r o n o i f r a m m e n t i d i fissione c o n u n a grossa c a m e r a d i i o n i z z a z i o n e . i n s e g u i l o l a fissione s p o n t a n e a dell ' u r a n i o f u s t u d i a l a d a n u m e r o s i a u t o r i (°)-( 1 0 ); r e c e n t e m e n t e l e c a r a t t e r i s t i c h e d e l f e n o m e n o p e r i l t o r i o , p r o t o a t t i n i o , u r a n i o e p e r i t r a n s u r a n i c i s o n o s t a t e i n d a g a t e d a s e g r è e d a l t r i ( n ) ( 1 2 ) : s o n o s t a l i così d e t e r m i n a t i i v a l o r i d e l l a v i t a m e d i a p e r fissione s p o n t a n e a d i q u e s t i n u c l i d i . 11 r e n d i m e n t o n e l p r o c e s s o d i fissione a r t i f i c i a l e d e l l ' u r a n i o p e r c i a s c u n o d e i p r o d o t t i d i fissione è o r m a i d a t e m p o c o n o s c i u t o ( 1 3 ). e n o t o f r a l ' a l t r o c h e t r a di essi si t r o v a n o a l c u n i i s o t o p i s t a b i l i d i gas r a r i e p r e c i s a m e n t e d e l k r i p t o n (sokr 8 3 , s„krs\ 3ekr86) e d e l l o x e n o n (54xe12", 5 4 x e 1 : n , 51x<? 134, r,4.x136). l ' a n d a m e n t o d e l l o y e h l d e l l a fissione s p o n t a n e a p e r c i a s c u n o d i q u e s t i n u c l i d i è s t a t o d e t e r m i n a t o d a macn a m a r a e t h o d e ( 1 4 ), i q u a l i h a n n o e s a m i n a t o la c o m p o s i z i o n e isotop i c a d e l k r i p t o n e d e l l o x e n o n e s t r a t t i d a l l a p e c b l e n d a . d a l c o n f r o n t o f r a l e a b b o n d a n z e r e l a t i v e d e g l i i s o t o p i d i fissione e q u e l l i n o n d i fissione d e l k r i p t o n e d e l l o x e n o n , essi h a n n o d e s u n t o l a p r o v a d e l l a c o m u n i c a z i o n e presentata al c o n v e g n o d e l l ' a s s o c i a z i o n e o e o f i s i c a italiana, t e n u t o a r o m a il 10-11 g i u g n o 1952. 552 c. f e s t a m . s a n t a n g e l f l p r e s e n z a di p r o d o t t i d i fissione n e i gas e s t r a t t i d a l l a p e c b l e n d a e, p r e c i s a m e n t e , dei t r e isotopi del k r i p t o n e dei c i n q u e dello x e n o n , m e n z i o n a t i p o c ' a n z i . h a n n o i n o l t r e r i s c o n t r a t o la p r e s e n z a d i gas atmosferici nella p e c b l e n d a ; q u e s t o si d e d u c e d a l f a t t o c b e l e a b b o n d a n z e r e l a t i v e degli isotopi n o n p r o v e n i e n t i dalla fissione sono i d e n t i c h e a q u e l l e a t m o s f e r i c h e . p a r t e n d o da q u e s t e c o n s i d e r a z i o n i , i d u e a u t o r i h a n n o p o t u t o calcolare lo y e l d della fissione s p o n t a n e a p e r le m a s s e : 83, 84, 86, e 129, 131, 132, 134, 136 ed h a n n o t r o v a t o c h e esso è, salvo a l c u n i s p o s t a m e n t i , a n a l o g o a q u e l l o della fissione i n d o t t a . 2. — lo s t u d i o dei gas r a r i h a u n g r a n d e i n t e r e s s e p e r le i n f o r m a z i o n i c h e dai r i s u l t a t i d i esso si possono t r a r r e r i g u a r d o al mecc a n i s m o di f o r m a z i o n e d e l l ' a t m o s f e r a e d e l l a crosta t e r r e s t r e . c o m ' è n o t o delti e l e m e n t i n o n sono i n f l u e n z a t i dalle r e a z i o n i c h i m i c h e , sicc h é la loro storia va s e m p r e i n t e r p r e t a t a i n f u n z i o n e dei f e n o m e n i fisici c b e possono avere a l t e r a t a l ' a b b o n d a n z a d e l l ' e l e m e n t o e degli i s o t o p i dei q u a l i esso si c o m p o n e . f r a questi f e n o m e n i v a n n o poste in p r i m o p i a n o l e d i s i n t e g r a z i o n i n u c l e a r i c h e possono c a u s a r e v a r i a zioni locali d e l l e a b b o n d a n z e di e n t i t à n o n t r a s c u r a b i l e . l ' a l t e r a z i o n e della c o m p o s i z i o n e isotopica d e l l ' a r g o n t e r r e s t r e ( l s ) e la a n o m a l a abb o n d a n z a di q u e s t o gas n e l l ' a t m o s f e r a , d o v u t e al d e c a d i m e n t o del k 1 0 p e r c a t t u r a e l e t t r o n i c a , costituiscono o r m a i u n e s e m p i o classico di simili v a r i a z i o n i . a l t r i f e n o m e n i del g e n e r e sono stali osservati e n e l n o s t r o p i a n e t a e n e l l e a t m o s f e r e s t e l l a r i (1(ì). u n p r o b l e m a b a s i l a r e p e r la i n t e r p r e t a z i o n e d e l l e regole di a b b o n d a n z a degli e l e m e n t i n e l cosmo e sulla t e r r a è a p p u n t o q u e l l o di conoscere la e n t i t à d e l l ' i n f l u s s o eserc i t a t o da simili processi sulle a b b o n d a n z e o r i g i n a r i e dei gas r a r i . p e r es., h . b r o w n ( ! 7 ) h a calcolato le a b b o n d a n z e c o s m i c h e di q u e s t i gas, f o n d a n d o s i sulla i p o t e s i c h e l o x e n o n e il k r i p t o n , p r e s e n t i nell ' a t m o s f e r a t e r r e s t r e , n o n siano s t a t i a l t e r a l i n e l t e m p o da a l c u n processo n u c l e a r e . p o i c h é , c o m e a b b i a m o visto p o c ' a n z i , f r a i n u c l i d i slabili p r o d o t t i d a l l a fissione s p o n t a n e a si trova u n b u o n n u m e r o di i s o t o p i di q u e s t i d u e gas, il p r e s e n t e l a v o r o è stato i n t r a p r e s o con lo scopo di i n d a g a r e se il f e n o m e n o della fissione s p o n t a n e a d e l l ' u r a n i o a b b i a p r o d o t t o u n ' a l t e r a z i o n e sensibile della l o r o a b b o n d a n z a o r i g i n a r i a . si p u ò p e r a l t r o osservare a p r i o r i c b e lo yeld della fissione p e r i singoli isotopi del k r i p t o n e d e l l o x e n o n r a g g i u n g e i l suo v a l o r e m a s s i m o in c o r r i s p o n d e n z a delle masse 86 e, r i s p e t t i v a m e n t e , 136, m e n t r e i n v e c e il krha e lo x e i 3 b n o n sono f r a gli isotopi p i ù a b b o n d a n t i del k r i p t o n e d e l l o s u l l a f i s s i o n e s p o n t a n e a d e l l ' u r a n i o t e r r e s t r e 553 x e n o n a t m o s f e r i c i . q u e s t o p o t r e b b e significare elle il processo origin a r i o di f o r m a z i o n e del k r i p t o n e dello x e n o n c b e si r i s c o n t r a n o nell ' a t m o s f e r a h a a v u t o u n c a r a t t e r e diverso da q u e l l o della fissione. i n a l t r e p a r o l e la fissione n o n d o v r e b b e a v e r e i n f l u i t o in m a n i e r a sensibile sulla f o r m a z i o n e dei gas r a r i d e l l ' a t m o s f e r a t e r r e s t r e . 3. •—l a q u a n t i t à di k r i p t o n e x e n o n g e n e r a t a d a l l ' u r a n i o t e r r e s t r e a p a r t i r e d a l l ' e p o c a di f o r m a z i o n e della t e r r a è l e g a t a al c o n t e n u t o a t t u a l e di u r a n i o n e l l a t e r r a ; p e r c i ò si è t e n t a t o c o m e p r i m a cosa u n a v a l u t a z i o n e a p p r o s s i m a t i v a di tale c o n t e n u t o . p e r q u a n t o r i g u a r d a il c o n t e n u t o di u r a n i o n e l l a crosta t e r r e s t r e esistono m i s u r e eseguite d i r e t t a m e n t e sulle rocce c b e la c o m p o n g o n o : si t r a t t a p e r ò , n e l l a m a s s i m a p a r t e dei casi, di m i s u r e del c o n t e n u t o p e r c e n t u a l e di r a d i o d a l l e q u a l i si risale al c o n t e n u t o di u r a n i o attraverso la n o t a r e l a z i o n e : u/ra = 2 , 8 x 10° v a l i d a p e r l ' e q u i l i b r i o r a d i o a t t i v o . sarà b e n e a v v e r t i r e fin da ora c b e l'esistenza d i q u e s t o e q u i l i b r i o n o n p u ò asserirsi con sicurezza p e r ogni t i p o di roccia (1 8). il c o n t e n u t o di u r a n i o n e i s e d i m e n t i m a r i n i ( l n ) è di 1 , 0 7 x 1 0 ° g / g e n e i r i m a n e n t i s e d i m e n t i (2 0) è di 1,25 x io'6 g / g . se si t i e n e c o n t o del f a t t o c b e il 70,8% dei s e d i m e n t i è c o s t i t u i t o da s e d i m e n t i mar i n i (2 1), il c o n t e n u t o m e d i o di u r a n i o n e l l o s t r a t o dei s e d i m e n t i risulta di 1,12 x io"6 g / g . a s s u m e n d o con g u t e n b e r g (2 2) p e r lo spessore m e d i o d e l l o s t r a t o dei s e d i m e n t i il v a l o r e di 5 k m e p e r la sua densità (2:l) 2,3 g cm' 3 , si p u ò c a l c o l a r e c b e in q u e s t o s t r a t o sono presen t i 7,9 x io21 <t di u r a n i o . lo s t r a t o del g r a n i t o si c o m p o n e (2 4) p e r il 52,5% di g r a n i t i , p e r il 39,3% di rocce i g n e e i n t e r m e d i e e p e r l ' 8 . 2 % di rocce basico-ignee. p o i c h é il c o n t e n u t o di u r a n i o (2r>) nei g r a n i t i è : 3,96 x io"11 g / g , n e l l e rocce i g n e e i n t e r m e d i e 2,61 x io'6 g / g e n e l l e rocce b a s i c o i g n e e : 0 , 9 6 x 1 0 ° g / g , il c o n t e n u t o m e d i o di u r a n i o n e l l o s t r a t o del g r a n i t o r i s u l t a p a r i a ea. 3,18 x 10 ° g / g . se si p r e n d e come spessore m e d i o c o n t i n e n t a l e i2 2) dello s t r a t o del g r a n i t o 20 k m e c o m e spessore m e d i o oceanico (2 3) 10 k m , t e n e n d o c o n t o del f a t t o c b e le a r e e c o n t i n e n t a l i ( 2 6 ) costituiscono il 29,2% d e l l a s u p e r f i c i e t e r r e s t r e ( 2 7 l, si p u ò p o r r e lo spessore m e d i o del g r a n i t o p a r i a 13 k m . a l l o r a , d a t o c b e la d e n s i t à dei g r a n i t i (2 3) vale c i r c a 2,65 g cm"3, si p u ò calcolare c b e l ' u r r n i o c o n t e n u t o n e l l o s t r a t o 554 c. f e s t a m . s a n t a n g e l f l del g r a n i l o a m m o n t i a 5,9 x io1" gc o n s i d e r a z i o n i a n a l o g h e a q u e l l e seguite p e r la v a l u t a z i o n e d e l l o spessore dello s t r a t o del g r a n i t o possono a p p l i c a r s i al calcolo dèlio spessore dello s t r a t o i n t e r m e d i o , c h e r i s u l t a d e l l ' o r d i n e di 16 k m . c o m e c o n t e n u t o di u r a n i o n e l l o s t r a t o i n t e r m e d i o si p u ò assum e r e il c o n t e n u t o di u r a n i o n e i b a s a l t i ( 3 0 ) e c i o è : 0,83 x 10'° g / g . con q u e s t i d u e v a l o r i , t e n u t o c o n t o del v a l o r e della d e n s i t à d e l l o s t r a t o i n t e r m e d i o (2 3) (2,87 g cm'3), si t r o v a c h e n e l l o s t r a t o i n t e r m e d i o sono p r e s e n t i 1,9 x io19 g di u r a n i o . i d a t i r e l a t i v i al c o n t e n u t o di u r a n i o nei singoli s t r a t i della crosta t e r r e s t r e , a d o p e r a t i n e l l e p r e c e d e n t i cons i d e r a z i o n i c o n c o r d a n o i n m o d o s o d d i s f a c e n t e con i d a t i r e l a t i v i al c o n t e n u t o di r a d i o n e g l i stessi s t r a t i , r i p o r t a t i da g u t e n b e r g (2 2). u n a v a l u t a z i o n e del c o n t e n u t o di u r a n i o n e l l e p a r t i p i ù i n t e r n e della t e r r a n o n p u ò o t t e n e r s i senza f o r m u l a r e ip o tesi, finora p r a t i c a m e n t e i n c o n t r o l l a b i l i , sulla c o m p o s i z i o n e c h i m i c a d e l l ' i n t e r n o d e l l a t e r r a . a t t u a l m e n t e l e ipotesi p i ù a c c e t t a t e r i g u a r d o alla n a t u r a del m a n t e l l o s o n o : 1) la c o m p o s i z i o n e c h i m i c a del m a n t e l l o è s i m i l e a q u e l l a d e l l e d u n i t i ( 2 8 ) ; 2) d a l l a b a s e della crosta fino a 1000 k m la c o m p o s i z i o n e c h i m i c a del m a n t e l l o è a n a l o g a a q u e l l a d e l l e a e r o l i t i , da 1000 k m fino alla s o m i t à del n u c l e o è a n a l o g a a q u e l l a d e l l e sideroliti. nella p r i m a ipotesi si a v r e b b e n e l m a n t e l l o u n c o n t e n u t o di u r a n i o ( 3 0 ) p a r i a 0,013 x 10"® g / g . se fosse v e r a la s e c o n d a il c o n t e n u t o d i u r a n i o (3 1) s a r e b b e di 0,029 x 10 ® g / g fino a 1000 k m e di 0,012 x 10 ° g / g da 1000 k m alla b a s e del m a n t e l l o . con i d a t i relativi alla d i s t r i b u z i o n e delle d e n s i t à n e l l ' i n t e r n o del m a n t e l l o otten u t i da b u l l e n (2 s) si p u ò c a l c o l a r e c h e l ' u r a n i o c o n t e n u t o n e l m a n tello sia n e l l ' o r d i n e di 5,5 x io19 g n e l l a p r i m a i p o t e s i e di 5,3 x io19 g n e l l a seconda. l ' a d o z i o n e d e l l ' u n a o d e l l ' a l t r a i p o t e s i n o n h a q u i n d i i m p o r t a n z a p e r i fini c h e si p r o p o n e la p r e s e n t e r i c e r c a . la c o m p o s i z i o n e c h i m i c a d e l n u c l e o t e r r e s t r e è g e n e r a l m e n t e rit e n u t a s i m i l e a q u e l l a d e l l e s i d e r i t i (2 4) ( 2 0 ) ; esistono p e r ò a l t r e t e o r i e sulla n a t u r a d e l n u c l e o : la t e o r i a di k u h n e r i t t m a n n (3 2) secondo la q u a l e il n u c l e o s a r e b b e c o s t i t u i t o di m a t e r i a s o l a r e i n d i f f e r e n z i a t a , ossia p r a t i c a m e n t e di i d r o g e n o ; la t e o r i a di r a m s e y ( 2 3 ), c b e consid e r a il m a n t e l l o ed il n u c l e o c o m e f a s i t e r m o d i n a m i c h e d i f f e r e n t i di u n a i d e n t i c a sostanza. s c a r t a t a i n u n p r i m o t e m p o l a ipotesi di k u h n e r i t t m a n n , c h e è n o t o r i a m e n t e la p i ù discussa e c h e c o m u n q u e e s c l u d e r e b b e la p r e s e n z a di u r a n i o n e l n u c l e o , n e l l a p r i m a ipotesi (sid e r i t i ) il c o n t e n u t o di u r a n i o n e l n u c l e o (3 1) r i s u l t e r e b b e p a r i a s u l l a f i s s i o n e s p o n t a n e a d e l l ' u r a n i o t e r r e s t r e 555 0,003 x 10"fi g / g , m e n t r e n e l l a ipotesi di r a m s a y s a r e b b e u g u a l e a ( s o ) 0,013 x 10 ° g/gdi conseguenza c o n s i d e r a n d o c h e la massa del nucl eo, secondo i dati oggi p i ù a t t e n d i b i l i (23), è c o m p r e s a f r a 1,85 e 1,89 x io27 g5 si o t t i e n e u n c o n t e n u t o di u r a n i o n e l nucleo di 0,5 x io19 g p r i m a ipotesi e di 2,4 x io19 g n e l caso che sia verificata la ipotesi di r a m s e y . i n q u e s t ' u l t i m a ipotesi i dati relativi a l l ' u r a n i o p r e s e n t e nella terra sono quelli raccolti nella t a b e l l a i : essi c o r r i s p o n d o n o ad u n cont e n u t o totale di 1,65 x io20 g, ( m x 1020 g nell'ipotesi di k u h n e r i t t m a n n ) . tabella i crosta m a n t e l l o n u c l e o e x loio g 8,6 5,5 2,4 u«,b x i o 4 1 titolili 2,2 1,4 0,6 gli isotopi n a t u r a l i d e l l ' u r a n i o a t t u a l m e n t e conosciuti sono t r e : i/2 3 4, £/-3r' e l/ 2 3 8 . un c o n f r o n t o f r a le a b b o n d a n z e di questi n u c l i d i e i valori delle vite m e d i e p e r emissione <t e p e r fissione s p o n t a n e a di ciascuno di essi m o s t r a come nei p r o b l e m i di geofisica p u ò assum e r e u n a certa i m p o r t a n z a u n i c a m e n t e la fissione s p o n t a n e a del nuclide di massa 238. t e n u t o conto d e l l ' a b b o n d a n z a di questo isotop o (3 4) si h a che n e l l a t e r r a sono presenti 4,2 x io41 a t o m i di j723s, d i s t r i b u i t i c o n f o r m e m e n t e alla seconda riga della t a b e l l a i. ci si p o t r e b b e d o m a n d a r e a questo p u n t o se sia lecito e s t e n d e r e a l l ' i n t e r a t e r r a i valori delle a b b o n d a n z e isotopiche (3 4), m i s u r a t e su c a m p i o n i p r o v e n i e n t i dalla p a r t e p i ù superficiale di essa. sta di f a t t o c h e i dati — sia p u r e r e l a t i v a m e n t e scarsi — r i g u a r d a n t i le abbondanze isotopiche nelle m e t e o r i t i e nelle a t m o s f e r e stellari, p r o v a n o che, salvo a l c u n e eccezioni, tali a b b o n d a n z e si m a n t e n g o n o costanti n e l l ' u n i v e r s o (35). 4. —il n u m e r o di nuclei di massa a, p r o d o t t i nel t e m p o t dal]'[/23s j ) e r f j g s i o n e s p o n t a n e a è dato d a : 556 c. f e s t a m . s a n t a n g e l f l d o v e }f è l a c o s t a n t e d i d e c a d i m e n t o p e r fissione s p o n t a n e a (8,6 x io"17 an' 1 ), a a la c o s t a n t e d i d e c a d i m e n t o p e r e m i s s i o n e <i (1,5 x io"10 an"1) e à = / f ) x a è la c o s t a n t e di d e c a d i m e n t o t o t a l e ( p r a t i c a m e n t e u g u a l e a hi); y 4 è l o y e l d d e l l a fissione s p o n t a n e a r e l a t i v o a l l a m a s s a a. s e si p o n e t = 3,3 x io9 a n , v a l o r e c o m u n e m e n t e a c c e t t a t o p e r l ' e t à d e l l a t e r r a ( 3 6 ), e si a t t r i b u i s c o n o a d n»as, successiv a m e n t e , i s i n g o l i v a l o r i r i p o r t a t i n e l l a s e c o n d a r i g a d e l l a t a b e l l a i , si p u ò c a l c o l a r e i l n u m e r o d i n u c l i d i di m a s s a a g e n e r a t i d a l l ' u r a n i o n e i s i n g o l i s t r a t i d e l l a t e r r a d a l l ' e p o c a d i f o r m a z i o n e d i q u e s t a . 11 c a l c o l o è s t a t o e s e g u i t o p e r l e m a s s e 83, 84, 86, 129, 131, 132, 134, 136. i r i s u l t a t i s o n o r a c c o l t i n e l l a t a b e l l a i i : d a essi si d e s u m e c b e i n 3,3 x 10° a n , p e r la fissione s p o n t a n e a d e l l ' i / 2 3 8 c o n t e n u t o n e l l ' i n t e r a t e r r a , si s o n o f o r m a t i 3,5 x io3 3 a t o m i d i k r i p t o n e 2,5 x 10:!4 a t o m i d i x e n o n , p a r i r i s p e t t i v a m e n t e a l 0,004°/o d e l k r i p t o n e al 0 , 3 i / o d e l l o x e n o n a t m o s f e r i c i . u n r i s u l t a t o s i m i l e a q u e s t o s e m b r a a b b i a n o o t t e n u t o k h l o p i n e g e r l i n g ( 3 7 ) i n u n a r i c e r c a s u l k r i p t o n e lo x e n o n p r o d o t t i d a l l ' u r a n i o t e r r e s t r e . s i c c h é , t u t t o i l k r i p t o n e l o x e n o n , p r o d o t t i d a l l ' u r a n i o t e r r e s t r e p e r fissione s p o n t a n e a c o s t i t u i r e b b e r o u n a f r a z i o n e m o l t o p i c c o l a d e i c o r r i s p o n d e n t i gas a t m o s f e r i c i . tabella i i 83 81 86 129 131 132 134 136 n di atomi di massa a prodotti i n 3,3 x io9 an per fissione spontanea d e i r u 2 3 s c o n t e n u t o i n crosta mantello n u c l e o at x 1033 at x 1032 at x 1032 1,0 0,6 0,3 3,7 2,4 1,0 13,5 8,6 3,5 0,6 0,4 0,2 6,1 3,9 1,6 28,6 18,3 7,4 42,1 26,9 11,0 49,6 31,7 23,9 p u ò a v e r e u n c e r t o i n t e r e s s e il c o n f r o n t o d e l l e a b b o n d a n z e d e i s i n g o l i i s o t o p i d e l k r i p t o n e d e l l o x e n o n a t m o s f e r i c i c o n l e a b b o n d a n z e d e i s i n g o l i n u c l i d i g e n e r a t i d a l l ' u r a n i o t e r r e s t r e . q u e s t i d a t i son o r i p o r t a t i n e l l a t a b e l l a i i i d o v e c o n jv ° si i n d i c a il n u m e r o d i atom i di m a s s a a p r e s e n t i n e l l ' a t m o s f e r a e n j i c o r r i s p o n d e n t i p r o d o t t i d i fissione. s u l l a f i s s i o n e s p o n t a n e a d e l l ' u r a n i o t e r r e s t r e 5 5 7 5. — i l c o n f r o n t o f r a il k r i p t o n e l o x e n o n o r i g i n a t i d a l l a fissione ed il c o n t e n u t o degli stessi e l e m e n t i n e l l ' i n t e r n o della t e r r a a v r e b b e u n a i m p o r t a n z a d i g r a n l u n g a s u p e r i o r e a q u e l l o p r e c e d e n t e m e n t e istit u i t o con il k r i p t o n e lo x e n o n a t m o s f e r i c i . esso s e r v i r e b b e a s t a b i l i r e d a u n l a t o i l c o n t r i b u t o della fissione s p o n t a n e a alla f o r m a z i o n e del k r i p t o n e d e l l o x e n o n c o n t e n u t i n e l l ' i n t e r n o della t e r r a e l'influsso d a essa e s e r c i t a t o s u l l a l o r o c o m p o s i z i o n e isotopica, e d a l l ' a l t r o le rel a z i o n i f r a q u e s t i gas e i c o r r i s p o n d e n t i n e l l ' a t m o s f e r a , le q u a l i inter e s s a n o il p r o b l e m a del m e c c a n i s m o di f o r m a z i o n e d e l l ' a t m o s f e r a stessa (1 5). p u r t r o p p o a t t u a l m e n t e u n s i m i l e c o n f r o n t o è i r r e a l i z z a b i l e p e r la m a n c a n z a di d a t i s p e r i m e n t a l i a t t e n d i b i l i s u l c o n t e n u t o d i k r i p t o n e x e n o n n e l l e rocce. tabella i i i a 83 84 86 129 131 132 134 136 n a f at x 10-™ 0,2 0,7 2,6 0,1 1,2 5,4 8 10 na« at x 1038 12,5 61,3 18,3 2,2 1,8 2,3 0,9 0,7 u n a v a l u t a z i o n e di t a l e c o n t e n u t o è s t a t a t e n t a t a d a b r o w n ( 1 7 ) : si t r a t t a i n v e r i t à di u n a e s t r a p o l a z i o n e f o n d a t a su a l c u n e m i s u r e d i c a d y e c a d y (3 8). q u e s t i u l t i m i b a n n o d e t e r m i n a t o l a q u a n t i t à di kr -jxe p r e s e n t e i n a l c u n i gas n a t u r a l i , o t t e n e n d o v a l o r i c b e scart a n o n o t e v o l m e n t e l ' u n o r i s p e t t o a l l ' a l t r o . n o n si sa, q u i n d i , q u a n t o sia l e c i t o a s s u m e r e l a m e d i a di q u e s t i v a l o r i c o m e il c o n t e n u t o di kr -(xe i n t u t t i i gas n a t u r a l i . c o m u n q u e , dai d a t i r i p o r t a l i n e l l a t a b e l l a v d e l c i t a t o l a v o r o di b r o w n , s e g u e n d o il m e t o d o s v i l u p p a t o d a l b r o w n stesso, si r i c a v a c b e n e l l a crosta t e r r e s t r e si t r o v e r e b b e r o 5,3 x io35 a t o m i di k r i p t o n e 4,2 x io34 a t o m i d i x e n o n . se q u e s t o fosse v e r o il k r i p t o n e lo x e n o n , p r o d o t t i in 3,3 x io9 a n p e r fissione s p o n t a n e a d e l l ' u r a n i o c o n t e n u t o n e l l a crosta, r i s u l t e r e b b e r o p a r i ris p e t t i v a m e n t e al 0,3% d e l k r i p t o n e a l 3 0 % d e l l o x e n o n , c o n t e n u t i n e l l a crosta m e d e s i m a : i l c o n t r i b u t o della fissione s p o n t a n e a n o n sar e b b e i n tal caso t r a s c u r a b i l e . è ovvio c b e q u e s t e v a l u t a z i o n i , la cui i n c e r t e z z a è già s t a t a r i l e v a t a , h a n n o u n v a l o r e p u r a m e n t e i n d i c a t i v o : servono t u t t a v i a a d i m o s t r a r e q u a l e i n t e r e s s e a v r e b b e la d e t e r m i n a 558 c. f e s t a m . s a n t a n g e l f l zione d e l l e a b b o n d a n z e e della c o m p o s i z i o n e isotopica d e l k r i p t o n e dello x e n o n n e l l e rocce e n e i gas n a t u r a l i . roma — istituto nazionale di geo fisica — agosto 1952. riassunto parteiulo dai dati più recenti sulla vita media della fissione spontanea dell'uranio, gli a. tentano una valutazione delle quantità di k r e xe generate in questo processo dall'uranio contenuto nella terra a partire dall'epoca di formazione della crosta terrestre. il confronto delle abbondanze dei singoli isotopi dei due elementi contenuti nell'atmosfera con le abbondanze degli stessi isotopi generati per fissione, porta a. concludere che il contenuto della fissione spontanea delz'u238 alla formazione del k r e xe atmosferici è trascurabile. dal confronto tra le quantità di k r e xe di fissione e il contenuto degli stessi elementi nella crosta terrestre, calcolato secondo brown, risulterebbe invece che in tal caso la fissione spontanea dell'uranio non sarebbe da trascurare. si prospetta infine, a conclusione, l'interesse di eseguire determinazioni delle abbondanze e della composizione isotopica del k r e xe nelle rocce e nei gas naturali. summary a rotigli determination of the k r and xe generated by the spontaneous fission of the terrestrial uran:um during the last 3,3 billion years (eartli's age) is attempted in this icorli. the calculations li ave been based on the most recent data for the spontaneous fission meati life of u 2 3 8 . from a comparison between the abundances of each isotope in the atmospherical k r and xe and the abundances of the sanie isotope in the fission k r and xe, one can estimate that the contributi on of the spontaneous fission of u238 to the production of the atmospherical k r and xe is negligeable. the fission k r and xe and tlie k r and xe content in the earth's crust fcalculated following the brown's metliod) are also compared. the spontaneous fission of u seems not negligeable in this case. the great interest of the determination of the k r and xe content in rocks and naturai gases and its isotopie composition is prospectexl. s u l l a f i s s i o n e s p o n t a n e a d e l l ' u r a n i o t e r r e s t r e 559 b i b l i o g r a f i a ( ! ) h a h n o . u n d strassmann f . : n a t u r w . 2 7 , 1 1 ( 1 9 3 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( u ) m a c n a m a r a j. and t h o d e h . g . : phys. rev., 8 0 , 4 7 1 ( 1 9 5 0 ) . ( i " 1 ) b o a t o g , c a r e r i g . e s a n t a n g e l o m . : n u o v o c i m e n t o 9 , 4 4 ( 1 9 5 2 ) . (i") gamow g. and c.ritciifield c. l.: tlieory of alomic nueleus and atomic energy sources. o x f o r d (1950). (1") brown h . : in the almosphere of the eartli and planets. ed. g. kuiperchicago (1949) pag. 260-268. ( j 8 ) b a c h e l e t f . , f e r r e t t i s f o r z i n i m . e i p p o l i t o f . : n u o v o c i m e n t o ; 8 , 851 (1951). (i«) urry w . d . : a n i . j o u r n . sci., 239, 191 (1941). (20) evans r . d . and goodman c.: b u l l . g e o l . ani., 52, 459 (1941). ( 2 1 ) k u e n e n p h . h . : marine g e o l o g y . n e w y o r k ( 1 9 5 0 ) . (22) gutenberg b . : the internai constitution of the earth. n e w y o r k (1951). (23) buixen k . e . : an introduction to the theory oj seismology. c a m b r i d g e (1947). (--•) daly r . : lgneous rocks and the depili oj the earth. n e w y o r k (1933). (2r.) senftle f. e. and keevil n . b . : trans. a n i . g e o p h y s . u n . 28, 732 (1947). (2«) kalle k . : der stojfhauslialt des meeres. p r o b l e n i s der k o s n i . p h y s . l e i p z i g (1943). ( 2 7 ) birch f . , schaiber f . a n d spicer h . c . : iiami. of phys. const. g e o l . s o c . ani. p a p . 36 (1942). < 2 s ) b i r c h f . : j . g e o p h . res. 5 6 , 3 7 9 ( 1 9 5 1 ) . f2») urey h. c..: g e o d i , and cosm. acta. 1. 209 (1951). (30) davis g. l . : ani. journ. s c i , 245, 677 (1947). (31) davis g. l . : a m . j o u r n . s c i , 248, 107 (1950). (32) k u h n w . und r i t t m a n n a . : geol. r u n d s c h a u . 32, 2 1 5 ( 1 9 4 1 ) . (33) ramsey w . h . : m.n.r.a.s. g e o p h . s u p p l . 5, 409 (1949). (3-») g f . n . e l . r e s . l a b . : chart of n u c l i d e s ( 1 9 5 0 ) . (3-r.) brown h . : r e v . m o d . p l i y s . , 2 1 , 6 2 5 ( 1 9 4 9 ) . 560 c . f e s t a m . s a n t a n g e l f l (30) alpher r . a . a n d herman r . c . : p h y s . r e v „ 84, 1111 (1951). (37) khi.opin v . g. a n d geri-inc e . k . : r a d i u m i n s t . a c . sci. u . s . s . r . (1948). 6 p ; c h e m . a b s t r , 43, 527 (1949). ( 3 8 ) c a d y g . h . a n d c a d y h . p . : i n d . e n g . c h e m . a n a l . e d , 1 7 , 7 6 0 ( 1 9 4 5 ) . ( 3 0 ) seaborc g . t . a n d p e r l m a n n m . l . : j o u r n . a m . c h e m . s o c , 7 0 , 1 5 7 1 (1948). v a r i a z i o n e diurna d e l l a g r a v i t à in e u r o p a c a r l o morelli nota 3": soluzione pratica 8. conclusioni pratiche. — a s e g u i t o d i q u e s t o e s p o s t o n e l l a n o t a p r e c e d e n t e ( 7 ) , l a s e g u e n t e p r o p o s t a s e m b r a l a p i ù r a z i o n a l e p e r o t t e n e r e c o n u n a v i s i o n e s i n t e t i c a l a s o l u z i o n e d e l p r o b l e m a i n o g g e t t o , c o n l a p r e c i s i o n e oggi r i c h i e s t a d i + 0 , 0 1 m g a l : a) r i p o r t a r e , su u n u n i c o g r a f i c o e p e r l a l o n g i t u d i n e c e n t r a l e e u r o p e a an — 15" est g r e e n w i c h , l e c u r v e d i v a r i a z i o n e d i u r n a p e r d i f f e r e n t i l a t i t u d i n i ; h) p e r l a t i t u d i n i c h e d i f f e r i s c a n o d a q u e s t e p i ù d i 2°.5, i n t e r p o l a r e g r a f i c a m e n t e ; c o n v i e n e p e r c i ò c h e l e l a t i t u d i n i d i r i f e r i m e n t o n o n d i s t i n o p i ù d i 10", e c h e l ' i n t e r p o l a z i o n e g r a f i c a sia f a t t a , q u a n d o o c c o r r e , s u l l e l a t i t u d i n i i n t e r m e d i e e q u i d i s t a n t i ; e) p e r l o n g i t u d i n i d i v e r s e d a x0 = 1 5 ° e. g r . , i n u n i n t e r v a l l o x0 = 1 5 ° + 60" l e c u r v e p r e d e t t e s o n o s e m p r e v a l i d e p u r c h é i t e m p i s i a n o i n t e s i i n « t e m p o l o c a l e » . s i o t t e n g o n o c o s ì i s e g u e n t i v a n t a g g i : ci) soluzione grafica, c h e c o n s e n t e d i s e g u i r e d i r e t t a m e n t e l ' a n d a m e n t o d e l f e n o m e n o i n e s a m e ; e) grafico unico, p e r t u t t e l e l a t i t u d i n i c o m p r e s e f r a q u e l l e e s t r e m e ( d e l r e s t o a m p l i a b i l i a d a r b i t r i o ) ; f ) interpolazione immediata: d i f a t t i , i l g r a v i m e t r i s t a c h e o p e r a s s e a l a t i t u d i n i d i v e r s e d a q u e l l e già s t a m p a t e , d o v r e b b e s e m p l i c e m e n t e d i s e g n a r s i c o n l a m a t i t a l a c u r v a d e s i d e r a t a , il c h e è p o s s i b i l e c o n e s t r e m a f a c i l i t à . c o m e e s e m p i o , si r i p o r t a n o l e c u r v e p e r il 1° t r i m e s t r e 1 9 5 3 , c a l c o l a t e p e r le l a t i t u d i n i 10" i n 10" f r a 2 5 ° e 6 5 ° n : se o c c o r r e s s e c o n o s c e r e la v a r i a z i o n e d i u r n a d e l l a g r a v i t à a 4 1 ° n, b a s t e r e b b e d i s e g n a r e p e r i n t e r p o l a z i o n e l a c u r v a c o r r i s p o n d e n t e a 4 0 ° n. in t a l g u i s a , c o n 3 g r a f i c i d i q u e s t o t i p o p e r o g n i m e s e , c a l c o l a t i p e r e s e m p i o p e r l e l o n g i t u d i n i d i 1 0 5 ° w . g r . ( a m e r i c a s e t t e n t r i o n a l e ) , 15" e. g r . ( e u r o p a ) , 1 0 5 ° e. g r . ( a s i a ) , si p o t r e b b e c o n o s c e r e l a v a r i a z i o n e d i u r n a p e r q u a l u n q u e p u n t o d e l l ' e m i s f e r o n o r d . osservatorio geofisico di trieste variazione diurna d e l l a gravita' in gennaio 1 9 5 4 ampiezzt in c®nr»simi di mgal. in 'tanpo locala ' la curva valgono naif intarvallo a • ; 90' europa o s s e r v a t o r i o g e o f i s i c o di t r i e s t e variazione diurna d e l l a gravita' in europa v a r i a z i o n e d i u r n a d e l l a g r a v i t à i n e u r o p a 5 8 3 p e r s a g g i a r e l a p r a t i c i t à d e l m e t o d o e l a p r e f e r e n z a f r a q u e s t o o il m e t o d o t a b e l l a r e , s e g u i t o d a i c o l l e g h i f r a n c e s i , l a e u r o p e a n a s s o c i a m o l i o f e x p l o r a t i o n g e o p h y s i c i s t s h a p r o p o s t o ( n e l c o n g r e s s o d i m i l a n o , d i c e m b r e 1 9 5 3 ) u n r e f e r e n d u m : esso v e r r à p u b b l i c a t o su « geophysical exploration » , c o n i g r a f i c i p e r gli a l t r i m e s i d e l 1 9 5 4 . istituto nazionale di geo fisica —• osserv. di trieste — dicembre 1953. riassunto per ottenere in un solo quadro la variazione diurna della gravità per veuropa, esalta a + 0,01 mgal, essa è stata calcolata per la longitudine centrali' "/.„ = 15" est gr. e per le latitudini di 25", 35", 45", 55" e 65" n. riportate queste curvi• graficamente, è allora possibile ottenere direttamente per interpolazione grafica la curva desiderata, per una qualunque latitudine intermedia. per longitudini diverse, nell'intervallo /„ 60" le curve restano valide purché il tempo sia quello « locale ». sono allegate le curve per il 1" trimestre 1954. summary in order to obtain in un unique map the diurnal variation of gravity for europe, with accuracy 1 0.01 mgal, it lias been calculated for the centrai longitude , „ = 15" e. gr. and for the latitudes 25", 35", 45", 55", 65" n. when the curves ari• grafically plotted, it becomes possible to obtain directly by graphical interpolation the desired curve for every latitude between those. for different longitudcs, the curves ari' il'orth in the interval /.„ + 60" '/ the time is the «locai » one. graphs for january-march 1954 are enclosed. b i b l i o g r a f i a ( 7 ) morelli c . : variazione diurna della gravità in europa. n o t a 2 " : fonda• menti teorici. a n n a l i ili g e o f i s i c a , \ i. 2, r o m a 1 9 5 3 . microsoft word 1_ferreira_appendix__ba.docx annals of geophysics, 64, 3, se321, 2021; doi:10.4401/ag-8412 appendix to: what scientific information on the seismic risk to nonstructural elements do people need to know? part 1: compiling an inventory on damage to nonstructural elements mónica amaral ferreira1, fabrizio meroni2, raffaele azzaro3, gemma musacchio2, rajesh rupakhety4, bjarni bessason4, sòlveig thorvaldsdottir4, mário lopes1, carlos sousa oliveira1, stefano solarino5 1 instituto superior técnico, ceris, university of lisbon, lisbon, portugal 2 istituto nazionale di geofisica e vulcanologia, milano, italy 3 istituto nazionale di geofisica e vulcanologia, catania, italy 4 earthquake engineering research centre, faculty of civil and environmental engineering, university of iceland 5 istituto nazionale di geofisica e vulcanologia, ont, roma, italy annex 1. photographic documentation the main result of the non-structural damage (nsd) analysis in terms of risk communication tools relevant for the mt. etna pilot area is the publication of a booklet giving an overview of the observed nsd during recent earthquakes at etna volcano. the photographic documentation, with brief explanatory descriptions, outlines the most common types of damage that frequently occur during moderate earthquakes, but which may be limited by adequate technical design or even simple precautions. figure a1. cover of the booklet devoted to nsd risk communication outreach in the mt. etna pilot area, and example of a page inside [azzaro et al., 2016]. annex 2. data collected for each lisbon parish table a1 data collected for each lisbon parish lisbon parishes broken windows and out-of-frame doors water infiltration / minor cracking collapse masonry chimneys fall of appendages cracks, spalling of plasters, ceilings; damage in roofs and landslides moderate structural damage, in some cases requiring evacuation total s. francisco xavier 1 1 2 s. joão deus 1 2 3 2 8 s. vicente fora 24 20 2 9 55 sé 12 19 1 5 37 s. josé 19 25 2 8 9 63 santos-ovelho 47 63 2 6 18 136 s. nicolau 22 47 2 9 2 82 s. justa 2 24 27 5 4 11 73 s. mamede 18 24 2 5 49 s. isabel 7 4 1 1 13 s. condestável 24 25 1 6 9 65 s. doming. benfica 12 6 2 20 n.s.pena 2 30 22 2 5 10 71 s. jorge arroios 1 79 73 4 15 10 182 alvalade 4 5 2 11 charneca 1 2 3 n.s.graça 51 42 7 8 108 s. paulo 1 21 45 3 4 7 81 s. engracia 8 8 1 2 6 25 s. sebast. pedr. 12 11 1 4 28 beato 18 9 6 7 40 alto de pina 2 5 4 11 s. estevão 1 15 14 2 2 9 43 n.s.mártires 4 12 4 3 1 24 s. tiago 10 6 1 2 19 campolide 12 6 1 10 29 alcântara 16 21 3 7 5 52 penha de frança 37 35 1 9 9 91 castelo 1 8 6 1 7 23 socorro 22 15 1 5 3 46 s. joão 1 26 27 1 10 4 69 s. maria olivais 11 1 2 14 lumiar 8 3 2 13 coração de jesus 18 17 6 5 3 49 s. miguel 28 10 4 14 56 anjos 2 30 58 2 18 18 128 mercês 20 41 6 10 77 n.s.fátima 3 22 47 3 14 89 ajuda 16 6 2 4 9 37 ameixoeira 1 1 1 1 4 encarnação 21 31 2 9 7 70 c.grande 7 3 3 13 s.joão brito 2 4 2 4 12 marvila 1 8 8 4 14 35 sacramento 10 9 2 2 1 24 s.catarina 35 42 3 2 5 87 madalena 7 6 1 14 lapa 21 34 3 14 72 carnide 4 1 5 benfica 11 7 1 5 24 prazeres 6 11 1 2 5 25 s. cristovão s. lourenço 6 16 1 3 7 33 s. maria belém 9 6 1 2 18 totals 19 893 986 55 189 316 2458 p r o s p e t t o d e l l e r i c e r c h e p e r « f o r z e e n d o g e n e » i n v a r i e r e g i o n i d e l m o n d o d u r a n t e l ' u l t i m o t r e n t e n n i o ( * ) m e r c i rino s a p p a in d u e m e m o r i e p u b b l i c a t e nel 1928 e nel 1937. a . r i t t m a n n d a v a n o t i z i e s u i l a v o r i e s e g u i t i , p e r r i c e r c h e d i f o r z e e n d o g e n e , i n i t a l i a , n e g l i s t a t i u n i t i e n e l l e i n d i e o l a n d e s i . in q u e s t i d u e u l t i m i p a e s i le r i c e r c h e , c h e si s v o l s e r o in a r e e v u l c a n i c h e , d e t t e r o r i s u l t a t i s o d d i s f a c e n t i . in i t a l i a , n e l l a r e g i o n e b o r a c i f e r a t o s c a n a , lo s f r u t t a m e n t o d e l v a p o r e e n d o g e n o a v e v a già r a g g i u n t o , a l l o r a , un a l t o l i v e l l o ind u s t r i a l e . d a l 1928 a d o g g i l e r i c e r c h e di e n e r g i a e n d o g e n a s o n o s t a t e intens i f i c a t e e d e s t e s e a m o l t e a l t r e r e g i o n i d e l m o n d o , con p a r t i c o l a r e r i g u a r d o a l l e a r e e v u l c a n i c h e . n e l l a t a b e l l a a c c l u s a a b b i a m o c e r c a t o di r i a s s u m e r e q u a n t o , nell ' u l t i m o t r e n t e n n i o , è s t a t o f a t t o , n e l l e v a r i e p a r t i del m o n d o , s i a p e r la r i c e r c a c h e p e r l ' u t i l i z z a z i o n e d e l l ' e n e r g i a g e o t e r m i c a . p u r t r o p p o i d a t i a d i s p o s i z i o n e , a t t i n t i d a l l a l e t t e r a t u r a c o n s u l t a t a e r i a s s u n t a in a p p e n d i c e , n o n s o n o s e m p r e r i s u l t a t i sufficienti a d i n q u a d r a r e la f e n o m e n o l o g i a e s a l a t i v o i d r o t e r m a l e ( c o n g e y s e r , f u m a r o l e ecc. i d e l l a r e g i o n e nel p r o p r i o a m b i e n t e g e o l o g i c o g e o f i s i c o . c o m e r i s u l t a d a l l a t a b e l l a , la p i ù a l t a t e m p e r a t u r a finora incont r a t a n e l m o n d o con le p e r f o r a z i o n i è q u e l l a c h e è s t a t a m i s u r a t a nel s o t t o s u o l o dei c a m p i f l e g r e i ( c i r c a 300" a 1700 m in a c q u a i . d a i l a v o r i c o n s u l t a t i a p p a r e e v i d e n t e c o m e , in (piasi tutti i c a m p i finora e s p l o r a t i , s i a n o s t a t e i n c o n t r a t e n o t e v o l i difficoltà n e l l ' i n t e r p r e t a z i o n e d e i r i s u l t a t i . p e r a l c u n i c a m p i si h a n o t i z i a di r i t r o v a m e n t i di v a p o r e s u r r i s c a l d a t o , m a s e m b r a c h e la p r e s e n z a d e l l ' a c q u a sotterr a n e a c o m p l i c h i n o t e v o l m e n t e sia la s u a r i c e r c a c h e il s u o s f r u t t a m e n t o . n e l l a m a g g i o r p a r t e d e l l e a r e e e finora i n v e s t i g a t e il fluido r i n v e n u t o è r i s u l t a t o e s s e r e a c q u a , in a l c u n i c a s i a d e l e v a t a t e m p e r a t u r a . l ' u t i l i z z a z i o n e di t a l e a c q u a a s c o p o i n d u s t r i a l e è s t a t a resa dif( * ) c o m u n i c a z i o n e p r e s e n t a t a a l l ' a s s e m b l e a d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , t e n u t o a r o m a il 28-29 m a g g i o 1 9 5 4 . mkim i kino s a p i ' a ficile d a l l a f o r m a z i o n e , d u r a n t e l ' e r o g a z i o n e p e r e b o l l i z i o n e , di i n c r o s t a z i o n i ( p r i n c i p a l m e n t e c a r b o n a t i c h e e s i l i c e e ! c l i c in b r e v i s s i m o temp o h a n n o o s t r u i t o i p o z z i . t a l e i n c o n v e n i e n t e h a a n c h e n o t e v o l m e n t e c o m p l i c a t o l e o s s e r v a z i o n i n e c e s s a r i e p e r l a r i c e r c a , i m p e d e n d o d i ten e r e i n e r o g a z i o n e i p o z z i p e r t e m p i s u f f i c i e n t e m e n t e l u n g h i . n o n o s t a n t e i b u o n i r i s u l t a t i r a g g i u n t i i n a l c u n i c a m p i e s a l a t i v i , n o n s e m b r a c h e , finora, s i a n o s t a t e o t t e n u t e r e a l i z z a z i o n i p r a t i c h e p a r a g o n a b i l i a <|uclle d e l l a r e g i o n e b o r a c i f e r a t o s c a n a ( d e t t a di l a r d e r e l l o l , l a q u a l e c o n s e r v a t u t t o r a i n c o n t r a s t a t o d o m i n i o i n q u e s t o c a m p o . roiìki ( s . pietro in vincoli), istituto di geologia applicata e di giacimenti minerari dell'università (facoltà ingegneria). 28 maggio 1954. riassunto si schematizzano i metodi ed i risultati delle indagini e ricerche eseguite nell'ultimo trentennio, in varie regioni del mondo, per captare ed utilizzare il vapore naturale sotterraneo per sco/ìi industriali (termoelettrici). si osserva, fra l'altro, come, (piasi dovunque, le acque sotterranee complichino non solo la utilizzazione, ma anche l'esatta interpretazione dei fenomeni riscontrati. in alcuni campi fumarolici i pozzi trivellati hanno rinvenuto vapore surriscaldato anche dove, nello stesso sottosuolo, si incontrano pure acque termali. nonostante i buoni risultati ottenuti in qualche campo esalativo, non sembra che, finora, siano state ottenute realizzazioni pratiche paragonabili a quelle della regione boracifera toscana (detta di larderello), la (male conserva tuttora incontrastato dominio in questo campo. dalla letteratura consultata non sempre risulta possibile inquadrare la fenomenologia esalativo-idrotermale (con geyser, fumarole ecc.) della regione nel proprio ambiente geologico-geofisico. resta, perciò, ancora alquanto problematica la classifica delle manifestazioni stesse sia dal punto di vista scientifico, che da quello applicativo. su m mary in this paper the a. outlines the metliods and the results on the investigations and researches iliade in the last tìlirty years in various i m i o s i ' e i i o d e l l e r i c e r c h e p e r « f o r z e e n d o g e n e )) parts oj the world for the exploitation and utilization of naturai underground steam for industriai purposes (thermoelectrical). among other things it ivas observved tliat almost exerywhere the underground waters complicate not only the utilization, but also the exact interpretation of the encountered phenomenons. in some fumar ole-fields the drilled wells have found superheated steam even where, in the sante subsoil, are also found thermomineralwaters. althoug the good resnlts accomplished in some exalation field, does not look tip to noie to have reached practicul results comparable to tliose of the boric-acid steam-jet districi of larderello /toscana), which stili maintains indisputed superiority in tliis field. from the consulted literature, does not seem possible to fit the exhalative-hydrothermal phenomenology of the region in its oivn geologica i-geopli isycai sorrounding. hence, remains stili quite problematic the classification of the baine manifestations either from a scientifie ]>oint of view or from a point of viete of application. b i b l i o g r a f i a s i r i a s s u m o n o , q u i d i s e g u i t o , i l a v o r i u t i l i z z a t i p e r r e d i g e r e l a t a b e l l a d e l l o t a t o d e l l e r i c e r c h e p e r f o r z e e n d o g e n e n e i v a r i p a e s i . p e r le a l t r e p u b b l i c a z i o n i i n p r o p o s i t o , si r i m a n d a a l l a l e t t e r a t u r a r i p o r t a t a n e l l a n o t a di f . penta ( r i c e r c h e e studi sui fenomeni esalntivo-idrotermali ed il problema delle forze endogene, c o n t e n u t a n e l l o s t e s s o f a s c i c o l o d i q u e s t i annuii ili geo fisica i. l i allf.n e . t . . negletteti factors in i?.e developement of thermal springs. p a p . f r o m t h e g e o p h y s i c a l c a r n e g i e i n s t i t u l i o n of w a s h i n g t o n , n. 8 5 2 , 1 9 3 4 . m e n t r e le c a u s e f o n d a m e n t a l i d e l l e s o r g e n t i t e r m a l i i a c q u a s o t t e r r a n e a c i r c o l a n t e r i s c a l d a t a d a v a p o r e m a g m a t i c o l s e m b r a n o e s s e r e d o v u n q u e le m e d e s i m e , c o n d i z i o n i a c c i d e n t a l i p o s s o n o t o t a l m e n t e c a m b i a r e il t i p o d i s v i l u p p o i d r o t e r m a l e . • o s i . p e r e s e m p i o , la p r e s e n z a d i c a l c a r i , n e l b a c i n o , d a r à l u o g o a s o r g e n t i c h e p r e c i p i t a n o e s c l u s i v a m e n t e c a r b o n a t o d i c a l c i o ' f o r m a z i o n e d i t r a v e r t i n o ! c o m e si v e r i f i c a a m a m m o t h s p r i n g s . n e l p a r c o d i ^ e l l o w s t o n e , i n v e c e , le s o r g e n t i t e r m a l i p r e c i p i t a n o i n s i e m e c a r b o n a t o d i c a l c i o e s i l i c e , s e n z a c h e il p r i m o p r e d o m i n i s u l l a s e c o n d a . l e a c q u e d i t a l i o r g e n t i s e m b r a c h e p r e n d a n o c a l c i o d a l l e v i c i n e r o c c e d i o r i t i c h e p e r u n p r o c e s s o d i a t t a c c o . s i f a o s s e r v a r e c h e la d e p o s i z i o n e d e l c a r b o n a t o d i c a l c i o è l e g a t a a l l a f a c i l i t à di l i b e r a z i o n e d i co., e q u i n d i a l l a v e l o c i t à d e l l ' a c q u a , a l l o s t a t o d e l l ' a l v e o • l i s c i o o s c a b r o s o ! a l l a p r e s e n z a o m e n o di c a s c a t e e d i r i s t a g n i . d o v e n o n si t r o v a n o c a l c a r i , l e a r e e t e r m a l i a v e n t i p i ù p i c c o l i b a c i n i i d r i c i m ehi 1 il l no saim'a s e p o l t i s o n o n e t t a m e n t e d i s t i n t e d a l l e a r e e a v e n t i b a c i n i p i ù e s t e s i . i.e a c q u e d e l l e p r i m e c o n t e n g o n o p r e v a l e n t e m e n t e s i l i c e , s o l f a t i m e t a l l i c i e ii., s o , e p r e c i p i t a n o o p a l e e m a t e r i a l i a r g i l l o s i . q u e l l e d e l l e s e c o n d e c o n t e n g o n o s i l i c e , b i c a r b o n a t o e c l o r u r o di s o d i o e d e p o s i t a n o s i l i c e . a l l e p r i m e a p p a r t e n g o n o le a r e e s o l f a t i c h e , il c u i c a r a t t e r e è d o v u t o a l l ' o s s i d a z i o n e d e l l ' / / . d a p a r t e d e l l ' o s s i g e n o a t m o s f e r i c o . a l l e s e c o n d e a p p a r t e n g o n o i b a c i n i 1 g e y s e r s , n e i « p i a l i le a c q u e e s o g e n e , p e n e t r a n d o a m a g g i o r i p r o f o n d i t à , a c q u i s t a n o u n c o n t e n u t o m i n e r a l e n e t t a m e n t e d i v e r s o d a l l e p r e c e d e n t i . 1 c l o r u r i a l c a l i n i , d i e r a p p r e s e n t a n o la m a g g i o r p a r t e d e l l e s o s t a n z e m i n e r a l i d i s c i o l t e n e l l e a c q u e d i t i p o a l c a l i n o , si r i t i e n e c h e s i a n o v o l a t i l i z z a t i d i r e t t a m e n t e d a l m a g m a . (21 allen e . t . . geyser hasins and igneous emanations. p a p . f . t h e g e o p h . l a b . c a r n e g i e i n s t . of w a s h i n g t o n , n. 8 5 9 , 1 9 3 5 . definizione di « hot springs ureo » . s i d e f i n i s c e a r e a a s o r g e n t i t e r m a l i l ' i n s i e m e c o s t i t u i t o d a l l e s o r g e n t i s t e s s e , c o n le l o r o c a r a t t e r i s t i c h e di a t t i v i t à t e r m a l e , d a l l e a c q u e c o n i l o r o m i n e r a l i d i s c i o l t i e c o n i g a s c h e d a e s s e g o r g o g l i a n o , d a l l e r o c c e d a l l e q u a l i s g o r g a n o le s o r g e n t i e d a i d e p o s i t i c h e si f o r m a n o . tipi di uree termali. s i a n a l i z z a il c a r a t t e r e d e l l e « e m a n a z i o n i » m a g m a t i c h e d e i b a c i n i a g e y s e r s a y e l l o w s t o n e e si a m m e t t e c h e t u t t e q u e s t e ( ( e m a n a z i o n i » s i a n o a l l o s t a t o a e r i f o r m e q u a n d o l a s c i a n o il m a g m a . i . e a c q u e d e i b a c i n i a g e y s e r s , c o n s i d e r a t e p e r l ' 8 5 / 9 0 % di o r i g i n e e s o g e n a , si r i t i e n e c l i c s c e n d a n o in p r o f o n d i t à fin d o v e l ' e v a p o r a z i o n e d i e s s e v i e n e c o m p e n s a t a d a l l a c o n d e n s a z i o n e d e l v a p o r e m a g m a t i c o . a q u e s t a p r o f o n d i t à l e a c q u e d i s c i o l g o n o i s a l i e s u c c e s s i v a m e n t e li p o r t a n o in s u p e r f i c i e . l e r o c c e c o s t i t u e n t i i b a c i n i g i o c a n o u n r u o l o i m p o r t a n t e n e l l o s v i l u p p o d e l l e s o r g e n t i a l c a l i n e , c e d e n d o a l l ' a c q u a la s i l i c e . ( 3 ) allen e . t . e day a . l . , steam t r e l l s and other thermal activity at the geysers. california. p a p . f. t h e g e o p h . l a b . , c a r n e g i e i n s t . of w a s h i n g t o n n . 3 7 8 , 1 9 2 7 . s i d a n n o n o t i z i e d e ! c a m p o d i c o a s t k a n g c in c a l i f o r n i a . in t a l e c a m p o si h a n n o , c o m e m a n i f e s t a z i o n i s u p e r f i c i a l i , f u m a r o l e c o n g a s c a l d i , g e y s e r s e s o r g e n t i t e r m o m i n e r a l i . f u r o n o p e r f o r a l i n. 8 p o z z i c o n p r o f o n d i t à v a r i a b i l e i r a m 67 e m 2 1 5 . i n u n o ili e s s i f u t r o v a l a la t e m p e r a t u r a ili 2 3 2 " c . v e n g o n o d e s c r i t t i i m e t o d i e g l i a p p a r e c c h i i m p i e g a t i p e r le m i s u r e e p e r la c a p t a z i o n e e d a n a l i s i d e i g a s . ( 4 ) a l l e n e . t . e day a . i... hot springs of the yellowstone mattonai park. p a p . t h e g e o p h . l a b . , c a r n e g i e i n s t . of w a s h i n g t o n n . 8 6 4 , 1 9 3 4 . l e s o r g e n t i t e r m a l i v e n g o n o c o n s i d e r a t e c o m e m a n i f e s t a z i o n i d e l l ' u l t i m a tasid e i v u l c a n e s i i n o e s e m b r a n o c o n n e s s e a f r a t t u r e . in t e r r e n i a r i d i l e s o r g e n t i t e r m a l i si t r o v a n o a s s o c i a t e c o n f u m a r o l e , a l c u n e d e l l e q u a l i e s a l a n o v a p o r e s u r r i s c a l d a t o . n e l p a r c o d i y e l l o w s t o n e e s i s t e c i r c a u n a d o z z i n a d i g e t t i d i v a p o r e s u r r i s c a l d a l o , c o n u n m a s s i m o di t e m p e r a t u r a di 1 3 8 " c . l a c o n d i z i o n e d i s u r r i s c a l d a m e n t o v i e n e s p i e g a t a a t t r i b u e n d o al v a p o r e o r i g i n e m a g m a t i c a , p u r a m m e t t e n d o u n a p o s s i b i l e « c o n t a m i n a z i o n e » d a p a r t e ili v a p o r e p r o v e n i e n t e d a l l ' e v a p o r a z i o n e ili a c q u e s o t t e r r a n e e . s i d à n o t i z i a d i d u e f o r i p e r f o r a t i n e l p a r c o d i y e l l o w s t o n e , i n u n o d e i q u a l i , n e l b a c i n o d i n o r r i s , a d u n a p r o f o n d i t à d i ni 70 è s t a t o i n c o n t r a l o v a p o r e a 2 0 5 " i . e 21 k g / c m a p o z z o c h i u s o . l ' e r o g a z i o n e di t a l i f o r i c o n t i n u a ila l u n g o t e m p o c o n p o r t a t a p r e s s a p p o c o c o s t a n t e . p r o s p e t t o d e l l e r i c e r c h e p e i ! (( foiize e n d o g e n e » l a p r e s e n z a (li v a p o r e s u r r i s c a l d a t o a p i c c o l a p r o f o n d i t à h a f a t t o p e n s a r e c h e l e a c q u e s u p e r f i c i a l i , a c a u s a d e l l a t e m p e r a t u r a , p e n e t r i n o p o c o n e l s o t t o s u o l o e c h e al d i s o t t o d e l l e a c q u e f r e a t i c h e g i a c c i a u n a z o n a d i v a p o r e s u r r i s c a l d a t o c h e g i u n g e f i n o al m a g m a . s e c o n d o l e o s s e r v a z i o n i e s e g u i t e , r i s u l t a c h e l o s p e s s o r e d e l l e a c q u e s u p e r f i c i a l i v a r i a d a u n ' a r e a a l l ' a l t r a , e , in u n a m e d e s i m a a r e a , d a p u n t o a p u n t o . l e a r e e t e r m a l i d e l l ' a r c o di y e l l o w s t o n e v e n g o n o s u d d i v i s e i n t r e t i p i : 1) a r e e s o l f a t i c h e ; 2 | a r e e a l c a l i n e , c o n u n s o t t o t i p o d i a r e e m i s t e ; 3) a r e e a t r a v e r t i n o . d a m i s u r e d i p o r t a t a a b b i n a t e a d a n a l i s i c h i m i c h e , r i s u l t a c h e d a t u t t e l e u r g e n t i d e l p a r c o di ^ e l l o w s t o n e l e a c q u e t e r m a l i p o r t a n o v i a , o g n i g i o r n o , 39(1 t o m i , d i m i n e r a l i d i s c i o l t i . l a q u a n t i t à d i c a l o r e e m e s s a al s e c o n d o è d i 2(mi.0(10 l u l o c a l o r i e . \ p r o p o s i t o d e l l ' e v o l u z i o n e d e l l e s o r g e n t i t e r m a l i , s e m b r a c h e l e s o r g e n t i s o l f a t i c h e p r e c e d a n o , n e l t e m p o , q u e l l e a l c a l i n e . \ i e n e d a t a , s o m m a r i a m e n t e , u n a s p i e g a z i o n e d e l l ' a t t i v i t à g e y s e r i a n a , b a s a n d o s i l i l l e o s s e r v a z i o n i e s e g u i t e s u i 2 0 0 g e y s e r s d e l p a r c o . i n c a r a t t e r e n o t e v o l e d i m o l t i g e y s e r s di y e l l o w s t o n e r i s u l t a e s s e r e la p r e s e n z a , in s u p e r f i c i e , d i a c q u a s u r r i e a l d a t a , c i o è d i a c q u a c h e si t r o v a a 2 " 3 ° c al d i s o p r a d e l p u n t o d i e b o l l i z i o n e •i q u e l l a p r e s s i o n e . d i q u e s t o f e n o m e n o v i e n e d a t a u n ' i n t e r p r e t a z i o n e . s i d à i n f i n e n o t i z i a d e l l e f l u t t u a z i o n i e d e s t i n z i o n i d i g e y s e r s d e l p a r c o , c e r c a n d o d i s p i e g a r e il f e n o m e n o . (5l barth t . f . w . , / ' r i s i i n e and contaminateti rock magma and thermal water. b u l l . \ o l e . ; s . 11. v i , 1 9 4 0 . s i d à n o t i z i a d i m i s u r e di [di. e f f e t t u a l e n e l l e a c q u e d i a l c u n e s o r g e n t i c a l d e in u n ' a r e a v u l c a n i c a d e l s w d e l l ' i s l a n d a , s e c o n d o l e q u a l i r i s u l t a c h e il ;>// a u m e n t a c o n l ' a l l o n t a n a r s i d e l l e s o r g e n t i d a l l a z o n a d e l l e g r a n d i f r a t t u r e . v i e n e i n o l t r e d a t a u n a s p i e g a z i o n e d i t a l e f e n o m e n o . i 6l barth t . i ? . \ v „ craters and fissare eruptions at myvatn in iceland. s a e r t z y k k o v n o r s k g e o g r a f i s k t i d s s k r i f t , '), 2, 1 9 4 2 . d o p o u n a d i s c u s s i o n e g e n e r a l e r i g u a r d a n t e l e c a u s e d e l v u l c a n e s i m o n e l l ' i s l a n d a , v i e n e d e s c r i t t a la r e g i o n e d e l l a g o m y v a t n n e l l ' i s l a n d a s e t t e n t r i o n a l e . s p e c i a l e c o n s i d e r a z i o n e è d a t a al r a g g r u p p a m e n t o d e i p i c c o l i c r a t e r i d i m y v a t n , . ila d e p r e s s i o n e di d i t n m u h o r g i r , al i l v e r f j a l l , c h e è il p i ù i m p o r t a n t e c o n o p i r o • l a t i r o d e l l ' i s l a n d a e al l . u d e n ! e h r a u n b u n g a , e n t r a m b i c o n i v u l c a n i c i . s e g u e u n a d e s c r i z i o n e d e l g r a n d e s i s t e m a di f r a t t u r e c h e si e s t e n d e p e r 35 k m d a l b l a f j a l l n e l s u d a l e i r h u n u k u r d e l n o r d . l a t e t t o n i c a m o s t r a c h e in c e r t e l o c a l i t à il m a g m a , a n z i c h é f u o r i u s c i r e , ila s p i n t o i n a l t o z o l l e d e l t e t t o . è d a t o u n r e s o c o n t o d e l l a c a t a s t r o f i c a e r u z i o n e p e r f r a t t u r e c h e e b b e l u o g o i n t o r n o a l m y v a t n n e l p e r i o d o 1724-29 e si c e r c a i n f i n e d i s p i e g a r n e il m e c c a n i s m o . <7l barth t . f . vi .. some unttsiial grottndwater phenomena in iceland. s a e r t z y k k o v n o r s k g e o g r a f i s k t i d s s k r i f t ; 9 , i. 1 9 4 1 , o s l o . \ i e n e d e s c r i t t a la g e o l o g i a d e l l ' i s l a n d a e p i ù d e t t a g l i a t a m e n t e è d i s c u s s a l ' i d r o l o g i a s o t t e r r a n e a d e l l ' i s o l a . < ; i i s t u d i e s e g u i t i h a n n o p e r m e s s o d i s t a b i l i r e l ' a n d a m e n t o d e l l a s u p e r f i c i e d i s e p a r a z i o n e f r a a c q u a m a r i n a e a c q u a f r e a t i c a n e l s o t t o s u o l o d e l l ' i s o l a . a t a l e s c o p o v i e n e a p p l i c a t a la f o r m u l a m e d i a n t e la q u a l e è p o s s i b i l e c a l c o l a r e la p r o f o n d i t à d e l l a s u p e r f ì c i e d i s e p a r a z i o n e f r a le d u e a c q u e i n f u n z i o n e d e l l a q u o t a s u l m a r e d e l p e l o l i b e r o d e l l a f a l d a . 4 5 6 m e i i c u r i n o s a p p a s i d à n o t i z i a (li u n a s o r g e n t e d i a c q u a m a r i n a b o l l e n t e , s u c c e d u t a a d u n g e y s e r . t a l e g e y s e r , e s s e n d o a l i m e n t a t o d a a c q u a m a r i n a , è s t a t o c o n s i d e r a t o u n f e n o m e n o u n i c o a l m o n d o . l ' e b o l l i z i o n e è c o n s i d e r a t a l a c a u s a d e l m a n i f e s t a r s i d e l l e s u d d e t t e s o r g e n t i . l a f o r m a z i o n e d i b o l l i c i n e d i v a p o r e i n s e n o a l l ' a c q u a n e f a d i m i n u i r e l a d e n s i t à , p e r c u i l ' a c q u a m a r i n a r a g g i u n g e l a s u p e r f ì c i e a t t r a v e r s a n d o la s o v r a s t a n t e f a l d a f r e a t i c a d o l c e . ( 8 ) fenner c . n . , hydrothermal metamorphism in geyser-iiusins oj yellowstone park, as schoivn by deep drilling. p a p . f. t h e g e o p h . l a b . c a r n e g i e i n s t . of w a s h i n g t o n , t r a n s a c t i o n of t h e a n n . g e o p h . u n . f i f t e e n t h a n n . m e e t i n g , 1 9 3 1 . s i d à n o t i z i a d i d u e p o z z i p e r f o r a t i n e l p a r c o d i y e l l o w s t o n e , u n o n e l u p p e r g e y s e r b a s i n , c h e h a r a g g i u n t o m 122 i n c o n t r a n d o u n a t e m p e r a t u r a ili 180 " c , e l ' a l t r o n e l n o r r i s b a s i n , c h e h a r a g g i u n t o m 62 i n c o n t r a n d o u n a t e m p e r a t u r a di 2 0 5 " c . s u i c a m p i o n i p r e l e v a t i in q u e s t i d u e p o z z i s o n o s t a t e e s e g u i t e a n a l i s i c h i m i c h e e p e t r o g r a f i c h e m e d i a n t e l e q u a l i è s t a t o p o s s i b i l e s t a b i l i r e la t r a s f o r m a z i o n e s u b i t a d a l l e r o c c e e l e c a u s e c h e l ' h a n n o p r o d o t t a . f l u i d i e n d o g e n i r i c c h i d i si", k, ci ( s i l i c e , c l o r u r i e b i c a r b o n a t i d i k) r i s a l g o n o d a z o n e p i ù p r o f o n d e d i q u e l l e e s p l o r a t e c o n i s o n d a g g i . g i u n t i n e l l e d u c i t i e r i o l i t i d e p o s i t a n o s i l i c e e n e s o s t i t u i s c o n o il a a c o n k n e i s i l i c o a l l u m i n a t i . l a s i l i c e e c c e d e n t e e d i s a l i d i na v e n g o n o p o r t a t i a l l a s u p e r f ì c i e e d e p o s i t a t i . n o n è s e m p r e p o s s i b i l e d i s t i n g u e r e i n e o p r o d o t t i di s o s t i t u z i o n e i d r o t e r m a l e n e l l e v u l c a n i t i . ( 9 ) rittmann a . , die natzbarmachung i nikanischer kriifte. d i e n a t u r w i s s e n s c l i a f t e n , 16, o t t . , 1928. il c o n t e n u t o d e l l a n o t a è s t a t o u t i l i z z a t o n e l l a t a b e l l a . (101 rittmann a „ die dienstbazmachung ì'ulkanischer kriifte. n a t u r u n d a o l k : 6 7 , 1 9 3 7 . 11 c o n t e n u t o d e l l a n o t a è s t a t o u t i l i z z a t o n e l l a t a b e l l a . ( 1 1 ) sonder r . a , sludien iiber heisse quellen und tektonik in islaiul. i s t . v i l l e . j . f r i e d l a e n d e r : n. 2, z u r i g o , 1 9 1 1 . s i d à n o t i z i a , f r a l ' a l t r o , d e l l ' u t i l i z z a z i o n e d e l l e a c q u e t e r m a l i p e r r i s c a l d a m e n t o d i s e r r e . ( 1 2 ) steiner a _ hydrothermal rock alterution ut ìf'airakei. new zelanti. e r . g e o l . : 1, 1 9 5 3 . m e d i a n t e u n a c c u r a t o s t u d i o p e t r o g r a f i c o e c h i m i c o , e s e g u i t o su c a m p i o n i d i r o c c e p r e l e v a t i n e i p o z z i p e r f o r a t i , è s t a t o p o s s i b i l e a c c e r t a r e la p r e s e n z a , n e l s o t t o s u o l o d e l l ' a r e a t e r m a l e di w a i r a k e i , d i q u a t t r o z o n e ili a l t e r a z i o n e c h e , i n o r d i n e d i p r o f o n d i t à , s o n o le s e g u e n t i : 1) z o n a di a l t e r a z i o n e p e r a c i d o s o l f o r i c o ; 2) z o n a d i a r g i l l i f ì c a z i o n e ; 3) z o n a d i z e o l i t i z z a z i o n e ; il z o n a d i f e l d s p a t i z z a z i o n e . ( 1 3 1 w 1 1 i t e d . e . . b r a n n o c k w . \ v . , f i x p h . f _ g i a n e l i . a \ . p „ p r e l i m i n a r y geochemicul results at steamboal springs, nevada. t r a n s a c t i o n s a m e r . g e o p h y s i c a l u n i o n ; 2 9 , 2 , 1 9 4 8 . l e r o c c e a f f i o r a n t i p r e s s o l e s o r g e n t i di s t e a m b o a l s o n o d i o r i g i n e v u l c a n i c a e s e d i m e n t a r i o m e t a m o r f ì c a d i e t à i g n o t a ; in e s s e si è i n t r u s a u n a g r a n o d i o r i t e p r o s p e t t o d e l l e r i c e r c h e p e r '( f 0 1 ì z e e n d o g e n e » p r o b a b i l m e n t e a s s o c i a t a ai b a t o l i t e d e l l a s i e r r a n e v a d a . l e r o c c e p i ù r e c e n t i v a n n o d a l t e r z i a r i o a l l ' a t t u a l e . l e s o r g e n t i d e l l a z o n a ( c i r c a 50) s e m b r a n o c o n n e s s e a f r a t t u r e r e c e n t i e g e n e t i c a m e n t e a s s o c i a t e a l l e r i o l i t i , p u r e e s s e r e c e n t i . m o l t e s o r g e n t i si t r o v a n o a l l a t e m p e r a t u r a d i e b o l l i z i o n e c o r r i s p o n d e n t e a l l a l o r o q u o t a s u l m a r e ( n i 1 1 3 0 1 2 5 0 ) . il g r a d i e n t e g e o t e r m i c o è di c i r c a 2 , 2 2 ° c p e r m e t r o . e s s o è s t a t o m i s u r a t o nei p o z z i p e r f o r a t i i n . 29 f r a m 3 0 e m 5 0 ) . i n q u e s t i p o z z i si s o n o f o r m a t e , d u r a n t e l ' e r o g a z i o n e , i n c r o s t a z i o n i c a r b o n a t i c b e . in u n p o z z o è s t a t o t r o v a t o u n m a s s i m o d i t e m p e r a t u r a d i 138" a 50 m e t r i di p r o f o n d i t à . l ' a n a l i s i c h i m i c a r i v e l a u n ' a c q u a m a d r e s a l i n a d i l u i t a d a a c q u e a p i ù b a s s a c o n c e n t r a z i o n e , f o r s e d i o r i g i n e m e t e o r i c a . il c o n t e n u t o d i s o s t a n z e m i n e r a l i è d i 2 0 0 0 4 2 5 0 0 p . p . n i . , i n m a s s i m a p a r t e na, ci, c o . , , / / c o , , s i o . „ s o , , li. i d e p o s i t i s o n o s i l i c e i c o n p i c c o l e q u a n t i t à d i ca co:t. l e f o r t i p i o g g e f a n n o s e n t i r e i l o r o e f f e t t i s u l l a p o r t a t a , s u l l a s a l i n i t à e s u l l a t e m p e r a t u r a d e l l e s o r g e n t i . l a p o r t a t a r i s e n t e a n c h e d e l l a p r e s s i o n e a t m o s f e r i c a . l a t e m p e r a t u r a , i n g e n e r e , a u m e n t a c o n la p o r t a t a . i g a s p r e s e n t i s o n o p r i n c i p a l m e n t e c o . , , h.,s, j \ „ e, i n q u a n t i t à m i n o r i , o s s i d i l o , i d r o g e n o e d a r g o n . n e i v a p o r i p i ù c a l d i è p r e s e n t e m e r c u r i o . c i n a b r o , s t i b i n a , p i r i t e e d a l t r i s o l f u r i si t ' o r m a n o a p o c h i m e t r i d i p r o f o n d i t à . a l c u n i f a n g h i s i l i c e i , d e p o s i t a t i d a l l e a c q u e d e l l e s o r g e n t i , c o n t e n g o n o o r o , a r g e n t o , m e r c u r i o , r a m e e d a r s e n i c o . l ' a c i d i t à e l ' a l c a l i n i t à d e l l e a c q u e s e m b r a n o e s s e r e in s t r e t t a r e l a z i o n e c o n la t e m p e r a t u r a , v e r i f i c a n d o s i u n a m a g g i o r e a l c a l i n i t à n e l l e a c q u e a t e m p e r a t u r a p i ù e l e v a t a . è a t t u a l m e n t e in c o r s o u n a a l t e r a z i o n e di t i p o a c i d o d e l l e r o c c e c o m p r e s e t r a la s u p e r f i c i e e d il p e l o l i b e r o d e l l a f a l d a f r e a t i c a ; si r i t i e n e c h e e s s a s i a c a u s a t a d a a c i d o s o l f o r i c o p r o v e n i e n t e d a l l ' o s s i d a z i o n e d e l l ' / / . , . s d a p a r t e d e l l ' o s s i g e n o a t m o s f e r i c o . p r o b a b i l m e n t e u n a l t r o t i p o d i a l t e r a z i o n e è i n c o r s o al d i s o t t o d e l p e l o l i b e r o d e l l ' a c q u a , d o v e l e r o c c e s o n o in c o n t a t t o c o n a c q u e n e u t r e o a l c a l i n e . l i t i w h i t e 1). e . , sandberc. c . h . , biiannock w . \ \ \ . geocliemicnl and geophysical approaches lo the problem oj utilization oj hot aprine: water and beat. p r o c e d i n g s o f t h i r d n e v a d a w a t e r c o n f e r e n c e , 1918, p p . 112-125, c a r s o n c i t y , n e v a d a . 1 9 1 9 . s i d à n o t i z i a d i p o s s i b i l i i m p i e g h i d e l c a l o r e n a t u r a l e . p e r la r i c e r c a di a r e e t e r m a l i si p r o p o n g o n o il m e t o d o d e i p o t e n z i a l i n a t u r a l i . il m e t o d o r e s i s t i v i m e t r i c o , il m e t o d o m a g n e t o m e t r i c o e q u e l l o a r e o m a g n e t o n i e t r i c o . si e s p o n g o n o i p r o b l e m i g e o c h i m i c i c h e si p r e s e n t a n o n e l l a u t i l i z z a z i o n e d e l v a p o r e e d e l l e a c q u e d e l l e s o r g e n t i c a l d e e si p r o p o n g o n o i m e t o d i p e r l a l o r o s o l u z i o n e . l . e s o r g e n t i <c n o r m a l i » v u l c a n i c h e d e p o s i t a n o q u a r z o f i b r o s o q u a n d o l a l o r o t e m p e r a t u r a è p r o s s i m a o c o i n c i d e c o n q u e l l a d i e b o l l i z i o n e , n o n d e p o s i t a n o q u a s i n i e n t e a t e m p e r a t u r a i n f e r i o r e . n e i p o z z i i n e r u z i o n e , i n v e c e , è s t a l a c o n s t a t a t a la f o r m a z i o n e ili i n c r o s t a z i o n i di ca c o : 1 . 11 d i f f e r e n t e c o m p o r t a m e n t o f r a s o r g e n t i i n e r o g a z i o n e e p o z z i in e r u z i o n e v i e n e s p i e g a t a c o n il f a t t o c h e i n u n p o z z o i n e r u z i o n e la p r e s s i o n e e l a t e m p e r a t u r a si a b b a s s a n o r a p i d a m e n t e e la l i b e r a z i o n e d i c o . , v i e n e q u i n d i f a v o r i t a . p e r e v i t a r e l e i n c r o s t a z i o n i c h e n e d e r i v a n o , l ' e r o g a z i o n e d i u n p o z z o d o v r e b b e a v v e n i r e s o t t o p r e s s i o n e i n m o d o d a e v i t a r e l a m k h c r i l l n o s a p p i l i b e r a z i o n e d i co.,; le a i q u e , p e r ò , d i v e r r e b b e r o m o l t o a g g r e s s i v e . s e , c o s ì f a c e n d o , si e l i m i n a l a d e p o s i z i o n e d i cu co , <i p u ò p e r ò v e r i f i c a r e la p r e c i p i t a z i o n e d e l l a sil i c e . a n c b e s e si r i e s c e a i m p e d i r e la p r e c i p i t a z i o n e d e l l a s i l i c e , si i n c o n t r a l ' i n c o n v e n i e n t e d e l l a c o r r o s i o n e p r o d o t t a d a il.,s e co., in s o l u z i o n e . s e c o n d o g l i a u t o r i , il m e t o d o m i g l i o r e p e r e v i t a r e q u e s t ' u l t i m o i n c o n v e n i e n t e s a r e b b e q u e l l o d i a l i m e n t a r e il ph d e l l ' a c q u a c o n a g g i u n t a ili a l c a l i , l o c a l i z z a n d o la p r e c i p i t a z i o n e d e l caco.. i n v a s c h e d i s t a b i l i z z a z i o n e e c o n v e r t e n d o vh.,s i n s o l f u r o a l c a l i n o e il b i c a r b o n a t o i n c a r b o n a t o . s i p r o p o n e a n c h e l ' u s o d i s c a m b i a t o r i d i c a l o r e in m o d o d a f a r c i r c o l a r e e n t r o i t u b i d e l l a r e t e di d i s t r i b u z i o n e a c q u a p u r a c h e 11011 g e n e r a i n c r o s t a z i o n i . c o n t r o l ' a t t a c c o d a p a r t e d e l l ' / / . . s o ( si p r o p o n e l ' i m p i e g o d i t u b i d i a c c i a i o i n o s s i d a b i l e . p r o s p e t t o d e l l e r i c e r c h e p e r « f o r z e e n d o g e n e » in v a r i e r e g i o n i del m . s a p p a 28 m a g g i o 1954 m o l l i l o ( t u r ; • a n t e l ' u l t i m o t r e n t e n n i o . n a z i o n e c a m p o regione terreno affiorante manifestazioni superficiali ricerche gcofisiche s o n d a g g i 11. (prof, minm a x in ni) fluidi rinvenuti t e m p e r a in slip. lire m a x in prof. pressione m a x in k g / c m portate (massime) per un singolo pozzo impieghi autore i t a l i a viterbese (lazio) vulcanico c sedimentario sorgenti t c r m o m i ncrali e m a n i f e stazioni di gas elettrica, sismica (a rifrazione) n" 5 (in 135-590) acqua calda risaliente con c o „ , h 2 s 7s» — 1 0 0 1 /sec — b . c o n f o r t o f . p e n t a c a m p i flefumarole (alcune a 1 7 5 m 4 8 " — 100 ton/h (acqua + vapore) in esperimento ad ischia una centrale g e o t e r m i ca a c l o ruro di e tile (300 k w ) f . p e n t a f . p e n t a e b . c o n f o r t o grei ed isola d'ischia (campanili) vulcanico surriscald a t o ) , s o r genti term o m i n e rali . elettrica, sismica (a rifrazione) n° h6 (m 10-1840) acqua calda con c 0 2 , h „ s , c h , , h 2 1 7 5 m 4 8 " — 100 ton/h (acqua + vapore) in esperimento ad ischia una centrale g e o t e r m i ca a c l o ruro di e tile (300 k w ) f . p e n t a f . p e n t a e b . c o n f o r t o isola di vulcano (sicilia) vulcanico fumarole — n» 3 (in 230) acqua calda con c o , 2 , h j s 110» — — 1 ton/h — c i . d e s s a u l . s l c a r d i sciacca (sicilia) sedimentario sorgenti t e r m o m i nerali e f u m a r o l e — il" 1 (in corso) — — — — — — g . f . g i n o e c . s o m m a r u g a c . a u m e n t i p r o s p e t t o d e l l e r i c e r c h e p e r « f o r z e e n d o g e n e » in v a r i e r e g i o n i d e l i n o n d o d u r a n t e l ' u l t i m o t r e n t e n n i o . m . s a p p a 28 m a g g i o 1954 n a z i o n e m a n i f e s t a r i c e r c h e s o n d a g g i n fluidi t e m p e r a t u r e m a x pressione p o r t a t e (massime) i m p i e g h i a u t o r e c a m p o regione affiorante zioni superficiali g e o f i s i c h e ( p r o f , m i n m a x ì l i 111) rinvenuti in s u p . in p r o f . m a x in k g / c m 2 per un s i n g o l o p o z z o i m p i e g h i a u t o r e larderello ( toscana) s e d i m e n tario « s o f f i o n i » (getti di v a p o r e surriscaldato) elettrica ; a s s a g g i sis m o l o g i c i , g r a v i m e trici, m a g n c t o m c trici, r a d i o a t t i v i n» 2 5 0 (di p r o d u zione) v a p o r e surris c a l d a t o c o n c o o , h „ s , ch4". k , " n . „ n h 3 , h e , ar, n e , h 3 1 ì 0 3 184"-241° — 17 a p o z z o c h i u s o 3,6 a p o z z o a p e r t o 3 0 0 t o n / h p r o d u z i o ne di energ i a elettrica. d u e miliardi di k w h all ' a n n o s o l e r , 1 3 o a g a , g . g i n o r i c o n t i f . p e n t a a . m a z z o n i c . d e s t e f a n i d . l e n z i b . l o t t i g r a v i m e n u o v a z e l a n d a w a i r a k e i t a u p o v u l c a n i c o s o r g e n t i t e r m o m i ncrali, geysers tria, m a g n e t o m e t r i a , e l e t t r i c a , s i s m i c a (a rifrazione) n" 13 (ili 3 0 0 4 » l) a c q u a calda c o n hos, h „ — 2 3 4 " — 9 0 t o n / h (acqua + v a p o r e ) c e n t r a l e da 10. 000 k w j . h e a i . y e r . w . f o s t e r a . s t e i n e r s t a t i u n i t i c o a s t r a n g c (california) v u l c a n i c o e s e d i m e n t a r i o f u m a r o l e g e y s e r s , s o r g e n t i t c r m o m i ncrali — i l " 8 (ili 66-215) v a p o r e s a t u ro e surris c a l d a t o con co2,n„,ch4 189" a 12,6 k g / c m 2 a p o z z o chiuso 2 3 2 " 21 a p o z z o chiuso 6 2 t o n / h di v a p o r e — e . t . a l i e n e a . l . d a y s o r g e n t i t e r m o m i nerali, lasscn park (•california) v u l c a n i c o g e y s e r s , f u m a r o l e a vapores a t u r o e surriscald. e . t . a l l e n p r o s p e t t o d e l l e r i c e r c h e p e r « f o r z e e n d o g e n e » in v a r i e r e g i o n i del m o n d o d u r a n t e l ' u l t i m o t r e n t e n n i o . m. s a p p a 28 m a g g i o 1954 n a z i o n e c a m p o regione t e r r e n o affiorante m a n i f e s t a zioni superficiali r i c e r c h e g e o f i s i c h e s o n d a g g i n. ( p r o f , m i n m a x ìli 111) fluidi rinvenuti t e m p e r a ili sup. ure m a x in p r o f . pressione m a x in k g / c m 2 p o r t a t e ( m a s s i m e ) per un s i n g o l o p o z z o i m p i e g h i a u t o r e y e l l o w stone park (wyoming) v u l c a n i c o s o r g e n t i t e r m o m i ncrali, geysers, f u m a r o l e a v a p o r e surriscald a t o — n" 2 (in 62-122) v a p o r e surriscaldato 138" 2 0 5 " — — — e . t . a l l e n e . t . a l l e n c a . l . d a y c . n . f e n n e u t . f . w . b a u t i i e f. d . bt.oss s t e a m b o a t s p r i n g s (nevada) v u l c a n i c o e s e d i m e n tario m e t a m o r f i c o s o r g e n t i t e r m o m i nerali con c o „ h 2 s , n . „ " h„". o " , a i " f u m a r o l e a v a p o r e s a t u r o e surriscald a t o — u" 29 ( m 32-50) a c q u a calda con c o . , , h 2 s , h 2 : n „ 100" 138" — r i s c a l d a m e n t o di fabbricati d . e . w h i t i i w . w . b r a n n o c k p i i . f. f i x v . i ' . g i a n f i i . a c . h . s a n d e r e r c k a t m a i (alaska) v u l c a n i c o f u m a r o l e a v a p o r e surriscald a t o — — — — — — — e . t . a l i e n g i a p p o n e k y u s h u , b e p p u v u l c a n i c o f u m a r o l e g e o t e r m i c h e n" -1 (in ho) a c q u a calda 139° a 3 , 9 k g / c m 159° — — c e n t r a l i n a s p c r i m c n t . (30 k w ) n e l 1954 è stata p r o gettata una centrale da 3 0 0 0 k w 1. h a y a s a k a p r o s p e t t o d e l l e r i c e r c h e p e r « f o r z e e n d o g e n e » in v a r i e r e g i o n i del m o n d o d u r a n t e l ' u l t i m o t r e n t e n n i o . m. s a p p a 28 m a g g i o 1954 n a z i o n e c a m p o regi cne t e r r e n o m a n i f e s t a r i c e r c h e s o n d a g g i 11. t e m p e r a t u r e m a x pressione p o r t a t e (massime) i m p i e g h i n a z i o n e c a m p o regi cne affiorante zioni superficiali geofisichc (prof, m i n m a x in 111) rinvenuti ili slip. in p r o f . m a x 111 k g / c m 2 per un s i n g o l o p o z z o i m p i e g h i a u t o r e indie o l a n desi k a w a h k a m o d i j a g (gìtien) v u l c a n i c o — ii° 4 ( m a x 111 128) v a p o r e saturo nei pozzi m e n o p r o f o n d i . v a p o r e surriscaldato nei pozzi pili p r o f o n d i . a m b e d u e con h 2 s — — 2 a p o z z o a p e r t o — — a . r i t t m a n n j . r . van b e m m e l e n islanda v u l c a n i c o geysers, s o r g e n t i t c r m o m i nerali, f u m a r o l e — — — — — r i s c a l d a * m e n t o di fabbricati t . f . w . b a r t h r . a . s o n d e r el s a l vador ( ° ) v u l c a n i c o f u m a r o l e a v a p o r e saturo e surriscald a t o — — — — — — — f . p e n t a a . pf.rozzi h . m a y e r a u i c h b . a . s t i r t o n e w . k . g e a l e y uganda ( ° ) t o r o s e d i m e n tario s o r g e n t i t c r m o m i nerali — — — — — — — — antili e ( ° ) s. lucia — — — — — — — — — n i c a ragua ( ° ) v u l c a n i c o — — — — — — — — — cile ( " ) v u l c a n i c o — — — — — — — — — s o n o stati p r o g r a m m a t i l a v o i i di ricerca. improvements on near real time detection of volcanic ash emissions for emergency monitoring with limited satellite bands annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6598 improvements on near real time detection of volcanic ash emissions for emergency monitoring with limited satellite bands torge steensen1*, peter webley2, jon dehn2 1 leibniz universität hannover, germany, 2 university of alaska fairbanks, usa *steensen@ipi.uni-hannover.de abstract quantifying volcanic ash emissions syn-eruptively is an important task for the global aviation com munity. however, due to the near real time nature of volcano monitoring, many parameters important for accurate ash mass estimates cannot be obtained easily. even when using the best possible estimates of those parameters, uncertainties associated with the ash masses remain high, especially if the satellite data is only available in the traditional 10.8 and 12.0 µm bands. to counteract this limitation, we developed a quantitative comparison between the ash extents in satellite and model data. the focus is the manual cloud edge definition based on the available satellite reverse absorption (ra) data as well as other knowledge like pilot reports or ground-based observations followed by an application of the volcanic ash retrieval on the defined subset with an ra threshold of 0 k. this manual aspect, although subjective to the experience of the observer, can show a significant improvement as it provides the ability to highlight ash that otherwise would be obscured by meteorological clouds or, by passing over different surfaces with unaccounted tem peratures, might be lost entirely and thus remains undetectable for an automated satellite approach. we show comparisons to volcanic ash transport and dispersion models and outline a quantitative match as well as percentages of overestimates based on satellite or dispersion model data which can be converted into a level of reliability for near real time volcano monitoring. i. introduction perational monitoring of erupting volcanoes and their ash emissions is crucial for aviation safety [prata, 1989]. space observations traditionally use satellite bands spectrally located around 10.8 and 12.0 µm as these have been found to have the ability to distinguish volcanic ash from meteorological clouds based on the reverse absorption (ra) feature discussed in prata (1989). o traditionally, this method has a threshold of 0 k which is often reduced to a negative value to avoid false alarms [see steensen et al, 2013]. a quantitative analysis of ash emissions detected with the ra method, the volcanic ash retrieval (var), has been developed by wen and rose [1994] and applied to different case studies around the world [e.g. rose et al., 1995; gu et al., 2005]. var uses the ra maps as base to estimate the ash masses. important input parameters are surface and ash temperatures. in 1 annals of geophysics, xxi, 1, 2015 addition, different volcanic ash transport and dispersion (vatd) models have been developed and are used globally to track and predict ash movement [e.g. puff (tanaka, 1991); name (‘nucelar accident model’; ryall and maryon, 1998) and hysplit (‘hybrid single-particle lagrangian integrated trajectory; draxler and hess, 1997)]. nonetheless, the accuracy of satellite analyses and vatd predictions depends on their respective input parameters. in addition, ideal satellite observations are limited to clear-sky conditions as meteorological clouds under or overlying the ash will hamper the measurements by altering the defacto ‘surface’ temperature and by shielding the ash from space-borne detection [wen and rose, 1994]. more sophisticated approaches exist but require more spectral bands which are not present on all satellites yet (e.g. pavolonis, 2010). we developed a new approach combining the advantages of ra-based retrievals and model calculations while acknowledging their respective uncertainties. this also includes a non-automated step in which the user can alter the auto-determined ash extent, determined by the ra approach with the 0k threshold, based on additional information like pilot reports or ground-based data sets, which are not recognized in current automated analyses but can provide valuable information regarding the extent of the volcanic ash. our case study is the eruption of kasatochi, a volcano in the central aleutian islands of alaska (fig. 1a). it erupted violently in august 2008 for the first time in recorded history and emitted ash in three different events. the ash reached heights of 18 km above sea level and travelled eastwards across the north pacific ocean [waythomas et al. 2010]. this ash cloud was observed with data from the geostationary operational environmental satellite (goes) and modelled with puff. the parameters we chose for the puff run are listed in table 1. as time frame, we chose the initial eruption sequence of the three main events and ignored the following continuous phase as ash heights and the durations for specific heights, essential parameters for vatd runs, have not been recorded. ii. methods the critical success index (csi) is a quantitative measurement tool developed by stunder et al. [2007] to evaluate the degree of overlap between the volcanic ash extents derived from satellite data and predicted in vatd model data. it is defined as: csi = o/(s+o+m) (1) where ‘s’ and ‘m’ are the ash extents only determined in the satellite and modelled data, respectively, and ‘o’ is the overlap between both data sets. in the ideal case, the csi equals ‘1’, i.e. a 100% overlap between both ash extents is shown. originally, the csi value has been developed to evaluate the performance of the hysplit vatd model against satellite data. in order to expand this to compare modelled and observed data sets against each other, without assuming either to be perfectly accurate, we added two further values, the satellite excess (se) and the model excess (me) which are defined, analogously to the csi, as follows: se = s/(s+o+m) (2) me = m/(s+o+m). (3) note that the observed data set (s+o) comprises the satellite data as used by stunder et al. [2007] and additional data like pilot reports, ground measurements and in-situ sampling. while such sources are scarce in a near real time setting, they will become important in ret2 annals of geophysics, fast track 2, 2014 roactive analyses using this method. we use the spatial extent of the puff calculations as input for our analysis. in addition, we apply the ra method to each satellite image to outline the theoretical ash extent with a 0 k threshold in the goes data. this raw outline is later manually modified to reflect information from other data sources. after obtaining the two extents separately, we can compare them quantitatively by calculating the overlap as well as the respective excess values defined in (2) and (3). the ratio between the three values will change over the course of the eruption, which allows us to draw conclusions about the accuracy of the input data sets and, in turn, their input parameters. the higher the csi will be, the more reliable the forecasted ash movement is. if the se is relatively large, the satellite images with external information show ash where the model doesn’t predict it. this can be due to erroneous input parameters for the model data which directly affect the calculated ash extent (e.g. chosen wind pattern or its resolution, height of the ash, etc.). similarly, a high me value stands for misrepresentations of the ash in satellite and other data sets. in this case, the vatd model predicts ash but the satellite products as well as other sources can’t confirm it. such errors can be caused by meteorological clouds blocking the ash from being detected or ash concentrations dropping below the detection limit. due to this behaviour, it is possible to draw conclusions about the stage of the eruption as well as the general accuracy of the respective input parameters: a) during the later stages of an eruption, an increase of me with a decrease of se will likely be observed as ash concentrations will drop below the detection limit of the respective sensor while they are still present in the model run. this could potentially be exploited to measure the sensitivity of the sensor and to adjust vatd models to predict ash movement more accurately. table 1: parameters chosen for puff and the volcanic ash retrieval [* as described by waythomas et al. (2010); ncar=national center for atmospheric research] puff start time 22:00 utc start date august 7, 2008 end time 05:00 utc end date august 8, 2008 number of separate events 3* vertical particle distribution poisson wind model ncar number of particles 100,000 eruption rate based on sparks et al., 1997 particle size distribution based on mastin et al., 2009 amount of fines (<63µm) 40% b) in cloudy settings, ra images often show a high number of false alarms, especially with a threshold of 0 k [prata, 1989]. when manually interpreting the results, it cannot be ruled out that there is ash beneath the meteorological clouds. an inclusion of the respective pixels in the ra data set can raise the se value. high numbers of se and me values can therefore be indicative of cloudy scenes. c) similar shapes of se and me values on opposite ends of the ash cloud can indicate a shift of the ash, potentially due to an uncorrected parallax in the 3 annals of geophysics, xxi, 1, 2015 satellite data or to incorrect wind information in the model settings. the ideal case is a csi value of 1. when this is not the case, both data sets need to be evaluated regarding the possible cause of this offset. the introduction of me and se values can prove valuable in addressing this problem. iii. results an example of the puff runs for the kasatochi 2008 eruption can be seen in figure 1a. the model output at 23:00 utc on august 8, 2008, is colour-coded by height above sea level and disperses across the north pacific ocean to the east. the accompanying se, csi and me values for each time step show the development of the ash cloud in reference to the model and observational data input. figure 1b represents the time-coinciding values of the ash dispersal depicted in figure 1a. the analysis shows a large me area to the east of the cloud that coincides with the lower parts of the modelled ash cloud (approximately below 12 km above sea level) as shown in figure 1a. the red area represents the se value, which is relatively large and goes back to the volcano and further west where, to the best of our knowledge, some ash still existed. the csi area is located between both excess values. when comparing the different values over time (figure 1c), an initial high percentage of me is quickly lowered by noon of august 9 before it peaks at over 90% shortly before the end of our analysis (august 10, 23:00 utc). the se value, on the other hand, has its peak at the end of august 8, roughly coinciding with figures 1a and b. by the end of the three day window, this value has reached 0%. the csi value increases at the beginning of the eruption up to 60% (august 9, 15:00 utc) but decreases in the later stages to about 10%. the overall trend during the eruption shows more fluctuation during the first few hours, with se values of 0 shortly after the onsets of new events, than during the end where comparably steady inand decreases of the different values can be observed. 4 figure 1: location of the kasatochi volcano in the alaskan aleutian islands (black cross). (a) snapshot from the results of the puff run from the kasatochi eruption in 2008. the color-coding is based on the ash height. (b) se, csi and me values corresponding to the puff scene in (a). note that the me values covers the lower part of the cloud, approximately up to 10 km above sea level.(c) development of se, csi, and me values over the course of the whole eruption. towards the end, me dominates and se decreases to 0.the timing of s and b is highlighted by the solid black line. arrows indicate the onset of different events as described by waythomas et al. (2010). annals of geophysics, fast track 2, 2014 iv. discussion the high se values in the earlier parts of the eruption likely represent an overestimate based on satellite and auxiliary data. an example of this can be seen in figure 1b where the expected ash extent goes back to the volcano while the model does not predict this. theoretically, this can also be caused by wrong model parameters but, due to the patchy character of the area, false alarms based on meteorological cloud cover are more likely. figure 1c also shows an se value of 0 shortly after the onsets of each event. this is due to the ash breaching the meteorological cloud cover. it reaches heights where it can easily be identified in satellite data. however, the larger part of the ash cloud beneath the meteorological cloud cover was not detected in satellite data, causing high me values. the fluctuations in the values at the beginning of the time series are due to a small plume where a few pixel differences can add up to large percentages. high me values at the end of the analyzed period stem from an expanding ash cloud in the model data which could not be identified in the observed data sets. towards the end of the ash dispersion, the ash concentration is likely too low for satellites to determine it and auxiliary data sets for this time did not exist. these findings suggest that an analysis of the se, csi and me values can quantify the accuracy of the data sets and predict the ash movement. problems can be caused by meteorological cloud cover, low ash concentrations and small ash extents. it is important to note that high se or me values do not point directly at incorrect input parameters of a specific method, but rather show a discrepancy between the compared techniques. this disagreement can then be analysed and corrected. using this qualitative analysis of the atmospheric ash extent, we not only incorporate additional data sets an automated approach cannot analyze, we’re also taking a step back from a quantization of ash masses in the atmosphere. while it is important to work towards an accurate assessment of volcanic ash, a mass analysis with limited satellite bands and default input parameters for satellite and vatd model data can artificially alter the masses by up to +/ 50% [steensen and webley, 2012]. for these scenarios, an approach as described here is the best option to evaluate the ash dispersal and to predict the ash movement. following up on this method, var can be applied to the csi area instead of the whole model extent, where the ash might be underneath meteorological clouds causing false readings. references [casadeval, 1994] casadeval, t. j. 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(1994). retrieval of sizes and total masses of particles in volcanic clouds using avhrr bands 4 and 5. j. of geoph. res., 99, 5421-5431. 6 22musson.qxd 597 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical earthquakes – seismicity – earthquake catalogues – united kingdom 1. introduction this paper is not a study of the history of british earthquakes, which can be found in musson (1994, 1996a, 2002). it is a study of the history of the history of british earthquakes, who wrote it, why and how. although britain is not an area much subject to earthquakes (maximum observed magnitude 6 ml; an earthquake of 4.7 ml has a return period of ten years) it is an area with a long recorded history; and a long intellectual history. this means that conditions for recording the occurrence of earthquakes in britain have been relatively good throughout the last 1000 years or so, and particularly in the last 300. the fact that earthquakes are quite uncommon, and that many british people may go throughout their life without ever feeling one, also means that people are more likely to take note of weak shaking than would be the case in other countries. in discussions on earthquake intensity (for example, in context of the workshop meetings of the esc working group «macroseismic scales», grünthal, 1998) the opinion is sometimes expressed that intensity 2 is a useless measure because it is never recorda critical history of british earthquakes roger m.w. musson british geological survey, edinburgh, u.k. abstract this paper reviews the history of the study of historical british earthquakes. the publication of compendia of british earthquakes goes back as early as the late 16th century. a boost to the study of earthquakes in britain was given in the mid 18th century as a result of two events occurring in london in 1750 (analogous to the general increase in earthquakes in europe five years later after the 1755 lisbon earthquake). the 19th century saw a number of significant studies, culminating in the work of davison, whose book-length catalogue was published finally in 1924. after that appears a gap, until interest in the subject was renewed in the mid 1970s. the expansion of the u.k. nuclear programme in the 1980s led to a series of large-scale investigations of historical british earthquakes, all based almost completely on primary historical data and conducted to high standards. the catalogue published by bgs in 1994 is a synthesis of these studies, and presents a parametric catalogue in which historical earthquakes are assessed from intensity data points based on primary source material. since 1994, revisions to parameters have been minor and new events discovered have been restricted to a few small events. mailing address: dr. roger m.w. musson, british geological survey, west mains road, edinburgh eh9 3la, u.k.; e-mail: rmwm@bgs.ac.uk «the third form of reflective history is the critical ... it is not history itself that is here presented. we might more properly designate it as a history of history; a criticism of historical narratives and an investigation of their truth and credibility. its peculiarity in point of fact and of intention, consists in the acuteness with which the writer extorts something from the records which was not in the matters recorded». g.w.f. hegel, the philosophy of history (1837) 598 roger m.w. musson ed in practice. this is absolutely not the case in the britain. a notable example is the damaging colchester earthquake of 1884. because this event was so newsworthy, even at the extreme limit of the felt area people were anxious to recount their experiences so as to be part of the historic occasion (musson et al., 1990). an extreme case is the person in scotland in 1841, who had a stone monument erected in his garden to record the occurrence of an earthquake that was only heard at his location (musson, 1993). in the survey that follows, a chronological approach will be adopted, starting with the earliest studies of historical earthquakes and continuing to the present day. 2. the earliest studies of british earthquakes (pre 18th century) the first traceable catalogue of earthquakes published in britain is that of fleming (1580). on 6 april 1580 a large (about 5 3/4 ml) earthquake occurred with epicentre in the dover straits, which caused considerable damage in england, france and the low countries, and a few deaths (neilson et al., 1984a; melville et al., 1996). it was strongly felt in london, where two children were killed by falling stones. such was the impact of the event in the english capital, that a number of pamphlets were rushed to the press, describing the earthquake, and generally exhorting people to reform their ways, given this warning of god’s displeasure (ockenden, 1936). one of these was a lengthy treatise by abraham fleming, the full title of which reads «a bright burning beacon, forewarning all wise virgins to trim their lampes against the comming of the bridegroome. conteining a generall doctrine of sundrie signs and wonders, specially earthquakes both particular and generall: a discourse of the end of this world: a commemoration of our late earthquake, the 6. of april, about 6. of the clocke in the evening 1580. and a praier for the appeasing of gods wrath and indignation» (fleming, 1580). the emphasis of this work is religious in nature. however, chapter 12 is entitled «a contemplation of wonderfull accidents, and principally of earthquaks, as well particular as generall, which have happened in the realms of england, ireland, and scotland, from the time of k. william the conquerour, to the reigne of our sovereigne lady and gratious queene elizabeth, & c». for those not familiar with english history, the «start» of it has often been dated to the norman invasion of william i (william the conqueror) in 1066, which marked an abrupt change in dynastic succession from the earlier saxon kings. the catalogue is presented in the form of free text, moving from one event to another, rather than as any sort of table. because of the interest of this very early catalogue, it may be as well to list the events in full. the earthquakes mentioned are as follows: – «reign of eugenius» london. – march 1077 all england. – 1084 unspecified. – 1165 ely, norfolk, suffolk. – 1179 oxenhall, near darlington. – monday the week before easter 1185 all england; damage at lincoln. – january-february 1199 scotland. – 1222 warwickshire. – 1247 london. – 1248 bath and wells. – 1250 st albans. – 1266 ireland. – 1274 various places in england. – 1275 all england, damage at glastonbury. – 1382 two earthquakes in various places, especially kent. – 1563 lincoln and northampton. – 6 april 1580 london. of these, the first is probably mythical, and the 1179 event is not an earthquake. no sources are given, but it is clear that the entries are mostly derived from medieval chronicles, directly or indirectly. it is noteworthy that there is only one event between 1382 and 1580, when one might expect the author to be better informed about his own time than about the 13th century. fleming’s failure to mention a large english earthquake in 1575, only five years previously, is particularly surprising. his principal source would seem to be the general historical work of 599 a critical history of british earthquakes holinshed (1577), hardly surprising when one finds fleming listed as a contributor to the second edition of holinshed et al. (1587). holinshed is also more or less the only source (albeit secondary) known for the scottish earthquakes of 1202, which fleming misdates, and the most accessible source for the irish earthquake of 1266 (which is confirmed by the annals of clyn and dowling; butler, 1849). the dating problem for the scottish swarm, probably either at comrie or the ochil hills, can be resolved here in passing, especially necessary since musson (1994) follows fleming and gives the wrong date. holinshed (1577) gives a specific date of 1199 for other occurrences, then states that about three years later a papal legate visited scotland, and the january following this, the earthquakes occurred. fleming evidently just picked the date written in numerals in the margin. the visit of the papal legate can be dated from other sources to 1201, so the earthquakes happened in 1202. fleming’s paragraphs are padded out with descriptions of damage which don’t appear in any of the sources now available to us, and which were probably fanciful inventions of fleming to embellish his tale. other early surveys of historical earthquakes occur within the context of wider compendiums, including either universal studies of earthquakes or general lists of wonders. thus fourteen british earthquakes between 1085 and 1580 get a mention in batman (1581), a work falling into the latter category. some of these works are early examples of the world earthquake catalogue; bonito (1691) is a famous example. an interesting and little-known example is the work of goad (1686), who compiled a list of earthquakes, including a number of british events between 1551 and 1682, in the interest of studying them from an astrological perspective. members of the royal society took an active interest in british earthquakes in the late 17th century and 18th century, but the work produced in this context, while providing useful data on earthquakes of that period (e.g., boyle 1666, wallis 1666), did not involve studies of historical earthquakes. 3. the 18th century the situation in the 18th century is similar to the 17th, with regard to anecdotal catalogues produced out of antiquarian interest rather than with scientific intent. in most cases these are accounts of world earthquakes, which nevertheless also include british events. a typical example is burton (1734). the occurrence of an abnormal number of british earthquakes in the year 1750, including two in central london, gave a boost to this sort of production, five years before the great lisbon earthquake had a similar effect in the rest of europe. the short catalogue of anon (1750) is exclusively concerned with british events, and the general study of grey (1750) places english earthquakes in a separate appendix. there is no attempt at source criticism, and landslips are frequently mixed up with earthquakes. (the word earthquake was still being used for landslip occasionally in the early 19th century). however, many works of this period do attempt to explain earthquakes scientifically, rather than attributing them to divine intervention. one study that requires particular attention because of its impact on later studies is that of short (1749). thomas short was a derbyshire doctor with a strong interest in mineral waters; he was also interested in the possible link between epidemic diseases and the weather. he spent many years collecting data from chronicles and histories, and published his «general chronological history of the air, weather, seasons, meteors &c» in two thick volumes. the bulk of the text consists of a chronological compilation of accounts of plagues and epidemics, together with any sort of weather information relating to the same years, treating «weather» in the broadest possible sense. his objective, scientifically enough, was to look for any sort of correlation between one type of data and another. however, at the back of the second volume appears a series of chronological tables for different phenomena, the first appearing being earthquakes, which presents a list of dates and places starting with an earthquake at babylon in 2407 am (anno mundi – in other words 1597 b.c.) and ending with a sussex earthquake in 1734. (the second list is for comets, and there 600 roger m.w. musson are further ones for floods, heavy snowfalls and some other less usually encountered phenomena). the striking thing about this table is that it contains numerous earthquakes from very early periods that appear nowhere else, for example, dumbarton being «greatly shaken» in 12 a.d.; «terrible at canterbury» in 193 a.d., and many more. these were duly transcribed by many later cataloguers of earthquakes, but it is impossible to imagine what sources short could have drawn on for these data, and it is rather unlikely that dumbarton even existed in 12 a.d. the other lists have similar problems, with a comet over london in 42 a.d., floods in edinburgh in 155 a.d. (rather unlikely given early edinburgh’s hilltop location) and so on. it is also notable that none of these dubious data appear in short’s main text, and where earthquakes do appear in the main text, they are referenced, and always genuine. a possible hypothesis is that some practical joker handed short a list of fake events, and short, while distrusting it too much to include it in his main text, nevertheless could bring himself to discard it entirely. these spurious events have proved quite difficult to kill, partly because they were copied by so many other authors, including, for example, milne (1911). even as recently as tiedemann (1991) the 1400 fatalities at st andrews in the fake 811 a.d. earthquake reappear with a warning that british seismicity may be stronger than it appears; another legacy of short’s table. 4. the 19th century the middle years of the 19th century were a period of great industry as regards historical research into earthquakes generally. the great catalogues of von hoff (1840) and mallet (18531855) are well known, as are the regional and annual catalogues of perrey, who published his study of historical earthquakes in the british isles in 1849. these catalogues are familiar to all in the field of historical seismology, and it is not necessary to discuss them further here. less well known is the work of david milne (no relation to john milne), later david milnehome, whose work was influential on later british studies. milne can fairly be stated to have been the first seismologist: he was involved in setting up a local instrumental network (in 1840-1841!), in conducting macroseismic surveys of recent events, in historical seismology, and in theoretical speculation. he also proposed the word «seismometer», from which all other «seismo-» words are back-formations. his theoretical ideas were in some respects quite advanced for his time; he wrote very well on the subject of focal depth, and was able to propose that the long axes of elliptical isoseismals should be aligned with major faults even though he had no idea of the causal relation between faults and earthquakes. his historical catalogue (milne, 1842) eschews the usual scrapings from the monastic chronicles via the likes of holinshed (1577) and starts instead in 1608, running forward to 1839, and then merging with contemporary observations from 1839 onwards. he gives sources where these are periodical publications such as gentleman's magazine or the transactions of the royal society of edinburgh, but many earthquakes are listed with dates and descriptions but no references. it seems to be the case that most of these are taken from newspaper data. this catalogue therefore rests on primary source data to a much greater extent than many catalogues of this period. in compiling this catalogue, milne made a number of dating errors that can often be traced back to newspaper practices. it was often the case that one weekly newspaper would publish a paragraph that would start «extract of a letter from ulverston – last tuesday many people observed a smart shock of an earthquake ...». the editor of the weekly newspaper in a neighbouring town would see this item and copy it verbatim, without any acknowledgement, in the next issue of his newspaper. someone finding only the second publication of the paragraph would miscalculate the date of the earthquake from the words «last tuesday» taking the second publication date as reference. a number of times, milne (1842) assigns dates to earthquakes that are one week later than the correct date, and sometimes the earthquake is duplicated and appears with both dates. many later studies simply copied milne’s entries without checking, and replicated these errors. 601 a critical history of british earthquakes two other individual catalogues from this period merit mention here. meldola and white (1885) compiled a remarkable book-length study of the damaging 1884 colchester earthquake, and included a historical introduction, including a list of the more prominent british earthquakes to date. it relies mostly on mallet (1852-1854), short (1749) and some other secondary references. it would not be remarkable of itself, but it has been quoted a number of times by authors referring to this work for its account of the 1884 earthquake. it includes the memorable mistake of listing a damaging earthquake in ireland in 1734; this can be traced back to a misreading of a reference to an earthquake in iceland. the catalogue of roper (1889) is a pamphlet of 46 pages (including index) of earthquakes in the british isles from the beginning of the christian era up to 1889. it is one of the fuller such works to appear, and is referenced throughout, but is based mostly on short (1749), milne (1842) and mallet (1852-1854), with some information from other sources such as gentleman’s magazine, and other periodicals including newspapers (usually the lancaster gazette, since roper lived in or near lancaster). 5. charles davison up to this point, all the catalogues discussed have been based on a process of compilation of historical data more or less without any critical scrutiny. this begins to change with the work of davison, a mathematics teacher who, in 1889, began to devote himself to the study of earthquakes as a hobby. from 1889 to 1926 (intermittently in the latter years) he conducted singlehandedly macroseismic surveys of british earthquakes, publishing the results in a long series of papers. he also published a number of books on various seismological subjects. he is best known, though, for his catalogue of british earthquakes, published in 1924 after 28 years of research. (the period of gestation is known because he printed an appeal for information about historical british earthquakes in several newspapers in 1896). this catalogue (davison, 1924) is a work of 416 pages, and includes a synopsis of the results of his various investigations since 1889, accounts of historical earthquakes back as far as he felt records were reliable (which is to 974 a.d.), and final sections of discussion on the distribution of british earthquakes in space and time, the causes of earthquakes, sound phenomena, and other topics. the main section of the work is divided into chapters according to region, starting with the north of scotland and moving south. within each chapter, earthquakes are divided up according to originating «centres». thus the chapter on earthquakes in south wales has four sections: pembroke earthquakes, carmarthen earthquakes, swansea earthquakes, and «earthquakes of unknown epicentres in south wales». for each centre, events are listed in chronological order, distinguishing between events considered doubtful or uncertain, and those better attested. in fact, davison is usually unduly pessimistic in which events he considers to be uncertain; in the majority of cases subsequent research has shown them to be genuine. earthquakes are assigned intensities where possible, using davison’s own modification of the rossi-forel scale, better considered as a scale in its own right (davison, 1900). the scale is unusual in having one test only for each degree, and is hard to correlate with other scales. for example, intensity 5 is defined simply as «the observer’s seat was perceptibly moved or raised». felt areas are estimated by davison wherever possible. this is therefore the first attempt at anything like a parametric catalogue of british earthquakes. however, he does not give epicentres as co-ordinate values, only the names of the nearest town. approximate numerical epicentres could easily be reconstructed from this, however, and he does include a map, shown here as fig. 1. davison recognised the difficulties inherent in the chronology of the medieval earthquakes and that previous catalogues contained duplicates; however, his success in improving the situation was limited. he was the first author to recognise that the very early earthquakes listed in short (1749) are all fakes, and also made an effort to eliminate other fake events, for instance, those due to landslips. he was therefore the first 602 roger m.w. musson fig. 1. map of earthquake «centres» in the u.k., with estimated fault orientations, according to davison (1924). 603 a critical history of british earthquakes historian of british earthquakes to make a clear attempt to apply critical standards to source material. however, working with limited resources and training, he still perpetuated a number of errors, such as dating errors in milne (1842). for many years this work remained the definitive account of historical british earthquakes. davison’s work of conducting macroseismic surveys of british earthquakes was continued by a.t.j. dollar of the university of glasgow (later of birkbeck college, london). dollar’s record in publishing the results of his macroseismic surveys was much poorer than davison’s; however, at least the data collected by dollar were preserved for posterity by his widow, unlike davison’s widow, who burnt all his papers after his death in 1941. dollar (1950) published a catalogue of scottish earthquakes 1916-1949, but did not engage in historical earthquake studies. one researcher who did follow in davison’s footsteps for historical investigation was a.e. mourant, a medical doctor and amateur seismologist. he compiled a history of earthquakes in the channel islands (mourant, 1931) based on a variety of historical source material including primary and secondary works (especially mallet). each event is given a description, and a maximum intensity value (davison scale) and felt area where possible. 6. modern historical earthquake studies in the mid 1970s there was a reawakening of interest in historical earthquake studies both in the british geological survey (then the institute of geological sciences, igs) and at imperial college, london (ambraseys and melville, 1983). 6.1. lilwall’s catalogue one of the first products of this was the work of lilwall (1976) who attempted to create a properly parametric catalogue of u.k. earthquakes for the first time. this was largely done by taking davison’s catalogue, assigning latitudes and longitudes to the various seismic «centres», and making some preliminary assumptions about magnitudes, including setting a default «minimum probable» magnitude for use where no other information existed. this was extended forwards in time by adding data from dollar (1950) and tillotson (1974). the report by lilwall (1976) presents some analysis based on the preliminary version of the catalogue that never went beyond being a working file; it was not published in full by lilwall because it was not regarded as a finished work. a copy of the file nevertheless fell into other hands, and was (as «the igs file») subjected to a dissection it was not intended to bear (e.g., soil mechanics, 1982). this file also seems to be the basis for the listing of u.k. seismicity in van gils (1988). the file was published by burton et al. (1984a) in order to provide a proper citable reference for the catalogue. 6.2. the inverness report a study of earthquakes in the vicinity of inverness (northern scotland) by browitt et al. (1976) seems to be the first published study in the u.k. in which historical earthquakes are revaluated from original source data. this study reproduces verbatim historical accounts from original newspaper descriptions, and assesses epicentral intensity values (mm and msk), felt areas, and gives approximate magnitudes, but how these are derived is not discussed. the study makes the point that earthquakes in the mountainous north of scotland have tended to «migrate» to settled regions; in other words, the population distribution affects the distribution of felt reports and this has caused misperceptions as to where the epicentres lay. this study provides the first example in britain of detailed historical work being undertaken to show that an event was a fake. this is the case of the 14 november 1769 inverness earthquake, which supposedly threw down houses and killed several persons. this appears in various catalogues, including davison (1924) and can be traced back to a single contemporary record in a periodical published in london (the london magazine) in which the information is given on the authority of a letter from york. browitt et al. (1976) examined specifically the scottish sources that should have reported such an occurrence and found no local evidence that 604 roger m.w. musson this earthquake ever occurred. although no explanation has ever been found (one suggestion is that it relates to some other inverness, perhaps in the americas), all subsequent investigations of this earthquake have agreed with browitt et al. (1976) conclusion that the event is fake. 6.3. principia mechanica at the beginning of the 1980s a large expansion in historical earthquake studies occurred in the uk, in order to inform seismic hazard studies for the u.k. npp programme. three major studies were commissioned at the same time, from two consultancies, principia mechanica ltd. and soil mechanics ltd., and from bgs. a fourth study was completed in the same time frame (ambraseys and melville, 1983). these will be taken in turn. the scope of the principia (1982) study was ambitious – to produce a complete catalogue of historical british earthquakes from original sources, in one year. the final report consists of three volumes. the first contains a wide-ranging discussion of british seismicity including subjects such as magnitude calibration and attenuation of strong ground motion, concluding with a descriptive (rather than parametric) list of british earthquakes from the earliest times up to the 1970s (the list is deliberately incomplete for modern events). the remaining two volumes give detailed accounts of each earthquake. one volume deals with the 19th century (and selected earlier events), the other with the 20th century. for each earthquake a list of the sources used is given, some brief notes of maximum effects, maximum intensity (using a home-made scale of eight degrees), radius of felt area, and a map of data points. larger earthquakes are also given isoseismal maps. tables of intensity data points (idps) are not given, and were not even made, the intensity values being entered directly onto the maps. the amount of information processed in this study is impressive, although inevitably, in the attempt to do so much so quickly, mistakes were made in terms of under-rating some earthquakes, over-rating others, and failing to identify fake events. many earthquakes receive cursory attention. numerical parameters are estimated only for some larger earthquakes, and not in the catalogue, but in a table in the text of the first volume. 6.4. soil mechanics the study by soil mechanics (1982) was conducted in parallel to the principia (1982) one. rather than producing any catalogue, this study took a representative sample of 72 earthquakes from all parts of the u.k. and all time periods, and subjected each to a more thorough historical study, using more sources, than was possible within the scope of the principia (1982) report. intensities are assessed using the msk scale, and magnitudes are estimated using seven different formulae taken from the literature. the parameters presented for the sample are quite detailed, including full sets of isoseismal radii, and depths. depths are estimated using the kövesligethy (1906) method (though other authors are referenced). soil mechanics counsel caution in handling macroseismic depth determinations; not surprising when their derived depth for the 1863 hereford earthquake is 105 km. the situation with regard to idps is no different to the principia (1982) study. 6.5. burton, musson and neilson the bgs study that was conducted at the same time as principia (1982) and soil mechanics (1982) was spread over three years instead of one, and published as burton et al. (1984a). in outline it was similar to the study of soil mechanics in taking a sample of events; however, the sample was larger (102 against 72; one turned out to be a fake event) and contained most of the larger earthquakes after 1700. this study was different in two important respects. first, the original source data on which intensity assessments were based were transcribed and reproduced verbatim, arranged by place. this means that the reports issued by this project contain the actual evidence of what occurred during each earthquake alongside the interpretations that are given in the form of intensity values. secondly, the idps (msk val605 a critical history of british earthquakes ues) for each earthquake are listed in full. this means that further analysis of the data collected by burton et al. (1984a) is relatively easy, whereas for principia (1982) and soil mechanics (1982) it is practically impossible. parameters other than epicentral intensity were not estimated for earthquakes in this study. owing to the sheer volume of the verbatim source material, the initial product of this study numbered fifteen volumes (burton et al., 1984ab, musson et al. 1984a-f; neilson et al., 1984b,c). further volumes, in more or less the same style, were issued in subsequent years, covering additional earthquakes (musson et al., 1986a,b; musson and redmayne, 1986; neilson et al., 1986; musson, 1987, 1989a,b, 1991, 1998). 6.6. ambraseys and melville the study by ambraseys and melville (1983) is concerned principally with earthquakes in the eastern part of the u.k., and only with earthquakes before 1800. parameters are not given, except for some imax values and radius at which events were perceived at intensities between 3 and 4 msk. idps are not listed. the important feature of this study is that it disentangled the complexities of dating of medieval earthquakes, and thus cleared up many of the fake earthquakes (from dating errors) from the earlier part of the u.k. catalogue once and for all. 6.7. other studies while the foregoing paragraphs encompass the major studies of historical earthquakes in the u.k. that were completed in the 1980s, a number of other noteworthy studies were also completed and/or published. all of these meet a high standard in use of primary source material and critical attention to sources. some of these were conducted as part of npp safety cases, were not published, and cannot be referred to. amongst published studies one may note melville (1985), which discusses geographical factors affecting the transmission of historical earthquake data and gives a catalogue of 18th century earthquakes in england. parameters include epicentral co-ordinates, imax (msk) and the radii of isoseismals 3 and 4 msk. melville (1986) presents descriptions for some nw england earthquakes. a study by musson (1986) discusses in a general way the use of newspaper data for studies of historical british earthquakes. ambraseys (1985) includes intensity maps for some of the larger historical earthquakes in nw europe (including the u.k.) and a parametric catalogue for 1800-1984, giving instrumental magnitudes (only), i0 (msk), depth and felt area. this was further developed in ambraseys (1988) in which a parametric catalogue for the u.k. is presented for the period 1700-1984, for events with magnitude larger than 4.0 ms. 6.8. synthesis at the end of the 1980s, the situation with regard to the understanding of historical british earthquakes was completely transformed with respect to what it had been ten years before. the amount of data researched and published was now very large, and based almost entirely on primary historical source material. earlier catalogues were used only as starting points for research. what was now needed was synthesis of the data already uncovered in the form of complete numerate catalogues. the first attempt at a synthetic catalogue was that of ove arup (1993), which draws on all the studies already listed, and also some unpublished investigations from site-specific safety cases by other authors. this study derives parameters (excluding depth) for events larger than 4 ms since 1000. maximum intensities are given as msk values, and areas for isoseismal 4 msk are also listed. some earthquakes below 4 ms are included (down to 3.7 ms) and the total number of events is 106. a problem with this catalogue is that, in the drive to make it fully numerate, the authors proceeded to estimate parameters for some earthquakes on data so slender that no weight can be given to the results. one earthquake in the 11th century is given a magnitude, an imax, 606 roger m.w. musson an area of isoseismal 4, and an epicentre to five kilometres, when all that is known about it is the date and that it happened «in england» and was «great». the text states that these values are «very approximate»; the problem is that it often happens that a parametric catalogue becomes separated from its accompanying text, and that someone will attempt to draw conclusions from these parameters regardless of the fact that the uncertainty is so high that the epicentre could be almost anywhere in england. the second such catalogue is that of musson (1994), which supplements the published studies listed above with archive data. it can be divided into sections as follows: for the period before 1700 only events considered to be larger than 4 ml are listed, and parameters are not estimated where these are very uncertain. after 1700 all events above 3 ml are listed, and some events less than 3 ml where these are particularly interesting. this extension to lower magnitudes means that this catalogue contains many more parametric data than the previous ones listed, fig. 2. current u.k. seismicity map (historical and modern data combined). symbol size is proportional to magnitude as shown in the key; shading is related to depth (where known), with shallower events having lighter shading. events with very uncertain epicentres (most medieval earthquakes) are not plotted. 607 a critical history of british earthquakes and is thus better for purposes such as estimating magnitude recurrence. it also integrates with data from modern microseismic monitoring. the derivation of parameters is described by musson (1996b). magnitudes are derived from analysis of felt area or outer isoseismals. depths are estimated by the kövesligethy (1906) method. intensities are cited as ems values. in some cases these are msk values converted on the basis of one-to-one, sometimes with split values (e.g., 6-7 msk) rounded down. the number of earthquakes in the catalogue is 502, including modern data. the weaknesses of this catalogue are two in number. firstly, more could be said about medieval and early earthquakes, the focus of the catalogue being really on the period after 1700. in particular, the distinction between medieval events deemed to be probably above 4 ml (and therefore included) and those probably not (and therefore excluded) is rather subjective. secondly, the documentation of sources, where entries are not derived wholly from published studies, leaves much to be desired. a report on source material for the beecd project (musson, 1996c) gives some improvement, but an expansion of musson (1994) to include more detailed, referenced descriptions is still needed. an unpublished extension of musson (1994) includes data since 1994, revisions to historical parameters and historical earthquakes discovered since 1994, and also parameters for many historical earthquakes smaller than 3 ml. this version is integrated with the bgs earthquake database (walker, 2000). the seismicity of the u.k., as shown in this combined historical and modern data set, is shown here as fig. 2. following the beecd project (stucchi et al., 1999) a database of idps for almost all the major british earthquakes now exists. the idea was proposed some years ago that a comparative database of idps should be compiled, including intensity estimates made in the context of other historical studies made in the u.k.. however, it turned out that (as already mentioned), most of the other studies entered data points directly onto maps. the task of trying to construct tables of idps from maps is difficult and attempting it unrealistic; thus for the foreseeable future the only available idps are those from the various bgs studies. 6.9. outlook further historical earthquake research in the u.k. is becoming a matter of diminishing returns. it is still possible to uncover reports of previously unknown earthquakes, but all those recently found have been small events, usually 3 ml or less. a recent example can be given of the 1809 cowbridge earthquakes, which turned up in the course of archive work in 2001. these two earthquakes, on 15 july 1809 and 11 august 1809, are described in a collection of miscellaneous mss in the national library of wales (ms reference 13121, ff 440, 453). the second of the two was strong enough to cause the fall of plaster. the felt area is described in detail, is sw of cardiff, and is about 230 km2 in size. the prospect of major discoveries in historical british earthquake studies is very low, and the parametric record of british earthquakes seems to be as good as it is ever likely to be, thanks to the quantity and quality of historical research accomplished in the last 25 years. references (*) ambraseys, n.n. (1985): intensity-attenuation and magnitude-intensity relationships for northwest european earthquakes, earthquakes eng. struct. dyn., 13, 733-778. ambraseys, n.n. (1988): engineering seismology, earthquakes eng. struct. dyn., 17, 1-105. ambraseys, n.n. and c. melville (1983): the seismicity of the british isles the north sea (serc marine technology centre, london). anon (1750): account of earthquakes in england, gent. mag., 20, p. 56. batman, s. (1581): the doome, warning all men to the judgemente (east london). bonito, m. (1691): terra tremante overo continuatione dè terremoti dalla creazione del mondo fino al tempo presente (parrino and mutii, naples), pp. 822. boyle, r. (1666): a confirmation of the former account touching the late earthquake near oxford, and the concomitants thereof, philos. trans. r. soc. london, 1, 179-181. browitt, c.w.a., p.w. burton and r. lidster (1976): seismicity of the inverness region, igs global seismol. un., rep. no. 76. (*) bgs reports are available from the bgs bookshop, kingsley dunham centre, keyworth, nottingham, ng12 5gg; or sales@bgs.ac.uk 608 roger m.w. musson burton, p.w., r.m.w. musson, and g. neilson (1984a): studies of historical british earthquakes, bgs global seismol., rep. no. 237. burton, p.w., r.m.w. musson and g. neilson (1984b): macroseismic report on historical british earthquakes iv: lancashire and yorkshire, bgs global seismol. rep. no. 219, 2 vols. burton, r. (1734): the general history of earthquakes: being an account of the most remarkable and tremendous earthquakes that have happened in divers parts of the world, from the creation to this time; as they are recorded by sacred and common authors; and particularly those lately in naples, smyrna, jamaica and sicily (bettesworth, london). butler, r. (1849): the annals of ireland by friar john clyn and thady dowling (irish archæological society, dublin). davison, c. (1900): scales of seismic intensity, philos. mag. 5th ser., 50, 44-53. davison, c. (1924): a history of british earthquakes (cup, cambridge). dollar, a.t.j. (1950): catalogue of scottish earthquakes, 1916-1949, trans. geol. soc. glasgow, 21 (2), 283-361. fleming, a. (1580): a bright burning beacon, forewarning all wise virgins to trim their lampes against the coming of the bridegroome, conteining a generall doctrine of sundrie signes and wonders, specially earthquakes both particular and generall: a discourse of the end of this world: a commemoration of our late earthquake, the 6 of april, about 6 of the clocke in the evening 1580, and a praier for the appealing of gods wrath and indignation (denham, london). goad, j. (1686): astro-meteorologica (blagrave, london). grey, z. (1750): a chronological and historical account of the most memorable earthquakes that have happened in the world from the beginning of the christian period to the present year 1750, with an appendix, containing a distinct series of those that have been felt in england, and a preface, seriously address’d to all christians of every denomination (bentham, cambridge). grünthal, g. (editor) (1998): european macroseismic scale 1998 (ems-98), in cahiers du centre europèen de gèodynamique et de séismologie (luxembourg), 15, pp. 99. hoff, k.e.a. von (1840): chronik der erdbeben und vulkanausbrüche, i. theil (justus, perthes, gotha), pp. 470. holinshed, r. (1577): the firste volume of the chronicles of england, scotlande, and irelande, conteyning, the description and chronicles of england, from the first inhabiting vnto the conquest, the description and chronicles of scotland, from the first originall of the scottes nation, till the yeare of our lorde 1571, the description and chronicles of yrelande, likewise from the firste originall of that nation, vntill the yeare 1547 (bishop, london). holinshed, r., w. harrison, stanyhurst, r. and i. hooker (editors) (1587): the first and second volumes of chronicles: comprising 1. the description and historie of england, 2. the description and historie of ireland, 3. the description and historie of scotland: first collected and published by raphaell holinshed, william harrison, and others; now newlie augmented and continued (with manifold matters of singular note and worthie memorie) to the yeare 1586 by iohn hooker alias vowell gent. and others; with conuenient tables at the end of these volumes (denham, london). kövesligethy, r. de (1906): a makroszeizmikus rengések feldolgozása, math. és természettudományi értesítõ, 24, 349-368. lilwall, r.c. (1976): seismicity and seismic hazard in britain, igs seismol. bull., 4 (hmso, london). mallet, r. (1853-1855): catalogue of recorded earthquakes from 1606 b.c. to a.d. 1850, brit. ass. rep., 1853, 1-176; 1854, 118-212; 1855, 2-236. meldola, r. and w. white (1885): report on the east anglian earthquake, essex field club spec. mem., 1. melville, c.p. (1985): the geography and intensity of earthquakes in britain the eighteenth century, in earthquake engineering in britain (telford, london). melville, c.p. (1986): historical earthquakes in northwest england, trans. cumb. west. ant. arch. soc., 86, 193219. melville, c.p., a. levret, p. alexandre, j. lambert and j. vogt (1996): historical seismicity of the straits of dover-pas de calais, terra nova, 8, 626-647. milne, d. (1842): notices of earthquake-shocks felt in great britain, and especially in scotland, with inferences suggested by these notices as to the causes of the shocks, edinburgh n. phil. j., 31, 92-122. milne, j. (1911): a catalogue of destructive earthquakes a.d. 7 to a.d. 1899, baas (london), pp. 92. mourant, a.e. (1931). earthquakes of the channel islands and neighbouring countries, société jersiaise (st helier). musson, r.m.w. (1986): the use of newspaper data in historical earthquake studies, disasters, 10 (3), 217-223. musson, r.m.w. (1987): seismicity of southwest scotland and northwest england; with a catalogue of earthquakes within 75 km of chapelcross, bgs global seismol., rep. no. 316. musson, r.m.w. (1989a): seismicity of cornwall and devon, bgs global seismol., rep. no. wl/89/11. musson, r.m.w. (1989b): seismic hazard assessment for fort william, bgs seismol., rep. no. wl/9/39. musson, r.m.w. (1991): macroseismic effects of the 17 march 1871 kendal earthquake, bgs global seismol., rep. no. wl/91/17. musson, r.m.w. (1993): discovery of a curious seismological monument from 19th century scotland, terra nova, 5, 513. musson, r.m.w. (1994): a catalogue of british earthquakes, bgs global seismol., rep. no. wl/94/04. musson, r.m.w. (1996a): the seismicity of the british isles, ann. geofis., xxxix (3), 463-469. musson, r.m.w. (1996b): determination of parameters for historical british earthquakes, ann. geofis., xxxix (5), 1041-1048. musson, r.m.w. (1996c): roots and references for the u.k. earthquake catalogue, bgs global seismol., rep. no. wl/96/03. musson, r.m.w. (1998): historical seismicity of the western frontiers area, bgs global seismol., rep. no. wl/98/26. musson, r.m.w. (2002): historical earthquakes of the british isles, in international handbook of earthquake quakes v: midlands, bgs global seismol., rep. no. 228, 2 vols. neilson, g., r.m.w. musson and p.w. burton (1984c): macroseismic reports on historical british earthquakes vi: the south and southwest of england, bgs global seismol., rep. no. 231. neilson, g., r.m.w. musson and p.w. burton (1986): macroseismic reports on historical british earthquakes xi: 1931 jun 7 north sea, bgs global seismol., rep. no. 280. ockenden, r.e. (1936): thomas twynès discourse on the earthquake of 1580 (pen-in-hand publishing, oxford). ove arup (1993): earthquake hazard and risk in the u.k., (dept. of environment, london). perrey, a. (1849): sur les tremblements de terre dans les iles britanniques, ann. sci. phys. nat. (soc. nat. d’agr. de lyon), 2nd series, 1, 115-177. principia mechanica ltd. (1982): british earthquakes (pml, cambridge). roper, w. (1889): a list of the more remarkable earthquakes in great britain and ireland during the christian era (t. bell, lancaster). short, t. (1749): a general chronological history of the air, weather, seasons, meteors, etc. (longman and miller, london). soil mechanics ltd. (1982): reassessment of u.k. seismicity data (sml, bracknell). stucchi, m., p. albini, r. camassi, r.m.w. musson and r. tatevossian, (1999): main results of the project «beecd»: a basic european earthquake catalogue and a database for the evaluation of long-term seismicity and seismic hazard, rep. to the ec, project ev5v-ct94-0497. tiedemann, h. (1991): catalogue of earthquakes and volcanic eruptions, (swiss re, zürich). tillotson, e. (1974): earthquakes, explosions and the deep underground structure of the united kingdom, j. earth sci., 8, 353-364. van gils, j.m. (1988): catalogue of european earthquakes and an atlas of european seismic maps, cec rep. no. eur 11344 en. walker, a.b. (2000): u.k. earthquake monitoring 1999/2000: bgs seismic monitoring and information service, eleventh annual report, bgs technical rep. no. wl/00/03. wallis, j. (1666): a relation concerning the late earthquake neer oxford; together with some observations of the sealed weatherglass, and the barometer, both upon that phaenomenon, and in general, philos. trans. r. soc. london, 1, 166-171. 609 a critical history of british earthquakes and engineering seismology, edited by w.h.k. lee, h. kanamori, p.c. jennings and c. kisslinger (academic press, san diego). musson, r.m.w., g. neilson, and p.w. burton (1984a): macroseismic reports on historical british earthquakes i: northwest (england and southwest scotland), bgs global seismol., rep. no. 207, 2 vols. musson, r.m.w., neilson, g. and p.w. burton (1984b): macroseismic reports on historical british earthquakes ii: (north wales and irish sea), bgs global seismol., rep. no. 208. musson, r.m.w., neilson, g. and p.w. burton (1984c): macroseismic reports on historical british earthquakes iii: (central and west scotland), bgs global seismol., rep. no. 209. musson, r.m.w., g. neilson, and p.w. burton (1984d): macroseismic reports on historical british earthquakes vii: (north scotland), bgs global seismol., rep. no. 232. musson, r.m.w., g. neilson, and p.w. burton (1984e): macroseismic reports on historical british earthquakes viii: (south wales), bgs global seismol., rep. no. 233, 2 vols. musson, r.m.w., g. neilson and p.w. burton (1984f): macroseismic reports on historical british earthquakes ix: dover straits. bgs global seismol., rep. no. 234. musson, r.m.w., g. neilson and p.w. burton (1986a): macroseismic reports on historical british earthquakes xii: 1927 january 24 (north sea), bgs global seismol., rep. no. 281. musson, r.m.w., g. neilson and p.w. burton (1986b): macroseismic reports on historical british earthquakes xiii: 1852 november 9 (irish sea), bgs global seismol., rep. no. 283. musson, r.m.w. and d.w. redmayne (1986): historical earthquakes in the dunoon area, bgs global seismol., rep. no. 312. musson, r.m.w., g. neilson and p.w. burton (1987): macroseismic reports on historical british earthquakes x: the great glen, bgs global seismol., rep. no. 347. musson, r.m.w., g. neilson and p.w. burton (1990): macroseismic reports on historical british earthquakes xiv: 22 april 1884 colchester, bgs seismol., rep. no. wl/90/33. neilson, g., r.m.w. musson and p.w. burton (1984a): the «london» earthquake of 1580 april 6, eng. geol., 20, 113-142. neilson, g., r.m.w. musson and p.w. burton (1984b): macroseismic reports on historical british earthannals 47, 5, 2004 1617 annals of geophysics, vol. 47, n. 5, october 2004 key words mediterranean – precipitation – principal component – canonical correlation – trajectory 1. introduction the mediterranean is characterised by hot, dry summers and warm, wet winters. the winter precipitation variability patterns of the mediterranean region, which extends 3600 km from the iberian peninsula in the west to israel in the east, show marked differences between the north-western and the south-eastern areas (e.g., conte et al., 1989; douguedroit, 1998). this variability can be described as a teleconnection; an anti-correlation between the rainfall in iberia and that in the near east. what influences the operation of the mediterranean precipitation teleconnection? convergence of moisture and regional precipitation is controlled by the dominant atmospheric pressure structures. the major sources of water close to the mediterranean region are the atlantic ocean, the mediterranean sea itself, and further east the arabian sea. consequently pressure systems over the north atlantic, europe and north africa must all be considered in answering the above question. hurrell (1995) has shown that changes in the north atlantic pressure system, indicated by extremes of the north atlantic oscillation (nao), strongly alter the moisture transport across the atlantic ocean and onto the european landmass. the nao is a measure of a pressure teleconnection and is defined as a large-scale almediterranean precipitation and its relationship with sea level pressure patterns roy thompson and david n. green school of geosciences, the university of edinburgh, u.k. abstract the relationship between mediterranean precipitation and north atlantic and european sea level pressure fields has been studied using statistical techniques to investigate the variability within the data. a principal component analysis shows the major winter precipitation variability is described by a see-saw fluctuation between the western and eastern mediterranean. the pressure-precipitation relationships indicate that a highly variable, pressure region situated to the south of britain dominates this major precipitation pattern. the large-scale pressure fields which facilitate the precipitation patterns have been isolated using a canonical correlation analysis. although the well-known major pressure centres of action in the north atlantic are important, pressure changes in the east are found to also control the transport of moisture across the mediterranean to a large degree, as the presence of a large high over kazakhstan causes meridonial flow and impedes the passage of moisture across the mediterranean. the pressure-precipitation relationships are found to be very consistent over multi-decadal, seasonal, monthly and daily time-scales with trajectory analysis confirming many of the features of the average seasonal pressure charts. this steadiness and regularity indicates that the mediterranean precipitation teleconnection is a robust phenomenon that is affected by large-scale pressure changes to both the east and west. mailing address: prof. roy thompson, school of geosciences, the university of edinburgh, grant institute, the king’s buildings, west mains road, edinburgh eh9 3jw, u.k.; e-mail: roy@ed.ac.uk 1618 roy thompson and david n. green ternation of atmospheric mass between regions of subtropical high pressure (the azores high) and subpolar low pressure (the icelandic low) in the north atlantic (e.g., lamb and peppler, 1987). the nao can be expected to contribute to explaining the dominant variations in the western mediterranean (e.g., zorita et al., 1992; storch et al., 1993; rodo et al., 1997; trigo and palutikof, 2001), however there may well be other pressure systems influencing the precipitation in the mediterranean basin. the spatio-temporal precipitation patterns of local regions within the mediterranean have been described in some detail. good summaries of precipitation and its relationships with circulation are available for the central (maheras et al., 1992; bordi et al., 2001; bordi and sutera, 2002), northwestern (zhang et al., 1997; ulbrich et al., 1999) and eastern (kuteil and paz, 1998; kuteil et al., 2002) mediterranean. these studies point to the importance of both regional and local topographic influences. here we include analyses of the less well-studied areas of northern africa, so increasing the geographical coverage, and concentrate on mediterranean wide effects. we are primarily concerned with long-term (multi-decadal) changes and variability and so only analyse sites with published precipitation data that extend as far back as at least 1905. the main objective of this paper is to study the extent and temporal behaviour of mediterranean precipitation patterns and their relation to sea level pressure situations over europe and the north atlantic. the main techniques employed are canonical correlation and principal component analysis. 2. methodology an overview of the steps employed in our analyses is given in the flow chart of fig. 1. monthly values of precipitation for stations around the mediterranean region spanning the time period 1905-1990 were obtained from the global historical climatology network database (vose et al., 1992). the winter months, december through to march, were chosen for study as these are the classic months for mediterranean precipitation and because more pronounced precipitation and pressure variability occurs in the winter season (hurrell, 1995). sites were selected on the basis of completeness of the records and on the need to achieve a wide geographical spread. forty-four satisfactory sites were found. we consider this data set to be the best obtainable, providing a good balance between geographical extent, completeness and series length. the collection of long-term precipitation data is fraught with difficulties and many records are discontinuous, due to either a break in recording or a change of recording site. missing data can cause severe problems in correlation and time-series analysis. this is especially true for the statistical techniques used in this project, principal component analysis and canonical correlation analysis, as both require complete data sets. 15 100 mean monthly precipitation values were available to us, however on account of the long time period under analysis there were inevitably many missing values. to form comfig. 1. the major stages of the analysis. ghcn = = global historical climatology network; cru = climate research unit, university of east anglia. 1619 mediterranean precipitation and its relationship with sea level pressure patterns plete records an imputation was executed. the most conservative approach is to use the mean of the available data. however we have fair correlation between the mean monthly precipitation at nearby sites (r 2 typically 0.55) and so a less conservative approach was preferred. we considered different geographical regions separately and then used a median polish technique (velleman and hoaglin, 1981) to estimate the missing data month by month. in practice for regions with poor site correlation (e.g., north africa; r 2∼ 0.3) our interpolation method will give results similar to using the time-series mean, while for regions with good spatial coherence (e.g., iberia; r 2∼ 0.8) results will be similar to those obtainable by multiple regression. the solid circles in fig. 2c,d are typical examples of our imputation. missing monthly means for two recent years were imputed for lisbon, while missing data for eight war years and one recent year were imputed for limassol. the winter monthly mean-sea-level pressure data were obtained on the 5° latitude by 10° longitude grid of jones (1987). we selected data extending from 50°w to 60°e and from 20°n to 65°n covering the north atlantic, eufig. 2a-d. a) the 1st canonical function (precipitation) index. b) the 1st precipitation principal component index. c) the precipitation record at lisbon, portugal. d) the precipitation record (scale inverted) at limassol, cyprus. a b c d 1620 roy thompson and david n. green rope, north africa and the western extent of asia. the data were initially checked for outlying values, which were removed. the pressure data grid was almost complete (< 1% data missing) and any outliers were replaced using the imputation scheme outlined above. to allow comparison between individual station records for both sets of data a normalisation is required to remove the bias due to some regions having larger mean values for the variables than others. as we are only concerned in the variability of precipitation and pressure, dividing each record through by the mean value of that time-series provides a simple normalisation with the advantage that all values are still positive reducing problems in the subsequent analysis. it should be noted that the normalised pressure and precipitation variables still have different ranges. in particular the coefficient of variation (standard deviation of time-series/ arithmetic mean of time-series) is approximately two orders of magnitude larger for the precipitation data compared to that of the pressure data. this causes no mathematical problems in the subsequent analyses except that certain values, as noted in the text below, are not directly comparable. the two data sets are moderately large and therefore for an efficient analysis they required a reduction in size, whilst maintaining the maximum information content possible. principal component analysis (pca) provides the ideal candidate for such data compression (e.g., jolliffe, 1990) and was applied individually to both the precipitation and pressure data sets. pca worked well on our two data sets allowing us to study only the first four principal components of the pressure data and the first three principal components of the precipitation data. the dimensions of the analysis were considerably reduced and yet 83.5% of the pressure variance and 51.2% of the precipitation variance was retained. to gain insight into the relationship between the two sets of data, pressure and precipitation, a canonical correlation analysis (cca) was performed. cca quantifies the strength of the relationship between two data sets by maximising the correlation between linear combinations of the variables (e.g.,, hair et al., 1995). the problem is to find the linear combination of precipitation variables that has maximum correlation with a linear combination of pressure variables. to simplify proceedings it is beneficial to use the two data sets in their principal component co-ordinates. from the cca a time-series can be constructed which emphasises the years at which the pressure and precipitation patterns are highly or inversely correlated. we also found it helpful to consider only the six most extreme years, for both the high and low canonical function values. for these extreme years mean pressure maps were constructed. these maps can be easily interpreted, alongside meteorological records, to show which large scale features of the pressure field give rise to the mediterranean precipitation teleconnection. storch and zwiers (1999, p. 382) provide us with a formal definition of a teleconnection pattern. the mean pressure maps can also be compared with maps of individual events (see section 3.3). it is important to emphasise that a statistical analysis based solely on principal component and canonical correlation analysis basically only serves to show that patterns of the analysed fields covary. by itself it does not demonstrate the patterns of major statistical weight are dynamically significant (being, for example, linear or non-linear normal modes); not that there is a cause and effect connection between the covarying variables; or that the dominant patterns of co-variation (as identified, by the canonical correlation analysis) are dynamically significant. so in addition to analysing mean monthly data, daily and sub-daily records were studied for specific synoptic conditions. the daily records chosen for comparison with the mean monthly charts were selected as objectively as possible. the year and month of an event were selected from the exceptionally high and low values of the first cca timeseries. the day of the event was chosen as that with the highest rainfall at either lisbon (portugal) or paphos (cyprus), depending on which polarity of the precipitation teleconnection was being considered. lisbon and paphos were chosen to represent typical west and east mediterranean sites respectively. surface pressure maps were constructed to give a synoptic overview of the conditions over 1621 mediterranean precipitation and its relationship with sea level pressure patterns europe, at the time of the precipitation event, and back and forward trajectories were produced using the british atmospheric data centre trajectory service to give an indication of the direction and speed of airflow towards and away from the two sites. for this analysis an initial pressure level of 700 mb was chosen over the two recording stations as a compromise corresponding very approximately to an average height of precipitation formation while being removed from surface boundary layer levels. the height along the trajectory path was calculated from the pressure level data using the hypsometric equation. 3. results 3.1. the mediterranean precipitation teleconnection a spatial view of the mediterranean precipitation teleconnection is provided by the 1st pca loadings map (fig. 3) giving an indication of the geographical extent of the first-order precipitation variability. the zero loading line trends south-west to north-east, with the loading size increasing with distance from the zero contour. the time series of the first principal component (fig. 2b) indicates how the variability in the precipitation across the mediterranean, shown in fig. 3, has changed over the 85 year timespan since 1905. the pca index is centred on a value of one, therefore high index years such as 1936 show unusually high precipitation in the west and low rainfall in the east, whereas low index years such as 1989 show the reverse situation. the index shows a slight tendency to rising values prior to the 1960s/1970s, and falling values thereafter. 3.2. cca and the synoptic pressure situations the canonical correlation analysis took the precipitation pca discussed above and a pca of the corresponding pressure field and calcufig. 3. the geographical form of the 1st precipitation principal component. the contours represent equal component loading values and indicate that the regions of maximum variability occur towards the west and southeast extents of the mediterranean. the smooth contours were drawn by fitting a lowess smoothing function (cleveland, 1979) to the loadings of the first principal component. black dots represent the forty-four precipitation recording stations used in the analysis. 1622 roy thompson and david n. green lated the strength of the relationship between the two sets. the resulting time series (fig. 2a) is centred on a value of 0.24, and shows the variability of the first canonical function. this cca time series is extremely well correlated with the precipitation first principal component (r = 0.86), giving an indication that the precipitation teleconnection is highly influenced by the pressure situation. the geographical patterns of the cca showing the positions of maximum response between the variability in the pressure and precipitation fields over the 85 year study are shown in fig. 4a,b. the first precipitation canonical function is remarkably similar to the first precipitation principal component indicating that the predominant mediterranean precipitation variability is well correlated with the pressure changes over fig. 4a,b. a) the geographical form of the first precipitation canonical function. the contours indicate equal loading values. increasing loading values represent increased correlation between the variations of precipitation at that locality and the pressure field. b) the geographical extent of the first pressure canonical function. the contours indicate equal loading values. increasing loading values represent increased correlation between the variations of pressure at that locality and the whole precipitation field. thus regions of high loading are particularly responsive. n.b.: the loading values in a) and b) are not directly comparable due to the choice of normalisation (see text for further details). a b 1623 mediterranean precipitation and its relationship with sea level pressure patterns the area chosen. the first pressure canonical function map indicates in which regions the pressure variability contributes most to the precipitation changes. the form of the canonical function is at first sight unexpected. rather than two centres of action over the east and west mediterranean or a single maximum lying in between the two foci of precipitation variability, major regions of pressure variation well to the north are found. one elongated focus, centred at an approximate position of 80°n and 10°w, extends from south greenland across the north pole to northern russia. it is similar in shape to the area of high correlation between portuguese rainfall and north atlantic sea level pressure as given by ulbrich et al. (1999), but with a promontory extending towards siberia. the negative polarity of loadings indicates that pressure variations in a high index year (wet in the west/dry in the east) will tend to be unusually high pressure over greenland and over northern russia. the second focus is situated over the approaches to the english channel (fig. 4a,b) and is of opposite (i.e. positive) polarity indicating that the pressure variations in this area are anticorrelated with those to the north. in high index years higher pressure than usual over the english channel guides maritime air and precipitating weather systems north westwards away from iberia. the form the canonical function takes can be clarified by examining the six most extreme years for both polarities of the index. figure 5a,b plots the mean winter sea-level pressure maps for these two sets of extreme conditions. figure 5a presents the situation causing high precipitation in the western mediterranean and low precipitation in the east. the azores high and icelandic low pressure systems are displaced south of their mean positions causing a series of isobars to run west-east directly across the iberian peninsula, providing a route for moisture from the atlantic ocean to impinge upon spain and portugal. towards the east of the mediterranean, a large high-pressure ridge (a western bulge of the siberian high-pressure fig. 5a. the mean winter pressure map associated with the six highest canonical function years (precipitation in the west, dry in the east). all contours labelled in millibars. 1624 roy thompson and david n. green system), referred to here as the kazakhstan high, provides a block causing meridional airflow. the col on the ridge between the azores and kazakhstan highs lies to the south of the mediterranean over libya. figure 5b indicates the situation leading to increased precipitation in the east and dry conditions in the west. the azores high pressure system is displaced to the north and extends over the iberian peninsula preventing airflow across the atlantic and onto the western mediterranean where below normal precipitation is recorded. consequently tight isobars trend north eastwards from the atlantic across northern scotland and then onwards across scandinavia and finland. situated in the eastern mediterranean is a distinct pressure minima, the cyprus low. this region has been linked to mediterranean cyclones (alpert et al., 1990), the associated cyclonic flow causes high precipitation in israel and the surrounding area (e.g., zangvil and druian, 1990). further east the kazakhstan high is more subdued and displaced somewhat to the south. the ridge between the azores and kazakhstan highs now lies to the north of the mediterranean with a broad col over hungary and romania. 3.3. individual events in addition to examining the 6-year mean winter circulation of the extremes of our mediterranean precipitation teleconnection, we have also analysed individual precipitation events to see how the sub-daily dynamic situation compares to the long-term average. a high rainfall event recorded in lisbon on 27th january 1979 was chosen as an example of behaviour in a year of high cca index. january 1979 was wet across iberia and dry, or moderately dry, in the eastern mediterranean (completely dry at cairo). the synoptic pressure map, trajectory path and trajectory height for this one event can be seen in fig. 6a. the pressure map for 26th january 1979 (the 24-h perifig. 5b. the mean winter pressure map associated with the six lowest canonical function years (precipitation in the east, dry in the west). all contours labelled in millibars. (i) (ii) (iii) 1625 mediterranean precipitation and its relationship with sea level pressure patterns fig. 6a. i) the pressure map for 26th january 1979. ii) the forward and back trajectories for the 700 mb level at lisbon (crosses) and paphos (open circles). the markers are placed at 6 hourly intervals. iii) the trajectory heights for lisbon (crosses) and paphos (open circles). solid symbols 00:00 on 27/01/1979. 1626 roy thompson and david n. green fig. 6b. i) the pressure map for the 13th february 1990. ii) the forward and back trajectories for the 700 mb level at lisbon (crosses) and paphos (open circles). the markers are placed at 6 hourly intervals. iii) the trajectory heights for lisbon (crosses) and paphos (open circles). solid symbols 00:00 on 14/02/1990. (i) (ii) (iii) 1627 mediterranean precipitation and its relationship with sea level pressure patterns od leading up to the precipitation event) shows a well formed high in the east extending from kazakhstan restricting the mediterranean airflow and generating anticyclonic circulation across the eastern mediterranean sea. the icelandic low pressure system and the azores high are displaced much further to the south than their mean positions so allowing depressions to pass across the atlantic along more southerly tracks than normal onto the iberian peninsula. the 26th sees lows passing over the canaries and the faeroes and a cold front approaching lisbon. the 3-day back trajectory for lisbon shows cyclonic circulation and rising air arriving from the sw (ii and iii in fig. 6a). the trajectory is consistent with rainfall in lisbon and with the air passing low over the atlantic gathering moisture before rising to condense and precipitate over the land. the forward trajectory from lisbon shows the air continuing to ascend and being carried north-west away from the mediterranean illustrating how the moisture from the atlantic does not reach israel or cyprus in such high index years. the corresponding trajectory path for paphos, cyprus for the same six-day period shows dry air circulating around the northern edge of an anticyclone before being transported off the sahara and passing towards the north-east. the flow corresponds well with the winter trajectories studied by dayan (1986). turning to a low canonical function year. paphos experienced a high rainfall event on the 14th february 1990 (fig. 6b). the pressure map shows a large area of high pressure centred over the canaries and extending over the iberian peninsula with depressions crossing to the north of scotland and a low pressure system extending into the eastern mediterranean. there is a high pressure system to the east extending from the persian gulf to kazakhstan, however this is much smaller than in the example above. a series of lows pass through the east mediterranean around this time. one formed over north italy on the 11th and during the 13th to 15th another passed to the north of cyprus, an associated cold front producing the torrential rain in paphos on the 14th. the trajectory paths show air movement consistent with this pressure situation. air crossing the atlantic is forced around the high pressure system, passing over the bay of biscay into the western mediterranean. this air has been at a low height across the ocean gathering up water, as it passes into the mediterranean it rises slowly until it reaches approximately 3000 m over cyprus, becoming the source for the precipitation event. the air passing over the iberian peninsula at this time also travels across the atlantic, however it moves slowly and descends as it reaches land due to it being trapped within the azores high pressure system. this descending air has no potential for precipitation. 4. discussion 4.1. pressure influences on the teleconnection the principal component analysis shows that the mediterranean precipitation teleconnection is the dominant form of precipitation variability across the whole region in the winter months. the canonical correlation analysis provides two synoptic pressure situations that give a first order explanation of this variability. high precipitation in the western mediterranean occurs due to the moisture transport off the north atlantic impinging upon the western seaboard of north africa and iberia. this is a direct consequence of the southward shift of the icelandic low and azores high pressure systems (fig. 5a) comparable to the motions described in the work of sahsamanoglou (1990). at the same time, the kazakhstan high provides a block for any westerly airflow across the mediterranean, forcing meridional flow northwards over eastern europe and russia and allowing the eastern mediterranean to remain dry. the meridional flow causes air temperatures to be some 1.5°c warmer than average in the region extending from rumania to eastern turkey. at the same time, in north europe, a zone extending from iceland across the northern baltic to northern russia experiences air temperatures 1.5°c below average. these temperature anomalies further demonstrate the regional extent and consistency of the pressure pattern producing the mediterranean precipitation teleconnection. 1628 roy thompson and david n. green conversely with an extended high pressure system over the iberian peninsula, as in fig. 5b, spain and portugal remain dry as moisture carried off the atlantic is advected north westwards onto northern europe. air temperatures some 2.0°c above normal are produced over much of sweden and north finland by the flow from the south-west. in contrast air temperatures over eastern turkey are some 2.0°c cooler than average. the isobars surrounding the extended azores high exhibit some curvature across northern europe leading to the possibility of moisture entering the eastern mediterranean either by transportation from the atlantic or from secondary depressions formed within mediterranean areas in the lee of the alps (e.g., buzzi and tibaldi, 1978). the trajectory of fig. 6bii indicates that the former occurred on the 13th of february 1979. the cyprus low in fig. 5b indicates that depressions and cyclonic flow are present in the east which remove considerable heat and moisture from the water (shay-el and alpert, 1991). this is illustrated by the rising airmass in the trajectory of fig. 6b as it approaches the cyprus region. the areas of pressure variability which affect the precipitation teleconnection most are highlighted by the large loading regions in the first canonical correlation map (fig. 4b). the central area of high variability correlation, centred over the english channel approaches, is linked to the translation of the azores high pressure system. the english channel centre of action is the position that is affected most by this movement. when the azores high is prominent and moves northward from its mean position (approximately 30°n and 35°w: sahsamanoglou, 1990) this region underlies high pressure (e.g., fig. 5b) but when the azores high and the icelandic low move southwards the region is overlain by a low-pressure system. as explained above this alters the moisture transport across the iberian peninsula. as this variability affects the movement of moisture into and across the western mediterranean, so the pressure variability in northern russia affects the eastern end of the teleconnection. as the pressure increases in northern russia the kazakhstan high blocks any east mediterranean storm tracks and airflow across the mediterranean (fig. 5a). this coincides with the southward migration of the azores high causing low pressure to be present in the english channel. this difference in pressure between the western and eastern centres of action is illustrated by the difference in polarity between the canonical function loadings of the two centres of action in fig. 4b. a year in which there is unusually high pressure in northern russia corresponds to a year of unusually low pressure in the english channel approaches. the opposite situation occurs when the kazakhstan high wanes and the azores high strengthens and pushes northward over the english channel region. calculation of the canonical function pair of fig. 4a,b involves a long series of matrix manipulations including scaling, extraction of principal components and estimation of canonical loadings. the whole sequence is then followed in reverse to generate the individual canonical components. a very instructive exercise, which can be used to confirm that the above matrix algebra has been performed correctly, is to subtract the pressure field of fig. 5a (the six highest index years) from that of fig. 5b (the pressure field of the six lowest index years). here the result, as expected, is an almost identical pattern to that of fig. 4b. recently palutikof et al. (1996) and quadrelli et al. (2001) have investigated the relationship between mediterranean precipitation and 500 hpa geopotential height. although quadrelli et al.’s (2001) results only spanned 17 years, and so they were in doubt about the long-term robustness of their results, our 85year sea level pressure/precipitation relationships are in excellent agreement with their linkages to large-scale upper-air circulation anomalies. by contrast, palutikof et al. (1996) emphasise the 500 hpa surface over the mediterranean sea itself as an indicator of atmospheric pressure which relates to mediterranean precipitation variability. 4.2. the second canonical pair the second canonical pair highlights the major correlation between the pressure and rainfall data sets under the constraint that this 1629 mediterranean precipitation and its relationship with sea level pressure patterns correlation is orthogonal to the first canonical pair. the second precipitation canonical function has high loadings over the north african sites and loadings close to zero over portugal to the west and israel to the east. the area of high loadings is centred over the region in which there is most difference between the first principal component (fig. 3) and the first precipitation canonical function (fig. 4a). this can be explained due to the fact that the second canonical function, which perturbs the consistency between the canonical function and the principal component is describing a second order effect. the first canonical function has a r 2 value of 0.79 while the second has a r 2 value of only 0.22. the second pressure canonical function shows that the area of large pressure changes which is predominantly correlated with the second-order precipitation variation in north africa is situated well to the north of the mediterranean, with a centre over northern scandinavia. the zero contour of the map of this second pressure canonical function passes along the whole length of the mediterranean sea. so again, we find it is the broad regional or far field, pressure variations that principally orchestrate mediterranean precipitation variability. 4.3. the link between monthly and daily analyses the pressure charts in fig. 5a,b are the average of six mean winter (djfm) pressure charts chosen due to their high or low canonical index value. therefore, they are an extremely smoothed version of the actual pressure situation at any particular time within the winter period. nevertheless the main pressure patterns are robust, representative features. for example the low-pressure centre that lies between crete and cyprus, in fig. 5b, occurs as a closed contour feature in three out of the four individual winter months (december through march). a distinctive, closed contour, eastern mediterranean low is also present in five of the six individual winters that were averaged to make up fig. 5b. furthermore the mean pressure charts of fig. 5a,b can be compared with the pressure maps and trajectory analyses of fig. 6a,b that shows plots representing actual events. these type events show broadly similar features to those of the monthly analysis. the most important dynamical features consistent with high precipitation in the west and low precipitation in the east are the high over kazakhstan linked with low pressure over the atlantic, both of which are present in the daily chart (fig. 6a). this situation causes a strong east-west pressure gradient and air to flow off the ocean onto the iberian peninsula, before it passes north as the block creates meridonal flow. the equivalent air mass passing over cyprus at the same time has its origins in the dry regions of the northern sahara. the back trajectory stays at an approximately constant height allowing no advection of moisture. dayan (1986) classified this type of flow as of ‘inland north african type’. consistency between the seasonal mean and daily charts is also apparent for the opposite situation in which the eastern mediterranean is subject to higher rainfall. the high pressure over portugal in fig. 6b causes the atlantic moisture to travel north and then turn around the pressure maximum and back into the mediterranean. the low pressure system then extends into the eastern region generating cyclonic flow and transportation of moisture over israel, cyprus and coastal regions of egypt. thus as discussed by zangvil and druian (1990) rainfall over israel is largely affected by the trajectory of air prior to the rain event. maritime air trajectories are linked with rain events. however, our canonical pressure patterns would suggest that flow from the atlantic is also of importance in addition to zangvil and druian’s (1990) postulated source as moisture evaporation from the mediterranean. at these times there is also airflow across the atlantic onto lisbon, however it is travelling extremely slowly within the high pressure system and is sinking as it passes over the land (fig. 6biii), thereby allowing no precipitation. once again the fact that the same features can be identified in both the seasonal average and daily synoptic charts suggests that a stable recurrent phenomenon orchestrates the mediterranean precipitation teleconnection. 1630 roy thompson and david n. green 4.4. mediterranean precipitation and pressure teleconnections it has been well documented by previous workers that the nao causes precipitation anomalies in the western mediterranean area (e.g., lamb and peppler, 1987; hurrell, 1995). however, the present study indicates that pressure variability in the east, far removed from the north atlantic, has a large influence on the passage of moisture through the entire mediterranean region. the relative importance of the kazakhstan high and cyprus low have possibly been previously underestimated in terms of their effect on mediterranean precipitation. the moisture which forms the precipitation comes from the west as seen in all the trajectories (fig. 6a,b), however it is the presence of a block or weakened anticyclone in the east which determines the extent of any meridonial flow, and therefore the extent of any eastern mediterranean precipitation. the influence of this eastern precipitation can be seen clearly on the time series (fig. 2a-d). comparing the cca index (fig. 2a) to the two individual precipitation records (fig. 2c and 2d) it can be seen that different temporal sections have distinct features from one or either of these rainfall time-series. for example the variation in the pca index (fig. 2b) around 1945 closely follows that of the lisbon precipitation record whereas the variation around 1970 has more in common with the paphos time series, these variations can then be followed into the cca time-series. rodgers (1997) suggests that the nao has little influence on the north atlantic storm tracks and that the major changes in these tracks are consistent with sea level pressure changes in the bay of biscay region. this part of the atlantic is identifiable in fig. 4b as having a large influence on the passage of moisture from the atlantic into the mediterranean, as seen by our ‘english channel’ centre of action centred slightly northwards of rodgers’ in the bay of biscay. when there is low pressure over the bay of biscay (fig. 5a), the azores high is pushed to the south allowing moisture to pass over the atlantic and onto the iberian peninsula causing high precipitation. high pressure over the bay of biscay extends across spain and portugal reducing the precipitation in this area. the storm tracks from the atlantic in this situation pass to the north of the bay of biscay and then turn south into the mediterranean, as seen by the trajectories in fig. 6b. the influence of pressure changes over the bay of biscay is described in terms of a series of north atlantic teleconnections summarised by rodgers (1990). recently kutiel and benaroch (2002) have identified and described the upper level atmospheric teleconnection pattern (ncp) at the 500 hpa level between the north sea and the caspian areas as the most significant in the european area. the associated circulation patterns are found to have a major and direct impact on the mean surface air temperature over the eastern mediterranean. however the impact of the ncp on rainfall is more complex (kutiel et al., 2002). the strength of our canonical correlation between mediterranean precipitation variability and the large scale pressure field over an 85 year period, coupled with a very similar relationship on annual, monthly and even daily time-scales, suggest it is a robust phenomenon that would be worth investigating further. in particular more work needs to be done on the influence of pressure variations in the east, including the siberian high, before a complete picture of the link between mediterranean precipitation and sea-level pressure changes is obtainable. winter precipitation persists further east through iraq, iran, turkestan and on through tashkent. however, the precipitation teleconnection patterns are rather weak through this area. to the south winter precipitation patterns change rapidly and there is less scope for additional studies. we were unable to use records on the desert margins (even sites such as cairo on the nile valley proved unworkable) i.e. our analyses in ne africa were largely restricted to the classic mediterranean zone which only occupies a very narrow coastal strip where the libyan and egyptian deserts encroach on the mediterranean sea (quezel, 1981). the strong canonical pressure loadings over greenland and russia, both areas well to the north of the mediterranean, point to the need of also extending the geographical coverage of the sea-level pressure variations and precipitation patterns northwards. indeed we have 1631 mediterranean precipitation and its relationship with sea level pressure patterns subsequently found that winter precipitation along the eastern coast of the black sea correlates inversely with precipitation in northern spain and southern france in a very similar fashion to that of the east and west mediterranean. more speculatively, links with other teleconnections could be explored. interestingly we find a significant correlation between the first canonical function and the indian ocean dipole. this link, with a pattern of sea surface temperatures within the indian ocean (saji et al., 1999), further suggests that an interaction from the east plays an important role in creating the correct conditions for the mediterranean precipitation teleconnection. the well-documented southern oscillation shows no link with the first canonical function but does have a significant relationship with the second. this connection, harnessed with the knowledge that precipitation over north-west africa is correlated with the southern oscillation (ropelwski and halpert, 1987) and that spring precipitation in spain shows the most significant relationship with the el niño-southern oscillation in europe (rodo et al., 1997), suggests that less pronounced mediterranean precipitation variations may have links with various global pressure phenomena. 4.5. statistical identification of pressure influences the findings of this paper are based on the identification of correlations between two large data sets. the multivariate statistical methods used provided an extremely powerful tool, allowing clear pictures to be obtained of the variability. the consistency of results throughout are remarkable, with multi decadal (86-year), winter, monthly and daily pressure charts all showing the same broad features and highlighting the phenomena giving rise to the precipitation teleconnection. the consistency is such that even smaller local features such as the cyprus low are identifiable even on maps of 6year (24-month) winter averages. consistency between independent statistical techniques is also found. the first precipitation principal component indicates the major pattern of internal variability within the rainfall data (fig. 3). this produces an extremely similar variability map to that of the first canonical function (fig. 4a), which describes the correlation of the variability between the precipitation and pressure data. these two types of analysis do not necessarily produce the same geographical patterns as the former derives from climatic variability within a climatic parameter, while the later derives from the relationship between parameters. 5. conclusions the major pattern of precipitation variability in the mediterranean is shown to be orientated such that it is dipolar with maximum fluctuations in the north-west and south-east. the zero variability line in the first principal component strikes in a south-west/north-east direction separating the iberian peninsula and northern europe from the eastern mediterranean and northern africa. canonical correlation analysis shows that the european surface pressure field and mediterranean precipitation field variations are highly correlated with a north-west/south-east precipitation see-saw and an anomalous, highly variable, pressure region situated to the south of britain. pressure maps for the high index years of the canonical function show that a high pressure system over west kazakhstan causes meridional flow, blocking precipitation from the eastern mediterranean. a simultaneous translation of the azores high to the south allows westerly flow from the atlantic to pass over the iberian peninsula increasing precipitation. pressure maps for the low canonical function show the azores high translated northwards over the iberian peninsula, thereby reducing precipitation in the western mediterranean. these average pressure maps also indicate the presence of a depression, the cyprus low, situated in the eastern mediterranean. its location is in good agreement with the findings 1632 roy thompson and david n. green of zangvil and druian (1990) who linked the phenomenon with high precipitation in israel. the winter pressure and precipitation interaction in the mediterranean is correlated with the pressure variations across the north atlantic i.e. the north atlantic oscillation. a further link with the indian ocean as described by the indian dipole mode is also found to exist. our results show excellent consistency between all four individual winter months and the total season (djfm) over the 86-year period studied, as well as for single precipitation events. the 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(received december 15, 2003; accepted march 4, 2004) layout 6 1 annals of geophysics, 63, 5, pa551, 2020; doi:10.4401/ag-8096 electromagnetic contribution to the resilience improvement against the vrancea intermediate depth earthquakes, romania dragoș armand stănică1, dumitru stănică*,1, monica valeca2, ștefan iordache3 (1) institute of geodynamics of the romanian academy, jean-lois calderon, str. 19-21, bucharest, romania, www.geodin.ro, armand@geodin.ro, dstanica@geodin.ro (2) institute of nuclear research, câmpului str.1, ro-115400, mioveni argeș, romania, www.nuclear.ro, monica.valeca@nuclear.ro (3) university of bucharest, faculty of geography, n. bălcescu blvd. 1, romania, www.unibuc.ro, st.iordache@gmail.ro article history: received january 18, 2019; accepted january 27, 2020 abstract in this paper we used the geomagnetic data, collected in real time on the intervals augustseptember and november-december, 2016, to emphasize possible relationships between the anomalous behavior of the normalized function bzn and the both m5.7 and m5.6 earthquakes, generated at 72 km and, respectively 71 km depth, in the seismic active vrancea zone on september 24 and december 28, 2016. daily mean distributions of the bzn and its standard deviation (sd) are obtained for the both time-intervals, in the ulf frequency range 0.001hz 0.0083hz, by using the fft band-pass filtering. we investigate the singularities of the pre-seismic anomalous signals related to the m5.7 and m5.6 earthquakes applying a statistical analysis based on a standardized random variable equation, and the results are presented as bz* time series performed on the new time intervals 1-30 september and 1-31 december, 2016. finally, two pre-seismic anomalous signals are observed: first one on september 21, with values greater than 5 sd, what means a lead time of 3 days before the onset of m5.7 earthquake; the second one, with values larger than 4 sd, which was identified on december 21 with 7 days prior to m5.6earhquake. in conclusion, as the work-station has specific programs for data processing, analyses and real time (daily) data display on the institute website, it may be used as an early warning system able to provide useful information for resilience improvement against the vrancea intermediate depth seismicity. keywords: electromagnetic pre-seismic signals, mw 5.7 and mw 5.6 earthquakes, vrancea zone, romania, resilience improvement. 1. introduction it is well known that the assessment of natural hazard and risk generally aims at analyzing potential impact of specific processes to a rather well balanced system, in order to emphasize to what extent it might be affected in the future. in this context, the application of the electromagnetic (em) methods to identify the precursory parameters related to the earthquakes could be very useful tool for the resilience improvement. thus, the basic features of the earthquake preparation stage related to the em pre-seismic anomalous signals have been analyzed, and some of them are presented farther on. according to gufeld et al. [1999], the degassing model of the earth could be one of main actors controlling seismicity processes and energy transfer in the lithosphere. this model, based on laboratory experiments, includes the ascending diffusion effect of helium, hydrogen and possible other gases belonging to the crystalline structure of the rocks. although, the origin of the ulf geomagnetic signal is not well-known yet, the following generation mechanisms may be also considered: a) magneto-hydrodynamic effect, which supposes that the conducting fluid flow, in the presence of a magnetic field, generates a secondary induced component [sasay, 1991]; b) piezo-magnetic effect, based on the idea that a secondary magnetic field is induced by changes in ferromagnetic rocks magnetization, due to an applied stress [fitterman, 1978]; c) electrokinetic effect, based on electric currents flow at the interface solid-liquid boundaries, which in turn may generate a magnetic field [fitterman, 1978; varotsos at al., 1986]; d) piezo-stimulated current and current generated by charged dislocation [varotsos at al., 1986]. another concept [freund, et al.,1999, freund, 2000], according to which the most of rock forming the mineral composing lithosphere can emanate molecular hydrogen, as result of the impurity of ohin their crystalline structures. some authors [nomikos et al. 1997; nomikos and vallianatos, 1997, 1998; vallianatos and tzanis, 1998, 1999; vallianatos et al. 2004; tramutoli et al. 2005;varotsos, 2005; yen, et al., 2004 and ouzounov et al., 2007] suggest that in the earth’s lithosphere, the well conducting channels (deep faults) do exist and may generate continuous intersecting geotectonic systems, so that the research on the relation between spatial temporal changes of the electrical conductivity and seismic events play an important role in hazard evaluation. possible scaling laws between the electric earthquake precursors and earthquake magnitude, as well as between spectral properties and ulf signals have been done by vallianatos and a. tzanis, [2003 and 2004]. it was just demonstrated [ismail-zadeh et al., 2000; stanica et al., 2004; stanica and stanica, 2012], that before an earthquake initiation, the high stress reached into the vrancea seismogenic volume may generate dehydration of the rocks and fracturing processes, followed by release of electric charge which are propagated along the faulting systems, acting as high conductive path known as the carpathian electrical conductivity anomaly (ceca), what lead to resistivity changes [pinna et al., 1992]. on the base of the em parameters carried out throughout the ulf range, these changes of resistivity will be analyzed in this paper [stanica and stanica, 2010, 2011; and stanica et.al, 2015]. taking into account that the seismic-active vrancea zone is one of the “hot” subjects in this direction, a specific em methodology able to emphasize the short–term em precursory parameters, associated to the two intermediate depth earthquakes of mw 5.7 and mw 5.6 generated in 2016, on september 24 and december 28, respectively, will be applied. according to the em information acquired in a span of several years correlated with the vrancea seismicity, it was relieved that: (i) some days before the earthquake occurrence, the daily mean distribution of the em parameters has an anomalous behavior marked by a significant increase/decrease value versus its normal trend identified in non-seismic conditions; (ii) applying a statistical analysis, based on the standardized random variable equation, it was created possibility to identify with high accuracy the pre-seismic anomalous intervals, some days before the onset of the both seismic events (mw 5.7 and mw 5.6); (iii) the topic of resilience could not be fully understood analyzing only by one methodology or by disciplines separately, due to the inherent complexity of the vrancea earthquakes generation mechanisms and the need to solve societal problems. consequently, in this paper some innovative contributions to the resilience improvement against the intermediate depth earthquakes by using em methodology will be promoted. 2. geodynamics and seismicity of the vrancea zone 2.1 geodynamic models the seismicity of the vrancea region, placed in the carpathians bending zone, is characterized by the occurrence of intermediate depth earthquakes in a specific narrow epicentral area and hypocentral volume and, starting with the end of the last century up to now, several tomographic images have been elaborated. inverting teleseismic events recorded by the romanian earthquake network, a low velocity structure situated between 40 and 60 km depth have been identified [oncescu et al., 1984]. dragoș armand stănică et al. 2 high velocity material situated between 80 and 170 km depth, representing the level of intermediate depth seismicity, was located [oncescu, 1984; koch, 1985]. the oceanic lithosphere breaking off took place after the beginning of the collision and thus the resulting slab, almost sub-vertical, have been emphasized [constantinescu and enescu, 1984]. analyzing the seismic behavior of the vrancea zone, a model based on the existence of two active zones located at depths of 80 -110 km and 120-170 km has been proposed [trifu and radulian,1989], and the both zones characterized by local stress inhomogeneity are capable of generating large earthquakes. an image of the entire mediterranean area including se europe reveals very important feature as a phenomenon of slab detachment in thyrrenian area, and weak indications of slab detachment are also visible for the carpathian arc bend [wortel and spakman, 1992; spakman et al., 1993]. with a larger set of regional earthquakes and records it was confirmed that the intermediate seismogenic volume is characterized by high velocity [fan et al., 1998]. taking into consideration that the geometry of the subduction zone was not unequivocally defined, four possible configurations for the vrancea zone are proposed (crc group 461, 1999): (a) subduction beneath the suture zone; (b) subduction beneath the fore-deep area; (c) two interacting subduction zones; (d) subduction beneath the suture, followed by delamination. the results carried out by magnetotelluric tomography [stanica et al., 2004; stanica and stanica, 2012] emphasized: (a) a continental origin for the seismogenic body; (b) the changes with depth of its strike orientation could be the result of a geodynamic torsion process. 2.2 vrancea seismicity and ceca the vrancea zone is located at the carpathian bending area being bounded to the east by the east european platform (eep), to the south by the moesian platform (mp), and westwards by the transylvanian basin (tb). the intermediate depth earthquakes are placed in a well-defined volume 70 km long, 30 km wide, about 220 km high, and the epicentral area is delimited at nw and se by ceca. the frequency of vrancea earthquakes can be assessed from the available catalogues, and their occurrence is about 2-10 events/month of magnitude m<4 and, about 1-4 strong events/century of magnitude m≥7. the ruptured zones in seismogenic volume jump from 150-180 km for the mw 7.8 earthquake in 1940, to 90-110 km for the mw 7.5 earthquake in 1977, to 130-150 km for the mw 7.2 earthquake in 1986, to 70-90 km for mw 6.9 earthquake in 1990. the crustal activity is weak, often occurred in clusters and mainly located in the depth interval of 10 40 km. ceca represents a conglomerate of sedimentary rocks, filled with hot highly-mineralized pore fluids, formed only at a contact zone between two continental plates and it is also quite possible that the geoelectric parameters remain fairly constant throughout its entire length [pinna et al., 1992; stanica and stanica, 2010, 2011, 2012; stanica et al., 2018]. the mw 5.7 and m5.6 earthquakes struck the vrancea zone on september 24 and december 28, 2016. their epicenters have been located at the geographic coordinates 45:71n; 26.62e and 45.72n; 26.61e, with a focal depth at 92 and 91 km, being placed at about 150 km ne of bucharest, as both of them have been determined by the euro-mediterranean seismic centre (http://www.emsc-csem.org). although the two earthquakes were moderate as magnitude, they have been felt in bucharest city and create panic through its inhabitants. 3. methodology and results 3.1 base relations of the electromagnetic precursors it was just demonstrated that at the earth’ surface, the vertical geomagnetic component (bz) expresses totally a secondary field and its existence represents an immediate indicator of the lateral inhomogeneities. furthermore, for a 2d geoelectrical structure, bz is essentially produced by the horizontal geomagnetic component perpendicular to the geoelectric strike (b┴) and, as a sequence, the normalized function bzn having the form (1): 3 resilience improvement to vrancea earthquake http://www.emsc-csem.org dragoș armand stănică et al. 4 (1) must be time invariant and it becomes unstable when the geodynamic processes are associated with the resistivity changes, due to the intermediate-depth seismicity in vrancea zone [word, et al., 1970; stanica and stanica, 2010]. to explain the connection between the earthquake generation and the anomalous values of the bzn (f ), the following relations that allow its assessment in terms of resistivity are introduced: (2) where: 𝜌z is vertical resistivity [vma-1], eii is electric field parallel to the geoelectric strike [vm-1]; 𝜌ii is resistivity parallel to the 2d structure [vma-1] and f is frequency [hz]. based on the relations (2), the normalized function bzn may be writen as: (3) with the relation (3), it is demonstrated that normalized function bzn may be associated with the resistivity changes before the onset of the seismic event and, consequently, bzn may be used to emphasize the pre-seismic em anomalies associated with the seismic activity in vrancea. to find the adequate monitoring site (on the 2d geoelectric structure), we have taken into account that between the electric components (ex and ey) and magnetic components (bx and by) of the natural em field there is the following relation of interdependence: (4) where: ex, ey and bx, by, are cartesian components of electric [vm-1] and magnetic [vsm-1] fields; zxx, zxy, zyx, zyy are the tensor impedance elements [va-1]. then, to identify a 2d geoelectric structure, in addition to the condition that dimensionality parameter skewness < 0.3 [stanica and stanica, 2010], the following conditions must be fulfilled: (5) with the other words, for a 2d geoelectric structure the electric and magnetic fields are mutually orthogonal and bzn may be associated with e-polarization mode, which in terms of the electromagnetic field components ex, by, and bz, describe electrical currents that are propagated parallel to strike (x direction), as it is shown in relation (6): ∂ex/∂y = ∂bz/∂t = iwbz; ∂ex/∂z = ∂by/∂t = -iwby; ∂bz/∂y ∂by/∂z = μσex, (6) where: w is angular frequency [s-1], μ is magnetic permeability [vsa-1m-1 = hm-1], σ is conductivity [sm-1], ex is electric field parallel to strike [vm-1]. to satisfy the relations (1) and (6), the gops was placed on the ceca at about 100 km far from the seismic active vrancea zone. the reliability level of the em methodology, related to the seismic activity, could be verified by analyzing the daily mean distribution of the bzn in correlation with intermediate depth earthquakes (mw ≥ 5.0) for long time intervals. to investigate the range effect of stain-related the em precursors, in this paper we used the relation given by [morgunov and malzev, 2007] and, consequently, the radius of the preparation zone for the two earthquakes (mw 5.7 and mw 5.6) was estimated to be about 300 km. as the distance between gops and earthquakes epicenter is 100 km, the existence condition of the em precursors is fulfilled. bzn (f ) = bz (f ) b┴ (f ) 𝜌z (f ) = � �2 and1 5 f eii (f ) bz (f ) eii (f ) b┴ (f ) 1 5 f 𝜌ii (f ) = � �2 ∣bzn (f )∣ =�𝜌ii(f ) 𝜌z (f ) � � = � � · � �ex ey bx byzxx zxy zyy zyy zxx = ‒zyy zxy ≠ ‒zyx 3.2 data collection and processing the ground-based monitoring system for em data collection (figure 1) is installed at gops and consists of: • magnetotelluric (mt) system (mtu data logger, three magnetic induction coils hx, hy and hz and nonpolarizable electric sensors) is used to validate the frequency range for a 2d geoelectric structure; • geomagnetic system (mag03dam data logger and three-axes fluxgate magnetic sensor) suitable for the geomagnetic time series bx, by and bz collection with a sampling rate of 60s; • computer with programs dedicated to the data acquisition, storage and daily transfer to the institute of geodynamics of the romanian academy (ig-ra). the automatic system for real-time data processing and analysis is installed at ig-ra and has the following components: • computer (server) used for receive and storage the geomagnetic data records; • work-station with specific programs for data processing, analyzing and display results every day on the institute webpage. 5 resilience improvement to vrancea earthquake figure 1. em monitoring system, data transfer and central computer for real-time data processing and analysis. in this study, the daily mean distributions of the normalized function bzn and its standard deviation (sd) are computed on the span of time intervals 01-30 september and 01-30 december, 2016, by using the following procedures: • fft band-pass filtering (fft-bpf) analysis in the ulf range applied to bzn time series, for two successive time windows of 1024 samples, with about 30% overlapping on 1440 data acquired each day, see figure 2 and table 1; dragoș armand stănică et al. 6 table 1. time series of b┴, bz, bzn, bzn (mean), sd, bzn (fft-bpf), bzn (fft-bpf) (mean), sd obtained only for 17 samples (δt = 60s) on september 21, 2016 b┴ [μt] bz [μt] bzn [μt] bzn (mean) sd bzn (fft-bpf) bzn (fft-bpf) (mean) sd 22.193 42.888 1.9325 1.933 0.0003 1.9341 1.934 0.0001 22.192 42.888 1.9326 1.9341 22.189 42.887 1.9328 1.9341 22.187 42.887 1.933 1.9341 22.187 42.887 1.933 1.9341 22.186 42.887 1.933 1.9341 22.185 42.887 1.9331 1.9341 22.184 42.887 1.9332 1.9342 22.184 42.887 1.9332 1.9342 22.184 42.887 1.9332 1.9342 22.184 42.887 1.9332 1.9342 22.184 42.886 1.9332 1.9342 22.185 42.886 1.9332 1.9342 22.184 42.886 1.9331 1.9342 22.183 42.886 1.93334 1.9343 22.183 42.886 1.9334 1.9343 22.182 42.886 1.9335 1.9343 figure 2. graphs with fft bpf (red line) applied on the bzn time series (blue line) for a time windows of 1024 samples (δt=60s) recorded on september 21, 2016. • a statistical analysis based on the standardized random variable equation (7) was applied for a clear evidence of the pre-seismic anomalies emphasized by the bzn time series and to eliminate the seasonal variations (stanica et.al. 2015). bzn* = (x-y)/w (7) where: • x is the mean value of the normalized function bzn obtained for 1day; • y is 30 days running average of bzn carried out before x; • w is 30 days running average of sd carried out before x; • bzn* represents the threshold for anomaly using sd. 3.3 results and discussions according to relation (1) and equation (6), the daily mean distributions of bzn and bzn* obtained on the frequency range 1e-3 hz to 0.0083hz, by using the fftbpf, should be constants or have small variations in nonseismic conditions and have consistent anomalous variations versus their normal trend, some days before the onset the both seismic events (mw 5.7 and mw 5.6). thus, as final results, this paper presents the contribution to the development of the pre-seismic em methodology, by using the geomagnetic data carried out for the two intervals: a) september 2016 with the following graphs: daily mean distributions of bzn (figure 3); b┴ and bz time series recorded for 7days (figure 4); bzn* time series, represented in absolute values (figure 5). on the interval september 19-26, a significant anomalous domain of minimum, with values ranging from 1.933 to about 1.942 is observed on bzn distribution (figure 3). this anomaly is generated by the increased value of b┴ (figure 4), due to the high stress reached in vrancea seismogenic volume, followed by the released electrical charges that are propagated along the 2-d geoelectric structure (ceca), before the onset of the mw 5.7 earthquakes. 7 resilience improvement to vrancea earthquake figure 3. daily distribution of the bzn (blue bar) and sd (red bar) on september 2016; red full circle is mw 5.7 earthquakes on september 24; ratio 5.7/92 is magnitude/hypocenter depth. figure 5 presents a graph with the bzn*abs time series which emphasizes a very large anomaly extended on the interval september 21-25. this anomaly with magnitude higher than 5∎sd (red dashed line), represents a pre-seismic em signature which starts on september 21, 3 days prior to the onset of mw 5.7 earthquake. dragoș armand stănică et al. 8 figure 4. bperp (red line) and bz (blue line) time series of the geomagnetic field carried out on the time interval september 19-25, 2016 (here bperp is b┴ in text). figure 5. bzn*abs time series (abs is absolute value) on september 2016; red full circle is mw 5.7 earthquake on september 24; red dashed line is threshold for anomaly using sd. b) december 2016 with the subsequent graphs: daily mean distributions of bzn (figure 6) bzn* time series, represented in absolute values (figure 7). on the second interval, a very large anomalous domain of maximum with values ranging from 1.951 to 1.955 is also observed on bzn distribution, placed on 10 days’ interval in december 19-29 (figure 6). the graph of bzn* time series (figure 7) emphasizes a very large anomaly of maximum which is extended on the interval december 19-29. this anomaly, with magnitude higher than 4∎sd (red dashed line), which started on december 21, represents an em precursor associated to the mw 5.6 earthquake generated on december 28, 2016. 9 resilience improvement to vrancea earthquake figure 6. daily mean distribution of bzn (blue line) and sd (red bar) on december 2016; red full circle is mw 5.6 earthquake on december 28; ratio 5.6/91km is magnitude/hypocenter depth. figure 7. bzn*abs time series (blue bar) on december 2016; red full circle is mw 5.6 earthquake on december 28, red dashed line represents threshold for anomaly using sd. 4. conclusions in this paper, the ulf geomagnetic data recorded on september and december 2016 are analyzed with the aim to detect possible em precursors associated with the mw 5.7 and mw 5.6 earthquakes and the following results are inferred: • two very clear anomalous intervals of minimum, extended from september 19 – september 26 and december 19 – december 29, have been identified on bzn distributions (figure 3 and figure 6); • these anomalous domains are also well defined in the both bzn* time series (figure 4 and figure 7) obtained after applying to the bzn distributions a statistical analysis based on equation (7); • no doubt, the above information demonstrates that variability of bzn observed in both situations are not randomly, these being two significant and reliable pre-seismic anomalous effects related to mw 5.7 and mw 5.6 earthquakes; • the results obtained suggest that the pre-seismic anomalous intervals of the bzn*, emphasized by using threshold for anomaly (red dashed line), have been triggered 3 days (bzn*= 5 in figure 5) and, respectively, 7 days (bzn*= 4 in figure 7) before the onset of the seismic events; • as the work-station installed at ig-ar has specific programs for data processing, analysis and real time (daily) display such information on the institute website, this may be used as an early warning system able to provide useful information for the resilience improvement against the vrancea intermediate depth earthquakes. taking into account that the major earthquakes may create considerable emergency situations, with disastrous consequences, it is very important to have reliable information prior to their occurrence, what this em methodology already showed that it is possible. therefore, any information related to a coming major earthquake, transmitted in time to the authorities responsible in this domain, can contribute to a better resilience and, consequently, to the prevention, management or decrease of the catastrophic risks. acknowledgements. the authors thank to the institute of geodynamics of the romanian academy for making electromagnetic data available to the user community. the authors are grateful to the anonymous reviewers for their comments and useful suggestions. references constantinescu, l. and d. enescu (1984). a tentative approach to possibly explaining the occurrence of the vrancea earthquakes, rev. roum. geol., geophys., geogr., 28, 19-32. crc group 461 (1999). tectonics, hazard and risk mitigation, 2-15. fan, g., t.c. wallace and d. zhao (1998). tomographic imaging of deep velocity structure beneath the eastern and southern carpathians, romania: implication for continental collision. j. geophys. res., 103, 2705-2723. fitterman, d.v. 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(1991). tectonomagnetic modeling on the basis of the linear piezomagnetic effect, bull earthquake res inst. univ. tokyo, 66, 585-722. spakman, w., s. van der lee and r. van der hilst (1993). travel-time tomography of the european-mediterranean mantle down to 1400 km, phys. earth planet. inter. 79, 3-14. stanica d., m. stanica, l. piccardi, e. tondi, g. cello (2004). evidence of geodynamic torsion in the vrancea (eastern carpathians), rev. roum. geophysique, 48, 15-19. stanica, d. and d. a. stanica (2010). constraints on correlation between the anomalous behavior of electromagnetic normalized functions (enf) and the intermediate depth seismic events occurred in vrancea zone (romania), terr. atmos. ocean. sci., 21(4), https://doi.org/10.3319/tao. 2009.09.09.01 (t), 675-683. stanica, d. and d. a. stanica (2011). anomalous pre-seismic behavior of the electromagnetic normalized functions related to the intermediate depth earthquakes occurred in vrancea zone, romania, nat. hazards earth syst. sci., 11, doi:10.5194/nhess-11, 3151-3156. stanica, d. and d. a. stanica (2012). earthquakes precursors, in “earthquake research and analysis-statistical studies, observations and planning” book 5, edited by: dr. sebastiano d’amico, intech open access publisher, chapter 4, doi:10.5772/2461d, 71-100. stanica, d, a., d. stanica and n. vladimirescu (2015). long-range anomalous electromagnetic effect related to m9 great tohoku earthquake, earth sciences, 4(1), http://www.sciencepublishinggroup.com/j/earth, doi:10.11648/j. earth. 20150401.13, 31-38, stanica, d. a. and d. stanica, j. blecki, t. ernst, w. jozwiak, j. slominski (2018). pre-seismic geomagnetic and ionosphere signatures related to the mw 5.7 earthquake occurred in vrancea zone on september 24, 2016, acta geophys. 66, https://doi.org/10.1007/s11600-018-0115-4, 167-177. tramutoli, v., v. cuomo, c. filizzola, n. pergola, c. pietrapertosa (2005). assessing the potential of thermal infrared satellite surveys for monitoring seismically active areas. the case of kocaeli (yzmit) earthquake, august 17th, 1999, remote sens. environ. 96, 409-426. trifu, c. i. and m. radulian (1989). asperity distribution on percolation as fundamental of earthquakes cycle, phys. earth planet. inter., 58, 277-288. vallianatos, f. and a. tzanis (1998). electric current generation associated with the deformation rate of a solid: preseismic and coseismic signals, phys. chem. earth, 23/9-10, 933-939. vallianatos, f. and a. tzanis (1998). a model for the generation of precursory electric and magnetic fields associated with the deformation rate of the earthquake focus” in m. hayakawa (ed.), seismic atmospheric & ionospheric electromagnetic phenomena, terra scientific publishing co., tokyo. vallianatos and a. tzanis (1999). on possible scaling laws between electric earthquake precursors (eep) and earthquake magnitude, geophys. res. lett., 26, 13, 2013-2016. vallianatos and a. tzanis (2003). on the nature, scaling and spectral properties of pre-seismic ulf signals”, nat. hazards earth syst. sci., 3, 237-242. vallianatos, f, d. triantis, a. tzanis, c. anastasiadis, i. stavrakas, (2004). electric earthquake precursors: from laboratory results to field observations”, phys. chem. earth, 29, 339-351. 11 resilience improvement to vrancea earthquake callto:1,%2057-60,%201998 https://doi.org/10.3319/tao http://www.sciencepublishinggroup.com/j/earth https://doi.org/10.1007/s11600-018-0115-4 callto:9-10,%20933-939,%201998 callto:13,%202013-2016,%201999 callto:237-242,%202003 varotsos, p., k. alexopoulos, k. nomikos, m. lazaridou (1986). earthquake prediction and electric signal, nature, 322, 120. varotsos, p. a. (2005). the physics of seismic electric signals, terrapub, tokyo, 338. word, r.d., h.w. smith and f. x. bostick jr. (1970). an investigation of the magnetotelluric tensor impedance method, electronic research center, the university of texas, austin, texas 78712, 242. wortel, m. j. r. and w. spakman (1992). structure and dynamics of subducted lithosphere in the mediterranean region. proceed. konn. ned. acad. van wetenschappen, 95, 325-347. yen h.y., c. h. chen, y. h. yeh, j. y. liu, c. r. lin and y. b. tsai (2004). geomagnetic fluctuations during the 1999 chi-chi earthquake in taiwan, earth planet and space, 56, 39-45. *co r r e s po n d i n g a u t h o r: dumitru s tă n i că, institute of geodynamics of the romanian academy, jean-lois calderon, str. 19-21, bucharest, romania; e-mail: dstanica@geodin.ro © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved dragoș armand stănică et al. 12 mailto:dstanica@geodin.ro i l v e n t o s u l l ' a l t i p i a n o c a r s i c o d i t r i e s t e silvio polli 1. premessa. — l a c i t t à di t r i e s t e , i l suo g o l f o e il r e t r o s t a n t e soll e v a m e n e c a r s i c o s o n o soggetti, p e r l a l o r o p a r t i c o l a r e p o s i z i o n e , ad l i n o s p e c i a l e e c a r a t t e r i s t i c o r e g i m e d i v e n t i . n e l s e m e s t r e estivo, con c o n d i z i o n i m e t e o r i c h e n o r m a l i , si h a n n o r e g o l a r i b r e z z e di m a r e e di t e r r a . nel s e m e s t r e i n v e r n a l e q u e s t e s o n o s o p r a f f a t t e , q u a s i i n c o n t i n u a a l t e r n a n z a , d a l v i o l e n t o e secco v e n t o da e n e ( b o r a ) e dai m o d e r a t i e u m i d i v e n t i da s e (scirocco). 11 c a r s o , c h e ad u n ' a l t e z z a d i c i r c a 3 0 0 n i si l e v a i m m e d i a t a m e n t e a n e a ne d e l l a città, p i ù c h e o s t a c o l a r e l a discesa d e l l a b o r a , l a f a v o r i s c e , sia c o m e soglia a p e r t a sul m a r e , sia c o m e e f f e t t i v a c i r c o s t a n z a c o n c o m i t a n t e a l l a sua g e n e r a z i o n e . l a b o r a si p r e s e n t a con l a m a s s i m a i n t e n s i t à n e i m e s i i n v e r n a l i , m a n o n è r a r a , m e n o v i o l e n t a p e r ò , a n c h e n e i m e s i estivi. si p r e c i p i t a d a l l ' a l t i p i a n o m a n t e n e n d o d i r e z i o n e m o l t o costante. a s s u m e m a s s i m a v i o l e n z a l u n g o i v a r c h i a v e n t i l a m e d e s i m a d i r e z i o n e . p u r r i s c a l d a n d o s i n e l l a c a d u t a p e r c o m p r e s s i o n e a d i a b a t i c a , r i m a n e s e m p r e u n v e n t o f r e d d o e secco. l o s t u d i o d e l l a b o r a e r a stato eseguito s u l l a base d e l l e r e g i s t r a z i o n i a n e m o m e t r i c h e e s e g u i t e a t r i e s t e e d e l l e o s s e r v a z i o n i s t i m a t e e s e g u i t e sul carso. q u e s t e u l t i m e , f a t t e senza l ' a i u t o di s t r u m e n t i , si l i m i t a n o a l l e t r e o s s e r v a z i o n i g i o r n a l i e r e d e l l e o r e 7, 1 4 e 1 9 . h a n n o v a l o r e p u r a m e n t e i n d i c a t i v o b a s a n d o s i esse sul p r e s u p p o s t o c h e l a b o r a s u l l ' a l t i p l i a n o f o s s e p i ù i n t e n s a c h e s u l l a c i t t à . si p r e s e n t a v a p e r ciò s e m p r e di g r a n d e i n t e r e s s e l a s i s t e m a z i o n e di a n e m o g r a f i s u l l ' a l t i p i a n o c a r s i c o e sul suo m a r g i n e . ciò, p e r v a r i e c i r c o s t a n z e , n o n è stato p o s s i b i l e r e a l i z z a r e p r i m a ili q u e s t i u l t i m i a n n i . a p p e n a a l l a fine d e l 1 9 4 7 , il l o c a l e ufficio a g r i c o l t u r a d e l g o v e r n o m i l i t a r e a l l e a t o , v e n u t o a c o n o s c e n z a d e l l a q u e s t i o n e , dispose s u b i t o p e r c h é f o s s e r o messi a l c u n i a n e m o m e t r i s u l l ' a l t i p i a n o e n e i d i n t o r n i d i t r i e s t e . l ' i d e a l e s a r e b b e stato p o t e r e d i s p o r r e di a p p a r e c l i i r e g i s t r a t o r i d e l l a direz i o n e , d e l l a v e l o c i t à m e d i a e d e l l e r a f f i c h e . l a spesa r i s u l t a v a t r o p p o f o r t e e p e r c i ò f u r o n o messi i n f u n z i o n e dei s e m p l i c i a n e m o m e t r i a c o p p e , t o t a l i z z a t o r i , a l e t t u r a d i r e t t a . p r e s e n t i a m o i n q u e s t a n o t a i r i s u l t a t i di d u e d e l l e c i n q u e sta2 8 6 s i l v i o l ' o l l i z i o n i a t t u a l m e n t e in f u n z i o n e . essi r a p p r e s e n t a n o l e p r i m e m i s u r e anem o m e t r i c h e eseguite sul carso. i v a l o r i o t t e n u t i sono r i g o r o s a m e n t e c o n f r o n t a b i l i f r a di l o r o e con q u e l l i di ogni a l t r o a n e m o g r a f o . i ris u l t a t i sono p e r t a n t o di n o t e v o l e i m p o r t a n z a sia s c i e n t i f i c a c h e p r a tica. in u n c e r t o senso essi sono a n c h e s o r p r e n d e n t i . s i riteneva inf a t t i c h e l a b o r a fosse s u l l ' a l t i p l i a n o p i ù f o r t e clic i n città, ciò c h e i n v e c e n o n r i s u l t ò d a l l ' e s a m e d e l l e s e r i e di m i s u r e eseguite. la ques t i o n e e r a d o v u t a p u r a m e n t e a i m p r e s s i o n i psicologiche. i n d u b b i a m e n t e , p e r q u a n t o si è d e t t o p r e c e d e n l e i n e n t e , la b o r a è sul c a r s o p i ù f r e d d a c h e al m a r e . i n o l t r e , l a m a n c a n z a di r i p a r i , i l s u o l o c o n cop e r t u r a i n v e r n a l e spesso gelata, l ' i m p l a c a b i l e costanza d e l l a d i r e z i o n e c h e i l paesaggio n u d o n o n r i e s c e ad a l t e r a r e , s o n o t u t t e c i r c o s t a n z e c h e f a n n o a p p a r i r e , a c h i g e n e r a l m e n t e v i v e n e l l a città in siti r i p a r a t i , p i ù f o r t e di q u a n t o i n r e a l t à n o n l o sia la v e l o c i t à del v e n t o sul carso. d a i dati c h e si p r e s e n t a n o in questa n o t a si d e d u c e i n v e c e c h e la v e l o c i t à m e d i a d e l l a b o r a è i 3/4 di q u e l l a di t r i e s t e a b a s o v i z z a , e i 2/3 a v i l l a opicina. i n o l t r e , da s o p r a l u o g h i eseguiti in p e r i o d i di b o r a i n v a r i e z o n e d e l l ' a l t i p i a n o , r i s u l t ò p u r e c h e l e r a f f i c h e sono m e n o m a r c a t e c h e in città. l a b o r a sul c a r s o ha c a r a t t e r i s l i r h e di m a g g i o r e u n i f o r m i t à . la m i n o r e v e l o c i t à e s o p r a t t u t t o la r i d o t t a v o r ticosità l a s c i a n o i n t r a v e d e r e la p o s s i b i l i t à di s i s t e m a r e s u l l ' a l t o p i a n o degli efficienti a e r o p o r t i , la cui c o s t r u z i o n e r i s u l t a i n v e c e i m p o s s b i l e in c i t l à . 2. la stazione di opicina. — è s i t u a t a a s e del paese, in posiz i o n e l e g g e r m e n t e e l e v a t a r i s p e t t o al n u c l e o c e n t r a l e d e l l a l o c a l i t à , n e l l a v i l l a a due p i a n i di v i a del r i c r e a t o r i o n. 16. l o s t r u m e n t o è s i s t e m a t o sul t e r r a z z i n o t e r m i n a l e d e l l ' e d i f i c i o . l e c o p p e s o n o a 2 , 5 m s o p r a di esso e a 1 7 m sul suolo. l e c o o r d i n a t e g e o g r a f i c l i e d e l l a staz i o n e s o n o : l a t . 45° 4 1 ' 0 9 " n, l o n g . 13° 4 7 ' 2 2 " e g r . , 1 " 2 0 ' 1 4 " e m. m a r i o ( r o m a ) , altezza del s u o l o m 3 3 0 sul l i v e l l o m e d i o m a r i n o . l ' a p p a r e c c h i o r i s u l t a in p o s i z i o n e d o m i n a n t e il p a e s e ed è suffic i e n t e m e n t e l i b e r o a tutti i v e n t i . i l s o l l e v a m e n t o m a r g i n a l e carsico, c l i e si e l e v a 6 0 ni p i ù a l t o a circa 8 0 0 m v e r s o s w , r i p a r a a l q u a n t o t u t t a l a l o c a l i t à dai v e n t i da s u d e da s w . l o s t r u m e n t o è u n a n e m o m e t r o t o t a l i z z a t o r e s a l m o i r a g h i a 3 c o p p e . l a distanza dell'asse di r o t a z i o n e d a l c e n t r o d e l l e c o p p e è di cm 1 7 , i l d i a m e t r o d e l l e c o p p e è di c m 8. esso i n d i c a il n u m e r o dei m e t r i p e r c o r s i dal v e n t o . l e l e t t u r e si eseguono g i o r n a l m e n t e a l l e rl v e n t o s u l l ' a l t i p i a n o c a r s i c o di t r i e s t e 2 8 7 tabella i — velocità media del vento a opicina. a n n o , i l i n iv v vi v i i v i l i ix x xi x i i a n n o 1 9 4 8 5,6 1 1 , 2 9,6 9,8 7,8 6 , 9 7 , 9 7,4 v 10.1 1 1 . 9 1 3 . 6 9.0 1 9 4 9 1 0 . 8 9 . 7 1 4 4 6.4 8.3 7,9 5 9 8 3 7 1 1 3 2 1 2 5 7 6 9.3 1 9 5 0 1 4 . 7 8.4 1 0 . 7 8 . 1 8 . 5 7.5 8 3 6,4 6 9 9 9 7.4 1 0 . 4 8 9 1 9 5 1 4.5 1 2 0 1 0 . 6 9.7 6 . 6 6 . 9 7.4 6.7 9 1 1 7 . 1 7.2 6 . 1 9.1 1 9 5 2 7.4 7 . 0 7.4 8.0 6.2 7 . 1 8.3 5 4 6.9 6.7 5.9 1 5 . 4 7 6 media 9 . 6 9 7 1 0 , 5 8.4 7.5 7 . 3 7 . 6 6 8 7.3 1 1 . 4 9.0 1 0 . 6 8 8 t a b e l l a i l -velocità media del vento nei singoli quadranti. a n n o j i i l i n iv | v vi v i i v i l i ix x xi x i i a n n o 1 °q. 1 9 8 7 . 1 1 2 . 9 1 1 . 8 1 0 . 9 10 0 7.6 9 . 6 1 2 . 1 8 3 1 2 . 6 1 3 . 0 1 5 . 3 1 0 . 9 1 9 4 9 1 2 7 1 3 . 2 1 5 . 7 9.8 1 3 <> 1 0 . 8 7 7 1 1 . 1 1 0 . 5 15 6 1 6 1 9.1 1 2 . 2 1 9 5 0 1 8 . 9 1 0 . 8 1 5 2 9 2 1 3 . 0 1 0 . 8 9 8 8.4 8.1 17-2 1 0 9 1 2 . 3 1 2 . 1 1 9 5 1 1 6 . 8 1 2 6 1 4 8 1 7 4 8 1 10.1 9 . 6 9 . 1 1 2 . 7 1 9 . 1 9 . 3 1 0 3 1 2 . 5 1 9 5 2 1 0 . 0 1 1 9 1 1 . 6 1 2 . 5 9 6 7 6 9.7 6.7 9.2 8.7 8.7 2 0 . 4 1 0 . 6 media 1 3 . 1 1 2 . 3 1 3 . 8 1 2 . 0 1 0 9 9 . 1 9.3 9.5 9.8 1 4 . 6 1 1 . 6 1 3 . 5 1 1 . 7 l l ° q . 1 9 4 8 6 5 2.8 6 7 8.0 6 . 8 : 4 8 . 1 5 5 5 . 1 5,0 6.2 5 4 6 . 1 1 9 1 9 1 1 . 3 5 5 0.0 7.2 4 3 5.1 5.2 9.4 4.2 6 3 8 . 1 5.8 6 . 0 1 9 5 0 3 2 7 . 8 4.9 6.8 7.0 7.1 0.0 5.4 5.6 5.4 6 5 6.4 5 . 5 1 9 5 1 5.9 1 1 8 8.3 6.0 5.7 4 6 5 . 9 5.2 5.2 3 9 6 . 7 4 1 6 . 1 1 9 5 2 4 2 4 . 4 3 . 1 6.8 5.5 6.5 7.6 5.8 6 1 5 5 4 3 7.4 5.6 m edia 6.2 6.5 4 . 6 7.0 5.9 6.1 5.4 6.3 5.2 5,2 6.4 5.8 5,9 1 i i ° ( , ) . l ' ' 4 8 1.8 2.7 0 0 0.0 0 0 9 8 0 0 5.9 4.1 0.0 0.0 4.0 2.4 1 9 4 9 1 . 6 0.0 0.0 6.7 4 . 9 0 0 4 9 l'.o 0.0 8.9 7.2 2.6 3 1 1 9 5 0 1 . 9 0 0 0 0 9 1 9 4 6 . 8 4 . 8 4.8 0.0 2.8 0.0 0 0 3.2 1 9 5 1 3 . 1 3 5 6.5 5 0 4 9 5.8 4 8 5 5 6 1 0 0 2 6 3.9 4 . 3 1 9 5 2 0.0 2 8 3 0 3 9 4 2 7.0 8.1 4.4 4 . 7 6.9 1 . 1 4 5 4.2 media 1.7 1 . 8 1 . 9 4.9 4.7 5.9 4.5 4 1 3 . 0 3.7 2.2 3.0 3.4 1v°(,) 1 9 1 8 2 . 3 2.6 6 . 1 5.3 5 0 5.2 3.8 4 . 5 4.3 2.9 0.0 0.0 3.5 1 9 4 9 4 . 9 3 . 4 5 4 4 8 4 6 4 . 5 4.5 4 . 7 4 . 1 4.3 6.7 2.0 4 5 1 9 5 0 9.3 4 . 8 4.4 5 3 5.0 4 2 6 8 4.2 3 . 4 0 0 3.0 0 0 4.2 1 9 5 1 2 2 0 0 5.7 6 . 8 4 5 0 0 0 0 6 6 0 . 0 0.0 0.0 5.7 2.6 1 9 5 2 0.0 4 7 2 7 2 5 4 0 7.5 5.4 4 6 4 4 3.8 3.2 0 . 0 3.6 m e d i a 3.7 3.1 4 . 9 4 . 9 4.6 4 3 4 . 1 4.9 3.2 2,2 2.6 1 . 5 3.7 t a b e l l a i i i — velocità media della bora. a n n o 1 ii iv v vi vii v i l i ix x xi x i i a n n o 1 9 4 8 9 . 4 1 7 . 2 1 2 1 1 1 . 4 1 1 2 7.7 1 0 1 1 8 . 0 9.7 1 8 . 1 1 6 . 1 1 8 6 1 3 3 1 9 4 9 13 4 1 4 . 8 17 7 1 0 , 3 1 4 9 1 1 . 2 8,2 1 2 . 1 1 1 . 9 1 7 . 9 2 0 . 0 1 3 . 4 13 8 1 9 5 0 2 6 3 1 1 7 1 0 . 3 1 1 . 1 1 3 . 0 12.4 1 0 . 1 8.4 8.5 20.4 1 2 0 1 4 7 1 3 . 7 1 9 5 1 1 9 . 0 1 5 . 8 16.2 1 7 . 4 1 0 , 8 1 1 5 1 0 . 1 9.3 1 5 . 0 2 0 8 1 2 . 0 1 0 8 1 4 . 1 1 9 5 2 1 0 . 9 13.7 14.4 1 5 . 8 1 2 . 5 7 . 8 1 0 . 2 7.8 9.7 9 . 9 1 0 , 2 2 2 . 6 1 2 . 1 media 1 5 . 8 1 4 . 6 15 3 | 1 3 . 2 1 2 . 5 1 0 . 1 9.7 1 1 : 1 1 1 . 0 1 7 4 1 4 . 1 1 6 . 0 1 3 . 4 2 8 8 s i l v i o l ' o l l i o r e 8. o s s e r v a t o r e è il m a e s t r o g i o v a n n i p a u l i c h e esegue p u r e l e a l t r e o s s e r v a z i o n i m e t e o r i c h e . 3. la stazione di basovizza. — è s i t u a t a a s e del paese, al n. 1 0 9 , presso la casa f o r e s t a l e d e l l a p r o v i n c i a . i l sito è p i a n e g g i a n t e e a p e r t o a t u t t i i v e n t i . i l m a r g i n e carsico, s i t u a t o 1 5 0 0 ni v e r s o s e , è di p o c h i m e t r i p i ù e l e v a t o del paese e p e r t a n t o i n f l u i s c e poco, q u a l e o s t a c o l o , sul r e g i m e l o c a l e del v e n t o . l e c o o r d i n a t e g e o g r a f i c h e d e l l a s t a z i o n e s o n o : lat. 45" 3 8 ' 3 0 " n, long. 13° 5 1 ' 3 3 " e g r „ 1 ° 2 4 ' 2 5 " e m. m a r i o ( r o m a ) , l a q u o t a al suolo è di 3 7 5 m sul l i v e l l o m e d i o m a r i n o . l o s t r u m e n t o è fissato a l l a s o m m i t à di u n p a l o con l e c o p p e a m 6,05 d a l suolo. è u n a n e m o m e t r o s a l m o i r a g h i t o t a l i z z a t o r e a 3 c o p p e di a l l u m i n i o . la distanza dall'asse di r o t a z i o n e al c e n t r o d e l l e c o p p e è di cm 1 7 , il d i a m e t r o d e l l e c o p p e è di cm 8. l e l e t t u r e s o n o eseguite ogni g i o r n o a l l e o r e 8. 4. la stazione di trieste. — l o s t r u m e n t o è s i s t e m a t o s u l l a t o r r e d e l l ' i s t i t u t o t a l a s s o g r a f i c o , s i t u a t o n e l l a p a r t e s w d e l l a città, a l n. 2 del v i a l e r o m o l o gessi. l a p o s i z i o n e è b e n e a p e r t a ai v e n t i d e l i, i i i e i v q u a d r a n t e , è i n v e c e l e g g e r m e n t e r i p a r a t a dai v e n t i del i i q u a d r a n t e dagli a l t i e d i f i c i c o s t r u i t i n e g l i u l t i m i a n n i sul p r o m o n t o r i o di s. a n d r e a . l e c o o r d i n a t e g e o g r a f i c h e d e l l a t o r r e sono : l a t . 45° 3 8 ' 3 7 " n, l o n g . 13° 4 5 ' 0 9 " e g r . , 1° 1 8 ' 0 3 " e m. m a r i o ( r o m a ) . i l m u l i n e l l o a c o p p e è fissato a l l ' e s t r e m i t à di u n t u b o d'acciaio, 7 ni s o p r a l a t e r r a z z a t e r m i n a l e , a 3 8 ni d a l suolo e a 4 5 ni sul l i v e l l o m e d i o del m a r e . l o s t r u m e n t o è u n a n e m o g r a f o u n i v e r s a l e s . i . a . p . a 3 c o p p e e con i l t u b o di p i t o t sull'asse d e l l a b a n d e r u o l a , c o n t r o v e n t o . esso r e g i s t r a con c o n t i n u i t à i k m p e r c o r s i , l a d i r e z i o n e e l e r a f f i c h e d e l v e n t o . il r a g g i o dall'asse di r o t a z i o n e al c e n t r o d e l l e c o p p e è di cm. 2 1 , 8 , i l d i a m e t r o e s t e m o d e l l e c o p p e è di cm 16,0. l o s c o r r i m e n t o d e l l a c a r t a d e l d i a g r a m m a è di 2 2 , 5 m m p e r 1 o r a . 5. elaborazione delle misure. — d a l l e l e t t u r e eseguite a l l e o r e 8 si d e d u s s e r o l e v e l o c i t à m e d i e g i o r n a l i e r e c o r r i s p o n d e n t i a l l e 2 4 o r e p r e c e d e n t i . d a l l e m e d i e g i o r n a l i e r e si o t t e n n e r o p o i q u e l l e m e n s i l i . queste p e r t a n t o si r i f e r i s c o n o al v e n t o p e r c o r s o d a l l e o r e 8 d e l giorn o 1 di ciascun m e s e a l l e o r e 8 del g i o r n o 1 del mese seguente. a n c h e i d a t i di t r i e s t e c o r r i s p o n d o n o agli stessi i n t e r v a l l i di rl v e n t o s u l l ' a l t i p i a n o c a r s i c o di t r i e s t e 2 8 9 t a b e l l a i v — velocità media del vento a basovizza. a n n o , i l i l i iv v v i v i i v i i i i x x x i x i i a n n o 1 9 4 1 5 1 1 2 . 4 9 . 5 1 1 8 7 . 7 8 3 7 . 7 7 . 2 6 . 3 1 1 . 7 1 2 . 7 1 4 . 2 9 . 6 1 9 4 9 10 6 9 . 3 1 5 . 1 9 2 8 . 6 8 . 9 6 . 0 8 . 3 6 8 1 4 . 3 1 3 . 3 7 . 3 9 8 1 9 5 0 1 3 6 8 9 1 0 . 2 1 0 . 2 9 . 6 7 . 4 7 9 6 1 6 . 8 1 2 2 9 . 9 1 2 8 9 . 6 1 9 5 1 1 0 . 3 1 2 . 3 1 1 . 1 1 2 5 7 . 1 6 . 5 7 . 3 6 . 6 9 . 6 1 9 . 7 8 5 5 . 6 9 . 8 1 9 5 2 8 . 2 8 . 6 6 2 8 0 6 . 0 6 . 9 8 . 0 4 . 5 6 . 3 6 . 6 5 . 1 1 6 . 6 7 . 6 m e d i a 9 6 1 0 . 3 1 0 4 1 0 . 3 7 8 7 . 6 7 4 6 . 5 7 . 2 1 2 . 9 9 . 9 1 1 . 3 9 3 t a b e l l a v -— velocità media del vento nei singoli quadranti. a n n o i ii i n iv v v i v i i v i l i i x x x i x i i a n n o l°q. 1 9 4 3 6 . 5 1 4 . 2 1 1 . 3 1 3 . 6 1 1 . 6 7 . 8 1 0 5 1 1 . 8 7 . 6 1 4 . 1 1 4 . 0 1 5 . 7 1 1 . 6 1 9 4 9 1 2 0 1 3 . 5 1 6 . 4 1 5 . 4 1 4 . 4 1 2 . 5 8 . 2 6 . 3 9 . 7 1 7 . 0 1 6 . 8 8 . 7 1 2 6 1 9 5 0 1 7 . 3 1 1 . 7 1 4 . 6 1 1 . 3 1 6 . 8 1 0 . 8 9 . 8 8 . 3 7 . 9 2 1 . 6 1 7 . 1 1 3 . 8 1 3 . 4 1 9 l 1 6 . 8 1 3 . 8 1 4 . 8 2 4 . 8 9 . 0 9 . 8 9 . 9 8 . 9 1 3 . 0 2 2 . 1 9 . 4 9 . 4 1 3 . 5 l ' i 5 2 1 0 . 9 1 5 . 4 9 . 1 1 0 . 1 9 . 4 7 . 5 9 . 7 6 1 8 . 1 8 . 9 7 . 3 2 1 . 6 1 0 . 3 m e d i a 1 2 . 7 1 3 . 7 1 3 . 2 1 5 . 0 1 2 . 2 9 . 7 9 . 6 8 . 3 9 . 3 1 6 . 7 1 2 . 9 1 3 . 8 1 2 . 3 u°<,>. 6 . 3 1 9 4 8 6 9 2 . 7 6 . 9 9 6 6 . 7 8 . 8 8 . 1 5 . 6 4 . 7 4 . 4 5 9 5 . 4 6 . 3 1 9 4 9 1 0 . 2 4 . 9 0 . 0 7 . 3 4 1 5 . 6 7 . 7 1 1 . 1 4 . 5 7 . 3 9 0 5 . 7 6 . 5 19 ' 0 3 5 8 . 1 5 . 2 9 . 9 6 . 1 8 5 0 0 7 . 1 6 . 3 6 4 7 . 8 1 0 . 6 6 . 6 1 9 5 1 7 . 0 1 1 . 0 9 . 1 <'.5 6 . 3 4 8 5 . 6 5 . 0 6 . 2 4 . 2 8 . 7 3 . 9 6 . 5 1 9 5 2 4 . 9 5 . 4 3 . 7 8 , 7 5 8 6 6 7 . 6 5 0 6 . 0 5 . 6 4 2 9 . 4 6 . 1 m e d i a 6 . 5 6 . 4 5 . 0 8 . 4 5 8 6 . 9 5 . 8 6 . 8 5 . 5 4 . 7 7 . 1 7 . 0 6 . 4 l l i o y . 2 3 1 9 4 8 3 . 1 2 . 7 0 0 0 . 0 0 0 1 0 . 7 0 . 0 5 . 6 3 . 0 0 0 0 . 0 2 . 7 2 3 1 9 4 9 1 . 8 0 . 0 0 0 7 . 0 4 . 4 0 . 0 5 . 0 0 . 0 0 0 8 . 9 8 . 1 2 . 6 3 . 2 1 9 5 0 2 . 6 0 . 0 0 . 0 1 1 . 4 1 0 . 8 6 . 4 3 . 9 4 0 0 . 0 3 . 1 0 . 0 0 . 0 3 . 5 1 9 5 1 4 . 0 4 . 0 8 . 2 4 8 5 . 0 4 . 3 4 . 2 5 . 4 6 . 1 0 . 0 2 . 5 3 4 4 . 3 1 9 5 2 0 . 0 3 . 2 2 . 5 5 . 1 2 . 7 5 . 8 7 . 0 3 . 5 4 . 3 5 . 5 0 . 9 5 . 1 3 . 8 m e d i a 2 . 3 2 . 0 2 . 1 5 . 7 4 6 5 . 4 4 . 0 3 . 7 2 . 7 3 5 2 3 2 . 8 3 . 4 iv 0 q. 3 . 4 1 9 1 8 3 . 7 2 . 3 6 . 9 6 . 1 5 . 1 4 . 5 2 . 8 3 . 5 3 . 2 2 . 6 0 . 0 0 . 0 3 . 4 1 9 4 9 5 . 8 2 . 7 6 . 4 6 . 8 4 . 4 4 . 4 4 . 0 3 . 5 4 . 0 2 . 9 7 . 2 3 2 4 . 6 1 9 5 0 8 . 9 5 . 0 3 . 5 5 . 7 4 . 5 3 . 8 5 . 9 3 . 4 2 . 7 0 . 0 5 . 9 0 . 0 4 . 1 1 9 5 1 7 . 1 0 . 0 6 . 1 8 . 1 7 . 3 0 . 0 0 . 0 7 . 1 0 . 0 0 0 0 . 0 6 . 3 3 . 5 1 9 5 2 0 . 0 3 . 4 3 . 7 2 . 0 3 . 6 6 . 3 5 . 0 3 . 1 3 . 9 3 . 8 1 . 6 0 . 0 3 . 0 m e d i a 5 . 1 2 . 7 5 . 3 5 . 7 5 . 0 3 . 8 3 . 5 4 . 1 2 . 8 1 9 2 . 9 1 9 3 . 7 t a b e l l a v i •— velocità media della bora. a m i o i i i i n i v v v i v i i v i l i i x x x i x i i a n n o 1 9 4 1 9 . 1 1 9 . 4 1 1 . 3 1 4 4 1 3 . 1 8 . 3 1 1 . 1 2 0 . 5 9 . 6 2 j 8 1 6 . 8 1 9 . 5 1 4 . 5 1 9 4 9 1 4 . 7 1 5 . 9 1 8 . 2 6 . 7 1 6 . 0 1 3 . 3 8 . 6 1 2 . 9 1 1 . 5 1 8 1 2 0 . 6 1 2 0 1 4 . 9 1 9 5 0 2 4 . 0 1 2 8 1 5 . 7 1 3 . 4 1 6 . 8 1 2 . 4 9 6 8 . 3 8 . 6 2 6 . 0 1 3 . 4 1 4 9 1 4 . 7 1 9 5 1 1 9 . . ; 1 7 . 6 1 6 . 4 2 4 . 8 1 2 . 3 1 1 4 1 0 . 4 6.1 1 4 . 8 2 4 . 2 1 2 . 4 9 . 8 1 5 . 0 1 9 5 2 1 1 . 4 1 8 . 2 1 0 . 3 1 3 . 0 1 0 . 0 7 . 9 1 0 . 1 7 . 8 8 . 8 9 . 9 8 . 0 2 3 1 1 1 . 5 m e d i a 1 5 7 1 6 . 8 1 4 . 4 1 6 5 1 3 . 6 1 0 . 7 1 0 . 0 1 1 2 8 . 9 1 9 8 1 4 . 2 1 5 . 9 1 4 . 0 2 9 0 s i l v i o l ' o l l i t e m p o e p e r t a n t o i v a l o r i d e l l e t r e s t a z i o n i sono, da q u e s t o p u n t o di vista, r i g o r o s a m e n t e c o n f r o n t a b i l i . l e t a b e l l e 1, 4 e 7 p r e s e n t a n o i v a l o r i m e d i m e n s i l i e a n n u i d e l l e v e l o c i t à , espresse in k m / o r a , p e r c i a s c u n a d e l l e t r e l o c a l i t à . s o n o cioè i q u o z i e n t i f r a il n u m e r o dei k m p e r c o r s i da t u t t i i v e n t i n e l l e s i n g o l e stazioni ed il n u m e r o d e l l e o r e c o n t e n u t e n e l mese. i v e n t i , secondo l e l o r o d i r e z i o n i di p r o v e n i e n z a , f u r o n o aggrupp a t i in q u a d r a n t i secondo l o s c h e m a s e g u e n t e : i q u a d r a n t e , c o m p r e n d e i v e n t i da nne ad e, i i q u a d r a n t e , c o m p r e n d e i v e n t i da e s e a s, i i i q u a d r a n t e , c o m p r e n d e i v e n t i da s s w a w , i v q u a d r a n t e , c o m p r e n d e i v e n t i d a w n w a n. le t a b e l l e 2, 5 e 8 d a n n o l e v e l o c i t à m e d i e in k m / o r a c o r r i s p o n d e n t i ai v e n t i a v u t i secondo i q u a d r a n t i o r a d e f i n i t i . p a r t i c o l a r e i m p o r t a n z a assume la conoscenza del c o m p o r t a m e n t o d e l l a b o r a s u l l ' a l t i p i a n o i n c o n f r o n t o a q u e l l o c h e essa m a n i f e s t a a t r i e s t e e sul golfo. la sua i n d i v i d u a z i o n e è m o l t o f a c i l i t a t a dal f a t t o c h e essa p r o v i e n e da e n e e m a n t i e n e m o l t o costante l a sua d i r e z i o n e . s u l d i a g r a m m a essa figura p e r c i ò b e n d e t e r m i n a t a . è p e r t a n t o poss i b i l e r i c a v a r e da q u e s t o d a t i m o l t o p r e c i s i sulla stia v e l o c i t à e f r e q u e n z a . n e l l e t a b e l l e 3, 6 e 9 sono date in k m / o r a , l e v e l o c i t à m e d i e del v e n t o da ene, cioè d e l l a b o r a . esse sono i q u o z i e n t i f r a il n u m e r o di k m p e r c o r s i d a l l a b o r a in ciascun mese ed i l c o r r i s p o n d e n t e num e r o di o r e d u r a n t e l e q u a l i essa h a soffiato. n e l l e u l t i m e t a b e l l e ( 1 0 , 1 1 e 1 2 ) sono r i p o r t a t e , p e r ogni local i t à , l e m e d i e q u i n q i i e n n a l i e ciò p e r a v e r e l a possibilità di e s e g u i r e u n i m m e d i a t o c o n f r o n t o t r a i v a l o r i d e l l e t r e stazioni. 6. considerazioni conclusive. —• a) s u l l ' a l t i p i a n o carsico la velocità m e d i a del v e n t o r i s u l t a m i n o r e c h e a t r i e s t e e n e l l a z o n a del p o r t o . a b a s o v i z z a essa è i l 7 8 % di q u e l l a r e g i s t r a t a a t r i e s t e , a opicina il 7 4 % . a d e t e r m i n a r e t a l e f a t t o c o n c o r r o n o t u t t i i p r i n c i p a l i v e n t i d e l l a r e g i o n e : l a b o r a , l o scirocco e l e brezze. s i v e d r à p i ù a v a n t i p e r c h é questi v e n t i sono m e n o f o r t i sul c a r s o c h e a t r i e s t e . b) p e r l a b o r a , cioè il v e n t o da ene, questi r i s u l t a t i sono a n c o r a p i ù m a r c a t i . i c o r r i s p o n d e n t i v a l o r i p e r c e n t u a l i sono p e r b a s o v i z z a 7 1 % e p e r o p i c i n a 6 8 % . nella p r i m a l o c a l i t à la v e l o c i t à d e l l a b o r a è circa i 3/4 di q u e l l a di t r i e s t e , n e l l a s e c o n d a c i r c a i 2/3. ciò è d o v u t o i n rl v e n t o s u l l ' a l t i p i a n o c a r s i c o di t r i e s t e 2 9 1 t a b e l l a v i i — velocità media del vento a trieste. a n n o 1 il i n i v v v i v i i vili i x x x i x i i a n n o 1 9 1 8 0.0 1 5 . 0 1 2 . 3 1 2 . 6 9 . 1 9 . 5 1 1 . 7 1 0 . 3 8 5 1 6 . 0 1 5 . 3 1 7 . 0 1 1 . 9 1 9 4 9 1 2 . 8 1 0 . 9 1 8 . 3 6 . 7 1 2 6 1 0 . 7 7 . 6 l l . ( ) 9 . 6 2 0 . 5 1 9 . 0 9 . 0 1 2 . 4 1 9 5 0 2 1 . 3 9 . 1 1 3 . 4 9 4 1 0 . 8 1 0 . 1 1 1 . 7 8 . 0 9 . 5 1 4 . 8 1 0 . 1 1 5 . 1 1 1 . 9 1 9 5 1 1 3 . 1 1 4 . 3 1 4 . 1 1 2 9 1 0 . 4 9 . 4 1 0 . 5 9 . 8 1 2 , 7 2 2 . 9 1 0 1 8 . 6 1 2 . 4 1 9 5 2 1 0 . 9 1 1 . 0 9 . 6 1 1 . 2 9 . 3 1 0 . 2 i l 2 8 . 1 1 1 . 2 1 0 7 8 . 7 2 1 . 4 1 1 . 1 m e d i a 1 2 . 8 1 2 . 1 1 3 . 5 1 0 . 6 1 0 . 4 1 0 . 0 1 0 . 5 9 6 io.3 1 7 . 0 1 2 , 6 1 4 2 1 1 . 9 t a b e l l a v i l i —velocità media del vento nei singoli quadranti a n n o i u in iv v v i v i i vili i x x x i x i i a n n o l ° ( , ) . 1 9 4 8 7 . 6 1 7 . 7 1 6 . 1 1 4 . 2 1 2 . 6 1 1 . 2 1 6 . 2 1 9 . 8 1 2 . 4 2 2 . 4 1 7 . 6 1 9 . 9 1 5 . 6 1 9 4 9 1 7 . 5 1 6 . 4 2 0 . 2 1 3 . 5 2 4 . 2 1 6 . 9 1 1 . 2 1 8 . 4 1 6 . 8 2 5 . 5 2 7 . 9 1 1 . 9 1 8 . 4 1 9 5 0 2 8 . 4 1 6 . 4 2 0 . 6 1 0 . 9 2 0 . 3 1 7 . 1 1 5 . 7 1 2 . 2 1 2 . 7 2 6 . 5 1 5 . 3 1 7 . 4 1 7 . 8 1 9 5 1 2 3 . 1 1 5 . 4 1 9 . 8 2 3 . 6 1 4 . 9 1 2 . 6 1 3 . 9 1 2 . 0 1 7 . 5 2 5 . 5 1 2 . 8 1 5 . 0 1 7 . 2 1 9 5 2 1 3 . 8 2 0 . 3 1 4 . 5 1 7 . 2 1 2 . 5 1 1 . 2 1 4 . 0 1 0 . 1 1 4 . 5 1 3 . 8 1 2 . 1 2 8 . 1 1 5 . 2 m e d i a 1 8 . 1 1 7 . 2 1 8 . 2 1 5 . 9 1 6 . 9 1 3 . 8 1 4 . 2 1 4 . 5 1 4 . 8 2 2 . 7 1 7 . 1 1 8 . 5 1 6 . 8 l l 0 o . 1 6 . 8 1 9 4 8 6 . 7 1 . 7 6 . 9 1 1 . 0 7 . 4 1 1 . 4 7 . 9 6 . 8 5 . 2 2 . 7 3 . 9 1 . 7 6 . 1 1 9 4 9 7 . 3 3 . 7 0 . 0 6 . 8 :\2 4 . 8 7 . 5 1 5 . 0 2 . 9 8 . 0 7 . 0 4 . 4 5 . 9 1 9 5 0 1 . 9 5 . 6 3 . 7 8 . 9 •'.4 7 . 8 0 . 0 6 . 0 4 . 9 7 . 6 8 . 8 1 0 . 2 5 . 9 1 9 5 1 . . 9 1 3 . 6 1 0 . 9 7 . 6 7 . 8 7 . 2 7 . 9 8 . 1 7 . 8 4 . 9 9 . 6 5 . 5 8 . 2 1 9 5 2 7 . 3 6 . 6 4 . 4 1 0 . 3 9 . 0 9ì5 8 . 0 9 . 6 1 0 . 1 8 . 9 7 . 0 1 0 . 8 8 . 5 m e d i a 6 . 2 6 . 2 5 . 2 8 . 9 6 . 6 8 . 1 6 . 3 9 . 1 6 . 2 6 . 4 7 . 3 6 . 5 6 . 9 ill°<>. 1 9 4 8 2 . 1 3 . 6 0 . 0 o.o 0 . 0 1 6 . 3 0 . 0 7 . 9 1 . 4 0 . 0 0 . 0 0 . 4 2 . 6 1 9 1 9 1 . 5 0 . 0 0 . 0 5 . 7 4 . 0 0 . 0 3 . 7 0 . 0 0 . 0 6 . 0 7 . 4 0 . 8 2 . 4 1 9 5 0 2 . 3 0 . 0 0 . 0 9 . 6 1 1 . 9 7 . 8 3 . 1 5 . 7 0 . 0 4 . 7 0 . 0 0 . 0 3 . 8 1 9 5 1 4 . 8 6 . 0 9 . 9 6 . 7 6 . 9 8 . 4 6 . 8 8 . 6 8 . 1 0 . 0 2 . 5 5 . 3 6 . 2 1 9 5 2 0 . 0 3 . 4 3 . 9 5 . 3 7 . 0 8 . 0 1 1 . 3 6 . 5 8 . 6 1 2 . 8 2 . 4 6 . 6 6 . 3 m e d i a 2 . 1 2 . 6 2 . 8 5 . 5 6 . 0 8 . 1 5 . 0 5 . 7 3 . 6 4 . 7 2 . 5 2 . 6 4 . 3 i v 1 9 1 8 1 . 6 1 . 5 8 . 1 5 . 2 4 . 9 4 . 9 2 . 7 4 . 2 2 . 0 2 . 3 0 . 0 0 . 0 3 . 1 1 9 4 9 3 . 7 1 . 3 5 . 8 3 . 6 4 . 3 3 . 2 5 . 0 3 . 0 3 . 4 2 . 0 9 . 9 1 . 0 3 . 9 1 9 5 l 1 0 . 5 1 . 7 3 . 2 3 . 3 4 . 2 3 . 5 7 . 4 3 . 4 2 . 5 0 . 0 3 . 1 0 . 0 3 . 6 1 9 . 1 2 . 6 0 . 0 6 . 0 8 . 7 7 . 5 0 . 0 0 . 0 1 0 . 5 0 . 0 0 . 0 0 . 0 9 . 0 3 . . 1 9 5 2 0.1) 3 . 7 4 . 9 3 . 6 5 . 8 1 1 . 1 7 . 3 5 . 6 6 . 5 4 . 5 5 . 1 0 . 0 4 . 8 m e d i a 3 . 7 1 . 6 5 . 6 4 . 9 5 . 3 4 . 5 4 . 5 5 . 3 2 . 9 1 . 8 3 . 6 2 . 0 3 . 8 t a b e l l a i x — velocità media della bora. a n n o i ii in iv v v i v i i vili i x x x i xll a n n o 1 9 1 8 1 1 . 5 2 .8 1 6 . 5 1 5 . 4 1 5 . 4 1 1 . " i ; . 8 3 2 . 5 1 5 . 5 3 4 . 6 2 2 . 5 2 5 . 4 2 0 . 3 1 9 4 9 1 9 . 1 1 9 . 3 2 2 . 9 1 4 . 3 2 .8 1 7 . 6 1 2 . 0 2 0 . 7 2 0 . 0 2 9 . 3 3 6 . 3 1 8 . 9 2 1 . 5 1 9 5 0 4 0 . 6 1 7 . 9 2 2 . 8 1 4 . 6 2 0 . 3 2 0 . 5 1 5 . 9 1 2 . 2 1 3 . 3 3 2 . 0 1 5 . 3 2 0 . 8 1:0.5 1 9 5 1 2 6 . 6 1 9 . 4 2 2 . 1 2 3 . 6 1 8 . 1 1 4 . 9 1 4 . 7 1 1 . 7 2 0 . 5 2 7 . 9 1 5 . 5 1 5 . 8 1 9 . 2 1 9 5 2 1 5 . 6 2 3 . 5 il .3 2 1 . 9 1 5 . 8 1 1 . 8 1 4 . 8 1 1 . 9 1 5 . 3 1 5 . 7 1 3 . 4 3 1 . 8 1 7 . 5 m e d i a 2 2 . 7 2 1 . 0 2 0 . 5 1 8 . 0 1 9 . 5 1 5 . 3 1 5 . 0 1 7 . 8 1 6 . 9 2 7 . 9 2 0 . 6 2 2 . 5 1 9 . 8 2 9 2 silvio r o l l i t a b e l l a x — velocità media del vento (1948-52). l o c a l i t à i l ì i n iv v v i v i i v i l i i x x x i x i i a n n o o p i c l n a 9 . 6 9 . 7 1 0 5 8 . 4 7 . 5 7 . 3 7 . 6 6 . 8 7 . 3 1 1 . 4 9 . 0 1 0 . 6 8 . 8 b a s o v i z z a 9 . 6 1 0 . 3 1 0 , 4 1 0 . 3 7 . 8 7 . 6 7 . 4 6 . 5 7 . 2 1 2 . 9 9 . 9 1 1 . 3 9 . 3 t r i e s t e 1 2 . 8 1 2 . 1 1 3 . 5 1 0 . 6 1 0 . 4 1 0 . 0 1 0 . 5 9 . 6 1 0 . 3 1 7 . 0 1 2 . 6 1 4 . 2 1 1 . 9 tab. x i — velocità media del vento nei singoli quadranti (1948-52) l o . a i t à ì i i i n iv v vi v i i v i l i ix x x i x i i a n n o i ° q u a d r opic>fla 1 3 . 1 1 2 . 3 1 3 . 8 1 2 . 0 1 0 . 9 9 . 4 9 . 3 9 . 5 9 . 8 1 1 . 6 1 1 . 6 1 3 . 5 1 1 . 7 b a s o v i z z a 1 2 . 7 1 3 . 7 1 3 . 2 1 5 . 0 1 2 . 2 9 . 7 9 . 6 8 . 3 9 . 3 1 6 . 7 1 2 . 9 1 3 . 8 1 2 . 3 i r i e s l e 1 8 . 1 1 7 . 2 1 8 . 2 1 5 . 9 1 6 . 9 1 3 . 8 1 4 . 2 1 4 . 5 1 4 . 8 2 2 . 7 1 7 . 1 1 8 . 5 1 6 . 8 l l ° q u a d r . o p i c i n a 6 . 2 6 . 5 4 . 6 7 . 0 5 . 9 6 . 1 5 . 4 6 . 3 5 . 2 5 . 2 6 . 4 5 . 8 5 . 9 b a s o v i z z a 6 . 5 6 . 4 5 . 0 8 . 4 5 . 8 6 . 9 5 . 8 6 . 8 5 . 5 4 . 7 7 . 1 7 . 0 6 . 4 t r i e s t e 6 . 2 6 . 2 5 . 2 8 . 9 6 . 6 8 . 1 6 . 3 9 . 1 6 . 2 6 . 4 7 . 3 6 . 5 6 . 9 i i n u a d r . u p i c i n a 1 . 7 1 . 8 1 . 9 4 . 9 4 . 7 5 . 9 4 . 5 4 . 1 3 . 0 3 . 7 2 . 2 3 . 0 3 . 4 b a s o v i z z a 2 . 3 2 . 0 2 . 1 5 . 7 4 . 6 5 . 4 4 . 0 3 . 7 2 . 7 3 . 5 2 . 3 2 . 8 3 . 4 t r i e s t e 2 . 1 2 . 6 2 . 8 5 . 5 6 . 0 8 . 1 5 . 0 5 . 7 3 . 6 4 . 7 2 . 5 2 . 6 4 . 3 l m u a d r . u p i c i n a 3 . 7 3 . 1 4 . 9 4 . 9 4 . 6 4 . 3 4 . 1 4 . 9 3 . 2 2 . 2 2 . 6 1 . 5 3 . 7 b a s o v i z z a 5 . 1 2 . 7 5 . 3 5 . 7 5 . 0 3 . 8 3 . 5 4 . 1 2 . 8 1 . 9 2 . 9 1 . 9 3 . 7 ì r . e s l e 3 . 7 1 . 6 5 . 6 4 . 9 5 . 3 4 . 5 4 . 5 5 . 3 2 . 9 1 . 8 3 . 6 2 . 0 3 . 8 t a b e l l a x i i —velocità media della bora (1948-52). l o c a l i t à i i i i i i iv v vi v i i v i l i i x x x i x i i a n n o o p i c l n a 1 5 . 8 1 4 . 6 1 5 . 3 1 3 . 2 1 2 . 5 1 0 . 1 9 . 7 1 1 . 1 1 1 . 0 1 7 . 4 1 4 . 1 1 6 . 0 1 3 . 4 b a s o v i z z a 1 5 . 7 1 6 . 8 1 4 . 4 1 6 . 5 1 3 . 6 1 0 . 7 1 0 . 0 1 1 . 2 8 . 9 1 9 . 8 1 4 . 2 1 5 . 9 1 4 . 0 t r i e s t e 2 2 . 7 2 1 . 0 2 0 , 5 1 8 . 0 1 9 . 5 1 5 . 3 1 5 . 0 1 7 . 8 1 6 . 9 2 7 . 9 2 0 . 6 2 2 . 5 1 9 . 8 p a r t e al f a t t o c h e s u l l ' a l t i p i a n o i l m o t o d e l l e masse d'aria a v v i e n e su u n a z o n a estesa, secondo d i s l i v e l l i e g r a d i e n t i , sia t e r m i c i c h e b a r o m e t r i c i , d i s c r e t i ; i n p a r t e all'essere l a b o r a u n v e n t o f r e d d o di cad u t a c h e p r e c i p i t a n d o dal m a r g i n e d e l l ' a l t i p i a n o al m a r e a u m e n t a g r a d a t a m e n t e l a sua v e l o c i t à . ne consegue c h e s u l l a città e sul p o r t o l a b o r a assume c a r a t t e r i di v o r t i c o s i t à c h e s u l l ' a l t i p i a n o d i f f i c i l m e n t e possono r e a l i z z a r s i . p e r l ' i n s i e m e dei v e n t i del i q u a d r a n t e v a l g o n o v a l o r i p e r c e n t u a l i poco d i v e r s i , dato c h e l a b o r a n e è il v e n t o p r e d o m i n a n t e f r a essi. c) p e r i v e n t i d e l ii q u a d r a n t e , c h e c o m p r e n d o n o q u e l l i da scirocco, si h a n n o p u r e v e l o c i t à m e d i e m i n o r i s u l l ' a l t i p i a n o r i s p e t t o a l l a rl v e n t o s u l l ' a l t i p i a n o c a r s i c o di t r i e s t e 2 9 3 c i t t à e al m a r e . a o p i c i n a e b a s o v i z z a essi r i s u l t a n o r i s p e t t i v a m e n t e l ' 8 5 e il 9 3 % di q u e l l i r e g i s t r a t i a t r i e s t e . i v e n t i sciroccali, p r o v e n i e n t i d a l m a r e , c o l p i s c o n o d i r e t t a m e n t e l a città e si s m o r z a n o lent a m e n t e n e l s o l l e v a r s i s u l l ' a l t o p i a n o e n e l l ' a l l o n t a n a r s i d a l m a r e . d) c i ò v a l e p u r e p e r i v e n t i d e l i i i q u a d r a n t e , c h e c o m p r e n d o n o q u e l l i da l i b e c c i o e l e b r e z z e di m a r e e di t e r r a . queste u l t i m e , allont a n a n d o s i d a l l a z o n a c o s t i e r a , si i n d e b o l i s c o n o r a p i d a m e n t e . d a t o i l p r e d o m i n i o d e l l e b r e z z e , si h a c h e i v a l o r i m a s s i m i d e l l e v e l o c i t à c a d o n o n e i m e s i estivi, i m i n i m i in q u e l l i i n v e r n a l i . e) t v e n t i del i v q u a d r a n t e c o m p r e n d o n o q u e l l i d e l n o r d , c h e p r o v e n e n d o d a l r e t r o t e r r a s c e n d o n o al m a r e d o p o a v e r s o r p a s s a t o l e d u e l o c a l i t à carsiche. l e v e l o c i t à m e d i e figurano a n c o r a l i e v e m e n t e i n f e r i o r i s u l l ' a l t i p i a n o . l a discesa l u n g o i p e n d i i m a r g i n a l i f a acquis t a r e a q u e s t i v e n t i f r e d d i u n a m a g g i o r e v e l o c i t à , p e r c u i a r r i v a n o s u l l a c i t t à con u n a v e l o c i t à i n c r e m e n t a t a del 3°/o circa. /) t r a l e d u e l o c a l i t à c a r s i c h e c o n s i d e r a t e risulta s e m p r e p i ù v e n tosa q u e l l a di b a s o v i z z a . ciò d i p e n d e d a l f a t t o c h e essa si t r o v a ad t i n a q u o t a m a g g i o r e e i n u n a z o n a l e g g e r m e n t e p i ù esposta. o p i c i n a i n v e c e giace i n p o s i z i o n e l e g g e r m e n t e c o n c a v a e p e r t a n t o p i ù r i p a r a t a . trieste — istituto talassografico — gennaio 1953. riassunto si presentano i risultati di cinque anni di rilievi anemometrici eseguiti nelle località di opicina e basovizza, sull'altipiano carsico di trieste. si confrontano i dati, con quelli di trieste mettendo in particolare evidenza il regime della « bora ». risulta che questa, contrariamente all'opinione generale, è meno intensa e meno vorticosa sul carso che al mare. su \1 mary the results of fi ve years of anemoni etric reliefs carried out in opicina arici basovizza (carsic tableland of trieste) are presented here. tliese data are confronted with the data of trieste, making especially evidente the regimen of the « bora ». the result is that this, ori the contrary of the general opinion, is less intens and less ivhirling ori the carso tliat ori the sea. 2 9 4 s i l v i o p o l l i b i b l i o g r a f i a s. polli, 100 anni di osservazioni meteorologiche eseguite a trieste. p a r t e i v (la velocità del vento), p . v . (la direzione del vento), p . v i (le raffiche del vento); b o l l . società a d r i a t i c a di s c i e n z e n a t u r a l i , t r i e s t e , v o i . x l i v ( 1 9 4 8 ) , x l v ( 1 9 4 9 ) , x l v i ( 1 9 5 0 ) . s. polli, le correnti aeree al suolo e in quota a trieste. r i v . di m e t e o r o l o g i a a e r o n a u t i c a , a. i v , n. 2, r o m a 1 9 4 0 . s. polli, dati climatici di trieste e dintorni. p u b b l . n. 284, i s t i t . t a l a s s o g r . , t r i e s t e 1 9 5 2 . geo 1269 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 3 ionospheric effects on terrestrial communications: working group 3 overview jan laštovička (1) and alain bourdillon (2) (1) institute of atmospheric physics, academy of sciences of czech republic, prague, czech republic (2) institut d’electronique et télécommunications de rennes, université de rennes 1, france telecommunications via ionospheric reflection of radio signals of ground-based transmitters are a traditional area. however, this technique is still in use in telecommunications, broadcasting, etc. various problems have not yet been solved and some of them were studied in working group 3 (wg 3). structure of wg 3 and the terms of reference of its four working packages are described in the introductory paper by zolesi and cander (2004). here we describe the main results achieved in cost 271 in the following areas: i) large-scale fluctuations of planetary and gravity waves; ii) development of a new type of hf channel simulator; iii) geomagnetic storm effects on the f1-region ionosphere; iv) the sporadic e-layer and spread-f phenomena; v) the hf radio wave propagation over northerly paths; vi) how to increase the bit rate in ionospheric radio links. in general, substantial progress was achieved but some problems remain open for future investigations. 3.1. introduction in spite of the rapid development of gps-type techniques, tropospheric microwave communications and other modern techniques, ionospheric radio propagation plays an important role in terrestrial communications, radio location, radar and remote sensing applications. working group (wg) 3 joined scientists dealing with such problems with emphasis on the european area, i.e. middle and high latitudes. the activity of wg 3 was defined by the following terms of reference: establishing the ionospheric effects (i.e. large-scale as well as small-scale ionospheric fluctuations and irregularity effects) on the behavior of the ionosphere and terrestrial communications, including remote sensing, radio location and radar techniques. investigations were organized into four working packages (wp): wp 3.1 – effects of large-scale ionospheric fluctuations on terrestrial communications, including remote sensing, radio localization and radar. wp 3.1 covered investigations of effects of gravity and planetary wave effects on the ionosphere and radio wave propagation, and part of high latitude, off great circle propagation studies. wp 3.2 – effects of small-scale ionospheric irregularities, interference and noise on terrestrial systems. wp 3.2 investigated effects of ionospheric irregularities on the hf channel by a theoretical approach with the objective to realize an hf channel simulator. 1270 jan laštovička and alain bourdillon wp 3.3 – mid-latitude ionospheric features in radio propagation models. wp 3.3 dealt with the impact of geomagnetic storms on the f1-region, with sporadic e-layers and examination of the spread-f phenomenon, all preferentially at european middle latitudes. wp 3.4 – development of methods and algorithms to minimize the deleterious effect of the ionosphere on terrestrial communications. wp 3.4 considered the problem of hf propagation over northerly paths. also a method was proposed to counteract the deep fading that affects the skywave propagation so as to increase the bit rate. more detailed information can be found in the five papers by altadill, zernov, bencze, warrington and perrine, which describe in this issue the research and the results achieved in individual areas of wg 3. why did we deal just with the above problems? planetary wave type effects on the ionosphere had been studied relatively little in the past, because planetary waves cannot penetrate directly into the ionosphere. however, such oscillations significantly affect the ionosphere and, thus, the radio wave propagation on time scales of days to weeks. as for gravity waves, effects of those of auroral origin have been broadly studied but much less activity was devoted in the past to gravity waves of different origin, namely meteorological origin and those related to solar terminator. they are important for ionospheric variability on time scales of tens of minutes to hours and, therefore, for forecasting. the ionosphere is substantially influenced by geomagnetic storms. while many authors studied storm effects on the f2-region, those on the f1-region have been studied very little (e.g., buonsanto, 1999). but, during geomagnetic storms, the f1-region plays a more important role in radio wave propagation, and under the g-conditions, when the weak f2-layer is screened by the f1-layer, the f1-region dominates the ionospheric radio wave propagation. the radio propagation is also remarkably affected by two types of ionospheric irregularities studied in wg 3, by sporadic e-layers and by the spreadf phenomenon. wide deviations in the direction of arrival of ionospherically propagating radio signals from the great circle path, caused particularly at northerly paths by the presence of the ionospheric trough, can substantially deteriorate the quality of radio propagation and, therefore, have to be examined. the hf ionospheric channel simulators had been developed in the past as empirically based models (e.g., mastrangello et al., 1999). within wg 3 activities, a physically based software simulator has been developed for the hf ionospheric reflection fluctuating channel of radio propagation, which is qualitative progress in the field of hf channel simulators, necessary for hf radio link planning and good performance. hf ionospheric radio signal propagation needs much less infrastructure than satellite links and can achieve a very long distance transmission. therefore methods to improve the quality of the hf ionospheric transmission of digital information are studied. the investigations made in the framework of wg 3 resulted in the significant progress in some areas, as shown below. 3.2. effects of gravity and planetary waves gravity and planetary waves are responsible for a part of the uncertainty of ionospheric radio wave propagation predictions and forecasts for telecommunication purposes. if we succeed in predicting their effects on the ionosphere, it would be possible to noticeably improve the accuracy of predictions and forecasts. this is the motivation of cost 271 investigations into planetary and gravity wave effects on the ionosphere. gravity waves, oscillations with periods from several minutes to several hours, reach to the midlatitude ionosphere either from the auroral zone in connection with geomagnetic and auroral activity, or from below where they are excited by various meteorological and other processes in the neutral atmosphere, or they are excited in situ by a solar terminator or solar eclipses. their effects on the mid-latitude f-region ionosphere were studied in collaboration between the institute of atmospheric 1271 ionospheric effects on terrestrial communications: working group 3 overview physics in prague and observatorio del ebro in roquetes (e.g., boška and šauli, 2001; šauli and boška, 2001; boška et al., 2003). the most important result of gravity wave studies for radio wave propagation is probably the regular occurrence of morning and evening wave bursts. they are regularly observed in the f-region ionosphere, particularly in the morning, when they occur so regularly that they should be included into short-term forecasts. these gravity waves are related to the solar terminator passage and related large and rapid changes in atmospheric temperature. the typical duration of wave events is about 4-6 h, their amplitudes in electron density or heights of levels of constant electron density show a high degree of variability, these gravity waves originate in the transition region between the f1and f2-layers at heights of about 180-200 (220) km, and they propagate upwards and downwards simultaneously from that region. the gravity waves excited by the solar terminator remain in the spectra also during days of geomagnetic storms and display a similar vertical structure as during quiet days, while additional strong wave enhancements of auroral origin occur. the latter however do not exhibit vertical propagation. the sunrise terminator-related gravity waves are good candidates for inclusion in ionospheric predictions. a similar vertical structure was observed with gravity waves excited by solar eclipses. the border of the eclipse shadow acts as a solar terminator. eclipses are well predicted in advance, so their effects can be included in radio wave propagation forecast, but they occur very rarely, thus their practical importance is low. whereas ionospheric effects of gravity waves of auroral origin have been widely studied in the past, and the waves excited by solar terminator and solar eclipses have also been studied to some extent, the effects of waves of meteorological origin have been studied very little by others. the specific gravity wave events are those caused by meteorological cold front passages. the ionospheric response to such gravity waves is well pronounced and displays specific features, but it is less regular than that of the solar terminator. the dominant feature is remarkable strengthening of gravity wave activity in the period range of about 60-80 min. the lifetime of such events is up to 6 h, and their energy propagates upwards at an average rate of a few ms-1. planetary waves are large-scale to global oscillations with periods of about 2-30 days, which are predominantly of tropospheric origin. when we deal with the planetary wave type oscillations in the ionosphere, it is better to use the term planetary wave signatures, because part of these oscillations is caused by quasi-periodic changes of geomagnetic activity, not only by planetary waves coming from below (altadill and apostolov, 2003). planetary waves of tropospheric origin can penetrate to f-region heights only indirectly, for example through planetary wave modification of upward propagating tides in the mesosphere and lower thermosphere (mlt) region. the planetary wave signatures in the f-region were studied in collaboration between observatorio del ebro in roquetes, the institute of atmospheric physics in prague, and the geophysical institute in sofia (e.g., altadill and apostolov, 2001, 2003; laštovička et al., 2003). the planetary wave signatures in the f-region occur as bursts of duration of several wave cycles. an important, albeit rather negative finding concerns the persistence of such events. the typical occurrence of planetary wave signatures with periods from 2 to 16 days is from 12 to 35% of the time respectively, and their typical duration ranges from 4 to 3 wave cycles respectively. however, the spectral distribution of duration of events is too broad to allow for a reasonable prediction of event duration based only on f-region measurements. this means that planetary wave signatures remain unpredictable from ionospheric measurements themselves, and we have to search for appropriate predictors both in the mlt region parameters and in geomagnetic activity. the planetary wave signatures in the midlatitude f-region are limited both temporally, as shown above, and spatially, even though they are large-scale phenomena. their typical longitudinal size was found to range from 80º for periods of 2-3 and 5-6 days to 180º for periods of 16 days. as mentioned above, some planetary wave signatures in the f-region are of geomagnetic activity origin. it is of some importance (also for possible predictions) to know seasonal variations of the 1272 jan laštovička and alain bourdillon role of different causes of planetary wave signatures. solar flux variations were found to be responsible for a very small contribution. the planetary wave signatures related to the geomagnetic activity tend to occur during summer half year. those related to the planetary wave activity in the mlt region tend to occur during summer half of the year for planetary waves with shorter periods, and those with longer periods tend to occur during the winter half year. 3.3. the hf ionospheric fluctuating channel to properly characterize the ionospheric reflection hf fluctuating channel of propagation on a physical basis, a comprehensive solution has been constructed for high frequency wave propagation in the 3d inhomogeneous (rigorously also anisotropic) ionosphere with local random inhomogeneities embedded. this entails a comprehensive solution to the problem of wave propagation in a random medium for the most general case of a 3d inhomogeneous dispersive medium with fluctuations of the parameters including the case of strong scintillation (strong fluctuation of the field amplitude), or that of the saturated regime of propagation. although a rigorous solution to this problem is not currently available, the best available solution has been considered, based on the complex phase method (in classical terms, rytov’s method), which has been extended to the case of an inhomogeneous medium and a point source of the field. first, zernov (1980) extended this method to the case of a plane-layered background medium and a point source. this extension was intensively employed in a series of papers (gherm and zernov, 1995, 1998; gherm et al., 1997a,b, 2001a,b, 2002, 2003) to study effects due to fluctuations of the electron density in the plane-stratified ionosphere. gherm and zernov (1995) studied the statistical moments of the phase and log-amplitude of the hf field, in particular, the effect of fresnel filtering. subsequent papers (gherm et al., 1997a,b) were devoted to the effects on propagation of both narrowand wideband pulses in the fluctuating ionosphere. finally, the most general theory was developed in the scope of the project «wideband hf and uhf simulators for ionosphericaly reflected and transionospheric channels», (under the financial support of the u.k. epsrc visiting fellowships programme) where the effects of the anisotropy of the ionosphere (including the background ionosphere) were taken into account. this project was performed in collaboration between the university of st. petersburg, st. petersburg, russia (nnz and veg) and the university of leeds, leeds, u.k. (hjs). this investigation resulted in producing a physically based software simulator for the hf ionospheric reflection fluctuating channel of propagation, which overcame limitations of empirically based models. the wideband hf simulator constructed is based on a detailed physical model. it can generate an output giving a time realisation of the hf channel for any bandwidth (up to a mhz) and for any given time, path and conditions. to characterise the hf channel of propagation, an analytic-numerical technique has been used, which employs the complex phase method that was recently extended to the case of a 3d inhomogeneous and even anisotropic medium. 3.4. geomagnetic storms and f1-region ionosphere the f1-region and its response to geomagnetic storm-induced disturbances have been studied much less than the f2-region, mainly because of its relatively small importance for radio wave propagation under undisturbed conditions. however, during geomagnetic storms under the g-conditions, when the f2region electron density is so small that it is screened by the f1-region, the role of the f1-region is principal. these two above facts are the motivation of the f1-region studies in cost 271. these studies were done prevailingly in the institute of atmospheric physics in prague with small contributions from spanish and hungarian partners (e.g., burešová and laštovička, 2001, 2003; burešová et al., 2002). 1273 ionospheric effects on terrestrial communications: working group 3 overview it was found that irrespective of the sign of the geomagnetic storm effect on nmf2 (maximum electron density in the f2-region), the effect on daytime electron density at the f1 heights at european higher middle latitudes was always negative (electron density depletion), if any at all. at european lower middle latitudes, the effects at f1-region heights were weaker and less regular. there is a significant seasonal variation of the geomagnetic storm effect on the f1-region with no significant effect of geomagnetic storms on electron density at the f1 heights in the range of 160-190 km during the summer half of the year except for the moderate-to-minor effect of the super storm. on the other hand, in the winter half of the year at higher middle latitudes there is a well-pronounced effect of magnitude increasing with height. a much larger effect is found in autumn and winter versus spring and summer for station chilton in england. the pattern of the geomagnetic storm effects on the f1region electron density does not appear to change with solar activity (solar cycle). the explanation of the observed effects is not sufficiently clear, but the upward motion of the boundary between the region dominated by molecular ions and the region dominated by atomic ions evidently plays a role. another interesting result is finding that for ionospheric storms with the positive phase, the maximum of storm effect is located well below the maximum of the f2-region. 3.5. sporadic e-layer and spread-f phenomena the radio wave propagation by reflection from well-developed sporadic e(es-) layers has been used in radio communication and radar location/ranging, and the spread-f phenomenon deteriorates the quality of radio signals propagating via the ionosphere. therefore it was necessary to study these two phenomena and their occurrence. the studies were realized in the geodetical and geophysical institute in sopron (e.g., bencze and bakki, 2002; bencze and märcz, 2002). the ionosphere depends substantially on the 11-year solar cycle. however, it was found that there was no change in h′es (height of the es-layer) exceeding the accuracy of ionosonde scaling (5 km) between the solar activity maximum and minimum years (bencze and märcz, 2002). this means that the change in the average es-layer height with solar cycle is not very important for radio wave propagation predictions. bencze and bakki (2002) analysed measurements from uppsala (59.8°n) and lannion (48.45°n). the daily variation of the simultaneous occurrence of spread-f and geomagnetic ae index enhancements displayed night time maximum. the number the simultaneous occurrence attains at the time of the morning is a maximum 30% of the total number of spread-f occurrences in uppsala and 23% in lannion. a delay of about 4 h was observed between the morning maximum of the simultaneous occurrence of spread-f and ae enhancements at the mid-latitude lannion station as compared with uppsala station. this delay may be connected with the propagation of travelling ionospheric disturbances from auroral latitudes. based on the results that concern travelling ionospheric disturbances as a source of ionospheric irregularities in the f-region, the second source, instabilities, may contribute during the morning maximum by about 70% to the development of irregularities. thus, instabilities appear to be the dominant source of irregularities and spread-f. however, it seems that the role of travelling ionospheric disturbances may not be neglected. 3.6. hf radio wave propagation over northerly paths wide deviations in the direction of arrival of ionospherically propagating radio signals from the great circle path (gcp) have serious implications for the planning and operation of communications and radiolocation systems operating within the hf-band. perhaps the most obvious example lies in the operation of radiolocation systems which usually operate by measuring the direction of arrival at 1274 jan laštovička and alain bourdillon several receiving sites. the location of the transmitter is then estimated from the intersection of the individual lines of bearing from each receiving site, and deviations from the gcp will therefore adversely affect the estimate of the transmitter location. the importance of off-great circle propagation extends beyond radiolocation to almost any hf system which employs directional antennas. with these systems, there is a significant possibility that performance will be degraded at times when the supported propagation path is in directions well displaced from the main lobe of one or both of the transmitting or receiving antennas. in addition to the large scale tilts which cause gross deviations of the signal from the great circle direction, irregularities in the electron density distribution cause signals associated with each propagation mode to arrive at the receiver over a range of angles in both azimuth and elevation. such directional spread of the received signal energy is an important parameter to be considered in the design of multi-element receiving arrays and the associated signal processing methods used, for example, in radiolocation or adaptive reception systems. it is often assumed in the design of such systems that the signal environment comprises a limited number of specularly reflected signals arriving at the antenna array from well-defined directions. however, for northerly paths, this is often not the case, and azimuthal standard deviations of several tens of degrees have been measured over polar cap paths (warrington, 1998). a common feature of northerly hf propagation is the large doppler and delay spreads imposed on the signal. the magnitude of these effects is such as to severely limit the data throughput achievable in hf communications systems due to current technological limitations in modem design (angling et al., 1998). the large doppler spreads are often associated with directional spreading and recent work (warrington et al., 2000) has indicated that adaptive beam/null steering from an array of antennas can be employed to exploit the directional spreading effects to reduce the apparent doppler spread at the modem input. various measurements of off great-circle propagation effects over a range of northerly paths and their interpretation have been undertaken over a number of years. significant progress has been made over the cost 271 period. this work was done by a team of the university of leicester. the important results of this research, to consider work in progress aimed at further improving our understanding of the high latitude propagation mechanisms, and to report on methods being developed for taking these propagation effects into account when designing and operating hf radio systems. 3.7. a way to increase the bit rate in ionospheric radio links hf waves (3-30 mhz), when propagated through the ionosphere, can achieve very long distance transmissions with a minimal infrastructure compared to satellite links, for example. this multipath and multimode channel strongly degrades transmissions (mainly fading and frequency selectivity). for this reason, the data rate of standard scalar hf modems does not exceed 4.8 kbps in 3 khz bandwidth. the originality of this study is to use a heterogeneous array to improve the hf transmission. indeed, it has been shown that such a device could achieve the direction finding regarding the incoming polarization as a decorrelation factor and, consequently, that the diversity of the spatial responses could, to some extent, replace the space diversity (erhel et al., 1998, 2004). the array processing performs on a set of four active collocated antennas or a circular array and the corresponding coherent receiving channels. the spatio-temporal equalization resorts to lms (least mean square) algorithm as the synchronization is based on a «zero crossing detector». it has been shown that a heterogeneous array can greatly improve the hf transmission, now offering the possibility to transmit images through the ionospheric channel. the general principle of the transmission system is described in the paper and experimental results are provided to show that this technique can increase the data rate, reaching 15 khz within a 3 khz bandwidth (qam-64) without 1275 ionospheric effects on terrestrial communications: working group 3 overview coding or interleaving. that work was realized at université de rennes1 and centre de recherches des ecoles de coëtquidan. 3.8. conclusions tasks of the four working packages of wg 3 have been defined by the four terms of reference presented in the introductory paper by zolesi and cander (2004). as for wp 3.1, much more was done than required by the terms of reference. the percentage contribution of investigated oscillations to the variability of fof2 has been established, plus the percentage of time when these oscillations occur and persistence of wave events have been estimated, main sources of wave oscillations have been identified including their relative role, and information on predictability for ionospheric predictions has been obtained. the task of wp 3.2 was modified. some information about the effects of irregularities was obtained, but the most important scientific result and practical product, the new physically based hf channel simulator, was constructed out of the original terms of reference. on the other hand, work done in wps 3.3 and 3.4 essentially corresponds to the terms of reference. significant progress was achieved in knowledge of the effects of large-scale irregularities on the ionosphere, particularly geomagnetic storm effects on the f1-region. considerable progress has also been reached in understanding the hf radio wave propagation along northerly paths, strongly affected by the ionospheric trough. major progress has been achieved in the ionospheric hf transmission of digital information based on the utilization of a heterogeneous array. this progress now allows us to transmit via the ionosphere pictures over distances of several hundred kilometers without loss of quality. in spite of the considerable progress achieved, various open questions must be addressed in future investigations. for instance, the planetary and gravity wave effects on the ionosphere and radio propagation require better quantification and further research into possible predictions, and the investigations need to be broadened by effects of ionospheric infrasonic waves and by studies of possible impacts on gnss-based techniques. further improvement of knowledge on digital radio systems would be very valuable, particularly as concerns predictions and methods of reliability estimating. the extension of hf simulators to the mf band is needed. increased capacity of the hf radio links should be a valuable output of future investigations. the space weather impact on the ionosphere and radio propagation and its prediction based on new space weather research products is another challenging area of investigations. acknowledgements the work of j.l. has been supported by the ministry of education, sports and youth of the czech republic via grant oc.271.10, and by the academy of sciences of the czech republic via grant s3012007. references altadill, d. and e.m. apostolov (2001): vertical propagating signatures of wave type oscillations (2and 6.5-days) in the ionosphere obtained from electron density profiles, j. atmos. solar-terr. phys., 63, 823-834. altadill, d. and e.m. apostolov (2003): time and scale size of planetary wave signatures in the ionospheric f-region. role of the geomagnetic activity and mesosphere/lower thermosphere winds, j. geophys. res., 108 (a11), 1403, doi:10.129/2003ja010015. angling, m.j., p.s. cannon, n.c. davies, t.j. willink, v. jodalen and b. lundborg (1998): mea1276 jan laštovička and alain bourdillon surements of doppler and multipath spread on oblique high latitude hf paths and their use in characterising data modem performance, radio sci., 33 (1), 97-107. bencze, p. and p. bakki (2002): on the origin of mid-latitude spread-f, acta geod. geophys. hung., 37, 409-417. bencze , p. and f. märcz (2002): a study of the variation of the transparency of es-layers, acta geod. geophys. hung., 37, 227-238. boška, j., and p. šauli (2001): observations of gravity waves of meteorological origin in the f-region ionosphere, phys. chem. earth (c), 26 (6), 425-428. boška, j., p. šauli, d. altadill, g. solé and l.f. alberca (2003): diurnal variation of the gravity wave activity at midlatitudes of the ionospheric f-region, studia geophys. geod., 47, 579-586. buonsanto, m.j. (1999): ionospheric storms – a review, space sci. rev., 88, 563-601. burešová, d. and j. laštovička (2001): changes in the f1-region electron density during geomagnetic storms at low solar activity, j. atmos. solar-terr. phys., 63, 537-544. burešová, d. and j. laštovička (2003): geomagnetic storm and f1-region over europe, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece (on line: http://www.cost271.rl.ac.uk/). burešová, d., j. laštovička, d. altadill and g. miro (2002): daytime electron density at the f1region in europe during geomagnetic storm, ann. geophysicae, 20, 1-15. erhel, y.m., l. bertel and f. marie (1998): a method of direction finding operating on an array of collocted antennas, in ieee-ap international symposium, atlanta, u.s.a. erhel , y.m., d. lemur, l. bertel and f. marie (2004): hf radio direction finding operating on a heterogeneous array: principles and experimental validation, radio sci., 39, rs1003 10.1029/ 2002rs002860. gherm, v.e. and n.n. zernov (1995): fresnel filtering in hf ionospheric reflection channel, radio sci., 30, 127-134. gherm, v.e. and n.n. zernov (1998): scattering function of the fluctuating ionosphere in the hfband, radio sci., 33, 1019-1033. gherm, v.e., n.n. zernov, b. lundborg and a. vastberg (1997a): the two-frequency coherence function for the fluctuating ionosphere; narrowband pulse propagation, j. atmos. solar-terr. phys., 59, 1831-1841. gherm, v.e., n.n. zernov and b. lundborg (1997b): the two-frequency, two-time coherence function for the fluctuating ionosphere; wideband pulse propagation, j. atmos. solar-terr. phys., 59, 1843-1854. gherm, v.e., n.n. zernov, b. lundborg, m. darnell and h.j. strangeways (2001a): wideband scattering functions for hf ionospheric propagation channels, j. atmos. solar-terr. phys., 63, 1489-1497. gherm, v.e., n.n. zernov and h.j. strangeways (2001b): scattering functions for multimoded wideband hf channels, in icap, manchester, u.k., conference publication no. 480, 393-396. gherm, v.e., n.n. zernov and h.j. strangeways (2002): multipath effects in wideband fluctuating hf channels, acta geod. geophys. hung., 37 (2/3), 253-259. gherm, v.e., n.n. zernov and h.j. strangeways (2003): wideband hf simulator for multipath ionospsherically reflected propagation channels, in twelfth international conference on antennas and propagation (icap 2003), iee cp 491, vol. 1, 128-131. laštovička, j., p. krizan, p. šauli, and d. novotna (2003): persistence of the planetary wave type oscillations in f 0f2 over europe, ann. geophysicae, 21, 1543-1552. mastrangelo, j.f., j.j. lemmon, l.e. vogler, j.a. hoffmeyer, l.e. pratt and c.j. behm (1999): a new wideband high frequency channel simulation system, ieee trans. commun., 45, 26-34. šauli, p., and j. boška (2001): tropospheric events and possible related gravity wave activity effects on the ionosphere, j. atmos. solar-terr. phys., 63, 945-950. warrington, e.m. (1998): observations of the directional characteristics of ionospherically propa1277 ionospheric effects on terrestrial communications: working group 3 overview gated hf radio channel sounding signals over two high latitude paths, iee proc. microwaves, ant. prop., 145 (5), 379-385. warrington, e.m., c.a. jackson and b. lundborg (2000): directional diversity of hf signals received over high latitude paths and the possibility of improved data throughput by means of spatial filtering, iee proc. microwaves, ant. prop., 147 (6), 487-494. zernov, n.n. (1980): scattering of waves of the sw-range in oblique propagation in the ionosphere, radiophys. quantum electron., 23, 109-114. zolesi, b. and lj.r. cander (2004): cost 271 action – effects of the upper atmosphere on terrestrial and earth-space communications: introduction, ann. geophysics, 47 (suppl. to no. 2/3), 915-925 (this volume). s u l l ' a u m e n t o d i t e m p e r a t u r a n e l m a n t e l l o d e l l a t e r r a p e r c o m p r e s s i o n e a d i a b a t i c a paolo emilio valle 1. — l'aumento della temperatura nell'interno della t e r r a , dovuto a compressione adiabatica, ha una notevole importanza in diverse questioni di carattere geofisico (*). la sua stima è stata eseguita da diversi autori, estrapolando all'interno della t e r r a i valori del rapporto fra la dilatazione termica e il calore specifico a pressione costante, determinati in laboratorio per varie rocce ignee. solo recentemente e stato fatto un tentativo per una valutazione in base alla teoria dei solidi ( 2 ). questa nota ha l o scopo di mostrare come si possa pervenire facilmente ad una relazione che lega la densità, l a temperatura e la velocità delle onde longitudinali e trasversali di un solido ideale, sottoposto ad una particolare trasformazione adiabatica e a quali risultati conduce la sua applicazione all'interno della t e r r a . 2. — è noto che l'agitazione termica in un solido si può pensare dovuta ad onde longitudinali e trasversali completamente diffuse ( 3 r*), per le quali esiste un limite inferiore alla lunghezza d'onda, in conseguenza della struttura discontinua del solido stesso. l o spettro delle frequenze dei due tipi di onde è quindi limitato superiormente. se si usano gli indici l e t per distinguere le grandezze che si riferiscono rispettivamente alle onde longitudinali e trasversali, la frequenza limite per ciascun tipo di onda è data da dove vm è la velocità, supposta indipendentemente dalla frequenza, ed tv il numero di atomi contenuti nel volume v. un solido così definito, cioè come un sistema continuo che ammetta un limite superiore per le frequenze delle onde elastiche, è il solido isotropo ideale. [1] (m = l,t) 4 7 6 p a o l o e m i l i o v a l l e i l numero complessivo delle oscillazioni è uguale ad n per le onde longitudinali e a 2 n per le onde trasversali. pertanto, il solido va trattato come un sistema di 3 n oscillatori armonici, dei quali 2v hanno la frequenza limite v, e 2 n la frequenza limite v,. i n questa schematizzazione la meccanica quantistica permette il computo statistico dell'entropia s (3~6). la sua variazione elementare risulta data da • ^ " ( ^ r j r b 1 2 1 (m = z, f, t) nella quale r è la costante dei gas e d(xm) = —-— f -f— xm [ 3 ] xms j et1 8 o se si introducono le temperature di dehye ©„,, ossia si pone e m = j t l [ 4 ] (m = l,t) dove h è costante di plank e k la costante di boltzmann, risulta xm = [ 5 ] essendo t la temperatura assoluta. i l secondo membro della [ 2 ] è quindi funzione della temperatura assoluta e del volume tramite le 0 m . si supponga ora di eseguire sul solido una trasformazione adiabatica reversibile, sottoponendolo ad una variazione isotropa ed omogenea di volume. si avrà dd d dx x ii dxm=0 [ 6 ] (m = j, t, t) se la temperatura t è molto inferiore alle temperature di debye, cioè x m è molto grande, si ha ^ t * " ^ m e si ottiene, tramite le [ 6 ] , [ 5 ] , [4] ed [1], s u l l ' a u m e n t o di t e m p e r a t u r a n e l m a n t e l l o della terra 477 t 3 / 1 2 -| ] = c o s t a n t e [ 8 ] e \ nella quale p è la densità, mentre se xm è piuttosto piccolo si può usare lo sviluppo — + [ 9 ] 20 1680 e risulta in modo analogo, per 0 m < < l ' 20 t , j o p t i v t ! =costante [lfl] è molto importante il fatto che la [ 8 ] e la [10] siano indipendenti dalla composizione chimica del solido. la [ 8 ] e la [10] si identificano se si pone v\=] 3 vt. 3. — nell'interno della terra, il valore della temperatura di dehye relativa alle onde longitudinali è presumibilmente intorno ai 1700" k alla profondità di 1000 km e ai 2000° k alla profondità di 2898 km, ossia alla base del mantello. le temperature di debye, relative alle onde trasversali, sono circa 1/y 3 volte le precedenti. è noto che fra queste due profondità si può ritenere che i materiali presentino l'omogeneità richiesta per l'applicazione della [8] o della [10], e che l'andamento della densità sia adiabatico ("). i presunti valori delle temperature di debye sopracitati in relazione al probabile valore della temperatura esistente alla profondità di 1000 km, richiedono l'uso della [ 1 0 ] , in base alle grandezze contenute nella tabella i (6~7), t a b e l l a i profondità t'i q km km sec 1 km sec 1 g c m ~ 3 1000 11,42 6,36 4,68 2600 13,50 7,21 5,54 2898 13,64 7,30 5,68 4 7 8 paolo e m i l i o v a l l e l dalla [10] si ottiene dalla [ 8 ] si otterrebbe t — 1 23 t 1 2808— j 1 1000 cioè la temperatura alla base del mantello sarebbe circa 1,24 volte la temperatura a 1000 km, se il materiale fosse stato compresso adiabaticamente. è ' interessnte il confronto con il calcolo eseguito da j . verhoogen ( 2 ). la [ 8 ] e la [10] forniscono rispettivamente 1̂000 ^s6oo = 1.22 7 1 0 0 0 mentre il predetto autore ottiene t s t 0 = 1 > 1 8 7\ooo roma — istituto nazionale di geofisica — dicembre 1951 riassunto viene stabilita una relazione fra la densità, la temperatura e la velocità delle onde longitudinali e trasversali relativa ad una particolare trasformazione adiabatica di un solido ideale. l'equazione, applicata al mantello della. terra, conduce al risultato che la temperatura in prossimità del nucleo sarebbe circa 1,24 volte, la temperatura alla profondità di 1000 km, se la compressione dei materiali fosse avvenuta adiabaticamente. b i b l i o g r a f i a ( ' ) benfield a . e . : temperature in an accreting earth. trans. amer. geophys union 31, 53-57 (1950). ( ) verhoocen j . : the adiabatic gradient in the mantle. trans. a m e r . geophys. u n i o n 32, 41-43 (1951). ( 8 ) briliouin l . : teuseurs en mècanique et en èlasticité. masson, paris (1946). ( 4 ) brillouin l . : [fave propagation in periodic structures. me gran-hill. newy o r k (1946). ( s ) slater j . c . : introduction to chemical physics. me. gran-hill.. new y o r k (1939). ( ° ) bullen k . e . : the problem of the earth'x density variation. bull. seism. soc. am. 30, 235-250 (1940). ( 7 ) bullen k . e . : introduction of the teory of seismology. cambridge university press (1947). layout 6 1 annals of geophysics, 63, 6, gd667, 2020; doi:10.4401/ag-8372 detecting ionospheric tec disturbances by three methods of detrending through dense cors during a strong thunderstorm mohamed freeshah1,4,5, xiaohong zhang*,2,3,4, jun chen2, zhibo zhao2, nahed osama1, mohammed sadek5,6, nana twumasi1,7 (1) state key laboratory of information engineering in surveying, mapping and remote sensing, wuhan university, 129 luoyu road, wuhan 430079, china (2) school of geodesy and geomatics, wuhan university, 129 luoyu road, wuhan 430079, china (3) key laboratory of geospace environment and geodesy, ministry of education, 129 luoyu road, wuhan 430079, china (4) collaborative innovation center for geospatial technology, 129 luoyu road, wuhan 430079, china (5) department of surveying engineering, faculty of engineering at shoubra, benha university, 108 shoubra st., cairo 11629, egypt (6) school of human settlements and civil engineering, xi’an jiaotong university, xi’an 710049, china (7) civil engineering department, wa polytechnic, box 553, wa, upper west region, ghana article history: received october 15, 2019; accepted september 17, 2020 abstract satellite navigation and communications system can substantially be disturbed by ionospheric perturbations. consequently, monitoring ionospheric anomalous has great significance. in this study, we focus on the short-term irregular disturbances through a strong thunderstorm in wuhan city, hubei, china by using ground-based gnss observations from dense continuously operating reference stations (cors) with a sampling rate of 1s. the total electron content (tec) was used to find possible perturbations after biases have been calibrated for the derived tec. additionally, the geomagnetic conditions and the state of solar radiation was checked in the study period to recognize the causes for the ionospheric disturbances. the maxima and minima values of tec deviations were ~2.5 and 0.5 tecu, respectively. three methods of detrended fluctuations analysis (dfa) were applied to assess the ionospheric disturbances over gnss cors stations; “multi-step numerical difference”(msnd), “6th order polynomials fitting” (pf), and “one-week average difference”(ad). the analyzed results showed that msnd has the lowest performance. meanwhile, the fitted tec data with 6th order polynomials technique presented an improvement and a discrepancy related to msnd. to resolve this discrepancy, we proposed ad technique, it accomplished the best performance related to the tec disturbances and was compared with the other two techniques. the research findings showed that ionospheric disturbed electrons can be generated with various rates and different velocities through lightning influences. keywords: ionosphere; total electron content (tec); thunderstorm; detrend; gnss. 1. introduction reliable and fast knowledge about ionospheric variations has become increasingly important. gnss users of single-frequency receivers need accurate corrections to remove signal degradation caused by the ionosphere. the ionospheric variations can be categorized as systematic and non-systematic. systematic variations either takes place in cycles or does not. consequently, it can be pre-modulated or predicted with reasonable accuracy [rama rao et al., 2006]. on the other hand, irregular variations which happen due to some phenomena such as earthquakes, tsunamis, and underground nuclear explosions (une) could not be expected [amieri-simkooei and asgari, 2012; jain, vijay, and gwal, 1996; park et al., 2011; salem et al., 2015; savastano et al., 2017; yu et al., 2015; zhang and tang, 2015]. there are many studies dealing with ionospheric disturbances which respond to the aforementioned hazards [amieri-simkooei and asgari, 2012; calais & minster, 1995; jain et al., 1996; park et al., 2011; salem et al., 2015; savastano et al., 2017; yu et al., 2015; zhang and tang, 2015]. the common factor for these hazards is a unique epicenter, where we can obtain the epicenter and determine its starting time clearly according to the different research centers. in addition, it is a chance to focus on the coupling between the solid-earth and atmosphere [liu and jin, 2019]. on the other side, ionospheric disturbances could be associated with lightning. however, it is quite difficult to deal with this issue where lightning has multi epicenters and different starting times. although the ionospheric anomalies have been observed through gps tec for about three decades [calais and minster, 1995], insufficient studies have been conducted on dealing with ionospheric response during lightning and their thunderstorm. with the increase of continuously operating reference station (cors), it supplies not only abundant measurements to observe the pattern and evolution of ionospheric disturbances, but also a magnificent opportunity for studying more details about the coupling between the tropospheric activities and higher atmosphere. the electromagnetic fields produced by natural thundercloud activities energy can transfer from the troposphere mainly through wave activity, i.e., gravity waves (gws) and infrasonic waves [blanc, 1985; chimonas, 1971; sentman et al., 2003]. therefore, the studies of ionospheric disturbances during lightning activity have been interested in exploring as well as monitoring the effect of this phenomenon through the ionosphere, where the lightning activity could disturb the tec in the ionosphere. a statistical study was conducted on it in 2005 which found a relationship between lightning activity and the sporadic e layer intensification [davis and johnson, 2005]. during the lightning, the ionosphere was substantially disturbed by electrical activity, which can cause variations in electron density through heating the electrons [layet al., 2013]. consequently, barta et al. [2013] have shown that the electron density of the sporadic e layer has decreased significantly associated with lightning based on infrared maps and lightning observations over the ionospheric station of rome. on the other side, lay et al. [2013] have fitted the gps total electron content (tec) data with a 6th order polynomial, therefore, the variations are calculated by subtracting this fit from the data. so, anomalous tec changes were closely associated with space and time to the large underlying thunderstorms. similar to the method described above, 6 dual frequency gps stations have been used with lightning data obtained from worldwide lightning location network (wwlln) over southern africa. the results have indicated that ~ 1 tecu varied in tec which corresponding to periods of lightning activity intensification in the studied regions [amin, 2015]. however, fitting method acts more sensitive only for the higher-frequency variations. additionally, it acts to filter out the lower-frequency changes [lay et al., 2013]. studying the response of the ionosphere to thunder provides a good way to understand the upper atmospheric coupling, where lightning or tropospheric activities may have transferred to the high atmosphere, and then cause ionospheric disturbances [zhang and tang, 2015]. it is well known that detrending techniques is the most common method to analyze ionospheric disturbances. park et al. [2011] used the “numerical difference method” to detect the ionospheric fluctuations using gps observations. afterward, [zhang and tang, 2015] presented a modified “numerical difference method” with multiple time steps to detect traveling ionospheric disturbances (tids) induced through une at north korean by using gps dual-frequency data. the effect of the time step is very relevant to the detectable period of the spectrum. the previous works have concluded that detrending methods may have a weakness for one or more of these reasons: (1) skips detectable disturbance, (2) time step could be relevant to the detectable period, (3) filter out lower-frequency variations [park et al., 2011; salem et al., 2015; x. zhang and tang, 2015]. it is worthy to indicate that we applied the first two detrending methods (1) multi-step numerical difference (msnd) and (2) six order polynomials fitting (pf), in order to assess the influence of thunderstorm activities on ionospheric perturbation through the local steady-state nighttime ionospheric over a small area by a dense gnss mohamed freeshah et al. 2 cors. the defects of the two detrending techniques have been confirmed through our research findings. for this reason, we have found the rationale to propose a third superior approach “one-week average difference (ad)” to detrend the data and it can avoid the aforementioned weaknesses reasons. where the proposed method does not depend on time step, it working with the whole time period, so weakness (1) and (2) will be avoided. the third weakness happens during using polynomial equations in detrending tec, so our proposed method can be more sensitive for the lower-frequency variations, especially, in the quiet geomagnetic conditions and the stable state of solar radiation over mid-latitude areas, where it depends on the average value. accordingly, using different detrended fluctuations analysis (dfa) techniques could increase the verification chances of the detectable span for perturbation duration. the structure of the next part of this paper is as follows: section 2 describes datasets and methods for calculating the slant tec (stec), and the detrending methods, section 3 covers the results and discussion of stec and detectable dtec perturbations through three different detrending methods: multi-step numerical difference, 6th order polynomials fitting, and one-week average difference. eventually, section 4 declares summarized conclusions. 2. data and methods 2.1 dataset materials in wuhan, precipitation is a characteristic of the city, about 6.2 months for a wetter season from february 22 to august 30, versus 5.8 for a drier season from august 30 to february 22 for more details we can refer to (https://weatherspark.com/y/128408/average-weather-in-wuhan-china-year-round). among the rainy days, we differentiate between those that experience rain alone or a mixture of rain and thunderstorms. the common form of precipitation throughout the year is rain alone. wuhan residents received a warning message from wuhan meteorological observatory at 16:26 (lt) where (lt = ut + 8 hours) on march 2 (doy61), 2018. wuhan citizens were notified for being careful of 6 hours of sudden lightning activity with rainfall 20-40 mm over wuhan city. the lightning was started at 17:00 am and continued to 23:00 pm according to local time. besides, the strong thunderstorm over wuhan city has been reported by the “time and date” website (table 1 shows the weather and the strong thunderstorm hit wuhan city started at 17:00 local time (9:00 utc)), where wuhan time = utc + 8hrs), taking into consideration the light rain, rain and fog at the same day happened without existing of thunderstorms and the lightning for more details we can refer to (https://www.timeanddate.com/weather/china/wuhan/ historic?month=3andyear=2018). the time period analyzed for this study is 8 nights from 22nd february to 2nd march (doy54-doy61) 2018, where our proposed method needs tec values for one week before the event day to subtract from tec of the event day in order to assess the ionospheric disturbance. it is widely known that geomagnetic and solar activities can affect the ionosphere. figure 1 displays the geomagnetic index dst, kp, and solar activity index f10.7 from doy53 to doy65 of 2018 around the wuhan thunderstorms. the data of dst, kp are available from the national aeronautics and space administration (nasa) with following: https://omniweb.gsfc.nasa.gov/form/dx1.html. the gnss dataset used in this study was provided by hbcors with a sampling of 1s. this kind of sample rate has a high time-resolution. so, it may be more useful in detecting short-time irregular ionospheric disturbance. meanwhile, the sampling rate of 1s used occasionally may be based on data availability, long time in processing and/or large storage rates. hbcors was constructed by a certain number of continuously operating reference stations (cors). it uses satellite positioning technology, computer network technology, communication technology and many other technologies to provide accurate positioning in case of real-time, and space navigation positioning. moreover, a service system for moving target-location information to help and support users and persons in charge. the average distance between each station is about 50 ~ 70 km. among them, about 21 reference stations are built on bedrock and the others’ reference stations are built in the soil layer with relatively stable tectonic activity. the receivers include trimble series r7 gnss, netr8, netrs, in addition to leica 1200. the observation uses a high precision geodetic antenna with a sampling interval of 30 s /1 s for cors stations. all the reference stations have an automatically operating system, in addition, the observation data are transmitted to the relevant data center through the network [(he et al., 2011; nie et al., 2013; zhang et al., 2013; zhang et al., 2013]. this provides us a very good opportunity to detect ionospheric disturbances with high spatial and time resolution. 3 tec disturbances by cors at thunderstorm https://weatherspark.com/y/128408/average-weather-in-wuhan-china-year-round https://www.timeanddate.com/weather/china/wuhan/historic?month=3andyear=2018 https://www.timeanddate.com/weather/china/wuhan/historic?month=3andyear=2018 https://www.timeanddate.com/weather/china/wuhan/historic?month=3andyear=2018 https://omniweb.gsfc.nasa.gov/form/dx1.html 2.2 stec calculation ionospheric observables have been extensively discussed by mannucci et al. [ciraolo et al., 2007; mannucci et al., 1998]. in fact, the ionosphere can cause an ionospheric delay on the gnss signals, depending on their frequencies ƒ as well as stec values extended along the line of sight (los) between the receiver and satellite. in this study, the stec was derived from the dual-frequency gnss observation from hbcors as follows: (1) where 𝑆 and 𝑟 indicate satellite and receiver, respectively; tec is the stec along los; 𝐿 and 𝑃 are the carrier phase and pseudorange measurements, respectively; 𝑓1 and 𝑓2 are the frequencies of the carrier phases of 𝐿1 and 𝐿2, respectively; bs and br are the satellite and receiver hardware delay of the pseudorange code, respectively; bs and br are the satellite and receiver hardware delay of carrier phase, respectively; 𝑁�� ,1 and 𝑁�� ,1 are carrier-phase ambiguity. although the ionospheric observations can be derived directly from both pseudorange code or carrier phase measurement, the pseudorange measurement contains more noise than that of carrier phase. therefore, the � 𝐿�� ,4 = 𝐿�� ,1 ‒ 𝐿�� ,2 = ‒ + (𝑏�,₁ ‒ 𝑏�,₂) + (𝑏�₁ ‒ 𝑏�₂ ) ‒ (𝑁�� ,1 ‒ 𝑁�� ,2) 40.3 × 𝑇𝐸𝐶 𝑓1² 40.3 × 𝑇𝐸𝐶 𝑓2²𝑃�� ,4 = 𝑃�� ,1 ‒ 𝑃�� ,2 = ‒ + (𝐵�,₁ ‒ 𝐵�,₂) + (𝐵�₁ ‒ 𝐵�₂ )40.3 × 𝑇𝐸𝐶 𝑓1² 40.3 × 𝑇𝐸𝐶 𝑓2² mohamed freeshah et al. 4 figure 1. dst, kp, and f10.7 indices in the period from doy53 to doy65 of 2018. horizontal axis represents doys number while vertical axis shows the dst, kp, and f10.7 indices, respectively. 5 tec disturbances by cors at thunderstorm precision of the carrier phase is more accurate than that of the pseudorange. moreover, it presents the additional problem of biased by unknown ambiguities [mannucci et al., 1998]. therefore, the so-called “leveling carrier to code” algorithms in this work are applied to minimize the noise of the pseudorange and to remove the ambiguity of the carrier phase [ciraolo et al., 2007]. the differential carrier phase increases measurements precision of stec while pseudorange gives the absolute scale of it [ugur and ibrahim, 2013]. this form can be represented as: ŀi,arc = li,arc – (li,arc pi)arc = tec + br + bs + (εp)arc + εl, (2) where ŀlarc is the ionospheric observable of carrier-phase “leveled” to the code-delay one, li,arc is the ionospheric observable obtained from carrier-phase; br and bs are the same as eq. (1), εp and εl are the noise and multi-path for the code-delay measurements and for the carrier-phase ionospheric observations, respectively. based on eq. (2), taking leveling process into consideration: “(1) ifbs for carrier-phase are replaced by the corresponding code-delay ifbs” [ciraolo et al., 2007]; (2) multi-path and noise presented in the code-delay measurements could affect the leveled carrier-phase ionospheric observable, in case these do not average to zero over the same continuous arc. in this study, we employed gps data observed above 10° cut off angle in order to have the longest arc as possible to check ionospheric disturbances with non-ionospheric related tracking errors. after processing all gnss data, we can plot the stec variation values versus time to detect the perturbations in the ionosphere without detrending. the previous method is conducted with an assumption that ionospheric perturbations have the same systematic error or stec includes constant bias [x. zhang and tang, 2015]. the spatial distribution of tec has been modeled by the spherical harmonic expansion based on single layer model as follow: (3) where 𝛼 and 𝜆 are the geomagnetic latitude and solar-fixed longitude for ionospheric pierce points (ipp), respectively; 𝑉𝑇𝐸𝐶(𝛼,𝜆 ) is the vertical ionospheric tec at ipp. vtec is converted from stec using a mapping function (mf) [deng et al., 2016; schaer, 1999]. nmax is the max degree of the spherical function; 𝑃��� represents the fully normalized legendre function of degree n and order m; 𝐶��� and 𝑆��� are the fully normalized spherical coefficients of degree n and order m [zhang and zhao, 2019]. before estimating stec from gps observations, the turboedit algorithm was used to detect cycle slips. the established algorithm by blewitt, can identify cycle slips, correct cycle slips wherever possible, even more, delete data outliers [blewitt, 1990]. based on the limited number of receivers and the study area, the expansion of spherical harmonics with order 3 is applied. the shell (single layer) has an altitude of about 450km. in this method, we have computed the differential code biases (dcbs) as a constant value per day as well as zero-average conditions are adopted for all observed satellites of each system in order to disconnect the dcbs of satellites and receivers from each other. therefore, the cleaned stec are extracted after separating the dcbs which can be applied for measuring and analyzing the ionospheric disturbances through this work. after obtaining the corrected stec, we can get a stec time series as follows: stec1, stec2,⋯, stecn. where the subscripts 1, 2, …, n introduce the epoch number started from one at midnight based on ut and end at 86400, where the total number of epochs represents for a whole day by seconds. 2.3 detrending methods 2.3.1 multi-step numerical difference for a given stec sequence, hernandez-pajares et al., [2006] have extracted ionospheric disturbances by applying a numerical difference method with an order of 1. recently, other researchers have applied the multi-step multi-order with higher orders 2 and 3 [park et al., 2011; zhang and tang, 2015]. the multi-order numerical difference is defined: 𝑉𝑇𝐸𝐶 (𝛼,𝜆 ) =� � 𝑃���(𝑠𝑖𝑛𝛼)�𝐶���cos(𝑚𝜆) + 𝑆���sin(𝑚𝜆)�,���� �₌₀ � �₌₀ δk(t) = δk(t) – (δk(t + τ) + δk(t – τ)) / 2, (4) where t is the sampling rate of the data, τ is the time step, which was chosen to detect the possible variations (δk) in tec with a specific time difference. apparently, the ionospheric disturbances may skip detectable disturbance because the time step could be relevant to the detectable period. hereinafter, to solve this problem, we chose multiple time periods (30, 90, 300) s. obviously, these different time steps increase the chances of verification the detectable span for perturbation duration. in such kind of detrending, as shown in the next equation we have two different amplitudes a and a' for δlj and ionospheric disturbances, respectively, which rest on (t), is the ionospheric disturbances period [hernandez-pajares et al., 2006]. (5) assuming tec trendlines have linear changes during a short period. in other words, the applied method will detect ionospheric disturbance periods of 600s (a'=2a) with significant sensitivity, and periods between (400-1200)s where (a'=a), will be quite efficient [hernandez-pajares et al., 2006; zhang and tang, 2015]. mostly, this type of problems happens in all detrending procedures commonly, so in this study, we investigate the suitability of a deterministic technique to display the strong thunderstorm fluctuations by employing two more of detrending methods; they are described in sections 2.3.2 and 2.3.3, respectively. 2.3.2 six order polynomials fitting in this strategy, the tec data are fitted with a 6th order polynomial as a second method to detrend the data, in order to sensitively observe tec fluctuations. (6) 6th order being the highest that can be displayed with the trend line. the disturbed tec is computed by subtracting this fitting from the stec data. we got that ionospheric perturbations as a function in tec variations are closely correlated in space and time during the strong underlying electrical storm. the aforementioned process removes the gross offset, as well as the detrended values, are mostly related to the carrier phase tec around 0.01 to 0.1 tecu accuracy [burrell et al., 2009]. 2.3.3 one-week average difference the following is the proposed method employed to detect ionospheric disturbances based on the average of one week before the phenomena day. subtraction of the average simultaneous observations from a certain day can lead to an observable residual, in other words, removing an aspect from the dataset arise some kind of distortion or disturbance can appear in the behavior of the output. as mentioned above, the mathematical process for this kind of detrending can be described in the next simple equations. ḱ = (kdoy54 + kdoy55 + …. + kdoy60) / 7, (7) v = kdoyi ḱ, (8) where ḱ is the average value of stec for period one week before electrical storm, v is the residuals between a certain day and average value. in the assumption, tec trendlines have linear changes during a short period. furthermore, the dst index, geomagnetic kp index, and the solar radio flux f10.7 index during the study period were quite steady, based on figure 1. so, the values of tec for the average simultaneous observations supposed to be almost close to each other. the difference between tec value for average and a certain day should be smooth a' = 2a . sin2 𝜏 .𝜋 т 𝑃(𝑥)=∑ 𝑎�𝑥��⁻¹ �₌₀ mohamed freeshah et al. 6 7 tec disturbances by cors at thunderstorm or/and have a constant shift. the average value calculated from doy54 to doy60, afterward, the mean values subtracted from doy60 (day before phenomena) and doy61 (day of phenomena), respectively. the next section represents the results and their discussion 3. results and discussion 3.1 geomagnetic index dst, kp, and solar activity index f10.7 the disturbance storm-time (dst) index, geomagnetic kp index, and the solar radio flux f10.7 index during 12 days (doy53-doy65) are depicted below in figure 1. the periods between the red lines indicate the time when a strong thunderstorm hit wuhan. the kp index value is an indication of mid-high latitude geomagnetic activities. in general, values less than 4 can indicate that there is a steady-state of geomagnetic activity. as we can see, during the whole period, no presence of strong geomagnetic activity or solar activity is observed. low geomagnetic activity (kp<4), except doy54 kp≤4.3 and doy58 has the highest kp ~5.3, indicating a moderate geomagnetic activity. except for doy54, 58, the geomagnetic conditions are quiet with dst < −35nt, and kp≤4. the solar radiation is in a stable state, according to the f10.7 index variation, within 0.3 and 2.5% of its average value (from doy53 to doy65). the dst index was quite steady and varied from -31nt to 10nt as min and max value, respectively. overall condition of geomagnetic index dst, kp, and solar activity index f10.7 are in steady-state during the day of phenomenon (doy 61). 3.2 disturbances by stec observations according to wuhan meteorological observatory, the lightning warning classified as yellow signal besides their strong thunderstorm occurred at 09:00 ut (05:00 pm lt) on 2nd march 2018 as shown in table 1 for more details we can refer to (https://www.timeanddate.com/weather/china/wuhan/historic?month=3andyear=2018). based on code-delay and dual-frequency carrier phase gps observations are combined to obtain ionospheric observables represented as the stec along the los between satellite and receiver. both the transmitting and the receiving hardware cause differential code biases (dcb) could have an effect on observations [ciraolo et al., 2007]. these biases have been estimated and eliminated from the data to adjust the experimental stec. however, assuming stec still contains bias, it is supposed to have a constant bias [zhang and tang, 2015]. according to this assumption, we analyzed the stec to detect the ionospheric disturbances then verified the results with vtec. finally, the ionospheric disturbances are assessed with three detrending methods. table 1. the weather conditions over wuhan on march 2nd, 2018 from 12:00 pm to 9:00 pm (local time). (www.timeanddate.com) conditions time temp weather 12:00 pm 50° f fog. 2:00 pm 55° f haze. 3:00 pm 55° f haze. 4:00 pm 55° f scattered clouds. 5:00 pm 52° f strong thunderstorms. fog. 6:00 pm 52° f thunderstorms. fog. 7:00 pm 52° f light rain. fog. 8:00 pm 52° f light rain. fog. 9:00 pm 52° f light rain. fog. https://www.timeanddate.com/weather/china/wuhan/historic?month=3&year=2018 http://www.timeanddate.com figure 2 displays the ionospheric stec (which contains constant bias) derived from the dual-frequency gps data of prn 28 over station wh04. since the ionospheric presence depends on solar radiation, a big part of ionospheric change due to the earth relative position with respect to the sun. in addition, zenith angle and earth rotation/24 h, widely contribute to the daily ionospheric variations [amieri-simkooei and asgari, 2012]. we selected the satellite which has observations at nighttime, where it covered the time before and during the lightning. generally, the behavior of the ionosphere is in steady-state during the night. some ionospheric layers going to disappear, so stec supposed to decrease or at least become a fixed value in normal conditions [zolesi and cander, 2014]. consequently, the ionospheric disturbances in these times may be caused by lightning and their thunderstorms effects, especially this phenomenon happened after the sunset with respect to china local time (ut+8h). the red arrows in figure 2 show the disturbed tec during 14:20–15:03ut separately, to distinguish the ionospheric disturbances arrival time. the two arrows mark the larger instabilities in stec. the arrow “a” marks the first arrival disturbances in stec, indicates the first longest disturbed period of approximately 15 to 20 min by the “black double arrow”. the arrow “b” shows the extreme deviation started with positive disturbance. afterward, stec has decreased and deviated from fitted value through the aforesaid duration, within 3-6 min. furthermore, ionospheric stec obtained by gps observations of satellite prn 28 over wh04 (figure 2) coincide with another 5 cors stations named as xf01, xf05, xg01, xg04 and xn01, shown in figure 3. meanwhile, all the 6 cors stations have stec perturbations synchronize with the lightning duration. as we can see, the disturbances appearance differs within a few mins over various stations depending on some elements such as the location of each station, ipp position as well as shell altitude. according to figure 3, the red arrows indicate the extreme ionospheric disturbances found after a few hours of the start-time of thunderstorms. this may depend on the electrical energy that have been generated by the lightning (lightning is generated in electrically charged storm systems) [nasa facts, 2006] which affect vertically in troposphere layer, takes some time to trigger ionosphere taking into account thunderstorms have lasted for several hours. in addition, lightning phenomenon consists of multi-epicenters with different locations with various influence for only the surrounding areas close to it. based on the previous reasons, in addition, the stec have been calculated by gps along los and only for distinct ipp. logically, this factor may associate with the delayed time for the detectable ionospheric disturbances as well as it can cause some undetectable ionospheric disturbances. on the other hand, the corresponding routes of the ipp created from los intersect the shell (single layer) with 450km altitude, which represents a distinct longitude and latitude with a specific time for each ipp point as well as these satellites have different elevation angles. logically, some ipp can be located in the disturbed area, therefore, use in sensing the ionospheric disturbance within the time of ionospheric fluctuations. mohamed freeshah et al. 8 figure 2. the ionospheric stec obtained by gps observations of the satellite prn 28 at station wh04. the two arrows indicate the arrival time of ionospheric disturbances. 9 tec disturbances by cors at thunderstorm figure 3. the ionospheric stec obtained by gps observations of satellite prn 28 at stations xf01, xf05, xg01, xg04 and xn01, panels (a-e) present disturbance series. figure 4. shows the location of wuhan city center (the red star) and the 6 cors stations (red circles). the ionospheric stec series also contains a significant disturbances during 15:00–15:45ut, as shown in figure 3 the perturbations can be considered as extreme values of ionospheric disturbances which happened up to 45min separately over most of the ground-based stations. based on figure 2, 3 we can see that ionospheric disturbances over wh04 moved quicker than other points. where the “b” arrow refers to an earlier time of ionospheric disturbances than the red arrows for all panels in figure 3. by referring to figure 4, we can interpret ionospheric disturbances moved quickly in n-e direction over wh04 located 58km close to the city center of wuhan. where the turbulence reached the highest value at 15:03ut. however, the xg01 10km closer than wh04 to the city center, but the highest perturbations have delayed about 11min at 15:14ut. for the n-w direction, the detectable large ionospheric disturbances were nearly 40min overdue than wh04, where the furthest station named as xf05 located about 323km in the same direction from wuhan city center. in accordance with the above results, the detected ionospheric disturbances have different horizontal velocity, period, direction, waveform and observed time compared with the reflected lightning and their thunderstorms time period. furthermore, a possible recurrent ionospheric disturbance has been checked, we also verified that there were not significant perturbations for one week before the day of the phenomenon at the same study times. based on the main concept of tec during the night, it should be discharged and decrease because of nonexistance of solar radiation to recharge it. however, solar radiation is the major element which associates in recharge ionospheric particles. as it is known, through the nighttime there are some ionospheric layers such as d and e which disappear [zolesi and cander, 2014]. therefore, in the steady states, the tec values decrease. however, tec values have increased during nighttime which is likely to support our interpretation that electrical energy generated by lightning could contribute in ionospheric fluctuations by vertical coupling between troposphere activities and higher atmosphere. as a result, the tec has been disturbed. these results can confirm that the observed ionospheric disturbances in figure 3 is triggered by the electrical energy that have been generated by lightning waves for the next reasons (1) the detected ionospheric disturbances happened during the nighttime where no solar radiation which is the main factor to recharge ions in the ionosphere. (2) as we can see in figure 1, during the whole day of the event (doy 61), no presence of strong geomagnetic activity or solar activity is observed so no effect came from dst, kp, and f10.7. in the next section, we used detrended methods to remove the effects of accumulated errors to display individually the ionospheric changes in tec values. 3.3 results of detrended methods in order to assess the ionospheric disturbances explained in section 3.2. it is worth considering the removal of the effects of accumulated errors to display individually the ionospheric changes in tec values. therefore, multiple detrending techniques are applied in the next sections which show their results, discussion, and analysis listed below. 3.3.1 multi-step numerical difference based on the results shown in figure 5, two types of ionospheric disturbances were detected by the gps station xg04 after the wuhan strong thunderstorms. ionospheric disturbances in panel (c) are a high-frequency wave with periods of approximately (time) 1 to 2min, whereas ionospheric disturbances shown in (a) and (b) are low-frequency waves with periods of approximately 2 to 5min. as a result, in thunderstorm could produce ionospheric disturbances with different frequencies and time period. in addition, 300 s is better than the other time steps (30 and 90) s in order to detect the high ionospheric disturbances. three other stations wh04, xf01 and xf05 also observed high ionospheric disturbances by employing 300 s as a time step figure 6. nevertheless, the (30 and 90) s are not good enough over the same stations. it is interesting to note that using time step 300 s is more sufficient to detect the ionospheric disturbance which is an indication of ionospheric response to a strong thunderstorm [zhang and tang, 2015]. as we can see in figure 5, 6 the detectable ionospheric disturbances for the four stations xg04, wh04, xf01, and xf05 are coincident with the time of lightning over wuhan city. the few minutes difference between disturbance for the aforementioned stations may be due to their different locations. the detectable ionospheric disturbances time range is started around 15:00 ut and has ended about 15:40 ut. mohamed freeshah et al. 10 11 tec disturbances by cors at thunderstorm figure 6. detecting ionospheric disturbances series by multi-step numerical difference technique over other five stations, the order is set to 1, using 300s time step 300 s. figure 5. detecting ionospheric disturbances series by multi-step numerical difference technique at station xg04, the order is set to 1, using three different time steps (30, 90 and 300) s, respectively. panels (a–c) show the results. it is acknowledged that the chosen time step could be relevant to the detectable period, so a certain time step may skip detectable perturbations. using the same criteria, there are no detectable perturbations over the other two stations xg01 and xn01, although there are detectable ionospheric disturbances over the same stations in figure 3. as a result, we need to apply other techniques, where the pre-analysis mentioned based on figure 3 indicates the aforementioned stations have disturbance common with the time period of lightning over wuhan. consequently, multi-step numerical difference is not enough to assess the detectable ionospheric turbulences response to the studied phenomena overall disturbed stations. 3.3.2 six order polynomials fitting hereinafter, the results of detrending by polynomial fitting for tec have displayed through the same 6 cors stations. the disturbed tec is computed by subtracting this fitting from the stec data. the satellite has the given gps pseudo-random number (prn) 28. in figure 7, the measured stec has large deviations from the fitted curve between ~14:30 and 16:00 utc. the vertical green lines show the period of the stec deviations from the fitted curve. this time range indicates that may detectable ionospheric disturbances can be found based on the large deviations. in addition, the corresponding time ionospheric disturbances which detected by the first method “multi-step numerical difference”. the applied method is more reliable for higherfrequency variations, in other words, it acts to filter out the lower-frequency changes [lay et al., 2013]. by deducting the observed tec from the best fit 6th order polynomial, the deviation can be detected easily. the performance of this kind of detrending for the six disturbed stations has more efficiency than the method of “multi-step numerical difference” which mentioned separately in section 3.3.1. the ionospheric disturbances over the stations xg01, xg04, wh04, xf05, and xg04 are successfully detected by the current method as shown in figure 8. however, the first two stations (xg01, xg04) are non-detectable by “multi-step numerical difference” method, and the last three stations (wh04, xf05, and xg04) are detected by both techniques. although the current technique using best fitting polynomials apparently has a better performance than the first one. “multistep numerical difference” has found a detectable ionospheric disturbance over the station xf01, which was not observed by subtracting the fitted-curve method from the data. polynomial fitting acts to filter out the lowerfrequency changes so it cannot detect the lower values disturbances. for this reason, we proposed a third detrending method to avoid these defects. mohamed freeshah et al. 12 figure 7. the deviations between the measured tec and the 6th order polynomial fitted curve for the given gps pseudo-random number (prn) 28 over station wh04. 13 tec disturbances by cors at thunderstorm 3.3.3 one-week average difference in the assumption, tec trendlines have linear changes during a short period. furthermore, the dst index, geomagnetic kp index, and the solar radio flux f10.7 index during the study period were quite steady, based on figure 1. so, the values of tec for the average simultaneous observations supposed to be almost close to each other. the average value calculated from doy54 to doy60, afterward, the mean values subtracted from doy60 (day before phenomena) and doy61 (day of phenomena), respectively. the results of these experiment shown in figure 9 below. the anomalous tec changes are found closely associated in space and time to the strong underlying lightning and their thunderstorms. the tec variations are observed with maxima and minima to be ~2.5 and 0.5 tecu, respectively, through typical hours of darkness background value of numerous tecus. the changes close to the thunderstorm appear to have a quick response than those away from the storm. in addition, there was a highfrequency content over the nearest neighbors’ stations, especially in the n-e direction from wuhan city center with periods of minutes to tens of minutes. the ionospheric disturbances lasted for a longer time than south direction by a few minutes, based on the results over xg04 and xn01 in the north and south of wuhan city center, respectively. the tec perturbations have ended in the n-w direction over xf05 station later than other stations around the area of interest (aoi). these findings are warranted by the pre-analysis over the same station which located about 323km (all distances measured from wuhan city center). indeed, the turbulences are coming later comparing to other stations, taking into consideration the direction and the farther distance away from wuhan city center. with ingesting deep, the n-w direction seems to have the last detectable large ionospheric disturbances, was nearly 40min overdue than wh04. the detectable localized tec variations for xf05 is observed at night of the strong thunderstorm. it was found that the tec perturbations finished with minutes to tens of minutes after other stations. emphatically, both results of the large and last ionospheric figure 8. differences between the measured tec and the 6th order polynomial fitted curve for the given gps pseudo-random number (prn) 28 over 6 ground-based stations. disturbances which can be detectable coincide with the ionospheric disturbances moved quickly in n-e direction on the other side, shifted little faster in the n-w direction. as a result, multiple epicenters of the lightning and their thunderstorm might be concentrated near to the center of the city and to n-e direction during that night. moreover, ionospheric disturbances can travel with different velocities in the case of thunderstorms depending on the direction as well as the remoteness from the electrical storm. by comparing the results of the second and third detrending technique, where both techniques agree in more than 80% of the detectable ionospheric disturbances. around 5 cors stations have detected ionospheric disturbances except for xf01 station discovered only by the third method. based on the results for both methods, the 6th order polynomial technique delayed within mins to tens of mins from the mean method for detecting ionospheric disturbances. the only non-detectable ionospheric disturbance for station xf01 by the fitting technique which has variations less than 1tecu during the common time with phenomena effect. emphatically, this finding agrees with the concept of fitting is acted more sensitive only for the higher-frequency variations [lay et al., 2013]. it is worthy of mentioning that under large ionospheric electron disturbances based on lightning and their thunderstorms, significant ionospheric disturbances maybe in some detrending styles. on the basis of this study results, and according to table 2 the third method called “one-week average difference” has the best performance agree with our pre-analysis. where the third method has detected the ionospheric disturbances over the 6 cors stations. this type of detrending technique is more significant especially in mid-latitude locations with quite steady solar radio flux f10.7 and moderate geomagnetic storm kp based on research findings. it is known that the tropospheric activities like thunderstorms could disturb the lower ionosphere of height range 60 to 90 km through lightning-generated electric fields [(cheng et al., 2007; davis and lo, 2008; lu, 2006; pasko et al., 1997; mende et al., 2005] as well as the convective atmospheric gravity waves (gws) [lay and shao, 2011; sentman et al., 2003; taylor and hapgood, 1988]. besides, [davis and johnson, 2005; yu et al., 2015] have found that the ionospheric sporadic e of altitude ∼100 km could be affected by lightning. recently, researchers have reported that the roles of ionization enhancements could happen through the lightning, and thunderstorms can trigger atmospheric gravity waves (agws) and propagate upwards to the ionosphere [yue and lyons, 2015]. it is known that gws are considered as a crucial part of the atmosphere’s dynamics process. gws influence temperature and atmospheric winds distribution where it mainly contributes to moving momentum and energy between the different layers of the atmosphere. once gws arrive at the upper atmosphere and interact with the plasma of ionosphere, the density of plasma is disturbed, electron density is also changed according to the law of conservation of charge, and causes the ionospheric disturbances. in 2017, yu et al. have tried to find the relation between earth’s atmosphere and ionosphere interactions, their results indicated that electric field effects and gws likely the mechanisms responsible enhancement of na layer [yu et al., 2017]. subsequently, yu, b. et al. have concluded that the dynamical process of the ionosphere’s sporadic e layers and na layer dominantly effect by atmospheric tides. their findings indicated that metallic layered has been intensified by atmospheric tides or in other words the troposphere-mesosphere-ionosphere coupling [yu et al., 2019]. mohamed freeshah et al. 14 table 2. shows the final status of detected and undetected ionospheric disturbances over 6 cors stations by three detrending techniques. cors station name method 1 method 2 method 3 msnd pf ad wh04 detected detected detected xf01 detected undetected detected xf05 detected detected detected xg01 undetected detected detected xg04 detected detected detected xn01 undetected detected detected 15 tec disturbances by cors at thunderstorm figure 9. detecting ionospheric disturbances series by one-week average difference technique over the six stations, (left) differences of the day before the event day, (right) differences of the event day. in this paper, the present results can confirm the ionospheric disturbance was observed in the tec through three methods, the mechanism responsible for these disturbances above the strong thunderstorms in wuhan could be the generated electric fields from lightning and convective agws produced by thunderstorms. it is unfortunate we lack information about simultaneous ionosonde observation associated with the lightning in our data. subsequently, since stec derived from gnss observation is only total electron content along the path from station to satellite, it is not easy to determine accurately the height of the occurrence of ionospheric disturbance resulting from a lightning strike. yue et al., [2016] have concluded that the ionospheric disturbance is mostly observed in the e and f region based on combined gnss rays from leo-based gnss and the ground-based receivers to get good global resolution in the topside ionosphere during a geomagnetic storm. as the f region is located too high to be associated with lightning, meanwhile, it is worthy of indicating that the ionospheric perturbations were preceding revealed in the ionospheric es region. in 2005, davis and johnson have concluded that electrical discharge is likely responsible for any ionospheric enhancement and the lightning is concerned with the disturbance ionospheric es layer which represented as the localized enhancement of electron density [davis and johnson, 2005]. a dominant response after few hours of lightning has existed. it was a result of gws generated by the thunderstorm which has a large vertical wavelength. however, the waves’ vertical component is small compared with their horizontal velocity [chimonas, 1971; johnson and davis, 2006]. in 2008, davis and lo have investigated the response to lightning strokes in the ionospheric es layer by a superposed epoch study. it has been revealed that the dominant cause of the observed ionospheric disturbance in the sporadic layer was related to cloud-to-ground lightning [davis and lo, 2008]. although [johnson and davis, 2006] have discussed the potential mechanisms summarized by [rycroft, 2006] in relation to the enhancement of the es layer at 100 km is associated with sprites. while [davis and lo, 2008] suggested that this unlikely whereas their results were lack to the peak current which accompanying the lightning. recently, [yu et al., 2017] have followed the conclusion by [inan et al., 1996] which indicated that the local ionosphere can be effected by lightning over a distance of up to 150 km to study the connection between neutral metal na layer and thunderstorms. the few minutes difference between disturbance for the aforementioned cors stations may be due to their different locations where the geographic location of lightning affect the influence of thunderstorms on the ionosphere. [chimonas, 1971] was expected the formation of the lower and higher es layers depend on the eastwest gravity waves and northerly waves, respectively. in 2006, johnson and davis have extended the limits of the data set, used by them in 2005, to 400 km from chilton where it was revealed a direct effect of the lightning on the ionosphere beyond the previous limits (200 km). in addition, the enhancement in the es layer revealed not only related to the location geographic of lightning but also the position of lightning as well such as cloud-to-ground or inter-cloud lightning [davis and lo, 2008]. 4. conclusions the influence of lightning and their thunderstorms could transfer to the high atmosphere so, it is a good way to understand the upper atmosphere coupling by detecting the response of the ionosphere to this phenomenon. in this study, we investigated the ionospheric response by using ground-based gnss observations from dense cors during strong thunderstorms over wuhan city. before estimating stec from gps observations, the turboedit algorithm was used to detect cycle slips, correct cycle slips wherever possible and delete data outliers [blewitt, 1990]. the stec was derived from the dual-frequency gps observation from hbcors. the code-delay and dual-frequency carrier phase were combined to obtain ionospheric observables represented as the stec along the los between satellite and receiver. assuming stec still contains a constant bias after calibrating it [zhang and tang, 2015], the stec were checked to find the possible ionospheric disturbances with respect to maxima and minima values to check the periods of extreme disturbance and first arriving of lightning influence on the ionosphere. the research has found that the ionospheric disturbances have different velocities based on thunderstorm activities. based on the distance and direction of each cors station from wuhan city center. the assessment part was conducted by three methods of detrended fluctuations analysis: (1) multi-step numerical difference, (2) 6th order polynomials fitting, (3) one-week average difference and was compared with stec disturbances. the three techniques which were applied in this study represented a fundamental and substantial mohamed freeshah et al. 16 17 tec disturbances by cors at thunderstorm mechanism in the detrended dataset during strong thunderstorms. applying only one method have some defects, so we used different methods to avoid these defects as possible. thereby, this issue is very worthy of discussing because maybe misleading results could do dissipation or impediment in detecting the ionospheric fluctuations depending upon the applied detrending technique. on the other hand, considering one day as a reference for subtracting from the day of phenomenon maybe be not enough for the detectable ionospheric tec disturbances, where it depends mostly upon the latitude of the study area, especially in low latitude regions where ionospheric variability is more dynamic and complex as compared to mid-latitudes [maruyama et al., 2007]. the analyzed results for the three detrending methods showed that the msnd method has the lowest performance. meanwhile, the fitted tec data with 6th order polynomials technique presented an improvement and a discrepancy related to the msnd method. the ad method has satisfied the best performance comparing with tec disturbances. based on results of the three detrending methods, further studies are therefore necessary to evaluate more ways with different phenomena, investigating various case studies for the further improvement could lead to a fast and efficient algorithm for detecting of duration and frequency of ionospheric perturbations can be proposed, where it will be more suitable for certain phenomena and regions. funding. this research was funded by the national science fund for distinguished young scholars (no. 41825009), the national natural science foundation of china (no. 41904026), the hubei province natural science foundation of china (no. 2018cfa081). acknowledgments. we are very grateful to hbcors for providing gnss data. the numerical calculations have been done on the supercomputing system in the supercomputing center of wuhan university. we are very grateful to the crustal dynamics data information system (cddis) data center for providing observation data and navigation file by the following ftp server: ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/ and the observation data from a geodetic survey of hubei cors (restricted). in addition, the dst index, kp index and f10.7 index are from national aeronautics and space administration (nasa) with following: https://omniweb.gsfc.nasa.gov/form/dx1.html. we would also like to express our gratitude to dr. xiaodong ren for his help in reviewing the article. the authors thank for the valuable comments of the reviewers and their promising suggestions to improve this paper. references amieri-simkooei, a. r., and j. asgari (2012). 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road, wuhan 430079, china and key laboratory of geospace environment and geodesy, ministry of education, 129 luoyu road, wuhan 430079, china and collaborative innovation center for geospatial technology, 129 luoyu road, wuhan 430079, china, e-mail: xhzhang@sgg.whu.edu.cn © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 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https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 https://doi.org/10.1029/2005ja011474 mailto:xhzhang@sgg.whu.edu.cn s u l l a c u r v a t u r a e t o r s i o n e d e l c a m p o d i g r a v i t a ' antonio makussi 1. -—• c o n s i d e r a n d o d u e p u n t i p i e po d e l c a m p o e s t e r i o r e di grav i t à t e r r e s t r e , a b b a s t a n z a p r o s s i m i f r a d i l o r o , e d i d u e v e r s o r i (vert i c a l i ) ili e d n2 d i r e t t i v e r s o l ' a l i o clie sono t a n g e n t i alle l i n e e di f o r z a i n pj e p<, gli assi d i t a l i d u e v e r s o r i n o n s a r a n n o , i n g e n e r a l e , c o m p l a n a r i f r a di l o r o . q u a n d o i d u e p u n t i g i a c c i o n o s u l l a m e d e s i m a s u p e r f i c i e e q u i p o t e n z i a l e i ! , t a l e d i f e t t o d i c o m p l a n a r i t à si t r a d u c e , c o m ' è b e n n o t o , n e l l a p r e s e n z a d e l l a t o r s i o n e g e o d e t i c a di n e l l a d i r e z i o n e pi p>; pos to p r e c i s a m e n t e pu — pi = st, con t v e t t o r e u n i t a r i o , ed s l u n g h e z z a d e l s e g m e n t o pi p» c h e s u p p o r r e m o p i c c o l a d e l p r i m o o r d i n e , si h a d a l l e f o r m u l e di f r e n e t g e n e r a l i z z a t e — — = y.a t + xnb , [1] ,s d o v e b è u n v e r s o r e t a l e c h e tt, b, n) f o r m i u n a t e r n a p o s i t i v a d e s t r o r s a , "/.„ è la p r i m a c u r v a t u r a o flessione d e l l a s u p e r f i c i e n e l l a d i r e z i o n e t c o n s i d e r a t a , e r a la t o r s i o n e g e o d e t i c a di iì n e l l a stessa d i r e z i o n e . l ' i n d i c e a c a r a t t e r i z z a poi l a d i r e z i o n e t, p e r l a r a g i o n e c h e a p p a r i r à i n seguito. m a , a l l o stesso m o d o , c o n s i d e r a n d o i d u e p u n t i p i e pu n o n p i ù s u l l a m e d e s i m a s u p e r f i c i e e q u i p o t e n z i a l e , m a c o m u n q u e n e l l o s p a z i o , s a r à p o s s i b i l e d e f i n i r e q u e l l e c h e p e r a n a l o g i a p o t r e m o c h i a m a r e anc o r a curvatura e torsione del campo di gravità, n e l l a d i r e z i o n e c a r a t t e r i z z a t a d a l v e r s o r e a t a l e c h e p>—pi — sa. s o n o q u e s t e c a r a t t e r i s t i c h e c h e ci p r o p o n i a m o di e s a m i n a r e , m o s t r a n d o i n p a r t i c o l a r e c o m e la s e c o n d a (la t o r s i o n e ) sia p i e n a m e n t e d e t e r m i n a t a m e d i a n t e la b i l a n c i a di e ò t v o s di s e c o n d o t i p o , c o n g i u n t a con la m i s u r a d e l l a g r a v i t à , e r i c e r c a n d o poi l e d i r e z i o n i di c u r v a t u r a e di t o r s i o n e n u l l a del c a m p o . 2. —• p e r q u e s t o , p o n i a m o a — s i n z . t -fcos z . n [2] 2 0 2 a n t o n i o m a r u s s i d o v e z r a p p r e s e n t a d u n q u e l a d i s t a n z a z e n i t a l e d i a, e t la d i r e z i o n e d e l l a s u a p r o i e z i o n e su s u p e r f i c i e e q u i p o t e n z i a l e p a s s a n t e p e r p i . p e r lo s p o s t a m e n t o s u p e r f i c i a l e d a px a p p i e d e d e l l a l i n e a d i f o r z a p e r po, s p o s t a m e n t o l a c u i g r a n d e z z a è s s i n z, si h a d a l l a f o r m u l a p r e c e d e n t e — ^ l = k a t + r a b ; l 3 ] n e l m e n t r e p e r l o s p o s t a m e n t o l u n g o l a l i n e a d i f o r z a , l a c u i g r a n d e z z a è s cos z, si h a p e r cose b e n n o t e _ —/ g r a d v log g = / b , [ 4 ] s cos z o v e / è l a c u r v a t u r a ( e s s e n z i a l m e n t e p o s i t i v a ) d e l l a l i n e a d i f o r z a , e b la s u a n o r m a l e p r i n c i p a l e ; / r a p p r e s e n t a o v v i a m e n t e a n c h e il m o d u l o d e l l a c o m p o n e n t e o r i z z o n t a l e d e l g r a d i e n t e l o g a r i t m i c o d e l l a g r a v i t à . s c o m p o n e n d o b s e c o n d o t e b si h a p o i b = cos i|>. t + sin iji. b [ 5 ] e d in d e f i n i t i v a , s o m m a n d o l e [ 3 ] e [ 4 ] —-• 1 = (v.n sin z + / c o s z cos t(>) t + ( t a s i n z -f f cos 2 s i n iji) b. s [6] d e f i n i a m o d u n q u e p e r a n a l o g i a c o m e curvatura ca e torsione t a d e l c a m p o d e l l a d i r e z i o n e a l e : c„ = -,ca s i n z + / cos z cos \|i ^ t a = t a sin z + f cos z sin i|i m a a q u e s t e f o r m u l e si p u ò d a r e a l t r a f o r m a p i ù e s p r e s s i v a , f a c e n d o a p p a r i r e solo e l e m e n t i a s s o l u t i . i n d i c a t a p r e c i s a m e n t e c o n a l ' a n o m a l i a d i t r i s p e t t o a ct, c{ e s s e n d o l a d i r e z i o n e p r i n c i p a l e d i c u r v a t u r a xi, si h a p e r l a f o r m u l a d i e u l e r o ~/.a = y.j cos 2 a -f y.s s e n 2 a ; [ 8 ] e p o i sin 2 a * « = £ — < * , * . ) [ 9 ] ap = •{> — a s u l l a c u r v a t u r a f. t o r s i o n e del c a m p o di cravità 2 0 3 i> essendo l ' a n g o l o o r i z z o n t a l e f r a b e c\. e d u n q u e ca = (v.j cos 1 a + x2 sin 2 a) sin z -f j cos z cos (•&—a) sin 2 a sin z [ 1 0 ] / . = (xj -— x() + / c o s z sin (fi—a). 2 sono q u e s t e l e e s p r e s s i o n i c e r c a t e . 3. -—ci si p u ò o r a c h i e d e r e : esistono d i r e z i o n i n e l l o spazio sec o n d o le q u a l i la c u r v a t u r a d e l c a m p o è n u l l a ? o p p u r e secondo l e q u a l i la t o r s i o n e è n u l l a ? od i n f i n e d i r e z i o n i s e c o n d o le q u a l i l a tors i o n e e la c u r v a t u r a sono e n t r a m b i n u l l e ? r i s p o n d e n d o alla p r i m a d o m a n d a , e p o n e n d o c'u = 0, si h a l ' e q u a z i o n e t g z = = j _ c o s j f t — r i ) _ ; [ n ] x ( cos" « -f x2 sin" a c h e r a p p r e s e n t a u n sistema oc 1 di d i r e z i o n i . q u e s t e t a g l i a n o , sulla s f e r a r a p p r e s e n t a t r i c e , u n a c u r v a , d e l cui a s p e t t o ci p o s s i a m o r e n d e r e s u b i t o c o n t o o s s e r v a n d o i n t a n t o c h e il m i n i m o v a l o r e del d e n o m i n a t o r e è u g u a l e al m i n o r e x f r a x, e x„ : ed il m a s s i m o v a l o r e del n u m e r a t o r e è u g u a l e ad /. d u n q u e la t a n g e n t e d i z si m a n t i e n e s e m p r e i n f e r i o r e al r a p p o r t o — e cioè, se si v u o l e , al r a p p o r t o f r a il raggio x m a s s i m o di c u r v a t u r a della s u p e r f ì c i e e q u i p o t e n z i a l e , ed il raggio di p r i m a c u r v a t u r a della l i n e a di f o r z a . t a l e r a p p o r t o è n o r m a l m e n t e assai p i c c o l o ; p e r il c a m p o n o r m a l e a 45°, esso v a l e circa 0,0053; e d u n q u e , in q u e s t o caso, la d i s t a n z a z e n i t a l e della l i n e a di c u r v a t u r a n u l l a si m a n t i e n e i n f e r i o r e a 18'. a n n u l l a n d o s i z, a t e n d e a i> + 9 0 ° ; e cioè la c u r v a a m m e t t e i n z = 0 u n a t a n g e n t e di a n o m a l i a ft + 9 0 ° , p e r p e n d i c o l a r e d u n q u e alla c o m p o n e n t e o r i z z o n t a l e del g r a d i e n t e della g r a v i t à . a n c h e la c u r v a si trova t u t t a da u n a p a r t e di q u e s t a t a n g e n t e , e p r e c i s a m e n t e d a l l a p a r t e o p p o s t a a q u e l l a verso la q u a l e t a l e c o m p o n e n t e è rivolta. n e l caso p a r t i c o l a r e i n cui x 1 = x s , l ' e q u a z i o n e r a p p r e s e n t a , sul p i a n o tang e n t e alla s f e r a d e l l e d i r e z i o n i n e l l o zenit, u n piccolo c e r c h i o (vedi le figure c h e i l l u s t r a n o t r e casi d i s t i n t i , dove le d i m e n s i o n i della c u n a sono s t a t e f o r t e m e n t e esagerate). 4. — le d i r e z i o n i di torsione n u l l a si o t t e n g o n o p o i d a l l ' e q u a z i o n e ta = — (x. — x.) sin n cos a sinz + f cos z sin (ft — a) = 0 ; [12] 2 0 4 antonio m a r u s s i l e tre l i g u r e r a p p r e s e n t a n o , i n p r o i e z i o n e s t e r e o g r a f i c a z e n i t a l e , i l u o g h i «li c u r v a t u r a n u l l a ( l i n e a tratteggiata) e q u e l l i «li t o r s i o n e n u l l a ( l i n e a c o n t i n u a ) d e l c a m p o di g r a v i t à , tracciati s u l l a sfera u n i t a r i a d e l l e d i r e z i o n i . 11 c e r c h i o e s t e r n o r a p p r e s e n t a l ' o r i z z o n t e ; 7. lo z e n i t . ci e c» r a p p r e s e n t a n o l e d i r e z i o n i p r i n c i p a l i di c u r v a t u r a d e l l a sup e r f i c i e e q u i p o t e n z i a l e . le d i m e n s i o n i d e i l u o g h i di c u r v a t u r a n u l l a s o n o fort e m e n t e e s a g e r a t e . fig. 1 l a t i t u d i n e 4 5 ° ; c a s o n o r m a l e / » = 0 ; y.\ = 157,1 ; / = 0,83 ; x.—x.,= 0,53 ; — = 1,566 x, x. s u l l a c u r v a t u r a f. t o r s i o n e del c a m p o di cravità 2 0 5 tolti i casi {} = 0, il = 90", o p p u r e x1 = x2, c h e s a r a n n o e s a m i n a t i a p a r t e , q u e s t a ci d à : f i s i n i) c o s i ) \ j-13-j sin a cos a p e r {)•—0, i) —̂ 90", x , = x2, si h a n n o invece le s o l u z i o n i : / ("xj — x2) cos a .1 p e r 9 = 0 : a = 0 , t g z — per a = 90": a = 90° , tg z = (xj — x2) sin a p e r x , = x 2 a. = il , s = 90° clic r a p p r e s e n t a n o : la p r i m a , il p i a n o v e r t i c a l e c o n t e n e n t e c\ ed il p i a n o ad esso n o r m a l e c h e f o r m a con l ' o r i z z o n t e u n a n g o l o la cui t a n g e n t e è u g u a l e a —-—— (vedi fig. 1 c h e raffigura il caso n o r m a l e a 45° di l a t i t u d i n e ; qui / = 0,830, v.l — x„ = 0,530; " = 0 , 6 3 9 , le c u r v a t u r e essendo m i s u r a t e in io"'1 km"1); la s e c o n d a , il p i a n o verticale c o n t e n e n t e c< ed il p i a n o ad esso p e r p e n d i c o l a r e i n c l i n a t o c o m e si è visto s o p r a ; la terza i n f i n e , l ' o r i z z o n t e ed il p i a n o v e r t i c a l e c h e c o n t i e n e la c o m p o n e n t e o r i z z o n t a l e d e l g r a d i e n t e g r a v i m e t r i c o . n e l caso g e n e r a l e s i n t e t i z z a t o d a l l a [ 1 3 ] , la c u r v a sferica r a p p r e s e n t a t r i c e h a l ' a s p e t t o r a f f i g u r a t o in p r o i e z i o n e s t e r e o g r a f i c a ( l i m i t a t a m e n t e alla m e t à s u p e r i o r e della s f e r a r a p p r e s e n t a t r i c e ) n e l l e figg. 2 e 3, dove s o n o a l t r e s ì r i p o r t a t i i p a r a m e t r i a d o t t a t i . 11 r a m o della c u r v a c h e passa p e r lo zenit, h a q u i c o m e t a n g e n t e il v e r t i c a l e di a n o m a lia f}, c o m e s u b i t o si vede. 5. — r i s p o n d e n d o alla t e r z a d o m a n d a , e tolto il caso s i n g o l a r e d e l l o zenit, n e l q u a l e c u r v a t u r a e t o r s i o n e p e r d o n o significato, a p p a r e c h e u n a sola d i r e z i o n e è s i m u l t a n e a m e n t e di t o r s i o n e e di c u r v a t u r a e n t r a m b i n u l l e ; l u n g o q u e s t a d i r e z i o n e le v e r t i c a l i si m a n t e n g o n o d u n q u e p a r a l l e l e f r a di loro, ed essa r a p p r e s e n t a p e r c i ò la l i n e a isozenitah;. q u e s t a p u ò a n c h e o t t e n e r s i u g u a g l i a n d o le [11] e [ 1 3 ] ; ciò c h e f o r n i s c e t& a — t e il 1 1 t>r et t^ il ^ ;k„) x ' ' " 1 [ u ] tg a x, 4x, tg2 « 2 0 6 a n t o n i o m a r u s s i e q u a z i o n e c h e si s o d d i s f a p o n e n d o tg a = —tg fr . [ 1 5 ] q u e s t a f o r n i s c e d u n q u e l ' a n o m a l i a della l i n e a i s o z e n i t a l e . 6. — p e r q u a n t o c o n c e r n e l ' e f f e t t i v a d e t e r m i n a z i o n e della curv a t u r a e d e l l a t o r s i o n e d e l c a m p o di g r a v i t à , c h e p o s s o n o essere pros p e t t a t e sia d a u n p u n t o d i vista r e g i o n a l e , sia d a u n p u n t o d i vista locale, i m e t o d i a s t r o n o m i c o g e o d e t i c i r i s p o n d o n o , a l m e n o t e o r i c a m e n te, al p r i m o p u n t o d i v i s t a ; e q u e l l i b a s a t i s u l l ' i m p i e g o d e l l a b i l a n c i a d i t o r s i o n e , al secondo. o c c o r r e p e r ò o s s e r v a r e s u b i t o c h e , n e l m e n t r e la b i l a n c i a d i e ò t v ò s d e l s e c o n d o t i p o , i n t e g r a t a con la d e t e r m i n a z i o n e d e l l a g r a v i t à , definisce p i e n a m e n t e la t o r s i o n e del c a m p o , n o n cosi acc a d e p e r la c u r v a t u r a , a m e n o d i r i c o r r e r e a b e n n o t i p r o c e d i m e n t i d i i n t e g r a z i o n e . n e l m e t o d o a s t r o n o m i c o g e o d e t i c o , d e t e r m i n a t e n i ed n2 in p i e p2 m e d i a n t e m i s u r e a s t r o n o m i c h e d i l a t i t u d i n e e di l o n g i t u d i n e , l ' a z i m u t a s t r o n o m i c o a e la d i s t a n z a z e n i t a l e z di po s u l l ' o r i z z o n t e d i pu e la d i s t a n z a r e a l e s f r a i d u e p u n t i (cosa q u e s t ' u l t i m a c h e si p u ò o t t e n e r e con g r a n d e a p p r o s s i m a z i o n e d a l l e t r i a n g o l a z i o n i o, con t u t t o r i g o r e , m e d i a n t e il g e o d i m e t r o ) , sarà n o t o il v a l o r e « r e g i o n a l e » ca e t a della c u r v a t u r a e della t o r s i o n e n e l l a d i r e z i o n e a c o n s i d e r a t a ; p o s t o a l l o r a a = a + v ; x = f> + v [16] con c b e v e / sono r i s p e t t i v a m e n t e gli a z i m u t a s t r o n o m i c i della direzione di c u r v a t u r a x4, e del g r a d i e n t e s u p e r f i c i a l e d e l l a g r a v i t à , si h a n n o le d u e e q u a z i o n i c„ = [x.j cos2 (a — v) -(x2 sin 2 (a — v)] sin z + f cos z cos (/_ — a) _ s i n 2 (a — v ) s i n z , v . . t. = i ! (x.„ — x j + 7 cos z sin ('/ — a) [17] 2 n e l l e 5 i n c o g n i t e x ( , x2, j , v, y; o n d e t r e osservazioni di q u e s t o t i p o o p p o r t u n a m e n t e scelte, f a t t e i n t o r n o al p u n t o 1 1, c o n s e n t o n o teoricam e n t e di d e t e r m i n a r e t u t t e le i n c o g n i t e c e r c a t e . p e r q u a n t o si r i f e r i s c e p o i alla d e t e r m i n a z i o n e l o c a l e m e d i a n t e la b i l a n c i a d i eòtvos, b a s t a r i f l e t t e r e c h e le i n d i c a z i o n i d i q u e s t o strum e n t o sono d o v u t e alla n o n c o m p l a n a r i t à d e l l e l i n e e d i a z i o n e della gravità agli e s t r e m i d e l b i l a n c i e r e ; e p r e c i s a m e n t e il m o m e n t o tors u l l a c u r v a t u r a f. t o r s i o n e del c a m p o di cravità 2 0 7 c e n t e r i s p e t t o alla v e r t i c a l e è d a t o , i n u n a b i l a n c i a i d e a l e f o r m a t a d a d u e m a s s e m u g u a l i p u n t i f o r m i d i s p o s t e secondo i l v e r s o r e a p i ù v o l t e c o n s i d e r a t o , collocate ad u n a d i s t a n z a a , d a mtt = m a2 g t u sin z . [18] n o t o a d u n q u e g, e m i s u r a t o il m o m e n t o con i c o n s u e t i proc e d i m e n t i s t r u m e n t a l i , r i m a n e p i e n a m e n t e d e t e r m i n a t o t„; q u a t t r o osservazioni ( a l l e q u a l i se n e deve a g g i u n g e r e , c o m e b e n n o t o , p e r r a g i o n i s t r u m e n t a l i u n a q u i n t a ) d e t e r m i n a n o p e r c i ò le q u a n t i t à x „ — x , , f , v e e con esse l a t o r s i o n e d e l c a m p o i n u n a d i r e z i o n e q u a l u n q u e ; e q u i n d i a n c h e il l u o g o d e l l e d i r e z i o n i di t o r s i o n e n u l l a consid e r a t o i n p r i n c i p i o . trieste — istituto di topografia e geodesia dell'univ.— giugno 1952. riassunto partendo dalle analoghe entità sulle superfici, si generalizza per il campo di gravità terrestre la nozione di curvatura e di torsione; si studiano in particolare i luoghi di curvatura e di torsione nulla, e si mostra in qual modo si possano determinare mediante misure astronomico-geodeticlie o mediante la bilancia di eótvòs del secondo tipo queste quantità. su m mary the defìnitions of curvature and torsion are extended to the gravity field of the earth; the loci of nuli curvature and torsion are sought; the theoretical possihility of determining the quantities so defined by means of astronomico-geodetic observations or by the eótvós balance of the second kind is shoivn. c o n t r i b u t o alla c o n o s c e n z a d e l l e c a r a t t e r i s t i c h e s i s m i c h e d e l l ' i r a n s e t t e n t r i o n a l e p . p e r o n a c i r i c e v u t o il 5 n o v e m b r e 1959 in una precedente nota abbiamo accennato alle carateristicke sismiche dell'altipiano iraniano con particolare riguardo alla sua sismicità, ed iniziato lo studio del forte terremoto del 13 dicembre 1957 del quale abbiamo determinato le coordinate ipocentrali e il tempo origine. nel presente lavoro ci siamo preoccupati di determinare la natura fìsica del movimento tellurico all'ipocentro, e abbiamo iniziato lo studio di un altro terremoto avvenuto nello stesso anno a nord di teheran nel massiccio del demavend. n a t u r a fisica della scossa all'ipocentro d e l terremoto d e l 1 3 d i c e m b r e 1 9 5 7 . ricordiamo che la possibilità di indagare sulla natura fìsica della causa che ha prodotto un movimento sismico, è legata all'esame degli impulsi iniziali registrali in un certo numero di stazioni distribuite secondo tutte le possibili direzioni intorno all'epicentro; partendo da questi dati sono state sviluppate varie teorie che hanno fornito risultati del massimo interesse circa il tipo di forza che origina un terremoto. notevole a tale riguardo è la teoria sviluppata da byerly per terremoti superficiali basata sulla ipotesi di mia terra omogenea, e quindi che implica la sostituzione della traiettoria curvilinea del raggio sismico con la rettilinea, e successivamente estesa da di filippo ai terremoti profondi. il metodo è molto noto per cui ci limiteremo a riportare i risultati da noi ottenuti applicandolo ai due terremoti su accennati. partiti dalle coordinate ipocentrali da noi ricavate analiticamente per il terremoto del 13 dicembre 1957 cp = 34° 36'53" n ± 0° 0'47" 1 = 47° 48' 43" e ± 0° 0' 52" h = 7 ± 5 , 3 km 5 2 4 f . p e r o n a c i abbiamo determinato la distanza epicentrale a e l'azimut a rispetto all'epicentro, di ognuna dede 65 stazioni a nostra disposizione, elementi necessari per passare ada rappresentazione cartografica, che nel nostro caso è la stereografica polare con epicentro coincidente con l'ipocentro, polo di proiezione l'antiepicentro, e piano di proiezione il piano equatoriale corrispondente. successivamente abbiamo calcolato la distanza epicentrale a' corrispondente al tragitto rettilineo del raggio sismico, dato che d modedo di terra a cui si appdca il metodo è supposto omogeneo, e siamo pervenuti (tabeda i) agli elementi che consentono di riportare in proiezione stereografica le posizioni dede varie stazioni rispetto ad'epicentro. come è noto, per d calcolo di a' è necessario appdcare la relazione di benndorf. v0 cos e = k ove v0 è la velocità ad'ipocentro, v„ la velocità apparente del raggio sismico in corrispondenza deda distanza epicentrale relativa alla stazione considerata. a questo riguardo le v„ sono state determinate dade dromocrone di jeffreys per terremoti con ipocentri superficiau mentre per v0 si è assunto d valore di 7,77 km/sec. dato che i calcod sono stati riferiti ada terra privata deda crosta terrestre e gd nnpulsi studiati si riferiscono ade onde p . t a b e l l a i s t a z i o n e im pu ls o in iz ia le a (km) a (gradi) vn (km/sec) cos e t a n g 1 a' 1 goris + 532 346,1 7,81 0,99488 0,102 2 n a c h i c h e v a n + 533 336,9 7,81 99488 0,102 3 k i r o v a d a n + 671 349,5 7,87 98729 0,161 4 s t e p a n o v a n + 752 337,4 7,90 98354 0,184 5 b o g d a n o v a + 798 334,1 7,91 98230 0,191 6 tiflis + 814 342,0 7,92 98106 0,197 7 a k h a l k a n a t i 824 333,9 7,92 98106 0,197 8 a b a s t u n i a n + 886 332,1 7,93 97882 0,204 9 zugdidi + 939 328,3 7,94 97858 0,210 10 a s o h k a b a d — 993 68,7 7,95 97735 0,216 11 e r e v a n + 1079 345,7 7,96 97613 0,222 12 k s a r a 1094 269,9 7,97 97494 0,228 13 p i a t i g o r s k + 1107 339,8 8,04 96641 0,266 14 b o r i o m + 1194 342,0 8,10 95925 0,295 15 sochi + 1200 326,9 8,10 95925 0,295 16 f e o d o s i a + 1549 320,7 8 , 4 3 92170 0,421 c o n t r i b u t o a l l a c o n o s c e n z a d e l l e c a r a t t e r i s t i c h e s i s m i c h e , e c c . 5 2 5 tabella i s t a z i o n e | i m pu ls o in iz ia le a (km) a (gradi) vn (km/sec) cos e t a n g -a' a 2 17 sinferopoli + 1622 318 2 8 47 91735 0 434 18 e l w a n + 1630 255 9 8 47 91735 0 434 19 s a m a r c a n d a — 1777 66 8 8 64 89930 0 486 20 i s t a n b u l + 1789 299 1 8 72 89105 0 509 21 q u e t t a — 1866 99 1 8 74 88901 0 515 22 s t a l i n a b a d — 1915 71 1 8 92 87107 0 564 23 k u l i a b — 1994 73 1 8 95 86816 0 572 24 t e s k h e n d — 2012 62 5 9 02 86141 0 589 25 k i r n i n e v + 2085 316 2 9 02 86141 0 589 26 k a r a k i — 2140 114 8 9 63 80685 0 732 27 b u c a r e s t + 2149 308 0 9 65 80518 0 736 28 w a r s a k — 2170 90 1 9 67 80351 0 741 29 a t e n e + 2183 286 2 9 68 80269 0 743 30 j a s i + 2189 315 8 9 68 80269 0 743 31 n a m a n g a n 2198 64 3 9 70 80103 0 747 32 a n d i z h a n — 2252 65 7 10 99 70700 1 000 33 l a o r e — 2495 93 2 11 21 69313 1 040 34 sverdlosk + 2633 17 7 11 44 67919 1 081 35 s c a l n a t e pleso + 2774 314 2 11 68 66521 1 122 36 d e r h a d u n 2833 92 3 11 80 65847 1 143 37 v a r s a v i a + 2877 320 8 11 82 65736 1 146 38 b r a t i s l a v a + 2940 310 8 12 06 64427 1 187 39 v i e n n a + 2998 309 8 12 19 63741 1 201 40 b o m b a y 3022 118 8 12 20 63688 1 210 41 r o m a + 3165 295 3 12 32 63068 1 231 42 trieste + 3170 301 2 12 33 63017 1 233 43 p r a g a + 3209 312 7 12 33 63017 1 233 44 p a v i a + 3400 302 1 12 61 61617 1 278 45 s t r a s b u r g o + 3625 307 4 12 68 61277 1 289 46 copenaghen + 3670 336 2 12 75 60941 1 330 47 clermont + 3940 303 3 13 05 59540 1 349 48 algeri + 4010 286 8 13 21 58819 1 375 49 k i r u n a + 4085 343 4 13 21 58819 1 375 50 kew + 4210 307 2 13 23 58730 1 378 51 t a m a r r a s e t + 4292 276 3 13 25 58641 1 381 52 d u r a m + 4500 310 4 13 50 57555 1 421 53 a s t r i d a + 4542 208 3 13 53 57386 1 427 54 lwiro + 4544 206 2 13 53 57386 1 427 55 toledo + 4565 292 9 13 54 57385 1 427 56 malaga + 4695 289 2 13 55 57343 1 428 57 i r k u t s k + 4829 48 1 13 70 56715 1 452 58 k i a k a + 4968 51 0 13 72 56632 1 455 59 k a b a n s k + 4976 48 6 13 72 56632 1 455 60 t a n a n a r i v e 5970 179 8 15 20 51118 1 681 61 uglegorsk + 7463 45 2 17 90 43408 2 093 62 i u z n o . s. + 7613 47 1 18 04 43071 2 094 63 college + 8877 6 7 20 74 37463 2 475 64 brébeuf + 9346 322 9 21 98 35350 2 646 65 o t t a w a + 9464 323 9 21 99 35334 2 647 5 2 6 f. p e r o n a c i nella fig. 1 sono state riportate in proiezione stereografica la posizione delle 65 stazioni e il primo impulso è stato rappresentato con una freccia verso l'ipocentro se dilatazione e con una freccia diretta in senso contrario se compressione. esaminando la distribuzione ottenuta è facde tracciare con buona approssimazione i due cerchi normali previsti dalla teoria, passanti per c o n t r i b u t o alla c o n o s c e n z a d e l l e c a r a t t e r i s t i c h e s i s m i c h e , e c c . 5 2 7 l'epicentro e che separano le compressioni dalle dilatazioni. le due rette tangenti ai cerchi e passanti per l'epicentro ci forniscono le tracce dei due possibili piani di frattura, ne è possibile come è noto, desumere dalla teoria quale dei due piani sia quello effettivo. il terremoto è quindi dovuto ad una frattura avvenuta secondo uno dei due piani formanti con il nord geografico gli angoh di 126° e 216° rispettivamente. per quanto riguarda l'angolo che i piani formano rispetto al raggio terrestre per l'epicentro è sufficiente (fig. 2) tracciare un cerchio corrispondente affa sezione meridiana, riportare sulla traccia del piano equatoriale a partire dal centro i diametri dei due cerchi e unire l'estremo dei due segmenti con il centro di proiezione. le corde che uniscono l'epicentro con i due punti di intersezione f ed f' rappresentano le traccie dei due possibili piani di f r a t t u r a e ci consentono di misurare gli angoh che i piani stessi formano con il raggio terrestre per l'epicentro. dalla figura si sono ottenuti gli angoh di 66° e di 16°. resta ora da decidere quale dei due piani possibili sia effettivamente il piano di frattura. come è noto, a questo riguardo è necessario l'ausilio o di f a t t i precedentemente accertati o la conoscenza della geologia e della tettonica della zona ove è avvenuto il terremoto. nella fig. 3 abbiamo riportato schematicamente la carta della regione nord-occidentale dell'iran dal punto di vista dell'orogenesi, secondo le indicazioni contenute nel t r a t t a t o « l'evolution de la lithosphère » di termier; abbiamo inoltre riportata su essa la posizione dell'epicentro. i n base a tale carta si nota che le catene di monti che recingono la parte meridionale dello scudo iraniano (di origine prepaleozoica) sono per la massima parte formate da una successiva copertura di altaidi che pur elaborate in epoche precedenti, si sono sovrapposte alla fine del secondario se non nel terziario. la catena dell'alwand che è compresa tra hamadan e kermanshah è invece formata in prevalenza da rocce metamorfiche del paleozoico e quindi depositate in epoca precedente alla co5 2 8 f . p e r o n a c i pertura delle altaidi. l'epicentro cade neda zona di separazione tra le due formazioni e se si adotta per piano di frattura quedo che forma con il meridiano un angolo di 126° si nota che tale piano risulta praticamente parade] o non solo alla linea eli demarcazione tra le due formazioni ma anche all'andamento dede catene montuose di origine recente che recingono d golfo persico. riteniamo quindi ragionevole pensare che la natura fisica della scossa che ha determinato d grande terremoto del 13 dicembre 1957 sia uno scorrimento tra due formazioni geologicamente di epoche diverse, e clie il piano di frattura sia quello che forma con d meridiano per l'epicentro un angolo di cn-ca 126°, e con d raggio terrestre passante per l'epicentro un angolo di 66° circa. se si tiene presente il fatto che la zona è già stata sede anche negli anni precedenti di altri movimenti tellurici, l'ipotesi di una orogenesi ancora in atto non è da escludersi. i l terremoto d e l 2 luglio 1 9 5 7 . alle ore 00 4-2™ c. del 2 lugdo .1957 suda regione montana a nord ili teheran si è verificato un violento terremoto avvertito almeno di ii 0 -iii° c o n t r i b u t o a l l a c o n o s c e n z a d e l l e c a r a t t e r i s t i c h e s i s m i c h e , e c c . 5 2 9 tinche nella capitale, l'area macrosismica molto estesa ha interessato qualche centinaio di villaggi molti dei quali hanno subito danni ingenti. la magnitudo del terremoto stesso ha raggiunto il valore di 7,4 e quindi il movimento tellurico è da classificarsi tra i più intensi avvenuti in iran in questo ultimo cinquantennio. numerose anche in questo caso sono state le repliche, t u t t e di minore entità, come si può riscontrare dall'esame dei bollettini deu'u.e.s.s. la determinazione dei dati microsismici relativi alla posizione dell'ipocentro e il tempo origine è stata da noi eseguita utilizzando le registrazioni delle 20 stazioni riportate nella tabella ii, le cui distanze dall'epicentro sono comprese tra 800 e 2800 km e distribuite nel modo più uniforme possibile intorno all'epicentro stesso. il metodo applicato è quello seguito per la determinazione dei dati ipocentrali del terremoto di farsinaj per cui rimando direttamente alla pubblicazione a riguardo. siamo partiti dai seguenti dati provvisori: ottenuti modificando leggermente la longitudine e il tempo origine calcolati per via provvisoria da mosca, che ha impiegato per la determinazione approssimativa le stazioni più prossime all'epicentro. per quanto riguarda la profondità abbiamo assunto h = 33 km dato che in base alle registrazioni, il terremoto ha le caratteristiche di un terremoto a profondità normale. la prima approssimazione ci ha fornito le seguenti correzioni da apportare ai dati di partenza. ove l'unità di misura per ò li0 è pari a 0,01 del raggio della terra privata della crosta terrestre (63,4 km). con queste correzioni le coordinate più probabili dell'epicentro, e il tempo origine divengono <p = 36,1 nord x = 52,5 est e = 00 42m 26,5s òcp0 = + 0°,1173323 ò h0 = + 0 ,13299 ò x„ = + 0°, 218567 òt0 = + 0s, 74533 "o <p0 = 36° 07',04 n x0 = 52° 43', 11 e li = 41,4 km. e = 00h 43m 27s,2 t a b e l l a i i s t a z i o n e a (gradi) 1 r e g i s t r a ! "empo di i g i t t o = 33 k m t e m p o di origine a/ tn + / ( <po [to + s t a z i o n e a (gradi) 1 r e g i s t r a ! 'j. t r p e r h i g i t t o = 33 k m t e m p o di origine 3 a ti (sec) a h (sec.) tn + / ( <po [to + 1 e r e v a n 07° 20' ,50 00h 4 4 m 1 8 s ,4 0 1 m 4 7 s , 6 0 0 h 4 2 m 3 0 s ,8 + 14s ,0 is ,2 + 4s ,3 2 tiflis 08 17 ,50 44 26 ,6 02 00 ,8 42 25 ,8 + 13 ,9 1 ,6 0 ,7 3 b a k u r i a n i 09 03 ,15 44 38 ,9 02 11 ,4 42 27 ,5 + 13 ,8 — 1 ,9 + 1 ,0 4 b o r j o m 09 10 ,60 44 41 ,3 02 13 ,0 42 28 ,3 + 13 ,8 1 ,9 + 1 ,8 5 p i a t i g o r s t 10 50 ,00 45 01 ,1 02 31 ,3 42 29 ,9 + 13 ,6 2 ,4 + 3 ,4 6 q u e t t a 13 25 ,43 45 35 ,0 03 10 ,6 42 24 ,4 + 13 ,1 3 ,0 — 2 ,1 7 k s a r a 13 47 ,85 45 43 03 15 ,4 42 27 ,6 + 13 ,1 — 3 ,1 + 1 ,1 8 o b u r d a m 13 57 ,71 45 40 ,4 03 17 ,6 42 22 ,8 + 13 ,1 — 3 ,1 — 3 ,7 9 gerusalem. 14 56 ,75 46 01 ,0 03 30 ,4 42 30 .6 + 12 ,9 — 3 ,3 + 4 ,1 10 k o r o g 15 21 ,63 45 57 ,7 03 35 ,8 42 21 ,9 + 12 ,8 — 3 ,4 — 4 ,6 11 w a r s a k 15 35 ,33 46 06 03 38 ,8 42 27 ,2 -j12 ,8 — 3 ,5 + 0 ,7 12 k a r a k i 16 44 ,35 46 19 03 53 ,4 42 25 ,6 + 12 ,5 — 3 ,7 — 0 ,9 13 mirzad 17 15 ,22 46 24 ,6 03 59 ,9 42 24 ,7 + 12 ,5 3 ,8 1 ,8 14 l a o r e 18 40 ,70 46 40 04 17 ,5 42 22 ,5 + 10 ,6 — 4 ,7 4 ,0 15 i s t a n b u l 19 00 ,20 46 46 04 21 ,5 42 24 ,5 + 10 ,6 — 5 ,0 — 2 ,0 16 d h e r a d u n 22 04 ,91 47 18 ,0 04 53 ,7 42 24 ,3 + 9 ,9 — 5 ,7 2 ,2 17 a t e n e 23 02 ,33 47 31 05 03 ,2 42 27 ,2 + 9 ,7 — 5 ,9 -l 0 ,7 18 b o m b a y 24 47 ,42 47 47 ,1 05 20 ,3 42 26 ,8 + 9 ,4 — 6 ,0 + 0 ,3 19 p o o n a 25 45 ,38 47 57 05 29 ,3 42 27 ,7 + 9 ,4 — 6 ,1 + 1 ,2 20 b e l g r a d o 25 48 ,31 47 59 ,0 05 29 ,8 42 29 ,2 + 9 ,4 — 6 ,1 2 ,7 media 42 26 ,5 c o n t r i b u t o alla c o n o s c e n z a d e l l e c a r a t t e r i s t i c h e s i s m i c h e , e c c . 5 3 1 siamo passati quindi alla seconda approssimazione i cui risultati sono stati i seguenti: ò <p0 = + 0",0534367 8 10 = + 0°,0095157 8 h0 = — 0, 235679 8 t0 = — 0*,851355 quindi in base a questa approssimazione la profondità ipocentrale passa da 41,4 km a 26,4 km con un errore standard di ± 15 km ca. questo ultimo, risultato da noi ritenuto non soddisfacente ci ba consigliata l'esecuzione di una ulteriore approssimazione compiuta impiegando solo 14 stazioni, e cioè eliminando dai calcoli el stazioni di poona, ksara, warsak, laore, is tanbul, korog, che comportavano scarti superiori al secondo rispetto al tempo origine provvisorio. 5 3 2 f . p e r o n a c i la tabella i i i riporta i dati necessari per quest'ultima approssimazione che ha condotto alle seguenti correzioni: ò cp0 = + 0°,0082715 ò x0 = + 0°,0366099 ò h0 = — 0 ,079396' ó t0 = — 0s,289722 tabeua i i i a ;rac t e m p o di tn — -[i>0 + s t a z i o n e a a ;rac li) t r a g i t t o h = 26,4 k m origine + /(fl»< -[i>0 + 1 e r e v a n 7° 38' ,75 01 52 ,4 0 0 h 4 2 m 2 6 s ,0 0 ,8 2 tifflis 8 19 ,00 02 01 ,8 42 24 ,8 — 0 ,9 3 b a k u r i a n i 9 06 ,00 02 12 ,7 42 26 ,2 + 0 ,5 4 b o r j o m 9 13 ,60 02 14 ,4 42 26 ,9 + 1 ,2 5 p i a t i g o t s k 10 50 ,17 02 36 ,5 42 24 ,6 — 1 ,1 6 q u e t t a 13 19 ,57 03 10 ,0 42 25 ,0 — 0 ,7 7 o b u r d a m 13 44 .43 03 15 ,4 42 25 ,0 — 0 ,7 8 g e r u s a l e m m e 15 10 ,50 03 34 ,3 42 26 ,7 + 1 ,0 9 k a r a k i 16 42 ,38 03 53 ,8 42 25 ,2 — 0 ,5 10 mirzad 17 02 ,25 03 58 ,0 42 26 ,6 + 0 ,9 11 d e r a d u n 21 55 ,55 04 53 ,1 42 24 ,9 — 0 ,8 12 a t e n e 23 11 ,36 05 05 ,6 42 25 ,4 — 0 ,3 13 b o m b a y 24 45 ,42 05 20 ,7 42 26 ,3 + 0 ,6 14 belgrado 25 53 ,33 05 31 ,4 42 26 ,6 + 0 ,9 abbiamo quindi proceduto al calcolo dell'errore medio dell'unità di peso e degli errori medi dei valori più probabdi delle incognite, per cui in definitiva i valori più probabili dede coordinate epicentrad (coordinate geografiche), della profondità e del tempo origine sono risultati i seguenti (p0 = 36" 11' 11",2 nord ± 0° 00' 53" lo = 52° 46' 22",3 est ± 0° 01' 28" jio = 21,4 km ± 6,5 km e = 00h 42m 54s,4 ± 0,5 sec. l'epicentro è quindi leggermente spostato a ne rispetto alla parte centrale del massiccio del demavend e ad una profondità che interessa la zona inferiore dedo strato del granito. anche per questo terremoto abbiamo voluto eseguire un tentativo per determinare la natura fisica della causa che ha originato il movimento sismico; a questo riguardo abbiamo impiegato gli impulsi iniziad delle registrazioni dede stazioni riportate neda tabella iv, nella quale sono contenuti i dati necessari per la rappresentazione cartografica in proiezione stereografica polare con epicentro coincidente con l'ipocentro c o n t r i b u t o alla c o n o s c e n z a d e l l e c a r a t t e r i s t i c h e s i s m i c h e , e c c . 5 3 3 t a b e l l a i v stazioni p ri m o im pu ls o a (gradi) a (gradi) 1 tiflis e — 8° 19 314°25' 2 p i a t i g o r s k i 10 50 40 06 3 s a m a r c a n d a i 11 49 68 19 4 q u e t t a i + 13 20 112 43 5 k s a r a 14 01 265 17 6 koros i 15 09 79 04 7 g e r u s a l e m m e i 15 10 281 46 8 k a r a k i i + 16 42 128 40 9 mirzad 17 02 75 57 10 l a o r e ei + 18 31 98 09 11 h e l w a n i + 18 39 256 29 12 i a s i 21 28 307 41 13 d e r a d u n i + 21 55 98 14 14 b u c a r e s t 21 35 300 41 15 a t e n e i + 23 11 283 13 16 b o m b a y i + 24 45 129 04 17 p o o n a e + 25 39 127 35 18 b r a t i s l a v u a ei — 28 30 306 02 19 reggio c. i 29 11 285 07 20 t r i e s t e } + 30 25 300 18 21 b e l g r a d o 25 53 299 30 22 r o m a i + 31 29 293 52 23 b o l o g n a i + 32 04 297 55 24 f i r e n z e i + 32 08 296 34 25 u p p s a l a i + 32 25 327 46 26 c o p e n a g h e n i + 33 20 318 19 27 coirà i + 33 25 302 10 28 p a v i a i + 33 37 299 10 29 m e s t e t t e n i + 33 54 304 33 30 s t o c c a r d a 33 48 305 38 31 s t r a s b u r g o i + 34 43 305 08 32 basilea i + 34 47 303 19 33 toledo i + 44 11 292 31 34 lwiro i + 44 19 214 55 35 matzuiro i + 66 25 61 10 36 de bilt i + 36 43 320 53 vn (km/sec) cos e t a n g — a' ° 2 8,00 97125 0,245 8,10 95925 0,295 8,22 94525 0,345 8,46 91843 0,431 8,48 91627 0,437 8,58 90559 0,368 8,59 90454 0,471 8,80 88295 0,532 8,90 87303 0,559 9,02 86141 0,589 9,02 86141 0,589 10,07 77159 0,824 10,09 77006 0,828 10,08 77083 0,826 11,40 68157 1,073 11,44 67919 1,080 11,78 65959 1,140 12,40 62611 1,244 12,42 62560 1,247 12,47 62309 1,256 11,80 65847 1,143 12,61 61618 1,279 12,70 61181 1,293 12,70 61181 1,293 12,75 60941 1,309 12,88 60326 1,322 12,89 60279 1,324 12,90 60232 1,325 12,92 60139 1,329 12,91 60185 1,327 12,93 60092 1,330 12,93 60092 1,330 13,87 56020 1,479 13,87 56020 1,479 17,34 44809 1,996 13,21 58819 1,374 e piano di proiezione al piano equatoriale corrispondente. applicando il procedimento già eseguito per il terremoto precedente, e illustrato dalla fig. 4, abbiamo potuto stabilire che il movimento sismico è stato originato da una frattura; i due possibili piani di frattura previsti dalla teoria formano con il nord geografico gli angoh di 51° e di 141° circa, mentre gli angoh di inclinazione dei piani stessi, rispetto al raggio terrestre passante per l'epicentro sono rispettivamente = 38°, /s2 = 52°. 5 3 4 f . p e r o n a c i riassunto proseguendo nello studio delle caratteristiche sismiche del massiccio iraniano, si determinano i dati ipocentrali e l'ora origine del terremoto del 2 luglio 1957 avvenuto a n di teheran. si è quindi indagato sulla natura del movimento sismico giungendo alla conclusione che trattasi di una frattura-, di essa sono stati calcolati gli elementi caratteristici. analogo studio condotto per il terremoto di farsinaj del 13 dicembre 1957, ha mostrato che anche per esso trattasi di una frattura. abstract following up the study of the seismic features of the iranian massif, the hypocentral data and time of the origin of the earthquake which toolc place north of teheran on july 2 1957, were tvorked out. then the nature of this seismic movement was inquired into, reaching the conclusion that a fracture was involved. salient features of this fracture were worked out. a similar study carried our in the case of farsinaj earthquake of december 13 1957, demonstrated that it was also due to a fratucre. b i b l i o g r a f i a peronaci f . , sicmicità dell'iran, « a n n u a l i di geofisica », x i , 1, (1958). byerly p . , the nature of the first motion in the chilean earthquake of november 11, 1922, « a m e r i c a , j o u r n . science», x v i , 9 3 , (1928). byerly, p . , the earthquake of july 6, 1934. amplitudes and first motion « b u l l . seis. soc. a m e r . » , 2 8 , (1938). di filippo, d., sulla rappresentazione in superfìcie della natura dinamica di una scossa all'ipocentro, « a n n a l i di geofisica», i i i , 2, (1950). caloi, p . , peronaci, f . , il terremoto del turkestan, « a n n a l i di geofisica», i , 2, (1948). termier, h.-termier, g., l'évolution de la lithosplière, p a r i s 1957. furon, r., géologie du plateau iranien, mémoire du muséum n a t i o n a l d ' i i i s t o i r e n a t u r e l l e , p a r i s , 1941. scismi city of matrmshiro micro carthquake swarm (*) j. c. j)lukopoulos (**) h. n. sltrvastava (***) t. tera~hl.ma ricevuto il il! agosto 1\)70 su~d1alty. lllll'illg uio jjrak swarjll adivity at ~rat";ll,.;hiro. micro. cal'thqllakes were rc('oj'(le,l at ~llzaka a)l(1 l'irathllkhiro (m f2\f magndie ,lata rp'(·ol',lcr. 'l'ilo (iuta 80 obbillc,1 wé'l'l' ilt.ilizeil lo ,.;tllil.v l.he val'iatio](~ or b ill r,lltenberg-h.khto·r'~ fn;(lllp]((·y·h(ilji; nitlldll rnln.tion~hijj givcll by log]o.' = " i -/131, whl'j'c m i~ the magllitn,1c all<i n is uio 1l((jllbcl' or t'arthlj1jake.~. it was [oullil thal thc vulnc or b in .\latsu"hiro rw1u'lll i8 hlon'· or lll.~,.; (~onktaht in ;;llort inlcl'vul" oi lilllo bllt ,.)mllgl's , .. ignificli.lltly for two dilfl'l'nht jl('rio,lsof olmnrvation>\ of ~ilzak!l.. tlle vahw of /, from tiri' oh,.;erval,ion" al ;\ia\..~ilhhiro has al"o hmm follml lo h(~ l'al ho!' ;\ji(ali ak "olllpar('([ lo that at s11)~uka. rrassun 'i'(). diil'allte l'illtell,''o perioilo ili attivitiì. mi(~ro,.;ikmica avvlllll1to a l'irat ";ll"hiro, le mino";"o";he fllrono nl/.:i"t.ratn a sil7.aka p 2\i'athilshil'u con iln n'gidl'atorl' a hij.stro ma,t:'lwlko f~j. i ,lali "ohi oltclluli ful'oilo utilizzati l'c!' ht1!,li~i.!,t· lo variazioni oli b. ,lalla rorljl\tla ili gllhmlwrg-h.kllt.llr ,.)11' ll'ga la [n' iiiwnza alla. mag-nitlhlo 10g.n = /[ -".ll ,10\'(' ..il i.~ la wagllilwlo c,l x i.~ il lllllllc!'o ,ici tcl'i'cmoli pl'csi in e.~alllc. dalle of;81l1'vazionl fatte a ;\iat.~ll~hiro (, .~tat.o trovato dw il valore di /, pe!' brevi int('rva.i1i (li tempo (, pii't o mfno ('o,.;tante, mentre yarin. hfn,.;ibilmehte a su7.aka qnij.h,lo "i (,oh~i,]prano ,i\h~ ,li\'i'hi perio,li si~mit'i. ilal collo frollto (1,01]', osrervaziolll ~i è polllto ('ollslatare t'hl' il valol'c di li rdalivo a mahnhhll'o è allj1jallto pii't piet'olo (li ij1h'llo ('akolato per ~";t,aka. l. tn'riwductto~. 8eismicity of 1htsllshil'o ea l'tltqnake f!warm ha.~ beell t.lte subject of illt.el'est for the bst rew yen,l's. some seinnologisth nmllely sllyehiro, (.) intel'hij.tiohal tnstit.llte of seismology and eart luluakl' engineering tokyo, japall. (u) scislilolo!-(ical lahoratory. atlwns l:llivcrsily, grcccc. ( ••• ) !!lilia ;\ici.co!'ological dcjjarllllclll, lo,li roa,l, }l'cw d,oihi 3. i![{!ia. 178 j. c. drakopoulos ii. ~. "rivastava t. terashdia et n.1 (i) studied tho se.ismicit,y of t,he region for celt earthquakes or ea rt,hqllakes of ratlwr large ll1agnitnde. in a series of ljal)ers, hamada and jtn.giwn.rn. (~) reljorted det,ailed re8lllts of tlw micro and ultra ll1icroea rt,hqnakefl in the a rea of ::\iu,t,slirhiro ilfling high sensit,ivity trivart,ite observations recorded on magnetic data recorder. 'l'hey fonlld t,hn.t, the valne of b in gutenbcrg-llichter's relation8hip (3) did not, clmnge signiflca.ntly with respect to dificrent intervals of time. :further, the valne of b dul"ing ulc abnormaliy high activity wa.s flill1ilar to that for normal seismie l1ctivit,y. they also found that the ishimot,o-lida's coclftcient "in (4) examined with res}jeet to nltrn. li1icro-en.rlhqnh,kes showod la rger v a.lnc before two elimaxef! of flei:muc activity n.t the bcginning of aprii and t,he end of august and the smh,lier vh,lue l11)1)earcd jnflt a fter the climaxef!. thc party for soiflmographir. obflervn.tion and soiflmological section (~) obtained the valne of "in for overy lo days and tho valuos of b for each month flince 1ifal'ch 1966. the value ofm at eh,ch fltation of obflcrvn.tion showed a sill1ilar belmvionr. the object of the ih'{\f!{lnt palmr if! to fltudy thc seismicity in lmrt,icuhlol" t,he variations of b in gntenberg-richtf>.r's l'elationshil) (3) within flllorter int.erval of timo than studicd hithcrto with reference to t,he micro-earthqnakef! recorded at smmka and :i\tatsushil'o lmd coml1!h'e t,ho reslùts with t,llose of other inve8t,igators. ,, ~w .... ,, ~,~t. '"~ ,.~.ult. fig. l block dia.gram for micro-carthqllake o1.jservatiolls. seimiici t y 01' matsusju1w mic1w eartliquake swarm 1 io 'l'able l 8uzaka l (196,:;) munitur perioù 01 ob~(\r\'ation i xum1jer of n° pl'oill '1'0 eal' t1hiuakes i ]kl'. 20 , 1411 1 om dlw. 20, l till 3"m 12 2 1 ti ,i2 23 2" " 3 dee. 21, oh 00 ilee. 21, 12 42 31 4 18 42 20 40 2 ,i 23 45 ilrw. 2') -, 00 48 o " ilox'. 22, 00 .51 01 44 o ; 01 '" 08 l" 1 35 8 08 17 12 31 " u 14 io 16 55 33 lo i i 3i 18 51 o ]] 18 51 20 23 o 12 20 38 lltw. 23, ])]) 27 37 13 ilox'. 23, 00 30 07 00 "' 14 oli 00 il 53 21 l,:; 12 32 18 00 52 lfi 18 o,i dee. 24, 00 40 33 17 due. 24, 00 41 00 "h 3 18 01 io 07 30 40 io 08 oli 14 33 37 20 14 34 21 16 52 21 21 i; d('i\ . 2,;, 00 06 18 22 de!'. 2,;, 00 14 07 30 41 23 08 34 13 o,i 26 24 13 13 16 12 i; 25 io 17 in 47 32 26 ili 48 00 ,i.5 61 2; de!:. 26, 00 ,,6 dee. 26, 07 30 85 suzaka ]] (1066) 50 jun. 13, 011l12m ,j a ll. 13, 08"00,n 02 51 lo 23 12 23 14 52 12 2.5 1550 15 ,:;3 16 o" 17 lo 8 ,)4i; iii 21 36 i 60 ,i,i 21 ,,8 ,j an. 14, 00 lo 1 ,i6 56 .tali. 14, 00 il og 45 i tii 5i i lo 05 12 14 7 58 12 io 1 h ,:;2 i 1.50 ,:;1) i i n 30 21 mj i4 60 21 51 23 35 n3 1 1 matsush iro (10g6) i i may 21, 00"10m i may 21, 04h44m i 510 180 ll ii akoi'our.os .r. c. s iilvastava il . s. 'l'eltas il d!a t. 2. inf:ltltu!lfenta'l'lo-", and collection oi<' nata. 'l'ihl experimental ,~i'i'}~llgellwllt for t.he observatioll of lili(li'o-eart.hqw~kes is giv()1l in -fig. 1. 'fhe oulpllt signalh frolli t,hn't' moving ooil transducers oi 1 seo fnlo ileriod wit.h it =!=o.(i5 ,vere roeorde(l hy fm magnetic tape l'eco!'der after alllpw-ieation. 'rhe .~ensititit,y or thc transdueer ih 3.5 yoi1:/kine. tlw respollse of lhe dat.,~ of ff.i was l\at irom nc to 250 eil,~ with a shal1) cut ort. 'plte gain of t.he aml)lifier was variahle dup01uling upon tlte grollllfl 1l00se. 'rlte t.'"i)es wci'e repi'(l(luced h'ypen galvallometer~ of natural frequency 30 cp.~ witlt a papm spee(l oi 30 lllm/.~ec. tihi l)erio(l of observation alld the nulnber of lliiero-e,~rlhqm1kes reeorded at, slizalm ami mat.sushiro are given in 'r,~hle 1. during t,he period oi observat,ioll at ì\iat.~lishiro approxilllately 2 eartltquakes were recorded per lliinllt~l. 3. cat.gv"lation of ]'l\gni'l'1'df~<; oi<' l\iicho-ea1~'l'l1lìliat;:e.<;. 'rlte ea.lcuh~t.jon oi nmgnitude of lliiel'o-ea.l-lhqm1kes is bamd 011 t.he eoilhidera tioll or atwnuatioll of seislllie waves with tlte epieentral dista.uee. mumm atu et al (s), (lerive(l tlw following equatioll fol' c,~lellla.t,illg the magnitude frolll lhe ohservations oi miel'o-lmrlhquakeh in the vieillit,y of gifu prefeetl1l'e. .ili = 1.25 log ~i" (kine) 12.5 log t (lnu) + 1.0 [lj whel'e a" i,~ uw velocily muplitude ,~jl(l t is llw h.ypocentml dihtauce. hecllllt.l.y tera~hillla (7) dl:'.1'ived the following equation iol' the enlculat.ion oi magnitude.~ of llliero-eal't.hquakes. m = log a"(t) + 1.5 log t + 2.35 [2] v'h have iis0il ellua.t.ion [2j in the pre8ellt stlllly fo!' lhe ealeulatioll oi mngnitlt(le.~ of miero-earthqua ke8. substitut.ing t.he inst.l'umental eollhtanth and the hyl)()eelltral (listanees in equa tion [2j we get: 0.-1-1 + 10ga -fm' suz,~ka monit.oring" recm'der sensitivit.'y,2011unimvolt [3j seis~iicity ù"'" ~j,\'rsljsllmll mlero llailtiiqua10: swaidi 1s1 ji = 0.84+ log a ..ili 3,10 jlog a 1<'01' suzaka lllollitol'ing reeorder ~en8it.ivity, 50mm,im yolt. [4] l·'or )htsushil'o monit.ol'ing ]'n',ol'del' ~en8it.ivity, 2mm/lij \'olt; [5j whel's .il is the doublo amplitllde as moaslll'ed on tllo st>.if!lllogl'am", ! ! i l" \' , " 'l'i ! i i i , , , , i , ,i. .. i , , , , i i i . i i i l ; , , i , i i i i ' i • i i i i i i i i i i , i i i i ' " ... _. " , i i , fig, 2 micro-eartlul!jakes reeonled at ~uzaka 011 ja11. 14-, 1\.166, i fig, 3 mkl'o-cal'thquakcs 1'6001'<1e<1 at matsushiro on may 21, 1966, ali tho oarthquakos of trace amplitude greater than l mm recorded 011 ~eismograms were ntilized for calclliating t.he magnitllde~ of lllierooal'thqllftkes_ 'l'he ea.rthqllake~ of m agnitndo largol' tlmn that col'l'espondingto tho traco ami)litude of 50 mm had to be omittod as t.he defloctions were too largo to be "\yit.hin the recol'ding papero the dlll'ation of p a mi.'; int.orval at sllzaka wa~ appl'oxima tely 2 second~. r and 8 phasos wore not quite cloar at thhtsushiro due 182 d u akoro u los j. c. s ih vastava i i. n. tima1i il dia t. to the location of the f\eif\mometers in the el)icentral region. typical micro·ea rt.hquaokes rccordecl at suzaka and j\j atsushiro are shown in fignres 2 aond 3 respectively. ,1. rbsults and discussion. it is weli known th a t, the nnmber j'l of eart,hqluvke8 during an a rbitmry interval of t,ime in a given seif\mic rl-lgion is l'elatomi to mlv' gnitude jf by log n = a'-bm. [lì] many investigl~torh use t,hif\ equa tion in the form, log n = ali + b (8-m) . [7j in the above equations the const anw a', ali l~llli b are coefficients which flore determined from the da,t,a by t,}ie least sqluvl'e met,}liid and the v a.l'iances a re alf\o given. the values of a' or ali vary wit,h the cluration of timc of the data, geographical location and choice of .di'il 'l'he valne of b i8 reh~t,ively uniform. liu,,,ever, collsidemble diffol'ence of uijiniun exists regarding the v a riations of b with the focal depth, geogmphical looa.t,ion, foresllllck and aftershock seqnences, ea l'thquakel sw al'ms ,lnll vi)lc,oni ? eal't,}lqm~ke.'!. 'l'he value of b increases as the (legree of hettlrogencity incl'ea.'!efl and a.'! tlw degree of symmetry of the al)plied stress decrcases. .a delcreasc of b with depth h a s been fonnd by fleveml sei.~mologists. it has been fonnil ( ~ .u) that thel v'olncr of b ,ore lal'ger for 'oftcl'shockr th an for foreshocks. 'l'he constants bi and b" of the foreand after· shock scqucnccs of the s,ome main shock are connectl-lll by the l'el ation(o) bi '"'"" (0.11 ± 0.13) + (0.65 ± 0.11) ba . [s] mogi (l~) 8howel(1 exveriment,oliy that when foreshocks and aft,ershoch occui' in the same volume the v altle b is sml~ller fol' fol'eshocks than for aftershocks because the main shock ca.nsef\ new era cks an(l thuh the number of irregula.l' voints increasf\,~. the value b for earthquali:e swarmf\ is larger t,}uvn fol' aftershocks or normal seismic a.otivity. ishimoto ami iirla c') 1)rol)08ed l~ fol'mnhv n(a) da ~ h-m da [9j seis~iicity 01' )iatsusiiiiw )liciw eautliqu.\ke ~w.lidl 183 where n is t,he illllnher of e'1.l'thquakes and a is the m.1.ximulll tmee a.lllplitude. 'fhe coeffieients k andm are detel'lllined by t,he ie't\st square method. for..ti, it i~ suffieient, t,o oonf\idel' t,lw llla.xiul1jln tmee t\mplit,nde of an arbit,rary chosen standal'd f\ei::!lllograph. coi'i'i\etiolls for dist a noe must ilo llla.de fl'om the point of observat.ion to tho opicentrai a rea. jk-llj]{akul'uulu~ j. c, shi\'astava il, ~. te]l\~i!du t. sincfl m -::: log a for ollicolltral llista,llee eqll a l 1.0 100 klll a lld da = adlli it (!)"m be slwwn tlmt tll(' conhklo llt,s /; a llli iii in tlw forllln!as [o] a llli [hj a ro rolated by /; = '11/.-1, [lo] 'l'lw 10g',.rilhm o( the onllln la.t,iye fw1llll']wy funct,ioll n(j'u) and tlw lllagnitllde of mic]'o-e,w llt1lllaoko ref'onlod :~t. t)m:a,ka dllring tlte lwl'ioù ~oth j o ::!oth i k.fll'llllwr 1965 is showll in tlw fig. -l 'l'ho 1llolh m: lol' kt'd with lejtl'],,~ a, 11, c, n, g a,lld 1<' indiclbt,e ilitii>l'ellt intol"yah o( time, it lll a y lw ,~eell tlmt, f~ ll tho six el1lteh hi'o lj,pp]'oxilnn,t,efy p, lo]'a lllll wit.,h (', wh otlwr im}llyill)..!," that /; ylj,lnoh mllulillh noilkl a llt ill ~ho]'t, illte]'ya l of (ime. 'l'1m f]'('(lllcney llm)..!,"llihule ]'olat.ioll~hip fo]' the~e <hli'y('s is (\x.iji'i~,~h('d l)y ( ,l) log n , .li5 .0:") jl! (ii) log j..v , .-w .0:") ;li (o) log -.n :l .::!.1 . o!! m ( li) log -.n . !]h .06 111 [11] (e) log n 1 . !ii! 1 .05 ;li (1") lo}.!; n 1 .. 1\) il .!w m li'])..!,". (r)) giyeh lite m ag"llitllde-froqul'liey l'dn,t,ioll for nllotllor 1h\]'ioli of oh,~(~rvalioll i, c,13th t.o l-it.h :\lh,~r 1!hw a l, snzaka . aiulonglt t.,lle yahlo oi b ih sliglruy dificrellt., f]'oill t1w )ji'evioll8 set. oi ohhorva t.iolls (fig. -i), tho v a lne of li 18 almof\1, ht a tioll a l'y dll]'ing" difiorl\nt int,crvafi; or tinte. '['[l(' ]'ef<lot.ioilh hd,\n~i'1l lo}.!; n vel'hils ;li a re givoll a.s iollow~: (a) log n 2 .~9 1 .19 m (ii) log n ::! .18 1 .~\l m (o) log n , .o~ .1 !! al ( n) log n 1 .,so 1 .18 111 li::!] (el log n 1 .:3/ 1 .1g 111 (p') log ii 1 .39 .13 m fn t.his case wc jmd to oxefulle t,he lllagllilihle slll a lllw lfl all -o .!) lwemlm' dlll'ing th is iwrioii l[m sellhilivil,y 01 uw (b ta rc'eonlor wa.'! nlmllg(~11 with lite 1'(~,~ilil th a t.. uw muth(jlm kes of wmller llmgllituùos ('onld bot he roool'ùed tlm.j\1ghout tllo :pol'ìoii of o!jsel'v a tioll. rl'hi,~ i8 the l'ea.soll why tlw cm'ves ùo not, et. t,he dal,a fo]' t1le lll agnilndes lowm' t,ll a ll o.:"), 'l'he plot o[ log n ve]"~ilh .m a l }h tf\ll8hi]'() is giveu ill :fig. n, 'l'he yalno of b l'mnains statiollal',y dul'illg ùiilerent intervuls of lime • " ~ • .. • • • • " " " " " • • • • • ~ ~ v p " • o • • o • ", ~ • • • • .. ~ " • ~ • • • • , .. • -.. '. • • • • , .. ", • ~ " nt'ì ~ <! .. " • • r .. .. ~ • , " 2 ~ .. ~ 'lo ... ,.,"1 • ... ~ . h u ~ , 'ii ~ ., •• ~ • , \o." . ... lo ~ • \ ~ o ·~i· ~ '-' • • , 'lo:. ' .h • ,., , .. ~ o.,'" , • o • > u • • o , <l , o ' .0 5 , ., • • , . , • • 1 • c '.' o f ,. • " ,., o < ~ • o • > o o • " ~ • " • • > '.' o • • < > o • lo , c , , , , o., o " " " ' ·0 > ,. • o < o., > ; .. • -' .'l -) .1 -}.o -1." -'. -1,·1 -., -z. .~ -1.'1 · 1·\ -1."1. -'l·t -'1..-" l.' -n ·u -" 1.'1 " , ]i'ig. 6 i •• ,. v e r ... us ji j<1r !l iffere nl. inl.e rvtl.l,. 0 1 ti nl(' te<'onl c'tl " i' jahm!ihi ro ( 196h ). sej~~licjty 010' ~iat~u~j-jjro migro l~arthq,uake ~w."'ib[ 18i foj' this period of obsel'vation although the value of b is srnallel' than t,lu~t a t suzaka. thc least squarlìs fit blìtween iog" n ltnd . .:lf fol' different bme int,ervals (fig. 6) is given below: (a) log ],t 0.10 0.8..1m (b) log ],t () .1h o .. ~3 . .:lf (c) log n () h o . 86 . .:lf (d) log n () 57 0.92 m [ 13] ( l<j) log n () .88 0.97 n (f) log n 1 o" ."' 0.86 n (a) log ],t 1.38 o 86 m . in table ii tlw cllmula tive fl'equency of n1rthquakes for ali cases is given. 'l'abl .. il cu~iulatj\'e frequexcy of ]<:ar1'iiqu.\kes • vrolil sm;aka i fi'olil :-luzuka il frolll .\iat..4u~hiro :.r ago cuil!. i :\fag. culli. "rag. culli. m freq. ][ freq. ,l{ frl'q. --o . .j. h4!j -o . h 610 -3 2 510 -0 . 3 7.j.2 -0 . 7 ;ii8 -3 . i 467 -0 . 2 h4!j -o . ti 545 i -3 o .j.36 0 . 1 555 -o . fi ii22 -2 . d 3im 0 . 0 4iih -oa 47h -2 . 8 2% () . 1 :170 -0 . 3 3ii .. 2 , 23i o. 2 2ui -0 . 2 320 -2 li 18ti o. 3 235 -0 . 1 261 -2 , l4i 0 . 4 lill 0.0 20.j. -2 4 128 0 . 5 l3ti o. j 158 -2 . 3 114 o. i! 10h il 2 130 -2 . 2 87 o. i 83 0.3 08 -2 . 1 i3 () . 8 70 il 4 tiu -2 . 0 ti3 o u 53 o 5 .j.ii -i . 0 ii 1 1.0 '" il li 3;; i . 8 43 1.1 " il 7 21! i , 34 1.2 19 o 8 \;j i .i! 24 1.3 5 i o 9 7 i . 5 iii 1.11 2 1.4 2 1.1 i i i,'\x 1j 1i akopolilos .r. c. -~ u n-astay,\ ii. :\". terasnnla t. frolll 1;hc u,hovc result~ it may be sccn that, thc valuc or b is sta tionary in short intcl'val or time in earthqm~ke swarmi>. 'rhis result is simil a r to tlmt ohtainc!1 foi" l ~ft,(wshock si'qlwnclls (ii). ho\\'lwci", in thcsc 1'1ll"thqnakc swarms, thc vu,hw of li changcs significh. nt,]y in dilhwenl mont·h" 01" lli1rm'l'nt ])crioll of ohmli""va tion n. o. ri) .8 . .s 2.:j ftd,. lo :\ial". roi"l'·aht'.r~ll(wk~, ~wal"lll ' l liol"liial s\'i"mit'itj' :\[lil~ii~llìl"o i'w:l1"1ii i u l.n i ,api". lo ,llil!. [.s <).j. i ,i iii. to .\11g-. 1l.!liii . .j.o 1/1, 2. [ !ili f~ lo. i ~hi!i 1.1 o. :j,") 0.7(; o.iiii±0.05 1.13±o.o.j. o.lj lo i.:j r')l"e"ho('k~' :\rllt.~l1~hil'o aftel""[lot~k~ rol"c~ij<,('k,,' ciiiii':lil artei"hij<)('k.~ l'urlh,[. lal"g-e pil1't.hq. ili ,liit\'l'l'lit, ~i'i~mit~ n'.g-ioji~ onlilllll"y parlll,[uakt,t; ! allll aft'i't><h,,,'k ad,ivit.y aiiiiiiii"" il1illif\!la .'\: ilag-iwal"a i !llili, l't. i liajll~\!la .'\: liag-iwara inno, l't. 2 [laijlf\!lli so. iili!.!,"ìwlil"a i !llili, l't, 3 ' l'al"t.y foi" spi~jllog-l"a[)hi(' ()h~t'.rl'at.ioll ([\jtl7) [[ol"i (ihtr"i) sl1,vl'hiro. l't al. ([ho.j.) ~ll:>'f\hii"o (i ~hi!i) glltmllh'tg $.: ri"litl1r (i h;j.j.j" :-ìllzuki ([ ~j,")~j) in tanc hl, thc valucs or li l ~ll(lm ohta inc!l hy diltm'ellt investigators are g-iven ror jlatsushiro rcgion. tt nmy be .~mltl t,hat thc vh.lllc of li com])al'lls wnli with thc cllrlicl' \vorkers. suyllhil'o (l) noliec!l a ~llhstl ~llt.ial diltenmcc betwcen thc forcshoeks and aftcrshocks in tlw b vahw whieh was folllld to he 0.35 for tlw foreshocks l~nd 0,7(; for thc l ~ft(\rsll()(lb or l ~ sllutll lh'i'cc])tible seis)li('ity ()f ~l\tsltsiitno ~llcno eah'i'iiqua"ke ~waioi jhfi oarthqllako in tlw )ia,tsnshiro rogiml. comparmi with them hw v a hw of li i~ larger l ~~ is expeetod far tllo oal'thqllakc' ,~warlll,~ al!!! aho snggostml by the oxporilllonts of i\iol!i (i~). h howovor, tlle v a lno o[ b for )iatsnshiro l'el!iou l[nl'iul! file hwlu'm i,~ colllp a rod with t!mt of genomi soislllicity ovoi' dil1(1!'(1ut p!;\e!\i; in tho world (13), it is rathel' diflicult to 00110h1(10 tllat t!w ya!no i,~ iliglw~t. dnl'ing tlw swu,rlll. 01](\ of tlle pos~ihl0 l'oason,~ far thih llis(,l'epancy is a,t,trihlltnd to t!w dittm'(mt gmphieal all(i nmtlh'lllmicallllotllods mpd [or detl1t'lllining b va!n(','l, )h1sidph tlw 11.~e or dilferellt lltagnitude sl·all's. 5. coè'fclttslon. '[']w abovll stlj([y hl'inl!s to light tlw iuterc~tilll! collelmiou thajiu il[l,rthqllfl,ko swlulil tho valno ai b l'olllaim station1h.v iu a sliort lime interval l~lthough it nmy cha,ngl' ovoi' dilferont ]wriod ai observat.illm. g. ackxowledgei'iien'l':'l. 'l'he a.nthol's l~re ~'tatpfu! to dr. s. ()illote, ;\diul! llirnctor of lj1tol'w~tioua,! illstitute of seislllo!ogy l ~ihl eal'thqw~lw enl!inoering, tokyo, for eucollral!lllll(mt and far providing tllo lh'('esslw~' [a,c,ili1il's ior the ih't\~ml1 inv('stil!ation. (') syuehiiw s., a.~aih t. & :'il uiltak.:, pon's/lfjckr (imi ai/rr".lwck.~ "-lecolllf!lwyiny il page}j/.ible h'arl,hquakl' iii celi/mi .!ajjall-, "pa]lllr~ ,rlltt'o!". geoph.y~. 1'okyo ",15, 71·88, (hhi-1-). (' ) ilul\da k &; t. hagiwai(a, lijgh sell".i/i!'i!.l! 1'ri}jar/.itc o/"<frm/iol, 01 jlii/~i/.".hiro barlhgihlke~, ,. bull. eartlhl. r('~. in~t.." puri 1, 121:1123.-., 1'111"1. 2, 1 2:111· i 21i1i. l'art, :{, hiii7·ih'-'7, (i!llili), l'art -1-, li!l· !\hi (i !hii). (") gljl'llnileiw il. & c. hictltei\, 8r:isjjl.fcil!l o/11m.' 11'111'/11. imd a~socilll,l'd f'hr'1ii11i1c/i{i .• prìncctoll l~niver~ity pm;;;; ", (1005). (') [siii}ioto .\1. & lilla k., ()/lsl'rl'lllillll~ il'l/)" [rs srisi!li'n hllre(!i~lrls }j/l1" k jl icroseismoyrn}jhl' (!r/ll.n/rll il, f)rrll-ihl'llicllt" il 1111. eurt li 'i. r('~. [n~l 17, .j-.j-3 . .j-7k, (111311). 190 fh ! akopoulos .1. c. s il ivastava ii. n. terasiiima t. (') purly for sei~mogruphic observatioll of ][atsus/dro eal'thqnakcs and heiwliulmetrieul hcction, .l[alsu~hil'o bul'lhquakes obscl't'ed wilh a ternpo. rary seismogra!,hic network, l'art 4,45, 887-9l7. (i9h7). (') ;-'tulta)iatu i., et al. o!jsc1tutioll of .l[icro-earlhqll(lke~ in j[ino dislricl in oifu prefect.ure, ceti trai .!o/lun "j. of l'hp\. of the l';arth ", 11, 35·49, (191i3). (1) terasnbia t., .l[agnillldc oj ]ficro'l'urthquuke alld tlw hpectra oj jiicro· mlrulqu(lk e ivavel,' ,. bull. liltorn. ill~t. scislil. eal'tllq. engin ", 5, 31·101:!, (hl(jl:!). (") suy.~:l!jiw ~., j)ioel'mlc~ !j~tlt'emi ajll'1"8/wcks (llid f01'es/wekli in lhc ri~. l.alionship (lf ][agni/udc to l<'rcq-lil'ne!! oj occ-urrcllce lor the gl'eal chilwn b'urlhquuke io 1%0 "bull. spislll. soc. alil.", 56, 185·200, (190h). (') ])ltakopoulos ,j., ghamc/eri~iie l'araml'lrr~ of fore (llid allcl'6hock hc· quenecs iii ihl' hegi(ln oj (frecce, "di88ertatioll ill athen8 univf'.rsity" ( 19g8). (ic) ;-'togi il, h/ud!! (lf e/rullie 8hod:s caused b!!lhe fraclurc 01 iiell'l·o.qeneo-us .l[alel'i(ll~ all{i iis re/.alirj!l.s lo bar/hquakc phcuollle/la, "bull. eal'thq. rer. 1118t.. ",40. 125·174 (19h2h). (l') utsu t., on the n(i/ll1'e oj thrl'e a/.a8krw aliershocks 1'3cquellees (li 19ii7 anri 19,;8, " bull. ~eh\ll'. so!'. am. ", 52, 279·297 (l962). ('2) ;-'togi k., home ])i8c-us~irhls rm ai/e1"8hock8 f01"cshoeks al/d earlhq·uakc hworms 'l'he fmcture oj a hmni·lnfinile body callseti by ali inner stress all~l ils rdatio-//. lo thfearlhquakfphcnomcna, "bull. i(nrlhll. res. fast. ", 41, lili)·658, (l963b). (l~) isacks b. an,] ul!y.~:1t j., scismie iv(wes with freqllf-ncies irom 1 lo 100 r;ycll'8 pa 8eeoml reconlcrlin a deep jline iu northern nc1j' ./crsc!!, "bull. ::)ei8nl. ::)01:. am. ", 54. 1941·1979, (1904). (h) hom m., .l[al8ushiro burlhquake slt'unn auri j/s perij!hcral scis1iiicily, "bull. l';arthq. rei<. last. ", 45, 489-503, (l967). (' 0) suzuki a., ,1 slalislical sturiy on ihe oecurre/lec oj hmall bar/hquakcs, "::)ci. rept. tohoku uaiv., ::)el'. 5, geollhys." il. 10·54, (1959). s0553 annals of geophysics, 60, 5, 2017; s0553; doi: 10.4401/ag-7399 gps seismology for a moderate magnitude earthquake: lessons learned from the analysis of the 31 october 2013 ml 6.4 ruisui (taiwan) earthquake huang-kai hung1, ruey-juin rau2,*, elisa benedetti3, mara branzanti3, augusto mazzoni3, gabriele colosimo3, mattia crespi3 1 national science and technology museum, kaohsiung, taiwan 2 department of earth sciences, national cheng kung university, tainan, taiwan 3 geodesy and geomatics division, department of civil, building and environmental engineering, university of rome “la sapienza”, rome, italy article history received february 25, 2017; accepted june 11, 2017. subject classification: high-rate gps, measurements and monitoring, instruments and techniques, ground motion, gps seismology . abstract the 31 october 2013 ml 6.4 ruisui earthquake was well recorded by twelve 50-hz, four 20-hz and thirteen 1-hz gps receivers, and twenty-five strong motion stations located within the epicentral distance of 90 km in eastern taiwan. kinematic positioning solutions estimated by four gnss software (track, rtklib, gipsy, vadase) are used to derive the seismic waveforms and the co-seismic displacements for this event; strong motion accelerometers are used to verify the capability of high rate gps to detect seismic waves generated by this earthquake. results show that the coordinate repeatability of the gps displacements time series are ~6 mm and ~20 mm standard deviation in the horizontal and vertical components respectively, after applying spatial filtering. the largest co-seismic displacement derived from high-rate gps is nearly 15 centimeter at 5 km northeast of the epicenter. s waves and surface waves are successfully detected by motions of high-rate gps and double-integrated accelerometers within the 15 km epicentral distance. for the first time twelve 50-hz and four 20 hz gps observations for seismological study were used and analyzed in taiwan; a clear benefit was evidenced with regard to the seismic waves features detection, with respect to the 1-hz gps data, so that ultra-high rate (> 1-hz) observations can compensate the sparse coverage of seismic data, provided proper monuments for the gps permanent stations are realized. spectra analysis between co-located gps and strong motion data further suggests that the optimal sampling rate for high-rate gps seismology study is 5 hz. the 2013 ruisui taiwan earthquake recorded by the high-rate gps permanent stations network in taiwan demonstrates the benefits of gps seismology for a moderate size earthquake at a local scale. 1. introduction high-rate gps has become an important sensor for the seismic wave detections and earthquake warning systems for moderate and large earthquakes. the true ground displacements retrieved from the processing of high-rate gps observations provide precise seismic waveforms to identify the wave property, the arrival time of the body waves and the surface wave [kouba 2003, larson et al. 2003, bock et al. 2004, langbein and bock 2004, kouba 2005, ohta et al. 2006, larson et al. 2007, bilich et al. 2008, larson and miyazaki 2008, miyazaki and larson 2008, davis and smalley 2009, larson 2009, shi et al. 2010, avallone et al. 2011, ohta et al. 2012, branzanti et al. 2013, li et al. 2013, benedetti et al. 2014, li et al. 2014a,b]. for large disastrous earthquakes, such as the 2004 sumatra and the 2011 tohoku-oki earthquakes, highrate gps gives the possibility to provide information about fault rupture processes, focal mechanism determinations, and tsunami warning and monitoring in near real-time [ohta et al. 2006, blewitt et al. 2009, crowell et al. 2009, colosimo et al. 2011a,b, ohta et al. 2012], also considering the remarkable advantage of the no-clipping and no-tilting features with respect to the strong motion and broadband seismometers [bilich et al. 2008, zheng et al. 2012]. significant surface wave propagation and attenuation are observed by regional dense continuous gps permanent stations (cgpss) network [davis and smalley 2009, hung et al. 2 grapenthin and freymueller 2011, hung and rau 2013]. however, the traditional sampling interval of the high-rate gps (1-hz) cannot always describe the seismic waveforms completely due to waveforms of higher frequency in near-field earthquakes [avallone et al. 2011]. efficient sampling rate of the position time series for the waveforms monitoring are controlled by the nyquist-shannon sampling theorem, which states that the recording sampling rate should be twice larger (in practice, it should be at least four times larger) than the main frequency for the interesting signals. some moderate earthquakes recorded in near-field with 5-hz and 10-hz sampling rate demonstrate the benefit of the very-high-rate gps for the waveforms recover [avallone et al. 2011, zheng et al. 2012, lou et al. 2014]. on the other hand, differently from the reconstruction of the entire waveforms, it was showed that co-seismic displacements can be accurately detected by the combination of the 1-hz gps solutions and co-located accelerometer [bock et al. 2011, melgar et al. 2013]. as regard the present study, since 2010 a dense cgpss network of nearly four hundred sites has been established at 10-20 km spacing in taiwan from various institutions. about 250 stations established mainly by central weather bureau (cwb), central geological survey (cgs) and national land surveying and mapping center (nlsc) are available for high-rate gps recording for both geodetic and geophysical applications. the receivers installed more recently are characterized by a larger storage and a higher sampling rate capacity [yeh et al. 2012, hung and rau 2013]. these features were exploited to collect gps observations at high-rate and very high sampling rate for this study, where we used 1-hz, 20-hz and 50-hz cgps observations for the investigations on the moderate ruisui earthquake (ml 6.4) occurred on 31th october, 2013 in eastern taiwan, which was already object of other seismological researches [e.g., lee at al. 2014, wen et al. 2016]. in details, thanks to the availability of both cgps network, broadband seismometers and strong motion accelerometers, the goals of the work was to develop a deep comparison about the solutions supplied by these different instruments (also considering different gps software) as regards seismic waveforms and the co-seismic displacements, and to verify the capability of high-rate gps to detect body waves and surface waves. in section 2 the used data and software are introduced; in section 3 the main results are presented and discussed; in section 4 some conclusions are outlined. 2. data and method cgps dual-frequency observations at 1-hz, 20-hz and 50-hz sampling rate were collected to estimate the epoch-by-epoch position time series during the 2013 ruisui earthquake in eastern taiwan (figure 1) for the seismic waveforms and co-seismic displacements monitoring. detailed information for the four 20-hz gps stations from national cheng kung university and twelve 50-hz gps stations from central weather bureau, including the types of the gps receivers and antennas are listed in table 1. observations of broadband seismometers (bb) were collected from bats (broadband array in taiwan for seismology, http:// bats.earth.sinica.edu.tw/) and strong motion data were collected from the central weather bureau, taiwan. note that some near-field seismic waveforms obtained from broadband seismometer stations hgsd and yulb are clipped (figure 2). figure 1. map of cgps and seismic stations around the epicenter of the 2013 ruisui earthquake, eastern taiwan. focal mechanism of the ruisui earthquake is shown in lower-hemisphere projection. yellow, green and red dots denote cgps at 50-hz, 20-hz and 1-hz frequency sampling, respectively, cyan dots are strong motion stations; blue diamonds are broadband seismic stations. stations mov1 and wust located at the western taiwan are selected as the base stations for the composition of the common mode filters. station pnhu was the fixed station for the gps processing using software track. gps seismology for a moderate magnitude earthquake 3 gps epoch-by-epoch displacements were estimated from phase measurements in kinematic positioning [hofmann-wellenhof et al. 2006], following three different approaches and using four gnss software: differential positioning (dp), using track software [herring 2009]. the reference station used, pnhu, as shown in figure 1 is located 200 km west of the epicenter, outside the deforming region with stable coordinate time series. precise point positioning (ppp), using gipsy-oasis ii [zumberge et al. 1997] and rtklib software [takasu 2011]; precise orbits and clock error corrections were generated by jet propulsion laboratory for gipsy-oasis ii, and from final international gnss service (igs) solutions for rtklib. variometric approach, using vadase software [colosimo et al. 2011a, branzanti et al. 2013, benedetti et al. 2014]; here solutions were obtained in post-processing mode, but in order to simulate its real-time capabilities standard broadcast orbits and clocks were used. to reduce the position errors due to the multipath effect in the cycles of the sidereal day, modified sidereal filtering (msf) was constructed and applied to the original positions [choi et al. 2004, hung and rau 2013]. subsequently, a spatial filter (sp) were applied to each position time series to remove the common-mode error resulted from unmodeled effects during data processing [wdowinski et al. 1997, bilich et al. 2008, yin and wdowinski 2013], where the stations mov1 and wust (figure 1) located about 150 km away from the epicenter are used to construct the spatial filter. these two stations were checked for their data quality and if there is any disturbance induced by the earthquake. the displacements after applying spatial filtering are analyzed for the investigations of their precisions and waveform propagations. the main features and parameter settings for the processing with all gnss software are listed in table 2. cgps longitude (degree) latitude (degree) receiver type antenna type monument type original sampling rate zen3 121.6067 24.1025 trimble netr8 trm55971.00 roof 20-hz fosa 121.4917 23.8463 trimble netr8 trm55971.00 roof 20-hz sifu 121.4297 23.6362 trimble netr8 trm55971.00 roof 20-hz wujn 121.3229 23.2788 trimble netr8 trm55971.00 roof 20-hz huap 121.7463 24.3106 trimble netr8 trm55971.00 roof 50-hz chnt 121.6573 24.1482 trimble netr8 trm55971.00 roof 50-hz hnsn 121.3079 24.3398 trimble netr9 trm57971.00 ground 50-hz huan 121.2687 24.1423 trimble netr9 trm57971.00 ground 50-hz tunm 121.4922 23.9647 trimble netr9 trm57971.00 ground 50-hz yenl 121.5972 23.9024 trimble netr9 trm57971.00 ground 50-hz slin 121.4381 23.8133 trimble netr9 trm57971.00 ground 50-hz shul 121.5608 23.7884 trimble netr9 trm57971.00 ground 50-hz jsui 121.4206 23.4934 trimble netr9 trm57971.00 ground 50-hz chun 121.3892 23.4516 trimble netr9 trm57971.00 ground 50-hz dchu 121.2773 23.2144 trimble netr9 trm57971.00 ground 50-hz yul1 121.3002 23.3213 trimble netr9 trm57971.00 ground 50-hz table 1. information of 50-hz and 20-hz cgps stations used. hung et al. 4 as regard the strong motion data (smt), the 200-hz sampling rate observations were transformed into displacements by double integration using software tspp established by usgs [boore 2010]. baseline corrections were applied to smt using a zeroth-order-correction by removing the mean of 8-seconds segment before the earthquake origin time. instrument responses were removed by deconvolution of the low-cut (0.02 hz) and high-cut (100 hz) filters in the acceleration space. for the broadband observations, the displacements were generated by single integration after removing the instrument response given the poles and zeros of the instrument [hung and rau 2013]. therefore, a comparison among the different gnss software, the strong motion and broadband integrated solutions was possible at the level of epoch-by-epoch displacements. 3. results and discussion several aspects were investigated considering the results obtained using different software, with different observations sampling rate and at different cgpss. hereafter the main results are presented and discussed. 3.1 seismic waveforms vs gps observations with different sampling rate to investigate the reconstruction of the seismic waveforms from gps at different sampling rate, we firstly considered the effectiveness of the waveforms sampling interval at 0.2-, 1-, 5-, 10-, and 20-hz. figure 3a shows that the clear seismic shaking occurs after 5 seconds with respect to the earthquake origin time for all the ground motion time series. the sampling rate at 0.2-hz and 1-hz truncate the seismic signals and cannot retrieve the complete waveforms; also 1-hz sampling rate apparently is not high enough to retrieve the complete patterns of the ground vibrations. as a matter of fact, spectral analysis show that the major energy in this earthquake is located at 0-1.5hz interval (figure 3b). this is also why waveforms retrieved from 10-hz and 20-hz sampling rates (figure 3a) indicate similar ground motions, therefore in the following we refer to the 10-hz gps solutions only instead of the original sampling rate. notice that the frequency spectra of sampling rate of 5 hz, 10 hz and 20 hz become flat at the frequency lower than about 2.5 hz. figure 2. seismic waveforms generated from broadband seismometer station hgsd, yulb, and nacb in all the three components in ruisui earthquake. waveforms are clipped for stations hgsd (all the three components) and yulb (east and north components). software strategy orbits clocks observation interval track differential precise (igs) 2 h gipsy ppp precise ( jpl) jpl 30 sec 2 h rtklib-ppp ppp precise (igs) code 5 sec 2 h vadase-l3 variometric broadcast broadcast 2 min table 2. main feature of different adopted gps processing strategies. gps seismology for a moderate magnitude earthquake 5 3.2 software comparison the seismic displacements derived from 10-hz observations at cgps sifu for software track, gipsy-oasis ii, rtklib and vadase are displayed in figure 4, as an example. the coordinate repeatability of these 10-hz solutions for the 20 seconds interval before the earthquake was estimated for the four software (track, gipsy-oasis ii, rtklib and vadase (table 3)). it is clear that similar waveforms were obtained with the different software at cgps sifu in all the three components during 5-30 seconds after the earthquake origin time (figure 4). vadase software, whose solutions were obtained using broadcast ephemeris and clocks only, was able to reconstruct waveform features similar to those of other software using precise orbits and clocks information. albeit with its 2 times larger uncertainties (table 3), solutions with vadase software confirmed the approach reliability. spectral analysis also indicates similar results in the frequency domain among the solutions obtained with the four different software (figure 5). similar results confirming the good agreement among the software were obtained for other cgpss. it has to be recalled that here all the solutions were obtained in post-processing mode, but all the approaches are able to supply real-time solutions, albeit with different technological figure 3. (a) displacements time series at cgps sifu in sampling interval from 0.2 to 20 hz in the east component using software vadase (t=0 sec is the earthquake origin time). (b) power spectral density (psd) of the displacements time series at the station sifu with the 20-hz (brown), 10-hz (green), 5-hz (purple) and 1-hz (blue) sampling interval in the east component; the major energy content of the seismic waveforms is located at 0-1.5 hz. table 3. coordinate repeatability of resampled 10-hz gps using various software (computed from 20 seconds before and to the origin time). software σn (mm) σe (mm) σu (mm) track 5.5 3.1 10.9 rtklib 3.1 8.0 10.9 gipsy 3.0 6.7 13.0 vadase 13.7 11.3 20.7 figure 4. displacements time series at cgps sifu generated from software track (brown), gipsy (green), rtklib in the ppp solution (blue), and vadase (grey) in all the three components. hung et al. 6 requirements (gnss receiver features, types of satellite orbits and clocks) [bock et al. 2000, colosimo et al. 2011, li et al. 2014a,b]. the filtered displacement time series after applying spatial filters (sp) composed by stations mov1 and wust indicate apparent consistency (figure 6). larger motion drifts occur for vadase solutions, which can be reduced after applying sp. the noise reduction effect are similar for the four software, except for the higher drifts for track solutions, which may be resulted from longer baseline estimations (around 200 km). 3.3 gps vs strong motion figure 6 shows that the ground motions have excellent agreement between gps and co-located double-integrated smt data (table 4). correlation coefficients in horizontal components between both measurements are 0.6-0.75 after applying 0.2-hz high-pass f ilter (figure 7). the need to high-pass the smt data is to remove the effects of low frequency bias, introducing trends after integration. spectral analysis shows that both observations have similar patterns in the frequency span of 0.3-2.0 hz (figure 8). the ground motion sensitivity limit is about -20db for gps observations for a frequency higher than about 2.5 hz, and above this frequency the noises of gps are always larger than those of the corresponding smt data (figure 8). this suggests that a minimum sampling rate of 5 hz is required for optimum high-rate gps observations. energ y in smt solutions at the frequency larger than 2.5-hz decreases rapidly and remains at the flat noise level for the gps results. the noise level in the up component is always larger than those in the horizontal component, especially for the station far away from the epicenter, such as station chnt (figures figure 5. power spectral density (psd) of the displacements time series at cgps sifu generated from software track (brown), gipsy (green), rtklib in the ppp solution (blue), and vadase (grey) in all the three components. figure 6. gps displacement time series retrieved after applying spatial filters using four gps software. stations mov1 and wust are used to compose spatial filters. brown, green, blue, and purple lines show the motions of mov1 and wust calculated using vadase, rtklib, track, and gipsy, respectively. time series of station sifu after applying spatial filtering are shown at the bottom. gps smt latitude (degree) longitude (degree) inter-distance (km) chnt hwa025 24.16 121.66 1.6 slin hwa020 23.81 121.44 0.4 shul hwa001 23.79 121.56 0.2 chun hwa037 23.45 121.39 0.4 table 4. global information of the approximately co-located cgps and strong motion stations (average). gps seismology for a moderate magnitude earthquake 7 7 and 8). spectrum coherence analysis between 60 seconds time series of gps and smt station pairs indicate large coherencies of near 1 in the frequency span of 0.3-2 hz, especially for the station pairs slin-hwa020 and shul-hwa001 in the horizontal components (figure 9). 3.4 waves propagation and attenuation displacements for all the high-rate gps of 10-hz and 1-hz sampling interval in the transverse and radial components show clear directivity effects (figure 10). apparent group velocities of about 4.0 km/s are shown in both components for waves propagating northward. the wave amplitudes in the northern direction are apparently larger than those toward south. seismic wave are hardly detected at the epicenter distance larger than 90 k ilometer. displacements of 1-hz gps (green line) improve the spatial coverage of the network for the estimation of the apparent group velocity (figure 10). 3.5 co-seismic displacements co-seismic displacements for each cgps in all the three components were determined by the difference between the coordinate averages of 100150 seconds after the earthquake orig in time and figure 8. power spectral density (psd) of the displacements time series from cgps and co-located strong motion data after double integration and application of 0.2-hz high-pass filter. blue is gps, and red is smt. figure 7. displacements time series in all the three components from resampled 10-hz cgps and co-located strong motion data after double integration and application of 0.2-hz high-pass filter. cc is the correlation coefficient. hung et al. 8 50 seconds before the earthquake. ground motions were estimated from the 1-hz high-rate gps measurements just using software track software, considering the proven good agreement among the four software; after applying the spatial f iltering, around 2-12 centimeters co-seismic displacements at the stations within 5-40 km to the epicenter were detected (figure 11a and figure 11b). the precision of the co-seismic displacements are 3 mm and 5 mm in the horizontal and vertical components, respectively. figure 12a and 12b show that the largest co-seismic displacements are located about 10 km ne of the epicenter. for the four gps stations with noticeable co-seismic displacement (horizontal component > 16 mm), we compare their co-seismic displacements estimated from 1-hz and daily solutions (the coordinate average of 3 days after the earthquake subtract the 3 days average before the earthquake), respectively (figure 12a and 12b). we found that the two stations (dsin and gufu) closer to the epicenter have larger co-seismic displacements estimated from the daily solutions than those derived from 1-hz solutions, while the other two stations (sifu and fenp) showing similar results. this indicates that the nearer stations may have early post-seismic deformation. on the other hand, peak ground displacement (pgd) of the stations show the largest value of about 200 mm for both horizontal and vertical components (figure 12a and 12b), and pgd distributions indicate clear attenuations of the amplitude with respect to the epicentral distances. figure 9. spectrum coherence in three components for the 60 seconds times series of station pairs (figure 7) of 10-hz gps and co-located strong motion data after double integration and 0.2-hz high-pass filter. figure 10. resampled 10-hz displacements (red line) and 1-hz displacements (green line) time series in the transverse and radial components with respect to the epicentral distance (an epicentral distance is positive northward with respect to the epicenter). gps seismology for a moderate magnitude earthquake 9 3.6 ground vs. roof monuments we selected three pairs of the cgps to investigate how the waveforms computation is affected by the different types (ground, roof ) of monuments (table 5, figure 13). ground-type monuments are established by four steel piles on the ground, and the roof-type antenna are mounted on the roof using the single-steel piles (figure 13). results show that the waveforms (1-hz sampling rate) have no significant differences for pair height difference lower than 10 meters. pair dsin-gufu, around 40 figure 11a. time series highlighting the co-seismic displacements of 1-hz cgpss using track software, after applying the spatial filtering in all the three components. hung et al. 10 meter height difference, has around 3 cm motion difference at 7-10 second after the earthquake origin time (figure 14). since the inter-station distance of dsin-gufu is around 3 kilometers, so that the waveform differences can be due also to this different location. 3.7 trapped waves in the longitudinal valley seismic waveforms obtained from cgpss and smts display regional inconsistence in eastern taiwan. seismic waveforms triggered by the ruisui earthquake are significantly different among stations in the central range figure 11b. time series highlighting the co-seismic displacements of 1-hz cgpss using track software, after applying the spatial filtering in all the three components. gps seismology for a moderate magnitude earthquake 11 (cer), longitudinal valley (lv), and coastal range (cor) (figures 15, 16, 17). the later phases, ~15 seconds after the s waves, have clearly larger amplitudes in lv than in cor and cer (where basically cannot be detected), especially in the radial component. this is also confirmed by the spectral analysis of the later phases, highlighting a significantly stronger energy in the frequency range 0.5-1 hz within lv than in the other two regions, with ~1-2.5 cm peak-to-peak amplitudes (figure 18). therefore, the joint analysis of the cgpss and smts derived waveforms suggest that later phases are trapped within the longitudinal valley, being this likely due to the fact that this is a suture zone composed of holocene thick sediment deposits. 4. conclusions in this study, the first twelve 50-hz, four 20-hz and thirteen 1-hz cgps epoch-by-epoch displacements in taiwan are derived for seismological applications with respect to the 31 october 2013, ml 6.4 ruisui earthquake, taiwan. the importance of gnss seismology, better if in real-time, can be well understood considering that, even for a moderate magnitude earthquake like the ruisui one, waveforms generated from the broadband seismometer are clipped for some of the near-field stations within 80 km epicentral distances. these truncated waveforms may cause severe mistakes for the estimation of the focal mechanism and the earthquake magnitude. gps derived waveforms can provide quite useful information to compensate figure 12. (a). horizontal co-seismic displacements derived from the 1-hz and daily cgps measurements. the 1-hz displacements were determined by the difference between the coordinate averages of 100-150 seconds after the earthquake origin time and 50 seconds before the earthquake. the largest horizontal co-seismic displacement was detected at sifu, approximately 6 cm toward se. peak ground displacements (pgd) of the stations are shown in horizontal and vertical components, respectively. (b). vertical co-seismic displacements derived from the 1-hz and daily cgps measurements. see the figure caption of figure 12a. gps ground gps roof latitude (degree) longitude (degree) inter-distance (km) height difference (m) slin waro 23.81 121.44 0.1 7.6 fenp fonb 23.60 121.52 0.2 11.6 dsin gufu 23.62 121.39 2.8 37.9 table 5. pairs of co-located (ground and roof ) cgps stations (average). figure 13. examples of cgps antennas in ground type and roof type monuments. hung et al. 12 the weakness of the seismic measurements close to the epicenter; in particular, observations at sampling rate as high as 50and 20-hz demonstrated their capability in retrieving reliable seismic waveforms for the near-fault observations. we used several types of positioning approaches and related software to evaluate their reliability for seismological applications. at first, results showed that the displacements estimated by ppp have a similar precision as those derived from the dp. furthermore, the basic feature of ppp is the displacements/waveforms estimation without reference station(s); that can avoid spurious effects due to the motion of the reference station(s) required in dd approach, in case these stations are affected by the earthquake too. on the other hand, real-time ppp solution is achievable using dual-frequency (geodetic class) receivers only. secondly, seismic displacements estimated using vadase supplied similar waveforms in comparison with results derived using other software implementing dp and ppp approaches. the three approach solutions are consistent at the level of 1-2 centimeters in the frequency range 0.08-3 hz. moreover, vadase approach supplies a more eff icient data processing with respect to the dd and ppp, since no convergence time for initial phase ambiguities is required, so that the observations just collected during the earthquake are suff icient to provide the solution (in our case, 2 minutes instead of 2 hours observations were used). in addition, since vadase is suitable to process single frequency observations, it actually represents a good alternative solution for the low-cost and real-time gps seismolog y. on the other hand, vadase solutions can suffer for unmodeled effects common to close cgps, which have to be removed by a spatial f ilter; anyway, this requirement is not a severe drawback and can be acknowledged if (as standard) all the solutions of a cgpss network are managed within a centralized data center. the high-rate gps waveforms were compared with those derived through double integration from approximately co-located smt, within an inter-distance within 2 k ilometers. a good agreement, with correlation coeff icients 0.7 in the frequency span of 0.3-2 hz, conf irmed that 50and 20-hz gps can be an alternative with respect to accelerometers in the moderate or large earthquake. however, we found that, as expected, the accuracy of the gps waveforms relies on the type of the monument (ground or roof ); anyway, if the height above the ground is small (let say, within 10 meters), the effect is still limited, within 1-2 centimeters. this fact must be accounted for since in the urban areas of taiwan, most of the cgps stations are established on the roof of the buildings due to the poor gps satellite geometry on the ground. figure 14. comparison between approximately co-located groundtype and roof-type 1-hz cgpss (detailed information about these pairs are shown in table 5 and figure 13). figure 15. location map of the 50-, 20-hz gps, 1-hz gps, and strong motion measurements at the central range (cer), longitudinal valley (lv), and coastal range (cos). gps seismology for a moderate magnitude earthquake 13 results of spectra analysis between co-located gps and strong motion data indicate that the ground motion sensitivity limit for gps is about -20db for a frequency above 2.5 hz, and thus we suggest that the optimal sampling rate for high-rate gps is 5 hz for seismolog y research, which is also shown by galetzka et al. [2015]. the performed analyses proved that waveforms derived from 50-, 20-, and 1-hz cgpss and strong motions measurements are substantially coherent at very few centimeters level. their combination was proven to be effective: when jointly analyzed for seismological interpretation, the waveforms generated from both high-rate gps and smt could give information about figure 16. seismic waves in radial component generated from resampled 10-hz cgps (brown), 1-hz cgps (green) and strong motion observations (blue) at zone cer, lv, and cos (an epicentral distance is positive northward with respect to the epicenter). figure 17. seismic waves in transverse component generated from resampled 10-hz cgps (brown) and strong motion observations (blue) at zone cer, lv, and cos (an epicentral distance is positive northward with respect to the epicenter). hung et al. 14 significant differences among various tectonic settings in eastern taiwan (central range, longitudinal valley, and coastal range), where the derived waveforms suggest that later phases are trapped within the longitudinal valley. so, dense seismic and gps network can contribute to more precise results for the interpretation of changing of the waveform features with different tectonic regime. acknowledgements. we thank ministry of the interior, institute of earth sciences of academia sinica, central weather bureau, and central geological survey of taiwan for the gps data. we are grateful to diego melgar and one anonymous reviewer for critical comments. figures were generated using the generic mapping tools (gmt), developed by wessel and smith [1991]. this research was partially supported by taiwan most grant 105-2116-m-006-003. references avallone, a., m. marzario, a. cirella, a. piatanesi, a. rovelli, c. di alessandro, e. d’anastasio, n. d’agostino, r. giuliani, and m. mattone (2011), very high rate (10 hz) gps seismology for moderate-magnitude earthquakes: the case of the mw 6.3 l’aquila (central italy) event, j geophys res, 116, b02305, doi:10.1029/2010jb007834. benedetti, e., m. branzanti, l. biagi, g. colosimo, a. mazzoni, and m. crespi (2014), global navigation satellite systems seismology for the 2012 mw 6.1 emilia earthquake: exploiting the vadase algorithm, seismol res lett, 85(3), 649-656. bilich, a., j. f. cassidy, and k. m. larson (2008), gps seismology: application to the 2002 mw 7.9 denali fault earthquake, b seismol soc am, 98(2), 593-606. blewitt, g., w. c. hammond, c. kreemer, h.-p. plag, s. stein, and e. okal (2009), gps for real-time earthquake source determination and tsunami warning systems, j geodesy, 83(3-4), 335-343. bock, y., r. m. nikolaidis, and p. de jonge (2000), instantaneous geodetic positioning at medium distances with the global positioning system, j. geophys. res., 105(b12), 28,223–28,253, doi:10.1029/2000jb900268. bock, y., l. prawirodirdjo, and t. i. melbourne (2004), detection of arbitrarily large dynamic ground motions with a dense high-rate gps network, geophys res lett, 31(6). bock, y., d. melgar, and b. w. crowell (2011), real-time strong-motion broadband displacements from collocated gps and accelerometers, b seismol soc am, 101(6), 2904-2925. boore, d. m. 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(2011), rtklib: an open source program package for gnss positioning. wdowinski, s., y. bock, j. zhang, p. fang, and j. genrich (1997), southern california permanent gps geodetic array: spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 landers earthquake, j. geophys. res., 102(b8), 18057-18070. wen, y.-y., y.-t. yen, s. wen, s.-j. lee, c.-h. kuo, and y.-y. lin (2016), hybrid ground motion simulation for the 2013 ml 6.4 ruisui, taiwan earthquake, terr. atmos. ocean. sci., 27, 3, 407-414. wessel, p., and w. smith (1991), free software helps map and display data, eos trans. agu, 72 (441), 445-446. yeh, t., b. chao, c. chen, c. chen, and z. lee (2012), performance improvement of network based rtk gps positioning in taiwan, surv rev, 44(324), 3-8. yin, h., and s. wdowinski (2013), improved detection of earthquake-induced ground motion with spatial filter: case study of the 2012 m= 7.6 costa rica earthquake, gps solutions, 1-8. zheng, y., j. li, z. xie, and m. h. ritzwoller (2012), 5hz gps seismology of the el mayor—cucapah earthquake: estimating the earthquake focal mechanism, geophys j int, 190(3), 1723-1732. zumberge, j. f., m. b. heflin, d. c. jefferson, m. m. watkins, and f. h. webb (1997), precise point positioning for the efficient and robust analysis of gps data from large networks, journal of geophysical research: solid earth, 102(b3), 5005-5017. *corresponding author: ruey-juin rau department of earth sciences, national cheng kung university, tainan, taiwan; email: raurj@mail.ncku.edu.tw 2017 by istituto nazionale di geofisica e vulcanologia. all rights reserved layout 6 special issue: from the sun to the earth’s interior preface the international association of geomagnetism and aeronomy (iaga) is one of the eight associations of the international union of geodesy and geophysics (iugg). research activities in the related fields are developed in italy at several universities and major national research organizations, such as istituto nazionale di geofisica e vulcanologia (ingv), istituto nazionale di astrofisica (inaf), consiglio nazionale delle ricerche (cnr). the iaga italian committee (http://www.iagaitalia.it/it/organi.asp), in cooperation with ingv, cnr and area di ricerca in astrogeofisica, organized the national conference "from the sun to the earth’s interior" (21−22 february, 2018) with the specific intent to stimulate the participation of young researchers, their mutual knowledge, the possibility to meet and discuss with more experienced researchers. to this end, the committee has also provided a prize for the best scientific contributions presented by young researchers. the conference was attended by about eighty researchers. some of the papers presented during the conference are collected in this special issue. they deal with a variety of subjects. some stud− ies have analysed magnetic field measurements with different time resolutions, from milliseconds to hours, to locate the position of polar cusp [marzocchetti et al., 2019], to reconstruct the plasmaspheric cold plasma density distribution [del corpo et al., 2019], to an− alyze the effects on the ground generated by the geomagnetically induced currents during the occurrence of geomagnetic storms [tozzi et al., 2019], to study the scaling properties of the geomagnetic field’s spatial fluctuations at both high latitudes (in the northern and southern hemisphere) and low and mid latitudes [de michelis et al., 2019], but also to investigate, along with electric field measure− ments, the anomalous electromagnetic signals generated in response to the propagation of a mechanical perturbation within the subsoil [romano et al., 2019]. magnetic measurements are also used by ghezzi et al. [2019] to perform an analysis of the errors that affect mag− netic anomaly data as regards the archaeological geophysics. other studies are instead based on ionospheric measurements to investi− gate how the f2−layer critical frequency varied over rome during the last three solar minima [ippolito, 2019], to study both the dynamical properties of the electron density and the scaling properties of the electron density fluctuations at high latitudes in response to changes in the geomagnetic activity [giannattasio et al., 2019], and to carry out an interhemispheric comparison between high−latitude ionos− pheric observations in response to peculiar solar wind conditions [d’angelo et al., 2019]. floating potential time series measured by the demeter satellite are instead used by materassi et al. [2019] to characterise plasma irregularities in the equatorward boundary of the auroral oval, relying on the recently proposed alif method. last but not least, a statistical approach for the computation of kinematic propagation of interplanetary coronal mass ejections has been developed and presented in order to forecast in real−time their propaga− tion and arrival at earth [del moro et al., 2019]. the hospitality of the ingv, the support of the area di ricerca in astrogeofisica, and the endorsement of the cnr are gratefully acknowledged. thanks are also due to the editorial board of annals of geophysics for the publication of this volume dedicated to the conference. annals of geophysics, 62, 4, gm457, 2019; 10.4401/ag-8166 references d’angelo g., m. piersanti, l. alfonsi, l. spogli, i. coco, g. li, n. baiqi (2019). the response of high latitude ionosphere to the 2015 june 22 storm. annals of geophysics, this issue, doi: 10.4401/ag-7780. de michelis p., g. consolini, r. tozzi, f. giannattasio, v. quattrociocchi, i. coco (2019). features of magnetic field fluctuations in the ionosphere at swarm altitude. annals of geophysics, this issue, doi: 10.4401/ag-7789. del corpo a., m. vellante, b. heilig, e. pietropaolo, j. reda, j. lichtenberger (2019). observing the cold plasma in the earth's magnetosphere with the emma network. annals of geophysics, this issue, doi: 10.4401/ag-7751. del moro d., g. napoletano, r. forte, l. giovannelli, e. pietropaolo, f. berrilli (2019). forecasting the 2018 february 12th cme propagation with the p-dbm model: a fast warning procedure. annals of geophysics, this issue, doi: 10.4401/ag-7750. ghezzi a., a. schettino, l. tassi, p. p. pierantoni (2019). magnetic modelling and error assessment in archaeological geophysics: the case study of urbs salvia, central italy. annals of geophysics, this issue, doi: 10.4401/ag-7799. giannattasio f., p. de michelis, g. consolini, v. quattrociocchi, i. coco, r. tozzi (2019). characterising the electron density fluctuations in the high-latitude ionosphere at swarm altitude in response to the geomagnetic activity. annals of geophysics, this issue, doi: 10.4401/ag7790. ippolito a. (2019). fof2 variations measured by the rome observatory during solar minimum in the last three solar cycles. annals of geophysics, this issue, doi: 10.4401/ag-7762. marzocchetti m., s. lepidi, p. francia, l. cafarella, d. di mauro (2019). the longitudinal polar cusp displacement from geomagnetic measurements in antarctica. annals of geophysics, this issue, doi: 10.4401/ag-7779. materassi m., m. piersanti, g. consolini, p. diego, g. d’angelo, i. bertello, a. cicone (2019). stepping into the equatorward boundary of the auroral oval: preliminary results of multi scale statistical analysis. annals of geophysics, this issue, doi: 10.4401/ag-7801. romano g., m. balasco, a. siniscalchi, a. e. pastoressa, v. lapenna (2019). robust analysis for the characterization of the seismoelectromagnetism signals observed in southern italy. annals of geophysics, this issue, doi: 10.4401/ag-7811. tozzi r., i. coco, p. de michelis, f. giannattasio (2019). latitudinal dependence of geomagnetically induced currents during geomagnetic storms. annals of geophysics, this issue, doi: 10.4401/ag-7788. paola de michelis giuseppe consolini ingv | istituto nazionale di geofisica e vulcanologia, inaf | istituto nazionale di astrofisica e planetologia spaziali, rome, italy rome, italy michael pezzopane umberto villante ingv | istituto nazionale di geofisica e vulcanologia, università degli studi dell'aquila, l'aquila, italy rome, italy << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 2 dm211, 2022; doi:10.4401/ag-8758 o p e n acc e s s national epos initiatives and participation to the epos integration plan kuvvet atakan*,1, massimo cocco2, beata orlecka-sikora3, ronald pijnenburg4, jan michalek1, christian rønnevik1, dorota olszewska3, beata górka-kostrubiec3, martyn r. drury4 (1) department of earth science, university of bergen, allégt.41, 5007 bergen, norway www.uib.no/geo; e-mail: kuvvet.atakan@uib.no (2) istituto nazionale di geofisica e vulcanologia, roma, italy www.ingv.it; e-mail: massimo.cocco@ingv.it (3) institute of geophysics, polish academy of sciences, warsaw, poland www.igf.edu.pl; e-mail: orlecka@igf.edu.pl (4) department of earth sciences, utrecht university, netherlands www.uu.nl/en/research/department-of-earth-sciences; e-mail: r.p.j.pijnenburg@uu.nl article history: received november 17, 2021; accepted march 1, 2022 abstract european plate observing system (epos) is designed on a three-level architecture. the national research infrastructures (nris) constitute the backbone of the epos delivery framework, where data are generated, processed, analyzed and archived. these data are then integrated by thematic core services (tcs) and distributed through the centralized integrated core services (ics). in this architecture, data provision from the nris is an essential element for the sustainable operation of the epos research infrastructure (ri). national epos initiatives in various countries in europe are developed thanks to the increased awareness of the importance of fair (findable, accessible, interoperable and reusable) data management in science. as such, out of the 14 countries (13 members and one observer) that constitute the epos european research infrastructure consortium (epos-eric), 11 have dedicated epos consortia established and included in the national roadmaps for research infrastructures. moreover, there are in total 24 countries involved in the epos delivery framework where 10 are not yet members of epos-eric. however, the diversity of regulations and procedures adopted in different countries, hampers the development of dedicated epos consortia contributing to sustainability. in this paper, the national epos initiatives are discussed in order to emphasize synergies achieved and the shared efforts to build the epos ri during its life-cycle (the design, preparation, implementation, and pilot operational phases), tackling the challenge of sustainable operation. keywords: european plate observing system (epos); research infrastructure (ri); earth science; solid earth science http://www.uib.no/geo mailto:kuvvet.atakan@uib.no http://www.ingv.it mailto:massimo.cocco@ingv.it http://www.igf.edu.pl mailto:orlecka@igf.edu.pl http://www.uu.nl/en/research/department-of-earth-sciences mailto:r.p.j.pijnenburg@uu.nl kuvvet atakan et al. 2 1. introduction the architecture of the european plate observing system (epos) relies on a federated approach to govern data integration. since its conception phase, epos is tackling the challenge of making openly accessible the enormous wealth of available solid earth science data generated and collected by national and international research infrastructures. epos is established as one of the european research infrastructure consortia (eric) in 2018 with its headquarters in rome, italy [epos-eric, 2021]. currently, 14 countries are involved in epos-eric (13 of which are formal members, and one is an observer). furthermore, in total 24 countries are involved in epos, most of which are committed to provide data, data products, software, and services (ddss) to be integrated by 10 thematic subdisciplines of solid earth science. epos entered the esfri roadmap in 2010 after a careful evaluation undertaken by international experts. at that time, 13 countries were participating the epos ri, with a further five more countries that joined during the european strategic forum for research infrastructures (esfri) evaluation and negotiation process (greece, turkey, romania, norway, poland). the european commission funded the epos preparatory phase project [epos pp, 2010-2014] aimed at the design of the epos architecture and governance model. epos pp was coordinated by ingv (italy), and it included 20 partners from 18 countries and 2 international research organizations. it was decided in epos pp to include only one research organization for each participating country and to start the further engagement of other national research organizations. the epos implementation phase was then started in 2015 when the european commission funded the epos ip project that lasted four years (2015-2019) and involved 47 partners and four international organizations. during the epos-ip project the thematic core services (tcs) were created, and the implementation of the epos functional architecture was achieved. epos represents a collaborative framework aimed at joining efforts and resources, sharing existing experiences and skills of domain scientists and e-scientists to develop a federated approach for integrating research data and scientific products ensuring data management and interoperability through e-science innovation [see bailo et al., 2022, this volume]. this federated approach relies on the construction of the thematic core services (tcs), where data and metadata are further quality-controlled and standardized, as well as of the integrated core services (ics) where data, metadata and services are integrated to make them usable fostering crossand multi-disciplinary science. the epos delivery framework is composed of the data integration system represented by the tcs and the ics and it is governed by epos eric [see cocco et al., 2022, this volume]. the epos architecture is founded on the data provided by national research infrastructures (nris) integrated into a centralized system “integrated core services (ics)” organized through 10 thematic communities and their services (thematic core services – tcs). this architecture (figure 1) relies not only on the services provided by the thematic communities but also to a large extent to the underlying data provision by the national research infrastructures. this provision is ensured by the engagement of national research organizations belonging to the 24 countries involved in epos. nearly 140 research organizations are engaged in the epos ri. among them, 65 research organizations and 4 international organizations are formally engaged in the tcs consortia, belonging to 21 countries out of the 24 involved in epos. the data and service provision are currently ensured by 35 service providers, which are making available nearly 198 validated data, data-products, software and services (ddss elements) provided through 239 services. the long-term sustainability of the entire epos delivery framework is therefore dependent on the entire supply chain of data provision starting from the national level (nris) to the service provision by the thematic communities (tcs) and finally to the fully integrated data, data products, software, and services (ddss) at the centralized european level (ics). the sustainable operation of the epos ri requires that the data and the services are intact, reliable, and robust at any given time [see saleh-contell et al., 2022, this volume]. in this study, we look deeper into the data supply chain and focus on the integration of data provision from the nris or other national level initiatives, which constitute the foundation of the epos architecture. a summary of status for the various national epos initiatives is given together with a few selected examples that are described in more detail to highlight both the challenges that are faced, as well as the success stories, where national initiatives are coordinated in a well-organized structure combining a variety of observational data from continuously monitoring, highly distributed, permanent sensor networks, data collected through temporary installations and campaign based measurements, as well as advanced data products stored, archived and curated at various national institutions. national epos initiatives 3 2. overview of national epos initiatives the status of epos related national initiatives was investigated during the epos implementation phase (eposip project) (see table a1 in appendix). according to this analysis, 24 countries were monitored for the status of national initiatives. among them, four countries had established epos consortia and successfully awarded support from national funding agencies through dedicated contracts. these countries are finland, italy, norway and poland. in 13 countries epos consortia (or joint research units) were established, however no dedicated contracts were yet signed with the national funding agencies. in addition, seven countries had no epos related consortia established and there was no support from the national funding agencies. in 2021, within the framework of the epos sustainability phase (epos-sp) project, updated information was collected both from the epos eric member countries, as well as other countries that are involved in the data provision to the epos delivery framework but not yet included as members of epos-eric. a summary of this is given in table 1. in total, 24 countries were included, among which 14 (13 full members and 1 observer) were members of epos-eric. out of the 14 member countries, 11 have epos included in their national roadmap for research infrastructures. for the remaining 10 countries that are not yet members in epos-eric, only one country (finland) has epos included in its national roadmap for infrastructures. in another case (germany), epos is only mentioned in a note to the national roadmap for research infrastructures. since 2017 there is a significant improvement in the success of the national epos initiatives, with an increase from four in 2017 to 11 in 2021. obviously, formal establishment of epos-eric may have played a role here. in the following, four example cases from italy, norway, the netherlands, and poland are presented in more detail. figure 1. epos architecture showing the three layers of the data supply chain, starting at the bottom (left) national research infrastructures (nris), in the middle thematic core services (tcs) and integrated core services (ics) at the top (right). kuvvet atakan et al. 4 country involvement status involvement details epos presence in national roadmaps belgium epos eric member 1) epos is included in the national ri roadmap; 2) there is national funding foreseen to modernize existing services to be integrated in epos, but the services themselves are not included in a national priority list. na belgian roadmap under preparation denmark epos eric member 1) epos is included in the national ri roadmap. yes danish roadmap for research infrastructures 2015 france epos eric member 1) epos is included in the national ri roadmap. there is an ongoing update to the 2016 version; 2) there is a cluster of french contributions to esfri in earth sciences and environment (data terra); 3) ongoing actions to secure funding of epos services are: funding proposals, requests for more permanent it staff. yes french national strategy on research infrastructure 2016 edition greece epos eric member 1) epos is included in the national ri roadmap; 2) ongoing actions to secure funding of epos services are: requests for national funds. na roadmap updated in 2014 iceland epos eric member 1) epos is included in the national ri roadmap (epos ísland was accepted as one of 6 projects on the iceland ri roadmap in january 2021; 2) epos services are prioritised through of the inclusion on the ri roadmap; 3) ongoing actions to secure funding of epos services are: a proposal for funding the first year of build-up of e-infrastructures (submitted april 2021 to national ri roadmap funds). na no roadmap available italy epos eric member 1) epos is included in the national ri roadmap; 2) epos italia is organised in a joint research unit (2017); 3) ongoing actions to secure funding of epos services are: community proposals to eu programs; alignment of epos-eric and national objectives. yes programma nazionale per le infrastrutture di ricerca (pnir). roadmap updated in 2017 the netherlands epos eric member 1) epos is included in the national ri roadmap; 2) epos-nl in place and recognised in the ri roadmap; 3) identification of more interested parties to increase contributions in kind, participation in new proposals. yes national roadmap for large-scale scientific infrastructure. roadmap updated in 2016 norway epos eric member 1) epos is included in the national ri roadmap; 2) epos-norway (epos-n) consortium is established in 2009; 3) epos-n ri is now in its operational phase providing services, and an application for an expansion in phase-ii is under evaluation. yes norwegian roadmap for research infrastructure. roadmap updated in 2016 national epos initiatives 5 country involvement status involvement details epos presence in national roadmaps poland epos eric member 1) epos is included in the national ri roadmap; 2) services are linked to research priorities in the country; 3) ongoing actions to secure funding of epos services are: request of increased support at ministerial level. yes polish roadmap for research infrastructures. roadmap updated in2020 portugal epos eric member 1) epos is included in the national ri roadmap; 2) c4g (https://www.c4g-pt.eu/) is the portuguese node of epos; 3) ongoing actions to secure funding of epos services are: outcomes of request of renewal of c4g project. yes portuguese roadmap of research infrastructures. roadmap updated in2020 romania epos eric member 1) epos is included in the national ri roadmap. yes romanian roadmap of research infrastructures (2017). roadmap update is in preparation slovenia epos eric member no reply to wp8 survey (not sure who was invited, if any) na slovenian roadmap of research infrastructures. roadmap updated in 2016 united kingdom epos eric member 1) no information on existence/presence of national ri roadmap. na ukri/bgs is the representing entity for epos in uk and contributes as one of the hosting organizations of epos ics. switzerland epos eric observer 1) epos is included in the national ri roadmap 2019. yes swiss roadmap for research infrastructures 2021-2024 bulgaria nacb member na yes bulgaria national roadmap for research infrastructure 2017-2023 finland nacb member 1) epos is included in the national ri roadmap. yes finland’s strategy and roadmap for research infrastructures 2014-2020 germany nacb member 1) epos is present in a note of the national roadmap for ris. na report of the sciencebased evaluation of large research infrastructure projects for a national roadmap, updated in 2019 https://www.c4g-pt.eu/ kuvvet atakan et al. 6 country involvement status involvement details epos presence in national roadmaps ireland nacb member na na national roadmap for research infrastructures 2007 slovak republic nacb member na na no roadmap available spain nacb member na na national roadmap for research infrastructure 2013 austria nacb observer na na austrian research infrastructure action plan 2014 hungary nacb observer na yes national research infrastructure roadmap 2018 czech republic nacb observer na na roadmap of large research infrastructures of the czech republic for the years 2016-2022 turkey nacb observer na na national roadmap for research infrastructures under preparation table 1. summary of the status on national epos initiatives in 2021. among the 24 countries involved in epos, there are 13 countries that are members of the epos eric and an additional one as observer. more information on status of the national roadmaps for infrastructure can be found at: https://www.esfri.eu/national-roadmaps. epos eric: european plate observing system, european research infrastructure consortium; nacb: national agencies coordination board; na: not available. 3. selected examples of national epos initiatives 3.1 epos-italy the italian participation to epos started in 2007 when the italian ministry decided to submit to the european forum on research infrastructures (esfri) the proposal to include the european plate observing system (epos) in the esfri roadmap. the italian proposal assigned to the istituto nazionale di geofisica e vulcanologia (ingv) the role of coordinating the european initiative, which at that time included 6 other countries (france, united kingdom, switzerland, germany, the netherlands, and denmark). this first step is relevant because it is an action promoted by the italian ministry for university and research (mur) that involved ingv as the coordinating research organization. epos entered in the italian roadmap for research infrastructures in 2011 with ingv as the coordinating institution. the italian solid earth science community was invited to participate to the working groups established https://www.esfri.eu/national-roadmaps national epos initiatives 7 in epos pp to design the thematic services for the different scientific domains included in the designed epos architecture. this national participation was promoted by ingv as epos coordinator, and it essentially concerned national research infrastructures and facilities (observatories and laboratories) to be involved in the data and service provision. the engagement process of italian research organizations in epos followed a bottom-up approach driven by the choice to prioritize the engagement of data providers and it experts, according to the epos strategic plan, and was supported by dedicated communication initiatives. it is important to emphasize that the relationships with the italian ministry and the national scientific community started following the epos preparatory phase. during the preparatory phase ingv and the italian ministry submitted the bid to host the epos eric legal seat in rome, at the ingv headquarter, which was subsequently selected by the epos board of governmental representatives. in parallel with the epos ip project, where six italian research organizations were formally engaged in the project activities, the italian community started to design and build a joint research unit (jru) to foster and facilitate the participation to the epos activities. the conceptual design of the jru was accompanied by dedicated communication activities to share the epos vision and mission, to inform about the epos implementation strategies and achievements, as well as to make the italian solid earth science community aware of the potential benefits of the epos data integration plan. the discussions to establish the jru concerned national research organizations, universities, foundations dealing with seismic risk and e-science, as well as the italian geological survey. since the beginning, epos has been presented to the italian solid earth science community with the ambition to clarify the different ways to participate in the initiative: as contributors by hosting service provides and national hubs, as data providers by sharing data and services to support open science, and as users accessing to the epos multidisciplinary data and services. in 2017 the joint research unit epos-italy was signed for the first time. since then, 10 scientific organizations have joined the jru: 4 research organizations (ingv, centro nazionale delle ricerche – cnr, osservatorio geofisico sperimentale ogs trieste, istituto superiore per la protezione e la ricerca ambientale ispra), 4 universities (university of naples federico ii, university of rome tre, university of genoa and university of trieste), and 2 foundations (eucentre pavia for seismic risk, and cineca bologna for supercomputing and e-science). the jru epos-italy has been acknowledged by the italian ministry in 2018. the scope of the jru epos italy is to coordinate and support the italian participation to the epos delivery framework governed by epos eric. the italian team participates to the tcs activities and the research organizations participating to the jru have signed consortium agreements to be engaged in the tcs boards and scientific committees. the jru epos-italy has developed its own annual activity plan, which is organized into three main objectives: support to the national nodes (service providers) identified by tcs, support the further implementation of data provision and service providers not yet validated by epos eric, support international cooperation and global scientific initiatives. the italian ministry provides funding to both epos eric and the jru epos-italy. the funds for epos eric concern the annual membership fee and the contribution to host the legal seat of epos eric (host premium). according to the selected bid to host the legal seat in rome, ingv is providing further in-kind contributions (and also human resources) to epos eric, as reported in the financial statements. the funds provided to the jru are managed by ingv and redistributed to the other participants to support the activities declared in the annual activity plan. these resources can be declared in epos eric as a further in-kind contribution provided by italy to support the tcs service provision. the research organizations committed to host national nodes (service providers) further provide in-kind resources necessary to operate the involved national research infrastructures and facilities. the in-kind contributions provided by the jru, and the in-kind resources provided by the committed research organizations are mapped in the tcs cost book and contribute to increase the sustainability of the epos delivery framework. thanks to the collaboration with cineca, epos participated in several ec projects dealing with e-science [eudat, 2012-2018; verce, 2011-2015; egi engage, 2015-2018; eosc pilot, 2017-2019; eosc hub, 2018-2021; cheese, 2018-2022; among several others]. the italian participation to the epos integration plan of data and services for solid earth science is effectively harmonized with the epos eric architecture and governance model. several italian research organizations have taken commitments to host service providers and/or coordinate the tcs on behalf of the thematic community, therefore supporting the epos federated approach to data and service integration. the interactions among the policy maker (i.e., the ministry), the representing entity nominated in the epos eric statutes, and the italian team engaged in epos activities are structured and organized in an effective way fostering design and planning. the italian civil protection agency is also informed about epos activities and engaged as a stakeholder for develkuvvet atakan et al. 8 oping scientific products for geo-hazards and risk. this might represent a good practice for harmonizing national priorities and strategies with pan-european initiatives and efforts to operate a distributed research infrastructure. 3.2 epos-norway (epos-n): norwegian node of epos parallel with the epos preparatory phase project, in 2009 seven norwegian institutions, university of bergen (uib), christian michelsen research (cmr), university of oslo (uio), norsar, national mapping authority (nma), geological survey of norway (ngu) and norwegian geotechnical institute (ngi), representing the key research organizations dealing with data provision in earth sciences, jointly established a norwegian national epos consortium (nnec). the main goal of nnec was to build the norwegian node of epos, through a coordinated effort with contributions from the distributed monitoring sensor networks in seismology (norwegian national seismic network and norwegian array stations) operated by uib and norsar, and geodesy (national gnss permanent network) operated by nma, as well as the geological and geophysical databases provided by ngu. in 2016, the six members of the nnec, uib, norce (former cmr), norsar, ngu, nma and uio, have jointly been awarded with a grant from the research council of norway to establish the norwegian node of epos, epos-norway [epos-n, 2021]. during the period 2016-2021, these six institutions have focused on three main activities: (i) improving the monitoring capacity in the arctic through new installations of seismic and geodetic (gnss) stations, (ii) developing a national e-infrastructure integrating all solid earth relevant data, data products, software and services in norway, and (iii) establishing a solid earth science forum to engage the user communities and for getting feedback on the development of an integrated data portal (epos-n portal). since 2018, epos is included in the national roadmap for research infrastructures in norway and through the establishment of the norwegian node of epos, epos-n, norway has entered the epos-eric as one of founding members. during the period 2016-2021, epos-norway has successfully accomplished installation of 23 new stations in the arctic region, in norland area in northern norway and the arctic islands of jan mayen, bear island and svalbard (figure 2). in addition to continuous seismological and gnss data from these stations, ocean bottom seismographs were used in several campaign-based data collections which provided new insights to the critical offshore regions figure 2. seismological and gnss station installations at svalbard as part of epos-norway arctic monitoring ri (photo: felix halpaap). picture in the middle shows the installations at verlegenhuken, northernmost part of spitsbergen. photographs on the left and right give snapshots of the logistic support for transport of equipment to the site provided by a helicopter and the royal norwegian navy vessel – k/v svalbard, respectively. national epos initiatives 9 in the arctic, including the mohns and knipovich ridges and the offshore area south of spitsbergen in storfjorden, where the source region of the continuous earthquake activity which started with the m=6.1, 2008 earthquake was monitored. in a dedicated survey new high-resolution aeromagnetic data were collected mapping the entire knipovich ridge shedding light to the complex mid-oceanic ridge opening history in the northern atlantic ocean. seismological data collected through, both the already existing stations of the norwegian national seismic network (nnsn) and norsar arrays, as well as the newly installed stations within the framework of epos-n, are now integrated into the norwegian eida node, which provide data from the arctic regions to epos. the second major achievement of epos-n is the development of a national data portal in solid earth science in norway, which is now completed, and the epos-n portal provides now open access to more than 300 different data sets that are integrated (table 2a and 2b). epos-n portal [epos-n data portal, 2021] (figure 3) provides open access to a wide-range of data sets from several sub-disciplines of earth sciences and offers advanced search and visualization services to the scientific community. furthermore, the advanced visualization services are now being integrated into a european level service and a prototype of an ics-d (integrated core services – distributed). partner institutions research infrastructures (ris) integrated in epos-norway in phase-i and new ris to be integrated in phase-ii geo-uib norwegian national seismic network (nnsn) geo-uib norwegian national ocean obs pool* to be established by epos-n phase-ii consortium norsar norwegian seismic and infrasound arrays and 3c stations and the infrasound and seismic array in finnmark* to be established by the phase-ii consortium norsar national broad-band seismometer pool norsar national short period seismometer pool* to be established by the phase-ii consortium nma norwegian geodetic (gnss) networks rpl-uit norwegian rock physics laboratories* (rpl-uit) tgo-uit norwegian geomagnetic stations* (tgo-uit) ngu paleomagnetic laboratory* ngu norwegian geological and geophysical databases ngu national transient electromagnetic (tem) measurement capacity* ngu, norce, nma detailed surface deformation (insar) data for norway* ngi tsunami modelling capacity* norce, geo-uib epos-norway portal (national e-infrastructure) providing access to solid earth science data in norway, with advanced data visualization techniques table 2a. research infrastructures (ris) already integrated in epos-norway in phase-i and new ris planned to be integrated in phase-ii (marked with *). geo-uib: department of earth science, university of bergen; norsar: norwegian seismic array; nma: national mapping authority; rpluit: rock physics laboratories, the arctic university of norway, tromsø; tgo-uit; tromsø geophysical observatory, uit; ngu: geological survey of norway; ngi: norwegian geotechnical institute; norce: norwegian research centre. kuvvet atakan et al. 10 table 2b. various integrated data, data products, software and services (ddss), accessible through the epos-norway (epos-n) portal, including also a number of new data planned to be integrated during the phase-ii of epos-norway. a new proposal for a phase-ii of epos-norway was submitted to the research council of norway in 2021, that included enhanced data integration as well as new physical installations (table 2a and 2b). despite the outstanding score (7 out of 7) achieved in the evaluation process, the project was not funded. this illustrates the sustainability challenges as a consequence of changing national priorities. the epos-n phase-ii proposal will be re-submitted in 2023. national epos initiatives 11 3.3 epos netherlands (epos-nl): access to dutch research facilities and data dutch universities and knowledge institutions have a long history of operating advanced laboratory facilities and producing and maintaining unique laboratory data, pertaining to research on georesources and geohazards. however, these facilities and data are often less than optimally accessible for external users. epos-nl (european plate observing systemnetherlands) was formed to overcome this limited accessibility and to further develop the dutch research infrastructure for solid earth sciences [epos-nl, 2021]. epos-nl is a partnership between utrecht university, delft university of technology and the royal netherlands meteorological institute and is financed through the national roadmap program for large-scale research infrastructure of the dutch research council (nwo). epos-nl provides free of charge (trans-)national access (tna) to a unique cluster of large-scale, geophysical facilities at utrecht university (uu) and delft university of technology (tu delft). access can be requested by any earth scientist, whether in the netherlands, or anywhere else in the world, by applying to one of our biannual calls [see epos-nl, 2021]. epos-nl facilities that can be accessed include two facilities. the first one of these is the earth simulation laboratory at uu and the petrophysics laboratory at tu delft, where, research can be performed figure 3. epos-norway (epos-n) portal integrates and provides open access to more than 300 data sets and offers advanced search and visualization capacities. epos-n portal is accessible at https://epos-no.uib.no/eposndata-portal/. https://epos-no.uib.no/eposn-data-portal/ https://epos-no.uib.no/eposn-data-portal/ kuvvet atakan et al. 12 to investigate: (i) the deformation and transport behavior of (fault) rock, through experiments at stresses and temperatures simulating crustal or upper lithosphere conditions at depths <40 km, and (ii) the fundamental processes related to the km-scale, tectonic deformation of the crust and lithosphere of the earth, through state-of-the-art analogue models. the second one is the multi-scale imaging and tomography (mint) facilities, distributed over uu and tu delft. mint allows imaging and microchemistry mapping of earth materials in 2d and 3d, at an unprecedented range in scales, from decimeters down to nanometers (figure 4). submitted applications to get access to these facilities are reviewed by an external committee based on excellence and have a high chance of being awarded. in 2021, during which access was largely provided remotely (send-in-sample analysis) due to covid-19 restrictions, 80% of the proposals received were awarded. epos-nl has committed to providing access in this way up to 2025 but strives actively to provide tna even beyond 2025. epos-nl helps to further develop the above research facilities to which it provides access. moreover, it provides a key contribution to developing the dap geothermal well, which in the coming years will be installed on the delft university of technology campus. this 2.2 km deep well will serve as a heat source for the university and surroundings, and simultaneously as a large-scale research facility to study the effects of geothermal energy production. understanding the response of the subsurface to human activities such as these is key in our search for alternative geo-energy resources. epos-nl further contributes actively to the data infrastructure provided by epos, and to two tcs specifically. the first one is the epos multi-scale labs (msl) community, where earth scientific laboratory data stored in europe are made centrally discoverable. epos-nl contributes data to msl and largely finances the current coordination and it development needed to further improve msl data discoverability, through the msl data catalogue epos tcs-msl, [2021]. this catalogue is currently being developed and is aimed at becoming the top access point for solid earth scientific lab data in europe, in 2023. the second tcs where epos-nl contributes is the epos seismology community. the orfeus data centre [orfeus, 2021] at the royal netherlands meteorological institute (knmi) is a highly mature provider of seismological data in europe, and one of the main pillars on which the epos seismology data services are based. 3.4 epos-poland (epos-pl) in 2012, polish research centers in solid earth sciences and ict: institute of geophysics, polish academy of sciences (igpas), institute of geodesy and cartography (igc), institute of geological sciences, polish academy of sciences (igspas), central mining institute (cmi), wrocław university of environmental and life sciences (wuels), the jarosław dąbrowski military university of technology (mut), the polish geological institute – national research institute (pig-nri, agreement terminated in 2016), academic computer center cyfronet of the agh unifigure 4. 2d and 3d imaging in the epos-nl multi-scale imaging and tomography (mint) facility, distributed across utrecht university and delft university of technology. national epos initiatives 13 versity of science and technology (acc cyfronet agh), and representatives of the private sector dealing with exploitation of geo-resources: polska grupa górnicza sp. z o.o. (pgg, former name kompania węglowa sa – coal company sa, kwsa), signed the letter of intent in 2012, transformed into the consortium agreement in 2015, and formed epos-pl. in 2019 the following three institutions joined epos-pl: space research centre, polish academy of sciences (srcpas), warsaw university of technology, faculty of geodesy and cartography (fgc wut), and the institute of environmental engineering, polish academy of sciences (ieepas). the main goal of epos-pl was to enhance the innovation potential of solid earth sciences in poland and europe based on modern it technologies, develop the advanced integrated research infrastructure of the solid earth sciences, and integrate the research and development community in the field of solid earth sciences. responding to the needs of the polish research community and following a recommendation by esfri concerning national ri roadmaps, the ministry of science and higher education initiated in september 2009 a selection procedure of projects for the polish ri roadmap. during this process, epos was recognized as a very ambitious undertaking of great socio-economic significance and has been included in the polish roadmap for research infrastructure since 2011, with the updates of the document in 2015 and 2020. since 2012, the ministry has taken part in the works being undertaken at the international level. the ministry launched the funding mechanisms for the projects included in the ri roadmap. under the present funding scheme, the ministry supports polish participation in european and international research infrastructures by covering annual membership fee – directly to epos eric and the national contribution – to the polish epos-pl consortium. additional sources of financing include (1) call for proposals dedicated to the research infrastructures included in the polish roadmap from european structural funds, (2) call for proposals for new equipment launched by the ministry every year. apart from integrating research infrastructure available at polish academic and research institutions and large investment projects financed with structural funds, polish participation in the epos project focused on building a european center for studies of triggered and induced seismicity associated with geo-resources exploitation. since 2018 thematic core service anthropogenic hazards (tcsah) [lasocki et al., 2022, this issue] has become operating in epos-eric. consequently, in 2013 igpas, with three other polish partners in the epos wg10 “infrastructures for georesources”: ack cyfronet, cmi and coal company, applied for funding the project “is-epos: digital research space for epos purposes”. the project was accepted for co-financing by the european regional development fund (erdf) as part of the operational program innovative economy, 2013 – 2015, with a budget of 4 m€. within this project, the prototype of the episodes research platform (former name is-epos), integrating research infrastructure in the field of the anthropogenic geo-hazards associated with exploitation of georesources, was built with the e-data node (figure 5), both presently being the primary services of tcs ah. in 2016, polish consortium applied for the project epos-pl: epos – european plate observing system, 2016-2022, with a budget of 15 m€, [epos-pl, 2021]. the main results of this undertaking are the innovative services that contribute to minimizing the environmental impacts and risks of the raw materials extractive industry, which will help secure continuity and sustainability of exploitation and help minimize the undesirable impacts of mining-induced deformation on surface infrastructure. this includes: (1) multi-parameter monitoring polygons for the integrated observation of geodynamic processes in two areas of georesources exploitation in poland – the interdisciplinary muse episodes (multidisciplinary upper silesia episodes) are integrated and accessible through the episodes platform of tcs ah; (2) six research infrastructure centers (ric) – lithospheric ric, analytical laboratories – paleomagnetic and environmental magnetism, gnss data ric, geomagnetic and magnetotelluric observations ric, gravimetric observations ric; (3) ric system software to organize, integrate and manage the data and their metadata; (4) e-research platform of tcs ah [see lasocki et al. 2022, this issue]; (5) on-line visualization in gis3d, with reference maps, numerical models of topology and 3d buildings; (5) swip-5 software for seismogram analyses; (6) service for monitoring tropospheric refractivity and zenith path delays in near real time and in real time; (7) igik–tvgmf: a matlab package for computing and analyzing temporal variations of gravity/mass functionals from grace satellite based global geopotential models [godah, 2019]; (8) system fosrem fibre-optic system for rotational events & phenomena monitoring; (9) sos-stress in-situ strain and stress measurement tool for boreholes using its overcome system, incorporated into geotechnical investigations and tested through sos-stress 16 network in the ruda-bielszowice coal mine during epos-pl project, with the patent application submitted; (10) structural monitoring system – a system for measuring the opening and changes of fractures on the buildings, tested on miroszewski palace in będzin; (11) service for modelling of de-stressing blasting in mines and a laboratory for testing materials at high strain rates. kuvvet atakan et al. 14 in 2019 the epos-pl consortium proposed epos-pl+ project for creating the satellite data ric in poland, the geophysical safety system for mine support pillars and epos-ai platform – a new functionality of e-research platform of tcs ah, utilizing an artificial intelligence service. for these goals, the consortium was awarded by the epos-pl+ project with a budget of 12.5 m€ and the project will finish in 2023. 4. discussion and conclusions epos by its nature is a highly distributed research infrastructure (ri) covering the entire european tectonic plate. data, data products, software and services are collected by nearly 140 research organizations in 24 countries in europe. national research infrastructures constitute therefore the backbone of epos delivery framework and are essential elements for the sustainability of epos. over the last decade the involvement of a large group of organizations during the design, preparation and implementation phases of epos, have helped raising awareness on the importance of the national epos initiatives. a significant increase in the number of established national epos consortia with dedicated funding achieved from national funding agencies, from four in 2017 to 11 in 2021, is a manifestation of this. in this development, the national roadmaps for research infrastructures that are established by the national agencies (various funding agencies, research councils, ministries, etc.), had an important role. out of the 32 countries in the european strategic forum for research infrastructures (esfri) list [esfri, 2021], 24 have established roadmaps for research infrastructures, which are updated in different time frames. the remaining eight countries in the esfri list have not yet established a roadmap for ris, among which three are in the process of developing. even if epos has succeeded in the inclusion in 11 national roadmaps for ris, there is still a significant challenge to be tackled. the process of inclusion of any research infrastructure (as epos) in a national roadmap differs from country to country. in some cases, approval of inclusion in the roadmap is subject to the specific national consortium succeeding in a competitive infrastructure call open for all fields of science for funding. this was the case of norway for example, where the norwegian national epos consortium (nnec) was established in 2009, however, epos was figure 5. an example of various visualizations from an episode from the epos-pl episodes platform. episodes portal is accessible at https://tcs.ah-epos.eu/#episodes:. national epos initiatives 15 not included in the roadmap until 2016. only after succeeding in the competitive call for research infrastructure support from the national research council in 2016, was epos considered eligible to be included in the national roadmap. in other cases, inclusion in the national roadmap is a pre-requisite for funding such as in czech republic and sweden. participation in epos is not only through the establishment of a national epos consortium. in some cases, participation is primarily coordinated by a single organization such as cnrs in france, gfz in germany and ukri-bgs in uk. in italy, on the other hand, a joint research unit (jru) is established for epos, with ingv coordinating the participation of further nine organizations in epos. in some rare cases, participation is granted through ministerial initiatives, such as the case in greece. it is also important to note the various synergy effects in participating in epos. one of the immediate effects is the development of national observational capacity and its geographical coverage. examples of such new investments done within the context of the national consortia for enhanced observational capacity exists (e.g., improved arctic monitoring achieved in epos-n). other synergies are also seen in the interactions between the national initiatives and some of the epos thematic core services (tcs). a striking example here is the developments within the national epos initiative in poland which have had important synergies in developing and building the tcs anthropogenic hazards (tcs-ah) community. other examples of synergies exist as well, such as raised awareness in national consortia for their data provision in various tcs due to the effect of participating in epos. another important synergy was achieved by the engagement of geological surveys in epos. this involvement has significantly contributed to the integration of geological and geophysical datasets and have helped bringing the two communities closer. furthermore, the three geological surveys in france, uk and denmark (brgm, ukri/ bgs and geus, respectively), with a coordinated and joint effort have now the responsibility of hosting the integrated core services central hub (ics-c). brgm and ukri/bgs provide hosting of the ics-c system installed both in france and uk, securing safety and redundancy, and supported by geus providing the monitoring system. a further synergy effect promoted by the development of national initiatives is certainly the community building and the user engagement. out of the nearly 140 research organizations involved in epos, 65 organizations are formally participating to tcs by signing the consortium agreement. the remaining organizations are engaged in national initiatives or are following the construction of the epos ri. they ensure and prove the existence of an involved community capable of promoting an informed use of the infrastructure as well as of strengthening the data and service provision in the near future. in this context, the financial support to national initiatives, provided coherently to the architecture and the epos eric strategic planning, enables epos to tackle the challenge of long-term sustainability including the further development of the epos delivery framework. for this to happen, national initiatives need to be harmonized with the epos architecture and strategic planning including financial planning. this is a key aspect of financial sustainability as the epos financial model is based on the provision of host premium and in-kind contributions from epos eric members [see kauzar-contell et al., 2022 this issue]. the support at the national level makes it possible to increase the provision of in-kind contributions to tcs to be included in the tcs cost book. in addition to the national contributions to epos coordinated through dedicated consortia or through individual institutions and associated research infrastructures, there are also other national initiatives that are developed independently within the same scientific domain. one such example is the dataterra [2021] initiative in france. such initiatives can be found also in few other countries. these developments can on one side be complementary to epos if coordinated well but may also be in competition with epos or replicating epos at a national scale if not coordinated. there is a high potential for positive synergies between these independent national initiatives and the national epos activities. another example of positive synergies comes from regional collaborations including several countries in a specific region. current collaboration established among the nordic countries (finland, norway, denmark, sweden and iceland) through a dedicated project nordic-epos [2021] supported by the nordic ministries is a good example in this regard. after its first year there are already significant synergies appearing in the form of harmonization and standardization of data provision and fair (findable, accessible, interoperable and reusable) data management. further to the fact that nris are the backbone of the epos delivery framework, another important consideration where nris can play a role, is the funding resources for the membership fees for individual countries’ participation in epos-eric. even if individual country’s membership to epos eric is secured by the relevant ministries in each country, membership fees are not necessarily directly provided by the same national authorities. in many cases, payment of the membership fee is delegated to either the national consortia or to individual institutions kuvvet atakan et al. 16 that are mandated to participate in epos as a representative entity on behalf of the relevant ministries. this is unfortunately not a sustainable solution, as many organizations and consortia have already pointed out the difficulties in prioritizing the epos eric membership fee among an ever-increasing number of other research priorities. with reduced budgets experienced in many of these public organizations during the last years, the sustainability of the epos eric membership over time may become challenging. it is therefore extremely important that national funding agencies carefully consider a sustainable solution to this issue, which is common for all eric’s. the evolution of the landscape for national participation in epos and contribution of data, products, and services from nris creates a complexity, which is challenging for the sustainability of the data provision in the wider epos delivery framework. it is obvious that it is not possible to have a unified approach for addressing this issue. more efforts are needed in finding appropriate mechanisms for participation in epos, taking into account the existing landscape with various regulations and procedures in different countries. despite this evolving landscape, there is one common element in participating in epos, which is the engagement of the research communities in each country. without this engagement and dedication of scientists both as individuals and as research groups, it would be difficult to build a complex research infrastructure as epos. the current engagement through 10 existing thematic communities included in epos, and especially the interest shown by new thematic communities in joining epos, are promising signs that this engagement was achieved and is continuing. data sharing and resources. this paper is focused on the processes of data provision from the national research infrastructures and therefore no data is used in this paper. acknowledgements. this work has benefited greatly from the individuals participating in various national epos teams in 24 countries in europe contributing to the development of epos related initiatives and data provision over the years. authors wishes to thank especially the participants of the italian joint research unit, epos-norway, epos-netherlands, and epos-poland consortia. we acknowledge the support from the following epos related projects from eu and various national funding agencies. – epos-pp project no. 262229, funded by the h2020 program. – epos-ip project no. 676564, funded by the h2020 program. – epos-sp project no. 871121, funded by the h2020 program. – epos-n project no. 245763, funded by the research council of norway (rcn). – is-epos digital research space of induced seismicity for epos purposes (poig.02.03.00-14-090/13-00) project framework funded by the polish national center for research and development. – epos-pl (no poir.04.02.00-14-a003/16) co-financed by the european union from the funds of the european regional development fund (erdf). – epos-pl+ (poir.04.02.00-00-c005/19-00) co-financed by the european union from the funds of the european regional development fund (erdf). references bailo, d., k.g. jeffery, k. atakan, l. trani, r. paciello, v. vinciarelli, j. michalek, a. 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(2022). the epos research infrastructure: a federated approach to integrate solid earth science data and services. ann. geophys., this volume. eudat (2012-2018). eudat collaborative data infrastructure – https://eudat.eu/ – last accessed 13 february 2022. egi engage (2015-2018). egi foundation, egi engage project – https://www.egi.eu/tag/egi-engage/ – last accessed 13 february 2022. https://www.data-terra.org/en/homepage-english/ https://cheese-coe.eu/ https://eudat.eu/ https://www.egi.eu/tag/egi-engage/ national epos initiatives 17 eosc hub (2018-2021). european open science cloud hub – https://www.eosc-hub.eu/ – last accessed 13 february 2022. eosc pilot (2017-2019). european open science cloud pilot – https://eoscpilot.eu/ – last accessed 13 february 2022. epos-eric (2021). european plate observing system – european research infrastructure consortium (eposeric) – https://www.epos-eu.org/ – last accessed 16 november 2021. epos-n (2021). european plate observing system – norway (epos-n) – https://epos-no.uib.no – last accessed 16 november 2021. epos-n data portal (2021). european plate observing system – norway (epos-n) data portal – https://epos-no. uib.no/eposn-data-portal/ – last accessed 16 november 2021. epos-nl (2021). european plate observing system – netherlands (epos-nl) – www.epos-nl.nl – last accessed 16 november 2021. epos-pl (2021). european plate observing system – poland (epos-pl) – https://epos-pl.eu/ – last accessed 16 november 2021. epos tcs-msl (2021). european plate observing system thematic core services – multi scale laboratories (epos tcs-msl) – https://epos-msl.uu.nl/ – last accessed 16 november 2021. esfri (2021). european strategic forum for research infrastructures (esfri) – https://www.esfri.eu/nationalroadmaps – last accessed 10 november 2021. godah, w. (2019). igik–tvgmf: a matlab package for computing and analysing temporal variations of gravity/ mass functionals from grace satellite based global geopotential models, computer geosci., 123, 47-58. lasocki, s., and b. orlecka-sikora, et al., (2022). sharing data and services for anthropogenic hazards, ann. geophys., in press. nordic-epos (2021). nordic – european plate observing system (nordic-epos) – https://www2.helsinki.fi/en/ infrastructures/nordic-epos/fair-nordic-epos-data-hub – last accessed 16 november 2021. orfeus (2021). observatories & research facilities for european seismology (orfeus) data centre – https:// www.orfeus-eu.org/ -last accessed 16 november 2021. saleh contell, k., k. karlzén., h. pedersen, m. cocco, k. atakan, d. bailo, o. lange, d. mercurio, g. maracchi, a. sangianantoni, d. piras, m. fredella and c. freda (2022). long-term sustainability of a distributed ri: the epos case. annals of geophysics, this volume. verce (2011-2015). virtual earthquake and seismology research community e-science environment in europe – http://www.verce.eu/ – last accessed 13 february 2022. *co r r e s po n d i n g a u t h o r: kuvvet ata ka n, department of earth science, university of bergen, bergen, norway, e-mail: kuvvet.atakan@uib.no, www.uib.no/geo https://www.eosc-hub.eu/ https://eoscpilot.eu/ https://www.epos-eu.org/ https://epos-no.uib.no https://epos-no.uib.no/eposn-data-portal/ https://epos-no.uib.no/eposn-data-portal/ http://www.epos-nl.nl https://epos-pl.eu/ https://epos-msl.uu.nl/ https://www.esfri.eu/national-roadmaps https://www.esfri.eu/national-roadmaps https://www2.helsinki.fi/en/infrastructures/nordic-epos/fair-nordic-epos-data-hub https://www2.helsinki.fi/en/infrastructures/nordic-epos/fair-nordic-epos-data-hub https://www.orfeus-eu.org/ https://www.orfeus-eu.org/ http://www.verce.eu/ mailto:kuvvet.atakan@uib.no http://www.uib.no/geo volcanic ash and so2 retrievals using synthetic modis tir data: comparison between inversion procedures and sensitivity analysis annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6616 volcanic ash and so2 retrievals using synthetic modis tir data: comparison between inversion procedures and sensitivity analysis stefano corradini1*, sergio pugnaghi2, alessandro piscini1, lorenzo guerrieri2, luca merucci1, matteo picchiani3, marco chini4 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy, 2 università degli studi di modena e reggio emilia, dipartimento di scienze chimiche e geologiche, modena, italy, 3 tor vergata university, rome, italy, 4 luxembourg institute of science and technology, environmental research and innovation department, belvaux, luxembourg *stefano.corradini@ingv.it abstract in this work the volcanic ash and so2 retrievals obtained by applying three different procedures (lut look up table, nn neural network and vpr volcanic plume removal) on modis thermal infrared (tir) synthetic measurements have been compared. the synthetic measurements are generated using modtran radiative transfer model (rtm) for defined volcanic cloud configurations. the results, presented as the per centage difference between the retrieved ash and so2 total masses and the true values used for the synthetic data generation, indicate maximum differences of +/15% and +/10% for all the procedures and for ash and so2 retrievals respectively. a sensitivity analysis has been also realized to investigate the influence of volcanic cloud altitude and water vapour profile on so2 retrievals at 7.3 and 8.6 µm. results confirm the high sensitivity of the 7.3 µm retrieval to the volcanic cloud altitude and show that the so 2 total masses estimated at 7.3 and 8.6 µm separately can be used to improve the information on the plume height. finally, the water vapour profile is used to compute the minimum altitude over which the 7.3 µm retrieval is effective. i. introduction he 2010 eyjafjallajokull eruption made evident the need to obtain more accurate volcanic ash and so2 quantitative estimations in a near real time basis [zenher, 2010]. to answer this pressing requirements the scientific community is forced to develop novel, fast and reliable retrieval approaches. in this paper the volcanic ash and so2 retrievals are realized by using the well established lut (look up table) procedure [corradini et al., 2009], and the novel vpr (volcanic plume removal) [pugnaghi et al., 2013] and nn (neural network) approaches [picchiani et al., 2011; piscini et al., 2014]. the results are compared 1 t annals of geophysics, fast track 2, 2014 and discussed and will give us information on the reliability of the inversion schemes presented. as test cases different modis (moderate resolution imaging spectroradiometer) tir (thermal infrared) synthetic measurements have been generated using modtran radiative transfer model (rtm) considering defined volcanic cloud configurations. a sensitivity analysis has also been realized to investigate the influence of volcanic cloud altitude uncertainty and the effect of the atmospheric water vapour (wv) profile on so2 retrieval. the paper is organized as follows: in sections ii and iii the retrieval procedures and the synthetic modis data generation are described. section iv shows the results of the ash and so2 retrievals comparison and in section v the sensitivity analysis is realized. the conclusions are drafted in section vi. ii. retrievals procedure description the lut, vpr and nn procedures have been designed for the simultaneous volcanic ash and so2 retrievals using multispectral satellite data in the tir spectral range. in this work the different approaches have been applied to modis synthetic measurements. modis, aboard the nasa terra and aqua polar satellites with a repetition cycle of 1 to 2 days, acquire data in 36 bands from vis to tir with a spatial resolution of 1 km2 [http://modis.gsfc.nasa.gov/]. the tir channels 28-29 (centered at 7.3 and 8.6 µm) and 31-32 (centered at 11 and 12 µm) are used for the volcanic so2 and ash retrievals respectively. in the following the retrieval approaches considered are described. lut approach: the ash detection and retrievals (aerosol optical depth – τ, effective radius – re and total mass – mt) are based on the exploitation of the different spectral ash signature around 11 and 12 µm. the so2 retrievals are realized considering the two wide spectral signature around 7.3 and 8.6 µm and using a χ2 procedure. the ash effect on so2 estimations has been taken into account considering the correction procedure developed by corradini et al., [2009]. all the retrievals needed rtm simulations computed for specific atmospheric profiles, volcanic cloud geometry, surface parameters and ash type. the main advantage of the lut approach is the precise characterization of the local atmosphere while the main drawback is the non-real time processing. vpr approach: by linearly interpolating the radiances surrounding a detected volcanic cloud, the vpr procedure computes the radiances that would have been measured by the sensor in the absence of the volcanic cloud itself, and reconstructs a new image without it. the new image and the original data allow the computation of the plume transmittance in the modis tir bands 29 (t29), 31 (t31), and 32 (t32) by applying a simplified model consisting of a plume at a constant altitude, temperature and thickness. the transmittances are then refined with a polynomial relationship obtained by means of modtran simulations adapted for the geographical region, ash type, and atmospheric profiles. τ, re and mt are computed from t31 and t32 while so2 is retrieved from t29 and t31. the main advantages of vpr is that it works in real time and that, once the polynomial coefficients are computed (for a specific volcano, ash type and sensor), the volcanic cloud altitude and temperature are the only input parameters required to run the procedure. the main drawback is that the surface under the volcanic cloud should be homogeneous enough. nn approach: is based on the coefficients computation of the relationship existing between the top of atmosphere (toa) spectral radiance and the volcanic ash and so2 parameters. the nn coefficients estimation is realized in the learning phase by examples 2 annals of geophysics, fast track 2, 2014 composed of training patterns, i.e. modis spectral radiances samples, and the corresponding ash (, re, mt) and so2 values. the topologies for the ash and so2 parameters estimations are established using the modis tir channels 28-31-32 and 29-31-32 as inputs and ten neurons for the single hidden layer of each nn. the main advantage of the nn procedure is the real time processing while the main drawback is that it requires a good training samples statistical distribution. iii. modis synthetic measurements generation the use of synthetic data instead of real measurements is due to the need to test the inversion schemes on known volcanic cloud configurations. the ash and so2 retrieved from the different approaches can be compared directly with the 'true' values used for the modis tir synthetic data generation. these images are produced by using the modtran rtm, for the modis tir channels 28, 29, 31 and 32, and considering specific surface characteristics, atmospheric profiles, plume geometry, ash type and different quantities of ash and so2. the volcanic cloud configurations are representative of mt. etna (sicily-italy) emissions. as atmospheric profiles (pressure, temperature, water vapour – ptwv) the trapani wmo data collected the 23 october at 12:00 utc, the 24 october 2011 at 00:00 utc and the 15 november 2011 at 12:00 utc have been considered. these profiles have been selected because representative of different wv content (31, 23 and 15 mm respectively). the volcanic cloud top altitudes has been set to 5, 6 and 7 km with 1 km of thickness and as ash type, the volz [1973] ash refractive index has been chosen. the volcanic cloud shape is considered ellipsoidal, the ash τ and so2 distributions are gaussian with peaks at 1.25 and 10 g/m2 respectively in different positions with different standard deviations. the effective radii are randomly distributed with values equally spaced in a logarithmic scale from 0.8 to 10 µm. figure 1 shows the ash τ, re and so2 distribution used for the generation of the 9 test cases (3 atmospheric profiles and 3 volcanic cloud altitudes). finally, the view zenith angle (vza) of the synthetic image vary between 22° and 34° (typical for modis images collected on etna volcano). figure 1: volcanic cloud configuration. τ (plate a), re (plate b) and so2 (plate c). for each of the 9 test cases (named in the following 'standard' test cases), a perturbation has been also applied to take into account the modis toa radiance variability of the real images due to change in surface temperature, emissivity and path radiance. for each pixel, the perturbed radiance at the sensor is a random number of normal distribution with mean equal to the radiance without perturbation and standard deviation corresponding to the 0.5% of the same value. these cases will be named in the following 'perturbed' test cases. the lut procedure runs with the same modtran simulations used to generate the synthetic measurements, the vpr is applied by knowing the right volcanic cloud altitude and temperature and the nn is trained by considering the 50% of the volcanic cloud of each im3 annals of geophysics, fast track 2, 2014 age and tested on the remaining 50%. therefore, because no uncertainties can be attributed to the input parameters, the discrepancy between the retrieved ash and so2 masses and the truth, for the standard images, represents the intrinsic error of the linearization process for the different retrieval schemes. the tests realized by using the ‘perturbed’ images will give us the sensitivity of the inversion schemes on the toa radiance variability. iv. results the results are presented as the percentage difference between the total ash and so2 masses retrieved (mretr) and the 'true' values (mtrue) used to generate the modis synthetic images (d%=100*[mretr–mtrue]/mtrue). figure 2: ash mass percentage differences. lut (top left), vpr (top right) and nn (bottom) retrievals. in figure 2 are shown the results for the ash retrievals obtained from the different procedures and for all the 18 test cases. the lut percentage difference (top left plate) is limited to 5% for the standard cases and increases to about 10% for the perturbed cases. the vpr differences (top right plate of figure 2) are all within +/15% and are not affected by the toa radiance perturbation. as vpr also the nn results are not affected by the perturbations and the % differences reach about the +/10%. figure 3 shows the results for the so2 retrievals obtained from all the procedures for all the test cases. as the top plates show the lut retrievals have been realized by considering both the 7.3 and 8.6 µm modis channels separately. figure 3: so2 mass percentage differences. top plates from left to right: lut 8.6 µm and lut 7.3 µm retrievals. bottom plates from left to right: vpr and nn retrievals. for both the lut retrievals the percentage difference is limited to +/5% for all the standard cases. it increases until +/10% for the 8.6 µm considering the perturbed test cases. as for ash the vpr and nn so2 retrievals are not affected by the toa radiance perturbation and the mass differences are all within the +/10%. as figures 2 and 3 show, and as expected, the lower differences are obtained with the lut inversion for the 'standard' cases. the perturbation increases meaningfully the discrepancies. on the contrary the vpr and nn inversions give approximately the same results for both standard and perturbed test cases. these latter are simplified algorithms and consequently they give less accurate results compared to lut in the standard cases. but at the same time, vpr and nn are less sensitive to small radiance variations introduced in the 'perturbed' cases. 4 annals of geophysics, fast track 2, 2014 v. sensitivity analysis for so2 retrievals the results shown in figure 3 for the so2 lut retrievals at 7.3 µm can be misleading. due to the strong water vapour absorption around 7.3 µm, the so2 estimation on this channels is strongly dependent on the wv profile then on volcanic cloud altitude [corradini et al., 2009; 2010]. for all the previous test cases these two parameters were completely known (see section iii). to quantify the so2 retrieval error due to the uncertainty on volcanic cloud altitude the lut procedure has been applied considering a specific synthetic image (generated using the 24 october 2011 pth profiles and a plume altitude of 6 km), and processed using different volcanic cloud altitudes. figure 4 shows the percentage differences between true and retrieved so2 values at 7.3 and 8.6 µm by considering the volcanic cloud at 4, 5, 5.5, 6, 6.5, 7 and 8 km. as can be seen, an uncertainty of 500 m on the plume altitude leads to an error on so2 retrievals of about the 50% and 10% for 7.3 and 8.6 µm retrievals respectively (a similar trend can be also found in campion et al., 2010, see figure 4). the analysis emphasize also that the two retrievals give the same results for the correct volcanic cloud altitude. when the volcanic cloud altitude considered for the retrievals is greater than the true value the 7.3 µm retrievals is always lower than the 8.6 µm retrieval and vice versa. the knowledge of the wv profile can also be used to investigate the minimum volcanic cloud altitude over which the 7.3 so2 retrieval is effective. to do it the noise equivalent spectral radiance – nesr (i.e. the minimum radiance variation detectable by the sensor) for the modis channel 28, is compared with the toa radiance variations induced by different values of so2 abundance for volcanic cloud at different altitudes. figure 4: so2 retrievals at 7.3 and 8.7 m: percentage differences between true and retrieved values for volcanic clouds at different altitudes. these latter values are computed from the difference between the modis channel 28 toa radiance in a free atmosphere (l0) and the toa radiance considering a volcanic cloud at different altitudes with different values of columnar abundance ls(cs). instead of the vertical column density (wv) more significant is the slant column density (wv*) that takes into account the vza i.e. the real path crossed by the atmospheric radiation (wv*=wv/cos(vza)). figure 5 shows the 3d plots of l0-ls(cs) for the atmospheric profiles of 23 (top plates) and 24 (bottom plates) october 2011, volcanic cloud at 4 km and considering mean vza equal to 28° (left plates) and 50° (right plates). the horizontal flat surface is the nesr threshold (0.0173 w/m2/sr/m). as figure shows for higher wv* and lower volcanic cloud altitudes (upper right plate), the most of the so2 is undetectable. for a volcanic cloud at the same altitude and lower wv* value (bottom plates) the so2 is almost all detectable. vi. summary and conclusion in this work the volcanic ash and so2 estimations are realized by using different retrieval 5 annals of geophysics, fast track 2, 2014 approaches applied to modis tir synthetic data. because no uncertainties can be attributed to the input parameters, for the standard images the discrepancy between the retrieved and true values represent the intrinsic error of the linearization process of the different retrieval schemes. figure 5: 3d plots of l0-ls(cs) at 7.3 µm for different atmospheric profiles and vza. the horizontal flat surface is the 7.3 µm nesr. in ordinate and abscissa the pixel number. results indicate that the maximum discrepancy between retrievals and truth are all within +/15% and +/10% for all the procedures and for ash and so2 retrievals respectively. the data perturbation affects significantly only the lut retrievals. the sensitivity analysis shows that the the 7.3 and 8.6 µm so2 retrievals realized separately can improve the information on volcanic cloud altitude. finally, it is shown how to use the water vapour profile to compute the minimum volcanic cloud altitude over which the 7.3 µm retrieval is effective. acknowledgemets the work of s. corradini was founded by the european union's seventh framework programme (fp7/2007-2013) through the project aphorism under grant agreement n. 606738. references [campion et al., 2010] campion, r., salerno, g. et al. (2010). measuring volcanic degassing of so2 in the lower troposphere with aster band ratios, jvgr, 194, 42–54. [corradini et al., 2009] corradini, s., merucci, l., and prata, a. j. (2009). retrieval of so2 from thermal infrared satellite measurements: correction procedures for the effects of volcanic ash, amt, 2, 177–191. [picchiani et al., 2011] picchiani, m., chini, m., et al. (2011). volcanic ash detection and retrievals from modis data by means of neural networks, amt, 4, 2619-2631, 2011. [piscini et al., 2014] piscini, a., picchiani, m., chini, m., et al. (2014) a neural network approach for the simultaneous retrieval of volcanic ash parameters and so2 using modis data, amtd-2014-57. [pugnaghi et al., 2013] pugnaghi, s., guerrieri, l., corradini, s., et al.. (2013). a new simplified procedure for the simultaneous so2 and ash retrieval in a tropospheric volcanic cloud, amt, 6, 1315–1327 [volz, 1973] volz, f. e. (1973). infrared optical constants of ammonium sulfate, sahara dust, volcanic pumice and fly ash, ao, 12, 564–568. [zenher, 2010] zehner, c. (2010). monitoring volcanic ash from space. proceedings of the esa-eumetsat workshop on the 14 april to 23 may 2010 eruption at the eyjafjoll volcano, south iceland. frascati, italy, 26-27 may 2010. esa-publication stm-280. 6 geo 1279 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 4 space plasma effects on earth-space and satellite-to-satellite communications: working group 4 overview sandro m. radicella (1) and ersin tulunay (2) (3) (1) the abdus salam international centre for theoretical physics (ictp), aeronomy and radiopropagation laboratory, trieste, italy (2) department of electrical and electronics engineering, middle east technical university, balgat, ankara, turkey (3) tubitak marmara research center, information technologies research institute, gebze, kocaeli, turkey this paper summarizes the the activities carried out by working group 4 of cost 271 action. the structure of this working group included four working packages that were dealing with different aspects of the same overall problem related to space plasma variability and irregularities effects on advanced satellite systems. general comments about the most relevant achievements and possible future lines of research are given. 4.1. introduction at present to assess the effect of space plasma variability and irregularities on satellite systems is becoming an important part of planning of advanced applications of such systems. this aim was chosen as the key objective of working group 4 of the cost 271 action. the structure of this working group included four working packages dealing with different aspects of the same overall problem covering ionospheric scintillations observations and modelling, development of algorithms for the forecasting and nowcasting of total electron content (tec), study of ionospheric disturbances generated by natural and anthropogenic processes, and the effects of electron density and tec gradients on earth-space and satellite-to-satellite communications. the details of these investigations and the results achieved can be found in the respective papers that correspond to each of the working packages of wg 4. general comments about the major achievements and possible future lines of research dealing with the same topics are given below. 4.2. effects of space plasma variability and irregularities on earth-space and satellite-to-satellite communication channels the propagation of satellite signals through ionospheric irregularities generates modifications of the signals like intensity and phase fluctuations and rapid variations of angle of arrival. these phenomena are known as ionospheric scintillations and are observed particularly at low and high latitudes. 1280 sandro m. radicella and ersin tulunay the gism model has been further developed (beniguel, 2002). it uses the multiple phase screen technique and gives statistical characteristics of transmitted signals, fade duration and other parameters like signal loss of lock probability. maps of scintillation index s4 and phase standard deviations can be produced. a new hybrid propagation model to describe ionospheric scintillations has been developed that includes generating a 2d random screen by means of the complex phase method (gherm et al., 2002). this can result in a full shaped model able to extract not only scintillation indices, but also additional spectral information and model time series particularly suited for system-oriented problems. both gism and the new model use as background ionosphere the 3d and time dependent nequick model of electron density that is one of the ionospheric models developed in the framework of the previous cost 251 action (radicella and leitinger, 2001). at present, it is widely accepted that at high latitudes phase scintillation activity is high and dominant while intensity scintillation is low and negligible. this scenario is based on several scintillation experiments done by means of gps scintillation monitors. forte and radicella (2002) have shown that data detrending, when the separation of deterministic and stochastic components is done using a fixed cut-off frequency, can lead to a misleading data interpretation. an experiment for measuring these effects on ionospheric scintillation information pointed out from gps signals has been set up at auroral latitudes being run at the sodankyla geophysical observatory (university of oulu). a gps receiver with a sufficiently high sampling rate has been used to measure the carrier phase and signal-to-noise ratios at l1. the preliminary result of the experiment seems to confirm the theoretical mechanism suggested as an explanation for the occurrence at high latitudes of high phase scintillation against low intensity scintillation. with these results new ways of describing scintillation effects at system level can be determined. an effort was also made by forte et al. (2002) to interpret experimental scintillations intensity data in a way useful to determine the impact of these phenomena on satellite navigation systems. attention has also been given to the comparison between experimental data and models predictions. it can be stressed that the models fail to describe the patchy structure of irregularities, a typical feature of ionospheric irregularities. 4.3. development of algorithms and software to treat disturbances in earth-space and satellite-to-satellite communications unpredictable variability of the ionospheric parameters due to disturbances related to the ionosphere-magnetosphere system limits the efficiency of communications affected by the ionosphere by causing serious technological problems including range errors, scintillations of satellite signals and others propagation problems. it is known that in such cases mathematical modeling of the variability based on first physical principles is extremely difficult. therefore data driven models such as neural networks based models can be used to forecast ionospheric parameters like tec. the only requirement for the success of data driven models is the availability of reliable data, which can represent the characteristics of the process to be modeled. the middle east technical university neural networks (metu-nn) technique to forecast 10 min values of the vertical total electron content (tec) values up to 24 h ahead during high solar activity in the current solar cycle has been applied to tec data evaluated from gps measurements from 2000 to 2001 at chilbolton (51.8 n, 1.26 w). an additional validation has been performed using an independent data set. the construction work of the neural network based model is carried out in the development mode. it is composed of «training phase or learning phase» and «test phase» (tulunay et al., 2004a,b). the good results of this investigation indicate the potential of the neural networks techniques to forecast local vertical tec values. the possible extension of the same tech1281 space plasma effects on earth-space and satellite-to-satellite communications: working group 4 overview nique to forecast two dimensional tec maps could provide a very useful tool to forecast the effects of the ionosphere on satellite navigation. since 1995, dlr/ikn has been operating a new system for regularly processing data and producing tec maps over the european region based on gps measurements by the international gps service (igs). the existing large database, containing data from all solar/geomagnetic conditions, is a very useful tool for the validation of all types of ionospheric effects calculations particularly for highly disturbed ionospheric conditions where the use of other measurement techniques (e.g., ionosondes) is limited. furthermore, dlr/ikn has developed the software modules for deriving ionospheric grid errors in the egnos system testbed (estb) in real-time. autoand cross-correlation procedures have recently been developed for predicting both the critical frequency of the f2-region and the tec, strongly relating the short-term forecast to present and future geomagnetic activity. preliminary results of these methods/procedures have already been tested and reported for the one-dimensional case when forecasting is performed at a given location based on gps-tec measurements, solar and geomagnetic activity indices. extending such a prediction to several locations in a given region, instantaneous maps of the forecast can be constructed covering the region of interest. other investigations on tec and critical frequency of f2 variability have been carried out together with the application of clustering techniques to characterize f2 critical frequency behavior. 4.4. application of theoretical considerations to the study of space plasma effects a series of investigations have been carried out to study different aspects of the plasma characteristics in the magnetosphere-ionosphere-thermosphere system. electromagnetic emissions at an ample range of frequencies measured on-board of low orbiting satellites have shown that during strong geomagnetic storms a series of phenomena are observed in same regions of the satellite orbital environment (b ⁄lecki et al., 2003). a study was done on the impact of magnetospheric disturbances on the daily critical frequency of f2 and considerations on the use of both topside and ground vertical soundings to this purpose have been made (rothkaehl et al., 2003). also hf electromagnetic noise in the ionosphere has been investigated using data from the coronas-i satellite. the results show that the electromagnetic noise introduced by broadcast stations can disturb the ionospheric environment particularly at night (rothkaehl and klos, 2002). the possible connection between earthquakes and disturbances in electromagnetic emissions has been studied. also the interaction between the body of the satellite and the surrounding ionospheric plasma has been investigated. it has been shown that the enhancement of radiation between the local plasma frequency an the f2 critical frequency can be correlated to interaction between the oscillation of electrostatically trapped particles in the wake of a spacecraft and superthermal electrons. 4.5. effects of the vertical and horizontal gradients of electron density on earth-space and satellite-to-satellite communication the ionosphere introduces a group delay in satellite signal and is at present the main source of error for satellite navigation systems that operate at single frequency. the possible dependence of these errors on different parameters has been described in the research done by nava (2000) with a systematic approach. nequick model has been used to assess this dependence and tests with experimental data have been performed. the results obtained suggest a way to «build-up» a correcting error function that applied to vertical tec could «bind» the error on slant tec after 1282 sandro m. radicella and ersin tulunay «back conversion». appropriate model simulations could indicate the influence of thin shell curvature, latitudinal and longitudinal gradients on conversion error as a function of location and satellite elevation and azimuth. some ionospheric features observed during geomagnetic disturbed periods, were investigated using particular sets of gps derived slant total electron content measurements: «supertruth» data. these data are high quality slant total electron content measurements obtained at 5 s sampling rate, using 25 ground stations located in the u.s.a., during specific time periods. the coinciding pierce point technique used has shown that a strong correlation between dst index variation and mapping function errors magnitude exists. since mapping function errors reflect the presence of strong electron density and tec gradients, the coinciding pierce point technique indicated that in some cases it is possible to detect some ionospheric storm effects in near-real-time (nava and radicella, 2003). information about the topside electron concentration distribution is not obtainable from groundbased measurements. in past decades only few satellites were equipped with ionosondes to sound the ionosphere above the f2 maximum (topside sounders). ionospheric electron density models have their topside formulation based on old databases or different kinds of measurements. nequick and iri models have been used for comparisons with topside sounder profiles (coïsson, 2002). from the study it appears that the nequick model performance is strongly affected by its topside thickness parameter, indicating the need for an improvement of its formulation. the study also shows that the topside formulation of iri leads to an overestimation of the electron density in the upper part of the profiles, and consequently an overestimation of tec. with the increasing use of radio systems for practical applications, particularly satellite-based navigation and positioning, interest is now turning to propagation effects on satellite-to-satellite paths. one specific aspect driving such investigations is concern over future requirements for the stationkeeping of satellites in the commercially very important, but potentially congested, geostationary orbit. geostationary satellites are currently tracked using a network of ground stations. a future possible development could involve positioning making the use of a satellite navigation system, like gps. however, since the gps system was designed primarily for terrestrial users the satellites have nadir-directed antennas. in consequence, reception in geostationary orbit would involve the use of signals transmitted by gps satellites on the far side of the earth. the model simulation study for gps-to-geostationary satellite ray paths has shown that very large tec values may be encountered (bailey and kersley, 2003, in radicella et al., 2004). it must be noted that for some extreme situations where the trajectory traverses the equatorial ionosphere at grazing incidence the tec estimates could give rise to positioning errors in excess of 0.5 km. it is clear from the study that in any use of gps signals for the station-keeping of geostationary satellites it would be necessary to exclude any dependence on signals where the ray-path geometry is close to eclipse, particularly close to the equatorial plane. references béniguel, y. (2002): global ionospheric propagation model (gim): a propagation model for scintillations of transmitted signals, radio sci., mai-juin. b ⁄lecki, j., s. savin, n. cornilleau-wehrlin, m. parrot, h. rothkaehl, k. stasiewicz, r. wronowski, o. santolikand and j.-a. sauvaud (2003): fine structure of polar cusp as deduced from the plasma wave and plasma measurements, adv. space res., 32 (3), 315-322. coïsson, p. (2002): uso di dati sperimentali per la convalida di modelli ionosferici applicati al disegno di servizi e sistemi satellitari, thesis (università degli studi di trieste). forte, b. and s.m. radicella (2002): problems in data treatment for ionospheric scintillation measurements, radio sci., 37 (6), november. forte, b. and s.m. radicella (2004): comparison of ionospheric scintillation models with experimental data for satellite navigation applications, ann. geophys. (in press). 1283 space plasma effects on earth-space and satellite-to-satellite communications: working group 4 overview forte, b., s.m. radicella and r.g. ezquer (2002): a different approach to the analysis of gps scintillation data, ann. geophysics, 45 (3/4), 551-561. gherm, v.e., n.n. zernov, s.m. radicella and h.j. strangeways (2002): propagation model for signal fluctuations on transionospheric radio links: recent developments, acta geod. geophys. hung., 37 (2/3), 271-280. nava, b. (2000): modello di densità elettronica della ionosfera applicato alla navigazione satellitare, thesis (università degli studi di trieste). nava, b. and s.m. radicella (2003): ionosphere response to geomagnetic storm as seen by gps derived total electron content data, in proceedings of 2003 international symposium on gps/gnss, tokyo, japan, 325-329. radicella, s.m. and r. leitinger (2001): the evolution of the dgr approach to model electron density profiles, adv. space res., 27 (1), 35-40. radicella, s.m., b. nava p. coïsson, l. kersley and g. j. bailey (2004): effects of gradients of the electron density on earth-space communications, ann. geophysics, 47 (suppl. to no. 2/3), 1227-1246 (this volume). rothkaehl, h. and z. klos (2002): broadband hf emissions as an indicator of global changes within the ionosphere, adv. space res., 31 (5), 1371-1376. rothkaehl, h., i. stanis ⁄lawska, j. b ⁄lecki and z. zbyszyński (2003): low altitude cusp-cleft region signatures of strong geomagnetic disturbances, cosmic res., 41 (4), 340-344. tulunay, y.k., e. tulunay and e.t. senalp (2004a): the neural network technique-1: a general exposition, adv. space res., 33 (6), 983-987. tulunay, y.k., e. tulunay and e.t. senalp (2004b): the neural network technique-2: an ionospheric example illustrating its application, adv. space res., 33 (6) 988-992. perturbazioni g e o m a g n e t i c h e , aurorali, i o n o s f e r i c h e e dei raggi cosmici : i n t e r d i p e n d e n z e e relazioni c o n l'attività solare f . m a r i a n i ( * ) f . m o l i n a ( * * ) r i c e v u t o il 30 aprile 1966 riassunto. — in relazione all'esistenza, a c c e r t a t a s p e r i m e n t a l m e n t e (cap. v a), di c a m p i magnetici i n t e r p l a n e t a r i diversi dal c a m p o geomagnetico e alla ipotesi delle nuvole di p l a s m a come causa di essi, si p r e n d o n o in esame i c a m p i m a g n e t i c i solari, in vista della possibilità di una loro estensione nello spazio. siccome con i c a m p i solari più intensi sono associati i fen o m e n i clie d a n n o luogo a e f f e t t i t e r r e s t r i , si descrivono, f r a tali f e n o m e n i , gli e v e n t i di radioemissione, che h a n n o s t r e t t e relazioni sia con i s u d d e t t i c a m p i m a g n e t i c i sia con r e m i s s i o n e di p l a s m a . p r i m a di discutere i d a t i s p e r i m e n t a l i , d i r e t t i e i n d i r e t t i , relativi alle n u v o l e di p l a s m a emesse dal sole, si p a s s a n o in rassegna gli e v e n t i connessi con l ' a r r i v o n e l l ' a t m o s f e r a t e r r e s t r e di p r o t o n i solari di 10-100 mev, i cos i d e t t i pca (polar cap absorption), di cui si esamina la morfologia, la dis t r i b u z i o n e geografica e le relazioni con gli e v e n t i solari. si descrivono infine i r i s u l t a t i finora o t t e n u t i delle misure d i r e t t e dei p r o t o n i solari effett u a t e sia a mezzo di palloni sia a mezzo di satelliti. summary. — e x i s t e n c e of i n t e r p l a n e t a r y m a g n e t i c fields o t h e r t h a n t h e geomagnetic field is now well established. t h e y t r a v e l as frozen-in fields inside t h e solar p l a s m a clouds: m a g n e t i c fields on t h e sun are t h e n discussed. f u r t h e r l y radioemission p h e n o m e n a are considered f r o m t h e v i e w p o i n t of t h e i r association a n d correlation w i t h t h e i n t e r p l a n e t a r y mag n e t i c field a n d solar p l a s m a p h e n o m e n a . morphology a n d geographical d i s t r i b u t i o n of p o l a r cap a b s o r p t i o n e v e n t s a n d t h e i r correlations w i t h solar p h e n o m e n a are reviewed. a t last e x p e r i m e n t a l results on solar p r o t o n s e v e n t s are discussed. (*) i s t i t u t o di fisica d e l l ' u n i v e r s i t à di r o m a . (**) i s t i t u t o nazionale di geofìsica, r o m a . 2 8 8 f. m a r i a n i f . . m o l i n a 5 . s t a t o f i s i c o d e l l o s p a z i o i n t e r p l a n e t a r i o (*) b . c a m p i m a g n e t i c i e b a d i o e m i s s i o n e s o l a r i p r o t o n i s o l a r i 5.5. c a m p i m a g n e t i c i s o l a r i . nei p a r a g r a f i precedenti a b b i a m o s t u d i a t o i campi m a g n e t i c i esis t e n t i nello spazio e x t r a t e r r e s t r e , quali si sono p o t u t i d e t e r m i n a r e o con inferenze i n d i r e t t e b a s a t e s o p r a t t u t t o sul c o m p o r t a m e n t o dei raggi cosmici nelle vicinanze del n o s t r o p i a n e t a , o con m i s u r e d i r e t t e eseguite m e d i a n t e satelliti artificiali e sonde spaziali. a b b i a m o così visto che n o n esiste solo il campo magnetico t e r r e s t r e t e n d e n t e a d annullarsi a g r a n d i distanze, m a che lo spazio i n t e r p l a n e t a r i o a p p a r e p e r m e a t o di c a m p i m a g n e t i c i p i u t t o s t o irregolari e disordinati nei quali il c a m p o magnetico t e r r e s t r e è immerso. ci p r o p o n i a m o ora di studiare e v e n t u a l i campi magnetici solari p e r p o t e r e in seguito discutere la possibilità di u n a loro estensione in m o d o p i ù o m e n o o r d i n a t o nello spazio i n t e r p l a n e t a r i o . lo studio dei campi magnetici sul sole è p i ù facile di quello dei campi i n t e r p l a n e t a r i , e di f a t t o si è iniziato verso il princìpio di questo secolo, utilizzando l ' e f f e t t o z e e m a n . 5.5.1. principio della misura. — s c h e m a t i c a m e n t e , il principio della m i s u r a dei campi m a g n e t i c i solari è il seguente. i n u n campo m a gnetico le righe s p e t t r a l i di u n a t o m o si scindono in più componenti (effetto zeeman); nei casi più semplici, osservando lungo la direzione delle linee di f o r z a del campo si n o t a una. scissione in due componenti di lunghezza d ' o n d a s p o s t a t a di ± a l d a quella a0 della riga i n d i s t u r b a t a , polarizzate circolarmente l ' u n a in verso opposto a l l ' a l t r a (effetto longitudinale); osservando invece p e r p e n d i c o l a r m e n t e alle linee di f o r z a del campo, si h a u n a scissione in t r e c o m p o n e n t i l i n e a r m e n t e polarizzate (effetto trasversale); la componente centrale h a la stessa lunghezza (*) questo lavoro costituisce la seconda p a r t e del capitolo 5. i primi q u a t t r o capitoli e la p r i m a p a r t e del capitolo 5 sono p u b b l i c a t i r i s p e t t i v a m e n t e in «ann. g-eof.» 12, 297 e 389 (1959); 13, 85 e 135 (1960); 16, 161 (1963). p k k t u k b a z i o n i g e o m a g n e t i c h e . e c c . 2 8 9 d ' o n d a 1 0 della riga i n d i s t u r b a t a , ed è polarizzata p a r a l l e l a m e n t e alla direzione del campo, m e n t r e le due componenti laterali, a distanza i al dalla componente centrale, sono polarizzate p e r p e n d i c o l a r m e n t e a q u e s t a . infine, osservando o b l i q u a m e n t e alle linee di forza, l ' e f f e t t o trasversale si t r a s f o r m a nel senso che le due componenti laterali risult a n o polarizzate ellitticamente l'una in verso opposto a l l ' a l t r a . l a differenza al di lunghezza d ' o n d a f r a le componenti e la riga i n d i s t u r b a t a è d a t a dalla espressione al e t 4 n m c' 3 = c g 10 e [5.9] d o v e e è la carica dell'elettrone, m la sua massa, h, la lunghezza d ' o n d a , h l ' i n t e n s i t à del campo magnetico e g il cosidetto f a t t o r e di l a n d e , funzione dei n u m e r i q u a n t i c i di m o m e n t o orbitale e di spin, che p u ò assumere valori compresi t r a 0 e 3. e s p r i m e n d o nella [5.9] le varie grandezze in u n i t à c.g.s. (e in u.e.s.), si ottiene per la costante c il valore 4,7.10"5. p e r la [5.9], dalla misura di al si può risalire alla i n t e n s i t à del campo magnetico esistente nella regione dove la riga ha. origine. p e r avere u n a idea dell'entità dell'effetto, si noti che p e r 1„ = 6000 a, g = 3, h = 1000 oersted, si h a al = 0,05 a . si comprende quindi come solo per campi molto intensi si possa osservare u n effettivo s d o p p i a m e n t o delle righe; p e r i n t e n s i t à p i u t t o s t o basse si nota soltanto u n a l l a r g a m e n t o , e in t a l caso la misura è resa possibile solo estinguendo a mezzo di opp o r t u n i analizzatori l ' u n a o l ' a l t r a delle due componenti. 5.5.2. campi magnetici delle macchie. — le macchie solari sono centri di intensi campi magnetici. la scoperta risale ad h a l e (1908), il quale f u c o n d o t t o alla ricerca di tali campi dalla struttura, della cromosfera a t t o r n o alle regioni delle macchie, s t r u t t u r a che lo a v e v a f a t t o pensare alla esistenza di linee di forza magnetiche. molto più t a r d i h a l e e mcholson (1938) pubblicarono i risultati di m o l t i a n n i di studio, dai quali essi e numerosi altri a u t o r i h a n n o p o t u t o t r a r r e v a r i e conclusioni. i campi magnetici delle macchie sono g e n e r a l m e n t e p i u t t o s t o intensi, dell'ordine delle centinaia o delle migliaia di oersted. esiste ima c e r t a relazione t r a l ' a r e a delle macchie e il valore massimo em del c a m p o 2 9 0 f . m a r i a n i f . . m o l i n a magnetico da esse p r e s e n t a t o : secondo h o u t g a s t e van sluiters (1948) tale relazione è espressa dalla formula 3m = 3 7 0 0 a / ( a + 66) oersted dove a è l'area espressa in milionesimi dell'emisfero visibile. nicholson (1933) aveva precedentemente dedotto una relazione che dava per ii valori u n po' minori della espressione s u d d e t t a . n a t u r a l m e n t e si t r a t t a di relazioni empiriche che forniscono valori medi dei campi, i quali possono a volte raggiungere valori ben più elevati; sono s t a t i m i s u r a t i inf a t t i campi fino a 4500 oersted. i n realtà il valore hm che viene effettivamente osservato presenta u n a sistematica dipendenza dalla posizione della macchia sul disco solare: esso è minore per le macchie presso il bordo che per quelle vicine al centro. ciò potrebbe indicare una diminuzione del campo con l'altezza nell'atmosferasolare al di sopra delle macchie, m a potrebbe anche t r a t t a r s i solo di u n effetto apparente, dovuto alla influenza pert u r b a t r i c e della scintillazione e delladiffusione della luce fotosferica sulle osservazioni dell'allargamento delle righe spettrali. ma-ttig (1958) sembra a t t r i b u i r e l'effetto a questa sola causa p e r t u r b a n t e . i l campo magnetico è massimo al centro della macchia, e va diminuendo fino a scomparire al bordo della penombra; inoltre, al centro, esso è diretto radialmente al sole, cioè perpendicolarmente alla superficie solare, m a l'angolo tra la direzione radiale e il campo v a progressivamente a u m e n t a n d o quando ci si allontanadal centro, fino a raggiungere u n valore p r a t i c a m e n t e uguale a nj'2, al limite della penombra. secondo broxson (1912), la relazione tra l'intensità del campo h e ladistanza r dal centro di una macchiaconsiderata approssimativamente circolare, il raggio della citi penombra sia b, è e = hm (1 — v-/b-) [5.10] questa relazione viene confermatada thiessen (1953), il quale giudica inesatta l'espressione di mattig (1953). h = h m (1 — r 4/b4) <r2'2/62 in q u a n t o nel dedurre quest'ultima non si sarebbe tenuto conto di un sistematico effetto ottico nelle osservazioni, tale dadare valori t r o p p o bassi per l'effetto zeeman e quindi per l'intensità del campo. i n ogni caso formule del tipo ora citato rappresentano semplicemente u n and a m e n t o medio del campo magnetico all'interno di una macchia; in p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 2 9 1 r e a l t à il campo presenta spesso una s t r u t t u r a fine p i u t t o s t o complessa, come è s t a t o messo in rilievo da severny (1905 b). il flusso magnetico a t t r a v e r s o l'area di una macchia è d a t o d a 0 = i 3 da, dove da è l'elemento di superficie; noto h,„. 0 si p u ò calcolare a p p r o s s i m a t i v a m e n t e dalla [5.10]. l'ordine di grandezza del flusso magnetico delle macchie è di lo-'-lo-oersted-cm 2 . a t t r a v e r s o l'esame delle osservazioni di monte wilson, cowling (1946) h a p o t u t o tracciare nelle grandi lince la « storia magnetica » delle macchie solari. d u r a n t e i primi giorni della formazione della macchia sia l ' a r e a che il campo magnetico a u m e n t a n o con notevole r a p i d i t à ; raggiunto un massimo verso il decimo giorno, l'area comincia l e n t a m e n t e a diminuire, m e n t r e il campo magnetico rimane pressoché costante fin verso la line della vita della macchia, quando anche esso diminuisce r a p i d a m e n t e . i n t e r m i n i di flusso, si ha perciò un notevolissimo a u m e n t o iniziale, d o v u t o all'incremento contemporaneo del campo e dell'area, seguito da una diminuzione p r a t i c a m e n t e proporzionale alla diminuzione dell'area, e infine un calo rapido fino alla estinzione. l'esistenza dei campi magnetici nelle macchie ha condotto alla classificazione dei gruppi di macchie in t r e categorie, d e n o m i n a t e rispett i v a m e n t e a, ¡3, y. i g r u p p i a h a n n o u n c a r a t t e r e unipolare, cioè i loro campi magnetici p r e s e n t a n o un solo polo; i g r u p p i ¡3 sono bipolari, costituiti, nei casi più semplici, da due macchie di polarità opposta, m a in generale da più macchie, di cui quelle situate nella, regione ovest h a n n o t u t t e una polarità opposta a quelle della regione est; i gruppi y, infine, presentano a m b e d u e le polarità, m a distribuite in modo assai irregolare. p e r farsi una, idea della frequenzarelativa delle t r e categorie, si noti che dalla statistica di richardson (1918) risulta che su 6387 gruppi di macchie m i s u r a t i a monte wilson tra il 1917 e il 1946 ben 5814, cioè il 91,0% erano bipolari; 24, p a r i allo 0,4% erano di tipo y, m e n t r e 549 (8,6%) erano unipolari. si conclude quindi che la grandissima maggioranza dei gruppi presenta u n a regolare bipolarità; ed è inoltre d a n o t a r e che molti dei gruppi classificati come unipolari sono in r e a l t à il residuo di gruppi bipolari ben s v i l u p p a t i ; essi sono spesso accompagnati d a u n a regione fa,colare p r e s e n t a n t e u n notevole campo magnetico di polarità opposta nella posizione in cui dovrebbe t r o v a r s i la m a c c h i a principale ormai dissolta (« macchie invisibili », secondo la denominazione di hale). un fenomeno molto i m p o r t a n t e è quello noto sotto il nome di legge della polarità: t u t t e le macchie ovest dei gruppi bipolari di u n certo 2 9 2 f . m a r i a n i f . . m o l i n a emisfero solare (nord o sud) h a n n o la stessa p o l a r i t à , opposta nei due emisferi; tale polarità e a m b i a segno all'inizio di ogni n u o v o ciclo solare. dal p u n t o di vista magnetico si ha d u n q u e u n ciclo non più di 11 anni, ma di 22 anni. nel ciclo or ora chiuso, iniziatosi nel 1954, le macchie ovest a v e v a n o polarità n nell'emisfero nord del sole, p o l a r i t à 8 nell'emisfero sud. le eccezioni a questa legge della p o l a r i t à sono molto poche: richardson h a t r o v a t o i n f a t t i che dei 5814 gruppi bipolari di cui si è d e t t o sopra solo 180, cioè il 3 , 1 % non seguivano tale legge. 5.5.3. campi magnetici fotosferici. — i l costante progresso nella tecnica di osservazione dei campi magnetici solari, c u l m i n a t o nel eosid e t t o « m a g n e t o g r a f o solare » ideato da babcock (1953, 1955) e perfezion a t o specialmente da severny (1962, 1964), ha permesso di s t u d i a r e lo s t a t o magnetico di t u t t a la superficie solare, e non solo delle regioni delle macchie. nella fascia di l a t i t u d i n i eliografiche i 50° esistono numerose regioni c a r a t t e r i z z a t e da campi magnetici localizzati. tali regioni non sono p e r m a n e n t i , m a h a n n o u n a vita che va da circa u n ' o r a a qualche mese; l ' i n t e n s i t à dei campi magnetici è g e n e r a l m e n t e compresa t r a qualche frazione e parecchie decine di oersted. le regioni m a g n e t i c h e possono classificarsi in due tipi: regioni bipolari (bm) e regioni unipolari (um). le regioni bm sono le più numerose; sembra clic le macchie solari si f o r m i n o solo in esse, ma c e r t a m e n t e non ili t u t t e . è i n t e r e s s a n t e il f a t t o che le regioni bm seguono la stessa legge di polarità delle macchie in modo quasi rigoroso. le regioni bm hanno dimensioni assai variabili; le p i ù estese possono giungere a più di un decimo di emisfero visibile, le più piccole non superano il millesimo. secondo le osservazioni dei babcock vi è u n a s t r e t t a corrispondenza t r a regioni c a r a t t e r i z z a t e da f o r t e emissione cromosferica nella riga k (?. = 3133,7) del c a l i (facole) e intense regioni bm. con u n m a g n e t o g r a f o di babcock modificato, h o w a r d (1959) ha misurato, n e l l ' a u t u n n o 1957 e nell'estate 1958, cioè al massimo dell ' a t t i v i t à solare, i campi magnetici in varie regioni di macchie, t r o v a n d o che m o l t o spesso presso le macchie m a al di f u o r i di esse esistono campi maggiori di 75 oersted (limite di linearità dello s t r u m e n t o ) ; ciò n a t u r a l m e n t e p o r t a a modificare le espressioni del tipo della [5.10] sulla distribuzione del campo. u n confronto con gli s p e t t r o l i o g r a m m i in c a l i c o n f e r m a la corrispondenza già t r o v a t a dai babcock t r a campi magnetici e facole cromosferiche; più e s a t t a m e n t e , secondo h o w a r d il confine p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 2 9 3 delle facole coincide con buona approssimazione con la linea di contorno di 10 oersted. una ulteriore conferma di ciò è d a t a dalle osservazioni di leighton (1959), il quale, da misure eseguite nell'agosto e settembre 1958, conclude che vi è uno stretto accordo fra la forma delle facole in luce di c a l i e quella delle regioni magnetiche, e che in corrispondenza delle facole il campo magnetico raggiunge spesso valori di 100 -f-200 oersted. secondo simon (1963) tale stretta correlazione si estende fino ai campi magnetici più deboli (circa 2 oersted). sembra inoltre che l'intensità del campo presenti una notevole correlazione con l'intensità di emissione delle facole, cosicché una rappresentazione della s t r u t t u r a cromosferica potrebbe veni]' considerata come la mappa solare del valore assoluto del campo magnetico (leighton, 1965). le regioni um sono zone, generalmente molto estese, della superficie solare in cui si osserva un campo magnetico di una unica polarità. l'intensità del campo è bassa, da frazioni di oersted a circa 3 oersted. malgrado siano meno numerose delle regioni bm, le regioni um non sono t u t t a v i a dei fenomeni rari. scoperte dai babcock (1955) dur a n t e la fase di declino del ciclo solare (1952-54) e considerate allora come eventi di carattere piuttosto eccezionale, esse sono state nuovamente osservate da howard (1965) nel periodo compreso fra l'estate del 1959 e la fine del 1962, in una fase del ciclo solare praticamente corrispondente a quella analizzata dai babcock. howard ha studiato più di 100 regioni um di notevoli dimensioni; in esse il campo magnetico non è distribuito con continuità, ina è concentrato in piccole zone. cont r a r i a m e n t e a quanto si pensava dopo le prime osservazioni, anche le regioni um sono associate a manifestazioni cromosfei'iche; i n f a t t i , secondo howard, gli spettrogrammi di calcio presentano, in corrispondenza di u n a regione um, un aumento della emissione di fondo. le regioni um si formano dalla disintegrazione di regioni bm ben sviluppate nell'ultima fase di esistenza di queste. dopo un tempo variabile a partire dalla loro formazione, ma dell'ordine di qualche mese, queste regioni bm si espandono, con diminuzione di campo, e infine si disintegrano, lasciando due regioni um di polarità opposte, che tendono a migrare in direzione dei poli. la vita delle singole regioni um può essere a sua volta notevolmente lunga: la più importante delle regioni unipolari osservate dai babcock è d u r a t a almeno sei rotazioni solari, dal maggio all'ottobre 1953 (v. anche 1.5.5.1.). nella valutazione di questi risultati occorre t u t t a v i a tener conto di un f a t t o molto importante, messo in luce da severny (1965 a): aumen2 9 4 f . m a r i a n i f . . m o l i n a t a n d o il potere risolutivo dello strumento, una regione unipolare può apparire magneticamente molto più complessa, con s t r u t t u r a multipolare. lo studio delle regioni bai e um permette di completare ed estendere la « storia magnetica » delle macchie solai! descritta da cowling f i g . 5.37 e s e m p i di m a g u e t o g r a m m i solari o t t e n u t i da b a b c o c k : sono visibili regioni bm e um, noneliò p o l a r i t à o p p o s t e e p i u t t o s t o u n i f o r m i nelle d u e c a l o t t e polari (secondo babcock). (v. 5.5.2); esso anzi suggerisce che il fenomeno fondamentale dell'attività solare sia la vita e l'evoluzione delle regioni magnetiche, m e n t r e macchie, brillamenti e facole non sarebbero che episodi di tale evoluzione. i n f a t t i le regioni magnetiche nascono qualche giorno prima delle macchie e delle facole, durano per molto tempo dopo che queste manifestazioni sono scomparse, e rappresentano infine l'estremo residuo dei centri di a t t i v i t à . p e r t u r b a z i o n i g e o m a g n e t i c h e e c c . 2 0 5 5.5.4. brillamenti e campi magnetici fotosferici. — è interessante s t u d i a r e q u a n t o avviene nei campi magnetici osservabili nella fotosfera, nelle regioni al disopra delle quali h a n n o luogo i brillamenti. i m p o r t a n t i informazioni sono fornite dalle osservazioni di gopasyuk e coli. (19(53), i quali h a n n o esaminato i campi magnetici di gruppi di macchie prima e dopo 52 brillamenti di varia importanza f r a il luglio 1957 e la line del 1961. risulta da tali osservazioni che i brillamenti h a n n o luogo in regioni dove esistono forti gradienti magnetici; l ' i m p o r t a n z a del brill a m e n t o cresce al crescere della intensità dei gradienti. i n seguito al b r i l l a m e n t o i gradienti diminuiscono n e t t a m e n t e , in misura t a n t o maggiore q u a n t o maggiore è l'importanza del brillamento stesso. alcuni casi particolari, s t u d i a t i in modo dettagliato, h a n n o f o r n i t o ulteriori informazioni. ad esempio, le osservazioni combinate di h o w a r d e s e v e r n y (1963) a monte wilson e all'osservatorio astrofisico di crimea in occasione del brillamento di i m p o r t a n z a 3 -f del 16 luglio 1959 h a n n o m o s t r a t o che in seguito ad esso si è verificata u n a variazione non solo dei g r a d i e n t i m a anche della intensità del campo, che è diminuito di un f a t t o r e 3; la diminuzione di energiamagnetica è s t a t a valutata, a. circa io32 erg. è i n t e r e s s a n t e il f a t t o che in realtà solo i campi magnetici di v a l o r e elevato sono s t a t i modificati in t a l e e v e n t o ; le osservazioni di m o n t e wilson, r i p o r t a t e da h o w a r d e babcoek (1960), h a n n o i n f a t t i m o s t r a t o che per i valori non superiori a 10 oersted (limite superiore dello s t r u m e n t o usato) i campi magnetici fotosferici nella regione del b r i l l a m e n t o non h a n n o p r e s e n t a t o d u r a n t e il fenomeno variazioni degne di rilievo; a n c h e la s t r u t t u r a filamentosa cromosferica a t t o r n o al brill a m e n t o , indicatrice di linee di forza magnetiche nella cromosfera, è r i m a s t a pressoché i n v a r i a t a . variazioni p i u t t o s t o complesse nella configurazione dei campi magnetici sono s t a t e osservate da severny (1965 c) in occasione del brill a m e n t o di i m p o r t a n z a 2 del 22 giugno 1962; in questo caso non si p u ò p a r l a r e di u n a r a p i d a diminuzione del campo in generale, m a p i u t t o s t o di f o r t i deformazioni locali. 5.5.5. campi magnetici nella cromosfera c nella corona. — i campi di cui si è p a r l a t o nei n u m e r i precedenti sono quelli esistenti nella fotosfera, e sono i soli campi magnetici solari per i quali è possibile la m i s u r a d i r e t t a . è n a t u r a l e supporre che i campi fotosferici si estendano anche al di sopra della fotosfera, nella cromosfera e nella corona. questa supposizione è c o n f e r m a t a d a osservazioni di vario tipo, in p r i m o luogo d a quelle del m o v i m e n t o di materia ricadente dalle p r o t u 2 9 6 f . m a r i a n i f . . m o l i n a beranze solari (osservate al bordo) nella f o t o s f e r a . e s s e n d o la m a t e r i a delle p r o t u b e r a n z e f o r t e m e n t e ionizzata, essa t e n d e r à a seguire, nel suo moto, le linee di forza di e v e n t u a l i campi. correli e coli. (1956, 1958), s t u d i a n d o i m o t i della materia di due p r o t u b e r a n z e presso il b o r d o del sole il 26 febbraio 1946 e il 20 aprile 1950, h a n n o concluso che tali m o t i a v v e n i v a n o sotto l'azione di un campo di dipolo, il cui centro era rispett i v a m e n t e 0,2 e 0,03 raggi solari al di sotto della superficie. l a p r o t u b e r a n z a del 20 aprile 1950 si è prodotta al di sopra di un i m p o r t a n t e g r u p p o di macchie, nel quale la massima i n t e n s i t à del campo magnetico a v e v a raggiunto i 4200 oersted. le t r a i e t t o r i e del m o t o della m a t e r i a , supposte coincidenti con le linee di t'orza del c a m p o magnetico, si estendono tino a circa 7io4 k m al di sopra della fotosfera. è quindi e v i d e n t e che i campi delle macchie si estendono a. notevoli altezze nella cromosfera; essi giungono anche nella corona, come induce a pensare la f o r m a degli archi coronali al di sopra dei centri di a t t i v i t à . a q u e s t ' u l t i m a conclusione si perviene anche in base alla osservazione che (come v e d r e m o più e s t e s a m e n t e in 5.6) la radioemissione solare di lunghezza d ' o n d a a t t o r n o al m e t r o , che ha c e r t a m e n t e una origine coronale ed è particol a r m e n t e associata ai centri di a t t i v i t à , è g e n e r a l m e n t e polarizzata, indicando l'esistenzadi notevoli campi magnetici. a questo proposito è interessante uno studio di cohen (1961) su di u n f e n o m e n o di radiopropagazione (e quindi i n d i p e n d e n t e dalla n a t u r a della sorgente) nell'atmosfera solare: in molti bursts di radioemissione nella gamma, delle microonde si osserva una polarizzazione circolare, in cui il senso di rotazione si in verte a u n a certa f r e q u e n z a . questo fenomeno, i n t e r p r e t a t o in t e r m i n i di teoria magnetoionica, suggerisce che il c a m p o magnetico ad una altezza di circa io5 k m al di sopra di una regione a t t i v a ha un valore compreso fra 2 e 6 oersted. 5.5.6. campo magnetico generale del sole. — una questione di g r a n dissimo interesse è infine quella della esistenza o m e n o di u n c a m p o magnetico generale del sole, campo che, in analogia con quello t e r r e s t r e , p o t r e b b e in linea di principio essere approssimato con u n campo di dipolo. tale ipotesi sembra c o n f e r m a t a dalla f o r m a dei raggi coronali polari, che si possono osservare verso il minimo della a t t i v i t à solare; essi h a n n o i n f a t t i l ' a s p e t t o delle linee di forza di u n dipolo, il cui asse coincide a p p r o s s i m a t i v a m e n t e con l'asse di rotazione solare. misure acc u r a t e eseguite in occasione di varie eclissi h a n n o m o s t r a t o che in r e a l t à i raggi divergono in m i s u r a maggiore di q u a n t o f a n n o le linee di forza i ' k k t u hb a z i o n i u r o m a c i n k t i c i 1 k , kc< 2 9 7 di un dipolo, e sembra anche che tale divergenza vari durante il ciclo solare (van de iiulst, 1953). i primi tentativi di misura diretta del campo magnetico generale del sole furono opera di hale e dei suoi collaboratori (hale, 1913; seares, 1913; hale e coli., 1918). essi conclusero che il campo solare è un campo di dipolo, il cui asse è inclinato di pochi gradi rispetto all'asse di rotazione, e il cui momento magnetico ammonta a 8.4-io33 oersted• cm3, il che comporta per il campo polare un valore di 50 oersted; l'orientazione del dipolo rispetto all'asse di rotazione è analoga a quella terrestre, cioè al polo eliografico nord corrisponde una polarità sud e viceversa. successive misure eseguite in anni più recenti con dispositivi sempre più perfezionati da von kluber e da tliiessen non hanno t u t t a v i a confermato tali conclusioni; l'intensità del campo polare è risultata di soli pochi oersted, con indicazioni che l'orientamento dell'asse del dipolo è opposto a quello del dipolo terrestre. ii magnetografo solare di babcock ha fornito un ulteriore import a n t e contributo alle osservazioni del campo magnetico generale del sole (babcock, 1955). secondo le osservazioni eseguite negli anni 1952-54 tale campo è rilevabile solo a latitudini eliografiche maggiori di circa ± 55°. l'intensità della componente nella direzione di osservazione è, in vicinanza dei poli solari, circa 1 oersted; assumendo come direzione delle linee di forza in tali regioni quella dei raggi coronali, si può ammettere che il valore medio dell'angolo tra le linee di forza e la direzione di osservazione sia attorno ai 60°, da cui risulta per il campo polare un valore dell'ordine dei 2 oersted. la polarità è risultata opposta a quella del dipolo terrestre. sono estremamente interessanti le misure eseguite in seguito. inf a t t i , sempre secondo babcock (1959), tra il marzo e il luglio del 1957 il campo nella regione del polo eliografico sud è dapprima diminuito di intensità lino ad annullarsi, poi ha cambiato verso; si sono così avute due polarità dello stesso segno (nord) lino al novembre 1958, quando quasi improvvisamente anche il campo della calotta nord ha cambiato polarità, ricostituendo in tal modo il dipolo, questa volta orientato come quello terrestre. dalle misure di howard (1965) a monte wilson, eseguite con continuità a partire dal 1960, risulta confermata la nuova polarità del campo generale del sole, almeno fino al marzo 1961; a questa epoca l'intensità del campo nei pressi del polo sud solare è diminuita fino a risultare praticamente nulla, e tale sembra essere rimasta fino all'estate del 1963, d a t a della comunicazione di howard. è di quest'ultima epoca 2 9 8 f . m a r i a n i f . . m o l i n a una osservazione di severny effettuata con un potere risolutivo assai elevato, resa pubblica nella discussione sulla nota di howard; tale osservazioni; conferma sostanzialmente quelle di howard, precisando tuttavia che al polo sud solare si notavano zone di campo positivo in numero uguale a quelle di campo negativo, e questa era la ragione per cui il risultato dava in media intensità nulla. dalle osservazioni di howard e di severny risulta che nelle zone alle latitudini più elevate e perciò al di fuori della fascia dei centri di attività, e quindi capaci di fornire informazioni più chiare sul campo generale del sole, il campo magnetico non è distribuito uniformemente sulla fotosfera, ma è concentrato in piccola aree, che in pratica coincidono con i flocculi polari in luce di c a l i ; si t r a t t a della stessa caratteristica osservata, come abbiamo visto, da howard nelle regioni unipolari. è importante notare che serie riserve sono state sollevate (alfvén, 1956; alfvén e lehnert, 1956) sulla attendibilità delle deduzioni attorno a deboli campi magnetici solari fatte sulla base dell'effetto zeeman. secondo alfvén, la turbolenza della fotosfera rivelata dalla presenza della granulazione deve produrre un campo magnetico turbolento di alcune centinaia di oersted; la misura dell'effetto zeeman viene necessariamente eseguita, a causa dei limiti della risoluzione strumentale, su di una regione fotosferica contenente migliaia di granuli e nelle conclusioni fin qui t r a t t e si è supposto clic, il risultato desse semplicemente un valor medio del campo magnetico in questa regione. manon è detto che l'influenza del campo turbolento sia in medianulla, poiché esso deve essere necessariamente « accoppiato » con la densità e la temperatura, dalle quali dipende l'assorbimento di una riga dello spettro, e può darsi che il modo di accoppiamento sia t a l 1 da falsare sistematicamente la misura del campo basata sull'effetto zeeman. in assenza di una teoria esatta dell'effetto zeeman in condizioni fìsiche quali quelle della fotosfera non è possibile, allenila alfvén, ritenere conclusive le misure su di esso basate. a questa interessante obiezione si possono tuttaviacontrapporre due dati di f a t t o : 1) i deboli campi delle regioni bipolari obbediscono alla legge di polarità, come abbiamo visto in 5.5.3., e ciò è un forte argomento a favore della attendibilità dell'uso dell'effetto zeeman; 2) le misure della polarizzazione delle componenti delle righe usate proverebbero anch'esse che si t r a t t a di un autentico effetto zeeman. è t u t t a v i a necessario tener conto dell'influenza del potere risolutivo dello strumento sui risultati, influenza messa in luce soprattutto da severny. agli effetti del campo magnetico generale del sole sono da l ' è k t [ ik li a 7.10 x i g e o m a g n e t i c h e , ecc. 2 9 9 citare a questo proposito le misure di stellilo (1966) col m a g n e t o g r a f o dell'osservatorio di crimea con f e n d i t u r e di varia grandezza, dalle quali risulterebbe nelle regioni polari del sole una i n t e n s i t à del campo crescente al diminuire delle dimensioni della f e n d i t u r a e quindi a l l ' a u m e n t a r e del p o t e r e risolutivo, intensità che sarebbe dell'ordine dei 5 oersted, con una p o l a r i t à parallela a quella del campo t e r r e s t r e . 5.6. r a d i o e m i s s i o n e s o l a r e . da, q u a n t o d e t t o nei precedenti capitoli, in particolare in 1 e 2, e nella p r i m a p a r t e del presente capitolo, è chiaro che l'ipotesi più imm e d i a t a riguardo alla causa prossima delle t e m p e s t e magnetiche e fen o m e n i concomitanti è quella, di u n a nuvola di particelle emessa dal sole; p e r la categoria delle tempeste s.c. tale emissione dovrebbe a v e r luogo con c a r a t t e r e esplosivo d u r a n t e u n brillamento. l a f o r t e influenza del campo geomagnetico sul corso e sulla distribuzione delle p e r t u r b a z i o n i , nonché la produzione dei fenomeni aurorali, rendono inoltre evidente che si t r a t t a di particelle elettricamente cariche. questa ipotesi apre o v v i a m e n t e notevoli problemi sulla n a t u r a delle particelle, sul processo di emissione e sulla loro propagazione nello spazio i n t e r p l a n e t a r i o . occorre quindi cercare di conoscere qualche cosa di più preciso su tale radiazione corpuscolare. a questo scopo può servire sia u n a eventuale osservazione d i r e t t a della nuvola di particelle cariche (plasma) nel t r a g i t t o dal sole alla t e r r a , sia, con procedimento più indiretto ma di più facile disponibilità, lo studio a p p r o f o n d i t o della connessione t r a i fenomeni solari e gli eventi t e r r e s t r i . in i zi a mo da questo secondo procedimento, passando in rassegna i principali r i s u l t a t i o t t e n u t i negli ultimi anni, dopo la pubblicazione del capitolo 1. e come primo passo illustreremo b r e v e m e n t e le n u o v e conoscenze acquisite sui fenomeni di radioemissione solare, limit a n d o c i t u t t a v i a alla cosidetta radioemissione del sole attivo, fenomeni che si sono d i m o s t r a t i di grandissima i m p o r t a n z a nello studio della interdipendenza f r a a t t i v i t à solare ed e v e n t i geofisici. 5.6.1. fenomeni di radioemissione durante un brillamento. — i n 1.5.4.3. e 1.5.4.4. abbiamo messo in evidenzache u n a caratteristica f o n d a m e n t a l e di un brillamento solare p r o d u t t i v o di t e m p e s t a magnetica è la esistenza di intensa radioemissione, ed abbiamo accennato alla esistenza, di varie specie di radioemissione, caratterizzate ciascuna da u n 3 0 0 f . m a r i a n i f . . m o l i n a particolare a s p e t t o dello s p e t t r o di f r e q u e n z a (bursts di tipo i i , i i i e i v ) . r i p r e n d i a m o qui l ' a r g o m e n t o , descrivendo in m o d o più p a r t i c o lareggiato q u a n t o si è fino ad oggi scoperto su questi tipi di radioemissione. 5.6.1.1. outbursts (li tipo ii. — sono compresi in questa categoria gli outbursts, della d u r a t a dell'ordine dei 10 m i n u t i , costituiti d a u n a i n t e n s a radiazione in u n a b a n d a p i u t t o s t o r i s t r e t t a di f r e q u e n z e , b a n d a che si sposta l e n t a m e n t e verso le f r e q u e n z e più basse (slow-drift bursts). i n questo s p o s t a m e n t o , gli outbursts spazzano t u t t e le g a m m e d ' o n d a / > » / / 0 50 100 150 200 250 f r e q u e n z a ( m h z ) fig. 5.38 d e r i v a di f r e q u e n z a degli outbursts di t i p o ii (secondo maxwell e t h o m p s o n ) . dalle onde centimetriche alle onde decametriche (cioè d a circa io4 a 10 m h z ) ; p a s s a n d o dalle alte alle basse f r e q u e n z e il f e n o m e n o si p r e s e n t a con u n a i n t e n s i t à r a p i d a m e n t e crescente: m e n t r e alle onde centim e t r i c h e l ' i n t e n s i t à delvoutburst è dell'ordine del 5 1 0 % del livello di base, essa sale al 1 0 0 % alle onde decimetriche, a un valore di 10-10* volte quello del livello di base alle onde m e t r i c h e , a a d d i r i t t u r a a u n valore io6 volte maggiore alle onde decametriche. la temperatura equiv a l e n t e ( t e m p e r a t u r a di un corpo nero che e m e t t e su t a l i f r e q u e n z e con la d a t a intensità) giunge ad u n valore dell'ordine di 101 0-10u ° k ; ciò rende e v i d e n t e che la radioemissione di tali outbursts non p u ò essere di origine t e r m i c a . l a d e r i v a di f r e q u e n z a è s t a t a s t u d i a t a d a maxwell e t h o m p s o n (1962) su sessanta e v e n t i fra il 10 o t t o b r e 1956 e il 31 dicembre 1960; essa a p p a r e come u n a funzione crescente della f r e q u e n z a (fig. 5.38), con dfjdt ^ — 0,3 mhz/sec a t t o r n o ai 100 m h z , e — 1,5 mhz/sec sui 200 m h z . u n a c a r a t t e r i s t i c a i n t e r e s s a n t e degli outbursts di tipo i i è che g r a n p a r t e di essi (circa l'80 p e r cento secondo maxwell e thompson) sono p k r t1 " k li a z i o n i ( ; e ( im a ci n ioti l ' i l i ' . , e c l 3 0 1 costituiti da una radioemissione su di una frequenza f o n d a m e n t a l e acc o m p a g n a t a da una seconda armonica, che presenta u n a i n t e n s i t à p r a t i c a m e n t e uguale a quella della frequenza f o n d a m e n t a l e . anche questo f a t t o p o r t a ad escludere che la radioemissione in esame sia di origine t e r m i c a . gli outbursts di tipo i i p r e s e n t a n o una strettissima associazione con i brillamenti, in particola]modo con quelli più i m p o r t a n t i , nel senso che essi s e m b r a n o verificarsi solo in occasione di questi; solo pochi sono s t a t i osservati senza che alcun brillamento venisse segnalato, ma, non è chiaro se ciò sia d o v u t o solo a mancanza di osservazione ottica solare nord • • f v 9 0 ° " " est • • • * s u d 4 0 . * • • v • • • • • • • ovest • • 4 0 ° fig. 5.39 d i s t r i b u z i o n e eliografica di b r i l l a m e n t i associati con outbursts di t i p o l i (secondo maxwell e t h o m p s o n ) . in quelle occasioni. i n fìg. 5.39 è r i p o r t a t a la posizione in l a t i t u d i n e e longitudine eliografiche di 106 brillamenti associati ad outbursts di tipo i i osservati d a maxwell e t h o m p s o n (1962); tali b r i l l a m e n t i sono ugualm e n t e distribuiti su t u t t e le longitudini, il che rivela, che la radiazione di tipo i i viene emessa in coni molto larghi. l ' i n t e r p r e t a z i o n e 51ìù attendibile degli outbursts di tipo i i sembra essere quella, che attribuisce tale radioemissione a oscillazioni di plasma nella cromosfera e nella corona solare. è noto i n f a t t i che in un gas ionizzato, soggetto ad u n a rapida perturbazione, si possono p r o d u r r e oscillazioni con u n a frequenza f o n d a m e n t a l e f„ = *]nf*jn~m [5.11] dove n è la densità elettronica in cm-3, e è la carica dell'elettrone in u.e.s., m la m a s s a dell'elettrone in gr; al cessare della perturbazione, l'ampiezza delle oscillazioni diminuisce esponenzialmente con u n a cos t a n t e di t e m p o p a r i a 1/v, dove v è la f r e q u e n z a di collisione t r a elet3 0 2 f . m a k í a x í f . m o l i n a t r o n i e ioni positivi. u n a frazione della energia delle oscillazioni del p l a s m a si t r a s f o r m a in radiazione e l e t t r o m a g n e t i c a . aggiungiamo che secondo westfold (1949) in presenza di un campo magnetico 77, oltre alla f r e q u e n z a di oscillazione /«,, si possono p r o d u r r e le f r e q u e n z e u = l i f . + (/h/2) 2 + /h/2 u = h ^ w f u i ^ dove fh è la girofrequenza per gli elettroni, d a t a eia fh = eli¡(2 ti me) . a questo f a t t o p o t r e b b e essere d o v u t a la s t r u t t u r a complessa della c o mp o n ente f o n d a m e n t a l e e della, armonica di m o l t i ovtbursts di tipo i i , con la loro suddivisione in due sottocomponenti (wild 1950; maxwell e t h o m p s o n , 1962). ciò p e r m e t t e r e b b e di d e t e r m i n a r e l ' i n t e n s i t à dei campi magnetici alle varie altezze solari, che sarebbero dell'ordine di qualche oersted (v. 5.5.5.). r o b e r t a (1959) lia però a v a n z a t o m o l t e obiezioni a t a l e i n t e r p r e t a z i o n e . come si è d e t t o , l ' i n t e n s i t à della radiazione e l e t t r o m a g n e t i c a decade esponenzialmente con una costante di t e m p o 1/v. secondo s m e r d e w e s t f o l d (1949) (v. anche p a w s e y e s m e r d (1953)) la f r e q u e n z a di collisione degli elettroni è data, da 4 c w ' & i r u [ 5 1 2 ] 3 (2 in)1!» (k t)3'* dove jc è la costante di b o l t z m a n , z la carica ionica in u n i t à di carica elementare, ni la densità degli ioni positivi ed a la funzione a = in 4 l t z en assumendo u n a atmosfera solare costituita da. idrogeno completam e n t e ionizzato, a v r e m o z = 1 e n i = n . p e r u n a f r e q u e n z a di oscillazione di 200 m h z e per t = io5 °k, si h a , dalle [5.11] e [5.12], v = 27 sec1; p e r u n a f r e q u e n z a di 20 m h z , v za 0,27 sec1. ciò significa che l'oscillazione si smorza in t e m p i dell'ordine del secondo al cessare della causa eccitatrice. come è noto, la f r e q u e n z a f0 è anche la f r e q u e n z a critica p e r u n a d a t a densità elettronica n: l'indice di rifrazione p e r u n ' o n d a elettrop k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 0 3 magnetica di f r e q u e n z a /„ diviene, per quella d a t a n , uguale a 0, m e n t r e per u n a densità superiore esso diviene immaginario. se si a m m e t t e che la densità elettronica della cromosfera e della corona diminuisca con l'altezza, dalla regione s o t t o s t a n t e alla d a t a densità a non può sfuggire quindi alcuna radiazione elettromagnetica di frequenza uguale o inferiore a f„. l a deriva di frequenza degli outbunts di tipo i i fa pensare che essi siano dovuti a oscillazioni di plasma in regioni di densità elettronica via via minore, cioè ad altezze via via maggiori. la sorgente che p e r t u r b a il p l a s m a si sposterebbe quindi verso l'alto. conoscendo la variazione di n con l'altezza, dalla deriva di frequenza df/dt si può d e d u r r e la velocità della sorgente. p o n e n d o q = r/r0, dove r è la distanza dal centro del sole e iib il raggio della fotosfera, una forma attendibile della funzione n (o) nella corona solare, è d a t a dalla espressione di baunibach-allen (alien, 19-17). n = io8 (1,55 ìj-9 + 2,99 o-1') cui" 3 . [5.13] è d a n o t a r e t u t t a v i a che tale modello si riferisce a d u n a corona n o n p e r t u r b a t a , m e n t r e è noto d i e in corrispondenza dei centri di a t t i v i t à la corona p r e s e n t a delle condensazioni, nelle quali la densità elettronica p u ò a u m e n t a r e di un f a t t o r e 10 rispetto ai valori d a t i dalla [5.13]. i n effetti maxwell e t h o m p s o n (1962), utilizzando numerose misure interferometriche dell'altezzadelle sorgenti di radioemissione sopra centri di a t t i v i t à a varie frequenze, t r o v a n o che la distribuzione della frequenza (cioè della densità n ) in altezza si accorda bene, almeno per frequenze inferiori a 500 m h z , con un modello di corona in cui a è d a t a dalla [5.13] moltiplicata per 10. con questa modifica, della [5.13], da essa e dalla [5.11] si deduce la velocità radiale vr della sorgente della perturbazione: r, • 1,9 • io-3 « f ' " ì km/sec . [5.14] 9,3 0"' + 47,8 o~u dt ' 1 1 d a i valori di dfldt d e d o t t i dalla fig. 5.38 si ricava la velocità radiale in funzione della frequenza, (o dell'altezza), come m o s t r a t o in fig. 5.40. d a essa si n o t a che la velocità è dell'ordine d i i 1000-1500 km/sec, cioè maggiore per u n f a t t o r e 3 di quella d e d o t t a in precedenza da altri a u t o r i che si b a s a v a n o sul modello di baunibacli-allen non modificato. i n u n o studio analogo a quello di maxwell e thompson, weiss (1965), basandosi sullo spettro dinamico (deriva di frequenza) di 21 outbursu di tipo i i osservati f r a il 1952 e il 1963, e usando, anziché 3 0 4 f . m a r i a n i f . . m o l i n a il modello coronale di baumbach-allen moltiplicato p e r 10, il modello d i newkirk (1961) per la densità elettronica lungo l'asse di u n raggio coronale moltiplicato p e r 2, giunge a conclusioni n o n m o l t o dissimili a quelle di maxwell e thompson m a con velocità leggermente inferiori; egli f a t u t t a v i a n o t a r e l'influenza della scelta del modello coronale sui r i s u l t a t i . a l t e z z a s u l l a ( o t o s f e r a ( i n io5 k m ) 5 0 100 150 200 f r e q u e n z a ( m h z ) f i g . 5.40 velocità r a d i a l e media delle p e r t u r b a z i o n i origine degli outbursts di t i p o i i (secondo maxwell e t h o m p s o n ) . è probabile che le oscillazioni del plasma siano p r o d o t t e dal passaggio a t t r a v e r s o la cromosfera e la corona di u n a n u v o l a di particelle, verosimilmente e l e t t r o n i e ioni positivi, in particolare p r o t o n i , emessi d u r a n t e il complesso fenomeno che h a come manifestazione ottica più e v i d e n t e il brillamento. p i ù e s a t t a m e n t e , l'eccitazione delle oscillazioni sarebbe p r o d o t t a dal f r o n t e di u n ' o n d a d ' u r t o i d r o m a g n e t i c a d o v u t a al f a t t o che la velocità della nuvola di particelle è superiore alla velocità del suono e che la regione in cui l ' e v e n t o ha luogo, cioè in prossimità di u n brillamento, è c a r a t t e r i z z a t a in generale dall'esistenza di campi magnetici. l'esistenza della seconda armonica, rivela u n a non linearità nel meccanismo delle oscillazioni. q u a n t o alla polarizzazione della radioemissione, essa sembra, p i u t t o s t o scarsa e in ogni caso senza, un c a r a t t e r e sistematico. mediante misure i n t e r f e r o m e t r i c h e (weiss e sheridan, 1962) sono s t a t e s t u d i a t e anche le dimensioni della sorgente di u n oubursts di t i p o i i ; è r i s u l t a t o che il d i a m e t r o della regione di emissione è dell'ordine dei 10 p r i m i di arco. 5.6.1.2. bursts di tipo i i i . — anch'essi, come quelli di tipo ii, p r e s e n t a n o u n a c a r a t t e r i s t i c a deriva di f r e q u e n z a , la quale però è m o l t o maggiore che per gli outbursts in precedenza descritti; si h a i n f a t t i p b k t i ' k b a z i o n i « ì k o m a o n k t i c i i k . e c c . 3 0 5 dfjdt = — fl±,5 spc.-i (wild, murray e rowe, 1954). la d u r a t a è molto più b r e v e : i bursts isolati ]>ossoiio d u r a r e una decina di secondi, ma molto spesso essi si presentano in gruppi della d u r a t a complessiva di pochi m i n u t i . tali bursts sono molto più f r e q u e n t i degli outbursts di tipo i i . benché la loro forma sia più semplice, la loro i n t e n s i t à è però dello stesso ordine e anche maggiore, con una t e m p e r a t u r a equivalente che raggiunge i io11 °k. anche i bursts di tipo i i i sono p r o b a b i l m e n t e d o v u t i a oscillazioni di p l a s m a ; col metodo p r e c e d e n t e m e n t e descritto si può dedurre la velocità della sorgente della perturbazione, che risulta compresa f r a i 60.000 e i 150.000 km/sec. ciò sembra c o n f e r m a t o da misure i n t e r f e r o m e t r i c h e (wild, sheridan e t r e n t , 1959), secondo cui la sorgente si m u o v e con velocità dell'ordine dei io5 km/sec. l'opinione, espressa per la prima v o l t a d a de j a g e r (1960), che si t r a t t i di u n fascio di elettroni di tali velocità, sembra ora generalmente a c c e t t a t a (v. ad es. s t e w a r t , 1965; t a k a k u r a , 1966). p e r q u a n t o riguarda la polarizzazione, le osservazioni degli ultimi anni (komesaroff, 1958; cohen, 1959; a k a b a n e e cohen, 1961 ; bhonsle e mcnarry, 1964; gopala r a o , 1965) hanno a c c e r t a t o che i bursts di tipo i i i sono i n d u b b i a m e n t e polarizzati, anche se il grado di polarizzazione è in generale moderato. come per il tipo i i , anche per questa radiazione le dimensioni della sorgente sono dell'ordine dei 10 primi d'arco, e sembra che esse a u m e n t i n o con l ' a u m e n t a r e della lunghezza d ' o n d a (coutrez, 1960). u n a categoria particolare di bursts di tipo i i i sembra essere costit u i t a dai eosidetti bursts a « u », così chiamati a causa della f o r m a dello s p e t t r o dinamico (maxwell e swarup, 1958). in altre parole, essi presentano all'inizio una deriva di f r e q u e n z a negativa, a p p r o s s i m a t i v a m e n t e della stessa grandezza dei normali bursts di tipo i i i ; r a g g i u n t a u n a f r e q u e n z a minima, generalmente a t t o r n o ai 100-150 mhz, la deriva d i v e n t a positiva, con u n df/dt p r a t i c a m e n t e uguale in valore assoluto alla p r i m a p a r t e dell'evento. questo ha nel suo complesso u n a durata, g e n e r a l m e n t e minore di 10 secondi. nella interpretazione d a t a per i bursts di tipo i i e i i i si t r a t t e r e b b e di una sorgente di oscillazioni di p l a s m a che d a p p r i m a sale ad elevata velocità nell'atmosfera solare per poi ridiscendere. secondo maxwell e swarup (1958) e hughes e h a r k n e s s (1963) questa sorgente seguirebbe le linee di forza di un campo magnetico f r a due regioni di polarità opposta; secondo t a k a k u r a e h a d d o c k (v. takakura, 1966) sarebbe preferibile pensare che la nuvola elettronica emessa verso l'alto non abbia in tali casi (al contrario di q u a n t o avver3 0 6 f . m a r i a n i f . . m o l i n a r e b b e nei t i p i i i i normali) velocità sufficiente a s u p e r a r e i campi m a gnetici, e dopo a v e r « piegato » le linee di forza v e r r e b b e r e s p i n t a verso il basso (fìg. 5.41). t = o n u v o l a di elettroni v c / 3 (a) 7 7 7 7 7 7 7 7 7 7 7 7 7 7 (b) pig. 5.41 modello di burst di tipo lj : in a) inizio del burst e m o t o obliquo della sorgente verso l ' a l t o ; in b) fase di m a s s i m a a l t e z z a (sec o n d o t a k a k u r a e haddoclc). 5.6.1.3. outbursts «li tipo iv. — come si è a c c e n n a t o in 1.5.4.4., nn a l t r o t i p o di radioemissione associato ai b r i l l a m e n t i è s t a t o individ u a t o d a boischot (1957) m e d i a n t e osservazioni a l l ' i n t e r f e r o m e t r o di n a n g a y sulla f r e q u e n z a di 169 m h z , e da lui è s t a t o c h i a m a t o di tipo i v . si t r a t t a di un f o r t e a u m e n t o di flusso della d u r a t a di alcune decine di m i n u t i e a n c h e di alcune ore, spesso con una polarizzazione prevalent e m e n t e circolare. questi outbursts di tipo i v sono s t a t i s t u d i a t i d a molti a u t o r i , s o p r a t t u t t o per la loro associazione con i m p o r t a n t i e v e n t i geofisici, come vedremo più a v a n t i ; si è t r o v a t o che essi si estendono in una vastissima g a m m a di frequenze, dalle onde c e n t i m e t r i c h e alle onde m e t r i c h e , e sono sostanzialmente costituiti da u n o s p e t t r o continuo di f r e q u e n z a , di i n t e n s i t à dell'ordine, e anche maggiore, di quella degli outbursts di tipo i l l a g r a n d e v a r i e t à di a s p e t t i degli eventi di radioemissione ai quali, dopo la comunicazione di boischot, è stataa t t r i b u i t a la denominazione '.< tipo i v », h a reso necessario un accordo per u n a definizione n o n t r o p p o vaga e generica. al congresso i n t e r n a z i o n a l e di k y o t o del s e t t e m b r e 1961 è s t a t a scelta la seguente definizione (wild, 1962): si assume come burst di t i p o i v u n evento di lunga d u r a t a e s p e t t r o continuo in u n a qualsiasi g a m m a di radioemissione, seguente a un brillamento. con questa definizione, voutburst di tipo i v si p r e s e n t a in genere in u n a f o r m a assai complessa. vi si possono distinguere varie compon e n t i , sia secondo il criterio della successione cronologica, sia secondo il criterio delle caratteristiche fisiche. p e r q u a n t o r i g u a r d a la successione cronologica degli eventi, si possono distinguere due p a r t i (boischot e p i c k g u t m a n n , 1962; f o k k e r , 1> e li ' l ' l i : b a z1 ( ) n i o e o m a ( ; n e t h ' i l k. e c c . 3 0 7 1963). la prima p a r t e è costituita da un largo spettro continuo, dalle onde centimetriclie alle onde metriche, della durata da 10 mimili a circa 2 ore a seconda della intensità del fenomeno. essa ha spesso un inizio piuttosto rapido ed è di frequente associata con bursts di tipo i i ; in tali casi la radiazione di tipo i v di frequenza inferiore ai 100 mhz inizia dopo gli outbursts di tipo i i , mentre alle frequenze superiori li precede (thompson e maxwell, 1962). alle frequenze superiori della, g a m m a eentimetriea, attorno ai 1500 mhz, l'evento procede in modo del t u t t o simile alle diverse frequenze, e generalmente consiste ili un aumento e successiva diminuzione di intensità, con a n d a m e n t o piuttosto calmo; alle frequenze inferiori si hanno fluttuazioni di intensità più vivaci e più diversificate da frequenze a frequenza. talvolta, dopo l'estinzione della radiazione eentimetriea e decimetrica, sulle onde metriche l'intensità permane, anche molto intensa, per alcune ore; ciò costituisce la seconda parte. la distinzione in due parti è giustificata non solo dal f a t t o clic quella che ora abbiamo chiam a t o seconda parte è costituita quasi esclusivamente di onde metriche, ma anche e s o p r a t t u t t o dalle diverse caratteristiche fisiche, come vedremo più avanti, e di direttività: mentre infatti la prima p a r t e ha una direttività molto bassa, in quanto essa può essere rivelata in eventi che hanno luogo sia, al centro che ai bordi del sole, la seconda presenta u n a direttività piuttosto spiccata, essendo osservata solo raramente in associazione con brillamenti presso il bordo. recenti studi (takakura e kai, 1961; takakura, 1962; k a k i n u m a e tanaka, 1961; tanaka e kakinuma, 1962; kundu, 1962; wild, 1962; v. più in generale fokker, 1963) hanno dimostrato clic una n e t t a distinzione fisica esiste anche f r a le varie gamme di frequenza nella, prima parte dell'evento; questa distinzione si manifesta nella posizione della, sorgente, nelle sue dimensioni e nella polarizzazione della radiazione emessa. si noti clic quest'ultima caratteristica rivela l'influenza di campi magnetici sull'evento di radioemissione. la sorgente delle onde centimetriche è in generale ad un livello piuttosto basso, attorno ai 0,05 r0 {r0 raggio della fotosfera), ed è di piccole dimensioni: da 1' essa tende ad espandersi fino a circa 4'. il grado di polarizzazione si aggira f r a il 10 e il 40%; il verso della polarizzazione corrisponde alla propagazione del raggio straordinario, assumendo come polarità del campo magnetico quella della macchia « ovest » del gruppo associato al centro di a t t i v i t à . alle onde decimetriche, la sorgente è situata a un livello più elevato, raggiungendo 0,3 r 0 al di sopra della fotosfera, e sembra fissa, o 3 0 8 i ' . m a k í a n i f . m o l i n a quasi, nella sua posizione; le sue dimensioni raggiungono alcuni p r i m i d i arco. la polarizzazione è (li verso opposto a quella della radiazione c e n t i m e t r i c a , cioè corrisponde alla propagazione del raggio ordinario, e raggiunge il 1 0 0 % . infine, alle onde metriche la sorgente, di dimensioni f r a i 6' e i 12', si sposta verso l'alto alla velocità di circa 1000 km/see, e si t r o v a nella corona f r a 0,5 e 5 jt„ al di sopra della fotosfera. la polarizzazione è in m e d i a p i u t t o s t o debole, ma s o p r a t t u t t o molto irregolare (fokker, 1963), ed è difficile stabilire se essa corrisponda alla propagazione del raggio ordinario o del raggio s t r a o r d i n a r i e . la radioemissione metrica della seconda p a r t e delvoutburst di tipo i v a p p a r e fisicamente d i s t i n t a , come già si è accennato, dalla radiazione me trica della p r i m a p a r t e ; l'elemento f o n d a m e n t a l e della distinzione è r a p p r e s e n t a t o dalla polarizzazione, molto più i n t e n s a nella seconda p a r t e che nella p r i m a . sulla sorgente e sui suoi m o t i n o n si h a n n o ancora conoscenze sufficienti; m a i n d u b b i a m e n t e qualche differenza fìsica notevole è assai probabile che vi sia, come è suggerito dalla sensibile differenza nella d i r e t t i v i t à delle due radiazioni. boischot e p i c k g u t m a n n (1962) assumono che la radioemissione della seconda p a r t e sia p r a t i c a m e n t e dello stesso tipo di quella delle cosidette « t e m p e s t e d i radioemissione », cioè di quella radiazione nella g a m m a delle onde m e t r i c h e , di notevole i n t e n s i t à e della d u r a t a di parecchie ore o anche d i giorni, che non è necessariamente associata ai b r i l l a m e n t i m a solo ai centri di a t t i v i t à ; la sorgente di tale emissione è p i u t t o s t o bassa nella corona (0,2-0,4 r 0 a 169 mhz) e h a u n d i a m e t r o di solo 3 ' . gli outburst di tipo i v non possono essere a t t r i b u i t i a oscillazioni di p l a s m a . i n n a n z i t u t t o essi si estendono ad una v a s t a g a m m a di f r e q u e n z e e h a n n o u n a d u r a t a assai più lunga della manifestazione o t t i c a del brill a m e n t o ; ora, le oscillazioni di p l a s m a devono essere l i m i t a t e a d u n a r i s t r e t t a b a n d a di f r e q u e n z e , e devono cessare, per la [5.12], contempor a n e a m e n t e alla causa eccitatrice, che è plausibile assumere di d u r a t a n o n superiore a quella del b r i l l a m e n t o . i n o l t r e , come si è d e t t o preced e n t e m e n t e , le sorgenti degli outbursts di tipo i v che e m e t t o n o anche sulle onde metriche («a 100 mhz) raggiungono spesso altezze di alcuni raggi solari al di sopra della fotosfera, alle quali altezze è molto poco probabile, a causa della r e l a t i v a m e n t e bassa densità elettronica, che si possano p r o d u r r e , secondo la [5.11], oscillazioni di f r e q u e n z a cosi e l e v a t a . p e r spiegare la radioemissione degli outbursts di tipo i v è quindi necessario f a r e ricorso, secondo il suggerimento di boischot e denisse (1957), alla ipotesi che si t r a t t i di radiazione di sincrotrone, emessa da p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 56 elettroni a v e n t i energia dell'ordine dei mev in u n campo magnetico dell'ordine degli oersted. u n a particella di carica « e di energia e > m0 c2 in u n campo magnetico h perde energia (si v e d a ad es. a l f v é n e herlofson, 1950), e m e t t e n d o onde elettromagnetiche, secondo la espressione — deldt = (2e2/3c) c r a2 0 con aio = ehim0 c (girofrequenza per la massa di quiete), e a = ejm0 e". l e onde emesse h a n n o u n a frequenza eoo a2 / = = 2,8 . 10* h a2. i n e l e t t r o n i di 5 mev in un campo magnetico di 1 oersted d a n n o quindi luogo ad u n a radioemissione di frequenza di circa 270 mhz. è evidente che u n o s p e t t r o di energia molto vasto, quale è c e r t a m e n t e quello delle particelle interessate dal fenomeno del brillamento, e campi magnetici di valore compreso in u n intervallo p i u t t o s t o ampio, come esistono nelle regioni di un centro di a t t i v i t à , possono d a r luogo ad u n o s p e t t r o cont i n u o di radioemissione compreso e n t r o u n a amplissima g a m m a di frequenze. t u t t a v i a boiscliot e p i c k g u t m a n n (1962) e denisse (1960) ritengono che la seconda p a r t e degli outbursts non si possa spiegare med i a n t e radiazione di sincrotrone; è da n o t a r e che tale supposizione si b a s a sulla ipotesi, da boiscliot e p i c k g u t m a n n a m m e s s a , che la radiazione della seconda p a r t e sia uguale a quella delle t e m p e s t e di radioemissione, come a b b i a m o accennato dianzi. secondo tali autori, i n f a t t i , e l e t t r o n i di alcuni mev all'altezza della sorgente di tale radioemissione, che a b b i a m o visto essere poco elevata, perderebbero energia molto rap i d a m e n t e p e r collisione e irraggiamento; inoltre anche le caratteristiche di polarizzazione sarebbero a sfavore della ipotesi della radiazione di di sincrotrone, poiché questa u l t i m a non d à emissione secondo il raggio ordinario, come avviene invece, secondo boiscliot e p i c k g u t m a n n nella radioemissione in esame. denisse ha a v a n z a t o l'ipotesi che si t r a t t i di emissione d o v u t a ad effetto cercnkov nel plasma. 5.6.1.4. bursts di tipo tr. — si t r a t t a di radioemissione su di una larga b a n d a di f r e q u e n z a , m a p r e v a l e n t e m e n t e inferiore ai 200 m h z . che accompagna molto spesso i bursts di tipo i i i ; in sostanza il tipo v è un « continuo » associato al tipo i i i così come il tipo i v è u n continuo associato al tipo i i (wild e coli., 1959: thompson e maxwell. 1962). 3 1 0 f . m a r i a n i f . . m o l i n a t u t t a v i a il tipo v ha u n a d u r a t a molto minore del t i p o i y (pochi minuti) anche se la sua i n t e n s i t à è dello stesso ordine, o poco minore. secondo weiss e s t e w a r t (1965) la sorgente dei bursts di t i p o v è p e r lo più stazionaria; t u t t a v i a in u n certo n u m e r o di casi si n o t a n o dei m o t i della sorgente a t t r a v e r s o il (liscio solare con velocità maggiori di 2000 km/sec. g e n e r a l m e n t e la sorgente del tipo y si t r o v a nelle vicinanze di quella del tipo i i i ad esso associata, a d i s t a n z a non m a g giore di rol3. p u ò darsi che il tipo v, come il tipo i v , sia d o v u t o a radiazione di sincrotrone; secondo il modello di weiss e s t e w a r t (1965) esso p o t r e b b e essere t u t t a v i a p r o d o t t o d a oscillazioni di p l a s m a g e n e r a t e nella corona e n t r o un v a s t o intervallo di altezze (e quindi in u n v a s t o i n t e r v a l l o di frequenze) d a elettroni a p p a r t e n e n t i alla stessa nuvola che lia p r o v o c a t o il burst di t i p o i i i , m a che sono s t a t i c a t t u r a t i dai c a m p i m a g n e t i c i del centro di a t t i v i t à e g u i d a t i lungo le loro ¡linee di f o r z a f r a p u n t i di riflessione (in modo analogo alle particelle nelle cinture di v a n allen). 5.6.1.5. a b b i a m o fin qui descritto singolarmente alcuni degli a s p e t t i della radioemissione associata ad un b r i l l a m e n t o ; agli e f f e t t i di u n a visione completa del fenomeno è t u t t a v i a necessario s t u d i a r e la concatenazione degli e v e n t i descritti e la loro successione t e m p o r a l e . l ' a n d a m e n t o tipico della radioemissione associata ad u n b r i l l a m e n t o di notevole i m p o r t a n z a è all'incirca il seguente. c o n t e m p o r a n e a m e n t e all'inizio del b r i l l a m e n t o si h a u n b r e v e m a intenso fiotto di radioemissione, che alle onde metriche è costituito da u n g r u p p o di bursts di tipo i i i , seguito d a emissione di tipo v; questo p r i m o e v e n t o h a u n a d u r a t a di pochi m i n u t i . a questa p r i m a fase segue, spesso dopo u n a p a u s a di qualche m i n u t o , u n outburst di tipo i i , della d u r a t a di alcune decine di m i n u t i , che al t e r m i n e si confonde con u n intenso « continuo » di t i p o i y il quale, come già si è d e t t o , p u ò persistere p e r v a r i e ore. questo a n d a m e n t o , che p o t r e m m o c h i a m a r e « completo », non è t u t t a v i a m o l t o f r e q u e n t e : assai spesso m a n c a n o una o p i ù delle fasi descritte. i n p a r t i c o l a r e non è d e t t o che esista la combinazione t i p o i i t i p o i y : vi possono essere outbursts di t i p o i i senza la emissione di tip o i v e viceversa. lap e r c e n t u a l e delle diverse « esclusioni » non è a n c o r a b e n p r e c i s a t a ; i valori d a t i dai vari a u t o r i dipendono evident e m e n t e dal p a r t i c o l a r e periodo preso in esame. secondo maxwell, t h o m p s o n e g a n n i r e (1959), f r a il p r i m o o t t o b r e 1956 e il 30 s e t t e m b r e 1958 solo s e t t e di sessanta outbursts di tipo i i erano seguiti (entro u n periodo di 15 m i n u t i ) d a radioemissione di t i p o i v ; secondo k u n d u p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 1 1 (1962) nel quadriennio 1957-60, su 61 outbursts di tipo i v , 35 erano associati a outbursts di tipo i i . q u e s t ' u l t i m o a u t o r e rileva inoltre delle significative differenze f r a eventi solari, in relazione alla associazione o alla m a n c a t a associazione f r a tipi i i e tipi i v : q u a n t o maggiore è l ' i m p o r t a n z a ottica del brillamento, t a n t o maggiore è la p r o b a b i l i t à che esso sia seguito da u n tipo i v con t i p o i i , e a l t r e t t a n t o si p u ò dire della i n t e n s i t à della radioemissione sulle onde centimetriche associata al tipo i v , nel senso che q u e s t ' u l t i m a è meno i n t e n s a per i tipi i v senza tipo i i che per quelli associati. r i s u l t a t i molto simili sono stati o t t e n u t i da thompson e maxwell (1962), in seguito alle osservazioni di un quinquennio, d a l l ' o t t o b r e 1956 a l l ' o t t o b r e 1961: secondo questi autori il 6 5 % degli outbursts di tipo i v è associato con outbursts di tipo i i , e l ' 8 5 % con bursts di tipo i i i . 5.6.2. radioemissione a variazione lenta. — oltre alla radioemissione concomitante ai brillamenti e di cui a b b i a m o finora p a r l a t o , e alle :< t e m p e s t e di radioemissione » associate con i centri di a t t i v i t à più che con i brillamenti (v. 5.6.1.3.), esiste anche una componente a variazione l e n t a , osservata specialmente sulle onde decimetriche. tale radiazione p r e s e n t a molto spesso il caratteristico periodo di 27 giorni; d a determinazioni di posizione d u r a n t e eclissi solari e m e d i a n t e m e t o d i interferometrici si è p o t u t o stabilire (de j a g e r , 1959; smerd, 1964) che le sorgenti di questa radioemissione corrispondono in generale alle facole di ca, e sono costituite da regioni a densità superiore al normale che si estendono r a d i a l m e n t e a p a r t i r e dalla fotosfera a t t r a v e r s o le facole cromosferiche fino a varie decine di migliaia di k m al di sopra della fotosfera, vale a dire nella corona. la t e m p e r a t u r a equivalente di questa radiazione è dell'ordine di io6 °k; a p p a r e quindi probabile d a questo f a t t o , dalla ubicazione delle sorgenti e dalla lentezza della loro evoluzione temporale, che si t r a t t i di radioemissione di origine t e r m i c a , anche se la f o r m a del suo s p e t t r o h a p o t u t o destare qualche perplessità (v. sm erd, 1964, p. 80). 5.7. p r o t o n i s o l a r i . 5.7.1. potar gap absorption (pga). — i n 3.6.4.2 si è accennato al f e n o m e n o dei blaclc-outs polari distinti dai black-outs aurorali, c a r a t t e rizzati dal f a t t o che, p u r essendo s t r e t t a m e n t e d i p e n d e n t i dai brillam e n t i solari, come le t e m p e s t e magnetiche e le aurore, il loro svolgi3 1 2 f . m a r i a n i f . . m o l i n a m e n t o a p p a r e p r a t i c a m e n t e i n d i p e n d e n t e dallo svolgimento di q u e s t e ; in particolare, essi h a n n o inizio g e n e r a l m e n t e diverse ore p r i m a delle t e m p e s t e magnetiche. come si è allora d e t t o , si t r a t t a di fenomeni di assorbimento delle radioonde da p a r t e della ionosfera; p e r la loro c a r a t t e r i s t i c a distribuzione geografica, su cui t o r n e r e m o in seguito, si è d a t o a tale e v e n t i il n o m e di polar gap absorption, p i ù b r e v e m e n t e indicato dalla sigla pca. i l f e n o m e n o consiste nella a t t e n u a z i o n e che u n ' o n d a e l e t t r o m a g n e tica subisce n e l l ' a l t r a versare s t r a t i ionosferici dove la f r e q u e n z a di collisione degli e l e t t r o n i liberi contro le molecole dei gas atmosferici è ril e v a n t e . l ' i n t e n s i t à dell'onda è d a t a dalla espressione i = ioe-ims j-5jl5j dove l'integrale è calcolato sul percorso dell'onda. i l coefficiente di assorbimento k si deduce dalla teoria magnetoionica; se la f r e q u e n z a dell'onda è sufficientemente elevata da p o t e r considerare t r a s c u r a b i l e r i s p e t t o ad essa la girofrequenza, ed inoltre è l o n t a n a dalla f r e q u e n z a critica, si h a per k l'espressione a p p r o s s i m a t a 2 n e'1 n v k = ; r [5.16] me co-fvdove n è la densità elettronica, to la pulsazione dell'onda incidente, v la f r e q u e n z a di collisione degli elettroni ed e, m sono r i s p e t t i v a m e n t e la carica e la m a s s a dell'elettrone. dalle [5.15] e [5.16] si rileva che l ' a s s o r b i m e n t o espresso in d b è a p p r o s s i m a t i v a m e n t e proporzionale al p r o d o t t o nv; valori a n o r m a l m e n t e g r a n d i dell'assorbimento si h a n n o perciò q u a n d o ad u n a determ i n a t a altezza, cioè con b u o n a approssimazione per u n a d e t e r m i n a t a f r e q u e n z a di collisione, la densità elettronica cresce al di sopra del suo valore n o r m a l e a causa di qualche jjrocesso di ionizzazione straordinario. i p c a vengono s t u d i a t i con v a r i m e t o d i . u n o di essi è il m e t o d o tradizionale del sondaggio verticale, m e d i a n t e la m i s u r a della m i n i m a f r e q u e n z a riflessa /min; la [5.16] m o s t r a i n f a t t i che l ' a s s o r b i m e n t o a u m e n t a al d i m i n u i r e della f r e q u e n z a dell'onda incidente; in p r a t i c a vengono assorbite c o m p l e t a m e n t e t u t t e le frequenze inferiori ad u n a f r e q u e n z a limite /min, funzione crescente del p r o d o t t o wv, del quale diviene u n a m i s u r a i n d i r e t t a . oltre all'inconveniente che il sondaggio verticale è di n a t u r a sua i n t e r m i t t e n t e e quindi discontinuo, questo m e t o d o pres e n t a a n c h e quello di u n a t o t a l e assenza di echi riflessi q u a n d o l'assorb i m e n t o è m o l t o f o r t e , come avviene spesso d u r a n t e i pca, nel qual caso c e r t i l i 11a z i o ni c e o m a ( i n e t i c h e . e c c . 3 1 3 esso diviene semplicemente un rivelatore q u a l i t a t i v o dell'esistenza del f e n o m e n o . l'inconveniente ora citato si presenta più r a r a m e n t e nel m e t o d o della « diffusione in a v a n t i » (foricard scatter) delle onde ad elevata frequenza (dell'ordine di parecchie decine di mhz) per t r a g i t t i di alcune migliaia di chilometri; tali onde, u s a t e nelle radiocomunicazioni, incidono o b l i q u a m e n t e sulla ionosfera e vengono diffuse e riflesse in a v a n t i d a irregolarità locali di densità elettronica, p r o d o t t e p e r la maggior p a r t e d a scie di meteoriti, cioè in un intervallo di altezza f r a gli 80 e i 100 k m . r e c e n t e m e n t e gregory (1962) ha utilizzato la retrodift'usione di onde di m e d i a f r e q u e n z a (2,3 mhz). i l m e t o d o che a p p a r e migliore è quello del riometro (relative ionospheric opacity meter). questo dispositivo è sostanzialmente mi ricevitore di r u m o r e cosmico su frequenza fissa, g e n e r a l m e n t e a t t o r n o ai 30 m h z ; l'assorbimento viene rivelato e m i s u r a t o dalla a t t e n u a z i o n e del segnale, che viene ricevuto con continuità. p e r misure e s a t t e occorre in r e a l t à calcolare il r a p p o r t o f r a l'intensità del segnale cosmico e f f e t t i v a m e n t e ricevuto e quella del segnale che si registra allo stesso tempo siderale in condizioni ionosferiehe calme. il riometro è un dispositivo che p r e s e n t a un certo grado di « selett i v i t à »: dalla [5.16] si deduce i n f a t t i clic, per una d e t e r m i n a t a densità elettronica a , l'assorbimento è massimo a quelle altezze alle quali la f r e q u e n z a di collisione v è uguale alla pulsazione ro dell'onda r i c e v u t a . p e r u n segnale di 30 m h z questa altezza è a t t o r n o ai 50 k m . 5.7.1.1. morfologia. — la continuità di registrazione del riometro h a consentito di studiare più a fondo i p c a; si e c o n s t a t a t o che essi si distinguono dai black-out aurorali non solo per la differente relazione con l ' a t t i v i t à magnetica, e aurorale (v. 3.6.4.1 e 3.6.4.2), ma anche p e r le caratteristiche della registrazione. i pca si p r e s e n t a n o con u n and a m e n t o molto più liscio e lento dei black-out aurorali (fig. 5.12), e questo è un elemento che p e r m e t t e p i u t t o s t o facilmente la individuazione dei due tipi di eventi, a differenza del tradizionale sondaggio verticale. t u t t a v i a , u s a n d o opportuni criteri suggeriti dalle caratteristiche dei p c a emerse già dai p r i m i casi c h i a r a m e n t e riconosciuti, è possibile utilizzare il gran n u m e r o di d a t i f o r n i t i dal sondaggio verticale p e r estendere al massimo il n u m e r o di eventi suscettibili di studio. la descrizione che ora d a r e m o del fenomeno è a p p u n t o d e d o t t a dagli s t u d i m e d i a n t e ionosonde n o r m a l i di h a k u r a , t a k e n o s h i t a e o t s u k i (1958), h a k u r a e goh (1959), obayashi e h a k u r a (1960), collins, j e l l y e matthew» 3 1 4 f . m a r i a n i f . . m o l i n a (1961), e m e d i a n t e riometro da reid e collins (1959) e da reid e l e i n b a c h (1959), m e n t r e baily (1957 e 1959) ha descritto un unico e v e n t o (23 febbraio 1956) m e d i a n t e l'uso dei vari metodi sopra citati, ivi compreso il jorward scatter. 1s00 1a00 1200 1000 0800 0600 0 4 0 0 1 5 0 0 1300 1100 fig. 5.42 e s e m p i di assorbimento anomalo di r u m o r e cosmico: in a) r a p presentazione di un sid associato con un b r i l l a m e n t o ; in b) di un assorbimento aurorale; in c) di un pca. w è la pot e n z a del r u m o r e cosmico in u n i t à a r b i t r a r i e (secondo reid e collins). p e ut u h i ! a zio.ni ci e ( im a ( ; \ e t i c11e, e c c . 3 1 5 i p c a iniziano in generale qualche ora dopo il b r i l l a m e n t o solare a cui sono associati, con un a u m e n t o di assorbimento p i u t t o s t o rapido; l'assorbimento raggiunge il massimo dopo alcune ore, e diminuisce grad u a l m e n t e p e r d u r a n d o per qualche giorno. a questo a n d a m e n t o generale si sovrappone una n e t t a variazione d i u r n a , con un massimo di assorbimento a mezzogiorno e un minimo a m e z z a n o t t e , e con un notevolissimo a u m e n t o al sorgere del sole a livelli ionosferici e di diminuzione al t r a m o n t o . si è già accennato in 3.6.4.2 alle linee generali di u n a spiegazione di tale variazione d i u r n a : la produzione di ionizzazione, che t e n d e ad a u m e n t a r e la densità elettronica a , sarebbe continua sia di giorno clic di n o t t e ; d u r a n t e le ore n o t t u r n e però, a diminuirebbe a causa di una ricombinazione degli elettroni liberi m e n t r e d u r a n t e il giorno la fotoionizzazione da p a r t e della radiazione solare m a n t e r r e b b e a a valori elevati. non è noto se della ricombinazione siano responsabili le molecole di o,, con formazione di ioni o^ (bailey, 1959), o se invece si abbia a che f a r e con un processo più complicato implicante la formazione di molecole di n o (herzberg, i960); ma questo problema non è di interesse i m m e d i a t o in questa sede. la caratteristica più evidente di questo tipo di assorbimento consiste nella sua distribuzione geografica; come dice il suo nome, esso è l i m i t a t o alle calotte polari. secondo i d a t i e s a m i n a t i da bailey (1959) e da sliapley e r o b e r t s (1957), d u r a n t e l ' e v e n t o successivo al grande brillamento del 23 febbraio 1956 (il primo pca registrato e riconosciuto) solo osservatori s i t u a t i a l a t i t u d i n i geomagnetiche superiori ai 60° h a n n o segnalato u n assorbimento n o t t u r n o ; dai 60° ai 70° l'assorbimento aum e n t a v a r a p i d a m e n t e di intensità, m e n t r e al di sopra dei 70° esso rim a n e v a p r a t i c a m e n t e costante. i n t r e eventi p r o d o t t i da brillamenti solari nel luglio-agosto 1958 i l u l t q v i s t (1959) t r o v a che l'assorbimento m i s u r a t o con riometro a k i r u n a (hit. geoniagn. 65,3° n) veniva regis t r a t o come black-out anche dalla ionosonda dell'osservatorio di lycksele (lat. geomagn. 62,5°) ma con d u r a t a minore, m e n t r e ad uppsala (lat. geomagn. 58,1°) non veniva osservato alcun black-out. d a l canto loro reid e leìnbach (1959), utilizzando le registrazioni dei riometri in varie stazioni artiche (alaska), h a n n o osservato nel periodo maggio 1957-luglio 1959 24 pca; la d u r a t a e la intensità di questi erano pressoché uguali a thule (lat. geomagn. 89,0° n) e a b a n w (lat. geomagn. 68,6° n), m a minori a college (64,5° n), m e n t r e a farewell, che è a soli 3,3° geomagnetici a sud di college il fenomeno era assai debole o a d d i r i t t u r a m a n c a n t e . ciò indica che i m m e d i a t a m e n t e a sud di college si t r o v a il lim i t e della zona interessata dai pca, limite che a p p a r e molto n e t t o . 3 1 6 f . m a r i a n i f . . m o l i n a in generale si può affermare che i pca iniziano alle latitudini più elevate, per poi estendersi gradualmente, anche se spesso rapidamente, a t u t t a la calotta polare fino ai 60° di latitudine geomagnetica. è imp o r t a n t e notare che l'intensità dell'assorbimento sembra indipendente dalla longitudine geomagnetica, almeno nella fase di massimo dell'evento (bailey, 1959). all'inizio della tempesta magnetica, che quasi invariabilmente segue questi eventi, l'assorbimento ha praticamente invaso t u t t a la calotta polare entro i 60° di latitudine. la perturbazione magnetica, dalla quale lo svolgimento temporale del pca nell'interno di questa calotta è indipendente (come si è già accennato), può t u t t a v i a creare condizioni nuove presso i confini della calotta polare, spostando questi verso latitudini inferiori; anche osservatori generalmente immuni da pca possono allora registrare, sia pure per breve tempo, un sensibile assorbimento. secondo reid e leinbaeh (1959), nei 24 casi sopra citati solo alcune volte l'assorbimento è stato osservato fino a 7° a sud di college, e in queste occasioni sempre dopo l'inizio di forti tempeste. talvolta invece può accadere il fenomeno inverso: h u l t q v i s t , aarons e ortner (1959) registrando a kiruna mediante riometro l'assorbimento successivo al brillamento del 7 luglio 1958, assorbimento che poco prima della tempesta magnetica iniziata il giorno 8 aveva raggiunto il valore di — 1 7 db, hanno constatato una rapida variazione di esso da — 1 7 a — 5,2 d b subito dopo l'inizio brusco. i n 3.6.4.2 si è accennato che h a k u r a e coli. (1958) avevano cons t a t a t o d u r a n t e gli eventi anteriori alle intense tempeste magnetiche del 13 settembre 1957 e 11 febbraio 1958 che l'assorbimento aveva luogo contemporaneamente nelle due calotte polari artica e a n t a r t i c a . chapman (1960) osserva la stessa cosa per il pca del 7 luglio 1958, notando t u t t a v i a che nella regione a n t a r t i c a l'inizio dell'assorbimento sembra rit a r d a t o di qualche ora rispetto alla regione artica ; egli avanza la ipotesi che si t r a t t i di un effetto dovuto alla spiccata variazione diurna di tale fenomeno, in quanto all'epoca dell'evento in esame la calotta polare nord era permanentemente illuminata dal sole, mentre quella sud era in ombra. jelly e collins (1962) da uno studio mediante ionosonde in azione nelle regioni polari durante l'anno geofísico internazionale confermano la simultaneità generica dei pca nei due emisferi, m a senza u n a assoluta contemporaneità nell'inizio, e anch'essi attribuiscono tale circostanza alla caratteristica influenza della illuminazione solare sui pca" è interessante infine notare u n ' a l t r a caratteristica morfologica dei pca. basandosi sull'andamento temporale dell'assorbimento nei sinp e r t u k b a z i 0 x 1 g e o m a l i x h t k ' i i e , e c o . 3 1 7 goli eventi, sinno (1961) classifica i pc a in due classi: i pca a inizio brusco (tipo s) e i pca ad inizio graduale (tipo g); la fig. 5.43 rapp r e s e n t a u n esempio di ciascuna classe. i pca del tipo s raggiungono il massimo dell'intensità in un t e m p o r e l a t i v a m e n t e breve (meno di due ore), m e n t r e quelli di tipo (} impiegano diverse ore. l'interesse di o l i 'wm r * r ' l 3 l l u g l i o ® 4 l u g l i o s.s.c. 5 l u g l i o i 9 s 7 ^ x z c e f i g . 5.43 e s e m p i di pca di tipo s (sopra) e di tipo g (sotto) (secondo sinno). q u e s t a classificazione è dovuto, come vedremo, alla particolare correlazione degli eventi delle due classi con la caratteristiche morfologiche dei fenomeni solari che li precedono. 5.7.1.2 altezza dell'assorbimento. — già dalla [5.16], che m o s t r a come l'assorbimento sia a p p r o s s i m a t i v a m e n t e proporzionale al p r o d o t t o av, si p u ò arguire che esso sia sensibile ad altezze r e l a t i v a m e n t e poco elevate, dove v assume valori notevoli. i t r e m e t o d i di osservazione sopra citati c o n f e r m a n o questa conclusione, senza t u t t a v i a p o t e r dare indicazioni molto precise sulle regioni atmosferiche e f f e t t i v a m e n t e interessate all'assorbimento. i l m e t o d o del sondaggio verticale, con la misura di /mm, mostra semplicemente che l'assorbimento ha luogo ad altezze inferiori alla regione e-, in analogia a q u a n t o avviene d u r a n t e un s i d (effetto dellinger), responsabile del fenomeno è d u n q u e la regione d. i l m e t o d o del formard scatter fornisce la stessa indicazione, p e r m e t t e n d o inoltre di precisare meglio u n limite superiore; il f a t t o che vengano assorbite onde elettromagnetiche che subiscono la diffusione da p a r t e delle scie dei m e t e o r i t i m o s t r a i n f a t t i che l'assorbimento ha luogo ad altezze inferiori alla zona dei meteoriti, cioè, come si è già d e t t o , g e n e r a l m e n t e al di sotto degli 80 k m . i l riometro, infine, con le sue p r o p r i e t à selettive f a ritenere 3 1 8 f . m a r i a n i f . . m o l i n a possibile, e forse probabile, che la regione assorbente si estenda fino ai 50 k m . conferme e ulteriori precisazioni delle indicazioni p r e c e d e n t i vengono f o r n i t e da osservazioni in p a r t e i n d i p e n d e n t i dai m e t o d i succitati. ad esempio, l a n d m a r k (1958), m e d i a n t e u n sondaggio v e r t i c a l e a impulsi di f r e q u e n z a fissa (2,3 mhz), ha osservato in norvegia degli echi d a altezze di 65-70 k m d u r a n t e periodi di f o r t e assorbimento d i u r n o ; ciò indica la presenza di u n a densità elettronica a n o r m a l m e n t e elevata a quei livelli. i n o l t r e in una esperienza m e d i a n t e razzi d u r a n t e u n pca di i n t e n s i t à m o d e r a t a seddon e j a c k s o n (1958) h a n n o t r o v a t o a churchill u n a densità elettronica n o t e v o l m e n t e elevata ano ad u n a altezza di circa 55 k m . 5.7.1.3 relazione con gli eventi solari. — si è già r i p e t u t a m e n t e accennato, fin d a 3.6.4.2, che i pca sono s t r e t t a m e n t e associati a brill a m e n t i solari; già d a allora, q u a n d o erano s t a t e p u b b l i c a t e solo le p r i m e ricerche in questo campo, in particolare quella di h a k u r a e goli (1959), si profilava u n a chiara relazione f r a i p c a e i b r i l l a m e n t i acc o m p a g n a t i d a outbursts di tipo i v . questa relazione è s t a t a n e t t a m e n t e c o n f e r m a t a (reid e leinbach, 1959; thompson e maxwell, 1960; k u n d u e h a d d o c k , 1960; maeda e coli., 1962; w a r w i c k e h a u r w i t z , 1962; obayashi, 1962). s e m b r a o r m a i a c c e r t a t o che almeno l ' 8 5 % dei p c a è associato con outbursts di tipo i v . i v a r i a u t o r i m e t t o n o in rilievo, agli effetti di q u e s t a associazione, la presenza neivoutburst di una particolare g a m m a di f r e q u e n z e : m e n t r e t h o m p s o n e maxwell si riferiscono alle onde m e t r i c h e , reid e leinbach e in m o d o speciale w a r w i c k e h a u r w i t z attribuiscono notevole import a n z a alle onde d e c a m e t r i c h e ; a loro v o l t a k u n d u e h a d d o c k p r o p e n d o n o a r i t e n e r e essenziali come causa dei p c a le onde centimetriche. d a t e le differenze negli intervalli di t e m p o prese in esame dai v a r i a u t o r i ' e, t a l v o l t a , nella individuazione dell'evento solare causa del pca, non è a n c o r a possibile decidere con sicurezza se ci sia u n a g a m m a di f r e q u e n z e p r e f e r i t a e quale essa sia; ciò che a p p a r e certo è l'associazione di tali f e n o m e n i con eventi solari di f o r t e i n t e n s i t à . incertezze sussistono anche r i g u a r d o alla e v e n t u a l e influenza della posizione dei b r i l l a m e n t i sul disco solare: reid e l e i n b a c h (1959) e t h o m p s o n e maxwell (1960), basandosi r i s p e t t i v a m e n t e su 18 e 10 casi sicuri, t r o v a n o u n a n e t t a p r e v a l e n z a di b r i l l a m e n t i nell'emisfero ovest del sole associati con i pca. questa prevalenza a p p a r e d i m i n u i t a nello studio di w a r w i c k e h a u r w i t z (1962) c o m p r e n d e n t e 38 casi, e gli a u t o r i p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 1 9 propendono a ritenerla non significativa. anche obayaslii dallo studio di 62 p c a conclude che non a p p a r e u n a chiara dipendenza della probabilità di occorrenza dell'evento dalla posizione del b r i l l a m e n t o sul sole. w a r w i c k e h a u n v i t z e obayashi dedicano la loro attenzione anche alla e v e n t u a l i t à che dei pca siano p r o d o t t i da eventi solari nell'emisfero invisibile del sole; essi giungono alla conclusione, n o t a n d o che quasi t u t t i pca possono venire associati con b r i l l a m e n t i e f f e t t i v a m e n t e osservati, che la p r o b a b i l i t à di casi del genere è assai scarsa. i r i t a r d i f r a gli eventi solari e l'inizio dei pca finora osservati v a n n o d a circa 20 m i n u t i a circa 50 ore. è interessante cercare se tali r i t a r d i dipendano d a particolari caratteristiche di quegli eventi. sakurai e maeda (1961) e maeda e coli. (1962) ritengono di p o t e r concludere che esistono due categorie distinte di pca riguardo al r i t a r d o di inizio rispetto a l l ' e v e n t o solare: i pca di « t i p o r a p i d o » , aventi un r i t a r d o minore di 5 ore, e quelli di « tipo lento », con u n r i t a r d o maggiore di 5 ore. i pca r a p i d i appaiono s t r e t t a m e n t e connessi con flussi molto elevati di radioemissione sui 3000 m h z (flussi superiori a 200 u n i t à io-22 w ni'hz_1), ment re quelli lenti sono associati con flussi inferiori alle 100 u n i t à ; nessuna relazione a p p a r e invece con la i n t e n s i t à della radioemissione sui 200 mhz. anche k u n d u e h a d d o c k (1960) t r o v a n o u n a s t r e t t a relazione f r a il r i t a r d o dei p c a e l ' i n t e n s i t à degli outbursts di tipo i v nella g a m m a centimetrica: i r i t a r d i m i n o r i di 5 ore sono generalmente associati con flussi maggiori di 500 u n i t à sulla i n t e r a g a m m a di lunghezza d ' o n d a di 3-30 cin, m e n t r e con flussi minori i rit a r d i sono maggiori di 5 ore. secondo maeda e coli, i due tipi di pca corrispondono ai due tipi morfologici s e g di sinno, che a b b i a m o citato in 5.7.1.1. è interessante n o t a r e che per a m b e d u e i tipi vi è u n a m a r c a t a dipendenza del r i t a r d o dalla, posizione del b r i l l a m e n t o sul sole: il r i t a r d o diminuisce s i s t e m a t i c a m e n t e a l l ' a u m e n t a r e della distanza del brillamento dal bordo est; in altre parole, vi è u n notevole e f f e t t o ovest nella distribuzione dei r i t a r d i . a differenza della «efficienza» dei b r i l l a m e n t i nel p r o d u r r e pca, la dipendenza del ritardo dalla posizione dell'evento sul sole sembra p i u t t o s t o chiara; su questo esiste accordo f r a la maggior p a r t e degli autori. come f a n n o n o t a r e mccracken e p a l m e i r a (1960), dai d a t i di reid e l e i n b a c h si deduce con notevole evidenza u n a diminuzione del t e m p o di r i t a r d o dell'inizio dei pca dagli e v e n t i solari allo spostarsi della posizione di questi u l t i m i verso ovest. è significativa la conferma di t a l e effetto nello studio di obayashi (1962); la fig. 5.44, che r i p o r t a i t e m p i di r i t a r d o dei pca d a l l ' e v e n t o solare e la loro d u r a t a , m o s t r a 3 2 0 f . m a r i a n i f . . m o l i n a con chiarezza l'esistenza dell'effetto. inoltre obayashi n o t a u n a differenza anche nel corso generale dei pca fra. quelli prodotti da brillamenti a est e quelli prodotti da brillamenti a ovest: i primi iniziano più t a r d i e hanno uno sviluppo più lento, raggiungendo un massimo di intensità dopo circa 25 ore dall'oittòifj'si, mentre gli altri, oltre ad iniziare prima, 100 5 0 4 0 3 0 .e 20 1 0 1 9 0 est 1 r —n—i—i i i i i "~l • i i "t 1i • • p 1 e i ,1 1 t i | 1 4 | i i i i i 60 3 0 30 6 0 9 0 a o v e s t f i g . 5.44 d i p e n d e n z a del t e m p o di r i t a r d o t d o p o l'outburst d a l l a l o n g i t u d i n e eliografica a. l e linee c o n t i n u e r a p p r e s e n t a n o i p c a ; i c i r c o l e t t i n e r i sono s.s.c. d i t e m p e s t e m a g n e t i c h e d i t i p o singolo, i c i r c o l e t t i c h i a r i s o n o s.s.c. di t e m p e s t e d i t i p o c o m p l e s s o , ( s e c o n d o o b a y a s h i ) . presentano anche uno sviluppo più rapido; dalla fig. 5.45 sembra anzi che la differenza nello sviluppo sia anche più evidente della differenza nel ritardo di inizio. l a fig. 5.45 m e t t e inoltre in luce u n ' a l t r a distinzione f r a i pca: essi vengono suddivisi dall'autore nelle due categorie di «singoli» e « complessi »; i « singoli » rappresentano serie isolate di eventi, costituiti da un solo brillamento e da un solo pca, con u n a sola t e m p e s t a magnetica associata, m e n t r e i « complessi » sono costituiti da più eventi p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 2 1 sovrapposti, dovuti ad u n susseguirsi piuttosto rapido di brillamenti di elevata intensità. è assai interessante, per quanto si dirà in seguito, la differenza di andamento delle due categorie, per cui la seconda è caratterizzata da un brusco aumento di intensità dei pca all'istante di inizio della prima tempesta, magnetica della serie. b r i l l a m e n t o n e l l ' e m i s f e r o e s t \ ' a s.c. b r i l l a m e n t o n e l l ' e m i s f e r o o v e s t t e m p o ( i n o r e ) fig. 5.45 variazione t e m p o r a l e m e d i a dei pca singoli (figg. a, b) e complessi (fig. e) (secondo obayaslii). t u t t a v i a , warwick e haurwitz mettono in dubbio il suddetto effetto ovest dei ritardi; essi affermano che, se mai, i ritardi minori sono associati con brillamenti vicino al meridiano centrale del sole. i l loro dubbio sembra sostanzialmente basato sul f a t t o che il coefficiente di correlazione f r a la distanza del brillamento dal bordo est e la velocità apparente dell'agente produttore dei pca è solo 0,23. di f a t t o , la fig. 5.44 m e t t e in luce la possibilità dell'esistenza dell'effetto ovest senza però che si abbia come conseguenza un elevato coefficiente di correlazione f r a le due q u a n t i t à sopra citate, che richiederebbe, come requisito aggiuntivo alla asimmetria di distribuzione, u n a relazione di linearità. i n ogni caso è u n f a t t o che, come fanno notare warwick e haurwitz, sia sufficiente u n a differenza di interpretazione nell'associazione di un pca a un brillamento in relativamente pochi casi per m u t a r e l'impressione offerta dalla fig. 5.44. gli autori mettono invece in evidenza che il ritardo sembra dipendente dalla fase del ciclo solare: i ritardi 3 2 2 f . m a r i a n i f . . m o l i n a diminuiscono dal massimo di a t t i v i t à solare verso il m i n i m o (fig. 5.46). i n r e a l t à la differenza f r a i valori m e d i è p i u t t o s t o piccola r i s p e t t o alla dispersione, t u t t a v i a la sistematicità d e l l ' a n d a m e n t o della fig. 5.46 sem| 3 0 0 e c 200 < 100 0 0 10 20 3 0 ¿0 5 0 m e s i d o p o il m a s s i m o d i a t t i v i l a ' f i g . 5.46 a n d a m e n t o del r i t a r d o at t r a b r i l l a m e n t o e inizio del p c a , in f u n zione della f a s e del cielo solare (secondo w a r w i c k e haurvvitz). b r a a b b a s t a n z a p r o b a n t e ; il coefficiente di correlazione f r a il t e m p o dal massimo della a t t i v i t à solare e il r i t a r d o dei p c a è •— 0,69, la prob a b i l i t à che t a l e valore risulti c a s u a l m e n t e è solo io 4 . 5.7.1.4 natura dell'agente produttore. — p e r q u a n t o r i g u a r d a la nat u r a d e l l ' a g e n t e ionizzante che d à origine ai p c a , si p u ò subito escludere qualsiasi f o r m a di radiazione e l e t t r o m a g n e t i c a . u n a p r i m a ragione di t a l e esclusione è c o s t i t u i t a dal r i t a r d o nell'inizio dei pca r i s p e t t o all'evento solare di c a r a t t e r e straordinario cui si a t t r i b u i s c e la causa dell'assorbimento; in linea di principio si p o t r e b b e a n c h e a m m e t t e r e che la sorgente della emissione solare responsabile del f e n o m e n o terr e s t r e fosse diversa dal b r i l l a m e n t o , m a è b e n difficile giustificare u n a t a l e ipotesi. u n ' a l t r a ragione è la p a r t i c o l a r e dipendenza dei p c a dalla l a t i t u d i n e , dipendenza che non si p u ò spiegare a l t r i m e n t i che con l'ipotesi di fasci di particelle cariche, controllate quindi dal campo geomagnetico. p e r d e t e r m i n a r e la n a t u r a di queste particelle occorre t e n e r e presente t r e f a t t o r i f o n d a m e n t a l i : la l a t i t u d i n e geomagnetica a cui a v v i e n e il fenomeno, l'altezza della regione in cui h a luogo la ionizzazione e il r i t a r d o f r a e v e n t o solare ed inizio del p c a . questi d u e u l t i m i f a t t o r i p e r m e t t o n o di t r a r r e conclusioni sulla energia delle particelle. i i i l ' k k t u r b a z i o n i o e o m a g x e t i c i i e , e c c . 3 2 3 un esame di tali elementi sulla base di quanto descritto nei numeri precedenti p e r m e t t e di concludere con certezza quasi assoluta che non si t r a t t a di elettroni. i n f a t t i , assumendo come tempo di percorrenza a t ( i n o r e ) fig. 5.47 altezza di p e n e t r a z i o n e per e l e t t r o n i , p r o t o n i e ioni iie+ in f u n z i o n e della r i g i d i t à m a g n e t i c a p, della velocità v e dell'energia cinetica t. sulla scala di d e s t r a è r i p o r t a t a la l a t i t u d i n e g e o m a g n e t i c a m i n i m a amln cui possono giungere le particelle di r i g i d i t à p-, sulla scala in alto t e m p o di t r a n s i t o at dal sole alla t e r r a su percorso rettilineo (secondo bailey). del t r a g i t t o sole-terra 30 minuti, che è il tempo minimo finora osservato, e a m m e t t e n d o che tale tragitto venga percorso in linea r e t t a , si trova c,he, se le particelle fossero elettroni, esse avrebbero u n a energia di 25 kev; dalla, fig. 5.47, che riporta il diagramma di bailey (1959), si deduce che in questo caso tali elettroni potrebbero giungere solo ad altezze poco inferiori ai 100 k m e a latitudini geomagnetiche non inferiori a 87°. anche supponendo che il tragitto dal sole alla terra non fosse rettilineo ma in qualche modo incurvato e allungato, si avrebbe una insufficiente diminuzione della latitudine di taglio; inoltre l'ipotesi di u n a deformazione del campo geomagnetico tale da permettere l'arrivo degli elettroni a latitudini inferiori è fuori causa poiché, come si è detto, 3 2 4 f . m a r i a n i f . . m o l i n a i pca avvengono prima delle p e r t u r b a z i o n i m a g n e t i c h e associate ai b r i l l a m e n t i solari. se invece le particelle in questione fossero protoni, p e r un t e m p o di percorrenza di 30 m i n u t i esse a v r e b b e r o u n a energia di circa 50 mev, giungerebbero fino ad u n a altezza di poco più di 40 k m e a v r e b b e r o u n a l a t i t u d i n e di taglio di 65°-70°; queste sono p r o p r i o le condizioni che r i s u l t a n o s p e r i m e n t a l m e n t e dallo studio dei pca. possiamo quindi concludere che il f e n o m e n o dei p c a è r i v e l a t o r e della emissione da p a r t e del sole, d u r a n t e i b r i l l a m e n t i a c c o m p a g n a t i d a f o r t i outbursts di tipo i y , di p r o t o n i di energia dell'ordine di 10-100 mev. d a l l ' i n t e n s i t à dell'assorbimento ionosférico si p u ò v a l u t a r e l'ordine di grandezza del flusso dei p r o t o n i incidenti, che a m m o n t e r e b b e (reid e collins, 1959) a circa io3 cm-2sec_1, equivalente ad u n a densità del fascio di circa 2 i o 8 cirr 3 . 5.7.2 osservazioni dirette di protoni solari. — l e precedenti deduzioni sulla n a t u r a delle particelle c a u s a n t i i p c a sono s t a t e c o n f e r m a t e d a osservazioni d i r e t t e . di queste osservazioni, eseguite a mezzo di palloni, di razzi e di satelliti in concomitanza ai pca, ne esiste o r m a i u n n u m e r o notevole: sono p i ù di u n a quindicina gli e v e n t i di questo t i p o r i l e v a t i d i r e t t a m e n t e n e l l ' a l t a a t m o s f e r a e nello spazio circostante alla t e r r a . l a bibliografìa su tali osservazioni è assai a b b o n d a n t e , p e r cui r i n u n c i a m o qui a r i p o r t a r l a p e r esteso, r i m a n d a n d o ad alcuni lavori sull'argomento, in p a r t i c o l a r e a quello di p i e p e r e coli. (1962) e di a k a s o f u e coli. (1963) e inoltre alla rassegna di bailey (1964). cercheremo qui invece di riass u m e r e le caratteristiche generali della radiazione in esame d e d o t t e d a t u t t e le n u m e r o s e osservazioni eseguite. i n t u t t i i casi osservati tale radiazione s e m b r a costituita p r e v a l e n t e m e n t e d a p r o t o n i ; come accenneremo p i ù a v a n t i , u n a certa f r a z i o n e è f o r m a t a t u t t a v i a da nuclei p i ù p e s a n t i , in p a r t i c o l a r e d a particelle a. i l flusso di particelle v a r i a n o t e v o l m e n t e da caso a caso: quello finora rivelato dai palloni, a q u o t a dell'ordine dei 30 k m , h a oscillato d a circa 0,8 particelle c m ^ s e c ^ s t e i 1 pei' energie superiori a 100 mev (evento del 22 agosto 1958: anderson e coli. 1959), a circa 240 p a r t i c e l l e c m ^ s e c 1 ster1 p e r energie superiori a 88 mev (evento del 15 luglio 1959: winckler, b h a v s a r e p e t e r s o n (1961)). le misure su satelliti h a n n o r i v e l a t o flussi di v a r i ordini di grandezza maggiori: il 13 luglio 1961 il satellite i n j u n 1 h a m i s u r a t o u n flusso di 33000 p r o t o n i cm-2sec-1ster-1 p e r energie t r a 1 e 15 mev e u n b e n scarso flusso di p r o t o n i di energia superiore a 40 mev (il flusso omnidirezionale di questi era di circa 12 cm-2sec-1), p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 2 5 m e n t r e il 18 luglio successivo, di f r o n t e a un flusso unidirezionale di 1000 cm-ssec^stei1 per p r o t o n i f r a 1 e 15 mev veniva rivelato u n flusso omnidirezionale di 900 cm-2sec_1 per p r o t o n i di energia superiore a 40 mev (pieper e coli., 1902). d a i valori indicativi r i p o r t a t i risulta clic i flussi osservati sono di v a r i ordini di grandezza superiori a quello di circa 0,1 protoni c m ' s e c ^ s t e r 1 della radiazione cosmica g a l a t t i c a . p e r u n confronto f r a i v a r i casi occorrerebbe invece t e n e r conto sia della l a t i t u d i n e di osservazione, sia dello s p e t t r o energetico della radiazione incidente. 5.7.2.1 spettro energetico e di rigidità. — i n generale si è cercato di r a p p r e s e n t a r e lo spettro differenziale m e d i a n t e u n a espressione della f o r m a dw = lct~y dt, oppure, m a più r a r a m e n t e , dn = h'p-" dp, dove t e p sono r i s p e t t i v a m e n t e l'energia cinetica e la rigidità m a gnetica delle particelle. l'esponente y dello spettro di energia è risultato assai variabile d a caso a caso, e, come vedremo più a v a n t i , variabile col t e m p o nei singoli eventi. esso oscilla in generale da 6 a 3; nell'evento del 4 maggio 1960 esso h a r a p p r e s e n t a t o il valore eccezionalmente basso di 1 (biswas e freier, 1961). a p a r t e questo valore eccezionale, lo s p e t t r o dei protoni solari r i s u l t a quindi in generale n e t t a m e n t e più ripido sia di quello dei p r o t o n i della fascia i n t e r n a di v a n alien (v. 5.3.3.1) sia di quello della radiazione cosmica galattica (v. 4.1.2), cioè è maggiore il r a p p o r t o f r a il n u m e r o delle particelle di bassa energia e quelle di energia e l e v a t a . i n generale, d a t o l'intervallo p i u t t o s t o r i s t r e t t o di energie che in ogni singola, osservazione p u ò essere esaminato, la rappresentazione secondo s p e t t r i di p o t e n z a si accorda sempre a b b a s t a n z a bene con le osservazioni. ma se. l'intervallo di energia e s a m i n a t o si estende, o se si vogliono accordare p e r uno stesso e v e n t o osservazioni eseguite ad altezze n e t t a m e n t e differenti e quindi interessanti intervalli di energia m o l t o diversa (osservazioni con palloni e osservazioni con satelliti), l'uso dello s p e t t r o di p o t e n z a richiede che l'esponente y (o n) sia u n a funzione dell'energia (o della rigidità); in generale y e n diminuiscono al diminuire dell'energia e della rigidità. 3 2 6 f . m a r i a n i f . . m o l i n a p e r s t u d i a r e p i ù a fondo la f o r m a dello s p e t t r o , p r e i e r e w e b b e r (1963) h a n n o e s a m i n a t o sei eventi i m p o r t a n t i di p r o t o n i solari, ricav a n d o i d a t i sperimentali dalle caratteristiche dei corrispondenti pca d e d o t t e m e d i a n t e riometri p e r energie f r a 1 e 50 mev, d a i r i s u l t a t i delle osservazioni m e d i a n t e palloni p e r l ' i n t e r v a l l o 70-500 mev, e dalle registrazioni dei n e u t r o n i al suolo per energie superiori a 500 mev. secondo gli a u t o r i lo s p e t t r o integrale dei p r o t o n i solari è r a p p r e s e n t a b i l e assai bene, p i u t t o s t o che come s p e t t r o di p o t e n z a , come s p e t t r o esponenziale di rigidità della f o r m a _ _p_ n = n0 e po dove p0 è u n a e f f e t t i v a costante (e non più u n a f u n z i o n e di p), che det e r m i n a la ripidità dello spettro. s o t t o q u e s t a f o r m a , la variabile r i p i d i t à degli s p e t t r i da caso a caso viene m a n i f e s t a t a da u n a notevole v a r i a b i l i t à di p0 \ nei sei casi e s a m i n a t i d a p r e i e r e w e b b e r p0 oscilla t r a il valore di 60 mv (spettro m o l t o ripido) osservato il 12 luglio 1961 e il valore d i 3 7 5 m v (spettro molto p i a t t o ) del 15 n o v e m b r e 1960. 5.7.2.2 variazioni temporali. — u n a i m p o r t a n t e c a r a t t e r i s t i c a generale d e l l ' a n d a m e n t o t e m p o r a l e di u n evento di p r o t o n i solari (che d ' o r a in poi indicheremo con s p e , solar protons event), è la sua d u r a t a , che r i s u l t a sempre molto più lunga di qualsiasi manifestazione osservabile d e l l ' e v e n t o solare r i t e n u t o come origine. ciò del r e s t o era s t a t o messo in luce dallo studio dei pca. l'esempio più notevole di e v e n t o di l u n g a d u r a t a è quello r e g i s t r a t o d a anderson e e n e m a r k (1960) nel luglio 1959 m e d i a n t e lanci di palloni a resolute b a y , a 150 k m dal polo m a g n e t i c o n o r d (inclinazione m a g n e t i c a di 90°). nel c a m p o di energia esplorato (fra 85 e 400 mev), il flusso di p r o t o n i associato al b r i l l a m e n t o di i m p o r t a n z a 3 + del 16 luglio era ancora registrabile il giorno 27 ; si n o t i p e r di più che, secondo u n a i n t e r p r e t a z i o n e di bailey (1964) dei p c a verificatisi in quel periodo di f o r t e a t t i v i t à solare car a t t e r i z z a t o d a t r e intensi b r i l l a m e n t i il 10, 14 e 16 luglio, i p r o t o n i r e g i s t r a t i d a anderson e e n e m a r k d o v e v a n o considerarsi associati al b r i l l a m e n t o del 10. l a l u n g a d u r a t a degli e v e n t i indica che la propagazione dei p r o t o n i f r a il sole e la t e r r a è u n f e n o m e n o assai complesso, la cui c a r a t t e r i s t i c a p i ù i m p o r t a n t e è la accumulazione delle particelle in qualche regione dello spazio e x t r a t e r r e s t r e ; ciò è e v i d e n t e m e n t e connesso con lo s t a t o p k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 2 7 fisico dello spazio interplanetario. di conseguenza è utile, agli effetti delle conoscenza di tali condizioni fisiche, studiare il decadimento temporale del flusso protonico incidente sulla terra, ossia lo svuotamento delle regioni di accumulazione. risultati su questo punto si hanno, in misure dirette di proioni solari, per gli eventi del luglio 1959 (anderson e eneinark, 1960), dell'aprile 1960 (arnoldy e coli. i960) e del 4 maggio 1960 (winclder, masley e may, 1961). p e r l'evento del luglio 1959 anderson e e n e m a r k trovano un decadimento temporale dell'intensità protonica fra 85 e 400 mev che si accorda assai bene con la legge contando i tempi a p a r t i r e da 16 ore dopo il brillamento del 16 luglio, bell'aprile 1960 si sono p o t u t e confrontare le intensità dei protoni solari registrate contemporaneamente da un pallone a minneapolis (lat. geomagn. 55° ít) e dalla sonda pioneer y a 5.000.000 km dalla terra. in ambedue i casi il decadimento avveniva secondo la legge i~2. la stessa legge di decadimento è s t a t a dedotta da winclder, masley e may per l'evento del 4 maggio 1960, osservato su pallone a minneapolis; t u t t a v i a in questo caso l'andamento poteva anche essere rappresentato nella forma i-1-5. i n realtà la rappresentazione del decadimento temporale dei protoni secondo u n a leggi1 del tipo ora citato ha senso solo se essa, viene riferita a protoni di una determinata energia, o, in altre parole, ad una r i s t r e t t a banda dello spettro energetico dei protoni incidenti. le numerose misure effettuate hanno infatti portato a concludere che in generale il decadimento delle particelle più energetiche è più rapido di quelle di energia minore, ossia lo spettro energetico dei protoni diventa col tempo più ripido. era gli spe importanti, solo quello del 10-12 maggio 1959 presenta una diminuzione dell'esponente y nello spettro differenziale di potenza da 5 a 2 in un intervallo di 11 ore nel corso dell'evento (winclder e bhavsar, 1960); ma tale andamento è condizionato, come si vedrà, da altre cause che si sovrappongono al semplice decadimento del flusso dei protoni provenienti dallo spazio. particolarmente indicativi sull'andamento temporale del flusso di protoni in funzione dell'energia fra 2 mev e 600 mev sono i risultali o t t e n u t i dal satellite explorer 12 nel settembre 1961 (bryant e coli., 1962). i n u n a serie di osservazioni a pochi minuti di distanza l'una dall'altra, a p a r t i r e da circa 25 minuti dopo il massimo del brillamento di importanza 3 del 28 settembre, è stato notato un rapido aumento del flusso di protoni; ma mentre inizialmente i protoni di energia attorno ai 600 mev presentavano una intensità maggiore di quella dei protoni meno energetici, nel tempo di un'ora il flusso di questi ultimi 3 2 8 f. m a r i a n i f . . m o l i n a era diventato n e t t a m e n t e superiore. le successive osservazioni, eseguite ad intervalli di tempo maggiori, hanno chiaramente m o s t r a t o l'inizio del decadimento dei protoni di energia superiore ai 100 mev quando il flusso di quelli di 3-10 mev era ancora in aumento; tale aumento si è p r o t r a t t o fln verso la fine del giorno 30. rappresentando lo spettro energetico con una espressione di potenza, si ottiene nell'intervallo da 3 a 600 mev per lo spettro differenziale y = 2 il giorno 29 e y = 3 il giorno 30. si noti che la novità nelle osservazioni con satelliti rispetto a quelle con palloni è costituita dalla possibilità di ottenere informazioni sulle energie molto basse, al di sotto dei 10 mev. se si confrontano i risultati sull'andamento temporale del flusso protonico durante gli s p e o t t e n u t i mediante satelliti e mediante palloni a latitudini diverse, si notano delle evidenti differenze. i lanci di palloni a latitudini molto elevate, quale quella di f t . churchill (lat. geomagn. 68,7° ij") hanno rivelato l'arrivo di protoni poco tempo dopo il brillamento solare associato all'evento; esempio tipico è quello del 22 agosto 1958 (anderson e coli., 1959), in cui il flusso di protoni si è f a t t o notevole circa u n ' o r a dopo il brillamento, praticamente in coincidenza con il pca e in assenza di perturbazioni magnetiche e aurorali. come si deduce dalle osservazioni di anderson e enemarlc (1960) per uno degli eventi del luglio 1959, osservazioni e f f e t t u a t e con palloni la-nciati a resolute bay, il decadimento del flusso dopo l'intensità massima avviene assai regolarmente, in maniera del t u t t o indipendente dalla t e m p e s t a magnetica successiva al brillamento. d ' a l t r a p a r t e nei lanci effettuati a minneapolis (winckler, 1960; winckler, b h a v s a r e peterson, 1961) intensi flussi di protoni solari si sono osservati solo d u r a n t e la fase principale delle tempeste magnetiche; un f a t t o assai interessante in questi casi è che venivano rivelati protoni di energia dell'ordine di 90 mev, cioè ben al di sotto della, normale energia di taglio per minneapolis, di circa 350 mev. risulta quindi evidente che d u r a n t e la perturbazione magnetica l ' a n d a m e n t o delle linee di forza a t t o r n o alla terra viene def o r m a t o in misura assai sensibile; ciò del resto è in accordo con quanto già t r o v a t o per i pca (v. 5.7.1.1), i quali si estendono verso le basse latitudini al momento della tempesta magnetica associata all'evento solare che h a dato origine al pca stesso. alla luce di questa constatazione si può spiegare la diminuzione dell'esponente nello spettro di potenza dei protoni nell'evento del 12 maggio 1959 osservato con palloni da winckler e bhavsar (1960). l'osservazione veniva e f f e t t u a t a a minneapolis, dove i protoni risultarono prep k k t u kb a z i o n i g e o m a g n e t i c h e . e c c . 3 2 9 senti solo d u r a n t e la tempesta magnetica; riprendendo il campo magnetico gradualmente verso la fine della tempesta la sua configurazione normale e ricrescendo le energie di taglio, ci si a t t e n d e effettivamente che il flusso di protoni di energia minore diminuisca più rapidamente che per quelli di energia più elevata, riducendo con ciò la ripidità dello spettro. i n realtà il taglio geomagnetico per protoni solari è risultato anche in condizioni magneticamente calme minore1 di quello calcolato sia in base alla teoria, di stòrmer per un campo di dipolo sia in base ad una migliore approssimazione del campo geomagnetico; ad esempio pieper e coli. (1962) mediante strumenti a bordo del satellite i n j u n i osservano protoni da 1 a 15 mev 18 ore prima dell'inizio brusco della tempesta magnetica del 13 luglio 1961 a una latitudine geomagnetica di 63°, dove secondo la teoria di stòrmer potrebbero giungere solo protoni di energia superiore a circa 200 mev. le osservazioni con satelliti hanno rivelato u n ' a l t r a differenza nell ' a n d a m e n t o temporale del flusso di protoni, rispetto a quello ottenuto con i palloni ad elevate latitudini. si è poco fa ricordata l'osservazione di anderson e e n e m a r k (1960) sulla indipendenza, dalle perturbazioni magnetiche dell'intensità dei protoni misurati con palloni a resolute b a y ; tale risultato vale per la banda di energia osservabile, cioè fra 85 e 400 mev. nel campo delle energie minori (da 1 a 15 mev), che i satelliti possono rivelare, tale indipendenza non sussiste più. i l satellite i n j u n i h a registrato durante la fase iniziale della, tempesta magnetica del 13 luglio 1961 un fortissimo aumento nel flusso dei protoni entro quell'intervallo di energia; un rapido decadimento dell'intensità ha avuto luogo all'incirca durante la transizione dalla fase iniziale alla fase principale della tempesta. una diminuzione analoga, anche se nel complesso l'intensità del fenomeno era minore, è stata osservata f r a la fase iniziale e quella principale della tempesta del 20 luglio. anche il satellite explorer x i i h a registrato un forte aumento della intensità dei protoni di energia inferiore a 15 mev in corrispondenza all'inizio della tempesta del 30 settembre 1961. sulla interpretazione di tali risultati torneremo in seguito in 5.8. u n altro aspetto importante delle variazioni temporali dei protoni solari è costituito dalla isotropia delle particelle incidenti sulla terra. già descrivendo la morfologia dei pca abbiamo visto che essi si distribuiscono con grande uniformità su tutta, la calotta polare, con u n a intensità che appare indipendente dalla longitudine geomagnetica; ciò è u n chiaro indice della isotropia delle direzioni di incidenza, delle parti3 3 0 m a h i a x i f . m o t . i x a celle. ma le osservazioni di e x p l o r e r x i i h a n n o f o r n i t o informazioni d i r e t t e sulla formazione della isotropia d u r a n t e l ' e v e n t o del 28 s e t t e m b r e 1961. nell'intervallo di energie f r a 100 e 600 mev nelle primissime fasi dello s p e le particelle p r e s e n t a v a n o u n a m a r c a t a anisotropia, che è anelata g r a d u a l m e n t e d i m i n u e n d o per scomparire c o m p l e t a m e n t e dopo circa u n ' o r a e mezza dal b r i l l a m e n t o , p r i m a che l ' i n t e n s i t à del flusso raggiungesse il suo massimo. nel campo delle basse energie, f r a 2 e 15 mev, l'anisotropia si è p r o t r a t t a per circa sette ore, dopo il brill a m e n t o , e, a differenza dei p r o t o n i più veloci, anche dopo a v e r raggiunto il massimo di i n t e n s i t à . le direzioni preferenziali di provenienza delle particelle nei due intervalli di energia n o n a p p a r i v a n o coincidenti. 5.7.3 particelle a e nuclei più penanti. — p e r u n a decina di eventi sono s t a t i s t u d i a t i i n t e n s i t à e s p e t t r o di eventuali nuclei diversi dai p r o t o n i presenti insieme a questi ultimi (fichtel e guss, 1962; biswas, f i c h t e l e guss, 1962; biswas, f r e i e r e stein, 1962; n e y e stein, 1962; biswas, 1962; f r e i e r , 1962 e 1963; biswas e coli., 1963; f r e i e r e w e b b e r , 1963). tale studio è s t a t o ('seguito m e d i a n t e emulsioni nucleari lanciate con palloni e con razzi e successivamente r e c u p e r a t e . i d a t i s p e r i m e n t a l i raccolti n o n sono in r e a l t à molto n u m e r o s i e non p e r m e t t o n o a n c o r a u n a descrizione sistematica delle c a r a t t e r i s t i c h e dei nuclei p e s a n t i d u r a n t e gli s p e . t u t t a v i a è a c c e r t a t a la presenza d u r a n t e m o l t i di questi e v e n t i di particelle a, spesso con flussi notevoli. ii r a p p o r t o m i s u r a t o f r a l ' i n t e n s i t à dei p r o t o n i e quella delle p a r ticelle a v a r i a evidentemente, a seconda, del p a r a m e t r o che si sceglie come r i f e r i m e n t o pei' il c o n f r o n t o : si possono i n f a t t i p a r a g o n a r e i flussi di p r o t o n i e particelle a nello stesso intervallo di energia p e r nucleone (il che equivale allo stesso intervallo di velocità), o p p u r e nello stesso intervallo di rigidità. è tuttavia, s p e r i m e n t a l m e n t e stabilito che il r a p p o r t o protoni/particelle a è e s t r e m a m e n t e variabile da e v e n t o ad e v e n t o ; m e n t r e in qualche evento (come in quelli del n o v . 1960) l ' i n t e n s i t à delle particelle a h a p r e s e n t a t o , almeno p e r certi intervalli di t e m p o , valori p r a t i c a m e n t e uguali a quelli della i n t e n s i t à protonica, in a l t r i essa scende a qualche u n i t à p e r cento. yi sono inoltre indicazioni di u n d e c a d i m e n t o col t e m p o più r a p i d o per le particelle a che per i p r o t o n i . secondo f r e i e r e w e b b e r (1963) anche le particelle a p r e s e n t a n o , come i p r o t o n i , u n o s p e t t r o esponenziale di rigidità a v e n t e la stessa c o s t a n t e p0 dello s p e t t r o dei protoni. i n qualche evento (luglio 1959, s e t t e m b r e 1960, n o v e m b r e 1960) sono s t a t i s t u d i a t i anche i nuclei di n u m e r o atomico maggiore, ragp e r t u r b a z i o n i « f . o m a g s e t i c h e . e c c . 331 g r a p p a t i nelle classi di nuclei leggeri (be, b), nuclei medi (c, n, o) e nuclei pesanti (z > 10). il rapporto fra l'intensità dei nuclei medi e quella dei protoni è in generale dell'ordine di io"3. secondo fielitel e guss (1962) e biswas, fichtel e guss (1962) che hanno studiato accurat a m e n t e gli eventi del novembre i960, la composizione dei fasci corpuscolari, ossia il rapporto fra le 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masley a . j . , may t . c . , journ. geoph. bes. 6 6 , 1 0 2 3 ( 1 9 6 1 ) . layout 6 1 annals of geophysics, 62, 2, vo219, 2019; doi: 10.4401/ag-7791 “assessment of the dual-band method by an indoor analog experiment„ leonie pick1,2, valerio lombardo3, klemen zakšek*,1,4 (1) university of hamburg, hamburg, germany; (2) gfz german research centre for geosciences, jacobs university, bremen, germany (3) istituto nazionale di geofisica e vulcanologia, roma, italy (4) university of ljubljana, ljubljana, slovenia 1. introduction the discipline of satellite thermal remote sensing provides physical insight into the processes governing volcanic activity and has become a valuable tool of vol− canology since the pioneering paper on the topic was published [gawarecki et al., 1965]. the volcanological community is confronted with the problem, that none of the currently operational satellite sensors have been designed primarily for volcanological purposes. conse− quently, the available data, mostly from meteorological satellites, is not ideal for volcano surveillance in two ba− sic regards. first, the characteristics of the spectrora− diometers themselves, essentially the three resolutions in the spatial, spectral, and radiometric domain, are not optimally balanced. secondly, the satellite orbits are not well adapted, affecting the satellite volcano viewing ge− ometry and temporal resolution. apart from these data limitations, there is the general difficulty that the radi− ance measured by the sensor differs from the radiance emitted by the ground heat source, mainly due to surface emissivity, atmospheric and geometric influences. to derive higher−level estimates of volcanic activity, such as lava discharge rate [harris et al., 2005; calvari article history receveid may 31, 2018; accepted november 17, 2018. subject classification: thermal anomaly; lava flow; remote sensing; thermal cameras; experiment. abstract the evaluation of infrared satellite images over active lava flows assists the identification of potentially threatened areas and thereby the overall lava inundation hazard assessment. the estimation of the lava flow’s size and temperature is not trivial as the lava occupies only a small fraction (< 1 %) of a typically resolved target pixel (e.g., from landsat 7−etm+; eos−modis). conventionally, this is solved by processing observations in at least two separate infrared spectral wavebands. we investigate the resolution limits of the dual−band (db) method by means of a uniquely designed indoor analog experiment. a volcanic thermal anomaly is simulated by an electrical heating al− loy of 0.5 mm diameter installed on a plywood panel. satellite observations are simulated by two thermographic cameras with wavebands comparable to those available from satellite data. these range from the short−wave infrared (swir) over the mid−wave infrared (mir) to the thermal infrared (tir). in the conducted experiment, the hotspot’s pixel fraction (p) was successively reduced by increasing the cam− era−to−target distance from 2 m (p = 41.7 %) to 38 m (p = 2.2 %). we carried out three experiments with different wire temperatures and compare different db setups. in the case of the hottest wire (1019 k), the standard method gives relative deviations between the observed and theoretical hotspot area fractions below 16 % for about one third of the cases (i.e., distances). et al., 2010], the flow’s area and temperature have to be estimated first, usually using the so called dual−band (db) method [dozier, 1981]. this method allows thermal unmixing of a pixel composed of two temperature com− ponents [crisp and baloga, 1990; dozier, 1981; rothery et al., 1988]: 1) a hot component with temperature th covering the pixel fraction p, and 2) a background component with temperature tb covering the pixel frac− tion 1–p. with sensors of coarse spatial resolution (e.g., moderate resolution imaging spectroradiometer – modis, advanced very high resolution radiometer – avhrr) this is an unrealistic assumption, as each pixel might consist of several further components with dif− ferent thermal properties, which renders an error of more than 100 k in th possible [oppenheimer, 1993; mouginis−mark et al., 1994; wright et al., 2000; lom− bardo and buongiorno, 2006; vaughan et al., 2010, za− kšek et al., 2013]. the influence of the interplay between all these components on the results of the db method is still difficult to estimate. here, we focus on the influence of the fractional area of an anomaly within a pixel (p). to systematically examine this effect, it needs to be iso− lated from the other influences. for this purpose, the satellites’ measurement situation is simulated in an ex− periment, using a steady heat source, which is observed by thermal cameras in spectra commonly found for satellite sensors. 2. experiments 2.1 the lava simulator the following list summarizes requirements for the simulated heat source: − target should be a two component mixed pixel with a hot component of at least th = 600 k compara− ble to crusted lava [lombardo et al., 2012] and a background component of significantly lower tem− perature tb ≈ 330 k. − target should be mobile to allow different camera− to−target distances and thereby different pixel sizes. − hot component’s area percentage (p) should ulti− mately be comparable to those from satellite mea− surements, being ≈ 1 %. − temperatures should be constant during the mea− surement campaign. − influences of the target’s surroundings should be minimized. the “lava simulator’s” heat source is a single heating wire alloy (figure 1). electricity is converted into heat through joule heating, with the heating power depend− ing on the electrical current and the resistivity. consid− ering different materials, we decided to use an alloy with a 0.5 mm diameter made of isachrom 60 (nicr6015) be− cause of its high specific electrical resistivity and oxida− tion resistance. a laboratory power supply unit is used as a power source with a total maximal output of 400 w. 2.2 experimental setup the pixel fraction p occupied by the hot wire decreases with increasing distance between the cameras and the lava simulator. the simulator is seated on a trolley, that is pushed away from the cameras in a 40 m long hallway. the distance between the cameras and the lava simula− tor was increased by 2 m at each step, with the closest measurement at a distance of 2 m (p = 41.7 %) and the furthest at 38 m (p = 2.2 %). at each step, we recorded mean temperatures of 10 s lasting observations with two cameras, producing a swir, mir and tir image. the ex− periment was repeated three times, each time with a dif− ferent temperature of the alloy, i.e., amperage of the power supply. we call the runs low (small th), med (medium th) and hot (large th). 2.3 cameras we used two different cameras of the company in− fratec. variocam (vc) operates mainly in the thermal in− frared spectrum (tir; centered at 10.3 μm). imageir 8300 (ir) is equipped with two filters covering the short−wave (swir; centered at 2.36 μm) and mid−wave infrared (mir; centered at 3.90 μm) spectra. in both cameras, the detector elements are arranged in a focal plane array, including 640×480 (vc) or 640×512 (ir) single detectors. vc has a lens with 30 mm focal length (angle resolution 0.8 mrad) and ir has a lens with 25 mm focal length (an− gle resolution 0.6 mrad). both cameras provide observa− tions with 1 % accuracy. the cameras are fixed on a tri− pod by custom−made housing in order to keep the viewing directions parallel. 3. methods 3.1 swir emissivity estimation the temperature observations require an accurate de− termination of the emissivities of the emitting surfaces. thus, in a darkroom environment, we collected spectra from the operating lava simulator using the field− pick et al. 2 portable spectroradiometer asd fieldspec pro (fs). fs is composed of three spectrometers which measure the spectral radiation energy in different portions of the wavelength spectrum from 0.35 to 2.5 μm with a spec− tral resolution of 2–12 nm. the electromagnetic radiation emitted from the surface is collected by the instrument entry optics and projected into a holographic diffraction grating. fs is provided with a bare fiber optic and a conical field of view (fov) of 35 . however, the bare fiber fov has been further reduced us− ing alternative fore optic tubes of 1 and 3 . given the temperature heterogeneities observed by the lava simulator, a new algorithm has been developed to take into account the radiance contributions from surfaces radiating at different temperatures. we assume a two thermal components model for the lava simula− tor. the radiance r detected by fs at wavelength λ is the average of the radiances emitted by the hot (at th) and cold (at tb) surfaces, weighted by their corresponding pixel fractions: r(ti, λ) = ε(λ) [p⋅r(th, λ) + (1 – p)⋅r(tb, λ)] ( 1 ) r is the radiance of a blackbody at temperature t and wavelength λ according to planck’s law, ti is the inte− grated temperature and ε(λ) is the spectral emissivity. the draping algorithm allows the simultaneous estimation of th, tb, p, and ε(λ) through the following steps: − identification of the maximum measured radiance rmax = max[r(λ)] across all wavelengths. − creation of a lookup table of spectra derived from equation 1. only spectra respecting the condition r(ti, λ) ≥ rmax are considered. this guarantees that ε(λ) ≤ 1. 3 experimental assessment of dual-band method figure 1. lava simulator setup. a: front view. 1) 50 cm × 50 cm plywood panel, 2) high emissivity (0.95) area colored with senotherm spray, 3) isachrom 60 alloy, 4) tensioning mechanism, 5) weights to support alloy tensioning, 6) revolvable and tiltable base, 7) electrical supply unit, 8) trolley. b: side view. c: close−up of heated alloy. pick et al. 4 − calculation of the spearman rank correlation coef− ficient (ρ) between the measured radiances and each spectrum of the lookup table. − maximum ρ identifies the spectrum r(ti, λ) derived from equation 1 that best matches the measured ra− diances r(λ) and therefore fixes the parameter triplet th, tb, p of the underlying two thermal components model. − spectral emissivity: ε(λ) = r(λ) / r(ti, λ) (2) figure 2 shows the emissivity spectrum derived from the radiance measurements. the emissivity is seen to be 0.93 at the central wavelength of ir’s swir filter (dashed lines). the iterative method (section 3.2) gives a very sim− ilar value of 0.95. we keep the latter value to account for the fact that the draping algorithm determines a lower bound of the swir emissivity as well as ensuring con− sistency with the emissivity calculations in mir and tir. 3.2 mir and tir emissivity calculations as we had emissivity observations available only for the swir spectrum, we had to determine the emissivity of the metal alloy in the mir and tir differently. we car− ried out an iterative procedure based on the camera ob− servations from all distances. first, we estimated the theoretical anomalous pixel fraction ptheo as a function of the distance between the lava simulator and the cameras, see figure 3a. then, we performed various runs of the db method using our swir and mir camera observations to calculate p (and th, see section 4.1). for these runs, we set the swir emissivity to be constant (0.95, see section 3.1) and var− ied the mir emissivity between zero and one in steps of 0.05. a mir emissivity of 0.85 gave the p−solution with the smallest cumulative deviation from ptheo across all distances and was finally chosen. we per− formed the same procedure for swir and tir camera observations, which resulted in an emissivity of 0.25 for the alloy in the tir. the emissivity of the sprayed background (black, see figure 1) was set to 0.95 for all wavelengths, following the senotherm spray spec− ifications. 4. results 4.1 db method: tb is known db needs two observations and one assumption to solve a system with three variables. typically, one as− sumes a background temperature tb and solves for the temperature of the anomaly th and its fraction p (“standard method”). with three independent obser− vations (section 2.2), we can produce different solu− tions, shown in figure 3 for the hot setup. overall, the solutions involving swir observations are most accurate with regard to the root mean squared error (rmse, panel a). the smallest relative figure 2. spectral emissivity of the hot wire derived with the draping algorithm. gray area: swir filter of ir cam− era. dashed lines: emissivity (0.93) at filter’s central wavelength (2.36 μm). figure 3. solution of the db method (a: pixel fraction p, b: th) for the hot setup and three band combinations with assumed tb = 370 k. dashed lines indicate the expected values for p (known) and th (mean of solutions over distances, values annotated). 5 experimental assessment of dual-band method error in p of ~1 % is reached by the mir−swir com− bination at a distance of 16 m (p = 5.2 %). at the max. distance of 38 m (p = 2.2 %), the mir−swir solution gives a relative error of 28.7 %. calculated th values vary notably around their means (panel b) with a min− imum rmse of 96.1 k for the mir−swir combina− tion. they seem to converge only at distances 26 m (p = 3.2 %). 4.2 db method: p is known our experimental setup allows to evaluate a less com− mon variation of the db method (“assumed p method”). we provide the theoretical hot pixel fraction p (figure 3a, black) and estimate th and tb. figure 4 shows the solutions for the experimental se− tups hot, med & low using the tir−mir band combi− nation, most sensitive to low temperatures (i.e., tb). the background temperature is most robustly determined for the hot setup (tb = 372 k, rmse = 9.7 k, panel a). vari− ations originate primarily from close distances 6 m (p = 13.9 %), beyond which the results converge towards the mean (dashed). the variability in th is remarkably re− duced compared to the results of the standard method. hot gives th = 1054 k and the lowest rmse = 39.2 k, while the runs with lower wire temperatures give rmses more than twice as large due to one outlier in med and two in low (noisiest, panel b). 4.3 tri-band method: no assumptions according to figure 4a, the assumption of a con− stant background temperature may result in significant errors. a better solution for the two thermal compo− nents case can be achieved by using the observations of all three wavebands [flynn et al., 1994]. then, no assumption is needed and p, th and tb are simultane− ously estimated (figure 5). the rmses for p (panel a) and th (97.4 k) are neg− ligibly larger than those of the swir solutions using the standard method (figure 3, orange & red). how− ever, improvements compared to the standard method are registered: first, the smallest relative error in p has reduced to 0.6 % at 16 m. secondly, the relative errors at 38 m, the case most relevant for satellite thermal re− mote sensing, have decreased to 24.9 % for p and 3.6 % for th. the mean of tb (372 k) is in excellent agree− ment with that calculated by the best assumed p method (see figure 4a, yellow). likewise, the mean of th (1019 k) agrees well with that given by the best standard method (see figure 3b, red). 5. discussion overall, the presented solutions obtained by the dif− ferent methods are coherent, lending support to the ser− figure 4. solution of the db method (a: tb and b: th) for the tir−mir band combination and three runs of the ex− periment with given pixel fraction p. dashed lines in− dicate the mean values for tb and th (annotated). figure 5. solution of the db method for the hot setup (a: p, b: th, and c: tb). dashed lines indicate the expected values for p (known), th and tb (mean of solutions over distances, values annotated). viceability of the experimental setup and the relevance of the results. subpixel characterization techniques are piv− otal in remote sensing of active lava flows. although the reliability of the db method has frequently been chal− lenged in this context [e.g., wright et al., 2003; harris, 2013], it is still widely used until today [e.g., aufaristama et al., 2018] due to the lack of satellite sensors with mul− tiple infrared channels across the spectrum. to our knowledge this study is the first that system− atically evaluates the dependence of the db method on the hot pixel fraction p under “controlled” measurement conditions. it complements studies that test the db method using synthetic data, as in murphy et al. [2014]. the authors calculate synthetic radiance spectra in the swir from two thermal components pixel and conclude that db is statistically incapable of reliably, i.e., with 95 % confidence, constraining p, th or tb (deviations ≥ 11 %). in our experiment, we use different infrared wavelength bands and succeed in estimating p with de− viations down to ~ 1% (mir−swir, figure 3a). however, the standard db method (assumed tb) will likely result in large p deviations (≥ 29 %) for small pixel fractions (≤ 2.2 %). best results are obtained by using swir ob− servations, for which we determined a reliable wire emis− sivity of 0.95. the iteratively estimated emissivities in tir (0.25) and mir (0.85) are smaller and relatively uncertain, as they were not validated independently based on spec− trometer measurements (section 3.1). the tir−mir com− bination thus gives the least accurate p results. they can not be transferred to an active lava flow, for which one would expect a far greater emissivity above 0.9 across the infrared spectrum. a higher accuracy of the p estimate can be achieved if observations in tir, mir and swir wavebands are available (figure 5a), as for instance at the visible infrared imaging radiometer suite (viirs). how− ever, the spatial resolution of viirs may be considered as too coarse. a preferable option is hyspiri mission sched− uled for launch in 2022. meanwhile, the fusion of high resolution optical data with that from thermal sensors can boost the accuracy of the fractional area estimate. subpixel temperatures th and tb can be calculated most robustly (rmses 10 k, 40 k) if p is known (figure 4). such a measurement setup is provided today by nu− merous high resolution satellite data (e.g., sentinel 1 and 2, geoeye, worldview, planet). their integration in a standard workflow based on, e.g., modis data can sig− nificantly increase the quality of the results. the db method operates best in the case of hot anomalies (figure 4). a sensor similar to the advanced spaceborne thermal emission and reflection radiome− ter – aster (with an additional mir band) or firebird [zakšek et al., 2015; with an additional swir band] is therefore desirable in terms of db method accuracy. 6. conclusions we implemented a simulation environment for the quantitative estimation of subpixel sizes and tempera− tures. it is important to understand quantitative relation− ships between instrumental spectral and radiometric characteristics and data exploitable for lava flow subpixel features. our analyses show that the standard db method leads to a large percent deviation of ~29 % for small p (2.2 %). this can be mitigated by − using mir, swir and tir wavebands simultane− ously. this confirms simulations by lombardo et al. [2012], stating that observations in three appropri− ate, independent spectral bands are necessary to obtain reliable solutions. − a more accurate determination of surface emissiv− ities, possibly using a spectral library approach [murphy et al., 2014]. − the fusion of thermal data with high−resolution optical / radar data. − additional swir and mir sensors. the above points suggest improvements in future payload development regarding the dynamic range and band wavelengths. this research has been supported by a grant from the german science foundation (dfg), number za659/1-1. we would like to thank joachim bülow and thomas schön for supporting the construction of the lava simulator as well as matthias hort for his valuable comments regarding the design of the whole experiment. references aufaristama, m., a. hoskuldsson, i. jonsdottir, m.o. ulfarsson and t. thordarson (2018). new insights for detecting and deriving thermal properties of lava flow using infrared satellite during 2014– 2015 effusive eruption at holuhraun, iceland. remote sens. 10(1), 151. doi: https://doi.org/ 10.3390/rs10010151. pick et al. 6 calvari, s., l. lodato, a. steffke, a. cristaldi, a.j.l har− ris, l. spampinato and e. boschi 2010. the 2007 stromboli eruption: event chronology and effu− sion rates using thermal infrared data. j. geophys. res. solid earth 115, b04201. crisp, j., s. baloga (1990). a method for estimating eruption rates of planetary lava flows. icarus 85, 512–515. dozier, j. (1981). a method for satellite identification of surface temperature fields of subpixel resolu− tion. remote sens. environ. 11, 221–229. flynn, l.p., p.j. mouginis−mark and k.a. horton (1994). distribution of thermal areas on an active lava flow field: landsat observations of kilauea, hawaii, july 1991. bull. volcanol. 56 (4), 284–296. gawarecki, s.j., r.j.p. lyon and w. nordberg (1965). in− frared spectral returns and imagery of the earth from space and their application to geological problems: scientific experiments for manned or− bital flight. am. astronaut. soc. sci. technol. ser. 4, 13–133. harris, a.j.l. (2013). thermal remote sensing of ac− tive volcanoes: a user’s manual. cambridge university press. isbn: 9780521859455. harris, a., j. dehn., m. patrick, s. calvari, m. ripepe and l. lodato (2005). lava effusion rates from hand−held thermal infrared imagery: an exam− ple from the june 2003 effusive activity at stromboli. bull. volcanol. 68, 107–117. lombardo, v. and m.f buongiorno (2006). lava flow ther− mal analysis using three infrared bands of remote− sensing imagery: a study case from mount etna 2001 eruption. remote sens. environ. 101, 141–149. doi: https://doi.org/10.1016/j.rse.2005.12.008. lombardo, v., m. musacchio, m.f. buongiorno (2012). error analysis of subpixel lava temperature measurements using infrared remotely sensed data. geophys. j. int. 191, 112–125. mouginis−mark, p.j., h. garbeil and p .flament (1994). ef− fects of viewing geometry on avhrr observations of volcanic thermal anomalies. remote sens. env− iron. 48, 51–60. murphy s.w., c. oppenheimer and c.r. souza filho (2014). calculating radiant flux from thermally mixed pixels using a spectral library. remote sens. environ.142, 83–94. oppenheimer, c. (1993). thermal distributions of hot volcanic surfaces constrained using three infrared bands of remote sensing data. geophys. res. lett. 20, 431–434. doi: https://doi.org/10.1029/93gl00500. rothery, d.a., p.w. francis and c.a. wood (1988). volcano monitoring using short wavelength in− frared data from satellites. j. geophys. res. 93, 7993–8008. vaughan, r.g., l.p. keszthelyi, a.g. davies, d.j. schneider, c. jaworowski and h. heasler (2010). exploring the limits of identifying sub−pixel thermal features using aster tir data. j. vol− canol. geotherm. res. 189, 225–237. wright, r. and l.p. flynn (2003). on the retrieval of lava−flow surface temperatures from infrared satellite data. geology 31, 893–896. doi: https://doi.org/10.1130/g19645.1. wright, r., d.a. rothery, s. blake and d.c. pieri (2000). improved remote sensing estimates of lava flow cooling: a case study of the 1991–1993 mount etna eruption. j. geophys. res. 105, 23681–23694. doi: https://doi.org/10.1029/2000jb900225. zakšek, k., m. shirzaei and m. hort (2013). constrain− ing the uncertainties of volcano thermal anomaly monitoring using a kalman filter technique. geol. soc. lond. spec. publ. 380, 137–160. doi: https://doi.org/10.1144/sp380.5. zakšek, k., m. hort, and l. eckehard (2015). satellite and ground based thermal observation of the 2014 effusive eruption at stromboli volcano. re− mote sens. 7(12), 17190–17211. doi: https://doi.org/10.3390/rs71215876. *corresponding author: klemen zakšek, university of hamburg, hamburg (ger); now at university of ljubljana, ljubljana, slovenia email: klemen.zaksek.geo@gmail.com © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 7 experimental assessment of dual-band method << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 62, 1, vo06, 2019; doi: 10.4401/ag-7655 “volcanic and volcano-tectonic activity forecasting: a review on seismic approaches„ ramón ortiz*,1, alicia garcía1, josé manuel marrero2, servando de la cruz-reyna3, roberto carniel4, josep vila5 (1) instituto de geociencias, igeo, csic-ucm, madrid, españa (2) asesoría investigación y desarrollo en riesgos y peligros naturales y antrópicos (repensar), quito, ecuador (3) instituto de geofísica, universidad nacional autónoma de méxico, c. universitaria, méxico (4) laboratorio di misure e trattamento dei segnali, dpia, università degli studi di udine, udine, italy (5) institut d’estudis catalans, barcelona, españa. currently at the preparatory commission for the comprehensive nuclear-test-ban treaty organization (ctbto), vienna, austria 1. introduction forecasting volcanic eruptions is one of the most controversial issues in volcanology. historically there are evidences that people abandoned hazardous areas before eruptions, even before the existence of modern instrumental monitoring methods. for example, the excavations conducted by marinatos since 1967 in the greek island santorini found that little or no loss of lives occurred in the settlement of akrotiri during the theran eruption that destroyed the city around 1627 bc, thus suggesting that the town was evacuated before the eruption [pellegrino, 1991]. similarly, the relatively small number of corpses found in herculanum, destroyed by the plinian eruption of mt. vesuvius in 79 ac, suggests a similar situation [scandone and giacomelli, 2014]. a move to prevent a tragedy such as abandoning endangered settlements depends on three basic factors: the capacity of observation to recognize significant changes in the patterns of volcanic activity, the perception that eruptive activity may cause fatal damage, and the capability of making a political decision resulting in a complex move as in the case of the evacuation of a community. failure in any of these article history receveid may 22, 2018; accepted december 13, 2018. subject classification: forecasting volcanic activity; volcano-tectonic earthquakes; precursory seismicity; volcanic unrest; hazard assessment. abstract forecasting volcanic activity is a difficult problem that is being addressed worldwide from different perspectives. significant advances have been made after the introduction of non-linear dynamical systems theory and the use of power-law distributions of different geophysical parameters in the earth sciences. in particular, frequency-magnitude power-law statistics evidences the scale-invariance and self-organization of seismicity, and brittle fracture models show that under certain conditions, a precursory causal evolution characterized by an accelerating strain rate culminates in a catastrophic failure of the system under stress. the precursory organization of the seismicity and the distinct characteristics of the seismic events have allowed the development of forecasting tools. in this work we present some examples of forecasting methods based on seismic observations at different volcanoes in the world, and how results and experiences has been used to improve both hardware and software tools developed for short-term forecasting of volcanic and seismo-volcanic activity. three factors may lead to a complete disaster, as it was the case in st. pierre (martinique) in 1902 [tanguy, 1994], or in armero (colombia) in 1985 [voight, 1990]. these three basic factors have been addressed for a long time in japan. for example, the 1910 usu volcano eruption was studied by scientists at that time, in particular by omori and satō [omori, 1903, 1911, 1912; satō, 1913; yokoyama and matsushima, 2018], and the management of the emergency situation was properly handled. with this background noguchi and kamiya [1963], mogi [1963] and minakami [1964] purported the concept of volcano observatory as a mean for forecasting volcanic eruptions. similarly, the analysis of the 1975 eruption of tolbachik [fedotov and markhinin, 1983], the successive eruptions of etna [e.g. vinciguerra et al., 1999] and the 1980 eruption of mt. st. helens [lipman and mullineaux, 1981], provided evidence in setting some physical basis on the nature of eruptions and their precursors [de natale et al., 1999]. an important boost of word-wide research works on forecasting and modeling of pre-eruptive processes from different perspectives followed afterwards. the theory of nonlinear dynamic systems and the power-law distribution attributed to the energy released by different geophysical phenomena have greatly helped to deal with and model the complex nature of forecasting hazardous activity [e.g. pshenichny et al., 2003, 2009]. it is very important to emphasize that the nature of the causal forecasting methods based on the concatenated increase of a geophysical observable implies dynamic time scales that decrease as the monitored system evolves (hours-days). in contrast, non-concatenated causal phenomena only provide “stationary” information on the state of a geophysical system which can only be used for the long-term (years-decades) assessment of the related hazard [jones, 1996]. in a volcanic region, this may be exemplified as the difference between an accelerated release of seismic energy, probably leading to some type of volcanic or seismo-volcanic manifestation, and a volcanic hazards map. in this work we aim to describe the evolution of the precursory unrest activity of active volcanoes and its relevance in forecasting eruptions through causal forecasting models and methods based on changes in the seismicity observed. our experience is focused mainly on the measurement and data processing of volcano seismicity, and to a lesser extent, in real-time deformation measurement using gps l1 and l2. in addition, we believe that these seismic models and methods are adequate for volcanoes where the assessment of new unrest detected with provisional monitoring equipment are critical for risk management, but due to the lack of resources, the deployment and maintenance of a comprehensive volcano monitoring network is not always possible. 2. methods based on changes in seismicity as stated in the previous section, historical evidence indicates that the perception of a relationship between seismic and eruptive activity may be as old as humanity. however, in the xx century the advent of electronics, computers, more sensitive motion transducers, and the capabilities to program elaborate signal analysis tools, allowed the recognition of multiple types of signals produced by internal processes of active volcanoes, most of them with precursory value [omori 1911, 1912; schick and riuscetti, 1973; schick 1981]. for example, during the 1982-1984 campi flegrei bradyseismic crisis [barberi et al., 1984], the introduction of real-time digital instruments [ortiz et al., 1984; 1992] not only permitted a follow-up of the ongoing activity, but also initiated the development of hardware and software specifically designed for volcano monitoring that is still used [barberi et al., 2008]. in general, seismic forecasting methods may be grouped in two types: those based on the time evolution of volcano-tectonic earthquakes, and those based on the analysis of waveform signals of volcanic origin. 2.1 precursory time evolution of volcano tectonic earthquakes the rapid increase of the rate of volcano tectonic (vt) earthquakes before an eruption is one of the earliest manifestations recognized as an eruption precursor [benoit and mcnutt, 1996]. however, since many earthquake swarms in volcanic areas are not followed by eruptive activity, the causality between these signals and eruptions is weak [gudmundsson, 2002; garcía et al., 2014a]. migration of seismic foci has also been used since the 1960's as a forecasting technique but sometimes eruptions are not preceded by migrations and vice versa [wiemer et al., 1998]. the time evolution of the seismic energy release by vt earthquakes and parortiz et al. 2 ticularly its cumulative value provides a more reliable tool for the eruption forecasting. yokoyama [1988] found that vt cumulative seismic energy reaching a threshold of 1011 j has significant forecasting value. it is also a relatively simple observable to evaluate because it requires only a basic seismic catalog with a magnitude of completeness of the catalogue mc=2 or lower [garcía et al., 2014a]. examples of such behavior are shown in figure 1, where a clear increase of the seismic activity near the teide volcano (tenerife, canary islands, spain) in 2004 contrasts with the uniformity of the seismicity at regional level. similarly, figure 2 shows an example of how different seismic parameters evolved before and after an eruption during the el hierro 2011 precursory unrest. the daily data processing allowed forecasting the eruption and the higher magnitude vt events [garcía et al., 2014a,b; 2016] using some of the methods described below. the daily data processing using different methods of analysis ((mean recurrence time mrt, garcía et al. 2016, real-time seismic amplitude measurement rsam, endo and murray, 1991, etc.) allowed to forecast the volcanic activity and provide the authorities and relevant bodies with pertinent technical input [garcía et al., 2014b]. we implemented a variety of methods because the appropriateness (and inherent precision) of a given method depends on the type of changes in the volcanic behavior. the mrt based in the seismic catalogue is the most used method. 2.2 magnitude distribution the distribution of earthquake magnitudes in a volcanic region may also be described by the frequency– magnitude distribution of tectonic earthquakes based on the ishimoto and iida [1939] and the gutenberg-richter [1944] law log10 n = a bm where n is the cumulative number of earthquakes with magnitude ≥m, and a and b are constants that describe the power law decay of occurrence frequency with increasing magnitude. the b-value is the slope of the line which best fits the earthquake-magnitude distribution data above the magnitude of completeness mc, defined 3 forecasting volcanic activity figure 1. near and far field seismicity at tenerife island during the 2004 teide volcano seismic crisis: the cumulative seismic energy release plots reveal a significant difference between the local vt seismicity and the seismicity at distance greater than 25 km from the volcano. ortiz et al. 4 figure 2. evolution of different geophysical parameters at el hierro during the volcano-tectonic and eruptive activity in 2011-2014 [in garcía et al., 2016]. figure 3. evolution of the mrt from 01 september to 12 october 2011. the calculated value of the mrt remains stable (about 103 days) during the first 10 days of september, and, rapidly decays afterwards reaching a value of about 10 days on 28 september, when a warning was issued. the eruption began on 11 october 2011. 5 forecasting volcanic activity as the lowest magnitude at which 100% of the events are detected [rydelek and sacks, 1989; taylor et al., 1990; wiemer and wyss, 2000]. the b-value mostly depends on the properties of the seismic medium and on the nature of the stress source causing the earthquakes. areas where seismicity is caused by extensive regional stress show b-values near 1.0 [frohlich and davis, 1993]. stress concentration leading to clustering of seismicity and fracture over distances of a few kilometers seems to have a strong effect on the value of b [ogata and katsura, 1993; wiemer and mcnutt, 1997; wiemer and wyss, 1997]; particularly, stress concentration produced by increased thermal gradients, or rock fracturing caused by hydraulic stresses [grasso and sornette, 1998] cause a significant increase in b [warren and latham, 1970]. both of the former processes are common in volcanic regions, and b-values larger than 1 are commonly associated to some type of internal volcanic activity [de la cruz-reyna et al., 2008]. the analysis of the b-values calculated over short time windows in a volcanic region permits calculating the changing “mean recurrence time” (mrt), a parameter measuring the probability of a vt earthquake exceeding a given (relatively high) magnitude. an application of this methodology is illustrated in figure 3. the methodology was developed as a forecasting tool for the expected largest-magnitude earthquake in each magma injection process during the el hierro seismo-volcanic crisis [garcía et al., 2016]. combining the mrt with an associated alert level procedure (val) provides a robust method to objectively communicate the seismo-volcanic hazard [garcía et al., 2014b]. 3. methods based on changes in seismic waveforms 3.1 analysis of continuous seismic signals the seismic signals recorded by a seismometer result from the superposition of multiple signals from different sources, including those generated by the volcanic activity. a main goal is to discriminate the signals generated by the volcanic activity from the rest. this is done by means of decomposition techniques such as singular value decomposition [carniel et al., 2006a], independent component analysis and non-negative matrix factorization [see carniel, 2014 for a review]. this task can be particularly difficult at the onset of volcanic activity that is, when the signals of volcanic origin are relatively weak. in such a situation, relevant information may be disregarded as “noise". the analysis of noise acquires then a particular importance [peterson, 1993]. recent developments in noise analysis based on continuous assessment of noise levels for quality control purposes [sleeman and vila, 2007], variations of signal-to-noise ratios [bormann, 2002; mcnamara et al., 2009], or specific applications to geology [ladina, 2012], resulted in useful tools for the recognition of subtle changes in the internal state of a volcano. changes systematically altering the amplitude and/or predominant frequencies of the background seismic noise can be detected using these methods of analysis in a given volcanic region. it is particularly important to mention the base level noise seismic signal (blnss) [vila et al., 2006, 2008], which proved to be very useful in monitoring the 2003-2005 unrest episode of the teide volcano. through this method it was possible to report changes in the internal process of the teide volcano. as an example, figure 4 shows the base level noise spectra computed for different days. a significant increase of noise energy in the 1-10 hz frequency band from 2004-12-01 through 2004-1203 preceded by nine days a new fissure with fumarole emissions that appeared in la orotava valley in the northeastern flank of teide volcano, which returned to the normal levels a few hours after the fissure aperture [see garcía et al., 2006 for details]. to assess the capabilities of the method, we performed blnss analyses on other active volcanoes such as soufriere hills in montserrat, west indies, and in llaima and villarrica in chile [vila et al., 2006]. generally speaking, the blnss proved to be a powerful tool that also provides relevant information on the status of volcanic unrest. it is important to emphasize that the methods based on the characterization of background noise detect changes in the evolution of a volcanic seismic signal but they do not provide any a-priori information about the sources, nor about the structures traveled through by transitory seismic waves. however, new developments adapted from neurosciences like the self-organizing maps, som [kohonen, 1982] have been recently applied in geophysics along with decomposition techniques such as the singular spectrum analysis to identify specific evolutionary patterns [carniel et al., 2006b, 2009; roden et al., 2015]. similar techniques were applied in volcanology to identify states of the volcanic activity from continuous tremor data [carniel et al., 2006a; langer et al., 2009; carniel et al., 2013]. the geostatistical approach at different time scales is also promising [jaquet and carniel, 2003; jaquet et al., 2006]. the failure forecast method (ffm; voight 1988) provides a simple tool that under certain conditions (closed volcanic systems) permits forecasting volcanic activity from the precursory accelerating release rate of different strain-related parameters. some of such (analogous) parameters are the rsam [real-time seismic amplitude measurement; endo and murray, 1991], the rsem [real-time seismic energy measurement; de la cruz-reyna and reyes-davila, 2001], the ssam [spectral seismic amplitude measurement; stephens et al., 1990], the ssem [spectral seismic energy measurement; tarraga et al., 2008] and the ssw [similar seismic waveforms; salvage and neuberg, 2016]. in particular, the analysis of accelerating rsam rates permitted real-time forecasting of eruptions at tungurahua volcano, ecuador (figure 5a) [de la cruzreyna et al., 2010], and at villarrica volcano (chile) [ortiz et al., 2003] (figure 5b). also in villarica, some results with the seismic signal spectrogram showed the open-close transitions of the conduit systems (figure 6a and 6b), and interactions between tectonic events, seismic noise and volcanic activity [ortiz et al., 2003; tarraga et al., 2006; carniel et al., 2006a; tarraga et al., 2012]. similarly, during the 2004-2005 seismic crises at teide volcano (tenerife island), the analysis of ssem series allowed us to forecast local earthquakes [tarraga et al., 2006]. the rsem analysis used at colima volcano in mexico [de la cruz-reyna and reyesdavila, 2001; reyes-davila and de la cruz-reyna, 2002], allowed the identification of the nature of the stress evolution at the source. the forecasting accuracy of these methods has been improved with the use ortiz et al. 6 figure 4. evolution of the base level noise seismic signal (blnss) in tenerife during the 2004 seismo-volcanic crisis (2004-1125 to 2004-12-09. ). seismic energy release in the band 1-10 hz significantly increases between 25 november and 03 december 2004, and decreases afterwards. on 03 december 2004 a fracture with fumaroles opened close to benijos village (28º34’ 20’’ n;16º 11’ 14’’ w) (garcia et al, 2006; vila et al, 2006, 2008). of kalman filtering [kalman, 1960], particularly when applied to other strain-rate related parameters such as the terrain deformation, as it was the case in the recent volcanic process at the canary island of el hierro in 7 forecasting volcanic activity figure 5. a) evolution of the inverse low-pass filtered ssem at tungurahua volcano (ecuador). at 10 hours of 20 aug 2003 a m 4.5 regional tectonic earthquake located at 34.5 km from the volcano (isolated dot near the center of the plot) marks the onset of a rapid decrease of the inverse ssem pointing to the time of the eruption [de la cruz-reyna et al., 2010]. b) two moderate explosions at villarrica volcano (chile) on 27 and 29 october 2000 were forecasted with the inverse ffm. the upper plot shows the radiance peaks caused by the explosions (hot spot monitoring program , hawaii institute of geophysics and planetology. [harris et al., 2001]. 2011-2014 [prates et al., 2013; garcía et al., 2014a,b]. the choice of the method to be used depends in ortiz et al. 8 figure 6. a) 24-hour spectrogram of a seismic signal at villarrica volcano (chile) obtained on 02 oct 2000. a sharp transition of the dominant frequency may be clearly seen around hour 21:00. this type of transition is attributed to sudden openings or closings of the volcano conduit. b) the seismic signal spectrogram of this eruptive activity shows that the dominant frequency of a closed conduit (red) is twice the frequency of an open conduit (blue and green). each case on a preliminary assessment of the physics controlling the volcanic activity and the type of available data. this sometimes requires developing new methods to deal with particular situations [see e.g. salvage and neuberg, 2016]. a review of other methods of data reduction to characterize volcanic activity can be found in carniel [2014], where particular emphasis is put on approaches based on the time evolution of parameters that consider the volcano as a non-linear dynamical system. 3.2 analysis of seismic waveforms and recognition of event families since late xix and through the xx century, earthquakes of volcanic origin have been differentiated from tectonic earthquakes [e.g. mercalli, 1883; omori, 1903, 1911, 1912; davison, 1921; noguchi, 1963; mogi, 1963; minakami, 1964], and different source mechanisms have been associated to each type. the identification and follow up of the evolution of different volcanic signals may have a high forecasting value. however, monitoring and recognition of the different signals is a difficult task, particularly during unrest periods, when signals of many coexistent processes are recorded. recently, some methods of automatic recognition and selection of families of signals in real or quasi-real time have been developed. for example, the hidden markov models, that have been applied to several volcanoes such as merapi [ohrnberger, 2001; wassermann and ohrnberger, 2001], las cañadas caldera [beyreuther et al., 2008], popocatepetl [cortes et al., 2009], stromboli, etna [ibáñez et al., 2009] and volcán de colima, where the results of automatic recognition accuracy, reported by the group in charge of the monitoring activities amount to 82% [cortes et al., 2009; arámbula-mendoza et al., 2018]. other examples of signal classification methods are the artificial neural networks [falsaperla et al., 1996] applied for example to vesuvius volcano [scarpetta et al., 2005], the support vector machines [masotti et al., 2006], parallel system architectures based on hmm [cortés et al., 2014], deep neural networks [titos et al., 2018a] and recurrent neural networks [titos et al., 2018b]. currently, the eu-funded project vulcan.ears (volcano-seismic unsupervised labelling and classification embedded in a real-time scenario) aims to develop a volcano-independent [cortés et al., 2019], , hmms-based software to classify events (https://cordis.europa.eu-/project/rcn/210584_en.html). 4. a case history: el hierro 2011-2014 volcanic activity after a long repose period the recent volcanic and seismic unrest observed at el hierro (canary islands; 2011-2015) provided with an opportunity to observe its complex evolution since its onset in july 2011 using some of the methods described in previous sections, and collect relevant forecast information data to assist decision-making. seismic data, including earthquake magnitudes and locations were obtained from the on-line official catalog of the spanish geographical institute (http://www.ign.es/ign/layoutin/sismoformulariocatalogo.do). to process this information, we first implemented a real-time calculation of the cumulative seismic energy and a stepwise estimate of the gutenberg-richter b-value [gutenberg and richter, 1944] on intervals showing stability in the rate of occurrence of different earthquake magnitudes [garcía et al., 2016]. from the beginning of the seismic analysis, the evolution of the seismic energy conformed to the yokoyama criterion [yokoyama, 1988] showing significant increments in the number and magnitude of earthquakes when the cumulative energy exceeded 10¹¹j (figure 7). kalmanfiltered data were used to track the ssem evolution. in the initial stages of unrest, the b-value provided information about the process of magma migrations causing the unrest, but difficulties with the stability of the low-cut-off magnitude made it difficult to continuously track changes of the b-value. the kalmanssem based forecasting of major earthquakes provided useful results until the onset of the submarine eruption (2011-10-10), when the strong tremor caused by the eruption masked most of the other signals. nevertheless, the ssem method still allowed forecasts using distant stations. volcano-tectonic seismicity at el hierro stopped when the eruption began, resuming some days later, on 2011-10-29, when a new seismic swarm and significant deformations developed in a way similar to the preeruptive unrest. the new unrest episode culminated with the largest earthquake with magnitude 4.6. figure 8 shows the improved inverse ssem forecast of this new unrest episode using the kalman-filtered data. this activity could be explained by a new deep magma injection process. afterward, a sequence of similar episodes of seismicity and deformation were recorded, with temporal and spatial evolutions that are consistent with the 9 forecasting volcanic activity model of magma injections along paths defined by stress controlled conical surfaces [de la cruz-reyna and yokoyama, 2011; garcía et al., 2014a]. the model allows detection of shallow regions of increased seismic and related landslide hazard. the similarities among the different episodes of seismicity motivated the definition of a new family of magma-migration related seismic swarms [farrell et al., 2009; garcía et al., 2016]. 5. volcanic activity forecasting and risk management the assessment and management of developing volcanic activity evolve in two parallel lines; one is specifically scientific, while the other is focused on the management of the risk posed by the activity. the latter must provide decision factors in near real time and include criteria and communication procedures to advise authorities and warn the population sufficiently in advance of a critical situation. timely needs are determined by the type of necessary preventive actions (e.g. hours in the case of closing of a road in a zone of probable landslides, or days in cases requiring to evacuate). marrero et al. [2010] presented a new tool for simulating and optimizing large-scale evacuation processes: the variable scale evacuation model (vsem), which allows simulating an evacuation considering different strategies for diverse impact scenarios. the decision to evacuate is one of the critical issues in managing a crisis. in order to make such an important decision, it is essential to estimate the potential loss of lives for each of the expected impact scenarios. to improve the emergency response planning and facilitate the decisionmaking process we also developed a methodology to estimate the number of potential fatalities, which has been applied to the central volcanic complex of tenerife and to el chichón volcano (méxico) [marrero et al., 2012, 2013]. employing several of the above mentioned forecasting tools, and according to a time window defined by the expected impact scenarios, it was possible to design an automatic “watchdog” system capable to detect actual data trend changes, and reject spurious signals reortiz et al. 10 figure 7. evolution of the cumulative seismic energy from the onset of unrest at el hierro island, to the onset of the submarine eruption. the rate of cumulative seismic energy release (blue thick line) clearly accelerates after reaching the yokoyama threshold level of 10¹¹ j. the purple line represents the inverse of the smoothed derivative of the cumulative seismic energy (inverse energy release rate), which clearly shows the increased rate of energy release just above (below in the inverse curve) that energy threshold. lated to instrumental failure. such “watchdog” system operates in steps according to the following routine: 1. automatic and/or visual daily examination of the available seismic data 2. computation of earthquake magnitudes. 3. vt hypocentral/epicentral locations or at least estimation of distance to station. 4. cumulative seismic energy calculation, and assessment of the seismic energy release rate. 5. 24-hour spectral analysis of the continuous signal. 6. rsam calculation and/or derived methods: ssam, ssem, blnss. 7. mrt calculation using vt located at less than 1020 km from the volcano. 8. identification of vt families. in this way unrelated signals are rejected, and only changes related to volcanic activity may be translated into a specific volcanic alert system (vas) that is easy to understand and use by scientists, technicians, and decisionmakers [garcía et al., 2014b]. although this procedure has been tested, its effectiveness is highly dependent on the cultural, political and social background of the area in which a crisis develops [marrero et al., 2015]. 6. discussion there are indeed many more methods for volcano monitoring and eruption forecasting with increasing degrees of sophistication. however, a common problem, particularly in developing countries, or in regions with volcanoes with low rates of high-magnitude eruptions, is the difficulty and sometimes impossibility to obtain funding for extensive and permanent monitoring, particularly when other basic social and economic issues must be addressed. the methods discussed here may render useful results even with data obtained with low-cost instruments, like those designed for educational purposes. for example, the arietta (www.acmesystems.it), raspberry (www.raspberrypi.org) or arduino (www.arduino.cc) embedded micro-computers allow assembling seismic monitoring stations at very low cost. affordable 4.5 hz geophones with a pre-amplifier can generate valuable data extending their signal response to 5 seconds [ortiz et al., 2001; havskov and alguacil, 2006]. data may be transmitted to a processing center by gsm, ethernet or wifi in quasi-real time or by specific timeperiods depending on the available links, and following 11 forecasting volcanic activity figure 8. inverse kalman-filtered ssem forecasts issued at el hierro about one day and about half day before the m 4.6 vt earthquake of 11 nov 2011. the procedure described in the previous section. these methods have been used at el hierro and el chichón volcanoes with good results 7. conclusions awareness of manifestations precursory to eruptions has existed for a long time. disasters happen when precursors are not recognized, or misunderstood, or ignored, and a wrong decision follows. it is thus necessary to increase the reliability of the precursor's recognition and interpretation, and improve the communication of the hazards and the confidence of the decision-makers. this task is not simple, as the number of parameters involved in a volcanic process exceeds the number of measurable quantities that can be accurately measured at the surface. it is now recognized that even simple dynamical systems can show a complex behavior when they become non-linear [lorenz, 1963; mandelbrot, 1977; turcotte, 1987; newman, 2011] and this triggered the introduction of novel methods to characterize [carniel, 2014] and model [pshenichny et al., 2003, 2009] the time series representing the dynamics of volcanic systems. for instance, the determination of powerlaw distributions for different parameters, in particular frequency-magnitude power-law statistics, may reveal some general scale-invariance and self-organization characteristics of the magmatic systems. such information can help to recognize a process that will culminate as a catastrophic failure. considering the technical developments of sensors, transducers, computers and communication devices, and the persistently increasing capabilities of software to process that type of information, it is presently feasible to monitor volcanoes and maintain a sound forecasting capability at an affordable cost. what is needed is to promote the political will to invest in such systems. acknowledgements. this research has been funded by projects from the csic (2011-30e070) and mineco (cgl2011-28682c02-01). the authors are grateful to the dgapa-papiit-unam program for their support (project in109616). we used seismic data from the ign public website (http://www.02.ign.es/ign/layout/sismo.do, public website, © instituto geográfico nacional). references arámbula-mendoza, r., reyes-dávila, g., vargas-bracamontes, d.m., gonzález-amezcua, m., navarroochoa, c., martínez-fierros, a. and ramírezvázquez, a. 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(2018). understanding active volcanoes: the case of usu volcano, japan, with emphasis on the 1977 summit eruption. geofísica internacional 57(1): 13-37. d.m. johnston, j.w. cole, j.b. wardman, g. wilson and s.t. barnard (2012). volcanic ash impacts on critical infrastructure. physics and chemistry of the earth, 45–46, 5–23. wilson, g., t.m. wilson, n.i. deligne and j.w. cole (2014). volcanic hazard impacts to critical infrastructure: a review. journal of volcanology and geothermal research, 286(1), 148–182. corresponding author: ramón ortiz, instituto de geociencias, igeo, csic-ucm madrid, spain email: ramon.ortiz@@csic.es © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved 17 forecasting volcanic activity layout 6 continuous so2 flux measurements for vulcano island, italy fabio vita1,2,*, salvatore inguaggiato1, nicole bobrowski1,3, lorenzo calderone1, bo galle4, francesco parello2 1 istituto nazionale di geofisica e vulcanologia, sezione di palermo – geochimica, palermo, italy 2 università di palermo, dipartimento scienze della terra e del mare, palermo, italy 3 university of heidelberg, istitute for environmental physics, heidelberg, germany 4 chalmers university of technology, department of radio and space sciences, gotenburg, sweden annals of geophysics, 55, 2, 2012; doi: 10.4401/ag-5759 abstract the la fossa cone of vulcano island (aeolian archipelago, italy) is a closed conduit volcano. today, vulcano island is characterized by sulfataric activity, with a large fumarolic field that is mainly located in the summit area. a scanning differential optical absorption spectroscopy instrument designed by the optical sensing group of chalmers university of technology in göteborg, sweden, was installed in the framework of the european project "network for observation of volcanic and atmospheric change", in march 2008. this study presents the first dataset of so2 plume fluxes recorded for a closed volcanic system. between 2008 and 2010, the so2 fluxes recorded showed average values of 12 t.d —1 during the normal sulfataric activity of vulcano island, with one exceptional event of strong degassing that occurred between september and december, 2009, when the so2 emissions reached up to 100 t.d —1. 1. introduction geochemical studies of fumarolic exhalations from active volcanoes indicate that the gas emissions are directly fed by degassing magma. over the last 20 years, many geochemical investigations have focused on the study of fumarolic gases, as tools to quantify the degassing processes during both active and quiescent phases of volcanic activity [e.g. italiano et al. 1997, aiuppa et al. 2005, allard et al. 2005, inguaggiato et al. 2011, inguaggiato et al. 2012a]. in particular, so2 fluxes measured for the plume of volcanic systems appear to be a promising parameter to study these degassing processes [e.g. casadevall et al. 1983, allard et al. 2005, mcgonigle 2005]. with the aim to study the so2 fluxes of vulcano island (aeolian archipelago, italy), a scanning differential optical absorption spectroscopy (doas) instrument was installed at palizzi (on the west side of vulcano island, near the crater) in march 2008. this instrument acquired a dataset of so2 fluxes in continuous mode directly from the summit crater. vulcano island is one of the currently active volcanoes of the aeolian island arc, located north of sicily, and it is characterized mainly by explosive eruptions. the last eruption from la fossa crater occurred between 1888 and 1890. since that time, the crater has been affected by intense degassing of fumarolic fluids at high temperatures (up to 700 ˚c in 1993). there is also fumarolic activity at forgia vecchia, a few hundred meters north of la fossa crater, where low-pressure fumarolic vapor escapes at 100 ˚c, and also at porto di levante beach, where the fluids are fed by the boiling of a hydrothermal system, with their temperature buffered at 100 ̊ c. la fossa crater fumaroles show large variations in their flow rates, temperatures and compositions over time. increasing fumarolic activity preceded the 1888 eruption, but no eruption occurred after a later increase in the fumarole temperatures, from 200 ̊ c in 1913, to 615 ̊ c in 1923. a more recent phase of increasing heat output started in 1977, with a temperature rise in the crater fumaroles from 200 ̊ c to 220 ̊ c, to about 700 ̊ c in 1993. detailed studies of the geochemistry of the fumarolic fluids of vulcano island started in the late 1970's, concurrent with the rapid changes that were happening at la fossa crater. at present, the main exhalative activity is concentrated in the crater area (see figure 1), with a wide fumarolic field, and with temperature ranging from 100 ˚c to 450 ˚c. moreover, strong soil degassing that is dominated by co2 occurs all around the volcanic edifice, and particularly where the active tectonic discontinuities drive the gases (mainly water vapor and co2) to the surface [e.g. capasso et al. 1997, diliberto et al. 2002]. finally, numerous co2 thermal wells (at up to 80 ̊ c) in the vulcano porto area testify to the presence of a geothermal system with an equilibrium temperature of around 200 ˚c [federico et al. 2010]. over the last few decades, vulcano island has shown indications of renewed activity due to increases in deeper article history received may 1, 2011; accepted february 7, 2012. subject classification: so2 , differential optical absorption spectroscopy, vulcano island, network for observation of volcanic and atmospheric change. 301 data and experiment descriptions fluid fluxes that have been highlighted by increases in the temperatures and volatile fluxes of the fumarolic area [chiodini et al. 1996, paonita et al. 2002], and strong variations in the chemical and isotopic compositions of the crater fumaroles and thermal waters [capasso et al. 1997, 1999]. over the last 10 years, la fossa crater has been used as a natural laboratory to test new remote and in-situ sensing techniques. in particular, mori et al. [1995] measured the so2/hcl ratio of the vulcano island plume using fourier transform infrared (ftir) spectroscopy, and aiuppa et al. [2004] carried out an inter-comparison on the plume of vulcano island using different methodologies (ftir, filterpacks, direct sampling). o'dwyer et al. [2003] performed realtime measurements of volcanic h2s and so2 concentrations with a prototype instrument based on ultraviolet (uv) spectroscopy. as vulcano island is a closed conduit volcano with only sulfataric activity and no real 'plume', telemetric measurements of so2 fluxes of the plume have not been performed systematically in the past. in the framework of the "network for observation of volcanic and atmospheric change" (novac), a worldwide network of permanent scanning doas instruments [galle et al. 2010] was installed for 19 volcanoes around the world, to measure real-time volcanic so2 emission fluxes. within this context, a scanning doas instrument was installed at vita et al. 302 figure 1. vulcano island, aeolian arcipelago, italy. main exhalative activity areas are indicated: crater fumaroles, beach fumaroles, and thermal wells. c) a 2009 google) b a) b) figure 2. the uv-scanning doas equipment installed in the palizzi area on the south-east flank of vulcano island. (a) map showing the positions of the uv-scanning doas and the wlan bridge (map © 2009 google). (b) photograph of the scanning doas station. (c) photograph of the wlan bridge for data transfer. ! thermal wells area beach fumaroles crater fumaroles ! ! ! ! ! ! ! ! ! ! ! 303 palizzi (the west side of vulcano island, near the active crater) in march 2008 (see figure 2). 2. methods a uv-scanning doas mark1 instrument developed as part of the novac project [galle et al. 2010] was installed in the palizzi area in march 2008, together with a weather station, on lentia hill at the same elevation (350 m a.s.l.) as la fossa crater (figure 3). the weather station provided wind speed and direction, rainfall, temperature, relative humidity, and uv, visible and infrared radiation. from the measurement station, these data arrived at the vulcano observatory through a wireless system. from vulcano observatory, the data were transmitted in a real time via the internet to the istituto nazionale di geofisica e vulcanologia (national institute of geophysics and volcanology, ingv, palermo). the scanning doas instrument was installed very close to the source, at about 700 m from an emission area of so2 where the slant column is high enough to be detected (50 to 250 ppmm), and therefore providing the possibility to calculate the fluxes (figure 4). the system was equipped with a single spectrometer from the ocean optics company (s2000 spectrograph), an embedded personal computer, a global positioning system receiver, a timer, optical fiber, and a telescope. a technical report relating to this continuous monitoring with the scanning doas system, including the instrument design, station networking, and measurement geometries, is described in more detail by e.g. zhang [2005], galle et al. [2010] and johansson et al. [2009]. uv light from the sun that is scattered from aerosols and molecules in the atmosphere is partly absorbed in passing the volcanic plume and then collected using the telescope with a quartz lens that defines a field-of-view of 16 mrad. the light is then transferred from the telescope to the spectrometer via the optical quartz fiber. the spectrometer used a 2,400 lines/mm grating combined with a 50 nm slit, which provided an optical resolution of ca. 0.6 nm over a wavelength range of 280 nm to 390 nm. the telescope automatically moved from horizon to horizon using a stepper motor. for further details, see galle et al. [2010]. the settings of the station at vulcano island were characterized by moving the telescope from 90˚ to —90˚ (horizon in the nnw direction, to horizon in the sse direction). for every elevation angle, 15 spectra were collated for one measurement. every full measurement cycle contained 53 spectra: one zenit sky spectra, one dark current spectra, one offset spectra, and 50 measurement spectra. this usually took about 10 min, depending on the light conditions of the sky. the so2 fluxes were calculated with the novac software [johansson et al. 2009]. the doas station worked only for the time span from 07:30 hours to 16:30 hours (gmt), because of light limitations. during the first months of observations, a traditional flat scanner system was used, which scanned a vertical plane that cut the transport direction of the plume; however, unfortunately, the plume was rarely hit. this was improved by changing this to a conical scanner system at the end of june 2008. the plume was then scanned along a conical surface, rather a vertical one. the new method significantly increased the wind direction range that was covered by a single scanning instrument, and at the same time, it reduced problems related to multiple scattering effects [galle et al. 2010]. as there were low so2 concentrations in the emissions of vulcano island, these so2 fluxes were usually recalculated (see the doas analysis below), to ensure the quality of the data. 3. analysis of the data evaluation of the flux 3.1. evaluation of the spectra the collected spectra were analyzed according to the doas procedures, which are based on the beer lamberts law. each spectrum was first corrected for the electronic noise (offset and dark current). the optical density was obtained by dividing each spectrum measurement by a reference spectrum, as a spectrum taken outside the plume, and then a logarithm was taken of the result of this calculation. a polynomial was used to take account of any broadband extinction structures that were caused by broadband absorption of trace gases [platt and perner 1983, platt 1994]. a high-pass filter was applied to the absorption cross-sections to use only the remaining high frequency structure, which is unique for any trace gas, and which can therefore be used to determine its abundance. shift and squeeze were allowed for the absorption crosssections [bogumil et al. 2003], to compensate for any small shifts, caused mainly by the variations in the temperatures of the spectrometer and the detector unit. to improve and to find the best solutions for the data evaluation, several tests were performed on data from this continuous so2 flux measurements for vulcano island, italy figure 3. the weather station in the lentia area (same altitude of la fossa crater), less than 2 km from the vulcano crater. 'scanning doas' on vulcano island. the dataset of august 14, 2008, was chosen to carry out various sensitivity studies on the so2 evaluation. on this day, the volcanic gas emissions were clearly inside the geometrical plane of the instrument. for all of the tests performed, the novac software was used, and the data of all of the scans from this day are compared below. the results presented, discussed and shown in the various figures, are always based on and compared to an evaluation carried out in the 305.26 nm to 318.22 nm range, which includes so2, ozone (o3), a ring spectra, and a polynomial of the 5th degree. in the first tests, the wavelength evaluation was slightly expanded from the range of 305.26 nm to 318.22 nm, to 305.26 nm to 321.40 nm. the second test was performed by shifting the evaluation range to 310.00 nm to 320.00 nm. figure 5a shows the results expressed in kg.s—1 so2 over the day analyzed for these wavelength regions. the different data obtained in the analysis for the various wavelength ranges are shown in figure 5a with different colors and symbols. neither the slight shift nor the larger shift analyzed showed clear differences in the so2 fluxes, although a systematic increase can be noted for the shift to the larger wavelength region. figure 5b shows the results (again displayed with different colors) from applying various polynomial degrees (2nd, 3rd and 5th degrees) to remove the broadband extinction structures from the spectra. no differences can be seen in these data. figure 5c shows the results considering the correction or no correction for the ring effects, as well as taking into account or not the o3 absorption. if only the ring effects are considered (figure 5c, red circles), the so2 fluxes are generally larger; instead, considering only the o3 presence (figure 5c, blue triangles) and leaving the ring effects out of the consideration, leads to comparable results as when considering both (ring effects and o3 absorption). figure 5d shows the last sensitive analysis carried out for this study; two different so2 references were used (273 k, 293 k), and the results were compared. the data obtained are almost overlapping, in contrast to earlier studies that were performed by bobrowski [2005], where a difference of 7% was observed. the most notable difference is the effect of the o3 absorption, and this is shown in more detail in figure 5e. vita et al. 304 a) b) shown the column density of so2 (ppmm), white point represent the peak intensity of the light and t figure 4. two examples of the distribution of the plume on two measurements made under conditions of different degassing activities. the graphs were produced by the novac software, and shows the positions of the telescope during a single measurement, from 90˚ to —90˚ (x axis; horizon to horizon), for a total 50 position for one complete measure, and the column density of so2 (y axis; ppmm). the white dots represent the peak intensities of the light, and the red bars the column density for each position of the telescope during the measurement. (a) measurement during 2008 (normal activity, ca. 12t.d—1). (b) measurement on november 30, 2009 (increased activity, ca. 60t.d—1). 305 continuous so2 flux measurements for vulcano island, italy figure 5. so2 fluxes as kg/s so2 over the duration of one day (a-d), and so2 plume scans as ppmm on august 14, 2008, as a function of the telescope elevation angle, scanning from horizon 0˚ to horizon 180˚ (e). (a) a small shift in the wavelength region does not show significant changes. if the shorter wavelength range shifted to a longer wavelength, the so2 fluxes are slightly higher. (b) applying various polynomial degrees (2nd, 3rd and 5th degrees) to remove the broadband extinction structures from the spectra. no significant difference can be seen. (c) with correction or no correction for the ring effects, and taking into account or not the o3 absorption, this leads to lower so2 fluxes, especially at the beginning of the day. (d) using two different crosssections of so2 (taken as 273 k and 293 k; bogumil et al. [2003]), with very little difference noted between these evaluations. (e) the so2 plume-scan values are general higher by fitting only a so2 reference to the spectra, especially for telescope elevation angles close to the horizon. in the afternoon (lower panels) this effect gets even stronger (difference of up to 10 ppmm between the two evaluations). this is as would be expected, because of an increased ozone column due to a longer light path in the stratosphere. this shows two plume scan examples, one at midday and the other in the afternoon, and compares the fit of the results obtained without and with the ozone (o3) included. it can be noted here that without including o3 in the evaluation process, the so2 values obtained are generally higher, especially when the fraunhofer reference spectra has a longer time distance to the measurement spectra in the end of a measurement cycle (scan). in the afternoon, this effect gets even stronger, showing a difference of up to 10 ppmm. this is expected, because of an increased ozone column due to a longer light path in the stratosphere when the sunlight enters the earth atmosphere under a higher solar zenith angle [hönninger et al. 2004]. 3.2. error analysis the errors identified in these measurements are mainly due to spectroscopy, atmospheric scattering, and wind parameters. spectroscopic error is related to nonlinear absorption, errors in the absorption cross-section, stray light, and temperature changes of the spectrometer. while the errors in absorption cross-section and nonlinear behavior in beer's law are a general nature of this technique and have been well documented [e.g. bobrowski 2005, platt and stutz 2008, kern 2009], the major spectroscopy errors in our study came from stray light and changes in temperature. these two causes differ in size for each spectrometer [e.g. bobrowski 2005]. to reduce the stray light error we used a filter (hoya u330), and to account for the temperature effects, we chose the fraunhofer reference spectra at a similar temperature and a spectra for the dark correction about every 10 min. furthermore, a squeeze for the absorption cross-section was allowed. overall the spectroscopic error was estimated to be in the range of 10% [galle et al. 2010]. 3.3. atmospheric scattering errors solar light is scattered by molecules and particles in the air, like volcanic ash, condensed water and other molecules, and this can give an extended or reduced path length in the plume. the scattering error strongly depends on the meteorological conditions, volcanic activity and emissions, and the distance between the instrument and the plume [kern et al. 2010]. in the present study, we did not correct for the atmospheric scattering errors. we assumed an error not larger then 20%, as the instrument was relatively closely situated to the plume, at less than 800 m. also, strong condensation rarely took place and volcanic ash was not emitted during this time period. 3.4. wind parameter error to calculate the flux, the concentrations of the molecules in a cross-section perpendicular to the direction of propagation of the plume must be multiplied by the wind speed. to reduce the so2 flux error, the wind speed and the wind direction must be measured directly in the plume above the instrument, or at least at the same elevation. therefore the meteorological station installed at lentia hill showed an estimated error always below 20%. 4. results and conclusions the so2 fluxes for the 2008 to 2010 period are reported in figure 6. these were filtered from values that contained less than 80% of the plume in the scanned plane. it is worth noting that after the change from flat to conical scanning optics, the frequency that the plume was inside the scanning plane increased significantly. sensitivity studies for the analysis of so2 showed that the chosen algorithm was relatively stable for these low so2 values. even the evaluation over the low wavelength range with low photon statistics did not appear vita et al. 306 figure 6. so2 plume flux (t.d —1) time variations, for the 2008 to 2010 period of the continuous dataset. vulcano so2 flux t day -1 0 20 40 60 80 100 120 140 05 /0 1/ 20 08 08 /0 9/ 20 08 11 /1 7/ 20 08 02 /2 5/ 20 09 06 /0 5/ 20 09 09 /1 3/ 20 09 12 /2 2/ 20 09 04 /0 1/ 20 10 07 /1 0/ 20 10 10 /1 8/ 20 10 307 to influence the data significantly. however, if o3 is excluded from the fitting process, the so2 values can be slightly overestimated, especially in the later afternoon hours. figure 6 shows the so2 flux as a function of time, where we can observe a relatively constant value of the so2 flux of around 12 t.d—1 in the period from august 2008 to august 2009. in mid-september 2009, there was a large increase in so2 emissions recorded, with fluxes up to 100 t.d —1 reached in november 2009. then, the values of this so2 flux decreased to the normal values of about 12 t.d—1, although high scattering of the so2 flux values was recorded in the spring of 2010 (up to 40 t.d—1). it is very interesting to see that these anomalous values of the so2 fluxes that showed an increase up to about one order of magnitude occurred in the same period as the anomalous degassing processes of the soil co2 recorded by a continuous monitoring station located at the summit of la fossa crater [inguaggiato et al. 2012b]. indeed, in the september to december 2009 period, the measurement of the soil co2 flux at the summit of the area of la fossa cone recorded huge variations, passing from mean fluxes of 1,600 g.m2.d—1 to values that in a few days reached 15,000 g.m2.d—1. this indicated that independently measured parameters like co2 soil flux and 'plume' so2 flux give the same geochemical indications, and reveal an increase in the volatiles 'masses' involved in the sulfataric degassing processes at vulcano island. this huge increase in both of these volatiles, so2 and co2, can be interpreted as a magma depressurization of the deep volcanic system. these contemporaneously recorded variations in co2 and so2 from the summit 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(2005). wireless sensor network for volcano monitoring, masters thesis, chalmers university of technology. *corresponding author: fabio vita, istituto nazionale di geofisica e vulcanologia, sezione di palermo geochimica, palermo, italy; email: : f.vita@pa.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. vita et al. 308 t e m p i d i t r a g i t t o d e l l e o n d e p * e s p e s s o r e d e l l o s t r a t o d e l g r a n i t o n e l l ' i t a l i a c e n t r a l e d. di filippo l . marcelli in un lavoro precedente ( j ) abbiamo effettuato uno studio su di un terremoto con epicentro nella zona del gran sasso d'italia. intendiamo ora valerci di alcuni dei risultati ivi conseguiti, allo scopo di eseguire ulteriori ricerche su alcune caratteristiche sismiche interessanti l'italia centrale. i l terremoto, avvenuto il 5 settembre 1950, risultò avere le seguenti coordinate geografiche epicentrali: <p=42° 30',8 + 2',0 n x = 13° 1 9 ' , 6 ± 5 ' , 4 e ed il tempo origine f f = 0 4 h 0 8 n , 5 7 ' , 5 ± 0 , , 5 profondità ipocentrale con il metodo dell'angolo di emergenza delle pg. — p e r quel che riguarda la profondità ipocentrale, eravamo giunti alla conclusione che essa dovesse essere molto piccola (al di sotto dei 10 km). i l metodo di inglada applicato alle onde pg per la stazione di r o m a ci aveva infatti dato un valore di h = 3,2 km. altre considerazioni inoltre confermavano il piccolo valore trovato per h : il tempo origine calcolato con la dromocrona delle pg coincideva quasi con quello calcolato con la dromocrona delle sg. abbiamo voluto in questo lavoro trovare una ulteriore conferma ai risultati ora esposti, e per questo ci siamo valsi di un metodo dato da caloi ( 2 ), e che si basa sulla conoscenza dell'angolo di emergenza delle onde longitudinali dirette. caloi nel suo lavoro applica tale metodo ad alcuni terremoti dell'europa centrale (avendo calcolato in precedenza ( 3 ) gli angoli d'emergenza delle onde pg in funzione delle distanze epicentrali, relativamente a detta zona). non esporremo la teoria del metodo per la quale rimandiamo all'opera citata ( 3 ) ; riportiamo direttamente la formula che permette > * u w " jm ~ r v i r ^ \ e v i 129 d. d i f i l i p p o l . m a r c e l l i di realizzare tale metodo, poiché dà il valore dell'angolo e di emergenza delle onde pg. ti[l + x— c ( l — x+ 2 x s i n s f t ) + ( i xyx sin1 lil u 1/ f 2 ]'x y i c (1 x)—cjx sili2 ti cos fl] h « 3 0 km i t a l i a c e n t r . ( m a r c e l l i ) e u r o p a c e n t r a l e ( c a l o i ) l ì » 3 0 km h . 2 s h.20 h. ìs h » io h= 5 h 1 5 5 0 150 fig. i 2 0 0 3 0 0 km la) i n essa h è la profondità ipocentrale, a la distanza epicentrale in rs k m e {} la stessa grandezza in misura angolare: x — — essendo r 0 t e m p i di tragitto d e l l e onde p * e s p e s s o r e dello strato del granito 5 8 1 582 d. di f i l i p p o l . m a r c e l l i il raggio terrestre ed r la distanza dell'ipocentro dal centro della t e r r a : inoltre c è una costante, per dati tipi di onde, caratteristica per una data zona. mediante la [ 1 ] conoscendo c, si possono calcolare i valori di e in corrispondenza di determinate distanze epicentrali, per ogni prof i g . 2 fondita ipocentrale. in un lavoro pubblicato da uno di noi ( 4 ) si trovò per l'italia centrale il valore c = 14,6531. in detto lavoro, per contingenze relative allo stato di guerra, non comparve la tabella n. 1 cbe qui riportiamo: di essa ci si deve servire per l'applicazione del f i g . 3 metodo nella zona suddetta. la fig. 1 rappresenta graficamente la tabella. in essa gli angoli di emergenza per le varie profondità e per varie distanze epicentrali compaiono a confronto con quelli cbe erano stati calcolati da caloi per l'europa centrale. è evidente in entrambe le serie di curve, come l'angolo di emergenza sia una funzione sensibilmente variabile con la profondità ipocentrale e con la distanza t e m p i di tragitto d e l l e onde p * e s p e s s o r e dello s t r a t o del granito 5 8 3 epicentrale, quest'ultima grandezza influenzando l'angolo e in maniera molto più sensibile per l'italia centrale che non per l'europa centrale. è chiaro quindi come una buona determinazione di e possa condurre ad individuare h con molta attendibilità. poiché tra l'angolo vero di emergenza e e quello apparente e sussiste la relazione il problema si riduce a trarre dai sismogrammi il valore di 5 (supponendo naturalmente di conoscere di velocità delle onde longitudinali dirette in superficie e v* velocità delle onde trasversali). il valore di e è dato direttamente dalla relazione dove x2, xsì xkt sono le componenti verticali e orizzontali rispettivamente dello spostamento del suolo (espressi in micron) relativi al primo impeto. l'applicazione di un tale metodo esige molto scrupolo nella determinazione delle costanti strumentali e nella misura dei periodi e delle ampiezze degli impulsi iniziali delle onde longitudinali dirette. offre d'altro canto il vantaggio di essere indipendente dagli eventuali errori dei tempi di osservazione. un'altra accortezza occorre usare nell'impiegare tale metodo : il raggio sismico, sotto l'azione degli strati superiori può subire una deviazione, più o meno marcata, la quale altererebbe l'angolo di emergenza. siccome però detta alterazione diviene quasi trascurabile quando la stazione sismica poggia su stratificazioni geologiche antiche e profonde, è necessario non valersi dei dati di stazioni poggianti su stratificazioni più recenti, alluvionali. è per questa ragione che dal nostro esame abbiamo dovuto escludere la stazione di roma — che pur ha delle bellissime registrazioni — in quanto il suo sottosuolo è costituito da una spessa coltre di tufo litoide: e per analoga ragione abbiamo dovuto scartare i begli impulsi iniziali ottenuti nell'osservatorio di rocca di papa che poggia su uno spesso strato di roccia lavica. fortunatamente abbiamo potuto usare gli elementi forniti dalla stazione sismica di firenze-arcetri (la quale poggia su una zona in cui prevalgono l'eocene e il pliocene terziario -) distante dall'epicentro 217,346 km. dalle registrazioni, individuata esattamente l'onda pg. abbiamo effettuato con la maggior precisione possibile le misure del[2] 5 8 4 d. di f i l i p p o l . m a r c e l l i le tre componenti relative al primo massimo dell'onda stessa, ottenendo i seguenti risultati: * a = 4 , 0 v r z = l ' , 7 2 * k = 6 , 7 2 tk=2 ,1 * „ = 4 64 t n = 2 ,1 di qui l'angolo di emergenza apparente risulta, applicando la [ 3 ] e = 26° 36' 1 3 " con la [ 2 ] ? assumendo per vi e t)« i valori da noi calcolati nella precedente nota ( ] ) (v, = v,,e — 5,46 km/sec, v2 = v,.g = 3 , 0 1 km/sec) abbiamo trovato per l'angolo di emergenza vero il valore e = 17° 36' 39",74 conosciuto in tal modo e, e nota la distanza epicentrale di fi1 4 0 i sec i 1 2 0 80 4 0 ~ ? 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 — k m fig. 4 lenze, si deduce immediatamente dalla tabella n. 1 e dalla fig. 1, il valore della profondità ipocentrale: si ha infatti li r= 5 km il risultato conseguito si accorda in maniera addirittura sorprendente con il valore di 3,2 km cbe era stato trovato precedentemente applicando il metodo di inglada alla stazione di roma, specie se si tien conto del fatto già altrove sottolineato che quando la profondità ipocentrale è molto piccola tutti i metodi risentono notevolmente degli errori di osservazione, o strumentali. la bontà del metodo usato trod r o m o c r o n a p * t e r r e m o t o d<zl s r a n 3 a « o ( 5 l x 5 0 ) t e m p i di tragitto d e l l e onde p * e s p e s s o r e dello s t r a t o d e l granito 5 8 5 va quindi brillante conferma; tanto più soddisfacente se si osserva cbe per una profondità ipocentrale di 5 km il limite di ricezione delle pg è di km 279,7; come si vede dalla tabella i i nella quale sono riportate tutte le distanze limiti a cui possono essere ricevute le pg per varie profondità ipocentrali. da questa tabella, calcolata altrove per l'italia centrale (4), risulta chiaro che la distanza epicentrale di firenze (km 217,3) è ben lontana dal limite calcolato di 280 km, e quindi sono da escludere gli eventuali errori di margine. t a b e l l a i i h in km distanze limiti delle pg 00 292,4 km 01 290,0 » 05 279,7 » 10 265,7 » 15 249,7 » '20 230,8 » 25 205,9 » dromocrona delle onde p * . — dallo spoglio dei sismogrammi abbiamo rilevato che in diverse stazioni abbastanza vicine all'epicentro la fase delle onde p * — generalmente poco visibile — era piuttosto chiara. ne abbiamo approfittato per calcolarne la dromocrona onde poter confrontare la velocità di propagazione con i risultati trovati da altri ricercatori. le figg. 2-3 mostrano esempi molto chiari di p* nelle stazioni di chur e stuttgart. le stazioni adoperate per questo studio sono 11. nella tabella i i i t a b e l l a i i i stazioni a in km tempi di arrivo delle p * ôb8. ^calc. 1 p a d o v a 341,265 55",7 + 0,2474 2 trieste 349,382 56 ,4 — 0,3245 3 t a r a n t o 397,747 64 ,7 + 0,3970 4 pavia 445,741 71 ,4 — 0,4250 5 messina 515,431 82 ,7 — 0,0462 6 cliur 567,068 91 ,2 + 0,3618 7 b e o g r a d 627,446 101 ,1 + 0,7999 8 ziirieh 655,556 103 ,9 — 0,8052 9 w i e n n 679,630 107 ,7 — 0,7778 10 basel 717,408 114 ,5 + 0,1010 11 stuttgart 765,463 122 ,4 + 0,4712 5 8 6 d. di f i l i p p o l . m a r c e l l i accanto a ciascuna ili esse sono riportati i tempi di arrivo delle p w , espressi in secondi, riferiti per comodità e per uniformità con le dromocrone delle altre fasi calcolate precedentemente, ad un tempo origine 04" 09"" 00s l'equazione più probabile per la dromocrona delle p * , ottenuta con il solito metodo dei minimi quadrati è risultata : t = ( 0 , 1 5 6 7 + 0 , 0 0 1 2 ) a + (1,9728 + 0,6618) che si può anche scrivere t= ( ( 1 , 9 7 + 0 , 6 6 ) 6,38 + 0,05 risulta di qui che la velocità delle p* nell'italia centrale è u p « = 6 , 3 8 km/sec riassumiamo nel seguente specchietto i valori trovati per questa velocità da altri ricercatori : gutenberg — germania meridionale 7,1 km/sec conrad — tauri — 6,29 » conrad — schwardorf vienna — 6,47 » griife — tirolo settentrionale — 6,7 » caloi — prealpi carniche — 6,4 » caloi — altipiano del cansiglio — 6,61 » rosini — garfagnana — 6,63 » di filippo-marcelli — italia centrale — 6,38 » il valore da noi ottenuto è molto prossimo al valore 6,45 che caloi ha trovato per la velocità vera delle p* ("). nella fig. 4 abbiamo tracciato la dromocrona delle p* e nella tabella i i i abbiamo riportato gli scarti fra i tempi osservati e quelli calcolati. la somma dei quadrati degli scarti è [vv] = 2,76316 coincidente quasi con [ii.2] = 2,76743 come è richiesto dalla teoria degli errori. spessore dello strato del granito. — poiché l'onda p * è dovuta all'esistenza di una prima superficie di discontinuità, limitante inferiormente lo strato del granito, abbiamo ritenuto opportuno servirci delle caratteristiche sismiche rilevate nello studio di questo terremot e m p i di tragitto d e l l e onde p* e s p e s s o r e dello s t r a t o del granito 5 8 7 to per calcolare lo spessore di tale strato in corrispondenza dell'italia centrale. il metodo più usato per questo calcolo si basa su semplici considerazioni geometriche ove si supponga che i raggi sismici si propaghino in maniera rettilinea (5). nella fig. 5 sia is il tragitto delle onde dirette pg, ei la profondità ipocentrale li, iabs il tragitto delle onde rifratte, d lo spessore dello strato del granito, in cui si propagano direttamente le pg. se fi è la differenza fra i tempi di registrazizone in s delle pg e delle p * , se si indicano con vpg e rh» le velocità di propagazione delle onde stesse e se si trascura il valore di h" rispetto a quello di a2 (per stazioni sufficientemente lontane), si perviene alla seguente formula di immediata applicazione pratica a _ a _ 6 tvg t v * fig. 5 2 dz _l_\s «... i v,„ + h la tabella i v contiene i calcoli effettuati per 6 stazioni. i valori delle grandezze che compaiono nella formula sono quelli da noi ricavati in questo lavoro e nel precedente ( j ) . t a b e l l a i v stazioni tempi di arrivo delle pg d p a d o v a 60,7 23,620 trieste 62,0 21,580 t a r a n t o 70,0 29,900 pavia 80,4 17,048 messina 91,2 29,390 chur 101,7 26,020 la media dei risultati è dunque 24,593 km. osserviamo però che il valore ottenuto per pavia differisce notevolmente dagli altri; tuttavia ricordiamo che la natura particolare del sottosuolo di questa stazione 588 d. di f i l i p p o l . m a r c e l l i influenza indubbiamente la propagazione dei vari tipi di onde abbassandone molto la velocità (°). il coefficiente di poisson che fu calcolato ivi da aliverti e solaini, risultò addirittura 0,45, mentre dalle velocità da noi trovate risulta essere 0,28. abbiamo perciò ritenuto lecito non tener conto del valore di d dato da pavia: in questo modo il valore medio risultante è d = 26,102 km caloi nello studio dell'appennino tosco-romagnolo dell'11-2-1939 ( 8 ) aveva trovato per lo strato del granito nell'italia centrale uno spessore compreso tra i 25 e i 30 k m : rosini per il terremoto della garfagnana ( 9 ) trova uno spessore medio di 32 k m : notiamo però che quest'ultimo aveva ottenuto una velocità delle p * di 6,63 km/sec. mentre caloi aveva assunto un valore medio di 6,4 km/sec. cioè molto più prossimo al nostro. infine, con il metodo della dispersione delle onde superficiali, valle e festa ( 1 0 ) hanno trovato come media generale per l'italia uno spessore 25,9. i l nostro risultato, quindi, conferma pienamente l'ordine di grandezza dello spessore del granito in corrispondenza dell'appennino. roma — istituto nazionale di geofisica — ottobre 1951. riassunto proseguendo lo studio del terremoto del gran sasso d'italia del 5 settembre 1950 si rilevano alcune caratteristiche sismiche dell'italia centrale. si determina la profondità ipocentrale con il metodo dell'angolo di emergenza. indi si costruisce la dromocrona delle p* e si calcola lo spessore dello strato del granito relativamente alla zona. b i b l i o g r a f i a ..t-iì f1) di filippo d. marcelli l. : uno studio del terremoto del gran sasso d'italia del 5 settembre 1950. annali di geofisica, voi. iv, n. 2 (1951). ('-) caloi p . : calcolo delle profondità ipocentrali in funzione della distanza epicentrale e dell'angolo di emergenza delle onde p. l a r i c e r c a scientifica, anno v , voi. ii, n. 3-4 (1934). t e m p i di t r a g i t t o d e l l e onde p * e s p e s s o r e dello s t r a t o del granito 5 8 9 ( s ) caloi p. : nuovo metodo per calcolare le profondità ipocentrali. la ricerea scientifica, anno i v , voi. i, n. 8 (1933). ( 4 ) marcelli l . : caratteristiche fondamentali delle onde longitudinali dirette nell'italia centrale (toscanai. la r i c e r c a scientifica, anno x i i i , voi. xx, n. 10 (1942). ("') caloi p . : ricerche su terremoti ad origine vicina: scosse del consiglio. la r i c e r c a scientifica, anno ix, voi. i i , n. 7-8 (1938). (0) aliverti g. solaini l . : sulla velocità di propagazione delle onde sismiche su brevi percorsi. annali di geofisica, voi. i l i , n. 4 (1950). ( " ) caloi p . : sulla velocità di propagazione delle onde p' e sullo spessore dello strato del granito nell'europa centrale. la r i c e r c a scientifica, anno x i , n. 11 (1 940). ( 8 ) caloi p . : caratteristiche sismiche dell'appennino tosco-romagnolo. la r i c e r c a scientifica, anno xi, n. 4 (1940). ( b ) rosini e . : il terremoto della garfagnana. la r i c e r c a scientifica, anno x i , n. 7-8 (1940). ( 1 0 ) festa c. valle p . : lina valutazione dello spessore dello strato del granito nel mediterraneo centro-occidentale. annali di geofisica, voi. i, n. 4 (1948). 32karcz.qxd 759 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical seismology – paleoseismology – israel – dead sea rift 1. introduction catalogues of historical earthquakes in the holy land include numerous events with inflated implied damaging intensities, which reflect decisions of the cataloguers rather than actual felt reports. locally written historiographic materials are scarce and earthquake echoes in religious, homiletic and literary texts are difficult to identify and elucidate. since in the past two millennia the country was under the yoke of different regional powers, cataloguers tended to rely on chronicles and documents written in distant administrative and cultural centers of the day, and extrapolated earthquakes reported to have occurred there to palestine and jordan. such decisions to extend the spread of damage to the holy land, often relied on a tenuous correlation to some supposed seismic literary motive in one of the texts written locally, or on a more sweeping assumption that the original chroniclers simply did not bother to record damage sustained by a distant and strategically and economically depressed province. moreover, some early cataloguers (a detailed list is given in table i) seem to have viewed the dead sea rift (dsr) and the anatolian fault zone as an interconnected seismogenic system that affects the whole near east simultaneously. implications of some early jewish sources for estimates of earthquake hazard in the holy land iaakov karcz 14 shay agnon, jerusalem, israel abstract for the past two millennia the holy land was under the yoke of successive invaders and oppressors, not a fertile ground for growth of historiographic traditions. consequently, earthquake cataloguers had to rely largely on chronicles and texts written at distant administrative and cultural centers of the day, where earthquake destruction suffered by a culturally and economically depressed province may have been overshadowed by damage in more important parts of the empire. on this assumption, and aided by an implicit notion that the lands bounded by the dead sea rift and anatolian fault systems are seismically contiguous, early cataloguers often extended the impact of earthquakes documented in nearby east mediterranean countries to the holy land. once published, such reports of supposed destructive intensities in israel were used by judaic scholars and archaeologists to date poorly defined, often metaphoric, literary seismic echoes, and to justify assigning seismic origin to equivocal signs of damage, asymmetry, or abandonment at archaeological sites of corresponding age. the spread of damage and intensity portraits are therefore enhanced and distorted, and so is their application in palaeoseismic analysis. four test cases are presented, illustrating the use and misuse of local judaic sources in identifying destructive intensities supposedly generated in the holy land by earthquakes of 92 b.c., 64 b.c. and 31 b.c., and in postulating a regional seismic catastrophe in 749 a.d.. mailing address: dr. iaakov karcz, 14 shay agnon, jerusalem 93589, israel; e-mail: karcz@zahav.net.il 760 iaakov karcz this paper focuses on the possible implications of some of the locally written ancient local jewish sources in study of the holy land earthquakes. the contents are presented in a series of test cases, which examine the use and misuse of such documents. in each case a very brief explanation of the character, advantages and shortcomings of each document or group of documents is given. the issue of 92 b.c. earthquake shows how perception of seismic, historic and geographic elements in ancient texts affects the implied damaging intensity estimates for the 198/154/ca.140/130 and the 92 b.c. israel catalog entries; the earthquake of 64 b.c. illustrates the circuitous way in which a distant earthquake not only was extended to jerusalem, but also how on strength of palaeoseismic correlations its epicenter was moved from north syria-anatolia to the dead sea; 31 b.c. earthquake presents the problem of geographic perception of spread of damage, and draws attention to the self enhancement of catastrophic image through adhesion of supposed literary earthquake motives; and 747-749 a.d. earthquakes illustrate the pitfalls of fusion of felt reports dated in different time-counting author type location sources comments perrey (1850) cat lev prim-sec 4th-19th cent. tholozan (1879) l s ns fragmentary diener (1886) l le ns fragmentary arvanitakis (1904) cat isr sec-prim blankenhorn (1905) l isr ns willis (1928, 1933) cat isr catl sieberg (1932) cat isr, s nss shalem (1949, 1951) isr prim-sec-catl fragmentary amiran (1950-1951) cat isr catl-sec-prim grumel (1958) l b prim-sec 4th-15th cent. plassard and kogoj (1962) cat le, s cat ben menahem (1979) cat isr, le, s, catl-sec-prim-arch taher (1979) cat a prim-sec 6th-18th cent. poirier and taher (1980) l s ns al hakeem (1987) l s ns alsinawi (1988) l a ns degg and doorenkamp (1989) cat isr catl ghawanmeh (1992) l lev catl-sec 8th-18th cent. al tarazi (1992) l j ns cat – catalog; l – list; loc – area of prime interest to the author; isr – israel, j – jordan; s – syria; le – lebanon; lev – levant (is, jor, le, sy); a – arabia; b – byzantine; catl – catalog data; sec – secondary sources; prim – primary sources; arch – archeological data; nss – no specific sources; ns – no sources. table i. catalogues and lists of historical earthquakes in the holy land. 761 implications of some early jewish sources for estimates of earthquake hazard in the holy land frames into a single earthquake, which then grows through adhesion of reports of supposed archaeoseismic damage. 2. seismotectonic framework israel lies near the junction of the eurasian, african and arabian plates (fig. 1). its tectonic pattern is dominated on the south by the arabonubian craton, on the west by the syrian arc a sigmoidal belt of late cretaceous-neogene folds and faults that extends from the sinai peninsula up to palmyra in syria – and on the east by the dead sea rift (dsr) – a more than 1400 km long complex transform fault along which the arabian plate slips northwards relative to the sinai-israel part of the african plate at a mean long term rate of 5 to 10 mm/yr (for references: e.g., garfunkel, 2001; garfunkel et al., 1981; salamon et al., 2003; westway, 2003). rather than a sensibly straight continuous dislocation, the dsr consists of colinear and offset segments, and shows local restraining and releasing bends and stepovers with associated transpressive blocks and transtensional pullapart basins. most studies assume a post eocene interplate displacement of 105 km, that occurred in two stages: first a continuous northerly movement of 75 km, and then, due to a change in pole of plate motions 4 myr ago, a diagonal extension with normal faulting and ultimate development of rift morphology. obviously, ongoing motion along dsr and the convergence of eurasia and africa would require rotations, displacements, and space readjustments also across the syrian arc structures. geological evidence of such differential vertical and tilting movements was found but the inferred rates and sense of motion differ widely. while distribution of recent seismicity (fig. 2) confirms that dsr is the major seismogenic element in the region, activity along individual segments is uneven with a strong clustering along the gulf of elat and dead sea segments, and conspicuous quiescence along the intervening arava and jordan valley segments. it appears that the segmentation would lead mainly to moderate magnitude earthquakes say ml = 5.5-6.5, with intensities determined by depth and site conditions. seismic activity was documented also along dsr’s nw aligned subsidiary fault zones, particularly the carmel-tirza zone, and some low magnitude events were identified also farther away along the syrian arc, suggesting that some reactivation of tectonic elements may have taken place in the past. the early attempts to quantify the regional seismic regime relied on scanty instrumental data, on some 20th century macroseismic observations, on historic earthquake catalogues and on the general distribution of the main tectonic elements. arie (1967) attributed most of fig. 1. tectonic sketch map (after picard, 1943; bartov, 1990). 762 iaakov karcz the significant regional activity to dsr main fault, discounting the ne-sw trends and the nw-se trends suggested respectively by the isoseismal maps of sieberg (1932) and shalem (1951), and suggested magnitude-frequency and magnitude-intensity relationships logn = = 4.9 − 0.8 m and m = 0.5 imax + 1.8, with a recurrence interval of about 40 year for m = 6 and 220 year for m = 7. ben-menahem et al. (1976), and ben-menahem (1979, 1991) used the historic catalogue data base far more extensively, estimating magnitudes and locations along the main seismotectonic zones of the east mediterranean region. a uniform b = 0.86, and a values of 3.35 for the mediterranean offshore zone, (with mmax = 7.2 every 2600 year), 3.1 for the israel inland segment of dsr (with mmax = = 7.3 every 1500 year), and 3.75 for the syrian segment (mmax = 8 every 1400 year) have been calculated. with small modifications these values were accepted also for jordan (abou karaki, 1987; kovach, 1989; kovach et al., 1990). following two decades of countrywide seismological monitoring and a more detailed structural analysis with emphasis on prospective postpliocene faults, shapira and shamir (1994), shamir et al. (2001), and shapira and hofstetter (2002) proposed more detailed frequency-magnitude relationships. they identified 17 seismotectonic zones in israel and 7 around it, for which they obtained a uniform b = 0.96, except for 1.07 for suez, and 0.98 for cyprus, and mmax values of 7.5-7.75 for dsr, 6.5 for the carmeltirza fault and for the offshore zone and 5.5-6 for a greater part of the syrian arc structures. the catalogue is taken to be complete for m = 6.5 at least since 100 a.d., and for m = 6 since 1000 a.d.. the estimated recurrence intervals for the part of the dsr across the densely populated part of israel and jordan, between south lebanon to the dead sea, are 75 and 960 years respectively for m = 6 and m = 7. already north (1974, 1977) and ben-menahem et al. (1976) drew attention to the apparent difference between the total slip of 5-10 mm/yr along the dsr, which was inferred from long term geological evidence, and the seismic slip of 1-3 mm/yr computed from the short term instrumental and historical data. the deficit was attributed to some form of aseismic creep, but so fig. 2. seismicity of israel from 1984 to 2002 (israel geophysical institute, seismological division, 2003). 763 implications of some early jewish sources for estimates of earthquake hazard in the holy land far a decade of geodetic monitoring of the galilee-golan segment of the dsr indicated that no consistent motion in excess of 1mm/y could have taken place. an obvious alternate explanation was that the available historic record is too short to include large magnitude earthquakes (or clusters of earthquakes) with long recurrence interval. whatever the mechanism, the very idea of slip deficit seems to have led to intuitive overestimates of earthquake magnitude, and to a tendency to assign seismic origin to signs of abandonment, damage, and construction asymmetry at archaeological sites, as well as to features of soft sediment deformation and mass movement. it appears however, that some of the updated plate tectonics models suggest that the slip deficit is much smaller, with a total slip of only 4 mm/yr. even more extreme in this context are the conclusions of sneh (1996) that the distribution of neogene formations younger than 16 myr across the dsr is continuous and shows no lateral offset, and of horowitz (2001) who discounts the concept of the 105 km interplate displacement altogether. in the past twenty years studies of tectonics and seismicity of the eastern mediterranean and the dsr, too numerous to list here, attempted to incorporate past earthquakes in analysis of plate kinematics and in identification and characterization of active structures. unfortunately, unlike the rapid progress in collection and processing of instrumental data, in modeling of plate motions, and in field studies of individual faults, historic input changed very little and still relies on the collective wisdom of largely unverified and incestuous earthquake catalogues (table i). in the 20th century, more than two dozens of earthquake catalogues and lists were published, creating an impression of a complete and reliable data base, which was repeatedly researched and verified. rather than retrieve and cross verify the primary historical, literary and sacred sources, however, each cataloguer relied largely on his predecessors and on assorted secondary sources. thus, fragile or erroneous evidence propagated from one catalogue to another, gaining stature and credibility. in some cases indiscriminate copying of slightly different descriptions of the same event from different older catalogues led to doublets and even triplets in the newer ones, while at the same time different events were fused merely on the strength of assumed proximity in time or location. transmission of evidence through the catalogues is uneven, and changes in dates, location and extent of damage or even elimination of events are left unexplained. such disregard of historical methodology and inadequate retrieval and analysis of primary sources, resemble the historical seismology of europe half a century ago. in our case, however, shortcomings of the data base are treated as an unavoidable evil, since the century-long earthquake cataloguing is assumed to have exhausted all possible sources of information. consequently, too often this residual uncertainty provides a license to tailor historic magnitudes and epicenter locations (including import of distant earthquakes), to the chosen tectonic concept, with the result that vague statements on the «overall historic seismicity» are invoked in support of contradictory claims, such as a threefold deficit of seismic slip on one hand, and a close fit between the total and the seismic slip on the other. catalogue data are used also in paleoseismic correlations to confirm the earthquake origin of alleged sedimentary «seismites» (e.g., ken-tor et al., 2000; migowski et al., 2004), yet the choice of «seismite» producing events from the catalogues appears to be guided by frequency and sequence of the seismites to be confirmed. 3. local historical sources most of the catalogued information about historical earthquakes in the holy land comes from chronicles and texts written outside. this is not surprising, since for two millennia (except perhaps for the crusader kingdom), this area was not autonomous and was governed from distant capitals be they roman, byzantine, ummayad, abbaside, mamluk or ottoman (table ii). shifting political fortunes, invasions and conquests, and internal clashes driven by differences in religion, traditions, and political allegiance, arrested the economic and cultural development for extensive periods. in the major centers of the day, where chronicles were written and archives were kept, the attitude to the holy 764 iaakov karcz land and jerusalem was marked by a separation between the heavenly and the earthly and mundane. places sacred whether to jews, christians or moslems, were revered and extolled in mystic religious terms, but except for drastic political changes and acts of persecution, daily events, were given little weight. the chroniclers may have been lax in verifying possibly distorted or exaggerated news from afar, but also may have not bothered to report a destructive earthquake in this depressed area, either because it was overshadowed by damage across the more important parts of the empire, or because it was of little interest to the writers and readers alike. on the latter assumption the modern cataloguers extrapolated to israel and jordan the impact of earthquakes that hit the levant as well as turkey, armenia and greece. since the implied damaging and destructive intensities not only inflate the local macroseismic record, but also increase the radius of damage (and thus also the inferred magnitudes) to vast proportions, verification based on materials written locally or focused specifically on the holy land is called for. one little explored group of such sources, which includes the jewish and samaritan texts forms the main topic of this paper. from the outset it should be clear that these materials differ in character from the roman, byzantine and moslem chronicles. the unhappy political history of the jewish people after the destruction of the second temple in 70 a.d. by the romans and the end of independent statehood, discouraged the writing of historical chronicles. except for a few works, such as those of josephus flavius (the 1st century a.d. jewish-roman historian), the books of maccabees (a part of the roman catholic canon), and megillat taanit (a pre-talmudic, 1st century a.d. or earlier, text with a later interpretative scholion), the jewish writers stressed the universal theological, legal and moral aspects of contemporary happenings, with little attention to factual details. only sketchy chronological data are available about even the leading figures, whose writings and oral rulings were embedded in the talmud and in post-talmudic literature and molded the future spiritual life of the nation. similarly shrouded in uncertainty is history of the early jewish poetry and chronology of the leadings poets. because of emphasis on argumentative, legal and philosophical approach, and the convolute language full of biblical allusions and metaphors, it is often hard to perceive whether the text refers to a real event. the scriptures as well as the later homiletic and poetic works frequently use earthquake motives as a descriptive tool in depiction of divine wrath and retribution, in extolling heavenly and worldly might, in theophanies and in apocalyptic visions. elucidating the true intention of the writers is complicated further by the lack of punctuation and phonological signs, which introduce further ambiguities in perception of texts written in the 22 letter vowel-less alphabet. usually, the later interpreters preferred to assume that the text reflects a true earthquake since this provided a possible chronological indicator of the time of the author. first, the broad time frame of the text was inferred from characteristics of language and style and then the suspect earthquake echo was attributed to the most prominent or closest shock reported in that time range by one of the local or regional catalogues. once made, such tenuous correlations often bounced back into seismological literature as a confirmed fact, indicative of local earthquake intensity. y table ii. dominant regimes in the holy land. hellenistic period early hellenistic 332 b.c.-167 b.c. late hellenistic 167 b.c.-37 b.c. roman and byzantine periods early roman 37 b.c.-132 a.d. late roman 132 a.d.-324 a.d. byzantine 324 a.d.-638 a.d. early arab to ottoman periods umayyad 638-750 abbasside 750-1099 crusader / ayyubid 1099-1291 late arab 1291-1516 ottoman 1516-1917 765 implications of some early jewish sources for estimates of earthquake hazard in the holy land caution is needed when attempting to identify an earthquake by checking whether a date given or implied in an ancient jewish text falls on the day and month date of an earthquake indicated in one of the occidental or oriental texts which use one of a myriad of calendars and eras followed by other communities of the region (e.g., grumel, 1958; bickerman, 1968; samuel, 1972; meimaris, 1992). the jewish lunisolar calendar (months according to the moon and the years according to the sun, thus ensuring that festivals celebrated on fixed dates fall also in fixed seasons) is based on a 19 years cycle, with 12 years of 12 months and 7 leap years of 13 months (for bibliography: e.g., mahler, 1916; akavia, 1953, 1976; frank, 1956; finegan, 1998; stern, 2003). in early times all calendar decisions (start of a new month, festivals, intercalation of the 13th month to compensate for the difference between the lunar and solar years) were in the hands of the religious leadership (the sanhedrin) and were based not only on observation of the moon and the sun and on a secret system of calculations, but mainly on the ripeness of the crops. when growth was arrested by unfavorable conditions it was common to intercalate another month, to delay the spring and the celebration of passover. the order of intercalations therefore was not fixed as it is now (years 3, 6, 8, 11, 14, 19 in each 19 years cycle) and indeed according to one of the traditions, once rabbi akiva (1st century a.d.) had to intercalate three years in succession. while the general calendar rules were published by rabbi hillel ii in mid-4th century, order of intercalation was not unified until the final sealing of the calendar practices in the times of rabbi saadia hagaon about 920-930 a.d. thus, attempts to harmonize pre-10th century felt reports on strength of coincidence of the day and month dates given in jewish and christian sources may be misleading, if based on current conversion tables, such as those of akavia (1976). further chronological indicators used in jewish texts include the era of adam, which starts in am 3760 with creation of adam, creation era which starts a year before the era of adam in am 3761, destruction era that starts from the destruction of the second temple by the romans in summer 70 a.d., the era of contracts synonymous with seleucid era (babylonian count starting spring 311 and macedonian count starting autumn 312 b.c.) and a continuous sabbatical count, which marks a fallow year every seventh year. though ostensibly the starting points are well defined, the texts indicate some deviations, whether as a result of confusion between the eras of adam and creation or because of different perception of the month on which reckoning starts, nissan in the spring, or tishri in the autumn (so that even the hebrew date for destruction was assigned alternately to 3828, 3829 and 3830 of creation era). in palestine, the era of contracts starts in autumn 312 b.c., am 3449 (for references: e.g., meimaris, 1992; di segni, 1997), however some texts (including 1 maccabees) follow the babylonian system and start the count in spring 311 b.c., while frank (1956) reports also evidence for start in autumn 313 b.c.. even the count of sabbatical years, may have not been followed uniformly (for references: e.g., wacholder, 1973; stern, 2003) in spite of its far reaching economic and social implications. thus detailed calibration of a first millennium hebrew year date, becomes meaningful only when three such independent chronological indications are given. examples of analysis and use of the early hebrew sources outlined below include the pre-1st century a.d. quasihistorical scroll of fasts and its later scholia and their implications in analysis of the 198/∼ 140/92 b.c. earthquake reports; the 1st century a.d. historical works of josephus flavius and pre-6th talmudic tractates for the 64 and 31 b.c. earthquakes and a liturgical poem, a 10th century prayer book and two samaritan chronicles for the 747-749 a.d. earthquakes. a voluminous literature developed around each such text or group of texts. to keep the bibliography within reasonable limits only a few works are cited in each case, usually those with the most extensive list of references on the topic. 4. 92 b.c. this event illustrates the role of locally composed jewish texts in analysis of several 2nd century b.c. earthquakes that were import766 iaakov karcz ed into the israeli catalogues from elsewhere in eastern mediterranean. catalogues used in assessment of seismic hazards in israel (e.g., ben-menahem, 1979, 1991; degg and doorenkamp,1989; amiran et al., 1994), list a major earthquake (ml = 7.1) and a tsunami, that caused extensive damage in syria, israel, cyprus and egypt in 92 b.c.. they rely on entries in the older catalogues of willis (1928, 1933) and of plassard and kogoj (1962). the former copies mallet (1853), who in turn copies von hoff’s (1840) undocumented «erdbeben in syrien und auf der insel cypern» and the latter follow sieberg’s (1932) undocumented «in syrien ein schweres erdbeben das bis agypten gefult wurde». the third source is an entry in the pre-70 a.d. megillat taanit (scroll of fasts) retrieved by shalem (1956). classical sources that report an earthquake that hit apamea in phrygia about that time, do not mention any effects in egypt, israel, levant nor cyprus (e.g., guidoboni, et al., 1994; ambraseys and white, 1996, 1997). admittedly, at first ambraseys et al. (1994) inferred from a greek inscription at magdolum that an earthquake hit egypt between 97 and 94 b.c., however it turned out that «seismos» in the inscription probably refers to «extortion» (mazza, 1998) or «disturbance» (ambraseys and white, 1996). therefore, the scroll of fasts remains the only primary record of damaging intensities and tsunami in this region and thus merits further attention. 4.1. evidence of megillat taanit (scroll of fasts) this text, written before the destruction of the second temple in 70 a.d., is a summary of earlier oral traditions which lists 35 day and month dates of joyous events in early jewish history, mainly during the hasmonean period (167 b.c.-37 b.c.), on which mourning and fasts were prohibited (for references: noam, 2003; for non-hebrew text: e.g., grossberg, 1905; zeitlin, 1922; lichtenstein, 1931). emphasis is on round the year observance and not on historiography, so that events are listed not in a chronological order but by month and day on which they happened. though these rulings were observed only for a few centuries, the text was transmitted and preserved in the jewish canon. scroll of fasts includes the scroll (megilla), which is very brief (since at the time of writing the commemorated events were still fresh in memory) and three versions of later addenda (gemara = scholion), named after the three main surviving manuscripts, which explain the reasons for rejoicing on the listed dates. this elucidatory process, continued through the talmudic period (2nd-6th century) until the late middle ages, but the source, veracity and sense of many of the addenda, which were appended at different times and by different persons, are not always clear. while many of the explanations are historically correct and reflect information transmitted orally or gathered from unspecified textual sources, others raise doubts as to the competence of the commentator. scribal errors are common, particularly in copying of geographic and personal names. the problem is particularly vexing since the text survived only in few late copies which differ conspicuously from each other. the three principal versions known are the parma manuscript (copied in the 14th century), the oxford manuscript (copied in 15th century) and the common («vulgata») version, which is the latest. the parma and oxford manuscripts differ in all bar four interpreted events and must have evolved separately, the former from the babylonian and the latter from the jerusalem talmud. the common version, which appears to combine the former two, adds some further details of unknown provenance. evidently, this blending of information was not accomplished in one go but represents a process recently shown to have started before the 11th century and to have ended in 1519 a.d. when first printed version was published in mantova (noam, 2003). the scholia represent therefore an aggregate of additions appended at different times by different editors and must be treated with caution. the relevant parts of the entry for the 17th of the month adar (the 33rd entry), in the scroll and in the three versions of the scholion are presented in table iii in literal unedited translation (even if the english text is unwieldy) indicating where a word may have two meanings. the 767 implications of some early jewish sources for estimates of earthquake hazard in the holy land scroll and the parma version do not mention earthquake nor inundation, oxford version may refer to an earthquake and the common version to an earthquake and a tsunami or inundation. though the word «raash» may stand also for «clamor» or «noise», the oxford version seems to indicate that 17th adar commemorates the rescue of scribes (sages) by a god sent disaster. the common version, derived from the parma and oxford texts, is more equivocal. the word «zia» («earthquake», «shock» but also «scare» or «tumult») is used to describe the blow dealt to the scribes and the fortuitous escape of survivors from tumult of attack without invoking any cataclysmic divine intervention. on the other hand, this version adds a report by rabbi hidka (one of the 2nd century a.d. sages) of a destructive inundation at the time «when the natives rose to kill the sages», implying that it interfered with the attack. the oxford and the common texts either reflect an otherwise unknown earthquake and tsunami reported in now extinct local traditions, or present echoes of events already recorded in non-judaic chronicles and texts. thus details of location and time of the 17th adar events are identified first and then are compared with the available earthquake records. the consensus is that the misspelled location names belikos, belikot (scroll), chalbos (parma), cholbos, chalikus (oxford) coselicos and calicos (common) as well as beit zabdin (scroll), beit zabdai (parma), beit zabadi (oxford and common), stand respectively for chalcis and zabadea in lebanon-syria (for references, e.g., hampel, 1976; noam, 2003) (fig. 3). of the many localities in antiquity named chalcis two were in syria. chalcis sub libanum (thereafter chalcis sl), present anjar, ayn al jarr, was in the beqaa in lebanon, about 10 km se of present baalbek, near zabadea, a narrow plain with present zabadani, 25 km nnw of damascus and ancient kaprazabadion present qafr zabad, 25 km sw of chalcis sl (dussaud, 1927; millar, 1987). it seems less probable, that the texts refer to chalcis ad belum (thereafter chalcis ab), present hadir and kinnisrin in north syria, less than 100 km table iii. comparison between the original text of the «scroll of fast» and the three versions of the scholion to the 33rd entry. scroll of fast «parma» scholion «oxford» scholion «common» scholion «... on the 17th the natives attacked the remnant of scribes in the country of belikos and beit zabdai and a salvation came to the house of israel». «... and there was a release since the natives wanted to kill the scribes of israel and they went to beit zabadi and sat there ... and the day they escaped was made into a holiday». «... and there was salvation when king janaeus saw killed bukinos and bukius his brother and escaped from him and went to syria and in the country of blikus all natives gathered to kill them and he [the almighty] sent onto them a great «raash» [earthquake, clamor] and there was a great blow amongst them». «... and when janaeus came down to kill the scribes they escaped from him and went to syria and stayed in country of koselikos and the gentiles there rose to kill them and they «heziu» them a great «zia» [shocked them a great shock, scared them great scare] and they struck them a great blow and left some survivors and they went to bet zabadi ... rabbi hidka says the day the natives wanted to kill the scribes of israel the sea upwelled and destroyed a third in the settled land». 768 iaakov karcz se of antioch (e.g., pauly and wissowa, 1899, iii, c. 2078; grainger, 1990, 1997; barrington atlas, 2000). none of localities with names akin to zabadea (zebed, zebadi on the orontes and zabboude) near chalkis ab appears to have a source in antiquity (doussaud, 1927). admittedly, chalkis sl, chalkis ab and zabadea are away from the levant coasts, but rabbi hidka’s report may imply only that the news of a catastrophic inundation along the littoral diverted the attackers. judaic scholars suggested two possible time frames for the events of 17th adar in the oxford and the common versions. one follows the references to king alexander janaeus (103-76 b.c.), whose persecution of pharisee opponents led to their flight to syria (josephus, wars 1.4.6: thackeray, 1976). the second, denies that the reference to janaeus has any historical significance and assumes that the widely detested cruelty of the king (josephus, antiquities 13.13.5; 13.14.2: marcus, 1976; josephus, wars, 1.4.3-6: thackeray, 1976), led the later day interpreter to blame him also for the misfortunes of the «remnant of scribes». according to this interpretation, the term refers to survivors from oppression and slaughter that followed one of the political upheavals in the 2nd century b.c. levant, when segments of the multicultural population were paying heavy price for shifts in political fortunes and shortlived alliances. indeed, in the oxford version the reference to janaeus seems to be garbled and inserted out of context and «bukius and bukinos» are regarded as a colloquialism rather than real persons (efron, 1987). instead, the 17th adar salvation of the scribes is thought to have taken place during the victorious campaign of jonathan maccabaeus (160-142 b.c.) against demetrius nicator and then against the zabadeans: «jonathan pursued them, but he did not overtake them, for they had crossed the eleutherus river, so jonathan turned aside against the arabs who are called zabadeans and he crushed them and plundered them». (1 maccabees, 12,30-31). fig. 3. orientation map for the 198-92 b.c. earthquakes. 769 implications of some early jewish sources for estimates of earthquake hazard in the holy land 4.2. evidence of classical sources classical sources report earthquakes during the reign of both kings. janaeus’s persecution of his religious opponents and their flight to syria agree roughly with the ca. 90 b.c. date estimated by guidoboni et al. (1994), and ambraseys and white, (1996, 1997) for earthquakes that destroyed apamea in phrygia (apamea kibotos, present kelenai on the meander river in turkey). according to athenaeus’s extract from 1st century nicolaus of damascus (deipnosophists, 8.332: gulick, 1957), the earthquake was accompanied by flooding, change of river courses, appearance and disappearance of lakes and ingression of sea waters, while strabo (geog., 12.8.18: jones, 1917), tells of earthquake destruction of apamea and the gift of mithidrates eupator to rebuild it. since apamea kibotos is a 100 km of hilly country away from the coast, catastrophic river floods along meander and marsyas and changes in nearby lake levels are more likely than a marine ingression. floods were common in this region and schurer (1979) comments on the noah’s elements in the local lore and points out that the term kibotos, was usually employed for noah’s arc. during jonathan’s reign, between tyre and ptolemais in phoenicia a «wave from the ocean lifted itself to extraordinary height and dashed upon the shore engulfing all men and drowning them...», event extracted from the now extinct works of poseidonius by athenaeus (deipnosophists, 8.332: gulick, 1957) and by strabo (geog., 16,2.26: jones, 1917). since the event followed the battle between diodotus trypho (grainger, 1997; baldus, 1970) and the seleucid army, the date is constrained by the start of trypho’s mutiny in 145/144 b.c. and his suicide in 138/137 b.c. jonathan’s campaign against the zabadeans took place when he was still allied with trypho, probably in 143/142 b.c. this same event may be responsible for 1st century b.c. poseidonius’s report of a disaster along the levant littoral, transmitted by strabo (geog. i, 215: jones, 1917): «city above sidon collapsed into the sea and nearly two thirds of sidon itself was engulfed...and the shock extended over whole of syria though with only moderate strength». since the shock, amongst other violent occurrences, was described together with the famed emergence of the island of hiera in 198 b.c., the two were assumed to be concurrent. strabo’s text, however, is a discourse on unusual natural phenomena, rather than a report of interrelated contemporaneous events (cf. kidd, 1988, 1999; ambraseys and white, 1996). indeed, both 1st century a.d. seneca (nat.quest. 6.24.6: corcoran, 1972) and 1st century b.c. lucretius (de rerum natura, 6,535-607: bailey, 1947 and rouse, 1966) refer to the earthquake in sidon, without mentioning hiera and other marvels of nature. yet another account of an earthquake in jonathan’s timeframe appears in 6th century john malalas (chronicle 8.25: jeffreys et al., 1986) who reports that in the days of antiochus grandson of grypos who ruled for nine years: «antiochia the great suffered from ‘wrath of god’ in the 8th year of his reign in the time of macedonians 152 years after the original building of the wall by seleucus nicator at the 10th hour on 21st of peritios february. it was completely rebuilt as dominos the chronicler has written. it was 122 years after the completion of the walls and the whole city it suffered it was rebuilt better». as downey (1938, 1961, 1963) pointed out, the dates are inconsistent. 152 years from the foundation of the city by seleucius nicator in 300 b.c., indicates the year 148 b.c., but the only antiochus who ruled for 9 years was antiochus vii sidetes (138 b.c. to 129 b.c.) so that his 8th regnal year would have fallen in 130 b.c. having failed to reconcile the dates and having found the reference to 122 years not indicative (different quarters of the city were built and walled at different times, none on which corresponds to either of the two indicated dates), he attributed the confusion to malalas’s mistaken chronology of the seleucid monarchs and suggested that the entry lumps together at least two different earthquakes, one in 148 b.c. and the other in 130 b.c. while malalas often errs in reporting events much before his time (e.g., jeffreys et al., 1986; jeffreys, 1990; croke, 1990) and nothing is known about his source «domninos the chronicler», an alternate explanation downrated by downey (1938) may be more reasonable. up to the death of 770 iaakov karcz demetrius i soter in 150 b.c. malalas’s chronology of seleucid rulers is parallel to that indicated by other sources. for the next 12 years, however, the accounts differ and converge again at the ascent of antiochus vii sidetes (138-129 b.c.). misinformed on rulers and on events of this turbulent decade, malalas assumes that the power was in the hands of one «antiochus, descendant of grypus and son of laodice and aristhrates». it is not clear whom he had in mind, but since antiochus sidetes was the only descendant of grypus who ruled for nine years, downey assumed that malalas referred to him and dated the earthquake to the 8th year of his reign i.e. 130 b.c.. however, irrespective of who ruled and for how long, the 8th year from the year of demise of demetrius i soter in 150 b.c., a date on which all chroniclers agree, falls in 141/142 b.c., in close agreement with the estimated date of jonathan’s campaign and events of 17th adar in megillat taanith and the tidal wave at ptolemais. the month and day dates, this of 21 peritos in malalas and that of 17th adar in megillat taanit fall in the same season, very close to each other and whatever the intercalation practices were, would have differed by five weeks at outmost. for example, had the current system been followed in the 150-140 b.c. decade (which it definitely was not) the two dates would have coincided in 140 b.c. and would have differed only by 3 days in 146 b.c. though reduced from 18 years to 6, the inconsistency between dates given by malalas still casts a shadow over this attempt to tie the antioch earthquake with the events at chalkis-zebedani, ptolemais and sidon. 4.3. summary the uneven quality of textual evidence and obtuse presentation may of course reflect the confusion of the compilers of megillat tanit scholion as to the true nature of the 17th adar events and the possibility that their explanations are more inventive than informed, can not be ignored. whatever the case however and whatever the time frame chosen, the overall tenor of megillat taanit makes it most unlikely that a day on which a severe earthquake hit the holy land would be observed as a joyous day on which fasting and mourning are prohibited. in fact one would expect just the opposite, i.e. a fast to memorize the shock and its woes. thus the scholion should not be used to document any damaging earthquake intensities and tsunami damage in israel, whether in ca. 90 b.c. or ca. 140 b.c. festive commemoration, however, would make sense, if the earthquake affected a more distant region in syria, without harming the dilapidated and oppressed jewish communities there. comparison of the literary and historic characteristics of the oxford and common versions with descriptions of earthquakes and possible tsunamis in chronicles and texts, tips the scales slightly in favor of king jonathan’s period. so far no evidence was found of impact of the ca. 90 b.c. apamea kibotos earthquake on the egyptian-israeli-levant coastal cities, 750 km away. on the other hand, the reported tidal wave between tyre and ptolemais, 75 km from chalcis sl-zabadea, the earthquake and submergence at nearby sidon and the earthquake at antioch 250 km away all occurred during jonathan’s reign, about mid 2nd century b.c. though the time frame is the same and all affected localities lie along a narrow strip between the littoral and the seismically active northern segment of dsr, attributing all reports to one single earthquake and, more important, to one seismogenic element would run ahead of the available evidence. nevertheless, it is noteworthy that the recent paleoseismic study of the nearby serghaya fault, along the northern restraining bend of dsr revealed indication of only one event in the age range of 170 b.c.-20 a.d. (gomez et al., 2003). 5. 64 b.c. this event presents a further example of earthquakes imported into the israeli catalogues on flimsy evidence. in this case the damage to the temple compound and city walls of jerusalem, supposedly reported in the talmud projected an image of intensities high enough to locate the epicenter in the dead sea area. ben-menahem (1979, 1991) and amiran et al. (1994) report serious earthquake damage in 771 implications of some early jewish sources for estimates of earthquake hazard in the holy land jerusalem in 64 b.c., on the authority of previous cataloguers (arvanitakis, 1904; willis, 1928; sieberg, 1932; amiran, 1950-1951 and plassard and kogoj, 1962), as well as of the talmud. all these catalogues related the damage to a devastating earthquake of ml = 7.7 in north syria and antioch (fig. 4). since this earthquake was reviewed in detail by guidoboni et al. (1994) and by traina (1995) and was examined also by ambraseys and white (1996, 1997), only a brief summary is presented here. their analysis builds on the statement of justinus, 2nd century a.d., (epit. phill. hist. 40.2.1: seel, 1985) that a devastating earthquake hit syria towards the end of the rule of tigranes the great, causing widespread destruction and leaving 170 thousand dead. apparent disagreement over the duration of tigranes’s reign in syria led traina (1995) and guidoboni et al. (1994) to date the earthquake to 65 b.c. and ambraseys and white (1996, 1997), to 69 b.c. yardley and heckel (1997) warned however that justinus often misunderstood, confused and misdated events and characters in the original text of pompeius trogos. damaged locations are not specified, but 5th century a.d. orosius (hist. 6.5.1: deferrari, 1964) writing more than four centuries later, mentions an earthquake that took place while mithridates was attending a festival of ceres in the bosphorus (presumably the cimmerian bosphorus at sea of azov and crimea), but does not say whether the king experienced the earthquake or just heard about it (guidoboni et al., 1994). the southward extent of damage is equally uncertain. though malalas (chronicle 8.30: jeffreys et al., 1986), reports that pompey rebuilt the bouleuterion in antioch, established by antiochus epiphanes, «that has fallen», this does not necessarily indicate that the whole city was destroyed, as later catafig. 4. orientation map for the 64 b.c. earthquake. 772 iaakov karcz loguers assumed. thus the extensive damage to the city walls and the temple compound in jerusalem, 550 km to the south, allegedly reported in the talmud, would be very significant, since it would indicate an earthquake of truly catastrophic proportions. 5.1. evidence from the talmud talmud (from «study», «teaching») is a vast body of texts accumulated over seven centuries, that deal with religious and legal norms to be followed by jews. though the laws that govern jewish personal and community life were laid down in the pentateuch (tora), their practical application under ever changing realities required interpretation of the spirit of the written law. the new decrees were transmitted orally and the expanding legal code was known as the oral law (oral tora). in wake of political misfortunes, when religious and legal structures were disrupted, rabbi yehuda the prince (2nd century b.c.) compiled the mishna, a compendium of rules of conduct developed and transmitted by several generations of sages («tanaim»: the «repeaters»). since mishna is very concise and occasionally cryptic, it led to an interpretative corpus, gemara, of elucidative commentaries, explanations and legal discussions by the later generations of sages (called amoraim: «speakers»). one version developed in palestine during the 3rd-early 5th centuries and the other developed in the babylonian diaspora during the 3rd-6th centuries. while free of historiographic orientation and interests, this enormous body of texts, which spans 7 centuries of intense intellectual activity, includes numerous references to specific events. usually, chronological details must be deduced from the context. nevertheless, in absence of chronicles and histories, such material is very important, but its veracity and accuracy differ from one case to another and require a great deal of caution. the jerusalem talmud is often fragmentary and difficult to follow, but deals with some questions inapplicable to diaspora and is regarded by many to be more reliable for palestine. the babylonian talmud is better edited (by «savoraim» – the «thinkers») and is more comprehensive and innovative, but some elements may have been glossed over by the editors. accounts of the supposed damage in jerusalem emanate from the babylonian talmud, (tractates sota, 59 b: cohen, 1936; menahoth 64 b: cashdan, 1935; baba kama, 82 b: kirzner, 1935) which tells of fight for power between the sons of queen salome alexandra (d.67 b.c.), hyrcanus and aristobulus, the last kings of the hasmonean dynasty. hyrcanus and his nabatean allies laid siege to jerusalem, but at passover time agreement was reached on supply of sacrificial animals to the temple. a big basket with golden coins was lowered over the city walls, to be filled by the attackers and be hauled up. the attackers broke the agreement, took the gold but sent a pig instead and «when it reached halfway up the wall it stuck its claws into the wall and the land of israel was shocked over a distance of 400 parasangs by 400 parasangs» (a parasang, persian mile is about 4000 yard long). josephus (antiquities, 14.2.2: marcus, 1976) presents a more detailed account of the events but does not mention an earthquake nor a hog. instead, the vehicle of divine punishment for preventing and mocking the passover services is, «a strong and vehement storm of wind, that destroyed the fruits of the whole country, till a modius of wheat was then bought for eleven drachmae». likewise, classical texts record the war between the last hasmoneans (e.g., dio’s roman history 37.15.3-16.3: cary, 1969), without mentioning the earthquake. thus it is possible that the talmudic text refers not to a real earthquake but to the general shock over the ungodly breech of agreement, which disrupted the services in the temple and indeed this seems to be the approach in the jerusalem talmud. the earthquake motive appears to have been raised by graetz (1888) who commented that the talmudic text may contain an echo of the heavy earthquake that «according to dio hit asia about that time». this remark (which is glossed over in the later english and hebrew editions of graetz’s memoir), was included in another context by neubauer (1868) in his book on geography of 773 implications of some early jewish sources for estimates of earthquake hazard in the holy land the talmud, from which it was extracted by arvanitakis (1904) and included in his catalog. since the temple and the city walls were mentioned in the text, arvanitakis assumed that they were damaged. other cataloguers followed suit and in some cases increased the severity of the damage. 5.2. assessment in light of other sources the timeframe of the events described in the talmud is indeed compatible with the record of the 64 b.c. earthquake in classical sources. the siege took place after the death of queen salome alexandra in 67 b.c. and before the fall of jerusalem to pompey in 63 b.c., shortly after mithridates’s death. at the time of the conflict in judaea, pompey was engaged in war with tigranes, which ended in late 66 b.c. and then, according to 1st century b.c. livy (summ. 101, 102: schlesinger, 1967); and 2nd a.d. appian (mithr. wars, 15,16: white, 1962), turned to pontus. one of his commanders, aemilius scaurus, was sent to damascus, from where he hurried on to judea to take advantage of the civil war there. offered comparable bribes by both sides, he ruled in favor of aristobulus and the siege was lifted (josephus, antiquities, 14.2.3: marcus, 1976; wars, 1.6.3: thackeray, 1976) the talmudic report corresponds to passover of 65/64 b.c. both passover and the festival of ceres mentioned by orosius are celebrated at the same time in the spring. even if the fanciful talmudic report is accepted, the image of extensive damage should not be. in fact, nothing was said about actual damage to city walls nor to the temple, event that would have made a profound impression across the country. instead, it is the cessation of services that is stressed. josephus’s fairly detailed account of the siege does not mention any disruption of the city defenses due to a sudden breech or collapse of the walls. resistance continued unabated until the siege was lifted as a result of intervention by aemilius scaurus, who was swayed to the side of the defenders since «it was not the same thing to take a city that was exceeding strong and powerful, as it was to eject out of the country some fugitives, with a greater number of nabateans» (josephus, antiq.14.2.3: marcus, 1976). robustness of jerusalem fortifications at that time (i.e. after the damage was supposed to have taken place) was mentioned also by strabo (geography, 16.2.40: jones, 1917) and josephus (antiquities, 14.4.1-3: marcus, 1976) in their account of difficulties that faced pompey when he laid siege to the city shortly thereafter. thus, even if the 64 b.c. shock was felt in jerusalem, it is unlikely that local intensity was much in excess of human perception. recently however, ken-tor et al. (2000) correlated 8 soft sediment deformation horizons («mixed layers») in the holocene sediments of nahal zeelim in the dead sea region, with a succession of 8 earthquake events listed in the catalogues and concluded that the deformation was seismic in origin. the earliest such «mixed layer», with a calibrated radiometric age in the range of 200-40 b.c., was attributed to the 64 b.c. earthquake. since this deformation unit was found in sediments differing in facies and at locations about 30 km apart, the authors assumed that such pervasive soft sediment deformation suggests that the epicenter was in the dead sea area and that the earthquake damaged the walls and the temple in jerusalem and that in syria it was felt «as far as antioch». 5.3. summary the implied damaging intensities assigned to the 64 b.c. event in israel seem to be misleading and to rely on a fallacious hermeneutical chain, which proceeds as follows: a devastating earthquake in north syria and turkey is invoked in support of a questionable talmudic story taking place in jerusalem; inflated elements of the story enter a catalogue as evidence of earthquake damage in jerusalem; the entry, repeatedly copied by the later cataloguers is correlated to a «mixed layer» of comparable time range in the dead sea area; and finally, the occurrence of «mixed layer» in sediments of different facies, 30 km apart confirms the seismic origin of the «mixed layer» and places the epicenter in the dead sea area. 774 iaakov karcz 6. 31 b.c. on the authority of josephus flavius, the 1st century a.d. jewish-roman historian, all earthquake catalogues list a very strong earthquake that hit the holy land in 31 b.c. and assign a magnitude ml ranging from 7 to 8 (e.g., benmenahem, 1979, 1991; kuran, 1980). this earthquake attracted the attention of judaic scholars and of archaeologists, who claimed to have found its manifestations in ancient literary texts as well as in features of damage at archaeological sites. such findings, in turn, were used to document the extensive spread of damage and the high earthquake intensities. a closer look at the original report suggests however that the earthquake magnitude is overestimated and that at least some of the interdisciplinary correlations are incestuous. 6.1. earthquake evidence of josephus josephus flavius reports: «at this time it was that the fight happened at actium between octavius caesar and anthony in the seventh year of the reign of herod and then it was also that there was an earthquake in judea such a one as had not happened at any other time and which earthquake brought a great destruction upon the cattle in that country. about ten thousand men also perished by the fall of houses, but the army which lodged in the field received no harm. when the arabians were informed of this and when those that hated the jews and pleased themselves with aggravating the reports, told them of it, they raised their spirits as if their enemy’s country was quite overthrown and the men were utterly destroyed and thought there now remained nothing that could oppose them. accordingly they took the jewish ambassadors who came to them after all this happened to make peace with them and slew them and came with great alacrity against their army...» (josephus, antiquities 15,5.2: marcus, 1976). a similar description is found in josephus (wars 1.19,3: thackeray, 1976) but the number of casualties is 30 000. since josephus does not mention any localities that were destroyed or damaged, it is obvious that the high magnitudes were assigned on the strength of the «catastrophic» language of description. the contents, however, are not entirely consistent. josephus reports tenor thirty thousands of dead in the built areas and immense losses in «cattle» (rather «herds», prof. steve mason, pers.comm.), stressing that the army that camped in the open was spared. at that time, however, also the herds were kept in the open or in primitive pens, so that the army may have escaped harm not because it was in the field but rather because it was far away from the earthquake epicentre. similarly, the account of the arab invasion from the east implies that the arab forces were not hit by the earthquake and that they were too far from the disaster area to have had a first hand knowledge of the extent of damage in judea (fig. 5). josephus does not say where exactly the two armies were at the time of the earthquake, but their approximate disposition may be inferred from his description of military engagements immediately prior to the earthquake (joseph., antiquities 15.5.1: marcus, 1976). these included herod’s victory at dion and his subsequent defeat at kanatha, after which the jewish forces retreated into the mountains (galilee or samaria) from where they waged a highly successful guerilla warfare against the arab armies. consensus is that kanatha and dion lie east of the dsr: kanatha is about 100 km east of tiberias, but for dion two alternate locations were offered: one less than halfway from tiberias to kanatha (e.g., schurer, 1973; tavo, 1980) and the other in jordan, about 25 km northeast of pella (ptolomey, in schurer, 1979). josephus’s description suggests therefore a modest spread of damage, so that judea in his account must have been the historic kingdom of judah, rather than the much larger judea of the hasmoneans and romans, which according to josephus (wars 3:35-38, 13.3.4-5: thackeray, 1976) and pliny (nat.hist.5.70, 5.73: rackham, 1958) included amongst others the galilee, samaria, perea, idumea and golan. geographic descriptions of josephus however are not always consistent (for references e.g., schalit, 1968, boettger, 1879, shahar, 1996) and in this case he uses judea both in the narrow and the wide sense. extent of the damage to the south is uncertain since in barren 775 implications of some early jewish sources for estimates of earthquake hazard in the holy land areas of the negev and arava even a severe impact might have escaped notice. all that may be said is that josephus and other chroniclers who record the triumphant advance of augustus across syria and palestine to egypt after the battle of actium, do not mention any signs of a supposedly devastating earthquake that took place only about a year earlier. moreover, the lavish reception and excessive supplies herod provided to the roman army on their way to egypt and back (josephus, wars, 1,20.3: thackeray, 1976; antiquities 15.6.7: marcus, 1976) are out of line with the reported earthquake devastation of the country, even if herod was desperate to win favor with ceasar. 6.2. earthquake evidence from malalas of the roman and byzantine chroniclers, only john malalas (chronicle, 10.3: jeffreys et al., 1986) mentions earthquake damage in fig. 5. orientation map for the 31 b.c. earthquake. 776 iaakov karcz palestine during the second half of 1st century b.c.: «during the reign of augustus city in palestine, named salamine suffered from the wrath of god augustus rebuilt the city and called it diospolis». the entry is rather vague since augustus reigned for more than half a century and the location of salamine-diospolis is not clear. in palestine only two cities bore the name diospolis: dion, already mentioned before (either in golan or in jordan) renamed with other cities of decapolis after the conquest of pompey in 64/63 b.c. and lod (lydda) in the coastal plain, renamed in 199 a.d., by septimius severus (schwartz, 1991). admittedly, malalas may have extracted the information from some post-200 a.d. source, that used the new name, but neither diospolis-dion nor diospolis-lod are known to have replaced a previous salamin, or to have had any connection to augustus. in the levant, however, very many location names were derived from salem or salam, so that one finds for example on the outskirts of diospolislod a deir abu salame, a hitherto unexplored site with remnants of greek inscriptions (peress, 1952 and vilnay, 1974) and next to diospolis-dion in jordan, an unexplored roman military settlement, shalem or salem salumnias, (e.g., tsafrir et al.,1994). it seems that even if malalas refers to the same event as josephus did, the conclusion regarding the size of the earthquake would not change. both diospolis/lod and diospolis/ dion-jordan are within the suggested reduced spread of damage. no ancient derivative of salem was found for dion/golan. of locations outside palestine, it is improbable that malalas lumped together salamiyya (salamias or salaminias) on the orontes in syria, with either dion-golan about 250 km away, or with diopolis north of laodicea, 100 km away (dussaud,1927). also, since he knew of salamin in cyprus (malalas, chronicle, 12:48: jeffreys et al., 1986, reports that salamin was destroyed by an earthquake in the days of constantinus chlorus) it is unlikely that he located it in palestine while copying some early report of an earthquake that hit cyprus. the only possible, albeit tenuous, suggestion that the spread of damage extended farther to the north, is implicit in another report of malalas, which he did not relate to the palestinian earthquake. malalas (chronicle 9.21: jeffreys et al., 1986) tells that during his visit to antioch in about 15 b.c., agrippa ordered the removal of collapse rubble, which accumulated in the old hippodrome in antioch as a result of «former earthquake/s». traina (1995) and downey (1938) assume that these were post-64 b.c. earthquake repairs, the same as the rebuilding of the bouleuterion by pompey. agrippa’s order implies therefore that either the work was not completed, or that the repaired hippodrome was damaged once again by a subsequent shock sometime between 64 and 15 b.c. in this time window guidoboni et al. (1994) and ambraseys and white (1996) list three earthquakes: in palestine in 31 b.c., in phrygia in 27 b.c. and on cyprus in 17 b.c., but there are no grounds to relate any of these shocks to damage in antioch. 6.3. evidence from other sources so far, the ongoing study of possible seismic echoes in contemporary judaeo-christian sources provided no clues to analysis of this earthquake’s parameters. it appears that numerous scholars traced its echoes in some of the apocrypha and pseudoepigrapha composed in the first few centuries a.d. (e.g., stone, 1990 found them in 4 ezra: klijn, 1983, in baruch 2: mignana, 1917 and charlesworth, 1983, in book of shem), however none are explicit and all lack distinct historic clues. the texts may refer to a shock or tumult, or may use the earthquake only as a metaphor. thus for example, modern editors of the talmudic tractate megilla (3.1: simon, 1938) saw an echo of the 31 b.c. event in the report: «the targum [translation] of the prophets was composed by jonathan ben uzziel from the mouth of the prophets haggai, zacharia and malachi and the land of israel quaked over an area of 400 parasangs by 400 parasangs and a voice came forth and exclaimed who is this that has revealed my secrets to mankind». the timeframe is right, since jonathan ben uzziel lived in 1st-2nd century a.d. and so is the biblical allusion since haggai (2.6-7; 2:21) and zacharia (14:4-5) include 777 implications of some early jewish sources for estimates of earthquake hazard in the holy land explicit and malachi (1:3) veiled references to earthquakes. the text, however, may be referring not to an earthquake but to the shock and distress caused by disclosure of holy texts to non-jews. similarly far-fetched was mignana’s (1917) and charlesworth’s (1983) correlation of the prediction in book of shem (7:20) that «when the year starts in libra [september 24october 23] there will be in galilee a severe earthquake» to the account of josephus. such uncertainties are particularly striking in the case of the dead sea scrolls (dss), a rich corpus of more than 800 documents, found in the caves at the northwestern end of the dead sea and associated with the sectarian community that occupied khirbet qumran during 2nd century b.c.-1st century a.d. (e.g., garcia martinez, 1994; vermes, 1995; wise et al., 1996; garcia martinez and tiglechaar, 1997). this collection of documents and the nearby archaeological site represent one of the most extensively studied and certainly the most hotly debated topics and produced a voluminous literature (for references: e.g., schiffman and vanderkam, 2000; schiffman et al., 2000; flint and vanderkam, 1999; wise et al., 1996). about one fourth of the retrieved dss materials includes texts specifically related to the community, one third includes biblical texts and the rest are non sectarian texts (for bibliography: e.g., dimant, 1995, 2000). none of these documents includes information on when the materials were composed or copied, by whom and where and only several include elements that are chronologically meaningful. the still ongoing analysis of texts identified 16 explicit references to earthquake disasters, 6 general literary allusions and further 5 which extol the importance of earthquake safe construction (e.g., «... this is the tested rampart the precious cornerstone that will not allow foundations shake or tremble from their place» (rule of community viii:78); or «for you place the foundation upon rock and the beams to correct size and a true plumb line to stretch out, tested stone for a strong building which will not shake...» (hodayot, xiv:26, cf. book of psalms 104:5). none of these motives is unequivocally related to the 31 b.c. earthquake and they may reflect echoes of other earthquakes preserved in the collective memory at the time of writing. the better known biblical echoes of ancient seismic disasters in israel and levant, relevant here, include for example the destruction of sedom-gomorrah (21-17 century b.c.) and jericho (15-12 century b.c.), the apheq (century 854 b.c.) and uzziahu (758 b.c.) earthquakes and the lebanese coast tsunamis (590 b.c., 138 b.c.) and possibly also the supposed allegoric crucifixion earthquake (33 a.d.). the number of earthquake motives in the dss is too small and dating of the texts is not precise enough to determine whether frequency of seismic metaphors in the texts increased after the 31 b.c. event. in fact the ratio of earthquake echoes to other natural phenomena used in linguistic imagery of that time (sea storms, floods, winds and thunders, fire), is fairly constant for texts similar in character. in poetic and apocalyptic texts, where the use of imagery is most frequent, earthquake echoes represent about 1020% (this is also the ratio in the book of psalms). none of the texts shows a pervasive use of literary earthquake motives that might suggest that the writer went through the trauma of a seismic disaster. though contemporary textual echoes are scarce, numerous instances of seismic damage, abandonment and rebuilding related to the 31 b.c. earthquake were reported from archaeological sites in israel. the implicit damaging intensities reinforce the devastating image of this event. the first and best known, was identification of earthquake destruction and related conflagration at khirbet qumran, the assumed center of the dss community (for references de vaux, 1973; humbert and chambon, 1994; magness, 2002). correlation of supposed signs of seismic damage (a n-s running «earthquake fault» across the entire site with a 35cm downthrow across one of the ritual baths, massive breakage of jars under the fallen ceiling of a pantry, damage to the main tower, collapse of rooms at the southeastern part of the site and signs of abandonment of the site) with josephus’s account, provided de vaux with a critical chronological marker for the site. at the same time it provided the geologists (starting with zeuner, 1955), with evidence for ongoing deformation along the dsr. subsequently, all in778 iaakov karcz dividual elements of this archaeoseismic evidence were shown to be ambiguous or unacceptable. features described by de vaux were attributed either to static geotechnical instability effects (e.g., zavislock and gibb in steckoll, 1967 and 1969; karcz and kafri, 1978); erroneous interpretation of elements of construction (hirshfeld, 2003) and a later collapse of upper part of the building (milik, 1959), or to destruction whether by aristobulus ii or hyrcanus ii (in 67-63 b.c.), gabinius (in 57 b.c.), the parthians (in 40/39 b.c.), or antigonus (in 4037 b.c.). the numismatic evidence for desertion of the site was rejected (e.g., leonard, 1997; magness, 1995, 1998). moreover a series of gpr traverses across the «earthquake fault», conducted by a. beck and i. karcz in august 1995, revealed no displacement in excess of survey resolution and the ns fault line marked by de vaux is neither continuous nor disruptive. at this time however de vaux’s hypothesis is so firmly entrenched that geophysicists have assumed that it was josephus himself who described the details of seismic damage at qumran (nur, 1991). similarly, once the strike slip nature of motion along the dsr became known, archaeologists have changed the nature of displacement along the «earthquake fault» from vertical to horizontal (e.g., shanks, 1998a,b). in the footsteps of the «qumran earthquake», several other locations across the country were thought to have been hit in the 31 b.c. event, amongst them massada and jericho along the dead sea, agappias (probably anthedon, between gaza and askalon), askalon, antipatris (arethousa) and stratton’s tower along the mediterranean and recently also the city of tiberias. it was also assumed that at least some of the intense building activities of herod were spurred by the need for post earthquake repairs (e.g., roller, 1998). archaeological evidence is tenuous at best, in fact netzer (1991, 1997) in his detailed analysis of architectural complexes of massada has shown that the signs of a possible seismic damage there are much later than 31 b.c., while mazar et al. (1966), gichon (1993) and fischer et al. (2000) found no 31 b.c. seismic effects even at ein fesha and ein boqeq, next to qumran. the claim that 31 b.c. earthquake was amongst those that according to historical sources caused severe damage to tiberias (marco and agnon, 2004) would have implied that the city was destroyed fifty years before it was built by herod antipas in 18 a.d. named to honor tiberius (who ruled between 14-37 a.d.). 6.4. summary it appears that the textual evidence reflects a relatively modest earthquake with magnitude in the range of 6-6.5, rather then a major catastrophe. the earthquake motives found in assorted texts and tentatively attributed to the 31 b.c. earthquake are non indicative and may represent literary metaphors, or echoes of any real events preserved in the collective memory at the time of writing. while there is no doubt that this area was repeatedly shaken by earthquakes, in itself the archaeological evidence found at qumran is equivocal and the signs of alleged seismic damage may reflect static and geotechnical damage, or destruction and damage inflicted by man. 7. 749 a.d. all local and regional earthquake catalogs list one or more earthquakes that in mid-8th century a.d. hit the middle east and caused severe damage in egypt, palestine, jordan, syria and iraq. recently however, all multiple year dates given in ancient texts were attributed to inconsistent use of different calendars and eras in dating the same event of 18th january 749 a.d. (e.g., tsafrir and foerster, 1989, 1992; guidoboni et al., 1994). this reduction of all felt reports to the same denominator implies destruction that extended from egyptian littoral, to iraq and iran (fig. 6) with ml estimates in the range of 7.3 to 8. this image was strengthened even further by reports of alleged signs of seismic destruction at a myriad of archaeological sites in israel, jordan and syria. in israel, reduction of all dates to a common denominator relied on chronological clues found in two ancient hebrew texts which describe a fast that allegedly commemorates a large earthquake on 18th january 749 a.d., as well as on archaeological and 779 implications of some early jewish sources for estimates of earthquake hazard in the holy land numismatic evidence. these findings were generally accepted and incorporated in regional earthquake catalogues. a closer examination shows however that such clues rely on highly questionable assumptions and, even more important, that the byzantine and arab chronicles and traditions clearly report at least two discrete events up to three years apart and hundreds of kilometers apart. moreover, comparison of texts that describe only one earthquake, shows that the difference between the reported dates, locations and descriptions of damage is too striking to be attributed merely to erroneous date calibrations. under such circumstances intensity maps with mercalli modified scale values freely assigned to both textual and archaeological indications and attributed to one single event (e.g., marco et al., 2003) would be misleading. 7.1. evidence of multiple earthquakes theophanes, d.818, (turtledove, 1982; mango and scott, 1997), one of the major source for events of that period, lists two earthquakes that hit palestine and the levant in mid-8th century a.d. the first entry, the day and month of which is cited by nearly all later authors and catafig. 6. orientation map for the 747-749 a.d. earthquakes. 780 iaakov karcz loguers, dates the earthquake to january 18th, a.m. 6238 (747 a.d., according to the indiction-based calibration of ostrogorsky, 1928): «... a great earthquake in palestine, by the jordan and in all of syria on 18 january in the 4th hour. numberless multitudes perished, churches and monasteries collapsed especially in the desert of the holy city». the second event occurred in am 6241 (749/750), but the day and month are not given. the record was entered immediately after that of the birth of leo, emperor constantinus’s son, on 25 january 750 a.d.: «... there was an earthquake and terrible destruction in syria as a result of which some cities were entirely destroyed others partially so while others slided down entire, with their walls and houses, from positions on mountains to low laying plains a distance of 6 miles or thereabout. eyewitnesses affirmed that the ground in mesopotamia was split along two miles and that out of the chasm was thrown a different soil very white and sandy, in the midst of which they say there came up an animal like a mule quite spotless that spoke in a human voice...». though dimensions are scaled up, the implied landslides, surface rupture and sand boils sound authentic (in a semiarid area they would be more common during the wet season) and the fantasy part is restricted to the mule-like oracle. 9th century nicephorus (bekker, 1837; de boor, 1880; mango, 1990) presents an almost identical description of the 750 a.d. shock but not of the 747 shock. he used the same sources as theophanes and though he was more selective (whitby and whitby, 1989), in this case the absence seems to reflect a lacuna or an illegible paragraph (as marked in the extant text). more tenuous evidence of two events comes from the great chronographer (e.g., whitby and whitby, 1989) and the minor chronicles (schreiner, 1979) which report the 18 january 747 earthquake and then a series of «wondrous events» after the birth of leo in 750 a.d. strangely enough, even though both earthquakes are listed also in later chronicles, such as those of 9th century anastasius bibliothecarius (migne, 1879), paulus diaconus (muratori, 1861) and the 12th century georgius cedrenus (bekker, 1839), as well as in the more recent compilations of baronius (1593), de muralt (1855), perrey (1850) and grumel (1958), the second earthquake of theophanes is still ignored by many. further evidence of more than one earthquake comes from old and modern catalogs based on moslem sources such as those of 15th century jalal ad-din as-suyuti (nejjar, 1973) and poirier and taher (1980), who mention two distinct earthquake events that in ah 130 (747/748) and ah 131 (748/749 a.d.) damaged jerusalem and damascus, while caetani (1912) lists references to earthquakes in muslim and christian chronicles under the years ah 129, 130 and 131. as-suyuti cites the eye witness evidence of abdalla al katir, a well known historian and scientist (d. ah 196, 811/812 a.d.), transmitted by al wadai (d. ah 716, 1316/1317 a.d.), who reports «we were hit in damascus by an earthquake in ah 130 that led the inhabitants to leave the city. in this earthquake the poultry market collapsed under the fallen rocks» and continues «...and they told me that a devastating earthquake in ah131 split the roof of the mosque so that one could look through it at the skies. thereafter another earthquake took place and the crack had closed». it is probable that these two successive earthquakes are responsible for the hesitant and possibly confused accounts of 13th century sibt ibn al jawzi, d.1257 (a. elad, 1991, pers. comm.) followed by 15th century ibn tagri birdi (shaltut, 1929), which report strong earthquakes (plural) in syria in ah 130, with heavy damage in jerusalem, in the wake of which people of damascus fled into desolate areas for 40 days and add «and it was said that the earthquakes took place in ah 131». obviously a discrepancy exists between the two pairs of dates, the 18 january 747 a.d. and early 750 a.d. of theophanes and ah 130 (747/748 a.d.) and ah 131 of the arab chronicles. both sources were written within fifty years from the events. arab texts were composed locally, while theophanes, though far from infallible (e.g., proudfoot, 1974; mango and scott, 1997; burgess, 1999) is believed to have drawn information about palestine and south syria from a local melekite chronicle written about 780 a.d. and brought by monks 781 implications of some early jewish sources for estimates of earthquake hazard in the holy land who fled persecution in palestine (brooks, 1906; proudfoot, 1974; mango and scott, 1997). information about iraq and north syria may have come to theophanes from a local source that used the babylonian count of seleucid years starting in spring 311 b.c. rather than in fall 312 b.c., leading to the date of early 750 a.d., rather than early 749 a.d. no explanation was found so far for the gap of a year between the 747 a.d. of theophanes and the 130 ah date of the arab sources. 7.2. single-event reports inclusion of an earthquake record in a chronicle, depends on whether the writer regarded it important enough relative to other events of that period and on the interest the shock-hit area held for him and for his prospective readers. it is difficult therefore to determine whether an event is not included in a chronicle because it was unknown to the author, or because it was dismissed as irrelevant. some byzantine and later arab chroniclers such as 12th century michael glycas, 13th century leo grammaticus and the 10th century said ibn batriq (eutychius), ignore the mid-8th century earthquake altogether. others follow theophanes, like the 9th century gregorios hamartolus (de muralt, 1863), who repeats verbatim the 749/750 a.d. entry in theophanes, with a comment on the «incredible» mule-like oracle and still others record the earthquake without any identifying details. 14th century zonaras (pinder, 1897) for example, says nothing about the time, location and extent of damage, but enters the event after the fall of germanikeia into constantinus’s hands, (745/746 a.d. according to theophanes) so that he may be referring to either of the two shocks. even less definite are reports such as that of the 10th century mukadassi (le strange, 1887) who writes that «earthquake in the days of the abbasids threw down the sanctuary except the part round the mihrab», which may refer to the 750 a.d. event (i.e. after marwan’s death and ascent of abbasides) but also to one of the later earthquakes, or the statement in «commemoratorium de casis dei» (808: tobler and mollinier, 1880) that the church of maria nea is still in ruins after having been damaged by an earthquake, but it is not clear which earthquake of the 8th century it was. once a text includes only one mid-8th century earthquake the critical questions are which one it is or whether the text almagamates two or more events into one report. answer is not always possible, because of uncertainties in definition of the era used and because translators of chronicles expanded, calibrated and even «corrected» the details of time and location in the original text. moreover in some oriental chronicles, the sequence in which events are described does not always reflect their true chronological order. finally, all single-event reports give either the day and month, or the month, or the year of the earthquake but none gives the full date, which obstructs the analysis, particularly since also the later event of theophanes may have taken place in or shortly after january. the reports which do supply pertinent information about either the date, location or intensity of damage sustained in a single mid-8th century event are reviewed below. 7.2.1. oriental christian sources the most extensive information about the spread of damage comes from oriental christian sources, but it does not necessarily refer to one and the same event. the four major syriac chronicles, i) pseudo dionysius of tel mahre (chabot, 1895; witakowski, 1987, 1996; harrak, 1999), now attributed to anonymous 9th century monk from zuqnin monastery, ii) 11th century elias of nisbin (brooks, 1910; chabot, 1910), iii) 12th century michael the syrian (chabot, 1899) and iv) mid 13th century «chronicon 1234» (chabot, 1937) provide (or imply) only year dates for the earthquake. despite some confusion between the macedonian and babylonian reckoning of seleucid years, it seems that they all refer to the later of the two theophanes’s events. elias of nisbin, the only one with a specific interest in chronology, dates the shock to ah 131 (i.e. 748/749 a.d.), which he says, lasted from 30 ab as 1059 to 20 ab 1060. the attached chronological tables show however that his as years started in the spring, 782 iaakov karcz in as 1059 on 25 jummada ah 130 (spring 748) and in 1060 on 7 ragab, ah 131 (spring 749 a.d.). «chronicon 1234» (mid 13th century a.d.) written about half a century after michael but using the same sources, dates the earthquake to as 1060 (with an erroneous parallel of ah 134, which may be a copying or printing error for ah 131). michael provides no year date (unlike russell, 1985, thought), but in a footnote appended to the translation, chabot (1899) dates it to year 741 of alexandrine africanus era (i.e. am 6241, as 1060, 749/750 a.d.). in the accompanying tables, chabot corrected michael’s chronology pulling the as and a.d. dates one year back i.e. to as 1059 = 748/749 a.d., but and it is not clear whether the correction should apply also to his own note. both he and others (for references: e.g., weltecke, 2003) indicate that michael's chronology is full of errors, whether of the author or of the copyists. pseudo dionysius, placed the earthquake in as 1059, with equivalent date 747/748 a.d. (suggestive of macedonian conversion). indication, albeit tenuous, that this earthquake is not the one of 18 january, is that none of the chroniclers mentions the near coincidence of the earthquake with the feast of mary for the seeds (blessing of crops), one of the principal feasts of the virgin in syrian tradition (observed on 15 january, which in 749 fell on sunday, so that it would have been celebrated a day or two later). pseudo dionysius provides no details of the damaged localities except for maboug and refers archly to a «grand et violent tremblement de terre dans la region occidentale». chabot (1894) pointed out however that amongst the jacobite writers, the terms «western» and «eastern» territories reflected the two ecclesiastic provinces with centers in antioch and tagrit and had no strict geographic significance. the other three chronicles list numerous localities that suffered, mainly in areas to the east of dsr. in palestine only tiberias, mt.tabor and jericho are mentioned, but significantly nothing is said about any damage or destruction in jerusalem, in striking contrast to the arab chronicles. this major difference, as well as lack of reference to the 18 january 747 event of theophanes confirm the post 746 a.d. divergence between michael and theophanes (e.g., brooks, 1906; proudfoot, 1974). elias refers only to displacement of a village near mt. tabor (with no damage nor casualties) and to damage in the church of maboug (a distance of about 500 km), whereas michael and the «chronicle 1234» record intense damage at localities from maboug in northernmost syria to baalqa and moab areas east of the rift, listing ghautah, dareiya, bosra, nawa, derat, baalbek, damascus and beit qoubaye, which may be a corrupted form of koubaiyat or qoubaiat at the foot of jebel akkar about 45 km southwest of homs (ancient edessa) and 25 km south of crac de chevalliers. on the other hand, their account of damage, subsidence and inundation along seashores does not mention any of the mediterranean coastal cities and town by name and refers instead to stormy seas and submersion of a yemenite coastal fortress in the area of baalqa and moab, which lie east of dsr (along rhomb shaped depressions of lake kineret, dead sea and gulf of aqaba) and to destruction of tiberias and lateral displacement of springs near jericho along the western side. it is not impossible that a local tsunami-like storm wave was generated in the dead sea whether due to a nearby fault, massive local slide, or by a temporary halt of flow along the jordan river and a subsequent dam-break wave at the inflow into the dead sea. a similar association of earthquake-induced flooding or inundation along the coast and an «enormous death toll and destruction in tiberias» is made by 10th century a.d. agapius of menbidj (vasiliev, 1912). here however, the earthquake was placed in january (kanoun), without a year date, but after the record of constantinus’s incursion into syria (i.e. the conquest of germanikeia) and prior to abu muslim’s revolt (ramadan ah 129, may 747). this would suggest that he refers to the earlier of theophanes’s two earthquakes and this is supported by the lack of any reference to damage in menbidj. severus ibn al muqaffa (evetts, 1947) refers to damage along the mediterranean coast, saying first that the shock was felt strongly in egypt, but caused no damage nor casualties, except for damietta and then proceeds to report the destruction of six hundred 783 implications of some early jewish sources for estimates of earthquake hazard in the holy land towns and villages, «from city of gaza to the farthest extremity of persia». again no location names are given, with a comment that no orthodox churches nor monasteries were damaged. in addition severus mentions that many ships were lost at sea. the earthquake took place on 21 tuba (17 january) on the day of our lady, dormition day, which in coptic tradition was celebrated between 16 and 18 january, unlike in the byzantine empire where since 6th century it was celebrated on 15 august (e.g., garritte, 1958; gamber, 1984). year of the earthquake is not given, but closely related events (imprisonment and release of patriarch michael, nubian incursion into egypt etc.) are dated by severus to ah 130 (747/748 a.d.). his description of events prior and after the earthquake, however, is confused particularly with respect to marwan campaigns and the abasside mutiny, which contrary to what russell (1985) assumed admit the earlier date of ah 129 (746/747 a.d.). the 13th century compilers al makin (erpenius, 1625) and petrus ibn rahib (cheiko, 1903) report a widespread destruction of cities and loss of life under the ruins and flooding along the coast, but do not mention any specific localities. the day and month date of 21 tuba follows severus, but the year appears to have been misrepresented. the text suggests that rather than the year of the earthquake, ah 120, 460th year of diocletianus (two incompatible dates of 737/738 a.d. and 744/745 a.d.) refers not to the earthquake but to the year of ascent of patriarch abnachajil (kail, michael), in course of whose 23 year long tenure the earthquake took place. while the diocletianus year agrees with the official chronology of patriarchs of alexandria, the hejira dates of al makin and erpenius do not. indeed michael’s ascent is given as ah 120 (737/738 a.d.) rather than the now known date of 743 ce and the ascent of the preceding patriarchs cosmas and theodoros is respectively ah 108 (726/727 a.d.) instead of 729 a.d. and ah 109 (727/728 a.d.) instead of 730. in view of al makin’s coptic background this may seem strange, but similar apparent inconsistencies appear also in ibn rahib chronicle, who tabulates the succession of patriarchs using the alexandrine-africanus era. the dates of ascent and duration of service, differ from both al makin and from the official chronology of the patriarchs and chronological calibration of the dates is inconsistent. finally the mekhitar d’airavanq chronicle (brosset, 1869) mentions an earthquake in 751 a.d., in times of constantine copronymus. this later date is supported also by lack of reference to the day and month date of 18 january, which is the day on which the armenians in the holy land celebrate christmas (12 days later than elsewhere). 7.2.2. arab sources near contemporary local arab reports of an ah 130 earthquake (or possibly two), traced back to 8th century, appear in two 11th century compilations of traditions transmitted by the diarists of the prominent jerusalem family abd el rahman, the 11th century cousins al wasiti and ibn al murajja (hasson, 1979; elad, 1995). the eastern and western parts of al aqsa compound in jerusalem were damaged and repairs were ordered by the caliph abu jafar al mansur (754-775) during his visit in ah 141 (757/758). once completed, however, the mosque was hit by another earthquake (presumably in 757 a.d.) to be repaired only in the days of caliph al mahdi (probably following his visit in ah 163). one of the traditions in ibn al murajja transmitted by thabit ibn istanibiyadh reports that the «first» earthquake occurred on a cold and rainy night of ramadan ah 130 (i.e. in may) and includes a vivid description of a crack that opens and on a command from heaven closes instantaneously. this wondrous motive appears also in description of some other earthquakes in the moslem literature and it is just possible that the holy month of ramadan is invoked to lend reliability to the story (elad, 1995 and pers. comm.). yet another tradition that emerged from jerusalem and was carried over to the later day chronicles, reports damage across whole syria (bilad al sham) but emphasizes the destruction in jerusalem and injuries to the descendants of shadad al aws, one of the prophet’s companions. 784 iaakov karcz 7.2.3. samaritan sources damage in jerusalem is not mentioned in two local sources that emanate from the samaritan community near nablus (schechem) only 50 km away. samaritans, now a tiny remnant of once large and powerful community with strong jewish affinities, trace their origin to one or more of the twelve tribes of israel (manasse) and see themselves as torchbearers of the original values, who kept to the written law (the torah) through millennia of uninterrupted habitation in the holy land (for references: e.g., crown, 1989, 2001). the jews, however, believe that samaritans descend from people brought into the holy land in 8th century b.c. by the assyrian conquerors, to replace the jews who were expelled to assyria. while subsequently the newcomers accepted the principal tenets of judaism, they were accused of having kept idolatrous practices (e.g., 2 kings 17:33) and were not accepted into the fold. samaritans strictly observe the written law, the samaritan toras and holy books are in the ancient hebrew script and tradition of passover sacrifice has continued. they reject, however, the later parts of the scriptures and the later holy days and festivals, regard moses as the only prophet and messaiah to come and reject the sanctity of jerusalem in favor of mt. gerizim near nablus. it is possible, therefore, that the lack or reference to mid-8th century damage in jerusalem reflects lack of interest and disdain. over the years the rift between samaritans and the jews ranged from uneasy coexistence to bitter rivalry, hatred and armed conflicts. since samaritan historical materials are scarce, much of the information about such disputes, wars and fates under the rule of various masters of the holy land comes from extrinsic, evidently biased sources. even more serious is the problem of provenance and authenticity of the available samaritan texts, since in some cases allegations were made that the supposedly faithful copies of ancient manuscripts were fabricated or expanded by copyists to sell to travellers and scholars. chronicle adler (adler and seligsohn, 1902), thought to represent a relatively recent compilation, reports: «a l’epoque de merwan un grand tremblement de terre et lieu, jamais il ny eu eut d’aussi terrible.» somewhat more informative is the 14th century chronicle of abu l’fath, which exists in many manuscript versions and «was plagiarized, summarized, abstracted, paraphrased and edited for several other chronicles which were then presented as different old chronicles» (stenhouse, 1989). the chronicle has a shorter original version which brings the text up to the rise of mohammed and an expanded version to bring it more up to date. abu l’fath wrote the chronicle in 1355, following a discussion he had with the high priest in 1352 lamenting the virtual absence of materials on history of the samaritans (payne-smith, 1863; vilmar, 1865; stenhouse, 1981). he used some extinct (or not found) samaritan sources and is thought to have used extensively materials then available in damascus and gaza. thus it is not clear whether the following description of an earthquake is based on primary notes: «in the days of marwan an extraordinarily powerful earthquake struck everywhere. houses collapsed on their inhabitants and untold numbers of people perished. it was a terrible earthquake that had no precedent. those who survived it stayed out in the open for many days while the earth was still shaking underneath them» (dr. paul l. stenhouse, pers. comm.: from ms samaritain, bibliothèque nationale de france, paris). the year of the earthquake is in question. before recording the earthquake the chronicler says: «marwan bin muhammad the last of ummayads who ruled five years and two months. from the beginning of islam to this time was 131 years and three months». marwan ruled between 744-750 a.d., so it is possible that the 131 ah and 3 months date refers to the earthquake of 748/749 a.d.. however, after the earthquake entry, the chronicle records the ummayad-abbaside conflict and the rise of abu muslim, which occurred earlier (ramadan 747 a.d.). 7.2.4. judaic texts one of the two jewish documents used in description and dating of the mid-8th century earthquake is a liturgic poem (piyyut) called «seventh earthquake». liturgic poetry was used in synagogues to expand, enliven and re785 implications of some early jewish sources for estimates of earthquake hazard in the holy land place some of the fixed prayers and break the monotony of the services, not only on holidays, but also on sabbaths, days of fast and on special occasions. the use of this literary form was widespread in palestine since the days of the talmud and the structure and style did not change much over the ages, so that the date of composition can not be deduced from literary form or language alone. the other text used comes from a 10th-11th century book of prayers and accompanying occasional texts, that was found in the famed cairo depository (genizza). this poem (zolai, 1937; margalioth, 1941) laments an earthquake that caused a widespread destruction and extensive casualties in tiberias («rage in fear and dark chaos will capital tiberias») and a catastrophic flooding in the plain of sharon («in wrath and anger sunk crowds in plains in sharon valley»). in modern times sharon refers exclusively to the central coastal plain of israel, but in the 4th century it was used by eusebius also for a part of jordan and yizrael valleys between mt.tabor and tiberias (weitz, 1939; brawer, 1940). the poet adds: «i heard how disaster befell the city and the old and young in it have perished», where it is just barely possible that «the city» refers to jerusalem rather than to tiberias. the language includes elements of habakkuk, ch.3.12, (earthquake significance is lost in king james version which gives «...though didst march through the land in indignation..» rather than the literal «...in wrath shall march the land...» where «wrath» may stand for «earthquake» similar to the byzantine chronicles). the poem repeatedly refers to a fast in memory of this earthquake, observed on the 23rd of shvat. zolai (1937) was unable to decide if the title of the poem refers to a seventh shock in course of the same earthquake swarm, or to a seventh earthquake in a series of events preserved in some extinct tradition. in his opinion, however, the form and style dated the poem to 10th-12th century, a period during which tiberias was damaged only in 1033/1034 a.d. and in 1202 a.d. since in end 11th century the jewish community in tiberias was too small for its misfortunes to trigger a nationwide day of fasting, he concluded that the fast of 23rd of shevat commemorated the earthquake that in 1033/1034 hit tiberias, jerusalem, ramle and other towns and villages. this date was rejected by margalioth (1941), who argued that the fast of 23rd shvat was mentioned already by pinneas the poet, who in a 10th century text was mentioned amongst «ancient» authors and that the poem includes a veiled reference to moslem rulers. he assumed therefore that the earthquake should be backdated and placed between the arab conquest (about mid7th century) and the beginning of 9th century, a period he regarded as consistent with the literary form and style of the poem. having found no evidence that successive earthquakes that hit the holy land were counted in numerical order, he read the title of the poem as «earthquake of the seventh (feminine)» rather than «seventh (masculine) earthquake». the «seventh» (feminine) stands for a sabbatical (fallow) year and margalioth indicated that in the above time range only the earthquakes of 712/713 a.d. and 747/748 a.d. occurred in a sabbatical year. having found no details about the former he dismissed it as unimportant and dated the earthquake to 23rd shvat (28 january), 748 a.d. in agreement with two late arab chronicles of mukaddasi (d.14th century) and ibn tagri birdi (d.15th century) who transmit news of an earthquake in ah 130 (747/748 a.d.). twenty years later, margalioth (1960) found a reference to the 23rd shvat fast in a 10th-11th century book of prayers found in the cairo genizza depository: «on 23 shevat a fast to the land of israel, since the land trembled and many cities fell and sages and pious and the just and the [etc.]... died under the ruins. and it is referred to in texts ‘in wrath the earth will pace ahead’ and since destruction of jerusalem to the date it happened in land of israel the count of in wrath [in wrath = b z a’ m]». margalioth (1960) recognized that [«b z a’ m»] is a gematric expression in which letters of alphabet were substituted for numbers. this form of codification is quite common in old jewish texts and varies in complexity (for references: e.g., levias, 1903; scholem, 1971; lipiner, 1989). the simplest code proceeds in alphabetical order with the first nine letters equivalent to 1-9, the next nine to 10-90, the next four to 100-400 and the end letters (five letters in the hebrew alphabet: chav, 786 iaakov karcz mem, nun, peh, tsadik, assume different form when at end of a word) to 500-900. there are, however, more than 70 gematric conversion codes, some relatively simple and some based on complex permutations or multiple-tier systems. the ancient and modern numerologists applied such diverse techniques to discover hidden meanings of words and expressions, for example by treating the numerical value of a word as a clue to another word with the same numerical value. occasionally the code is selected so that the number, word or expression would lead to a specific biblical verse or moral. consequently, gematric hints must be treated with caution, since it is not always clear what code the writer used. margalioth used one of the most common codes (b = 2, z = 7, a’ = 70, end m = 600) to arrive at the 679th year since the destruction of the temple (70 a.d.) and obtained a final earthquake date of 23rd shvat, 17 january 749 a.d., one year later than his former interpretation. in support of this ameliorated date margalioth cited theophanes’ (d.818) account of an earthquake in palestine on 18 january 747 and michael the syrian’s (d.1199) account of an earthquake in syria and palestine in 749 a.d.; combining the day and month of the former with the year of the latter. unfortunately, neither the 748 nor the 749 a.d. date is fully consistent with margalioths own arguments. on one hand, he showed that in 749 a.d. the jewish and julian day and month dates of 23rd shvat and 18 january nearly coincided, whereas in 748 a.d. they were 10 days apart, but on the other hand 748/749 a.d. was not a sabbatical year, whereas 747/748 a.d. was. margalioth opted for the 749 a.d. year date, assuming that the 8th century count of sabbatical years followed an obscure system that lagged a year behind that used currently (wacholder, 1973). tsafrir and foerster (1992) took the same stand, reinforced by their find of a mint condition 748 a.d. coin underneath seismic collapse rubble in the excavations of beth shaan. a somewhat similar situation was reported by tsafiris (1989) from cappernaum, where the last in a hoard of coins found under collapse rubble was minted in 744 a.d.. cushioned by the great authority of margalioth, one of the leading judaic scholars of his time, the 18 january 749 a.d. earthquake took roots in the local historical, literary and archaeological literature and is occasionally referred to as the «great earthquake». particularly numerous are reports of damage and destruction it caused across byzantine-ummayad complexes in israel, jordan and syria and of the far reaching implications it had. thus for example, archaeologists (e.g., ben dov, 1985, 2002; wightman, 1993) report extensive damage in jerusalem, particularly southwest of the temple mount, dated by them to 747 a.d.. they claim that the damage, particularly that to a central ummayad administrative building, still under construction at the time of the earthquake, was responsible for marwan’s decision to abandon his plans to move the center of power from damascus to jerusalem. this suggestion, while plausible, ignores the fact that the arab chronicles reporting the ah 130 and 131 earthquakes record similar damage and destruction in syria and damascus itself. furthermore, it is not altogether clear, what was the structural effect of the punitive removal of fortifications of jerusalem, damascus and baalbek ordered by marwan in about 744/745 in retaliation for their sympathies with his opponents (wellhausen, 1973) and whether the city walls were dismantled or demolished with a resulting weakening of the adjacent buildings. the ingenious proposal of margalioth relied on some questionable assumptions and was not entirely consistent in chronological analysis as well as in perception of the texts (karcz and elad, 1992; berberian, 2001). since the creation, destruction and possibly also the sabbatical year counting customs varied at that time, identification of a year date in a jewish text is fully meaningful only if the years are given in all three reference systems and are consistent. similarly, the present conversion tables that margalioth used are not applicable in detailed calibration of pre10th century month and day dates, when calendar and intercalation practices were not fully fixed. thus the choice of a year date using coincidence of the julian and jewish month and day, which was behind the decisions of margalioth to move the day and month of theophanes’s 18 january 747 earthquake from 747 to 749 a.d. and to move the sabbatical count so that the fallow year falls in 749 rather than in 748, may be misleading. berberian (2001) raised further weighty objections to margalioth’s methodology. first, he drew attention to a piyyut, which counts in nu787 implications of some early jewish sources for estimates of earthquake hazard in the holy land merical order 10 earthquakes reflecting largely allegoric seismic echoes in the bible and denied that the «seventh» refers to a fallow year and even more important, that the «seventh earthquake» may refer to an allegoric rather than a historical event. second, since the gematric interpretation of the hebrew expression «b z a’ m» follows a method, which is not unique but one of many, the figure of 679 years after destruction is equivocal and would change according to the chosen conversion code. finally, berberian pointed out that the form and style of the poem can not indicate its age even with a resolution of several centuries (indeed the intuitive dating of zolai and margalioth spans almost six centuries) and suggested that the poem may have been composed much earlier than mid-7th century. 7.3. summary it appears that margalioth’s assumptions are questionable and do not provide a solid enough basis for harmonization. part of the problem was that he was unaware of most of the mid-8th century earthquake records in byzantine, eastern christian and moslem chronicles and texts, which indicate that difficulty in harmonizing the day, month and year dates stems from there have been more one event. in fact, the unresolved question is whether there were only two such earthquakes or more. whatever the case, it is apparent that the image of widespread damaging intensities produced by a single high magnitude earthquake is misleading. it is probable that the intense damage in northern israel and jordan (possibly also in jerusalem and damascus) includes the collapse of structures weakened in the previous earthquake with epicenter more to the south. 8. conclusions a two stage, circuitous interdisciplinary mechanism leads to overestimates of historical earthquake intensities in the catalogues of earthquakes in the holy land. the first stage starts with a tentative identification, usually by judaic scholars or by archaeologists, of supposed seismic evidence either in literary or homiletic texts, or at archaeological sites. usually such evidence is equivocal and admits explanation and interpretation that are unrelated to any seismic effects. the supposed literary earthquake motives may represent linguistic allusions or metaphors that reflect biblical events and quasi-historical traditions preserved in the collective memory at the time the text was written. equally the word or a phrase with supposed seismic connotations, may in fact refer to tumult, fear, or horror. usually, the prevailing argument for seismic significance rests on a correlation to an earthquake indicated in one of the earthquake catalogs within a time-window tentatively defined for the text. such correlations, though obviously biased, first by the quality of the catalogue used and then by the vested interest of the scholar in obtaining a terminus post quem date for the text, often involved earthquakes that were documented only more than five hundred kilometers away. in a similar fashion, early archaeologists in the holy land justified the assigning of seismic origin to features of «total disaster», structural damage and repairs, architectural asymmetries and site decay and desertion. since the same features may result from indifferent design and workmanship, static decay of buildings or willful destruction by man (e.g., karcz and kafri, 1978, 1981; rapp, 1986; stiros and jones, 1996), it is the correlation to historic records that tips the scale in favour of seismic interpretation. also in this case, in addition to a sense of drama, seismic interpretation provides a chronological marker that aids in dating the site and relating it to a discrete historical event. once such correlations, textual or archaeological are published, they are accepted by earth scientists as independent evidence of the time, extent and even earthquake intensity and all simplifying assumptions on which correlation rests are ignored. in the next stage of the process, the supposedly reliable earthquake impact undergoes self enhancement by adhesion of further textual seismic echoes or further archaeoseismic interpretations. it should be clear that this is not to say that the local textual, archaeological or palaeoseismic evidence, is of no value, in fact just the reverse is 788 iaakov karcz true. to realize its full potential, however, analysis of such evidence must be accompanied by a detailed budget of assumptions and uncertainties inherent in each and every interpretation. acknowledgements this is to thank max stucchi and paola albini for support and endless forbearance, to paola albini and nick ambraseys for editorial and critical comments. the remaining blunders are mine. i am very grateful to amikam elad and paul stenhouse for extracts from their unpublished work and to beni begin for fig. 2. references the short quotation in the text explicitly mentions the source and the used edition of the source. the latter is fully referenced in the following. abou karaki, n. 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(1937): liturgical poems in memory of various events: an earthquake in tiberias and other places, institute for study of hebrew poetry in berlin, 3, 151-162. c o n s i d e r a z i o n i s u i m e t o d i d i h o l m e s e d i j e f f r e y s p e r l a d e t e r m i n a z i o n e d e l l ' e t à d e l l a c r o s t a t e r r e s t r e (*ì g . imbò f . s. gaeta l e a n a l i s i i s o t o p i c h e di m i n e r a l i di p i o m b o e p a r t i c o l a r m e n t e di g a l e n e , eseguite da n i e r e c o l l a b o r a t o r i , h a n n o m e s s o i n e v i d e n z a u n a v a r i a z i o n e della c o m p o s i z i o n e con l ' e t à del m i n e r a l e . è stato d i f a t t i r i s c o n t r a t o p e r l e g a l e n e , a c o m p o r t a m e n t o n o r m a l e , c h e l e a b b o n d a n ze degli isotopi di o r i g i n e r a d i o a t t i v a : pb->oa, pb-2or, pb-2w r i s p e t t o a q u e l l a del pb2o4 di o r i g i n e non r a d i o a t t i v a , d i m i n u i s c o n o al c r e s c e r e della età. i n e f f e t t i l ' a b b o n d a n z a r e l a t i v a di c i a s c u n i s o t o p o di p i o m b o di o r i g i n e r a d i o a t t i v a v a r i a n e l t e m p o n e l senso c h e a l l ' a b b o n d a n z a o r i g i n a r i a v i e n e a g g i u n t a q u e l l a d e r i v a n t e n e l decorso i n t e r v a l l o dalla t r a s m u t a z i o n e r e l a t i v a a l l a f a m i g l i a r a d i o a t t i v a i l cui u l t i m o m e m b r o , s t a b i l e , c o r r i s p o n d e a l l ' i s o t o p o c o n s i d e r a t o . i c a p o s t i p i t i delle t r e fam i g l i e r a d i o a t t i v e r e l a t i v e agli isotopi pbooe, pb207, pb 203? sono rispett i v a m e n t e gli e l e m e n t i : u23s, u235, tìioug. n o t e p e r t a n t o l ' e t à del min e r a l e (t m ), n o n c h é l ' a b b o n d a n z a r e l a t i v a , r i s u l t a f a c i l e s t a b i l i r e u n a r e l a z i o n e n e l l a q u a l e figurano a n c o r a t r e i n c o g n i t e e p r e c i s a m e n t e : l ' e t à della c r o s t a ( i 0 ) e l e a b b o n d a n z e o r i g i n a r i e d e l l ' i s o t o p o di piomb o n o n c h é del r e l a t i v o c a p o s t i p i t e della f a m i g l i a r a d i o a t t i v a . e pert a n t o , se s ' i n d i c a n o l e c o m p o s i z i o n i i s o t o p i c h e r e l a t i v e di p i o m b o : con re, b, c q u e l l a r e l a t i v a a l l a g a l e n a c o n s i d e r a t a , e con x, y , z l'orig i n a r i a i n c o g n i t a ; ed i n o l t r e con u 0 , v0, tt'o l e a b b o n d a n z e dei t r e corr i s p o n d e n t i c a p o s t i p i t i , si deducono, d i s t i n t a m e n t e p e r re, b, c, tre r e l a z i o n i , di cui p e r s e m p l i c i t à si d a n n o solo l e espressioni i n d i c a t i v e ; e c i o è si h a e v i d e n t e m e n t e : fltm — / ( x^ to tm) ed a l t r e due a n a l o g h e . s e da u n c a n t o la c o n s i d e r a z i o n e di u n a e v e n t u a l e v a r i a b i l e a b b o n d a n z a i n i z i a l e dei t r e c a p o s t i p i t i r e l a t i v a m e n t e a l l e s i n g o l e gal e n e l a s c e r e b b e r i l e v a r e l ' i m p o s s i b i l i t à di d e d u z i o n e d e l l e r i c h i e s t e ( * ) comunicazione presentata al convegno dell'associazione geofisica italiana, tenuto a r o m a il 17-18 giugno 1953. 4 1 8 c . i m b ò . f . s . gaeta i n c o g n i t e ; d'altro canto l'ammissione della variazione u n i f o r m e nel tempo della composizione isotopica, nel senso di costante composizione in corrispondenza di a r b i t r a r i m a medesimi tempi, indipendent e m e n t e pertanto dalla località e dalle vicende subite, con l a conseguente deduzione di una costante composizione isotopica iniziale, consente di poter avere l'espressione per il rapporto r = vo/uo, in base alla conoscenza del valore attuale di esso n o n c h é delle costanti di disintegrazione relative ai due considerati isotopi. nella detta espressione evidentemente figura con l'età del m i n e r a l e a n c h e l'età della crosta. r i s u l t a n d o ovvia l'impossibilità di sottoporre ad esame l'espressione in c, wa, per ciascuna galena, si h a p e r t a n t o u n a unica relazione, la quale, sempre per semplicità, è data dalla espressione gen e r i c a : fftm v, to) = 0. p e r evidenti subite c o n t a m i n a z i o n i non tutte le analisi relative a l l e 25 galene esaminate possono essere prese in considerazione. e pertanto, pur servendosi dei medesimi dati, i diversi studiosi c h e h a n n o voluto applicare l ' e n u n c i a t o metodo, q u a l i : holmes, c h e n e è stato l'introduttore, e successivamente f . g. houterinans, e . c. b u l lard, j . p. stanley, h. jeffreys, h a n n o ottenuto valori p e r l'età della crosta non sempre concordanti, in relazione sia ai c r i t e r i di calcolo, sia ancora alle analisi considerate. a d esempio l o j e f f r e y s , m e d i a n t e l ' e l i m i n a z i o n e delle relazioni corrispondenti a due galene dalle tredici prese i n i z i a l m e n t e in considerazione, asserisce di essere passato dal valore di 2,6 x 10u anni all'altro di c i r c a 3,3 x k>9. p e r un'indagine critica del metodo si è reso quindi indispensabile una r i e l a b o r a z i o n e dei calcoli, l i m i t a n d o l i alle galene per le quali, oltre alla conoscenza dell'età entro t o l l e r a b i l i l i m i t i , rilievi su a n d a m e n t i qualitativi delle abbondanze h a n n o convinto dell'effettiva o della pratica assenza di contaminazioni o c o m u n q u e dell'assenza di un comportam e n t o anomalo. sono state quindi assunte l e sole undici galene c h e , nell ' e l e n c o del nier, si riferiscono rispettivamente ai n u m e r i : 1, 3, 6, 13, 16, 17, 18, 20, 22, 23, 25, l e cui età in 10° anni, risultano rispettivamente per le prime tre (1, 3, 6) : 0 , 0 2 5 ; per le successive quattro (13, 16, 17, 18) : 0 , 2 2 0 ; per l a 20 : 0 , 8 0 0 ; per l e 22 e 23 : 1,2 ed infine 1,33 per la 25. p e r la deduzione di t0 si è innanzitutto applicato il metodo c h e può dirsi dei valori discreti. a tale scopo si sono assegnati al valore incognito del tempo (t) n distinti valori in modo da aversi, in base alla soluzione col metodo dei m i n i m i quadrati degli n indipendenti j l l 1,5 2,0 2, ,5 3,0 8,0 (1.0) 13,0 5,5 ( 1,0) 11,0 11,0 19,0 (2,0) 10,0 (3,0) 10,0 (1,0) 6,0 (3,0) 5.5 )4,0) 3.0 (1,5) i 0,0 13,0 6,5 11,0 11,0 21,0 13,0 11,0 9,0 9,5 4,5 1,5 3.0 2,0 (1,0) 6,0 5,0 5,0 8,0 (3,0) 4,0 (1,0) 7,0 (2,0) 10,5 (3,0) 21,0 (2,5) 3 1,5 3,0 3,0 6,0 5,0 5.0 11,0 5,0 9,0 13,5 23,5 4.5 (1,0) 1,0 1,0 — 6,5 2,0 6,0 (2,0) 4,0 6,0 (1,0) 10,0 (3,0) 17.5 (1,0) 6 5,5 1,0 1,0 6,5 2,0 8,0 4,0 7,0 13,0 18,5 2,0 (1,0) 3,0 3,5 5,0 (1,0) 7,5 (1,0) 18,5 (1,0) 8,0 9,5 5,0 (2,0) 1,0 (6,0) 5,5 (2,0) 13 3,0 3,0 3,5 6,0 8,5 19,5 8.0 9,5 7,0 7,0 7,5 4,0 (1,0) — 1,0 3,0 2.0 4,0 4.5 (7,0) 3.5 (1,0) 2,5 2,5 ( 7,0) 8,0 ( 2,0) 16 5,0 — 1.0 3.0 2,0 4,0 11.5 4.5 2,5 9,5 10,0 7,5 10,5 6,5 (1,0) 7,5 4,0 6,5 4.5 (8,0) 5,5 5.5 (1,5) 11,0 (2,5) 15,0 (1,0) 17 7,5 10,5 7.5 7,5 4,0 6.5 12.5 5,5 7 0 13,5 16,0 — 3,0 1,0 3.0 (1,0) 3.0 (1,0) 2.0 (1,0) 5,0 (1,0) 6.0 (1,0) 4,0 (1,5) 9,5 (1,5) 16,5 (1,0) 18 — 3,0 1.0 4,0 4,0 3.0 6.0 7.0 5,5 11,0 17,5 7.0 1,0 3,5 7,5 8.5 (2,0) 10,5 8.0 (2,0) 12.0 12,0 (1,0) 5,0 (1,0) 7,5 (1,0) 20 7,0 1,0 3.5 7,5 10,5 10.5 10.0 12,0 13,0 6,0 8,5 1,0 1,0 3,0 8.0 (2,0) 7,0 4,0 6 5 (2.0) 9.0 3,5 (0.5) 10,0 (2,5) 7.0 22 1,0 1,0 3,0 10,0 7,0 4.0 8.5 9.0 4,0 12,5 7,0 4,5 7,5 6.0 (1,0) 10,0 8,5 21.0 11.5 (2.0) 9,0 8.5 (1,5) 8.0 (1,5) 16,5 (2.0) 23 4,5 7,5 7,0 10,0 8,5 21.0 16.5 9.0 10,0 9,5 18,5 4,0 7,0 5,0 9,0 7,0 21.5 13.5 (2.0) 6.0 8.0 9,5 (2,0) 16,0 25 4,0 7,0 5,0 9,0 7,0 21,5 15,5 6.0 8,0 11,5 16,0 44,0 (4,0) 50,0 38,0 (4,0) 70,0 (4,0) 70,0 (4,0) 114,0 (4,0) 88,5 (32,0) 78,5 (4,0) 68,0 (14,0) 82,5 (34,0) 133,5 (14,0) 48,0 50,0 42,0 74,0 74,0 118,0 120,5 82,5 82,0 116,5 147,5 tabella i 3,5 4,0 7,5 (8,0) 5,5 (0,5) 5,0 (1,0) 1.5 (2.0) 5,0 15,5 6,0 6,0 3,5 5,0 17,0 (7,0) 13.5 (2,5) 21.5 (1,0) 13.5 ( 3,0) 4,0 (1,0) 24,0 16,0 22,5 16,5 5,0 11,0 (1,0) 9.0 (2,0) 7,5 (2,0) 10,5 11,0) 11,0 (1.0) 12,0 11,0 9,5 11.5 15,0 8,5 ( 1,0) 6,0 ( 1,0) 5,0 (3,0) 11,0 (1,0) 13,0 12,5 7.0 8,0 12,0 13,0 5.0 (3,0) 12,0 (1,0) 19,5 (2,0) 8,0 (3,0) 4,5 (1,0) 8.0 13,0 21,5 11,0 5.5 13,0 (3,0) 4,0 (1,0) 7,0 (3.0) 5,5 4,5 16,0 5,0 10,0 5,5 4,5 16,0 (4,0) 8,0 (1,0) 6,0 1,5 (2,0) 5,0 (1,0) 20,0 9,0 6,0 3,5 6,0 15,5 (2,0) 10,0 9,0 10,0 7.0 17,5 10,0 9.0 10,0 7,0 5,0 (3,0) 10,0 (1,0) 4,5 (2,0) 5,0 (2,0) 6,0 8,0 11,0 6,5 7,0 6,0 14.0 (3,0) 6,0 11,0 (2,0) 5.0 (2,0) 8,5 17,0 6,0 13,0 7,0 8,5 13,5 (2,0) 3,0 14,0 8,5 8,5 15,5 3,0 14,0 8,5 8,5 126,0 (40,0) 87,0 (10,0) 110,0 (16,0) 80,0 (16,0) 80,0 (4,0) 166,0 97,0 126,0 96,0 84,0 4,5 5,0 5.5 1,5 (1,0) 5,5 (2,0) 11,5 (2,0) 5,0 8,0 4,0 7,0 (1,0) — 169,0 (31,0) 2,5 7,5 13,5 5,0 8,0 4,0 8,0 — 203,0 3,0 (2,0) 5.5 (8,0) 3,5 ( 1.0) 5.0 (4,0) 9,0 11.0 2,0 (1,0) 1,0 182,5 (43,0) 5,0 13,5 4,5 9,0 9,0 11,0 3,0 1,0 225,5 8,5 (1,0) 8.0 ( 8,0) 1,5 (3.0) 3.0 (2.0) 1,0 — — 1,0 133,5 (29,0) 9,5 16,0 4,5 5,0 1,0 — — 1,0 162,5 9,0 (1,0) 10,0 (4,0) 2,0 5,0 4,0 (2,0) 11,0 3.0 (1.0) 1.0 157.0 (31,0) 10,0 14,0 2.0 5,0 6.0 11.0 4,0 1,0 188,0 2,5 (1,0) 5,5 (3,0) 8,0 2.0 (1,0) 2,0 3.0 6.0 1,0 114.0 (33,0) 3.5 8.5 8,0 3,0 2,0 3.0 6.0 1,0 147,0 2,0 1.5 (3.0) 0,5 3,0 — — — — 125,0 (24,0) 2,0 4,5 0.5 3,0 — — — — 149,0 3,0 7.0 (2,0) 6,5 5,0 (1,0) 5.0 4,0 2,0 (1,0) — 122,0 (21,0) 3,0 9,0 6,5 6.0 5,0 1.0 3.0 — 143,0 4,0 6,5 5,5 2,0 (2,0) 8,0 1,0 1,0 162.0 (11,0) 4,0 6,5 5,5 2,0 2.0 8,0 1,0 1,0 173,0 6,5 (1,0) 7,5 (2,5) 4,5 (2,5) 4,0 (1,0) 10.0 (1,0) 7,0 5,0 1,0 136,0 (23,0) 7,5 10,0 7,0 5,0 11,0 7,0 5,0 1,0 159,0 3,5 (1,0) 4.5 i l , 5 ) 7,5 (3,5) 6.0 (1,0) 4.0 0 , 0 ) 5,0 3,0 1,0 193,0 (23,0) 4,5 6,0 11,0 7,0 5,0 5.0 3,0 1,0 216,0 4,5 4,5 (2,0) 7,0 6,0 7,0 3.0 3,0 1,0 190,0 (8,0) 4,5 6,5 7,0 6,0 7,0 3,0 3,0 1,0 198,0 48,0 (8,0) 66,0 (36,0) 58,0 (12,0) 46,0 (10,0) 50,0 (6,0) 56,0 32,0 (4,0) 8,0 1684,0 (280,0) 56,0 102,0 70,0 56,0 56,0 56,0 36,0 8,0 1961,0 c o n s i d e r a z i o n i s u i m e t o d i di h o l m e s e di j e f f r e y s 4 1 9 sistemi di equazioni l i n e a r i in x ed y, per ciascun sistema l ' e r r o r e q u a d r a t i c o medio. s i assume come valore più p r o b a b i l e di t0, quel valore di l cui corrisponde il valore m i n i m o del detto errore. con l'applicazione di tale metodo si è ottenuto a l l ' i n c i r c a il v a l o r e : to = 2,7 x io9 anni. si è voluto i n o l t r e applicare anche il metodo c h e può dirsi delle intersezioni. s e difatti s'indicano con cf:t e cpj le relazioni corrispondenti alla i-ma ed alla j m a galena, risulta agevole e l i m i n a r e tra esse la x o la y , ottenendosene in effetti l e due altre r e l a z i o n i : x ; j {x, t) = 0 yis (y, t) = 0 rispettivamente in x, t ed in y, t. i n tal modo, escludendo evidentemente gli a c c o p p i a m e n t i tra le galene coetanee, si h a n n o 45 relazioni in x, t ed a l t r e t t a n t e in y, t. l a rappresentazione grafica di entrambi i gruppi di curve h a consentito i n o l t r e la deduzione delle coordinate dell'eventuale punto d'intersezione di ciascuna curva con tutte l e altre curve del medesimo gruppo. l a constatazione di parallelismi di curve fino alla pratica o totale sovrapposizione r i m a n e solo parzialmente giustificata spesso dalla coetaneità dei due accoppiamenti necessari per l a deduzione dell'incrocio. m e n t r e difatti da un canto p r a t i c i parallelismi si h a n n o anche per coppie distintamente costituite da galene non coetanee, d'altro c a n t o si h a n n o netti incroci a n c h e in casi nei quali, a quanto è stato detto, non si s a r e b b e r o dovuti presentare. in effetti su 1980 incroci teorici, complessivamente per i due gruppi, nell'intervallo, di t, espresso in 10° : 1,5 -f5,5, se n e osservano a l l ' i n c i r c a solo il 5 0 % di cui c i r c a i l 7 % corrispondenti ad i n c r o c i a n o m a l i , m e n t r e solo presso a poco i l 4 3 % relativi ad i n c r o c i n o r m a l i . l ' a l t r o 4 4 % degli i n c r o c i n o r m a l i corrisponde o ad andamenti p r a t i c a m e n t e paralleli o ad i n c r o c i verificantisi all'esterno dell'indicato intervallo. d a l l ' a n d a m e n t o delle f r e q u e n z e degl'incroci (vedi fig. 1) si nota u n a distribuzione irregolare con la presenza di t r e distinti massimi di cui il centrale ( a l l ' i n c i r c a per t = 3,5 x 10° anni), per i l n u m e r o relativamente cospicuo dei punti c h e ad esso si r i f e r i s c o n o e p e r la pratica regolarità dell'andamento della distribuzione delle f r e q u e n z e verso i valori crescenti di t, può ritenersi il principale. degli altri due il massimo per circa 2,5 x io9 presenta in 418 c. i m b ò . f . s . gaeta quanto a distribuzione delle f r e q u e n z e contigue, s e b b e n e con n n n u m e r o r e l a t i v a m e n t e ridotto di punti, c a r a t t e r i s t i c h e n o r m a l i , l ' a l t r o invece, a c i r c a 4,4 x io9, è costituito da u n a lieve gobba c h e risulta 7 5 r i 4.5 5.0 5.5 maggiormente evidente p e r l'azione determinata da un n u m e r o relativamente rilevante di i n c r o c i anomali. nella figura gl'incroci anomali sono rappresentati dalle altezze dei rettangoli punteggiati, m e n t r e quelli n o r m a l i dalle altezze dei rettangoli tratteggiati. p e r l'individuazione i n o l t r e dello eventuale c o n t r i b u t o delle singole galene a i tabella 11 y 1 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 1 1 7,0 (0,5) 5,0 6,0 8,0 (2,0) 16,5 (1,0) 19,0 (0,5) 15,5 (3,5) 8,0 (1,0) 2,5 (3,0) 9,0 (4,0) 3,0 (1,0) 5,5 (6,5) 10,0 (2,5) 5,0 (3,0) 1,5 4,5 (2,0) 0,5 (1,5) 11,0 (2,0) 10,5 (1,5) 10,5 4,0 (1,0) — 1,0 1,0 164,5 (36,5) 7,5 5,0 6,0 10,0 17,5 19,5 19,0 9,0 5,5 13,0 4,0 12,0 12,5 8,0 1,5 6,5 2,0 13,0 12,0 10,5 5,0 — 1,0 1,0 201,0 3 1,0 1,0 (2.0) 4,0 4,5 5,5 5,0 (0,5) 7,5 (3,5) 1,5 (1,0) 7,0 (2,0) 19,0 (3,0) 12,0 (2,0) 13,0 (5,5) 21,5 (4,5) 21,5 (3,0) 9,0 (1,0) 6.0 (2,5) 5,0 (1,5) 6,5 (7,0) 7,0 (2,0) 4,5 (2,0) 4,0 (1,0) — 2,0 1,0 169,0 (44,0) 1,0 3,0 4,0 4,5 5,5 5,5 11,0 2,5 9,0 22,0 14,0 18,5 26,0 24,5 10,0 8,5 6,5 13,5 9,0 6,5 5,0 — 2,0 1,0 213,0 6 4,0 (1,5) 2,0 4,5 3,0 (1,0) 7,0 (2,0) 4,0 4,5 (2,0) 4,0 8,5 (1,0) 12,5 (1,0) 11,5 (1,0) 8,5 16,0 (2,0) 8,5 (2,0) 9,0 (1,0) 18,5 (2,5) 4,5 (0,5) 9,0 (7,5) 2,5 (3,5) (1,0) — — 2,0 1,0 145,0 (29,5) 5,5 2,0 4,5 4,0 9,0 4,0 6,5 4,0 9,5 13,5 12,5 8,5 18,0 10,5 10,0 21,0 5,0 16,5 6,0 1,0 — — 2,0 1,0 174,5 13 4,0 2,5 (1,0) 2,0 (1,0) 5.5 (1,5) 12,5 (3,5) 15,5 (1,0) 12,5 8,0 2,5 (2,5) 6,0 (7,0) 6,0 (1,5) 4,5 (2,0) 10,0 (1,0) 4,5 (2,0) 8,5 (1,0) 17,5 (2,0) 6,0 (1,0) 12,0 (2,5) 9,5 (0,5) 1,5 2,0 (1,0) — 2,0 2,0 157,0 (32,0) 4,0 3,5 3,0 7,0 16,0 16,5 12,5 8,0 5,0 13,0 7,5 6,5 11,0 6,5 9,5 19,5 7,0 14,5 10,0 1,5 3,0 — 2,0 2,0 189,0 16 4,0 (0,5) 1,0 1,0 4,0 (1,0) 2,0 6,5 (0,5) 6,5 (6,5) 3,0 (1,0) 1,5 6,5 (7,5) 6,5 (1,5) 5,0 (2,0) 16,0 (1,5) 17,5 (3,5) 7,5 (1,0) 4,0 (1,0) 4,0 (1,5) 9,5 (1,0) 6,0 (0,5) 8,0 (1,0) 2,0 — — 1,0 123,0 (31,5) 4,5 1,0 1,0 5,0 2,0 7,0 13,0 4,0 1,5 14,0 8,0 7,0 17,5 21,0 8,5 5,0 5,5 10,5 6,5 9,0 2,0 — — 1,0 154,5 17 6,0 (0,5) 2,5 (2,0) 7,0 7,0 (1,0) 7,0 (1,0) 6,5 (0,5) 8,5 (7,5) 5,5 (1,0) 9,0 (1,0) 13,5 (4,0) 9,0 (1,0) 8,5 (2,0) 12,0 0 , 0 ) 5,5 (2,0) 4,0 3,5 (1,0) 3,0 1,5 (2,0) — 1,0 1,0 — — — 121,5 (27,5) 6,5 4,5 7,0 8,0 8,0 7,0 16,0 6,5 10,0 17,5 10,0 10,5 13,0 7,5 4,0 4,5 3,0 3,5 — 1,0 1,0 — — — 149,0 18 1,5 (1,0) 1,5 (1,0) (1,0) 3.0 (1,5) 3,0 (2,5) 3,5 (1,0) 7,5 (1,0) 3,0 (2,0) 5.0 (1,0) 15,5 (2,0) 9,0 (1,0) 15,0 (2,0) 13,0 (1,5) 5,5 (1,5) 2,5 5,0 (2,0) 3,0 8,5 (2,0) 9,5 2,5 (1,0) 1,0 (1,0) — 2,0 2,0 122,0 (26,0) 2,5 2,5 1,0 4,5 5,5 4,5 8,5 5,0 6,0 17,5 10,0 17,0 14,5 7,0 2,5 7,0 3,0 10,5 9,5 3,5 2,0 — 2,0 2,0 148,0 20 4,0 4,5 (2,0) 6,0 (2,0) 10,5 (2,0) 9,5 8,5 12,0 (2,0) 9,0 9,0 10,5 (2,0) 6,0 11,0 (2,0) 17,0 7,5 7,5 8,0 5,5 4,0 (2,0) 7,0 4,0 1,0 — — 4,0 166.0 (14,0) 4,0 6,5 8,0 12,5 9,5 8,5 14,0 9,0 9,0 12,5 6,0 13,0 17,0 7,5 7,5 8,0 5,5 6,0 7,0 4,0 1,0 — — 4,0 180,0 22 2,0 5,0 3,5 5,0 9,5 (1,0) 6,5 (2,5) 9,5 (2,5) 4,0 (1,0) 7,5 (1,0) 14,0 (5,0) 5,0 6,0 14,5 (2,5) 3,0 (2,5) 2,0 7,5 (0,5) 5,0 (1,5) 8,5 (4,0) 8,0 (2,5) 6,0 (0,5) 5,0 — 1,0 2,0 140,0 (27,0) 2,0 5,0 3,5 5,0 10,5 9,0 12,0 5,0 8,5 19,0 5,0 6,0 17,0 5,5 2,0 8,0 6,5 12,5 10,5 6,5 5,0 — 1,0 2,0 167,0 23 6,0 7,0 4,0 7,0 13,0 (3,0) 24,5 (0,5) 20,5 (2,5) 5,5 (1,0) 7,5 (1,0) 13,0 (5,0) 8,5 11,0 (1,0) 14,0 (1,5) 9,0 (2,5) 6.0 6,5 (0,5) 4,0 (0,5) 9,0 (2,0) 12,0 (3,5) 6,0 (0,5) 2,0 — 1,0 — 197,0 (25,0) 6,0 7,0 4,0 7,0 16,0 25,0 23,0 6,5 8,5 18,0 8,5 12,0 15,5 11,5 6,0 7,0 4,5 11,0 15,5 6,5 2,0 — 1,0 — 222,0 25 8,5 4,0 6,0 2,5 12,5 22,5 (1,0) 21,5 (1,0) 4,5 8,5 10,0 (1,0) 11,5 (1,0) 11,0 10,0 (2,0) 14,5 8,5 11,0 3,5 6,5 (2,0) 6,0 4,0 2,0 — 1,0 2,0 192,0 (8,0) 8,5 4,0 6,0 2,5 12,5 23,5 22,5 4,5 8,5 11,0 12,5 11,0 12,0 14,5 8,5 11,0 3,5 8,5 6,0 4,0 2,0 — 1,0 2,0 200,0 48,0 (4,0) 36,0 (8,0) 44,0 (4,0) 60,0 (10,0) 98,0 (14,0) 122,0 (8,0) 126,0 (32,0) 56,0 (8,0) 68,5 (12,5) 129,5 (41,5) 88,0 (10,0) 99,0 (23,0) 154,0 (20,0) 102,0 (22,0) 66,0 (4,0) 92,0 (14,0) 44,0 (8,0) 86,0 (34,0) 78,0 (14,0) 48,0 (6,0) 24,0 (4,0) — 12,0 16,0 1697,0 (301,0) 52,0 44,0 48,0 70,0 112,0 130,0 158,0 64,0 81,0 171,0 98,0 122,0 174,0 124,0 70,0 106,0 52,0 120,0 92,0 54,0 28,0 — 12,0 16,0 1998,0 h o l m e s e di j e f f r e t s galena 6 s u i m e t o d i 1)1 c o n s i d e r a z i o n i galena 1 fig. 2 418 c . i m b ò . f . s . gaeta tre indicati massimi si è a t t r i b u i t o il tempo ottenuto a ciascuna delle quattro galene costituenti l e due distinte coppie da cui si è rilevato l'incrocio. n e l caso in cui n e i due accoppiamenti u n a galena è figurata due volte, a n c h e i l tempo è stato assegnato ad essa due volte. si sono così avuti i valori r i p o r t a t i n e l l e u n i t e t a v o l e : la tavola i relativa alle curve x, t; l a tavola i i relativa alle curve y, t ; m e n t r e la tavola i i i si r i f e r i s c e alla somma dei corrispondenti v a l o r i delle due tavole precedenti. nelle tavole sono i n d i c a t i distintamente i valori relativi agl'incroci n o r m a l i e quelli ( t r a parentesi) relativi agl'incroci f i g . 3 anomali n o n c h é i totali. p e r ciascuna galena, salvo per la 13, per la quale presentano quasi identica importanza ambedue i massimi estremi, si nota u n a decisa prevalenza di uno solo dei tre indicati massimi. s i riproducono n e l l a figura 2 i soli a n d a m e n t i relativi alle galene 1, 3, 6 pei quali si n o t a n o i massimi rispettivamente, per t in io9, i n t o m o a 2 , 4 ; 3 , 7 ; 4,1. avendo considerato c o m e si è detto, principale i l massimo c e n t r a l e si è r i t e n u t o opportuno di rieseguire i medesimi precedenti calcoli, escludendo l e galene per l e quali nella distribuzione delle frequenze si h a un deciso c o n t r i b u t o pei massimi estremi. s o n o state quindi prese in considerazione l e sole sei galene i n d i c a t e coi n u m e r i : 3, 16, 17, 18, 20, 22. a n c h e in t a l e caso i due distinti m e t o d i : q u e l l o dei valori discreti e l ' a l t r o delle intersezioni, non danno per t0 i l medesimo valore. e precisamente m e n t r e in base al p r i m o si h a t0 = 2,6 x io9 anni, la distribuzione della frequenza r t a b e l l a i i i 1,5 2 , 0 2 , 5 3 , 0 3,5 4 , 0 4,5 5 , 0 5 , 5 1 5 , 0 ( 1 , 5 ) 1 8 , 0 1 1 , 5 ( 1 , 0 ) 1 9 , 0 ( 2 , 0 ) 2 7 , 5 ( 1 , 0 ) 3 8 , 0 ( 2 , 5 ) 2 5 , 5 ( 6 , 5 ) 1 8 , 0 ( 2 , 0 ) 8 , 5 ( 6 , 0 ) 1 4 , 5 ( 8 , 0 ) 6 , 0 ( 2 , 5 ) 1 3 , 0 ( 1 4 , 5 ) 1 5 , 5 ( 3 , 0 ) 1 0 , 0 ( 4 , 0 ) 3 , 0 ( 2 , 0 ) 9 , 5 ( 2 , 0 ) 2 , 0 ( 2 , 5 ) 16,5 ( 4 , 0 ) 2 2 , 0 ( 3 , 5 ) 15,5 1 2 , 0 ( 1 , 0 ) 4 , 0 8 , 0 ( 1 , 0 ) 1 , 0 3 3 3 , 5 ( 7 0 , 5 ) 16,5 1 8 , 0 1 2 , 5 2 1 , 0 2 8 , 5 4 0 , 5 3 2 , 0 2 0 , 0 1 4 , 5 2 2 , 5 8 , 5 2 7 , 5 1 8 , 5 1 4 , 0 5 , 0 1 1 , 5 4 , 5 2 0 , 5 2 5 , 5 1 5 , 5 1 3 , 0 4 , 0 9 , 0 1 , 0 4 0 4 , 0 2,5 4 , 0 ( 2 , 0 ) 6 , 0 ( 1 , 0 ) 1 0 , 5 1 0 , 5 1 0 , 0 ( 0 , 5 ) 1 5 , 5 ( 6 , 5 ) 5 , 5 ( 2 , 0 ) 1 4 , 0 ( 4 , 0 ) 2 9 , 5 ( 6 , 0 ) 3 3 , 0 ( 4 , 5 ) 3 0 , 0 ( 1 2 , 5 ) 3 5 , 0 ( 7 , 0 ) 4 3 , 0 ( 4 , 0 ) 2 2 , 5 ( 4 , 0 ) 1 0 , 0 ( 3 , 5 ) 8 , 0 ( 3 , 5 ) 12,0 ( 1 5 , 0 ) 10,5 ( 3 , 0 ) 9,5 ( 6 , 0 ) 1 3 , 0 ( 1 , 0 ) 1 1 , 0 4 , 0 ( 1 , 0 ) 2 , 0 3 5 1 , 5 ( 8 7 , 0 ) 2,5 6 , 0 7,0 1 0 , 5 10,5 1 0 , 5 2 2 , 0 7,5 1 8 , 0 3 5 , 5 3 7 , 5 4 2 , 5 4 2 , 0 4 7 , 0 2 6 , 5 1 3 , 5 1 1 , 5 2 7 , 0 13,5 15,5 1 4 , 0 1 1 , 0 5 , 0 2 , 0 4 3 8 , 5 8,5 ( 2 , 5 ) 3 , 0 5,5 3 , 0 ( 1 , 0 ) 1 3 , 5 ( 2 , 0 ) 6 , 0 1 0 , 5 ( 4 , 0 ) 8 , 0 1 4 , 5 ( 2 , 0 ) 2 2 , 5 ( 4 , 0 ) 2 9 , 0 ( 2 , 0 ) 1 9 , 5 ( 1 , 0 ) 2 5 , 0 ( 4 , 0 ) 1 6 , 0 ( 4 , 0 ) 1 9 , 5 ( 2 , 0 ) 3 2 , 5 ( 3 , 5 ) 1 3 , 0 ( 1 , 5 ) 1 7 , 0 ( 1 5 , 5 ) 4 , 0 ( 6 , 5 ) 3 , 0 ( 3 , 0 ) 1 . 0 — 2 , 0 2 , 0 2 7 8 , 5 1 5 8 , 5 ) 1 1 , 0 3 , 0 5 , 5 4 , 0 1 5 , 5 6 , 0 1 4 , 5 8 , 0 1 6 , 5 2 6 , 5 3 1 , 0 2 0 , 5 2 9 , 0 2 0 , 0 2 1 , 5 3 6 , 0 1 4 , 5 3 2 , 5 10,5 6 , 0 1 , 0 — 2 , 0 2 , 0 3 3 7 , 0 6,0 ( 1 , 0 ) 5 , 5 ( 1 , 0 ) 5 , 5 ( 1 , 0 ) 1 0 , 5 ( 2 , 5 ) 2 0 , 0 ( 4 , 5 ) 3 4 , 0 ( 2 , 0 ) 2 0 , 5 1 7 , 5 7,5 ( 4 , 5 ) 7 , 0 ( 1 3 , 0 ) 1 1 , 5 ( 3 , 5 ) 1 3 , 0 ( 6 , 0 ) 1 6 , 0 ( 2 , 0 ) 9 , 5 ( 5 , 0 ) 1 9 , 5 ( 2 , 0 ) 3 0 , 5 ( 2 , 0 ) 1 5 , 0 ( 2 , 0 ) 2 2 , 0 ( 6 , 5 ) 11,5 ( 0 , 5 ) 6 , 5 6 , 0 ( 3 , 0 ) 1 1 , 0 5 , 0 ( 1 , 0 ) 3 , 0 3 1 4 , 0 ( 6 3 , 0 ) 7,0 6,5 6,5 1 3 , 0 2 4 , 5 3 6 , 0 2 0 , 5 1 7 , 5 1 2 , 0 2 0 , 0 1 5 , 0 1 9 , 0 1 8 , 0 1 4 , 5 2 1 , 5 3 2 , 5 1 7 , 0 2 8 , 5 1 2 , 0 6,5 9 , 0 1 1 , 0 6 , 0 3 , 0 3 7 7 , 0 8 , 0 ( 1 , 5 ) 1 , 0 2 , 0 7 , 0 ( 1 , 0 ) 4 , 0 10,5 ( 0 , 5 ) 1 1 , 0 ( 1 3 , 5 ) 6,5 ( 2 , 0 ) 4 , 0 9 , 0 ( 1 4 , 5 ) 1 4 , 5 ( 3 , 5 ) 1 0 , 0 ( 5 , 0 ) 2 8 , 0 ( 2 , 5 ) 3 7 , 0 ( 5 , 5 ) 1 5 , 5 ( 4 , 0 ) 8,5 ( 2 , 0 ) 6,5 ( 2 , 5 ) 1 5 , 0 ( 4 , 0 ) 1 4 , 0 ( 0 , 5 ) 1 0 , 0 ( 2 , 0 ) 4 , 0 3 , 0 6 , 0 2 , 0 2 3 7 , 0 ( 6 4 , 5 ) 9,5 1 , 0 2 , 0 8 , 0 4 , 0 1 1 , 0 2 4 , 5 8 , 5 4 , 0 2 3 , 5 1 8 , 0 1 5 , 0 3 0 , 5 4 2 , 5 1 9 , 5 1 0 , 5 9 , 0 1 9 , 0 14,5 1 2 , 0 4 , 0 3 , 0 6 , 0 2 , 0 3 0 1 , 5 13,5 ( 0 , 5 ) 1 3 , 0 ( 2 , 0 ) 1 3 , 5 ( 1 , 0 ) 1 4 , 5 ( 1 , 0 ) 1 1 , 0 ( 1 , 0 ) 1 3 , 0 ( 0 , 5 ) 1 3 , 0 ( 1 5 , 5 ) 1 1 , 0 ( 1 , 0 ) 1 4 , 5 ( 2 , 5 ) 2 4 , 5 ( 4 , 5 ) 2 4 , 0 ( 2 , 0 ) 2 1 , 5 ( 5 , 0 ) 1 6 , 0 ( 2 , 0 ) 1 2 , 5 ( 5 , 0 ) 9 , 5 8 , 0 ( 1 , 0 ) 5 , 0 3 , 0 ( 5 , 0 ) 0,5 4 , 0 1 , 0 — — — 2 6 4 , 5 ( 5 1 , 5 ) 1 4 , 0 1 5 , 0 14,5 15,5 1 2 , 0 13,5 2 8 , 5 1 2 , 0 1 7 , 0 3 1 , 0 2 6 , 0 2 6 , 5 1 8 , 0 1 7 , 5 9,5 9 , 0 5 , 0 8 , 0 0,5 4 , 0 1 , 0 — — — 2 9 8 , 0 1,5 ( 1 , 0 ) 4 , 5 ( 1 , 0 ) 1,0 ( 1 , 0 ) 6 , 0 ( 2 , 5 ) 6 , 0 ( 3 , 5 ) 5 , 5 ( 2 , 0 ) 12,5 ( 2 , 0 ) 9 , 0 ( 3 , 0 ) 9 , 0 ( 2 , 5 ) 2 5 , 0 ( 3 , 5 ) 2 5 , 0 ( 2 , 0 ) 3 1 , 0 ( 6 , 0 ) 2 1 , 0 ( 2 , 5 ) 1 1 , 5 ( 1 , 5 ) 4 , 0 ( 2 , 0 ) 1 0 , 0 ( 3 , 0 ) 6 , 0 15,5 ( 4 , 0 ) 1 6 , 0 7,5 ( 2 , 0 ) 6 . 0 ( 1 . 0 ) 4 , 0 4 , 0 ( 1 , 0 ) 2 , 0 2 4 4 , 0 ( 4 7 , 0 ) 2,5 5 , 5 2 , 0 8 , 5 9 , 5 7,5 1 4 , 5 1 2 , 0 1 1 , 5 2 8 , 5 2 7 , 5 3 7 , 0 2 3 , 5 1 3 , 0 6 . 0 1 3 , 0 6 , 0 19,5 1 6 , 0 9 , 5 7,0 4 , 0 5 , 0 2 , 0 2 9 1 , 0 11,0 5,5 ( 2 , 0 ) 9 , 5 ( 2 , 0 ) 1 8 . 0 ( 2 , 0 ) 1 8 , 0 ( 2 , 0 ) 1 9 , 0 2 0 , 0 ( 4 , 0 ) 2 1 , 0 2 1 , 0 ( 1 , 0 ) 15,5 ( 3 , 0 ) 1 3 , 5 ( 1 , 0 ) 2 6 , 5 ( 4 , 0 ) 2 7 , 0 1 6 , 5 1 7 , 5 1 5 , 0 9 , 5 10.5 ( 2 , 0 ) 12,5 6 , 0 1 . 0 ( 2 , 0 ) 8 . 0 1 , 0 5 , 0 3 2 8 , 0 ( 2 5 , 0 ) 1 1 , 0 7,5 1 1 , 5 2 0 , 0 2 0 , 0 1 9 , 0 2 4 , 0 2 1 , 0 2 2 , 0 18,5 1 4 , 5 3 0 , 5 2 7 , 0 1 6 , 5 1 7 , 5 1 5 , 0 9,5 12,5 12,5 6 , 0 3 . 0 8 . 0 1 , 0 5 , 0 3 5 3 , 0 3,0 6 , 0 6,5 1 3 , 0 ( 2 , 0 ) 1 6 , 5 ( 1 , 0 ) 1 0 , 5 ( 2 , 5 ) 1 6 , 0 ( 4 , 5 ) 1 3 , 0 ( 1 , 0 ) 1 1 , 0 ( 1 , 5 ) 2 4 , 0 ( 7 , 5 ) 1 2 , 0 1 1 , 0 ( 3 , 0 ) 2 4 , 5 ( 3 , 5 ) 7,5 ( 4 , 5 ) 7 , 0 ( 2 , 0 ) 1 3 . 5 ( 0 , 5 ) 1 1 , 5 ( 2 , 5 ) 1 6 . 0 ( 6 , 5 ) 12,5 ( 5 , 0 ) 1 0 , 0 ( 1 , 5 ) 1 5 . 0 ( 1 . 0 ) 7.0 6 , 0 3 , 0 2 7 6 , 0 ( 5 0 , 0 ) 3,0 6 , 0 6 , 5 1 5 , 0 1 7 , 5 1 3 , 0 2 0 , 5 1 4 , 0 12,5 3 1 , 5 1 2 , 0 1 4 , 0 2 8 , 0 1 2 , 0 9 , 0 1 4 , 0 1 4 , 0 2 2 . 5 17,5 11,5 1 6 . 0 7 . 0 6 . 0 3 , 0 3 2 6 , 0 10,5 1 4 , 5 1 0 , 0 ( 1 , 0 ) 1 7 , 0 2 1 , 5 ( 3 , 0 ) 4 5 , 5 ( 0 , 5 ) 3 5 , 0 ( 4 , 5 ) 1 4 . 5 ( 1 , 0 ) 1 6 , 0 ( 2 , 5 ) 2 1 , 0 ( 6 , 5 ) 2 5 , 0 ( 2 , 0 ) 2 5 . 0 ( 4 , 0 ) 2 0 , 0 n , 5 ) 2 0 , 0 ( 4 , 5 ) 1 1 , 0 ( 2 , 0 ) 1 5 , 0 ( 0 , 5 ) 7,5 ( 1 , 5 ) 13.5 ( 3 , 5 ) 19.5 ( 7 , 0 ) 1 2 , 0 ( 1 . 5 ) 6 . 0 ( 1 , 0 ) 5 . 0 4 . 0 1 , 0 3 9 0 , 0 ( 4 8 , 0 ) 10,5 1 4 , 5 1 1 , 0 1 7 , 0 2 4 , 5 4 6 , 0 3 9 , 5 1 5 , 5 18,5 2 7 , 5 2 7 , 0 2 9 , 0 2 1 , 5 2 4 , 5 1 3 , 0 1 5 , 5 9 , 0 1 7 . 0 2 6 , 5 1 3 , 5 7.0 5 . 0 4 . 0 1,0 4 3 8 , 0 12,5 1 1 , 0 1 1 , 0 1 1 , 5 1 9 , 5 4 4 , 0 ( 1 , 0 ) 3 5 , 0 ( 3 , 0 ) 1 0 , 5 1 6 , 5 1 9 , 5 ( 3 , 0 ) 2 7 , 5 ( 1 , 0 ) 2 4 , 5 ( 2 , 0 ) 1 3 , 0 ( 2 , 0 ) 2 8 , 5 1 7 , 0 1 9 , 5 8 , 0 1 1 , 0 ( 4 . 0 ) 1 3 , 0 1 0 , 0 9 . 0 3 , 0 4 . 0 3 , 0 3 8 2 , 0 ( 1 6 , 0 ) 12,5 1 1 , 0 1 1 , 0 1 1 , 5 1 9 , 5 4 5 , 0 3 8 , 0 1 0 , 5 1 6 , 5 2 2 , 5 2 8 , 5 2 6 , 5 1 5 , 0 2 8 , 5 1 7 , 0 1 9 , 5 8 , 0 1 5 , 0 1 3 , 0 1 0 , 0 9 . 0 3 . 0 4 . 0 3 , 0 3 9 8 , 0 9 2 , 0 ( 8 , 0 ) 1 0 0 , 0 8 6 , 0 ( 8 . 0 ) 9 4 , 0 8 2 , 0 ( 8 , 0 ) 9 0 , 0 1 3 0 , 0 ( 1 4 , 0 ) 1 4 4 , 0 1 6 8 , 0 ( 1 8 , 0 ) 1 8 6 , 0 2 3 6 , 0 ( 1 2 , 0 ) 2 4 8 , 0 2 1 4 , 5 ( 6 4 , 0 ) 2 7 8 , 5 1 3 4 , 5 ( 1 2 , 0 ) 1 4 6 , 5 1 3 6 , 5 ( 2 6 , 5 ) 1 6 3 , 0 2 1 2 , 0 ( 7 5 , 5 ) 2 8 7 , 5 2 2 1 , 5 ( 2 4 , 0 ) 2 4 5 , 5 2 2 5 , 0 ( 6 3 , 0 ) 2 8 8 , 0 2 4 1 , 0 ( 3 0 , 0 ) 2 7 1 , 0 2 1 2 , 0 ( 3 8 , 0 ) 2 5 0 , 0 1 4 6 , 0 ( 2 0 , 0 ) 1 6 6 , 0 1 7 2 , 0 ( 1 8 , 0 ) 1 9 0 , 0 9 2 , 0 ( 1 6 , 0 ) 1 0 8 , 0 1 5 2 . 0 ( 7 0 , 0 ) 2 2 2 , 0 1 3 6 . 0 ( 2 6 . 0 ) 1 6 2 , 0 9 4 . 0 ( 1 6 , 0 ) 1 1 0 , 0 7 4 . 0 ( 1 0 . 0 ) 8 4 . 0 5 6 . 0 5 6 . 0 4 4 . 0 (4,01 4 8 . 0 2 4 , 0 2 4 , 0 3 3 8 1 , 0 ( 5 8 1 , 0 ) 3 9 6 2 , 0 f c o n s i d e r a z i o n i s u i m e t o d i di h o l m e s e di j e f f r e y s 123 degli incroci, sempre nell'intervallo considerato (fig. 3), consente di ottenere par t„ i l valore di circa 3,45 x io9 anni. p e r questi i n c r o c i si h a in effetti un comportamento analogo a quello indicato preced e n t e m e n t e : p r e c i s a m e n t e su 132 incroci teorici se n e osservano in detto intervallo solo c i r c a il 4 5 % di cui presso a poco il 9 % corrispondenti ad incroci anomali. spiegazione identica a quella già data in precedenza si ha nei riguardi dell'assenza dei prevedibili i n c r o c i normali. l a diversità nei valori ottenuti seguendo uno o l'altro dei due metodi viene però giustificata in base alla constatata irregolarità n e l l e distribuzioni e p a r t i c o l a r m e n t e in base al f a t t o che la gran parte degli inosservati i n c r o c i n o r m a l i , a m m o n t a n t i all'incirca al 4 4 % nell'esame delle 11 galene ed al 5 0 % nell'esame invece delle 6 galene, si verifica, senza presentare alcun particolare addensamento, per valori di t i n f e r i o r i ad 1,5 x 10" anni, finanche negativi, in modo da determ i n a r e nel valore medio ( c b e dovrebbe presso a poco coincidere col valore ottenuto m e d i a n t e il metodo dei valori discreti) un valore più basso di quello cui corrisponde il valore del massimo di frequenza. in effetti, come si osserva specie dal grafico della figura 1, m e n t r e a 5,5 x 10" il valore della frequenza degl'incroci è già presso a poco nullo, ad 1,5 x 10" invece si h a già un n u m e r o d'incroci relativamente rilevante, c h e solo gradualmente si riduce col diminuire di t. s e dal punto di vista m a t e m a t i c o la diversità dei valori risulta del lutto giustificata: la dissimmetria e la irregolarità n e l l ' a n d a m e n t o delle frequenze, n o n c h é la presenza di a l m e n o due massimi di frequenza esigono invece una interpretazione fisica. si osserva innanzitutto c h e l ' e n u n c i a t o metodo di holmes si poggia sull'assunzione c h e nella disintegrazione relativa alle tre f a m i g l i e radioattive, alla riduzione di ogni singolo atomo del capostipite corrisponde l ' a u m e n t o di un atomo del relativo isotopo di piombo. una tale condizione potrebb e essere r i t e n u t a sicuramente soddisfatta solo n e l caso in cui tra i m e m b r i di transizione delle varie famiglie non vi fossero elementi volatili. n e l l ' a l t r o caso risulta ovvia la possibilità di fuga o di apporto di atomi del m e m b r o volatile con l a conseguente presenza di un n u m e r o di atomi radiogenici di p i o m b o diverso da quello che si s a r e b b e avuto in base all'ammissione teorica. questa considerazione e precisamente l'ammissione, ancora giustificata, di una prevalenza di fughe con l a conseguenza ovvia di una riduzione n e l n u m e r o presente di atomi di p i o m b o radiogenici e quindi a n c h e nel valore di t, dà c o n t e m p o r a n e a m e n t e la spiegazione delle osservate anomalie. oltre al constatato andamento delle frequenze può ritenersi una prova di u n a c . i m b ò . f . s . gaeta effettiva alterazione n e l l e ottenute abbondanze, l'entità del medio scostamento rispetto a l l ' a n d a m e n t o m e d i o dei v a l o r i : abbondanza-età, v a r i a b i l e a seconda dell'isotopo di p i o m b o considerato. a n c h e soffermandosi ad u n ' i n d a g i n e qualitativa, può dirsi c h e esso risulta relativamente grande p e r i l pb2oo, esiguo p e r i l pb20i, i n t e r m e d i o per i l pb2os5 in accordo con i valori del periodo d e l l ' e l e m e n t o volatile da cui essi p o t r e b b e r o avere origine e cioè del rn, dell'are, del tn, ai quali corrispondono per il periodo, r i s p e t t i v a m e n t e i v a l o r i : 3b, 8 2 3 ; 54s, 5 ; 3% 92. l o stesso j e f f r e y s h a i n f a t t i constatato p e r ciascun gruppo di galene coetanee c h e , m e n t r e per il pb^oe,; e per i l pb2os si h a n n o variazioni nei valori delle a b b o n d a n z e relative in senso concorde e pressoché l i n e a r e ; p e r i l pb207 si h a invece u n ' i n a t t e s a esigua variazione. anzi j e f f r e y s c h e h a creduto di s t a b i l i r e un m e t o d o p e r la r i c e r c a dell'età della crosta, basato sulla deduzione di detti medi scostamenti, in q u a n t o r i t e n u t i derivanti da dispersioni n e l l e abbondanze i n i z i a l i dei capostipiti delle f a m i g l i e radioattive n e l l e differenti parti della crosta primitiva, h a dovuto p r o p o r r e di l i m i t a r e l'esame solamente p e r i l pfoooc e per i l pb2og. l e precedenti considerazioni, con l ' a t t r i b u i r e l e n o t a t e deviazioni in tutto od a n c h e in p a r t e ad altra origine, l a s c e r e b b e r o però escludere la possibilità di applicazione del metodo di jeffreys. i n quanto al metodo di holmes può concludersi invece sulla necessità di u l t e r i o r i analisi isotopiche, spinte a n c h e a galene più antiche, p r i m a di poter avere in base ad esso un a c c e t t a b i l e valore per l'età della crosta. i n effetti p o t r e b b e r o presentarsi due c a s i : o c h e n e l l ' e v e n t u a l e b i l a n c i o di perdite e di acquisti vi sia stato un p e r f e t t o compenso ed in t a l caso il metodo, pur con q u a l c h e lieve ritocco, può essere applicato nel m o d o anzidetto con prevedibile accordo tra i valori o t t e n u t i ; oppure, c o m e è presumibile, c h e vi sia stata u n a prevalenza di fuga sugli apporti (e t e o r i c a m e n t e è ammissibile a n c h e i l viceversa) ed in quest'altro caso, prima dell'applicazione, è necessario apportare ai dati originali opportune correzioni, di cui si vede l a possibilità di deduzione attraverso indagini c r i t i c h e degli osservati c o m p o r t a m e n t i . c o n s i d e r a z i o n i s u i m e t o d i di h o l m e s e di j e f f r e y s 125 riassunto in base alle analisi isotopiche di 13 galene viene dedotta l'età della crosta terrestre, seguendo i metodi dei valori discreti nonché delle intersezioni. il disaccordo dei risultati, nonché la pluralità dei valori ottenuti col secondo metodo, sono attribuiti a diverse cause tra le quali l'eventualità di fughe o di apporti di elementi volatili, alteranti pertanto le condizioni basilari del problema. le effettuate considerazioni lascerebbero da un canto escludere la possibilità di applicazioni del metodo di jeffreys e d'altro canto dedurre per l'uso del metodo di holmes vintroduzione di particolari correzioni ai dati originali o tindispensabilità di un numero di analisi isotopiche notevolmente maggiori rispetto all' attuale. summary on the basis of isotopie analysis of 13 samples of lead ores, the age of the earth's crust has been determined jising the method of discrete• values as ivell as the intersections. the disagreement in the results, as ivell as the diversity of values obtained with the second method, is attributed to various causes among which are the possibilities of escape or adsorbtion of volatile elements, thus altering the basic conditions of the problem. these considerations ivould exclude on one hand the possibility of applying the method of jeffreys, and on the other hand, in order to apply the method of holmes, ivould require the introduction of particular corrections to the originai data, or a number of isotopie analyses notably greater than that actually obtained. layout 6 chemical and isotopic composition of fumarolic gases at iwate volcano, japan, during and after seismic activity in 1998: implications for the modification of ascending volcanic gases takeshi ohba1, kenji nogami2, jun-ichi hirabayashi2, takeshi sawa2, kohei kazahaya3, noritoshi morikawa3, michiko ohwada3 1 tokai university, school of science, department of chemistry, kanagawa, japan 2 volcanic fluid research center, tokyo institute of technology, gunma, japan 3 geological survey of japan, national institute of advanced industrial science and technology, tsukuba, japan annals of geophysics, 54, 2, 2011; doi: 10.4401/ag-5182 abstract in 1998, there were many volcanic earthquakes recorded at iwate volcano, japan. although an eruption was anticipated, it never occurred. fumarolic gases were sampled at the volcano on six occasions during 1998 and 1999, and were analyzed for their chemical compositions and isotope ratios. the fumarolic gases were again sampled in 2004 and 2006, after the period of seismic activity. the hcl concentrations and isotope ratios of the h2o in the gas samples collected from an active geothermal area at the volcano were high in 1998 and 1999, but decreased significantly after 2004, irrespective of the relatively stable concentrations of co2 and sulfurbearing gases. a notable feature of the fumarolic gases is the high isotope ratio of h2o, similar to that of pure magmatic gas, which is typical of andesitic volcanoes, although the temperature at the outlet was less than 146°c in the present case. these features can be explained if the enthalpy of the magmatic gas was equivalent to that of water vapor at 252°c to 370°c under a pressure of 1 bar. the correlations observed among co2/h2o and hcl/h2o, and the δ 18o values of the fumarolic gases suggest partial condensation of h2o vapor during the ascent of the volcanic gas to the surface. the estimated co2/h2o molar ratio of the magmatic gas was 0.008, which is less than the 0.03 reported previously for magmatic gas sampled during an effusive eruption at unzen volcano, japan. these data suggest that the magma at iwate volcano is depleted in volatiles, because co2 is preferentially degassed from the silicate melt relative to h2o. such depletion in co2 might explain the failed eruption at iwate volcano in 1998. 1. introduction fumarolic gases generally include volatiles that are emitted from degassing magma; consequently, the chemical and isotopic compositions of such gases can provide information on the degassing magma. however, the chemical and isotopic compositions of the gases can be modified during the ascent to the surface, due to, for example, contamination by ground water, condensation of water vapor, and removal of acidic components, such as hcl. in 1998, many volcanic earthquakes occurred at iwate volcano, japan. however, although an eruption was anticipated, it never occurred. during and after this period of volcanic seismicity, we sampled fumarolic gases at the volcano and measured their chemical and isotopic compositions. in this study, we investigate the processes that acted to modify the chemical compositions and isotope ratios of these fumarolic gases, and we estimate the co2/h2o ratios of the magmatic gases. the modification process is quantitatively modeled assuming simple physicochemical processes, which successfully reproduce the observed variations in the chemical and isotopic compositions of these fumarolic gases. 1.1. iwate volcano iwate volcano is an active volcano in northern honshu, japan (figure 1). the volcano has two topographic peaks and it is divided into its western and eastern parts, which are centered around onawashiro crater (figure 1, o) and yakushidake crater (figure 1, y), respectively. itoh and doi [2005] investigated the development of iwate volcano. the eastern part is younger than the western part, and its surface layer was deposited over the past 7,000 years. an eruption occurred at the outer rim of the yakushidake crater in 1732, which produced lava flow. the eastern part contains limited geothermal activity. a weak and diffusive steam discharge occurred at point h in figure 1, without any apparent sulfurbearing gases. the western part is relatively old, with the most recent lava from the onawashiro crater estimated to be older than 30 ka [itoh and doi 2005]. the western part is characterized by intensive geothermal activity that is article history received december 9, 2010; accepted may 16, 2011. subject classification: gases, magmas, volcano monitoring, hydrothermal systems. 187 concentrated in four areas (figure 1, j, k, u and a). at site j, in particular, there was a strong smell of sulfur-bearing gas. as the most recent volcanic activity, a steam driven explosive eruption was observed at site j in 1919. about 300,000 people live in morioka city, which is located 20 km southeast of iwate volcano, and a north–south express highway is located 9 km east of iwate volcano. during an eruption at yakushidake crater in 1686, houses near iwate volcano were affected by ash deposits and a lahar. any future eruption at iwate volcano poses serious risk to nearby residents and infrastructure. iwate volcano is monitored continuously by the japan meteorological agency, in terms of its seismicity and crustal deformation (tiltmeter and gps), but its fumarolic gases are not monitored. in terms of the geochemistry of the fumarolic gases at iwate volcano, only noguchi et al. [1961] have reported on the chemical composition of the gas and sublimate within the yakushidake crater. miura et al. [2000] reported deformation of the volcanic body at iwate volcano in 1998, which was attributed to the intrusion of magma at a depth of 3 km beneath the western part. from may to september 1998, many volcanic earthquakes were recorded beneath the region covered by points o, j, k and u in figure 1 [miura et al. 2000]. the earthquake hypocenters were concentrated in a zone that trended e–w. tanaka et al. [2002] reported a region of anomalously high seismic velocity beneath the onawashiro crater (figure 1), which was attributed to a body of solidified magma. a similar model was proposed by aizawa et al. [2009] based on a magnetotelluric survey. 2. sampling and analysis of gas and water most of the fumarolic gases at sites h, j, k, u and a (figure 1) were sampled following the method proposed by ozawa [1968], using a glass syringe in which 20 ml of 5 m koh solution was filled, to absorb acidic gases. this method results in only a small amount of air contamination in the syringe. the koh solution obtained was subjected to wet chemical analysis, according to ozawa [1968]. the gas phase collected in the syringe, which included the «residual» composition of the gas (e.g., n2, o2, h2, ch4, ar, he and ne), was analyzed using two gas chromatographs (gcs) with ar and o2 as the carrier gases, through which the components other than ar and o2 were analyzed. the gc data were then combined to yield the complete composition of the residual gas. fumarolic gases at site j were sampled in 2004 and 2006 for he isotope analysis, using an evacuated glass bottle [giggenbach and goguel 1989] that included 20 ml 5 m koh, to prevent air contamination. at site j, hot spring water was also sampled in 1998 and 1999. two fumaroles at site j were selected for repeated sampling. one series of samples (table 1, 188 ohba et al. figure 1. topographic map of iwate volcano (contour interval, 100 m; altitude, meters above sea level). inset, location of the volcano. dashed curves o and y, outer rims of the onawashiro and yakushidake craters, respectively. shaded areas lo and ly, distribution of the lava flows from the onawashiro and yakushidake craters, respectively. letters h (higashi iwate), j (ojigokudani), k (kurokura), u (ubakura), and a (amihari), locations of geothermal areas from which the fumarolic gases were collected. at site j (39°51¢05ʺn, 140°58¢21ʺe), hot spring water was also collected. the area with altered soil at site j measured about 100 m (north–south) by 75 m (east–west). intense fumarolic activity was noted at j, including a strong smell of sulfur-bearing gases (also detected at site a). at sites h, k and u, in contrast, sulfur-bearing gases were not smelt when sampling the fumarolic gases. 189 fumarolic gases at iwate volcano, japan l o ca ti o n c o d e d at e t em p °c h 2o c o 2 h 2s so 2 h c l h e h 2 o 2 n 2 c h 4 a r δ d sm o w ‰ δ 18 o sm o w ‰ δ 13 c p d b ‰ 3 h e/ 4 h e r / r a 4 h e/ 20 n e j ja 1 19 98 /6 /2 13 5 98 02 00 13 60 0 52 40 26 8 95 8 0. 03 2 94 ** 10 7 0. 32 0. 29 -3 1 3. 2 -3 .2 j jb 1 19 98 /6 /2 11 0 98 41 00 11 90 0 32 10 16 6 61 2 0. 02 8 1. 2 ** 82 .7 0. 08 1 0. 25 4 -2 7 3. 1 -3 .5 j ja 2 19 98 /7 /2 4 13 4 98 65 00 92 80 34 10 35 7 11 70 0. 01 5 2. 3 ** 45 .8 0. 29 0. 17 -2 6 4. 4 -3 .3 j jb 2 19 98 /7 /2 4 10 6 98 69 00 95 40 23 50 27 9 92 6 0. 01 9 0. 41 ** 52 .8 0. 05 2 0. 21 -2 5 4. 4 -3 .2 j ja 3 19 98 /9 /1 0 13 5. 6 98 49 00 96 20 38 00 54 5 22 80 0. 01 4 2. 5 ** 52 .2 0. 02 5 0. 18 -2 2 5. 4 -3 .0 j jb 3 19 98 /9 /1 0 10 9 98 62 00 10 50 0 26 10 46 5 44 7 0. 02 3 2. 4 ** 67 .8 0. 05 7 0. 26 -2 5 4. 4 -3 .0 j ja 4 19 99 /5 /1 2 14 3 98 33 00 84 20 31 50 74 3 39 90 * 34 5 ** 89 .0 0. 37 * -2 2 5. 2 -3 .5 j jb 4 19 99 /5 /1 2 10 2. 6 98 79 00 91 70 20 00 52 0 39 0 0. 01 5 0. 61 ** 51 .6 0. 05 5 0. 34 -2 5 4. 8 -3 .2 j ja 5 19 99 /8 /1 7 13 9 98 17 00 10 11 0 35 70 96 6 34 90 * 67 .2 ** 62 .3 0. 17 * -2 4 4. 8 j jb 5 19 99 /8 /1 7 98 98 85 00 88 00 19 30 40 6 29 8 0. 00 65 0. 27 ** 25 .1 0. 02 3 0. 14 -2 2 4. 8 j ja 6 19 99 /1 0/ 14 14 6 98 15 00 11 00 0 50 00 12 40 11 10 * 72 .3 ** 79 .7 0. 24 0. 43 -3 0 3. 4 j jb 6 19 99 /1 0/ 14 10 1 98 63 00 10 40 0 27 30 55 2 u d 0. 02 3 1. 03 ** 64 .9 0. 11 0. 23 -2 9 3. 5 j ja 7 20 04 /9 /2 2 11 1. 3 97 91 00 15 60 0 41 10 50 8 66 3 0. 00 59 1. 1 1. 8 35 .3 0. 25 0. 16 -3 4 0. 0 -3 .9 5. 01 46 .4 j jb 7 20 04 /9 /2 2 96 .3 96 72 00 28 90 0 37 20 91 u d 0. 00 66 0. 8 1. 4 40 .1 0. 29 0. 22 -4 6 -2 .7 -3 .3 4. 91 31 3 j ja 8 20 06 /8 /1 11 6 25 41 00 14 20 0 58 30 17 30 u d u d 4. 0 12 70 00 59 00 00 u d 68 00 -5 3 -3 .5 j jb 8 20 06 /8 /1 96 .6 98 41 00 11 20 0 44 60 10 5 u d 0. 02 1 1. 6 u d 74 .7 0. 00 01 2 1. 09 -8 1 -1 0. 3 -2 .7 4. 82 31 .6 u u 1 19 98 /9 /1 0 94 .6 87 07 00 12 93 00 -1 48 -2 6. 0 -3 .5 u u 2 20 06 /8 /2 95 .6 47 51 00 13 30 0 3. 9 10 10 00 40 60 00 u d 46 60 -1 35 -2 5. 5 -4 .9 1. 08 0. 34 k k 20 06 /8 /2 92 .4 84 51 00 44 00 u d 29 50 0 12 00 00 u d 13 90 -1 21 -2 0. 8 -4 .2 1. 12 0. 33 a a 20 05 /9 /5 12 1. 3 98 81 00 78 50 36 70 28 .0 u d * 95 21 20 6 50 .7 1. 70 -6 1 -4 .3 -4 .4 4. 32 82 .9 h h 1 20 05 /9 /1 83 .8 31 20 00 66 80 3. 8 13 70 00 53 80 00 u d 62 70 -1 12 -1 7. 1 -7 .6 1. 04 0. 34 h h 2 20 05 /9 /1 83 .6 49 13 00 13 90 0 9. 8 97 80 0 39 20 00 u d 45 90 -1 39 -2 2. 0 -5 .9 1. 11 0. 33 y y 1 19 60 /9 /2 9 14 4 98 30 00 14 60 0 10 90 81 7 37 6 y y 2 19 60 /9 /2 9 18 3 98 20 00 18 40 0 11 70 15 20 18 5 t ab le 1 .c he m ic al a nd is ot op ic c om po si ti on s of t he f u m ar ol ic g as es a t iw at e vo lc an o. c he m ic al c on ce nt ra ti on s as μ m ol /m ol . u d, b el ow d et ec ti on li m it . u nt il o ct ob er 1 99 9, t he g as es w er e sa m pl ed u si ng a g la ss s yr in ge a s th is m et ho d w as e as y to a pp ly a nd s u ffi ci en tl y ac cu ra te in t er m s of t he m aj or c he m ic al c om po ne nt s, e xc ep t h e an d a r. a ft er 2 00 4, t he g as es w er e sa m pl ed u si ng a n ev ac u at ed g la ss b ot tl e to m ea su re t he h e is ot op e ra ti o. * h e co nc en tr at io n no t m ea su re d du e to in te rf er en ce fr om a la rg e pe ak o f h 2 in th e ga s ch ro m at og ra ph . * *o 2 su bt ra ct ed fr om th e ra w d at a as su m in g th e o 2 de te ct ed w as c on ta m in at ed b y ai r tr ap pe d in th e gl as s sy ri ng e u se d fo r sa m pl in g. s u bt ra ct io n of th e at m os ph er ic co m po ne nt w as p er fo rm ed fo r n 2 an d a r, b as ed o n th e co nc en tr at io n of o 2. e m pt y ce lls , n o an al ys is . r , 3 h e/ 4 h e m ol ar r at io . r a, r v al u e of n or m al a ir a s 1. 4 × 10 –6 . t he c he m ic al c om po si ti on s of t he g as es w it hi n th e ya ku sh id ak e cr at er ( y 1, y 2) a re fr om n og u ch i e t al . [ 19 61 ]. ja1–ja8) was collected at the fumarole with the highest outlet temperature. a second series (table 1, jb1–jb8) was sampled at a fumarole located 32 m from the first. in addition, hot spring waters were sampled at site j for analysis of their anionic compositions by ion chromatography. to measure the δd values of h2o in the fumarolic gases, we sampled the gas condensates. part of the condensates was converted to h2 gas with metallic zn [coleman et al. 1982] for analysis using mass spectrometry (finnigan mat 252). the same procedure was used for measuring the δd value of h2o in hot spring water. the δ18o values of h2o in the condensate and water sample were measured using the conventional co2-h2o equilibration method [epstein and mayeda 1953], using mass spectrometry. to measure the δ13c value of co2 in the fumarolic gases, the co2 in the gases was fixed as a baco3 precipitate at the sampling site [kusakabe 2001]. the precipitate was converted to co2 in the laboratory for analysis by mass spectrometry. the analytical errors were 1.0‰, 0.1‰, and 0.1‰ for δd, δ18o and δ13c, respectively. the gas collected in the headspace of the evacuated bottle was analyzed noble mass spectrometer (mm5400), to obtain the rare gas composition and 3he/4he ratio. the analytical error of the 3he/4he measurements was less than ±0.1 ra. analyses of chemical and stable isotopes were performed at the kusatsu–shirane volcano observatory at gunma prefecture, japan, and those of he isotopes were performed at the geological survey of japan at tsukuba city, japan. 3. results 3.1. major components of fumarolic gases tables 1 and 2 list the chemical and isotopic data for the fumarolic gases and hot spring waters, respectively. the major components of the fumarolic gases are h2o, co2, h2s, so2 and hcl. figure 2 shows a ternary diagram of the h2o, co2 and s concentrations in the fumarolic gases (where s = h2s + + so2). except for the samples jb7 and ja8, the gases at site j are clustered in the center of the diagram, along with the sole sample from site a. the gases at sites u, k and h are depleted in s; as such, they lie along the right-hand edge of the ternary diagram in figure 2. the co2/h2o ratios of samples from sites h and u are distinctly (although only slightly) higher than those of the cluster (except for sample u1). sample u1 is strongly depleted in h2o, whereas the sample from site h is enriched in h2o. gases at yakushidake crater, as sampled in 1960, were enriched in s, similar to the composition of gases at site j. 3.2. temporal variations in the compositions of the fumarolic gases at site j figure 3 shows the temporal variations in the chemical compositions and isotope ratios of the gases from site j. the co2 and h2s concentrations were relatively low in 1999, and they showed slight increases in 2004 and 2006. the so2 concentrations in the ja samples increased from june 1988 to october 1999, followed by a decrease in 2004 and an increase in 2006. except for 2006, the temporal variations observed for the jb samples are similar to those for the ja samples. the hcl concentrations in the ja samples show an increase from june 1998 to may 1999, followed by a decrease. the hcl concentrations in the jb samples were relatively high in july 1998, but decreased gradually thereafter. the h2 concentrations in the ja samples were high in may 1999, moderate in late 1999, and low in 2004 and 2006. the ch4 concentrations in the ja and jb samples show no clear temporal trends. there is no correlation between the values of the ja and jb series. the δd and δ18o values were high until august 1999, but decreased thereafter. ohba et al. 190 code date temp °c ph δdsmow ‰ δ18osmow ‰ cl– mg/liter so2–4 mg/liter w1 1998/7/24 75.7 2.4 -69 -10.4 290 825 w2 1998/7/24 30.6 2.5 -73 -11.5 26 1820 w3 1998/7/24 70.1 1.0 -21 4.9 9300 4050 w4 1998/7/24 95.3 2.1 -64 -8.9 711 2520 w5 1998/9/10 94.8 1.2 -31 0.2 4309 2230 w6 1998/9/10 78.2 2.3 -68 -9.8 472 800 w7 1998/9/10 96.7 2.0 -53 -6.2 489 2211 w8 1998/9/10 90.9 2.1 -64 -9.3 442 730 w9 1999/5/12 74.2 1.9 -73 -10.9 14 1700 w10 1999/5/12 94.4 2.0 -60 -7.5 160 997 w11 1999/5/12 95.1 0.9 -18 4.8 8330 4790 table 2. chemical and isotopic compositions of the hot spring waters sampled at site j. figure 2. h2o-co2-s ternary diagram (s = h2s + so2) showing the compositions of the fumarolic gases (molar basis). 191 3.3. isotope ratios of h2o in the gases and water in 1999, the δd and δ18o values of h2o in the gases at site j were high, similar to those of magmatic water (typical of andesitic volcanoes) [taran et al. 1989], as shown in figure 4. the gases with high isotope ratios contained high concentrations of hcl (e.g., ja4). in figure 4, the gases from site j are distributed along the line connecting the general magmatic water (figure 4, square at top right) and local meteoric water, as reported by kazahaya et al. [2000], who analyzed seven water samples from altitudes of 1,300 m to 1,800 m on the iwate volcano. the average δd and δ18o values of the seven samples were -73‰ and -11.5‰, respectively. some of the hot spring waters (table 2, w3 and w11) sampled at site j yielded high isotope ratios, close to those of magmatic water. the concentration of cl– in samples w3 and w11 was 9,300 mg/l and 8,330 mg/l, respectively, which is equivalent to 4,720 mmol/mol and 4,230 mmol/mol as the cl/h2o molar ratio (similar to the hcl/h2o molar ratio for sample ja4: 4,060 mmol/mol). in figure 4, the isotope ratios of the hot spring waters are also distributed along the line connecting the magmatic vapor (mv) and the local meteoric water (lmw). the δd and δ18o values of h2o in the gases from sites u, k and h are much lower than those of the lmw, and the isotope ratios of the gases are much lower than those of the vapors equilibrated with the lmw at 100 °c. 3.4. residual gas composition figure 5 shows the compositional data for the fumarolic gases at site j plotted on a n2-he-ar ternary diagram. the data fumarolic gases at iwate volcano, japan figure 3. temporal variations in the compositions (μmol/mol) and isotope ratios (‰ relative to smow) of the water in the fumarolic gases from site j (as indicated). solid and dotted lines, ja and jb series, respectively. figure 4. the δd and δ18o values of water in the fumarolic gases and hot spring waters. square, upper right, general range of the values for the typical mv of an andesitic volcano above a subduction zone. elliptical region, local meteoric water (lmw) at the summit area of iwate volcano [kazahaya et al. 2000]. small filled dots, isotope ratios of the hot spring water. dashed horizontal and vertical lines, bottom left, δd and δ18o, respectively, of the water vapor equilibrated with the lmw at 100 °c. points are distributed in the region of mixing between the end member with a high n2/ar ratio and the atmospheric end members. the end member with a high n2/ar ratio can be represented by the gas of the circum-pacific volcanoes (figure 5, cpvs) [giggenbach 1997]. the 3he/4he ratios in the fumarolic gases are plotted in figure 6 ( ja7, jb7, jb8 and a), where the 3he/4he ratio is normalized by the ratio of air (= r/ra). the r/ra and 4he/20ne ratios of u, k and h are almost identical to the atmospheric values. the data points for ja7, jb7 and jb8 plot along the mixing line, the end members of which are air and a component with a high 4he/20ne ratio. the r/ra ratio of the end member is estimated to have a value of 5, which is slightly lower than the value of 6 reported for he in magmatic gas from the japanese islands [poreda and craig 1989]. fumarolic gas from site a lies beneath the mixing line in figure 6, suggesting an additional contribution of a 4he-enriched component that probably originated in the crustal rock. figure 7 shows the relationships between the δ13c value of co2 and the co2/ 3he molar ratios, indicating that the origin of the co2 is the fumarolic gases [e.g., marty et al. 1989, sano and marty 1995]. co2 in volcanic gases can originate from the upper mantle (e.g., volatiles in the midoceanic ridge basalt; figure 7, morb), the sea-floor sediment (figure 7, s), or limestone (figure 7, l). the co2 in fumarolic gases at site j originated mainly from limestone, with a minor contribution of morb-type co2. 4. discussion 4.1. apparent equilibrium temperature the apparent equilibrium temperature (aet) is an index of volcanic activity [giggenbach 1987], and it can be regarded as the quenched temperature of volcanic gas. as h2 gas is the most reactive component in fumarolic gases, and because h2 included in the following reaction has a stoichiometric coefficient of 3, the aet for the reaction: (1) is expected to be a proxy of gas temperature in the conduit connecting the gas source and the outlet of the fumarole. the temperature of the gas conduit is initially high, as it is similar to the temperature of the magma at depth, but it decreases with the ascent of the gas. at high temperatures, the equilibrium of equation (1) shifts to the left side of the reaction. during the ascent of the gas within the conduit, the temperature decreases, resulting in a shift in the 3 2so h h s h o2 3 2 2+ = + ohba et al. 192 figure 5. n2-he-ar ternary diagram showing the compositions of the fumarolic gases at site j (molar basis). circum-pacific volcanoes (cpv) and m, end members typical of the cpv gases and gases originating in the mantle, respectively. asw , composition of the water saturated with air. the data are plotted from table 1. figure 6. 3he/4he versus 4he/20ne ratios of the fumarolic gases. r and ra, 3he/4he ratios of fumarolic gas and air (1.4 × 10–6), respectively. dashed curve, mixing line between air and an end member with an r/ra of 5 and a 4he/20ne of 1000. figure 7. co2/ 3he ratios and δ13c values of co2 within the fumarolic gases from site j. l, s, end members typical of oceanic sediment (s) and oceanic limestone (l), respectively. dashed curves, mixing between s and morb and between l and morb. 193 equilibrium of equation (1) to the right side. however, the reaction takes some time to reach completion, and if the gas ascends rapidly, the shift in the reaction is incomplete, resulting in a high aet. in this case, the increase in the aet is interpreted as reflecting a rapid ascent of the gas. aet was calculated for the composition of the ja and jb series, as follows: (2) where lq is the logarithm of the quotient, (3) where ci is the concentration of gas species i. equation (2) was obtained from the temperature dependence of the standard equilibrium constant of formation, as compiled by giggenbach [1987]. the total pressure of gas is assumed to be the saturated pressure of water vapor at the calculated aet (see table 3). figure 8 shows the temporal variations in the aet and the outlet temperature of the gas. the aet of the ja series showed a significant increase in 1999, over 1998. the outlet temperature of ja increased slightly in 1999 compared with 1998. miura et al. [2000] reported that the seismic activity beneath onawashiro crater was high until september 1998, and decreased thereafter. the increase in aet for the ja series lags behind the period of active seismicity. the aet values for the jb series show no clear temporal trends. the short distance between fumaroles ja and jb indicates that the processes governing the temporal variations in the aet of ja was active at shallow depths. 4.2. magmatic vapor at site j the h2o isotope ratios of the ja and jb series fall along a line connecting the point for the lmw and the water vapor with a magmatic origin (figure 4, mv). the δd and δ18o values of the mv are similar to those for the ja and jb series; consequently, the line connecting the lmw and the mv passes through the cluster of data points near the mv. the δ18o value of the mv was tentatively assumed to be +7‰ relative to the smow, which is in the range of typical values for andesitic volcanoes above a subduction zone [giggenbach 1992]. the δ18o value of the mv is limited to less than +8.5‰, because the upper limit for δd of water in andesitic magma is -10‰ [giggenbach 1992]. the δ18o value of ja3 is +5.4‰, close to that of the mv. ja3 contains high concentrations of hcl. if liquid water encounters a gas similar to that of sample ja3, most of the hcl in the gas would be absorbed by the liquid phase, making the vapor phase depleted in hcl. during september 1998 at iwate volcano, there was relatively little contamination by water, and consequently there was a higher hcl content in the gas phase. after contamination, no liquid phase remains because it absorbs hcl in the gas phase. for the above case, the temperature of the mv was estimated according to the following equations: (4) ( ) 1.242 273.15 8924 aet °c lq = lq log c c c c c c1 3 h o h s h o so h o h 2 2 2 2 2 2 = a a ak k k% / f f1f mv lmwd d d= + -^ h fumarolic gases at iwate volcano, japan code temp (outlet) °c aet °c ja1 135 464 jb1 110 229 ja2 134 263 jb2 106 201 ja3 136 271 jb3 109 273 ja4 143 642 jb4 103 223 ja5 139 482 jb5 98 193 ja6 146 486 jb6 101 240 ja7 111 236 jb7 96 206 ja8 116 379 jb8 97 230 table 3. apparent equilibrium temperatures (aets) for the chemical compositions of ja and jb. figure 8. temporal variations in the apparent equilibrium temperature (aet) calculated for the chemical compositions of the ja (solid lines) and jb (dashed lines) series (upper), and for the outlet temperatures of the gases (lower). (5) where δ is the δ18o value of the gas and the water; the subscripts f, mv and lmw indicate the fumarolic gas, mv and lmw, respectively; f is the molar fraction of h2o derived from the mv; h is the enthalpy of gas and water; δmv is assumed to be +7‰ or +8.5‰; δlmw is assumed to be -11.5‰; and δf gives f according to equation (4). the f obtained is substituted into equation (5). hf was assumed to be the enthalpy of saturated h2o vapor at the temperature of the fumarolic gas. hlmw was 42 kj/kg, which is the enthalpy of liquid water at 10 °c. solving equation (5), hmv can be obtained. the temperature of the mv was estimated assuming that the mv is a super-heated water vapor under a pressure of 1 bar (the gas pressure beneath the surface is expected to exceed 1 bar). the temperature of the mv obtained is the potential temperature at the surface when the volcanic gas expands, with the enthalpy of water vapor kept constant. the temperature of the mv depends on δf and on the temperature of the fumarolic gas, although the effects of temperature are small relative to those of δf. the calculated temperature is shown in figure 9. the temperature is lowest for ja3, for which the δf value is highest. as discussed below, the ja series is affected by the partial condensation of h2o vapor, resulting in reduced δf values. the calculated temperature of the mv at ja3 is 252 °c and 370 °c for the δmv values of +7‰ and +8.5‰, respectively. these temperatures are much lower than the general temperatures of degassing magma. it is therefore necessary to invoke a cooling process during the ascent of the mv from the source to the surface. 4.3. correlation between δ18o values of h2o and co2/h2o the correlation between the chemical and isotopic compositions of fumarolic gases can be explained by a simple model [ohwada et al. 2003, ohba 2007, ohba et al. 2010] in the case of the tatun volcanoes (taiwan), the izuoshima volcano ( japan) and the kusatsu–shirane volcano ( japan), for which the fumarolic gas is the vapor phase that is generated between high-temperature magmatic gas and low-temperature groundwater after mixing. as shown in the following equations, a simple mixing between fluids is the basis of the calculations of the composition of the vapor phase, assuming the conservation of total enthalpy with the amount of substance, and the partitioning equilibrium between the vapor and liquid phases. (6) (7) (8) (9) (10) where c is the molar concentration of co2 or hcl; the subscripts ps and pw indicate the vapor phases (primary steam; ps) and the co-existing liquid phase (primary thermal water; pw), respectively, generated from the mixing of the mv and the lmw; and f and g denote the mixing fraction of the mv and the generating fraction of the ps, respectively. in general, g is not equal to f. equations (6), (7) and (8) describe the conservation of enthalpy, the isotope ratio, and the chemical composition (co2/h2o or cl/h2o ratio) for the generation of the ps and the pw, respectively. equations (9) and (10) describe the equilibrium between ps and pw in terms of stable isotopes, co2, or the distribution of hcl. α is the isotope fractionation factor of h2o [horita and wesolowski 1994], and b is the distribution coefficient of co2 [giggenbach 1980]. the partitioning of hcl between the vapor phase and the liquid phase is complicated relative to that of co2, because hcl is ionized into h+ and clin the liquid phase. the calculated composition of ps is shown in figure 10, along with data points for the fumarolic gases. the x axis in figure 10 is the scaled fraction (scf) of co2 in the co2-h2o binary system, defined by: . (11) the scf is convenient in terms of plotting a wide range of co2/h2o ratios on a single calibration axis. given the degree of freedom in f, the composition of ps is expressed as a curve. smaller values of f give lower δ18o values on the ps curve. the ps curve originates from the mixing curve c c clor co pw ps 2b = 1000 1000 ps pwa d d = + + f f g gc c c c1 1lmw pwmv ps+ = + -^ ^h h f f g g1 1lmw pwmv psd d d d+ = + -^ ^h h 1fh h h hf g g1lmw pwmv ps+ + -=^ ^h h ( ) 100 scf co c c c100 2 co h o co 2 2 2 = + f f1h h hf mv lmw= + -^ h ohba et al. 194 figure 9. estimated temperatures of the mv. filled, open circles, estimates assuming δ18o for magmatic vapor of +7‰ and +8.5‰, respectively. 195 between the mv and the lmw, and its location depends on the enthalpy of mv. in figure 10, the enthalpy of mv was assumed to be equivalent to that of vapor heated to 400 °c at 1 bar. in constructing the ps curve, the co2/h2o ratio of the mv was assumed to be 0.008, which was chosen to explain the composition of the gas at site a. the correlation between δ18o and co2/h2o in gases at site j is not compatible with the composition of ps. as shown above, the gas at site j with a high δ18o value could be a simple mixture of the mv and the lmw. the generation of a liquid phase is unlikely. because of the high hcl concentration in the gas at site j. figure 10 shows a wide range of co2/h2o ratios. gases with a high co2/h2o ratio have a lower δ 18o value, which can be explained if the gas undergoes partial condensation of the water vapor, modeled by the rayleigh process. the composition of the gas phase after the partial condensation of the water vapor is expressed as follows: (12) (13) where f is the fraction of water vapor remaining after the condensation of the water vapor, r is the co2/h2o ratio of the gas phase (molar ratio), and the subscript i represents the initial value of the gas before condensation. the gases from site j lie around the trend of condensation at 100 °c or 120 °c, except jb8 (figure 10). the gas samples from u, k and h lie far from the ps curve in figure 10. the δ18o values of the gases are much lower than that of ps. even if partial condensation is assumed for ps, the δ18o values would still be too high; however, the δ18o value can be reduced by introducing a vapor phase that originated from the lmw. if groundwater is heated in the shallow crust within a geothermal area, some of the water would be vaporized. the isotope ratio of the vapor generated (the local meteoric vapor; lmv) is shown on the y-axis in figure 10. given the freedom of the fraction of vaporized water, the δ18o value of the lmv is expressed as a line. the addition of the lmv to a gas phase (e.g., the gas at a) produces a gas phase with a low co2/h2o ratio, as shown by the shaded region in figure 10. if such a gas undergoes partial condensation, the composition of the remaining gas phase would approach the composition of the gas from sites u, k and h. 4.4. correlation between δ18o values of h2o and hcl/h2o the hcl concentrations in the vapor phase (after partial condensation) can also be calculated under the assumption of a rayleigh process. two different cases are assumed in terms of condensation. in the first case, the condensed liquid phase is a pure hcl solution. the concentration of h+ in the condensed liquid is equal to that of the clions. in this case, the hcl concentration in the vapor phase is given by: (14) where m is the molarity of hcl in the vapor phase, and k is the partitioning factor of hcl between the vapor and liquid phases, as defined by: r rfi 1 1 = b a k f1000 1000i 1d d= + -a-^ ^h h km f f m k 1 1 1 i 2 = +c m fumarolic gases at iwate volcano, japan figure 10. correlation between the δ18o values of h2o and the co2/h2o ratios in the fumarolic gases. bold dashed curve, mixing line between the mv and the lmw. the lmv represents the vapor generated from the lmw at 100 °c. shaded region, mixture between the lmv and a gas similar to that from site a. solid curve (labeled ps), composition of primary steam. thin dashed curves and solid curves, trends of the partial condensation of the water vapor phase, for which f is the fraction of water vapor remaining after partial condensation. figure 11. correlation between the δ18o values of h2o and the hcl/h2o ratios in the fumarolic gases from site j. bold horizontal line, trend during the partial condensation of the vapor, where the condensed liquid contains h+ and cl– ions with a common molarity. thin dashed curves, trends of partial condensation, for which the ph of the condensed liquid phase is fixed at –0.3 or –0.9. bold dashed curve, mixing line between the mv and the lmw. (15) where mh+ and mcl– are the molarities of the h + and clions in the condensed liquid phase, respectively. the temperature dependence of k was given by simonson and palmer (1993). the temperature during condensation was assumed to be 120 °c. the changes in the δ18o value of h2o and in the hcl/h2o ratio of the vapor phase are shown as a subhorizontal line in figure 11, in which the x-axis is the scf of hcl in the hcl-h2o binary system, which is defined by: (16) hcl in the vapor phase is effectively removed by the condensed liquid phase. most of the hcl was lost by the time that 20% of the water vapor had condensed. the above compositional trend does not always explain the distribution of data points for the gases from site j. we assume the following condition for the second case of condensation. within a conduit filled with volcanic gas, a condensed liquid phase is likely to be dominant. the liquid phase is likely to be strongly acidic due to the absorption of hcl from the vapor phase; consequently, the removal of hcl from the vapor phase is suppressed because k is the equilibrium constant of the following reaction: . (17) two cases of condensation were assumed, where the ph of the coexisting liquid phase is -0.3 or -0.9. when the ph of the condensed liquid phase is fixed, the change in the hcl/h2o ratio of the vapor phase is given by: (18) where, . (19) the trend in the composition with progressive condensation is given in figure 11. most of the data points for the gases from site j are distributed within the two curves. the composition of the hot spring waters at site j is also shown. most of the data points for the hot spring water are distributed along the direct mixing curve between the mv and the lmw. 5. conclusions based on the composition of the gases from site j, the aet reached a peak in may 1999. this period with high aet followed the period of intense earthquakes, which suggests that magma movement was followed by magma degassing, which may have increased the flux and speed of the gas ascent through the conduit to the surface. the temperature of magmatic vapor at site j was estimated at 252 °c to 370 °c, based on the enthalpy and isotope ratio of the gas. during the ascent, contamination by groundwater was limited, which suggests the existence of a welldeveloped inner wall of the gas conduit, which prevents the intrusion of groundwater. during the ascent of the magmatic gas to the surface, the enthalpy of the gas would have been lost by conduction through the wall of the conduit. the magmatic gas would have ascended a large distance from the deep source to the surface, which is consistent with the findings of a magnetotelluric survey [aizawa et al. 2009] that revealed an electrically resistive body beneath the geothermal area at site j, at depths of 0.5 km to 3.0 km. the resistive body was interpreted to be a solidified intrusive magma. the gas was probably transported through the solidified magma. the correlations among the δ18o values of h2o, co2/h2o and hcl/h2o suggest that the gases at site j experienced partial condensation of the water vapor. during condensation, a strongly acidic liquid phase would have coexisted with the vapor phase, preventing the absorption of hcl in the vapor phase by the liquid phase. the strongly acidic liquid phase might have originated from the condensed liquid phase. the co2/h2o ratio of the magmatic gas was estimated as 0.008, which is much lower than the 0.028 observed for magmatic gas sampled from the erupting unzen volcano in 1992 [ohba et al. 2008]; this suggests that the degassing magma beneath onawashiro crater is depleted in co2 relative to the magma of unzen volcano. the fumarolic gas sampled at site a might be a steam phase generated by the mixing of magmatic gas and groundwater. the fumarolic gases at sites u, k and h showed significant condensation of water vapor. the addition of water vapor derived from groundwater would have occurred prior to the condensation. the n2-he-ar compositions of the gases from site j suggests that the magmatic component is identical to that of andesitic volcanoes above a subduction zone. the correlation between the 3he/4he and 4he/20ne ratios in the fumarolic gases suggests that the gases at site j contain an end member for which the 3he/4he ratio is as high as 5 (r/ra). the correlation between the δ13c value of co2 and the co2/ 3he ratio suggests that the origin of the co2 in the gases from site j was mainly limestone, with a minor component of morbtype co2. the hot spring waters discharged at site j comprise a direct mixture of mv and lmw. acknowledgements. we sincerely thank dr. mauro martelli for his constructive review of the manuscript, and dr. salvo inguaggiato for the editorial handling of the manuscript. k 10 1 ph c = fm mi= c ( )h cl hcl gas+ = + ( ) 1000 scf hcl c c c1000 hcl h o hcl 2 = + k m m m h cl = + ohba et al. 196 197 references aizawa, k., y. ogawa, m. mishina, k. takahashi, s. nagaoka, n. takagi, s. sakanaka and t. miura (2009). structural controls on the 1998 volcanic unrest at iwate volcano: relationship between a shallow, electrically resistive body and the possible ascent route of magmatic fluid, j. volcanol. geotherm. res., 187, 131-139. coleman, m.l., t.j. shepherd, j.j. durham, j.e. rouse and g.r. moore (1982). reduction of water with zinc for hydrogen isotope analysis, anal. chem., 54, 993-995. epstein, s. and t.k. mayeda (1953). variation of 18o content of waters from natural sources, geochim. cosmochim. acta, 4, 213-224. giggenbach, w.f. 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(in russian). *corresponding author: takeshi ohba, tokai university, school of science, department of chemistry, kanagawa, japan; e-mail: takeshi_ohba@tokai-u.jp. © 2011 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. fumarolic gases at iwate volcano, japan articolo wasburn.pdf key words paleoseismology – altyn tagh fault – strike-slip faults – india-eurasia collision 1. introduction the sinistral altyn tagh fault (atf) traverses the northern boundary of tibet for > 1500 km and is a major structure in the india-eurasia conpaleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china zack washburn (1), j ramón arrowsmith (2), guillaume dupont-nivet (3), wang xiao feng (4), zhang yu qiao (4) and chen zhengle (4) (1) geomatrix consultants inc., costa mesa, ca, u.s.a. (2) department of geological sciences, arizona state university, tempe, az, u.s.a. (3) paleomagnetic laboratory «fort hoofdijk», faculty of earth sciences, utrecht university, netherlands (4) institute of geomechanics, chinese academy of geological sciences, beijing, china abstract although the altyn tagh fault (atf) is thought to play a key role in accommodating india-eurasian convergence, little is known about its earthquake history. studies of this strike-slip fault are important for interpretation of the role of faulting versus distributed deformation in the accommodation of the indiaeurasia collision. in addition, the > 1200 km long fault represents one of the most important and exemplary intracontinental strike-slip faults in the world. we mapped fault trace geometry and interpreted paleoseismic trench exposures to characterize the seismogenic behavior of the atf. we identified 2 geometric segment boundaries in a 270 km long reach of the central atf. these boundaries define the westernmost wuzhunxiao, the central pingding, and the easternmost xorxol (also written as suekuli or suo’erkuli) segments. in this paper, we present the results from the camel paleoseismic site along the xorxol segment at 91.759°e, 38.919°n. there evidence for the last two earthquakes is clear and 14c dates from layers exposed in the excavation bracket their ages. the most recent earthquake occurred between 1456 and 1775 cal a.d. and the penultimate event was between 60 and 980 cal a.d. combining the camel interpretations with our published results for the central atf, we conclude that multiple earthquakes with shorter rupture lengths ( 50 km) rather than complete rupture of the xorxol segment better explain the paleoseismic data. we found 2-3 earthquakes in the last 2-3 kyr. when coupled with typical amounts of slip per event (5-10 m), the recurrence times are tentatively consistent with 1-2 cm/yr slip rates. this result favors models that consider the broader distribution of collisional deformation, rather than those with northward motion of india into asia absorbed along a few faults bounding rigid blocks. tinental collision (e.g., tapponnier and molnar, 1977; peltzer et al., 1989; yin and harrison, 2000; tapponnier et al., 2001; fig. 1). two competing end-member models have developed that describe how northward convergence of india is accommodated by tibet and southern asia. the first model divides this region into a few rigid blocks bounded by major faults (i.e. altyn tagh and kunlun faults, fig. 1) (e.g., avouac and tapponnier, 1993; tapponnier, et al., 2001). the second model has convergence accommodated by crustal thickening and along numerous minor and a few major faults (e.g., england and molnar, 1997). models of the first type are con1015 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. j ramón arrowsmith, department of geological sciences, arizona state university, tempe, az 85287-1404 u.s.a.; e-mail: ramon.arrowsmith@asu.edu 1016 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle china tibet altyn tagh faul t 80° 100° 120° 40° 20° kunlun fault red river fault himalayan frontal thrustsindia fig. 1. overview map showing major structures and topography in the india-eurasia continental collision zone. inset shows location of fig. 2. figure generated from gtopo30 1 km digital elevation data set using gmt (wessel and smith, 1995). sistent with higher slip rates ( 3 cm/yr for the atf) while lower slip rates are consistent with the second type because deformation is distributed instead of localized along a small number of discrete fault planes. global seismicity databases (cnss) and the chinese catalog of historic strong earthquakes (2300 b.c. to 1911 a.d.) record no major earthquakes along the atf. however, we know that large earthquakes have occurred because of moletracks seen on satellite imagery, surface breaks shown by ge et al. (1992), and limited field investigation by western scientists (molnar et al., 1987). despite the clear evidence of active faulting, slip rate determinations are discrepant. over timescales of 10 s-100 s of kyr, slip rates are inferred to be 2-3 cm/yr. peltzer et al. (1989) inferred 3 cm/yr rates in the area of 84°e-92°e based on an assumption of initial holocene age for offset landforms. recent work has focused on determining the slip rate by dating offset quaternary landforms using cosmogenic methods. meriaux et al. (1999, 2000) determined 3 cm/yr between 85°e and 89°e and 2-3 cm/yr near 90°e. merieaux et al. (2003) determined a minimum of 20 ± 3 mm/yr at 94°e. chinese earthquake geology studies (mapping, paleoseismic trenching, and 14c) inferred a minimum rate of 5 mm/yr along the entire atf (ge et al., 1992). studies of deformation in the indiaeurasian collision over geodetic timescales (5-10 years; bendick et al., 2000; shen et al., 2001; wang et al., 2001) indicate that distributed deformation in tibet is an important process because faults such as the atf have slip rates on the order of 10 mm/yr (9 ± 5 mm/yr, bendick et al., 2000; 9 ± 2 mm/yr, shen et al., 2001). holt et al. (2000) also used a rate of 10 ± 8 mm/yr along the atf in their analysis of the velocity field in asia. two major strike-slip earthquakes have recently occurred in northern tibet. the 1997 mw 7.6 manyi earthquake ruptured for a distance of about 170 km with peak left-lateral slip of about 7 m, apparently reactivating a quaternary fault (fig. 2; peltzer et al., 1999). the 2001 mw 7.8 kokoxili or central kunlun earthquake had a 300-400 km long rupture zone and sinistral slip up to 16.3 m (lin et al., 2002; klinger et al., 2003). it ruptured the eastern portion of the kunlun fault (van der woerd et al., 2002). these earthquakes draw attention to the style of earthquake rupture along the great strike-slip faults of asia and also to their response to the geodynamic setting. because the historic record in this region is only complete for about the last 100 years, paleoseismological methods are important for investigating the timing of large ruptures like these and those along neighboring faults. earthquake displacements, surface rupture lengths, and recurrence intervals along major strike-slip faults such as the atf have implications for which geodynamic models better explain continental deformation and for regional earthquake sequences in continental settings. therefore, we conducted a study of the earthquake geology of the atf to characterize the seismogenic behavior of this major strike-slip fault. in this paper, we first highlight the geopaleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china 80 85 90 95 35 40 kunlun fault tibet 6 7 8 white < 2500 years bp ssb data historic earthquake m scale 5 6 7 8 cnss data black > 2500 years bp 1997 mw 7.6 manyi earthquake tarim basin qaidambasin qimen tagh milan ruoqiang na tf huatago lapequan alty n ta gh f ault 1924 m>7 earthquakes 1951 ~m 6 earthquake altyn tagh altyn tagh fault akato tagh minfeng subei sulamu tagh tula 2001 mw 7.8 kokoxili earthquake ? fig. 2. overview map showing the central altyn tagh fault (atf), location of study area (rectangular box, fig. 3), instrumental seismicity (council of the national seismic system (cnss); http://quake.geo.berkeley.edu/cnss/catalog-search.html), historic earthquakes (chinese catalog of historic strong earthquakes 2300 b.c. to a.d., 1991), paleoearthquakes (state seismological bureau (ssb); ge et al., 1992), and major towns. the ge et al. (1992) data are plotted at the reported locations with their inferred magnitude and color-coded by age range. the events most relevant for this study are < 2500 years bp (white). the low seismic activity is unexpected because the atf is thought to accommodate a significant portion of india-asia convergence. the historical record also shows fewer earthquakes than anticipated. north altyn tagh fault (natf) mapping from cowgill et al. (2000). manyi earthquake surface rupture from peltzer et al. (1999) and kokoxili earthquake location and approximate surface rupture are from van der woerd et al. (2002) and lin et al. (2002). base figure generated from gtopo30 1 km digital elevation data set using gmt (wessel and smith, 1995). 1017 metric segmentation of the central atf by dividing it into three sections (westernmost wuzhunxiao, the central pingding, and easternmost xorxol-also written as suekuli or suo’erkuli). secondly, we present new paleoseismic results from the xorxol segment at 91.759°e, 38.919°n at the camel paleoseismic site. finally, we review results from washburn et al. (2001) and washburn (2001), to develop earthquake rupture scenarios for the central atf. the geometric boundaries and 14c dates defining age ranges for paleoearthquakes imply that one large earthquake rupturing all 3 segments (270 km) is unlikely. furthermore, multiple earthquakes with shorter rupture lengths ( 50 km) rather than complete rupture of the xorxol segment better explain the paleoseismic data. when coupled with typical amounts of slip per event (5-10 m), the recurrence times are consistent with 1-2 cm/yr slip rates. 2. geomorphic and geologic framework the study area is located in xinjiang, qinghai and gansu provinces of western china between 89.619°e, 38.302°n and 92.523°e, 39.138°n (figs. 1, 2 and 3). this is an arid region of alluvial fan and playa environments around 3000 m in elevation with neighboring glaciated peaks rising up to 5000 m. in the western part of the study area, the atf passes through a 4000 m high, axial valley south of liweiqiming shan and descends across alluvial fans and saline lakes of the wuzhunxiao area (figs. 2 and 3). from here, the atf traverses bedrock and small perched alluvial basins (< 0.5 km2) within an isolated mountain range, marked by the 4800 m pingding shan. from gobiling, the atf descends into the alluvium filled strike-slip rift of xorxol valley. this straight and distinct tectonic landscape continues east for zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle 39.00 e, 91.75 n 38.50 e, 89.75 n 38.50 e, 91.00 n 39.00 e, 92.50 n liweiqiming shan kulesayi to milan magnei lake wuzhunxiao arkateng dayang shan pingding shan huatago (huatougou) old road to milan ku shui he gobiling camel site figure 19 figure 18 bitter seato milan to lapequa n wuzhunxia o segment pingding segment xorxol seg ment xorx ol va lley figure 4 explanation strike-slip fault active normal fault qm normal fault debris flow jeep road active trace (well located, inferred) qm fault trace tectonic lineament interpreted trace river fig. 3. fault trace map of central altyn tagh fault (atf) showing geometry of most recent and late quaternary rupture traces. major roads, location of geographic landmarks, and towns are also shown for reference. this portion of the atf is predominantly left lateral and has well-developed strike-slip topography (e.g., xorxol rift valley). however, it has not produced instrumentally recorded earthquakes > m 6. the active trace traverses alluvial flats around lake wuzhunxiao and the bitter sea and bedrock in the arkateng and pingding mountain ranges. two geometric boundaries are identified: the 7 km wide arkateng restraining bend and the 4 km wide pingding releasing step. these geometric boundaries de fine the westernmost wuzhunxiao, the central pingding, and the easternmost xorxol segments. the boundaries serve as a framework for interpreting offset data and estimating rupture extent because such discontinuities commonly act as rupture barriers. 1018 over 110 km (figs. 2 and 3). proterozoic intrusive rocks and slivers of paleozoic-mesozoic granitoid, ultramafic, marine carbonate, and ophiolitic rocks comprise the mountains in the study area (cowgill et al., 2003). thick neogene boulder gravels to silts encircle the proterozoic-mesozoic mountains and fill adjacent basins. intense folding of these strata in the qaidam basin and throughout the field area indicate a contractional regime (regionally) following deposition of the neogene units. relatively thin quaternary gravels and silts (piedmont deposits and primary and reworked loess) overlie the neogene strata. 3. fault trace mapping and paleoseismic site identification in the field, we mapped the fault traces that comprise the individual faults that have had recent rupture based on geomorphic age of the scarps, disruptions of holocene deposits and landforms. the active trace designation generally refers to surface breaks created by the last 1-3 earthquakes. we have identified the active traces and most quaternary faults, but may not show all of the early quaternary faults because we are primarily concerned with the geometry and earthquake history for the last few kyr. in fig. 3, faults labeled qm are similar in age to or slightly younger than surfaces and deposits of late pleistocene and early holocene inferred age. these were mapped as unit qm and are the most extensive of the quaternary deposits and form broad floors that depositionally abut adjacent hill sides and often de fine a sharp break in slope. they generally have a few to 10 s of m incision and may be covered with varnished clasts. tectonic lineaments refer to suspicious geomorphic alignments that are most likely faults older than qm. our mapping approach was to inspect the principal active traces as identified on corona imagery. we mapped the main fault traces, the quaternary units discussed above, and occasionally obtained bedding or other structural attitudes. we typically covered a 5 km wide zone centered over and parallel to the active trace to identify relevant structures and geologic relations. we made many small excavations with shovels to clarify stratigraphic relationships and expose bedding surfaces. these smaller excavations helped add confidence to our geomorphic and stratigraphic-based paleoseismic site decisions. we logged natural and hand-dug exposures, and documented offset (both vertical and horizontal) landforms and strata. while mapping the fault trace, we looked for sites where landforms indicated that steady accumulation of distinctive sediments would preserve evidence for past ruptures. in a number of excavations at such sites, broken and tilted beds overlain by continuous beds and upward terminations of fault traces identify past earthquakes in excavations. disruption seen at the same stratigraphic level at several locations and on both trench walls qualifies as a distinct event (after grant and sieh, 1994). organic matter within the stratigraphy was dated using 14c and fine silt was dated by infrared stimulated luminescence (irsl) (forman, 1999). we use the 2 calibrated sample ages and oxcal v3.5 (ramsey, 2000) for timing constraints. we also use a monte carlo program that uses ordering constraints to «trim» probability distributions of radiocarbon samples and then yields probabilities for events (hilley and young, 2003). this approach allows for more objective event correlation between paleoseismic sites. 4. fault trace geometry a small-scale representation of the active traces that we mapped is shown in fig. 3. for convenience, we refer to locations along the mapped traces in terms of distance from the western end of the study area. washburn et al. (2001) described the western 150 km of the area shown in fig. 3 and established the geometric segmentation. our subsequent mapping added another 120 km toward the east (washburn, 2001). a view along the strike of the active trace in fig. 3 shows a prominent right bend (65 to 85 km) and left step (97 km) in the generally straight central altyn tagh fault. we followed the conventions of depolo et al. (1991) and knuepfer (1989) and identified these discontinuities as geometric bound1019 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china aries which in turn define the westernmost wuzhunxiao segment, the central pingding segment, and the eastern xorxol segment. these segments are not necessarily rupture segments; however, this approach is useful for characterizing fault zones because geometric boundaries may be locations of significant change in slip or moment release (e.g., wesnousky, 1989; harris and day, 1993, 1999). historical rupture data show that rupture endpoints commonly coincide with geometric boundaries and these boundaries appear to retard rupture (knuepfer, 1989). the major geometric boundary in the study area is the pingding releasing step. here, the active trace jumps from the southern side of pingding shan 4 km north to the active trace that continues to the xorxol valley (fig. 3; washburn, 2001; washburn et al., 2001). the 3-4 m vertical offset of holocene-age fan on the southeast side of pingding shan, a prominent scarp on the east face of pingding shan, and the presence of recent landslides indicate that the normal fault that links the bounding strike-slip faults ruptured recently. our mapping stops at 268 km in a straight and simple portion of the xorxol segment (fig. 3). this 070° striking simple geometry continues eastward, but just off the eastern edge of fig. 3, around 300 km, a prominent northern strand develops (fig. 2). we infer that the xorxol geometric segment is at least 200 km long and remarkably straight (washburn, 2001). 5. xorxol segment paleoseismic investigation overview seismogenic properties such as fault straightness and maximum displacement during the last earthquakes at specific sites along the central atf were presented above and in washburn et al. (2001) and washburn (2001). the next steps in characterizing the seismogenic behavior of the central altyn tagh fault are determinations of recent earthquake timing and estimates of magnitude. in order to establish the timing of past earthquakes, we made 10 excazack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle table i. 14c data for the camel paleoseismic site. all analyses are standard ams dates from cams facility at llnl. .13c values are the assumed values according to stuiver and polach (1977). values measured for the material itself are given with a single decimal place. the quoted age is in 14c years using the libby half life of 5568 years and following the conventions of stuiver and polach (1977). 14c concentration is given as fraction modern, d14c, and conventional 14c age. field sample cams # .13c fraction ± d14c ± 14.c age ± 2 calibrated name modern age (*) (cal a.d.) c 1 69687 – 25 0.8923 0.0035 – 107.7 3.5 920 40 1024-1209 c 7 69688 – 25 0.9592 0.005 – 40.8 5 330 50 1456-1650 c 8 69689 – 25 0.9855 0.0048 – 14.5 4.8 120 40 1676-1939 c 10a 69690 – 25 0.7921 0.0034 – 207.9 3.4 1870 40 60-240 c 16 69691 – 25 0.8912 0.0043 – 108.8 4.3 930 40 1022-1191 c 20 69692 – 25 0.8639 0.0047 – 136.1 4.7 1180 50 717-980 c 22 69693 – 25 0.9803 0.0039 – 19.7 3.9 160 40 1662-1950 c 24 69694 – 25 0.7376 0.0042 – 262.4 4.2 2440 50 762-403 b.c. c 25 69695 – 25 0.9874 0.0039 – 12.6 3.9 100 40 1678-1937 c 26 69696 – 25 0.8929 0.0045 – 107.1 4.5 910 50 1022-1221 c 32 69697 – 25 0.6212 0.003 – 378.8 3 3820 40 2404-2140 b.c. c 45 69698 – 25 0.5745 0.0027 – 425.5 2.7 4450 40 3337-2924 b.c. sl rabbit 69699 – 25 1.1245 0.0054 124.5 5.4 > modern > 1950 sl camel 69700 – 25 1.2074 0.0058 207.4 5.8 > modern > 1950 sl stick 69701 – 25 1.1898 0.0052 189.8 5.2 > modern > 1950 (*) determined with calib 4.2 (stuiver et al., 1998). does not include curve intercepts comprising < 2% of the probability distribution. 1020 1021 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china 195 197.5 200 21o 19o 10o ~30o 12o playa hill 10o n o trench site n explanation strike-slip fault qm normal fault active trace qm fault trace jeep road along-fault distance (km) fig. 4. fault trace map showing main strike-slip rupture cutting playa hill, the eastern end of the playa, small alluvial fans at the base of north facing escarpment, and the camel paleoseismic site. note the 058°-striking secondary strikeslip fault, which intersects the main trace at (n) and dies out within 2 km along strike to the southwest. the qm normal faults located along the piedmont and range front to the south have 1-8 m high degraded scarps indicating they have not ruptured recently. the only exception is a slightly sharper scarp seen at (o). however its more distal position may explain this morphologic difference. base image is a rectified corona photograph. see fig. 3 for location of this site. vations somewhat evenly spaced along the field area (fig. 3). interpretations of logs of these excavations are presented in washburn (2001). in addition, early results from 1998 and 1999 excavations are presented in washburn et al. (2001). here, we present observations and inferences from the camel site where the record of repeated ground rupture was best preserved (figs. 3 and 5). all of the 14c data are presented in table i. 5.1. camel site setting the camel excavation has an understandable relationship with neighboring faults, reveals a clear stratigraphy, and preserves abundant organic material. this site yields the most robust timing constraints for recent earthquakes of all our paleoseismic sites. a secondary 058° striking, strike-slip fault intersects the active trace one kilometer west of the camel trench site and normal scarps cut the qm piedmont 2.5 km south of this site (fig. 4). these secondary structures could allow for an incomplete earthquake record in excavations across the active trace. fortunately, degraded surface rupture implies that the 058° strands probably did not rupture during the last earthquake and the normal faults also probably have not ruptured during the last several earthquakes. additionally, it is unlikely that the 058° system would rupture independently because of its close proximity to the active trace. in support of this assumption, the portion closest to the active trace has sharper scarps (shown in red on fig. 4) and preserves geomorphic offsets (moderately reliable 3.5 m), while farther west there is no evidence for recent surface rupture. therefore, we hypothesize that the last earthquake on the main trace activated the only the easternmost part of the 058° fault. furthermore, because the site emphasizes earthquake timing, a broader rupture zone should provide better and redundant constraints than a narrow one. see washburn (2001) for more detailed geometric observations and further 1022 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle 0 25 m n camel trench siterectified kite photos n contour interval 1.0 m relative to arbitrary datum 0 25 m trench fault trace topographic breakline channel strandline a b c c b b b a b b strandline 1000 1005 1010 1015 1020 a b fig. 5a,b. a) rectified kite aerial photograph mosaic showing the geomorphic setting and geometry of surface rupture traces at the camel trench site. note the playa strandline at the north end of the trench, small dunes just south of the strandline on the east side of the trench, neogene hills at the southern end of the trench, and alluvial fans at the base of these hills. b) topographic map quantifying the landforms at this site. the letters (a, b, c) on both figures identify corresponding fault traces discussed in the text. review of secondary structures at the camel site. four geomorphic processes interact to create an environment that is conducive to paleoseismology (fig. 5a,b). on the northeast side of the trench, small dunes are visible. they create distinctive cross bedded deposits comprised of well rounded, medium to coarse-grained sand (fig. 9). small alluvial fans on the west side of the trench deposit muddy sand and fine gravels units seen in the trench. to the south, low hills of indurated neogene silts comprise the «basement» units exposed in the southern part of the trench. the playa edge contributes fine silt and muddy strata to the northern end of the trench and introduces organic material at regular intervals. individual pieces of wood and dung are sporadically exposed throughout the trench strata. in addition, playa strandlines are also preserved in numerous places in the strata. a modern strandline is seen in fig. 5a,b; it formed by shallow inundation of the playa and subsequent rafting of organic material towards the leeward end of the playa. regular seasonal winds blow to the east and have subsequently concentrated woody material, flower parts, camel dung, and rabbit dung in < 1 m wide by 5 cm high by 10 s of m long deposits ringing the playa edge. radiocarbon dating of the modern strandline allows us to evaluate the residence time of these deposits and therefore possibility for detrital ages from the trench strandlines (table i). although 2 unconformities were identified within the trench, their planar contact implies that erosion was not significant. furthermore, the presence of strandlines at different stratigraphic levels in the trench generally implies that nearly continual deposition has been occurring at this site. this is key to preservation of a complete earthquake record. the intermixing of these various sedimentary deposits results in distinctive, interpretable, and datable stratigraphy at the camel trench, which in turn allows us to determine earthquake timing at this site (figs. 6, 7a,b and 9). the active trace near the trench sites consists of two prominent fault traces, 12-15 m apart that have been almost entirely degraded in places (figs. 4 and 5a,b). a possible third trace breaks along the neogene hill front. we excavated a 34 m long trench across all three of these traces to minimize the potential for missing an event. the trench stratigraphy closely corresponds with the geomorphic processes at this site (figs. 6, 7a,b and 9). playa and dune sediments thicken toward the northern end of the trench, while coarser sand and gravel (alluvial fan) units and neogene bedrock dominate the southern end. we identified 6 fault zones with recurrent disruption of the stratigraphy and number these starting with fault zone 1 in the north end of the trench. we also number stratigraphic units upward and show their contact relationships in fig. 7a,b. projection of preserved portions of active trace indicate that the northern trace (a on fig. 5a,b) corresponds with fault zones 1 and 2 and the southern trace (b on fig. 5a,b) forms fault zones 3 and 4 (fig. 9). 5.2. camel most recent earthquake (mre ) faults in zones 1-4 cut units 100 through 130 and sometimes 140, however fractures in fault zones 1 and 2 cut higher units (fig. 9). these fractures show no offsets across them and the often show downward terminations at medial stratigraphic levels. these characteristics and proximity to the playa show the fractures are probably desiccation-mud cracks and non-tectonic. from fault zones 1 to 4, the silts comprising unit 140 grade to coarse sand and fine gravel and erode the top of dune unit 130 (figs. 7a,b and 9). on the east wall, fault zone 1 shows faults terminating in the middle of unit 140, while zone 2 shows faults capped by unit 140. in portions of fault zones 3 and 4, unit 140 is missing. the west wall shows the same general trend of thinning and eventual disappearance of unit 140 south of fault zone 4. we interpret unit 140 as a time transgressive unit that was deposited before and after the last earthquake based on these cross-cutting relationships and because the faults seen in zones 1-4 ruptured the surface based on the fault traces on either side of the trench. unit 140 is probably a sheet flow deposit coming from runoff-driven sediment transport on the small alluvial fan. the base of 150 is the event horizon for the last earthquake in these fault zones. 1023 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china on the east wall of zone 5, a single fault surface cuts unit 130 and is covered by unit 160 (fig. 9). unit 150 pinched out north of this point, so these cross cutting relationships are similar to those in fault zones 1-4. on the west side, unit 123 is disrupted by the same fault, however a < 3 cm wide fissure implies higher rupture through unit 127. in fault zone 6, a single fault cuts to the same level (unit 127). south of the zone 6, numerous faults cut neogene basement and strongly indurated quaternary gravels. compared to fault zones 1-4, the smaller fault zones and amount of disruption suggest that fault zones 5 and 6 slipped only a small amount and may be reactivated older principal rupture zones. the consistent level of upward fault terminations indicates that the base of 150 is the event horizon for the last earthquake. units 130 and 140 represent the ground surface at this time. the southern end of the trench has thinner units and thus does not record the surface ruptures as well as the lower gradient northern end. note the gently convex warp in the strata on east wall of fault zone 3. this is apparently a buried moletrack formed during the last earthquake and subsequent erosion beveled the top off. below it gently folded strata are strata deformed prior to the last earthquake. erosion over the top of fault zone 6 makes correlation of fracture events there difficult. however, fractures do terminate at the base of a unit correlated to 140 in the northern portion of the trench. 5.3. camel penultimate earthquake different levels of induration and an irregular contact sometimes showing centimeter deep rills identify an unconformity between units 105 and 100. the west side of fault zone 2 shows folded and faulted strata of unit 100 overlain by undeformed silts and sands of unit 105. this relationship is also seen on the other side of this fault zone and indicates the penultimate rupture 1024 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle 170 160 150 140 mre 130 127 125 123 115 fig. 6. photograph of fault zone 1 on east wall of camel trench (fig. 9). note the metric tape measure on the trench wall for scale. photograph shows, from top to bottom, units 170, 160, 150, 140, 130 (thick crossbedded sand), 127, 125, 123, and 115. note that the faults have upward terminations between the top of unit 130 and the base of unit 150. this relationship shows rupture resulting from the last earthquake at the camel site. 1025 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china (fig. 9). additional evidence for the penultimate event is seen in heavily deformed strata overlain by less deformed strata in fault zone 3. both sides of fault zone 4 show upward fracture terminations and increased local erosion at the 100-105 unit contact. fault zone 5 shows a clear north side up sense of offset below the unconformity. therefore unit 100 is the youngest unit cut by the penultimate event and the unconformity is the penultimate event horizon. as noted above, the mre and the penultimate events produced local highs (moletracks) that are especially well manifest on the east side of fault zone 3, but also evident in fault zones 4 and 5. each time they formed, they were erosionally beveled. n2 x x x x x c22 1662-1950 a.d. c25 1678-1937 a.d. c7 1456-1650 a.d. c26 1022-1221 a.d. c16 1022-1191 a.d. c20 717-980 a.d. c1 1024-1193 a.d. c10 60-240 a.d. c24 762-403 b.c. c32 2404-2140 b.c. c45 3337-2924 b.c. uncf uncf uncf 170 160 150 140 130 125 123 127 115 110 105 100 90 85 80 c8 1676-1939 a.d. younger than mre older than mre/younger than pen.x radiocarbon samples older than penultimate event uncf unconformity 115 unit number 4000 cal b.c. 2000 cal b.c. cal bc/cal a.d. 2000 cal a.d. c22 160 ± 40bp c25 100 ± 40bp c8 120 ± 40bp c7 330 ± 50bp c26 910 ± 50bp c16 930 ± 40bp c1 920 ± 40bp c20 1180 ± 50bp c10 1870 ± 40bp c24 2440 ± 50bp c32 3820 ± 40bp c45 4450 ± 40bp calibrated dates radiocarbon dates and calibrated curves from oxcal 3.5unit # fig. 7a,b. a) stratigraphic column of camel trench showing relative unit thickness, unit number, major unconformities, and stratigraphic location of radiocarbon samples. diamonds show dates above the mre, xs show dates older than the mre, and os are older than the penultimate event. weak evidence in the trench near fault zone 2 suggests an event at the level of unit 125. b) probability distributions for calendar dates determined from the radiocarbon age for each sample using oxcal v3.5 (ramsey, 2000). a b 5.4. other possible earthquakes in the camel trench there is limited evidence for a possible event occurring before the mre, but after the penultimate event. two 50° south dipping fault traces apparently terminate in unit 125 and 130 on the east wall of fault zone 2 (fig. 9). the die out of the tip in unit 130 is most likely due to dilatation of the sand unit. the base of unit 127 could be a rupture horizon that lies between the mre and penultimate event horizons. however, we do not see similar relationships on the west wall. there, the two faults have tips within unit 130; therefore it is likely that the tipline of the faults plunge eastward. a fault trace splays into three strands that are apparently overlain by continuous bedding of unit 130 on the east wall of fault zone 3 (fig. 9). this potential rupture horizon could also represent an intermediate earthquake. the corresponding fault on the west wall ruptures to a higher level, supporting the idea of upward die out or plunging tip line (fig. 9). we also see a fault trace that ruptures to an intermediate level in the southern part of fault zone 4. on the east wall this fault is capped by unit 110, but breaks to the top of 110 on the west wall (fig. 9). this situation is similar to fault zone 3 and could also represent an intermediate event. however, the different rupture levels could also result from a plunging tip line of an mre fault trace. if the intermediate level terminations seen in fault zones 2, 3 and 4 represent an intermediate event, then it produced only small amounts of disruption of the trench strata. in contrast, the mre and penultimate events produced intense disruption of the strata along the entire > 30 m length of the trench. therefore, the limited deformation, seen as only 3 faults, indicate that this intermediate event was different and had lower magnitude than the mre and penultimate events. we favor an interpretation of upward die out and/or plunging tip line of mre fractures because the faulting relationships are not the same on both trench walls and the much smaller degree of deformation compared to ruptures seen in other trenches in the study area. more substantial evidence for a pre-penultimate event is seen 1 m north of and in the bottom of fault zone 5. here, faulted and tilted strata are overlain by flat lying bedded sand and fine gravel of unit 85. these relationships are suggestive of an earthquake before the penultimate event, however the trench was not deep enough to confirm this event. fault zone 6 preserves evidence of the failure of large blocks of the local neogene basement indurated silts into mostly eolian sands. the blocks are strongly fractured and in combination with the windblown sands represent a slightly different environment than the eolian sand and alluvial fan muddy sands and gravels to the north, further making correlation of events with the rest of the trench difficult. 5.5. camel 14c dating we collected 57 samples of organic material from the trench and dated 12, as well as 3 from a modern strandline. because much of the organic material in the camel trench was represented in modern strandlines, and often deposited as strandlines, we dated the modern ones to assess their ability to define the depositional age. the 3 strandline samples are all modern. thus, the 14c dates in the trench probably do not include a significant pre-depositional age. the 14c data are presented in table i and the probability distributions from calibrations using oxcal (ramsey, 2000) are shown in fig. 7a,b. an 8 by 5 by 3 mm piece of wood with intact bark (c8) from unit 150 provides an upper bound of 1676-1939 cal a.d., while a rabbit dropping (c7) yields a lower bound of 1456-1650, for the last earthquake in this area. likewise, an elongate rabbit dropping (c20) places an upper bounds of 766-980 cal a.d. and 10 small pieces of woody debris were mixed (c10) to provide a lower bounds of 60-240 cal a.d. for the penultimate event. the oldest strata exposed in the trench are between 3337-2924 cal b.c. based on a radiocarbon date from piece of wood (c45). because of the complete stratigraphic agreement within all 12 samples, we used oxcal 1026 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle 1027 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china to refine the constraints on event timing (ramsey, 2000). this program uses stratigraphic ordering constraints to eliminate overlapping age probabilities, which refines our earthquake timing brackets. we used 1950 a.d. as an upper event boundary because the world wide seismic network was able to detect moderate sized earthquakes by this time. oxcal is most useful for samples that are closely spaced stratigraphically and yield similar radiocarbon ages. c22 is located near the top of unit 160 while c25 in the lower part of this unit and c8 is located in unit 150. these samples all yield similar calendar ages (table i), however their stratigraphic position allows oxcal to cut off overlapping portions of their probability calendar age distributions. this is useful because this part of the calibration curve results in a wide age range (e.g., 1676-1939 for c8). after running oxcal, c8 yields a narrow upper bound of 1676-1775 for the camel mre. the lower bound of 1456-1650 cal. a.d. remains unchanged because the next lowest sample (c26 1022-1221 cal a.d.) is much older and therefore probability distributions do not overlap. the lower bound on the penultimate earthquake is also unchanged for the same reason. above the penultimate event horizon, unit 125 contains 2 radiocarbon samples, however they are spaced over an 8 m horizontal distance and sit at the approximate same stratigraphic level. therefore we cannot apply ordering constraints to these samples, although c26 in unit 127 does shift their age distributions 30 years older. figure 8a,b shows the earthquake timing constraints at a number of sites along the central atf. the ages displayed in fig. 8a are calibrated but otherwise not refined with strati0 50 100 150 200 250distance (km) c al en d ar y ea rs -2500 -1500 -500 500 1500 k ul es ay i 2617 2618 c am elexplanation 14 mre x x x most recent event younger than mre older than mre/younger than pen. older than penultimate event earthquake from active altun fault monograph (ge et al.) 2 sigma calibrated age of c (solid line) and irsl (dotted line) samples older than earliest event nosym. -2500 -1500 -500 500 1500 camel probability 0.01 0.0 c al en d ar y ea rs explanation mre penultimate event fig. 8a,b. a) earthquake timing constraints from stratigraphic relationships and 14c and infrared stimulated luminescence (irsl) samples (from this study and washburn et al., 2001). note the active trace map above plot a for reference. these are the calibrated ages without oxcal or other refinements from stratigraphic ordering. b) monte carlo probability plot for the camel paleoseismic site earthquake timing data (hilley and young, 2003). the area under the curves represents the probability distribution of the age of past earthquakes. sharper peaks represent well-constrained events and show the refinement of the camel event ages. a b zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle 1028 fig. 9. trench logs for the camel paleoseismic site at 198 km. most faults terminate at the upper blue line, the event horizon for the last earthquake at this site. the penultimate earthquake is identified by upward fault terminations and folded bedding overlain by continuous strata, which is marked by the lower blue line. this penultimate event horizon coincides with a small unconformity (yellow line) identified by different levels of induration and an irregular contact with cm deep rills in places. stratigraphic column of camel trench at lower left shows relative unit thickness, unit number, major unconformities, and stratigraphic location of radiocarbon samples. see discussion in text for further interpretation and timing constraints on earthquakes seen in this trench. n2 x x x x x c22 1662-1950 a.d. c25 1678-1937 a.d. c7 1456-1650 a.d. c26 1022-1221 a.d. c16 1022-1191 a.d. c20 717-980 a.d. c1 1024-1193 a.d. c10 60-240 a.d. c24 762-403 b.c. c32 2404-2140 b.c. c45 3337-2924 b.c. uncf uncf uncf 170 160 150 140 130 125 123 127 115 110 105 100 90 85 80 c8 1676-1939 a.d. stratigraphic column unit # sample calibrated date younger than mre older than mre/younger than pen. older than penultimate event x radiocarbon samples uncf unconformity 115 unit number key explanation matrix supported massive sand gravel clast supported gravel bedded sand eolian sand silt sandy silt prominent strand line event horizon unconformity 14c sample fault, fracture neogene bedrock east wall c32 (2404-2360, 2354-2140 cal b.c.) c26 (1022-1221 cal a.d.) c25 (1803-1907, 1678-1744 cal a.d.) c16 (1022-1191 cal a.d.) c1 (1024-1193 cal a.d.) west wall fault zone 1fault zone 2 fault zone 3 fault zone 4 fault zone 5 fault zone 6 c24 (762-678, 671-608, 599-403 cal b.c.) c7 (1456-1650 cal a.d.) c8 (1802-1939, 1676-1765 cal a.d.) c20 (766-980 cal a.d ) c22 (1662-1711, 1717-1886, 1911-1950 cal a.d.) c45 (33373208, 3194-3150, 3140-3008, 2987-2924 cal b.c.) c10 (60-240 cal a.d.) 0 5m prominent strand line 14c sample explanation event horizon unconformity fault, fracture matrix supported clast supported bedded sand massive sand gravel gravel eolian sand silt sandy silt neogene bedrock 120 130 south north unit numbersh unit numbers 170 160 155 140 130 123 115 105 100 170 160 130 115 110 150 140 90 120 130 100 105 170 160 155 140 130 123 115 105 100 unit numbers 80 90 100 110 160 170 fig. 6. (continued) 1029 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china graphic ordering constraints using oxcal. hilley and young (2003) have developed a 14c age refinement method that is similar to oxcal and produces probability curves for events with a monte carlo-based analysis of 14c age calibration and stratigraphic ordering. we show the probability curves for the last two well-defined camel events determined using the hilley and young (2003) method in fig. 8b. 6. discussion 6.1. earthquake ruptures along the central altyn tagh fault correlation of earthquakes between paleoseismic sites can lead to misinterpretation because not all strands in wide fault zones may be activated during an earthquake. we have eliminated most of the potential for this scenario by thorough evaluation of paleoseismic sites and excavation across only the sharpest, most active traces. however, «dogtail» rupture terminations in the middle of geometric segments and may not rupture through all excavations in question (e.g., ward, 1997). therefore trenches showing overlapping geochronologic earthquake constraints that are only 10 s of kilometers apart may not record the same earthquake. we therefore utilize other observations to help clarify the rupture history. we know the peak surface displacement at a given site from the offset data (washburn, 2001; washburn et al., 2001) and we have constrained earthquake timing at a number of paleoseismic sites. with these data, we can determine crude recurrence intervals, because we have only 1-3 events in each trench. we can also make magnitude estimates using the displacement data. better estimates of moment magnitude (mw) can be achieved by using surface rupture length (srl) (wells and coppersmith, 1994). we estimate surface rupture length by utilizing geometric constraints, offset distribution, surface rupture morphology, timing constraints from the trench sites. figure 8a,b shows that the last earthquake at kulesayi, 2-6-17, 2-6-18, and the camel site all occurred at the approximate same time, between 1215-1775 cal a.d. (data from washburn et al., 2001, and this paper). however, it is unlikely that one large earthquake ruptured the entire field area between 1215-1775 cal a.d. in such a case, the mre would have jumped two geometric boundaries and ruptured from the kulesayi trench (23 km) to at least the camel trench (199 km) and likely farther based on the offset distribution and sharp surface rupture that continues to > km 240. this scenario yields a srl of > 220 km and mw > 7.8. however, consideration of details of the age control from the excavations and the geometric boundaries makes it unlikely, and the relatively high slip implied ( 10 m) is not consistent with our inferred offset distribution (maximum offset 7 m; washburn, 2001; washburn et al., 2001). it is possible that the 4 km pingding left step barred rupture. the arkateng restraining bend and associated complexities around dayang shan are also potential rupture barriers fig. 3. furthermore, the discontinuous and degraded surface rupture around these boundaries argues for smaller earthquakes that did not rupture the whole field area. therefore we separate wuzhunxiao and pingding events from xorxol events based on surface rupture morphology and geometric boundaries. consequently, the last earthquake at kulesayi occurred between 1215 and 1750 a.d. and may have produced the 3-5 m offsets on the west side of lake wuzhunxiao (fig. 8a,b; washburn et al., 2001). along the xorxol segment, the timing constraints imply that the last earthquake ruptured its western portion before the eastern portion broke. the last earthquake in the west ruptur ed 1270 and 1430 cal a.d. (from bitter sea published results of washburn et al., 2001; fig. 8a,b). this rupture may go from western end of the xorxol segment to around the bitter sea ( 50 km srl) with peak displacement of 7 m (washburn, 2001; washburn et al., 2001). the younger camel mre occurred between 1456 and 1775 cal a.d. (fig. 8a,b). it ruptured from somewhere east of the bitter sea through the camel trench and likely farther based on the rounded and semi continuous moletracks between 247 km to the edge of the study area (268 km, fig. 3). the degraded nature of the surface rupture suggests that this reach is either the eastern terminus of the xorxol mre or that erosion rates, which are detectably higher in this area, led to the more degraded surface rupture. higher erosion rates are plausible because of the position of the eastern xorxol valley relative to the «mouth» of the tarim basin only 7 km north of the playa at 265 km on fig. 3. the mouth allows more frequent storms and intense daily winds to blow through eastern xorxol valley and more animal access and potential degradation. in summary, the last earthquakes along the central altyn tagh fault: 1) occurred between 1215 and 1775 cal a.d.; 2) had maximum displacement of 5 m on the wuzhunxiao and pingding segments and 7 m on the xorxol segment (washburn, 2001; washburn et al., 2001), and 3) had surface rupture lengths between 50 and > 150 km with corresponding magnitudes of low mw 7 s to high mw 7 s, respectively. 6.2. implications for slip rate based on the size and frequency of earthquakes in the last 3 kyr although the earthquake data do not tightly constrain the surface rupture length or mw of the mre, the recurrence times from trenches show good agreement. we find 2 earthquakes in the last 0.8-2.0 kyr at the camel site, 3 events in the last 2.4-3.0 kyr at the bitter sea, and 2 kulesayi events in the last 0.8-2.2 kyr (this paper; washburn, 2001; washburn et al., 2001). although we have a limited number of events for robust recurrence calculations, taking averages from these data give repeat times of 0.7 ± 0.3 kyr for the camel site, 0.9 ± 0.1 kyr for the bitter sea trenches, and 0.75 ± 0.35 kyr for the kulesayi site. these frequencies are consistent with the 2 earthquakes in the past 3 kyr reported by ge et al. (1992) in the study area and are generally similar to the 800-1000 year recurrence times estimated for the kunlun fault (van der woerd et al., 1998). given the limited number of earthquakes in this study, we emphasize the tentative results derived from relating earthquake recurrence to slip rates. however, systematic underinterpretation or rupture location variability must be invoked for significantly higher recurrence rates or slip/event. if a large earthquake occurred in the near future, our recurrence rates would be maxima and any inferred slip rate would increase. to compare these paleo-earthquake data with other findings along the atf we cast slip rate in terms of average recurrence intervals by using 5 and 10 m of slip per event because these values bracket the peak offsets recorded for the mre(s) (washburn et al., 2001). using: 1) 10 mm/yr determined from repeat gps surveys (bendick et al., 2000; shen et al., 2001) yields recurrence intervals of 0.5 and 1.0 kyr for 5 and 10 m of slip per event, respectively; 2) 3 cm/yr determined by large scale post glacial offsets and offset terraces and moraines in areas west of the study area (peltzer et al. 1989; meriaux et al., 2000) yields recurrence times of 0.2 and 0.3 kyr. figure 10 shows a compilation of the various slip rates that have been inferred for the central atf. comparison of the different recurrence intervals show that the paleoearthquake data most closely agree with a 1-2 cm/yr slip rate because we would expect to see 2-3 times the number of earthquakes in our paleoseismic investigations and the faulting and sedimentary record in them is not consistent with so many missing earthquakes. an important issue in slip rate comparisons is the observed eastward decrease in slip rate along the atf (fig. 10). peltzer et al. (1989) inferred 3 cm/yr rates in the area of 84°-92°e. meriaux et al. (1999, 2000) determined 3 cm/yr between 85° and 89°e and 2-3 cm/yr near 90°e. ryerson et al. (2003) report a well constrained 18 mm/yr at the pingding shan (90.5°e). meriaux et al. (2003) determined a minimum of 20 ± 3 mm/yr at 94°e. ryerson et al. (2003) also report 12.5-15 mm/yr at 95°e. meyer et al. (1996) concluded that slip rates east of 96° were 4 ± 2 mm/yr, consistent with decreasing slip rate along the atf and accommodation of the slip gradient by thrusting along the south side in the qaidam basin (also suggested by peltzer et al., 1989; chen et al., 1999). meyer et al. (1998) observed that the thrusts branch from the atf and are thus kinematically linked; and van der woerd et al. (2001) showed the decrease in slip rate and geometry of the fault junction at the altyn tagh fault-tanghenan shan thrust triple junction. 1030 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle this study covers the portion of the atf from 89.6°e to 92.5°e. it is within the area covered by the higher rates in peltzer et al. (1989) and spans the meriaux et al. (1999) and ryerson et al. (2003) site near the pingding shan (18 mm/yr). our best earthquake recurrence data come from the camel site is at about 91.75°e, so we may begin to observe a decrease in slip and thus recurrence rate due to interaction between the atf and the adjacent active thrusts of the qaidam basin (for example, note that the active northwest striking yousha-shan thrust fault of chen et al. (1999) approaches the atf near the gobiling). although the results of ryerson et al. (2003) and our inferred slip rate from near the pingding shan overlap (fig. 10), the 20 ± 3 mm/yr minimum rate at 94°e (meriaux et al., 2003) implies a higher earthquake recurrence rate than we inferred for the xorxol segment. the geodetic investigations are not sufficiently dense to image variations in slip rate along the central atf. given their great breadth (100 s of km perpendicular to the atf), near fault locking and seismic cycle variation in velocity cannot explain the slip rate discrepancy (fig. 10; thatcher, 1990; shen et al., 2001). shen et al. (2001) also point out that because there is no major convergence parallel to the atf on its south side across the qimen tagh 1031 paleoseismology of the xorxol segment of the central altyn tagh fault, xinjiang, china 84 86 88 90 92 94 96 98 0 0.5 1 1.5 2 2.5 3 3.5 4 longitude (°e) a t f p a ra lle l s lip r a te ( cm ) ge et al. (1992) shen et al. (2001) bendick et al. (2000) meriaux et al. (1999, 2000) peltzer et al. (1989) meriaux et al. (2003) ryerson et al. (2003) this study ryerson et al. (2003) meyer et al. (1996) fig. 10. inferred altyn tagh fault-parallel slip rates from long-term studies (blue), geodesy (red), and earthquake geology (black) compiled from the literature and this study. the eastward decrease in slip rate indicated by the long-term studies is consistent with the activity of numerous thrust faults splaying from and to the south of the atf (e.g., meyer et al., 1996, 1998; tapponnier et al., 2001; although not consistent with the geodetic results from shen et al., 2001). in general, the geodetic results show lower slip rates relative to the long-term studies. our earthquake geology results span the two other methods in both the time scale of measurements (a few kyr versus decadal and > 10 kyr) and the resulting inferred rates. mountains at the southern margin of the qaidam basin, the slip rate along the atf must not decrease significantly between 80° and 92°e. one explanation for the discrepancy in recurrence times from gps site velocities and the earthquake record and geology is that a major earthquake along the central atf is overdue. another intriguing explanation is that the long term loading rate along the atf has decreased in the past 100 kyr and strain is now accommodated by other structures in central asia. for example, the faults that produced four m 8 earthquakes in mongolia in the past century released significant strain in an area of relatively lower inferred long term deformation rate (baljinnyam et al., 1993). earthquakes, such as the 1997 mw 7.6 manyi earthquake that ruptured 170 km along the kunlun fault (peltzer et al., 1999) and the 2001 mw 7.8 kokoxili earthquake (van der woerd et al., 2002; lin et al., 2002) may indicate that the western portion of the kunlun fault and other central and north tibetan strike-slip faults may be more active than previously thought. finally, shortening within the qimen tagh and other contractional structures in northern tibet may now accommodate a higher percentage of india-eurasia convergence (yin and harrison, 2000; cowgill, personal communication). 7. conclusions in this paper, we have characterized the fault trace geometry and paleoseismology of the central altyn tagh fault, emphasizing its 230 km long and remarkably straight xorxol geometric segment. trace geometry, timing of paleoearthquakes, and inferences of rupture magnitudes contribute to the global catalogue of strike-slip fault systems and provide information about its geodynamic role in the indiaeurasian collision. the trace geometry of the central atf is dominated by the arkateng restraining bend, pingding releasing step, and the notably straight xorxol segment. the geometric boundaries are probable earthquake rupture boundaries based on surface rupture continuity, distribution of small geomorphic offsets, and historical data. major holocene deformation along the altyn tagh fault system is confined to a narrow zone less than 3 km wide. the geometric boundaries and timing constraints probably preclude one large earthquake rupturing all 3 segments (270 km). furthermore, multiple earthquakes with shorter rupture lengths ( 50 km) rather than complete rupture of the xorxol segment better explain the paleoseismic data. we found 2-3 earthquakes in the last 2-3 kyr. when coupled with typical amounts of slip per event (5-10 m), the recurrence times are tentatively consistent with 1-2 cm/yr slip rates. this result favors models that consider the broader distribution of collisional deformation, rather than those with northward motion of india into asia absorbed along a few faults bounding rigid blocks. acknowledgements we thank p. molnar, r. bilham, p. tapponnier, m. hamburger, r. bürgmann, a. yin, s. graham, and j. freymueller for early reviews of this research, our colleagues at the institute of geomechanics in beijing, and s. holloway, m. baillie, s. selkirk, e. catlos, c. schrein, and s. zhang for help in the lab, field, and the office. constructive reviews by k. okamura and f. galadini and the guidance of daniela pantosti for annals of geophysics were also very helpful. r. ryerson arranged our 14c dating at lawrence livermore national laboratory/university of california center for accelerator mass spectrometry where we were guided by g. sietz, m. caffee, j. southon, and their colleagues. g. hilley provided important advice on interpretation of 14c data. the northern california earthquake data center (ncedc), and the member networks of the council of the national seismic system (cnss) provided the seismicity data shown in fig. 2. we are particularly grateful to an yin for inviting us to join the project and to eric cowgill for introducing us to field work in xinjiang. this work is part of a collaborative research effort supported by the u.s. national science foundation continental dynamics program (grant ear-9725780). 1032 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle references avouac, p.j. and p. tapponnier (1993): kinematic model of active deformation in central asia, geophys. res. lett., 20, 895-898. baljinnyam, i., a. bayasgalan, b.a. borisov, a. cisternas, m.g. dem’yanovich, l. ganbaatar, v.m. kochetov, r.a. kurushin, p. molnar, h. philip and y.y. vashilov (1993): ruptures of major earthquakes and active deformation in mongolia and its surroundings, geol. soc. am. mem., 181, pp. 62. bendick, r., r. bilham, j. freymueller, k. larson, g. peltzer and g. yin (2000): geodetic evidence for a low slip rate on the altyn tagh fault and constraints on the deformation of asia, nature, 404, 69-72. chen, w-p., c-y. chen and j.l. nábelek (1999): presentday deformation of the qaidam basin with implications for intra-continental tectonics, tectonophysics, 305, 165-181. cowgill, e., a. yin, x. wang and q. zhang (2000): is the north altyn fault part of a strike-slip duplex along the altyn tagh fault system?, geology, 28, 255-258. cowgill, e., a. yin, j.r. arrowsmith, w. xiaofeng and s. zhang (2003): the akato tagh bend along the altyn tagh fault, nw tibet, 1. cenozoic structure, smoothing by vertical-axis rotation, and the effect of topographic stresses on borderland faulting, geol. soc. am. 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(1989): seismicity and the structural evolution of strike-slip faults, in fault segmentation and controls of rupture initiation and termination, edited by d. schwartz and r. sibson, u.s. geol. surv. openfile rep. 89-315, 409-431. wessel, p. and w.h.f. smith (1995): new version of the generic mapping tools released, eos, trans. am. geophys. un., 76 (33), p. 329. yin, a. and m. harrison (2000): geologic evolution of the himalayan-tibetan orogen, annu. rev. earth planet. sci., 28, 211-280. 1034 zack washburn, j ramón arrowsmith, guillaume dupont-nivet, wang xiao feng, zhang yu qiao and chen zhengle palombo_correctedok_layout 6 annals of geophysics, 56, 3, 2013, rv0326; doi:10.4401/ag-6270 rv0326 on the recovery and analysis of historical seismograms barbara palombo1,*, nicola alessandro pino2 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, rome, italy 2 istituto nazionale di geofisica e vulcanologia, osservatorio vesuviano, naples, italy abstract the analysis of historical seismograms has proven to be a fundamental tool to help with the definition of the seismic risk in specific regions. indeed, modern quantitative reappraisal of relevant earthquakes that occurred before the 1960’s; i.e., prior to both the developments of modern recording instruments and the theoretical progress, has been essential for the assessment of the seismic potential of a source area. however, due to the characteristics (transducing and recording) of the old analog seismographs, the data available are affected by intrinsic uncertainties, and errors can be introduced during the processing of waveform digitization. these drawbacks can seriously influence the quality and reliability of an investigation. in general, no standard technique can be applied when dealing with historical seismograms. thus, specific tests and cross-checks have to be designed to estimate the limits of each specific analysis. here, we aim to provide an overview of the whole procedure while focusing on the most crucial steps, from the seismogram recovery to the application of modern techniques for the retrieval of the seismic source information. we also suggest possible checks for the robustness of the data and for the available instrument characteristics, with a description of the effects of various uncertainties on the results that can be obtained. we thus provide useful indications for the analysis of historical seismograms, and also for the correct interpretation of the resulting characteristics of the seismic source. 1. introduction major faults represent stable geological structures where huge amounts of strain energy can accumulate in response to stress produced by tectonic movements. this energy is then released in a few seconds, giving rise to major earthquakes, which is a process that occurs recursively, and often in a similar way on the same fault. as it takes a long time to accumulate large amounts of energy, the biggest earthquakes have long return periods, which range in many cases from several hundreds to thousands of years. knowing the energy released by major earthquakes, and the slip kinematics, size of the ruptured surface, and distribution of displacement on the fracture is fundamental for the definition of the potential of the causative seismogenic faults, and thus for the assessment of the associated seismic hazard. currently, seismic source studies are mainly based on the analysis of the large amounts of digital data that are collected with modern instruments. these data allow for detailed reconstructions of a seismogenic processes. on the other hand, modern instrumental seismology was developed only about 50 years ago, so since the 1960’s, when modern electromagnetic seismometers and, shortly later, digital recorders were designed and installed all over the world. with recurrence times of hundreds or thousands of years, detailed information on the characteristics of an earthquake source is only available for a relatively small number of significant events. on these grounds, investigations into historical earthquakes are fundamental, and, indeed, they have become progressively more important in recent years. overall, research into historical earthquakes can be grouped into three main distinct fields that are characterized by the differences in both the types of data and the methods of investigation: paleoseismology, macroseismicity (studies of intensity data) and analysis of coseismic geophysical data. this last approach is based on objective measurements of variables that relate to the whole space–time evolution of the fracturing processes, and it represents the most appropriate framework for analysis of the characteristics of a seismic source. usually, instrument data of historical earthquakes comes from geodetic measurements of static surface displacement or from recordings of transient ground motion. to quantitatively estimate the permanent surface deformation generated by a seismic event from geodetic data, a reference measure in the source area should to be available, which should be taken from before the earthquake happened [e.g., bilham and england 2001, nyst et al. 2006]. unfortunately, this is not often the case, article history received december 18, 2012; accepted march 12, 2013. subject classification: historical seismology, waves and wave analysis, earthquake source and dynamics. and in particular for events older than a few decades. unlike static displacement, seismic waves generated by strong earthquakes produce effects that are detectable over the entire planet. thus, considering that since the beginning of the 20th century a relatively large number of seismic stations have been operating globally, historical seismographic data can potentially provide a source of precious data for investigations into major seismic events that occurred before the electronic instrument era. indeed, depending on the quantity and geographical distribution of the recording stations at the time of a specific earthquake, there might be a sufficient number of seismograms to be used to reconstruct the associated process of rupture. however, quantitative seismological analyses and modern perspectives of a seismic source that allow the description of a rock fracture in the earth crust starting from the analysis of the recorded seismograms were only developed from the early 1960’s. thus, to gain more information on a source of an important earthquake that occurred from 1900, the original historical seismograms must be gathered and re-analyzed. even though they are analogous to modern recordings, at least in principle, these seismograms have fundamental differences relative to digital waveforms, and special care is needed for their collection and analysis. several studies have been published that have described the general procedures for quantitative analysis of historical seismograms [e.g., batlló et al. 2008, pino 2011], which have also shown several case histories. in the present study, within a general overview on the recovery, processing, and study of historical seismograms, we focus on some of the possible difficulties, the results attainable, and the intrinsic limitations of the analysis. the aim is to provide both useful indications for these kinds of investigations, and valuable information for the interpretation of the results of the analysis of historical seismograms. 2. retrieval of historical seismograms the first step in the study of historical seismograms is the data retrieval. since the early 20th century, the total number of seismological observatories worldwide greatly exceeded 100 [schweitzer and lee 2003], and many sites have operated more than a single instrument. at the same time, the number of seismograph stations rapidly increased in the following years [johnston 1996]. seismographs were based on either mechanical or electromagnetic equipment, and up to the last decades of the 20th century, analog data were recorded on plain, smoked or photographic paper. in most cases, the paper sheets were changed daily, therefore a large amount of paper began to accumulate at observatories very rapidly, which became a problem due to both the need of suitably large spaces for storage and the perishable nature of the support. over time, with decreasing interest, with wars, and with the closing (or relocation) of observatories, these original recordings have been lost and damaged. moreover, at that time, the analyses were performed on the original recordings, and in particular, for the most significant earthquakes the seismograms were often loaned all over the world, and were not always returned to the original observatory. consequently, many recordings were destroyed or lost. fortunately, in recent decades, the scientific community has become increasingly aware of the importance of preserving this huge amount of data, and several international initiatives have been proposed with this aim [lee and benson 2008]. in particular, specific projects for the collection, recovery and storage of historical seismograms have been implemented [e.g., seismoarchives, http://www.iris.edu/seismo/projects; sismos: michelini et al. 2005; euroseismos: ferrari and pino 2003], which have generally made the scanned images available. to be analyzed by means of modern techniques, historical seismograms need to be converted into waveforms that are analogous to present digital recordings. this process is generally accomplished through separate, successive steps, starting with the scanning of the original image. thus, waveform digitization can be performed with automatic or semi-automatic procedures. several studies have developed computer programs for the digitization of historical seismograms [e.g., baskoutas et al. 2000, bromirski and chuang 2003, pintore et al. 2005] and more codes are still being developed [s.r. taylor and x. yang: http://www.rdss.info/library box/mrr/mrr2010/papers/08-07.pdf]. digitization techniques based on the manual selection of the points are mainly characterized by distinct methods of interpolation between the single points, to obtain a waveform that can then be resampled at a constant time interval. a crude linear interpolation would give a polygonal line that would account for the main signal features, but would also strongly alter its frequency content. on the other hand, more sophisticated interpolation methods that are based on the global properties of the sample data (such as splines or fourier-based techniques) or on the local characteristics of the signal (such as those proposed by akima [1970] or wiggings [1976]) would reproduce the signal more faithfully, although a careful check is still required to avoid undesired oscillations that are not present in the original recordings. as for automatic or semi-automatic techniques, the start and end points are usually palombo and pino 2 3 selected, and thus the single points in-between are searched according to a chosen sampling rate. this process is generally based on color analysis of the images [e.g., bromirski and chuang 2003] or even making use of neural networks [e.g., pintore et al. 2005]. overall, the definition of a best technique is not possible, as it will depend on both the features of each specific seismogram and the expectations of the operator. waveform digitization is a critical step in data processing, whether it is manual or automatic: signals generated by actual ground motion can be taken out, or conversely, inconsistent features can be introduced that were absent in the recording. such drawbacks are more frequent when the original seismogram is not very clear or is partially missing (e.g., signal thickness is too large compared either to the wave amplitude or to the distance between successive oscillations; holes in the waveform due to time marks; very fast oscillations of the needle that prevented clear writing) (figure 1). among those especially developed for seismic waveforms digitization, most codes also allow for the correction of possible distortion due the finite arm length and inclination (figure 2), with the transformation of each generic plane curve obtained from the vectorization into a single valued time function for the amplitude. in general, these corrections are made with numerical algorithms that require the estimation and/or assumption of some parameters, thereby introducing elements of subjectivity. figure 3 shows a comparison of two seismograms derived from the same original recording and processed by two different operators using the same procedures. the difference between the waveforms is evident and becomes macroscopic when a low-pass filter is applied to the seismograms. these differences can have important effects on the results. finally, when the original recordings are being transformed into digital waveforms, some caution must be exercised with the determination of the paper speed. this can be achieved by carefully checking the distance between the time marks, as historical seismographs can sometimes experience significant changes in paper speed over a few minutes. 3. the analysis of historical seismograms although there are the limitations and possible errors as described in the previous section, seismograms from this processing are analogous to the signals recorded by modern digital stations, in that they represent traces recorded by seismographs in response to ground motion at specific sites. thus, modern techniques of waveform analysis and modeling can be applied to the study of the seismic source and to the structure of the earth interior. the shape and the amplitude of a seismogram is determined by the transduction characteristics of the instrument; i.e., the way in which the chain formed by the sensitivity to ground motion, the damping apparatus, and the writing system transforms the oscillations produced by the seismic waves. in modern seismometers based on electronic technology, the instrument response is determined by the characteristics of the circuit components that make up the various parts of the instrument, and these are well known and extremely stable, and can be verified with accuracy. on this basis, the actual ground displacement can be reconstructed with negligible error. on the recovery and analysis of historical seismograms figure 1. (a) the seismogram of the june 7, 1910, irpinia (southern italy) earthquake (m = 5.8), recorded at the cheb (czech republic) seismic station using a belar zlatorog seismograph. in this optical recording, the signal is too thick compared to the amplitude and period of the first wave-trains, masking the details and making any reading very problematic. (b) the september 7, 1920, garfagnana (central italy) seismic event (m = 6.5), as recorded by the wiechert seismograph operating at uppsala (sweden): breaks in the waveform due to the time marks prevent a complete reconstruction of the actual ground motion. (c) the january 13, 1915, marsica (central italy) earthquake (m = 7) recorded at the same station. here, several segments of the waveform are missing, due to the fast oscillation of the arm (from pino [2011]). figure 2. comparison between the original seismogram (left) and the corresponding digitized waveform (right), relative to the 1908 messina straits (southern italy) earthquake (m = 7.1), as recorded by the wiechert seismograph installed in plauen (germany). the original waveform shows distortions produced by the finite length and inclination of the arm. the analytical form of a frequency response can be derived from the instrument characteristics. for example, in the case of an old mechanical instrument (such as a wiechert seismograph), this depends on the natural period of the pendulum, the damping coefficient, and the static magnification (electromagnetic seismographs, like galitzin seismographs, also have a galvanometer natural period). the instrument parameters are usually reported in the station logs or in the periodical station bulletins, and so this is why these records are extremely important: no waveform analysis is possible without knowing the characteristics of the seismograph. however, even when these bulletins are available, in some circumstances problems can arise from possible instrument instability. indeed, especially for seismographs used during the last decades of the 19th and early 20th centuries, the responses to ground motion were very unsteady, and usually the parameters were not tested very frequently. at potsdam, for instance, the natural period of the horizontal wiechert seismograph that corresponded to 14 s in november 1905 for both components, turned out to be 19.8 s and 12.5 s for the n and e components, respectively, in september 1910 [hurtig and kowalle 1988]. at the same time, in addition to the pendulum response, other unpredictable, albeit less important, factors can affect the recordings, such as defective decoupling between the two horizontal components, or friction between the stylus and the paper; in common practice, unless they evidently affect the recordings, these latter are usually neglected. the knowledge of the pendulum parameters allows the reconstruction of a theoretical analytical form of the instrument response (figure 4). however, this is only an approximation of the true response, which can be more or less accurate, and which in general remains unknown and can be very different. in particular, this is the case either at lower frequencies, where small differences in the seismogram amplitude correspond to large differences in the ground displacement, or close to the natural period of the instrument, where the response function is very sensitive to the damping constant h (figure 5). of note, the pendulum damping is usually reported in terms of either the damping ratio f (i.e., the ratio between the amplitude of two succeeding peaks, for free decaying oscillations) or the damping constant h (sometimes also called the damping ratio, representing the ratio of the actual to the critical damping). f and h are related by the equation (richter [1958], see also kanamori [1988], for this and further useful information on the instrument response of historical seismometers). sometimes, there can be confusion as to which parameter is referred to in the bulletins and station books; therefore, some caution should be used in their use. palombo and pino 4 figure 3. comparison between two digitized seismograms of the 1909 lambesc (southern france) earthquake (m = 6.0) derived from the same original recording on a wiechert instrument in hamburg (germany), but processed by different operators. clear differences in the digitized waveforms are seen, which are even more evident in the low-pass filtered waveforms, showing that data processing might have influences on the final seismograms, and thus on the analysis results. the numbers on the lower plots indicate the filter corner frequency. /exp h h1 2f r= -^ h 5 due to the uncertainties on the actual response of the seismometer described above, in general, the reconstruction of the ground motion by deconvolution of the theoretical response from the original recording is certainly not advisable. anyway, some tests of the possible effects that uncertainty in the instrument parameters can have on the results of waveform analysis should always be performed. 3.1. seismic source investigation from the analysis of historical seismograms 3.1.1. source locations locations of historical earthquakes were often plagued by poor station distributions or lack of data from existing stations (seismograms and/or bulletins) at the time of the event. also, arrival-time data can suffer from on the recovery and analysis of historical seismograms figure 4. examples of response functions for seismographs operating in europe at the beginning of the 20th century. capital and small letters indicate station codes and seismograph component, respectively. the curves represent theoretical reconstruction, based on the instrument characteristics reported in the relevant bulletins. all the instruments were wiechert seismographs, except for dbn-b (bosch-omori) and ebr (vicentini). figure 5. response functions obtained by varying the damping constant h (left) and the pendulum free period t0 (right), starting from initial values of h = 0.456, t0 = 14 s, and static magnification a = 130. as evident, small uncertainties in these parameters can produce large differences in the estimation of the true ground motion, and in turn, in the evaluation of the earthquake source characteristics. the presence of large outliers, which might derive from timing errors, erroneous phase identification, or difficulties in reading arrival times on analog, possibly smoked, paper recordings. the presence of outliers, in combination with the use of one-dimensional (1-d) velocity– depth models that can be poor approximations of the real earth structure, results in inadequately constrained locations, with very large errors. using regionalized and 3-d models can really help in reducing the uncertainty; however, outliers may still represent a major drawback. for these reasons, modern standard inversion methods for source location are usually not appropriate for old earthquakes. in recent years, lomax [2005] developed a location code (nonlinloc) that is based on the use of an equal-differential-time cost function that gives an efficient and quasi-automatic identification of outliers. the algorithm performs an iterative procedure, which uses an importance-sampling method, which then provides a final location in terms of a probability density function in 3-d space. the lomax [2005] code has already been used for the study of the sources of historical earthquakes, and it has proven to be very effective in providing well-constrained results. pino et al. [2008], for instance, applied nonlinloc to the 1930 irpinia, (southern italy) earthquake (mw = 6.6). here they obtained a reliable hypocenter using a total of 63 readings, with uncertainty comparable to the regional/ teleseismic location of modern events, (38 p and 25 s) from 42 stations. 3.1.2. earthquake magnitude magnitude is one of the fundamental parameters for the characterization of an earthquake source. this was introduced for the first time in 1935 by c. richter, to determine as quantitatively as possible the strength of an earthquake from the recorded ground motion. thus, since the first seismographic records, several decades passed before a quantitative estimate of the earthquake strength could be obtained from the information contained in the seismograms. the most common approach to earthquake magnitude determination is to consider it as a function of the logarithm of the maximum value of the a/t ratio, where a is the amplitude of true ground motion and t is the period of the corresponding oscillation; i.e., it depends on the size and period of the recording, and therefore it can be seriously affected by both a poor match between theoretical and actual frequency response functions and possible errors introduced in the waveform processing (e.g., digitization, curvature correction). in particular, a wrong estimate of a/t by a factor of 2 results in a 0.3 error in magnitude estimation. the effect of different values for damping constant h and pendulum free period t0 on the determination of the magnitude is evident from figure 5: the largest sensitivity to h is at periods around the natural period of the pendulum, as expected, while magnitudes estimated at wave periods around or larger than the instrument free period t0 (lower frequencies) can be affected by the assumption of an inappropriate value for t0. as is obvious, uncertainties in the magnitude evaluation can be statistically reduced progressively for the increasing numbers of available seismograms [bath 1981]. 3.1.3. spectral analysis as stated above, a knowledge of the reliable values for the instrument parameters is crucial for any analysis of seismograms, and sometimes the value reported by original bulletins can be inaccurate. the theoretical response function constructed from the listed constants can be tested, at least qualitatively, by performing spectral analysis of the associated seismogram. in principle, the spectrum of the pulse of the p-wave palombo and pino 6 figure 6. comparison between the spectra of seismograms (green) relative to the recordings of the 1930 irpinia earthquake at ebro (spain) (mainka horizontal seismographs) and the theoretical spectra computed using different the damping constants h, reported. small letters indicate the components (north, east). apparently, the damping values reported in the bulletins – 0.40 and 0.28, respectively for the north and the east components – appear to be slightly overestimated. 7 recorded at teleseismic distances can be considered a good approximation for the spectrum of the seismic moment rate released by the source. therefore, to assess the reliability of the seismograph theoretical response, the spectrum of the recorded p-wave pulse can be compared to theoretical spectra that are calculated for different source models and are corrected for propagation attenuation and instrument response. this procedure can also be applied to test instrument responses when only regional seismograms are available, and for magnitude (m) <7.0 to 7.5 earthquakes (and source duration < ~20 s), and even for crustal sources. indeed, at epicentral distances between 8° and 10° and 18° and 20° or beyond, the spectrum of the pwave train for crustal earthquakes is complicated by the superposition of the pl-wave, upper mantle triplications and depth phases. typical periods for pl phases are usually larger than 20 s to 30 s, and the depth phases [lay and wallace 1995, p. 375], as well as the triplication phases, add amplitude at low frequency, below both the source corner frequency and the instrument natural period for most historical seismographs. it is worth noting that for event–station couples for which the source duration and instrument natural period are distinct from each other, this kind of spectral analysis can also be a viable technique to check the tabled damping constant (figure 6), at least qualitatively. in particular, this can be useful when there is no information at all available for h. overall, while providing a good check on the instrument parameters, the spectral analysis can also provide information on the characteristics of the earthquake source, such as the seismic moment m0 and the apparent duration of the source, as demonstrated by pino et al. [2000] (figure 7). alternatively, the seismic moment and, thus, the moment magnitude, can be directly derived from the spectral amplitude, using an analytical source model. for instance, teves-costa et al. [1999] and badal et al. [2000] used this second method (by applying the brune [1970] source model) to estimate the seismic moment of the 1909 benavente (portugal) earthquake, of mw = 6.0, and of some small events (3.64 < < mw < 4.85) that occurred in spain over the period from 1923 to 1961, from the analysis of historical seismograms recorded at local and regional distances. 3.1.4. focal mechanism today, the source mechanism of modern moderate and large earthquakes is routinely obtained by automatic inversion of three-component, high-quality, digital, broadband waveforms, in real-time or quasireal-time, by matching the data with synthetic green’s on the recovery and analysis of historical seismograms figure 7. comparison between spectra of seismograms (red) relative to recordings of the 1908 messina straits earthquake and theoretical (green) ones (redrawn after pino et al. [2000]. station code (capital) and seismograph component (small) are reported on top of each plot. the seismic moment m0 obtained by scaling the synthetic spectra to fit the data is also indicated. this kind of test provides a first rough estimate of m0 (for comparison, pino et al. [2000] report m0 = 5.38 × 10 19 nm as the preferred solution, derived from body waveform inversion); but is also useful as a check for the theoretical instrument parameters. in the synthetic spectra, the peak at 0.067 hz corresponds to the inverse of the assumed source duration (15 s), while the other peak that is visible in some of the plots, is due to the instrument free period. functions (i.e., moment tensor inversions). usually, instrument response is deconvolved from the recordings and the horizontal components are rotated along the radial and transversal directions. in these analyses, the waveforms are commonly low-pass filtered (below a few hundredths of a hz, at least) to reduce the effects of minor lateral structural heterogeneities, thus allowing the adoption of 1-d models for the inner earth in computing the synthetics. in principle, these methods can be applied to historical seismograms (see, for instance, huang et al. [1994]). however, specific limitations arise from the availability and the quality of the data, including the information on the instrument response, as uncertainties in one or more of the instrument responses will seriously affect the performance of the whole inversion. thus, joint inversion at multiple stations is not always feasible or suitable. for these reasons, for older events, single-station inversions are very frequent [e.g., huang et al. 1998] and sometimes only attempts with two components and/or a portion of the seismograms can be made. in general, it would be better not to manipulate the original waveforms, thus applying to the synthetics both the correction for instrument response and the rotation of the horizontal components along the seismograph recording directions, rather than the opposite. nevertheless, several studies have obtained reliable source mechanisms also by deconvolving the response from digitized data [e.g., ichinose et al. 2003, stich et al. 2005]. a simpler method for deriving a source mechanism is the inversion of the first motion polarities, which can be effectively applied to the analysis of historical seismograms [e.g., pino et al. 2008]. in this case, the use of sh-wave polarity, albeit restricted to naturally rotated components, can greatly help in constraining the result (figure 8). depending on the relative locations of the available stations and nodal planes, on the focal sphere, sometimes a very few sh-polarity readings can be enough to constrain the solution unequivocally. although the inversion of the first motion polarities gives less accurate solutions, sometimes the application of moment tensor inversions to historical seismograms can be problematic, inappropriate or even not possible. indeed, filtering of historical waveforms at low frequencies can produce very unstable results, which seriously affect the estimated moment tensor. at the same time, the seismograms of the strongest earthquakes, especially when recorded at local or regional distances, can be clipped (i.e., the oscillations induced by the ground motion are greater than the maximum allowed by the seismograph and part of the amplitude is cut out) or truncated (i.e., the recording is stopped because the stylus went off scale and did not returned back) (figure 9). a further advantage of first motion inversion is that this method can also be applied when the pendulum characteristics are poorly or not known at all. moreover, as bulletins with indications of the firstarrival polarities are often available, solutions can be attempted for the focal mechanism even without having the original seismograms. finally, when the data available are not enough for any inversion, which might be because of number or quality, some information on the seismic source also regarding magnitude and focal depth can be obtained by visually comparing the waveforms with the recordings of recent events with similar locations of the source and receivers, and with known source characteristics [e.g., kanamori et al. 2010] or through qualitative comparisons between recorded waveforms and synthetics computed for a number of suitable sources with different characteristics [baroux et al. 2003] (figure 10). in this second circumstance, if a reliable earth model is available, the relative amplitude of the different wave trains can be very diagnostic and palombo and pino 8 figure 8. the focal mechanism obtained by pino et al. [2008] for the 1930 irpinia (southern italy) earthquake (m = 6.6), by inversion of the first motion polarity of p-waves and sh body waves. a few sh polarities might be enough to constrain the solution; however, only sh on the naturally rotated (transversal axes of stations located at cardinal directions from the source) components can be used, as artifacts are likely to occur in reconstructing sh motion from the composition of both of the horizontal components, at stations located along other directions. 9 can help in defining a preferred source mechanism. as is obvious, all of the above analyses are based on reliable knowledge of the orientation and polarity of the seismograph components, which are usually reported in the station logs or bulletins. no check is possible on the accuracy of these parameters, unless seismograms from the same station are available for an earthquake with a well-known source mechanism. when no reference seismic event can be used, tests can only be performed on the inner consistency among the various components and the seismic ray back-azimuth. 3.1.5. rupture history waveform inversion for deriving the slip distribution with time on the whole fault plane from seismograms recorded at local, regional and teleseismic distances is a well-established technique for the analysis of modern earthquakes. due to the limitations described above, the application of this method to historical recordings is very problematic. in general, when the data allow the use of inversion procedures, several constraints must be imposed to have reasonable results, otherwise solutions are likely to be affected by artifacts and/or poor resolution. several studies have been published describing the applications of these methods to historical seismograms. batlló et al. [2008] cite and give an extensive description of a large number of these studies, and therefore we will not mention them further here. more often, the quantity and/or quality of the available recordings do not permit the use of such techniques, and ad-hoc analysis should be designed. in most cases, single stations inversion is the best that can be done, with final comparisons of the results from the useable stations for derivation of reliable source images on the recovery and analysis of historical seismograms figure 9. (a) seismogram of the 1930 irpinia (southern italy) earthquake, recorded by the vicentini seismograph installed in piacenza (northern italy). the waveform is clipped for by 20 min. (b) seismogram of the same event at the firenze (central italy) omori-alfani instrument. the recording is suddenly interrupted because the large oscillation takes the needle off the scale. figure 10. source geometry estimation for the 1909 lambesc earthquake, by simple qualitative comparison of data (dashed), as recorded by the göttingen (germany) wiechert vertical and horizontal seismographs, with synthetics (continuous) computed for different focal mechanisms (after baroux et al. [2003]). in some cases, this technique can provide useful and reliable constraints on the source geometry, even when only a single station is available. (figure 11). as is obvious, in this case, no inversion can be performed to retrieve a variable slip function on the fault plane, and only an apparent source time function can be obtained. this is usually done from the inversion of body waves, and in particular, on the first-arrival pwaves. it is worth recalling that for moderate magnitude historical events, in particular, most of the available recordings are at regional distances, due to both the sparse stations locations and the prevalent concentration of observatories in some regions. under these circumstances, the role of the earth model used to compute the synthetic seismograms is crucial, and due to the heterogeneity of the upper mantle, it is very difficult to define a structural model that can reproduce the ground motion at regional distances, with the details needed for a reliable determination of the source time function, especially for moderate earthquakes. thus, the effects of the model on the results should also be checked, to avoid the incorrect interpretation of artifacts that derive from unmodeled structural features (figure 11). despite the basic nature of the technique, wellconstrained and very useful information can still be derived from the results. if the useable stations provide sufficient azimuthal coverage, for instance, the rupture length, orientation, and velocity can be estimated by an investigation of the angular variation of the apparent moment rates (figure 12), and m0 can also be evaluated by integrating the source time functions. in the case of earthquakes with only a narrow angle that are spanned by the source–receiver paths (i.e., all of the stations in a restricted area), specific analyses can still be performed to get important indications of the rupture palombo and pino 10 figure 12. example of determination of fault plane parameters from apparent duration of the source time functions. in this particular case, pino et al. [2008] inverted ~20 moment rate durations m(t) derived from seismograms of the 1930 irpinia (southern italy) earthquake, and derived rupture length, velocity, and direction using a grid search technique, coupled with a simple inversion scheme. in particular, they assumed any possible rupture direction and, for each one, computed the variance of the observed source durations with respect to the expected ones for the best fitting rupture length and velocity (a). the minimum variance corresponds to a rupture occurring for 32 km, toward n100°e, at about 2 km/s. these values provide a good fit to the data (b) and are in good agreement with other independent seismological and geological evidence, supporting the validity of the technique. figure 11. moment rate functions m(t), with each one derived from single-station inversion, from pino et al. [2000] for the 1908 messina straits earthquake. for all available stations, these authors also computed two source–time functions (shown with distinct line styles), using two different structural models. comparisons among all of the results allows the identification of the prominent source features. 11 characteristics. indeed, considering the equation for the apparent duration ta = (1/or − cosjr/ci ) [aki and richards 2002], exploiting the dependence of ta on both the velocity of the seismic waves considered (ci ; i = p,s) and the angle between the rupture propagation and the source–receiver path jr , the fracture propagation direction can be inferred, and in turn, so can the true moment rate function m(t) (figure 13). no matter what method is used to derive m(t), in some cases this function can be used to obtain an approximate picture of the slip distribution along a fault. indeed, for prevalent unilateral rupture propagation, and assuming constant fracture velocity, the source time function corresponds to the slip integral along the fault width, as a function of the rupture length. thus, following the method introduced by kanamori et al. [1992] in their analysis of the 1992 landers earthquake, a rough approximation of the along-strike slip distribution can be derived by simple algebra (figure 14). it should be noted that when applied to modern events [e.g., kanamori et al. 1992, pino and di luccio 2009], for which source models have also been obtained from inversions of high-quality huge datasets, this method has been proven to provide reliable and very useful results, albeit crude and much less resolved than modern standard inversions. the method described above relies on the determination of moment rate functions. these can be affected by the characteristics of the instruments that are used to convolve the synthetic seismograms, which are different from the actual ones. indeed, small variations in the pendulum natural period, the damping coefficient, or the paper speed can significantly distort the apparent source time functions, which will produce errors in all of the results that were derived from these moment rate functions (figure 15). in particular, while no dramatic effects are expected on the source duration, the amplitude of the moment rate that results from the deconvolution of a synthetic green’s function from the recorded waveform is sensitive to small variations in the considered parameters. it should be considered that errors in the seismic moment would map in the m0 estimate, and in turn, in the amplitude of the retrieved slip function. as shown in figure 15, deviations as large as those reported by hurtig and kowalle [1988] for the wiechert seismograph in potsdam (see section 3) can result in a ~0.2 error in magnitude and a more significant inaccuracy in the estimated slip amplitude. we note that this description is meant to provide only a general picture, as the accuracy of the results obtained from the analysis of historical seismograms should always be checked, case-by-case, using tests on the robustness of the single estimates. 4. concluding remarks for any region, quantitative investigations of earthquakes that occurred in the early stages of the seismological instrument era can provide invaluable information on the modes of stress release in this area. today, both the theoretical advances and the powerful technological and computational tools made available on the recovery and analysis of historical seismograms figure 13. although only a narrow azimuth interval was covered by the available stations for the 1908 messina straits earthquake, pino et al. [2000] suggested that useful information on the source directivity can be obtained by the comparison of the p-wave and s-wave pulses. in particular, once the p-wave apparent source–time functions were derived, they assumed unilateral propagation and computed the expected s-wave apparent source durations for the two possible rupture propagation directions, along the fault strike, oriented n-s. the synthetics computed for these two hypotheses were then compared with the data, which clearly indicated that the northward direction is highly preferred for the waveform similarities, and also for the consistency of m0 with those obtained from p-wave modeling (around 5 × 10 19 nm). palombo and pino 12 figure 14. along the strike-slip distribution for the 1908 messina straits earthquake (continuous line; the shaded area indicates the uncertainty), as derived by pino et al. [2000]. these authors applied the simple method introduced by kanamori et al. [1992] for computing the slip distribution from source–time functions. for comparison, slip profiles derived from geodetic modeling performed by boschi et al. [1989] (dashed-dotted) and de natale and pingue [1991] (dotted) are also shown. the clear similarity between the seismological and geodetic results indicated the effectiveness of the technique. in this specific case, the seismic data allowed the exclusion of the sharp source peak, indicated by one geodetic solution, corresponding to messina harbor. figure 15. inappropriate instrument characteristics affect the resulting source functions. (a) the result of figure 14 is initially assumed here and several inversions have been performed for varying free period (left), damping (middle), and paper speed (right). the effects on the shape of the moment rate function is quite evident: according to what is shown in figure 14, these would map in erroneous slip distributions, and as is obvious, (b) on m0 and mw estimates. here, circles, diamonds, and triangles correspond respectively to the left, middle, and right panels in (a). 13 by progressive advances over the last decades have allowed the reappraisal of significant past earthquakes, which can lead to the definition of important source characteristics for these events. however, due to intrinsic uncertainties, the analysis of historical seismograms presents several added possible sources of error, which derive from both the recording procedures themselves and from the waveform processing. thus, special care needs to be taken in the handling of these kind of data and also in the interpretation of the results. in the whole procedure, the reliable knowledge that the instrument parameters are as correct as possible has a crucial role. for this reason, as well as recovering the original seismograms, the retrieval of observatory bulletins and station logs is fundamental for allowing any analysis of historical seismograms. the continuous evolution of seismic instruments and the introduction of long-distance transmission of digital data have meant that the seismic observatories that were distributed in considerable numbers worldwide until a few decades ago are now progressively obsolete. within this framework, the instruments, data, and documents kept in these structures have rapidly deteriorated, and in many cases they have been destroyed or lost, together with their huge content of information. the invaluable activities being carried out by the projects mentioned above are of great value, as they are specifically aimed at the recovery, preservation, and distribution of the historical seismic data. at the same time, renewed interest in historical seismograms has giving an impulse to the development of specific techniques of processing and analysis. all of these initiatives have great scientific, and more in general, cultural significance, and they deserve increased support, also considering the possible outcomes for seismic-hazard evaluation and risk mitigation. here, we have tried to give an overview of the whole procedure, from recovering seismograms to the application of modern techniques for retrieving seismic source information, with a focus on the most crucial steps. we have also suggested possible checks for the robustness of the data and for the available instrument characteristics, with a description of the effects of various uncertainties on the results obtained. we believe this provides useful indications for the analysis of historical seismograms, and also for the correct interpretation of the resulting characteristics of the seismic source. acknowledgements. the authors thank the anonymous reviewers for their useful comments and suggestions. references aki, k., and p.g. richards (2002). quantitative seismology, 2nd edition, university science books, sausalito, california. akima, h. 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(1958). elementary seismology, freeman, san francisco, california. schweitzer, j., and w.h.k. lee (2003). old seismic bulletins to 1920: a collective heritage from early seismologists, in: p.c.j. william, h.k. lee, h. kanamori and c. kisslinger (eds), international handbook of earthquake and engineering seismology, international geophysics, part b, vol. 81, academic press, 1665-1723. stich, d., j. batlló, r. macià, p. teves-costa and j. morales (2005). moment tensor inversion with single-component historical seismograms: the 1909 benavente (portugal) and lambesc (france) earthquakes, geophys. j. int., 162, 850-858; doi: 10.1111/j.1365-246x. 2005.02680.x. teves-costa, p., j.f. borges, i. rio, r. ribeiro and c. marreiros (1999). source parameters of old earthquakes: semi-automatic digitization of analog records and seismic moment assessment, nat. hazards, 19, 205-220; doi:10.1023/a:1008140908258. wiggins, r.a. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice a p r o p o s i t o d i u n a q u e s t i o n e s u l l e o n d e m i c r o b a r i c h e c a r a t t e r i s t i c h e d e i f r o n t i f . m o s e t t i n e l l e s v a r i a t e r i c e r c h e s u l l e o n d e m i c r o h a r i c h e c h e da q u a l c h e a n n o s t i a m o c o n d u c e n d o , a b b i a m o f a t t o via via u s o di s t r u m e n t i s e m p r e p i ù p e r f e z i o n a t i . d a p p r i m a a b b i a m o a d o p e r a t o il r u d i m e n t a l e , s i a p u r e i n g e g n o s o , s t r u m e n t o i d e a t o da p a d r e all'ani, p i ù t a r d i q u e l l o del b o s s o l a s c o , e n t r a m b i a r e g i s t r a z i o n e s u c a r t a a f f u m i c a t a ; infine strum e n t i a r e g i s t r a z i o n e f o t o g r a f i c a da noi stessi c o s t r u i t i . a b b i a m o b e n p r e s t o c e s s a t o d i u s a r e il m i c r o b a r o g r a f o a l f a n i e f u m m o indotti a ciò d o p o a v e r visto c h e q u e s t o s t r u m e n t o e r a utile s o l t a n t o a l l a r e g i s t r a z i o n e di b r e v i o s c i l l a z i o n i d e l l a p r e s s i o n e (fino a q u a l c h e m i n u t o di p e r i o d o | d i p e n d e n t i m a s s i m a m e n t e d a l v e n t o . q u e s t o s t r u m e n t o p u ò v e n i r u s a t o con p r o f i t t o c o m e t u r b o g r a f o , p e r lo s t u d i o d e l l a t u r b o lenza a t m o s f e r i c a in r e l a z i o n e a v a r i e s i t u a z i o n i m e t e o r o l o g i c h e m a m a l si p r e s t a a l l a r e g i s t r a z i o n e d e l l e fluttuazioni b a r i c h e a l u n g o per i o d o . l e o n d e p i ù l u n g h e ( d i p e r i o d o d e l l ' o r d i n e d i a l c u n e d e c i n e di m i n u t i i, c h e s o n o di g r a n l u n g a le p i ù i n t e r e s s a n t i , o non v e n g o n o reg i s t r a t e s u f f i c i e n t e m e n t e a m p i e d a l l ' a l f a n i , c a u s a la s u a c o s t a n t e valv o l a r e e l e v a t a ( l ' a m p l i f i c a z i o n e , p e r i m i c r o b a r o g r a f i ("l ha un a n d a •meilto p r o p o r z i o n a l e a co | co" a2, p i ù e l e v a t o è a più p i c c o l o , a p a r i t à di p e r i o d o , risulta l ' i n g r a n d i m e n t o ) , o la l o r o r e g i s t r a z i o n e n o n è c o m o d a da s e g u i r e c a u s a il t r o p p o v e l o c e s c o r r i m e n t o del reg i s t r a t o r e . s u i n o s t r i f o t o m i c r o b a r o g r a f i è m o l t o p i ù f a c i l e e c o m o d o r i l e v a r e le o n d e l u n g h e e si p u ò c h i a r a m e n t e n o t a r e l a l o r o f o r m a e il loro c o m p o r t a m e n t o . c o s i a b b i a m o visto ( 1 ) c h e , o g n i q u a l v o l t a pass a v a s u l l a s t a z i o n e , o n e l l e v i c i n a n z e di e s s a , u n f r o n t e , f o s s e esso c a l d o , f r e d d o o d o c c l u s o , si n o t a v a l a c o m p a r s a di o n d u l a z i o n i d i p e r i o d o tra 15 min e 1 li c i r c a , i r r e g o l a r i e f r a s t a g l i a t e , di solito m o l t o a m p i e , c h e si s o v r a p p o n e v a n o al s u b s t r a t o o s c i l l a n t e ' n o r m a l e ' d o v u t o ad a l t r i f a t t o r i . t u t t e le v o l t e , cioè, c h e si n o t a , sul b o l l e t t i n o sin o t t i c o d e l l o s t a t o d e l t e m p o ( 4 ) il p a s s a g g i o di u n f r o n t e s u u n a d a t a s t a z i o n e (o t u t t e le v o l t e c h e , da q u e s t o b o l l e t t i n o , il t e m p o di p a s s a g g i o d e l l a l i n e a f r o n t a l e p u ò e s s e r d e d o t t o in b a s e a l l e s u e 4 6 4 i". m o s e t t i p o s i z i o n i p r e c e d e n t i e s e g u e n t i r i s p e t t o a l l a s t a z i o n e ) , si t r o v a n o a l m i e r o b a r o g r a f o l e c a r a t t e r i s t i c h e o n d e f r o n t a l i a l u n g o p e r i o d o . l a figura ì a è u n c h i a r o e s e m p i o di a l c u n e d i q u e s t e o s c i l l a z i o ni s c e l t e a c a s o d a l l a n o s t r a r i c c a r a c c o l t a . s i v e d e c h i a r a m e n t e s p e c i e n e l l e r e g i s t r a z i o n i d e i g i o r n i 28-29 s e t t e m b r e 1 9 5 1 e 6-7 m a r z o 1952 c o m e s i p a s s a , i n c o r r i s p o n d e n z a a l t r a n s i t o di u n a l i n e a f r o n t a l e i fig l b i , d a l l i s c i o e r e g o l a r m e n t e o n d u l a t o m i c r o b a r o g r a m m a ' n o r 1 * a —a tjf y v 2 , ? 128 e 2 9 9 5 1 * • i r <j" i" 5 9 5 2 j 1 l 6 e 7 3 5 2 p ? t 0" 1" 2" 3" 4" 5" 6' 7" 20" 21" 22" 25" 0" u! j j i i ? t 2 9 9 5 2 ^ i 23" 0" 1" 2" y 4t i 5' 6" i in" 20" 21* 22" 23" 0" 1" 2" i—trv 1 ^ 1 j | ^ 2 4 e 2 5 * 8 5 2 il * * 1 1 a l m i h i u > * ^ del fronte a — — 8 tratti interessati da oscillazioni .normali" cr0bar0grammi in due stazioni distanti a ventina 01 km i in" 20" 21* 22" 23" 0" 1" 2" i—trv 1 ^ 1 j | ^ 2 4 e 2 5 * 8 5 2 il * * 1 1 dj c-" "-0 tratto interessato 0a vento forte f i g . l a m a l e ' , a l m i c r o b a r o g r a m m a ' p e r t u r b a t o ' , i n v e r t e n d o le p r e m e s s e di ( p i a n t o a b b i a m o d e t t o p o s s i a m o a s s e r i r e c h e : ogni ipialvolta sulla traccia microbarografica si riscontra il gruppo (o i gruppi) d'onde caratteristico del fronte si ha effettivamente il passaggio di una tale discontinuità sulla stazione. r e t i di s t a z i o n i m i c r o b a r o g r a f i c h e a n n e s s e a l l e n o r m a l i s t a z i o n i m e t e o r o l o g i c h e p o t r e b b e r o c o s i r i l e v a r e , m o l t o c o m o d a m e n t e , il p a s s a g g i o d e l l a l i n e a f r o n t a l e l e . s p e s s o , a n c h e p r e v e d e r l o ) c o n i n d u b b i a u t i l i t à p r a t i c a ( 7 i ( s i ("'). m o l t o s p e s s o , s e p u r n o n s e m p r e , il p a s s a g g i o d i u n f r o n t e è acc o m p a g n a t o d a l l o s c a t e n e r s i di v i o l e n t i t e m p o r a l i , a l t r e v o l t e , i n v e c e , n o t i a m o al m i c r o b a r o g r a f o o n d e d a f r o n t e a n c h e c o n t e m p o n o n eca p r o p o s i t o di u n a q u e s t i o n e s u l l e o n d e m i c r o b a r i c h e d e i f r o n t i 4 6 5 e e s s i v a m e n t e b r u t t o t a n t o c h e , se non t r o v a s s i m o c o n f e r m a s u i bollettini s i n o t t i c i , d u b i t e r e m m o q u a s i noi stessi d e l l ' a v v e n u t o p a s s a g g i o . il t e m p o r a l e di p e r se s t e s s o , a n c h e s e n o n è c o l l e g a t o a d u n f r o n t e , p r o v o c a t u r b a m e n t i m i c r o b a r i c i a b r e v e e b r e v i s s i m o p e r i o d o ( q u a l c h e s e c o n d o e m e n o ) e a n c h e a f o r t e a m p i e z z a , c h e ne r a p p r e s e n t a n o la c a r a t t e r i s t i c a p r i n c i p a l e ( 7 ) ; tali r e g i s t r a z i o n i si o t t e n g o n o col mic r o b a r o g r a f o all'ani c h e è lo s t r u m e n t o i d e a l e p e r la p e r c e z i o n e di o n d e b r e v i . q u a n d o il t e m p o r a l e è l e g a t o a l p a s s a g g i o di u n f r o n t e , le o n d e l u n g h e c a r a t t e r i s t i c h e d i q u e s t o p a s s a g g i o si m a n i f e s t a n o anc h e s u l l o s t r u m e n t o all'ani, b e n c h é n o n c h i a r a m e n t e , p e r c h é m o l t o r i d o t t e d ' a m p i e z z a e p i ù o m e n o m a s c h e r a t e d a l l e o s c i l l a z i o n i a b r e v e p e r i o d o . q u e s t e o n d e l u n g h e , d e l r e s t o , si n o t a n o a n c h e sui n o r m a l i b a r o g r a f i . a b b i a m o i n f a t t i ( 1 0 | s c o m p o s t o i n cieli s e m p l i c i ( p r e v i o i n g r a n d i m e n t o ) a l c u n i b a r o g r a m m i o t t e n u t i d u r a n t e t e m p o r a l i e abb i a m o visto, m e s c o l a t i e m a s c h e r a t i f r a le a l t r e p e r i o d i c i t à , a l c u n i g r u p p i di o n d e f o r t e m e n t e e s a l t a t e in a m p i e z z a c h e p r e s e n t a n o i me4 é ( i f . m o s e t t i d e s i m i p e r i o d i o s s e r v a t i d i r e t t a m e n t e al m i e r o b a r o g r a f o n e l l a m e d e s i m a c i r c o s t a n z a . s e g n o , d u n q u e , c a r a t t e r i s t i c o , d e l p a s s a g g i o di u n t e m p o r a l e è l a r e g i s t r a z i o n e m i c r o b a r i c a di p a r t i c o l a r i o n d e o d i m p u l s i di b r e v i s s i m o p e r i o d o e f o r t e a m p i e z z a . t a l i o n d e d e r i v a n o d a l v e n t o p i ù o m e n o i m p e t u o s o , s o f f i a n t e q u a s i s e n z a d i r e z i o n e p r i v i l e g i a t a d u r a n t e il t e m p o r a l e , e si n o t a n o a n c h e s e n z a il c o n t e m p o r a n e o p a s s a g g i o d e l f r o n t e . q u a n d o i n v e c e p a s s a u n f r o n t e s u l l a s t a z i o n e , a n c h e s e q u e s t o n o n è a c c o m p a g n a t o d a t e m p o r a l i , v e n g o n o r e g i s t r a t e d a l m i e r o b a r o g r a f o le o n d e l u n g h e a g r u p p i i r r e g o l a r i e f r a s t a g l i a t i c h e a b b i a m o d e s c r i t t o . l e o n d e b r e v i d e l t e m p o r a l e s i m e t t o n o b e n e in e v i d e n z a c o l m i c r o b a r o g r a f o a l l a n i , a d a t t o a p p u n t o a l l a r e g i s t r a z i o n e d i b r e v i p e r i o d i c i t à ; q u e l l e l u n g h e , s e g n a l a n t i il p a s s a g g i o d e l f r o n t e , si not a n o c o n a l t r i m i c r o b a r o g r a f i p i ù s p e c i f i c i p e r l a r e g i s t r a z i o n e d i o n d e l u n g h e . d u e s t r u m e n t i m e t t o n o d u n q u e in e v i d e n z a d u e d i v e r s i a s p e t t i d e l l o s t a t o a t m o s f e r i c o . c h e p a r t i c o l a r i o n d e l u n g h e s i a n o l e g a t e s i g n i f i c a t i v a m e n t e ai f r o n t i , lo a b b i a m o d e l r e s t o m e s s o in e v i d e n z a a n c h e in a l t r a m a n i e r a . 11 p a s s a g g i o d i f r o n t i s u l m a r e i n g e n e r a , i n f a t t i , ( ) ("} m i c r o s i s m i ili p a r t i c o l a r e p e r i o d o , a m p i e z z a e f o r m a . esisti' un chimo legame l ' i di interdipendenza tra questi microsismi e le onde microbariche a lungo periodo che abbiamo definito come dipendenti dai fronti, mentre non ne esiste alcuno tra i microsismi e le oscillazioni barometriche a breve o brevissimo periodo. i n p r o p o s i t o a l l a d e s c r i z i o n e d a n o i e f f e t t u a t a l ' i di o n d e m i c r o b a r i c h e a l u n g o p e r i o d o l e g a t e a l p a s s a g g i o di f r o n t i , a b b i a m o letto u n l a v o r o ( l u ) d e l l o z a n o n , o v e l ' a u t o r e si d i c e n o n in a c c o r d o coi n o s t r i c o n c e t t i . s u l l e n o s t r e a s s e r z i o n i n o n a b b i a m o d a r i c r e d e r c i . l o z a n o n p i u t t o s t o , in a l c u n i l a v o r i p r e c e d e n t i (1:m ( 1 4 ) i n c u i t r a t t a v a d e l l ' a p p l i c a z i o n e p r a t i c a d e i m i c r o b a r o g r a f i , p u r u s a n d o gli s t r u m e n t i a l f a n i ( a s s a i p o c o s e n s i b i l i , c o m e si d i s s e , p e r le o n d e l u n g h e ) , e r a r i u s c i t o a d i n t r a v e d e r e , e d e s c r i s s e p u r e l u i , 1111 l e g a m e f r a q u e s t e o s c i l l a z i o n i a t m o s f e r i c h e a l u n g o p e r i o d o e il p a s s a g g i o di f r o n t i l o . p i ù g e n e r i c a m e n t e , c o n l a p r e s e n z a di t e m p o r a l i e di z o n e p e r t u r b a t e 1. a p p a r e c o m u n q u e c h i a r o d a q u e s t i p r i m i l a v o r i d e l l o z a n o n l ' i m p o r t a n z a c h e l ' a u t o r e d à a l l e o n d e di l u n g o p e r i o d o e la n a t u r a cont i n g e n t e e a f f a t t o c a r a t t e r i s t i c a g i u s t a m e n t e a t t r i b u i t a a l l e b r e v i o s c i l l a z i o n i . i n u n s u o u l t i m o l a v o r o ('"'i, i n v e c e , p u r t r a t t a n d o in p a r t e p r o p r i o s u g l i s t e s s i c a s i g i à p r e c e d e n t e m e n t e d e s c r i t t i , l ' a u t o r e dia p r o p o s i t o di i m q u e s t i o n e s i i . l e o n d e m i c r o b a r i c h e d e i f r o n t i 1 6 7 m e n t i c a q u a s i l ' e s i s t e n z a d e l l e i n t e r e s s a n t i s s i m e o n d e l u n g h e e si p o n e a v o l e r d i m o s t r a r e la p r e s e n z a di r e l a z i o n i t r a o n d e m i c r o b a r i c h e a b r e v i s s i m o p e r i o d o e il p a s s a g g i o di u n f r o n t e , e c i ò p e r c o n f e r m a r e a n a l o g h e a s s e r z i o n i di d u e a u t o r i a m e r i c a n i (m ( " | c h e r i c e r c a r o n o c o n m i c r o b a r o g r a f ì e l e t t r o m a g n e t i c i d e l t i p o di q u e l l o c o s t r u i t o d a l m a c e l w a n c ( 8 ) . q u e s t e c o r r e l a z i o n i t r a o n d e b r e v i e p a s s a g g i o d e l f r o n t e s o n o , a n o s t r o a v v i s o , d e l t u t t o i l l u s o r i e . e s s e n o n d i p e n d o n o d a u n a c a r a t t e r i s t i c a p r o p r i a d e l l a d i s c o n t i n u i t à m a s o l o d a l v e n t o c h e , con m a g g i o r e o m i n o r e i n t e n s i t à , l ' a c c o m p a g n a . g l i s t r u m e n t i e l e t t r o m a g n e t i c i u s a t i d a g l i a m e r i c a n i s o n o , a n c o r p i ù s t r e t t a m e n t e d e l m i c r o b a r o g r a f o a l f a n i , l e g a t i a l l a s o l a r e g i s t r a z i o n e di o n d e b r e v i e n e c e s s a r i a m e n t e q u i n d i , i p r e d e t t i a u t o r i non p o t e r o n o i n d i v i d u a r e il l e g a m e e s i s t e n t e t r a il p a s s a g g i o del f r o n t e e i v i s t o s i g r u p p i di i m p i l i s i m i c r o b a r i c i c o n p e r i o d o t r a 15 min e 1 lì c i r c a c h e ne s o n o l a c a r a t t e r i s t i c a e s s e n z i a l e . e s s i si s p i n s e r o q u i n d i in p o c o s i g n i f i c a t i v e r i c e r c h e s u l l e b r e v i o s c i l l a z i o n i d a v e n t o c h e a v o l t e a c c o m p a g n a n o il p a s s a g g i o d e l l a d i s c o n t i n u i t à m a non r a p p r e s e n t a n o a f f a t t o u n a c a r a t t e r i s t i c a d e g n a d i n o t a . n o i s i a m o c e r t i o r m a i , d o p o a v e r c o n t r o l l a t o q u a l c h e c e n t i n a i o di c a s i , d e l l e n o s t r e a s s e r z i o n i e s p o s t e in ( ' l e (pia m e g l i o d e f i n i t e . anzi, insistiamo che venga applicato con finalità pratiche (di indubbia importanza) lo studio e l'interpretazione di queste oscillazioni a lungo periodo cos'i significativamente legate al passaggio della linea frontale. i n fig. 1 a r i p o r t i a m o a l c u n i c a s i di o n d e m i c r o b a r i c h e d a i r o n i e e i n figura 1 b , p e r c o n f r o n t o , l e p o s i z i o n i d e i f r o n t i r e l a t i v e a l l a staz i o n e d i t r i e s t e a l l e o r e 1 d e l g i o r n o s e g n a t o ( 4 ) . c o n f r o n t a n d o c o n q u e s t e p o s i z i o n i il t e m p o d i r e g i s t r a z i o n e d e g l i i m p u l s i m i c r o b a r i c i q u a d e s c r i t t i , o e s t r a p o l a n d o d a q u e s t e p o s i z i o n i il p r o b a b i l e t e m p o d i p a s s a g g i o d e l f r o n t e , si p u ò o s s e r v a r e s e n z a o m b r a di e r r o r e la perf e t t a c o n c o r d a n z a c o n l e n o t r e p r e c e d e n t i a s s e r z i o n i . istituto \a:, di geofisica — osserv. geofisico di trieste — maggio 1954. riassunto poiché alcuni autori, usando apparecchiature poco idonee allo scopo, credettero di trovare un certo legame fra le onde microbariche n breve periodo e il passaggio di fronti sulla stazione registratrice, • 1 6 8 i". m o s e t t i abbiamo (/uà insistilo sul fatto che l'aspetto microbarografico caratteristico del passaggio di un fronti• è rappresentato da particolari onde a lungo periodo. summary silice some autliors, using instruments few idoneous to tlie aim, believed to find a certain connexion between microbaric waves of short periodi> and the transit of fronts over the registering station, ice have hier insisted on the fact that the microbarographic aspect of the i>assage of a front is represented from particulares waves of long periodi'. b i b l i o g r a f i a l 1 ) clark. r . i ) . m . , atmospheric microoscilations. j o u r n . m e t e o r . 7 ( 1 9 5 0 ) . ( 1 d'henry g . e morelli c . , sulle emise ilei microsismi. a n n . g e o f . , i i , 2 ( 1 9 4 9 ) . ( : ! ) m a c e l w . a n e j . l ì . e r a m i r e z e . , the electroinngnetic microbarograph and ils performance. t r a n s . a m e r . g e o p h . u n i o n . 19 ( 1 9 3 8 1 . ( 4 1 m i n i s t e r o d e l l ' a e r o n a u t i c a , bollettino quotidiano d'informazioni del servizio meteorologico delfaeronautica. r o m a 1 9 5 1 5 2 . (•"') morelli ('.., contributo allo studio dei microsismi. a n n . g e o f . i , 4 ( 1 9 1 8 ) . ( " ) mosetti f . , teoria del microbarografo alfani. a n n . g e o f . i v , 1 ( 1 9 5 1 ) . ( " ) mosetti f . , le onde microbariche. r i v . m e t e o r . , a e r o n . 12, h i ( 1 9 5 2 ) . ( s l mosetti f . , primi risultati sulla localizzazione delle perturbazioni atmosferiche in base allo studio delle onde microbariche. a n n . g e o f . , \ i , 1 ( 1 9 5 3 ) . ('•') mosetti f . , ulteriori elementi per la localizzazione delle perturbazioni atmosferiche in base alla direzione di provenienza delle onde microbariche. r i v . m e t e o r . a e r o n „ x i v , 2 ( 1 9 5 4 ) . ( ' " > mosetti f . g . , alcune componenti dell'oscillazione barometrica. ( in s t a m p a ) . ( n ) r o s e l i k f . h . , the relation between air-pressure microoscillation and concurrenl synoptic patterns. j o u r . m e t e o r . 9 ( 1 9 5 2 1 . c1'-) vercelli f . e mosetti f . , osservazioni sui rapporti tra onde microbariche. oiule microbarograficlie e microsismi. a c a d . p o n t . s c i e n t . s e m a i n e d ' é t u d e s u r l e p r o b l è m e d e s m i c r o s e i s m e s , n o v . 1951. ( 1 ; i ) zanon f . s . , pulsazioni della pressione atmosferica a periodo brevissimo. a n n . g e o f . i i , 4 ( 1 9 4 9 ) . ( 1 4 ) zanon f . s . , fronti aerologici e stalo biologico. a r d i . o s p e d . a l m a r e , i v , 1 9 5 0 . ( 1 3 ) zanon f . s . , il microbarografo alfani ed i fronti aerologici. g e o f . p u r a e a p p i . , 2 7 ( 1 9 5 4 ) . crust and upper mantle velocity structure of the northwestern indian peninsular shield from inter-station phase velocities of rayleigh and love waves annals of geophysics, 58, 2, 2015, s0215; doi:10.4401/ag-6674 s0215 crust and upper mantle velocity structure of the northwestern indian peninsular shield from inter-station phase velocities of rayleigh and love waves gaddale suresh1,*, sankar n. bhattacharya1, satbir s. teotia2 1 india meteorological department, seismology division, new delhi, india 2 kurukshetra university, department of geophysics, kurukshetra, india abstract we measure the inter-station rayleigh and love wave phase velocities across the northwestern indian peninsular shield (nw-ip) through crosscorrelation and invert these velocities to evaluate the underneath crust and upper mantle velocity structure down to 400 km. we consider a cluster of three stations in the northern tip of the peninsula and another cluster of eight stations in the south. we measure phase velocities along 28 paths for rayleigh waves and 17 paths for love waves joining two stations with one from each cluster and using broadband records of earthquakes which lie nearly on the great circle joining the pair of stations. the phase velocities are in the period range of 10 to 275 s for rayleigh waves and of 10 to 120 s for love waves. the isotropic model obtained through inversion of the phase velocities indicates 199.1 km thick lithosphere with 3layered crust of thickness 36.3 km; the top two layers have nearly same velocities and both constitute the upper crust with thickness of 12.6 km. the upper crust is mafic, whereas the lower crust is felsic. in the mantle lid, velocities increase with depth. the velocities of mantle lid beneath nw-ip is lower than those beneath south indian peninsula showing the former is hotter than the later perhaps due to large phanerozoic impact on nw-ip. the significant upper mantle low velocity zone beneath nw-ip indicates high temperature which could be attributed to the past existence of a broad plume head at the west-central part of the peninsula. 1. introduction the indian peninsula is a combination of a few precambrian cratonic nuclei and is one of the oldest archean shield regions of the world. the amalgamation of the cratons occurred at the end of the archean and all the nuclei stabilized at about 2.5–2.6 ga [meert et al. 2010]. the juxtaposition along the central indian tectonic zone (citz, figure 1) took place during the earliest paleo-proterozoic [stein et al. 2004]. radhakrishna [1989] proposed citz as an ancient suture between north and south protocontinents. the south block consists of the north dharwar craton, its northern part is covered by the deccan trap flows and heart of the deccan volcanic province (dvp), which is result of interaction between the mantle plume and the overriding continental lithosphere at ~65 ma [white and mckenzie 1989]. the north block consists of the aravalli-bundelkhand craton which constitutes the basement beneath the lesser himalayan successions, and the quaternary alluvium of the indo-gangetic plain [kailasam 1976]. to the northwest of this block, the rajasthan orogenic belt had evolution in the archean and continued to the end of phanerozoic. the north and south cratonic blocks have a dominantly proterozoic cover with properties of archean lithosphere [mahadevan 2013]. early use of surface waves to find lithospheric velocity structure beneath indian region has been reviewed by bhattacharya [1992]. however, such studies for 1d structure were based on analog data and generally of periods < 100 s. with the availability of broadband digital data, we can now prepare 2d maps of surface wave velocities; however, such maps have been prepared with period only up to of 70 s [acton et al. 2010]. inter-station phase velocity measurements using waves from distant earthquakes generate longer period surface wave velocity data and such long period data allow us to evaluate not only the lithosphere but also deep structure of the uppermost mantle; further, the measured inter-station velocities are nearly independent of the errors in epicenter and origin time determinations [mitra et al. 2006, bhattacharya et al. 2013]. many cratons over the globe are underlain by lithospheric roots which extend to depth > 250 km in contrast to the oceanic area and phanerozoic contiarticle history received september 29, 2014; accepted january 27, 2015. subject classification: cratons, surface waves, lithosphere-asthenosphere boundary, indian peninsular shield. nents [mahadevan 2013]. these deep roots may perturb the surrounding mantle and the mantle convection [thompson et al. 2011]. however, the loss of these roots beneath indian cratons might have been caused by the deccan plume [lehmann et al. 2010]. thus the topology of lithosphere-asthenosphere boundary (lab) i.e. the bottom of the mantle lid, beneath the indian peninsular shield will allow us to model its formation and evolution. further studies of upper mantle velocity structure beneath the peninsular shield using long period surface waves are limited [mitra et al. 2006, bhattacharya et al. 2009]. the topology of lab has been estimated by priestley and mckenzie [2006] by converting s-wave velocity (vs) into temperature profiles and then fitting the geotherms beneath eurasian plate. they showed lab depth is ~160 km in central india and increasing northward to ~250 km in the northern tip of the indian peninsula. in recent studies, lithosphere thickness beneath the indian peninsular shield has been estimated with variations of 70 to 140 km thick on one side using receiver function studies [kumar et al. 2007, kumar et al. 2013a]; and 150 to 200 km on the other side using rayleigh wave phase velocities for periods up to 200 s by mitra et al. [2006], gravity studies by arora et al. [2012], and receiver function analysis by bodin et al. [2014]. these results underline the need for studying the uppermost mantle structure further. so far the upper mantle structure of the northwestern part of the indian peninsular shield is poorly resolved due to limited broadband earthquake records in the northwestern part of the shield region. with the installation of a few broadband stations recently, we measure inter-station phase velocities of both love and rayleigh waves to evaluate the velocity structure of the northwestern indian peninsular shield (nw-ip) region down to 400 km. surface wave dispersion mainly depends on s-wave velocity structure, which is reliably obtained from surface wave studies. the dispersion depends least on density. left aside density, rayleigh wave dispersion depends both on p-wave velocity and s-wave velocity structure, while love wave dispersion depends on s-wave velocity structure only. thus use of both love and rayleigh waves, as has been done here, will give a reliable result on s-wave velocity structure. further, a discrepancy to fit an isotropic elastic model to both love and rayleigh wave dispersion will show a radial anisotropy, if any, in the structure. the derived results are used to study (i) crustal structure (ii) nature of mantle lid and depth of lab; and (iii) characteristics of upper mantle lvz. suresh et al. 2 figure 1. precambrian cratonic blocks and associated mobile belts in and adjoining regions of present study (figure modified from mahadevan [2013]). abbreviations: cbr, cambay rift; citz, central indian tectonic zone; dvg, damodar valley graben; cgd, chotanagpur gneissic-granitic domain; egmg, eastern ghat mobile belt; gg, godavari graben; wgb, west ganga basin; egb, east ganga basin. locations of observatories (triangles) and the inter-station wave paths used in this study are shown. 3 2. data and methodology 2.1. data we consider records of broadband seismic stations operated by the india meteorological department (imd) and the national geophysical research institute (ngri) to measure inter-station surface wave dispersion over broad period range. we consider two clusters of stations: the south cluster, consisting of 8 stations, is located in koyna-warna region in the nw dharwar craton (in the western peninsula) and the north cluster, consisting of 3 stations, is in the northern tip of the peninsula (figure 1). the information on the stations is given in table 1. for inter-station phase velocity measurement, we consider the records of two earthquakes (table 2) each of which lies nearly in the same great circle joining a pair of stations with one from each cluster. the angle between the two great circles (1) joining the two stations and (2) the longer of the two great circles joining each station and the epicenter, is considered to be less than 3° [bhattacharya et al. 2013], in order to minimize the influence of the structure between the earthquake and nearest station; this also minimizes the errors in azimuth arising from refraction, but such errors introduce only second order effects in phase velocity [yao et al. 2005]. we process 28 inter-station wave paths for rayleigh waves and 17 paths for love waves (table 3) after careful examination for good quality records consisting of least scattered waves and only those waveforms having a signal to noise ratio ≥ 2.5. we consider each wave path between one of the stations of north cluster and another of the stations of south cluster (table 3). these wave paths are close to each other and mainly cross north-dharwar craton, narrow central crust-mantle of nw indian peninsula s. no. station station code station coordinates instrumentation latitude °n longitude °e elevation (m) sensor + data logger frequency response of sensor 1 new delhi ndi 28.683 77.217 230 sts-2 + kinemetrics q330 120s 50hz 2 lodi road ldr 28.583 77.217 200 -do-do3 j.n university, new delhi jnu 28.540 77.166 210 -do-do4 karad kad 17.307 74.183 582 sts-2 + quanterra q680lvg -do5 pune poo 18.530 73.849 560 trillium-240 + taurus 240s 32hz 6 ambegar abg 17.275 73.829 835 reftek 151-120 + reftek 130b-01 120s 50hz 7 kokrud kok 17.007 73.980 555 -do-do8 furus fur 17.338 73.618 116 -do-do9 sakarpa skp 16.930 73.700 100 -do-do10 gokul gkl 17.397 73.727 762 guralp cmg-3esp + reftek 130b-01 -do11 warna war 17.123 73.897 518 -do-doearthquake number date origin time (utc) latitude longitude depth (km) magnitude 1 2012 jun 29 21:07:33.90 43.43 84.70 18 6.3 (mb) 2 2012 oct 28 03:04:08.82 52.79 -132.10 14 7.8 (mw) table 2. the hypocentral parameters of the earthquakes used (from neic, usgs). table 1. list of stations used. indian tectonic zone (citz) in the middle and then northwestern cratonic block. epicentral distances of event 1 (table 2) are around 1770 km from the northern cluster of stations and around 3070 km from southern cluster of stations. while those of event 2 (table 2) are around 10,560 km from southern cluster of stations and around 11,860 km from northern cluster of stations. 2.2. measurement of inter-station phase velocities measurement of inter-station phase velocities is based on cross-correlation method [yao et al. 2005]. a brief description of the methodology implemented through matlab is as follows: (a) recorded data are corrected for its instrument response to get true ground motion records. (b) the n-s and e-w components are rotated to obtain radial and transverse components; we use the vertical component for rayleigh wave and the transverse component for love wave. (c) for each seismogram, we obtain group arrival times (tgi = d/ui ) of a given period ti, where d is epicentral distance and ui is the group velocity at that period ti. we obtain ui through frequency time analysis (ftan: dziewonski et al. [1969], bhattacharya [1981, 1983]). now consider two stations with a pair of vertical seismograms (for rayleigh wave) and another pair of transverse seismograms (for love wave). using a given pair of seismograms, we evaluate inter-station phase velocity at period ti as described in the next three steps: (d) each seismogram in time domain is passed through cosine tapering window w(t,ti,tgi) centered at tgi, where where m > n. we have normally selected m = 4.5 and n = 4.0; we have kept a small difference between m and n to have small tapering parts on both sides of the window. this window effectively removes the noise of higher mode waves and other spurious signals. (e) each windowed seismogram is now filtered with a narrow frequency band centered at frequency 1/ti. for filtering, we use wavelet transform with morlet wavelet [wu et al. 2009]. (f ) finally we cross correlate a pair of such filtered seismogram, one from the north cluster and another from the south cluster. the correlated waveform shows maximum amplitude (crest) at a time when the phase difference is zero; this time corresponds to the phase arrival time and is used to compute phase velocity (= inter-station distance/ arrival time) of period ti. thus, computed phase velocities for various periods have been plotted as phase velocity image. on this image, at higher periods phase velocities marked by amplitude peaks (crest) are easily discernible and phase w t, ti, t giq v = 1 cos r 2 m nq vti t t gi nti # & 0 z [ \ ] ] ] ] ] ]] ] ] ] ] ] ]] , if nti 2 t t gi $ 0 if mti 2 t t gi $ nti if t t gi $ mti suresh et al. 4 e ve n t n o. 1 paths ndiabg ndifur ndigkl ndikrd ndikok ndiskp ndiwar waves used lr, lq lr, lq lr, lr, lq lr, lq lr, lq lr, lq e ve n t n o. 2 paths ndiabg ndifur ndigkl ndikad ndikok ndipoo ndiskp ndiwar ldrabg ldrfur ldrkok waves used lr, lq lr, lq lr lr, lq lr, lq lr, lq lr, lq lr lr lr lr, lq paths ldrgkl ldrkad ldrpoo ldrskp ldrwar jnuabg jnufur jnugkl jnukad jnuskp jnuwar waves used lr lr, lq lq lr, lq lr, lq lr lr lr lr lr lr table 3. station pairs used. note: lr: rayleigh waves (28 wave paths); lq: love waves (17 wave paths). (1) 5 velocities of available period range are obtained in a continuous manner. as an example, figure 2 shows a pair of vertical component of seismograms of ndi and skp from the event 2; the cross correlated plot of this pair as in the steps above is shown in figure 3. 2.3. observed data and comparison with data of the peninsular india the measured phase velocity extends for periods from 10 to 275 s for rayleigh waves and from 10 to 120 s for love waves (table 4, figure 4). we could get phase velocities of surface waves of lower period range (10 to 52 s) from event 1 and higher period range (26 to 275 s) from event 2. phase velocities of love waves are restricted up to 120 s due to insufficient signal-to-noise ratio at longer periods. love wave phase velocity with period is not so smooth (figure 4); from 70 s onward the phase velocity is nearly flat with an upward jump at 100 s. such anomalous part may not be due to noise, because we have obtained these data with many wave paths and the standard deviations at this part are small. we feel that this part is likely to be due to some lateral variation along the wave path. yoshida [2000] has demonstrated that a lateral variation of moho depth (at about 35 km) gives rise to a flat phase velocity of love wave around the period 30 s. in the present case the flat phase velocity observed between 74-100 s indicates that such a lateral variation along the wave path is occurring at a greater depth. as per the partial derivatives of phase velocities of love waves with respect to s-wave velocity [bloch et al. 1969], this period range is most affected by a layer at a depth ~200 km. a down going lab was noted by priestley and mckenzie [2006] from south to north across citz. such anomalous part of phase velocity for rayleigh wave is not discernible because effect of lateral heterogeneity is small on rayleigh waves than on love waves [lavender 1985]. we shall model the phase velocity data with a laterally homogenous structure assuming that the effect of lateral heterogeneity in a small period range may not affect the result significantly. for the bastar craton, on the eastern part of the peninsula, crust and upper mantle structure was obtained earlier through inversion of inter-station phase velocities of rayleigh and love waves [bhattacharya et al. 2009]. the theoretical curves for this model closely agree with data of love wave, but not that of rayleigh wave (figure 4). the theoretical phase velocity curves are obtained here for an isotropic elastic layered spherical earth following bhattacharya [1996, 2009]. the crustal structure of dvp was evaluated earlier [prajapati et al. 2011, suresh et al. 2014] through inversion of group velocities of rayleigh and love waves; the region lies close to the southern part of the wave paths considered here. based on this crust of nw-dvp and upper mantle crustal structure of the bastar craton, we developed an initial model for which dispersion curves are close to the present phase velocity data. we shall obtain the crust-upper mantle model through nonlinear inversion around these initial model parameters. crust-mantle of nw indian peninsula figure 2. vertical components of seismograms of ndi and skp from event 2 (table 2). the start time of both the waveform is 03h 04m 08.82s. the phase velocities obtained through cross correlation of this pair of seismograms are shown in figure 3. figure 3. an example of rayleigh wave phase velocity measurement between ndi and skp using the vertical records (figure 2) from the event 2 (table 2). white and black areas show positive and negative amplitude part in the cross correlated waveform at each period. a peak (crest) is shown by a symbol, which is properly chosen as phase velocity. 3. inversion and results we perform a nonlinear iterative search of a model using genetic algorithm (ga) [suresh et al. 2008]. in ga, we obtain a model minimizing the misfit between observed and theoretical phase velocities. we define misfit as { = max(misfitr, misfitl) where misfitr =(ri di r)/nr and misfitl = (rj dj l)/nl and di r is the absolute difference between the observed and suresh et al. 6 period (s) rayleigh wave love wave c (km/s) sd (km/s) no. of data c (km/s) sd (km/s) no. of data 10.00 3.480 0.069 7 3.691 0.072 6 11.48 3.529 0.063 7 3.738 0.069 6 13.18 3.594 0.057 7 3.785 0.042 6 15.14 3.643 0.051 7 3.844 0.017 6 17.38 3.711 0.056 7 3.916 0.014 6 19.95 3.768 0.060 7 3.965 0.016 6 22.91 3.807 0.088 7 4.013 0.016 6 26.30 3.853 0.084 10 4.084 0.012 6 30.20 3.932 0.071 13 4.255 0.061 16 36.47 3.985 0.069 15 4.299 0.048 16 39.81 4.012 0.035 24 4.330 0.030 16 45.71 4.041 0.016 23 4.433 0.035 17 52.48 4.057 0.024 23 4.451 0.029 17 60.26 4.066 0.011 21 4.466 0.031 17 69.18 4.088 0.008 21 4.481 0.036 16 79.43 4.094 0.009 21 4.506 0.043 13 91.20 4.132 0.023 21 4.617 0.063 12 104.71 4.186 0.030 21 4.730 0.061 12 120.23 4.188 0.034 21 4.773 0.065 11 138.04 4.206 0.046 20 158.49 4.260 0.052 19 181.97 4.352 0.068 19 208.93 4.429 0.089 19 239.88 4.534 0.117 19 275.42 4.615 0.168 18 figure 4 (left). observed mean phase velocities of rayleigh and love waves are shown by small circles and triangles; vertical grey lines through these symbols show ± one standard deviation. the theoretical curves for crust-upper mantle of bastar craton (ip11ba, bhattacharya et al. [2009]) and those of north indian peninsula (nw-ip) (present study) are shown. the theoretical curves of love waves for ip11ba and nw-ip (present study) are close to each other. table 4. mean (c) and standard deviation (sd) of observed rayleigh and love wave phase velocities. 7 theoretical phase velocity of a given model at period ti (i =1, 2, …, nr) for rayleigh wave; dj l are corresponding values of period tj (j = 1, 2, …, nl) for love wave. we have used misfit as the maximum of misfitr and misfitl to give equal importance to rayleigh and love waves. in this case, in the ga solution of a model, the values of misfitr and misfitl are nearly equal. we consider a layered structure down to 400 km with 3 layered crust, below which there are 8 layers. the model parameters vs, vp/vs and thickness in each of the top 10 layers are considered as variables with possible upper and lower limits as search ranges (table 5). the ga begins with a random k models. in each successive generation, we create k new models through crossover and mutation as well as through a few (e.g. 2) elite models, which replace the worst models of current generation of models with the best individuals of previous generation. after l number of generations, we retain the best solution (model). with 30 variables (table 5), we consider k = 600 models during each generation and best solution is obtained after 200 generations. in each ga operation, we get best solution for 30 variables. ga operation is repeated 24 times and mean of the 24 best solutions is considered as the final accepted solution (table 5). the theoretical curves for the accepted model are in figure 4; the misfit { for this model is 0.0288. the model parameters along with standard deviations of the accepted model are given in table 5. the accepted model shows that lab is the top of the 7th layer. from each of the 24 solutions, we have obtained the depth of lab as 199.1 ± 9.6 km. it is well known that rayleigh wave dispersion largely depends on vs compared to vp and love wave dispersion is independent of vp. the dependence of rayleigh wave phase velocity to vs and vp at different depths of the accepted model is shown by relative sensitivity in figures 5 and 6. the relative sensitivity of phase velocity of love waves to vs is shown in figure 7. the relative sensitivity of love wave is maximum in the crust and decreases with depth. the crustal vs affects phase velocity of rayleigh waves of period below ~30 s; above this period, rayleigh wave sensitivity to vs has peaks in the uppermost mantle (figure 5). the sensitivity to vp has maximum in the crust at all periods and decrease with depth (figure 6). in figures 5 and 6, we have plotted relative sensitivity to vs and vp only for 5 representative periods. we may also draw sensitivity for all the 25 periods (table 4) considered for ga inversion and get two figures each with 25 curves instead of 5 as in figures 5 and 6: one for vs and other for vp. from each figure, for a given depth, we evaluate the maximum sensitivity among all 25 curves. figure 8 shows the depth-wise variation of maximum sensitivities to vs and also to vp. in figures 5 and 6 each curve shows the relative sensitivity of a particular period. while the two curves of maximum sensitivities in figure 8 show the relative sensitivities to vp and vs in the period range 10–275 s. in this figure we also plot depthcrust-mantle of nw indian peninsula layer no. search range accepted structure thickness (km) vs (km/s) vp/vs thickness (km) vs (km/s) vp (km/s) depth of top layer (km) 1 04 08 3.0 3.4 1.7 1.85 05.9±0.6 3.335±0.031 6.155±0.058 0.0 2 05 08 3.4 3.6 1.7 1.85 06.7±0.7 3.493±0.043 6.395±0.100 05.9 3 @ 3.8 4.0 1.68 1.8 23.7±1.1 3.871±0.029 6.517±0.051 12.6 4 55 70 4.3 4.7 1.68 1.8 62.5±2.1 4.548±0.012 7.650±0.022 36.3 5 55 70 4.3 4.7 1.68 1.8 65.1±0.9 4.588±0.019 7.726±0.038 98.8 6 20 40 4.3 4.7 1.68 1.8 35.2±2.4 4.676±0.021 7.912±0.068 163.9 7 28 48 4.3 4.7 1.68 1.8 41.6±3.8 4.528±0.058 7.642±0.111 199.1 8 50 70 4.3 4.7 1.68 1.8 59.8±3.0 4.434±0.041 7.494±0.085 240.7 9 50 70 4.3 4.7 1.68 1.8 62.4±2.6 4.383±0.054 7.404±0.101 300.5 10 10 30 4.3 4.7 1.68 1.8 18.0±4.9 4.632±0.051 7.868±0.119 362.9 11 $ 4.7 1.732 19.1 4.70 8.60 380.9 12 ∞ 5.08 1.842 ∞ 5.08 9.10 400.0 table 5. the search range of the 12-layered model and the accepted structure. vp : p-wave velocity; vs : s-wave velocity. values after ‘±’ show the corresponding standard deviations. @: thickness of 3rd layer = crustal thickness sum of thickness of top two layers. search range of crustal thickness 35-40 km. crustal thickness of the accepted structure is 36.3 km. $: thickness of 11th layer = (400 sum of the thickness of top ten layers) km; depth of lab = top of 7th layer = 199.1 ± 9.6 km. wise variation of ratio of maximum sensitivity = (maximum sensitivity to vp)/ (maximum sensitivity to vs). at depths below 10 km the ratio is about 0.1. thus for rayleigh waves, nearly at all depths the sensitivity to vp is about 10 per cent of the sensitivity to vs in the period range considered here. this shows that the dependence of rayleigh wave phase velocity to vp is relatively small but not negligible. thus using the limits of vp/vs, we have inverted vp, whose determination is less reliable than that of vs. 4. discussions 4.1. the crust along the present wave paths we have considered a 3-layered crust namely shallow, upper and lower crust based on the findings for dvp [prajapati et al. 2011, suresh et al. 2014]. the vs as well as vp in the top two layers are close to each other (table 5); thus considering these top two layers as an upper crust, we find the thickness of this part of the crust as 12.6 km, while the lower crust is 23.7 km. such thin upper crust is seen in a northsouth profile crossing whole of the indian peninsula [bhattacharya 1974]. further, rao et al. [2002] obtained 13.8 km thick upper crust for central india in a grid search method through ray tracing using the data of body waves at regional distances from 1997 jabalpur earthquake occurred in central india. the southern part of the present wave paths are close to those used by prajapati et al. [2011] across nw-dvp, where upper crust is 15.2 km thick; in addition there is a top deccan trap layer of thickness 1.6 km (figure 9). here, we have ignored the thin deccan trap since the present wave suresh et al. 8 figure 5. relative sensitivity of phase velocity (c) of rayleigh waves to s-wave velocity (vs) versus depth of the accepted model (table 5) at various representative periods. the sensitivity shows partial derivatives ∂c/∂vs in a layer of 1 km centered at the given depth. figure 6. relative sensitivity of phase velocity (c) of rayleigh waves to p-wave velocity (vp) versus depth of the accepted model (table 5) at various representative periods. the sensitivity shows partial derivatives ∂c/∂vp in a layer of 1 km centered at the given depth. figure 7. relative sensitivity of phase velocity (c) of love waves to s-wave velocity (vs) versus depth of the accepted model (table 5) at various representative periods. the sensitivity shows partial derivatives ∂c/∂vs in a layer of 1 km centered at the given depth. 9 paths are partly across dvp where the deccan trap is thinner; further, the periods considered here are 10 s and above where the effect of a top thin layer may not be significant. in the upper crust of nw-ip, the vp/vs is between 1.83 and 1.85. such mafic crust was noted in adjoining west ganga basin [mitra et al. 2011]. however, at nwip, the lower crust is felsic (vp/vs ~1.68). felsic lower crust has also been seen earlier in the indian peninsular by bhattacharya ([1981], ip11, vp/vs = 1.68), singh et al. ([1999], vp/vs =1.60) and rao et al. ([2002], vp/vs = 1.70). zhou et al. [2000] explained such felsic crust in central india as possible compositional differences between archean and proterozoic crust. the lower crust has vs (= 3.87 km/s) similar to that of nw-dvp (vs = 3.89 km/s). such vs at lower crust is higher than the corresponding values in the adjoining regions like west ganga basin [mitra et al. 2011] and indus block crust [suresh et al. 2008], where the crust is thicker due to sedimentary deposits (figure 9). 4.2. the mantle lid down to 164 km, the mantle lid of nw-ip is similar to that of the bastar craton (figure 10). below this depth there is a slight increase of vs down to lab, which is at 199 km depth. such increase is consistent with other observations in cratons, where vs reach higher values with depth in the lithosphere [lebedev et al. 2009]. for south-ip (s-ip), a significant high velocity crust-mantle of nw indian peninsula figure 8. rayleigh wave maximum relative sensitivities to vs and vp at a given depth considering sensitivities of all the periods (table 4) considered in inversion. these two curves show relative sensitivities in the period range 10 to 275 s. the figure also shows depth-wise variation of ratio of maximum sensitivities = (maximum sensitivity to vp)/ (maximum sensitivity to vs). at depths below 10 km the ratio is about 0.1. figure 9. the crust and sub-crustal s-wave velocity (vs) structure of nw-ip (present study) compared with that of the surrounding regions: (1) nw-dvp: north west dvp [prajapati et al. 2011], (2) ib11: indus block [suresh et al. 2008], (3) wgb: west ganga basin [mitra et al. 2011] and (4) nw-ip (present study). the grey area around the nw-ip model shows its standard deviations in inversion. figure 10. the upper mantle s-wave velocity (vs) structure of nwip (present study) compared with those of south indian peninsula (s-ip, mitra et al. [2006]) and bastar craton in east indian peninsula (ip11ba, bhattacharya et al. [2009]). the grey area around the nwip model shows its standard deviations in inversion. was noted in mantle lid with lab at 155 km depth [mitra et al. 2006]. thus the mantle beneath s-ip is colder than that beneath nw-ip. the phanerozoic impact was feeble in s-ip compared to nw-ip making sip lighter and colder lithosphere [mahadevan 2013]. in the recent study by bodin et al. [2014] clarified that the lab at depth about 110 km obtained from s receiver functions by kumar et al. [2007] and kumar et al. [2013a] is a mid-lithospheric discontinuity and the actual lab is seen at depth between 150 and 200 km in central india. in the western side of dharwar craton, gravity data shows lab is as deep as 165 km [arora et al. 2012]. considering maximum negative vs gradient as an indicator of lab, the velocity structure of the bastar craton shows lab depth as 160 km beneath this craton [bhattacharya et al. 2009]. while in nw-ip, we find lab is at 199.1 ± 9.6 km depth. thus, the lab depth is increasing from south to north agreeing with the findings of priestley and mckenzie [2006]. in the mantle lid beneath nw-ip, vp/vs ~ 1.69. low values of vp/vs ~ 1.70 are noted in the mantle lid beneath central india [singh et al. 1999] and beneath southern africa [wang et al. 2008]. low velocity ratios were also noted in the stable cratons around the tibet plateau [pei et al. 2011]. in the mantle lid, lower aluminium content might have caused increase in clinopyroxene and orthopyroxene and decrease in garnet and thus lowering velocity ratio in the mantle lid [wang et al. 2008]. since an isotropic model satisfied both the measured rayleigh and love wave velocities, the radial anisotropy in the peninsular shield is nearly absent in the crust and uppermost mantle. 4.3. the lvz of upper mantle beneath nw-ip, the 164 km thick upper mantle lvz extend from 199 to 363 km depth (figure 10); the lowest velocity in lvz is 4.383 km/s, which is 6 per cent decrease of vs in lvz. although the lvz thickness closely agrees with other cratons but the decrease of vs in lvz is slightly larger compared to 4-5 per cent beneath other cratons [priestley and mckenzie 2006, lebedev et al. 2009]. kumar et al. [2013b] reported delays in p-to-s conversions from the 410 km discontinuity below the indian shield and suggested low shear wave speeds in the lithospheric and sub-lithospheric mantles due to higher temperatures. approximately 10 per cent reduction of vs in lvz has been noted for tanzanian craton [weeraratne et al. 2003]; such reduction indicated high temperature and the presence of melt which are due to plume head beneath tanzanian craton. for the indian peninsular shield, rao and lehmann [2011] postulated past existence of a large plume head of diameter 2000-2500 km centered at the middle of the peninsula. partial melt only decreases vs and not vp [wang et al. 2008]. in lvz beneath nw-ip, we find vs has decreased along with vp (table 5). thus we do not consider the existence of partial melt in lvz, which is hot due to past existence of a plume head. in lvz also we find vp/vs ~1.69, which is below normal value. low velocity ratio may be caused by lower aluminium content as indicated above for the mantle lid. 5. conclusions 1. the 3-layered crust is 36.3 km thick. the top two layers have velocities close to each other and these two layers constitute upper crust, which is 12.6 km thick and is mafic. however, the lower crust is felsic. 2. the theoretical curves of the isotropic elastic structure satisfy the observed phase velocities of both rayleigh and love waves. this indicates that the radial anisotropy either is absent or weak in the crust and uppermost mantle. 3. the lithosphere is 199.1 km thick and vs increases downward below the crust. however, vs in mantle lid of nw-ip is lower than s-ip. relatively low vs in mantle lid shows its high geothermal gradient. this low vs shows that the lithosphere in nw-ip is hotter than s-ip perhaps due to large phanerozoic impact on nw-ip compared to s-ip. 4. strong lvz is interpreted as positive thermal gradient due to which thermal mantle convection may not occur [eaton et al. 2009]. the high temperature in lvz is likely due to past existence of a broad plume head centered at middle of the indian peninsula. 5. the important result of crust and upper mantle structure obtained here will help to study the evolution and stability of lithosphere and to obtain upper mantle thermal convection model beneath cratons of nw-ip. acknowledgements. we are grateful to the director general of meteorology and head (seismology), imd for providing necessary facility to carry out the study and permission to publish the paper. we also acknowledge the support of director, national geophysical research institute (ngri), hyderabad for permission to utilize the data of ngri’s network in koyna region. also, many thanks to t.m. mahadevan for his insight and helpful suggestions. references acton, c.e., k. priestley, v.k. gaur and s.s. rai (2010). group velocity tomography of the indo-eurasian collision zone, j. geophys. res., 115, b12335; doi:10. 1029/2009jb007021. arora, k., v.m. tiwari, b. singh, d.c. mishra and i. grevemeyer (2012). three dimensional lithospheric structure of the western continental margin of india constrained from gravity modelling: implication for tectonic evolution, geophys. j. int., 190, 131-150; suresh et al. 10 11 doi:10.1111/j.1365-246x.2012.05506.x. bhattacharya, s.n. 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planets space, 52, 393-402. zhou, l., w.-p. chen and s. özalaybey (2000), seismic properties of the central indian shield from broadband p-sv conversions at hyderabad, b. seismol. soc. am., 90, 1295-1304. corresponding author: gaddale suresh, india meteorological department, seismology division, new delhi, india; email: gsureshimd@yahoo.com. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. suresh et al. 12 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 24mayer.qxd 621 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words switzerland – historical earthquakes – ecos – seismic hazard 1. introduction descriptions of historical earthquakes in switzerland can be divided up into three categories: i) monographs on single prominent earthquakes by one author (e.g., heusser, 1856), ii) chronological collections of earthquakes and observed phenomena (e.g., volger 1857-1858), iii) catalogues with source parameters deduced from the descriptive information (e.g., mecos, 1999). the third category, especially after 1975, is almost exclusively prepared for input to seismic hazard assessment. their contents and quality are therefore specially geared to the required input and the desired output of such projects. certain events in history have often initiated or intensified the investigation of historical earthquakes in switzerland. such an effect certainly occurred e.g. after the destructive lisbon and valais earthquakes in 1755, the establishment of the swiss earthquake commission (sec) in 1878, or even by demand of the regulatory body for improved seismic hazard assessments, as in 1974 and 1998 for nuclear installations. the introduction of forel’s intensity scale in 1880, or the installation and continuous operation of the first seismographs between 1913-1922 and the introduction of efficient computer-based networks and data evaluation from 1970 also significantly accelerated the investigation of earthquakes. the foundation of the swiss earthquake commission in 1878 caused probably the most remarkable progress in the history of earthon historical earthquakes in switzerland: summary of compilations and investigations dieter mayer-rosa (1) and gabriela schwarz-zanetti (2) (1) schwandenholz 260, ch-8046 zürich, switzerland (2) swiss seismological service, eth-honggerberg, zürich, switzerland abstract studies of historical earthquakes in switzerland are contained in monographs, chronological collections of effects and parametric catalogues. the systematic collection of macroseismic material started with the creation of the swiss seismological commission in 1878. all parametric catalogues since 1975 have been prepared for seismic hazard assessment. the most up-to-date investigation of macroseismic data and compilation into a catalogue (ecos) was made in the 2002 in context of the re-assessment of seismic hazard for nuclear sites. mailing address: dr. dieter mayer-rosa, schwandenholz 260, ch-8046 zürich, switzerland; e-mail: dmrosa1@bluewin.ch «it is the investigation of the properties and forces of nature, / which was all the time one of the most pleasant and useful efforts». j.j. scheuchzer, historia naturalis (1716) 622 dieter mayer-rosa and gabriela schwarz-zanetti quake investigation in switzerland (früh, 1911). the sec was active until 1914, when its duties officially were taken over by the swiss meteorological central institute (schweiz. meteorologische anstalt), and funded by the swiss government. this transition also marks the onset of instrumental observations, another landmark in the history of seismology in switzerland. for completeness it should be mentioned here that the earthquake service was transferred again in 1956 to the swiss institute of technology (eth-zürich). on the other hand, the investigation of historical earthquakes as such was never regarded as a prime task in swiss geosciences after ww ii. it was usually left to individuals with a strong motivation to work on this «side track of real science». this summary concentrates on the enumeration of significant works completed in different epochs in switzerland and, if appropriate, describes the connected projects. historical aspects and methods are also briefly touched upon. 2. the pre-1878 period lycosthenes (1557) wrote one of the earliest chronicles on earthquakes, mixed with all sorts of other noteworthy observations in nature, as was usual at that time. scheuchzer (1716-1718), the famous swiss scientist (medical doctor and mathematician) published his books on historical earthquakes, which are «felt in switzerland from time to time» in 1706 and following years. he also identified active zones in northern switzerland, e.g. the border area to germany, the region of basel and the canton glarus. a rich compilation of historical earthquakes is found in the books of bertrand (1756, 1757) with special focus on the 1755 visp (valais) earthquake. fig. 1. realistic drawing of an unknown artist from a typical building (mazot) near stalden/vs, damaged by the earthquake of 25 july 1855 (in: heusser, 1856). 623 on historical earthquakes in switzerland: summary of compilations and investigations systematically collected and archived with rigorous standards. from 1881 the commission published 33 annual bulletins (jahresberichte, 1881-1914) until 1914 within the annals of the swiss meteorological central agency (sma) in zürich. the public was requested to send earthquake observations on a form with prepared questions in german, french and italian. soon after the establishment of the commission, forel created his 10-graded macroseismic scale and in 1882, the updated «swiss-italian» rossi-forel intensity scale followed. it became very popular among seismologists and the media and was the preferred scale until 1964, when the msk-64 scale was introduced. it is noteworthy that even 20 years later candreia (1905), a librarian, totally ignored the new macroseismic scales in his chronology of earthquakes between 823 and 1897 in the canton graubünden. on the other hand, he already used a historicocritical investigation of sources, at least for a part of his work. the period 1856-1879, just before the creation of the commission, was covered by wanner (1932) in his earthquake catalogue for switzerland. the swiss seismological service was finally created in 1913 within the sma, and the idea of installing a new seismograph with 21 tons mass was realized in 1922 in zürich (station zur). this instrument, in conjunction with other similar instruments in neuchâtel and basel, initiated another new era of seismology in switzerland. the systematic collection of macroseismic data did not come to a halt; it was now seen as complementary to regular and continuous instrumental recordings. in 1956, the swiss seismological service was integrated in the new geophysical institute at the federal institute of technology and issued annual seismological bulletins from 1913 to 1964 with instrumental and macroseismic data (sed, 1913-1964). the isoseismal maps published in the bulletins served as the main databases for the compilation of catalogues, since the original macroseismic data have not been preserved. the strongest swiss earthquake in this century occurred on 25th january 1946 near the heusser (1856) published the first objective description on the visp 1855 earthquake. he also included personal observations in the epicentral area together with four realistic drawings of earthquake scenes, created on the spot by an unknown artist (fig. 1). these drawings can be seen as direct precursors of photographic pictures taken from the second half of the 19th century onwards. they are certainly realistic and of interest with respect to the question of whether artistic depictions can help to evaluate the severity of historical events. volger (1857-1858) produced an impressive compilation on historical earthquakes for switzerland between 800 and 1855. the second volume of three contains a large collection of felt earthquakes with detailed descriptions of locations with observed phenomena and – most important – with references to many documents used. this must be considered the first compilation in switzerland, which met to a certain extent the historical-scientific requirements on completeness and objectivity a milestone for its time. however, he made no attempt to classify the observations according to any scale. in the third part of this work volger describes in detail the effects of the prominent earthquake activity of 1855 in the visp valley in the canton of valais. he used statistical figures to describe these earthquakes, their geographical distribution and correlation with meteorological observations. for the cartographic presentation of the macroseismic observations of the strongest four shocks in 1855 he developed his own 7-graded scale, which was never used after that by anybody else. another noteworthy manuscript of mayer (ca. 1860) lists earthquakes between 1350 and 1859 in chronological order. 3. the 1878-1975 period the swiss earthquake commission, founded in 1878, was the first commission of this kind in the world, before italy in 1879 and japan in 1880 followed with similar organisations. this fact probably caused the most drastic change in the style of earthquake investigation in switzerland in this century. macroseismic data and related documents hereafter were 624 dieter mayer-rosa and gabriela schwarz-zanetti city of sion (valais), about 50 km west of the visp 1855 activity. it was originally assigned intensity ix (rf-scale) and caused about 7 mio fr damage (corresponding to io (ems) = viii and about 80 mio today’s value). more than 500 aftershocks followed the two main shocks on 25 and 30 january. unfortunately all of the original macroseismic data were lost and only generalized isoseismal maps survived. however, in a re-evaluation of this event many of the macroseismic data points in and around switzerland could be reconstructed (schibler, 2000). an illustrative map of earthquake centres between 1295 and 1971 was created by pavoni (1977), depicting felt and damaging earthquakes using a simplified scheme of 4 categories for the observed effects. 4. recent catalogue studies after 1975 a comprehensive earthquake catalogue (scat, 1975) covering the time period between 1300 and 1974 was compiled for the first probabilistic seismic hazard maps of switzerland (saegesser and mayer-rosa, 1978). most earthquake parameters of this catalogue are based on the evaluation of existing descriptive compilations listed in chapters 3 and 4, and complemented by the catalogues of montandon (1942), sieberg (1940), sponheuer (1952) and karnik (1969-1971). it can be regarded as up-to-date in terms of completeness and macroseismic parameters (msk-64) for that time. besides the evaluation of the very heterogeneous material, considerable efforts had to be invested in the conversion of the different scales used in these publications into the msk-64 scale (medvedev et al., 1965). it should be mentioned that the original intensity assessment of montandon was found to be systematically too high by 1-2 degrees in terms of msk-64/ems-98 figures. for the first time also a unified uncertainty scheme for all parameters was introduced in the scat (1975) catalogue. the works of sieberg and sponheuer have been mainly compiled for earthquakes in germany, which nevertheless was of importance for events near the border having effects in switzerland. 4.1. the mecos earthquake catalogue in the early 1990s a revision of the scat (1975) catalogue was undertaken. for this purpose about 6000 macroseismic observations have been associated with uncertainty figures, to be used in the construction of attenuation relationships for a new probabilistic seismic hazard study (rüttener, 1995). in particular, the use of discrete probability distributions to characterise uncertainties was regarded as an important step compared to the former use of normal error distributions. half-grade intensity values were also eliminated in this way. source evaluation procedures developed in the ec project rhise were used for the strongest events (albini et al., 1989). the internet catalogue of 1997, named mecos (macroseismic earthquake catalogue of switzerland), represented a part of the rüttener catalogue containing all earthquakes with intensity v and higher. epicentral (msk-64) intensity was used as main source parameter for the earthquake size. no attempt was made to convert intensity into a homogeneous magnitude m of some kind. however, for internal use, a simplified conversion of the form m = 0.5 io + 1.5 was taken when (approximated) magnitude figures of historical events were requested. this corresponds to the well-known relationship by karnik (1969-1971) for europe and 10 km average depth of focus. a joint seismic hazard study for the german speaking countries germany, austria and switzerland (dach-project) was carried out between 1996 and 1998 (grünthal and mayerrosa, 1998). for this study a homogenisation of national earthquake catalogues was required, especially for major earthquakes in the border areas between these countries. the hazard map is presented in terms of (ems-98) intensities for 475 years average recurrence time. the common earthquake catalogue includes many events which had to be re-assessed in terms of intensity and location. the geoforschungszentrum in potsdam/germany hosts the dach catalogue. 4.2. the ecos earthquake catalogue in the context of a new hazard assessment for nuclear sites (pegasos) and new seismic 625 on historical earthquakes in switzerland: summary of compilations and investigations the eth group, starting in eclogae geologicae helvetiae (vol. 3, 2003), will cover the investigations of the 1021, 1295, 1584, 1601, 1755, 1796, 1855 and 1946 events. figure 2 shows for reference purposes the largest earthquakes (io ≥ vii or mw ≥ 5) of the catalogue. this map is also representative for the catalogues mentioned in this section, with exemption of the event in 1524, which was added only recently. it is interesting to note that the 1117 verona earthquake had damaging effects in switzerland. hazard maps for zoning purposes (swiss building codes 261), the swiss seismological service undertook a substantial historico-critical and seismological revision of the total macroseismic database. a new earthquake catalogue (ecos, 2002a) was established by fusion of all historical and instrumental catalogues relevant for the larger swiss region. the revision took place in several parts: i) the early period up to 1878, ii) the period 1879 -1963 (wössner 2001) and iii) from 1964 up to today. a number of publications by fig. 2. epicenter map of swiss earthquakes 1200-2000 with io ≥ vii (ems-98) or mw ≥ 5, including bordering areas. 626 dieter mayer-rosa and gabriela schwarz-zanetti the new ecos catalogue includes i) events located within or outside switzerland, which produced significant effects (i ≥ v) in switzerland. they have been substantially re-evaluated with respect to size and macroseismic effects. ii) earthquakes (io ≤ vi) located within a distance of 200-300 km to the swiss borders, for which the original parameters have been adopted from the original catalogues, iii) earthquakes (io ≤ vi) inside switzerland, in general based on the mecos catalogue. prehistoric times could also be considered, thanks to several paleoseismic studies carried out in the northern part of switzerland (becker et al., 2000; meghraoui et al., 2001). for the definition of the quality and reliability of the macroseismic information, the standards of the european beecd project (albini et al., 2000) have been followed where considered appropriate. also a root classification procedure, introduced in the beecd project (albini and stucchi, 1997), was used in this study for certain events. all intensity points were assessed in – or transformed into – ems-98 values. finally, and most important, a homogeneous magnitude (mw) throughout the catalogue was introduced. both procedures are described in the downloadable ecos report (2002b). another finding during the preparation of ecos was that the generic uncertainties in location are generally larger than previously anticipated. for events with intensity smaller than v this uncertainty was set to 5-10 km before 1878, 10-20 km before 1750 and 20-50 km before 1650. the uncertainties for larger events have been assessed individually from the distribution of maximum observed intensity points. 5. historical aspects for ecos, earlier compilations served as guidelines for intensive searches in libraries and archives for primary (eyewitness) sources such as annals, chronicles written by clergy and laymen, private and official documents, newspapers, journals etc. for assessing the reliability of sources, the historico-critical method developed in the 19th century (e.g., droysen 1857) was employed by the historical research group at the eth zürich, referred to as external and internal criticism. the external criticism defines the author and his background, the place of observation, the date of the event and the context of the writing. this allows qualifying the source as: a – eyewitness; b – very close to the event; c – uncertain document; f – fake. the internal criticism focuses on the description itself. also aspects of mentality and writing skills are considered here. crosschecking between different (i.e. at least three) sources was intended for deriving reliable statements, but was not always achievable in switzerland for the earlier centuries. the goal was to obtain a provable historical minimumstatement. one condition for declaring an io ≥ vi earthquake was the explicit mentioning damage in the primary documents. otherwise, the contemporary use of the language and religious or scientific/political intentions could lead to considerable misinterpretation. in early and high middle ages the historical information on earthquakes is generally very poor («an earthquake happened on the day of ...»). the first confirmed event in the new ecos catalogue is one in 849, besides of a rockslide (possible earthquake) in the wallis in 563. the first event with mentioned damage appeared in 1117 (verona!). at the end of the 13th century a few annals gave better descriptions, as for example the annales colmarienses for the event of 1295 in churwalden (graubünden), which – in previous studies always regarded a questionable event – could definitively be located at this place. on the other hand the event 1021 of basel was confirmed to be a real fake. in the 14th century, the first town-chronicles appeared in switzerland giving general descriptions of natural phenomena, probably motivated by a series of disasters in the middle of this century, e.g., the 1348 «villach» earthquake. the famous 1356 basel event (mayerrosa and cadiot, 1979; wechsler, 1987) is hence described in several chronicles, letters and other documents (e.g., das rote buch von basel, 1357; die verweser des bistums von konstanz, 1356; senn von münsingen, 1362; closener, 1362; mueller 1380). for the region 627 on historical earthquakes in switzerland: summary of compilations and investigations of basel the oldest known diary with nature observations started in 1399 (anonymous, 1399). it contains only two earthquakes besides weather and astronomical observations. in the middle of the 15th century a gap in information exists, probably because of different local war-activities during this time. in the 16th and 17th centuries the frequency and quality of information in chronicles, diaries, official registers and in scientific and religious documents increased substantially. but even the most important events of 1584 (11 march, aigle/vs, mw = 6.4, io = vii) and 1601 (18 september, unterwalden, mw = 6.3, io = = vii) in this period still show large geographical gaps of the reported places. a damaging earthquake in 1524 (april, ardon/vs, mw = 6.4, io = viii) was only recently found, probably because it was described only by one (reliable) witness. this also shows the poor database for this time period. in the first half of the 18th century science in switzerland was progressing significantly. many handwritten documents on several topics of natural phenomena such as earthquakes were found. the 1755 visp earthquake in the valais attracted particular attention among reporters because of the lisbon earthquake only one month before. in the second half of the century scientists apparently lost interest in earthquakes. for this time only private documents such as journals, letters, etc. are available. the production of newspapers is only starting in the last part of this period. appearing in about 1780 a monthly newspaper (neue zürcher zeitung) frequently reported on earthquakes in northern switzerland. in the 19th century the production of newspapers increased enormously and most information on earthquakes for ecos is taken from this kind of documents, but also from scientific papers e.g., published by the naturforschende gesellschaft zürich (published from 1746). for the 20th century three different types of documents were available: newspapers, the annual bulletins of the swiss earthquake commission (from 1879) and different official questionnaires (meldekarten) of the sed (since 1964). existing macroseismic data-files could be crosschecked with foreign databases, especially with respect to major near border earthquakes. the completeness of events over the centuries is by no means satisfactory (table i), which is true also for the neighbouring countries. it is found that the political situation often is regarded as more important for a chronicler than anyway relatively rare earthquakes (i.e. the french and swiss revolution at the end of the 18th century). on the other hand, there were periods of time, when interest in science emerged again, e.g., 1510-1525 or 1650-1750, and for which scientists and chroniclers left rich material. in general, earthquakes were not in a broad focus (for different reasons) in the second half of the 18th century and therefore have been reported rather seldom. besides assessing the completeness of a data set by relatively standardized statistical methods, it is also interesting to illuminate this aspect table i. estimated «completeness» of major earthquakes (intensity ≥ vii) in the new ecos 2002 catalogue from the historian’s point of view. century completeness background situation and comments early and high middle ages 5-20% eyewitness reports are only found in few annals of monasteries in ne switzerland. 12th and 13th 5-25% interest in science increased in general, first two earthquakes with damage descriptions 14th and 15th 25-50% basel 1356-event found widespread public attention; sensitivity of the observers increased in the towns; improved education; paper mills and printing introduced 16th and 17th 50-75% lack of sources for mountain areas (e.g., graubünden, central switzerland, ticino and valais), where reliable sources appear only slowly study was to obtain for the first time homogeneous and comparable output parameters for all nuclear sites in switzerland. with the availability of instrumental data and also the use of sophisticated computer codes in 1995 an update of seismic hazard assessment for 10 selected cities in switzerland could be made (rüttener, 1995). the main point in this study was the explicit implementation of uncertainties into probabilistic hazard calculations. the principal output was defined by seismic intensity as primary ground motion. for the re-evaluation of macroseismic data in the respective catalogue (including the production of attenuation relationships for strong earthquakes) about 1 personal year was scheduled. this study showed unexpected large uncertainties in the resulting ground motions, especially for recurrence times larger than 1000 years. it also demonstrated the importance of a careful evaluation of the uncertainties in the basic catalogue data and hence the importance of an intensified and refined investigation of historical earthquake material in general. the hitherto most sophisticated seismic hazard assessment project (pegasos) was started in 2000, initiated by the swiss agency for the safety of nuclear installations (hsk). the spectral ground motion accelerations have to be assessed for much larger return periods (≥ 1mio years) than considered previously. this required a new effort in historical earthquake investigation, which was performed by a specialized group of historians and seismologists at the swiss seismological service (ecos, 2002b). numerous groups and individuals have completed a number of most sophisticated studies of historical earthquakes in europe in the past two decades. they have brought this field to a very high scientific standard. however, a deficit in some points may still be recognized: – the full harmonization of historical earthquake data and catalogues across political borders in europe is not yet achieved. – the common assessment of intensities through internationally coordinated field observations is still in its infancies, certainly on european level. – the proper implementation and practical use of common intensity scales (ems-98) must be enhanced. 628 dieter mayer-rosa and gabriela schwarz-zanetti from a historian’s point of view. however, this method is much more complex, since local historical aspects make this assessment very uncertain for the whole country. the historians of the ecos group do, therefore, not feel very comfortable with the generalized table i. very helpful in the evaluation of sources were the comments of alexandre (1990), who examined critically the printed primary sources of central european earthquakes, which are believed to be as complete as possible for the time covered between 394-1250. a special problem was the association of identical events in documents of different cantons, due to the varying calendars used in parallel in switzerland between the 16th and 19th century. some catholic cantons had already adopted in 1584 the gregorian change of 1582, others followed in 1587 (unterwalden) or 1655 (valais). the protestant cantons changed much later between 1701 and 1724 and the last one in 1812 (graubünden). to make it even more complicated for the analyst, private sources changed at will and sometimes without any indication. this obviously led to duplications in earlier catalogues, which could be eliminated in ecos. 6. conclusions the investigations of historical earthquakes in switzerland in the past 25 years were mainly triggered and sponsored by seismic hazard projects. among them are the requests of regulatory or government bodies to assess the hazard and risk for nuclear installations, hydroelectric plants and zoning in connection with engineering building codes. the expected output of such projects has set the main guidelines for the composition of parameters in the catalogues. seismic intensity was the primary ground motion mapping parameter for the 1978 seismic hazard project (saegesser and mayer-rosa, 1978). therefore the main efforts have been spent to produce reliable intensity values in the catalogues and all attenuation relationships were elaborated in this way. for the compilation of the first comprehensive catalogue for the 1978 hazard study only 6 personal months could be invested. the main goal of this hazard 629 on historical earthquakes in switzerland: summary of compilations and investigations torique et diplomatique de la ville de strasbourg, t.1, 155-157, in w. wackernagel, 1856. das rote buch von basel (1856) (the red book of basel) (1357). handwriting of the town archive of basel, in w. wackernagel, 1856. die verweser des bistums von konstanz (1356): document at the state archives in basel, in w. wackernagel, 1856. droysen, j.g. (1857): historik. rekonstruktion der ersten vollständigen fassung der vorlesungen, edited by p. leyh, 1977, stuttgart. ecos (2002a): earthquake catalogue of switzerland (on line: http://histserver.ethz.ch). ecos (2002b): ecos report to pegasos, version 31 march 2002 (sed, zürich), (on line: http://histserver. ethz.ch/download/ecos.pdf). früh, j. (1911): ueber die 30-jährige tätigkeit der schweiz. erdbebenkommission. verh. der schweiz. naturforschenden gesellschaft in solothurn 1911 (genève, soc. gén. d’ imprimerie), pp. 24. grünthal, g. and d. mayer-rosa (1998): einheitliche erdbebengefährdungskarte für deutschland, österreich und die schweiz, report, schweizerischer pool für erdbebendeckung, bern. heusser, chr. (1856): das erdbeben im vispertal vom jahr 1855, orell füssli, zürich. jahresberichte der schweiz. erdbebenkommission (18811914): archives of the swiss seismological service (eth-zürich). karnik, v. (1969-1971): seismicity of the european area. reidel, part i (d. reidel pubbl. comp., dordrecht, holland), pp. 220; part ii (d. reidel pubbl. comp., dordrecht, holland), pp. 218. lycosthenes (wolfharth), c. (1557): prodigiorum ac ostentorum chronicon, xiii, basel, pp. 672. mayer, d. (ca.1860): verzeichnis von erdbeben und vulkanischen ausbrüchen besonders mit bezug auf die schweiz, ms., vadiana, st. gallen. mayer-rosa, d. and b. cadiot (1979): a review of the 1356 basel earthquake, tectonophysics, 53, 325-333. mecos (1999): macroseismic earthquake catalogue of switzerland (on line: http://seismo.ethz.ch/products/catalogs/). medvedev, s.v., w. sponheuer and v. karnik (1965): seismic intensity scale version msk 1964, akad. nauk sssr, geofiz. com. (moscow), pp. 10. meghraoui, m., b. delouis, m. ferry, d. giardini, p. huggenberger, i. spottke, and m. granet (2001), active normal faulting in the rhine graben and paleoseismic identification of the 1356, io = ix-x, basle earthquake (central europe), science, 293 (5537), 2070-2073. montandon, f. (1942): les séismes de forte intensité en suisse, rev. pour l’etude des calamités, geneve, fascicules 18-21. mueller, e. (1380): yearbook of the town of zürich, in l. ettmueller, zürich, 1844, p. 88; in w. wackernagel, 1856. pavoni, n. (1977): erdbeben im gebiet der schweiz, eclog. geol. helv., 70 (2), 351-370. rüttener, e. (1995): earthquake hazard evaluation for switzerland, materiaux pour la géologie de la suisse, geophysique, 29 (commission suisse de géophysique, zürich). saegesser, r. and d. mayer-rosa (1978): erdbebenge– the unification of methods and procedures used in historical investigations in europe should be enforced. the investigation of historical earthquakes deserves to be put on a new level of intensified cooperation in europe, starting with the joint collection of damage data just after an earthquake up to the final consensus how to use such data for the benefit of the public. the european seismological commission provides one very suitable platform for these objectives. acknowledgements the authors appreciate the fruitful discussions with the other members of the historical earthquake group of the swiss seismological service at the eth zürich, and also the permission to include some of their important findings in this summary. the helpful comments of two reviewers are gratefully acknowledged. references albini, p. and m. stucchi (1997): a basic european earthquake catalogue and a database for the evaluation of long-term seismicity hazard (beecd), in seismic risk in the european union, edited by a. ghazi and m. yeroyanni (brussel-luxembourg), vol. 1, 53-77. albini, p., e. chignola, m. ferrari and m. stucchi (1989): revision of the seismicity of north-western alps in the first half of the xviii century. the 1755, december 9, valais event, progress report, pp. 165. albini, p., c. mirto, m. stucchi and r. tatevossian (2000): contribution to the improvement of the supporting dataset of the most significant earthquakes (1000-1899) for the swiss territory, rapporto tecnico irrs-cnr, milano, pp. 64. alexandre, p. (1990): les séismes en europe occidentale de 394 à 1259. nouveau catalogue critique, in sér. géophys., observ. r. belgique (bruxelles), pp. 267. anonymous (1399): library of the university of basel, e. vi. 26. becker, a., c.a. davenport, w. haeberli, c. burga, r. perret, a. flisch and w.a. keller (2000): the fulnau landslide and former lake seewen in the northern swiss jura mountains, eclog. geol. helv., 93, 291-305. bertrand, m.e. (1756): mémoire sur les tremblements de terre, p.a. chenebie, vevey, pp. 218. bertrand, m.e. (1757): mémoires historiques et physiques sur les tremblements de terre, la haye, pp. 328. candreia, j. (1905): zur chronik der erdbeben in graubünden bis zum jahre 1879, wyss, bern, pp. 120. closener, f. (1362): chronicles of strasbourg: code his630 dieter mayer-rosa and gabriela schwarz-zanetti fährdung in der schweiz, schweizerische bauzeitung, heft 7, pp. 16. scat (1975): swiss earthquake catalogue, swiss seismological service (eth, zürich), computer file. scheuchzer, j.j. (1716-1718): historische beschreibung aller erdbidmen, welche in dem schweizerlande von zeit zu zeit gespürt worden, in beschreibung der natur-historie des schweizerlandes, 3 parts. schibler, r. (2000): makroseismische untersuchung der erdbeben von 1946 im mittelwallis, diploma thesis, (inst. of geophysics, eth, zürich), pp. 76. sed (1913-1964): seismologische jahresberichte des schweiz. erdbebendienstes, archives swiss seismological service (eth, zürich). senn von münsingen, joh. (1362): codex diplomaticus urstisii, public library in basel, in w. wackernagel, 1856. sieberg, a. (1940): beiträge zum erdbebenkatalog deutschlands und angrenzender gebiete für die jahre 58 bis 1799, mitt. deutsch. reichs-erdbebendienst, h. 2 (jena), pp. 112. sponheuer w. (1952): erdbebenkatalog deutschlands und angrenzender gebiete für die jahre 1800 bis 1899 (academie-verlag, berlin), pp. 195. volger, g.h.o. (1857, 1858): untersuchungen über das phänomen der erdbeben in der schweiz, justus perthes, gotha, vol. i, pp. 367; vol. ii, pp. 522. wackernagel, w. (1856): das erdbeben von 1356, in basel im 14. jahrhundert, basler historische gesellschaft, h. georg’s verlag, 211-253. wanner, e. (1932): erdbebenkatalog der schweiz für die jahre 1856-1879, jahresberichte des schweizerischen erdbebendienstes 1932, 16-21. wechsler, e. (1987): das erdbeben von basel 1356, teil 1. historische und kunsthistorische aspekte, sed publication series (eth, zürich), 102. wössner, j. (2001): revision of the macroseismic catalogue and database for switzerland in the time period 1879 to 1963, swiss seismological service, zürich, internal report. s u l l e p o s s i b i l i t à o f f e r t e d a l t e r r i t o r i o d e l l a r e p u b b l i c a d i e l s a l v a d o r n e l l ' a m e r i c a c e n t r a l e n e l c a m p o d e l l e « f o r z e e n d o g e n e » ( * ) nota preliminare francesco penta i s p i r a n d o s i p r i n c i p a l m e n t e alle realizzazioni della r e g i o n e dei soffioni b o r a c i f e r i i n t o s c a n a , il g o v e r n o s a l v a d o r e n o aveva già deciso di i n t r a p r e n d e r e i l a v o r i p e r t r a r r e e n e r g i a dal v a p o r e a c q u e o rivel a l o , d i r e t t a m e n t e o i n d i r e t t a m e n t e , dalle n u m e r o s e m a n i f e s t a z i o n i fum a r o l i c h e e s o l f a t a r i c h e diffuse n e l l a r e g i o n e . a n c h e con l ' i n t e n z i o n e di e s a m i n a r e , in p a r i t e m p o , l e e v e n t u a l i a l t r e p o s s i b i l i t à i n d u s t r i a l i c o n n e s s e a q u e s t e m a n i f e s t a z i o n i , il g o v e r n o invitò l o s c r i v e n t e ad e s p r i m e r e un p a r e r e p r e v e n t i v o s u l l ' i n t e r e s s e p r a t i c o d e l l e m a n i f e s t a zioni stesse ed a s u g g e r i r e i c r i t e r i da seguire n e l l e ricerche, u n a volta c h e q u e s t e fossero r i s u l t a t e c o n s i g l i a b i l i . p e r r i s p o n d e r e ai q u e s i t i si rese n e c e s s a r i o osservare « de visu » l e m a n i f e s t a z i o n i e v i s i t a r e l a r e g i o n e della r e p u b b l i c a . l a visita fu e f f e t t u a t a nel g e n n a i o ( 1 2 2 1 ) del c o r r e n t e a n n o dallo s c r i v e n t e accomp a g n a t o dal dott. ing. a d o l f o p e r o z z i , assistente presso l ' i s t i t u t o , e dal dott. h e l m u t m e y e r a b i c h , geologo del m i n i s t e r o dei l a v o r i p u b b l i c i di e i s a l v a d o r . i l t e r r i t o r i o di e1 s a l v a d o r è quasi e s c l u s i v a m e n t e d ' o r i g i n e vulc a n i c a p r e v a l e n t e m e n t e e s p l o s i v a ; s o l t a n t o in u n a l i m i t a t a a r e a , n e l l a zona di m e t a p a n , c o m p a i o n o i s e d i m e n t i m e s o z o i c i e f o r s e a n c h e il p a l e o z o i c o m e t a m o r f o s a t o ; p i c c o l i affioramenti di r o c c e g r a n i t o i d i ( n e l l a zona ad e della p a l m a ) c o s t i t u i s c o n o a l t r a e c c e z i o n e . f r a i terr e n i v u l c a n i c i , d a t a b i l i dal p o s t c r e t a c i c o ( a n o r d ) a l l ' a t t u a l e , si i n t e r c a l a n o a n c h e depositi l i m n o v u l c a n i c i c o n n e s s i a l l e v a r i e fasi di sprof o n d a m e n t i l o c a l i ( v u l c a n i c i ) o a v e n t i c a r a t t e r e r e g i o n a l e ( t e t t o n i c i ) . d a q u a n t o h a n n o p o t u t o s t a b i l i r e gli studiosi e s p e c i a l m e n t e più di r e c e n t e h e l m u t m e y e r a b i c h , i l v u l c a n i s m o s'è spostato n e l t e m p o . ( * ) comunicazione presentata al convegno dell'associazione geofisica italiana, tenuto a r o m a il 17-18 giugno 1953. 3 1 0 f r a n c e s c o p e n t a grosso modo, da nord verso sud ed ila c o n s e n ato c a r a t t e r e « p a c i f i c o » con i prodotti varianti dal basaltico al r i o d a c i t i c o (meyer-aliich). allineamento di boccile eruttive, morfologia e rigetti d i r e t t a m e n t e osservabili rivelano più giaciture di f r a t t u r e : una di queste f r a t t u r e o fascio di fratture è di origine tettonica regionale ed è q u e l l a lungo la quale si allineano i vulcani « a t t i v i » attuali od a ricordo s t o r i c o : la fascia cioè comprendente izalco, s. salvador, s. v i c e n t e , s. miguel. altre fratture intersecano il t e r r i t o r i o ; ad esse si aggiungono le numerose discontinuità piii o m e n o arcuate e tortuose accusanti sprof o n d a m e n t i locali vulcanici (caldere). dalle osservazioni effettuate dai geologi d i e h a n n o studiato la zona e stando a quanto lui potuto constatare d i r e t t a m e n t e il dott. meyer-abich (nei suoi due anni d'intenso lavoro scientifico e applicativo sul posto), non sembra che n e l l e vulcaniti affioranti delle varie età siano presenti proietti o b l o c c h i rigettati asportati dai sedimenti. d e i sedimenti f o r m a n t i la p i a t t a f o r m a sulla quale giacciono i rilievi vulcanici sepolti od emergenti, non si ha n o t i z i a ; non è perciò possibile precisare la profondità di tale basamento sedimentario. stando alla morfologia della superficie, all'intensità dei rigetti più giovani ed alla mancanza di proietti sedimentari rigettati (nelle vulcaniti più giovani) è da stimare poco p r o b a b i l e . che lo spessore delle vulcaniti stesse nel sottosuolo della regione e specialmente nella parte meridion a l e sia inferiore a due-tremila m e t r i . non può escludersi clic oggi il tetto del bacino magmatico o delle sue apofisi attive sia costituito da prodotti vulcanici delle eruzioni più antiche. comunque, b e n c h é il carattere esplosivo delle eruzioni pare a b b i a predominato e predomini tuttora, non m a n c a n o affioramenti di colate e cupole laviche molto potenti ed estese per c h i l o m e t r i . è molto p r o b a b i l e che rocce lapidee del genere si trovino con f o r t i potenze anche in profondità. l ' i z a l c o in fase di attività r i t m i c a , l'eruzione del s. salvador del 1917 ed il tipo di attività del s. miguel all'estremo o r i e n t a l e dell ' a l l i n e a m e n t o su ricordato, testimoniano c h e persiste ancora una enorm e capacità esplosiva nel sottostante magma. l e manifestazioni f u m a r o l i c h e , s o l f a t a r i c h e ed i d r o t e r m a l i distrib u i t e nella fascia c o n f e r m a n o tale stato, a n c h e se molte di esse sono da ritenersi secondarie, legate, cioè, in parte ad acqua sotterranea di o r i g i n e meteorica. s u l l e p o s s i b i l i t à d e l l e « f o r z e e n d o g e n e » n e i . e l salvador 3 1 1 d a l l e osservazioni (*) risultò che le possibilità offerte dalla regione salvadorena sono r i d u c i b i l i ai due t i p i : possibilità di caplare vapore ed utilizzabilità dell'h20 calda ( l i q u i d a o allo stato di vapore) c i r c o l a n t e nel sottosuolo non molto profondo delle aree attualmente sedi di manifestazioni f u m a r o l i c h e e s o l f a t a r i c h e ( l o c a l m e n t e delte ausoles) e possibilità di rinvenire, mediante r i c e r c h e profonde, vapore acqueo in aree non rivelanti manifestazioni f u m a r o l i c h e . nel sottosuolo della striscia lungo la q u a l e si allineano i vulcani storicamente attivi, è da ritenersi, infatti, altamente p r o b a b i l e il rinvenimento di vapore acqueo a temperatura superiore ai 200". quivi, allo slato delle conoscenze, si prevede che si dovranno raggiungere profondità non i n f e r i o r i ai 2.500-7-3.000 m e t r i ed eseguire per lo m e n o due coppie di sondaggi prima di dichiarare esaurita l a ricerca esplorativa. f u egualmente consigliata una campagna di r i c e r c h e con sondaggi n e l l e aree delle f u m a r o l e e delle manifestazioni solfatariche. l a loro condotta si discosta da quella relativa alle trivellazioni prof o n d e : la differenza sta nella m i n o r profondità da esplorare e nella necessità di impostare i successivi fori di una stessa serie, partendo dalle vicinanze delle manifestazioni e spostandosi verso la zona n e l l a quale si desumerà c h e avvenga l'afflusso sotterraneo di fluidi caldi. s i avvertì c h e non sempre ciò risulta agevole. si f o r n i r o n o a n c h e i suggerimenti necessari per l a condotta dei sondaggi e delle relative osservazioni sistematiche e per la sicurezza delle persone. circa le altre possibilità m i n e r a r i e legate alle manifestazioni attuali, risultò non i m p r o b a b i l e la coltivabilità dello zolfo « solfatarico », già in altri paesi utilizzato. si precisò c h e con i sondaggi previsti per la r i c e r c a di h^o calda si potrà procedere anche all'accertamento delle « p o t e n z i a l i t à » dei depositi s o l f i f e r i : analisi e determinazioni sistematiche c h i m i c h e e p i r o g r a f i c h e dei campioni da estrarre dal sottosuolo solfatarico p e r m e t t e r a n n o non solo u n a valutazione del tenore di zolfo, m a a n c h e il riconoscimento di altre sostanze event u a l m e n t e utili (min. di as, allumi, silice l i b e r a ecc.) ed utilizzate i n altre località simili. gas utili e specialmente gas rari, se presenti in ( ! ) ! risultati delle osservazioni sui posti sono contenuti in questo stesso fascicolo degli annuii di geofilica in una nota corredata di cartine e fotografie redatta da adolfo perozzi. 3 1 2 f r a n c e s c o p e n t a q u a n t i t à discrete, potranno essere a n c h e essi oggetto di utilizzazione. l e determinazioni e l e misure quantitative da effettuarsi durante i lavori daranno modo di valutare a n c h e queste altre possibili risorse. così, per es., l'f/os, una volta riconosciuto in quantità sufficiente, potrà costituire, per suo conto, m a t e r i a prima per la produzione di hnsoi. analogamente per la c o j da destinare alle diverse sue utilizzazioni. si concluse col r i p o r t a r e l e manifestazioni f u m a r o l i c h e e solfatar i c h e della regione salvadoreiia per b u o n a parte al tipo di quelle attualmente in fase di studi e di « r i c e r c a » n e l l e aree v u l c a n i c h e in italia (napoletano, v i t e r b e s e , isola v u l c a n o ) , in giappone, nuova zelanda, ecc. alcune sono evidentemente legate ad a c q u e sotterranee. di regola, rispetto alle simili manifestazioni dei distretti vulcanici italiani, esse si presentano in migliori condizioni, p e r c h é c o p r o n o aree di gran lunga maggiori e sono legate ad uno dei vulcanismi più aitivi conosciuti. altro fattore favorevole di i n e s t i m a b i l e valore è la quasi certezza che f r a le acque sotterranee non siano da annoverare q u e l l e di origine m a r i n a ; in e1 salvador non sembra, cioè, si debba temere l'influenza del m a r e sui fluidi profondi a l i m e n t a n t i , d i r e t t a m e n t e o indirettamente, le manifestazioni visibili in superficie. i n t o r n o ed entro alle aree di queste manifestazioni conviene intensificare le r i c e r c h e sia nella speranza di captare vapore con pressione, t e m p e r a t u r a e portata sufficiènti, sia n e l l ' i n t e n t o di utilizzare, in via subordinata, le acq u e più calde come sorgenti di calore. con i sondaggi da eseguire in conseguenza si potranno studiare a n c h e l e altre possibilità ( s , as, gas v a r i e gas r a r i , radioattività ecc.). l e r i c e r c h e stesse potranno accertare se (come sembra verosimile, per es., nelle f u m a r o l e di alta q u o t a : 1280 m sul l.m.m. di a h u a c h a p a n con 125° c e al t r o n a d o r a q. 800 in sul l.m.m. con 107° ci il vapore esalante dal suolo sia già direttam e n t e utilizzabile, sempre, s'intende, col ricorso a sondaggi alquanto approfonditi. l ' a l l i n e a m e n t o di vulcani attivi (izalco, s. salvador, s. miguel per restare in e1 salvador) e la lontananza (decine di c h i l o m e t r i ) dal m a r e r e n d o n o p r o b a b i l e il rinvenimento n e l sottosuolo più profondo di imp o r t a n t i fonti di energia termica in corrispondenza s p e c i a l m e n t e della striscia nella quale si allineano i suddetti vulcani attivi. quivi fu, perciò, consigliata l'esecuzione di sondaggi da spingere fino a raggimis u l l e p o s s i b i l i t à d e l l e » f o h z e e n d o g e n e » n e l f.l salvador 3 1 3 gere, in profondità, formazioni lapidee, p e r m e a b i l i in grande ed idonee al c o n t e n i m e n t o ed alla circolazione di vapore acqueo industrialm e n t e utilizzabile. roma (s. pietro iti vincoli) — istituto di geologia applicata e di giacimenti minerari della facoltà d'ingegneria — giugno 1953. riassunto dall'esame dei luoghi ed in considerazione delle caratteristiche geologiche della regione si riportano le manifestazioni fumaroliche e solfatariche della regione salvadorena per buona parte al tipo di quelle attualmente in fase di « ricerca » nelle aree vulcaniche in italia (napoletano, viterbese, isola vulcano), in giappone, nuova zelanda, ecc. alcune sono legate anche ad acque sotterranee. di regola, esse si presentano in ottime, condizioni, perché coprono aree molto vaste e sono legate ad uno dei vulcanismi più attivi conosciuti. fattore favorevole è la quasi certezza che fra le acque sotterranee non siano da annoverare quelle di origine marina. intorno ed entro alle aree delle manifestazioni conviene intensificare le ricerche sia nella speranza di captare vapore utile, sia nell'intento di « sfruttare », in via subordinata, le acque più calde come sorgente termica. con i sondaggi da eseguire in conseguenza si potranno studiare anche le altre possibilità (s, as, gas vari e gas rari, radioattività ecc.). prescindendo da tali manifestazioni superficiali, esalative ed idrotermali, l'allineamento di vulcani attivi (izalco, s. salvador, s. miguel ecc.) e la lontananza dal mare rendono probabile il rinvenimento anche nel sottosuolo più profondo di importanti fonti di energia termica in corrispondenza specialmente della « linea » di dislocazione lungo la quale si succedono i vulcani attivi stessi. summary from the emination of some of its areas, considering the geologie characteristics of the region, ice roport ori the exlialation of smoke and sulphur gases of the salvadorena region, for the most part typical of those actually being investigated in the volcanic areas of italv (napolitan, viterbian, and the isola vulcano) and of japan, new 3 1 4 francesco penta zealand, etc. some of these are also connected to subterrnnean ivaters. as a ride these are in optimum condition because they cover very vast areas and belong to one of the most active vulcanisms known. a favorable jactor is that almost certainly one can exclude the admixing of water of marine origin with the subterranean waters in this region. around and within the areas showing these manifestations it ivould be valuable to intensify the research, either in the hope of capturing useful vapors or, as a secondary purpose to talee advantage of the liottest ivaters as a source of heat. with the soundings that should be carried out, one could study also otlier possibilities (s, as, various gases includilig rare gases, radioactivity, etc.). prescinding from such manifestations, the release of gases and hot ivater through the soil, the alignment of the active voi can os (izalco, s. salvador, s. miguel, etc.), and the distance from the sea, it is quite probable that also in the deeper lying soils there are important sources of tliermal energy corresponding especially to the dislocation « line » along ivìch these sanie active volcanos zie. b i b l i o g r a f i a rock p., izalco (salvador). zt. f. vulk., 10, 2, 1926. coleman s . n.? volcanoes neiv and old. museum press limited, london, 1949. eabdley a . j . , structiiral geologv oj norlli america. h a r p e r e brotli. p u b i . ; new y o r k , 1951. mercalli g., i vulcani aitivi della terra. hoepli, milano. 1907. meyer-abich consideracionex geolópicas acerca de la pianta eléctrica proyectada en el lu^ar cliorrera del guayabó en el rio lemjxi. comunic. d. instituto t r o p i c a l des investigaciones cientificas, n. 1 ; san salvador, 1952. meyer-abich h , el problema de mctapun. il>. n. 1, 1952. meyer-abich h_ terremoto de jucuapa en el salvador (america central) 6-7 de mayo de 1951. id. n. 2. 1952. meyer-abich h., el instituto tropical de investigaciones cientificas de la universitad de el salvador. i b , n. 3, 1952. meyer-abich ii., el puerto de acajutla, el salvador. ib^ n. 3, 1952. meyer-abich h., das erdbeben von jucuapa in el salvador (zentralamerilca) 6 und 7. mai 1951. neues j b . geol. u. palaontol. 95, 1952. sapper k . , die mittelamerikanisclien vulkane. ergiinzungsli., 178 zìi petfkmanns mitt.; gotha, 1913. sapper k., mittelamerikas. in « geologisclie jaliresberichte » ii v o i . ; berlin, 1940. shaffer f . x., geologische liinderkunde. f r a n z deuticke, w i e n , 1941. stirton r . a . e gealey w . k., reconnaissance geology and vertebrate /mleontology oj el salvador, central america. bull. geol. soc. a m . ; 60, pag. 1731-1754; 1949 ( c i t . d a e a r d l e y ) . ag_56.05.13_helman_finalonline:layout 6 annals of geophysics, 56, 5, 2013, g0564; doi:10.4401/ag-6184 g0564 earth electricity: a review of mechanisms which cause telluric currents in the lithosphere daniel s. helman california state university long beach, department of geological sciences, long beach, ca, united states abstract telluric currents are natural electrical phenomena in the earth or its bodies of water. the strongest electric currents are related to lightning phenomena or space weather. earth electricity can cause damage to structures, and may be useful for earthquake forecasting and other applications. thirty-two distinct mechanisms that cause earth electricity are described, and a broad selection of current research is highlighted. 1. introduction several phenomena that can generate telluric currents have been described in scientific specializations whose members may not communicate with each other regularly. the study of this topic is both intriguing and challenging: electrical signals do not carry much of a marker to indicate how they are generated, beyond magnitude, frequency, and polarization. attenuation and new phenomena caused by transmission complicate signal characteristics. a wide range of possible applications for telluric data exists in different fields (seismolog y, hydrolog y, mineral prospecting, geother mal prospecting, planetary science, etc.) for example, seismic electric signals may occur in a period leading to increased seismic risk, and understanding the causes of purported seismic electric signals is critical to characterizing any extant mechanism related to electricity and earthquake phenomena [varotsos et al. 2011]. as another example, dissolved ions in groundwater increase rock conductivity, and the motion of the groundwater itself creates an electrical signal [corwin and hoover 1979]. this text is a brief selection of research in the subject, meant to be a resource for further study. along with artificial signals, earth electrical phenomena are summarized in table 1, and table 2 lists telluric currents by frequency, magnitude and signal duration. thirty-two causes of earth electricity are described in the text that follows. telluric currents were originally defined as natural electric currents passing through the earth’s soil or rock layers or bodies of water, as opposed to its atmosphere. artificial currents were not included. for the purposes of this paper, any electric current in a planet or on it may be classed as a telluric current. 2. space phenomena 2.1. geomagnetically-induced currents, gic the ionosphere is composed of charged particles and located 85 to 600 km above the earth’s surface. electrical phenomena are caused as the solar wind or space weather impact the ionosphere. the solar wind and space weather create ionospheric electromagnetic phenomena in the radio spectrum, and these disrupt communication. eddies in the ionosphere also occur, and these create electric current in situ, from the motion of ions. this electric current affects the geomagnetic field, and the resulting geomagnetic anomalies induce telluric currents in the ground [boteler et al. 1998]. this is gic. geomagnetically-induced currents cause corrosion in pipes and pipelines, and are a problem at high latitudes, where the earth’s magnetic flux lines point towards (or away from) the surface of the earth. ore, or other rock bodies, or human-made structures, such as pipes and cables, respond to electrical changes in the ionosphere, so that a telluric current is induced in the ground. osella et al. [1998], everett and martinec [2003], constable and constable [2004], pulkkinen et al. [2007], and others have studied this phenomenon. cycles are related to space weather, and are dominated by the influence of the sun’s eleven-year sunspot cycle, whose period predicts emissions of gas from the solar surface. diurnal variations within this larger cycle have been recorded [diodati et al. 2001]. article history received august 9, 2012; accepted june 27, 2013. subject classification: magnetosphere, magnetic storms, mineral physics and properties of rocks, exploration geophysics, magnetic and electrical methods, geology, space and planetary sciences, solar-terrestrial interaction. the ongoing diurnal currents are responsible for the corrosion of pipelines and cables in some locations, especially at high latitudes, and have been studied extensively in scandinavia [viljanen et al. 2006]. gic typically are on the order of 200 amperes (a) in man-made conductors, with durations of approximately 10 seconds [kappenman et al. 1981, viljanen et al. 1999, pulkkinen et al. 2008]. the oscillating frequency is typically 0.01 helman 2 name cause cycle mechanism space phenomena gic cosmic particle flux planetaty magnetic field plasma solar cosmic cosmic events (magnetotail) 11/24hr not known not known 28d electromagnetic induction charge transmission charge transmission atmospheric phenomena tid lightning strikes lightning strikes induction whistler induction whistler plasma volcanic lightning strikes storm charging atmospheric disturbavce lightning lightning lightning lightning volcanic lightning weather not known seasonal seasonal seasonal seasonal not known seasonal electromagnetic induction charge transmission electromagnetic induction electromagnetic induction charge transmission charge transmission electrostatic induction oceanic phenomena electrochemical effects ocean transport induction oceanic charging metabolic electrochemistry ocean currents ocean currents ocean electric currents microbes and algae seasonal seasonal seasonal not known charge transmission electromagnetic induction electromagnetic induction charge transmission surface phenomena artificial signals metabolic electrochemistry exo-electron emission industry microbes and plants primed material various 24hr not known electromagnetic induction charge transmission charge transmission groundwater phenomena electrochemical effects electrokinetic effects seismic dynamo induction radioactive ionization fluid flow fluid flow in porous media seismic waves radioactive decay seasonal various not known not known charge transmission electrostatic induction charge transmission charge transmission other terrestrial phenomena volcanic em signals seismic em signals seismic electric signals fractoemission defect charging piezoelectric effects thermoelectric effects pyroelectric effects magma electrochemistry radioactive emission volcanism earthquakes earthquakes fracture materiale defects crystal lattice geometry temperature gradient temperature gradient magma processes radioactive decay not known not known not known not known not known not known not known not known not known not known not known not known not known charge transmission charge transmission domain rearrangement charge transmission domain rearrangement charge transmission charge transmission deep terrestrial phenomena geomagnetic jerk geodynamo not known electromagnetic induction table 1. causes and periods of earth electricity. gic are geomagnetically induced currents, tid are traveling ionospheric disturbances, and em is an abbreviation for the term electromagnetic. 3 to 0.001 hz [price 2002]. peak current can be on the order of 2000 a, and these occur about 10 to 100 times in 100 years [pulkkinen et al. 2008]. diurnal flux rates at the sub-auroral latitudes are on the order of a few millivolts per kilometer (mv km-1) [mather et al. 1964]. the strongest oscillation frequency of these diurnal signals is 0.4 hz, and is widespread at different latitudes [mather et al. 1964]. at high latitudes, the motion of charged particles also creates a distinct radio signal, termed the polar chorus, with a characteristic frequency of 300 hz to 2 khz [barr et al. 2000]. polar chorus is associated with the solar wind, and the peak intensity is around 50 μv m-1 as recorded from stations on the ground in antarctica. it typically exhibits a diurnal variation [salvati et al. 2000]. telluric currents (and specifically gic) were first documented in the 1840s with the invention of the telegraph. buried telegraph lines are electrical conductors, and susceptible to electrical induction. geomagnetically-induced currents caused interference during telegraph transmission, so that the telegraph needles hung frozen by the signals of the gic. at first this phenomenon was attributed to weather causes, but it was soon recognized that the hung needles coincided with the occurrence of aurora borealis and magnetic storms [walker 1861]. 2.2. cosmic-particle flux direct bombardment by high-energy charged particles and radiation coming from solar, stellar, and cosmic sources, act generally to form gic. for example, a gamma-ray flare from a neutron star 23,000 light years away was reported in 1999 as causing vlf amplitude changes of more than 20 decibels from interaction with the ionosphere. the lyrid, delta-aquarid and perseid metoer showers have caused phase variations in a 16 khz signal due to gic [barr et al. 2000]. for planetary bodies with no atmosphere, this flux of cosmic particles creates telluric currents directly [madey et al. 2002]. cosmic ray ions have an energy flux of about 6 × 109 ev cm-2 s-1 for bodies in the solar system. for mars, protons and neutrons strike the surface with energy fluxes of around 6000 and 1400 mev cm-2, respectively [molina-cuberos et al. 2001]. this process is not occurring on the earth’s surface at present; the atmosphere intervenes. 2.3. planetary magnetic-field plasma if ultraviolet and x-ray emissions from a star encounter a magnetic field, the interactions will create a plasma of energetic electrons. such a plasma is created in the earth’s magnetosphere from solar radiation, and strikes the moon’s surface as it passes through the earth’s magnetotail. as described in stubbs et al. [2007]. the magnitude of the charging can be several thousand volts [halekas and fox 2012]. a magnetotail is the distal part of an oblong magnetic field, caused in this case by the solar wind. 3. atmospheric phenomena 3.1. traveling ionospheric disturbances, tid atmospheric compression (i.e. acoustic waves) from a sudden event, such as an earthquake, tsunami, volcanic eruption, severe weather or rocket launches can create traveling ionospheric disturbances (tid) [georges 1968, johnston 1997, afraimovich et al. 2001], and these tid can induce telluric currents in the ground via the geomagnetic field. tid are themselves a category of gic. the ionosphere also has resonant electrical phenomena, called schumann resonances, at a fundamental frequency of 10.6 hz, with overtones at 18.4, 26.0, 33.5 and 41.1 hz [barr et al. 2000]. the background amplitude of measured shumann resonances is about 1.0 picoteslas [schlegel and füllekrug 1999]. in addition to the above examples, tid can form as the result of gravity waves at the troposphere-ionosphere interface [georges 1968]. a gravity wave is one where buoyancy or gravity (or both) act to oppose the displacement. a common example of a gravity wave is the wind-generated wave forms one sees at the ocean at the ocean-air interface. some tid are akin to these, occurring at the troposphere-ionosphere interface. no quantitative reports of the magnitudes of telluric currents resulting from tid are extant, to the best of the author's knowledge, though qualitative magnitudes are known. space weather events are the strongest tid, and then, in descending order, daytime signals, atmospheric compression events, and gravity waves [georges 1968]. the effect of tid within the ionosphere is for the disturbance to develop a potential on the order of 1 millivolt per meter [shiokawa et al. 2003]. frequency for the ionospheric dynamo region is modeled to be on the order of 10-6 to 10-7 hz [kaladze et al. 2003]. higher frequencies are also present [munro 1958]. short-term changes (on the order of hours) to the earth’s magnetic field may be caused by ionospheric activity. kaladze et al. [2003] have modeled ionospheric activity that matches the magnitude and timing of ground observations of changes to the geomagnetic field. the ionosphere is studied with dedicated groundbased facilities, such as the high frequency active auroral research program (haarp) and with satellites. a network of satellites measuring ionospheric disturbances are in place. a 1996 space experiment with a nearly 2 km long conducting line gathered electrical data in the ionosphere, and then compared these with earth electricity: a review of mechanisms satellite data. the accuracy of modeled ionospheric activity between satellites is low. modeled electrical data are off by as much 140% [szuszczewicz et al. 1998]. on the ground, haarp has been in operation in alaska since 1993 [bailey and worthington 1997]. that facility is designed to transmit radio-frequency electromagnetic radiation into the ionosphere for communication with submarines, with the electrolytes in the ocean acting as an antenna. haarp is also suited for ionospheric studies. haarp experiments are designed to study the structure of the ionosphere, and for determining practical applications of wave propagation, such as radio signaling. results have included techniques to produce very low frequency and extremely low frequency (vlf/elf 30 hz to 30 khz) radio waves [cohen et al. 2008]. lightning channels broadcast electromagnetic radiation in the vlf range, and haarp can duplicate this vlf. haarp has also been used for magnetotelluric surveying. in a magnetotelluric survey, both electrical and magnetic fields are used for remote sensing, to determine the electrical resistivity of an area, and variations within it, according to an empirical equation (1) where t is the resistivity in ohm meters (ω m), f is frequency in hertz (hz), e is the electric field tensor in volts per meter (v m-1), and b is the magnetic field tensor in nanoteslas (nt) [wescott and sentman 2002]. haarp can generate magnetotelluric signals, and was used as the transmitter for a proof-of-concept controlled-source audio-magnetotelluric survey (csamt) in alaska in 1999 and 2000, prospecting for petroleum [wescott and sentman 2002]. this is a new trend. most magnetotelluric surveys have historically used natural fields [simpson and bahr 2005]. simultaneous measurements of the geomagnetic field and of telluric currents are used to calculate a value for the electrical impedence at depth, to explore subsurface features. 3.2. lightning strikes electrical charge is transferred between the ground and the atmosphere during lightning strikes, and the tops of stormclouds close an electrical circuit with the ionosphere. lightning discharge is energetic and creates plasma that we see. the first pulse of lightning occurs as charges within the cloud consolidate to form a strike leader, and plasma from the ground rises up to meet the leader in that cloud. the next pulse comes from the cloud to the ground. the process repeats, with alternating pulse initiations between ground and cloud. a lightning strike is a combination of about 30 pulse events, each lasting nanoseconds, and its overall duration is on the order of milliseconds [uman 1994]. the bulk result is a negative charge given to the ground. peak electric current is 99 ± 7 ka, measured by quantifying remanent magnetization of the ground and calculating the peak magnetic field [verrier and rochette 2002]. oscillation signal frequencies are on the order of 10-3 mhz to 103 mhz, or higher [uman and krider 1982]. the previous data have been normalized to a 10 km distance, and higher frequency signals are known to attenuate. with some dry lightning and strikes which ignite fires, a positive charge is given from the cloud to the ground. the magnitude of charge carried by positive cloud to ground strikes is increased by the presence of aerosols and smoke [nichitiu et al. 2009]. 3.3. lightning-strike induction lightning strikes can also cause transient changes to the geomagnetic field [verrier and rochette 2002]. lightning can occur in various weather conditions, including thunderstorms, dust storms and tornadoes [barr et al. 2000]. telluric currents arising from this phenomenon ought to have frequencies of 10-3 mhz to 103 mhz, based on the ns to ms variations recorded in lightning phenomena. these are the same frequencies that a direct flash of lightning will display. induction is localized, and portions of large buildings and towers are frequently subject to induction when they are struck. hussein et al. [2003] compare data from several structures, and the average magnitudes are between 7 and 12 ka, with peak magnitudes from 20 to 100 ka. 3.4. whistler induction lightning discharge heats the air and creates plasma. the entire lightning channel radiates electromagnetic energy. if in the radio frequency, it is called a radio atmospheric signal, or sferic. if the plasma from lightning dishcarge travels along geomagnetic lines, the resulting radio-frequency disturbances are termed “whistlers” and are named for the sound which this interference makes in telephone lines, as first described in 1919 [schlatter 2008]. the sound was attributed to lightning phenomena in 1953. whistlers typically occur in the elf/vlf range of 3 hz to 30 khz [barr et al. 2000]. for example, observations made from antarctica at 22.3 khz show common changes in amplitude of 3 decibels to an artificially transmitted signal, with duration of around 30 seconds. these changes were associated with whistler activity [helliwell et al. 1973]. the propogation of whistlers along geomagnetic flux lines can induce changes to local magnetic fields, and these can cause induction of human-made conductors and ore bodies. helman 4 1/5 f e/b ,t = 26 @ 5 3.5. whistler plasma whistlers are caused when plasma from lightning travels along the geomagnetic flux lines. the magnitude of the electron density striking the earth is on the order of 104 electrons per cm3 during whistler events at the equator. poleward densities should be higher, as the flux lines there are more inclined to the earth’s surface [schlatter 2008]. 3.6. volcanic lightning strikes an electrical response in the ground as lightning strikes during volcanic eruptions has been described in [aizawa et al. 2010]. they describe magnetotelluric data from sakurajima volcano in kyushu, japan, from may 2008 to july 2009. magnetotelluric pulses were recorded coincident with several strikes. generally, volcanic plume heights where volcanic lightning has been observed are distributed bimodally: plume heights 1 to 4 km, and plume heights 7 to 12 km. in the former, volcanic lightning is due to vent processes, and in the latter volcanic lightning is due to stratospheric processes [mcnutt and williams 2010]. occurrence of volcanic lightning increases with height in the stratospheric plumes, and peak currents greater than three thousand amperes have been observed [bennett et al. 2010]. ash erupted from a volcano is electrically charged. whether a circuit is made between charged ash and the ionosphere has not yet been reported. low frequency (30 khz to 300 khz) sferics are reported, as with meteoroligical lightning events, but the emission spectra of the flash itself has not been. likewise, the author has not found reports of the magnitude of electrical discharge during volcanic lightning. james et al. [2000] have described a mechanism for ash charging. in a series of experiments, ash-sized particles were ground from several crustal rock samples in a non-conducting sample holder. the materials developed charges of both polarities, generally based on the chemical composition of the particles, with the net charge of ≈10-5 to 10-6 coulombs per kilogram (c kg-1). their data are consistent with measurements taken previously within ash fall plumes [james et al. 2000]. they attribute the charge in the ash to fracture-charging, also called fractoemission, where electrical charge is caused in a fracture as electrons are distributed unevenly during fracture processes. lightning in volcanic plumes is controlled by topography and wind direction, with negative strikes (negative charge carried to the ground) and positive strikes (positive charge carried to the ground) evolving over the course of an eruption [hoblitt 1994] or occurring simultaneously. the role of water in the magma and its influence in volcanic lightning is still an open question, though the occurrence of volcanic lightning is not related to latitude, and, hence, is likely not related to the ability of the air to hold water [mcnutt and williams 2010]. 3.7. storm charging terrestrial electrical f ields occur during storm activity. a vertical f ield on the order of 100 v m-1 and a current density at the surface of roughly 2 × 10-2 a m-2 has been measured during thunderstorm activity, with water droplet interactions in clouds perhaps serving as the source of the initial (negative) charge buildup. these may induce a positive charge in the ground. in thunderstorm clouds, the negative charge at the bottom of the cloud is offset by a positive charge in the top portion, that together form a conductive circuit flowing upward through the stratosphere, also called a thundercloud cell. in reality, negative charge is flowing downward. the current has been experimentally determined to be around 0.7 a per thundercloud cell [troshichev et al. 2004]. current flow in a thundercloud cell accounts for about 96% of the electrical activity of a storm, while lightning discharges account for a minority (≈ 4%). electrical charg ing of storm clouds is offset by charging of the ocean surface, where daily electrical ocean surface variations are consistent with the daily change of the total area occupied by thunderstorms [troshichev et al. 2004]. electrical frequency spectra in the g round from electrostatic storm charg ing are not reported, to the author's knowledge. 4. oceanic phenomena 4.1. electrochemical effects in the ocean in the oceans, different layers of water will be stratif ied by temperature and salinity, and each influences density. both of these gradients influence electrical conductivity, and create variations in electric currents in the oceans [chave and luther 1990]. the signals are low-frequency (30 khz to 300 khz) or lower, typically. voltages from temperature and salinity variations in the ocean are less than a few mv (silver/silver-chloride electrodes were used) and the differences in salinity and temperature were less than a few parts per thousand and a few degrees celsius, respectively, between electrodes [larsen 1992]. the electrode material affects the observed voltage. internal waves (within the stratified ocean) are measurable electrically in their vertical component as gradients are crossed [chave 1984]. earth electricity: a review of mechanisms 4.2. ocean transport induction electrical induction in the oceans occurs by three processes: transport of seawater across the geomagnetic field (treated in this subsection); the influence of gic on saltwater, a conductor (treated in the subsections above dealing with gic and tid); and variations in sea water due to variations in salinity and temperature (treated above.) bulk water transport was first measured electrically by faraday in 1832, at the waterloo bridge with electrodes placed in the thames river, but sunspot activity (unfortunately) masked the periodic influence of the gulf stream [larsen 1992]. induced voltage due to transport of saline water has been observed successfully, with a magnitude on the order of 25 mv per kilometer, measured on a cable fitted with electrodes in the straits of florida [larsen 1992]. the gic (with peaks up to about 50 mv km-1 but with typical values of 10 to 20 mv km-1) had been subtracted out of the data by hand. the voltages occur at frequencies from 10-3.8 to 10-7.0 hz and are incomplete, and tidal variation and other outliers create peaks around 10-5 hz. 4.3. oceanic charging two sources of electric currents in the ocean already described in this text are: storm clouds charging the ocean surface (above); and processes to charge water strata in the ocean itself. electricity from both of these may be transmitted to the rock with which it is in contact via electrostatic induction [cox 1981]. the oceanic lithosphere receives a quasi-static charge from the ocean. due to the high metal content of the rock, both electrostatic and electromagnetic induction will occur if major changes to electric current in the oceans or to the geomagnetic field also occur. 4.4. metabolic electrochemistry in the ocean the metabolic action of microand macrobiota in the oceans may contribute to an electrical signal that is measurable. bohlin et al. [1989] describes how fish are attracted to electric signals; this phenomenon might be related either to physiology or to food sensing. brahic [2010] describes how an extensive network of microbial electric currents may exist in oceanic mud. atekwana and slater [2009] introduce the study of microbial geophysical signatures in a comprehensive manner; biogeophysics is an emerging field, and more research is warranted. 5. surface phenomena 5.1. artificial signals earth electric currents may come from the transmission of electricity or electromagnetic radiation emanating from human-made sources [keller 1968, pham et al. 1998] and also from on-ground activity, such as from electric trains. telluric currents may come from electrical fields set up intentionally as, for example, from a direct-current (dc) electrical field designed to remove contaminants from soils [probstein and hicks 1993]. electroremediation can be accomplished with a field strength of about 150 v m-1. a complexing agent is added to the groundwater, and contaminants are attracted to wells for removal [wong et al. 1997]. the magnitude of artificial telluric currents depends directly upon the generation process. extremely low frequency radio waves are generated by heating the ionosphere, and are used by the us military to communicate with submarines, for example. nuclear explosions above ground also create ionospheric vlf radiation, with frequencies of 10 khz to 15 khz [barr et al. 2000]. 5.2. metabolic electrochemistry in soil the daily action of plants, fungi, bacteria, lichen or algae that inhabit soil and rock fissures may produce electrical signals from electrochemical processes related to metabolism. some evidence exists that soil microbes respond to changes in the geomagnetic field [jie li et al. 2009], though the converse has not been shown. abdel aal et al. [2010] report that the imaginary component of measured conductivity in sand is increased linearly as pseudomonas aeruginosa are introduced to the grains. the imaginary component of conductivity is a measure of its dissipation, part of the field equations that model oscillating or alternating current. abdel aal et al. [2010] used low frequency (0.1 to 1000 hz) signals for their study. no change to the real component of the conductivity was observed. regarding plants: despite the existence of diurnal electrical variations measured in sapwood [gilbert et al. 2006], and in leaves and leaf stems [gil et al. 2008], and also despite the invention of functional electrical circuitry powered by plants and trees [himes et al. 2010, yamaguchi and hashimoto 2012], no diurnal soil-root signal from plants has been detected [love et al. 2008]. a new sensor for these signals has recently been developed [gurovich 2009], but no evidence of diurnal signals has been published yet. 5.3. exo-electron emission the process of stress relaxation may release electrons after an initial priming, as can the addition of heat or photons to a previously stressed sample [oster et al. 1999]. these and related processes are termed exo-electron emission if the energy of the electrons is low (less than or equal to one electron volt), to distinguish it from high-energy electron emission phenomena, such as frachelman 6 7 toemission. exo-electron emission functions by means of traps and defects and requires a solid-gas or solid-vacuum interface for its action: it acts at a surface. exo-electron emission may occur in the vadose zone or on the surface of the crust [oster et al. 1999, freund 2011]. exoelectron flux is observed as less than or equal to 108 electrons (e-) per square centimeter [oster et al. 1999]. 6. groundwater phenomena 6.1. electrochemical effects in groundwater as ionically-charged fluids travel in porous rock, an electric current is created by the motion of the suspended ions [corwin and hoover 1979]. this is the principle behind household chemical batteries, and is common in nature. the electrochemical effect found in ore bodies, for example, is akin to commercial electrochemical batteries in magnitude (a few volts) [lile 1996]. while the chemistry of the fluid determines the voltage, the signal frequencies are controlled by the motion. 6.2. the electrokinetic effect just as the motion of ionically-charged fluids in porous rock creates an electrochemical current, so too the interaction of the charged fluid with the bounding rock creates a complementary charging in the rock itself. at the fluid-rock interface, a single layer of adsorbed ions attracts a second layer of the opposite sign, and these are sufficient to create an electrical potential over a distance. this so-called streaming potential, caused by an electrokinetic effect, involves electrostatic induction by moving ions. self potential is a combination of streaming potential (based on the electrokinetic effect) and of the diffusion of the ions themselves. typically, self potential is present in groundwater flows [ aubert and atangana 1996, birch 1998, revil et al. 2003, jardani et al. 2006], but can also be found in many geologic settings, such as sulphide ore bodies [lile 1996] and other mineral deposits, including graphitic deposits [stoll et al. 1995], and on volcanoes, where the phenomenon is due to hydrothermal activity, changes in groundwater flow, and magma displacement [zlotnicki and nishida 2003]. in hydrothermal settings, streaming potential from electrokinetic effects is much stronger than associated thermoelectric effects [corwin and hoover 1979]. a streaming potential of up to 30 mv can be generated from a groundwater change of 50 cm, if the fluid resistivity is 102 ω m and the rock permeability is 10-12 m2 [jouniaux and pozzi 1995]. streaming potential variations occur, with pulses in amplitude of 15 to 40 mv, and a frequency of 0.1 to 0.5 hz [jouniaux and pozzi 1997]. 6.3. seismic-dynamo induction rock is displaced as seismic waves pass through. groundwater in the pore space is displaced as well, as are ions in the groundwater. the motion of ions relative to the geomagnetic field creates circularly or elliptically polarized electric fields, with opposite orientations for positive and negative ions. this effect was reported in 2009, and was observed both for artificial seismic waves from blasting and for natural seismic waves [honkura et al. 2009]. the magnitude of the seismic-dynamo effect is on the order of μ v to mv, and frequency depends upon the ions in the groundwater, with observed values between approximately 10 and 50 hz. cyclotron frequency is the name given for charged particles moving in circular motion perpendicular to a magnetic field. each charged particle has a cyclotron frequency based on its charge, mass and velocity relative to the magnetic field. the observed seismic-dynamo effect reported in honkura et al. [2009] shows electric frequencies that may be interpreted as resonances of the cyclotron frequency of particles and the geomagnetic field, with bicarbonate, chloride, sodium and calcium taken as constituents. these vary in abundance by location, and account for differences of orientation in the observed electric fields. 6.4. radioactive ionization radionuclides release energy as they decay, and that energy can ionize surrounding material. radon gas is one example. the most common isotope of radon (222ra) has a half-life of 3.8 days [jordan et al. 2011]. several thousand scientific publications have described the presence of radon as co-seismic with major events. radon at the earth's surface ionizes particles in the air, and the motion of these ions creates atmospheric electrical phenomena linking the surface to the ionosphere [pulinets 2007]. co-seismic ionospheric anomalies might be attributed to the action of ions created as radon is released. studies of radon occurrence as an earthquake precursor often look for radon concentrations in groundwater [jordan et al. 2011]. it is plausible to assume that radon ionizes other atoms in groundwater, and that the motion of these ions can create an electric signal. other radionuclides could do the same. 7. other terrestrial phenomena 7.1. volcanic electromagnetic signals hata et al. [2001] report detection of consistent electromagnetic signals during the izu-miyake volcanic eruption of 2000 in japan. the signals preceded the eruption by a week, and were associated with changes to the surface of the earth from magma dike growth. earth electricity: a review of mechanisms the exact mechanism of the signal generation is unknown. the observational apparatus was set to detect extremely low frequency radio waves (between 10 hz and 300 hz). below 10 hz, ionospheric and other geomagnetic signals predominate, and above 300 hz, lightning noise predominates. the full spectrum of the occuring radiation is not known. 7.2. seismic electromagnetic signals matsumoto et al. [1998] report television signal interference associated with the 1995 kobe earthquake in japan. the electromagnetic radiation preceded the earthquake by 6.5 hours, and was characterized as having a magnitude of a few tens of mv m-1, a frequency in the 217 mhz range with microsecond duration. other reports of electromagnetic phenomena during earthquakes are common, typically involving ionospheric disturbances [davies and baker 1965, pulinets and boyarchuk 2004, popov et al. 2004, singh and singh 2007, pulinets 2007, perrone et al. 2010, heki 2011, zolotov et al. 2012]. 7.3. seismic electric signals three types of signals have been reported from a network of monitoring stations in greece [varotsos et al. 1993]. first, a gradual variation in the electric field of the earth (gvef) has been recorded, on the order of weeks or more before an earthquake, and with voltage an order of magnitude higher than other purported precursory ses. these occur rarely. second, presumed seismic electric signals, on the order of hours to 11 days before an earthquake, with an order of magnitude in the millivolt range, occur commonly. third, a short duration pulse, 1 to 4 minutes precedent to seismic waves, with an order of magnitude in the volt range, occur rarely [ralshovsky and komarov 1993, varotsos et al. 1993]. all three are low-frequency signals, less than or equal to 1 hz [varotsos et al. 2011]. similar seismic signal data have been reported in japan using the same method [uyeda et al. 2009]. 7.4. fractoemission fractoemission, in which electrons escape from a freshly cleaved surface, has been described in james et al. [2000]. their observations show electrical charging of about 10-5 coulombs per kilogram for volcanic ash. fracture experiments have recorded up to 10 parts per million (ppm) ozone production from the crushing of typical terrestrial crustal rock, with the ozone being generated by electricity from physical charge separation during fracture [baragiola et al. 2011] the energy of the emitted electrons can be very high, up to tens of thousands of electron volts (10 kev) or higher. the electrons are produced as the surface attains and maintains a charge, often pulling electrons from deep inside, termed the malter effect [oster et al. 1999]. electromagnetic emission resulting from a single crack in ice under various stress regimes has a frequency of 103 to 105 hz, with a change in potential of about 2 mv [shibkov et al. 2005]. 7.5. defect charging pressure can cause defects in materials. defects can both liberate charge carriers, such as ions or electrons, and create charge acceptors, such as holes or lattice vacancies. pressure changes can also result in the reorientation and charging of lattice defects, called defect charging. these processes have characteristic electromagnetic emissions, with frequencies in the range 102 to 106 hz for crystals with predominantly ionic bonds [shibkov et al. 2005]. defect charging has been studied as a candidate for the cause of electrical signals associated with earthquake phenomena [varotsos et al. 1998, freund 2011]. takeuchi and nagao [2013] demonstrate an electromotive force of 80 mv in gabbro with 50 mpa of load. freund [2011] proposes that peroxy defects present in silicate rocks, where the tetrahedral silicate bonds are o3si oo sio3 instead of o3si o sio3, can be a source of mobile charge carriers. peroxy bonds commonly break under pressure. the new structure can accept an electron from a neighboring silica-oxygen tetrahedron. that transfer, in turn, creates a positive hole in the electron donor. positive holes can also interact with negative ions liberated from the lattice by changes in pressure, e.g. from seismic waves. this condition may form paths for electric current to flow. such defect phenomena are fundamental to how semiconducting materials work, and are probably widespread in nature. general terms for the process described above are "charge-vacancy coupling" or "defect and charge transport" [raymond and smyth 1996]. peroxy bonds exist in silicate rocks in enough numbers to create measurable electricity. the process of charge-vacancy coupling is nontrivial in all rocks, given the right conditions. typical electric currents from defect charging in rock are on the order of 1 nanoampere at 20 megapascals of pressure [freund 2011]. 7.6. the piezoelectric effect the piezoelectric effect (electric field or charge caused by applied pressure) has been modeled as a crystal lattice effect, as deformation from stress or strain displaces the positions of shared electrical bonds [cady 1946, mason 1950]. this is the mechanism first described by voigt [voigt 1910, katzir 2006]. stress is an helman 8 9 internal pressure of particles acting on each other, caused by external load. strain is a change to the shape of a material, caused by stress. piezoelectricity is based on the symmetry of a crystal. a more in-depth treatment of the ideas in the following paragraph can be found in sands [1994]. crystals can have three types of symmetry. if the coordinates of a crystal lattice are hypothetically reflected through a point, a new inverse lattice with new inverse coordinates is created. if the inverse lattice is identical to the original crystal lattice, the crystal is centrosymmetric. if the inverse lattice is not identical to the original crystal lattice, but the inverse lattice can be rotated to match the original lattice, then the crystal is non-centrosymmetric. if the inverse lattice is not identical to the original crystal lattice, and the inverse lattice cannot be rotated to match the original lattice, then the crystal is chiral, also called enantiomorphic. the terms “chiral” and “enantiomorphic” are synonyms and refer to handedness. these crystals occur in both left-handed and right-handed forms. for a more in depth treatment of the following descriptions of piezoelectricity, see cady [1946]. a centrosymmetric crystal lattice ought not allow for any electrical charge to build up under pressure. every bond displaced will be countered by another bond whose displacement can cancel the charge of the first. however, some minerals with centrosymmetric crystal lattices, such as zeolites and topaz, express electricity under stress and strain. no explanation has yet been proposed for this anomaly. many chiral and non-centrosymmetric minerals display the piezoelectric effect. the most well-known is quartz, and the most intuitive application at one time was in record needles or microphones, to change variations in pressure into an electrical signal. the electrical signals in these devices are very small, on the order of 10-12 coulombs per newton (c n-1) for a single crystal. piezoelectric data for some minerals can be found in springermaterials, an online version of the landoltbörnstein database [springer 2012]. 7.7. the thermoelectric effect a homogeneous conductor expresses a voltage when there is a temperature gradient, with electrons (the more negative) at the cold end. this is called the seebeck effect (or the thermoelectric effect), and is directly observable as electric current when two dissimilar conductors are connected to each other under a thermal gradient [goupil et al. 2011]. thermocouples are based on this effect. for more information on the following discussion, see von baeckmann et al. [1997]. corrosion of bridges and other metal structures where dissimilar metals are found is caused by this phenomenon. in the crustal materials of the earth, the thermoelectric effect is important in ore bodies, and in regions with high heat flux. the magnitude is on the order of 10-5 volts per degree kelvin [springer 2012]. geologic case studies are abundant. shankland [1975] describes measurements of thermoelectricity from rock samples in a laboratory setting. leinov et al. [2010] describe thermoelectric effects in brine-saturated sandstone in situ. the thermoelectric effect from ore minerals, for example from pyrite during computeraided resistivity surveys for gold (called pyrite-thermoelectric surveying), has also been described [cao ye et al. 2008, zhang yun-qiang et al. 2010], as has thermoelectricity from magnetite grains in the earth’s crust, and especially in the middle-lower crust [junfeng shen et al. 2010]. this widespread effect is similar to defect charging (described in the defect charging subsection, above) in that both processes mobilize charge carriers and holes (acceptors), the one with a temperature gradient, the other with pressure. note that both of the temperature effects listed in this section (i.e. the thermo-and pyroelectric effects) have sometimes been lumped together as the thermoelectric effect. they have been described as such by corwin and hoover [1979], who treat temperature effects as unwanted signal noise in self potential surveying. they are unwanted if one is looking for electrical indications of water flow (from the motion of ion-rich water, and from the electrokinetic effect) for geothermal use. 7.8. the pyroelectric effect water is a polar molecule. any material whose structure has an axis with dissimilar ends, and whose ends are of uneven electrical charge, is a polar material. the dissimilar ends are called a permanent electric dipole. in polar minerals, electric charges located at the ends of the permanent electric dipole are rapidly neutralized by the environment under normal conditions. during heating or cooling, however, the charges do not have time to dissipate, and are detectable. this phenomenon is called pyroelectricity, or the pyroelectric effect [bhalla et al. 1993]. it is sensitive to both the change in temperature and the rate of change in temperature of a polar material. tourmalines are common minerals that exhibit this effect [hawkins et al. 1995]. typical magnitudes are on the order of 10-6 coulombs per square meter per kelvin for single crystal samples. 7.9. magma electrochemistry the motion of magma during volcanic processes and also of volatiles can hypothetically create an elecearth electricity: a review of mechanisms tric signal. volatiles can in some instances ionize surrounding materials, and magma itself can be rich with ions. no study of these natural electrical phenomena is found in the literature, to the author's knowledge. toramaru and yamauchi [2012], in trying to create an analog to layered dikes and sills, used an externally applied electric field to create cyclically-layered structure in an artificial material, pbi2. 7.10. radioactive emission electric current can hypothetically be caused directly by the motion of charged particles released by the breakdown of radionuclides. for example, α -particle emission is a steady source of charged particles, and therefore creates an electric signal. significant radioactive decay has been reported in natural fission reactors as having occurred in the past [gauthier-lafaye 1997, jensen and ewing 2001, stille et al. 2003]. electrical observations of this phenomenon are not in the published literature, to the author’s knowledge. 8. deep terrestrial phenomena 8.1. geomagnetic jerk short-term changes to the second derivative of the geomagnetic field are termed geomagnetic jerks, and arise from electrical signals traveling through the mantle during deep (core) events [nagao et al. 2003]. transmission of electricity from the upper mantle to the lower crust is likely, but has not been observed. separate geomagnetic jerks of limited extent have been modeled as having been caused by single events originating in and traveling through earth’s core, as described in chulliat et al. [2009]. the physical models suggest that quantifying mantle conductivity is still an open question [malin and hodder 1982]. the electrical conductivity of the deep mantle is two orders of magnitude higher than that of the shallow mantle, with a transition depth of 670 km. another region of higher conductivity transition occurs at 2700 km, the d’’ layer, so named as part of keith bullen’s earth taxonomy from the 1940s [chao 2000, constable and constable 2004, duffy 2008, ohta et al. 2008]. the increased conductivity has been modeled using a combination of proton conduction if hydrogen is present and polaron conduction, which is electron hole hopping between fe2+ and fe3+ ions in minerals that contain iron [yoshino 2010]. both of these are semiconductor phenomena. a wet mantle is not generally required to fit the observations [yoshino et al. 2008]. the increase at the d’’ layer is also related to the transition from perovskite, an orthorhombic mineral, to post-perovskite, a sheet mineral, that occurs at this depth [duffy 2008]. bulk rock responses to local changes in the geomagnetic field caused by changes in the earth’s core ought to affect ore bodies or other rock with high conductivity. no articles are apparently available that report long-term telluric currents caused by fluctuations in the geomagnetic field originating in the earth’s core. 9. producing electricity several models depict electricity-generation processes. four of these are useful in conjunction with the study of telluric currents. these are listed in table 3, with a brief explanation for each. charged particles, if they change location, transfer charge. charged particles, such as electrons or ions, are termed charge carriers. the motions of ions or electrons are examples of the direct transfer of electric charge. a deeper treatment of the ideas in the following description may be found in jonassen [2002]. electrostatic induction is a special case of charged particle transfer. an external charge elicits an electrical response from a second material containing mobile charge carriers. the charge carriers move to neutralize the applied field. if the external charge is positive, for example, then negative charge carriers will migrate within the second material to the site of the external charge. this model is termed electrostatic induction because the charge transfer is induced within one of the materials, but no charge is transferred between them. a more thorough treatment of the ideas in the following description may be found in schieber [1986]. electromagnetic induction occurs in any electrical conductor where a change in a magnetic field occurs, so that the magnetic flux lines pass through the conductor. the combination of mobile charge carriers and magnetic flux creates an electromotive force. this is the principle behind electrical power generation, for example. a coil of wire moves through a magnetic field, and the motion creates electric current in the wire. electricity is generated. if the charge carriers in a material are not mobile, electricity can be generated by deformation of the material, or some other process that changes the configuration of the domains carrying electric charge. the rearrangement of electrical domains can generate electricity. 10. monitoring telluric currents the geomagnetic field is currently monitored by an extensive network of government-run observatories and includes a near-real-time international data repository [kerridge 2001, intermagnet 2012]. the same is not the case for earth’s telluric phenomena, but some helman 10 11 earth electricity: a review of mechanisms model explanation direct transfer of charge charge carriers such as ions or electrons change location, and their motion transfers electric charge electrostatic induction external electric charge creates an electromotive force within a material as particles move to neutralize the external charge electromagnetic induction relative motion of a magnetic field creates an electromotive force on electric charge carriers within it rearrangement of electrical domains deformation creates electricity as it changes the position of immobile charge carriers in a material table 3. electricity-generation models. name magnitude duration frequency lightning strikes lightning strike induction volcanic lightning strikes artificial signals gic (space weather) 105 a (peak) 105 a (peak)/104 a > 3ka (peak) various 200 a (metal) 50 μv m-1 (chorus) not reported (γ-rays) not reported (meteors) 1 ms 1 ms not reported various ≈ 10s various various various 10-3 to 103 mhz 10-3 to 103 mhz not reported various 10-3 to 10-2 hz 300 hz to 2 khz vlf 16 khz planetary magnetic field plasma storm charging electrochemical effetcs electrokinetic effects ocean transport induction gic (diurnal) cosmic particle flux whistler plasma seismic em signals fractoemission seismic electric signals seismic dynamo induction thermoelectric effects pyroelectric effects defect charging exo-electron emission volcanic em signals tid (schumann resonances) geomagnetic jerk magma electrochemistry radiaoctive emission radioactive ionization 5000 v (lunar surface) 10-2 a m-2 5 v 100 mv km-1 25 mv km-1 (metal) 1 mv km-1 (ionosphere) 6000 mev cm-2 (mars) 104 ecm-3 (surface) ≈ 20 to 50 mv m-1 10-5 c kg-1 (ash) 1 to 10 mv μv to mv 10-5 v k-1 10-6 c m-2 k-1 (single crystal) 10-9 a ≤ 108 ecm-2 1 to 5 pt 1.0 pt not reported not reported not reported not reported ≈ 1 week hrs to days various ongoing ongoing ≈ 1 d various not reported ≈ 1 μs various various various ongoing ongoing various various various ongoing ongoing ongoing not reported not reported not reported not reported < 300 khz (ocean) 0.1 to 0.5 hz some from 10-7.0 to 10-3.8 hz 0.4 hz not reported not reported 217 mhz 103 to 105 hz (ice) ≤ 1 hz 10 to 50 hz not reported not reported 102 to 106 hz not reported radio frequencies 10.6, 18.4, 26.0, 33.5, and 41.1 hz not reported not reported not reported not reported oceanic charging tid (compressive event) tid (gravity wave) whistler induction metabolic electrochemistry not reported not reported not reported not reported not reported ongoing various ongoing ≈ 30 s not reported (hypothetical) not reported not reported 10-7 to 10-6 hz 10 hz to 30 khz not reported table 2. magnitude, duration and transmission frequency of earth electricity, arranged by magnitude. gic are geomagnetically induced currents, tid are traveling ionospheric disturbances and eis electrons. national institutions and systems are in place and usually produce freely-available data. china, russia, south africa, japan, greece, the united states and canada, for example, all have networks of magnetotelluric stations to monitor seismic events as they sometimes correlate with electrical and magnetic signals. references or websites exist for china [xuhui shen et al. 2011], russia [istp sb ras 2012], south africa [fourie 2011, facebook 2012], japan [kawase et al. 1993, uyeshima et al. 2001, geospatial information authority of japan 2010], greece [varotsos et al. 1993], and the u.s. and canada [zhdanov et al. 2011, incorporated research institutions for seismology 2012]. no global correlation network of electric signal data exists in real time, though the mtnet, maintained by a working group of the international association of geomagnetism and aeronomy will perhaps assume this role [mtnet 2012]. this association houses research results and data, acts as an international forum, and hosts workshops and conferences. likewise, a working group of the international union of geodesy and geophysics (iugg) hosts conferences and workshops on electromagnetic studies of earthquakes and volcanoes [emsev 2013], and they may have a strong interest in creating this type of network. 11. discussion thirty-two distinct causes of telluric currents have been listed above. here are some highlights that might serve to promote further research. these have been selected as affecting either public health, the economy, or human progress. – lightning phenomena, gic, tid and the thermoelectric effect are mechanisms that have been of the greatest interest to society, since these can disrupt communications or destroy structures and equipment. except for lightning, these act via corrosion, and are slow. – the references to self-potential and the electrokinetic effect may deserve more interest, especially the work out of china, looking at sulfide ores and gold. rare earth elements also seem to be of strategic economic and political importance. – seismic electric phenomena may have immense practical applications for public safety. distinguishing between purported seismic electric signals and other electrical events might allow for successful earthquake forecasting. – the papers on using plant metabolism to power circuits and batteries may be of interest for alternative energy research. harnessing plants directly for electricity would attack anthropogenic climate change on two levels: by providing carbon sequestration and energy production. it is hoped that there will be more progress in these areas, and that tables 1 and 2 will be critically assessed and added to. electrical energy is one of the hallmarks of life, and of space. it is critical for the transfer of information, as well. as more of human culture takes advantage of electronic media, it seems logical and benign to extend human knowledge of electricity in the earth itself. for example, remote sensing is now available for our homes, and we can turn on lights in a room just by moving towards it. how much more important is it, now, to build a more complete system of remote sensing for our global home, the earth, and its environs? missing from this study is a thorough review of data related to the electrical conductivity of the various features of the lithosphere, and the connection between resistivity and rock types, tectonic regimes, seismic activity, geochemistry, and similar geologic parameters. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 09altez_grases.qxd 451 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words venezuela – earthquake catalogue – seismological methodology – historical seismology 1. introduction: the interpretative transformation the 26th of march 1812 was holy thursday in venezuela. a few months before, in july of 1811, the first republic had been declared. this was to rupture with the colonial past. on the afternoon of the 26th, with cathedrals filled with people, a slightly confused society was preparing itself to profess its faith and, perhaps, to kneel in prayer for its uncertain destiny. in the mean time, the new republican authorities were nervously discussing how to confront a spanish army that was already approaching the capital from the west. a few minutes after four o’clock in the afternoon, in caracas, bishops and worshippers observed how the churches collapsed, burying thousands. simultaneouslpy, the cities of barquisimeto, la guaira and san felipe were converted into a pile of ruins. almost one hour later, mérida saw a similar scene, when at five o’clock in that same afternoon, everything turned into dust and rubbish. over a distance of about 800 km, and with an imperceptible, at that time, difference of approximately one hour, two great earthquakes followed by several aftershocks tumbled the ambitions of the rebel criollos and, at the same time, turned out to be the last earthquakes of the colonial record and the first of the republican era. this was not only a historical landmark, but also a hermeneutic landmark. with the coming of modernity, american societies were not only shaken institutionally, but also underwent a deep ideological transformation. with the advent of the nineteenth century, new forms of thinking and new behaviours appeared. for that reason, latin america’s excolonial societies began early on to readjust their societal relationships and to re-legitimise a brief history of a restless vision: seismological research in venezuela rogelio altez (1) and josé grases (2) (1) escuela de antropología, universidad central de venezuela, caracas, venezuela (2) academia de ciencias físicas, matemáticas y naturales de venezuela abstract seismological research in venezuela has managed to survive in the twentieth century in spite of having produced only two catalogues for public consultation in a period of sixty years, the last of them being published in 1999. however, great investigators, national and foreign, gave their best years to scientific knowledge. through their restlessness and attitudes, the perspective of the science of earthquakes was constructed. watching through their eyes, and by means of the 1812 earthquakes, the most controversial and important in this country, the history of the venezuelan seismology is revealed in this paper. amidst discussions and ruins, opening footpaths on destructive earthquakes, venezuelan seismology began to build its own vision. mailing address: dr. rogelio altez, escuela de antropología, facultad de ciencias económicas y sociales, universidad central de venezuela, residencias vargas ii, los chaguaramos, caracas, venezuela; e-mail:ryaltez@cantv.net 452 rogelio altez and josé grases powers. this process, occurring from the inside out, was totally focused on political and economic issues, so that the observation of the development of a scientific mindset was far from being a priority in the first decades. thus, the observation and study of nature had little space in daily life in the convulsive postcolonial republics. the venezuelan situation fits accurately within this description. the 1812 disaster, which had foiled the first emancipating attempt of the criollos, at the same time, ended a way of life and a way of understanding nature’s acts and therefore, a way of constructing reality. the ideological transformation of the colonial society into a liberal society, which lasted throughout the nineteenth century, turned out to be a political project, and counted among its priorities only institutional consolidation and an unattainable economic stability, over any other type of development or social growth. intellectual activities were, at that time, so involved with the political process that academic life or scientific thought had little to do. in that sense, the observation of nature seems displaced by the more immediate problems. this situation, which captured society’s attention in the early years of the republic, began to change, and spaces opened up for academic life in the second half of the nineteenth century. the development of natural sciences, imposed perhaps by an eagerness for status and guided by the rhythm of phenomena, made important steps following great events. concerning seismic ones, after 1812 there were no destructive earthquakes until 1853, when the city of cumaná suffered one of the most important quakes in venezuelan history. some comments, attempts at erudite explanations, came to the public light in those days. nevertheless, it was only with the appearance of the sociedad de ciencias físicas y naturales (physical and natural sciences society), whose origin dates back to 1866, that hypotheses about the behaviour of nature had a formal frame for their discussion. thus, the eyes that observed the natural phenomena initiated their modern reading of reality. 2. from the enlightenment to the academy many were the travellers who walked the american roads between the eighteenth and the nineteenth century. men of the enlightenment and other adventurers lived years of astonishment in the new world. everything nature offered became an exotic window for interpretation and experimentation. armed with an exuberant and romantic rhetoric, their narratives drew scenes that tread between the magic and the descriptive. for that reason, natural phenomena caught their attention, and their efforts to explain them were quickly transformed into technical manuals, methodological guides and objects of discussion and contrast. the most obvious case is that of the german scholar alexander von humboldt, who generously contributed to the knowledge of the equatorial regions. his travels in venezuela allowed him to witness several interesting events, among them, some remarkable earthquakes. nevertheless, his most remarked opinions on the matter can be found in his work «viaje a las regiones equinocciales del nuevo continente» (humboldt, 1956) – the first edition of which is dated 1814 – on the earthquake of 1812, in spite of his not having been present at that time in venezuela. generally, humboldt’s text should be considered a credible secondary source of those phenomena, since it contains accounts from direct witnesses who, later, would have their own say on the matter. it could be said that it was with the german scholar that observations on nature in these latitudes were inaugurated. his intellectual contributions on natural phenomena, geology and geography served as the departure point for many later followers or detractors. nevertheless, the text of manuel palacio fajardo (1816) regarding the events of 26 march 1812 must be considered to be the first venezuelan contribution to the discussion of earthquakes. he not only personally gave humboldt a manuscript that described the earthquakes, during an encounter in paris in 1813 (altez, 1999), which in fact, is one of the references that the german scholar used in his work, but he also published his own conclusions in «an account of the earthquake of caracas» (palacio fajardo, 1816). in addition to being an 453 a brief history of a restless vision: seismological research in venezuela open supporter of the independence movement, palacio fajardo was the chair of the medical department of the real colegio de mérida (real college of mérida), in 1808, and counted among his diplomas those of medical doctor and doctor of civil and canonical law (urbani, 1990). certainly, he was a well informed criollo who initiated, without knowing it, a long discussion of almost two hundred years on the 1812 earthquakes, and who, like a strategic bridge, helped to cross the boundary between pre-modern thought and modernity. that boundary was being crossed not only by venezuelan thinkers, but also by all western culture, which began to understand itself and its natural surroundings, from a standpoint more concrete than mystical, more physical than metaphysical, and more scientific than religious. for that reason, when on 15 july 1853 the city of cumaná was levelled by an earthquake, the few accounts mentioned in newspapers of the time were from the pen of a very reliable witness: daniel beauperthuy (1807-1871), who, at the time, lived in that city and who later discovered the mechanism for the transmission of yellow fever (lemoine and suárez, 1984). the first commentaries on this event, as well as the first references to seismic issues in venezuela, was written by alejandro ibarra (1813-1880), physicist, mathematician and philosopher, the person who inaugurated with his writings the venezuelan seismological vision (fig. 1). ibarra would publish, in the newspaper diario de avisos, an article titled «temblores y lluvias en caracas» (earthquakes and rains in caracas, 1854), in which he attempted to explain the reasons for the occurrence of several natural phenomena with a rigorous academic style. his quality as academician and intellectual authority of the university of caracas, gave his discourse a formality that was already demanding space and recognition in a growing academic and intellectual context. later, in 1862, his work, in ten articles entitled temblores y terremotos (tremors and earthquakes, 1862), was published in the newspaper el independiente (caracas), and, as a collection, would become a historical document on the venezuelan scientific reflections of the nineteenth century. a few years later, and as an outgrowth of this initiative, the sociedad de ciencias físicas y naturales de caracas was founded, under the initial direction of another german of repute: adolfo ernst. under the auspices of that scientific society, the most qualified discussions took place on anything that could be appreciated as a physical phenomenon, especially the subjects of mineralogy, meteorology and geology, unavoidably tied, by then, to the current explanation of earthquakes. systematic quantification and measurements on the behaviour of nature captured the attention of the scientists, and thanks to their indefatigable work, today we can count on important data for cataloguing natural phenomena. the academic attempts of these thinkers went beyond their scholarly discussions to be published in their own journal: vargasia (with only seven editions, between 1868 and 1870) (fig. 2). from there, and in the space offered by newspapers of the time, enlightened researchers spoke of their scientific conclusions. after alefig. 1. alejandro ibarra 1813-1880, seismologist (from leal, 1994). 454 rogelio altez and josé grases jandro ibarra, the qualified members of the sociedad de ciencias naturales continued the seismological discussion. arístides rojas (18261894), an active member, was one of the most outstanding in this area. his writings on humboldt are famous (for instance, humboldtianas, 1942), always accompanied by empirical efforts to contrast the hypotheses of the german, and his numerous articles in venezuelan newspapers. even more, rojas maintained correspondence with scientists from all over the world, and saved their letters and exchanges of information, keeping an archive in which he filed more than fifty articles from national and international newspapers, on earthquakes, volcanoes and the origin of the earth. this archive is conserved today in the academia nacional de la historia de venezuela (national academy of history), in addition to thousands of other papers that at one time were part of his personal file. foreign scientists who spent time in the country, also contributed to the development of these discussions, as, for example, another german, wilhelm sievers, who dared to formulate interesting reasoning on 1812, published by mittheilungen der geographischen gesellschaft zu hamburg (1884), following diligent field work in the mountains of caracas. he introduced an element unappreciated at that time as a valid resource: the reading of documents contemporary to the disaster. his work received the angry answer by his compatriot ernst (1886), in the nederlands aardrykskund genoostrah, following which the discussion on that afternoon of 1812 was intensified and the debate on venezuelan seismology command. sievers (sievers and friedrichsen, 1895; sievers, 1905) was to write again on earthquakes in venezuela, after the great earthquakes of 1894 in los andes and 1900 in caracas. the nineteenth century ended with a certain academic maturity in the equatorial regions of the enlightened humboldt. mixed between formality and amateur collecting, other tireless venezuelans would contribute with important information in their works to the future of seismology in venezuela. for example, tulio febres cordero (1860-1938), editor, bequeathed invaluable data on the earthquake in the andean region (table i), becoming, perhaps consciously, a historical collaborator in earthquake research (febres, 1929 and 1991). by then, at the doors of the twentieth century, both great earthquakes mentioned before (1894 and 1900), had underlined the need for the construction of an exclusive and official space, and of systematic actions, to take care of the problems generated by a recurrent natural behaviour that was soon to be known as «disaster». 3. the necessary specialization when on 28 april 1894, the venezuelan city of mérida collapsed once again in history (fig. 3), under one of the most devastating earthquakes of the region, the editorial office of the newspaper el lápiz, edited by tulio febres cordero, collapsed. miraculously, febres cordero saved his life, unlike more than 300 people, or el lápiz (which did not continue publication). the damage of that violent earthquake to churches, buildings, railways and the landscape in general, exposed to a great extent the impotence of all, spectators and victims, in the face of attacks by nature. six years later, on fig. 2. vargasia’s cover. the journal was published between 1868 and 1870 in caracas. 455 a brief history of a restless vision: seismological research in venezuela 29 october 1900, caracas and a vast extension of the coast to the north of the city, facing the caribbean sea, suffered the last earthquake of the nineteenth century and first of the twentieth century. once again, that symbolic boundary illustrated important transformations in the venezuelan society of the times. while caracas and its surroundings succumbed during the 1900 earthquake, two great researchers of earthquakes initiated methodology for advancing knowledge of earthquakes, going so far as to mention the responsibility of geological faults as their origin. the scholars at issue are j. milne (1850-1913) and montessus de ballore (1851-1923), and the methodological resource, of course, was the cataloguing of events. milne compiled information on destructive events, which «...probably were the result of the formation or extension of geologic faults» (grases, 1986), in a catalogue (milne, 1911) presented at the meeting of the baas (the british association for the advancement of science), in 1911, held in portsmouth. montessus de ballore (1915-1919), at the same time, put together an extensive archive of manuscripts with a combined total of 90 feet of shelves, containing information on earthquakes totalling 171 434 entries (grases, 1986). it was a time of precursors. in caracas, and thanks to the earthquake of 1900, the first map with intensity contours elaborated by a venezuelan was published in the newspaper la linterna table i. descriptive and parametric catalogues in venezuela authors funded by access date of edition time-window area tulio febres cordero autofinancial public 1929 1611-1929 andean region josé félix soto sociedad venezolana de ciencias naturales public 1931 1802-1918 caracas melchor centeno graü academia de ciencias físicas, matemáticas y naturales public 1940 and 1969 1530-1949 venezuela observatorio cagigal national government public 1959-1978 1959-1978 venezuela edelca (electricidad del caroní) privado restricted 1987-1999 1987-1999 guayanese region josé grases pdvsa-intevep restricted 1979 1530-1977 eastern region josé grases pdvsa-intevep restricted 1980 1530-1977 western region funvisis national government public 19831983venezuela josé grases, rogelio altez and miguel lugo academia de ciencias físicas, matemáticas y naturales, universidad central de venezuela public 1999 1530-1998 venezuela 456 rogelio altez and josé grases mágica (caracas, 15 november 1900). the author, engineer melchor centeno graü (18671949), steered the necessary course: that of specialists. considered the pioneer of seismology in venezuela, his greatest contribution was, indeed, the first earthquake catalogue published in this country (centeno graü, 1940) (table i). previously, important «lists» had been elaborated by other contemporary researchers. one of those was that of his colleague josé félix soto, titled «la sismología en venezuela» (the seismology in venezuela) and published by the boletín de la sociedad de ciencias naturales (bulletin of the natural sciences society), in 1931. nevertheless, the work of centeno exceeded all the previous work made in this country. with the title of «estudios sismológicos», (seismological studies) in 1940, centeno graü published a catalogue preceded by an introductory study in which he carefully explained all the current theories on earthquakes, accompanying them with his own hypotheses on the matter. he reviewed all the earthquakes that he was aware of, not having been a documental researcher. he illustrated his work with pictures, figures and maps, to which he added details that had not been taken into account before, such as the assessment of damage and the time of the events. he tried to establish periods of recurrence, calculating cycles of destructive earthquakes, based on chronological criteria. he used the scale of mercalli and concentrated on earthquakes between intensity vii and x. he established bonds with geology, volcanism and other risks (grases et al., 1999). he also discussed the 1812 earthquake, indicating that it was a «polycentric seism» and of simultaneous action (altez, 2000b). the academia de ciencias físicas, matemáticas y naturales de venezuela (academy of physical, mathematical and natures sciences) would publish in 1969 a second, amplified edition of centeno’s catalogue (centeno fig. 3. damage caused by the earthquake of 1894 in mérida, venezuela (from romero, 1894). 457 a brief history of a restless vision: seismological research in venezuela graü, 1969), with the addition of some details that were taken from his notes after his death. that study was the basis of the knowledge of seismic history in venezuela for decades, and the only published catalogue until 1999 (table i). meanwhile, another german, günter fiedler, was to contribute to venezuelan geological research. having arrived in caracas after world war ii, fiedler entered the scientific scene with his presentation in the iii congreso geológico venezolano (iii geological congress of venezuela, 1961), «areas afectadas por terremotos en venezuela» (areas affected by earthquakes in venezuela) (1961). later, in the iv congress of the same discipline, in his paper on «la liberación de energía sísmica en venezuela, volúmenes sísmicos y mapas de isosistas» (the liberation of seismic energy in venezuela, seismic volumes and isoseismal maps) (1969), like many of his predecessors, he would give his opinions on the earthquake of 26 march 1812 (fig. 4), affirming that there were three earthquakes with different epicentres, where one could have triggered the others, with an imperceptible difference between them (fiedler, 1969). in the aftermath of the most destructive earthquakes in the history of venezuela, seismological research inserted its discourse and made its advancements. the best example, perhaps, is that of 1812, which even today generates serious controversy. fiedler would be an important protagonist in setting this course in the science of earthquakes in venezuela. later, in 1967, when the last destructive earthquake of caracas occurred, the german thinker would be in charge of the observatorio cagigal (cagigal observatory), and would observe, with frustration, how the force of the movement caused the collapse of the instruments located there. again, an earthquake marked great changes in seismological observation in venezuela. after 1967, the earthquake issue would become an official matter. fig. 4. map of intensities for 1812, from fiedler (1969). 458 rogelio altez and josé grases 4. instruments and institutions the appearance of applied technology, and its uses in research and knowledge, was one of most important advances in seismological research at the turn of the 20th century. in venezuela, with the creation of the observatorio cagigal, in 1889, the formal monitoring of natural phenomena was made official. in 1901 two seismographs were installed; a pair of rudimentary instruments, agamennone and ewing (salas, 1990), unfortunately no bulletins of their performances have been found. however, hubschmann (1988) alleged the existence of an instrumental registration of seismic movement by means of a horizontal pendulum used by henri pittier, prior to the seismographs of the observatory. in any case, the new century began to give signs of what would be its fundamental characteristic: technology. instrumental seismology, the fundamental resource for research and monitoring of earthquakes, was now being carried out in an accurate institutional context. advancement in this field obviously depended on governmental decisions. only after the occurrence of a destructive earthquake in the capital of the country, were the necessary steps taken towards the consolidation of seismological research and the financing of technological development. soon after the 1967 earthquake, the fundación venezolana de investigaciones sismológicas (funvisis, venezuelan foundation of seismological investigations) was created (1972). in this way, a specialized institution was born, exclusively dedicated to observation and seismological analysis. funvisis would be in charge of the creation of alarm networks, and for the installation of observation stations all over the country, as well as the production of knowledge, through research and explorations. the occurrence of a destructive event was necessary for the seismic history of venezuela to be remembered. probably, this experience has been repeated around the world; nevertheless, even more alarming than the fact that this is a common variable, is the fact that it has reccurred in spite of previous experiences. it is for that reason that it took so long for the political and scientific world to understand that the problem is not one of assistance, but the prevention of disasters. 5. researchers, criteria and discussions after the work of fiedler, venezuela has counted on the contributions of investigators of long standing, some of them still producing. over the last decades, the research of andré singer stands out; he is, perhaps, the most brilliant geomorphologist in the country. french of birth, singer become funvisis president, after many years as chair of the departamento de ciencias de la tierra (department of sciences of the earth). there he was in charge of promoting documentary research, initiating a research project that has not yet found an heir: the inventario nacional de riesgos geológicos (national inventory of geological risks), which produced a preliminary publication in 1982 (singer et al., 1982). innumerable articles enrich the shelves of funvisis through his research. another venezuelan of german heritage, carlos schubert, geophysicist, also dedicated years of his life to the study of earthquakes, and standing out among his more than eighty scientific writings, is his book «los terremotos en venezuela y su origen» (the earthquakes in venezuela and their origin, 1982), a didactic manual for general consultation. also notable are the contributions of josé grases, venezuelan engineer who initiated his study of earthquakes after his observation of the effects of the 1967 earthquake on the buildings of caracas (grases, 1979, 1980, 1986 and 1990; grases et al., 1999). the catalogue by grases et al. (1999) deserves a special commentary. as we indicated earlier, the catalogue of centeno was the only published catalogue available in venezuela following its first edition in 1940, illustrating the lack of attention in this country regarding seismological cataloguing, and the official uninterest in historical and documentary research. in effect, even though in funvisis efforts were made to include in their publications historical knowledge of venezuelan geological activity, those pieces of research only figured under the title of «historical seismicity», and only attained descriptive levels. only occasionally were those levels extended to embrace an interpretative scope. the elaboration of a seismological catalogue, naturally, has to be the duty of the official institution that systematically in459 a brief history of a restless vision: seismological research in venezuela vestigates earthquakes; nevertheless, the initiative was developed outside its offices and depended on external academic financing. at the same time, the catalogue published by grases et al. (1999) is also a revision and update of the pioneering work of centeno, taking it as its basis, displaying information, correcting and extending if necessary, and adding the results of years of documentary research. at the present time, the researchers related to historical seismology in all parts of the country, are following up an initiative that took its first step in 1997, when in the andean city of trujillo, the primeras jornadas de sismicidad histórica (first meeting of historical seismicity) was held. today, following three of those seminars, they have changed their name to jornadas de sismología histórica (meeting of historical seismology), in recognition that the works presented go beyond the descriptive levels and attempt to produce systematic knowledge. these efforts are supported by funvisis, the universidad de los andes (university of los andes), the universidad central de venezuela and the sociedad venezolana de historia de las geociencias (venezuelan society of history of the geosciences). interestingly, 26 march 1812 is still being discussed there... at the first event of 1997, an anthropologist presented a work titled «cronometrización extemporánea, los sismos del 26 de marzo de 1812 en caracas y mérida» (untimely chronometry, the earthquakes of march 26th of 1812 in caracas and mérida) (altez, 1998), where he demonstrated that not one earthquake destroyed so many cities at the same time, but that there had been two earthquakes with one hour difference occurring, the first at 04:07 p.m. in caracas (fig. 5), and the second about five o’clock in the afternoon in mérida. these conclusions were based on documentary research supported by the knowledge of the seismologists of funvisis. after so many decades of discussion and with debates that span three different centuries, the events of 1812 continue to provoke controversy, establishing a transhistorical dialogue among researchers who follow the steps of humboldt or sievers, fiedler or ernst, victims or witnesses. fig. 5. map of intensities for the 1812 earthquake in caracas (altez, 2000b). 460 rogelio altez and josé grases 6. catalogues and sources it has been shown above that cataloguing has received very little stimulus in venezuela. from the listings and chronologies of the nineteenth century, elaborated by amateurs and collectors, through the academic attempts to obtain knowledge of nature’s behaviour from the repetition of the events, catalogues have been isolated efforts with no continuity (table i). in fact, although centeno’s work is known as a «catalogue», it is an inventory of dates and data, that can be called «chronology». certainly, a chronology, or «chronological list», it is based on the idea of presenting the repetition of natural phenomena in an orderly way. that is the quality of the works that were elaborated before the understanding of the meaning of the cataloguing. however, centeno’s work changed the theoretical support of chronologies and gave the first steps toward a new understanding of historical earthquakes. among lists and amateur studies, in 1929, tulio febres cordero would publish his «cronicón sísmico» (seismic chronicle); later, as already mentioned, josé félix soto would publish «la sismología en venezuela» (1931); and centeno would initiate his «estudios sismológicos» (1940), that would take years to be followed up; also, the observatorio cagigal would publish the result of its activities in its boletines sísmicos mensuales (monthly seismic bulletins) (19591978), which covered the period between 1959 and 1978. nevertheless, it must be noted that at least from 1979, the intention has existed to catalogue the seismic activities of the country in some official institutions. this intention however, has not necessarily been in the public domain or available for consultation. as an example, pdvsa, one of the main companies in charge of oil exploration in venezuela, and one of the main financial supporters of seismological research, has sponsored studies, through its branches, fundamentally intevep. they produced a pair of important catalogues previous to the one of 1999, but they were not available to the public: both were the work of grases (1979 and 1980), and are reserved for specialised consultation for official state research. later, the institutions would understand the importance of cataloguing, at least from the instrumental bulletins, thus producing information of utility for their interests. consequently, lists were elaborated of instrumental bulletins without interpretative intent and strictly designed for technical use. the boletines sísmicos mensuales de la red de guri (monthly seismic bulletins of guri network) (edelca, 1987-1999), the boletín sismológico de venezuela (seismologic bulletin of venezuela) (funvisis, from 1983), and the catálogo de sismos registrados instrumentalmente con epicentro en venezuela (catalogue of instrumentally-registered earthquakes with epicentres in venezuela) (funvisis-intevep, 1993), are examples that illustrate this situation. these are catalogues based only on instrumental monitoring and not integral cataloguing that includes historical information. in fact, it would appear that a certain epistemological criterion underlies the elaboration of these lists of registrations, which seems to acknowledge a split between the «instrumental» and «historical», as if they existed in different and disconnected worlds. from this stance, the «historic» is perceived as irrelevant and documentary registers are interpreted as failed interpretations from a context already surpassed discursively and theoretically. in 1886, in answer to the efforts of sievers to interpret the 1812 earthquake, adolfo ernst replied forcefully: «i have looked in vain in the local archives for more documents and i have only found in the palacio arzobispal some data sent by the parish priests from diverse localities to the superior ecclesiastical authorities; nevertheless, for the geological history of the earthquake, they completely lack value» (ernst, 1886, p. 481, translation by the authors). in spite of the temporal distance, these methodological criteria still seem to be held by many of the researchers of the present time. probably, one of the main reasons behind that position, although it is not assumed as transparently as by the german scholar of the nineteenth century, has to be the practical ignorance of documentary handling. it is difficult to find geologists, seismologists, engineers or geophysicists, with adequate preparation in paleography or knowledge of primary sources, in spite of the interest they may profess. this is an unavoidable limit to the seismological perspective. 461 a brief history of a restless vision: seismological research in venezuela there are only a few occasions in venezuela when it is possible to appraise the effort to integrate several methodological resources and different epistemological tools. the already mentioned jornadas de sismología histórica, have provided spaces for the effort of the researchers concerning the historical knowledge of earthquakes. these gatherings promote discussions among investigators coming from different branches of science: geophysicists, geomorphologists, historians, anthropologists, engineers, geologists, geographers, topographers and many students. in the last few years, the more stimulating papers have been a good example of it: christl palme, geophysicist, with her research on the earthquakes of 1674 (palme, 1993; palme and altez, 2002); the works of the geomorphologist carlos ferrer and also the geophysicist jaime lafaille on the events of 1610 and 1812 (1998 and 2000); one of the works of andré singer, «contribución de simón rodríguez a la emancipación científica independentista latinoamericana...» (2000); the research of altez on 1812 (1998, 1999, 2000b and 2000c); the catalogue of grases et al., (1999); and the article by the engineer jose antonio rodriguez on the earthquake of cumaná of 1929 (1998). separate mention must be made of the dedication and the contribution of franco urbani, in charge of the sociedad venezolana de historia de las geociencias, which has published dozens of historical documents related to geology, seismology and the geography of venezuela, through the boletín de historia de las geociencias en venezuela (bulletin of venezuelan society of history of geo-sciences), with 71 volumes from its creation in 1984 (e.g., urbani, 1990). this effort to coordinate an interdisciplinary vision is, certainly, a multifaceted link between natural sciences and primary sources. the knowledge of historical processes and the knowledge of the behaviour of the active faults have been interlinked. also, the need to know possible periods of recurrence forced the investigators to look at the colonial past and the nineteenth century, telluric places of irretrievable data for the physical sciences. for that reason, the information contained in centeno’s catalogue provided initial, although often insufficient, historical background for seismological research. even in centeno’s catalogue there are very few references for the first four centuries of history of the region. between the first entry of that work, referring to 1 october 1530, and 1900, there are only 36 pages of information. in the catalogue by grases et al. (1999), those four centuries fill 245 pages with data collected in primary and direct sources, all of them accompanied by their documentary references. this difference between the two catalogues is, above all, qualitative. it is the result of the concrete interest and the need to know those processes better. works previous to the catalogue peeked into that past, sometimes gropingly, and others with certainties rarely reached among historians. punctual research, such as that financed by the oil companies, provided documentary revelations on events of the past. the engineers doing these researches were forced to go through piles of mouldy, dusty manuscripts and indecipherable handwriting. at that time, the best solution was to contract documentary specialists. however, from the discussion between social and natural scientists, the idea was born to create its own space, such that of the jornadas. this was a methodological recognition that singer noted in his words of opening: «before world war ii, in their own field and for restricted global scales of time incomparably more restricted, historians faced the same problem confronted by the seismologists of the necessity of an analogous widening of the «time window» at the time of the development of the well-known historical school of annals of economic and social history arises in france, headed by marc bloch, lucien febvre and fernand braudel. this school of thought contributed to the interdisciplinary bases necessary to unify the understanding of long term historical phenomena relative to the trends the economic fluctuations, and the daily vicissitudes of micro-history, corresponding to the development of the diverse forms of life of the men, policy, military, religious and artistic». (singer, 1998, pp.18-19, translation by the authors). however, the handling of those sources still continues being the art of specialists. but knowing the documentary repositories does not mean 462 rogelio altez and josé grases necessarily an understanding of the semantic content of texts, nor the ability to interpret them in a seismological way. even more, information production, which is directly determined by the historical contexts, is elaborated and structured semantically in relation to hermeneutic senses different from those in the present. this is why what is reflected in a text from the past, literally means what it exposes. in order to understand it, an interpretative effort that knows much more than the palaeographic techniques is needed: it must differentiate the discourse of the context, which implies, therefore, knowledge of the mindset of that context: «the elaboration of information that a society produces daily, either from its official instances, or from its own protagonists, is always in coherence with the fundamental structures of that society. in that sense, the information has particular characteristics that identify it with the historical processes to which it belongs. for that reason, the structural differences that separate the colonial model from the republican, for example, also will create substantial distances in information production» (grases et al., 1999, p. 16, translation by the authors). therefore, information is the result of putting into practice the reading of the reality that its context contains; consequently, the quality and the amount of that information also are determined by the same circumstances. for that reason, the common direction that has been used for the search of information in the past on the part of the researchers has been, generally, one that uses as a starting point the date of the event, when in fact, to arrive at the event, the direction has to be inverse: it must go in the first instance, to the context of the society of the time, which places the information produced in relation to the event. it is not the fact that guides the information, but its context. before there was awareness of this methodological strategy, the seismic silence of the region in the first centuries of history was much greater. in the case of the colonial past, much of the information that appeared for the 1999 catalogue, was the outcome of a two-vectored search: one of them concentrated in the information on geographical relations, prospective mineral searches (both coincident with the spanish strategy of precious metals) and travellers; the other, aimed toward the history of the churches. the latter proved richer documentarily and equipped with a determining variable for the case: they were the only buildings of the time, that is to say, they were the best witnesses of damage and ruins. more surprising is the seismic silence from the nineteenth century, where there was better access to the interpretation. nevertheless, except for the most destructive earthquakes, almost nothing was known of other earthquakes. again, the knowledge of the contexts facilitated the search: «it is for these reasons, fundamentally, that the first decades of the republican life of venezuela show a seismic «silence» that surely cannot be taken as a faithful testimony of the geologic reality. if the earth moved and it did not destroy that which remained after the war and the earthquake of 1812, who would care? it would be necessary to wait for a few decades so that the newspapers proliferated, diversified their contents, and promoted columns on the opinions of scientists and students of nature. the negligible information on earthquakes found for the years between 1812 and 1853, is a faithful witness to this» (altez, 2000a, p. 471, translation by the authors). in the catalogue of centeno, between 1812 and 1853 there are only 13 entries in three pages; in the 1999 catalogue, for the 1812 earthquakes alone there are 26 pages. this evidence regarding information is the reflection of the need for a methodological transformation in the search of data. it is, finally, the recognition of an interpretative change, or a hermeneutic turn, in seismological research. it is a necessary turn that gently moves to unite the multiple visions of the sciences, perhaps an insight leading to the inescapable idea of only one science. references altez, r. 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(1990): terremotos destructores del caribe, 1502-1990 (unesco-relacis, montevideo), pp. 132. grases, j., r. altez and m. lugo, (1999): catálogo de sismos sentidos o destructores: venezuela, 1530-1999 (academia de ciencias físicas, matemáticas y naturales/facultad de ingeniería, ucv, caracas). hubschmann, k. (1988): observatorio cagigal, cien años de historia y de ciencia (lagoven, caracas). humboldt, a. von (1956): viaje a las regiones equinocciales del nuevo continente (ediciones del ministerio de educación, caracas). ibarra, a. (1854): temblores y lluvias en caracas, diario de avisos (caracas), 2 september, p. 2. ibarra, a. (1862): temblores y terremotos, el independiente, (caracas), march-april. lafaille, j. and c. ferrer (2000): el terremoto del jueves santo en mérida: año 1812, ii jornadas de sismicidad histórica (mérida) (forthcoming). leal, h. (1994): la enseñanza de la física en la universidad central de venezuela, 1827-1880, fuentes para la historia republicana de venezuela (academia nacional de la historia, caracas). lemoine, w. and m.m. suárez (1984): beauperthuy, de cumaná a la academia de parís (ucab-ivic, caracas). milne, j. (1911): a catalogue of destructive earthquakes a.d. 7 to a.d. 1899, baas (london), pp. 92. montessus de ballore, f. de (1915-1919): bibliografía general de temblores y terremotos, sociedad chilena de historia y geografía (imprenta universitaria, santiago de chile). observatorio cagigal (1959-1978): boletines sísmicos mensuales (caracas). palacio fajardo, m. (1816): an account of the earthquake of caracas, quarterly j. sci. (london), 3, 337-348. palme, c. (1993): los terremotos de los años 1674, 1775 y 1886 en trujillo (departamento de física y matemáticas, universidad de los andes, caracas). palme, c. and r. altez (2002): los terremotos de 1673 y 1674 en los andes venezolanos, interciencia (caracas), 27 (5). rodríguez, j.a. (1998): de la sismicidad histórica a la sismicidad instrumental: el terremoto de cumaná de 1929, rev. geogr. ven. (facultad de ciencias forestales y ambientales, instituto de geografía y conservación de recursos naturales, ula, mérida), 189-202. rojas, a. (1942): humboldtianas (editorial cecilio acosta, caracas-buenos aires). romero g., p. p. (1894): fotografías de daños ocasionados por el gran terremoto de los andes, colección venezuela siglo xix, archivo audiovisual, biblioteca nacional (caracas). salas, h. (1990): primeros sismógrafos en venezuela, boletín de historia de las geociencias en venezuela (sociedad venezolana de historia de las geociencias, caracas), 39, 1-16. schubert, c. (1982): los terremotos en venezuela y su origen (lagoven, caracas). sievers, w. (1884): das erdbeben von 26 märz, 1812, an der nordküste südamerikas, mittheilungen der geographischen gesellschaft zu hamburg, 265-271 (a facsimile copy). sievers, w. (1905): das erdbeben in venezuela von 26 oktober 1900, jahrb. veroffnet. geogr. ver. (bonn), 35-50 (a facsimile copy). sievers, w. and l. friederichsen (1895): das erdbeben 464 rogelio altez and josé grases in venezuela von 28 april 1894, mittheilungen der geographischen gesellschaft zu hamburg, 10, 237-244. singer, a. (1998): palabras de apertura a las primeras jornadas de sismicidad histórica, rev. geogr. ven., (mérida), 39 (1-2), 17-21. singer, a. (2000): contribución de simón rodríguez a la emancipación científica independentista latinoamericana: su papel como precursor de los estudios de microzonificación sísmica en concepción, chile, ii jornadas de sismicidad histórica, mérida, (forthcoming). singer, a., c. rojas y m. lugo (1982): inventario nacional de riesgos geológicos, estado preliminar, (funvisis, caracas). soto, j. f. (1931): la sismología en venezuela, bol. soc. ven. cien. nat., 2, 37-89. urbani, f. (1990): manuel palacio fajardo (1784-1819) y su contribución a las ciencias naturales, bol. hist. geoc. (caracas), 40, 24-30. s u l l e o s c i l l a z i o n i l i b e r e d e l l a g o d i a l b a n o p . caloi m . giorgi m. g i o r g i (*) lia f a t t o l a d e t e r m i n a z i o n e d e g l i e l e m e n t i f o n d a m e n t a l i d e l l e sesso d e l l a g o di a l b a n o , a p p l i c a n d o al m e d e s i m o i m e t o d i d i h i d a k a e d i d e f a n t . l a c u r v a n o r m a l e d e l l a g o (fig. 1) c o n s e n t e p e r ò l ' a p p l i c a z i o n e di lino d e i m e t o d i d i c h r y s t a l , e s p o s t o e d esteso d a c a l o i a d a l c u n i l a g b i i t a l i a n i (2"3'4). l a c u r v a n o r m a l e d e l l a g o di a l b a n o p u ò i n f a t t i e s s e r e s c h e m a t i z z a t a c o m e d a fig. 2. 1. vediamo come il problema può essere risolto analiticamente. —r i f e r i a m o l a c u r v a n o r m a l e ad u n s i s t e m a d i assi c a r t e s i a n i con o r i g i n e i n o i , e s s e n d o v e r s o at il senso p o s i t i v o d e l l e ascisse e v e r s o m q u e l l o d e l l e o r d i n a t e . se f a c c i a m o o j ai = «, , o, an ~ a2 , oim — h , 0 , p = p , otq = q , l e e q u a z i o n i d e l l e r e t t e 1 e 2 d i v e n g o n o hl(x) = h[ 1 ^ ) » h(*) = h(l + j ) . p o n i a m o 2 ' r si h a [ 2 ] f a , 1 g h i g h 1 a i v gh v « 2 kh ) 'gh \ u t u t m j aj^wj+b^aj j sin n (t-t) h ( ì ^ 1 / , f c o 1 ) + b t y o k ) j 8 ì n n ( r | / , ( < » , ) + » , ! > > , ) | s i n n ( t t) n ( t r ) , [ 2 ] 2 4 8 p . c a l o i m . g i o r g i dove j e 1 sono s i m b o l i d i f u n z i o n i di bessel e di n e u m a n n rispett i v a m e n t e . n e l l e [ 2 ] s o n o da d e t e r m i n a r e l e c o s t a n t i a,, blf a-z, b2, n val e n d o s i d e l l e c o n d i z i o n i ai l i m i t i . delle p r i m e q u a t t r o , c o n s i d e r i a m o i r a p p o r t i a/b. l e c o n d i z i o n i ai l i m i t i cui d e b b o n o s o d d i s f a r e le [2] c o n s e g u o n o « c o . ! \ >6 | "p « <8 5? 60 v( x) f i g . 1 d a l l ' a n n u l l a r s i di ; agli e s t r e m i p e q d e l lago e d a l l a legge di cont i n u i t à . si b a : x=p i ?i=° ; *= -<] i ?»=o i x = = 0 , ^ — ìo , t 4 = t 2 , w ^ r t ' i , , m„ — na„ . ̂ [3] p e r le p r i m e d e l l e [ 3 ] , c o n s e g u e d a l l e [2] — e p e r l e [1] — a j a n f o + b * y1(n|32) = 0 a b„ w . ) j{ (n 3g) [ 4 ] [5] p e r * = 0 , iol=nal , c o „ = r e « s . p e r u n a p r o p r i e t à delle f u n z i o n i d i bessel lini j,(( o) cn->-0 y { (o>) = 0 . [6] s u l l e o s c i l l a z i o n i l i b e r e d e l l a c o d i a l b a n o 2 4 9 o r a , p e r p — * a u q—>-«2, jifi,—>0, / i p 2 — 0 ; q u i n d i , d a l l e [ 4 ] , [ 5 ] e p e r l e [6] : a v r e m o p e r t a n t o b , = £ , = 0 . £,0), —-aljl(col) sin n(t— t) . _ li — aj(.(coj) sin n(t—t) esm„ =a2j1((»2) sin t) li 2 a. " a„j0((ùs) s i n n ( « — t ) p e r il s e c o n d o g r u p p o d e l l e [ 3 ] , c o n s e g u e d a l l e [ 7 ] (n et,)(nn„)=0 i a l , . . a , [7] m l ' e l i m i n a z i o n e di a,, an c o m p o r t a l ' a n n u l l a r s i d e l d e t e r m i n a n t e dei coefficienti = 0 ./,(««,) —/,(rarts) /0(ra«t) / 0 ( r e u 2) l ' e q u a z i o n e dei p e r i o d i è q u i n d i / . ( n o , ) u n n j + u n u j / 1 ( n a , ) = 0 .«" d a l l a [ 7 ' ] si d e d u c e a„ ./, (rea,) [8] , i 2 = yli . «1 j , (na 2 ) (§) la [ 8 ] poteva d e d u r s i d i r e t t a m e n t e da u n caso p i ù c o m p l e s s o trattato da c a l o i , s t u d i a n d o l e sesse del l a g o di l e v i c o (5). r i f e r i a m o c i alla [ 1 3 ] di q u e l lav o r o e o s s e r v i a m o che p e r n a , v o è 7 0 ( u a 3 ) = l , j , ( ( i a , ) = 0. q u e l l a e q u a z i o n e d i v i e n e allora ma per u f i , — 0 è lin i j ( ( « p , ) . ' v , ( n ^ , ) 0 ; la (*) q u i n d i v i e n e a c o i n c i d e r e c o n la [ 8 ] , 2 5 0 p . c a l o i • m . g i o r g i si p o s t o c = — / , ( « « , ) , a v r e m o [ 9 ] ^ jt(mt) • / ; f1co1 = c o 1 — — sin n ( f — t ) ji ("«,) ch /,(«>,) . cj = — s i n n(t — t) 2 (ria,) r ji k ) • „ , fsft)g = c a s —5 s i n n(/ — i) " a (raag) r h / 0 ( o > „ ) . c „ = c — — sin nlt—t) 2 / . ( n o . ) i p e r o g n i p e r i o d o 7' = — t r a t t o d a l l a [ 8 ] , l a [ 9 ] dà l ' a n d a m e n t o n degli s p o s t a m e n t i o r i z z o n t a l i e v e r t i c a l i . i p u n t i di z e r o d e l l a f u n z i o n e jo{(o) d a n n o la p o s i z i o n e dei n o d i p e r o g n i p e r i o d o d ' o s c i l l a z i o n e . si o t t e n g o n o i n o d i n e l t r a t t o 0] a], a 2 q p a, r i s p e t t i v a m e n t e ot a2, m e d i a n t e le r a d i c i d e l l ' e q u a z i o n e 7 o ( c o ) = 0, c h e s i a n o m i n o r i d i n « j , r i s p e t t i v a m e n t e d i ria*. 2. applichiamo la teoria esposta al lago di albano. —• c o n i d a t i c h e h a n n o c o n s e n t i t o d i c o s t r u i r e l a c u r v a n o r m a l e , si s o n o o t t e n u t e l e e q u a z i o n i d e l l e r e t t e 1 e 2. l ' i n t e r s e z i o n e d i 1 e 2 ( p u n t o m) è ris u l t a t o di c o o r d i n a t e x = 179,418 . io4 m 2 y = 64090,5 . io4 i n 3 . a v r e m o p e r t a n t o «i = 179,4.10 4 m 2 ; ax = 428,1.10 4 m 2 ; h = 64090,5.10 4 m 3 45,25 sec ; a > = 107,98 sec s u l l e o s c i l l a z i o n i l i b e r e d e l l a c o d i a l b a n o 2 5 1 periodi. le p r i m e t r e s o l u z i o n i d e l l a [ 8 ] c o r r i s p o n d o n o , con g r a n d e a p p r o s s i m a z i o n e , a ri = oy02936; n = 0,0515016; n — 0,0714. avrem o q u i n d i p e r i p e r i o d i d e l l ' u n i , hie t r i n o d a l e i v a l o r i r , = 214s , to = 122s , t3 = 88" . nodi. d a l l a 1" d e l l e [ 1 ] r i s u l t a l _ , "\tv 2 71(1 , i v a l o r i «li di e « i , figurano l e [ 1 0 ] ; p e r cui, p e r i n o d i n e l t r a t t o oi a i , a v r e m o * = 1 7 9 , 4 1 — di, t 2 8 4 . 3 d o v e 0)4 è il v a l o r e c h e a n n u l l a jo(o>,), t a l e c h e r i s u l t i — c o m e si h a d a l l a [ 1 ] -—• ( i ) , < n « 1 . s e m p r e d a l l e [ 1 ] , si h a , p e r il ( r a t t o oya2, x— — a„ 1 d>„ r 2 71(1., p e r i v a l o r i di a 2 e a 2 d a t i d a l l e [ 1 0 ] , la f o r m u l a r e l a t i v a ai n o d i n e l t r a t t o oi an d i v i e n e o». t v x = — 428,1 16 7 8 , 5 [12] d o v e co2 s o n o r a d i c i d e l l ' e q u a z i o n e 7o((r>„) = 0, c h e s o d d i s f a n o alla c o n d i z i o n e o i 2 < w r „ . p e r l ' u n i n o d a l e è « a , = 1,329 . l e p r i m e r a d i c i d e l l ' e q u a z i o n e / 0 ( ( d ) = 0 s o n o 2,4048 ; 5,5201 ; 8,6537 ; 11,7915 ; ecc. p o i c h é n e s s u n a d i t a l i r a d i c i è m i n o r e d i n ctj, n e s e g u e c h e l ' u n i n o d o non è t r a t t o oi an . s e m p r e p e r l ' u n i n o d o si h a n ciò = 3,170 ; l ' u n i n o d o r i s u l t a q u i n d i n e l t r a t t o ox an, p o i c h é 2,4-048 < m i n. p o n i a m o n e l l a [12] co2 = 2,4048; p o i c h é 1 \ = 214 sec, o t t e r r e m o .v = — 181,81 . io4 in 2 . l'ascissa d e l l ' u n i n o d o v a l e p e r t a n t o v(x,) = 361,23 . io4 in 2 2 5 2 p . c a l o i m . g i o r g i l ' i m i n o d o c a d e q u i n d i t r a le sezioni 11" e 12" e dista d a l l ' o r i gine (lato m o n t e cavo) m 2009. p e r i b i n o d i si h a : n u i — 2,330; n a 2 = 5,561. n e s s u n b i n o d o si h a q u i n d i n e l t r a t t o oi a i . p o s t o n e l l a [12] t 2 = 122 s e co2 = 5,5201 e co2 = 2,4048 rispettiv a m e n t e , si t r o v a c h e i d u e b i n o d i h a n n o ascisse v (.to') = 185,73 . io4 ; v(x2") — 5 2 7 , 4 8 . io4 e d i s t a n o m 1191 e 2850 r i s p e t t i v a m e n t e d a l l ' e s t r e m o origine. p e r i t r i n o d i è n n i ~ 3,231, ; i a 2 = 7,71: u n o dei n o d i è q u i n d i n e l t r a t t o 0,alt gli a l t r i d u e c a d o n o n e l t r a t t o oi an. f a t t o n e l l a [11] coi = 2,4048, essendo t3 = 88% si t r o v a c h e il p r i m o t r i n o d o c a d e t r a la 3" e la 4" sezione, a v e n d o un'ascissa v ( * 3 ' j = 80,00 . io4 m 2 , e dista d a l l ' e s t r e m o o r i g i n e .va' = m 703 . la p o s i z i o n e degli a l t r i d u e t r i n o d i si o t t i e n e con la [ 1 2 ] , fac e n d o i n essa t3 — 88s e ro2 = 5,5201; co 2 = 2,4048 r i s p e t t i v a m e n t e . si t r o v a così c h e il t r i n o d o m e d i o e il t e r z o t r i n o d o h a n n o l e ascisse 3 8 8 , 0 8 . io4 ni 2 e d i s t a n o d a l l ' e s t r e m o o r i g i n e ni 2125 e m 3097 ris p e t t i v a m e n t e . andamento degli spostamenti verticali. — l ' a n d a m e n t o d e l l e ampiezze i n senso v e r t i c a l e si d e t e r m i n a a p p l i c a n d o la 2" e la 4" d e l l e [ 9 ] . la c o n t i n u i t à degli s p o s t a m e n t i n e l passaggio d a l t r a t t o ax oh al t r a t t o oi ao si assicura m o l t i p l i c a n d o i coefficienti delle r e l a z i o n i nom i n a t e p e r j j ( n « i ) . ecco i r i s u l t a t i . p e r la sessa uninodale: s e z i o n i 0 1 2 3 ! 4 5 6 6 bis 7 •-! s p o s t a m e n t i relativi ],00 0.968 0.925 0.870 0.808 0.724 0.645 0.605 0.492 s e z i o n i 8 9 10 11 uninodo 12 13 11 15 16 s p o s t a m e n t i relativi 0.415 0.305 0.182 0.041 00 — 0 . 1 4 4 —0.344 —0.527 — 0 . 7 6 0 —1.041 s e z i o n i 17 18 19 20 s p o s t a m e n t i r e l a t i v i — 1 . 2 3 4 —1.453 — 1 . 6 0 4 —1.692 p e r la sessa binodale : s e z i o n i 0 1 2 3 4 5 6 6|]js ] bin 7 8 s p o s t a m e n t i relativi 1 0.903 0.779 0.626 0.470 0.275 0.113 0.039 0 —0.049 —0.181 s u l l e o s c i l l a z i o n i l i b e r e d e l l a c o d i a l b a n o 2 5 3 s e z i o n i 9 l o 1 1 1 2 13 14 15 2 bwo 1 6 17 18 s p o s t . r e i . —(1.336 — 0 . 4 6 8 — 0 . 5 7 1 — 0 . 6 3 4 — 0 . 6 1 5 — 0 . 0 . 5 2 4 — 0 . 3 1 0 0 — 0 . 0 2 4 0.453 0.945 s e z i o n i 19 2 0 s p o s t a m e n t i r e l a t i v i 1,336 1,583. \ \ t i / \ v \ x 9 10 ii > jz 15 11 15 ,16 17 1920 n / \ v j \ \ \ 1 \ \ l \ f i g . 3 p e r la sessa trinodale: s e z i o n i 0 1 2 3 4 p trinodo 5 6 6 bis " 8 9 s p o s t . r e i . 1 0.818 0.597 0.348 0.120 0 — 0 . 1 2 0 5 — 0 . 2 7 4 — 0 . 3 2 8 — 0 . 3 5 6 — 0 . 4 0 1 — 0 . 3 9 6 s e z i o n i 10 1 1 2'trin. 1 2 13 1 1 15 16 it 3 tpin18 19 s p o s t . r e i . — 0 . 3 2 5 — 0 . 1 9 0 0 0.028 0.263 0.436 0.544 0.465 0.147 0 — 0 . 4 1 2 — 0 . 9 6 5 s e z i o n i 20 s p o s t . r e i . 1.357. le figg. 3, 4 d a n n o la r a p p r e s e n t a z i o n e grafica dei v a l o r i o t t e n u t i . andamento degli spostamenti orizzontali. — l ' a n d a m e n t o d e l l e a m p i e z z e i n senso o r i z z o n t a l e si d e t e r m i n a a p p l i c a n d o l a l a e la 3a 2 5 4 p . c a l o i m . g i o r g i d e l l e [ 9 ] , con u n p r o c e d i m e n t o analogo a q u e l l o seguito p e r l ' a n d a m e n t o degli s p o s t a m e n t i in senso v e r t i c a l e . si o t t e n g o n o i seguenti v a l o r i : p e r la sessa uninodale: s e z i o n i 0 1 2 3 4 5 6 (ibis spostamenti relativi 0 0.177 0.266 0.343 0.405 0.168 0.511 0.528 s e z i o n i 7 8 9 10 11 molla 12 13 s p o s t a m e n t i relativi 0.585 0.685 0.799 0.898 0.983 1.00 1.06 1.11 sezioni 11 15 16 17 18 1 ) 20 spostamenti relativi 1.12 1.10 1.03 0.898 0.687 0.432 0 ^ ' / "m „• * — \ \ 'f ^ / i ~~\7 li) 19 l ) -x \ 8 -̂o io ii i? n l) ~h 7~ \ \ \ v. y fig. 4 p e r l a binodale: sezioni 0 1 2 3 4 5 6 spostamenti relativi 0 0.300 0.430 0.520 0.567 0.581 0.557 sezioni (ibis i binodo 7 8 9 10 11 12 spostamenti relativi 0.534 0.539 0.545 0.543 0.497 0.407 0.273 0.067 sezioni 13 14 15 2 blhodo 16 17 13 19 spostamenti relativi —0.171 —0.386 —0.637 —0.822 —0.832 —0.920 —0.841 —0.587 s e z i o n i 20 spostamenti relativi 0. p e r l a trinodale: sezioni 0 1 2 3 4 | trinodo 5 6 spostamenti relativi 0 0.396 0.531 0.581 0.558 0.519 0.456 0.323 s u l l e o s c i l l a z i o n i l i b e r e d e l l a c o d i a l b a n o 2 5 5 s e z i o n i 6 bis 7 8 9 i o i l li irinodo 1 2 s p o s t a m e n t i r e l a t i v i 0.249 0.196 0.071 — 0 . 0 9 5 — 0 . 2 5 4 — 0 . 3 8 5 — 0 . 4 6 2 — 0 . 4 6 6 s e z i o n i 1 3 1 1 1 5 16 1 7 li] irlnodo 1« 1 9 s p o s t a m e n t i r e l a t i v i — 0 . 4 3 6 — 0 . 3 1 2 — 0 . 0 4 7 0.301 0.629 0.705 0.789 0.645 s e z i o n i 20 s p o s t a m e n t i r e l a t i v i 0. c o n f o r m e m e n t e alla t e o r i a , i m a s s i m i s p o s t a m e n t i n e i d u e sensi •— v e r t i c a l e e o r i z z o n t a l e — si v e r i f i c a n o n e l l a p a r t e del lago, dove si osservano le m i n o r i p r o f o n d i t à (figg. 3 e 4). roma •— istituto nazionale, di geofisica — maggio 1952. riassunto si espone la pratica applicazione del metodo di chrystal (dia determinazione degli elementi fisici (spostamenti e periodi) relativi alle sesse del lago di albano. i risultati che si ottengono sono in buon accordo con quelli determinati con altri metodi e, conformemente alla teoria, i massimi spostamenti verticali ed orizzontali si verificano nella parte del lago dove si osservano minori profondità. summary the practical application of crystal's method for determination of pliysical elements (displacements and periods) of the seiches of albano lal>e is exposed. the results ichich have been obtained agree enough ivith those determinai by other methoils and according to the theory higliest vertical and horizontal amplitudes occur u-here smaller depths are observed. b i b l i o g r a f i a o ) giorgi m., studio delle s e s s e del ingo ili albano. a n n a l i di g e o f i s i c a , v o i . i, 3 ( 1 9 4 8 ) . ( 2 ) caloi p., le sesse del lago di garda. p a r t e i i . a n n a l i di g e o f i s i e a , i, 2 ( 1 9 4 8 ) . ( 3 ) caloi p . , sui periodi di oscillazione libera del verbano. a n n a l i di g e o f i s i c a , i, 3 ( 1 9 4 8 ) . ( 4 ) caloi 1'., di filiito d . e spadea m . c., ulteriore studio sulle oscillazioni libere del lago di scanno. a n n a l i di g e o f i s i c a , v , 1 ( 1 9 5 2 ) . ( 5 ) caloi i \ , oscillazioni libere del lago di levico. a n n a l i di g e o f i s i c a , i v , 2 ( 1 9 5 1 ) . geo 1287 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 the cost 271 action: conclusions and the way ahead ljiljana r. cander (1) and bruno zolesi (2) (1) rutherford appleton laboratory, chilton, didcot, oxon, u.k. (2) istituto nazionale di geofisica e vulcanologia, roma, italy a brief summary is given of the major achievements of the cost 271 action. new challenges have been identified that open the way for a proposal, which is outlined, for a follow-on to the cost 271 action within the cost telecommunications, information science and technology framework. 1. introduction the expanding need for new communications services, especially those involving ionospheric hf communications, satellite communications and navigational systems, imposes increasing demands for the continuous monitoring and better understanding of the propagation effects imposed by the earth’s upper atmospheric that play important roles in determining the characteristics and reliability of radio systems. there is a requirement to develop means to predict regular behaviour and to minimise disturbing effects over a wide range of propagation conditions. within this context, the cost 271 action on «effects of the upper atmosphere on terrestrial and earth-space communications» has made a significant impact in a number of areas: a) progress has been made in the examination of the need for information on propagation effects on both currently-operational and future-anticipated terrestrial and earth-space radio systems that can be affected by space-weather phenomena, with emphasis being placed on characterisation of the relevant phenomena and their associated consequences. b) specific studies have been undertaken in response to the terms of references of cost 271 action work packages. c) the results from these investigations have been discussed at three workshops and published to the considerable benefit of both participating and other organisations. d) new data have been collected and mathematical methods and computational tools developed. e) dedicated studies have made a significant input to the emerging science of the impact of space weather on the terrestrial environment and advanced technology. f) results of significance have been made available to the itu-r study group 3 for incorporation into international recommendations. full details of these achievements have been given in the previous sections of this report and placed on the cost 271 web site (www.cost271.rl.ac.uk). in addition, the cd containing this final report includes the full bibliography, addresses of participants and relevant web links. the major achievements of the cost 271 action are summarised below, with new challenges being identified and incorporated into a proposal for a follow-on to the action that is outlined. 1288 ljiljana r. cander and bruno zolesi 2. the major achievements of cost 271 – a survey has been carried out of modern radio systems in different frequency bands, within the context of international regulations and the propagation phenomena that can lead to system impairments, together with the space-weather effects that can have significant adverse impacts on propagation conditions that need to be modelled. – ionospheric stations over europe have made a vital contribution in maintaining the high standard of vertical-incidence measurements and providing data on a regular basis. collection and distribution of the historical, new and prompt ionospheric data has been carried out at both the rutherford appleton laboratory (http://www.wdc.rl.ac.uk/cgi-bin/digisondes/cost_database.pl) and the space research centre (http://www.cbk.waw.pl/rwc). in addition, there has been continuing support of the limited database of past ionospheric observations that was developed under the previous cost action 251 (http://cost251.ictp.trieste.it/). – a database of eiscat observations from 1981 to 1999 have been developed at the university of grenoble (http://www-eiscat.ujf-grenoble.fr). – operational services for nowcasting and forecasting the state of the ionosphere over europe are now available at the regional warning center in warsaw (http://www.cbk.waw.pl/rwc) and the rutherford appleton laboratory (http://ionosphere.rcru.rl.ac.uk). – modelling techniques have been formulated, leading to experimental and real-time operational services involving new mathematical methods and computational tools for the forecasting and regional mapping of ionospheric characteristics. – improvements have been made to existing understanding of the physical mechanisms responsible for possible long-term trends in the earth’s ionospheric and atmospheric parameters that may possibly link such trends with anthropogenic activities. – studies have been carried out to advance knowledge of the structure and dynamics of the upper atmosphere and provide tools needed for the investigation of thermospheric-ionospheric interactions under various geophysical conditions. – models, and other tools to assess ionospheric effects in non-ionospheric applications of gnss signals, have been investigated at the university of graz (http://www.uni-graz.at/igamwww/cost271/) including a) ionospheric influences in the use of gnss occultation for stratosphere / troposphere applications and b) the effect of higher-order ionospheric propagation errors in advanced ground-based applications, like water vapour retrieval. – gps-based regional maps and the teceda data bank are now available at the deutsches zentrum für luft und raumfahrt (http://www.kn.nz.dlr.de). – collection has been started of examples of ionospheric extremes and anomalous cases with unusually high or low electron content values, anomalous gradients and variations that could be attributed to the influence of magnetic storms, travelling ionospheric disturbances (tids) and other as yet partially unexplained effects. – investigation has been made of gravity and planetary waves at mid-latitudes, responsible for some of the residual uncertainty in ionospheric radio-wave propagation predictions, with a view to identifying patterns to improve the accuracy of forecasts for telecommunication purposes. – a physically-based software simulator has been developed for the hf ionospheric reflection channel and uhf simulators for transionospheric channels, which overcome the limitations of existing empirically-based models. – studies have been carried out leading to a better understanding of the behavior of the ionospheric f1-region, sporadic e (es) and spread-f phenomena at european middle latitudes under geomagnetic storm conditions. – measurements and simulations have been made and results of importance to applied radio systems obtained, relating to the propagation of hf radio waves over northern european paths, where ionospheric effects impose large doppler and delay spreads on the propagating signal. 1289 the cost 271 action: conclusions and the way ahead – a heterogeneous array has been investigated to improve hf transmission, offering the possibility to transmit images via the ionospheric channel by providing an increase of the data rate of 15 khz within a 3 khz bandwidth (qam-64) without coding or interleaving. – improvements to the gism model include use of the multiple phase screen technique to give statistical characteristics of transmitted signals; fade duration and other parameters like the probability of signal loss of lock. maps of scintillation index s4 and standard deviations of the phase fluctuations can be produced. – algorithms and software tools have been developed to treat disturbances in earth-space and satellite-to-satellite communications. – studies have been made of plasma effects in the magnetosphere-ionosphere-thermosphere system generated by different natural processes and by human activity. – information has been gathered about effects of the vertical and horizontal gradients of the electron density on earth-space and satellite-to-satellite communication. – model simulation studies have been made of extreme propagation effects on gps-to-geostationary satellite ray paths. 3. proposal for a follow-on action since 1991, the cost 238, cost 251 and cost 271 actions have been highly successful in bringing together most of the available european expertise in the field of ionospheric/plasmaspheric research and the application of such knowledge to the improvement of practical radio systems. the important results stemming from these collaborative projects can be found in reports of the earlier actions and in the current volume for cost 271. the results obtained from the measurements and modelling within these actions have aided radio-systems planners and designers in two primary respects. not only is there better understanding of the upper atmosphere environment over europe and its effects on terrestrial and earth-space communications and navigational systems, but also it has been possible to determine with greater accuracy the propagation effects imposed by the atmosphere on received signals and consequent interference levels. in addition, each successive project has raised new questions that have been taken up in the next action. cost 271 is no different in this respect. indeed, it is inevitable that wide-ranging studies of this magnitude, together with the advance of the technological requirements of new generations of communications systems, will raise different technical issues that cannot be properly addressed or completed within the prescribed timeframe of the action. however, the cost framework provides a unique european forum for the progression of such matters. in consequence, the management committee of the cost 271 action has formulated a memorandum of understanding of a new four-year study to follow, in which such questions can be taken up. while cost 271 was concerned with identification and investigation of the propagation effects imposed by the upper atmosphere, the new proposal (miers) now plans to deal directly with the mitigation of such effects on practical radio systems. 3.1. mitigation of ionospheric effects on radio systems (miers) with the following main objectives: – to support and enhance the existing european facilities for historical and real-time digital ionospheric data collection and to exchange information needed on methods and algorithms to mitigate the effects of ionospheric perturbations and variations on advanced terrestrial and space-based communication services by creating an effective computing infrastructure. – to develop an integrated approach to ionospheric modelling, create the mechanism needed to ingest processed data into models, extend and develop suitable mitigation models and define the protocols needed to link models together. 1290 ljiljana r. cander and bruno zolesi – to make applicable results available to the itu-r and to promote the research aspects to funding agencies like esa, esf and fp 6/7. – to strengthen the areas of expertise that already exist by stimulating closer cooperation between scientists and users, focusing the scope of all the previous cost ionospheric related studies to the mitigation of ionospheric effects on specific radio systems, which are in operational use or in the development stage into new systems. the work will be arranged under three topic headings and will progress within each of these areas throughout the duration of the action (four years) as follows. 3.1.1. wg 1 – ionospheric monitoring and modelling wp 1 – near-earth space plasma monitoring – monitoring near-earth space by vertical incidence and oblique sounding networks and gnss techniques (retrospective and real-time). – ionospheric data networking. – tec: model-assisted techniques and measurements from the global network. wp 2 – data ingestion and assimilation in ionospheric models – data collection and processing. – model adaptation for data ingestion and assimilation. wp 3 – near-earth space plasma modelling and forecasting – real-time near-earth space plasma modelling. – modelling and forecasting of ionospheric perturbations. – tomographic imaging for model validation. – channel modelling by neurofuzzy techniques. wp 4 – climate of the upper atmosphere – long-term ionospheric trends. – gravity and planetary wave effects on propagation and infrasonic waves. – ionospheric space weather. 3.1.2. wg 2 – advanced terrestrial systems wp 1 – radar and radiolocation – ionospheric effects on surface-wave radar. – frequency management of ground-wave and sky-wave radars. – angle of arrival measurements for sky-wave signals. wp 2 – hf/mf communications – digital radio systems – predictions, methods of estimating reliability, etc. – wideband propagation modelling and development of a hardware simulator. – increased capacity of hf links through mimo techniques – experimental measurements and simulation. – extension of existing wideband hf simulators to the mf band. wp 3 – spectrum management – use of gps to improve hf communications management. 1291 the cost 271 action: conclusions and the way ahead – adaptive waveform management. – effects of infrasound on radio propagation. 3.1.3. wg 3 – space-based systems wp 1 – space plasma effects – ionospheric effects on space-based remote sensing systems. – gravity wave effects on gnss. – space plasma media and security implications. wp 2 – mitigation techniques – ionospheric structures, their physical nature and impact on gnss signals. – improved accuracy of gnss by better ionospheric correction. – errors due to ionospheric perturbations. wp 3 – scintillation monitoring and modelling – scintillation effects, their physical nature and impact on ionospheric radio systems, gnss signals in particular. – high-latitude scintillation effects: experimental work and modelling. – equatorial scintillation effects: experimental work and modelling. 4. conclusions the cost 271 action has been very successful in bringing together in collaborative studies many researchers who collectively represent much of the available european expertise on the effects of the upper atmosphere on radio systems. the significant results and advances in knowledge and understanding stemming from cost 271 have been summarized here. in addition, a proposal has been outlined for new investigations within the cost telecommunications, information science and technology framework into the mitigation of such propagation effects on practical communications systems. le s e s s e ( s e i c k e s ) dell'adriatico s. polli 1 . p r e m e s s e il mare adriatico, come ogni bacino acqueo, è soggetto ad oscillazioni libere, aventi periodi determinati, dipendenti dalla forma del bacino e dal modo di oscillazione. il fenomeno si genera quando una causa, generalmente meteorica, produce un temporaneo dislivello in una zona del mare. il ristabilimento dell'equilibrio avviene mediante una successione di oscillazioni che gradualmente si smorzano. queste fluttuazioni sono dette anche sesse (seiches, free oscillations, eigenschwingungen) perchè sotto tale nome furono per la prima volta osservate e studiate nei grandi laghi svizzeri. nell'adriatico l'oscillazione principale avviene longitudinalmente, l'acqua cioè oscilla attorno ad una linea nodale, trasversale all'asse del bacino. sono possibili oscillazioni con la linea nodale all'estremità aperta del mare, con il nodo al centro, con più linee nodali ed anche oscillazioni trasversali. lo studio delle sesse può essere considerato sotto diversi aspetti che, pur essendo strettamente collegati tra di loro, possono essere esaminati anche separatamente. i principali sono: a) determinazione empirica delle sesse dalle registrazioni del movimento verticale del livello acqueo; b) determinazione teorica delle sesse date le dimensioni del bacino; e) determinazione delle cause che producono le sesse. il primo problema presenta la difficoltà di dover isolare le singole sesse dalle oscillazioni di marea, dalle altre sesse e dalle fluttuazioni accidentali. i ricercatori che sinora si occuparono di esso si limitarono ad esaminare direttamente i mareogrammi o ad esaminare la curva-differenza fra la curva mareografìca reale e quella teorica. i n questo caso, se la sessa appare pura e bene sviluppata, si può ricavare il periodo, in modo però approssimato ed incerto. occorre notare che per il mare adriatico le due sesse principali hanno periodi molto prossimi a quelli delle maree, e ciò rende la separazione delle oscillazioni maggiormente difficile. di questo primo problema si occupò particolarmente il kesslitz sulla base dei mareogrammi di pola e di eagusa. il secondo problema è stato trattato e sviluppato da diversi autori con vari procedimenti teorici, che hanno portato a risultati ben concordanti tra loro e con le determinazioni mareografìche. per l'adriatico si occuparono della questione il defant ed altri autori indicati nella bibliografìa. il terzo problema non è stato ancora trattato direttamente. esso richiede un lungo e complesso lavoro, dapprima di analisi delle sesse e degli elementi meteorici che possono produrle, poi di correlazione tra singola causa e singolo effetto ed infine un lavoro di sintesi tra le varie cause considerate contemporaneamente e gli effetti risultanti. oltre alla difficoltà dovuta alla complessità del problema, occorre disporre di molto materiale mareografico e meteorologico, il quale sinora è insufficiente, specialmente per quanto riguarda le registrazioni mareografìche. della questione si è occupato sopratutto il kesslitz, senza arrivare però a risultati conclusivi. 2 . p r o p o s i t o in questa nota considereremo solamente il primo dei tre problemi indicati, cioè quello della determinazione dei periodi e delle ampiezze delle singole sesse sulla base delle registrazioni mareografìche. di queste con70 s. p o l l i fig. 1. analisi periodale della c u r v a differenza t r a m a r e a reale e calcolata, r e l a t i v a al p o r t o di trieste, p e r il periodo 29 gennaio-2 f e b b r a i o 1948. (non a p p a r e apprezzabile in questo i n t e r v a l l o di t e m p o uè la sessa di ore 7,7 nè quella di ore 4,3). sidereremo solamente quelle ottenute a trieste; si noti che questo porto si trova proprio all'estremità longitudinale chiusa del bacino adriatico, dove le sesse assumono la massima ampiezza, cioè nelle condizioni più favorevoli per eseguire la ricerca ora esposta. 3 . p r o c e d i m e n t o il metodo adottato consiste essenzialmente nell'applicare alla curva mareografìca due successivi procedimenti selettivi, in maniera da ottenere separate dalle maree e separate fra di loro t u t t e le sesse presenti in un dato intervallo di tempo. questa doppia selezione è necessaria sia perchè, come già si disse, i periodi delle due principali sesse adriatiche sono molto prossimi a quelli delle maree diurne e semidiurne, per cui risulta molto difficile distinguere sul mareogramma le sesse; sia perchè le varie sesse risultano sovrapposte, componendosi in una curva complessa. l e sesse ( s e i c h e s ) d e l l ' a d r i a t i c o 71 fig. 2. analisi periodale della c u r v a differenza t r a m a r e a reale e calcolata, r e l a t i v a al p o r t o di t r i e s t e p e r il periodo 3-6 giugno 1948. ( r i s u l t a n o e v i d e n t i t u t t i i tipi di sesse). 72 s. p o l l i la prima selezione consiste nel togliere dalla curva effettivamente registrata quella della marea teorica, cioè calcolata. rimane una curva residua che è la somma delle eventuali sesse e fluttuazioni accidentali. se la sessa è presente ed è unica, essa appare allora evidente; ma spesso risulta fortemente alterata da altre fluttuazioni accidentali o meglio dalla coesistenza di più sesse; occorre allora separarle. la seconda e definitiva selezione si ottiene applicando alla curva-differenza un metodo di analisi periodale. conveniente risulta quello sviluppato dal vercelli, sia perchè si adatta molto bene ai caratteri delle curve da selezionare, sia perchè è molto pratico. tutte le sesse presenti, di ampiezza apprezzabile, ne risultano così separate. è evidente che la somma delle curve analizzate riproduce la curva di partenza, e in ciò si ha un controllo del procedimento. praticamente si opera nel modo seguente: a) per il periodo che si vuole esaminare, per esempio un anno (e in un anno si possono trovare appena due o tre casi di sesse ben sviluppate), si calcolano, col metodo della sintesi armonica, le ordinate orarie della marea teorica. occorrono almeno le costanti armoniche delle maree m2, s2, n2, k2, k l , 01, p i , m4, ms4, per avere una sufficientemente precisa marea teorica. b) si traccia sul foglio del mareogramma reale la curva della marea teorica. c) si fa la differenza tra le ordinate delle due curve. la curva-differenze metterà in evidenza, se esiste, la curva risultante dalla sovrapposizione di più sesse. talvolta questa curva avrà un andamento corrispondente a quello della sessa predominante. d) a questa curva, riportata in scala opportuna, si applica il procedimento di analisi periodale, sino ad ottenere isolate le singole sesse, t u t t e nella stessa scala di riduzione. risulta allora immediata la determinazione del periodo, dell'ampiezza e della fase delle singole oscillazioni. per eseguire l'analisi periodale si prenda per guida una delle chiare pubblicazioni del vercelli indicate nella bibliografia. e) nello svolgimento del lavoro si può procedere graficamente, operando cioè sulle varie curve tracciate in scala conveniente; oppure si può procedere col calcolo, operando sui valori numerici delle corrispondenti ordinate orarie. il primo procedimento è più conveniente perchè offre la possibilità di una visione immediata dello svolgimento e dei risultati delle successive operazioni. 4 . e s p o s i z i o n e d e i r i s u l t a t i per le tre annate 1948,1949 e 1950 si tracciò sistematicamente la curva ottenuta quale differenza tra quella della marea registrata e quella della marea calcolata. risultarono convenienti alla ricerca solamente quattro periodi di tempo, con sesse notevolmente sviluppate. negli altri casi, una ventina complessivamente, lo sviluppo delle oscillazioni libere risultò o debole, o incerto, o breve, in ogni modo non sufficiente a ricavare dati sicuri. la scarsa frequenza delle sesse nell'adriatico dimostra che occorrono condizioni atmosferiche particolari, azioni intense e ad effetto concordante, specialmente nei due estremi del bacino, per provocare oscillazioni notevoli in tutto il mare. le singole sesse selezionate si presentano come oscillazioni sinusoidali che si smorzano, in generale, rapidamente, cioè dopo due o tre oscillazioni. ciò si è avuto per la sessa di ore 21,20 dei giorni 3-4 giugno 1948; di ore 21,50 dei giorni 18-22 ottobre 1949; e di ore 21,00 dei giorni 15-17 aprile 1950. in situazioni più rare, lo smorzamento risulta più lento e l'oscillazione può comprendere anche una serie di 10-15 onde. in questi casi l'ampiezza rimane inalterata per lungo tempo, e può anche subire aumenti dopo le prime onde. le oscillazioni appaiono come onde forzate in risonanza con una causa limitatamente persistente. ciò si è avuto per le sesse di ore 21,36 dei giorni 27 gennaio-8 febbraio .1948; di ore 12,20 dei giorni 1-6 giugno 1948; di ore 12,20 dei giorni 18-23 ottobre 1949. questo tipo di sesse permettono determinazioni molto precise del periodo. dato l'interesse che presentano questi diversi casi si è ritenuto opportuno specificarli singolarmente. nella seguente tabella presentiamo i risultati delle analisi ottenuti per ognuno dei quattro intervalli di tempo considerato. il l e sesse (seiches) d e l l ' a d r i a t i c o 73 periodo ore e r r o r e medio n u m . onde a m p . cm i n t e r v a l l o di t e m p o 2 1 , 3 6 ± 0 , 6 7 12 94 26 . 1 8 . i i 1948 12,19 0 , 2 3 13 20 30 . 1 6 . i i )) 7 , 6 8 0 , 3 0 12 12 28 . 1 5 . i i )) 4 , 2 5 — t 3 10 27 . 1 )) 2 1 , 2 0 1 32 3 4. vi 1948 12,20 0 , 3 2 10 28 1 6. vi 1948 • 7 , 6 1 0 , 1 4 10 9 1 6. vi » 4 , 3 0 — 4 7 3 4. vi » 2 1 , 5 0 — 3 11 18 22 . x 1949 12,20 0 , 1 6 10 22 18 23 . x » 7 , 6 0 0 , 3 6 3 8 18 19 . x » 4 , 1 0 — 7 6 20 21 . x » 2 1 , 0 0 — 2 20 15 17 . i v 1950 12,17 0 , 2 8 3 18 15 17 . i v » 7 , 3 3 0 , 2 4 7 1 1 15 18 . i v » 4 , 2 5 — 4 10 15 16 . i v » periodo è dato in ore e centesimi di ora; nei casi più significativi si è calcolato l'errore medio. | n — 1 di ogni gruppo di onde della sessa selezionata: s è lo scostamento della media aritmetica dei periodi delle singole onde di una stessa sessa; n è il numero di onde di ciascuna sessa selezionata. l'ampiezza in cm è quella massima registrata nell'oscillazione completa. in ciascun intervallo di tempo considerato sono state trovate sempre oscillazioni dello stesso tipo, che sono apparse però con caratteri di ampiezza, persistenza e smorzamento diversi. nei giorni 26 gennaio-8 febbraio 1948 predomina per ampiezza e persistenza l'oscillazione di ore 21,36. essa assume la massima ampiezza nei giorni 26-31, si smorza poi progressivamente. deboli e meno regolari appaiono l'onda di ore 12,19 e le altre di periodi minori. la settimana 1-6 giugno 1948 è invece caratterizzata dalle sesse di ore 12,20 e di ore 7,61; che persistono ampie per tutto l'intervallo di tempo, decrescendo appena verso il giorno 8. la sessa di ore 21,20 comprende invece una sola ampia oscillazione che si smorza rapidamente e irregolarmente in un paio di giorni. nei giorni 17-24 ottobre 1949 prevale, con carattere di notevole regolarità, l'onda di ore 12,20. essa persiste con inalterata ampiezza per più di una settimana smorzandosi appena verso il giorno 25. le altre oscillazioni risultano t u t t e più deboli e meno regolari. l'intervallo 14-19 aprile 1950 è caratterizzato da un ampio sviluppo iniziale di t u t t i i quattro tipi di sesse, che vanno poi progressivamente smorzandosi. nel giorno 16 le oscillazioni subiscono un nuovo rin74 s. p o l l i fig. 3. analisi periodale della c u r v a differenza t r a m a r e a reale e calcolata, r e l a t i v a al p o r t o di trieste, per il periodo 15-18 aprile 1950. (risultano evidenti t u t t i i tipi di sesse). l e s e s s e ( s e i c h e s ) d e l l ' a d r i a t i c o 75 forzo, e ciò avviene quasi contemporaneamente per t u t t e le quattro sesse. nelle seguenti due tabelline presentiamo i periodi ottenuti per ogni tipo di sessa e la loro media ponderata, cioè la media aritmetica dei periodi di t u t t e le singole onde dei quattro casi considerati. nella parentesi è indicato il numero di onde considerate in ciascuna sessa. p e r i o d i m e d i e m e d i e p o n d e r a t e (in o r e e c e n t e s i m i d i o r a ) queste sono le sesse registrate nel triennio 1948-1950 nel porto di trieste; altri tipi di oscillazioni non risultano in modo chiaro o apprezzabile. la sessa di 21,33 ore dovrebbe corrispondere ad un'oscillazione del mare con nodo nello stretto di otranto o prossimo a questo; quella di 12,19 ore corrisponderebbe invece all'oscillazione con nodo verso il centro del bacino. il periodo della sessa di 7,57 ore è circa di un terzo di quello dell'oscillazione fondamentale; potrebbe corrispondere ad una sessa binodale. più complesso è invece il caso della sessa di 4,20 ore, in quanto che varie sono le ipotesi che si possono fare su di essa: potrebbe essere una sessa traversale, una oscillazione armonica di una delle due sesse principali. analisi di sesse adriatiche, simili a questa, ma eseguite in altri porti dell'adriatico, sia nello stretto d'otranto che nella parte centrale, contribuirebbero certamente alla soluzione completa della conoscenza delle sesse effettive di questo mare. trieste, 2 novembre 1957 riassunto dai mareogrammi di trieste si determina la curva-differenza tra la marea osservata e la marea calcolata. questa curva è composta essenzialmente dalle sesse dell' adriatico. ad essa si applica vanalisi periodale del vercelli. risultano chiaramente selezionate le seguenti sesse: ore 21,33-, ore 12,19-, ore 7,57-, ore 4,20. abstract from mareograms of trieste, it may he determined the differential curve between the observed and the reckoned tide. said curve is composed essentially of the seiches of the adriatic sea. the vercélws periodai analysis may be indeed applied thereto. the following seiches have been selected clearly: 21.33 h. p. m., 12.19 hours, 7.57 h. a. m., 4.20 h.a.m. b i b l i o g r a f i a (x) caloi, p . , sesse dell'alto adriatico con particolare riguardo al golfo di trieste. c o m i t . t a l a s s o g r . i t a l . , m e m o r i a c c x l v i i , ven e z i a , 1938. (2) defant, a., tìber die periodendauer der eigenschwingungen des adriatischen meeres. « a n n a l e n d e r h y d r o g r . u n d m a r i t . met e o r . », x x x i x , ( m à r z , 1911). (3) — \v. v . kessutz, die gezeiten im adriatischen meere. « a n n a l e n d e r h y d r o g r . u n d m a r i t . m e t e o r . », (aprii, 1914). (4) —neue metliode zur ermittlung der eigenschwingungen (seiches) von abgeschlossenen wassermassen (seen, bucliten, usw). « a n n a l e n d e r h y d r o g r . u n d m a r i t . m e t e o r . », ( f e b r u a r , 1918). 2 1 , 3 6 ( 1 2 ) 1 2 , 1 9 ( 1 3 ) 7,68 (12) 4,25 ( 3) 21,20 ( 1) 1 2 , 2 0 ( 1 0 ) 7,61 (10) 4,30 ( 4) 21,50 ( 3) 1 2 , 2 0 ( 1 0 ) 7,60 ( 3) 4,10 ( 7) 21,00 ( 2) 12,17 ( 3) 7,33 ( 7) 4,25 ( 4) 2 1 , 3 3 (18) 1 2 , 1 9 (36) 7 , 5 7 ( 3 2 ) 4 , 2 0 (18) p e r i o d i m e d i e m e d i e p o n d e r a t e (in o r e e m i n u t i ) . li m h m h m h m 21 22 12 11 7 41 4 15 21 22 12 12 7 37 4 18 21 30 12 12 7 36 4 06 21 00 12 10 7 20 4 15 2 1 2 0 1 2 1 1 7 3 4 4 1 2 76 s. p o l l i ( 5 ) k e s s l i t z , w . , das gezeitenphànomen im hafen von fola. « m i t t e i l . a u s d e m g e b i e t e dea s e e w e s e n s », b a n d 38, h . v y i , p o l a , 1910. (6) — das gezeitenphànomen im hajen von ragusa, « m i t t e i l . a u s d e m g e b i e t e d e s s e e w e n s e n s », p o l a , 1912. (') — die gezeitenerscheinungen in der adria, i . teil, die beobachtungsergeben. der flutstationen. « d e n k s c h r . d. a k a d . d . w i s s . in w i e n », b d . 96, w i e n , 1919. (8) sterneck, r . , die gezeitenerscheinungen in der adria, 2. t e i l . « d e n k s c h r . d . a k a d . d . w i s s », w i e n , 1919. (9) vercelli, f . , sulla previsione dei termini di correzione nelle maree, « r i v . m a r i t t i m a » f e b b r a i o , r o m a , 1922. (10) —ciò che sappiamo delle maree dell' adriatico, « l a r i c e r c a s c i e n t i f i c a » , i i i , 9-10, r o m a , 1932. ( n ) — metodi pratici per l'analisi delle curve oscillanti. « l a r i c e r c a s c i e n t i f i c a », v, 7 (1934). (12) — schemi di calcolo per l'analisi dei diagrammi oscillanti. « l a r i c e r c a s c i e n t i f i c a » serie i i , v i i i , i i , 11-12, r o m a , 1938. (13) —analizzatore meccanico delle curve oscillanti. « p o n t i f . a c c a d . delle s c i e n z e », c o m m . i l i , 19, (1939). (14) — guida per l'analisi delle periodicità nei diagrammi oscillanti. « c o m i t . t a l a s s o g r . i t a l . », m e m . 285, v e n e z i a , 1940. s u l l a m i s u r a d e l l ' a s s o r b i m e n t o i o n o s f e r i c o piero dominici 1. — è b e n n o t a la g r a n d e i m p o r t a n z a c h e h a n n o assunto, sia n e l c a m p o scientifico, sia n e l c a m p o p r a t i c o , l e q u e s t i o n i r i g u a r d a n t i la i o n o s f e r a . d a l l o n t a n o 1902, q u a n d o , k e n n e l l y , h e a v i s i d e , e n a g a o k a i p o t i z z a r o n o l'esistenza n e l l ' a l t a a t m o s f e r a di r e g i o n i f o r t e m e n t e ionizz a t e e d o t a t e di p o t e r e r i f l e t t e n t e p e r l e r a d i o o n d e , gli s t u d i ionosferici sono stati s e m p r e p i ù s v i l u p p a t i , ed ai n o s t r i g i o r n i si p u ò d i r e c h e n o n vi è p a e s e civile in cui n o n ci si d e d i c h i con g r a n d e i n t e r e s s e a q u e s t e r i c e r c h e . p a r a l l e l a m e n t e allo s v i l u p p o degli s t u d i t e o r i c i , v e r a m e n t e cospicuo è s t a t o a n c h e il p r o g r e d i r e ed il raffinarsi d e i m e t o d i s p e r i m e n tali. a p p l e t o n , n e l 1925, con u n a c e l e b r e e s p e r i e n z a , m e t t e v a i n evid e n z a , p e r l a p r i m a volta, l'esistenza della i o n o s f e r a , ed a l t e m p o stesso i n d i c a v a il p r i m o m e t o d o d i s t u d i o s p e r i m e n t a l e della stessa. i l successivo m e t o d o d o v u t o a b r e i t e t u v e , b a s a t o sui so n d ag g i verticali d e l l e regioni i o n o s f e r i c h e a m e z z o d i b r e v i t r e n i d ' o n d a (met o d o degli i m p u l s i ) , h a p e r m e s s o , col c o n t i n u o p r o g r e d i r e della tecnica e l e t t r o n i c a , la c o s t r u z i o n e delle m o d e r n e i o n o s o n d e , a p p a r a t i di grand e efficienza, di n o t e v o l e sicurezza ed a f u n z i o n a m e n t o c o m p l e t a m e n t e a u t o m a t i c o . è n o t o c h e la p r e s e n z a di e l e t t r o n i l i b e r i n e l l a i o n o s f e r a p o r t a ad u n a m o d i f i c a z i o n e d e l l ' i n d i c e di r i f r a z i o n e , c h e , t r a s c u r a n d o l'azion e del c a m p o g e o m a g n e t i c o , vale : dove e, m s o n o la carica e la massa d e l l ' e l e t t r o n e , n la densità elett r o n i c a , / la f r e q u e n z a d e l l ' o n d a . se la f r e q u e n z a d e l l ' o n d a emessa ad i m p u l s i è tale da a n n u l l a r e n , in r a p p o r t o alla d e n s i t à e l e t t r o n i c a di 1 le-n u n c e r t o s t r a t o i o n o s f e r i c o , cioè se è f = / — si h a la riflessione \ ix. m d e l l ' o n d a sullo s t r a t o , e gli i m p u l s i t o r n a n o verso la t e r r a . q u i vennmj562 pif.ro d o m i n i c i gono c a p t a t i d a u n r i c e v i t o r e s i n t o n i z z a t o col t r a s m e t t i t o r e , poi rivel a t i ed a p p l i c a t i al r e g i s t r a t o r e , c h e i n g e n e r e è u n t u b o a raggi catodici d i cui si f o t o g r a f a c o n v e n i e n t e m e n t e l o s c h e r m o . i l t e m p o int e r c o r s o f r a l ' e m i s s i o n e di u n i m p u l s o e la r i c e z i o n e della sua eco, r i c a v a b i l e con u n a t e c n i c a p a r t i c o l a r e del sistema r e g i s t r a t o r e , ci d à l ' a l t e z z a r i s p e t t o al suolo d e l l o s t r a t o c h e h a riflesso il segnale. questa altezza, r i c a v a t a s u p p o n e n d o c h e l ' o n d a viaggi n e l l a i o n o s f e r a ad u n a velocità c o s t a n t e p a r i a q u e l l a della l u c e n e l v u o t o — il c h e n o n è — si c h i a m a « altezza v i r t u a l e » d e l l o s t r a t o ed è m a g g i o r e d e l l ' a l t e z z a v e r a . a u m e n t a n d o la f r e q u e n z a d e l l ' o n d a , lo s t r a t o ad u n c e r t o m o m e n t o cessa di essere r i f l e t t e n t e , e l ' o n d a lo a t t r a v e r s a . i l v a l o r e della f r e q u e n z a m a s s i m a a n c o r a riflessa si c h i a m a « f r e q u e n za critica » d e l l o s t r a t o , ed i n c o r r i s p o n d e n z a si h a il v a l o r e mass i m o della densità e l e t t r o n i c a i n q u e l p a r t i c o l a r e s t r a t o , p a r i a ar„ = 1,26 . 1 0 4 ( / ) 2 m h z e l e t t r o n i / c m 3 . i n g e n e r e il c a m p o di f r e q u e n z a d e l l e i o n o s o n d e c o m u n e m e n t e adop e r a t e negli osservatori i o n o s f e r i c i è d a 1 a 20 mhz ca., e q u e s t o p e r l a difficoltà di g e n e r a r e e r i c e v e r e i m p u l s i di m i n o r e f r e q u e n z a . ciò significa c h e l e i o n o s o n d e sono a t t e ad e s p l o r a r e r e g i o n i i o n o s f e r i c l i e c o n d e n s i t à e l e t t r o n i c a n o n m i n o r e di circa 2 . io4 e l e t t r o n i / c m 3 , v a l e a d i r e gli s t r a t i e ( q u o t a circa 100 k m ) ed f ( q u o t a f r a 200 e 400 k m ) , e n o n possono d i r c i n u l l a , a l m e n o con sicurezza, delle r e g i o n i i n f e r i o r i ( s t r a t o d). i n r e a l t à gli s t r a t i e ed f , p a r t i c o l a r m e n t e q u e s t o u l t i m o sono gli s t r a t i d e c i s a m e n t e p i ù i n t e r e s s a n t i , e q u e l l i c h e det e r m i n a n o l e c o n d i z i o n i della r a d i o p r o p a g a z i o n e a g r a n d e d i s t a n z a , t a n t o che, s i n c h é si sono r i g u a r d a t i solo p r o b l e m i d i p r o p a g a z i o n e d e l l e o n d e corte, è stata del t u t t o t r a s c u r a t a l ' i m p o r t a n z a di u n event u a l e o di e v e n t u a l i s t r a t i a p i ù bassa q u o t a . q u e s t a p o s i z i o n e si è d o v u t a p e r ò r i v e d e r e n o n a p p e n a si è v o l u t o a f f r o n t a r e il p r o b l e m a d e l calcolo d e l l ' i n t e n s i t à di c a m p o a g r a n d e d i s t a n z a . si sa c h e u n ' o n d a p r o p a g a n d o s i n e l l a i o n o s f e r a si a t t e n u a . se l ' i n t e n s i t à di c a m p o e r a en in p a r t e n z a , a l l ' a r r i v o , d o p o u n c e r t o trag i t t o nella i o n o s f e r a , essa v a r r à e„ e~f, e t si c h i a m a assorbimento ionosferico r e l a t i v o a q u e l c e r t o t r a g i t t o . i n u n a r e c e n t e m e m o r i a ('1 ) il p r o f . b o l l e ed io a b b i a m o t r a t t a t o , l i m i t a n d o c i al c a m p o delle o n d e c o r t e , la q u e s t i o n e d e l calcolo d e l l ' a s s o r b i m e n t o i o n o s f e r i c o . a b b i a m o d i m o s t r a t o c h e p e r le o n d e c o r t e si p u ò r i t e n e r e , dal p u n t o di vista d e l l ' a s s o r b i m e n t o , c h e il c a m p o g e o m a g n e t i c o n o n h a a p p r e z z a b i l e i 1 ) a n n a l i di g e o f i s i c a , v o i . v , n. 3, 1952. s u l l a m i s u r a d e l l ' a s s o r b i m e n t o i o n o s f e r i o o 563 i n f l u e n z a , ed a b b i a m o poi r i c a v a t o u n serie d i f o r m u l e c b e d a n n o l ' a s s o r b i m e n t o s t r a t o p e r s t r a t o , ed a s e c o n d a c b e l ' o n d a si r i f l e t t a sullo s t r a t o o lo a t t r a v e r s i . i n q u e s t e f o r m u l e , c b e , essendo del t u t t o t e o r i c h e , d o v r a n n o poi a v e r e la c o n t r o p r o v a d e l l ' e s p e r i e n z a , comp a i o n o c o m e p a r a m e t r i d u e v a l o r i c a r a t t e r i s t i c i d e l l ' o n d a , cioè la f r e q u e n z a e l ' i n c l i n a z i o n e del r a g g i o c o n s i d e r a t o , e d u e c a r a t t e r i s t i c i d e l l o s t r a t o , cioè la f r e q u e n z a d e l l e collisioni f r a e l e t t r o n i l i b e r i e m o l e c o l e n e u t r e (legata alla pressione) e la m a s s i m a d e n s i t à elett r o n i c a (legata a l l ' a g e n t e i o n i z z a n t e , cioè alla r a d i a z i o n e solare). facc i a m o u n e s e m p i o t i p i c o : u n ' o n d a c h e si r i f l e t t a sullo s t r a t o f e c h e q u i n d i a t t r a v e r s i lo s t r a t o e e lo s t r a t o d. l ' a s s o r b i m e n t o complessivo f sarà la s o m m a d e l l ' a s s o r b i m e n t o in f, r „ f ( a s s o r b i m e n to d e l l a r e g i o n e deviatrice), p i ù l ' a s s o r b i m e n t o i n e ed in d, r i e + r i d ( a s s o r b i m e n t o n e l l e r e g i o n i n o n deviatrici). o r a , l ' a s s o r b i m e n t o complessivo f si p u ò m i s u r a r e con u n o dei m e t o d i c h e descriverò p i ù a v a n t i , e, c o m e b o detto, gli a s s o r b i m e n t i r e l a t i v i agli s t r a t i e e f si possono c a l c o l a r e a p a r t i r e dai d a t i f o r n i t i d a l l e i o n o s o n d e . si p u ò i s o l a r e cosi il t e r m i n e c o m p e t e n t e d e l l ' a s s o r b i m e n t o dello s t r a t o d, e da q u e s t o r i s a l i r e alle c a r a t t e r i s t i c h e dello s t r a t o stesso. ciò è di g r a n d e a i u t o p e r la p r e v i s i o n e d e l l ' i n t e n s i t à di c a m p o a l l ' a r r i v o n e l l e c o m u n i c a z i o n i a g r a n d e d i s t a n z a . v e r a m e n t e le c a r a t t e r i s t i c h e dello s t r a t o d si possono r i c a v a r e a n c h e con la tecnica dei s ondaggi ad i m p u l s i , a d o p e r a n d o a p p o s i t e a p p a r e c c h i a t u r e , a l q u a n t o d i v e r s e d a l l e c o m u n i i o n o s o n d e , m a com u n q u e f a c i l m e n t e r e a l i z z a b i l i n e i n o s t r i l a b o r a t o r i . i l m o t i v o p r i n c i p a l e c h e h a d e t e r m i n a t o l ' i n s e r i m e n t o delle m i s u r e s i s t e m a t i c h e dell ' a s s o r b i m e n t o i o n o s f e r i c o n e l p i a n o di l a v o r o del g r u p p o i o n o s f e r i c o d e l l ' i s t i t u t o n a z i o n a l e di geofisica è la s t r a o r d i n a r i a u t i l i t à di q u e l l e m i s u r e p e r l o s t u d i o delle r e l a z i o n i t r a f e n o m e n i i o n o s f e r i c i e f e n o m e n i g e o m a g n e t i c i e solari. mi l i m i t o a r i c o r d a r e la s t r e t t a conness i o n e già d a t e m p o r i l e v a t a f r a le p e r t u r b a z i o n i g e o m a g n e t i c h e ad inizio r a p i d o e l e e v a n e s c e n z e b r u s c h e d e l l e r a d i o c o m u n i c a z i o n i , e f r a la c o m p a r s a di m a c c h i e solari e l ' i r r e g o l a r e a n d a m e n t o d e l l a i o n o s f e r a . q u e s t i sono d u e e s e m p i tipici di q u e s t i o n i c h e sono difficilmente stud i a b i l i s p e r i m e n t a l m e n t e con le n o r m a l i i o n o s o n d e , m a c h e si possono r i l e v a r e con g r a n d e c o m o d i t à d a l l ' a n d a m e n t o d e l l ' a s s o r b i m e n t o i o n o s f e r i c o . 2. — e s a m i n e r ò o r a b r e v e m e n t e i m e t o d i c o m u n e m e n t e i m p i e g a t i p e r la m i s u r a d e l l ' a s s o r b i m e n t o i o n o s f e r i c o . 564 p i e r o d o m i n i c i i l p r i m o , e p i ù a n t i c o , c h e c h i a m e r ò « m e t o d o a d o n d a c o n t i n u a », f u u s a t o l a p r i m a v o l t a d a a p p l e t o n n e l 1930. q u e s t o m e t o d o si b a s a s u l b e n n o t o f e n o m e n o d e l l ' e v a n e s c e n z a ( f a d i n g ) , d o v u t o all ' i n t e r f e r e n z a f r a o n d a d i r e t t a e o n d a s p a z i a l e . u n a s t a z i o n e s i t a i n a (fig. 1) e m e t t e d e l l e r a d i o o n d e di f r e q u e n z a c o s t a n t e l e q u a l i g i u n g o n o a l p o s t o r i c e v e n t e c — a p o c h e d e c i n e d i c h i l o m e t r i d i d i s t a n z a — s e g u e n d o i n p a r t e i l p e r c o r s o ac ( o n d a d i r e t t a ) e d i n p a r t e i l p e r c o r s o abc ( o n d a s p a z i a l e ) c o n rifless i o n e i n b s u l l a i o n o s f e r a . s u l l ' a n t e n n a d e l r i c e v i t o r e s o n o p r e s e n t i c o n t e m p o r a n e a m e n t e s e g n a l i d o v u t i a l l e d u e o n d e , d i r e t t a e s p a z i a l e ; q u e s t ' u l t i m a è v a r i a m e n t e a t t e b f i g . 1 n u a t a l u n g o i l p e r c o r s o i o n o s f e r i c o e d h a u n a c e r t a d i f f e r e n z a di f a s e <p, v a r i a b i l e n e l t e m p o , r i s p e t t o a l l ' o n d a d i r e t ta. i l r i c e v i t o r e è p r o v v i s t o d i u n m i s u r a t o r e o r e g i s t r a t o r e d i u s c i t a a c c u r a t a m e n t e t a r a t o i n m i c r o v o l t / m e t r o d e l l a t e n s i o n e a r a d i o f r e q u e n z a a l l ' a n t e n n a . se l a i o n o s f e r a f o s s e u n r i flettore p e r f e t t o , l ' i n t e n s i t à d i c a m p o e i d e l l ' o n d a s p a z i a l e i n c s a r e b b e solo i n v e r s a m e n t e p r o p o r z i o n a l e a l l a d i s t a n z a p e r c o r s a d, s e c o n d o u n a c o s t a n t e k d i p e n d e n t e solo d a l l a p o t e n z a e f f e t t i v a m e n t e i r r a d i a t a d a l l ' a n t e n n a t r a s m i t t e n t e . i n r e a l t à l o s t r a t o n o n è u n r i f l e t t o r e p e r f e t t o , m a h a u n coefficiente di riflessione p m i n o r e d e l l ' u n i t à . o s s e r v o s u b i t o c h e p è l e g a t o a l l ' a s s o r b i m e n t o r d a l l a r e l a z i o n e t = — l o g p . si h a j? ' h • « d < f d< r f ' £ , = — p , c i o è : q=~-eì—~f0 , o , k k f„ d o v e e0 è l ' i n t e n s i t à d i c a m p o d e l l ' o n d a d i r e t t a , f a c i l m e n t e m i s u r a b i l e e l i m i n a n d o i n u n m o d o o n e l l ' a l t r o e u a d e s e m p i o , c o m e f e c e a p p l e t o n , m i s u r a n d o l a d i n o t t e , q u a n d o cioè, n e l l e c o n d i z i o n i d e l l a s u a e s p e r i e n z a ( x = 491 m ) m a n c a v a l ' o n d a s p a z i a l e . i l r a p p o r t o ei/e„ bì ricava o s s e r v a n d o i m a s s i m i e d i m i n i m i d e l l a c u r v a d e l r e g i s t r a t o r e d i u s c i t a . i n f a t t i l ' i n t e n s i t à di c a m p o a l l ' a e r e o r i c e v e n t e è : l* = e*+ e*+2 f0el cl-s cp s u l l a m i s u r a d e l l ' a s s o r b i m e n t o i o n o s f e r i o o 565 cioè, p e r <p — 0,em=e„-\/*.', ( v a l o r e massimo), p e r rp = n, em—e„ — e] ( v a l o r e m i n i m o ) si h a c o s ì : e, _ l 'u e e +h j o m ' m n o l o c h e la e i n o n si m a n t i e n e c o s t a n t e m a v a r i a a sua v o l t a seg u e n d o l e v a r i a z i o n i della i o n i z z a z i o n e . t u t t a v i a è f a c i l e d i s t i n g u e r e da e v e n t u a l i evanescenze da a s s o r b i m e n t o l e evanescenze d i f a s e u t i l i p e r l a d e t e r m i n a z i o n e del r a p p o r t o e i / e a , sia p e r la l o r o m a g g i o r e a p p a r i s c e n z a , sia p e r il l o r o c a r a t t e r i s t i c o a n d a m e n t o r e g o l a r e . conosciuto il r a p p o r t o e-^/e,,, n o t a e0 a s s u n t o p e r eli il v a l o r e 2 h', dove li' è l'altezza dello s t r a t o r i f l e t t e n t e — cosa lecita se t r a s m e t t i t o r e e ric e v i t o r e n o n sono m o l t o l o n t a n i — , calcolala k in b a s e alle c a r a t t e r i s t i c h e d e l l a stazione t r a s m i t t e n t e , r i m a n e d e t e r m i n a t o p o, c h e è l o stesso, r . u n a causa di gravi e r r o r i i n q u e s t o m e t o d o è c o s t i t u i t a d a event u a l i v a r i a z i o n i della p o t e n z a emessa dal t r a s m e t t i t o r e o da v a r i a z i o n i n e l l a s i n t o n i a t r a t r a s m e t t i t o r e e ricevitore. p r e c a u z i o n i c o m u n e m e n t e u s a t e n e l l e m o d e r n e a p p a r e c c h i a t u r e di q u e s t o t i p o — u n e s e m p i o n e è l ' a p p a r a t o del n a t i o n a l b u r e a u of s t a n d a r d s d i w a s h i n g t o n (2) — consistono n e l l ' u s o di tensioni di a l i m e n t a z i o n e a s s o l u t a m e n t e stabilizz a t e e n e l l ' u s o di cristalli di q u a r z o p e r il c o n t r o l l o della f r e q u e n z a sia n e l t r a s m e t t i t o r e c h e n e l r i c e v i t o r e . u n s e c o n d o m e t o d o è q u e l l o « ad i m p u l s i », deriv a t o dalla o m o n i m a t e c n i c a di s o n d a g g i o i o n o s f e r i c o . a n c h e q u i u n t r a s m e t t i t o r e m o d u l a t o ad i m p u l s i l a n c i a verso l ' a l t o dei b r e v i t r e n i d ' o n d a di u n a c e r t a f r e q u e n z a e gli e c h i riflessi d a l l a i o n o s f e r a v e n g o n o capt a t i da u n r i c e v i t o r e sito poco l o n t a n o , ed a p p l i c a t i ad u n t u b o a raggi catodici, il cui asse tiei t e m p i è s i n c r o n i z z a l o con la c a d e n z a degli i m p u l s i . sullo s c h e r m o del t u b o a p p a r i r à u n a t r a c c i a sul t i p o di q u e l l a s c h e m a t i z z a t a n e l l a fig. 2, f i g . 2 (2) n . b . s . circolare n. 462 w a s h i n g t o n , 1948. 566 p i e r o d o m i n i c i i n cui si n o t a n o il segnale d o v u t o a l l ' i m p u l s o di p a r t e n z a t, e q u e l l i s' s" s"' d o v u t i al p r i m o s e c o n d o t e r z o eco ecc. ecc. r i f e r e n d o c i alla fig. 1, s e g u i t i a m o a c h i a m a r e d„ la d i s t a n z a f r a t r a s m e t t i t o r e e r i c e v i t o r e e d i , a l solito p r e s o u g u a l e al d o p p i o dell'altezza d e l l o s t r a t o , la l u n g h e z z a del t r a g i t t o d e l l ' o n d a spaziale. t, s', s" ecc. s i a n o le m i s u r e in m i n d e l l ' a l t e z z a del segnale d o v u t o al segnale d i r e t t o , al p r i m o eco ecc. £ e v i d e n t e m e n t e t = — , dove a è u n a costante. q u a n t o ad s\ è d 0 s' =z p , dove (3 è u n ' a l t r a costante. così p e r s" = —-— p 2 , a v e n d o dt 2d{ p r e s o u g u a l e a l l ' u n i t à il coefficiente d i riflessione del suolo t e r r e s t r e . si h a a l l o r a : i l r a p p o r t o s"/s' si m i s u r a d i r e t t a m e n t e sullo s c h e r m o dell'oscill o g r a f o e p e conosciuto. i n p r a t i c a la cosa n o n è così s e m p l i c e . anc h e a m m e s s o c h e la n a t u r a del suolo sia t a l e da n o n i n f l u i r e a p p r e z z a b i l m e n t e s u l l a riflessione d e l l ' o n d a , r i m a n e p u r t u t t a v i a la difficoltà g r a n d i s s i m a di r i l e v a r e s " , c h e g e n e r a l m e n t e è p i u t t o s t o d e b o l e , dall ' i n t e n s o f o n d o di i n t e r f e r e n z e e d i s t u r b i c h e a f f e t t a n o r m a l m e n t e m i s u r e d i t a l g e n e r e . p r e z i o s a è a t a l r i g u a r d o l ' e s p e r i e n z a f a t t a rec e n t e m e n t e (19501 dai r i c e r c a t o r i del b u r e a u l o n o s p h é r i q u e f r a n c a i s , c h e a v e v a n o c o s t r u i t o u n p r e g e v o l e a p p a r a t o ad i m p u l s i con u n reg i s t r a t o r e e l e t t r o n i c o c a p a c e di c a l c o l a r e a u t o m a t i c a m e n t e il r a p p o r t o s"/s\ si è stati p e r ò costretti a m o d i f i c a r e l ' a p p a r a t o a causa della p r a t i c a i m p o s s i b i l i t à di r i l e v a r e con sicurezza l'eco s". w l i i t e e b r o w n i n u n a l o r o e s p e r i e n z a d e l 1935, a g g i r a r o n o questa difficoltà s e r v e n d o s i solo dell'eco di p r i m a riflessione. i n f a t t i , in« s' t r o d o t t o il coefficiente di trasmissione k = , si h a p = kd{ ; m o t s" . 2 s" t m a è a n c h e o = 2 , q u i n d i k = s' <il y y q u e s t o coefficiente k n a t u r a l m e n t e si d e t e r m i n a u n a volta p e r t u t t e , e q u i n d i p e r la m i s u r a si o p e r a solo con t ed s', c h e sono seg n a l i sicuri e f a c i l m e n t e r i l e v a b i l i . 3. — i l m e t o d o ad i m p u l s i è s e n z ' a l t r o p i ù c o m p l e t o e preciso c h e n o n il m e t o d o ad o n d a c o n t i n u a , i n n a n z i t u t t o p e r c h é in p r a t i c a è e s e n t e da i n t e r f e r e n z e d o v u t e ad emissioni di a l t r e stazioni sulla m e d e s i m a s u l l a m i s u r a d e l l ' a s s o r b i m e n t o i o n o s f e r i o o 567 f r e q u e n z a , ed i n s e c o n d o l u o g o p e r c h é p e r m e t t e di m i s u r a r e separat a m e n t e l ' a s s o r b i m e n t o s u b i t o da o g n u n a d e l l e d u e c o m p o n e n t i mag n e t o i o n i c h e d e l l ' o n d a , cosa q u e s t ' u l t i m a c h e , a m e n o d i d elicati artifici, n o n si p u ò o t t e n e r e col m e t o d o ad o n d a c o n t i n u a . t u t t a v i a l'esigenza c h e si è di p i ù c e r c a t o di s o d d i s f a r e n e l l a esec u z i o n e d e l l e n o s t r e m i s u r e , è la r e g i s t r a z i o n e dei d a t i , e m i p a r e clic, dei d u e m e t o d i , q u e l l o p i ù efficiente e c o m o d o d a q u e s t o p u n t o di vista, è il p r i m o , n e l q u a l e l ' a p p a r a t o r e g i s t r a t o r e si r i d u c e semp l i c e m e n t e ad u n v o l t m e t r o s c r i v e n t e od a r e g i s t r a z i o n e f o t o g r a f i c a , c o n v e n i e n t e m e n t e a p p l i c a t o al c i r c u i t o del c o n t r o l l o a u t o m a t i c o d i s e n s i b i l i t à del r i c e v i t o r e . 11 p r o g e t t o di m a s s i m a p e r l ' e s e c u z i o n e d e l l e n o s t r e m i s u r e di a s s o r b i m e n t o è, salvo v a r i a n t i i m p o s t e d a i m p r e v e d u t e necessità, il s e g u e n t e . n e l l ' o s s e r v a t o r i o di s. alessio dell'i.n.g., presso r o m a , sarà istall a t o u n t r a s m e t t i t o r e d i m o d e r a t a p o t e n z a ad o n d a n o n m o d u l a t a di f r e q u e n z a sui 3 m h z , con c o n t r o l l o a q u a r z o della f r e q u e n z a emessa. p o c o l o n t a n o , s e m p r e n e l l ' a m b i t o d e l l ' o s s e r v a t o r i o , u n s e m p l i c e reg i s t r a t o r e di i n t e n s i t à di c a m p o di piccola sensibilità f o r n i r à la q u a n tità e„. u n v a n t a g g i o n o t e v o l e n e l l a i n t r o d u z i o n e di q u e s t o p r i m o r e g i s t r a t o r e è d i svincolarsi d a l l a necessità dell'assoluta costanza della p o t e n z a e m e s s a : q u e s t a c o n d i z i o n e n o n è c o m p l e t a m e n t e r a g g i u n g i b i l e n e a n c h e con la i n t e g r a l e s t a b i l i z z a z i o n e delle t e n s i o n i di a l i m e n t a z i o n e — cosa già a b b a s t a n z a difficile — a causa d e l l ' e v e n t u a l e deper i m e n t o dei t u b i o di a l t r i c o m p o n e n t i dei c i r c u i t i . la e i i n v e c e s a r à m i s u r a t a n e l l ' o s s e r v a t o r i o dell'i.n.g. di r o c c a di p a p a , a 10 k m ca. in linea d ' a r i a , m e d i a n t e u n sensibile r i c e v i t o r e a c c o r d a t o seguito d a l complesso v o l t m e l r i c o di r e g i s t r a z i o n e . l e a n t e n n e p r o b a b i l m e n t e sar a n n o a t e l a i o p e r s e p a r a r e m e g l i o e v e n t u a l i i n t e r f e r e n z e . ogni 10 m i n u t i il t r a s m e t t i t o r e cesserà di e m e t t e r e p e r u n b r e v e t e m p o e ogni o r a p e r u n t e m p o p i ù l u n g o . così, o l t r e ad avere sulle r e g i s t r a z i o n i u n a c o n n i n e b a s e dei t e m p i , si p o t r a n n o d i s c e r n e r e d a l segnale u t i l e i segnali i n t e r f e r e n t i e i d i s t u r b i di v a r i a n a t u r a . il c o n f r o n t o f r a e i e„ e e„ f o r n i r à f al m o d o già visto, c i o è : t = — l o g — — . roma — istituto nazionale di geofisica —• giugno 1952. 568 p i e r o d o m i n i c i riassunto relativamente alla misura dell'assorbimento ionosferico, se ne illustra brevemente l'importanza ed il significato. vengono quindi esaminati i metodi di misura comunemente usati. summary concerning the measurement of ionosplieric absorption and a brief outline of its importance and significante. the measurement methods commonly used are examined and discussed. articolo fenton.pdf key words paleoseismicity – active faulting – thailand 1. introduction due to a lack of large, damaging earthquakes during historical time, thailand has not been considered to be a seismically active country. although there are a number of accounts of historical earthquake damage (nutalaya et al., 1985), the locations and sizes of most of these events are not well constrained. it is likely that some historical earthquake damage in thailand may have been the result of large earthquakes on distant faults such as the sagaing fault in myanmar and faults in southern china, which have been the locus of several magnitude (unspecified scale) > 6.0 earthquakes during historical time (le dain et al., 1984). recent seismicity in thailand has been confined to low to moderate levels with no clear association with existing mapped faults (bott et al., 1997). in areas like thailand, where there is no reliable, long-term earthquake record and an absence of historical fault surface ruptures, it is necessary to examine the geologic and geomorphic record, in order to quantify the activity on suspected active faults, and thereby determine their contribution to the seismic hazards of the region. a fault is deemed active and is considered to be a potential source of future earthquakes if the fault: 1) displays geomorphic features indicative of recent fault activity; 2) there is evi957 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. clark h. fenton, urs corporation, 500 12th street, suite 200, oakland, ca 94607, u.s.a.; e-mail: clark_fenton@urscorp.com recent paleoseismic investigations in northern and western thailand clark h. fenton (1), punya charusiri (2) and spencer h. wood (3) (1) urs corporation, oakland, ca, u.s.a. (2) department of geology, chulalongkorn university, bangkok, thailand (3) department of geosciences, boise state university, boise, id, u.s.a. abstract recent paleoseismic investigations have identified a number of active faults in northern and western thailand. northern thailand is an intraplate basin and range province, comprised of north-south-trending cenozoic intermontane grabens and half grabens, bounded by northto northwest-striking normal to normal-oblique faults and northeast-striking left-lateral strike-slip faults. the basin-bounding normal faults are marked by steep, linear range fronts with triangular facets and wineglass canyons and have slip rates of 0.1 to 0.8 mm/yr. based on limited data, the average vertical displacement-per-event is about 1.0 to 1.5 m. these faults are characterized by recurrence intervals of thousands to tens of thousands of years and are capable of generating earthquakes up to moment magnitude (m) 7, and larger. the northeast-striking strike-slip faults are marked by shutter ridges, and deflected drainages. slip rates are 3 mm/yr or less. western thailand is dissected by a number of northwestand north-northwest-striking, right-lateral strike-slip faults related to the sagaing fault in myanmar. although showing much less activity than the faults in neighboring myanmar, these faults display abundant evidence for late quaternary movement, including shutter ridges, sag ponds, and laterally offset streams. the slip rate on these faults is estimated to be 0.5 to 2.0 mm/yr. these faults are considered capable of generating maximum earthquakes of up to m 71/2. dence for displacement in young (late quaternary) deposits or surfaces; and/or 3) is associated with a moderateto large-magnitude historical earthquakes or a pattern of microearthquakes suggestive of an active fault. in this paper, we summarize the results of several recent investigations of active faults in northern and western thailand. the evidence for active faulting, illustrated by examples from the thoen, pua, mae chan, and three pagodas faults, and the characteristics of these faults are discussed. 2. tectonic setting the contemporary tectonic framework of southeast asia is a consequence of the interaction between the indo-australian and eurasian plates (fig. 1), and the more distant philippine and west pacific plates. the eurasian plate, within which thailand is situated, is surrounded by convergent margins, including the andaman thrust and sunda arc, to the west and south, respectively. australia is moving northward (along a vector of 010° to 020°), towards southeast asia, with a convergence rate of 65 to 70 mm/yr (mccaffrey, 1996). in addition, southeast asia may also be moving towards eurasia at a rate of 10 mm/yr. deformation across these plate margins is diffuse. partitioning of oblique convergence along the andaman and sumatra-java margins gives rise to broad, complex zones of deformation (fig. 1) involving both subduction and transform faulting (malod and kemal, 1996), the most notable of the latter being the sumatra fault. persistent deformation within the eurasian plate is illustrated by the number of seismogenic faults in this region (molnar and deng, 1984). thailand occupies an intraplate setting within the eurasian plate. the present tectonic stress regime in thailand is one of transtension, with opening along north-south oriented basins and right-lateral and left-lateral slip on northwestand northeast-striking faults, respectively (polachan et al., 1991; packham, 1993). this regime of transtensional faulting was initiated sometime between the late cretaceous and early tertiary (polachan et al., 1991; mccabe et 958 clark h. fenton, punya charusiri and spencer h. wood fig. 1. major tectonic elements in southeast asia and southern china. arrows show relative directions of motion of crustal blocks during the late cenozoic. mpfz mae ping fault zone; ntfz northern thailand fault zone; tpfz three pagodas fault zone; ufz uttaradit fault zone. modified from polachan et al. (1991). al., 1988). the cenozoic tectonics of thailand, and southeast asia as a whole, are a consequence of collision of india with eurasia. collision began about 50 ma (middle eocene) and has resulted in 2000 to 3000 km of shortening across the himalayan orogen (peltzer and tapponnier, 1988). as india drove into the southern margin of eurasia, indochina was rotated clockwise about 25° and extruded to the southeast by approximately 800 km along the red river and wang chao-three pagodas fault zones during the first 20-30 million years of collision (peltzer and tapponnier, 1988). extrusion migrated northwards onto the altyn tagh fault as collision progressed. rotation of indochina continued, reversing the sense of motion of the red river fault from left-lateral to right-lateral (allen et al., 1984; peltzer and tapponnier, 1988). differential slip between the main strike-slip faults, from north to south, the red river, mae ping, three pagodas, and sumatra fault zones (fig. 1), created a transtensional regime that resulted in the opening of the tertiary basins in southeast asia (ducrocq et al., 1992). the tertiary basins of thailand are grabens or half grabens, typically bounded by north to northwest-striking normal faults (polachan et al., 1991; lorenzetti et al., 1994). the location and geometry of the basins are controlled by the north-south structural grain in pre-triassic rocks and pre-existing northwest-striking, strikeslip faults (o’leary and hill, 1989). basin evolution throughout thailand follows a roughly similar sequence of events. the main phase of strike-slip tectonism, resulting in rapid extension, with widespread fluvial sedimentation (oligocene-early miocene), was followed by lacustrine sedimentation as basins became increasingly isolated (early-middle miocene). lacustrine sedimentation ended, and there was an influx of coarse terrigenous clastics suggesting a period of rapid, localized uplift (middlelate miocene). finally, following a brief period of basin inversion that resulted in a widely recognized late miocene unconformity, fluvial sedimentation, generally coarsening upward, resumed (late miocene-recent) (polachan et al., 1991; remus et al., 1993; alderson et al., 1994). the climax of extensional tectonism is marked by the eruption of late tertiary and early to middle quaternary alkaline basalts (hoke and campbell, 1995). paleomagnetic data, showing localized rotations during the tertiary, indicates that the opening of these basins appears to have been fairly complex (mccabe et al., 1988, 1993; richter et al., 1993). with the exception of the lignite-bearing mae moh basin (e.g., ratanasthien, 1986), few non-proprietary studies have been carried out to determine the detailed tectonic development of these basins. the total amount of extension across the region is unknown; olinstad et al. (1989) estimate about 50 km of cenozoic extension at the northern end of the gulf of thailand. in addition, the thickness of basinfill sediments is poorly constrained and the geometry of the basin-bounding faults are not well known. many of the basins, however, are bounded by linear escarpments that display a number of geomorphic features suggesting that they may still be active structures (siribhakdi, 1986). 3. seismotectonic provinces we have modified the seismotectonic zones of nutalaya et al. (1985), incorporating geologic, heat flow, and fault activity data, to better represent the seismotectonic character of thailand (fig. 2). most of northern thailand falls within what, on account of its tectonic similarities with the basin and range province of the western united states, fenton et al. (1997) have called the northern basin and range province. this is a region of extended crust, forming basin and range topography (macdonald et al., 1993). heat flow is presently moderate to locally high and probably has been since the initiation of rifting in the early tertiary (raksaskulwong and thienprasert, 1991). to the west the northern basin and range is bounded by the western highlands, a region of lower heat flow, comprising uplifted and complexly folded and faulted metamorphic rocks. this province is traversed by a number of northwestto north-northwest-striking rightlateral strike-slip faults, including the three pagodas and mae ping faults, which splay from the sagaing fault in eastern myanmar (fig. 1). to the east of the northern basin and range is the loei or eastern fold belt, a transitional zone between the extended crust of the northern basin and range and the korat plateau, an uplifted plateau region with subdued topography, low heat flow, and apparent tectonic quiescence. to the south the northern basin and range transitions into the central plain (also known as the chao praya basin) which in turn transitions into the gulf of thailand. each of these provinces is a 959 recent paleoseismic investigations in northern and western thailand region of extended crust, distinguished by an increasing amount of extension and subsidence towards the south. whereas the northern basin and range comprises a number of distinct basins, separated by intervening ranges, the central plain has extended to the point where it has created a single, broad, alluvium-filled topographic basin. north of the northern basin and range is a region dominated by left-lateral strike-slip deformation. this is called the mae chan province after the active strike-slip fault that appears to mark the southern boundary of this deformation. 4. historical seismicity contemporary seismicity in the northern basin and range province is diffusely distributed, of low to moderate levels, does not appear to be associated with currently mapped faults (fig. 3), and is probably confined to the upper 10 to 20 km of the crust (bott et al., 1997). although the historical earthquake record extends back to at least 1300 a.d. (year 1843 b.e., thai calendar), the largest known earthquake in thailand has probably not exceeded richter magnitude (ml) 61/2. associating seismicity with specific geologic structures, particularly mapped faults, is extremely difficult in northern thailand because of the large location uncertainties of individual earthquakes (bott et al., 1997). in fact, in common with the basin and range province in the western united states, much of the widely distributed seismicity in northern thailand is probably associated with faults that have no clear surface expression (bott et al., 1997). in the basin and range province, these so-called background earthquakes can be as large as m 6 to 61/2 (depolo, 1994). 5. previous fault studies with the exception of limited ground cracking accompanying the 1983 ml 5.9 reservoirinduced earthquake at srinagarind reservoir (klaipongpan et al., 1991), there are no reports of historical surface faulting events in thailand. nutalaya (1994) and hinthong (1995, 1997) initially compiled data on active faults in thailand, identifying 22 active, potentially active, or suspected active faults, based primarily on geomorphic expression and thermoluminescence ages obtained from fault gouges. subsequent investigations have added to the inventory of active and suspected active faults in thailand (fig. 4). thiramongkol (1986) presented evidence for holocene movement on the bang pakong fault on the eastern margin of the central plain. le dain et al. (1984) suggested that the northweststriking papun (moei or mae ping) fault might also be active, based on recent seismicity recorded in the myanmar-thailand border region 960 clark h. fenton, punya charusiri and spencer h. wood fig. 2. seismotectonic provinces in thailand (after woodward-clyde federal services, 1996). 961 recent paleoseismic investigations in northern and western thailand fig. 3. late cenozoic faults and historical seismicity (1362 to 1996) of the northern basin and range seismotectonic province. figure modified from bott et al. (1997). (fig. 1). earthquake focal mechanisms suggest that this fault is a right-lateral, strike-slip fault (le dain et al., 1984). woodward-clyde federal services (1996) carried out extensive fault investigations in northern thailand that showed recent activity on the mae chan, pua, phrae, phrae basin, thoen, nam pat, long, and phayao faults (fig. 3). rhodes et al. (1996) reported young faulting within cenozoic sediments along the margin of the chiang mai basin, west of the city of chiang mai. perez et al. (1999) inferred recent movement on the mae kuang fault, northeast of chiang mai, from the offset of three tributaries of the mae kuang river. wood (1995, 2001) provided strong geomorphic evidence for late quaternary left-lateral displacement along the mae chan fault. recent trenching investigations have confirmed late quaternary faulting on the mae chan fault (rymer et al., 1997). woodward-clyde federal services (1998) highlighted evidence for recent movement on the three pagodas fault in western thailand and also on the tavoy, tenasserin, and kungyaungale faults in neighboring peninsular myanmar. 6. approach of recent investigations the identification of active faults in thailand has been hampered by: 1) the comparative lack of fault studies; 2) extensive weathering of bedrock and extremely active erosion acting together to prevent the preservation of all but the most resistant geomorphic features; 3) large areas of thick forest vegetation, and 4) the probable low slip rates of the intraplate faults in thailand. combined, these factors result in a lack of recognizable, long-lived surfacefaulting geomorphic features (fenton et al., 1999). despite these shortcomings, paleoseismic investigations in thailand have been successful and indicate that late quaternary deformation is ongoing and that these active faults pose a significant seismic hazard (e.g., wong et al., 1997). lacking a well-defined late quaternary framework for northern thailand, recent studies (e.g., fenton et al., 1997) concentrated on the geomorphic expression of faulting, and the comparison with the features observed along other active faults in similar tectonic settings worldwide. these features included fault scarps, faceted spurs, and offset terraces and drainages. these geomorphic data were supplemented by 962 clark h. fenton, punya charusiri and spencer h. wood fig. 4. active and suspected active faults in thailand. modified from hinthong (1995). paleoseismic data obtained from limited trenching and fault zone exposures (woodward-clyde federal services, 1996; fenton et al., 1999). ideally, investigations of active faults progress from a regional, through a local, to a sitespecific scale. in areas where there is some data on the regional neotectonic setting, the emphasis of such studies is on the local and site-specific scale. in thailand, where damaging earthquakes are infrequent and possibly there are no surface-faulting events in historical time, the regional, as well as local seismotectonic setting is not well understood. therefore, investigations have to start at a regional level, and progress through a series of logical steps towards detailed site-specific studies. initial studies involved remote sensing interpretation, starting at small scales with satellite imagery, progressing to detailed stereoscopic aerial photographs to locate zones of active deformation (wood, 1995, 2001; fenton et al., 1997). once these zones were identified, detailed mapping of quaternary landforms and deposits along a zone of deformation was carried out. the quaternary stratigraphic framework was then established and landform styles identified. based on the estimated ages of these features and/or deposits, they were used to measure the amount and the rates of displacement across faults (wood, 1995, 2001). geomorphic mapping aided in evaluating the location of active faults and the nature of the faulting, identifying the number of paleo-events, and the approximate size and timing of these events (fenton et al., 1999). finally, when there was sufficient knowledge of the local quaternary stratigraphy and geomorphology with which to evaluate the results of detailed trench logging, we excavated and logged a number natural fault zone exposures. a number of the faults investigated, even after detailed geomorphic studies, were not considered suitable for trenching studies because of the lack of preservation of young deposits along the fault zones. these faults, although having very prominent geomorphic expressions, usually as topographic escarpments with faceted spurs with oversteepened bases (hamblin, 1976), are rarely expressed in young geologic materials. these studies have allowed us to estimate a number of fault parameters including: the total amount of offset across fault zones; slip-perevent; fault slip rates; recurrence intervals; fault segmentation, and bracket the age of faulting events. the geomorphic evidence for late quaternary faulting in thailand is discussed in the following sections, illustrated by examples from the thoen, pua, mae chan, and three pagodas fault zones. 7. thoen fault the 120 km long thoen fault zone (fig. 3) is a series of northto northeast-striking faults that traverse the region between the phrae basin to the east and the lampang, mae moh, and thoen basins to the west (piyasin, 1974; charoenprawat et al., 1994). two faults within this recent paleoseismic investigations in northern and western thailand fig. 5. simplified map of the thoen fault system showing fault segments and localities mentioned in the text. 963 system display a number of structural, stratigraphic, and geomorphic features indicating recent movement. they are the fault bounding the eastern margins of the mae moh, lampang, and thoen basins, herein called the thoen fault, and the fault bounding the western side of the long valley, the long fault (fig. 5). on aerial photographs and landsat tm satellite imagery, the thoen fault is a well-defined, sharp lineament. the predominantly vertical offset across the fault is highlighted by the contrast between the deep incision by streams in the uplifting footwall compared to the lack of incision on the same streams in the subsiding footwall, i.e. denudation and aggradation in the footwall and hangingwall, respectively, of an active fault. the thoen fault can be divided into four segments, based on geomorphic expression, structural style, and sense of offset. from north to south these are the ban mai, doi ton ngun, mae tan, and thoen segments (fig. 5). over much of its length, the thoen fault forms a northwest-facing bedrock escarpment that varies in height from 400 to 600 m. the escarpment comprises a series of faceted spurs interrupted by several benches or erosional pediment remnants (fig. 6). hamblin (1976) has shown that such features were the result of episodic fault movement; the facets forming during periods of fault movement and the benches during periods of tectonic stability, erosion and fault scarp retreat (fig. 7). studies of faults in the mae moh basin, immediately to the west of the thoen fault, have shown that extension began sometime in the early oligocene (vella, 1983; ratanasthien, 1986). these faults show a complex history of movement throughout the tertiary, however, they now appear to be relaclark h. fenton, punya charusiri and spencer h. wood fig. 6. triangular faceted spurs along the thoen fault escarpment east of ban san pa pao. note the oversteepened base and the development of several erosional benches along the escarpment. 964 progressive steepening towards the base of a fault scarp or escarpment shows repeated fault movement, the steeper bevels representing more recent (less eroded fault scarps) faulting episodes (mccalpin, 1996). several streams crossing the ban mai segment exhibit wine-glass canyon profiles. this results from renewed uplift or an increased rate of uplift of the footwall in a normal fault system (fig. 8). in order to equilibrate their profiles across a fault-produced knickpoint, streams may have to cut down rapidly when fault movement either resumes after a period of tectonic quiescence or when the rate of movement increases. the result of this rapid down-cutting is the formation of a narrow slot canyon or stem of the wine-glass. the presence of wineglass canyons in addition to the marked steepening of the faceted spurs at the base of the escarpment, indicate that, at least along the ban mai segment, the thoen fault is undergoing renewed or increasing vertical displacement. vertical offset has also resulted in a sudden widening of drainage valleys as they cross from the footwall into the hangingwall. these valley floors and the adjacent ridge crests also show left-lateral offset across the fault. valley walls are offset in a leftlateral sense by approximately 200 to 400 m. at ban mai, the thoen fault crosses the active floodplain of the mae mai river (fig. 5) and vertically offsets an alluvial terrace surface, down-to-the-northwest, by approximately 6 m. the absolute age of this terrace surface is unknown; however, from the preservation of the terrace and the relatively immature soil development on the terrace surface (a bw horizon with only limited rubification), we consider this surface to be holocene in age. assuming approximately 6 m of vertical offset during the holocene, this gives a vertical displacement rate of about 0.6 mm/yr for the ban mai segment. a road cut about 1 km west of the main escarpment at ban mai (fig. 5) exposes a normal fault offsetting, down-to-the-west, a sequence of alluvial gravels and lacustrine clays. the fault dips about 80°w and strikes n48°e. total vertical displacement exposed in the cut is 1.3 to 1.6 m. the number of events and the amount of slip-per-event is unknown, as the top 965 recent paleoseismic investigations in northern and western thailand fig. 7. development of faceted spurs produced by episodic vertical tectonic movement. i undissected fault scarp. ii development of faceted spurs by streams cutting across the fault scarp. iii period of tectonic quiescence with slope retreat, and development of a narrow pediment. iv renewed fault movement. v dissection of the new fault scarp by major streams and streams developed on the faces of the faceted spurs developed at stage ii. vi a new period of tectonic quiescence, with the development of another narrow pediment within the footwall block at the base of the range front. vii renewed fault movement. viii dissection of the fault scarp produced at stage vii resulting in a line of small faceted spurs at the base of the range front. remnants of narrow pediments (benches) are preserved at the apices of each set of faceted spurs. progressive slope retreat is accompanied by a decrease in the slope angle of the faceted spurs. modified from hamblin (1976). tively inactive (ratanasthien, 1986). the most recent movement, therefore, appears to have been confined to the margins of the basin along the main thoen fault. along the ban mai fault segment, the lowest faceted spurs are about 250 to 350 m high. these facets themselves are not planar, but comprise at least three beveled planes that steepen towards the base of the escarpment. of the section has been eroded. young, unfaulted gravel (probably holocene in age) has been deposited across this eroded surface and is not offset by the fault. faulting is assumed to be late quaternary in age on account of the poorly developed soils both in the unfaulted unit crossing the fault and also within the upper offset units. the relationship between this fault and the main thoen fault is unknown. its location and geometry suggest that it must be a synthetic fault, i.e. it has the same dip and sense of movement as the main fault. a lack of geomorphic expression and the pre-holocene offset, however, indicates that it is less active than the main basin-bounding fault. with the exception of these two localities, no other faulting is found in young deposits along the ban mai segment. the lack of observed recent faulting over much of the northern segment of the thoen fault is due to the relative absence of young deposits along the fault trace. where present, mainly along modern drainage courses, these deposits are very young, most likely less than a few thousand years old (woodward-clyde federal services, 1996). in addition, rice farming has extensively modified these deposits. the high rates of erosion in northern thailand result in very little aggradation along active drainages in upland regions (kiernan, 1991). the 25 km long east-west-striking doi ton ngun fault segment forms a 300 m high range front with well-developed triangular facets and wine-glass canyons (fig. 5). the facets have three well-developed bevels: the lowest is about 3 to 5 m high and dips 45°; the intermediate bevel is about 25° to 30° and about 100 m high; and the upper bevel is 15° to 20° and extends to the crest of the range. the lowest bevel corresponds to a prominent scarp observed on aerial photographs (fenton et al., 1997). all streams crossing this fault splay show marked incision on the footwall, indicating vertical offset. no evidence for lateral offset is observed. with the exception of some minor faulting and tilting of alluvial gravels in a borrow pit at the eastern end of this segment, no faulting was observed in young deposits. displacement appears to die out rapidly westwards. the late pliocene-early pleistocene kho kha basalts (sutthirat et al., 1995) are offset by less than 1 m, and the fault does not appear to cut through the entire sequence of basalt flows. the mae than segment of the thoen fault is comprised of a number of splays that strike northeast to east-northeast (fig. 5). these splays bound the tertiary mae than basin. the main fault, bounding the southeastern side of the basin, forms a 600 m high, northwest-facing escarpment characterized by well-developed triangular facets and wineglass canyons (fig. 6). a well-developed bench is observed at the apices of 280 m high triangular facets. these facets show progressive steepening towards their bases. drainages crossing the fault show extensive footwall incision, but no evidence for lateral displacement. these drainages appear to 966 clark h. fenton, punya charusiri and spencer h. wood fig. 8. formation of a wineglass canyon. from woodward-clyde federal services (1996). 1 tributary drainage crosses normal faulted basin margin at grade. 2 normal faulting event. tributary drainage offset vertically. 3 drainage erodes down to new base level. 4 repeated fault movement. scarp height increases, as does depth of slot canyon if rate of faulting is greater than erosion. 1 2 3 4 1 2 3 4 have been ponded within the graben as shown by the sudden widening of their respective flood plains as they flow in and then narrow again as they flow out of the graben. no fault exposures were found in young alluvial deposits. exposures in the mae than lignite mine (fig. 5) show that none of the faults on the northwest side of the graben have been active during late quaternary time. south of mae than, at the base of the main fault escarpment, at least two, possibly three, late cenozoic, down-to-the-northwest faulting events are preserved as stacked colluvial wedges in a sequence of alluvial fan gravels exposed in a stream cut-bank. the age of these deposits is unknown; however, the degree of reddening and cementation of these gravels almost certainly precludes them from being holocene. clasts size, mainly cobbles and boulders, is also incompatible with the size of the present drainages issuing from the footwall block. we conclude, therefore, that these gravels are pliopleistocene in age. the thickness of individual colluvial wedges, approximately 1.0 to 1.5 m, indicates faulting events with comparable amount throw. assuming a similar 0.6 mm/yr vertical displacement rate, as calculated for the ban mai segment, this would suggest that the return period for the mae than segment is about 2500 years. to the south of sop prap (fig. 5), some indication of the total vertical offset across the mae than segment is shown by the displacement of a pre-tertiary peneplain surface. assuming the 600 to 800 m vertical displacement of this surface has occurred since the initiation of normal faulting in northern thailand, approximately 33 ma (charusiri, 1989), this gives a long-term vertical displacement rate of 0.02 mm/yr. this value, however, averages older, slower rates of activity with the higher, more recent rate that has produced the faceted spurs and wine-glass canyons. therefore, this slip rate must be considered a minimum value. south of sop prap, the thoen fault steps right and follows the base of the doi mae puan range front (fig. 5). this segment of the fault, although marked by a linear range front, is much more subdued than the other fault segments, suggesting that it has a lower slip rate. 8. pua fault the pua fault is a 68 km long, north-striking, west-dipping normal fault bounding the eastern margin of the tertiary pua basin (figs. 3 and 9). on aerial photographs and landsat tm satellite imagery, the fault forms a very prominent west-facing escarpment that gradually decreases in height and definition from north to south. the fault has three distinct segments: the linear northern thung chang segment; the concave-west central pua segment; and the linear southern santi suk segment (fig. 9). the most prominent tectonic geomorphology is observed along the northern and central segments where the fault is marked by a steep, west-facing escarpment with triangular facets and wineglass canyons. the base of the escarpment is progressively oversteepened. drainages crossing the escarpment are deeply incised in the footwall and have little or no incision in the hangingwall. terrace fragments are preserved along several drainages within the footwall block. the geomorphic expression of the southern segment of the fault is less pronounced. the fault is still marked by a prominent oversteepening at the base of the escarpment. the escarpment gradually decreases in height toward the south until the fault no longer has any topographic expression. the fault may continue to the south for another 5 km as a prominent bedrock fracture striking north-northeast – southsouthwest. the thung chang segment (fig. 9) of the fault is marked by a linear escarpment, comprising triangular faceted spurs and wine-glass canyons. the facets show a marked steepening towards the base of the escarpment and have at least two bevels at higher elevations. the steepened section at the base of the escarpment is up to 10 m high. where alluvial fans are better developed, this steepening is manifest as a warping, down-to-the-west, of the fan surface creating low-angle (about 10 to 15°) scarps (fig. 10). a number of drainages along this segment of the fault have been beheaded, suggesting that there is also a lateral component of slip. the offset of these drainages is ambiguous, thus, the sense of lateral slip is not clear. the offsets of terrace margins along one drainage appear to be 967 recent paleoseismic investigations in northern and western thailand the 24 km long pua segment is concave to the west (fig. 9). the central part of this fault segment displays the highest and steepest triangular facets found along the pua fault, and also displays some of the best-developed wine-glass canyons in northern thailand (fig. 11). this increased definition of the fault escarpment is coincident with a widening of the pua basin (fig. 9). the base of the escarpment is oversteepened and small terrace fragments appear to have been stranded along tributary drainages as a result of footwall uplift. assuming that these uplifted terrace fragments were formed at similar elevations to the alluvial fan surfaces within the basin (hangingwall), then the terrace fragments along the khun and khwang rivers have been offset vertically by at least 50 m. the age of these terrace surfaces, however, is unknown and therefore they cannot be used to determine the fault slip rate. no evidence for lateral movement has been found along this segment of the fault. at ban nam khrai, the fault again changes orientation to a more north-northwest strike (fig. 9) and the height of the escarpment decreases markedly. this bend in the fault corresponds with an intrabasin high and it also marks the boundary between the pua and santi suk segments of the fault. the santi suk segment (fig. 9) comprises the most subdued topography along the pua fault, although the fault escarpment displays the same geomorphic features indicative of active faulting found along the other two fault segments. this segment is marked by an escarpment that decreases in height from about 200 to 100 m over a distance of about 12 km. at its northern end, this escarpment has well defined triangular facets with oversteepened bases. the oversteepened bevel at the base of the escarpment is about 6 m high. the low faceted spurs at the southern end of the santi suk segment show at least three bevels, the lowest and steepest (approximately 22°) of which is about 6 m high. this bevel is uniformly well developed along the entire santi suk segment. the age of alluvial surfaces offset by this bevel is unknown; however, the lack of incision and preservation of these surfaces suggests that they are likely to be holocene in age (fenton et al., 1997). assuming that the bevel at 968 clark h. fenton, punya charusiri and spencer h. wood fig. 9. simplified map of the pua fault system showing fault segments and localities mentioned in the text. right-lateral. the amount of lateral offset, however, is relatively small (< 10 m) compared with the vertical offset. the southern end of the thung chang segment is marked by a prominent fault bend where the strike of the range front changes from north to northwest. recent paleoseismic investigations in northern and western thailand fig. 10. low angle fault scarp on surface of late quaternary alluvial fan, thung chang segment, pua fault. fig. 11. wine-glass canyon developed where the nam khun crosses the pua fault. compare canyon profile with fig. 8. 969 the base of the escarpment has formed during the holocene, this 6 m offset implies a minimum vertical displacement rate of about 0.6 mm/yr. further evidence for holocene movement on the santi suk segment is the exposure of a 30 m wide fault zone offsetting young soil horizons developed in a sequence of alluvial gravels and lacustrine clays in a gravel pit at ban hua na (fig. 12). the complex nature of faulting exposed in the gravel pit, involving both normal and reverse faulting, along with the mismatch of units across the faults suggest that movement on the santi suk segment involves a significant lateral component. the youngest deposit offset by faulting is a soil horizon developed in clayey gravels, immediately below the modern soil. offset of this soil is about 0.3 m down-to-the-east on a west-dipping fault, i.e. reverse movement, and about 0.5 m down-tothe-west on a west-dipping fault. the offset soil horizon is considered to be holocene in age on account of its lack of reddening and poorly developed rubification (woodward-clyde federal services, 1996). although this gravel pit exposes faults offsetting very young deposits, there are no stratigraphic markers that can be used to determine the amount or true sense of offset. the westdipping nature of these deposits suggests downto-the-east movement on an east-dipping normal fault, i.e. a fault antithetic to the main pua fault. a short east-dipping fault is observed on the opposite side of the basin on aerial photographs, although the geomorphic expression of this fault does not suggest that it has been a particularly active structure. 9. mae chan fault the mae chan fault is an east-west to northeast-southwest striking left-lateral strike-slip fault that extends for about 140 km from nam mae chan valley in northernmost thailand near the border with myanmar, across the mekong river and into laos (fig. 3). hinthong (1995) first recognized deflected drainages, shutter ridges, and sag ponds indicating active strikeslip faulting. older streams, entrenched in bedrock, are offset by as much as 600 m (wood, 2001). younger streams channeled into quaternary sediments are offset by a few meters. preliminary trenching investigations indicate that the most recent movement has been late quaternary and possibly holocene (rymer et al., 1997). some of the best examples of deflected drainages are found near ban su fu at the western part of the fault (fig. 13a-d). here older, more deeply incised streams are offset by up to 600 m, while younger, shorter streams are offset by 100 to 300 m (wood, 2001). as well as deflected drainages, wood (2001) also mapped a series of offset ridge crests, creating conspicuous shutter ridges and uphill-facing scarps (fig. 14a,b). using a denudation rate for similar regions of southeast asia (0.1 mm/yr), wood 970 clark h. fenton, punya charusiri and spencer h. wood modern soil pebble gravel clayey gravel stoneline fractured pebbles clayey gravel clayey gravel clayey sand modern soil bedding pebble-cobble gravel aligned clasts mottled clay mottled clay slump 356° to 013° vertical fractures numerous small fractures with small offsets clay gouge 80°/209° 73°/334° 90°/040° 90°/043° water seep along fault mottled clay buried soil stoneline modern soil colluvial wedge? pebble-cobble gravel 50 mm 20 mm 0.4 mm 0.2 mm 0.2 mm slump clayey sand l l k h g0,8 m k 0 5 mottled clay g 0.75 m b b b e f d d b d l l aa b b a c c l j fig. 12. log of south-facing exposure of faulted alluvial gravels and lacustrine clays at the southern end of the pua fault. overall sense of displacement is down-to-the-west. scale in meters. 971 recent paleoseismic investigations in northern and western thailand fig. 13a-d. a) landsat image of the mae chan fault and mekong river in northernmost thailand. river flows from top of image to the southwest (image processed and furnished by michael j. rymer, u.s. geological survey, menlo park, california). b) offset streams, shutter ridges, and water gaps along the western mae chan fault. c) detail of stream offsets near ban su fu. d) graph of offset versus stream length. one group of streams 0.5-to-0.7 km long is offset 100 to 300 m, while another group 1.1-to-1.8 km long is offset about 600 m. c d a b (2001) calculated that a time of 200 ka is required to erode a valley extending 1.4 km into the saprolite-mantled granitic upland south of the fault. this assumes a trapezoid-shaped volume of sediment removed to form the valley 60 m deep and 500 m wide at the range front. these streams are typically offset 600 m (fig. 13d), implying a slip rate of 3 mm/yr. because of the many uncertainties in our geomorphicbased estimate of valley age and fault-slip rate, at this point in our studies we believe the slip rate is between 0.3 and 3 mm/yr. the mekong river crosses the mae chan fault (fig. 13a). from the top of the image, the river flows southward to the golden triangle in a straight bedrock canyon 600 m deep that forms the laos-myanmar border. fifteen kilometers north of the mouth of the kok river the valley floor of the mekong river broadens to an alluvial plain about 4 km wide. at the mouth of the kok river, there is a ne-sw elongate (14 km by 5 km) marshy depression known as wiang nong that extends southwest along the north side of the fault (fig. 13a). we interpret this depression as a tectonic pull-apart basin or sag consistent with left-lateral motion of the fault. east of the mouth of the kok river, the river course deflects east 25 km (in a left-lateral sense) forming two broad loops north of the fault. the first loop is in an alluvial plain about 8 km wide, and the second is in a bedrock canyon about 400 m deep. the offset river then resumes a southerly course in the bedrock canyon and crosses the fault about 10 km north of chiang khong, thailand (fig. 13a). we agree with lacassin et al. (1998) that at this crossing the river course appears left-laterally deflected no more than 1.5 km. the fault has clearly affected the course of the mekong river, but we do not know if the entire 25 km deflection is a result of tectonic shift. at this point in our studies we do not have a detailed explanation for the abrupt change in the river course at the mouth of the kok river and the s-loops north of the fault. as the river interacted with the fault it must have continued to flow in the canyons previously incised 300 m through the northeastsouthwest-trending ranges. if a significant part of the 25 km deflection is tectonic, it is difficult to explain why the actual fault crossing shows only a 1.5 km deflection. it is possible that an older course of the mekong river lies to the 972 clark h. fenton, punya charusiri and spencer h. wood fig. 14a,b. a) ridge crest offset creating shutter ridge along the mae chan fault 3 km west of ban su fu. b) looking north along the crest of the offset ridge crest. a b east on the south side of the fault, but is now covered by a quaternary-aged basalt field that covers a 15-square-km area in laos just south of where the fault crosses the mekong. a k-ar age on the basalt is 1.7 ma (barr and macdonald, 1981). lacassin et al. (1998) interpret the unusual hairpin loops of the mekong river crossing the active nam ma fault 60 km upriver to be the result of late cenozoic slip-sense inversion. they suggest that the nam ma fault changed from a right-lateral to a left-lateral slip at some time between 5 and 20 ma. they noted the 1.5 km shift in the mekong on the mae chan fault crossing, but did not address the larger deflection of 25 km noted here. slip on the order of 25 km since late cenozoic is consistent with the slip rate of 3 mm/yr we derived for the small streams near ban su fu. this is corresponds to the upper bound slip rates calculated by lacassin et al. (1998) from river channel restoration along similar strike-slip faults to the north in myanmar, laos, and yunnan province, china. 10. three pagodas fault the three pagodas fault zone (tpfz) is a 350 km long, 25 km wide, northwest-striking zone of distributed faulting extending from just south of moulmein, myanmar, across the three pagodas pass, along the mae nam khwae noi/mae nam khwae yai (river kwai) drainage basin, and towards the gulf of thailand (fig. 15). at its northern end, the tpfz either merges with the sagaing fault system (fig. 1), or a splay of that fault, in the andaman sea at about 16°n (fig. 15). tapponnier et al. (1982) proposed that the tpfz and the mae ping fault (mpfz in fig. 1) to the north accommodated the majority of movement along the malay peninsula during the early phase (50-20 ma) of india-eurasia collision. during this phase of tectonism, movement on these faults was left-lateral, with a total offset of approximately 300 km (peltzer and tapponnier, 1988). recent thermochronologic studies in western thailand have revealed that this phase of left-lateral slip ended about 36 to 33 ma on the tpfz (lacassin et al., 1997). the subsequent change in the plate motion between india and eurasia causing a reversal in the rotation of southeast asia initiated east-west extension across central and northern thailand (peltzer and tapponnier, 1988). coeval with this, movement on the northwest-striking faults on the malay peninsula reversed a right-lateral displacement. right-lateral movement on the tpfz began about 24 ma (lacassin et al., 1997). earthquake focal mechanisms in the region indicate that right-lateral strike-slip faulting persists to the present day (le dain et al., 1984). initial investigations of the tpfz using landsat imagery concluded that the fault was inactive (ebasco, 1984). however, shrestha (1987) identified a number of suspected active fault traces along the mae nam khwae noi (fig. 16), including one that was claimed to offset an alluvial fan by 1.5 km. hetrakul et al. (1991) suggested that reservoir-induced seismicity in the area of khao laem reservoir may have occurred on a segment of the three pagodas fault. the yunnan seismological bureau (1997) identified three fault splays along the tpfz that have been active since middle pleistocene (240-160 ka) and estimated a slip rate of 0.19 mm/yr. woodward-clyde federal services (1998) carried out reconnaissance investigations of the tpfz between three pagodas pass and kanchanaburi (fig. 16). using aerial photographs and satellite imagery and field reconnaissance, they identified numerous geomorphic features indicative of active faulting, including lateral drainage deflections, scarps on alluvium, shutter ridges, and faceted spurs. between three pagodas pass and sangkhlaburi (fig. 16), the fault has a well-defined trace only a few hundred meters wide. south of the international border, the fault zone comprises two parallel strands. the western strand appears to offset a number of small streams in a right-lateral manner. the amount of offset is on the order of several meters to several tens of meters. about 6 km north of sangkhlaburi, the eastern fault trace forms a prominent break-inslope and side-hill along the northeastern side of a fault line valley. streams crossing this fault splay show a minor amount of right-lateral offset. 973 recent paleoseismic investigations in northern and western thailand at sangkhlaburi, the strike of the fault changes to a more north-south direction. immediately north of khao laem reservoir the fault is marked by a prominent west-facing escarpment that steepens towards its base. aggradation of small alluvial fans at the base of this escarpment and incision of the drainages running out of the escarpment are indicative of a component of vertical down-to-the-west displacement. the northern end of this escarpment forms a shutter ridge that deflects drainage in a right-lateral manner. further south, below khao laem dam, the easternmost trace of the tpfz forms a southwest-facing escarpment that bounds a small alluvium-filled basin. this escarpment displays well-developed triangular facets with a stepped or benched profile indicating by hamblin’s 974 clark h. fenton, punya charusiri and spencer h. wood fig. 15. active faults in western thailand and myanmar. (1976) criteria that this section of the fault has undergone episodic uplift. at its confluence with the huai mae nam noi, the mae nam khwae noi is deflected in an apparent right-lateral manner (fig. 16). several reaches of the river downstream also appear to be fault controlled, as are reaches of the mae nam khwae yai (woodward-clyde federal services, 1998). over most of its length, the tpfz is expressed in bedrock, or bedrock covered with a thin veneer of late holocene alluvium. there are very few exposures of the fault in late cenozoic deposits. along strike from a bedrock scarp, near rong rian yang thon (fig. 16), plio-pleistocene lacustrine and alluvial deposits exposed in a gravel pit are warped and increase in dip from 10° to 21° to the west. there is no 975 recent paleoseismic investigations in northern and western thailand fig. 16. simplified map of the three pagodas fault zone showing localities mentioned in the text. indication of slope movement or any other gravitational instability, therefore this increase in dip, beyond the angle of repose for alluvial sediments, is most likely tectonic in origin. based on deflected drainages near sangkhlaburi, where assumed late pleistocene channels are laterally displaced by up to 10 m, the slip rate on the tpfz is estimated at about 0.1 mm/yr (woodward-clyde federal services, 1998). across the border in myanmar, the stronger geomorphic expression of the fault and greater deflection of streams indicates that the slip rate may be as high as 2 mm/yr (woodward-clyde federal services, 1998). this is similar to the level of activity observed on other faults in peninsular myanmar (woodward-clyde federal services, 1998). 11. discussion recent investigations have identified a number of faults in northern and western thailand that all display evidence for late cenozoic movement. these late quaternary faults are considered capable of producing large earthquakes. the thoen, pua, mae chan, and three pagodas fault zones show abundant geomorphic and stratigraphic evidence for late quaternary activity. fault scarps on quaternary alluvial and colluvial deposits; vertical displacement of quaternary terrace surfaces; faulting in recent alluvial gravels; well-developed triangular facets along fault escarpments with progressive steepening towards the base of the escarpments; wineglass canyon profiles; and lateral offset of stream courses and ridge crests are all testament to late quaternary movement along these faults. many of these features are also found along the phrae, phrae basin, nam pat, long, and phayao faults (fenton et al., 1997). the predominant displacement on the basinbounding faults of northern thailand, with the exception of the mae chan fault, is normal dipslip while in western thailand the predominant fault movement is strike-slip. the pua fault and the ban mai segment of the thoen fault, however, show evidence for lateral displacement. displacement along the mae chan fault in northernmost thailand is predominantly leftlateral. we believe that this fault lies within a different seismotectonic province, dominated by strike-slip faulting. with the exception of the phrae basin fault, and small fault splays along the thoen fault zone, recent faulting has been confined to relatively narrow zones along range fronts (woodward-clyde federal services, 1996). this is similar to the style of faulting that is observed in the basin and range province of the western united states (depolo et al., 1991). the structural complexity and geomorphic heterogeneity of these basin-bounding faults suggests that, like the normal faults in the basin and range province, they are segmented. the thoen fault is comprised of four segments (fig. 5). the northern (ban mai) segment shows a subordinate left-lateral component of slip while the remainder of the fault shows no apparent lateral offset. the three remaining fault segments are defined on fault geometry (strike and dip of the fault plane) and on the geomorphic expression of the fault (fault scarp heights, scarp angles, etc.); better-defined fault geomorphology indicates a more recently active fault segment. the segmentation of the other faults in northern thailand (e.g., the pua fault) is also based on similar evidence (fenton et al., 1997). although most seismicity appears diffuse (fig. 3), bott et al. (1997) consider that only the 1980 earthquake sequence south of phrae and the 1994 phan earthquake may be associated with mapped faults; the phrae and phayao faults, respectively. the basin and range province in the western united states shows a similar relationship between mapped faults and contemporary seismicity (e.g., smith and arabasz, 1991). only three focal mechanisms have been computed to date for earthquakes in northern thailand (bott et al., 1997). these mechanisms are consistent with the geologic data presented in this paper, showing that this region is undergoing east-west to northwest-southeast extension on northto northeast-striking normal or normal-oblique faults. focal mechanisms from western thailand indicate strike-slip faulting (le dain et al., 1984). the holocene slip rates for the basin-bounding normal faults in northern thailand range between 0.1 and 0.6 mm/yr (table i). the mae 976 clark h. fenton, punya charusiri and spencer h. wood chan strike-slip fault may have a slip rate as high as 3 mm/yr, however this has not been confirmed by paleoseismic trenching (wood, 2001). slip rates were calculated from the offset of the youngest geomorphic features or stratigraphic units. long-term slip rates for these faults, calculated from the cumulative throw across the basin margins (i.e., the sum of sediment thickness within the basin and the height of the range front escarpment along the basin margin), are between 0.02 and 0.07 mm/yr. the average long-term slip rate is, therefore, about an order of magnitude less than the contemporary slip rate. this suggests that the late quaternary has been a period of increased fault activity or renewed activity following a period of quiescence. this is supported by the presence of wineglass canyons and downward-steepening facets along the fault escarpments of prominent range fronts. from these slip rate values and the relatively rare slipper-event data for faults in northern thailand, we estimate that the recurrence for large earthquakes on these faults is of the order of thousands to tens of thousands of years. slip rates for faults in western thailand are less well constrained, but appear to be of the same order of magnitude as those in northern thailand (table i). the slip rate for the three pagodas fault is about an order of magnitude less than the recently identified active strike-slip faults across the border in peninsular myanmar (woodwardclyde federal services, 1998). maximum magnitudes estimates for each fault are based on empirical relations between expected rupture dimensions (wells and coppersmith, 1994). maximum magnitude is commonly estimated through a comparison of fault rupture length and magnitude. however, considerable uncertainty often exists in the selection of the appropriate rupture length to be used in the analysis (schwartz et al., 1984). rupture lengths of past surface-rupture events on a specific fault may provide direct evidence. no such data exists for thailand. in the absence of definitive data to the contrary, we conservatively assume that the recent paleoseismic investigations in northern and western thailand table i. seismic source parameters of active faults in northern and western thailand. fault length (1) age of most recent long-term slip rate slip-perrecurrence maximum (km) movement (2) slip rate (3) (mm/yr) (4) event (5) (6) credible (mm/yr) (m) (kyr) earthquake (7) (m) long 56 late pleistocene 0.04 0.1 – – 7 nam pat 35 late pleistocene 0.04 0.1 – – 7 phayao 28 late pleistocene 0.05 0.1 – – 7 phrae 51 late pleistocene 0.07 0.1 – – 7 phrae basin 59 late pleistocene 0.07 0.1 1.2-1.5 12-15 7 pua 68 holocene 0.06 0.6 – – 71/4 thoen 120 holocene 0.02 0.6 1.0-1.5 1.7-2.5 71/2 mae chan 140 holocene – 0.3-3.0 – – 71/2 three pagodas 350 (8) holocene? – 0.5-2.0 – – 71/2 (1) fault length exhibiting recent faulting. (2) relative age estimated from geomorphic relationships and pedologic development. (3) total vertical displacement across the fault (range height + basin depth) divided by time since basin opening, approximately 33 ma (charusiri, 1989). uncertainties on these slip rate values are at least ± 50%. (4) total offset of youngest geomorphic feature or stratigraphic unit divided by its relative age. uncertainties on these values are at least ± 50%. (5) determined from paleoseismic trench excavations (6) estimated by dividing slip-per-event by slip rate. (7) estimated moment magnitude from empirical relationships among rupture length, rupture area, and magnitude (wells and coppersmith, 1994). uncertainties in magnitude values are ± 1/4. (8) total fault length. the complex geometry of this fault suggests that it is very unlikely that the entire fault ruptures in a single event. maximum magnitude is based on the segmentation model of woodward-clyde federal services (1998). 977 maximum rupture length corresponds to the maximum length of the mapped active fault trace or trace of identified fault segments. maximum magnitude estimates also are computed based upon rupture area, which incorporates the maximum rupture length and maximum rupture width. the dip of the fault and thickness of the seismogenic crust are required to estimate down-dip width of the rupture plane. neither of these properties are well constrained in northern thailand. however, because of the similarities between the tectonic setting and crustal properties of northern thailand and the western united states, we use data from the latter to estimate the dip of active faults and the thickness of the seismogenic layer. doser and smith (1989) determined that the approximate average dip of normal faults in the interior of the western united states is about 60°, with a range from about 40 to 90°. for faults in the northern basin and range province of northern thailand, we used a dip of 60° ± 15° in order to calculate rupture areas. moderate-to-large basin and range province earthquakes occur at or near the base of the seismogenic crust, and nearly all of the events have focal depths of 15 km or less (doser and smith, 1989). many continental areas with youthful mountains appear to be characterized by a seismogenic thickness of 15 ± 5 km, which corresponds with crustal temperatures less than 350 ± 100°c, the upper limit for brittle failure and earthquake generation (chen and molnar, 1983). thus, we assumed 15 ± 5 km as the thickness of the seismogenic crust for northern thailand (bott et al., 1997). the empirical relationships for surface rupture length and rupture area used in this maximum magnitude assessment are those developed by wells and coppersmith (1994). the maximum moment magnitude (m) for each fault is given in table i. 12. conclusions recent geologic studies show late quaternary activity along a number of northto northeast-striking cenozoic basin-bounding faults in the northern basin and range seismotectonic province in northern thailand and along strikeslip faults in the western highlands and mae chan provinces. geomorphic indicators of active faulting, including fault scarps on colluvium/alluvium, faceted spurs, wine-glass canyons, linear range fronts, stream knickpoints, and faulted late quaternary deposits, are found along six major faults in northern thailand. the sense of slip along these faults is predominantly normal dip-slip. the pua and thoen faults, however, have minor components of lateral slip. these geomorphic data, and the evidence from limited paleoseismic excavations, show that the most recent surface rupturing events on these faults are either late pleistocene or holocene. from the slip-perevent along the thoen and phrae basin faults, approximately 1.5 m, and the range of slip rates calculated for these faults, we infer that the return period for surface rupturing events on these faults is on the order of 2500 to 15.000 years (table i). empirical relationships between rupture length and earthquake magnitude indicate that these faults are capable of generating earthquakes as large as m 7. thus, like the tectonically-similar basin and range extensional province of the western united states, northern thailand is characterized by faults with low slip rates, long recurrence intervals, and large magnitude paleoearthquakes. the mae chan fault in northernmost thailand and the three pagodas fault zone in western thailand are both marked by scarps on alluvium, deflected drainages, shutter ridges, and sag ponds. displacement of geomorphic features indicates slip rates between 0.1 mm/yr to 3 mm/yr for these strike-slip faults. based on fault segment lengths, both of these faults are capable of generating large, damaging earthquakes of up to m 71/2. acknowledgements this paper arose from several seismic hazards studies carried out for proposed and existing dam sites in northern and western thailand. the financial and management support of the royal irrigation department (rid), the department of mineral resources (dmr), and 978 clark h. fenton, punya charusiri and spencer h. wood the electricity generating authority of thailand (egat) are gratefully appreciated. chaiyan hinthong, apichard lumjuan, and suwit kosuwan of dmr, and supawan klaipongpan of egat are thanked for facilitating these investigations and for providing stimulating discussions on the tectonics of thailand and southeast asia. c. fenton thanks his many colleagues at urs who participated in various aspects of these investigations. s. wood thanks the faculty of the department of geological sciences of chiang mai university for help and hospitality during his sabbatical leave there. the efforts of ivan 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(1995): late cenozoic faulting in mountainous regions of low but persistant historic seismicity: hells canyon (nw u.s.a.) and northern thailand and the meaning of «active fault», in iugg xxi general assembly, abstracts week b, b344. wood, s.h. (2001): slip-rate estimate from offset streams, valley volumes, and denudation rate: mae chan fault, northern thailand, eos, trans. am. geophys. un., 82, 932. woodward-clyde federal services (1996): seismic hazards evaluation, environmental impact assessment: geological aspect, kaeng sua ten dam project, changwat phrae, unpublished report prepared by gmt consultants and others for department of mineral resources, 4 vols. (english and thai). woodward-clyde federal services (1998): preliminary seismic hazard evaluation of khao laem and srinigarind dams, thailand, unpublished report prepared for electricity generating authority of thailand, bangkruay, nontaburi 11000. yunnan seismological bureau (1997): report on three pagoda fault zone from downstream of khao laem dam to sai yok waterfall: seismological bureau of yunnan province, china, unpublished report prepared for electricity generating authority of thailand, pp. 29. 981 recent paleoseismic investigations in northern and western thailand .ajjglkjshfljkshfg gli « annali di geofisica » a chi conosco le grilli, spesso insormontabili difficoltà in cui si dibattono attualmente le attività ili ricerca e la stampa scientifica in particolare, parrà forse alto (li eccessiva audacia quello di ilare oggi l'ila ad un nuovo periodico scientifico. l'istituto nazionale di geofisica ha tuttavia intrapreso la pubblirazione di questa nuova rivista destinata ad accogliere scritti di geofìsica. sia per aderiipiere ad uno dei propri compiti fondamentali. sia per andare incontro alle esigenze di molti cultori italiani della geolisica che giustamente desiderano veder riuniti, per (pianto è possibile, in una rivista specializzata i lavori inerenti a questo importante ramo della scienza; lavori che attualmente si pubblicano in riviste diverse, dedicate a disci/ìline affini e pertanto già impegnate in programmi redazionali nei quali gli studi gcofisici non possono inserirsi con facilità. la dispersione della produzione scientifica, oltre a costringere lo studioso che voglia mantenersi n giorno dei progressi compiuti in italia dalla geofisica, atl uria complessa ricerca bibliografica, impedisce sjiesso che, sia in italia che all'estero, si abbia conoscenza di una notevole parte dei nostri lavori in tale campo. ciò è tanto più dannoso al progresso della scienza, in quanto il lavoro scientifico raggiunge completamente il suo scopo solo quando se tic di [fondano ampiamente i risultati in modo da offrirli alla libera critica ed alla considerazione dei ricercatori. le alte finalità della scienza non jiossono essere conseguite compiutamente e rapidamente se non con l ausilio di una esiliente stampa periodica che consenta, in tempo rclativamnte breve, di dare notizia dei lavori compiuti. a questi concetti si è ispirato l'istituto nazionale di geofisica nel prendere l'iniziativa di pubblicare questo periodico, il quale, per il fallo di essere l'organo dell' istituto, si può considerare — nel campo della geofisica — l'unica rivista in italia clic promani da un corpo scienti fico speci ali zzalo. 1 g l i « a n n a l i i l i g l o i ' i s i c a » sino uri ora i utiori compitili dall'istituto (lavori dei quali in questo stesso fascicolo è riportalo l'ciclico) sono apparsi, nella loro grande maggioranza, sulla rivista del c.\.r. "ricerca scientifica e ricostruzione". a questo proposito, la redazione di ' annali ili gcofisica" desidera, (piale suo primo ulto, rivolgere un caldo ringraziamento al consiglio nazionale delle ricerche, il quale, attraverso la propria rivista ed ora anche mediante l'erogazione di notevoli contributi ai principali periodici, ha svolto e svolge operu altamente meritoria a vantaggio (iella diffusione ilei pensiero scientifico. gli annali di geo fisica , clic sono aperti alla collaborazione di filili gli studiosi senza eccezione, ospiteranno lavori relativi a tutti vari rami della geofisica, ed in particolare a quelli corrispondenti ai reparti in cui. è suddivisa l'attività dell istillilo: sismologia, elettricità atmosferica, radiazioni e fenomeni ottici atmosferici, ionosfera, magnetismo terrestre, radioattività della lena, prospczione del sottosuolo e vulcanologia. essi pubblicheranno inoltre lavori inerenti a ricerche di meteorologia ed in genere a ricerche su tpiei fenomeni naturali il cui studio /tossa interessare, la geofisica. neil"intraprendere la propria fatica la redazione degli "annali", desiderosa soltanto di servire gli interessi della scienza, esprime il volo che i cultori di geofisica vogliano incoraggiare la sua opera con la loro collaborazione, seguirla con il loro consenso, correggerne le mende con la loro vigile critica. u n m e t o d o p e r l a m i s u r a a s s o l u t a d e l l ' a c c e l e r a z i o n e d i g r a v i t à : i l r o t o g r a v i m e t r o ( * ) enrico medi p e r e s e g u i r e una m i s u r a assoluta di gravità si possono, a l m e n o in t e o r i a , seguire diverse vie s p e r i m e n t a l i . una espressione m a t e m a t i c a c h e c o n t e n g a la g, p e r m e t t e di r i c a v a r e questo dato q u a n d o si possano d e t e r m i n a r e s p e r i m e n t a l m e n t e le a l t r e grandezze fisiche. cosi, p e r e s e m p i o , la v e l o c i t à finale di un c o r p o , c h e c a d e l i b e r a m e n t e nel v u o t o , è u n a f u n z i o n e della gravità e d e l l ' a l t e z z a di c a d u t a ; la f o r m a della t r a i e t t o r i a p e r c o r s a da un m o b i l e n e l l o spazio, senza a t t r i t o con l ' a r i a , è u n a f u n z i o n e della g r a v i t à ; le o s c i l l a z i o n i di un p e n d o l o b a n n o un p e r i o d o l e g a t o alla g da una espressione ben nota. questo u l t i m o t i p o di e s p e r i e n z e è q u e l l o p i ù c o m u n e m e n t e usato per la d e t e r m i n a z i o n e assoluta di g. s o n o noti gli a r t i f i c i n e c e s s a r i affinché la m i s u r a sia c o r r e t t a da err o r i . p e r e l i m i n a r e tali e r r o r i bisogna t e n e r e conto della e l a s t i c i t à dei m a t e r i a l i , degli effetti di t e m p e r a t u r a , delle p e r t u r b a z i o n i dovute agli a t t r i t i e c c . p e r o t t e n e r e u n a m i s u r a a d e g u a t a m e n t e precisa è n e c e sario d e t e r m i n a r e le l u n g h e z z e e i p e r i o d i in m o d o da r i c a v a r e alm e n o la q u i n t a c i f r a significativa esente da e r r o r i , il c h e c o m p o r t a il dover r i p e t e r e u n n u m e r o sufficiente di volte le m i s u r e . in partic o l a r e , p e r c o n o s c e r e con la dovuta esattezza il p e r i o d o , è necessario f a r c o m p i e r e al p e n d o l o un grande n u m e r o di o s c i l l a z i o n i . d a t e t a l i difficoltà si è pensato di r i c o r r e r e ad un artificio sperim e n t a l e diverso da q u e l l i c o m u n e m e n t e usati, atto a f o r n i r e il v a l o r e della gravità p a r t e n d o d a l l a conoscenza di un p e r i o d o e di una distanza. d e t e r m i n a b i l i con m a g g i o r e f a c i l i t à e p r e c i s i o n e ed e v i t a n d o m o l t e cause di e r r o r e , s o p r a t t u t t o q u e l l e p r o v o c a t e dagli effetti della t e m p e r a t u r a . i l m e t o d o consiste nel f a r r u o t a r e un l i q u i d o i n t o r n o ad un asse p a r a l l e l o alla d i r e z i o n e della gravità. se il l i q u i d o è p e r f e t t o , la sua s u p e r f i c i e l i b e r a diventa u n a superficie e q u i p o t e n z i a l e : la r i s u l t a n t e della a c c e l e r a z i o n e c e n t r i f u g a e della gravità è in ogni p u n t o della 1*1 ii m e t o d o è stato e s p o s t o d a l l ' a . a l l ' a s s o e i a t i o n i n t e r n a t i o n a l e de g e o d é s i e n e l l a x a s s e m b l e a g e n e r a l e d e l l ' u . g . g . i . r o m a , 1 9 5 4 . •188 e n r i c o m e d i superficie, n o r m a l e alla superficie stessa. essendo l ' a c c e l e r a z i o n e centrifuga proporzionale alla distanza dall'asse, la superficie è un paraboloide di rivoluzione avente come asse di s i m m e t r i a l'asse di rotazione. l ' e q u a z i o n e del p a r a b o l o i d e è la seguente, essendo il polo all'origine degli assi, come si vede c h i a r a m e n t e essa è funzione di un rapporto fra accelerazioni e non fra le forze, quindi gli eil'etti della temperatura sono nulli. qualunque sia la densità del liquido, purché perfetto, la l'orma del paraboloide e le sue c a r a t t e r i s t i c h e sono identiche. con semplici passaggi si trova la distanza locale del p a r a b o l o i d e e quindi il valore della accelerazione di gravità l'espressione ricorda quella del pendolo, pur non avendo nulla in c o m u n e dal punto di vista della impostazione del metodo. p e r avere g, è sufficiente d e t e r m i n a r e il periodo di rotazione e la distanza focale. l a p r i m a misura è facilitata dal fatto di essere in presenza di un moto rotatorio a velocità angolare costante. ciò p e r m e t t e di usare un sistema a moltiplicazione di frequenza, e di c o n f r o n t a r e la frequenza così ottenuta con un oscillatore c a m p i o n e . p e r la determinazione della frequenza di rotazione si possono usare diversi m e t o d i : la scelta del metodo più adatto è suggerita dai risultati delle esperienze in corso. è necessario innanzi tutto disporre di un misuratore di tempi, atto a dare la precisione al decimillesimo di secondo. l o strumento adatto a ciò è l'orologio a quarzo, o un oscillatore tarato su quello. nella rotazione il disco sostenente il liquido, mediante 1111 sistema di accoppiamenti a rulli a frizione, fa r u o t a r " un asse con una frequenza avente l'ordine di grandezza di alcune centinaia di rotazioni al secondo. i l rapporto fra la frequenza di rotazione del disco e quella dell ' u l t i m o asse è f a c i l m e n t e d e t e r m i n a b i l e innestando su questo un contagiri. l ' u l t i m o asse porta una bobina di dieci spire sfasate fra loro or x u n m e t o d o p e r l a m i s u r a a s s o l u t a d e i . l * a c c e l e r a z i o n e i i i g r a v i t a 4 8 9 di trentasei g r a d i ; ruotando si genera in un indotto fisso una corr e n t e sinusoidale avente una frequenza di alcune migliaia di hertz. questa modulazione è amplificata e confrontata con la frequenza campione mediante sistemi ben noti. un altro artificio consiste nell'eseguire fotografie del conta giri e di un cronografo campione, mediante illuminazioni di brevissima durata. a p p a r e ben c h i a r o come con alcune centinaia di rotazioni è possibile ottenere una precisione più c b e soddisfacente nella determinazione del periodo. b a s t a n o alcune diecine di minuti, invece delle m o l t e ore di sperimentazione necessarie col sistema pendolare, per fare la misura. p e r la determinazione della distanza focale si può usare uno dei vari metodi suggeriti dall'ottica per la misura delle distanze focali dei sistemi ottici. si ricordi che, nel caso del paraboloide di rotazione, per il fuoco non esiste aberrazione di sfericità e astigmatismo. il fuoco è rigorosamente stigmatico per un punto all'infinito. t u t t i i raggi paralleli all'asse, dopo la riflessione, passano per il fuoco e viceversa. la individuazione della posizione del fuoco può essere eseguita con il seguente dispositivo. una sorgente luminosa puntiforme posta nel fuoco dà origine dopo la riflessione nella superficie del paraboloide ad un fascio di raggi paralleli che incontrano la superficie di uno specc h i o piano posto n o r m a l m e n t e all'asse. i raggi riflessi da questo specchio sono ancora paralleli e danno luogo, dopo l ' u l t i m a riflessione sulla superficie del paraboloide, ad una immagine reale, clic coincide esattamente con la sorgente. un microscopio, dotato di m i n i m a profondità di fuoco, assicura la coincidenza della sorgente con la sua immagine r e a l e : quando si realizza questa condizione, è individuata la posizione del fuoco e si è certi che lo specchio piano è normale all'asse. s t a b i l i t a la posizione del fuoco, si d e t e r m i n a , con il medesimo microscopio, la posizione del vertice della parabola, unico punto fisso nella rotazione. l a distanza fra le due posizioni è la distanza focale cercata. p e r ottenere una misura ancora più precisa e indipendente da campioni di misura, si fa funzionare il paraboloide ruotante come uno specchio di un i n t e r f e r o m e t r o di miclielson e quindi si può determinare, in lunghezze d'onda di una radiazione nota, la distanza focale, come differenza fra due c a m m i n i ottici. compensando tale differenza si riottengono le frange di ordine zero o centrali. 4 9 0 e n r i c o m e d i quanto è qui esposto parte dall'ipotesi c h e si t r a t t i di un liquido p e r f e t t o , nel quale cioè siano da t r a s c u r a r e le forze di interazione molecolare. i n realtà ogni liquido h a una certa tensione superficiale che p e r t u r b a la forma della superficie del paraboloide. si usano perciò nella esperienza liquidi di bassissima tensione di vapore e conseguentemente di piccola tensione superficiale. r e a l i z z a n d o queste condizioni la p e r t u r b a z i o n e è trascurabile. i n ogni modo, in una nota immediatamente seguente questa, è p u b b l i c a t o il calcolo rigoroso della perturbazione. un risultato f o n d a m e n t a l e del calcolo è che bastano due determinazioni sperimentali per conoscere la p e r t u r b a z i o n e e q u i n d i correggere l ' e r r o r e da essa apportato. date le sue c a r a t t e r i s t i c h e al dispositivo è dato il nome di « rotogravimetro ». i n un prossimo lavoro saranno p u b b l i c a t i i p a r t i c o l a r i s p e r i m e n t a l i e i p r i m i risultati delle misure. rortm — istituto nazionale di geofisica — settembre 1954 riassunto è esposto un metodo per la misura assoluta della gravità, basato sulla forma assunta dalla superficie libera di un liquido posto in rotazione, attorno ad un asse parallelo alla direzione dell' accelerazione di gravità nel luogo di osservazione. misurando la distanza focale del paraboloide di rivoluzione che così si ottiene, e il periodo di rotazione del sistema, è possibili' ricavare il valore di g. il metodo fa sì che gli effetti dovuti alla temperatura non influiscano sulla misura e che siano eliminate altre cause di errore, presenti in altri metodi. su m mary a method is discussed jor an absolute measurement of the constant of gravity. this method is based on the forni taken by a liquid set in rotation around an axis parallel to the direction of the acceleration of gravity at the place of observation. by measuring the foca! distance of the revolution paraboloid tluis obtained, and the period of rotation of the system, il is possible to obtain a value for g. tliere are no temperature effects with this method measurement, and further sources of errors, normally present in other methods, are eliminated. ag_56.06.13_finocchio_final:layout 6 annals of geophysics, 56, 6, 2013, s0673; doi:10.4401/ag-5806 s0673 interpreting the interseismic deformation of the altotiberina fault (central italy) through 2d modelling debora finocchio1,2,*, salvatore barba2, stefano santini1, antonella megna2 1 università degli studi di urbino “carlo bo”, dipartimento di scienze di base e fondamenti, urbino, italy 2 istituto nazionale di geofisica e vulcanologia, sezione roma 1, rome, italy abstract the altotiberina low-angle normal fault in central italy has been a focus of many recent studies. although the existence of this fault has long been known, its seismicity and relationship to other faults are still debated. we present a 2d elastoplastic finite-element model that reproduces the interseismic deformation of the altotiberina fault. the model predictions are compared to observed geodetic velocities, stress orientations and geological data. the influence of the altotiberina fault on interseismic evolution is tested by building several models with different boundary conditions. the best model is 180 km long, 40 km deep and contains two layers with different rheological parameters, two ramps, two faults and four freely slipping segments. the main factors contributing to the largescale interseismic deformation include basal traction, rheology and the altotiberina fault itself, whereas the local, small-scale variations are due to two secondary high-angle faults. 1. introduction the interseismic phase is the period in the seismic cycle following post-seismic relaxation and preceding the next large earthquake. during the interseismic phase, it is useful to identify locked faults because they have the potential to be seismogenic [e.g., chamotrooke and le pichona 1999, simoes et al. 2004]. the locked portion of a fault can accumulate stress, which is later released when the fault slips during an earthquake [e.g., subarya et al. 2006, konca et al. 2008, moreno et al. 2010]. several studies have described the interseismic phase, mainly by using geodetic or geophysical data [e.g., mazzotti et al. 2000, simoes et al. 2004, brooks et al. 2011], which have different time scales. geodetic data record deformation at time scales that are much shorter than the intervals between large earthquakes [e.g., wei et al. 2010]. the stress field derived using borehole data and the strain field derived using earthquakes describe present-day tectonic activity [e.g., zoback 1992]. geological observables such as vertical velocities from uplift and subsidence rates [e.g., cook and royden 2008, huang et al. 2010] record several seismic cycles and help constrain tectonics over longer periods of time. interseismic deformation is often studied through analytical and numerical methods. faults are embedded into an elastic medium while allowing the deeper portion of the fault to slip aseismically [savage 1983, ruegg et al. 2009]. alternatively, the fault can be embedded into a crustal structure that is modelled as elastic upper crust overlying viscous lower crust or elastic crust overlying the upper mantle [e.g., vergne et al. 2001, correa-mora et al. 2008, huang et al. 2010, doglioni et al. 2011]. typically, these models are used for convergent or transform plate boundaries [e.g., marshall et al. 2009, vigny et al. 2009], where the alongstrike length and the depth extent of the fault are both very large. these models are able to reproduce the interseismic deformation for large thrust and strike-slip faults, but they are not able to reproduce deformation induced by low-angle normal faults (lanfs). indeed, lanfs activity is not predicted by traditional anderson-byerlee fault mechanics. many authors have explained the existence of the lanfs by using mechanical theories with deformation localised at the interface of elastic and viscous layers [e.g., melosh 1990] or by using an additional horizontal force [yin 1989, westaway 1999]. among lanfs, the altotiberina fault (atf) is an example of a basal detachment in the extensional area of central italy as shown in the crop03 deep-crust reflection profile [barchi et al. 1998]. the existence of the atf is well documented in geological [e.g., brozzetti 1995, brozzetti et al. 2000] and geophysical data [e.g., mirabella et al. 2011, and references therein]. the atf article history received may 28, 2012; accepted december 19, 2012. subject classification: low-angle normal faults, earthquakes, numerical model, rheology. special issue: earthquake geology borders the west side of the tiber basin with a nnwsse strike for approximately 70 km (figure 1) and dips 10°-30° eastward to a depth of approximately 12 km [barchi et al. 1998], which suggests a downdip extent of 35 km or more. the displacement along the atf ranges from 5 to 8 km [mirabella et al. 2011] and has developed since 3 ma (late pliocene) [brozzetti 1995]. the tiber basin is a 6-km-wide tectonic depression filled by plio-pleistocene deposits and holocene lacustrine and fluvial deposits [boncio et al. 2000]. at the surface, the atf produces a number of low-angle (30°) synthetic splays [boncio and lavecchia 2000a, brozzetti et al. 2009]. a series of antithetic high-angle (30°-60°) normal faults (hanfs), such as the colfiorito fault (cff), the gubbio fault (gf), and the umbria valley faults (vufs), are located in the hanging-wall block of the atf (figure 2). these faults are related to shear stresses at the base of the brittle upper crust beneath the apennines, which is caused by eastward mantle flow [doglioni et al. 1998]. as the atf belongs to the large etrurian fault system, which extends approximately 350 km, lanfs may be widespread in central italy [boncio et al. 2000]. earthquakes of moderate magnitude, such as the norcia 1979 (m=5.9), gubbio 1984 (m=5.2), and colfiorito 1997 (m=5.9) earthquakes, have occurred within the hanging-wall of the atf [barba and basili 2000, boncio and lavecchia 2000b, boncio et al. 2000]. the present-day activity of the atf is characterised by small-magnitude earthquakes [boncio et al. 1998, carminati et al. 2001, chiaraluce et al. 2007], mostly in its southern section [piccinini et al. 2003]. based on the lack of microseismicity in the 20-km-long northern section of the atf, piccinini et al. [2003] suggest the existence of a locked portion along the atf, which may be considered a seismic gap and thus may be hazardous. other authors have assumed an andersonian state of stress and model the atf as creeping [e.g., collettini and barchi 2002] and nearly aseismic. the choice between these different interpretations is debated and is critical as it affects the hazard estimates of the area. the atf has been modelled for various purposes. using a two-dimensional viscoelastic model of apennine subduction along the crop03 seismic profile, carminati et al. [2001] modelled the atf as a structural and rheological discontinuity embedded within viscoelastic and elastoplastic stratifications. they computed the stress directions and intensities surrounding the atf and the dynamic topography along the modelled section. by comparing the model predictions with the dip of the t axes of earthquakes and the earthquake distribution at depth, carminati et al. [2001] proposed that the atf re-orients the v3 stresses at depth, increasing their dip and becoming nearly parfinocchio et al. 2 figure 1. a map showing gps data (arrows), wsm data (grey bars), the tiber basin (grey area), and principal faults: altotiberina fault (atf), gubbio fault (gf), mt. san vicino fault (svf), monte vettore fault (mvf), colfiorito fault (cff), and norcia fault (nf). labelled dashed lines are geological sections from the literature (a: brozzetti et al. [2009], b: boncio et al. [2004], c: doglioni et al. [1998], see figure 2). trace a-a’ is the section used in our numerical model. 3 allel to the atf. these results provide a locking mechanism for the atf and suggest that the local stress field is non-andersonian. pauselli and federico [2003] developed a two-dimensional elastic stratified finite-element model in which the faults are modelled as areas of decreased young’s modulus. this model allows the stress directions to be computed along the crop03 seismic profile for different stratifications and assuming different hypotheses about the gubbio fault. pauselli and federico [2003] found that the inclusion of the gubbio fault along with the atf in the model is essential to reproducing the spatial pattern of seismicity at depth and the moderate-magnitude earthquakes. hreinsdòttir and bennett [2009] modelled the atf as a variable-dip fault in an elastic half space. by using a constrained random search algorithm, they determined the locking depth of the atf and the slip rate that best reproduce the vertical and fault-perpendicular components of the horizontal velocities of continuous gps sites in the region. in this paper, we assumed that shear stresses at the base of the plastic lower crust are the cause of fault motion, following westaway [1999]. the aim is to demonstrate how the shear basal tractions contribute to large-scale interseismic deformation. we applied this model to the atf and associated hanfs, and we implemented the main characteristics highlighted in previous studies. we developed an elastoplastic finiteelement model where the atf is embedded as a discontinuity as proposed by carminati et al. [2001], and introduced antithetic normal faults, as implied by pauselli and federico [2003]. we compared the model predictions with the interseismic fault-perpendicular component of the horizontal velocities of continuous gps sites in the region, similarly to hreinsdòttir and bennett [2009], and with stress directions and geological data. we adopted two layers with elastoplastic rheology and a realistic subsurface geometry. given the time scale of interest (a few years), we neglected the poroelastic effects and focused on crustal heterogeneities. this allowed us to determine the locked (or unlocked) state of the faults. in this paper, we present different models, and we discuss the geological implications of the best fit. altotiberina fault 2d interseismic model figure 2. three geological sections of the altotiberina fault (atf) and other high-angle normal faults, modified from (a) brozzetti et al. [2009], (b) boncio et al. [2004], (c) doglioni et al. [1998]. gf: gubbio fault; mvf: monte vettore fault; c-nfs: colfiorito-norcia faults; bdt: brittle-ductile transition; mf: mt. martani fault: vufs: umbria valley faults; s: base of the sedimentary layer; m: moho; and rm: regional monocline. 2. data and model we aimed to reproduce the interseismic data through finite-element numerical modelling. starting from an initial model, we varied the rheological parameters, fault parameters and boundary conditions, and we chose the combination that best reproduced the geodetic velocities, stress orientations and surface geology. in all models, an unlocked fault is represented by a freely slipping segment. 2.1. data a dataset of 182 continuous gps velocities was analysed by caporali et al. [2011]. forty-seven velocity data points are within the study area and are expressed with respect to the eurasia-fixed reference system defined by devoti et al. [2008]. the horizontal gps velocity data were projected along model section a-a’, shown in figure 1, and the section-parallel component of motion (hereinafter referred to as vh) was considered. by selecting subsets of gps data located at different distances from the section (20, 40, 50, 70, and 150 km), we verified that the gps velocities are largely homogeneous. thus, we selected all data within 150 km of the section (figure 1). the orientations of the maximum horizontal stress (shmax), available from the world stress map 2008 (wsm) [heidbach et al. 2010], were projected along model section a-a’, and data were plotted at the same ranges used for the gps velocities (20, 40, 50, 70, and 150 km). we used data within 70 km of the section, which includes 150 shmax values (figure 1). we filtered both the stress azimuth and the sectionparallel components of the horizontal gps velocities using spatial gaussian smoothing, i.e., by convolution with a gaussian function of a certain width. we used a 50-km width for both the gps and wsm data (figure 3). the vh values increase to the ne, and the shmax data indicate extension corresponding to the tiber basin. the interseismic model should predict the location of the subsiding area of the tiber basin (figure 1). thus, we used the data of mirabella et al. [2011] to constrain the direction of vertical displacement in the model. 2.2. geological section and model geometry from the geological sections shown in figure 2, we built the model based on a simplified configuration of the principal faults and stratigraphy. we divided the model into a lower layer (ll), which includes most of the acoustic basement, and an upper layer (ul) representing the triassic-miocene stratigraphic sequence of the umbria-marche. the interface between these two layers represents the elastoplastic transition (ept). the ept is shallower than the brittle-ductile transition below the atf and corresponds approximately to the brittleductile transition elsewhere. this geometry allows the atf to be within the brittle (elastic-frictional) layer. its depth increases to 15 km to the east, in agreement with brozzetti et al. [2009] and carafa and barba [2011]. the atf is modelled by a low-angle (20° east-dipping) fault along the ept that is either locked or unlocked. the locked case corresponds to an interface with elastic-frictional properties (normal friction; e.g., µ = 0.6-0.8). the unlocked case corresponds to an interface with free-slip properties to represent low-friction aseismic slip. following doglioni et al. [1998], we finocchio et al. 4 figure 3. (a) section-parallel gps velocity data (vh) projected along model section a-a’. black and grey circles represent the data located within 40 km and 150 km of the section, respectively. the black and grey lines represent the result of gaussian smoothing (50 km width) applied to the black and grey circles. (b) the azimuths of the wsm data (shmax) projected along model section a-a’. black circles and grey circles show the data within 20 km and 70 km of the section, respectively. the black and grey lines represent filtered versions of the datasets. 5 included the regional monocline (rm) between the apennines and the adriatic sea (figure 2c). we modelled two of the principal hanfs (represented as faults a and b in figure 4) and a series of tentative faults that vary in dip and are considered freely slipping segments. the hanfs cut through the ul and do not offset the ll. 2.3. numerical model we developed a 2d finite-element model using msc.marc 2010 [msc.software corporation 2010]. the displacement was computed assuming elastoplasticity and plane strain. the model was run for the 11 years, corresponding to gps time span. our model is 180 km long and 40 km deep. the two layers have elastoplastic rheology, and the yield stress in the ul is greater than in the ll (table 1). this choice lets the ul reach the plastic limit at localised points along the slipping segments and lets the ll be close to the plastic limit to exhibit a small amount of diffuse plastic strain. the grid includes four-node quadrangular elements. the number of elements ranges from 7000 to 8000 (the area of each element is ~1 km2), and there are 7500 to 8500 nodes, depending on the faults considered. the faults are modelled through duplicate nodes on both sides of the interface [melosh and raefsky 1981]. in the unlocked case, the fault nodes allow relative movement with zero friction between the footwall and hanging-wall. reaction forces prevent the interpenetration of the footwall and hanging-wall (see e.g., megna et al. [2005]) except for a negligible amount that is required for numerical reasons. the boundary conditions for the right and left sides are containment edges, i.e., the material cannot leave the model, but it is free to move inside it. the nodes along the bottom of the model are locked vertically and freely slip horizontally. a shear basal traction is applied to the bottom of the model with values from 1 to 10 mpa. these tractions generate flow in the plastic lower continental crust, which is transmitted to the upper crust. the model is subject to gravity (9.8 m/s2). the poisson’s ratio (h) is 0.3, and the density (t) is 2300 kg/m3 for both layers. the yield stress (y) ranges from 10 to 200 mpa, and the young’s modulus (e) ranges from 1 to 50 gpa. altotiberina fault 2d interseismic model tested values best model layer z (km) e (gpa) h t (kg/m3) y (mpa) l (km) sh (mpa) z (km) y (mpa) l (km) e (gpa) sh (mpa) ul l:5-10 r:16-21 1-50 0.3 2300 30-200 100-200 1-10 l:5 r:16 50 180 10 4.3 ll l:24-29 r:35-40 1-10 10-40 l:24 r:35 20 9 figure 4 (right). the model geometry showing the tested faults (dashed lines) and the principal faults (a and b). ul: upper layer; ll: lower layer; atf: altotiberina fault; ept: elastoplastic transition; and rm: regional monocline. table 1. the parameters used for the lower layer (ll) and the upper layer (ul) in the tests and the best model. z: the approximate depth range of the layer; e: young’s modulus; h: poisson’s ratio; t: density; y: yield stress; l: model length; l: left side; r: right side; and sh: shear basal traction. figure 5. four tests: models a, b, c, and d. model a contains two plane-parallel layers, model b includes the atf, model c includes fault a dipping to the ne and model d includes fault a dipping to the sw (see figure 4 for acronym definitions and the text for descriptions of the models). we developed a series of models by varying the rheological parameters, geometries, and faults to test the effects of each parameter on the surface velocities and vertical displacements. we varied y from 30 to 200 mpa in the ul and 10 to 40 mpa in the ll. the values of e ranged from 1 to 50 gpa for the ul and from 1 to 10 gpa for the ll. we then varied the dips of the hanfs from 30° to 60° (figure 4), imposing different widths of free slip along each fault. we also tried different values of the model length (100 to 200 km), the model thickness (20 to 40 km) and the distance of the atf ramp from the left edge of the model (10 to 60 km, see table 1). for simplicity, we show geometries (figure 5) and results (figure 6) of four end-member models. in model a, we tested a simple crustal structure incorporating two plane-parallel layers (the ul and ll), the containment edge boundary condition, and no faults. in model b, we introduced a low-angle ramp along the ept at the left edge of the model. we built models c and d to verify the effect of the opposing fault dip (ne vs. sw). these models include a free-slip portion along the ept and rm. we calculated the root mean square (rms) for each model. the rms is an indicator of the difference between the filtered gps data and the model predictions. the lowest rms indicates the best-fitting model. we obtained the horizontal velocities at the surface from the horizontal displacements versus the time factor at the top edge of the models and compared these curves with filtered vh and shmax data. the time factor corresponds to the duration of the gps data (11 years). the vertical displacement was calculated at the surface of the model and was compared with the position of the area of subsidence. thus, the best-fitting model was chosen, and the model curve that is closest to the data represents the best model. to better evaluate the significance of the model results, we computed the vertical and horizontal displacement fields in the model section due only to the boundary conditions, in the pre-stressed model, and for the total stress and plastic strain, which includes the gravity load. 3. results by varying the model parameters (table 1), we ran approximately 200 models and chose the one that best fit the data. we focused on the locking state of the faults in the best model, together with the rheology in the interseismic period. of all the tests, we show only the most relevant outcomes and describe the contributions to the surface velocities, the vertical displacements of the ept, the altotiberina fault (atf), detachments, unlocked faults, and the rheological parameters. model a shows that the horizontal velocities increase in the sw half of the section (figure 6); this is due to the presence of the ept (the interface of the two layers), together with the shear basal traction (4.3 mpa). we observe a wide area of slight downward displacement at the left edge of the model (from 0 to 60 km; figure 6b). in model b, the presence of the atf controls the area of subsidence and the horizontal velocities (figure 6a,b). we observe a step in the horizontal velocity and subsidence, in the left portion of the section. in model c, the dip of the faults was reversed (from sw to ne), causing a reversal in the patterns of local subsidence and uplift (figure 6b) that does not produce observable variations in the horizontal velocities (figure 6a). the lowest rms (0.1 mm/yr) was obtained for model d, which is deemed the best model. model d shows that the free slip on the two highangle faults (faults a and b) produces steps in the horizontal velocities near each fault (at 20 km and 60 km from the origin) (figure 6a). in the other tests, the rheological parameters control the surface velocities and vertical displacements. by changing the yield stresses finocchio et al. 6 figure 6. (a) the horizontal velocities and (b) vertical displacement for models a, b, c, and d. (c) the yield stresses for the best model (model d). the yield stresses for the tests are as follows: for test y1, ul: y=30 mpa and ll: y=10 mpa; for test y2, ul: y=50 mpa and ll: y=20 mpa; and for test y3, ul: y=100 mpa and ll: y=25 mpa. the red line represents the filtered gps data from within 150 km from the model section (see grey line in figure 3). the rms values represent the mismatch between the model horizontal velocities at the surface for models a-d and the red line. 7 for the upper and lower layers, we obtained an increase or a decrease in velocity (figure 6c). in particular, the velocities increase if the ll (i.e., y = 10 mpa) is more plastic than the ul (i.e., y = 30 mpa). in model d, free slip is also predicted along the ept at 90 km from the origin, where the ept is horizontal. this result represents the relative displacement between the upper and lower layers along the brittle-to-ductile transition, where it coincides with the ept. the free slip found on the rm suggests a mechanism to facilitate thrust propagation with a low seismic coupling factor. the predictions of the best-fitting model closely agree with the data. in the first 80 km, both the modelled horizontal velocities and the filtered vh increase to 2.5 mm/yr, indicating that the model is under extension (figure 6a). the velocities decrease at greater horizontal distances, suggesting an area of compression, also in agreement with the wsm stress data (figure 7a,b). the modelled vertical displacements show subsidence from 0 to 60 km (figures 7 and 9a), with the largest amount of subsidence in the tiber basin. the high-angle faults (unlocked faults a and b) reproduce the irregularities in the filtered vh located at 20 km and 60 km from the origin (figure 7b). at these two points, the filtered vh is steeper, increasing by 0.5 mm/yr within 10 km. furthermore, the vertical displacements of faults a and b increase the subsidence of each fault by 25 m (figure 7c). in summary, the best model is characterised by a yield stress of 50 mpa for the ul and 20 mpa for the ll, a young’s modulus of 9 gpa for the ul and 10 gpa for the ll, a fully locked atf, and a 16-km-wide freely slipping fault for the rm. along the ept, a 21-km-wide freely slipping segment is located 45 km from the origin. the best model includes two high-angle faults in the ul: an unlocked 9-km-wide freely slipping fault that dips 50° se (fault a) and an unlocked 20-km-wide fault that dips 40° se (fault b) (figure 8). the position of fault a corresponds to the umbria valley faults (vufs) near the surface trace of the atf (shown in figure 2b), and fault b corresponds to the colfiorito fault (cff). the geometric and elastic parameters for the best model are listed in table 1. the vertical and horizontal displacements, the stress, and the plastic strain (figure 9) suggest that the faults and the basal traction influence the model results. the vertical displacements (figure 9a) highlight local subsidence in agreement with the position of the tiber and colfiorito basins (0 to 60 km from the left edge of altotiberina fault 2d interseismic model figure 7. the results for the best model. (a) the azimuths of the wsm (shmax) filtered data are in grey for the extensional area and black for the compressional area. the dashed line represents the results for the best model. (b) the horizontal velocities (vh) for the filtered data and the best model. (c) the vertical displacements and corresponding areas of subsidence in the tiber basin, shown within the square. figure 8. the geometry and grid for the model with the best-fitting boundary conditions, with the free slip segment (grey rectangle). the circles represent nodes that are locked in the vertical direction, horizontal and dipping bars represent the containment bars, and arrows indicate the direction of the shear basal traction (4.3 mpa). ul: upper layer; ll: lower layer; atf: altotiberina fault; rm: regional monocline; gf: gubbio fault; cff: colfiorito fault; and vufs: umbria valley faults. the model). the horizontal displacements (figure 9b) increase at depth, corresponding to the lower tip of the atf. the stress (figure 9c) primarily accumulates in the ul and the plastic strain (figure 9d) increases with depth, with perturbations induced by the slip along the unlocked segments. in the ul and the upper part of the ll, the model mainly behaves elastically, with a neglectable component of plastic strain. 4. discussion and conclusions the shear basal traction appears to be an essential factor to modelling the large-scale interseismic pattern in the presence of the low angle fault. this traction allows the reproduction of extension and compression rates for central italy as also shown by barba et al. [2008] in a larger-scale model. the lack of a clear discontinuity in the fault-perpendicular horizontal velocities indicates that the atf is at least partly locked. on the other hand, the high-angle faults induce local variations in the vertical velocities. the locked state of the atf and the prior knowledge that it is located within a brittle layer indicate that the atf is possible seismogenic. in the sense of scaling laws, the locked width corresponds to earthquakes up to magnitude 7. the main limit of our method is that it is two-dimensional. the rotations of the horizontal velocity vectors and the stress axes that occur far from the modelled sections may be due to local features in the sub-crustal structure or to the presence of minor or unfavourably oriented faults. the two-dimensional geometry also does not allow the discrimination between parallel faults of similar size; in our case, the gf is parallel to the cff, and in practice, the resulting unlocking width of the cff may also depend on the gf. additionally, modelling a simplified stratigraphy induces long-wavelength patterns, especially in the predicted vertical displacements. in our case, the predicted subsidence is wider than the width of the tiber basin. consequently, our model may underestimate the locked width of the vufs. in regards to the relationship between the locked width and the potential for earthquakes, we make note of two points: the first is related to historical earthquakes, and the second is related to the seismogenic character of a locked fault. chiaraluce et al. [2007] noted that if the atf is seismogenic, historical seismic catalogues should report large-magnitude earthquakes finocchio et al. 8 figure 9. the results for the best model (model d). (a) vertical displacement, (b) horizontal displacement, (c) stress, (d) plastic strain (see text for descriptions). dashed lines represent the unlocked portions of modelled faults. 9 that could be associated with such a large fault although this point is debated. historical earthquakes have large uncertainties in location and magnitude. moreover, large earthquakes are not periodic, so comparing the seismic catalogue to the recurrence time of large earthquakes does not provide sufficient evidence to conclude that the atf is aseismic. for the future behaviour of a locked fault, it is impossible to know if a locked fault will rupture as one large earthquake, as several smaller ones, or through episodes of silent slip, as observed in the central apennines [amoruso et al. 2002, scarpa et al. 2008]. therefore, we suggest considering the steeper segments of the atf in estimates of seismic hazard, as suggested by other authors as well [e.g., piccinini et al. 2003, brozzetti et al. 2009, diss working group 2010]. the slip along the unlocked portions of the vufs, gf, and cff as well as the small plastic strain below the atf may correspond to the frequent microseismicity observed in the study area [boncio et al. 1998, carminati et al. 2001, chiaraluce et al. 2009]. the release mechanism of a small amount of accumulated stress into a small amount of plastic strain is an alternative to the one proposed by collettini and barchi [2002], essentially differing because of the non-andersonian state of stress. the gps data used to calibrate the model span a time interval of approximately 11 years, from 1998 to 2008. this period follows the colfiorito earthquake, which occurred in 1997. the unlocked state of the cff thus corresponds to the behaviour after the 1997 earthquake and is not representative of the times prior to 1997, when the fault may have been locked. including tentative faults in the model geometry allows for evaluation of the sensitivity of the model to the fault geometry. the numerical model helps address some characteristics of the faults and detect unmapped buried faults. this approach is indispensable when modelling areas where little is known about the fault geometries. the results of the model are reasonably good in central italy, which is one of the best-studied areas of normal faulting in the central mediterranean. calibrating the method along the atf allows generalising it for applications to other areas where the faults are poorly known. for the atf, the model presented here constitutes a starting point to determine the slip rates of the modelled faults. our results provide insight to the locked state of faults. the model that best fits the observed data indicates that the atf is entirely locked due to the lack of a discontinuity in the horizontal and vertical interseismic velocities. the presence of a discontinuity in the horizontal and vertical velocities suggests that the cff and vufs behave as unlocked. acknowledgements. we thank francesco brozzetti for providing a high-quality map of the quaternary basins of the area and alessandro pignatelli for developing a matlab interface for marc output files. paolo boncio and an anonymous reviewer helped improve the manuscript with their constructive suggestions and comments. references amoruso, a., l. crescentini, a. morelli and r. scarpa (2002). slow rupture of an aseismic fault in a seismogenic region of central italy, geophys. res. lett., 29 (24), 2219; 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 4, dm423, 2022; doi:10.4401/ag-8761 o p e n a c c e s s improved generalized s‑transform deconvolution for non‑stationary seismic data chao sun1, dengke he*,2, lijun shen1 and liang sun2 (1) shandong provincial research institute of coal geology planning and exploration, jinan 250104,china; (2) college of geoscience and surveying engineering, china university of mining and technology (beijing), beijing 100083, china article history: received november 20, 2021; accepted march 23, 2022 abstract improving the vertical resolution is one of the significant tasks for seismic data processing. most traditional resolution-enhancement techniques assume that the seismic wavelet is time-invariant. however, the seismic wavelet varies with seismic wave propagation in the subsurface. to solve this issue, a new spectral-modeling method is proposed to extract the time-varying wavelet using improved generalized s‑transform (igst) and higher‑order fourier series. the igst based on modified time‑window function can effectively improve the resolution of the time‑frequency (t‑f ) spectrum. the high‑order fourier series is used to fit on the logarithm t‑f spectrum and achieve the high‑precision time‑varying wavelet. the proposed method is composed of four steps in the implementation. firstly, the seismic data is decomposed by the igst and converted to the logarithm t‑f domain. secondly, the time‑varying wavelet spectrum is modeled at each time sample using a higher‑order fourier series. thirdly, the boxcar smoothing method is used to smooth the time‑varying wavelet spectrum and extract the time‑varying wavelet with hilbert transform. finally, using the time‑varying wavelet spectrum to spectrally balance seismic data to flatten the seismic response. synthetic and field data examples demonstrate the feasibility of the proposed method in improving the signal‑to‑noise ratio and enhancing the vertical resolution. keywords: deconvolution; time-varying wavelet; improved generalized s-transform; high-order fourier series; spectral-modeling method 1. introduction improving vertical resolution plays a significant role in seismic processing [vardar and alpar, 2016; zhou et al., 2016; hao et al., 2019, lari et al., 2021]. many methods, such as wiener deconvolution [robinson and treitel, 1967], spectral modeling [rosa and ulrych, 1991; xue et al., 2021; li et al., 2022], and sparse‑spike deconvolution [velis, 2007; pereg et al., 2017], have been proposed to improve the vertical resolution. these methods tend to be according to that the wavelet is time‑invariant, which meets the condition of the stationary convolutional model. in fact, the seismic wave propagates through a heterogeneous, viscous-elastic media of subsurface which brings both dispersion chao sun et al. 2 and attenuation effects, leading to the seismic wavelet’s variation during seismic wave propagation, forming a non‑stationary seismic data [margrave et al., 2004; radad et al., 2015; zhang et al., 2019]. many researchers [bickel and natarajan, 1985; hargreaves and calvert, 1991; hargreaves 1992; wang, 2002, 2006; zhang et al., 2007; zhao et al., 2019] have provided the inverse q filtering technique to address the non‑stationary seismic data. but evaluating the accurate q‑factor for the inverse q filter is not easy [corrales et al., 2014; zhang et al., 2015]. margrave et al. [2011] presented a t‑f deconvolution methodology that calculates the time‑varying wavelet spectra smoothing the gabor spectrum of the non‑stationary seismic data. the gabor transform, a t‑f analysis technology, is used to analyze the non‑stationary signal in the t‑f deconvolution method. in fact, numerous t‑f analysis technologies [gabor, 1946; morlet et al., 1982; stockwell et al. 1996], such as the wavelet transform (wt), short-time fourier transform (stft), and s-transform (st), can depict the frequency variation with time of the signal. the st, which avoids many disadvantages of the gabor transform, stft, and wt, is a popular t‑f analysis technique. moreover, st is multiresolution, linear, reversible and lossless. due to the mother wavelet of s‑transform is fixed, it is difficult to adjust it as needed. mansinha et al. [1982] defined mother wavelets of st according to the seismic data requirements and expands it to the generalized s‑transform (gst). inspired by the study work of rosa and ulrych [1991], the reflectivity is generally white and the seismic wavelet spectrum is smooth. the time‑varying wavelet spectrum can be estimated by the spectral modeling method after smoothing the t‑f spectrum. zhou et al. [2016] used a polynomial to smooth the t‑f spectrum at each time sample for estimating the time‑varying wavelet. wang et al. [2017] used a filter to smooth the secondary t‑f spectra of non‑stationary seismic data. the spectrum can become smooth after being constrained by these spectral modeling methods. in this paper, we proposed an improved generalized s-transform (igst) to calculate the t-f component of seismic data based on the non‑stationary hypothesis. then the time‑varying wavelet spectrum is estimated using the higher‑ order fourier series in the logarithm t‑f domain. the two techniques are employed to deconvolve the non‑stationary seismic data for improving resolution. 2. the improved generalized s‑transform (igst) fourier transform, an important tool for seismic signal analysis, establishes the relationship of the signal’s time and frequency domain. however, they are independent of each other, and cannot represent the amplitude spectrum and phase spectrum at a specific time. to deal with the problem, gabor [1946] proposed that the signal can be divided into different time windows, and each time window can be analyzed by fourier transformation, which is gabor transform and can be written as: (1) where is the input signal in the time domain, means the signal in the t-f domain, 𝜏 and 𝑓 are the parameters of time shift and frequency value respectively, and represents the gaussian function or the window function and can be expressed as (2) however, once 𝜎 was selected, the time resolution and frequency resolution are invariable. stockwell et al. [1996] proposed the st by setting and then (3) non-stationary seismic deconvolution 3 the inverse st can recover the signal and is expressed as (4) due to the mother wavelet of st is fixed, the low‑frequency signal’s time resolution is generally poor. therefore, if we denote (5) and substitute equation (5) and equation (2) into equation (1), we can obtain the improved generalized s‑transform (igst) (6) the parameter of 𝜆 can influence the time width and bandwidth of the window. a larger parameter of 𝜆 the larger the time width, the smaller the bandwidth and the higher the frequency resolution. the parameter of 𝑝 can enhance the impact on the frequency parameter 𝑓 for 𝜆. the special parameter 𝑏 can influence the position on the frequency coordinates, which can improve the time frequency resolution low frequencies. when 𝜆 = 1, 𝑝 = 1, 𝑏 = 0 in equation (6), the igst becomes the s‑transform. similarly, when 𝑝 = 0, 𝑏 = 0 in equation (6), the s‑transform turns into gabor transform. the igst overcomes the restriction of st where the gaussian window function cannot be changed, which can improve the time resolution of the low‑frequency signal by adjusting the parameters 𝜆, 𝑝, and 𝑏. here, we produce a synthetic signal 𝑥(𝑡) to illustrate the performance via varying the parameters of the igst. the synthetic signal 𝑥(𝑡) contains four kinds of cosine functions, and their frequencies are 20 hz, 40 hz, 80 hz, and 100 hz, respectively. the synthetic signal 𝑥(𝑡) is expressed as (7) figure 1 shows the synthetic signal 𝑥(𝑡). figure 2a represents the actual t‑f spectrum, which precisely describes the t‑f distribution of the synthetic signal. figure 2b displays the t‑f spectrum using the st. figure 2c is the t‑f spectrum using the igst. the frequency resolution and time resolution of the st is poorer compared with the actual t‑f spectrum. the low‑frequency signal in figure 2b has lower time resolution than that in figure 2c. the high‑frequency signal in figure 2b has lower frequency resolution than that in figure 2c. thus, the igst has better t‑f resolution than st. moreover, we can choose igst parameters flexibly according to our own needs to obtain the relatively optimum t‑f spectrum. figure 1. the synthetic signal with four main frequencies. chao sun et al. 4 figure 2. the t‑f analysis example. left: the true t‑f spectrum. center: the t‑f spectrum by st (𝜆 = 1, 𝑝 = 0, 𝑏 = 0). right: the t-f spectrum by the igst (𝜆 = 1, 𝑝 = 0.85, 𝑏 = 6). 3. methods 3.1 non‑stationary seismic data in logarithm t‑f domain based on the igst, we can get the suitable t-f spectrum of the non-stationary data by: (8) where is the t-f components of the non-stationary data 𝑥��, igst[.] denotes igst by the equation (6). the amplitude spectrum and phase spectrum are respectively expressed as (9) (10) where and indicate the real and the imaginary part of the t-f spectrum . next, we convert the amplitude spectrum to the log domain (11) where denotes the logarithm t‑f spectrum. ln[·] means the natural logarithm of the amplitude spectrum . non-stationary seismic deconvolution 5 3.2 spectral modeling of non‑stationary seismic data in logarithm t‑f domain the spectrum modeling method was proposed by rosa and ulrych [1991] for extracting the wavelet in the frequency domain of seismic trace. it is assumed that the wavelet’s amplitude spectrum is a smooth unimodal curve, and the reflectivity’s amplitude spectrum is generally white due to the geological structures commonly meet the statistical model. the amplitude of the wavelet can be acquired by smoothing the amplitude spectrum of stationary data in the logarithm frequency domain. since the non‑stationary seismic trace can be considered stationary at each time sample in the t‑f domain. we can extract the wavelet’s amplitude spectrum at each time sample of the logarithm t‑f domain by smoothing the amplitude spectrum of the non‑stationary data at each time sample. finally, the time‑varying wavelet’s amplitude spectrum can be obtained. rosa and ulrych [1991] provided a low order polynomial to fit the wavelet amplitude spectrum. inspired by fourier analysis, the fourier series is utilized to fit the wavelet’s amplitude spectrum in logarithm t‑f domain, which can be expressed as (12) where (13) is the l2 norm, 𝜀 indicates the misfit error waited to be optimized, 𝑎0, 𝑎�, and 𝑏� are coefficient of fourier series varied with time, n is the order of fourier series, 𝑘� is a positive constant. the 𝑎0, 𝑎� and 𝑏� can be calculated by the nonlinear least square methodology. equation (12) represents the fsf method, which can estimate the time‑ varying wavelet spectrum. 3.3 3.3. spectral smoothing of non‑stationary seismic data in t‑f domain the estimated amplitude spectrum of the time-varying wavelet shows that some components are inconsistent with that of the exact amplitude spectrum. the boxcar smoothing method and hyperbolic smoothing method are used to deal with the problem. the boxcar smooth method can be regarded as a 2d boxcar convolution with the estimated amplitude spectrum, which is: (14) where 𝐴 is the estimated amplitude spectrum, 𝐵 represents the convolution operator, 𝑀 represents the length of time series, 𝑁 indicates the length of frequency series. the convolution operator smooths along with the time and frequency directions and forms a transition zone at the discontinuous points of the amplitude spectrum. the hyperbolic smoothing method uses the constant q‑value model. in the case of the constant q‑value model, the attenuation interface of the amplitude spectrum is distributed along the hyperbola, if the amplitude spectrum is divided into n parts according to hyperbola, each part corresponds to different attenuation interface, the middle part of a two‑phase hyperbola is called a hyperbolic belt. thus, the result of the hyperbolic smoothing method can express as [margrave et al., 2011] (15) where 𝐼� indicates the nth hyperbolic belt, 𝑏(𝑓0) is a convolver and • is an convolution operation in frequency domain. chao sun et al. 6 3.4 deconvolution in t‑f domain after getting the estimated amplitude spectrum of the time-varying wavelet, it is necessary to design the deconvolution operator. we design a deconvolution operator at each time sample in the t‑f domain. firstly, the maximum 𝐴max at each time sample window is extracted, which is expressed as (16) secondly, according to the minimum phase hypothesis, the phase of the estimated time-varying wavelet can be represented as: (17) where 𝐻[·] represents the hilbert transform, 𝜇 is a very small constant value, which ensures the stability of deconvolution. thirdly, the deconvolution operator is (18) finally, the deconvolution result in the t-f domain can be obtained, which is expressed as (19) then the time‑domain trace can be reconstructed by the inverse transformation of igst. 4. synthetic data example for the purpose of demonstrating the advantage and effectiveness of the igst deconvolution methodology proposed in this paper, we design a synthetic model. the model consists of a series of horizontal strata and a deeper wedge, which velocity range is between 1600 m/s and 2800 m/s. figure 3 shows the vertical reflection coefficient and figure 3. reflection coefficient model (left) and its non‑stationary seismic section (right). non-stationary seismic deconvolution 7 its non‑stationary seismic data that is convoluted by the vertical incidence reflectivity, the minimum phase wavelet which center frequency is 45 hz, and attenuation function (q = 50). the trace interval is 1 m, and the time interval is 1 ms. in order to compare the processing effect, the synthetic section is implemented by the gabor deconvolution and the igst deconvolution respectively. seeing in the figure 4, the result section by the igst deconvolution has better resolution than that by the gabor deconvolution. the wavelength about the coda wave from the event after the gabor deconvolution is longer than that from the event after the igst deconvolution. the comparison indicates the igst deconvolution is more effective than the gabor deconvolution for non‑stationary seismic data. figure 4. the result section of gabor deconvolution (left) and igst deconvolution (right). 5. field data testing to examine the effectiveness of the igst deconvolution methodology, post‑stack seismic data is provided in this paper. after implemented the mainly process techniques including static correction, surface‑consistent correction, surface wave suppression, normal move‑out and stack, the resulting seismic profile is dealt by the gabor deconvolution and the igst deconvolution respectively for comparison. seeing in the left subplot of the figure 5, the original seismic events within a yellow box cannot be well distinguished, which means that the resolution of the original seismic profile is low. the gabor deconvolution is used to process the original seismic section, and the outcome is displayed in the center subplot of the figure 5b, which has a higher resolution than the original section. by comparison, the result section after igst deconvolution, which is shown in the right subplot of the figure 5, has the highest resolution than that after the gabor deconvolution, especially the events within the yellow box are more detailed and smoother. figure 5. original seismic section (left), result sections after gabor deconvolution(center) and after igst deconvolution (right). chao sun et al. 8 6. conclusions the igst deconvolution method is proposed to evaluate and remove the effect of time-varying wavelets on non-stationary seismic data, which can improve the signal-to-noise ratio and enhancing the vertical resolution of seismic section. according to the new method, the igst is firstly used to calculate the t‑f distribution of non‑stationary seismic data, and then convert that to logarithm t‑f domain. secondly, the higher‑order fourier series fitting is used to smooth the amplitude spectra at every time sample in the logarithm t‑f domain and estimate the time‑varying wavelet. finally, the deconvolution operator will be executed. due to employ the high resolution features about the igst for time frequency analysis and the higher‑order fourier series for wavelet fitting, the new deconvolution method proposed can enhance the vertical resolution and compensate attenuation effect effectively, which is tested and exhibited by the synthetic seismic data and actual seismic data in the paper. author contributions: all authors are equal contributors to this work. funding: this research was funded by the natioanl 111 project (b18052), china university of mining and technology (beijing) yueqi distinguished scholar program (2019jca01) and shandong province geological exploration fund project (lukanzi (2019) no. 13, (2020) no. 19). references bickel, s.h. and r. r. natarajan (1985). plane‑wave q deconvolution, geophysics, 50, 1426‑1439. corrales, á., f. cabrera and l. montes (2014). enhancing time resolution by 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spectral modelling, geophys. j. int., 205, 203‑219. *co r r e s po n d i n g a u t h o r: dengke h e, college of geoscience and surveying engineering, china university of mining and technology (beijing), beijing 100083, china e‑mail: 201120@cumtb.edu.cn mailto:201120@cumtb.edu.cn trace elements mobility in soils from the hydrothermal area of nisyros (greece) annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6623 trace elements mobility in soils from the hydrothermal area of nisyros (greece) kyriaki daskalopoulou1, sergio calabrese2*, silvia milazzo2, lorenzo brusca3, sergio bellomo3, walter d’alessandro3, konstantinos kyriakopoulos1, franco tassi4, francesco parello2 1 department of geology and geoenvironment, university of athens, greece, 2 dipartimento di scienze della terra e del mare, university of palermo, italy, 3 istituto nazionale di geofisica e vulcanologia palermo, italy, 4 department of earth sciences, university of florence, italy *sergio.calabrese@gmail.com abstract nisyros island, greece, is a stratovolcano known for its intense hydrothermal activity. on june 2013, during a multidisciplinary field campaign, soil samples were collected in the caldera area to determinate the main mineralogical assemblages and to investigate the distribution of trace element concentrations and the possible relationship to the contribution of fluids of deep origin. soil samples were analysed with xrd and for the chemical composition of their leachable (deionized water) and pseudo total (microwave digestion) fraction both for major and trace elements. the results allow to divide the samples in 2 groups: lakki plain and stefanos crater. the latter, where a fumarolic area is located, shows a mineralogical assemblage dominated by phases typical of hydrothermal alteration. their very low ph values (1.9 – 3.4) show the strong impact of fumarolic gases which are probably also the cause of strong enrichments in these soils of highly volatile elements like s, as, se, bi, sb, tl and te. i. introduction race elements are considered to be the chemical elements whose concentration is less than 1000 ppm of rock’s composition. in soils, they originate from natural and anthropogenic sources. consequently, their concentrations can vary considerably. the trace element loading in soil is a function of the parent material plus subsequent atmospheric or water-borne deposition. high concentrations of trace elements in soil can affect the soil’s fertility and may represent an ecological and human health risk if they enter the food chain or leach into receiving waters. however, the impact of trace t metals on soil and the surrounding environment often cannot be predicted simply by measuring the total concentration, since only the soluble and mobile fraction has the potential to leach or to be taken up by plants and enter the food chain [robinson et al., 2005]. the mobility, solubility and bioaccumulation of trace elements depend on various factors, as well as their properties. a strongly acidic environment can favour mobility. such conditions can be the result of natural processes, such as magmatic/geothermal degassing or oxidation of sulfide minerals. natural processes generating metal-rich acid waters have highlighted the importance of metal-sulfate minerals, forming upon 1 annals of geophysics, fast track 2, 2014 evaporation, oxidation and neutralization processes [alpers et al., 2000]. metal sulfate salts are common around active crater lakes, fumaroles and acid hot springs [delmelle and bernard, 2000; fulignati et al., 2006; nordstrom et al., 2009]. in these environments, h2s is oxidized to elemental s, which accumulates and is further, oxidized to form sulfuric acid [jambor et al., 2000]. the acid reacts with the surrounding bedrock, leading to the formation of a variety of metal-sulfate minerals. some of these minerals are highly soluble and store metals and acidity only temporarily, whereas others are insoluble and improve water quality by removing metals from solutions. sometimes geothermal/volcanic activity is, therefore, responsible for elevated fluxes of toxic elements through ground and surface-waters [delmelle and bernard, 2000; varekamp et al., 2009]. the study area of this paper, nisyros, is a currently active volcanic area. the geothermal field of nisyros has been characterized as the hottest and most dynamic field in greece, due to the presence of magma chamber at shallow depths. the high geothermal anomaly of nisyros is manifested at surface by the occurrence of hot springs and fumaroles [marini and fiebig, 2005]. the major fumarolic vents are located in the craters of kaminakia, mikros polyvotes, megalos polyvotes and stefanos [caliro et al., 2005]. in this article, the results of mineralogical and chemical analyses of soil samples collected in the caldera of nisyros island are presented. this dataset summarizes the results of field observations and experimental measurements, regarding the distribution of trace element concentrations related to the contribution of fluids of deep origin. ii. study area nisyros is the easternmost island of the south aegean active volcanic arc in greece. it is an active stratovolcano, mainly consisting of quaternary volcanic rocks and alternations of lava flows, pyroclastic deposits and lava domes. the island has an area of 47 km2 and forms a truncated cone with a base diameter of 8km and a 4km wide central caldera [hunziker and marini, 2005]. the northern part of the caldera is named lakki plain and the southern ramos plain. no historical magmatic activity is known on nisyros and the most recent activity was of hydrothermal character [marini et al. 1993]. fumarolic emissions are located in the southern lakki plain and on the southeastern flank of the lofos dome both within the caldera. hydrothermal eruptions formed a series of hydrothermal explosion craters whose ages decrease from southeast to northwest. the last phreatic eruptions took place in 1871–1873 and 1887 partially destroying the small lofos dome. two deep explorative geothermal wells drilled in the lakki plain revealed the existence of two distinct hydrothermal aquifers. the shallowest at about 500 m depth has temperatures around 150 °c while the deeper one (> 1500 m) reaches temperatures up to 340 °c [brombach et al., 2003]. iii. materials and methods on june 2013, 33 samples of top-soils were collected in the caldera area (16 within the stefanos crater and 17 in the lakki plain at different distance from the fumarolic areas). all samples were collected in areas covered either by reworked volcanic material (mainly deriving from the prophitis ilias post-caldera lava domes) or by the products of the most recent (1871-1887 a.d.) hydrothermal explosion activities (annex 1). moreover, 1 soil sample was collected outside the caldera as local background, for comparison. in the laboratory, all soil samples were dried at temperatures below 36 °c and powdered in an 2 annals of geophysics, fast track 2, 2014 agate mortar. then, the powder was divided in 3 aliquots for the following analysis (xrd, leaching and microwave digestion). detailed description of the analytical methods can be found in the electronic annex (annex 2). iv. results and discussion soil acidity: ph is one of the most important parameters that influences soil alteration and, consequently, the geochemical mobility of elements. the values measured in the sampled soils display a wide range (from 1.9 to 7.2), with lower values found where the fumarolic activity is more intense. the good inverse correlation between s concentration and ph (fig. 1) is related to h2s degassing. h2s, one of the main components of the fumarolic gases at nisyros [marini and fiebig, 2005], in the shallow oxidizing environment produces h2so4. basing on ph values, it is possible to distinguish the soils in two main groups and a reference value: i) the soils of the active fumarolic field of stefanos crater (1.9 3.4), ii) the soils from the lakki plain (4.2 5.8) and iii) the reference value of the background soil (7.2) (fig. 1). the results obtained from the xrd analysis showed that in the majority of the lakki plain soils the main mineralogical assemblage is quartz, feldspar and gismondine (ca2al4si4o169h2o). mineralogical composition: the latter is a zeolite that derives from the hydrothermal alteration of plagioclase. in some samples, stellerite (caal2si7o187h2o) is appearing as a result of hydrothermal alteration, whilst wollastonite (casio3) is also found as a product of thermally metamorphosed siliceous carbonates. in the crater soils, because of the low ph values (1.9-3.8) and the relatively high temperatures of the fumaroles (about 100°c), the main assemblages mostly comprises hydrothermal alteration minerals. quartz, sulfur, wollastonite, gypsum and gismondine were found in most of the samples. 2 4 6 8 ph 10 2 10 3 104 10 5 10 6 s ( µ g /g ) stefanos lakki plain local background figure 1: ph vs. s in soils binary plot. the lower amount of feldspars with respect to the lakki plain soils can be justified by the high percentage of gismondine, their alteration products. voltaite (k2fe(ii)5fe(iii)3al(so4)1218h2o) was also found in some of the samples as a product of hydrothermal alteration of si deposits. close to the fumaroles, the occurrence of alunogen (al2(so4)317h2o), a sulphide alteration mineral found in fumarolic environments, is also noticeable. the mineralogical assemblages in the caldera of nisyros are presented in annex 3. geochemical mobility of major and trace elements: the results of the analysed major and trace elements are reported in box and whisker plots (fig. 2) and in the tables of annex 3. the data obtained from leaching and microwave digestion were both sorted according to the median value of the pseudo–total composition of the soils. furthermore, the local background was added in the graphs and presented as black dash lines, in order to allow a comparison. the concentration of local background shows sometimes values differing up to two orders of magnitude with respect to the soils of stefanos crater and lakki plain (fig. 2a). these differences are caused by the hydrothermal activity that is or has been present in the lakki plain area. some of major and trace elements display strong enrichments. 3 annals of geophysics, fast track 2, 2014 most of these elements are highly volatile (s, as, se, bi, sb, tl and te). on the contrary, a few elements show strong depletions in the stefanos and lakki plain soils with respect to the background soil (ca, mg, ti, mn, ni and li). these elements have high solubility, thus they were likelyleached out of the soils. this could be related to the low to very low ph values, which enhance their mobility. the most soluble elements, such as ca, mn, zn, ni, li, be, cd, are highlighted in figure 2c where the 104 10 2 10 0 10 2 104 106 p se ud o -t ot al c o nc en tr at io ns (µ g/ g) 10 4 10 2 100 102 104 10 6 l ea ch ab le c on ce nt ra ti on s (µ g /g ) 0 20 40 60 80 100 l ea ch ab le (% ) a l s fe k n a ca mg si ba ti sr pb v mn a s co zn cu la rb cr b y th n i li mo bi se sb cs tl u te b e cd median max q3 q1 min figure 2: box and whisker plots of (a) pseudo-total concentration, (b) leachable concentration and (c) solubility (% of leachable with respect to pseudo-total content). the black dash lines are the values of local background. the elements are ordered basing on decreasing median pseudo-total concentrations. percentages of the leachable element with respect to the pseudo-total concentration of the same element are shown (fig. 2). the enrichment factor (ef) is a useful tool to show the enrichment or depletion degree of a given element in a geochemical medium relative to a reference material and has been used in this study in order to distinguish elements with different geochemical mobility. the enrichment factor of an element x can be calculated as: efx=(cx/cr)sample/(cx/cr)rm (1) where (cx/cr)sample is the concentrations ratio of an element x normalized to a reference element (r) in the sample and (cx/cr)rm is the corresponding ratio in a reference material (rm). the results obtained (annex 1), highlight enrichments or depletions with respect to the “normal” soils of the island and therefore we used the background soil samples as reference material. th being characterized by a scarce mobility was chosen as reference element (r). basing on the calculated efs, the elements analysed in the soils of nisyros can be subdivided in 3 groups: group a: enriched elements, e.g. s, tl, bi, pb, te, sb, se, as, ba; (efs > 10) group b: slightly enriched to slightly depleted elements, e.g. la, sc, be, al, ti, v, zn, y, u, sn, mo, k; (10 >efs > 0.1) group c: depleted elements e.g. fe, mn, mg, cd, ca, na, b, li (efs < 0.1). the geochemical behaviour (enrichment or depletion) of the elements mainly depends on their mobility. the relative mobility within the soils is mainly controlled by their water 4 annals of geophysics, fast track 2, 2014 solubility that can be, in the present case, estimated from leachable % shown in fig. 2. the most depleted elements (group c) have generally higher solubility especially in the most acidic soils and are easily leached out leaving the soils depleted. 10-3 10-2 10-1 10 0 101 102 103 e f ( t h ) 10-2 10-1 100 10 1 102 103 104 e f ( t h ) 2 4 6 8 ph 0 20 40 60 80 100 % l ea ch ab le 2 4 6 8 ph 0 20 40 60 80 100 % l ea ch ab le figure 3: graphs of the efs of the major (a) and trace (b) elements normalized with respect to the immobile element th and their relative mobility gained from their leachability (c and d). the elements of group b have intermediate solubility and tend to remain in the soils, whereas that of group a show a relatively low solubility and, consequently, accumulate in the soils. due to the fact that the mobility of the elements strongly depends on the soil ph, the highest enrichments or depletions are therefore found close to the most active fumarolic area of stefanos where the soils reach the lowest ph (fig. 3). most of the elements of group a are highly volatile. fulignati et al. [2006] found the same elements strongly enriched in the rock coatings of the fumarolic area at vulcano island in italy. but while in that case fumarolic temperatures up to nearly 700 °c could justify a gaseous transport of these elements, in the present case, lower fumarolic temperatures (< 100 °c) make such transport less probable, although some of these elements could be present as gases (h2se, h2te and ash3). further gaseous species of as (planer-friedrich et al., 2006) and other metals (ti, v, mo, w, ni, cu, zn, cd, ga, si, ge, pb and sb planerfriedrich and merkel, 2006) have also been identified at low temperature hydrothermal features at yellowstone (u.s.a.). at nisyros enrichment of some element (ba, pb, as, se, bi, tl, sb and te) with respect to the background soil could be also due to the fact that most of the soils in lakki plain and the stefanos crater area derive from the explosive activity of the xix century. such activity deposited hot and hypersaline muds deriving from the deep hydrothermal system enriched in these elements. furthermore the highest s contents, mainly in the form of sulphate, contribute to the accumulation in these areas of the elements that form highly insoluble sulphate minerals (e.g. pb, ba etc.) especially where h2s is continuously supplied by fumarolic activity. v. conclusions the island of nisyros is characterized by a wide fumarolic field inside the lakki plain. the soils of stefanos, one of the main active 5 annals of geophysics, fast track 2, 2014 craters of the lakki plain caldera, showed low ph values, high concentrations of s and anomalous temperatures that reach the boiling point of water. due to these conditions, the main assemblage mostly comprises hydrothermal alteration minerals. quartz, sulfur, wollastonite, gypsum, alunogen and gismondine are found in most samples from stefanos crater, whereas in the soils of lakki plain, where the conditions are not so acidic, primary minerals are more abundant. the geochemical behaviour of the investigated elements depends mainly on their mobility, which in turn mainly depends on soil ph and therefore from distance of the soils from the main fumarolic emissions. the most soluble elements were fe, mn, mg, cd, ca, na, b and li. the high solubility contributes in leaching out the elements and leaving the soil depleted, while elements with low solubility tend to accumulate in the soils. references [alpers et al., 2000] alpers, c.n., jambor, j.l., nordstrom, d.k. eds., (2000). sulfate minerals –crystallography, geoch. and environment significance. rev. min. geochem. 40, 608pp. [brombach et al., 2003] brombach, t. caliro, s., et al. 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[varekamp et al., 2009] varekamp, j.c., ouimette, a.p. et al., (2009). naturally acid waters from copahue volcano, argentina. appl. geochem., 24, 208-220. 6 microsoft word 7401-18212-1-ed_bohle-ellis-rev daniela.doc annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7401 1 furthering ethical requirements for applied earth science martin bohle* directorate general research and innovation european commission martin.bohle@ec.europa.eu ellis c. erle university of maryland, baltimore county, baltimore, usa ece@umbc.edu abstract to face the challenges that anthropogenic global change poses to societal development, the ethical framework for earth science needs to be robust and both accepted and personally adopted by geoscience professionals. geoscience and engineering are increasingly called upon to inform societies about anticipated social and environmental outcomes based on scientific understanding. some professions are regulated to maintain the quality of work, professional integrity, and the trust of the stakeholders. in less applied professions, the role of training in research integrity is important. an oath or promise would strengthen the ethical framework for applied earth sciences going beyond the existing professional codes relating to research integrity, public regulations or professional charters that refer especially to engineering works. 1. introduction he ongoing processes of anthropogenic global change pose challenges to societal development, nature conservation, and how people value these [monastersky, 2015]. a framework beyond sound professional practice for handling ethical issues in earth science is needed [lynn, 2000; potthast, 2015]. earth scientists are intimately involved in discussions about climate change between researchers, diplomats, lawmakers, and industrialists that can result in action to modify humankind's engineered systems for production and consumption of goods. therefore, modern earth science and engineering disciplines and related social sciences and humanities need to continue developing their (geo)ethical frameworks in support of professionals [ellis and haff, 2009]. this essay is not about improved professional codes, codified practices or geoethics as a discipline nor about geoethics within the combined context of philosophy, politics and sciences [jamieson, 1996; proctor, 1998; cutchin, 2002; martinez-frias et al., 2011; peppoloni and di capua, 2015; riede et al., 2016; bobrowsky et al., 2017]. however, these issues provide context for us to discuss and understand geoethical concerns. “what is geoethics, and what does it add to our moral understanding...? [geoscience] has much to contribute to ethical theory, discourse, and action. the contribution emphasized by geoethics is the importance of context... which generates situated understandings of moral problems” [lynn, 1998 p. 282]. existence of a vibrant geoethical identity throughout our professional community can only serve to strengthen public support for our work and professional recommendations. in this essay, we discuss the idea described in ellis and haff [2009] or in matteucci et al. [2014] of an 'oath' or 'promise', respectively. 2. context a core responsibility of earth science and engineering disciplines is to guide societies towards improved social and environmental outcomes based on robust scientific understanding and technological know-how [ellis et al., 2016]. ext annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7401 2 amples include how to engineer humangeosphere intersections, how to use earth science to effectively address regional problems, or how production systems can be made more efficient at lower environmental costs [ellis, 2011; bohle, 2016]. while earth science and engineering disciplines have specific professional frameworks relating to scientific methods and technological challenges, their general professional frameworks for the social or political consequences of their work are less developed. tackling these difficulties involves scientific, technical, economic and other social concerns that all require professional handling of ethical issues beyond the integrity of research, deontological codes or sound engineering practice. many applied geoscience studies are especially value-laden; for example, the exploration, discovery, and extraction costs of natural resources compared with expected profits. therefore, professional organizations of engineering and chartered or licensed geoscientists have developed ethical frameworks [e.g., iapg, 2017a]. likewise, some other professions are self-regulated to maintain the quality of work, professional integrity, and the trust of stakeholders through adherence to aspirational statements, codes and standards published by their respective professional societies [e.g., iapg, 2017b; aag, 2017]. earth science as a whole needs a well-developed framework of meta-professional ethics of generic content and simple form (a meta-profession is a profession that is built upon the foundation of another profession, as geoscience teaching is built on the base of geoscience [arreola et al., 2001]). such professional and meta-professional ethical frameworks to handle value-loaded issues in societal context seem less familiar with scientists working in earth science [almeida and vasconcelos, 2015; peppoloni and di capua, 2016]. however, such frameworks are needed, either because of the complexity of handling anthropogenic global change [waters et al., 2016] or because of the number of different scientific professions and individuals involved [tumbman and escobar-wolff, 2016]. likewise, the role of training in research integrity and in other aspects of applied ethics is important [mayer, 2015]. professionals, researchers, and scholars in earth science have acquired particular understanding and talents that enable them to investigate, manage, and intervene on various physical, biological and social components of earth systems for human dwelling and acquisition of resources. these capabilities entail responsibilities with respect to serving the private and public good in the most just manner possible. also, it sets perspectives for the interaction with colleagues, laypersons, decision takers, and when aggregated, with society, nature and the planet. to outline these responsibilities and perspectives, a geoscientist's promise (as the following) or, alternatively, an oath could be instrumental: i promise… … i will practice geosciences being fully aware of the societal implications, and i will do my best for the protection of the earth system for the benefit of humankind. … i understand my responsibilities towards society, future generations, and the earth for a sustainable development. … i will put the interest of society foremost in my work. … i will never misuse my geoscience knowledge, resisting constraint or coercion. … i will always be ready to provide my professional assistance when needed, and will always make my expertise available to decision makers. … i will continue lifelong development of my geoscientific knowledge. … i will always maintain intellectual honesty in my work, being aware of the limits of my competencies and skills. … i will act to foster progress in the geosciences, the sharing of geoscientific knowledge, and the dissemination of the geoethical approach. … i will always be fully respectful of earth processes in my work as geoscientist i promise! (geoethical promise, from the cape town statement on geoethics, [di capua et al., 2016]). annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7401 3 3. discussion human communities face significant challenges that will only expand in the future. applying a wider knowledge base than natural sciences or embracing a multidisciplinary approach to environmental problems this sets challenging ethical and social perspectives [bonneuil and fressoz, 2013; hamilton et al., 2015]. to cope with these challenges of anthropogenic global change, the role and responsibility of earth scientists, engineers, and scholars should include concern for the societies and environments in which the individuals reside, the individual scientist's particular conscience, and relationships with colleagues [bobrowsky et al., 2017]. only by guaranteeing the intellectual freedom of researchers, scholars, professionals, and practitioners to apply robust professional standards can geoscientists follow appropriate ethical approaches and strive for excellence (peppoloni and di capua, this volume). by increasing awareness of the ethical implications of applied work earth science will be a force for public good, such as addressing the exploitation of geo-resources, land and sea management, mitigation of natural and human-made geo-hazards, and climate change adaptation and mitigation. in these works, ethical and social issues are central: for scientific or professional debates and public opinion, trustworthy handling of data, risk communication, education, integrity, anti-harassment/discrimination, gender or inclusion. geoethics is not a specialist domain, and a mere promise or oath would help to understand the multi-dimensionality of earth science professionalism. it includes the dimensions of individual behaviour, social responsibility, and viewing earth from various angles as a home for many, and it offers different points of equilibria. these multiple equilibria would reflect different philosophical, geoscience, and economic or social concerns, so that sound individual and professional behaviour can prosper in different societal settings. the single response could dwell on shared professional responsibility, know-how, mutual understanding of diversity, and intellectual honesty. as 'bedrock' to an oath or promise the regulated geoscience professions provide a robust setting of professional codes and regulations to support professionals [bobrowsky, 2013]. given a geoscientist's promise or oath, such codes find their extension in softer means, such as through recognized shared best practices. such means would further professional frameworks and geoethical thinking, and they would provide reference and the space for the habitual diversity of individual, professional, social, economic, and environmental situations within earth science. a geoscientist's promise or oath offers an intermediate level of codification. it should enable earth scientists and scholars to cope better with the economic, social, cultural, and geologic differences between countries that affect their work [stewart and gill, 2017]. it should also hope them deal more effectively with the vast diversity in geoscience concerns ranging from (geo)heritage sites through all of the forms of applied geoscience, whether relevant to a particular country, region, or to the entire planet. it appears further that a geoscientist's promise or oath also helps to address communication, outreach, and advocacy on issues of geoethical concerns in a case-dependent manner that is balanced yet has a standard foundation. the well-founded-ness of these activities is critical to handle the breadth of earth-system complexities such as non-linear behaviour, uncertainties, incomplete sets of observations, counter-intuitiveness of many natural processes [pievani, 2015], and human habits of perceiving these [levin et al., 2015]. finally, a geoscientist's promise or oath will make earth scientists aware of issues such as the need to earn public trust, to respect the limits of our knowledge, and to confront the denial of scientific evidence. a ‘geoscientists promise' or ‘oath for earth scientist' should further responsible application of science and engineering. the personal commitment enshrined in them anchors in the professional sphere of geosciences and their application contexts. specialist's work and practitioners activities will further these commitments when addressing matters such as the ethics of the professional at work, an inclusive composition of the workforce, and annals of geophysics, 60, fast track 7, 2017; doi: 10.4401/ag-7401 4 integrity of the individual. thus, the general ethical framework for applied earth sciences can be strengthened by an "oath for earth scientists" or a "geoscientist's promise" that aggregates a wider context. in this manner, the concern of riede et al. [2016] is addressed, namely: "while we shy away from drafting an ‘environmental archaeology promise’... a discussion on the pros and cons of such a document might be a vital step along the path towards structured action rather than individual activism." 4. conclusion summarizing an 'ethical requirement' of a promise or oath offers a flexible framework for aggregate professional ethics for a broad range of application cases. it includes the constraints of earth sciences, engineering and scholarship, such as limited understanding of humangeospheric processes and cultural biases in their valuation, and it relates to education, training and professional practices. the ethical requirements of an oath or promise could offer an additional level of scientific respectively cultural standards above existing professional codes relating to research integrity and public regulations and professional charters. it remains the issue how to integrate the promise or oath into vocational education. finally, it is to consider whether the notions like ‘oath' or ‘promise ' convene the appropriate level of individual commitment. within the (western) cultures and languages of the authors' preference is for the notion ‘promise'. however, we are curious what term other cultures and languages may offer. references aag – american association of geographers (2017). statement on professional ethics. http://www.aag.org/cs/resolutions/ethics (last access: 24 july 2017). almeida a. and vasconcelos c. 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(2016). the anthropocene is functionally and stratigraphically distinct from the holocene. science 351: aad2622-aad2622. doi:10.1126/science.aad2622. disclaimer*: all views expressed herein are entirely of the author, do not reflect the position of the european institutions or bodies and do not, in any way, engage any of them. acknowledgement: the authors declare no conflict of interest. this paper is the extended version of the authors' contribution to the 35th international geological congress, 2016 cape town. the authors like to thank the anonymous reviewers. annals of geophysics, 55 (3), 2012: limaye observing geoethics in mining and in ground-water development: an indian experience shrikant daji limaye director, ground water institute (ngo), pune, india annals of geophysics, 55, 3, 2012; doi: 10.4401/ag-5573 abstract two famous quotes from ‘mahatma gandhi’, the indian philosopher, are helpful in describing the foundation of geoethics: (a) “god has provided enough resources for everyone’s need but not for everyone’s greed” and (b) “before starting a development project, first think about the effects which the project would have on the poorest of the poor people in the society”. geoethics is relatively a new branch in geosciences. it lies at the meeting point of geosciences, sociology and philosophy. much earlier in the history of sciences such as nuclear physics, biology or chemistry, ethical principles attracted the attention of social leaders and philosophers while using laboratory research for field trials and applications on a large scale. using the research not for atomic bombs but for atomic energy and not for chemical weapons but for chemical products and drugs was the priority of social leaders. in italy, geoethics originated with philosophers and later on got the support from geologist. in other countries geoscientists working primarily in mining industry promoted geoethics for ensuring socially responsible behavior on the part of mining companies. 1. introduction two famous quotations from mahatma gandhi, the indian philosopher, are helpful in providing the ‘foundation’ for geoethics since 1930s: “god has provided enough resources for everyone’s need but not for everyone’s greed”, and “before starting a development project, first think about the effects which the project would have on the poorest of the poor people in the society.” geoethics is a relatively new branch of the geosciences. it lies at the meeting point of geosciences, sociology and philosophy. much earlier in the history of science, such as with nuclear physics, biology or chemistry, ethical principles attracted the attention of social leaders and philosophers while using laboratory research for field trials and applications on large scales. using the research not for atomic bombs but for atomic energy, and not for chemical weapons but for chemical products and medicines, was the priority of philosophers and social leaders. in italy, in late 1960’s, felice ippolito, who was a geologist and engineer with philosophical inclinations, began to reflect on the philosophical, sociological and historical aspects of the geosciences, considering the ethical value of geological knowledge, while using nature or the natural resources to meet the needs of society [ippolito 1968]. this can be interpreted as the beginnings of geoethics in europe. ippolito developed philosophical topics related to the geosciences, such as the relationships between man and nature; between the scientific community, nature and society; and the position of geology between exact sciences like mathematics, and natural sciences like biology. he had realized that scientific research is not necessarily associated with ethical values. however, the application of such research must ensure ethical considerations about the effects on society. his reflections on the ethical and social implications of the geological activities showed for the first time the importance of the role played by geoscientists in promoting and observing geoethical principles and values. in other countries, geoscientists who were working primarily in the mining industry promoted geoethics to ensure socially responsible behavior on the parts of the mining companies. since 1996, regular symposia on geoethics have been organized at the international geological congresses (igc), and more frequently at the regular meetings of the pribram mining conferences (czech republic) due to efforts of vaclav nemec and lidmila nemcova. over the last three years, silvia peppoloni from the instituto nazionale di geofisica e vulcanologia (ingv), rome, has been organizing sessions on geoethics at the biennial geoitalia conferences, and recently she introduced a session on geoethics at the european general union, general assembly 2012 [european geosciences union general assembly 2012]. 2. the multidisciplinary nature of geoethics as it is at the confluence of three faculties, as mentioned above, geoethics has been defined in various ways, depending on the background of its proponents. philosoarticle history received february 14, 2012; accepted february 21, 2012. subject classification: mining, groundwater, geoethical practices, india. 379 phers prefer the ‘precautionary principle’, which emphasizes the necessity of putting restrictions on those engaged in the application of a new technology or a new activity on a larger scale, without evaluating the long-term effects of such an application. the sociologists consider geoethics from the point of view of the specific needs of society, if any, which can be fulfilled by the application of a new technology or a new activity on a larger scale. geoscientists employed in any new development at field level or in the application of any new technology view geoethics as a set of decisions to be made at field level, through which the new development will be friendly to society and to the environment, while being economically viable to the developer. geoscientists engaged in teaching or in government offices view geoethics as a way to guide such new developments through directives and regulations and also to educate society about the value of natural resources and their prudent use. we thus have various stakeholders who are interested in the theory and practice of geoethics, including philosophers, sociologists, geoscientists, environmentalists, teachers, developers and businessmen promoting a new activity. there are also the participants in the activity, the regulators and policy makers in the government, and finally, the civil society that is likely to be affected by the new activity. even in the absence of any such new development for resource use, geoethical activities of geoscientists would include: (a) ethical behavior of geoscientists within their own community; (b) efforts to educate civil society, and especially the younger generation, regarding prudent and eco-friendly use of natural resources; (c) pursuit of scientific research and dissemination of results which might be of interest or beneficial to civil society; (d) a role in educating the public about geohazards, like floods, landslides, earthquakes, droughts, and volcanic activity, and in promoting preparedness in society to face such hazards; (e) an active role in predicting geohazards, whenever possible, and in post-hazard remediation; and (f ) contributions to the decision-making processes at various levels, and especially in mining development, ground water use and geohazards. regarding this last point, geoethical behavior expects geoscientists to ensure that the process of decision making is transparent and involves all of the stakeholders, and that the decisions are eco-friendly and can be implemented at the field level. 3. geoethics as applied to the mining industry for many years, mining was considered as a nonsustainable activity. this is because mining includes the physical removal of a non-renewable resource from one site for further processing and use. the resource itself is thus not sustainable. sustainability of mining is a relatively recent concept, which embodies geoethical behavior towards the society close to the mining area. the society close to mines is considered to have several types or aspects of ‘capital’, such as natural resources capital, educated people or human resources capital, infrastructure, employment opportunities, health services, productive farms, a clean environment, and social institutions. all these are what we can call the ‘social capital’, which contributes to a stable social order and community well-being [rajaram et al. 2011]. due to mining activities in its neighborhood, the natural resources capital is permanently lost, with possible partial loss in other aspects of capital, like a clean environment. sustainable mining incorporates compensation for the society towards these losses by increasing several other capital assets mentioned above. during the operation of mining, various aspects of the capital of the society can be enhanced by the mine developers, like education for children, job opportunities for elders, health services at a company hospital, and the infrastructure for an electricity supply, roads and markets. these would continue during the several years or decades of the mining operation. however, even after active mining stops, sustainability for the society can be achieved by effective rehabilitation processes. some of the cement companies in india have taken up such rehabilitation processes by using old mine pits for creating large water storage to promote fisheries, forestry, and irrigation for agriculture. the ‘self-help groups for village women’ that were created during active mining continue to work as ‘value addition’ to agricultural produce or production of handicrafts in the rehabilitation phase. in large cement factories, active mining goes on in new sectors, while rehabilitation takes place in old, usedup sectors. geoscientists have an important role in this rehabilitation. one of the coastal cement factories in india used a farmer friendly approach in ground water use. the farmers in the surrounding villages were worried that the factory would pump a lot of ground water from its mining lease areas at high levels, for the construction and operation of the factory, thereby promoting sea-water intrusion into irrigation wells of the farmers located at the lower levels closer to the sea. the factory authorities declared that they would not pump any ground water from the mining area. the factory entered into an agreement with the farmers that the farmers should supply water to the factory from their wells rather than using it for irrigation and the factory would pay them in cash on a daily basis, which would be much more than what they could earn from the irrigated crops. after the factory started working at full capacity, it used hot gases in the chimney for sea water desalination for industrial use. the factory also promoted ground water recharge by using the lowest levels shrikant daji limaye 380 381 of the mine pits as ground-water recharge ponds during the monsoon rainy season. this increased the water availability in the wells of the farmers, and also checked the intrusion of saline water in the aquifer. 4. geoethics in ground-water use in india, and also in several other low-income countries, the ownership of ground water lies with the land owner. therefore, the farmers have a strong sense of ownership of whatever ground water there is within their farms. suppose a farmer has a good well on his farm. then his neighbor also digs or drills a well on his own farm, not far from this good well, with the result that the yield of the good well is considerably reduced. here, there is no court case, law suit or ethical considerations. each farmer knows that it is perfectly legal to dig or drill a new well on their tiny farmland, irrespective of any consideration of the effects that the new well might have on the yields of existing wells in the vicinity. in a mini-watershed of about 100 ha, suppose a farmer has a 1 ha farm in which he is lucky to have a very good well, so that he can pump out large quantity of ground water and irrigate his whole farm of 1 ha with ‘high water demand’ crops, like banana or sugarcane. in the view of geoethics, the farmer’s equitable right to the ground water resources of the mini-watershed is just 1%, according to the area of his farm. however, just because he is lucky, how much of the share of ground water in the watershed can he rightfully pump from his well? five percent? ten percent? this is a very difficult question to answer at the village level. if the farmer is not ready to reduce his pumping, the geoethical behavior on his part will be to make provision for increasing the ground water recharge during the monsoon rainy season. this could be done by collecting the runoff water in a farm pond, allowing it to settle and then filtering and putting it into the good well. moving further from the scale of a farm to the scale of a watershed, we find that in many watersheds the pumping of ground water has increased considerably in the past 30 years, and the water table has become depleted. increasing the recharging of the ground water has been traditionally considered as the responsibility of the government. only recently have some of the village councils taken a lead in increasing the recharging by convincing the farmers that ethically it is their duty to recharge the aquifer because they have depleted it through excessive pumping. pollution of ground water by industries is another issue in which there is a need to stress the principle of ‘the polluter must pay’ upon the industries. however, the best practice is to prevent the pollution from occurring in the first place, rather than trying any remediation of a polluted aquifer. 5. conclusions although it is a relatively new branch in the faculty of geosciences, geoethics is gradually assuming importance because observing ethical behavior while developing geo-resources or while dealing with geohazards, has become an urgent need of the society. geoscientists have a duty to educate society on prudent and eco-friendly use of these resources, and also to increase the preparedness of society in dealing with geohazards. the success of the session on geoethics at the geoitalia conference in 2011 in turin [geoitalia 2011] was a good indication that italian geoscientists are increasingly becoming aware of this duty, and are poised to take a leading position on the global stage. as a low-income country, some of the nongovernmental organizations and business houses in india are also taking an active interest in this field. references european geosciences union general assembly (2012). vienna, austria; http://www.egu2012.eu/home.html. geoitalia (2011). viii italian forum on earth sciences, turin, italy; http://www.geoitalia.org/index.php?action 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(1968). la natura e la storia, milano, vanni scheiwiller (in italian). rajaram, v., s. dutta and k. parameswaran (2011). sustainable mining practices – a global perspective, aa balkema publishers, 370 pp. corresponding author: shrikant daji limaye, director, ground water institute (ngo), pune, india; e-mail: limaye@vsnl.com © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. india – geoethics in mining and groundwater use << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice articolo madeira.pdf key words azores – neotectonics – active faulting – paleoseismology – seismic hazard 1. introduction the studied region is located in a unique tectonic setting (fig. 1). the central and eastern groups of the azores islands stand on a broad sheared region that absorbs the deformation induced by the eastward differential displacement of the eurasian and african plates, east of the azores triple junction. the volcanism that built the islands and submarine volcanoes is tectonically controlled and is a result of the active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) josé madeira and antónio brum da silveira departamento de geologia and laboratório de tectonofísica e tectónica experimental, faculdade de ciências da universidade de lisboa, portugal abstract the neotectonics of the islands of faial, pico and s. jorge (azores) is presented. preliminary paleoseismology data from trench exposures across three active fault zones (lomba do meio, lagoa do capitão and pico do carvão faults) complement the information. radiocarbon age constraints of paleoearthquakes suggest clustering of surface rupturing events. slip rates deduced from paleoseismology analysis range from 0.10 to 0.40 cm/year and validate long-term slip rates obtained by neotectonic studies (using pleistocene markers). the studied faults allowed a preliminary seismic hazard assessment: magnitudes of the largest paleoearthquakes, determined from slip per event range from mw = 6.9 to 7.1, and maximum expected magnitudes, estimated from rupture length or rupture area, vary from mw = 6.4 to 6.8. the former mw estimates are in closer agreement with the magnitude of the major historic and instrumental seismic events in the archipelago, even though the used empirical relations between magnitude and rupture parameters may not be the most adequate due to the unique tectonic setting of azores. transtensile regime acting on that portion of oceanic lithosphere. the deformation is represented by widespread active faulting of the islands and the surrounding seafloor. this work presents the description of the tectonic structure of three islands from the azores central group: faial, pico and s. jorge. the faults were mapped through fieldwork and vertical airphoto interpretation. kinematics was deduced from rare striated fault surfaces and displaced stratigraphic and morphological markers. determination of fault separations of k/ar dated volcanostratigraphic units (at the scale of the island) by cartographic methods allowed the estimation of slip rates for a time scale of the order of tens or hundreds of thousand years. in order to verify the validity of the obtained slip rate values, and to observe the faults at different time and geometry scales, exploratory paleoseismology studies were performed across three known active faults, one from each island. the 733 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. josé madeira, departamento de geologia and laboratório de tectonofísica e tectónica experimental, faculdade de ciências da universidade de lisboa, edifício c2, 5° piso, campo grande, 1749-016 lisboa, portugal; e-mail: jmadeira.@fc.ul.pt paleoseismological analysis was to complement the neotectonic study of the three islands and, therefore, must be understood as a preliminary and exploratory work. the choice of the trenched faults and their location depends on geological and morphological reasons, including: the size of the fault scarps (many of the main faults present scarps several tens to hundreds of metres high with the base covered by thick talus deposits); their topographic position (scarps located in depressed areas are unfavourable as they are either partially fossilized by recent deposits or amplified by stream erosion); morphologically fresh aspect of scarps suggesting recent surface rupture and/or knowledge of historic surface rupturing events; nature of the materials exposed in the scarps (where possible, scarps cut in pyroclastic deposits were preferred to those that expose the lava flow basement). paleoseismology results allowed the determination of age of paleoearthquakes, estimation of recurrence intervals, and comparison of slip rates for different time scales. 2. geological setting the azores archipelago is located at the north america, eurasia and africa triple junction (fig. 1). the mid atlantic ridge separates the american from the eurasian and african plates; east of the rift, the azores-gibraltar fault zone is the boundary between eurasia and africa. the islands of flores and corvo lie on the american plate, while the central (graciosa, terceira, s. jorge, faial and pico) and eastern (s. miguel, santa maria and formigas islets) groups of islands are located along the western segment of the azores-gibraltar fault zone (madeira, 1998; lourenço et al., 1998), whose morphologic expression is the azores plateau. the region is under right lateral transtension as a result of higher rates of spreading north of the triple point and to the obliquity of the azorean segment of the eurasian-african plate boundary relatively to the spreading direction. 734 josé madeira and antónio brum da silveira fig. 1. tectonic setting of the azores archipelago (modified from laughton and whitmarsh, 1974; lourenço et al., 1998; madeira, 1998). the mid atlantic ridge (mar) separates the north american plate (na) from eurasia (eu) and africa (af). the azores oblique spreading centre (grey area) and gloria fault (gf) separate eurasia from africa. the east azores fault zone (eafz) is an abandoned segment of the eurasia-africa boundary. the archipelago islands are corvo (c), flores (f), graciosa (g), terceira (t), s. jorge (sj), faial (fa), pico (p), s. miguel (sm) and santa maria (sma). the azores plateau is enclosed by the 1600 m isobath (thin line). inset shows location of archipelago in the atlantic and its relation with the mid-atlantic ridge and the azores-gibraltar boundary. 735 table i. historic tectonic seismicity in azores. date location intensity estimated effects of earthquake (mcs-17; magnitude mm-56) oct. 22, 1522 s. miguel x (1) voluminous landslides; > 5000 deaths estimated june-july, 1571 d. joão de vi/vii seismic swarm castro bank may 24, 1614 terceira x/xi; viii (2) > 6.3 > 8 km of surface rupture on lajes fault; 93 deaths (3) june 9, 1647 terceira v/vi oct. 19, 1656 s. miguel iv/v march 29, 1690 central group v alternative date is april 5, 1690 nov. 15-25, 1698 terceira iv seismic swarm dec. 8, 1713 s. miguel vi/vii july 9, 1757 s. jorge x/xi 7.1; 7.4 (4) deaths: 1034 in s. jorge; 11 in pico; small tsunami observed dec. 1759 to may 1760 faial vi seismic swarm aug. 1791 terceira iv/v jan. 21, 1837 graciosa viii; ix (2) probable rupture in serra das fontes fault; 3 deaths june 15, 1841 terceira x/xi; ix (2) > 1 km of surface rupture in cruz do marco fault; 5500 homeless oct. 30 to nov. 8, 1848 s. miguel vi/vii seismic swarm april 16, 1852 s. miguel vii/viii; vi (2) 9 deaths sept. 21 to dec. 10, 1862 faial v/vi seismic swarm march 16, 1920 faial v feb. 9, 1924 faial v/vi aug. 31, 1926 faial viii/ix; x (2) 8 deaths (1) machado (1966); (2) nunes (1991); nunes et al., (1992); (3) coeval texts refer three different numbers: 29, 93 or > 200 deaths; 93 is the most credible number; (4) machado, (1949). active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) as a result of its location on an active plate boundary, the archipelago is subject to frequent seismic activity. although most are low to moderate magnitude seismic isolated events or swarms, the islands are occasionally struck by high magnitude (m 7) earthquakes (table i; fig. 2) exemplified by the october 22nd, 1522, s. miguel earthquake ( 5000 victims), the july 9th, 1757, s. jorge event ( 1000 deaths) and the january 1st, 1980, terceira earthquake (64 casualties) (mckenzie, 1972; udías et al., 1976, 1986; hirn et al., 1980; udías, 1980; grimison and chen, 1986; buforn et al., 1988; moreira, 1991; mezcua et al., 1991; nunes et al., 1992; miranda et al., 1998). published focal mechanisms (table ii; fig. 2) indicate dominant normal, dextral and sinistral strike-slip solutions on wnw-ese and nnw-sse structures. volcanism has also been observed since the settlement of the archipelago in mid 14th century. at least 27 eruptions (fig. 3; table iii) were reported in the islands of s. miguel, pico, s. jorge, faial and terceira, and at sea between them, although several eruptions may have been unnoticed (zbyszewski, 1963; weston, 1963-1964; forjaz, 1992; queiroz et al., 1995; madeira, 1998). the nature and style of eruptions is variable and include trachyte plinian or subplinian events, basaltic hawaiian/strombolian, shallow surtseyan or deeper submarine eruptions, and basic or acid hydrovolcanic explosions. 3. neotectonics of faial, pico and s. jorge two main fault systems were recognized in the studied islands. the dominant structures, striking wnw-ese (n60-80w) and dipping 60° to 90° to nne or to ssw, control the general shape of the islands. the second fault system, trending nnw-sse (n10-30w) and dipping 60° to 90° to wsw or to ene, although important, is less frequent (figs. 4, 11 and 14). the size of the islands limits the observation of the full length of the faults. volcanism is controlled by the tectonic structure: intersection of the two fault systems determines the location of polygenetic volcanoes, while monogenetic volcanoes lie mainly on wnw-ese structures (madeira and ribeiro, 1990; madeira, 1998). splays, duplexes, en échelon structures, and frequent changes in trend, portray the map geometry of the faults. in section, the fault planes display variations in dip, sometimes alternating almost vertical with less steep inclinations. at the outcrop scale, the non-planar geometry of the fault surfaces results in the formation of open spaces along strike as well as along dip. 736 josé madeira and antónio brum da silveira fig. 2. seismicity in the azores: main historic tectonic earthquakes (triangles) and published focal mechanisms of instrumental events (circles) (data from mckenzie, 1972; udías et al., 1976, 1986; hirn et al., 1980; udias, 1980; grimison and chen, 1986; moreira, 1991; nunes, 1991; miranda et al., 1998). numbers relate to table ii. 737 table ii. seismic events with published focal mechanisms. n. date magnitude location depth references 1 may 8, 1939 7.1 37.4n-23.9w 2, 4, 6, 7 2 sept. 6, 1964 4.9 38.3n-26.6w 2, 4, 6, 7 3 sept. 18, 1964 5.5 39.8n-29.7w 2, 4, 6 4 july 4, 1966 5.3-5.4 37.51n-24.75w 12 ± 5 km 1, 2, 4, 5, 6, 7 5 july 5, 1966 5.1-5.0 37.6n-24.7w 2, 4, 6, 7 6 april 20, 1968 4.8 38.3n-26.7w 7 7 april 20, 1968 5.0-5.1 38.30n-26.77w 15 ± 5 km 5, 6, 7 8 sept. 1, 1968 4.8 39.16n-29.93w 6 9 nov. 23, 1973 4.9 38.52n-28.37w 6 10 dec. 11, 1973 5.0 38.7n-28.7w 6, 7 11 jan. 1, 1980 7.2-6.4 38.82n-27.78w 10 km 3, 5, 6, 7 12 feb. 12, 1981 5.3 38.5n-26.7w 7 13 sept. 9, 1984 5.0 36.9n-24.6w 7 14 oct. 16, 1988 5.1 37.70n-25.20w 8 15 nov. 21, 1988 5.8 37.9n-26.2w 7 16 jan. 21, 1989 5.4 38.04n-26.28w 8 17 june 26, 1989 5.7 39.23n-28.34w 8 18 july 22, 1992 3.2 38.15n-26.65w 9 19 july 22, 1992 3.4 38.15n-26.65w 9 20 july 9, 1998 5.8 (md) 6.2 (mw) 38.63n-28.52w 10, 11 1 mckenzie (1972); 2 udías et al. (1976); 3 hirn et al. (1980); 4 udias (1980); 5 grimison and chen (1986); 6 udías et al. (1986); 7 moreira (1991); 8 nunes (1991); 9 miranda et al. (1998); 10 vales et al. (2000); 11 madeira et al. (1998a). fig. 3. location of reported historic eruptions in the azores (data from zbyszewski, 1963; weston, 1963/64; forjaz, 1992; queiroz et al., 1995; madeira, 1998). submarine eruptions occurred at depths shallower than – 400 m. numbers relate to table iii. active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) the kinematics of the faults, deduced from striated fault surfaces, as well as from the displacement of morphologic (e.g., spatter cones, streams and ridges) and stratigraphic markers, indicates normal dextral slip on the wnw-ese fault system and normal sinistral displacement in the nnw-sse conjugate system. small pullapart basins and push-ups are related to releasing and restraining bends, respectively. morphologic features such as fault scarps, sag ponds, alignments of volcanic cones and craters, displaced streams and craters, characterize the fault traces. major faults usually have well-developed scarps with metric to hectometric heights. however, the fault scarps height may not correspond directly to the accumulation of successive surface rupture. often, differential erosion of the disrupted materials (lava flows versus pyroclasts) or partial fossilization of the scarp by volcanism may amplify or attenuate the tectonic slope. on the up-thrown block, secondary fault scarps, showing en échelon geometry, are frequent. observation of striated fault surfaces indicates that, besides oblique slip, strain partitioning may occur at the fault zone scale: parallel slip surfaces from a fault zone displaying either dip slip or strike slip slickensides, show that strike and normal components may be separated in time or space. the occurrence in the archi738 josé madeira and antónio brum da silveira table iii. historic eruptions in the azores. n. year location structure 1 1439? s. miguel pico da ferraria sete cidades 2 1439 or 43 s. miguel pico do gaspar furnas 3 1562/64 pico pico do cavaleiro (presently cabeços do fogo) 4 1563/64 s. miguel lagoa do fogo, pico do sapateiro (queimado), pico de da guiomar, monte escuro 5 1580 s. jorge riba do almeida, mistério da queimada, riba do nabo 6 1630 s. miguel lagoa seca furnas 7 1638 off shore ponta da candelária (s. miguel) 37°49 02 n-25°52 03 w 8 1652 s. miguel picos do fogo i and ii 9 1672/73 faial cabeço do rilha boi (presently cabeço do fogo) and pincarito 10 1682 off shore between s. miguel and d. joão de castro bank 38°07 n-26°08 w 11 1713 s. miguel pico das camarinhas 12 1718 pico lomba de fogo, cabeço de cima and de baixo, off shore s. joão 13 1720 pico cabeço do soldão (or cabeço do fogo) 14 1720 off shore formed a temporary island on d. joão de castro bank 38°14 n-26°39 w 15 1761 terceira picos das caldeirinhas, mistério negro 16 1800 off shore ponta do topo (s. jorge) 38°27 07 n-27°22 07 w 17 1808 s. jorge craters w of pico do pedro, entre ribeiras and areias de santo amaro 18 1811 off shore ponta da ferraria (s. miguel) 37°51 04 n-25°51 08 w; a temporary island (sabrina) formed 37°52 04 n-25°51 06 w 19 1867 off shore wnw of ponta da serreta (terceira) 38°47 04 n-27°27 00 w 20 1902 off shore w of ponta do topo (s. jorge) 38°31 02 n-27°26 08 w 21 1907 off shore close to south coast of s. miguel 37°41 05 n-25°48 00 w 22 1911 off shore mónaco bank 37°36 n-26°52 w 23 1957/58 off shore capelinhos; phreatic explosion in caldeira (faial) 24 1963 off shore probable eruption nw of cachorro (pico) 25 1964 off shore probable eruption w of velas (s. jorge) 26 1981 off shore probable eruption at mar de prata (sw of s. miguel) 27 1998/00 off shore wnw of ponta da serreta (terceira) modified from madeira (1998), based on zbyszewski (1963); weston (1963/64); forjaz (1992), queiroz et al. (1995) and coeval written accounts. pelago of earthquakes with normal and strike slip focal mechanisms in parallel faults, separated by a few kilometers or tens of kilometers, suggests that strain partitioning may also occur at the regional scale (madeira, 1998). 3.1. faial 3.1.1. geological and structural setting the island of faial is 21 km long, has a maximum width of 14 km, and reaches an altitude of 1043 m at cabeço gordo (fig. 4). two central volcanoes and two fissural volcanic zones form the sub aerial part of the island (serralheiro et al., 1989; madeira, 1998; pacheco, 2001). the ~.800-580 ka old (féraud et al., 1980) ribeirinha shield volcano constitutes the eastern region of faial. it is predominantly composed of sub aerial hawaiitic lava flows mapped under the designation of ribeirinha volcanic complex (rb in fig. 4). the caldeira stratovolcano was built on the western flank of the ribeirinha volcano and constitutes the main relief of faial. it is approximately 15 km in diameter at sea level; the summit is truncated by a 2 km wide, 400 m deep, caldera. the products of this volcano range from basalts to trachytes and are divided into two volcanostratigraphic units. the lower unit, cedros volcanic complex (cd in fig. 4), is mainly composed of sub aerial basalt to benmoreite lava flows. the available ages for this unit range from .470 to 11 ka (féraud et al., 1980; baubron, 1981 in chovelon, 1982). the upper unit, caldeira formation (p and i in fig. 4), corresponds to a holocene sequence of trachytic products, emplaced by explosive eruptions. radiocarbon ages from caldeira formation products range from .10 to 1 ka (madeira et al., 1995). a basaltic fissural unit, the almoxarife formation (al in fig. 4), overlies the southeastern slope of caldeira volcano and is covered by caldeira formation pyroclasts. it comprises hawaiian/strombolian scoria cones and lava flows and one surtseyan cone. the only available age for this formation is a low quality k/ar date of 0.03 ± 0.02 ma (féraud et al., 1980). the western region of faial is formed by a dextral en échelon alignment of basaltic hawaiian/strombolian and surtseyan cones and lava flows, the capelo volcanic complex (cp in fig. 4), built on the west slope of caldeira volcano. it is younger than 10.000 years (as it overlies the oldest pyroclasts of caldeira formation), and includes two historical eruptions (1672 and 1957 in fig. 4). as already mentioned, the active faults control the general shape of the island (fig. 4). the eastern part of faial island is characterized by a wnw-ese trending graben structure (pedro miguel graben) composed of seven normal dextral faults: ribeirinha (r) (fig. 5), chã da cruz (cc), lomba grande (lg), ribeira do rato (rr), rocha vermelha (rv), espalamaca (e) and flamengos (f) faults. the graben is partially fossilized by holocene pyroclastic fall and flow deposits from the caldeira formation in the central area of the island. other wnw-ese important faults are located south of the caldera (lomba do meio lm, and lomba de baixo lb, faults) and in the western part of the island (ribeira do adão ra; ribeira das cabras rc; ribeira funda rf, and capelo c, faults). the conjugate fault system, trending nnwsse to nw-se, composed of normal left lateral faults, is secondary in importance: geomorphic expression is less developed and lengths are smaller. between the southern rim of the caldera and the lomba do meio fault these structures are more abundant (inset in fig. 4) displacing and controlling the smaller order drainage. the longer faults are those of águacutelo (ac), salão (s), and cedros (cd). 3.1.2. historical seismicity the 1958 earthquake swarm there are no records of earthquakes felt in faial until the 1614 event that struck terceira island. however, significant local earthquakes only occurred during the 20th century, almost 500 years after settlement. the 1924 earthquake, the 1926 seismic swarm, and the 1998 event caused important damage in eastern faial. in 1958, a seismic swarm, contemporary of the capelinhos eruption, caused buildings 739 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) 740 josé madeira and antónio brum da silveira f ig . 4 . s im pl if ie d to po gr ap hy a nd g eo lo gy o f f ai al i sl an d (a da pt ed f ro m s er ra lh ei ro e t a l. , 19 87 , an d m ad ei ra , 19 98 ). v ol ca no s tr at ig ra ph ic u ni ts : r b r ib ei ri nh a v ol ca ni c c om pl ex ; c d c ed ro s v ol ca ni c c om pl ex ; a l a lm ox ar if e f or m at io n; p p um ic e fa ll , ph re at ic , ph re at om ag m at ic , br ec ci a an d su rg e de po si ts f ro m c al de ir a f or m at io n; i 1 20 0 ye ar s b p p um ic e fa ll , ig ni m br it e an d as so ci at ed l ah ar s fr om c al de ir a f or m at io n; c p c ap el o v o lc an ic c o m p le x , in cl u d in g 1 7 5 2 a n d 1 9 5 7 h is to ri c er u p ti o n s. i d en ti fi ca ti o n o f fa u lt s: r r ib ei ri n h a; c c c h ã d a c ru z; l g l o m b a g ra n d e; r r r ib ei ra d o r at o ; r v r o ch a v er m el h a; e e sp al am ac a; f f la m en g o s; l b l o m b a d e b ai x o ; l m l o m b a d o m ei o ; r a r ib ei ra d o a d ão ; c c ap el o ; r c r ib ei ra d as c ab ra s; r f r ib ei ra f u n d a; a c á g u ac u te lo ; c d c ed ro s; s s al ão . in se t sh ow s d et ai l o f l o m b a d o m ei o fa u lt z o n e an d l o ca ti o n o f tr en ch ( tr ia n g le s ar e m ai n g eo d et ic m ar ke rs : c g c ab eç o g o rd o ; l m l o m b a d o m ei o ). c o n to u r li n es e ve ry 2 0 0 m . collapse and ground deformation along faults in central and western areas of the island. the 1958 seismic swarm started in may 12, and continued until june. during the first three days of most intense seismic activity, 450 earthquakes, some of which reaching intensities viii and ix (mcs-17 scale) in the epicentral region (western faial), were felt. seismicity caused total collapse of 508 houses and heavy damage in 1000. housing was (and still is in many rural areas) characterised by low quality, highly vulnerable, one or two storeys high, stone buildings (a and b vulnerability classes according to the 1998 european macroseismic scale; grünthal, 1998). the stronger earthquakes caused surface deformation along several faults (capelo, lomba do meio, lomba de baixo, ribeira do adão, ribeira das cabras, lomba grande, and espalamaca); mainly ground cracking but, locally, 0.5 to 1 m vertical displacements were reported. photographs of ground cracks also suggest right lateral slip (fig. 6a,b; zbyszewski and ferreira, 1959). widening or narrowing of ground fissures during the first two weeks of the seismic swarm is also referred (tazieff, 1959; zbyszewski and ferreira, 1959). however, these effects were never studied in detail or mapped because attention was focussed on the ongoing eruption. at the epoch only 3 seismic stations existed in the archipelago (installed in 1902 in s. miguel and faial, and in 1932 in terceira; silveira, 2002), but the equipment in faial was inoperative at the time. presently a few tension cracks still persist along fault scarps in the up-thrown block of lomba do meio fault. 3.1.3. geometry, kinematics and geomorphic expression of lomba do meio fault the southern slope of the caldeira volcano is crossed by the north dipping lomba do meio (inset in fig. 4; fig. 7) and lomba de baixo faults. to the east, the lomba do meio fault links with the espalamaca fault through a restraining bend that follows the west and north slopes of the cabeço gordo height, which is 741 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) fig. 5. fault scarp of ribeirinha fault, in ne faial, viewed from the west. ribeirinha village in the down threw block gives the scale. the observed maximum height of scarp is 120 m in the centre of photograph. interpreted as a push-up structure. to the west, it links to capelo fault, a fracture system defined by the alignment of cones and craters of capelo volcanic complex (< 10 ka), displaying right lateral en échelon geometry, which extends to capelinhos volcano in the western tip of faial. in this particular area, capelo fault is expressed by tens of sub-vertical fractures, concentrated in a 250 m wide zone, cutting trough the pyroclastic sequence of a surtseyan cone exposed in a paleo-sea cliff east of capelinhos volcano. these fractures display an accumulated ne down throw of 50 cm. thus, the capelo, lomba do meio and espalamaca faults constitute three 742 fig. 7. fault scarps of lomba do meio fault, in central faial, viewed from the east. the main fault scarp is 50-60 m high, but is amplified by fluvial incision. notice dextral en échelon secondary scarps in up-thrown block. trench was open in front of the most distant electric pole (left side of photo). josé madeira and antónio brum da silveira fig. 6a,b. examples of surface deformation produced during the may 1958 seismic swarm in the island of faial: a) pull apart tension cracks (a) in dextrally sheared dirt road in praia do norte. nnw-sse conjugate system (b) is also present; b) surface rupture along fault scarp in cabeço gordo area. note normal dextral displacement of surface irregularities (a) and fractures with the orientation of conjugate system (b). similar features were observed in faial after the july 9th, 1998, earthquake. photographs reproduced from zbyzewski and veiga ferreira (1958) with kind permission of instituto geológico e mineiro. a b sections of the same tectonic structure, characterized by normal dextral slip. the lomba do meio fault geomorphic expression is a 50 m high, north facing, fault scarp coupled with a set of right lateral en échelon secondary fault scarps on the up-thrown block (fig. 7). a trench was open across one of these secondary en échelon scarps, where surface rupture was reported in the 1958 seismic swarm. other reasons for this location were the size of the main lm fault scarp, fresh aspect and pyroclastic constitution of the secondary fault scarps. 3.1.4. paleoseismic analysis of lomba do meio fault the trench exposed a sequence of pyroclastic deposits and colluvial wedges displaced by a fault zone composed of three main fault planes (a1, b and c) defining a graben and horst structure (figs. 8 and 9). fault plane b is responsible for the fault scarp, which is slightly degraded. stratigraphy is composed of trachyte (1, 2, 3, 4, 5, 6, 7) pyroclastic fall and flow deposits, basaltic ash fall layers (9, 11), and colluvial wedges (8, 10, 12, 13); the sequence ends with the topsoil. only one radiocarbon age was derived from the trench; however, it was possible to correlate some of the volcanic deposits exposed in the trench with volcanic layers radiocarbon dated outside the trench (madeira et al., 1995; table iv). on the basis of these stratigraphic correlations, 2. intervals of dendrochronologically calibrated radiocarbon ages of units 4, 5 and 7 are: unit 4-1878 to 1420 cal b.c.; unit 5-259 to 536 cal a.d.; unit 7-890-1040 cal a.d. for unit 6 a radiocarbon age of 1500 ± 50 bp was determined, but no calibrated dates are available. unit 12 was dated using a fragment of wood preserved in the base of colluvial wedge that yielded a 2. calibrated age interval of 12901640 cal a.d. the evolution of the fault zone was deduced from geometric analysis, nature, and composition of the deposits exposed on the trench, and indicates the following events (graphically represented in fig. 10a-o): a) deposition of pyroclastic sequence 1 to 5 (deposit 5 is 259 to 536 cal a.d.). b) surface rupture at fault planes a1, a2 and c (seismic event 1.-.se1) after deposition of pyroclast 5 and before the erosive episode that created the unconformity on which rests pyroclasts 6 (1500 ± 50 bp). se1 is marked by different thickness of layer 5 preserved below layer 6: layer 5 was totally eroded from the higher block (north of fault c) and partially removed south of fault c, presenting different thickness in the central (50 cm) and southern (at least 65 cm) tectonic blocks. this indicates that normal separation on the faults was at least 50 cm in fault c and 15 cm in fault a. the event horizon (the top of 5) marking this rupture was not preserved due to erosion after se1. c) erosion of fault scarps with levelling of topography. deposition of pyroclast 6. formation of a soil on 6. deposition of layer 7 (between 890 and 1040 cal a.d.). d) surface rupture on fault plane a1 and a2 with normal separation of 80 + 15 cm (se2). the 80 cm displacement in fault a1 was determined from the separation in the base of layer 7, taking into account the thickness of layers 5 and 6, preserved between faults b and c, and missing in the block between faults a1 and b; the 15 cm separation were measured in lithic rich layers in pumice 4 displaced by fault a2. e) erosion degrades fault scarp and trun cates layer 7. f ) surface rupture on fault plane b (se3)(*); normal separation must have exceeded 2 m in order to expose pumice deposit 4 (which will contribute to the formation of colluvial wedge 8) in the fault scarp; this value was obtained from the thickness of layers 5, 6, 7 added to the preserved thickness of colluvial wedge 8. 743 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) (*) there is no preserved event horizon for se2 (the top of layer 7), which we believe was eroded between se2 and se3. in fact, there is no direct evidence to separate the two seismic events. if we consider surface rupture in both faults (a1 and c), a narrow horst structure would be created and a synthetic fault, generating a fault scarp further south, would be needed to root fault a. there is no evidence of such a scarp, which suggests that se2 is an earlier event. furthermore, horst structures are not observed in any of the up-thrown faulted blocks in the archipelago. . . . . . . . . fig. 8. simplified and reduced (originally at the 1/10 scale) map of the lomba do meio trench west wall showing three main faults related to lm fault zone. fault b is related to the present fault scarp, which is slightly degraded. stratigraphic sequence exposed in the trench is composed of: 1 trachyte hydromagmatic explosion breccia; 2 trachyte explosion breccia (or colluvium?); 3 trachyte weathered ash layer (or colluvium?); 4 trachyte pumice fall deposit, with a massive structure except for a few lithic rich layers; 5 trachyte massive phreatomagmatic pyroclast composed of fine ash and lithics; 6 trachyte weathered pyroclast grading from lapilli to fine ash overlying an erosive surface cut on 5 and 4; 7 trachyte phreatomagmatic surge deposit with alternating fine ash and lapilli layers; the base is an erosive surface; 8 wedge shaped colluvium made of pumice and lithics from level 4; the base is an erosive surface; 9 basaltic fine ash fall layer deposited on an erosive surface; the ashes fell into, and partially filled, open cracks below; 10 wedge shaped colluvium made of pumice and lithics from level 4 and basaltic ash from level 9; the base is an erosive surface; 11 discontinuous basaltic fine lapilli layer; it rests on 4, 7, 9 and 10 trough an erosive surface; 12 wedge shaped colluvium containing pumice and lithics from level 4 and basaltic lapilli from level 11; 13 wedge shaped colluvium containing pumice and lithics from level 4 and turfaceous material similar to that of the soil developed on 12; the sequence ends with the present turfaceous soil which was displaced by fault plane b during the 1958 seismic swarm. inset shows slope profiles across the fault and location of trench: a smaller scale profile across lomba de baixo (lb) and lomba do meio (lm) faults (vertical and horizontal scales equal; vertical scale indicates altitude in metres); b profile of main lomba do meio fault and associated secondary faults in up-thrown block (dashed lines represent original topography before erosion). radiocarbon calibrated dates of deposits (2 intervals) indicated: dates obtained outside the trench, attributed by correlation of deposits, in grey rectangles. josé madeira and antónio brum da silveira 744 745 g) fault scarp retreat and formation of colluvial wedge 8, composed of pumice of layer 4 and basaltic lithics from layers 5, 6 and 7. deposition of a basaltic ash layer (9) correlative of a basaltic eruption. erosion and partial removal of basaltic ashes. h) surface rupture in fault planes b and c (se4); in fault b the value of normal separation is unknown, but must have exposed pumice of layer 4 in the fault scarp; in fault plane c vertical separation was equal or smaller than 9 cm. i) fault scarp retreat and formation of colluvial wedge 10, composed of ashes from layer 9, pumice from 4, and lithics from 5, 6 and 7. deposition of basaltic ashes (11) correlative of a basaltic eruption. partial erosion of basaltic ashes that became discontinuous. j) surface rupture of fault plane b with normal separation of unknown value (se5). k) fault scarp retreat and formation of colluvial wedge 12, composed of ashes from 11, pumice from 4, and basaltic lithics. a soil developed on the colluvium. calibrated radiocarbon age of a wood fragment from the base of the colluvium is 1290-1640 cal a.d. l) surface rupture in fault planes b (unknown separation) and c (vertical separation of 11 cm) (se6). m) fault scarp retreat and formation of colluvium 13, composed of pumice, basaltic ashes, basaltic lithics and turfaceous soil fragments from layer 12. n) erosion truncates colluvia 12 and 13; colluvium 13 became discontinuous. development of present top soil (14). fig. 9. oblique view lomba do meio trench west wall from outside the trench. a small graben, in the centre of the trench, is bound by faults b and c. darker deposit in centre of graben is basaltic ash 9. to the left (south) of fault b is pumice 4. to the right of fault c deposits 6, 7 and basaltic ashes 9 and 11 are easily distinguishable. a pen (enclosed by circle) gives scale. active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) 746 josé madeira and antónio brum da silveira t ab le iv . r ad io ca rb on a ge s of s eq ue nc es t re nc he d in f ai al , p ic o an d s . jo rg e. l ab r ef . is la nd f ie ld r ef . m at er ia l a ge c al ib ra te d da te 1 in te rv al 2 in te rv al t re nc h (y ea rs b p ) un it ( *) ic e n -9 61 f ai al fa 4a 2 p ea t in p al eo so l 33 20 ± 90 16 21 c al b .c . (a ) 17 37 -1 71 3 ca l b .c . 18 78 -1 71 3 ca l b .c . 4 17 07 -1 71 6 ca l b .c . 18 25 -1 72 9 ca l b .c . 17 90 -1 42 0 ca l b .c . ic e n -9 94 f ai al fa 8 w oo d 16 60 ± 60 41 2 ca l a .d . (b ) 34 835 4 ca l a .d . 25 929 1 ca l a .d . 5 37 643 0 ca l a .d . 32 153 6 ca l a .d . ic e n -1 11 5 f ai al fa 19 p al eo so l (e xt ra ct ) 15 00 ± 50 nd nd nd 6 ic e n -9 51 f ai al fa 9 c ha rc oa l 10 40 ± 60 99 9 ca l a .d . (a ) 96 810 21 c al a .d . 89 010 40 c al a .d . 7 ic e n -9 50 f ai al fa 11 w oo d 46 0 ± 90 14 37 c al a .d . (a ) 14 03 -1 48 1 ca l a .d . 12 90 -1 64 0 ca l a .d . 12 ic e n -1 19 1 p ic o p i1 c ol lu vi um ( ex tr ac t) 24 10 ± 13 0 nd nd nd 7 ic e n -1 19 2 p ic o p i2 c ol lu vi um ( ex tr ac t) 11 70 ± 35 0 nd nd nd 5 ic e n -1 19 9 p ic o p i3 c ol lu vi um ( ex tr ac t) 52 50 ± 70 nd nd nd 3 ic e n -1 19 7 p ic o p i4 f au lt f il li ng ( ex tr ac t) 89 0 ± 40 nd nd nd f au lt a ic e n -1 19 6 p ic o p i4 f au lt f il li ng ( re si du e) 11 40 ± 90 nd nd nd f au lt a ic e n -1 19 0 p ic o p i5 p ea t (e xt ra ct ) 16 60 ± 60 nd nd nd 4 ic e n -1 19 8 p ic o p i6 p ea t (e xt ra ct ) 19 10 ± 50 nd nd nd 6 ic e n -1 19 5 p ic o p i6 p ea t (e xt ra ct ) 25 60 ± 60 nd nd nd 6 ic e n -1 19 3 p ic o p i7 p ea t (e xt ra ct ) 16 0 ± 12 0 nd nd nd 8 ic e n -1 19 4 p ic o p i7 p ea t (e xt ra ct ) 87 0 ± 16 0 nd nd nd 8 s ac -1 35 0 s . jo rg e s j1 6 p al eo so l (e xt ra ct ) 37 40 ± 50 21 37 c al b .c . (c ) 21 97 -2 11 0 ca l b .c . 22 86 -1 97 5 ca l b .c . 2 20 89 -2 03 8 ca l b .c . s ac -1 32 8 s . jo rg e s j5 p al eo so l (e xt ra ct ) 16 60 ± 60 41 2 ca l a .d . (b ) 34 044 0 ca l a .d . 24 854 5 ca l a .d . 3 s ac -1 33 0 s . jo rg e s j7 p ea t in c la y (e xt ra ct ) 75 0 ± 45 12 82 c al a .d . (b ) 12 55 -1 29 2 ca l a .d . 12 21 -1 30 3 ca l a .d . 5 s ac -1 32 9 s . jo rg e s j6 p al eo so l (e xt ra ct ) 70 0 ± 70 12 93 c al a .d . (b ) 12 77 -1 31 0 ca l a .d . 12 21 -1 40 6 ca l a .d . 4 13 53 -1 38 5 ca l a .d . s ac -1 32 3 s . jo rg e s j1 a w oo d 51 0 ± 10 0 14 26 c al a .d . (b ) 13 21 -1 34 0 ca l a .d . 12 90 -1 63 8 ca l a .d . 5 13 93 -1 44 3 ca l a .d . s ac -1 34 3 s . jo rg e s j9 a w oo d 38 0 ± 40 14 83 c al a .d . (b ) 14 52 -1 52 0 ca l a .d . 14 40 -1 64 1 ca l a .d . 8 15 69 -1 62 7 ca l a .d . s ac -1 34 4 s . jo rg e s j9 b p ea t (e xt ra ct ) 15 0 ± 40 16 86 ,1 73 8, 18 09 , 19 29 , 16 75 -1 77 7 ca l a .d . 16 63 -1 95 5 ca l a .d . 8 19 54 c al a .d . (b ) 17 98 -1 82 2 ca l a .d . 18 33 -1 88 2 ca l a .d . 19 12 -1 94 4 ca l a .d . 19 53 -1 95 4 ca l a .d . r ad io ca rb on a ge s by i ns ti tu to t ec no ló gi co n uc le ar ( fo rm er i ns ti tu to d e c iê nc ia s e e ng en ha ri a n uc le ar – i c e n ), s ac av ém , p or tu ga l. c al ib ra ti on cu rv es : (a ) s tu iv er a nd p ea rs on ( 19 86 ); ( b ) s tu iv er a nd p ea rs on ( 19 93 ); ( c ) p ea rs on a nd s tu iv er ( 19 93 ); n d no t de te rm in ed . (* ) l oc at io n of s am pl es in m ad ei ra e t a l. ( 19 95 , 19 98 b) a nd m ad ei ra ( 19 98 ). o) surface rupture on fault plane b (vertical separation of 15 cm?). this event (se7) is assigned to the may 1958 seismic swarm. thus the lomba do meio trench revealed 7 surface rupturing seismic events in 1606 ± 138 years that can be summarized as follows: 1) the older event occurred between 259 to 536 cal a.d. and about 400 to 500 years before deposition of layer 7 (890 to 1040 cal a.d.) involving fault planes a1, a2, and c and an accumulated normal separation exceeding 65 cm. 2) the following four events occurred between 890 to 1040 cal a.d. and 1290 to 1640 cal a.d.; the second event reactivated fault planes a1 and a2 and produced vertical separations of 80 and 15 cm respectively; the third event produced rupture in fault plane b and a separation exceeding 2 m in order to expose the pumice deposit in the fault scarp; the fourth event reactivated fault planes b and c with unknown normal separation (probably > 70 cm, deduced from the preserved thickness of colluvium 10) in fault b and 9 cm in fault plane c, producing 60 cm of down-throw to the north; the fifth event involved fault plane b producing normal separation of unknown value (probably > 60 cm) and may be related to deposition of colluvial wedge 12 (between 1290 and 1640 cal a.d.); this event preceded settlement. 3) the sixth event reactivated fault planes b (unknown normal separation, possibly 50 cm) and c (vertical separation of 11 cm). there is no historic written record of this event, which probably occurred just before settlement (around 1450 a.d.). 4) the last event involved fault plane b during the may 1958 seismic swarm, producing a vertical separation of 15 cm at the trench site. time distribution of the 7 events shows one cluster of 5 events in a 510 ± 100 calendar years period, separated by 500 years intervals between the first and the last events. the average recurrence interval is 229 ± 20 years, but the recurrence interval during the . . . . . . . . fig. 10a-o. sequence of events (tectonic, depositional and erosional) leading to present day geometry exposed in lomba do meio trench. radiocarbon-dated deposits and surface rupturing seismic events (se 1-7) indicated. full description in text. a ecb d f jhg i k oml n 747 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) cluster is 102 ± 20 years with clusters separated by approximately 500 years intervals. the exposed fault planes are certainly linked to the main lomba do meio fault zone at depth, in which antithetic faults develop in relation to synthetic splays of the main plane. fault plane c should branch into fault plane b, and fault planes a1 and a2 may be linked to an undetected synthetic fault further south. accumulated slip in the sampled 1600 years period is > 1.10 m in fault planes a1.+.a2, in fault plane b it was 4.20 m, and 0.70 m in fault plane c. as planes a and c are antithetic to the lomba do meio main fault, the trenched portion of the fault zone produced a net downthrow of 2.40 m to the north in 1600 years. however, after 890 to 1040 cal a.d., fault plane b accumulated 4.20 m, while in fault plane c only 0.20 m of antithetic normal slip occurred, thus yielding 4 m of down throw to the north in a 1113 to 963 years period. slip rate deduced from lomba do meio trench data is 3.6 to 4.2 mm/year. considering that the lomba do meio main fault produced 50-60 m down throw in pyroclasts from caldeira formation in the last 10-15 ka a longerterm slip rate of 3.3 to 6.0 mm/year is obtained. both slip rate values are of the same order of magnitude. the detected displacements are just the observable normal component of slip (no markers for strike slip component were found), in just a few strands of the entire fault zone. total net slip is certainly larger. 3.2. pico the island of pico is 46 km long, has a maximum width of 15.8 km and reaches an altitude of 2351 m at ponta do pico (pico volcano). the sub aerial part of the island is composed of two central volcanoes and one fissural volcanic zone (madeira, 1998; nunes, 1999). the older shield volcano (topo volcano) is partially dismantled by landslides, displaced by faulting, and covered by younger volcanics. it is composed of ankaramitic and basaltic lava flows assembled under the designation of lajes volcanic complex (lj in fig. 11); a single published k/ar date yielded an age of 250.±.40 ka (baubron, 1981, in chovelon, 1982). an intermediate volcanostratigraphic unit, the calheta do nesquim volcanic complex (cn in fig. 11), comprises fissural basaltic strombolian scoria cone alignments and associated lava flows; this unit outcrops in several areas on the eastern part of pico. three k/ar ages are available for the calheta do nesquim volcanic complex ranging from 270 ± 150 to < 25 ka (féraud et al., 1980; baubron, 1981 in chovelon, 1982). the madalena volcanic complex is the younger unit in pico, which includes three historic eruptions in 1562, 1718 and 1720; very fresh morphology suggests a holocene age which is confirmed by radiocarbon dates younger than 6000 years (madeira, 1998; nunes, 1999). structurally it can be divided into two sub-units: the pico volcano and the east fissural zone members (respectively mp and mf in fig. 11). pico is a stratovolcano displaying a pit crater on its summit. the east fissural zone is composed of several alignments of hawaiian/strombolian scoria cones and related lava flows; these flowed over cliffs cut in older units and originated lava deltas. tectonic structure is characterized by two fault systems (fig. 11). the main wnw-ese trending structures are the normal dextral lagoa do capitão (lc) and topo (t) fault zones that progressively merge to the east; these define a graben structure, narrower and shallower than pedro miguel graben (in faial island). the western portion of the graben is completely covered by the recent pico stratovolcano (< 10 ka) and to the east it is partially filled by lava flows and cones of the east fissural zone member of madalena volcanic complex. most structures on the eastern plateau of pico, whose morphology is marked by volcanic alignments and short fault scarps, belong to this system. the nnw-sse trending conjugate faults are less abundant and are marked, mainly, by volcanic alignments. although scarce, there is evidence of normal left lateral, oblique slip. the lomba de fogo-s. joão (lf) fracture, along which the 1718 volcanic eruptions occurred, and the santo antonio (sa) volcanic alignment are the most obvious structures. at the outcrop 748 josé madeira and antónio brum da silveira 749 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) f ig . 11 . s im pl if ie d to po gr ap hy a nd g eo lo gy o f p ic o is la nd ( ad ap te d fr om m ad ei ra ,1 99 8) . v ol ca no s tr at ig ra ph ic u ni ts : l j l aj es v ol ca ni c c om pl ex ; c n c al he ta d o n es qu im v ol ca ni c c om pl ex ; m ad al en a v ol ca ni c c om pl ex : m p p ic o v ol ca no m em be r; m f e as t f is su ra l z on e m em be r; d at es i nd ica te h is to ri c er up ti on s. i de nt if ic at io n of m ai n fa ul ts : l c l ag oa d o c ap it ão ; t t op o; l f l om ba d e f og os . jo ão ; s a s an to a nt ón io . c ur ve d st ru ct ur es a re a nc ie nt l an ds li de s ca rs a t s . m at eu s an d a rr if e. d as he d li ne s re pr es en t hi dd en f au lt s an d ra di al f ra ct ur es o f p ic o vo lc an o. i ns et s ho w s de ta il o f l ag oa d o c ap it ão f au lt z on e an d lo ca ti on o f tr en ch . c on to ur l in es e ve ry 2 00 m . scale, several faults from this system were observed at cabeço vermelho (cv) quarry and in lagoa do capitão trench. most scoria cones occur along wnw-ese structures, but the larger cones and pico volcano are located at the intersection of these with the nnw-sse conjugate faults. at least two large gravitational (flank collapse) structures were observed on the flanks of the pico stratovolcano and topo shield volcano: the landslide scars of s. mateus and arrife. 3.2.1. geometry, kinematics and geomorphic expression of lagoa do capitão fault the lagoa do capitão fault zone runs wnw-ese for at least 30 km in the central and eastern areas of the island of pico. it has complex map geometry, with duplexes and splays, and presents a set of south facing, 9 km-long, fault scarps (inset in fig. 11) that reach a maximum height of 20 m. towards the east, the scarps disappear under recent scoria cones, one of which formed during the 1562-1564 eruption, but the fault trace is marked by alignments of craters. several sag ponds are associated with this fault zone, the most important of which is lagoa do capitão (captain’s lake). to the west, the holocene volcanic pile of pico stratovolcano buries the fault. the lagoa do capitão fault zone displaces right laterally a few older scoria cones (for example cabeço do caveiro; fig. 12a,b), indicating an oblique, dextral normal, slip. 3.2.2. paleoseismic analysis of lagoa do capitão fault an 8.5 m long trench was open across the main strand of lagoa do capitão fault, about 400 m east of the lake (fig. 13). the sequence exposed is composed of alternating layers of basaltic pyroclasts and colluvium, overlying a basement of basaltic lava flows: 1 pyroxene/ olivine rich basalt aa lava flow; 2 pyroxene rich basalt scoria and lapilli fall layer; 3 colluvium deposit containing basalt clasts (from 1?) and lapilli (from 2?) in clay matrix; a paleosol developed on this deposit is preserved in places; 4 discontinuous, strongly weathered, pyroxene rich, basalt lapilli fall layer; 5 colluvium deposit containing lapilli from 4 in clay matrix; 6 basalt lapilli fall layer similar to 4; 7 colluvium containing lapilli from 4 and 6; 8 fine, pyroxene rich, basalt lapilli layer, containing 750 fig. 12a,b. vertical aerial photograph of cabeço do caveiro region in the island of pico showing an example of strike-slip evidence: a) uninterpreted photo; b) interpreted photo showing base (dotted lines) and craters (arcs) of scoria cones and fault traces. cabeço do caveiro cone is right laterally displaced. josé madeira and antónio brum da silveira a b dispersed scoria fragments; the sequence ends with the present soil. all contacts between layers are erosive. the face of the scarp is cut in lava flows; the same four layers of lapilli are found overlying the lava flows in the up thrown block, but there they are separated by paleosols instead of colluvia (inset b, fig. 13). the topographic position of the colluvial deposits (higher than the depressed region located to the south) and its absence in the up-thrown block confirms that they derive from the fault scarp, although it is not possible to relate them to scarp forming events. the irregular geometry of the colluvial deposits is a consequence of erosive episodes that occurred during intervals between deposition of successive lapilli layers and colluvia. neither the original geometry of colluvial deposits, nor that of pyroclastic layers is preserved. the trench revealed two main faults, one directly related to the scarp (fault a), and another (fault b), 4 m to the south, without any significant geomorphic expression (fig. 13). each fault is composed of numerous slip surfaces, predominantly vertical, sometimes with open voids, presenting small vertical separations that suggest dominant strike slip component associated to down throw to the south. active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) fig. 13. simplified and reduced (originally at the 1/10 scale) map of the lagoa do capitão trench west wall showing two faults (a and b) associated to lc fault zone. the fault scarp, slightly degraded, is related to the northern fault. stratigraphic sequence is composed of alternating layers of basaltic pyroclastic fall deposits (2, 4, 6, 8) and colluvial wedges (3, 5, 7; in grey), overlying a basement of basaltic lava flows (1); the sequence ends with present topsoil. all contacts are erosional. inset a: slope profile across the fault and location of trench. inset b: pyroclastic sequence (2, 4, 6, and 8), separated by dated paleosols and overlying lava flow 1, observed in up-thrown block. radiocarbon dated deposits indicated (in years bp). 751 samples for radiocarbon analysis were collected from the colluvial layers in the trench and the paleosols separating the lapilli layers in the up-thrown block (fig. 13; table iv). the results revealed radiocarbon/relative age incompatibilities, and no age correlation between colluvia and paleosols separating equivalent lapilli layers; nonetheless, all samples yielded holocene ages ( 5300 -1000 years old; madeira, 1998), including material fallen into voids in fault a (1140 ± ± 90 bp), indicating recent surface faulting. this agrees with the observation that the topsoil seems displaced by fault a. no paleoearthquakes were deduced from the trench because of absence of reliable stratigraphic markers and cumulative vertical separations from the exposed sequence. the pyroclastic and colluvial layers, where they are cut by the fault planes, show variable separations and thicknesses suggesting dominant strike slip component. the vertical separation between correlative lapilli layers exposed in the trench and outcropping on the up-thrown block, is apparent, as the pyroclasts were probably emplaced over a pre-existing scarp cut on lava flows. 3.3. s. jorge the island of s. jorge is a 55 km long fissural active volcano, with a maximum width of 6.7 km, and reaching an altitude of 1053 m at pico da esperança. the volcanic products in s. jorge, all basaltic in nature, are divided into three stratigraphic units (forjaz and fernandes, 1970, 1975; forjaz et al., 1970, 1990; madeira, 1998). the older topo volcanic complex (tp in fig. 14) constitutes the eastern part of the island. it is a fissural volcanic zone composed of scoria cones and lava flows. marine erosion already destroyed the northern half of the lava pile and most of the axial cones, creating steep cliffs 400 to 800 m high. cones are only preserved in the east tip of s. jorge. four published k/ar ages (féraud et al., 1980) range from 550 ± 60 ka in lavas from the base of the pile, at sea level, to 110 ± 70 and 140 ± 50 ka in the higher lava flows. the rosais volcanic complex (ro in fig. 14) constitutes the basement of the western part of the island and is similar to topo volcanic complex. the scoria cones in the rosais axial zone are less degraded and its products less weathered than those of topo suggesting that this unit is younger. no ages are available for this unit and, because younger lavas cover the contact between rosais and topo complexes, it is not possible to establish a field relation in order to confirm the assumed stratigraphic relation. the younger unit is manadas volcanic complex (mn in fig. 14). it is also a fissural volcanic unit, composed of two alignments of hawaiian/ strombolian and surtseyan cones (one along the axis of the island and another diverging from the former towards the south coast) and the related lava flows. three eruptions occurred in historic times: two on land in 1580 and 1808 and a probable submarine event in 1964 (fig. 14). the manadas lavas flowed over cliffs cut in rosais volcanic complex creating lava deltas (locally called fajãs). radiocarbon dates indicate a holocene age for this unit; ages of 10 prehistoric volcanic events range from 5310 ± 80 to 700 ± 70 years bp (madeira et al., 1998b). two main normal dextral wnw-ese fault zones dominate the younger western half of the island: picos (p) and pico do carvão (pc) fault zones (figs. 14 and 15). on the older eastern half, a set of faults with the same direction defines a graben structure; the most important tectonic structure in that region is the urze-s. joão (usj) fault which displays a 10 km long continuous scarp. the two regions are separated by the left lateral normal nnw-sse ribeira seca (rs) fault represented by a west facing very degraded scarp. the volcanic axis of the island is left laterally offset by at least 3 km, although the fault seems presently inactive. three other nnw-sse structures are marked by alignments of scoria cones west of ribeira seca fault. 3.3.1. historical seismicity the island of s. jorge has been struck by two high magnitude earthquakes since settlement: the july 9th, 1757, with an estimated magnitude m = 7.4 (machado, 1949) and the january 1st, 1980, m = 7.2 event (hirn et al., 752 josé madeira and antónio brum da silveira 753 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) f ig . 1 4. s im pl if ie d to po gr ap hy a nd g eo lo gy o f s . jo rg e is la nd ( ad ap te d fr om m ad ei ra , 19 98 ). v ol ca no s tr at ig ra ph ic u ni ts : t p t op o v ol ca ni c c om pl ex ; r o r os ai s v ol ca ni c c om pl ex ; m n m an ad as v ol ca ni c c om pl ex ; da te s in di ca te h is to ri c er up ti on s. i de nt if ic at io n of m ai n fa ul ts : p p ic os ; p c p ic o do c ar vã o; r s r ib ei ra s ec a; u sj u rz es . jo ão . in se t sh ow s de ta il o f p ic o do c ar vã o fa ul t zo ne a nd l oc at io n of t re nc h. t he c ra te r of p ic o do c ar vã o co ne i s de xt ra ll y di sp la ce d. c on to ur l in es e ve ry 2 00 m . 1980). in both cases the rupture occurred on offshore faults. the 1980 earthquake, with epicentre between s. jorge, terceira and graciosa, produced important destruction on the eastern half of the island and some tens of deaths. the 1757 historic event was even more destructive: all the eastern region of s. jorge, from the village of calheta to topo, was completely destroyed, killing 1000 people in a population of 5000. this earthquake has been related to fault rupture in s. jorge channel (between s. jorge and pico islands) (machado, 1949), surface rupture in pico do carvão fault (ribeiro, 1982), or a fault rupture close to the north coast of the island (madeira, 1998). the last hypothesis is favoured because a small tsunami, referred in contemporary texts, was observed in terceira and graciosa islands, located north of s. jorge (fig. 2). 3.3.2. geometry, kinematics and geomorphic expression of pico do carvão fault the pico do carvão fault zone is composed of three parallel strands, which cross the island of s. jorge in a n75w direction. it extends for 12 km, from pico do carvão cinder cone, where it merges with pico fault zone (figs. 15 and 16), to morro grande (velas) surtseyan cone; to the west it probably extends to the site of the 1964 submarine eruption, adding, at least, 5 km more to its length. en échelon secondary scarps and a displaced crater indicate that the fault is an oblique, normal dextral, structure. holocene volcanism from the manadas volcanic complex (madeira et al., (1998b) occurred along this structure and its products are locally displaced by the fault. as a result, pico do carvão fault zone exhibits a well developed and 754 josé madeira and antónio brum da silveira fig. 15. oblique aerial photograph of central part of the island of s. jorge, above the axial chain of cones, seen from the west. picos fault zone is marked by alignment of cones, craters and scarplets. pico do carvão fault zone is represented by continuous scarp extending to pico do carvão cone where it merges with picos fault. fig. 16. pico do carvão fault scarp seen from the west, with pico do carvão scoria cone in background. persons (encircled) give scale and mark the location of trench. 15 16 continuous geomorphic expression, represented by alignments of volcanic cones and craters and a fault scarp at the eastern sector; the north facing scarp is 1750 m long and 2.5 to 10 m high (inset in fig. 14; figs. 15 and 16). 3.3.3. paleoseismic analysis of pico do carvão fault an 11.5 m long trench was opened across the pico do carvão fault scarp (fig. 17). it revealed a faulted sequence of basalt lapilli fall deposits (layers 1, 2, 3, 4 and 9) with paleosols developed on top, colluvial deposits from the erosion of the fault scarp (layers 4a, 5, 6, 7 and 8b) and carbonaceous clay sediments interpreted as sag pond deposits (layer 8). the lapilli layers display southward dips, parallel to the slope. the sedimentary deposits related to the fault scarp are gently dipping to the north, or filling the tectonic depression. most of the deposits were radiocarbon dated (table iv; madeira et al, 1998b): layer 2 younger than 2286-1975 cal b.c. (age of the paleosol developed on 1); layer 3 younger than 248-545 cal a.d. (age of the paleosol developed on 2); layer 4 younger than 1221-1406 cal a.d. (age of the paleosol developed on 3); layer 4a older than 1221-1303 cal a.d. (soil developed on 4a); layer 5 1290-1638 cal a.d. (wood fragment in the base of 5); layer 8 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) . . . . . . . . . . . . . . . . fig. 17. simplified and reduced (originally at the 1/10 scale) map of pico do carvão trench east wall. the exposed sequence is composed of: 1 pyroxene rich, basalt lapilli fall layer; 2 pyroxene rich, basalt scoria and lapilli deposit, including a lower massive coarser zone (2a), a well stratified central deposit (2b), and a upper massive zone, altered by pedogenesis (2c); 3 sanidine and pyroxene rich basalt lapilli fall deposit; 4 lithic rich basalt lapilli fall layer; 4a colluvial wedge formed by clay supported lapilli, pyroxene and sanidine crystals from layers 3 and 4; 5 colluvial deposit formed by clay supported lapilli and scoria; 6 colluvial deposit similar to layer 5; 7 colluvial deposit, similar to 5, containing intercalated thin layers of fine basaltic ash; 8 sag pond clayey sediment containing wood fragments, discontinuous peat horizons, and a basaltic fine ash layer (from 1580 eruption), passing laterally to colluvium closer to the fault scarp (8b); 9 discontinuous basalt lapilli fall layer (probably from 1808 eruption). the sequence ends with a thin soil. inset shows slope profile across the fault and location of trench. radiocarbon calibrated dates of deposits indicated (2 intervals). 755 1440-1641 cal a.d. (wood fragment from the base of 8) to 1663-1955 cal a.d. (peat layer some centimetres above an ash layer of the 1580 a.d. eruption); layer 9 lapilli of the 1808 a.d. eruption. the fault plane strikes n63-81w, dips 75°-85° to the nne, and presents an anastomosed geometry (figs. 17 and 18). no striated surfaces were observed. the fault produces 3.5 m of vertical displacement of levels 2 to 4, indicating surface rupture following deposition of layer 4, about 750-700 years ago. since then the fault scarp receded about 1.8 m but maintains a steep inclination, supported by fast growing vegetation. millimetric displacements of thin ash layers in colluvium 7 indicate upward propagation of the fault plane during the 700 to 380 years bp time interval. there are two possible interpretations for the recent tectonic activity on the trenched pico do carvão fault strand. one is that only one major surface rupture occurred 750-700 years ago, producing the 3.5 m of displacement observed on stratigraphic references from layers 2, 3 and 4. another interpretation is that the 3.5 m separation is the sum of displacements produced by more than one surface rupturing events in the period between 700 ± 70 bp to 380 ± 40 bp. there is no way of verifying if layers 5 and 6 accumulated different amounts of normal separation because they are only preserved in the downthrown block. however, the propagation of the fault plane to colluvium 7 is indirect evidence that those colluvia may be tectonically displaced (figs. 17 and 18). different estimates for the average recurrence interval between seismic events producing sur756 josé madeira and antónio brum da silveira fig. 18. photograph of central part of the east wall of pico do carvão trench. fault plane is sharp vertical contact below arrow. to the right (south) of fault plane is lapilli deposit 1 and to the left (north) are deposits 3, 4 and colluvial wedge 4a. above the fault trace is colluvium 7. face displacement derive from the two hypotheses. the first interpretation is that one major earthquake occurred 782-597 years ago, after a time interval of at least 3000 years without surface rupturing events; this is deduced from the fact that levels 2, 3 and 4 show the same amount of normal separation. the second interpretation is that, after 3000 years without surface rupture, more than one event (possibly two, represented by colluvial wedge 4a and colluvium 6?) occurred within an interval of less than 420 years (from 1221-1406 a.d. to 1440-1641 a.d.), producing an accumulated separation of 3.5 m. this may indicate the occurrence of short periods of seismic clustering, as observed in lomba do meio trench, separated by longer periods without surface rupture. recurrence interval between single events or clusters of events is equal or larger than 3000 years. the slip rate in the trenched strand of the pico do carvão fault, taking into account that 3.5 m of slip accumulated in an interval of 3700 years, is of the order of 1 mm/year for the normal component of the fault. 4. conclusions: neotectonics and paleoseismology in this area of the azores plateau, fieldwork observations identified a faulting pattern characteristic of three-dimensional strain (reches, 1983; reches and dieterich, 1983), represented by two main fault systems with oblique slip, each composed of two sets dipping in opposite directions. the fault geometry and kinematics indicate a stress field with the minimum horizontal compressive stress axis ( .3) trending ne-sw. however, permutations between maximum ( .1) and intermediate ( .2) compressive stress axis (nw-se horizontal, and vertical, respectively), may originate transtensile or tensile regimes (reches, 1983). this may explain a history of alternating phases of tectonic activity and phases of volcanism, observed in the archipelago since settlement. in the studied islands, all faults can be considered active (in the sense of boschi et al., 1996) as they displace volcanic sequences younger than 100 ka (féraud et al., 1980; chovelon, 1982). long-term slip rates range from tens of millimetre to some centimetres per year (madeira, 1998) and nearly 70% of the estimates are in the order of 0.10 to 0.40 cm/year. paleoseismic slip rates are in the same order of magnitude (table v). average recurrence intervals were estimated in both lomba do meio and pico do carvão trenches (table v). in lomba do meio, surface rupturing events suggest clustering separated by gaps of roughly 500 years. recurrence interval within cluster is 102 ± 20 years. in pico do carvão, information is less reliable but also suggests that clustering may occur; the time gap before first paleoearthquake is 3000 years, and two earthquakes may have occurred in less than 420 years. in lagoa do capitão the age of deposits filling voids in fault plane a, suggests that the most recent surface rupturing event may have occurred about 1000 years ago, providing a minimum value for the recurrence interval. slip per event values, determined in lomba do meio trench, vary from 0.15 m to more than 2 m (se1 > 0.65 m; se2 0.95 m; se3 > 2.0 m; se4 > 0.60 m; se5 > 0.60 m; se6 > 0.40 m; se7 0.15 m). one, or two, surface rupturing events accumulated 3.5 m of normal slip in pico do carvão trench; in the second hypothesis, considering that colluvial wedge 4a had an original thickness of 1 m at the fault scarp, the two events may have produced 2 m and 1.5 m of slip, respectively. according to the available parameters (slip rate, recurrence interval and slip per event), the degree of fault activity, in the studied islands, is high to moderate (classes 2 to 4 according to slemmons, 1982). 5. discussion and conclusions: seismic hazard assessment seismic hazard assessment in the azores is a complex task. one problem is the determination of strike slip component in oblique slip faults; often, in the lack of markers for strike slip, only the dip slip component is detectable, leading to underestimated net slip values. another complexity is that geologic record 757 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) does not allow the discrimination between coseismic, post-seismic and aseismic slip; this problem is being currently assessed by gps monitoring of a network of 60 markers, strategically implanted on the three islands (displazor, 2002). an additional difficulty is that the interaction between volcanism and faulting complicates the interpretation of paleo surface ruptures; there is evidence that surface deformation associated to eruptions may concentrate along pre-existing faults, leading to surface displacement events that are difficult to distinguish from those produced by coseismic deformation. furthermore, the available empirical relations between seismic rupture parameters and magnitude are not adequate for the particular tectonic environment in the azores plateau (oceanic crust in a transtensile regime), where lithospheric structure is not well understood, and a thin oceanic crust interacts with magmatism. testing the wells and coppersmith (1994) equations with rupture parameters of two recent earthquakes shows that the magnitudes obtained do not differ significantly from those assigned by seismology methods. the 1st january 1980, 6.9 to 7.2 magnitude, earthquake (hirn et al., 1980; moreira, 1991) produced a 50-60 km long surface rupture, based on the epicentral distribution of aftershocks (hirn et al., 1980). applying the surface rupture length (srl) to the srl/m correlation [mw = 5.08 + 1.16 log (srl)] yields 758 josé madeira and antónio brum da silveira table v. estimated slip rates and recurrence intervals of faults from faial, pico and s. jorge. fault / island separation (m) age of slip rates (cm/year) average marker (ky) recurrence interval normal strike net normal strike net (years) ribeirinha 120 80 144 580 or 0.02 or 0.01 or 0.03 or faial 73-40 0.16-0.30 0.11-0.20 0.20-0.36 lomba grande 200 73-40 0.37-0.50 faial rocha vermelha 150 73-40 0.21-0.38 faial 23 50-10 0.05-0.23 espalamaca 145 73-40 0.20-0.36 faial flamengos 115 73-40 0.16-0.29 faial lomba do meio 50-60 15-10 0.33-0.60 500 faial 4 (*) 1.113-0.963 0.36-0.42 lomba de baixo 30-50 15-10 0.2-0.5 500 faial capelo 0.49 1 0.05 faial lagoa do capitão 60 270-25 0.02-0.24 > 1000 pico or 10 or 0.6 topo-pico 44 600 602 290-210 0.015-0.02 0.20-0.29 0.21-0.29 picos-s. jorge > 10 5.3-5.6 > 0.2 pico do carvão 3.5 (*) 3.7 0.1 3000 s. jorge urze-s. joão > 20 < 100 > 0.02 s. jorge separations measured in main volcano-stratigraphic units by cartographic methods, from displaced morphologic markers (madeira, 1998), or in trenches (*). magnitudes mw = 7.05 to 7.14, inside the magnitude interval attributed by the cited authors. magnitude estimation from rupture area (ra) [mw = 4.07 + 0.98 log (ra)] has to deal with unknown, and probably variable, schizosphere thickness in the azores. hirn et al. (1980) indicate a 14 km thickness based on the depth distribution of the 1980 aftershocks, grimison and chen (1986) consider 10 km, while luís et al. (1998), based on gravity data, suggest 7 km. magnitudes estimated from m/ra using the thickness value indicated by hirn et al. (1980) are in better agreement with those assigned to the 1980 event (mw 6.9). the same verification was made relatively to the 6.2 magnitude, july 9, 1998, event that produced a surface rupture estimated as 8-10 km, based on the epicentral distribution of aftershocks (vales et al., 2000). magnitude estimates from srl range from mw = 6.13 to 6.24, while using ra (for a thickness of 14 km) mw = 6.08 to 6.17 are obtained. both srl and ra derived estimates are also significantly close to the assigned magnitude. because of its submarine location, maximum displacement (md) is unknown in both earthquakes so md/m correlation cannot be tested. in the lack of a more appropriate equation, it is assumed that estimation of magnitude from slip per event, using equations such as the wells and coppersmith’s, is the only available for seismic hazard assessment. the mw values obtained, considering total rupture of the known length of the espalamaca/ lomba do meio/capelo (19.5 km), lagoa do capitão (30 km), and pico do carvão (17 km) faults and a seismogenic thickness of 14 km (hirn et al., 1980), range from mw = 6.5 to 6.8 and mw = 6.4 to 6.7 using the m/srl and m/ra relations, respectively. higher, less credible, magnitudes are obtained applying the maximum displacement (md) and moment magnitude (mw) regression equation [mw = 6.69 + 0.74 log (md)] (wells and coppersmith, 1994). considering the observed displacements per event in the lomba do meio trench (0.15 m > 2.0 m), moment magnitudes range from mw = 6.1 to 6.9. pico do carvão trench data suggest either a single, mw = 7.1, event (producing 3.5 m of normal slip) or two, mw = 6.9 and 6.8, events (generating 2 and 1.5 m of slip, respectively). both the md and srl values are underestimated, leading to under evaluation of maximum expected magnitudes. md values, determined in trenches, do not take the strike slip component in account, and may not represent maximum displacements, as they were obtained in a single cross section of the fault. on the other hand, the total length of faults is certainly larger because they extend to offshore regions for which there are no detailed bathymetry. furthermore, averaged recurrence intervals do not express seismic behaviour of the faults through time because of suggested clustering of events; this makes it difficult to assess the probability of future earthquakes. however, the high number of active faults in each island is clear evidence for a high seismic potential in the area. acknowledgements we thank late prof. josé ávila martins of azores university, dr. renato leal, mayor of horta, and dr. manuel vargas, director of serviços de desenvolvimento agrário, in faial, eng. josé ferreira, director of secretaria regional da habitação e obras públicas, and manuel paulino costa, mayor of lajes, in pico, and eng. alfredo alvura and miss isabel jorge, directors of secretaria regional da habitação e obras públicas, and frederico maciel, mayor of velas, in s. jorge, for considerable logistical support. fieldwork was financed by sispor (peam/c/rnt/35/91) and displazor (pocti/32444/cta/2000) research projects. editorial committee of instituto geológico e mineiro kindly authorized the reproduction of photographs in fig. 7. we also acknowledge the contribution of the reviewers and volume editor daniela pantosti to significant improvements in the manuscript. this is a contribution from lattexlaboratório de tectonofísica e tectónica experimental research team. references boschi, e., g. domenico, d. pantosti, g. valensise, r. arrowsmith, p. basham, r. bürgmann, a.j. crone, a. hull, r.k. mcguire, d. schwartz, k. sieh, s.n. 759 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) ward and r.s. yeats (1996): new trends in active faulting studies for seismic hazard assessment, ann. geofis., 39 (6), 1301-1307. buforn, e., a. udías and m.a. colombas (1988): seismicity, source mechanisms and tectonics of the azores-gibraltar plate boundary, tectonophysics, 152, 89-118. chovelon, p. 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(1963/64): list of recorded volcanic eruptions in the azores with brief reports, bol. mus. lab. mineral. geol. fac. ciênc. lisboa, 10 (1), 3-18. zbyszewski, g. (1963): les phénomènes volcaniques modernes dans l’archipel des açores, comunicações dos serviços geológicos de portugal, 47, pp. 227. zbyszewski, g. and o. veiga ferreira (1959): rapport de la deuxième mission géologique sur le volcanisme de l’île de faial, mem. serv. geol. port., 4, 29-55. 761 active tectonics and first paleoseismological results in faial, pico and s. jorge islands (azores, portugal) ........... t h e r i g i d i t y o f t h e e a r t i i ' s i n n e r c o r e k . e . b u l l e n 1. the purpose of tliis paper is to examine and assess, in llie light of recent evidence, the theory lliat the earth's inner core 'ìas a significant rigidity. the presenee of an inner core in the earth is revealed from observations of the seismie pliase pkp in the « sliadow zone » for which the epicentral distance a lies in the range 105" < a < 143". miss i. lehmann ( r ) in 1936, followed by gutenberg and richter ( 2 ) in 1938, atlrihuted these observations to tlie presence of an inner core; and jeffreys ( 3 ) in 1939 applied airy's theory of diffraetion near a caustic to sliow that the alternative theory of diffraetion round the outer boundary of the centrai core was not capable of explaining tlie observations in the shadow zone. the existence of the inner core has been f a i r l y generallv accepted sinee tliis ealculation of jeffreys. 2. the work of jeffreys ( 4 ) on llie p—-velocity, « say, in the centrai core favours tlie view that the inner core is composite, consisting of an outer region f ( 5 ) occupying the range 1250 k m < r < 1390 km, wliere r denotes tlie distance from the eentre, and an inner region g of radius 1250 k m ; (the region e refers to the outer part of the core, for wliich 1390 km < r < 3470 k m ; and the region d to that part of the mantle for which 3470 km < r < 5400 km, approx.). in the solution given by jeffreys, the value of « in f decreases with increase of depth from 10.44 km/sec to 9.5 km/sec (the latter figure corresponds to a rate of decrease of a taken arbilrarily to he linear by jeffreys), while, in g, u increases slowly from 11.16 km/sec at the boundary to 11.3 km/sec at the centre; across ! he boundary between f and g. tliere is a discontinuous j u m p in <t. the existence of the region f in which the velocity gradient is negative is not accepted by ali authors, for example gutenberg (°ì; (the existence of f rests on a hypothesis of reflection at the inner core 2 k . e . b u l l e n boundary whicli jeffreys considers may well he true in the liglit of various supporting evidence). gutenberg's velocity distribution, on the other band, gives a steady increase of a from 8.0 km/see at the outer boundary of the centrai core to 10.1 km/sec at r = 1250 k m , tlien a sharp increase in the gradient of a , w i t h no discontinuous cliange in a itself, resulting in a value of oa eqnal to 11.2 km/sec at about r = 1000 k m ; from bere to the centre, gutenberg fiiicls to he nearly c o n s t a n t . 3. following calculations ( 7 ) on the pressure and density distributions in the earth, the present writer noted ( 8 ) in 1946 that the indicated values of dk/dp, wliere k denotes the incompressi])ility and p the pressure, were not sensibly different in the outer part of the centrai core from the values in the lower mantle down to a depth of 2700 km. (this result is indicated most clearly by consideration of the relation dk/dp = 1 — g-'d0/dr , which applies to a homogeneous material, where g denotes the acceleratimi due to gravity, 0 = — 4 fi -/3, and denotes the s — velocity). moreover, the change in k itself was so small as to be not necessarily significant. ali well-established geophysical data could in fact be fìtted by assuming that k and dk/dp change smoothly between the mantle and the core. at the sanie tiine, it was noted that the materials of the mantle and core are widely different in respect of the density q and rigidity [i. further, in 1946, it was widely accepted that the chemical compositions of the mantle and core are distinct, that the mantle is coinposed of ultrabasic rock, and the outer core of iron or nickel-iron. and, in addition, laboratory investigations at high pressure had sliown that, in general, the differences in the compressibilities of different materials diminish with increasing pressure. it these circumstances, the writer proposed as a trial working hypothesis ("'• 8' n) t h a t : a) the compressibility of a m a t e r i a l and its pressure gradient are independent of chemical composition at pressures and temperatures of the order of tliose prevailing in the earth's deep interior (say below a deptli of 1000 km). 4. when the hypothesis (o) was first set down, it had not been envisaged that there would soon be a serious challenge to the view t h e itlcidity of t h e e a r t h ' s inner core 3 that the inantle and outer core are chemically distinct. following work of kronig, de boer and korringa ( i 0 ), however, ramsey ( n ) and the writer (1 2), on the basis of planetary data, have lield that there is a probability that the region e consists essentially of a pressure modification of ultrabasic rock; (ramsey, but not the present writer, holds that the inner core, also, is caused by pressure). several authors, notably elsasser (1 3), urey ( 1 4 ) and bircb ( l a ) , have put forward opposing arguments to the modified-utrabasic-rock core theory, but the opposing arguments themselves have been shown ( 17) to he subject to several weaknesses; the present writer's view is that it is stili at least moderately probable that the region e and the mantle are not sharply distinct chemically. in these circumstances, the originai hypothesis (a) is possibly too broad a generalisation, so that it becomes necessary ( i 8 , page 58) to consider the less general hypothesis tliat : b) a phase transition occurs at the boundary between the regions d and e of the earth and leaves the compressibility and its pressure gradient unchanged on the two sides of the boundary. at the same tiine it needs to be remarked tliat k, if determined by the relation fc/y = dp/do, would be discontinuous at the transition point itself, so that there are no a priori physical grounds for expecting (b) to hold. thus, on the emprical evidence stated in section 3, ( a ) may well stili be a f a i r approximation for a class of materials whose representative atomic number is close to tliat of the region e. 5. on the hypothesis (a), it is to be expected tliat the values of k and dk/dp in the regions f and g are close to those at the base of e. tliis would, however, be impossible on the results of jeffreys for llie p — velocity a if the inner core is fluid; (we ignore the extremely improbable happening that the density decreases witli increase of depth in tliis part of the earth). on the jeffreys data, a increases suddenly at r = 1250 km by 15 per cent, so that if tlie inner core were composed of fluid for which k = there would be at least a 32 per cent j u m p in k across the boundary. if, on the other band, the inner core possesses significant rigidity u , witli k -f4 n/3 = qa~ , then k will remain continuous across the boundary if u has a value equal to about 25 per cent of that of k in the inner core. the hypothesis (a), in conjunction with the data of jeffreys, thus leads to the theory, stated in 1946 and developed in 1949 and 1950 (8, 9, 18), 4 k . e. b u l l e n tliat the earth's inner core is solid in the sense that description of its elastic hehaviour under stresses of periods of a few seconds requires the presence of a rigidity parameter of order 3.6 x io12 dyn cin-. tliis conclusion lias received support in f u r t h e r work of ramsey ( l t t ). the diminution of a with increase of depili found by jeffreys in the region f, in conjunction with tlie relation k = per, incidentally entails that, in f, eitlier q increases r a p i d l y or else k diminishes rapidly. on tlie hypothesis (a), the former alternative is preferred, the total increase in p in f being 25 per cent. also tlie relatively low gradient of a inside g entails tliat the average density gradient in g is more tlian twice as great as the density gradient in the region e. 6. a consequence of rigidity in the inner core is that tlie inner core coulcl transmit s seismic waves. on tlie basis of the jeffreys data and the hypothesis («}, theoretical tables for a seismic phase pkjkp bave been constructed (2 0), where the symbol j corresponds to s waves in the inner core. companion energy calculation ( 2 1 ) show, however, that s u d i a phase could be only 011 the border of observability. a comprehensive survey by burke-gaffney ( 2 2 ) of the whole series of seismograms recorded at the riverview college observatorv between 1909 and 1952 shows that none of these seismograms quite conforms to the theoretical requirements for a finn identification of the phase pkjkp, thougli a few observations could contribute to a wider statistical examination wliich would include data from a number of observatories. further, the unknown character of the region f leaves it possible that the energy in anv s waves generated in g would be even less tlian the energy calculated ( 2 1 ) 011 the assumed boundary condilions. it follows that the rigidity of the inner core can at best be established by ibis direct seismic metliod only tlirough intensive studies of seismograms followed by the use of searching statistical significance tests. 7. in the meantime, it is desirable to examine the sources of evidence for a solid inner core in some f u r t h e r detail. points on wliich further investigation can usefully be iliade at present i n c l u d e : £) a comparison of ihe effeets of using gulenberg's and jeffreys' observational data on « in the inner core; ii) questions arising from the forms that the compressibilitypressure hypothesis miglit l a k e : t h e itlcidity of t h e e a r t h ' s i n n e r core 5 iii) the hearing on the problem of quantum-mechanical (lata at extreinely high pressures. 8. in considering (i), it will he sufficient to malte an approxiinate calculation hased on gutenberg's velocity data quoted in section 2. we shall represent tliis data by a 2 = 11.22 — a(r—1000)2 , [ 1 ] wliere a = (11.22 — 10.12)/2502 , taking cla/dr to be zero at r = 1000 km, and taking the k m and sec as units. the equation [ 1 ] yields 0.19 km/sec2 for the magnitude of da2/dr wlien r is just less tlian 1250 km, as against 0.019 km/sec2 when r is just greater than 1250 k m , and implies that the magnitude of d j (/c — 4 [i/3)/p{ dr suddenly increases (as the depth increases) by a factor of ten at r = 1250 km. on the hypothesis (a), it would follow that, while il and its gradient are zero in the fluid region above r = 1250 km, |d(4 jx/3p)/o!r| clianges to the value 0.17 km/sec2 just below; i.e. di u j oìdr suddenly rises from zero to 0.13 km/sec2. assuming for (32 ( = u/o) a forni similar to [ 1 ] , we tlien obtain for 1000 k m < r < 1250 km p2 = po2 — b (r — 1000)2, wliere b = 0.13/500, and |30 is the s — velocity at r = 1000 km. since is bere being taken as zero at r = ]250 km, we tlien have (30 evi 4.0 km/sec at r = 1000 km. previous work using the data of jeffreys baci yielded p0 = 4.8 km/sec. the jeffreys data had involved an increase in q from 12 to 15 g/cin:i in the region f (see section 5). if we ignore the possibility of an abnormal increase in density in the range 1000 k m < r < 1250 km ( s u d i increase is neiiber indicated nor precluded on gutenberg's data), tlien the relation = p (32 would, on the hypothesis (a) and gutenberg® data, y i e l d a value of u at r = 1000 km equal to 2.0 x io12 dyn/cm2, approximately, i.e. 0.6 of the value yielded on the jeffreys data. if an abnormal density increase were present, the yielded rigidity would of course he greater. thus, on the hypothesis (a) and gutenberg's data, the minimum rigidity in the innermost 1000 k m of the earth appears to be about 2.0 x io12 dyn/cm2. tliis value is stili comparable witli the rigidity of the mantle, and is appreciably greater than that of surface rocks. 6 k . e. bullen between r = 1000 km and tlie centre, gutenberg's data would, on the hypothesis (a), y i e l d a slightly higher density gradient tlian the jeffreys data because of the less velocity gradient, but the density reaclied at tlie centre need not be much greater than 15 g/cm3. this is closer to the figure preferred by birch (1 5), altliough gutenberg's data do not preclude an appreciably higher value. a further point is that whereas the inain change in ti would take place suddenly on the jeffreys data, it is only the gradient of ;i that would change suddenly on the gutenberg data, the change in u being spread over 250 km. the boundary conditions of the inner core would tlien be very different from tliose assumed in the energy calculations ( 2 1 ) for pkjkp, and the energy transformed into s waves in the inner core would be very much less. tlius, if gutenberg's velocity distribution is approximately correct, the phase pkjkp is quite unobservable, and it would not be possible to establisli the r i g i d i t y of the inner core directly by observations of tliese waves. 9. the point ( « ) of section 7 on the possible limitations of the hypothesis ( « ) of section 3 w i l l now be considered in relation to the inner core. if the view of elsasser, urey and birch that the region e consists of iron is substantiated, then the hypothesis (a) remains, as in 1946, an important working hypothesis. it is l i k e l y to be reliable at least to the extent that, for atomic numbers between about 18 and 29, k and dk/dp at pressures of 1.3 million atmosplieres differ by not more than the uncertainties in k and dk/dp in the writer's earth model a ( 5 i 18). in model a, the formai solution gave a reduction (not necessarily significanti of 5 per cent in k at the boundary between the regions d and e. ramsey ( 1 2 , l f t) prefers a greater change in k at this boufidary, but his argument involves deviation from homogeneity in the region d to an extent that birch ( 1 5 ) regards as improbable. ramsey (1 b) finds dk/dp for the mantle and core to be about the same, namely 3.7 and 3.5-3.8, respectively, wliile the writer's estimates y i e l d 3.2-3.3 near the base of the mantle, and the sanie or a little less at the top of the core. birch (1 5), on independent grounds, finds the value in the mantle to be 3.3. in these circumstances, application of the hypothesis (a) to the inner core probably does not lead to errors in k and dk/dp of more than the order of 5 per cent. on the assumption that the region e consists of iron, and in the t h e i t l c i d i t y of t h e e a r t h ' s i n n e r core 7 light of cosmic abundance data for elements, it is strongly probable that any deviation from the hypothesis (a) will involve changes of k between e and g smaller than between d and e, and therefore much smaller than would be required to bring the value of ;i obtained in section 5 down to zero. a 5 per cent j u m p in k near the boundary of g would stili require the rigidity |i in the inner core to be nearly 3.1 x io12 dyn/cm2 on the jeffreys data. if, on the other band, the region e consists of modified ultrabasic rock, the remarks in section 4 become relevant. on the view of ramsey that the region g is not chemically distinct from either e or d, the expected change in k between e and g would not be greater than between d and e, and the argument favouring rigidity in g is essentially unchanged. but the case favoured by the writer on present evidence ( ' " • 2 3 , 2 4 ) , in which g consists of iron or denser materials while e lias the same chemical composition as the ultrabasic-rock material of d, is more difficult. the remark i n the last paragraph of section 4 shows that there is stili some probability that the value of k in g is very close to that in the lower part of e, but a less general hypothesis than (a), closer to (b) of section 4, would not lead of itself to any inference on the rigidity of g, and furtlier considerations are desirable. 10. such further considerations come from the point (iii) of section 7; tliis supplements the argument in section 9 in cases in which an estimate of the rigidity is yielded, and also goes some distance towards filling the gap in the more difficult case. several authors, including jensen (2 5), feynman, metropolis and teller (2fi), have considered the compressibility-pressure relations for thomas-fermi-dirac gas models. jensen, elsasser ( 1 3 ) and the writer ( 1 7 ) have discussed the hearing of relevant parts of the results on the composition of the earth's core. elsasser quotes data at pressures of the order of 10 to 30 million atmosplieres, according to which k increases by 8 per cent when the atomic number z changes from 18 to 36. the pressure, pi say, at the boundary of the region g is approximately 3.2 million atmospheres, and elsasser's comparison of experimental results of bridgman witli the tlieoretical high-pressure results indicates about the same proportionate change of k witli z at the pressure pi as at the liiglier pressures. the writer has amended certain parts of elsasser's work, and fìnds that bridgman's results, 8 k . e. b u l l e n geophysical data and tlie feynman data are w.ell fitted together by taking smaller values of k at the pressure pi than those assumed by elsasser; this procedure raises the proportionate change in k when z changes from 18 to 36 to about 12 per cent at the pressure pi. most theories of the composition of the earth's core require the difference in z between the outer part of the region g and the base of e to be nearly zero; the writer's theory, referred to in section 9, gives the biggest difference, of order seven units. tiius the quantuin-mechanical calculations indicate that any j u m p in k between e and g is of tlie order of 5 per cent or less, and that, on the jeffreys velocity data, the rigidity of the inner core is stili not less than the minimum value arrived at in section 9. this argument is of course subject to uncertainties in the interpolations between bridgman's data and the quantum-mechanical data, and again falls well short of deductive proof, but, in combination with the earlier discussion, leaves it most probable tliat the inner core is significantly rigid. 11. gutenberg's data show appreciably smaller changes in a than jeffreys' data in the vicinity of the boundary of the inner core, and may be reasonably taken as indicating the least allowable changes in a . taking tlien the lowest estimate of [i in section 8, diminished by a possible 0.5 x io12 dyn/cm2 in accordance with the discussions in sections 9 and 10, we arrive at the value 1.5 x io12 dyn/cm2 as the least value of the rigidity in the inner core that is reasonably compatible with ali present evidence. this value is less one-half the value 3.6 x io12 dvn/em2 involved in earlier calculations of the writer ( 1 s i 20, 21), but is stili about twice the rigidity of stecl at zero pressure. as in earlier papers, temperature effects bave been disregarded in ali the foregoing discussions. birch's recent work ( 1 5 ) gives further confirmation that this would not lead to serious error. if the rigidity of the inner core were in actual fact as low as the above suggested m i n i m u m value of 1.5 x io12 dyn/cm2, there would be repercussions on the calculations of the travel-times of the phase pkjkp, and the time of the j rays would need to be increased by a factor of 1.3, approximately. this would entail increases ranging from zero u p to about 2% minutes in the calculated times. if the phase pkjkp arrived as late as this, however, the theoretical traveltime table ( 2 0 l would be devoid of practical value, since, 011 gutenberg's data (see section 8), the phase would not then be observable. but from the discussion in sections 9 and 10 it follows tliat, even on 9 the leffreys data, the limes of the j rays may need to he increased fly 11 fad or of 1.08, approximately, this would entail increases in the ('.ilculalcd times for pk]kp, reaching nearly 40 seconds at j. = 180". it follows that, in seeking evidence on the occurrence of the phase i'kjkp, it is desirahle to consider readings on seismograms rangin;! up 10 perhaps a minute later than the limes in the calculated table. tih~ existence of the phase could he estahlished only if sufficient of sllch readinp:s fitted to adc(iuale precision 11 travel-time curve po~­ ;<e~~in;! the rip:ht p:radienl this cousideration emphasises further that the cxi~lenee of the phase pkjkp can he estahlished only hy statio stical exatllination of 11 large numher of readin:rs. ' summary the i'tjidf'nce for the existence of significant rigidity in the earth's il!l!f'r ("orl' is slllll11lf1risf'ci (iiul discussed in the lif!,ht of recent work. quantum-mf'chanical calculfltions, based on a thomfls-fermi·dirac model, ,m{!,w'st that (ill f'flrlif'r f'stimatl' of 3.6 x ltjl~ dyn/cm~ for this ri{!,idi/.i" mfly lieed to bl' reduced by 0.5 x 1012 dyn/cm~. tiu' tlworetical travl'i·ti/ih's of the phase pkjkp would then need to be increased b.\· fl1llou/lis rrmg':'!{!. from zero to 40 sf'conds. it is shown that tlm use of gii/i'1i1h'rg's seismic da/a in place of that of leflreys would reduce thl' i,s/imfl/f'd righlity of the inner .core fry a furthl'r 1.6 x loi~ dynicm 2 , approximatl'ly. it is su{!,w~sted that glitenfjerg's data lead to the lowest likl'ly lm[m' for till' rip:idity of the inner core, nallh,lv 1.';xi01~ dyn " cill~ , so tfw/, 011 all prr'sent eljidence, it is improbable that tlw inner core is [('ss ihall about tldce as rigid as steel is at zero pressure. bibliography iq lud!.\"" 1.: p'. pu!.!. blir. ccnlr. 5eislll. int"rnat .•• erie a, h, 1936, pp. :l-:11. (~i grn:"nep.(; b. ant.! rici!uii c. f.: p' rwd ihe earlh' .• c()re, ?i.n.r.a.s., g"nphy.,. sui'pi., ·l, 1938,1'1'.363·372; 0" .• "i .• ",ic ,,·"l·e.'. g"rland" i)dlr. geophy • ., ;,\, 1939, pp, 91.j:j6. (:lj jehghs h.: the li",!!.' of the rote h'",·e.'. ?i.n.r.a.s., geophy., sul'l'l~ -1" 1939, 1'1'. 5·lb·56j. 1.·') jf:haeys 1-1.: ti,e li",e.' of the cot~ u'flt'e .• {s!!('()1ld //fijjerl. :\i.n,r.a.s., (;~ol'h)", sui'i'i~ -l. 1939, pp. 591.6u. k . e. bullen ( 5 ) bullen k . e . : an introduction to the theory of seismology. c a m b r i d g e , 1 9 4 7 , c h . x i i i . ( ° ) gutenberg b . : pkkp, p'p' and the earth's core. t r a n s . a m e r . g e o p h y s . u n i o n , 32, 1 9 5 1 , p p . 3 7 3 3 9 0 . ( 7 ) bullen k . e . : the problem of the earths density variation. b u l l . s e i s m . s o c . a m e r , 30, 1 9 4 0 , p p . 2 3 5 2 5 0 ; the density variation of the earth's centrai core. i b i d „ 32, 1 9 4 2 , p p . 1 9 2 9 . ( 8 ) bullen k . e . : a hypothesis on compressibility at pressurcs oj the order of a million atmospheres. n a t u r e , 1 5 7 , 1 9 4 6 , p . 4 0 5 . («) bullen k . e . : compressibility-pressure hypothesis and the earth's interior. m . n . r . a . s . , g e o p h y s . s u p p l „ 5, 1 9 4 9 , p p . 3 5 5 3 6 8 . ( 1 0 ) kronig r . , de boer j . a n d korrinca j . : on the internai constitution of the earth. p h y s i c a , 1 2 , 1 9 4 6 , p p . 2 4 5 2 5 6 . ( n ) ramsey w . h . : ore the constitution of the lerrestrial planets. m . n . r . a . s , 1 0 8 , 1 9 4 8 , p p . 4 0 6 4 1 3 ; ore the nature of the earth's core. m . n . r . a . s , g e o p h y s . s u p p l . , 5, 1 9 4 9 , p p . 4 0 9 4 2 6 . ( 1 2 ) bullen k . e . : on the constitution of venus. m . n . r . a . s . , 1 0 9 , 1 9 4 9 , p p . 4 5 7 4 6 1 . (13) elsasser w . m . : quantum-theoretical densities of solids of extreme compression. s c i e n c e , 1 1 3 , 1 9 5 1 , p p . 1 0 5 1 0 7 . ( 1 4 ) urey h . c . : the planets, their origin and development. y a l e , 1 9 5 2 . ( 1 5 ) birch f . : elasticity and constitution of the earth's interior. j . g e o p h y s . r e s . , 5 7 , 1 9 5 2 , p p . 2 2 7 2 8 6 . ( 1 6 ) bullen k . e . : cores of terrestrial planets. n a t u r e , 1 7 0 , 1 9 5 2 , p p . 3 6 3 3 6 4 . ( 1 7 ) bullen k . e . : o n density and compressibility at pressures tip to thirly million atmospheres. m . n . r . a . s , g e o p h y s . s u p p l . ( i n c o u r s e of p u b l i c a t i o n ) , 1 9 5 2 . ( 1 8 ) bullen k . e . : an earth model based on a compressibility-pressure hypothesis. m . n . r . a . s , g e o p h y s . s u p p l . , 6 , 1 9 5 0 , p p . 50-59. ( 1 0 ) ramsey w . h . : ore the compressibility of the earth. m . n . r . a . s . , g e o p h y s . s u p p l , 6, 1 9 5 0 ; p p . 4 2 4 9 . ( 2 0 ) bullen k . e . : theoretical travel-times of s u a v e s in the earth's inner core. m . n . r . a . s , g e o p h y s . s u p p l , 6, 1 9 5 0 , p p . 1 2 5 1 2 8 . ( 2 1 ) bullen k . e.-.theoretical amplitudes of the seismic phase pkjkp. m . n . r . a . s , g e o p h y s . s u p p l , 6, 1 9 5 1 , p p . 1 6 3 1 6 7 . ( 2 2 ) bullen k . e. a n d burke-gaffney t . n . : detection of s uaves in the earth's inner core. nature, 1 7 0 , 1 9 5 2 , p . 4 5 5 . ( 2 3 ) bullen k . e . : ore the constitution of mars. m . n j i j v . s , 1 0 9 , 1 9 4 9 , p p . 6 8 8 6 9 2 . ( 2 4 ) bullen k . e . : venus and the earth's inner core. m . n . r . a . s , 1 1 0 , 1 9 5 0 , p p . 2 5 6 2 5 9 . ( 2 5 ) jensen h . : das druck-dichte diagramm der elemente bei liòheren drucken am temperaturnullpunkt. z e i t s c h . f . p h y s i k , 1 1 1 , 1 9 3 8 , p p . 3 7 3 3 8 5 . ( 2 6 ) feynman r . p , metropolis n . a n d teller e . : equations of state of eiements based on the generalized fermi-thomas theory. p h y s . r e v , 75, 1 9 4 9 , p p . 1 5 6 1 1 5 7 2 . on the inclusion of data from solar cycle 19 to estimate f2 layer characteristic long term trends annals of geophysics, 58, 4, 2015, a0441; doi:10.4401/ag-6676 a0441 on the inclusion of data from solar cycle 19 to estimate f2 layer characteristic long term trends blas f. de haro barbas1,*, ana g. elias1,2 1 universidad nacional de tucumán, departamento de física, lab. física de la atmosfera, tucumán, argentina 2 consejo nacional de investigaciones científicas y técnicas, conicet, argentina abstract the effect of including solar cycle 19 (1954-1964) in ionospheric trend estimation is assessed using experimental fof2 values. the dominant influence on the f2 layer is solar euv radiation. in fact, around 90% of inter-annual variance of ionospheric parameters, such as fof2, is explained by solar euv proxies such as the sunspot number, rz, and solar radio flux at 10.7 cm, f10.7. this makes necessary to filter out solar activity effects prior to long term trends estimation, which is reduced at most to the remaining 10% variance. in general solar activity is filtered assessing the residuals of a linear regression between fof2 and rz, or between fof2 and f10.7. solar cycle 19 is a strong cycle during which rz and f10.7 exceeded the values beyond which the ionosphere does not respond linearly to a further increase in euv radiation. this effect, called saturation, implies a break down of the linearity between fof2 and euv, and results in persistent negative residuals during this period. since solar cycle 19 is at the beginning of the time series, trends result to be positive, or less negative, than trends without considering this period. in this case the filtering process is generating a “spurious” trend in the filtered data series which may lead to erroneous conclusions. hmf2 that do not present a saturation effect is also analyzed. 1. introduction to understand and predict the variability of the f2 region of the ionosphere it is essential to understand euv variability which explains around 90% of the variation of parameters such as the critical frequency of the f2 layer, fof2, and the height of peak electron concentration, hmf2 [chen et al. 2012]. in the study of long-term trends it is necessary then to apply filtering processes in order to filter out solar activity effect and make trends detectable. ionospheric trends have become a main subject since the beginning of the 1990’s when the study of upper atmosphere trends gained importance in the context of global climatic change (see lastovicka et al. [2012], and references there in). several studies link ionospheric trends with the middle and upper atmosphere cooling due to an increase in greenhouse gases [roble and dickinson 1989, rishbeth 1990, upadhyay and mahajan 1998]. a doubling in co2 concentration would produce a cooling of 30-40 k in the thermosphere, a 20%-40% decrease in air density between 200-300 km, a ~15-20 km lowering of hmf2 and a worldwide decrease in fof2 less than 0.5mhz [roble and dickinson 1989, rishbeth 1990, rishbeth and roble 1992]. however, the global pattern of experimental hmf2 and fof2 of several worldwide stations is highly complex and cannot be entirely reconciled with the greenhouse hypothesis. some reasons for this are, on one side the existence of other sources of upper atmosphere trends (such as geomagnetic activity long-term variation and earth’s magnetic field secular variations) which act jointly with the greenhouse effect. on the other side, the methods used to extract trend values differ from author to author, and usually rely on some filtering process that may bias the trend results. filtering is a standard mathematical operation which in the case of ionosphere is used to enhance features, as it is a long-term trend, otherwise not visibly apparent in the data. however, filtering can have unexpected consequences such as the introduction of spurious oscillations or a spurious trend [chandler and scott 2011]. a special care is therefore required when making inferences from filtered time series. since there are not long-term continuous measurements of solar euv, different indices are used to describe the variations of solar euv irradiance and to filter ionospheric parameters. among these indices, f10.7 and rz are the most widely used. saturation takes place when rz or f10.7 exceeds certain value, beyond which the ionosphere seems not respond to a further increase in euv radiation [balan et article history received september 30, 2014; accepted june 18, 2015. subject classification: ionospheric trend, euv proxies, forecasts, dynamics. al. 1994, liu et al. 2003, ma et al. 2009]. it is linked to a break down of the linearity between fof2 and euv (or solar index), as reported in liu et al. [2006, 2011]. saturation is dominant in the equatorial ionization anomaly regions during all seasons, while at mid-latitudes it is weaker in local winter but it is still present in other seasons [chen and liu 2010]. figure 1 shows fof2 measured at slough in term of f10.7 for different months, representing different seasons. fof22, which is proportional to nmf2, is also shown. the saturation effect can be clearly noticed for this mid-latitude station during local summer. during winter fof2 nearly increases linearly with f10.7 in agreement with chen and liu [2010] results for mid-latitudes. filtering methods are based on linearity, so the saturation effects may strongly influence the long-term trend reached. this problem can be overcome in some cases using fof22 (∝nmf2) instead of fof2. but since this is not always the case another possibility is to exclude the period from the analysis. this situation results in persistent negative residuals which will affect the trend value according to where this period of persistence is located within the whole period of analysis. if it is in the middle, it may not cause any spurious results, but if is close to an extreme it will depend if it is at the beginning, inducing a spurious positive trend, or at the end inducing a spurious negative trend. in particular, solar cycle 19 (1954-1964) has a very high maximum compared to other cycles, which presents rz > 160 and f10.7 > 200. since it is at the beginning of most ionosphere time series so it should be expected a positive, or a less negative trend, than the trend without considering this period. in the case of hmf2, the results obtained by chen and liu [2010] indicate that a linear fit can well present its variation with f10.7 in the whole range of solar activity level. this means that, contrary to what happens with fof2, trend values should not change with the period considered. four stations were studied: rome, slough, townsville and kokobunji. the purpose of the present study is to see the impact of solar cycle 19 in estimating trends. 2. data analysis fof2 and m(3000)f2 monthly median data for the month of june at 12 lt from slough, rome, ottawa and townsville were obtained from damboldt and suessmann database [2012] (ftp://ftp.ips.gov.au/wdc-data/ iondata/medians/damboldt/), for the period 1954-1995. de haro barbas and elias 2 figure 1. fof2 (black circles) measured at slough, and fof22 (red circles) in terms of f10.7 for (a) june, (b) september, (c) december, and (d) march. dotted lines correspond to a polynomial fit of order 2. the fof2 data are monthly median values for period 1954-1995. a b c d 3 the first step in order to assess fof2 trends is to filter out solar activity. this is done estimating the fof2 residuals from the regression between the experimental values and the solar activity index f10.7 that is: fof2res = fof2exp − (a f10.7 + b) (1) where a and b are constants determined with least squares. mielich and bremer [2013] analyzed more than 100 stations in order to detect the best index to filter solar activity from fof2 and hmf2, and conclude that f10.7 is a better index than rz. then a regression of fof2res with time is adjusted with least squares so that fof2res = a t + b (2) where a and b are the regression coefficients, and a (in mhz/year) is the trend we are interested in. trend values were calculated for three different periods: (1) complete period available, 1954-1995, which includes solar cycle 19, (2) period 1954-1995 excluding periods where f10.7 exceeded 200, (3) period 1964-1995 that excludes solar cycle 19. since the peak electron density, nmf2, in the ionosphere is linked to fof2 through: nmf2 = 1.24 × 104 (fof2)2 (3) the same filtering process was also applied to fof22. fof22res was estimated as fof22res = fof22exp − (a’ f10.7 + b’) (4) followed by fof22res = a’ t + b’ (5) where a’, b’, a’ and b’ are constants determined with least squares, and a’ (in mhz2/year) is fof22 trend. figure 2 shows as an example fof2 residuals in the case of slough together with the linear trends for the three cases considered. trend values, a and a’, for all the station analyzed are listed in table 1. only in the case of townsville the trend becomes positive when solar cycle 19 is not included. this is due to a relatively big positive residual in 1960, and in a time series of 39 data it makes a difference when it is ~7 times the average absolute value of the residuals. in this case fof22res = 40 mhz2 while the average of |fof22res| is 6 mhz2. this difference is less evident when fof2 is not squared. when only periods where f10.7 exceeded 200 are excluded, the 1960 value remains and the trend results negative. in the case of hmf2, it was first estimated using the formula of shimazaki [1955]: trends were assessed for slough, rome and townsville in the same way as in the case of fof2. otsolar cycle 19 and f2 long term trends (6) geographic fof2 trend a [mhz/year] fof22 trend a [mhz2/year] station lat. long. 1954-1995 without f10.7>200 1964-1995 1954-1995 without f10.7>200 1964-1995 ottawa 45.4 284.1 0.002 0.000 -0.006 0.023 -0.006 -0.070 rome 41.8 12.5 -0.002 -0.012 -0.022 -0.010 -0.134 -0.272 townsville -19.3 146.7 0.004 -0.001 0.003 0.011 -0.014 0.094 slough 51.5 359.4 0.002 -0.003 -0.008 0.012 -0.034 -0.088 table 1. trend values: fof2 and fof22 residual trends for four ionospheric stations calculated for three periods: 1954-1995 that includes solar cycle 19, 1954-1995 excluding periods where f10.7 exceeded 200 and 1964-1995 that do not includes cycle 19. residuals assessed using f10.7 as solar euv proxy. figure 2. fof2 residual ( june, 12 lt) for slough assessed for the periods: 1954-1995 (black line), 1964-1995 (red line), 1954-1995 excluding periods where f10.7 exceeded 200 (green line). dotted lines are the corresponding linear trends. tawa was excluded in this analysis due to data gaps. hmf2 trend values are listed in table 2. except for the case of slough, differences in hmf2 trends are relatively small as expected. 3. discussion and conclusions filtering is a crucial step in ionosphere long-term trend estimation which can lead to “spurious” trends when not properly applied to the time series under analysis. it can be noticed from table 1, that in all the cases, except fof22 for townsville, trends become negative, or less positive, when solar cycle 19 (1954-1964) is excluded from trend estimation. fof2 trend values for ottawa, townsville and slough are in close agreement with the values obtained by upadhyay and mahajan [1998] who analyzed the period 1957-1990. these positive trends, together with other stations, made them conclude: “trends were negative for some stations and positive for other stations. we conclude that ionospheric data do not provide any definitive evidence of a global long term trend”. clilverd et al. [2003] analyzed the impact of residual periodicities on the trend values considering that filtering from a time series a periodical signal result in a damped periodical signal. they did this for hmf2, considering the first 10 years of the residual time series and then adding the following year and computing a new de haro barbas and elias 4 geographic hmf2 trend a [km/year] station lat. long. 1954-1995 without f10.7>200 1964-1995 rome 41.8 12.5 -0.28 -0.24 -0.27 townsville -19.3 146.7 -0.23 -0.22 -0.19 slough 51.5 359.4 -0.21 -0.03 0.04 table 2. trend values: hmf2 residual trends for three ionospheric stations calculated for three periods: 1954-1995 that includes solar cycle 19, 1954-1995 excluding periods where f10.7 exceeded 200 and 1964-1995 that do not includes cycle 19. residuals assessed using f10.7 as solar euv proxy. figure 3. fof2 trend estimated for fof2 residual time series starting during (a) 1954 solar minimum, (b) 1964 solar minimum, (c) 1957 solar maximum, and (d) 1969 solar maxim considering the first 10 years and then adding the following year for a new data and adding one year at a time. the year corresponding to each trend value is the final year considered for trend estimation. a b c d 5 trend, obtaining a trend values in terms of the length of the series considered. when the process began at a solar maximum the first value was negative and when it began in a solar minimum it was positive. both trend series should result in a ringing which is gradually damped out, tending both curves to the same final value. we tested this effect in our case for fof2 and hmf2, following their procedure [clilverd et al. 2003], that is considering the first 10 years of the residual time series and then adding the following years one at a time and computing a new trend successively. figure 3 shows fof2 trends beginning in two minimum epochs: 1954 (cycle 19) and 1964 (cycle 20), and also in two maximum epochs: 1957 (cycle 19) and 1969 (cycle 20). as expected from our results, it makes a clear difference including or not solar cycle 19, especially when the fof2 residual time series begin at a maximum. we obtain similar results to those of clilverd et al. [2003] when solar cycle 19 is not included. when beginning at solar minimum, our results are not so clear but the ringing effect can still be noticed. in the case of hmf2, depicted in figure 4, we do not obtain very clear results. when hmf2 residuals begin at the 1954 minimum, trends begin with a positive value, in coincidence with clilverd et al. [2003] results. however when we shift the beginning to 1964, trends begin with a negative value. in the case of solar maximum, our results do not agree with clilverd et al. [2003]. a possible cause of this disagreement may be due to the fact that we analyzed just one month, and probably seasonality has some effect which is smoothed when annual data is considered. the difference found between stations deserves further analysis. in conclusion, in the case of solar cycle 19, including it in the period of analysis and filtering using a linear fit between the ionosphere parameter and the solar euv proxy, may result in less negative or even positive trends. this may erroneously lead to a disagreement with the expected effects of the increasing concentration of greenhouse gases, in favor of other mechanisms. when trends are assessed with experimental fof2 data including solar cycle 19 care must be taken then with the filtering method used. in conclusion, at least in the case of fof2 the underlying “true” trend value can be affected by the filtering process due to solar euv emissions during solar cycle 19 are not well represented by usual proxies such as f10.7. references balan, n., g.j. bailey, b. jenkins, p.b. rao and r.j. moffet (1994). variations of ionospheric ionization and related solar fluxes during an intense solar cycle, j. geophys. res., 99, 2243-2253. solar cycle 19 and f2 long term trends figure 4. hmf2 trend estimated for hmf2 residual time series starting during (a) 1954 solar minimum, (b) 1964 solar minimum, (c) 1957 solar maximum, (d) and 1969 solar maximum considering the first 10 years and then adding the following year for a new data and adding one year at a time. the year corresponding to each trend value is the final year considered for trend estimation. a b c d chandler, r.e., and e.m. scott (2011). statistical methods for trend detection and analysis in the environmental sciences, john wiley & sons, london. chen, y., and l. liu (2010). further study on the solar activity variation of daytime nmf2, j. geophys. res., 115, a12337; doi:10.1029/2010ja015847. chen, y., l. liu and w. wan (2012). the discrepancy in solar euv-proxy correlations on solar cycle and solar rotation timescales and its manifestation in the ionosphere, j. geophys. res., 117; doi:10.1029/2011j a017224. clilverd, m.a., t. ulich and m.j. jarvis (2003). residual solar cycle influence on trends in ionospheric f2layer peak height, j. geophys. res., 108 (a12), 1450; doi:10.1029/2003ja009838. damboldt, t., and p. suessmann (2012). consolidated database of worldwide measured monthly medians of ionospheric characteristics fof2 and m(3000)f2, inag bulletin on the web, inag-73; http://www.ips.gov.au/ipshosted/inag/web73/2012/damboldt_consolidated_database.pdf. lastovicka, j., s.c. solomon and l. qian (2012). trends in the neutral and ionized upper atmosphere, space sci. rev., 168, 113-145. liu, j.y., y.i. chen and j.s. lin (2003). statistical investigation of the saturation effect in the ionospheric fof2 versus sunspot, solar radio noise, and solar euv radiation, j. geophys. res., 108 (a2), 1067; doi:10.1029/2001ja007543. liu, l., w. wan, b. ning, o.m. pirog and v.i. kurkin (2006). solar activity variations of the ionospheric peak electron density, j. geophys. res., 111, a08304; doi:10.1029/2006ja011598. liu, l., w. wan, y. chen and h. le (2011). solar activity effects of the ionosphere: a brief review, chinese sci. bull., 56, 1202-1211; doi:10.1007/s11434-0104226-9. ma, r., j. xu, w. wang and w. yuan (2009). seasonal and latitudinal differences of the saturation effect between ionospheric nmf2 and solar activity indices, j. geophys. res., 114, a10303; doi:10.1029/2009ja 014353. mielich, j., and j. bremer (2013). long-term trends in the ionospheric f2 region with different solar activity indices, annales geophysicae, 31, 291-303. rishbeth, h. (1990). a greenhouse effect in the ionosphere?, planetary and space sciences, 38, 945-948. rishbeth, h., and r.g. roble (1992). cooling of the upper atmosphere by enhanced greenhouse gases. modeling of the thermospheric and ionospheric effects, planetary and space sciences, 40, 1011-1026. roble, r.g., and r.e. dickinson (1989). how will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere?, geophys. res. lett., 16, 1441-1444. shimazaki, t. (1955). world wide daily variations in the height of the maximum electron density in the ionospheric f2 layer, j. radio res. labs., japan, 2, 85-97. upadhyay, h.o., and k.k. mahajan (1998). atmospheric greenhouse effect and ionospheric trends, geophys. res. lett., 25, 3375-3378. corresponding author: blas f. de haro, universidad nacional de tucumán, departamento de física, tucumán, argentina; email: blasdeharo2000@yahoo.com.ar. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. de haro barbas and elias 6 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 annals of geophysics, 61, 3, se337, 2018; doi: 10.4401/ag-7656 1 “coseismic displacements on ischia island, real-time gpspositioning constraints on earthquake source location„ roberto devoti1,*, prospero de martino2, grazia pietrantonio1, mario dolce2 (1) istituto nazionale di geofisica e vulcanologia, osservatorio nazionale terremoti, roma, italy (2) istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, napoli, italy 1. introduction gps data have proven the potential of early tsunami warning [blewitt et al., 2007; sobolev et al., 2007; singh et al., 2012; hoechner et al., 2013; savastano et al., 2017]. in literature, so far, the focus is mainly on the advantages of the gps data in recovering large earthquake parameters, but recently there is a growing interest in developing automated algorithms for real-time determinations of seismological parameters and deformation analysis for smaller seismic events [melgar et al., 2015; crowell et al., 2016]. the real-time monitoring of ground deformations is not a technically demanding effort because of the broad spreading of gps data, but may be extremely useful for assessing seismic location estimates, or for cross-checking purposes with other geophysical data in the early emergency phase. in this study we investigate the capability for singleepoch gps positioning to characterize small to medium earthquake sources. we also assess the latency for the rapid seismic source location using only geodetic data for a recent event occurred on ischia island. ischia is a volcanic island located in the northern sector of the gulf of naples, italy. thermal activity characterizes diffusively the island, fumarolic and thermal fluids emissions are widespread [chiodini et al., 2004], mostly concentrated along the faults boarding mt. epomeo, a resurgent dome in the center of the island. the seismic activity is rather low and generally with low magnitude, however the historical record shows that ischia island was struck by several earthquakes, a few of them causing severe damages and fatalities. the most outstanding in 1883 in which more than 2,000 people were killed and causing the total destruction of the village of casamicciola [carlino et al., 2010]. on august 21th, 2017, an earthquake (md=4; mw 3.9) occurred,damaging a limited portion of the island in the casamicciola area, reaching a maximum intensity of 8 on the european macroseismic scale (ems) and causing two fatalities [azzaro et al., 2017]. the neapolitan volcanic area is constantly monitored by the ingv osservatorio vesuviano (ingvov) institution. it operates a geodetic monitoring system, which includes a continuous gps network of 37 permanent stations [tammaro et al. 2013; de martino et al. 2014]. at present, 6 of these gps stations are located on ischia island (figure 1). an additional station on the island, part of the regione campania gnss article history receveid february 14, 2018; accepted march 29, 2018. subject classification: kinematic gps solution; coseismic offset; real-time detection; ischia earthquake; geodetic earthquake location. abstract we use gnss data to simulate the early seismic source location of a mw 3.9 event occurred in the island of ischia, italy. the study suggests that real-time gnss data can support the seismic location system in the early stage of the emergency phase. we demonstrate that this very shallow earthquake, triggered significant displacements at a few stations in less than half an hour. using exclusively gps data, the first location of the hypocenter was possible with a latency of only 20 minutes. early upgrades of the offset field in the first two hours confirm the source location confined within 1-2 km in the horizontal plane and less than 1 km depth. network, is also considered in this work (isch in figure 1). the gps observations were used to compute the coseismic displacements caused by the earthquake. we demonstrate that the coseismic offsets could be available in less than half an hour after the earthquake and could be useful in constraining the hypocenter position even for such small events. the proposed method is especially effective in volcanic or geothermal areas or for those events that are poorly constrained by the observing seismic network (e.g. on islands). 2. gnss data analysis the gps data are sampled at 30 seconds and transmitted to the processing center at ingv-ov at naples every 24 hours. station positions are routinely computed and updated at daily rates using the bernese v.5.0 software [beutler et al., 2007]. the routine precise processing demands standard products delivered by the international gnss service (igs) with latency of about 2 weeks, the most relevant being, precise orbits, satellite clock corrections and earth rotation parameters. less devoti et al. 2 figure 1. map of the horizontal (blue arrows) and vertical (red arrows) coseismic displacement field of the ischia gnss network. the purple square box displays the source location obtained from the inversion of the gnss displacement. precise and predicted products are also released four times per day for real time and near real time use, these are the so called rapid and ultra-rapid igs products. in particular the ultra-rapid predicted products are available in real time and allow adequate position estimates, virtually at every observation epoch, relative to a control point located hundreds kilometers away. in this work we estimate gps time series using both ultra-rapid and precise products assessing the real-time capability to detect coseismic displacements. 2.1 precise position solution two different precise station position solutions are available from the ingv analysis centers, hereafter labeled ov and rm, both processing phase double differences. these two solutions are used to estimate a combined offset field for the seismic event occurred on the 21st of august, and used here as the reference offset field for kinematic solution assessing. the standard processing scheme for dual frequency gps receivers uses the ionosphere-free linear combination (l3) of phase observables while the phase ambiguities are solved using the qif (quasi ionosphere free) strategy [mervart, 1995]. the troposphere delay is modeled using the dry-niell model for the hydrostatic troposphere and the wet-niell mapping function is used for the estimation of a hourly zenith delay correction [niell, 1996]. to model tropospheric azimuthal asymmetries, an additional daily horizontal gradient at each station is also estimated. tidal ocean loading is modelled using the fes2004 coefficients [lyard et al., 2006] provided by the ocean tide loading web service (http://holt.oso.chalmers.se/loading/). at each station, the igs08 absolute antenna phase corrections are applied. gps satellite orbits and earth rotation parameters are fixed to the combined official igs precise products. the reference frame realization differs substantially for the two solutions. the rm solution is obtained as loosely constraint estimates and subsequently projected on the itrf2008 reference frame by a 4-parameter (3 translations and a scale factor) helmert transformation. the ov solution instead, is obtained imposing three nonet translation conditions on a set of itrf2008 reference stations (minimum constraint solution). the two precise positioning solutions are used to assess the relative quality of the individual solutions and in particular, to build a merged (in a least squares sense) solution for the coseismic offsets. the combined “consensus” solution is a rather common procedure for the geodetic product generation (see e.g. www.iers.org). a rationale for this approach is that the consensus solution minimizes the chance of systematic errors and permits an assessment of the repeatability of the estimates. 2.2 kinematic position solution we use the same processing software (bernese v. 5.0) to build the kinematic solution. we select 7 stations at ischia island as rover stations using 3 additional stations as reference sites, located far from the rover stations in the neapolitan area. to simulate a real-time application we use the igs ultra-rapid products, processing data at different sampling rates (30s, 180s and 300s). the processing scheme may be summarized by the following steps: cycle-slip screening and outlier removal using the ionosphere-free linear combination (l3); ambiguity resolution using a sigma dependent strategy [beutler, 2007] and finally the epoch wise position estimation based on l3 single differences whereas the reference sites are fixed to itrf2008 coordinates. the dry part of the troposphere is modeled using the dry-niell a-priori model and estimating two troposphere zenith delay parameters per day, using the wet-niell mapping function. 3. precise coseismic offset combination and source location estimation two coseismic displacement fields (ov and rm) are estimated using the daily precise time series of the gps stations. the coseismic offsets are computed as differences between the average positions, respectively 15 days before and 4 days after the event. thus, the two displacement fields are combined in a least squares sense [devoti, 2012], obtaining a “consensus" solution that minimizes possible sources of systematic errors. figure 1 shows the displacements observed at the gps stations for the horizontal and vertical spatial components. among all stations, only two of them (oscm and mepo, casamicciola observatory and mt. epomeo, respectively) show significant horizontal offsets, characterized by a slightly divergent northward displacement, approaching each other in the s-n direction, whereas the vertical offsets are generally negligible. the average errors associated to the offsets in the vertical and horizontal components are respectively 8 mm and 2.5 mm. figure 2 displays the residuals of the two input solutions with respect to the combined solution, respectively labeled rm and ov. it turns out that the residuals are well below their uncertainties and are unbiased, thus testifying an adequate repeatability and reproducibility of the displacement field. acknowledg3 gps constraints on earthquake location devoti et al. 4 ing this offset field and assuming a normal fault source in an elastic half-space with area proportional to the given magnitude, the estimated source location is provided in figure 1. unfortunately this conclusions can be drawn a few days after the seismic event, depending on how many daily positions are averaged out and when the rapid products are available. we will now push the real-time potentiality of the method, exploring the feasibility of estimating the offsets as soon as they become significant, given that real time data connection is available. 4. real time displacement detection we have to face the problem of detecting a transient signal in a growing time series, when an instantaneous change of the position coordinates (i.e. offsets) occurs. in this validation case we exploit the advantage of knowing the epoch at which the offset may occur, i.e. we recognize a trigger event after which we should be able to detect positions changes as soon as they will become reliable. due to the noise process affecting the position estimates, the displacement can be measured only when an adequate number of observations are available. to quantify this time span, we implement a student's t-test to check the null hypothesis that the population mean of the offset estimated after the trigger event, is equal to zero. in this context, the offsets are computed at each subsequent epoch providing the sample of offsets, that will be tested against the hypothesis of zero offset. the test will reject the null hypothesis as soon as the offset sample becomes significantly distinct from zero. the amount of time needed to detect an offset (i.e. the time at which the null hypothesis will be rejected) is the latency for the gps observations to provide a significant displacement at the chosen significance level. obviously the latency depends strongly on the signal-to-noise ratio (i.e. the displacement magnitude over the noise), the larger the signal (e.g. higher magnitude of the seismic event or shorter distance from the epicenter), the shorter will be figure 2. residuals of the two displacement fields (ov and rm) with respect to the combined field at each gnss station. 5 gps constraints on earthquake location the latency. figures 3 and 4 show the updated offsets for mepo and oscm stations respectively, the two stations showing the most stable offsets. similar offset plots for the other stations are reported in supplementary figures: a key point is in fact not only the matching of the significant offsets in the off-line and real-time approach, but also the lacking of false positive values in the real-time approach. the solid red line shows the offset estimated every 180 seconds after the trigger event in the three spatial components (up, east and north) and the inset on top of each graph shows the ratio (t/t1%) of the test statistics over the critical value (1% significance level), so that the region of rejection is t/t1%>1. we recall that rejection of the null hypothesis implies the detection of a significant offset, thus when t/t1% passes the unit threshold the displacement is significantly different from zero. due to their impact on the latency of the estimations, we investigate three different time sampling intervals for the position estimates, namely the 30 sec (the gps receiver sampling rate), 180 sec and 300 sec. the three time sampling do not differ much in the low frequency band (supplementary figure s1), evidenced by the nearly equal efficiency to detect the same offset, but differ in high frequency noise which implies different latency time for the offset detection. the mepo station shows a clear northward offset of 1.0±1.2 cm, recognized after 47 minutes, whereas the vertical and east components do not show any significant displacement, albeit a persistent negative easting offset of a few mm (i.e. towards west) is noticeable and the vertical is trending towards negative values. the only other station that shows any significant displacements is oscm located about 2km north of mepo. all three components show a positive offset, of 5.7±2.4, 0.5±0.5 and 1.4±0.7cm, respectively in the vertical, east and north directions. the vertical uplift, initially of about 6 cm (significant after 17 minutes) reduces gradually in the following hours to zero (0.3±2.3cm) and the test statistics, after first overtaking the unit threshold, rebounds back to the region of acceptance (no detectable offset). the east and the north offsets, instead, show more stable estimates detected at about 30 and 20 minutes respectively after the trigger event. the only other station of the ischia network that shows a non zero offset is fori, where the east component is disfigure 3. real-time progression of the offset detection algorithm at mepo station (mt. epomeo). the thinner blue line shows the significance ratio (t/t1%) whereas the heavy red line shows the offset estimated at each epoch in millimeters. the grey bar indicates the epoch at which the offsets become significant (t/t1% >1), next to it, the value of the offset is given along with its 1-sigma error. −! " ! ∆ #$ % &'() ∆v=0±3.4 " * ! +,+ *−* " * ! ∆ #' ./ + ∆e=0±0.8 " * ! +,+ *" 0" *!" *1" !2" 3"" " * ! 3 ∆ #4 56 +7 '8.%/9:#;<=9#>=<?@ ∆n=1.0±1.1 (47min) 1.1±1.2 1.1±1.2 0.9±1.3 0.8±1.2 " * ! +,+ *devoti et al. 6 placed by a small amount (0.2±0.6cm) that becomes significant about 3 hours after the trigger event (see supplementary figure s2). 5. assessment of the real-time gnss seismic source for the test case the coseismic displacement obtained from the daily gps time series is noticeable only in two stations of the ischia gnss network, at the mt. epomeo station (mepo) and at the casamicciola observatory station (oscm). the offsets are directed towards n (15.6 mm) and nne (10.3 mm), respectively, whereas the vertical displacements are negligible apart from a slight subsidence observed at mepo station. the observed displacements are compatible with the coseismic subsidence measured by the insar technique that shows a localized (~1 km2) subsidence up to 4 cm, sw of casamicciola (gruppo di lavoro ingv sul terremoto dell’isola di ischia, 2017, de novellis et al., 2018]. both results can be achieved only after a time delay that depends on data collection issues and products availability for the respective techniques. to prove the usefulness of the gps data at the early stage of the earthquake detection, we perform a fast computation of the hypocenter by inverting the significant gps displacements assuming a rectangular fault source [okada, 1985]. in the apennines and tyrrhenian area normal faulting is quite a common tectonic style [e.g. malinverno and ryan, 1986] but in volcanic domains also a point-like mogi inflating/deflating source may be appropriate [mogi, 1958]. to study the more general problem we use a simple square fault plane and try to minimize the number of source parameters to be estimated. we fix the source dimensions according to the given magnitude using the relationship of the seismic moment m0=μad [aki, 1966], where μ is the rigidity modulus, a is the area of the fault surface and d is the slip dislocation. the surface area a is chosen according to the empirical relationship of wells and coppersmith, 1994 and fixed to 1 km2. the rake is fixed to normal faulting and the location, depth, dip and strike are chosen as the unknown parameters. the source parameters are solved using a nonlinear minimum search function fmincon [matlab, 2012a]. figure figure 4. real-time progression of the offset detection algorithm at oscm station (casamicciola). the thinner blue line shows the significance ratio (t/t1%) whereas the heavy red line shows the offset estimated at each epoch in millimeters. the grey bars indicate the epochs at which the offsets become significant (t/t1% >1), next to it, the value of the offset is given along with its 1-sigma error. ! " # ∆ $% & '()* ∆v=5.7±2.4 (17min) 2.3±2.4 1.7±2.3 1.1±2.2 0.3±2.3 ! + " ,-, +. −+ ! + " ∆ $/ 01 , ∆e=0.5±0.5 (29min) 0.3±0.5 0.3±0.4 0.3±0.4 0.3±0.4 ! + " ,-, +. ! 2! +"! +3! "#! 4!! ! + " 4 ∆ $5 67 ,8 /90&1:;$<=>:$?>=@a ∆n=1.4±0.7 (20min) 0.8±0.7 0.7±0.7 0.7±0.7 0.5±0.7 ! + " ,-, +. 7 gps constraints on earthquake location 5 shows the projections of the source surfaces estimated at increasing time spans after the earthquake (20, 40, 60 and 120 minutes) using the pertinent offset field estimated at that epochs. the weighted root-mean-squared (wrms) of the misfit residuals, in all three spatial components, are on the order of 3 mm. the source locations are mainly scattered along the n-s direction, being roughly confined in a 1x3 km2 area. the early source depth never exceeds 0.5 km, a result which is constrained by the few stations that show a non-zero offset. the estimated dip is very low, indicating a nearly horizontal source plane. however the geometry of the source (strike and dip) is weakly constrained, since the actual displacement field is quite under-sampled by the sparse gps stations. 6. conclusions the estimated gnss positions using 24h of satellite observations are highly accurate, stable and replicable under different processing approaches and currently used for calibration purposes in conjunction with insar data; however the potentiality of continuous and real-time nature of gnss data is not commonly fully exploited. to assess the potentiality of a real-time positioning system, we set up a simulated kinematic solution and a procedure suited for offset detection that checks for a significant offset at any epoch step. the algorithm, based on classical statistical hypothesis testing, demonstrates the possibility of revealing displacements on the order of 1 cm in less than 20 minutes (50 and 90 minutes for different sampling rates) after the earthquake. the results concerning the intermediate sampling rate (180s) for the position time series are shown in this paper, enlightening also that different sampling rate does not affect substantially the estimates (e.g. see supplementary figure s1), but affects the latency at which the estimate becomes reliable. this is an expected behavior since the noise does not decrease much as the sampling lengthens, providing less data points per unit of time and therefore allowing a longer latency. the real-time displacements are very close to the precisely estimated offsets but they figure 5. estimated source locations at subsequent epochs (in black) after the earthquake, as obtained from the real-time detected offsets. the purple square box displays the estimated source location shown in figure 1. are not identical. the differences may arise mainly for two reasons, first because of the high noise affecting the kinematic solution, possibly biased by orbital and tropospheric modeling errors; secondary because the kinematic solution provides the instantaneous timevarying displacement, whereas the static solution describes the long lasting deformations. in conclusion, gps kinematic positioning if linked with real-time data transmission, is suitable for detecting sudden displacements in the time series down to 1 cm or even below with sufficient reliability. we demonstrate that even a small magnitude earthquake, because very shallow, can trigger detectable gps displacements useful for characterizing a preliminary source. in this case, the location of the hypocenter using exclusively gps data, is possible as early as 20 minutes after the earthquake and is persistent after that time, thus being a valuable piece of information to be cross-validated with classical seismological inferences. this result testifies the capability of gps data to provide quick evidence of earthquake source location, which can be crucial in assisting decision makers during the emergency management. in the italian area the hypocentral depth is on average deeper (8-12 km), so that higher magnitudes (>m5) will cause the same amount of displacements and the proposed method could be applied as well to monitor the low to moderate seismicity in other areas of the italian peninsula. we evaluate that earthquake frequency in italy reaches a few events per year for magnitude higher than m5 (and about 1 event per year for mw > 5.4) [gasperini et al., 2013]. thus it could be worthwhile to pursue real-time gnss monitoring at national scale, or in special areas (e.g. alpine and apennines orogens) where major seismogenic structures have been identified [diss working group, 2015) with the aim to possibly detect geodetic-derived seismic sources in the very first few minutes of the emergency response phase. acknowledgements. we thank the ingv-ov staff involved in the management and maintenance of the gps network. references aki k., (1966) generation and propagation of g waves from the niigata earthquake of june 16, 1964. part 2. estimation of earthquake moment, released energy, and stress-strain drop from the g wave spectrum, bulletin of earthquake research institute, tokyo university, vol. 44, pp. 73-88. azzaro r., s. del mese, g. martini, s. paolini, a. screpanti, v. verrubbi a. tertulliani (2017), questrilievo macrosismico per il terremoto dell’isola di ischia del 21 agosto 2017, rapporto interno, doi:10.5281/zenodo.849091 (in italian). available at: https://ingvterremoti.files.wordpress.com/2017/08/casamiccio la-report-prelimquest.pdf beutler, g., h. bock, r. dach, p. fridez, a. gäde, u. hugentobler, a. jäggi, m. meindl, l. mervart, l. prange, s. schaer, t. springer, c. urschl, p. walser, (2007). bernese gps software version 5.0, edited by r. dach, u. hugentobler, p. fridez, m. meindl, astronomical institute, university of bern, january 2007. blewitt, g., c. kreemer, w. c. hammond, h.-p. plag, s. stein, and e. okal (2006), rapid determination of earthquake magnitude using gps for tsunami warning systems, geophys. res. lett., 33, l11309, doi:10.1029/2006gl026145. carlino s., e. cubellis, a. marturano, (2010), the catastrophic 1883 earthquake at the island of ischia (southern italy): macroseismic data and the role of geological conditions, nat. hazards, 52, 231-247, doi:10.1007/s11069-009-9367-2. chiodini, g., r. avino, t. brombach, s. caliro, c. cardellini, s. de vita, f. frondini, d. granirei, e. marotta, g. ventura, (2004), fumarolic and diffuse soil degassing west of mount epomeo, ischia, italy, j. volcanol. geotherm. res., 133, 291-309, doi:10.1016/s0377-0273(03)00403-7. crowell, bw, bock y, liu z. 2016. single-station automated detection of transient deformation in gps time series with the relative strength index: a case study of cascadian slow slip. journal of geophysical research-solid earth. 121:9077-9094. de martino p., umberto tammaro, francesco obrizzo (2014), gps time series at campi flegrei caldera (2000 2013). ”, annals of geophysics., 57, 2, 2014, s0213, doi:10.4401/ag-6431s0213 de novellis, v., carlino, s., castaldo, r., tramelli, a., de luca, c., pino, n. a., et al. (2018). the 21 august 2017 ischia (italy) earthquake source model inferred from seismological, gps, and dinsar measurements. geophys. res. lett., 45, 2193-2202, doi:10.1002/2017gl076336 devoti, r. 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(1985), surface deformation due to shear and tensile faults in a half-space, bull. seismol. soc. am.,75, 1135–1154. savastano, g., a. komjathy, o. verkhoglyadova, a. mazzoni, m. crespi, y. wei, a. j. mannucci, (2017). realtime detection of tsunami ionospheric disturbances with a stand-alone gnss receiver: a preliminary feasibility demonstration. sci. rep.7, 46607, doi: 10.1038/srep46607. singh, s. k., x. pérez-campos, a. iglesias, d. melgar (2012) a method for rapid estimation of moment magnitude for early tsunami warning based on coastal gps networks. seism. res. lett., 83(3), 516530, doi: 10.1785/gssrl.83.3.516. sobolev, s. v., a. y. babeyko, r. wang, a. hoechner, r. galas, m. rothacher, d. v. sein, j. schröter, j. lauterjung, and c. subarya (2007), tsunami early warning using gps-shield arrays, j. geophys. res., 112, b08415, doi:10.1029/2006jb004640. tammaro, u., de martino p., obrizzo f., brandi g., d’alessandro a., dolce m., malaspina s., serio c., pingue f. (2013). somma vesuvius volcano: ground deformations from cgps observations (2001-2012), special issue “mt. vesuvius monitoring: the state of the art and perspectives”, annals of geophysics 56, 4, 2013, s0456; doi:10.4401/ag-6462 wells d.l., coppersmith k.j. (1994), new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. bull. seismol. soc. am., 84 (4), 974-1002. *corresponding author: roberto devoti, istituto nazionale di geofisica e vulcanologia,, osservatorio nazionale terremoti, roma, italy email: roberto.devoti@ingv.it © 2018 the istituto nazionale di geofisica e vulcanologia. all rights reserved. 9 gps constraints on earthquake location supplemental material for coseismic displacements on ischia island, real-time gps positioning constraints on earthquake source location roberto devoti1, prospero de martino2, grazia pietrantonio1, mario dolce2 (1)istituto nazionale di geofisica e vulcanologia, osservatorio nazionale terremoti, roma, italy (2)istituto nazionale di geofisica e vulcanologia,sezione di napoli, osservatorio vesuviano, napoli italy figure s1. comparison of different data sampling rate (blue 30 sec, green 180 sec, red 300sec) for the kinematic position solution of station mepo for the vertical (upper panel) east and north (midand lower panels) components. the time series span 24 hours of august, 21st 2017, the red vertical bar indicates the epoch of the mw 3.9 earthquake. 11 13 gps constraints on earthquake location figure s2. real-time offset detection algorithm worked out at five remaining stations of ischia island. the thinner blue line shows the significance ratio (t/t1%) whereas the heavy red line shows the offset estimated at each epoch in millimeters. the grey bars indicate the epochs at which the offsets become significant (t/t1% >1), next to it, the value of the offset is given along with its 1-sigma error. the sampling rate of the kinematic solution is 180 sec. valensise et al_annals_second version_formatted annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 1 fossil landscapes and youthful seismogenic sources in the central apennines: excerpts from the 24 august 2016, amatrice earthquake and seismic hazard implications gianluca valensise1*, paola vannoli1, roberto basili1, lorenzo bonini2, 1, pierfrancesco burrato1, michele matteo cosimo carafa3, umberto fracassi1, vanja kastelic3, francesco emanuele maesano1, mara monica tiberti1, gabriele tarabusi4 1istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy 2dipartimento di matematica e geoscienze, università di trieste, italy 3istituto nazionale di geofisica e vulcanologia, sezione roma 1, l’aquila, italy 4istituto nazionale di geofisica e vulcanologia, sezione roma 1, bologna, italy *gianluca.valensise@ingv.it abstract we show and discuss the similarities among the 2016 amatrice (mw 6.0), 1997 colfiorito-sellano (mw 6.0-5.6) and 2009 l’aquila (mw 6.3) earthquakes. they all occurred along the crest of the central apennines and were caused by shallow dipping faults between 3 and 10 km depth, as shown by their characteristic insar signature. we contend that these earthquakes delineate a seismogenic style that is characteristic of this portion of the central apennines, where the upward propagation of seismogenic faults is hindered by the presence of preexisting regional thrusts. this leads to an effective decoupling between the deeper seismogenic portion of the upper crust and its uppermost 3 km. the decoupling implies that active faults mapped at the surface do not connect with the seismogenic sources, and that their evolution may be controlled by passive readjustments to coseismic strains or even by purely gravitational motions. seismic hazard analyses and estimates based on such faults should hence be considered with great caution as they may be all but representative of the true seismogenic potential. i. introduction the 24 august 2016, amatrice earthquake (mw 6.0), which devastated a portion of the central apennines causing nearly 300 victims and substantial property damage, occurred in an especially well monitored region. hence it has taken merely two days to realize that the amatrice earthquake, although sharing the general characteristics of other apennines extensional events, exhibits peculiarities that took many earthquake geologists by surprise. scientists were once again confronted with a complex rupture, too deep and too shallow-dipping to connect with the active faults known in the region, and breaking across what many considered a major regional tectonic boundary (the monti sibillini thrust). the amatrice earthquake is the last of three disastrous, similarly-sized earthquake sequences, all caused by normal faulting along planes dipping 40°-50° to the southwest. the 2016 sequence follows the 6 april 2009 l’aquila earthquake (mw 6.3), annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 2 with its strong 9 april aftershock in the campotosto-monti della laga area (mw 5.4), and the 26 september and 14 october, colfiorito-sellano earthquakes (mw 5.7, 6.0 and 5.6), respectively located to the southeast and to the northwest of amatrice along the apennines crest (fig. 1). these earthquakes delineated a tectonic style that is largely at odds with the views on the active tectonics of the central apennines that dominated the scientific debate over the past thirty years (see discussion in vannoli et al. [2012]). after summarizing the established facts concerning the epicentral region (§2) we discuss the constraints arising from the tectonic and landscape evolution of the central apennines (§3) and from insar evidence for coseismic strains in recent extensional earthquakes (§4). finally we focus on the implications of our findings for an improved assessment of the regional seismic hazard (§5). the three sequences share the availability (a) of numerous and reliable earthquake data collected by the permanent ingv network and by temporary networks, and (b) of good quality insar data. the latter in particular proved fundamental for understanding the details of the source processes and unraveling the puzzle of their associated surface coseismic deformation, represented by limited or ambiguous surface breaks for all three earthquakes. insar observations (fig. 2) are by far the best tool available for modern earthquake geology, as they: carry a multi-wavelength signal that can be interpreted, with appropriate filtering, to investigate the properties of the main seismogenic source separately from the associated secondary and gravity-driven effects; provide a framework for interpreting the geology and landscape evolution resulting from repeated faulting; are sensitive to the fault patches that experienced the largest slip and insensitive to finer details of the coseismic ruptures. figure 1. geographic setting and seismogenic sources discussed in text. active faults from various sources are shown in red. sources in green are from the diss database [diss working group 2015]. the two subsources proposed for the amatrice earthquake, in purple, are from gruppo di lavoro irea & ingv [2016]. a dashed line shows the regional watershed. ii. previous and established knowledge similarly to the l’aquila and colfiorito regions, the area that spawned the amatrice earthquake has long been known as earthquake-prone. yet, the understanding of the exact location and characteristics of its seismogenic sources has always been limited. common characteristics of the 1997, 2009 and 2016 sequences are (see fig. 2 and table 1): faulting occurred between 3 and 10 km depth on southwest-dipping faults striking parallel to the local trend of the apennines; little or no slip was observed above 3 km depth, either by seismological methods or by inversion of gps and insar data; annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 3 figure 2. panels a-d: comparison of insar models. (a) 1997 colfiorito earthquake [moro et al. 2007]; (b) 2009 l’aquila earthquake [esa, 2009]; (c) 2016 amatrice earthquake, solution obtained from sentinel-1 data [gruppo di lavoro irea & ingv 2016]; (d) 1995 dinar, turkey earthquake [douglas and funning 2016]. the observed or presumed – depending on cases surface expression of the fault is shown by a thick black line. panels e-f: comparison of the "coseismic signature" obtained for two distinct model faults, showing the extent of coseismic uplift and subsidence per unit slip: (e) is an average of the models obtained for the 1997, 2009 and 2016 earthquakes, (f) reproduces the 1995 dinar earthquake. a thick red line shows the surface projection of the fault plane. see also tables 1 and 2. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 4 name, date and origin time (utc) ref strike (deg) dip (deg) rake (deg) fault length (km) fault width (km) fault top/bottom depth (km) avg slip (m) mw surface breaks (ref) colfiorito i 1997, 26 sep, 00:33 1 148 40 277 9.0 6.0 4.0/7.8 0.38 5.7 none (a) colfiorito ii 1997, 26 sep, 09:40 1 143 40 275 14.0 8.0 3.4/8.5 0.33 6.0 none (a) sellano 1997, 14 oct, 15:23 1 144 40 260 6.0 6.0 2.5/6.5 0.28 5.6 minor breaks (b) l'aquila 2009, 6 apr, 02:32 2 133 43 275 14.0 9.5 3.0/9.5 0.60 6.3 a few cm along paganica fault (c) campotosto 2009, 9 apr, 00:52 3 135 45 275 5.9 5.2 7.3/11.0 0.10 5.4 none amatrice 2016, 24 aug, 01:35 4 165-175 39-51 295-290 8.0 12.0 8.0 5.0 3.0/8.0 2.5/6.4 0.70 6.0 up to 20 cm along vettore fault (d) dinar (turkey) 1995, 1 oct, 17:57 5 148 54 277 11.4 8.6 1.4-8.0 1.44 6.1 (ms) up to 30 cm (e) table 1. fault parameters for the 1997, 2009 and 2016 central apennines earthquakes, and for the 1995 dinar, southern turkey earthquake: (1) diss working group [2015]; (2) vannoli et al. [2012]; (3) this work; (4) gruppo di lavoro irea & ingv [2016]; (5) wright et al. [1999]. surface breaks reports: (a) basili et al. [1998]; (b) cinti et al. [2000]; (c) vannoli et al. [2012]; (d) emergeo working group [2016]; (e) wright et al. [1999]. notice that the amatrice fault was modeled using two distinct planes (parameters from gruppo di lavoro irea & ingv [2016]). seismological, insar and gps observations combined indicate that the dip of the seismogenic fault is in the range 39°-51°, much lower than the dip commonly reported for central apennines active faults [e.g. vannoli et al., 2012] but coherent with a recent elaboration on the fault dip distribution of global normal faulting earthquakes [basili and tiberti, 2016]; as a result of the limited width of the rupture plane and of coseismic slip (<10 km and <1 m, respectively), the three earthquakes also exhibit a relatively small magnitude. similarly to thrust faults, however, the shallow dip of the fault may allow for substantially larger ruptures; in all cases the rupture appeared to be vertically confined by the interference with thrust planes dating to the build-up of the apennines fold-and-thrust belt (fig. 3). iii. relations with surface structures: landscape features over the past two decades various investigators have pointed out that the extensional tectonic regime responsible for the normal faulting earthquakes that occur along the crest of the apennines is relatively young. for example, the age of inception of one of the main and oldest extensional structures in the central and northern apennines, the etrurian fault system, dates back to the late pliocene-early pleistocene (see pucci et al. [2014], and references therein). a straightforward implication of this finding is that, given the current rates of extension in the apennines, the amount of topography that has been created at any point after the inception of this new stress field is only a fraction of the extreme topography seen along the chain axis; examples of that are the western side of the mt. vettore-mt. bove range, the northern side of the gran sasso range, and the velinosirente, majella and matese massifs as a whole. in their turn most of the intervening basins do not show the architecture that would result from sustained extensional faulting. the colfiorito plain, the castelluccio plain, the amatrice basin, and the mid-aterno valley, the main depressions that lie in front of the three normal faults being analyzed in this work (from north to south), are consistently too shallow, too narrow, and too short to be the long-term effect of the seismogenic faults that lie next to them; faults which should generate halfgraben architectures that are not seen. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 5 at first glance the three earthquakes do appear to have affected lowlands where population and infrastructures naturally concentrate lying next to fault-controlled ranges. yet a closer inspection of the insardetected elevation changes reveals that: (a) most of the regions undergoing coseismic subsidence are areas of high topography, not basins or depressions; and (b) the strain field associated with these typical apennines earthquakes induces significant but relatively localized subsidence (15-30 cm for the earthquakes being considered here) and very mild uplift (2-4 cm) over a much broader region. as for (a), it should be recalled that the observed topography reversal is the expected outcome and in fact a proof of the current extensional regime being rather youthful (see valensise and pantosti [2001] and references therein). as for (b) one should recall that bagnaia et al. [1992] pointed out on a purely geological basis that the central apennines are currently dominated by "gravity faults" that are disrupting and lowering the axial portions of the fold-andthrust belt. finally, all three insar models show that the causative faults of the three investigated earthquakes tend to create a basin-and-range architecture whose wavelength perpendicular to the chain is at least twice the spacing between existing basin-bounding faults. notice that any intervening afterslip or post-seismic relaxation would increase this wavelength, not reduce it. all these observations strongly suggest that the current landscape is largely a fossil terrain that is being slowly dismantled by the erosion caused by the well documented regional-scale uplift of the entire chain, and reshaped by the action of newly incepted normal faults such as those responsible for the three earthquake sequences investigated here (e.g. cavinato et al. [2002] and santo et al. [2014]). dealing with a fossil landscape implies also that several faults that punctuate it have ceased their activity and participate only passively to the ongoing deformation (see bonini et al. [2014] for a thorough discussion on these circumstances). one of many such cases is that of the pettino fault, near l’aquila: seen by many as a major active fault, it turned out to be “floating” in the hanging wall of the fault that caused the 6 april 2009, l’aquila earthquake [bonini et al., 2014]. iv. relations with surface structures: faults insar data provide crucial evidence on the role played by any surface fault occurring in the region hit by a significant earthquake. recall that insar observations record static slip not just dynamic slip over an interval of a few days before and after the event, and hence are suitable also to document postseismic readjustments, if any. figs. 2a, 2b, 2c show that for the 1997, 2009 and 2016 earthquakes most if not all interferometric fringes describing the subsidence associated with the mainshock are neatly closed in front of the presumed surface expression of the primary fault, implying that slip in the uppermost portion (up to 2-4 km depth) of the fault was much smaller than in its lower portions. in contrast, fig. 2d shows the case of the 1995 dinar (turkey) earthquake (ms 6.1), where dynamic slip was modeled up to a depth of 1.4 km and where a 30 cm-high scarp was observed along the upward prolongation of the model fault: in this latter case the fringes are broken against the scarp, or not resolvable due to diffuse deformation, thus testifying that at least part of coseismic slip propagated all the way to the surface. this different behavior is illustrated in figs. 2 e, 2f and in table 2. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 6 figure 3. geological sections across the causative faults of the 6 april 2009, l’aquila, 26 september 1997, colfiorito (based on data from chiaraluce et al. [2005]), and 9 april 2009, campotosto (based on data from bigi et al. [2012]) earthquakes. the seismogenic sources are shown by a red line; black dots show aftershock locations. the sections illustrate the vertical confinement of the seismogenic sources within pre-existing thrusts, locally illuminated by minor aftershocks (from bonini et al. [2014], modified). source strike (deg) dip (deg) rake (deg) length (km) width (km) top/bottom depth (km) d1 (km) d2 (km) d3 (km) smax umax rdisp rsym central apennines, italy 148 43 270 12.0 8.5 3.0-8.8 4.7 9.5 8.3 0.370 0.025 14.8 2.0 dinar, turkey 148 54 270 12.0 8.5 1.4-8.3 2.6 12.4 4.6 0.450 0.075 6.0 4.8 d1: distance of maximum subsidence from intersection of fault prolongation with surface d2: distance of max subsidence from zero displacement line in hanging wall d3: distance of site of max subsidence from site of max uplift smax: max subsidence (for unitary slip) umax: max uplift (for unitary slip) rdisp: smax/umax rsym: d2/d1 table 2. parameters used for the comparison of the "coseismic signatures" shown in fig. 2 e-f. (see also table 1). the 24 august 2016 earthquake generated a 20 cm-high scarp that could be followed over a distance of nearly 5 km along the vettore fault, a highly visible and highly recognized element in the central apennines landscape [emergeo working group, 2016]. in the first few days after the earthquake many workers argued that this scarp is evidence for primary slip at seismogenic depth; others contended that the scarp is merely a surface effect unrelated to slip at depth. crucial observations to settle this issue include: 1) seismological and geodetic evidence show that, largely due to its shallow dip, the plane that ruptured in the 24 august mainshock projects to the surface over 5 km ene of the surface expression of the vettore fault (see fig. 2 in michele et al. [2016] and fig. 2 in bonini et al. [2016]: see also our figs. 2c, 2e); annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 7 2) the elaboration of all of the three available insar datasets by different and independent groups (the japanese alos-2, sentinel-1 by the european copernicus program, and the italian cosmo-skymed, developed by agenzia spaziale italiana) and the slip model obtained by tinti et al. [2016] from the inversion of strong motion records clearly shows that neither dynamic nor static slip occurred during the mainshock or in the following few days in the uppermost 3 km of the crust; 3) the vettore fault lies at the very northern end of the 2016 rupture: no sizable breaks have been observed along the central part of the rupture, for example along the wellknown gorzano fault. the observed 5 kmlong scarp is in an awkward position relative to the length of the earthquake source and is too short for a mw 6.0 quake. 4) the vettore fault lacks a basin/depocenter representative of its long-term activity, knowing that the castelluccio plain is a karst basin elongated almost perpendicular to the mt. vettore ridge and filled with a thin veneer of continental deposits; 5) the surface breaks seen along the vettore fault have been shown to be largely the result of landsliding and sediment compaction [gruppo di lavoro irea & ingv, 2016], as testified also by the recent geomorphological evolution of the portion of the castelluccio plain that lies at the base of the vettore mt. [aringoli et al., 2014, and references therein]. for these reasons we maintain that the surface breaks seen along the western flank of the vettore mt. do not represent the primary expression of fault slip at depth, but rather some form of shaking-induced sliding along the vettore fault. this case is not isolated. in a large report written following the 13 january 1915, avezzano-fucino basin (central apennines) earthquake (mw 7.0), oddone [1915] described a large landslide affecting the talus deposits lying over the large and well known serrone fault. he described the landslide as clearly separated from the shaking. all buildings of the village of gioia dei marsi, unwisely built on these deposits, were totally destroyed, and every single wall was reported overturned. this suggests that their foundations had become loose under the combined action of shaking and gravity sliding. similarly to the vettore fault, the serrone fault lies at the very end of the 1915 rupture, to the southwest of the fucino basin. the literature of the central apennines reports a number of faults that are considered both active and seismogenic. vannoli et al. [2012], however, pointed out that these faults are too many, generally too small and too closely spaced to root in at seismogenic depth all at once. what is the evidence that they indeed play an active role in the current evolution of the region? key observations include: 1) most, if not all, of the bedrock normal faults are closely associated with thrusts that were active between the late miocene and the pliocene: if we admit that the onset of the current extensional regime is not older than the early pleistocene, these older normal faults may well be prone to be (passively?) reactivated, but they can hardly be seen as the primary expression of the current stress field; 2) the consistent lack of slip in the uppermost 3 km of the crust either coseismic or slow/post-seismic testified by insar data (fig. 2) and consistent with the spatial distribution of aftershocks [michele et al., 2016], indicates that the currently active seismogenic faults act independently from the structure and evolution of the shallowest portion of the crust; 3) basic knowledge on the structure of the apennines fold-and-thrust belt aided by analogue models (see bonini et al. [2015]) shows a sort of “vertical segmentation” of large active normal faults; these develop and annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 8 grow in-between major pre-existing subhorizontal discontinuities – e.g. large thrust planes that effectively isolate them from the surface; 4) insar evidence combined with the results of analogue modeling suggests that such discontinuities do not prevent the upward propagation of coseismic strain, but also that they do act as effective barriers for fault propagation, thus contributing to decouple the deeper from the shallower faults. iv. conclusions and sha implications we have provided and discussed evidence showing that, similarly to the 1997 colfiorito, and 2009 l’aquila earthquakes, the 24 august 2016 amatrice earthquake has been generated by a shallow-dipping, blind fault. as such this fault bears no relationships with the presumed active faults mapped in the region over the past 40 years. while this paper was under revision two additional damaging earthquakes occurred on 26 october near visso-ussita (mw 5.9) and on 30 october near norcia (mw 6.5). preliminary analyses of these two events suggest that they were also generated by shallow-dipping faults, relatively deep faults, although the 30 october event appears to have caused a complex pattern of brittle surface deformation. once again the central apennines seismogenic sources demonstrated to be youthful faults lying beneath a mattress formed by older thrusts and their accompanying extensional faults. the investigation of such deeper faults requires a new paradigm that finally acknowledges these circumstances and develops new and appropriate investigation strategies. our findings have four main categories of implications that need to be carefully considered to avoid gross misrepresentations of the local earthquake potential: a) the complex structure of the central apennines fold-and-thrust belt causes the effective decoupling between the normal faults that affect the uppermost 2-4 km of the crust and current extension accommodated by larger faults lying beneath them. as a result, surface faults are crucial for land planning but carry little information on seismogenic processes at depth (see #4 in previous section); b) for the same reasons, the evolution of large-scale, long-term landscape features e.g. the evolution of basin architectures and the distribution of fluvial or marine terraces should be regarded as the main and most faithful indicators of ongoing long-term strain at depth; c) as the current geodynamic and geomorphic evolution of the apennines is controlled more by regional uplift than by localized faulting, and in consideration of the above #a), one has to be extra careful not to interpret the largely climateand gravitycontrolled evolution of steep apennines mountain slopes as evidence for continuing activity of surface faults (an ample discussion on this topic is supplied in kastelic et al. [2016]); d) for exactly the same reasons, and based on the recent evidence from the vettore fault, from the serrone fault in the fucino basin and from a number of large landslides controlled by older surface faults (see moro et al. [2007] and moro et al. [2011], respectively for the 1997 and 2009 earthquakes), extreme care should be taken when trenching scarps of unproven origin; since they may represent only the accumulation of deposits mobilized by shaking, they may carry information suitable for dating the shaking itself but not for deriving meaningful slip rates or mw estimates. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 9 vi. acknowledgements we wish to thank an anonymous reviewer for a thoughtful revision. the work was supported by the project “abruzzo” (code: rbap10zc8k_001; rbap10zc8k_003), funded by the italian ministry for education, university and research (miur), and by the university of trieste, program “finanziamenti di ateneo per progetti scientifici – fra 2015”, p.i. lorenzo bonini. references aringoli, d., and 17 others (2014). morphotectonic characterization of the quaternary intermontane basins of the umbria-marche apennines (italy). rend. fis. acc. lincei, 25, 111-128, doi: 10.1007/s12210014-0330-0. bagnaia, r., a. d’epifanio and s. sylos labini (1992). aquila and subequan basins: an example of quaternary evolution in central apennines, italy, quat. nova, 2, 187209. basili, r., v. bosi, f. galadini, p. galli, m. meghraoui, p. messina, m. moro and a. sposato (1998). the colfiorito earthquake sequence of september–october 1997: surface breaks and seismotectonic implications for the central apennines, j. earthq. eng. 2, 291-302. basili, r., and m. m. tiberti (2016). new statistics of earthquake-fault dip angles. paper esc2016-447, presented at the 35th general assembly of the european seismological commission, trieste, 4-10 september 2016. bigi, s., p. casero, c. chiarabba and d. di bucci (2012). contrasting surface active faults and deep seismogenic sources unveiled by the 2009 l’aquila earthquake sequence (italy), terra nova, 25, 1, 21-29, doi: 10.1111/ter.12000. bonini, l., d. di bucci, g. toscani, s. seno and g. valensise (2014). on the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 april 2009 l’aquila (central italy) earthquake (mw 6.3), solid earth, 5, 389-408, doi: 10.5194/se-5-389-2014. bonini, l., r. basili, g. toscani, p. burrato, s. seno and g. valensise (2015). the role of pre-existing discontinuities in the development of extensional faults: an analog modeling perspective, j. struct. geol. 74, 145–158. doi: 10.1016/j.jsg.2015.03.004. bonini, l., f. e. maesano, r. basili, p. burrato, m.m.c. carafa, u. fracassi, v. kastelic, g. tarabusi, m.m.tiberti, p. vannoli and g. valensise (2016). imaging the tectonic framework of the 2016 amatrice earthquake: new roles for old players? annals of geophysics, 59 (5), doi: 10.4401/ag-7229. cavinato, g. p., c. carusi, m. dall’asta, e. miccadei and t. piacentini (2002). sedimentary and tectonic evolution of pliopleistocene alluvial and lacustrine deposits of the fucino basin (central italy). sedimentary geology 148:29-59 doi: 10.1016/s0037-0738(01)00209-3. chiaraluce, l., m. r. barchi, c. collettini, f. mirabella and s. pucci (2005). connecting seismically active normal faults with quaternary geological structures in a complex extensional environment: the colfiorito 1997 case history (northern apennines, italy), tectonics 24, tc1002. doi:10.1029/2004tc001627. cinti, f. r., l. cucci, f. marra and p. montone (2000). surface tectonic effects vs seismological data of 1997 umbria-marche earthquakes (italy), j. seismol., 4, 333-343. diss working group (2015). database of individual seismogenic sources (diss), version 3.2.0: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas. http://diss.rm.ingv.it/diss/ © istituto nazionale di geofisica e vulcanologia, doi: 10.6092/ingv.it-diss3.2.0. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7215 10 douglas, b., and g. funning (2016). imaging active tectonics with insar and lidar. getsi's earth-focused modules and courses for the undergraduate classroom, http://serc.carleton.edu/124395. emergeo working group: pucci, s., and 46 others (2016). coseismic effects of the 2016 amatrice seismic sequence: first geological results. annals of geophysics, 59, fast track, 5, doi: 10.4401/ag-7195. esa (2009). an envisat advanced synthetic aperture radar interferogram interpretation by ingv: http://www.esa. int/spaceinimages/images/2009/04l_aquil a_earthquake_envisat_interferogram. gruppo di lavoro irea & ingv (2016). sequenza sismica di amatrice: aggiornamento delle analisi interferometriche satellitari e modelli di sorgente. doi: 10.5281/zenodo.61682. kastelic, v., p. burrato, m. m. c. carafa and r. basili (2016). repeated surveys reveal non-tectonic exposure of supposedly active normal faults in the central apennines, italy. submitted to j. geophys. res. michele, m., r. di stefano, l. chiaraluce, m. cattaneo, p. de gori, g. monachesi, d. latorre, s. marzorati, l. valoroso, c. ladina, c. chiarabba, v. lauciani and m. fares (2016). the amatrice 2016 seismic sequence: a preliminary look at the mainshock and aftershocks distribution, annals of geophysics, 59 (5), doi: 10.4401/ag-7227. moro, m., m. saroli, s. salvi, s. stramondo and f. doumaz (2007). the relationship between seismic deformation and deep-seated gravitational movements during the 1997 umbria–marche (central italy) earthquakes, geomorphology, 89, 297– 307, doi:10.1016/j.geomorph.2006.12.013. moro, m., m. chini, m. saroli, s. atzori, s. stramondo and s. salvi (2011). analysis of large, seismically induced, gravitational deformations imaged by high-resolution cosmo-skymed synthetic aperture radar, geology, 39(6), 527–530, doi:10.1130/ g31748.1. oddone, e. (1915). elementi fisici del grande terremoto marsicano-fucense del 13 gennaio 1915, boll. soc. sismologica italiana, 19, 71-217. pucci, s., f. mirabella, f. pazzaglia, m. r. barchi, l. melelli, p. tuccimei, m. soligo and l. saccucci (2014). interaction between regional and local tectonic forcing along a complex quaternary extensional basin: upper tiber valley, northern apennines, italy, quat. sci. rev., 102, 111–132, doi: 10.1016/j.quascirev.2014.08.009. santo a., a. ascione, g. di crescenzo g., e. miccadei, t. piacentini and e. valente (2014). tectonic-geomorphological map of the middle aterno river valley (abruzzo, central italy), journal of maps, 10 (3), 365378, doi: 10.1080/17445647.2013.867545. tinti, e., l. scognamiglio, a. michelini and m. cocco (2016). slip heterogeneity and directivity of the ml 6.0, 2016, amatrice earthquake estimated with rapid finite-fault inversion, geophys. res. lett., doi: 10.1002/2016gl071263. valensise, g., and d. pantosti (2001). the investigation of potential earthquake sources in peninsular italy: a review, j. of seismology, 5, 287-306. vannoli, p., p. burrato, u. fracassi and g. valensise (2012). a fresh look at the seismotectonics of the abruzzi (central apennines) following the 6 april 2009 l’aquila earthquake (mw 6.3), ital. j. geosci., 131, 309-329, doi: 10.3301/ijg.2012.03. wright, t. j., b. e. parsons, j. a. jackson, m. haynes, e. j. fielding, p. c. england and p. j. clarke (1999). source parameters of the 1 october 1995 dinar (turkey) earthquake from sar interferometry and seismic bodywave modelling, earth and planetary science letters, 172, 23-37, doi: 10.1016/s0012-821x(99)00186-7. layout 6 1 special issue when volcanologists meet archaeologists and other disciplines foreword this special issue of annals of geophysics entitled: “when volcanologists meet archaeologists and other disciplines: relationships between eruptions and human communities” originates from a session (s13) of the rittmann international conference which took place in catania on february 13th 2020, having as its main theme the history of volcanology and the impact of volcanic activity on humans. the twelve articles collected in this special issue reflect the aims and contents of the reports presented by some participants at this session of the rittmann conference. volcanological science already had the characteristic of a strongly interdisciplinary field in the vision that alfred rittmann developed during the course of the twentieth century. current technology allows us to investigate volcanic phenomena by acquiring more and more data from different archives that make their processing and integration increasingly complex. the study of the relationships between eruptions and human communities, together with the social, economic and urban consequences, embraces numerous disciplines ranging from volcanology, geology, geomorphology and geophysics, to archaeology, anthropology, sociology and history. nevertheless, interdisciplinary studies between these various field of study are limited, both for the lack of a common language and for purely cultural reasons. among the other interdisciplinary studies, a fruitful exchange of information between volcanologists and archaeologists and optimal integration of data from these two disciplines is necessary. the main purpose of the papers collected in this special issue is to stimulate innovative and interdisciplinary approaches aimed at studying the history of volcanology and the relationships between eruptive phenomena and the communities that have interacted through time with volcanic phenomena, and also to improve the assessment of volcanic hazard and risks. the twelve articles included in this special issue can be divided into two main groups. the largest group (eight contributions) deal with the eruptive history of some famous italian volcanoes, and the impact that their activity had on the destiny of local communities, since prehistoric times. the other group (four contributions) deals with the volcanic context and its materials both as a resource for human communities and as a means of archaeological, historical and anthropological investigation. below there is a brief review of the contents and authors of each work. eruptive history and human communities the human presence on the slopes of mt etna volcano is reconstructed by branca et al. starting from the first neolithic settlements, together with the impact of the different eruptive manifestations and the volcano’s geological changes on the prehistoric and proto-historic communities. volcanological and historical investigations on the 1971 major flank eruption of etna and the formation of a new summit crater, the southeast crater, are presented by branca et al. a multimedia exhibition, created to illustrate the complex interactions between human settlements and the geological dynamics of the island of ischia is presented by de vita et al. the exhibition is hosted in the geological annals of geophysics, 64, 5, 2021; doi:10.4401/ag-8764 o p e n acc e s s section of the archaeological museum of villa arbusto, at ischia, dedicated to the archaeologist giorgio buchner. 6000 years of human presence and abandonments at stromboli volcano and an overview on the whole aeolian archipelago are described by di renzoni et al. the authors conclude that the human presence is related more to the volcanic and island environment than to the volcanic activity. di vito et al. present recent geo-archaeological discoveries which allow a detailed picture of prehistoric human settlements during the succession of intense activity and quiescence of the campi flegrei and vesuvius volcanoes. giacomelli et al. reconstruct the effects on buildings, vegetation, animals and human of various phenomena occurred during the 79 a.d. plinian eruption of vesuvius. the authors focus the study on the site of pompei. historical and cartographic documents are re-examined by manni and rosi. they confirm the emergence of a new island north of vulcano, at the turn of the year 1000 ad, which played a prominent role in favoring the creation of the flat area of vulcano porto and a sandy isthmus that definitively connected vulcanello to the main vulcano island. volcanological, archaeological and historical data on one of the lesser known phases of the eolian archipelago’s history are presented by martinelli et al. this volcanism occurred after at least 6000 years of quiescence and was followed by a demographic crisis between the 6th and 11th century ad. the authors pay particular attention to the last eruption in the 13th century during the early middle age. volcanic landscape and materials as resources foresta martin and furlani investigate how geomorphology, volcanic materials and other local resources influenced the choice of early human settlements sites, their development and their end in the small volcanic island of ustica (palermo, sicily). lava millstone covering a period from the phoenicians to the romans are investigated by santi et al. the authors propose a set of discriminating geochemical parameters to identify the four main volcanic areas of italy used in the antiquity to produce millstones: in the surrondings of orvieto, etna, hyblaean mountains and pantelleria. the discovery of a new neolithic settlement in the island of ustica (palermo, sicily) is described by speciale et al. the authors characterized materials and tools found during excavation and established their possible provenance. environmental and climatic changes during the late pleistocene at the monte amiata volcano (southern tuscany, italy) are reconstructed by vezzoli et al. using paleontological samples quarried since 16th century from small extinct lake basins of late pleistocene in the foothills of monte amiata. they are part of the ezio tongiorgi paleontological collection consisting of fossil diatoms exposed in the museum of natural history of the university of pisa. the guest editors and the authors are grateful to the managing editor for the opportunity to publish this special issue of annals of geophysics, and to the reviewers whose contribution has been fundamental to improve the quality of the articles. references branca, s., f. privitera, o. palio and m.turco (2021). prehistoric human presence on mount etna (sicily) in relation to the geological evolution, ann. geophys., this volume, doi:10.4401/ag-8667. branca, s., d. musumeci and l. ingaliso (2021). the significance of the 1971 flank eruption of etna from volcanological and historic viewpoints, ann. geophys., this volume, doi:10.441/ag-8669. de vita, s., m.a. di vito, d. barra, g. aiello and c. gialanella (2021). disseminating the knowledge on the complex interactions between humans and volcanoes: the geological section of the villa arbusto archaeological museum at lacco ameno (ischia, naples – italy), ann. geophys., this volume, doi:10.4401/ag-8666. di renzoni, a., s.t. levi, a. renzulli, m. rosi and d. yoon (2021). should i stay or should i go? 6000 years of human presence and abandonments at stromboli volcano and an overview on the whole aeolian archipelago (southern tyrrhenian sea, italy), ann. geophys., this volume, doi:10.4401/ag-8664. di vito, m.a., p. aurino, g. boenzi, e. laforgia and i. rucco (2021). human communities living in the central campania plane and eruptions of vesuvio and campi flegrei since neolithic, ann. geophys., this volume, doi:10.4401/ag-8708. foresta martin, f. and s. furlani (2021). geomorphology and prehistoric settlements on a volcanic island: the case 2 of ustica (palermo, italy) (2021). ann. geophys., this volume, doi:10.4401/ag-8703. giacomelli, l., r. scandone and m. rosi (2021). the loss of geological memory of past catastrophes: the case of pompei (2021). ann. geophys., this volume, doi:10.4401/ag-8631. manni, m. and m. rosi (2021). origins of vulcanello based on the re-examination of historical sources (vulcano, aeolian islands), ann. geophys., this volume, doi:10.4401/ag-8670 martinelli, m.c., m. manni and m. coltelli (2021). resilience and adaptation to volcanoes in late middle ages in lipari island (aeolian, italy), ann. geophys., doi:10.4401/ag-8648. santi, p., t. gambin and a. renzulli (2021). the millstone trade from the most exploited italian volcanic areas: an overview from the phoenicians to the roman period, ann. geophys., this volume, doi:10.4401/ag-8647. speciale, c., g. montana, r. mentesana, c. forgia, f.mantia, g. battaglia, m.a. di vito, s. vassallo and s. de vita (2021). materials and tools across volcanoes: exploitation of obsidian and other volcanic rocks in ustica (italy) during prehistory, ann. geophys., this volume, doi:10.4401/ag-8684. vezzoli,l., c. principe and c. sorbini (2021). the paleo-lacustrine diatomaceous deposits of monte amiata volcano (tuscany, italy) and the ezio tongiorgi paleontological collection in the museum of natural history of the university of pisa, ann. geophys., this volume, doi:10.4401/ag-8634. g u e s t e d i to r mauro antonio d i v i to istituto nazionale di geofisica e vulcanologia sezione di napoli, osservatorio vesuviano napoli, italy franco f o r e s ta m a rt i n laboratorio museo di scienze della terra isola di ustica, palermo, italy maria clara m a rt i n e l l i parco archeologico delle isole eolie museo luigi bernabò lipari, italy 3 layout 6 1 annals of geophysics, 62, 2, vo216, 2019; doi: 10.4401/ag-7859 “advances in the rheology of natural multiphase silicate melts: importance for magma transport and lava flow emplacement„ daniele giordano*,1,2,3 (1) università degli studi di torino, dipartimento di scienze della terra, torino, italy (2) istituto nazionale di geofisica e vulcanologia−sezione di pisa, pisa, italy (3) institute of geoscience and earth resources (igg−cnr), italian national research council (cnr), pisa, italy 1. introduction the transport of magmas and volcanic materials is characterized by very dynamic, interdependent and com− plex, physical and chemical processes that all are affected by and affect the materials physical properties. under− standing the dynamic processes operating during magma ascent and eruption and the timescales and mechanisms of emplacement, welding and remobilization of frag− mental or massive volcanic deposits, constitutes one of the main challenges in the earth sciences [dingwell, 1996; papale, 1999; sparks, 2004; russell and quane, 2005; giordano et al., 2005]. accurate description of these processes requires the characterization of a wide range of transport and thermodynamic properties for the melt or magma (e.g. viscosity, density, enthalpy, entropy, heat capacity, thermal conductivity, solubility of volatile phases). these properties play crucial roles at micro− to macroscopic scale and many are correlated, in a non−lin− ear manner [e.g. richet et al., 1984; giordano et al., 2008a; russell and giordano, 2017]. lava flow dynamics are strongly governed by sub− surface buoyancy forces, resulting from the density con− trast with the host rock, which push the magma toward the surface [e.g. wilson and head, 2016a; wieczorek et al., 2001] and by the evolving internal and external frictional forces (e.g. with dyke, conduit wall and topography) that oppose to the movement of magmas and lavas [e.g. nemeth, 2010; cañón−tapia, 2016, dragoni, 1993; drag− oni et al., 2005; giordano et al., 2007; cashman et al., 2013; kolzenburg et al., 2016a,b; 2018a,b; hulme, 1974; hiesinger et al., 2007; chevrel et al., 2013, 2015; cas− truccio et al., 2014]. the rheological properties of magmas undergo tremendous changes from transport in the subsurface to eruption or emplacement at the surface and to final de− article history receveid july 17, 2018; accepted march 18, 2019. subject classification: multicomponent and multiphase silicate melts; rheology. abstract a review of recent advances in the field of rheology of multicomponent silicate melts and multiphase silicate melt and multiphase silicate melt and analogue material suspensions is presented. is presented here. the advances include the development of new experimental de− vices and field and remote sensing methods for measuring the rheological properties of natural melts and magmas as well as new mod− elling strategies. these promising approaches combine laboratory experiments, theoretical models, numerical simulations and remote sens− ing data derived from ground, airborne and satellite−based tools. each of these sub−disciplines has evolved rapidly in recent years and the growing range of complementary data appears now to provide an opportunity for the development of multi−disciplinary research. ul− timately, these multidisciplinary initiatives seek to provide near−real−time forecasting of hazardous volcanic processes such as lava flow field evolution. the results and approaches described here focus on multiphase (i.e. melts, bubbles, crystals) rheology of natural systems and are pertinent to the effusive emplacement of lavas, dykes and sills, as well as, to the eruption dynamics attending explosive eruptions. position and cooling. these changes are caused domi− nantly by the evolving of thermo−chemical and defor− mational conditions, imposing phase transitions and therewith heterogeneous textural and morphological variations of the magmatic and volcanic suspensions which evolve in space and time. the complex rheologi− cal evolution of lava flows can tentatively be constrained by carrying out laboratory measurements under con− trolled conditions, simulating natural systems, and by monitoring flow emplacement at the field−scale and via satellite−based platforms. in parallel with this, the so− phistication of physical models of lava flows and domes have improved significantly and are capable of provid− ing fast simulations [see, amongst the others, costa and macedonio, 2003, 2005; del negro et al., 2008, 2013, 2016; melnik and sparks, 1999, 2005; melnik et al., 2009; kilburn 2015 for reviews on this topic]. these models are increasingly informed by, or validated by, satellite−derived parameters such as lava flow discharge rate or periodic updates on flow advance/geometry. to− gether these capabilities represent an emergent strategy that may provide timely reliable projections of lava flow field evolution and derive information for hazard assess− ment and mitigation measures. yet, to date they do not always provide coherent results reproduced in nature. this highlights the necessity to estimate the rheolog− ical properties of magmas and volcanic materials at con− ditions pertinent to nature and to investigate the effect of each variable over the range of relevant environmental conditions (e.g. pressure, temperature, volatile contents) during varying thermodynamic (equilibrium and non− equilibrium) conditions, and deformation regimes. our understanding of the single− and multi−phase (liquid+crystals+bubbles) rheology of magmas and vol− canic products has greatly improved in the last two decades. this can largely be attributed to the growing availability of empirical data from the following sources (each of which will be reviewed in detail below): 1) laboratory experimentation on natural and simpli− fied silicate melts. these data support the creation of robust models for predicting the newtonian vis− cosity of pure liquid natural melts as a function of temperature (t), pressure (p) composition (x), volatile content (xv) and structural features (see chapter 2). 2) the rheological experimentation and modelling of non−reactive multiphase suspensions (liquid+bub− bles; liquid+crystal and liquids+bubbles+crystals) constituted by analogue materials or simplified or natural silicate melts mixtures (chapter 4); 3) dynamic cooling rheological measurements on nat− ural multiphase suspensions at non−isothermal and non−equilibrium conditions to explore the interde− pendent effects of composition, cooling−rate, shear− rate and oxygen fugacity acting during magma and lava transport in nature (chapter 5); 4) rheological measurements of actively flowing lava. these represent snapshots of actual lava flow rheol− ogy at specific conditions and provide data that helps to constrain the conditions required to be reproduced in systematic laboratory studies (chapter 6). 5) studies on the 3d and 4d evolution of lava flows at increasing spatial and temporal resolution and con− temporary estimates of effusion rate and flow de− velopment from satellite data. these provide data for cross correlation and benchmarking of labora− tory measurements (appendix a1) and to re−visit long standing methods for deriving rheological pa− rameters from morphologic data (chapter 6). these studies document that the effective viscosity of natural silicate melts and magmas can span more than 15 orders of magnitude (10 1 – 1014 pa s), primarily in re− sponse to variations in melt composition (x), dissolved volatile content (xv), temperature (t), pressure (p), as well as the proportions, size, and shape distributions of sus− pended solid and/or exsolved fluid phases (i.e. crystals and bubbles). the deformation rate, which in nature would depend on the discharge rate will determine whether flow behavior will be newtonian (i.e. one for which there is a linear relationship between stress and strain rate; or spatial variation of velocity) or non−newtonian [e.g. caricchi et al., 2007, 2008; costa et al., 2007a, 2009; vona et al., 2011; hess et al., 2009]. deformation rate also ex− erts an influence on the crystallization kinetics [vona et al., 2013; kouchi, 1986; kolzenburg, 2018]. it may fur− ther determine whether the melt will deform viscously or elastically and, therewith, whether or not it will eventu− ally fracture giving origin to effusive rather than explo− sive eruptive styles [dingwell, 1996]. combined the above experimental data and computational models form a ba− sis from which to understand the flow behavior of nat− ural magmatic and volcanic suspensions. in the following i present a review of the research ad− vances in the rheological characterization of pure silicate melts and multiphase silicate mixtures (i.e. lavas and magmas) achieved in the past decades. i follow the struc− ture of points 1−5 outlined above to group the individ− ual fields. in the appendices (a1−a3) i summarize the daniele giordano 2 most commonly employed experimental devices and technological advances to measure the single and mul− tiphase silicate melts also reporting the most common equations used to describe the viscosity variation as a function of p, t, x (appendix a1) as well as suspended solids phase and/or porosity (appendices a2 and a3). i conclude with a discussion on how new laboratory de− velopments together with the growth in complementary datasets (e.g. remote−sensing; drone technology; high− speed calculation facility) is providing greater under− standing of magma and lava transport on earth. 2. pure liquid melt newtonian viscosity experiments and models 2.1 t – dependent models for predicting melt viscosity the first step toward characterizing multiphase rhe− ology of natural silicate melts mixture is the knowledge of multicomponent viscosity of pure liquids as a function of their composition (including dissolved volatile species such as h2o, c and s –species, f, cl) temperature (t) and pressure (p). early models for predicting the viscosity of silicate melts were developed using data that spanned rel− atively small ranges of temperature (t) and viscosity (η). these experimental data, restricted to superliquidus tem− peratures and a narrow compositional range, showed a nearly linear trend of viscosity in reciprocal temperature space. thus, early models adopted an arrhenian formu− lation of the temperature−viscosity relationship [shaw, 1972; bottinga and weill, 1972]. expansion of the melt viscometry database over a wider range of compositions and temperatures exposed the limitations of arrhenian models. with the emergence of viscometry data closer to the glass transition temperature (tg) (i.e. the temperature of transition between a liquid−like and a solid−like be− havior) [e.g. angell, 1991, giordano et al., 2005], the ar− rhenian models proved unsuitable to describe the tem− perature dependence of silicate melt viscosity. these measurements were enabled by experimental devices that allow very small displacements to be monitored (e.g. linear voltage displacement transducers) and, the pro− duction of quenched glasses, freezing in the crystal free melt structure. in these experiments, supercooled glasses are reheated above tg, where the “relaxed melt” viscos− ity [e.g. angell, 1991; scherer, 1984] could then be mea− sured. these experiments are performed at timescales shorter than phase transitions timescale, therewith al− lowing anhydrous and hydrous pure liquid viscosity measurements [angell, 1991; scherer, 1984; giordano et al., 2008b]. based on the large number of experimental studies [e.g. richet et al., 1995, 1996; hess and dingwell, 1996; whittington et al., 2000, 2001; giordano et al., 2009 amongst the others], models of melt viscosity were de− veloped [e.g. avramov, 1998; angell, 1991; russell et al., 2003; giordano and dingwell, 2003a, b; russell and giordano, 2005; giordano and russell, 2007; hui and zhang, 2007; giordano et al., 2006, 2008a,b; ardia et al., 2008; mauro et al., 2009], also accounting for the non− arrhenian viscosity behaviour [e.g. vogel, 1921, fulcher, 3 advances in rheology: application to lava flows emplacement figure 1. the figure shows the variation of viscosity as a function of the reverse of temperature for the anhydrous melts (a) and, per comparison, the anhydrous and hydrous melts (b) as reported by giordano et al. (2008). the curves in a) represent the most arrhenian (continuous line in a))(strong) and the least arrhenian (dashed curve in a)) (fragile) melts amongst those reported in panel of figure 1a. the effect of water is that of significantly reducing viscosity and the fragility (de− viation from arrhenian behavior) of the melts [details in giordano et al., 2008]. daniele giordano 4 1925; tammann and hess, 1926; adam and gibbs, 1965]. these models describe the p−t−x dependence of the viscosity of silicate melts. some of the most relevant em− pirical and theoretical formulations describing the t− dependence of silicate melts and the relationships between constitutive parameters are reported in appendix a2. the growing database and the new models show that silicate melts display various degrees of non−arrhenian behavior, from strong to fragile [angell, 1991; russell et al., 2002, 2003], which depend on composition and dis− solved volatile content (figure 1). all these models provide viscosity predictions based on composition commonly expressed in terms of oxide abundances or combination of oxides and a range of ad− justable parameters. of the various models only the hz model [hui and zhang, 2007] and the grd model [gior− dano et al., 2008a] accounts for the effects of dissolved volatile species (h2o, f). the grd model is based on the well−known vft (vogel−fulcher−tammann) equation, such that: log [η (pa s)] = avft+bvft/(t−cvft) (1) where avft is the pre−exponential factor, bvft is the pseu− do−activation energy and cvft is the vft−temperature. in contrast the hz model uses a purely empirical t−de− pendent viscosity formulation of non straightforward cor− relation with thermo−physical amounts. the grd mod− el has gained support due to its simplicity and direct cor− relation of constitutive parameters (i.e. appendix a2) to others important physical and structural properties such as the glass transition (tg), the fragility (m) (i.e. the rate at which viscosity varies with temperature, that is an in− dication of melts capacity to store energy), the calorimetric properties (configurational entropy, sconf and the config− urational heat capacity cp conf; see equation a2.4) [e.g. gior− dano and dingwell, 2003a; giordano et al., 2008b; chevrel et al., 2013; giordano and russell, 2017; russell and giordano, 2017] and the structural properties (e.g. qn− species and raman ratio) [i.e. le losq and neuville, 2017; giordano and russell, 2018; giordano et al., 2019]. these models show that, to a first approximation, the viscosi− ty of silicate melts and the descriptive parameters of equa− tion 1) can be correlated at constant temperature to em− pirical, composition−based pseudo−structural parameter (i.e. the sm − structural modifiers − and the nbo/t − i.e. the non bridging oxygen over tetrahedra − parameters). the nbo/t and sm parameters are commonly assumed as proxies for the degree of polymerization of silicate melts and glasses [e.g giordano and dingwell, 2003a, b; gior− dano and russell, 2018; giordano et al., 2019] (figures 2,3). compositions with low values of the sm−parameter (or low nbo/t values) are associated to strong (arrhenian− like) rheological behavior, i.e. a linear behavior in the logη− 1/t space, and more polymerized melts. on the other hand high values of sm (or high nbo/ts) are related to more depolymerized melts which show fragile rheological be− havior (i.e. the logη vs 1/t paths are significantly non− linear) [e.g. angell, 1991; giordano and dingwell, 2003] (figures 1,2). russell et al. [2003], in agreement with ear− ly theoretical studies [e.g. angell, 1991 amongst others], showed that the pre−exponential factor of the vft and ag formulations, i.e. the viscosity at infinite temperature (appendix a2), is a constant independent of compositions [russell et al., 2003; giordano et al., 2008a]. in addition, approximately 1 logunit distinguishes the avft and the aag values. the current models are applicable within the compositional space that they are based upon, but some compositional regions (e.g. peralkaline compositions) still remain unmapped and the models struggle to repro− duce measured viscosity values [giordano et al., 2006, 2008a, di genova et al., 2017]. those formulations also put in evidence that the role of water (h2o) dissolved in the melt is counterintuitive being opposite to that of net− work modifier cations. in fact, although dissolved h2o strongly decreases the viscosity of silicate melts (figure 1b), the parameters describing the t−dependence of vis− cosity (e.g. bvft and cvft in equation 1) are differently af− fected by h2o and by the most common structure mod− ifiers (figure 2). 2.2 p – dependent models for predicting melt viscosity measuring the effect of pressure (p) on the viscosity of melts is a complex experimental task and, as a result, has not been investigated extensively. a short summa− ry of applied techniques and technological advances is reported in appendix a1, together with some of the main results. largely, and oppositely to silica−rich melts, the viscosity of silica−poor melts increases as pressure in− creases [liebske et al., 2005; ardia et al., 2008 and ref− erences therein]. however, the available data suggest that the effect of p is negligible at near surface conditions per− tinent to explosive and effusive volcanism. as a con− sequence this effect will not be discussed any further in this contribution. figure a2.1 shows for the ab−di sys− tem what is the effect of p which changing composition in the binary system, by using fitting procedure as adopt− 5 advances in rheology: application to lava flows emplacement ed by ardia et al. [2008]. this system is considered to show what is the effect of p on polymerized (ab) to de− polymerized (di) synthetic compositions from low to high p. similar behaviours is expected for natural composi− tions, but, as shown by previous authors [e.g. giordano et al., 2008b; chevrel et al., 2013; whittington et al., 2009], simplified systems (e.g. an, di, ab) should not be considered as proxies for natural compositions. 2.3 toward a structural model for geological melts more recently, le losq and neuville [2017], giordano and russell [2018] and giordano et al. [2019], following different approaches, showed that the viscosity of sim− ple and multicomponent anhydrous silicate melts over a temperature interval of ~ 700 to 1600°c, can be predicted from the raman spectra obtained from the correspond− ing glasses (i.e. fast quenched melts). these methods prove to be very promising methods for in situ rheolog− ical investigations and may have great importance for planetary sciences studies [angel et al., 2012; giordano and russell, 2018]. le losq and neuville [2017] developed a 13 − parameters model for melt viscosity in the simple system sio2−na2o−k2o which connects the transport and thermodynamic properties of these simple melts ex− plicitly to the structural state of the melt expressed via the figure 2. relationships between constitutive parameters of the grd model (giordano et al., 2008), based on the vft formulation (equation 1), as a function of the modified sm (structure modifiers) parameter (giordano and dingwell, 2003). the role of increasing sm on the constitutive parameters of anhydrous melts (black symbols) is that of decreasing bvft and in− creasing cvft (figure 2a, b) while increasing the fragility (m) (figure 2e). on the other hand adding h2o to the melt struc− ture (gray symbols) results in decreasing bvft while decreasing cvft , the glass transition temperature tg (as taken at a viscosity of 1012 pas) (figure 2d) and the fragility (m). this observation put in evidence that the structural role of h2o is different from that of those cations which simply modify silicate melts structure (giordano et al., 2008, 2009). abundances of qn −species recovered from raman spec− tral analysis of the glasses. giordano and russell [2018] first and giordano et al. [2019], later, the presented a first order model predicting the viscosity of multicomponent natural melts by the employment of the so−called raman ratio (r) [giordano and russell, 2018] and normalized ra− man ratio (rn) [giordano et al., 2019] derived by raman spectra measured on the corresponding glasses as defined by mercier et al. [2009, 2010]. as shown in figure 3 a strong relationship exists between bvft and cvft param− eters and r which allows the viscosity of anhydrous multicomponent natural melts to be predicted with a great accuracy. although, the model requires expansion to use of the structural information of volatile−bearing melts, it allows accurate description of the viscosity of anhydrous melts by the employment of a simple equa− tion with 6 adjustable parameters and the measured r. also the sm and nbo/t parameters, calculated from compositions, are shown to be strongly correlated with r. 3. from pure liquids to multiphase analogues and magmas: advantages and disadvantages of the different experimental approaches being magmas and volcanic materials complex, texturally evolving (reactive) mixtures of crystals and vesicles suspended in a silicate melt phase which evolve as a function of the evolving p, t and compo− sitional variations and dynamic regimes, the descrip− tion of effect of suspended phases on the viscosity of these natural suspensions has followed different ap− proaches. the early models devoted to describe the multiphase rheology were historically based on the in− vestigation of analogue materials [e.g. einstein, 1906; einstein and roscoe, 1952]. more recently, the basis for the description of natural multiphase suspensions has been largely developed using natural and simplified silicate melts mixtures at experimental conditions at around thermodynamic equilibrium [e.g. campagnola et al., 2016; chevrel et al., 2015; robert et al., 2014; sehlke et al., 2014; soldati et al., 2016; vona and ro− mano, 2013; vona et al., 2011, 2013]. as such, their ap− plication to natural environments requires extrapola− tion into the thermal and deformational disequilibrium state at which magmatic and volcanic processes com− monly operate. this is only possible to a limited ex− tent, as natural magmatic and volcanic processes of− ten operate quite far from equilibrium. recent studies on the disequilibrium rheology of crystallizing natu− ral silicate melts have documented that deformation− rate and cooling−rate may significantly affect the phase transitions of magmatic mixtures so to forcing the material toward a thermal and mechanical dise− daniele giordano 6 figure3. model vft parameters bvft (r)(a) and cvft (r)(b) as defined by giordano and russell (2018) (panels a, b) and relationships between pseudo−structural parame− ters (sm, nbo/t)(c), as a function of the raman ratio (r). according to the above mentioned authors: bvft (r) = b1r b2 and cvft (r)=c1r c2+c3 where b1, b2, c1, c2, c3 are adjustable parameters. quilibrium state [e.g. giordano et al., 2007; kolzenburg et al., 2016, 2017a, b; 2018a,b; arzilli and carroll, 2013]. following, the experimental efforts aimed at re− trieving information of the multiphase rheology of natural silicate mixtures have been broadly summa− rized by subdividing it into, experiments on non reac− tive materials (chapter 4) or reactive silicate melts mixtures undergoing variable thermal or deforma− tional variation (chapter 5). these kinds of experiments can be further subdivided into three main categories: a) experimentation on analogue materials; b) experi− ments on simplified silicate mixtures and c) experi− ments on natural volcanic products. each of these ex− perimental approaches has different advantages/disadvantages which are listed below. 3.1 analogue materials multiphase analogue materials are commonly con− stituted by non−reactive mixtures of mono− or poly− disperse particles and/or bubbles, with varying content and shape and size distributions, immersed in some newtonian synthetic fluid (e.g. silicon oil, syrup, liq− uid paraffin), which can be investigated at room tem− perature. these multiphase mixtures can normally be investigated at room temperature conditions and therefore their rheological characterization is simpli− fied as it does not involve the need for high tempera− ture or pressure equipment and the sample texture can readily be controlled (e.g. the solid/bubble pro− portion or variation). these kinds of experiments are commonly performed on transparent multiphase mix− tures and therefore allow observing and characteriz− ing strain partitioning processes occurring amongst the phases during the deformation. the main disadvantage of this kind of experimentation is that it cannot re− produce neither the transient disequilibrium processes occurring in natural mixtures (e.g. crystallization or degassing stages) nor the natural dynamic physical properties of silicate melts (e.g. viscous and cohesive forces between the natural residual melt and sus− pended particles and bubbles). the largest part of these studies investigate two phase suspensions of either liquid and solid particles (simulating crystal bearing magma) or liquid and bubbles or vesicles (simulating the exsolution of volatile gases). 3.2 multiphase silicate melt suspensions the experiments of categories b) and c), above men− tioned, require significantly more complex experimental infrastructure and are substantially more complex to be characterized in terms of textural parameters (crystal and bubble content; crystals and bubbles size and shape dis− tributions) but they offer the opportunity to perform measurements on materials with direct application to the earth sciences. the inherent inhomogeneity of geo− materials and the large variations of the size– and shape−distributions found in natural products can, to date, not be captured in a satisfactory manner by the available theoretical or empirical models. experiments on natural materials at controlled conditions have the advantage of being representative of natural scenar− ios and, in most cases, they allow retrieving, at least, the final stage of textural evolution as a function of the imposed environmental conditions (i.e. isothermal; non−isothermal; isobaric; non−isobaric) as well as varying deformation regimes (i.e. constant or varying stress and/or strain rate). this allows the reconstruc− tion of the rheological parameters in a tightly con− strained parameter space, however it requires unique experimental characterization for each studied sce− nario. when volatile free samples are investigated, this kind of experiments can be performed, using a va− riety of experimental techniques (e.g. rotational con− centric cylinder/patterson deformation rig and uniax− ial compression and/or micropenetration and parallel plates techniques; see appendix a1 for details), over the entire temperature−viscosity interval from super− to sub−liquidus conditions that are characteristic of natural environments. for volatile−bearing natural melts and suspensions, this becomes more complex as limited experimental infrastructures exist to date to measure at the elevated pressures required to maintain volatiles in solution. there have been some recent ad− vances which take advantage of the metastable liquid state close to tg or by using devices which allow the sample to be pressurized [paterson, 1978; paterson and olgaard, 2000; caricchi et al., 2007; 2008; ardia et al., 2008; robert et al., 2008a, b; piermarini et al., 1978]. a further advancement is the 4d characteriza− tion of the sub liquidus evolution of natural melts is represented by experiments within synchrotron facil− ities which allow real time monitoring of the textural evolution of samples of volcanological interest during crystallization and/or degassing [e.g. ohtani et al., 2005; pistone et al., 2015; pleše et al., 2018; polacci et al., 2018; polacci et al., 2010; song et al., 2001]. these techniques are starting to be coupled with devices for 7 advances in rheology: application to lava flows emplacement rheometry, which may in the future allow for in situ es− timates of both the crystallization kinetics and the rhe− ological response of evolving natural systems [coats et al., 2017; dobson et al., 2015; dobson et al., 2016; raterron and merkel, 2009]. the results of experi− mental campaigns and modelling of the multiphase rheology of natural magmatic suspensions performed on natural or analogue silicate melts at high temper− atures will be presented in section 4.2. 4. experiments and models of non-reactive multiphase mixtures following the results obtained on isothermal bub− ble−bearing or particle−bearing suspensions rheology of analogue materials, simplified silicate melt mixtures and natural melts and magmas are here first intro− duced. finally i summarize what is known on the ef− fect of the presence of bubbles+crystals on suspension rheology measurements performed at constant tem− perature. 4.1 models of bubble suspension rheology early studies estimating the effect of void spaces within natural and simplified silicate melts [e.g. bag− dassarov and dingwell, 1992, 1993; lejeunne et al., 1999; vona et al., 2016; ryan et al., 2019] or synthetic analogues [e.g. manga et al., 1998; llewellin et al., 2002a, b; llewellin and manga, 2005] has been car− ried out by several authors. those investigations showed, largely, that, two end member cases can be considered: 1) bubbles behave as rigid objects (capil− lary number ca<1); 2) bubble are deformed (ca>1) [llewellin et al., 2002a,b]. for the different regimes various empirical equations were proposed [bagdas− sarov and dingwell, 1992; lejeunne et al., 1999; llewellin et al., 2002; llewellin and manga, 2005] (details in appendix a3) which suggested that, dur− ing steady flow: a) an increase in relative viscosity in the case of the first end−member (ca<1) and b) a decrease of the relative viscosity in the case of sec− ond end member condition (ca>1) can be observed. additional complexities are introduced, as discussed in appendix a3, for non−steady flow for which the definition of a dynamic capillary number (cd) is re− quired. the same authors [e.g. lejeune and richet, 1996; bagdassarov and pinkerton, 2004, llewellin et al., 2002, llewellin and manga, 2005] also provided important attainments concerning the understanding of the effect of closed and opened voids on liquid viscosity [e.g. lejeune et al., 1999; bagdassarov and dingwell, 1992; quane and russell, 2004; llewellin et al., 2002a, b; llewellin and manga, 2005; mader et al., 2013; vona et al., 2016; ryan et al., 2019]. a summary of recent formulations and works related to both crystal bearing and bubble bearing rheological studies is reported in appendix a3. 4.2 from shear-rate independent to shearrate dependent particle suspension rheology models of analogue materials early studies on the rheological behavior of mul− tiphase suspensions [e.g. einstein, 1906; roscoe, 1952; krieger and dougherty, 1959; gay et al., 1969; pinkerton and stevenson, 1992] suggested a thresh− old in solid fraction, the so−called crystal maximum packing fraction (φc), that separates a liquid domi− nated rheology from a solid−dominated rheology. for dilute suspensions of solid mono−disperse spher− ical particles (φ < 3 vol%) einstein [1906] proposed that the relative viscosity ηr (i.e. the ratio between the vis− cosity of the particle−bearing suspension and that of the particule−free melts) could be calculated as: ηr = (1+ bφ), where b is a constant depending on object ge− ometrical features (b=2.5 for spheres). roscoe [1952] extended einstein’s expression to higher concentration of spheres, by first defining the maximum crystal packing fraction (φm) and providing for the relative viscosity the following expression: ηr = (1 + φ/φm) −2.5. different φm values were proposed by different authors depending on crystal geometry (see appendix a3 for more details). later, krieger and daugherty [1959] generalized the previous expressions as it follows: ηr = (1 + φ/φm) −beφm where be is a constant called the einstein coefficient (kd model). others similar ex− pressions were formulated for which different value of the be coefficients were determined (see appendix a3). although widely applied, a limitation of those empir− ical or semi−empirical laws is that they do not account for neither the strain−rate dependence nor the exis− tence of, although still debated, yield strength [moitra and gonnermann, 2015] of multiphase mixtures typ− ical of non−newtonian fluid (see appendix a3 for more details). for a review on the two phase rheology of particle bearing analogue suspensions the reader can refer to mader et al. [2013], who presented a compre− hensive review on this topic daniele giordano 8 4.3 non-newtonian models for particle suspension rheology of simplified silicate mixtures concerning magma−equivalent suspensions, more recently, caricchi et al. [2007], costa et al. [2007a, 2009], based on the available experimental data obtained at constant temperature, presented models describing the non−newtonian strain−rate−dependent rheological ef− fects of crystals in the range of solid fractions from 0 to 0.8 and over. these models cover the transition from the regime where the deformation behavior is controlled by melt viscosity up to the beginning of the regime where the deformation behavior is controlled by a solid frame− work of interlocking particles. the most detailed and comprehensive model to date proposed by costa et al. [2009] model (cm) (eqs. a3.3−a3.4.), describes the rel− ative viscosity ηr (i.e. the viscosity of a crystal melt mix− ture (ηmix) divided by the viscosity of the melt phase (ηr)). the cm model is the result of the combined math− ematical and experimental efforts condensed in the works of costa [2005], costa et al. [2007a] that was used by caricchi et al. [2007] to describe their experimental data. compared to previous models [e.g., einstein− roscoe, 1952; costa, 2005; caricchi et al., 2007], the cm model accounts for the strain−rate dependent changes in the rheology of liquid+crystal mixtures. the model in particular shows that the strain rate dependence of the relative viscosity at varying crystal volume fractions follows a sigmoid curve with exponential increase above a critical solid fraction (φc ~ 0.3−0.4) which is also a function of strain rate and crystal shape. this model is consistent with the early einstein−roscoe equation [ein− stein, 1906; roscoe, 1952] for crystal fractions in the range of 0 to 0.1−0.3 depending on crystal shape and size [e.g. cimarelli et al., 2011]. a summary of the main results obtained by the employment of the cm and a summary of its original formulation are reported in appendix a3. extension of cm devoted to characterize the effect of crystal size and shape distribution and sus− pended particle ratio and particle roughness are dis− cussed in appendix a3. 4.4 non-newtonian strain-rate dependent models for particle suspension rheology of natural mixtures the fermenting production of studies [shaw et al., 1968; lejeune and richet, 1995; giordano et al., 2007; caricchi et al., 2007, 2008; ishibashi, 2009; vetere et al., 2010, 2017; vona et al., 2011; pistone et al., 2012, 2016; chevrel et al., 2015, 2017; campagnola et al., 2016] de− voted to the characterization of the isothermal viscos− ity evolution of silicate melts at subliquidus tempera− ture as a function of presence and size and shape distributions of crystals and bubbles and deformation regimes of the last twenty years has permitted extraor− dinary advances that are condensed in empirical and theoretical models of suspension rheology [saar et al., 2001; caricchi et al., 2007; costa et al., 2009; mueller et al., 2011; vona et al., 2011; moitra and gonnermann, 2015]. according to the comprehensive model of costa et al. [2009] (cm) (see section 3.1), inspired by the pre− vious work of costa [2005], costa et al. [2007a] and car− icchi et al. [2007], the relative viscosity of two−phase mixture increases following a sigmoid curve with ex− ponential increase above a critical solid fraction (φc) corresponding to the first (phi~0.3−0.4) inflection point. a second inflection point (φm) at phi ~0.6−0.7 is deter− mined by the beginning of crystal dominated rheology (figure a3.3). since the seminal contributions of caricchi et al. [2007] and costa et al. [2009], numerous scientists pro− vided new and more complete formulation of the crit− ical crystal fraction (φc) for the natural variability in of crystal size and shape distribution which would also ac− count for new variables (e.g. crystal surface roughness) [e.g. mueller et al., 2011; klein et al., 2018]. the em− ployment of these critical contributions have allowed interpreting, based on model calculations, the effect of rheological constraints on eruptive behavior. 4.5 models for particles and bubbles suspension rheology complex three−phase suspensions (i.e. liquid+bub− bles+crystals) have been investigated in only a few studies [cordonnier et al., 2009; robert et al., 2008a, b; lavalleé et al., 2007, 2008; vona et al., 2013, 2017; campagnola et al., 2016; pistone et al., 2012, 2015, 2016]. given their complexity only a fewer studies have provided preliminary models describing the complex rheology of three−phase mixtures [pistone et al., 2012, 2013, 2015, 2016]. the viscosity data presented in those studies are the same as those presented in pistone et al [2012], but the authors apply their results to different geological context by showing that size− and shape− distributions of crystals and bubbles may significantly vary while undergoing certain stress−strain regimes. the experiments by pistone et al. [2012] were per− formed at pressurized and isothermal temperature con− 9 advances in rheology: application to lava flows emplacement ditions in a paterson device (appendix a1) on samples for which the liquid+crystal rheology was characterized by caricchi et al. [2007]. they show that bubbles strongly affect the rheological properties of crystal− rich mushes. by presenting a comprehensive review of existing literature and performing new measurements, they estimated that a decrease of up to 4 orders of magnitude is observed by the addition of only 9 vol% of bubbles to a liquid+crystals suspension containing 70 vol% of crystals. they also established that two non− newtonian deformation regimes originate as a conse− quence of the bubble and crystal interaction: i) a shear thinning behavior result of the crystal size reduction and shear banding due to strain localization [also observed by caricchi et al., 2008] which is typical of magmas which are transported and emplaced in earth’s crust and may feed eruptions; ii) a shear thickening behavior which is the consequence of crystal interlock and flow blockage which they argue locks plutonic rocks in the lower and upper crust, inhibiting eruptions. more details of the results obtained by the works of pistone and coauthors are provided in appendix a3.3. 5. non-isothermal cooling-rate and strain-rate dependent rheology of volcanic materials efforts to systematically describe and predict magma migration and lava flow behavior rely heavily on these experimental measurements to derive empirical models. however, during migration and transport of silicate melts in the earth’s crust and at its surface magma/lava can ex− perience varying cooling and deformation conditions which may drastically influence its thermorheological evolution; see for example rhéty et al. [2017] and robert et al. [2014]. as a consequence, data intended for appli− cation to the natural environment will have to account for the disequilibrium behavior of natural magmatic sus− pensions. cooling rates of basaltic lavas, measured at the surface and within active lava channels during emplace− ment range from ~0.01 to 15 c/min [cashman et al., 1999; flynn and mouginis−mark, 1992; hon et al., 1994; wit− ter and harris, 2007; kolzenburg et al., 2017]. these val− ues are largely representative for the exterior part of lava flows or for the initial cooling of newly emplaced dikes. they can, therefore, be taken as maximum cool− ing rates that are expected to be lower in the interior of the lava flow or a cooling dike. the importance of vary− ing thermal conditions on the crystallization kinetics and textural development of silicate melts has been recognized for decades and inspired disequilibrium experimentation in petrology and volcanology [e.g. walker et al., 1976; arzilli and carroll, 2013; coish and taylor, 1979; gam− ble and taylor, 1980; hammer, 2006; lofgren, 1980; long and wood, 1986; pinkerton and sparks, 1978; giordano et al., 2007; vetere et al., 2013]. these studies highlight that significant differences in textures and par− agenesis emerge when moving from equilibrium to dis− equilibrium conditions that can, in turn, affect the flow behavior. albeit a growing experimental disequilibrium database is becoming available no models for the dise− quilibrium phase dynamics of natural silicate melts have been developed to date. understanding the rheological evolution of crystalliz− ing melts, magmas and volcanic products requires direct measurement of the flow properties of investigated ma− terials at such disequilibrium conditions in the field or in the laboratory. in such environments, the studied mate− rials are degassed and undergoes transient increases in viscosity as it is increasingly undercooling until a “rhe− ological cut−off temperature” [giordano et al., 2007; kolzenburg et al., 2016, 2017, 2018a, b, c; 2019] is reached and the lava rheologically solidifies. this transient rhe− ological gradient, which occurs in all natural, non− isothermal environments, governs the lavas emplacement style. in recent years, the first sets of measurements were presented that constrain the rheological evolution of natural silicate melts under temperature− and deforma− tion−conditions pertinent to the transport of silicate melts on the earth’s surface and in shallow magma plumbing systems. the recovered data show a strong dependence of composition [kolzenburg et al., 2017, 2018a], cooling−rate [giordano et al., 2007; kolzenburg et al., 2016, 2017], oxygen fugacity [kolzenburg et al., 2018a] and shear−rate [kolzenburg et al., 2018] on the thermorheological evo− lution of natural silicate melts. they represent the first contributions to a growing database of lava rheology un− der natural conditions. however, significant experimen− tal effort in this field is required to expand the range of available data to cover the most relevant compositions and to experimentally map the range of parameters per− tinent to flow of natural silicate melts under disequilib− rium. such a database would then allow deducing the un− derlying processess and expanding these into a theoretical description of the flow behavior of magma and lava. so far, the main limitation of this kind of studies is the dif− ficulty to monitor, and therefore extend, the results to daniele giordano 10 non−degassed materials and therefore the application to intra−crustal magmatic or explosive volcanic processes. according to previous authors [e.g. melnik and sparks, 1999, 2005; costa and macedonio, 2003, 2005; costa et al., 2007b; hess et al., 2008; cordonnier et al., 2012] an additional complexity could be due to the effects of non− linear thermal effects, potentially generated by viscous dissipation and loss by conduction at the contact between the molten material and the hosting rock, in conduits, and channels or tunnels after eruption to the surface. the nonlinear behaviour of thermal effect are mainly gov− erned by specific non−dimensional numbers (graetz; nahme; prandtl; reynolds regimes), which according to costa et al. [2007b], amongst the others above mentioned, may determine the necessity to distinguish between three main regimes − a conductive−heat−loss−dominated regime, an intermediate regime and a viscous−heating− dominated − may have significant effects for the defini− tion of the rheological behaviour and emplacement dy− namics of lava flows and lava domes. figure 4 shows a summary of rheological data recov− ered using a variety of experimental methods on etna melts. the melt compositions, albeit stemming from dif− ferent eruptions, are similar for most major oxides with the exception of the sample from vona et al. [2017], that is more rich in silica and poor in iron and, as a result, more viscous than the samples in vona et al. [2011] and kolzenburg [2018]. for the investigated degassed materials these data summary highlights a number of effects acting during the transport of magma and lava at sub liquidus conditions. comparison of the pure liquid viscosity of the remelted bulk rock and the separated groundmass [vona et al., 2017]; triangles documents that, for basaltic melts, crys− tallization induced changes in melt composition result in relatively small changes in the viscosity of the liquid phase of the evolving suspension. therefore, the variations of the flow behavior of crys− tallizing basalts are controlled by variations in the vol− umetric fractions of crystals and bubbles. these data also reflect the measurement limits of the respective methods that are described in more detail in kolzenburg et al. [2016a]. concentric cylinder suspension viscometry for these etnean lavas is confined to <104 pa s and shows that the measured viscosities at constant temperature (i.e. at or near thermodynamic and textural equilibrium) are commonly higher than non−isothermal measurements at the same temperature. this is due to the fact that un− der dynamic thermal conditions, the crystal nucleation and growth kinetics lag behind the equilibrium state and commonly produce lower crystal contents. the non− isothermal viscosity data from kolzenburg et al. [2018c] document that both cooling−rate (blue circles vs. red squares) and shear rate (open vs. filled symbols) exert a modulating effect on the disequilibrium rheology of the etna melt. measurements beyond the mechanical limit of concentric cylinder (cc) viscometry were presented in vona et al. [2017] who employed parallel plate (pp) vis− cometry via unconfined uniaxial deformation (open black stars) to measure the viscosity of three phase magmatic suspensions. the data form an apparent continuing trend with respect to the concentric cylinder viscometry mea− surements but document lower lava viscosities than ex− trapolation from the two phase measurements would suggest. this is likely a result of the differences in sam− ple texture, where all cc data are restricted to bubble free two phase suspensions of crystals and melt, whereas the pp data are measured on three phase (i.e. crystal and bub− ble bearing) suspensions. in summary, the rheological evolution of lava at sub− liquidus conditions can be reconstructed neatly by com− bination of datasets from differing sources. this is also 11 advances in rheology: application to lava flows emplacement figure 4. summary of available melt and crystal−suspension vis− cosity data on remelted etnean lava as a function of temperature. melt viscosity measurements (open black diamonds, black circles and black triangles) were per− formed via concentric cylinder viscometry, micro penetration and differential scanning calorimetry; sub liquidus viscosity measurements were preformed using 1) concentric cylinder viscometry at constant tem− perature (black circles), 2) concentric cylinder viscometry at varying cooling− and shear−rates (open and filled red squared and blue diamonds) and 3) parallel plate viscometry via unconfined uniaxial deformation (open black stars). k&' 18; v& '11 and v& '17 refer to work published by kolzenburg et al (2018), vona et al. (2011) and vona et al (2017). shown in figure 3 in kolzenburg et al., [2019, this issue], where laboratory and field estimates of lava rheology at emplacement conditions are compared and the respective data fall within a range of similar values. this highlights the potential of cross correlation of data from different experimental and field sources and the need to expand the available experimental database in order to generate a holistic view of the dynamics of magma and lava trans− port. 6. alternative ways of retrieving rheological information from remote sensing groundor satellite-based techniques besides laboratory viscometry (i.e. the direct mea− surement of melt / suspension viscosity under controlled conditions) there are several other sources of rheological information that are useful to place the laboratory mea− surements in context of the natural environment. this kind of information is important as it allows accounting for the multiphase nature of lava bodies and can serve to place the laboratory measurements within the framework of conditions relevant in natural scenarios. however, to date such data only represents a very limited source of in− formation of the rheological evolution of lava flows, in space and time. this is largely due to large logistical and financial efforts required for some of these measurements and to the uncertainties associated. broadly these ap− proaches can be separated into: 1. direct measurement of viscosity on active lava flows via penetration− or rotational−viscometry [einars− son, 1949; gauthier, 1973; panov et al., 1988; pinker− ton and sparks, 1978; belousov et al., 2015; belousov and belousova, 2018; shaw et al., 1968; pinkerton and norton, 1995; pinkerton and wilson, 1994; chevrel et al., 2018]. these represent snapshots of actual lava flow rheology at specific conditions and provide data that help to constrain the conditions re− quired to be reproduced in systematic laboratory studies. however, such measurements are quite dif− ficult and require significant logistical effort and manpower. further, the available devices [e.g. be− lousov and belousova, 2018; chevrel et al., 2018] for such measurements are only slowly advancing to be able to measure all relevant parameters sufficient− ly well to recover high quality viscosity data (ap− pendix, a1.5). 2. calculation of the apparent viscosity based on jef− freys’ equations [e.g. jeffrey, 1925; hulme, 1974] (ap− pendix 4, smo) using flow rate measurements of ac− tive lavas in channelized flows [naboko, 1938; nichols, 1939; minakami, 1951; einarsson, 1966; walker, 1967; gautier, 1973; moore, 1978; andreev, 1978; fink and zimbelman, 1986; vande−kirkov, 1987; panov, 1988; soldati, 2016; belusov and be− lousova, 2018]. such data are still few due to the dif− ficulty of accessing active lava flows. however, the development of affordable unmanned aerial vehi− cles (uav’s) in recent years appears to be promis− ing making this method widely applicable with the opportune considerations. in fact, the above men− tioned approach has strong limitations as it is based on the assumption of parabolic velocity profile that is not generally valid because of thermal effects [e.g. costa and macedonio, 2003, 2005; costa et al., 2007b; filippucci et al., 2013 and filippucci et al., 2019, this issue] (details at section 3.1.2). such aspect is still never considered to describe the nonlinear dynam− ic of lava flows and lava domes rheology [melnik and sparks, 1999, 2005; melnik et al., 2009]. to an adequate analysis of this contribution for specific cas− es, it is recommended to refer, for instance, to the above mentioned works and e.g. filippucci et al. [2019, this issue]. 3. ties between lava flow geometry and viscosity. mor− phological−derived rheological parameters (i.e. vis− cosity and yield strength) are commonly obtained in planetary sciences [heisinger et al., 2007; cas− truccio et al., 2010 and chevrel et al., 2015] provide excellent reviews of the employed equations and re− sults). rheological information is obtained by re− trieving, in the field or remotely also from satellites, length, width, thickness and slope of emplacement of lava flows. this methodology has also been ap− plied based remote sensing data collected during ac− tive flow emplacement [james et al., 2015; farquarson et al., 2015; kolzenburg et al., 2018a]. also in this case, the emplacement of lava flows is commonly modelled using a single rheological parameter (ap− parent viscosity or apparent yield strength) calcu− lated from morphological dimensions using jeffreys’ and hulme’s [jeffrey and acrivos, 1976; hulme, 1974] equations. the rheological parameters are then typically further interpreted in terms of the nature and chemical composition of the lava (e.g., mafic or felsic). chevrel et al. [2013, 2015] employing this daniele giordano 12 methodology has shown that providing an unique factor to describe rheology of lava flows is definitely far from being representative of the real emplace− ment dynamics of lava flows. as above mentioned (point 2), given the nonlinear dynamics of lava flows and domes, which may determine significant ther− mal effects, significant limitations may be observed and should be carefully considered before applying to any natural context [e.g. costa and macedonio, 2003, 2005; costa et al., 2007b; filippucci et al., 2013 and filippucci et al., 2019, this issue]. 4. ties between the intensity of thermal anomalies generated by actively flowing lava and its silica content and therewith discharge rate of lavas [e.g. coppola et al., 2013, 2017]. this approach takes ad− vantage of the fact that low viscosity lavas are readily able to spread into thin sheets during flow, whereas high viscosity lavas usually retain low− er aspect ratios. since the heat loss of a lava is largely governed by its surface to volume ratio, its spreading ability (i.e. viscosity) can, empirically, be correlated to the measured heat loss. over the last decades such satellite−based remote sensing and data processing techniques have proved well suited to complement field observations and to al− low timely eruption detection, as well as for flow tracking. 7. concluding remarks and outlook the present review shows the extraordinary improved knowledge of rheological properties of multicomponent and multiphase silicate melts occurring in the last twenty years. such knowledge advancement has been due to the necessity of constraining natural processes and parallel the development of new technological advances, fre− quently obtained to face specific problems. it has been observed that the continuously evolving rheology of magmas and eruptive products during their ascent, erup− tion and emplacement can be described with increasing accuracy and specifically applied to geological issues with improved confidence. the observed transition be− tween newtonian to strongly non−newtonian rheologi− cal behaviour is typical of both simple liquids and/or multiphase natural mixtures. these transitions govern the observed eruption dynamics and the eruption dynamic transitions, potentially determining also whether an erup− tion will be effusive or explosive. the employment of the rheological flow laws for mul− ticomponent and multiphase silicate melts find a very promising application to constraining the advancement and halting of lava flows. for these superficial phenom− ena, the opportunity of monitoring important variables such as the discharge rate and the topography of em− placement provide fundamental advantage for the em− ployment of numerical simulations tools. these have al− lowed showing that more accurate estimates of the effects of crystals and bubbles during lava flow emplacement can be obtained only by real−time monitoring of lava flows through field and remote sensing methods paralleled by a proper experimental campaign, which in particular would account and would be related to the non−equilib− rium, non−isothermal rheology of multiphase mixtures. this progress in understanding the mechanisms of ad− vancement and emplacement of lava flows and domes has also been made possible recently thanks to the recent em− placement of large long−lasting silicic to basaltic effusive eruptions. prior to 2008, for instance, no rhyolite lava flow−forming eruptive event was observed or docu− mented. hence, the real−time observations of active rhy− olitic flow and dome emplacement at the chilean volca− noes of chaitén [carn et al., 2009; lara 2009; bernstein et al., 2013; pallister et al., 2013] and puyehue−cordón caulle significantly developed our knowledge of rhyolitic lava emplacement [castro et al., 2013; schipper et al., 2013; tuffen et al., 2013; bertin et al., 2015; farquhar− son et al., 2015; magnall et al., 2017]. analogously the long lasting 2014−2015 basaltic eruption at holuhraun, bardarbunga system, iceland [e.g. pedersen et al., 2017], offered the opportunity to establish/calibrate, through the contemporaneous employment of field work, remote sensing techniques [kolzenburg et al., 2018a] and labo− ratory experimentation [kolzenburg et al., 2017], which allow retrieving thermal properties, estimates of effusion rate [coppola et al., 2013, 2017] and evaluate the effect of bubbles by comparison with experimental campaign on liquid+ crystals material as collected during eruption [kolzenburg et al., 2017, 2018 a,b]. worth mentioning is also the integrated field, remote sensing, physical prop− erties and physical modelling and numerical simulations studies performed in the recent years for intermediate compositions producing effusive activities [chevrel et al., 2013 a,b; 2015]. for andesitic domes huge progresses in understanding the non−linear thermal effects which determine non−linear eruption dynamics has been made by previous authors [e.g. costa et al., 2007b; melnik and sparks, 1999, 2005; melnik et al., 2009]. although the 13 advances in rheology: application to lava flows emplacement purposes of this paper is to mostly describe the develop− ment of rheological properties in relationship to the em− placement of lavas, most of the general results obtained here, and in particular those related to the effect of crys− tals and vesicle on multiphase rheology, can be extended to eruption dynamics of explosive phases. some of the main results deduced by application of the existing rheological models and experimental stud− ies, supported by petrological analysis and field work, al− lowed to unequivocally show that lava flow emplace− ment may be a long lasting process also for silicic magmas and that flow may continued also unrooted from the vent for long times [e.g. farquarson et al., 2015] and that extremely voluminous silicic lava flows may be emplaced in relatively short time without giv− ing origin to significant explosive stages [tuffen et al., 2013; farquarson et al., 2015; giordano et al., 2017; polo et al., 2018a, b]. in addition, kolzenburg et al [2016, 2017, 2018] showed that disequilibrium, cooling− and shear− rate controlled rheological properties may have funda− mental influence in determining the effective length of basaltic lava flows. although the results evidenced by performing non− equilibrium, non−isothermal, transient rheology of basaltic lava flows, are promising and provided a first understanding of lava flow rheology under natural con− ditions, it is possible to anticipate that future studies will require performing this kind of experiments also to a wider range of effusive products. i would like to thank stephan kolzenburg and kelly russell for sharing ideas and help in the reorganization of the manuscript. fabio arzilli and an anonymous reviewer are also acknowledged for the careful revision of the manuscript. daniele giordano acknowledges the university of turin for support provided by the local research funds (2017/2020). references adam g., gibbs j.h. 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(2007). field measurements of heat loss from skylights and lava tube systems. j. geophys. res.: solid earth 112, 1978–2012. *corresponding author: daniele giordano, università degli studi di torino, dipartimento di scienze della terra torino, italy email: daniele.giordano@unito.it © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved daniele giordano 24 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 what can we learn from the january 2012 northern italy earthquakes? marco massa*, gabriele ameri, sara lovati, rodolfo puglia, gianlorenzo franceschina, francesca pacor, lucia luzi, paolo augliera istituto nazionale di geofisica e vulcanologia, sezione milano-pavia, milan, italy annals of geophysics, 55, 1, 2012; doi: 10.4401/ag-5574 this note focuses on the ground motion recorded during the recent moderate earthquakes that occurred in the central part of northern italy (panel 1), a region that is characterized by low seismicity. for this area, the italian seismic hazard map [stucchi et al. 2011] assigns a maximum horizontal acceleration (rock site) of up to 0.2 g (10% probability of being exceeded in 50 yr). in the last 4 yr, this region has been struck by 9 earthquakes in the magnitude range 4 ≤ mw ≤ 5.0, with the three largest located in the northern apennines (the mw 4.9 and 5.0 parma events, in december 2008 and january 2012) and on the po plain (the mw 4.9 reggio emila event, in january 2012). we have analyzed the strong-motion data (distance <300 km) from these events as recorded by stations belonging to the istituto nazionale di geofisica e vulcanologia (rais, http://rais.mi.ingv.it; rsnc, http:// iside.rm.ingv.it) and the department of civil protection (ran, www.protezionecivile.it; http://itaca.mi.ingv.it). the 2008 and 2012 parma events were both characterized by reverse focal mechanisms (http://cnt.rm. ingv.it/), with depths of 27 km and 60 km, respectively. the deeper event produced a maximum peak ground acceleration (pga) of 97 cm/s2 at novellara station (nvl, ec8 c class; 70 km from the epicenter). the january 25, 2012, event had a depth of 34 km and produced a maximum pga of 114 cm/s2 at sorbolo station (srp; 7 km from the epicenter). preliminary analyses show the following: 1) peculiar ground-motion attenuation of the deep parma event with respect to the shallow event. in panel 2, the pgas for the two parma events are plotted as functions of the hypocentral distance and compared with the global ground motion prediction equation (gmpe) calibrated by cauzzi and faccioli [2008] using events with depths <30 km. the different distance decays of the pgas for the deep event (blue for a class of ec8, and red for b and c classes) [cen 2003] are evident, in particular for distances up to 100 km. on the other hand, the pgas of the 2008 parma crustal event (gray) are well explained by this gmpe. in panel 3, the pgas for the deep 2012 event are grouped for the ec8 classes and are compared with the national gmpe calibrated by bindi et al. [2011] using crustal events and epicentral distances. also in this case, the gmpe underestimates the pgas up to 200 km. although most of the class c sites (red) show the largest pgas, the underestimation cannot be completely ascribed to site effects. the large pgas from the parma deep event, with respect to the shallow one, can be explained in terms of source effects (e.g. large stress-drop values that enhance the high-frequency radiation). in addition, as explained by castro et al. [2008], the different attenuation in the lower and upper crust could explain the large pgas recorded for the 2012 deep event. 2) seismic amplification at the po plain sites. in panel 4, the pgas of the january 25, 2012, mw 4.9, reggio emilia event are plotted as a function of the epicentral distances, together with the bindi et al. [2011] gmpe. in general, the largest amplitudes occur at the po plain sites (red), which suggests possible peculiar site responses. an overall increase in the pgas is seen around 100 km, in agreement with the results of bragato et al. [2011], who studied the regional influence of moho s-wave reflections in the area. an example of the site response is shown in panel 5. this considers the treg (class c) and zen8 (class a) stations (panel 5a), which are located at 88 km from the reggio emila epicenter. the rotational standard spectral ratio (panel 5b) for 10 s of s waves shows polarized amplifications around 2 hz, which were also detected at other po plain sites (not shown), as well as amplification on the vertical component. the points discussed above should to be interpreted as a warning for future applications dealing with groundmotion estimations in the aftermath of an earthquake in this area (e.g. shakemap calculation): the currently used gmpes, which are based on different events and site characteristics, can lead to significant bias in the final results. article history received february 16, 2012; accepted february 27, 2012. subject classification: ground motion, waves and wave analysis, data processing, seismological data. 1 scientific news references bindi, d., f. pacor, l. luzi, r. puglia, m. massa, g. ameri and r. paolucci (2011). ground-motion prediction equations derived from the italian strong motion database, b. earthq. eng., 9, 1899-1920. bragato, p.l., m. sugan, p. augliera, m. massa, a. vuan and a. saraò (2011). moho reflection effects in the po plain (northern italy) observed from instrumental and intensity data, b. seismol. soc. am., 101, 2142-2152. castro, r.r., m. massa, p. augliera and f. pacor (2008). bodywave attenuation in the region of garda, italy, pageoph, 165, 1351-1366. cauzzi, c., and e. faccioli (2008). broadband (0.05 to 20 s) prediction of displacement response spectra based on worldwide digital records, j. seismol., 2, 453-475. cen (2003). pren 1998-1-eurocode 8: design of structures for earthquake resistance. p1: general rules, seismic actions and rules for buildings. draft 6, doc cen/tc250/sc8/n335, jan 2003, brussels. stucchi, m, c. meletti, v. montaldo, h. crowley, g.m. calvi and e. boschi (2011). seismic hazard assessment (20032009) for the italian building code, b. seismol. soc. am., 101, 1885-1911. *corresponding author: marco massa, istituto nazionale di geofisica e vulcanologia, sezione milano-pavia, milan, italy; email: marco.massa@mi.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. massa et al. 2 3 what can we learn from the january 2012 northern italy earthquakes? panels. 1: instrumental seismicity of the area from 2008 to 2012 (mw ≥4, http://cnt.rm.ingv.it/tdmt.html). yellow insets, analyzed events. 2: comparison between the pgas recorded for the january 27, 2012, mw 5.0, event and the cauzzi and faccioli (2008) gmpe. gray symbols, pgas recorded for the 2008 parma, mw 4.9 event. 3: comparison between the pgas recorded for the january 27, 2012, mw 5.0, event and the bindi et al. [2011] gmpe. 4: comparison between the pgas recorded for the january 25, 2012, mw 4.9, event and the bindi et al. (2011) gmpe. 5a: ns component of the january 25, 2012, event (mw 4.9) as recorded at the treg (c class) and zen8 (a class) rais stations. 5b: rotational standard spectral ratio (ssr) calculated on 10 s of the s-phases for the january 25, 2012, mw 4.9 event as recorded at the treg station (with zen8 as reference). dashed black line, vertical ssr. italiano_2_corretto_layout 6 815 geochemical features and effects on deep-seated fluids during the may-june 2012 southern po valley seismic sequence francesco italiano1, marcello liotta1, mauro martelli1, giovanni martinelli2, riccardo petrini3, anna riggio4, andrea luca rizzo1, francesca slejko3, barbara stenni3 1 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy 2 università di trieste, dipartimento di matematica e geoscienze (dmg), trieste, italy 3 agenzia regionale prevenzione e ambiente (arpa) dell'emilia romagna, sezione di reggio emilia, reggio emilia, italy 4 istituto nazionale oceanografia e geofisica sperimentale (ogs), sgonico (trieste), italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6151 1. introduction a periodic sampling of the groundwaters and dissolved and free gases in selected deep wells located in the area affected by the may-june 2012 southern po valley seismic sequence has provided insight into seismogenic-induced changes of the local aquifer systems. the results obtained show progressive changes in the fluid geochemistry, allowing it to be established that deep-seated fluids were mobilized during the seismic sequence and reached surface layers along faults and fractures, which generated significant geochemical anomalies. the may-june 2012 seismic swarm (mainshock on may 29, 2012, m 5.8; 7 shocks m >5, about 200 events 3 > m > 5) induced several modifications in the circulating fluids. this study reports the preliminary results obtained for the geochemical features of the waters and gases collected over the epicentral area from boreholes drilled at different depths, thus intercepting water and gases with different origins and circulation. the aim of the investigations was to improve our knowledge of the fluids circulating over the seismic area (e.g. origin, provenance, interactions, mixing of different components, temporal changes). this was achieved by collecting samples from both shallow and deep-drilled boreholes, and then, after the selection of the relevant sites, we looked for temporal changes with mid-to-long-term monitoring activity following a constant sampling rate. this allowed us to gain better insight into the relationships between the fluid circulation and the faulting activity. the sampling sites are listed in table 1, along with the analytical results of the gas phase. 2. geo-hydrological background the po sedimentary basin is filled by sediments of alpine and apennine origin. north-verging apennine fronts that are buried under this cover overthrust the padanian basement. along with the north apennine outcropping units, these buried structures represent a seismically active belt, where stresses connected in a complex way to the neogene or earlier deformation history are still active [boccaletti et al. 2011]. plio-quaternary stress field evaluations indicate the presence of a tectonic regime that is characterized by a n-ne compressive axis, and average deformation rates that range from 4 mm/year to 7 mm/year have been measured in the area [cenni et al. 2012]. the local tectonics drove the sedimentation processes, allowing the filling of the sedimentary basin by alternate layers of sand and clay [ori 1993, and references therein]. gravel is only present near the apennine chain in the alluvial fan areas, while the bottom of the quaternary sediments is located at depths of 100 m to 700 m. these multilayered sequences constitute the po aquifer system [regione emilia-romagna and eni-agip 1998]. in the part of the basin close to the apennine chain (the belt of alluvial fans), aquifers are located in unsatured sediments and are subjected to seasonal recharge processes. north of this area, the hydrogeological features are quite different: fine sediments host confined and/or semiconfined aquifers that are characterized by low circulation velocities (10 m/year). isotopic data of 18o/16o and 2d/1h indicate that the deep groundwaters in this area are of alpine origin, while 3h, 14c and 36cl data indicate ages of 15,000 to 30,000 years [martinelli et al. 1998, and references therein]. these ancient groundwaters are found mainly in sediments of alpine origin, from 100 m in depth to the quaternary basement [dondi et al. 1982, regione emilia romagna and eniagip 1998]. field data indicate that inside these sediments that are located at 100 m to 300 m in depth, the hydrostatic load increases linearly with depth, following a regular hydraulic gradient. on the other hand, at depths shallower than article history received july 24, 2012; accepted august 22, 2012. subject classification: fluids, geochemistry, faults, temporal changes. 2012 emilia earthquakes 50 m, load fluctuations are observed in connection with rainfall and barometric pressure, although lenses of confined aquifers are known at shallow depth too. the above-mentioned features suggest that the meteorological effects on the piezometry of deep aquifers can be considered to be negligible in the part of the basin that borders the po river in the north and the belt of the apennine alluvial fans in the south [albarello and martinelli 1994, and references therein]. the chemical composition of the aquifers of the emiliaromagna plain is largely due to the stratigraphy of the plioquaternary sedimentary successions, where the great prevalence of cahco3 groundwaters characterizes the plain at the foot of the apennine range. the positive buried structure of the mesozoic limestones, known as the ferrara anticline, largely affects the composition of the shallow aquifers above it, which become of the nahco3 type all along the structure, due to the precipitation of calcite or to water–rock interaction processes with al-silicates. the shape of this structure is clearly reflected by the chemical composition of the aquifers of the emilia-romagna plain, where nahco3 type waters follow the wnw-ese structural trend, as already established through seismic surveys [pieri and groppi 1981]. saline, nacl, type waters are limited to the northeastern sector of the plain, and to the area north of modena. they appear to be related to evaporitic material deposited in lagoonal areas north of the ferrara anticline during the pliocene [conti et al. 2000] or to the uprising of marine brines trapped in pliocenic layers. the o-h isotopic composition of the nacl waters has a clear meteoric signature, and no inflows of present day seawater appear necessary to justify such a composition, although mixed phenomena between meteoric originated groundwaters and brines of the pliocenic or pre-pliocenic layers cannot be excluded. to better understand the role of deep-seated fluids during the mayjune southern po valley seismic swarm, a periodic sampling was carried out, including the groundwaters, dissolved gases, and free gases. six wells, which include phreatic (ca. 10-20 m deep) and deep groundwaters (boreholes drilled to depths >100 m; table 1) were considered during the first sampling campaigns. three of the six wells sampled, which represent deep groundwaters, were considered for regular periodic sampling. these wells are located in the municipalities of mirandola (depth 190 m, characterized by weak bubbling activity), concordia [agip mineraria 1977, gorgoni and tosatti 2004] (1,700 m deep agip well characterized by intense bubbling activity), and san felice sul panaro (190 m deep well, characterized by a little bubbling activity). the results from one of these (the vallalta site) are shown in table 1 the waters were sampled using pvc bottles, while the dissolved gases were sampled in 250-ml glass vessels. when available, the bubbling gases were collected using an inverse funnel, driving the gas to a two-way sampling bottle. when free gas was not available, 240 ml water samples were collected in glass bottles sealed by a silicon rubber septa, using special pliers to extract the dissolved gas phase. all of the samples were collected, taking care to avoid even tiny bubbles, to prevent atmospheric contamination (details in italiano et al. [2009]). 3field work and laboratory results 3.1. groundwaters the groundwaters were characterized by a circum-neutral ph, variable ionic strengths (electrical conductivity in the range 1-20 ms/cm) and a redox environment ranging from anoxic to slightly oxic. based on their major constituent anions (cl–, so4 2–, hco3 –) and cations (mg2+, ca2+, na+, k+), most of the waters belong to the nacl hydrofacies, with the exception of the sample from camposanto menozzi station, which belongs to the nahco3/cl water-type (figure 1a). this is consistent with the saline signature reported for waters from wells in the northeastern sector of the emilia-romagna plain and north of modena [martinelli et al. 1998]. na and cl show a linear correlation, with waters from the mirandola site characterized by higher na and cl contents. on the basis of the molar concentrations, this trend deviates from a seawater dilution trend and from the 1:1 line as expected for halite dissolution. waters appear to distribute along a progressive dilution pattern for saline (65 g/l) waters found in fractured mesozoic carbonates or in pliocenic or pre-pliocenic layers. this characterizes the structural high in the modena-ferrara low-plain area, which suggests possible admixing of a deep-seated component at different extents. in terms of the oxygen and hydrogen isotopic compositions (table 2), the collected waters deviate from the line of meteoric precipitation reported for apennine catchments and the easternmost part of the region (ravenna plain), while they plot close to the meteoric water line for northern italy [longinelli and selmo 2003]. this indicates a meteoric origin, as already suggested by martinelli et al. [1998] and a long circuit for the local groundwaters. however, waters from the mirandola site appear to deviate slightly towards a freshwater–seawater mixing line, which supports the possible contribution of formation waters of marine origin. preliminary sr isotopic ratios have been measured on waters from the mirandola, concordia and san felice sul panaro sites. the mirandola groundwaters sampled at different times overlap within the experimental error (87sr/86sr in the range of 0.708793 to 0.708848). slightly lower 87sr/86sr values were obtained for san felice sul panaro (87sr/86sr, 0.708762), while the concordia samples are characterized by a more radiogenic character (87sr/86sr, 0.709081). assuming that the srisotopic signature in the saline waters was not significantly altered by late-stage diagenetic processes, and that the highitaliano et al. 816 817 fluids geochemistry and seismic crisis si te i. d . si te b o re h o le d ep th m u n ic ip al it y d at e d is so lv ed g as es d 13 c o 2 d 13 c h 4 r / r a h e/ n e (m ) h e (c m 3 s t p /l ) h 2 (c m 3 s t p /l ) o 2 (c m 3 s t p /l ) n 2 (c m 3 s t p /l ) c o (c m 3 s t p /l ) c h 4 (c m 3 s t p /l ) c o 2 (c m 3 s t p /l ) 1 b as so li 70 fi na le e m ili a 29 /0 5/ 20 12 1. 9e -0 3 bd l 6, 5e -0 6 2, 1e -0 3 bd l 6, 6e -0 3 3, 8e -0 3 n. m . n. m . 0, 05 1, 06 3 b el le nt an i 19 0 fi na le e m ili a 29 /0 5/ 20 12 5. 3e -0 4 6. 4e -0 3 2. 0e -0 6 1. 3e -0 3 0. 0e + 00 3. 7e -0 3 1. 0e -0 3 n. m . n. m . n. m . n. m . 20 /0 6/ 20 12 1. 4e -0 4 bd l 2. 7e -0 6 1. 1e -0 3 4. 6e -0 5 3. 2e -0 3 6. 6e -0 4 n. m . n. m . n. m . n. m . 4 m ir an do la 19 0 m ir an do la 22 /0 5/ 20 12 bd l bd l 1. 0e -0 6 2. 2e -0 4 9. 3e -0 5 4. 2e -0 3 1. 0e -0 3 n. m . n. m . 0. 55 0. 50 " 20 /0 6/ 20 12 bd l bd l 1. 4e -0 6 1. 7e -0 4 2. 7e -0 4 2. 5e -0 3 5. 2e -0 4 n. m . n. m . n. m . n. m . 5 m in oz zi 20 c am po sa nt o 26 /0 5/ 20 12 bd l bd l 8, 2e -0 6 8. 5e -0 4 bd l 6. 4e -0 4 9. 8e -0 3 n. m . n. m . 0. 74 0. 34 08 /0 6/ 20 12 2. 9e -0 4 bd l 2. 8e -0 6 9. 4e -0 4 0. 0e + 00 6. 3e -0 4 8. 2e -0 3 n. m . n. m . 0. 14 3. 32 6 fe do ss a 20 c am po sa nt o 26 /0 5/ 20 12 3. 3e -0 4 bd l 3. 1e -0 6 1. 6e -0 3 0. 0e + 00 3. 1e -0 4 7. 1e -0 3 n. m . n. m . 0. 15 2. 88 7 fr eg ni 20 c am po sa nt o 26 /0 5/ 20 12 bd l bd l 5. 9e -0 6 1. 2e -0 3 bd l 4. 7e -0 3 5. 0e -0 3 n. m . n. m . 0. 79 0. 32 m ia no 10 40 c or ni gl io 30 /0 5/ 20 12 5. 0e -0 4 bd l 3. 6e -0 6 2. 7e -0 4 5. 7e -0 4 2. 1e -0 3 2. 6e -0 4 n. m . n. m . 0. 11 4. 76 07 /1 0/ 20 04 bd l bd l 1. 6e -0 5 1. 5e -0 4 1. 9e -0 4 1. 0e -0 3 2. 1e -0 4 n. m . n. m . n. m . n. m . b . r om ag na 0 b . r om ag na 09 /0 6/ 20 04 2. 3e -0 4 1. 2e -0 3 4. 2e -0 6 4. 6e -0 4 2. 1e -0 4 2. 6e -0 3 1. 4e -0 4 n. m . n. m . n. m . n. m . b u b bl in g ga se s h e (p pm ) h 2 (p pm ) o 2 (v ol % ) n 2 (v ol % ) c o (v ol % ) c h 4 (v ol % ) c o 2 (v ol % ) 2 v al la lt a 17 25 c on co rd ia 27 /0 5/ 20 12 4 bd l 0. 01 11 .2 bd l 86 .8 3 0. 68 -9 .1 6 n. m . 0. 15 1. 94 29 /0 5/ 20 12 6 3 0. 07 11 .1 1 84 .0 5 0. 76 -7 .3 8 n. m . 0. 17 2. 53 01 /0 6/ 20 12 3 16 0. 07 11 .4 1 87 .4 0 0. 66 n. m . -7 4. 4 0. 22 2. 75 07 /0 6/ 20 12 3 bd l 0. 01 11 .1 0. 4 87 .2 6 0. 90 n. m . -7 4. 9 0. 24 2. 69 18 /0 6/ 20 12 4 12 0. 01 11 .2 bd l 84 .9 4 0. 79 n. m . -7 4. 7 n. m . n. m . 20 /0 6/ 20 12 4 27 0. 03 11 .2 1. 8 85 .3 4 0. 74 n. m . -7 4. 8 n. m . n. m . 26 /0 6/ 20 12 4 7 0. 02 10 .8 0. 6 84 .1 4 1. 23 n. m . n. m . n. m . n. m . 03 /0 7/ 20 12 5 15 0. 02 11 .1 0. 3 86 .3 6 0. 79 n. m . n. m . n. m . n. m . 13 /0 7/ 20 12 3 15 0. 02 10 .9 0. 7 85 .7 4 0. 91 n. m . n. m . n. m . n. m . t ab le 1 .c he m ic al a nd c ar bo n an d he liu m is ot op ic c om po si ti on o f di ss ol ve d an d bu bb lin g ga se s. c ar bo n is ot op es a re r ep or te d in d ‰ v er su s th e in te rn at io na l v ie nn a pe e d ee b el em ni te (v -p d b ) s ta nd ar d. h e is oto pe s ar e sh ow n as r a va lu es ( r a = a tm os ph er ic 3 h e/ 4 h e ra ti o = 1 .3 9 × 1 0– 6 ) . u ni ts o f m ea su re fo r bo th d is so lv ed a nd b u bb lin g ga se s ar e sh ow n. b dl , b el ow d et ec ti on li m it s; n .m ., no t m ea su re d. salinity component mostly contributed to the sr budget in the waters studied, the isotopic composition of the mirandola waters would correspond to a paleo-seawater component of mio-pliocene age. 3.2. gases the analytical results show that both bubbling and dissolved gases have low atmospheric contamination (table 1). the oxygen content is negligible, and ch4 is always the main gas species. he is enriched in all of the gas samples, and particularly in the dissolved gases, where its concentration is greater than the equilibrium with air saturated water, by ca. 40-fold at the san felice sul panaro well, 50-fold at the concordia well, and ca. 10-fold at mirandola, which shows the deep provenance of the fluids feeding the selected sites. both dissolved and bubbling gases are plotted in the triangular diagram of figure 1b, which shows the relative gas contents. it is worth noting the ch4 content, which is always the dominant species, includes the gases dissolved in shallow waters, as a consequence of diffuse ch4 degassing at a regional scale that can remove the atmospheric components from the recharged waters. the carbon isotopic composition of ch4 (–79.7‰< d 13c < –74.4‰) collected at the deep boreholes (table 1) highlights a biogenic origin for that methane from carbonate reduction. co2 is always detectable in the bubbling gases, with a concentration ranging between 0.66% and 0.9% at concordia, ca. 1% at massa finalese, and 0.645% at san felice sul panaro. while the isotopic composition of co2 from massa finalese is –17‰, the gases at concordia displayed more positive values, from –9‰ to –7‰ (table 1). considering together the information of the geochemical features of the gas phase, we argue that they originate from deep gas reservoirs, for which we exclude contributions from the mantle or mantle-derived sources, considering the isotopic ratio of he (in the range of 0.15-0.4 ra). the significant amount of he, which is mainly composed of 4he, originates from radioactive decay of the parent radioactive elements distributed in the thick sedimentary layers of the po plain, and concentrated in the hydrocarbon reservoirs responsible for the large release of ch4. high radon activity has never been detected, however, which includes the preliminary determinations carried out during the geochemical prospection. in contrast, temporal changes have been detected in other gas species, including he, co2, n2 and ch4. 4. earthquake-induced variations the crustal deformative processes associated with the occurrence of the seismic sequence induced a squeezing of deep-seated groundwaters associated with brines of the miocenic-pliocenic geological structures of the ferrara buried anticlines. italiano et al. 818 figure 1a. water geochemical features. piper diagram for the water samples. triangle, mirandola; circle, concordia; diamond, bellentani; square, menozzi; cross, bassoli; star, miano. 819 in the same sampling sites, and in particular during the month of may 2012, analysis of the dissolved gases and free gases detected a deep component that was characterized by high he concentrations, as above the atmospheric contribution, and by 13c linked to deep geological layers. in coincidence with the may 29, 2012, m 5.8 event, the ch4 concentration in the bubbling gases from the deep well of the vallalta site dropped sharply, with an increase in co2 (figure 2a). the co2-rich component is also the main carrier for he; thus, considering the radiogenic origin for the he component, we argue that the deep crustal component is represented by a ch4-dominated (>80% by volume), co2-rich (1% to 5% by volume) gas phase. after the occurrence of the main earthquakes, the gas composition underwent modifications, which were probably due to the long-term seismicity of the area. a behavior marked by a slow recovery of the former equilibrium conditions will help to define the background level for the area in terms of natural venting of deep and shallow components and their mixing proportions. we expect that the general behavior might show similar effects as those already observed in several sites around the epicentral area of the 1997-98 umbria seismic sequence [italiano et al. 2004] preliminary speciation calculations at the concordia site indicate that the waters underwent an abrupt calcite undersaturation pattern, followed by a trend towards new saturation fluids geochemistry and seismic crisis figure 1b. gas geochemical features. triangular diagram showing the relative concentrations of he, co2 and ch4 for both bubbling (crosses) and dissolved (empty triangles) gases. samples from two sites located far away from the epicentral area (m, miano di corniglio; br, bagno di romagna) are shown for comparison. sample i.d. as in table 1. site name date d18o dd bassoli 29/05/2012 -10.03 -68.29 bellentani 01/06/2012 -10.79 -74.93 07/06/2012 -10.81 -74.63 29/05/2012 -10.81 -74.09 minozzi 08/06/2012 -8.42 -54.55 mirandola 29/05/2012 -7.71 -52.99 01/06/2012 -8.12 -55.73 07/06/2012 -8.68 -60.29 vallalta 29/05/2012 -10.68 -73.81 01/06/2012 -10.70 -74.96 07/06/2012 -10.69 -74.99 table 2. isotopic composition of stable isotopes (o2 and deuterium) for some waters collected during the seismic crisis. data in d‰ versus standard mean ocean water (smow) international standard. conditions (figure 2a). this can be attributed to calcite dissolution in contact with co2-charged waters in a closed system, which indicates possible transients of co2-rich gases. after june 15, the deep component lowered together with the water flow rate observed at the san felice sul panaro sampling site. the only way towards the contemporary generation of both groundwater chemical anomalies and gaseous anomalies is the existence of faults that cross the quaternary and mio-pliocenic deep layers. this hypothesis is supported by the preliminary data on gas and water geochemistry changes. longer time series are required to confirm the proposed model of fluid/fault relationships. further investigations will better highlight possible contributions of pre-miocenic groundwaters hosted in carbonate reservoirs. italiano et al. 820 figure 2. temporal changes that occurred in the liquid and gas phases. (a) temporal changes in the calcite saturation state between may 29, 2012, and may 20, 2012. symbols as in figure 1a. (b) temporal changes recorded in the gas phase (bubbling gases of the 1,725-m-deep vallalta borehole). variations in co2 and ch4 concentration (vol%) with respect to the seismicity of the area (bar chart based on iside-ingv data). crosses, co2; empty circles, ch4. the may 29, 2012, m 5.8 event occurred in coincidence with the main changes recorded in the chemical compositions of the vented gases. the inverse correlation shown by co2 (solid line) and ch4 (dashed line) is of note. a b 821 5. conclusions this geochemical prospection and the periodic sampling of both liquid and gaseous phases in selected groundwaters of the southern po valley have revealed the occurrence of tectonic discontinuities that can allow the fast uprising of deep-seated fluids. the fluids are of deep origin and their movement towards the surface is enhanced during seismic sequences. the lowering with time of the main deep-originated components of the gaseous and water phases followed the lowering of the seismic rate and magnitudes. further sampling will allow a better description of the ongoing tectonic process. acknowledgements. the authors wish to thank prof. g. yuce and prof. g. bianchini for their useful comments and suggestions. the authors are indebted to giovanni martinelli who carried out the field work at his own expense. fausto grassa, igor oliveri, aldo sollami, mariano tantillo and francesco salerno from ingv-palermo are acknowledged for their contributions to the analytical work. references agip mineraria (1977). temperature sotterranee. inventario dei dati raccolti dall'agip durante la ricerca e la produzione di idrocarburi in italia, san donato milanese (milano), 1390 pp. albarello, d., and g. martinelli (1994). piezometric levels as possible geodynamic indicators: analysis of the data from a regional deep waters monitoring network in northern italy, geophys. res. lett., 21, 1955-1958. boccaletti, m., g. corti and l. martelli (2011). recent and active tectonics of the external zone of the northern apennines (italy), int. j. earth sci., 100, 1331-1348. cenni, n., e. mantovani, p. baldi and m. viti (2012). present kinematics of central and northern italy from continuous gps measurements, j. geodyn., 58, 62-72. conti, a., e. sacchi, m. chiarle, g. martinelli and g.m. zuppi (2000). geochemistry of the formation waters in the po plain (northern italy): an overview, appl. geochem., 15, 51-65. dondi, l., f. mostardini and a. rizzini (1982). evoluzione sedimentaria e paleogeografica nella pianura padana, in: g. cremonini and f. ricci lucchi (eds.), guida alla geologia del margine appenninico-padano, guida geologica regionale, s.g.i, bologna, 47-58. gorgoni, c., and g. tosatti (2004). emissioni di metano e fanghi salmastri da un pozzo dismesso in comune di concordia sulla secchia (provincia di modena), atti della società dei naturalisti e matematici modenesi, 135, 155-174. italiano, f., g. martinelli and a. rizzo (2004). geochemical evidence of seismogenic-induced anomalies in the dissolved gases of thermal waters: a case study of umbria (central apennines, italy) both during and after the 19971998 seismic swarm, g-cubed, 5, 11; doi:10.1029/2004 gc000720. italiano, f., p. bonfanti, m. ditta, r. petrini and f. slejko (2009). helium and carbon isotopes in the dissolved gases of friuli region (northeastern italy): geochemical evidence of co2 production and degassing over a seismically active area, chem. geol., 266, 76-85; doi:10.1016/j. chemgeo.2009.05.022. longinelli, a., and e. selmo (2003). isotopic composition of precipitation in italy: a first overall map, j. hydrol., 270, 75-88. martinelli, g., a. minissale and c. verrucchi (1998). geochemistry of heavily exploited aquifers in the emilia-romagna region (po valley, northern italy), environ. geol., 36, 195-206. ori, g.g. (1993). continental depositional systems of the quaternary of the po plain (northern italy), sediment. geol., 83, 1-14. pieri, m., and g. groppi (1981). subsurface geological structure of the po plain (italy), cnr, progetto finalizzato geodinamica, 414, 1-13. regione emilia-romagna and eni-agip (1998). riserve idriche sotterranee della regione emilia-romagna, g.m. di dio (ed.), s.el.ca., firenze. *corresponding author: francesco italiano, istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy; email: f.italiano@pa.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. fluids geochemistry and seismic crisis << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice articolo he.pdf key words holocene earthquakes – zemuhe fault – southwestern china 1. introduction the active zemuhe fault in western sichuan province, china, is a part of a left lateral fault system, which is at least 1400 km long and extends northwest along the southeastern margin of the tibetan plateau. this fault system (fig. 1, thick lines in insert), referred as the kangding fault system (e.g., molnar and tapponnier, 1978) or the xianshuihe-xiaojiang fault zone (e.g., li, 1993; wang et al., 1998), comprises five main holocene earthquakes on the zemuhe fault in southwestern china honglin he (1) (2) and jinwei ren (2) (1) active fault research center, national institute of industrial science and technology, ibaraki, japan (2) institute of geology, china seismological bureau, beijing, china abstract the zemuhe fault is a prominent active fault in southwestern china. seven ravines along a 5 km long fault scarp indicate seven large magnitude earthquakes in the holocene. the youngest four ravines were abandoned during four large magnitude earthquakes, the age of which are constrained by radiocarbon data: ravines 7, 6, and 4 formed in association with the earthquakes at a.d. 1850 and a.d. 814, b.c. 4477 ± 240 or older, and ravine 5 to a paleo-event between b.c. 4477 ± 240 and a.d. 814. three trenches excavated by earlier workers together with a trench excavated and analyzed here revealed 3 or 4 earthquakes, which are consistent with those indicated by the youngest five ravines. these radiocarbon-dated earthquakes mainly occurred within two temporal clusters: the older cluster of two paleoearthquakes occurred approximately between b.c. 4250 and b.c. 6000, and the younger cluster includes two historical earthquakes of the a.d. 814 and a.d. 1850. each cluster lasted about 1000-2000 years. a tranquil period of about 5000 years separates the two clusters, during which only one large magnitude earthquake occurred. moreover, the average recurrence interval of large magnitude earthquake in the holocene is about 1400-1700 years. comparison of the maximum horizontal displacement of the a.d. 1850 earthquake, and the 85 ± 5 m cumulative lateral offset over the last 13-15 ka gives the average recurrence interval of 1000-1360 years. the different estimates may arise because moderate and small earthquakes produced a quite high cumulative lateral displacement along the zemuhe fault during the holocene. faults: ganzi fault, xianshuihe fault, anninghe fault, zemuhe fault, and xiaojiang fault. these are amongst the most active faults in china, and clearly deserve comparison with other highly active strike-slip fault systems worldwide, such as san andreas fault in california, the north anatolian fault of turkey, and alpine fault of new zealand (allen et al., 1991). each of the main faults has accommodated at least one large historical earthquake. according to the local chronicles, the zemuhe fault produced two large earthquakes of m 7 in a.d. 814 and a.d. 1850 (huang, 1979; gu, 1983; chen, 1993a; ren and li, 1993). since the 1950’s the neotectonics, seismogeology and seismicity of the zemuhe fault and in its surrounding area have been studied (e.g., southwest investigation team of earthquake intensity, china seismological bureau, 1979), and the slip rate since the last glacial 1035 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. honglin he, institute of geology, china seismological bureau, beijing 100029, china; e-mail: honglinhe@hotmail.com 1036 honglin he and jinwei ren fig. 1. map of the zemuhe fault. thick lines indicate the fault traces, large-unfilled circles are the epicenters of three historic earthquakes of m 7 (gu, 1983), and the dashed ovals show the isoseismal lines of the a.d. 1850 earthquake (ren and li, 1993). the unfilled rectangles across the zemuhe fault with numbers indicate the trench locations excavated by fen et al. (1995), while the filled rectangle indicates the location of our trench. inset box shows the sketch of tibetan tectonics and the zemuhe fault zone location. thick line is the kangding fault system: 1) ganzi fault; 2) xianshuihe fault; 3) anninghe fault; 4) zemuhe fault and 5) xiaojiang fault. wide arrows indicate the movement directions of crustal blocks within the tibetan plateau, while thin arrows show the slip of faults. data come from li (1993). epoch for the zemuhe fault has been reasonably well constrained at 4.9 mm/yr (ren, 1986) or 5.8-8.6 mm/yr (he et al., 1999). the active trace produced during the a.d. 1850 earthquake on the zemuhe fault was first recognized in 1985 (ren and li, 1989). analysis of the faultscarp morphology suggests that the rupture length of this earthquake may have been ca. 30 km (chen, 1993a) or up to 90 km long (ren and li, 1993). trenching across the fault by fen et al. (1995) reveals 6 events during the holocene, however, these data provide an incomplete record of earthquakes for this period of time. here we use data from the displacement of dateable landforms and from trenches across the fault to constrain the earthquake history of the zemuhe fault. these data are combined with information from an exceptionally long historical record, and produce what we believe a complete data set of large magnitude earthquakes in the holocene. 2. historical earthquakes on september 16th, a.d. 1850, a severe earthquake struck xichang and its surrounding area, resulting in the deaths of more than 23.526 people and significant damage to infrastructure (fig. 1) (gu, 1983). the local chronicles show that two other large earthquakes of m 7 also occurred in a.d. 814 and a.d. 1536 near xichang, and these events can tentatively be correlated to the zemuhe fault (e.g., huang, 1979; chen, 1993a; ren and li, 1993). the a.d. 1536 earthquake may have occurred on the anninghe fault (fig. 1) (e.g., chen, 1993b; wen et al., 2000), while the epicenter of the a.d. 814 earthquake was located near the intersection of the zemuhe and anninghe faults (gu, 1983). there is no strong geological evidence to suggest which of these two faults produced the a.d. 814 event. based on historical records, most researchers preferred to locate it on the zemuhe fault (e.g., ren and li, 1989). the epicenters of historical earthquakes in china are defined as the center of the meizoseismal area (xie, 1957). the meizoseismal area is delineated based on the descriptions of earthquake damage recorded in local chronicles. magnitude of historical earthquake is usually deduced from the seismic intensity in the meizoseismal area. generally, both seismic intensity and magnitude can be estimated from historical records. initially, the meizoseismal area of the 1850 earthquake was located at xichang city, the intensity in the meizoseismal area was determined to be x on the new chinese seismic intensity scale (xie, 1957), and the magnitude inferred to be 7.5 (kunming geophysics institute, 1965 in gu, 1983; huang, 1979). the new chinese seismic intensity scale was made in 1957 and revised in 1980 based on the modified mercalli scale in association with characteristics of chinese buildings. the two seismic intensity scales are very similar. historical records indicate that during the earthquake some damage occurred south of the qionghai lake, extending the meizoseismal area southeastward, and shifting the epicenter to between the xichang and ningnan (lin and gan, 1980). furthermore, by reanalyzing historical earthquake records, huang (1985) suggested the epicenter of this earthquake to be south of xichang, with intensity in the meizoseismal area of x+ on the chinese seismic intensity scale, and inferred the magnitude of this earthquake to be 7.7. this earthquake not only produced damage to buildings and heavy loss of life, but also produced significant deformation of the ground surface, which was recorded in local chronicles more than 140 years after the earthquake the surface erosion had modified the active trace. however, many relics of this earthquake remain and can be identified in the field today. in 1985, the research group of seismic risk in panxi region, sichuan seismological bureau identified a 30 km long rupture of this earthquake from south of qionghai lake to the tuomugou (chen, 1993a), and estimated the magnitude to be 7.5. in contrast, ren and li (1993) suggest a 90 km long rupture from lijianbao to songxin (fig. 2) with a maximum lateral displacement of 7 m (fig. 3), and a modified pattern of isoseismal lines (dashed ovals on fig. 1). they located the epicenter of the earthquake near the daqing village, south of qionghai lake, and proposed m 8 based upon the rupture length and maximum displacement. based on our fieldwork in 1997 and 1998, we concur with the conclusions of ren and li (1993) for the reasons shown below. 1037 holocene earthquakes on the zemuhe fault in southwestern china 1) we have identified a number of tectonic features between daqing and qiaowuo (figs. 1 and 2), such scarplets, fault trough, sag pond, offset landform, fissure and filled wedge, etc., as the landforms observed by ren and li in 1985 and 1986 (ren and li, 1993). 2) at lijinbao village north of the qionghai lake (figs. 1 and 2), a fissure created in the a.d. 1850 earthquake, which is filled by a wedge of material, was described by wu (1982). we named this kind of wedge-like filled material as filled wedge. although no rupture phenomenon was found near the xichang city presently, many landslides and fissures are recorded in the local chronicles. these fissures and landslides may be largely absent today because of intensive human activity, which has resulted in landform modification. 3) near songxin village (figs. 1 and 2) to the south a series of scarplets and tensional fissures were observed, which are arranged in a left-stepping en échelon pattern. the height of these scarplets ranges from 0.3 m to 6 m, and the depth and width of these tensional fissures from 0.5-10 m and 1-10 m, respectively. 4) along the rupture zone, lichen samples (caloplaca sp. and acarospora sp.), taken from the fault scarp or from talus in 9 sites including the scarplets and tensional fissures mentioned above, were dated at between 114 ± 46 and 164 ± ± 35 years bp (ren, 1986). these data suggested that most of the observed fault-scarp was produced during the a.d. 1850 earthquake. in addition to these two large historical earthquakes, there were 10 moderate earthquakes of 5 m < 7 recorded on the zemuhe fault since a.d. 116 (gu, 1983). of these moderate size earthquakes, five events are between magnitude 5 and 6, and the other five between magnitude 6 and 7. 3. paleoearthquakes recorded by the offset of ravines along the fault scarp the daqingliangzi uplift is the watershed between the anninghe river basin and the zemuhe river basin (fig. 1). along the zemuhe fault, uplift has produced a 5 km long fault scarp (fig. 4a,b). the fault scarp reveals that the uplifted region comprises the daqing forma1038 honglin he and jinwei ren fig. 2. map of surface rupture associated with the a.d. 1850 earthquake. thick lines show the location of the active trace, while the thin lines show the drainage of the zemuhe river. fig. 3. near the sijiabushi village a 50 cm wide fissure due to the a.d. 1850 earthquake offset sinistrally a small pressure ridge by 7 m, view to north. tion gravel of late pleistocene age. daqing formation principally consists of pebbles derived from purple-gray quartz sandstone and rhyolite of sinian age. the diameters of the pebbles vary between 18 and 35 cm. the southeast dip of imbricated pebbles in the lower layer of the gravel suggests a paleo flow direction of zemuhe river to the northwest towards qionghai lake in the early late pleistocene (huang, 1979). it further suggests that the daqingliangzi uplift started to grow no earlier than the late pleistocene. the height of the fault scarp decreases gradually from north to south, and diminishes at the bend in the highway near sijiabushi (fig. 4b). in the north, the scarp height is about 40 m and its slope is about 50°, however, these values changes to 20 m and 70° in the south. the 0.5-2 m high surface rupture produced by the a.d. 1850 earthquake developed along the base of the northern segment of the fault scarp, and it merges with, and cannot be differentiated from, the fault scarp in the south. seven ravines are cut into the top of the daqingliangzi uplift, on the upthrown eastern side of the fault scarp (figs. 4b and 5). the average length of these ravines is about 1000 m, and the width varies between 15 and 20 m. ren measured the geometry of the features of these ravines in 1986 (table i). we suggest that formation of the fault scarp not only resulted in abandonment of the ravines across the top of the daqingliangzi uplift, but also separated the modern zemuhe river into two branches. the eastern branch flows on the east of the uplift, whereas the western branch flows along the base of the fault scarp. east of the uplift all of these ravines are occupied by streams which flow southeastward into the eastern branch. west of the fault scarp on the downthrown wall 1039 holocene earthquakes on the zemuhe fault in southwestern china b a fig. 4a,b. 5 km long fault scarp from daqing to sijiabushi (location shown in fig. 2). a) a pair of air photographs; b) field photograph, view to east. of the zemuhe fault, there are several minor streams flowing southeastward into the western branch (figs. 4a and 5). ravines on the eastern, upthrown, side of the fault are all much deeper and wider than those on the downthrown wall, suggesting much greater flow in those ravines. moreover, the ravines are all parallel to, and similar in shape to, the modern channel segment of the western branch north of sijiabushi, which flows eastwards at the southern end of the fault scarp (figs. 4b and 5). these data suggest that the ravines were, in all probability, cut by the main channel segment of the western branch, and later we hypothesize that they were abandoned due to repeated uplift events on the fault. the geomorphologic expression of these ravines indicates that the northern ravines are older than the southern ravines. the most direct expression is that slope angle and depth of the northern ravines are less than those of the southern ravines (table i). the three northernmost ravines (1 to 3 from north) on the daqingliangzi uplift are subtle features and not easily identifiable. these ravines are shallow troughs with gentle (less than 10°) sideslopes, which are indicative of significant post-abandonment erosion of the ravines. the shape of the southernmost four ravines (4 to 7) is characterized by steep walls. the steep bank slopes of the southernmost four ravines also suggest that the ravines young from north to south (table i). 1040 fig. 5. map of the fault scarp, offset ravines and drainage system on the top of the daqingliangzi uplift. thick line is the fault scarp with ticks on the downthrown side of the fault. thin lines show the drainage system. gray zones show the ravines; the grayscale from dark to light show the age of ravines from young to older. arrows in the gray zones indicate the following directions. black squares indicate the locations of the c14 samples. table i. slope angle of the bank and the depth of the seven ravines. southern slope northern slope d (m) as ls (m) hs (m) an (m) ln (m) hn (m) (hs + hn)./.2 1 ca. 5 2 10° 61 10.6 7° 45 5.5 8.1 3 12° 64 13.3 10° 39 6.8 10.1 4 44° 31 21.5 25° 17 7.2 14.4 5 55° 30 24.6 30° 22 11.0 17.8 6 60° 15 12.9 40° 30 19.3 16.1 7 72° 14 13.3 45° 21 14.9 14.1 as, and an are angles of the southern and northern slopes; ls, and ln are lengths of the southern and northern slopes; hs, and hn are heights of the southern and northern slopes, hi = li * tan ai (i = s, n); d is the depth of ravine, d = (hs + hn)./.2. honglin he and jinwei ren fig. 6. sketch showing the evolutionary scenario of ravines on the top of the daqingliangzi uplift formed in association with large magnitude earthquakes on the zemuhe fault. all of these ravines are cut into, and post date, gravel of the daqing formation of late pleistocene age. a layer of yellow colored loess-like silt soil occurs on the top of the daqingliangzi uplift. since loess-like silt layer envelopes the three northernmost ravines, the daqing formation gravel is not exposed. this suggests that the northernmost ravines 1-3 formed after deposition of the daqing formation gravel and the change in flow patterns of the zemuhe river, but before deposition of the loess-like layer. in the floor and sides of ravines 4-7 the daqing formation gravel crops out and the gravel of the modern river is exposed and identified in the bottom of ravines 6 and 7. this suggests that ravines 4-7 must have formed after deposition of the loess-like silt. we therefore propose that the ravines formed in sequence from north to south across the top of the daqingliangzi uplift and that shifts in the position of the active stream channel were triggered by uplift during large magnitude earthquakes (fig. 6). in the early part of the late pleistocene, the zemuhe river flowed northward into the qionghai lake (huang, 1979), while in the latter part of the late pleistocene it changed its flow direction from northward to southward. after the change in river course, the zemuhe river flowed as a single channel, flowing along the present-day eastern branch with a series of small tributaries flowing into the channel from each side. formation of the fault scarp during large magnitude earthquakes resulted in the abandonment of small tributaries, which formally crossed the daqingliangzi uplift. the water from these abandoned minor tributaries subsequently flowed southward along the base of the fault scarp, and produced the western branch of the zemuhe river. the newly formed western branch changed flow direction from southward to eastward at the southern end of the fault scarp producing a new channel or occupying a preexisting minor tributary. during each large magnitude earthquake, the fault scarp extended southward, uplifting the area east of the fault and forcing western branch further south around the fault scarp. simultaneously, the old eastward flowing segment of the western branch was uplifted and abandoned, producing a ravine immediately north of holocene earthquakes on the zemuhe fault in southwestern china 1041 the active channel. therefore, the recurrence of large magnitude earthquake on the zemuhe fault is interpreted to produce the ravines on the top of the daqingliangzi uplift (figs. 4b, 5 and 6). for such a model seven large magnitude earthquakes could produce the observed ravines. the evolutionary model outlined above is consistent with the southward decrease in daqingliangzi uplift. as a result, the fault scarp is a cumulative vertical displacement along the higher in north than in the south. we postulate that each ravine was abandoned due to episodic tearing of the fault during large magnitude earthquakes. therefore, the time of abandonment of each ravine may indicate the date of each large magnitude earthquake. the dating of three c14 samples, taken in ravines 4, 6 and 7, implied the ages of three earthquakes corresponding to ravines 4, 6 and 7 (table ii, samples’ location shown in fig. 5). of all of the ravines the southernmost one is the most similar in shape to the modern river channel at the southern end of the uplift. the difference between the two is that many trees and shrubs are growing on the two side-slopes of the latter, while the former contains many dead roots and trunks with little vegetation. these many dead roots and trunks in the southernmost ravine should have been growing before abandonment; their death is due to abandonment. a piece of a dead root (r7 in table ii) taken from the floor of ravine 7 was radiocarbon dated as younger than 200 years bp. based on this date, we infer that ravine 7 was abandoned during the a.d. 1850 earthquake. at the floor of ravine 6 two hundred meters east of the fault scarp we observed a piece of paleo-flood plain. on the paleo-flood plain covers a layer of dark brown peat. we infer that the peat layer developed in a mash pond after abandonment. a sample (r6 in table ii) taken from the dark brown peat layer was radiocarbon dated at 745 ± ± 85 years bp, implying that this ravine was abandoned at some time older than a.d. 1279 ± 123. there is no historical record in local chronicles of an earthquake in this region near a.d. 1279 ± ± 123. we consider it unlikely that an earthquake of the magnitude as large as the a.d. 1850 event would not be recorded in local chronicles had it occurred near a.d. 1279 ± 123. therefore, we hypothesize that ravine 6 was abandoned during the a.d. 814 earthquake and that the young radiocarbon age may indicate that some of the dated carbonaceous material accumulated in the ravine well after abandonment. near the fault scarp on the base of northern convex slope of ravine 4 found a layer of black peat, which has been radiocarbon dated at 5615 ± 115 years bp (r4 in table ii). also considering the black peat layer developed in a mash pond in the ravine after abandonment, we can infer another earthquake event have occurred at, or older than b.c. 1042 table ii. radiocarbon data. no. location material c14 age (years bp) cal age (yr)# lab. r7 ravine 7 a piece of wood < 200 < a.d. 1750 lab. 1 (1) r6 ravine 6 peat 745 ± 85 a.d. 1279 ± 123 lab. 1 (1) r4 ravine 4 peat 5615 ± 115 b.c. 4477 ± 240 lab. 1 (1) tro-1 our trench a piece of wood 3150 ± 100 b.c. 1422 ± 216 lab. 1 (2) tr1-1 trench 1 wood fragments 500 ± 100 a.d. 1412 ± 119 lab. 2 (3) tr1-2 trench 1 wood fragments 1200 ± 120 a.d. 828 ± 211 lab. 2 (3) tr1-3 trench 2 peat 3612 ± 106 b.c. 1969 ± 243 lab. 2 (3) tr2-1 trench 2 peat 5528 ± 95 b.c. 4386 ± 167 lab. 2 (3) tr3-1 trench 3 wood fragments 670 ± 145 a.d. 1259 ± 235 lab. 2 (3) tr3-2 trench 3 peat 6700 ± 155 b.c. 5602 ± 244 lab. 2 (3) tr3-3 trench 3 peat 12.015 ± 216 b.c. 11.979 ± 462 lab. 2 (3) tr3-4 trench 3 wood fragments 4993 ± 137 b.c. 3778 ± 270 lab. 2 (3) lab. 1 new date laboratory, institute of geology, csb; lab. 2 sichuan seismological bureau. # calculated by calib radiocarbon calibration (vision 4.3). (1) ren (1986); (2) this study; (3) fen et al. (1995). honglin he and jinwei ren 4477 ± 240. in addition to the three inferred events occurred at a.d. 1850, a.d. 814 and b.c. 4477 ± 240 or older, four more events might be indicated by the observed ravines. for such a scenario the large magnitude earthquake that resulted in abandonment of ravine 5 would date at between a.d. 814 and b.c. 4477 ± 240. the remaining three ravines are inferred to be older than b.c. 4477 ± 240, and also older than the loess-like silt. the silt has not been dated and we suggest that ravines 1-3 are most likely to be early holocene in age. this would require seven large magnitude earthquakes on the zemuhe fault in the last 10 kyr. 4. paleoearthquakes recorded in trenches five hundred meters south of the southern end of the fault scarp a stream is offset in a leftlateral sense by about 85 m. on the eastern side of the fault the stream has been abandoned and formed a relict tail-cut stream. a new alluvial fan has formed on the eastern side of the fault and a small gully has been displaced left-laterally by 16 m (fig. 7a). north of the abandoned tailcut stream a small pressure ridge is developed on the eastern side of the fault (fig. 7b), probably due to a slight right step of the fault traces. north of the small pressure ridge, we excavated a 10 m long, 1.5 m wide and 2.5 m deep trench (location shown by a filled box across the zemuhe fault in fig. 1, and a unfilled box in fig. 7b). based on mapping of the trench the log was divided into 15 units (fig. 8). this trench revealed two earthquakes of the zemuhe fault. it is, however, difficult to identify the displacement of each event, because no sedimentary layers could be correlated across the fault surface on the trench log. the wedge deposits of unit 2 and unit 8 are likely to indicate the two seismic events. figure 9 presents a possible scenario of the events in the trench. the older event displaced and flexed sediments of unit 10 to unit 15 (stage 2). after this event, a colluvial wedge (unit 8) was deposited in the depression formed by the flexure. the younger event cut this colluvial wedge and dragged the peat sediment of unit 7 down (stage 4). another wedge deposition (unit 2) was formed after the second event. a piece of wood (tro-1 in table ii) enclosed by sediments of unit 11 to unit 14 was radiocarbon dated at 3150 ± 100 years bp. this dating result indicates that unit 11 is, or older than b.c. 1422 ± 216. the fault trace does not reach the ground surface, and the sediments of units 2 to 6 are not displaced. moreover, the 90 km long surface rupture of the a.d. 1850 earthquake was found to go through east of the trench along the fault scarplet (fig. 7b; ren, 1986). we therefore infer that the two events interpreted in this trench should have occurred in the period between b.c. 1422 ± 216 and the a.d. 1850 earthquake, and the younger one may be consistent with the a.d. 814 earthquake. on the same section of the fault that we trenched, fen et al. (1995) excavated four trenches in 1995 (locations shown in fig. 1, unfilled boxes across the zemuhe fault with numbers). because the timing of paleo-events in the southernmost trench was not constrained by dating (trench 4), we only use data derived from the remaining three trenches. trench 1 was excavated north of the daqing village across the fissure zone formed in the a.d. 1850 earthquake. this trench revealed a multifissure zone, which is filled by three wedges of different materials. two c14 samples of wood fragments (tr1-1 and tr1-2 in table ii) taken from the two youngest filled wedges produced dates of 500 ± 100 years bp and 1200 ± 120 years bp. the newest stratum, which is cut by the boundary fault of the oldest filled wedge, is ca. 1.5 m thick light yellow sediment of silt with pebbles. in the middle of the stratum there is a thin peat layer of ca. 1.75 m long and 5 cm thick, which was radiocarbon dated at 3612 ± ±106 years bp (tr1-3 in table ii). if the three filled wedges resulted from three seismic events, the three seismic events should all be younger than b.c. 1969 ± 243. if the wood fragments taken from the two youngest filled wedges died prior to being incorporated into the wedges, the event corresponding to the newest filled wedge should be the a.d. 1850 earthquake (younger than a.d. 1412 ± 119), and the event corresponding to the middle filled wedge should be a.d. 828 ± ± 211, which is consistent with the historical 1043 holocene earthquakes on the zemuhe fault in southwestern china honglin he and jinwei ren fig. 7a,b. a) geomorphologic map of zemuhe fault (location shown in fig. 2). fault scarp is marked by the thick line; ticks are on the downthrown side of the fault. eighty-five-meter lateral displacement of abandoned stream is shown. contour data (10 m intervals) are from 1:50 000 topographic maps. b) detailed map of part of fault. topographic contours of 1 m were constructed from surveying and the thick lines with teeth show fault scarplet and the rupture of the a.d. 1850 earthquake. b a 1044 earthquake of a.d. 814. the oldest event recorded in this trench must be younger than b.c. 1969 ± 243. these conclusions are similar to inferences from our trench (fig. 8). trench 2 crossed a scarplet of the a.d. 1850 earthquake. this trench revealed five fault surfaces (f1, f2, f3, f4, and f5) and a colluvial wedge. although fen et al. (1995) proposed that there are five seismic events recorded in this trench, only four of these events, including the 1850 earthquake, are unambiguously represented (fig. 10). the oldest event is indicated by f1, which is covered by the colluvial wedge. the sediment layer displaced by f1 was dated at 5528 ± 95 years bp using a sample of peat taken from this layer (tr2-1 in table ii). this implies that the oldest event is younger than b.c. 4386 ± 167. the colluvial wedge, which occurs in the northeast of f2 and covers f1, demonstrates an earthquake younger than the oldest event. a sediment layer was deposited over the colluvial wedge and f2, f3. the fault surface f4 cut into this sediment layer and indicates another event younger than the two events noted above. the newest event is the a.d. 1850 earthquake that produced a modern scarplet (f5) and a sag pond northeast of the scarplet. trench 3, which is about 100 m south of our trench, also crossed a fissure of the 1850 earthquake. fen et al. (1995) identified six events in this trench, but in our opinion two of these events are equivocal. the remaining four events in this trench will be summarized here (fig. 10). a multi-fissure zone filled by two wedges of different materials and two fault surfaces (f1 and f2) within the trench we interpreted to have formed in four earthquakes. the younger filled wedge is composed of yellow sandy silt, from which a sample of wood fragments (tr3-1 in table ii) was dated to be 670 ± 145 years bp. considering that the wood fragments possibly died hundreds of years prior to being incorporated into the wedge, we infer that the younger filled wedge formed after the a.d. 1850 earthquake (much younger than a.d. 1259 ± 235). the older filled wedge is composed of gray sandy silt including some pebbles. a sample of peat taken from the older filled wedge (tr3-2 in table ii) was dated to be 6700 ± 155 years bp. this radiocarbon date demonstrates another earthquake occurred at b.c. 5602 ± 244 or older. the event indicated by the fault surface f1 is the oldest one revealed in this trench. a peat sample (tr3-3 in table ii) taken from the 1045 holocene earthquakes on the zemuhe fault in southwestern china fig. 8. log of the southern wall of the trench excavated by us for this study. the location of the trench is shown in figs. 1 and 7b. 1 light yellow boulder gravel bed; 2 mixed deposit wedge of gray soil and gravel; 3 dark brown soil used for cone cultivation; 4 black peat lens in unit 3; 5 black peat bed; 6 brown humus-rich silt bed; 7 black peat bed; 8 deposit wedge of sand gravel; 9 black peat bed like unit 7, but having poor stratification; 10 dark brown humus-rich silt bed, thicker than unit 6; 11 light yellow gravel sand bed with rich pieces of root and root hair; 12 black peat with rich pieces of root and root hair; 13 yellow sand bed with rich pieces of root and root hair; 14 dark brown humus-rich silt bed with rich pieces of root and root hair; 15 gray gravel-bearing sand bed, its top layer has rich pieces of root and root hair. honglin he and jinwei ren fig. 9. a possible history for the trench in fig. 8, giving 5 stages, in which stage 2 and stage 4 correspond to two large magnitude earthquakes. 1046 surface of f1 was dated to be 12.015 ± 216 years bp. if inferring that the peat formed in surrounding sediment before and was drawn onto the surface of f1 during this oldest earthquake, the oldest event should be younger than b.c. 11.979 ± 462. another layer of sandy gravel deposited over the fault surface f1 was cut by another fault surface f2. this sandy gravel was dated to be 4993 ± 137 years bp using wood fragments (tr3-4 in table ii) taken from the gravel. the event indicated by f2 should be therefore younger than b.c. 3778 ± 270. 5. discussion the radiocarbon dates indicate four large magnitude earthquakes since some time older than b.c. 4477 ± 240, which were associated with the formation of the southernmost four ravines (4-7). the trench we excavated recorded two earthquakes, probably in the period from b.c. 1422 ± 216 to a.d. 1850, in addition to the latest historic earthquake of a.d. 1850. the three trenches excavated by fen et al. (1995) suggest three, four and five events. we summarize these results as in fig. 10. although the trench data are incomplete in the holocene, it is likely that the seven events for the ravines include all large magnitude earthquakes as strong as the a.d. 1850 earthquake or a.d. 814 earthquake. of the seven large magnitude earthquakes, the trenches reveal the youngest five. the paleoearthquake younger than b.c. 3778 ± 270 revealed in trench 3 is an independent event, no relation to any large event indicated by ravine data (fig. 10). considering the inference that the seven events for the ravines are all large magnitude earthquakes of the zemuhe fault in the holocene, we suggest this paleo-event younger than b.c. 3778 ± 270 is a moderate earthquake. moreover, trench 3 recorded a late pleistocene earthquake younger than b.c. 11:979 ± 462. seven large magnitude earthquakes indicated by ravine data suggest an average recurrence interval of large magnitude earthquakes on the zemuhe in the holocene of about 1400-1700 years. however, collectively the trench and ravine data indicate that the recurrence interval of large magnitude earthqua1047 fig. 10. seismological history of the zemuhe fault based upon historic and paleoseismological data. tr1, tr2 and tr3 are three trenches excavated by fen et al. (1995), tour is the trench excavated by us in 1997. arrow with left direction show the upper age limit of paleoearthquakes, while arrow pointing rightwards indicates the lower age limit of paleoearthquake. the unfilled arrow indicates that the seismic event dated by radiocarbon sample is actually a historical earthquake. dark gray rectangles show the errors on the radiocarbon dating. question marks within dark gray rectangles indicate an inferred paleoearthquake without dating. double-arrow shows temporal cluster of seismic activity. light gray rectangles show preferred ages for the paleoearthquakes; thin vertical lines shows possible age bound of the paleoearthquake, while dashed line shows no age bound of the paleoearthquake. holocene earthquakes on the zemuhe fault in southwestern china kes in the holocene along the zemuhe fault is not uniform (fig. 10). except for the oldest two holocene large paleoearthquakes with no radiocarbon date, the remaining five large radiocarbon-dated events present obviously two temporal clusters. the older cluster occurred between b.c. 4250 and b.c. 6000 including two large magnitude earthquakes (large events 4 and 5), and the younger cluster since a.d. 814 including historical earthquakes of a.d. 1850 and a.d. 814. the recurrence interval in the older cluster is similar to the average, 1400-1700 years, while the recurrence interval in the younger cluster is only 1000 years, shorter than the average. the two temporal clusters are separated by a tranquil period of about 5000 years, during which only one large magnitude earthquake (large event 3) occurred. the majority of quaternary sediments in the zemuhe river valley deposited during deformation were derived from a local source. local transport of these sediments resulted in the formation of three river terraces the zemuhe river valley (t1, t2, and t3 from low to high). the most extensive terrace is the lowest (t1) (fig. 11), which consists of a unitary deposit of boulder gravel with a sand and mud matrix. the sizes of framework boulders range from several centimeters to several meters. this gravel is polymictic, comes mainly from the following sources: 1) sinian-age purple sandstone, tuff, conglomerate, and gray dolostone and limestone; 2) triassic-age gray quartz sandstone and limestone, and 3) jurassic-age purple sandstone. fresh exposures of the gravel are light purple because most of the source rocks are purple. a silt and fine sand sample, taken from the lowest terrace (about 1 m deep), has been dated at 13.290 ± 104 years bp using a thermoluminescence method (he et al., 1999). ren (1986) radiocarbon dated a piece of wood in the terrace deposit to be 15.630 ± 990 years bp. therefore, the lowest terrace (t1) is estimated to have in the last glacial epoch, about 10-15 kyr bp. between sijiabushi and tuomugou, we have measured five offsets of streams or creeks at four sites (85 m, 88 m, 80 m, 90 m and 80 m), which are 1048 fig. 11. map showing the fault trace and terraces of the alluvial fan in the zemuhe valley between sijiabushi and tuomugou. five offsets of streams developed on the alluvial fans of the first terrace (t1) have been measured at four sites. three boxes above show detailed offsets of streams. honglin he and jinwei ren cut into the lowest terrace (t1) (fig. 11). therefore, the average 85 ± 5 m offset is younger than the formation of the lowest alluvial fan terrace, and was produced by a series of earthquakes since the last glacial epoch. the maximum lateral displacement in the a.d. 1850 earthquake was identified to be 7 m (fig. 3). eleven to thirteen large magnitude earthquakes with lateral displacement of 7 m would be required to produce 85± ± 5 m cumulative offset. the average recurrence interval would be 1000-1360 years. this result is shorter than 1400-1700 years estimated by historic earthquakes and dated paleoearthquakes. regardless of the error of the radiocarbon dating, the fact remains that quite a high part of total accumulated displacement along the zemuhe fault was probably not produced by large magnitude earthquakes. we infer that quite a large part of total accumulated displacement along the zemuhe fault was accommodated by moderate and small earthquakes. according to the local chronicles, 10 moderate earthquakes of magnitude between 5 and 7 have occurred on the zemuhe fault since a.d. 116 (gu, 1983). trench 3 reveals a moderate paleoearthquake younger than b.c. 3778 ± 270, which should have produced surface rupture. the xiaojiang fault, another active fault of the kangding fault system as the zemuhe fault (inset in fig. 1), has similar features. the average recurrence interval estimated using the rate of strike slip is less than that estimated from paleoearthquake data (shen et al., 1997). shen et al. (1998) proposed a model similar to ours to explain this kind of difference of recurrence intervals. table i shows the geometric features of those ravines on the top of the daqingliangzi uplift. the depths of the ravines are all greater than the 5 m maximum vertical displacement of the 1850 earthquake (ren, 1986) and the depths of the zemuhe river channel or other nearby streams. this suggests much stronger river erosion on the daqingliangzi uplift than nearby area. we infer the much stronger river erosion is caused by the vertical displacement of the daqingliangzi uplift during those moderate earthquakes. therefore, we improve the evolutionary model proposed above as fig. 6 shows. initially, the western zemuhe river channel flowed southward along the fault scarp and turned to east around the southern end of the fault scarp (fig. 12a). secondly, small displacement during moderate and small earthquakes occurred. these displacements were not large enough to force the eastward flowing channel segment to be abandoned, but were sufficiently large to produce river down cutting. simultaneously these displacements make the fault scarp extended southward (fig. 12b). thirdly, when a large magnitude earthquake occurred, it contributed slip to the fault scarp and resulted in southward extension of the western zemuhe river channel. a new eastward flowing channel segment developed at the new southern end of the fault scarp; the old eastward flowing channel segment at the old southern end of the fault scarp was abandoned simultaneously (fig. 12c). 6. conclusions in this paper we describe and summarize earthquakes on the zemuhe fault recorded by abandoned ravines along an uplifted fault scarp, holocene earthquakes on the zemuhe fault in southwestern china fig. 12a-c. model to illustrate the development the daqingliangzi fault scarp and ravines on the scarp top. .l and d are the horizontal and vertical offsets during large magnitude earthquakes; .la and the depth of the abandoned ravine da are the horizontal and vertical offsets accommodated by moderate and small earthquakes. 1049 a b c by paleoearthquake recorded in trenches and by examining historical records. seven abandoned ravines on the daqingliangzi fault scarp are interpreted to indicate seven events as large as the a.d. 1850 earthquake or a.d. 814 earthquake. the youngest four earthquakes were radiocarbon dated at a.d. 1850, a.d. 814, b.c. 4477 ± 240 or older, and between b.c. 4477 ± 240 and a.d. 814, while the older three are only inferred. the seven events indicated by seven ravines should be all the large magnitude earthquakes in the holocene. the trenches excavated by previous workers and by us reveal 2-4 earthquakes in the holocene, most corresponding to those demonstrated by youngest five ravines on the daqingliangzi uplift. we conclude that the average recurrence of large magnitude earthquake on the zemuhe fault in the holocene is about 14001700 years, with a minimum of about 1000 years. in detail, however, recurrence intervals were variable with clusters in the holocene. the first cluster occurred between b.c. 4250 and b.c. 6000, and the second younger than a.d. 814. each cluster lasted for about 1000-2000 years. between the two clusters, there was a long tranquil period of about 5000 years, during which only one large magnitude earthquake (corresponding to ravine 5) occurred. furthermore, using the maximum horizontal displacement of the a.d. 1850 earthquake and the 85 ± 5 m cumulative lateral offset that accrued over the last 13-15 ka, we inferred another average recurrence of 1000-1360 years, shorter than 1400-1700 years. this difference may arise because moderate and small earthquakes produced a quite high part of cumulative lateral displacement along the zemuhe fault during the holocene. acknowledgements we thank mr. song fangmin, ms. zhang wanxia and dr. li chuanyou for their kind help in field investigation. we are grateful to dr. kenji satake for his constructive criticism of the manuscript. thanks are also due to dr. a. nicol for critical reviews that greatly improved this paper. honglin he acknowledges the financial support partly by the sasakawa scientific grant from the japan science society. references allen, c.r., z. luo, h. qian, x. wen, h. zhou and w. huang (1991): field study of a highly active fault zone: the xianshuihe fault of southwestern china, geol. soc. am. bull., 103, 1178-1199. chen, s. (1993a): 1850 earthquake of magnitude 7.5 between xichang and puge, in the appendix ‘representative earthquakes in sichuan’ of active faults and earthquakes in sichuan province, edited by r. tang et al. (seismological press, beijing, china), 20-28 (in chinese). chen, s. (1993b): 1536 earthquake of magnitude 7.5 in xinhua, in the appendix ‘representative earthquakes in sichuan’ of active faults and earthquakes in sichuan province, edited by r. tang et al., (seismological press, beijing, china), 4-7 (in chinese). fen, y., p. du, y. jiang, s. liu, y. huang and t. den (1995): synthetical report of mapping and investigation of the zemuhe fault (working report of sichuan seismologocal bureau, chengdu, china), (in chinese). gu, g. (1983): earthquake catalog of china (scientific press, beijing, china), 8-266 (in chinese). he, h., f. song and c. li (1999): topographic survey of micro-faulted landform and estimation of strike slip rate for the zemuhe fault, sichuan province, seismol. geol., 21 (4), 361-369 (in chinese). huang, s. (1985): preliminary study of the features of strong historic earthquakes and the assessment information of the general seismic trend, j. seismol. res., 8 (5), 485-495 (in chinese). huang, z. (1979): inquiry on the relationship between the tectonics of fault block and seismicity in xichang, sichuan, in bulletin of seismogeological investigation in strong earthquake area in yunnan and sichuan provinces, edited by southwest intensity investigatory team, china seismological bureau (seismological press, beijing, china), 104-120 (in chinese). li, p. (ed.) (1993): xianshuihe-xiaojiang fault zone (seismological press, beijing, china), pp. 267 (in chinese). lin, x. and x. gan (1980): a study ningnan-xichang earthquake of 1850, j. sichuan univ., 1, 96-103 (in chinese). molnar, p. and p. tapponnier (1978): active tectonics of tibet, j. geophys. res., 83 (b11), 5361-5375. ren, j. (1986): seismotectonics of xichang earthquake of 1850 and paleo-earthquake in zemuhe fault zone, m.s. thesis of institute of geology (china seismological bureau, beijing, china), (in chinese). ren, j. and p. li (1989): earthquake-caused landforms and paleoseismic study on the northern segment of the zemuhe fault, seismol. geol., 11 (1), 27-34 (in chinese). ren, j. and p. li (1993): the characteristics of surface faulting of 1850 earthquake in xichang, sichuan, seismol. geol., 15 (2), 97-106 (in chinese). shen, j., y. wang and f. song (1997): the grouping of strikeslip data and seismicity of the xiaojiang active fault, earthquake res. china, 13 (4), 366-375 (in chinese). shen, j., y. wang and f. song (1998): relative creep rate characteristic earthquake recurrence interval-example from the xiaojiang fault zone in yunnan, china, seismol. geol., 20 (4), 328-331 (in chinese). 1050 honglin he and jinwei ren southwest investigation team of earthquake intensity, china seismological bureau (ed.) (1979): bulletin of seismogeological investigation in strong earthquake area in yunnan and sichuan provinces (seismological press, beijing, china), pp. 141 (in chinese). wang, e., b.c. burchfiel, l.h. royden, l. chen, j. chen, w. li and z. chen (1998): late cenozoic xianshuihexiaojiang, red river, and dali fault systems of southwestern sichuan and central yunnan, china, geol. soc. am., pap. 327, pp. 108. wen, x., p. du and d. long (2000): new evidence of paleoearthquakes and date of the latest event on the xiaoxiangling mountain segment of the anninghe fault zone, seismol. geol., 22 (1), 1-8 (in chinese). wu, d. (1982): identification of a paleoearthquake relic in lijinbao village, xichang and its implication, sichuan seismol., 4, 22-24 (in chinese). xie, y. (1957): a new scale of seismic intensity adapted to the conditions in chinese territories, acta geophys. sin., 6 (1), 35-47 (in chinese). 1051 holocene earthquakes on the zemuhe fault in southwestern china g g g g g agitazioni microsismichf. o r i g i n a t e da a t t i v i t à " vulcanica p . caloi a . l o surdo g . ponte e ' n o l o l'ile t u l l e le stazioni s i s m i c h e del i n o n d o r e g i s t r a n o , in m i s u r a p i ù o m e n o a c c e n t u a l a , a s e c o n d a d e l l e stagioni e d e l l a posiz i o n e g e o g r a f i c a , un c o m p l e s s o di p e r t u r b a z i o n i r i t m i c h e , di o r i g i n e non p r o p r i a m e n t e s i s m i c a , n o t e sotto il n o m e g e n e r i c o di a g i t a z i o n e m i e r o s i s m i c a . a n c h e c a t a n i a r e g i s t r a o v e n l e periti i l l a z i o n i r i t m i c h e di q u e s t o g e n e r e . s t u d i p r e c e d e n t i a t t r i b u i s c o n o d e t t e p e r t u r b a z i o n i a l l ' u r l o dei m a r o s i sulle c o s t e c i r c o s t a n t i . e ' noto |>erò c h e l ' u r l o localo p u ò caus a r e solo p e r t u r b a z i o n i r a p i d i s s i m e , m a i c o m u n q u e d e l l ' o r d i n e di 2 4 s e c o n d i , q u a l i sono i p e r i o d i p r e d o m i n a n t i n e l l ' a g i t a z i o n e m i e r o s i s m i c a di c a t a n i a . n 5 aam/w^/wv/ww" */wvvwsvmma"ww/\/w1 e w fig. i tratto ili r e g i s t r a z i o n e c o n t e m p o r a n e a a l l ' a t t i v i t à esplosiva dell e t n a ( • 23 h 31'" del 30 giugno 1942) ma q u e s t o a r g o m e n t o esula dal n o s t r o a s s u n t o . \ o g l i a m o qui l i m i t a r c i a d a r n o t i z i e di un p a r t i c o l a r e tipo di a g i t a z i o n e m i e r o s i s m i c a c l i c ha a c c o m p a g n a t o l ' a t t i v i t à esplosiva cent r a l e d e l l ' e t n a i n c o m i n c i a t a d o p o cessata la b r e v e e r u z i o n e l a t e r a l e del 3 0 giugno e r i p e t u t a s i il 5 l u u l i o . p. c u o i \. i o s i rino g. ponte vccenneremo a due f a l l i c a r a t t e r i s t i c i . .si c h e l ' a b i t a z i o n e m i c r o s i s m i c a dovuta a l l ' a z i o n e dei m a r o s i e , più g e n e r a l m e n t e , ad agenti m e t e o r o l o g i c i , può d u r a r e i n t e r e s e m i n a n e : ad ogni m o d o , non è m a i di d u r a t a i n f e r i o r e a q u a l c h e o r a . piando la causa è del lutlo l o c a l e (nel qttal eu<o p e r ò si m a n i f e s t a con periodi ili 1-2 secondi al m a s s i m o ) . o r b e n e , il 3 0 giugno 1 9 4 2 , sullo sfondo di una d e b o l e a g i t a z i o n e ( t — 2 sec c a . ) di a m p i e z z a p r e s s o c h é u g u a l e sulle d u e c o m p o n e n t i orizzontali e m i l a v e r t i c a l e , quasi d " i m p r o \ v i s o , a l l e 231' 32"' 30" i n i z i a u n ' a m p i a r e g i s t r a z i o n e ili oscillazioni r e g o l a r i , ili p e r i o d o m e d i o pari a 3 , e per la durala di soli 7 minuti. d a n o t a r e c l i c d e l l a r e g i s t r a zione interessa quasi e s c l u s i v a m e n t e la c o m p o n e n t e e w (fig. i). k' n o t e v o l e il f a l l o c h e , alla stessa o r a , da p e r s o n e c h e si t r o v a vano a l l ' a l b e r g o « ktna » ( 1 7 0 0 m in) fur o no visti ( ' ) l'orti b a g l i o r i a c c o m p a g n a t i da boati sul c r a t e r e c e n t r a l e , la <'iii attività si affievolì presto. dalle 3" 46m a l l e 31' 5 1 " del i " luglio -i lui la r e g i s t r a z i o n e -n l i t t l e e tre le c o m p o n e n t i di due d e b o l i scosse, di p r o b a b i l e o r i g i n e e t n e a . dalle 91' a l l e io1' -i ha quindi una r i p r e s a d e l l ' a g i t a z i o n e del t i p o su r i f e r i t o , clic i n t e r e s s a a n c h e questa volta quasi e s c l u s i v a m e n t e la c o m p o n e n t e e w . c o n m a s s i m i i n t o r n o a l l e 8 , 4 0 . o u e l c d u e m a n i f e s t a z i o n i i s o l a t e di a g i t a z i o n e m i c r o s i s m i c a r i v e lano l ' a c c i d e n t a l i t à d e l l a loro o r i g i n e , c h e esula d a l l e cause a cui e-sa c o m u n e m e n t e è dovuta ed è l e c i t o p e r t a n t o a t t r i b u i r s i a l l ' a t t i v i t à de! vicino v u l c a n o . notevole i! f a t t o , a riprova d e l l ' e c c e z i o n a l i t à d e l l a causa d e l l ' a g i tazione c i t a l a , c h e m e n t r e l e p e r t u r b a z i o n i c o m u n i c h e f a c e v a n o da fondo a i l c agitazioni in p a r o l a avevano s e n s i b i l e c o m p o n e n t e v e r t i c a l e , tale c o m p o n e n t e è nulla p e r l ' a g i t a z i o n e sismica i n e r e n t e a l l ' a t t i v i t à del v u l c a n o . l ' e r u z i o n e i n i z i ò il 3 0 giugno a l l e 10h l ó m ; nel p o m e r i g g i o del 1 luglio l ' e r u z i o n e di lava e i a quasi f i n i t a . .nei giorni 2 , 3 e i il c r a t e r e c e n l i a ì e m a n t e n n e piuttosto t r a n q u i l l o ; solo fu vista q u a l c h e 1 , 1 risulta ila indagini fatte -ni posto dal prof. pont.-. dirottole dell ' m i l u l o vulcanologico dell'i niver-ità di catania e dell'osservatorio sismico dcll ' i . n . g . di catania. ir. jia^lt a g i t a z i o n i m i c h o s i s m i c h e o r i g i n a t e 1)a a t t i v i t à v l ' l c a m c a p i c c o l a f u m a t a c a r i c a (li c e n e r e . a l t r e m a n i f e s t a z i o n i si e b b e r o n e l l a n o l l e del 4 . i s i s m o g r a f i di c a t a n i a , p e r m o l i v i d i p e n d e n t i d a l l o «lalo di guerra, non p o t e v a n o in q u e i g i o r n i f u n z i o n a r e i n m o d o c o n t i n u o . m a n c a n o così le r e g i s t r a z i o n i t r a il 1" ed il 5 l u g l i o . ii g i o r n o 5 l u g l i o , r i m e s s i a l l e 8h 5m i l o g l i ai s i s m o g r a f i , q u e s t i r i v e l a r o n o u n ' a g i t a z i o n e i n a t t o del t i p o a n z i d e t t o , r e g i s t r a t a d a l l a n s aaaàmaawì vwvvv-vv-aawvvxrt/l/w-'; ,wvvwma/1/^~vwvw-./vv'1—^aaa/^^aaaaa^'^"/»' ì fi e w fig. 2 t r a i l i di agitazioni* mierosismica in r e l a z i o n e con la forte attività esplosiva dell etna dalle 7h alle l l ! l del 5 luglio 1942 c o m p o n e n t e e w con a m p i e z z e n o t e v o l i , m e n t r e sulla c o m p o n e n t e n s , c o m e i n p r e c e d e n z a , e r a a p p e n a a c c e n n a t a . i a l e a g i t a z i o n e , a l i p o p a r o s s i s t i c o , d u r ò v i o l e n t a fino a l l e i 4 h , c o n a m p i e z z e elle ragg i u s e r o , s u h ' e w . c i r c a 18 li : p o i . quasi b r u s c a m e n t e , ; i a c q u i e t ò (fig. 2). v a m e s s o n e l l a dovuta evidenza il f a l l o c h e a n c h e q u e s t a a g i t a z i o n e dei sismografi di c a t a n i a fu in p e r f e t t a c o i n c i d e n z a con i a t t i v i t à esplos i v a d e l c r a t e r e c e n t r a l e del v u l c a n o . e c c o i n f a t t i co-a r i l e v a v a il p r o f . p o n t e : « a l l e 6 , 5 5 del 5 l u g l i o i n c o m i n c i a r o n o ad essere a v v e r t i t i da c a t a n i a e dai paesi c i r c u m e t n e i l'orti c c o n t i n u i b o a t i p r o v e n i e n t i dal c r a t e r e c e n t r a l e , m e n t r e a b b o n d a n t e s a b b i a c a d e v a in tutto il v e r s a n t e t a l e del v u l c a n o . i ' . caloi a. lo sl'udo ( ; . l ' o n t e l'ale intensa attività esplosiva (pliniana) durò senza tregua fino alle j.3; poi andò diminuendo e cessò completamente ulte ore 14 ». le c a r a t t e r i s t i c h e di questa nuova, più lunga r i p r e s a dell a g i t a zione in e-arne -mio -sempre le s t e s s e ; p e r i o d o p r o s s i m o a 3", registrazione m o l l o a m p i a stilla c o m p o n e n t e k-w e p r e s s o c h é i n e s i s t e n t e -lille c o m p o n e n t i n-s e v e r t i c a l e . s e , c o m e tutlo lascia r i t e n e r e , detta agitazione -i deve a t t r i b u i r e al lavorio del v u l c a n o , p o i c h é la b o c c a c e n t r a l e di questo -i trova p o c h i s s i m o discosta dal m e r i d i a n o ili c a t a nia ({\x. ."<). possiamo s e n z ' a l t r o a d e r i r e c h e l e o s c i l l a z i o n i r e g i s t r a t e sono trasversali-tangenziali, cioè o n d e s u p e r f i c i a l i di l o v e . questo è uno degli a s p e t t i p i ù i n t e r e s s a n t i del f e n o m e n o . r e s t a da s t a b i l i r e >c si t r a t t a di un l'alto i s o l a l o , o p p u r e se a c c o m p a g n a s e m p r e l ' a t t i v i t à esplosiva del v u l c a n o ; cosa c h e ci p r o p o n i a m o di f a r e n o n a p p e n a sarà p o s s i b i l e , c o n sistem a t i c h e osservazioni i n t o r n o al cono dell e t n a . r e g i s t r a z i o n i di t r e m i t i vulcanici f u r o n o o t t e n u t e , in modo s i s t e m a t i c o , nel 1 9 3 8 ila t a k a h a s i , jnagala e h i r a n o ( ' ) con 1 a u s i l i o ili 19 s t a z i o n i , m u n i t e di m i c r o s i s m o g r a f i con p e r i o d o p r o p r i o di 2s poste i n t o r n o c ' vulcano m i l i a r a , n e l l ' i s o l a di o o s i m a ( g i a p p o n e ) . f u r o n o f a t t e n u m e r o s e cu di f r e q u e n z a dei p e r i o d i , s i a per le c o m p o n e n t i nella d i r e z i o n e del c r a t e r e , sia p e r le c o m p o n e n t i a sii". il p e r i o d o p r e d o m i n a n t e risultò di 0 , 3 2 0 , 3 0 s e c o n d i , r e l a t i v o a t r e m i l i g e n e r a t i sul l'ondo del c r a t e r e sotto l'orma di o n d e l o n g i t u d i n a l i , p r o p a g a n t i s i in tutta l ' i s o l a di o o s i i n a . gli autori ritengono p o s s i b i l e la g e n e r a z i o n e di o n d e a p i ù lungo p e r i o d o , del tipo l o v e : le registrazioni r e l a t i v e a l l ' e t n a , c h e f o r m a n o oggetto di questa nota, s e m b r a n o c o s t i t u i r e la t e s t i m o n i a n z a di d e l l a p o s s i b i l i t à . a g i t a z i o n i m i c h o s i s m i c h e o r i g i n a t e da a t t i v i t à m m a n i c a 0 riassunto a r i periodi di attività esplosiva del cratere centrale, verificatisi duratile l'eruzione dell'etna dal 30 giugno al 5 luglio 1912. i sismografi ili catania hanno registrato agitazioni ritmiche con periodo dell'ordine di 3". si prova che dette agitazioni consistono in onde di l o v e , ciò che costituisce una singolare caraneristica che viene messa in evidenza per hi prima volta. b i b l i o g r a f i a t j ) r . t a k a h a s i . f . .nagala e in. mirano « bulletin e a r l h q u a k e r e * e a r r h limitine», voi. xvi. 1938. pag. 87. ag_56.04.13_cover+front+index+intro+ackn_final_layout 6 [special issue_56_04_2013] vesuvius monitoring and knowledge: state of the art and perspectives. edited by francesca bianco, stefano caliro, mario la rocca and umberto tammaro. annals of geophysics [special issue_56_4_2013] vesuvius monitoring and knowledge: state of the art and perspectives preface francesca bianco, stefano caliro, mario la rocca, umberto tammaro s0433 articles flood hazard of the somma-vesuvius region based on historical (19-20th century) and geomorphological data giuliana alessio, melania de falco, giuseppe di crescenzo, rosa nappi, antonio santo s0434 analytical models of volcanic ellipsoidal expansion sources antonella amoruso, luca crescentini s0435 thirty years of precise gravity measurements at mt. vesuvius: an approach to detect underground mass movements giovanna berrino, vincenzo d’errico, giuseppe ricciardi s0436 p-wave velocity and density structure beneath mt. vesuvius:a magma body in the upper edifice? paolo capuano, guido russo, roberto scarpa s0437 the 2nd to 4th century explosive activity of vesuvius: new data on the timing of the upward migration of the post-a.d. 79 magma chamber raffaello cioni, claudia d’oriano, antonella bertagnini, daniele andronico s0438 felt index, source parameters and ground motion evaluation for earthquakes at mt. vesuvius elena cubellis, aldo marturano s0439 the first long period earthquake detected in the background seismicity at mt. vesuvius paola cusano, simona petrosino, francesca bianco, edoardo del pezzo s0440 seismic activity and thermal regime of low temperature fumaroles at mt. vesuvius in 2004-2011: distinguishing among seismic, volcanic and hydrological signals paola cusano, simona petrosino, paolo madonia, cinzia federico s0441 the recent seismicity of mt. vesuvius: inference on seismogenic processes luca d’auria, antonietta m. esposito, domenico lo bascio, patrizia ricciolino, flora giudicepietro, marcello martini, teresa caputo, walter de cesare, massimo orazi, rosario peluso, giovanni scarpato, ciro buonocunto, marco capello, antonio caputo s0442 inside mt. vesuvius: a new method to look at the seismic (velocity and attenuation) tomographic imaging edoardo del pezzo, francesca bianco s0443 new insights into mt. vesuvius hydrothermal system and its dynamic based on a critical review of seismic tomography and geochemical features edoardo del pezzo, giovanni chiodini, stefano caliro, francesca bianco, rosario avino s0444 electric effects induced by artificial seismic sources at somma-vesuvius volcano rosa di maio, gianpaolo cecere, prospero de martino, ester piegari s0445 neural analysis of seismic data: applications to the monitoring of mt. vesuvius antonietta m. esposito, luca d’auria, flora giudicepietro, teresa caputo, marcello martini s0446 annals of geophysics groundwater geochemistry of the mt. vesuvius area: implications for volcano surveillance and relationship with hydrological and seismic signals cinzia federico, paolo madonia, paola cusano, simona petrosino s0447 wavefront distortion across mt. vesuvius observed by a seismic array danilo galluzzo, mario la rocca s0448 level of carbon dioxide diffuse degassing from the ground of vesuvio: comparison between extensive surveys and inferences on the gas source domenico granieri, maria luisa carapezza, rosario avino, stefano caliro, carlo cardellini, giovanni chiodini, marco donnini, carmine minopoli, massimo ranaldi, tullio ricci, luca tarchini s0449 the seismic monitoring network of mt. vesuvius massimo orazi, luca d’auria, anna tramelli, ciro buonocunto, marco capello, antonio caputo, walter de cesare, flora giudicepietro, marcello martini, rosario peluso, giovanni scarpato s0450 spatial and temporal distribution of vertical ground movements at mt. vesuvius in the period 1973-2009 folco pingue, milena bottiglieri, cataldo godano, francesco obrizzo, umberto tammaro, teodoro esposito, claudio serio s0451 vesuvio civil protection exercise mesimex: survey on volcanic risk perception tullio ricci, rosa nave, franco barberi s0452 19 years of tilt data on mt. vesuvius: state of the art and future perspectives ciro ricco, ida aquino, sven ettore p. borgstrom, carlo del gaudio s0453 the automated infrared thermal imaging system for the continuous long-term monitoring of the surface temperature of the vesuvius crater fabio sansivero, giovanni scarpato, giuseppe vilardo s0454 volcanic precursors in light of eruption mechanisms at vesuvius roberto scandone, lisetta giacomelli s0455 somma vesuvius volcano: ground deformations from cgps observations (2001-2012) umberto tammaro, prospero de martino, francesco obrizzo, giuseppe brandi, andrea d’alessandro, mario dolce, santa malaspina, claudio serio, folco pingue s0456 preface vesuvius monitoring and knowledge: state of the art and perspectives mt. vesuvius is one of the most hazardous volcanoes in the world, due to the highly populated surrounding area, where more than 800,000 people live. it experienced various regimes of eruptive activity, from effusive to plinian. its last eruption, a mild effusive event, occurred on march 1944. after that the volcano started a period of quiescence, characterized by low seismicity, low deformation pattern, widespread fumaroles emissions accompanied by diffuse co2 degassing in the crater area and co2-rich groundwater along the southern flank and in the adjacent plain. despite the low level of activity, vesuvius is one of the best-monitored volcanoes in the world. during the last decades many researchers have been involved in the volcano monitoring, covering a wide range of topics, in order to discover any signals that could contribute to understand the volcano dynamics. however, in spite of so much effort, an exhaustive comprehension of the volcano system, aimed at a reliable prediction of the future activity, is far from having been reached. on the other hand, the fast technological evolution makes new instruments and methodologies available for a more sensitive monitoring in the future. the objective of this special issue, that appears 70 years after the last eruption, is to draw a precise picture of the state of the art about the monitoring of vesuvius volcano, with detailed description of instruments, tools and methods used in the past and at present, the results achieved, and new ideas to develop in the future. thanks to the researchers that submitted their contributions, this special issue of annals of geophysics provides a more comprehensive picture of the vesuvius status, its monitoring system, the current knowledge and the most innovative techniques currently applied and to be further developed in the near future. in particular, contributions are presented on the recent developments and open problems related to mt. vesuvius monitoring, from in-situ methods – such as seismic, geodetic and geochemical networks, geochemical survey – to remote sensing methods. the arguments treated in the various contributions cover a wide range of topics as briefly described below. the most common precursors of volcanic eruptions recognized worldwide regard seismicity, ground deformations and increase and variations of gas emissions, while other anomalies such as ground temperature, electric, magnetic and gravity variations, have also been observed but are less documented. a continuous monitoring of any geophysical and geochemical parameters is therefore necessary for a high risk volcano, and it is currently realized at mt. vesuvius. inside the special issue, one may found contributions describing the monitoring networks and the scientific results obtained from such advanced networks. the seismic monitoring network of mt. vesuvius is described by orazi et al. the station distribution provides appropriate coverage of the area around the volcanic edifice, allowing for the location of seismic events with m<1. the real time broadcast of seismic data to the monitoring center of the osservatorio vesuviano (istituto nazionale di geofisica e vulcanologia) dates back to 1972. therefore a detailed catalog of local earthquakes during the last 40 years is available, and it is the starting point for many detailed studies. a deep insight into the main features of the recent seismicity of mt. vesuvius, with particular emphasis on the period 1999-2012, is furnished by d’auria et al. two main seismogenic volumes inside the volcanic edifice have been recognized: one with hypocenters clustered below the mt. vesuvius crater at depths mostly between 1-6 km, and another, shallower, with hypocenters clustered within the gran cono. the energy transported by hydrothermal fluids and dissipated by hot soils generally represents a main term of the total energy released from quiescent volcanoes. the monitoring of the thermal energy release through the ir automatic system, which operates at vesuvius, and its technical evolution furnish a suitable tool for volcanic surveillance. sansivero et al. describe the first results obtained by this evolution of the ir automatic system. granieri et al., on the base of a measurement campaign of co2 soil diffuse degassing from the cone of vesuvius, and comparing the data with those of a previous study, estimate the total budget of co2 released by diffuse degassing and describe the relationships with the main geologic structures. beside the eruptive phenomena, other causes may increase the hazard in volcanic environments. among these, annals of geophysics, 56, 4, 2013, s0433; doi:10.4401/ag-6439 s0433 floods and mudflows produced by heavy rain exacerbated by unconsolidated volcanic material constitute the highest risk at a number of volcanoes worldwide. a thorough and highly useful compilation of past rainfall data and related hazard for the somma-vesuvius region, represent a preliminary approach to mitigate the hazard posed by rain-induced floods (alessio et al.). the ground deformation topics are broadly described. a detailed description of the monitoring by classical leveling methods carried out for almost four decades is provided by pingue et al., who describe also the year by year enlargement of the monitoring circuits. they observe that vesuvius area is affected by a general subsidence that becomes more significant at the center of the volcano. the somma caldera rim seems the limit between two zones with different subsidence velocities. since the leveling measurements give information only about the elevation, several tiltmeters were installed on the volcano flanks to fill, at least partially, the gap left by leveling measurements (ricco et al.). when the new gps technology became available, a permanent gps network was established, and now it ensure a continuous monitoring of the 3d ground deformations (tammaro et al.). results of gps measurements carried out during the last decade indicate that somma-vesuvius is affected by a predominant slow contraction phase in the gran cono area. the analysis of ground deformation data collected during the last four decades by different methods indicates that mt. vesuvius is not affected by ground deformations attributable to magmatic sources. the interpretation of ground deformations is not a trivial matter, and its comparison with theoretical patterns is important to establish the expected deformations in case of future re-awakening of the volcano. amoruso and crescentini describe a method for estimating the volume change and the corresponding superficial displacements of a finite pressurized ellipsoidal source immersed in an elastic medium. they present equations not available in the common scientific literature, that can be implemented in computer codes. tightly related with ground deformations are the observations of the gravity field and its variations. an important contribution to our understanding of mt. vesuvius has been furnished by the 30 year high precision gravity measurements on the volcanic system (berrino et al.). the extremely comprehensive gravity data set has allowed for both spatial and temporal analysis of changes that, without the baseline data, may have been misinterpreted as precursors of renewed eruptive activity. the fact that gravity changes cannot be easily explained by the deformation signal and therefore are better supported by fluid migration is an important contribution. di maio et al. describe how the natural electric field pattern under the volcanic edifice showed significant variations in conjunction with the last active seismic tomography survey (tomoves experiment). studies on the seismic waveform features allow for a better knowledge of both the dynamics related with the hydrothermal system, as well as of the volcanic structure. many active volcanoes show seismic signals different from “classical” shear failure earthquakes. cusano et al. (the first long period earthquake...) describe detailed analysis of the first local seismic event classified as lp earthquake, recorded by the local network in 2003. the classification of seismic events may be a demanding task for seismologists, particularly when the seismicity rate is high (e.g. hundreds of events per day). fortunately modern software can provide a great help in such cases, as pointed out by esposito et al., who describe the most advanced automatic analysis of seismic data based on neural network, able to recognize and classify many kinds of seismic signals. a wavefront distortion across mt. vesuvius, observed by a temporary deployed seismic array, has been attributed to the propagation through a non-homogenous velocity structure (galluzzo and la rocca) by computing synthetic wavefront propagation in two different velocity models, one homogeneous, the other heterogeneous. the heterogeneous model gives better results, also in agreement with the results of previous tomographic studies. the continuous improvement of computing power, analysis tools and graphical software give important support to the earth sciences, permitting new analysis and new methods of displaying, and hence interpreting the results. an example in this special issue is provided by del pezzo and bianco, who show new interpolations of tomographic pictures. several contributions on multidisciplinary approaches, which are becoming more and more important for the comprehension of volcanic systems, are included in this special issue. the contribution by cusano et al. (seismic activity and thermal regime...) is an interesting attempt to study the correlation between the seismicity and the features of gases emitted by fumaroles. on the other hand, a geochemical-geophysical approach based on data of preface 2 groundwater from the vesuvius aquifer collected from 1998 to 2011 and geophysical signals, provide a more comprehensive conceptual model of the fluids circulation in the volcanic system and the possible relations with the stress field variation (federico et al.). del pezzo et al. analyze tomographic images of vp and q, the hypocenter distribution of volcano-tectonic earthquakes and geochemical measurements. the authors suggest an interaction between patches of magma and the hydrothermal system to explain the behaviour of seismic and geochemical parameters. capuano et al., through a simultaneous inversion of travel times, hypocentral parameters of local earthquakes and gravity data, provide a high-resolution image of the shallow (300-500 m) edifice of mt. vesuvius as well as a critical review of a number of previous studies focused on the detection of a shallow magma chamber and interpretation of the present local seismicity. descriptions of the past activity and discussion on the possible future dynamical behavior are commented inside the issue. the analysis of historical earthquakes is also important to evaluate the maximum expected magnitude in the area. cubellis and marturano provide an interesting analysis of historical seismicity based on the earthquake effects described by old sources. a comprehensive understanding of the volcano status and its possible evolution cannot be separated from detailed study of past eruptions. accordingly, cioni et al. analyze the eruptive period preceding the explosive eruption of a.d. 472. understanding the dynamics of past eruption is crucial for a correct interpretation of possible precursors, as pointed out by scandone and giacomelli. the last eruption, occurred in 1944, was not much different or larger than others before, and it occurred at the end of a long period characterized by a decreasing trend of eruption explosivity. most of the uncertainty on the duration of the present stage and character of a future renewal of activity derives from the basic questions regarding the nature of the current repose: due to a diminished supply of magma, related with structural condition, or to a sealing of the upper ascent path to the surface? a debate on this question is enclosed (scandone and giacomelli). the volcanic risk perception is also commented, since a survey on this topic, described by ricci et al., was carried out during the october 2006 european civil protection exercise mesimex (somma vesuvio mesimex‚ major emergency simulation exercise). the obtained results strongly suggest that a continuous and effective effort has to be done by both scientific community and civil protection, also turning particular attention to education and outreach activities, as well as in involving people in risk mitigation procedures, also through more frequent exercises. the 23 papers included in this special issue of annals of geophysics present a clear view of any monitoring networks, tools and methodologies currently used to survey vesuvius volcano. any fields of possible research are discussed, from very technological topics to human related questions, from events occurred 20 centuries ago to very recent ones. moreover, in these papers the reader will find exhaustive references to the most important papers regarding mt. vesuvius as well as to general topics. the editors appreciate the efforts of the many colleagues who provided critical reviews of the papers included in this special issue, significantly contributing to increase quality and contents. without their efforts, the volume could not have been completed in the present form. francesca bianco, stefano caliro, mario la rocca, umberto tammaro istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy annals of geophysics, 56, 4, 2013, s0433; doi:10.4401/ag-6439. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. preface 3 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 precambrian tectonic inheritance control of the ne brazilian continental margin revealed by curie point depth estimation jefferson tavares cruz oliveira*,1, josé antonio barbosa1, david lopes de castro2, paulo de barros correia1, josé ricardo gonçalves magalhães3, osvaldo josé correia filho1 and bruno varela buarque1 (1) federal university of pernambuco, geoquantt research in geosciences, department of geology, avenida acadêmico hélio ramos, s/n, cidade universitária, recife pe, brazil, 50740.530 (2) federal university of rio grande do norte, department of geology, campus universitário s/n, lagoa nova, natal rn, brazil, 59072-970 (3) federal university of recôncavo, exact and technology sciences center, campus, cruz das almas ba, brazil, 44380-000 article history: received january 30, 2020; accepted december 7, 2020 abstract an investigation of curie point depths (cpd) based on spectral analysis of airborne magnetic data was carried out in the ne brazilian continental margin. the studied region represents a narrow hyper-extended margin with three sedimentary basins. regional geothermal gradient and heat flow were also calculated. cpd results were integrated with interpretation of 2d deep seismic data and with estimated isostatic moho depths. the results reveal that the narrow hyper-extended crust is 150 km wide in the southern sector and 80 km wide in the north, with a narrow ocean-continental transition (oct) zone that varies from 50 km wide in the south sector to 30 to 20 km wide in the north. the cpd isotherm showed the strong influence of the three main continental blocks of borborema´s shield in the tectonic evolution of the three marginal basins. the cpd analysis corroborated models provided by gravimetric data and successfully demonstrated the sharp control of basement compartments on the thermal properties of the marginal basins domains. keywords: curie point depth; rift margin evolution, basement inheritance; transitional crust; borborema province. 1. introduction magma-poor hyper-extended margins are characterized by a hyper-thinned zone of continental crust, which involves deep faulting within the crust and formation of mantle-rooted detachment zones [reston, 2009; pérezgussinyé, 2013; sibuet and tucholke, 2013; franke, 2013; guillot et al., 2015; gillard et al., 2017]. those margins show little syn-rift magmatism and a continent-ocean transition formed by mantle exhumation, leading to serpentinization, which preceded the oceanic spreading. two characteristics are most distinctive [reston, 2009]: annals of geophysics, 64, 2, gt213, 2021; doi:10.4401/ag-8424 o p e n acc e s s 1 extreme crustal thinning hyper-extended crust (~30 km to a few kilometers over a distance of 100-200 km), 2 a sector of anomalous basement, between the last identifiable continental crust and the true oceanic crust. this zone of “intermediate” crust is formed by the process of mantle exhumation with a variable degree of serpentinization, which is named ocean-continent transition zone (oct). the eastern continental margin of northeast brazil represents part of the central segment of the south atlantic ocean (figure 1). this region experienced low spreading rates (~4mm/y) during the rift evolution [brune et al., 2014], which influenced the formation of narrow continental margins. their model also suggests that these margins are magma-poor type and prone to the formation of hyper-extended crust. in contrast with the expressive knowledge accumulated for the adjacent regions of the study area, such as the sergipe and alagoas marginal basins [mohriak and rosendhal, 2003; aslanian et al., 2009], the lack of geophysical data (seismic and nonseismic) has hampered a better understanding of the geotectonic evolution of the eastern border of the borborema province (bp in figure 1). this research focuses on the three basins that formed in this marginal region: the pernambuco basin (peb) and its distal portion formed by the pernambuco plateau (pep), the paraíba basin (pab), and the natal platform (nap) [magalhães et al., 2013; buarque et al., 2016] (figure 1). curie point depth (cpd) is the theoretical depth where most magnetic minerals lose their magnetic properties due to the increased temperature in the subsurface [tanaka et al., 1999; thébault et al., 2010]. the cpd obtained through the analysis of geomagnetic data can be used as a proxy for inferring the thermal characteristics of the crust (e.g., heat flow and geothermal gradient) [tanaka et al., 1999; ates et al., 2005; bektas 2013; hsieh et al., 2014; li et al., 2013, 2017; idarraga-garcía and vargas, 2018; aydemir et al., 2019; gaudreau et al., 2019; quintero et al., 2019]. cpd investigation is a complementary method to assess crustal characteristics that can be constrained by deep seismic surveys or heat flow and geothermal gradient data measured in boreholes [ross et al., 2006; li et al., 2010; petrunin et al., 2013; hsieh et al., 2014]. one of the applications of cpd estimation is the assessment of the relationship between the moho surface (inferred crust-mantle boundary) and the cpd isotherm. some studies have shown that the relationship between these two boundaries is complex, and the curie isotherm can be shallower or deeper than the estimated moho boundary. therefore, the cpd-moho relationship is influenced by the geotectonic context: continental shields [maule et al., 2005; guimarães et al., 2014; leseane et al., 2015; chopping et al 2015; aboud et al., 2016], fold belts [dolmaz et al., 2005; rajaram et al., 2009), subduction zones [manea and manea, 2010; hsieh et al., 2014; manalo et al., 2016], volcano-tectonic zones [bektaş et al., 2008; bektaş, 2013]. some works have also pointed out that cpd estimation represents a useful tool for the investigation on mantle magnetization, caused by serpentinization, beneath oceanic crust [li et al., 2010, 2013; ravat et al., 2011], continental shields and subduction zones [blakely et al., 2005; manea and manea, 2011; wang and li, 2015]. 2. geological setting the bp comprises the northeastern portion of the south american platform [almeida, 1981; van schmus et al., 2008; santos et al., 2010; araujo et al., 2013; neves et al., 2015] (figure 1) and is bounded to the south by the são francisco craton, to the west by the large intracratonic parnaíba basin, and to the north by the potiguar, ceará and barreirinhas marginal basins [matos, 1999; de castro et al., 2012]. the bp consists of a complex combination of continental masses [almeida et al., 1981; santos et al., 2010], and its evolution involved a series of precambrian orogenic events [santos et al., 2010; neves et al., 2012, 2015] and late rifting stages that culminated with a continental breakup during the cretaceous [chang et al., 1992; matos, 1999]. the bp is divided into three main domains, or continental blocks, that are bounded by continental-scale shear zones: the pernambuco-alagoas complex, known as the south domain (sd); the central domain, known as the transversal zone (tz), and the north domain (nd) [vauchez et al., 1995; neves and mariano, 1999; brito neves et al., 2002, ferreira et al., 2008, medeiros et al., 2011; araujo et al., 2013; neves et al., 2015] (figures 1a and 1b). the most important shear zones within the bp are the pernambuco shear zone (pesz) and the patos shear zone (pasz), both trending mainly e-w and wswene (figure 1). the formation of marginal basins was strongly controlled by this compartmentation of the bp and the large shear zones, which acted as boundaries to the studied basins (figures 1 and 2) [matos, 1999; magalhães et al., 2013]. the studied area encompasses the pernambuco, paraíba and natal platform basins (figures 1 and 2) [mabesoone and alheiros, 1993; barbosa and lima filho, 2006; barbosa et al., 2007; magalhães et al., 2014b; buarque et al., 2016]. jefferson tavares cruz oliveira et al. 2 the continental breakup in this region is proposed at the lower-middle albian [matos, 1999; almeida et al., 2005; turner et al., 2008]. this segment of the brazilian continental margin has been suggested to be a magma-poor margin type [reston, 2009; reston and manatschal, 2011; haupert et al., 2016]. geophysical studies have indicated that the studied region is characterized by a narrow margin, probably formed by hyper-extended crust [blaich et al., 2008; magalhães et al., 2013]. the narrow extended margin was a consequence of the low spreading rates observed for this segment of the central south atlantic (~30mm/yr), in comparison to >50 mm/yr for the southeast margin of brazil, around 120 m.y (brune et al., 2016). this segment also presents an asymmetry in terms of extension compared to the african conjugate margins, which experienced more extension [blaich et al., 2008; aslanian et al., 2009; brune et al., 2016]. 3 cpd unveil ne brazilian margin framework figure 1. a) location of the study area in northeastern brazil and eastern portion of borborema tectonic province. b) studied area formed by three marginal basins: pernambuco, paraíba and natal platform, and the adjacent continental basement. white numbers in black circles refer to boundaries between these basins. purple, light-green, orange, blue, and dark-green areas represent the coastal zones of the marginal basins. the red line indicates the position of the continent-ocean boundary [magalhães et al. 2014a, 2014b]. black dotted lines indicate major fracture zones. dark blue lines (l1 and l2) represent the deep seismic sections that were used for the integration of geophysical transects. b) topography and bathymetry of the studied area. the yellow line represent coastline and states boundaries. white lines indicate the isobaths. pesz = pernambuco shear zone, pasz = patos shear zone. jefferson tavares cruz oliveira et al. 4 the pernambuco basin (peb in figure 1b) linked to the bp south domain (sd) is separated from the alagoas basin to the south by the maragogi-barreiros high and from the paraíba basin (pab in figure 1b) to the north by the pesz (figure 1b). this basin exhibits a narrow coastal zone that was implanted over a narrow graben, which formed an interior basin, in the continental platform [maia, 2012a; barbosa et al., 2014; buarque et al., 2016]. the platform break is marked by an outer hinge, the maracatu high, which runs parallel to the coastline (figure 2). the distal offshore region of peb is mainly formed by the pernambuco plateau (pep) [fainstein and milliman, 1979; gomes, 2000; oliveira, 2013], which developed over hyper-extended continental crust [magalhães et al., 2014b] and extends to isobaths of approximately 4000 m (figures 1 and 2). the pep comprises at least four main basement lows and a central outer high, the gaibu high (figure 2). another important outer high, the itamaracá high, nwse trending, is located on the northeastern border of the plateau (figure 2). figure 2. a) structural map of marginal basins in the eastern sector of brazilian margin and onshore precambrian blocks of the borborema province (nd north domain, tz transversal zone, sd south domain). the thin blue line represents the coastline; the thick red line represents the continent-ocean boundary [after magalhães et al., 2013]; red lines a-a´ and b-b´ represent 2d seismic sections. b) integrated geological section based on seismic sections a-a’ and b-b’, showing the architecture of the pernambuco basin [modified from buarque et al., 2016, 2017]. depth is in time (seconds). the main sets of structures that controlled the peb are mostly ne–swand e–w-trending ductile precambrian shear zones, which have been reactivated as strike-slip and normal faults during the rifting process. secondarily, there are oblique transfer faults trending nw–se [polônia, 1997; lima filho, 1998]. additionally, normal faults trending n–s, wnw–ese, and nnw–sse, which formed during basin opening (figures 1 and 2). the interpretation of seismic and potential field data has provided evidence of a significant sedimentary column in the plateau regions, which possibly reaches up to 6 to 7 km in thickness, over the main depocenters of the pep [oliveira, 2013; magalhães et al., 2014a] (figure 2b). the coastal zone of the pernambuco basin records a magmatic event that occurred during the upper albian and generated a series of volcanic rocks of the ipojuca magmatic suite [sial et al., 1988; lima filho, 1998; nascimento, 2003] (figure 2). the age of the main magmatic pulse is 102-103 my (long et al., 1986 by rb-sr isochrons, and nascimento, 2003 by ar39/ar40 zircon fission tracks). buarque et al. [2016] proposed that this magmatic event continued in the pep regions, forming extrusive and intrusive structures during the post-rift period (figure 2b). the paraíba basin (pab in figures 1 and 2) represents the second basin in the studied area and is linked to the tz block of the bp. it is bounded by the two major shear zones pesz to the south and the pasz to the north (figures 1 and 2). the pab continental platform is narrow, with an abrupt shelf break. the maximum thickness of the sedimentary cover is approximately 0.4 km onshore, reaching a maximum of 1.5–2.0 km over the continental platform (barbosa and lima filho, 2006; lima filho and barbosa, 2010). the evolution of the pab was controlled by precambrian shear zones trending ne–sw, ene–wsw, and e–w, which were reactivated as normal and oblique faults (figs. 1 and 2). secondarily, nw-se-oriented transfer faults, and n–sand nnw–sse-trending normal faults formed during the basin opening (barbosa and lima filho, 2006; bezerra et al., 2014). the natal platform basin (nap in figure 1) is the northernmost marginal basin within the studied area and represents the narrowest continental margin sector (figs. 1 and 2), with an even more abrupt shelf break (lima filho and barbosa, 2010; magalhães et al., 2014b). this basin is bounded to the south by the pasz and to the north by the touros high. this basin is related to the north domain (nd) of bp (figure 1b). its onshore sedimentary cover reaches a maximum thickness of 0.25 to 0.4 km, and offshore up to 2.0 km [barbosa et al., 2007]. the evolution of nap was controlled by precambrian shear zones trending ene–wsw and normal faults formed during the rift phase trending to wnw–ese, nnw–sse and n–s (figures 1 and 2). 3. materials and methods the approach integrated the estimation of isostatic moho accomplished through the processing of data obtained with satellite gravimetry missions and the estimated curie depths map through the processing of airborne acquired magnetic data. this approach allowed us to observe the behavior of these two boundaries from the adjacent basement of the margin to the continent-ocean limits. topographic and bathymetric data composed regional topography of the study area. the deep seismic profiles helped to constrain the behavior of the seismic basement and moho, and the location and general characteristics of the crustal transition from the preserved continent zone to the oceanic domains. 3.1 satellite gravimetric dataset we have used regional free-air and bouguer gravity anomaly grids obtained from the earth gravitational model (egm 2008) through the national geospatial-intelligence agency (nga) [pavilis et al., 2012]. egm2008 is a spherical harmonic model of the earth’s gravitational potential, developed by a least-squares combination of the itggrace03s gravitational model and its associated error covariance matrix [pavlis et al., 2012]. this product represents a 1´x 1´equiangular global bouguer anomaly grid which was computed by spherical harmonic analysis of global topography-bathymetry data (etopo 1), up to the degree and order of 10,800, and it has considered the boundaries and densities of largest lakes and interior seas, polar caps, and land areas below sea level [balmino et al., 2011]. the topographic correction was computed with a distance of 167 km, by using the 1´x 1´ from the etopo1 elevation model. the value for correction density used for the bouguer anomaly calculation was 2,670 kg/m3 [fullea et al., 2008; balmino et al. 2011]. the processing of these datasets provided the free-air, bouguer, and regional topography maps used in this research (figures 1c, 3a, and 3b). 5 cpd unveil ne brazilian margin framework 3.2 isostatic moho the isostatic mohorovičić (moho) discontinuity depth map (figure 3c) is elaborated by isostatic modeling of the topographic load on the lithospheric plate using the airy-heiskanen isostatic compensation model [heiskanen and moritz, 1967]. the calculation requires the estimation of three parameters, a depth compensation for sea-level elevation ds, a density contrast along the bottom of the root load δ𝜌, and the density of the topographic load 𝜌𝑡 [simpson et al., 1986]: (1) where elevation e represents the elevation of the topographic surface. in oceanic crustal domains, the water column represents a negative load at the top due to the presence of water density: (2) thus, 𝑑𝑤 represents the bathymetry, and 𝑝𝑤 represents the density of seawater. the crustal thickness (ds) of 30 km we used for the zone between onshore and offshore regions was based on an estimation made by previous works using seismic and potential field methods data, which showed that the average thickness in the narrow coastal zones of studied margins is about 30-32 km [lima et al., 2015; oliveira and medeiros, 2012]. the other parameters were based on moho depths estimated by blaich et al. [2008, 2011], and magalhães et 𝜌𝑡 δ𝜌 𝑑 = 𝑑𝑠 + 𝑒 � � 𝑑 = 𝑑𝑠 − 𝑑𝑤 � �𝜌𝑡‒𝜌𝑤 δ𝜌 jefferson tavares cruz oliveira et al. 6 figure 3. free-air (a) and bouguer (b) gravity anomaly and isostatic moho depth (c) maps elaborated on the ne brazilian marginal basins derived from satellite data. white line: coastline; light blue lines: bathymetry; thin black lines: main faults and shear zones in continental domains. white lines (l1 and l2) represent 2d deep seismic sections used to build integrated geophysical transects (figures 9 and 10). domains of the bp: sd: south domain; tz: transversal zone; nd: north domain. al. [2014b], for marginal basins of ne brazil: density of water, 𝜌𝑤 = 1.03 g/cm3, the density of uniform crust (basement rocks and sediment cover), 𝜌𝑡 = 2.67 g/cm3, and the contrast at the base of the interface crust-mantle, δ𝜌 = 0.45 g/cm3. we assume a density for lithospheric mantle = 3.3 g/cm3 estimated isostatic moho depths in the continental region adjacent to marginal basins present values ranging from 29 to 35 km (figure 3c), which is consistent with values of isostatic moho depth found by lima et al. [2015] based on seismic refraction data analysis, and oliveira and medeiros [2012], based on gravimetric data. 3.3 airborne magnetic dataset the dataset used in this study (figure 4) represents an integration of four airborne magnetic surveys obtained from the exploration and production data repository (bdep) of the brazilian national agency of petroleum, natural gas and biofuels (anp), and from the brazilian geological survey (cprm). each dataset was processed separately before integration. the plataforma continental nordeste survey (s1) was concluded in 1970 with ne–sw-oriented flight lines, a spacing of 5 km, and flight height of 700 m. the lasa company acquired the platô de pernambuco survey (s2) 1988 with nw-se (n30° w)-oriented flight lines, a spacing of 3 km, and a flight height of 500 m. the maragogi–canavieiras survey (s3) was concluded in 1986 with nw-se (n30° w)-oriented flight lines, a spacing of 3 km, and flight height of 500 m. the projeto aerogeofísico da borda leste do planalto da borborema survey (s4), which covers the bp basement adjacent to the eastern margin, was carried out in 2007, with n-s-oriented flight lines spaced 500 m, e-w-oriented control lines spaced 10 km, and a flight height of 100 m. furthermore, the international geomagnetic reference field (igrf) was subtracted from each dataset separately considering their period of acquisition (1970 – s1, 1985 – s2 and s3, 2005 – s4) using the oais-montaj software. the four surveys were interpolated using the bidirectional gridding method with a 750 m cell size. the data processing included spike removal, microleveling, and upward continuation. the airborne magnetic data were leveled using directional cosine and butterworth filters to attenuate leveling artifacts [johnson et al., 1999]. we have integrated the four grids by considering an altitude of 700 m, which was achieved by using the upward continuation technique in surveys s2, s3, and s4. thus, by considering the overlapping of the four survey areas we chose the suture method to merge the grids [johnson et al., 1999]. the merged total magnetic intensity (tmi) map covers the northeastern most brazilian continental platform region to the 2,000 to 3,000 m isobaths and most of the pep domains (figures 4a and 4b) to the isobaths at 4,000 to 5,000 m. this map also covers 80 to 100 km of the adjacent onshore basement formed by the bp, which allowed us to analyze the transition between the preserved and the extended segments of the continental crust. 3.4 curie point depth (cpd) estimation the estimation of cpd can be based on an analysis of airborne magnetic data as proposed by bhattacharyya and leu [1965, 1975a, 1975b, 1977], spector and grant [1970], and okubo et al. [1985], and tanaka et al. [1999]. according to this method, the magnetic anomalies are divided into rectangular regions, for which the depths of the top (zt) and centroid (z0) of the magnetic sources can be defined by using the logarithm of the power spectrum. the depth to the bottom (zb) can be deduced from zt and z0 [okubo et al., 1985]. the analysis is based on three main steps: 1 the division of data in overlapping squared windows; 2 the calculation of the radially averaged logarithm of the power spectrum for each window; and 3 the estimation of the cpd from the calculation of the centroid and top depths of the magnetic source for each subregion. the upper limit of a magnetic source is assumed to feature a vertical dimension that is far smaller than the magnetic source’s horizontal dimension, and the magnetization m(x,y) is a random function of x and y [spector and grant, 1970; bhattacharyya and leu 1975b, 1977]. furthermore, according to the model that was proposed by blakely [1995], the power density spectra of the observed magnetic anomaly 𝜙�� is given by: 𝜙��(𝑘�,𝑘�) = 𝜙�(𝑘�,𝑘�) · 𝐹(𝑘�,𝑘�) (3) 7 cpd unveil ne brazilian margin framework 𝐹�𝑘�,𝑘�� = 4𝜋² 𝐶²�|θ�|² |θ�|² 𝑒⁻²|�|���1‒ 𝑒⁻|�|⁽��⁻��⁾�² (4) where 𝜙� is the power density spectrum of the magnetization; cm is a proportionality constant; and θm and θf represent the magnetization direction and geomagnetic field direction, respectively. we can simplify this equation by considering that all the terms except |θm|2 and θf |2 are radially symmetric, that the radial average measures of θm and θf are constant, and that m(x,y) is constant for random and uncorrelated source distributions. therefore, we can postulate that φm (kx, ky) is constant. thus, the radial average of 𝛷�� is given by 𝜙�� (|𝑘|) = 𝐴𝑒⁻2|�|���1 ‒ 𝑒⁻|�|⁽��⁻��⁾²� (5) where a is a constant and k is the wavenumber. jefferson tavares cruz oliveira et al. 8 figure 4. a) airborne total magnetic intensity (tmi) map of the ne brazilian marginal basins created by the integration of 4 aeromagnetic surveys (s1, s2, s3, and s4). b) tmi map with locations of the cpds, and representation of square windows used in the test of spectral analysis (50, 100, 150, 200 km), numbered 1 to 4, respectively. the green polygon indicates the s1 survey area, blue polygon = s2 survey area, yellow polygon = s3 survey area, and brown polygon = s4 survey area. for very thick magnetic bodies, the second part of equation 4 is used to define only the top depth, so equation 5 can be reduced to ln[𝜙�� (|𝑘|)¹/²] = ln𝐵 ‒ |𝑘|𝑍� (6) where b represents a constant. thus, the upper limit of the studied magnetic source zt can be estimated by assuming that the signals from the source tops dominate the power spectrum. equation 5 can be further simplified to compute the centroid depth z0 of the magnetic source from the long wavelengths, or the low-frequency portion of the power spectrum: (7) thus, the slope of the average amplitude spectrum allows us to estimate the top and centroid of the magnetic body by tracing straight lines over the curve of the radially averaged power spectrum, dividing the area into two trends. the short wavelengths, or high frequencies, indicate the top depth of the body, and the long wavelengths, or low frequencies, mark the depth positions of the centroid [okubo et al., 1985; tanaka et al., 1999]. this information is obtained from equations 6 and 7; hence, the depth of the lower limit of the magnetic body, zb, which is assumed to be the depth at which the minerals lost their ferromagnetic properties, possibly the cpd [okubo et al., 1985; okubo and tsu, 1992], can be obtained from equation 8 (figure 5a): 𝑍� = 2𝑍0 ‒ 𝑍� (8) the power spectrum was computed by using a 2d fast fourier transform. we have determined the depth of the centroid, z0, and the depth of the top, zt, of the magnetic sources via two trends: 1 the slopes of the frequencyscaled power spectrum; and 2 the average amplitude spectrum for the deep sources from the first segment and the shallow sources from the second segment. we use the wavenumber range of 0.005 to 0.04 rad/km for the calculation of z0, and 0.04 to 0.10 rad/km for the zt. 3.4.1 window size an important debate about the cpd estimation involves the definition of the effective width of the sampling window, which is an essential part of the centroid method [bouligand et al., 2009; li et al., 2010; abraham et al., 2014a, 2014b; nwankwo and shehu, 2015]. some authors argue that large square windows are necessary to ensure that the response of the deepest magnetic layers is captured and that the use of windows with small widths cannot efficiently resolve deep sources. previous works have proposed that an optimal square window dimension is approximately 10 [chiozzi et al., 2005; ravat et al., 2007] or 10 to 15 [bouligand et al., 2009; chopping and kennet, 2015] times the expected bottom depth of the deepest magnetic layers. in general, the curie isotherm is expected to be above or near the moho [salem et al., 2014]. however, the position of the bottom of the magnetic layer can vary significantly according to the geotectonic context. in thinned lithospheric regions which experienced higher geothermal gradients and heat flow in the mantle lithosphere, it is expected that the curie isotherm is shallower than the moho [maule et al., 2005; jaupart and mareschal, 2007; thébault et al., 2010; salem et al., 2014; leseane et al., 2015]. in regions where the mantle is magnetized the curie isotherm can be below the moho [li et al., 2016]. according to magalhães et al. [2014b], and our calculation of the isostatic moho depth, the crustal thickness at the proximal region of the studied basins, near the coastline, is approximately 30 km. the crustal thickness is reduced along the margin and reached approximately 10-12 km in the continent-ocean boundary. the crustal thickness in the adjacent basement (bp) of the studied margin varies from 30 to 35 km, which is in accordance with the analysis of deep seismic refraction data [lima et al., 2015]. to ensure the optimal size of the window sampling we performed a test with a single point in the central region of the pep with four different square windows (50 x 50, 100 x 100, 150 x 150, 200 x 200 km2) (figure 4b). the test showed that the detection of the depth to the base of the magnetic signal stabilizes in windows greater ln � � = ln𝐶 ‒ 𝑘𝑍0𝜙�� (|𝑘|)½ |𝑘| 9 cpd unveil ne brazilian margin framework than 150 km (table 1), which suggests that it represents a window size capable of resolve the detection of the bottom of the magnetic layer. thus, we choose the square window of 150 km to perform the spectral analysis. we divided the magnetic map into 162 overlapping 150-km-wide square windows (figure 4b). the rate of overlap between the windows was 75%. after the distribution of the sampling area centers, windows with more than ~25% missing data, such as some windows near the boundaries of the map, were removed to avoid errors and artifacts. the chosen sampling window size was the same used to perform the analysis of the curie depths in the bp through the spectral analysis of airborne magnetic data [correa et al., 2016]. the estimated crustal thickness of bp varies from 25 to > 45 km [correa et al., 2016]. jefferson tavares cruz oliveira et al. 10 figure 5. a) curie point depth map of the studied region. b) map of the estimated error for the value of the base of the magnetic layer (z0). black lines in the adjacent continental zone represent major faults and shear zones. light blue lines represent the bathymetry. black lines (l1 and l2) represent 2d deep seismic sections used for integration with geophysical maps. domains of the bp: sd: south domain; tz: transversal zone; nd: north domain. blue lines represents isobaths. 11 cpd unveil ne brazilian margin framework 3.4.2 cpd uncertainty control we have performed a statistical evaluation of obtained z0 values, which is important to define the error generated by the range of wavenumber in the analyzed spectrum because the error is expected to be higher where the wavenumber is lower. the error estimation [okubo and matsunaga, 1994] is calculated based on the misfit between the radially calculated spectrum (kcalculated) and the straight line obtained through the numerical adjustment (ktheoretical) (eqs. 6 and 7). thus, the misfit (r) is calculated was calculated with equation 9: (9) where n is the number of points in the calculated spectrum. the map showing the error for the cpd estimation is shown in figure 5b. 3.4.3 geothermal gradient (gg) and heat flow (hf) after calculating the cpd, we proceed to obtain the geothermal gradient (gg), which can be estimated from the variation between a given cpd, where the local temperature is assumed to be ~580 °c, and the temperature at the earth’s surface by considering a vertical direction and the distance between them [eppelbaum et al., 2014]. in this case, we have not considered the effect of radiogenic heat. we have considered the value of 5° c for the surface temperature in the offshore region, according to beardsmore and cull [2001]. this estimation is obtained by considering the relationship between the depth of the seabed, and the latitude of a given marine region, with an error of ± 2° c. the same value was considered by li et al. [2017] for oceanic regions to build a global cpd model. we have considered 5° c also as the surface temperature of the continental region because the estimate of this parameter for large onshore sectors is very complex due to the many aspects that control this information (altitude, climate, and lithology). the gg distribution was then obtained through equation 10: (10) after estimating the gg values in °c/m, we estimated the heat flow (hf) values by applying equation 11: (11) where q represents the hf, given in mw/m2; l represents the thermal conductivity constant (w/m°c); and represents the gg (°c/m). due to the lack of information about the conductivity properties of the crust in the 𝑅 = � ��(𝐾���������� ‒ 𝐾����������)²1 � 𝜕𝑇 𝜕𝑍 δ𝑇 δ𝑍 = = 580° 𝐶 ‒ 5°𝐶 𝑍� 𝑞 = ‒ 𝜆 𝜕𝑇 𝜕𝑍 𝜕𝑇 𝜕𝑍 table 1. values of zt (top) and zb (bottom) of the magnetic layer according to the window size sampling showed in figure 4b, from 1 to 4, respectively. square window (km) top depth (zt; km) base depth (zb; km) 50 2.48 20.03 100 3.81 26.66 150 5.89 34.13 200 6.67 34.16 offshore studied regions, we have adopted the proposition of kappelmeyer and haenel [1974], who proposed a general estimation for the conductivity of the earth’s crust with a factor range between 2 to 2.5 w/m°c. thus, we assume an intermediate value for l of 2.25 w/m°c [eppelbaum et al., 2014]. jefferson tavares cruz oliveira et al. 12 figure 6. a) geothermal gradient map calculated after the estimation of the cpd in the study area. b) heat flow map estimated for the studied region. black lines in the adjacent continental zone represent major faults and shear zones. blue lines represent the bathymetry. domains of the bp: sd: south domain; tz: transversal zone; nd: north domain. blue lines represents isobaths. 13 cpd unveil ne brazilian margin framework after the calculation of gg and hf estimates, based on the 162 cpd points, the data were interpolated in a 30km-spaced grid to produce gg and hf maps (figure 6a and 6b). to evaluate the cpd derived gg and hf data, we have used heat flow data from 20 wells located in the onshore and offshore domains of the studied area available in the global heat flow database of the international heat flow commission (ihfc, 2018) for comparison and further discussion (figure 7). figure 7. heat flow (hf) map of the ne brazilian marginal basins with locations of the hf measurements in exploratory wells (ihfc, 2018). yellow dots mark boreholes, and the numbers indicate the heat flow measurements in mw/m2. 3.5 2d multichannel seismic dataset a set of 143 2d multichannel, time-migrated, seismic sections was studied to define some of the main tectonic characteristics of the region (figure 8a). the data was obtained from the bdep-anp. the seismic surveys consist of a variety of legacy seismic data with different qualities and imaging depths (8 s to 11 s), representing a total length of 10,360 km. interpretation of the surveys allowed the creation of a contour map of the seismic basement with the depth in time (twt), and we have created a second version with the depth in meters (figure 8b). due to the lack of offshore wells, a simple layer-to-layer velocity model was used for the depth conversion. we have considered estimations of the velocities for the three main sequences interpreted in seismic data (rift, cretaceous post-rift, cenozoic post-rift) (figure 2b). for the time-to-depth conversion, we have used velocities described for similar sequences in wells from the neighbor alagoas basin (figure 8). the estimation of basement depth and its topography, as well as the thickness of the sedimentary cover, was used to help to estimate the crustal properties in the studied region, especially for the pernambuco plateau region and paraíba basin (figs. 9 and 10). jefferson tavares cruz oliveira et al. 14 figure 8. a) distribution of 143 2d multichannel seismic sections (grey lines) used to interpret the basement contour and the sedimentary cover in the pernambuco and paraíba basins. black lines represent major continental shear zones and faults; light blue lines represent bathymetry, b) depth to basement map that shows the main structural domains of the pernambuco basin and the paraíba platform. the dotted black line marks the interpreted limit of the continent-ocean boundary [magalhães et al., 2013]. the dashed black line indicates the extension of pesz to the platform. blue lines represent isobaths at 200 m, 2000 m, 3000 m, and 4000. structures in the continent: thin black lines: lineaments. l1 and l2 represent regional deep seismic sections used to create the geophysical transects. 15 cpd unveil ne brazilian margin framework figure 9. geophysical transect l1. a) gravity and magnetic anomalies, b non-interpreted, and interpreted seismic sections located in the offshore distal region of the pab, c) isostatic moho, and cpd surface. moho 1 = isostatic moho estimated in this research, moho 2 = crust-mantle boundary extracted from the global earth crust thickness project crust 1.0, curie surface 1 = cpd estimated by the present research, curie surface 2 = cpd extracted from the cpd global model of li et al. [2017]. jefferson tavares cruz oliveira et al. 16 figure 10. geophysical transect l2. a) gravity and magnetic anomalies, b) non-interpreted and interpreted seismic sections located in the offshore distal region of pernambuco basin, c) isostatic moho, and cpd surface. moho 1 = isostatic moho estimated in this research, moho 2 = crust-mantle boundary extracted from the global earth´s crust thickness project crust 1.0, curie surface 1 = cpd estimated by the present research, curie surface 2 = cpd extracted from the cpd global model of li et al. (2017). 17 cpd unveil ne brazilian margin framework 3.6 integrated 2d geophysical transects two regional 2d multichannel, time-migrated, deep seismic sections (11 s twt) acquired by the brazilian navy leplac program were also integrated into the dataset to define the characteristics of the crustal transition in the north and south sectors of the studied margin. the seismic sections (l1 and l2 in figure 9b and 10b) are 170 and 220 km long, respectively. interpretation of these sections was integrated with gravity and magnetic anomalies to improve the characterization of crustal domains. we have compared the estimation projected by this research about the moho topography and the curie isotherm with data from global models calculated with satellite-based gravity and magnetic surveys, and seismic data. data from the global model crust 1.0, earth´s crust thickness, were collected and plotted in the l1 and l2 transects. crust 1.0 represents a crustal thickness model based on a grid of 1° cell size calculated from active source seismic studies and receiver function studies and constrained with gravity data (https://igppweb.ucsd.edu/~gabi/crust1.html). this model followed the same procedures applied to the creation of the earth´s crust model crust 2.0 [tenzer et al., 2009; reguzzoni et al., 2013]. we have also extracted data points from the global model of cpd provided by li et al. [2017]. this model represents a global 10´cell-size grid with the estimation of curie isotherm calculated with the fractal magnetization method that used the global magnetic field anomaly model emag2. this approach used three sampling windows with a size of 98.8 x 98.8 km2, 195 x 195 km2, and 296.4 x 296.4 km2. the final grid represents the average cpd obtained from the three windows for each sampled zone. we have plotted data of cpd from this model in the l1 and l2 transects (figures 9 and 10) for comparison with our results. 4. results 4.1 crustal thermal properties the cpd values in the studied margin vary from 21 to 38 km, and they are deeper in the continental region and shallower towards the oceanic region. the tz continental block shows the main concentration of deepest cpds, which varied from 35 to 29 km. the shallower cpd values are related to the two plateaus located to the north, touros, and to the south, pernambuco, limits of the studied region. the southern sector of the studied area, that represents the peb and its plateau, as well the southern limits between the peb and alagoas basins, exhibits cpd values varying from 32 to 26 km (figure 5a) in the continental region. in the offshore region of peb the cpd values vary from 28 to 23 km. the northern part of the pep exhibits values varying from 28 to 25 km (figure 5a). the southern part of the plateau presents cpd values ranging from 25 to 21 km, with shallower values in the southeast border. the cpd isotherm is roughly oriented ene-wsw and e-w (figure 5a). the central block of bp (tz in figs 5 and 6), bounded by the pasz and pesz, shows deeper cpd values, ranging from 38 to 29 km. cpd in the marginal region, which represents the domains of the pab, ranges from 39 to 28 km (figure 5a). offshore cpd isotherm trend ene-wsw and e-w, whereas they are oriented nw-se to nnw-sse in the continental domain. the north domain of bp (nd in figs. 5 and 6), which is linked to the nap basin, shows a more complex cpd relief, with values up to 39-km-deep in the zone domain (figure 5a). to the east of this block cpd values in the coastal zone ranges from 27 to 23 km. in the offshore zone of the north domain (nd in figs. 5 and 6), including the nap domains and its limits with the potiguar basin, across the touros plateau, exhibit cpd values ranging from 29 to 21 km. the main cpd isotherm in the offshore region is oriented ene-wsw, e-w, and nne-ssw (figure 5a). the gg and hf maps show that the continental region adjacent to the peb exhibits values of 18 to 22 °c/km and 38 to 50 mw/m2, respectively (figs. 6a and 6b). the offshore counterpart of the peb presents values ranging from 19 to 26 °c/km and 44 to 60 mw/m2 for the gg and hf, respectively. higher-gg and -hf zones occur in the southeastern border of the pep, with 23 to 28 °c/km and 47 to 63 mw/m2, respectively (figures 6a and 6b). the continental region adjacent to the pab, in the tz continental block, presents relatively homogeneous behavior, with values of 18 to 20 °c/km, and 37 to 46 mw/m2, for gg and hf, respectively. the offshore region of the pab presents values of gg ranging from 16 to 22 °c/km and values of hf ranging from 37 to 50 mw/m2 (figures 6a and 6b). the continental region adjacent to the nap exhibits values of gg ranging from 16 to 23 °c/km and hf values ranging from 32 to 54 mw/m2. the offshore region of nap presents values of gg varying from 20 to 28 °c/km and values of hf varying from 45 to 65 mw/m2 (figures 6a and 6b). to evaluate the effect of prediction of thermal properties based on the estimated cpd, a set of 20 heat flow measurements collected in boreholes (table 2) was used to make a comparison with values extracted for the same sites in the hf map (figure 7). the data encompass 13 wells in the onshore region of nap, touros high, and the eastern limits of potiguar basin (nd); 1 well in the distal southeast border of the pep; and 6 wells in the limits of the onshore alagoas basin (figure 7). no measurement points are available for the onshore pernambuco and paraíba basins, neither for the offshore domains of the pab and nap. the comparative analysis of values shows that the average difference between the calculated hf values is approximately 20% lower than values measured in wells (table 2). differences can arise from different variables that influenced local measurements (depth of acquisition, geological factors, sampling techniques), being the thermal modeling results for the hf satisfactory for the regionalscale study. 4.2 seismic basement interpretation the depth to basement map of the offshore basins is shown in figure 8b. the boundary between the pernambuco and alagoas basins is evidenced by the maragogi-barreiros high, an nw-se-trending regional hinge. this high is linked with the maracatu high, which forms the outer hinge of the pep, trending nne-ssw, and it is linked to the pesz in the north. to the north of the pesz, the continental platform of the pab and nap is dominated by a structural high trending n-s and nnw-sse. the expression of this structural high that formed the continental platform is evidenced in the free-air anomaly map (figure 3a) as a positive elongated anomaly extending from the southern limit of peb to the northern limits of the touros plateau. jefferson tavares cruz oliveira et al. 18 well no. lat s long e hf (mw/m²) hf calc (mw/m²) difference % 1 5°21´21´´ 36°07´58´´ 73 68 9 2 5°23´56´´ 36°01´34´´ 25 50 -100 3 5°26´02´´ 36°01´42´´ 63 52 17 4 5°09´14´´ 35°52´43´´ 38 51 -34 5 5°13´20´´ 35°48´58´´ 77 48 38 6 5°23´47´´ 35°51´57´´ 78 49 37 7 5°31´09´´ 35°49´49´´ 55 48 13 8 5°24´11´´ 35°29´42´´ 83 48 42 9 5°37´55´´ 35°35´20´´ 71 49 31 10 5°48´05´´ 35°12´55´´ 91 54 41 11 5°21´07´´ 35°21´18´´ 46 53 -15 12 5°53´00´´ 35°14´26´´ 57 53 7 13 6°00´09´´ 35°09´34´´ 91 51 44 14 9°33´11´´ 34°14´06´´ 69 50 28 15 9°37´08´´ 35°43´59´´ 51 49 4 16 9°39´49´´ 35°47´06´´ 65 49 25 17 9°52´57´´ 35°59´03´´ 53 49 8 18 9°49´11´´ 36°06´03´´ 70 52 26 19 9°47´04´´ 36°06´59´´ 65 52 20 20 9°78´33´´ 36°13´33´´ 59 52 12 average 64 51 20 table 2. comparison between heat flow (hf) measured in the boreholes (ihfc, 2018) and values extracted from the hf map calculated from cpd modeling. 19 cpd unveil ne brazilian margin framework the pep framework is composed of deep grabens (tamandaré, guadalupe, and itapuama), and outer highs formed over the continental thinned crust (figure 7b). two main outer highs dominate the central region of the plateau: the nw-se-trending itamaracá high, approximately 70 km long, which forms the plateau northern border, and the gaibu high, a semicircular structure in the central region. the xaréu outer high, named in this work, trending to nne-ssw, forms the boundary between the central region of the plateau and its most distal border. the xaréu outer high also forms the first escarpment of the plateau (see seismic deep section crossing the plateau in figure 10). the itamaracá high is separated from the platform of the peb by a structural strait named here as the artax strait (figure 8b). the seismic basement map allows us to define that the transect l2 (figure 10) is positioned over a series of grabens, which top basement depth varies from 7 to 8 km below the sea level, and it also allowed to confirm that the first escarpment of the plateau is formed by a structural high that marks the limit of the transitional crust. this information was integrated in the transect l2 (figure 10). the basement map also showed that the distal regions of these basins form a deep domain in contrast to the external hinge of the continental platform, which contributed to the changes in gravimetric anomalies observed along the crustal transition in the transects l1 and l2 (figures 9 and 10). 4.3 integrated 2d deep seismic sections and geophysical transects we used information provided by interpretation of deep 2d seismic sections and data extracted from the calculated isostatic and thermal maps (figures 3, 5, and 6) two integrate two geophysical transects (figures 9 and 10). the transect l1 represents a 220-km-long dip section located in the distal region of the pab (figures 1, 3, and 8). in this area, the continental margin is very narrow, with a necking zone of approximately 40 to 60 km wide. based on the interpretation of seismic data we proposed that a sector of hyper-thinned crust possibly formed an oct zone of approximately 30 to 40 km wide (figure 9). the shelf break is abrupt, with no proper slope, which caused the sedimentary deposits of the platform to fall over a very steep gradient to the distal basin (figure 9). from the shelf break to the oceanic crust, the margin is approximately 80 to 100 km wide. the value of free-air anomaly changes abruptly after the shelf break from -40 mgal to -66 mgal in about 28 km, and then it increases to -24 mgal in the next 40 km across the beginning of the oceanic crust. after the beginning of the ocean crust, the free-air anomaly shows an increase to 8 mgal, which is related to a seamount (figure 9). the free-air anomaly shows relative flat behavior related to the oceanic crust with values about -23 mgal (figure 9). the bouguer anomaly exhibit a constant increase in values from the shelf break, 50 mgal, to the next 100 km oceanward, to 265 mgal across the beginning of the oceanic crust. in the ocean domain, the bouguer anomaly exhibits a gentle increase from 270 to 320 mgal to the end of the transect (figure 9). the magnetic anomaly varies from -22 nt in the shelf break to -35 nt in the next 10 km oceanward, then it changes abruptly to 14 nt in the next 12 km, and then it varies abruptly again to -4 nt in the next 8 km. the magnetic profile is limited to about 58 km due to the covering of the aeromagnetic survey in this region. the integration of data allowed us to corroborate the beginning of the oceanic crust at approximately 80 to 100 km from the shelf break (figure 9). along the necking zone, the isostatic moho depth varies from ~27 km ascending eastward to 20 km below the interpreted oct zone and approximately 15 km below the oceanic crust (figure 9). the cdp surface is approximately 2 km below the isostatic moho in the shelf break (figure 9), and it ascends gently to 26 km below the oct zone (figure 9). the cdp surface is approximately 8 km below the isostatic moho along the interpreted oct zone (figure 9). the thickness of the continental crust in the narrow oct zone is estimated at 10-12 km, and the thickness of the oceanic crust beyond the oct zone is approximately 10 km (figure 9). the transect l2 represents a deep seismic section that crossed the distal region of the peb (figure 1) and extends from the shelf break region across the southern part of the pep to the oceanic crustal domain, approximately 170 km long (figure 10). the interpretation of seismic data shows that the continental margin in the pep sector is wider than observed in the pab (figure 9). the integration of data suggests that the necking zone is 30 to 40 km wide, and the hyper-extended crust is approximately 80 to 90 km wide (figure 10). a narrow region forms an outer high between the first inner escarpment and the escarpment that marks the border of the plateau and the beginning of the oceanic crust. we have interpreted this sector as an oct zone, possibly a continuation of the oct zone identified at the pab, which is approximately 40 to 50 km wide in the pep. the free-air anomaly show values which vary abruptly from 36 mgal in the shelf break to -60 mgal in the 20 km oceanward. that behavior is associated with the necking zone. the free-air value then ascends to about -22 to -25 mgal in the next 60 km oceanward (figure 10). this point marks the beginning of the sector interpreted as an oct zone. in the oct zone, the free-air curve varies from -30 mgal to -50 mgal and again to -30 mgal in the next 60 km oceanward, forming a concave behavior. in the ocean domain, after the oct zone, the free-air shows values ranging from -30 mgal to -12 mgal (figure 10). the bouguer anomaly curve shows values from 100 to 110 mgal in the shelf break, which ascends gently along the necking zone, 35 km oceanward, to values about 125 mgal. after the necking zone, the bouguer anomaly ascends constantly and gently in the next 90 km oceanward to values about 280 mgal, to the point that marks the beginning of the ocean domain. bouguer anomaly values in the oceanic crust show a very gentle increase oceanward from 280 mgal to 310 mgal at the end of the transect (figure 10). the magnetic anomaly curve varies from about -100 nt in the shelf break region to -75 nt in the next 20 km oceanward, which is related to the necking zone (figure 10). from this point, the curves ascend from -75 nt to about -2 nt in the next 20 km. the magnetic anomaly curve exhibit a sine shape with values varying from about -1 nt to 100 nt, and descending again to about -17 nt (figure 10). this part of the magnetic anomaly curve is 40 km wide and corresponds to the center of the pep. the next part of the curve presents a gentle variation with values ranging from -15 nt to 35 nt in the next 20 km oceanward, which ends in the first escarpment of the plateau (figure 10). along the next 35 km, the magnetic anomaly curve shows a sine shape with values varying from -15 nt to 70 nt. this part of the curve is related to the interpreted oct zone, and at the beginning of the ocean crust, the magnetic anomaly varies from -20 nt to -94 nt (figure 10). considering the estimated basement and the isostatic moho depths, we infer that the crustal thickness in the central region of the pernambuco plateau is approximately 12 km, and in the oct zone, it is about 9 to 10 km (figure 10). interpretation o seismic and potential fields data allowed us to propose that the beginning of the oceanic crust domain is approximately 140 to 150 km from the shelf break (figure 10). in this transect, the isostatic moho is approximately 28 km deep after the shelf break at the beginning of the necking zone, and it ascends to approximately 22 km beneath the hyper-extended crust (figure 10). the isostatic moho ascends eastward to 18 km in the oct zone and is approximately 16 km beneath the oceanic crust (figure 10). the cpd isotherm is approximately 3 km above the isostatic moho after the shelf break, in the necking zone, and it passes to below the isostatic moho at the end of the necking zone. the cpd isotherm ascends gently below the moho to approximately 22 km in the hyper-extended crust zone and 20 km in the oct zone (figure 10). the cpd isotherm is approximately 3 to 4 km below the isostatic moho along the hyper-extended crust sector, and 4 to 5 km below the isostatic moho along the oct zone. the model also shows that the cpd isotherm is approximately 5 km below the oceanic crust eastward (figure 10). comparison of depths of moho boundary calculated by this research (isostatic moho) and moho depths estimated by the crust 1.0 project (earth’s crust thickness) along the geophysical transects showed that both curves present similar behavior in the paraíba basin region (transect l1) (figure 9). the isostatic moho is below the global moho in the shelf break region, about 4 km, and at the beginning of the oceanic crust, about 5 km, and they present similar depths in the narrow margin and the oceanic crust (figure 9). in the pernambuco basin (transect l2) (figure 10), the global moho and the isostatic moho present similar behavior, but the first one is positioned above the isostatic moho, and the difference between them varies from 3 to 6km (figure 10). the differences observed possibly are relative to the scale of the grids, the smoothing applied in global models, and differences in parameters adopted. the differences are within the predicted uncertainties estimated for the crust models [tenzer et al., 2009]. comparison of cpd calculated by the present research and data provided by the global cpd model calculated by li et al. (2017) showed that in the paraíba basin (transect l1) (figure 9), the global cpd is near our cpd isotherm in the shelf break region, but it is below our cpd curve in the narrow hyper-thinned margin. the difference between the two curves in this region varies from 7 to 7,5 km (figure 9). in the pernambuco basin region (transect l2) (figure 10), the curve extracted from the global cpd model (li et al., 2017) is below our cpd curve, and they converge for a similar depth in the oceanic crust region (figure 10). the difference between the two cpd curves in the hyper-extended margin region varies from 7 to 7,5 km (figure 10). differences observed between the two cpd models can be attributed to the different methodologies adopted, and to the scales of the grids created. local complexities can influence cpd estimation, and we interpret that general behavior is correlated. jefferson tavares cruz oliveira et al. 20 21 cpd unveil ne brazilian margin framework 5. discussion analysis of the seismic data (figures 9 and 10), together with gravity maps (figure 3), reveals that the northern part of the studied ne brazilian marginal region is formed by a very narrow continental platform, with an abrupt shelf break and a narrow hyper-thinned continental crust, which encompasses the pab and nap basins (figures 3, 5, and 8). otherwise, the southern sector presents a narrow continental platform and a wider hyper-extended continental crust section that formed the peb and its plateau (figures 8 and 10). the continental platform of these three basins is linked and constitutes a regional basement high that dominates the margin (figures 3a and 8). the framework of pep exhibits deep grabens and outer highs that formed a complex morphology, including a ~50 km wide transitional crust zone sector (figure 9). the major grabens within the pep reach 8 to 9 km below the sea level. the origin and crustal nature of the large itamaracá and gaibu outer highs (figures 8 and 10) are still unknown, but they are linked to magmatic activities [buarque et al., 2016] and crustal heterogeneities within the hyper-extended crust [magalhães et al., 2014]. our interpretation regarding the occurrence of a short marginal extent, characterized by a hyper-thinned crust and the lack of structures like seaward-dipping reflectors corroborate previous studies that have suggested that the central segment of the south atlantic ocean shows affinity to the magma-poor model [reston et al., 2009; haupert et al., 2016]. comparison of cpd values obtained in this study for the adjacent bp basemen of the marginal basins and curie isotherm estimated by correa et al. [2016] for the bp by performing spectral analysis of airborne magnetic data show good correlation. they found that calculated cdp values in the bp continental shield vary from 18 to 59 km. they also produced a heat flow map for the bp, whose values range between 25 and 259 mw/m2, with an average of 67 mw/m2. according to these authors, however, higher hf values were likely caused by shallow hydrothermal fluids. a few investigations have been published on crust thermal properties of the south american platform, focusing on southeastern continental margins [gomes and hamza, 2005; cardozo and hamza, 2014] and cratonic regions [blum, 1996; hurter and pollack, 1996; guimarães et al., 2013; correa et al., 2016]. a study of the thermal gradient in the onshore region of the potiguar basin, based on 164 stratigraphic wells, showed geothermal gradients ranging from 24 to 74 °c/km, with a mean value of 37° c/km [souto filho, 1984]. according to the latter study, the values calculated for the eastern border of the potiguar basin vary from 26 to 46 °c/km, which is consistent with our results in the northern portion of the nd (continental region between the touros high and potiguar basin) (figures 6b and 6c). hamza et al. [2018] elaborated the gg and hf maps for the passive brazilian margin and its adjacent basement using bottom-hole temperature (bht) data and sea-bottom temperature measurements, and comparison of their results show that the gg and hf distributions and intensities are quite similar to the values found in the present study for the eastern margin of the bp (figure 6). the most important finding of the cpd estimation in this study is the strong control of the bp sub-domains (sd, tz, and nd) on the thermal characteristics of the continental platform and marginal basins. analysis of the thermal maps (cpd, gg, and hf) reveals that the basement inheritance controlled the evolution of the marginal basins in the studied area. the regional distribution of the cpd values of the three blocks that form the bp show correlation with the cpd values in the three marginal basins. the cpd isotherms in the sd are bounded by the pesz to the north, and they show intermediate-depth values, between 32 and 19 km, with a shallower zone in the southeast border of the pernambuco plateau (figure 5a). an nne-ssw-trending elliptical cpd low, approximately 50 km long, with depth values varying from 25 to 21 km, is located southwest of recife in the coastal zone. this shallow cpd zone is spatially associated with the albian magmatic pulse in the coastal zone of the peb. the tz exhibits deeper cpd values than the nd and sd, ranging from 39 to 28 km in the adjacent continental region, continental platform, and distal basin (figure 5a). it is remarkable how the ene-wsw-trending cpd distribution is bounded by the pasz within the tz to the north. the nd presents a more complex distribution of cpd anomalies, with deeper values in the adjacent continental region, ranging from 39 to 27 km, and shallower values, ranging from 27 to 21 km, in the basin border of the continental platform and towards the marginal basin over the touros plateau (figure 5a). therefore, the regional analysis of gg and hf maps show that the peb and related sd present more intense geothermal flux compared to the pab and the related tz (figures 6a and 6b). the nd presents less intense thermal flux in the adjacent continental region but higher values in the marginal basin, including the nap and the touros plateau (figure 6c). this warmer condition of the offshore region of the nd could be linked to the influence of a thermal mantle anomaly located beneath the eastern equatorial brazilian margin that generated a series of alkaline magmatic pulses (volcanic plugs and flows) during the cenozoic [knesel et al., 2010]. this region was also affected by tholeiitic magmatism during the rifting process, possibly associated with the santa helena hotspot (130 to 120 myr) [sial et al., 1981; archanjo and araújo, 2002]. estimated cpd isotherm showed that the southern block of bp seems to represent a warmer and younger crustal block to the tz and nd, which impacted the evolution during the margin formation. the lithological composition certainly plays an important hole in the thermal properties of the crust. the bp basement mainly comprises paleoproterozoic orthogneisses dating from 2.2 to 2.0 ga, with exposures of archean and early paleoproterozoic crust occurring in the nd and tz [neves et al., 2012]. the nd consists of metasedimentary and metavolcanic rocks dominated by schists and metarhyolites. the tz domain is composed by a series of terrains, which contain greenschist metapelites, metasedimentary and bimodal metavolcanics. the sd is divided in two sectors: northern, which contains high-grade, migmatitic orthogneisses and metasedimentary rocks (over which the peb was formed), and southern, with metasedimentary units. unknown deep features of the continental blocks could influence their thermal properties. further analyses are necessary to address the influence of the thickness and physical nature of each bp sub-domain on their thermal properties. the study of lima et al. [2015], based on a 900 km seismic refraction/wide-angle reflection profile across the borborema shield, showed that the bp present a smooth moho topography and that the velocities in the lower and upper crust of the tz are slightly lower than what was found for the sd and the nd. this work proposed that there is no deep root, or crustal underplating, associated with the higher topography of tz, whose underlying crust is thicker than the crust beneath sd. this research also showed that the moho depth is around 31.5 in the nd, 32 to 33 km in the tz, and it is around 35 km in the sd. there is a good correlation between the information on the relative thickness of the bp domains and the thermal characteristics we found for each domain in our cpd investigation: the intermediate-thickness sd (intermediate cpd, gg, and hf), the thicker tz (deeper cpd), and the thinner nd (shallower cpd in the marginal basins and adjacent basement). analysis of the relationship between the calculated isostatic moho depths and the cpd isotherm surface (figures 9, 10 and 11) shows that the cpd is overall above the isostatic moho in the onshore and proximal continental margin, except for a small area in the tz. in the onshore region, the vertical difference between the curie isotherm and moho depths vary from 2 to 5.5 km in the sd, from 2 to 11 km in the tz, and 5.5 to 9 km in the nd (figures 9, 10 and 11). however, in two regions of the continental basement, the cpd isotherm is deeper than the isostatic moho: the northern portion of the tz, near the coastal zone, and in the southern portion of the nd (figure 11). the deepening of the cdp in the continental region forms a ene-wsw-trending valley, aligned with the pasz (figure 11). the distance between the coast line and the point where the cpd isotherm passes below the isostatic moho surface varies from 72 km in the southern area of the peb to approximately 118 km in the north-central region of the peb. in these areas the coverage of both data are similar which support the interpretation of their relative position (figure 11c). our estimated depths of the moho boundary show similar behavior compared to the global moho relief estimated by the crust 1.0 project (figures 9 and 10). our isostatic model showed deeper values for the moho, but they are within the variation considered for this global grid. the values we have estimated for the cpd show differences compared to depths extracted from the global cpd model created by li et al. [2017] over the l1 and l2 transects (figures 9 and 10). however, the global cpd was below both the moho relief of the crust 1.0 model and the isostatic model (figures 9 and 10). this comparison showed that despite differences observed in the estimated depths of both boundaries, the aspects provided by the study of their relationship are useful to the understanding of the margin evolution. the positioning of the cpd below the moho has important implications for the geotectonic history and the present framework of the studied margin. cold magnetic mantle (< 600 °c) can exist below archean and proterozoic shields, subduction zones, and old oceanic plates [ferré et al., 2014; ismaie et al., 2019]. in some geological settings, the cold and magnetic uppermost mantle can exhibit a thickness of tens of kilometers [ferré et al., 2013]. in this sense, the curie point will lie below the crust-mantle boundary [thébault et al., 2010; correa et al., 2016]. no direct correlation exists between the depth of the isostatic moho and the depth of the curie isotherm. the moho represents a seismic discontinuity influenced by many factors, such as overand underplating, the mineralogy, and the temperature. the depth of this boundary can change throughout time over cratonic regions and is transitional in noncratonic regions [o’reilly and griffin, 2013; wang et al., 2013; thybo and artemieva, 2013]. the curie isotherm depends on the mineralogical composition of the crust and upper mantle, the crustal thickness, the tectonic environment, and other physical parameters, such as pressure and temperature [wasilewsk and mayhew, 1992; tanaka et al., 1999; rajaram et al., 2009; thébault et al., 2010; ferré et al., 2014]. idárraga-garcía and vargas [2018] jefferson tavares cruz oliveira et al. 22 23 cpd unveil ne brazilian margin framework studied the behavior of cpd on the south american continent, based on satellite data, and verified that the curie surface occurs below the moho in the stable central and north sectors of the brazilian shield as well in some parts of the andean cordillera. one cause associated with this process is the serpentinization of the mantle beneath cratonic regions, subduction zones and oceanic plates [manea and manea, 2010; li et al, 2013, 2016; ravat et al., 2011; guillot et al., 2015; wang and li, 2015; andrés et al., 2017]. indeed, correa et al. [2016] found that the curie isotherm is deeper than the moho in some regions of the bp, and they attributed this phenomenon to serpentinization of mantle below the uplifted bp. the studied margin was subject to the lowest spreading rates during the central atlantic formation [brune et al., 2014; 2016]. as a tectonic result, an asymmetric conjugate margin developed, where the northeast brazilian margin corresponded to the narrow side of a hyper-extended plate. the hyper-extended model proposed for this segment of the brazilian margin suggested that the continental margin could present an extension of 60 to 80 km wide [brune et al., 2014], which is consistent with what we found for the pab and nap. however, the additional extension observed in the pep, more than 150 km, could be associated with the oblique rifting that affected this region. serpentinization is a process that can occur with the formation of hyper-extended magma-poor margins [lundin et al., 2011; sutra and manatschal, 2012; pérez-gussinyé, 2013; guillot et al., 2015; ros et al., 2017], with the development of serpentinized mantle bodies below the necking zone across the oct zone, due to the mantle exhumation, and consequent ocean water and lithospheric mantle interaction. the geophysical evidence gathered here corroborates the present hypothesis, as the region presents the expected characteristic of a hyper-extended margin, especially a narrow oct zone where the curie isotherm is below the isostatic moho. this oct zone is 20 to 30 km wide in the paraíba and natal platform regions (figure 9) and approximately 40 to 50 km wide in the pernambuco plateau region (figure 10). figure 11. 3d visualization of the relationship between the cpd isotherm and the isostatic moho surface in the studied region. thin black line: coastline for reference. a and b) 3d view of the surfaces (v.e. = 2.5). c) plan view of the maps showing the position where the curie isotherm passes under the isostatic moho boundary. further studies like seismic-derived velocity models are necessary to consolidate the proposed model and the nature of the oct zone. however, the estimation of cpd is useful to the study of marginal regions where geophysical data is scarce as is the case for the studied region. the inference that the cpd is below the moho in a scenario of hyper-thinned crust corroborates the hyper-extended model for this margin, and it also helps to suggests that this margin possibly was affected by serpentinization associated with the formation of a narrow oct zone [li et al., 2010; salem et al., 2014]. 6. conclusions cpd estimation together with the estimation of isostatic moho depths and interpretation of deep seismic sections provided an important overview of the lithospheric characteristics of the eastern continental margin of northeast brazil. the region comprises a narrow continental margin formed by a hyper-extended crust that is 150 km wide in the peb, 80 km wide in the pab, and approximately 60 km wide in the nap. the pep, the larger sector of the studied area, presents deep grabens with basement depths varying from 8 to 9 km below the sea level. analysis of deep seismic sections allowed us to suggest that the margin presents an ocean-continent transitional crust (oct) zone that is approximately 40-50 km wide in the peb basin and 30 to 40 km wide in the pab and nap. the oct zone is characterized by a thinner crust sector, which was possibly affected by serpentinization. the oct was also interpreted based on the analysis of gravimetric data. analysis of cpd, as well the gg and hf maps, shows that the thermal properties of marginal basins were strongly controlled by the nature and structure of continental blocks that formed the bp, which also controlled the tectonic evolution of each basin. the peb and its adjacent basement exhibit intermediate to shallow cpd values and higher values of gg and hf, which are bounded by the pesz. the pab and its adjacent basement show deeper cpd values, as well as lower gg and hf values. the regional trend of the pab is bounded by the pasz. the nap is characterized by shallower cpd values in the adjacent border and continental platform of this basin. the continental basement of the nap presents deeper cpd values, as well as lower gg and hf values. these differences possibly are related to the crustal thickness and lithological nature of each continental block. analysis of the relationship between the cpd isotherm and isostatic moho suggests that the cpd can be below or near the moho boundary in the marginal region. this aspect could be caused by the serpentinization process that occurs in hyper-extended continental margins with the consequent magnetization of the lithospheric mantle. however, further studies are necessary to prove this interpretation. acknowledgements we wish to thank the sinochem petróleo brasil ltda., which funded this research through the cooperation agreement fade/ufpe/sinochem “seasound” project. we 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and lithospheric geothermal state of western north america and their implications for a magnetic mantle, tectonophysics, 638, 112–125. *co r r e s po n d i n g a u t h o r: jefferson tava r e s c r u z o l i v e i ra, federal university of pernambuco, geoquantt research in geosciences, department of geology, avenida acadêmico hélio ramos, s/n, cidade universitária, recife pe, brazil, 50740.530, e-mail: jefferson.tavares@ufpe.br; jf.ufpe@gmail.com © 2021 the author(s). all rights reserved. open access. this article is licensed under a creative commons attribution 3.0 international jefferson tavares cruz oliveira et al. 30 mailto:jefferson.tavares@ufpe.br c o n v e g n i e c o n g r e s s i c o m m i s s i o n e s i s m o l o g i c a e u r o p e a riunione di utrecht (8-12 aprile 1958) dall'8 al 12 aprile si sono svolti a u t r e c h t (olanda) i lavori della 4a assemblea generale della « commission séismologique européenne ». vi presero p a r t e u n a sessantina di delegati dei diversi paesi europei, oltre ad un certo numero di invitati. le sedute si tennero nell'aula magna del senato accademico della università di utrecht, sotto la presidenza del prof. pietro caloi, dell'istituto nazionale di geotisica di r o m a . f r a i partecipanti ricorderemo il prof. sir harold j e f f r e y s , dell'università di cambridge, presidente dell'associazione internazionale di sismologia e fisica dell'interno della terra, il prof. j . p. r o t i i é di strasburgo, segretario dell'associazione stessa, il prof. r. s t o n e l e y , dell'università di cambridge, la prof. i. l e u m a n n di kebenhavn, il prof. av. killer, dell'università di stoccarda, il prof. g a s s m a n n di zurigo e il prof. v . b e l o u s s o v , del comitato di geodesia e geofisica dell'accademia delle scienze di mosca. f u r o n o presentate circa cinquanta comunicazioni, su argomenti vari di sismologia, con particolare riguardo alla costituzione della crosta terrestre, alla c a r t a sismo-tettonica d ' e u r o p a , agli apparecchi, alle onde canalizzate in europa, all'interpretazione dinamica delle osservazioni sismiche, ecc. da p a r t e italiana, nel suo indirizzo presidenziale il prof. caloi ha accennato al meccanismo dei terremoti all'ipocentro, alla interpretazione delle registrazioni provocate da grandi esplosioni, a scopo di prospezione profonda, al problema delle onde canalizzate in e u r o p a e all'uso dei sismografi a brevissimo periodo. dopo il saluto alle autorità olandesi presenti ed agli intervenuti, il prof. caloi h a così proseguito: « non posso non pensare che mi trovo in olanda: come italiano, come veneto, ciò richiama alla mente la stupenda fioritura pittorica che — quasi contemporaneamente — nel veneto e nei paesi bassi accomunò nel seicento t a n t i spiriti nella stessa commozione artistica. f u u n a fusione di europei, operata dall'incanto dell'arte, nella quale ci si riconosceva e ci si a m a v a . d e t t o questo, non posso non meditare sulla diversa f o r t u n a che accompagna l'opera artistica dalla produzione scientifica. l'arte, quando è tale, non conosce tramonti, nè frontiere, nè oblìi. omero, virgilio, dante hanno f a t t o dei loro poemi fonti di emozioni per t u t t e le genti civili. gli è che la poesia è il solo aspetto della realtà che sentiamo vero nel nostro cuore. « ben diverso il destino dell'opera scientifica, sempre imperfetta, sempre approssimativa, sempre incompleta: verità che sfugge senza posa nella sua ultima essenza. se l'incertezza può costituire uno sprone per gli spiriti avventurosi — e la scienza è la più affascinante delle a v v e n t u r e —, talvolta può condurre allo scoramento, se non a d d i r i t t u r a all'acquiescenza. di qui alla pedanteria il passo è breve. senza giungere a ciò, accade sovente di irretirsi in schematismi dogmatici, nei quali la moda e l'inerzia mentale hanno gran peso. p e r limitarci agli argomenti che sono all'ordine del giorno dell'attuale riunione, vediamo i). es. cosa si pensa oggi del meccanismo che è all'origine dei terremoti. p e r la quasi totalità dei cultori di sismologia, 150 convegni e congressi oggi è assiomatico che u n t e r r e m o t o non può essere provocato che d a f r a t t u r a (" f a u l t i n g " ) . e p e r provarlo, non si esita di ricorrere alle t e s t i m o n i a n z e più sospette, quali quelle d e r i v a n t i dalle onde riflesse longitudinali o alle onde trasversali. posso o b i e t t a r e semplicemente che non possono r i e n t r a r e in questa categoria i t e r r e m o t i originanti negli appennini, che p r e s e n t a n o un modello c a r a tteristico della sorgente u n i c a positiva (sollevamenti), m e n t r e dello stesso t i p o (ma con senso contrario) si m o s t r a n o i t e r r e m o t i nel vicino adriatico. « oggi si ritiene che solo le g r a n d i esplosioni possano risolvere i problemi i n e r e n t i alla crosta t e r r e s t r e . riconosco il g r a n d e interesse che p r e s e n t a n o tali esperienze. r i t e n g o però che esse possano servire d a utili c o m p l e m e n t i agli s t u d i sismici ordinari, senza potersi sostituire ad essi. lo p r o v a n o le g r a n d i discordanze cui conducono esperienze del genere. l'eccessiva superficialità dell'origine p o r t a a complicazioni nella propagazione delle onde sismiche, tali d a condurre ad incertezze forse maggiori di quelle che si riscontrano nell'ora origine dei t e r r e m o t i . le difficoltà sono poi accresciute dall'elevata f r e q u e n z a degli s t r u m e n t i di cui si f a uso in queste ricerche, che si t r a d u c e in u n a successione int r i c a t a di impulsi, m o l t o spesso p r o v o c a t i d a discontinuità di 2 a specie o c o m u n q u e accidentali, prossime alla superficie. a n c h e qui l ' u n i l a t e r a l i t à dei giudizi è t u t t ' a l t r o che giustificata. « u n altro a r g o m e n t o all'ordine del giorno è quello delle onde canalizzate. argom e n t o di particolare interesse p e r me, se non a l t r o p e r essere s t a t o il p r i m o a parlare di questo tipo d'onde, c a r a t t e r i s t i c h e degli str a ti a flessione di velocità. n o n vorrei però che lo schematismo, di cui si faceva cenno più sopra, proprio di u n a ment a l i t à o r i e n t a t a dalla sola intelligenza, non portasse qualcuno ad esagerare, nel senso di vedere d o v u n q u e onde canalizzate. p e r es., a mio parere, un'estrapolazione di tali o n d e nelle stratificazioni continentali europee, v a a c c e t t a t a n o n senza f o r t i riserve: non d i m e n t i c h i a m o che il nostro continent e è f o r t e m e n t e inciso d a geosinclinali e d a anticlinali montagnose, incisioni che vengono a r o m p e r e o, in ogni caso, a modificare, la c o n t i n u i t à delle stratificazioni e l ' a n d a m e n t o della velocità nelle stesse, t a n t o d a r e n d e r e p r o b l e m a t i c a l'esistenza — in p i ù zone — del " canale " di propagazione e da ritenere illusorie le cifre che d a n n o la velocità, spinta fino ai centesimi di chilometro. i n v a s t e zone di e u r o p a , forse più che di onde " canalizzate " è giusto p a r l a r e di o n d e " g u i d a t e " , nel senso che si d à a questo t e r m i n e in elett r o m a g n e t i s m o . « u n ' a l t r a considerazione, sull'uso degli a p p a r e c c h i a b r e v e periodo. oggi molti ritengono tali a p p a r e c c h i i soli v e r a m e n t e necessari nello studio dei t e r r e m o t i di non l o n t a n a origine. i o ritengo che a n c h e qui si esageri. voi s a p e t e che, in corrispondenza di discontinuità f r a mezzi di analoghe c a r a t t e r i s t i c h e elastiche, possono formarsi onde di stoneley o di sezawa e k a n a i . gli s t r a t i prossimi alla superficie esterna della t e r r a , del q u a t e r n a r i o o del terziario — dai sedimenti sciolti, ai conglomerati o a d a l t r e stratificazioni più consolidate — contengono n u m e r o s e di t a l i discontinuità, a n c h e semplicemente meccaniche, in corrispondenza delle quali si possono f o r m a r e , e si f o r m a n o r e a l m e n t e , onde di a l t a freq u e n z a che i sismografi a b r e v e periodo ricevono ed esaltano. avviene così — come è t e s t i m o n i a t o dalle registrazioni di sismografi di diverso periodo in u n a stessa stazione sismica —, che le onde provenienti dalle stratificazioni più p r o f o n d e vengano m a s c h e r a t e d a quelle di più e l e v a t a frequenza, f o r m a t e s i nelle stratificazioni più esterne. solo s t r u m e n t i con periodi propri superiori ad 1 sec possono r e n d e r e trascurabile l ' e f f e t t o degli s t r a t i sedimentari. « n o n mi dilungo oltre, per n o n tediarvi. f o r s e a v r e m o occasione di i n t r a t t e n e r ci su questi problemi d u r a n t e i prossimi lavori. ad ogni modo, p e r t o r n a r e al car a t t e r e d ' i n c e r t e z z a che è alla base delle n o s t r e f a t i c h e , devo confessare che — talv o l t a — , v i n t o dalla preoccupazione di sbagliare, di p e r d e r t e m p o in cose superflue, di s o f f e r m a r m i su questioni ovvie, di perd e r m i in a p p a r e n z e senza valore, senza domani, mi viene s p o n t a n e o di r i f u g i a r m i nei valori e t e r n i dell'arte. ma non si t r a t t a di diserzione, che ritorno ben presto all'abituale f a t i c a . a n c h e il nostro lavoro h a il 151 convegni e congressi suo fascino: q u e s t a incertezza è stimolante per gli spiriti decisi; il t r o v a r s i sempre ai confini f r a il noto e l'ignoto, ai m a r g i n i di u n mistero che più si allarga più t i accanisci a d i p a n a r e , h a le sue invincibili a t trazioni. sento di aderire p r o f o n d a m e n t e , in questo, a ciò che a f f e r m a l e s s i n g — e i miei colleghi tedeschi c e r t a m e n t e ricordano — in u n celebre passo della sua " replica " : " n o n il possesso della v e r i t à — dice l e s s i n o —, m a lo sforzo sincero che ciascuno h a f a t t o per giungervi determina il v a l o r e dell'individuo. il possesso rende spesso inerti, pigri, superbi: solo la ricerca sviluppa le forze dell'uomo, solo m e d i a n t e la ricerca si a t t u a incessantemente la sua perfezione " . « i n questo spirito — ne sono certo — noi svolgeremo i nostri lavori, allo scopo di contribuire, sia p u r e m i n i m a m e n t e , al progresso delle nostre conoscenze ». d a p a r t e italiana, vi f u r o n o inoltre comunicazioni del d o t t . f . p e r o n a c i , dell ' i s t i t u t o nazionale di geofìsica di r o m a , sulla sismicità della regione i r a n i a n a ; del d o t t . m . d e p a n f i l i s , dello stesso i s t i t u to, s u l l ' a t t i v i t à sismica in i t a l i a nel quinquennio 1953-1957 ; del prof. m a r u s s i , dir e t t o r e d e l l ' i s t i t u t o di geodesia dell'università di trieste, sulle correlazioni f r a ricerche sismiche e g r a v i m e t r i c h e nella zona del p a m i r c a r a c o r u m e del p r o f . e . p e r r i , del politecnico di torino, sopra u n nuovo tipo di o n d a superficiale. seguono — nel testo francese — le risoluzioni prese al t e r m i n e del convegno: r é s o l u t i o n le. la commission séismologique e u r o p é e n ne, réunie à u t r e c h t le 8 avril 1958, a étudié en détail le p r o j e t de grandes explosions qui doivent è t r e exécutées dans les alpes frangaises a u eours de l'été 1958; de nombreuses équipes de plusieurs p a y s européens (allemagne, f r a n c e , grande-bretagne, i t a l i e , suède, etc.) doivent p a r t i c i p e r aux enregistrements séismiques. la commission séismologique e u r o p é e n ne a t t i r e l ' a t t e n t i o n des a u t o r i t é s gouvernementales sur l ' i n t é r è t scientilìque de ces expériences qui doivent constituer la deuxième phase de l ' é t u d e entreprise en 195g sur la s t r u c t u r e profonde des alpes. r é s o l u t i o n 2 e . l a commission séismologique européenne, réunie à u t r e c h t le 8 avril 1958, a p r i connaissance avec g r a n d intérét d u p r o j e t d'explosion i m p o r t a n t e e t profonde qui doit è t r e réalisée en belgique. l a commission séismologique européenne est résolue à p r è t e r son a p p a i p o u r l'organisation scientifìqiie et pour l'observat i o n séismique de c e t t e expérience. r é s o l u t i o n 3 e . 1. l a commission séismologique e u r o péenne r e c o m m a n d e q u ' u n symposium sur les problèmes séismotectoniques e t sur les m é t h o d e s de la composition de la c a r t e séismotectonique de l ' e u r o p e soit organisé p e n d a n t la réunion prochaine de la commission séismologique e u r o p é e n n e . i l est r e c o m m a n d é que le p r o g r a m m e du symposium comprende: a) les r a p p o r t s des r e p r é s e n t a n t s des divers p a y s concernant les é t u d e s sur les relations e n t r e la séismicité et la tectonique régionale; b) la p r é s e n t a t i o n des principes mét h o d i q u e s de la composition de la c a r t e séismotectonique de l ' e u r o p e et la discussion de ces principes; c) la p r é s e n t a t i o n (les propositions t e c h n i q u e s concernant la composition de la c a r t e séismotectonique (le contenti, la légende, l'échelle, e t c . . . ) et leur discussion; d) la discussion des questions d'organisation et l'élaboration des mesures p r a t i ques p o u r assurer le développement du t r a v a i l . 2. l a commission séismologique européenne considère cornine nécessaire l'établissement des relations directes avec la commission de la c a r t e t e c t o n i q u e d u congrès géologique i n t e r n a t i o n a l p o u r le succès de ce travail. 3. considérant que dans l ' u . r . s . s . il y a d é j à une expérience dans le domaine 152 convegni e congressi de la composition des cartes séismotectoniques et q u e la commission de la c a r t e t e c t o n i q u e a son secrétariat à moscou, la commission séismologique e u r o p é e n n e charge le professeur b e l o u s s o v : a) d ' é t a b l i r le c o n t a c t indiqué ci-dessus a v e c la commission de la c a r t e t e c t o n i q u e e t d ' é t u d i e r ics f o r m e s des relations effectives avec clles; b) de f o r m u l e r les principes de la composition de la c a r t e séismotectonique de l ' e u r o p e et les faire circuler p a r m i les p a y s p a r t i c i p a n t s d a n s la commission séismologique e u r o p é e n n e ; c) de p r é p a r e r , en collaboratimi a v e c le p r o f e s s e u r bath, le symposium sur la composition de la c a r t e séismotectonique de l ' e u r o p e à la procliaine session en t a n t (pie r a p p o r t e u r s . r é s o l u t i o n -1<-'. considèrant les g r a n d s a v a n t a g e s présentés p a r des j e u x de séismographes homogènes p o u r la solution d e n o m b r e u x problèmes séismologiques, la. commission séismologique e u r o p é e n n e r e c o m m a n d e que: 1 ) t o u t e s les s t a t i o n s séismologiques qui possèdent d é j à des séismographes gah t z i n e ou galitzine-wilip, ou d ' a u t r e s séism o g r a p h e s à g r a n d e période, complètent leur é q u i p m e n t p o u r obtenir u n jeu de 3 composantes réglées de fagon aussi homogène que possible (mèmes caractéristiques de f r é q u e n c e p o u r les 3 composantes); 2) que les stations indiquées ci-dessous complètent leur é q u i p e m e n t ou acquièrent des séismographes à g r a n d e période sensibles afin d ' o b t é n i r u n jeu d ' a p p a r e i l s homogènes: spitzberg, r e y k j a v i k , bergen, dublin, p r a g u e , les agores et alger. il nuovo consiglio di presidenza della «commission séismologique e u r o p é e n n e » è risultato così composto: presidente prof. p i e t r o caloi (roma) — r i c o n f e r m a t o —; segretario i n g . e . peter,schmitt (strasburgo) — r i c o n f e r m a t o — ; primo vice-presidente j . b o n e l l i r u b i o (madrid); secondo vice-presidente e . t. s a v a r e n s k y (mosca). si è stabilito di t e n e r e la prossima riunione della c.s.e. ad alicante (spagna), nella seconda quindicina di o t t o b r e del 1959. p . c. direttore: p r o f . enrico m e d i p r o f . p i e t r o caloi responsabile t i p o g r a f i a p i o x r o m a v i a d e g l i e t r u s c h i , 7 — 1 2 1 1 1 9 5 8 sull' influenza solare nelle esperjenze di gtrointerazione g. righl ni • g. godoli le e slle rienze di g i roinle r az io ne c he l 'is iîlul o di fiaica teçni ca de ll ' uni ven i l à di napo li h a o rganizza le, so ll a la direzion e di m. cu lo lo, da l 1946 h anna 'nloe t rato che il feno meno s i ve rili ea so lt anto in moùo salluario. tale fatto a,,'e,,'a indott o i ricercatori a pen sare c he le perturba. zioni cromosferiche, connes8e con l'aui'\.·ità general e e sporadica del bo le, f osrefo responsahili di queslo ; è proprio per ind agare u na c\'en1 " .-~~~ , -, ---_ ...... , , .0.4 , , , , , , , , , ". , , , , 0.0 , , -, '\....-. _ -....... / -0. 4 , , , "---/ lual e influenza dei so le sull e esp erîenze di ~ i roin[ erazione che ab· hiamo i n tra preso queslo lu voro su richiesta dei ce nt ro slu di pet l a r afliollfopa!!3z ione dc ll 'u nh'enit il di napo li. ji prol.l c m a , o ltre a l sil o inle reese s recul a lî vo, rîves lîv a un a ce rl a ifll ilorian za iira li u în vis la di uhe r io ri e s perienze: data inra lli la lo ro co ill(llessilà, qua lo r a fone 61ata messa i n luee un a q ualc he in1 2 c . r i c h 1 n i • g. godoli fluenza solare diretta od indiretta del fenomeno, ci si sarebbe astenuti dall'eseguire ricerche nei giorni di maggiore attività solare. ci siamo limitati a prendere in considerazione le esperienze dello campagne del 1948-1950 eseguite nei giorni contenuti nella tabella i : tali esperienze erano effettuate con omogeneità di metodo e risullavano le più attendibili. nella tabella sono sottolineati i giorni in cui l'interazione non ha avuto luogo. + 0 . 4 , 0.0 0 . 4 4 2 0 4 2 + 4 e f i g . 2 p r o t u b e r a n z e al m e r i d i a n o . per le influenze solari dirette, dovute all'attività generale del sole, è stato studiato il comportamento delle macchie e delle protuberanze al meridiano centrale del sole mentre per le influenze dirette, dovute ai fenomeni sporadici, abbiamo preso in considerazione i brillamenti solari. l'influenza indiretta del sole è stata studiata prendendo in considerazione gli indici triorari k, che caratterizzano l'attività magnetica generale, le perturbazioni ad inizio brusco se, che sono una conseguenza della radiazione corpuscolare di origine solare, ed infine i « crochet » magnetici o perturbazioni brusche di breve durata che sono direttamente connessi ai bruschi aumenti di radiazione ultra-, t.4.i1ell a i giorni ilf,i '1uali j/lrollo esef!./iife esperieflze di giroiflferflziflfw. soli o sof/olil!('llfi i giorfli iii i:ui l'illterf/ziolle 11011 iw flv/lto ll/ogo. 19·18 12 ... v 15 w 2:1 22 25 27 :::') :a l , .. vi 3 5 " 1(41) 18 .. . vi 19 21 23 28 ... \"11 -2 ... viii 3 " " 9 13 15 t aiiella ii ]1).10 lo .. . v 2. 21 3. 1 .. . vi l 7 lo 13 17 23 1 ... vii 3 ... vlii grflllr/i';;;ze iissuflff' ller i l'llri jellomelli collsirlerali fenomeno macehie l'rotu!jel'anze al mel'idiano brillamenti indiei k se crochet fre(jllenza criti('a dello ~trato f, frequenza minima l'i. grandezza .:\11111(''1"0 fii \voh (11. anivitil delle protuljeranze al hordo (nei giorni opportnnil e~pre~li-a in unità di pro· tuberanza (21. numero giornaliero (31. somma del!li 8 valmi giornalieri (41. 1\"umero giornaliero (51. somma i!iornaliera dei prodoni il. b. c (61 dove l/ = numero fii o!'hen'atori regi!'tranti il fenomeno b _ durata 15'" (' = i !'e il fenomeno non è stato con· fermalo da osservazioni solari od iono!'feri('he (' 2 !'e il fenomeno è li-tato confero mato. ju f., alle ore 12 (71. f1e~~a dallo ~trato d j d alle ore 12 (81. a,,~or"imento dello strato d ,4d alle ore 12 (9). 1 4 g. r 1 g h i n i c . godoli fig. 3 b r i l l a m e n t i . violetta solare. questi u l t i m i sono strettamente correlati con i brillamenti solari. per lo stato della ionosfera abbiamo preso in considerazione la frequenza critica dello strato f 2 che indicheremo con fo f-2, quella minima riflessa dallo strato d che indicheremo con { d ed infine l'assorbimento dello strato d che indicheremo con ad. per ognuno dei fenomeni considerati è stata scelta u n a grandezza che ne sintetizzasse l'attività g i o r n a l i e r a ; le grandezze assunte sono elencate nella tabella ii. indicando generalmente con m la grandezza relativa ad un certo fenomeno e con ms il valore che tale grandezza assume in un giorno in cui si osserva il fenomeno della girointerazione e con mn il valore che essa assume in un giorno in cui non si osserva il fenomeno, abbiamo calcolato le medie seguenti: s u l l ' i n f l u e n z a s o l a r e n e l l e e s p e r i e n z e iii giro interazione 1 5 1 1 -p 1 1-9 m — ì ; „,.(') m a = — v, „,„(') p 1 1 ì-p+'l m = — n <9 clic ci (lamio rispettivamente il valor medio di m jter i p giorni di osservazione positiva (si); i l valor medio di m per i q giorni di osseri + 0 . 8 + 0 . 4 0 . 0 0 . 4 0 . 8 _ / t o + 3 + 4 fig. 4 i n d i c i k . vazione negativa (no); ed infine il valore medio di m per tutti i p -(q giorni di osservazione. l'indice statistico / che si deduce dalla j = m ci dà l'influenza del fenomeno considerato sulla girointerazione. esso evidentemente dovrebbe essere piccolo o nullo se il fenomeno non influisse sulla girointerazione; positivo se il fenomeno agisse come inibitore e negativo nel caso contrario. 1 6 c . r i g h i m i c . godoli per tener eonto anche degli eventuali sfasamenti f r a causa ed effetto abbiamo calcolato / non soltanto per il giorno in cui si è fatta l'esperienza di girointerazione ma anche per alcuni giorni precedenti e seguenti. abbiamo cioè applicata a questa indagine il metodo delle f i g . 5 se. epoche sovrapposte rappresentando / in funzione dell'epoca e. per il giorno dell'osservazione si avrà e = 0 ; per il giorno precedente e = •—1, per quello seguente e = + 1 e così via. 11 i + 3.0 +2,0 " , ' , \ , \ i , , , i , , , , +1.0 , i i , \ i 1\ , \ , , , i \ \ 0,0 \ i \ i i i '\ i , \, , v ( l,o \ i , i \ ,,_---_ j \ ' i '\ ""-2.0 v il ''0 i 3.0 4 -2 o +2 +4 e 1 8 c . r i f i l i ini g. godoli i grafici 1 r 7 danno l'andamento di / in funzione di e per i diversi fenomeni. sono stati tracciati tre grafici separati per gli anni 1948 4 9 ' ) 50 (———' ed inoli re un quarto grafico estendendo le medie a tutto i l triennio (—-—). fig. 8 id. 2 0 + 2 f i g . 9 a d . + 4 e s u l l ' i n f l u e n z a solare n e l l e e s p e r i e n z e iii giro interazione 1 9 il materiale ha ancora un valore statistico molto basso per poterne trarre delle conclusioni sicure e dettagliate; possiamo però asserire clie in linea generale le varie perturbazioni solari ed i fenomeni geofìsici connessi, non sembrano responsabili dei risultati negativi delle esperienze di girointerazione. firenze — osservatorio astrofisico di arcetri centro di astrofisica del c.n.r. — febbraio 1952. riassunto nella presente nota si mostra come sia estremamente improbabile una. influenza solare nelle esperienze di girointerazione. su m mary in the present paper ice shoivn that it is extremely improbable that there is any solar influence on gyro-interaclion. b i b l i o g r a f i a ( ! ) e ( 3 ) q u a r t e r l y b u l l e t i n on solar a c l i v i t y . ( 2 ) o s s e r v a z i o n i di a r c e t r i e catania r i d o t t e ad a r c e t r i e c o n s e r v a t e n e g l i arc h i v i d e l l ' o s s e r v a t o r i o . (4) (5) (gj j o u r n a l of g e o p h y s i c a l r e s e a r c h . (7) (8) (9) s . p j . m . o b s e r v a t i o n s i o n o s p l i e r i q u e 6 . s t a t i o n de f r i b o u r g . geo 1067 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 8 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging leonard kersley (1), daniel malan (1), s. eleri pryse (1), ljiljana r. cander (2), ruth a. bamford (2), anna belehaki (3), reinhart leitinger (4), sandro m. radicella (5), cathryn n. mitchell (6) and paul s.j. spencer (6) (1) university of wales, aberystwyth, u.k. (2) rutherford appleton laboratory, chilton, didcot, oxon, u.k. (3) national observatory, institute for space applications and remote sensing, p. penteli, athens, greece (4) institute of physics, institute for geophysics, astrophysics and meteorology (igam), university of graz, austria (5) the abdus salam international centre for theoretical physics (ictp), aeronomy and radiopropagation laboratory, trieste, italy (6) university of bath, u.k. the paper reports on a series of studies carried out within the cost 271 action relating to the measurement and use of total electron content (tec) of the ionosphere over north west europe. total electron content is a very important parameter for the correction of propagation effects on applied radio systems so that it is vital to have confidence in the experimental measurements and the resultant products derived as aids for the practical user. comparative investigations have been carried out using tec values from several different sources. it was found that in general there was broad statistical agreement between the data sets within the known limitations of the techniques, though discrepancies were identified linked to steep ionospheric gradients at the onset of geomagnetic storm disturbance and in the vicinity of the main trough. the paper also reviews recent progress in the development of tomographic inversion techniques that use total electron content measurements to image the ionosphere as an aid to various radio systems applications. 8.1. introduction total electron content is a key parameter in the mitigation of ionospheric effects on radio systems. in numerous applications involving radio links between satellites and ground, which play vital roles in the modern technology of communications, navigation and surveillance, the propagation departs from free space conditions in various ways with important consequences for the reliability and accuracy of the service. the phase advance and doppler shift of a carrier, group retardation and time delay of a modulation, faraday rotation of the polarisation and wedge refraction of the wave direction are all dependent in some way on the line integral of the electron density along the ray path through the ionised atmosphere, the socalled total electron content. the ability to make allowance for the effects on the propagation of the electron content and its temporal and spatial variations thus becomes an essential component in the reliable 1068 leonard kersley et al. operation of the system. indeed, adequate correction for the effect of ionospheric propagation may represent a limiting error for the accuracy of some uses of the technology. investigations of total electron content are thus central to fulfilment of the objectives of the cost 271 action. total electron content has been measured experimentally since the 1960s using a variety of different techniques. while there have been relatively few purpose-built satellites for such observations, it has often been possible to make use of radio signals from satellites launched for another primary purpose. some early measurements involved determination of the polarisation rotation of vhf transmissions from the first-generation of geosynchronous communications satellites. later it became possible to exploit the signals from the various satellite navigation and positioning systems, like nnss and more recently gps. a vast body of data and information about total electron content has been amassed over the years from numerous locations giving almost worldwide coverage. in addition, products have been developed in the form of regional and global models yielding total electron content, while more recently maps are now routinely available of the parameter on the internet. another recent advance has been the application of tomographic techniques to invert electron content measurements to obtain images of ionospheric electron density. the present paper reports collectively some results from a number of separate studies carried out within the cost 271 project that have been directed towards comparative investigations of total electron content derived from various sources and the use of the parameter for ionospheric imaging. the broad aim was to demonstrate confidence in some of the basic products of the project and to identify possible areas and circumstances where further study and development are warranted. 8.2. total electron content (tec) most measurements and applications of total electron content are concerned with arbitrary slant ray paths between a satellite and a ground station. however, for comparative purposes it is necessary to define the observations in a more standard way. in consequence, the convention was adopted at an early stage to apply a simple geometrical construction, usually based on an assumed thin-shell ionosphere at a fixed height, to rotate the actual slant measurement to obtain an equivalent vertical total electron content. in practice, most estimates of total electron content (usually abbreviated to tec and measured in units where 1 tecu ≡ 1016m-2) are based on this equivalent vertical parameter. 8.2.1. gps-tec transmissions from the gps navigation satellite system now provide the main source for most experimental measurements of tec currently being made. the satellites are at heights of some 20 200 km, in orbits with inclination of 55° to the equatorial plane with periods of about 12 h, and so move slowly across the sky in patterns that repeat accurately from day to day in sidereal time when viewed from a given ground station. the resultant measured phase and group delays of the l-band signals received along the slant ray paths using segments of data from many different satellite passes can be combined in an algorithm to yield values of equivalent vertical total electron content. the diurnal variation of the equivalent vertical tec in the vicinity of a receiving station can be estimated by means of such analysis and calibration procedures. papers by lunt et al. (1999a-d) have discussed a number of issues concerning one such process. the analysis used in the present work to estimate tec from gps transmissions was carried out by means of the procedure developed by ciraolo (1993) and used by ciraolo and spalla (1997). the studies described here made use of data both from gps signals monitored in u.k. and also from other stations that form part of the igs network throughout europe. values of tec obtained from gps observations now form the basis for most current applications of ionospheric corrections to practical radio systems and derived products like maps. it is thus important to try to understand the 1069 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging reliability and limits to the accuracy of such measurements. the situation is complicated further for measurements at locations in u.k. by two factors, geometrical and geophysical respectively. first, the non-polar inclination of the gps satellite orbits means that there is incomplete sky coverage for the ray paths, particularly for ground stations at higher latitudes. furthermore, the main trough, bounding the mid-latitude and auroral sectors is a regular feature of the nighttime ionosphere over northern u.k. the present studies have been directed towards addressing some of the resultant issues, particularly with regard to tec measurements and products in the vicinity of u.k., but within the wider european context of the cost 271 action. 8.2.2. nims-tec while gps observations give tec values as a function of time above a given location, measurements with respect to latitude at particular times can also be made. some satellites in 1100 km polar orbits, in what was the former nnss constellation, now known as nims, provide signals for ionospheric measurements. an experimental programme using nims satellites has been carried out in u.k. as part of the current project to provide independent measurements of tec determined by the differential carrier phase method. the analysis of the phase observations to obtain absolute equivalent vertical tec was done using an extension to multiple stations of the well-established procedure described by leitinger et al. (1975). many of the results outlined here are based on these nims observations that yield tec as a function of latitude at the time of the satellite pass. 8.3. accuracy of tec measurements some initial studies were carried out to investigate the limitations imposed by the equivalent vertical assumptions involved in most tec measurements. in converting from slant to vertical electron content, the parameter that is necessary for comparison purposes, it is usual to assume a mapping function based on a thin-shell ionosphere at a chosen height. the magnitudes of the errors introduced by an inappropriate choice of height were investigated in these preliminary studies, using data from a chain of 5 stations in u.k. (kersley et al., 2002). the assumed centroid height of the ionosphere was varied between 250 km and 450 km and the consistency of the resultant latitudinal profile of the tec investigated. it was found that for the lowest assumed mean ionospheric height of 250 km the tec values were marginally lower in the vicinity of the stations and significant differences opened up between the records for the individual stations both to the south and the north. conversely for an assumed ionosphere at 450 km, the tec values were higher in absolute terms with the largest differences between the station records again at the extremities of latitude. the limitations to the accuracy of tec measurements in any practical experimental situation can be appreciated from studies of this kind. while the 200 km range in the uncertainty of the centroid height of the ionosphere used may be over-large in general, nevertheless it was concluded from such investigations that the accuracy of the resultant equivalent vertical tec measurements cannot be taken to be better than a few tec units, even in the vicinity of the stations, and it could be much worse well outside the latitudinal range of the observing locations. additional studies, using data from pairs of stations within the set showed that to maintain accuracies of a few tecu attention must be confined to within a limited latitudinal span some 5° to 7° beyond that of the stations. it was also noted from further studies that the inversion of the data set to create a two dimensional tomographic image of electron density through which the vertical total electron content was calculated by integration is the optimal method for the estimation of the vertical tec over the latitudinal range. use of a tomographic image was of particular importance in regions of steep gradients, like those associated with the main trough that is often to be seen in observations 1070 leonard kersley et al. over northern u.k. it can be noted that the use of tec observations to image the electron density by tomographic reconstruction will be discussed in more detail later in this report. from these initial investigations it was concluded that the estimation of equivalent vertical tec from slant measurements is likely always to be limited to an accuracy of a few tecu when a simple mapping function based on a thin-shell ionosphere is used. this inherent limitation must be kept in mind when comparisons are made of tec measurements from different experimental techniques. 8.4. comparisons of tec measurements 8.4.1. hirac campaign two studies were undertaken to compare values of tec obtained by two completely independent techniques. the first involved observations made during a campaign of intensive measurements of tec between 23 and 29 april 2001, which was coordinated by dr. n. jakowski (drl, germany) under the auspices of the cost 271 project and known by the acronym hirac. the motivation was to create a database of measurements near vernal equinox at solar maximum, a time when the tec values would be expected to be very high. it was fortuitous that the campaign included an extended period of geomagnetic quiet followed by an impulsive storm so that the data are representative of a wide range of conditions at solar maximum. the gps observations used for the current work were obtained from a receiver located at chilbolton (52.1°n) in southern england. in addition, latitudinal plots of tec were obtained from some 44 high-elevation passes of nims satellites monitored at sites in u.k. during the campaign period. these were used to determine the equivalent vertical tec at 52.1°n at the times of the passes, for comparison with the gps data from chilbolton. fig. 8.1. comparison of equivalent vertical tecs obtained using the gps observations at chilbolton and those for the same latitude from nims satellite passes during the hirac period. the best-fit line is plotted bold, while equality is shown by the faint line of unity slope. 1071 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging a comparison of the two data sets is shown in fig. 8.1. it can be seen that the estimates of tec from the two experimental techniques are in broad agreement, with the slope of the best-fit line close to unity. in general, the departures from agreement between individual estimates are at the level of a few tec units, confirming the inherent limitations discussed above. it must be noted that there is a positive intercept of the best-fit line of some 1.7 tecu, indicating that the gps measurements are inherently larger than the nims values. this difference can be attributed to the differing ray-path geometries in the two situations, with the nims measurements confined to 1100 km altitude, while those from gps extend to some 20 000 km and encounter additional plasma on protonospheric flux tubes. the results confirmed the measurements reported in the papers by ciraolo and spalla (1997) and by lunt et al. (1999a-d), cited above. the former had found the gps-tecs to be larger by some 3 tecu from analysis of a very extensive data set of nims-type observations in italy. the latter workers demonstrated that gps estimates from a station in u.k. were greater by about 2 tecu for ray paths to the south, but equivalent to the nims values when looking north, in keeping with current understanding of the physical mechanisms controlling the plasma on the protonospheric flux tubes. other studies carried out here failed to reveal any additional systematic cause for the differences between the data sets. thus it was concluded that, in general, there was broad agreement within the experimental errors between the measurements of equivalent vertical tec over u.k. from the independent measurements made during the hirac period. however, it was noted that one apparently rogue point can be seen in fig. 8.1, where the gps estimate is almost double that of the nims measurement. further investigation showed that this discrepancy arose on the afternoon of the first day of an impulsive geomagnetic storm during the collapse of the positive phase. the kp index was generally less than 3 from 24-27 april 2001, but had three periods with values of 5 following the onset of the storm on 28 april. the diurnal plots of tec from the gps observations showed that the average daytime maximum was about 40 tecu during the geomagnetic quiet times, but exceeded 55 tecu on the day of the storm. these very high tecs were followed by the dramatic collapse of the ionisation, which is characteristic of such conditions, to reach a pre-dawn minimum of only 5 tecu, less than half that of earlier nights. this apparent temporal collapse in the ionisation densities and hence tec values has consequences, not only for the longitudinal structure, but also for the latitudinal gradients in the plasma. the dramatic changes can be appreciated from the tec observations from the two successive nims passes. the first at about 16:13 ut on 28 april 2001, showed a very steep latitudinal gradient, with tec values exceeding 57 tecu below 50°n, but less than 15 tecu at latitudes greater than 65°n. however, only one hour later the ionosphere had a very different structure. from a pass at about 17:19 ut, the tec at 50°n was now only some 27 tecu, with less than 5 tecu being found at 65°n. it was concluded that there were very steep spatial gradients in both the meridional and zonal directions around this time, in an ionosphere that was also changing rapidly in a temporal sense. the observations contributing to the outlying point in fig. 8.1, which corresponded to the 17:19 ut pass on 28 april, can thus be placed in context. both of the measured values were estimated from slant observations through ionospheric regions containing significant gradients. for the nims-tec, the results were obtained from a single satellite as it passed, essentially following a line of longitude, in this case slightly to the west of the receiver stations with the ray paths traversing the changing ionospheric gradients. by contrast, the gps-tec was estimated by a complex procedure that made use of multiple satellite observations with many different, and changing, observational geometries. when steep gradients are present the ray paths from the various spacecraft may encounter rather different ionospheric conditions that will depend also on the exact form of the constellational geometry available for use at any particular time. it was concluded that the two independent estimates that contribute to the outlying point in fig. 8.1 may be equally valid measurements, with the difference between the values being accounted for by the ionospheric conditions prevalent at the time and the particular procedures required for the derivation of the equivalent vertical tec from the basic observations. the current study highlights the 1072 leonard kersley et al. need for caution in the use of tec values for operational applications under such circumstances. it is clear that at times of substantial spatial and temporal gradients in the ionosphere, as on the first afternoon of an impulsive geomagnetic storm, different measurement techniques may give rise to very different estimates of the equivalent vertical tec, well beyond the accepted experimental errors of the individual results. 8.4.2. igs site data a later study (kersley et al., 2003) was also carried out to compare measurements of tec made independently at sites in u.k. using different techniques. data were selected from about 100 passes of nims satellites, monitored at three sites spanning u.k. latitudes, and chosen to cover a wide variety of geophysical conditions in autumn 2002, including days of geophysical disturbance. comparisons was made with tec measurements obtained by analysis of rinex files from the routine gps observations taken at the international geodetic survey (igs) site in u.k., hailsham (50.0°n, 0.3°e). care was taken to reduce all known possible sources of error in the data for the comparison. the nims-tec values were determined by vertical integration through tomographic images (fig. 8.2). the comparative plot shows good statistical agreement between the data sets, with a variance of 0.91. the best-fit line has a slope of 0.91 and an intercept of 2.7 tecu, the latter in broad agreement with the earlier estimates of the protonospheric contribution to gps-tec measurements in u.k. in an examination of possible sources for the residual scatter several factors were considered. the difference between the individual gps and nims-tec estimates is generally only a few tec units and it must be recalled that an offset of some 2-3 tecu would be expected from the different altitude limits of the two sets of measurements. however, the difference appears to show a slight dependence on time of day. figure 8.3 shows that cases where the tec determined from the 1100 km altitude nims satellites apparently exceeds that from the 20 200 km gps satellites are concentrated in the afternoon hours, while the highest positive differences are found in the morning. the effect can fig. 8.2. comparison of equivalent vertical tec estimated from rinex files recorded at the igs-gps site at hailsham with values obtained by vertical integration through tomographic images obtained from analysis of nims observations at a chain of stations in u.k. 1073 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging also be seen in the differences expressed as a percentage of the gps-tec. however, it again must be noted that protonospheric contribution introduces a bias that, while small during the day, may represent a significant fraction of the total content at night, resulting in a few very large positive percentage differences. the predominance of positive tec differences in the morning and negative values in the afternoon may be linked to longitudinal gradients. the plot shown in fig. 8.4 of the tec difference versus the longitudinal gradient (obtained from the gps temporal observations) shows that, allowing for the small positive mean difference between the data sets, the largest positive values occur when there is an ionospheric gradient rising to the east, while negative differences are associated with gradients increasing to the west. fig. 8.4. absolute difference between gps-tec and nims-tec estimates as a function of the longitudinal tec gradient. fig. 8.5. absolute difference between gps-tec and nims-tec estimates as a function of the latitudinal tec gradient. fig. 8.3. difference between gps-tec and nims-tec estimates as a function of ut expressed in absolute and percentage terms. 8.4 8.5 1074 leonard kersley et al. another study investigated possible effects of gradients in latitude on the difference between the two tec data sets (kersley et al., 2003). the latitudinal gradient in the u.k. sector, determined from the nims observations, normally decreases to the north. allowing for the known positive offset between the gps and the nims measurements, there is some limited evidence in fig. 8.5 to suggest that less steep and even reversed mid-latitude gradients may give rise to increasing residual differences between the gps and nims-tecs. it was concluded that the tec measurements from the two independent techniques agree well on a statistical basis within the limits of experimental observation. no seriously outlying points were found in the data sets, even through the observations encompassed times of geomagnetic disturbance. the residual scatter was generally at the level of a few tecu. there was some evidence to suggest that differences of a few tecu may be linked to gradients in the ionosphere. in particular, the gps technique may give rise to small overestimates with respect to the nims value during the morning longitudinal gradient with corresponding small underestimates at the time of the afternoon decline in the ionisation. it is possible that the fixed height assumption of the thin-shell ionosphere used in the analysis procedures contributes to this effect, with the consequences being magnified for the gps results that are obtained from much longer slant paths through the ionisation gradient. the effects of latitudinal gradients are more complex, involving not only the bias towards the south of the content on the protonospheric flux tubes, but also the incomplete coverage of the sky for the ray paths from gps satellites, in 55° inclination orbits, when viewed from a site at u.k. latitudes. however, it must be stressed that the residual effects found here, linked to ionospheric gradients, are at the level of only a few tecu. 8.5. gps-tec maps a product available on-line from the ral group are the maps of tec over europe created from measurements made using ftp rinex files from the network of gps receiving stations operated for the international geodetic survey (igs). contour maps of tec are generated after applying a kriging gridding procedure, in which regional variable theory is used to calculate the correlation between measurements with a weighted smoothing dependent on their separations (samardjiev et al., 1993). these maps are updated at 10 min intervals to provide an indication of the changing large-scale structure of the tec on a europe-wide basis (cander and ciraolo, 2002). at the time of the hirac campaign the coverage of the available igs stations providing data was limited, nevertheless it was thought useful to try to make a first assessment of the reliability of the tec maps by comparison with independent measurements. once again the equivalent vertical tec measurements from the nims passes were used, with values now being estimated for different latitudes (kersley et al., 2002). selected comparative plots between the gps-tec from the maps at u.k. longitudes and the nims observations, for latitudes of 50°n, and 60°n, are shown in fig. 8.6a,b. a consistent pattern emerged from the study, with the slope of the best-fit line generally being about 0.8. while the gps-tec estimates for low magnitudes exceeded those from nims as expected, at higher levels the values were much closer. indeed, there was evidence to suggest that the situation may be reversed at the lower latitude of the study, in contradiction to the known physical mechanisms governing the observations. thus, while the broad agreement between the data sets provides confirmation for the validity of the map tecs, nevertheless it is clear that there are limitations to the technique, at least in the preliminary form in which it was used at the time of the hirac campaign. more detailed comparisons showed that the form of the latitudinal gradient in the gps maps was often less steep than that found by the nims observations. however, the general agreement was encouraging for the future development of tec mapping techniques, particularly when it is noted that the station coverage in the vicinity of u.k. was very limited at the time of hirac. in an extension to the study the form of the tec maps during the collapse of the positive phase of the geomagnetic storm on 28 april 2001 was also investigated (kersley et al., 2002). the latitudifig. 8.7a,b. plots of equivalent vertical tec as a function of latitude from gps maps (dash/dot notation) and nims observations at aberystwyth and hawick, for a satellite pass on 28 april 2001 at about (a) 16:15 ut and (b) 17:15 ut. a b 1075 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging nal gradients in the tec in the vicinity of u.k., for two selected times in the afternoon, are plotted as the dashed lines on fig. 8.7a,b. for comparison, the corresponding latitudinal gradients obtained from nims pass observations are also shown. the much shallower nature of the map gradients is confirmed, with the magnitudes of the two measurements being similar at about 60°n for the earlier time. nims passes at 15:05 ut and 16:15 ut (the latter is plotted here in fig. 8.7a) showed very similar strong gradients, though the maps tecs, while much less steep, were also replicated at the two fig. 8.6a,b. comparison of equivalent vertical tecs from gps maps and nims observations at latitudes of (a) 50°n and (b) 60°n. ba 1076 leonard kersley et al. times. it can also be noted that the values from the maps at about 52°n were comparable to the independent measurements from the chilbolton receiver discussed earlier. the results at 17:15 ut (fig. 8.7b) show a similar map gradient to that found earlier, albeit with tec magnitudes lower overall, being some 10 tecu less than the previous observations. by contrast, the nims-tecs are greatly reduced, as was reported in the earlier discussion. it is clear that again the gps-based observations seem unable to yield the details of the collapse of the ionisation levels. it was concluded that, at the time of the hirac campaign, the tec maps gave a general indication of the magnitude of the equivalent vertical tec likely to be encountered in the vicinity of u.k. however, the gradients present in the ionosphere did not appear to be very well represented in the maps and, in particular, the steep spatial and temporally changing gradients driven by geomagnetic storm process were not followed. however, it was stressed that the coverage of the data from the igs stations was somewhat restricted during hirac and, in addition, the mapping techniques used were at a very early stage of development. the positive nature of the present results encouraged further development, together with subsequent assessment by means of independent comparison following the methods used here, though such analysis has not been possible under the present project for a variety of reasons. 8.6. tec in the vicinity of the main trough an important aspect of the project has been an ongoing programme of experimental observations directed towards obtaining definitive information on tec variations in the vicinity of the main ionospheric trough in the u.k. sector (kersley et al., 2004). the main trough represents a significant feature of the ionosphere in the sub-auroral region. it forms at the interface between the high-latitude regime, where the dynamics of the plasma are in the control of processes driven by space-weather interactions, and the mid-latitudes where production by solar euv radiation is a key factor. the trough forms in the magnetic post-noon sector near the boundary between the sunwards convection of flux tubes at high latitudes and the co-rotating flow of the mid-latitude plasma. as local time progresses during the night it moves to lower latitudes, so that in the sector of the ionosphere over north-west europe it represents the dominant large-scale structure. the trough and the associated gradients present particular problems for ionospheric models, developed for use in the mitigation against propagation effects on practical radio systems, because of the dynamic and variable nature of the feature. many models simply ignore the trough, but even for those where the structure is included, locating the depletion in the wrong place or having the incorrect form may represent a worse description of the actual ionosphere than complete omission. there have been many attempts to investigate the trough over several decades using various types of experimental observations and with the results being used to try to characterise different aspects of behaviour. while some understanding has been gained of aspects of the fundamental responses to different geophysical influences that play roles in controlling the mechanisms responsible for the location and form of the feature, definitive information necessary for the testing of models has been lacking. the present project was aimed at providing data from experimental measurements in a consistent and reliable form that could be used for the validation of ionospheric models applicable to the western european sector. the approach was to characterise the trough location and shape in terms of a set of closely defined parameters, building up a database of values from the observations. the parameters were chosen and defined so that it would be possible to obtain estimates of their values in any given situation directly from a model, hence enabling direct comparison with the actual experimental measurements. the basic experimental measurements were of tec, obtained from nims transmissions monitored at three ground stations spanning u.k. latitudes, aberystwyth, hawick and reay. observations for the complete year from september 2002 to august 2003 have been analysed, with vertical tec as a function of latitude being obtained by integration tomographic images. parameters characteris1077 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging fig. 8.8a. median tec versus latitude profiles of trough shape for successive two-hour bands throughout the night under quiet geomagnetic conditions. ing the location and shape of the trough were estimated from the observations resulting in a database of more than 600 examples of trough form and behaviour. a detailed analysis demonstrated that some of the parameters underwent systematic variations with local time and/or levels of geomagnetic disturbance. many previous studies had concentrated essen1078 leonard kersley et al. fig. 8.8b. median tec versus latitude profiles of trough shape for successive two-hour bands throughout the night under moderate geomagnetic conditions. tially on the location of the trough minimum. however, the present work has highlighted other aspects, including the sensitivity of the gradient in tec equatorwards of the minimum in the afternoon and evening to geomagnetic disturbance (kersley et al., 2004). it was found that median values of most of the fundamental parameters in the database, for both tec and latitude, in two-hour bands throughout 1079 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging fig. 8.8c. median tec versus latitude profiles of trough shape for successive two-hour bands throughout the night under disturbed geomagnetic conditions. the night within three ranges of geomagnetic activity, showed stable and consistent variations. in consequence, plots of tec versus latitude were produced to represent how the location, structure and latitudinal shape of the trough varied with time for three different bands of kp. the results are presented in figs. 8.8a-c for low, medium and high kp levels respectively. the plots provide a reliable and con1080 leonard kersley et al. fig. 8.9. comparison of the malvern radar profile (dark line) taken on 18 february 1970 at 14:57 ut, with the profile (grey line) extrapolated according to the reinisch and huang (2001) method. fig. 8.10. comparison between the itec and tec700 parameters calculated using the electron density profiles measured by the malvern radar during nighttime in 1968. 8.9 8.10 sistent dataset with which to test the validity of ionospheric models in the trough region and it is believed that, within the defined criteria, these tec versus latitude profiles represent the best available description of trough form and behaviour in the north-west european sector. 8.7. the itec parameter in general, tec is obtained experimentally from slant measurements between a satellite and ground. however, reinisch and huang (2001) proposed a new technique to estimate the tec of the ionosphere up to 1000 km from ground-based digisonde ionograms. since ionograms only contain direct information about the vertical electron density profile up to the peak of the f2-layer, the proposed method approximated the profile above the peak to an a-chapman function with constant scale height (ht), derived from the bottomside shape at the peak. a new parameter itec was defined to represent the content obtained by integration through the resultant profile. a study was undertaken within cost 271 to try to assess the potential of the itec parameter as a proxy for the total electron content (belehaki and kersley, 2003). a data set of some 4000 height profiles of ionospheric electron density up to 700 km was used. these had been measured by a prototype incoherent scatter radar (isr), which was operated at the mid-latitude site of malvern (52.1°n, 2.3°w) in u.k. for several years around 1970. each bottomside electron density profile was extrapolated to the topside using the reinisch and huang (2001) method and the itec parameter estimated from the integral of the reconstructed electron density with height up to 700 km. in addition, the actual total electron content (tec700) was estimated from the integral of the observed electron density up to the same height. a representative example of the resultant profiles is presented here in fig. 8.9. the scale height at the peak of the f2-layer for this specific case was found to be 50.7 km, a value then used for the scale height in the topside ionosphere. the difference between itec and tec700 1081 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging for this example was found to be only ~1%, though it can be seen that differences in profile shape below and above some 500 km act to cancel each other in the integration. in a statistical comparison between the two parameters good agreement was found, especially at night (belehaki and kersley, 2003). figure 8.10 gives a representative example of such a comparison for profiles measured during nighttime in 1968, a year close to solar maximum. the slope of the bestfit line is very close to unity and the intercept negligibly small. corresponding results for daytime observations showed reasonable statistical agreement, but with a wider scatter of the data points. an earlier study by belehaki and jakowski (2002) had already noted that the itec parameter was subject larger fluctuations during the day than at night. the topside ionosphere undergoes a composition change at some height, where the o+ dominance lower down gives way to the essentially hydrogen-based plasma of the protonosphere. while the transition height has wide variations it is generally higher during daytime. further analysis showed that the differences between the observed and assumed profiles found in fig. 8.9 were more representative of nighttime conditions, with differences in one sense immediately above the peak essentially compensating for those of opposite sign higher up in the integrated itec measurement up to 700 km. the results from the study were encouraging. while some two thirds of the electrons generally reside in the topside, nevertheless the investigation indicated that information obtained from an ionosonde probing the bottomside mid-latitude ionosphere may be of use in obtaining a first-order estimate of total electron content of the whole profile to 700 km, particularly at night. an ongoing study is underway to try to understand the details underlying the correspondence and the limitations to the applicability of the assumption of a constant scale height for the topside profile and the validity of the itec parameter. 8.8. comparisons with nequick tec the studies reported to date have been concerned with experimental measurements of tec. however, an ultimate aim of such investigations is to support the development of models that can be used for the mitigation of propagation effects on practical radio systems. one of the most important aspects of the cost 271 action and its predecessors has been the ongoing development by the groups at graz and trieste of a family of ionospheric models (hochegger et al., 2000; radicella and leitinger, 2001). nequick, is a quick-run model that is particularly tailored for transionospheric propagation applications. it has been used by the esa-egnos project for assessment analysis and is being proposed for single-frequency operation in the european galileo project. it has been adopted by recommendation p531-6 of the international telecommunication union-radiocommunication sector (itu-r) as a suitable method for tec modelling. the basic input parameters of the model are geographic coordinates, epoch, solar activity index and values of fof2 and m(3000)f2, while the output is electron density as a function of height, geographic coordinates and epoch in universal or local time. the model is continuous in all spatial first derivatives, a condition that is necessary in applications like ray tracing and location finding. in addition, it allows the calculation of electron density along arbitrarily chosen ray paths and vertical or slant tec, including the case of satellite-to-satellite links. the topside of nequick is an approximation to a height-aligned diffusive equilibrium. other members of the family of models developed at trieste and graz provide more sophisticated approaches. for example, costprof uses a height-aligned o+-h+ diffusive equilibrium, neuog-plas has a magnetic field aligned plasmasphere above 2000 km and is especially tuned to satellite-to-satellite applications. all three members of the model family can be modulated to allow for smaller-scale static or dynamic structures like the main trough, travelling ionospheric disturbances and other types of depletions and enhancements (leitinger et al., 2002). a version of the nequick model was made available to the cost 271 community for evaluation purposes in mid-2003. a brief study was undertaken to assess the applicability of the model to the u.k. sector by comparing the tec predicted with actual experimental measurements. while the in1082 leonard kersley et al. put drivers for the model are limited to location, month, time and monthly average solar flux, the experimental data set was that from the autumn of 2002 used for the investigations described earlier, covering a wide range of geomagnetic conditions (kersley et al., 2003). the tec estimated by the nequick model up to 20 000 km showed good statistical agreement with the experimental values from the gps observations at the hailsham site (fig. 8.11). the variance was 0.81 and the best-fit line gradient 0.72, though an intercept of 8.3 tecu indicated that there may be a small positive bias to the model estimates. however, given that the model is essentially representative of monthly conditions with no account taken of geomagnetic disturbance, while the actual observations covered a wide range of activity, the broad agreement is encouraging. the study was extended to compare the model output tec up to 1000 km with the experimental measurements from the nims observations covering the latitude range of u.k. the results, replicated in fig. 8.12a,b, showed that for a latitude of 50°n the correlation coefficient was 0.75, with a gradient of 0.64. the intercept of 7.3 tecu was in broad agreement with the conclusion from the study with the gps results that there may be a small positive offset of a few tecu in the nequick model estimates. however, the slope of less than unity appears to indicate that the model may underestimate the tec at the higher levels observed. the comparison between the model output and the nims measurements for 60°n latitude (fig. 8.12b) showed a greater scatter than that for the lower latitude, particularly for the smaller tec values at night. the correlation coefficient was now only 0.48 and the best fit gradient 0.62. however, the increased scatter was consistent with a location in northern u.k. that is in the vicinity of the main trough on most nights. the problems of accurate modelling of the ionosphere in the trough region have already been noted. the intercept of 8.9 tecu was again broadly consistent with those found from the comparisons at lower latitude. it was concluded from this limited study that within the limitations imposed by changing geomagnetic activity the model output showed similar trends to the experimental measurements. a broad correspondence was demonstrated, particularly in the results for 50°n, though it was clear from the spread of the data points that the performance was degraded in the vicinity of the trough further north. fig. 8.11. comparison of tec up to 20 000 km estimated by the nequick model with experimental measurements obtained from gps observations at hailsham in u.k. 1083 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging while there were indications from the intercept of the best-fit lines that the model values may have a small positive bias of a few tecu, the gradients were indicative of a tendency to underestimate the higher levels of tec. 8.9. radio tomographic imaging one of the recent advances in the use of total electron content measurements has been the development of techniques for tomographic imaging of the ionised atmosphere. in a wider sense, the application of tomographic reconstruction techniques has been one of the scientific achievements of the final decades of the twentieth century. while the mathematical basis of the method was formulated early in the century, the advances in computing technology in the 1960s gave practical realization to the use of tomography as an imaging tool. the technique is now hailed primarily for transforming medical diagnostics, but it has also given rise to significant progress in many other fields, including several areas of geophysics. a number of authors have reviewed the application of computerised reconstruction to radio tomographic imaging of the ionised atmosphere. early developments in the field are documented in special issues of the journals international journal of imaging systems and technology (1994) and annales geophysicae (1996), while in further reviews leitinger (1996, 1999) discussed the theoretical basis and the limitations of the ionospheric application. more recently, ionospheric tomography, including descriptions of a number of theoretical formulations, has been the subject of a book by kunitsyn and tereshchenko (2003). in addition, pryse (2003) has reviewed experimental results from the application of radio tomography to ionospheric imaging, concentrating in the main on use of the method in investigations directed towards understanding of the underlying geophysics. another comprehensive review within the context of the cost 271 action (kersley, 2004) focussed on aspects of the use of tomographic imaging as an aid to practical radio science applications. material included in that review will be outlined here in summary only. fig. 8.12. comparison of tec up to 1000 km estimated by the nequick model with experimental measurements obtained from observations of nims satellites in u.k. for latitudes of (a) 50°n and (b) 60°n. a b 1084 leonard kersley et al. the radio tomographic method involves measurement of the slant electron content, the line integral of the electron density, along ray paths from a satellite to a chain of ground stations approximately aligned with the satellite orbital track. inversion of the data set in a reconstruction algorithm yields a pixelised image in two dimensions of the spatial distribution of the electron density throughout the region of the raypath intersections. while many groups worldwide have been involved in the development of radio tomographic imaging and the technique has now been used in many situations, the results presented here of relevance to the cost 271 action are concerned in the main with investigations of the potential usefulness of the method to applications related to practical radio systems. the experimental programme involved reception of the phase coherent signals from satellites in the nims system, in polar orbits at about 1100 km altitude, monitored at a chain of receiving locations aligned essentially in longitude, but separated in latitude. the observations have been made for the most part at locations instrumented by the uwa group in u.k. and scandinavia. the former sites have been used for studies of structures in the mid-latitude and sub-auroral regions, including the main trough, while the measurements at high latitude have been concerned with investigations of signatures of space-weather processes in the auroral and polar ionospheres. the development work carried out in recent years has demonstrated that radio tomography has now matured into a powerful experimental technique capable of providing images of large-scale spatial structures in a wide area of the ionosphere from a limited number of ground stations. the spatial nature of the observations is complementary to those from many other types of experimental measurements of the ionosphere. the limitations to the method were thoroughly investigated and well understood from early simulation studies. these result primarily from the limited-angle geometry of the satellite-to-ground observations so that there is incomplete information in the raw measurements about the vertical profile. however, even at an early stage of the development, algorithms were formulated capable of reproducing ionospheric layers at different heights throughout the image (fremouw et al., 1993). an additional limitation is that the temporal coverage is dependent on the number and orbits of the available satellites with appropriate beacon transmitters. the possible relevance of radio tomographic techniques to a number of areas of practical radio systems is outlined below. a comprehensive account of the most important results has been given in the recent review (kersley, 2004), while further details can be found in the reference cited therein. 8.9.1. tomography and ionosondes the ionosonde remains the basic experimental tool used to monitor the state of the ionised atmosphere for radio systems applications. significant advances have been made in the development of the instrument in recent decades, not just in the electronic hardware and digital processing of the sounding signals, but also in inversion software that can yield electron density versus height profiles below the layer peak in near-real-time. however, ionosondes can only provide measurements for specific locations at discrete times. the spatial imaging capability of the radio tomography technique forms a natural complement to the ionosonde for routine monitoring, with synergies of mutual benefit to both methods. the addition of information about the horizontal structure of the ionosphere from tomographic imaging can be used to place the point measurements from the sounder in their wider spatial context, while inclusion of input from an ionosonde in the reconstruction algorithm can be used to mitigate the limited capability of the tomographic technique to determine heights. a case study was discussed by kersley et al. (1997) that demonstrated the complementary nature of the measurements by the two techniques, which involved observations in the main trough region over europe at a time of extreme geomagnetic disturbance. measurements by ionosondes have also been used to improve the representation of the vertical profile in tomographic reconstructions. discussion of the role of ionosonde input in reconstruction algorithms, used as complementary information to the tec observations, can be found in papers by raymund et al. (1993) and heaton et al. (1995). the former was also the first to include verification of the resultant image of electron density using independent results from the eiscat incoherent scatter radar. 1085 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging 8.9.2. tomography and the validation of ionospheric models a powerful potential application of radio tomographic imaging is in the validation of ionospheric models. dabas and kersley (2003) discussed comparisons between tomographic images over u.k. and corresponding output from three different ionospheric models: the international reference ionosphere (iri-95) (bilitza et al., 1993), the parameterised ionospheric model (pim) (daniell et al., 1995) and the european regional model developed during the cost 238 action (bradley, 1999). the experimental results from observations at a tomographic chain of stations in u.k. in an example presented showed an ionosphere with a clear trough and also a mid-latitude nighttime enhancement to the south. the output from the iri-95 model for the same geophysical conditions failed to replicate either feature, while that from pim did show a trough, but with no localized enhancement to the south. the results from the cost 238 model did appear to show a trough, though because this particular model was designed specifically to cover the european region below 55°n latitude, with default matching to the iri model above that latitude, it was concluded that the feature found was likely to be an artefact of this process. while the work outlined above involved the testing of models that had been developed specifically for radio-systems applications, tomographic images have also been used to assess the ability of a physical model of the coupled ionosphere/thermosphere system to replicate conditions in the high-latitude ionosphere. experimental observations from a chain of satellite stations in the scandinavian sector were used to provide ‘ground truth’ information for comparison with output from the sheffield university coupled thermosphere ionosphere plasmasphere (suctip) model in a study reported by idenden et al. (1998). a subsequent investigation by balthazor et al. (2002) compared the locations of the trough minimum, determined experimentally from tomographic images made under quiet geomagnetic conditions, with those predicted by the model. it was found that the model tended consistently to place the trough at slightly lower latitudes than those identified experimentally from the tomographic observations. 8.9.3. tomography and the mapping of ionospheric parameters maps of ionospheric parameters, on a regional or global basis, provide another tool used to correct for propagation effects in practical radio systems. dabas and kersley (2003) also investigated the potential use of tomographic images in such mapping. they demonstrated that improvements could be achieved to the reliability of maps of the f2-layer critical frequency and peak electron density over europe by incorporating information from experimental tomography into the mapping process. in essence, the electron densities at the layer peak, determined from the image and containing information about latitudinal structures, were mapped in longitude, with zonal gradients from the model being used to create a revised map that was more realistic of the actual conditions. verification of the improvement afforded by the use of tomographic observations in the mapping procedure was obtained by comparing the mapped values with actual measurements of fof2 from the network of ionosondes throughout europe. figure 8.13 shows the results of such comparison for the pim model. it can be seen that inclusion of tomographic input with the pim model improved the correlation coefficient between the mapped nmf2 and actual ionosonde measurements to 0.92, while a smaller coefficient of 0.83 was found when the pim model was used alone. 8.9.4. tomography and oblique ionograms the possible role of radio tomography as an aid to oblique ionospheric sounding was investigated by heaton et al. (2001). a network of oblique sounders was established in u.k. during an experimental campaign, covering the region spanned by a chain of tomographic receivers. the results demonstrat1086 leonard kersley et al. ed that the maximum density at the mid-point of the oblique path estimated by inversion of the oblique ionogram was in reasonable agreement with the density at the corresponding height and latitude in the tomographic image. it was concluded from the study that images of path conditions from the tomographic technique could have a role to play in assessing the applicability of the assumption of spherical symmetry in the reduction of oblique incidence ionograms. 8.9.5. tomography and hf ray tracing the observations from the oblique incidence sounder network in u.k. were also used in a study of the potential of radio tomography as an aid to the ray tracing of hf propagation paths (rogers et al. (2001). the study investigated the effects of various types of input information that can be used to constrain the vertical electron density structure in the tomographic reconstructions. it was found that the use of a fine height resolution and incorporation of input from one vertical ionosonde in the reconstruction process made significant improvements to the overall reliability of the tomographic image. as expected, e-layer propagation was better defined using a climatological model than by the tomographic method. however, the use of tomographic images reduced the rms error in the determination of the f2-layer maximum useable frequency (muf), yielding significantly smaller errors than those obtained from three independent ionospheric models (faim, pim and iri-95). 8.9.6. tomography and hf direction finding warrington et al. (2002) presented observations from an hf direction-finding experiment, on a link between uppsala in sweden and leicester in u.k., where the propagation path was aligned approximately along the orientation of the main ionospheric trough. examples were found on many nights of significant deviations from the great-circle path, with consequent increased times of flight and doppler frequency shifts. in a particular case studied the azimuth of the received signals shows a fig. 8.13. correlation of nmf2 from tomography plus pim maps (left panel) and pim alone maps (right panel) with corresponding values from ionosondes (from dabas and kersley, 2003, reproduced by permission of the american geophysical union). 1087 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging sudden transition from the great-circle direction, followed by a smooth rotation and reversal over a period of several hours before returning to the original direction. in a subsequent study, reported in kersley et al. (2004), images of electron density from the u.k. tomography chain from a succession of satellite passes, revealed a trough with a steep poleward wall that advanced equatorwards during the early part of the night before receding again northwards. the anomalous propagation found in the results from the hf experiment could be explained in terms of reflection by the enhanced densities seen in the tomographic images polewards of the trough minimum. 8.9.7. tomography and space weather many of the processes that can have adverse effects on the performance of radio systems are of particular significance during times when disturbed space weather impacts on the terrestrial environment. radio tomographic imaging of ionospheric structures found at high latitudes has been shown to provide insight into space-weather processes. the convergence of the geomagnetic field close to the earth results in flux tubes containing very different plasma populations, which are characteristic of widely separate regions of space, being brought into close spatial proximity in the dayside cusp ionosphere. in consequence, structures in the ionospheric plasma represent signatures of physical processes that are operating far out in space. tomographic imaging of such spatial features has demonstrated that they contain footprints of important mechanisms in solar-terrestrial coupling. several papers have addressed such issues, including two that illustrate the signatures in the dayside cusp ionosphere of the reconnection processes that link the interplanetary magnetic field (imf) carried by the solar wind to the geomagnetic field. walker et al. (1998) used radio tomography to image features resulting from reconnection near the equatorial plane when the imf was southwards, while pryse et al. (1999) discussed structures linked to lobe reconnection under northwards imf. 8.10. 4d imaging the radio tomographic imaging reported above uses signals from satellites in low-earth orbit, which when received at a chain of stations provide the electron content data from the intersecting ray paths for the inversion process. a recent development has been the extension of the underlying principles of the fig. 8.14. (left panel) frame from a movie of vertical tec (in tecu) over europe obtained from the inversion of gps data recorded about 22 ut on 12 august 2000; (right panel) corresponding plot of electron concentration (×1011m-3) at 15°e longitude. 1088 leonard kersley et al. method to gps satellites. simultaneous measurements are made by a single receiver along the slowly varying ray paths from several gps satellites. while the ionosphere can be taken to be stationary during the period of a few minutes of data acquisition from a leo satellite, this assumption is no longer valid for gps. however, a method has been developed that allows for inversion of the greatly increased quantity and angular coverage of measurements from gps signals monitored at a widely distributed network of receiving sites. the midas (multi-instrument analysis technique) inversion method has been described in detail by mitchell and spencer (2003). in summary, the technique uses measurements of slant electron content obtained from dual-frequency differential phase observations of gps satellites at a network of receivers to produce hour-long movies of the electron concentration, in what is essentially a 4d inversion. the inversion uses a linear, least-squared approach, with the algorithm incorporating a priori information about the evolution of the electron concentration during a specified period of time. an example of output from the method is shown in fig. 8.14. the left panel gives the vertical tec over much of europe, estimated by vertical integration of the electron concentration in the image of one particular frame. a longitudinally extended depletion, corresponding to the main trough, can be seen over the u.k. and northern europe at this time. the right panel shows the corresponding height versus latitude plot of the electron concentration along the 15°e longitude. it can be seen that the location of the trough minimum was well to the south at about 55°n, consistent with the significant geophysical disturbance on the day in question. 8.11. conclusions total electron content is a key parameter for the mitigation of ionospheric effects on practical radio systems. the paper has reviewed recent progress in studies carried out within the cost 271 action on the estimation of tec and the use of such measurements in ionospheric imaging. results have been outlined from a number of investigations directed towards the validation of observations and estimates by different techniques with a view to demonstrating consistency and hence confidence in the products. most of the work has been concerned with the ionosphere over north-west europe where the main trough represents a significant feature on many nights. the term total electron content is often used to refer to the equivalent vertical tec obtained from slant ray path measurements between a satellite and ground using a geometrical conversion based on an assumed thin-shell ionosphere at given height. studies demonstrated that the choice of centroid height is critical to such measurements, leading to the conclusion that the accuracy of such equivalent vertical tec estimates can never be relied on to better than a few tec units. integration through tomographic images was found to yield marginally more consistent values of vertical tec, particularly in regions of steep ionospheric gradients like the vicinity of the main trough. comparisons were made between tec values estimated from nims observations with those obtained from gps satellites, both in dedicated monitoring and use of routine data from igs sites. the results demonstrated a broad consistency between the measurements within the known limits, allowing for the contribution from the protonosphere on the much longer gps ray paths. however, a significant departure was noted and attributed to differences in observational geometry through steep gradients linked to the collapse of the ionisation on the first afternoon of an impulsive storm. inability accurately to replicate such gradients was also noted as a limitation to the current state of development of maps of tec over europe available for applications purposes. in another study, the regular gradients associated with the morning rise and evening fall in ionisation and also the normal latitudinal structure during daytime were shown to be linked to discrepancies in tec estimates at the level of a few tecu. it is clear from this series of investigations that in general the gps and nims techniques yield consistent values, though further study is needed of the circumstances leading to potentially serious errors at times of severe geomagnetic disturbance when localised gradients associated with storm enhanced density structures are found over north-west europe. 1089 total electron content – a key parameter in propagation: measurement and use in ionospheric imaging a brief study was made of the itec parameter using historic ionospheric profile data from an incoherent scatter radar operated in u.k. it was found that there was reasonable agreement between the actual tec and that estimated by the itec method up to 700 km for nighttime measurements, but poorer agreement by day. there were indications that the assumption of a constant scale height resulted in differences in the profile that were in a particular sense immediately above the peak, but which cancelled with opposite errors higher up in the integrated estimate. however, the study indicted that application of the itec method to ionosonde data had the potential to provide first order information about the tec magnitude. in another limited investigation comparison was made between actual measurements of tec in the u.k. sector and estimates from the nequick model. reasonable correspondence was found at 50°n in the essentially mid-latitude ionosphere. however, the agreement between model and experiment was less good at 60°n, in a region where the main trough is a dominant feature of the ionosphere on many nights. a systematic programme of experimental observations was carried out in u.k. as part of the project to determine the latitudinal structure of the tec and its variations in the vicinity of the trough. the approach was to characterise the form of the trough in terms of a set of defined parameters that could be replicated in a model for comparative purposes. median values of the parameters from measurements made over a complete year have been use to create plots of tec versus latitude for different times throughout the night within three bands of geomagnetic disturbance. this data set is now available as an output from cost 271 for the testing of models in the trough region. an important application of total electron content measurements, which has been developed in recent years, has been radio tomographic imaging of ionospheric electron density. the paper reviewed progress in the use of the technique, particularly in relation to applications as an aid to practical radio systems. the potential roles of radio tomographic observations as complementary to ionosondes, validating both empirical and physical ionospheric models and improving the mapping of structures in the regional ionosphere, were discussed. additional results were also described, involving possible uses of the technique to aid oblique sounding and hf ray tracing and direction finding, together with applications a high latitudes in mapping signatures in the ionosphere of space weather processes. a recent extension of radio tomographic inversion to use data from a network of gps observing stations to create moving images of the ionosphere in three spatial dimensions and time was described briefly, with an example frame being shown from the european sector. the various studies outlined here, carried out under the broad remit of the measurement and use of total electron content, have demonstrated a broad consistency between results from the different types of measurement, within the known limitations. however, further work is needed in key areas, particularly in relation to the reliability of both measurements and resultant products at times of steep gradients in the mid-latitude ionosphere during the onset phase of geomagnetic storms. in addition, the ability of models to replicate conditions in the vicinity of the main trough also needs investigation, though the present project has been able to provide a data set that is now available for validation. acknowledgements the assistance of many organisations, groups and individuals in the investigations described here is acknowledged with grateful thanks. particular mention must be made of financial support towards cost 271 activities given by the u.k. radiocommunications agency (now part of ofcom). references balthazor, r.l., m.h. denton, l. kersley, s.e. pryse, i.k. walker, r.j. moffett and g.j. bailey (2002): the high-latitude dayside trough under quiet geomagnetic conditions: a comparison of modelled and experimental observations 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(2004): ionospheric tomography and its applications in radio science and geophysical investigations, ann. geophysics (in press). kersley, l., s.e. pryse, i.k. walker, j.a.t. heaton, c.n. mitchell, m.j. williams and c.a. willson (1997): imaging of electron density troughs by tomographic techniques, radio sci., 32, 1607-1621. kersley, l., d. malan, s.e. pryse, lj.r. cander and r.a. bamford (2002): progress towards verification and validation of tec observations over europe, in proceedings of the cost271 workshop, 2-4 october 2002, faro, portugal. kersley, l., d. malan, s.e. pryse and lj.r.cander (2003): further investigations on the verification of tec measurements by different techniques, in proceedings of the 3rd cost 271 workshop, 23-27 september 2003, spetses, greece. kersley, l., d. malan and s.e. pryse (2004): study into the effect of the upper atmosphere on terrestrial and earth-space communications, report to u.k. radiocommunications agency/ofcom, university of wales, aberystwyth (in preparation). kunitsyn, v.e. and e.d. tereshchenko (2003): ionospheric tomography (springer-verlag). leitinger, r. 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doi: 10.4401/ag-7801 “stepping into the equatorward boundary of the auroral oval: preliminary results of multi scale statistical analysis„ massimo materassi*,1, mirko piersanti2, giuseppe consolini3, piero diego3, giulia d’angelo4, igor bertello3, antonio cicone5 (1) istituto dei sistemi complessi, consiglio nazionale delle ricerche, firenze, italy (2) istituto nazionale di fisica nucleare – sezione di roma “tor vergata”, rome, italy (3) istituto di astrofisica e planetologia spaziali, rome, italy (4) dipartimento di matematica e fisica, università degli studi “roma tre”, rome, italy (5) disim, università degli studi dell’aquila, l’aquila, italy 1. introduction auroral ovals (ao) are the regions located in the northern and southern high latitude ionosphere where the main particles coming from magnetosphere precipitate [feldstein, 1963; feldstein et al., 1969; akasofu, 1966]. these particles interact with the atmospheric atoms and molecules giving rise to luminosity in both visible and ultraviolet spectra. at ionospheric altitudes, the aurorae appear as continuous and luminous bands surrounding both geomagnetic poles [frey, 2007]. moving from the large number of observations and models, feldstein [1963] and khorosheva [1967] suggested the concept of “auroral oval”, that high-latitude region between 65°n(s) and 78°n(s) where aurorae are generally observed. essentially, the ao consists in two parts: the discrete oval and the diffuse oval. the last one lies mainly equatorward of the discrete oval, while diffuse precipitation, although often not visible, appears also poleward of the discrete oval. the discrete arcs are more dyarticle history receveid june 4, 2018; accepted january 29, 2019. subject classification: space weather; plasma physics; dynamics; statistical analysis; data processing. abstract this work aims at showing how analysis techniques, designed to highlight the multi−scale structure of a signal, may be of help in de− scribing fluctuations of the ionospheric medium. in particular, the technique used here is the alif (adaptive local iterative filter). we have applied alif to the characterisation of plasma irregularities in the equatorward boundary of the auroral oval. the data used are from the leo satellite demeter (detection of electro−magnetic emissions transmitted from earthquake regions) while it crosses the equatorward boundary of the oval. the time series analysed are those from the electric field instrument ice (instrument de champ electrique) on board of demeter, and the identification of the equatorward boundary in the time series of in situ data is based on the comparison between the satellite trajectory and the picture of the auroral oval from the simultaneous auroral imaging by the dmsp satellite. in detail, we pre− sent an analysis of the multi−scale features and dynamics of ionospheric plasma along the demeter trajectory in different regions, which seem to be characterized by plasma irregularities of different nature. a clear change of the multi−scale nature of plasma fluctuations is observed in the correspondence of the irregularities originated by particle precipitation in the equatorward boundary of the auroral oval. namic in space (size, shape and location) and time (from second to minutes) than the diffuse oval [borovsky, 1993]. naturally, the dynamics and the position of the aos strictly depend on the geomagnetic activity level. in fact, its equatorward boundary (ewb) location depends on both the energy of the precipitating particles and on the magnetospheric electric and magnetic fields [kauristie et al., 1999; villante and piersanti, 2010; piersanti and villante, 2016; piersanti et al., 2017]. notwithstanding, its poleward boundary is defined as the separation between the closed field lines and the polar cap. the boundaries of the ao can be determined either by ground-based observations [moen et al., 2004; aikio et al., 2006] and by low earth orbit (leo) satellite observations [wang et al., 2005]. in addition, global view snapshots of the ao can be provided through optical images taken from high-altitude satellites [østgaard et al., 2007]. on this topic, using the dmsp (flowing at around 850 km at sunsychronous orbit) data, newell et al. [2009] modelled the monoenergetic discrete aurora and the ao boundaries. closely related to the ao location, a current system, which is directly driven by the interaction between the solar wind and the plasma magnetically bounded to the earth, continuously flows along geomagnetic field lines into and out of the ionosphere (field aligned currents – facs). iijima and potemra [1978] found that fac sheets are aligned with the boundary of the ao during quiet geomagnetic conditions, while distortions of this alignment occur during disturbed periods. xiong et al. [2014a] developed an efficient algorithm to detect the auroral boundaries from fac signatures by using 10 years observations of the magnetic field from champ (challenging minisatellite payload). in a successive statistical analysis, based upon the same champ dataset, xiong et al. [2014b] developed an empirical model able to efficiently determine both the equatorward and the poleward boundaries of the ao. given its high variability and richness on many space and time scales, the ao can be conceived as a spaceand time-multiscale complex dynamical structure. still, it is well localised conceptually in terms of geomagnetic line morphology: in fact, its ewb is where the geomagnetic lines change between dipolar lines and those stretching several earth radii into the geomagnetic tail of the earth. the ao poleward boundary (pwb), instead, is where the geomagnetic lines stop to be closed (relatively near the earth), and open deeply in the near-earth space and solar wind, reconnecting with the interplanetary magnetic field. crossing the ewb, as one moves poleward from the region of dipolar geomagnetic field lines to that where geomagnetic lines stretch into the earth’s magnetic tail, the local physical quantities φ(x,t) describing the ionospheric plasma (e.g., the electron and ion densities n e (x,t), n i (x,t), or their local temperatures t e (x,t) and t i (x,t)) will change. specifically, the quantities φ(x,t) pass from the values characteristic of the plasma populating the low and mid latitude ionosphere (or plasmasphere, depending on the height of the trajectory one describes (vellante et al., 2014a,b)), to those pertaining to the plasma coming from the magnetic tail. moreover, in the correspondence of the ewb, the static plasma of the dipolar region gives place to plasma fluxes arriving from the tail, so a strong shear in plasma velocity is expected, possibly producing instabilities, and hence strong fluctuations. thus, across the ewb, local plasma quantities φ will change not only in their “background” value φ 0 but also in their “fluctuating” part δφ, so that one has φ = φ 0 + δφ. the local proxy will change in terms of fluctuation-to-background ratio δφ/φ 0 and, possibly, of the statistical distribution of these δφ values. plasma boundary layers, as the ewb, are characterised by a major relevance of irregularities (i.e., the ratio δφ/φ 0 ) because turbulent regions are met: one often refers to turbulent boundary layers, indeed. boundary layers are very interesting “mixed” regions. in fact, because of the high gradients to which the plasma undergoes, conditions of strong non-linear coupling between many degrees of freedom hold. this makes the plasma motion definitely nonlaminar and instabilities give rise to irregularities at many different time and space scales. as the dynamical features of different time and space scales (e.g., particle and kinetic effects, turbulent structures, continuum excitations) interact with each other, single approach treatments may result unsatisfactory: this is the real complexity of such an environment. the characterization of plasma proxies φ = φ 0 + δφ with fluctuations behaving differently at different scales, i.e. a multi-scale statistical analysis (mssa) of the δφ local fields, may open new perspectives in the comprehension of this complex dynamics. in the present paper, we study the behaviour of different scale components of a suitable plasma proxy measured along the trajectory of a leo satellite, as specified below. as part of the selected proxy time series φ(t) of the in situ measurements lays within the ewb of the ao, and part outside of it, our approach allows for a comparison between the two plasma regions in terms of mssa. in detail, we examine some intervals of in situ data relative to the floating potential v float collected by the ice (instrument champ electrique, berthelier et al., (2006)) experiment onboard of the demeter (detection of electro-magnetic emissions transmitted from earthquake regions) satellite [lagoutte et al., 2006], when the materassi et al. 2 3 multi scale analysis of the auroral oval satellite was entering the northern ao crossing the ewb plasma. this physical quantity (i.e., our φ(t) = v float (t)) is an indirect proxy of the local plasma features and dynamical state. the floating potential v float (t), even if not trivially related to the local plasma quantities (more directly to n e and t e ), is expected to be rather faithful to them, both in terms of its trend behaviour and fluctuations. moreover, the high rate sampling of v float (t) allows to study the tiny details of the irregularities met. this grants to highlight how the passage between the topsideplasmaspheric environment and the ewb plasma is neither a sudden jump nor a smooth transition, but rather a midland in which it is possible to meet, sporadically but clearly, plasma features and structures characterizing both the two regions. if the use of a pictorial image is allowed, this is roughly what the driver meets when the car starts to enter a rainy region, and sees the first droplets on the windshield before the pouring rain envelops the car (of course, in the case of a satellite entering the ewb, the droplets are the first irregular localized plasma structures of the ewb). as already anticipated, the proxy v float (t) is multi-scale analysed via the alif technique [piersanti et al., 2018]. we remark that the analysis in this work is not intended to diagnose the entrance of the satellite into the ewb, but rather to introduce a tool to compare the multi-scale statistical dynamics of the plasma in different points of the trajectory that are presumed to be located in different plasma regions. in fact, the identification of the ewb in the v float (t) time series is based on the comparison between the satellite trajectory and the picture of the ao from the simultaneous auroral imaging of the dmsp satellite, see section 3. the paper is organized as follows. in section 2 the data analysed and the method used are presented. the quantities calculated for the analysis proposed are illustrated in some detail. section 3 thoroughly describes the 4 case events at hand, giving detailed explanations of the general appearance of the time series v float (t) analysed, and sketching the near-earth space context from which it comes. in section 4, the results of our analysis are presented and duly commented; then, some conclusions are drawn and future research directions sketched. 2. data and methods in this section we describe the dataset used in our analysis, which are relative to the demeter mission, launched on june 29, 2004 and devoted to study the earth’s ionosphere and the disturbances due to seismic activities. let’s shortly remind that the demeter satellite orbits on a sun-synchronous orbit, located at 10:30 lt (local time) and at 22:30 lt, with an inclination of about 98.23° and an altitude of about 700 km. the demeter satellite performs 14 orbits per day and measures continuously between -65° and +65° of geographical latitude, and its payload includes, among others, electric field sensors (ice), magnetic sensors (imsc), and langmuir probes (isl). here, we make use of the ice experiment data. 2.1 ice demeter measurements in order to investigate the high latitude ionospheric fluctuation features we analyse data from the electric potential of the ice (instrument champ electrique) probe on demeter while crossing the northern high latitude ewb layer [berthelier et al., 2006; badoni et al., 2018]. in addition, the correspondent langmuir probe measurements are used to detect plasma parameters (ion n i and electron n e density, and electron temperature t e ) which at the demeter orbit according to the international reference ionosphere (iri) model should range as follows: plasma density n i = n e between 5·109 and 5·1011 m-3; electron temperature t e between 2000 k and 3500 k. ice probes acquire the floating potentials (v float ) in ulf band at a frequency of 39 hz [berthellier et al., 2006]. in general, the floating potential follows the fluctuations of plasma potential (v pl ). in the case of the ice probes, four contributions are relevant: electron collection, ion collection, photoelectron emission, and the current injected to the probe by the embedded current bias source. the behaviour of v float is modelled by the rates of particles collections as described by the equations reported in diego et al. (2017). in an ideal case, plasma density variations do not affect v float since ion and electron collections have equal and opposite dependence from n, and bias and photoelectron contributions could be considered almost negligible at locations along the demeter orbit. however, in case of strong plasma density reductions, such contributions could be of the same order of electron current and, therefore, more effective in moving positive the v float . demeter ice probes have been developed for an accurate (of the order of µvs) measurement of the floating potential with the aim to detect electric fields in the range dc-4 mhz. sensors are designed to operate at plasma densities greater than 1010 m-3. for this reason, ice time series are very suitable for the study of rapid changes in ionospheric environment, such as the discontinuities of auroral zone. on the other hand, langmuir probe measurements remain of good reliability but are not adequate for the monitoring of such small structure in the high latitude plasma. 3 materassi et al. 4 2.2 data segments analysed the four case events considered are the ones reported in table 1 below, namely: january 1, 2005; january 1, 2008; june 1, 2010; july 2, 2010. we choose this data segments, because of their almost homogeneous geophysical conditions (in terms of ae and dst indices), ensuring demeter satellite passing through the ao. 2.3 multiscale statistical analysis (mssa) in many dissipative dynamical systems outside of equilibrium, the observed fluctuations are very large and result from the interaction of a high number of degrees of freedom. when such an interaction yields multi-scale and chaotic spatio-temporal field fluctuations, and involve a nonlinear transfer of energy among the different spatio-temporal scales (e.g., via a direct and/or inverse cascading mechanism as for instance the richardson’s cascade), the system is generally said to display turbulent dynamics. more in general, quoting falkovich [2008], a turbulent state can be defined “as a state of a physical system with many interacting degrees of freedom deviated far from equilibrium. this state is irregular both in time and in space and is accompanied by dissipation”. the present analysis of the time series of the ionospheric proxy v float (t) is inspired by the tools used in the study of turbulent media, which manifest large amplitude multiscale fluctuations and high spatial gradients due to the couplings of their components [frisch, 1995]. the study of signals collected probing turbulent media often deals with highly erratic time series x(t) = x 0 (t) + δx(t), and hence must be treated statistically (in the foregoing expression, x 0 (t) is the “low pass component” of x(t), while fluctuations δx(t) are generally considered to be the “high pass component”). the statistical analysis are well based for data intervals within which the statistics of the signal fluctuations δx(t) is stationary. hence, the first step to apply this reasoning to the study of our fluctuating time series v float (t) is to select sub-intervals of the whole time series within which fluctuations δv float (t) have a stationary statistics. as a suitable interval i for our analysis is fixed, the statistics of the fluctuations δx(t) are studied at different time resolutions, in order to single out those different scale-fixed degrees of freedom interacting non-linearly in the complex medium. in particular, the differences δ τ x(t) = δx(t+τ) – δx(t) are studied separately for each τ , so to obtain the probability distribution functions p i (δ τ x) for different values of τ. the dependence on τ of the statistics of the values δ τ x(t) will give information on the physical mechanisms governing the fluctuations δx(t). in the following, we will refer to this study of the dependence of p i (δ τ x) on τ as multi-scale statistical analysis (mssa) of fluctuations. as a first step, the mssa requires a multi-scale decomposition of the signal δx(t), giving rise to the series δ τ x(t). this decomposition may either be done through the simple definition δ τ x(t) = δx(t+τ) – δx(t), or via some more formal decomposition of δx(t) into “independent” components δ τ x(t) with characteristic variability length ≈ τ, being 〈ψ 〉 the time average of the quantity ψ(t) over the time interval of stationary statistics for ψ. such decomposition into scale-fixed independent modes is generally obtained as ( 1 ) the collection of functions w τ,t0 is a set of independent modes, each of has ≈ τ, with support centred in t 0 . the coefficients g(τ,t 0 ) give the relevance of the -sized mode in a neighbourhood of t 0 to the composition of δx(t). without loss of generality, the expression (1) may be constructed with orthonormal w τ,t0 , so that: (2) (the deltas in (2) are kronecker symbols, being the set {w τ,t0 } necessarily countable in data analysis). the mssa consists then in the characterization of how the distribution of those values depends on τ, i.e. how p i (δ τ x) changes with τ. the functions w τ,t0 (t) in (1) may be of various nature, basically equivalent. adaptive local iterative filtering (alif), where w τ,t0 (t) is a set of signal-adapted functions, was used in piersanti et al. [2018] to obtain multi-scale filters as (2) for gps radio scintillation signals. in that work the authors put in evidence ẇ τ,t0 w τ,t0 δ τ,x δ τ,ẋ table 1. a table resuming the helio−geophysical proxies of the case events analysed. case event ut ae index dst index january 1, 2005 6.3 < ut < 6.48 500 nt -13 nt january 1, 2008 14.5 < ut < 14.7 125 nt -17 nt june 1, 2010 6.7 < ut < 7.0 310 nt -30 nt july 2, 2010 14.2 < ut < 14.5 425 nt -16 nt x(t)=ʃ t 0 g(τ,t 0 )w τ,t0 (t), ∫ w*τ,t0 (t)wτ′,t ′0(t)dt=δt0,t ′0δτ,τ′ tf ti δx(t) = ʃ τ δ τ x(t) δ τ x(t) = ʃ t 0 g(τ,t 0 )w τ,t0 (t) 5 multi scale analysis of the auroral oval how mssa from discrete wavelets and alif give roughly the same diagnosis for the turbulent signal examined. in the present paper, the alif technique has been adopted to construct all the -dependent statistical functions (hereafter called intrinsic mode components imcs). we have however to remark that using the above definition of δ τ x(t) is not exactly the same of that used in the classical approaches to the study of turbulence media (frisch, 1995). indeed, here we do not deal with increments at a fixed scale τ but, rather, with the fluctuation contribution at a certain characteristic scale τ. this approach is quite different and it has been already applied in the framework of turbulent space plasma studies via similar decomposition techniques [see, e.g., consolini et al., 2017]. the distributions p i (δ τ x) can be characterized via some quantities (cumulants) describing their shape: each statistical parameter obtained from p i (δ τ x) will depend on the scale τ at hand (and on the interval). in order to check the τ -dependence of p i (δ τ x), its first statistical momenta are usually considered: in particular, the second cumulant and the fourth normalized one: the variance σ i 2 (τ) and the kurtosis k i (τ). the normalized kurtosis, or better the difference k i ex(τ) = k i (τ)–3, indicated as excess of kurtosis, represents how the distribution tails differs from those of the gaussian distribution. it may also make sense to calculate the shannon entropy (3) which can provide a measure of the “degree of randomness” of the fluctuations δ τ x. furthermore, in order to highlight how the τ -th component δ τ x stands out at a particular time t in terms of “energy”, the ratio ε rel (t,τ) between the quantity |δ τ x(t)|2 and its integral over the scales is calculated: ( 4 ) the quantity, named local intermittency measure (lim) [farge, et al., 1990], is widely use in turbulent studies to identify the signal structures which are responsible for the occurrence of intermittency. in order to stress even more the (time) intermittency of the series δx(t), the square of lim is calculated, and simply named lim2: ( 5 ) the two quantities, ε rel (t,τ) and lim2, allow for locating the sites where the fluctuation squared amplitude (energy) exceeds the average one or the intermittent events (lim2 > 3). thus, while the variance, kurtosis and shannon information of p i (δ τ x) are related to global features of the signal in the interval i as seen at the scale τ, the lim ε rel (t,τ) in (4) and the lim2 in (5) rather define local properties of the δ τ x at time t. in what follows, the mssa is applied to the time series v float (t) collected by the ice instrument on board of demeter. in detail, we show a collection of results from the different tools mentioned above. furthermore, each time series v float (t) is first subdivided into two intervals: the “pre” interval, pertaining to the plasma within the dipolar geomagnetic field; the “post” interval, where the satellite crosses tailward stretched geomagnetic lines. two sub-series v (t) and v (t) are selected accordingly, and the mssa difference between their features are highlighted. 3. results of the analysis in this section we present in detail the results about multi-scale analysis of the case events mentioned in session 2. 3.1 the january 1, 2005 case event figure 1 shows the ice and isl observation during the demeter orbit segment between 5.88 ut and 6.48 ut. the offset observed between n i and n e is an instrumental effect. in fact, langmuir probes (lp), performs the various plasma particle collections at different polarization level (i.e. current collected along a sweeping voltage applied) to obtain the so called i-v characteristic from which it is possible to derive plasma parameters (chen, 1977). since the relevant currents values are known to have different slope, the computational uncertainty of v pl detection gives rise to a computational difference between n i and n e . in general, it is expected that v float (t) (panel 3) follows the theoretical floating potential (v f,th , panel 4), obtained through the plasma data measured by lp as reported in diego et al. (2017). for the event under investigation it does not happened. in fact, while before 6:43 ut (vertical purple dashed line), v float (t) is characterized by an almost monotonichal increase and presents an unexpected growth at about 6.2 ut, v f,th shows an almost constant value (≈ 1v). this inconsistency is related to the concurring contribution of highly plasma variations (see the large enhancement of about 3v, occurring between 6.2 ut and 6.43 ut), that changes the particles collection mode of the probe, and to the motion of the satellite across pre float post float lim2(t , τ) = |δ τ x(t)|4 〈|δτ x(t)|2〉 2 t ii (τ) = –ʃ δ τ x pi (δτ x)log2 pi (δτ x) ε rel (t , τ) = |δ τ x(t)|2 ∫ dτ′|δτ x(t)|2 an increasing geomagnetic field. the first effect increases the ratio λ d /r probe , invalidating the above mentioned thick sheath approximation. the second effect reduces the electron gyroradius, causing a no more isotropic collection of electrons by the probe. the consequence is a positive growth of v float (t). interestingly, between 6.3 ut and 6.43 ut, the plasma densities (panel 2, blue and green lines) and the electron temperature (panel 2 red line) show a sharp decrease and a sharp increase, respectively. this behaviour is explained by the crossing of demeter satellite through the “ionospheric trough” [hunsucker and hargreaves, 2007]. finally, the sharp decrease of plasma density appearing at 6.2 ut, is clearly a seasonal effect caused by the equator crossing of the satellite, passing from summer to winter [whitteker et al., 1978; sojka, 1982]. the same but more marked situation appears in v float (t) materassi et al. 6 figure 1. demeter ice and isl data relevant to the 5th orbit of january 1th, 2005. top panel shows satellite latitude and lon− gitude, second panel display the ion and electron density (blue and green curve respectively), the electron temperature and its standard deviation (red curve and red dotted curve respectively) computed every minute. third panel shows one ice probe floating potential (black), along with plasma potential (orange) and s/c potential (red). bottom panel shows the theoretical curves obtained with each current computed with the correspondent density (i e computed by using n e , and i i by using n i ). time is reported in decimal units ut. the vertical purple dashed line represents the time of 6.43 ut. after 6.43 ut. in fact, it is first characterized by fluctuations whose amplitudes increases with increasing latitude, and then, it shows huge and sudden variations. at the same time, the relative plasma parameters show a strong decrase of the electron temperature, coupled with a decrease in the plasma density. t e depends on the transition region slope as well as the probe-plasma impedance. thus, since the current of particle collection directly depends on their number, it is normal to observe enhanced and noisy temperature values in an extraordinary lowdensity plasma. on the other hand, in such condition the computational differences in n e and n i tend to disappear as can be seen in figure 1 (third panel), since the electron and ion currents slopes become similar, and the uncertainties of δi/δv ratio near the inflection point (v pl ) are similar as well. the peculiar behaviour of v float , n e and t e after 6.43 ut can be related to the entrance of the satellite into a ionospheric boundary. the geographical position of the satellite between 6.43 ut and 6.48 ut renders it sensible to expect the satellite to be entering the ewb of the ao, as confirmed by figure 2: there, the superposition of the demeter orbit (red line) on the ao observations detected by dmsp satellite is reported. the selected orbit arc (figure 2) refers to the last 2 minutes (about 800 km) of the analysed data segment, precisely the interval corresponding to the irregularity in the lp data. comparing the trace of the red arc in figure 2 to the time series of v float (t) reported in figure 1, it is visible that the occurrence of the peaks of v (t) takes place exactly at the latitudes of the outermost auroral oval. a point has to be clarified in this respect. the demeter satellite flies at 700 km, that is well above the region from which auroral emission comes. that emission is produced by the particle precipitation along the geomagnetic field lines: we believe that the irregular structure met in the time series of data from its lp v float (t) corresponds to the plasma irregularities upstream the emission origin region, caused by the same particle precipitation switching on the aurora below. on on the basis of the v float (t) behavior, we divided our observations into two time series: the first one (v (t)), which starts at 6.3 ut and terminates at 6.43 ut (outside the ao); the second one (v (t)), which begins at 6.43 ut and finishes at 6.48 ut (inside the ao). 3.2 mssa results to analyze the multi-scale statistics of the in situ irregularities of the medium corresponding to the pouring particle precipitation determining the auroral oval, v float (t) between 6.3 ut and 6.48 ut is considered, concentrating, however, on v (t) (figure 3). as the time series v float (t) is reported in panel (a) of figure 3, its subseries v (t) and v (t) are singled out by glance. when entering the ewb the v float becomes more spiky, as it is clear from the middle panel (b) where v (t) is reported, and the difference between adjacent elements of time, δv (t), reported in the bottom panel (c) (that is useful to highlight apparent intermittency). time-scale analysis is possible for all these time series, which turns into being a space-scale analysis because of the satellite-plasma relative motion. it is worth specifying that a trend has been subtracted from the time series of figure 3 in order to highlight the fluctuations, whose analysis represents the core of this paper. in addition, we can neglect the baseline behaviour in because all its physical proprieties are already identifiable in its detrended version. the plot of the time series v float (t) in figure 3 (panel a) indicates at a glance how an irregular structure follows the smoother part of the series. a first simple but effective way of comparing the two data segments v (t) and v (t) is to calculate their power spectral density (psd) via fourier analysis showed in the panels (a) and (b) of figure 4, respectively. the spectrum of v (t) in figure 4a is characterised by a strong peak at 1 hz on the top of a smooth power spectrum background representing rather weak noise; the 1 hz peak is due to an experimental artefact of the langmuir probe (lebreton et al. 2006; logoutte et al., 2005). post float pre float post float post float post float pre float post float post float post float pre float pre float 7 multi scale analysis of the auroral oval figure 2. the trace of demeter’s orbit (red) drawn on the top of the auroral oval map on january 1, 2005. the arc of orbit spans from ut 06:23:56 to ut 06:28:29, and it is superimposed on the map of auroral oval emis− sion (in kilo−röntgen) taken at ut 06:27. the figure 4b plot represents instead the psd of v (t): the time series collected in the ewb plasma is a double power-law spectrum with two distinct regimes, i.e., a lower frequency power law with exponent α 1 = -1.48±0.42, and a higher frequency power law with exponent around α 1 = -2.69±0.22. this kind of behaviour, definitely pointing towards multi-scale dynamics, is not easily recognizable in terms of well-known and accepted theories. this is a preliminary indication that in the ewb it is very likely that some plasma instability may generate a multi-scale power-law spectrum. furthermore, the power-law character of psd suggests that scale invariance may be a peculiar feature of such fluctuations. however, this point requires more analysis that are outside the scope of this work and are demanded to a future more detailed study (see section 4 for more comments). when v float (t) is alif-decomposed, a change of regime is observed at t* = 6.43 ut, becoming apparent as a multi-scale element. figure 4c and figure 4d, report the relative energy ε rel (f,t) for the whole time series v float (t) and lim2 (f,t), as functions of the equivalent instantaneous frequency f (assuming f = τ -1, see piersanti et al. (2018)) and time t, respectively. the ε rel (f,t) behaviour (figure 4c) shows that the energy distribution is definitely different before and after t*. before t* the whole segment is characterized by the strong f =1hz component, while after t* a strong f =2hz component appears. looking at figure 4d, reporting the lim2 (f,t) of the v (t) time series, at around 10 hz, smaller and sporadic plasma structure appear where the intermittency is highly concentrated. the proper analysis of multi-scale statistics of v float (t) is performed by analyzing the two aforementioned segments v (t) and v (t). each segment is assumed to be statistically homogeneous (which is true in first approximation, and however their internal statistical variability is much less that the difference between the two ones), post float post float pre float post float materassi et al. 8 figure 3. from top to bottom: the whole time series (panel a) of the floating potential, the sub−series v post (t) (panel b) collected inside the ewb, and the difference between adjacent elements of time, δv post (t) (panel c). float float float float allowing its alif decomposition and mmsa analysis in a time-independent way. then, the statistical parameters ε rel (f), k(f) and i(f) are calculated for each component, of equivalent fourier frequency f, and reported as functions of f. the resulting plots for the smooth part of the v (t) series are reported in figure 5, while, in figure 6, the results for the irregular part of the series v (t) are shown. here, the time does not appear as a variable anymore, because the statistics of the whole (presumed homogeneous) time series v (t) and v (t) have been considered together. by comparing figures 5 and figure 6, three main features may be highlighted as important characteristics of the v (t)-versus-v (t) “transition”. first of all, as already visible in figure 4, the energy becomes differently distributed along the scales, so that smaller structures appear in the irregular portion of the crossed medium. moreover, while in figure 5 the function k(f) is almost flat, with an increase between 3x10-1 hz and 1 hz, in figure 6 k(f) is growing monotonically for frequencies higher than 1 hz, indicating that the smaller the structures is considered, the less-gaussian distribution they show. in the presence of fully developed turbulence, which is not ascertained to be the case here, this would be safely stated as a condition of anomalous scaling, i.e., of intermittency [frisch, 1995]. last but not least, in the comparison between the multi-scale statistics of the smooth and the irregular parts of the medium (figures 5 and figure 6, respectively), the i(f) variability as a function of frequency f has to be considered. an important feature is that the kf) and i(f) slopes seems to be “anti-correlated” in both cases. in addition, in both cases, i(f) are different according to the difference between the two excesses of kurtosis. the “anti-correlation” of i(f) with respect to k(f) denotes that, as the f-fixed fluctuations depart from the gaussian distribution and their kurtosis grows, they become less random showing a smaller shannon entropy. this anticorrelation suggests that the shape of the pdfs of the fluctuations acquires a more leptokurtotic feature by decreasing the scale τ (as shown in figure 7). last focus on the irregular behaviour of plasma at the ewq of the auroral oval is reported in figure 7, where the pdfs p(x τ ) distributions is reported for the various τ scales. here, to compare the shape of the pdfs at different scales, the fluctuations δ τ x(t) have been scaled to unit variance, i.e., where σ is the standard deviation. these pdfs are clearly leptokurtotic showing fat tails and a very pronounced probability peak around zero in the core of the distribution. pre float post float post float pre float post float pre float xτ = δ τ x σ 9 multi scale analysis of the auroral oval figure 4. upper panels: results of the spectral analysis of v post (t) (panel a) and v post (t) (panel b). the red and black dashed lines represent the best fit of the power spectrum. lower panels: the result of the mssa analysis on the whole time series v post (t), see the text float float float the semilog plot clearly indicates that the pdfs do not collapse on a unique master shape. this behaviour indicates that the statistics of fluctuations of v is not merely scale invariant, but conversely there is a change of the pdf shape from long to short timescales. in turbulent media, this is one of the signatures of intermittency, i.e. the crowd of localized features shows a scale dependence that is point dependent, and gives rise to multifractal features. this is another indication of the highly irregular and complex dynamics weaving the small scale texture of the ewb. 3.3 the other events (january 1, 2008; june 1, 2010; july 2, 2010). figure 8 shows, on the left, the trace of demeter’s orbit (red) drawn on the top of the auroral oval map on january 1, 2008 (top), june 1, 2010 (middle) and july 2, 2010 (bottom), as in figure 2. on the right, figure 2 reports the psd analysis for the corresponding events. as for the january 1, 2005 event, it can be seen that, as demeter flew in/out of the ao, the psds of each v time series collected in the ewb plasma is a double powerlaw spectrum with two distinct regimes (a lower frequency power law and a higher frequency power law), suggesting again that scale invariance may be a peculiar feature of such fluctuations. when each v float (t) is alif-decomposed, a peculiar t* appears, at which a change of regime is observed, becoming apparent as a multi-scale element. figure 9 shows a summary plot of the mssa applied to the july 2, 2010 event. left panels show the statistical parameters ε rel (f), k(f) and i(f) calculated for each alif component of v as functions of the frequency f. right panels shows the lim2 parameter (top) and the probability density function of v for 1 hz < f < 10 hz. the analysis confirms the results obtained for the january 1, 2005. in particular, the relative energy peaks at several different scales, indicating the presence of multi-scale structures, while the kurtosis grows up with f indicating the same “intermittent appearance” seen in the v segment studied in §§ 3.1 and 3.2. in the same way, the fluctuations with higher kurtosis appear to be less shannon-entropic, i.e. likely to be more deterministic, more “organized”. last but not least, the multi-scale pdf plot clearly indicates a non-collapse of the reduced distribution, probably indicating a not-self-similar behaviour of turbulent dissipation in the plasma in the ewb. the for the other 3 case events, that may be consulted in the supplementary material of the paper, just confirm the same diagnosis about the ewb plasma studied via multi-scale statistics. post float post float post float post float post float materassi et al. 10 figure 5. the result of the alif analysis on the pre−boundary time series v pre (t). from top to the bottom: the rel− ative energy ε rel ( f ), the kurtosis k( f ) and the shan− non entropy i( f ) of the alif components as func− tions of the equivalent frequeny f. figure 6. the result of the alif analysis on the post−bound− ary time series v post (t). form top to the bottom: the relative energy ε rel ( f ), the kurtosis k( f ) and the shannon entropy i( f ) of the alif components as functions of the equivalent frequeny f. float f 4. conclusions in this work we have presented the results of applying mssa to the high latitude fluctuations observed by ice experiment on-board of demeter, while crossing the ewb of the northern auroral oval. indeed, thanks to the availability of the high rate v float (t) time series, one is able to perform a mssa of the signatures of the ionospheric plasma irregularities on electric field measurements. in particular, the present paper addresses the mssa of data collected in the outermost border of the ao, by applying the alif technique to multi-scale decompose the v float (t) time series, which is able to highlight the irregular structures sitting along the geomagnetic lines, approximately 700 km higher than the ao. the observed region should be that of the trails of particle precipitation giving rise to auroral emission. our analysis shows that fluctuations produced by the particles re-entering from the magnetotail along the geomagnetic lines are characterized by features pointing towards spatio-temporal intermittency. this statement is true under the reasonable hypothesis that the medium remains approximately steady while the probe crosses it (namely, the taylor hypothesis, see frisch (1995)). the take home message suggested by our results is the following. the most external part of currents flowing in the auroral region is an intermittent distribution of irregular, localized structures. their presence produces fluctuations in v float (t) that show higher and higher kurtosis distributions as smaller and smaller space details are examined (in figure 6, indeed, k(f) grows with f, as also for k(f) in figure 9, and in the respective plots in the supplementary material). this suggests that, at the ewb of the ao, the ionospheric plasma medium can be in a turbulent state displaying patchy multi-scale structures. a further characterization of the small scale fluctuations of the plasma at the ewb is done by checking the shape of their reduced pdf at different scales (frequencies), seeing that, due to the fat tail behaviour, those probability distributions do not collapse. in practice, this means that fluctuations at different scales show more and more leptokurtotic statistics as smaller and smaller scales are considered. if one wanted to envis11 multi scale analysis of the auroral oval figure 7. the standardized pdf for the fluctuations of v post (t) at frequencies between 1 hz and 5 hz. figure 8. left panels: the trace of demeter’s orbit (red) drawn on the top of the auroral oval map on january 1, 2008 (top), june 1, 2010 (middle) and july 2, 2010 (bottom). the arc of or− bit is superimposed on the map of auroral oval emission (in kilo−röntgen). right panels: the psd of each “inside−the− ewb” segment v post, always showing a double power−law regime (see the text). float float age a theoretical picture justifying such results, it would be necessary to state that from scale to scale the behaviour of dissipation field is not scale-invariant (nonself-similar, multifractal). all in all, at the ewb the plasma shows a nearly turbulent state, the origin of which could be related to the growing of some type instabilities (e.g., kelvin-helmotz, kruskal-shafranov, etc) in the boundary regions where large shear velocity gradient could be present. in figure 2 this irregular nature of the ao boundary structure seems to appear already in the distribution of emissions registered by dmsp. however, when the fluctuations of the plasma are directly monitored through the in situ proxy v float (t), this “irregular appearance” turns out to be the real texture of that ionospheric region. remarkably, such irregular texture occurs where a “boundary” is well known to exist (in this case, the ewb of the ao), suggesting that ionospheric “boundaries” may appear much richer and more complex if studied in detail. the results presented in this paper are just preliminary ones, due to the limited amount of data analysed. when a much wider statistics is available, it will be possible to study how the plasma dynamics does influence and produce the statistics registered here, which is the real physics challenge of this kind of study. anyway, the results obtained here completely agree with tam et al. [2005], who performed intermittency analyses on the electric field data obtained by the sierra sounding rocket in the auroral zone. they obtained that the electric field fluctuations are more intermittent at smaller scales. in addition, the authors think that these results, obtained on a very limited collection of data, however, point towards the convenience of systematically applying mssa to high rate in situ data collected by satellites through the whole ionosphere. collecting a serious “zoology” of similar ionospheric “details”, and dressing them with the plasma physics motivation, will be an unprecedented progress in near-earth space science. this work is based on observations with the electric field instrument ice embarked on the satellite demeter launched by cnes (centre national d’etudes spatiales). the author thanks j.j. berthelier the pi of this instrument for the use of the data, and cdpp (centre des données de la physique des plasmas) for the provision of these data. space weather data from the dmsp satellites are kindly provided by the space vehicles directorate of the air force research laboratory. m. piersanti, g. consolini, p. diego and i. bertello thank the italian space agency (asi) for the financial support under the contract asi “limadou scienza”. materassi et al. 12 figure 9. left panels: the result of the alif analysis on the post−boundary time series v post (t). form top to the bottom: the relative energy ε rel ( f ), the kurtosis k( f ) and the shannon entropy i( f ) of the alif components as functions of the equivalent fre− queny f; right panels: lim2 of v post (top) and the standardized pdf for the fluctuations of v post at frequencies between 1 hz and 10 hz. f float float references aikio, a. t., t. pitkänen, a. kozlovsky, and o. amm (2006). method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event, ann. geophysicae, 24, 1905-1917, doi:10.5194/angeo-241905-2006. akasofu, s.-i. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 12acosta.qxd 487 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words earthquakes – mexico – catalogues – seismological compilations – pictograms 1. introduction the mexican republic is located in a highly seismic region of the planet, mainly because the cocos and the rivera plates subduct below the north american plate, and the pacific plate separates from the north american plate along the baja california gulf spreading center. besides, the country is crossed by the trans-mexican volcanic belt, part of the pacific ring of fire. numerous earthquakes have occurred throughout mexican history (fig. 1). one of the earthquakes with the greatest magnitude has been the one that occurred on 19 september 1985. that year constituted a breaking point in mexican seismological history in more than one sense. in fact, catalogue production has to be understood before and after 1985. a definitive interest in rescuing mexican seismological history with a holistic perspective has acquired special importance since then. up to 1985 there existed two types of seismological catalogues or compilations: a) parametrical ones produced with an exclusive interest for seismologists, corresponding mainly to the instrumental period of mexican seismology and, as such, covering basically the 20th century; b) earthquake compilations produced mainly during the 19th century, as part of the greatest amount of enlightenment production linked to the growing interest in understanding and explaining nature. since 1985 a joint interest developed among historians, anthropologists, ethnohistorians and seismologists to construct a reliable and multidisciplinary catalogue. our task started having a genhistorical earthquakes in mexico. past efforts and new multidisciplinary achievements virginia garcía acosta centro de investigaciones y estudios superiores en antropología social (ciesas), mexico d.f., mexico abstract the 1985 mexican earthquakes demonstrated that knowledge concerning their history was still scarce and precarious. in fact those earthquakes acted as triggers, because it was then when a new field of research began to develop: disaster historical research. an initial task was to retrieve the history of earthquakes in mexico in order throughout to produce an exhaustive inventory. the main result was a paradigmatic catalogue, published some years ago as the book los sismos en la historia de méxico (earthquakes in mexican history). it contains information about every event along 450 years of mexican seismological history. this paper will focus on the background of this seismological compilation and its characteristics, addressing mainly methodological items concerning sources, qualitative and/or quantitative data, the importance of joint and multidisciplinary efforts, and the research they have inspired on historical earthquakes investigation in mexico. mailing address: dr. virginia garcía acosta, centro de investigaciones y estudios superiores en antropología social (ciesas), juárez 87, tlalpan, cp 14000, méxico d.f., mexico; e-mail: vgarciaa@juarez.ciesas.edu.mx 488 virginia garcía acosta eral knowledge on catalogues or compilations from other countries (china, the caribbean, japan) and particularly, the magnificent 19th century work of the french count montessus de ballore on the seismological history of the andes. for mexico there existed some 19th century compilations which had omissions and lack of precision, but in fact they were the starting point. a short time later, in 1987, the joint effort yielded its first product with the edition of the book «y volvió a temblar», which means «and it trembled again» (rojas-rabiela, et al., 1987). this title reminds us of the first sentence of many colonial documents that reported earthquake occurrence. one year later, in 1988, the second great product appeared (garcía-acosta et al., 1988). research continued for several years, during almost one decade. in 1996 the most recent effort appeared with the edition of «los sismos en la historia de méxico» («earthquakes in mexican history»), which covers more than 450 years of mexican seismological history, from 1455 up to 1913 (garcía-acosta and suárez-reynoso, 1996). 2. methodology historical earthquake research was virtually a new field in mexico. as such, we did not have a guide or an accurate indication to know what path we should follow. for this reason, the methodology of this work was developed step by step. some methodological paths we followed in this retrieval of the seismological history of mexico will be mentioned below: a path we may baptize as methodological lessons learnt. 2.1. multidisciplinary team one of the main methodological elements was working in a team, as a team, with a team. fig. 1. 20th century mexican earthquakes with m ≥ 6.5 (courtesy of servicio sismologico nacional, instituto de geofisica, unam, mexico). 489 historical earthquakes in mexico. past efforts and new multidisciplinary achievements this is absolutely necessary in studies that, like this one, wish to be exhaustive. 2.2. bibliographical sources to guide the search in libraries, we used a series of published bibliographies, some of them even coming from 19th century, and we also used published indexes, mainly to guide our search in archives that were really useful before leaving mexico on our way to the archivo general de indias located at seville in spain. 2.3. temporal frame source availability determined the temporal frame. as such, the oldest date goes as far as the 15th century, while the most recent date was 1912. this period corresponds to what is called the non-instrumental era of mexican seismology, because the instrumental era began in 1910 when the national seismological network was set up. thus, our compilation ended in 1912 because in this year there occurred an earthquake with specific characteristics and important effects in mexico city, for which it was particularly interesting to compare the new quantitative data coming out from the instruments with the qualitative one. 2.4. spatial frame next, we had to define spatial limits. besides covering mexico city, and the valley of mexico in which mexico city is located, the spatial limits were defined with the seismologists choosing the 10 mexican states (estados in spanish) that had two characteristics: they have been historically identified by their great seismic activity and, offered archives, libraries and depositories which could be explored and used. the ten mexican states that were chosen were, from north to south the following: nayarit, jalisco, colima, michoacán, veracruz, estado de méxico, puebla, guerrero, oaxaca and chiapas. 2.5. critical route what we called a «critical route» or work route had to be defined, mainly to know when to continue and when to stop. i will mention two examples of what we did with journals and archives. the large number of 19th century mexican journals that exist in journalistic depositories imposed certain modifications to the task method we had employed with bibliographical sources. the criterion had to be more selective, and the route had to be more specific. at the beginning we tried to find out the most important earthquakes, and went through the journals for the following 30 days, reviewing different newspapers at diverse dates. but this route had to be changed because the information related to a certain quake varied from one newspaper to another. the variation was not only in quantitative terms but also in qualitative ones. we then had to choose the «little red riding hood long way» and decided to examine simultaneously several newspapers for the same date. once we had finished with the search for strong and important earthquakes, and having identified important gaps, we made a daily systematic review of the newspapers to fill the gaps. this path led us to find out some great earthquakes not previously identified as such, and now well documented in our compilation with journalistic information. that is the case of the event known as «earthquake of all saint’s day» which occurred on mexico’s saint’s day, 2 november 1894. the second example comes from the archives, and specifically from the archivo general de indias, located so far away from mexico and where the research time had to be optimized. we chose four criteria: i) to look through all the legajos or sections where we knew there were useful data on earthquakes, as those including documents asking for tax-exemption; ii) to fill the gaps we already had, that is to search specifically for the years for which we had no data (this criterion proved useless and took up too much of our valuable time); iii) to search for specific dates of specific earthquakes we wanted to document in detail (this criterion was also discouraging; the «un490 virginia garcía acosta grateful search» began to threaten the research); iv) previous experiences in mexican archives led us to look for documents with certain characteristics that gave us wonderful results related to building reconstruction and, as such, probably related with earthquake occurrence. we placed special emphasis on documents coming from sections called propios and reales novenos (garcíaacosta and suárez-reynoso, 1996). this one was a more fruitful choice in several senses. we just mentioned «ungrateful» or «useless search». as usual, we had to face it. it represents what our italian colleagues call «negative sources» (gazzini, et al., 1991). one extreme example comes from emilio del castillo negrete’s masterpiece «méxico en el siglo xix» («mexico in 19th century»). only in one of its 29 volumes did we find useful data. as the useless research in bibliographical sources grew, we abandoned them and went to journals, when this one grew, we jumped to archives, and so on. sometimes an ungrateful search is continuous and discouraging. that is why we decided to help other professionals who adopted this technique, and tell them explicitly where and what not to look for (garcíaacosta and suárez-reynoso, 1996). 3. sources according to their contents, sources can be divided into primary and secondary. according to their origin, sources are archives, bibliographical, old newspapers and iconographic. let us take a brief look at these main sources. 3.1. archives undoubtedly, the main source of information for historians are the archives. they require a considerable investment of time, and the participation of experienced professionals in archive work, in paleography, and in other specialities such as pictographic writing, as we will see later. these depositories lack specific sections to look at. among all the archives we visited, only two of them in mexico had specific sections dealing with earthquakes: the historical archives of jalisco, with a section named «natural phenomena», and the diocesan archives at san cristóbal de las casas, chiapas, with a file dealing with earthquakes and volcanic eruptions in the section asuntos civiles-asuntos regionales (civil affairs-regional affairs). we worked in private and public archives. the latter are organized in mexico at different levels, according to the political-administrative division of the country, so they can be municipal, state or federal archives. finally, we had a splendid opportunity to search in the archivo general de indias in seville (spain), looking for data from colonial mexico that were not found in mexican depositories. other colleagues gave us information coming from other archives, such as data from the southern mexican border coming from the general archives of central america. archives and codices were the sources that provided the earliest data about historical earthquakes. 3.2. bibliographical sources 200 bibliographical sources provided primary as well as secondary data for the catalogue, going from codices written in pictographic characters, to 20th century published books, passing through 19th century manuscripts. a good example came from codices, the pictographic documents where mesoamerican people recorded the most important events of their daily life. they are called «painted books» because the writing system used is pictographic, in which each element is determinant: shape, size, color, position of every glyph. the recording of earthquakes, combining written and calendar systems, was especially important because of mesoamerican cosmovision. earthquakes were recorded combining the earth and movement glyphs (fuentes, 1987). as such the earthquake pictogram is the result of the association of both glyphs (fig. 2). next to the earthquake pictogram there always appears a chronological glyph showing the date of the event, that is to say the date when the earthquake or any other natural, social or political phenomenon occurred 491 historical earthquakes in mexico. past efforts and new multidisciplinary achievements (fig. 3). codices offer scattered information, but in some cases it is the only one we have for the pre-hispanic period. manuscripts are other important bibliographical sources, like the one written by martínez gracida (1890), found in the general archives of oaxaca, which includes nationwide as well as world-wide information on earthquake occurrence from 1507 to 1885. 3.3. old newspapers in our research we used 36 newspapers, half of them printed in mexico city and the other half outside the capital, mainly in the state capitals. most were printed during the nineteenth century, when mexican journalism was established. early mexican newspapers were associated with the initial development of printing in new spain, between 1536 and 1540. the first forms of printed information were called hojas volantes or papeles sueltos (flyers or «loose sheets»). the first flyer that appeared in mexico city, printed by the italian juan pablos (considered the first printer in mexico), was a description of the earthquake felt during the night of 10-11 september 1541. it was signed by the notary juan rodríguez as a witness. research in old newspapers and in archives requires the greatest amount of time. we had to go through the newspapers on a daily, monthly fig. 3. pictograms coming from pre-columbian or early post-conquest codices. hailstorm ollin movimento movement tlalli tierra earth date t h re e e a rt h q u a ke s vulcanism fig. 2. earth and movement (tlalli and ollin in nahuatl language) glyphs and their combination meaning earthquake (tlalollin). 492 virginia garcía acosta and yearly basis to find out a few pieces of information. first we went through the few colonial newspapers, then the 19th century and early 20th century, which definitely yielded more information. data from newspapers cover a period of almost 200 years (1722, 1728 to 1742 and 1784 to 1912). 3.4. iconographical sources iconographic sources were not explored or used enough in our research on mexican earthquakes. they represent an enormous field to look at, and were used more exhaustively in our last research dealing with what we call agricultural disasters in mexican history, that is droughts, floods, heavy rains, and so forth (garcía-acosta et al., 2003; escobar, 2003). we used ex votos or retablos for the covers of the two volumes of «earthquakes in mexican history». these retablos are «an old and deepseated expression of mexican popular religiosity» (rodríguez-lapuente, 1995), and they are frequently associated with disasters. primary information used in the compilation came from archives, newspapers and over 60% from bibliographical sources. the introductory chapter to the book «earthquakes in mexican history» contains a detailed analysis of each source for the reader to understand its characteristics and the type of information it provides. this source analysis led to the following four criteria: i) type of source; ii) type of material; iii) how the material is presented; iv) reliability and deficiencies. newspapers provided the bulk of earthquake information, especially for the 19th century. however, in an excellent article on the use of information taken from british newspapers, musson found that one of their main deficiencies is that the information was not treated with scientific interest, so it might be incomplete; it often happens that they do not always describe or even mention some earthquakes found in other sources. hence the recommendation is to use, as we did, as many different newspapers as possible (musson, 1986). so, even when journalistic information is considered primary (except when it deals with prior events), it has to be compared with information coming from other sources in order to determine its accuracy and reliability. the details on how the information was extracted, ordered and systematized appear in the introduction to «earthquakes in mexican history». the required characteristics of any pre-instrumental earthquake compilation or, given the case, any historical long-term catalogue are four: i) homogeneity; ii) continuity; iii) complementarity; iv) reliability. those four requirements have to be accomplished with sources, include textual information, as well as all the information obtained in the documents. by combining several sources in a complementary way, we managed to cover almost all our study period. this proves the usefulness of this method not only in this type of long-period research, but in any exhaustive study. comparing several sources also helps us to solve dating problems, such as counting a single earthquake twice, mistaking an earthquake for its aftershock or its foreshock, confusing two different earthquakes, and so forth. it also solves toponymic problems, such as the town names in indigenous languages, towns that changed names, and so forth. thus, complementing and comparing different sources really matters. only in this way will the reader, user, and researcher be able to use the information, to have confidence, and to choose or discriminate it for his research purposes. they are the minimal requirements for this type of research that has a multidisciplinary origin and objectives. it is so because potential users are not only specialists in earth sciences and engineering, but also historians, ethnohistorians, anthropologists, sociologists, art historians, architects, urbanists and stakeholders in general whose work and decisions are related to these issues. for example, diverse earthquake-related studies developed by historians, ethnohistorians and anthropologists have increased during recent years. an example of this are the contents of the second volume of «earthquakes in mexican history» (garcía-acosta, 2001) that include analysis on topics like: the evolution of 493 historical earthquakes in mexico. past efforts and new multidisciplinary achievements seismic records throughout centuries; scientific thoughts about the origin of earthquakes; historical social responses to earthquake occurrence throughout mexican history. 4. catalogues and seismological compilations before and after 1985 catalogues and seismological compilations include primary and secondary material. the first ones were developed in the 19th century mainly by engineers, geologists, meteorologists and several members of scientific societies (table i). many of those coming from the 19th century have serious mistakes and omissions. for this reason, they have to be checked and compared with original data. that is, they do not have the homogeneity, continuity, complementarity and reliability required for these sources. the same happens with the work of juan orozco y berra (1887-1888) «efemérides seísmicas mexicanas» («mexican seismic ephemeris»), which until recently was the only reference for specialists, although it contains imprecisions, dating errors and omissions. this interest to maintain the historical memory of earthquakes was reborn in mexico after the 1985 earthquake, a breaking point in this type of research as i mentioned before. after that event, many local compilations were made especially for mexico city (cf. amerlinck, 1986; gonzález-polo, 1985 and manzanilla, 1985). there have also been many regional, state or local efforts, such as the work of juan vizcaíno (1986) who studied the seismic history of his native town zapotlán or ciudad guzmán in jalisco, a city damaged by many earthquakes including the 1985 one. it is also the case of césar luna bauza (1992), who studied the whole state of veracruz. each one covers different periods: 1543-1985 for zapotlán and 1900-1994 for veracruz. an even greater work was the catalogue of garduño et al. (1998) which covers a large western region including three mexican states: jalisco, colima and michoacán, or the regional coverage made by molina del villar (1988) for the northwestern part of mexico. some previous efforts were the works of gonzález-ramírez (1981) on puebla, superficially covering the 1575-1980 period, and núñez-cornú and ponce (n.d.) on oaxaca, which covers only the 19th century with a selection of the strongest oaxacan earthquakes. many of these compilations and catalogues have used the same methodology as «earthquakes in mexican history». undoubtedly, these local, state and regional efforts will improve the existing databases, which will lead to a new national catalogue with local, state and regional addenda to «earthquakes in mexican history». what have these compilations on historical earthquakes in mexico inspired? what type of work was produced after them? how do seismologists use this qualitative information carefully collected and systematized by historians? table i. 19th century nation-wide earthquake compilations. edition date author and title coverage 1837 c.m. bustamante «temblores de méxico...» 1620-1835 1861 j.g. romero «noticia de los terremotos...» 1532-1861 1864 j.n. adorno «memoria acerca de los terremotos de méxico» 1532-1864 1887 j. orozco y berra «efemérides seísmicas mexicanas» 1460-1887 1890 (ms.) m. martínez gracida «catálogo de terremotos...» 1507-1885 1893 e.r. de zayas «los estados unidos mexicanos...» 1460-1884 494 virginia garcía acosta what have seismologists done since the edition of «earthquakes in mexican history» and other derived catalogues? according to the national university (unam) and the national polytechnic institute (ipn), seismologists are still generating instrumental catalogues going back only as far as 1900, the instrumental period of mexican seismology (zúñiga, et al., 2000). the seismic instrumental catalogues for the mexican republic have been used, for example, to divide the country into four seismic zones (fig. 4), in order to design anti-seismic strategies. this type of catalogues should include homogeneous historical intensities and other parameters useful for hazard and risk studies. although there has not been a sequel to «earthquakes in mexican history» for the period after 1912, seismologists interested in this subject acknowledge that this work has allowed them to study specific historical earthquakes. nevertheless, even today some seismologists express uncertainty in the use of past earthquake records. they accept that it is very important to have such records, but they regret that some data are not always accurate and reliable. sometimes the information is diffuse and it is not easy or feasible to estimate the magnitude, assign the intensity, locate the epicentre. what has been done with instrumental records has been very useful to understand pre-instrumental ones. it is important to know, for example in mexico city, how large and how frequent the earthquakes are, where they have done most damage, how intense the movements are, and make comparisons between past and present records. but for a seismologist it is crucial to identify the epicentre, and it is not always easy to find out where it was. isoseismal maps have been made with preinstrumental records, based mainly on 19th century records, but the weakness of qualitative data for some purposes persists. looking after information coming from more local sources may help to avoid these gaps. nevertheless past earthquakes records have led to major advances in seismology, as well as in so34.00 32.00 30.00 28.00 26.00 24.00 22.00 20.00 18.00 16.00 14.00 –118.00 –114.00 –110.00 –106.00 –102.00 longitude l a t it u d e –98.00 –94.00 –90.00 –86.00 fig. 4. mexican seismic zones: a = no historical earthquake records for the last 80 years; b and c = intermediate zones; d = large historical earthquakes recorded (courtesy of servicio sismologico nacional, instituto de geofisica, unam, mexico). 495 historical earthquakes in mexico. past efforts and new multidisciplinary achievements cial sciences, or in multidisciplinary products (suárez et al., 1994). by using historical catalogues, several seismologists have carried out case studies on certain earthquakes and with very concrete objectives, such as the work by the centro de geociencias (earth sciences center) at unam. this center has studied mainly tectonics and its effects on central mexico in states such as aguascalientes, guanajuato and zacatecas, in contrast to the enormous amount of research developed for the coastal or subduction zone. another example comes from the research done also with information coming from «earthquakes in mexican history» on the 1887 earthquake that hit the north-western sonoran area with intensity of 8 to 9, which seems to exemplify what is known in seismology as «triggering». an effort including these type of analyses will be achieved with the future publication of the third volume of «earthquakes in mexican history». if volume one of this series includes the seismological compilation and volume two the analysis social scientists made with that information, forthcoming volume three will offer the seismological perspective emerging out from the same information included in volume one. the construction of historical catalogues with earthquake records constitutes an effort that has been made all over the world since several centuries. current state-of-the-art of similar studies in latin american countries during the last 50 years look very encouraging. past earthquakes have been recorded employing several methodologies and sources, but they are not so different from one another. these ideas lead us to some unanswered questions we already posed ourselves at the beginning of this workshop. some of these questions are the following: how should historical earthquake catalogues be made? should they be all-inclusive with the information coming from every event ever reported? should fakes be excluded or not? which is the best methodology for a really complete catalogue? how to deal with these specific historical sources? how can different sources be treated? can a world catalogue be built? how? is it feasible? we still have many questions, but we should agree on at least one thing. the starting point that encourages this search is the same that has inspired the construction of catalogues with information coming from the past, and this starting point is based on the idea that historical records should help protect from earthquakes. acknowledgements i wish to thank shri krishna singh (unam), ramón zúñiga (unam) and fernando angulo brown (ipn) for the information they offered on current uses of qualitative data for seismological purposes. references adorno, j.n. (1864): memoria acerca de los terremotos en méxico escrita en octubre de 1864 por ... (imprenta de mariano villanueva, mexico). amerlinck, m.c. (1986): relación histórica de movimientos sísmicos en la ciudad de méxico (1300-1900) (socicultur, mexico). bustamante, c.m. (1837): temblors de méxico y justas causas por que se hacen rogaciones públicas (imprenta de luis abadiano y valdés, mexico). escobar, a. (2003): desastres agrícolas en méxico. catálogo histórico (ciesas, fondo de cultura económica, mexico), vol. 2 (in press). fuentes, s. (1987): estudio del glifo ‘temblor de tierra’, in y volvió a temblar... cronología de los sismos en méxico. de 1 pedernal a 1821, coordinated by t. rojasrabiela, j.m. pérez-zevallos and v. garcía-acosta (ciesas, mexico), 173-196. garcía-acosta, v. (2001): los sismos en la historia de méxico (fondo de cultura económica, ciesas, unam, mexico), vol. 2. garcía-acosta, v., r. hernández, i. márquez, a. molina, j. m. pérez, t. rojas and c. sacristán (1988): cronología de los sismos en la cuenca del valle de méxico, in estudios sobre sismicidad en el valle de méxico (ddf, pnud, mexico), pp. 409-498. garcía-acosta, v. and g. suárez-reynoso (1996): los sismos en la historia de méxico, (universidad nacional autónoma de méxico, centro de investigaciones y estudios superiores en antropologia social, fondo de cultura económica, mexico), vol. 1, pp. 718. garcía-acosta, v., j.m. pérez-zevallos and a. molina del villar (2003): desastres agrícolas en méxico. catálogo histórico (ciesas, fondo de cultura económica, mexico), vol. 1 (in press). garduño, v.h., a. cuevas muñiz and r.j. escamilla torres (1998): descripción histórica de la sismicidad en colima, jalisco y michoacán (universidad michoacana, universidad de colima, morelia, mexico). gazzini, m., p. margaroli and m. p. zanoboni (1991): fonti 496 virginia garcía acosta per lo studio di terremoti in area padana nei secoli xi-xv, in atti del convegno gndt, edited by p. albini and m.s. barbano, macrosismica, bologna, vol. 2, 37-45. gonzález-polo, i. (1985): temblores y terremotos en la ciudad de méxico. siglo xviii, boletín de la sociedad mexicana de geografía y estadística, cxxxvi, i, 31-47. gonzález-ramírez, z. (1981): historia sísmica del estado de puebla, ba thesis geography, (unam, mexico). luna bauza, c. (1992): sismos en el estado de veracruz (gobierno del estado de veracruz-llave, mexico). manzanilla, l. (1985): relación de los sismos ocurridos en la ciudad de méxico y sus efectos, revista mexicana de sociología, 48 (2), 265-282. martínez gracida, m. (1890): catálogo de terremotos desde 1507 hasta 1885, in cuadro sinóptico, geográfico y estadístico de oaxaca, ms. molina del villar, a. (1988): cronología de sismos en el noroeste de méxico, siglos xviii y xix, in memoria del xv congreso de historia y antropología de sonora (instituto de investigaciones históricas, universidad de sonora, hermosillo), i, 253-272. musson, r.m.w. (1986): the use of newspaper data in historical earthquake studies, disasters, 10 (3), 217-223. núñez-cornu, f. and l. ponce (n.d.): zonas sísmicas de oaxaca, méxico: sismos máximos y tiempos de recurrencia para el periodo 1542-1988, ms. orozco y berra, j. (1887-1888): efemérides seísmicas mexicanas, memorias de la sociedad científica «antonio alzate», i, 303-541. rodríguez-lapuente, m. (1995): foreword, in miracles on the border. retablos of mexican migrants to the united states, edited by j. durand and d. s. massey (the university of arizona press, tucson), pp. xi-xii. rojas-rabiela, t., j.m. pérez-zevallos and v. garcíaacosta (editors) (1987): y volvió a temblar... cronología de los sismos en méxico. de 1 pedernal a 1821 (ciesas, mexico). romero, j.g. (1861): noticia de los terremotos que se han sentido en la república mexicana, desde la conquista hasta nuestros días, boletín de la sociedad mexicana de geografía y estadística, viii, 468-470. ssn servicio sismológico nacional (on line: http://www. ssn.unam.mx). suárez, g., v. garcía-acosta and r. gaulon (1994): active crustal deformation in the jalisco block, mexico: evidence for a great historical earthquake in the 16th century, tectonophysics, 234, 117-127. vizcaíno, j.s. (1986): crónicas de zapotlán. historia de los temblores, 1543-1985 (archivo municipal, zapotlán, mexico). zayas, e.r. (1893): los estados unidos mexicanos, sus condiciones naturales (oficina tipográfica de la secretaría de fomento, mexico). zúñiga, f.r., c.m. valdés-gonzález and m.a. reyes (2000): a general overview of the catalog of recent seismicity compiled by the mexican seismological survey, geofísica internacional, 39, 161-170. data insertion in volcanic ash cloud forecasting annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6624 data insertion in volcanic ash cloud  forecasting kate l. wilkins 1*, shona mackie1, i. matthew watson1, helen  n. webster2, david j. thomson2, helen f. dacre3  1 university of bristol, 2 met office, exeter, 3 university of reading *kate.wilkins@bristol.ac.uk abstract during the eruption of eyjafjallajökull in april and may 2010, the london volcanic ash   advisory   centre   demonstrated   the   importance   of   infrared   (ir)   satellite   imagery   for   monitoring   volcanic   ash   and   validating   the   met   office   operational   model,   name.   this   model is used to forecast ash dispersion and forms much of the basis of the advice given to   civil aviation. name requires a source term describing the properties of the eruption plume   at the volcanic source. elements of the source term are often highly uncertain and significant   effort   has   therefore   been   invested   into   the   use   of   satellite   observations   of   ash   clouds   to   constrain them. this paper presents a data insertion method, where satellite observations of   downwind   ash   clouds   are   used   to   create   effective   ‘virtual   sources’   far   from   the   vent.   uncertainty in the model output is known to increase over the duration of a model run, as   inaccuracies   in   the   source   term,   meteorological   data   and   the   parameterizations   of   the   modelled   processes   accumulate.   this   new   technique,   where   the   dispersion   model   (dm)   is   ‘reinitialized’ part­way through a run, could go some way to addressing this. i. introduction atellite imagery is an important tool for  detecting and monitoring volcanic ash  clouds. ir satellite sensors are used for  imaging   ash   day   and   night   and  performing   retrievals   of   the   physical  properties   of   ash   clouds   [e.g.   wen   and   rose,  1994; francis et al., 2012; pavolonis et al., 2013].  we   present   a   novel   method   to   combine   ir  observations   and   dispersion   modelling   for  volcanic   ash   forecasting,   and   give   a   brief  overview   of   some   of   the   current   methods.  here,   the   term   eruption   plume   is   used   to  describe   a   column   of   ash   erupting   from   the  volcano   vent.   ash   which   has   been   passively  transported by wind is described as downwind  or distal.  when   using   volcanic   ash   dms   such   as   the  numerical   atmospheric­dispersion   modelling  environment   (name)   [jones   et   al.,   2007],  many of the eruption source parameters (esp)  contributing   to  the   source   term   (e.g.   eruption  plume  height,  mass  emission   rate)  need  to  be  estimated,   often   with   high   uncertainty,   which  can  cause large uncertainties in  model output  [devenish et al., 2012a]. furthermore, errors in  the simulated ash cloud location in space and  time   can   result   from   uncertainties   in   the  driving   meteorology   [eckhardt   et   al.,   2008],  which   have   been   shown   to   cause   cumulative  uncertainties   in   model   output   [dacre   et   al.,  2011]   and  can   lead   to  errors  in   calculated   ash  column loading (acl). model validation & data assimilation s francescocaprara typewritten text francescocaprara typewritten text operationally,   some   of   the   effects   of   esp  uncertainty   are   addressed   through   validation  of   model   output   against   remotely   sensed   and  in­situ   observations   prior   to   a   forecast   being  issued   [webster   et   al.,   2012].   millington   et   al.  [2012] developed a method to enable like­with­ like comparison of ir satellite imagery and dm  output  using  the fast  radiative transfer model  rttov to simulate images from the spinning  enhanced   visible/infrared   imager   (seviri)  sensor   on­board   the   meteosat   second  generation satellite.  data assimilation (da) is used to estimate the  true state of a system where both model output  and  observations  are uncertain.  this  can  then  be   used   as   an   initial   state   from   which   to  produce   a  forecast  for  some  later  time.  da in  this   field   has   been   approached   a   number   of  ways.   denlinger   et   al.   [2012]   presented   a  bayesian method to forecast uncertainty in ash  dispersion.   a   genetic   algorithm   variational  approach is presented by schmehl et al. [2011]  to minimize differences between modelled and  observed   ash   concentrations.   inversion  modelling is a da method that has been used  to  estimate   optimized   esps,   for   both   volcanic  ash   [e.g.   stohl   et   al.,   2011;   kristiansen   et   al.,  2012]   and   so2  [e.g.   eckhardt   et   al.,   2008;  flemming and inness, 2013; boichu et al., 2014],  using   later   observations   to  reduce   esp  uncertainty   and   hence   improve   dm  predictions. the   above   methods   have   been   shown   to  improve   esp   estimation;   however,   even   if   the  source term is known perfectly, uncertainties in  model output can still increase with run time.  a   simple   method   is   presented   here   using  satellite   observations   to   initialize   dm  simulations at a series of time­steps. this may  halt   the   propagation   of   some   uncertainty   via  sequential   updates   of   the   simulations   instead  of esp correction, and requires little to no prior  knowledge of the eruption source term. ii. methods the   viability   of   using   retrievals   from   ir  satellite observations of downwind ash clouds  to initialize name dispersion and sequentially  update   name   output   (data   insertion)   is  examined.     no   uncertainty   analysis   has   yet  been undertaken; this is an initial investigation  which   assumes   negligible   uncertainty   in   both  the model and observations (see section iv for  a short discussion). we use acl and ash cloud  height   (ach)   estimates   from   the   13­14   may  2010   eyjafjallajökull   ash   cloud   retrieved   from  the seviri sensor following the 1d­variational  (1d­var)   method   of   francis   et   al.   [2012],   and  meteorological data from the global version of  the met office unified model (metum).  table1:  volcanic ash psd used operationally in name   and distal psd based on table 1 from dacre et al. [2013],   derived from faam observations on 14 may 2010. particle  diameter  (µm) fraction of total mass operational  name measured distal 0.1­0.3 0.001 0.000 0.3­1.0 0.005 0.006 1.0­3.0 0.050 0.235 3.0­10.0 0.200 0.535 10.0­30.0 0.700 0.224 30.0­100.0 0.044 0.000 data insertion each pixel from the seviri image identified as  ash  is treated as a  source in  name. together  they contribute to a ‘virtual source’ where they  are   released   into   the   model   atmosphere  instantaneously. the   retrieval   provides   no   information   on   the  vertical  structure  of  the  ash,  so  the  entire  ash  cloud is estimated to be 1 & 2 km thick, based  on   uk   facility   for   airborne   atmospheric  measurements   (faam)   bae­146   aircraft   lidar  and   cloud   and   aerosol   spectrometer   (cas)  measurements on 14 may 2010 [johnson et al.,  2012;     marenco   et   al.,   2011],   with   a   normally  distributed   vertical   ash   profile.   the   ash  concentrations   are   calculated   using   the  retrieved   acls   and   the   estimated   source  volumes.  the   particle   size   distribution   (psd)   used  operationally   in   name   (table   1)   includes  particle   sizes   outside   the   range   detectable   by  seviri (~1­35 µm diameter [stohl et al., 2011]).  both the operational psd and one based on the  distal   psd   from   table   1   in   dacre   et   al.   [2013]  derived   from   faam   measurements,   which   is  more   representative   of   the   seviri   detection  thresholds,   are   used   for   the   simulations.   a  time­series   of   simulations   are   then   combined  to produce a forecast. the presence of water in  the   atmosphere   as  clouds   can   obscure   ash   by  overlying it, by ash  particles becoming coated  in ice, or where an ash cloud has a high water   content   [rose   et  al.,   1995],   so   by   combining   a  series   of   model   runs   initialized   using   the  observations,   it   is   more   likely   that   features  obscured   by   atmospheric   water   in   one   image  are  captured  in   others  and   evolved   with   time  in the dispersion model. iii. results fig. 1a) shows a seviri ‘dustrgb’ false color  composite image at 1315 utc on 14 may 2010  where   ash   shows   as   a   pink   streak   extending  northwest from scotland toward iceland and as  a   yellow   region   west   of   iceland   [devenish   et  al.,   2012b],   b)   is   a   name   simulation   for   that  time   using   a   source   term   based   on   the   met  office’s   “best   guess”   of   the   plume   height,  calculated   eruption   rate   and   the   operational  psd in table 1 [webster et al., 2012], and c) is  retrieved acl. the data insertion forecast d) is  a combination of the greatest acl values (for  each   grid   cell)   of   four   name   simulations  ending   at   1315   utc   14   may,   initialized   using  retrievals from 2115 utc 13 may, 0115, 0515 &  0915   utc   14   may,   assuming   1   km   layer  thickness   and   normal   vertical   distribution   at  the time of insertion. both the “best guess” and  data   insertion   simulations   are   in   good  agreement   with   the   ash   position   in   1a   &   c),  however   1b)   shows   lower   acl   over   the  northeast   and   north   of   iceland   and   much  higher   values   west   of   iceland   than   either   the  retrieval or data insertion simulation.  fig.   1d)   shows   a   large   patch   of   ash   over  greenland which is not visible in 1c) nor in 1b).  it   is  possible   to  make   out   a   pink   area   in   that  region   in   1a)   which   may   be   ash;   there   is   a  negative   brightness   temperature   (bt)  difference   in   this   region   of   the   image   (bt   at  10.8   µm   wavelength   ­   bt   at   12.0   µm  wavelength) [not shown] which could indicate  the   presence   of   ash,   but   it   is   not   clearly  distinguishable   from   background   values.   fig.  13c)   in   johnson   et   al.   [2012]   shows   lidar   and  cas  derived   ash   concentration   profiles  for  14  may at 59.6°n 7.0°w. the ash layer has a peak  concentration at ~800 µg m­3 and is ~2 km thick  extending from ~5­7 km altitude.  figure 1:  eyjafjallajökull ash cloud at 1315 utc 14   may 2010. a) seviri ‘dustrgb’ false color composite   image.  b) “best guess” name source term estimate   (acls >10 g m­2 shown as 10 g m­2). c) 1d­var acl   retrieval.  d) data insertion forecast. only acls >0.01 g   m­2 shown in 1b & d, color bar is for 1b­d. fig. 2 shows “best guess” and data insertion (1  &   2   km)   modelled   profiles   for   the   same  location at 1315 utc using both psds in table  1,   and   a   shaded   region   showing   the  approximate   extent   of   the   observed   ash   layer  shown   in   johnson   et   al.   [2012].   all   of   the  modelled   peak   concentrations   are   in  reasonable   agreement   with   the   measurement­ derived   profiles,   with   those   from   the   data  insertion runs slightly higher. these layers are  also ~2 km below the measured altitude (at a  concentration of 200 µg m­3), possibly reflecting  an   underestimation   of   heights   in   the   1d­var  retrieval. however, the thickness of these layers  is   2­3   km   and   therefore   in   better   agreement  than   the   “best   guess”   source   term   estimate,  which is ~4 km. figure 2: ash concentration profiles for 1315 utc   14 may at 59.6°n 7.0°w.  the solid line is the “best   guess” name source term estimate. other lines   are the data insertion (di) method assuming 1 and   2 km thick ash layers using the measured distal (m)   and operational (o) psds shown in table 1. the   line “di 1 km psd(m)” is from fig. 1d). the filled   box shows the approximate observed concentration   profile from fig. 13c) in johnson et al. [2012]. iv. discussion by assuming that the ash forms a single 1­2 km  thick layer distributed normally in the vertical,  potential multi­layered ash clouds are ignored.  also,   ash   particles   with   sizes   outside   the  seviri   detection   range   and   ash   obscured   by  overlying   meteorological   cloud   for   long   time  periods   may   not   be   included   in   the   ‘virtual  source’.   therefore   the   data   insertion   method  may   produce   less   conservative   forecasts   than  where   the   ash   is   initialized   at   the   eruption  plume.   however,   the   vertical   depth   and  distribution   of   the   ash   layer   can   be   updated  according   to  observations  from   satellite­based  lidar   (e.g.   caliop)   or   research   flights   when  they   become   available.   these   observations  could   also   be   used   to   validate   the   1d­var  height   estimates,   which   correspond   to   the  height of radiative emission and therefore may  be below the top of the ash cloud for a sparse  cloud [francis et al., 2012]. evaluation   of   both   model   and   observation  accuracy   is   challenging   as   neither   constitute  the   “truth”;   the   model   is   not   a   true  representation   of   reality   and   in   cases   of  disagreement   it   is   therefore   unclear   which   is  correct.   a   more   sophisticated   implementation  of   this   scheme   could   address   this   by  implementing   weights   that   account   for   the  model and observation uncertainties specific to  each   model   and   observation   location.   for  historical events, epistemic  uncertainties could  be informed using other available data, but in  an operational setting when a dm must be run  in   near­real   time,   these   may   be   unknown   or  poorly   constrained.   uncertainties   can   be  provided   by   the   1d­var   scheme,   however  accurate   and   reliable   model   uncertainties   are  more   difficult   to   calculate.   there   is   ongoing  work   in   the   dispersion   modelling   community  to better characterize and quantify both model  and observational uncertainty and future work  will focus on including these. in this paper a novel approach  using the met  office   name   dm   and   a   1d­var   ir   satellite  retrieval   algorithm   is   suggested.   a   data  insertion method is shown, using retrievals of  downwind   ash   clouds   within   name   as   a  snapshot   from   which   to   reinitialize   ash  dispersion instead of using the eruption source  term. an example shows good agreement with  ash position and column loading from satellite  imagery,   and   layer   thickness   and   peak  concentrations   agree   reasonably   with   aircraft  measurements,   however   layer   altitude  estimates   are   too   low.   a   more   sophisticated  sequential update assimilation scheme  should  allow   known   uncertainties   in   the   model   and  observations   to   be   well   characterized   and  tracked   throughout   to   provide   a   forecast  indicating uncertainty. acknowledgements this   work   was   supported   by   the   natural  environment   research   council   [consortium  on   risk   in   the   environment:   diagnostics,  integration,   benchmarking,   learning   and  elicitation   (credible)   and   cooperative  awards   in   science   &   technology   (case,   met  office);   grant   ref.   ne/j017450/1].   thanks   to  the   reviewers   for   their   helpful   comments,   to  pete francis for providing code for the 1d­var  method and great help implementing it, claire  witham,   james   hocking   (met   office),   and  natalie   harvey   for   helpful   input.   we  acknowledge   use   of   the   name   atmospheric  dispersion   model   and   associated   nwp  meteorological   data   sets   made   available   to  us  by   the   met   office,   and   eumetsat   for   the  provision   of   seviri   data   via   the   earth  observation portal. references [boichu et al., 2014] boichu, m., clarisse, l., et  al.   (2014).   improving   volcanic   sulur   dioxide  cloud   dispersal   forecasts   by   progressive  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kyongju-ulsan area, some of which have caused fatalities and produced liquefaction. some of the earthquakes destroyed houses, created ground fissures, surface depressions, and paleoseismology of the yangsan fault, southeastern part of the korean peninsula jai bok kyung department of earth sciences, korea national university of education, kangnae, cheongwon, korea abstract the nne-trending yangsan fault (ysf) is the most prominent fault in the southeastern part of the korean peninsula and has a continuous trace about 200 km long. activity on this fault was recently investigated using aerial photographs, topographic analysis, and trenching. the geomorphologic evidence of late quaternary faulting is clearly recognized on both the northern (yugyeri and tosung-ri areas) and southern parts (eonyang to tongdosa area) of the fault. the main ysf is marked by a zone of shattered rock that is tens of meters wide and zone of fault gouge. during the late quaternary, right-lateral movement occurred mainly on the southern part, as shown by lowangle striations on the fault plane, elongation of deformation features in the fault gouge. the estimated vertical slip rate is about 0.02-0.07 mm/yr, and the lateral slip rate may be several times larger than the vertical rate. the most recent event occurred prior to deposition of holocene alluvium. in the northern part, the fault locally changes trend to almost n-s, dips to the east and has reverse movement. the average vertical slip rate is estimated less than 0.1 mm/yr. the most recent event probably occurred after 1314 cal. years bp (a.d. 536). landslides, and caused liquefaction and sand blows. eight of these large historical earthquakes occurred in and around the kyongju area (a.d. 34, 100, 123, 304, 471, 510, 630, 779) (lee, 1998). in particular, the kyongju earthquake in a.d. 779 caused great damage, killed approximately 100 people, and is described in the historical literature ‘samguksaki’. the greatest earthquake (mmi x) in the korean peninsula occurred in the ulsan area in 1643 (lee, 1998), had a large felt area, triggered a tsunami, caused liquefaction along the coast, and destroyed some beacon lighthouses and castel. faults that could have caused these earthquakes include the nne-ssw-trending yangsan fault and nnw-sse trending ulsan fault (fig. 2). of these two structures, the ysf has a strong linear expression and is continuously traceable for about 200 km (yoon and kim, 1990). the surface expression of the ysf is marked by a fault-controlled valley containing wide, nnessw shatter zones, and sub-horizontal striations (reedman and um, 1975). the fault plane is 983 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. jai bok kyung, department of earth sciences, korea national university of education, kangnae, cheongwon, chungbuk 363-791, korea; e-mail: jbkyung@knue.ac.kr almost vertical based on exposures in outcrops and geophysical data (e.g., kim and lee, 1988). this study clarifies the history of the late quaternary movement on the ysf based on geomorphological studies, topographic mapping, and trenching. the study concentrated on two sites, one on the northern part of the fault at yugye-ri area, and one on the southern part in the eonyang to tongdo-sa areas (tn and ts in fig. 2). 2. general tectonic setting the korean peninsula is part of the amuria plate and is an important link between the continental blocks of north and south china and the island arcs of japan. major orogenic events in the korean peninsula occurred mainly in late paleozoic and mesozoic time (e.g., reedman and um, 1975; lee, 1987; kim, 1996; ree et al., 1996). in late permian to early triassic time, the songrim orogeny produced strong deformation and metamorphism, and compressional deformation occurred again in the early to late jurassic (daebo event). dextral ductile shearing associated with thrusting and folding continued in the mid-southern korean peninsula due to northwestward subduction of the izanagi plate under the asian continent (maruyama et al., 1997; chough et al., 2000). in cretaceous time, the northward subduction of izanagi plate formed strike-slip basins and the emplacement of volcanic and granitic rocks (chough et al., 2000). non-marine sedimentary rocks in these basins and volcanic rocks are mainly present in the southeastern part of the peninsula (kyongsang basin) (chun and chough, 1992). figure 2 shows the general geology of the eastern part of the kyongsang basin. in tertiary time, the southeastern margin of the korean peninsula experienced back-arc opening, resulting in deposition of tertiary rocks in and around pohang (pohang basin) in association with back-arc opening in the east sea (sea of japan) (chough et al., 1990; yoon and chough, 1995). no prominent tectonic movements occurred during the quaternary except for some isolated volcanic events. however, evidence of uniform uplift exists for much of the southern half of the 984 jai bok kyung 126°e 40°n 40°n 36°n 36°n 126°e 130°e 130°e 1810 1597 1518 1681 100, 510, 779 123, 630 304 34, 471 1001 1757 1455 1643 1757 766 ulsan 1643 kyongju 502 1214 east sea (sea of japan) index fatalities liquefaction ground deformation (ground fissure, landlside, surface depression) ground fissure, fatalities ground fissure, liquefaction ground deformation, liquefaction, fatalities fault yellow sea 1298 1311 89 380 china korea ix x fig. 1. epicenters of large historical earthquakes (mmi equal to or greater than ix) which caused destructive damage in the korean peninsula from a.d. 27 to 2002. the number in the figure represents the occurrence time (in years) of earthquake. korean peninsula (oh and choi, 2001). uplifted marine terraces are developed along the eastern coast (e.g., oh, 1981; choi, 1997, 2001; oh and choi, 2001). the average uplift rate of the eastern coast is estimated to be about 0.1 m/ka since the last glacial culmination (oxygen isotope stage 5e) (choi, 2001). some faults, such as the yangsan and ulsan faults (fig. 2) 985 paleoseismology of the yangsan fault, southeastern part of the korean peninsula fig. 2. geological map (after korea institute of energy and resources, 1981) and the yangsan fault system in the shaded relief map of southeastern part of the korean peninsula. 1 tertiary basin deposits; 2 late cretaceous to early tertiary igneous rocks; 3 late mesozoic kyongsang basin deposits; 4 late mesozoic yucheon volcanics; 5 tertiary volcanics; 6 fault; m moryang fault; y yangsan fault; d dongrae fault; i ilgwang fault; u ulsan fault. tn and ts indicates the sites in the northern (yugye-ri) and southern sectors (eonyang to tongdo-sa), respectively. were active in the late quaternary, even though their slip rates are estimated to be less than 0.1 m/ka (okada et al., 1994; kyung et al., 1999a,b; kyung and im, 2001; okada, 2001). 3. yangsan fault system the yangsan fault system consists of several faults, including the moryang fault, the tongrae fault, the ilkwang fault and the ulsan fault (fig. 2). the ysf is one of the faults in the fault system and is a prominent dextral, strikeslip fault that has a clear, linear topographic expression for about 200 km. it consists of two or three closely spaced, sub-parallel faults. the width of the fault zone varies in a regular, cyclical patterns every 25-35 km along strike (chang, 2001). along the northern part of the fault zone, its maximum width is about 5 km and along the southern part, the maximum width is about 6 km. the significance of these regular changes in fault zone width are not known. the ysf contains evidence of multiple deformation events through geologic time. initial movement on the ysf probably occurred after 45 ma (eocene) based on radiometric dating of volcanic rocks along the northern part of the fault (chang et al., 1990). paleo-stress analysis (chae and chang, 1994; chang and chang, 1998; chang, 2001) indicates that movement on the ysf has been right-lateral strike-slip caused by ne-sw compression since eocene time, including during its most active period in the late miocene. later, the sense of movement changed to left-lateral under a nw-se compressional stress regime at the end of the miocene. in the pliocene, n-s compression affected the fault, and in the quaternary, the fault was reactivated by a right-lateral strike-slip movement in an e-w to ene-wsw compressional stress regime. this is the present stress regime in the korean peninsula (jun, 1991). the total amount of dextral displacement on the fault is estimated to be about 35 km in the north (chang et al., 1990) and about 25 km in the south, most of which occurred between 14 and 42 ma (reedman and um, 1975; choi and park, 1985). to document the fault’s quaternary history, it is necessary to quantify the deformation of quaternary deposits, but quaternary deposits are not abundant or widespread on the korean peninsula. locally, quaternary deposits are present along the ysf, although these are largely restricted to small streams and valleys. we have studied these deposits at several sites to clarify the late quaternary faulting history of the yangsan fault (okada et al., 1994; kyung et al., 1999a,b; kyung and chang, 2001). 3.1. northern part of the ysf (tn in fig. 2) in the yugye-ri area on the northern part of the ysf (fig. 2), fluvial terraces along the seojeong stream can be divided into four levels: qt1, qt2, qt3 and qt4 (fig. 3.). here, qa indicates the deposits of undifferentiated alluvium of the latest pleistocene to holocene. the fault was exposed in two excavations during construction of a dam. in this area, terrace qt2 is cut by several fault splays, which are buried with alluvial deposits (unsorted pebble to boulder gravel). a fault scarp on the qt2 terrace has a down-to-the-west vertical displacement of about 1.5 m (fig. 3; profile d-d and e-e ). the fault plane forms the boundary between alluvial deposits on the west and cretaceous sedimentary rocks on the east. it strikes almost n-s, dips to the east about 45°, and reverse-slip striations plunging about 40°e with hanging wall up-tothe west, resulting in right-lateral reverse movement. the fault has a gouge zone that is as wide as 1 m and a cataclastic zone several meters wide. a trench at site a (fig. 3) exposed, alluvium consisting of unweathered pebble to boulder gravel with silty lenses (fig. 4). this alluvium was strongly deformed and overturned near the fault zone. the long axis of the slender gravel clasts near the gouge zone has been rotated and dips to the east, sub-parallel to the fault plane. charcoal in silty sediments in the gravel yielded calibrated radiocarbon ages of 7182 (snu 99-136) years bp, 11 230 (snu 99-135) years bp, and 25.900 ± 730 (snu 99-118) years bp (table i) from the upper to the lower parts of the gravel. a humic layer in the lower of the terrace 986 jai bok kyung 987 paleoseismology of the yangsan fault, southeastern part of the korean peninsula 125 125 1 5 0 100 100 100 100 n qt1 qt1 qt2 qt2qt2 qt3 qt4 qt4qt4 faultqa qa 129°18' 3 6 ° 1 1 '4 6 " 3 6 ° 1 2 '1 3 " n 129°18'26'' e 0 200 m terrace surface yugye-ri s eojung stream a a' b b' c c' d e a b c d e a' b' c' a a' f b b' f c c' f 8 m d d' f 1.6 m e e' f 1.5 m d' e' d' e' site a site b site a site b 0 20 40 60 80 100 120 140 160 180 (m) 10 8 (m) 6 4 2 0 20 40 60 80 100 120 140 160 180 (m) 10 8 (m) 6 4 2 0 20 40 60 80 100 120 140 160 180 (m) 10 8 (m) 6 4 2 0 20 40 60 80 100 120 140 160 180 (m) 8 (m) 6 4 2 0 20 40 60 80 100 120 140 160 180 (m) 8 (m) 6 4 2 fig. 3. geomorphic map around the yungye-ri area and topographic profiles across the ysf. a and b are the locations of excavations across fault. the lines from a-a to e-e indicate topographic profiles. table i. c14 dating result in the northern part of the ysf. sample n. lab. n. radiometric calibrated age 2 age (calendar year): years bp (yr) nysf-1 snu 99-137 1550 ± 90 1314 (a.d. 536) a.d. 261-660 nysf-2 snu 99-138 1930 ± 90 1875 (a.d. 75) b.c. 163-a.d. 321 nysf-3 snu 99-116 2410 ± 90 2359 (b.c. 409) b.c. 796-b.c. 233 nysf-4 snu 99-115 3060 ± 80 3298 (b.c. 1348) b.c. 1504-b.c. 1050 nysf-5 snu 99-136 6250 ± 150 7182 (b.c. 5232) b.c. 5482-b.c. 4801 nysf-6 snu 99-135 9870 ± 340 11.230 (b.c. 9280) b.c. 10.845-b.c. 8323 nysf-7 snu 99-118 25.900 ± 730 na – nysf-8 snu 99-133 36.600 ± 780 na – nysf-9 snu 99-134 39.400 ± 570 na – * na: not applicable, calibrated data set only extends back to 24.900 years bp (stuiver et al., 1998). deposits on both sides of the fault has ams radiocarbon dates of 36.600 ± 780 (snu 99133) years bp and 39.400 ± 570 (snu 99-134) years bp, but these data cannot be calibrated because the calibration of the data extend only back to 24.900 years bp (stuiver et al., 1998). a trench at site b, which was about 10 m south from site a, exposed gravelly alluvium covered by humic, silty sediments (fig. 5). the gravel deposits and humic silt were displaced by reverse faulting. the fault plane is steep in bedrock and becomes less steep upward to the west. calibrated radiocarbon dating on organic material in the humic layer was dated at 1314 (a.d. 536) (snu 99-137) years bp, 1875 (a.d. 75) (snu 99-138) years bp, 2359 (b.c. 409) (snu 99-116) years bp and 3298 (b.c. 1348) (snu 99-115) years bp from the upper to the lower layer, as shown in table i. the humic silt and gravel are vertically displaced about 50 cm down to the west. the four radiocarbon ages from the humic silt suggest that deposition of this unit began before about 2400 years bp and ended after 1314 years bp. however, the humic material in this silt was from samples that were buried only about 0.75-1.25 m below the surface. at such shallow depths in a humid climate such as southeastern korea, it is possible that some of the carbon in these samples could be from younger sources such as soluble humic acids. if these samples are contaminated by younger carbon, then the apparent age of this deposit could be too young. if the samples are not contaminated, then the radio988 jai bok kyung 0 1 3 4 5 6 7 89 (m) 2 0 1 2 3 5 4 qt2 qa qt2 colluvium11 230 (b.c. 9280) 7182 (b.c. 5232) 25 900±730 silt fault gouge gravel basement rock recently active fault old fault in bed rock legend m 36 600±780 fault west east 2 m 2 m fig. 4. log of the northern wall across the ysf at site a in fig. 3. carbon ages indicate that the silt was faulted less than 1314 (a.d. 536) years bp, which is the maximum age of the most recent event at this site. 3.2. southern part of the ysf (ts in fig. 2) in the eonyang to tongdo-sa area on the southern part of the ysf, fluvial terraces formed by eastward-flowing streams are well developed in a relatively wide (a few km) fault valley (fig. 6). the terraces can be divided into four levels (qt1, qt2, qt3, qt4), as shown in fig. 6. following geomorphologic and geological studies in the area, we excavated several trenches across the ysf in a 10 km long section between eonyang to tongdo-sa area. the trenches exposed clear fault traces and steeply dipping fault contacts between old terrace gravels and the basement rocks. 3.2.1. trench t1e and t1w studies of the topography and aerial photographs show evidence of three sub-parallel fault strand between kyodong-ri and shinhwa-ri (fig. 6). the eastern strand has a clear scarp about 0.6 km long and 12 m high. the trench across this scarp exposed a sharp fault contact between alluvial gravel and cretaceous rocks of taegu formation (fig. 7a) (kyung et al., 1997). the fault strikes n40°e and dips northwestpaleoseismology of the yangsan fault, southeastern part of the korean peninsula fig. 5. log of the northern wall across the ysf at site b in fig. 3. 989 fig. 6. geomorphic map and topographic profiles in the southern part of eonyang area, showing active faults and trench sites. the lines from a-a to f-f indicate topographic profiles. jai bok kyung 990 ward about 80°n. the fault was covered with highly weathered reddish soil. the total vertical displacement on the fault is about 12 m from the topographic profile. although the fault has been active since deposition of qt4 terrace gravel, its activity was probably not so high, judging from narrow cataclastic and gouge zone. a road sub-parallel to the central strand of the fault made it impossible to make a trench. however, the fault scarp is as high as about 8 m with up-to-the-east displacement (okada et al., 1994). several trenches were dug across the western strand of the fault, which has a 5 m high scarp. these trenches did fig. 7a,b. sketches of the northern wall across the ysf at site t1e (a) and t2 (b) in fig. 6. paleoseismology of the yangsan fault, southeastern part of the korean peninsula 991 northern wall northern wall a b not expose a clear fault contact, but rather surficial reddish soil and highly weathered gravels were gently warped in to a westward-dipping monocline that formed after deposition of the qt4 terrace gravels (kyung et al., 1999b). the total vertical displacement on all three strands of the fault in this area is about 25 m. 3.2.2. trench t2 south of trench t1, we identified two quaternary fault splays in qt4 terrace deposits that form a subtle monoclinal scarp (fig. 6). the eastern strand seems to be the main ysf and has a clear scarp and a zone of shattered rock that is tens of meters wide with a prominent fault gouge. although the ground surface is modified, it shows vertical displacement of about 10 m on the fault. a highway close to the eastern strand made it impossible to trench here. so we excavated trench t2 across the western strand. on this strand, the surface still has an eastside-up scarp that has a few meters of relief even though the surface has been modified artificially. this subsidiary strand is parallel to and about 300 m west of the eastern fault. in the trench, the fault dipped 70-85°e and had a strike of n2030°e. it defines the contacts between gravel of qt4 deposits and granitic basement rock (fig. 7b) (kyung et al., 1999a). the fault is marked by a zone of clay fault gouge that is as wide as 20 cm and a cataclastic zone of about 1 m wide. striations on the fault plunge 20-30°n. elongate gravel clasts near the fault are rotated parallel to the fault plane, and fault breccia in the gouge zone is elongated in the strike direction of fault. the en échelon arrangement of fault clay in the gouge zone indicates that recent fault movement has been predominantly rightlateral. the total amount of vertical displacement at this site is estimated to be about 10 m based on the offset of a gravel layer found in a drill hole. thermoluminescence dating of the fault gouge suggests that the most recent movement on the fault is younger than about 100 ka (kyung et al., 1999a). but we could not find datable material in the terrace gravel to farther constrain the age of the faulting. the total vertical displacement on these two strands of the fault in this area is more than 20 m. 3.2.3. trench t3 south of trench 2, we excavated a trench across the fault where it crosses qt1 terrace deposits (qt1), which are present only adjacent to the sangcheon stream. the fault in this trench strikes n25°e and dips 80°se. it separates highly weathered gravels (probably qt4 alluvium) from cretaceous granitic rock (fig. 8a). in the trench, the faulted qt4 gravel is buried by about 1 m of unweathered gravel, which extends across the fault zone and is not displaced. based on its unweathered character, this gravel is thought to be holocene in age, but we could not find datable materials in this deposit. the topograhic profile does not indicate a meaningful deformation across the fault strike. the stratigraphic relations in this trench show that the weathered gravel of probable late quaternary age is faulted, but younger holocene (?) gravel is not faulted. the zone of fault gouge in this trench is as wide as 20 cm and the zone of cataclastic rock is several meters wide. stereographic projection of the long axes of elongate gravel clasts adjacent to the fault are oriented subhorizontally (angle of 0-30°), which is subparallel to striations in the fault gouge. these observations indicate that the predominant sense of motion on the fault has been right-lateral with a minor component of up-to-the-east reverse slip. 3.2.4. trench t4 at site t4, qt4 gravel across the estimated fault was excavated by okada et al. (1994). here, the up-to-the-east vertical displacement is estimated about 12 m based on a topographic profile. in this trench, the fault plane separates terrace gravels and basement rocks, strikes n25°e, and is nearly vertical (fig. 8b). the trench exposed a shattered zone several meters wide and typical fault gouge. the attitude of clasts in the gravel and elongation of clay fabric and breccia in the gouge zone suggest mainly strike-slip movement on the fault. 992 jai bok kyung 4. discussion and conclusions the ysf is a major structure in the southeastern korea, and the data presented here provides new insight into late quaternary movement on the fault. the fault’s linear trace, its orientation with respect to the current compressive stress field, and slip indicators in several of the trenches described here all show that recent motion on the fault is dominantly right-lateral paleoseismology of the yangsan fault, southeastern part of the korean peninsula t3 t4 1 2 3 4 5 6 7 8 9 10 0 -1 -2 -3 (m) northern wall 0 1m 1 2 3 4 5 6 7 8 9 10 0 -1 0 1m fresh gravel highly weathered gravel fault gouge basement rock northern wall artificial fill gravel t brecciafelsic igneous rock light colored part within fault gouge fault gouge andesite fig. 8a,b. sketches of the northern wall across the ysf at site t3 (a) and at site t4 (b) (after okada, 1994) in fig. 6. 993 a b strike-slip. however, the presence of prominent scarps locally along the fault and the evidence of reverse slip documented in several trenches indicates a significant component of compression on the fault. this combination of lateral and reverse faulting is common in many major strike-slip fault systems (e.g., van dissen and yeats, 1991). 4.1. northern yansang fault the northern part of the ysf (yugye-ri area) is characterized by several fault strands, zones of shattered bedrock tens of meters wide, and a prominent zone of fault gouge as wide as 1 m. eastward-plunging striations and stretching of gravel beds parallel to the fault plane indicate that recent movements have been mainly reverse slip. on terrace qt2, reverse slip has formed a 1.5 m high scarp (fig. 3), and holocene alluvial gravels are vertically offset about 0.5 m (fig. 5). based on the analysis of aerial photographs and field mapping, we recognize four terraces along this part of the ysf. the ages of individual terraces are only generally known, but estimating the age of these deposits permits us to generally estimate slip rates on the fault. the formation of major alluvial terraces is probably related to significant climate events in the quaternary. the formation of marine terraces along the adjacent parts of the korean coastline have been correlated with climatically related high stands of sea level, and the climatic conditions that produced these high stands may have also produced conditions favorable for the formation of alluvial terraces inland. according to the general classification of river terrace in south korea (park and kong, 2001), the high terrace surface formed during the mindel glacial, the middle terrace surface during riss glacial, and the lower terrace surface during the last glacial. based on the relative height from the level of stream, the degree of weathering the gravel, and the characteristics of soil development, we can generally correlate the terraces as follows: qt1 last glacial (30-10 ka), qt2 early last glacial (30-70 ka), qt3 late riss glacial (?), qt4 late mindel glacial (about 450 ka?). on the basis of the estimated ages of the terraces, the amount of vertical displacement on those terraces, the vertical slip rate on the northern section of the fault is in the range of about 0.03-0.05 mm/yr. we do not have data that permit us to estimate the amount of quaternary lateral slip or a lateral slip rate. in the trench at site b (fig. 5), a humic silt that has an age of about 1314 cal years bp, has been offset by recent faulting. thus the most recent event on this part of the fault occurred about a.d. 536 (table i; 2 of a.d. 221-a.d 667). historical accounts describe several strong, damaging earthquakes that have occurred in this part of the korean peninsula. these accounts indicate that strong earthquakes occurred in a.d. 304, a.d. 471, a.d. 510, and a.d. 630 (fig. 1). although the two-sigma range of ages for this sample spans the time range for many of these events, the preferred calibrated age for this event (a.d. 536) may correspond to the historical accounts of a damaging earthquake in a.d. 510 that caused fatalities. 4.2. southern yangsan fault locally, the southern part (eonyand to tongdo-sa) of the ysf consists of at least three strands. fault scarps of different heights on quaternary deposits of different ages are evidence of repeated late quaternary movements. the trace of the fault in this area is very linear, and the fault plane exposed in trenches is high angle to vertical. fault motion in this area is mainly rightlateral strike-slip as shown by rotated clasts adjacent to the fault zone and striations on the fault plane. a component of reverse slip has elevated the eastern side of the fault and resulted in scarps that are more than 10 m high on terrace qt4. based on the analysis of aerial photographs and field mapping, we recognize several sets of prominent terraces along parts of the ysf. the ages of individual terraces are only generally known, but estimating the age of these deposits permits us to establish general estimates of the slip rate. jo (1997) approximately correlated the higher terrace (qt4) to the mindel period or mindel/riss interglacial period. okada et al. (1994) estimated an age of 250-500 ka for terrace 994 jai bok kyung qt4, about 100 ka for terrace qt3, and 30-60 ka for terrace qt2. based on the relative height of terraces above stream level, weathering of the gravel and characteristics of soil development, we can approximately correlate the terraces to the classification of river terraces (park and kong, 2001) as follows: qt1 last glacial (30-10 ka), qt2 early last glacial (30-70 ka), qt3 late riss glacial (?), qt4 late mindel glacial (about 450 ka?). the cumulative amount of vertical offset on qt4 deposits is about 25 m on all strands, and scarps as high as 2-7 m are present on qt3 deposits. scarps as high as about 2 m are present on qt2 deposits. using the height of scarps on terraces of different ages, the vertical slip rate is in the general range of 0.02-0.07 mm/yr. the lateral slip rate may be several times larger than the vertical rate, based on the observation of near the horizontal striation for the part of the fault between eonyand and tongdo-sa. the most recent event on this part of the fault is thought to be pre-holocene in age based on the evidence of unfaulted deposits of probable holocene age (fig. 8a) and the absence of any scarps in holocene deposits. acknowledgements i thank anthony crone for critical advices and reviews that led to substantial improvement of this paper. i am also grateful to two anonymous referees for their constructive comments on the original manuscript. the work was supported in part by kosef (korea science and engineering foundation) and by kigam (korea institute of geoscience and mineral resources). references chae, b.g. and t.w. chang (1994): movement history of yangsan fault and its related fractures at chonghayongdok area, korea, j. geol. soc. korea, 30 (4), 379-394. chang, c.j. 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(1996): mesozoic tectonics in korea, j. southeast asian earth sci., 13, 251-265. kim, y.w. and k. lee (1988): a geoelectric study on the structure of the yangsan fault in the south of kyeongju, j. geol. soc. korea, 24, 47-61. korea institute of energy and resources (1981): geological map of korea (scale 1:1000 000). kyung, j.b. and t.w. chang (2001): the latest fault movement on the northern yangsan fault zone around the yugye-ri area, southeast korea, j. geol. soc. korea, 37, 563-577. kyung, j.b. and c.b. im (2001): active fault study in the southeastern part of the korean peninsula, in ten years of paleoseismology in the ilp: progress and prospects, december 2001, kaikoura, new zealand, program and abstracts, 75. kyung, j.b., k. lee, a. okada, k. takemura, m. watanabe, y. suzuki and t. naruse (1997): active fault study in the central part of the yangsan fault, southeastern part of korea, in tectonic-evolution of eastern asian continent: geological society of korea 50th anniversary int’l symp., edited by y.i. lee and j.h. kim (geological society of korea), 33-38. kyung, j.b., k. lee, a. okada, m. watanabe, y. suzuki and k. takemura (1999a): study of fault characteristics by trench in the sangchon-ri area in the southern part of the yangsan fault, southeastern korea, j. korean earth sci. soc., 20, 101-110 (in korean). 995 paleoseismology of the yangsan fault, southeastern part of the korean peninsula kyung, j.b., k. lee and a. okada (1999b): a paleoseismological study of the yangsan fault analysis of deformed topography and trench survey, j. korean geophys. soc., 2, 155-168 (in korean). lee, d.s. 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(1981): marine terraces and their tectonic deformation on the coast of the southern part of the korean peninsula, bull. dep. geogr. univ. tokyo, 13, 1-61. oh, g.h. and s.g. choi (2001): marine terraces of korea, in the quaternary environment of korea, edited by y.a. park and w.s. kong (seoul national university press), 159-191 (in korean). ree, j.h., m. cho, s.t. kwon and e. nakamura (1996): possible eastward extension of chinese collision belt in south korea: the imjingang belt, geology, 24, 10711074. reedman, a.j. and s.h. um (1975): the geology of korea (geology and mining institute of korea), pp. 139. stuiver, m., p.j. reimer, e. bard, j.w. beck, g.s. burr, k.a. hughen, b. kromer, f.g. mccormac, j. plicht and m. spurk (1998): intcal98 radiocarbon age calibration 24.000-0 cal bp, radiocarbon, 40, 1041-1083. van dissen, r. and r.s. yeats (1991): hope fault, jordan thrust, and uplift of the seaward kaikoura range, new zealand, geology, 19, 393-396. yoon, s. and j.y. kim (1990): tectonic history of the yangsan fault, southwestern part of the korean peninsula, in pacific neogene events-their timing, nature and interrelationship, edited by r. tsuchi (university of tokyo press), 151-160. yoon, s.h. and s.k. chough (1995): regional strike-slip in the eastern continental margin of korea and its tectonic implications for the evolution of ulleung basin, east sea (sea of japan), geol. soc. am. bull., 107, 83-97. 996 jai bok kyung s u l l a a p p l i c a z i o n e d e l m e t o d o d e l l e e m u l s i o n i n u c l e a r i a l l o s t u d i o d e l l a r a d i o a t t i v i t à d e l l e l a v e v e s u v i a n e ( * ) m . ferretti-sforzini c. f e s t a f . ippolito è s e m b r a t o i n t e r e s s a n t e a p p l i c a r e il m e t o d o delle e m u l s i o n i nuc l e a r i a l l o studio della r a d i o a t t i v i t à delle lave del v e s u v i o . t r a le i n f o r m a z i o n i c h e s e m b r a v a n o r a g g i u n g i b i l i con questo metodo e r a n o l e s e g u e n t i : a) c o n f e r m a della v a r i a z i o n e del c o n t e n u t o in e l e m e n t i rad i o a t t i v i c o n la data di e r u z i o n e ; b i identificazione dei c e n t r i di emiss i o n e delle sostanze a l f a e m i t t e n t i ; c) d e t e r m i n a z i o n e della esistenza d e l l ' e q u i l i b r i o r a d i o a t t i v o n e l l a f a m i g l i a d e l l ' u r a n i o e, n e l caso in c u i t a l e e q u i l i b r i o non esista, d e t e r m i n a z i o n e del t e m p o di r o t t u r a d e l l ' e q u i l i b r i o stesso. l e r i c e r c h e sono stale eseguite con l a s t r e i l f o r d c2 da 50 [i. esposte a l l ' i r r a g g i a m e n t o di sezioni sottili di lava (spessore s u p e r i o r e a 50 [i. ) a p p a r t e n e n t i ad e r u z i o n i di d i f f e r e n t e età. è n o t o c h e una v a r i a z i o n e del c o n t e n u t o in e l e m e n t i r a d i o a t t i v i con la data di e r u z i o n e è stata r i s c o n t r a t a da vari a u t o r i ( r ) . t a l i misuro sono da m e t t e r e in r e l a z i o n e con q u e l l e eseguite da h o l m e s ( 2 ) e più r e c e n t e m e n t e da k e e v i l ( 3 ì sui g r a n i t i . l ' i n t e r e s s e della r i c e r c a sta n e l c e r c a r e di s t a b i l i r e le cause c h e a b b i a n o p o t u t o dar luogo a f e n o m e n i q u a l i q u e l l i osservati, i q u a l i non t r o v a n o r i s c o n t r o n e l l e osservazioni a n c h e di r e c e n t e eseguite su p r o d o t t i lavici di a l t r i vulcani c o m e ad e s e m p i o d e l l ' e t n a ( 4 ) . d ' a l t r o c a n t o la n o t e v o l e difficoltà di p r o c u r a r s i d i r e t t e i n f o r m a z i o n i sui complessi f e n o m e n i della evoluzione e della d i f f e r e n z i a z i o n e del m a g m a somma-vesuviano, i l l u s t r a t i da r i t t m a n n ( 5 ) fino d a l 1933, spinge ad a p p r o f o n d i r e alcun poco queste r i c e r c h e il cui interesse m a g m a l o g i c o e g e o c h i m i c o è di p e r sé evidente. * * * i r i s u l t a t i delle nostre r i c e r c h e c o n f e r m a n o l ' a n d a m e n t o crescent e d e l l ' a t t i v i t à con la data di e r u z i o n e . l ' a t t i v i t à g l o b a l e m e d i a dei ( * ) c o m u n i c a z i o n e presentala alla « a s s o c i a t i o n i n t e r n a t i o n a l e de s é i s m o l o g i e et p h y s i q u e de f i n t é r i e u r de la t e r r e » nella x a s s e m b l e a g e n e r a l e d e l l ' u . g . g . i . r o m a 195-1. r 5 i o hi. f e r r e t t i s f o r z i ni c . f e s t a f . i p p o l i t o t r e c a m p i o n i di lava appartenenti rispettivamente alle eruzioni del 79 d. c., 1760, e 1872 è risultata pari a : 8,5 x io'1 tracce alfa per era2 e per sec. per la lava del 79 d. c. 31,5 x 10"' t r a c c e alfa per cm 2 e per sec. per la lava del 1760 38,7 x 10"' tracce alfa per cm 2 e per sec. per la lava del 1872. nell'ipotesi che l'uranio contenuto nella lava sia in e q u i l i b r i o con tutti i suoi discendenti da questi valori si desume una c o n c e n t r a z i o n e di uranio pari a : 0,85 x io'5 g di v per g di lava per la lava del 79 d. c. 3,02 x io"5 g di u per g di lava per la lava del 1760 3,71 x io"3 g di u per g di lava per la lava del 1872. « » # i risultati parziali relativi al punto b) sono già stati p u b b l i c a t i in una nota di b a c h e l e t , f e r r e t t i s f o r z i n i e ippolito (l !ì. è stala allora esaminata una lava proveniente dalla colata di castel c i s t e r n a , a t t r i b u i t a , come è noto, alla eruzione « p l i n i a n a » del 79 d.c. si t r a t t a di una t e f r i t e a leucite a struttura porfiriea, con massa f o n d a m e n t a l e olocristallina a grana fine. i fenoeristalli (delle dimensioni di qualc h e m m ) sono costituiti essenzialmente da augite e da l e u c i t e ; quelli di augite, più abbondanti e un po' più sviluppati (fino a 2.5 mm), presentano generalmente una forma prismatica e contengono quasi sempre numerosi i n c l u s i ; quelli di leucite per l o più abbastanza l i m p i d i contengono in genere pochissimi inclusi piuttosto piccoli. v a infine n o t a t a la presenza di qualche fenocristallo di olivina, c o m p l e t a m e n t e alterato, o quasi, in ematite e limonite. l a massa f o n d a m e n t a l e è costituita da plagioclasi (del tipo labrador-bitowniticoi, augite, leucite e m i n e r a l i opachi (magnetite); subordinatamente si notano m i c r o l i t i di sanidina, olivina, biotite, orneblenda e apatite. i n p a r t i c o l a r e nella sezione esaminata la biotite risulta piuttosto r a r a e per lo più priva di inclusi. l'esame della distribuzione delle tracce delle p a r t i c e l l e alfa indica anzitutto che in questa lava non esistono m i n e r a l i specifici di u r a n i o o di torio, come del resto era prevedibile in base a considerazioni magmalogiche. si è inoltre riscontrata l'esistenza di zone n e l l e quali si h a concentrazione di m a t e r i a l e non attivo e zone n e l l e quali il m a t e r i a l e attivo è invece p a r t i c o l a r m e n t e concentrato rispetto al s u l l a a p p l i c a z i o n e del m e t o d o d e l l e e m u l s i o n i n u c l e a r i 5 4 1 resto della lastra. l e aree di densità minore corrispondono ai fenocristalli : ciò indica che nei fenocristalli intratellurici, formatisi nella massa magmatica prima della sua estrusione, non vi è stata una sensibile sostituzione isomorfica di elementi radioattivi con altri di prossimo raggio ionico. dobbiamo quindi ammettere cbe le sostituzioni isomorfiche di altri metalli da parte di elementi radioattivi avvengano in presenza della fase gassosa del magma, all'atto della eruzione e della successiva rapida consolidazione della lava. questo fatto si inquadra perfettamente col noto comportamento geochimico degli e l e m e n t i radioattivi che tendono a restare nella fase gassosa, ed a dare, nel caso di completa solidificazione, minerali specifici negli stadi da pneumatolitico a idrotermale. l e concentrazioni di particelle alfa superiori alla media sono pres u m i b i l m e n t e da attribuire alla presenza nei minerali essenziali (in fenocristalli o in microliti del feltro) di qualche incluso (zircone, ilmenite, titanite) i quali, come è nolo, sono relativamente r i c c h i di uranio e torio in sostituzione isomorfogena. l e zone r i m a n e n t i , con la loro distribuzione di particelle alfa abbastanza omogenea, ci autorizzano a r i t e n e r e che gli elementi radioattivi abbiano sostituito isoi n o r f i c a m e n t e altri ioni di prossimo raggio ionico più o meno in tutti i m i n e r a l i della massa f o n d a m e n t a l e ; p r o b a b i l m e n t e l'uranio avrà sostituito il calcio. p e r quanto riguarda il punto c) si può osservare anzitutto che la evoluzione del magma somma-vesuviano ha p r o b a b i l m e n t e dato luogo a fenomeni geochimici selettivi, per azione delle masse magmatiche sulle rocce di contatto. in conseguenza di tali fenomeni può essersi verificata una alterazione dell'equilibrio tra gli elementi delle famiglie r a d i o a t t i v e ; alterazione che nel mutato chimismo del magma, passato da « ottavianitico » a « vesuvitico » (dal 79 al 18721 potrebbe trovare una ovvia spiegazione. d'altra parte nello studio di fenomeni di. siffatta natura occorre considerare che gli elementi delle tre famiglie radioattive, clic via via si formano, h a n n o caratteristiche geochimiche affatto diverse dall'elemento originario ( l 3 8 , t/23'' o th-i2) e possono quindi concentrarsi in particolari minerali eli contatto o p a r t e c i p a r e a processi differenziativi, essendo in tal modo sottratti al successivo sviluppo dei fenomeni della massa magmatica. p e r queste ragioni ci si è proposti di studiare se dalla misura degli spettri di energia delle particelle alfa si possono dedurre infor5 4 2 m . f e r r e t t 1 s f 0 r z i n i c . f e s t a f . i p p o l i t o inazioni sullo stato di evoluzione delle f a m i g l i e radioattive nei mater i a l i e r u t t a t i dal vesuvio. i l p r o b l e m a riguarda u n i c a m e n t e gli elem e n t i della famiglia dell'uranio, perché si può r i t e n e r e senz'altro c h e il torio sia in e q u i l i b r i o con i suoi discendenti in t u t t i e tre i campioni di lava esaminati. supponiamo che altrettanto si possa dire della famiglia d e l l ' u r a n i o dallo ionio in poi. si tratta allora di determinare la concentrazione dell'uranio e dello ionio n e l c a m p i o n e di l a v a : dai valori di queste concentrazioni si può d e t e r m i n a r e 1111 l i m i t e superiore del tempo al quale è avvenuta la rottura d e l l ' e q u i l i b r i o , ragionando c o m e segue. s u p p o n i a m o che la rottura d e l l ' e q u i l i b r i o sia avvenuta al tempo — f ; il caso più p r o b a b i l e è che al tempo — t , in conseguenza di q u a l c h e processo geochimico, il m a t e r i a l e a b b i a perduto una c e r t a quantità di torio e quindi di ionio. sia ri la lunghezza m i n i m a misurabile al microscopio ed r„ la proiezione orizzontale del massimo range residuo delle particelle alfa emesse da ul e da un: se con z ( r ) si indica il numero di tracce per cm 2 e p e r sec. la cui proiezione orizzontale è maggiore o uguale ad r si h a : z ( r l ) = ^ [ i / ] „ / i l + b l i ( r 1 ) + t ! , / o r j e / i o ( r 1 ) [1] 2 (r,)" = + / . [ , „ ] . / , „ ( ' • , ) dove ò è la p e r m e a b i l i t à alfa del m i n e r a l e , [f/]„ è la concentrazione attuale dell'uranio, [ i o ] ( . la concentrazione di ionio in e q u i l i b r i o con l'uranio. jul-\-ullè la funzione di distribuzione delle proiezioni orizzontali dei ranges delle particelle alfa emesse da ul e un, jul è la funzione di distribuzione delle proiezioni orizzontali dei ranges delle particelle alfa emesse dallo ionio in e q u i l i b r i o con i suoi discendenti, e jo è una funzione del tipo 1 — ) in cui àio è la costante di decadimento dello ionio e t un parametro maggiore o uguale a zero (è c h i a r o c h e quest'ultimo caso si verifica quando lo ionio è stato c o m p l e t a m e n t e asportato al tempo — t ) . d a l l e [1| si può agevolmente ricavare /„ e quindi t -jf. è evidente che analoghe considerazioni valgono nel caso in cui la rottura d e l l ' e q u i l i b r i o si sia verificata a partire dal radio, anziché dallo ionio. le funzioni jn^ + uj, sono già state calcolate da b u t t l a r e houtermans ( ' ; nella ipotesi che l'attività del minerale si ripartisca in modo omogeneo sulla superficie in esame, e che il m i n e r a l e sia costituito da cristalli o molto grandi o molto piccoli rispetto al range delle part i c e l l e alfa entro il minerale stesso. i campioni di lava sono stati pers u l l a a p p l i c a z i o n e d e l m e t o d o d e l l e e m u l s i o n i n u c l e a r i 543 ciò studiati mediante integratore presso l'istituto di geologia applicata della università di napoli per stabilire le dimensioni medie dei cristalli in essi contenuti. si è potuto stabilire che un buon 5 0 % di essi h a dimensioni paragonabili al rango, delle particelle alfa nella lava, il che p o r t e r e b b e ad escludere la possibilità di applicare il metodo di b u t t l a r e houterinans. s i può però osservare che la condizione sopra detta non è essenziale per l'applicazione del metodo, purché sia soddisfatta la condizione c h e il valore medio dell'attività su tutta la superficie s esaminata non dipenda sensibilmente dalla profondità z dello strato attivo. si avranno tuttavia delle fluttuazioni attorno alla media dovute alle dimensioni finite dei cristalli. queste fluttuazioni possono tuttavia essere calcolate e si può tener conto dell'errore da esse causato nel modo seguente: se diciamo p(n,z) l a p r o b a b i l i t à di trovare /: c e n t r i emittenti alla profondità z si può scrivere: [ 2 ] ss 3 p con d2 = -—— e t = ns dove \ è uguale a una costante dell'ordine delle dimensioni lineari medie dei cristalli, y è la concentrazione, s la superficie media ed n il numero dei cristalli attivi su un piano normale a . l'integrale di [ 2 ] è : i r — r„v — 4 u — ?„i/j2 [ 3 ] ed esprime la probabilità di trovare una attività totale fzdz ad una profondità compresa fra z e z dz, nota la attività totale f„z 0 dz 0 ad una profondità compresa fra zt, e z0 + dza. ora si ha, usando i soliti simboli e seguendo il metodo di b u t t l a r c h o u t e r m a n s : z(r1) = a i dr f p(r,z)y dz dove r i è la m i n i m a proiezione orizzontale misurata, a è una costante e p(r,z) la p r o b a b i l i t à che alla profondità z si a b b i a una proiezione orizzontale del rango residuo uguale ad r. t e n u t o conto della [ 3 ] si 5 4 4 ai. f e r r e t t i s f 0 r z i n 1 c. f e s t a f . i p p o l i t o può allora dimostrare elle l'errore quadratico medio relativo su z i r i ) r i s u l t a : z(rl) \xn~~ dove z — è dell'ordine del range, e quindi dello stesso ordine delle dimensioni l i n e a r i medie, 10 dei cristalli. essendo n, n u m e r o medio dei c r i s t a l l i attivi, molto grande, come risulta dall'esame mineralogico eseguito, l ' e r r o r e su z ( r t | risulta m i n o r e dell'errore statistico dovuto alle nostre condizioni sperimentali e quindi può essere trascurato. # * * l a studio del problema d e l l ' e q u i l i b r i o r i c h i e d e però la eliminazione delle t r a c c e dovute al torio eventualmente presente nella lava. ora il metodo delle emulsioni fotografiche risulta assai poco sensibile per la determinazione del rapporto dei contenuti in uranio e torio. i n f a t t i se si contano io1 tracce per c a m p i o n e di lava esaminato e non si trovano tracce con proiezione orizzontale maggiore di 39 [i. si può tutt'al più concludere c h e il rapporto numero tracce t h / numero tracce u non è superiore a circa il 1 0 c / c . essendo poi a p a r i t à di conc e n t r a z i o n e l'attività del torio circa 1 3 di quella dell'uranio, se ne potrà dedurre che, in tal caso, il rapporto concentrazione t h concentrazione l non supera verosimilmente il 30c/c, risultato assai modesto data la necessità di contare almeno 1 0 ' tracce. ammesso però di conoscere il rapporto th/u per altre vie e con precisione migliore, si potrebbe applicare vantaggiosamente il metodo delle emulsioni nucleari allo studio d e l l ' e q u i l i b r i o radioattivo perché, c o m e si è detto, lo studio di questo p r o b l e m a è ricondotto alla determinazione del rapporto io/u e la situazione dal punto di vista statistico è in questo caso molto più favorevole. occorre però t e n e r presente che, se n e l l ' i n t e r n o della lava si verifica una diffusione non trascurabile del radon, l ' e q u i l i b r i o potrebbe essere rotto per questa ragione e sarebbe necessario tenerne conto. determinazioni sperimentali per assodare l'entità di questa correzione sono tuttora in corso. v o g l i a m o vivamente ringraziare il p r o f . e . medi, d i r e t t o r e dell'istituto nazionale di geofisica, per aver messo a nostra disposizione i mezzi necessari a questa ricerca. istituto di fisica dell'università, roma — istituto nazionale di geo fisica, roma — istituto di geologia applicata dell'università„ napoli •— luglio 1954. s u l l a a p p l i c a z i o n e d e l m e t o d o d e l l e e m u l s i o n i n u c l e a r i 5 4 5 riassunto il metodo delle emulsioni nucleari è stato applicato allo studio delle lave vesuviane. in particolare, per quanto riguarda il problema dell'esistenza o meno dell'equilibrio fra gli elementi della famiglia dell'uranio presente nelle lave, un esame più approfondito della questione ha mostrato che, per ottenere informazioni sufficientemente attendibili sul rapporto torio /uranio, è necessario ridurre gli errori statistici ad una misura molto più piccola di quella generalmente ammessa dagli aa. che hanno lavorato in questo campo e si è cos'i stabilito che non si potevano contare meno di diecimila tracce per campione esaminato. occorre però tener presente che, se nell'interno della lava avviene una diffusione non trascurabile del radon, l'equilibrio potrebbe rompersi per questa ragione. determinazioni sperimentali per assodare l'entità di questa correzione sono tuttora in corso. su m mary the metliocl of the nuclear emulsion has been ajìplied to the study of the vesuvian lavas. in particular ive hoped to get the follouing informations: al whetlier the variation of the radioactive content of sudi lavas witli the date of the eruption ivas confìrmed; bl whether it ivas possible to localize the alplia-emitters centers; ci whether il iras fiossi bl e to find a metliod for deciding the existence of the radioactive equilibrium between the elements of the u-family inside the lavas, and, if not, to determine a superior limit for the time of rupture. for the solution of tlie problem ci tre have seen that it is importuni to have statistical errors smaller than those generally admitted by ollier allthors, in order to get the vaine of the ratio th/tj irith a reasonable good approximation. experimentals controls in order to decide whetlier a possible rn-diffusion through surface layers of the lavas should he responsible for a rupture of the equilibrium are stili in jìrogress. b i b l i o g r a f i a ( 1 ) joly j . : « pilli. m a g . » , 18, 5 7 7 1 1 9 0 9 ) . imiìò g. e casertano l . , « a n n . geof. » 6, 315 ( 1 9 5 3 ) . ( 2 ) gutenberg i ! . : internai constitution oj the eartli, n. y o r k ( 1 9 5 1 ) , p. 154. ( 3 ) u . g . g . i . , c o m i t é p o u r l e t u d e de la p h y s i q u e de l ' i n t é r i e u r de la t e r r e . 5 4 6 m . f e r r e t t i s f o r z i n i c. f e s t a f . i p p o l i t o comptcs rendus des séances tenues à bruxelles du 16 au 29 aoiit 1951: « a n n . g é o p h . », 8, 112 ( 1 9 5 2 ) . ( 4 ) b a r b e r a l . , c u r a t o l o m . , addahio i n d o v i n a m . m . , p a l u m b o d . e s a n t a n celo m., « a n n . g e o f . » , 6, 161 ( 1 9 5 3 ) . ( 5 ) rittmann a . , « z e i t . f. v u l k . » , 15, 8 ( 1 9 3 3 ) . ( « ) b a c h e l e t f . , f e r r e t t i s f o r z i n i m . e i p p o l i t o f . , « n u o v o c i n i . » , 8 , 8 5 1 ( 1 9 5 1 ) . ( " ) v. buttlar h . iind houtermans f . g., « g e o d i , et c o s m o c h . a r i a », 2, 43 ( 1 9 5 1 ) . convegni e congressi a s s o c i a z i o n e g e o f i s i c a i t a l i a n a convegno del 10-11 giugno 1952 n e i g i o r n i 10-11 g i u g n o u. s., si è t e n u t o i n r o m a , p r e s s o l a sede d e l l ' i s t i t u t o n a z i o n a l e d i geofisica, il 11° c o n v e g n o d e l l ' a s s o c i a z i o n e geofisica i t a l i a n a p e r t r a t t a r e e d i s c u t e r e , i n a r m o n i a c o n l e finalità s t a b i l i t e n e l l o s t a t u t o , a l c u n i p r o b l e m i geofisici d i a t t u a l e i n t e r e s s e . s o n o i n t e r v e n u t i ed b a n n o p a r t e c i p a t o ai l a v o r i d e l c o n v e g n o i s e g u e n t i s o c i : p r o l i . g i u s e p p i n a a l i v c r t i , c a r m e l o a q u i l i n a , r a u l bil a n c i n i , g i o v a n n i b o a g a , a n t o n i o b o l l e , r e n a t o c i a l d e a , m a r i o d e p a n f i l i s , p i e r o d o m i n i c i , r a m i r o f a b i a n i , g i o r g i o f e a , m a u r i z i o gioi^ gi, l i l i a n a m a r c e l l i , e n r i c o m e d i , f r a n c o m o l i n a , c a r l o m o r e l l i , g u i d o p a n n o c c h i a , f r a n c e s c o p e n t a , f r a n c e s c o p e r o n a c i , e z i o r o s i n i , mar i a n o s a n t a n g e l o , s t e l i o s i l l c n i , l u i g i s o l a i n i , m a r i o t e n a n i , p a o l o e m i l i o v a l l e , g u g l i e l m o z a n o t e l l i . n o n h a n n o p o t u t o p a r t e c i p a r e alla r i u n i o n e m a h a n n o i n v i a t o l a l o r o p i e n a a d e s i o n e al c o n v e g n o i p r o f f . : f r a n c e s c o v e r c e l l i , g i n o cassinis, p a o l o d o r è , g i u s e p p e i m b ò , silvio p o l l i , p i e t r o caloi, gius e p p e t r i b a l t o , e n z o b e n e o , e d o a r d o a r n a l d i , g i o r g i o r o n c a l i , dom e n i c o di f i l i p p o , c a m i l l a f e s t a . a t u t t i i c o n v e n u t i l ' o n . p r o f . e . m e d i , b a r e c a t o il c o r d i a l e b e n v e n u t o d e l l ' i s t i t u t o n a z i o n a l e di geofisica, f o r m u l a n d o l ' a u g u r i o c h e u n o s p i r i t o di s e m p r e p i ù s t r e t t a c o l l a b o r a z i o n e a n i m i i geofisici ital i a n i p e r il m i g l i o r a v v e n i r e d e l l a r i c e r c a s c i e n t i f i c a i n i t a l i a . l ' a s s e m b l e a n e l l a sua p r i m a r i u n i o n e , p r e s i e d u t a d a l p r o f . f r a n cesco p e n t a , h a a p p r e s o c o n vivissimo r a m m a r i c o la f o r z a t a assenza, p e r m o t i v i di s a l u t e del p r e s i d e n t e d e l l ' a s s o c i a z i o n e p r o f . f r a n c e s c o v e r c e l l i , ed h a es pres s o u n a n i m e m e n t e il d e s i d e r i o di f a r g l i p e r v e n i r e gli a u g u r i ed i s a l u t i p i ù a f f e t t u o s i d a p a r t e di t u t t i i soci r i u n i t i . l e v a r i e r i u n i o n i sono s t a t e p r e s i e d u t e s u c c e s s i v a m e n t e d a i p r o f e s s o r i f r a n c e s c o p e n t a , g i o v a n n i b o a g a , e r a u l b i l a n c i n i . i l a v o r i si sono svolti s e c o n d o il s e g u e n t e p r o g r a m m a già i n p r e c e d e n z a s t a b i l i t o : assemblea generale dei soci. martedì 10 giugno: ore 10. a) c o m u n i c a z i o n i d e l l a p r e s i d e n z a ; l>) e s p o s i z i o n e e d i s c u s s i o n e d e l l e c o m u n i c a z i o n i s c i e n t i f i c h e p r e s e n t a t e d a i soci sul t e m a « le c o n o s c e n z e a t t u a l i s u l l a c o s t i t u z i o n e d e l l a c r o s t a t e r r e s t r e ». 3 1 2 c o n v e g n i e c o n g r e s s i 1) r e l a z i o n e d e l p r o f . silvio polli: « m i s u r a d e i l e n i i m o v i m e n t i d e l l a s u p e r f i c i e t e r r e s t r e ». 2 ) r e l a z i o n e d e l p r o f . silvio polli: « g l i a t t u a l i m o v i m e n t i v e r t i c a l i d e l l e coste c o n t i n e n t a l i ». 3 ) r e l a z i o n e d e l p r o f . carlo morelli: « p r i m o c o n t r i b u t o p e r u n a r e t e g r a v i m e t r i c a f o n d a m e n t a l e i n i t a l i a ». 4 ) r e l a z i o n e d e l p r o f . luigi solaini: « c r i t e r i e p r o b l e m i cos t r u t t i v i d i s t r u m e n t i p e r m i s u r e di r e s i s t i v i t à ». martedì 10 giugno: ore 16. 5l r e l a z i o n e d e l p r o f . carmelo aquilina: « l e p r i n c i p a l i i p o tesi s u l l a isostasia ». 6 ) r e l a z i o n e d e i p r o f f . camilla festa e mariano santangelo: « s u l l a fissione s p o n t a n e a d e l l ' u r a n i o n e l l a c r o s t a t e r r e s t r e ». martedì 10 giugno: ore 18. g i t a o r g a n i z z a t a d a l l ' i s t i t u t o n a z i o n a l e di g e o f i s i c a a l l ' o s s e r v a t o r i o g e o e l e t t r i c o d i « s. a l e s s i o » e a l l ' o s s e r v a t o r i o g e o f i s i c o di « rocca d i p a p a ». martedì 10 giugno: ore 20,30. r i c e v i m e n t o a m o n t e c a v o o f f e r t o d a l l ' i s t i t u t o n a z i o n a l e d i g e o f i s i c a . mercoledì 11 giugno: ore 9,30. 7) r e l a z i o n e d e l p r o f . enrico medi: « s u l l ' e l e t t r i c i t à d e l l ' a t m o s f e r a i n r e l a z i o n e c o n i f e n o m e n i di e v a p o r a z i o n e e c o n d e n s a z i o n e ». 8) r e l a z i o n e d e i p r o f f . pietro caloi e liliana marcelli: « s u l l a c o s t i t u z i o n e g e o l o g i c a d e l s u b a t l a n t i c o ». 9| r e l a z i o n e d e l p r o f . renato cialdea: « l a s p e d i z i o n e d e l l ' i s t i t u t o n a z i o n a l e di g e o f i s i c a a k a r t h o u m i n o c c a s i o n e d e l l ' e c l i s s e t o t a l e d i sole d e l 2 5 f e b b r a i o 1952 » . 10) r e l a z i o n e d e i p r o f f . antonio bolle, piero dominici e carlo alberto tiberio: « l e n u o v e a p p a r e c c h i a t u r e i o n o s f e r i c b e d e l l ' i . n . g . » . ili r e l a z i o n e d e i p r o f f . maurizio giorgi, antonio bolle, franco molina e carlo alberto tiberio: « o s s e r v a z i o n i i o n o s f e r i c b e e m a g n e t i c h e i n o c c a s i o n e d e l l ' e c l i s s e s o l a r e d e l 2 5 f e b b r a i o 1952 » . 12) r e l a z i o n e d e i p r o f f . maurizio giorgi, enrico medi e franco molina: « r i l i e v o m a g n e t i c o d e l s e c o n d o o n l i n e d e l l a s i c i l i a » . mercoledì 11 giugno: ore 16. 13) r e l a z i o n e d e l p r o f . guglielmo zanotelli : « u n m e t o d o p e r i l s o n d a g g i o a c u s t i c o d e l l e n u b i ». 14) r e l a z i o n e d e l d o t t . piero dominici: « s u l l ' a s s o r b i m e n t o d e l l e o n d e e l e t t r o m a g n e t i c h e n e l l a i o n o s f e r a ». 313 convegni e c o n g r e s s i ci e l e z i o n e dei n u o v i consiglieri. r/| a p p r o v a z i o n e del b i l a n c i o 1951 e v a r i e . t u t t i i soci h a n n o a t t i v a m e n t e preso p a r t e alle discussini r e l a t i v e ai v a r i t e m i p r e s e n t a t i . a l l e esposizioni delle v a r i e n o t e h a n n o f a t t o seguito i n t e r e s s a n t i e p r o f i c u e discussioni, le q u a l i h a n n o a n c o r a u n a volta m o s t r a t o q u a n t o sia o p p o r t u n a la c o l l a b o r a z i o n e f r a gli studiosi d e l l e v a r i e d i s c i p l i n e geofisichc. l ' a s s e m b l e a h a d e l i b e r a t o c h e t u t t e le c o m u n i c a z i o n i scientifiche c h e sono s t a t e oggetto di s t u d i o del convegno v e n g a n o p u b b l i c a t e n e l l a rivista « a n n a l i di geofisica ». p r i m a della c h i u s u r a del convegno, con le m o d a l i t à s t a t u t a r i e , sono stali eletti d u e n u o v i c o n s i g l i e r i ; in tal m o d o il consiglio dir e t t i v o r i s u l t a a t t u a l m e n t e così c o s t i t u i t o : presidente: p r o f . f r a n c e s c o v e r c e l l i . consiglieri : p r o f . c a r m e l o a q u i l i n a . p r o f . r a u l b i l a n c i n i . p r o f . g i u s e p p e i m b ò . p r o f . f r a n c e s c o p e n t a . p r o f . l u i g i solaini. p r o f . m a r i o t e n a n i . segretario : p r o f . m a u r i z i o giorgi. 11 s e g r e t a r i o h a c o m u n i c a t o all'assemblea l o stalo d e l l e p r a t i c h e già i n o l t r a t e p e r il r i c o n o s c i m e n t o g i u r i d i c o dell'associazione e h a q u i n d i sottoposto a l l ' a t t e n z i o n e dei soci il b i l a n c i o p e r l ' a n n o 1951 il q u a l e è s t a t o q u i n d i a p p r o v a t o . a c h i u s u r a del convegno il p r o f . b o a g a h a r i v o l t o a l l ' a s s e m b l e a da lui p r e s i e d u t a p a r o l e di vivo c o m p i a c i m e n t o p e r il c o n t r i b u t o portato dei soci t u t t i n e l l o s v o l g i m e n t o dei l a v o r i ed b a espresso u n vivo r i n g r a z i a m e n t o all'on. p r o f . e. m e d i p e r la l a r g a o s p i t a l i t à o f f e r t a ai c o n v e n u t i d a l l ' i s t i t u t o n a z i o n a l e di geofisica. l ' o n . p r o f . e. m e d i h a , a sua volta, i n d i r i z z a t o u n caloroso saluto all'associazione e s p r i m e n d o l e l ' a u g u r i o di u n a s e m p r e p i ù f e c o n d a a t t i v i t à . l ' a s s e m b l e a alla fine del convegno h a i n v i a t o u n f e r v i d o teleg r a m m a di o m a g g i o al p r e s i d e n t e p r o f . f r a n c e s c o v e r c e l l i . riassunto delle comunicazioni scientifiche presentate al convegno 315 convegni e c o n g r e s s i s i l v i o p o l l i 1) « misura elei lenti movimenti verticali della crosta terrestre ». si i n d i c a u n p r o c e d i m e n t o c h e p e r m e t t e d i m i s u r a r e i l e n t i mov i m e n t i v e r t i c a l i della crosta t e r r e s t r e m e d i a n t e d e t e r m i n a z i o n e di livelli m e d i m a r i n i . esso vale t a n t o p e r z o n e r i s t r e t t e c h e vaste. si applica d i r e t t a m e n t e alle r e g i o n i costiere e m e d i a n t e u n a livellazione d i p r e c i s i o n e alle z o n e i n t e r n e . esso si basa sulla d e t e r m i n a z i o n e della d i f f e r e n z a f r a le velocità del m o v i m e n t o m e d i o v e r t i c a l e del livello d i t u t t i i m a r i della t e r r a e di q u e l l o c o r r i s p o n d e n t e ad u n a d e t e r m i n a t a s t a z i o n e . t a l e d i f f e r e n z a dà lo s p o s t a m e n t o v e r t i c a l e d e l sito costiero n e l l ' i n t e r v a l l o c o n s i d e r a t o . dai d a t i di t u t t e l e stazioni m a r e o g r a f ì c h e della t e r r a si c a l c o l a n o i v a l o r i dello s p o s t a m e n t o v e r t i c a l e del livello m e d i o m a r i n o p e r ogni 10 a n n i . q u e l l o c o r r i s p o n d e n t e al sito i n esam e si d e d u c e d a l m a r e o g r a f o o dai m a r e o g r a f i posti l u n g o la costa. la p r e c i s i o n e del p r o c e d i m e n t o è di 1 nini di s p o s t a m e n t o v e r t i c a l e i n 10 a n n i . 2) « gli attuali movimenti verticali delle coste continentali ». m e d i a n t e il p r o c e d i m e n t o delle d i f f e r e n z e dei livelli m e d i mar i n i si d e t e r m i n a , p e r le p r i n c i p a l i coste c o n t i n e n t a l i , il v a l o r e dello s p o s t a m e n t o v e r t i c a l e s u b i t o da esse negli u l t i m i 70 a n n i . i r i s u l t a t i sono p r e s e n t a t i i n t a b e l l e e in c a r t i n e geografiche. f i g u r a n o in fase di a b b a s s a m e n t o : d e l l ' e u r o p a , le coste d a l l a g e r m a n i a alla f r a n c i a a t l a n t i c a e q u e l l e m e d i t e r r a n e e f r a n c e s i e i t a l i a n e ; d e l l ' a f r i c a , le coste s e t t e n t r i o n a l i , o r i e n t a l i e o c c i d e n t a l i ; dell'asia, le coste centro-orientali con le isole g i a p p o n e s i e q u e l l e m e r i d i o n a l i con le isole e q u a t o r i a l i ; d e l l ' a m e r i c a , le coste u.s.a. e q u e l l e d e l l ' a r g e n t i n a m e r i d i o n a l e . s o n o i n f a s e di i n n a l z a m e n t o : d e l l ' e u r o p a , l e coste s c a n d i n a v e , l e inglesi s e t t e n t r i o n a l i , le i b e r i c h e e l e s i c i l i a n e ; dell'asia, le coste del b e n g a l a ; d e l l ' a m e r i c a , le coste s e t t e n t r i o n a l i ; d e l l ' a u s t r a l i a , le coste o r i e n t a l i e della n u o v a z e l a n d a . i n g e n e r a l e si h a u n a b b a s s a m e n to nella fascia c o m p r e s a t r a la l a t i t u d i n e 20° e 45° ed i n n a l z a m e n t o n e l l a r e g i o n e s o p r a la l a t i t u d i n e di 45° n. c a r l o m o r e l l i « primo contributo per una rete gravimetrica fondamentale in italia ». m e d i a n t e d u e g r a v i m e t r i w o r d e n p r e v i a m e n t e t a r a t i con g r a n o d e p r e c i s i o n e (0,35','e) e c o n v e n i e n t e m e n t e i m p i e g a t i ( t r a s p o r t o aereo, coll e g a m e n t i c h i u s i n e l m i n o r temilo possibile, ecc.), l ' i s t i t u t o n a z i o n a l e 3 1 6 c o n v e g n i e c o n g r e s s i (li geofisica h a i s t i t u i t o i n i t a l i a u n a p r i m a r e t e f o n d a m e n t a l e di stazioni di r i f e r i m e n t o , d i s t r i b u i t e o p p o r t u n a m e n t e i n l a t i t u d i n e . i c o l l e g a m e n t i eseguiti c o s t i t u i s c o n o t u t t i p o l i g o n i c h i u s i , e gli e r r o r i di c h i u s u r a n o n s u p e r a n o m a i 0,12 m g a l . c o m e e s e m p i o della possibilità di c o m p e n s a z i o n e , è stato a p p l i c a t o il m e t o d o dei m i n i m i q u a d r a t i al q u a d r i l a t e r o c o m p l e t o r o m a n a p o l i c a t a n i a p a l e r m o : l'err o r e m e d i o di u n c o l l e g a m e n t o è r i s u l t a t o + 0,04 mgal. tn conseg u e n z a d e l l ' a l t a p r e c i s i o n e della r e t e , u n a q u a l u n q u e delle stazioni f o n d a m e n t a l i p u ò essere u t i l i z z a t a n e i c o l l e g a m e n t i i n t e r n a z i o n a l i . p e r i f u t u r i c o l l e g a m e n t i aerei n a z i o n a l i e i n t e r n a z i o n a l i , sono s t a t e inolt r e i s t i t u i t e i n q u a s i t u t t i gli a e r e o p o r t i civili stazioni sussidiarie, coll e g a t e con g r a n d e c u r a con q u e l l e p r i n c i p a l i . sono state i n f i n e eseguite, con la p r e c i s i o n e del 0,4%<-, su basi di t a r a t u r a e c o n t r o l l o p e r i grav i m e t r i a piccola scala (3 n e l l ' i t a l i a sett., 1 n e l l ' i t a l i a c e n t r a l e e 2 i n sicilia) ed u n a p e r i g r a v i m e t r i a g r a n d e scala ( p a g a n e l l a ) . l u i g i s o l a i n i « criteri e problemi costruttivi di strumenti per misure di r>>sistività ». si d i s c u t o n o le v a r i e s o l u z i o n i o p e r a t i v e e s t r u m e n t a l i r e l a t i v e alla esecuzione di g r a n d i so n d ag g i e l e t t r i c i v e r t i c a l i . si a u s p i c a la a d o z i o n e di c o r r e n t i a l t e r n a t e a bassa f r e q u e n z a e di i n t e n s i t à n o n t r o p p o elevata e di s t e n d i m e n t i eseguiti con s o n d e p o t e n z i o m e t r i c h e fisse e r a v v i c i n a t e . si p a s s a n o finalmente in r a s s e g n a le possibili soluzioni p e r gli s t r u m e n t i di m i s u r a . c a r m e l o a q u i l i n a « le principali ipotesi siili'isostasia ». l ' a u t o r e r i c h i a m a le t r e p r i n c i p a l i i p o t e s i sull'isostasia, e cioè q u e l l a d o v u t a a p r a t t , q u e l l a di a i r y e q u e l l a di v e n i n g m e i n e z . n e l f a r n e il c o n f r o n t o e la c r i t i c a p r o p o n e , t r a l ' a l t r o , di p r e n d e r e i n c o n s i d e r a z i o n e le e v e n t u a l i s p i n t e , secondo il p r i n c i p i o di p a s c a l , d o v u t e alle a z i o n i dei pesi nei f o r t i dislivelli topografici. c. f e s t a m. s a n t a n g e l o « sulla fissione spontanea dell'uranio nella crosta terrestre ». la d e t e r m i n a z i o n e d e l l a c o m p o s i z i o n e i s o t o p i c a dei gas r a r i c h e si t r o v a n o n e l l ' a t m o s f e r a e di quelli c o n t e n u t i n e l l a crosta t e r r e s t r e , p u ò d a r e i n f o r m a z i o n i sui processi di f o r m a z i o n e di esse. t r a le cause c h e d e t e r m i n a n o v a r i a z i o n i i s o t o p i c h e vi sono q u e l l e 317 convegni e c o n g r e s s i d o v u t e a r e a z i o n i n u c l e a r i : q u e s t o ad es. il caso d e l l ' a r g o n il cui isot o p o di massa 40 v i e n e g e n e r a t o p e r c a t t u r a k d a l p o t a s s i o 40. n e l caso dello xe e kr ci si d o m a n d a se e q u a n t o il processo d i fissione s p o n t a n e a dell'j72 3 8 c o n t e n u t o n e l l a t e r r a c o n t r i b u i s c a a var i a z i o n i della c o m p o s i z i o n e isotopica di q u e s t i e l e m e n t i . se si accett a n o i d a t i sulla vita m e d i a di q u e s t o processo r e c e n t e m e n t e trovati, e si calcola l ' u r a n i o totale i n base alle i p o t e s i c o r r e n t i , si t r o v a elio lo xe ed il kr g e n e r a t i p e r fissione s p o n t a n e a d e l l ' u r a n i o t e r r e s t r e i n 3 , 3 . 10:) a. ( v a l o r e di h o l m e s ) r a p p r e s e n t a u n a piccola f r a z i o n e d i t u t t o lo xe. e kr c o n t e n u t i n e l l ' a t m o s f e r a . si p r o s p e t t a q u i la possibilità di r i s c o n t r a r e d e l l e v a r i a z i o n i locali d e l l a c o m p o s i z i o n e isotopica di q u e s t i e l e m e n t i , connesse con le conc e n t r a z i o n i di u r a n i o . e n r i c o m e d i « siili'elettricità dell'atmosfera in relazione con i fenomeni di evaporazione e condensazione dell' acqua ». e s p o n e d e t t a g l i a t a m e n t e u n complesso di r i c e r c h e a v e n t i l o scopo d i e s a m i n a r e a l c u n i p a r t i c o l a r i a s p e t t i c o n c e r n e n t i i passaggi di stato d e l l ' a c q u a . q u e s t e r i c e r c h e i n t e r e s s a n o lo s t u d i o dei f e n o m e n i m e t e o rologici e l e r e l a z i o n i f r a la carica elettrica spaziale d e l l ' a t m o s f e r a e la f o r m a z i o n e d e l l e gocce d ' a c q u a i n seno alla m e d e s i m a . n e l l e esper i e n z e eseguile i n u n vasto a m b i e n t e , a p p o s i t a m e n t e a t t r e z z a t o , sono s t a t e o t t e n u t e d e n s i t à di carica s p a z i a l e ( p o r t a t a d a l l e gocce di a c q u a i , p a r t i c o l a r m e n t e elevate. lo s t u d i o del c o m p o r t a m e n t o di tale carica, i n f u n z i o n e del t e m p o e delle p a r t i c o l a r i c o n d i z i o n i d e l l ' a m b i e n t e , è s t a t o seguito con a c c u r a t i e precisi dispositivi s p e r i m e n t a l i . i r i s u l t a t i o t t e n u t i h a n n o messo in evidenza degli a s p e t t i s i n g o l a r i d i tali complessi f e n o m e n i sulle cui p a r t i c o l a r i t à l ' a u t o r e r i f e r i s c e a l c u n i dettagli, e si p r o p o n e di p u b b l i c a r e , al p i ù p r e s t o , u n p r i m o r e n d i c o n t o . e g l i h a a n c h e f o r m u l a t o u n a ipotesi di l a v o r o sulla i n t e r p r e t a z i o n e di q u a n to è s t a l o osservato s p e r i m e n t a l m e n t e . p . c a l o i l. m a r c e l l i « sulla costituzione geologica del sub-atlantico ». i n p r e c e d e n t i l a v o r i a b b i a m o p r o v a t o c h e il b a c i n o d e l l ' a t l a n t i c o ( a l m e n o n e l l a sua p a r t e c e n t r o s e t t e n t r i o n a l e ) si c o m p o r t a , dal p u n t o d i vista d e l l a t r a s m i s s i o n e delle o n d e s u p e r f i c i a l i , in m a n i e r a del t u t t o a n a l o g a al b a c i n o del pacifico. si p o r t a n o n u o v i a r g o m e n t i a f a v o r e di d e t t a c o n c l u s i o n e , r i s p o n d e n d o ad a l c u n e o b i e z i o n i f a t t e da j . p . r o l l i é . si r i a s s u m o n o i n o l t r e le t e s t i m o n i a n z e , f a v o r e v o l i ai n o s t r i ri318 c o n v e g n i e c o n g r e s s i s u l t a t i , t r a t t e dal l a v o r o d e l geologo t e i x i e r a e dei geofisici e w i n g e p r e s s . si r i p e t e il t e n t a t i v o di d e t e r m i n a z i o n e d e l l o spessore d e l l o s t r a t o s u p e r f i c i a l i sotto l ' a t l a n t i c o , a p p l i c a n d o la t e o r i a di j e f f r e y s sulla p r o p a g a z i o n e delle o n d e s u p e r f i c i a l i trasversali, c h e c o n s i d e r a il s e c o n d o s t r a t o e t e r o g e n e o . p a r t e n d o d a l l e c u r v e di d i s p e r s i o n e d e l l e o n d e l e p r e s c i n d e n d o d a l l o s t r a t o del « g r a n i t o », r i t e n u t o m a n c a n t e o di ent i t à t r a s c u r a b i l e , si trova u n o spessore del s o t t o s t a n t e s t r a t o p a r i a 19 k m ca., a c o n f e r m a di q u e l l o o t t e n u t o p r e c e d e n t e m e n t e e d e l l ' o r d i n e di q u e l l o s o p p o r t a n t e il p a c i f i c o . r e n a t o c i a l d e a «"la. spedizione dell'istituto nazionale di geofìsica a kharthoum in occasione dell'eclissi totale di sole del 25 febbraio 1952 ». si r i f e r i s c e sulla p r e p a r a z i o n e della s p e d i z i o n e o r g a n i z z a t a dall ' i . n . g . allo scopo di e f f e t t u a r e r i l e v a m e n t i geofisici in u n a z o n a d i t o t a l i t à d u r a n t e l'eclissi t o t a l e di sole. v e n g o n o poi descritti a l c u n i n u o v i a p p a r a t i a d o p r a t i p e r tali ric e r c h e ; si r i f e r i s c e i n f i n e s u l l ' i m p i a n t o di u n o s s e r v a t o r i o t e m p o r a n e o f u n z i o n a n t e a k h a r t o u m d a l 10 f e b b r a i o al 7 m a r z o 1952. a. b o l l e c. a. t i b e r i o p . d o m i n i c i « apparecchiature ionosfericbe ». l ' i . n . g . sta a l l e s t e n d o 3 i o n o s o n d e di m o d e l l o p e r f e z i o n a t o con i p i ù r e c e n t i c r i t e r i e o s t r u t t i v i ; esse a n d r a n n o i n s t a l l a t e r i s p e t t i v a m e n t e n e g l i osservatori d i p a v i a , di s. alessio e di g i b i l m a n n a in sicilia. p e r la c o s t r u z i o n e di d e t t i a p p a r a t i i o n o s f e r i c i ci si è avvalsi dell ' e s p e r i e n z a di 4 a n n i di f u n z i o n a m e n t o di u n m o d e l l o s p e r i m e n t a l e i n efficienza d a l 1948 e della conoscenza degli a c c o r g i m e n t i tecnici con cui sono r e a l i z z a t e le i o n o s o n d e del b u r e a u of s t a n d a r d di wash i n g t o n o r m a i quasi u n i v e r s a l m e n t e a d o t t a t i . a. b o l l e m. g i o r g i f . m o l i n a c. a. t i b e r i o « osservazioni ionosfericbe e magnetiche in occasione dell'eclissi solare del 25 febbraio 1592 ». sono i n corso di p u b b l i c a z i o n e i d a t i i o n o s f e r i c i r e g i s t r a l i i n r o m a d u r a n t e l'eclissi solare del 25 f e b b r a i o 1952. si è r i l e v a t o u n abb a s s a m e n t o della f r e q u e n z a critica l e g g e r m e n t e i n f e r i o r e a q u e l l o p r e 319 convegni e c o n g r e s s i v e d i b i l e d a l c o m p u t o della d i m i n u z i o n e di i n t e n s i t à d i r a d i a z i o n e i o n i z z a n t e d o v u t a a l l ' o s c u r a m e n t o della s u p e r f i c i e solare. p e r ciò c h e c o n c e r n e la v a r i a z i o n e del c a m p o m a g n e t i c o t e r r e s t r e d a l l e r e g i s t r a z i o n i degli osservatori di g i b i l m a n n a ( s e n s i b i l i t à : h — 4,9 y / m m ; d = 8,1 y / m m ; z = 8,0 y / m m ) , b e l v e d e r e (sensibil i t à z— 1,95 y / m m ) e c o r i n a l d o (sensibilità z = 4,0 y / m m ) , m a l g r a d o fosse d u r a n t e l'eclissi in corso u n a n o t e v o l e p e r t u r b a z i o n e m a g n e tica, si p u ò c o n c l u d e r e p e r l'assenza di e f f e t t i d i r e t t i dell'eclissi sul c a m p o m a g n e t i c o t e r r e s t r e . m. g i o r g i e. m e d i . f. m o l i n a « rilievo magnetico del secondo ordine della sicilia ». n e l q u a d r o di u n p i a n o già e l a b o r a t o d a l l ' i s t i t u t o n a z i o n a l e di geofisica r e l a t i v o all'esecuzione di u n a r e t e geofisica g e n e r a l e su t u t t o il t e r r i t o r i o n a z i o n a l e c o m p r e n d e n t e m i s u r e sismiche, g r a v i m e t r i c h e , m a g n e t i c h e e r a d i o a t t i v e è stala i n i z i a t a , p a r a l l e l a m e n t e al l a v o r o grav i m e t r i c o , la e s e c u z i o n e del rilievo m a g n e t i c o del secondo o r d i n e di t u t t a la r e g i o n e siciliana. la s q u a d r a m a g n e t i c a b a già eseguito su u n t e r r i t o r i o di complessivi 4000 k i n 2 c o r r i s p o n d e n t i a circa u n q u i n t o di t u t t a la s u p e r f i c i e della sicilia. la r e g i o n e r i l e v a t a c o m p r e n d e il massiccio m o n t a n o d e l l e madon i e o . la zona di l a v o r o è q u e l l a c o m p r e s a t r a s. a g a t a di m i l i t e l l o , t e r m i n i i m e r e s e e c a l t a n i s e t t a . a l c e n t r o della base di q u e s t a zona si trova il g r a n d e osservatorio geofisico di g i b i l m a n n a del q u a l e è già i n f u n z i o n e il r e p a r t o m a g n e t i c o p e r le m i s u r e v a r i o m e t r i c h e ed il r e p a r t o della e l e t t r i c i t à . la r e t e c o m p r e n d e complessive 150 stazioni di m i s u r e r e l a t i v e , eseguite con b i l a n c e a s k a n i a e l t u s k a , p e r le d u e c o m p o n e n t i ii e z del c a m p o . la m i s u r a c o n t i n u a d e l l e v a r i a z i o n i t e m p o r a l i degli e l e m e n t i m a gnetici d u r a n t e la c a m p a g n a è stata eseguita n e l l ' o s s e r v a t o r i o di gib i l m a n n a con il complesso v a r i o m e t r i c o r u s k a e in u n a stazione camp a l e con b i l a n c e r e g i s t r a t r i c i a s k a n i a . i d a t i sono a n c o r a in e l a b o r a z i o n e m a i p r i m i r i s u l t a t i c o n f e r m a n o u n a sufficiente o m o g e n e i t à di t u t t a la z o n a r i l e v a t a . g u g l i e l m o z a n o t e l l i « un metodo per il sondaggio acustico delle nubi ». si e s p o n e e si d i s c u t e u n m e t o d o p e r l ' e s p l o r a z i o n e acustica d e l l e n u b i , i d o n e o a d e t e r m i n a r e il n u m e r o d e l l e goccioline di a c q u a cont e n u t e p e r u n i t à di v o l u m e ed il loro raggio. 11 m e t o d o si b a s a sulla riflessione d e l l e o n d e s o n o r e della g a m m a u d i b i l e p e r azione s i m u l t a 3 2 0 c o n v e g n i e c o n g r e s s i n e a del f e n o m e n o m e c c a n i c o di t r a s c i n a m e n t o d e l l e goccioline n e l l o s p o s t a m e n t o s o n o r o , e di q u e l l o t e r m o d i n a m i c o di a l t e r n a c o n d e n s a zione ed e v a p o r a z i o n e d e l l e m e d e s i m e p e r e f f e t t o delle oscillazioni di p r e s s i o n e acustica. n e l l a v o r o si d e t e r m i n a il coefficiente d i riflessione acustico di u n b a n c o di n u b i , e si m o s t r a c h e t a l e coefficiente d i p e n d e in m o d o c a r a t t e r i s t i c o d a l l a f r e q u e n z a s o n o r a , ed è f u n z i o n e del r a g g i o e del n u m e r o delle gioccioline, o l t r e c h e della t e m p e r a t u r a e d e l l a p r e s s i o n e ; t u t t a v i a l ' i n f l u e n z a eli q u e s t i u l t i m i p a r a m e t r i p u ò r i d u r s i n o t e v o l m e n t e scegliendo o p p o r t u n a m e n t e certi val o r i d e l l e f r e q u e n z e s o n o r e d ' o s s e r v a z i o n e , c o r r i s p o n d e n t i a p u n t i car a t t e r i s t i c i i n cui m u t a l ' a n d a m e n t o del coefficiente di riflessione. p e r q u a n t o solo l ' e s p e r i e n z a possa a c c e r t a r e i l i m i t i e la p o t e n z i a l i t à del m e t o d o p r o p o s t o , è l e c i t o p r e v e d e r n e u n a p r a t i c a a p p l i c a z i o n e , spec i a l m e n t e i n c o n s i d e r a z i o n e del f a t t o c h è l ' i n t e r p r e t a z i o n e dei r i s u l t a t i è f o n d a t a sul s e m p l i c e e s a m e della f o r m a delle c u r v e del coefficiente i n f u n z i o n e d e l l a f r e q u e n z a s o n o r a , d a l l a q u a l e f o r m a p u ò d e s u m e r s i il r a g g i o d e l l e goccioline c h e o p e r a n o la riflessione, e p u ò a n c h e stim a r s i il l o r o n u m e r o . i n o l t r e m e d i a n t e l ' o s s e r v a z i o n e degli s p o s t a m e n ti c h e d e t t e c u r v e s u b i s c o n o al t r a s c o r r e r e d e l t e m p o p u ò seguirsi l'evol u z i o n e degli e l e m e n t i c o s t i t u e n t i la n u b e i n c o n s e g u e n z a degli eventi m e t e o r o l o g i c i . direttore: p r o f . e n r i c o m e d i p r o f . p i e t r o c a l o i responsabile istituto grafico t i b e r i n o v i a gaeta, 14 r o m a ( o f f i c i n e grafiche, t i v o l i ) layout 6 1 annals of geophysics, 63, 1, se110, 2020; doi: 10.4401/ag-8443 special issue 1968-2018: fifty years after the belice’s earthquake. geological, geophysical, geochemical and territorial aspects of this earthquake and its heritage for the italian society foreword the aim of this special issue of “annals of geophysics” is to follow the seismic thread that, starting with the belice earthquake (14-15 january, 1968), crosses the recent history of our country until the days of the 2016 amatrice seismic sequence. we interpreted the belice earthquake like the “zero event” for the developing of the connections between the italian society and the seismic catastrophes, striking italy in the post-war period. we considered the territory as a unit, made up of geographical, geological, social, historical and urban aspects, proposing nine papers with reflections and readings on these aspects. these reflections concern the geological and structural context in which the belice earthquake occurred, such as the novelties proposed by the most recent research in geological, geophysical, geochemical and geodetic fields, but also themes and reflections on the impact that the earthquake has had on the italian and sicilian societies, on their housing and infrastructural contexts, on the history of our country. this issue fosters a contamination among different disciplines, reflecting the complexity of the impact that an earthquake has on any territory. in barreca et al., we find new insights from the analysis of geological and geodetic data, aimed to the definition of the active faults potentially involved in the 1968 seismic events and, probably, also involved in two earthquakes hitting the ancient city of selinunte. the monitoring of aquifers in the south-western sicily is debated by favara et al., who explored the potential of geochemical monitoring of the santa ninfa karst system, and by madonia and madonia, with a specific investigation of the santa ninfa cave. another interesting aspect is depicted by camarda et al., who studied the co2 flux from the soil as a useful instrument for the detection of active faults and fractures in south-western sicily. azzaro et al, in the paper titled “a reappraisal of the 1968 valle del belìce seismic sequence (western sicily): a case study of intensity assessment with cumulated damage effects”, discuss an important argument about the difficulties in determining the correct intensities when repeated earthquakes occur in the same area, and apply a technique of investigation already used for the 2016-2017 sequence in central italy. in the paper of azzaro et al. titled “earthquakes and ghost towns in sicily: from the valle del belice in 1968 to the val di noto in 1693. the first stage of the virtual seismic itinerary through italy” the authors describe a virtual tour (contained in a dvd) through the “ghost towns” in sicily, which are the villages destroyed by natural events and later abandoned. in de lucia et al. we find a pilot study on the demographic perspective of the impact of 1968 belice and 1980 irpinia-basilicata earthquakes on local communities, with interesting arguments about the relationship between the marginality of these territories and their vulnerability. the two remaining papers report specific investigation on the hard and controversial reconstruction of the valle del belice villages destroyed by the earthquake. in pappalardo and martinico we find a general investigation on the underutilization of the reconstructed dwellings, with some reflections on the guidelines for the revitalization of these area, from a town planning perspective, and taking into account the present and future challenges for residents and local authorities, facing the risk of a progressive abandonment of these settlements. finally, in caponetto and d’urso it is possible to find an innovative approach to the revitalization of an abandoned village (poggioreale) in a touristic perspective. references barreca g., v. bruno, g. dardanelli, f. guglielmino, m. lo brutto, m. mattia, c. pipitone, m. rossi, (2020). an integrated geodetic and insar technique for the monitoring and detection of active faulting in southwestern sicily, ann. geophys., this issue, doi:10.4401/ag-8327. favara r., m. cangemi, f. grassa (2020). chemical and isotopic signature of groundwater in the santa ninfa karst system and possible inferences on neotectonics, ann. geophys., this issue, doi:10.4401/ag-8253. madonia p. and g. madonia, the santa ninfa cave (belice valley): hydrogeochemical features and relationships with neotectonics, ann. geophys., this issue, doi:10.4401/ag-8321 camarda m., s. de gregorio, r. m.r. di martino, r. favara, v. prano, (2020). relationship between soil co2 flux and tectonic structures in sw sicily, ann. geophys,, this issue, doi:10.4401/ag-8264. azzaro r., m. s. barbano, a. tertulliani, c. pirrotta (2020). a reappraisal of the 1968 valle del belice seismic sequence (western sicily): a case study of intensity assessment with cumulated damage effects, ann. geophys., this issue, doi:10.4401/ag-8308. azzaro r., m. cascone, a. amantia (2020). earthquakes and ghost towns in sicily: from the valle del belìce in 1968 to the val di noto in 1693. the first stage of the virtual seismic itinerary through italy, ann. geophys., this issue, doi:10.4401/ag-8175. de lucia m., f. benassi, f. meroni, g. musacchio, n. a. pino, s. strozza (2020). seismic disasters and the demographic perspective: 1968, belice and 1980, irpinia-basilicata (southern italy) case studies, ann.geophys., this issue, doi:10.4401/ag-8298. pappalardo v. and f. martinico, (2020). gibellina, salaparuta, poggioreale and montevago: about built environment underutilization and possible urban future, ann.geophys., this issue, doi:10.4401/ag-8333. caponetto r. and s. d’urso, (2020). ancient poggioreale: an opportunity for reflection on the topic of postearthquake territory abandonment, ann. geophys., this issue, doi:10.4401/ag-8280. mario m at t i a ingv, osservatorio etneo, catania, italy paolo m a d o n i a ingv, sezione di palermo, palermo, italy 2 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8308 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8308 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8308 ingv data lifecycle management system performances during mw 6.0 2016 amatrice earthquake sequence annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 ingv data lifecycle management system performances during mw 6.0 2016 amatrice earthquake sequence stefano pintore, fabrizio bernardi, andrea bono, peter danecek, licia faenza, massimo fares, valentino lauciani, francesco pio lucente, carlo marcocci, donatella pietrangeli, matteo quintiliani, salvatore mazza, alberto michelini istituto nazionale di geofisica e vulcanologia stefano.pintore@ingv.it abstract at 01:36:32 utc on august 24, 2016 an earthquake of magnitude 6.0 occurred in central italy, affecting many small towns and municipalities in the lazio, umbria, marche and abruzzo regions . the event caused severe damages, many victims and 299 fatalities. only 21 seconds after the beginning of the earthquake, the first automatic location of this earthquake was available and stored in our earthquakes database. the first magnitude estimate followed 68 seconds after the origin time. few seconds later the ingv seismologists on duty in accordance to the agreed protocols provided the first alert to the italian civil protection department (dipartimento di protezione civile, dpc) and thereby triggered the seismic emergency protocol. subsequently, they elaborated the data in order to produce the first manually reviewed hypocenter, which was published o n the institute’s website at 01:53:18 utc. the sequence following this mainshock generated thousands of earthquakes in the epicentral area, which the ingv automated localization system processed and detected along with the usual seismic activity in the rest of the italian territory. in this paper we analyze the behavior of the automated system and of the data lifecycle management procedures in such extraordinary condi tions. in particular we want to measure the capability of the system to manage the huge data flow, in terms of frequency and size of seismic events and its ability to remain fairly responsive and accurate in accomplish ing its duty in the expected time. this will help us to identify potential problems and to suggest necessary improvements to better serve the ingv mission for civil protection. i. introduction he information system aida was built to collect, process, archive and distribute seismic data in near real-time. it became fully operational in may 2012, when it substituted the former main earthquake detection system at ingv. its core components are the earthworm software for the real-time earthquakes detection [johnson et al., 1995], seiscomp3 package [sc3] for the exchange and archiving of seismic waveforms and a mysql database to store earthquakes data. in order to meet the specific requirements of the institute’s mission, the system features many custom modules, tools and applications develt oped in house. for a detailed description of the overall system architecture and a previous evaluation of its performance, refer to [mazza et al., 2012]. since its initial deployment, the aida system has been continuously developed further and gradually improved, to make it more accurate and performing. considerable work was made to refine the software proce dures and increase hardware performances, enabling the system to respond in a few seconds when triggered by an earthquake. at the same time, the load on the system has progressively increased. this is due both to the volume of data to be processed and to the con stantly increasing amount of requests and queries for various types of seismic data. the complexity of the whole system justifies the ac1 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 tual impossibility to perform an overall test, including the human interactions with the system. only few parts of the system, like the insertion into the database of the earthquakes data or the localization system, or the websites were tested singularly. the seismic sequence starting with the august 24, 2016 magnitude 6.0, is considered a “real life stress test” and we illustrate how this sequence and the large amount of detected seismic events impacted the whole processing and data dissemination system. we analyze various aspects in order to assess the performance of the aida system, and we highlight some of the strengths and weaknesses of the current system. this analysis should provide tangible actions to be proposed for future developments of the system. ii. impact on the processing system the ingv earthworm implementation involves four different systems running in parallel to perform the event detection. each one provides, for each earthquake detected, a package of sac waveform files used by the software for the interactive revision [bono, 2008]. we show in figure 1 the amount of data pro duced by a single earthworm server during 2016. at the moment of this writing the september column is not complete. the monthly average data is about 74 gb during the first seven months of 2016 averaging to a rate of approximately 360 mb/day. figure 1: data produced by a single earthworm server. in figure 2. we show that during the first days of the seismic sequence, the daily data rate ran up to 73gb, almost the same value of the average monthly data rate seen before. the backup routines were quickly modified to avoid risk of disk full on the various systems. figure 2: earthworm data archived in august 2016. the data volume growth needs to be seriously taken into account for future system upgrades. our database is filled by all the seismic events data calculated by the earthworm systems and by the manual reviewers. during the first month of the seismic sequence starting from day 24 august, we stored 220k localizations with corresponding 3.3m phases and 2.7m amplitudes. to better understand the difference with normal activity, notice that this amount of data is comparable with what was recorded by the system during the whole past year. iii. real-time performance evaluation the real-time processing and localization system has been properly working during the sequence. in particular, we have been able to give prompt information to our counterparts and comply with all agreements and obligations, taking also into account that in agreement with dpc the magnitude threshold for immediate phone communications was increased soon after the mainshock. to meet those obligations, the automatic results must be available before the time limits reported in table 1. 2 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 table 1: automatic solutions communication rule minutes after origin time 2 5 2.5<m<3.0 none final automatic location and magnitude m>=3.0(*) location and magnitude estimate final automatic location and magnitude (*) the threshold was raised to 4.0 one hour after the mainshock to limit the overload for the seismologists on duty (and the counterparts at dpc) caused by continuous phone calls. the comparison between the number of events meeting the criteria and the number of actually recorded ones is shown in table 2. table 2: locations matching the rules minutes after origin time 2 5 first hour 27/28(*) 28/28 after 48/49(**) 49/49 in the first hour after the mainshock, we recorded 28 quakes to be notified to dpc; only one (*) of those locations, belonging to the sequence, was affected by an 11 seconds delay. searching until september 22, 2016 we discovered that only another (**) record, always belonging to the sequence and occurred the first day, was delayed of about 44 seconds. figure 3: map showing the distribution of the earthquakes for which the automatic and revised localizations are compared. red circles indicate the earthquakes (approximately 11,000) occurred in the “during–the–sequence” period, i.e. from 24th august onward. the approximately 11,000 blue circles indicate earthquakes occurred on italian territory before that date, backward to april 2015. a zoomed view of the seismicity in the area affected by the amatrice seismic se quence is in the top right inset of the map. circles are scaled based on magnitude of the earthquakes. 3 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 figure 4: statistical distributions of the differences between pairs of hypocentral parameters belonging to automatic and revised earthquakes localizations. in panels a–d, on the top row, the differences between revised and automatic (a) origin time, (b) epicenter, (c) depth and (d) local magnitude ml are shown, respectively, for earthquakes occurred before the seismic sequence onset (blue histograms). in panels e–h, on the bottom row, the differences between revised and automatic (e) origin time, (f) epicenter, (g) depth and (h) local magnitude ml are shown, respectively, for earthquakes occurred during the sequence (red histograms). also, in panels e–h, the corresponding differences computed only for the events belonging to the seismic sequence (red circles in the top right inset of figure 3) are shown as smaller insets for comparison (green histograms). iv.accuracy of the automatic localization system in this section we assess the quality and accu racy of about 11,000 localizations and magnitude estimates generated by the automatic system during the amatrice seismic sequence by comparing them to the ones which were successively revised by the operator on duty (red circles in figure 3). also, to gain insights on the behavior of the system in such a “stress condition” we perform the same comparison on a set of as many pairs of localizations and magnitude estimates (automatic and revised) for earthquakes occurred before the onset of the sequence (blue circles in figure 3). even though both the sets of hypocentral parameters pairs are distributed over the whole italian territory, those in the “during–the–sequence” period are prevalently related to the ongoing seismic sequence (see top right inset in figure 3). in figure 4 we compare four main hypocentral parameters, the origin time, the epicenter on surface, the depth and the local magnitude ml for the “before–the–sequence” and the “during–the–sequence” localization pairs. results of this exercise demonstrate that the behavior of the system is not affected by the data load increase during the sequence. rather, the “during–the–sequence” localizations pairs show smaller differences between automatic and revised hypocentral parameters, hence are characterized by an overall better automatic localization. only the automatic magnitude estimate is slightly worse in the “during–the–sequence” period. this is due to the frequent presence of multiple earthquakes in the same time window, which may lead to wrong automatic associations of the maximum amplitude of the seismic signal to the right event. v. system detection capability a critical issue to face in the aftermath of a major earthquake is the magnitude completeness of the aftershock catalog. this issue arises from the under-reporting of short-term aftershocks, especially smaller ones in earthquake catalogs, 4 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 simply because systems are not able to distinguish them in time windows containing larger events [enescu et al., 2007]. however this “under-reporting effect” may affect the whole seismic catalog, because of possible deficiencies in recording capabilities by the system under heavy load condition. to evaluate how the heavy load of data generated by the amatrice seismic sequence affected the detection capability of the system we compute the magnitude of completeness (mc) as a function of time for two different earthquake catalogs: earthquakes occurred inside the seismic sequence area (the area represented by the top right inset in the map of figure 3); earthquakes occurred outside this area. we perform the calculation for the two catalogs in two time windows: (1) from january 2015 to the end of september 2016; (2) from the onset of the seismic sequence (24 au gust 2016) to the end of september 2016. results of these calculations are shown in figure 5a e 5b for the “inside” and the “outside” catalogs, respectively. we compute the mc vs time relationships for the selected earthquakes catalog on running windows of 500 events with 50% overlap (figure 5a,b). on each sample, we determine mc as the magnitude at which 90% of the data can be modeled by a power law fit [wiemer and wyss, 2000]. figure 5: (a) mc as a function of time for the “inside–catalog” from january 2015. continuous dark gray line represent the mc values computed on running windows of 500 events, dashed gray lines indicate the standard deviation. in the top left inset of panel (a), the continuous red line represents the mc vs time for the “inside–catalog” when only data from the start of the sequence (24 august 2016) are considered in the calculation, dashed red lines indicate its standard deviation. (b) mc as a function of time for the “outside–catalog” from january 2015. in the top left inset of panel (b) the mc vs time for the “outside–catalog” is shown, when only data from the start of the sequence (24 august 2016) are con sidered in the calculation. symbols and color in panel (b) have the same meaning as in panel (a). the green bar marks the time of occurrence of the ml 6.0 amatrice earthquake. calculations shown in this figure are made using the zmap code [wiemer, 2001]. 5 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 uncertainties on the mc values were calculated by bootstrapping each sample with 200 realizations, and indicated with dashed lines in figure 5a,b. we observe that, before the ml 6.0 amatrice earthquake in the sequence area (inside catalog, figure 5a), the mc is always below 1.5, with upward oscillation due to the higher weather–related noise level in the winter months. the mc rises to 2.7 immediately after the main shocks (green bar in figure 5), and then decreases again below the 1.5 threshold in few days. this can be observed in better detail looking at the red line in the inset of figure 5a, where only data from the start of the sequence (24 august 2016) are considered in the calcula tion. the trend of the mc for the “outside–catalog” is characterized by more oscillations, related to the existence of multiple different conditions in an area as large as the whole italian territory (proximity to the coasts, anthropic–related noise, weather–related noise, etc.), even though it remains always below the 1.5 threshold. no significant variations are observed at the time of the occurrence of the ml 6.0 amatrice earthquake (green bar in figure 5) when data from january 2015 are considered. a slight increase of mc up to 1.8 is observed for few days if only the data from the start of the sequence (24 august 2016) are considered in the calculation (red line in the inset of figure 5b). summing up, we can conclude that the heavy load of data generated by the amatrice seismic sequence did not significantly affect the detection capability of the system, neither inside the area affected by the sequence nor in the whole italian territory. vi. data sharing and dissemination the cnt website (http://cnt.rm.ingv.it) is our main seismic parametric data sharing and earthquake information portal. it received more than one million contacts the day of the mainshock. see figure 6 for a more detailed time series. our hosting provider blocked the traffic immediately after the sudden increase of connections only few minutes after the mainshock, assuming that this amount and pattern of http requests was corresponding to a distributed cyber attack. figure 6: connections to cnt web site. the web portal iside (http://iside.rm.ingv.it) [mele et al. 2016] is another instrument for data dissemination targeting more specifically users from the research community. although it registered more than 50,000 accesses shortly after the mainshock, accessibility was not affected. we experienced very good performances, and many new users (around 25% of all contacts) were able to connect, to register to the portal and to browse and request data. see figure 7 for more details. figure 7: contacts to iside web portal. our webservice server (http://webservices.rm.ingv.it) provides methods to access raw data and data products programmatically through standardized and public apis. these data in clude parametric event data, station information and metadata, as well as raw seismogram waveform data. parametric earthquake data like events, magnitude, phases and picks are distributed in the standardized quakeml format (https://quake.ethz.ch/quakeml/), a flex6 http://cnt.rm.ingv.it/ https://quake.ethz.ch/quakeml/ http://webservices.rm.ingv.it/ http://webservices.rm.ingv.it/ http://iside.rm.ingv.it/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 ible, extensible and modular xml representation of seismological data. figure 8: data downloaded via web-services. during the amatrice earthquake, the number of requests and the amount of downloaded data are increased dramatically. nonetheless, the service remained always available and guaranteed access to the requested data. this service is also used to provide the input data in json format (http://www.json.org) for ingv’s mobile applications (apps) called ingvterremoti for the ios and android operating system. the number of requests made by these apps increased from a value of about 2,000 from the previous day to almost 150,000 on august 24, 2016. the development of the traffic can be observed in figure 8. all these figures (fig. 6 fig. 8) show an immediate increase of sessions and traffic in the night of august 24, 2016, followed by a slow and constant decrease over the following days. vii. conclusions the 2016 amatrice earthquake sequence has severely tested our automatic and interactive processing systems. it generated an heavy load of new data in a relatively short time period, a load which was several times larger than usual in terms of number of events, archived data, bandwidth and requests by users and automated processes. nevertheless, the system’s behaviour and response was satisfactory, in terms of event processing speed, detection capability and accuracy, and service uptime and responsiveness, although the system needed some extra work to remain efficient without running out of storage space. this experience teaches us that we need to continuously up grade the hardware and notably disk space, in order to keep up with the constant growth of the seismic networks and constantly improve detection capabilities. moreover we should try to reduce the amount of data written by the system, reducing or completely eliminating the use of sac waveforms during the manual revision in favour of the use of time series webservices like the irisws-timeseries service (http://service.iris.edu/irisws/timeseries/1) . the procedures for the insertion of seismic data into our database systems, even if satisfactory, would benefit from further improvements, in particular some fine tuning of the database server and data insert optimizations should be done, in order to obtain even better performances. quality of automatic magnitudes during the sequence is slightly worse than in the usual scenario. this will be further investigated later on, as a finer tuning of the time window used to search the maximum amplitude may arguably guarantee some improvement in the automatic calculation of the magnitude, limiting the cases of wrong associations of amplitudes to seismic signals. dis semination of information and data to the public has been very successful with millions of request fulfilled by our websites. however, new and improved solutions for even more requests and higher data volumes should be prepared and established, because we have to an ticipate the continuous growth of the internet population over the coming years. acknowledgements this study benefited from funding provided by the italian presidenza del consiglio dei ministri, dipartimento della protezione civile (dpc); scientific papers funded by dpc do not represent its official opinion and policies. references [bono, a. 2008] sispick! 2.0 sistema interattivo per l’interpretazione di segnali sismici, rapporti tecnici ingv, n.59 7 http://service.iris.edu/irisws/timeseries/1/ http://www.json.org/ annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7218 [johnson et al., 1995] johnson, c. e., a. bittenbinder, b. bogaert, l. dietz, and w. kohler (1995). earthworm: a flexible approach to seismic network processing, incorporated research institutions for seismology (iris) newsletter 14, 1–4 [enescu, b., j. mori, and m. miyazawa, 2007], quantifying early after-shock activity of the 2004 mid-niigata prefecture earthquake (mw6.6), j. geophys. res., 112, b04310, doi:10.1029/2006jb004629. [wiemer, s., and m. wyss, 2000] minimum magnitude of complete reporting in earthquake catalogs: examples from alaska, the western united states, and japan, bull. seismol. soc. am., 90, 859–869, doi:10.1785/ 0119990114. [mazza et al., 2012] mazza s., basili a., bono a., lauciani v., mandiello a., marcocci c., mele f., pintore s., quintiliani m., scognamiglio l. and selvaggi g. (2009). aida – seismic data acquisition, processing, storage and distribution at the national earthquake center, ingv, annals of geophysics (2012), vol. 55, no. 4. [mele et al., 2016] iside working group (2016) version 1.0, doi: 10.13127/iside [quakeml] danijel schorlemmer, adrian wyss, silvio maraini, stefan wiemer, and manfred baer: quakeml an xml schema for seismology, orfeus newsletter, volume 6, no 2, october 2004 (html) [sc3] http://www.seiscomp3.org [wiemer, s., 2001] a software package to analyze seismicity: zmap. seismological research letters, 72(3), 373-382. 8 i. introduction iii. real-time performance evaluation iv.accuracy of the automatic localization system v. system detection capability vi. data sharing and dissemination vii. conclusions references 25grunthal.qxd 631 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words historical earthquakes – fake quakes – germany – historical earthquake research – earthquake catalogues 1. introduction this is a short treatise on the meta-history of earthquakes in germany, i.e. on the history of collecting and evaluating information on earthquakes. it is not a history of the broad field of literature about hypotheses or theories on earthquakes. the seismicity of germany can be regarded as relatively low, although the largest in europe north of the alps. earthquakes of 5.1 ml (4.8 mw) have a mean return period of ten years and events of 5.8 ml (5.5 mw) of 50 years. the maximum magnitude of known earthquakes in germany is 6.1 ml (5.8 mw). events of this size are associated with a mean return period of about 100 years. an exception is the basel earthquake in 1356 with 6.6 mw (grünthal and wahlström, 2003), which occurred in what is today switzerland but next to the german border and should have been strongly felt over a large area of germany. the seismicity in germany is concentrated along the upper rhine graben from basel in the south to frankfurt/main in the north as well as to the lower rhine embayment in the area between liège (belgium) and köln (fig. 1). another subregional seismicity concentration occurs in a narrow source zone south of stuttgart. this zone released the largest earthquakes in germany in the 20th century. the seismotectonic province of eastern thuringia and western saxony, with pronounced swarm quakes in a highly localized area, is anoththe history of historical earthquake research in germany gottfried grünthal geoforschungszentrum, potsdam, germany abstract the paper summarizes the history of collecting and evaluating information on earthquakes in germany. a rich literature mentioning historical and contemporary earthquakes has existed since the 16th century. early earthquake catalogues began to appear in the middle of the 16th century, some of which report earthquakes in germany dating back to the 9th century. modern seismological views were introduced in connection with intense philosophical analysis of the 1755 lisbon earthquake, which was largely observed in central europe. the 19th century was characterized by a tremendous increase in detailed earthquake studies as well as earthquake compilations in the form of catalogues. the most comprehensive non-parametric catalogues were created in the middle of the 20th century, while the first digital parametric catalogues were published in the 1980s. this was also the time when critical studies on the re-interpretation of historical earthquakes began. only in the 1990s was such analysis made in a systematic manner resulting in numerous publications and the current development of a modern earthquake catalogue. mailing address: dr. gottfried grünthal, geoforschungszentrum, telegrafenberg c3, d-14473 potsdam, germany; e-mail: ggrue@gfz-potsdam.de «it would be desirable / that one would collect complete information about all natural occurrences / since this is the only way to reach a thorough cognition of nature». j.j. scheuchzer (1746) 632 gottfried grünthal er seismicity area in germany. furthermore, there is scattered seismicity over large parts of the country with occasionally even damaging events outside of the primary seismic zones. the rapid and effective distribution of news, including information on earthquakes, was to a large extent connected with the advent of printed books, illustrated broadsheets or «new tidings», especially in germany. these new developments have a strong link to the dramatic expansion of trade and commerce in central and northern europe since the late middle ages and the early modern times. obviously, there was a lively interest by german urban dwellers in the outside world. moreover, the printer-publishers made use of the appetite of the german-speaking customers for news and entertainment and had, especially with the broadsheets, a quick financial return. when certain phenomena, like earthquakes, were not so frequent in germany itself, such information was collected from other parts of the known world, not seldom connected with sensationalism. the large distribution of printed material (including earthquake reports) can be traced back to the beginning of the 16th century, while illustrated news-sheets and chronicles first appeared at the middle of this century. they even began to appear on a daily basis when the conditions had stabilized after the thirty years’ war. examples of woodcuts or engravings produced in and distributed from germany are reproduced in e.g., deresewicz (1982) or kozák and thompson (1991). many of them depict mediterranean earthquakes. a negative consequence of this practice is that a certain portion of such reported «earthquakes» are fake and partly included in modern earthquake catalogues. in particular, this concerns cases where the «earthquake epicentres» are associated with towns where the reports originate (grünthal and fischer, 2000; grünthal and fischer, 2001). 2. developments during the last half millennium 2.1. the 16th century and early 17th century profiting from the invention and introduction of the art of printing with moveable metallic letters by johann gutenberg about 1450, with the famous prototype print of the bible in 1454/1455, an invasion of illustrated broadsheets (in german flugblätter or fliegende blätter «flying sheets» or einblattdrucke «single-leaf prints»; prints from a single block existed already half a century earlier) and chronicles of all kinds began making their appearance in germany in subsequent decades. the broadsheets are usually printed on one side of a single sheet of paper and consist of a woodcut illustration, frequently coloured, combined with text, often columns of verses. the earliest known printed earthquake report, at least in germany, concerned the strong 1511 slovenia friuli venezia earthquake and was published shortly after by an unknown bavarian writer issued by a münich printer (günther, 1890). among the early prints, usually in paragraphs entitled «miscellaneous», there are frequent compilations of natural events including earthquakes, which often go back to ancient times. it would be beyond the frame of this short contribution to give an even roughly complete overview of these references; the following are a few examples of relevant historical literature where contemporary and historical earthquakes are mentioned with their effects: nausea (1532), fritschius (1555), sleidanus (1556), lycosthenes (1557), fabricius (1569), spangenberg (1572), dresser (1596a,b) and peckenstein (1608). one of the most popular of those compilations were obviously the «prodigies and apparitions» by conradus lycosthenes (1557). it served as a major source of earlier times for mallet’s (1853-1855) catalogue. lycosthenes’ work contains almost 200 earthquake reports and more than 1400 illustrations on 670 largeformat pages. among these illustrations, there are six distinct woodcuts illustrating earthquake effects which appear repeatedly, altogether 134 times. one of the six woodcuts, already used in sebastian münster’s cosmographia (in the issue by münster, 1550 but not the one by münster, 1544), is the one which some modern authors associate with the 1356 basel earthquake (fig. 2). lycosthenes uses this picture not only together with the basel eartquake, but also with 633 the history of historical earthquake research in germany like dressl (1559), who compiled conflagrations, floods, storms, lightening and thunder, plague of locusts, frost, etc. – and, what is of special interest here, earthquakes. early earthquake catalogues, i.e. chronicles focussing on earthquakes, are those by, e.g., ragor (1578), many other events. with the dramatic excitement this woodcut expresses, it was apparently one of the most popular already by lycosthenes, regarding the frequency of its use. other studies, profitable as well, give compilations of «all sorts of god’s punishments», fig. 1. seismicity of germany and adjacent areas (in yellow; after grünthal and wahlström, 2003) and a selection of fake quakes in germany with a former maximum intensity larger than or equal to vi. the latter are differentiated into hoax events and mix-ups with real earthquakes (in red) mostly happening outside of germany, misinterpreted storms, landslides or collapses (in blue), and events relocated by more than 100 km distance with an intensity deviating by more than one degree (orange). these findings were subject of papers by grünthal together with fischer or meier from the last ten years (cf. text). 634 gottfried grünthal rasch (1582a,b, 1591), beuther (1601), sigwart (1613) and bernhertz (1616). ragor’s (1578) earthquake catalogue contains about 100 earthquakes from 22 a.d. up to 1577. his first entries on german earthquakes date back to 880 a.d. on an event experienced in mainz (correct date is 30 december 880 or 881). before that he reports mostly about greek earthquakes. the earthquake history by rasch (1591) was written after experiencing the fairly strong (intensity ix) earthquake in 1590 east of vienna, the last entry of his catalogue. his compilation, starting in 19 a.d., contains several hundred events – up to the 8th century mostly in the eastern mediterranean area. with the beginning of the 9th century his catalogue becomes specific with respect to german earthquakes. it would be beyond the scope of this paper to mention the numerous chronicles with local concern only. many of these are not printed and exist only in one or a few handwritten copies. this prosperous period was interrupted during the thirty years’ war, a dark period in cultural and scientific life, within which strikingly few earthquakes were reported. 2.2. late 17th century and early 18th century besides the rich literature of municipal chronicles including earthquake reports, e.g. pusch (1713) and vogel (1714), there are several studies with detailed and broad regional scope from the late 17th century, in some cases solely dedicated to earthquakes. examples are mpsac (1670), höpfner (1691) and the anonymous disaster-chronicon (1692). 2.3. mid and late 18th century: post 1755 lisbon earthquake studies the lisbon earthquake on 1 november 1755 gained so much attention in central europe, not least in germany, that it can be regarded as the starting point to an early seismological science there, connected with the abandoning of many previous myths associated with this natural phenomenon. on the other hand, the lisbon earthquake became the initial element of disillusion of the period of enlightenment. this event had a tremendous effect on the fig. 2. illustration (münster, 1550) of a dramatic situation in which a town is severely affected by an earthquake. lycosthenes (1557) used this woodcut in connection with his description of the 1356 basel earthquake – but for many other earthquakes as well (cf. text). it is therefore not likely that the town basel its depicted, as assumed by some modern authors. 635 the history of historical earthquake research in germany in this conjunction it becomes clear that a number of pure earthquake chronicles, e.g., mjaw (1756), seyfart (1756) and gottfried (e.g., 1759), were prompted by this remarkable series of events. they contain material published in europe including «flying-sheets» and newspapers. 2.4. 19th century and early 20th century in the 19th century great attention and interest were paid to the general description of nature and natural phenomena like earthquakes. this favoured the ability and knowledge to make detailed treatments and analyses not only of seismic events occurring in that time but also of historical events. earthquake catalogues created can be differentiated into those covering large regions, at least country-wide and those containing more detail and focussing on a local or sub-regional areas. to the group of large regional studies belong two less reliable sources, schnurrer (1823-1825), where earthquakes are mentioned chronologically among all kinds of diseases, meteorites and other «plagues», and keferstein (1827). much more comprehensive are the catalogues by von hoff (1840, 1841), in two volumes published after his death by his friend, the famous geographer and cartographer heinrich berghaus. the first volume extends from 3460 b.c. up to 1759 a.d. (von hoff, 1840) and the second from 1760 to 1805 and from 1821 to 1832 (von hoff, 1841). this valuable work covers events of what was known worldwide with a precise and methodic study based on cited sources. another earthquake catalogue from 786 a.d. up to 1846 is that by boegner (1847). an outstanding compilation of earthquake data from about 1000 b.c. up to 1897 is the manuscript by lersch (1897), handwritten in a narrow gothic style and using many abbreviations, not all of which could be deciphered. it amounts to some 7000 pages (sieberg, 1902). this huge work was presented at the first international seismological conference, 11-13 april 1901, at the imperial main station for earthquake research in strasbourg (lersch himself, already 83 years old, was not present). the conpeople. it caused a large revision in the philosophy of life initiating a new direction of thinking, european pessimism. the lisbon earthquake was the motive for immanuel kant to write two famous treatises on the nature of earthquakes (kant, 1755, 1756; cf. also kant, ed. in 1910). in a critical scientific polemic he defended the explanation of these events on the basis of natural causes contrary to speculative unscientific and clerical concepts (e.g., in jaem, 1756). an early poetic requittal with the up to that time dominating optimistic philosophy is voltaire’s «poem sur le désastre de lisbonne» (voltaire, ed. in 1931, 1994; he moved to potsdam in 1749 at the invitation of the king of prussia «frederik the great») as well as «candide ou l’optimisme» from 1759 (in voltaire, ed. in 1993). goethe reflects on the perplexity provoked by the news from the ruin of the city in «das erdbeben von lissabon (the earthquake of lisbon)» in «dichtung und wahrheit» (goethe, ed. in 1964). kant’s polemic found fertile ground in the socalled educated public, since central europe was at that time in a real earthquake hysteria. this was not only due to the news from lisbon, where exactly on all saints’ day the prosperous city had been laid in ruin with about 60 000 deaths, but also due to the widely recognized long periodic effects in all central europe, like swinging candelabras in the crowded churches during the morning services, seiches in lakes and similar phenomena at coast lines on this highly religious holiday. the chroniclers themselves could not flee from this hysteria. everybody who had experienced the shaking became part of its grip. this situation became even worse due to real new strong earthquakes. so, felt effects were mixed with news on strong aftershocks striking lisbon. the felt effects were due to the damaging earthquake on 9 december 1755, 6.1 ml in the wallis (switzerland), also strongly felt in southern germany, and a sequence of damaging earthquakes in the lower rhine embayment between december 1755 (the largest shock 5.1 ml on 26 december) and february 1756 (the main shock 6.1 ml on 18 february being the strongest event in the lower rhine embayment in historical times). 636 gottfried grünthal ference expressed its highest appreciation of this monumental catalogue and suggested its use in the future earthquake research (polis, 1902/1903). consequently, it served as the basis for the two german earthquake catalogues finished half a century later; i.e. sieberg (1940) and sponheuer (1952). from today’s perspective, the lersch catalogue can therefore be regarded as essentially exhausted by these two catalogues. more locally oriented catalogues are, e.g., nöggerath (1848, 1870), pfaff (1850), eisel (1863), lasaulx (1874), laube (1874), von gümbel (1889, 1898), langenbeck (1892, 1895), pauls (1893), doß (1898), knett (1899), laska (1902), schorn (1902), günther und reindl (1903), reindl (1903, 1905a,b), regelmann (1907) and botzong (1912). the historical earthquake studies in this period benefited much from the systematic general historical research connected with the edition of reprints of many mediaeval sources. although not subject of historical research, it is worth mentioning that countless studies on contemporary earthquakes from the 19th and early 20th century exist with systematic and detailed collections of macroseismic effects, which have a high quality even from today’s perspective. earthquakes of this time period have been an excellent basis for detailed modern studies (e.g., grünthal, 1992 and fischer et al., 2001). the improved timing accurate to within a few seconds in connection with the country-wide telegraph system as part of the railway network facilitated the first attempts of «instrumental» localization of earthquakes with the method of homoseisms after hopkins (1847). this method was applied e.g. by von seebach (1873) for a damaging earthquake in eastern thuringia in 1872 and by von lasaulx (1874) for an event of similar size in the lower rhine embayment in 1873. filled by faith in the technique, some investigators trusted instrumental locations more than well established macroseismic ones. to this productive period belong also contemporary catalogues covering several years or a few decades, e.g., fuchs (1886) with about 10 000 entries of seismic events for the years 1865-1884, credner (1902, 1903, 1904, 1907) covering the intense vogtland swarm quakes from 1900-1906 with several thousands of felt earthquakes and etzold (1919) listing the continued swarm activities from 1907-1915, which were even more intense and largely recorded instrumentally (for further references cf. grünthal, 1988). the comprehensive work on the intense seismic activity in sw germany, initiated by the so called central european earthquake in 1911, with many contemporary studies (e.g., gutenberg, 1915) is not further referred to here. another culmination of historical studies of the early 20th century are the two volumes by gießberger (1922, 1924) entitled «the earthquakes of bavaria». in spite of the title, they contain much more than bavarian earthquakes. they cover the time period from 169 a.d. up to 1905, the year when the instrumental recordings started in bavaria by the earthquake service. this is not an earthquake catalogue, but an exhaustive annotated chronological extract from original sources in their original wording. since this «raw material» has not been critically analysed and commented (except for some footnotes), the author gives a corresponding warning for its use, since he was aware that several references do not have a real background in earthquakes or are fake in other respects. overcritical readers might have overlooked this warning and for no reason discredited this work. 2.5. mid 20th century the non-parametric catalogues by sieberg (1940) and sponheuer (1952) represent fundamental milestones. as mentioned, both rely to a certain extent on the monumental work by lersch (1897). sieberg (1940) covers the time span 58 a.d.-1799 using 58 referred historical sources, while sponheuer (1952) continues with the period 1800-1899 based on 93 contemporary or topical quotations. one should be indulgent with both authors that their verbal descriptive catalogues are, besides the already expressed doubts on several events by themselves, not free from misinterpretations. many events have been subject to reevaluations in the last ten years. the high seismic activity in the chronic swarm quake area of the vogtland (in the triangle 637 the history of historical earthquake research in germany where saxony, bavaria and bohemia meet) with several thousands of felt shocks, mainly in 19011911, was obviously a too large burden for sponheuer and other workers at that time to continue the catalogue into the early 20th century. it was not until the grünthal (1988) catalogue that this part was covered down to small magnitudes. in this respect the catalogue by kárník et al. (1956) for czechoslovakia, terminating in 1956, can be mentioned, since it contains information for the western adjoining parts of germany. the significant seismic activity in sw germany in the years 1800-1950 was the subject of the phd thesis by fiedler (1954). however, this catalogue contains many errors and gives little indication on exact source references. quotations are often given without relation to the accompanying data (fischer, pers. comm.). further work for this area and time period is presented in several diploma theses (e.g., vogel, 1955). an early attempt to squeeze the verbal data into a parametric format is the catalogue by rothé and schneider (1968) for the rhine graben area, which belongs both to germany and france. it covers the time span 1021-1965 and makes intense use of sieberg (1940), sponheuer (1952) and fiedler (1954). earthquake chronicles with emphasis on the lower rhine area are given by schwarzbach (1951) and, for the period 1958-1970, by ahorner (1964, 1972). a not so critical reflection on the damaging earthquakes of the federal republic of germany is given by ahorner et al. (1970). in this publication, many currently known misinterpretations with respect to earlier catalogues remain untouched, as was common in large parts of europe at that time. 2.6. late 20th century up to 1995 2.6.1. 1975-1988 with the rapid development and broader use of computers starting in the 1970s and the need to perform probabilistic seismic hazard assessments, a new generation of country-wide catalogues had to be prepared. following initial internal versions, two catalogues were published, by leydecker (1986) for the western and southern parts of germany for the time period 1000-1981 and by grünthal (1988) for central-eastern germany for the time span 8231984 (*). leydecker’s catalogue has 423 entries before 1900 and 63 between 1901 and 1910, while grünthal gives 536 events before 1900 and 3466 between 1901 and 1910. concerning the historical events, both catalogues are strongly based on sieberg (1940) and sponheuer (1952). although the personal resources were limited and the time pressure high to prepare these catalogues, attempts were made to improve the data on historical events compared to the earlier catalogues. leydecker (1986) refers additionally to 20 and grünthal (1988) to 154 historical sources or sources about pre1910 earthquakes. the latter refers to 80 isoseismal maps based on given intensity data points for the strongest events since the 18th century. also leydecker (1986) could make use of such data, giving isoseismal radii for a considerable number of events. the catalogue by grünthal (1988), later extended up to 1990, has been assimilated with that by leydecker (1986) in 1990 as an implication of the reorganization of scientific activities in the united germany, where the catalogue work is continued by leydecker. besides the catalogue work, early probabilistic and deterministic seismic hazard assessments were performed from 1975 up to about 1988. also maps of maximum observed intensities for different time spans were derived. part of these studies also concerned maximum effects and areal extension of certain intensities of historical key events. with some exceptions that time was not ripe for separate publications on historical events. most of the findings were simply assimilated into the ongoing catalogue work. studies of a more general nature are e.g., schmidt (1980) and kozák and schmidt (1981); one more specific study on a certain quake is by sponheuer and grünthal (1981). (*) germany was after ww ii separated into two states. logue would have profited had it had feedback from seismologists. still it is regrettable that no continuation has been published by the author. detailed studies on single historical events based on contemporary sources are wolf and wolf (1989), on the phantom earthquake in 1062; grünthal (1992), on the reinterpretation of the central german earthquake in 1872, the strongest known in eastern thuringia with extensive descriptions of felt effects from 409 localities (see also above); meier and grünthal (1992), reinterpreting a key event in 1770 in nw germany; and vogt and grünthal (1994), on another quake in that area from 1612. the two latter studies resulted in distinctly new source parameters. the 1612 event is one of the few early german earthquakes being subject to a contemporary illustration (fig. 3). this engraving, appearing on a contemporary broadsheet, clearly reflects the effects described by 638 gottfried grünthal 2.6.2. 1989-1995 in the late 1980s and early 1990s there were several concerted actions in europe to systematically study the historical seismicity, e.g., by stucchi et al. (1991) and stucchi (1993). with respect to germany, the studies by vogt (1993a,b) on rhenish earthquakes in 1737 and 1787, respectively, resulted from the project rhise (review of historical seismicity in europe). further publications from this project are vogt (1994) on the 1640 lower rhine embayment quake and alexandre and vogt (1994) on the 1755-1762 sequences in the same area. the mediaeval sources of western europe from 394 a.d. to 1259 (including germany) are part of the exhaustive, comprehensive and methodologically rigorous study by alexandre (1990), a study non-historians would usually not be able to master themselves. in spite of its qualities this catafig. 3. contemporary engraving showing the effects of the 1612 bielefeld earthquake frightened people, falling pewter ware and damaged buildings (cf. vogt and grünthal, 1994). 639 the history of historical earthquake research in germany et al. (1999), fischer et al. (2001), grünthal and fischer (2001) and grünthal and fischer (2002a,b). the paper by grünthal and fischer (2000) is a summary of findings on erroneously alleged damaging events in central european catalogues as a result of confusion with distant large earthquakes. they are illustrated in fig. 1. the mentioned references include only a limited part of the new findings on historical earthquakes in germany. the current knowledge on the state of the art of interpretations of historical earthquakes in germany has been incorporated as part of the earthquake catalogue being the basis for ongoing probabilistic seismic hazard assessments and related studies (grünthal and wahlström, 2003). detailed descriptions are given on the data cleaning, i.e. to separate fake and non-tectonic events and to find duplicates, especially connected with the use of different calendars. special emphasis is given to the calculation of unified moment magnitudes mw involving the derivation of reliable conversion relations of earthquake strength parameters (stromeyer et al., 2004). a previous re-evaluation of earthquake catalogues for germany, switzerland and austria with neighbouring regions was performed in 1995-1997 in connection with a joint project on seismic hazard assessment for these countries (d-a-ch project, see grünthal et al., 1998; grünthal and mayer-rosa, 1998). 3. macroseismic scales used in germany apart from attempts to classify the severity of shakings of single events, the earliest recognizable use of intensity in a generalized way was introduced by egen (1828), who specified five degrees. this scale was applied to rhenish earthquakes in the early 19th century. based on the later denoted mercalli-cancani scale (mc; cancani, 1903-1904), with 12 degrees, sieberg (1912) introduced a more extensive definition for the single intensity degrees. a french translation of this, published in sieberg (1917), was later (but never by sieberg) called the mercalli-cancani-sieberg scale (mcs-1917). a slightly changed version was published by sieberg (1923). a strongly improved version by sieberg several independent and reliable sources contrary to a large portion of «flying sheets» reflecting exaggerations. a catalogued i = vi-vii event in an almost aseismic area in bavaria in 1822 proved to be simply a newspaper hoax (bachmann and schmedes, 1993). using contemporary sources grünthal and meier (1995) showed that the former key earthquake in ne germany (i0 = vii in 1410) was a quake in 1409, i0 = v-vi, with the focal area more than 100 km to the south. worth mentioning is also the phd thesis by meidow (1995) on the most striking damaging earthquakes in the lower rhine embayment 1755-1878 largely based on newspaper reports. unfortunately, this study does not make use of the rich material collected by alexandre and vogt (1994). according to the intention of this issue and to limit its extent, only earthquake studies in the last 15 years will be mentioned, which attempt to retrieve and interpret contemporary sources not used in a decisive way in earlier interpretations and which resulte in distinctly deviating source parameters. 2.7. recent studies after 1995 fundamental work on retrieval and classification of roots of historical sources was carried out within the project beecd (basic european earthquake catalogue and data base 19951998; albini and stucchi, 1997). numerous detailed studies within this project could be performed by j. fischer and g. grünthal, largely based on the root classifications for the events with catalogued i0 =vi or vi-vii in the period 1300-1900. the sources were traced back, whenever possible, to contemporary or eyewitness reports. it is beyond the scope of this contribution to mention all the references here. the publication policy since beecd has been to make the user community familiar with the most striking changes in interpretations of historical events. examples are formerly catalogued «damaging earthquakes» turning out to be fake. a selection of these analyses are grünthal and fischer (1998), grünthal and fischer (1999, 2000), grünthal 640 gottfried grünthal (1932) resembles, with its defined use of frequencies of effects, much later developed scales in the 1960s. all three versions by sieberg were applied up to the mid 1960s without usually specifying which of the versions was actually used. the medvedev-sponheuer-kárník scale (medvedev et al., 1965; sponheuer, 1965) was quickly adopted in germany. after an early attempt to update the msk-scale, initiated by the european seismological commission (esc) in 1980 (thanks to dieter mayer-rosa), it took another twelve years, until the test version of the european macroseismic scale (the ems-92) appeared (grünthal, 1993), again on behalf of the esc. the roermond earthquake in 1992 was one of the prominent events used to test the new scale. later, the ems-98 (grünthal, 1998), adopted by the general assembly of the esc for use in the seismological practice in europe, was applied. 4. conclusions much progress in earthquake catalogue production and connected historical studies has been made in the last 20 years. the first parametric catalogues from the mid 1980s could be significantly improved in the last ten years. an essential basis for this was the beecd project. the most striking false interpretations of historical key earthquakes have also likely been found and corrected, at least for those occurring in time periods used for hazard assessments due to the completeness criteria. still, there may exist historical material on earthquakes that has not yet been retrieved and made available for analysis and so, further new interpretations of historical earthquakes with respect to their localization and strength can be expected. although not studied in detail, our experience from beecd is that the amount of earthquakes whose information was hidden so far in the chronicles is limited. all newly found events were rather small and most occurred in time periods not used for seismic hazard assessments. however, no doubt is a better and more complete view necessary to improve our understanding of the seismicity. of special interest could be areas with high current seismicity but lacking data in the past and national border regions, where limited data could be improved by joint efforts of specialists from the different countries. acknowledgements the author is indebted to rutger wahlström and dieter mayer-rosa, the latter as a referee, for critical reading and valuable comments. references ahorner, l. 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acocella et al., 2016]. from 2011 to 2017, numerous eruptive episodes occurred; most of them characterized by the emission of lava fountains, pyroclastic material, and short-lasting lava flows that mostly spread within the valle del bove [cappello et al., 2018; ganci et al., 2018]. on the evening of may 17, 2016 an intense strombolian activity started at the north-east crater (nec), followed by the emission of volcanic ash. this eruptive activity ended the following day, when the voragine (vor) began to erupt, producing a pulsating lava fountain and a small lava flow from the western rim of the voragine-bocca nuova depression (figure 1). on 18 may, a new eruptive vent opened at the eastern base of the nec, near the two cones that formed during the july-august 2014 activity, which fed a lava flow that expanded towards the northern wall of valle del bove. this lava flow remained active until the early article history receveid august 8, 2018; accepted september 7, 2018. subject classification: etna volcano; satellite remote sensing; pléiades imagery; digital elevation model. abstract during basaltic eruptions, the average rate at which lava is erupted (effusion rate) is one of the most important factors controlling the evolution, growth and extent of the flow field. this has implications both for forecasting purposes, highlighting the importance of the effusion rate as input parameter of physics-based numerical models, and to advance knowledge on the shallow feeder system by constraining the supplied mass. satellite remote sensing provides a mean to estimate the average effusion rate by applying a direct conversion from the measured radiant heat loss by an active lava flow. this conversion relies on a set of parameters of lava (e.g. rock density, heat capacity, vesicularity, emissivity, etc.) and suffers of multiple sources of uncertainties and measurements errors, whose quantification is still an open problem. here we constrain the volume of lava emitted at mt etna on 17-25 may 2016 and emplaced out of the summit craters, by using pre-eruptive and post eruptive digital elevation models (dems) obtained processing satellite images acquired by the pléiades constellation, which provides images at 50 cm resolution in stereo and tri-stereo mode. the 3d processing of the tri-stereo pléiades imagery (acquired on 24 december 2015 and 18 july 2016), performed using the free and open source micmac photogrammetric library, provides estimations of the distribution of thickness and the bulk volume emitted. the integration of multi-platform remote sensing products represents a new potential of merging capabilities to enable a more comprehensive response to effusive crises. morning of 19 may. during the morning of 19 may a new paroxysm occurred at the vor, with the emission of a new lava overflow toward west, which descended over the lava of the previous evening. the sequence of summit eruptions at etna in may 2016 ended with two episodes of intense strombolian activity at the nec during the night of 22-23 may, and at the vor between 24 and 25 may, which fed multiple flows that covered the lavas emplaced during the 18-19 may events. mapping these volcanic deposits constitutes a critical component for constraining the average rate at which lava is erupted, controlling lava flow morphological parameters, and giving insights into emplacement processes [del negro et al., 2016]. however, the calculation of the amounts of lava erupted is still an open issue, and problems arise when trying to quantify the associated uncertainties [ganci et al., 2018]. photogrammetry, using both aerialand satellite-imagery, has been proven to be useful in monitoring volcanic activity and mapping volcanic hazards [baldi et al., 2000; gwinner et al., 2000; james et al., 2014; neri et al., 2017; smets et al., 2017; carr et al., 2018]. the recent availability of high spatial resolution data acquired in stereo or tri-stereo configuration (e.g., pléiades) gives new opportunities for frequently updating the topography and modeling the 3d shape of volcanic products by differencing successive topographies. here we employed pléiades data, which provide images at 50 cm in stereo and tri-stereo mode, to derive digital elevation models (dems) and map the products emitted during the eruptive episodes occurred at mt etna between 17 and 25 may, 2016. we also computed some morphometric parameters (area, maximum length, thickness, and volume) from the three-dimensional distribution of the volcanic deposits obtained by dem difference. ganci et al. 2 figure 1. elevation change obtained by differencing the two dems derived from pleiades images acquired before (24 december 2015) and after (18 july 2016) the may 2016 etna eruptions. colors indicate the flow thickness in meters inside the lava flow fields. flow a was emitted during the 18-19 and 22-25 may eruptive episodes occurred at bn and vor. flow b is the lava flow fed on 18-19 may by the eruptive vent opened at the eastern base of the nec, near the july-august 2014 cones. the inset shows the zero-peaked histogram of the terrain residuals, proving the two dems are properly aligned. by nuth and kääb co-registration, the standard deviation of elevation changes was reduced from 5.3 to 1.6 m. 2. methods we produced two dems at a spatial resolution of 2 meters, starting from pléiades primary images acquired on 24 december 2015 (pre-eruptive) and 18 june 2016 (post-eruptive) in tri-stereo mode containing rpc (rational polynomial coefficient) files. both images have been acquired with minimal cloud percentage (i.e. less than 5%) and cover an area of about 15 km × 15 km, which includes the summit area of etna volcano and almost all the valle del bove. the processing of the tri-stereo pléiades imagery was performed using the free and open source micmac software (multi-images correspondances, méthodes automatiques de corrélation) [rupnik et al., 2017] that performs: (i) tie points recognition and matching between images; (ii) calibration and orientation, recognizing relationships between viewpoints and objects; (iii) correlation, producing dense matching for 3d scene reconstruction. to retrieve areas, volumes and thickness distribution of the lava flows emplaced out of the summit craters on 17-25 may 2016, the difference between post eruptive and pre eruptive dems was computed. a crucial step for deriving this difference is the coregistration between the two dem datasets, since misaligned dems could result in a wrong estimation of elevation change. we applied the co-registration method introduced by nuth and kääb [2011]. this methodology firstly finds iteratively the horizontal shift between two dems, based on a slope-aspect method, and removes it. then, a potential elevation-depended error is checked and removed. finally, higher-order biases are checked and corrected, by rotating the coordinate axis, if necessary (e.g. along/cross track corrections). corresponding corrections, which result in a polynomial function, can be then applied. 3. results by subtracting the 2015 dem from the 2016 one, we obtained the topographic changes due to the volcanic deposits emplaced during the eruptive activity of etna occurred on may 2016 (figure 1). in this way, we estimated the distribution of thickness, areas and volumes (table 1) in correspondence of the flows emplaced during the 18-19 and 22-25 may eruptive episodes occurred at bn and vor (flow a), and the flow emitted on 18-19 may by the eruptive vent opened at the eastern base of the nec, near the julyaugust 2014 cones (flow b). we found that flow a exhibits an aerial extension of 1.14 km2 with a volume of 4.25 × 106 m3, while flow b has an area of 0.24 km2 with a volume of 0.4 × 106 m3 (table 1). considering both lava flow fields, we found a cumulative area of 1.38 km2. in order to quantify errors associated with volume estimation, we computed the histogram of the elevation differences outside the lava flow fields (inset of figure 1) finding a standard deviation of 1.6 m, which results in an error of 2.2 × 106 m3. the total volume is thus 4.65 ± 2.2 × 106 m3. 4. discussion and conclusions we quantified the topographic-derived volume and associated uncertainties of the lava flows erupted out of the summit craters of mount etna on may 2016. previously published results related to this eruption provided an estimation of 5.6 – 8.9 × 106 m3 for the cumulative volume infilling the bn-vor depression and the western flow [edwards et al., 2018]. our estimation for the western flow (flow a in figure 1) falls within this range, permitting to infer a volume of 3 ± 3 3d lava flow mapping at etna volcano lava flow area (km2) thickness (m) lava volume (m3) min average median max flow a 1.14 1.0 3.7 3.3 16.4 4.25 × 106 flow b 0.24 0.5 1.7 1.4 7.7 0.4 × 106 table 1. areas, thicknesses and volumes estimated for the two lava flows emplaced during the 18-19 and 22-25 may eruptive episodes occurred at bn and vor (flow a), and the flow emitted on 18-19 may by the eruptive vent opened at the eastern base of the nec (flow b). ganci et al. 4 1.6 × 106 m3 for the bn-vor depression. moreover, we found that the thickness distribution of flow b is in good agreement with typical field measurements related to the small lava flows emplaced on valle del bove [behncke et al., 2014]. indeed, by dividing flow b in five sectors based on the topography average slope, thickness ranges between: 0.9 and 2.3 m in the upper part of the flow (average slope: 7.4°), 0.5 and 1.5 m in the second sector (average slope: 11.3°), 0.9 and 2.9 in the third sector (average slope: 7.5°), 0.5 and 2.9 m in the fourth section (average slope: 13°), 1.1 and 3.1 in the last sector, which is the flattest area (average slope: 5.5°). the total volume estimated from dem differencing independently bounds the one derived from thermal satellite data processed via the hotsat system [ganci et al., 2011b] (figure 2). a cloud index has been computed considering the percentage of cloudy pixel in the volcanic area [ganci et al., 2011a; 2016] since seviriand modis-derived radiant heat flux curves (respectively blue squares and red dots in figure 2), are heavily affected by clouds (dashed grey curve). by applying the cooling curve method introduced by ganci et al. [2012], we inferred a total volume of 2.12×106 m3 expressed as dense-rock equivalent (dre). applying a correction of 25% for vesicularity to the topographic-derived bulk volume [swainston, 2017], it yields 3.48×106 m3 of dre volume. the volume difference of 1.36×106 m3 is mainly due to cloud coverage on ir satellite data. the presented methodology provides new merging capabilities of multi-platform remote sensing products to monitor effusive eruptions, enabling a rapid response during effusive crises. in particular, the comparison with hotsat derived volumes provides a first order validation to the eruption rate derived from thermal satellite data, opening new perspectives in studying the conversion between radiant heat flux and time averaged effusion rate, which is still an open problem [garel et al., 2012]. the 18-25 may eruption at mt etna was characterized by a small lava flow field, involving quite high relative errors associated with volume estimation. nevertheless, since relative error decreases when increasing the thickness of the lava flow, this figure 2. radiant heat flux derived from seviri (blue squares) and modis (red diamonds) data, and cloud coverage in percentage (dashed grey line) at mt etna during 18-23 may 2016. 5 3d lava flow mapping at etna volcano methodology would be particularly efficient for longlasting eruptions. further research will include new study cases involving both pléiades and infrared satellite data, in order to find new constraints to the radiant heat flux eruption rate conversion, as eruptive conditions change. acknowledgements. thanks are due to eumetsat for seviri data. pléiades data were available through the svop project. this work was developed within the framework of tecnolab, the laboratory for technological advance in volcano geophysics of the ingv in catania (italy) and was partially supported by the dpc-ingv 2012–2021 agreement. this paper benefited from the comments and suggestions of two anonymous reviewers. references acocella v., n. neri, b. behncke, a. bonforte, c. del negro and g. ganci (2016). why does a mature volcano need new vents? the case of the new southeast crater at etna. front. earth sci. 4:67, doi:10.3389/feart.2016.00067. baldi, p., s. bonvalot, p. briole and m. marsella (2000). digital photogrammetry and kinematic gps applied to the monitoring of vulcano island, aeolian arc, italy. geophysical journal international, 142, 3, 801-811, doi: 10.1046/j.1365246x.2000.00194.x. behncke, b., s. branca, r.a. corsaro, e. de beni, l. miraglia and c. proietti (2014). the 2011–2012 summit activity of mount etna: birth, growth and products of the new se crater. j. volcanol. geoth. res., 270. doi: 10.1016/j.jvolgeores.2013.11.012. cappello, a., g. bilotta, m. neri and c. del negro (2013). probabilistic modelling of future volcanic eruptions at mount etna, j. geophys. res. solid earth, 118. doi:10.1002/jgrb.50190. cappello, a., ganci, g., bilotta, g., herault, a, zago, v., del negro, c., (2018) satellite-driven modeling approach for monitoring lava flow hazards 1 during the 2017 etna eruption, ann. geophys, (this issue) doi: 10.4401/ag-7792. carr, b.b., a.b. clarke, j.r. arrowsmith, l. vanderkluysen and b.e. dhanu, (2018). the emplacement of the active lava flow at sinabung volcano, sumatra, indonesia, documented by structurefrom-motion photogrammetry, j. volcanol. and geotherm. res., doi:10.1016/j.jvolgeores.2018.02.004. del negro, c, a. cappello and g. ganci (2016). quantifying lava flow hazards in response to effusive eruption. geol. soc. am. bull. 128, 752–763. doi: 10.1130/b31364.1. matthew, j.e., l. pioli, d. andronico, s. scollo, f. ferrari and a. cristaldi (2018). shallow factors controlling the explosivity of basaltic magmas: the 17–25 may 2016 eruption of etna volcano, j. volcanol. and geotherm. res. 357, 425-436, doi:10.1016/j.jvolgeores.2018.05.015. ganci g, vicari a, bonfiglio s, gallo g, del negro c (2011a). a texton-based cloud detection algorithm for msg-seviri multispectral images. geomatics, natural hazards & risk, vol. 2, doi: 10.1080/19475705.2011.578263. ganci, g., vicari, a., fortuna, l., del negro, c. 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(2017). the simulation and risk analysis of mt etna’s may 2016 eruption. master thesis, school of earth and environmental science bsc (hons) geological hazards, university of portsmouth. *corresponding author: gaetana ganci, istituto nazionale di geofisica e vulcanologia, sezione di catania, osservatorio etneo catania, italy email: gaetana.ganci@ingv.it © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved ganci et al. 6 sugli effetti delle esplosioni n e l l e r o c c e e s u l l e c o n s e g u e n t i alterazioni del relativo c a m p o elastico p . c a l o i m . c . s p a d e a r i c e v u t o il 5 maggio 19(j(5 riassunto. — si riassumono i r i s u l t a t i finora conseguiti nello s t u d i o delle a l t e r a z i o n i del c a m p o elastico, p r o v o c a t e nelle i m m e d i a t e vicinanze del p u n t o di scoppio, in mezzi solidi in cui siano f a t t i brillare esplosivi conv e n z i o n a l i o nucleari. t a l i a l t e r a z i o n i si c o m p e n d i a n o nella successione: zona f u s a , zona f r a n t u m a t a , zona f r a t t u r a t a (con formazione di correnti plastiche) e, finalm e n t e , zona elastica indefinita. si d i s c u t o n o infine le conseguenze di tali r i s u l t a t i sulla d e t e r m i n a z i o n e del m o d u l o elastico di un sistema roccioso. summary. — the results achieved a b o u t the elastic field a l t e r a t i o n s c a u s e d in solid m e d i u m s , nearest to t h e shot-point by c o n v e n t i o n a l or nuclear explosions, are here r e s u m e d . such a l t e r a t i o n s may subdivided in various zones: a ) melt zone — b) c r u s h e d zone — c) cracked zone (where plastic c u r r e n t s form) — i)) ind e f i n i t e elastic zone (s. pig. 5). t h e r e are e x a m i n e d , at last, the effects of a b o v e m e n t i o n e d a l t e r a t i o n s in c o n n e c t i o n w i t h elastic modulus d e t e r m i n a t i o n in a rocky s y s t e m . 1. è n o s t r o p r o p o s i t o r i a s s u m e r e b r e v e m e n t e q u a n t o è o r m a i n o t o s u l m e c c a n i s m o delle esplosioni e sulle p r o f o n d e a l t e r a z i o n i d e l c a m p o e l a s t i c o c h e si o s s e r v a n o n e l l ' i n t o r n o del p u n t o d i s c o p p i o . s e b b e n e le r i c e r c h e a q u e s t o r i g u a r d o n o n s i a n o n u m e r o s e , q u a n t o si è f i n o r a o t t e n u t o è s u f f i c i e n t e a d e l i n e a r e l e p r i n c i p a l i v i c e n d e a t t r a v e r s o le q u a l i u n m e z z o solido p a s s a n e i p r e s s i della c a m e r a d i s c o p p i o . t r e f a s i , s o p r a t u t t o , v e n g o n o d i s t i n t e , c o r r i s p o n d e n t i a t r e d i v e r s e r e g i o n i n e l l o s p a z i o : f a s e d i s c o p p i o , delle o n d e d ' u r t o e d e l l e o n d e e l a s t i c h e , r i s p e t t i v a m e n t e l e g a t e a l l a z o n a di d e f l a g r a z i o n e , alla z o n a di f r a n t u m a z i o n e e a l l a z o n a e l a s t i c a . p e r s e m p l i c i t à , c o n s i d e r i a m o u n m e z z o solido i n d e f i n i t o , o m o 3 3 6 p . c a i . o i m . c . s p a d e a geneo e isoti-ujio. i n esso, u n a c a v i t à sferica sia riempita (li esplosivo con d e t o n a t o r e centrale. a d u n i s t a n t e d e t e r m i n a t o , inizi nella carica esplosiva u n ' o n d a di detonazione: essa si p r o p a g a con u n a velocità specifica, che costituisce uno dei p a r a m e t r i caratteristici dell'esplosivo u s a t o . poiché l'onda d e t o n a n t e è il disturbo più rapido che p u ò essere prop a g a t o nell'esplosivo, il mezzo in cui l'esplosivo è posto s a r à in u n a condizione i n d u s t u r b a t a fino al m o m e n t o in cui t u t t o l'esplosivo è s t a t o c o n v e r t i t o in gas a r d e n t e sotto a l t a pressione. questo è il p r i m o stadio, corrispondente alla zona di detonazione, di volume p a r i a quello occupato dall'esplosivo. le tensioni p r o d o t t e dai gas in r a p i d a espansione, sono n e t t a m e n t e maggiori dell'estrema, fermezza (e s t r e n g t h ») del mezzo: la s t r u t t u r a del solido viene d i s t r u t t a e r i d o t t a allo s t a t o di polvere, a t t r a v e r s o u n complicato sistema di f r a n t u m a z i o n e . perciò, un'esplosione ilei suolo f o r m a u n a zona di compressione a t t o r n o al centro della carica: il suolo viene violentemente spostato sotto l'effetto di u n ' u l t e r i o r e espansione dei gas dalla zona di compressione. ricerche compiute dai russi perm e t t o n o di ritenere che il volumi; di questa zona non ecceda 250 v o l t e quello della carica, per t e r r e n i di m e d i a f e r m e z z a , e dieci v o l t e p e r rocce solide. secondo esperienze compiute da diversi studiosi, a t t r a v e r s o il mezzo viene t r a s m e s s a , in questa fase, u n ' o n d a d ' u r t o con velocità maggiore di quella del meccanismo di f r a n t u m a z i o n e , come si verifica p e r i gas e per i liquidi. la velocità dell'onda d'urto è inoltre maggiore di quella delle onde elastiche proprie del mezzo, è condizionata dall'intensità dell'esplosione e decresce dal crescere della distanza (forse a d essa è l e g a t a la dispersione a n o m a l a scoperta da caloi) ( l ). dopo la formazione della zona di compressione, il mezzo circostante subisce s p o s t a m e n t i e deformazioni p e r e f f e t t o dello stesso processo dell'espansione di gas, d a n d o luogo a fessurazioni radiali e t r a s v e r s a l i . l a zona di questi; p e r t u r b a z i o n i nella c o n t i n u i t à del mezzo è c h i a m a t a zona di distruzione. a seconda delle p r o p r i e t à tisiche del mezzo, l'estensione della zona di distruzione varia e n t r o i limiti f r a 2000 e 6000 volte il volume della carica esplosiva. secondo sadovskii e b e l y a e v (2), le dimensioni della regione in parola sono regolate dalla legge di s i m m e t r i a , secondo la f o r m u l a r = k q1'* dove r r a p p r e s e n t a il raggio di distruzione e q il peso della carica esplosiva. y a qui sottolineato che l'energia dell'esplosione, d u r a n t e il suo sprigionarsi, inizia ad essere i m m e d i a t a m e n t e spesa p e r compressione s l t ì l . i e f f e t t i d e i . i . e e s c l u s i o n i n e i . l e l i o c c f . 3 3 7 del mezzo e simultanea trasformazione nell'energia di u n ' o n d a elastoplastica. ulteriori processi nel mezzo si verificano per effetto dell'energia della così d e t t a onda d'esplosione (onda d ' u r t o ) . i n questo caso, il mezzo è soggetto a g r a n d e deformazione, a c c o m p a g n a t a , ad una certa dis t a n z a d a l centro della, carica, da f r a t t u r a z i o n e . questa regione del mezzo è interessante sia come regione di formazione delle onde sismiche, sia come regione di residue deformazioni, con notevoli s p o s t a m e n t i e velocità delle particelle del suolo. l ' e f f e t t o delle onde d'esplosione (o d ' u r t o che dir si voglia) non si l i m i t a alla compressione e alla deformazione residua; in più r e m o t e zone del mezzo, l'onda d'esplosione causa deformazioni che sono govern a t e dalle leggi della teoria dell'elasticità. in queste zone, l'onda d'esplosione (d'urto) si m u t a in onde sismiche elastiche. i n altre parole, a u n a c e r t a distanza critica la tensione che decresce con l ' a u m e n t a r e della distanza d a l l ' o r i g i n e e g u a g l i a i limiti elastici del mezzo e la p e r t u r b a z i o n e diviene u n ' o r d i n a r i a onda elastica, che si propaga con velocità condizionata dalle costanti elastiche del mezzo. è questa la regione così d e t t a classica, poiché ad essa, può essere applicata la classica (lineare) teoria dell'elasticità. ciò che conia m e t t e r e bene in evidenza è clic, nelle i m m e d i a t e adiacenze dell'origine dello scoppio, non possono essere applicate le leggi valevoli per un mezzo p e r f e t t a m e n t e elastico. i n d e t t o spazio, il mezzo è soggetto al violento a u m e n t o dell'elevata pressione dell'esplosione, che provoca nel mezzo varie specie di anormali fenomeni. il più rilevante è quello concernente la relazione tensione-deformazione (« stress-strain »), che in queste condizioni non p u ò più essere considerata lineare: in conseguenza di ciò — come si è verificato più sopra — si osservano, nella, zona, su d e t t a , onde (l'urto o onde elasto-plastiche in luogo di onde elastiche. le loro caratteristiche sono t a n t o complicate da lasciare scarse possibilità sulla completa soluzione del fenomeno. qualche anno fa, g-. morris (») presentò una teoria sulle esplosioni, in cui venivano considerate t r e delle zone sopra citate: la zona di detonazione, la zona, di compressione (con distruzione del mezzo a t t r a v e r s o u n complicato sistema di f r a n t u m a z i o n e ) e la zona così d e t t a classica. nella seconda zona, come si è visto, la velocità di propagazione è molto più e l e v a t a della velocità delle onde elastiche e l'intensità tende a cadere d m a n t e la propagazione, sia per il progressivo a u m e n t o del volume del mezzo occupato dall'onda d ' u r t o , sia per perdite di energia nella f r a n tumazione del mezzo. 3 3 8 i ' . c ' a l o ! 31. c . s p a d e a il secondo stadio termina perciò quando il fronte d ' u r t o è giunto alla distanza critica, decrescendo in intensità al limite elastico del mezzo. morris ritiene che caratteristica fondamentale della regione classica, a p a r t e la costante velocità di propagazione, sia che l'ampiezza delle onde elastiche decresce secondo la legge dove u è lo spostamento elastico, c una costante, r la distanza dal centro della carica, t il tempo e v la velocità delle onde elastiche. esiste però u n a zona di transizione in cui le onde elastiche si propagano con velocità costante, m a nella quale la legge testé r i p o r t a t a sulla diminuzione di ampiezza cade in difetto. in questa zona di transizione, i n f a t t i , l'ampiezza a) -b)m 3* f i g . 1 decresce molto più rapidamente, secondo la legge ljrk, con le < 1 . indicando con a la distanza critica f r a i limiti della regione d ' u r t o (o d'esplosione che dir si voglia) e la regione classica, si v a l u t a che la zona di transizione sopra accennata, nel caso di onde dell'ordine n = 0, abbia ima ampiezza ¡lineare r = 3a (fig. 1). p e r onde dell'ordine n = 1 questa estensione è già r = 19« (4). l'esistenza della zona di transizione nella regione classica, modifica alquanto il modello morris per le esplosioni in materiali solidi. l a regione d ' u r t o (« shock region ») è di decisiva importanza ai fini della formazione delle onde elastiche. la sua delimitazione, esige la conoscenza della distanza critica a. morris propose la determinazione di questo limite, determinando la minima distanza a p a r t i r e dalla quale è valida la [1] (a p a r t i r e dalla quale cioè la legge di decrescita delle ampiezze, conforme a 1/r, comincia ad esser valida). l'esistenza della zona di transizione nella regione classica toglie attendibilità a questo metodo, s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 3 9 clie condurrebbe comunque a valori approssimati per eccesso della dis t a n z a a. l'esistenza di tale zona, d'altronde, è s t a t a q u a l i t a t i v a m e n t e p r o v a t a dai risultati delle esperienze condotte da ivirillov (5) sugli eff e t t i sismici delle esplosioni. determinazioni q u a n t i t a t i v e di a richiedono speciali indagini sismiche. i l giapponese k a s a h a r a (e) a f f r o n t a il problema p a r t e n d o da u n a teoria di k a w a s u m i e y o s i y a m a ( ' ) sulla propagazione delle onde sismiche, provenienti d a un'origine sferica. i n questa teoria era s t a t o p r o v a t o che la f o r m a d ' o n d a è funzione del raggio della superficie, sulla quale avveniva il brusco c a m b i a m e n t o di tensione («stress»). k a s a h a r a ritiene giustificata l'estensione di d e t t a teoria al caso delle esplosioni, a condizione di ignorare l ' i m p e r f e t t a elasticità d o v u t a all'enorme pressione delle esplosioni. sia quello r a p p r e s e n t a t o nella fig. 2 il modello per origine esplosiva. sia r = a0 il raggio di u n a superfìcie sferica in u n mezzo omogeneo, soggetta a normale tensione p(t). allora, lo spostamento potenziale <j>(i) ad u n a certa distanza r, assume, nella teoria di k a w a s u m i , l'espressione + a + co n„ c /"'mi-i '-«o/c) /' «j, (t) = j t7 ^ a p ¿ v ) « p d co [2] 2 710 r j (p — a)(p—p) j 00 — co dove: . 2 i>2 v \ l (v\. .2v2 n v i / (v\* a = t — + 2 / 1 — ; /s = t — — 2 — 1 / 1 — i — , a0v a„ w) a0v a0 | \vj e q, v, v sono la densità, la velocità delle onde longitudinali e quella delle onde trasversali. con riferimento alla equazione del pendolo: r + 2 £ f + n& = p (t) , 3 4 0 p . c'ai.oi m . c . s l ' a d e a p u r c h é si faccia a = i e + v«2 — e" > fi = * e — v"2 — e~ > <facile prov a r e che risulta: _ na„ l p ~ r ' e v n ^ v [3] nell'ipotesi che la pressione di detonazione sia del tipo a gradino, si v e d e che lo s p e t t r o dell'onda sismica c a u s a t a da u n a f o r z a origine f (ito) è del tipo: 8{oj) = y0(co)f(o))(o , d o v e y0 (co) è la risposta impulsiva del pendolo equivalente. p e r f (co) = 1 /co (fig. 3 a ) , si h a : 8(w) = y0(oj) . ( à ) pct) t 0 — > t f i g . 3 si osserverà quindi uno s p e t t r o simile a quello della f i g . 3b. n o t o y0(co), p e r questa v i a sarà possibile o t t e n e r e a„, inteso come raggio a p p a r e n t e della zona origine. n o n si h a n n o notizie precise circa il m u t a m e n t o di tensione a l l a superfìcie della sfera i i n a g i n a t a . k a s a h a r a lo r a p p r e s e n t a con u n a f u n zione del t i p o a gradino (unit s t e p f u n c t i o n ) (fig. 3«,). d a l c o n f r o n t o con gli s p e t t r i analizzati d a l l ' a u t o r e e riassunti in fig. 3b, si n o t a u n o s t r e t t o accordo f r a essi. i n questo modo, t0, d e t e r m i n a t o dal ris u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 4 1 spettivo spettro, è uguale al periodo che corrisponde alla massima intensità dell'ultima curva teoretica. i l raggio apparente, determinato per questa via, è confrontato con la carica degli esplosivi: risulta chiar a m e n t e la tendenza di ig a0 ad aumentare proporzionalmente a ig iv (fig. 4). fig. 4 e a g g i o a p p a r e n t e per o n d e elastiche d'origine esplosiva (°), raggio e q u i v a l e n t e delle zone f r a n t u m a t e (*) in f u n z i o n e della carica esplosiva, secondo k a s a l i a r a ; — x — x-— relazione f r a w e a*, secondo gli e s p e r i m e n t i di t. m u r a t a ; © stessa relazione nel caso di esplosioni n u c l e a r i : di h a r d h a t (1 ) e d i s a l i n o l i ( 2 ) . trascurando l'effetto legato alle diverse qualità degli esplosivi, k a s a h a r a ottiene la seguente relazione f r a w e a-0: ig a. = a+p lg 1f , m dove a = 0,52 e fi = 0,38. il valore di a„ potrebbe essere facilmente ottenuto, qualora esso coincidesse con il raggio della regione sferica, costituita da roccia perf e t t a m e n t e f r a n t u m a t a . nella fig. 4, a* indica il raggio di u n a sfera di volume uguale al volume totale della roccia f r a n t u m a t a . dalla figura risulta che a* cresce con a0 e che quest'ultimo è parecchie volte più grande del primo. questo f a t t o suggerisce una stretta relazione esistente fra 3 4 2 f . c a l o i m . c . s p a d e a le d u e q u a n t i t à , di chiaro interesse ai fini della conoscenza del meccanismo di generazione delle onde di origine esplosiva. 2. q u a n t o è s t a t o esposto i m m e d i a t a m e n t e sopra, vale nell'ipotesi che il mezzo sia p e r f e t t a m e n t e elastico. questa ipotesi è applicabile ai fenom e n i che insorgono a sufficiente distanza dall'origine. p e r f e n o m e n i vicinissimi all'origine, l'ipotesi non è più applicabile, se noi escludiamo dalla discussione quel complesso dì complicati fenomeni, che v a n n o — come si è visto — dalla completa distruzione del m a t e r i a l e ad u n a diffusa f r a t turazione. i n tale spazio, il mezzo è soggetto al violento a u m e n t o dell ' e l e v a t a pressione di detonazione, che p r o v o c a nel mezzo v a r i tipi di a n o r m a l i fenomeni. i l più notevole è. quello concernente la « stress-strain relation », che si p r e s e n t a non più lineare nelle s u d d e t t e condizioni, a v e n d o come conseguenza l'osservazione di onde d ' u r t o o plastiche (« shock waves or plastic waves ») in luogo di onde elastiche, nella regione su c i t a t a . si è d e t t o degli almeno t r e s t a d i conseguenti a d un'esplosione. i l secondo stadio, c a r a t t e r i z z a t o — come propose g. morris — d a veloc i t à di propagazione più g r a n d e di quella delle onde elastiche, t e r m i n a q u a n d o il f r o n t e d ' u r t o raggiunge la distanza critica e la sua i n t e n s i t à decresce al livello del limite elastico del mezzo. il meccanismo in questo terzo stadio è soggetto alla teoria classica della propagazione ondulat o r i a ; p e r t a n t o , k a s a h a r a ritiene ivi applicabile la teoria di k a w a s u m i e y o s i y a m a , che condurrebbe —• come si è visto — alla d e t e r m i n a z i o n e della d i s t a n z a di formazione di f r o n t i d ' o n d a , da ritenersi come raggio a p p a r e n t e dell'origine p e r esplosione (pig. 4). (a questo riguardo, morris h a s p e r i m e n t a l m e n t e t r o v a t o che la d i s t a n z a critica p e r u n a l i b b r a di carica esplosa in a r e n a r i a è dell'ordine di q u a t t r o piedi). p e r q u a n t o concerne la velocità nella seconda regione, esperienze di o k a w a , h i r a y a m a e k i y o t a (8) h a n n o d i m o s t r a t o che la velocità delle onde d ' u r t o è e f f e t t i v a m e n t e maggiore della velocità delle onde elastiche nelle i m m e d i a t e vicinanze dell'origine. ad ogni modo, anche k a s a h a r a conclude che il raggio a p p a r e n t e origine v a inteso nel senso della d i s t a n z a critica della, teoria, di morris, più che il raggio dello spazio in cui la roccia viene finemente f r a n t u m a t a . 3. deformazione della forma d'onda dovuta alla viscosità del mezzo. q u a n t o è s t a t o d e t t o finora, specie sulla scorta delle ricerche di k a s a h a r a , ci autorizza a ritenere il raggio-origine delle onde elastiche s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 4 3 due o più volte maggiore di quello della regione f r a n t u m a t a , purelié si tenga debito conto dell'alta pressione della detonazione. le stime eff e t t i v e del raggio origine sono, d'altronde, parecchie volte maggiori di a*, così da rendere insufficiente la spiegazione della disparità sopra menzionata. u n ' a l t r a causa appare legata alla viscosità della roccia. basandosi sulla teoria di ricker [geophys., 15 (1950, 348], kasaliara calcola l'apparente aumento del periodo caratteristico t0 con la distanza, basandosi sullo spettro s(co) t r a t t o dalle sue ricerche e valendosi, per ottenere lo spettro alla distanza x — x, della formula avendo f a t t o q = 2 (a titolo di prova), e le curve essendo state tracciate per diversi valori del parametro fig. 5 visione s c h e m a t i c a delle alterazioni in un mezzo solido elastico, c o n s e g u e n t i a d esplosioni. regione a ) c a v i t à di scoppio; b ) roccia f r a n t u m a t a ; g) regione anelastica (plastica), f r a t t u r a t a ; d) zona elastica. l a linea t r a t t e g g i a t a limita l ' a p p a r e n t e raggio-origine delle onde elastiche, quale risulta dai periodi c a r a t t e r i s t i c i . al crescere di % la frequenza predominante dello spettro diviene minore. k u b o t e r a valuta per le p (crosta terrestre) £ = 7 . 9 x lo-'cmr 1 ; ne deriva per un valore dell'ordine di 0.4 per p u n t i d'osservazione dis t a n t i 5 k m dal p u n t o di scoppio. i l raggio origine a0 viene stimato dal 40 al 50% più grande, in conseguenza dell'aumento subito dal periodo caratteristico t0. non ostante le brevi distanze, le più alte frequenze delle onde subiscono u n a influenza relativamente grande in conseguenza della viscosità. 3 4 4 i \ c a i . o i m. c . s p a d h a k a s a h a r a t r a e , dalla discussione dei due f a t t o r i e s a m i n a t i , la seg u e n t e imagine per le origini delle p e r t u r b a z i o n i d o v u t e ad esplosioni (fig. 5). all'esterno della regione p e r f e t t a m e n t e f r a n t u m a t a (b) esiste u n a regione (c), in cui l'intensità delle tensioni cade al limite elastico della roccia d u r a n t e la sua propagazione verso la d i s t a n z a critica (r = ac). le onde elastiche prendono quindi origine a questa d i s t a n z a , p e r cui non a* bensì ac corrisponde all'a 0 di k a w a s u m i e y o s i y a m a . le onde elastiche p r o p a g a n t e s i da essa sono soggette a l l ' e f f e t t o della viscosità della roccia, il che si t r a d u c e in u n a p p a r e n t e a u m e n t o delle dimensioni-origine, come da c i t a t a figura. k a s a h a r a dà la seguente s t i m a approssimativa di questo raggio a p p a r e n t e , a' = 1,5 «<¡ = 3 ~ 5 a* . [1] 4. spettro delle onde sismiche e magnitudo dei terremoti. è s t a t a p r o v a t a la t e n d e n z a p e r la quale più g r a n d e è la m a g n i t u d o dei t e r r e m o t i , p i ù lungo diviene il periodo delle onde sismiche g e n e r a t e i o ( * e c ) f i g . 6 all'origine. g u t e n b e r g e r i c h t e r , p. es., h a n n o proposto la seguente f o r m u l a : lg t = — 0,82 + 0,22 m (t = periodo onde di m a s s i m a ampiezza: m = m a g n i t u d o ) . [5] i l periodo dell'onda sismica dipenderebbe p e r t a n t o dalle dimensioni dello spazio-origine. s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 4 5 k a s a h a r a studia, con il metodo dell'analisi spettrale, la fase iniziale (onde p) di 21 terremoti d i s t r u t t i v i . il risultato delle sue indagini (riass u n t o nella pig. 6) lo ha condotto a ritenere pressoché lineare la relazione f r a la m a g n i t u d o e il logaritmo di t0: lg t0 = a + j8 m , [5'] con a e /? costanti che, nel caso specifico, assumevano i valori a = — 2,59 ; = + 0,51 . p r e c i s a m e n t e , k a s a h a r a ha o t t e n u t o : p p e r t e r r e m o t i superficiali — 0,78 0,28 ± 0,042 d » profondi — 2,19 0,43 ± 0,047 complessivamente (superficiali e profondi) — 2,59 0,51 ± 0,033 . allo scopo di chiarire il significato fisico di t0, k a s a h a r a esamina il caso di u n a zona origine a f o r m a sferica, risultante in u n a radiazione a z i m u t a l e di tipo a q u a d r a n t i , quale è s t a t a s t u d i a t a da i n o u y e . p a r t e n d o dalla teoria di inouve, e sulla base dei risultati delle sue personali ricerche, k a s a h a r a ottiene la seguente relazione f r a la velocità delle onde longitudinali vp e il raggio origine a„ di tali onde, 2 re (io nr. , . n _ to vp ~ 0,7 da cui «„ ~ 0,i . vp tu ~ — 2 ti p e r q u a n t o r i g u a r d a l'energia accumulata nella zona origine, k a s a h a r a osserva che essa, con riferimento alle onde r a d i a t e per il supposto modello, è n • volte la differenza f r a la energia di deformazione (« strain energy ») d i s t r i b u i t a p r i m a e dopo il terremoto, n essendo u n a costante d i p e n d e n t e dalla r a p i d i t à del m u t a m e n t o di tensione (« stress change ») — valendo 1 per un processo rapidissimo e 0 per uno e s t r e m a m e n t e lento — . l'energia di deformazione per u n i t à di volume può essere scritta s o t t o la f o r m a : e = ^ [(a + 2 p) (err + em + e^f + p [e^ + e^ + er& — 4 , — ±e<p<pe-rr—±erre&&)} ' e f f 3 4 6 p . c a i . (ii m . (,'. s p a d e a considerata ad un p u n t o (r, &, tp), essendo a e fi le c o s t a n t i di l a m é . k a s a h a r a ottiene (fig. 7) l ' a m m o n t a r e di energia in un guscio sferico (a0 < r < r0) secondo la f o r m u l a p e r t a n t o l'energia t o t a l e di deformazione, a c c u m u l a t a nello spazio esterno alla superficie origine, diviene: p e r semplicità, k a s a h a r a suppone che la distribuzione delle tensioni si verifichi sulla superficie origine i s t a n t a n e a m e n t e ; allora l'energia t o t a l e secondo la f o r m u l a [6], viene t u t t a r a d i a t a come onde sismiche. i n a l t r i t e r m i n i , supposta la superficie-origine soggetta ad u n a det e r m i n a t a iniziale distribuzione di tensioni e l i b e r a t a d ' u n subito d a essa, l'energia di deformazione d i s t r i b u i t a a t t o r n o alla superficie-origine scompare, e — in sua vece — v e r r a n n o osservate onde sismiche, la cui energia uguaglia la scomparsa, energiadi deformazione. k a s a h a r a considera il r a p p o r t o erjem , nella sua dipendenza da r„, come i n d i c a t o r e p e r l'accumulo di energia sismica. nella fig. 8 è disegnata la relazione f r a er0 e r0, nelle r e l a t i v e scale. d a essa r i s u l t a che l ' 8 0 % dell'energiat o t a l e è a c c u m u l a t a in u n a cor o n a sferica posta f r a a„ e 1 . 7 a„. i l volume della corona sferica così definita, p u ò essere r i t e n u t o l ' e f f e t t i v o volume d ' a c c u m u l o p e r l'energia. nei limiti della v a l i d i t à di d e t t e approssimazioni, la densità dell'energia d ' a c c u m u l o (valori medi) p u ò essere r a p p r e s e n t a t a d a : per x = ¡i ed essendo a e b costanti. fig. 7 a c c u m u l a z i o n e di d e f o r m a z i o n i i n t o r n o ad una origine sferica. [7] s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 4 7 dove ve = 71 a0 x 4 . [8] er0 ¡ek % •loo to/do f i g . 8 volume di un guscio sferico i n t o r n o all'origine ed energia in esso a c c u m u l a t a (secondo k a s a h a r a ) . f r a le t a n t e formule che danno e in funzione di m (magnitudo), quella di gutenberg e richter si esprime ig e = a + b m , dove a = 11,8 , e 6 = 1,5 . o t t e n u t o e , p. es. con questa formula, e dedotto ve dalla conoscenza di a0, si può facilmente trarre e. kasahara riporta i risultati dei suoi calcoli nella fig. 9; da essa si ricava che i valori di e appaiono distribuiti entro un largo margine fra 3 x io2 erg/c.c. e 3 x 104erg/c.c. 0.8 e/ve (cg-s) • i o 6 ì0ui o 1 4 0 ° 4 5 6 ? 0 f i g . 9 a p p a r e n t e a c c u m u l a z i o n e della d e n s i t à di energia a t t o r n o all'origine sismica, p e r t e r r e m o t i superficiali (•) e p r o f o n d i ( x ). (secondo k a s a h a r a ) . *x yx *x * ' ^ a n « 3 4 8 ] ' . c a l o i m . c . s i ' a d e a è questo, n a t u r a l m e n t e , s o l t a n t o l'ordine di grandezza. a d ogni modo, esso risulta attendibile e rientra f r a i valori o t t e n u t i da tsuboi, sulla base di s t u d i geodetici compiuti sulle deformazioni della crosta. a pag. 345 è s t a t a d a t a u n a relazione fra, t0 ed m, con riferimento alle onde spaziali [5']. altri sismologi (aki, j o n e s , k a n a i . . . ) h a n n o f o r n i t o relazioni del genere, riassunte nel seguente specchietto: tipo (li t e r r e m o t o superficialissinio (aki) superficiale-locale (jones) m o d e r a t a m e n t e p r o f o n d o ( k a n a i ) onde i> 7 ' 3 cg m t sé » ti x ~ 2'3 — m: t ~ j / ° . 5 d a n d o lo stesso peso a relazioni o t t e n u t e con m e t o d i diversi, s e m b r a di p o t e r a f f e r m a r e che i periodi delle onde p ed 8 sono proporzionali a m°'r> in caso di t e r r e m o t i profondi, e a d m i n caso di t e r r e m o t i superficiali, a p p r o s s i m a t i v a m e n t e . 5. comportamento del mezzo sotto vazione di esplosioni nucleari. l a l e t t e r a t u r a sull'argomento comincia ad arricchirsi di c o n t r i b u t i sempre più interessanti, sopra t u t t o da p a r t e — per ovvie ragioni — di americani e di russi.| ci limiteremo, ai fini della presente discussione, ad alcuni lavori p u b blicati sull'esplosione nucleare s o t t e r r a n e a di 5-kiloton 1), provoc a t a nel g r a n i t o ad i l a r d h a t . theodore e . b u t k o v i c h (9) dimostra la possibilità di calcolare gli e f f e t t i dell'onda d ' u r t o di un'esplosione nucleare s o t t e r r a n e a , usando u n modello teorico, sfericamente simmetrico, idrodinamico, elasto-plastico. l'accordo f r a calcolo e osservazione è t a n t o p i ù s t r e t t o q u a n t o p i ù a c c u r a t a la conoscenza delle p r o p r i e t à elastiche e dinamiche dei m a t e r i a l i coinvolti n e l l ' e v e n t o . q u a n d o queste prop r i e t à sono conosciute, l'accordo nella determinazione degli impulsi di pressione, delle velocità delle particelle (« peak particle velocities »), dei t e m p i d ' a r r i v o dell'ondad ' u r t o e della f o r m a degli impulsi di pressione, r i e n t r a nei limiti delle incertezze delle misure. lo scoppio f u p r o v o c a t o a 290 m (286.2 in) sotto la superfìcie, al f o n d o di u n a p r o f o n d a c a v i t à verticale, r i c a v a t a nel g r a n i t o (ne v a d a ) . s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e rocce 3 4 9 l ' a p p a r e c c h i a t u r a i n t o r n o ad h a r d h a t (un vero « array ») h a consentito l'indagine in u n ' a r e a s t e n d e n t e » dalla regione idrodinamica a quella elastica, f u r o n o f a t t e misure di t e m p i d ' a r r i v o dell'onda d ' u r t o nello spazio f r a 7.35 e 24.1 m [cliabai a n d bass, 1963]. misure di im' + 50\ pulso di pressione in due località: u n a di 450 in 5 . 5 1 , ed u n ' a l t r a di 664 ± 40 k b a m 4 . 8 5 . — 25 k b (kilobars) a li 5. 100 l dejimiiaziom 10 i \ — accurato. § " 9 s s s £ 1 100 1 0 + i q 0,01 v w o 13 n n • »-i p• j p \ dclimiiaziam v ^ approssimati fuso f r a n i u m a ì o i ^ i fatturato' m e t r i j 5 i o ? o s o 1 0 0 2 0 0 s o o disianza, radiale, dal centro di esplosione fig. 10 e v e n t o nucleare sotterraneo eli h a n l h a t . nella figura, l'espressione « f r a n t u m a t o » sta per « f r a n t u m a t o e finemente crepaeciato »: la zona relativa, nella sua p a r t e esterna, contiene anche il guscio sferico caratterizzato da corrente plastica. l a r e t e di osservazioni sismiche predisposta f r a 78 m e 460 m h a permesso di calcolare in 5526 m/sec la velocità media dell'onda: d ' u r t o . xon possiamo r i p o r t a r e , per ovvie ragioni, i risultati delle indagini in parola. osserveremo che, oltre a tensioni radiali ( n a t u r a l m e n t e le p i ù forti) sensibili tensioni tangenziali f u r o n o osservate a m 61,8 d a l c e n t r o di detonazione. riescono qui di particolare interesse i risult a t i dell'osservazione, relativi alle zone i n v e s t i t e dall'esplosione (fig. 10). i l mezzo figura vaporizzato in u n raggio di ca 3 m , fuso da 3 a 6 m ca, 3 5 0 p . 0 a i . o i m. c . s p a i ) k a f r a n t u m a t o o finemente f r a t t u r a t o da 6 a 75 m ca, erepacciato da 75 fino oltre 400 m e t r i (limite della zona investigata). p e r q u a n t o concerne le misure di pressione, f u m i s u r a t o un massimo di 4 k b ca, come tensione compressiva radiale, a 62 m ca di distanza (pari, in t e m p o , a ca 11 m sec) e 1,2 k b pari a 110 m ca (24 m sec circa). f i g . 11 l ' e v e n t o di h a r d l i a t s c h e m a t i z z a t o nelle sue p r i n c i p a l i manifestazioni. u n a l t r o articolo interessante sul « h a r d h a t e v e n t » è quello di nicolas m. short (10). l'esplorazione post-esplosione comprende l'esecuzione di q u a t t r o fori a p p r o s s i m a t i v a m e n t e radiali alla regione di scoppio (pig. 10). e s e m p l a r i di roccia f u r o n o e s a m i n a t i per i n d a g a r e su variazioni nelle m i c r o s t r u t t u r e , nella formazione plastica e nelle p r o p r i e t à fisiche, quali l a f e r m e z z a , le velocità soniche, la p e r m e a b i l i t à ecc. in f u n z i o n e delle pressioni d ' u r t o f r a 2, e 110-kb su distanze prescoppio di t r e cav i t à radiali. i l raggio della c a v i t à era di 19 m e t r i . il m e t o d o di studio consiste nell'indagare sulla, risposta del mezzo a l l ' u r t o di pressione, in zone d istr ib u ite concentricamente i n t o r n o al p u n t o di esplosione. le p r o p r i e t à peculiari di d e t t e zone, riflettono lo s t a t o di pressione e i modi di deformazione a cui furono soggetti. così lo s t a t o vitreo c a r a t t e r i z z a la zona m a z z o : granito c a r i c a : 5±ikf profondai: 286. z m * lìmik approssimativo > \ in r i , i / . t . residuo di materiale fuso radioattivo o 30 m suiu.i h f k k t t i d e i . i . e k . < l ' ! . 0 . « i c > x i s k i . l . e r o c c e 3 5 1 « idrodinamica », in q u a n t o il vetro solidifica da roccia f u s a o v a p o r i z z a t a : la zona « plastica » contenente roccia d i e m o s t r a distorsione di grani e scorrimenti (« slip displacements ») nell'interno dei grani stessi: la zona «frantumata», contenente miriadi di m i c r o f r a t t u r e . queste u l t i m e s f u m a n o g r a d u a l m e n t e in f r a t t u r e più l a r g a m e n t e spaziate, che divengono via via m e n o f r e q u e n t i avvicinandosi al limite esterno della zona « fratturata ». 8.0 5 ^ 7 0 !ö 6.0 <d 4.0 2.0; i i ! | media £7.75']—»-4 (presso la baie della ceviiàj • / / camino zona franhimata 49 1 3 6 5 4 1 20 3 0 3 5 5 5 6 0 5 10 15 disfama dalla, perforazione b (in metri) (evento di fiardhat) fi». 12 variazione dell'indice di frattura nell'evento di llardliat (secondo short). esistono parecchi metodi per correlare le microfratture in rocce policristalline, con s t a t i conosciuti di tensioni imposte sotto condizioni specifiche di peso. generalmente l'indie di frattura è b a s a t o sulla stima del n u m e r o di f r a t t u r e in gruppi di cinque o multipli di cinque, in ogni centinaio di grani osservati al microscopio. l ' a u m e n t o della pressione provoca, un generale a u m e n t o nel numero delle f r a t t u r e . le differenze di tensione nello s t r a t o di compressione o estensione (decompressione), d e t e r m i n a un analogo a u m e n t o di m i c r o f r a t t u r e oltre i valori critici della tensione, con orientazione p r e d o m i n a n t e imposta dalle principali tensioni agenti. nel caso specifico, f u r o n o e s a m i n a t i esemplari di grano-diorite p r i m a dello scoppio: risultò una generale assenza di f r a t t u r e nel quarzo e un n u m e r o minimo nei feldspati. i grani di minerali variano 3 5 2 ] ' . c a i . o i m. (.'. s p a i ) k a in d i a m e t r o f r a 0,2 e 5,0 inni. le misure compiute in più luoghi dopo lo scoppio, rivelarono i c a m b i a m e n t i p r o v o c a t i dalle tensioni i n d o t t e dal passaggio delle ondi; d ' u r t o . così l'indice di f r a t t u r a , definito — come si è d e t t o — come media della frequenza di f r a t t u r e p e r sezione sottile, m o s t r a r o n o il grado di n i i c r o f r a t t u r a z i o n e subito dal mezzo a t t r a v e r s a t o . la f r a t t u r a z i o n e cresce g r a d u a l m e n t e da u n indice dì 3.0 p 16 ^ 8 . o c 4 j qruppi sottolineati si riferiscono ai campioni esaminati (i numeri indicano disianza dai fori trivellati} j numeri in parentesi sono valori dell'indice difrcrftara co -ci co -xì 10 ì y u ^.oo'ìf n 7 ¿ 7 1 o! mi o 00 k>| ol m "o cd m | u-j 2 • •+i ol 00 0> n i n o ni m i 1 in id o) o o o» o o» <o o <si «0 00 » cm cvj co 4 0 30 2 0 io . q ! cs ^ disianza dal pvnto di scoppio (in metri} (èveilio di ¡lardimi) fig. 13 variazione dell'indice di frattura nell'evento di hardliat (secondo short). pre-scoppio a 7.5, sopra un intervallo di 36 m e t r i , soggetto a pressioni che raggiunsero 50-kb a 18 metri dal p u n t o di scoppio (fig. 12). presso la c a v i t à (a m 10 dal p u n t o di scoppio), l'indice cresce tino a 16.2 nel p u n t o p a r i a 100-kb di pressione (fig. 13). f r a t t u r e estese nel quarzo, m o s t r a n o o r i e n t a m e n t i preferenziali presso la cavità, t e n d e n d o a divenire casuali, «stornamente.. grani di quarzo in granodiorite in zone interessate da p u n t e di tensioni con pressione sopra 50-kb, contengono d a 2 a 5 serie di superfìcie parallele (« planar ») per grano. queste appaiono svilupparsi p e r a d a t t a m e n t i plastici in roccia i n t e n s a m e n t e scossa. correnti plastiche risultano q u a n d o viene s u p e r a t o un valore critico delle tensioni, al quale corrisponda u n s u p e r a m e n t o del limite elastico dinamico d u r a n t e il carico o del limite elastico statico d u r a n t e lo scarico. sicché alle pressioni radiali, in a t t e n u a z i o n e con la distanza, corrispondono e f f e t t i divergenti e n t r o piani normali alla direzione di p r o p a g a zione delle onde. q u a n d o la fermezza trasversa del mezzo è n e t t a m e n t e s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 5 3 s u p e r a t a , il materiale n o n può resistere alle tensioni differenziali e perciò t e n d e a fluire i d r o d i n a m i c a m e n t e (fig. 14). nella zona f r a t t u r a t a , la fermezza tensile del mezzo controlla il collasso. k b disfama dalpunìa di scoppio (mefrjj (evento di ilardhat) fig. 14 l a simmetria sferica di propagazione e formazione dei fenomeni è n a t u r a l m e n t e soltanto u n ' u t i l e semplificazione: in r e a l t à , l'esistenza di inomogeneità e discontinuità nel mezzo, altera notevolmente la su d e t t a s i m m e t r i a , di modo che, alla stessa distanza, su differenti direzioni, si possono osservare differenti effetti. anche l'orientazione dei piani cristallografici p u ò influire sulla m i c r o f r a t t u r a z i o n e nei grani, specie per pressioni sotto 50-kb, nonché le r i s u l t a n t i delle differenze di tensione fra grano e grano. alle pressioni p i ù elevate i singoli cristalli esercitano u n controllo a t t e n u a t o perchè t u t t i i grani rispondono in senso unitario ad una direzione d o m i n a n t e della tensione imposta all'aggregato i n t e r g r a n u l a r e . la fermezza tensile di ogni g r a n o viene allora così largamente superata, che il collasso può verificarsi pressoché a livello atomico, indipendentemente dalvorientazione. butkovicli (luogo citato) calcola il limite plastico esterno ad h a r d h a t sulla base di u n limite dinamico elastico (corrispondente a deformazione e s t r e m a ) di 40-kb, ad una distanza di ni. 19.5. sulla base eselusiva del 3 5 4 p . c a i . d i m . ( ' . s p a i ) e a p r i m o a p p a r i r e della serie di f r a t t u r e « p l a n a r » nel quarzo, considerato come sintomo per l'inizio di corrente plastica, la t e s t i m o n i a n z a p e t r o g r a f i a pone il limite plastico a p p r o s s i m a t i v a m e n t e a 111 .15.5 (60-kb). sulla base dell'accartocciamento (« kinking ») della biotite, t a l e limite sarebbe vicino a m 22.5 (30-kb). il limite elastico-inelastico, p e r un valore prelim i n a r e di o-kb, è a 66.5 metri. se si ritiene che la tensione di f r a t t u r a zione segni l'inizo della zona inelastica, la distanza alla quale la microf r a t t u r e cominciano a crescere sopra i valori di prescoppio è a 56.0 m e t r i . ciò concorda ragionevolmente con i in 66.5 o t t e n u t i da b u t k o v i e h . l ' e n t i t à delle variazioni nel grado di m i c r o f r a t t u r a è quindi tecnica a d e g u a t a per la stima della posizione di questi limiti, i n d i p e n d e n t e m e n t e da altri m e t o d i . l a zona delle correnti plastiche p u ò quindi esser d e l i m i t a t a frani 19 e m 65 ca. p e r q u a n t o concerne la velocità delle onde longitudinali, alla p r o f o n d i t à di 297.5 m e t r i — prima dello scoppio —• q u e s t a era di 5800 m/see e di 5910 m/see a 267.1 metri. l ' i n d i c e di f r a t t u r a a d e t t e p r o f o n d i t à — sempre prescoppio — era di 2.70 e 3.20 r i s p e t t i v a m e n t e . d o p o lo scoppio — p e r p r o f o n d i t à d a 4.5 a 18 m e t r i circa — la velocità scendeva d a 5800 m/sec a 4400 m/sec (kb-15; indice di f r a t t u r a 4.34) fino a 21 m di p r o f o n d i t à , e a 3950 m/sec p e r 31.0 m e t r i di p r o f o n d i t à (kb-32; indice di f r a t t u r a 4.42). alla p r o f o n d i t à di 32.6 m e t r i si era già in zona anelastica (kb-60; indice di f r a t t u r a 11.73); così a 34.0 m (kb-83; indice di f r a t t u r a 13.88). i n a l t r a direzione, a m 58.7 ( k b 4 3 ! ; indice di f r a t t u r a 7.50) la velocità era ancora 2900 m/sec, così p u r e a 61 m (kb-45?; indie,e di f r a t t u r a 7.88). f i g u r e notevoli sono le 10, 11, 12, 13, 14, riassunto delle indagini compiute da b u t k o v i e h e da s h o r t . u n a l t r o esperimento di esplosione nucleare s o t t e r r a n e a ha consentito, r e c e n t e m e n t e , l ' e f f e t t u a z i o n e di preziose indagini. si t r a t t a del « salmon seismic e x p e r i m e n t » realizzato il 22 o t t o b r e 1964 vicino a h o t t i e s b u r g (mississippi), nel p u n t o di coordinate 31°08'31",6 n ; 89°34'11",8 w . u n a carica nucleare di 5,3 ± 0,5 ivt f u f a t t a esplodere nel duomo di sale ( n ) t a t u m , ad u n a p r o f o n d i t à di 827,8 m . d a l p u n t o di v i s t a sismologico, l'esplosione f u registrata nel raggio di 2500 k m come u n telesismadi m a g n i t u d o 4,35. le onde longitudinali p m o s t r a r o n o a s i m m e t r i a nella propagazione, con maggiori ampiezze, maggiori velocità apparenti e più elevate frequenze (a conferma di q u a n t o era s t a t o o t t e n u t o d a caloi p e r le piccole distanze (*)) verso il nord e il nord-ovest che n o n s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 5 5 verso ovest. l a propagazione delle onde sismiche venne s t u d i a t a sotto diversi a s p e t t i (12), (13), (14), (15), (16). su di essa, n a t u r a l m e n t e , noi qui non ci i n t r a t t e r r e m o , sebbene ci sembri di notevole interesse particolarm e n t e lo studio di archanibeau, f l i n n e l a m b e r t (15), in cui, f r a l'altro, vengono calcolate le velocità delle onde longitudinali nell'interno della t e r r a , a t t e a giustificare i t e m p i osservati in corrispondenza del « salmon e v e n t ». l a fig. l o , che riassume i r i s u l t a t i dei calcoli, mostra chiaram e n t e la zona a flessione di velocità, al di sotto della crosta terrestre, t a n t o a lungo s t u d i a t a da gutenberg, e nella quale avviene la canalizzazione dell'energia sismica (canale astenosferico), come h a p r o v a t o • ™dft pa. sa (1953). errata corrige. — a pag. 355, riga 7", leggasi fig. 16 invece di 15. a pag. 357, riga leggasi fig. 17 invece di 16. a pag. 357, riga 8", leggasi fig. 15 invece di 17. i fig. 15 e v e n t o di salinoli: configurazione nei pressi del p u n t o di scoppio. s u c i . i e f f e t t i 1 ) e e i , e e s p l o s i o n i n e l l e r i n v i verso ovest. la, propagazione delle onde sismiche venne studiata sotto diversi aspetti ('-), (13), (l4), (15), (16). su di essa, naturalmente, noi qui non ci i n t r a t t e r r e m o , sebbene ci sembri di notevole interesse particolarmente lo studio di archambeau, plinn e l a m b e r t (15), in cui, fra l'altro, vengono calcolate le velocità delle onde longitudinali nell'interno dellaterra, a t t e a giustificare i tempi osservati in corrispondenza del « salinovi event ». la fig. l o , che riassume i risultati dei calcoli, mostra chiaramente la zona a flessione di velocità, al di sotto della crosta terrestre, t a n t o a lungo studiata da gutenberg, e nella quale avviene la canalizzazione dell'energia sismica (canale astenosferico), come ha provato caloi con la scoperta delle onde fa, sa (1953). ma per ciò che a noi qui più interessa, merita di essere citato, oltre al lavoro di vverth e raudolph ( u ) , anche quello di rawson, randolph, b o a r d m a n e wheeler (1g). da queste ricerche, risulta che nella zona dello scoppio si è f o r m a t a u n a cavita, di forma sferica di un volume di circa 20.000 m3, corrispondente ad una sfera di 17 metri di raggio. calcoli più accurati precisano per (presto raggio il valore di metri 17,1 ± 0,0. (si ritiene che il raggio ( f i g . 15 e v e n t o d i s a l i n o l i : c o n f i g u r a z i o n e n e i p r e s s i d e l p u n t o di s c o p p i o . 356 ,\i. c. si'adjca 11.0 velocità onde di cottipmjione fig. 16 e v e n t o di salinoli: variazione della velocità delle onde longitudinali con la p r o f o n d i t à . in n e t t a evidenza a p p a r e la zona a flessione di velocità (gutenberg), in cui avviene la canalizzazione dell'energia sismica (caloi) cavifd, postscoppio (m 17 raggio) frantumato e t cte.f>o.ccùato (m so n ) cbrente plastica ( m /so r) onde, sismiche fig. 17 e v e n t o di salmon: successione delle q u a t t r o principali fasi, associate all'esplosione. s u g l i e f f e t t i d e l l e e s p l o s i o n i n e i . l e r o c c e 3 5 7 a p p a r e n t e della c a v i t à n e l l ' a t t i m o dell'esplosione sia s t a t o di 24 m, c o n t r a t t o s i poi a 17 m ea). come p e r l ' e v e n t o di h a r d h a t , anche per questa esplosione si è osservato l'ordine a b i t u a l e di alterazione del mezzo: fusione, frantumazione e fratturazione, corrente plastica e, da ultimo, zona propriamente elastica (onde sismiche). tale successione è riassunta nella fig. 16. l'esplosione ha fuso circa 5 . 4 x io6 kg di roccia, p a r t e raccolta sul fondo in sale ricristallizzato (fig. 17). gran p a r t e dell'energia di esplosione f u t r a s f o r m a t a in calore t u t t o a t t o r n o al p u n t o di scoppio, per un raggio di 45 in. la roccia i n t o r n o alla c a v i t à f u f o r t e m e n t e m i c r o f r a t t u r a t a , p u r contenendo alcune m a c r o f r a t t u r e . la porzione maggiormente f r a t t u r a t a a t t o r n o alla c a v i t à f u osservata nella regione f r a 39 e 50 m sotto il p u n t o di scoppio. 1 volumi di roccia, interessati dalle successive alterazioni, f u r o n o n e t t a m e n t e superiori a quelli osservati nella esplosione di h a r d h a t . ciò è d o v u t o in piccola p a r t e alla carie,a maggiore usata nell'evento di salinoli, m a , s o p r a t u t t o , alla diversità del mezzo in cui si verificarono i due eventi: g r a n i t o p e r h a r d h a t , duomo di sale per salmon. i n questo secondo e v e n t o , p . es., il limite esterno della zona elasto-plastica è s t a t o più che doppio (150 in) di quello osservato per h a r d h a t (67 m). 6. d a q u a n t o precede, si possono t r a r r e le seguenti conclusioni, con particolare riferimento alla determinazione del modulo di elasticità di u n a roccia-, o di un sistema roccioso. p r e s c i n d i a m o dai m e t o d i di laboratorio (applicabili a campioni di roccia), i quali possono avere indubbio interesse scientifico nella ricerca delle caratteristiche dinamiche di u n d e t e r m i n a t o tipo di roccia, legate alla sua costituzione cristallina, ai suoi piani di sedimentazione, ecc., m a i risultati dei quali non possono e v i d e n t e m e n t e essere estrapolati agli a m m a s s i rocciosi « in sito ». p e r questi, è giuocoforza ricorrere ai m e t o d i dinamici. questi però possono essere usati con vantaggio solo p r o v o c a n d o nella roccia esplosioni di piccola intensità, e registrando a non immediata, distanza. sono i metodi b a s a t i sul rilevamento della velocità delle onde longitudinali e trasversali, da uno di noi i n t r o d o t t i in i t a l i a fin dal 1948: m e t o d i semplicissimi, per nulla dispendiosi, facilm e n t e ripetibili sugli stessi t r a g i t t i (17). v a n n o invece senz'altro a b b a n d o n a t i quei m e t o d i macchinosi che richiedono la creazione di gallerie o cunicoli, con l'uso f o r z a t o di enormi q u a n t i t à di esplosivo. e v a n n o a b b a n d o n a t i non t a n t o per il loro costo — i n c o m p a r a b i l m e n t e più elevato di quello che si a v v a l e del semplice rileva3 5 8 p . c a l o ! ni. c. i p a d k a m e n t o d e l l a v e l o c i t à d e l l e o n d e e l a s t i c h e — o p e r a l t r i g r a v i i n c o n v e n i e n t i g i à i n p r e c e d e n z a r i l e v a t i (18), m a s o p r a t t u t t o p e r la l o r o i n i d o n e i t à . i n t a l i m e t o d i , i n f a t t i , si s p e r i m e n t a s u l l a r o c c i a l a s c i a t a l i b e r a d a l l e e s p l o s i o n i , r o c c i a c h e — c o m e si è v i s t o — r i e n t r a i n p i e n o n e l l a r e g i o n e i n c u i l ' e l a s t i c i t à è s t a t a p r o f o n d a m e n t e a l t e r a t a , f i n o a l l i v e l l o m o l e c o l a r e . i l m o d u l o d i y o u n g d i t a l e r o c c i a r e s i d u a n o n p u ò m a n i f e s t a m e n t e e s s e r e e s t e s o a t u t t o l ' a m m a s s o c i r c o s t a n t e , d o v e il m o d u l o s t e s s o ris u l t e r à , i n g e n e r a l e , e n o r m e m e n t e p i ù e l e v a t o . b i b l i o g r a f i a (*) caloi p . , sulla dispersione delle onde sismiche dell'ambito delle altissime frequenze. « a n n a l i di geofisica a, x (1957). caloi p . , dispersione delle onde sismiche nell'ambito delle altissime frequenze. « r e n d . a c c a d . n'az. lincei », ci. sc. fts., m a t . e n a t . , serie v i i i , x x i v , 3 (1958). (2) belyaev a. f. e sadovskii m. a., explosion. e n c i c l o p e d i a s o v i e t i c a , 7, 626 (1951). (3) .morris g., geophysics, 15, (1950). ( 4 ) v a n e k j . , 'transitional zone in the classical region for explosions in solid materials. « czechosl. j o u r n . p l i v s . », 4, (1954). (5) kirillov f . a., effetti sismici di un'esplosione (in russo). t r u d y seism. i n s t . a n s s s r , 121, (1947). ( ' ) kasaiiaiìa k . , the nature of seismic origins us inferred from seismolog•ieal and geodetic observations (1). « b u l l . e a r t h q . r e s . i n s t . », univ e r s i t y of t o k y o . x x x v , 3, (1957). (7) kawasumi ii. a n d yosiyama r., on an elastic wave animated by the potential energy of initial strain. « bull. e a r t h q . res. i n s t . », u n i v e r s i t y of t o k y o , x i i i , 3, p. 497, (1935). (8) okavva t . , iiiraya.ma m . a n d kiyota k . , journ. industr. explos. soc., japan, 18 (1957) (in g i a p p o n e s e ) . ( 9 ) b u t k o v l c i i t h e o d o r e r . . oalewlati-o» of the shock wave from an underground nuclear explosion in granite. « j o u r n . geopliys. r e s . » 70, 4, (1965). (10) short nicholas m., effects oj shock pressures from a nuclear explosion on mechanical and optical properties of granodiorite. « j o u r n . g e o p l i y s . res. », 71, 4, (1966). ( u ) werth gl. a n d randolph ph. the salmon seismic experiment. « j o u r n . geopliys. r e s . » 71, 14, 3405-3413 (1966). ( 1 2 ) w a r r e n d a v i t h . , h e a l y j . h . a n d j a c k s o n w a y n e h . , grustal seismic measurements in southern mississippi. « j o u r n . geopliys. r e s . , » 71, 14, 3437-3458 (1966). •sugli e f f e t t i d e l l e e s p l o s i o n i -nelle rocce 359 springer donald l., calculation of first-zone p ware for salmon event and for decoupled sources. « ,lourn. geopliys. res. ». 71, 14, 34593467, (19««). jordan j a m e s x . , mickey w e n d e l l v . , hei.terbran w a y n e , a n d clark don m., travel times and amplitudes from the salmon explosion. « .tourn. geopliys. r e s . » , 71, 14, 34(39-3482, (19(56). archambeau <_'. b . , flinn l i . a . a n d lambert 1). u . , detection, analysis, and interpretation of teleseismic signals (1966). « j o u r n . geopliys. res. », 71, 14, 3483-3501, 1. gompressional phases from the salmon event. r a w s o n d . , r a n d o l p h i ' . , boardman c . , a n d w h e e l e r v . , postexplosion environnement resulting from the salmon event. « j o u r n . geopliys. r e s . » , 71, j4, 3507-3521, (1966). caloi p . , spadea m. 0 . , principali risultati conseguiti durante l'osservazione geodinamica, opportunamente estesa nel tempo, di grandi dighe di sbarramento, e loro giustificazioni teoriche. « a n n a l i di geofìsiea ». x i x , 3, (1966). semenza c., su un sistema di valutazione comparativa delle caratteristiche elastiche delle masse rocciose. •< a t t i 1st,. ven. sei. l e t t , ed ar. ». tomo c x i i i , 1954-1955, ci. scien. m a t . e n a t . articolo perea.pdf key words paleoseismicity – trenching – normal fault – catalan coastal ranges – el camp fault 1. introduction paleoseismologic methods developed in high seismicity areas for high slip rate faults are increasingly used for improving the calculations of seismic hazard on faults with very low slip rates (< 0.1 mm/yr). this is the case of some fault systems in europe, such as the rhine 763 annals of geophysics, vol. 46, n. 5, october 2003 paleoseismological data from a new trench across the el camp fault (catalan coastal ranges, ne iberian peninsula) hector perea (1), paula m. figueiredo (2) (3), jesús carner (1), stefano gambini (4), kirsty boydell (5) (1) departamento geodinàmica i geofísica, universitat barcelona, spain (2) lattex, laboratório de tectonofísica e tectónica experimental, faculdade de ciências da universidade de lisboa, portugal (3) icte, instituto de ciências da terra e do espaço, universidade de lisboa, portugal (4) school of earth sciences, the university of leeds, u.k. (5) department of civil and environmental engineering, london, u.k. and participants in the europaleos course paola albini, pedro alfaro, raquel amores, ramon arrowsmith, kuvvet atakan, milos bavec, kelvin berryman, tamer yigit duman, khalilallah feghhi, matthieu ferry, alessandro fontana, chrysa gountromichou, robert hus, juan miguel insua, ramon julià, fidel martín, eulàlia masana, mustapha meghraoui, vasso mouslopoulou, bruno pace, niko palyvos, daniela pantosti, gwendolyn peters, stefano pucci, alexander radulov, josé sánchez-cabañero, pere santanach, gordon seitz, yonathan shaked, dimitra tsoukala and gulsen ucarkus abstract the el camp fault (catalan coastal ranges, ne iberian peninsula) is a slow slipping normal fault whose seismic potential has only recently been recognised. new geomorphic and trench investigations were carried out during a training course across the el camp fault at the la porquerola alluvial fan site. a new trench (trench 8) was dug close to a trench made previously at this site (trench 4). with the aid of two long topographic profiles across the fault scarp we obtained a vertical slip rate ranging between 0.05 and 0.08 mm/yr. at the trench site, two main faults, which can be correlated between trenches 8 and 4, make up the fault zone. using trench analysis three paleoseismic events were identified, two between 34.000 and 125.000 years bp (events 3 and 2) and another event younger than 13 500 years bp (event 1), which can be correlated, respectively, with events x (50.000125.000 years bp), y (35.000-50.000 years bp) and z (3000-25.000 years bp). the last seismic event at the la porquerola alluvial fan site is described for the first time, but with some uncertainties. mailing address: dr. hector perea, universitat barcelona, departamento geodinàmica i geofísica, zona universitària de pedralbes, barcelona 08028, spain; e-mail: hector@geo.ub.es graben, the catalan coastal ranges and the betic cordillera. in fact, the recognition of the seismogenic characteristics of a fault, i.e. the evaluation of its slip rate, the size of the maximum expected earthquake and the age of the most recent surface faulting earthquake can substantially change the perception of seismic hazard in regions traditionally considered to be stable or not very active. the «europaleos field training course in paleoseismology» focused on the el camp fault (catalan coastal ranges, ne iberian peninsula) and was held in cambrils (spain) in february 2001. the aim of this course was to provide training for young researchers in paleoseismology. the course centred on the el camp fault for the following reasons: a) earlier studies have demonstrated its potential for paleoseismic investigation; b) some uncertainties concerning the fault geometry and its seismic behaviour remained to be resolved; c) it is a good example of a low slip rate fault that could be encountered in other parts of europe and around the mediterranean, and d) the area is characterised by high seismic vulnerability. this paper seeks to gain an insight into the slip rate and paleoseimic history of the fault, account for change in the scarp direction visible at the trench site and to present the results obtained during the europaleos course. 2. the el camp fault the el camp fault is located on the southeastern flank of the catalan coastal ranges (fig. 1) which are bounded by the valència trough basin on the west. the catalan coastal ranges are characterised by an en échelon array of ne-sw faults, where the main faults are listric, dip to the se and have the detachment level at a depth of 15 km (roca and guimerà, 1992; roca, 1996). the catalan coastal ranges are the result of an e-w extension, which affected the eastern part of the iberian peninsula during the neogene (mauffret et al., 1973; fontboté et al., 1990; banda and santanach, 1992; roca and guimerà, 1992; roca, 1996). this extension has been interpreted as a product of an extensional back-arc basin related to the apennine subduction (doglioni et al., 1997, 1999; gueguen et al., 1998). earlier studies on the catalan coastal ranges suggest that these ranges have been in a post-rift stage with weak tectonic activity since the middle miocene (roca and guimerà, 1992; roca, 1996). the el camp fault is made up of two en échelon faults (north and south segments). from commercial and deep seismic profiles, it has been shown that the el camp fault is a normal fault, with a dip of 60° and the main detachment level located at 13-15 km depth (roca, 1992; roca and guimerà, 1992; sàbat et al., 1997). the el camp fault constitutes the north-western limit of the el camp basin. near the town of reus, the detrital sedimentary infill of the el camp basin has a thickness of 1400-2000 m, which extends from the miocene to the present time (nuñez et al., 1980; medialdea et al., 1986). the el camp fault has not shown significant historical or instrumental seismic activity. however, recent paleoseismologic studies based on detailed geomorphological and trench analyses have shown that the el camp fault is active, particularly its southern segment. this 764 hector perea, paula m. figueiredo, jesús carner, stefano gambini, kirsty boydell and participants in the europaleos course me dite rran ean sea el camp fault studied area neogene basins faults under neogene extension pyrenees cat alan coa stal ran ges 1° e 2° e 3° e 42°n 41° n barcelona tarragona va lè nc ia tro ug h fig. 1. location maps: a) location of the catalan coastal ranges within the iberian peninsula. the map shows the neogene basins and the faults with neogene extension; b) location of the el camp fault within the catalan coastal ranges. the map shows the en échelon array of nw-se neogene listric faults in the catalan coastal ranges. ba segment is approximately 24 km long, bearing in mind that the fault extends under the sea for a distance of 10 km. the segment is characterised by: a) a slip rate ranging between 0.02 and 0.08 mm/yr; b) surface faulting earthquakes of mw = 6.7 maximum magnitude; c) an average recurrence interval of 30.000 years, and d) the occurrence of the last event 3000 years ago (masana, 1995, 1996; masana et al., 2000, 2001a,b; santanach et al., 2001). seven trenches located at different sites in the southern segment of the el camp fault enabled these authors to describe three seismic events during the last 125.000 years: event x, event y and event z. event x, between 125.000 and 50.000 years bp, has been clearly described at two sites (site of trenches 1 and 2 and site of trench 4). event y, between 50.000 and 35.000 years bp, has only been described at the site of trench 4. event z, between 25.000 and 3000 years bp, has been described at two sites (at trenches 1 and 2 strongly based and at trench 3). 3. geomorphologic and topographic survey the geological and geomorphological study of the southern part of the el camp basin (fig. 2) reveals the presence of recent alluvial fans of four generations: g1, g2, g3 and g4 (villamarín et al., 1999; santanach et al., 2001), which were mapped and surveyed in the field during europaleos. fans belonging to the g2 and g3 generations stretch from the mountain range to the sea, whereas those corresponding to the g4 generation are located at the foot of the mountain range or in the lowlands between the fans of the older generations. according to different dating methods (u/th, thermoluminescence, paleomagnetic studies and correlation between different fan generations and sea level highstands), minimum ages of 300.000 and 125.000 years have been attributed to the top of the g2 and g3 generations, respectively (villamarín et al., 1999; santanach et al., 2001), the g4 generation being younger than 125.000 years in age. the oldest fans belonging to the g1 generation were not considered in this study since they do not interact with the fault where it is exposed. the geomorphological map (fig. 2) shows a discontinuous fault scarp offsetting alluvial fans belonging to the g2, g3 and g4 generations. the el camp fault scarp offsets the g4 generation fans between the la porquerola and the les planes fans and south of the les planes fan. these observations indicate that this segment of the fault has been active during the last 125.000 years. for logistical reasons, we focused on one portion of the la porquerola fan, where the fault affects the top of the g3 generation fan. this fan is highly cemented and therefore difficult to erode in contrast to the unconsolidated materials in the lower part of the scarp. consequently, the scarp is clearly visible at the surface and a change in the direction is recognisable. the scarp is the result of a normal fault-propagation fold with a small scarplet at the bottom. locally, a small wall constructed along the foot of the scarp operates as a sediment trap and has produced some sharp morphologies that are visible on the trench walls. to the ne of trench 4, the scarp shows a change in direction close to a gully that is entrenched in the scarp. using a total station (leica 1700), we made a microtopographic map of the selected area (fig. 3) and five topographic profiles. this information together with the geomorphology helped us to select the most favourable site for trenching. we decided to dig the trench in the zone where the fault scarp changes its direction (fig. 3) in an attempt to determine whether the change in the scarp direction was due to a change in the fault direction or to erosion evidenced by the small gully located there. in addition, the location of the new trench in the vicinity of the gully would enable us to obtain a more complete section of recent sediments on the downthrown wall than at trench 4. thus, the possibility of detecting the most recent paleoseismic event, which had not been observed at this site, is greater. only two topographic profiles (fig. 4) were long enough to obtain a preliminary evaluation of the slip rate at this location. the offset of the 125.000 years fan surface measured in the profiles is between 6.7 and 10.5 m. this provides a maximum vertical slip rate between 0.05 and 0.08 mm/yr. although these results are consistent with the slip rates obtained in earlier studies on the el camp fault, which vary from 0.02 to 0.08 mm/yr (masana, 1995, 1996; masana 765 paleoseismological data from a new trench across the el camp fault (catalan coastal ranges, ne iberian peninsula) hector perea, paula m. figueiredo, jesús carner, stefano gambini, kirsty boydell and participants in the europaleos course fig. 2. the el camp basin geomorphological map (modified from masana et al., 2001b). location is shown in fig. 1. the map shows the distribution of the different alluvial fan generations as well as the zones where the fault scarp intersects the different alluvial fans. montroig les planes casalot l’ hospitalet de l’infant almadrava rifà creek ermo t miam i p orquerola c reek montroig alluvial fan porquerola alluvial fan les planes alluvial fan l la stre s c re e k trench site tr6 tr5 tr8 tr4 tr1 tr2 tr3 tr7 0°55 e 1°00 e 41°05 n 41°00 n fluvial channel coastal scarp triangular facet fault scarp trench site alluvial fan generation 4 (g4) alluvial fan generation 3 (g3) alluvial fan generation 2 (g4) alluvial fan generation 1 (g1) substratum 0 5 km tr7 766 et al., 2000, 2001a,b; santanach et al., 2001), the 125.000 years fan surface offset obtained by these authors in the same zone was 6.5 m, which is more similar to the lower values obtained from our profiles. therefore, the lower slip rate would be more realistic. on the assumption that the fault is purely normal with a dip of 60° at depth, a dip slip of 0.06 to 0.09 mm/yr is calculated. 4. trench analysis the trench dug through the scarp located at the la porquerola fan (fig. 5) was the eighth trench dug along the el camp fault (masana et al., 2000, 2001a,b; santanach et al., 2001). the trench had a wnw-ese orientation, a length of approximately 23 m and a depth of approximately 6 m. the trench 8 logs (fig. 6) show a succession of sedimentary layers, conglomerates and gravels with different degrees of calcrete development. in the fault zone, the sediments are strongly weathered because of fluid circulation through the faults, cracks and paleoseismological data from a new trench across the el camp fault (catalan coastal ranges, ne iberian peninsula) 900 950 1000 1050 1100 850 900 950 1000 1050 1100 a a b b trench 8 profile 1 profile 3 0 50 100 easting (m) n o rt h in g (m ) i i i i i i i i i i i i i i i i i i i i i i i i i i i i trench 4 -20 0 20 40 60 80 100 120 1 40 160 180 200 distance along profile (m) 88 92 96 100 104 r e la tiv e e le va tio n (m ) -20 0 20 40 60 80 100 120 1 40 160 180 200 distance along profile (m) 88 92 96 100 104 r e la tiv e e le va tio n (m ) 9.1 m 7.7 m a ' 10.5 m 6.7 m b trench 8 a 'b profile 1 profile 3 fig. 3. microtopographic map of the trench site. the map shows the location of trenches 8 and 4 and the long profiles 1 and 3. the fault trace is also shown. the contour line interval is 0.5 m. fig. 4. topographic profiles 1 and 3. a maximum and a minimum offset are plotted for each profile, taking into account that the original fan slope surface could correspond to the surface on the downthrown (dotted line) or upthrown block (dashed line). the position of trench 8 is projected on profile 1. the location of the two profiles is shown in fig. 3. the vertical scale is exaggerated. 767 ture f2 has a subhorizontal dip and affects unit f. f2 is an open fracture produced by a dip slip along a normal fault (f1) and by the obstruction of the shallower part of the downthrown block of this fault (santanach et al., 2002). this kind of movement could have produced the opening of the hollow visible at the trench along f2. the subsequent collapse of the obstructed part could have closed the hollow and could have produced a «reverse» fault geometry (fig. 8). two minor faults are also present, f3 and f4, the former being more clearly defined than the latter. fault f3, a small antithetic fault, affects the hard layer located at the top of unit k with a maximum offset of 20 cm. f4 is defined by the 60 cm offset at the top of unit k, despite being badly constrained at lower levels. unit j shows a mixture of materials, and clasts with a vertical preferred orientation, which could be the result of the f4 movement. one question we attempted to answer with the opening of the trench was whether the change in the scarp direction is due to: a) a shift in the direction of the fault, or b) to the erosion of the scarp. the analysis of the relationship between f1, the main fault, and the surface scarp, at trench 8, shows that the change in the scarp direction is attributed to the shift in the fault direction since the scarp follows the fault direction (fig. 6). however, erosion controls the trench sedimentary architecture and could highlight the change in the scarp direction. eight samples were collected for radiocarbon dating. only three of them contained sufficient material to be dated and only two (ert8-2 and ert8-6) provided a measured age (table i). samples ert8-2 (charcoal) and ert8-6 (a shell) were collected from unit l1, ert8-2 near the top of the unit and ert8-6 near the limit between l1 and l. these datings indicate that unit l1 was deposited between 34.000 and 13.000 years bp. the other available age constraint is the age of the top of the g3 generation (villamarín et al., 1999). therefore, the top of unit f is ca. 125.000 years old and all the units exposed on the upthrown block are the same age or older with the exception of m, which is the most recent unit. 768 hector perea, paula m. figueiredo, jesús carner, stefano gambini, kirsty boydell and participants in the europaleos course fault fault fig. 5. trench 8 photograph taken from the southeast. the people in the photograph are on the upthrown block of the fault. hollows, which hinders the distinction between the different layers (fig. 7). on the upthrown block, units b, c, d and f are planar and sub-parallel to each other. only unit f, which corresponds to the top of the g3 generation alluvial fan, can be followed across the fault zone. on the downthrown block, seven units (g, h, j, k, l, l1 and m) overlie unit f near the scarp, but f appears at the surface again at a distance of approximately 40 to 50 m from it. thus, units g, h, j, k, l, l1 and m infill a local depression produced by the fault movement. the geometry of the sedimentary units on the downthrown block is more complex than on the upthrown block. units h to m cover part of the eroded scarp produced by the faulting and bending of unit f. the geometrical relationships between these lithological units will be discussed below in terms of paleoseismic events. the fault zone is composed of a fault (f1) and a fracture (f2). fault f1 is located along unit w2 and offsets units b, c, d and f. the fault plane has a subvertical attitude or dips strongly to the wnw (fig. 6). the offset top of unit f along f1 on the south wall shows a buried scarp that is 1.3 m high. however, this is the minimum offset since the scarp had probably been eroded. to the east of f1, fracpaleoseismological data from a new trench across the el camp fault (catalan coastal ranges, ne iberian peninsula) fig. 6. logs of the trench 8 walls. unit b unconsolidated, well sorted sitly-clay, yellow-brown in colour, includingsome centimetre size carbonaticclasts. unit c strongly cemented, poorly sorted, matrix-supported conglomerate. clasts are carbonatic, angular to subangular and range from a few centimeters to a few decimeters in size. matrix is silty and reddish in colour. unit d poorly consolidated, well sorted, very fine sandstone reddish in colour, including some centimeter-size carbonatic clasts. unit f strongly cemented, poorly sorted, matrix-supported conglomerate. clasts are carbonatic, subangular to subrounded and range from a few centimeters to a few decimeters in size. matrix is silty and reddish in colour. unit w1 poorly cemented and sorted, matrix-supported conglomerate. clasts are carbonatic, subangular to subrounded, and range from a few centimeters to a few decimeters in size. matrix is silty and is light-yellow in colour. unit w2 poorly cemented and sorted, matrix-supported conglomerate. clasts are carbonatic, angular to subangular, and centimetric in size. matrix is silty and is pink in colour. unit g moderately consolidated, poorly sorted, matrix-supported gravel. clasts are carbonatic, subangular to subrounded and centimetric in size. matrix is silty and light brown in colour. unit h poorly consolidated and sorted, matrix-supported gravel. clasts are carbonatic, subangular to subrounded and centimetric in size. matrix is silty-clay and brown-orange in colour. unit j poorly consolidated and sorted, matrix-supported gravel. clasts are carbonatic, subangular to subrounded and centimetric in size. matrix is silty and brown-orange in colour. unit k poorly consolidated and sorted, matrix supported gravel. clasts are carbonatic, subangular and range from a few centimeters to a few decimeters in size. matrix is silty and brown in colour. at top there is a hard layer. unit l unconsolidated, poorly sorted, matrix-supported gravel. clasts are carbonatic, subrounded and range from a few centimeters to a few decimeters. matrix is silty and reddish-orangein colour. unit l1 unconsolidated, well sorted, fine sandstones reddish-orange in colour, including some centimeter-size carbonate clasts. unit m unconsolidated, brown silt, including few centimeter-size (max. 2 cm) carbonate clasts. at the top there is 10 cm width actual soil. fault possible fault and fault direction hollow 769 5. paleoseismic events at trench 8 we found evidence of at least one individual surface-faulting earthquake. furthermore, evidence for two more events will be discussed. briefly, we found evidence for three possible events (fig. 6): a) event 1 at the top of l1 (uncertain); b) event 2 at the top of k (good), and c) event 3 at the top of h (uncertain). we shall discuss the events beginning with the most certain event and ending with the most uncertain one. it was not possible to obtain the single event displacement at trench 8. displacements along f3 (and f4?), which reach the top of unit k, and the greater warp of this unit with respect to the overlying units suggest an event horizon, event 2, at the top of unit k. units l and l1 represent the post event deposit. evidence for event 1, albeit with some uncertainty, is provided by: a) the erosional truncation of unit m over l and l1, and b) the geometrical relations of unit m with respect to units l and l1. units l and l1 filling a gentle depression appear to join the original slope, which is not consistent with the present one. moreover, the origin of unit m differs from that of l and l1 in that it is much higher on the scarp. this could be explained by a change in the slope/scarp caused by a displacement along the fault (an earthquake?) and a consequent sinking of the downthrown block. event 3 could be located at the bottom of unit k based on: a) unit j, a deformed unit, discussed above, is covered by unit k that shows less deformation, and b) unit k, which lies unconhector perea, paula m. figueiredo, jesús carner, stefano gambini, kirsty boydell and participants in the europaleos course fig. 7. photographic assembly showing the fault zone on the south wall of trench 8. the photograph shows the complexity of the fault zone as well as the weathering degree of the units located close to the fault. the main features and sedimentary units are shown (see the legend in fig. 6). ese wnw 1 m1 m 1 m 1 m 6 5 4 3 2 1 789101112131415 3 4 5 6 m f d c b m l1 l k hollows f2 f2 f1 f1 1 m dip slip: 0.5 m «reverse» offset 0.17 m there is not movement along this part of the fault blockade blockblock collapse fig. 8. this figure shows how a tensional fracture with an associated hollow and a post-collapse reverse geometry on a strongly vertical normal fault is produced (explained in the text). semicircles identify homologue points and arrows indicate the movement of the block. 770 formably over unit f, sealing the subhorizontal fracture and associated collapse breccia. given the low number of available datings, it has not been possible to determine with precision the time bracket for the three paleoseismic events. with our data, the three possible events would be younger than 125.000 years bp. the oldest events (2 and 3) are older than 34.000 years bp, whereas the youngest one (event 1) would be younger than 13.500 years bp. 6. comparison with earlier studies by comparing the data obtained from this new trench (trench 8) across the el camp fault with the paleosesimological information obtained from earlier studies on the same fault (masana, 1995, 1996; masana et al., 2000, 2001a,b; santanach et al., 2001), we were able to gain a new insight into the fault behaviour. the comparison between trenches 8 and 4 (fig. 9) shows that f1 and f2 have the same dip tendency and relative location with respect to the fault scarp at both trenches and could, therefore, be interpreted as the same faults. accordingly, at trench 8 it is assumed that f2 becomes a vertical fault at a certain depth and branches into f1. the accommodation space at trenches 4 and 8 due to the fault movement should be very similar because of their proximity, resulting in a similar sedimentary thickness. nevertheless, the sedimentary thickness at trench 8 is greater than at trench 4. moreover, the top of unit f is approximately 2 m deeper at trench 8 than at trench 4 (fig. 10). this modification of the accommodation space can be attributed to a marked erosion in the vicinity of trench 8 evidenced by the small gully located to the north of it. the modification of the accommodation space should be pointed out to avoid interpreting the sedimentary relationship between units f, g and h as another event. three seismic events have been described in relation to the el camp fault in the last 125.000 years (masana, 1995, 1996; masana et al., 2000, 2001a,b; santanach et al., 2001): a) event x has a time bracket between 125.000 and 50.000 years bp and is the oldest; b) event y is between 50.000 and 35.000 years bp, and c) event z occurred between 25.000 and 3000 years bp. at trench 8 we also found 3 events, although some uncertainty exists for events 1 and 3. as stated above, the two oldest events at trench 8 (3 and 2) occurred in a time bracket between 125.000 and 34.000 years bp, whereas event 1 is younger than 13.500 years bp. therefore, events 3 and 2 could correspond to events x and y, respectively, and event 1 could correspond to event z. thus, trench 8 could yield evidence of the last event, which has not been recorded at trench 4. although it was not possible to obtain the single displacements for events x, y and z at trench 8, they have been reported in earlier works (masana et al., 2000, 2001a,b; santanach et al., 2001): a) a displacement ranging between 1.4 and 2.0 for event x, but the authors suggest that this displacement could correspond to the accupaleoseismological data from a new trench across the el camp fault (catalan coastal ranges, ne iberian peninsula) table i. trench 8 dating results. sample 13c fraction ± d14c ± 14c age ± calibrated ± name modern age (*) ert8-2 .05 mgc – 25 0.2483 0.0076 – 751.7 7.6 11 190 250 13153 bp 657 ert8-6 shell 0 0.0143 0.0005 – 985.7 0.5 34 130 280 ert8-8 – 25 1.2301 0.0078 230.1 7.8 > modern (*) stuiver and reimer (1993). 13c values are the assumed values according to stuiver and polach (1977) when given without decimal places. values measured for the material itself are given with a single decimal place. the quoted age is in radiocarbon years using the libby half life of 5568 years and following the conventions of stuiver and polach (1977). radiocarbon concentration is given as fraction modern, d14c, and conventional radiocarbon age. sample preparation backgrounds have been subtracted, based on measurements of samples of 14c-free coal for ert8-2 and ert8-8, and of 14c-free calcite for ert8-6. backgrounds were scaled relative to sample size. comments: the material dated was acid-alkali-acid treated charcoal. the large uncertainty for ert8-2 is due to the small sample size. 771 hector perea, paula m. figueiredo, jesús carner, stefano gambini, kirsty boydell and participants in the europaleos course 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 0 1 2 3 4 5 6 7 8 9 wnw ese m m f surface on trench 4 northern wall f surface on trench 8 southern wall f surface on trench 8 northern wall fault zone ? ? eroded eroded fig. 9. logs of trench 4 walls (modified from masana et al., 2001b). strongly cemented, poorly selected, matrix-supported conglomerate. clasts are carbonatic, subangular to subrounded, and range from a few centimeters to a few decimeters in size. locally weathered at the top and along the joints. laminar caliche at the top. unconsolidated, well selected silt and clay light brown orange in color, including some decimeter-size, carbonatic clasts. highly cemented, matrix-supported conglomerate. clasts are carbonatic, little selected, angular, and range from a few centimeters to a few decimeters in size. matrix range from clay to silty and is pinky in color. laminar caliche at the top. the north wall shows a clay-bed interlayered in the conglomerate. it is built up by light brown clay containing subangular clasts up to a few centimeters in size. unconsolidated clay, dark brown in color, containing some clasts ranging from 2 to 10 cm in size. the upper part of the unit corresponds to the current soil. unconsolidated, matrix-supported gravel. clasts are very scarce. the matrix range from clay to silt. slightly consolidated, matrix-supported gravel. clasts are carbonatic, subangular, and moderately selected. the matrix range from clay to silt and is light brown in color. moderately consolidated, heterometric, matrix-supported conglomerate. clasts are derived from units f and c. their size reaches up to 40 cm, and are strongly angular. the matrix range from clays to silts and is light brown in color. loose matrix-supported gravel with silty and sandy matrix. fig. 10. the sketch shows the differences in height between the top of unit f on the northern wall of trench 4 and the southern and northern walls of trench 8. the different f surfaces are placed according to the f1 location and taking into account that the height of unit f on the upthrown block is similar (see fig. 3). 772 mulation of two events; b) 0.4 m of displacement for event y, and c) a displacement ranging between 0.7 and 1.0 m for event z. these displacements could correspond to a maximum earthquake with an mw ranging between 6.3 to 6.8 and, consequently, a rupture surface length between 13 and 24 km (wells and coppersmith, 1994), which is in agreement with the length of the el camp fault. 7. conclusions two long topographic profiles made across the el camp fault at the la porquerola alluvial fan allowed us to establish an offset bracket for the top of the g3 generation fan. this offset bracket ranges between 6.7 and 10.5 m. based on these data, the vertical slip rate for the last 125.000 years ranges between 0.05 and 0.08 mm/yr and the dip slip between 0.06 and 0.09 mm/yr. we consider the lower values to be more realistic when these are compared with the earlier studies. the change in the scarp direction visible at the la porquerola site is attributed to the change in the fault direction, although the erosion evidenced by the adjacent gully influenced the architecture of trench 8. the stratigraphic and structural analyses of the different units at trench 8 constrain one clear paleoseismic event and two less certain ones. the datings obtained give an age for the two oldest paleoseismic events (3 and 2) ranging from 125.000 to 34.000 years bp and an age younger than 13.500 years bp for the last one (event 1). events 1, 2 and 3 at trench 8 can be correlated with events z (3000-25.000 years bp), y (35.000-50.000 years bp) and x (50.000100.000 years bp) described in earlier studies. event z had not been described at the la porquerola site to date. acknowledgements the data presented in this report are the result of the europaleos course (hpcf-ct2000-00077). we wish to thank all those who attended the europaleos course and especially daniela pantosti, pere santanach and eulalia masana for organising the course. the reviews by a. meigs and p. villamor greatly helped to improve the manuscript. references banda, e. and p. santanach (1992): the valencia trough (western mediterranean): an overview, tectonophysics, 208, 183-202. doglioni, c., e. gueguen, f. sàbat and m. fernández (1997): western mediterranean extensional 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(1995): l’activitat neotectònica a les cadenes costaneres catalanes, doctoral thesis, universitat de barcelona, pp. 444. masana, e. 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(1996): la evolución geodinámica de la cuenca catalano-balear y áreas adyacentes desde el mesozoico hasta la actualidad, acta geol. hipánica, 29 (1), 3-25. roca, e. and j. guimerà (1992): the neogene structure of the eastern iberian margin: structural constrains on the crustal evolution of the valència trough (western mediterranean), tectonophysics, 203, 203-218. sàbat, f., e. roca, j.a. muñoz, j. vergés, p. santanach, m. sans, e. masana, a. estévez and c. santisteban (1997): role of extension and compression in the evolution of the eastern margin of the iberia: the escivalència trough seismic profile, rev. soc. geol. españa, 8 (4), 431-448. santanach, p., e. masana, j.a. villamarín, j. sanchez cabañero and j. plaza (2001): proyecto datación, publicaciones csn, pp. 159. santanach, p., e. masana and h. perea (2002): structure of the montroig scarp (el camp fault, ne iberian peninsula): paleoseismological implications, in xxviii general assembly of the european seismological commission (esc), 1-6 september 2002, genoa, italy, book of abstracts (university of genoa), pp. 204. stuiver, m. and h. polach (1977): discussion: reporting of c-14 data, radiocarbon, 19 (3), 355-363. stuiver, m. and p.j. reimer (1993): extended (super 14) c data base and revised calib 3.0 (super 14) c age calibration program, radiocarbon, 35 (1), 215-230. villamarín, j.a., e. masana, t. calderón, r. julià and p. santanach (1999): abanicos aluviales cuaternarios del baix camp (provincia de tarragona): resultados de dataciones radiométricas, geogaceta, 25, 211-214. wells, d.l. and k.j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, bull. seismol. soc. am., 84, 1-24. 774 hector perea, paula m. figueiredo, jesús carner, stefano gambini, kirsty boydell and participants in the europaleos course d i e r h e o l o g i s c h e n e i g e n s c h a f t e n der e r d k r u s t e vorn s t a n d p u n k t der n e u e n e r k e n n t n i s s e iiber d e n h e r d v o r g a n g bei g r o s s e n e r d b e b e n (*) (: ) l. h i e r s e m a n n 1. e i n l e i t u n g und d e f i n i t i o n der r h e o l o g i e . die geophysik, also aucli die seismologie, ist niclit selbstzweck, sondern h a t das ziel, geologische gedankengànge, avo immer es moglich ist, m i t einem exakten physikalischen i n h a l t zu erfallen. die neuen erkenntnisse iiber den herdvorgang bei erdbeben liefern ein kriterium, an dem m a n die richtigkeit geologischer yorstellungen iiber das rheologische yerbalten der erdkruste kontrollieren kann. der n a m e rheologie h a t sieh seit dem jahre 1928 fiir den zweig der physik eingebiirgert, der sieh m i t dem verhalten der materie u n t e r dem einfluss von formveràndernden k r a f t e n beschàftigt. leider werden auch in der geoplivsik die rheologischen begrifje wie z. b. festigkeit, fliessen, krièchen spròdes oder plastisehes verhalten nicht immer einheitlich und oftmals u n e x a k t gebraueht. es sei daher gestattet, an h a n d einiger allgemeiner bemerkungen iiber arbeitsweise und arbeitsgebiet der rheologie die mòglichkeiten zur rheologischen deutung seismischer daten aufzuzeigen. (*) z u s a i n m e n f a s s e n d e r v o r t r a g a u f d e r t a g u n g d e r c o m m i s s i o n s e i s m o l o g i q u e e u r o p e e ne v o m 8 . 1 2 . 4. 1 9 5 8 i n u t r e c h t i i b e r d i e s c h o n i n i f r e i b e r g e r f o r s c l i u n g s h e f t c 24 (1956) u n d i n d e r g e o l o g i s c h e n r u n d s c h a u 4 6 , (1957) v e r ò f f e n t l i c l i t e n a r b e i t . (**) v e r ò f f e n t l i c h u n g n r . 56 d e s i n s t i t u t s fiir a n g e w a n d t e g e o p h y s i k d e r b e r g a k a d e m i e f r e i b e r g , d i r e k t o r : p r o f . d r . 0 . meisseb. 2 . d i e rheologische d e u t u n g seismologischer d a t e n . 2.1 allgemeines. fiir die d e u t u n g der rheologischen eigenschaften der e r d k r u s t e kònnen die bek a n n t e n seismischen messdaten wie einsatzzeiten, a m p h t u d e n und amph'tudenrichtungen sowie die aus ihnen abgeleiteten grossen herangezogen werden. sie bilden das mehr oder weniger gesicherte tatsachenmaterial, das die sonst in der rheologie iiblichen messgrossen ersetzen muss, weil laborversuche, wie sie bei der materialpriifung wirtschaftlich wichtiger baustoffe moglich sind, wegen des meist u n b e k a n n t e n modellfaktors nicht in frage kommen. ausserdem gilt es zu beachten, dass die erdkruste ein historisch gewordenes gefiige darstellt, in dem friihere beanspruchungen irreversible ànderungen der inneren s t r u k t u r verursachten, wodurch eine wiederholung des vorganges mit dem gleichen ausgangspunkt unmòglich gemacht wird. schliesshch spielen sieh die deformationen des krustenmaterials unter bedingungen ab, die im labor nur selir schwer und meist gar nicht reproduzierbar sind. diese bedingungen sind nicht nur die temperatur und der hydrostatische drude sondern voi' allem auch die dauer der krajteinwirkung, die den menschlichen e r f a h rungsbereich und damit natiirlich auch den seismischen beobachtungsschatz weit iiberschreitet. die auf grund seismischer daten gewonnenen erkenntnisse liefein aus den erwalinten griinden mehr oder weniger n u r ein augenblicksbild, dem keine absolute giil114 l. hiersemann tigkeit fiir die vergangenheit u n d die zuk u n f t z u k o m m t . e s ist jedoch eine auch in geologischen kreisen weit v e r b r e i t e t e auffassung, dass e r d b e b e n als f o r t s e t z u n g schon lange in gang befindlicher tektonischer vorgànge anzusehen sind, so dass sie in der ursàchlich v e r b u n d e n e n k e t t e einen ersten a n h a l t s p u n k t a u c h fiir die deut u n g v o n e r d k r u s t e n d e f o r m a t i o n e n f r u h e r e r e p o c h e n hefern. 2.2 strukturmodelle. z u m s t u d i u m u n d z u r beschreibung des rheologischen y e r h a l t e n s v o n baustoffen, wie s t a h l u n d b e t o n b e n u t z t m a n als erste annàlierung an die w i r k h c h k e i t sogenannte wellen zeigt. sie k a n n also in erster nàlierung d u r c h das g r u n d e l e m e n t der elastischen f e d e r beschrieben werden. die makroskopischen b e o b a c h t u n g e n liefern zusatzlich den beweis, dass e r d b e b e n als folge tektonischer bewegungen m i t permanenten deformationen a u f t r e t e n . diese e i g e n s c h a f t wird zweckmàssigerweise durch das rheologisclie g r u n d e l e m e n t eines reibend gegen seine u n t e r l a g e verschiebbaren gewichtes dargestellt. die bessere annàherung a n die wirklichkeit ist also eine kombination der feder mit dem geivicht zu einem s t r u k t u r m o d e l l , das in d e r bheologie fiir das plastische fliessen b e k a n n t ist. bei einer k r a f t , die kleiner ist als die festigkeit der k r u s t e , i m modell also kleiner als die bei" w q— '/sm a b c a b b . 1 a) v o l l k o m m e n e l a s t i s c h e f e d e r . — b) p e r f o r i e r t e r k o l b e n , d e r sich i n e i n e m m i t e i n e r v i s k o s e n f l i i s s i g k e i t g e f i i l l t e n z y l i n d e r b e w e g t . — c) g e w i c h t , d a s r e i b e n d g e g e n s e i n e u n t e r l a g e v e r s c l i o b e n w e r d e n k a n n . s t r u k t u r m o d e l l e . sie beschreiben das rheologische y e r h a l t e n des materials m i t hilfe v o n 3 rheologischen grundelementen, die entweder h i n t e r e i n a n d e r oder parallel zu strukturmodellen k o m b i n i e r t werden k ò n n e n . die bezeichnung « s t r u k t u r m o d e l l » darf n i c h t zu d e r irrigen yorstellung f ù h r e n , dass seine g r u n d e l e m e n t e reelle gefiigebestandteile darstellen. die s t r u k t u r m o d e l l e beschreiben als ganzes den i n t e g r a l e f f e k t des rheologischen yerhaltens, ohne dass ihre e l e m e n t e reelle analoga besitzen miissen. e s erscheint auch fiir das s t u d i u m der rheologischen e i g e n s c h a f t e n der e r d k r u s t e zweckmkàssig, ein s t r u k t u r m o d e l l zu flnden, dass den b e o b a c h t e t e n seismischen d a t e n gerecht wird. u n t e r erdkruste sei im folgenden stets der bereich bis ca. 700 k m tiefe v e r s t a n d e n , in dem noch e r d b e b e n herde a u f t r e t e n . fiir k u r z a n d a u e r n d e k r à f te, die ihre festigkeit n i c h t ùberschreiten, v e r h à l t sich die e r d k r u s t e vollkommen elastisch, wie die a u s b r e i t u n g der seismischen b u n g des gewiclites, wird das krustenm a t e r i a l n u r elastisch d e f o r m i e r t , bei einer gròsseren plastisch. natuxlich stellt dieses s t r u k t u r m o d e l l d e r e r d k r u s t e , wie iiberh a u p t alle modelle eine yereinfachung der t a t s a c h l i c h e n verhàltnisse dar. a b b . 2 s t r u k t u r m o d e l l f i i r p l a s t i s c h e s f l i e s s e n . u n t e r b e r u c k s i c h t i g u n g d e r eigenschaft der elastischen nacìiwirkung, d. li. also eines zeitabhàngigen deformationsriickganges, h a t benioff fiir seine krieclitheorie der naclibeben ein strukturmodell der erdkruste entworfen, das d e n seismischen beobacht u n g e n weitgeliend gerecht wird. d i e rheologischen eigensciiaften d e r e r d k r u s t e , usw. 115 in seinem modell entspricht / dem gewicht bzw. d e r verschiebungsflàche in der jìatur. — e s sei a n dieselstelle e r w à h n t , dass iiber das im herd wirkende kraftsystem beute nock zwei auffassungen bestehen, die aus den sckwierigkeiten bei dei' b e s t i m m u n g k ò r p e r m i t elastischer nachwii'kung entspricht. d a s paraliel zum ganzen liegende viskose e l e m e n t e , liat einen grosseren w i d e r s t a n d als r 2 u n d w i r k t sàkular. e s b e s t i m m t die d e f o r m a t i o n des g e s a m t e n systems. 62 i ri 61 m ri — 3 } a b b . 3 b e n i o f f s s t r u k t u r m o d e l l f i i r d i e t i e f e r e n r e g i ò h e n d e r e r d k r u s t e . von a m p h t u d e u n d e i c h t u n g des ersten einsatzes der transversalwellen resultieren. geologische e n v à g u n g e n , wie a u c h die arbeiten u n s e r e r sowjetischen kollegen m a chen es jedoch wahrscheinlich, dass der verscmebungsmechanismus, d. h. also ein i m herd wirkendes kràftepaar mit moment bei 2.3 deformationscharakteristiken und empirisclie kriechfunktionen. 2.3.1 allgemeines. auf der g r u n d l a g e seines s t r u k t u r m o d e l l s h a t benioff eine theorie iiber die e n t s t e h u n g der n a c h b e b e n entwickelt, die es a b b . 4 «) d e f o r m a t i o n s e h a r a k t e r i s t i k f i i r r e i n e l a s t i s c h e s v e r h a l t e n ( i l o o k e ' s c h e r f e s t e r k ò r p e r ) . — 6) d e f o r m a t i o n s e h a r a k t e r i s t i k f i i r flussiges v e r h a l t e n ( n e w t o n ' s c h e f l i i s s i g k e i t ) . den meisten b e b e n iiberwiegt. die liier v o n benioff e i n g e f u h r t e a n n a h m e wird also auch dui-eli die n e u e r e n u n t e r s u c h u n g e n iiber den herdvorgang g e r e c h t f e r t i g t . — d a s plastische s t r u k t u r m o d e l l ist bei benioff m i t einem modell in e e i h e k o m b i n i e r t , das einem erlaubt, d e f o r m a t i o n s c h a r a k t e r i s t i k e n fiir die unzugànglichen teile der e r d k r u s t e zu gewinnen. dieso d e f o r m a t i o n s c h a r a k t e r i s t i k e n sind z u s a m m e n m i t den empirischen k r i e c h f u n k t i o n e n eine weitere mòglichkeit, die rheologischen e i g e n s c h a f t e n des k r u 116 l. hiersemann s t e n m a t e r i a l s zu beschreiben. d a b e i verstehe ich u n t e r einer deformationscliarakteristik ein d i a g r a m m , in dem die d e f o r m a t i o n als f u n k t i o n der zeit a u f g e t r a g e n ist. bei diesen k u r v e n h a t i m m e r eine ganze schar gemeinsame merkmale, die c h a r a k t e ristisch fiir die betreffende a r t der d e f o r m a tion sind und durch eine empirische kriech funktion m a t h e m a t i s c h beschrieben werden konnen. so unterscheiden sich z. b. die deformationscliarakteristik en fiir plastisches v e r h a l t e n v o n denen fiir viskos fliissiges verhalten d a d u r c h , dass nacli u b e r s c h r e i t e n der f e s t i g k e i t , oder g e n a u e r der st. venantnachgebespannung, die steigung der deforeines bebens proportional der elastischen deformation des k r u s t e n m a t e r i a l s ist, die dem beben v o r a u s g e h t . d a b e i ist vorausgesetzt, dass die d e f o r m a t i o n durch die herdverschiebung vollstàndig auf nuli zurùckgeht. w i r d die d e f o r m a t i o n erst allmahlich durch eine beihe v o n n a c h b e b e n be3eitigt, so stellt die q u a d r a t w u r z e l s u m m e der wellenenergie, wenn m a n sie als f u n k t i o n der zeit a u f t r à g t , den allmàhlichen deformatiomruckgang des k r u s t e n m a t e r i a l s dar. ein solches d i a g r a m m e n t s p r i c h t einer deform a t i o n s c h a r a k t e r i s t i k u n d gibt aufschluss iiber das rheologisclie v e r h a l t e n der begionen, in d e n e n e r d b e b e n a u f t r e t e n . die a b a b b . 5 a) d e f o r m a t i o n s c l i a r a k t e r i s t i k . b) s t r u k t u r m o d e l l f i i r d e n k e l v i n ' s c h e n f e s t e n k ò r p e r . m a t i o n n i c h t k o n s t a n t ist. die v o n griggs (1939) g e f u n d e n e n k r i e e h f u n k t i o n e n enthalt e n neben einem k o n s t a n t e n glied meist noch ein glied, das sich m i t d e m logarithm u s der zeit a n d e r t . die in dem benioff'schen s t r u k t u r m o d e l l e n t h a l t e n e k o m p o n e n t e m i t elastischer n a c h w i r k u n g wiirde eine deformationscliar a k t e r i s t i k haben, bei der die steigung der d e f o r m a t i o n w a h r e n d der b e l a s t u n g allmahlich a b n i m m t , u n d die d e f o r m a t i o n nacli der e n t l a s t u n g langsam auf nuli z u r ù c k g e h t . 2.3.2 deformationscharakteristiken aus erdbebenmagnituden. d e f o r m a t i o n s c h a r a k t e r i s t i k e n fiir die tief e r e n begionen der e r d k r u s t e gewinnt m a n m i t hilfe der h e r d z e i t u n d der a m p l i t u d e n im s e i s m o g r a m m . e s làsst sich zeigen, dass die (juadrativurzel der ausgestraìdter energie energiewerte d e r beben g e w i n n t m a n aus der instrumentellen magnitudenslcala von gut e n b e r g und b i c h t e r (1942). sie sind natiirlich noch i m m e r m i t den b e k a n n t e n unsicherheiten b e h a f t e t , so dass a u c h die mit i h r e r h i l f e gewonnenen deformationschar a k t e r i s t i k e n noch nicht ganz fehlerfrei sind. ausserdem m u s s m a n b e d e n k e n , dass die methode, die riehtige ausivahl der zu einer verschiebungsflàche gehorigen nachbeben e r f o r d e r t , was zweifellos nicht i m m e r gewàhrleistet ist. schliesslich g e h t in die kurve noch das relaxationsfliessen, d. h. also ein allmàhlicher a b b a u der s p a n n u n g bei k o n s t a n t gehaltener d e f o r m a t i o n , ein, so dass die a n n a h e r u n g n u r danti vollstàndig ist, wenn sich das material wie ein fester kòrper m i t elastischer n a c h w i r k u n g v e r h à l t . die aus e r d b e b e n m a g n i t u d e n gewonnenen def o r m a t i o n s c h a r a k t e r i s t i k e n erlauben somit n u r eine e n t s c h e i d u n g , ob sich die e r d k r u s t e d i e rheologischen eigensciiaften der e r d k r u s t e , usw. 117 in den b e o b a c h t e t e n z e i t r à u m e n wie ein fester k ò r p e r oder wie eine viskose fliiss i g k e i t v e r h à l t ; d e n n das plastische verhalten ist in der k u r v e fiir den festen k ò r p e r mit e n t h a l t e n . k u r v e besser m i t d e n vorkin gezeigten char a k t e r i s t i k e n vergleielien zu kònnen, miisste m a n bei ilir eigentlich oben und u n t e n vert a u s c h e n . e s zeigt sieh d a n n sehr deutlich, dass der erste ast die elastisclie nachwirkung 2.3.3 die deformationseharakteristik der tiefenbeben von tonga-kermadec. i c h darf thnen liier als besonders anschauhches beispiel eine c h a r a k t e r i s t i k von benioff (1954) v o r f u h r e n . e s ha miei t sicli uni die beben m i t herdtiefen von 70-680 km des gebietes tonga-kermadec. auf der ordinate ist die q u a d r a t w u r z e l s u m m e der wellenenergie a u f g e t r a g e n , die sieh aus der magnitudengleichung ergibt u n d auf der abszisse die zeit. i n d e m d i a g r a m m sind zwei empirische formeln angegeben, die den deformationsriickgang besehreiben. u m die des materials darstellt. die tatsaehe, dass sieh dieser a s t d u r e h eine f u n k t i o n besehreiben làsst, die keinen terni e n t h à l t , der der zeit d i r e k t p r o p o r t i o n a l ist, bed e u t e t eine aufspeichei'ung elastischer def o r m a t i o n e n oline merkliches fhessen fiir ein zeitintervall v o n 24 j a h r e n . d e r zweite ast der charakteristik stellt die lineare bewegung der verschiebungsblòcke mit der zeit dar, n a c h d e m die blockierungsstelle m i t d e m grossen beben vom 26. mai 1932 gebrochen war. d a s soeben vorgefiihrte beispiel zeigte, dass das rheologische verhalten der erd1 1.1915 11 1925 1.1 1935 1 11945 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i m i i i i i i i i i i i ouuanisches datum 24 22.000 a b b . 6 d e f o r m a t i o n s e h a r a k t e r i s t i k i 1 i 2 426,000 2 430.000 2.434 000 d e r t i e f e n b e b e n v o n t o n g a k e r m a d e c . 118 l. hiersemann lcruste bis 680 km tiefe dem eines festen kórpers mit elastischer nachicirkung entspricht. w e n n diese e i g e n s c h a f t auch n u r fiir ein zeitintervall v o n 24 j a h r e n giilt.igkeit h a t , so zeigt sie doch, dass die e r d k r u s t e a u c h in gròsseren tiefen eine f e s t i g k e i t besitzt u n d a u c h bei geologisch langen k r a f t e i n w i r k u n g e n n u r noch plastisch d e f o r m i e r t werden k a n n . dies s t e h t im widerspruch zu vielen h y p o t h e s e n , die schon in geringen tiefen ein viskos fliissiges material a n n e h m e n . 2.4. vereinfachte und exalcte strukturtheorien 2.4.1 allgemeines. die d r i t t e mòglichkeit z u r b e s c h r e i b u n g des rheologischen y e r h a l t e n s der e r d k r u s t e sind vereinfachte und exalcte strukturtheorien, die schliesslich die beste a n n a h e r u n g a n die wirklichkeit darstellen. e s ist das ferne, wahrscheinlich n u r sehr schwer erreichbare ziel, die b e o b a c h t e t e n t a t s a c h e n , in unserem falle also die seismologischen messd a t e n , so m i t e i n a n d e r zu v e r k n ù p f e n , dass sie sich wieder aus wenigen e l e m e n t a r e n , m a t h e m a t i s c h f o r m u l i e r t e n gesetzmàssigkeiten ableiten lassen. w e n n wir bisher d u r c h s t r u k t u r m o d e l l e u n d d e f o r m a t i o n s c h a r a k t e r i s t i k e n glcichsam das «makroskopisclie» materialverhalten, d. h. die s u m m e des verhaltens der einzelnen k o m p o n e n t e n beschreiben k o n n t e n , so e r f o r d e r t die e x a k t e theorie die einfùlirung der gefilgebestandteile. die e r d k r u s t e ist nicht homogen isotrop, sondern sie stellt ein melirphasiges gefiige dar. e i n e theoretische b e h a n d l u n g i h r e r bheologie e r f o r d e r t also v o n der seismik die a u s w e r t u n g u n d h i n z u z i c h u n g der fortpfianzungsgeschwindigkeiten seismischer wellen u n d ihrer unstetigkeiten wie a u c h der r à u m l i c h e n anordn u n g d e r bebenlierde sowie die eindeutige b e s t i m m u n g der verschiebungsebenen u n d b i c h t u n g e n . die z u k u n f t s a u f g a b e der seismologie, die vielleicht a n uniibei'brùckbaren schwierigkeiten scheitern wird, b e s t e h t d a n n darin, diese skalaren u n d velctoriellen gefiigedaten in à q u i v a l e n t e n t e r m e n rheologisclier gleichungen auszudriicken, uni so z u n à c h s t iiber v e r e i n f a c h t e zu e x a k t e n s t r u k t u r t h e o r i e n zu gelangen. d a b e i vers t e h t m a n u n t e r rheologisclier gleichung eine gleichung zwischen d e m deformationstensor e u n d dem s p a n n u n g s t e n s o r p, die das rheologische v e r h a l t e n eines k o r p e r s charakterisiert. d a s rheologische v e r h a l t e n einphasiger gefiige làsst sich m i t ililfe der physikalischen k e n n t n i s s e iiber das ver h a l t e n der b e t r e f f e n d e n p h a s e mathematisch beschreiben. bei melirphasigen gefiigen k o m m t m a n m a n c h m a l m i t der zerlegung ideelle liomogene gefiigeelem e n t e aus. (ein beispiel dafiir sind che molir'schen spannungskreise). i m palle der e r d k r u s t e wird jedoch meist eine betracht-ung der reellen teile u n d ilires rheologischen verhaltens innerhalb inhomogener bereiche notwendig sein. 2.4.2. das system der lclassischen kòrper. d e n a u s g a n g s p u n k t fiir die m a t h e m a tisch e x a k t e b e h a n d l u n g des rheologischen verhaltens d e r kugelschale der festen erdk r u s t e , miissen also vereinfachte strukturtheorien bilden. e s gilt z u n à c h s t die rheologischen gleichungen von b e s t i m m t e n iclealisierten k ò r p e r n aufzustellen, u n d dann d u r c h eine k o m b i n a t i o n dieser gleichungen die d e r wirklich e x i s t e n t e n k ò r p e r immer m e h r a n z u n à h e r n . die e r s t e tabelle zeigt i h n e n das system der lclassischen k ò r p e r . sie w e r d e n so gen a n n t , weil sich die theorie der klassischen p h y s i k seit j e h e r m i t i h n e n b e s c h à f t i g t hat. l i n k s sind die f e s t e n u n d r e c h t s die flussigen k ò r p e r angegeben. die rheologische gleichung gelit fiir jeden k ò r p e r in zwei gleichungen fiir die hauptdiagonalglieder und die nichtdiagonalglieder des spannungsu n d d e f o r m a t i o n s t e n s o r s iiber. d a s f e s t e e x t r e m u m ist der starre oder eulclidisclie kòrper, dessen einzelne teile i m m e r gleichen a b s t a n d b e w a h r e n . unabhàngig von b e t r a g u n d b i c l i t u n g der k r à f t e e n t s t e h e n keine d e f o r m a t i o n e n . d e r def o r m a t i o n s t e n s o r e ist gleich nuli. nacli e u k l i d ist dieser k ò r p e r deshalb b e n a n n t , weil die e u k l i d i s c h e geometrie auf d e r liyp o t h e t i s c h e n e x i s t e n z eines solchen korpers b e r u h t . e s h a t sich allgemein eingebiirgert, die d e m unterschiedlichen rheologischen v e r h a l t e n e n t s p r e c h e n d e n k ò r p e r m i t n a m e n von wissenschaftlern zu belegen, die sie als erste n à h e r beschrieben liaben. die rheologischen eigensciiaften d e r e r d k r u s t e , usw. 119 das fliissige e x t r e m u m ist die sogenannte pascavsclie flussigkeit. e s ist ein k ò r p e r , der einer reinen p o r m à n d e r u n g (ev = 0) oline v o l u m e n à n d e r u n g keinen w i d e r s t a n d entgegensetzt. d a s f e h l e n der formelastitraditionelle definition d e r s e h e r u n g zur ù c k z u f u h r e n . die d e f o r m a t i o n s e h a r a k t e r i stik u n d das s t r u k t u r m o d e l l fiir d e n hooke'schen k ò r p e r zeigte ieli ilinen sclion in einem friiheren bild. die tlieorie d e r seistabelle 1. d a s s y s t e m d e b k i . a s s i s c i i e n k ò b p e e ( n a c i i r e i n e e ) . b e z e i c l m u n g des m a t e r i a l s tu h a u p t d i a g o n a l g l i e d e r n i c h t diagonalirlieder f u n d a m e n t a l e k o n s t a n t e n a b g e l e i t e t e k o n s t a n t e n f e s t f 1 i i s s i g e u k l i d ev= o e0 = o o o o o l l o o k e vrn = xev vo = 2 ye0 — y f i i r i = 9xy ! j« = oo ; 3x + y £* = 3y 3x-2y o* = v2° ~ <ìx+2y s te « n e w t o n p a s c a l vm = xev + + vvcv po = 2?/e0 c„ = 0 vo = 0 oo > o il ----v i s k o s i t a t (sclier-) il,, = v o l u m e n v i s k o s i t a t a = p o i s s o n ' s c h e k o n s t . a* = k o e f f l z i e i i t d e s v i s k o s e n z u g e s v o n t r o u t o n y = s o h e r u n g s m o d u l p o r m e l a s t i z i t i i t t o r s i o n s m o d u l s t a r r h e i t s m o d u l s c h i e b u n g s m o d u l r i g h e i t y. — k o m p r e s s i b i l i t i i t = v o l u i n e l a s t i z i t i i t = e l a s t i z i t à t s m o d u l e = y o u n g ' s c h e r m o d u l l i i n g e n e l a s t i z i t a t ( e l a s t i z i t i i t s m o d u l ) zitat b e w i r k t ein yerschwinden aller tangentialspannungen (p0 = 0). d a m i t k e n n e n wir die beiden e x t r e m a , zwischen d e n e n alle wirklich existenten kòrper, also a u c h die e r d k r u s t e liegt. e s sind physikalisck m a t h e m a t i s c h e ideahsierungen, die in d e r n a t u r nicht v o r k o m m e n . i n der tabelle sind noch zwei weitere klassische k ò r p e r angegeben. der hooke'sche feste kòrper geniigt dem gesetz, dass fiir eine lineare d e f o r m a t i o n die s p a n n u n g proportional der d e f o r m a t i o n ist. wegen der u n a b h à n g i g k e i t der h a u p t u n d nichtdiagonalglieder wird das elastisehe v e r h a l t e n durcli zwei u n a b h à n g i g e materialkonstanten, die k o m p r e s s i b i l i t à t x u n d den scherungsmodul y ausgedriickt. der f a k t o r 2 bei den nichtdiagonalgliedern ist auf die mischen wellen zeigt, dass wir die e r d k r u s t e in erster, grober n à h e r u n g als l l o o k e ' schen festen k ò r p e r auffassen kònnen. die neivton'sche flussigkeit ist ein k ò r p e r , bei d e m der fliesswiderstand proportional der relativgeschwindigkeit seiner teile ist. e i n solcher k ò r p e r verha.lt sieh u n t e r allseitig gleicliem, k o n s t a n t e m , h y d r o s t a tischem d r u c k wie ein i l o o k e ' s c h e r fester k ò r p e r , d. li. seine diclite n i m m t in u m k e h r b a r e r weise zu. e i n solches material lasst sieh wieder d u r c h zwei u n a b h à n g i g e k o n s t a n t e n beschreiben, die scherviskosita,t 1-j u n d die volumenviskositàt rjv. l e t z t e r e wird meist vernachlàssigt. d a s s t r u k t u r modell u n d die d e f o r m a t i o n s e h a r a k t e r i s t i k dieses k ò r p e r s zeigte icli i h n e n ebenfalls sclion ini bild. 120 l. hiersemann 2.4.3 die rheologischen kòrper im engeren sinne. d u r c h eine k o m b i n a t i o n der gleichungen fiir den ilooke'schen k ò r p e r und die newt o n ' s c h e fliissigkeit (auflòsung nach e0 u n d addition) e r h à l t m a n einen rheologischen a r t kombinieren. als kelvin''schen festen kòrper bezeichnet m a n ein material mit der charakteristischen e i g e n s c h a f t der elastischen nachwirkung. sein s t r u k t u r m o d e l l uncl seine d e f o r m a t i o n s c l i a r a k t e r i s t i k h a t t e ieh i h n e n ebenfalls selion in einem friiheren bild gezeigt. t a b e l l e 2 . d a s s y s t e m d e r l i n e a r e n k ò r p e r ( n a c h r e i n e r ) . b e z e i c l i n u n g d e s m a t e r i a l e s f e s t a l l g e m e i n e r 1 i n e a r e r k ò r p e r f 1 u s s i g b e z e i c l i n u n g d e s m a t e r i a l e s ® ® rq c3 h r/ì 5 <d m 0 <c o 00 'w cj 5 0 k e l v i n a l l g e m e i n e r 1 i n e a r e r k ò r p e r m a x w e l l 8 k la ss is ch e k ò rp er au s t ab el le 4 d e v i a t i o n s k o m p o n e n t e d e r r h e o l o g i s c h e n g l e i c h u n g f u n d a m e n t a l k o n s t a n t e n ® ® rq c3 h r/ì 5 <d m 0 <c o 00 'w cj 5 0 po = = 2ye0 + 2rìès0 &o + ax e0 + a 2 é0 + a 3 j>0 + a 4 p0 =-0 (•pa = & s t . y e n a n t k ò r p e r ) èo = _ po , po 2ye r 2 r, 8 k la ss is ch e k ò rp er au s t ab el le 4 d e v i a t i o n s k o m p o n e n t e d e r r h e o l o g i s c h e n g l e i c h u n g f u n d a m e n t a l k o n s t a n t e n ® ® rq c3 h r/ì 5 <d m 0 <c o 00 'w cj 5 0 y < f]s &o, av a2, a3, a, yc—^ v 8 k la ss is ch e k ò rp er au s t ab el le 4 a b g e l e i t e t e k o n s t a n t e n ® ® rq c3 h r/ì 5 <d m 0 <c o 00 'w cj 5 0 e = 3y 3 r]s = 3 ye a*= 3 ti t = t]/ye 8 k la ss is ch e k ò rp er au s t ab el le 4 & = n a c h g e b e s p a n n u n g k ò r p e r im engeren sinne, die s o g e n a n n t e maxwelvsche fliissigkeit. diese fliissigkeit zeigt die charakteristische e i g e n s c h a f t der eelaxation, d. h . der allmàhlichen a b n a h m e der s p a n n u n g bei k o n s t a n t e r d e f o r m a t i o n . f i i r die e r d k r u s t e b e d e u t e t das, beim y o r h a n d e n s e i n einer maxwell-komponente, dass die d u r c h blockierung der h e r d v e r schiebun'g aufgespeicherten s p a n n u n g e n , teilweise d u r c h e e l a x a t i o n a b g e b a u t werden kònnen. i c h e r w à h n t e schon, dass dieser f a k t o r in die d e f o r m a t i o n s c h a r a k t e r i s t i k e n aus e r d b e b e n m a g n i t u d e n m i t eingeht. man k a n n die rheologischen gleichungen fiir den h o o k e ' s c h e n k ò r p e r u n d die newt o n ' s c h e fliissigkeit noch in einer a n d e r e n die c h a r a k t e r i s t i k d e r t i e f e n b e b e n von t o n g a k e r m a d e c h a t t e u n s gezeigt, dass sich die e r d k r u s t e bei a n n a h m e eines im h e r d wirkenden k r à f t e p a a r e s m i t moment, fui1 zeiten in der gròssenordnung von 25 j a h r e n wie ein k e l v i n ' s c h e r f e s t e r k ò r p e r v e r h à l t , also d u r c h die in der tabelle angegebene gleichung m a t h e m a t i s c h besclirieb e n w e r d e n k a n n . eine verallgemeinerung der rheologischen gleichungen s t a m m t v o n h o h e n e m s e r und p r a g e r (1932): •&„ + c^ e0 + a2 è0 + a 3 po + a , p„ = 0. sie u m f a s s t alle bisher besprochenen rheologischen gleichungen u n d dariiber d i e rheologischen eigensciiaften der e r d k r u s t e , usw. 121 hinaus noch einige, auf die hier jedoch nicht nàher eingegangen werden k a n n . ĝ bis a4 sind die rheologischen koeffizienten (rj u n d rjs y und ye). w e n n sie wie bislier k o n s t a n t sind, so spricht m a n von linearen oder speziellen k ò r p e r n . meist sind sie jedoch bei den allgemeinen kòrpern m i t dem spannungsoder d e f o r m a t i o n s z u s t a n d variabel. das &0 in der gleichung von h o h e n e m s e r und p r a g e r b e d e u t e t die st. venant-nacha a b b . 7 a) d e f o r m a t i j j n s o h a r a k t e r i s t i k . m a x w e h ' s c h e f l u s s i g k e i t . gebespannung, weil die meisten k ò r p e r , wie auch die e r d k r u s t e , die m a n fiir ideine spannungen als f e s t e n k ò r p e r ansehen k a n n , anfangen n a c h z u g e b e n u n d plastisch zu fliessen, w e n n eine gewisse s p a n n u n g , eben diese nacligebespannung, erreicht ist. der speziatali p„ = & ist die rheologische gleichung fiir den st. venant-kòrper. w e n n die n a c h g e b e s p a n n u n g verschwindet, so h a t m a n die klasse der fliissigkeiten. die materialien, bei denen die spannung, die einen b r u c h bewirkt, vor dem n a c h g e b e p u n k t erreicht wird, bilden die klasse d e r spròden festen k ò r p e r . i m u m g e k e h r t e n p a l l e spricht m a n v o n plastichen k ò r p e r n . 3 . schlussbemerkungen. mein v o r t r a g solite i h n e n die mòglichkeiten der rheologischen d e u t u n g seismischer daten aufzeigen. bei dieser d e u t u n g spielt das s t u d i u m des herdvorganges t e k t o nischer e r d b e b e n eine wichtige rolle. e s ist bei b e k a n n t e m h e r d m e c h a n i s m u s moglich, d e f o r m a t i o n s c h a r a k t e r i s t i k e n fiir die e r d k r u s t e zu gewinnen, die w i e d e r u m die grundlage fiir v e r e i n f a c h t e s t r u k t u r theorien bilden. e x a k t e s t r u k t u r t h e o r i e n erforden die einbeziehung noeh anderer, seismischer u n d geologischer d a t e n , wodurch die problemstellung zur zeit noch zu kompliziert wird. aber a u c h die aus e r d b e b e n m a g n i t u d e n gewonnenen deformationscharakteristiken sind noch m i t erhebliclien unsicherheiten b e h a f t e t . e s empfìehlt sieh zur u b e r w i n d u n g dieser fehlerquellen: 1. die i n s t m m e n t e l l e magnitudenskala so weiter zu entwickeln, dass die v o n i h r gelieferten energiewerte als gesichert angesehen werden kònnen. 2. die lòsungen fiir die h e r d v o r g a n g e d u r c h a u s w e r t u n g der erstcn einsàtze der transversalwellen eindeutig zu maclien. 3. die p a r a m e t e r der herdverschiebungen (lage der yerschiebungsflàche, b e t r a g u n d b i c h t u n g des verschiebungsvektors) m i t hilfe der stereographisc.hen p r o j e k t i o n z u r statistischen e r m i t t l u n g zusammengehòriger beben heranzuziehen. w e n n a u c h die rheologische d e u t u n g der seismischen messergebnisse noch a m an— b) relaxationskurve. — c) straktiiiinodell fiir die 122 l. hiersemann f a n g s t e h t , so verspricht sie doch fiir die z u k u n f t , dem geologen ein umfangreiehes t a t s a c l i e n m a t e r i a l zu liefern, das ihn d a v o r b e w a h r t , falsehe a n s e h a u u n g e n iiber die d y n a m i k der e r d k r u s t e zu entwiekeln. riassunto grazie alla interpretazione reologiea dei dati sismici, lo studio dell'andamento di epicentro dei terremoti di carattere tettonico assume un'importanza particolare. è così possibile determinare grazie al conosciuto meccanismo di epicentro e grazie all'opportuna scelta di una superficie di spostamento i relativi terremoti successivi, e le caratteristiche di deformazione della grandezza dei terremoti superficiali per le regioni più profonde della crosta terrestre. queste caratteristiche di deformazione costituiscono insieme con i modelli strutturali e le funzioni empiriche di scorrimento le prime approssimative possibilità di descrizione del comportamento geologico dei corpi geologici. esse formano la base per le teorie strutturali semplificate. le esatte teorie strutturali necessitano l'apporto di ancora altri dati sismici e geologici, con il che la impostazione del problema viene, per il momento, ancora complicata. la valutazione finora fatta dei risultati delle misure sismologiche mostra che la crosta terrestre, in un periodo di venticinque anni, fino alla profondità di circa 700 lem, si comporta come un corpo solido di kelvin con effetti elastici. abstract thanlcs to the rheological interpretation of seismic data, the study of the trends of epicentres in earthquakes of tectonic character is acquiring particular importance. in this manner, owing to the known mechanism of the epicentre and to a suitable choice of the surface of smfting, it becomes possible to determine the subsequent eartliqualces involved and the features of the deformation of the size of superficial earthqualces in the deeper regions of the earth's crust. these deformation features, together with the structural models and empirical functions of shifting, constitute the first approximate possibility of describing the geological behaviour of geological bodies. tliey constitute bases for simplified structural theories. exact structural theories require the contributicni of other seismic and geological data, and, thus, the posting up of the problem is stili complex. the study of the results of seismic measurements, carried out so far, has shown that the crust of the earth, during the past 25 years, and to the depth of about 700 kilometres, has been beliaving as a kelvin's solid body with elastic effect s. z usa mmenfass ung bei der rheologischen deutung seismischer daten spielt das studium des herdvorganges telctonischer erdbeben eine ivichtige rotte. es ist bei belcanntem herdmechanismus und bei richtiger auswahl der zu einer verschiebungsflàche gehòrigen nachbeben móglich, deformationscharakteristiken aus erdbebenmagnituden fiir die tieferen regionen der erdkruste zu gewinnen. diese deformationscharakteristiken sind zusammen mit strukturmodellen und empirischen kriechfunktionen die erste angenàherte móglichkeit zur beschreibung des rheologischen verhaltens geologischer kòrper. sie bilden die grundlage fiir vereinfachte strukturtheorien. exakte strukturtheorien erfordern die einbeziehung noch anderer seismischer und geologischer daten, wodurch die problemstellung zur zeit noch zu kompliziert wird. die bisherige auswertung seismologischer messergebnisse zeigt, dass sich die erdkruste bis ca. 700 km tiefe in einem zeitintervall von 25 jahren wie ein kelvinscher fester kòrper mit elastischer nachwirkung verhàlt. l i t e r a t u r (') benioff, i i . , orogenesis and deep crusial structure-additional evidence from seismology. « b u l l . geol. soc. a m . », 6 5 , 5, 3 8 5 4 0 0 , (1954). ( d t . u b e r s . : b e r g a k a d e m i e 11 u n d 12, 4 8 8 4 9 5 u n d 542-54g) (1954). (2) •— earthqualces and rock creep. « b u l l . s e i s m . s o c . a m . », 4 1 , 1, 31-62 (1951). d i e r h e o l o g i s c h e n e i g e n s c i i a f t e n d e r e r d k r u s t e , usw. 123 (») griggs, d . , creep of rocks. « j o u r n . geol. », 4 7 , 2 2 5 2 5 1 , (1939). («) g u t e n b e r g , b . r i c h t e k , c . , earthquake magnitude intensity, energy and acceleratìon. « b u l l . s e i s m . soo. a n i . » , 3 2 , 163191, (1942). (6) henckt, h . , zur theorie plastischer deformation und der hierdurch im material hervorgerufenen nachspannungen. « z t s c h r . f. a n g e w . m a t h . u . m e c h . », 4 , 3 2 3 3 3 4 , (1924). ( 6 ) i i i e r s e m a n n , l . , geologiscli-geophysikalisehe theorien iiber den aufbau und die dynamik der erdkruste. « f r e i b e r g e r f o r s c h u n g s h e f t c 2 4 » , 154, (1954). (') — ober die bedeutung der rheologie fiir geopliysikalisch-geologisclie theorien. « geol. r d s e h . », 4:6, 69-86, (1957). (8) — die neuen erkenntnisse iiber den herdvorgang bei grossen erdbeben und ihre bedeutung. fiir die geotektonik. (zusammenfassendes beferat). « b e r g a k a d e n i i e » , 5 , (1958). ( 9 ) i i o h e n e m s e r , k . p r a g e r , w . , fundamental equations and definitions concerning the mechanics of isotropie continua. « j. r h e o l . », 3 , 16-22, (1932). (10) nadai, a., theory of flow and fraeture of solids. n e w y o r k (1950). ( u ) reiner, m., twelve lectures on tlieoretical bheology. a m s t e r d a m (1949). (12) scheidegger, a., examination of tlie physics of tlieories of orogenesis. « b u l l . geol. soo. a m . », 6 4 , 127-150, (1953). (13) timoshenko, s., theory of elastic stability. n e w y o r k , 1936. 33karakhanian.qxd 793 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words armenia – historical seismicity – volcanism – catalogue 1. introduction the volcanic uplands of the armenian highland are situated between the anatolian plateau and the iranian plateau and include areas of today’s armenia, eastern turkey, southern georgia, western azerbaijan and northwestern iran (fig. 1) (abich, 1882; mutafian, 1988; hewsen, 2001). the armenian highland falls in the central part of the zone of continental collision of the arabian lithosphere plate (jackson and mckenzie, 1988; dewey et al., 1986; taymaz et al., 1991; jackson, 1992; jackson and ambraseys, 1997). the north-anatolian fault, east-anatolian fault, levant fault, zagros fault and other active faults cut across the armenian highland. many strong earthquakes of m = 7.0-7.7 are associated with these faults (ambraseys and melville, 1982; jackson and mckenzie, 1988; ambraseys and jackson, 1998). clear estimates of maximum potential magnitudes and earthquake recurrence intervals for the armenian highland are lacking. the recorded parameters of recent strong earthquakes in the region determined unexpectedly large scales of destruction and high casualty rates, which later forced re-estimation of the seismic hazard level. among such strong earthquakes we can mention the chaldiran earthquake in 1976 (m = 7.1), norman earthquake in 1983 (m = 7.0), and bingel earthquake in 2003 (m = 6.4) in eastern turkey, spitak earthquake in 1988 (m = 7.1) in armenia, roudbar earthquake (m = 7.7) in iran, and racha earthquake in 1991 (m = 7.1) in georgia. collection of data and compilation of as much complete database on historical and prehistorical seismicity as possible represents one of the most important tasks to be addressed in seismic hazard assessment. evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) arkady karakhanian and yelena abgaryan georisk scientific research company, yerevan, armenia abstract this work presents a summary on the development of studies of historical earthquakes in armenia and adjacent parts of turkey and iran. since ancient times, this region has been an arena where active geodynamic and seismic history intermingled with no less active and dynamic evolution of human cultures and societies. a long-term historical record in this region beginning as early as the 8th century b.c. provides abundant evidence that can make an inestimable contribution to studies of historical seismicity and volcanism in the area. we discuss the main research methodology and sources used, and dwell on the principal catalogues of historical earthquakes compiled to date. mailing address: dr. arkady karakhanian, georisk scientific research company, 24a marshal baghramian ave., 375019 yerevan, armenia; e-mail: georisk@sci.am 794 arkady karakhanian and yelena abgaryan considering high rates of recent deformation in the centre of the arabian collision zone, the recorded manifestations of seismic activity in the 20th century may be insufficiently representative for a reliable estimation of the hazard. there is a bulk of historical and paleoseismological evidence indicating that many strong earthquakes occurred in this area in the past (stepanian, 1964; ambraseys, 1975, 1988; ambraseys and melville, 1982; ambraseys and adams, 1989; bommer and ambraseys, 1989; berberian, 1994, 1997; guidoboni and traina, 1995; guidoboni et al., 1996; shebalin et al., 1997; philip et al., 2001; karakhanian et al., 1997, 2002, 2003). from the 1st century b.c. till the 14th century a.d., the armenian highland was the land of historical armenian kingdoms (fig. 1) (mutafian, 1988; armenie, 1996; hewsen, 2001). after the failure of the last armenian kingdom, between the 14th and 19th centuries a.d., the armenian highland continued to be the centre of culture and activity of a large armenian population. the centres of armenian culture and christian monasteries in particular, have preserved plenty of recorded evidence on natural catastrophes that occurred in these areas during more than 1500 years. hittite, urartian, greek, persian, arab and turkish sources supplement the scenes of history of seismic and volcanic activity in the armenian highland. the main purpose of this work is to consider and analyze the present condition of studies of new historical evidence on natural disasters in the armenian highland. 2. sources most existing data on historical strong earthquakes in the armenian highland are derived from the analysis of written sources, among which armenian manuscripts and ecclesiastical materials include the largest share of information. monk chroniclers and historians provided detailed descriptions of many natural disasters in armenia, anatolia, syria, levant, fig. 1. the area of the anatolian semi-island in the armenian highland with historical hittite and armenian kingdoms from the 18th century b.c. till the 14th century a.d. (from hewsen, 2002) 795 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) western iran and southern caucasus from the 5th century to the 19th century a.d. these chronicles are of unique value considering the abundance and level of detail in the references to natural phenomena they contain. the main part of these manuscripts (more than 20 000) is kept in matenadaran, the institute of ancient manuscripts in yerevan (armenia), and about 9000 are in the archives of jerusalem, venice, vienna, nor jougha (iran), lebanon and paris. earthquake catalogues include only part of data from sources in armenia (matenadaran) and in venice (san lazzaro). recent studies have shown that manuscript sources of matenadaran and the armenian apostolic church archives may attest to earthquakes earlier unknown to scholars, or determine a substantial revision of parameters of already known events (haroutunian et al., 1993; haroutunian and karakhanian, 1997; karakhanian et al., 2002, 2003; guidoboni et al., 2003). in many respects, trustworthy interpretation of evidence from armenian sources written in old armenian depends on how well scholars know this language, posing considerable difficulties for translation. besides, the sources use the armenian calendar, different from other systems of chronology, and a 12-h day scale, which also complicate the analysis. available written evidence on natural disasters in the armenian highland is not limited solely to the armenian sources, but can be substantially added with information provided by greek sources of the 5th century b.c. 4th century a.d., syrian of the 2nd-6th centuries a.d., byzantine and persian of the 3rd-15th centuries a.d., and arab and turkish for the 11th-19th centuries a.d. moreover, important remarks can be extracted from notes of foreign travellers, who visited historical armenia since the 12th century a.d. use of evidence from various written sources allows cross-analysis and refining of earthquake-related data. the earliest written records of volcanic and seismic catastrophes in the armenian highland were found in rock inscriptions of hittite (the 26th and 18th centuries b.c.) and urartian (8th century b.c.) kings (ferraud, 1994; karakhanian et al., 2002). a petroglyph dated to the 6th millennium b.c. with five pictures of volcanic eruption was found in the south of lake sevan (karakhanian et al., 2002; avagyan et al., 2003). as archaeological and paleoseismological sources provide data on the early period events, not covered by written sources, they have been increasingly used in recent studies of seismic and volcanic catastrophes in the armenian highland (philip and karakhanian, 1999; karakhanian et al., 2002; avagyan et al., 2003). despite the limited scope of information derived from this kind of source, they certainly have a future. archaeological data and radiocarbon age estimates enabled identification of strong earthquakes and volcanic eruptions occurred in the south of armenia between 782 and 773 b.c., in the 18th century b.c., and in the 5th and 6th millennia b.c. (philip and karakhanian, 1999; philip et al., 2001; karakhanian et al., 2002). paleoseismic data helped to record earthquakes of m = 7.2-7.4 in the north and in the south of armenia and date them to 400 b.c., 2200 b.c. and 7500 b.c., and estimate ages of 10 500, 20 879 and 21 705 years b.p. for other three earthquakes (philip et al., 2001; karakhanian et al., 2004). new evidence on seismic disasters of the past can be derived from analysis of folklore sources. nikonov (1991) and berberian (1994) demonstrated that information in armenian and persian folk eposes can bear witness to earthquakes and volcanic eruptions. new folklore accounts from armenian and hittite eposes dated back to the 16th-12th and the 6th centuries b.c., respectively, are provided in appendix a. 3. descriptive and parametric catalogues the history of collection and analysis of strong earthquake data from armenian sources can be divided into several periods (table i). this division is rather conditional and based on both catalogue publication dates, and the objectives the compilers had to achieve. in periods i and ii, and at the early stage of period iii, catalogues reflected historical rather than seismological studies, while catalogues of the later two stages of period iii could serve databases for seismic hazard assessment we demarcate the boundary between the second and third stages in period iii at the occurrence of few 796 arkady karakhanian and yelena abgaryan strong earthquakes in the region during a short span (spitak 1988; roudbar 1990; racha 1991; erzinjan 1992), which aroused interest in seismic hazard assessment and compilation of earthquake catalogues. studying ancient manuscripts, l. khanlaryan (1983) discovered a chronicle of amiras yerzncatsi (amiras from yerznca, a chronicler of the 17th century), who refers to strong earthquakes in his native town of yerznca (an old armenian name of the town of erzinjan) (chronicle of amiras yerzncatsi, 1988). amiras entitled this chronicle the list of earthquakes: it actually contains a list of earthquakes from 1045 to 1584 in chronological order with short notes describing the damage they caused. we consider this work of amiras yerzncatsi the earliest known attempt to collect strong earthquake data from armenian sources and relate it to period i (table i). in general, the early period of collecting historical seismicity data from armenian sources can be related to the works of inčičean (1822, 1835), shahatounianc’ (1842), alishan (1881, 1890) and kostaneanc’ (1902) in the 19th-early 20th centuries (guidoboni and traina 1995; pirouzian et al., 1997). these authors were the first who integrated information about a large number of earthquakes, but never tried to estimate their parameters. the earthquake catalogue of the russian empire by musketov and orlov (1893) and studies of individual strong earthquakes by abich (1862a,b; 1882) represent more detailed studies in the early period. table i included here, and detailed analysis by guidoboni and traina (1995) and pirouzian et al. (1997), provide an idea of general trends of the later development of historical seismicity research. this work focuses on the most important points of historical seismicity studies undertaken in the region recently. the fundamental descriptive catalogue of stepanian published in 1964 was an important milestone in historical seismicity studies in armenia. stepanian’s catalogue contains detailed information on more than thousand strong historical earthquakes that occurred since 550 b.c. till the early 20th century a.d. in anatolia, syria, armenia, georgia, azerbaijan and iran. in the meantime, this catalogue is mainly descriptive. stepanian did not cite original sources of historical earthquake evidence, but just retold them liberally, without any critical analysis of the reliability of the data provided. in 1977, the new catalogue of strong earthquakes in the territory of the ussr (kondorskaya and shebalin, 1977) presented the first estimates of principal parameters of strong historical earthquakes in the region of the southern caucasus republics of the ussr (armenia, georgia and azerbaijan). the catalogue advised users to take into account the subjective character of information contained in historical sources, which, according to the authors of the catalogue, tended to exaggerate seismic hazard in the caucasus, where strong earthquakes could not occur (pirouzian, 1972; borisov et. al., 1975; kondorskaya and shebalin, 1977; borisov, 1982). later, the 1988 spitak earthquake in armenia and the 1991 racha earthquake in georgia, both with ms = 7.1, as well as the roudbar earthquake of 1990 (m = 7.7, ne iran) and erzinjan earthquake of 1992 (m = 6.9, e turkey) proved that strong seismicity in the armenian highland, attested by the historical sources, was the case. after these events, studies of historical seismicity in the region gained strong impetus. during the eight years from 1989 till 1997, 12 catalogues with historical seismicity data for anatolia, armenia, caucasus and iran were prepared and published (table i). among them, 9 catalogues were parametric, and 3 were only descriptive. to date, the parametric catalogue of shebalin and tatevossian (1997) has been the most complete and representative one. the catalogue prepared for the trans-caucasian region by berberian (1997) was the first attempt to summarize parameters estimated by different authors for the same earthquakes. for historical seismicity analysis, of most interest are descriptive catalogues of zeitounian (1991), guidoboni and traina (1995), and guidoboni et al. (1996), based largely on armenian sources. with its rich summary of historical accounts on strong earthquakes, zeitounian’s catalogue does not contain any comments or reliability analysis for the historical texts it presents. the catalogue itself has not been published yet. 797 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) p er io d c at al og ue t yp e c at al og ue n am e e . a na to li a a rm en ia , c au ca su s n w i ra n p er io d c ov er ed the 20th century,modern period stage iii p ar am et ri c an d de sc ri pt iv e ca ta lo gu es s he ba li n an d t at ev os si an ( 19 97 ) 20 00 b .c .19 90 a .d . p ir ou zi an e t a l. (1 99 7) 20 th 17 th m il le nn ia b p 1 98 8 b er be ri an ( 19 97 ) 24 76 5 b p -1 99 1 a .d . s oy sa l et a l. ( 19 97 ) 21 00 b .c .18 99 a .d . g ui do bo ni a nd t ra in a* ( 19 95 ) 2n d ce nt ur y a .d .11 90 a .d . g ui do bo ni e t a l. * (1 99 6) 12 20 a .d .17 03 a .d . b er be ri an ( 19 94 ) 4t h ce nt ur y b .c .11 90 a .d . m oi nf ar e t a l. (1 99 4) 30 00 b .c .19 92 a .d . z ei to un ia n* ( 19 91 ) 55 0b .c .19 88 a .d . k ar ap et ia n (1 99 1) 55 0 b .c .18 89 a .d . a m br as ey s an d a da m s (1 98 9) 86 3 a .d .19 86 a .d . stage ii k ar ap et ia n (1 98 6) 55 0 b .c .19 80 a .d . p ar am et ri c an d de sc ri pt iv e ca ta lo gu es a m br as ey s an d m el vi ll e (1 98 2) 33 0 b .c .19 79 a .d . k on do rs ka ya a nd s he ba li n (1 97 7) 55 0 b .c .19 74 a .d . s te pa ni an * (1 96 4) 55 0 b .c .19 53 a .d . stage i b yu s* ( 19 48 ) 13 9 b .c .19 48 a .d . d es cr ip ti ve ca ta lo gu es s te pa ni an * (1 94 2) 55 0 b .c .19 39 a .d . l eb ed ev * (1 93 1) 73 5 a .d .18 40 a .d . the 19th century early period c hr on ic le s an d de sc ri pt iv e ea rt hq ua ke ca ta lo gu es k os ta ne an c’ * (1 90 2) 85 8 a .d .11 56 a .d . m us ke to v an d o rl ov * (1 89 3) 73 5 a .d .18 40 a .d . a li sh an * (1 88 1, 18 90 ) 55 2 a .d .18 81 a .d . a bi ch * (1 86 2a ,b ) 10 45 a .d .18 40 a .d . s ha ha to un ia nc ’* ( 18 42 ) 16 22 a .d .18 40 a .d . in či če an * (1 82 2, 18 35 ) 73 5 a .d .18 th c en tu ry a .d . the 17th century ancient period c hr on ic le s of ea rt hq ua ke s a m ir as y er zn ka ts i* ( 17 th c en tu ry ) 10 45 a .d .15 84 a .d . (* ) d es cr ip ti ve c at al og ue s. t ab le i. c hr on ol og y of e ar th qu ak e ca ta lo gu es f or t he r eg io n of t he a rm en ia n h ig hl an d. 798 arkady karakhanian and yelena abgaryan the first critical analysis of ancient armenian manuscript texts, providing earthquake-related data, can be found in guidoboni and traina (1995), and guidoboni et al. (1996). the authors identified cases of duplication and even triplication by historical seismicity catalogues of many seismic events (e.g., dvin earthquakes of the 9th and 10th centuries), and revealed several false events and previously unknown strong earthquakes. the works of zeitounian (1991), guidoboni and traina (1995), and guidoboni et al. (1996) can be considered the next important milestone after the studies of stepanian in 1964. unfortunately, the interest with respect to historical seismicity information has been declining in the region since 1997 and today studies in this field are episodic (haroutunian and karakhanian, 1997; guidoboni et al., 2003). less interest in seismic hazard problems fifteen years after the last disastrous earthquake in armenia (spitak, 1988) is explicable, but, along with general cuts of research financing today, it is one of the main reasons of the limited scope of historical seismicity studies in armenia. 4. reappraisal of some earthquakes our recent studies revealed some typical errors of the catalogues, and proved that certain data on historical earthquakes in the armenian highland can be complemented and corrected. among frequent errors is the wrong identification of old geographic place-names reported by the sources, which considerably affects the accuracy of the catalogues. analyzing the report of armenian chronicler samuel anetsi on the collapse of mount koghatu and resulting blockage of the euphrates river in the earthquake of 802 a.d., guidoboni and traina (1995) locate the epicenfig. 2. the map of the armenian highland with indications of geographic place-names, historical earthquakes and volcanic eruptions discussed in the text: 1– historical settlements and provinces; 2 – epicentres of historical earthquakes, for which new parameter estimates have been made as discussed in the text; 3 – historical volcanic eruptions. 799 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) tre of the event in mesopotamia and exclude this event from the catalogue of earthquakes in ancient armenia. we know, however, that the koghatu mountains are situated in the upper riches of the euphrates, 30 km to the southwest of the city of erzinjan, near the towns of anikamakh and akn (see the next paragraph) (fig. 2). therefore, it is early to exclude the considered earthquake from the list of historical armenian earthquakes. the catalogues of kondorskaya and shebalin (1977), karapetian (1986, 1991), papazachos and papazachou (1997), nssp (1998) and noaa all mention two earthquakes with the date of 1045 (or 1046 in some catalogues), suggesting that one of them with a magnitude of m = 6.0-7.0 destroyed yerznca (erzinjan), while the second event of m = 5.5 struck the city of ani, 350 km east of yerznca. our analysis of original chronicle sources shows that apart from yerznca, destruction in 1045 was recorded not in ani, but in the town of ani-kamakh (today’s city of kamakh in eastern turkey), 30 km to the southwest of yerznca (fig. 2). originally published in the acts of the caucasian archeographic commission meeting (1873), this wrong location of the earthquake was repeated by other catalogues for 100 years onwards, and resulted in the duplication of the 1045 event. another example of errors related to ancient geographic names is illustrated in appendix b. such errors can be found in many earthquake catalogues. six volumes of the toponymic dictionary of armenia and adjacent areas by hakobian et al. (1986-2003) may help researchers in locating armenian place-names correctly. useful information can be found also in the historical atlases of cilicia (mutafian, 1988) and in armenia: a historical atlas, a fundamental work of hewsen (2001). catalogues often misestimate earthquake parameters due to incomplete or imprecise translation from the old armenian language. discussing information available for the armenian earthquake of 1406 (or 1407 in some catalogues), guidoboni et al. (1996) cite the original report from manuscript matenadaran 9247a and provide the following translation: «... now let me tell the calamity of those who have doubts and of the grief of this country. because of the quantity of our sins, the earthquake happened on the earth, and because of it the fundaments trembled and the stones quarried and the earth was trembling the whole year. and while this tributation the premature death reached the countries of jˆula and sˇ amb». interpreting this account written in the tatev monastery, guidoboni et al. (1996) conclude that it tells about a low-intensity earthquake in the tatev monastery region (fig. 2). they also comment that the mention in this text of «premature death reaching the countries of jula (jougha) and samb (shamb)» shall not be related to the earthquake of 1406, but rather to the death of few monks in those regions. actually, the translation cited by guidoboni et al. (1996) omits two phrases – one in the middle part of the passage, and one in its end. below we present the passage in full, translated according to zeitounian (1991), and emphasize the omitted parts in italics. «now let me tell the calamity that caused perturbation and grief in these places. because of the quantity of our sins, the earthquake happened on the earth, the fundaments of the earth trembled and the firm natural stones crashed, and many corps of numerous dead were found, since the earth was trembling the whole year constantly. before that time, we had banished anxiety out of our mind, and then premature death reached /the people/ in jˆula and sˇ amb, and today smbat’s brother, lord broutel with his sons passed away in christ, and also smbat’s son and many others that are countless. and who can describe the sorrowful cries and the yell of women who lost their children?». the omitted part of the text allows us to suggest a strong earthquake with numerous victims, and to identify the area of greatest destruction it caused, since we know that lord broutel was not a monk, but the prince of sunik. a gravestone inscription on his tomb in the vorotnavank monastery tells about his death and the death of his children and people of his suite in 1406. death found them all in the vorotnavank monastery, on their way home from the funeral of some ozbek, when the monastery was destroyed by a strong earthquake. a detailed account of the earthquake in 800 arkady karakhanian and yelena abgaryan 1406 can be found in a verse by the poet arakel suniketsi, a contemporary of the event. some phrases in his verse suggest that an epidemic and migration of the population followed the earthquake. the vorotnavank monastery, reconstructed in 1438, is situated 13 km to the east of the tatev monastery, 17 km to the southeast of shamb, and 55 km to the north of jougha (fig. 2). the tatev monastery was also damaged severely. in his manuscript of 1411, chronicler movses writes: «o venerable fathers and brethren, i beg you to pray the lord pleading him to have mercy on us and restore anew what have been destroyed by the earthquake». the intensity of the earthquake is attested also by the following comment of tovma mekopetsi, a scribe from the tatev monastery: «glory to your mercy and grace, jesus, my lord, who saved us of this earthquake, since my house trembles like a leaf. o, an astonishing earthquake that trembles the tatev monastery of surb arakelots, the belltower, and the gavazan. it was on the 29th of november» (translation according to zeitounian, 1991). actually, in the tatev monastery, the gavazan was not a wooden or metal crosier, or staff used during liturgy, as commented in guidoboni et al. (1996), but an 8m-high stone obelisk (fig. 3). this gavazan obelisk was erected in 904 as an octahedral column of stones crowned with an ornamented cornice and kachkar stone stele. the gavazan obelisk could serve to record seismic oscillations (khalpakhchian, 1962, 1980). from seismic ground shaking, or even a strong strike with one’s hand, this gavazan can tilt and return back to its original position like a pendulum thanks to the hinged connection of the column with the stylobate. the 1931 earthquake with estimated intensity 9 in the area of the tatev monastery (msk-64) destroyed its churches, and had macroseismic impacts on the «gavazan» obelisk similar to those recorded in 1406 (gorshkov, 1932, 1933; karakhanian et al., 2003). apart from destruction in the tatev monastery, the 1931 earthquake caused damage to the «vorotnavank monastery», and severe destruction and casualties in shamb and jougha, for which intensity rates are estimated at 9 and 8 by msk-64, respectively (gorshkov, 1932, 1933; karakhanian et al., 2003). therefore, the macroseismic effects of the 1931 earthquake (m = 6.5) can be commensurable with the effects of the 1406 event. studying historical sources, earthquake catalogue authors often translate, analyze, and later use information from only one short passage from the text, which directly refers to an earthquake, while the rest of the text is disfig. 3. gavazan obelisk in the tatev monastery, situated in the south of today’s armenia (photo by a. karakhanyan). 801 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) regarded. in the meantime, reading and analyzing of the whole text of the source is essential and can help to estimate earthquake parameters accurately (see examples in appendices a and b). as illustrated by the example below, another typical error is misinterpretation as earthquakes of historical evidence attesting to other natural disasters. beginning from the catalogue of musketov and orlov (1893), many recent catalogues (stepanian, 1964; kondorskaya and shebalin, 1977; karapetian, 1986, 1991; nssp, 1998) have recorded intensity 6-7 events in 1658 and 1622 in an area near the village of ailidzor (fig. 2) in the province of sunik (south of armenia). however, historical sources cited by arakel davrizhetsi and zackaria sarcavagi are dated to 1658, and not 1622, and clearly describe a landslide near the ailidzor village, but not an earthquake. therefore, these events should be unambiguously excluded from earthquake catalogues (haroutunian et al., 1997) numerous descriptions of volcanic eruptions in armenian manuscripts indicate that volcanic hazard and risk in this country are by no means lower than the hazard and risk of strong earthquakes (karakhanian et al., 2002; haroutunian, 2001). matteos uorkhaetsi, an armenian chronicler of the 12th century, reports about the phenomena observed near lake van: «severe destruction occurred the same year of 1111 in the mentioned area. in armenia’s province of vaspourakan, in winter, the vault of heaven opened wide and fire poured down into lake van. waves surged on the shore with a terrible noise. waters and land trembled with horrible crash. lake water turned red. the flame tore the solids of the abysses. in the morning, people saw that multitude of fish had died. a stench filled all around. very deep clefts cut through the earth in many places». abich (1882), stepanian (1964), ergin et al. (1967), karapetian (1986, 1991), nssp (1998), and many other catalogues interpret this report as evidence of a strong earthquake. guidoboni and traina (1995) believe that was a landslide. we think the passage rather describes a volcanic eruption in the lake water area, than an earthquake, or a landslide (fig. 2). 5. discussion and conclusions the armenian highland and areas adjacent to it were densely populated throughout the historical period, and even in the pre-historical time. this was an arena of emergence, development and decline of many large cultures of the past. archaeological monuments for a period from the 5th millennium b.c. till the 18th century a.d., rock paintings of the 6th-3rd millennia b.c. (so called petroglyphs), folk epics and legends, cuneiform inscriptions of the hittite and urartian cultures (19th 6th century b.c.), and antique and medieval chronicles (2nd-18th centuries a.d.) provide a huge amount of descriptions of natural calamities and their social impacts almost for the entire period of the holocene. one would think that such a long record of natural disasters in the armenian highland should facilitate seismic hazard assessment studies for this region, where historical seisimicity studies were started late in the 19th century. however, in armenia of the 1960s-1980s these studies were not a priority, their findings were considered subjective and disregarded in seismic hazard assessment. after the earthquakes of 1988-1991, 12 seismic catalogues compiled for the area of the armenian highland (table i) included much new evidence on historical earthquakes. in the meantime, even most complete parametric catalogues (shebalin and tatevossian, 1997; berberian, 1997) often did not include new historical evidence collected in descriptive ones (zeitounian, 1991; guidoboni and traina, 1995; guidoboni et al., 1996). moreover, there is certain confrontation between historians and seismologists, who, as a rule, are authors of descriptive and parametric catalogues, respectively. historians consider that seismologists lack relevant competence for historical studies (zeitounian, 1991), while seismologists avoid presenting original historical sources they have used to estimate earthquake parameters. despite proponents of joint research in both camps (guidoboni and traina, 1995), there still has not been a successful attempt in this direction. one common shortcoming of both descriptive, and parametric catalogues has been the ab802 arkady karakhanian and yelena abgaryan sence of information from archaeological and paleoseismological sources. the 1988 spitak earthquake in armenia and 1991 racha earthquake in georgia occurred in zones for which the catalogues had not recorded strong historical seismicity. the paleoseismic and archaeological studies conducted in the zone of spitak earthquake indicate that recurrence interval of strong earthquakes in this region may range from 5000 to 17 000 years (philip et al., 1992, 2001, karakhanian et al., 2003). while historical analysis is still applicable for a date of 5000 years ago, although with nearly extreme resolution of this method, historical record would appear too short to help in assessing seismic hazard in armenia in case of an interval of 17 000 years. therefore, archaeoseismological and paleoseismological methods must be applied along with historical analysis. paleoseismological evidence of strong earthquakes in the zones, for which no historical seismicity data had been available, were found for georgia as well (khromovskikh et al., 1979). after 1997, historical seismicity studies in armenia almost ceased due to the grave social and financial crisis started in the southern caucasian countries. since 2001, we have attempted to create a new database on historical earthquakes, volcanic eruptions and other natural disasters in the armenian highland. the study has been conducted by an integrated group including historians, seismologists, geologists and archaeologists. in coming years, jointly with our colleagues, we plan to prepare a new catalogue of strong earthquakes and volcanic eruptions in the armenian highland. this catalogue will contain detailed description of historical, archaeological, and paleoseismological sources attesting to the disasters of the past, as well as all principal parameters necessary to assess seismic and volcanic hazards. acknowledgements the authors are grateful to dr. andranik zeitounian from the matenadaran institute of ancient manuscripts, dr. vladimir trifonov from the institute of geology of the russian academy of sciences, dr. herve philip from the montpellier-ii university (france), dr. massimiliano stucchi (italy), dr. ruben jrbashyan and dr. ruben haroutunian from the institute of geological sciences of the national academy of sciences of armenia, and also to dr. suren arakelian, dr. haik baghdassarian, and dr. ara avagyan from the georisk scientific research company. we would like to thank vardan makaryan from the mother see of holy echmiatsin for arranging translations of texts from old armenian. we thank also dr. christine chataigner (maison de l’orient méditerranéen, lyon, france), and dr. ashot pilipossyan, dr. ruben badalyan and dr. pavel avetissyan from the institute of archaeology and ethnography of the national academy of sciences of armenia for their inestimable help and contribution to the realization of this work. appendix a. the hittite and armenian myths as potential sources of information about natural disasters of the ancient time to date, there has not been an attempt to analyze old legends and myths and identify information they may contain about strong earthquakes, or natural calamities in armenia and in neighboring countries. nikonov (1991) just noticed briefly that the armenian epic of sassountsi david (david from sassoun), or sasna tsrer, could contain reference to an old earthquake in armenia. a more detailed analysis of persian and some armenian myths in relation to seismic and volcanic disasters belongs to berberian (1994). we present here two examples demonstrating that ancient legends and traditions constitute one nearly untouched source of useful information about natural calamities of the past. 803 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) fig. a.1. bas-relief found in the city of malatya, the southeastern turkey: a god fighting a dragon (gurney, 1966). a. 1. the hittite myth of the slaying of the dragon and the myth of the missing god during most of the 2nd millennium b.c., the ancient hittite kingdom occupied the area of the anatolian semi-island (fig. 1). one of the principal myths in the hittite mythology is the myth of the fight between the dragon illuyankas, living on a mountain, and the weather-god (a prototype of teishouba, the urartian thunderstorm-god) dated to the 16th-12th centuries b.c. in various versions of this myth, either the dragon, or the god wins this fight (gurney, 1966). a bas-relief picture found near malatya (eastern anatolia, fig. 2) may depict a dragon fighting the weather-god. part of this picture resembles a lava-flow front: there are clear piles of burning stone blocks and tongues of flame behind anguine rings of the dragon’s body (fig. a.1). the second group of hittite myths is related to a disaster that paralyzed all life on earth caused by the disappearance of the god of fertility. gurney (1966) provides translation of a passage from the text of this myth that describes the disaster: «dust (?) clouds beset the window, smoke (?) besets the house, the embers on the hearth were choked (?), the gods stifled [in the temple], the sheep stifled in the fold, the oxen stifled in the stall, the ewe spurned her lamb, the cow spurned her calf. […] barley and emmer wheat throve no more, oxen, sheep and humans ceased to conceive, and those who were pregnant could not bear. trees withered and the meadows and springs dried up. so there was a famine, and both gods and men began to starve». the presented passage can be interpreted as a description of volcanic disaster effects, accompanied by emission of toxic gases, and of catastrophic environmental consequences and famine started as a result of the volcanic eruption. the ancient east history (1988) contains another translation of the same passage, where the word corresponding to «stifled» is interpreted rather as «squeezed», or «pressed down». by the second translation, we may imply that a strong earthquake accompanied the volcanic eruption in the 16th-12th centuries b.c. the disaster could have stricken the eastern and central parts of anatolia and reflected in the myth of the dragon illuyankas and of his fight with the weather-god. other available evidence of historical eruptions in the region is dated to 6500 b.c. (mellaart, 1967). ferraud (1994) presents reports from hurrian texts describing eruptions of a volcano near the city of parsahanda, which made farming not possible during the life of one generation. according to gurney (1966) and the ancient east history (1988), the town of parsahanda (parsanda, or bouroush’khatoun) was located on the southwestern spurs of the taurus ridge. we may assume that apart from domestic and external factors of military, political and economic character, environmental problems could also contribute have to the abrupt decline of the hittite kingdom in the 11th century b.c. a.2. armenian legends and myths of the war of tigran and azhdahak and of the birth of vahagn movses khorenatsi, an armenian historian and chronicler of the 5th century, provides the following report on an earthquake that caused landslip from the ararat mountain slope in the 6th century b.c. 804 arkady karakhanian and yelena abgaryan «tigran settled anouish, the above-mentioned spouse (of azhdahak) and her sons in a safe place: (remains of) the slide from the great mount said to happen from a terrible earthquake (start to) stretch from there. this is recounted by travellers who, by the order of ptolemaeus, traveled long to measure in stadia not only the inhabited areas, but partly seas and uninhabited lands, from the torrid belt to the cimmerion…» many scholars have analyzed this report to define the time and the place of the mentioned earthquake. kovalevsky and markov (1903), kondorskaya and shebalin (1977), karapetian (1991), and pirouzian et al., (1997) all record two earthquakes on mount ararat: one with the date of 550 b.c. and the other with the date of 139 a.d. (table i), although none of these authors presents any supporting arguments. guidoboni and traina (1995) believe that the above report of movses khorenatsi stems from the tradition in the myth of argonauts telling that jason opened up a passage from the arax river to the caspian sea (sources in jessen and tatevossian, 1916, col. 769) and therefore should not be regarded trustworthy evidence. haroutunian et al., (1997), shebalin, (1997) and berberian (1997) record one earthquake only, suggesting that the earthquake mentioned by khorenatsi should be related not to 550 b.c., but to 139 a.d. in the discussed report, movses khorenatsi gives two indications of the time, and one indication of the place of the earthquake. we can with confidence identify the place of the earthquake as mount ararat, considering that since the time when ararat was within the ancient armenia till today, armenians have called ararat the great mount. other reports khorenatsi presented in relation to the events of 550 b.c. also help to locate the event. khorenatsi mentions that tigran settled azhdahak’s spouse anouish and many other captives «behind the eastern ridge of the great mount, up to the limits of gokhtn, namely in tambat, oskiol, and dazhagouik, as well as in the province of vranjounik near the bank of the river (arax), in front of the fortress of nakhchavana» (p. 44, movses khorenatsi, 1893). nakhchavana was an ancient armenian town: today it is a city of nakhichevan (azerbaijan) in the ararat valley, near the mount of ararat (fig. 2). the areas listed by khorenatsi are also located in the ararat valley. so, the great mount is the ararat volcano. by khorenatsi’s reference to the «(remains of) the slide from the great mount stretching from there» we can define the place where anouish was settled more accurately, because the northern slope of ararat, in the gorge of akory, comprises deposits of huge debris flows triggered by strong earthquakes, the latest of which occurred in 1840. definition of the date of this earthquake is a much more difficult task. dragon azhdahak in the armenian mythology corresponds to the iranian azhi-dakhaka (dakhaka dragon) and is equivalent to astiag, the name of mydian king (at rule since 585 till 550 b.c.), who was dethroned by persian king cyrus the 2nd (559-530 b.c.). according to the armenian tradition cited by khorenatsi, the mydian kingdom and the king astiag (azhdahak) were defeated by cyrus the 2nd and his ally, armenian king tigran the 1st yervandian in 550 b.c. in his cyropedia, greek historian xenophon (6th-5th century b.c.) confirms this relation, telling that cyrus and the armenian king tigran were friends and allies in the military campaigns of cyrus (emin, 1893; sarkissyan, 1990). ptolemaeus claudius worked in alexandria in 127-151 a.d. and died in 168 a.d. therefore, the two indications of earthquake date in the citation from movses khorenatsi are separated by a period of almost 700 years: – anouish, azhdahak’s spouse, was settled at the edge of the slide approximately in 550 b.c. – the travellers sent by ptolemaeus claudius could visit armenia between 127 and 151 a.d., i.e., within the period of ptolemaeus’ activities. studies of an earthquake in 550 b.c. and/or 139 a.d. have been limited to the analysis of the passage from the text of khorenatsi presented above. our analysis of the whole text, including the story of tigran’s victory over king azhdahak in khorenatsi’s history of armenia, allows new interpretation of this information. immediately next to the description of the place where azhdahak’s spouse was settled, khorenatsi mentions legends about dragons living on the slopes of azatn massis (an armenian name of ararat) (p. 45, movses khorenatsi, 1893). then khorenatsi tells that before his battle with tigran, azhdahak had a dream of himself standing beside a high mountain with a snow-covered summit in the land of haikides (armenians). in his dream, azhdahak saw a purple-dressed woman standing on the summit and giving birth to three sons. one of her sons, riding a lion, rushed to the sunset, the other son on a leopard went 805 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) to the midnight, and the third one on a huge dragon pounced upon azhdahak and his country, destroying them and shedding a sea of blood (p. 40, movses khorenatsi, 1893). the legend of the birth of three sons to tigran the 1st yervandian, azhdahak’s winner, is of particular interest. according to this legend, vahagn, the third son, was an old armenian dragon-fighter divinity. the legend about vahagn’s birth reads: «the earth and the heaven were in travail; and the purple sea was in travail; the sea gave birth to a reddish reed; smoke was coming out of the neck of the reed; flame was coming out of the neck of the reed; a young man was coming out of the flame fire was his hair; flame was his beard, and his eyes were /as two/ suns.» the legend about the prophetic dream of azhdahak is almost identical to the one of the birth of three sons to tigran the 1st. in poetical manner, both legends link the third son to a dragon he defeats. in the armenian folklore, the name of azhdahak is usually identified with a dragon. many ancient asian legends refer to volcanic lava flows, or earthquake-triggered landslides, as to dragons (khromovskikh, 1984; ferraud, 1994). for instance, the ancient iranian epic shahnameh of ferdausi-tusi tells how khrouden (feridoun), a legal successor to the throne, dethrones the evil king zohak (a dragon-equivalent name) and imprisons him shackled in a deep sulfuric pit (crater) of the active damavand volcano in northern iran (sarkissian, 1990; berberian, 1994). a direct identification of the azhdahak dragon, confined in a mountain, with the eruption of sipan volcano (displaying solfataric activity recently) can be found in a manuscript of muslim traveller eulia chelebi dated to the middle ages (haroutunian, 2001): «in the north of lake van, the seven-headed dragon azhdahak is confined in the interior of mount sipan and his voice is heard once each 40-50 years, and for 5-10 days each 70 80 years he shows his tail from the mount sipan». the principal sanctuary of vahagn divinity has been located near lake van (turkey), on the mount of nemrout, which is also an active volcano. berberian (1994) considers the epic of vahagn as an argument to suggest possible volcanic eruption in armenia. the discussed legends provide consistent poetical descriptions of an event that could be an earthquake (the earth and the heaven in travail), volcanic eruption of lava, or pyroclastic flow (red reed with flame and smoke rising out of it, etc.) or lahar-like landslide (a dragon rushing down, dragons living on ararat) taking place on ararat. in the meantime, these legends indirectly support a historical tradition about an earthquake and slide from ararat mentioned by khorenatsi in his report about the place where azhdahak’s spouse anouish was settled. the legends of the birth of vahagn, azhdahak’s dream, and captivity of artavazd were for the first time collected and recorded by khorenatsi in the 5th century a.d. cited only by khorenatsi, the vahagn’s birth legend can be older than the events khorenatsi related to 550 b.c. (petrosyan, 2002). although apart from the information provided by khorenatsi in relation to the names of azhdahak, tigran and ptolemaeus, we have no other data to define when and where these legends and traditions of an earthquake or volcanic eruption were created, it is important to consider that tigran the 1st yervandian and azhdahak (astiag mydian) are all historical personages of the first half of the 6th century b.c. considering the traditions and legends cited by khorenatsi, we might with certain caution suggest that they refer to real events earthquake, volcanic eruption, and lahar-landslide, all occurring on ararat during the opposition between tigran and azhdahak in the first half of the 6th century b.c. (about 550 b.c.). historical, archaeological and geological data attest to a few historical eruptions of the ararat volcano (karakhanian et al., 2002). the latest happened in 1840 and was triggered by a 806 arkady karakhanian and yelena abgaryan very strong m = 7.4 earthquake (ambraseys and melville, 1982). the earthquake was accompanied by volcanic eruption and large lahar-type landslide in the akory gorge. the frontal part of sediments in the gorge is about the place, where, according to the legend, azhdahak’s spouse anouish could be settled (karakhanian et al., 2002). the first part of the khorenatsìs report directly links the earthquake and the slide with the names of tigran, azhdahak, and his wife anouish, i.e., with historical personages of the first half of the 6th century b.c. (about 550 b.c.). the second part of the khorenatsìs report, however, does not contain any indication that the earthquake described by the travelers sent by ptolemaeus happened the time they visited armenia (between 127 and 151 a.d.). therefore, there are no serious grounds to assume that the earthquake reported by khorenatsi occurred not in 550 b.c., but rather in the 2nd century a.d. (between 127-151 b.c.) as earlier suggested by haroutunian et al. (1997) and shebalin and tatevossian (1997). nevertheless, the case of two separate events, although much less supported by khorenatsi’s reports, shall not be ruled out completely. appendix b. the earthquake of july 25, 454 a.d. in eastern iran, near the town of nishapur (the land of apar, town of nishapoukh). «the same day (the 27th day of the month of hrotic’ the 25th of july) in the evening, a strong earthquake suddenly happened in this place. a thunder-like noise and terrible voices were heard from the depth, masses of gathered clouds covered the earth; thundering and lightning shook the mountains. a column of light descended to the ground as a rainbow and wrapped around the bodies of martyrs. the guard was thrown down to the ground half-alive, no one knowing where his neighbor was: they went round and could not stand on their feet firmly, because the shaking caused by the strong earthquake was throwing them up and down to the ground» (lazar parpetsi, 1904). guidoboni and traina (1995) consider the above account of the armenian historian lazar parpetsi, who lived in late 5th-early 6th century a.d., in the context of historical seismicity. the historian includes his report to the story about the martyrdom of saint «associates of ghevond», who were armenian grandees and monks captured by persian king shapouh during the war of 450-453 a.d. after the captive armenian priests and princes refused to adopt zoroastrism, they were taken to the land of apar in the northeast of iran and kept in prison in the region of the town nishapouh (nishapur). later, some of them were killed, and some died in exile. by the time the earthquake happened, persian warders had guarded the captives. lazar parpetsi also adds that «the shaking continued one day and two nights». analyzing the report of lazar parpetsi, guidoboni and traina (1995) assume a link between the earthquake of 454 and the accounts about an earthquake in 461 in the locality of apahunik, contained in 13th century a.d. chronicles of hovhannes avageratsi (1951): «there was an earthquake at apahunik, and it destroyed many villages». assuming that the historians could confuse similar place-names of apar (lazar parpetsi) and apahunik (hovhannes avagheratsi), guidoboni and traina (1995) suggest that there could be a single earthquake. although with some reservations, guidoboni and traina (1995) identify the earthquake of 454 in apar with the earthquake in apahunik in 461 and include the latter in their earthquake catalogue. the province of apahunik was located to the west of lake van, in the middle course of the aratsani (mourat) river (figs. 2 and b.1). as this area was familiar to all armenian historians, it is difficult to suggest that they could confuse it with the land of apar, also a well-known locality in the east of iran (fig. b.1), close to the lut desert boundary. yeghishe, an armenian historian of the 5th century a.d., contemporary and eyewitness of the rebellion of armenians against persians in 451, adds information in support of locating the july 25 earthquake of 807 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) fig. b.1. historical geographical place-names related to the earthquake of 454 a.d. 454 in the vicinity of nishapur in the land of apar, according to lazar parpetsi. yeghishe (1971) wrote: «by the midnight, horrible sounds were heard again, and thunder-like rolls (that reached us) from beneath resembled earthquake rumble, the earth trembled under their feet, and sparkles from the glare of swords were flying around them». guidoboni and traina (1995) notice that yeghishe did not mention the place of the earthquake of 454, but we believe he actually did. in chapter 7 of his history of armenia, yeghishe clearly indicates the place, where armenian princes and six captive priests were kept guarded: «so, in the sixteenth year of the rule of the same king, he (the king) again went to the land of koushans for martial affairs. coming out of vrkan and entering the land of apar, he ordered to hold the nakharars /princes/ and priests in the same bonds – in the citadel of the town of nishapouh» (verse 142, p. 127, yeghishe, 1971). the king mentioned in the account was yezdegerd the 2nd sassanid, who reigned since 438 till 457 a.d. therefore, the sixteenth year of his rule corresponds to 454 a.d., which confirms the date of the earthquake. the land of koushans corresponds to the koushan kingdom (1st century b.c.-5th century a.d.), which covered the greater part of the middle asia, northern afghanistan, and northern india. in the 4th-5th centuries a.d., the sassanid iran led constant wars with koushanians. vrkan was an area to the southeast of the caspian sea (nowadays torkan, a city in iran). the land of apar covered an area in the northeast of iran: later it was named khorassan and nishapouh was its principal city. today, nishapouh corresponds to the city of nishapur in the northeast of iran, on the border of the lut desert. then yeghishe mentions: «having arrived at the (designated) deserted place, which appeared totally devoid of green vegetation and so wild and stony, that the three nakharars (princes) could not find a place even to take a seat ...» (verse 160, p. 141, yeghishe, 1971). «and these six died a saint and desired death on the 25th day of the month of hrotic’ (july), in the great desert of the land of apar near the town of nishapouh (nishapur)» (verse 182, p. 158, yeghishe, 1971). 808 arkady karakhanian and yelena abgaryan references abich, g. (1862a): distribution map for the ararat earthquake, notes of the caucasian division of the russian imperial geographic society, tbilissi, vol. 5. abich, g. (1862b): earthquakes in shemakha and erzroum of may 1859, notes of the caucasian division of the russian imperial geographic society, tbilissi, vol. 5. abich, g. (1882): geologie des armenischen hohlandes westhalfte, geologische forschungen in den kaukasichen ländern, wien, 479. acts of the caucasian archeographic commission meeting (1873), in archive of the caucasus governor’s principal administration, tbilissi, vol. 5. alishan, g. (1881): shirak, venice. alishan, g. (1890): airarat, venice. ambraseys, n.n. (1975): studies in historical seismicity and tectonics, geodynamics today (royal soc. publ., london), 7-16. ambraseys, n.n. (1988): engineering seismology, earthquake eng. struct. dyn, 17, 1-105. ambraseys, n.n. and r. adams (1989): long-term seismicity of north armenia, eos, trans. am. geophys. un., 70, 145-154. ambraseys, n.n. and j.a. jackson (1998): faulting associated with historical and recent earthquakes in the eastern mediterranean region, geophys. j. int., 133 (2), 390-406. ambraseys, n.n. and c.p. melville (1982): a history of persian earthquakes (cambridge university press, cambridge), pp. 219. ancient east history (1988): the ancient east history: origin of the earliest class societies and the early sources of slave-owning system civilizations, 2. front asia, egypt, edited by g.m. bongard-levina (principal editorial board of oriental literature, moscow), 619 (in russian). armenie (1996): tresors de l’armenie ancienne (des origines au ive siecle) sous la direction de jacques santrot, paris, pp. 288. avagyan, a., a. karakhanian and h. philip (2003): une éruption volcanique gravée dans le roc, pour la science, 305, 13-14. berberian, m. (1994): natural hazards and the first catalogue of iran. historical hazards in iran prior to 1900, int. inst. earthquake eng. seismol., 1, 603. berberian, m. (1997): seismic sources of the trans-caucasian historical earthquakes, in historical and prehistorical earthquakes in the caucasus, edited by d. giardini and s. balassanian, ilp publ. 333, nato asi ser. 28 (kluwer academic publishers), 233-311. bommer, j.j. and n.n. ambraseys, (1989): the spitak (armenia, ussr) earthquake of 7 december, 1988: a preliminary engineering seismology report, esee/eftu research rep. 89/1, eng. seismol., 85. borissov, b.a. (1982): about the importance of evidence of past earthquakes for seismic hazard assessment in the greater caucasus, bull. moip, geol. sec., 57, 1. borissov, b.a., g.i. reisner and v.i. sholpo (1975): identification of earthquake-prone zones in the alpine folding area (publishing house nauka, moscow), p. 138. byus, e.n. (1948): seismic conditions in the trans-caucasus. part i: chronology of earthquakes in the transcaucasus, tbilissi (in russian). chronicle of amiras yerzncatsi (1988): chronicle of amiras yerzncatsi, 17th century, historical journal of the armenian academy of sciences, yerevan, 4, 189-208. dewey, j.f., m.r. hempton, w.s.f. kidd, f. saroglu and a.m.c. sengor (1986): shortening of continental lithosphere: the neotectonics of eastern anatolia a young collision zone, in collision tectonics, edited by m.p coward and a.c. riea, geol. soc. london, spec. publ. 19, 3-36. emin, n.o. (1893): comments to the history of armenia edited by movses khorenatsi (lazarev’s institute of oriental languages, moscow), 412 (in russian). ergin, k.u., u. guclu and z. uz (1967): a catalogue of earthquakes for turkey and surrounding area (11 a.d. to 1964 a.d.), istanbul. ferraud, j. (1994): les volcans actifs de turquie: guide geologique et itineraries d’excursions, memoire de l’association volcanologique européenne, 2, 140 (in french). gorshkov, g.p. (1932): the zanghezour seismic expedition, in the volume on the zanghezour seismic expedition, edited by n.v. raiko, vestn. akad nauk sssr, (special issue of the ussr as expedition of 1931, leningrad) (in russian). gorshkov, g.p. (1933): geological settings of the zanghezour earthquake of april 27, 1931, tr. seismol. instit. akad nauk sssr, moscow-leningrad, 31 (in russian). guidoboni, e. and g. traina (1995): a new catalogue of earthquakes in the historical armenian area from antiquity to the 12th century, ann. geofis., xxxviii (1), 85-147. the place of the earthquake was repeatedly and clearly described by the historian yeghishe, who was contemporary to the events. besides, the full-flow river aratsani (mourat) and numerous springs in the mountainous landscape of the province of apahunik could hardly be confused with the description of the lut desert we read in yeghishe’s account. therefore, there are no grounds to suggest that the earthquake of 454 a.d. reported by parpetsi and yeghishe, the 5th century historians, and the earthquake of 461 a.d. reported by avageratsi, the 13th century historian, should be related to the single seismic event in 461 that happened at apahunik, west of lake van. the earthquake of july 25 in 454 occurred in the east of iran, in the region of the city of nishapur, most probably within the zone of the nishapur-binaloudi active fault. 809 evidence of historical seismicity and volcanism in the armenian highland (from armenian and other sources) guidoboni, e. et al. 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(1964): the earthquakes in the armenan upland and the surrounding region, yerevan, hayastan, pp. 342 (in armenian). taymaz, t., h. eyidogan and j. jackson (1991): source parameters of large earthquakes in the east-anatolian fault zone (turkey), geophys. j. int., 106, 537-550. yeghishe (1971): about vartan and the armenian war (translated from old armenian by i.a. orbeli), yerevan (in russian). zeitounian, a.s. (1991): earthquakes in armenia and neighbour countries (historical evidence from the ancient time till today), p. 383 (in russian). l a s t a z i o n e s i s m i c a d i m e s s i n a antonino girlanda uno degli scopi principali per cui fu istituito, verso il 1876, l'osservatorio di messina fu quello delle osservazioni sismiche. l'osservatorio, che in un primo tempo fu aggregato all'istituto nautico, ebbe inizialmente soltanto la possibilità di eseguire la determinazione astronomica del tempo, le ordinarie osservazioni meteorologiche e la regolazione dei cronometri delle numerose navi che toccavano il porto di messina. nel 1896 il comune, di messina, riconosciuta l'insufficienza e l'irrazionale ubicazione del locale fino allora destinato ad osservatorio, intraprese la costruzione di un nuovo edilizio sulla collina dell'andria, sorgente alla periferia della città, in località allora riconosciuta come la più idonea alle osservazioni geofisiche in genere, e a quelle sismiche in particolare. la costruzione ebbe termine nel 1902 e, l'anno successivo, il consiglio provinciale e il comune di messina, su proposta della facoltà di scienze, deliberarono di farne la consegna all'università e di affidare la direzione del nuovo osservatorio al professore titolare di fisica terrestre, l a cui cattedra era in corso di istituzione. nel 1904 l'osservatorio di messina divenne così l'istituto di fisica terrestre e di meteorologia dell'università. gli strumenti destinati alle prime osservazioni sismiche furono alcuni « sismoscopi avvisatori » di vario tipo e un sismografo a due componenti, costruito su un modello di agamennone. questo sismografo, che con i sismoscopi era stato fornito dall'ufficio di meteorologia e geodinamica, era costituito da un pendolo della lunghezza di 15 metri, con una massa di 300 k g ; la carta si svolgeva con una velocità di circa 40 centimetri all'ora e il tempo veniva segnato ogni mezz'ora. sin d'allora si era ravvisata l a necessità di attrezzare l'istituto di strumenti più sensibili e completi e, in seguito al grave terremoto, che l'8 settembre 1905 colpì duramente la calabria e parte della sicilia, venne accelerato l'impianto di un microsismografo vicentini, che iniziò i l suo regolare funzionamento il 1° gennaio 1906. verso la fine del 1908 era in via di realizzazione un notevole in54 a n t o n i n o « i r l a n d a • xii o • n r r n r z i r z r h e m 0 1 e v) i 1 b ^ b 0 g 1 b f t i i i l i f i g . 1 l a s t a z i o n e s i s m i c a di m e s s i n a 4 9 cremento dell'attrezzatura sismica con l'impianto di due pendoli orizzontali del mainka, costruiti a messina, e di un wiechert orizzontale da 200 kg, acquistato a gòttingen da splindler e hoyer. il grande terremoto, che all'alba del 28 dicembre di quell'anno distrusse quasi totalmente messina, paralizzò tutte le attività: i due mainka furono distrutti nel crollo di un'ala dell'edilìzio e il vicentini gravemente danfig. 2 neggiato. ii disastro che commosse l'opinione pubblica mondiale, destò, come avviene generalmente in simili occasioni, un grande interesse nei confronti dei problemi riguardanti la sismologia e numerosi furono i programmi e le proposte per l'organizzazione di un servizio sismico efficiente. così, la commissione reale, incaricata di designare le aree più adatte per la ricostruzione dei fabbricati, nel presentare il rendiconto dei lavori più urgenti da essa eseguiti nel 1° semestre del 1909. aveva formulalo una serie di proposte complementari tra le quali figurava la seguente: che in calabria sorgessero delle stazioni sismiche secondarie funzionanti sotto la direzione dell'osservatorio di messina. così, durante la terza riunione della commissione permanente della associazione sismologica internazionale, tenutasi a zermatt nel 1909, il delegato del governo italiano, prof. luigi palazzo, avanzò la propo5 0 a n t o n i n o « i r l a n d a sta di istituire, ira gli osservatori di messina, catania e gli osservatori in calabria, un servizio di u triangolazione sismica, con collegamento telegrcfico, per lo studio delle velocità di propagazione delle onde sismiche, conformemente a quanto si era già fatto in giappone e conformemente alle idee che in quel tempo andavano sviluppandosi per opera di kòvesligety. le vicende parlamentari del tempo e il susseguirsi di difficoltà di vario genere non permisero, né allora né in seguito, l'attuazione di questi brillanti disegni. f i g . 3 le osservazioni sismiche furono continuate con l'ausilio del sismografo vicentini, rapidamente riparato e collocato nel sotterraneo dell'istituto, che era rimasto quasi illeso. nel 1912 l'istituto fu dotato di un pendolo orizzontale conrad e (li due pendoli orizzonlali di alfani a registrazione meccanica. questa attrezzatura non fu sostanzialmente modificata fino al 1947, anno in cui l'istituto nazionale di geofisica, ravvisando la necessità di disporre in messina di un osservatorio facente parte della rete geofisica italiana, stipulava con l'università una convenzione in seguito alla quale veniva impiantata la nuova stazione sismica di messina. in un primo tempo fu impiantato un sismografo verticale di tipo wiechert avente la massa di 80 kg, costruito nelle officine dell'i.n.g. 9. ' 5 2 antonino cirlanda e dotato del nuovo tipo di smorzatore ad aria adottato dall'istituto nazionale di geofisica per i sismografi dello stesso tipo funzionanti negli osservatori della rete sismica italiana ( ' ) ; fu, inoltre, messo in efficienza c u r v e d ' i n g r a n d i m e n t o d i n a m i c o d e i s i s m o g r a f i d e t t a s t a z i o n e s i s m i c a di m e s s i n a fig. 6 il vecchio sismografo orizzontale wiechert con la sostituzione completa del meccanismo di registrazione e smorzamento. in seguito l'istituto geofisico ebbe assegnato, sui fondi del piano e.r.p., un sismografo verticale sprengnether a breve periodo, corredato di un galvanometro e di due registratori fotografici. tale sismografo, com'è noto, è essenzialmente costituito da un pendolo verticale di tipo galitzin. l'asse di rotazione è determinato da due coppie di laminette d'acciaio incrociate e la massa del sistema oscillante è distribuita in modo che la molla di sospensione sopporti il peso di poche diecine di grammi; questa circostanza permette l'uso di una molla di k antonin» cirlanda l a s t a z i o n e s i s m i c a 1)1 m e s s i n a 5 5 sospensione relativamente debole e, conseguentemente rende rapida ed agevole la regolazione e minimo l'effetto di temperatura sulle costanti elastiche della molla stessa. la bobina oscilla nel campo magnetico, non regolabile, generato da un magnete permanente ad elevata magnetizzazione, fissato alla base dello strumento. l'amplificazione, qualora se ne ravvisi la necessità, può essere ridotta per mezzo di un opportuno shunt magnetico. lo smorzamento è ottenuto per mezzo di un'aletta di rame oscillante nel campo magnetico regolabile di un secondo magnete anch'esso fissato alla base. l'istituto nazionale di geofisica ha completato, infine, la attrezzatura strumentale con l'invio di due sismografi orizzontali galitzin del tipo i.n.g. per avere la possibilità di impiegare i due registratori sprengnether, non si è potuto adottare il sistema speciale di registrazione introdotto nella stazione sismica di roma ( 2 ) che permette il funzionamento dei registratori nello stesso ambiente in cui sono collocati i sismografi. si è dovuto, pertanto, allestire un locale, adiacente alla sala dei sismografi, per adibirlo a sala di registrazione. per registrare le due componenti orizzontali galitzin su un unico foglio, è stato modificato uno dei due registratori fotografici sprengnether, dimezzando la velocità di rotazione del tamburo e munendolo di un secondo riflettore. le coordinate geografiche della stazione sono le seguenti: latitudine = 38" 11',8 n, longitudine = 15" 33',3 e, a l t i t u d i n e : ni. 15 sul livello del mare. il terreno su cui poggia la stazione è costituito da alluvione rossa quaternaria a facies sabbiosa, ghiaiosa, leggermente argillosa; l'invasamento è costituito da uno zoccolo scistoso cristallino. i locali destinati alle osservazioni sismiche e ai servizi, come si vede dalla planimetria riportata nella fig. 1, sono così distribuiti: a sala dei sismografi a registrazione meccanica, nella quale in a (fig. 2| è collocato il sismografo wiechert verticale, in b il sismografo wiechert orizzontale, in c il pendolo regolatore riefler per i segnali del tempo, in ci un apparecchio radio per la recezione dei segnali o r a r i ; b saletta per gli accumulatori; c corridoio di accesso ai locali destinati ai sismografi ad amplificazione elettromagnetica; d sala dei sismografi ad amplificazione elettromagnetica, nella 5 6 a n t o n i n o « i r l a n d a f i z . 8 1) quale in e, su un unico basamento, sono collocati i due sismografi galitzin (fig. 3) e il sismografo sprengnether (fig. 4); e saletta per la registrazione fotografica, nella quale in /, su apposita mensola, sono collocati i galvanonietri e in g, su appositi supporti, i due registratori fotografici (fig. 5); m r l i w w m i i m w i m m s w m p e s i i 4>tf . fig. 8 c f camera oscura per la preparazione delle carte dei sismografi a registrazione fotografica; g saletta dei clinometri il cui impianto è di prossima realizzazione; h camera oscura per lo sviluppo e il fissaggio delle carte fotografiche. i valori delle costanti strumentali, quali risultano nella ultima determinazione eseguita nell'agosto 1952, sono riportati nelle tabelle seguenti : c o m p o n e n t e t e t s v m k 11 a 1 i) g a l i t z i n 1 . n. g . n s 22",1 22»,9 8 0 0 e a 3 2 , 4 5 0 . 0 4 3 1 1 8 1 1 8 3 0 galit/in 1 . n. g . e w 20*. 7 19»,7 9 5 2 ™ 4 9 , 7 0 0 . 0 1 6 1 0 5 1 1 5 3 0 c o m p o n e n t e tg t s v m u k a l i) s p r e i i f i t i e t h e r v 1»,5 1«,5 3 . 9 0 0 0 , 0 0 0 8 . 0 0 0 6 0 la stazione s i s m i c a di m e s s i n a 58 a n t o n i n o « i r l a n d a c o m p o n e n t e to v„ s m o r z . h r/ts d w i e c h e r t 8 0 k g . v w i e c h e r t 2 0 0 k g . w i e c h e r t • n s 3»,7 1 2 1 , 8 a r i a 0 , 3 3 2 0 , 0 6 2 3 0 e w 3»,8 1 1 0 , 7 aria 0 , 2 7 0 0 , 0 5 9 3 0 v 4",3 1 1 2 , 5 aria 0 , 2 4 9 0 , 0 7 2 3 0 le corrispondenti curve di ingrandimento dinamico sono riportate nella fig. 6. la registrazione sistematica ha avuto inizio nell'agosto del 1947, in un primo tempo coi due soli sismografi wiechert. nel febbraio 1951 ha avuto inizio il regolare funzionamento dello sprengnether e, infine, nel dicembre dello stesso anno la stazione ha inizialo la sistematica registrazione in tutta la sua attuale efficienza. i risultati dello spoglio delle registrazioni vengono mensilmente pubblicati nel bollettino dell'istituto nazionale di geofisica. in tutto l'anno 1952 sono stati registrati complessivamente 673 terremoti, di c u i : 401 lontanissimi (uì, 28 lontani (ri, 138 vicini (vi, 87 locali (d), 19 non caratterizzabili. nelle figg. 7, 8 sono riportati alcuni esempi di registrazioni ottenute con i diversi tipi di strumenti. nella fig. 9 è riportata una carta di linee equidistanti ed isoazimutali, allestita allo scopo di determinare rapidamente gli epicentri provvisori usufruendo dei dati della sola stazione di messina, qualoi'a il carattere delle registrazioni ottenute lo consenta. la posizione della stazione sismica di messina, prossima a zone fortemente anomale dal punto di vista geofisico — in particolare sismico —, la rende di eccezionale interesse per lo studio dei fenomeni sismici e delle caratteristiche fisiche della crosta terrestre. è quindi giustificato il proposito di farne un caposaldo della rete sismica italiana. messina osservatorio dell'i.wg. — dicembre 1952. si danno ampie notizie sulla nuova stazione sismica di messina, facente parte della rete sismologica dell'istituto nazionale di geofisica. gli strumenti a registrazione meccanica (wiechert 200, wiechert 80 kg) e quelli a registrazione fotografica (galitzin e sprengnether) riassunto i-a s t a z i o n e s i s m i c a di m e s s i n a 59 fanno ch'ila stazione sismica eli messina imo dei caposaldi della rete si sin o logica il ci li ana. su m mary complete informatimi is giveii 011 the new seismic station of messina taking pari in the seismological net of the instituto nazionale di geo fisica. the mechanical registration instruments (wiechert 200, wiechert 80 kg) ancl tliose for photographic registration (galitzin and sprengnether) malie the seismic: station of messina one of the most important of the italian seismological net. crupi_final:layout 6 the role of radiation damping in the modeling of repeated earthquake events paola crupi1, andrea bizzarri2,* 1 università degli studi di bari, dipartimento di scienze della terra e geoambientali, bari, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy annals of geophysics, 56, 1, 2013, r0111; doi:10.4401/ag-6200 abstract we have investigated the role of the radiation damping term (rdt) on repeated earthquake ruptures by modeling the faulting process through a single one-dimensional analog fault system governed by different constitutive laws. the rdt expresses the energy lost by the seismic waves. the rdt is inherently accounted for in more elaborated, fully dynamic models of extended fault, whereas it is neglected in one-dimensional fault models. in this study, we adopt various formulations of the laboratoryderived rate-dependent and state-dependent friction constitutive laws: the dieterich-ruina law, the ruina-dieterich law and the chester and higgs law. our numerical results clearly indicate that the rdt significantly affects the system dynamics. more specifically, the more the rdt is effective, the more frequent the slip failures are (with a cycle-time reduction of ca. 30%). we also show that inclusion of the rdt tends to promote smaller but more frequent earthquake instabilities, irrespective of the choice of the governing law. our data shed light on the limitations implied by the conventional formulation of the equation of motion for the spring system, in which the energy radiation is ignored. 1. introduction one of the challenges of natural-hazard reduction and earthquake-risk mitigation is the need to understand the recurrence interval in seismic instability events. plate tectonics provide the theoretical background that is required to identify at least some of the specific seismic source regions where earthquakes have occurred in the past [allen 2007]. however, it is still difficult to predict large earthquakes, due to their complex occurrence patterns [abe and kato 2012]. scientists have been trying to understand the earthquake machine and the chemical and physical processes controlling it (as well as their complicated feedback and unknown parameters) from different points of view and with different approaches [panza et al. 2001, bizzarri 2011, panza et al. 2011, bizzarri 2012d, wyss et al. 2012]. the relationships between the frequencies of earthquakes and their different seismic moments – expressed in terms of the slip rate on a fault – is a possible approach to the study of the seismic activity that is likely to occur in a specific region [molnar 1979, allen 2007]. nevertheless, this assessment approach usually starts from the existing historical seismicity records, and many seismologists agree that the seismic records available are often not extensive enough to provide reliable assessment of earthquake frequency in a settled area [e.g., molnar 1979]. indeed, data collection ranging from 103–104 years might be required to determine, with reasonable confidence, a possible earthquake recurrence interval in regions with high seismic-motion variability [molnar 1979]. consequently, further methods of investigation are required to provide a broader insight into seismicity recurrence. this emphasizes the importance of numerical model simulations, which are a powerful tool to explore the complex physical conditions behind earthquake cycles and the occurrence of unstable slip episodes. in the ideal case where no remote events perturb the state of a given fault system, the elastic rebound theory [reid 1910] remains the basis of our understanding of the earthquake cycle, and this postulates that once the stress in a region exceeds some strength value, a rupture occurs. several physical models have been proposed to describe the properties of the earthquake cycle, among which there is the springslider system (namely, a harmonic oscillator), which can reproduce the seismic instability periodical patterns and the so-called stick-slip mechanism observed in laboratory experiments [bizzarri et al. 2011, and references therein]. in this study, we focus on the temporal evolution of the frictional properties of faults. more specifically, we present and discuss numerical experiments that are conducted using a mass-spring model, to explore how the radiation damping term (rdt) can affect the phenomenology of earthquake faulting and the seismic cycle of faults, through obeying different governing laws: i.e., the dieterich-ruina (dr) law [dieterich 1979]; the ruina-dieterich (rd) law [ruina 1983]; and the chester and higgs (ch) law [chester and higgs 1992]. article history received september 5, 2012; accepted january 28, 2013. subject classification: seismic cycle, earthquake recurrence, radiation damping, spring-slider model, fault rheology, computational seismology. r0111 research articles from a physical point of view, the radiation damping approximation [rice 1993] mimics the energy lost due to seismic-wave propagation. while this factor is inherently assumed in more elaborate fault models (such as continuum, three-dimensional [3-d] fault models), it is frequently ignored in the simplistic one-degree-of-freedom mass-spring dashpot model (to date, relevant exceptions are xu and knopoff [1994], beeler [2001, 2006], and bizzarri [2012a]). furthermore, we have seen that its inclusion within spring-slider dynamics leads to interesting results, which are widely illustrated within the next applicative sections. the novelty of the present study is that it is a systematic study of the effects on the analog fault dynamics that result from the introduction of the radiation damping factor, a concept as yet unexplored in previous studies (xu and knopoff [1994] and beeler [2001] did not consider rate and state friction, while bizzarri [2012a] considered only one single configuration with the rd law and did not consider the effects of the rdt in the context of earthquake recurrence). 2. modeling of the seismic cycle with the mass-spring system 2.1. the spring-slider model in this study, we consider the widely used 1-d fault model known as the spring-slider system (or the mass-spring system; see figure 1 in belardinelli et al. [2003]), in which a fault is modeled as a point of mass m (per unit surface area) that slides over a plane against a shear stress x and subject to a normal load (e.g., see rice and tse [1986], among many others). the system is loaded by a remote velocity vload that acts through the end of the spring on the block, which physically represents the velocity of a tectonic plate acting on a potentially seismogenic structure that is to be modeled. for the sake of simplicity, we will assume here that is constant over the whole faulting process; the possible variations in effective normal stress and other seismological implications have been quantitatively discussed elsewhere [bizzarri and cocco 2006a, bizzarri and cocco 2006b, bizzarri 2010b, bizzarri 2012c]. the spring-slider system equation of motion is a firstorder partial differential equation that describes a harmonic oscillator (see also bizzarri [2012d]): (1) where: (2) with: (3) in equations (1), (2) and (3) (see also equation 2 in beeler [2001], or equation 2 in beeler et al. [2002]), the overdot indicates the time derivative, u is the block displacement, v is its velocity, and t is the time. the fault stiffness k, which mimics the interactions with the elastic medium surrounding the fault, can be associated with the static stress drop and the total slip that develops during a failure event [walsh 1971]. the shear stress x expresses the fault governing law, as described in section 2.2. the last term in equation (3) defines the so-called rdt, in which constant c depends on the parameters of the medium where the fault is embedded [rice 1993]: (4) where g is the elastic medium rigidity, and vs is the s-wave velocity away from the fault plane. the rdt simulates the energy that is lost during the sliding phase, in terms of propagating seismic waves, and moreover, it reduces the shear stress on the fault plane by an amount that is directly proportional to the slip velocity [rice 1993, kato and tullis 2001, see also brune 1970]. in other words, the rdt defines how the local fault impedance relates the slip velocities to the stress, as a (1-d) strain wave propagates into the host rock [rubin and ampuero 2005]. indeed, equation (3) is a proxy of the true behavior of an extended fault that is embedded in a continuous medium, and it is deemed valid in the case of a fault with homogeneous properties. in the case of the rice [1993] simulation of a continuum medium with rate-dependent and state-dependent friction, is it possible to either perform a fully dynamic simulation or use the radiation damping approximation. without one or the other, the fault-slip velocities with rate-weakening are unbounded, and the model will blow up. on the contrary, such bounding is not needed for a spring-slider model that accounts for inertia; nonetheless, it allows for a more earthquake-like analog model. indeed, the use of the rdt in the slider-block equation is just a way to produce a simulation that has radiated energy loss. if we perform a spring-slider simulation with inertia alone, no radiation effects will be observed, and a complete overshoot will also result (by overshoot, we refer to the further traction drop that occurs in the decelerating phase of the system, during which the block decelerates due to the spring compression; see rice and tse [1986]). this behavior, however, is not consistent with natural earthquakes, or likely to occur with laboratory stick-slip experiments (no radiated energy is the complete overshoot condition). moreover, we also emphasize that the use of the rdt with inertia does not duplicate the rate dependence contained in the rate and state laws, as the explicit dependence of the velocity in the expression of frictional resistance and in the rdt has the opposite sign. finally, we mention that if, on the other hand, a numerical simulation of a slider block with radiation damping and no inertia is performed (as in segall [2010]), the result is zero overshoot. while this is a crupi and bizzarri 2 n effv n effv ,u f u t/o ^ h fand , ,u f u v m v u v t1/ / ^ h; ;e e and , , .v u f u v t kv t ku t cv tload// x-o ^ ^ ^h h h c v g 2 s / 3 good analog model of the orowan condition [orowan 1960] (see also gibowicz [1998]), which has often been used by kanamori to analyze earthquakes, it is generally still not consistent with natural earthquakes. the energy lost in the form of propagating seismic waves represented by the rdt is essentially assumed to be due to planar waves [rice 1993]. the particular choice made in equation (4) then states that all radiation results from shear waves. in equation (1), u formally defines the system state at a generic time t, in the conventional phase space (u,v). despite its obvious limitations in reproducing a fault system, either owing to its own finite size or to its being a single-degree-offreedom dynamic system [gu et al. 1984, carlson et al. 1994, bizzarri et al. 2011, among others], the spring-mass model has provided crucial insight into repeated seismic events. 2.2. governing equations stick-slip motion results from the interactions between an elastic system and a frictionally slipping surface [brace and byerlee 1966, gu et al. 1984]. several attempts have been made to find a realistic and adequate expression of a faultgoverning law [e.g., dieterich 1978, dieterich 1979, ruina 1983, chester and higgs 1992]. despite the advances in laboratory settings, we are still far from the formulation of a universal constitutive model for real-world faults (see discussions in bizzarri and cocco [2006c] and bizzarri [2011]). a wide class of earthquake source studies has been based upon the rate-dependent and state-dependent (rs) friction laws, in which the shear stress x depends on the slip rate v and on a state variable w (see bizzarri [2011] for a detailed review). in the present study, we adopt the following formulations of the laboratory-derived rs friction law models: (5) (6) (7) where n * and v * are a reference friction coefficient and a reference velocity, respectively, a and b are experimental constitutive parameters that express the direct and evolution effects, respectively, of friction, and l is the characteristic distance for the state variable evolution. moreover, is the effective normal stress, r is the universal gas constant, qa and qb are apparent activation energies pertaining to the direct and evolution effects, respectively, t * is a reference temperature, and t is the temperature developed from frictional heating. this last quantity, t, is computed as follows [bizzarri et al. 2011]: (8) in equation (8), t0 is the initial uniform temperature (i.e., at t = 0), t is the cubic mass density of the rock, w is the specific heat, l is the thermal diffusivity of the continuous medium in which the fault is embedded, the apex m denotes the discrete time, and t(0) = 0 and t(n) = t. equations (6) and (7) are the rd and ch laws, respectively, that are derived from the dr law (equation 5), where the state variable physically represents the average contact time of the sliding surface micro contacts [bizzarri 2011]. it is clear from equations (5) to (7) that x, which appears in equation (3), can be written (with the standard abuse of notations) as x = x (v(t),w(t)). the initial state of the system is given by , and the corresponding shear stress is x0 = x ss(v0); i.e., the steady-state, which is achieved when w = 0 in equations (5) to (7). 2.3. simulation strategy in the present study, we numerically solve a system of differential equations that is composed of an equation of motion (equation 3) that is combined with one of the three constitutive laws (equations 5, 6 or 7). the system of equations is solved by adopting the runge-kutta temporal integration technique with adaptive step-size control [press et al. 1992]. in line with the simulation strategy widely used and described by boatwright and cocco [1996] and belardinelli et al. [2003], in the present study, the slider equation of motion (equation 3) is solved quasi-statically for relatively low sliding velocities, and dynamically for high sliding velocities. the threshold that marks the limit between these two sliding regimes is the critical velocity vc, the reference value of which is assumed to be 0.1 mm/s in the present study (according to belardinelli et al. [2003], and references therein). moreover, as previously discussed by weeks [1993], we assume that the steady-state friction becomes independent of v at sliding velocities that exceed the critical value vc; this entails considering a ln(vc/v*) instead of a ln(v/v*) at high sliding velocities (namely, for v > vc; see [weeks 1993, boatwright and cocco 1996, belardinelli et al. 2003]). this formulation is known as the frozen approximation of the rs friction laws. first, we simulate the spring-slider evolution by using all of the three constitutive laws (the dr, rd and ch laws) with and without the inclusion of the rdt in the equation of motion (equation 3). subsequently, we compare the numerical results, to highlight the influence potentially exerted radiation damping in 1–d v v vw ln lna v dt d l v b l n effn w + +) ) ) 1w = ` `j j8 b* v v vw ln ln ln a v b l dt d l v l v n effn w w + +) ) ) 1w = ` ` ` j j j 8 b* v v vw ln ln ln a v b l a r q t t dt d l v l v r q t t 1 1 1 1 a n eff b n w w + + + + ) ) ) ) ) w =` ` ` ` ` j j j j j ; ; e e z [ \ ]] ]] n effv t t v t t t t1 mm n m m m 0 1 1 tw rl x = + -= -^^ ^ ^ ^ hh h h h/ vu0 0 0 = u8 b by the radiation damping approximation on the seismic cycle evolution, and consequently, on the recurrence time in seismic instabilities that are governed by the different constitutive laws. next, we perform further simulations with the ch constitutive law only, and use table 1, configuration a by varying the vc, so as to focus on the role that their different magnitudes can have in seismic-cycle evolution. 3. numerical results 3.1. the rdt influence: a stable behavior irrespective of the choice of the governing law in this section, we aim to analyze the time evolution of a spring-slider system that obeys three different constitutive laws (the dr, rd and ch laws; see section 2.2). more specifically, earthquake recurrence tends to be markedly affected by the nonobvious choice of the fault constitutive law [e.g., bizzarri et al. 2011]. in the present study, we present and discuss several numerical results that illustrate a set of further effects of different radiation damping coefficient values, and of critical velocity (see table 3) on controlling the temporal evolution of repeated earthquake ruptures. the comparisons will initially be conducted by running simulations without and with the inclusion of the rdt, and through a selection of the initial set parameters from both configurations a and b, as reported in table 1. although both of these configurations are velocity-weakening regimes (which explains the difference b − a > 0), configuration a is more unstable than configuration b because b = 2a (this defines a strong seismic regime, as per the terminology of boatwright and cocco [1996]). the results are reported in figures 1 (the dr law), 2 (the rd law) and 3 (the ch law). more specifically, the red lines in figures 1-3 pertain to configuration a, and the blue lines refer to configuration b. crupi and bizzarri 4 parameter value configuration a configuration b model parameters loading velocity, vload 3.17 × 10 −10 m/s 3.17 × 10−10 m/s machine stiffness, k 10 mpa/m 10 mpa/m tectonic loading rate, t0 = kvload 3.17 × 10 −3 pa/s 3.17 × 10−3 pa/s period of the analog freely slipping system, ta.f. = 2π 5 s 5 s radiation damping constant, c (reference) 4.5 mpa s/m 4.5 mpa s/m critical velocity, vc 10 −4 m/s 10−4 m/s threshold velocity defining seismic instability, vl 10 −1 m/s 10−1 m/s fault constitutive parameters effective normal stress, 30 mpa 30 mpa initial slip velocity, v0 3.17 × 10 −10 m/s (= vload) 3.17 × 10 −10 m/s (= vload) initial shear stress, x0 16.8 mpa (= n* ) 16.8 mpa (= n* ) initial temperature, t0 210 °c 210 °c logarithmic direct effect parameter, a 0.008 0.012 evolution effect parameter, b 0.016 0.016 characteristic scale length, l 0.01 m 0.01 m reference friction coefficient value, n * 0.56 0.56 reference sliding velocity value, v * 3.17 × 10−10 m/s (= v0) 3.17 × 10 −10 m/s (= v0) reference temperature value, t * 210 °c (= t0) 210 °c (= t0) table 1. parameters adopted in the present study. both configurations are velocity weakening regimes, but configuration a (with b = 2a) represents a more unstable fault with respect to configuration b. next page. /top: figure 1. comparisons between the numerical simulations for the dr law (equation 5) without and with the rdt (thick and thin lines, respectively). (a) time evolution of displacement. (b) slip velocity history on a logarithmic scale. (c) time history of the temperature developed by frictional heating, computed as in equation (3) by bizzarri [2010c], which is known to represent an upper bounds of the frictional heating. dt represents the difference between the temperature computed at the present time step and the initial temperature, which in this case also equals the reference temperature (see table 1). (d) phase diagram; i.e., normalized stress versus dimensionless slip velocity. the quantity a is defined as a = a . the vertical black line represents ln(vc/v * ), which is the threshold between the quasi-static and the full dynamic regimes (see section 2.3 for numerical details). the parameters are listed in table 1. red curves, configuration a; blue curves, configuration b. /bottom: figure 2. as for figure 1, here related to the rd law (equation 6). m k n eff v n eff v n eff v n eff v 5 radiation damping in 1–d figure 1 (caption on previous page). figure 2 (caption on previous page). moreover, the thick lines refer to the simulations without the rdt, and the thin lines refer to the rdt-aided simulations. interestingly, if we consider the rdt simulations (figures 1-3, thin lines), we can appreciate that the inclusion of such an approximation in the equation of motion (equation 3) has several important effects on the evolution of the system (in what follows, we will focus on the dr law model, as the results discussed are qualitatively the same for the rd law and ch law simulations). as an example, for configurations a and b, the slip developed during each seismic instability event is smaller than the slip where the rdt approximation has not been accounted for (see figure 1a). similarly, the velocity peaks reach a lower value in the rdt case, compared to the reference case without rdt (see figure 1b). moreover, the temperatures that result from the frictional heating are lower with the adoption of the rdt term (see figure 1c). figure 1d reports the phase diagram, which illustrates that the dynamic stress drop, which occurs within the close-to-failure accelerating phase, is the same in both cases, so without and with the rdt (figure 1d, thick and thin lines, respectively). on the contrary, the dynamic overshoot crupi and bizzarri 6 governing law recurrence time configuration a configuration b without rdt (years) with rdt (years) rdt difference (%) without rdt (years) with rdt (years) rdt difference (%) dieterich-ruina (figure 1) 170.1 118.6 30.3 128.8 90.3 29.9 ruina-dieterich (figure 2) 156.3 108.3 30.7 117.8 83.3 29.3 chester and higgs (figure 3) 73.7 54.02 26.7 78.05 57.01 26.9 figure 3. as for figure 1, here related to the ch law (equation 7). table 2. recurrence times for the different governing laws and the two configurations listed in table 1. the last row in the table reports the cycle time variation, computed as: 100 .t t t t cycle cycle without rdt cycle without rdt cycle with rdt d = ^ ^ ^ h h h 7 is rather different. indeed, the dynamic overshoot is larger in the case without rdt, and this difference explains why the seismic cycle is so different if the rdt is considered or neglected. in particular, by adding the rdt, there is a reduction in the dynamic overshoot of ca. 44%. this result is in agreement with previous estimates [mcgarr 1994, 1999; see also beeler 2001]. in all of the simulations, the seismic cycle that result from the rdt inclusion is shorter than the cycle calculated for the models that do not include the rdt (table 2). consequently, all of the observations made so far show that if we consider a spring-slider system that includes the rdt, it will take less time to reach a new instability phase, as it appears to require less stress compared to the case without rdt. the results discussed so far are the same, irrespective of the choice of the governing laws assumed. figures 2 and 3 show qualitatively identical features, except that the dynamic slip displacement, sliding velocity, temperatures and dynamic stress drop values are different in absolute terms, as expected. by looking at tables 2 and 3, we can appreciate that while the dr law and rd law simulations show shorter recurrence times for rupture events governed by configuration b, the opposite occurs for the ch law simulations, which provide a longer seismic cycle for configuration b, compared to that for configuration a. this result is discussed in more detail in appendix a. 3.2. sensitivity study to quantitatively assess the role of the rdt, we focus on a fault governed by the ch law (equation 5) and we change the value of parameter c, which modulates its importance in the equation of motion (equation 3). we selected different values of c by considering typical values of g and vs for depths comparable with our hypocentral depth (see equation 4). our numerical results clearly show that high values of c entail low co-seismic displacement (see figure 4a); from a physical viewpoint, this results in the simulated earthquake having a lower magnitude. moreover, high c values entail lower velocity peaks and higher velocity minima (figure 4b), lower temperature changes (see figure 4c), and smaller dynamic overshoots (see figure 4d). these observations support our conclusion that the rdt significantly affects the system dynamics by promoting smaller and more frequent earthquakes. we also explored the role of the critical velocity (vc). as mentioned in section 2.3, vc formally establishes the relative influence of the dynamic regime on each repeated instability. indeed, the rdt is considered in the equation of motion (equation 3) only in the dynamic behavior of the system, which corresponds to velocities greater than the vc. this is physically reasonable, as given the quasi-static stage of the radiation damping in 1–d figure 4. comparisons between the numerical simulations for the ch law (equation 7) with rdt for configuration a, using different values of c (the radiation damping coefficient from equation 3) at the reference value of vc. (a) slip displacement versus time. (b) time history of the slip velocity (on a logarithmic scale). (c) time history of the temperature developed by frictional heating. (d) phase portrait. black, reference configuration (see table 1). rupture, it is unlikely that energy is dissipated through seismic-wave emission. in figure 5, we keep parameters c unchanged (as in table 1), but we vary vc. it is apparent from figure 5 that the critical velocity is a key parameter that can significantly affect the temporal recurrence of repeated earthquakes. indeed, for those simulations where the critical velocity values are greater than the reference vc (figure 5, black lines ), we observe that the coseismic slip, the velocity peaks, and the temperatures developed decrease (figure 5ac, red lines). the opposite occurs in the case of a smaller value of vc compared to its reference value (figure 5a-c, blue lines). referring to figure 5d and to table 3, we can also appreciate that the higher the critical velocity, the shorter the resulting seismic cycle. in conclusion, we emphasize that the role of the rdt in the equation of motion (equation 3) can strongly affect the earthquake rupture recurrence time for a large class of constitutive equations and for different degrees of fault instability. in addition, we wish to note the importance of our numerical experiments, which demonstrate that high values of both the rdt and the vc control dramatically shorten the seismic cycle. 4. the ch law phase diagram if a physical system is not perturbed (i.e., an absence of stress perturbation effects due to another seismogenic fault), and if there is no occurrence of specific phenomena known to complicate earthquake cyclic patterns, such as wear production [bizzarri 2010b] or the evolution of permeability and porosity [bizzarri 2012b], the spring-slider system reaches its limit cycle [see gu et al. 1984, rice and tse 1986, belardinelli et al. 2003]. this section is aimed at detailing with the differences between the phase diagrams obtained for each of the constitutive laws. more specifically, a special phase diagram shape that occurred only for the ch law models, and was never observed for the other constitutive laws, emerged from the numerical simulations. indeed, if we focus on figures 3d, 4d and 5d, we can clearly observe that the ch law phase plane shows a 'figure-of-8' shape, which is not present for the dr law model or the rd law model (figures 1d, 2d) (see also figure 3 of belardinelli et al. [2003]). in other words, focusing on the ch law numerical results, we can see from figures 3d, 4d and 5d that a point in the phase space (x,v) is reached twice by the system trajectory during each single instability event. in figure 6, we have plotted an example of a ch law phase diagram that represents a single, generic limit cycle that was extracted from the whole simulation. we use a color scale to emphasize the temporal evolution of the dynamic system. in this case, a particular phase of the physical system can be associated with a particular portion of the limit cycle. as an example, we can easily understand that the fault analog system takes a very short time to enter the dynamic regime, to perform a seismic instability, and to decelerate to crupi and bizzarri 8 figure 5. as for figure 4, with changed vc and unchanged c, with respect to the reference value of table 1. black, reference configuration. 9 radiation damping in 1–d parameter value recurrence time (years) dieterich-ruina law ruina-dieterich law chester and higgs law conf. a conf. b conf. a conf. b conf. a conf. b parameter c in the rdt (vc = 1 × 10 -4 m/s) 2.700 × 106 pa s/m (*) 134.46 102.11 123.18 93.97 60.19 63.55 3.375 × 106 pa s/m 127.96 127.96 117.09 89.59 57.68 60.89 4.500 × 106 pa s/m (*) 118.65 90.35 108.33 83.30 54.02 57.01 5.400 × 106 pa s/m 112.35 85.66 102.38 79.02 51.52 54.39 8.500 × 106 pa s/m (*) 96.30 73.71 87.15 68.05 45.00 47.47 critical velocity vc (c = 4.5 × 106 pa s/m) 3 × 10-4 m/s (*) 114.31 83.37 104.36 77.06 50.92 51.79 1 × 10-4 m/s (*) 118.65 90.35 108.33 83.30 54.02 57.01 1 × 10-5 m/s 127.69 104.55 116.65 96.28 60.75 68.35 1 × 10-6 m/s 136.63 118.36 124.95 109.19 67.72 80.07 1 × 10-7 m/s (*) 145.49 131.86 133.25 122.09 74.92 92.12 table 3. recurrence times for the different governing laws and the two configurations listed in table 1 resulting from the sensitivity study (see section 3.2). values reported in bold represent the reference configurations. asterisks denote the simulations reported in figures 4 and 5. conf., configuration. figure 6. single limit cycle resulting from the ch law and with the rdt. the parameters are those of table 1. the colors express the relative duration of each single phase with respect to the total duration of the cycle. the count begins from the point marked by the symbol vc and proceeds clockwise. dashed black line, ln(vc/v * ) = 12.7; continuous black line, ln(vl/v * ) = 19.57, which formally defines the time occurrence of a dynamic instability. very low slip velocities. the fully dynamic phase of the seismic cycle is no more that 1% of the entire fault cycle (figure 6, red portion), as expected physically. following the occurrence of a seismic instability episode (figure 6, black vertical line), the system itself keeps on accelerating, even when the traction starts to rise. this fast restrengthening stage has already been observed by bizzarri [2010a] in an extended fault model that obeys the ch constitutive law. we can clearly see that the dynamic system runs through a rapid restrengthening phase just before reaching its peak: the shear stress reaches its minimum before the velocity attains its maximum value, vpeak. this is clearly shown in figure 7, where the vertical dashed line indicates the minimum in x (blue line) and the peak in v (red line). the same behavior has been reported in mechanical lubrication models [bizzarri 2012c]. as discussed by tinti et al. [2004], this result entails relevant seismological implications, in that we cannot use the v time integration to retrieve the (equivalent) slip-weakening distance up to the time when vpeak is attained, as suggested by mikumo et al. [2003]. 5. discussion and concluding remarks in the present study, we have modeled repeated instability events through a one-degree-of-freedom mass-spring fault analog model. the results obtained by using this analog fault model are appropriate to simulate the dynamic motion of a fault when a rupture propagates as a classical expanding crack, or when it is stopped by a strong barrier (for instance by a velocity strengthening region), when the material properties of the rupture surface are homogeneous, and when the spatial distribution of the stress on the fractured portion of the fault is either homogeneous or well represented by a spatial average. when these conditions are met, the springslider system is extremely effective in modeling repeated instabilities on the same fault structure (primarily because of its computational simplicity). we have considered different governing laws in the rs friction framework; namely the dr law, the rd law and the ch law. the first two of these laws are 'canonical', in that they have been largely used in the previous literature. we have also considered the ch law because it is simply a (laboratory-based) extension of the rd law. we emphasize that besides having been the subject of previous studies [e.g., marone 1998, rubin and ampuero 2005, 2009, bizzarri 2011], a systematic comparison of the variety of formulations of the governing laws is certainly beyond the scope of the present study. by conducting numerical simulations of seismic cycles, and by adopting different laboratory-derived friction laws and various physical configurations, we have essentially attained three key objectives. first, we show that including the rdt ( in equation 3) dramatically affects the evolution of a spring-slider system cycle. in particular, the inclusion of the rdt induces a ca. 30% reduction in the recurrence time of subsequent instability events, with respect to the reference case (see table 2). this reduction is a stable feature that does not depend on any specific governing law applied to the fault, nor on the fault instability level. moreover, we show that the rdt entails a decrease (by ca. 27%-30 %) in the co-seismic slip displacement that develops during each instability event (figures 1a, 2a, 3a). at the same time, we can also observe the decreasing velocity peaks during a seismic instability event (figures 1b, 2b, 3b), a drop in the developed temperature due to frictional heating (figures 1c, 2c, 3c), and finally crupi and bizzarri 10 figure 7. superimposition of the slip velocity and the shear stress time histories, in the ch law case within a very narrow time window. the velocity peak is reached only a few instants after the shear stress minimum is achieved. cuo 11 a dynamic overshoot reduction (figures 1d, 2d, 3d). in particular, neglecting the seismic radiation (i.e., without the rdt) results in a complete overshoot, while adding the rdt, results in a reduction in the dynamic overshoot of ca. 44% (in the case of the dr law and the rd law; see figures 1d, 2d) and of ca. 51% (in the case of the ch law; see figure 3d). these values are consistent with the exact estimates given by mcgarr [1994, 1999]; a simulation with inertia without rdt yields an overshoot of 1/2, while the introduction of the rdt (still with inertia) results in a dynamic overshoot of ca. 1/4 (so that the reduction is by a factor of 2; see also savage and wood [1971], beeler [2001] and segall [2010]). overall, this suggests that neglecting the rdt indicates stronger but less frequent earthquakes. secondly, we propose a parametric study aimed to show how different values of c and vc can affect the simulated cycle of the ch law model. our numerical experiments suggest that as long as values c and the critical slip rate values vc (controls the system dynamic phase onset) increase (see section 2.3 for further details), the duration of the seismic cycle decreases. thirdly, as a side result, we show a specific feature of the ch law model, which shows a phase diagram with a 'figureof-8'-shaped trajectory; in this case, a point in the phase space (x,v) is reached twice for each instability event during the system evolution (see figure 6). this phenomenon is closely related to the rapid restrengthening mechanism that occurs immediately after the dynamic stress release phase [see also bizzarri 2010a]. this rapid restrengthening is intimately related to the frictional heating, which is, in turn, proportional to the degree of instability of the fault. indeed, we have seen from figure a1c that a more unstable configuration leads to more pronounced rapid restrengthening. this in turn results in a different cycle time value; we observed that with the ch law, the strong seismic configuration shows shorter recurrence times than less unstable configurations, contrary to the general behavior expected in cases when the temperature does not explicitly enter into the analytical expression of the governing model (such as the canonical dr law and rd law). as an overall conclusion, we highlight that our data show the limits of the canonical, widely used, equation of motion formula for the 1-d spring-system, which neglects the rdt; i.e., the energy lost in seismic wave propagation. this contribution is inherently assumed in more elaborate 3d fault models [e.g., bizzarri and cocco 2005, day et al. 2005, among many others], or it is introduced in the so-called quasi-dynamic 3-d models [e.g., cochard and madariaga 1994, lapusta and liu 2009]. on the contrary, this contribution has been largely ignored in the literature that relates to the 1-d mass-spring model, with some relevant exceptions [xu and knopoff 1994, beeler 2001, beeler et al. 2002, beeler 2006, bizzarri 2012a]. undoubtedly, such an analog fault system is extremely efficient in modeling repeated instabilities on the same fault structure (primarily because of its computational simplicity). however, our findings emphasize that the conventional equation of motion can lead to biased results (regardless of the choice of a specific fault constitutive equation), and that the rdt should be correctly included in numerical models with a mass-spring system. in particular, the dynamic overshoot obtained in the framework of a spring-slider system with inertia and with the rdt is comparable with that which results from 3-d models, and consistent with observations from natural earthquakes. to conclude, we are aware that the spring-slider model cannot be regarded as a tool to obtain exact, deterministic earthquake predictions, and that even the concept of the seismic cycle is largely debated (see bizzarri [2012d] and references therein for a thorough review of the subject). nevertheless, the mass-spring model can provide some useful insight into a more elaborated (and realistic) extended fault model, provided the radiation loss is adequately included in the model. acknowledgements. the authors thank m. loddo and s. de lorenzo for constructive discussions and for their support. e. fukuyama and y. benzion are kindly acknowledged for some comments on an early, preliminary version of the manuscript. we are indebted to n. beeler for stimulating discussions related to the issue of the radiation damping term in the equation of motion of the 1-d fault model used here. finally, we wish to thank two referees and the associate editor for their constructive comments. references abe, y., and n. kato (2012). complex earthquake cycle simulations using a two-degree-of-freedom spring-block model with a rateand state-friction law, pure appl. geophys.; 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(1979). modeling of rock friction, 1, experimental results and constitutive equations, j. geophys. res., 84, 2161-2168. gibowicz, s.j. (1998). partial stress drop and frictional overshoot mechanism of seismic events induced by mining, pure appl. geophys., 153, 5-20. gu, j.c., j.r. rice, a.l. ruina and s.t. tse (1984). slip motion and stability of a single degree of freedom elastic system with rate and state dependent fiction, j. mech. phys. solids, 32, 167-196. kato, n., and t. tullis (2001). radiation damping as approximation to inertial term for dynamic slip, aces/gem meeting, maui, japan. lapusta, n., and y. liu (2009). three-dimensional boundary integral modeling of spontaneous earthquake sequences and aseismic slip, j. geophys. res., 114, b09303; doi: 10.1029/2008jb005934. marone, c. (1998). laboratory-derived friction laws and their application to seismic faulting, annu. rev. earth planet. sci., 26, 643-646. mcgarr, a. (1994). some comparisons between mining-induced and laboratory earthquakes, pure appl. geophys., 142, 467-489. mcgarr, a. (1999). on relating apparent stress to the stress causing earthquake fault slip, j. geophys. res., 104, 30033011. mikumo, t., k.b. olsen, e. fukuyama and y. yagi (2003). stress-breakdown time and slip-weakening distance inferred from slip-velocity functions on earthquake faults, bull. seismol. soc. am., 93, 264-282, doi:10.1785/012002 0082. molnar, p. (1979). earthquake recurrence intervals and plate tectonics, bull. seism. soc. am., 69, 115-133. orowan, e. (1960). mechanism of seismic faulting in rock deformation, geol. soc. am. mem., 79, 323-345. panza, g.f., romanelli, f. and f. vaccai (2001). seismic wave propagation in laterally heterogeneous anelastic media: theory and applications to seismic zonation, adv. geophys. 43, 1-95. panza, g.f., irikura, k., kouteva, m., peresan, a., wang, z. and r. paragoni (2011). advanced seismic hazard assesscrupi and bizzarri 12 13 ment, pure appl. geophys. 169, 1-9; doi:10.1007/s00024010-0179-9. press, w.h., s.a. teukolsky, w.t. vetterling and b.t. flannery (1992). numerical recipes in c: the art of science computing, 3rd ed., cambridge university press, new york. reid, h.f. (1910). the mechanics of the earthquakes, the california earthquake of april 18, 1906, report of the state investigation commission, 2, carnegie inst., washington, d.c., usa. rice, j.r., and s.t. tse (1986). dynamic motion of a single degree of freedom system following a rate and state dependent friction law, j. geophys. res., 91, 521-530. rice, j.r. (1993). spatio-temporal complexity of slip on a fault, j. geophys. res., 98, 9885-9907. rubin, a.m., and j.-p. ampuero (2005). earthquake nucleation on (aging) rate and state faults, j. geophys. res., 110, b11312; doi:10.1029/2005jb003686. rubin, a.m., and j.-p. ampuero (2009). self-similar slip pulses during rate-and-state earthquake nucleation, j. geophys. res., 114, b11305; doi:10.1029/2009jb006529. ruina, a.l. (1983). slip instability and state variable friction laws, j. geophys. res., 88, 10,359-10,370, doi:10.1029/83 jb088. savage, j.c., and m.d. wood (1971). the relation between apparent stress and stress drop, bull. seism. soc. am., 61, 1381-1388. segall, p. (2010). earthquake and volcano deformation, princeton univ. press, princeton, n. j., 424 pp. tinti, e., a. bizzarri, a. piatanesi and m. cocco (2004). estimates of slip weakening distance for different dynamic rupture models, geophys. res. lett., 31, l02611; doi: 10.1029/2003gl018811. xu, h.-j., and l. knopoff (1994). periodicity and chaos in a one-dimensional model of earthquakes, phys. rev. e., 50, 3577-3581. walsh, j.b (1971). stiffness in faulting and in friction experiments. j. geophys. res., 76, 8597-8598; doi:10.1029/jb076 i035p08597. weeks, j.d. (1993). constitutive laws for high-velocity frictional sliding and their influence on stress drop during unstable slip, j. geophys. res., 98, 17,637-17,648. wyss, m., a. nekrasova and v. kossobokov (2012). errors in expected human losses due to incorrect seismic hazard estimates, nat. haz.; doi:10.1007/s11069-012-0125-5. *corresponding author: andrea bizzarri, istituto nazionale di geofisica e vulcanologia, sezione di bologna, bologna, italy; email: bizzarri@bo.ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. radiation damping in 1–d appendix a in section 3.1, we saw that when we assume the dr law or the rd law, the cycle time resulting from configuration a is greater than that obtained with the parameters of configuration b, regardless of the adoption of the rdt (see also table 2). this is clearly visible in figures 1b and 2b. to better appreciate this result, we re-plotted figures 1d, 2d and 3d without normalization (see figure a1a, b, c, respectively), to avoid any possible masking effects due to normalization by constant a, which is different in the two configurations. it is clear from figure a1a, b that in the strong seismic configuration (i.e., configuration a) the stress release is greater than in configuration b: dx(a) > dx(b) (this makes sense physically, since a more unstable fault is known to produce a greater stress drop). moreover, the minimum velocity values are lower in configuration a compared to those from configuration b, especially in the case of the dr law simulation: vmin (a) < vmin (b) (this is also clear from figures 1b and 2b). from these conditions (dx(a) > dx(b) and vmin (a) < vmin (b)) we can see that at the end of the decelerating stage, the system has to recover more stress in the case of configuration a than in the case of configuration b. this is essentially the cause of the result tcycle (a) > tcycle (b). from table 2 (and from figure 3b), it emerges that the opposite occurs in the case of the simulations performed by adopting the ch law model, still regardless of the introduction of the rdt: tcycle (a) > tcycle (b) (although the difference is relatively small). this apparent paradox can be solved by looking at figure a1c; from this plot, we can clearly see that dx(a) < dx(b) and this (small) difference causes tcycle (a) to be (slightly) greater than tcycle (b) with the ch governing law. at this point, a question emerges: why do we have dx(a) < dx(b) with the ch law? this is essentially due to the explicit dependence of the temperature t that appears in the analytical expression of the constitutive model (equation 7). during the co-sesimic phase, the variations of t are important, and they result in the very special shape of the phase diagram at high speeds (this issue is discussed in detail in section 4). in the strong seismic case (i.e., configuration a, red curves), after the rapid re-strengthening, the shear stress level is higher compared to that attained in configuration b (blue curves). correspondingly, the final shear stress level after the dynamic overshoot is greater in configuration a than in configuration b (so that we have dx(a) < dx(b)). crupi and bizzarri 14 figure a1. phase diagrams without normalization for the reference configurations and for the three different governing models considered in this study. panels (a), (b) and (c) correspond to figures 1d, 2d and 3d, respectively. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice on amplitudes of p near the shadow zone (*) i. l e u m a n n some years ago i iliade a studv (lehm a n u 1953) in order to find out whetlier or not t h e r e was a sudden drop in t h e p amplitude a t 105° or another neiglibouring distance m a r k i n g clearly t h e beginning of the shadow zone. i made use p a r t l y of the i.s.s. d a t a and p a r t l y of copenhagen records. i t is well known t h a t p is often recorded at distances greater tlian 105°, b u t usually it is small and it is considered to be a diffracted wave. i found, however, t h a t in the i.s.s. t h e r e were sometimes series of p entries u p to 110° or more t h e residuals of wliich were consistenti enough to show t h a t t h e onset was sharp and therefore not likely t o be due to a diffracted wave. on the otlier h a n d i sometimes found t h a t p of a n e a r t h q u a k e had considerable uncert a i n t y al ready a t distances of a b o u t 95° or so and y e t was recorded beyond 105° as if no deerease of amplitude took place at t h a t distance. usingcopenhagen records i found also t h a t in some earthquakes p was very clearly recorded u p to and considerably beyond 105° while in otlier earthquakes of similar magnitude p became small and insignificant already a t distances smaller t h a n 100°. thus t h e variation of amplitude with distance is diiferent in differente earthquakes. the shadow zone seems to begin a t diiferent distances and it is usually not possible to say exactly wliere it begins; there is no sudden deerease of a m p l i t u d e to m a r k its boundaiy. a t t h e laniont geologica! observatory the surface waves of the samo aearthquake (*) l a m o n t g e o l o g i c a l o b s e r v a t o r y (col u m b i a u n i v e r s i t y ) c o n t r i b u t i o n n ° 319. 14 aprii 1957 were being studied witli t h e object of fìnding variations of phase velocity and hence of crustal thickness aerosa t h e united states and canada. 51 records of short-period vertical m s t r u m e n t s had been collected. they were from epicentral distances ranging from 71° to 114°. i t was suggested to me b y dr. ewing t h a t i examine t h e records for t h e greatest distances in order to see how p varied near t h e shadow zone. i t was immediately apparent t h a t there was a considerable drop in amplitude from t h e nearer t o the greatest epicentral distances, b u t it was clear also t h a t no precise determination of the variation of amplitude with distance could be iliade, for t h e instruments varied and their responso was not a c c m a t e l y known. also, t h e distances are subject to errors, an epicentre in t h e samoan region not being determinable with great precision. however, t h e two independent determinations of t h e u.s.c.g.s. and the central bureau at strasbourg differed by less t h a n 1°. the seismological stations are not evenly distributed over t h e continent. there are a great m a n y stations in the western p a r t and t h e y were a t epicentral distances ranging from 71° to 84°. there were 10 stations in the eastern p a r t a t distances greater t h a n 100° and t h e r e were not m a n y stations in between. p amplitudes were very large a t t h e western stations, t h e phase being marked by a group, or two groups, of large swings continuing for about a minute. the wave period was two seeonds or a little less. e a p i d city a t an epicentral distance of 87° stili had very large amplitudes thougli 154 i. l e u m a n n somewhat smaller t h a n tlie western stations. fayetteville a t 90°.4, however, hacl considerably smaller amplitudes though t h e phase was stili quite large. columbia a t d i s t a n t e 100°.2 was the nearest of t h e 10 stations a t distances greater t h a n 100° and halifax at 114° 6 t h e most distant. this station and palisades a t 106°.9 did not record p , b u t ali the others did record it. greater magniftcation t h a n the sprengnether i n s t r u m e n t in the period range in question, b u t tliat would be of no avail if t h e core b o u n d a r y were grazed b y tlie ray emerging at 105°. i t is obvious t h a t the p a t h to shawinigan falls has t h e sanie general character as tliose to t h e nearer stations and t h a t t h e waves recorded are not diferacted waves. the reason w h y the cleveland record is shown and not tlie one s a m o a e a r t h q u a k e r e c o r d e d a t c l e v e l a n d , a = 101°, 3 tlie cleveland record from epicentral distance 101°.3 is shown in the figure. i t is from a sprengnether short-period vertical i n s t r u m e n t . tlie p amplitudes are here quite small as compared with tliose a t the western stations. where and how quickly t h e amplitudes fall off cannot be said because of a lack of stations a t intermediate distances. the wave period is about 2 sec. as a t t h e smaller distances. the kirkland lake record from a distance of 103°.4 is almost exactly like t h e cleveland record. the p a t t e r n is the sanie and t h e amplitudes are equal. i t is also from a sprengnether instrument. chapel hill at 102°.2 has similar amplitudes b u t t h e p a t t e r n is dilferent; there is not the same division of t h e phase. i t is somewhat surprising to and t h a t shawinigan falls a t an epicentral distance of 108°.2 has a record similar to t h e cleveland record and only slightly smaller. tlie p a t t e r n is t h e sanie and t h e waves have period of about 2 sec. the seismograph is a willmore instrument and it m a y have from shawinigan falls is t h a t t h e trace of t h e latteiis a little fainter. of t h e other stations beyond 100° columbia a t 100°.2 has a much smaller p phase t h a n tlie stations mentioned, but t h e p a t t e r n is the same. this is also trae for ottawa a t 106°.0. montreal at 107°.5 has quite a small b u t distinct p. state college a t 103°.9 has also a clear but small p. w e have seen t h a t t h e p amplitudes are small in t h e 8° range of distance 100° to 108°. no precise determination of the variation of amplitude with distance can be iliade, not even for t h e distances of tlie m a n y western stations, but t h e decrease of amplitude between 84° and 100° is very great, undoubtedly f a r greater t h a n the decrease between 71° and 84°. the strong decrease of amplitude will be due to a m a r k e d decrease of velocity gradient at deptli. the lack of observations a t intermediate distances makes it impossible to say whether t h e decrease is graduai or abr u p t or to determine even roughly the deptli on a m p l i t u d e s of p n e a r t h e s h a d o w zone 155 at which it occurs. b u t our fìnding is in suppoi't of t h e assumption t h a t k . e . bullen's region d divides itself into two regions of wliich the inner one, d", is cliaracterised by a velocity gradient t h a t is very small and distinctly smaller t h a n t h a t of d' (see e. g. bullen 1956). the f a c t t h a t t h e time-curve straightens before t h e shadow zone is reached has led to this result. the jeffreys-bullen p curve straightens a t about 90° and has very little curvature from t h e r e onwards. the samoa p curve runs practically parallel to it a t these distances. i t does not, however, r u n parallel to the j.-b. curve ali t h e way f r o m 70° to 108°. taking t h e t i m e of occurrence of t h e earthquake to be 19:17:57 as determined by t h e u.s.c.g.s. we fincl t h e following mean residuals: 70° 75° 10 observations 0s.96±0s.17 75° 80° 8 » l s . 6 9 ± 0 s . l l 100° 108°.2 8 » 2s.39±0s.17 there is some scatter of t h e residuals near 85°. between 90° and 100° there are only 1 observations; t h e mean of their residuals is 2s.23 ± 0s.22. the azimuths of ali the observing stations is between 30° and 58° so it is evident t h a t the samoa earthquake p curve is steeper t h a n t h e j.-b. curve from 70° up to about 90°. the distances taken were geographie distances, b u t ellipticity corrections were applied to t h e transmission times. silice the amplitude of t h e direct p wave usually is quite small a t t h e greatest distances where it occurs it will not be easy to decide whether a p phase as recorded is due to a direct or to a diffracted wave and to iind where the shadow zone begins. there is as a rule no short period waves in distant p's, and this has been t a k e n to indicate diffraction. w h e t h e r it is a sure criterion is not known and it is not applicatale unless t h e recording instruments have a good response to short period waves. i t does not seem to have been a t t e m p t e d to fix the beginning of the shadow zone of an individuai earthquake by a method similar to the one applied here, by direct comparison of records. to obtain definite results it would be necessary to clioose earthquakes well recorded a t the distances in question by a dense net-work of stations as e. g. t h a t of western xortli america. stations equipped witli short period instruiiients would be required and it would be higlily desh'able t h a t the relative response of the instruments as mounted should be known. this lattei' requirement is not easily fulfìlled, b u t it m a y be indispensable if we are to succeed in deriving more detailed knowledge about t h e e a r t h ' s interior from seismological d a t a . unless the error in t h e epicentre is unusually large, shawinigan falls is a t an epicentral distance greater t h a n 105°, and t h e shadow zone of our samoan shock does not begin a t this distance. taking t h e direct p ray to reach out to different epicentral distances in diiferent shocks, and t h e relative amplitudes to differ, we shall have to look for a possible explanation, and we find t h a t slight regional variations in t h e coniposition of the region d" would account for it. the velocity gradient in d" is quite small and a small regional variation would alter slightly t h e curvature in d" of t h e rays having their deepest points in t h e region. the rays are long and therefore a small bending or straightening of t h e m in their deepest portion would give rise to an appreciable change in t h e epicentral distance a t wliich t h e y emerged. i t would alter also t h e spreading of the rays and therefore affect amplitudes. actually it is not known a t present whether t h e difference in a m p h t u d e variation is regionally conditioned, but it would be of great interest to have it investigated. my t h a n k s are due to dr. maurice ewing by whose kind invitation i worked a t t h e lamont geologica! observatory and who allowed me to use the records of t h e samoa earthquake collected there. silice t h e precedili»' was written the study by a. vogel: uber unregelmàssigkeiten der àusseren begrenzung des erdkerns (doctor's tliesis, not printed) has become known to me. observed regional variations in the transmission times of t h e ] 56 i. l e u m a n n waves reflected a t the core boundary are p u t down to irregularities in this boundary. undulations in t h e core boundary would account also for t h e sliadow zone for p beginning a t diflerent epicentral distances in different earthquakes, b u t not for t h e observed differences in amplitude variation they seem to require for their explanation t h e (additional) assumption of regional variation of velocity in d " . abstract 51 short-period vertical north american and canadian records of the samoa earthquake 14 aprii 1957 were examined. amplitudes of p ivere very largo at the western stations (a < 84°) and small at the stations to the cast, 8 of which recorded p at distances greater than 100°. there ivas a lack of stations at intermediate distances. at the greatest distance, 108°.2, the p waves were not diffracted ioaves, for the pliase had the same general character as at sliorter distances-, the period, ivas 2 sec. from 70° to about 90° the samoa p timedistance curve is steeper than the j.-b. curve. it had been found previously that from about 900 epicentral distance onivards the variation of the p amplitude with distance ivas not always the same and also that the sliadow zone seemed to begin at different distances. it is suggested that this be due to small regional differences of velocity in the deepest layer d" of the mantle. riassunto sono state prese in esame 51 registrazioni della componente verticale a corto periodo, relative al nord america e al canadà, del terremoto di samoa del 14 aprile 1957. le ampiezze della p sono molto grandi presso le stazioni occidentali (a < 84°) e piccole presso quelle orientali, 8 delle quali registrarono la p ad una distanza maggiore di 100 c'è stato un ritardo nelle stazioni a distanza intermedia. alla distanza massima, 108°.2, le onde p non furono onde diffratte, poiché la fase ha avuto lo stesso carattere generale di quella registrata a distanza minore-, il periodo fu di 2 sec. da 70° a circa 90° la curva tempo-distanza delle p del terremoto di samoa, è più ripida di quella j.-b. è stato precedentemente trovato che da una distanza epicentrale di circa 90° ed oltre, la variazione dell'ampiezza della p con la distanza, non era sempre la stessa e inoltre che la zona d'ombra sembrava cominciare a distanze diverse. si fa l'ipotesi che ciò sia dovuto a piccole differenze regionali nello strato più profondo d" del mantello. r e f e r e n c e s bullen, k . e . , seismology and the broad structure of the earth's interior. « p h y s i c s and c h e m i s t r y of t l i e e a r t h », 1, 68-93. p e r g a m o n press, 1956. leumann, i., on the shadow of the earth's core. « b u l l . seismol. soc. a i n e r . », 4 3 , 291306. 1953. sonde r a d i o a t t i v e a p e r c o r s o r i d o t t o r . clalilfa \. l o suiìdo l ' n o (lei m e l o d i più dillusi p e r la m i s u r a del c a m p o e l e t t r i c o lerr e s t r e è q u e l l o d e l l e sonde r a d i o a t t i v e . q u e s t e sono i n g e n e r a l e f o r m a t e da piccoli c o n d u t t o r i m e t a l l i c i stilla cui s u p e r f i c i e è un d e p o s i t o di sostanza r a d i o a t t i v a . i ila sonda v i e n e p o l a nel p u n t o ili cui si vuole m i s u r a r e il p o t e n z i a l e e l e t t r i c o e me—a in c o m u n i c a z i o n e con un e l e t t r o m e t r o : la sostanza r a d i o a t t i v a ionizza l ' a r i a c i r c o t a n t e e q u i n d i s t a b i l i s c e le c o n d i z i o n i p e r c h è la sonda a c q u i s t i il p o t e n z i a l e s p e t t a n t e al la sua p o s i z i o n e nel c a m p o . il p o t e n z i a l e di e q u i l i b r i o assunto da una sonda r a d i o a t t i v a non <' p e r ò in g e n e r a l e e s a t t a m e n t e q u e l l o c h e c o m p e t e r e b b e alla sua pos i z i o n e nel c a m p o . c i ò è dovuto p r i n c i p a l m e n t e al fatto c h e l ' a r i a i o n i z z a l a i n t o r n o a l l a ~onda c o s t i t u i s c e in sostanza un c o n d u t t o r e di i o r m a ed e s t e n s i o n e v a r i a b i l i e c h e la c a r i c a s p a z i a l e in esso c o n t e n u t a può a l t e r a r e il c a m p o . p e r r i d u r r e i e l i c i l o di q u c s l a causa p e r t u r b a t r i c e l e sonde di p o t e n z i a l e v e n g o n o a t t i v a l e quasi csclusiv a m e n l e con p o l o n i o o i o n i o , due sostanze c h e e m e t t o n o s o l t a n t o raggi e c h e n o n p r o d u c o n o e m a n a z i o n e ( ' ) ; cosi l ' a r i a v i e n e i o n i z z a l a s o l t a n t o in un p i c c o l o s p a z i o i n t o r n o al l a sostanza a d i v a . il p u n t o v i r t u a l e di presa del p o t e n z i a l e è q u i n d i più vicino a l l a s o n d a , p o n i l e ili tal m o d o -i e l i m i n a n o gli eccessivi scarti dal p o t e n z i a l e ed'ellivo dovuti alla p r e s e n za i n t o r n o a ll a sonda di un n o t e v o l e v o l u m e di a r i a i o n i z z a l a . c i ò avv i e n e (piando si i m p i e g a n o sostanze r a d i o a t t i v e c h e e m e t t o n o raggi e y, i (piali sono m o l t o più p e n e t r a n t i degli u . o p p u r e ( p i a n d o sviluppa e m a n a z i o n e c h e si d i f f o n d e r a p i d a m e n t e in m o d o i n c o n t r o l l a b i l e . e ' l e c i t o p e n s a r e q u i n d i clic le p e r t u r b a z i o n i d e l l e m i s u r e di pot e n z i a l e e s e g u i l e con sonde r a d i o a t t i v e possano essere r i d o t t e d i i n i ( r l ii percorso (lei ru;:j!ì (( 1 u ratine »"l n e l l ' a r i a in condizioni normali per il polonio è di cui 3.117 e per lo ionio di cui 3.13 ( j o u o t o mi;, f.c.s riulioélémcnis tinnirci*. h e r m a n n , p a r i s , 1916). k . c i a i . i i e a " a. 1.0 m 11110 u n e n d o la l u n g h e z z a del p e r c o r s o d e l l e p a r t i c e l l e i o n i z z a n t i . p a r l e n do da q u e s t a c o n s i d e r a z i o n e a b b i a m o e s e g u i l o a l c u n e r i c e r c h e , c l i c qui e s p o n i a m o , a l l o s c o p o di r e n d e r e p o s s i b i l e lilla m a g g i o r e a p p r o s s i m a z i o n e n e l l e m i s u r e del p o t e n z i a l e e l e t t r i c o d e l l ' a t m o s f e r a . c o m e è n o l o , non si d i p o n e ili sostanze c h e e m e t t a n o s o l t a n t o raggi e. a v e n t i p e r c o r s i n o t e v o l m e n t e più b r e v i di q u e l l i d e l p o l o n i o e d e l l o i o n i o : p e r t a n t o a l l o s c o p o di l i m i t a r e la i o n i z z a z i o n e p r o d o t t a dai raggi ti in uno s t r a t o di p e s o r e m o l t o più p i c c o l o , a b b i a m o i d e a t o i seguenti d i s p o s i t i v i c l i c c o n s e n t o n o di r i d u r r e il p e r c o r s o u t i l e dei raggi del p o l o n i o stesso. i) sonile <'on riduttore tubolare. i na sonda di q u e s t o t i p o è c o s l i t u i t a da un d i c o m e t a l l i c o coll o c a t o e n t r o un tulio o p a c o ai raggi ". : q u e s t o d i c o p r e s e n t a la sup e r f i c i e a t t i v a s r i v o l t a verso l ' a p e r t u r a a ( f ì g . i ) : la d i s t a n z a <1 f r a la s u p e r f i c i e n e la b o c c a del t u b o , è m i n o r e del p e r c o r s o r dei raggi ti.. c.on tale d i p o s i z i o n e , t r a s c u r a n d o l'citet!o d e l l a d i f f u s i o n e degli ioni p r o d o t t i nel! in ! t e r n o del t u b o . i a z i o n e d e l l a sonda -i può \ c o n s i d e r a r e c o m e p i e — a p o c o dovuta ad una s u p e r f i c i e a t t i v a c o i n c i d e n t e con l ' a p e r t u r a u elei t u b o , e m i t t e n t e raggi « di p e r c o r s o i » 1j r' ~ r d e n t r o un a n g o l o s o l i d o più r i d o t t o . ].j„ | le p r o v e e s e g u i t e in un c a m p o e l e t t r i c o u n i f o r m e a r t i f i c i a l e h a n n o m o s t r a t o , c o m e -i v e d r à in s e g u i t o , il n o t e v o l e v a n t a g g i o c h e -i può o t t e n e r e con q u e l o t i p o ili s o n d a , s p e c i a l m e n t e in a r i a c a l m a . si e o s s e r v a t o p e r ò , c h e il vento p u ò a v e r e a n c o r a una n o t e v o l e influenza sui valori del p o t e n z i a l e r i l e v a t i . il) sonde •ori riduttore ad assorbimento. l ' i n c o n v e n i e n t e c l i c si p r e s e n t a con le s o n d e per e f f e t t o del m o t o d e l l ' a r i a , ci ila i n d o t t o a s p e r i m e n t a r e un a l t r o d i s p o s i t i v o , col ( p i a l e , o l t r e a d i m i n u i r e il p e r c o r s o e s t e r n o u t i l e dei raggi <<, -i r i e s c e ad i m p e d i r e l ' i n t e r v e n t o degli ioni p r o d o t t i nel p e r c o r s o i n t e r n o . in questo d i s p o s i t i v o la v e l o c i t à dei ' a g g i (t v i e n e d i m i n u i t a per e l i c i l o del p a s s a g g i o a t t r a v e r s o ad una l a m i n a m e t a l l i c a di s p e s s o r e o p p o r t u n o : i e n e r g i a r e s i d u a posseduta d a l l e p a r t i c e l l e a a l l ' u s c i t a d a l l a l a m i n a c o n s e n t e la i o n i z z a z i o n e d e l l ' a r i a s o l t a n t o p e r un p e r c o r s o p i ù l i m i l a l o . n e l l e sonde di q u e s t o tipo da noi a d o p e r a t e il l ' a l l e n t a m e n t o dels o n d e h a i k o a t t i v e v l ' e l i c o h s o h 1 i i o t t o 5 1 lip a r t i c e l l e a è o t t e n n i o m e d i a n t e u n a f o g l i a di a l l u m i n i o : lo spess o r e di ló u, a d o t t a l o spesso n e l l e n o s t r e m i s u r e , r i d u e e il p e r c o r s o dei raggi « a poco p i ù di un c e n t i m e t r o ( j ) . la foglia di a l l u m i n i o q n e ra || l i > • > . . . ? ? ? m ' l, % i n c o n s i d e r a z i o n e d e l l a f r a g i l i t à d( sta v i e n e posta ira due l a m i n e t t c di o t t o n e l ( l ì g . 2 ) l'orale mi tutta la s u p e r f i c i e in m o d o c l i c i raggi ci nel l o r o p e r c o r s o , p e n e t r a n d o p e r i fori della l a m i n a /, possono u s c i r e l i b e r a m e n t e ali e s t e r n o pa.-sando p e r i c o r r i s p o n d e n ti fori d e l l a l . d o p o a v e r e a l l r a v e r a l o la foglia di a l i u i n i n i o i. le e s p e r i e n z e e s e g u i l e in un c a m p o u n i f o r m e a r t i f i c i a l e h a n n o m e s o in evid e n z a , con a r i a c a l m a , un c o m p o r t a m e n t o a n a l o g o a q u e l l o d e l l e sonde a r i d u t t o r e t u b o l a r e : p e r ò le p e r t u r b a z i o n i p r o d o t t e d a l l e c o r r e n t i di a r i a sono m o l l o m i n o r i . le s o n d e v e n i v a n o s p e r i m e n t a t e in un c a m p o u n i f o r m e c r e a l o arl i f i e i a l n i e n l c m e d i a n t e d u e g r a n d i e l e t t r o d i p i a n i m e t a l l i c i disposti par a l l e l a m e n t e , tra i quali v e n i v a a p p l i c a l a una d i f f e r e n z a di p o t e n z i a l e . ogni sonda era c o s t i t u i t a da un c u b e t t o m e t a l l i c o f o r a l o c o m e -i v e d e n e l l e l i g u r e : il foro era c h i u s o ad un e s t r e m o m e d i a n t e u n a l a m i na. m e t a l l i c a , e l a s c i a l o a p e r to (fig. i) o c o p e r t o d a l rid u t t o r e a s s o r b e n t e , n e l l a p a r ie o p p o s t a (fig. 2 ) . l ' i n s i e m e e r a fissalo ad un s o t t i l e t u b o di o t t o n e c o l l e g a t o a l l ' e l e t t r o m e t r o . q u e s t o tulio di sostegno v e n i v a d i s p o s t o d e n t r o il c a m p o p a r a l l e l a m e n t e ai due p i a n i m e t a l l i c i . a l l ' i n t e r n o del foro e r a c o l l o c a t o un piec o l o disco di o t t o n e a r g e n t a lo ed a t t i v a t o m e d i a n t e dep o s i t o e l e t t r o l i t i c o di p o l o , n i o su una d e l l e d u e f a c c e . t's ( f i g u r e i e 2 ) . irato di alluminio equivalerne ad un c u b i e , loc. cil.). f i n . 3 l ' i c o n i ' è nolo, lo spessore dello en li metro ili aria in condizioni normali è di 6 u (vedi joi. a 5 2 11. c i a i d i a " a . 1.(1 s c u d o dai d i a g r a m m i d e l l e figg. 1 e ó si rileva l ' a n d a m e n t o ù n i c o del p o t e n z i a t e di e q u i l i b r i o assunto d a l l e s o n d e , a p e r c o r s i n o r m a l e e r i d o t t i , n e i vari p u n t i del c a m p o e l e t t r i c o a r t i f i c i a l e da noi a d o p e r a l o . nella figura 4 sulle ascisse sono r i p o r t a t e le d i s t a n z e della sonda dall a r m a t u r a a p o t e n z i a l e z e r o e s u l l e o r d i n a t e le r e l a t i v e d i l l e r e n z e di p o t e n z i a l e m i s u r a l e tra la sonda e t a l e a r m a t u r a . la r e t t a / r a p p r e senta i valori e f f e t t i v i del p o t e n z i a l e . le l i n c e i i e /// r a p p r e s e n t a n o i v a l o r i del p o t e n z i a l e r i l e v a l i m e d i a n t e u n a c o m u n e sonda a polonio d i s p o s t a con la s u p e r f i c i e a t t i v a .s n o r m a l e a l l e l i n e e di forza d e l s o n d e r a d i o a t t i v e a p e r c o r s o r i d o t t o f>:5 0 jo zo 50 40 50 60 70 80 90 100 d f i g . 5 c a m p o , e p r e c i s a m e n t e p e r i dati r a p p r e s e n t a t i d a l l a linea ii la sup e r f ì c i e a t t i v a v e n i v a r i v o l t a verso l ' a r m a t u r a a p o t e n z i a l e z e r o , e p e r la l i n e a iii v e r o l ' a r m a t u r a a p o t e n z i a l e 100. l a l i n e a ii r a p p r e s e n ta i valori o t t e n u t i con la -tessa sonda q u a n d o essa v e n i v a d i s p o s t a con la s u p e r f i c i e a t t i v a p a r a l l e l a a l l e l i n e e di forza del c a m p o . nella figura 5 sono r i p o r t a t e m i s u r e a n a l o g h e e s e g u i l e c o n una sonda a r i d u t t o r e t u b o l a r e n e l l a q u a l e il p e r c o r s o u t i l e era di 12 m i n . ! v a i o l i o t t e n u t i n e l l e stesse c o n d i z i o n i m e d i a n t e una sonda ad a s s o r . b i i n e n t o il cui p e r c o r r o e s t e r n o u t i l e e r a p u r e di 12 i n n i , n o n sono stati qui r a p p r e s e n t a l i p e r c h è a p p r o i m a t i v a m c n t e c o i n c i d e n t i con q u e l l i d e l l a sonda a r i d u t t o r e t u b o l a r e . l a figura 6 dà a p p r o s s i m a t i v a m e n t e un i d e a dell a n d a m e n t o m e d i o della d i s t a n z a fra il c e n t r o d e l l a sonda ed il p u n t o v i r t u a l e di p r e s a del p o t e n z i a l e . i i f / / / ' / / _ — | /w vai 0 # i /w vai / / y / / •• / / s v / z' —i m r / ' / / // v r / s j f ' / a ' v1 = w yj \ l i . c i a i . l l k a " a . 1 . 0 s i i i i k ) [n as' i s c v e n g o n o r i p o r t a t e . c o m e al s o l i l o , le d i s i a n z e d e l l a -onda d a l l ' a r m a l u r a a p o t e n z i a l e z e r o ed in o r d i n a t e i v a l o r i del rapp o r t o — — dove ay i n d i c a la d i f f e r e n z a ira il j i o t e n z i a l e m i s u r a l o e q u e l l o e f f e t t i v o , ed e il c a m p o e l e t t r i c o . i a l e r a p p o r t o , elle v i e n e espresso in c m , risulta c o &v m e è e v i d e n t e , uguale a l l a d i s i a n z a , lungo le l i n e e di l'orza, fra il p u n t o v i r t u a l e di p r e s a del p o t e n z i a l e ed il ( e n t r o della s o n d a . l e l i n e e e /, -i r i f e r i s c o no a l l e m i s u r e f a t t e c o n ima sonda a p o l o n i o di t i p o com u n e d i s p o s t a c o n la s u p e r ficie a t t i v a r i s p e t t i v a m e n t e r i v o l t a verso i a r m a t u r a a p o t e n z i a l e z e r o e verso i arm a t u r a a p o t e n z i a l e mio. la iinca /, è r e l a t i v a a l l a -lessa sonda d i s p o s t a p e r ò con la « u p e r f i e i c a t t i v a p a r a l l e l a a l l e l i n e e di lor/.a del c a m p o . le l i n e e ///„, iii, e ///r si r i f e r i s c o n o ai v a l o r i o t t e n u t i con d i s p o sizioni a n a l o g h e m e d i a n t e la slessa s o n d a , m u n i t a p e r ò di r i d u t t o r e ad a s o r l > i m e n l o o r i d u t t o r e t u b o l a r e , con lo -lesso p c r c o i o u t i l e di 12 m i n , e q u i n d i , c o m e è eletto a v a n t i , p r o d u c e n t i in a r i a c a l m a lo stesso e f f e t t o ( '). r i c e r c h e p r e l i m i n a r i , l'ulte p e r s t u d i a r e l ' e f f e t t o di una c o r r e n t e d a r i a sul p o t e n z i a l e o t t e n n i o con le s o n d e r a d i o a t t i v e dei t r e t i p i , h a n n o c o n f e r m a l o la g r a n d e s u p e r i o r i t à , ai fini d e l l ' a p p r o s s i m a z i o n e o t t e n i b i l e n e l l e m i s u r e del p o t e n z i a l e e l e t t r i c o d e l l ' a t m o s f e r a , d e l l a sonda con r i d u t t o r e ad a s s o r b i m e n t o su q u e l l a con r i d u t t o r e tub o l a r e , la (piale p e r a l t r o p r e s e n t a un n o t e v o l e v a n t a g g i o -lilla sonda c o m u n e . fin. r, c i nel. diagramma si trovano anelli' lo linee lla //, o ii], relative a misuri analoghe eseguile -on -onde a riduttore dei d\ie tipi, avenii però un pereor-i utile di 21 inni. s o n i i k l u d i o a t t l v i : \ l ' k h c o l ì s o h1doito s o n o i n c o r s o u l t e r i o r i r i c e r c h e su q u e s t o a r g o m e n t o c l i c f a r a n n o o g g e t t o di u n a p r o s s i m a n o i a . roma, istillilo nazionale di ('colisicu • ottobre 1917. rlissu.mo in tjiu'stu 'mota sono desciillc due nuore sonde radioattive ideale dagli latori, con le (piali, utilizzando solo una parte del percorso delle particelle n emesse dal polonio, si può aumentare l'approssima, zinne nelle misure del potenziale elettrico dell' atmosfera. moretti_ok_corretto_layout 6 583 rapid response to the earthquake emergency of may 2012 in the po plain, northern italy milena moretti1,*, luigi abruzzese1, nasser abu zeid2, paolo augliera1, riccardo m. azzara1, carla barnaba3, lucilla benedetti4, andrea bono1, paola bordoni1, tobias boxberger5, augusto bucci1, stefano cacciaguerra1, marco calò6, fabrizio cara1, simona carannante1, vincenzo cardinale1, angelo castagnozzi1, marco cattaneo1, adriano cavaliere1, gianpaolo cecere1, claudio chiarabba1, lauro chiaraluce1, maria grazia ciaccio1, rocco cogliano1, gianfranco colasanti1, marco colasanti1, cécile cornou7, françoise courboulex8, fabio criscuoli1, giovanna cultrera1, ezio d'alema1, ciriaco d'ambrosio1, stefania danesi1, pasquale de gori1, alberto delladio1, giovanni de luca1, martina demartin1, giuseppe di giulio1, catherine dorbath9, emanuela ercolani1, licia faenza1, luigi falco1, andrea fiaschi10, pietro ficeli1, antonio fodarella1, diego franceschi1, gianlorenzo franceschina1, massimo frapiccini1, michel frogneux6, lucian giovani1, aladino govoni1,3, luigi improta4, eric jacques11, chiara ladina1, philippe langlaude12, valentino lauciani1, barbara lolli1, sara lovati1, francesco pio lucente1, lucia luzi1, alfonso mandiello1, carlo marcocci1, lucia margheriti1, simone marzorati1, marco massa1, salvatore mazza1, diego mercerat12, giuliano milana1, felice minichiello1, giancarlo molli13, giancarlo monachesi1, andrea morelli1, raffaele moschillo1, francesca pacor1, davide piccinini1, ulderico piccolini1, maurizio pignone1, stefano pintore1, silvia pondrelli1, enrico priolo3, stefania pucillo4, matteo quintiliani1, gaetano riccio1, marco romanelli3, antonio rovelli1, simone salimbeni1, laura sandri1, giulio selvaggi1, andrea serratore1, marcello silvestri1, luisa valoroso1, jerome van der woerd6, gianfranco vannucci1, lucia zaccarelli1 1 istituto nazionale di geofisica e vulcanologia, italy 2 università di ferrara, dipartimento di scienze della terra, ferrara, italy 3 istituto nazionale di oceoanografia e di geofisica sperimentale (ogs), sgonico (trieste), italy 4 centre européen de recherche et d'enseignement des géosciences de l'environnement, umr7330, cnrs/aix-marseille université, aix-en-provence, france 5 deutsches geoforschungszentrum gfz, potsdam, germany 6 ipgs-eost, umr7516 cnrs/université de strasbourg, strasbourg, france 7 isterre, université de grenoble 1, ird, cnrs, grenoble, france 8 géoazur, université de nice sophia-antipolis, cnrs, oca, france 9 ipgs-eost, umr7516 cnrs/université de strasbourg, and irdur54, strasbourg, france 10 fondazione prato ricerche, prato, italy 11 institut de physique du globe de paris, paris, france 12 cete méditerranée, laboratoire régional de ponts et chaussées, nice, france 13 università di pisa, dipartimento di scienze della terra, pisa, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6152 article history received july 24, 2012; accepted september 14, 2012. subject classification: surveys, measurements and monitoring, instruments and techniques, seismological data, emilia 2012 sequence, sismiko. 2012 emilia earthquakes 1. introduction rapid-response seismic networks are an important element in the response to seismic crises. they temporarily improve the detection performance of permanent monitoring systems during seismic sequences. the improvement in earthquake detection and location capabilities can be important for decision makers to assess the current situation, and can provide invaluable data for scientific studies related to hazard, tectonics and earthquake physics. aftershocks and the clustering of the locations of seismic events help to characterize the dimensions of the causative fault. knowing the number, size and timing of the aftershocks or the clustering seismic events can help in the foreseeing of the characteristics of future seismic sequences in the same tectonic environment. instrumental rapid response requires a high degree of preparedness. a mission in response to a magnitude (ml) 6 event with a rupture length of a few tens of kilometers might involve the deployment within hours to days of 30-50 seismic stations in the middle of a disaster area of some hundreds of square kilometers, and the installation of an operational center to help in the logistics and communications. when an earthquake strikes in a populated area, which is almost always the case in italy, driving the relevant seismic response is more difficult. temporary station sites are chosen such as to optimize the network geometry for earthquake locations and source study purposes. stations have to be installed in quiet, but easily reachable, sites, and for real-time data transmission, the sites might need to have optical intervisibility. the operational center can remain in a town if there is one within the damaged area, and it should coordinate the actions of the field teams and provide information to colleagues, the civil protection authorities and the general public. the emergency system should operate as long as the seismic rate remains high; the duration of any mission might also depend on the seismic history of the area involved. this study describes the seismic response following the may 20, 2012, ml 5.9 earthquake in northern italy, which included rapid deployment of seismological stations in the field for real-time seismic monitoring purposes, the coordination of further instrumental set-ups according to the spatial evolution of the seismic sequence, and data archiving. 2. the ingv rapid response system since 1990, the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology) has had an emergency structure ready to face the occurrence of damaging earthquakes. temporary seismic networks have been usually deployed for ml ≥5.0 crustal earthquakes that have occurred in the italian territory; these have been important monitoring and scientific tools to better understand the last major seismic sequences (e.g., carlentini in 1990, umbria-marche in 1997-1998, forlì in 2000 and molise in 2002). in 2008, a new emergency structure was co-founded with the civil protection department (dpc; dipartimento di protezione civile; see 2012-2021 decennial agreement between the dpc and the ingv; http://istituto. ingv.it/l-ingv/progetti/allegati-convenzioni-dpc/accordoquadro-2012-2021). this was tested for the first time during a regional seismic emergency simulation that was organized by the civil protection of the marche regione. this structure is composed of three operational units: (i) a real-time temporary seismic network; (ii) a stand-alone temporary seismic network; and (iii) the seismic emergency operational centre (coes; centro operativo emergenza sismica) [govoni et al. 2008]. the coes is the ingv facility that is devoted to seismic emergency management in an epicentral area [moretti et al. 2010a]: it is a mobile office equipped with satellite internet communication that can be rapidly installed in a disaster area. the coes supports the ingv staff operative needs, and helps in the coordination with other national and international institutes. it cooperates with the dpc by providing updated information 24 hours a day, which contributes to the decision-making stages during an emergency. furthermore, the coes serves as a reference information center for all of the people involved in the crisis management, and thus provides important practical, and also psychological, support [la longa and crescimbene 2010] to the rescuers and to the population affected by an earthquake. the experience of the l'aquila earthquake in 2009 moretti et al. 584 figure 1. ingv centers in the italian territory. differently colored stars, availability of both personnel and instruments or personnel only for emergency network deployments; white stars, other italian institutes that generally deploy temporary stations (e.g., prato ricerche, ogs and genova university). 585 taught us that coordination between national and international institutes in the deploying of seismic stations is fundamental to optimize material, human and management resources and scientific results [margheriti et al. 2011, chiaraluce et al. 2011]. today the ingv structure for rapid response to seismic emergency [moretti et al. 2010b] is based on the one that operated during the l'aquila earthquake, but with an increased number of temporary seismic networks. the different sets of temporary seismic stations (networks) that belong to different ingv centers can either be independently deployed or coordinated into a unique network. this integrated seismic network is composed of: 1. real-time stations equipped with both radio-satellite (re.mo.tel.) [abruzzese et al. 2011] and universal mobile telecommunications system transmission (re.mo.-umts), which then become part of the national seismic network (rsn; rete sismico nazionale) [amato and mele 2008]. this ensures substantial improvements in the real-time monitoring systems. 2. standalone recording stations (re.mo.-standalone and emersito) [bordoni et al. 2012, this volume] that guarantee the acquisition of high resolution data devoted to the improvement of our scientific knowledge (e.g., seismic hazard, seismotectonics, earthquake physics, site response, wave propagation studies). the ingv seismic stations and the people involved in the emergency deployment are distributed in different centres in the italian territory (figure 1: ancona, arezzo, bologna, catania, gibilmanna, irpinia, milan, naples, pisa and rome). in the last year, an internal ingv project was launched, named as 'sismiko', to coordinate the activities during these emergencies and to establish common procedures for station deployment, network maintenance and data archiving. after the emergency alarm to all sismiko participants, the deployment is coordinated by one central office that designs the possible network geometry and assigns the installation sites to the different groups in the field, according to the instrument availability and the achievable group reaction time. station installation and maintenance can be performed by teams composed of staff coming from different centers, to guarantee knowledge of the area, and of the instruments and their interchangeability. the data are archived in standard for the exchange of earthquake data (seed) format, and need to be available to the scientific community as soon as possible (they can be restricted up to 3 years duration for specific projects). in the case of the may-june 2012 emilia seismic sequence, the sismiko effort resulted in a much faster reaction than in the past for all of the ingv components, and in an easier management of the intervention. at the european level, within the framework of the 'network of european research infrastructures for earthquake risk assessment and mitigation (nera)' european project, a european rapid response network (ern) is being developed [margheriti et al. 2011]. four european institutes (ingv, italy; rapid response to the earthquake figure 2. photographs of the seismic deployments in the po plain, showing the standalone recording stations and the stations connected in real-time to the seismic monitoring center in rome. top to bottom: the seismic stations (digitizer, gps, battery and cables) are transported and installed inside a plastic case. the case is covered by a tarpaulin and a sign explains the instrument scope and gives contact information. most of the stations are powered with solar panels; the sensor is buried inside a plastic bag close to the station. stations transmitting data in real-time are equipped with antennas. gfz, german research centre for geosciences, germany; eth, eidgenössische technische hochschule, switzerland; cnrs, centre national de la recherche scientifique, france) are combining their efforts under the one umbrella, to set up a common european framework for the coordination of future post-seismic initiatives. this on-going project has facilitated the exchange of information at the european level (http://www.nera-eu.org/content/mm_files/do_798/nerawp4-webpage-update.pdf ), and the coordination of station deployment with european colleagues during the may-june 2012 emilia seismic sequence. 3. the emilia seismic sequenceand the rapid response timing at 02:03 utc (04:03 local time) on sunday, may 20, 2012, an earthquake of ml 5.9 hit northern italy (44.89 ˚n, 11.23 ˚e, 6.3 km in depth) which resulted in casualties and severe damage to the historical buildings and to the economical activities of the region. in the following 72 hours (may 20-22, 2012) the rsn located about 300 events, 68 of which had ml ≥3.0 (including 13 events with ml ≥4.0) (data source: 2005-2012 iside-italian seismological instrumental and parametric data-base) [mele et al. 2011]. after few minutes, the mainshock was followed by a ml 5.1 event, and after a few hours, by a second ml 5.1 earthquake (origin time, 13:18 utc). after the mainshock struck, contact among the different ingv centers started immediately, and early in the morning of may 20, 2012, the first ingv emergency rapid-response group was heading to the meizoseismal area. the first real-time station started operating (i.e., sending real-time data to the ingv monitoring system) at about 10:30 a.m. on may 20, 2012. the ingv groups from ancona, arezzo, bologna, milan, irpinia, pisa and rome installed 16 seismic stations (eight with real time transmission) within the first 48 hours (figure 2). the whole meizoseismal area is characterized by the presence of the po plain sediments, which did not allow the deploying of the stations on rock outcrops. in the meantime, the ingv group started the coordination with other italian and foreign institutes (prato ricerche; ogs, istituto nazionale di oceoanografia e di geofisica sperimentale; dpc; nera project participants). during the first days of the sequence, the rate of seismicity remained high, with more than 60 events per day. on may 29, 2012, at 07:00 utc, a new large earthquake, of ml 5.8, struck the area at the western edge of the ongoing seismic sequence (44.85 ̊ n, 11.09 ̊ e, 10.2 km in depth). this was followed by several earthquakes, including ml 5.2 and ml 5.3 events, at 10:55 utc and 11:00 utc, respectively, of may 29, 2012. the emergency seismic network continued to improve under the central coordination of the ingv. as soon as further stations became available, the network was expanded, following the spatial evolution of the seismic sequence (details and timing of the installation are described in moretti et al. [2012]). when a 5.1 magnitude event occurred on june 3, 2012, at 19:20 utc, about 70 seismic stations were operating for the emergency (figure 3), including stations managed by french groups. the 10 real-time stations (re.mo.tel and re.mo.-umts) integrated into the rsn contributed to the ingv event locations for monitoring purposes. the 15 dpc strong-motion stations made use of the moretti et al. 586 figure 3. map of the seismic stations deployed after may 20, 2012. triangles, ingv permanent (green) and temporary stations; squares, stations from other institutes; stars, earthquakes with magnitude >5.0. 587 general packet radio service data transmission to the dpc acquisition center [zambonelli et al. 2011]. the ca. 50 remaining stations, which were recording as standalone, from the ingv (re.mo.-standalone and emersito) and other institutes (ogs; università di ferrara; french ipgseost stations coordinated by insu, institut national des sciences de l'univers/cnrs) will provide excellent datasets for scientific studies. after this 5.1 magnitude earthquake on june 3, 2012, the seismicity was characterized by a progressive decrease in the seismic rate and seismic moment release (http://ingvterre moti.wordpress.com/category/i-terremoti-della-pianurapadana-emiliana/aggiornamento-sismicita/). the formal coes was activated at the agenzia della protezione civile regionale dell'emilia-romagna in bologna relatively late, on june 8, 2012, because the ingv center in bologna was previously fulfilling the coes tasks. the coes (figure 4) ensured continuous and direct communication with the dpc officers present in the epicentral area, 24 hours a day, providing formal updates on the seismic sequence twice a day. at the same time, the structure helped in providing education and information services to support the affected populations, and for the civil protection operators and rescue volunteers (http://ingvterremoti.wordpress.com/ category/informazione/). the duration of this rapid response mission will depend on the trend of the seismic rate and the seismic moment release, along with information drawn from historical seismic catalogs. the historical seismic catalogs indicate a relatively moderate seismicity in this area, as well as in neighboring rapid response to the earthquake figure 5. earthquakes located by the ingv monitoring system for the period january to june, 2012. black circles, towns and villages; colored triangles, seismic stations; circles and stars, earthquakes with magnitude >5.0. earthquakes and stations are color-coded according to occurrence/deployment time relative to the scale on the legend. figure 4. photograph of the coes room in bologna. areas in the po plain, which have been characterized by earthquakes with maximal estimated magnitudes close to 5.5 [i.e., selvaggi et al. 2001]. it is interesting to note that the seismic sequence that struck the city of ferrara (estimated magnitude of mw 5.5) on november 17, 1570, was a sequence of four strong shocks [dbmi11; locati et al. 2011]. it was a complex and long sequence, and it including an event on march 17, 1574, that produced damage in finale emilia (estimated magnitude of mw 4.7), one of villages most damaged by the 2012 seismic sequence. 4. data archives and preliminary results the ingv temporary network data, as defined in the sismiko agreement, are being stored within the european integrated data archive (eida; http://eida.rm.ingv.it/) waveform archive, to make it available to the scientific community [mazza et al. 2011, moretti and govoni 2011, mazza et al. 2012, this volume]. the continuous data are archived in seed format and can be requested by the scientific community in seed or sac format. data from the re.mo. and re.mo.tel stations (22 stations, named as t0800-t0828) are available on eida at the time of writing ( july 2012). all of both the real-time and standalone ingv stations are, or are going to be, registered at the international seismological centre (isc; http://www.isc.ac.uk/). some restrictions on data access will be applied to the emersito network. ogs colleagues will share their continuously recorded data, without restrictions, through the oasis portal (http://oasis.crs. inogs.it/) [priolo et al. 2012, this volume]; french colleagues will do the same through the resif portal (http://www. resif.fr/portal). dpc data consists of triggered events; the two mainshocks are available on the official department website, and other events recorded within the dpc-rete accelerometrica nazionale (ran; national accelerometer network), with magnitudes >5.0 will be released soon. a first report on the quality control procedures applied to the temporary network data, and to the very preliminary results obtained by an automatic analysis of the continuous waveforms, has been prepared by marzorati et al. [2012, this volume]. overall the station operation scores were satisfactory: by the end of june, the real-time stations had contributed to the location of about 2000 earthquakes by the rsn, by providing more than 5000 p-wave and more than 4000 s-wave pickings, allowing thus for enhanced completeness and accuracy of the earthquake list distributed by the ingv through iside (figure 5). refined magnitude estimations and earthquake location procedures need to take into account that all of the temporary stations were installed on the soft sediments of the po plain, an aspect that has not been rigorously dealt with in real-time monitoring due to the obvious lack of time. in figure 5, the aftershocks located by the real-time ingv monitoring system, and the stations in real-time, are colored according to their temporal evolution; this allows for a comprehensive view of the seismic sequence spatio-temporal evolution and the parallel rapid-response seismic network development. on the whole, the epicenter distribution was elongated about 50 km in the e-w direction, from the town of ferrara to the village of novi di modena. most of the seismic events and all of those with magnitude >5.0 were thrust events [scognamiglio et al. 2012, pondrelli et al. 2012, this volume], in agreement with the well-known geological structures buried under the sediments of the po plain [boccaletti et al. 2011] and with the ca. n-s compressional stress regime in the region [mariucci et al. 1999]. currently, several research groups are in the process of analyzing the continuous data stored in eida [e.g., marzorati et al. 2012, scognamiglio et al. 2012; govoni, personal communication] for the improvement of the locations, the magnitude estimations, and understanding of the seismic sequence. 5. conclusions rapid earthquake response is very challenging and requires a high degree of preparedness. the coordination projects launched within sismiko at a national scale and inside nera at the european level constitute an important first step towards being prepared. the contacts established in these projects significantly helped the rapidity and coordination of the seismic deployment. the national and european protocols established for data transmission and data archiving have succeeded in improving the real-time monitoring system and the data availability to the scientific community. further improvements are foreseen: at least once every year, there will be the organization of a seismic-risk emergency simulation, which will allow enhanced technical and scientific exchange among the groups, to lubricate the communication and coordination skills. acknowledgements. we want to thank the many people who contributed to the success of the seismic rapid-response in the earthquake area in emilia, and all of our colleagues involved in the sismiko coordination, from the arezzo, catania, gibilmanna, l'aquila and naples centers, who are always ready to take part in any new seismic emergency. we are grateful to elisa zambonelli and other dpc colleagues for the coordinating of our efforts during the deployment of the seismic stations. references amato, a., and f.m. mele (2008). performance of the ingv national seismic network from 1997 to 2007, annals of geophysics, 51 (2/3), 417-431. abruzzese, l., g. de luca, m. cattaneo, g. cecere, v. cardinale, a. castagnozzi, c. d'ambrosio, a. delladio, m. demartin, l. falco, d. franceschi, a. govoni, a. memmolo, f. migliari, f. minichiello, m. moretti, r. moschillo, m. pignone, g. selvaggi and l. zarrilli (2011). la rete sismica mobile in telemetrata satellitare (re.mo.tel.), rapporti tecnici ingv, 177, 32 pp. moretti et al. 588 589 boccaletti, m., g. corti and l. martelli (2011). recent and active tectonics of the external zone of the northern apennines (italy), int. j. earth sci., 100, 1331-1348; 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and magnitudes, annals of geophysics, 55 (4); doi:10.4401/ag-6159. selvaggi, g., f. ferulano, m. di bona, a. frepoli, r. azzara, a. basili, c. chiarabba, m.g. ciaccio, f. di luccio, f.p. lucente, l. margheriti and c. nostro (2001). the mw 5.4 reggio emilia 1996 earthquake: active compressional tectonics in the po plain, italy, geophys. j. int., 144, 1-13. zambonelli, e., r. de nardis, l. filippi, m. nicoletti and m. dolce (2011). performance of the italian strong-motion network during the 2009, l'aquila seismic sequence (central italy), b. earthq. eng., 9, 39-65. *corresponding author: milena moretti, istituto nazionale di geofisica e vulcanologia,centro nazionale terremoti, roma, italy; email: milena.moretti@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. moretti et al. 590 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice wrf-chem modeling of sulfur dioxide emissions from the 2008 kasatochi volcano annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6626 1 wrf-chem modeling of sulfur dioxide emissions from the 2008 kasatochi volcano sean d. egan1*, martin stuefer1, peter w. webley1, catherine f. cahill1, jonathan dean1 1 university of alaska fairbanks, geophysical institute *sdegan@alaska.edu abstract we simulate the dispersion and chemical evolution of the sulfur dioxide (so2) plume following the eruption of kasatochi volcano in alaska, usa, on august 7th, 2008 with the weather research forecasting with chemistry (wrf-chem) model. the model was initialized with the observed three distinct plumes, which were characterized by a total estimated so2 mass of 0.5 to 2.7 tg. wrf-chem modeled output was compared to remote sensing retrievals from the ozone monitoring instrument (omi), and the modeled plumes agreed well in shape and location with the omi retrievals. the calculated so2 column densities showed comparable dobson unit values with higher densities especially in the center of the distal plume over northern canada. we concluded from our analysis that wrf-chem derived a 9.1-day lifetime of the so2 when initialized with a 12km eruption height. sensitivity tests with varying eruption plume heights revealed significantly increased lifetimes of so2 up to 17.1 days for higher plumes. i. introduction asatochi volcano [52.169°n, 175.511°w] is a small (2.7 x 3.3 km, 314 m above sea level, a.s.l.), uninhabited stratovolcano in the aleutian arc of alaska (scott, nye, waythomas, & neal, 2010). on august 2nd, 2008 us fish and wildlife biologists reported small tremors and a sulfur odor while on assignment (waythomas et al., 2010). they were evacuated prior to a m5.8 earthquake on august 7th, 2008 at 2:00 pm akdt (22:00 utc), detected by instruments from the great sitkin seismic network. infrared satellite retrievals from the advanced very high resolution radiometer (avhrr) confirmed the presence of a volcanic plume situated over the volcano’s vent during this time (waythomas et al., 2010). two additional eruptions followed at 01:50 utc and 04:35 utc (scott et al., 2010). the ash and so2 emissions dispersed in a complex pattern due in large part to a low-pressure cyclogenesis situated nearly on top of the volcano (krotkov, schoeberl, morris, carn, & yang, 2010). coarse ash and fine lapilli deposited quickly while most of the fine ash and so2 initially dispersed to the southeast (waythomas et al., 2010). the resulting so2 plume eventually entered the jet stream and traveled into the continental united states and canada within a week (krotkov et al., 2010). the kasatochi eruption is unique for various reasons. it resulted in the largest injection of so2 into the atmosphere since the mount hudson eruption in chile, august 1991. initial estimates were between 1.20 to 2.7 tg (krotkov et al., 2010; prata, gangale, clarisse, & karagulian, 2010). by using inverse transport modeling, kristiansen et al. (2010) established a 1.7 tg k annals of geophysics, fast track 2, 2014 2 mass loading. in addition, plume altitudes exceeded the tropopause (maxima near 7 to 12 km with smaller emissions up to 20 km) introducing about 1.0 tg of so2 into the stratosphere (kristiansen et al., 2010). modeling so2 emissions is useful for various reasons. so2 is often collocated with volcanic ash and thus may be used as a proxy for ash where remote sensing is hindered by ice formation, water or cloud cover. additionally, wrf-chem studies of historical volcanic eruptions are motivated to test and provide source data and model parameterization schemes capable of predicting volcanic so2 and ash eruptions in an operational setting in near real-time. here, we use the well-defined kasatochi so2 eruption to study wrf-chem’s ability to model volcanic so2 transport and conversion. ii. background sulfur dioxide emissions from kasatochi have been modeled previously with particle dispersion models (d’amours et al., 2010; kristiansen et al., 2010). wang and others (2010) studied so2 dispersion and aerosol formation plume height sensitivity using the eulerian geoschem model by initializing the model domain with time-fitted so2 column densities from the ozone monitoring instrument (omi) using the extended iterative spectral fit (eisf) method. this wrf-chem study differs from the geoschem study in that it does not require plume column densities, only specific eruption source data such as location, height, emission rate and duration. these parameters are included in tabulated eruption source parameters (mastin et al., 2009), which may be used to initialize wrfchem for operational volcanic ash and so2 forecasts. sulfur dioxide converts quickly (on the order of days) to sulfate aerosols. in the stratosphere, where the majority of the kasatochi so2 converted to sulfate, the conversion process is dominated by the interaction of so2 with the hydroxyl radical (oh). production of oh begins with the generation of excited states of atomic oxygen from ozone and diatomic oxygen via photolysis. the hydroxyl radical oxidizes so2 in the stratosphere according to equation 1. 𝑆𝑂2 + 𝑂𝐻 + 𝑀 → 𝐻𝑂𝑆𝑂2 + 𝑀 ∗ (1) m* is a third body quencher required to remove excess energy from the reaction. this process was initially proposed to decrease the ambient amount of oh, and thus a second order rate equation would be needed to model it. however, as mentioned by mckeen, liu, & kiang (1984), there is a cycling of the hydroperoxy radical, ho2, and oh in the presence of nitrogen oxide species, nox (mckeen et al., 1984): 𝑂2 + 𝐻𝑂𝑆𝑂2 → 𝐻𝑂2 + 𝑆𝑂3 (2) 𝐻𝑂2 + 𝑁𝑂 → 𝑂𝐻 + 𝑁𝑂2 (3) this cycling ensures the regeneration of oh concentration. if we assume [oh] is constant and [m] varies only with pressure, we may solve the following differential equation to analytically calculate the change in concentration of so2 with time: 𝑑𝑆𝑂2 𝑑𝑡 = −𝑘3[𝑆𝑂2][𝑂𝐻][𝑀] (4) [𝑆𝑂2]𝑓 = [𝑆𝑂2]𝑖 𝑒 −𝑘3,𝑀[𝑂𝐻]𝑡 (5) here, k3[m] is the pseudo first order rate constant based on equation 3. sulfur dioxide and oh also interact via aqueous phase reactions. in such reactions, oh is produced by dissolved hydrogen peroxide in water, which then reacts with dissolved, aqueous so2. iii. methods annals of geophysics, fast track 2, 2014 3 the application of wrf-chem for simulating the transport and effects of volcanic emissions within the atmosphere has been described in (stuefer et al., 2013). importantly, wrf-chem has been proposed as an operational tool for volcanic emissions modeling. here, we test the feasibility of using wrf-chem to capture so2 emissions using the well-studied 2008 kasatochi eruption. the choice of eruption initialization parameters greatly impacts the ability of the model to predict volcanic ash and so2 transport (mastin et al., 2009; webley, stunder, & dean, 2009). table 1 lists the domain initialization parameters used in this study and table 2 provides the initialization parameters for the eruption. we utilized the global forecast system (gfs) final reanalysis (fnl) datasets as base meteorological fields (noaa, 2014). wrf-chem may use either default values for eruption source parameters (esp) or if available, source data from plume observations. karagulian and others (2010) discovered a minimum of 1.7 tg so2 from the kasatochi eruption using remote sensing data from the infrared atmospheric sounding interferometer (iasi). kristiansen and others (2010) utilized inverse transport modeling to establish a similar mass of 1.7 tg based on measurements from uv, ir and lidar data. in a recent geos-chem study by wang et al. (2013) a value of 2.0 tg so2 was used. herein, we initialized wrf-chem with a total of 1.7 tg of so2. this mass was gradually added to the model using a constant eruption rate of 23,600 kgs-1 over the course of the three eruptions, using eruption durations and times based on waythomas et al. (2010) (compare table . wrf-chem initializes, by default, volcanic ash and so2 plumes as an umbrella shape with 75% of erupted mass in the plume surrounding the specified plume height and 25% of the mass linearly detrained underneath (stuefer et al., 2013). for the eruption plume height, the esp implemented within the wrf-chem preprocessor as a default includes a height of 11km for kasatochi. however, in accordance with kristiansen et al. (2010), we chose 12 km a.s.l ± 4 km for this study in order to test the sensitivity of the model to its plume height source. for the example of our mean plume height of 12 km a.s.l, the umbrella will include 75% of the mass between 9 – 13 km a.s.l (peaking at 12 km a.s.l) and 25% below 9 km, linearly decreasing with height (stuefer et al, 2013). table 1: domain parameters for wrf-chem domain size 600 x 400 dx, dy 15 km x 15 km vertical levels 40, terrain following model height 2,000 pa projection lambert-conformal center lat/lon 50°n, -120°w kasatochi erupted over half of the so2 into the stratosphere. therefore, it is important to capture the gas-phase chemistry behind stratospheric so2 oxidation shown in equation 1. the model simulations utilized the second generation regional acid deposition model mechanism (radm2) for gas and aqueous phase reactions. the radm2 model includes the oxidation of so2 by oh as depicted in equation 1 using the first order kinetics in equations 4 and 5 (stockwell et al., 1990) as well as the treatment of the nox species in equations 2 and 3. gaseous precursors, such as no and oh, were loaded into the model using the prep-chemsource 1.4 preprocessor (freitas et al., 2011). as mentioned, so2 also converts via aqueous phase chemistry with oh. this scheme is also parameterized within wrf-chem radm2. annals of geophysics, fast track 2, 2014 4 table 2: eruption times and durations eruption date, time eruption duration eruption rate 8/7 22:00 utc 60 minutes 23600 kg/s 8/8 01:50 utc 30 minutes 23600 kg/s 8/8 04:35 utc 30 minutes 23600 kg/s global ozone and other trace gases, such as so2, are detected by the ozone monitoring instrument (omi), a nadir viewing, ultraviolet (uv)/visible spectrometer aboard the national aeronautical & space administration’s (nasa) earth observing system’s (eos) aura satellite. omi covers a spectral range of 264-504 nm, allowing measurements of ultraviolet and visible so2 signals. it provides global coverage once per day with a nadir pixel size of 13 x 24 km2 and swath width of 2,600 km and has been used in previous research studies for volcanic emissions analysis (kristiansen et al., 2010; krotkov et al., 2010; lopez et al., 2013; wang et al., 2013). for spatial analysis, we utilized nasa’s level 2 so2 product, columnamountso2_stl (from here on stl). column densities of so2 in dobson units (du) for this product are shown in figure 1. since the stl derived data may underestimate the total amount of so2 in plume areas of high concentration, we used values from krotkov et al. (2010) based on the extended iterative spectral fit (eisf) method for mass analysis. applications of this method to kasatochi so2 suggest that it may capture additional so2 that other algorithms might miss (krotkov et al., 2010; yang et al., 2010). iv. results figure 1 shows the dispersion of the plume as modeled by wrf-chem and derived by the omi stl product. the model captured the plume’s interaction with the meteorology well, as it dispersed over the north american continent. figure 2 shows a brief spatial analysis along two transects (105w and 145w) marked in red. plume so2 column densities were generally collocated with omi here, however the normalized masses peaked in different areas. to compare the change in mass in the domain, a linear correlation plot was constructed and presented in figure 3. we saw a high degree of correlation (> 0.9 r2) for all plume height test cases between the change in wrf-chem predicted so2 mass and those observed by omi. figure 1: dispersion of the kasatochi so2 plume as modeled by wrf-chem (left) and calculated by the omi stl product (right). the transects used for spatial analysis are shown in red on the august 11th plot. geopotential height and wind vectors are plotted on wrf-chem plots. a lifetime of so2 was established using linear regression analysis; the lifetimes significantly annals of geophysics, fast track 2, 2014 5 varied with height. the 12km eruption height yielded a 9.1 days lifetime (r2=0.74) while the 8km and 16km plumes resulted in longer lifetimes of 10.6 and 17.1 days (r2=0.72, 0.68), respectively. v. discussion wrf-chem generally predicted a more disperse plume than was observed by omi. in addition, there is a higher mass bias in the model results. this is markedly different from the work of wang et al. (2010) where goes-chem produced a low mass bias. in addition, the mass located in the distal plume trended higher than that in the proximal. this is likely a direct result of the chosen esp used for the modeled case as this varied with the plume height. the rate of so2 conversion agreed well with literature values. we used values from krotkov et al. (2010) using the eisf method and from kristiansen et al., (2010) to test wrf-chem output. in figure 3 we see that all three initialized eruption heights produced r2 values above 0.9. as mentioned, a range of values was produced for the lifetime using linear regression analysis. the 12km eruption produced the shortest lifetime, being about 9 days. this dependence of lifetime on plume height is most likely a direct result of different chemistry at the various levels of the atmosphere. the 12km eruption included so2 mass located mostly in the stratosphere where conversion based on equation 1 dominated, yet also included enough so2 in the troposphere where aqueous phase and heterogeneous chemistry can also occur. wrf-chem captured the dynamics and mass changes of the kasatochi plume according to these results. it is, therefore, a robust candidate for volcanic emissions modeling, especially in the operational setting where multiple unknowns such as a specific plume height are present. figure 2: spatial comparisons of so2 column densities from wrf-chem using 8, 12 and 16 km initialized plume heights and from the omi stl product. figure 3: correlation between wrf-chem (y-axis) and literature omi (x axis) so2 domain masses for 8, 12 and 16 km plume heights. references [d’amours et al., 2010] d’amours, r., malo, a., servranckx, r., bensimon, d., trudel, s., & gauthier-bilodeau, j.-p. 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[yang et al., 2010] yang, k., et al. (2010). direct retrieval of sulfur dioxide amount and altitude from spaceborne hyperspectral uv measurements: theory and application. journal of geophysical research: atmospheres, 115(d2). 1 annals of geophysics, 65, 2, dm214, doi:10.4401/ag-8768 o p e n acc e s s transnational access to research facilities: an epos service to promote multi‑domain solid earth sciences in europe richard wessels*,1, geertje ter maat1, elisabetta del bello2, lucia cacciola2, fabio corbi3, gaetano festa4, francesca funiciello5, george kaviris6, otto lange7, jörn lauterjung8, ronald pijnenburg1, giuseppe puglisi9, danilo reitano9, christian rønnevik10, piergiorgio scarlato2, letizia spampinato2 (1) utrecht university, department of earth sciences, princetonlaan 8a, 3584 cb, utrecht, the netherlands (2) istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, rome, italy (3) istituto di geologia ambientale e geoingegneria – cnr, p.le aldo moro 5, 00185, rome, italy (4) università degli studi di napoli federico ii, dipartimento di fisica, via cintia, 80126, naples, italy (5) università roma tre, dipartimento di scienze, largo san leonardo murialdo 1, rome, italy (6) national and kapodistrian university of athens, department of geology and geoenvironment, zografou, 157‑84, athens, greece (7) utrecht university, university library, heidelberglaan 3, 3584 cs, utrecht, the netherlands (8) geoforschungszentrum, helmholtz‑zentrum potsdam, helmholtzstraße 6/7, 14467 potsdam, germany (9) istituto nazionale di geofisica e vulcanologia, sezione catania, osservatorio etneo, catania, italy (10) university of bergen, department of earth science, allégaten 41, 5020, bergen, norway article history: received november 26, 2021; accepted april 6, 2022 abstract transnational access (tna) allows cross‑border, short‑term and frequently free‑of‑charge access to world-class research facilities, to foster collaborations and exchanges of experience. specifical‑ ly, tna aims to encourage open science and innovation and to increase the efficient and effective use of scientific infrastructure. within epos, the european plate observing system, the volcano observatories and multi‑scale laboratories communities have offered tna to their high‑quality research facilities through national and european funding. this experience has allowed the defi‑ nition, design, and testing of procedures and activities needed to provide transnational access in the epos context. in this paper, the epos community describes the main objectives for the provision of transnational access in the epos framework, based on previous experiences. it includes practical procedures for managing transnational access from a legal, governance, and financial perspective, and proposes logistical and technical solutions to effectively execute transnational access activities. in addition, it provides an outlook on the inclusion of new thematic communities within the tna framework, and addresses the challenges of providing market‑driven access to industry. keywords: epos; transnational access; tna; solid earth sciences; physical and remote access richard wessels et al. 2 1. introduction transnational access (tna) can broadly be defined as the process in which actors, such as researchers, are accessing services or organizations located in another country. the term transnational access has been in use for many decades [sommeren and tallberg, 2016] and an overview of the historical evolution of this term is described in jönsson [2010]. transnational access occurs in a broad range of contexts and actors, including governmental organizations (e.g., jönsson and tallberg, 2010), law [e.g., krämer, 2012] and justice [e.g., whytock, 2020], as well as scientific domains [e.g., journeau et al., 2003; fardis and biskinis, 2015; simell et al., 2019]. tna to scientific research infrastructures (ris) has gained significant traction through the european commission framework and h2020 programs [european commission, 2018]. research infrastructures are crucial for meeting the three main strategic priorities of the european commission (ec), namely open science, open innovation, and open to the world [european commission, 2016a]. the euro‑ pean research infrastructures aim at bringing together and opening up key national and regional facilities to all european researchers, from both academia and industry, ensuring their optimal use and joint development. in this framework, an increasing number of research infrastructure projects have offered access to their facilities: either transnational access, based on calls for proposals, or virtual access through online services. in particular, european research infrastructure projects relevant to the environmental (e.g., envri plus1), earth sciences (e.g., euro‑ volc2, sera3), biological and medical sciences (e.g., inext4), energy (e.g., eccsel5), and material and physical sciences (e.g., hemera6), have offered transnational access to various high‑quality research facilities7. epos, the european plate observing system, is a multidisciplinary, distributed european research infrastruc‑ ture consortium (eric) that facilitates the integrated use of data, data products, and facilities, within the solid earth sciences community in europe [cocco et al., 2022]. the epos community has particularly benefited from experience obtained within the horizon 2020 infrastructure projects envriplus and eurovolc, which allowed the definition, design, and testing of procedures and activities needed to provide transnational access. these expe‑ riences offered the research institutions the opportunity to expand their potential for contributing to the european research ecosystem, in terms of resources sharing, networking, and scientific collaboration. moreover, the resulting scientific collaborations have allowed the exchange of scientific expertise and skills within and across different scientific domains, thus promoting new scientific developments. in this paper, the epos community uses their previous experiences to define the main objectives for the pro‑ vision of transnational access in the epos framework. it proposes practical procedures for managing the accesses from a legal, governance, and financial perspective, and proposes logistical and technical solutions to effectively execute transnational access activities. 2. definitions within the context of epos, the following definitions are identified, which are based on the principles and guidelines formulated by the european commission [2016b]: – facilities: facilities, observatories, laboratories, stations, platforms, research vessels, equipment, resources and services that are used by the research communities to conduct research and foster innovation in their fields. they include, but are not limited to, major scientific equipment (or sets of instruments), knowledge-based resources such as collections, archives and scientific data, e-infrastructures, such as data and computing sys‑ tems and communication networks and any other tools that are essential to achieve excellence in research and innovation. 1 https://cordis.europa.eu/project/id/654182 2 https://cordis.europa.eu/project/id/731070 3 https://cordis.europa.eu/project/id/730900 4 https://cordis.europa.eu/project/id/653706 5 https://cordis.europa.eu/project/id/675206 6 https://cordis.europa.eu/project/id/730970 7 https://ec.europa.eu/info/files/research‑infrastructures‑offering‑free‑access‑eu‑support_en https://cordis.europa.eu/project/id/654182 https://cordis.europa.eu/project/id/731070 https://cordis.europa.eu/project/id/730900 https://cordis.europa.eu/project/id/653706 https://cordis.europa.eu/project/id/675206 https://cordis.europa.eu/project/id/730970 https://ec.europa.eu/info/files/research-infrastructures-offering-free-access-eu-support_en epos transnational access 3 – users: individuals, teams and institutions from academia, business, industry and public services, which are engaged in the conception or creation of new knowledge, products, processes, methods and systems and also in the management of projects. teams can include, but are not limited to, researchers, doctoral candidates, tech‑ nical staff and students participating in research in the framework of their studies. – providers: the research infrastructures hosting, offering and granting access to their own facilities to users. – access: the legitimate and authorized physical and remote admission to, interactions with and use of facili‑ ties and services offered by providers to users. such access can be granted, amongst others, to machine time, computing resources, software, data, data‑communication services, trust and authentication services, sample preparation, archives, collections, set‑up, execution and dismantling of experiments, education and training, expert support and analytical services. access is divided into physical and remote access: – physical access: involves hands‑on access of users to the facility (i.e., a physical visit) – access to limited resources for which competitive selection is needed. – remote access: user does not visit the facility physically – access to limited resources for which competitive selection is needed. this can involve remote operation or use of a facility, or remote service by an operator at a facility. within this context, two modes for facility access are identified within epos: – excellence‑driven access: excellence-driven access is exclusively dependent on the scientific excellence, originality, quality and technical, logistical, and ethical feasibility of an application that is evaluated through peer review conducted by internal or external experts. this access mode enables collaborative research and technological development efforts across geographical and disciplinary boundaries. – market‑driven access: market-driven access applies when access is defined through an agreement between the user and the facility that will lead to a fee for the access, which may remain confidential. 3. transnational access in epos transnational access (tna) to research facilities allows cross‑border short‑term, frequently free‑of‑charge access to world-class facilities, to foster collaborations and exchanges of experience. specifically, tna aims to encourage open science and innovation and to increase the efficient and effective use of instrumentation. in this chapter we provide the rationale for tna provision in the context of epos, explore the lessons learned from the experience by the epos thematic communities at the european and national levels, elaborate on the epos tna procedures, governance, and financial framework, and address the technical implications on operating tna. 3.1 rationale for tna provision modern geosciences have developed and operate research infrastructures containing an increasingly larger number of scientific instruments, facilities and experimental environments. often these facilities are highly spe‑ cialized and, in many cases, unique worldwide. a strong interaction and cooperation between research infrastructures, industry and public services, and users, helps build bridges between the public, commercial and research infrastructure worlds. this increases knowledge and technology transfer between these stakeholders and drives innovation. in addition to acting as a user, industry also plays an increasing role in the construction, operation and development of research infrastructures. tna provision moreover promotes interaction with industry and public services, in order to maximize the return on investment in research infrastructures, foster competitiveness and increase efficiency in terms of use of the scarce resources available. epos offers a large variety of high‑level and highly specialized facilities, such as analogue modelling laborato‑ ries, high‑pressure, high‑temperature melt, rock physics and mineralogical experiments, volcano and near‑fault observatories, deep boreholes, and instrument pools for field experiments and measurements. transnational ac‑ cess to these facilities provides the opportunity to a researcher or a group of researchers to access research infra‑ richard wessels et al. 4 structure facilities in a country foreign to their own affiliation. the purpose of this access is to allow cross-border short‑term access to world‑class facilities to foster new science, collaborations and exchange of experience. 3.2 lessons learned from previous experience in epos the epos community is organized in ten thematic core services (tcs), which represent sub‑communities within the solid earth sciences (e.g., seismology, volcanology, geodesy, laboratories, etc.). each tcs has its own dedicated governance framework, including a consortium board, which is responsible for the data and services, legal, and financial framework of the tcs. the e-infrastructure aspect of epos is organized in the integrated core services (ics), which allows access to multidisciplinary data, data products, and services. the key element of the ics in epos is a central hub (ics‑c) where users can discover and access data and data products. the epos european research infrastructure consortium (eric) coordinates the legal framework aimed at operating epos and providing effective governance to the tcs and ics8. for a more detailed description of epos and its various components, the reader is referred to the respective publications in this special issue [e.g., cocco et al., 2022; elger et al., 2022; puglisi et al., 2022]. within the context of epos, two tcs have already provided access to their facilities: the tcs multi‑scale labo‑ ratories (msl) and the tcs volcano observations (vo). in addition, the national program epos-nl has been using dedicated funding to provide free‑of‑charge physical and remote access to dutch research facilities since early 2021 and will continue to do so in the coming years. 3.2.1 experience from tcs msl tna calls the tcs multi‑scale laboratories (msl, figure 1) developed and implemented tna during three different call‑for‑proposals in 2017, 2018, and 2019, which served as a proof of concept for the epos tna general principles that were therein applied. the number of facilities involved in these tna calls increased from five in the first year, 8 https://www.epos‑eu.org/about‑epos/what‑we‑do figure 1. selection of epos tcs multi‑scale laboratories facilities. https://www.epos-eu.org/about-epos/what-we-do epos transnational access 5 to 22 in the second, and to 38 in the third year, which mirrors the success of the tna program. additional statistics can be found in table 1. participating laboratories were encouraged to provide free‑of‑charge access, in order to be more attractive to users. the 22% of laboratories that could offer full financial support to users during the second msl tna call in 2018 received 46% of the applications. epos msl employed user feedback forms to evaluate tna procedures and processes, which also presented a clear correlation between financial support and application to specific facilities. out of 18 feedback replies, 17 indicated that they would not have been able to carry out tna without the available financial support. these statistics clearly indicate the need for and the importance of available funding to make tna a success. statistics about length of the visits are also relevant for a funding organization, to assess possible access costs related to financial support, especially for accommodation. from the msl tna calls, 63% of granted projects had access periods up to 15 days, while only 20% needed 21 days or more for execution (table 1). a milestone in the success of msl tna was the publication in 2019 of the first paper and dataset that resulted directly from access under the epos tna framework (boulton et al., 2019). this positively demonstrated the effectiveness of the epos data policy, which is aimed at making data collected under the tna program openly accessible to the public. during the three msl tna calls the functioning and implementation of tna workflows were tested and refined, which are at the basis of tna provision within epos eric. 3.2.2 experience from the tcs vo tna calls the tcs volcano observatories (vo, figure 2) has experience in tna starting with the h2020 envri plus9 project (2015-2019), and subsequently in the h2020 eurovolc10 project (2018-2021). during the envri plus phase, the partners ingv and cnrs contributed to implement tna at observatories on mount etna and piton de la fournaise volcanoes. in the eurovolc project the portfolio of providers and the list of proposed services expand‑ ed. this project has granted hands‑on and remote access to research facilities in italy, iceland, french overseas territories, spain and portugal. during the first tna call in 2018, 12 out of 28 submitted proposals were funded. 9 https://cordis.europa.eu/project/id/654182 10 https://cordis.europa.eu/project/id/731070 figure 2. selection of epos tcs volcano observatories facilities. https://cordis.europa.eu/project/id/654182 https://cordis.europa.eu/project/id/731070 richard wessels et al. 6 during the second call that started in 2019, 27 out of 42 submitted proposals were funded, which is an increase of 44% in terms of submitted proposals. although the majority (75%) of the applications originated from eu‑coun‑ tries, a total of 25% of proposals were submitted from outside the eu, mostly from the americas. the statistics for the eurovolc calls are summarized in table 1. the tna call management was revised during the second call in response to lessons learned from the first call. specifically, the evaluation and application procedures were simplified by the use of a web portal developed by ingv catania, which handled these processes and allowed better promotion and clearer guidelines for proposal submission. also, the procedure for the evaluation of the access proposals was revised in order to optimize the feedback from the tna providers about the technical feasibility of the proposals and to improve their quality. these processes, including the web portal, are at the basis of the development of epos tna. 3.2.3 experience from a national program: epos‑nl epos‑netherlands (epos‑nl11) is the dutch research infrastructure for solid earth sciences and contributes with data, facility access, coordination, and it‑resources to epos. funded by the dutch research council (nwo), epos‑nl has been providing free‑of‑charge access to top dutch research facilities since early 2021 and will contin‑ ue to do so until at least 2025, and possibly beyond. throughout 2021, epos‑nl funded 24 out of 31 applications from both within and outside the eu. successful applicants were granted access, plus associated technical and scientific support, to the rock physics, analogue tectonic modelling, and microscopy and tomography facilities at utrecht university and delft university of technology. for additional statistics, see table 1. due to covid restrictions, access was frequently provided remotely, where successful applicants sent their sam‑ ples to the netherlands for analysis. it was generally experienced that such remote access was particularly effective in the case of short analyses (several hours), requiring high‑end apparatus operated locally by an experienced staff member. hence, access to the microscopy and tomography laboratories was in all cases provided remotely, which will remain a highly efficient mode of operation after covid restrictions relax. on the other hand, where it came to longer, more laborious analyses or experiments (taking longer than one week), physical access was typically preferred. in such cases, training a visiting researcher was found to be more time-efficient and beneficial for the exchange of knowledge, compared to letting a staff member from the hosting facility execute the project. physical access was typically preferred in the case of analogue tectonic modelling. epos tna statistics group year offered facilities/ instruments no. of applications applicant’s institute granted access type granted access days submitted granted eu non‑eu physical remote min avg max msl 2017 5/15 7 6 50% 50% 66% 33% 5 16 35 msl 2018 22/72 30 23 65% 35% 86% 14% 3 15 45 msl 2019 38/211 33 28 52% 48% 82% 18% 2 11 30 vo 2018 25/45 28 12 82% 18% 100% 0% 5 10 24 vo 2019‑2021 28/63 42 27 75% 25% 96% 4% 1 20 40 epos‑nl 2021 5/16 31 24 68% 32% 87% 13% 1 5 30 table 1. overview of epos tna statistics. statistics relevant to the vo tna from 2019 to 2021 refer to the same call that was launched in 2019 but extended until 2021 due to the covid pandemic. 11 https://meetingorganizer.copernicus.org/egu21/egu21‑2670.html https://meetingorganizer.copernicus.org/egu21/egu21-2670.html epos transnational access 7 3.3 epos tna governance and finance the epos tna governance involves a variety of stakeholders, each with their specific roles and relations. the financial model is based on the horizon 2020 tna regulations. the governance and financial structure as described here is specific for epos, but can be generically applied to other projects as well. 3.3.1 tna governance and legal framework the highest governing authority, represented by its executive committee, is epos eric. the eric is responsible for the validation of each tcs participating in tna and acts as a potential funding agency (figure 3). figure 3. tna stakeholders and governance structure. the consortium board (cb) of each tcs is responsible for managing tna within their thematic domain. the cb is responsible for the selection of tna providers, the appointment of the scientific evaluation committee (sec), and the appointment of the tna coordinator – i.e., the legal entity responsible for this specific tcs. across the different tcs, all tna coordinators are organized in a facility access board (fab), which liaises with epos eric on topics concerning tna. the tna coordinator signs a collaboration agreement with the eric in which the requirements and financial regulations for coordinating tna are set out. the tna coordinator is also responsible for collecting requirements from the tna providers and the subsequent harvesting of tna‑related metadata by the epos tna brokering ser‑ vice. in addition, the tna coordinator signs a provider agreement with the individual tna providers, in which the requirements and financial regulations for providing tna are set out. the tna providers, in turn, sign an access agreement with each user in which the legal requirements and finan‑ cial regulations for participating in tna are specified, involving topics such as finances, reimbursement schemes, schedules, access rules, health and safety rules, compliance with data policy, and privacy policy. the privacy policy ensures that all parties involved comply with the general data protection regulation (gdpr) and states which personal data is collected, how this is collected, where and for how long this is stored, who has access to the in‑ formation and how the user can request to remove this data. the data policy is aimed at users and sets out the regulations for the licensing, use, sharing, and exploitation and open access to publications, data, data products richard wessels et al. 8 and digital tools resulting from epos tna activities, in accordance with the findable, accessible, interoperable, and reusable (fair) principles [wilkinson et al., 2016]. 3.3.2 tna financial framework epos follows the horizon 2020 financial regulations12 to calculate the costs of access to a facility, which can be the unit cost method, the actual costs method, or a combination of both. upon usage of the facility by users in the tna context, the facility is reimbursed based on the before‑mentioned methods. tna users can, if budget allows, be reimbursed for expenses such as travel, accommodation, and sustenance. the flux of reimbursements can follow two routes, scenario 1 and scenario 2, as indicated in figure 4. the preferred route depends on the financial regulations of the different legal entities and countries involved. the necessary financial justifications follow the same route, but in opposite direction, towards the granting authority, which in fig. 4 is depicted as epos eric. figure 4. reimbursement schema for tna. 3.4 technical aspects of tna provision the goal of epos is to fully incorporate tna within its ecosystem. the wealth of heterogeneous data generated by epos tna activities must be able to flow through the federated epos services into the epos data portal, there‑ by ensuring the fairness of that data. facilities, instrumentation, and the services that are made available through epos tna need to be stored in databases and be findable through the epos channels. to achieve this interoperability, there is a need for a common metadata model describing both epos tna and the epos data services. in addition, the epos tna calls need to be advertised within the epos framework, which requires a dedicated tna brokering service. epos tna consists of the following components: a coordination module and database that is kept at the level of each individual tcs, and a brokering service that showcases the epos tna calls (figure 5). this section address‑ 12 https://ec.europa.eu/research/participants/data/ref/h2020/other/legal/unit_costs/unit‑costs_tna‑infra_en.pdf https://ec.europa.eu/research/participants/data/ref/h2020/other/legal/unit_costs/unit-costs_tna-infra_en.pdf epos transnational access 9 es the different elements that are needed to reach these goals, and provides an example of how this was accom‑ plished within the eurovolc project. 3.4.1 epos tna conceptual metadata model based on the experience from previous tna calls, the tcs msl and vo mapped-out the infrastructures and resources needed to operate tna. after this mapping a first harmonization step was made, which resulted in the identification of commonly used terminology, thereby providing a minimum of metadata that needs to be collect‑ ed for a successful tna implementation. the identification of dependencies between these assets resulted in the creation of a conceptual metadata model. a total of eight classes were identified, being research infrastructure, facility, equipment, measurement, sample, research data, person, and tna call information. this metadata model contains the minimum set of classes needed to describe a tna call, the roles of persons involved in the tna call, the participating infrastructures and equipment, the type of experiments that can be performed, the samples that can be acquired or analyzed, and the resulting data. 3.4.2 harmonization with the epos‑dcat‑ap model and cerif catalogue after identifying a conceptual metadata model for epos tna, a harmonization step towards the common meta‑ data model employed by epos was necessary. to enable semantic interoperability of information within its ecosys‑ tem, epos has created a metadata model built on the established w3c standard data catalog vocabulary (dcat) application profile (ap), termed epos-dcat-ap, which is represented in the resource description framework (rdf)/turtle format [bailo et al., 2020]. in addition, epos employs the common european research information format (cerif) catalogue to store relations between research entities. the tna conceptual model thus needed to be mapped onto the epos‑dcat‑ap and cerif models to be compliant with the epos ecosystem. most of the classes in the tna metadata model matched with entities already available in the epos metadata model, such as person, facility, and equipment, or otherwise required additional attributes. an example is the ad‑ dition of the attribute roles that a person can have within the tna process. in other cases, the decision was made to repurpose existing or multipurpose entities in the epos‑dcat‑ap, and extend these with additional attributes. an figure 5. epos tna components. richard wessels et al. 10 example of this was the modification of the service class in epos-dcat-ap with temporal and spatial attributes, to handle the complexities of the tna call information class. this mapping of classes from the tna conceptual metadata model onto the epos‑dcat‑ap and cerif models ensures that tna information can be seamlessly incorporated within the epos framework. 3.4.3 eurovolc tna portal and database information about the facilities and personnel participating in tna varies on a per‑call basis, which is therefore governed and stored by the individual tcs. prior to the completion of the harmonized tna metadata model, each tcs employed their own register structure to store this information. in addition, each tcs employed their own portal for announcing and handling tna calls. below is an example of the eurovolc tna application portal and database. lessons learned from the eurovolc experience are being implemented by all tcs for future tna calls. with the experience from the first eurovolc tna call in 2018, a software tool was implemented during the second call (2019) to manage and disseminate tna information. a dedicated tna portal was developed within the vo-tcs gateway i) to facilitate discovery of information related to facilities offered through the call, ii) to manage the proposal submission phase, through the implementation of a tna submission form and the development of a searchable relational database containing the relevant metadata, iii) to aid the proposal evaluation phase, and (iv) to enable statistical analysis of call‑related information. the front‑end of this software tool was a webpage that consisted of four main sections: 1) the discovery page of tna calls; 2) the user registration/access page for completing and revising the tna proposal submission form; 3) a submission guide and general information on how to use the portal, and 4) the f.a.q. section, dedicated to the most frequently asked questions. 3.4.4 the epos tna brokering service although the actual detailed management of a tna application is executed at the tcs level, information about epos tna possibilities has to be available at the ics‑c for end‑users (fig. 5). this information is to be offered through a so‑called tna brokering service, which was designed to provide a cross‑tcs catalogue of the tna possi‑ bilities offered by epos that underpin the various basic thematic services. this service will contain a graphical user interface that is embedded within the epos data portal, through which users can discover tna‑related services based on spatial and temporal constraints. more specific details can be found at the tcs level, thus a url link to the tcs tna pages is also required. this brokerage service will collect and expose relevant tna‑related information (e.g., structured metadata on facilities and equipment, and temporal and spatial information) by harvesting the tcs tna databases, and provid‑ ing redirection to applicable tna services for interested users. as such, users can search for relevant equipment and facilities through the epos ics‑c and simultaneously obtain direct access to the relevant tna information at the tcs level. 3.5 epos tna procedures the epos tna procedures describe the processes and interactions related to tna. these procedures are based on the principles of non‑discrimination and transparency, inspired to provide equal opportunities and encour‑ aging the access to new users. they build on previous tna experience in epos and on recommendations in the european charter for access to research infrastructures [european commission, 2016b] and envri plus d10.113 and d10.214 deliverables. 13 https://www.envriplus.eu/wp‑content/uploads/2015/08/d10.1.pdf 14 https://www.envriplus.eu/wp‑content/uploads/2015/08/d10.2.pdf https://www.envriplus.eu/wp-content/uploads/2015/08/d10.1.pdf https://www.envriplus.eu/wp-content/uploads/2015/08/d10.2.pdf epos transnational access 11 the epos tna procedures are organized in a workflow that consists of at least: a) preparatory phase, b) propos‑ al submission phase, c) review phase, d) access phase, and e) reporting phase. this workflow is described in detail below and shown in figure 6. during the call preparation phase, the tna coordinator collects requirements from the tna providers regard‑ ing the mode(s) of access, units of access, and available facilities for the specific call period. tna legal documents are also drafted in this period. the collected information is published through the tna brokering system and available on the dedicated call website. in the proposal submission phase, users create a scientific proposal that is submitted through an online applica‑ tion. interaction with the requested facility is advised at this time to discuss the technical and logistical feasibility of the project. during the evaluation phase, submitted proposals are first subjected to a check that verifies the eligibility of a user and their application, based on admissibility, legal, and ethical criteria. the tna coordinator is responsible for this check, but the verification may take place through self-assessment by the applicants acknowledging the criteria to which their application must adhere. subsequently, the feasibility of the proposal is checked based on lo‑ figure 6. access procedures for epos tna. richard wessels et al. 12 gistical, technical, and risk criteria. this check is managed by the tna provider and may result in an exchange with the applicant before the proposal is deemed feasible. the excellence‑driven access mode adopted by epos tna requires scientific peer-review of the proposals by a selected panel of experts, whose composition and functioning are based on principles of transparency, fairness and impartiality. the proposal will be reviewed using a predefined scoring system with criteria based on scientific excellence, originality and quality. additional evaluation criteria can be tailored to a specific tna call by the respective tcs. the outcome of the scientific review is a ranked list of all submitted proposals. the minimum threshold for acceptance can be adjusted to match the availability of resources in a call (e.g., budget, availability of instruments). if needed, the coordinator can agree with the facilities to re‑distribute successful proposals. the accepted projects are executed during the access phase. for physical access projects, the user receives the necessary safety training provided by the tna facility. for remote access projects, the user is informed about reg‑ ulations regarding the remote use of the facility and services where necessary. the access agreements between the user and the facility are signed before the beginning of the access period. the reporting phase for each tna call consists of the following components; i) setting the feedback require‑ ments, ii) collecting feedback from the users, facilities, and scientific review panel, iii) reporting call statistics and feedback to the relevant governing authorities, and iv) updating of tna procedures, when necessary. 4. future perspectives the epos tna landscape is continuously evolving. lessons learned from previous tna calls, changes in funder requirements, the identification of new potential user groups, as well as scientific communities that are not yet providing tna, all require refining and updating of the epos tna processes. in addition, the experiences gained under epos can be used to obtain dedicated tna funding through communityor country-specific funding calls. this last section addresses some of the challenges and opportunities that epos tna currently faces. these include, but are not limited to, i) connecting new thematic communities within the scope of tna, exemplified by the tcs near‑fault observatories (nfo) and candidate‑tcs tsunami, and ii) opportunities for market‑driven access to users from industry. 4.1 connecting new thematic communities – the nfo and tsunami example the successful implementation of tna by the epos tcs volcano observatories and the epos tcs multi-scale laboratories has drawn interest from other thematic communities within, or soon to be included within, epos. the goal is to provide tna to as broad an earth sciences community as possible. to this end, thematic communities in‑ terested in providing tna need to adhere to the rules, regulations, and technical requirements as outlined by epos eric. after approval these communities can then provide tna in the epos context. one of the challenges lies in the different temporal and spatial scales that each thematic community studies, and the associated complexities in experimental setups, which subsequently need to be included within the epos tna framework. the epos near‑fault observatories (nfo) consists of multidisciplinary research infrastructures aimed at mon‑ itoring chemical and physical processes occurring on faults, investigating the deformation processes and the me‑ chanical behavior of the observed geological structures, and detecting the precursory phase of large seismic events [chiaraluce et al., 2022]. the facilities range from multi‑instrument networks of seismological stations, gnss sta‑ tions, and fiber optics, such as the corinth rift lab (crl)15, to integrated facilities linked to seismic networks for real‑time testing, such as crew16. the candidate tcs tsunami is concerned with simulating and investigating the processes and consequences of tsunamis. examples of facilities within the tsunami community include the fast flow facility17 flume tank (figure 7) with auxiliary equipment operated by hr wallingford, aimed at studying the characteristics of a tsuna‑ 15 https://nfo.crlab.eu/ 16 http://lccepos.fisica.unina.it 17 https://www.hrwallingford.com/facilities/tsunami‑simulator https://nfo.crlab.eu/ http://lccepos.fisica.unina.it https://www.hrwallingford.com/facilities/tsunami-simulator epos transnational access 13 mi wave, and the cantabria coastal and offshore basin (ccob18) and wave‑current‑tsunami flume (cocotsu19) hydraulic facilities, integrating physical modelling with numerical simulations. a major challenge is to harmonize and integrate the heterogeneous facilities within epos, whilst maintaining the unique elements that each facility offers, and presenting them to the user communities. integrated physical and numerical facilities also provide an opportunity to test access to high‑performance computing installations that represent a limited resource for which access is based on a competitive selection of research proposals. figure 7. fast flow facility flume tank at hr wallingford. 4.2 market‑driven access to industry increasing the interaction and cooperation between the epos facilities, academic and industrial users, and public services, helps build bridges between these stakeholders and enhance their socio‑economic impact. in ad‑ dition, such dedicated initiatives will help increase the technology and knowledge transfer between academia and public and industrial services, thereby driving innovation. to support these efforts, epos is not only exploring the opportunity for allowing users from private sector and industry access the research infrastructures, but also for allowing academic users access to facilities offered by in‑ dustry. the development of a framework for a cooperative tna with the private sector is already part of the epos eric roadmap, and will also be included in the epos tna policy in the future. challenges related to this include i) establishing a regulatory concept to handle such partnerships, including a financial model, especially since access fees are anticipated to be involved, ii) understanding how the research output of this mode of tna with the private sector aligns with the epos data policy and the principles of fair open‑science, including the licensing of data, data products, and software, and iii) assessing the needs and inter‑ ests of key industrial sectors to promote epos as a tna facilitator. epos is currently addressing this challenging topic and anticipates to be able to provide tna in cooperation with the private sector in the near future. 18 https://ihcantabria.com/en/instalaciones‑experimentales/laboratorio‑de‑hidraulica‑costas‑y‑offshore/ 19 https://www.ictsmarhis.com/en/coco‑tsu https://ihcantabria.com/en/instalaciones-experimentales/laboratorio-de-hidraulica-costas-y-offshore/ https://www.ictsmarhis.com/en/coco-tsu richard wessels et al. 14 5. conclusions transnational access (tna) allows cross‑border short‑term access to world‑class research facilities, fosters col‑ laborations and the exchange of experiences. within epos, the volcano observatories and multi-scale laborato‑ ries communities have used national and european funding to offer tna to their high‑quality research facilities. this funding has proven vital to lower costs, or even provide free‑of‑charge access to researchers, who would otherwise not have been in the position to visit these facilities and utilize their services. the epos tna governance structure, which includes legal, financial, and operational stakeholders, has been responsible for the establishment of transparent and non‑discriminatory tna principles designed to provide equal opportunities to its users. the streamlined execution of these procedures has enabled a steady growth of partic‑ ipating facilities and submitted applications since the first call launched in 2017. the tna legal framework has ensured that the rights and obligations of the participating parties are well documented. the tna data policy has guaranteed the fair publication of research results, in turn providing maximum impact to the scientific commu‑ nity, while the privacy policy has guaranteed compliance with gdpr standards to safeguard the privacy of the researchers involved. harmonization of the metadata and the creation of a common metadata model capable of describing both epos tna and the epos data services has proven pivotal to achieving the interoperability needed to integrate tna within the epos ecosystem, while ensuring the fairness of the produced data. the provision of physical or remote access to research facilities under the excellence‑driven access mode has been well‑established within the epos framework and will be further expanded to include other epos thematic communities within the coming years, specifically the near-fault observatories (nfo) and the candidate tcs tsunami. the provision of market-driven access to industry, however, still remains a challenge, especially concerning the financial and regulatory aspects and the fair publication of obtained results. these challenges are currently being met and we anticipate providing tna for the private sector in the near future. references bailo, d., r. paciello, m. sbarra, r. rabissoni, v. vinciarelli and m. cocco (2020). perspectives on the implementation of fair principles in solid earth research infrastructures, front. earth sci., 8, doi:10.3389/feart.2020.00003. boulton, c., a. r. niemeijer, c. j. hollis, j. townend, m. d. raven, d. k. kulhanek and c. l. shepherd (2019). tempera‑ ture‑dependent frictional properties of heterogeneous hikurangi subduction zone input sediments, odp site 1124, tectonophysics, 757, 123‑139, doi:10.1016/j.tecto.2019.02.006. chiaraluce, l., g. festa, p. bernard, a. caracausi, i. carluccio, j. clinton, r. di stefano, l. elia, c. p. evangelidis, s. ergintav, o. ianu, g. kaviris, a. marmureanu, s. šebela and e. sokos (2022). the near fault observatory community in europe, ann. geophys., in press cocco, m., c. freda, k. atakan, d. bailo, k. saleh contell, o. lange and j. michalek (2022). the epos research infrastructure: a federated approach to integrate solid earth science data and services, ann. geophys., this volume. elger, k., g. ter maat, r. caldeira, c. cimarelli, f. cobri, s. dominguez, m. drury, f. funiciello, o. lange, a. ougi‑ er‑simonin, m. rosenau, r. wessels, e. willingshofer and a. winkler (2022). the epos multi‑scale laborato‑ ries: a fair framework for stimulating open science practices across european earth sciences laboratories, ann. geophys., in press. european commission, directorate‑general for research and innovation (2016a). open innovation, open science, open to the world: a vision for europe, publications office, doi:10.2777/552370. european commission, directorate‑general for research and innovation (2016b). european charter of access for research infrastructures: principles and guidelines for access and related services, publications office, doi:10.2777/524573. european commission, directorate‑general for research and innovation (2018). european research infrastruc‑ tures in motion: transnational access, integrated initiatives and coordination actions, publications office, isbn:92‑79‑00434‑4. fardis, m.n. and d. biskinis (2015). transnational access to european seismic research facilities, proceedings of the institution of civil engineers – structures and buildings, 168, 1, 775‑787, doi:10.1680/jstbu.13.00087. epos transnational access 15 jönsson, c. (2010). capturing the transnational: a conceptual history. in transnational actors in global gover‑ nance by c. jönsson and j. tallberg (editors), palgrave macmillan, london, doi:10.1057/9780230283220_2. jönsson, c. and j. tallberg (2010). transnational actors in global governance, springer, doi:10.1057/9780230283220. journeau, c., p. piluso, l. godin‑jacqmin, i. ivanov, i. mladenov, p. grudev, s. kalchev, h. alsmeyer and b. eppinger (2003). transnational access to the plinius prototypic corium experimental platform, proceedings of the 2003 fisa symposium. krämer, l. (2012). transnational access to environmental information. transnational environmental law, 1, 95‑104, doi:10.1017/s2047102511000070. puglisi g., d. reitano, l. spampinato, k. s. vogfjörd, s. barsotti, l. cacciola, a. geyer traver, d. s. guðjónsson, y. guehenneux, p. labazuy, a. lemarchand, r. nave, j. m. saurel, p. bachelery (2022). the integrated multi‑ disciplinary european volcano infrastructure: from the conception to the implementation, ann. geophys., in press. simell, b.a., o. m. törnwall, i. hämäläinnen, h. e. wichmann et al. (2019). transnational access to large pro‑ spective cohorts in europe: current trends and unmet needs, new biotechnology, 49, 98‑103, doi:10.1016/j. nbt.2018.10.001. sommerer, t. and j. tallberg (2016). transnational access to international organizations 1950‑2010: a new data set, international studies perspectives, doi:10.1093/isp/ekv022. wilkinson, m., m. dumontier, i. aalbersberg et al. (2016). the fair guiding principles for scientific data manage‑ ment and stewardship, sci. data, 3, 160018, doi:10.1038/sdata.2016.18. whytock, c.a. (2020). transnational access to justice, berkeley journal of international law, 38, 2, 154‑184. *co r r e s po n d i n g a u t h o r: richard w e s s e l s, utrecht university, department of earth sciences, princetonlaan 8a, 3584 cb, utrecht, the netherlands, email: r.j.f.wessels@uu.nl mailto:r.j.f.wessels@uu.nl o n estimation of the displacement in an earthquake source and of source dimensions v . k e y l i s b o r o k 1. introdtjction. the study of an earthquakes mechanism is usually fìnished by the determination of a fault piane and the direction of displacement along it. in many aspects of geodynamics and seismology it would be very important to estimate also the dimensions of a source and cliiefly — the magnitude of the displacement in it. this problem can be reduced to the comparing of fields of sfaticai stress before an earthquake and after it. the difference of energies of these two fields depends upon source properties in question; lience it is possible to determine dimensions of the source and the displacement in it if the forni of the source and the difference of the energies be known. the energy difference can be found from observations on the seismic waves energy. the idea of such an approach belongs to l. knopoff. he solved the two — dimensionai problem which. deals with the study of very strong earthquakes with long faults. the analogica! three — dimensionai problem is considered in this paper; it is necessary for studying of more numerous weaker earthquakes and investigation of some contiguous problema. 2. t h e o r y . the problem can be formally stated as following. before an earthquake a homogeneous field of tangentional tractions v exist xxy — p, with corresponding displacement u = cy, v = ex, c m being arbitrary. w e shall cali this field an initial field. in the moment of the earthquake tangentional tractions vanisli along the fault piane, x* + z2 < 62, y = 0. 2 0 6 v . k e y l i s b o r o k a zone of crusmng or intence plastic fìows can appear around the fault piane. this zone must be included in the source model because the tangential tractions in it deerease or vanish; that is why we shall consider the source as an ellipsoid. x2 -fz2 y2 ~b2 a2 0 < a < b [1] now we shall examine an final field of stress which is defined by two conditions: 1 displacements and stress tend to initial ones at infinite distances; 2 the source boundary is free from stress except the hydrostatic pressure. w e shall suppose the final field to appear after the earthquake with the source [1]. i neglect here plastic fiows and an inhomogeneity of the initial field which can really exist. besides, the source boundary in the above meaning can be really not sharp. thus idealized problem in its main part can be reduced to this one which is already solved by neuber in the theory of stress concentration. i omit here ealculations which are not complicated principally and shall give the final result. the neuber's solution is obtained in elhptical coordinates: x = chu sin v cos w, y = sliu cos v, z = chu sin v cos w; it can be represented by the formulas: normal stresses. v , g an = ~ sh 2 u sin v cos v cos iv + p -q (t shu — 2 chu) + + | cìl2 uq +•1 2 1>2 chu — 2 2 eh2 u0 2 eh2 u0 chu eh u eh2 u0 chu ci? u sin v cos v cos w [2a] v 7 „ • c cr„ = — sii 2 u sin v cos v cos w + p — • sh2 u„ o \ 2 2 ( ch2 uq t chu h f 1 \ chu •— q (t sh2 u — 2 chu) + •—chu | sin v cos v cos w [2b] 2 pg h2 1 — a chu eh2 u0 eh3 u sin v cos v cos w . [2c] on e s t i m a t i o n of t h e d i s p l a c e m e n t i n a n e a r t h q u a k e source, etc. 2 0 7 tangentional stresses. v i , eh 2 u sin2 v\ v „ rm = 7j2 [ch*u j cos w + — g a \ shu q (shu — t sh2 u) 2 shu — qt + ( eli2 u0 — —— 1cos2 v (tch 2u—2 shu) q a ) gii u . , a\ shu 2 shu , , , — cos2 v ( eh2 w0 — 0 h m — eli2 m„) cos w [3a] v v t = , s/rasini;sinw + •g li li qs + |c/ì2 sin » sin w [3b] v v xuw = c/mcos»sinw -j——•g h h q (thu — t cliu) + 2 shu eh2 u0 eli3 u [3c] displacements. . . . v c òu„ = — p a„ sin v cos v cos w òuv (av + av cos2 ®) 2 [a, h v òtjm = --g aw cos ® sin w . 2 11 [4a] [4b] [4c] energy-difference. òw = -p v i l i " , a [i \ b r • a„ sli2 u\ — + sc7f.2 uq ) — a v t s h u 0 + s(avsh2u0— avch2u0) -fav eh 2 u[ — + s sh2 uq a„j cliu 11 , 2 71 41 ) tri ^ + [5] bere e,,, r„„ are the stress components; ój7„ are the differences between the displacement in the initial and final fields; òvi' is the stress 2 energy difference (the last terni — n thu0 in [5] represents the energy o inside [1]). _ a + 2 fi t = are e tg (shu) ; li — ] sh2 u + cos2 v ; 2 0 8 v . k e y l i s b o r o k = t shu — 1 ; thu0 g _ eh2 u0 a b ' q = 3 sh2 u0 + 1 n(u0) ' a„ = (3 sh2 u + 1) (2 sste + t) — s/iw jv = q (sshu + t) — 2 s/tw0 . c/i.2 m„ eh2 u + a (sshu — t ) 0.2 0.f 0.6 oi (.0 a fiff. 1 the maximal tangentional stress, according to neuber is: tmax — txy i * = y = 0 — v « i f " 9 jst" shun i t is interesting to note, that tmax is always more than ^ 2p (fig. 1). the displaeements change quaintly from point to point. p o r the measure of displacement we shall take v — the maximal one on the ?/-axis, that is in the middle of the source boundary: chu ix = —2 n (k + 2 a s ) on e s t i m a t i o n of t h e d i s p l a c e m e n t i n a n e a r t h q u a k e source, e t c . 2 0 9 i n particular, it is interesting to consider the displacements at the source boundary (u = ua). if the source is thin (a = 0), we have: ò u x = ± j l 4 " i [i tc(2-\-a) xjr p a —1 òtjy = — x fi 2 + a òtjz = 0 here sings " + " and " — " correspond to displacements in upper and lower sides, so that the fault does exist in our model. i t can be seen from this that in elastic case the fault piane undergoes also a rotation (óuy ^ 0) which reestablishes the sfaticai equilibrium after the forming of a fault. i t is not clear whether this rotation will talee place in fact because the reestablishment of the equilibrium can be the result of imperfect elasticity. this will not change the order of quantities studied here. however the rotation will essentially distort the seismic waves, generated after the formation of a fault; therefore in constructing of the dynamic model of a fault it is necessary to allow for tlie plasticity near the source. (of course, even in elastic case the above formulae can not be used directly for construction of dynamic model because they represent only its zero-frequency component). 3. interpretation. the following formulae can be of the main interest for the interpretation of seismic data: 8 = òb* = 71w v t* vl3 pvs = òw t* rn 2/3 i òw le [6] [7] [8] where ò w is the difference of energies of the initial and final fields, v is our measure of displacement, 8 is the fault piane area and ls, rs are plotted on fig. 2. (rs = r-n-3'* ; ls = l ti 1/2 le = lr~lh). 2 1 0 v . k e y l i s b o r o k b y means of these formulae one can try to determine the fault piane area s and the displacement v in the source if the energy of seismic waves e is known. because of energy dissipation e is less than òw. e however the ratio —— is much more for earthquakes than for exploow sions and according to byerly e and aw are of the same order. tu = 1.5 i t is better to use these formulae in logarithmic forni which corresponds to the accuracy of e — determination. the values of the coefficient rs, ls are given in fig. 2 for the case a = 1.5, which corresponds to the poisson coefficient v = 0,25; the variation of v in reasonable limita (0.23-0.27) may be neglected. the dashed line corresponds to the case when the energy inside the source model (ellipsoid [1]) is not included in ò w and is considered as dissipated in the plastic flow hear the source. on e s t i m a t i o n of t h e d i s p l a c e m e n t i n a n e a r t h q u a k e source, e t c . 2 1 1 the stress p in the initial field is not known and this is the main difficili t v in using formulas [6-8], i t may be asserted only that p is less than the strengh limit as it corresponds to an average stress in a great region. h . benioff estimated p for kern county earthquake of 1951 from p2 the formula e = —— iif (h = thickness of the crust, f area of z fi aftershocks' zone); the formula is based on the assumption that the energy e of seismic waves was accumulated in the zone coinciding with the zone of aftershocks. he obtained p = 2.67 dyne/cm2. since e is known for this earthquake 5.io22 ergs) p can be estimated using [6]. the movement took place along the fault ~ 60 km long. designating the depth of the rupture in kilometres by h we obtain s = 6h io11 cm2. then with fi = 5.io11 cgs and a = 0 we get 5 • io22 ~ 6-10n)s/al,l 5 • io11 h is unlikely to be greater than 35 km. (the thickness of the crust in california) and less than 15 km. than p ~ (5-9).io7 cgs. since the source of this earthquake is greatly elongated if would be more reasonable to use formulas for two-dimensional problems. assuming again that ii = from 15 to 35 km, we obtain from the formulas of knopoff: p ~ (1.5-3.1).io7 cgs. the obtained values of p are of the sanie order; we shall try to use them for the study of other earthquakes. in conclusion it should be noted that in a number of paper the volume v of the region of stress accumulation was estimated from the energy of earthquakes e applying tlie formula: the value v should not be mixed up with the volume of the model [1]: the latteimay be equal to zero (with a = 0); although e remains finite. one can see from fig. 2 that e relatively little depends upon the volume of the source and is determined mainly by the area of a fault and by the initial stress p. formulas [2], [3] can be used also for the estimating of the infìuence of earthquakes on the stress field around the source. in a small region around [1] the tangentional stress diminishes near the #-axis and becomes maximal near z-axis (where the screw-deformation occurs). this 2 1 2 v . k e y l i s b o r o k can stimulate the expansion of the fault or the formation of new faults along direction, perpendicular to the motion direction x. but it would be especially important to take into account non-ideal elasticity when studying the stress in the final field. 4. comparison w i t h observations. unfortunately the greater part of observational data refers to strong earthquakes with surface faults which are certainly far from the circle (fault planes are unlukly to go much deeper than the earth's crust and their horizontal length amounts to hundreds of kilometres). for the san-francisco earthquake of 1906 the length of a fault was l = 435 km; assuming 8 = 13 we obtain with 3 = 35 km (the thickness of the crust in california) 8 = 1.5; io14. assuming a — 0, /1 = 3.io11 we obtain from formula [7] v ~ 3 m. this value is of the same order that the actual one. from formula [6] 8 ~ 0.7.io14 cm! which also agrees with actual data. i t would be interesting also to compare the analogous computations with the empirical relation between e, v and 8 communicated by don tocher at the toronto meeting; however he studied the strong earthquakes for which knopoff's formulas are more valid. in conclusion it is interesting to compare formulas [6]; [7] with the data on the frequency of occurence of earthquakes. b. gutenberg and c. richter showed, that with the decrease of energy of each earthquake by 100 the number of sucli earthquakes increases only by 10; therefore the part of weak earthquakes in the total amount of annually released seismic energy is very small. however with the decrease of e by 100 the total area of faults will decrease (after [6]) only by ìo1^ and the total movement (according to [7]) will even increase by ìo1^. certainly fault areas and movement values cannot be summed up in such direct way. however these estimates show that weak earthquakes can take an essential part in causing great faults or in movements along them. (the frequency of weak earthquakes is determined for short-period ones; each of them can correspond to the movement along a part of a great fault. f. press discovered also the long-period weak shoks, and explained them as a weak movement along a large faults as a whole). the author is greatly tliankful to prof. l . knopoff, who acqunainted him with bis paper in manuscript, and to g. pavlova for troublesome computations. on e s t i m a t i o n of t h e d i s p l a c e m e n t i n a n e a r t h q u a k e source, e t c . 2 1 3 abstract an ellipsoid of rotation — a round fault piane surrounded by a crushing zone — is taken as a source model. the area s of the fault piane and the displacement v along it can be estimated using formulas: h = sl £3/2 bs ; v = — i/s ls h /i where the energy e is known (fi is shear modulus; rs, ls coefficients given in fig. 2; p mean stress before the earthquake; the value p = 3.1 (p gives satisfactory results in several cases). these formulas are cornpared with the well-known fact that the frequency of occurrence of earthquakes is proportional to e°-5 — e0-6; it is shoivn that the weak numerous earthquakes can take a considerable part in the formation of large faults and movement along tliem, though their total role in releasing seismic energy is negligibly small. riassunto un élissoide di rotazione — un piano di faglia rotondo circondato da una zona di schiacciamento — è presa come modello fondamentale. l'area s del piano di faglia e lo spostamento v lungo questo, possono essere calcolati impiegando le formule: e = ; v = p l/sls h fi dove l'energia e è nota (/i è il modulo di taglio; rs, ls i coefficienti dati nella figura 2; p significa la forza prima del terremoto; il valore p = 3.io7 dà risultati soddisfacenti in diversi casi). queste formule sono confrontate con il fatto ben noto che la frequenza con cui si verificano i terremoti è proporzionale a e0-' — e0-6; si mette in rilievo che numerosi terremoti deboli possono assumere una considerevole pai-te nella formazione di grandi faglie e dei movimenti lungo di esse, sebbene la loro importanza totale nello sviluppo di energia sismica sia trascurabile. 2 1 4 v. k e y l i s b o r o k references b e n i o f f , h . , mechanism and strain characteristics of the white wolf fault as indicated by the aftershoek sequence. kern county, cai. earthquake of 1952, bull. 171, cai. depart. of mines, 1954. b y e r i . y , p., report on a ' 1 ^iss. uggì, toronto, 1 9 5 7 . i v n o p o f f , l., energy release in earthquakes, « geopliys. jorn. », 1, 44-52, (1958). lurje, a., theree-dimentimal problemi in elastostatics (in russian). m. 1955. love, a., malhematical theory of elasticity. n e u b e r , i l , kerbspannungslehre. berlin, 1 9 3 0 . starr, a., slip in a cristal and rupture in a solid due to shear, proc. « camb. phil. soc.», 2 4 , ( 1 9 2 8 ) . 11montero.qxd 477 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words central america – historical earthquakes – catalogues 1. introduction central america has an important seismic and volcanic activity due to the interaction of several plates (fig. 1). that activity has conditioned its economic and social development. in order to define the seismic hazard of this region, a necessary step is to collect seismicity data by means of «earthquake catalogues». in order to have a better understanding of the different earthquake sources and its activity rates, historical earthquake information must also be included. for this reason, during the last two decades important research efforts have been conducted in historical seismicity (pre-instrumental earthquakes) and on developing a digital catalogue for this region. before 1980 there were some valuable regional and national catalogues, although the information they contain was scattered in different reference sources. also, modern techniques were not applied to integrate the regional pre1900 earthquake information in a systematic and methodical way. however, since 1980 there has been a remarkable increase in the historical seismicity investigation at regional and local levels, and its results have been partially incorporated in a digital catalogue. in the next paragraphs we review the previous work with especial emphasis in the historical seismicity research topic (pre-1900 earthquake studies). 2. pre-1980 descriptive earthquake catalogues research and compilations of the central american destructive earthquakes which occurred since the spanish conquest in the 16th century up to 1900 are found in different works. current knowledge on the central america historical seismicity: an analysis of recent catalogues walter montero and giovanni peraldo escuela centroamericana de geología, universidad de costa rica, san pedro, costa rica abstract historical seismicity studies in central america have increased since 1980. currently, there is a well-organized regional digital catalogue database structured according to modern techniques. it contains more than 17 000 earthquakes which occurred between 1520 and 1992. a regional tsunami catalogue is also available. the historical seismicity research included a more complete earthquake documentation obtained from foreign and central american depositories. new intensity data point estimates and isoseismal maps allows among others, macroseismic magnitude calculations and seismogenic source determinations, for many pre-1900 earthquakes. future trends in central america historical seismicity research are discussed. mailing address: dr. walter montero, escuela centroamericana de geología, universidad de costa rica, apartado 214-2060 ucr, san pedro, costa rica; e-mail: wmontero@geologia.ucr.ac.cr 478 walter montero and giovanni peraldo at the central american level, the principal compilations came from montessus de ballore (1888), sapper (1925), díaz (1933) and grases (1974). at national level we mention gonzález (1910) for costa rica; lardé (1960) and martinez and maximiliano (1978) for el salvador and kirkpatrik (1920) for panamá. all the previous works were descriptive catalogues compiling historical earthquake information, but they do not include intensities or isosesismal estimates or define focal parameters obtained from macroseismic information. 3. current earthquake and tsunami catalogues currently, descriptive and parametric catalogues, and a mixture of both are found in central america. leeds (1974) compiled for nicaragua, the first published parametric catalogue including pre-1900 earthquake information for a central american country. after this publication, other parametric catalogues were compiled. sutch (1981) published one for honduras; white (1984) produced one for the polochic-motagua fault system of guatemala; miyamura (1980) and montero (1989) published catalogues for costa rica and víquez and toral (1987) published one for panamá. the previous catalogues also included instrumental seismicity. rojas (1993) and rojas et al. (1993) carried out the main seismological digital parametric catalogue currently existing at the regional level. they included information of around 17 000 earthquakes which occurred between 1505 and 1992. this catalogue gathered seismological information from the previous catalogues and other diverse reference sources, although no archival or any specific earthquake investigation was conducted. the catalogue merged the instrumental and macroseismic information applying the standard nordic format, which includes three files: 1) the main catalogue file includes the main focal parameters of the event, as well as the possibility to include several entrances for any event. it includes instrumental and macroseismic event information. 2) the catalogue file is a bridge that connects files 1 and 3. 3) extensional files include additional information of the event, such as macroseismic data and references, among others. for some earthquakes intensity data points are included. besides the parametric catalogues, national and regional descriptive or mixed catalogues were compiled. feldman (1985, 1986 and 1993) gathered from diverse archives a large number of new primary documents about destructive earthquakes which occurred in guatemala and in central america between 1500 and 1899. he compiled descriptive catalogues, not including intensity data or isoseismal maps. peraldo and montero (1999) published a mixed earthquake catalogue including information obtained from archives and other different reference sources. they studied central america destructive earthquakes which occurred from 1469 to 1899 and obtained new seismological interpretations such as intensity data points, isoseismal maps and macroseismic focal parameters. figure 1 shows the epicentre locations and the suggested seismogenic sources of the historical earthquakes studied by peraldo and montero (1999). in relation to 20th century central america seismicity studies, we briefly review two important recent works, among other valuable specific or national studies. we mentioned the work of ambraseys and adams (2001) who gathered instrumental and macroseismic information on central american earthquakes which occurred during the period 1898-2000. the study includes a regional descriptive and parametric catalogue, and maps with macroseismic locations but without intensities estimations. they also obtained new magnitude calculations for a considerable number of events. on the other hand, white and harlow (1993) studied the upper crust central american earthquakes which occurred since 1900. they obtained magnitudes for several of the shocks and compiled isoseismal maps for some damaging earthquakes. an important number of earthquake specific studies are also found in the literature but they will not be reviewed in this article. for a general overview see for instance peraldo and montero (1999) for pre-instrumental seismicity and ambraseys and adams for 20th century seismicity. the occurrence of tsunamis is another kind of earthquake hazard related phenomenon in 479 current knowledge on the central america historical seismicity: an analysis of recent catalogues central america. a recent tsunami catalogue was compiled for this region (molina, 1997; fernández et al., 2000). 4. scope, professional background and financial research agencies since 1980 as a result of the research carried out in historical seismicity of central america since 1980, now there are improved and more complete catalogues. they constitute the fundamental tool to obtain better seismic hazard estimates. seismologists and historians, working in separate or in interdisciplinary ways carried out the investigation. however, according to our experience, a previous work of interdisciplinary investigation, including the analysis of the historical context where the seismic event took place, favors better seismological interpretations. fig. 1. spatial-temporal historical seismicity along the different cocos-caribbean plate boundary convergent segments (blank circle events). also shown are the epicentre locations related to other regional seismogenic sources. the large number of earthquakes shown for northern central america is explained by its early colonization and its relatively larger development during the colonial period. the scarce seismic activity of the caribbean side of central america is also shown (modified after peraldo and montero, 1999). 480 walter montero and giovanni peraldo most of the investigation performed by historians has been focused toward finding new primary information in diverse archival sources. we here highlight the work carried out by the historian feldman (1993) at the regional level, investigation that received financial support of the agency for international development (aid) and of the united states geological survey (usgs). also, the historians hilje and alvarez (1994) carried out a similar research in costa rica, work financed by the pan-american institute of geography and history (pigh). in 1992, the geologist peraldo and the historian claudia quirós gathered primary historical central american seismological information from different archives. it was used together with other documents and references in the seismological interpretations carried out by peraldo and montero (1999). the historical context of the seismic events was considered in their interpretations. the previous investigators received financial support from the pigh through a project financed by the international development research centre of canada (idrc). also, they obtained local support from the university of costa rica (ucr). the seismological analysis carried out by these authors is somehow extensive and focused on studying most of the pre-instrumental destructive regional events. the digital regional parametric catalogue merged by the seismologists rojas (1993) and rojas et. al. (1993) was supported by the norwegian agency for international cooperation (norad). the 20th century seismicity study carried out by the seismologists ambraseys and adams (2001) was financed by the national scientific cooperation programme of brussels (cec) and the international seismological centre of england (isc). table i summarizes the characteristics of the recent regional catalogues. at the national level, peraldo and montero (1994) using primary and secondary sources studied the colonial seismicity of costa rica. this work, that includes seismological interpretations, is considered quite intensive, because the archival sources were intensely reviewed, although only few costa rican colonial time table i. main characteristics of central america seismological catalogues. authors year type period catalogue type format idp or intensity map intensity scale financial agency rojas 1993 parametric 15051992 compiled seismological information d partially mm norad, ucr feldman 1993 descriptive 15001899 primary historical information p no aid, usgs peraldo and montero 1999 descriptive and parametric 14691898 mixed p both mm pigh, idrc, ucr ambraseys and adams 2001 descriptive and parametric 18981995 summary of seismological information p no cec, isc d = digital, p = printed; idp = intensity data point; mm = modified mercalli. 481 current knowledge on the central america historical seismicity: an analysis of recent catalogues earthquakes were documented. the university of costa rica supported this work. in panamá, camacho and víquez (1993) studied the historical seismicity of the north panamá deformed belt, a seismic source area located on the caribbean side of panamá and costa rica (fig. 1). the pan-american institute of geography and history and the university of panamá supported this research. 5. primary information sources the documents found in archives and libraries including seismic phenomena descriptions from the spanish time up to 1821 (1903 for panamá as an independent republic), are chronicles, reports and correspondence elaborated by conquerors, governors, ecclesiastical authorities, among others. after 1821, municipal and government archive documents, ecclesiastical reports, newspapers reports, and naturalist and travellers’ chronicles, especially by foreign people, also record damaging earthquakes that affected the central american republics. that historical data baggage constitutes the basic primary information with which the diverse seismological central american catalogues have been elaborated since the end of the 19th century. the primary source reports have been obtained from archives and libraries of europe and america. they include, among others, the archivo general de indias and of simancas, museo naval of madrid and biblioteca nacional (spain), archivo general de la nación (méxico), archivo general de centro américa (guatemala), archivo nacional and archivo eclesiástico of león (nicaragua), archivo nacional and archivo eclesiástico of the curia metropolitana of san josé (costa rica). let us briefly review the depositories researched at the regional level. feldman (1993) carried out a profuse investigation in different archives and libraries, including several of those mentioned previously. peraldo and montero (1999) carried out their investigation based on documents obtained from archives of spain and central america, and compiled other primary and secondary documentary sources. ambraseys and adams (2001) gathered historical and seismological information published by diverse authors. at the national level, the depositories researched by hilje and alvarez (1994) and by peraldo and montero (1994) were the costa rican archives. camacho and víquez (1993) researched the canal of panamá library, the archivo nacional and the biblioteca nacional of the republic of panamá, the biblioteca simón bolivar of the university of panamá and the archivo nacional and the biblioteca nacional of the republic of colombia. 6. intensity scales used the scale used to estimate the levels of intensity in the diverse studies carried out on historical seismicity in central america has been the modified mercalli intensity scale (mm). exceptionally, the intensity scale msk (medvedev-sponheuer-kárník) or its later modifications was applied. rojas (1993) gathered some historical earthquake mm intensity data in digital format. peraldo and montero (1999) assigned mm intensity levels to central american earthquakes and drew isoseismal maps when enough estimates of intensity existed. otherwise, they only showed maps with intensity data points, as is the case of most of the earthquakes of the costa rican colonial period studied in peraldo and montero (1994). the mm intensity scale was also used by camacho and víquez (1993) and by white and harlow (1993), among others. 7. catalogue completeness according to rojas et al. (1993), the central american catalogue is reasonably complete for earthquakes of ms ≥ 5.5 from 1900 and of ms ≥ 7.0 from around 1820. ambraseys and adams (2001) consider their catalogue complete for earthquakes of magnitudes ms > 5.0 starting from 1920 and of ms > 6.0 starting from 1910. table ii shows the earthquake number per century included in different catalogues. figure 2 shows the historical accumulative 482 walter montero and giovanni peraldo earthquake number found in peraldo and montero (1999). a remarkable slope increment in the seismicity reported is shown around the beginning of the 18th century, probably related to the consolidation of the spanish colonial period and accordingly, to a better documentation of the historical earthquakes. 8. difficulties and solutions in historical research several difficulties were found during the historical seismicity research and among these we emphasize: i) manuscript reading difficulties related to the grammatical and calligraphic language evolution, especially from the old to the modern spanish. this problem has been partially solved with the participation of historians trained in paleography or in the case of professionals coming from other fields, by learning paleographic techniques; ii) physical deterioration of the documents caused by improper conservation, humidity damage, ink scattering, writing fading and paper destruction caused by fungi and diverse animals; iii) document losses due to such diverse causes as fires and robberies; iv) document misadministration including inadequate classification and improper organization of the historical material and inappropriate loan practices; v) scattered archival information, in some cases occurring in different document series in the same archive or between distinct depositories, many of them difficult to access; vi) changes in the nomenclature of towns and localities or wrong geographical locations; vii) physical disappearance or seat change of populations; viii) earthquake dating errors included in primary and secondary reference sources. an example of an error in the event occurrence date is found in gonzález (1910), who based on a municipal record, mentioned an earthquake which occurred on 3 april 1827, in nw costa rica. however, a primary source located by peraldo (1993) in the government file of the costa rican national archive referred the occurrence of an earthquake on 3 april 1826, in the same region. this leads peraldo (1993) to propose that the earthquake occurred in 1826 instead of 1827, something sustained by the new investigations carried out by montero and peraldo (work in progress). for instance, a municipal record dated 4 april 1826, from alajuela town, located east of the epicentral region, explicitly mentions the occurrence the day before of a strong earthquake that caused some damage. table ii. number of earthquakes included in the seismic catalogues. authors type period 16th c. 17th c. 18th c. 19th c. 20th c. total rojas (1993) parametric 1505-1992 92 300 16 698 17 188 feldman (1993) descriptive 1500-1899 37 45 66 217 365 peraldo and montero (1999) descriptive and parametric 1500-1899 16 22 65 70 173* ambraseys and adams (2001) descriptive and parametric 1898-1995 – – – 6 1823 1829 (*) this study investigated the earthquakes with intensities mm larger than vi. 98 483 current knowledge on the central america historical seismicity: an analysis of recent catalogues an important aspect that must be analyzed carefully in any historical earthquake research is the historical conditions prevailing at the time the documents were written, including the specific political and social situation. examples are the august and september 1717 and the july 1773 seismic sequences, that affected the city of antigua, guatemala (peraldo and montero, 1996). in both cases, a series of documents suggest widespread city destruction allowing some authorities to propose its relocation. on the other hand, another group of documents refers major damage although with remarkable exceptions and in this case other authorities argue that the city should remain in the same location. in this case two opposed political factions existed, with different concerns about the necessity or not of the city relocation, partially because they had different property damage level assessments. both theses also reflected a power fight between clergyman and the «real» authorities. the identification and opposition of the different document discourses and their historical explanation allowed a better estimate of the damage level produced by those earthquakes. 9. current knowledge status and future work the seismic history before the spanish arrival in central america shows an information gap because only few documentary sources have been identified. the principal ones are the popol vuh, the memorial de los cackquiqueles, and the interpretation of indigenous narratives, although they do not necessarily correspond to the pre-hispanic period (peraldo and montero, 1994; 1999). a possible solution to expand the historical seismicity record to the pre-spanish epoch would be to increase paleoseismological investigations with the objective to identify and date paleoearthquakes. another way would be to begin research in seismic archaeology, because it is possible that some earthquakes affected the physical and social structure of some central american indigenous cultures, especially those from the mayan period. in relation to the seismicity which occurred between 1500 and 1900, a first investigation phase in future research would be to evaluate the information contained in the catalogue of fig. 2. temporal variation of the accumulative number of events according to peraldo and montero (1999). the graph incorporates only well documented seismic events. the absence of appropriately documented damaging earthquakes is evident for the period between 1500 and 1700. some periods of apparent seismic quiescence are identified at regional level between 1540 and 1565, 1594 and 1607 and 1625 and 1648. starting from the 17th century the slope increases, indicating a better documentation due to the consolidation of the spanish colonization process. also, the periods of apparent seismic quiescence shorten. 484 walter montero and giovanni peraldo rojas (1993). a necessary step will be to update that catalogue with the new historical seismological information that has been generated since 1993 at regional and national levels. by doing so, the seismic parameters information incorporated in peraldo and montero (1994, 1999), ambraseys and adams (2001) and other newly published works including some intensive studies of earthquakes, can be incorporated in that catalogue. a second phase would consist in carrying out a systematic and careful evaluation of the historical records gathered in the diverse archives, libraries and reference sources explored. a good approach to proceed can be found in guidoboni and stucchi (1993), where very useful guidelines are found to review the historical documentation. with regard to the primary documentation of earthquakes from the conquest up to the end of the 20th century, it is considered that the investigation carried out until now by the diverse authors has been in general extensive and not intensive. the next objective will be the identification of the most important destructive events that still have not been studied intensively, in order to proceed to its investigation in promissory local, national and foreign little explored archives. one important step of any future historical intensive investigation should be to increase the completeness of the central america seismic catalogue, including the search for new events and a better documentation of those not well known. finally, the procedures followed in estimating earthquake parameters from macroseismic data must be reviewed and some homogeneous and robust criteria must be identified to obtain well-supported macroseismic earthquake parameters. finally, seismicity studies for the 20th century would benefit from macroseismic studies, such as defining intensities and isoseismals for specific damaging earthquakes, because this important research tool can better constrain the earthquake focal parameters. this is especially true for events which occurred during the first six decades of that century, because some damaging shocks before 1960 had a magnitude level below the world seismographic network detection capacity. for that reason, their instrumental magnitudes were poorly or not determined. another problem is that the instrumental epicentres were poorly constrained due to the scarce coverage of the world seismographic network before 1960. acknowledgements we are grateful to virginia garcía acosta, massimiliano stucchi and paola albini for inviting us to write this paper. references ambraseys, n.n. and r.d. adams (2001): the seismicity of central of america: a descriptive catalogue 18981995 (imperial college press, london), pp. 309. camacho, e. and v. víquez (1993): historical seismicity of the north panamá deformed belt, rev. geol. am. centr., 15, 49-64. díaz, v. (1933): conmociones terrestres de américa central, editorial josé ibarra, guatemala, pp. 267. feldman, l. (1985): disasters, natural and otherwise, and their effects upon population centers in the reino de guatemala, in estudios del reino de guatemala, duncan kinkead (escuela de estudios hispano-americanos, seville), 49-60. feldman, l. (1986): a master list of historic (pre-1840) earthquakes and volcanic eruptions in central america, in inv. nat. haz. lat. am. history, edited by robert h. claxton (ed. studies in the social sciences, west georgia college), vol. 25, 66-106. feldman, l. (1993): mountains of fire, lands that shake. earthquakes and volcanic eruptions in the historic past of central america (labyrinthos, california), pp. 295. fernández, m., e. molina, j. havskov and k. atakan (2000): tsunamis and tsunami hazards in central america, natural hazards, 22, 91-116. gonzález, c. (1910): temblores, terremotos, inundaciones y erupciones volcánicas en costa rica 16081910 (alsina, san josé), pp. 200. grases, j. (1974): sismicidad de la región centroamericana asociada a la cadena volcánica del cuaternario (instituto de geociencias-organización de estados americanos, caracas), vol. ii, pp. 253. guidoboni, e. and m. stucchi (1993): the contribution of historical records of earthquakes to the evaluation of seismic hazard, ann. geofis., xxxvi (3-4), 201215. hilje, b. and f. alvarez (1994): historia de los sismos y desastres naturales en costa rica, (observatorio volcanológico y sismológico de costa rica, heredia), pp. 292. kirkpatrik, r. (1920): earthquakes in panamá up to january 1, 1920, bull. seismol. soc. am., 10, 120-128. 485 current knowledge on the central america historical seismicity: an analysis of recent catalogues lardé, j. (1960): obras completas, (publicaciones del ministerio de cultura, san salvador), pp. 597. leeds, d. (1974): catalogue of nicaraguan earthquakes, bull. seismol. soc. am., 64, 1135-1158. martinez, h. and a. maximiliano (1978): cronología sísmica y eruptiva de la república de el salvador a partir de 1520 (ministerio de obras públicas, centro de investigaciones geotécnicas), pp. 40. miyamura, s. (1980): sismicidad de costa rica (editorial universidad de costa rica, san josé), pp. 190. molina, e. (1997): tsunami catalogue for central america 1539-1996, technical report no. ii 1-04, reduction of natural disasters in central america (university of bergen). montero, w. (1989): sismicidad histórica de costa rica, período 1610-1910, rev. geofis. int., 28, 531-559. montessus de ballore, f. (1888): sur les tremblements de terre et les éruptions volcaniques dans le centre amerique: depuis la conquête espagnole jusqu’ a nos jours, société des sciences naturelles de saone-etloire (ed. jobard, dijon), pp. 293. peraldo, g. (1993): el geólogo ante el discurso del documento histórico. rev. geol. am. centr., 15, 92-97. peraldo, g. and w. montero (1994): los temblores del período colonial de costa rica (editorial tecnológica, cartago), pp. 162. peraldo, g. and w. montero (1996): la secuencia sísmica de agosto a octubre de 1717 en guatemala, efectos y respuestas sociales, historia y desastres en américa latina, coordinated by v. garcía acosta, i, 295324. peraldo, g. and w. montero (1999): sismología histórica de américa central, instituto panamericano de geografía e historia (ipgh), méxico, publ. no. 513, pp. 342. rojas, w. (1993): catálogo de sismicidad histórica y reciente en américa central, desarrollo y análisis, tesis de licenciatura en geología (universidad de costa rica, san josé), pp. 91. rojas, w., h. bungum and c. lindholm (1993): historical and recent earthquakes in central america, rev. geol. am. centr., 16, 5-22. sapper, k. (1925): los volcanes de la américa central (max niemeyer, halle), pp. 116. sutch, p. (1981): estimated intensities and probable tectonic source of historic (pre-1898). honduran earthquakes, bull. seismol. soc. am., 71, 865-81. víquez, v. and j. toral (1987): sismicidad histórica sentida en el istmo de panamá. instituto panamericano de geografía e historia, rev. geofís., 27, 135-166. white, r. (1984): catalogue of historic seismicity in the vicinity of the chixoy-polochic and motagua faults, guatemala, u.s. geol. surv. open file rep. 84-88, pp. 24. white, r. and d. harlow (1993): destructive uppercrustal earthquakes of central america since 1900, bull. seismol. soc. am., 83, 1115-1142. source identification for situational awareness of the august 24th 2016 central italy event annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 1 source identification for situational awareness of the august 24th 2016 central italy event christian bignami*, cristiano tolomei, giuseppe pezzo, francesco guglielmino, simone atzori, elisa trasatti, andrea antonioli, salvatore stramondo, stefano salvi (1) (1) istituto nazionale di geofisica e vulcanologia *christian.bignami@ingv.it abstract on august 24, 2016, at 01:36 utc a ml 6.0 earthquake struck a portion of the central apennines between the towns of norcia and amatrice. the epicentre was located near the town of accumoli. prompt synthetic aperture radar (sar) acquisitions and the available scientific knowledge of the area allowed to elaborate a first interpretative framework of the ongoing seismic sequence only 30 hours after the mainshock and a second analysis, complete of several interferometric sar (insar) data within two weeks. through the inversion of insar data, we found that the seismogenic structure is oriented nnw-sse and extends about 20 km between the towns of norcia and amatrice with a width of about 10 km. the retrieved slip reaches a maximum value of more than 1.2 m, and stops at a depth of about 4 km. preliminary fault slip inversions suggest two main patches of co-seismic deformation located nw and se of the hypocenter. i. introduction entral apennines are known to be one of the most seismically hazardous area in italy [http://zonesismiche.mi.ingv.it/mappa_ps_apr04/italia.html]. the region surrounding the monti della laga, was struck by several earthquakes in historical times. in 1627 and 1639, the area was affected by two events with estimated magnitudes of about 5.3 and 6.2 respectively. a century later, in 1672, another mw 5.3 hit the same area, followed in 1703 by a strong event of magnitude higher than 6 [rovida et al., 2016]. the 24th of august 2016, at 01:36 utc an mw 6.0 earthquake hit an extensive portion of this area. c annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 2 the epicentre is located near the village of accumoli, between the towns of norcia and amatrice, at a depth of 8.00.2 km. the mainshock was followed by an m 5.3 earthquake at 02:33 utc, about 12 km nw from the first event, and it was the only m>5 aftershock registered. the aftershocks sequence evolved mainly to the nw towards norcia and the sibillini mountains. in this paper, we show the scientific productions obtained during the emergency phase. a first report was sent to the dpc (italian civil protection department) the 25th of august (doi: 10.5281/zenodo.60938, in italian). as soon as new sar (synthetic aperture radar) images from cosmo-skymed (csk) and sentinel-1a (s1a) became available, prompt data analysis and new fault inversions led to model insar (interferometric sar) images, with different line of sight (los) and orbits, producing a robust slip distribution just fourteen days after the mainshock. ii. first insar results the first post-event sar image was acquired the 24/08/16 by alos-2 (l-band instrument of the japan aerospace exploration agency, jaxa). the first insar (interferometric sar) analysis was performed considering three preevent (01/07/15, 09/09/15, 27/01/16) and one post event (24/08/16) alos-2 satellite acquisitions. among the three co-seismic interferograms, the most coherent and with minimum atmospheric influences, is the 09/09/15-24/08/16 (id-1, table 1). even if it includes the effects of all the seismicity encompassing the period, most of the deformation is due to the mw 6.0 mainshock (24/08/16 1:36 utc) and possibly the mw 5.3 greatest aftershock (24/08/16 2:33 utc). the measured surface displacement field was subsequently modelled in a first instance by means of a non-linear inversion providing the parameters (strike, dip, and rake) for a uniform slip fault plane. the heterogeneous slip was then obtained, by using a linear inversion algorithm exploiting the fault parameters previously obtained [e.g., atzori et al., 2009] and a preliminary model of the seismogenic fault was already released the day after the mainshock (doi: 10.5281/zenodo.60938). iii. insar analysis and fault modelling the insar analysis benefitted of a large number of multi-frequency images from different satellites having different characteristics and spatial resolutions: alos-2 (jaxa) operating at l-band, s1a (from the european program copernicus) at c-band, csk (agenzia spaziale italiana, asi), at x-band. the european space agency (esa) also provided sentinel-1b (s1b) data (the twin satellite of s1a, with the same sensor on board), although the satellite was still in the commissioning phase, and therefore not fully operative. with s1a and s1b images we were able to process cross-interferometric pairs, with a temporal baseline of 6 days only (respect to the 12 days revisiting time of the single satellite). table 1 shows the resulting co-seismic interferometric sar pairs and the sensors’ main the authors want to thank asi for providing cosmo-skymed data during the emergency and esa for furnishing sentinel-1b images even if the satellite is still in commissioning phase. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 3 characteristics, while some of the calculated interferograms are shown in figure 1. all the interferograms were unwrapped to retrieve the deformation field caused by the earthquake. the resulting displacement maps from all these interferograms show very similar patterns, characterised by a maximum displacement value of about 20 cm away from the respective sensor los. note that the incidence angle of the data is similar for all the available data, apart from the different orbits. to estimate 3d surface displacement maps, we applied the sistem (simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements) method [guglielmino et al., 2011a] to the available co-seismic gps [ingv working group “gps geodesy”, 2016] and to three insar data with the wider spatial coverage of the epicentral area. in particular, we used the two s1 (ascending and descending) and the descending alos-2 insar data, reported in table 1 with id-3, id-4, and id7, respectively. the sistem’s outputs are reported in figure 2. table 1: co-seismic interferometric pairs available and processed. s1 is sentinel-1, csk is cosmo-skymed. iws means interferometric wide swath. id sensor acquisition mode interferometric pair wavelength [cm] perpendicular baseline [m] orbit incidence angle [deg] id-1 alos-2 stripmap 09092015 24082016 23.6 -198 ascending 36.6 id-2 s1 iws 20082016 26082016 5.56 105 descending 39 id-3 s1 iws 21082016 27082016 5.56 79 descending 39 id-4 s1 iws 15082016 27082016 5.56 32 ascending 39 id-5 csk stripmap 20082016 28082016 3.1 101 descending 30.6 id-6 s1 iws 22082016 28082016 5.56 -29 ascending 39 id-7 alos2 stripmap 25052016 31082016 23.6 88 descending 32.9 annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 4 surface deformations revealed by sistem, show a predominant vertical deformation pattern (2 to 3 times higher than the horizontal ones) with a maximum value of -0.25 m. thanks to these 3d maps, it would be hopefully possible to identify further structures activated during the seismic sequence. the vertical component confirms the surface subsidence with the typical "spoon" shape, as already observed in past earthquakes in the apennine (e.g., mw 6.3 l’aquila earthquake, 2009). it extends for about 20 km along the nnw-sse direction. the maximum displacement value is in correspondence of the town of accumoli. figure 2. sistem results: east, north, and up components of the displacement field are reported, respectively. the dashed box refers to the inverted fault plain. figure 1. examples of insar wrapped maps from: a) s1 ascending pair (id-4 in table 1); b) csk interferograms. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 5 with respect to the first report, a new solution was delivered after all the interferograms listed in table 1 were processed. insar data used in the inversion procedure consist of about 19500 measurements obtained by sampling 5 unwrapped interferograms (id-1, id-3, id-4, id-5, and id-7 in table 1) and 107 measurements from the cgps [ingv working group "gps geodesy", 2016; cheloni et al., 2016]. the results of such inversion are shown in figure 3. the retrieved source model consists of a fault plane oriented nnw-sse. the slip spans an area of about 20 km length, and a width of 10 km, and stops at a depth of about 4 km. this area is located between the towns of norcia and amatrice. the fault slip is characterised by two main patches, one located between amatrice and accumoli, and one between accumoli and norcia. the maximum slip values are about 1 m for both patches. note that a relatively large slip is also present in the southern part of the fault plane, and it is associated to some uncertainties on the data. comparison of the fault plane with the relocated aftershocks distribution [michele et al., 2016] shows a good agreement between the structures constrained by the two different analysis. note that, in this preliminary result, two main patterns of seismicity are visible. one pattern is approximately laying around the fault plane, while a second one seems identify an antithetic fault east of norcia village. the last feature is still under investigation. in order to show how the slip of the mainshock modified the stress loading of the region, we computed the coulomb failure function [cff, e.g., harris et al., 1998] on nearby fault planes as listed in the diss [diss working group, 2010] data-base. in particular, we imaged the figure 3. modelling results. a) co-seismic fault slip distribution from the insar data modelling. the mainshock and the m 5.3 aftershock are reported with the red pentagon, while the m<5 aftershocks are shown with black dots. b) 3d view of the fault below the topographic surface. the view is from sse along the strike direction (sse-nnw, section a-a’ in panel (a)). the red dots are the largest events (m≥4) and black dots are the m<4 aftershocks. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 6 cff values on six fault planes surrounding the mainshock. positive cff variations can promote subsequent slip on adjacent faults, while negative cff values in principle inhibit it. our results show a decrease of cff value on the fault planes west of the main fault, while a positive increment of its value is present on the two planes aligned with the amatrice fault. in particular, a value relatively high compared with the mean stress drop on an earthquake of this size, is present in the northern portion of the gorzano fault. iv. conclusion we showed how, during the emergency phase of an earthquake, the use of insar geodetic data and their inversion for seismic source retrieval can play an important role for supporting civil protection authorities. the prompt availability of a large dataset of sar images and the fast deliver of displacement modelling and fault slip distribution demonstrated that insar, thanks to the present day sar constellation (s1, csk, alos-2, etc.) can rapidly issue scientific information. a quite robust solution of insar data inversion was provided only few hours after the main event. indeed, thanks to alos-2 data, the ingv team responsible for the analysis of the geodetic data was able to process and deliver a first seismic source model just one day after the mainshock occurrence. this first estimation was improved few days later, when additional sar images, acquired at different wavelengths and different viewing geometries, allowed to better constrain the fault parameters. the new fault model substantially confirmed the reliability of the first estimation, adding more details about the slip values and fault patches dislocation. furthermore, it confirms the agreement with the fault plane imaged by means of relocated aftershocks. v. references atzori, s., i. hunstad, m. chini, s. salvi, c. tolomei, c. bignami, s. stramondo, e. trasatti, a. antonioli, and e. boschi (2009). finite fault inversion of dinsar coseismic displacement of the 2009 l’aquila earthquake (central italy), geophys. res. lett., 36, l15305, doi:10.1029/2009gl039293. cheloni, d., serpelloni, e., devoti, r., d’agostino, n., pietrantonio, g., riguzzi, f., anzidei, m., avallone, a., cavaliere, a., cecere, g., d’ambrosio, c., esposito, a., falco, l., galvani, a., selvaggi, g., sepe, v., calcaterra, s., giuliani, r., mattone, m., gambino, p., abruzzese, l., cardinale, v., castagnozzi, a., de luca, g., figure 4. cff computations on the known faults nearby the amatrice fault. the stress variations are due to the mainfault slip distribution. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7233 7 massucci, a., memmolo, a., migliari, f., minichiello f., and zarrilli, l., (2016). gps observations of coseismic deformation following the 2016, august 24, mw 6 amatrice earthquake (central italy): data, analysis and preliminary fault model. annals of geophysics, vol. 59, fast track 5, 2016, doi: 10.4401/ag-7269, in press. guglielmino, f., nunnari, g., puglisi, g., spata, a. (2011). simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements (sistem) to obtain three-dimensional displacement maps. ieee trans. geosci. remote sens. vol. 49, 1815–1826. doi:10.1109/tgrs.2010.2103078. harris, r. (1998). introduction to special section: stress triggers, stress shadows, and implications for seismic hazard. j. geophys. res. 103, 24347–24358. ingv working group “gps geodesy (gps data and data analysis center)” (2016). preliminary co-seismic displacements for the august 24, 2016 ml6, amatrice (central italy) earthquake from the analysis of continuous gps stations, doi:10.5281/zenodo.61355. michele m., di stefano r., chiaraluce l., cattaneo m., de gori p., monachesi g., latorre d., marzorati s., valoroso l., ladina c., chiarabba c., lauciani v., fares m., the amatrice 2016 seismic sequence: a preliminary look to the mainshock and aftershocks distribution, annals of geophysics fast track, in preparation. rovida a., locati m., camassi r., lolli b., gasperini p. (2016). cpti15, the 2015 version of the parametric catalogue of italian earthquakes. istituto nazionale di geofisica e vulcanologia. doi:http://doi.org/10.6092/ingv.it-cpti15. working group, diss (2010). database of individual seismogenic sources (diss), version 3.1.1: a compilation of potential sources for earthquakes larger than m 5.5 in italy and surrounding areas. http://diss.rm.ingv.it/diss/. annals n.6/2003 ok 23/04 1209 annals of geophysics, vol. 46, n. 6, december 2003 key words aftershocks – seismic moment – scaling 1. introduction on 12th of october 1992 a moderate size earthquake of mw = 5.8 occurred at epicentral distance of about 25 km south west of cairo city downtown in dahshour region (fig. 1) at a depth of 22 km. this earthquake was felt from alexandria to aswan. it registered a maximum observed intensity of 7 + in msk intensity scale in dahshour region (maamoun et al., 1993). this shock caused considerable damage to the buildings in the eastern part of cairo, giza and the northern area of fayoum. significant damage was found mostly in adobe and old non-reinforced brick masonry buildings and non-engineering reinforced concrete buildings. in addition, extensive liquefaction of deltaic silty sand deposits (sand plumes) was observed in some cultivated fields of giza governerate located 19 km from the epicenter. according to eyewitnesses, sand and water continuously blew up to a height of 3 m for about 45 min (maamoun et al., 1993). the october 12, 1992 cairo earthquake struck a previously known low seismicity zone. the only significant earthquake reported in this region during the historical period occurred in 1847. this mainshock was followed by an aftershock sequence. within twenty two days after the occurrence of the cairo earthestimation of seismic moments from local magnitudes and coda durations for the cairo earthquake aftershocks recorded at kottamyia (keg) broadband station el-sayed m. abdelrahman (1), mohamed m. dessokey (2), hesham m. hussein (2) and mohamed f. abdelwahed (2) (1) cairo university, faculty of science, geophysics department, cairo, egypt (2) national research institute of astronomy and geophysics (nriag), seismology department, helwan, cairo, egypt abstract the spectral analysis of fifty-five keg vbb records from the october 12, 1992 cairo earthquake source region was performed to obtain the seismic moment. we obtained this parameter in turn to develop empirical local magnitude (ml), seismic moment (mo), coda duration (d) relations for that region. in this study the data consist of lg-waves on the vertical component seismograms for the recorded earthquakes with ml ranging from 1.7 to 4.7. the derived empirical relation between the seismic moment (mo) and magnitude ml for the aftershocks sequence with 1.7 ≤ ml < 3.5 is log (mo) = (0.96 ± 0.05) ml + (17.88 ± 0.13). we found a correlation between the coda duration (d) and log of the moment (log (mo)) as follows: log (mo) = (2.35 ± 0.27) log (d) + (16.33 ± 0.48). mailing address: dr. hesham m. hussein, national research institute of astronomy and geophysics (nriag), seismology department, helwan 11421, cairo, egypt; e-mail: hesham6511421@yahoo.com 1210 el-sayed m. abdelrahman, mohamed m. dessokey, hesham m. hussein and mohamed f. abdelwahed quake 55 shocks with 1.7 ≤ ml ≤ 4.7 were recorded by kottamiya (keg) broadband station. the event data are given in table i. figure 1 shows the distribution of the well located aftershocks of 1992 cairo earthquake. aftershocks sequence was recorded by a temporary seismic network installed by national research institute of astronomy and geophysics during the first twenty days of this sequence. details about the seismic network and the procedure for determination of the hypocenters were described by abou el enein et al. (2000). however, the pattern of the aftershocks reflects a cluster rather than a clear trend of seismicity. teleseismic body wave inversion of the mainshock (hussein, 1999) suggested a normal faulting mechanism with a small strike slip component on a plane striking ew to wnw-ese and dipping to the east (fig. 1). the purpose of this study is to calculate the seismic moments of cairo earthquake aftershocks recorded by the keg broadband station during the first three weeks after the occurrence of the mainshock using the ground displacement spectra of lg waves and to develop empirical formulas for estimating mo from both ml and coda duration. 2. data fifty-five cairo earthquake aftershock sequences recorded during the three weeks after the occurrence of the mainshock of the october 12, 1992 earthquake were used in this study. event data are given in table i. the local magnitude (ml) of these events varied from 1.7 to 4.7. all of these earthquakes are of crustal origin. figure 1 shows the location of keg station. keg station is a part of the mednet project (gardini et al., 1992). the seismic records were electronically digitized at a rate of 20 samples per second by sts-1 vbb velocity type sensor. for frequencies between 0.003 and 7 hz the velocity response is flat with a full dynamic range of 140 db. keg station was the only available digital recording station in northeast africa and egypt in 1992. 3. method of analysis in this work, a time window that started shortly before the lg arrival, depending on the size of earthquake was obtained from the vertical component seismometer of keg sts-1 vbb. the time window was nearly 10 s long. the digital data within the time window were corrected for the instrumental response, transformed to frequency domain using fast fourier transformation. this spectrum was then integrated to obtain the displacement spectra. typical spectrum examples are shown in fig. 2. the spectrum was corrected for attenuation by multiplying it by the transfer function e−γ r where r is the distance from the source to the receiver and γ is the coefficient of anelastic attenuation which is related to the quality factor q (nuttli, 1986) u q f $ $ =c r (3.1) f is the wave frequency and u is the lg-wave group velocity. dessokey et al. (2000) obtained the γ value of 0.001497 km– 1 for 1 hz lg in dahshour region. from the corrected spectrum, the low frequency spectral level was estimated. the low frequency level (ωo) was estimated visually by fitting a straight line at the low frequency spectra. fig. 1. aftershocks of dahshour earthquake of 1992 (open squares). the fault plane solution of the main shock (star) is also shown. 1211 seismic moments estimation table i. list of earthquakes used in this study. event date origin time lat. long. depth md ml no. y.m.d h min s °n °e (km) (hlw) 1 1992.10.12 13 50 9.65 29-46.94 31-13.66 10.93 2.5 2 1992.10.12 14 11 8.64 29-43.33 31-9.12 18.84 2.3 3 1992.10.12 14 15 7.43 29-57.23 30-57.28 25.09 2.8 4 1992.10.12 14 58 14.73 29-52.55 31-14.55 22.32 1.8 5 1992.10.12 15 07 42.05 29-53.52 31-6.80 26.85 2.3 6 1992.10.12 15 25 24.65 29-52.48 31-6.83 29.55 4.7 7 1992.10.12 15 52 50.77 29-51.24 31-9.94 22.77 2.3 8 1992.10.12 16 55 9.79 29-50.29 31-11.01 25.59 2.4 9 1992.10.12 18 31 42.36 29-47.89 31-9.82 21.14 2.2 10 1992.10.12 19 55 59.20 29-51.15 31-13.38 19.10 2.2 11 1992.10.12 21 31 34.22 29-40.00 31-7.06 20.73 4.2 12 1992.10.12 21 46 16.02 29-43.61 31-13.10 9.83 2.9 2.5 13 1992.10.12 23 34 22.50 29-55.72 31-16.90 17.10 3.0 2.7 14 1992.10.12 23 46 24.42 29-44.40 31-10.38 21.78 2.9 2.1 15 1992.10.13 18 09 8.14 29-52.44 31-10.98 22.39 3.4 3.7 16 1992.10.13 18 34 54.26 29-50.62 31-13.15 20.72 3.2 17 1992.10.13 23 27 56.39 29-53.00 31-7.37 32.02 2.4 2.2 18 1992.10.14 02 44 23.14 29-52.16 31-9.69 30.25 3.1 2.8 19 1992.10.14 03 50 14.53 29-46.63 31-6.43 29.49 3.4 2.8 20 1992.10.14 09 40 27.04 29-43.49 31-1.28 27.64 4.0 4.2 21 1992.10.14 10 41 57.34 29-46.67 31-7.99 22.95 2.6 1.9 22 1992.10.14 12 09 15.72 29-43.34 31-5.99 20.53 3.0 23 1992.10.14 13 46 39.47 29-50.45 31-10.37 28.51 3.5 3.2 24 1992.10.14 14 23 44.67 29-41.18 31-14.42 20.22 3.1 2.4 25 1992.10.14 14 31 27.90 29-43.41 31-6.98 24.97 3.3 2.9 26 1992.10.14 20 16 11.05 29-49.25 31-9.74 20.94 2.6 2.3 27 1992.10.15 12 13 41.41 29-48.25 31-9.02 25.52 2.6 2.0 28 1992.10.16 03 28 51.46 29-47.29 31-5.79 24.12 2.5 2.0 29 1992.10.16 05 56 11.84 29-47.31 31-3.17 30.70 3.1 2.9 30 1992.10.16 09 57 46.87 29-47.50 31-5.51 26.25 3.3 3.1 31 1992.10.16 18 07 53.12 29-51.50 31-7.49 24.20 2.6 2.4 32 1992.10.17 01 35 28.51 29-45.06 31-8.25 25.44 2.6 2.0 33 1992.10.18 08 12 16.12 29-43.38 31-8.40 20.74 3.4 2.9 34 1992.10.18 13 04 28.44 29-42.75 31-13.80 18.50 3.0 2.6 35 1992.10.19 10 46 30.95 29-42.67 31-9.58 21.58 2.9 2.2 36 1992.10.19 12 30 16.44 29-44.22 31-6.30 25.20 3.2 3.0 37 1992.10.19 14 59 50.43 29-44.50 31-8.87 18.68 3.3 3.2 38 1992.10.20 06 00 30.77 29-32.36 31-4.27 22.28 2.7 1.7 39 1992.10.20 17 28 28.44 29-40.39 31-9.24 20.62 2.5 2.0 40 1992.10.20 23 14 47.46 29-43.08 31-7.65 17.20 3.1 3.1 41 1992.10.21 18 09 27.53 29-46.47 31-2.00 14.65 2.7 2.0 42 1992.10.22 08 28 58.70 29-44.83 31-6.53 21.90 3.4 3.4 43 1992.10.22 17 38 57.30 29-40.62 31-5.68 22.88 3.9 4.6 44 1992.10.23 02 40 5.43 29-44.21 31-5.46 22.85 2.6 2.2 45 1992.10.23 15 12 10.08 29-41.39 31-10.00 22.40 3.8 3.3 46 1992.10.23 16 02 4.12 29-44.78 31-7.38 21.70 2.4 1.7 47 1992.10.25 09 05 4.64 29-40.09 31-10.71 22.05 3.1 2.8 48 1992.10.25 12 26 15.06 29-39.93 31-5.95 24.12 3.1 2.7 49 1992.10.25 16 21 05.28 29-38.35 31-6.70 22.92 2.9 2.3 50 1992.10.25 19 45 34.56 29-40.97 31-8.52 23.78 2.3 3.2 51 1992.10.26 06 45 23.96 29-52.40 31-6.49 27.45 3.4 3.3 52 1992.10.26 08 43 52.01 29-40.55 31-5.75 28.25 2.8 2.4 53 1992.10.28 06 20 54.90 29-37.30 31-3.63 24.63 2.9 2.5 54 1992.10.28 18 25 56.28 29-50.64 31-16.07 19.46 2.8 2.5 55 1992.10.30 14 08 11.19 29-40.70 31-5.09 25.71 3.0 2.7 1212 el-sayed m. abdelrahman, mohamed m. dessokey, hesham m. hussein and mohamed f. abdelwahed fig. 2. displacement spectra for lg-waves for some selected events. 1213 seismic moments estimation the seismic moments (mo) were derived for the 55 recorded events from the low spectral level (ωo) using street et al. (1975)’s formula that was subsequently derived by means of theoretical model by herrmann and kijko (1983) / < / m r r r r r r r 4 / o o o o o o o o o o 3 3 1 2 = rtb ω m r r r4 $= rtb ω ^ ^ h h (3.2) where ρ is the density of the medium, β is the shear wave velocity, r is the epicentral distance and ro is the reference distance. the reference distance ro is related to different kinds of lgwaves geometric spreading. 4. results 4.1. seismic moment coda duration and local magnitude the seismic moment mo is a measure of an earthquake strength defined in terms of parameters of double couple shear dislocation source model (aki, 1966). it provides a better description of the real size of an earthquake. on the basis of previous empirical results, the seismic moments of the events under study were estimated using eq. (3.2). constant values corresponding to cairo earthquake source region in eq. (3.2) were selected as follows: ρ = 2.7 gm/cm3, β = 3.5 km/s and ro = 100 km. the seismic moment estimated was found to range from 3.5 × 1019 to 2.5 × 1022 dyne⋅cm. the calculated seismic moments using eq. (3.2) are listed in table ii. the scatter for the events having a duration longer than 108 s (fig. 3) is due to the small number of longer duration shocks. to obtain a more appropriate relation between the moment and the duration, a mathematical regression between the reported seismic moments and the corresponding coda durations d was made for 30 ≤ d ≤ 108 s. the coda duration in seconds used in this study was obtained from the available analog seismographic stations installed at the time of the earthquake. it was taken from the p arrival to the point in the coda where the seismic amplitude is nearly twice the backtable ii. seismic moment of the events under study. event duration ml mo no. (s) (dyne ⋅ cm) 1 2.5 1.942783e + 20 2 2.3 2.267175e + 20 3 2.8 2.836813e + 20 4 1.8 5.468023e + 19 5 2.3 7.922314e + 19 6 279 4.7 2.267175e + 22 7 2.3 8.386375e + 19 8 2.4 1.225617e + 20 9 2.2 1.28611e + 20 10 2.2 1.520192e + 20 11 169 4.2 7.825914e + 21 12 57 2.5 1.021348e + 20 13 65 2.7 7.1504e + 20 14 2.1 8.689964e + 19 15 161 3.7 2.028367e + 21 16 1992 3.2 7.671786e + 20 17 37 2.2 6.70293e + 19 18 56 2.8 3.1557e + 20 29 2.8 4.717935e + 20 20 237 4.2 5.429151e + 21 21 1.9 5.667937e + 19 22 34 3.0 3.666106e + 20 23 3.2 4.157197e + 20 24 2.4 4.071058e + 20 25 69 2.9 4.769604e + 20 26 45 2.3 6.387174e + 19 27 35 2.0 6.550127e + 19 28 2.0 3.571662e + 19 29 108 2.9 5.07458e + 20 30 62 3.1 7.187676e + 20 31 2.4 2.237957e + 20 32 35 2.0 6.845436e + 19 33 78 2.9 4.614945e + 20 34 63 2.6 3.037181e + 20 35 39 2.2 9.024836e + 19 36 85 3.0 7.510935e + 20 37 79 3.2 6.659202e + 20 38 1.7 2.856025e + 19 39 30 2.0 5.832978e + 19 40 87 3.1 9.421127e + 20 41 2.0 6.235795e + 19 42 94 3.4 1.637872e + 21 43 4.6 2.522325e + 22 44 34 2.2 1.227609e + 20 45 3.3 1.490997e + 21 46 1.7 3.499786e + 19 47 55 2.8 4.527036e + 20 48 68 2.7 5.052129e + 20 49 38 2.3 1.011922e + 20 50 3.2 7.076904e + 20 51 99 3.3 1.602396e + 21 52 34 2.4 1.00893e + 20 53 59 2.5 2.729787e + 20 54 50 2.5 1.03903e + 20 55 2.7 3.834036e + 20 1214 el-sayed m. abdelrahman, mohamed m. dessokey, hesham m. hussein and mohamed f. abdelwahed fig. 3. seismic moment (mo)-coda duration relation of dahshour aftershocks. fig. 4. plot of seismic moment versus local magnitude. 1215 seismic moments estimation ground noise amplitude. the linear regression fitting of the data as shown in fig. 3 indicates the following relation: . . . . . log logm d d 2 35 0 27 16 33 0 48 30 108 o ! ! # # = + + ^ ^ ^ ^ ^ h h h h h (4.1) estimation of seismic moment from a simple parameter such as coda duration is very useful, as it allows the estimation of the source size of local earthquakes for the moment range 3.5 × 1019 to 1.6 × 1021 dyne ⋅ cm. our linear moment magnitude relation over the available duration range results in mean root square error of 0.05 in log (mo). the obtained relation for all the available moment range is insufficient for meaningful comparison with the published log (mo)-log (d) relations in different tectonic provinces. the seismic moment-local magnitude relation of the form log (mo) = cml + d should be carried out carefully (shapira and hofstetter, 1993). bakum (1984) and hanks and boore (1984) detected a change in c value in this relation at ml ≈ 3.5, usually undetected due to a large number of ml < 5.0 and relatively small number of ml > 5.0 earthquakes. using these recommendations, the moment magnitude relation is obtained for the selected 50 earthquakes in the range 1.7 ≤ ml < 3.5. in this work, a change in the moment magnitude relation at ml ≈ 3.5 could not be observed due to small number of larger events. figure 4 shows the seismic moment versus local magnitude ml. the least square fitting gives the following equation: . . . . . < . . m m m log 0 96 0 05 17 88 0 13 1 7 3 5 o l l ! ! # = + + ^ ^ ^ ^ h h h h (4.2) this relation is valid for earthquakes with 1.7 ≤ ≤ ml < 3.5 and seismic moment in the range 3.5 × 1019 to 1.6 × 1021 dyne⋅cm. the average root mean square difference in the estimated log (mo) is 0.028. the relation is in a fair agreement with log mo = 1.1 ml + 17.9 for ml ≤ 4.1, this relation was obtained by fletcher et al. (1984) for aftershocks of orville, california earthquake. performing a least square fitting of log mo versus ml for ml ≤ 4.1 yields almost identical values to eq. (4.2) log (mo) = (0.95 ± 0.05)ml + (17.90 ± ± 0.13). the slope of the least square fitting between log (mo) and ml is near one. a slope of about 1.0 for the moment-magnitude relationship seems to be similar to those obtained in many proposed moment magnitude relations for mammoth lakes, california (archuleta et al., 1982), oroville, california (fletcher et al., 1984), hawaii (savage and mayer, 1985), california (chen and chen, 1989), baltic shield (kim et al., 1989) and petlan, mexico (valdes et al., 1996). randall (1973), archuleta et al. (1982), and fletcher et al. (1984) demonstrated that the slope of 1.0 for moment magnitude relations can be explained by the response of the wood-anderson instrument since the corner frequency of small events is greater than the cutoff frequency of 1.2 hz wood-anderson instrument. 5. conclusions seismic moment mo, logarithm of the coda duration and the local magnitude empirical relations were derived for the aftershocks of 1992 cairo earthquake source area, providing a simple and straightforward way to quantify the source strength mo through such relationships, which is superior to any magnitude scale. the resulting estimate of seismic moment, from local magnitudes and coda duration imply the following: – seismic moment and local magnitude data are consistent with log (mo) = (0.96 ± 0.05) ml + + (17.88 ± 0.13). the rms precision in log (mo) is 0.028. – coda durations are related to seismic moment by the following linear relation: log (mo) = = (2.35 ± 0.27) log (d) + (16.33 ± 0.48) with a rms precision of 0.05 in log (mo). the derived empirical relationships are valid only for cairo earthquake source region (dahshour) as recorded at keg broadband station. their significance is only available in the 3.5 × 10 19 to 1.6 × 10 21 dyne⋅cm moment range and 1.7 to 3.4 local magnitude range. 1216 el-sayed m. abdelrahman, mohamed m. dessokey, hesham m. hussein and mohamed f. abdelwahed references abou el enein, k.m., h.m. hussein, a.s. abu el ata and e.m. ibrahim (2000): seismological aspects of the cairo earthquake, 12 october 1992, ann. geofis., 43 (3), 485-504. aki, k. (1966): generation and propagation of g waves from the niigate earthquake of june 16, 1964. part 2. estimating of earthquake movement, release energy, and stress-strain drop from g wave spectrum, bull. earthquake res. inst., tokyo univ., 44, 73-88. arculeta, r.j., e. crrnswick, c. muller and p. spudich (1982): source parameters of the 1980 mammoth lakes, california, earthquake sequence, j. geophys. res., 87, 4595-4607. bakum, w.h. (1984) : seismic moments, local magnitudes, and coda duration for earthquakes in central california, bull. seismol. soc. am., 74, 439-458. chen, p. and h. chen (1989): scaling law and its applications to earthquake statistical relations, tectonophysics, 166, 53-72. dessokey, m.m., h.m. hussein, e.s. abdelrahman and m.f abdelwahed (2000): local magnitude, duration magnitude and seismic moment of dahshour 1992 earthquake, ann. geofis., 43 (1), 95-103. fletcher, j.b., j. boatwright, l. harr, t.c. hanks and a. mcgarr (1984): source parameters of aftershocks of the oroville, california earthquake, bull. seismol. soc. am., 74, 1101-1123. gardini, d., e. boschi, s. mazza, a. morelli, d. ben sari, d. najid, h. benhallou, m. bezzeghoud, h. trabelsi, m. hfaidh, r.m. kebeasy and e.m. ibrahim (1992): very-broad-band seismology in northern africa under the mednet project, tectonophysics, 209, 17-30. hanks, t. and d. boore (1984): moment magnitude in theory and pratice, j. geophys. res., 89, 6229-6235. herrmann, r.b. and a. kijko (1983): modeling some empirical vertical component lg relations, bull. seismol. soc. am., 73, 157-171. hussein, h.m. (1999): source process of the october 12, 1992 cairo earthquake, ann. geofis., 42 (4), 665-674. kim, w.y., r. wahlstrom and m. uski (1989): regional spectral scaling relations of source parameters for earthquakes in the baltic shield, tectonophysics, 166, 151-161. maamoun, m.e., a. meghaed, a. hussein and m. marzouk (1993): preliminary studies on the dahshour earthquake (national research institute of astronomy and geophysics, cairo, egypt), abstract. nuttli, o.w. (1986 ): yield estimates of nevada test site explosions obtained from seismic lg-waves, j. geophys. res., 91, 2137-2151. randall, m.j. (1973): the spectral theory of seismic sources, bull. seismol. soc. am., 64, 1133-1144. savage, m.k. and r.p. meyer (1985): aftershocks of an m = = 4.2 earthquake in hawaii and comparison with long term studies of the same volume, bull. seismol. soc. am., 75 (3), 759-777. shapira, a. and a. hofstetter (1993): source parameters and scaling relations of earthquakes in israel, tectonophysics, 217, 217-226. street, r.l., r.b. herrmann and o.w. nuttli (1975): spectral characteristics of the lg-wave generated by the central united states earthquakes, geophys. j. r. astron. soc., 41, 51-63. valdes, c.m. and r.p. meyer (1996): spectral analysis and earthquake scaling of the petatan earthquake (ms = = 7.6) aftershocks, recorded at stations along the pacific coast and between the coast and mexico city, pure appl. geophys., 147 (4), 631-656. (received january 15, 2003; accepted november 4, 2003) i l t e o r e m a d e l v i r i a l e e l ' e q u i l i b r i o d i u n s i s t e m a a u t o g r a v i t a n t e i s o l a t o p . e . v a l l e r i c e v u t o il 2 luglio 1960 riassunto. — si stabilisce il t e o r e m a del viriale per i sistemi continui. m e d i a n t e questo t e o r e m a , si cerca di o t t e n e r e qualche informazione sull'equilibrio di un s i s t e m a a u t o g r a v i t a t i te isolato. l a teorìa, a v a n z a t a d a alcuni geologi, secondo la quale la t e r r a a v r e b b e a u m e n t a t o n o t e v o l m e n t e le sue dimensioni, nel corso delle ere geologiche, n o n è in c o n t r a s t o con le conclusioni cui si perviene. summary. — the viriale t h e o r e m for c o n t i n u o u s systems, is here e s t a b l i s h e d . t h e a., t h r o u g h t h i s t h e o r e m , tries to achieve some i n f o r m a t i o n a b o u t t h e e q u i l i b r i u m of t h e i n s u l a t e d a u t o g r a v i t i n g s y s t e m . a c c o r d i n g to t h e t h e o r y of some geologists, t h e e a r t h — d u r i n g t h e geologic e r a s — m a y be increased r e m a r k a b l y its own dimensions. t h e r e s u l t s a t t a i n e d in t h i s p a p e r are n o t clashing w i t h above ment i o n e d t h e o r y . p r e m e s s a . sembra che hi nozione di viriale, che ha il vantaggio di implicare globalmente le forze esterne ed interne agenti su un sistema, sia destin a t a ad avere una funzione prevalentemente in meccanica statistica. questa nota costituisce un tentativo di richiamare l'attenzione sul f a t t o che questa grandezza può essere utilmente introdotta in altre questioni, in particolare in talune questioni che riguardano la geofìsica. i l t e o r e m a d e l v i r i a l e p e r i s i s t e m i c o n t i n u i . si consideri un sistema continuo riferito, per semplicità, ad una terna assoluta 0 sia c la sua configurazione attuale, c una parte qualunque di g. 2 la superfìcie che costituisce il contorno di 0 e a la superfìcie che costituisce il contorno di c. 3 6 2 p . jo. v a l l e ii m o m e n t o (l'inerzia polare lagraugiano di c, r i s p e t t o al polo 0 , è definito dalla relazione io = ioopxopdc dove o è la densità di massa nell'intorno del generico p u n t o p di c. u n sistema continuo si dirà in equilibrio se, almeno in m e d i a , il valore di i 0 n o n v a r i a nel t e m p o . è questa u n a ovvia estensione della nozione di equilibrio. d a l l a p r e c e d e n t e si ottiene subito 1 (pio 2 d t 2 2 t + j oop x adc dove t è l'energia cinetica di c ed a l'accelerazione. se con f si indica il campo delle forze di massa e con & t j il tensore dello sforzo, r i s u l t a n o t o r i a m e n t e 3 o a = o f — s t —— ! a xi si h a p e r t a n t o \ =2 t + j ' ( h x i f i ) e d c 3 li>i x op\ dc il xi ora si può scrivere 3 ö 3 ò s i ^ ^ x o p = s i r — < xop) — h 1 ì>xi i t) ¿ci dove l 1 è l ' i n v a r i a n t e lineare del tensore dello sforzo, ossia il = si 0n ì d ' a l t r a p a r t e se n è la n o r m a l e i n t e r n a a a e q u n suo generico p u n t o , con qualche passaggio, si ha si " t " d ' i x o p) 1 o xi de = — oq x da i l t e o r e m a d e l v i r i a l e , e c c . 3 6 3 ove si i n t r o d u c a il viriate delle forze di massa v = j xì f^j q a g c risulta in definitiva 1 d2i0 /' f 7t j j = 2 t + v +1 ix dc + i oq x®n da c a sebbene questa relazione sia suscettibile di notevoli applicazioni, qui verrà m o s t r a t o s o l t a n t o come sia possibile ottenere da essa qualche informazione sull'equilibrio di u n sistema a u t o g r a v i t a n t e isolato. i l v i r i a l e d e l l e f o r z e d i .massa p e r un sistema a u t o gii avita n t e i s o l a t o . siano fjdg e o' d c' due masse elementari, poste r i s p e t t i v a m e n t e nei p u n t i p e p ' di un sistema continuo e soggette a m u t u a azione gravitazionale. i l viriale di t a l e azione, calcolato rispetto al polo 0 , sarà dv =:= m ' i c d c ' (p' p x 0 p' + p p' x 0 p) ri dove si è posto r = p p ' e con y si è i n d i c a t a la costante di gravitazione universale. ora p ' p x op' + p p ' x op = — r2 consegue qq' dg dc' d v = — y r se con u si indica il potenziale, si h a quindi v = — i ugdc la quale m o s t r a che il viriale delle forze di m a s s a di un sistema autog r a v i t a n t e isolato, coincide con l'energia gravitazionale e del sistema stesso. 364: p . e . v a l l e i l t e o r e m a d e l v i r i ale p e r u . \ s i s t e m a a u t o g r a v i t a n t e i s o l a t o . tenuto conto di quanto si è esposto nei paragrafi precedenti, dato che il sistema si suppone isolato e quindi si deve porre tjjn = 0 su risulta subito c la relazione c mostra che un caso interessante di equilibrio si verifica se è nulla la componente radiale, rispetto al polo 0, della velocità di ogni punto. comunque, se il sistema è in equilibrio, si ha necessariamente 0 = 2 t + e + j i1d g c conviene introdurre il valor medio di in g e scrivere 0 = 2 t + e + i , v la quale rappresenta una condizione necessaria per l'equilibrio di un sistema continuo autogravitante isolato. se l'equilibrio del sistema, è di tipo idrostatico, i! valor medio di 7x coincide col triplo del valor medio della pressione. la condizione stabilita pone in evidenza, t r a l'altro, il ruolo essenziale del valor medio dell'invarianti' lineare del tensore dello sforzo, nel caso di equilibrio di tipo non idrostatico, e del valor medio della pressione, nel caso di equilibrio idrostatico. se in un certo istanti' essa non fosse più. soddisfatta, si verificilerebbe un fenomeno assai simile ad un'esplosione oppure ad un collasso. si osservi però che la condizione in questione, non è sufficiente per assicurare l'equilibrio. si potrebbe verificare una variazione uniforme di i 0 , con conseguente possibile aumento o diminuzione delle dimensioni del sistema, a prescindere dalle cause di u n a simile eventualità, è interessante t u t t a v i a rilevare che la teoria, a v a n z a t a da alcuni geologi, secondo la quale la terra avrebbe a u m e n t a t o notevolmente le sue dimensioni, nel corso delle ere geologiche, non è in contrasto con queste considerazioni. 6677_pagani_final_ok:layout 6 annals of geophysics, 58, 1, 2015, s0108; doi:10.4401/ag-6677 s0108 a summary of hazard datasets and guidelines supported by the global earthquake model during the first implementation phase marco pagani1,*, julio garcia1, damiano monelli1,2, graeme weatherill1, anselm smolka3 1 gem hazard team, gem foundation, pavia, italy, hazard@globalquakemodel.org 2 now at tokyo millenium re, zurich, switzerland 3 secretary general, gem foundation, pavia, italy abstract the global earthquake model (gem) initiative promotes open, transparent and collaborative science aimed at the assessment of earthquake risk and its reduction worldwide. during the first implementation phase (2009-2014) gem sponsored five projects aimed at the creation of global datasets and guidelines toward the creation of open, transparent and, as far as possible, homogeneous hazard input models. these projects concentrated on the following global databases and models: an instrumental catalogue, a historical earthquake archive and catalogue, a geodetic strain rate model, a database of active faults, and set of ground motion prediction equations. this paper describes the main outcomes of these projects illustrating some initial applications as well as challenges in the creation of hazard models. 1. introduction the global earthquake model (gem) initiative is a public-private effort which currently involves sixteen countries and eleven private organizations from the engineering, insurance and public/governmental sectors. gem promotes an open and transparent assessment of seismic hazard and risk based on state-of-the-art datasets and methodologies. the first implementation phase started in 2009 and concluded at the end of 2014, saw gem sponsor five projects aimed at the creation of global, homogenous datasets supporting the development of new hazard models and, in general, research in the seismological sector. these projects called global components covered the most important research areas for the characterization of the earthquake process such as historical seismicity, instrumental seismicity, strain rate modelling, earthquake geology and ground motion modelling. five global projects were also promoted in the field of the physical risk assessment [crowley et al. 2013]. with the same aim of actively encouraging reproducibility and transparency in seismic hazard model creation process, gem also developed an extensive opensource library of tools for the preparation of probabilistic seismic hazard analysis (psha) models and for the calculation of hazard. this library includes the openquake hazard modeller’s toolkit (hmtk), the openquake ground motion toolkit (oq-gmtk) and the openquake-engine (which comprises the openquake hazard library; pagani et al. [2014]). the openquake hazard modeller’s toolkit [weatherill et al. 2012, 2014a] is a suite of tools for processing basic information, e.g., earthquake catalogues, active faults and geodetic strain, used for the characterization of the seismic source model (ssm hereafter). using the hmtk it is possible to create sources that can be incorporated into a source model for the calculation of hazard at a specific site. the openquake ground motion toolkit (oq-gmtk; weatherill et al. [2014b]) is the counterpart of the hmtk for the creation of the ground motion models (hereafter gmm) for application in a particular analysis. it contains tools for the construction of a ground motion database starting from a set of processed recordings, for the calculation of ground motion residuals given a ground motion prediction equation (gmpe) and a collection of processed strong motion recordings, for the ranking of gmpes using quantitative methods [e.g. scherbaum et al. 2009]. both toolkits integrate components of the openquakeengine into the software, ensuring full consistency with openquake-engine itself with regard to the application of gmpes, representation of fault geometry and geodetic distances and the generation of ruptures. in many areas of the world the results of the global seismic hazard assessment programme (gshap; giararticle history received october 2, 2014; accepted november 24, 2014. subject classification: earthquake faults, ground motion, seismic risk, global datasets, seismic hazard, global earthquake model. dini et al. [1999]) still represent a reference, or in some cases the only, quantitative assessment of hazard available, despite the progress made in the past twenty years in the comprehension of the earthquake process. the overall goal of gem during its first implementation phase was therefore to create a set of tools, databases and guidelines, which could serve the creation of new and more harmonized probabilistic seismic hazard analysis (psha) input models and consequently provide to the broader community more updated assessments of seismic hazard. the auspice is that in the near future, local communities at regional and national levels will harmonize the information included in gem’s global datasets with more specific knowledge and will work in a collaborative effort at testing, applying and improving the guidelines, the methodologies and the tools created by gem. ideally, this should be a collaborative process directed at the creation of reliable and consistent models as well as at the collective progress of science within the seismic hazard assessment field. during the first implementation phase some research projects operated as gem regional programs i.e. independent projects directed at the creation of new datasets and hazard and risk models. these projects were: the seismic hazard harmonisation in europe (share), the earthquake model for the middle east (emme) and the earthquake model for central asia (emca). share [giardini et al. 2014] produced an extensive group of updated datasets as well as a hazard model covering the entire european continent. the results have been released in 2013 and they are accessible though the project website (http://www.share-eu.org/, last accessed on september 2014). the emme project (sesetyan et al. [2014]; http://www.emme-gem.org/, last accessed september 2014) created a hazard model covering a region spanning from the western coast of turkey, though iran and until pakistan. danciu and giardini [2015, this issue] provide an overview of the share and emme projects emphasizing the issues connected with the harmonization of the two models at their mutual boundaries. finally, the earthquake model for central asia (emca; parolai et al. [2015, this issue]) created a community seismic hazard model for the area covering turkmenistan, kazakhstan, tajikistan, uzbekistan and kirgizstan (additional information available on the project website, http://www.emca-gem.org/, last accessed on september 2014). these projects had been an extraordinary opportunity for testing the use of the openquake-engine in the calculation of region-wide seismic hazard models. the sara (south america risk assessment) project, a new gem regional program, sponsored by the swiss re foundation, will be the first project in which gem tools and datasets will be extensively tested for the creation of psha input model; the collaboration with the local scientific community will certainly help on improving the current tools. this paper aims at illustrating the main datasets produced by the gem global components, at briefly illustrating some recent research based on this datasets and on discussing how they can be possibly used for the construction of seismic hazard analysis input models. 2. gem hazard global components three out of the five gem hazard global components had the primary goal of developing global homogenous datasets the global instrumental catalogue, global historical catalogue and active fault database while the other two worked on creating specific models (i.e. a strain rate model and a ground motion model). most of the gem hazard global components started their activities in 2010 and concluded their work between 2012 and 2013. the teams were composed by international groups of experts in order to guarantee to the extent possible global participation and the involvement of highly qualified scientists. in the following sections we briefly summarize the main achievements of the five gem global components. 2.1. global instrumental catalogue (isc-gem) the scientific project dedicated to the construction of a global uniform instrumental catalogue was led by the international seismological centre and saw the participation of a numerous and highly qualified group of international experts. the isc-gem catalogue [storchak et al. 2012, 2013, 2015] covers the entire instrumental period (1900-2009) and is compiled considering information included in a vast number of catalogues (e.g. isc bulletin, global cmt; ekström et al. [2012]), in the literature [lee and engdahl 2015], new information ported from iss paper bulletins into a digital format and new results produced within the project. during the period 1900-1917, the isc-gem catalogue contains events with magnitude larger or equal to 7.5 (ms ) worldwide, supplemented by a few tens of smaller shallow earthquakes in stable continental areas. from 1918 until 1959 the catalogue was compiled considering earthquakes whose magnitude was equal or larger than 6.25 (ms ). the most recent part of the catalogue (starting from 1960 until 2009) includes events equal or larger than 5.5 (ms ). the earthquakes comprised in this catalogue were relocated following a homogenous approach entailing a first stage, where the hypocentral depth is computed using the ehb methodology [engdahl et al. 1998], and a second stage where the epicentral location is obtained pagani et al. 2 3 with the application of the bondar and storchak [2011] algorithm. figure 1 shows the spatial distribution of the earthquakes (more than 18,800 in total) included in version 1.05 released in february 2014. the catalogue is updated by the isc every time changes are introduced; a log of changes is maintained on the isc website (http:// www.isc.ac.uk/iscgem/update_log/). the determination of magnitude for each earthquake followed a homogeneous methodology aimed at the compilation of a catalogue where the size of each event is defined in terms of moment magnitude. for this purpose seismic moment information already available (e.g. global cmt catalogue http://www.global cmt.org/) as well as data included in a vast body of literature was used to the largest extent possible; for the older events proxies, new regression models from other magnitude types were used as a primary method for magnitude harmonisation [see di giacomo et al. 2014]. michael [2014] analysed the isc-gem catalogue in order to define the region of the magnitude-time space within which this catalogue can be considered complete (i.e. containing all the events occurred). the results of michael show that the magnitude of completeness (mc) in the early instrumental era (i.e. between 1900 and 1917) for shallow seismicity corresponding to the events with hypocentral depth lower than 60 km is around 7.7. this threshold progressively decreases with time from 7.0 in the period between 1918 and 1939, going to 6.8 between 1940 and 1954. in its most recent part (i.e. between 2004 and 2009), the catalogue is complete above magnitude 5.7 mw. the green dotted line in figure 6 shows the completeness thresholds for different periods in a time-magnitude plot. christophersen et al. [2013] used the isc-gem catalogue to analyze at the global scale the statistics of large aftershocks i.e. earthquakes with a magnitude within 1.1 units from the mainshock occurring at relatively short distance from the mainshock. the study had the goal to analyze the darfield christchurch sequence in the context of global seismicity occurrence properties. hayes et al. [2013] analyze the earthquake and tsunamis potential of seismic sources in the caribbean using the isc-gem catalogue in combination with other catalogues such as pde (usgs), global cmt [ekström et al. 2012] and ehb [engdahl and villaseñor 2002]. pollitz et al. [2014] use the isc-gem catalogue to investigate global seismic quiescence periods in the framework of a study aimed at assessing the shortand long-term triggering of seismicity produced by the m 8.6 indian ocean event occurred in april 2012. geist [2014] uses the isc-gem catalogue together with other earthquake catalogues to investigate through an epidemic-type aftershock sequence model the temporal clustering of seismic activity from seismic sources capable of generating tsunamis. 2.2. global historical earthquake archive (ghea) and catalogue (ghec) the determination of earthquake locations and magnitudes through the investigation of historical sources yielded in the past and still provides useful gem first implementation phase hazard datasets and guidelines figure 1. geographic distribution of the epicenters included in the isc-gem catalogue. the color indicates the hypocentral depth. information for better constraining the properties of seismic occurrence [guidoboni 2002]. in many countries in the world the information produced by these investigations still constitutes the primary dataset used for the characterization of seismicity occurrence in psha input models. the global earthquake model sponsored a project called “global earthquake history” (geh; albini et al. [2014a, 2014b]) led by scientists of the istituto nazionale di geofisica e vulcanologia (national institute of geophysics and volcanology, ingv, italy) and the british geological survey (bgs, uk). this project produced an archive of historical seismicity studies and from this archive created a database of earthquakes occurring during the period 1000 c.e. and 1903 c.e. with magnitude equal or larger than 7. the global historical earthquake archive (ghea) contains 3175 records extracted from 239 published studies. each earthquake is associated with a list of studies providing relevant parameters needed for its comprehensive description from a seismological point of view, primarily epicentral position and magnitude. the archive is a source of information of extraordinary importance since it makes the procedure adopted for the construction of the historical catalogue completely transparent and reproducible, and it ensures its future expandability by benefitting from the knowledge already gathered. the ghec (version 1.0) catalogue contains 825 events of magnitude 7.0 and larger; this magnitude threshold had been lowered in stable continental regions in order to incorporate important earthquakes that otherwise would have been neglected. the global historical catalogue was compiled by taking for each event in the ghea archive the parameters provided by the preferred study selected considering the quality of the information used for the description of the earthquake and the methodology adopted for the determination of the earthquake parameters. figure 2 shows the geographic distribution of the events included in the catalogue. in the period between 1000 and 1490 the catalogue contains only events in europe, the middle east, china and japan, whilst from the end of the 16th century the catalogue contains also earthquakes occurring in the americas. estimates of earthquake parameters in historical catalogues are habitually affected by epistemic uncertainties that can be explained by the paucity of the information available and the relevant subjective component involved in the process which is indispensable for a comprehensive interpretation of the original information. in the construction of the geh catalogue, despite only a limited number of studies provided this information, special treatment was devoted to the characterization of uncertainty related to location and magnitude. albini et al. [2014a] explain that the majority of the earthquakes included in the catalogue are affected by an error in the estimation of the magnitude in the order of half a unit; according to the same authors only for a limited number of events this error exceeds one unit of pagani et al. 4 figure 2. geographic distribution of the epicenters included in the geh catalogue [albini et al. 2014a]. the color indicates the year of occurrence. 5 magnitude. in figure 3 we depict the distribution in the time-magnitude space of the earthquakes included in the geh catalogue as well as their cumulative curve with respect to time. 2.3. global strain rate model the contribution of remote sensing to the comprehension of the geodynamic processes increased exponentially over the last few years thanks to large technological improvements and the spread of more advanced instrumentation [tralli et al. 2005]. recognizing the importance of results deriving from the modeling of remotely sensed information, gem promoted a project, called the gem global strain rate model (gsrm2), which leveraged from the results of the global strain rate model project (http://gsrm.un avco.org/intro/, kreemer et al. [2003]). the project involved scientists from the university of nevada, the university of california at los angeles, the china earthquake administration and unavco. the goal of the gsrm2 was the creation of a model describing the secular deformation on a 0.2°× 0.25° mesh covering the deforming areas of the world. a considerable portion of the project activities was devoted to the creation and processing of an extensive database of global positioning system (gps) velocity observations, which ideally should cover homogenously the entire globe. the gsrm2 project used 22,415 velocities at about 18,300 locations; 17,567 velocities characterize deforming areas while the remaining part have been measured on rigid plates. the calculation of strain on this mesh was completed using the methodology proposed by haines and holt [1993] which computes the strain field as well as vortices and expected velocities through an interpolation methodology based on splines. figure 4 shows the spatial distribution of the second invariant of the strain tensor as defined in the most recent release of the gem global strain rate model. bird et al. [2015] used the gem global strain rate model for the construction of a hybrid earthquake forecast model, which combines the rates obtained by smoothing the seismicity within an instrumental catalogue with the ones obtained by applying the shift methodology (seismic hazard inferred from tectonics; bird and liu [2007]) to the results of gsrm2. sevilgen et al. [2014] utilize the global strain rate model to infer the activity rates of active faults within some countries in the balkans region. the comparison between the rates obtained using this new technique and the ones defined by earthquake geologists based on expert judgment overall show a good agreement. the overall goal in this case is to build a psha input model combining faults and distributed seismicity sources. gem first implementation phase hazard datasets and guidelines figure 3. time-magnitude space plotting of the earthquakes included in the geh catalogue. 2.4. active fault database earthquake geology is a critical source of information for the creation of state-of-the-art probabilistic seismic hazard input models. nevertheless, the manner by which geologically based information is assimilated into the construction of seismicity occurrence models for individual fault structures, as well as for actively deforming tectonic regions, is still the subject of scientific research [e.g. field and page 2011]. recognizing the importance of this discipline and the significance of improving and harmonizing the description and modelling of fault sources, gem sponsored a project named “faulted earth” (fe) which worked on the creation of a database of active faults with global coverage. gns science (new zealand) led this project, which involved experts from almost all the continents. the initial part of the project was predominantly focused on the construction of a database with a structure capable to incorporate information currently included in the most important national active fault databases publicly available. the information in the gem active fault database is divided into a number of levels organized hierarchically, ranging from punctual data collected on the field until the description of segments structure and the illustration of a fault source as it used in psha analysis. figure 5 shows an example for japan of the collection of active fault sections included in the gfe database. the second phase focused on populating this database with information included in the most recent and advanced databases of active faults available. the databases of active fault incorporated into the gem faulted earth (gfe) database are the following ones: the new zealand active fault database ( jongens and dellow [2003]; http://data.gns.cri.nz./af, last accessed on september 2014). the active fault database of japan (https://gbank. gsj.jp/activefault/index_e_gmap.html, last accessed on september 2014). the quaternary fault and fold database of the united states (machette et al. [2004]; http://earthquake. usgs.gov/hazards/qfaults/, last accessed september 2014). the australian neo-tectonic feature database (clark et al. [2011]; http://www.ga.gov.au/earthquakes/static pagecontroller.do?page=neotectonics, last accessed september 2014). the alaska quaternary fault and folds database (koehler et al. [2012]; http://www.dggs.alaska.gov/ pubs/id/23944, last accessed on september 2014). the faulted earth project also developed, in collaboration with the science and technology teams at the gem secretariat, a web based tool for the compilation of information describing active faults from punctual data collected in the field to fault descriptions that can be readily ported into an hazard input model. the gem faulted earth project promoted also a number of auxiliary initiatives aimed at creating a comprehensive framework for the incorporation of faults into hazard models. a special task group worked on the creation of a global database of subduction interpagani et al. 6 figure 4. geographic distribution of the second invariant of the strain tensor as defined in the version 2.1 of the gem global strain rate model [kreemer et al. 2014a, 2014b]. 7 face sources [berryman et al. 2014] while a second one worked at appraising the available magnitude scaling relationships available for different tectonic contexts [stirling et al. 2013]. 2.5. global ground motion prediction equations the pacific earthquake engineering research center (peer) coordinated the “global gmpes” (ggmpes) project, a project that had the primary goal of selecting a suite of ground motion prediction equations (gmpes) to be used for hazard calculation at global scale. a secondary, though not less important, objective of this project was to define a framework for the selection of gmpes for hazard analysis in different tectonic contexts and regions of the world. while the first goal has clearly a more immediate application given the high number of new gmpes published every year the second goal provides ways to maintain the suite of gmpes updated. the ggmpes project was organized into six tasks: 1. definition of a consistent strategy of modelling ground motion: (a) consistency in model parameters; (b) consistency in site parameters. 2. compilation and critical review of gmpes. 3. selection or derivation of a global set of gmpes. 4. inclusion of near-fault effects. 5. database of recorded waveforms. 6. specifications for compilation of a global database of soil classification. the first task provided guidance on the most suitable predictor variables to be used for the construction of empirical ground motion prediction equations (e.g. magnitude type, magnitude range commonly covered, rupture-site distances), as well as on the intensity measure types that should be used for representing the intensity of the shaking. the second task leveraged from the extensive review of the literature related to ground motion prediction equations, continuing the extensive archiving of gmpes initiated by douglas [2001] and updated in douglas [2011a]. starting from this large collection of models, and in order to obtain a doable number of models, the pre-selection criteria proposed by cotton et al. [2006] had been applied in a manner as to balance the need to cover, to the largest extent possible, the epistemic uncertainty within the smallest set of potential gmpes to be analysed within task 3. task 3 focused [stewart et al. 2013, 2015] on defining a general transparent strategy for the selection of ground motion prediction equations which is based on: gem first implementation phase hazard datasets and guidelines figure 5. example of fault section traces included in the gem faulted earth database. earthquake epicenters belong to the isc-gem catalogue [storchak et al. 2013]; the radius of the circles used to represent the epicenters is proportional to the rupture length while the face color indicates the hypocentral depth as explained in the legend. the study of the predicted values of ground motions in a multidimensional space composed by the most important predictor variables (e.g. magnitude, rupture-site distance etc.); the analysis of the functional form; the completion of several quantitative tests assessing the level of agreement between the predicted values of shaking and the values observed. the set of gmpes selected is summarized in table 1. the project highlighted issues related to gmpe selection in less well-constrained tectonic regions, such as volcanic areas, deep non-subduction areas (e.g. vrancea) and areas affected by earthquakes with propagation paths within oceanic crust. recognising the limitations of the current knowledge in such regions, however, the group refrained from providing a specific gmpe selection in those cases. stewart et al. [2013] suggest that nonlinear site amplification models should be preferred to the linear sitecorrection terms and provide guidance on how gmpes should be modified in this sense. task 4 addressed the problem of incorporating near source effects into the modelling of ground motion while task 5 compiled a collection of strong ground motion databases. finally, the last task of the project provided guidance for the construction of a database of soil information. campbell et al. [2013] combines the gmpes included in the gem global ground motion prediction equation to create a ground motion models for loss estimation in australia based on the hazard model published by geoscience australia in 2012. haendel et al. [2015] uses the set of gmpes preselected by douglas [2011b] for the subduction environment to create a mixture models for an area in the northern of chile. these authors prove that in the selected area the values of ground motion computed with mixture models when compared against observed data outperforms single gmpes. 3. challenges in using gem global datasets for seismic hazard assessment at regional and national scales in promoting global projects, gem’s principal aim had been the production of information supporting the development of new regional and national seismic hazard models. the results achieved by these projects are an unprecedented collection of information in terms of homogeneity, global coverage and overall quality. in the following we discuss some recommendations and signal some critical aspects that in our opinion should be considered in creation of hazard source models and ground motion models based on the outcomes of the gem hazard global components. 3.1. earthquake catalogues within probabilistic seismic hazard analysis the processing of catalogues is still the primary information used for the characterization of distributed seismicity. however, the use of earthquake catalogues in regional and national seismic hazard assessments still pagani et al. 8 reference gmpe acronym notes subduction sources abrahamson et al. [2015] aea12 atkinson and boore [2003] ab03 zhao et al. [2006] zea12 stable continental regions pezeshk et al. [2011] pea11 atkinson and boore [2006] ab06’ this model includes the updated proposed by atkinson and boore [2011]. silva et al. [2002] sea02 aleatory variability (i.e. standard deviation) model replaced by the one of epri [1993]. toro et al. [1997] tea97’ this model includes the updated proposed by toro [2002]. active crustal regions akkar and bommer [2010] ab10 chiou and youngs [2008] cy08 zhao et al. [2006] zea06 table 1. ground motion prediction equations selected by the global gmpes project for the calculation of seismic hazard at a global scale [stewart et al. 2013, 2015]. 9 presents at least two major issues: (1) the compilation of an homogenous catalogue starting from a set of non uniform catalogues and (2) the selection of the best algorithms (and the parameters needed for their use) for processing seismicity from the original seismicity catalogue until the calculation of the parameters characterizing the magnitude-frequency distribution. figure 6 shows a comparison between the share and isc-gem catalogues for a rectangle covering part of southern europe in a time-magnitude space. this plot emphasizes the necessity of complementing the iscgem catalogue with regional (and/or national) databases since the isc-gem alone does not provide sufficient information to comprehensively characterize seismicity occurrence in the intermediate and low magnitude range. the recent literature contains several interesting examples describing the creation of harmonized seismicity catalogues specifically created for seismic hazard analysis purposes. beauval et al. [2013] for example describe the process adopted for the construction of a composite catalogue starting from the information included in a number of national, regional and international datasets. the procedure described shows a possible example of how gem global catalogues can be combined with local catalogues for the construction of seismicity catalogue for hazard analysis. the catalogue processing is usually the result of a sequence of different operations: declustering, completeness analysis, magnitude-frequency distribution selection and calculation of the corresponding parameters via regression analysis. in this case, the recent literature contains few contributions discussing the impact on hazard results of the procedures adopted for the processing of seismicity. the documentation accompanying psha input models, seldom discusses the details of the procedure adopted to process the catalogue and to characterize the magnitude-frequency distribution. christophersen et al. [2011] discusses the impact of different declustering methods on the final values of hazard and conclude that the choice of the algorithm has limited impact on high values of shaking (above 0.4 g). the psha model used to complete this study is the 2010 new zealand model [stirling et al. 2012], which is a combination of faults and distributed seismicity sources, the latter modelled through grid sources created by smoothing seismicity. the hazard at low values of annual frequency of exceedance is most probably controlled by faults and this might justify the little impact of declustering observed for high values of hazard. the gem hazard modeller’s toolkit contains a large number of algorithms commonly adopted for the characterization of seismic sources based on earthquake catalogues. this suite of tools will hopefully help in making the psha input model preparation process more transparent and in understanding even further the impact of different algorithms on the final values of hazard. gem first implementation phase hazard datasets and guidelines figure 6. (left panel) time-magnitude plot showing the earthquakes included in the isc-gem and share catalogues. the earthquakes in this plot have magnitude larger than 4.0 and epicentral location included between longitude 0° and 25°e and latitude comprised between 38°n and 55°n. (right panel) histograms showing the distribution per magnitude bins (0.1 magnitude units width) of the two catalogues considered in this example. the green dashed line shows the completeness intervals proposed by michael [2014]. 3.2. active fault data state-of-practice seismic hazard models for active tectonic regions nowadays should include fault sources. however, the database of active fault produced by the gem faulted earth initiative does not cover large parts of the world. the plan is to integrate the gfe database with the information produced within regional projects such as in the case of share, emme, emca and sara. we also hope that people looking at the fault related information that will distributed publically through the openquake-platform will feel more eager to share information and therefore improve the overall spatial coverage of the gem databases. from a methodological perspective, one of the major issues related with the incorporation of faults sources into regional psha models pertains to the correct treatment of epistemic uncertainty and the inherent calculation complexity. information about faults can be collected using different methodologies [e.g. mccalpin 2009], though a majority of cases the basic information coming from observation is punctual i.e. it refers to a specific observation point of an active fault structure, therefore an effort in reconciling these observations in order to define an overall description of fault characteristics is generally asked to the earthquake geologist. the large epistemic uncertainties connected with this process and the necessity to formally account for epistemic uncertainties in the psha model building process indicate the importance of a strong interaction between experts of different disciplines and particularly between earthquake geologists and hazard modelers. 3.3. strain rate the use of strain modeling for probabilistic seismic hazard analysis is still a research topic. the scientific literature contains only few examples discussing the incorporation of strain in the construction of the psha seismic source model. mazzotti et al. [2011] use a logic tree methodology to incorporate seismicity rates inferred from a statistical analysis of catalogues with the ones obtained from strain using the classic kostrov [1974] formula. extensive research on the best approaches having the capacity to incorporate geodetic surface deformation information into a seismicity occurrence model had been performed in the framework of the ucerf projects (uniform california earthquake rupture forecast; field et al. [2009, 2014]). in ucerf2 for example the slip rate on some faults as well as the rates for type-c sources were constrained using strain values derived from geodesy. bird and liu [2007] proposed a methodology providing long-term seismicity forecasts using a kinematic model describing the recent tectonic process, the methodology used by bird et al. [2015] to compute a global earthquake forecast model. a forecast model is a first very important step toward the creation of a regional hazard model but technical speaking is not psha seismic source model (weatherill and pagani [2014] briefly discusses the differences between a forecast model and a hazard model). the shift methodology can be adapted to obtain the information needed for the creation of a hazard model and, according to authors’ opinion, it offers the most viable option for incorporating strain into psha regional models. 3.4. ground motion prediction equations the ggmpe project completed a comprehensive study and provided an initially recommended set of gmpes. recognizing the need to keep this set of gmpes updated with time they also suggested a methodology to be adopted for the selection of gmpes. the gem ground motion toolkit comprises most of the tools needed to implement the recommendations of stewart et al. [2015]. 4. conclusions we summarized the main achievements of the five global projects supported by the global earthquake model initiative during the first implementation phase (2009-2014) and we briefly discussed examples of applications as well as challenges in incorporating their results into new seismic hazard models. most of the datasets and guidelines produced by these projects constitute an improvement of our knowledge of the earthquake process and particularly of the most relevant components of regional probabilistic seismic hazard input models. the challenge is now to identify the missing gaps and promote mechanisms that will help maintaining these products updated and improved. the gem datasets currently do not contain information on site conditions, this is a gap that gem aims to incrementally fill in the second implementation phase mostly though accumulation of open knowledge that will be collected and produced by new projects we will be carrying out in different parts of the world. gem does not envision sponsoring for the second implementation phase projects similar to the ones described in this paper. cooperation with the scientific community remains nevertheless a primary goal of gem; 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(2009). paleoseismology, international geophysics series, academic press, vol. 95. michael, a.j. (2014). how complete is the isc-gem global earthquake catalog?, b. seismol. soc. am., 104, 1829-1837; doi:10.1785/0120130227. pagani, m, d. monelli, g. weatherill, l. danciu, h. crowley, v. silva, p. henshaw, l. butler, m. nastasi, l. panzeri, m. simionato and d. vigano (2014). openquake-engine: an open hazard (and risk) software for the global earthquake model, seismol. res. lett., 85 (3), 692-702; doi:10.1785/0220130087. parolai, s., j. zschau and u. begaliev (2015). preface, annals of geophysics, 58 (1): special issue “earthquake model central asia: seismic hazard and risk assessment in central asia”, s0101; doi:10.4401/ag6784. pezeshk, s., a. zandieh and b. tavakoli (2011). hybrid empirical ground-motion prediction equations for eastern north america using nga models and updated seismological parameters, b. seismol. soc. am., 101 (4), 1859-1870. pollitz, f., r. stein, v. sevilgen and r. burgmann (2014). the profound reach of the 11 april 2012 m 8.6 indian ocean earthquake: short-term global triggering followed by a longer-term global shadow, b. seismol. soc. am., 104, 972-984; doi:10.1785/012 0130078. scherbaum, f., e. delavaud and c. riggelsen (2009). model selection in seismic hazard analysis: an information-theoretic perspective, b. seismol. soc. am., 99 (6), 3234-3247. sesetyan, k., l. danciu, m.b. demircioglu, m. erdik and d. giardini (2014). emme-haz-2014: seismic hazard assessment results for the middle east region (abstract), in: proceedings of the 2nd european conference on earthquake engineering and seismology (istanbul, august 25-29, 2014). sevilgen, v., r.a. bennett, i. brlek, l. danciu, v. kastelic, s. kovacevic, c. kreemer, k. kuk, n. kuka, z. milutinovic, s. mustafa, b. sket-motnikar, r.s. stein and l. vucic (2014). balkans-oq: a collaborative seismic hazard assessment of the balkan countries using the openquake software and the gem strain rate model, in: proceedings of the 2nd european conference on earthquake engineering and seismology (istanbul, august 25-29, 2014). silva, w.j., n. gregor and r. darragh (2002). development of regional hard rock attenuation relations for central and eastern north america, report from pacific engineering and analysis, el cerrito, california, 57 pp. stewart, j.p., j. douglas, m.b. javanbarg, c. di alessandro, y. bozorgnia, n.a. abrahamson, d.m. boore, k.w. campbell, e. delavaud, m. erdik and p. stafford (2013). gem-peer task 3 project: selection of a global set of ground motion prediction equations, peer report 2013/22, december 2013, 251 pp. stewart, j.p., j. douglas, m. javanbarg, y. bozorgnia, n.a. abrahamson, d.m. boore, k.w. campbell, e. delavaud, m. erdik and p.j. stafford (2015). selection of ground motion prediction equations for the global earthquake model, earthq. spectra, 31 (1), 1945; doi:http://dx.doi.org/10.1193/013013eqs017m. stirling, m., g. mcverry, m. gerstenberger, n. litchfield, r. van dissen, k. berryman, p. barnes, l. wallace, p. villamor, r. langridge, g. lamarche, s. nodder, m. reyners, b. bradley, d. rhoades, w. smith, a. nicol, j. pettinga, k. clark and k. jacobs (2012). national seismic hazard model for new zealand: 2010 update, b. seismol. soc. am., 102 (4), 1514-1542. stirling, m., t. goded, k. berryman and n. litchfield (2013). selection of earthquake scaling relationships for seismic-hazard analysis, b. seismol. soc. am., 106 (3), 1-19; doi:10.1785/0120130052. storchak, d.a., d. di giacomo, i. bondár, j. harris, e.r. engdahl, w.h.k. lee, a. villaseñor, p. bormann and g. ferrari (2012). isc-gem global instrumental earthquake catalogue (1900-2009), gem technical report 2012-01 v1.0.0, gem foundation, pavia, italy, 128 pp.; doi:10.13117/gem.gegd.tr 2012.01. storchak, d., d. di giacomo, i. bondar, e.r. engdahl, j. harris, w.h.k. lee, a. villasenor and p. bormann (2013). public release of the isc-gem global instrumental earthquake catalogue (1900-2009), seismol. res. lett., 84 (5), 810-815; doi:10.1785/022013 0034. storchak, d.a., d. di giacomo, e.r. engdahl, j. harris, i. bondár, w.h.k. lee, p. bormann and a. villaseñor (2015). the isc-gem global instrumental earthquake catalogue (1900-2009): introduction, phys. earth planet. in., 239, 48-63; issn 0031-9201, http://dx.doi.org/10.1016/j.pepi.2014.06.009. toro, g.r., n.a. abrahamson and j.f. schneider (1997). model of strong ground motions from earthquake in central and eastern north america: best estimates and uncertainties, seismol. res. lett., 68 (1), 41-57. toro, g.r. (2002). modification of the toro et al. (1997) attenuation equations for large magnitudes and short distances, technical report, risk engineering, inc., louisville, co; available at http://www. riskeng.com/downloads/attenuation_equations. gem first implementation phase hazard datasets and guidelines tralli, d.m., r.g. blom, v. zlotnicki, a. donnellan and d.l. evans (2005). satellite remote sensing of earthquake, volcano, flood, landslide and coastal inundation hazards, isprs journal of photogrammetry & remote sensing, 59, 185-198. weatherill, g.a., m. pagani and d. monelli (2012). the hazard component of the gem modeller’s toolkit: a framework for the preparation and analysis of probabilistic seismic hazard (psha) input tools, in: proceedings of the 15th world conference on earthquake engineering, lisbon. weatherill, g.a., and m. pagani (2014). from smoothed seismicity forecasts to probabilistic seismic hazard: insights and challenges from a global perspective, in: proceeding of the 2nd european conference on earthquake engineering and seismology (istanbul, august 24-29, 2014). weatherill, g., l. rodriguez and m. pagani (2014a). the seismic hazard modelle’s toolkit: an open-source library for the construction of probabilistic seismic hazard models, in: proceedings of the 2nd european conference on earthquake engineering and seismology (istanbul, august 24-29, 2014). weatherill, g.a., m. pagani and j. garcia (2014b). openquake ground motion toolkit user guide. global earthquake model (gem), technical report; doi: 10.13117/gem.openquake.man.gmtk.01. zhao, j.x., j. zhang, a. asano, y. ohno, t. oouchi, t. takahashi, h. ogawa, k. irikura, h.k. thio, p.g. somerville, y. fukushima and f. fukushima (2006). attenuation relations of strong ground motion in japan using site classification based on predominant period, b. seismol. soc. am., 96 (3), 898-913. appendix useful links and on-line resources we provide herein a short summary of the resources accessible on-line for the different gem hazard global components. the personnel of the international seismological centre constantly update the isc-gem catalogue. the most recent version of the isc-gem catalogue can be downloaded from their website at the following link: http://www.isc.ac.uk/iscgem/download.php (last accessed on september 2014). the global historical earthquake catalogue and archive can be accessed at the project website (http:// www.emidius.eu/geh/, last accessed on september 2014) where the catalogue can be also downloaded. the gem global strain rate model (gsrm2) version 2.0 is accessible and downloadable from the unavco website (http://gsrm2.unavco.org/, last accessed on september 2014). version 2.1 of the gem global strain rate model is available on the gem website (http://www.globalquakemodel.org/what/seismichazard/strain-rate-model/, last accessed on september 2014). the final report of the gem global gmpes had been published as a joint peer-gem report and is available on the peer website (http://peer.berkeley.edu/ publications/peer_reports/reports_2013/webpeer2013-22-gem.pdf, last accessed on september 2014). the reports of the different tasks can be downloaded from the web page dedicated to this project on the gem website (http://www.globalquakemodel.org/what/ seismic-hazard/gmpes/, last accessed on september 2014). the reports produced by the gem faulted earth project can be downloaded from nexus, gem’s collaborative platform at the following link (http://www.nex us.globalquakemodel.org/gem-faulted-earth/posts). the gem hazard modeller’s toolkit and the gem ground motion toolkit can be downloaded from github at the following link: https://github.com/gem sciencetools (last accessed on september 2014). the openquake-engine and the gem hazard library are available on gem’s area on github: https:// github.com/gem (last accessed on september, 2014). all the gem datasets and psha input models will be accessible and downloadable from the gem openquake-platform, which will be publicly released at the end of 2014. *corresponding author: marco pagani, gem hazard team, gem foundation, via ferrata 1, pavia, italy; email: marco.pagani@globalquakemodel.org. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. pagani et al. 14 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 404 not found a b s o r p t i o n of stationary elastic vibrations ili r o c k s e . v. k a e u s a q u a n t i t a t i v e interpretation of t h e tinamical charaeteristics of seismic waves in the e a r t h , revealed by t h e study of real inedia strueture, is possible in cases, when the parameters of elastic waves attenuation in rocks are known. i t is also i m p o r t a n t for a correet understanding of phvsical nabration. the elastic power supplied by the coil-moved vibration equal to 50 w a t t . this vibrator is used to set up continuous vibrations a t frequencies ranging from 40 c/s t o 5.000 c/s. i t is known t h a t stationary elastic vibrations comprise t h e superposition of compressional, shear and surface (rayf i g . l a ture of propagation of elastic waves and study of medianica! properties of rocks. to measure parameters, characterizing divergence and absorption of seismic waves, the seismoacoustic method based on amplitude-and-phase measurement of elastic vibrations generated by a source of stationary harmonic vibration in rocks « in situ »: was used (1). for this purpose a special portable apparature was coiistructed. the elastic waves are generated by t h e electrodynamical vileigli) waves. [the dominant is the rayleigh wave (2)]. these vibrations excited in the rock can be registered with maximum magniflcation about 20.000.000. to raise the effective sensitivity of t h e receiving device a special filtering system is used. the response charaeteristics make us enable to filtrate the signal within t h e frequencv band of a b o u t 2 c/s for any frequency of excitation. fig. 1 gives an examplé of seismoacoustic observations in a mine a t a d e p t h of 250 metres. the observations were carried out 120 e. v. karus in d e c e m b e r of 1957, in silvinite p o t a s s i u m salt. t h e profile is 75 m e t r e s long. t h e observations were eonducted with t h e interval of 0.5 m e t e r . this resulted in t h e creat i o n of a system of reversed and overlapping a m p l i t u d e ( f i g . l a ) a n d p h a s e (fig.lb) curves, g r a p h s charaeterizing decrease of a m p l i t u d e w a v e l e n g t h a = 3 m . , t h e v a l u e of the phase velocity of t h e compressional wave d e t e r m i n e d b y t h e f o r m u l a : v<pr v<pp òxf ' where òx is t h e distance b e t w e e n t h e onduand phase s h o f t with distance. these exper ime n ta l d a t a can be used t o d e t e r m i n e b y t h e grapliical m e t h o d of difference t h e phase velocity of t h e d o m i n a n t surface w a v e v<pr, t h e p h a s e velocity of compressional waves, v<pp index of t h e divergence f u n c tion « n » a n d a b s o r p t i o n coefficient. i n our e x a m p l e t h e m e a n value of t h e p h a s e velocity v<pr = 2.280 m/sec., t h e lation e x t r e m a on t h e p h a s e c u r v e and a m p l i t u d e c u r v e gives t h e v a l u e of t h e compressional w a v e velocity v<pp = 4.200 m/sec. t h e index of t h e divergence f u n c t i o n « n » can b e d e t e r m i n e d b y a grapliical method of difference f r o m t h e s y s t e m of overlapping a m p l i t u d e curves (fig. l e ) . i n our case n = 0 . 5 ± 0 . 1 . pig. l e absorption op stationary elastic vibrations in rocks 127 the a b s o r p t i o n coeffieient can be deterthese seismoacoustic p a r a m e t e r s were demined b y a graphical w a y f r o m system of termined in homogeneous rocks occurring reverse curves (fig. 1 d). t h e absorption near t h e e a r t h ' s surface, such as loess-like sandy loams, porphyrites, gneisses. observations in mines helped t o d e t e r m i n e seismoacoustical p a r a m e t e r s of rocks occurring a t d e p t h s of 150-270 m., such as: a c m i t e cherts, sdvinite, jaspilite, different kinds of iron ore (magnetite, g e m a t i t e , m a r t i t e ) . r e s u l t s of d e t e r m i n i n g t h e seismoacoustical p a r a m e t e r s are as follows: 1) on fig 2 we can see t h e chaning of t h e p h a s e velocity of t h e d o m i n a n t w a v e with t h e f r e q u e n e v f o r different rocks. f o r t h e rocks wicli occur a t t h e e a r t h ' s surface velocity increases w i t h t h e f r e q u e n c y rise, i. e. t h e p h e n o m e n o n of « anomalous » depression is observed f o r d o m i n a n t waves which in seismoacoustics seem to b e surface waves. b u t t h e silvinites h a v e n o r m a l dispersion, i. e. t h e m a i n pliase velocity decreases with f r e q u e n c y . 2) t h e comparison of t h e given d a t a on fig. 3 leads t o t h e conclusion t h a t it is possible to d i f f e r e n t i a t e rocks b y a b s o r p t i o n of elastic waves. g r e a t a b s o r p t i o n is characteristic of loess-like s a n d y loams. gneisses h a v e g r e a t e r a b s o r p t i o n t h a n porpliyritic rocks. m e t a m o r p h y c a l rocks and silvinite occurring a t d e p t h s of 150-220 ni. a r e characterized b y lesser values of x a n d a t h a n those occurring n e a r t h e e a r t h ' s surface. mine pressure a n d absence of decay procoefficient here has t h e value x = 0.086 1/m. t h e absorption decrement, t h e absorption q u a l i t y f o r t h e wavelength a = 0 . 1 3 . vf[/sec] dtspersion of phase veeocity gy stationary excitatlon sitvinites porphyrites gneisses rl 2000 3000 4000 pig. 2 the seismoacoustic m e t h o d was used to measure t h e p h a s e velocities, divergence indices, coefficients a n d d e c r e m e n t s of t h e absorption of a r m o n i e elastic waves with frequencies r a n g i n g f r o m 80 t o 4.500 c/s. 128 e. v. karus cesses s t i p u l a t e m o r e elastieity and lesser values of a b s o r p t i o n p a r a m e t e r s a t a d e p t h . 3) d e t e r i n i n a t i o n s of v, o b t a i n e d f o r different roeks show t h a t in t h e overwhelming m a j o r i t y of roeks studied x increases with f r e q u e n c y b y a linear law within a wide f r e q u e n c y range, i. e. t h e value of t h e a b s o r p t i o n coefficient is p r o p o r t i o n a l to t h e f r e q u e n c y of excited oscillations: * = pf t h e value /9, t h e f u n c t i o n a l coefficient in this equation is in practice d e t e r m i n e d b y t h e slope angle of t h e s t r a i g h t line a p p r o x i m a t i n g t h e g r a p h of d e p e n d a n c e of x to /. this t a b l e given t h e results of d e t e r m i n ing /? f o r different roeks. t a b l e 1 n a m e of r o c k d e p t h of o c c u r r e n c e p ( s e c / m ) x io 4 g n e i s s e s . . 0 3 . 1 l ' o r p l i y r i t e s . 0 1 . 2 m a r t i t i c o r e . 262 2 . 0 j a s p i l i t e s . . 212 1 . 0 s i l v i n i t e s . . 247 0 . 6 m a g n e t i t o h e m a t i t i c o r e 267 0 . 2 a c m i t e c h e r t s 267 0 . 4 4) t h e values of t h e a b s o r p t i o n decrem e n t a of a rock di (ter, thougli t h e sensible law of c h a n g e of a w i t h f r e q u e n c y is observed (fig. 4). this can b e seem f r o m t h e d e n i o n s t r a t e d d i a g r a m where changes w i t h f r e q u e n c y a r e p l o t t e d f o r : gneisses, p o r p h y rites, silvinite. t h e correlation of average values of p h a s e velocity v<pr a n d absorption d e c r e m e n t a f o r different roeks shows (fig. 5) t h a t roeks possessing a g r e a t e r absorption coefficient a r e characterized b y a lovv p h a s e velocity, i. e. inverse proportion a l i t y is observed b e t w e e n a a n d vrpr. i n conclusion we w a n t to say, t h a t a t h o r o u g h investigati on of a b s o r b i n g properties of roeks w i t h t h e help of seismoacoustics m a y yield e x p e r i m e n t a l materials for s t u d y i n g t h e laws of t h e propagation of elastic waves in real m e d i a . i n particular, aesoretl.or\ decrement f i g . 4 absorption of stationary elastic vibrations in rocks 129 the (lata obtained b y us confimi t h e conclusion iliade b y b. v. deryagin who has explained t h e n a t u r e of seismic wave abp i g . 5 sorption based on t h e use of elastic a f t e r working f o r a speeially chosen f u n c t i o n of afterworking (4). abstract the seismoacoustic metliod based on amplitude and phase measurements of elastic oscillations caused by a source of stationary harmonic oscillations in rocks is used to study the character of divergence and absorption of seismic waves. the results of the experimental study of the absorbing properties of rocks by the seismoacoustic metlwd moke it possible to diffcrentiate rocks by absorption parameters. the greatest absorption is acquired by sedimentary rocks, the least — by erupted and metamorphized rocks. inverse dependance between the velocity of elastic waves and absorption decrement has been observed for different types of rocks. it ivas shown that for the prevailing majority of the rocks investigated the absorption coefficient increased with frequency under the linear late. it is pointed out that the experimental data obtained confimi the conclusions mode from the theory developed by b. v. derjagin based on the phenomenon of the elastic after-cffect. riassunto il carattere di divergenze e d'assorbimento delle onde sismiche è stato investigato mediante il metodo sismo-acustico basato sulle misurazioni di ampiezze e fasi delle oscillazioni quali sono eccitate nelle roccia da una sorgente d'oscillazioni armoniche stazionarie. i risultati d'investigazione sperimentale mediante il m. s.-a. d'assorbimento della roccia permettono di differenziare le roccie diverse secondo i parametri d'assorbimento. l'assorbimento massimo è proprio di roccia sedimentaria il minimo di roccia d'eruzione e di roccia sottoposta a un metamorfismo. la proporzionalità inversa è stata osservata tra la velocità delle onde elastiche e la diminuzione d'assorbimento per i tipi diversi di roccia. è dimostrato che il coefficiente d'assorbimento è aumentato linearmente con la frequenza. è osservato il fenomeno della dispersione anomala della velocità di fase. i dati sperimentali ottenuti confermano le conclusioni della teoria basata sul fenomeno d'elasticità in seguito a quella svolta da b. v. derjagin. b i b l i o g r a f i a f1) ivaiius, e . v . , p a s e c h n i k , i . p . , methods of studying elastic and absorbing properties of rocks in their naturai occurrence by the methods of seismoaconstics, i z v e s t i a , a c . s e . u s s r . g e o p h y s . s e r . , 6 ( 1 9 5 4 ) . (2 ) — on the nature of the elastic waves excited in real media by a harmonic source, i z v e s t i a a c . s e . u s s r , g e o p h y s . s e r . , 2 ( 1 9 5 5 ) . (3) — absorption of elastic waves in rocks by stationary excitation. i z v e s t i a a c . s e . u s s r , g e o p h y s . s e r . , 4 ( 1 9 5 8 ) . (4) l e r j a o i n , b . v . , damping of seismic and acoustic waves and its independence on frequency. « j o u r n a l of g e o p h y s i c s », 3 4 ( 1 9 3 1 ) . tne correfation of average vocues of ir, and a for different \ ì \ v porphyrltes \ si£nnites ^ v^f"*0''" tes • gematiittr^e— . . flcr'ìite caerts • magnetite ore geo 1093 annals of geophysics, supplement to vol. 47, n. 2/3, 2004 9 relations between electron content and peak density: regular and extreme behaviour reinhart leitinger (1), luigi ciraolo (2), leonard kersley (3), stamatis s. kouris (4) and paolo spalla (2) (1) institute of physics, institute for geophysics, astrophysics and meteorology (igam), university of graz, austria (2) istituto di fisica applicata «nello carrara» (ifac), firenze, italy (3) university of wales, aberystwyth, u.k. (4) department of electrical and computer engineering, aristotelian university of thessaloniki, greece electron content statistics offers important information for planning and operation of various application systems that make use of the transionospheric propagation of radio signals. electron content statistics meet with an important difficulty: the majority of data stem from observations on the radio signals of orbiting satellites. the database for vertical electron content derived from observations of radio signals emitted from geostationary satellites is only very small. therefore it is important to make use of statistics for peak density from ionosonde measurements. quantile statistics do not contain information about ionization extremes. therefore it is necessary to complement the statistics with case studies which reveal unusual and extreme electron content structures in space and in time. 9.1. introduction (total) electron content (tec) is an important ionospheric characteristic for planning and operation of satellite navigation and communication systems. thus the study of its behaviour and especially of its variability in time and space is essential. since we have very long experience with f2-layer peak density (gained from the square of critical frequency fof2) and since suitable ionospheric electron content data are scarce compared with the large number of fof2 data, it is very important to make use of peak density experience to gain better knowledge on temporal and spatial behaviour of tec. this can be done by means of a relation parameter, (equivalent) slab thickness t which is vertical electron content iv divided by (f2-) peak electron density nm: τ = iv / nm. usually this quantity is given in km. 9.1.1. notes on electron content (tec) in this paper electron content means the height integral of electron density. it cannot be measured directly but has to be estimated from propagation effects on radio signals transmitted from artificial satellites. therefore the primary observed quantity is approximately proportional to the integral of electron 1094 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla density along a slant ray path, slant electron content (stec). we gain an approximation to vertical electron content (vtec) by means of a projection from slant to vertical at a «pierce point» on the straight line from the satellite receiver to the ground station in a suitably chosen «mean ionospheric height». it is important to note that vtec is height integrated electron density from the ground to a «ceiling height» which depends on the orbital height of the transmitter and on the propagation effect used. the faraday rotation of linearly polarised signals was used in early studies of electron content. however, the relationship between the measured rotation and the content is not direct and it is necessary to assume a representative value for the longitudinal component of the electron gyrofrequency along the ray path in the analysis. in consequence, for the faraday effect observed on the vhf signals of geostationary satellites the effective ceiling height is 2000 km and the relative slant to vertical conversion error is about 5% for receiving stations in mid latitudes (titheridge, 1972). vtec gained by means of propagation effects on the signals of geostationary satellites suffers from a smoothing effect: in general the ray from the ground station to the satellite is not meridional but cuts through longitudes which gives data smoothing in local time. local time smoothing strongly affects the time interval around sunrise. unfortunately we have not any longer geostationary faraday beacons which had the considerable advantage of providing (nearly) constant observation geometry and an extremely good temporal resolution. of course variability and comparison with fof2 studies can also be made with tec data from other sources, e.g., with the latitudinal cross sections provided by vhf/uhf beacons in nearly polar orbits. in case of the former navy navigation satellites (nnss, now nims – navy ionosphere monitoring satellites) the ceiling height is the orbit height of about 1100 km. the propagation effect is the difference doppler effect. absolute vertical tec suffers from the need of «calibration» but for low zenith angles the tec differences over short distances are comparatively good data. the best way to reveal larger structures in vertical electron content is the combination of data from the receiving stations in meridional chains and «calibration» by means of the results of tomographic reconstruction. in principle, a large quantity of vtec data could be gained by means of the gps beacons. as a first guess one is tempted to take the orbit height of 20 000 km to be the ceiling height for vtec. since we are interested in ionospheric electron content we should correct for the contribution of the plasmasphere (see ciraolo and spalla, 1997, 1998; ciraolo et al., 2001). if we succeed with this we end up with a «ionospheric» ceiling height of 2000 km. for a critical review of gps-tec see jakowski et al. (2004). at least on a statistical basis the difference between gps-tec and nnss(nims)-tec should give a good indication on the plasmaspheric vertical electron content. some studies were initiated in the frame of cost 251 on a monthly median basis (leitinger and feichter, 2000) and continued in the frame of cost 271 (see, e.g., ciraolo and spalla, 1997, 1998; ciraolo et al., 2001). 9.1.2. notes on slab thickness ττ 1) most of slab thickness data were gained by combining vertical electron content derived by means of the faraday effect on the vhf signals of geostationary satellites with peak electron density derived from ionograms. therefore the slab thickness ceiling height is 2000 km and we have to expect smoothing around sunrise. since in general the geographic coordinates of the pierce point (in a mean ionospheric height of 400 or 420 km) differ from the geographic coordinates of the ionosonde station, we have co-location problems, especially around sunrise. if the transmitter satellite is not truly geostationary (inclination > 0°) we should account for a local time variation of the faraday rotation to stec and of the stec to vtec conversion factors and of the pierce point coordinates. 2) for mid latitudes several slab thickness models have been published (e.g., titheridge, 1973; hajeb-hosseinieh and kersley, 1975; kersley and hajeb-hosseinieh, 1976; huang, 1983; fox et al., 1991; davies and liu, 1991; spalla and ciraolo, 1994). they have in common: a) a slab thickness range between about 170 km and 320 km at low solar activity and between about 200 and 400 km at high solar activity; b) an increase in slab thickness with solar activity; c) for non-storm conditions on1095 relations between electron content and peak density: regular and extreme behaviour ly a slight positive (kersley and hajeb-hosseinieh, 1976) slab thickness dependence on magnetic activity. d) under (major) magnetic storm conditions, not including strong positive storm effects, which occur in the american east coast region, we can expect a slight daytime increase in slab thickness (of around 10 to 30 km). the pre-dawn slab thickness peak also seems to be a stable and regular feature, even if we allow for the interpretation problems we expect around sunrise. 3) neglecting solar activity, diurnal and seasonal variations the overall mean slab thickness for europe (around 42°n, 13°e) can be given as 230 km with a standard deviation of 50 km (spalla and ciraolo, 1994). for high solar activity an average around 300 km probably is more appropriate (fig. 9.1). the overall means for the american east coast sector are slightly higher: the mean from the model by fox et al. (1991) is 267 km for an intermediate level of solar activity of r12 = 75 (τ = 242 km for r12 = 0, τ = 292 km for r12 = 150). for the same set of data (refig. 9.1. annual variation of equivalent slab thickness for 1981 from the combination of vertical electron content measured by means of the faraday effect on the vhf signal of sirio at graz (latitude of ionospheric point 42.5°n) and peak electron density from the ionosonde data of roma (41.9°n, 12.5°e). upper panel: 0 lt; lower panel: 12 lt (leitinger, 1998). the different curves have been gained with the lower and upper deciles, lower and upper quartiles and with the medians. 1096 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla ceiving station sagamore hill, 42°n, 288°e) davies and liu (1991) get for low solar activity a noontime τ = 253 km and a midnight one of 260 km. the corresponding high solar activity (r12 = 150) values are τ = 318 km and τ = 300 km. figure 9.1 shows the annual variation of monthly mean slab thickness for 0 lt and 12 lt from the combination of faraday-tec measured on the vhf signal of sirio with peak density from the ionosonde station roma (41.9°n, 12.5°e). 4) in low latitudes we expect an increase in slab thickness towards the dip equator when the equatorial anomaly is on. however, this is a conclusion we have to draw from theoretical considerations: because of the configuration of the geomagnetic field much larger errors are involved in the interpretation of the faraday effect on the vhf signals of geostationary satellite received at ground stations in low latitudes. under solar minimum conditions klobuchar et al. (1991) calculate a slab thickness peak near the dip equator of about 460 km for «average drift conditions» and of about 560 km for «high drift conditions». the solar maximum values are around 510 km both for «average» and «high drift conditions». klobuchar et al. (1991) find that the region of enhanced slab thickness extends to dip latitudes around ± 34°. in experimental data the increase in τ with decreasing dip latitude is indicated by huang (1983). his graphics indicate an overall mean for the receiving station lunping (pierce point coordinates 23°n, 122°e, dip latitude 17°) of τ > 300 km (unfortunately he gives no numbers). 5) some slab thickness studies have been carried out with the purpose of assessing empirical and theoretical ionospheric models, some have been carried out to draw conclusions on vtec variability from peak density variability. 9.1.3. notes on «worst cases» and «extremes» assessment and feasibility studies for engineering applications are often based on «worst case» considerations. the probability that a «worst case» is exceeded must be very low (< 10−7 is often quoted). on the other hand it is not acceptable to take the smallest or largest observed value with a large margin to be «on the safe side» because this would mean low availability of an application system or unnecessarily high costs. the usual «worst case» concept does not work with nature data or events. ground and near ground environmental data use rarity of event considerations, e.g., river flood levels that should not be exceeded more often than once a century. the question of a «worst case» value or a «probably not more often than once per century exceeded» value of (slant and vertical) electron content needs qualification to obtain a reasonable answer. 1) the tec database from faraday effect and differential doppler observations is very small compared with the peak electron density database. furthermore, it is concentrated in a few regions of the world. 2) despite a large amount of dual frequency receiving stations for gps which have collected data over several years we have a very limited «tec from gps» data set only. this is because a) the receiving stations are far from being evenly distributed and especially for lower latitudes large regions like africa must be considered as «white spots»; b) reliable vertical electron content data have been produced by means of gps observations for a part of the present solar cycle only and this cycle must be considered a comparatively weak one which has not given really high solar activity values. 3) it is practically impossible to get a reasonably good prediction for the next solar cycle which means that experience from the present one might lead to wrong conclusions if the next solar maximum happens to be much stronger or much weaker than the 2000-2001 maximum. 4) strong magnetic storms have to be considered as singular events which do not allow us to draw conclusions on a statistical basis. there are only a few really strong storms per solar cycle. 5) from time to time nature tends to produce «rare and irregular» events which affect small areas only. there are no statistics for such events and the probability is high that most of them have not been observed. 1097 relations between electron content and peak density: regular and extreme behaviour the first qualification to the «worst case» question should be «exclude strong storms». for operational systems this qualification might be acceptable because we have reliable «storm indicators». the second qualification should be «exclude rare events» which might produce unexpected smaller scale ridges and troughs. collections of observed cases (see section 9.5) help in demonstrating possible effects on application systems. 9.1.4. notes on monthly and hourly statistics the ionosonde network has taken over the concept of «synoptic» observations which was developed first by gauss and weber for geomagnetic observations and later for the observation of ground temperature and other meteorological parameters. propagation effects on the signals of geostationary satellites too can be read from chart records or digitally in hourly or shorter regular intervals. this is not possible with orbiting satellites. some ionosonde stations have produced long series of monthly and hourly medians and quartiles. in principle, it is possible to derive the deciles too. however, with 28 to 31 data (not accounting for equipment failure) it is not possible to derive quantiles below the lower decile or above the upper decile. 9.2. electron content variability deduced from the variability of peak electron density concentrating on mid latitudes we should distinguish between a) short term variability, b) day to day variability, and c) storm effects. the use of slab thickness allows us to transfer nm(f2) variability to vtec variability. for small enough percent variability of critical frequency fof2 we can assume percent variability of nm(f2) to be twice the percent variability of fof2. except for pre-dawn conditions we can safely assume that within one hour τ does not change appreciably. therefore we expect short-term relative vtec variability to be twice the relative short-term variability of fof2. however, the day-to-day variability of slab thickness cannot be neglected. therefore we expect that the «worst case» day to day variability of tec is considerably higher than the corresponding variability of nm(f2). in mid latitudes a factor of 1.5 seems to be safe. in low latitudes we need a larger factor: the slab thickness values depend crucially on the status of the equatorial «fountain». to knowledge our no relevant studies have been made. from klobuchar et al. (1991) it follows that we have to expect slab thickness variability at the dip equator from about 200 km (no e × b drift, «fountain off») to about 560 km (high e × b drift) for equinox and more or less average conditions. a «worst case» factor of 3 to 4 seems to be appropriate. 9.3. results of the tec variability studies of cost 271 day-to-day and hour-to-hour variability is a permanent feature of the ionosphere and therefore meaningful predictions can only have statistical character, i.e. specify probabilities (rawer, 1993; rawer et al., 2003). the variability from hour-to-hour of any ionospheric characteristic is estimated from its hourly relative deviations, i.e. from the variation from hour-to-hour of the hourly daily values with respect to their corresponding monthly-median (e.g., kouris and fotiadis, 2002). in this study we used hourly daily tec data obtained observing the faraday rotation affecting the signals emitted from geostationary satellites, measured at florence from 1976 to 1982 and 1989 to 1991, as well as gps-tec-data measured at matera (40°n, 16°e) from 1993 to 1999. we also used hourly daily fof2 measurements made at the station of rome during the same periods. fig. 9.2a. upper and lower deciles in tec, fof2 and (fof2)2. faraday rotation data have been used. 1098 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla 1099 relations between electron content and peak density: regular and extreme behaviour fig. 9.2b. upper and lower deciles in tec, fof2 and (fof2)2. gps data have been used. 1100 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla fig. 9.3a. probability that a given level, positive (left) or negative (right), of variability in tec is exceeded at low solar activity (mid-latitudes). 1101 relations between electron content and peak density: regular and extreme behaviour fig. 9.3b. probability that a given level, positive (left) or negative (right), of variability in tec is exceeded at high solar activity (mid-latitudes). 1102 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla first we have evaluated the relative deviations of tec, fof2 and (fof2)2, respectively at each hour of each day/month/year. then, we counted the upper and lower deciles of relative deviations in each hour of each month/year, for each parameter. by examination of the obtained results we have observed (kouris et al., 2004) that the variability from hour-to-hour in tec is close to that of fof2 during periods of low/medium solar activity and overlaps with that of (fof2)2 at high solar activity. figure 9.2a,b as an example illustrates upper and lower deciles of variability in tec, fof2 and (fof2)2, respectively at some randomly selected months/years using faraday rotation data (fig. 9.2a) or gps data (fig. 9.2b). it is evident from this figure that the variability in tec lies between the variability of fof2 and that of (fof2)2 according to the solar activity and/or the disturbed state of the ionosphere. during very disturbed conditions, as for instance in february 1978 or october 1989 (fig. 9.2a), the variability in tec could be greater than that in maximum electron density. this might suggest that disturbances affect the topside more than the bottomside of the ionosphere. indeed, it is found (kouris and fotiadis, 2002) that the variability in the critical frequency of the e-layer is much less than that in fof2, supporting the above statement. moreover, the present investigation highlights that the variability exhibits its higher values mostly during nighttime hours, when disturbed conditions usually prevail and during winter-time (fig. 9.1). on the other hand, in summer time the variability seems to be rather independent of the time of day and of solar activity, although the lower levels of variability indicate a certain dependence on the phase of solar cycle. more investigation is needed. it is worth assessing the range of variability in tec and also the probability of occurrence of a given level of variability. the statistical analysis (kouris and polimeris, 2003) has shown that the upper levels of variability in tec, i.e. positive deviations (enhancements) are higher than the lower (negative deviations). the former may reach values nearly twice those of the corresponding monthly-median values whereas the latter usually reach values up to 50% or 60% lower than the corresponding monthly-median values and are independent of season. figure 9.3a,b shows curves of probability that a given level positive or negative could be exceeded at low solar activity (fig. 9.3a) and high solar activity (fig. 9.3b). figure 9.3a shows clearly that the probabilities are higher during the night-time hours and that especially in summer and equinoxes, whereas at high solar activity (fig. 9.3b) this is marked during winter. it is also evident that the probability of exceeding a positive or negative level of variability of the order of 60% higher of the corresponding monthly-median value is less than 2% except in winter at high solar activity (fig. 9.3b) where the probability of an enhancement of that order is just a little higher. from the same figures it can also be seen that the probability to exceed a given level is higher for positive deviations than for negative except in summer where there is practically no difference between enhancement and depression levels (fig. 9.2a,b). 9.4. some electron content statistics tec from faraday rotation data is very well suited for temporal statistics. the worldwide best data set is kept at the former istituto di ricerca sulle onde elettromagnetiche (iroe) at florence (spalla et al., 1983). the archive covers more than one solar cycle and contains vertical electron content with a time resolution of one minute. the relative, short-term accuracy is better than 2 × 1014 m-2 (0.02 tec units). here we show only a small sample from the statistics work done at florence. the complete material with statistics for time intervals from one minute to one hour is available in the form of power point presentations. figure 9.4a-d shows occurrence statistics (histograms) for vtec time derivatives from all data (fig. 9.4a), high solar activity data (fig. 9.4b), medium solar activity data (fig. 9.4c) and low solar activity data (fig. 9.4d) (ciraolo and spalla, 1999). the time derivatives were gained as, d i i 2t t t1 1= -+ -^ h , i being vertical tec. d is given in tec units per minute (1016 m-2 per minute). 1103 relations between electron content and peak density: regular and extreme behaviour fig. 9.4a-d. distribution of the time derivative of tec. b d a c the second important source of data for tec statistics is the latitudinal scans provided by the navy navigation satellite system (nnss, after decommissioning for shipboard navigation the remaining systems is called navy ionospheric monitoring system – nims). from the latitude dependence of vertical electron content we can derive latitudinal gradients as the basis of statistics. such studies have been done at florence and at graz. 9.5. a short overview on peculiar cases and rare events with no intention for completeness a few examples are given for peculiar electron content structures and rare events. it is very likely that the majority of peculiar cases and rare events have not been observed. it is also likely that data archives contain interesting structures and «extremes» and it would be worthwhile searching through the archives. the samples presented were collected at graz. the most interesting case is shown in fig. 9.5: a narrow peak in mid latitudes which was «seen» during 5 h in the night october 8 to 9, 1984. it had disappeared on the morning of october 9 but due to the lack of nnss passes the real duration of the event is not known. in longitude the peak was «seen» over more than 10°. figure 9.6 shows an example of large scale and large amplitude travelling ionospheric disturbances (tids), fig. 9.6 gives a strong hint on the longitudinal gradients which can be coupled to tids (narrow fan beam structure). 1104 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla fig. 9.5. unexpected mid-latitude peak (sharp mid-latitude gradients) on 8 october 1984, around 23 lt (leitinger, 1993). electron content in units of 1016 m-2 versus geographic latitude from nnss diff. doppler, receiving stations graz (47.1n, 15.4e, right hand curve) and uppsala (51.6n, 10.1e left hand curve). the midlatitude peak is clearly no artefact: it exists in 6 consecutive satellite passes recorded at graz between about 20 h lt on october 8 and 01 h lt on october 9. fig. 9.6. large amplitude tids from the faraday effect on the vhf signals of the geostationary satellite sirio. high pass filtered tec in units of 1015 m-2 versus time in hours ut. fig. 9.7. example for strong latitudinal and longitudinal gradients from tids (putz et al., 1990). high pass filtered electron content in units of 1016 m-2 versus geogr. latitude from nnss diff. doppler, receiving stations tours (47.4n, 0.7e), nançay (47.3n, 2.2e), besançon (47.3n, 6.0e), bonn (50.5n, 6.9e). 9.6 9.7 9.5 1105 relations between electron content and peak density: regular and extreme behaviour fig. 9.9. example for a very sharp trough region enhancement (leitinger, 1993). electron content in units of 1016 m−2 versus eccentric dipole latitude from nnss diff. doppler, receiving station uppsala (59.8n, 17.6e). simultaneous observations from lindau/harz (51.6n, 10.1e) prove that the uppsala observations are not an artefact. date: 28 november 1980; time: around 1:20 ut. fig. 9.8. example for a regional negative storm effect (leitinger et al., 1984). latitude dependence of vertical electron content from the differential doppler effect on the 150/400 mhz signals of navy navigation satellites (nnss). april 12, 1981, early afternoon (left) and late afternoon (right). upper panels: maps of ionospheric points in a height of 400 km and receiving station locations (crosses). receiving stations: effelsberg (50.5°n, 6.9°e), lindau (51.6°n, 10.1°e), bologna (44.6°n, 11.7°e), graz (47.1°n, 15.5°e), dionysos (38.1°n, 23.9°e). local time for reaching the ionospheric points in 42.5°n and station symbols: (left hand side) e – solid, 14:53; g – squares, 15:11; e – triangles, 14:07; g – circles, 14:17; d – dashed, 14:33. (right hand side) e – solid, 17:36; l – squares, 17:39; b – circles, 17:51; e – triangles, 16:48; g – plus signs, 17:02; d – dashed, 17:20. 1106 reinhart leitinger, luigi ciraolo, leonard kersley, stamatis s. kouris and paolo spalla figure 9.8 gives an example for a negative storm effect confined to a rather narrow longitude zone. the electron content depression occurred west of 10°e and east of the american east coast only. central and eastern europe was not affected at all. figure 9.9 shows a very sharp electron content enhancement over the minimum of the main f-region trough. the latitudinal gradient of nearly 7 tec units per degree can be considered as an «extreme of horizontal ionization gradient». 9.6. conclusions the studies undertaken in the frame of cost 271 demonstrate that in mid latitudes and for statistics we can successfully transfer experience from ionogram derived f2-layer peak density (nmf2) to vertical ionospheric electron content (vtec). as for fof2 or nmf2 we can derive monthly and hourly vtec quantiles from faraday rotation data on the signals of geostationary satellites if restricted to the range between the deciles. ionospheric vtec data from gps need an other approach: correction for the plasmaspheric contribution and the use of «voxels» (elements in geographic coordinates, local time and possibly also solar activity space with given intervals for each of these «coordinates»). the «voxel» approach was successfully used with latitudinal scans of vtec from the differential doppler effect on the 150/400 mhz signals of the polar orbiting nnss satellites (feichter and leitinger 1997) quantile statistics do not include the ionospheric signatures from «geophysical events», like magnetic storm effects, large scale and large amplitude tids, ridges, troughs and other structures which cannot be explained by means of external parameters like those for solar and geomagnetic activity. since such «ionization extremes» might be of considerable interest for the planning and operation of application systems that make use of transionospheric propagation of radio signals, efforts should be made to complement existing case collections. acknowledgements the work reported here was part of the cost 271/ wg 2 activities. thus the authors are grateful to the cost 271 members for participation and numerous helpful suggestions and discussion contributions. they also thank colleagues and collaborators in their institutions. references ciraolo l., and p. spalla (1997): comparison of ionospheric total electron content from the navy navigation satellite system and the gps, radio sci. 32, 1071-1079. ciraolo, l. and p. spalla (1998): some results on gps-nnss tec comparison, acta geod. geophys. hung., 33, 75-81. ciraolo, l. and p. spalla (1999): a statistics of time and space variability of ionospheric electron content at middle latitudes, in xxvi general assembly ursi, 13-21 august 1999, toronto, canada. ciraolo l., p. spalla and e. capannini (1994): 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(1993): wave propagation in the ionosphere (kluwer publ. co., dordrecht nl). rawer, k., s.s. kouris and d.n. fotiadis (2003): variability of f2 parameters depending on modip, adv. space res., 31, 537-541. spalla, p. and l. ciraolo (1994): tec and fof2 comparison, ann. geofis., xxxvii (5), 929-938. spalla p., e. capannini, l. ciraolo and r. ragionieri (1983): use of the vhf signal of the sirio for ionospheric investigations, in proceedings of the international symposium on beacon satellite studies of the earth’s environment, new delhi, edited by t.r. tyagi, 415-428. titheridge, j.e. (1972): determination of ionospheric electron content from the faraday rotation of geostationary satellite signals, planet. space sci., 20, 353-369. titheridge, j.e. (1973): the slab thickness of the mid-latitude ionosphere, planet. space sci., 21, 1775-1793. s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e d e l l a g o dt i d r o maria cecilia spadea dal lavoro « 1 laghi d'italia » di r. riccardi, si traggono le seguenti notizie circa i dati clic caratterizzano il lago di idro: lat. 45°47' n c i r c a ; long. 1°17' w ca. (monte mario); alt. ni 368; sup. kmq 10,87; jungh. mass, fra gli estremi km 10, lungo la linea di valle km 10,8; mass, largii, km 2 ; mass. prof. ìli. 122. questi ed altri dati vanno considerati come approssimativi, essendo soggetti a tante cause di variazione: diversità di metodi di misura, cambiamenti stagionali o secolari o accidentali, ecc. per la determinazione analitica delle fondamentali caratteristiche dei moti liberi del lago, si è fallo ricorso alla carta batimetrica del lago stesso pubblicata dal de agostini nellv atlante dei laghi italiani ». la carta è stata riportata al 25.000. il lago è stato diviso, lungo la linea di valle, in 32 sezioni di 300 in 300 ni salvo le prime due, per le quali l'equidistanza è stata di 150 m. dette sezioni vanno contate a partire dall'estremo nord (figg. 1-2). la larghezza b ( x ) delle singole sezioni in superficie, l'area s(x) delle sezioni stesse, l'area v{x) della superficie libera del lago f r a due sezioni consecutive, l'area v(x) f r a le varie sezioni e l'estremo nord del lago e i valori della funzione ni*) (uguale al prodotto s ( « ) • ò(.x)) per le 32 sezioni, sono riportati nella tabella i. si è provveduto quindi alla determinazione della curva n o n n a i e del lago, clic come è noto, si ottiene rappresentando a ( x ) in funzione di v(x). la figura 3 ne dà l'andamento. la complessità della curva normale ha portato ad eseludere l'applicazione di uno dei metodi suggeriti da chrystal, per la determinazione degli elementi caratteristici delle sesse del lago. si è fatto pertanto ricorso ai metodi di h i d a k a e di defant, l'applicazione dei quali prescinde dalla forma della curva normale. il metodo di hidaka, la cui teoria è stata esposta altrove, è stato applicato per il caso m = 2. tabella i — largii. r m = .£ a ( x ) s ( x ) b (x) < ( ) a ( x ) r a ( , ) a : , 7 . 2 ( 1 z 2 isz m /. m 1 2 m •/ • m it ' c x ni 1 0 3 ii. 2 in llll,2 h n . 2 [ h n . 3 ] u ( m n i 3 ) o (7.) m 1 2 m •/ • m it ' 0 0 0 0 0 0 0 0 0 0 1 1 5 0 2 9 , 7 5 1 4 1 2 2 2 , 2 9 7 2 2 , 2 9 7 4 2 , " 0 7 •0 9 9 3 9 , 2 8 4 • 0 1 9 9 3 • 1 8 5 0 3 • 0 0 3 6 9 • 0 0 0 0 7 • 0 0 0 0 0 2 • 0 0 0 0 0 0 0 1 2 3 0 0 9 6 , 2 5 1 8 2 5 4 8 , 6 1 9 7 0 . 9 4 6 1 7 5 , 6 5 6 • 0 6 3 4 1 2 0 , 0 9 6 • 0 4 3 4 8 • 8 7 3 7 7 • 0 5 5 1 1 • 0 0 3 5 1 • 0 0 0 2 2 • 0 0 0 0 2 3 1 3 3 , 0 0 1 8 7 5 5 7 , 4 3 2 1 2 8 , 3 7 8 2 4 9 , 3 7 5 •l 1 4 7 3 4 1 , 3 4 3 • 0 5 1 3 2 2 , 1 2 1 7 2 • 2 4 3 4 2 • 0 2 7 9 2 • 0 0 3 2 0 • 0 0 0 3 6 4 1 4 3 , 5 0 1 7 7 5 5 8 , 1 0 8 1 8 6 , 4 8 6 2 ~ 4 , 7 1 2 • 1 6 6 6 7 7 5 , 7 3 3 • 0 5 1 9 1 3 , 9 3 3 5 7 • 6 5 5 6 1 • 1 0 9 2 7 • 0 1 8 2 1 • 0 0 3 0 3 5 > 1 4 9 , 0 0 1 7 7 5 5 4 , 0 5 4 2 4 >,540 2 6 4 , 4 7 5 • 2 1 4 9 7 1 0 7 , 6 8 5 • 0 1 8 3 0 5 , 2 0 1 1 9 1 , 1 1 8 1 0 • 2 4 0 3 5 • 0 5 1 6 5 • 0 1 1 1 3 6 1 5 1 , 2 5 1 6 7 5 5 7 , 4 ì2 2 9 7 , 9 7 2 2 5 3 , 3 4 4 • 2 6 6 3 0 1 5 0 , 7 0 4 • 0 5 1 3 3 7 , 7 3 5 6 4 2 , 0 6 0 0 0 • 5 4 8 6 1 • 1 4 6 1 3 • 0 3 8 9 1 7 » 1 4 1 , 7 5 1 6 20 • 1 8 , 6 1 9 3 1 6 , 6 2 1 2 2 9 , 6 3 5 • 3 0 9 7 8 1 9 9 , 0 5 5 • 0 1 3 4 8 8 , 6 5 4 9 1 2 , 6 8 1 1 2 • 8 3 0 5 3 • 2 5 7 3 1 • 0 7 9 7 1 8 » 1 1 7 , 7 5 1 3 2 5 4 1 , 8 9 2 3 8 8 , 5 1 3 1 5 6 , 0 1 9 • 3 4 2 2 3 2 9 , 2 5 5 • 0 3 7 4 4 1 2 , 3 2 7 3 1 4 , 2 8 0 2 9 1 , 4 8 6 1 8 • 5 1 6 0 2 • 1 7 9 1 2 9 » 1 1 8 , 2 5 1 3 2 0 4 3 , 2 4 3 4 3 1 , 7 5 6 1 5 6 , c 9 0 • 1 : 8 5 8 7 3 5 9 , 7 9 2 • 0 3 8 6 5 1 3 , 9 0 5 9 6 5 , 3 6 5 8 9 1 , 0 6 0 7 0 • 7 9 9 0 4 • 3 0 8 3 0 1 0 » 1 1 1 , 7 5 1 5 1 5 4 \ 6 2 2 4 7 8 , 3 78 1 6 9 , 3 1 • 4 2 7 . 4 3 5 1 , 8 0 8 • 0 4 1 6 7 1 4 , 7 4 3 1 8 6 , 3 0 3 3 0 2 , 6 9 4 9 1 1 , 1 5 2 1 8 • 1 9 2 5 7 1 1 » 1 3 7 , 2 5 1 4 7 5 4 8 , 6 4 9 5 2 7 , 0 2 7 2 0 2 , 4 4 l • 4 7 1 0 1 3 0 6 , ( 5 3 • 0 4 ' 4 7 1 3 , 3 3 0 2 1 6 , 2 7 8 6 6 2 , 9 5 7 3 1 1 , 3 9 2 8 7 • 6 5 6 1 1 1 2 1 2 0 . 5 0 1 3 4 0 4 7 , 2 9 7 5 7 4 , 3 2 4 1 6 1 , 4 0 • 5 1 3 2 8 3 8 6 , 5 1 1 • 0 4 2 2 7 1 6 , 3 3 7 8 2 8 , 3 8 5 8 8 4 , 3 0 4 3 6 2 , 2 0 9 3 6 1 , 1 3 4 0 1 1 3 > 1 4 2 , 2 5 1 4 6 5 4 1 . 8 9 2 6 1 6 , 2 1 6 2 8 , 3 9 5 • 5 5 0 7 2 2 9 3 , 7 6 8 • 0 3 7 4 4 1 0 , 9 9 8 6 7 6 , 0 5 7 1 9 3 , 3 3 5 7 9 1 , 8 3 7 1 1 1 , 0 1 1 6 6 14 » 1 2 7 , 2 5 1 3 7 5 4 4 , 5 9 5 6 6 0 , 8 1 1 1 7 4 , 9 6 9 •590:"8 3 3 4 , 1 1 6 •03'>86 1 3 , 3 1 7 8 6 7 , 8 6 5 2 6 4 , 6 7 5 0 0 2 , 7 4 3 2 1 1 , 6 2 0 1 2 1 5 > 1 1 7 . 2 5 1 2 7 5 4 1 , 2 1 6 7 0 2 , 0 2 7 1 4 9 , 4 9 4 • 6 2 7 4 2 3 6 5 , 5 6 6 • 0 3 6 * 4 1 3 . 4 3 0 8 9 8 , 4 2 6 8 1 5 , 2 8 7 2 0 3 , 3 1 7 3 0 2 , 0 8 1 3 8 1 6 > 1 1 9 , 0 1 3 2 0 4 0 , 5 4 0 7 1-2,567 1 5 7 , 0 8 0 • 6 6 3 6 5 3 1 7 , 2 2 7 • 0 3 6 2 3 1 1 , 4 9 3 1 3 7 , 6 2 7 4 2 5 , 0 6 1 9 2 3 , 3 5 9 3 3 2 , 2 2 9 4 4 1 7 » 1 2 9 , 7 5 1 3 5 0 4 7 , 9 7 3 7 9 0 , 5 4 0 1 7 5 , 1 6 2 • 7 0 6 2 2 4 5 , 4 ' 0 • 0 4 ° 8 7 1 0 , 5 2 1 5 8 7 , 4 3 3 7 1 5 , 2 5 2 0 6 3 , 7 1 0 6 5 2 , 6 2 1 6 6 18 > 1 0 7 , 0 0 1 3 2 0 1 3 , 2 4 3 8 3 3 , 7 8 3 1 4 1 , 2 4 1 • 7 4 5 1 7 2 5 5 , 3 1 0 • 0 3 8 6 5 9 , 8 6 7 7 3 7 , 3 5 3 1 4 5 . 4 7 9 3 5 4 , 0 8 3 0 7 3 , 0 4 2 6 2 l'i > 5 5 , 2 5 9 5 0 3 2 , 4 3 2 8 6 6 , 2 1 5 5 2 , 4 8 7 • 7 4 1 5 5 8 2 , 4 3 0 • 0 2 8 9 8 1 6 , 8 7 8 7 9 1 3 , 0 6 6 7 2 1 0 , 1 1 5 6 3 7 , 8 3 1 0 8 6 , 0 6 2 3 5 2 0 7 0 , 5 0 9 1 5 2 8 , 3 7 8 8 9 4 , 5 9 3 6 4 , 5 0 7 • 7 9 9 5 2 3 9 8 , 2 5 1 • 0 2 5 3 7 1 0 , 1 0 3 6 3 8 , 0 7 8 0 5 6 , 4 5 8 5 4 5 , 1 6 3 7 6 4 , 1 2 8 4 4 2 1 5 2 , 5 0 8 1 5 2 9 , 7 3 0 9 2 4 , 3 2 3 4 2 , 7 8 7 • 8 2 6 0 9 4 8 2 , 3 8 9 • 0 2 6 5 7 1 2 , 8 1 7 0 8 1 0 , 5 8 8 0 6 8 , 7 4 6 6 3 7 , 2 2 5 5 0 5 . 9 6 8 9 1 2 2 » 4 1 , 2 5 7 1 0 2 2 , 9 7 3 9 4 7 , 2 9 6 2 9 , 2 8 7 • 8 4 6 6 2 5 7 5 , 6 8 2 • 0 2 0 5 3 1 1 , 8 1 8 7 5 1 0 , 0 0 5 9 9 8 , 4 7 1 2 1 7 , 1 7 1 8 5 6 , 0 7 1 7 6 2 3 » 3 9 , 2 5 7 4 0 2 2 , 2 9 7 9 6 9 , 5 9 3 2 9 , 0 4 5 • 8 6 6 5 4 4 6 0 , 3 2 0 • 0 1 9 9 2 9 . 1 6 9 5 7 7 , 9 4 5 8 0 6 , 8 8 5 3 4 5 , 9 6 6 4 6 5 , 1 7 0 1 7 2 4 » 2 4 , 5 0 6 5 0 2 1 , 6 2 2 9 9 1 , 2 1 5 1 5 , 9 2 5 • 8 8 5 8 7 6 4 2 , 3 8 6 • 0 1 9 3 3 1 2 , 4 1 7 3 2 1 1 , 0 0 0 1 3 9 , 7 4 4 7 4 8 , 6 3 2 5 2 7 , 6 4 7 3 3 2 5 » 1 5 , 2 5 5 5 0 1 8 . 9 1 9 1 0 1 0 , 1 3 4 8 , 3 8 7 • 9 0 2 7 8 9 1 8 , 0 8 8 • 0 1 6 9 1 1 5 , 5 2 4 8 5 1 4 . 0 1 5 5 2 1 2 , 6 5 2 9 1 1 1 , 4 2 2 7 2 1 0 , 3 1 2 2 3 2 6 > 1 1 , 5 0 5 9 0 1 6 , 2 1 6 1 0 2 6 , 3 5 0 5 , 7 5 0 • 9 1 7 2 7 1 0 0 1 , 7 3 9 • 0 1 4 4 9 1 4 , 5 1 5 2 0 1 3 , 3 1 4 3 6 1 2 , 2 1 2 8 0 1 1 , 2 0 2 4 0 1 0 , 2 7 5 6 0 2 7 8 , 7 5 5 9 0 1 8 , 2 4 3 1 0 1 4 , 5 9 3 5 , 1 6 2 • 9 3 3 5 7 7 4 3 , 8 9 8 • 0 1 6 3 0 1 2 , 1 2 5 5 4 1 1 , 3 2 0 0 4 1 0 , 5 6 8 0 1 9 , 8 6 5 9 5 9 , 2 1 0 5 6 2 8 » 1 8 , 2 5 6 4 0 1 9 , 5 9 5 1 0 6 4 1 8 8 1 1 , 6 8 0 • 9 5 1 0 9 1 8 4 , 9 3 1 • 0 1 7 5 2 3 , 2 3 9 9 9 3 , 0 8 1 5 2 2 , 8 3 0 8 0 2 , 7 8 7 4 6 2 . 6 5 1 1 2 2 9 » 4 , 5 0 3 7 5 1 6 , 8 9 2 1 0 8 1 , 0 8 0 1 , 6 8 7 • 9 6 6 1 8 6 2 8 . 3 3 4 • 0 1 5 0 9 9 , 4 8 1 5 6 9 , 1 6 0 8 9 8 . 8 5 1 0 4 8 , 5 5 1 7 0 8 , 2 6 2 4 2 3 0 > 1 , 7 5 2 4 0 1 0 , 8 1 1 1 0 9 1 , 8 9 1 0 , 4 2 1 1 , 7 8 1 • 9 7 5 8 4 1 3 0 9 , 5 2 4 • 0 0 9 6 6 1 2 , 6 5 0 0 0 1 2 , 3 1 4 3 8 1 2 , 0 4 6 0 9 1 1 , 7 5 5 0 1 1 1 , 4 7 1 0 2 3 1 > 3 , 7 5 4 7 5 8 , 7 8 4 1 1 0 0 , 6 7 5 0 , 4 2 1 1 , 7 8 1 • 9 8 3 6 9 1 4 5 . 9 8 5 • 0 0 7 8 5 1 , 1 4 5 9 8 1 , 1 2 7 2 9 1 , 1 0 8 9 1 1 , 0 9 0 8 2 1 , 0 7 3 0 4 3 2 0 0 1 8 , 2 4 3 1 1 1 8 , 9 1 8 0 1 0 0 1 , 1 2 7 2 9 1 , 1 0 8 9 1 1 , 0 7 3 0 4 3 2 0 0 1 8 , 2 4 3 1 1 1 8 , 9 1 8 0 1 0 0 3 1 0 , 8 6 8 4 3 l o 2 0 7 , 2 4 3 6 5 1 . 1 5 5 , 1 2 7 5 9 l o 1 2 4 , 2 0 4 0 9 1;[ 1 0 3 , 8 1 5 1 0 1 . 61 m a r i a c f . c 1 l i v s p a d e a sia a la superficie del lago, g l'accelerazione di gravità, x un parametro che risulta dalla risoluzione dell'equazione di iii grado (corrispondente al caso m = 2); i periodi delle oscillazioni l i b e r e si traggono dalla formula : r = 2x a/( g a)y 2 t i ] l'equazione che consente la determinazione dei valori di }. contiene opportune combinazioni di certe grandezze /o, /1, /j, i-s, 14, i valori delle quali vengono dedotti dai dati che caratterizzano la forma lezioni t + 6 jtz/oni 9 * !* bertoni /j m1 f i g . 2 del lago. poiché, come si è detto, l a curva normale è risultata di forma complessa, la relativa ecpiazione di chrystal è stata risolta mediante integrazione numerica. da tale risoluzione (vedi tabella i), si sono tratti i seguenti valori: /0 = 310,86843 /, = 207,24365 in 155,12759 /o = 124,26409 u — 103,81510 ne è venuta la seguente equazione di iii grado in x: 4166,937à3 — 63,009à2 -10,16997,'. —0,000095238 = 0 s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e del lago d i i d r 0 65 per la determinazione dei nodi va tenuto presente che in essi du/(lz=0. ciò comporta nel caso m = 2. 4 — 3 ( — l z2-j-2 ( 1 — | z = 0 [ 2 ] dove il valore dei rapporti a'/ao e ao/a0 può essere dedotto dal sistema di equazioni in ao, ai, una volta determinati i valori di io, ij, e per ogni valore di x. il metodo delle approssimazioni successive ha dato i tre seguenti valori per x : ai — 0.0007677 a 2 — 0,002515 /.:( — 0,011839 poiché « = 0,111892 mm-, conseguono dalla [ 1 ] ]ier i tre valori di x ottenuti, t\ = 13m,5 t 2 = 7m,5 7 ' . , = 3"',4 che rappresentano i periodi delle sesse uni-bi-trinodali del lago di idro. 66 m a r i a c e c i l i a s p a d e a avremo pertanto, con riferimento all'oscillazione libera uninodale l'equazione di iii grado in z : 17,40040 z3 — 17,'i2232 + 5,07468 2 — 1 = 0 . risolta tale equazione con il metodo delle approssimazioni successive, si è pervenuti al seguente valore per z. z = 0,71812 che è il solo dei valori di z compresi f r a 0 e 1. l'uninodo corrisponde quindi ad una sezione che dista dall'estremo nord 111 4890. il valore di x corrispondente all'oscillazione libera binodale è : x = 0,002515 pertanto la corrispondente equazione dei nodi viene : 58,28488 z3 — 75,69180 z+ 19,31876 = + 1 = 0 . le due radici di z comprese tra 0 e 1 sono le seguenti: si = 0,42578 z* = 0,91124 . le linee nodali della sessa binodale corrispondono perciò a sezioni trasversali che distano dall'estremo nord 111 2687 e 111 7375 rispettivamente. p e r la sessa trinodale si h a : x = 0,11839 . la corrispondente equazione dei nodi risulta pertanto: 10,5896 z3 — 18,3642 z2 + 8,9480 s — 1 = 0 . le tre radici di questa equazione sono tutte evidentemente comprese fra 0 e 1. eccone i v a l o r i : zi = 0,15876 z= 0,62764 z3 = 0,94777 a questi valori corrispondono tre sezioni trasversali (trinodi) che distano dall'estremo nord 111 854, 111 42-00, 111 8043 rispettivamente. s t u d i o p r e l i m i n a r e s u l l e o s c i l l a z i o n i l i b e r e d e l lago iii u ro tabella ii '/.ioni 1° uninodole 11° binodale 111° trinodale 0 — 1 — 1 1 1 — .90574 ] 3^532 — .82 91 2 — .74170 — 1.93529 — .50378 3 — .62341 — 2.30835 — .19o0s 4 .56319 2.38700 + .03023 5 — .55064 — 2.23400 -4. 18009 6 — .56969 — 1.87(51 + . 28050 7 .60170 — 1.45399 + .32451 8 . 63414 —.1.02226 + .3s621 9 — . 66596 053307 . 32681 10 .69171 + 0.02120 + .29642 11 — . 70112 + 0.60271 . 24701 12 .68499 + 1.14397 j . 18663 13 .64355 + 1.58202 + . 12694 l i — . 56518 — . 06069 )5 — . 45552 + 2.28007 + ,00ol3 i e — .30763 + 2.47996 — . 05457 i7 — .07157 + 2.57866 — .110 39 18 .19612 + 2.5112 1 — .12773 19 -j. 44012 -f 2.h65e8 — . 16561 20 + .68559 + 2.15050 — . i7'z72 21 + .97562 i ] cc1705 — . 17047 2 ì + 1,22430 ' 1 5°877 — . 16118 23 + 1,48708 + 1.17055 — .14525 24 -r 1,76280 + 0.76709 — .12302 25 + 2,02164 + 0.36471 — . 09742 26 + 2,25689 0.01747 — .07080 27 + 2,53668 — 0.48971 — .03551 28 + 2,85595 — 1.049f8 + .00920 29 + 3,14 "55 — 1.57598 + .05348 30 -f 3,34037 — 1.93497 + . 08472 31 43,502" 9 — 2.24013 + .11186 3 ì + 3,85276 — 2.91184 + . 17340 68 m a r i a c e c i l i a s p a d e . * —.—«—, . . o , • 16 17 la ip 70 h 7? n >1 ?1 n n n • i < 5 6 > « ò 10 ii 17 ij id £ è , -—) •*5 » j( 1? fig. 4 fu calcolato poi l'andamento dello spostamento verticale della superficie del lago, corrispondentemente alle tre oscillazioni libere studiate. le equazioni ebe danno l'andamento degli spostamenti verticali sono : t,' = 17,40040 z3 — 17,62232 z2 + 5,07468 z — 1 per la sessa uninodale xj' = 58,28488 z3 — 75,69180 z2 + 19,31876 z f i per la sessa binodale £'" = 10,5896 z 3 — 18,3642 z2 + 8,9480 z — 1 per la sessa trinodale 7 0 m a r i a c e c i l i a s p a d e a preso uguale ad 1 lo spostamento iniziale all'estremo nord, sono stali calcolati, di sezione in sezione, gli spostamenti verticali relativi ad ognuna delle tre sesse citate. la tabella 11 riporta i risultati dei calcoli e la lig. 4 ne dà il relativo grafico. metodo defant. — a scopo di controllo, ho applicato anche il metodo di defant. è ben nota la teoria di questo metodo. ali limito qui a dare il significalo dei simboli che figurano nella tabella iii. in essa è a=4tn~/gt-, con manifesto significato dei simboli; /a(a-) rappresenta l'intervallo fra sezione e sezione; 2 2 i]o esprimono le ampiezze delle oscillazioni, contate lungo una sezione longitudinale del lago, in senso orizzontale e verticale rispettivamente; 2 a . u dà l a variazione dello spostamento verticale da una sezione a l l ' a l t r a ; q = eprime il volume dell'acqua che nel tempo t/4, f r a il riposo ed il massimo spostamento di una particella liquida, passa attraverso una generica sezione quando il periodo t dell'oscillazione libera considerata coincide con il periodo dell'oscillazione del lago, in corrispondenza dell'ultima sezione si dovrà avere q —r 0. per l a sessa uninodale, il valore t = 14m,4 ha portalo al residuo q = 142,03 : io10 cm3. dopo un altro tentativo per t = 1 2 ' " , 2 il residuo è stato praticamente nullo (tabella iii). per quanto concerne l a sessa binodale un residuo praticamente nullo si è avuto con il periodo t = 7m,5 (tabella i l l ì . la fig. 5 dà la posizione dei nodi e l'andamento degli spostamenti lungo il lago, per le due oscillazioni libere considerate, ottenute con il metodo dì defant. la ricerca analitica condotta sul lago di idro ha portato ai risultati che qui ora riassumo o s c i l l a z i o n i l i b e r e p r i m o m e t o d o ( h i d a k a ) s e c o n d o m e t o d o ( d e f a n t ) u n i n o d a l e ( p e r i o d o j n o d o d a l l ' e s t r e m o f n o r d 13,5 4 8 9 0 m e t r i 12,2 1650 m e t r i b i n o d a l e l p e r i o d o % 1 b i n o d o ( i l b i n o d o 7,5 2 6 8 7 m e t r i 7375 m e t r i 7,5 2 7 0 0 m e t r i 7800 m e t r i t r i n o d a l e ( p e r i o d o 1 1 t r i n o d o ) i i t r i n o d o \ i i i t r i n o d o 3,4 8 5 4 m e t r i 4 2 0 0 m e t r i 8 0 4 3 m e t r i 700 -1 r i * i i 1 ' 1 < 5 6 7 » s? » * 11 " " * , * " » >9 x 21 2? a » « li 2? a 29 » j1 j? a 72 m a r i a c e c i l i a s p a d e a 1 due metodi hanno condotto a risultati in ottimo accordo, specie per quanto si riferisce all'oscillazione libera binodale. le osservazioni, che mi propongo di eseguire in seguito, permetteranno di stabilire, per quanto si riferisce all'oscillazione uninodale, quale dei due metodi ha portato a valori più rispondenti alla x-ealtà. conformemente alla teoria, nella parte sud del lago, stretta e poco profonda, si verificano i massimi spostamenti per le oscillazioni uni-binodali. anche per quanto si riferisce ad essi, l'osservazione permetterà di decidere sull'attendibilità dei risultati ottenuti dai due metodi. roma — istituto nazionale di geofisica — gennaio 1953. riassunto si riportano i risultati dello studio preliminare eseguito allo scopo di determinare, nelle loro fondamentali caratteristiche, le oscillazioni libere del lago di hlro. ottenuta la curva normale del lago, si sono applicati due diversi metodi di calcolo, che hanno consentito di pervenire ai periodi delle prime tre sesse (uni,-bi-trinodale) nonché ai corrispondenti sposta' menti verticali e orizzontali e alle linee nodali. le osservazioni, che ci riserviamo di fare, serviranno a stabilire il grado di attendibilità dei risultati ottenuti per via teorica. su m mary it e present here the results of a preliminarx study to determine the fundamental characteristics of the free oscillations of the lake of hlro. having obtained the normal curve for the hthe, tiro d i f f e r e n t . methods of calculation nere applied ichich have given the periods of the first three seiches (once, tii'ice and thrice-nodal), including the corresponding vertical and horizontal. displacements of the nodal lines. the observations, which ice intend to do, ni ii serve to establish the degree of reliability of the results obtained by theoretical methods. b i b l i o g r a f i a caloi p., le sesse del lago di garda. p a r t e i e ii, a n n a l i di g e o f i s i c a i, 1 e 2, 19 is. 35wang.qxd key words historical earthquakes – catalogues – intensity – isoseismals – china 1. collection of historical earthquake data from archives 1.1. brief on historical earthquake documents china is one of the countries with the longest history and abundant historical archives. among various ancient writings from remote time, there are a great number of earthquake narratives. the earliest one was in the 23rd century b.c., with the description of «earthquake and springs gushing». generally, early recordings were very simple. from the ying dynasty (16-11th century b.c.), there were official historians appointed by governors. besides political events, natural phenomena including astronomy, weather and earthquakes were recorded. in the qin dynasty (221-206 b.c.), a united country was formed firstly, with a 831 annals of geophysics, vol. 47, n. 2/3, april/june 2004 regime consisting of central and local governments (canton and county). meanwhile there was a reform and a standardization of the writing system. from the western han (206 b.c.24 a.d.), earthquakes had been recorded as «catastrophes» in official historical records. in the yuan dynasty (1271-1368 a.d.) there was a reform in the regime and provinces became the highest level of local government. from that time, the annals and varied records of differently ranked local governments became popular. earthquake records increased greatly in number and their contents became more detailed. 1.2. two large-scale collections of historical records the first collection of earthquake records appeared in 977 a.d. a total of 45 earthquake items between the 11th century b.c. and 618 a.d. were compiled in a chapter of a book, which named as taiping yulan (li, 1960). in the 1950s, immediately after the founding of the people’s republic of china, there were about two hundred engineering projects which needed on-site assessment of seismic intensity. for this purpose, historians and seishistorical earthquake investigation and research in china jian wang institute of geophysics, china seismological bureau, beijing, china abstract china is one of the countries with the longest tradition of culture and has suffered many earthquake disasters, so many earthquake documents have therefore been conserved. in this paper we try to outline some basic information of historical earthquake investigation and research in china, such as collection of historical earthquake data from archives, historical earthquake catalogues, seismic intensity scales. we introduce briefly the huge accomplishments of historical research and discuss some problems encountered. through examples, we illustrate the solutions to some typical problems. there are some suggestions on further work. mailing address: dr. jian wang, institute of geophysics, china seismological bureau, 5 minzu daxue nanlu, haidian district, beijing 100081, china; e-mail: wangjcsb@hotmail.com 832 jian wang mologists were organized by the academia sinica to collect historical earthquake records systematically. through more than two years' hard work together, the historians and seismologists searched through more than eight thousand items of historical literature. tens of thousands of historical earthquake records were found and picked out. after checking and analyzing, they compiled the records chronologically. a total of 26 provinces or municipalities were concerned. they put the results in a twovolume book: «chronological data of historical earthquakes in china», which was published in 1956 (historical earthquake working group of seismological committee of academia sinica, 1956). from 1977, historians and seismologists were organized again by the state seismological bureau (ssb, nowadays renamed as china seismological bureau, i.e. csb), the china academy of social sciences and the academia sinica. seismologist xie yu-shou and historian cai mei-biao were the editors in chief. this time they explored earthquake recordings in wider scope. based on «chronological data of historical earthquake in china», a large amount of supplementary information was added, and amendments were made. some earthquake documents were found and translated from the manwen (literature of qing dynasty) and zangwen (literature of zangzu, i.e. tibet) archives. after five years' hard work, a five-volume «compilation of historical materials of chinese earthquakes» was completed and published successively in eight books during 1983-1987 (xie and cai, 1983a,b, 1985, 1986, 1987). the historical records of the first volume covers the time period from the 23th century b.c. to 1367 a.d.; and the second and third volumes cover the period from 1368 a.d. to 1644 a.d., and 1645 a.d. to 1911 a.d. respectively. from 1912 a.d. there have been instrumental records and investigations on macro-intensity. meanwhile, most provinces, autonomous regions and metropolitan regions published their own regional compilation of earthquake materials. besides the two large-scale collections of historical earthquake records, there are some collections done by individuals, but these are small scale and generally in local regions. the work to find new earthquake records from historical archives has never ceased in china. 1.3. seismic field investigation since 1900 many field investigations of destructive earthquakes have been carried out. a huge amount of intensity data and photographs of earthquake disasters were compiled and published in a two-volume book: «investigation on china earthquakes» (shi et al., 1987, 1990). in the same period, instrumental recording has been gradually increasing. earthquakes with both intensity data and instrumental records are very important to build up relationships between intensity and magnitude. with these relations the parameters of historical earthquakes were estimated. 2. catalogues of historical earthquakes and seismic intensity 2.1. methods and principles of editing historical earthquake catalogues in china, we define «historical earthquakes» as earthquakes whose parameters were inferred only from written records of historical archives. generally, the methods of determining the parameters of historical earthquakes include the following steps: i) in a region, putting together all sites with historical records at the same time. according to the records of each site, assessing the intensity. ii) utilizing the events with instrumental parameters and intensities to work out the statistics or empirical relations between earthquake parameters and intensities, especially epicentral intensity. extrapolating the relations to infer the parameters of historical earthquakes. iii) generally, in the catalogue, the parameters of an earthquake include origin time, epicenter, maximum intensity and magnitude. if the data are enough, accuracy and depth may be inferred. 833 historical earthquake investigation and research in china iv) for earthquakes recorded in a large region with no damage whatsoever, the empirical relation between the magnitude and the equivalent radius of the intensity iv isoseismal is used to estimate their magnitudes. v) generally, the center of the isoseismals or the site with most damage reports will be regarded as the epicentre. vi) besides earthquake origin time, epicentre and magnitude, the places where historical earthquakes were recorded are listed. if possible, isoseismals are given. vii) doubtful material and earthquakes with indeterminate parameters are listed in an appendix for further study. 2.2. editions of historical earthquake catalogues in china there are many kinds of earthquake catalogues in china; here we mention only the catalogues compiled mainly with historical records. since the founding of the people’s republic of china, there have been four editions of the catalogue of chinese historical earthquakes. chinese seismologists have continually endeavored to study and revise the catalogues to make them more systematic and correct. the first edition of «the catalogue of chinese earthquakes» (cce) was published in 1960 (li, 1960). prof. li shan-bang (s.p. lee) was the editor in chief. this catalogue has two volumes. volume one is the catalogue of earthquakes with magnitude greater than 4.7, and covers the time period from 1189 b.c. to 1955 a.d. total 1180 earthquakes were listed. the parameters of earthquakes before 1900 were determined mainly according to the «chronological data of historical earthquakes in china». the parameters of earthquakes after 1900 were compiled based on investigations and instrumental records. volume two is the catalogue of counties. there were more than two thousand counties in china, while about one thousand and six hundred counties have historical records, most of them concentrated in the eastern part of china. the second edition of «the catalogue of chinese earthquakes» was published in 1971 (china seismological working group, 1971). this four-volume catalogue covered the time period from 1177 b.c. to 1969 a.d. the total number of earthquakes listed with magnitude greater than 4.7 is 2257. the third edition of «the catalogue of chinese earthquakes» was published in 1983 (gu et al., 1983). prof. gu gong-xu was the editor in chief, lin ting-huang and prof. shi zhenliang were vice editors in chief. this two-volume catalogue contains 3187 earthquakes, which covered the time period from 1831 b.c. to 1969 a.d. the fourth edition of «the catalogue of chinese earthquakes» comprises two parts. part one «the catalogue of chinese historical strong earthquakes» was published in 1995, with earthquake parameters determined mainly from historical records (min et al., 1995). this catalogue is arranged chronologically, beginning from the 23rd century b.c. and ending at 1911, with a time span of more than 4100 years. it contains 1034 earthquakes with magnitude greater than 4.7 (fig. 1). prof. min zi-qun was the editor in chief. part two «the catalogue of chinese present earthquakes» was published in 1999, with earthquake parameters determined from investigations and instrumental records (wang et al., 1999). this catalogue covers the time period from 1912 to 1990. prof. wang suyun, prof. wu ge and shi zhen-liang were the editors in chief. from fig. 1, we can notice that the distribution of historical earthquake documents is not uniform. most of them concentrated in the east part of china, especially in the huanghe river (yellow river) and the changjiang river (yangtse river) areas. the completeness and reliability of historical earthquake documents depended on many elements, such as dynasty changes and wars. in some cases, even individual inclinations of governors might have affected the documents. in order to compile the seismic intensity zoning map of china (1990), the concise catalogue of chinese earthquakes (ccce) was published in 1988, covering the time period from 78 b.c. to 1986 a.d. total 5142 earthquakes with magnitude greater than 4.7 were listed. prof. min zi-qun was the editor in chief. the former catalogues, four editions of cce and the ccce, were all sponsored by governments and recognized in the whole country. there are some catalogues edited by individuals and local branches of the china seismological bureau (csb), which are not included in this paper. 2.3. seismic intensity and atlas of isoseismals the china intensity scale is a twelve-degree system. it was created by prof. li shanbang (li s.b, 1954) and rewritten by prof. xie yu-sou (xie, 1957). when rewriting the new intensity scale, besides investigating historical records in china, prof. xie yu-sou also compared the data with various intensity scales in the world, including the intensity scale of the former soviet union in 1952 and the modified mercalli intensity scale. the china intensity scale was supplemented and simplified by prof. liu hui-xian in 1978 (liu, 1978), taking account of new kinds of modern buildings destroyed by strong earthquakes in recent years both in china and elsewhere in the world. in china intensity scale, seismic intensity was described according to four kinds of damage phenomena: houses; structures (including special ancient chinese buildings, such as memorial archways, pagodas, steles, ramparts, etc.); residual phenomena in the ground and changes of surface and subsoil water conditions, and other phenomena. accounts taken from historical records describing earthquake damage were included to help estimate intensity from archive documents (li, 1989). some atlases of isoseismals were compiled in china. prof. min zi-qun compiled an atlas of isoseismals including 64 earthquakes (min, 1957). as an important database for seismic intensity zoning, the atlas of chinese earthquake isoseismals was published in 1979 (compilation group of china seismic intensity zoning map ssb, 1979). total isoseismals of 151 earthquakes were delineated in this atlas, covering the time period from 1125 a.d. to 1976 a.d. more complete and detailed work was done by cooperation between seismologists from the institute of geophysics ssb and historians from the institute of chinese historical geography, fudan university. the three-volume «atlas of the historical earthquakes in china» was the result of cooperation of seismologists, historians and geographers (institute of geophysics, ssb and institute of chinese historical geography, fudan university, 1986, 1990a,b). 3. review and prospects of historical earthquake research 3.1. accomplishments of the past half century from the 1950s, a huge amount of work and research has been done in historical earthquake investigation and research. great accomplishments have been achieved. the investigations and researches can been summarized as follows: i) collection of historical earthquake documents; ii) besides the textual research, field investigations have been done on many important historical earthquake sites. much evidence was found on buildings, steles, pagodas and tombs; iii) research on the intensity scale. estimating seismic intensity and isoseismal distribution inferring careful from historical documents; iv) utilizing earthquakes with both instrumental parameters and intensities to get the statistics or empirical relations between intensity and earthquake parameters; v) compilations of historical earthquake catalogues and isoseismal atlases; vi) application: the historical earthquake data prolong the time and extend the space, in which seismicity can be analyzed more reliably. the results are applied to longto mid-term earthquake prediction, seismic hazard analysis, seismic intensity zoning, etc.; vii) institutional organization and cooperation: the china historical earthquake committee was established in 1987, as a branch of the china seismological association. every two years, there is an assembly of the china seismological association and historical earthquake is one of the subjects. besides the regular meetings, there are some seminars and 834 jian wang 835 historical earthquake investigation and research in china f ig . 1. d is tr ib ut io n of h is to ri ca l e ar th qu ak es i n c hi na . 836 jian wang workshops on special topics (min et al., 1989; guo et al., 1991). there are some special researches on historical earthquakes of japan and the korean peninsula (gao, 1991; lu, 1991). a cooperative project between seismologists of china and north korea on historical earthquakes was carried out from 1987 to 1988, supported by the two governments (shi and li, 1989). there were cooperation projects with the former soviet union, the united states, japan and korea on historical earthquake research. 3.2. main problems encountered owing to the long history and the huge area, there are various problems when we investigate and research historical earthquakes in china. generally language is the first problem. because the ancient word system is different from that used nowadays, special knowledge is needed to interpret historical documents. besides chinese characters, there are other writing systems of minorities, which need translation. earthquake records are widely scattered in historical documents, so it is very difficult to search and pick them out. in our long history there are countless dynasty changes and wars, which might cause historical records lost or destroyed. change of place-names is another problem, which may lead to misunderstandings. transformation of topography and physiognomy is also a very important element to consider. here we will illustrate the problems with a case of detailed investigation of an historical earthquake (wang et al., 1998). according to historical documents, an earthquake occurred in 1046 a.d. (song dynasty) in the jiaodong peninsula, shandong province. historical documents state that the earthquake occurred in the juyu mountain. while the earthquake was occurring, the stones of the juyu mountain were shaked and falling down directly into the sea. because of changes of dynasties and wars, the place name today is completely different from then. we cannot recognize where the juyu mountain is nowdays, so we cannot determine the epicentre of the earthquake. after about one month of field investigation on the jiaodong peninsula, we were lucky enough to find a stele of the song dynasty, which told us where the juyu mountain is. but another problem arose, for now the juyu mountain is about twenty kilometers from the coast. it is not possible that stones from the mountain fell down directly into the sea as described in the document. searching through the historical documents, we found that from the qing dynasty (1644 a.d.-1911 a.d.) there had been continuous filling of artificial land between the mountain and the sea. that means the land between the mountain and the sea was formed later. evidence from field investigation also proved this fact. with these problems solved, it is easy to determine the epicentre of the earthquake. finally the epicentre was put at the junction of the juyu mountain and the ancient sea. this example tells us that information from documents and field investigations are equally important. local documents and field investigations may be the keys to rediscover the ancient conditions, which are necessary to understand the historical documents precisely. 3.3. future work although a lot of work and researches have been carried out, more effort is still needed. here we give our suggestions about further work: i) collection of scattered historical documents: after two large-scale collections, there are still historical earthquake documents remaining. because these documents are scattered, it is more difficult to collect them. ii) detailed field investigations on historical earthquake sites. this kind of work is especially needed in the western part of china, where population and historical documents are sparse. with less man-made demolition and dry natural conditions, the traces of historical earthquakes can generally be maintained for longer. cooperation between seismologists and geologists on historical earthquake research will bring more rewards. iii) utilizing the information of historical documents synthetically, especially the records of earthquake disaster distributions. these data are very valuable for anti-seismic design and mitigation of earthquake disasters. 837 historical earthquake investigation and research in china iv) developing new methods to determine the parameters of historical earthquakes. continuously amending the catalogue of chinese earthquakes. compiling and publishing the catalogue of felt earthquakes, whose data have not previously been fully utilized. v) recording the experience and knowledge of elderly experts as a matter of urgency; some of them have already died and many are more than 70 years old. vi) making electronic versions of historical earthquake data, in case we lose what we already have; these are more convenient to use and safer to store. by translating and publicizing, let more experts world-wide be aware of and use this valuable information, which should belong to the whole world. chinese historical earthquake documents and researches are so abundant that it is not possible to cover them completely in one paper. in this short paper we just try to outline the main points. if there are some omissions or misunderstandings, please do not hesitate to contact me. appendix. table of chinese dynasties xia (21-16 cent. b.c.). shang (yin) (16-11 cent. b.c.). western zhou (11 cent.-771 b.c.). eastern zhou (770-221 b.c.). spring & autumn period (770-475 b.c.). warring states period (475-221 b.c.). qin (221-206 b.c.). western han (206 b.c.-24 a.d.). eastern han (24-220 a.d.). three kingdoms (220-280). western jin (265-316). eastern jin (317-420). southern & northern dynasties (420-589). sui dynasty (581-618). tang dynasty (618-907). five dynasties & ten kingdoms (907-979). song dynasty (960-1279). yuan dynasty (1271-1368). ming dynasty (1368-1644). qing dynasty (1644-1911). the republic of china (1911-1949). the people’s republic of china (founded in 1949). rererences china seismological working group (1971): the catalogue of chinese earthquakes (science publishing house, beijing), (in chinese). compilation group of china seismic intensity zoning map ssb (1979): atlas of chinese earthquake isoseismals (seismic publishing house, beijing), (in chinese). gao, j.g. (1991): five historical earthquakes in china and great earthquakes in japan, in special collection on china historical earthquake research (seismological publishing house, beijing), vol. 2, 114-116 (in chinese). gu, g.x., t.h. lin and z.l. shi (1983): the catalogue of chinese earthquakes (science publishing house, beijing), (in chinese). guo, z.j., z.q. min and z.l shi (1991): special collection on china historical earthquake research (seismological publishing house, beijing), vol. 2 (in chinese). historical earthquake working group of seismological committee of academia sinica (1956): chronological data of historical earthquakes in china (science publishing house, beijing), (in chinese). institute of geophysics, ssb and institute of chinese historical geography, fudan university (1986): atlas of the historical earthquakes in china: the ming dynasty (china cartographic publishing house, beijing), (in chinese). institute of geophysics, ssb and institute of chinese historical geography, fudan university (1990a): atlas of the historical earthquakes in china: the period from remote antiquity to the yuan dynasty, (china cartographic publishing house, beijing), (in chinese). institute of geophysics, ssb and institute of chinese historical geography, fudan university (1990b): atlas of the historical earthquakes in china: the qing dynasty period (china cartographic publishing house, beijing), (in chinese). li, q. (1989): assessments on china historical earthquake intensity, in special collection on china historical earthquake research (seismological publishing house, beijing), 1, 1-4 (in chinese). li, s.b. (s.p. lee) (1954): on the application of seismic scales, acta geophys. sin., 3 (1), 35-54 (in chinese with english abstract). 838 jian wang li, s.b. (s.p. lee) (1960): the catalogue of chinese earthquakes (science publishing house, beijing), (in chinese). liu, h.x. (1978): on the concept and application of earthquake intensity, acta geophys. sin., 21 (4), 340-351 (in chinese with english abstract). lu, z.h. (1991): the state and developing of historical earthquake research in japan, in special collection on china historical earthquake research (seismological publishing house, beijing), vol. 2, 203-211 (in chinese). min, z.q. (1957): study of strong earthquakes from historical records, acta geophys. sin., 6 (1), 49-58 (in chinese with english abstract). min, z.q., z.l. shi and y.c. li (1989): special collection on china historical earthquake research, (seismological publishing house, beijing), 1 (in chinese). min, z.q., g. wu, z.x. jiang, c.s. liu and y.l. yang (1995): the catalogue of chinese historical strong earthquakes (b.c. 23-a.d. 1911) (seismological publishing house, beijing), (in chinese). shi, z.l. and y.c. li (1989): retrospect and prospect on china historical earthquake research, special collection on china historical earthquake research (seismological publishing house, beijing), vol. 1, 1-4 (in chinese). shi, z.l., g.w. su and y.f. cheng (1987): investigation on china earthquakes (seismological publishing house, beijing), vol. i (in chinese). shi, z.l., g.w. su and y.f. cheng (1990): investigation on china earthquakes (seismological publishing house, beijing), vol. ii (in chinese). wang, j., y.l. yang, x.d. zhang and k.m. cai (1998): textual research and investigation on two doubtful historical earthquakes in jiaodong peninsula, shandong province, earthquake res. china, 14 (3), 43-50 (in chinese with english abstract). wang, s.y., g. wu and z.l. shi (1999): the catalogue of recent earthquakes in china (a.d. 1912-a.d. 1990) (china science and technology publishing house, beijing), (in chinese). xie, y.s. (1957): a new scale of seismic intensity adapted to the conditions in chinese territories, acta geophys. sin., 6 (1), 35-48 (in chinese with english abstract). xie, y.s. and m.b. cai (1983a): compilation of historical materials of chinese earthquakes (science publishing house, beijing), vol. 1 (in chinese). xie, y.s. and m.b. cai (1983b): compilation of historical materials of chinese earthquakes (science publishing house, beijing), vol. 5 (in chinese). xie, y.s. and m.b. cai (1985): compilation of historical materials of chinese earthquakes (science publishing house, beijing), vol. 2 (in chinese). xie, y.s. and m.b. cai (1986): compilation of historical materials of chinese earthquakes (science publishing house, beijing), vol. 4 (in chinese). xie, y.s. and m.b. cai (1987): compilation of historical materials of chinese earthquakes (science publishing house, beijing), vol. 3 (in chinese). amoruso_final_layout 6 annals of geophysics, 56, 4, 2013, s0435; doi:10.4401/ag-6441 s0435 analytical models of volcanic ellipsoidal expansion sources antonella amoruso*, luca crescentini università di salerno, dipartimento di fisica “e.r. caianiello”, fisciano (salerno), italy abstract modeling non-double-couple earthquakes and surficial deformation in volcanic and geothermal areas usually involves expansion sources. given an ensemble of ellipsoidal or tensile expansion sources and double-couple ones, it is straightforward to obtain the equivalent single moment tensor under the far-field approximation. on the contrary, the moment tensor interpretation is by no means unique or unambiguous. if the far-field approximation is unsatisfied, the single moment tensor representation is inappropriate. here we focus on the volume change estimate in the case of single sources, in particular finite pressurized ellipsoidal sources, presenting the expressions for the computation of the volume change and surficial displacement in a closed analytical form. we discuss the implications of different domains of the moment-tensor eigenvalue ratios in terms of volume change computation. we also discuss how the volume change of each source can be obtained from the isotropic component of the total moment tensor, in few cases of coupled sources where the total volume change is null. the new expressions for the computation of the volume change and surficial displacement in case of finite pressurized ellipsoidal sources should make their use easier with respect to the already published formulations. 1. introduction modeling non-double-couple (non-dc) earthquakes and surficial deformation in volcanic and geothermal areas usually involves expansion sources, e.g. microcracks, magma chambers, and dikes. observations of non-dc earthquakes include earthquakes related to landslides and volcanic eruptions, long-period volcanic earthquakes, short-period volcanic and geothermal earthquakes, earthquakes at mines, deep-focus earthquakes, and other shallow earthquakes [for a review, see e.g. miller et al. 1998]. as regards shallow earthquakes in volcanic or geothermal areas and mines, isotropic components are consistent with failure involving both shear and tensile faulting, which may be facilitated by high-pressure, high-temperature fluids, and with cavity closing in mines. also microearthquakes induced by hydraulic fracturing [e.g. sileny et al. 2009] may have significant non-dc parts including a positive isotropic component consistent with opening of cracks oriented close to expected hydraulic fracture orientation, and could be the result of crack opening by fluid injection. for mixed mode earthquakes, which combine shear and tensile dislocations on a planar fault that can possibly be opened or closed during the rupture process, the deviation of the dislocation vector from the fault, i.e. the tensility (or slope) of the source, can be determined uniquely [vavryčuk 2011]; volume change of the source δv is given by m/(m+2n/3), where m is the isotropic (iso) component of the moment tensor (one third its trace), and m and n are the lamé parameters. seismic moment tensors are not always consistent with combined shear and tensile faulting [see e.g. foulger et al. 2004]. as regards source mechanisms of deep low-frequency earthquakes, the isotropic component is often assumed to represent a spherical magma oscillation [e.g. julian et al. 1998, miller et al. 1998], and in this case δv is given by m/(m+2n). very-long-period earthquakes at volcanoes have sometimes been interpreted in terms of a mutual deflation and inflation of two connected magma chambers [e.g. nishimura et al. 2000] whose shape is not necessarily spherical. it is common practice to decompose the moment tensor into iso and trace-free parts, and obtain δv from the isotropic component only. on the other hand wielandt [2003] has demonstrated that δv cannot be obtained from iso without choosing a priori a source model. there is no general relationship between iso (m) and δv, but only between iso and the volume displaced through any spherical surface enclosing the source (i.e. the surface integral of the normal displacement over the spherical surface), which is given by m/(m+2n). this result is true for any position of the point moment tensor within the given sphere and thus remains valid for distributed sources. for a given iso, a article history received august 31, 2012; accepted april 16, 2013. subject classification: crustal deformations, volcano seismology, volcano monitoring, general or miscellaneous, data processing. special issue: vesuvius monitoring and knowledge source model with any volume change, positive or negative, can be hypothesized. only for a single expansion source (e.g., an ellipsoidal cavity) a positive iso can only result in a positive δv. it also follows that the surface integral of the normal displacement over a surface enclosing the source cannot be used to estimate δv, but in case the enclosing surface is exactly the boundary of a single expansion source, like a pressurized cavity or a tensile fault. unfortunately, displacements on the source boundary computed using the point moment tensor approximation of the source are different from the actual ones because the far-field assumption (implicit in the point moment tensor approximation) is obviously unsatisfied. these fundamental results are often not considered in research work. expansion sources are usually modeled as ellipsoids and tensile cracks. amoruso and crescentini [2009] have computed exact expressions for the volume change of a pressurized ellipsoidal cavity in an infinite homogeneous elastic medium that are approximate solutions for a homogeneous half-space. amoruso and crescentini [2009] have also shown that the moment tensor of earthquakes generated by a sudden magma exchange between two ellipsoidal cavities (e.g., because of the breaking of a barrier) can include all the three (iso, dc, compensated linear vertical dipole clvd) components, and the volume change obtained from the isotropic component only can be much smaller than that really involved in the magma exchange process. from the analysis of the clvd and iso components, it is possible to get clues on the source shapes and estimate their volume change. as regards surficial deformation, its analysis is a widely used tool for studying volcanoes, and in particular the critical stages prior to eruptions. the source is often modeled as a single pressurized cavity [e.g. amoruso et al. 2007, amoruso et al. 2008], and the density of the material entering the source may be inferred from gravity data, if the source geometry and volume change are known; this approach may allow discriminating between deformation episodes due to the hydrothermal system and those ascribable to magma intrusion. if the source dimensions are small with respect to depth, surficial deformation is the same as from a single (point) moment tensor, and what previously outlined for non-dc earthquakes still holds. if the source dimensions are not small with respect to depth, a further complication arises because of the small number of analytical solutions for finite sources [for a review, see e.g. lisowski 2006]. the point moment tensor representation is equivalent to the monopole term in the multipole expansion of any extended deformation source, but the resulting far-field approximation may be inappropriate. presently the library of available analytical (approximate or exact) solutions for surface displacements includes few finite cavity shapes [spheres, mctigue 1987; prolate spheroids, yang et al. 1988; fluid-pressurized closed pipes or dislocating open pipes, bonaccorso and davis 1999; circular horizontal cracks, fialko et al. 2001; finite triaxial ellipsoids, amoruso and crescentini 2011] and very small (with respect to depth) cavities [spheres, mogi 1958; generic triaxial ellipsoids, through its representation in terms of a single moment tensor, davis 1986]. assuming homogenous elastic conditions, general geometrical configuration of pressure sources can be obtained from surficial displacement data [e.g. vasco et al. 2002, camacho et al. 2011]. these sources are described as an aggregate of pressure point sources, which are able to model hydrothermal systems [e.g. rinaldi et al. 2010] but not expanding non-spherical cavities or aggregates of cracks. in this work we give closed analytical expressions for computing the volume change of finite pressurized triaxial ellipsoidal sources. we detail that, if a single source is assumed and the moment tensor eigenvalues are consistent with an ellipsoidal source or a mixed mode dislocation, the volume change is proportional to the moment tensor trace, but the proportionality constant depends on the source shape. we also discuss three examples of coupled cavities. at last, we give explicit analytical expressions for computing surficial displacements from a pressurized ellipsoidal cavity, under the quadrupole approximation. 2. estimation of the volume change of expansion sources in this section, we summarize some results about the computation of δv and the moment tensor representation of pressurized ellipsoidal cavities, and give explicit expressions some of which still unpublished. later on, we discuss how the volume change of each source can be obtained from the iso component of the total moment tensor, in few cases of coupled sources where the total volume change is null. 2.1. ellipsoidal cavity the elastic field due to an ellipsoidal inclusion in a homogeneous half-space has been treated by davis [1986], following eshelby’s [1957] approach but using the point force solution for a half-space rather than a full-space (kelvin force) as the fundamental green’s function. the solution satisfies boundary conditions exactly on the free surface but approximately on the ellipsoid, and is reasonably accurate if the depth to the ellipsoid center is larger than twice its dimension. davis’ far-field solution is the sum of displacements amoruso and crescentini 2 3 from three co-located double forces acting along the axes of the ellipsoid. far-field deformation from a pressurized ellipsoidal cavity is thus the same as from a moment tensor, whose eigenvalues are proportional to the product of the source pressure and volume pv and depend on the axis ratios; the normalized eigenvectors of the moment tensor represent the directions of the ellipsoid axes. although δv is of great importance, e.g., for estimating the intrusion density from gravity data, its link to pv is not trivial. if the reference axes are aligned with the ellipsoid axes (a, b, c), davis [1986] showed that far-field deformation from a pressurized ellipsoidal cavity is the same as deformation from the moment tensor ellipsoidal expansion sources m = v ⎛ ⎝ p t a 0 0 0 p tb 0 0 0 p tc ⎞ ⎠ δv = 1 − 2ν 2 (1 + ν) v p μ ( pt p − 3 ) = 1 − 2ν 2 (1 + ν) v p μ ( p ta p + p tb p + p tc p − 3 ) m = 2 1 + ν 1 − 2ν μ δv 1 pt /p − 3 ⎛ ⎝ p t a /p 0 0 0 p tb /p 0 0 0 p tc /p ⎞ ⎠ and gave expressions for computing pa t/p, pb t/p, pc t/p as a function of the ellipsoid axis ratios, in terms of elliptic integrals. amoruso and crescentini [2009] have shown that for an ellipsoidal inclusion where o is the poisson’s ratio, pt = pa t+ pb t + pc t is rotational invariant, being proportional to the trace, and can be calculated for any orientation of the cavity axes, and p is positive for overpressure. in terms of δv, the moment tensor can be written as thus, δv is proportional to the iso component (m) of the moment tensor that represents the far-field displacements from the ellipsoidal source, but the proportionality factor depends on the shape of the ellipsoidal cavity: for a given m, δv can vary up to a factor of about two, depending on the moment tensor eigenvalue ratios. if two of the axes of the ellipsoid are equally long (i.e., the ellipsoid is a spheroid), the eigenvalues of the moment tensor can be computed in terms of elementary functions (for details, see the work of eshelby [1957]). amoruso and crescentini [2009] obtained approximate results for very small or very large axis ratio using appropriate taylor expansions of equations (3.15) and (3.16) in the work by eshelby [1957]. here we give the details of the computations, in particular explicit expressions for pa t/p, pb t/p, pc t/p in terms of elliptic integrals and in algebraic form for a couple of limit cases. these expressions are not new, but are given in a form that might make their use easier with respect to previously published ones. we stress again that they are valid exactly in an infinite homogeneous elastic space and approximately in a half-space. following eshelby [1957] and davis [1986], pa t/p, pb t/p, pc t/p can be obtained by solving the linear system δv = 3 2 1 − 2ν 1 + ν 1 μ ( 1 − 3 p pt ) m a ⎛ ⎝ xy z ⎞ ⎠ = (1 − 2ν) ⎛ ⎝ 11 1 ⎞ ⎠ (1) (2) (3) (4) (5) where x = − pa t/p, y = − pb t/p and z = − pc t/p, is the carlson elliptic integral of the second kind (numerically evaluable, e.g. press et al. [1992]). by the way, there is a typo in both the equation (13) and the system of equations (14) in the work by davis [1986], where the right-hand side of the equation and the known term of the system of equations miss a division by 3. for a spheroid (a = b ≠ c), there are only two unknowns and the previous linear system becomes (since x = y) amoruso and crescentini 4 s11 = qa 2iaa + ria s12 = qb 2iab − ria s13 = qc 2iac − ria s21 = qa 2iba − rib s22 = qb 2ibb + rib s23 = qc 2ibc − rib s31 = qa 2ica − ric s32 = qb 2icb − ric s33 = qc 2icc + ric r = 1 8π 1 − 2ν 1 − ν q = 3 8π 1 1 − ν iab = iba = ib − ia 3 (a2 − b2) iaa = 4π 3a2 − iab − iac ia = 4 3 πabc rd ( b2, c2, a2 ) ic = 4 3 πabc rd ( a2, b2, c2 ) ib = 4π − ia − ic rd(α, β, γ) = 3 2 ∫ ∞ 0 dt (t + γ) 3/2 √ t + α √ t + β b ( y z ) = (1 − 2ν) ( 1 1 ) a = ⎛ ⎝ [(s11 − 1) − νs12 − νs13] [−ν (s11 − 1) + s12 − νs13] [−ν (s11 − 1) − νs12 + s13][s21 − ν (s22 − 1) − νs23] [−νs21 + (s22 − 1) − νs23] [−νs21 − ν (s22 − 1) + s23] [s31 − νs32 − ν (s33 − 1)] [−νs31 + s32 − ν (s33 − 1)] [−νs31 − νs32 + (s33 − 1)] ⎞ ⎠ ( (6) (16) (17) (18) (19) (20) (21) (22) (23) (7) (8) (9) (10) (11) (12) (13) (14) (15) and as in davis [1986] where 5 ellipsoidal expansion sources putting e = c/a, for a = b > c (e < 1, oblate spheroid) and a = b < c (e > 1, prolate spheroid), following eshelby [1957] we have taylor expansions of the expressions for ia = ib in case e <<1 give b = ( [(1 − ν) (s11 + s12 − 1) − 2νs13] [−ν (s11 + s12 − 1) + s13] [(1 − ν) (s31 + s32) − 2ν (s33 − 1)] [−ν (s31 + s32) + (s33 − 1)] ) ia = ib = 2πe (1 − e2)3/2 [ arccos e − e √ 1 − e2 ] ia = ib = 2πe (e2 − 1)3/2 [ e √ e2 − 1 − cosh−1 e ] ic = 4π − 2ia iac = ica = ibc = icb = 1 a2 (e2 − 1) ( ia − 4 3 π ) iab = iba = 1 4a2 (e2 − 1) ( −ia + 4 3 πe2 ) iaa = ibb = 3 4a2 (e2 − 1) ( −ia + 4 3 πe2 ) icc = 2 a2 (e2 − 1) [ −ia + 2π ( 1 − 1 3e2 )] ia = ib = π 2e ic = 4π − 2π2e iac = ica = ibc = icb = 1 a2 ( 4 3 π − π2e ) iab = iba = π2 4a2 e iaa = ibb = 3π2 4a2 e icc = 4π 3c2 b = ⎛ ⎜⎜⎝ ν − 1 − (1 + ν) (4ν − 3) 8 (1 − ν) πe ν − 1 + ν 8 (1 − ν)πe 2ν − 1 + ν 4 (1 − ν)πe −2 ν2 1 − ν − (1 + ν) (4ν − 1) 2 (1 − ν) πe ⎞ ⎟⎟⎠ −p t a p = (1 − 2ν) 4 (1 − ν) 2 (1 + ν) (1 − 2ν)2 1 πe [ −ν (1 + ν) 1 − ν ] = − 4 π 1 − ν 1 − 2ν ν 1 e −p t c p = (1 − 2ν) 4 (1 − ν) 2 (1 + ν) (1 − 2ν)2 1 πe [− (1 + ν)] = − 4 π (1 − ν)2 1 − 2ν 1 e (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) and respectively, and for both cases where taylor expansions of the expressions for ia = ib in case e >> 1 give amoruso and crescentini 6 we have seen that if the moment tensor obtained from seismic and/or deformation data represents a mixed mode dislocation on a fault plane or the expansion of an ellipsoidal cavity, δv is proportional to the iso component of the moment tensor, but the proportionality factor is not a constant. figure 1 shows the ratio (riso) of δv of the real source to the volume change (δviso) of an isotropic source sharing the same value of m. figure 1 also shows domains of possible ratios of eigenvalues for mixed mode (shear and tensile) dislocations on a planar fault (from pure compressive source, slope a = −90°, to pure shear source, slope a = 0°, to pure extensive source, slope a = 90°; vavryčuk [2011]) and ellipsoidal pressurized cavities (light gray area), embedded in an elastic medium with o = 0.25. in case of mixed mode dislocation, again it is possible to compute both δv and the moment tensor in terms of δv: ia = ib = 2π ic = 0 iac = ica = ibc = icb = 2π 3a2e2 iab = iba = π 3a2 iaa = ibb = π a2 icc = 0 b = ⎛ ⎜⎜⎝ −4ν 2 − 3ν + 1 2 (1 − ν) ν 2ν + 1 2e2 −1 − ν 2 (1 − ν) 1 e2 ⎞ ⎟⎟⎠ −p t a p = (1 − 2ν) 2 (1 − ν) (1 + ν) (1 − 2ν)2 [− (1 + ν)] = −2 1 − ν 1 − 2ν −p t c p = (1 − 2ν) 2 (1 − ν) (1 + ν) (1 − 2ν)2 [ − 1 + ν 2 (1 − ν) ] = − 1 1 − 2ν δv = v p μ m = 1 (1 − 2ν)μ δv ⎛ ⎝ 2 (1 − ν) 0 00 2 (1 − ν) 0 0 0 1 ⎞ ⎠ it is thus possible to compute both δv and the moment tensor in terms of δv: δv = (1 − 2ν) 2 (1 + ν) v p μ ( 4 π 1 − ν2 1 − 2ν 1 e − 3 ) � 8 3 a3 (1 − ν) p μ m = 2 (1 − 2ν)μ δv ⎛ ⎝ ν 0 00 ν 0 0 0 1 − ν ⎞ ⎠ (41) (42) (43) (44) (45) (46) (47) (48) (49) (50) (51) (52) (53) where 7 ellipsoidal expansion sources 2.2. coupled cavities when dealing with expansion sources, volumecompensating processes involving mass movements should often be taken into account. for example, in volcanic environments sudden magma movement between coupled cavities can originate seismic waves. amoruso and crescentini [2009] have already shown that source properties retrieved from far-field deformation data for interconnected equally pressurized ellipsoidal cavities are less reliable than for single sources. the case of a source region including ellipsoidal cavities interconnected by narrow fluid-filled conduits (whose contribution to the total volume change is negligible) can be treated under the assumption that the ellipsoidal cavities share the same pressure, and that the distance between cavity surfaces is generally larger than twice the cavity size (thus an ensemble of equally pressurized ellipsoidal cavities can be treated as if each cavity were isolated). under the far-field approximation, moment tensors associated with the cavities inside the volume source can be considered co-located, and the overall volume change is given by it is interesting to note that riso for ellipsoidal sources approaches riso for tensile faults as the ellipsoid degenerates into a penny-shaped crack. unfortunately, as previously mentioned, if the moment tensor cannot represent an ellipsoidal pressurized cavity or a mixed mode dislocation, no univocal estimation of δv is possible. as the ratio of δv of the real source to the volume change of an isotropic source sharing the same value of the product of pressure and volume. we get depends on the eigenvalue ratio and consequently on the ellipsoid shape. amoruso and crescentini [2009] obtained is constant and equal to 1.8 for o = 0.25, independently of the slope of the source. in case of ellipsoidal pressurized cavities riso = 3 1 − ν 1 + ν riso = 3 1 − ν 1 + ν ( 1 − 3 p pt ) rv p = 2 3 1 − 2ν 1 + ν ( pt p − 3 ) riso = 3 1 − ν 1 + ν rv p rv p + 2 (1 − 2ν) / (1 + ν) . δvtot = n∑ i=1 δvi = n∑ i=1 vi p 3k ( pti p − 3 ) = v p 3k ( n∑ i=1 vip t i pv − 3 ) = v p 3k ( pt p − 3 ) = 1 − 2ν 2 (1 + ν) v p μ ( pt p − 3 ) (54) (55) (56) (57) (58) where is also the trace of the moment tensor that represents the far-field displacements from the ellipsoidal sources [amoruso and crescentini 2009]; k = 2n (1 + o) / [3 (1 − 2o)] is the bulk modulus. here we just recall that, if the cavities have different shape and orientation, it is impossible to infer the volume change of an ensemble of ellipsoidal cavities from the equivalent moment tensor, if, as usual, pv cannot be estimated independently. the total moment tensor representation, its decomposition into iso, dc and clvd force systems, and its trace, have been widely used not only to interpret surface deformations but also seismic records in volcanic areas. the iso component is usually attributed to a spherical magma oscillation, the dc component to shear faulting, and the clvd to rapid movement of magmatic fluid [e.g. julian et al. 1998]. in this framework, amoruso and crescentini [2009] have considered the case of two coupled spheroidal cavities, and assumed that for some reason (e.g., fluid migration) volume of both cavities change and total volume change can be different from zero. they considered the case in which the symmetry axes of both cavities are supposed parallel to the z axis of the coordinate system and the case in which the symmetry axes of the two cavities are parallel to the z axis and y axis of the coordinate system respectively. the decomposition of the moment tensor into iso, dc and clvd force systems were performed using the method of knopoff and randall [1970], which makes the major axis of the clvd coincide with the corresponding axis of the dc. amoruso and crescentini [2009] have shown that the moment tensor of earthquakes generated by a sudden magma exchange between two spheroidal cavities (e.g., because of the breaking of a barrier) can include all the three (iso, dc, and clvd) components. in particular, while the iso component is given by the same expression (equation 21 in amoruso and crescentini [2009]), the dc component is null if the symmetry axes of the two cavities are parallel to the z axis and not null in the other case. although the interpretation of the moment tensor decomposition is by no means univocal, results in amoruso and crescentini [2009] can be used to interpret non-dc earthquakes. as an example, they re-examined three deep low-frequency earthquakes occurred beneath iwate volcano, japan, in 1998 and 1999 [nakamichi et al. 2003], and showed that volume-change values obtained from the isotropic component only can be much smaller than those really involved in the magma exchange process. similar computations can be performed also for the moment tensor obtained from the inversion of surficial deformation data and different coupled sources, like spheroidal magma chambers and cracks, but their application to real cases is a case-to-case problem and a-priori assumptions about the involved sources are always necessary. here we focus on the simpler problem of how the volume change of each source can be obtained from the iso component of the total moment tensor, in few cases of coupled sources where the total volume change is null. in other words, the volume change δv1 of the first cavity (here always an ellipsoid) and the volume change δv2 of the second cavity (a very thin oblate spheroid, a very elongated prolate spheroid, or a sphere) are such that δv1 = −δv2 = δv. if m1 and m2 are the iso components of the two sources, the apparent total volume change (δvapp) given by the total iso component is (59) independently of the orientations of the two cavities. in all the described cases, (60) we will see that δvapp is not zero, its sign can be positive or negative, and it is proportional to δv. 2.2.1. ellipsoid and very thin oblate spheroid this case schematizes magma transfer between a magma chamber and a dike. the iso component of the second source is (61) thus (62) figure 2 shows δvapp/δv for different shapes of the ellipsoidal chamber. in this case δvapp/δv is always positive. 2.2.2. ellipsoid and very thin prolate spheroid this case schematizes magma transfer between a magma chamber and a closed conduit. the iso component of the second source is (63) thus (64) figure 3 shows δvapp/δv for different shapes of the ellipsoidal chamber. in this case δvapp/δv can be either positive or negative. amoruso and crescentini 8 δvapp = m1 + m2 λ + 2μ m2 = − 2 3 1 + ν 1 − 2ν μ δv δvapp δv = 1 + ν 1 − ν 1 pt /p − 3 m2 = − 5 − 4ν 3 (1 − 2ν)μδv δvapp δv = 1 2 (1 − ν) 5 − 4ν − (1 − 2ν) pt /p pt /p − 3 m1 = 2 3 1 + ν 1 − 2ν μ pt /p pt /p − 3 δv vp v pt ii n i t 1= =/ 9 2.2.3. ellipsoid and sphere this case schematizes magma transfer between a magma chamber and a secondary chamber. the iso component of the second source is (65) thus (66) figure 4 shows δvapp/δv for different shapes of the ellipsoidal chamber. in this case δvapp/δv is always negative. ellipsoidal expansion sources figure 1 (left). ratio (riso) of δv of the real source to the volume change (δviso) of an isotropic source sharing the same value of the moment-tensor trace; o = 0.25. here m1 and m3 are the absolutely largest and smallest moment-tensor eigenvalues; m2 is the absolutely intermediate one. the light gray area indicates the domain consistent with ellipsoidal sources. the piecewise solid line indicates the domain consistent with mixed-mode planar faults, from pure compressive source (slope a = −90°) to pure shear source (slope a = 0°) to pure extensive source (slope a = 90°). note that |m2|/|m1| and |m3|/|m1| depend on |a| only. figure 2 (right). ratio of the apparent total volume change (δvapp) to the volume change (δv) of an ellipsoidal chamber in case of mass transfer to a dike, when the actual total volume change is null. here a>b>c are the axes of the ellipsoid; o = 0:25. m2 = −2 1 − ν 1 − 2ν μδv δvapp δv = 1 3 (1 − ν) 9 (1 − ν) − 2 (1 − 2ν) pt /p pt /p − 3 figure 3 (left). ratio of the apparent total volume change (δvapp) to the volume change (δv) of an ellipsoidal chamber in case of mass transfer to a closed conduit, when the actual total volume change is null. here a>b>c are the axes of the ellipsoid; o = 0.25. figure 4 (right). ratio of the apparent total volume change (δvapp) to the volume change (δv) of an ellipsoidal chamber in case of mass transfer to a spherical chamber, when the actual total volume change is null. here a>b>c are the axes of the ellipsoid; o = 0.25. 3. surficial displacement generated by finite ellipsoidal cavities in this section we give the expressions for surficial displacement (under the quadrupole approximation) generated by finite ellipsoidal cavities embedded in a homogeneous half-space [amoruso and crescentini 2011], after a brief recall of the necessary basic concepts. amoruso and crescentini [2011] mentioned that explicit expressions (in closed analytical form) for displacements and stresses can be given for homogeneous media but gave a more approximate solution through the combined effects of seven suitable point sources of appropriate location and strength, to make it usable in heterogeneous media. the external displacement field due to a pressurized ellipsoidal cavity (axes a ≥ b ≥ c, parallel to the coordinate axes) embedded in an infinite elastic medium, is the same as that due to a uniform distribution of moment tensors located inside the cavity itself. in this special coordinate system, moment density is [eshelby 1957] (67) for any other system it can be found by the usual law for tensor rotation. the equation is a practically useful approximation also for a semi-infinite medium, provided that the ellipsoid is small with respect to its depth [davis 1986]. thus, surficial displacement (vx, vy, vz) of an observation point (xs, ys, 0), due to a pressurized ellipsoidal cavity centered in (x, y, z) and embedded in a semi-infinite elastic medium (z < 0), is given (approximately) by the volume integration over the ellipsoid (68) where is the green’s function giving i-component of displacement due to a very small (point) ellipsoidal source (unitary pv product) located in = = (x, y, z) on a point at = (xs, ys, 0), and the z-axis is perpendicular to the free plane surface. similar equations also hold for stresses and strains, provided the proper green’s functions are used. taylor series expansion up to order 2 of about the ellipsoid center is amoruso and crescentini 10 vi = p ∫ v gi(�r − �rs, z) dv gi(�r − �rs, z) � gi(�r0 − �rs, z) + (xj − x0j) ( ∂gi(�r − �rs, z) ∂xj ) (�r,z)=(�r0,z0) + + 1 2 (xj − x0j) (xk − x0k) ( ∂2gi(�r − �rs, z) ∂xj∂xk ) (�r,z)=(�r0,z0) in a homogeneous half-space it is possible to express the displacement green’s functions, and thus their second derivatives , in analytical form. here we decompose the moment tensor representing a point ellipsoid into those representing three orthogonal point cracks (tensile faults) whose openings are parallel to the ellipsoid axes. other decompositions (e.g., in terms of double forces) can be used as well. crack potency are thus, since ∫v (xj − x0j) dv = 0 because of symmetry, vi � pv gi(�r0 − �rs, z0) + 1 2 pqjk ( ∂2gi(�r − �rs, z) ∂xj∂xk ) (�r,z)=(�r0,z0) qjk = ∫ v (xj − x0j) (xk − x0k) dv. πa = v p 2μ (1 + ν) ( p ta p − ν p t b p − ν p t c p ) πb = v p 2μ (1 + ν) ( −ν p t a p + p tb p − ν p t c p ) πc = v p 2μ (1 + ν) ( −ν p t a p − ν p t b p + p tc p ) dm = p ⎛ ⎝ p t a /p 0 0 0 p tb /p 0 0 0 p tc /p ⎞ ⎠ dv ,g r r zi s-^ h ,g r r zi s-^ h ,r z^ h ,r 0s^ h , ,r z r z0 0=^ ^^ h hh (69) (70) (71) (72) (73) (74) where ,g r r z x xi s j k 22 2 2-^ h 11 ellipsoidal expansion sources ii) crack perpendicular to axis b i) crack perpendicular to axis a where πa indicates potency of the crack perpendicular to axis a, and so on. we describe the spatial orientation of the ellipsoid by means of the three euler angles a, b, and c, following the convention of z-x-z co-moving axes rotations (x convention in goldstein et al. [2001]) for moving the (x, y, z) reference frame (here x eastward, y northward, z upward) to the (x, y, z) referred frame (x parallel to c, y to b, z to a). because of the ellipsoid symmetry, euler angle ranges can be restricted to −180° ≤ a < 180°, 0° ≤ b ≤ 90°, 0° ≤ c < 180°. direct computations show that strike (z, clockwise from north) and dip (d) of the three cracks are related to the euler angles as follows: where i = x, y, z, π is the tensile fault potency (hereinafter expressed as ∑δu, surface times opening), and ai, bi are given in equations (10) and (11) in okada [1985]. as regards the second derivatives of the displacement, after some tedious algebra we get the following expressions, grouped according to the component of the displacement. using this decomposition, surface displacements and their second derivatives are computable from equations 10, 11, and 12 in okada [1985]. for the sake of clarity we use the cartesian coordinate system by okada [1985], i.e. right-handed, x-axis parallel to strike and z-axis upward. we put r2 = (x0 − x) 2 + (y0 − y) 2 + z2 and q = y sin d − z cos d, where d is the dip of the tensile fault. final expressions can be can be easily implemented in computer codes and results can be transformed in any other coordinate system through proper axis rotation. the three components of the displacement due to a crack share the same structure iii) crack perpendicular to axis c φa = π 2 − α δa = β cos φb = − sin γ cos α + cos β sin α cos γ√ 1 − cos2 γ sin2 β sin φb = sin γ sin α − cos β cos α cos γ√ 1 − cos2 γ sin2 β cos δb = cos γ sin β ⎫⎪⎪⎪⎪⎬ ⎪⎪⎪⎪⎭ if 0◦ ≤ γ < 90◦ cos φb = sin γ cos α + cos β sin α cos γ√ 1 − cos2 γ sin2 β sin φb = − sin γ sin α − cos β cos α cos γ√ 1 − cos2 γ sin2 β cos δb = − cos γ sin β ⎫⎪⎪⎪⎪⎬ ⎪⎪⎪⎪⎭ if 90◦ < γ < 180◦ cos φc = cos γ cos α − cos β sin α sin γ√ 1 − sin2 γ sin2 β sin φc = − cos γ sin α + cos β cos α sin γ√ 1 − sin2 γ sin2 β cos δc = sin γ sin β ui = π 2π [ αi − μ λ + μ βi sin 2 δ ] (75) (76) (77) (78) (79) amoruso and crescentini 12 ∂2ux ∂x∂y = σ δu 2 π [ ∂2u (1) x ∂x∂y − μ λ + μ sin2 δ ∂2u (2) x ∂x∂y ] ∂2u (1) x ∂x∂y = 3q r5 [ 2 sin δ − 5 (y0 − y) q r2 + 5 (x0 − x)2 r2 ( −2 sin δ + 7 (y0 − y) q r2 )] ∂2u (2) x ∂x∂y = 3 (y0 − y) r2 [ − 1 r3 + 1 r (r − z)2 + 2 (r − z)3 − (y0 − y) 2 r (r − z)2 ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) + (x − x0)2 r ( 5 r4 − 3 r2 (r − z)2 − 6 r (r − z)3 − 6 (r − z)4 ) + (x − x0)2 (y0 − y)2 r2 (r − z)2 ( 5 r3 + 10 r2 (r − z) + 12 r (r − z)2 + 8 (r − z)3 )] ∂2ux ∂x∂z = σ δu 2 π [ ∂2u (1) x ∂x∂z − μ λ + μ sin2 δ ∂2u (2) x ∂x∂z ] ∂2u (1) x ∂x∂z = 3q r5 [ −2 cos δ + 5zq r2 + 5 (x0 − x)2 r2 ( 2 cos δ − 7zq r2 )] ∂2u (2) x ∂x∂z = 1 r2 [ 3z r3 + 1 r (r − z) + 1 (r − z)2 − (y0 − y) 2 r (r − z) ( 3 r2 + 3 r (r − z) + 2 (r − z)2 ) 3.1. x-component ∂2ux ∂x2 = σ δu 2 π [ ∂2u (1) x ∂x2 − μ λ + μ sin2 δ ∂2u (2) x ∂x2 ] ∂2u (1) x ∂x2 = − 15 q 2 (x0 − x) r7 [ 3 − 7 (x0 − x) 2 r2 ] ∂2u (2) x ∂x2 = 3 (x0 − x) r2 [ − 3 r3 + 1 r (r − z)2 + 2 (r − z)3 + (x − x0)2 r ( 5 r4 − 1 r2 (r − z)2 − 2 r (r − z)3 − 2 (r − z)4 ) − 3 (y0 − y) 2 r (r − z)2 ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) + (x − x0)2 (y0 − y)2 r2 (r − z)2 ( 5 r3 + 10 r2 (r − z) + 12 r (r − z)2 + 8 (r − z)3 )] (80) (82) (85) (86) (87) (84) (83) (81) where where where and and and 13 ellipsoidal expansion sources −(x − x0) 2 r ( 15z r4 + 3 r2 (r − z) + 3 r (r − z)2 + 2 (r − z)3 ) +3 (x − x0)2 (y0 − y)2 r2 (r − z) ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] ∂2ux ∂y2 = σ δu 2 π [ ∂2u (1) x ∂y2 − μ λ + μ sin2 δ ∂2u (2) x ∂y2 ] ∂2u (1) x ∂y2 = 3 (x0 − x) r5 [ 2 sin2 δ − 20 sin δ q (y0 − y) r2 − 5 q 2 r2 + 35 q2 (y0 − y)2 r4 ] ∂2u (2) x ∂y2 = 3 (x0 − x) r2 [ − 1 r3 + 1 r (r − z)2 + 2 (r − z)3 + (y0 − y)2 r ( 5 r4 − 6 r2 (r − z)2 − 12 r (r − z)3 − 12 (r − z)4 ) + (y0 − y)4 r2 (r − z)2 ( 5 r3 + 10 r2 (r − z) + 12 r (r − z)2 + 8 (r − z)3 )] ∂2ux ∂y∂z = σ δu 2 π [ ∂2u (1) x ∂y∂z − μ λ + μ sin2 δ ∂2u (2) x ∂y∂z ] ∂2u (1) x ∂y∂z = 3 (x0 − x) r5 [ −2 cos δ sin δ + 10 sin δ zq r2 + 5 (y0 − y) q r2 ( 2 cos δ − 7zq r2 )] ∂2u (2) x ∂y∂z = 3 (x0 − x) (y0 − y) r3 [ − 5z r4 − 3 r2 (r − z) − 3 r (r − z)2 − 2 (r − z)3 + (y0 − y)2 r (r − z) ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] ∂2ux ∂z2 = σ δu 2 π [ ∂2u (1) x ∂z2 − μ λ + μ sin2 δ ∂2u (2) x ∂z2 ] ∂2u (1) x ∂z2 = 3 (x0 − x) r5 [ 2 cos2 δ − 20 cos δ q z r2 − 5 q 2 r2 + 35 q2 z2 r4 ] ∂2u (2) x ∂z2 = 3 (x0 − x) r3 [ − 1 r2 − 2 r (r − z) + 1 (r − z)2 + 5 (y0 − y)2 r (r − z)2 ( 3 r − 2 (r − z) ) (88) (89) (90) (91) (92) (93) (94) (95) (96) where where where and and and amoruso and crescentini 14 ∂2uy ∂x∂y = σ δu 2 π [ ∂2u (1) y ∂x∂y − μ λ + μ sin2 δ ∂2u (2) y ∂x∂y ] ∂2u (1) y ∂x∂y = −15q (x0 − x) r7 [ 2 (y0 − y) sin δ + q − 7 (y0 − y)2 q r2 ] ∂2u (2) y ∂x∂y = − 3 (x0 − x) r2(r − z)2 [ 1 r + 2 (r − z) −3 (y0 − y) 2 r ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) −(x − x0) 2 r ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) + (x − x0)2 (y0 − y)2 r2 ( 5 r3 + 10 r2 (r − z) + 12 r (r − z)2 + 8 (r − z)3 )] ∂2uy ∂x∂z = σ δu 2 π [ ∂2u (1) y ∂x∂z − μ λ + μ sin2 δ ∂2u (2) y ∂x∂z ] ∂2u (1) y ∂x∂z = 15q(x0 − x) (y0 − y) r7 [ 2 cos δ − 7qz r2 ] ( ) + z2 r2 ( 5 r2 − 1 (r − z)2 ) + 5 (y0 − y)2 z2 r3 (r − z)2 ( 1 r + 2 (r − z) )] 3.2. y-component ∂2uy ∂x2 = σ δu 2 π [ ∂2u (1) y ∂x2 − μ λ + μ sin2 δ ∂2u (2) y ∂x2 ] ∂2u (1) y ∂x2 = −15 q 2 (y0 − y) r7 [ 1 − 7 (x − x0) 2 r2 ] ∂2u (2) y ∂x2 = 3 (y0 − y) r2 (r − z)2 [ − 1 r − 2 (r − z) + 6 (x0 − x)2 r ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) −(x0 − x) 4 r2 ( 5 r3 + 10 r2 (r − z) + 12 r (r − z)2 + 8 (r − z)3 )] (97) (98) (99) (100) (101) (102) (103) (104) (105) where where where and and 15 ellipsoidal expansion sources ∂2u (2) y ∂x∂z = 3(x0 − x) (y0 − y) r3(r − z) [ 3 r2 + 3 r (r − z) + 2 (r − z)2 −(x − x0) 2 r ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] ∂2uy ∂y2 = σ δu 2 π [ ∂2u (1) y ∂y2 − μ λ + μ sin2 δ ∂2u (2) y ∂y2 ] ∂2u (1) y ∂y2 = 3 r5 [ 2 sin2 δ (y0 − y) + 4 sin δ q − 20 sin δ q (y0 − y)2 r2 − 15 q 2 (y0 − y) r2 + 35 q2 (y0 − y)3 r4 ] ( ) ∂2u (2) y ∂y2 = 3 (y0 − y) r2 (r − z)2 [ − 1 r − 2 (r − z) + (y − y0)2 r ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) + 3 (x0 − x)2 r ( 1 r2 + 2 r (r − z) + 2 (r − z)2 ) −(x0 − x) 2 (y − y0)2 r2 ( 5 r3 + 10 r2 (r − z) + 12 r (r − z)2 + 8 (r − z)3 )] ∂2uy ∂y∂z = σ δu 2 π [ ∂2u (1) y ∂y∂z − μ λ + μ sin2 δ ∂2u (2) y ∂y∂z ] ∂2u (1) y ∂y∂z = 3 r5 [ −2q cos δ + 5q 2z r2 − 2 (y0 − y) sin δ ( cos δ − 5qz r2 ) + 5 (y0 − y)2 q r2 ( 2 cos δ − 7qz r2 )] ∂2u (2) y ∂y∂z = 1 r2(r − z) [ − 1 r − 1 (r − z) + (y0 − y)2 r ( 3 r2 + 3 r (r − z) + 2 (r − z)2 ) + (x − x0)2 r ( 3 r2 + 3 r (r − z) + 2 (r − z)2 ) −3(x − x0) 2 (y0 − y)2 r2 ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] ∂2uy ∂z2 = σ δu 2 π [ ∂2u (1) y ∂z2 − μ λ + μ sin2 δ ∂2u (2) y ∂z2 ] (106) (107) (108) (109) (110) (111) (112) (113) where where and and and amoruso and crescentini 16 ∂2uz ∂x∂y = σ δu 2 π [ ∂2u (1) z ∂x∂y − μ λ + μ sin2 δ ∂2u (2) z ∂x∂y ] ∂2u (1) z ∂x∂y = 15q(x0 − x) z r7 [ 2 sin δ − 7q (y0 − y) r2 ] ∂2u (2) z ∂x∂y = 3(x0 − x) (y0 − y) r3(r − z) [ 3 r2 + 3 r (r − z) + 2 (r − z)2 −(x − x0) 2 r ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] ∂2uz ∂x∂z = σ δu 2 π [ ∂2u (1) z ∂x∂z − μ λ + μ sin2 δ ∂2u (2) z ∂x∂z ] ∂2u (1) z ∂x∂z = −15q(x0 − x) r7 [ 2z cos δ + q − 7qz 2 r2 ] ∂2u (1) y ∂z2 = 3 (y0 − y) r5 [ 2 cos2 δ − 20 cos δ q z r2 − 5 q 2 r2 + 35 q2 z2 r4 ] ∂2u (2) y ∂z2 = 3 (y0 − y) r3 (r − z) [ 2 r − 1 (r − z) + z2 r2 (r − z) + 5 (x0 − x)2 r (r − z) ( − 3 r + 2 (r − z) ) −5 (x0 − x) 2 z2 r3 (r − z) ( 1 r + 2 (r − z) )] where 3.3. z-component ∂2uz ∂x2 = σ δu 2 π [ ∂2u (1) z ∂x2 − μ λ + μ sin2 δ ∂2u (2) z ∂x2 ] ∂2u (1) z ∂x2 = 15 q2 z r7 [ 1 − 7 (x − x0) 2 r2 ] ∂2u (2) z ∂x2 = 3 r2 (r − z) [ − 1 r − 1 (r − z) + 2 (x0 − x)2 r ( 3 r2 + 3 r (r − z) + 2 (r − z)2 ) −(x0 − x) 4 r2 ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] (114) (115) (116) (117) (118) (119) (120) (121) (122) (123) where where where and and 17 ellipsoidal expansion sources ∂2u (2) z ∂x∂z = 3(x0 − x) r5 [ 3 − 5 (x0 − x) 2 r2 ] ∂2uz ∂y2 = σ δu 2 π [ ∂2u (1) z ∂y2 − μ λ + μ sin2 δ ∂2u (2) z ∂y2 ] ∂2u (1) z ∂y2 = −3 z r5 [ 2 sin2 δ − 20 sin δ q (y0 − y) r2 − 5 q 2 r2 + 35 q2 (y − y0)2 r4 ] ∂2u (2) z ∂y2 = 1 r2 (r − z) [ − 1 r − 1 (r − z) + (x0 − x)2 r ( 3 r2 + 3 r (r − z) + 2 (r − z)2 ) + (y0 − y)2 r ( 3 r2 + 3 r (r − z) + 2 (r − z)2 ) −3 (x0 − x) 2 (y − y0)2 r2 ( 5 r3 + 5 r2 (r − z) + 4 r (r − z)2 + 2 (r − z)3 )] ∂2uz ∂y∂z = σ δu 2 π [ ∂2u (1) z ∂y∂z − μ λ + μ sin2 δ ∂2u (2) z ∂y∂z ] ∂2u (1) z ∂y∂z = 3 r5 [ 2z cos δ sin δ + 2q sin δ − 10qz 2 sin δ r2 − 5q (y0 − y) r2 ( 2z cos δ + q − 7qz 2 r2 )] ∂2u (2) z ∂y∂z = 3(y0 − y) r5 [ 1 − 5 (x0 − x) 2 r2 ] ∂2uz ∂z2 = σ δu 2 π [ ∂2u (1) z ∂z2 − μ λ + μ sin2 δ ∂2u (2) z ∂z2 ] ∂2u (1) z ∂z2 = 3 r5 [ −2 cos2 δ z − 4 cos δ q + 20 cos δ q z 2 r2 + 15 q2 z r2 − 35 q 2 z3 r4 ] ∂2u (2) z ∂z2 = 3 r3 [ 1 r − 1 (r − z) + z2 r2 (r − z) + 5 (x0 − x)2 r (r − z) ( − 1 r + 1 (r − z) ) −5 (x0 − x) 2 z2 r3 (r − z) ( 1 r + 1 (r − z) )] (124) (129) (130) (131) (132) (133) (128) (125) (126) (127) where where where and and and and these closed analytical expressions can be easily implemented in any computation code. they are valid only in the okada coordinate system, which is different from crack to crack, but can be used to obtain the correct results in any other coordinate system. the procedure is essentially the following: 1. given the euler angles of the ellipsoid, determine dip and strike of the cracks that are perpendicular to the ellipsoid axes; 2. determine crack potency for unitary pv; 3. for each crack, compute the components of the displacement vector and the elements of the secondderivative third-order tensor in the okada coordinate system using expressions in our work; 4. for each crack, compute the rotation matrix from the okada coordinate system to the reference one; 5. for each crack, use the rotation matrix from step 4 to compute the components of the displacement vector and the elements of the second-derivative third-order tensor in the reference coordinate system. if r is the proper rotation matrix from the cartesian coordinate system in okada [1985] to the reference one, the elements of the second-derivative third-order tensor can be transformed by using the expression (134) as previously mentioned, r depends on the euler angles identifying the ellipsoid orientation and is different from crack to crack. 4. conclusions with respect to already published material, here we have presented the explicit expressions for the computation of the volume change and the generated surface deformation for finite pressurized ellipsoidal sources, in a closed analytical form to make their use easier in comparison to previous formulations. we have also evidenced and discussed a few results from the literature, that are often ignored, on the computation of the volume change for ellipsoidal sources and mixed mode dislocations on a fault; ignoring these findings might bias derived estimates (e.g. intrusion densities for volcanic environments). with the same aims, we have discussed how the volume change of each source can be obtained from the iso component of the total moment tensor, in few cases of coupled sources where the total volume change is null. acknowledgements. we are grateful to verdiana botta and ilaria sabbetta for checking equations. some computations have also been checked using maxima (http://maxima.sourceforge.net). figures have been generated using open-source software (gmt, wessel and smith [1998], and grace, http://plasma-gate.weizmann.ac.il/grace). references amoruso, a., l. crescentini, a.t. linde, i.s. sacks, r. scarpa and p. romano (2007). a horizontal crack in a layered structure satisfies deformation for the 2004-2006 uplift of campi flegrei, geophys. res. lett., 34, l22313; doi:10.1029/2007gl031644. amoruso, a., l. crescentini and g. berrino (2008). simultaneous inversion of deformation and gravity changes in a horizontally layered half-space: evidences for magma intrusion during the 1982-1984 unrest at campi flegrei caldera (italy), earth planet. sci. lett., 272, 181-188; 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(2003). on the relationship between seismic moment and source volume; http://www.soft ware-for-seismometry.de/textfiles/theoreticalseis mology/volsource.ps. yang, x., p.m. davis and j.h. dieterich (1988). deformation from inflation of a dipping finite prolate spheroid in an elastic half-space as a model for volcanic stressing, j. geophys. res., 93, 4249-4257. *corresponding author: antonella amoruso, università di salerno, dipartimento di fisica, fisciano (salerno), italy; email: antonella.amoruso@sa.infn.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ellipsoidal expansion sources << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice annals of geophysics, 65, 6, se639, 2022; doi:10.4401/ag-8387 appendix to a refined catalogue of focal mechanisms for the intra-carpathian region of romania: implications for the stress field and seismogenic features assessment eugen oros1, mihaela popa1, 2, paulescu daniel1, anica otilia placinta1, cristian ghita1 1 national institute for earth physics, calugareni street, no. 12, magurele 077125, ilfov, romania 2 academy of romanian scientists, ilfov street, no. 3, sector 5, 030167 bucharest, romania table 3. the catalogue of focal mechanisms for the intra-carpathian region of romania. np1 and np2 = nodal planes; az and pl = azimuth and plunge of p, t and b axes, no pol and no s/p= the number of polarities and s/p waves ratio; q = the quality of the solutions; fs = faulting style; obs = details about the used procedures and methods, reference sources for imported mechanisms (details at the end of the table). date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1909 8 31 21 32 27.7 45.188 21.862 14.0 5.1 43 41 -149 108 70 -54 240 51 352 17 94 34 b ns o19 1910 10 11 11 54 3.3 45.005 22.479 16.2 5.2 75 82 -164 163 75 -8 300 73 208 5 101 73 b ss o19 1915 10 9 21 24 58.5 45.382 21.177 4.1 5.1 190 77 -163 97 74 -13 54 21 323 2 228 69 b ss o19 1915 10 19 8 43 54.6 45.388 21.109 12.7 5.4 100 80 -5 191 86 -170 56 10 325 4 214 79 b ss o19 1916 1 26 7 38 21 45.467 24.531 18.4 6.6 284 70 -158 185 66 -22 145 32 53 3 320 51 14 0 a ns 1936 9 6 4 48 59.8 45.660 21.098 8.4 5.1 81 52 -23 186 72 -140 50 41 309 12 206 46 a ns mti* 1938 7 8 6 32 56.5 45.942 20.874 16.8 5.2 348 33 -174 253 87 -57 193 39 316 34 72 33 8 0 a ss 1941 8 30 4 41 46.2 45.913 20.913 17.9 5.2 203 33 13 102 83 122 166 30 43 43 277 32 6 0 a ss 1959 5 27 20 38 26.2 45.629 21.072 8.6 5.3 245 58 -148 137 63 -36 99 44 192 3 285 46 29 0 a ns 1960 10 22 19 17 48.9 45.680 21.153 19.8 5.1 168 45 -148 54 68 -50 9 50 116 14 217 37 9 0 a ns 1 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1969 4 12 20 38 42.0 45.457 25.180 5.2 5.2 228 80 -162 135 72 -10 93 20 1 6 255 69 18 0 a ss 1974 4 17 1 31 34.8 45.992 21.062 17.2 5.1 173 71 -168 79 79 -19 35 21 127 5 230 68 15 0 a ss 1978 1 4 5 31 45.2 45.086 21.228 15.1 4.2 101 53 -148 350 65 -42 310 47 48 7 144 42 6 0 d ns 1978 1 17 2 29 31.3 45.777 21.104 13.4 4.1 350 59 157 92 70 33 219 7 314 37 120 52 6 0 c ts 1978 6 22 2 33 24.9 46.751 21.175 6.0 4.6 265 37 17 161 80 126 224 26 106 44 334 35 23 0 a ss 1978 6 22 2 57 55.6 46.779 21.134 14.4 4.0 175 56 168 272 80 35 39 16 139 31 286 54 9 0 b ss 1978 6 22 5 30 59.1 46.776 21.328 18.3 3.7 49 30 2 317 89 120 21 37 254 39 136 30 5 0 d ss 1978 6 30 1 15 30.5 47.668 23.198 9.0 4.0 128 33 122 271 62 71 15 15 145 67 280 17 5 0 d ts 1979 3 8 1 20 14.0 47.724 23.356 12.2 3.4 186 41 77 23 50 101 105 5 347 80 196 9 b tf co80 1979 3 30 15 56 21.3 47.832 23.679 12.8 4.7 110 86 138 204 48 6 165 25 58 32 285 47 15 0 a ts 1979 9 22 18 6 12.8 48.112 23.468 8.7 4.0 29 24 139 157 75 71 262 27 43 56 162 18 9 0 c ts 1981 3 3 15 41 50.0 47.483 24.189 12.6 3.9 251 83 86 101 8 120 345 38 156 52 252 4 b tf ra02 1982 7 1 5 50 2.0 48.309 22.267 9.5 4.5 22 82 157 115 67 9 70 10 336 22 183 66 19 0 a ts 1983 1 27 1 1 1.0 44.080 21.630 10.0 3.9 213 83 -94 63 8 -60 118 52 307 38 214 4 b nf ml14 1983 3 3 15 40 8.8 47.483 24.189 12.6 3.9 251 83 86 101 8 120 345 38 156 52 252 4 b tf ra02 1983 3 16 7 58 1.0 45.400 25.050 1.0 3.9 53 89 93 161 3 18 140 44 326 46 233 3 b tf ra02 1983 4 1 0 28 27.2 46.918 22.224 19.3 3.5 197 11 33 74 84 99 156 38 354 50 253 9 7 0 b ts 1983 5 11 4 37 35.8 48.014 23.914 14.5 2.8 254 58 -167 157 79 -33 111 31 209 14 320 55 8 0 c ss 1983 7 21 15 1 1.4 45.096 23.784 9.5 3.5 293 50 163 34 77 41 158 17 262 38 49 47 8 0 c ss 1983 9 4 13 39 20.4 45.157 23.777 13.3 3.7 347 43 21 241 76 131 301 20 191 44 48 39 11 0 b ss 1983 10 4 22 17 46.6 47.764 22.418 16.8 3.3 119 51 -142 3 61 -46 326 52 63 6 158 37 9 0 a ns 1983 11 5 6 56 51.8 46.959 22.223 14.8 3.1 242 42 177 334 88 48 97 30 210 33 336 42 10 0 c ss 1983 11 6 12 35 11.3 46.921 25.106 9.7 3.0 206 61 175 298 86 29 68 17 166 23 305 61 8 0 d ss 1983 11 14 16 22 1.0 45.200 25.350 48.0 2.7 53 66 -116 283 35 -46 284 60 162 17 64 24 11 b ns ra02 1984 5 20 20 58 14.6 45.537 24.381 15.8 4.5 141 50 -148 29 66 -45 348 48 88 10 187 40 16 0 a ns 1984 8 10 14 45 51.5 45.093 21.816 2.9 3.1 240 51 -137 120 58 -48 86 55 181 4 274 35 5 0 d ns 1984 9 7 0 43 46.1 43.750 21.270 10.0 5.1 10 81 -168 278 78 -9 234 15 144 2 47 75 a ss hrvd 1984 12 20 10 13 16.5 45.848 22.562 14.6 3.0 240 51 -108 87 42 -69 91 76 342 5 251 13 6 0 d nf 1984 12 20 12 42 39.6 47.107 25.172 5.0 3.0 355 58 131 116 50 43 57 5 320 56 150 34 9 0 a ts 1984 12 20 14 34 33.4 45.349 24.992 19.9 2.9 330 33 -169 231 84 -57 172 42 295 31 47 32 10 0 b ss 1985 2 1 6 35 1.0 45.200 25.400 50.0 2.8 268 49 -67 55 46 -114 247 73 342 2 73 17 b ts ra02 1985 3 13 14 18 21.0 45.204 23.109 17.4 2.8 312 19 72 151 72 96 236 27 70 63 329 6 10 0 b ns 2 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1985 9 12 8 1 3.6 47.003 22.564 5.0 3.2 106 24 -172 9 87 -66 302 43 78 37 188 24 9 0 b ss 1985 10 18 12 4 53.6 46.938 21.096 13.3 3.2 353 63 150 98 64 31 225 1 316 39 134 51 8 0 b ts 1985 10 21 18 32 57.9 45.515 22.595 15.8 3.1 15 83 -156 282 66 -8 241 22 146 12 29 65 8 b ns ra02 1985 11 11 14 58 33.0 45.056 25.184 15.0 3.1 298 26 -45 70 72 -109 314 59 175 25 76 18 7 0 c ns 1986 12 2 22 44 1.0 45.010 25.520 31.0 3.4 238 84 -98 111 10 -37 139 50 335 38 239 8 14 b nf ra02 1987 4 29 7 48 57.7 46.855 22.809 19.7 3.0 56 51 -8 151 84 -141 21 31 277 22 158 50 8 0 c ss 1987 7 1 15 59 15.5 45.109 25.053 10.8 2.8 42 43 -1 133 89 -133 8 32 257 30 134 43 8 0 b ss 1987 11 16 12 13 51.5 44.995 22.512 12.3 3.0 143 75 -147 43 58 -18 7 34 270 11 165 54 11 0 b ns 1988 3 12 23 10 0.0 45.260 25.480 50.0 3.3 98 86 -128 3 38 -6 335 37 218 31 100 38 20 a ns ra02 1988 4 22 1 23 22.4 45.498 20.967 5.0 3.8 127 24 23 16 81 112 87 32 310 49 192 22 8 0 b ts 1988 5 12 7 8 21.4 45.329 23.072 11.6 3.3 120 45 -176 27 87 -45 333 32 83 28 205 45 10 0 b ss 1988 5 27 14 18 1.0 44.150 21.530 13.4 4.6 10 49 45 247 58 129 310 5 212 57 43 32 b ts ma02 1988 5 28 2 23 1.0 44.050 21.710 10.0 3.6 271 48 -67 59 47 -113 253 73 345 1 75 17 b ns ml14 1988 7 20 17 32 38.7 45.416 24.845 10.1 2.3 267 12 49 129 81 98 212 36 48 53 308 8 20 0 a ts 1988 11 29 1 23 38.8 45.674 21.587 6.3 3.9 83 66 -169 348 80 -24 303 24 37 9 146 64 21 0 a ss 1988 12 21 3 54 50.8 47.751 22.466 15.5 3.0 82 51 174 176 85 39 302 23 47 30 181 50 10 0 b ss 1989 2 1 1 54 13.9 44.928 20.614 7.7 3.9 169 41 -151 56 71 -53 7 50 119 18 222 35 8 0 b ns 1989 2 25 15 43 54.8 47.539 22.295 10.3 4.4 130 82 -156 36 66 -9 355 23 261 11 147 64 10 0 a ss 1989 3 10 16 24 59.0 46.660 23.460 5.0 3.7 33 59 -13 130 79 -148 356 30 258 13 147 57 7 c ss ra02 1989 4 9 4 42 0.0 44.270 21.710 10.0 3.0 190 49 12 92 81 138 148 21 42 35 263 47 b ss ml14 1989 8 4 8 29 43.8 46.210 25.499 5.0 3.1 186 60 -168 90 80 -31 44 28 141 13 253 59 10 0 c ss 1989 8 12 19 29 26.9 46.663 22.350 11.6 3.1 217 22 33 96 78 109 171 31 28 53 272 18 12 0 b ts 1989 11 24 16 41 17.6 46.821 25.269 11.7 4.1 63 74 -179 333 89 -16 287 12 19 11 151 74 14 0 b ss 1989 12 6 0 48 56.4 46.724 25.083 15.3 4.3 170 69 -147 67 59 -25 32 38 297 6 199 51 21 0 a ns 1990 2 15 10 18 33.3 45.440 22.180 5.0 3.9 267 87 137 360 47 4 322 26 214 31 84 47 6 0 d ts 1990 4 17 4 35 23.0 44.905 22.881 14.2 3.1 258 41 23 150 75 129 212 21 99 46 319 37 14 0 b ts 1990 10 2 17 28 22.6 44.563 22.270 8.0 3.5 191 54 175 284 86 36 52 21 154 28 290 54 11 0 c ss 1990 12 8 4 38 44.3 45.681 24.269 17.6 3.0 350 36 -20 96 78 -124 331 46 212 25 104 33 8 0 c ss 1991 1 26 17 30 32.4 45.318 23.218 10.0 3.4 35 74 -137 291 49 -21 262 41 158 16 51 45 b ns ra02 1991 6 26 15 27 1.0 45.330 21.150 12.2 3.2 174 65 -173 81 84 -25 35 22 130 13 248 64 b ss mr10 1991 7 12 10 42 22.7 45.379 21.097 12.6 5.6 9 89 -168 279 78 -1 235 9 143 8 12 78 a ss hrvd 1991 7 12 11 12 15.7 45.406 21.172 11.0 2.8 234 44 -170 137 83 -46 84 36 194 25 310 44 7 0 d ss 3 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1991 7 12 12 9 31.5 45.325 21.158 14.3 2.8 314 36 9 217 85 126 278 31 159 39 34 36 6 0 d ss 1991 7 12 16 29 10.0 45.426 21.124 12.4 4.1 100 68 -171 7 82 -22 322 21 55 10 169 67 29 0 a ss 1991 7 12 20 42 8.8 45.365 21.263 6.3 3.6 160 59 -175 67 85 -31 19 25 118 18 240 58 9 0 c ss 1991 7 13 4 33 46.6 45.398 21.184 14.9 3.4 355 47 13 256 81 136 313 22 206 36 68 46 13 0 a ss 1991 7 13 5 10 34.4 45.398 21.141 13.8 3.4 198 61 -161 99 73 -30 56 33 151 8 253 56 7 0 d ss 1991 7 13 14 6 25.3 45.420 21.152 12.2 3.7 259 28 -77 64 63 -97 318 71 159 18 67 6 10 0 a nf 1991 7 13 17 27 35.9 45.392 21.179 16.1 4.0 256 30 -29 12 76 -117 251 52 123 26 19 26 25 0 a ns 1991 7 13 17 38 40.7 45.476 21.198 9.4 3.4 192 59 -154 88 68 -34 47 39 142 6 239 50 8 0 b ns 1991 7 13 17 56 58.6 45.374 21.224 14.4 3.6 321 36 -141 198 68 -60 147 57 266 18 6 27 18 0 a ns 1991 7 13 19 3 4.1 45.413 21.217 9.4 3.8 327 21 -19 75 83 -110 324 48 182 35 77 20 13 0 a ss 1991 7 14 17 3 27.8 45.412 21.177 9.9 3.5 209 30 177 302 89 60 58 37 185 39 303 30 16 0 a ss 1991 7 14 23 59 32.5 45.439 21.315 18.2 4.1 171 63 -151 67 64 -30 29 39 119 1 210 51 24 0 a ns 1991 7 15 15 45 4.0 45.390 21.121 12.2 4.0 225 26 -116 73 67 -78 4 66 154 21 248 11 11 0 b ns 1991 7 18 11 56 32.9 44.951 22.467 14.5 5.6 75 52 -124 303 49 -54 282 64 188 2 97 26 98 a ns hrvd 1991 7 18 12 3 31.3 44.768 22.290 14.9 3.2 298 55 -36 51 61 -139 267 49 173 4 80 41 15 0 b ns 1991 7 18 14 3 47.6 44.681 22.328 9.8 3.4 233 22 -79 41 68 -94 303 66 134 23 42 4 10 0 c nf 1991 7 18 15 13 55.3 45.466 21.191 11.5 3.1 250 26 -109 91 66 -81 18 68 174 20 267 8 6 0 d nf 1991 7 18 17 2 10.7 44.928 22.457 8.2 3.1 185 81 -170 93 80 -9 49 14 319 1 225 76 7 0 d ss 1991 7 18 21 51 6.3 44.981 22.477 13.2 2.9 210 57 -163 111 76 -34 66 34 164 12 271 53 9 0 c ss 1991 7 19 1 19 54.8 45.315 21.159 6.8 4.2 56 74 -146 315 57 -19 280 36 182 11 78 52 36 0 a ns 1991 7 19 1 27 34.3 45.282 21.140 6.1 5.1 199 77 -163 105 73 -14 63 21 332 3 234 69 53 a ss hrvd 1991 7 19 2 23 49.8 45.327 21.072 7.9 3.3 202 44 -175 108 87 -46 54 33 165 28 286 44 8 0 c ss 1991 7 19 2 43 21.7 45.334 21.235 5.0 3.8 182 30 -113 27 62 -78 323 70 108 16 201 11 10 0 b ns 1991 7 19 5 24 21.1 45.276 21.110 9.6 3.4 19 87 -149 287 59 -4 248 24 149 19 25 59 13 0 a ts 1991 7 19 8 6 34.6 45.401 21.190 7.5 3.5 165 61 174 258 85 29 28 16 125 24 267 61 9 0 b ss 1991 7 19 15 42 23.3 45.080 22.250 13.5 3.1 152 67 165 248 76 24 19 6 112 27 277 62 10 0 b ss 1991 7 19 16 19 28.0 44.988 22.392 8.2 3.1 350 19 -9 89 87 -109 340 45 196 39 90 19 9 0 a ss 1991 7 20 3 36 31.8 45.296 21.120 11.7 4.1 93 31 111 249 61 78 348 15 132 71 255 11 20 0 a tf 1991 7 20 3 58 47.9 45.337 21.145 7.6 4.1 188 72 -143 85 55 -22 52 39 313 11 210 49 21 0 a ns 1991 7 22 15 45 15.0 45.406 21.149 14.9 3.4 356 54 176 88 87 36 216 22 318 27 92 54 8 0 b ss 1991 7 29 20 46 35.0 44.823 22.437 15.9 3.3 181 50 -175 88 86 -40 37 30 142 24 264 50 7 0 c ss 1991 7 31 11 22 52.9 45.317 21.129 8.9 4.3 76 78 -141 336 52 -16 303 36 201 17 90 49 16 0 a ns 4 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1991 8 1 12 57 24.1 45.422 21.142 13.1 3.6 213 66 -151 110 64 -27 72 37 341 1 250 53 7 0 d ns 1991 8 1 17 30 2.2 45.381 21.083 11.3 3.2 337 20 -38 103 78 -106 353 54 206 31 106 16 6 0 d ns 1991 8 2 0 51 5.2 45.268 21.179 13.5 4.0 317 72 -173 225 82 -18 180 18 272 7 22 71 21 0 a ss 1991 8 6 15 4 46.2 45.424 21.133 5.0 3.9 251 67 179 341 89 23 114 15 208 17 345 67 15 0 a ss 1991 8 7 19 24 1.4 45.289 21.158 14.3 3.7 278 77 -153 181 64 -15 142 28 47 9 301 60 22 0 a ns 1991 8 11 21 25 50.7 45.445 21.217 8.7 3.4 227 43 -151 115 71 -51 68 48 177 17 280 37 6 0 d ns 1991 8 12 4 59 41.0 45.405 21.151 10.3 4.3 300 73 -174 208 84 -17 163 16 255 8 11 72 38 0 a ss 1991 8 14 23 36 2.6 45.455 21.336 9.7 4.8 255 47 69 104 47 111 180 0 90 75 270 90 21 0 b tf 1991 8 15 1 34 34.9 45.417 21.283 6.0 4.0 252 55 179 343 89 35 112 23 213 25 345 55 14 0 a ss 1991 8 16 19 35 23.3 45.425 21.278 13.0 3.8 293 32 -40 58 70 -116 294 57 167 21 67 24 10 0 b ns 1991 8 18 23 1 44.3 45.403 21.138 14.4 3.4 199 71 178 290 88 19 63 12 156 15 296 71 6 0 d ss 1991 9 2 10 12 2.5 45.367 21.116 16.6 3.7 273 30 29 157 76 117 226 26 98 52 330 26 7 0 c ts 1991 9 3 12 42 46.3 44.849 22.262 5.7 2.5 79 55 164 178 77 36 304 14 44 34 195 52 7 0 d ss 1991 9 4 18 13 14.8 44.879 22.393 13.6 3.2 152 73 171 245 81 17 18 6 109 19 271 70 8 0 b ss 1991 9 6 18 27 19.1 44.953 22.394 5.0 3.0 153 24 -35 276 77 -110 162 54 22 29 281 19 6 0 d ns 1991 9 7 7 28 50.4 45.453 21.231 19.2 3.3 200 26 -104 34 65 -84 316 70 119 20 211 5 10 0 c nf 1991 9 9 20 48 5.1 44.981 22.339 6.3 3.3 61 27 -134 288 71 -71 226 59 3 24 102 19 9 0 a ns 1991 9 11 22 10 11.8 45.334 21.048 11.4 3.2 325 35 27 212 75 122 278 23 157 50 22 30 9 0 c ts 1991 9 13 7 8 17.1 44.941 22.343 13.1 3.3 333 70 166 68 77 21 200 5 292 24 99 65 9 0 b ss 1991 9 13 12 12 45.0 45.438 21.165 13.8 3.1 285 43 -16 27 79 -132 259 41 148 22 38 41 9 0 c ss 1991 9 15 0 11 40.1 44.973 22.369 8.4 3.1 333 54 -29 81 67 -140 302 44 204 8 106 45 11 0 c ns 1991 9 15 18 26 28.5 44.914 22.388 16.5 3.7 131 65 -121 6 39 -42 358 58 243 15 145 28 14 0 a ns 1991 9 17 3 2 4.0 44.606 22.344 15.4 3.9 172 55 178 263 88 35 32 22 133 25 266 56 35 0 a ss 1991 9 18 7 45 18.4 45.462 21.223 15.0 3.3 132 53 2 41 88 143 93 24 350 27 218 52 b ss mr10 1991 9 18 17 7 2.8 45.318 21.153 12.4 3.5 340 77 -177 249 87 -13 204 11 295 7 57 77 7 0 d ss 1991 9 19 20 40 9.0 45.090 22.191 20.0 3.1 43 11 123 190 81 84 285 36 93 54 191 6 7 0 d ts 1991 9 25 11 30 7.1 45.247 21.128 12.3 3.3 202 76 -171 110 81 -14 65 16 156 3 256 74 7 0 c ss 1991 10 2 23 2 35.4 44.981 22.315 5.5 3.3 264 17 -107 102 74 -85 20 61 188 29 281 5 8 0 c nf 1991 10 8 8 51 46.4 45.479 21.230 9.5 4.0 189 50 173 284 85 40 50 23 155 31 290 50 13 0 b ss 1991 10 14 20 56 14.2 45.445 21.279 12.6 3.7 21 56 -156 277 70 -36 234 40 332 9 72 49 10 0 a ns 1991 10 17 14 37 53.8 45.425 21.149 6.1 3.6 178 69 -174 86 84 -21 40 19 134 10 250 68 9 0 b ss 1991 10 24 16 38 41.1 45.358 21.199 9.3 3.6 350 32 -6 85 87 -122 326 40 202 34 87 32 9 0 b ss 5 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1991 10 28 0 21 32.0 44.291 21.551 17.5 4.8 200 77 -169 107 79 -13 63 17 154 1 247 73 37 0 a ss 1991 11 1 12 29 4.2 45.181 22.956 20.0 3.3 131 64 68 354 34 128 237 16 3 64 141 20 14 a tf ra02 1991 11 18 12 48 45.5 44.263 21.416 2.9 3.9 294 66 -169 199 80 -24 154 24 248 9 357 64 16 0 a ss 1991 11 21 2 16 31.2 45.479 21.180 8.5 4.3 47 38 -153 295 74 -55 243 49 359 21 104 33 26 0 a ns 1991 11 23 23 36 44.2 45.930 21.586 17.1 3.5 85 69 -156 346 68 -23 306 31 215 1 123 59 8 0 b ns 1991 11 26 19 15 10.4 45.493 21.187 9.2 3.5 190 34 45 60 67 115 131 18 7 60 229 23 8 0 a ts 1991 12 2 8 49 41.8 45.436 21.219 7.5 5.5 103 72 -5 195 85 -162 60 16 328 9 210 72 47 a ss hrvd 1991 12 2 10 52 35.8 45.485 21.264 13.6 3.8 202 18 -111 43 73 -84 322 61 128 28 221 6 9 0 b nf 1991 12 2 14 17 26.1 45.713 20.786 12.5 3.4 292 2 -89 111 88 -90 21 47 201 43 291 0 6 0 d nf 1991 12 2 15 4 3.4 45.397 21.325 8.0 3.6 12 77 -151 275 62 -15 237 29 141 10 34 59 5 0 c ns 1991 12 2 18 27 42.3 45.466 21.087 7.8 3.4 205 36 -8 301 85 -126 179 39 60 31 304 36 9 0 c ss 1991 12 3 13 38 39.0 45.428 21.259 12.6 3.7 203 34 -128 66 64 -68 14 64 140 16 236 20 7 0 b ns 1991 12 8 8 3 5.3 45.289 21.218 14.8 3.4 220 17 12 119 87 107 194 40 46 46 298 16 7 0 c ss 1991 12 12 0 59 57.7 45.453 21.332 14.3 3.4 166 41 0 256 90 131 19 32 133 32 256 41 6 0 c ss 1991 12 12 9 27 22.9 45.429 21.211 12.9 3.2 223 41 -35 341 68 -125 208 53 96 16 355 32 6 0 d ns 1991 12 12 12 2 4.6 45.438 21.262 8.2 3.4 5 21 -118 215 72 -80 141 62 297 26 32 10 11 0 b ns 1991 12 17 6 22 38.7 45.403 21.138 14.2 3.0 1 38 -45 129 64 -119 356 60 240 14 143 26 11 0 a ns 1991 12 18 10 24 15.3 45.796 21.565 12.7 3.7 177 46 179 268 89 44 33 29 142 30 268 46 16 0 b ss 1991 12 19 3 12 24.1 45.941 21.628 18.6 4.8 152 62 -162 53 74 -29 10 32 105 8 207 57 55 0 a ss 1991 12 21 11 43 7.8 45.875 21.584 15.3 3.6 301 32 7 205 86 122 268 33 144 40 22 32 14 0 a ss 1991 12 23 13 16 47.6 45.181 24.013 18.1 3.4 132 84 -175 41 85 -6 356 8 87 1 184 82 9 0 c ss 1991 12 24 11 11 50.4 45.439 21.175 7.8 3.8 57 33 -156 307 77 -59 250 49 13 26 119 30 15 0 a ns 1991 12 26 3 13 15.6 45.455 21.299 15.7 3.3 297 52 -19 39 75 -140 265 38 163 15 56 48 9 0 c ss 1992 2 5 10 42 28.0 45.360 24.840 10.0 3.3 303 61 -70 86 35 -122 253 67 18 14 113 18 12 b nf ra02 1992 2 17 11 0 57.2 44.400 22.710 13.0 3.7 94 53 58 320 47 125 206 3 303 65 115 25 12 b ts ra02 1992 3 2 20 33 23.3 45.898 21.483 13.2 4.0 169 64 167 265 78 27 35 9 130 27 288 61 43 0 a ss 1992 5 24 15 46 44.8 45.905 21.577 11.1 3.6 246 90 -157 336 67 -180 199 16 293 16 66 67 9 0 b ns 1992 6 11 11 20 56.7 44.946 22.403 8.3 3.1 314 84 -174 223 84 -6 179 9 269 0 359 81 8 0 d ss 1992 7 5 3 20 8.9 48.118 22.726 15.3 3.0 329 53 156 74 71 40 198 11 298 41 96 47 6 0 c ts 1992 12 19 9 34 5.8 45.557 20.965 10.7 4.4 185 64 152 288 65 29 56 1 147 37 325 53 48 0 a ts 1992 12 23 21 5 13.7 45.503 20.992 6.5 4.3 272 59 38 160 58 143 36 1 126 48 305 42 28 0 a ts 1993 1 11 0 49 18.0 45.877 21.191 18.5 3.5 323 17 76 158 74 94 245 29 74 61 337 4 11 0 a tf 6 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1993 1 14 8 23 6.0 45.576 21.090 11.2 4.1 69 66 162 167 74 25 297 5 30 29 198 60 23 0 a ss 1993 2 25 4 37 47.1 44.430 21.126 18.1 2.7 1 55 29 253 67 141 309 7 212 43 46 46 10 0 c ts 1993 2 26 21 26 4.7 44.178 20.694 5.0 2.1 278 27 -48 53 70 -109 296 61 157 23 60 17 10 0 b ns 1993 3 16 7 22 27.6 44.186 21.992 20.0 3.0 54 90 136 144 46 0 108 29 360 29 198 0 13 0 a ts 1993 9 27 22 25 19.2 44.229 21.012 5.0 3.0 185 33 -140 60 70 -64 5 57 130 20 230 25 8 0 a ns 1994 4 8 15 7 10.9 45.383 25.054 5.0 3.1 170 55 11 74 81 145 127 17 26 31 241 54 7 0 d ss 1994 4 21 3 0 18.0 45.410 25.380 5.0 2.3 55 58 80 253 33 105 152 13 298 75 60 8 11 b tf ra02 1994 4 28 18 40 1.0 45.200 25.380 11.0 2.3 155 44 -94 341 46 -86 320 87 68 1 158 3 9 b nf ra02 1994 5 25 10 31 0.0 46.320 23.590 1.0 3.3 348 54 -175 255 86 -36 205 28 307 21 69 54 9 b ss ra02 1994 6 20 15 59 50.0 44.465 21.049 5.0 3.7 98 50 -136 336 58 -49 302 56 39 5 132 34 20 0 a ns 1994 7 21 5 13 50.7 45.506 21.136 18.7 4.1 231 73 -158 134 69 -18 93 27 2 3 266 63 5 0 d ss 1994 8 10 11 6 7.0 47.095 23.915 7.2 3.3 311 73 -46 58 47 -156 264 44 10 16 115 42 5 a ns ra02 1994 8 10 13 56 0.0 46.980 22.250 10.0 2.8 30 46 82 221 45 98 125 1 221 84 35 6 9 b tf ra02 1994 10 13 13 49 29.7 45.552 21.326 19.4 4.0 238 43 67 88 51 110 164 4 59 74 255 15 19 0 a tf 1994 10 13 23 31 30.2 45.510 21.276 14.6 3.9 272 39 82 102 51 96 188 6 47 82 279 5 10 0 b tf 1994 10 15 1 42 53.4 45.464 21.132 19.9 4.1 234 36 73 75 56 102 156 10 22 76 248 10 31 0 a tf 1994 11 12 18 50 33.8 45.444 21.132 7.4 4.3 177 73 153 275 64 19 228 6 134 31 328 58 b ts mr10 1994 11 12 19 34 41.9 45.507 21.211 17.7 3.9 74 55 -16 173 77 -144 39 34 299 14 190 52 15 0 b ss 1994 12 16 5 45 12.9 44.058 21.332 19.9 4.1 164 59 179 255 89 31 25 21 124 22 256 59 34 0 a ss 1995 1 28 7 31 58.4 45.502 25.077 15.3 3.1 327 32 129 103 66 68 209 18 338 63 112 20 8 0 a ts 1995 2 3 0 55 59.1 45.519 21.325 9.6 3.7 11 48 -170 274 83 -42 223 34 330 23 87 47 10 0 c ss 1995 2 12 6 5 58.1 45.406 21.075 15.9 3.9 93 50 -157 348 73 -42 302 41 45 14 150 46 8 0 b ns 1995 3 14 5 46 22.0 44.926 22.489 15.4 3.1 320 51 -155 214 71 -42 170 42 271 12 14 46 5 0 d ns 1995 8 24 15 14 35.4 45.410 21.174 8.4 4.0 71 29 16 327 82 118 34 31 265 46 142 28 19 0 b ss 1995 10 16 21 13 8.2 47.220 25.311 12.9 3.3 117 17 129 257 77 79 356 31 153 57 260 11 7 0 c ts 1995 11 23 4 51 28.4 45.507 25.233 8.7 2.5 117 26 -136 346 72 -71 283 58 61 25 160 19 7 0 c ns 1995 11 28 8 16 5.1 45.377 24.240 16.1 3.5 343 42 118 127 54 67 233 6 341 70 141 19 25 0 a ts 1995 11 30 13 29 31.3 45.258 24.016 5.0 3.0 88 17 -74 251 74 -95 154 61 345 29 252 5 8 0 b nf 1995 12 20 14 59 57.8 45.132 23.050 5.0 3.0 98 60 -142 347 58 -36 313 47 222 1 131 43 14 0 b ns 1996 3 6 10 50 18.6 46.305 23.536 12.6 3.4 23 69 176 114 86 21 247 12 340 18 125 68 12 0 a ss 1996 3 24 9 13 26.1 45.422 21.189 13.9 4.0 105 78 176 196 86 12 330 6 61 11 212 77 16 0 a ss 1996 9 29 19 1 49.0 47.186 21.362 17.2 3.4 92 18 27 336 82 106 52 35 264 50 154 16 9 0 b ts 7 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 1996 9 29 21 45 56.6 47.194 21.147 15.1 3.2 113 15 -11 213 87 -105 108 46 317 40 214 15 6 0 d ss 1996 9 30 0 42 17.4 47.297 21.768 8.9 3.3 270 31 -163 165 81 -60 105 46 231 30 340 29 8 0 c ss 1996 11 30 3 46 8.8 45.121 24.176 13.9 3.5 199 39 38 78 67 123 145 16 30 55 245 30 6 0 d ts 1997 2 10 8 38 15.2 45.382 23.718 12.6 3.1 189 30 -162 83 81 -61 22 46 150 30 258 28 10 0 b ss 1997 3 27 14 56 51.9 44.982 24.000 5.0 3.0 270 40 -153 159 73 -53 109 48 222 20 327 35 7 0 d ns 1997 6 17 13 33 45.4 46.528 20.718 6.1 3.3 215 35 -8 312 85 -125 190 40 70 31 315 35 10 0 b ss 1997 6 17 17 3 7.3 46.533 20.679 5.0 2.5 317 85 160 49 70 5 5 10 271 18 123 69 7 0 d ss 1998 1 16 5 13 19.2 45.166 23.483 6.5 3.5 101 82 150 196 60 9 152 15 54 27 268 58 17 0 a ts 1998 2 7 7 0 29.2 45.623 21.117 15.8 3.4 336 42 163 79 79 49 199 23 311 41 88 40 6 0 d ss 1998 4 22 13 35 37.9 46.089 20.193 8.9 2.8 189 25 -2 280 90 -115 167 40 33 40 280 25 8 0 c ss 1998 7 29 15 31 49.3 45.415 23.140 18.1 3.4 147 58 148 255 63 36 20 3 113 44 287 46 15 0 b ts 1998 7 30 4 54 55.5 45.408 23.252 10.9 3.4 178 51 -174 84 85 -39 33 30 138 23 259 50 17 0 a ss 1998 9 5 20 42 6.0 45.352 23.462 5.0 3.7 69 39 -137 302 64 -60 256 59 11 14 108 27 7 0 d ns 1998 9 26 11 15 15.4 44.935 23.873 2.3 3.0 293 22 71 133 69 97 217 24 55 65 310 7 11 0 c tf 1998 9 29 22 14 50.0 44.201 20.089 11.2 5.5 177 70 176 268 86 20 41 11 134 17 279 70 15 a ss hrvd 1998 9 29 22 34 33.7 45.792 21.792 17.4 3.3 326 50 -9 62 83 -140 292 33 187 22 70 49 12 0 a ss 1999 1 4 0 30 35.8 47.966 22.914 7.5 4.3 264 81 -141 167 52 -11 133 33 30 19 275 51 24 0 a ns 1999 1 7 7 40 40.0 43.760 21.120 20.0 5.2 221 76 -132 116 44 -20 91 43 341 20 233 40 a ns hrvd 1999 1 21 18 44 47.0 45.584 21.332 15.0 3.1 230 31 35 109 73 116 179 24 52 54 281 25 8 0 a ts 1999 6 29 2 0 31.3 46.671 21.165 18.5 1.9 72 18 46 297 77 103 17 31 223 56 114 12 6 0 d ts 1999 9 3 9 38 18.5 46.532 21.162 18.4 3.4 115 71 -177 24 87 -19 338 15 71 11 196 71 12 0 a ss 1999 10 3 4 43 41.2 45.835 21.053 15.1 3.2 124 50 -15 224 79 -139 92 36 348 19 236 48 13 0 a ss 1999 10 8 17 26 11.4 45.289 21.102 10.4 3.6 322 78 165 55 75 12 9 2 278 19 105 71 12 0 a ss 1999 10 9 1 1 8.8 45.375 21.070 10.1 3.5 139 47 -45 263 59 -127 119 58 19 7 285 31 12 0 a ns 1999 10 9 20 49 13.3 45.315 21.107 13.3 3.5 138 37 -11 237 83 -126 114 41 356 29 243 36 11 0 b ss 1999 12 20 23 40 33.3 44.557 21.768 13.4 4.5 172 86 150 264 60 4 222 18 124 24 345 59 14 0 a ts 1999 12 21 3 22 34.8 45.076 23.111 5.0 3.8 232 27 -163 127 82 -64 64 47 195 32 303 26 24 0 a ss 2000 3 2 6 15 42.4 47.067 21.178 12.4 3.8 64 17 7 327 88 107 41 41 254 45 146 17 8 0 c ss 2000 7 18 13 54 5.9 44.537 22.414 15.6 3.4 19 84 -149 285 58 -7 247 27 148 17 30 57 15 0 a ns 2000 7 22 7 23 37.0 45.741 21.399 12.0 3.6 327 40 178 59 89 50 181 32 296 34 60 40 12 0 a ss 2000 7 22 7 51 3.0 45.721 21.441 9.8 3.7 251 41 -9 347 85 -131 222 37 108 28 351 40 8 0 b ss 2000 8 16 22 13 59.1 45.500 21.271 17.0 4.0 105 53 -37 219 61 -137 76 50 340 5 246 40 21 1 a ns 8 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2000 8 21 10 28 9.4 45.715 21.395 10.7 2.9 103 26 -71 262 66 -99 155 68 359 20 266 8 8 0 b nf 2000 10 5 23 4 11.6 45.730 21.380 14.2 4.0 243 70 -152 143 64 -22 105 33 12 4 276 57 21 1 a ns 2000 10 30 16 53 24.8 45.747 21.405 13.3 3.8 130 58 -165 32 77 -33 346 32 84 12 192 55 12 1 b ss 2000 12 4 4 1 48.7 46.604 21.486 12.3 2.7 318 71 153 57 65 21 9 4 276 32 105 58 6 0 d ts 2001 1 3 8 8 18.5 44.907 22.274 13.7 3.5 282 86 -154 190 64 -4 149 21 53 15 290 64 10 1 a ns 2001 3 15 19 1 56.9 44.940 20.445 17.5 3.9 272 76 -121 159 33 -27 149 50 25 25 280 29 18 2 a ns 2001 6 6 11 13 53.0 44.367 20.903 9.4 4.1 77 54 -135 317 55 -46 286 55 17 1 108 35 11 2 a ns 2001 6 15 8 4 46.3 45.257 23.035 8.1 3.6 55 65 -119 288 38 -44 282 59 166 15 68 27 9 1 b ns 2001 6 20 15 44 47.7 45.730 21.420 10.8 3.9 126 36 -23 235 77 -124 110 47 350 24 243 33 10 1 a ns 2001 6 24 0 52 51.2 45.345 21.141 13.0 3.4 112 88 168 202 78 2 158 7 66 10 282 78 8 0 b ss 2001 8 2 21 50 45.3 45.483 21.107 6.8 3.8 264 32 -163 159 81 -59 100 45 225 29 334 30 18 0 a ss 2001 8 20 19 44 44.4 45.749 21.396 9.9 3.5 327 57 -19 68 74 -145 292 36 194 11 90 52 10 0 b ss 2002 2 17 16 27 37.8 44.892 22.423 4.4 3.1 76 55 -143 323 60 -41 287 49 21 3 114 41 6 1 c ns 2002 4 6 14 5 9.4 45.300 21.090 11.0 3.4 154 63 -178 63 88 -27 15 20 112 17 239 63 10 0 a ss 2002 5 3 13 10 24.4 44.729 21.609 8.3 3.6 146 40 -140 23 66 -57 337 56 90 15 194 17 8 0 b ns 2002 5 17 8 10 10.4 45.376 23.024 18.0 3.0 289 40 -66 79 54 -109 297 73 182 7 90 15 10 0 c nf 2002 5 23 3 26 0.9 44.717 21.567 6.5 3.9 73 39 -97 262 51 -84 204 83 348 6 78 4 26 1 a nf 2002 5 24 20 42 27.4 44.731 21.634 8.9 4.5 206 49 -148 94 66 -46 52 48 154 10 253 40 50 1 a ns 2002 6 22 13 26 45.7 44.110 21.250 9.6 4.0 189 50 179 280 89 40 47 26 152 28 281 50 24 0 a ss 2002 8 2 9 31 24.9 44.761 21.704 11.6 3.3 53 46 -150 301 69 -48 256 48 2 14 103 39 7 1 b ns 2002 8 2 9 37 19.2 44.737 21.628 7.7 4.2 250 70 -157 152 68 -21 111 30 21 1 289 60 41 0 a ns 2002 8 4 18 27 28.9 44.813 21.687 2.1 3.1 81 39 -114 290 55 -72 249 73 7 8 99 15 9 0 b ns 2003 3 1 20 9 46.2 45.553 20.056 12.5 3.0 359 23 124 143 71 77 243 25 33 62 147 12 7 1 b ts 2003 4 22 22 1 56.7 44.267 21.435 6.8 3.9 297 73 144 39 56 21 351 11 253 37 95 51 21 0 a ts 2003 6 19 18 2 47.6 44.933 22.456 16.6 3.7 123 88 -162 32 72 -2 349 14 256 11 129 72 15 0 a ss 2003 8 26 7 17 14.0 44.261 21.168 12.1 3.5 116 7 -106 312 83 -88 224 52 40 38 132 2 8 0 c nf 2003 12 17 1 59 37.1 45.405 19.978 9.4 3.2 7 18 92 185 72 89 275 27 94 63 185 0 11 0 b tf 2004 5 19 22 37 53.8 45.152 24.804 17.0 3.8 74 79 -141 335 52 -14 302 35 199 17 88 50 14 0 a ns 2004 9 2 16 40 30.1 45.517 20.496 15.0 3.1 195 46 31 82 68 131 143 13 38 49 243 38 11 0 b ts 2004 9 11 5 55 57.9 44.652 21.736 18.7 2.6 73 55 -8 168 83 -145 36 29 295 19 176 54 6 0 d ss 2004 10 22 5 37 4.9 44.913 22.248 18.6 4.0 223 74 180 313 89 16 87 11 179 12 315 74 22 0 a ss 2004 12 9 18 41 50.4 44.369 22.292 16.1 4.6 183 22 160 292 83 69 40 35 180 48 295 21 8 0 b ss 9 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2005 1 9 14 17 49.3 45.360 24.961 6.9 2.3 278 34 -128 141 64 -68 89 64 215 16 311 20 8 0 c ns 2005 2 14 23 37 1.9 45.438 22.469 13.9 3.4 153 34 -162 48 80 -57 351 45 112 28 222 32 9 0 b ss 2005 3 20 9 51 4.3 45.120 22.731 13.3 3.3 317 82 -143 221 53 -10 186 32 83 19 327 51 13 0 b ns 2005 4 20 8 17 55.8 44.869 22.197 11.5 3.0 104 67 -161 6 73 -24 324 29 56 4 153 61 9 0 c ss 2005 5 17 2 52 21.8 46.680 22.961 5.0 3.3 206 70 159 303 70 21 255 0 165 29 345 61 5 0 d ss 2005 5 28 10 28 1.8 45.312 23.186 5.0 3.6 359 43 -14 99 81 -132 331 39 220 24 107 42 5 0 d ss 2005 7 17 7 30 58.2 45.526 21.310 8.4 3.3 237 32 -26 349 77 -119 227 49 102 26 356 29 12 0 b ns 2005 11 11 11 18 30.6 45.086 22.795 4.1 3.0 37 58 -151 290 65 -36 251 42 345 4 79 48 10 0 a ns 2005 12 10 7 56 36.0 44.622 22.209 9.7 3.0 93 51 -164 353 78 -40 305 36 48 17 159 49 8 0 b ss 2006 2 5 10 52 59.0 44.775 21.681 12.8 3.4 122 70 118 245 34 38 191 20 68 56 291 26 10 1 a ts 2006 6 28 17 24 59.1 44.590 21.721 5.7 3.0 294 42 -139 171 64 -56 127 57 237 12 334 30 23 2 a ns 2006 7 1 3 59 20.9 45.070 23.653 7.2 2.7 318 18 117 110 74 82 207 29 8 60 112 8 7 0 d ts 2006 7 2 21 7 29.5 44.601 21.779 4.0 2.5 16 88 -133 284 43 -3 251 33 140 30 18 43 8 2 a ns 2006 8 10 7 19 9.2 45.283 23.150 4.6 3.4 44 84 173 135 83 6 90 1 359 9 186 81 11 0 a ss 2006 8 14 1 40 14.9 44.694 24.621 16.0 2.4 312 40 -1 43 89 -130 280 34 165 32 44 40 10 0 a ss 2006 11 15 18 9 38.4 48.159 22.621 6.7 4.3 66 79 -150 330 61 -13 292 28 195 12 84 59 13 0 b ns 2006 11 21 1 58 47.7 43.864 21.610 10.3 4.5 214 71 -168 120 79 -19 76 21 168 5 271 68 21 0 a ss 2006 11 23 7 15 21.2 48.214 22.489 11.0 4.2 45 71 162 141 73 20 273 1 3 26 181 64 25 0 a ss 2006 12 2 14 8 8.4 45.180 22.928 11.2 2.1 79 57 -45 197 54 -138 46 53 139 2 231 37 9 0 b ns 2006 12 2 18 13 42.8 44.070 21.400 12.3 3.7 19 50 153 127 70 43 249 12 351 44 147 43 b ts ml14 2006 12 8 16 32 33.6 46.710 22.840 19.1 2.6 281 86 -163 190 73 -4 147 15 54 9 294 72 5 0 c ss 2006 12 15 2 25 4.3 45.249 24.588 2.4 2.6 27 26 -146 266 76 -68 203 54 339 28 81 21 8 0 b ns 2006 12 20 11 51 46.8 44.423 21.746 13.9 2.4 71 83 -109 322 20 -20 321 49 177 35 74 18 7 b nf ml14 2007 1 7 2 42 20.1 45.012 22.498 3.7 2.3 78 59 -124 311 45 -47 295 60 192 8 98 29 5 2 a ns 2007 3 9 12 32 35.2 46.673 25.491 6.2 2.5 231 42 174 325 86 48 88 29 200 35 328 42 6 0 d ss 2007 4 21 19 35 43.9 46.109 20.515 15.2 2.3 132 25 -150 14 78 -68 310 52 86 30 189 22 7 0 d ns 2007 5 18 16 16 29.2 44.295 22.061 10.7 2.5 127 21 169 227 86 69 335 38 116 45 228 21 6 0 d ss 2007 5 20 11 9 7.4 44.063 21.710 6.6 3.6 81 54 153 188 68 39 312 9 50 43 213 46 21 0 a ts 2007 7 13 11 17 10.4 45.149 23.116 3.4 3.4 148 28 -179 57 90 -62 352 39 122 38 237 28 11 0 a ss 2007 7 30 20 51 24.6 45.579 20.514 15.8 3.2 98 67 140 206 54 29 155 8 57 44 253 45 21 2 a ts 2007 8 14 5 4 43.1 45.699 21.013 11.2 2.4 350 14 -147 228 82 -78 151 51 308 36 46 11 6 0 c ns 2007 8 22 14 39 30.1 46.985 25.061 2.3 2.7 118 52 172 213 84 38 339 21 83 31 221 51 7 2 b ss 10 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2007 9 20 22 46 7.0 45.592 20.670 15.4 3.7 2 78 162 96 72 13 50 4 318 21 150 69 23 6 a ss 2007 10 10 6 59 18.8 45.034 22.847 8.9 3.1 246 31 -162 140 81 -60 80 46 206 30 315 29 9 2 b ss 2007 10 17 13 30 54.4 47.008 25.080 3.5 2.2 299 37 -23 48 76 -125 282 47 164 23 58 34 7 0 d ns 2007 12 16 13 0 54.9 47.043 21.325 5.0 2.4 356 28 -156 245 79 -64 183 49 314 29 59 26 8 0 c ns 2007 12 19 8 48 28.9 45.749 22.796 9.0 1.5 138 38 -160 32 78 -54 338 45 95 24 203 35 5 0 d ss 2008 1 23 6 3 17.3 44.646 22.229 2.1 2.9 255 46 -19 358 76 -134 227 41 120 19 11 43 8 0 b ss 2008 2 7 16 45 31.7 45.877 20.797 13.1 3.6 236 85 -162 144 72 -5 101 16 9 9 251 72 29 2 a ss 2008 2 13 15 22 19.9 44.163 21.652 1.4 2.4 344 36 -160 238 78 -56 183 46 302 25 50 33 10 0 b ss 2008 3 17 1 49 4.3 45.044 21.789 6.1 2.1 234 29 -135 103 70 -69 43 59 177 22 275 20 10 0 b ns 2008 3 30 16 50 37.9 45.316 24.814 5.0 2.3 262 32 26 150 77 119 217 26 92 49 323 29 8 0 c ns 2008 3 30 17 32 43.8 45.241 24.828 2.9 2.5 290 60 163 29 75 31 157 10 253 32 52 56 12 2 a ss 2008 4 11 6 24 31.6 44.824 22.333 3.8 2.2 247 44 -124 110 55 -62 77 66 180 6 273 23 6 0 c ns 2008 4 28 2 25 38.8 44.921 22.438 3.5 2.3 172 22 -94 356 68 -89 269 67 85 23 176 1 7 0 d nf 2008 5 1 21 15 28.3 44.683 22.298 14.1 2.3 6 34 -121 222 61 -71 170 68 298 14 32 17 9 0 a ns 2008 5 25 21 51 44.1 44.857 22.389 8.9 2.6 213 33 -140 88 70 -64 33 57 158 20 258 25 6 0 c ns 2008 5 27 19 13 31.4 45.822 20.431 3.6 2.6 3 24 -52 141 72 -106 28 60 243 25 146 15 7 0 c ns 2008 6 22 13 35 43.4 44.440 21.537 5.0 3.4 280 77 159 15 70 14 328 5 236 24 69 65 14 0 b ss 2008 6 27 11 24 36.4 45.167 23.107 5.0 3.3 247 79 164 340 74 11 294 3 203 19 33 71 17 0 b ss 2008 7 8 1 25 21.3 45.344 24.546 4.6 1.7 47 73 -140 303 51 -22 273 41 170 14 65 46 5 3 c ns 2008 7 10 16 13 38.6 45.143 23.079 10.0 3.4 218 18 -152 101 82 -74 29 51 177 35 278 16 21 0 a ns 2008 7 20 12 42 18.4 45.497 20.354 7.7 1.9 122 33 -158 13 78 -59 316 48 79 27 186 30 7 0 c ss 2008 8 17 5 0 48.2 45.353 21.076 9.9 2.9 1 44 166 101 80 47 223 23 333 39 110 42 19 1 a ss 2008 9 15 20 41 33.1 47.831 24.105 7.2 2.7 208 49 144 323 64 47 82 9 183 51 345 38 8 2 a ts 2008 9 30 7 46 39.9 46.477 21.720 9.1 2.7 29 47 180 120 89 43 246 28 353 30 121 47 8 0 b ss 2008 9 30 18 41 4.2 44.947 22.348 6.8 3.0 74 48 178 165 89 42 291 27 38 29 166 48 21 0 b ss 2008 10 15 21 0 23.3 47.749 21.483 12.6 2.2 292 31 -179 201 89 -59 138 38 264 37 20 31 13 0 b ss 2008 10 28 14 23 29.9 45.606 22.787 12.2 2.0 335 30 13 234 84 119 300 32 173 43 51 30 5 0 d ss 2008 10 30 19 54 25.6 47.751 23.899 14.3 3.0 198 61 142 309 57 35 254 2 162 47 346 43 7 0 d ts 2008 10 31 12 44 24.1 47.858 23.922 3.9 2.5 63 82 141 159 51 10 117 20 13 33 233 50 8 2 a ts 2008 11 2 8 55 53.5 47.847 23.905 7.6 2.8 50 66 -159 311 71 -26 269 31 1 3 96 59 8 1 b ss 2008 11 11 5 56 21.4 46.755 20.930 12.9 3.0 28 54 -173 294 84 -36 245 29 346 20 106 54 11 1 b ss 2008 11 13 7 47 27.9 46.715 20.920 9.3 3.6 312 40 122 93 57 66 200 9 313 68 107 20 25 0 a ts 11 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2008 11 13 12 50 21.4 46.717 20.859 5.3 3.0 52 86 159 144 69 4 100 12 6 18 222 68 24 0 a ss 2008 11 20 11 16 11.7 45.739 22.847 9.7 2.2 227 38 -122 85 59 -68 41 68 159 11 253 19 6 0 d ns 2008 11 21 19 37 30.0 45.421 24.287 8.0 2.5 308 47 -177 216 88 -43 163 30 271 27 34 47 20 3 a ss 2008 11 21 21 56 13.0 45.206 22.280 5.0 2.7 322 83 164 54 74 7 9 6 277 16 119 73 12 0 a ss 2008 12 12 10 56 31.8 45.189 23.114 2.7 2.7 234 34 -138 106 68 -64 53 59 177 19 276 24 12 0 c ns 2009 1 16 11 28 0.0 44.665 22.221 6.1 2.3 281 80 141 19 52 13 336 18 233 34 89 50 5 2 b ts 2009 1 18 5 49 12.3 44.519 21.593 8.0 2.6 341 70 -153 241 65 -22 202 33 110 3 15 57 9 2 a ns 2009 1 18 22 47 11.5 45.273 22.506 8.4 3.5 184 57 -170 89 82 -33 41 29 141 17 257 55 42 5 a ss 2009 1 22 9 17 35.2 47.024 22.269 3.3 2.0 211 21 -19 319 83 -110 208 48 66 35 321 20 7 1 b ss 2009 2 1 12 23 39.0 44.512 21.598 7.5 2.8 202 54 -49 326 52 -132 173 58 264 1 355 32 17 0 b ns 2009 2 6 8 38 35.7 45.876 21.190 2.5 2.8 227 50 -160 124 75 -42 77 40 181 16 288 46 13 5 a ss 2009 2 13 1 46 5.5 45.971 21.132 2.4 2.8 95 43 -71 250 50 -107 97 77 352 4 261 13 8 2 a nf 2009 2 17 5 18 7.8 44.476 21.592 7.1 2.4 336 77 179 66 89 13 200 8 292 10 72 77 6 1 b ss 2009 2 27 2 12 20.4 47.841 23.923 6.1 3.0 159 64 120 286 39 44 228 14 113 59 325 27 9 7 a ts 2009 2 28 14 22 8.5 47.839 23.910 3.7 3.1 265 81 -164 172 74 -9 129 18 38 5 293 71 15 3 a ss 2009 3 6 0 29 38.2 45.431 24.756 5.2 2.0 83 71 -94 275 19 -79 347 64 176 26 84 4 8 6 a nf 2009 3 7 0 51 50.9 45.576 23.304 15.0 2.4 317 62 119 87 39 47 26 13 272 61 122 26 11 5 a ts 2009 3 26 10 24 18.6 44.529 21.619 18.2 2.5 280 56 -117 142 42 -56 137 67 29 7 296 22 6 1 c nf 2009 3 27 22 47 13.2 45.290 24.002 2.8 3.0 25 45 -155 277 73 -48 229 45 337 17 82 40 26 10 a ns 2009 3 27 23 7 41.8 45.258 23.965 5.0 2.2 198 78 162 292 72 13 246 4 154 21 346 69 8 12 a ss 2009 3 31 13 9 51.6 46.634 21.334 7.0 2.3 162 35 117 310 59 72 53 12 181 70 320 15 12 1 a ts 2009 4 12 14 49 34.3 45.433 24.328 6.9 3.0 131 73 139 235 51 22 188 14 85 41 293 46 17 3 a ts 2009 4 12 14 58 2.5 45.386 24.326 1.5 2.9 176 67 166 272 77 24 43 7 136 26 299 63 12 0 a ss 2009 4 13 5 41 54.3 45.299 21.091 5.6 2.4 182 88 -133 90 42 -3 58 33 305 30 184 42 11 4 b ns 2009 4 16 21 42 42.3 45.915 21.119 6.3 2.4 78 31 -176 345 88 -59 283 39 48 36 164 31 9 2 b ss 2009 4 17 12 39 5.9 45.751 21.062 15.2 1.9 332 73 146 73 58 20 25 10 288 36 128 52 4 2 b ts 2009 5 7 22 54 46.3 45.517 23.281 16.5 2.3 175 37 -146 57 70 -58 6 53 124 19 226 30 9 3 a ns 2009 5 23 4 5 13.4 46.544 21.303 7.9 3.0 142 50 146 255 65 45 15 9 116 49 277 39 23 2 a ts 2009 5 27 0 35 21.6 45.161 22.893 4.5 2.1 16 83 -134 279 44 -10 249 36 139 25 23 44 7 1 b ns 2009 6 5 23 20 11.8 45.570 21.020 5.4 1.7 212 72 169 305 80 18 78 6 170 20 332 69 5 1 c ss 2009 6 22 5 9 44.9 45.588 20.087 8.1 2.6 229 79 156 324 66 12 278 9 184 25 26 63 11 1 a ts 2009 7 1 0 54 51.7 45.355 22.635 16.1 2.8 7 52 -18 108 76 -141 335 37 233 15 125 49 7 2 b ss 12 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2009 7 4 2 51 1.8 48.195 22.472 13.7 2.6 51 62 -78 207 30 -111 347 71 132 16 225 10 5 2 b nf 2009 7 4 4 6 30.9 46.713 20.973 10.4 2.5 38 80 108 156 21 30 113 32 329 52 215 18 16 2 a tf 2009 7 11 6 54 51.3 48.035 23.766 4.8 3.2 255 14 148 16 83 78 117 37 273 51 18 12 16 1 a ts 2009 7 13 5 31 43.5 45.271 22.419 4.6 2.4 64 79 -127 320 38 -18 298 44 182 25 72 36 11 2 a ns 2009 7 31 4 33 48.4 45.229 23.227 13.5 2.2 205 68 105 349 26 57 284 22 139 64 20 13 19 3 a tf 2009 8 11 18 1 20.6 44.503 22.287 5.6 2.4 233 66 -95 65 24 -79 134 68 327 21 235 5 22 3 a nf 2009 8 20 12 57 28.1 47.129 24.856 5.3 2.5 66 62 -170 331 81 -28 285 26 21 13 135 61 18 1 a ss 2009 8 21 8 4 51.7 45.301 22.677 13.5 2.2 255 51 -142 139 61 -46 102 52 199 6 294 37 16 6 a ns 2009 8 23 9 34 21.2 44.703 21.701 3.6 1.7 348 10 -148 226 85 -82 145 49 308 39 45 9 13 3 a ns 2009 8 23 11 22 5.7 45.374 21.203 6.0 2.5 12 71 146 114 58 22 66 8 329 37 166 52 17 10 a ts 2009 8 24 23 24 56.0 47.055 21.120 10.4 2.6 333 53 147 84 64 42 206 7 303 47 110 42 21 4 a ts 2009 8 25 1 39 50.2 46.973 20.830 7.8 2.3 54 35 97 225 55 85 319 10 115 79 228 4 18 1 b tf 2009 8 30 9 27 49.4 45.043 22.583 15.1 2.2 282 81 100 54 13 43 4 35 203 53 101 9 20 4 a tf 2009 9 4 11 23 38.6 44.641 21.730 7.8 2.1 256 44 -161 152 77 -48 102 42 211 20 320 41 9 3 a ss 2009 9 14 3 32 10.2 45.068 22.076 10.1 1.6 280 27 -114 127 65 -78 59 68 208 19 302 11 12 6 a ns 2009 9 16 2 10 38.7 44.614 21.037 8.2 1.8 303 56 23 200 71 144 254 10 157 39 356 49 7 1 c ts 2009 9 21 7 54 53.8 45.289 20.822 9.8 3.2 209 70 162 305 73 21 77 2 168 27 343 63 36 13 a ss 2009 9 24 1 27 25.8 45.297 24.236 5.0 1.7 47 64 -91 229 26 -88 315 71 138 19 48 1 20 5 a nf 2009 9 30 3 18 52.2 46.525 24.637 3.4 1.5 97 75 179 187 89 15 321 10 53 11 190 75 7 0 b ss 2009 9 30 10 10 41.7 44.893 22.391 5.0 2.3 194 78 143 293 54 15 248 16 147 34 359 51 11 3 b ts 2009 10 2 3 34 9.6 45.531 23.699 16.1 1.9 160 52 156 265 71 40 29 12 129 42 286 46 12 4 a ts 2009 10 4 3 54 55.4 45.265 24.199 6.4 2.1 139 66 -112 4 32 -50 14 62 245 18 148 20 9 4 a nf 2009 10 5 19 6 5.3 48.402 22.271 14.4 3.2 181 59 -129 59 48 -43 37 57 298 6 204 32 16 1 a ns 2009 10 7 9 19 39.1 44.639 21.739 8.1 2.4 254 67 177 345 87 23 117 14 212 18 351 67 7 3 b ss 2009 10 9 10 34 35.7 45.261 21.631 7.4 1.6 241 44 -117 96 52 -67 67 71 170 4 261 18 13 3 a ns 2009 10 9 14 31 5.7 46.183 21.275 18.5 2.7 346 90 152 76 62 0 35 19 297 19 166 62 10 3 a ts 2009 10 17 8 51 39.9 46.148 20.321 14.8 2.3 13 48 135 137 58 52 253 6 352 58 159 31 20 5 a ts 2009 10 17 10 32 29.1 45.170 20.388 16.5 2.3 26 41 126 162 58 63 271 9 22 65 177 23 14 5 a ts 2009 10 20 16 8 44.9 46.164 20.285 14.2 1.8 24 39 -155 274 75 -54 222 47 337 21 83 35 6 2 b ns 2009 10 21 10 29 49.9 44.637 21.802 7.3 2.5 155 48 170 252 83 42 16 23 123 34 259 47 7 1 b ss 2009 10 24 8 15 11.6 44.600 21.757 4.1 2.2 216 11 119 7 80 85 102 35 271 55 8 5 8 4 b ts 2009 10 26 23 11 13.3 45.441 21.260 15.0 2.0 45 55 139 162 57 43 283 1 15 52 192 38 15 8 a ts 13 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2009 10 29 11 41 10.1 45.830 21.733 12.5 2.3 246 88 -152 155 62 -2 114 21 17 18 250 62 8 3 b ns 2009 10 30 0 51 27.0 45.022 22.420 2.9 2.9 4 89 -165 274 75 -1 230 11 138 10 7 75 41 7 a ss 2009 11 5 3 42 15.4 46.651 24.865 5.7 2.1 92 51 152 201 69 43 323 11 64 45 223 43 10 2 a ts 2009 11 5 23 59 11.5 44.602 22.135 9.6 2.1 153 77 167 246 77 13 200 0 110 19 290 71 9 4 a ss 2009 11 6 0 59 16.9 45.944 21.867 15.8 1.4 333 76 -108 206 23 -39 220 56 78 29 338 18 10 5 a nf 2009 11 6 2 54 12.2 45.290 24.209 5.0 2.3 258 47 -177 166 88 -43 113 30 221 27 344 47 22 8 a ss 2009 11 18 21 49 45.0 47.388 25.062 12.6 2.2 336 21 106 139 70 84 234 25 39 65 141 6 10 2 a tf 2009 11 19 13 56 38.5 45.824 21.725 8.5 2.7 38 53 -166 299 79 -38 252 34 354 17 106 51 13 5 a ss 2009 11 25 9 34 37.8 47.962 22.886 7.6 2.6 10 74 152 108 63 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45.606 22.862 8.1 3.1 235 68 147 339 60 26 289 5 195 38 25 52 53 8 a ts 2011 3 25 14 51 26.7 45.609 22.823 5.8 2.7 243 74 145 344 57 19 297 11 199 36 41 52 15 4 a ts 2011 3 25 15 12 24.9 45.606 22.830 6.7 3.4 242 76 164 336 74 15 289 1 199 21 22 69 75 6 a ss 2011 3 26 9 13 42.0 45.026 22.386 14.0 1.8 240 59 -178 149 88 -31 100 23 199 20 326 59 15 4 a ss 2011 3 26 23 37 3.6 45.601 22.867 8.8 1.8 265 71 158 2 69 20 314 1 223 29 46 61 12 5 a ss 2011 3 27 1 13 3.0 45.649 22.812 9.5 2.1 307 82 -94 154 9 -63 212 53 41 37 308 4 11 4 a nf 2011 3 27 19 38 30.3 45.601 22.850 8.3 1.9 258 82 -164 166 74 -8 123 17 31 6 282 72 13 8 a ss 2011 3 30 13 40 21.0 46.115 21.693 14.8 2.3 307 83 -157 214 67 -8 173 21 78 11 322 66 7 3 a ss 2011 4 1 20 41 35.9 45.489 21.249 12.5 2.1 300 78 -103 168 18 -43 193 55 41 32 303 13 10 6 a nf 2011 4 5 9 24 25.8 45.048 22.338 12.5 2.9 132 81 -156 38 66 -10 357 23 263 10 151 65 21 7 a ns 2011 4 5 23 5 46.1 45.010 22.766 10.0 2.0 349 89 94 93 4 14 75 44 263 46 169 4 20 7 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13 0 b nf 2011 4 22 23 4 44.1 45.271 23.571 16.3 2.3 131 66 -102 339 27 -65 18 67 230 20 136 11 10 3 a nf 2011 4 29 10 24 57.4 45.808 21.736 15.2 2.8 83 70 -175 351 85 -20 305 18 39 10 157 69 11 3 a ss 2011 4 29 12 32 55.3 45.444 22.458 5.5 2.2 23 54 -125 253 48 -52 233 62 137 3 45 28 19 6 a ns 2011 5 1 21 9 51.4 45.526 25.408 15.5 2.0 319 61 -176 227 87 -29 179 22 277 18 43 61 6 2 b ss 2011 5 19 10 25 25.1 47.746 23.835 16.9 2.7 10 70 145 113 57 24 64 8 327 39 164 50 6 2 b ts 2011 5 21 11 2 53.9 45.459 21.164 13.8 1.8 17 36 -146 258 71 -59 206 54 325 20 67 29 11 3 b ns 2011 5 21 13 8 31.7 44.623 22.335 15.8 2.4 229 64 -106 82 30 -61 109 67 331 18 236 14 17 9 a nf 2011 5 22 21 47 30.1 45.455 21.166 10.7 2.3 40 33 -163 296 81 -58 238 45 1 29 111 31 8 6 a ss 2011 5 23 11 18 45.7 45.355 21.800 9.6 2.0 20 35 -57 162 61 -111 32 67 267 14 172 18 14 8 a mf 2011 5 31 23 58 37.6 45.606 22.861 10.5 2.2 60 87 170 151 80 3 106 5 15 9 225 80 17 8 a ss 2011 6 2 17 1 14.2 45.808 21.099 11.3 1.3 288 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331 31 22 0 a ns 2017 9 23 12 52 39.4 45.016 25.200 16.4 2.5 70 57 -143 318 60 -39 283 48 15 2 107 42 18 0 a ns 2017 9 29 17 50 48.8 44.956 22.458 10.1 2.2 117 53 -178 26 88 -37 335 27 78 24 203 52 12 4 a ss 2017 10 9 4 51 58.8 44.957 22.473 8.9 2.6 100 41 -50 232 60 -119 93 63 342 10 247 25 13 9 a nf 2017 10 22 17 37 11.6 45.521 21.208 11.1 1.9 19 90 134 109 44 0 74 31 324 31 199 44 6 3 b ts 2017 11 23 2 8 19.1 45.454 21.303 17.7 2.3 13 71 -180 283 89 -19 236 14 330 13 101 71 9 4 a ss 2017 12 5 0 22 26.0 45.116 25.402 18.1 2.4 334 90 -104 244 14 0 230 43 78 43 334 15 13 7 a nf 2017 12 14 20 31 40.2 44.930 22.703 14.0 2.2 111 51 -130 344 53 -52 316 60 48 1 139 30 12 3 a ns 2017 12 18 3 32 3.5 45.337 22.920 17.0 2.0 146 86 163 237 73 4 193 9 100 15 313 72 7 4 a ss 2017 12 29 2 36 59.6 45.451 24.450 8.7 1.8 217 67 -111 81 31 -50 94 62 322 19 225 19 15 5 a nf 2018 1 9 22 15 50.8 46.075 21.638 6.3 1.7 235 69 -171 142 82 -21 97 21 190 9 302 67 7 3 a ss 2018 1 16 23 56 49.2 46.041 21.476 9.8 2.4 328 63 171 62 82 27 192 13 288 25 77 61 20 6 a ss 2018 1 21 20 20 25.2 45.292 22.742 9.6 2.5 64 75 -123 312 36 -26 298 49 179 23 74 32 28 9 a ns 2018 1 21 20 51 59.7 45.520 21.222 15.0 1.6 35 64 -148 289 61 -30 253 41 161 2 69 49 9 3 b ns 2018 1 22 4 15 31.1 45.749 21.215 15.3 2.0 251 59 -168 155 80 -32 109 29 207 14 320 57 12 3 a ss 2018 1 22 10 14 1.0 45.740 21.203 14.6 2.6 315 48 -164 214 78 -43 165 38 271 19 22 46 16 5 a ss 2018 1 23 22 18 38.4 45.665 21.480 12.6 1.5 114 42 171 211 84 48 333 27 85 37 216 41 7 5 a ss 2018 1 24 2 58 46.8 47.830 24.122 5.0 2.7 218 66 164 315 75 25 85 6 178 28 344 61 24 5 a ss 2018 2 10 2 55 50.1 45.204 23.224 7.9 2.7 12 48 -175 279 86 -42 227 31 333 25 95 48 21 7 a ss 2018 3 2 16 40 18.7 45.434 21.287 17.6 2.3 112 87 -153 20 62 -4 340 22 243 17 118 62 7 2 b ns 2018 3 8 9 26 14.2 46.934 22.336 9.3 2.1 170 68 -151 68 63 -25 30 36 298 3 204 54 15 5 a ns 2018 3 12 0 53 5.2 45.587 22.804 18.4 2.1 289 56 -112 145 40 -61 148 70 35 8 302 18 10 5 a nf 2018 3 29 5 18 4.4 45.670 21.475 16.0 1.4 105 51 -150 355 67 -43 313 46 53 10 152 42 9 1 b ns 2018 4 2 7 26 14.8 45.456 21.184 15.4 2.1 19 69 180 109 89 21 242 14 336 15 111 69 6 2 b ss 2018 4 9 1 27 37.8 45.362 23.341 5.0 1.5 129 69 107 269 27 53 206 22 65 62 303 16 12 1 a tf 2018 4 16 10 49 13.5 45.759 20.795 15.7 2.2 310 50 -97 141 41 -82 174 83 45 5 315 5 10 5 a nf 2018 4 17 12 3 9.2 46.000 21.922 9.1 1.9 266 73 114 29 29 37 338 24 206 55 79 23 9 3 a ts 2018 4 30 13 8 7.3 44.541 19.842 6.4 2.5 66 57 -166 328 78 -34 282 32 21 14 131 54 12 1 a ss 37 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2018 5 7 20 22 39.8 45.064 22.801 4.9 1.9 17 71 -175 285 84 -19 240 18 332 9 88 70 11 4 b ss 2018 6 2 21 37 2.6 44.897 22.441 17.6 2.6 284 53 -176 192 87 -37 141 27 244 23 8 53 18 7 a ss 2018 6 5 23 2 0.5 45.160 23.082 15.8 2.3 207 54 -101 45 37 -76 79 78 305 9 214 8 17 8 a nf 2018 6 5 23 13 52.1 45.127 23.080 19.1 2.1 291 55 97 99 36 80 16 10 228 79 107 6 9 2 a tf 2018 6 7 2 50 57.0 45.968 22.194 9.4 2.2 330 60 173 64 84 30 193 16 291 25 74 60 10 6 a ss 2018 6 25 23 14 17.5 45.527 21.506 17.7 1.5 295 80 -129 192 39 -16 169 42 54 25 303 38 5 3 a ns 2018 6 29 0 53 14.2 45.839 21.778 11.6 1.7 315 67 91 132 23 87 44 22 227 68 134 1 7 3 a tf 2018 7 8 0 12 4.2 45.153 22.414 9.6 1.8 80 70 -111 308 29 -46 320 60 186 22 88 19 11 5 a nf 2018 7 17 4 3 58.2 45.086 25.562 16.4 2.7 338 50 -160 234 74 -42 188 41 291 15 37 45 25 10 a ss 2018 7 17 8 45 20.2 44.912 22.282 5.0 2.6 91 46 156 198 73 47 319 17 65 44 213 41 6 2 a ts 2018 7 25 2 40 31.3 44.234 26.754 12.9 2.2 215 43 126 350 57 61 100 8 206 65 6 24 15 3 a ts 2018 8 2 0 43 49.0 44.320 20.974 4.0 2.3 98 90 -169 188 79 -180 52 8 144 8 278 79 17 2 a ss 2018 8 6 0 27 14.0 44.086 20.566 7.4 2.7 285 76 -108 158 23 -39 172 56 30 29 290 18 16 2 a nf 2018 8 15 17 22 49.3 45.082 23.047 10.9 2.3 268 27 -74 70 64 -98 323 70 166 19 73 7 14 3 a nf 2018 8 21 2 58 18.2 45.399 24.585 9.8 2.1 333 56 161 74 74 36 200 12 299 36 95 51 6 3 b ss 2018 8 25 0 59 52.6 44.887 22.429 16.5 2.6 97 53 -133 334 54 -48 305 57 36 1 127 33 21 7 a ns 2018 8 30 7 48 52.6 46.954 22.320 12.5 2.3 171 86 147 264 57 5 222 20 122 26 345 56 8 3 a ts 2018 8 31 2 24 55.7 45.990 21.980 16.2 1.3 283 75 94 88 16 76 10 30 199 60 102 4 8 4 a tf 2018 10 21 17 2 36.4 47.272 23.790 13.9 2.4 178 62 177 269 87 28 40 17 137 22 275 62 14 4 a ss 2018 11 3 0 59 1.2 45.200 23.945 7.6 1.9 48 65 100 205 27 69 130 19 338 68 223 10 8 2 a tf 2018 11 4 3 56 24.9 44.022 21.648 10.9 2.5 58 78 110 178 23 32 132 30 351 53 234 19 16 5 a tf 2018 11 7 0 2 55.3 46.083 21.605 11.1 2.0 355 60 -157 253 70 -32 211 37 306 6 44 52 7 4 a ns 2018 11 9 1 14 18.1 45.105 22.272 7.3 3.8 287 68 -105 143 26 -57 173 64 28 22 292 13 30 5 a nf 2018 11 14 23 22 39.9 45.348 24.249 5.5 1.7 236 68 -97 72 23 -75 135 67 331 23 239 6 9 5 a nf 2018 11 19 1 33 43.4 45.978 21.536 10.3 1.5 36 79 -122 289 34 -20 273 46 151 27 43 32 8 3 a ns 2018 11 19 2 18 9.8 45.473 20.884 10.7 2.2 88 84 -146 354 56 -7 317 28 216 19 96 55 17 3 a ns 2018 11 19 12 38 27.2 44.828 22.310 15.5 3.1 238 65 139 348 54 32 295 7 198 46 32 43 16 7 a ts 2018 11 19 19 7 24.8 45.783 21.201 14.0 1.9 89 60 155 192 69 32 319 6 53 37 221 52 5 3 a ts 2018 11 23 16 11 6.2 45.458 23.357 6.7 2.3 215 75 174 307 84 15 80 6 172 15 329 74 7 2 c ss 2018 11 24 11 6 50.9 45.135 22.806 14.0 2.9 5 46 164 106 79 45 228 21 336 39 117 44 9 5 a ss 2018 12 5 15 42 3.5 44.843 21.775 10.0 1.9 295 41 -150 181 71 -53 132 50 245 18 348 34 6 4 a ns 2018 12 11 6 55 11.8 44.886 22.424 16.3 2.0 333 75 -157 236 67 -17 196 27 103 5 3 62 8 3 b ns 38 date time latitude longitude depth mw np1 np2 p t b no pol no s/p q fs obs year month day hour minute second strike dip rake strike dip rake az pl az pl az pl 2018 12 13 3 50 7.2 46.823 23.072 3.7 2.0 34 84 115 137 26 14 103 34 329 45 211 25 9 4 a ts 2018 12 14 2 20 48.1 45.407 24.925 5.7 1.8 95 59 -92 279 31 -87 359 76 187 14 97 2 10 4 a nf 2018 12 22 14 55 53.6 45.158 20.990 5.0 2.1 356 65 -134 242 49 -34 217 50 116 9 19 39 7 3 a ns 2018 12 22 14 59 7.7 45.181 21.035 7.7 2.8 348 50 -134 224 57 -51 191 58 287 4 19 32 14 3 a ns 2018 12 22 16 0 26.9 45.203 21.098 3.9 1.8 278 31 -10 17 85 -121 258 42 132 33 20 30 9 0 b ss 2018 12 23 3 47 0.8 45.156 20.926 14.6 2.8 359 51 113 145 44 64 73 4 332 72 164 18 17 3 a ts 2018 12 23 4 29 41.2 45.196 21.115 10.2 1.8 351 58 -145 241 61 -37 205 46 297 2 29 44 7 3 b ns 2018 12 23 4 51 34.3 45.166 21.115 2.3 2.2 179 75 -107 47 22 -44 68 57 282 28 183 16 8 3 a nf 2018 12 23 4 58 20.6 45.181 21.031 6.1 2.3 351 62 -123 225 42 -44 213 59 104 11 8 29 11 3 a ns 2018 12 23 6 33 6.9 45.223 20.959 5.0 4.2 344 49 168 82 81 42 206 21 312 35 91 47 40 3 a ss 2018 12 23 6 41 44.6 45.203 21.074 6.7 2.1 350 53 172 85 84 37 212 21 314 30 92 52 7 3 a ss 2018 12 23 6 55 59.6 45.146 20.950 7.7 3.7 175 85 145 268 55 6 227 20 126 28 348 55 26 4 a ts 2018 12 23 7 37 43.6 45.186 21.064 12.2 2.9 354 76 171 86 81 14 220 3 311 16 120 74 11 3 a ss 2018 12 23 8 55 50.0 45.465 21.109 16.4 1.8 22 53 -173 288 84 -37 238 30 341 21 100 52 6 2 a ss 2018 12 23 19 3 40.1 45.183 21.052 15.3 2.1 353 52 -153 246 69 -41 203 43 303 11 44 45 5 3 a ns 2018 12 23 19 27 16.3 45.195 21.078 13.0 1.5 358 57 -141 244 58 -40 211 49 301 1 32 41 6 4 a ns 2018 12 23 19 54 8.0 45.188 21.101 13.3 2.4 353 50 174 87 85 40 213 23 318 31 93 50 10 4 a ss 2018 12 25 12 31 24.8 45.189 21.037 10.5 1.8 23 47 -163 281 78 -44 232 39 339 20 90 44 8 3 a ss 2018 12 26 20 14 39.8 46.110 20.677 4.9 2.1 194 55 148 304 64 40 67 5 163 46 332 43 10 2 a ts 2018 12 27 13 19 24.0 45.195 21.086 6.0 1.4 355 81 -117 248 28 -19 237 47 107 31 359 27 6 2 b ns 2018 12 28 13 45 19.8 45.195 21.085 13.5 1.7 351 51 -129 223 53 -52 195 61 287 1 18 29 7 2 a ns 2018 12 29 1 28 10.1 45.043 22.881 20.1 1.9 103 89 -166 13 76 -1 329 11 237 9 109 76 7 4 c ss 2018 12 30 8 4 59.0 44.506 21.500 9.9 3.1 215 78 139 315 50 16 271 18 167 37 22 47 25 6 a ts co80 cornea, i., g. polonic and v. steflea (1980). seismotectonic studies in baia mare-halmeu area, studii si cercetari de geologie, geofizica, geografie, geofizica, 19:166. hrvd isc bulletins, www.isc.ac.uk. ma02 marovic m. et al. (2002). neotectonics and seismicity of the southern margin of the pannonian basin in serbia. egu stephan mueller special publication series, 3, 277–295. ml14 mladenovic a., trivic b., antic, cvetkovic v., pavlovi r., radovanovic s., fuenschuh b. (2014). the recent fault kinematics in the westernmost part of the getic nappe system (eastern serbia): evidence from fault slip and focal mechanism data. geologica carpathica, 65, 2, 147—161 doi: 10.2478/geoca-2014-0010. mr10 malita z., radulescu f. (2010). focal mechanisms of some crustal earthquakes that occurred in the pannonian depression (arad – south timisoara area), the moesian platform and north-dobrogean orogen. revues roumaine de geophysique (romanian geophysical journal, 54, 19–37. 39 http://www.isc.ac.uk o19 oros e., constantinescu e., paulescu d., popa m., placinta a. o. (2019). using early instrumental data to determine the source parameters of the strongest historical earthquakes occurred in west romania (1900 1980). sgem 2019 conference proceedings, science and technologies in geology, exploration and mining, vol. 19, issue: 1.1, 991-998, doi: 10.5593/sgem2019/1.1/s05.122. pa14 pavlova a., hrystai o., malytskyy d. (2014). determining the focal mechanisms of the events in the carpathian region of ukraine. geosci. instrum. method. data syst., 3, 229–239, 2014, www.geosci-instrum-method-data-syst.net/3/229/2014/, doi:10.5194/gi-3-229-2014. ra02 radulian m., popescu e., bala a., utale a. (2002). catalog of fault plane solutions occurred on the romanian territory. romanian journal of physics, 47, 5-6, 663-685. mti* focal mechanism computed using moment tensor inversion method (isola software). 40 http://www.geosci-instrum-method-data-syst.net/3/229/2014/ a u r o r e b o r e a l i osservate dalla stazione astronomica di l o i a n o negli anni 1957-1958 p . m a f f e i negli anni 1957 e 1958 tre aurore polari venivano occasionalmente osservate dalla stazione dell'osservatorio astronomico universitario di bologna situata nei pressi di loiano. l'eccezionalità del fenomeno perla nostra latitudine ci induceva a compiere osservazioni visuali, sia pure sommarie, nonostante ciò esulasse completamente dall'indirizzo delle nostre ricerche. tutte le osservazioni venivano compiute dalla stazione stessa avente le seguenti coordinate geografiche: cp = + 44°15'23"; x = (m5»20s est di greenw.; h = 800 m gli istanti sono espressi in t. u. le misure espresse in gradi sono state compiute empiricamente a stima e risultano, semmai, approssimate per difetto. per la prima aurora le descrizioni che seguono venivano scritte nel corso del fenomeno; per le altre due sono state registrate mediante un magnetofono. questo secondo metodo si è dimostrato particolarmente efficace in questo genere di osservazioni, evitando perdite di tempo e distrazioni e dando all'osservatore la possibilità di mantenersi nel buio più completo, in modo da seguire ed annotare anche le luminosità più deboli e le variazioni più rapide. delle tre aurore la più imponente è stata certamente quella del 21 gennaio 1957, specialmente nella sua fase oltre le ore 22. pure notevole l'aurora del 1 settembre 1958 quantunque osservata nelle più avverse condizioni: all'inizio dietro una cortina di fitti veli, dopo le ore 21 con la luna sempre più alta sull'orizzonte. 21-22 gennaio 1957. il fenomeno veniva notato a 20h02m (t.u.), nel corso delle consuete osservazioni astronomiche, mentre la cupola era volta verso sud. da 2 1 6 p . m a f f e i quel momento ne veniva iniziata l'osservazione, favorita da un cielo limpido e completamente privo di nubi. diamo, qui di seguito, la descrizione delle varie fasi. la vistosità del fenomeno viene indicata con un punto esclamativo a sinistra dell'ora di osservazione. 20u02m la luce aurorale si estende per circa 45° a partire dalla zona della costellazione del cigno, immersa in una luce rossastra sfumante nel rosa e nel verde verso nord. bei raggi giallastri appaiono e scompaiono rapidamente, particolarmente nella zona di massima intensità. degno di nota uno di circa 20° di altezza, apparso intorno alla fine (20h05m) di questa fase. 20 10 la luce aurorale è pressoché scomparsa. 20 18 riappare una zona luminosa, ampia circa 25°, centrata esattamente a nord. 20 25 compaiono due intensi raggi larghi circa 5° e alti circa 20°. 20 29 rimane una zona rossastra di 20° di ampiezza in corrispondenza della costellazione del cigno. 20 34 compaiono due intensi e larghi raggi rossastri nella zona del cigno. 20 40 nessuna traccia di luce aurorale. 20 44 20 46 leggerissime luminosità appaiono e scompaiono nella zona nord. 20 50 un leggerissimo raggio appare e svanisce nella zona nord. 20 55 leggeri raggi appaiono e scompaiono nella zona nord. 21 02 leggeri raggi. 21 05 riappare, leggera, la colorazione rossastra. 21 12 leggerissima luce rosa nella zona del cigno. appare e scompare un raggio. 21 22 su tutto l'orizzonte settentrionale, per una lunghezza di circa 45°, si estende una debolissima luce rossa. 21 24 intensi e larghi raggi nella zona n n w , dove da poco è tramontata la costellazione del cigno. 21 35 ancora alcuni deboli raggi. 21 45 zona rossastra a n n w . ! ! ! ! ! 22h02m appaiono numerosi e intensi raggi giallastri con sfumature verde pallido su fondo unito rosa-rosso, disposti in uno splendido arco di circa 100° di lunghezza e 25° di spessore. l'arco si innalza al di sopra dell'orizzonte come uno splendido ponte luminoso; al di sotto di questo una zona, limitata alla base dall'orizzonte, splende di intensa luce bianca verdastra. ! ! ! 22 11 i raggi svaniscono. ! ! ! ! 22 12 i raggi riappaiono ! ! ! ! 22 20 tutto l'arco tende a svanire e restringersi; i colori si fanno di uu rosso sempre più cupo. ! ! ! 22 25 splendidi larghi raggi nella zona s s w . ! ! ! 22 30 due parti dell'arco rimangono intense, rosso cupo, solcate a momenti da qualche raggio. una a nord, leggermente spostata ad est e un'altra nella zona n w . !! 22 35 le due zone continuano ad essere visibili ma sempre più deboli e di un colore rosso cupo. 22 43 debolissima luce rossastra diffusa. 23 00 il fenomeno è completamente scomparso; rimane soltanto la luce della luna sorta da pochi minuti. 8-9 luglio 1958. il fenomeno veniva notato alle ore 21 (t. u.) quando, essendosi spenta la luce del crepuscolo, stavano per essere iniziate le consuete osservazioni astronomiche. 21h02m inizio dell'osservazione. luminosità generale nella zona nord. si notano, particolarmente intensi, due raggi in direzione n n w di circa 30° di lunghezza e 3° di larghezza. 21 05 appaiono, particolarmente intensi, due raggi rosa: uno a nord e l'altro a n w , che si accentuano sempre più. i l raggio a nord, successivamente, si allarga e si intensifica, poi si suddivide in tre raggi principali compatti ed uno secondario staccato dal resto di circa 4° verso ovest. il gruppo si intensifica colorandosi sempre più vivamente in rosa. poco dopo il raggio a n w scompare. dimensioni del gruppo di raggi nord: larghezza totale 22°, altezza sull'orizzonte 35°. la zona nord si colora sempre più 2 1 8 p . m a f f e i in rosa mentre torna a comparire anche la luminosità a j w . la luminosità nella zona nord non è più disposta secondo raggi ma ha un aspetto sfumato ed omogeneo simile, anche nella forma, a quello della luce zodiacale. 21h10m appare un doppio raggio leggerissimo della lunghezza di c. 30° e larghezza di c. 12° a n n w . 21 12 scomparsa la luminosità a n w . 21 14 leggera, tenue luminosità generale e due raggi debolissimi. complessivamente il fenomeno si è notevolmente indebolito. 21 16 si vedono solo tre raggi, sfumatissimi ed appena percettibili, convergenti verso il punto nord. poi appaiono due raggi appena visibili: uno in direzione n n w , un altro, leggermente più intenso, a n w . 21 22 leggera luminosità generale diffusa in tutta la zona nord. 21 23 appare a n w un debolissimo raggio lungo circa 22° e largo circa 4°. 21 26 ii raggio appare ora allargato e sfumato mentre si è riformato un raggio di minore intensità a n n w . intensità dei due raggi: circa come la luce zodiacale normale. 21 30 tutto scomparso, tranne una leggera luminosità appena percettibile a n w . 21 33 si intravedono nuovamente dei raggi disposti secondo un arco centrato a nord, dell'altezza massima sull'orizzonte di circa 10° e della lunghezza di circa 90°. 21 36 dal momento di quest'ultima apparizione i raggi delle varie zone dell'arco hanno subito rafforzamenti ed indebolimenti alterni e in questo momento sono piuttosto sfumati. 21 38 appare un raggio in direzione nord, debole quasi come gli altri, ma lungo circa 45°. 21 40 tutti i raggi sono pressoché scomparsi. rimangono due leggere luminosità in direzione nord e n w , la prima intensa come la normale luce zodiacale, la seconda appena percettibile. 21 42 pressoché scomparsa ogni traccia di luminosità. 21 48 appare a nord una zona (che si indovina formata da raggi) di 20° di ampiezza e 20° di altezza. a u r o r e b o r e a l i o s s e r v a t e , ecc. 2 1 9 21h50m scomparsa anche questa luminosità. 22 03 nessuna traccia di aurora fino a questo momento in cui tornano a vedersi due leggere luminosità, appena raggiate, in direzione n k w e n w . 22 05 rimane, molto debole, il raggio a n n w . 22 08 pressoché scomparsa ogni traccia di luminosità tranne una leggerissima luce aurorale in direzione n n w . 22 17 si forma a nord una debole zona luminosa larga circa 50° ed alta circa 15°. 22 22 scomparsa ogni luminosità. 22 28 continua a non vedersi più nulla, tranne un debole chiarore all'orizzonte intorno al punto nord. 32 10 non si vede più nulla da parecchio tempo. la luna è sorta da 45m. l'osservazione viene sospesa. 21 00 un ultimo controllo conferma la scomparsa di ogni traccia di luminosità, al di fuori del debole chiarore lunare. 4 5 s e t t e m b r e 1 9 5 8 . i l fenomeno veniva avvistato a 20 45 (t. u.). si manifestava sotto forma di una luminosità generale diffusa color porpora, nella zona nord, visibile nonostante il cielo velato o addirittura coperto. 20h50m appare un raggio alto circa 40° a n n w . la luce diffusa di fondo presenta la massima intensità, con colore sempre rossastro, nella zona n n e . poco dopo il raggio tende a sfumare e a confondersi con la zona di fondo. 20 52 i i raggio a n n w è pressoché scomparso; è diminuita anche la intensità generale del fenomeno. 20 54 appare in direzione nord un intenso raggio che presto si trasforma in una zona larga e diffusa mentre l'intensità generale del fenomeno aumenta. 20 55 raggi gialli molto intensi, larghi c. 10° e alti c. 45°. 20 57 i i fenomeno presenta la massima intensità in due zone diffuse a nord e a nord-est, prevalentemente nella prima. successiva2 2 0 p. m a f f e i mente, a nord-est, appare una serie di raggi, emergenti dalle nuvole, che si intensificano sempre più, giungenti all'altezza della stella mizar e della larghezza di 10°. 21h00m i raggi sono scomparsi; la luce aurorale generale si è notevolmente attenuata. dal momento dell'inizio del fenomeno la nebulosità è continuamente diminuita; a questo istante le nubi coprono una fascia di cielo di appena 10° sull'orizzonte. 21 05 ii fenomeno, ridotto ormai ad una leggerissima luminosità generale diffusa appena percettibile, viene ravvivato da un leggero raggio apparso esattamente a nord che poco dopo sfuma confondendosi con il fondo. è visibile la luna (in età di giorni 20) all'orizzonte. 21 08 la luminosità a nord, rimasta fino ad ora di intensità costante, sta ora diminuendo ulteriormente. leggerissima luminosità a n w . 21 10 non si vedono più tracce di luce aurorale. 21 12 torna a comparire, a nord, una leggera luminosità, larga 20° e alta 25°, che sparisce però nel minuto successivo. ogni manifestazione del fenomeno è praticamente scomparsa. 21 11 luminosità raggiata di color rosa scuro presente nella zona nord ed estendentesi verso n w , larga ed alta 30°. due raggi spiccano particolarmente. 21 17 compare anche un raggio piuttosto sfumato ad ovest, che scompare entro i due minuti successivi. 21 21 la luminosità a nord, che si è via via indebolita, è oramai appena percettibile. la luce della luna comincia a disturbare. 21 23 nessuna traccia di aurora. 21 30 appaiono luci raggiate rossastre nella zona nord e n n e . la luce della luna disturba molto. 21 32 compare a n n e una zona rossastra raggiata che si intensifica sempre più. 21 34 permane la luminosità rossastra a n n e ed appare un doppio raggio a n w . a u r o r e b o r e a l i o s s e r v a t e , ecc. 2 2 1 21 35 ii raggio a m w è pressoché scomparso. tutta la luminosità sì è affievolita, traccie sono appena visibili a n n e . 21 37 non si vedono più traccie di aurora. le nuvole sono completamente scomparse tino all'orizzonte. la luna disturba notevolmente. 22 05 non si vedono più traccie di aurora. ciò però può essere dovuto alla ormai preponderante luce lunare. 22 25 a n n w , sotto le tre stelle e, rj u ma, compare una zona rossa piuttosto intensa spostantesi in azimut verso nord. raggio molto intenso. 22 29 in direzione n n w compare un magnifico raggio, alto circa 15°, e che si intensifica sempre più. tutta la zona tra nord e n w , per una lunghezza di c. 30°, è colorata di un rosso intenso. zona e raggio si intensificano sempre più. 22 32 la luminosità tende a svanire pressoché completamente. 22 33 compare un raggio congiungente le due stelle a e /j u ma. il raggio si rinforza mentre ne compare un altro verso est e si rinforzano anche quelli a n w . i raggi a n w si rinforzano assumendo un colore giallastro. 22 36 i tre raggi cedono il posto a tre zone con luminosità diffusa rossastra. i l raggio giallo a n w è completamente sparito. 22 39 luminosità diffusa larga c. 60° ed alta c. 30°. 22 12 si intravede una luminosità generale debolissima. 22 16 luce rossastra a n e . 22 19 non si vedono più traccie del fenomeno. 22 53 leggerissima luce rossastra tra nord e n n e . 22 58 nessuna traccia del fenomeno. 23 15 nessuna traccia del fenomeno. 23 35 un'ultima occhiata al cielo non rivela alcuna traccia del fenomeno. è però da rilevare che la preponderante luce della luna rende impossibile l'osservazione di eventuali deboli luminosità. 2 2 2 p . m a f f e i r é s u m é e é d i g é s u e l e f o r m u l a i r e d e t h o m s e n 21-22 janvier 1957 mauro tent. u. forme conrdunnées remarques t. u. dono) remarques 20h02m g — 295/-, 340/ciel olair; début des observations; rougeàtre, estompé rose et verte vers le nord » r — — jaune 20 10 — — — absence d'aurore 20 18 g — 345/-, 15/— 20 25 2r — -/20,-/20, intense 20 29 g — 320/-,340/-, rougeàtre 20 34 d — 320/-,340/-, » 20 40 — — — absence d'aurore 20 44 g — o(centre) apparition e dispar. rapide 20 50 r — o(centre) » très faible 20 55 r — o(centre) » plusieurs rais faibles 21 05 g — o(centre) rougeàtre 21 12 g — 320/-,340/-, rose 21 22 g — 335/-,20/-, rouge faible 21 24 d — 340(centre) intense 21 35 d — 340(centre) faible 21 45 g — 340(centre) rougeàtre 22 02 r a i 300/0,0/60,60/0 r jaunes, estompés vert; fond: are rose-rouge de 25° de larg. » g i 0(centre)/15 au dessous de l'are: blanckeverdàtre très intense 22 11 ra q » » » g q » » » r d » » 22 12 ra q — — » g q » » » r i — — 22 20 ra d » rouge sombre 22 25 d — 340(centre)/20 — 22 30 2d d 310(oentre)/20 10(centre)/20 rouge sombre 22 35 2d d » rouge sombre, faibles 22 43 s — — rougeàtre 23 00 absence d'aurore, lueur de la lune fin des observations a u r o r e b o r e a l i o s s e r v a t e , ecc. 2 2 3 8 9 jtjellbt 1 9 5 8 heure t. u. forme tendence cnordonnées remarques 21h02m gr 01début des observations » 2r — 340/30 — 21 05 2r i 315/-,0/-, rose 21 09 d i 0(centre)/35 » » r d 315/— » s q 340(centre)/0(centre)/20 rose 21 10 r — 340/30, très legère 21 14 g — — très faible 21 16 r — — » 21 22 g — o(centre)/-, » 21 23 r — 315/20 » 21 26 2r — 315/-,340/-, faible 21 30 — — — absence d'aurore 21 33 r a — 330/0,0/10,30/0 faible 21 38 r — 0/45 » 21 40 2s 315(centre)/o(centre)/— 21 42 — — — absence d'aurore 21 48 s — 0(centre)/20 faible 21 50 — — — absence d'aurore 22 03 2d — 315(centre)/340(oentre)/faibles 22 05 d — 340(centre)/très faibles 22 08 — — — absence d'aurore 22 17 ' gr — 335/0,0/15,25/0 faible 22 22 24 00 — absence d'aurore; lueur de la lune; fin des observations 2 2 4 4 5 s e p t e m b r e 1 9 5 8 p . m a f f e i heuro t. u. forma tentlence coordnnées r e m a r q u e s 20h45m g 0( centro )/début des observations ciel voile ou couvert 20 50 r — 340/40 — » g — 20(centre)/rougeàtre 20 52 g d — — 20 54 r i ;rs 0 _ 20 55 r — — jaunes, très intenses larg. 10°, liaut. 45° 20 57 s — 01— » s — 45/li—. 20 58 r i 45/15 — 21 00 g d le ciel est toujours plus clair; les nuages couvrent à ce moment une bande de 10° de hauteur 21 05 g q o(centre)/très faible; lune visible à o(centre)/l'horizont 21 08 g d o(centre)/— 21 10 — • — — absence d'aurore 21 12 s i 0(centre)/25 faible 21 13 s d » » 21 14 d — 330/-, 0/30 rose sombre 21 17 r — 300/" — 21 21 s — o /très faible la lune gène 21 23 — — — absence d'aurore 21 30 d — 0/—,45/— rougeàtre; la lune gène beau20/eoup 21 32 d i 20/rougeàtre 21 34 d q 20/— » r — 315/— 21 35 g .—. — faible 21 37 — — absence d'aurore; ciel clair; la lune gène beaucoup 22 05 — absence d'aurore; la lueur de la lune est prépondérante 22 25 s w n — ; rouge » r — très fort 22 29 r i 340/45 — )) g i 330/-,0/rouge vif 22 32 g d )) — 22 33 r i 340/-,350/15,20/jaune 22 36 s — )) rougeàtre 22 39 g — 330/-,0/30,30/— 22 42 — — lueur gènerale (à nord) très faible 22 46 g — 45/rougeàtre 22 49 — — — absence d'aurore 22 53 g — 10/rougeàtre, très faible 22 58 — . • — . — absence d'aurore 23 15 — — • — — 23 35 absence d'aurore; la lune est très haute et la continuation des observations n'est plus possible; fin des observations a u r o r e b o r e a l i o s s e r v a t e , ecc. 2 2 5 riassunto vengono descritte tre aurore boreali osservate dalla stazione astronomica di loiano (bologna), le notti del: 21-22 gennaio 1957; 8-9 luglio 1958; 4-5 settembre 1958. abstract three auroras observed from the astronomicai observatory of loiano (bologna) in the nights of 21-22 january 1957; 8-9 july 1958; 4-5 septernber 1958, are described. b i b l i o g r a f i a del et terre, 73, 527 e 546; 74, 48. meletti_corretto_layout 6 623 seismic hazard in the po plain and the 2012 emilia earthquakes carlo meletti, vera d'amico, gabriele ameri, andrea rovida, massimiliano stucchi istituto nazionale di geofisica e vulcanologia, sezione di milano/pavia, milano, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6158 1. introduction the emilia earthquakes of may 20, 2012 (ml 5.9, ingv; mw 6.11, http://www.bo.ingv.it/rcmt/) and may 29, 2012 (ml 5.8, ingv; mw 5.96, http://www.bo.ingv.it/rcmt/) struck an area that in the national reference seismic hazard model [mps04; http://zonesismiche.mi.ingv.it, and stucchi et al. 2011] is characterized by expected horizontal peak ground acceleration (pga) with a 10% probability of exceedance in 50 years that ranges between 0.10 g and 0.15 g (figure 1), which is a medium level of seismic hazard in italy. the strong impact of the earthquakes on a region that is not included among the most hazardous areas of italy, and the ground motion data recorded by accelerometric networks, have given the impression to the population and the media that the current seismic hazard map is not correct, and thus needs to be updated. since the mps04 seismic hazard model was adopted by the current italian building code [norme tecniche per le costruzioni 2008, hereafter termed ntc08; http://www.cslp. it/cslp/] as the basis to define seismic action (the design spectra), any modification to the seismic hazard model would also affect the building code. the aim of this paper is to briefly present the data that support the seismic hazard model in the area, and to perform some comparisons between recorded ground motion with seismic hazard estimates and design spectra. all of the comparisons presented in this study are for the horizontal components only, as the italian hazard model did not perform any estimates for the vertical component. 2. reference seismic hazard map for the emilia area the reference italian seismic hazard model (mps04) was released in 2004, and it was adopted as an official document by italian law in 2006 (prime minister ordinance 3519/2006). this model provides the basic data to be considered to update the seismic zoning of municipalities, and it was used in the determination of the design spectra in the italian building code (ntc08). the mps04 seismic hazard model was obtained using the standard cornell probabilistic approach, by adopting a logic-tree strategy. this is conventionally used in probabilistic seismic hazard assessment (psha) as a tool to capture the epistemic uncertainty that is associated with the input elements of computational models. the main input elements were: the cpti04 earthquake catalog [cpti working group 2004], the zs9 seismic source zone model [meletti et al. 2008], the 2.0 version of the database of individual seismogenic sources (diss) [valensise and pantosti 2001], and a set of ground-motion predictive models based on italian and european data [ambraseys et al. 1996, sabetta and pugliese 1996, malagnini et al. 2000, malagnini et al. 2002, morasca et al. 2002]. the estimates produced by each logic-tree branch were then combined with the relevant weights to obtain the median value (assumed as the reference hazard value) and the percentiles that express the relevant uncertainty. the emilia 2012 earthquake sequence occurred in northern italy, inside the po plain, where thick sedimentary deposits cover the most external active fronts of the apennine belt that overlie the adria microplate. figure 2 shows the seismogenic zones for the study region from the abovementioned zs9 model. source zones 913, 914 and 918 correspond to the compressive structures (on the northeastern side of the apenninic relief ), source zones 915 and 916 include the extensional structures of the internal margin of the apennines, and source zones 912 and 917 correspond to the blind thrusts that characterize the most external portion of the apennines. these structural elements have been well known for many years, due to the huge subsurface exploration that was carried out by eni for oil and gas reservoir identification. the epicenters of the earthquakes reported by cpti04 catalog [cpti working group 2004] are also shown in figure 2. even if the seismicity was mostly concentrated inside the seismic source zones 913 and 914, strong earthquakes have also occurred in the past within source zone 912, such as the 1346 (mw 5.81), 1570 (mw 5.48) and 1688 (mw 5.88) earthquakes. for this seismogenic zone, the diss database of potential seismogenic sources (in the 2.0 version available in 2004) reported information on faults that can article history received july 24, 2012; accepted august 28, 2012. subject classification: 2012 emilia earthquakes, seismic hazard, ground-motion recordings, design code spectra. 2012 emilia earthquakes generate earthquakes up to magnitude 6.2. all of this information was considered to derive the seismicity rates and define the maximum magnitude (mmax) to be adopted in the assessment of seismic hazard. the seismicity rates adopted in mps04 for source zone 912 are shown in figure 3. two methods were followed for their computation: in the ar branch, the rates were estimated independently for each magnitude bin, while in the gr branch, the rates were fitted assuming the gutenbergrichter frequency–magnitude distribution. for both of these meletti et al. 624 figure 1. seismic hazard map for the emilia-romagna region. the color at each grid point represents the expected horizontal pga on rock with a10% probability of exceedance in 50 years (as indicated). red stars, epicenters of the events of the 2012 emilia sequence with magnitudes ≥5. figure 2. seismic source zones from the zs9 model (blue lines), potential seismogenic structures from the diss database (green boxes), and earthquakes from the cpti04 catalog (magnitudes mainly derived from historical data) in the northern apennines. red stars, epicenters of the two main events of the emilia earthquake sequence. 625 branches, the rates for the mmax (magnitude bin = mw 6.14 ±0.115) were derived from geological considerations only. figure 3 also shows the cumulated number of earthquakes in 1000 years (without any consideration about catalog completeness) reported by the cpti04 [cpti working group 2004] and cpti11 [rovida et al. 2011] catalogs, together with the seismicity rates assessed for the same source zone in the framework of the ongoing seismic hazard harmonization in europe (share) project (http://www.share-eu.org). the small differences between the cpti04 and cpti11 catalogs are mainly due to the updated procedures used in the cpti11 catalog for magnitude determination from macroseismic data points. anyhow, the seismic rates adopted in mps04 appear to be conservative with respect to the observations and to an independent estimate, such as that of the share project. the output of the reference mps04 seismic hazard model is given in terms of horizontal ground accelerations on rock for several return periods and several spectral periods, computed on a regular grid (5-km spacing) that covers the whole italian territory. for each grid point, the pga and uniform hazard spectra (uhs) are available for nine probabilities of exceedance in 50 years, ranging from 2% to 81%, and corresponding to return periods from 2475 years to 30 years. the data are accessible through a webgis application (http://esse1-gis.mi.ingv.it) [martinelli and meletti 2008]. figure 4 shows the uhs for the grid point id15173 (longitude, 11.0935˚; latitude, 44.8608˚), which is the closest node to the only permanent station of the national accelerometric network (rete accelerometrica nazionale, ran) located in the epicentral area of the first event (i.e., mrn, mirandola). the uhs computed for the nine return periods are plotted to represent the whole range of expected values. the 16th percentile curve for the return period of 30 years, and the 84th percentile curve for the return period of 2475 years, are also shown, to illustrate the wide uncertainty of the pshas. 3. comparison of probabilistic seismic hazard assessments and ground-motion recordings of the emilia earthquakes the two main shocks of the emilia seismic sequence of may 20 and 29, 2012, together with the april 6, 2009, l'aquila event (mw 6.3), are the strongest earthquakes to have occurred in italy since the release of the reference mps04 seismic hazard model and of the current italian building code (ntc08), which has been in force since july 1, 2009. of course, the seismological community and the media looked at the seismic hazard model in the area to test the model performance against the new observations, in the same way as happened after the l'aquila 2009 earthquake [see e.g., crowley et al. 2010, stucchi et al. 2010, masi et al. 2011]. in that case, the assumptions and modeling choices made in the mps04 hazard study were in line with the observations [see crowley et al. 2010, for a more detailed analysis]. as is well known from a statistical point of view, it is not possible to validate a probabilistic hazard estimate with data observed for a single earthquake: indeed, on one side, there is a ground-motion value expected with a certain probability of occurrence, while, on the other, there is just one observed value and we cannot know if it represents the maximum expected value, the most probable value, or something else. on the contrary, a meaningful way to assess the reliability of a psha map, e.g., for a 475-year return period, should be to observe whether, over a period of 50 years, more than 10% of the computational sites have experienced ground motions higher than those predicted [see e.g., albarello and d'amico 2008]; indeed, even several consecutive 50-year periods of observations would be required to make this test robust. what it is possible to do is to compare the recorded accelerations with the probabilistic hazard model (the range of expected values and their uncertainties) to understand if they are consistent with each other. furthermore, any comparisons must be performed by considering the same conditions. this means that the values for the same shaking parameter (in our case, the maximum horizontal acceleration) and for the same soil class should be compared. figure 5 shows the maximum horizontal pgas produced by the two emilia earthquakes up to an epicentral distance of 200 km. while the main shock was only recorded by ran permanent stations, for the second shock, data obtained by 16 temporary stations that were installed in the epicentral area in the aftermath of the first shock are also available [dolce et al. 2012]. the strong-motion data were manually post-processed, adopting the procedure used for the italian accelerometric archive (itaca) database and described by paolucci et al. [2011] and pacor et al. [2011]. the highest values were registered for the may 29, 2012, seismic hazard and the 2012 earthquakes figure 3. comparison between the cumulative number of earthquakes in the cpti04 and cpti11 catalogs and the seismicity rates for the gr and ar branches of mps04 and for the share project (as available on july 2012) for seismic source zone 912. all rates are normalized to 1000 years. event at the ran stations of cento (cnt, a newly installed permanent station) and of mirandola (mrn) (0.30 g and 0.29 g, respectively), and for the may 20, 2012, earthquake at the mrn station (0.26 g). as shown in figure 5, for the first event, only one record (i.e., at mrn) is available within 35 km of the epicenter, while for the second earthquake, 13 recordings were acquired, thus allowing the pga decay with distance to be better depicted. values larger than 0.15 g were meletti et al. 626 figure 4. acceleration uniform hazard spectra for several return periods (as indicated) considered in mps04 at the grid point closest to the mirandola (mrn) station. figure 5. larger horizontal pgas (in g) versus the epicentral distance recorded for the may 20 and 29, 2012, earthquakes. the ec8 [cen 2004] soil class of each station (as assigned based on the geological map) is also indicated. 627 only observed in the first 20 km, and the pga quickly decreased beyond this distance. the larger horizontal pgas recorded at mrn, which was the ran station nearest to both of the epicenters, are compared with the seismic hazard estimates computed at the node of the computational grid closest to the mrn station (figure 6). for this, we consider the hazard curve; i.e., the expected pga for different return periods (or the inverse; i.e., seismic hazard and the 2012 earthquakes figure 6. comparison between the hazard curves for soil types a and c, computed at the node of the computational grid closest to mirandola and the maximum horizontal pgas recorded at the mrn station for the may 20 and 29, 2012, earthquakes. the 16th and 84th percentile curves represent the epistemic uncertainty of the probabilistic seismic hazard estimate. figure 7. comparison between the horizontal acceleration response spectra recorded at the mrn station for the two main events of the emilia sequence, and the design code spectra prescribed by ntc08, for soil type c, for 30, 475 and 2475-year return periods (as indicated). the annual frequency of exceedance, afoe, of different pgas). to make this comparison meaningful, we applied the ntc08 coefficient for soil-type c sites (like the one of mrn station, see figure 5), to the original hazard curve of the mps04 model computed for soil class a (i.e., rock or very stiff soil, with vs30 ≥800 m/s). as shown in figure 6, the expected pga ranges from 0.05 g to 0.43 g (from the shortest to the longest return period), and the highest recorded pga (for the may 29, 2012, event) was 0.29 g, which was thus consistent with seismic hazard estimates. in figure 7, the horizontal acceleration response spectra (5% damping) recorded at the mrn station for the two main events of the emilia sequence are compared with the design code spectra corrected for soil type c, as prescribed by ntc08, for the shortest, the longest, and the most 'standard' return period considered (i.e., 30, 2475 and 475 years, respectively). both recorded spectra largely exceed the code spectrum for 475 years, as they look much closer to that expected for 2475 years, especially in the case of the we components, which always lie below this latter, except for the sharp peak at 0.1 s observed for the may 29, 2012, event. major differences emerge instead for the ns components, which go over the 2475-year design spectrum for several period ranges; i.e., at 0.27 s, around 0.6 s to 0.7 s, and at longer periods. in particular, the bulge observed at the intermediate-to-long periods (between 0.7 s to 2 s for the main shock, and beyond 1 s for the second shock) appears to be widely unpredicted by the code spectrum. however, preliminary site response analyses based on horizontal-to-vertical spectral ratios of noise measurements suggest remarkable site amplification for periods longer than about 1 s at mrn and other stations inside the po plain [bordoni et al. 2012, this volume; dolce et al. 2012]. such amplifications are also, in many cases, coupled with the generation of large-amplitude surface waves that affects the spectral amplitudes at long periods. moreover, the spectral accelerations at periods greater than about 2 s recorded at most of the stations inside the po plain are underestimated by the recent ground-motion predictive model developed for italy [bindi et al. 2011], whereas the opposite holds for stations outside the po plain [luzi et al. 2012]. finally, it is worth mentioning that the mrn station is located just few kilometers from the earthquakes epicenters, and the large amplitude difference between the ns and we components might be due to near-source directivity effects that typically increase the ground-motion amplitude in the strike-normal direction (i.e., ns for both earthquakes, given the fault-plane solutions by http://www.bo.ingv.it/rcmt/) [e.g., somerville et al. 1997]. except mrn and cnt (which only had a large peak at 0.3 s), all of the other stations that recorded the events of the emilia sequence showed response spectra below code spectra. however, if response spectra from single earthquakes are compared with seismic design code spectra, the conceptual difference between the two objects should be kept in mind. indeed, code spectra are based on uniform hazard spectra computed by psha (in particular, spectral shapes in ntc08 are those that more closely fit the uhs provided by the reference seismic hazard model) [see montaldo et al. 2007], where each spectral ordinate has the same return period (or probability of exceedance in a fixed time span). thus, for a given return period, different spectral ordinates of the uhs can be caused by different earthquakes taken into account by the psha; i.e., all possible events that can occur within a given area [see also crowley et al. 2010]. furthermore, in the case of ntc08, code spectra only consider the median computed uhs, thus neglecting information on the relevant large uncertainty. as shown in figure 4, for the 84th percentile curve for 2475-year return period, the expected acceleration on rock nearly reaches 0.9 g at 0.2 s, even without any soil correction (from class a to c), which would certainly raise the curve up to the peak values observed for the two emilia main events. in conclusion, even considering all of the mentioned issues concerning the comparison of a record from a single event with a probabilistic estimate, the analysis performed in our study shows that the accelerometric evidence from the 2012 emilia earthquakes are consistent with the reference seismic hazard model of italy that was adopted for the ntc08 building code. references albarello, d., and v. d'amico (2008). testing probabilistic seismic hazard estimates by comparison with observations: an example in italy, geophys. j. int., 175, 1088-1094. ambraseys, n.n., k.a. simpson and j.j. bommer (1996). prediction of horizontal response spectra in europe, earthq. eng. struct. d., 25, 371-400. bindi, d., f. pacor, l. luzi, r. puglia, m. massa, g. ameri and r. paolucci (2011). ground motion prediction equations derived from the italian strong motion database, b. earthq. eng., 9, 1899-1920. bordoni, p., r. azzara, f. cara, r. cogliano, g. cultrera, g. di giulio, a. fodarella, g. milana, s. pucillo, g. riccio, a. rovelli, p. augliera, l. luzi, s. lovati, m. massa, f. pacor, r. puglia and g. ameri (2012). preliminary results from emersito, a rapid response network for site-effect studies, annals of geophysics, 55 (4); 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available at http://www.protezionecivile.gov.it/jcms/it/ran.wp luzi, l., f. pacor, g. ameri, r. puglia, p. burrato, m. massa, p. augliera, g. franceschina, s. lovati and r. castro (2012). overview on the strong motion data recorded during the may-june 2012 emilia seismic sequence, seismol. res. lett., submitted. malagnini, l., r.b. herrmann and m. di bona (2000). ground motion scaling in the apennines (italy), b. seismol. soc. am., 90, 1062-1081. malagnini, l., a. akinci., r.b. herrmann, n.a. pino and l. scognamiglio (2002). characteristics of the ground motion in northeastern italy, b. seismol. soc. am. 92, 2186-2204. martinelli, f. and c. meletti (2008). a webgis application for rendering seismic hazard data in italy, seismol. res. lett., 79, 68-78. masi, a, l. chiauzzi, f. braga, m. mucciarelli, m. vona and r. ditommaso (2011). peak and integral seismic parameters of l'aquila 2009 ground motions: observed versus code provision values, b. earthq. eng., 9, 139-156; doi: 10.1007/ s10518-010-9227-1. meletti, c., f. galadini, g. valensise, m. stucchi, r. basili, s. barba, g. vannucci and e. boschi (2008). a seismic source zone model for the seismic hazard assessment of the italian territory, tectonophysics, 450, 85-108; 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(2011). cpti11, the 2011 version of the parametric catalogue of italian earthquakes. milano, bologna; http:// emidius.mi.ingv.it/cpti sabetta, f., and a. pugliese (1996). estimation of response spectra and simulation of nonstationary earthquake ground motions, b. seismol. soc. am., 86, 337-352. somerville, p.g., n.f. smith, r.w. graves and n.a. abrahamson (1997). modification of empirical strong ground motion attenuation relations to include the amplitude and duration effects of rupture directivity, seismol. res. lett., 68, 199-222. stucchi, m., c. meletti, a. rovida, v. d'amico and a.a. gómez capera (2010). historical earthquakes and seismic hazard of the l'aquila area, progettazione sismica 'speciale abruzzo' (special volume in english), 23-34. stucchi, m., c. meletti, v. montaldo, h. crowley, g.m. calvi and e. boschi (2011). seismic hazard assessment (20032009) for the italian building code, b. seismol. soc. am., 101, 1885-1911; doi:10.1785/0120100130. valensise, g., and d. pantosti (2001). database of potential sources for earthquakes larger than m5.5 in italy, annali di geofisica, supplement to vol. 44 (4), 180 pp., with cdrom. *corresponding author: carlo meletti, istituto nazionale di geofisica e vulcanologia, sezione di milano/pavia, milano, italy; email: carlo.meletti@pi.ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. seismic hazard and the 2012 earthquakes << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice prikryl_#6219_ok_corrected_layout 6 annals of geophysics, 56, 2, 2013, r0222; doi:10.4401/ag-6219 r0222 probabilistic forecasting of ionospheric scintillation and gnss receiver signal tracking performance at high latitudes paul prikryl1,*, veettil sreeja2, marcio aquino2, periyadan t. jayachandran3 1 communications research centre, ottawa, on, canada 2 university of nottingham, nottingham geospatial institute, nottingham, united kingdom 3 university of new brunswick, physics department, fredericton, nb, canada abstract at high latitudes, phase scintillation occurs predominantly on the dayside in the ionospheric footprint of the magnetospheric cusp, and in the nightside auroral oval. a new technique of probabilistic forecasting of phase scintillation occurrence relative to the arrival time of high-speed solar wind from coronal holes and interplanetary coronal mass ejections has recently been proposed [prikryl et al. 2012]. cumulative probability distribution functions for the phase-scintillation occurrence that are obtained can be specified for low (below-median) and high (above-median) values of various solar wind plasma parameters. recent advances in modeling of high-speed solar wind and coronal mass ejections, combined with the probabilistic forecasting of scintillation, will lead to improved operational space weather forecasting applications. scintillation forecasting and mitigation techniques need to be developed to avoid potential costly failures of technology-based global navigation satellite systems in the near future, in particular during the upcoming solar maximum. the global navigation satellite systems receiver-tracking performance during severe scintillation conditions can be assessed by the analysis of receiver phase-locked-loop jitter. tracking jitter maps [sreeja et al. 2011] offers a potentially useful tool to provide users with expected tracking conditions, if based on scintillation predictions as proposed above. scintillation indices are obtained from l1 gps data collected with the canadian high arctic ionospheric network. combined with high rate amplitude and phase data, they can be used as input to receiver tracking models to develop scintillation mitigation techniques. 1. introduction global navigation satellite systems (gnss), such as the us global positioning system (gps), the russian gnss (glonass) and the eu galileo, have become indispensable for precise positioning and time keeping for the support of a wide range of civilian and military applications. the main threat to reliable and safe operation of modern technology that relies on gnss is the variable propagation conditions that are encountered by radio signals as they pass through the ionosphere. variable ionospheric delays and signal scintillation can affect the performance of radio communications and navigation systems, and can severely degrade gps positional accuracy. scintillation, rapid signal amplitude fading, and phase fluctuation, are phenomena of great concern at low and high latitudes. as pointed out by conker et al. [2003], to determine the effects of scintillation on gnss receivers, including the gps/satellitebased augmentation system (sbas), "both predictions of scintillation activity in the ionosphere and residual effects of this activity after processing by a receiver" must be considered. these authors estimated the scintillation effects on the availability of gps and sbas for l1 (1575.42° mhz) c/a and l2 (1227.60° mhz) semicodeless receivers. in the present study, the benefits of a probabilistic forecasting of phase scintillation to applications relying on robust and accurate phase signal tracking are addressed. ionospheric scintillation, which is seen as fluctuations of radio signal amplitude and phase that are caused by ionospheric irregularities [aarons 1982, basu et al. 1998], is most intense at low latitudes during post sunset hours [basu et al. 2002], and at high latitudes in the nightside auroral oval and on the dayside in the region called the cusp [basu et al. 1998, spogli et al. 2009, prikryl et al. 2011a]. at high latitudes, ionospheric scintillation is largely controlled by the process of coupling between the solar wind, including the interplanetary magnetic field, and the earth magnetic field. in the nightside auroral oval, intense electric currents (auroral electrojets), various plasma instabilities, and energetic particle precipitation result in steep plasma density gradients and irregularities. it has been shown that scintillation strongly affects gps receiver performance in the article history received december 20, 2011; accepted january 13, 2012. subject classification: scintillation, space weather forecasting, gnss tracking performance. auroral zone [e.g., skone and de jong 2000]. in the cusp, just poleward of the dayside auroral oval, where intense ionospheric convection and formation of patches of enhanced ionization occur, scintillation occurrence is particularly enhanced. scintillation occurrence is more seasonally dependent in the cusp than in the nightside auroral oval [prikryl et al. 2011a], which is characterized by annual and semi-annual variations, respectively. it is also strongly enhanced during intense solar wind events, as either solar wind co-rotating interaction regions (cirs), or interplanetary coronal mass ejections (icmes). icmes and cirs on the leading edge of high-speed streams (hsss) from coronal holes are major causes of ionospheric disturbances. the strongest of these solar wind events result in major geomagnetic storms [e.g., borovsky and denton 2006, richardson et al. 2006] that have a global impact on the ionosphere. based on a statistical study of scintillation occurrence during such events from the years 2008 to 2010, a new method of probabilistic forecasting of phase scintillation was proposed [prikryl et al. 2012]. a summary of this method using a larger number of events is presented in the present study. scintillation indices, s4 and vu, are widely used as the measures of amplitude and phase scintillation, respectively. the s4 index is the standard deviation of the received power normalized by its mean value, and the phase scintillation index vu is the standard deviation of the detrended phase using a filter in the receiver with a 0.1 hz cut-off frequency. however, as pointed out by aquino et al. [2007], these scintillation indices do not fully characterize the actual instantaneous values of phase and amplitude fluctuations that affect gnss receiver performance. they combined scintillation indices and high rate gnss phase and amplitude data with state-of-the-art receiver tracking models, to study receiver tracking performance under scintillation conditions. this strategy was also applied for an intense scintillation event at low latitudes [sreeja et al. 2011], where the variance of the phase-locked-loop (pll) tracking error (tracking jitter) of a gps receiver was used to construct maps that can provide an indication of tracking conditions for that particular receiver. in this study, tracking-jitter maps are obtained for a highlatitude scintillation event. 2. the canadian high arctic ionospheric network at present, the canadian high arctic ionospheric network (chain) [jayachandran et al. 2009] consists of ten specialized gps receivers and six canadian advanced digital ionosondes that are distributed in the auroral oval and the polar cap (figure 1). the latitudes and longitudes of the stations can be found elsewhere [e.g., prikryl et al. 2011a, their table 1]. the receivers are gps ionospheric scintillation and total electron content (tec) monitors (gistms) gsv 4004b [van dierendonck and arbesser-rastburg 2004]. the gistms consist of a novatel oem4 dual frequency receiver with special firmware specifically configured to measure and log the power and phase of the gps l1 signal at a high sampling rate (50 hz). the receiver computes ionospheric tec from the gps l1 and l2 signals. the gsv 4004b can also automatically compute and record s4 and vu indices; 1-min values are used in the present study. each receiver can track and report scintillation and tec measurements simultaneously from up to 10 gps satellites in view. 3. occurrence of gps phase scintillation at high latitudes a scintillation climatology study for the solar minimum years of 2008 and 2009 showed that phase scintillation, as a function of magnetic local time and geomagnetic latitude, primarily occurs in the night-side auroral oval and on the dayside in the ionospheric footprint of the cusp [prikryl et al. 2011a]. as expected, these scintillation regions shift in latitude in response to varying geomagnetic activity. we have extended the climatology data set to 4 years. figure 2a, b shows the maps of the percentage occurrence of phase scintillation with vu that exceeds 0.1 radians as a function of magnetic local time (mlt) and corrected geomagnetic (cgm) latitude for 1,272 quiet and 189 disturbed days selected using the threshold of 3-h occurrence of kp >2 prikryl et al. 2 figure 1. canadian high arctic ionospheric network (chain). the loci of ipps at 350 km altitude for all gps satellites, as seen from iqaluit at elevation angles above 30° during one day. the cgm latitudes 70° and 80° (yellow) are superposed over the geographic grid in the dotted line. 3 for more than 60% of the day. the boundaries of the feldstein statistical auroral oval for quiet and moderately disturbed conditions are superposed on the maps [holtzworth and meng 1975], indicated by the value of the iq parameter. a dotted concentric circle in figure 2b shows the latitude of cambridge bay. the cambridge bay dataset samples the poleward edge of the auroral oval in the night [prikryl et al. 2010], but it is representative of the cusp during the day. the cambridge bay data time series are used to obtain cumulative probability distribution functions for the phase scintillation occurrence for the days before and after the arrivals of cir/hsss and icmes. figure 3a, b shows scintillation occurrence maps for the first two days of hss/cirs and for icme days, respectively. in total, 88 hss/cirs with maximum solar wind velocity exceeding 500 km/s are included. the updated catalog of near-earth icmes [richardson and cane 2010] includes 57 icme events from 2008 to 2011 (101 days in total). however, most of these icmes did not cause significant geomagnetic storms [e.g., prikryl et al. 2011b]. scintillation occurred predominantly in the cusp for both datasets, but the occurrence was higher for icme days than for hss/cir days. this is opposite to the 2008 to 2010 results [prikryl et al. 2012] and it is a sign that many more significant icmes occurred in 2011. also, comparing the total numbers of hss/cirs and icmes, they increased by 114% and 33%, respectively, from 2010 to 2011. 4. the superposed-epoch analysis of geomagnetic and phase scintillation indices arrivals of hsss originating from coronal holes are followed by significantly enhanced phase scintillation occurrence, particularly in the cusp, which usually continues for a few days. to obtain the mean response of the scintillation relative to a new cir arrival time, a superposed-epoch analysis on time series of scintillation occurrence (figure 4a) and solar wind parameters was applied, using the key times of 88 cirs. figure 4a shows that the mean phase scintillation occurrence steeply rises at the cir arrival day and tapers off a few days later, which is similar to the mean kp index and the quasi-invariant index, qi [osherovich et al. 1999]. also, the average number of gps cycle slips, which is nearly zero a few days before the cir key day, increases at the probabilistic forecasting of scintillation figure 2. phase scintillation occurrence maps for geomagnetically quiet (a) and moderately disturbed (b) days. figure 3. phase scintillation occurrence maps for days of cirs/hsss (a) and icmes (b). prikryl et al. 4 cir interface and remains elevated during the hss. the one-day periodicity in scintillation is due to the cambridge bay cusp location and is only partly smoothed out in this relatively small statistical sample of 88 cirs. figure 4b shows the results of the superposedepoch analysis for 39 icmes for which the dst index dropped below −10 nt, eliminating the weakest icmes. the qi increases more gradually but to higher values than for cirs. this is because many icmes are actually magnetic clouds, and the ratio of the magnetic pressure to ram pressure that the qi represents is higher inside magnetic clouds. the phase scintillation occurrence peaks at ~15% and the mean number of cycle slips peaks at more than 5 /h on the day of the icme arrival. a cycle slip [horvath and crozier 2007] is defined here as a jump in differential phase tec of more than or equal to 1.5 tec units in 1 s (1 tec unit corresponds to 1016 electrons/m2). if the superposedepoch analysis is further limited to only 15 of the strongest icmes (dst < −30 nt), the peak mean scintillation occurrence, the mean number of cycle slips, and the kp index are 22%, 15 and 4.3, respectively. 5. probabilistic forecasting of phase scintillation in the cusp based on the superposed-epoch analysis of solar wind cirs and icmes mcpherron and siscoe [2004] and mcpherron and weygand [2006] demonstrated a new promising technique to forecast geomagnetic disturbance indices relative to the stream interface between the fast and slow solar wind. in particular, they showed geoeffectiveness of what they called co-rotating compression ridges, their reference to cirs. this method is readily applicable to other manifestations of space weather in the geospace environment that are sensitive to arrivals of hsss, including ionospheric scintillation. figure 5a shows the cumulative probability distribution function (pdf) for the phase scintillation occurrence at cambridge bay for the days before and after the major cir/hsss arrivals. the probability that phase scintillation will exceed a given value is shown, as plotted on the abscissa. the pdfs are stepped by 2-day intervals from 6 days before and 6 days after cirs. figure 5b shows the pdfs for days before and after icmes. the probabilities are clearly highest for the first two days after cir and icme arrivals. these pdfs can be used to develop high-latitude scintillation forecasting techniques [prikryl et al. 2012]; e.g., scintillation occurrence in the cusp relative to the stream interface between the fast and slow solar wind and/or shocks associated with cirs and icmes. to gain further practical application of the probabilistic forecasting of scintillation occurrence, tracking-jitter maps can be constructed to assess gnss receiver signal-tracking performance under anticipated scintillation conditions [sreeja et al. 2011]. 6. tracking-jitter maps in a gnss receiver, the pll aims to minimize the error between the input phase and its estimated phase output. it is the magnitude of this error that determines the ability of the loop to remain locked. the variance of the error at the output of the pll (the tracking-jitter variance) increases during scintillation, and hence is a good measure of the effects of scintillation on the receiver. the tracking-jitter variance at the output of the pll can be calculated using the scintillation sensitive tracking model of conker et al. [2003]. the formula for the gps l1 carrier pll accounts for the effects of scintillation on the input phase and computes the trackingjitter variance , in radians2, as [conker et al. 2003]: (1) where are, respectively, the error variance components relating to the phase scintillation, the figure 4. superposed epoch analysis of qi, kp, number of cycle slips ncys and phase scintillation occurrence. the time series are keyed by the start times of cirs (a) and icmes (b). 2vz^ h , ands t osc 2 2 2v v v 2 2 2 2 s t osc v v v v= + +z z z z 5 probabilistic forecasting of scintillation thermal noise (amplitude scintillation), and the oscillator noise (assumed to be 0.01 radians2 in the receiver). in equation (1), the amplitude scintillation is modeled as an increase in the thermal noise, related to the decrease in the received signal power, in radians2, as [conker et al. 2003]: (2) where bn is the l1 third-order pll one-sided bandwidth (equal to 10 hz in this study), (c/n0)l1−c/a is the fractional form of the signal-to-noise density ratio, equal to 100.1c/n0, h is the predetection integration time (equal to 0.02s in this study), and s4(l1) is the amplitude scintillation index at l1 frequency. the phase scintillation component is modeled, in radians2, as [conker et al. 2003]: (3) where t is the spectral strength of the phase power spectrum density (pds) at 1 hz, p is the spectral slope of the phase psd, k is the order of the pll (equal to 3 in this study), and fn is the loop natural frequency (equal to 1.91 hz). for this study, the one minute scintillation indices and the high data rate (50-hz phase and amplitude fluctuations) for the gps l1 c/a signal recorded by the receivers in the chain network on november 1, 2011, were considered. a satellite elevation angle cut-off of 30° was applied to the data to reduce the impact of non-scintillation-related effects on the tracking errors (e.g., induced by multipath). the latitude of the ionospheric pierce point (ipp) for the different satellite-to-receiver links was calculated every 1 min, assuming a single-shell ionospheric model at an altitude of 350 km. the pll jitter for the different satellites in view with elevation angles greater than 30° at each epoch were calculated using the equations above. these pll jitter relate to the pll tracking error assumed in the slant direction of the line of sight between receiver and satellite. for practical use of the proposed pll jitter maps it is necessary that a verticalised version of this error is represented, so that the user can estimate the error for any specific line of sight by applying an appropriate mapping function. this mapping function, as explained in sreeja et al. [2011], is the subject of ongoing research funded by the uk engineering and physical sciences research council (epsrc). for the construction of the map presented in the present study, however, an approximation was used to convert the slant pll jitter to vertical pll jitter, by assuming a standard mapping function cos |, where | is the zenith angle at the ipp. the values of the verticalised pll jitter were then gridded in bins with a resolution of 1 min in time and 0.5° in ipp latitude, to produce the pll jitter map of figure 7 in the next section. figure 5. cumulative probability distribution functions for the phase scintillation occurrence for cambridge bay, stepped by 2-day intervals before and after cirs (n = 88) (a) and icmes (dst < −10 nt; n = 39) (b). b / 2 / c n s l c n s l 1 2 1 1 1 1 2 1 / / l c a n l c a2 0 1 4 2 0 1 4 2 t v h = + z ^ ^^ ^ ^^ h hh h hh; e tr k k p 2 2 1 n p 2 1 s v r = + -z sinkf 6 @c m prikryl et al. 6 7. the event of november 1, 2011 figure 6 shows 5-min averages of the solar wind velocity, vsw, the interplanetary magnetic field (imf) bz component in geocentric-solar-magnetospheric (gsm) coordinates, and the magnetic field magnitude, bt, from the omni dataset projected to subsolar bow shock (http://omniweb.gsfc.nasa.gov/). the arrival of an icme on november 1 (shown by a vertical dotted line) resulted in an ae index of about 1000 nt and the dst index dipped to −61 nt, intensifying a geomagnetic storm that started earlier after the arrival of another icme and a moderate hss stream on october 30. the hourly percent occurrence of phase scintillation index vu >0.1 rad was enhanced between 10:00 and 22:00 ut, as shown for the cambridge bay gps receiver. figure 7 shows the maps of vu values (top panel) and pll jitter (bottom panel) as a function of ut and ipp geographic latitude for the chain gps l1 signals, as described in the previous section. these maps were constructed using the data from all of the stations of figure 1, except station qikiqtarjuaq. it is also worth noting that for the construction of the maps in figure 7, not only the pll jitter have been verticalised, but also the vu values. the latter were verticalised using the mapping function described in spogli et al. [2009]. it can be observed from figure 7 that the regions of enhanced pll jitter generally coincide with enhancements in vu. the phase scintillation events at high latitudes occurred primarily in the cusp and dayside polar cap between ~10:00 and 20:00 ut, which also coincided with the occurrence of pll jitter enhancements. this indicates increased likelihood for the occurrence of cycle slips and loss of lock, which degrade the positioning accuracy. in figure 8 the percentage occurrence of scintillation and pll jitter are mapped as functions of magnetic latitude and mlt, to show the main regions of enhanced scintillation relative to the auroral oval. figure 8 shows the occurrence maps for s4 and the vu indices, as well as for pll jitter, on a coarser grid figure 6. solar wind magnetic field and plasma parameters from omniweb dataset combining available solar wind monitor data projected to the nose of the earth bow shock. the hourly percent occurrence of vu > 0.1 rad from cambridge bay is highlighted in blue. figure 7. phase scintillation index vu (a) and receiver pll jitter maps (b) as functions of ut and ipp geographic latitude for the chain gps l1 signals. 7 probabilistic forecasting of scintillation (compared to figure 7) of 1° × 0.25 h of mlt. the position of the statistical auroral oval for very disturbed conditions (iq = 6) [feldstein 1967, holzworth and meng 1975] is superposed. the phase, and to a lesser degree the amplitude, scintillation occurrence exceeds given threshold values maximized in the cusp and the polar cap, although significant scintillation also occurred at auroral and even at subauroral latitudes. the percentage occurrence of pll jitter exceeding 0.15 rad was significantly lower, although this generally mapped to the same regions as the phase scintillation; i.e., the cusp, the polar cap, and the auroral oval. figure 9 shows the 60-s values of vu and pll jitter obtained for the chain gps receiver in iqaluit on november 1, 2011. the scatter plot (figure 9c) shows that vu is moderately correlated with the pll jitter (pearson's correlation coefficient, 0.68). 8. discussion the climatology of gps phase scintillation at high latitudes [prikryl et al. 2011a] has been extended by two more years to a period from 2008 to 2011. this larger dataset has confirmed that scintillation predominantly occurs in the ionospheric footprint of the cusp. the primary driver of the cusp dynamics is the solar wind coupling to the dayside magnetopause, which peaks with arrivals of icmes and hss/cirs. based on the statistics of these solar wind events, using a superposed epoch analysis technique, a probabilistic forecasting method that focuses on the phase scintillation in the cusp has been proposed [prikryl et al. 2012]. solar activity has been gradually rising since 2010, and there was a significant increase in the number of moderate icmes in addition to hss/cirs. the number of icmes that resulted in dst < −10 nt doubled in 2011 alone, when compared with the period of extended solar minimum (2008-2010). the superposed-epoch analysis based on improved statistics of events shows consistently higher probability of scintillation occurrence in the cusp on the days of arrival of icmes and hss/cirs, in contrast to the low probability 1-2 days earlier. this pattern of significant increase in scintillation occurrence makes it possible to specify phase scintillation forecasts for anticipated arrival days of icmes and hss/cirs [prikryl et al. 2012]. while the scintillation activity in the polar cap was generally quite low from 2008 to 2010, during moderate to strong events, irregularities in a tongue of ionization or in polar patches that were drawn into the polar cap caused significantly enhanced scintillation occurrence in polar regions [prikryl et al. 2011b, prikryl et al. 2013a, this issue]. this depended on the phase of the season in either the southern or northern hemisphere. in particular, the event discussed in section 7 was associated with strong ionospheric convection across the polar cap and enhanced tec in a tongue of ionization [prikryl et al. 2013b]. fast drifting ionospheric irregularities within a structured tongue of ionization resulted in high occurrence of strong phase scintillation (figure 8b). under scintillation, maps of tracking jitter [sreeja et al 2011] can be used to assess the tracking conditions, the likelihood for the occurrence of cycle slips, and the loss of lock, which are known to degrade the gnss positioning accuracy. the pll jitter is correlated with vu and the enhancements of pll jitter appear to coincide, in general, with the occurrence of phase scintillation. figure 8. the occurrence of s4 index (a), phase scintillation (b) and receiver pll jitter (c) as functions of mlt and cgm latitude for november 1, 2011. the statistical auroral oval for very disturbed conditions is superposed. prikryl et al. 8 hence, applications relying on robust and accurate phase signal tracking should benefit from the probabilistic forecasting of phase scintillation. 9. conclusions phase scintillation occurrence maps from 2008 to 2011 for geomagnetically quiet and disturbed days, and more specifically for the prime days of hss/cirs or icmes, all show predominance of scintillation occurrence in the cusp. cumulative probability distribution functions for the phase scintillation occurrence that are a by-product of superposed epoch analysis of growing databases of cirs and icmes can be used in probabilistic forecasting of scintillation activity relative to the arrival times of these major solar wind disturbances. phase scintillation occurrence can impact on the gnss receiver tracking performance, which can be assessed by the tracking jitter maps. the pll jitter is correlated with the phase scintillation index and the regions of enhanced pll jitter approximately coincide with phase scintillation occurrence. hence the probabilistic forecasting of scintillation activity should be beneficial in assessing prevailing gnss tracking conditions at high latitudes, and particularly in the cusp. acknowledgements. infrastructure funding for chain was provided by the canada foundation for innovation and the new brunswick innovation foundation. chain operation is conducted in collaboration with the canadian space agency. research activities at nottingham geospatial institute, university of nottingham, are funded by the uk engineering and physical sciences research council project polaris (http://www.bath.ac.uk/elec-eng/polaris/) and the fp7 funded project transmit (http://www.nottingham. ac.uk/transmit/index.aspx). the solar wind data were obtained from goddard space flight center space physics data facility omniweb (http://omniweb.gsfc.nasa.gov/). references aarons, j. (1982). global morphology of ionospheric scintillations, proc. iee, 70, 360-378. aquino, m., m. andreotti, a. dodson and h. strangeways (2007). on the use of ionospheric scintillation indices as input to receiver tracking models, adv. space res., 40, 426-435. basu, s., e.j. weber, t.w. bullett, m.j. keskinen, e. mackenzie, p. doherty, r. sheehan, h. kuenzler, p. ning and j. bongiolatti (1998). characteristics of plasma structuring in the cusp/cleft region at svalbard, radio sci., 33, 1885-1899; doi:10.1029/98rs01597. basu, s., k.m. groves, su. basu and p.j. sultan (2002). specification and forecasting of scintillations in communication/navigation links: current status and future plans, j. atmos. sol.-terr. phys., 64, 1745-1754. borovsky, j.e., and m.h. denton (2006). differences between cme-driven storms and cir-driven storms, j. geophys. res., 111, a07s08; doi:10.1029/2005ja0 11447. conker, r.s., m.b. el arini, c.j. hegarty and t. hsiao (2003). modeling the effects of ionospheric scintillation on gps/sbas availability, radio sci., 38, 1001; doi:10.1029/2000rs002604. feldstein, y.i., and g.v. starkov (1967). dynamics of auroral belt and polar geomagnetic disturbances, planet. space sci., 15, 209-230. holzworth, r.h., and c.-i. meng (1975). mathematical representation of the auroral oval, geophys. res. lett., 2, 377-380. horvath, i., and s. crozier (2007). software developed for obtaining gps derived total electron content values, radio sci., 42, rs2002; doi:10.1029/2006rs00 3452. jayachandran, p.t., et al. (2009). canadian high arctic ionospheric network (chain), radio sci., 44 (1), rs0a03; doi:10.1029/2008rs004046. mcpherron, r.l., and g. siscoe (2004). probabilistic forecasting of geomagnetic indices using solar wind air mass analysis, space weather, 2, s01001; doi:10. 1029/2003sw000003. mcpherron, r.l., and j. weygand (2006). the solar wind and geomagnetic activity as a function of time relative to corotating interaction regions, in: b. tsurutani, r.l. mcpherron, w. gonzalez, g. lu, j.h.a. sobral and n. gopalswamy (eds.), recurrent magnetic storms: corotating solar wind streams, agu monograph, 167, 125-137. osherovich, v.a., j. fainbergand and r.g. stone (1999). solar wind quasi-invariant as a new index of solar activity, geophys. res. lett., 26, 2597-2600. prikryl, p., p.t. jayachandran, s.c. mushini, d. pokhotelov, j.w. macdougall, e. donovan, e. spanswick figure 9. phase scintillation index vu (a), receiver pll jitter (b), and correlation between vu and the pll jitter (c). 9 probabilistic forecasting of scintillation and j.-p. st.-maurice (2010). gps tec, scintillation and cycle slips observed at high latitudes during solar minimum, annales geophysicae, 28, 1307-1316. prikryl, p., p.t. jayachandran, s.c. mushini and r. chadwick (2011a). climatology of gps phase scintillation and hf radar backscatter for the high-latitude ionosphere under solar minimum conditions, annales geophysicae, 29, 377-392; doi:10.5194/angeo-29-377. prikryl, p., et al. 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midlatitude european regions, annales geophysicae, 27, 3429-3437. sreeja, v., m. aquino and z.g. elmas (2011). impact of ionospheric scintillation on gnss receiver tracking performance over latin america: introducing the concept of tracking jitter variance maps, space weather, 9, s10002; doi:10.1029/2011sw000707. van dierendonck, a.j., and b. arbesser-rastburg (2004). measuring ionospheric scintillation in the equatorial region over africa, including measurements from sbas geostationary satellite signals, proc. ion gnss 17th technical meeting of the satellite division, long beach, ca, usa, 316. *corresponding author: paul prikryl, communications research centre, ottawa, ontario, canada; email: paul.prikryl@crc.gc.ca. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice studio preliminare sulle oscillazioni libere del lago di sauris a due diverse quote (*) g . romualdi m . c. spadea r i c e v u t o il 30 maggio 1966 riassunto. — i n questa prima parte, viene applicato al l a g o di sauris il m e t o d o di h i d a k a — opportunamente esteso — allo scopo di studiarne le oscillazioni libere. l a ricerca è stata particolarmente interessante per i due seguenti m o t i v i : 1. si t r a t t a di un bacino artificiale f o r m a t o dallo sbarramento del torrente l u m i e i ( d i g a del luiniei alta 130 m). 2. si è potuto studiare il l a g o a due diverse quote (980 m, invaso pieno — e 939 m, quota allegerimento), f a t t o che è pressoché impossibile riscontrare in un lago naturale. sono stati così determinati i periodi, la posizione dei nodi e le ampiezze corrispondenti alla sessa unie bi-nodale alle due diverse quote. n e l l a seconda parte, di prossima pubblicazione, lo studio sarà esteso al confronto f r a i dati qui ottenuti per via teorica e quelli ottenuti per v i a sperimentale mediante registrazioni liinnografiche. summary. — i n the first stage concerning the f r e e oscillations of the sauris' l a k e , w e limited ourselves to the periods, knots and amplitudes determination of the uni and bi-nodal seiche, making use of the h i d a k a ' s method. t h e investigation is particularly interesting due to t w o particular reasons: 1. i t is an artificial reservoir, f o r m e d barring the l u m i e i torrent. ( l u m i e i d a m , high 130 m). 2. t h e l a k e has been studied at t w o different altitudes (whole basin, 980 mand 939 m ) , matter which in a natural lake is almost impossible t o find. i n the second part, the research will be exended to the comparison of the theoretical resultshere achievedw i t h the experimental results obtained b y limnographic records. ( * ) questo l a v o r o è stato compiuto con contributi del c . n . r . 368 a . k 0 m u a l i 1 i m. c. s p a d e a p a r t e i premessa. l'accordo fra teoria (applicazione di vari metodi matematici di chrystal, defant, hidaka, goldberg, alcuni dei quali opportunamente estesi) e osservazione (registrazioni ottenute con limnografi nello studio delle oscillazioni libere di laghi naturali, golfi, e t c . . . ) è stato più volte confermato, come risulta dai numerosi lavori già pubblicati. i l lavoro è stato diviso in due parti. nella prima, che concerne questa nota, il lago artificiale di sauris viene studiato dal punto di vista teorico; nella seconda parte di prossima pubblicazione lo scopo sarà quello di dimostrare che l'accordo sopra menzionato viene rispettato, anche nel caso di laghi artificiali quale è quello del lumiei, formatosi quest'ultimo dopo la costruzione della diga sul torrente omonimo, uno dei tanti affluenti del tagliamento. 1. l a diga del lumiei è sita in località maina di sauris (carnia) a 1000 m. di altezza s.m., del tipo ad arco-cupola (arco a doppia curvatura); essa è alta 136 m, con imo spessore di 16 ni alla base e di 3,5 m al coronamento (sezione maestra), ed ha creato un serbatoio della capacità utile di 70 milioni di m3. i l lago di sauris dopo un'ansa piuttosto larga a monte della diga, si restringe notevolmente assumendo ima forma molto allungata. l a profondità massima si ha appunto in corrispondenza di questa ansa, ed essa decresce rapidamente in concomitanza al diminuire della larghezza, tanto da essere minima all'estremo opposto. n e l caso specifico vanno sottolineati tre aspetti del lago: a) è un bacino artificiale; b) di forma irregolare (vedi curva normale: p i g g . 1-2); c) è soggetto a forti variazioni di livello. quest'ultima caratteristica ci ha permesso di studiare il lago di sauris in due diverse situazioni di invaso: a invaso pieno (m. 980) e ad ima quota di alleggerimento (m 939), quasi si trattasse di due laghi diversi, tali risultando dal punto di vista morfometrico. a nostro avviso il punto e) è il più significativo, dato che non è possibile usufruire in natura di un lago che, in epoche diverse, presenti differenze di quota di oltre 40 metri. studio p r e l i m i n a r i ; s u l l e o s c l l l a z r o n i liberi-; d e l lago 1)1 s a u r i s 3 6 9 l a prima jmrte di questa nota si riferisce appunto allo studio delle oscillazioni libere del lago di sauris a invaso pieno e a quota di alleggerimento (41 m e t r i sotto il livello normale). v a ancora messo in evidenza che la b a t i m e t r i a del lago è p e r f e t t a m e n t e conosciuta, a causa del rigoroso r i l e v a m e n t o topografico cui il bacino è stato sottoposto p r i m a della costruzione della diga; ciò che o v v i a m e n t e non a v v i e n e per i laghi naturali la cui batimetria è sovente inesatta, in ogni caso ottenuta con un t r o p p o l i m i t a t o numero di sondaggi, ripetuti soltanto a lunghi intervalli di t e m p o . l a g o d i sauris a quota 980. 2. i principali elementi m o r f o m e t r i c i del lago di sauris a bacino pieno sono: altezza m . 980 s.m. superficie km 2 1 . 7 6 5 . 1 2 2 p r o f o n d i t à massima m 140 . 3. d a t o il particolare t i p o di lago, abbiamo ritenuto opportuno applicare il m e t o d o di h i d a k a (2), il più idoneo per laghi irregolari, la cui curva normale risulti complicata ( f i g . 2). n e l caso in esame per ottenere i dati necessari alla determinazione di d e t t a curva ci siamo serviti di una carta b a t i m e t r i c a del lago 1 : 2000 (disegno f o r n i t o a suo t e m p o dalla s a d e ) . 370 <;. komuar.m m. c. spadka l'estensione del metodo (3), non solo consente di determinare i periodi delle oscillazioni libere, ma anche la posizione delle linee nodali, nonché l'ampiezza degli spostamenti verticali nelle singole sezioni trasversali, tracciate perpendicolarmente alla linea di valle dall'estremo 0 alla diga; dette sezioni distano una dall'altra 100 m (fig. 3). tutti i dati numerici necessari all'applicazione del metodo sono riportati nella tabella i . 4. metodo di hidaka. senza sollerrnarsi sulla teoria, già più volte ampiamente esposta e opport unamente ampliata (3), richiamiamo solo le formule fondamentali, necessarie a fornire i dati numerici per il nostro lavoro. d e t t o f lo spostamento orizzontale delle particelle liquide di ima x stessa sezione trasversale s(x) e posto u — i-s(x)-, v = f b(x)dx, dove ó b(x) rappresenta la larghezza variabile del bacino misurata alla superfìcie libera, l'equazione sul moto libero dei laghi (equazione di chrystal) (*) assume la forma: d2u 4ti2 i n essa, valgono le condizioni ai limiti u(o) — u(a) = 0 (a = superfìcie totale del lago). nella [1] <j(v) — s(x) • b(x) rappresenta la curva normale del lago (con a(v) che si annulla agli estremi in modo che sia a{o) = a{a) = 0) nella quale i valori di a(v) sono riportati in ordinata e quelli dell' j b(x) dx in ascissa. v ! posto a = la [11 diviene: a d'u x — + — u = 0 , [2] dza(z) con le condizioni ai limiti w(0) = m(1) = 0 [3] e a = [4] g t 2 l j g . r o m u a l d i .m. c. sl'adea fig. 3 s t u d i o p r e l i m i n a r i ; s u l l e o s c l l l a z r o n i l i b e r i ; d e l l a g o 1)1 s a u r i s 3 7 1 hidaka integrando la [2] con le condizioni [3], ha provato che questo equivale alla ricerca del valore minimo dell'integrale: 0 posto m u = sì ai z (1 —e) z< con a i — cost. , [6] o e sostituendo la [6] nella [4], determiniamo le m + 1 costanti a0, alt... am in modo che la [5] diventi un minimo, il che presuppone l'annullaci i mento dei rapporti delle derivate . ì) ai con opportuni calcoli si ottiene un sistema di m + 1 equazioni: j — i 0 ^ a 0 + y — i j a ^ x + f ~ — i t x j a t + = 0 l i + i h j 4 ^ + ( i r ' 4 a > + = 0 • eliminando le i costanti, si annulla il determinante dei coefficienti di a0, alt a2 , la cui soluzione dà l'equazione dei periodi. i n questa equazione si ha: ~ i ct(z) ds[8] ò poiché la soluzione diviene tanto più difficoltosa quanto maggiore è il valore di m, hidaka fornisce le soluzioni per m = 1 ed m = 2, soluzioni più che sufficienti per il nostro problema. per m = 1, dalla [6] risulta u = s (1 — z) ( + a^z), l'equazione del periodo della sessa uninodale si ottiene annullando il determinante formato dai coefficienti di j.„ e nelle prime due equazioni della [7]: i / . a ) ( l a a 0 ! 372 itomualni \r. c. m'\:t. \ sviluppando il quale si ha: ( z 0 j2 — j ^ ) — [ 9 ] sostituendo ad i 0 , l i valori numerici ricusati dalla tabella i , si ha un'equazione di 2° in a le cui radici sono: = 0,00952 r i c o r d a n d o la [4], d o v e 1\ = = 0,06171 . ti a t2 = ti a , siamo finalv i / a ' v f f i ^ m e n t e arrivati alla determinazione dei periodi della sessa uni e binodale: t1 = 6">03» = 2m 22s . [10] p e r m = 2, sempre dalla [6], si ha m = z (1 — z) {aa -r atz -ja2 z-) . l ' e q u a z i o n e dei periodi si ottiene sviluppando e annullando il determinante f o r m a t o dai coefficienti di a0, alt j.2> nelle prime tre equazioni del sistema [7]: •70a i o • i 2 a 1_ 6 15 1_ 10 i 2 a 73a 1 i o 1 10 3 35 — 72 a — h i — 7 4 a = 0 ( i l i — 7 7 — 7 7 4 2 1 1 i — 1 p \ 0 2 4 0 x3 1 4 ~ a1 x2 3 2 ] 7 1 7 2 + 7 1 7 3 — 7 0 rs 15 35 « v , a ì 0 ì 4 + i . i . + i . z . i ^ j ; [11] + 53 \700 ——j 1 — — r q.-.n i • 2100 " 350 30 60 a u 10500 = 0 . s t u d i o p r e l i m i n a r i ; s u l l e o s c l l l a z r o n i l i b e r i ; d e l lago 1)1 s a u r i s 3 7 3 dalla tabella i , sostituiti ad i 0 , i l t i 2 , i 3 , z4 i loro valori numerici i 0 = 27,91089 ; = 9,05917 ; i 2 = 3,95690 ; i 3 = 2,11514 ; i 4 = 1,30720 , abbiamo un'equazione di 3° in x: 188972 p — 34583 )? + 1337 / — 9,52381 = 0 . [12] mediante l'applicazione del metodo di n e w t o n per successive approssimazioni otteniamo la prima radice corretta di x = 0,00920): dividendo poi la [12] per la radice trovata si ha un'equazione di 2° in x, risolvendo la quale abbiamo i valori di 2x2 e 2/3: = 0,00920 , 2a2 = 0,04134 , 2a3 = 0,13247 . sostituendo nella [4] rispettivamente le tre radici trovate, si ricavano i periodi corrispondenti alla sessa uni-bi e tri-nodale: tx = 6® 09s t2 = 2™ 54s ts = l m 376 . [13] 5. nodi. p e r la determinazione dei nodi ci rifacciamo alla [6] limitandoci al caso m = 2. p e r m — 2, u = a0 z (1 —z) (l + z + da cui derivando rispetto a 0 e ricordando che nei nodi du dz = ° ^ otteniamo per risolvere la [15], ci serviamo ancora una volta della [7], ricavando —• da una qualunque coppia di equazioni del sistema — i ,valori numerici dei rapporti e ''j 2 ; ~ = —1,56949 ~ = + 0,85246 . sostituiti questi valori nella [15], otteniamo un'equazione di 3° in z, che risolveremo ricorrendo al metodo di n e w t o n e rammentando la condizione ai limiti 0 < z < 1 : 3,40984 s3 — 7,26585 s2 + 5,13898 s — 1 = 0 . [16] 374 g. k o m u a l d i m . c. s p a i } e a r i s o l v e n d o la [16], v e d i a m o elle l'unica radice possibile, delle tre ottenute, è & = 0,31223 , la quale rappresenta il valore dell'uninodo. l ' u n i n o d o cade quindi f r a le sezioni 21 e 22 a ca k m 2,14 dall'estremo 0 del l a g o ( v . t a b . i , f i g . 1). a a sostituendo nella [15] ai rapporti e —? i valori numerici ottenuti e servendoci del v a l o r e della radice 2a2r dalla [7] si ha: 16,25872 z3 — 27,76689 s2 + 12,38190 » — 1 = 0 . [17] l e radici utili, comprese cioè f r a 0 ed 1 sono: & = 0,10320 , 2z2 = 0,58381 . i due nodi, quindi, cadono rispettivamente f r a le sezioni 10 e 11 a d una distanza di k m 1,05 dall'estremo 0 del lago, e f r a le sezioni 29 e 30 a circa 1 k m dalla diga ( t a b . i e f i g . 1). 6 . ampiezze. p e r determinare le ampiezze, la formula rj = — ^ a (a = superficie totale del l a g o ) clz in particolare per il caso m = 2, si può scrivere: lago .savris curve ampiezze (q. 980) 0.60 o.'w 0 . 2 0 -0.20 -0.40 -0.60 o.eo 1.00 z 4 6 6 » li 14 16 16 . • zi z4 26 28 « iz 34 36 38 * • ' x * f i n . 4 s t u d i o p r e l i m i n a r i ; s u l l e o s c l l l a z r o n i l i b e r i ; d e l l a g o 1)1 s a u r i s 3 7 5 t a b e l l a i i sezioni ax 102m <9 r 0 0 — 1 — i 1 1 — 0,98807 — 0,97130 2 2 — 0,95317 — 0,88807 3 3 — 0,91840 — 0,79209 4 4 — 0,87962 — 0,71589 5 5 — 0,83618 — 0,61645 6 6 — 0,77915 — 0,48867 7 7 — 0,72741 — 0,37576 8 8 — 0,67509 — 0,26460 9 9 — 0,62624 — 0,16380 10 10 — 0,57328 — 0,05807 11 11 — 0,51583 + 0,05197 12 12 0,45910 + 0,15518 13 13 — 0,39637 i0,26254 14 14 — 0,33874 + 0,35382 15 15 — 0,28168 0,43606 16 16 — 0,22583 + 0,50756 17 17 — 0,18292 + 0,55513 18 18 0,13974 f0,59562 19 19 — 0,09181 + 0,62997 20 20 — 0,04618 f 0,65002 21 21 — 0,00306 i 0,65437 22 22 | 0,03462 + 0,64297 23 23 + 0,06911 + 0,61585 24 24 + 0,10125 + 0,57035 25 25 i0,12826 + 0,50973 26 26 + 0,15040 -f 0,43694 27 27 + 0,17004 + 0,34217 28 28 •h 0,18524 + 0,23325 29 29 + 0,19643 + 0,10667 30 30 f0,20417 — 0,05472 31 31 + 0,20706 — 0,26929 32 32 |0,20591 — 0,43875 33 33 + 0,20603 — 0,52217 34 34 + 0,20857 — 0,55308 35 35 + 0,21362 — 0,54943 36 36 f 0,22301 — 0,50833 37 37 + 0,25171 — 0,33369 38 38 + 0,27049 — 0,20990 39 39 + 0,27863 — 0,15540 40 39,7 + 0,28297 — 0,12627 370 l i . li' i m l'atei >1 m. c. s p a i i e a r i p r e n d i a m o i v a l o r i 2/kl e 2/2, corrispondenti alla sessa uni e binodale, rispettivamente, la [19] diviene: c = 3,10984 z3 — 7,26585 s2 -f5.13898 z — 1 [20] = 16,25872 s3 — 27,76689 s2 + 12,38190 g — \ . servendoci ora dei valori numerici di s dati dalla tabella i e sostituendoli nella [20], si ha l ' a n d a m e n t o delle ampiezze p e r le due sesse considerate. l a f i g . 4 dà una rappresentazione grafica di detto andamento, mentre i risultati del calcolo sono contenuti nella t a b e l l a i i . l a g o ni s a u r i « a quota 939. 7. l a possibilità di eseguire registrazioni nei periodi di svaso di un lago art iliciale, ci ha permesso di studiare il lagoj di sauris a q. 939 (quota di alleggerimento della diga), applicando lo stesso m e t o d o usato per il lago ad invaso pieno, allo scopo di determinarne i periodi, i nodi e le ampiezze della sessa unie binodale. v a premesso che la diminuzione del livello del bacino, e quindi della superficie libera del lago, lascia al di fuori dell'acqua t u t t a la zona compresa precedentemente fra le sezioni 0 . . . 13 ( f i g . 1). l a sezione 0 del lago a quota 939, coincide pertanto con la sezione 13 del lago a quota 980. i l grafico in f i g . 5 rappresenta la curva normale, quello in f i g . 6 l ' a n d a m e n t o delle sezioni verticali alla quota in esame. i dati e,he caratterizzano la forma del lago sono contenuti nella tabella i i i , dalla quale si traggono anche i v a l o r i numerici per l'applicazione del m e t o d o di itidaka: i 0 = 8 0 , 5 0 6 5 7 i j = 2 8 , 6 2 1 6 9 / 2 = 1 3 , 6 0 6 4 0 i 3 = 7 , 8 1 6 0 7 [21] i 4 = 5 , 1 2 8 6 6 , valori necessari al calcolo dei periodi, dei nodi e delle ampiezze. 8. periodi. trascurando il caso m = 0, per m = 1 l'equazione di 2° grado in x risulta: 2 7 6 , 2 0 3 4 5 6 x~ — 5 , 7 2 9 0 9 0 x + 0 , 0 1 6 6 6 7 = 0 , le cui radici sono: ìaj = 0 , 0 0 3 5 0 ^ = 0 , 0 1 7 2 4 . [ 2 2 ] suzioni virheuh fig. g 378 g. b o m l ' a l d i m. c. s l ' a l l e a sostituendo nella [4], i valori della [22], si hanno i periodi corrispondenti rispettivamente alla sessa unie bi-nodale: = 4m52s 1\ = 2ra12s . [23] per m ~~ 2, si ha la seguente equazione di 3° grado in /: 67,073751» /? — 3,212071 a2 + 0,038071 a — 0,0000952381 = 0 , risolvendo la quale si hanno le tre radici: 2/i = 0,003416 . ,à2 = 0,013361 , 2a3 = 0,031111 . [24] introdotti i valori della [24] nella [4], si ottengono i valori dei periodi per la sessa unibie trinodale del lago: t i = 4m 56s (uninodale) t2 -2m 29s (binodale) [25] rj\ = t » 38s (trinodale) . 9. nodi. s e i caso m 2, sostituendo nell'equazione dei nodi [15] il valore ^ = 0,003416, si ha un'equazione di 3° grado in z: 3,02960 s3 — 6,57654 z* -f 4.86956 z — 1 = 0 . [26] l'unica radice possibile (0 < z < 1) è quella minore dell'unità: 2si = 0,33021 . [27] che rappresenta il valore corrispondente all'uninodo; questo viene dunque a trovarsi fra le sezioni 25-26 e dista 1,24 km circa dall'estremo 0 del lago (v. fig. 1, tabella i i i ) . sostituendo poi nella [15] la radice 2x.> == 0,013361, si ha: 14,11492 s» — 24,92157 s* + 11,55692 — 1 = 0 , [28] le cui soluzioni valide sono: jsì = 0,11175 2z2 = 0,60362 . [29] i due binodi cadono, pertanto, fra le sezioni 19-20 e 30-31 e distano rispettivamente circa 0,6 km dall'estremo 0 del lago, e circa 0,9 km dalla diga (v. f i g . 1. tabella i i i ) . studio p r e l i m i n a r i ; s u l l e osclllazroni liberi-; del lago 1)1 s a u r i s 3 7 9 10. ampiezze. p e r m — 2, le equazioni delle ampiezze relative alle oscillazioni dell'uninodale e della binodale sono [6], [7]: c = 3,02960 z3 — 6,57654 z2 -f 4,86956 z — 1 £ " = 14,11492 z3 — 24,92157 z2 + 11,55691 z — 1 . sostituendo nella [30] i valori numerici di 0 dati dalla t a b . i l i , si ottengono le ampiezze delle oscillazioni in corrispondenza delle v a r i e t a b e l l a i v sezioni ax 102m r r 0 = 13 0 — i — i 1 = 14 1 — 0,95758 — 0,90005 2 = 15 2 — 0,88617 — 0,73515 3 = 16 3 — 0,79925 — 0,54087 4 = 17 4 — 0,72408 — 0,37897 5 = 18 5 — 0,63356 — 0,19268 6 = 19 6 — 0,53474 — 0,00192 7 = 20 7 — 0,42914 + 0,18456 8 = 21 8 — 0,33014 + 0,33899 9 = 22 9 — 0,23202 + 0,46720 10 = 23 10 — 0,14677 + 0,55187 11 = 24 11 — 0,08042 + 0,59438 12 = 25 12 — 0,02446 + 0,60882 13 = 26 13 + 0,03317 + 0,59467 14 = 27 14 + 0,08269 + 0,54842 15 = 28 15 + 0,12936 + 0,45756 16 = 29 16 + 0,16204 + 0,34627 17 = 30 17 + 0,18704 + 0,20961 18 = 31 18 + 0,21264 — 0,04301 19 = 32 19 + 0,22649 — 0,30613 20 = 33 20 + 0,23403 — 0,44033 21 = 34 21 + 0,24267 — 0,50290 22 = 35 22 + 0,25696 — 0,51025 23 = 38 23 + 0,28275 — 0,43331 24 = 37 24 + 0,29897 — 0,36042 25 = 38 25 + 0,31081 — 0,30807 26 = 39 26 + 0,31807 — 0,27256 27 = 40 26,7 + 0,32262 — 0,24973 380 ( i . h o m u a l ' d i m. c. s p a i ) ica sezioni del lago; i valori ottenuti — riportati nella tabella i v — ci hanno permesso di costruire graficamente le curve delle ampiezze (fig. 7). 0.60 0.40 0 . 2 0 0l a 6 0 -savris curve ampiezza ( q . 9 3 9 ) * h 1 1 '—t0 2 4 6 8 10 o.zo 0.40 f — 0.60 j. v 0 . 6 0 1.00 a ^ 14 16 18 20 22 24 26 fig. 7 conclusioni. i n questa prima parte del lavoro sulle oscillazioni libere di un lago1 artificiale, quale è il lago di sauris, ci siamo limitati a determinare i periodi della sessa uni e bi-nodale nei casi m = 1 ed m = 2, la posizione dei nodi e l'andamento delle ampiezze, alle [quote 980 (invaso pieno) e 939 (quota alleggerimento diga). d a i valori numerici si sono potuti ricavare i grafici relativi. l a seconda parte, che è in fase di compimento, sarà dedicata al; confronto fra i dati ottenuti per via teorica e quelli ottenuti per v i a sperimentale. , ringraziamo il prof. p. galoi per l'assistenza e per i preziosi consigli fornitici, i quili ci hmny permesso di portare a termine questa prima parte dello studio del lago di sauris. studio pjuolimixakk s u l l e o s c i i . l a z i o m l i b e r e del laoo 1)1 sauuis 3 8 1 b i b l i o g r a f i a f 1 ) ci-irystal g., on the hydrodyuaniical theory of seiches. transactions of the r o y a l society of edinburg. x l i (1905), x l v (1906), x l v i (1907-8). ( 2 ) hidaka k . , application of bite's variation to the determination of seiches in a lake. t h e memoirs of the imperial marine observatory, v i , 2, (1960). ( 3 ) caloi p . , le sesse del layo di garda. p a r t e i , « annali di geofisica », i , 1, (1948). ( 4 ) caloi p . , le sesse del lago di garda. p a r t e i i , « a n n a l i di geofisica», i , 2, (1948). volcanic ash cloud observation using ground-based ka-band radar and near-infrared lidar ceilometer during the eyjafjallajökull eruption annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6634 1 volcanic ash cloud observation using ground-based ka-band radar and near-infrared lidar ceilometer during the eyjafjallajökull eruption frank s. marzano 1,2, luigi mereu 1,2, mario montopoli 1,2, domenico cimini 3,2 and giovanni martucci 4 1 diet sapienza università di roma, rome, italy, 2 center of excellence cetemps, university of l’aquila, l’aquila, italy, 3 imaa, national research council (cnr) tito scalo, italy, 4 national university of ireland, galway, ireland (currently at meteoswiss, switzerland) *frank.marzano@uniroma1.it abstract active remote sensing techniques can probe volcanic ash plumes, but their sensitivity at a given distance depends upon the sensor transmitted power, wavelength and polarization capability. building on a previous numerical study at centimeter wavelength, this work aims at i) simulating the distal ash particles polarimetric response of millimeter-wave radar and multi-wavelength optical lidar; ii) developing and applying a model-based statistical retrieval scheme using a multi-sensor approach. the microphysical-electromagnetic forward model of volcanic ash particle distribution, previously set up at microwaves, is extended to include non-spherical particle shapes, vesicular composition, silicate content and orientation phenomena for both millimeter and optical bands. monte carlo generation of radar and lidar signatures are driven by random variability of volcanic particle main parameters, using constraints from available data and experimental evidences. the considered case study is related to the ground-based observation of the eyjafjallajökull (iceland) volcanic ash plume on may 15, 2010, carried out by the atmospheric research station at mace head (ireland) with a 35-ghz ka-band doppler cloud radar and a 1064-nm ceilometer lidar. the detection and estimation of ash layer presence and composition is carried out using a bayesian approach, which is trained by the monte carlo model-based dataset. retrieval results are corroborated exploiting auxiliary data such as those from a ground-based microwave radiometer also positioned at mace head. i. introduction olcanoes are among most important natural sources of ash, which may influence meteo-climatological conditions on large scales modifying the earth radiation budget [graf et al., 2007]. continuous monitoring of such phenomena is crucial for the initialization of ash dispersion models [degruyter and bonadonna, 2012].satellite visibleinfrared radiometric observations from geostationary platforms are usually exploited for long-range trajectory tracking and for measuring low-level eruptions [e.g., rose et al., 2000; corradini et al., 2011]. ground-based microwave radars represent an important tool to detect ash clouds [harris and rose, 1983; lacasse et al., 2004; marzano et al., 2006; gouhier and donnadieu, 2008; schneider and hoblitt, 2009; marzano et al., 2010]. the possibility of monitoring in all weather conditions at a fairly high spacetime resolution is the major advantage of using ground-based scanning weather radar systems at s, c and x band [marzano et al., 2013]. on the other v annals of geophysics, fast track 2, 2014 2 hand, ka band doppler radar can provide a higher sensitivity to medium size particles [madonna et al., 2010]. the physical-chemical properties of volcanic particles are modified during advection, and size sorting takes place due to aggregation, breaking and fallout [sparks et al., 1997]. multi-wavelengths lidars can be complementary systems useful to integrate the micron-sized particle measurement, especially if far from the volcanic vent [e.g., ansmann et al., 1992]. lidar techniques developed for aerosol particle detection and estimation can be properly adapted for the retrieval of ash clouds [gesteiger et al., 2011; martucci et al., 2012; scollo et al., 2012]. previous methodological studies investigated the possibility of using ground-based radar systems for the quantitative remote sensing of volcanic ash cloud [marzano et al., 2006; 2010]. a volcanic ash radar retrieval (varr) algorithm for singleand dual-polarization radar systems was proposed and applied to s-, cand xband weather radar data volumes [marzano et al., 2013; montopoli et al., 2014]. this forward and inverse model framework can be extended to include very fine particles and to ingest both ka band radars and multi-wavelength lidars. the paper is organized as follows. sect. ii illustrates the ground-based observations carried out at the atmospheric research station (ireland) on may 15, 2010, concerning the ash plume emitted by the eyjafjallajökull volcano. sect. iii summarizes the results and discusses the outlooks. the annex deals with ash microphysics, scattering and extinction models for microwave and near-infrared wavelengths. ii. vacr combined retrieval case study in order to fully exploit the multi-sensor multi-wavelength polarimetric forward scattering model for ash cloud remote sensing purposes, we can apply a bayesian metrics to combined radar-lidar data [marzano et al., 2013]. this approach will be briefly illustrated before discussing the eyjafjallajökull eruption case study with measurements taken from the mace head site on may 15, 2010. the mace head atmospheric research station supersite over the west of ireland in carna, county galway, is an example of integrated measuring site [martucci et al., 2012]. ground-based remote sensing of cloud microphysics is performed using ka-band doppler cloud radar (mira36), a jenoptik chm15k lidar-ceilometer at 1024 nm, and a rpg-hatpro multi-channel microwave radiometer combined with the synergistic analysis scheme. the radar mira36 radar is a monostatic magnetron-based pulsed ka-band doppler radar. linearly polarized signal at 35.5 ghz is transmitted, while coand cross polarized signals are received simultaneously to detect doppler spectra of the reflectivity and linear depolarization ratio. the radar is also equipped with a 3-d scanning unit even though it is usually zenith pointing with a vertical resolution up to 15 m. note that chm15k data are available only up to 8000 m and its sensitivity is lower than multi-wavelength research lidar [e.g., madonna et al., 2010]. however, our purpose is to show the potential and flexibility of the combined inversion methodology. ii.a combined radar-lidar retrieval algorithm similarly to the volcanic ash radar retrieval (varr) approach [marzano et al., 2006], the volcanic ash combined retrieval (vacr) utilizes 2 steps: i) ash classification; ii) ash parameter estimation. both steps are trained by the hapess forward polarimetric model, where particle distributions, density, and permittivity parameters are supposed to be constrained random variables within a monte carlo approach (see annex for details and symbols). within the vacr technique, ash classification is performed by means of maximum a posteriori probability (map) estimation criterion. the probability density function (pdf) of each ash class (ci), condiannals of geophysics, fast track 2, 2014 3 tioned to the measurement vector xm, can be expressed by bayes’ theorem (marzano et al., 2006). the map estimation of ash class ci, corresponds to the maximization with respect to c of the posterior pdf p(ci|xm). under the assumption of multivariate gaussian pdfs, the previous maximization reduces to minimizing a quadratic distance d(x,ci) with respect to ci:                     )(ln2)det(ln ),( 1 ixi ximxi t xim im cp cd c mxcmx x (1) where “t” is the matrix transpose, whereas mxi and cxi are the mean vector and covariance matrix, respectively, of the combined simulated vector x of the class ci. the a priori probability p(ci) can be used to weight the different classes on the basis of ancillary information and/or data. using hapess dataset, a regressive model can be used as a function of the class c to estimate both ash concentration cp and numberweighted mean diameter dnp of the class ci. using a log-linearized parametric model, it holds:         )(lnln )(lnln )( lnlnln )( ln )( )( lnlnln )( ln )( c xmxxxd c d c np c xmxxxc c c c p md mc mxcc mxcc 1 1 (2) where mlnc and mlnd are the log-value averages of cp and dnp, clnxc and clnxx are the log-value cross-covariance and auto-covariance matrices, whereas lnxm and mlnx are log-value measurement vector and its mean vector. in (2) the regression coefficient matrices are obtained assuming a zero-mean random noise due to instrumental and forward modeling uncertainties. as already done elsewhere for varr [marzano et al., 2010, 2012], a numerical analysis of the vacr retrieval uncertainty has been carried out using a synthetic simulated dataset in presence of noise. using 4 combined observables (as later on in sect. 4.2), results show that: i) the particle correct classification, using (1), has an average probability higher than 95%; ii) the regression root mean square error, using (2), is less than 25% with respect to the same parameter mean value. of course, these scores degrade if the noise increases due to instrumental causes or data pre-processing errors (e.g., path attenuation correction). ii.b available data and vacr results the eyjafjöll stratovolcano is located under the eyjafjallajökull small glacier within the icelandic east volcanic zone. the eruptions in 2010 lasted several weeks [guðmundsson et al., 2010; marzano et al., 2010]. geostationary satellite observations from seviri, combined with other sources, indicate that the eyjafjallajökull volcanic cloud covered much of ireland on may 15, 2010 [guðmundsson et al., 2012]. the ash cloud was indeed detected at the mace head site as documented in [o’dowd et al., 2012]. in order to apply the vacr algorithm, we have considered ground-based observations of the eyjafjallajökull volcanic ash plume on may 15, 2010 at mace head, carried out with the mira36 doppler cloud radar and chm15k lidar ceilometer. the mace head combined radar-lidar system provided both measured zhhm and ldrm at ka-band from mira36 and hhm at nir from chm15k. note that, being the lidar ceilometer system at single wavelength, hh is derived after inverting measured hhm data through an inversion algorithm assuming a proper lidar ratio at =1064 nm [ferguson and stephens, 1983; martucci et al., 2012]. in order to apply vacr to mace head data, the combined measurement vector is set to xm=[zhhm(36ghz) ldrm(36ghz) hhm(m) hhmm)]t which can be reduced to radar-only and lidar-only special cases (see annex). based on a priori information, the number of particle classes can be optimized by merging sub-classes and selecting annals of geophysics, fast track 2, 2014 4 two low concentration classes (vc and sc) and three orientations (to.2, oo, and po). being far from the volcano vent, we do not expect lapilli and the coarse particles over ireland, so that only 2 dispersed classes (va, fa) are considered here. in order to consider clouds located above the freezing level, we have assumed a mixed-phased refractive index with a balanced mixture of ash and ice (vam and fam). finally, in a cold region, pure ice crystals (ic) and dry snow (ds) may be expected and their microphysical modeling are derived from marzano et al. (2007). spherical ash particles are also included for va, fa, ca considering both ash and mixed-phase particles. this implies that the number of vacr classes has been set to 18. the eyjafjallajökull volcanic ash plume over mace head, discussed here, is related to the observation of may 15, 2010 from 20:00 till 24:00 utc. measurements of mira36 and chm15k have been aligned in time and spatially averaged in order to deal with the different sensor specifications. their time series have been resampled to every 30 s, whereas in the vertical direction both measurements have been averaged at 30-m resolution. figure 1: time evolution of liquid water path (in kg/m2), derived from hatpro microwave radiometer, and freezing level during may 15, 2010. the discrimination of ash clouds with respect to water clouds is still an open issue [martucci et al., 2012]. the detection of a water cloud is aided by the liquid water path (lwp) estimate derived from collocated hatpro microwave radiometer measurements. fig. 1 shows the time series of lwp, together with the the freezing level height estimated from the temperature profile derived from hatpro. note that lwp is increasing up to 4 kg/m2 from 15:00 till 22:00 being almost negligible after then, while the freezing level is below 2 km. colocated radar-lidar measurements are shown in fig. 2 in terms of 24-hour profile time series of copular reflectivity zhh and ldr from mira36 and backscatter coefficient hh from chm15k. figure 2: range-time section of copolar reflectivity zhh(36ghz) and linear depolarization ratio ldr(36ghz) from mira36 (upper and lower panel) and copolar backscatter coefficient hh(1064nm) from chm15k (middle panel) on may 15, 2010. the black circles indicate the ash signature. figure 3: (upper panel) classification of ash cloud observed at mace head may 15 2010, from 20:00 to 24:00 utc, using only the radar observables (zhh and ldr). (lower panel) same as upper panel, but using both radar and lidar observables (zhhldr, hh, hh), when available. note lidar-ceilometer data are not available above 8000 m. the signature before 20:00 below 5 km can be attributed to an incoming altostratus which is followed 0 5 10 15 20 25 0 2000 f re e z in g [ m ] mace head 20100515 time utc [h] 0 5 10 15 20 25 0 5 l w p [ k g *m -2 ] 0 5 10 15 20 25 0 2000 4000 f re e z in g [ m ] mace head 20100516 time utc [h] 0 5 10 15 20 25 0 0.5 1 l w p [ k g *m -2 ] time utc [h] r a n g e [ m ] radar classification 20 20.5 21 21.5 22 22.5 23 23.5 24 5000 6000 7000 8000 9000 10000 c_sky fa_oo fa_po fa_sp fa_to fam_oo fam_po fam_sp fam_to va_oo va_po va_sp va_to vam_oo vam_po vam_sp vam_to ds_kc ic_ab time utc [h] r a n g e [ m ] lidar classification 20 20.5 21 21.5 22 22.5 23 23.5 24 5000 6000 7000 8000 9000 10000 c_sky fa_oo fa_po fa_sp fa_to fam_oo fam_po fam_sp fam_to va_oo va_po va_sp va_to vam_oo vam_po vam_sp vam_to ds_kc ic_ab annals of geophysics, fast track 2, 2014 5 by a stratus clouds with some rainfall. between 20:00 and 22:00 the lower stratus is coexisting with a high cloud, as showed by the radar above 6 km, which is not detected by chm15k due to the stratus extinction. after 22:00, when the lwp gets almost negligible due to the dissipation of the lower stratus (see fig. 1), the lidar-ceilometer signal shows a peak at some 8 km. correspondingly, the radar signal shows a feature around 8 km, a bit weaker than before 22:00. in summary, the ash cloud lidar-radar combined signature can be clearly detected around 6.5-10 km between 20:00 and 24:00, as indicated by black ellipses in fig.2. results from the two-step vacr are presented in fig. 3, where the vacr classification is obtained through (1) by using radar and radar-lidar observables. using (2), fig. 4 shows the vacr estimate of ash concentration by employing only radar observables, only lidar observables, and combined radar-lidar observables. figure 4: estimation of ash concentration in the zoomed ash cloud region in fig. 3, using radar (upper panel), lidar-ceilometer (middle panel) and combined lidar-radar observables, when available (lower panel). fig. 3 and 4 indicate that the prevailing ash classes are fine ash fa-oo (about 77%) with fam-po (16%) and some fa-po, fa-to, fa-sp (less than 4%) and fam-oo, fam-to, vam-to (less than 3%). this means that oblate ash particles are coexisting with heterogeneous nucleation of ice crystals. lidar signatures are sensitive to micron-sized particles and reveal the presence of mixed very fine ash vam which are mainly detected at the upper edges of the ash cloud (notably after 22 utc in fig. 4). from fig. 4 it is interesting to note that estimated ash concentration in the middle of ash cloud can reach values of 100 mg/m3 and its vertical profile is far from being uniform. by vertically integrating the vacrbased estimates of ash concentration of fig. 4, we can derive the ash cloud columnar content shown in fig. 5. similarly to fig. 4, fig. 6 shows the mean diameter of ash particles retrieved by vacr using (2). the combination of radar and lidar data extends the capability of each to detect ash particles between 20:00 and 24:00. figure 5: columnar ash concentration derived from vacr algorithm by vertically integrating ash concentration derived from fig. 4. figure 6: as in fig. 4, but for particle mean diameter dn. estimated effective diameters goes from 5 microns at the ash cloud edge and in regions where the cloud radar: estimation of c a [mg/m 3 ]-20100515-20-24 20 20.5 21 21.5 22 22.5 23 23.5 24 6000 8000 10000 0 50 100 lidar: estimation of c a [mg/m 3 ]-20100515-20-24 20 20.5 21 21.5 22 22.5 23 23.5 24 6000 8000 10000 0 50 100 time utc [h] radar & lidar: estimation of c a [mg/m 3 ]-20100515-20-24 20 20.5 21 21.5 22 22.5 23 23.5 24 6000 8000 10000 0 50 100 20 20.5 21 21.5 22 22.5 23 23.5 24 0 0.5 1 1.5 2 2.5 x 10 4 time c o lu m n a r a s h c o n c e n tr a ti o n [ m g /m 2 ] radar: ash concentration estimation -20100515-20-24 0 100 200 300 400 500 600 700 800 900 1000 5000 6000 7000 8000 9000 10000 h e ig h t [m ] ash concentration mean value [mg/m 3 ] radar: ash concentration estimation -20100515-20-24 mean conc. m.c.+std max c. min c.radar: estimation of d n [m]-20100515-20-24 20 20.5 21 21.5 22 22.5 23 23.5 24 6000 8000 10000 0 10 20 30 40 50 lidar: estimation of d n [m]-20100515-20-24 20 20.5 21 21.5 22 22.5 23 23.5 24 6000 8000 10000 0 10 20 30 40 50 time utc [h] radar & lidar: estimation of d n [m]-20100515-20-24 20 20.5 21 21.5 22 22.5 23 23.5 24 6000 8000 10000 0 10 20 30 40 50 annals of geophysics, fast track 2, 2014 6 layer is thin (mainly detected by lidar) up to 40 microns in the cloud inner core (mainly detected by kaband radar). iii. conclusions microwave radars and multi-wavelength lidars are complementary instruments, providing a complementary view with respect to the satellite segment. this work has shown how dual-polarization groundbased weather radars and lidars can be merged for volcanic ash cloud dynamical monitoring and quantitative retrieval of ash category, concentration, and effective diameter. the expected accuracy of vacr algorithm is conditioned by the microphysical assumptions. the eyjafjallajökull volcanic ash plume over the mace head site on may 15, 2010 has been used for testing the vacr methodology using a ka band radar and nir lidar-ceilometer. results confirm the potential of the combined approach highlighting interesting features of the retrieved ash cloud in terms of concentration and mean diameters. future work shall be focused on in situ data for a systematic characterization of the vacr absolute error estimates. acknowledgments. this work has been supported by the european fp7 projects aphorism (fp7-spa2013, n. 606738) and futurevolc (fp7-env-2012, n. 308377). references [ansmann et al. 2012] ansmann, a., et al.. combined raman elastic-backscatter lidar for vertical profiling of moisture, aerosol extinction, backscatter and lidar ratio, appl. phys. b., 55, 18–28. [corradini et al., 2011] corradini, s., et al. (2011). volcanic ash cloud properties: comparison between modis satellite retrievals and fall3d transport model. ieee grsl, 8, 248-252. [degruyter and bonadonna, 2012] degruyter, w., and bonadonna, c. (2012). improving on mass flow rate estimates of volcanic eruptions, geophysical research letters, 39, l16308. [gasteiger et al., 2011] gasteiger, j., et al. (2011). volcanic ash from iceland over munich: mass concentration retrieved from ground-based remote sensing measurements, acp, 11, 2209-2223. [gouhier and donnadieu, 2008] gouhier, m., et al. (2008). mass estimations of ejecta from strombolian explosions by inversion of doppler radar measurements. jgr, vol. 113, pp. b10 202–b10 219. [graf et al., 2007] graf, h. f., et al. (2007). volcanic effects on climate: revisiting the mechanisms, atm. chem. phys., 7, 4503–4511. [guðmundsson et al., 2010] guðmundsson, m. t., et al. (2010). eruptions of eyjafjallajökull volcano, iceland. eos 91, 190–191. [harris and rose, 1993] harris, d. m, and rose w. i. (1983). estimating particle sizes, concentrations, and total mass of ash in volcanic clouds using weather radar, j. geophys. res., 88, c15, pp. 10969–10983. [lacasse et al., 2004] lacasse, c., et al. (2004). weather radar observations of the hekla 2000 eruption cloud, iceland, bull. vol., 66, 5, 457–473. [madonna et al., 2010] madonna, f., et al. (2010). observation of nonspherical ultragiant aerosol using a microwave radar, geophys. res. lett., 37, l21814, doi:10.1029/2010gl044999. [marticci et al. 2012] martucci, g., et al. (2012). impact of volcanic ash plume aerosol on cloud microphysics. atm. environment, 48, 205-218. [marzano et al. 2006] marzano, f. s., et al. (2006). volcanic ash cloud retrieval by ground-based microwave weather radar. ieee tgrs, 44, 3235-3246. [marzano et al. 2010] marzano, f. s., et al. (2010). monitoring sub-glacial volcanic eruption using cband radar imagery. ieee tgrs, 48, 1, 403-414. annals of geophysics, fast track 2, 2014 7 [montopoli et al. 2014] montopoli, m., et al. (2014) interpretation of observed microwave signatures from ground dual polarization radar and space multi-frequency radiometer for the 2011 grímsvötn volcanic eruption (2014), atm. meas. techniques, 7 (2), pp. 537-552. doi: 10.5194/amt-7537-2014. [rose et al. 2000] rose, w. i., et al. (2000). integrating retrievals of volcanic cloud characteristics from satellite remote sensors: a summary, phil. trans. r. soc. a, vol. 358, 1770, 1585–1606. [schneider and hoblitt, 2012] schneider, d. j., hoblitt, r. p. (2012). doppler weather radar observations of the 2009 eruption of redoubt volcano, alaska, j. of vol. and geoth. res. v. 259, p. 133-144. [scollo et al. 2012] scollo, s., al. (2012). monitoring etna volcanic plumes using a scanning lidar. bull volcanol, doi 10.1007/s00445-012-0669-y. [sparks et al. 1997] sparks, r. s. j., et al. (1997). volcanic plumes, wiley, new york (usa). contemporary stress field in the area of the 2016 amatrice seismic sequence (central italy) annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 1 contemporary stress field in the area of the 2016 amatrice seismic sequence (central italy) maria teresa mariucci, paola montone istituto nazionale di geofisica e vulcanologia, roma1 mariateresa.mariucci@ingv.it, paola.montone@ingv.it abstract we update the last present-day stress map for italy relatively to the area of 2016 amatrice seismic sequence (central italy) taking into account a large number of earthquakes occurred from august 24th to october 3rd, 2016. in particular in this paper, we discuss the new stress data from crustal earthquake focal mechanisms selecting those with magnitude ≥ 4.0; at the same time, we revise the borehole data, analyze the stratigraphic profiles and the relative sonic logs in 4 deep wells located close to the amatrice sequence along the apennine belt and toward east along the adriatic foredeep. from these data we consider the pwave velocity trend with depth and estimate rock density following an empirical relationship. then we calculate the overburden stress magnitude for each well. the new present-day stress indicators confirm the presence of prevalent normal faulting regime and better define the local stress field in the area, highlighting a slight rotation from ne-sw to ene-wsw of extension. the analysis evidences that the lithostatic gradient gradually changes from ~26 mpa/km in the belt to less than 23 mpa/km along the adriatic foredeep. finally, at a depth of 5 km we estimate the vertical stress magnitude varying from 130 mpa to 114 moving from the apennine belt to the adriatic foredeep. although the wells are very close each other they show different p wave velocities from the belt to the foredeep with values ~7km/s and ~4 km/s at 5 km depth, respectively. i. introduction his paper combines present-day stress data from 2016 amatrice seismic sequence (until october 3rd, 2016) and some information retrieved from the analysis of some deep wells in two different geologic context: the apennine belt, where the earthquakes occurred, and the contiguous eastward apennine foredeep. along the entire central apennine belt (fig. 1) the main active process is the ~ne–sw extension, well defined by several normal faulting focal plane solutions and fault data. ne-sw compression is limited to the external areas, along the northern apennine front (from the po plain up to ~42.5° latitude) and also toward the central adriatic sea, with a localized well-depicted n-s compression [montone and mariucci 2016]. this extensional-compressional pairing is nowadays t annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 2 well defined and recognizable through the distribution of earthquake focal mechanisms and other present-day stress data, mainly breakouts [frepoli et al., 1997; lavecchia et al., 2004; montone and mariucci, 2016]. in this paper we consider 4 deep wells, located in the 2016 amatrice area and along the contiguous foredeep (fig. 1). data from all wells include information relative to the stratigraphy and physical properties deduced from down-hole logs such as resistivity, gamma ray and sonic log. in particular, the sonic log curves display p-wave travel-times versus depth, from which we estimate the rock densities of the sequence crossed by each well; then we use density to determine the vertical stress magnitude. very close to the amatrice seismic sequence, two deep wells were analyzed [mariucci et al., 2010] in order to define the minimum horizontal stress orientation (shmin) through the breakout analysis (fig. 2). the boreholes, campotosto (c) and varoni (v), about 2500 m and 5700 m deep respectively, cross the entire umbria-marche succession from the flysch (arenaceous-clayey unit) belonging to “laga fm.” (lower messinian) to the middle-lower trias dolomites (the deepest one). the breakout zones were identified at a depth range between 1.4 and 4.6 km and the results show an shmin n81±22° and n74±10° oriented for v and c wells, respectively (fig. 2). in northern italy, the analysis of several deep wells [montone and mariucci 2015] allowed to estimate the vertical stress magnitude (sv) by the integration of derived rock densities from the surface to the bottom of each analyzed well. in the area mentioned above, the lithostatic gradient gradually changes from a maximum of 26 mpa/km in the northern apennine belt to a minimum of 21 mpa/km in the po plain and adriatic region. consequently, moving from the apennine area to the adriatic coast, values of lithostatic pressure from 130 to 105 mpa have been estimated for instance at a depth of 5 km. the estimated rock density in that sector of the apennine belt ranged from 2.65 to 2.33 g/cm3 (montone and mariucci 2015) in good agreement with the result found in central italy (umbria marche apennines) by other authors [federico and pauselli 1998; pauselli et al. 2010]. ii. data analysis and results ii.1 fault plane solutions horizontal stress orientations we have updated the recent italian presentday stress map [montone and mariucci, 2016] by adding the shmin orientations inferred from the focal mechanisms of m≥4 earthquakes of the ongoing sequence, with a maximum depth of 11 km (table 1). we have taken into account the available quick regional centroid moment tensors [http://autorcmt.bo.ingv.it/quicks.html, pondrelli et al., 2006] for homogeneity with the other focal mechanism data already published in the previous stress maps. ten moment tensors have been added up to october 3rd, 2016. to identify the minimum horizontal stress (shmin) azimuth and the tectonic regime we use the plunge of the p, t and b axes applying the criteria of zoback [1992]. following the world stress map guidelines [heidbach et al. 2010], all data are well constrained but they are assigned c quality as “single event solutions” [mckenzie, 1969; see also montone et al. 2012 for a detailed explanation]. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 3 figure 1. present-day stress in central italy. minimum horizontal stress (shmin) orientations [montone and mariucci, 2016) and the two mainshocks of the amatrice seismic sequence until october 3rd, 2016 [bsi, 2016]. normal faulting regime: shmin=sigma 3; strike-slip: shmin= sigma 3; thrust: shmin= sigma 2. location of the boreholes analysed in this study: a, atri; cc, colle casone; c. campotosto; v, varoni. the focal mechanisms are "normal-faulting" except a strike-slip mechanism in the eastern part of the seismic sequence area; the inferred shmin orientations point out a general extension perpendicular to the apennines with orientations from n48° to n76°. only one normal faulting event m4.4 shows a nw-se extension. the shmin orientations from earthquake data show a good agreement with those from borehole data although they refer to different depths. in fact as mentioned before, the reviewed borehole breakout data from the two wells available in the area (c and v wells) show an shmin ene-wsw oriented (n81° and n74°), only slightly different from the earthquake axes (fig. 2). annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 4 figure 2. the 2016 italian stress map and the new minimum horizontal stress (shmin) orientations inferred from the available focal mechanisms of ten earthquakes with mw≥4 [http://autorcmt.bo.ingv.it/quicks.html]. numbers as in table 1. bold circles are the event n. 1 and n. 2 of figure 1 and table 1. v and c stand for varoni and campotosto wells. see legend in figure 1. ii.2 sonic log p-wave velocity and vertical stress magnitude we have analysed sonic log data of the two deep boreholes within the area of the seismic sequence (v and c in fig. 1) and, for comparison, two wells to an easternmost location (a and cc in fig. 1), in the foredeep. sonic log data are among the most important in situ measurements of rock properties and provide a reliable image of in situ conditions at depth, notwithstanding their limits mainly due to the investigated rock volume and the tool working frequency. we have inferred the p-wave velocity (vp) values at depth from homogeneous sonic log (slowness) intervals and, using an empirical relation for sedimentary rocks based on borehole measurements [gardner et al., 1974], we have estimated rock density. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 5 table 1. earthquake focal mechanism data n date time utc mw depth (km) lat n lon e shmin q tr 1 24-08-2016 01:36 6.0 8 42,71 13,22 61 c nf 2 24-08-2016 02:33 5.4 9 42,79 13,15 48 c nf 3 24-08-2016 04:06 4.3 8 42,77 13,12 63 c nf 4 24-08-2016 17:46 4.4 10 42,66 13,22 72 c nf 5 25-08-2016 03:17 4.5 10 42,75 13,21 76 c nf 6 25-08-2016 12:36 4.4 10 42,60 13,29 56 c nf 7 26-08-2016 04:28 4.8 11 42,60 13,29 52 c nf 8 28-08-2016 15:55 4.4 9 42,82 13,24 124 c nf 9 03-09-2016 01:34 4.3 11 42,78 13,13 73 c nf 10 03-09-2016 10:18 4.5 9 42,87 13,21 65 c ss shmin: minimum horizontal stress; q: data quality; tr: tectonic regime; nf: normal faulting; ss: strike-slip faulting. see also figure 2. the two boreholes in the belt penetrate the whole sedimentary sequence of the region but the youngest units, whereas the two boreholes in the foredeep cross the plioquaternary sedimentary units only (fig. 3). in the first 5 km, the vp clearly shows this setting with values varying from around 4 km/s to about 7 km/s in the apennines; in the foredeep vp is about 2 km/s and reaches maximum values around 5 km/s. the values increase with depth irregularly with many inversions. it is noteworthy the abrupt vp increase in well cc around 1.5 km and the low velocity zone in v well at about 5.5 km. the magnitude of the vertical stress (sv) is usually estimated from the lithostatic pressure [e.g. cornet and rockel, 2012]. as suggested by the inversion of several earthquake focal mechanisms, we are confident that in the apennine belt the three principal stresses are perpendicular to each other with one being in a vertical orientation [e.g. frepoli and amato, 1997], then we have evaluated the magnitude of sv from the cumulated weight of overburden in the four selected wells. the sv values at depth show two different trends that reflect the different geological stratigraphic sequences (fig. 4): in the apennines the gradient is ~26 mpa/km whereas in the adriatic foredeep is ~23 mpa/km, resulting in a magnitude at 5 km around 130 mpa in the epicentral area and about 114 mpa close to the adriatic coast. annals of geophysics, 59, fast track 5, 2016; doi:10.4401/ag-7235 6 figure 3. stratigraphic sequence and p-wave velocity (vp) at depth along v, c, cc, and a wells (see figure 1 for location) as inferred from sonic log. iii. discussion and conclusions the new present-day stress data cover an area with very few information, characterized by a prevailing normal faulting regime with a ne-sw to ene-wsw extension. the focal mechanisms of the main events (with m≥4) of the amatrice seismic sequence agree with this tectonic regime, except one strike-slip mechanism, showing n48° to n76° shmin orientation [http://autorcmt.bo.ingv.it/quicks.html]. also the tdmt focal mechanisms of event m≥ 3.9 of the amatrice sequence [scognamiglio et al., 2016] agree with this trend. the earthquake focal mechanism forecast [roselli and mariucci, 2016], computed before the ongoing sequence, shows a general agreement among the computed and predicted horizontal stress axes. we have analysed in detail the sonic and stratigraphic logs of four deep wells, two located in the area of the amatrice seismic sequence and two close to adriatic coast. in particular, the sonic measurements provide a reliable image of in situ conditions that allow describing some physical properties of the annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 7 first kilometers of the crust, as p-wave velocity and vertical stress magnitude. along the boreholes, in the first 5 km, the vp shows values varying from around 4 km/s to about 7 km/s in the apennines, whereas in the foredeep vp is about 2 km/s reaching maximum values of ~5 km/s. vp increases with depth irregularly with many inversions, like the abrupt increase around 1.5 km in well cc and the low velocity zone at about 5.5 km in v well. the few available data do not allow obtaining a clear interpretation of the results although they could be related to different lithologies, or a different fluid content or highly fractured zones. in cc well the estimated velocity change correspond to different lithologies within the early pliocene succession. the low velocity interval, about 50 m thick, in v borehole is very reliable but not clearly associated to the lithologies: it occurs at the base of the dolomite interval, a highly fractured zone, then possibly characterized by a different fluid content. the pwave velocity variations with depth in the shallow crust are often neglected by 1d velocity models where a unique vp value is assigned to the layer down to about 5 km (e.g. bsi, 2016) concerning the estimated lithostatic gradient, we have found 26 mpa/km and 23 mpa/km in the belt and foredeep, respectively, in agreement with the values inferred in the northern apennine belt and in the po plain by montone and mariucci [2015]. moving from the apennine area to the adriatic coast, we observe a difference of ~15 mpa of lithostatic pressure, for instance at a depth of 5 km (fig. 4). this can imply that toward the foredeep, because the extensional tectonic regime is confined to the axial belt, even though the shmax gradient was still the same (within the range of the sv gradients relative to the apennines and the foredeep), the relative magnitude of the principal stress axes would change into a strike-slip or compressive tectonic regime. unfortunately no data are available in this sector of the belt to estimate the shmin magnitude, while further analysis could be performed in the foredeep area. very interesting is the comparison among the three different stress indicators available in this area: active fault measurements at surface, breakout data at intermediate depth and t-axes from earthquake focal mechanisms in the deep crust. earthquake focal mechanisms and breakouts are in good agreement also with the coseismic ruptures mapped for more than 5.2 km along the mt. vettore fault system characterized by n160°-striking normal faults, sw-dipping [emergeo, 2016], revealing a homogeneous stress field orientations for the entire crust along the depth. quaternary faults [e.g. galadini et al., 2001] together with the breakouts, the previous seismic sequences and the past seismicity indicate a small rotation of shmin orientation. in fact, from ~ne, in the southernmost part, for instance in the area of the 2009 l'aquila seismic sequence, shmin changes to ~ene in the northern sector (1979 norcia earthquakes), up to the wells and the 2016 amatrice sequence area. the previous hypothesis [mariucci et al., 2010] based only on deep well data has been confirmed: in this area occurs a slight rotation of horizontal stress axes with respect to the northern and southern sectors. the new focal mechanisms define the picture suggested in the past by only two boreholes enhancing the role of breakouts as reliable stress indicators in those areas where other information are few. annals of geophysics, 59, fast track 5, 2016; doi: 10.4401/ag-7235 8 figure 4. vertical stress (sv) estimated from sonic log data. 1, trend line of data from the wells in the foredeep (a and cc); 2, is relative to the wells in the apennine belt (v and c). references bsi, bollettino sismico italiano gruppo di lavoro amatrice (2016). rapporto preliminare sulle attività svolte dal gruppo bollettino sismico italiano a seguito del terremoto di amatrice mw 6.0 (24 agosto 2016, italia centrale). doi: 10.5281/zenodo.157545. cornet, f.h. and rockel, t. 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(2006). the italian cmt dataset from 1977 to the present. phys. earth planet. int., 159(34):286-303, doi:10.1016/j.pepi.2006.07.008. roselli, p. and mariucci, m.t. (2016). preliminary remarks on the earthquake focal mechanism forecasts applied in the amatrice sequence (central italy). submitted to annals of geophysics, this issue. scognamiglio l., tinti e. and quintiliani, m. (2016). the 2016 amatrice seismic sequence: fast determination of time domain moment tensors and finite fault model analysis of the ml 5.4 aftershock. submitted to annals of geophysics, this issue. zoback, m.l. (1992). firstand second-order patterns of stress in the lithosphere: the world stress map project. j. geophys. res., 97(b8):703–728. http://dx.doi.org/doi:10.1016/j.pepi.2006.07.008 13molina.qxd 497 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words earthquakes – mexico – 19th century – mexico city 1. introduction nineteenth century mexico was seriously affected by three large earthquakes that occurred on 8 march 1800, 7 april 1845 and 19 june 1858 (garcía-acosta and suárez, 1996). while the 1800 and the 1858 ones have already been studied (marquez, 2001; molina, 2001), the 1845 one, which seems to have been the strongest, still lacks a thorough investigation. these three earthquakes happened during the first half of the 19th century, a period characterised by the mexican transition from a colony (1521 onwards) to an independent nation (1821 onwards). this transition did not modify the way earthquakes were reported and recorded. the type of information, descriptions and even governmental responses were very similar in 1800, 1845 and 1858, which was due mainly to the fact that the three earthquakes took place in the long period known as «the age of enlightenment», which is part of a process of political and social secularisation that in mexico began in the 18th century and whose effects were more strongly felt throughout the following century. this paper is divided into three sections. first, we will present the political, social and economic context in which the three earthquakes occurred. in the second section, we will refer to the type of sources available for each event, whose information allowed us to know their effects and geographical reach. as the references are more abundant and more detailed for mexico city than for the rest of the country, the third section is dedicated to material and human damage of each of the three earthquakes in the capital. brief conclusions appear at the end of the paper, highlighting the main information obtained. 2. political, social and economic context of the three earthquakes in the late 18th century, the bourbon regime carried out several reforms in its colonies, whose main purpose was to undermine the 19th century earthquakes in mexico: three cases, three comparative studies américa molina del villar centro de investigaciones y estudios superiores en antropologìa social (ciesas), mexico d.f., mexico abstract this paper is a comparative study of the three strongest 19th century earthquakes in mexico and their effects and impact mainly in mexico city. the research is based on historical sources and previous investigations already published for two of the seismic events (8 march 1800 and 19 june 1858), with newly retrieved and analysed data for the 7 april 1845 earthquake. primary and secondary sources include reports on damage to city buildings, streets and neighbourhoods and allowed a detailed analysis of the national and local effects, impacts and responses to each of the three earthquakes, whose comparison allows a better understanding of specific aspects related to the investigation of historical earthquakes in mexico. mailing address: dr. américa molina del villar, centro de investigaciones y estudios superiores en antropologìa social (ciesas), juarez 87, tlalpan cp 14000, méxico, d.f., mexico; e-mail: avillar@juarez.ciesas.edu.mx 498 américa molina del villar power of the church. the implementation of these changes took many years and had several effects, among which and associated with other factors, were the revolts which at last ended with the declaration of mexico’s independence in 1821. the secularisation process as a whole culminated in the middle of the 19th century with the publication of the mexican laws of reform, whose main contents are related to the definitive separation between state and church, including the nationalisation of ecclesiastical properties and goods, the extinction of religious communities and the declaration of the cult of liberty. as part of the «enlightenment», the ideas related to progress and technological innovation gradually adopted scientific explanations of the origin of certain natural phenomena, giving up the religious ones. in dealing with earthquakes, these ideas and the struggle between the civil power and the church spawned through publications that appeared from the middle of the 19th century on, mainly in journals, bulletins and books published by several scientific societies. by the same token and as a consequence of this transition to an ideological secularisation, a gradual separation between ecclesiastical and governmental responses can be seen. processions organised together by civil and ecclesiastical authorities, as well as other religious manifestations dealing with the presence of earthquakes that were so common during previous centuries gradually ceased, notably after the 1845 earthquake. while the enlightened ideas were proliferating throughout mexico, several demographic, political and economic changes were happening. in 1800, mexico was going through economic and social prosperity. in 1802 the capital had 137 000 citizens; it was already larger than cities like new york, philadelphia or boston. one of the key interests of the authorities in these big cities was maintaining public works and cleaning the streets. after the 1800 earthquake, one of the viceroy’s immediate actions was inspecting the districts and repairing material damage. in 1821, after independence, there began a difficult period in mexican history. the first half of the 19th century was a long period of physical and demographic stagnation, also visible in mexico city. struggles between political factions, military interventions, foreign invasions and coups d’état were coupled with epidemics and other disasters that restrained the growth of the capital. between 1821 and 1870 the city’s population remained around 200 000. the constant governmental changes and lack of financial resources interrupted several urban projects and the capital maintained during this time its colonial characteristics. the city government did not have enough money to attend to basic urban works, such as repairing aqueducts, drainpipes and bridges. therefore the authorities had enormous difficulties to carry out the reconstruction works after the 1845 and 1858 earthquakes. a few months before the 1845 quake occurred, there was a liberal political upheaval against the conservative forces, with the specific objective of prosecuting an old dictator, antonio lópez de santa anna. the result of this movement was the change of president in december 1844, with the naming of an interim, josé joaquín herrera. herrera had to face the earthquake of april 1845 and carried out reconstruction works despite the economic difficulties and political problems. inner conflicts were intensified by the conflict with the united states over texas and california. the country was bankrupt and threatened by spain, supported by internal groups that tried to impose a monarchy. herrera’s government was very brief; in december 1845 he was removed by another coup d’état headed by a general and supported by the monarchists (diccionario, 1986; vázquez, 2000). the situation in mexico did not change and hard times continued. the war of reform, in which two political groups, liberals and conservatives, disputed power, had broken out a few months before the june 1858 earthquake. conflicts and political tension concentrated in mexico city. the fact that the federal powers resided in mexico city continued to diminish its scarce budget. at the beginning of 1858, the conservatives took the city and established a conservative government, while liberals established a rival government in veracruz, in the gulf of mexico. thus, in june 1858 there was not only a civil war going on in mexico, but two simultaneous governments. 499 19th century earthquakes in mexico: three cases, three comparative studies table i. main data for the 1800, 1845 and 1858 earthquakes. 1800 1845 1858 date, hour 8 march, 8:30 a.m. 7 april, 3:47 p.m. 19 june, 9:00 a.m. («some light movements (aftershock on 10 april). (two more light aftershocks a few hours after the on 20 and 23 june). earthquake»). most damaged area mexico city and mexico city and the mexico city and the states of guanajuato, the states of aguascalientes, the states of colima, oaxaca, puebla, guerrero, guanajuato, guanajuato, guerrero, and veracruz. hidalgo, jalisco, hidalgo, jalisco, mexico, michoacán, méxico, michoacán, morelos, oaxaca, puebla, oaxaca, puebla, querétaro, querétaro, san luis potosí, san luis potosí, veracruz and zacatecas. tlaxcala and veracruz. estimated magnitude (*) m 7.0 m 8.0 m 8.0 (*) according to bravo et al. (1988). 3. sources, effects and geographical reach table i shows the main data related to the earthquakes studied, including current estimates of their magnitudes (bravo et al., 1988), all of them 7 or above. areas affected by each earthquake are shown in fig. 1a-c. the three earthquakes affected mexico city and nearby states located on the pacific coast (oaxaca), in the centre of the country (puebla) and in the gulf of mexico (veracruz). only a few reports of damaged localities are known for the 1800 event; but the records for the other two are more abundant. this fact is due to the sources available, specifically, newspapers. in 1800 there was only one newspaper, gazetas de méxico, whose publication was erratic and mostly dedicated to what happened in mexico city. when the other two quakes happened there existed several newspapers with correspondents distributed in some parts of the country whose reports offer more spatial information. 3.1. 8 march 1800 earthquake this event is now better known thanks to a study by márquez published in 2001. as people used to name earthquakes according to the fig. 1a-c. areas and level of damage reported: a) 8 march 1800; b) 7 april 1845; c) 19 june 1858. a b c 500 américa molina del villar new spain mexico city primary sources bibliographical: bustamante (1852), lópez (1961). – archives: ahdf (1800), agnm (1800) (mayors and architects reports). – newspapers: la gaceta de méxico (1800). – bibliographical: bustamante (1852), castillo (1875), lópez (1961). secondary sources bibliographical: gómez (1864), martínez (1890), sedano (1880). – newspapers: el siglo diez y nueve (1858), diario oficial (1869). – bibliographical: figueroa (1963), gómez (1864), martínez (1890), sedano (1880), waitz (1920). christian calendar, this one is known as the «san juan de dios» earthquake because 8 march is the commemoration of this saint. earth movements were reported in several localities at the southern and central parts of new spain viceroyalty, mainly in those located at the pacific and gulf coast, like oaxaca city and the port of veracruz. they were also felt in the valleys of mexico and puebla, as well as in guanajuato, located 400 kilometres north from mexico city. the 8 march earthquake is the first in the garcía-acosta and suárez seismic catalogue (1996) with many references and detailed descriptions; in fact historical records on this earthquake are the richest throughout the colonial period. nevertheless, the type of sources available are richer for mexico city than for the rest of the country (table ii). for one of the most affected areas, the state of oaxaca, information comes from historians books, local chronicles, diaries and articles written mainly by historians and scientists. there is no specific information available for the states of puebla and veracruz; for the state of guanajuato it is even more scarce, for only the diario de bustamante whose editor carlos maría bustamante, one of the best known publishers and historians in those days, table iii. sources for the 1845 earthquake. country mexico city primary sources – archives: agnm (1845), ahmm (1845), app (1845) (governmental and ecclesiastical letters). – newspapers: el siglo diez y nueve (1845) and diario oficial (1845). – bibliographical: adorno (1864), böse and garcía (1908), malo (1948), ramírez and reyes (1873), romero (1972), waitz (1920), zayas (1893). – archives: ahdf (1845) (mayors and architects reports). – newspapers: el siglo diez y nueve (1845), diario oficial (1845). – bibliographical: adorno (1864), böse and garcía (1908), garcía cubas (1904), gómez de la cortina (1864), malo (1948), ramírez and reyes (1873), rivera (1883), romero (1972), waitz (1920), zayas (1893). secondary sources bibliographical: amerlinck (1962), figueroa (1963), orozco y berra (1887), reséndiz (1988), salas (1941). bibliographical: alfaro (1906), amerlinck (1962), figueroa (1963), galván (1950), mentz (1980), orozco y berra (1887), pérez (1904), reséndiz (1988), salas (1941). table ii. sources for the 1800 earthquake. 501 19th century earthquakes in mexico: three cases, three comparative studies mentioned it was slightly felt there (bustamante, 1852). figure 1a shows three areas with different levels and/or perceptions related to damage: 1) strongly damaged; 2) strongly felt, but no considerable material damage, and 3) felt, without reporting any damage. as can be appreciated, the most affected areas were located in mexico city and in la mixteca and la cañada regions in oaxaca. according to an earthquake chronology written almost one century later (martínez, 1890), in those areas the earthquake caused some damage. 3.2. 7 april 1845 earthquake it is amazing that this earthquake, considered by mexican seismologists as the strongest among those that occurred throughout the 19th century even similar or perhaps stronger than that of 19 september 1985, has not deserved more investigation up to now. according to recent estimates (bravo et al., 1988), the 1845 earthquake had a magnitude similar to that of the september 1985 earthquake: 8 or more. this earthquake was felt with great intensity in mexico city and in some localities of the states of michoacán and jalisco. as the 1985 earthquake, the epicentre of the 1845 one was located in the coast of guerrero, in the pacific ocean. unlike the scarce press information for the 1800 earthquake, newspapers and archival sources provide detailed information on the effects of the 1845 event in the country as well as in mexico city (table iii), which makes it one of the best documented for that century. the novelty in this case was that the district reports about damage were published in the newspapers el siglo diez y nueve (1845) and in the diario oficial (1845) which allows us to compare the information coming from two primary sources that recorded it. besides reports on damage, sometimes newspapers published edicts or decrees which help to analyse the response of government and society in face of these great earthquakes. the 7 april earthquake was strongly felt and caused damage in mexico city as well as in several mexican states (table i and fig. 1b) covering areas in the central part of the country spreading from the pacific to the atlantic coasts. more specific data is available for the cities of morelia (michoacán), guadalajara (jalisco), puebla (puebla), orizaba (veracruz), oaxaca (oaxaca) and guanajuato (guanajuato). one of the most affected areas was huamuxtitlán, where no building was left standing, nevertheless no deaths were reported. this town is located in the state of guerrero whose western limit is the pacific coast, where the epicentre was located (bravo et al., 1988). aftershocks were reported on the following day, and on 10 april in mexico city and the states of oaxaca, puebla and morelos. figure 1b shows the vast area affected by this quake using the same damage classification used for fig. 1a according to effects and impact on buildings and among the population: 1) strongly damaged: severely damaged areas, with collapsed buildings or temples, public buildings and houses at risk of collapsing; 2) strongly felt: areas with slight or imperceptible damage such as cracks, even though the movement was strongly felt; 3) felt: areas with no damage, where the main quake and its aftershocks were slightly or not felt at all. 3.3. 19 june 1858 earthquake this earthquake, named the «santa juliana», is best known after the study by molina (2001). for this event there are dozens of references with details on its effects and impact in the capital city as well as outside it, even in small towns (garcía and suárez, 1996). the latter, unmentioned in newspapers, were found in the archives of the michoacán bishopric, among which the magnificent ecclesiastical archives include letters that the priests of several towns sent to the bishop of michoacán describing the damaged ceilings, bell towers and façades of their churches. other primary sources like newspapers also offered a large number of records from outside the capital city (table iv). the press made several references to this earthquake, highlighting its impact outside the capital city. according to newspapers, the cities t p t b o c ( s ( ( 502 américa molina del villar country mexico city primary sources – archives: app (1858), ahcmr (1858, 1859), ahmm (1858) (ecclesiastical letters and council sessions). – newspapers: el siglo diez y nueve (1858). – bibliographical: romero (1972). – archives: ahdf (1858) (mayors and architects reports). – newspapers: el siglo diez y nueve (1858). – bibliographical: garcía cubas (1904) gómez de la cortina (1864), malo (1948), ramírez and reyes (1873), rivera (1883), waitz (1920). secondary sources bibliographical: munguía (1976), orozco y berra (1887), zayas (1893). bibliographical: orozco y berra (1887). most affected besides mexico city were located in the states of michoacán, jalisco, guerrero, mexico and hidalgo (table i and fig. 1c), with reports of collapse of temples, houses and buildings. the earthquake was strongly felt in other localities of michoacán and guerrero, though no information on economic damage is available. one of the most affected towns was pátzcuaro in michoacán, where the parish church showed severe damage, previously affected by the 1845 earthquake; its towers collapsed and their reconstruction took many years. in fact the only reference of deaths caused outside mexico city is for pátzcuaro, where 21 corpses were found, 5 of them at the parish church (app). other towns reported slight damage and only minor economic losses, like: iguala (guerrero), toluca and texcoco (state of mexico), san luis potosí (san luis potosí), pachuca (hidalgo), querétaro (querétaro), tlaxcala (tlaxcala) and oaxaca (oaxaca). 4. effects and impact in mexico city tables ii, iii and iv show that for the three studied earthquakes, the majority of historical references coming from archives, bibliography or newspapers, primary as well as secondary in origin, refer to the capital of the country. furthermore, archival records include file reports prepared by the mayors, who were in charge of surveying buildings street by street and house by house. these reports provide the richest data and most detailed information on the type of damage caused in mexico city, and made possible the construction of maps for the 1800 and the 1858 quakes identifying the type of damage in each street and in each building (márquez, 2001 and molina, 2001). 4.1. 1800 by the end of the 18th century, mexico city was divided into eight major districts in order to better administer power and public affairs. in each district the mayor was in charge of several tasks such as the security and well-being of the population, as well as attention in case of disaster, such as epidemics or earthquakes. whenever an earthquake occurred, the mayor, with the help of architects, councillors and the people in charge of each block, would report the damage caused. the 1800 earthquake was the first during the late colonial period to be described in these reports. figure 2, originally published by márquez (2002) shows the damage caused by this earthquake in the eight major districts mexico city was divided into. these districts are marked in fig. 2 with roman numerals following the code used in the original documents. after márquez (2001) another code is introduced in this figure to identify six different levels of damage. the 1800 earthquake affected 27 buildings in mexico city, located mainly in the first four major districts. the main public square, includtable iv. sources for the 1858 earthquake. 503 19th century earthquakes in mexico: three cases, three comparative studies ing the cathedral, the national palace (federal government) and the town hall was at the intersection of these four districts. many rich people lived there, and their houses were next to commercial and religious buildings, including convents, churches and religious schools. most damaged buildings were located precisely in these four districts. a general balance of the areas and buildings affected by this quake shows, as can be seen in fig. 2, that the most damaged area was the city centre, mainly north, south and south-westwards of the cathedral, corresponding to districts ii, iii and iv. this area produced more detailed descriptions because the majority of the population and important public, religious and commercial buildings were located here (agnm, obras públicas and correspondencia de virreyes, 1800). districts i, v, vi, and viii were not as severely affected; nevertheless there were reports related to ruined buildings and collapsed walls, cracks in public buildings and houses, as well as damage to drainpipes and archways. in spite of the severe damage, it is interesting to note that no human losses were reported. 4.2. 1845 as stated before, a detailed study of the effects of the 1845 earthquake in mexico city, as well as its mapping, has yet to be carried out, fig. 2. effects of the 8 march 1800 earthquake in mexico city. 504 américa molina del villar although it is one of the best documented events in mexican seismological history (table iv). here is a first approach to a future, deeper case study, such as those for the 1800 and 1858 events, already published (márquez, 2001 and molina, 2001). around the middle of the 19th century the most important buildings were still located in mexico city’s first four districts. the 1845 event was popularly known as «the earthquake of santa teresa», as it collapsed the dome of the church of santa teresa la antigua located in district iv. there are reports of 62 damaged public or religious buildings as well as 127 damaged houses, rooms and neighbourhoods (vecindades) in the first four districts. these are amazing numbers compared to the 27 damaged buildings reported in 1800 in the same area, even considering the estimated magnitude is higher, but there were also more people and more buildings there than in 1800, and hence greater exposure. archives and newspapers offer detailed descriptions of the church after which the earthquake was named, as well as for other churches, convents, schools, government buildings and hospitals that suffered different types of damage to towers, bell towers, walls, facades, doors and windows. the total number of damaged buildings identified with an actual address (at least street name) and therefore not including reports that mention «several houses in ruins or bad shape», was 210, with damage ranging from fig. 3. effects of the 7 april 1845 earthquake in mexico city. 505 19th century earthquakes in mexico: three cases, three comparative studies slight cracks to collapsed inner and outer walls, towers and domes. in addition the earthquake affected main aqueducts that brought water into the city, bridges and city gates. figure 3 shows the first mapping of the effects of this earthquake in mexico city. it is still necessary to identify the precise location of each building with its specific type of damage. nevertheless this approach allows one to realise that the affected area is very similar to that of the 1800 earthquake, with a considerably higher number of buildings affected mainly in districts i, ii, iii, iv, and viii. seventeen dead and injured were reported in mexico city, a rather low figure considering the quake’s intensity as well as its estimated magnitude. as for the economic damage, reports were prepared by the relevant mayors describing in great detail the neighbourhoods affected. newspapers, such as el siglo diez y nueve (1845) and the diario oficial (1845) published the reports. 4.3. 1858 even though only reports for five districts have been found for this earthquake, newspapers provide additional information on neighbourhoods not mentioned in archives. these considered districts vi and viii to be disaster areas in, where many destroyed houses were reported. historical documents and newspapers report fig. 4. effects of the 19 june 1858 earthquake in mexico city. 506 américa molina del villar damage to 31 public buildings, 106 cracked buildings and 63 in ruins in the city. the real figures must have been higher, for some districts offer only general comments, such as «in the neighbourhoods of san cosme and santa maría, located in district vi, many buildings were damaged» (ahdf and el siglo diez y nueve, 1858). as compared to the north, east and western parts of the city, the most affected areas were the southwest and southeast. damage was worse in district viii, southward of the most important park in town, the alameda central, a very popular park even nowadays. this district registered high population growth in the 19th century, and was mainly composed of urban and suburban housing quarters lacking public utilities. the population of viii district was not only numerous, but also heterogeneous, as it included native peoples, craftsmen and some rich merchants. this becomes evident in the type of reports, some thoroughly describing the blocks close to main square, and the rest barely and carelessly mentioning peripheral blocks. districts i and iv, severely affected in 1800, were not as damaged in 1858, for there were not as many people or houses, although many public buildings were still in the area, mainly in district iv (fig. 4, originally published in molina, 2001). damage was caused to poor neighbourhoods in the outskirts, where streets were irregular and most of the housing consisted of shantytowns, including adobe quarters. as always, the press only made general comments on these poor areas. this lack of reports and of district mayors’ visits reveals the limited access those inhabitants had to municipal assistance. the 1858 earthquake also affected the two main aqueducts in town: san cosme and salto del agua, as well as some drainpipes. 5. conclusions the study of effects and impact throughout the country and particularly in mexico city caused by the three strongest earthquakes recorded in early to mid 19th century mexico required a comparative focus that showed many constants and scattered changes. a comparative focus made it necessary to study in depth the historical contexts in which each quake occurred. comparison was also needed to evaluate the quality of available sources, i.e. to consider not only the amount of information, but also its type. the press has certainly been a relevant source for the historical analysis of earthquakes. not only has it provided information on the impact of the three earthquakes studied here, but has also proved particularly valuable when establishing their geographical reach. historical documentation coming from archives and primary historiographical bibliographical sources offered material related mainly to governmental actions dealing in particular with inspection and reconstruction, and this did not vary much from 1800 to 1858. after each earthquake, architects and experts carried out visits to each one of the eight major districts of mexico city, inspecting the state of each building. these reports allow us to draw figures for the mexican capital and help to compare the level of damage in each neighbourhood affected by the earthquakes. the first four districts of the city, an area which represented the core of the urban development within the metropolitan zone, where a considerable number of commercial stores and housings had been established, and in which were located the principal buildings and a great number of housing, were the most affected by the 1800 and 1845 earthquakes. during the 1858 event the major district number viii, south of la «alameda central», was the most affected. there are more references to damage outside the capital in the case of the 1845 and 1858 earthquakes; for the 1800 one there are only few detailed references and only for the state of oaxaca. in contrast, for the other two earthquakes several newspapers described in detail the localities affected. the 1858 earthquake is the most well documented of the three, if one takes the country as a whole, with a specific archival section that offered valuable information on the affected towns at the state michoacán, nearest to the epicentre. it is interesting to note that human losses during the three earthquakes were not high. in fact only two records mention deaths: mexico 507 19th century earthquakes in mexico: three cases, three comparative studies city in 1845 (17 dead and injured) and pátzcuaro in 1858 (21 deaths). the town of huamuxtitlán was completely destroyed by the 1845 earthquake, yet no deaths were reported. this suggests that, compared to our times, the population was not as exposed to the effects of seismic activity. population density and concentration were considerably lower. some colonial buildings were perhaps more resistant; if houses were not as crowded, the collapse of adobe walls did not threaten people’s safety so much. it is without doubt that the intensity of the event that has remained less studied, the 7 april 1845, was the highest in mexico in the first half of the 19th century, and maybe the highest before the 1985 event. it deserves a deeper investigation. all the information related directly to the earthquakes needs to be completed and compared with that from other historical sources in order to understand the specific context in which each one occurred. in this case the three earthquakes occurred in a period when the quality and even quantity of records were quickly developing and increasing. it was the «age of enlightenment», the waking up of scientific explanations for natural phenomena, the gradual giving up of religious manifestations associated with hazards, and the beginning of a new era for understanding earthquakes and disasters. acknowledgements a special acknowledgement to paola albini and virginia garcía-acosta, who revised the paper and made valuable comments and suggestions. references archival sources agnm (archivo general de la nación mexico-distrito federal) (1800): correspondencia de virreyes, gobernación indiferente, obras públicas, templos y conventos. ahdf (archivo histórico del distrito federal) (1800, 1845, 1858): historia temblores, fincas, edificios ruinosos . ahmcr (archivo histórico manuel castañeda ramírezmichoacán) (1858): negocios diversos. ahmm (archivo histórico municipal de morelia-michoacán) (1845, 1858): actas de cabildo, libro de sub-prefectura de pátzcuaro. app (archivo parroquial de pátzcuaro-michoacán) (1845, 1858): libro de composturas. newspapers diario oficial, 1845. el siglo diez y nueve, 1845, 1858. gacetas de méxico, 1800. papers and monographs adorno, j.n. (1864): memoria acerca de los terremotos en méxico escrita en octubre de 1864 por ... (imprenta de mariano villanueva, mexico). alfaro, m. (1906): reseña histórica-descriptiva del antiguo hospicio de méxico (imprenta del gobierno federal, mexico) amerlinck, t. (1962): sismos en la ciudad de méxico; ojeada histórica (editorial luz, mexico). böse, e. and j. garcía (1908): el temblor del 14 de abril de 1907, in parergones del instituto geológico de méxico, ii (4, 5, 6), 135-258. bravo h., g. suárez and e. zúñiga (1988): potencial sísmico en méxico, in estudios sobre sismicidad en el valle de méxico (departamento del distrito federal, programa de las naciones unidas para el desarrollo, mexico), 1-31. bustamante, c.m. 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(1893): los estados unidos mexicanos, sus condiciones naturales (oficina tipográfica de la secretaría de fomento, mexico). c r i t e r i e p r o b l e m i c o s t r u t t i v i d i s t r u m e n t i p e r m i s u r e d i r e s i s t i v i t à ' (*) luigi solaini 1. —• t r a i v a r i m e t o d i geofisici p e r l a e s p l o r a z i o n e d e l s o t t o s u o l o , q u e l l o d i r e s i s t i v i t à è u n o dei p i ù l u n g a m e n t e s p e r i m e n t a t i e m a g g i o r m e n t e c o n o s c i u t i e s e m b r a o r m a i c b e n o n r e s t i m o l t o d a d i r e , n é s u l l a c o s t r u z i o n e degli s t r u m e n t i d i m i s u r a , n é s u l l e m o d a l i t à o p e r a t i v e , n é su q u e l l e i n t e r p r e t a t i v e . p e r ò l ' i n c r e m e n t o d e l l a p r e c i s i o n e c b e si d e s i d e r a o t t e n e r e , c o m u n e a t u t t i i p r o c e d i m e n t i geofisici, e l a n o t e v o l e p r o f o n d i t à a cui si c e r c a d i s p i n g e r e l a i n v e s t i g a z i o n e e l e t t r i c a d e l t e r r e n o , p o n g o n o div e r s i p r o b l e m i di c a r a t t e r e o p e r a t i v o , c b e r i t e n g o u t i l e r i a s s u m e r e b r e v e m e n t e . q u a l u n q u e sia i l p r o c e d i m e n t o p r a t i c o di m i s u r a , a c c a d e s e m p r e c b e l a d i f f e r e n z a d i p o t e n z i a l e t r a gli e l e t t r o d i p o t e n z i o m e t r i c i , c h e c h i a m e r ò i n s e g u i t o b r e v e m e n t e s o n d e , d e c r e s c e q u a n d o a u m e n t a l a d i s t a n z a t r a gli e l e t t r o d i di c o r r e n t e e d i v i e n e m o l t o p i c c o l a p e r l u n g h i p r o f i l i , s p e c i a l m e n t e se il t e r r e n o è b u o n c o n d u t t o r e ; i l r a p p o r t o t r a l a d i f f e r e n z a d i p o t e n z i a l e v m i s u r a t a e l a c o r r e n t e i i n v i a t a n e l terr e n o s c e n d e a l l o r a a v a l o r i assai bassi. s e i n f a t t i ad e s e m p i o il sottov s u o l o h a u n a r e s i s t i v i t à i n e d i a d i 5 0 q m , i v a l o r i d e l r a p p o r t o j i n f u n z i o n e d e l l a d i s t a n z a d t r a gli e l e t t r o d i di c o r r e n t e è d a t a d a l l a s e g u e n t e t a b e l l i n a , n e l l e d u e i p o t e s i c h e l a d i s t a n z a a t r a l e s o n d e sia u n t e r z o ( m e t o d o w e n n e r ) o u n d e c i m o di d. d = 2 4 6 8 1 0 2 0 km — = 1 1 . 9 6 . 0 4 . 0 3 . 0 2 . 4 1 . 2 i o " 3 q — = — i d 3 — = 3 . 2 1 . 6 1 . 1 0 . 8 0 . 6 0 . 3 i o " 3 q — = — i d 10 p o i c h é l a r e s i s t i v i t à m e d i a d e l t e r r e n o p u ò e s e r e a n c h e i n f e r i o r e ( * ) c o m u n i c a z i o n e p r e s e n t a t a a l c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , t e n u t o a r o m a i l 1 0 1 1 g i u g n o 1 9 5 2 . 3 5 8 l u i g i s 0 l a 1 n i a 5 0 q ni, ciò significa c h e u n o s t r u m e n t o m o d e r n o d e v e essere cap a c e di m i s u r a r e i l r a p p o r t o —ycon a p p r o s s i m a z i o n e n o n i n f e r i o r e a io"5 q ; se si d e t e r m i n a n o s e p a r a t a m e n t e t e n s i o n e e c o r r e n t e , ammesso di i n v i a r e n e l t e r r e n o u n a c o r r e n t e di 1 a, i l v o l t m e t r o d e v e duncpie a v e r e u n a s e n s i b i l i t à di a l m e n o 1 0 ti v. la m i s u r a d e l l a corr e n t e n o n o f f r e i n v e c e difficoltà. q u a n d o si g i u n g e a s e n s i b i l i t à così e l e v a t e , d i v e r s i p r o b l e m i si a f f a c c i a n o di s o l u z i o n e t u t t ' a l t r o c h e f a c i l e . essi r i g u a r d a n o : a) l a scelta d e l t i p o di c o r r e n t e d a i n v i a r e , c h e p u ò essere cont i n u a o a l t e r n a t a o c o m m u t a t a ; n e l s e c o n d o caso la scelta d e l l a f r e q u e n z a p i ù o p p o r t u n a ; b) i l t i p o p i ù c o n v e n i e n t e di s t e n d i m e n t o ; c) l a c o s t r u z i o n e degli a p p a r a t i di m i s u r a . in questa b r e v e n o t a d e s i d e r o e s p o r r e a l c u n e c o n s i d e r a z i o n i r e l a t i v e ai t r e p u n t i c o n s i d e r a t i , p e r c h é p e n s o c h e l a l o r o discussione p r e senti u n a c e r t a u t i l i t à . 2. — è b e n n o t o c h e t a n t o l a c o r r e n t e c o n t i n u a q u a n t o q u e l l a a l t e r n a t a h a n n o d i v e r s i i n c o n v e n i e n t i . nei l u n g h i s t e n d i m e n t i , l e d i f f e r e n z e di p o t e n z i a l e d o v u t e a l l e corr e n t i t e l l u r i c h e , c a p r i c c i o s a m e n t e v a r i a b i l i , e i d i s t u r b i d o v u t i a corr e n t i a r t i f i c i a l m e n t e i m m e s s e n e l t e r r e n o , g i u n g o n o a v a l o r i d e l l o stesso o r d i n e di g r a n d e z z a di q u e l l e c h e si d e v o n o m i s u r a r e , p e r cui l e m i s u r e i n c o r r e n t e c o n t i n u a d i v e n g o n o o l t r e m o d o p e n o s e se n o n i m p o s s i b i l i . i n o l t r e l ' i m p i e g o degli e l e t t r o d i i m p o l a r i z z a b i l i non è del t u t t o s c e v r o da i n c o n v e n i e n t i , p e r c h é t r a i d u e e l e t t r o d i esiste s e m p r e u n a d i f f e r e n z a di p o t e n z i a l e c h e r a g g i u n g e e s u p e r a 1 m f e l e cui v a r i a z i o n i possono i n f l u i r e s u l l e m i s u r e d i p r e c i s i o n e . d ' a l t r a p a r t e , a n c h e o p e r a n d o i n c o r r e n t e a l t e r n a t a , se si e l i m i n a n o c o m p l e t a m e n t e l e i n f l u e n z e d e l l e f o r z e e l e t t r o m o t r i c i di c o n t a t t o , n o n ci si p u ò e s i m e r e d a l l a i n f l u e n z a d e l l e c o r r e n t i a l t e r n a t e c i r c o l a n t i n e l t e r r e n o p e r e f f e t t o d e l l e t e r r e d e l l e l i n e e di t r a s p o r t o d e l l a e n e r g i a ecc. in r e c e n t i p r o v e e f f e t t u a t e i n l o m b a r d i a , i n u n a z o n a t r a n q u i l l a , l o n t a n a da l i n e e e l e t t r i c h e , si è r e g i s t r a t a con u n o s c i l l o g r a f o u n a d i f f e r e n z a di p o t e n z i a l e a l t e r n a t a a 5 0 hz di q u a l c h e c e n t i n a i o di ti v , t r a d u e e l e t t r o d i posti a 3 0 0 m di d i s t a n z a . l ' i m p o r t a n z a di questi d i s t u r b i è l e g a t a a l l a f r e q u e n z a d e l l a corr e n t e c h e si i m p i e g a , l a q u a l e a sua v o l t a d i p e n d e d a l l a p r o f o n d i t à a cui si spinge la e s p l o r a z i o n e e da difficoltà s t r u m e n t a l i . queste u l t i m e c r i t e r i e p r o b l e m i c o s t r u t t i v i di s t r u m e n t i p e r m i s u r e di r e s i s t i v i t à 3 5 9 f a n n o p r e f e r i r e l a f r e q u e n z a p i ù e l e v a t a possibile, c o m p a t i b i l e con l a p r o f o n d i t à di p e n e t r a z i o n e ; p e r p r o f i l i l u n g h i a l c u n i c h i l o m e t r i , è s e n z ' a l t r o sufficiente u n a f r e q u e n z a n o n i n f e r i o r e a 2 0 hz, m e n t r e quand o si s u p e r a n o i 1 0 k m (cosa c h e i n v e r o a c c a d e assai di r a d o , p e r c h é il p o t e r e r i s o l u t i v o d e l l e m i s u r e di r e s i s t i v i t à d i v i e n e assai piccolo) è n e c e s s a r i o a d o t t a r e f r e q u e n z e di p o c h i h e r t z . nel p r i m o caso che, r i p e t i a m o , è di g r a n l u n g a i l p i ù c o m u n e , l a f r e q u e n z a d e l l a c o r r e n t e i m m e s s a n o n è m o l t o d i v e r s a da q u e l l a d e l l a c o r r e n t e i n d u s t r i a l e e q u i n d i l a e l i m i n a z i o n e d e i d i s t u r b i d i v i e n e u n p r o b l e m a c o m p l e s s o d a l p u n t o di v i s t a s t r u m e n t a l e . f i n a l m e n t e , si p o t r e b b e i m p i e g a r e c o r r e n t e c o m m u t a t a , ciò c h e d e l r e s t o si f a o r d i n a r i a m e n t e con a l c u n e a p p a r e c c h i a t u r e , m a p e r gli e r r o r i c h e q u e s t o p r o c e d i m e n t o c o m p o r t a , r i t e n g o l a s o l u z i o n e i n a d a t t a a m i s u r e di p r e c i s i o n e e a l u n g h i s t e n d i m e n t i , b e n c h é l a c o r r e n t e comm u t a t a a b b i a i l v a n t a g g i o di e l i m i n a r e gli i n c o n v e n i e n t i d e l l a c o r r e n t e c o n t i n u a senza d o v e r c a m b i a r e gli s t r u m e n t i , s a l v o l a i n t r o d u z i o n e dei c o m m u t a t o r i . c o n c l u d e n d o , si p u ò d i r e c h e nessun t i p o di c o r r e n t e è i d e a l e p e r i l u n g h i sondaggi e l e t t r i c i v e r t i c a l i ; t u t t a v i a r i t e n g o c h e la c o r r e n t e a l t e r n a t a a bassa f r e q u e n z a sia q u e l l a p i ù o p p o r t u n a , p e r c h é l a e l i m i n a z i o n e dei d i s t u r b i p u ò essere o t t e n u t a i n m o d o s o d d i s f a c e n t e con o p p o r t u n i d i s p o s i t i v i s t r u m e n t a l i e con l a scelta del t i p o di q u a d r i p o l o . p a r l a n d o p e r c i ò degli s t r u m e n t i di m i s u r a , ci l i m i t e r e m o a cons i d e r a r e i p r o b l e m i r e l a t i v i a l l ' i m p i e g o d e l l a c o r r e n t e a l t e r n a t a . 3. —• e s a m i n i a m o o r a b r e v e m e n t e l e m o d a l i t à o p e r a t i v e , m a solo d a l p u n t o di v i s t a d e l l a esecuzione d e l l e m i s u r e e n o n d a q u e l l o int e r p r e t a t i v o . l e q u e s t i o n i da c o n s i d e r a r e sono p r i n c i p a l m e n t e d u e e cioè l a i n t e n s i t à d e l l a c o r r e n t e da i n v i a r e n e l t e r r e n o e l a d i s p o s i z i o n e degli e l e t t r o d i . ' la t e n d e n z a p i ù r e c e n t e r e l a t i v a a l l a c o r r e n t e , è q u e l l a di i n v i a r e f o r t i i n t e n s i t à , a n c h e di a l c u n i a m p è r e , p e r m e z z o di g e n e r a t o r i di q u a l c h e c h i l o w a t t di p o t e n z a , i n m o d o da o t t e n e r e a n c h e n e g l i stend i m e n t i p i ù l u n g h i e n e i t e r r e n i m e n o r e s i s t i v i , u n a d i f f e r e n z a di p o t e n z i a l e s u f f i c i e n t e m e n t e e l e v a t a . s i c c o m e l e r e s i s t e n z e di c o n t a t t o degli e l e t t r o d i e q u e l l e d e l t e r r e n o n e l l e l o r o i m m e d i a t e v i c i n a n z e a m m o n t a n o g e n e r a l m e n t e a d i v e r s e c e n t i n a i a di o h m , p e r o t t e n e r e c o r r e n t i di a l c u n i a m p è r e , l e t e n s i o n i d e l g e n e r a t o r e d e v o n o a v e r e v a 3 6 0 l u i g i s o l a i n i l o r i e l e v a t i , c h e r a g g i u n g o n o a n c h e 1 0 0 0 v. l'uso di t e n s i o n i così f o r t i n o n è s c e v r o di i n c o n v e n i e n t i , sia p e r c h é r i c h i e d e c a v i m o l t o b e n e i s o l a t i i n t u t t a l a l o r o l u n g h e z z a , sia p e r c h é p u ò p r e s e n t a r e d e i p e r i c o l i p e r gli u o m i n i a d d e t t i agli s p o s t a m e n t i d e g l i e l e t t r o d i . s i p u ò p e n s a r e a l l o r a d i d i m i n u i r e l a i n t e n s i t à di c o r r e n t e aum e n t a n d o i n p a r i t e m p o l a s e n s i b i l i t à d e g l i s t r u m e n t i , m a si u r t a c o n t r o u n a d u p l i c e d i f f i c o l t à . l a p r i m a è di n a t u r a p r e t t a m e n t e strum e n t a l e , p e r c h é , m a l g r a d o i g r a n d i p r o g r e s s i r e a l i z z a t i n e l l a costruz i o n e di s t r u m e n t i e l e t t r i c i d i m i s u r a e i m i r a c o l i d e l l a t e c n i c a elett r o n i c a a c u i si p u ò a t t i n g e r e p e r s t u d i a r e m o d e r n e a p p a r e c c h i a t u r e di p r o s p e z i o n e , n o n è f a c i l e e s o p r a t t u t t o n o n è o p p o r t u n o s u p e r a r e i n s t r u m e n t i di c a m p a g n a , c e r t e s e n s i b i l i t à . a b b i a m o v i s t o c h e già con la c o r r e n t e di u n a m p è r e , l a s e n s i b i l i t à r i c h i e s t a al v o l t m e t r o è di p o c h i m i c r o v o l t a ; essa r a p p r e s e n t a a m i o p a r e r e presso a p o c o l a m a s s i m a s e n s i b i l i t à u t i l e i n s t r u m e n t i d i c a m p a g n a . l a s e c o n d a difficoltà è d e t e r m i n a t a d a l l a p r e s e n z a d e l l e c o r r e n t i v a g a n t i c h e sono t a l v o l t a t u t t ' a l t r o c h e t r a s c u r a b i l i , a n c h e i m p i e g a n d o c o r r e n t e a l t e r n a t a , m a l g r a d o gli e v e n t u a l i a c c o r g i m e n t i s t r u m e n t a l i a t t i a d i m i n u i r e l a l o r o i n f l u e n z a . q u a n d o a u m e n t a l a l u n g h e z z a d e l l o s t e n d i m e n t o , se l a r e s i s t i v i t à a p p a r e n t e è c o s t a n t e , da u n l a t o d i m i n u i s c o n o l e d i f f e r e n z e di pot e n z i a l e m i s u r a t e , d a l l ' a l t r o a u m e n t a i n e v i t a b i l m e n t e l a d i s t a n z a t r a l e s o n d e e q u i n d i l ' i n f l u e n z a d e i d i s t u r b i . o c c o r r e q u i n d i c h e l a pot e n z a d e l g e n e r a t o r e sia s u f f i c i e n t e m e n t e g r a n d e da a s s i c u r a r e i n ogni caso u n a d i f f e r e n z a di p o t e n z i a l e m a g g i o r e di q u e l l a c r e a t a d a l l e corr e n t i v a g a n t i ; t a l e p o t e n z a n o n p u ò q u i n d i s c e n d e r e al d i s o t t o di 1 k w p e r i p i ù l u n g h i p r o f i l i ; se ci si l i m i t a a p r o f i l i di 5 o 6 k m , p u ò essere sufficiente u n a p o t e n z a di m e z z o c h i l o w a t t . n a t u r a l m e n t e è p i ù c h e o p p o r t u n o , n e i l i m i t i d e l possibile, c h e l a s e n s i b i l i t à strum e n t a l e sia n o t e v o l e . c i r c a i l t i p o di s t e n d i m e n t o , r i t e n g o t r a t u t t i p r e f e r i b i l e q u e l l o a d o t t a t o d a l l a c o m p a g n i e g é n é r a l e d e g é o p h v s i q u e e da q u a l c h e t e m p o c o s t a n t e m e n t e i m p i e g a t o a n c h e d a l m i o i s t i t u t o . m a n t e n e r e fissi i n g e n e r a l e gli e l e t t r o d i o f f r e , i n f a t t i , o l t r e a q u e l l o di u n a m a g g i o r e s p e d i t e z z a di l a v o r o , i l v a n t a g g i o n o t e v o l e c h e l e mis u r e sono b e n p o c o i n f l u e n z a t e d a l l e e t e r o g e n e i t à s u p e r f i c i a l i ; a l c u n i sondaggi e l e t t r i c i , e f f e t t u a t i t a n t o con l o s t e n d i m e n t o w e n n e r , q u a n t o con q u e l l o a s o n d e fisse, m o s t r a n o c h i a r a m e n t e l a s u p e r i o r i t à di ques t ' u l t i m o p r o c e d i m e n t o . p o i c h é m a n t e n e n d o f e r m e l e sonde v a r i a da u n a m i s u r a a l l ' a l t r a i l r a p p o r t o t r a l a l o r o d i s t a n z a a e q u e l l a d t r a c r i t e r i e p r o b l e m i c o s t r u t t i v i di s t r u m e n t i per m i s u r e di r e s i s t i v i t à 3 6 1 g l i e l e t t r o d i di c o r r e n t e , si f a i n m o d o c h e a sia t a n t o p i c c o l o r i spetto a d, da p o t e r t r a s c u r a r e n e l l e f o r m u l e i n t e r p r e t a t i v e il rapp o r t o . l e d i f f e r e n z e di p o t e n z i a l e c h e si m i s u r a n o sono all o r a n e t t a m e n t e i n f e r i o r i a q u e l l e o t t e n i b i l i con l o s t e n d i m e n t o di w e n n e r , ad e s e m p i o , e r i c h i e d o n o u n a e l e v a t a s e n s i b i l i t à d e l l o strum e n t o . è b e n e t u t t a v i a f a r n o t a r e c h e l ' a u m e n t o d e l l a d i s t a n z a t r a l e s o n d e c r e a u n b e n e f ì c i o i l l u s o r i o r i g u a r d o a l l a p r e c i s i o n e di m i s u r a , p e r c h é c r e s c o n o c o n t e m p o r a n e a m e n t e l e d i f f e r e n z e di p o t e n z i a l e dov u t e a l l e c o r r e n t i v a g a n t i . a c o n c l u s i o n e di q u e s t e b r e v i c o n s i d e r a z i o n i , m i s e m b r a si possa s t a b i l i r e la necessità di u s a r e v o l t m e t r i di s e n s i b i l i t à p a r i a p o c h i m i c r o v o l t a , c o r r e n t i n o n e c c e s s i v a m e n t e e l e v a t e , m a t a l i da c o n s e n t i r e m i s u r e c o r r e t t e a n c h e i n p r e s e n z a di d i s t u r b i e di a d o t t a r e stendim e n t i con s o n d e fisse e r a v v i c i n a t e . 4. — p a s s i a m o finalmente i n rassegna l e d i v e r s e p o s s i b i l i soluz i o n i s t r u m e n t a l i , c o n s i d e r a n d o n e i v a n t a g g i e gli i n c o n v e n i e n t i . è evid e n t e a n z i t u t t o l a necessità di n o n p r e l e v a r e c o r r e n t e d a l l e s o n d e ; ciò p u ò essere o t t e n u t o o u s a n d o dei v o l t m e t r i e l e t t r o n i c i , o p p u r e m e d i a n t e o p p o s i z i o n e . l a p r i m a s o l u z i o n e , m o l t o s e m p l i c e e già a t t u a t a i n a l c u n i strum e n t i , h a i l g r a v e d i f e t t o d e l l a i n c o s t a n z a d e l l a a m p l i f i c a z i o n e d e l v o l t o m e t r o , il q u a l e d e v e essere t a r a t o ad ogni m i s u r a con u n a t e n s i o n e c a m p i o n e . l e m i s u r e sono p e r c i ò p i u t t o s t o l e n t e e l a p r e c i s i o n e ragg i u n g i b i l e n o n è m o l t o e l e v a t a , s p e c i a l m e n t e t r a t t a n d o s i di s t r u m e n t i di a l t a s e n s i b i l i t à e q u i n d i d o t a t i di a m p l i f i c a z i o n e p i u t t o s t o f o r t e . i n o l t r e è m o l t o d i f f i c i l e l a e l i m i n a z i o n e d e i d i s t u r b i , ciò c b e d e t e r m i n a gli i n c o n v e n i e n t i c h e a b b i a m o già e s a m i n a t i . l a s e c o n d a s o l u z i o n e , i n d u b b i a m e n t e m i g l i o r e , p r e s e n t a t u t t a v i a a l c u n e difficoltà. l a o p p o s i z i o n e d e v e essere f a t t a con u n a t e n s i o n e d e l l a i d e n t i c a f r e q u e n z a d e l l a c o r r e n t e i n v i a t a n e l t e r r e n o e in f a s e con essa; b i s o g n a p e r c i ò , o p r e l e v a r e u n a f r a z i o n e d e l l a c o r r e n t e e n e r g i z z a n t e stessa, o d i s p o r r e di u n p i c c o l o a l t e r n a t o r e a u s i l i a r i o c a l e t t a t o s u l l o stesso asse d e l g e n e r a t o r e di c o r r e n t e . nel p r i m o caso il p r e l e v a m e n t o d e l l a c o r r e n t e p u ò essere o t t e n u t o solo a t t r a v e r s o u n t r a s f o r m a t o r e o con a l t r i a r t i f i c i , p o i c h é a l t r i m e n t i si p o r t e r e b b e u n a sonda a l l o stesso p o t e n z i a l e di u n e l e t t r o d o di c o r r e n t e ; d a t a l a n o t e v o l e s e n s i b i l i t à degli s t r u m e n t i di m i s u r a , è diffìcile t u t t a v i a e v i t a r e con i 3 6 2 l u i g i s o l a i n i t r a s f o r m a t o r i p i c c o l e f u g h e d i n a t u r a e l e t t r o s t a t i c a , d a n n o s i s s i m e perc h é n o n si p u ò i n t r o d u r r e u n o s c h e r m o e l e t t r o s t a t i c o efficiente. s e m b r a p e r c i ò p i ù r a z i o n a l e l ' u s o d e l s e c o n d o g e n e r a t o r e , m o n t a t o con l a c u r a n e c e s s a r i a p e r e v i t a r e s f a s a m e n t i t r a l e d u e c o r r e n t i . la m i s u r a p u ò essere e f f e t t u a t a o con u n u n i c o s t r u m e n t o a b o b i n e i n c r o c i a t e , s e g u e n d o l o s c h e m a a d o t t a t o n e l m e g g e r geofisico, o p p u r e con u n a m p e r o m e t r o e d u n m i c r o v o l t m e t r o i n d i p e n d e n t i . n e l p r i m o caso è m o l t o d i f f i c i l e r a g g i u n g e r e u n a e l e v a t a s e n s i b i l i t à , p e r c i ò q u e s t a s o l u z i o n e s e m b r a da s c a r t a r s i s e n z ' a l t r o . n e l s e c o n d o caso, o l t r e a l l ' a m p e r o m e t r o , o c c o r r e u n m i c r o v o l t m e t r o di p r e c i s i o n e , o p p u r e d e l l e r e s i s t e n z e n o n i n d u t t i v e t a r a t e ; q u e s t ' u l t i m o m e t o d o è p r e f e r i b i l e al f i g . 1 p r i m o , p e r c h é l'uso di u n o s t r u m e n t o di m i s u r a a c o r r e n t e a l t e r n a t a e di e l e v a t a s e n s i b i l i t à è p a r t i c o l a r m e n t e d e l i c a t o . i n e n t r a m b i i casi è n e c e s s a r i o u n g a l v a n o m e t r o p e r s t a b i l i r e l ' e q u i l i b r i o t r a l a t e n s i o n e di o p p o s i z i o n e e q u e l l a e s i s t e n t e t r a l e s o n d e ; esso d e v e a v e r e u n a s e n s i b i l i t à v o l t m e t r i c a n o n i n f e r i o r e a q u a l c h e d i e c i n a di m i c r o v o l t a p e r p a r t e , cioè d e v e p o s s e d e r e p r a t i c a m e n t e l a m a s s i m a s e n s i b i l i t à o t t e n i b i l e i n s t r u m e n t i p o r t a t i l i a c o r r e n t e a l t e r n a t a . i l t i p o di strum e n t o c h e s e m b r a p i ù a d a t t o è i l g a l v a n o m e t r o a v i b r a z i o n e , t u t t a v i a a b b a s t a n z a i n g o m b r a n t e e m o l t o costoso. p o i c h é q u e s t i s t r u m e n t i sono m o l t o s e n s i b i l i a t e n s i o n i e l e t t r o s t a t i c h e , è o p p o r t u n o r i c o r r e r e al gen e r a t o r e a u s i l i a r i o p e r o t t e n e r e l a c o r r e n t e di o p p o s i z i o n e ; s i c c o m e t a l e c o r r e n t e è i n d i p e n d e n t e d a q u e l l a i n v i a t a n e l t e r r e n o , o c c o r r e u n c r i t e r i e p r o b l e m i c o s t r u t t i v i di s t r u m e n t i p e r m i s u r e di r e s i s t i v i t à 3 6 3 a p p o s i t o s t r u m e n t o c h e l a m i s u r i o c h e p e r m e t t a di r i d u r l a a d u n v a l o r e costante. l o s c h e m a di u n a p p a r a t o d i s e g n a t o secondo i c o n c e t t i e n u n c i a t i r i s u l t a p e r c i ò q u e l l o d e l l a fig. 1. p e r q u a n t o a b b a s t a n z a complesso, esso r a p p r e s e n t a u n a d e l l e m i g l i o r i s o l u z i o n i p e r l e m i s u r e di resistiv i t à i n c o r r e n t e a l t e r n a t a ; del resto è m o l t o s i m i l e ad a l c u n i s t r u m e n t i a c o r r e n t e c o n t i n u a c o m u n e m e n t e i n uso, s a l v o l a i n t r o d u z i o n e d e l g e n e r a t o r e go d i r e t t a m e n t e a c c o p p i a t o a q u e l l o p r i n c i p a l e g ì al posto d i u n a p i l a i n d i p e n d e n t e d a l l a b a t t e r i a e n e r g i z z a n t e . è solo o p p o r t u n o o s s e r v a r e c h e l a r e s i s t e n z a r è p a r t i c o l a r m e n t e d e l i c a t a , p e r c h é d e v e v a r i a r e da a l c u n e c e n t i n a i a di o h m a q u a l c h e d e c i m o d i o h m e d e v e essere l e t t a con l ' a p p r o s s i m a z i o n e di a l m e n o io"2 q ; i n p r a t i c a è n e c e s s a r i o r i c o r r e r e ad u n a t t e n u a t o r e . l a g r a n d e s e n s i b i l i t à n e c e s s a r i a p e r i l g a l v a n o m e t r o g p u ò essere d i m i n u i t a a n t e p o n e n d o g l i u n a m p l i f i c a t o r e ; siccome l o s t r u m e n t o serv e solo p e r s t a b i l i r e l ' e q u i l i b r i o t r a d u e t e n s i o n i o p p o s t e , l a i n t r o d u z i o n e d e l l ' a m p l i f i c a t o r e n o n d e t e r m i n a i n c o n v e n i e n t i sensibili, m e n t r e h a i l v a n t a g g i o di p o t e r e i n t r o d u r r e dei filtri atti a r i d u r r e n o t e v o l 10 2 0 3 0 4 0 5 0 6 0 |-| f i g . 2 46 l u i g i s o l a i n i m e n t e i d i s t u r b i c h e a b b i a n o f r e q u e n z a s e n s i b i l m e n t e d i v e r s a da q u e l l a d e l l a c o r r e n t e e n e r g i z z a n t e . l o s t u d i o di t a l e a m p l i f i c a t o r e e d e l filtro n o n è m o l t o s e m p l i c e , a causa d e l l a bassa f r e q u e n z a d e l l a corr e n t e i m p i e g a t a ; t u t t a v i a si p u ò o t t e n e r e u n a c u r v a di s e l e t t i v i t à c o m e q u e l l a r i p r o d o t t a in fig. 2 ; c o m e si v e d e , se l a s i n t o n i a si v e r i f i c a p e r l a f r e q u e n z a di c i r c a 2 0 hz, t u t t e l e f r e q u e n z e i n f e r i o r i a 5 hz s o n o p r a t i c a m e n t e t a g l i a t e , m e n t r e l e t e n s i o n i con l e f r e q u e n z e di 4 0 hz sono già r i d o t t e a 1/20 d e l l o r o v a l o r e . l ' a m p l i f i c a t o r e c o n s e n t e a n c h e di r e g o l a r e i n m o d o s e m p l i c e e c o n t i n u o l a s e n s i b i l i t à senza r i c o r r e r e a s h u n t ; p e r ò h a l ' i n c o n v e n i e n t e di r e n d e r e l o s t r u m e n t o e s t r e m a m e n t e s e n s i b i l e a l l e f u g h e elett r o s t a t i c h e . f i n a l m e n t e , m o l t o suggestiva è l ' i d e a di s e m p l i f i c a r e l'uso dell ' a p p a r a t o e di p o t e r m i s u r a r e d i r e t t a m e n t e i l r a p p o r t o t r a t e n s i o n e e c o r r e n t e ; l ' a p p a r a t o i d e a l e p e r l e m i s u r e di r e s i s t i v i t à s a r e b b e q u e l l o c h e possedesse s o l t a n t o u n i n d i c a t o r e di z e r o ed u n o s t r u m e n t o di v m i s u r a del r a p p o r t o — . b a s t a p e r q u e s t o t r a s f o r m a r e l o s c h e m a di fig. 1, i n m o d o da o t t e n e r e c h e l a t e n s i o n e f o r n i t a d a l g e n e r a t o r e g2 sia s e m p r e p r o p o r z i o n a l e a q u e l l a i n v i a t a da g ì n e l t e r r e n o . b i s o g n a a b b a n d o n a r e a l l o r a i d u e g e n e r a t o r i a c c o p p i a t i e p r e l e v a r e d a l circ u i t o p r i m a r i o u n a f r a z i o n e n o t a d e l l a cori-ente e n e r g i z z a n t e , p e r u t i l i z z a r l a p e r l ' o p p o s i z i o n e , e v i t a n d o p e r ò ogni a c c o p p i a m e n t o e l e t t r o s t a t i c o ; l e s o l u z i o n i t e o r i c h e n o n m a n c a n o , m a l a l o r o r e a l i z z a z i o n e m e c c a n i c a è m o l t o d e l i c a t a . v a l e l a p e n a t u t t a v i a di s p e r i m e n t a r e in q u e s t o senso, p e r c h é si r i u s c i r e b b e ad o t t e n e r e così u n o s t r u m e n t o di uso v e r a m e n t e p r a t i c o e di s e n s i b i l i t à m o l t o e l e v a t a . u n u l t i m o c e n n o sugli s t r u m e n t i a r e g i s t r a z i o n e f o t o g r a f i c a ; è e v i d e n t e che, v o l e n d o r i c o r r e r e al m e t o d o di m i s u r a p e r o p p o s i z i o n e , l a r e g i s t r a z i o n e p u ò a v v e n i r e solo d o p o l ' a z z e r a m e n t o del g a l v a n o m e t r o ; l ' o s c i l l o g r a f o sostituisce s e m p l i c e m e n t e l ' a m p e r o m e t r o e i l mill i v o l t m e t r o . il v a n t a g g i o d e l l a r e g i s t r a z i o n e è p e r c i ò m o l t o m o d e s t o , a m e n o c h e n o n si pensi a l l a p o s s i b i l i t à di r e g i s t r a r e a n c h e l a corr e n t e p a s s a n t e p e r i l g a l v a n o m e t r o , ciò c h e c o n s e n t i r e b b e di e f f e t t u a r e u n a o p p o s i z i o n e a p p r o s s i m a t a e di c o r r e g g e r e s u c c e s s i v a m e n t e l e ind i c a z i o n i d e l l a r e g i s t r a z i o n e v o l t m e t r i c a . milano — istituto di geofisica appi, del politecnico — giugno 1952. c r i t e r i e p r o b l e m i c o s t r u t t i v i di s t r u m e n t i p e r m i s u r e di r e s i s t i v i t à 3 6 5 riassunto si discutono le varie soluzioni operative e strumentali relative alla esecuzione di grandi sondaggi elettrici verticali. si auspica la adozione di correnti alternate a bassa frequenza e di intensità non troppo elevata e di stendimenti eseguiti con sonde potenziometriche fisse e ravvicinate. si passano finalmente in rassegna le possibili soluzioni per gli strumenti di misura. summary various operative and instrumentai solutions are discussed concerning the execution of great vertical electrical explorations. the author thinks preferable the alternating currents at low frequency and not too high intensity, as tvell as electrode arrangements ivitli fixed and at a short distance potentiometric electrodes. finally, possible solutions are revieived for the potential measuring circuits. die tiefen der erdbebenlierde in dentschland auf grund makroseismisclier berechnungen w. s p o n i i e u e lì zusa m menfassun (j ùber die tiefe der erdbebenlierde in deutscliland wabrisher n u r wenig b e k a n n t . lediglieli f u r das gebiet der schwàbischen alb konnten dank dem stationsnetz des wiirttembergischen erdbebendienstes die herdtiefen von neueren e r d b e b e n mikroseismisch bestimmt werden. f u r alle anderen herdgebiete deutselilands lagen bisher weder mikroseismisch noch makroseismiseh ermittelte tiefen von erdbebenherden vor. wegen der relativen seltenheit gròsserer erdbeben innerlialb deutschlands u n d des fehlens geeigneter f u r herdtiefenbestimmungen auswertbarer iiegistrierungen wurden naehfolgend n u r herdtiefen derjenigen erdbeben makroseisraiscli erniittelt, von denen einelsoseistenkarte gezeiehnet werden konnte. mit hilfe einer neuartigen methode (7) ist es mòglicli, niit geringem zeitaufwand eine gròssere zahl von erdbeben zu bearbeiten und ergebnisse zu erzielen, die m i t mikroseismisch erreclineten gut ubereinstimmen. wie den meisten bisher b e k a n n t e n yerfaliren liegt aneli dem hier b e n u t z t e n die gleichung von kòvesligethy (5) zugrunde. i 0 — i = 3 log '' + 3 a m ( r — h), [1] li oder, da nach abb. 1 r2 = h2 + s\ i — i 0 = 3 log • | h+ -s' 2 + + 3 a m ( | i 2 + s2 — h). [2] hierbei ist i 0 = i n t e n s i t à t ini e p i z e n t r u m [grad] i = i n t e n s i t à t in der e n t f e r n u n g s vom e p i z e n t r u m s = mittlere e n t f e r n u n g der isoseiste i vom e p i z e n t r u m (isoseistenradius)[km] h = herdtiefe [km] a = absorptionskoefflzient [km] 1 m = log e : 0,4343. herd abb. 1. i n einem diagramm nach gleichung [2] k a n n nun f u r einen festen w e r t des absorptionskoefflzienten a die herdtiefe li in abhangigkeit von der epizentralentfernung dargestellt werden (abb. 2). s t i m m t zufàllig der w e r t von der diesem diagramm zugrunde liegt, mit dem eines erdbebens iiberein, so muss sicli zu jedem isoseistenradius s und der zugehòrigen intensitàtsdiiì'erenz i „ — i„ ein konstant e r w e r t der herdtiefe li ablesen lassen. i n alien anderen fàllen, in denen der feste 158 w . s p o n i i e u e r w e r t a des diagramms von dem des e r d bebens verschieden ist, werden die abgelesenen herdtiefenwerte, die den e n t f e r n u n gen sn urid denlntensitàtsdifferenzen i0— i» entsprechen, mit grosser werdender epizent r a l e n t f e r n u n g entweder ansteigen oder fallen, wie es in abb. 2 f u r a = 0,02 und a h 150 100 sfl 0,04 gezeigt io-i = 1 wirc 2 l. 3 4 5 7 3 a h 150 100 sfl / a h 150 100 sfl / • / / a h 150 100 sfl * ne / / a h 150 100 sfl a h 150 100 sfl f\ i ; ( a h 150 100 sfl 7 ;/ / a h 150 100 sfl r 7 7 | / / / / / k a h 150 100 sfl i/i ! ! / / a / | •7t a h 150 100 sfl "7t "v / a h 150 100 sfl _ ! / "7t — v . poi 0 h 0 ¥ 100 150 • 2 00 a b b . 2. d a r s t e l l u n g v o n g l e i c h u n g [2] f ù r ,. f e s t e s a u n d g a n z z a h l i g e i n t e n s i t à t s d i f f e r e n z e n i 0 — 1. dieses steigen oder fallen der herdtiefenwerte hn entsprechend den isoseistenradien sn ist also ein kriterium dafiir, class der dem d i a g r a m m ziigrnnde gelegte absorptionskoefflzient a grosser oder kleiner als der des erdbebens ist, dessen herdtiefe untersucht werden soli. umgekehrt zeigen also k o n s t a n t e w e r t e der herdtiefe f ù r entsprechende epizentralentfernungen sn u n d intensitàtsdifferenzen i„ — / „ an, dass beide absorptionskoeffizienten, der feste des diag r a m m s u n d der des erdbebens, ubereinstimmen. aus der konstanz des abgelesenen herdtiefenwertes geht hervor, dass dieser der gesuchte richtige w e r t ist. hierbeiwird wie bei alien anderen makroseismischen herdt i e f e n b e s t i m m u n g s v e r f a h r e n s t i l l s c h w e i g e n d v o r a u s g e s e t z t , dass d e r a b s o r p tionskoeffizient eines erdbebens f ù r verschiedene h e r d e n t f e r n u n g e n sich niclit àndert. man k o n n t e n u n m i t hilfe der vorstehenden b e t r a c h t u n g eine methode zur bestimmung der herdtiefe gewinnen. man benòtigt dazu eine anzalil von diagrammen nach abb. 2 m i t hinreichend abgestuften festen w e r t e n des absorptionskoeffizienten a. durch probieren, also ini nàherungsverfahren, wird nun dasjenige diagramm herausgesucht, das einen konstanten, d. li. richtigen w e r t der herdtiefe f ù r jeden gegebenen isoseistenradius s u n d intensitàtsdifferenz i 0 — i ablesen lasst. dieser methode stehen lediglieli ini wege, dass einmal eine grosse zahl derartiger diagramme notwendig ist, zum anderen, dass die herstellung solcher diagrammo infolge der transzendenz der gleichung [2] selir mùlievoll und zeitraubend ist. uni diese schwierigkeit zu umgehen, wurde von ullmann (*) in gleichung [1] der absorptionskoeffizient a eliminiert, wodurch sich die gleichung in eine bequeme f o r m bringen làsst. e r stellt folgende definitionsgleichungen auf. rj = ah, q = a r, a = a s [3] oder die zuordnung dieser gróssen geht aus abb. 3 hervor, u n d m a n k a n n n u n [1] wie folgt schreiben. = log — + m (o — rj) . [4] 3 il nach der abb. 3 ist q_ 1 _ il sin <p ' und daher o — >i = g (1 — sin <p) . n u n wird ^0 = — log sin 99 + m q (1 — sin cp) [5] (*) d r . w . u l l m a n n , i n s t . f ù r b o d e n d y n a m i k u n d e r d b e b e n f o r s o l i u n g , j e n a . d i e t i e f e n d e r e r d b e b e n h e r d e i n d e u t s h l a n d , u s w . 150 daraus iln — i • \ 1 1 f log sin . . [61 3 5 ' ) m 1 — sin ip l j s e t z t m a n c <*> = m ! _ ' s i u , ' ^ — e (<p) log sin <p = / (<p)s [8] [9] i o i y = 3 ' dann ergibt sich fui[6] q = yc{<p) — f {<p) • [10] o ist nur defìniert fiir y log sin (p > 0 o d o r folglich ^ + in sin cp > 0 , sm w > e m mit e i l f e von gleichung [10] lassen sich fiir alle intensitàtsdifferenzen i 0 — / „ , die zweckmàssig uni halbe oder viertel grade abgestuft werden, k u r v e n berechnen, da die ausdriicke [7, 8, 9] leiclit tabelherbar sind und m u h s a m e nàherungsberechnungen fortfallen. e n t w e d e r kann m a n mit dem winkel cp u n d der gròsse q fiir jedes y eine k u r v e zeichnen, oder m a n b e n u t z t dazu die gròssen a und rj, die sich aus •)] • = o sin cp und ct2 = (?2 + v"' ergeben. die bestimmung der herdtiefe eines erdbebens mittels des a, ^-diagramnis geht nun folgendermassen vor sich: auf grund der isoseistenkarte sind die mittleren isoseistenabstande vom epizentrum s„ fiir die entsprechenden intensitàtsdifferenzen /„ —1„ sowie die i n t e n s i t à t ini epizentrum i„ gegeben. unter annahme eines geschatzten wertes fiir die absorption a erliàlt m a n durch einfache multiphkation mit den isoseistenradien s„ die gròssen a„. nini e n t n i m m t man dem diagramm zu jedem a bei der zugehòrigen intensitàtsdifferenz i0 — i das entsprechende rj. steigen oder fallen die ?;-werte, so wird der vorgang mit einem kleineren bzw. gròsseren w e r t von a solange, wiederholt, bis die r/w e r t e k o n s t a n t bleiben. aus rj erliàlt m a n durch division mit a die gesuchte herdtiefe li. selbstverstàndlicli muss in kauf genonunen werden, dass infolge der niemals idealen yerteilung der isoseisten die r\w e r t e streuen. jedoch làsst sich fast immer erkennen, ob sie eine fallende oder steigende tendenz haben. stàrker streuende w e r t e zeigen an, dass die zugehòrigen rsoseistenradien zu stark fehlerhaft und dalier nicht geeignet fiir eine herdtiefenbestimmung sind. da bei diesem verfahren die absoriition a zunàehst gescliàtzt werden muss, sind meist mehrere nàherungsschritte nòtig, uni den richtigen, d. li. konstanten w e r t der herdtiefe zu ermitteln. ferner geht aus gleichung [1] liervor, dass die epizentralintensitàt 10 mòglichst genau b e k a n n t sein muss, uni sichere ergebnisse zu erlialten. man kaim aber die zahl der nàherungsschritte stark herabsetzen, wenn der absorptionskoefììzient nicht beliebig gewàhlt, sondern in geeigneter weise abgeschàtzt wird. stelli m a n gleichung [2] so dar, dass die intensitàtsdifferenz /„ — i in abhàngigkeit vom isoseistenradius.s gebracht wird, so stelli diese f u n k t i o n (abb. 4) die i n t e n sitàtsabnahme mit der e n t f e r n u n g fiir die gleichung von kòvcsligethy dar. aber aucli fiir jedes erdbeben làsst sich mit den makroseismischen d a t e n eine derartige k u r v e zeichnen, wenn man i in abhàngigkeit von s a u f t r à g t . vergleicht m a n eine derartige i n t e n s i t à t s a b n a h m e k u r v e m i t den tlieoretischen i n t e n s i t à t s a b n a h m e k u r v e n etwa durch tibereinanderlegen der auf transparentem papier gezeichneten diagramme, so làsst sich einmal der w e r t des absorptionskoeffizienten « m i t genugendergenauigkeit abschàtzen, zum anderen zeigt die zu der bebenkurve passende theoretische abn a h m e k u r v e den extrapolierten w e r t der i n t e n s i t à t i a hinreichend genau an. ausserdem ist es in vielen fallen mòglich, aucli den ungefàhren w e r t der herdtiefe li aus dem diagramm zu entnehmen, so dass irrt ù m e r bei der herdtiefenberechnung ver158 w. s p o n i i e u e r rnieden werden konnen. mit hilfe des abgeschàtzten wertes von a k a n n die e r m i t t lung der herdtiefe naeli dem vorher beschriebenen verfaliren vorgenommen werden. fische mit a = 0,001. legt m a n die beiden k u r v e n so ùbereinander, dass die epizentralintensitàt von 6 grad mit dem xuup u n k t des koordinatensystems der theoretischen k u r v e zusammenfàllt, so liegen die abb. 4. abliàngigkeit der intensitiitsdifferenz i 0 i voli der entfernung s fùr verschiedene ilerdtiefen li und absorptionskoeffizienten a nacli gfleiclnmg [2], i i 10 20 30 w 50 60 10 80 so 100 120 •<k = q001 •d. = 0,01 (k = 0.025 ck = 0,05 das verfaliren moge an einem biebeobachteten epizentralentfernungen ungespiel erlautert werden. (erdbeben an der f à h r auf der theoretischen k u r v e fiir li = hornisgrinde vom 30 . 12 . 1935). 30 k m . (abb. 5). s [km] orad 10 30 50 70 700 150 200 250 8l abb. 5. abschàtzung von a und li durcli anpassen der makroseismischen daten an eine kurvenschar nacli gleichung [2], der isoseistenkarte von hiller( ') wurden folgende mittlere tsoseistenradien entnomm e n : i n 5,5 4,5 4 3,5 3 grad sn 32 85 120 180 230 k m tràgt m a n s in abhàngigkeit von i auf, so erhàlt m a n f ù r diese i n t e n s i t à t s a b n a h m e k u r v e die beste annàherung an die theorinach den makroseismischen beobachtungen ist allerdings i 0 hòher als (i grad, jedoch niedriger als 6,5 grad gewesen, so dass i 0 = 6,25 gesetzt wurde. dalier muss die herdtiefe geringer als 30 k m sein, wie sich auch aus dem kurvenvergleieh ergibt, wenn m a n die ordinate von 6,25 auf den ursprung der theoretischen k u r v e legt. mit a — 0,001 erhàlt m a n , da a = sa, f ù r die d i e t i e f e n d e r e r d b e b e n h e r d e i n d e u t s c h l a n d , u s w . 1(31 entsprechenden i n t e n s i t à t s d i f f e r e n z e n io — ini0 —1} 1 0,75 1,25 1,75 2,25 2,75 3,25 ak 0,032 0,05 0,085 0,120 0,180 0,230 d e r yorteii der aufgezeigten m e t h o d e liegt vor alleni darin, dass zu j e d e m w e r t der m i t t l e r e n isoseifltenentfernung ein h e r d tiefemvert b e t u n m t w i r d . s t r e u e n die erlialtenen w e r t e der h e r d t i e f e i n n e r h a l b eines diagramm zur ermittlung der herdtiefe 71 • f(tr) o qo2 o.o6 o.io an o.ie a?5 0,30 aas q«? a b b . 6. a, j j d i a g r a m m z u r b e s t i n u n u n g d e r h e r d t i e f e . h i e r z u liest m a n aus dem a, ^ d i a g r a m m (abb. 6) die zugehòrigen ^,,-werte a b , die durch a dividiert li ergeben. r\n 0,024 0,0225 0,025 0,024 0,026 0,024 hn 24 22,5 25 24 26 24 k m abgesehen v o n der u n v e r m eidhchen s t r e u u n g sind die h-werte als k o n s t a n t anzusehen, die h e r d t i e f e s c h w a n k t also in diesem f a l l zwischen li — 22,5 bis 26 k m . gròssen bereichs, so miissen wie sclion erw à h n t , die makroseismischen dateli als zu f e h l e r h a f t u n d d a h e r als nicht geeignet fiir eine h e r d t i e f e n b e r e c h n u n g v e r w o r f e n werden. dagegen wiirde die a n w e n d u n g eines der r e c h e n v e r f a h r e n , e t w a n a c h der met h o d e d e r kleinsten q u a d r a t e , in einem solchen f a l l e ein scheinbar gesichertes e r gebnis erbringen, dem in wirklichkeit d e r physikalische sinn f e h l t . n a c h d e n bisherigen e r f a h r u n g e n m i t der 158 w. s p o n i i e u e r neuen methode stimmen die liiermit errechneten herdtiefen gut m i t den mikroseismisch bestimmten uberein, wàhrend friilier oftmals erhebliche unstimmigkeiten zwischen mikroseismisch und makroseismisch ermittelten herdtiefen a u f t r a t e n . da tur viele auch makroseismisch beobachtete erdbeben wegen der umstàndlichkeit der bisherigen yerfahren die herdtiefe nicht bekannt war, konnte deren magnitude, wenn keine branchbaren instrumentellen aufzeichnungen registriert waren, nur mit hilfe der i n t e n s i t à ! im e p i z e n t r u m i„ und dei' gròsse des fuhlbaren sc.hùttergebietes b e t i m m t werden. nach gutenberg u n d richter (3) làsst sich die magnitude eines erdbebens aus makroseismischen d a t e n nach folgenden bezieliungen bestimmen: log e = 9,5 + 3,2 log h + 1,11 0 , [11] f e r n e r log e = 11,1 + 6,1 log rh — 3,2 log li [12] duch zusammenfassung der beiden gleichungen [11, 12] erhàlt m a n log e = 10,3 + 3,2 log r/( + 0,55 i 0 [13] eine beziehung, ili der die herdtiefe li nicht a u f t r i t t . mit [13] h a t fiedler (2) die magnit u d e n der makroseismisch beobachten beben sudwestdeutschlands bestimmt, wobei er die h v p o z e n t r a l e n t f e r n u n g rh durch die e p i z e n t r a l e n t f e r u u n g r ersetzte. e s lag n u n nahe, fiedlers ergebnisse u n t e r yerwendung von gleichung [11] nachzuprùfen, nachdem die herdtiefe der beben oline grossen a u f w a n d aus den makroseismischen daten erreclinet werden konnte. hierbei stellte sich heraus, dass mit ausnahme der in der tabelle m i t * bezeichneten beben n u r unbedeutende unterschiede bei der berechnung von log e nach [11] gegenùber der b e s t i m m u n g nach [13] a u f t r a t e n . allerdings muss hierzu b e m e r k t werden, dass nach der defìnition von gutenberg u n d richter r als der radius des scliiittergebietes angeselien wird, das von der isoseiste mit der i n t e n s i t à t i = 1,5° eingeschlossen wird, wàhrend sicli bei fiedler in der regel r auf einen seliutterbezirkbezieht, der von der isoseiste 3. grades eingeschlossen wird. die ubereinstimmung der ergebnisse spricht fiir fiedlers auffassung von r. ubrigens diirfte die festlegung der grenze des makroseismischen bereiciis ini sinne von gutenberg und richter heutzutage bei dem holien stoipegel durch industrie und verkehr n u r nodi in ausnahmefàllen mòglich sein. die herdtiefen in deutschlands z. zt. regsamstem erdbebengebiet, der schwàbischen alb, sind, wie eingangs bemerkt, teilweise auch mikroseismisch belegt. wie die tabelle zeigt, liegt ein grosser teil der herdtiefenwerte im intervall von 8 bis 10 km, n u r wenige liegen tiefer bis etwa zu 15 km. d a h e r ist es besonders auffàllig, dass das e r d b e b e n vom 21.2.1933 m i t sicherheit eine grossere h e r d t i e f e von li = 30 km aufweist. bei den mit einem * gekennzeichneten beben der schwàbischen alb ist die herdtiefenbestimmung, wie aucli schon peterschmitt (6) b e m e r k t h a t , nicht eindeutig. man erhàlt bis zu einer gewissen e]iizent r a l e n t f e r n u n g s, deren w e r t bei etwa 50 km liegt, einen konstanten w e r t der herdtiefe li fiir ein bestimmtes a. f ù r isoseistenradien grosser als etwa 50 k m ergibt sich eine bedeutend tiefere herdlage mit einem kleineren absorptionskoeffizienten. i n einigen derartigen fàllen konnte durch yergleich m i t mikroseismisch errechneten herdtiefen immer die kleinere der beiden sieli ergebenden herdtiefen als zutreffend b e s t i m m t werden. hier liegt die annahme nahe, dass bei e r d b e b e n m i t gròsserem schuttergebiet, denti n u r bei diesen treten zwei herdtiefenwerte auf, die ausbreitung der energie niclit, wie vorausgesetzt, ungefàhr geradlinig vor sich gegangen ist, sondern dass von einer gewissen e n t f e r n u n g an energie zur erdoberfìache gelangt, die an einer tieferen schicht entlang sich ausgebreitet h a t . e r d b e b e n m i t einer solchen eigentùmlichkeit f a n d e n sich innerhalb deutschlands bisher n u r in der schwàbischen alb und in oberschwaben. hier zeigte sich bei dem schadenbeben vom 27 . 6 . 1935 die vorerwàhnte eigentùmlichkeit. in ubereinstimmung m i t den mikroseismischen berechnungen wurde die herdtiefe zu etwa 9 k m ermittelt. die herdtiefe des bebens von 1898 ist dagegen weniger sicher, sie liegt zwischen 5 u n d 9 k m tiefe. d i e t i e f e n d e r e r d b e b e n h e r d e i n deutschland, u s w . 1(31 fiir die erdbeben des dornstetter graben und des albvorlandes wurde eine herdtiefe in der gròssenordnung von etwa 15 k m gefunden. ina gebiet sfiditeli der donali in oberschwaben ergaben sich herdtiefen von 8 bis 9 k m . . der h e r d des erdbebens vom 13 . io . 1886 ani nordufer des bodensees lag sehr flach in etwa 3 km tiefe, wàhrend derjenige des bebens vom 2 1 . 6 . 1917 bei konstanz die fiir deutschland sclion erhebhche tiefe von 20 k m aufweist. die drei erdbeben ini niederrheingebiet ani nordabfall der eifel bzw. bei aachen stimmen etwa mit ihren herdtiefen, die bei 20 km liegen, iiberein. ganz geringe tiefen haben die herde der beben zu herzogenr a t h und tollhausen. wenig unterscheiden sich aucli die herdtiefen der erdbeben der gegend von siegburg-ziilpich, der e r f t gegend und der von euskirchen, die einen bereich von 8 bis 12 km einnehmen. fiir das mitteldeutsche erdbeben mit dem e p i z e n t r u m ani ostrand des thiiringer foo 300 200 100 f m6 | m-55 • noo 25 50 75 1800 25 50 75 1900 25 50 a b b . 7. f r e i g e w o r d e n e s p a n n u n g s e i i e ì ' g i e als f u n k t i o n d e r z e i t fiir e r d b e b e n d e u t s c l i l a n d s m i t e i n e r m a g n i t u d e m — 3,3. die e r d b e b e n der mitleren schivarzivaldregion haben herdtiefen von 8 bis .13 k m , das beben vom 3 . 8 . 1 7 2 8 ili der gegendvon kenzingen eine solclie von 14 bis 17 k m . die grossten herdtiefen auf deutschem boden wurden in der gegend des siidlichen schwarzwaldes erreicht, hier wurden fiir die beiden kurz aufeinander folgenden erdbeben an der hornisgrinde 26 k m herdtiefe nachgewiesen. die erdbeben in der bheinebene liegen mit ausnahme des erdbebens vom 24 . 1 . 1880 bei langenkandel in ganz geringen tiefen von 3 bis 6 km. von derselben gròssenordnung sind sie herdtiefen der erdbeben in der gegend des kaiserstuhls, einer vulkanruine, wenn m a n das erdbeben vom 2 8 . 6 . 1926 bei sasbacli-endingen-riegel ausn i m m t . die herde der erdbeben ini gebiet des mittelrheins von ivoblenz bis grossgerau liecen in einem tiefenbereich von 6 bis 12 k m . beckens wurde eine herdtiefe von etwa 15 k m bestimmt, dagegen sind die beben von stadtroda u n d vom w e r r a t a l als einst-urzbeben anziisprechen und haben daher nur sehr geringe herdtiefen. fiir das erzgebirge stand n u r ein auswertbares erdbeben zur verfiigung, dessen herdtiefe 8 km b e t r à g t . ferner sind in der tabelle zwei e r d b e b e n in schlesien mit herdtiefen von etwa 9 k m a n g e f ù h r t . aus den zusammengestellten ergebnissen scheint hervorzugehen, dass abgesehen von wenigen ausnahmen jedes herdgebiet in deutschland eine gewisse ihm eigentiimliche herdtiefe aufweist. die gròsste bisher nachgewiesene tiefe liegt bei 30 k m . gròssere schwankungen, aneli innerhalb eines eng begrenzten herdgebietes, zeigt der absorptionskoefflzient a. beispielsweise liegen die w e r t e diesel' gròsse ini bereich der schwàbisehen alb zwischen a = 0,001 und 158 w. s p o n i i e u e r t a b e l l e d a t u m h e r d t i e f e h [ k m ] a b s o r p t i o n s k o e f f l z i e n t a [ k m ] 1 m a g n i t u d e m e p i z e n t r u m 1) se iwabische al b 23 9 1830 12 0 , 0 2 5 4 , 5 h a y i n g e n e g l i n g e n 24 11 1872 1 0 , 5 1 1 0 , 0 1 3 , 2 m ò s s i n g e n 13 7 1875 11-12 0 , 0 1 5 , 3 h e c h i n g e n m ò s s i n g e n 16 11 1911* 13-17 0,001 6 , 3 e b i n g e n o n s t m e t t i n g e n 20 7 1913 12-14 0,001 5 , 5 h e c h i n g e n 11 12 1924* 14-15 0 , 0 2 5 5 , 1 b i t z e b i n g e n 11 12 1931 8 9 0,01 5 , 0 e b i n g e n b a l i n g e n z i l l h a u s e n 22 12 1931 10 0 , 0 2 5 5 , 5 l o n s i n g e n k o h l s t e t t e n ( r e u t l i n g e n ) 21 2 1933 28-32 0 , 0 0 1 4 , 5 m a r g r e t h a u s e n b u r g f e l d e n p f e f f l n g e n 10 10 1933 9-12 0 , 0 0 1 3 , 3 e b i n g e n o n s t m e t t i n g e n p f e f f l n g e n 1 1 1934 5 7 0 , 0 0 5 5 , 5 jnòrdl. s c l i l a t t ( s u d w e s t a l b ) 24 3 1934 7 8 0 , 0 5 2 , 8 n ò r d l . g a m m e r t i n g e n 17 6 1937 11 0,001 3 , 1 b e i b r o n n e n 2 8 1938 8 1 0 0 , 0 0 1 4 , 1 t a i l f i n g e n o n s t m e t t i n g e n 1 3 1939 6 7 0 , 0 2 3 , 3 p f e f f l n g e n l a u t l i n g e n 6 8 1940* 9 1 1 0,001 2 , 9 o n s t m e t t i n g e n 2 5 1943* 7 8 0 , 0 2 5 5 , 8 o n s t m e t t i n g e n p f e f f l n g e n 28 5 1943* 6 7 0 , 0 1 6 , 2 3 o n s t m e t t i n g e n p f e f f l n g e n 27 12 1943 7-10 0 , 0 0 1 5 , 0 o n s t m e t t i n g e n 28 6 1947 9 1 0 0 , 0 1 5 , 6 o n s t m e t t i n g e n t a i l f i n g e n 14 9 1947 8 1 0 0 , 0 1 2 , 9 m a r g r e t h a u s e n 27 1 1948* 13-14 0,001 4 , 5 o n s t m e t t i n g e n 8 7 1949 7 , 5 8 0 , 0 5 3 , 2 t a i l f i n g e n 15 9 1949 5 0 , 0 2 5 2 , 8 o n s t m e t t i n g e n p f e f f l n g e n 6 11 1949 7 8 0 , 0 1 4 o n s t m e t t i n g e n 2) oberscliwaben 6 10 1898 5 9 0,001 3 , 9 s a u l g à u m e n g e n 27 6 1935* 8 , 5 9 , 5 0 , 0 2 5 6 , 0 s a u l g a u h e r b e r t i n g e n 3) dornstetter graben albvorland 28 11 1822 13-16 0 , 0 2 5 5,1 m u r g t a l ( d o r n s t e t t e r g r a b e n ) 19 4 1936 14 0 , 0 1 4 , 5 a l b v o r l a n d b e i t i i b i n g e n , b e b e n h a u s e n h a g e r l o c h 4) bodensee 13 10 1886 4 0 , 0 1 2 , 7 4 bei f r i e d r i c h s h a f e n , b o d e n s e e n o r d u f e r 21 6 1917 20 0 , 0 0 1 4 , 5 k o n s t a n z , w e s t l i c h e r b o d e n s e e 5) scliwarzwald 3 8 1728 14-17 0 , 0 0 1 5 , 3 k e n z i n g e n l a h r e r b r u c l i 24 1 1883 10-13 0 , 0 1 3 , 4 g o t t e n h e i m (siidlicher s c l i w a r z w a l d ) 21 4 1885 11-12 0 , 0 0 1 3,1 f e l d b e r g 7 6 1886 8 0 , 0 1 0 , 0 0 1 2 , 6 m i t t l e r e r s c l i w a r z w a l d b e i l a h r 13. 1 1895 8 , 5 1 0 , 7 0 , 0 1 3 , 5 b o n n d o r f e r g r a b e n 22 1 1896 13 0 , 0 0 5 4 , 8 n e u s t a d t k a p p e l s a i g 30 12 1935 26 0 , 0 0 1 5 , 1 i l o r n i s g r i n d e d i e t i e f e n d e r e r d b e b e n h e r d e in d e u t s c h l a n d , u s w . 1(31 d a t i m i h e r d t i e f e li [ k m ] a b s o r p t i o n s koeffizient [ k m ] 1 masrnitude 31 e p i z e n t r u m 6) oh, 'rrheintalgrab en 7. 2.1839 8 1 0 0 , 0 5 4 , 9 z a b e r e n z w u n n g e b i e t 24. 1.1880 8 9 0 , 0 0 5 3 , 4 l a n g e n k a n d e l i i . 6.1887 3 , 5 0 , 0 4 3 , 2 s c l i u t t e r n bei l a l i r 8. 2.1933 4 6 , 5 0 , 0 7 5 , 4 r a s t a t t 7. 6.1948 5 6 , 5 0,001 5 , 1 k a r l s r u h e 7) kaiserstulil 21. 11.1823 6 , 5 0 , 0 7 4 e n d i n g e n .24. 6.1884 4 , 5 5 , 5 0 , 0 4 2 , 9 e n d i n g e n j e c h t i n g e n s a s b a c h 3. 1.1886 3 0 , 0 5 2 , 5 2 s a s b a c h e n d i n g e n r i e g e i 9. 10.1886 3 0 , 0 5 3 , 6 kelil k a p p e l 17. 11.1891 5 0 , 0 2 5 3 o p f i n g e n 14. 2.1899 5 0 , 2 5 3 , 7 s a s b a c h w e s t l i c h e r k a i s e r s t u h i 28. 6.1926 11 0 , 0 0 5 5 , 5 s a s b a c h i h r i n g e n riegei 8) miltelrhein 2!). 7.1846 6 , 5 8 0 , 0 0 1 4 , 7 s t . g o a r , r b e i n t a l 2.13. 10.1869 8 1 0 0 , 0 0 1 3 , 9 e n g e r s k o b l e n z 31. 10.1869 6 0 , 0 5 4 , 3 g r o s s g e r a u 1. 11.1869 6 8 0 , 0 4 4 , 4 g r o s s g e r a u 1 11.1869 10 0 , 0 2 5 4 , 1 g r o s s g e r a u 2. 11.1869 8 , 6 1 0 , 6 0 , 0 5 4 , 3 g r o s s g e r a u 19. 6.1928 10-12 0 , 0 7 4 , 1 3 a n d e r n a c h n e u w i e d 9) niederrliein, aachener gebiet 20.27 . 12.1755 18-21,5 0,002 5 , 8 n o r d a b f a l l d e r e i f e l 18 2.1756 21-25 0,006 6 . 2 n o r d a b f a l l d e r e i f e l 22 10.1873 3 , 7 4 , 5 0 , 0 4 4 , 2 h e r z o g e n r a t h 24 6.1877 2 0,01 4 , 0 5 i l e r z o g e n r a t h 26 8.1878 4 5 0,001 5 , 0 5 t o l l h a u s e n , r h e i n i s c l i e b u c l i t 18 11.1881 22-24 0 , 0 1 4 , 7 a a c h e n 13 12.1928 1 0 , 5 1 1 , 5 0 , 0 2 4 . 1 e r f t r ò d i n g e n 11 7.1949 12 0 , 0 2 4 , 2 5 e u s k i r c l i e n k o l n 8 3.1950 6 , 5 8 , 5 ,01 4 , 4 e u s k i r c l i e n 14 3.1951 9 0,0010 5 , 2 e u s k i r c l i e n 10) milteldeutscliland 0 3.1872 1 4 , 5 1 6 , 5 0,001 5 , 6 o s t r a n d des t l i i i r i n g e r b e c k e n s , r o n -1 4 , 5 1 6 , 5 n e b u r g 28 1.1927 3 0 , 0 1 3 , 1 4 s t a d t r o d a (tliiir. b e c k e n , ò s t l i c h e r tel) 22 2.1953 1,1 0 , 0 4 3 , 5 7 w e r r a t a l 11) t ogtl. erzebirge 23 11.1875 8 0 , 0 0 1 3 , 4 4 p l a u e n 12) s chieste ir 31 1.1883 9-10 0,001 3 , 6 7 j t r a u t e n a u 6.1895 9 0 , 0 0 1 3 , 9 7 m i t t e l s c h l e s i e n : s t r e h l e n , r e i c h e n -0 , 0 0 1 ! bacìi 158 w. s p o n i i e u e r a — 0,05. nacli den vorliegenden e r f a h rungen ist a = 0,001 als unterei' grenzwert anzusehen. die obere grenze ist dagegen nicht ganz sicher. ini e a h m e n dieser untersuchung wurden die hòchsten werte fiir den absorptionskoeffizienten ini oberrheintalgraben gefunden, anscheinend bedingt durch die hier vorhandene grosse màchtigkeit der lockeren sedimente. w e r t e von a = 0,07, ini palle des bebens vom 1 4 . 2 . 1899 sogar a = 0,25, wurden hier erreicht. selbst wenn m a n diesen letzteren w e r t als unsicher, etwa durch besondere u m s t a n d e vorgetàuscht, ansieht, so bleibt dennoch fiir den absorptionskoeffizienten ein schwankungsbereich von ungefàhr a = 0,001 bis fast 0,1. es muss allerdings hierbei berùcksichtigt werden, dass der absorptionskoeffizient keine einfache physikalische gròsse, wie z b. die ausbreitungsgeschwindigkeit elastischer wellen, darstellt, sondern dass er von melireren f a k t o r e n abhàngt. abgesehen von den elastischen k o n s t a n t e n des mediums wirken hier n o d i die frequenz der bodenschwingung u n d die a r t der wellenausbreitung mit, die entweder ràumlich oder innerhalb einer schicht voi' sich gehen k a n n . die tabelle enthàlt die wichtigsten e r d beben vom j a h r e 1800 an sowie dariiber hinaus n o d i einige erdbeben aus frullerei' zeit. mit hilfe der dazu bereclineten magnit u d e n w e r t e ist es mòglich, nach dem verf a h r e n von benioff (l) die e r d b e b e n t à t i g k e i t i)eutschlands energiemàssig darzustellen. i n abb. 6 ist als ordinate die q u a d r a t wurzel aus der erdbebenenergie aufgetragen, wobei alle e r d b e b e n m i t einer magnitude von m ; 3,3 verwendet. wurden. aus dem d i a g r a m m gelit hervor, dass nach der jalirhundei'twende eine bedeutende z u n a h m e der seismizitàt in deutschland zu verzeichnen ist. jedoch ist dieser zuwachs gròsstenteils auf die e r d b e b e n t à t i g k e i t der schwàbischen alb seit1911 zui'ùckzufuhren. z usammenfass ung eine neue makroseismische metliode gestattet die schnelle und sichere ermittlung der herdtiefe fiir jeden mittleren isoseistenabstand vom epizentrum. hierbei wird ein diagramm benutzt, dem die gleichung von kóvesligethy zugrunde liegt. die auf diese weise erhaltenen werte der herdtiefen derjenigen erdbeben deutschlands, von denen isoseistenkarten gezeichnet werden konnten, zeigen eine gewisse ubereinstimmung fiir jedes epizentralgebiet. die aus epizentralintensitàt und radius des schiittergebietes bzw. aus epizentralintensitat und herdtiefe errechneten magnitudenwerte stimmen im allgemeinen uberein. die darstellung der ausgelòsten spannungen in abhdngigkeit von der zeit fiir alle erdbeben deutschlands mit einer magnitude von m ' 3,33 zeigt ein starkes scheinbares ansteigen der erdbebentàtigkeit seit 1911. abstract a new macroseismic method permits tlie guide and sure determination of the focal depth for each middle isoseismal distance from epicenter. herewith a diagram is used, whicli is founded on kóvesligethy''s eguation. the focal depth of earthquakes in germani/, determined in this manner, to which isoseismal maps could be drawn, shows a certain accord for each epicentral region. the mcignitudes computed from epicentral intensity and radius of proceptibility or from epicentral intensity and focal depth accord generally. the representation of strain release in function of lime for ali earthguakes of germany with m 3,3 shows an importuni apparent ascent since 1911. riassunto un nuovo metodo macrosismico permette la determinazione rapida e sicura della profondità ipocentrale per ogni distanza isosismica media dall'epicentro. a guest'uopo ci si serve di un diagramma ricavato dalla formula del kóvesligethy. i valori delle profondità dei focolari ottenuti con guesto metodo per guei terremoti della germania, di cui si potevano disegnare delle carte isosismiche, mostrano una certa conformità per ogni territorio epicentrale. i valori di magnitudo calcolati rispettivamente dall'intensità epicentrale ed il raggio del territorio di scossa oppure dall'intensità d i e t i e f e n d e r e r d b e b e n h e r d e in deutschland, u s w . 1(31 epieentrale e la profondità ipocentrale corrispondono generalmente. il grafico delle tensioni svincolate in dipendenza dal tempo, per tutti i terremoti della germania con una magnitudo di m 3,33 mostra un'apparente forte crescendo dell'attività sismica dal 7.977. l i t e r a t u r (') bexioff, il, (holxrf strain and belease as bevealed by great earthquakes. « geol. soc a m e r . », 6 2 . (1951 ). (2) fiedlek, g., die erdbebentcitigkeil in siidwestdeutschlcmd 1s00-1950. d i s s e r t a t i o n , s t u t t g a r t , j 954. (!) gutenberg; b. u n d richter, c. f . , earthquake magnitude iutensili/, energy and aceeleration. « b u l l . s e i s m . s o r . a m e r . ». 2 3 . 3( j u l i , 1943). (4) hiller, w . , seismisehe berichte der ìviirttembergischen erdbebenwarten, j a l i r g . 1935, s t u t t g a r t , 1930. (5) kòvesligethy, v. r., seismisclier starkegrad und intensitàt der beben. « g e r l a n d s b e i t r à g e z u r g e o p h y s i k », b d . v i l i , l e i p z i g , 1907. (6) petersciimitt, e . , sur la variation de l'intensile macroséismique aree la distance epieentrale. « p u b i . b u r e a u c e n t r a l séism o l . i n t e r a . •». serie a , 18 (1952). (7) sponheuer, w., methoden zur herdtiefenbestimmung in der makroseismik (als m a n u s k r i p t g e d r u c k t . ) 1958. layout 6 1 annals of geophysics, 62, 5, se564, 2019; doi: 10.4401/ag-7877 “recent seismotectonic stress regime of most seismicallyactive zones of gulf of guinea and its kinematic implications on the adjoining sub-sahara west african region„ ayodeji adekunle eluyemi1,2,3, santanu baruah3, sangeeta sharma3, saurabh baruah2,3,* (1) division of environment and earth science., centre for energy research and development (cerd), obafemi awolowo university (oau), ile−ife, nigeria (2) academy of scientific and innovative research (acsir), csir− north east institute of science and technology (csir−neist) campus, jorhat−785006 (3) geosciences and technology division, csir. north−east institute of science and technology, jorhat−785006, assam india 1. introduction the south atlantic ocean around the equator is known as the gulf of guinea. this gigantic water body borders most of the west african nations in the south and few of the central africa nations in the west. most of the countries located on the adjoining area of the gulf of guinea are mega cities / national capitals and are highly populated, having their various boundaries to the south-atlantic ocean (gulf of guinea) and are regarded as stable and aseismic in nature (figure 1). though, evidence of tectonic activities are numerous on the oceanic floor of this zone including submarine volcanic fields, seismicity and the mid atlantic ridges, the tectonic activities / seismicity within the atlantic ocean have a significant contribution to the intraplate seismic activities of the adjoining areas pertinent to the continental crust [shimazaki, 1976]. this has resulted into accumulation article history received august 6, 2018; accepted april 1, 2019. subject classification: gulf of guinea stress tensor inversion; migration of earthquakes; cameroun volcanic line; sub-sahara west african region. abstract the tectonic stress regime of gulf of guinea region has been studied by stress tensor inversion analysis for the area bounded by lati− tude −10.0 0 s to 4.0on and longitude −25 0 w to −11.0 0 e. a total of one hundred and four focal mechanism solutions, pertaining to the earthquake events which have occurred in this region, were used for this study. in order to decipher the stress pattern of this region, we have divided the region into four fracture zones namely romanche, chain−romanche, charcot and ascension fracture−zones based on seismicity clustering, tectonics and available focal mechanism solutions. the seismicity pattern indicates that none of the nearby coun− tries on the border line between the west africa region and the gulf of guinea is devoid of seismicity. simultaneously, the stress tensor inversion in the four subzones of investigation indicates different types of stress orientations. all of these zones are characterized by vary− ing principal axial directions. the orientation of the principal axial direction along romanche, chain−romanche, charcot and ascen− sion fracture−zones are along ne−sw, ne−sw, ene−wsw and ese−wnw respectively. the stress tensor inversion results indicate that chain−romanche, charcot and ascension fracture zones are characterized by extensional stress regime while romanche fracture zone is characterized by strike−slip stress regime respectively. these patterns imply sea floor spreading activities in and around the mid oceanic ridges and the general orientation of the extensional stress regime is found towards the continents, along the line of migration / pro− gression of earthquakes. of stress and successive migration / progression of occurrence of earthquakes toward the continental crust from its inherent secondary faulting. stress accumulated within the earth is often released in form of earthquakes, which take the form of foreshocks, main-shocks, aftershock and earthquake swarms [michael, 1987]. this has been reflected in the identified seismically active zones of the gulf of guinea, extending from the north of ascension island to the mid atlantic ridge province. the seismicity pattern indicates clusters of earthquakes that are well observed within this region (figure 1). this is substantiated by the evidence of earthquake migrations and successions. on the other hand, it was argued that the intra plate seismic activities on the continental crust of the west africa is greatly affected by the seismic activities within the oceanic crust which is still very active. in general, stress accumulation in the province of intra plate zones takes a longer period of time to accumulate, hence the recurrence time for intraplate earthquakes is often found to be longer than the inter plate zones shimazaki [1976] as a result, the continental crust of the west african sub region is seen to be seismically quiet (figure 1). consequently, the orientation of its seismotectonic stress regime is poorly known. over the years, the information of the stress regime of a particular region could be obtained from stress tensor inversion of focal mechanisms of given sets of earthquake events occurring from a particular region, which is based on determination of stress field orientation [martinez-garzon et al., 2014]. however, the conventional methods of stress tensor determination are through inversion of focal mechanisms. efforts towards writing up of a code and developing a more precise and accurate means of determining stress tensor have been carried out by various authors [michael, 1987; kiratzi, 1999; abers, 2001; yamaji, 2006; zalohar and vrabec, 2007; angelier, 2008; de vicente, 2008; delvaux and barth, 2010; tirifu, 2011; baruah et al., 2013 and martinezgarzon et al., 2014]. in recent past, seismotectonics of the intraplate earthquake of the west africa has been linked to the adjoining area of the gulf of guinea province [kutu, 2013 and akpan et al., 2014]. attempts to determine the stress tensor and its orientation through in-situ and mechanical methods within this region were made to ascertain the bore-hole lithological-wall stability of the asahnti deep gold mine in ghana [affam and achibald, 2012]. in-situ stress determination, through ocean drilling project (odp) integrated with focal mechanism solutions of earthquake events were carried out in order to have an eluyemi et al. figure 1. tectonic setting and epicentres plot of the gulf of guinea and the adjoining continental crust of the countries of sub−sa− hara west africa. in order to depict the seismicity pattern of the region of study, seismic data during the period of 1900− 2016 were utilized. the epicentres are shown in red to green circles (the red circles represent events with depth range of 0−33 km, and the green circles represent events with depth range of 33−70 km). dashed lines indicate the fault lines, the volcanic zones are depicted in yellow triangles. in-depth knowledge of the structural, thermal and sedimentary processes that take place at the transform margine [mascle et al., 1998]. since the region around the gulf of guinea is comprised of numerous geological features with active tectonics: including cameroun volcanic line (cvl), which is estimated to one thousand and eight hundred kilometers long and made up of continental and oceanic sectors tokam et al. [2010] and adams et al. [2015] seismically active mid atlantic ridge and the array of submarine volcanoes on the oceanic floor of the gulf of guinea. however, the adjoining region of the gulf of guinea (sub shara west africa) is far away from the plate boundaries but it has experienced devastating earthquakes on five occasions, thrice in ghana, once in togo and guinea. during the period of 18th december, 1636 (ms = 5.7), 1862 (ml ~ 6.5 and ms ≥ 6.5), 11 th february, 1907, on 22nd of june, 1939 (ms ~ 6.5 and mb ~ 6.4) accra earthquake in ghana killed sixteen (16) people and one hundred and thirty three (133) others were severely injured. on 22nd december, 1983 in conakry-guinea (mw ~ 6.3) caused the death of three hundred (300) people and several others were severely injured including loss of properties [juner, 1941; blundell, 1976; bacon and quah, 1980; yarwood and doser, 1990; amponsah, 2004 and ramdani et al., 1984]. similarly, in recent years, the region has also suffered from devastating volcanic eruption with estimated death of one thousand and seven hundred (1,700) people, only on 21st of august 1986 [fomine, 2011]. other similar volcanic eruptions took place in the year 1999 and 2000 [such et al., 2003] respectively. in this paper, we aim to interpret the active deformation of gulf of guinea bound by the latitude -10.0o to 4.0on and longitude -25o to -11.0oe through estimation of seismotectonic stress based on inversion of earthquake focal mechanism solutions, with emphasis on reconstruction of the consistent seismotectonic stress state. in fact, prior to stress estimation, we required delineating the major structural domain through an in-depth understanding of present geodynamics. thus, this study attempts to determine the stress tensor inversion of the investigated zones exclusively for: (1) the dimension of azimuthal variation of compressional axis related to existing geodynamics of the region and (2) the significance of seismotectonic mechanisms related to four major fracture lines. the aforementioned features are directly or indirectly connected to stress accumulation and discharge of the intra-plate earthquake on the adjoining areas of the gulf of guinea (continental crust of west africa), with the scope of seismic hazard assessment and its mitigation pertaining to the occurrence of future large earthquakes. we finally intend to better characterize the interplate deformation in gulf of guinea and its kinematic implications on the sub-sahara west african region, with emphasis on the west african rift system along the cameroun volcanic line (cvl). 2. tectonic settings freeth [1977] described the regional tectonics of the west africa and the gulf of guinea from a “membrane tectonics” perspective: the west africa and the gulf of guinea with peripheral “compression” was transformed into peripheral “tension”, as african plate moved across the equator having africa and south america split apart, where a major rift opened up into west africa rift system during hauterivian to turonian times [burke et al., 1971; burke and whiteman, 1973]. the rifts are the lower benue-rift and the yola rift [burke et al., 1972]. this involved the separation of continents and zones of major weaknesses in between the eastern ends of the yola rift and the red sea rift which is termed as the effective centre of the african plate known as “jebel marra” which is a major geographical feature. this “effective centre” moves towards the equator, as the portion of the plate on the same side of the equator known as the “effective centre” experiences peripheral compression, towards south of the equator during early cretaceous to eocene times [freeth, 1977]. post early cretaceous, to eocene times, “jebel marra” moved away from equator, therefore, peripheral extension was experienced, north of the equator. having acted upon the west african rifts are evidences of peripheral extension or compression, in which resulting membrane effects were therefore completely swamped by the stresses, which opened the atlantic and the west african rift systems. during the last 30 m.y. “jebel marra” has been moving northwards, away from the equator. this is the period of the formation of rifts opening up in west africa and the gulf of guinea, with major rift formation known as the cameroun volcanic line (cvl) extending up to one thousand and eight hundred kilometers (1,800 km), from the offshore (gulf of guinea) up to the central african republic [tokam et al., 2010 and adams et al., 2015]. cameroun volcanic line (cvl) is further explained as a subset of the pan african belt formed by the collision of sao francisco, congo and west african cratons during neoproterozoic formation of gondwana that lies within the obanguides belt, with multiple shear zones, consisting of central african shear zones, in close association with pernambuco lineament in brazil [brown and fairhead, 1983; dorbath et al., 1986 and adams et 3 tectonic stress regimes of the gulf of guinea eluyemi et al. 4 al., 2015]. cvl comprises of a chain of tertiary to recent alkaline volcanoes, which split into two arms. one running northward into the northeast of nigeria, forming biu plateau, while the other arm of cvl runs eastwards through nagoundere plateau of eastern cameroun [fitton, 1980] thereby forming a “y” shaped volcanic feature. based on long-baseline geodetic gps data from africa to eurasia kreemer et al. [2014] there is indication that within the framework of northerly extension of gulf of guinea (25 mm/yr), the present day deformation is statistically significant, relative to mainland africa which comprises of guinea, liberia, ivory-coast, ghana, togo, benin, nigeria and cameroun. 3. data used the global centroid moment tensor (gcmt) and the international seismological centre (isc) data catalogue revealed the presence of four types of available focal mechanism solutions (strikeslip, normal, thrust and oblique) that are present along the existing fracture zones of the gulf of guinea region. altogether 104 fms data used were obtained from global centriod moment tensor (gcmt) known as harvard cmt dziewonski et al. [1981]; ekström et al. [2012] and international seismological centre (isc) catalogue project where magnitude (mw) ranges from 4.8-7.1 and depth range (km) varies from 3.0-33.2 km. from the time period of 1977-2016. since different types of focal mechanism solutions are available for the study region, specific types of focal mechanism solutions are more prevalent than the others. therefore, the focal mechanism solutions are not uniform and neither are these focal mechanism solutions are likely to be controlled by a single fault system with similar orientations. the epicentral pattern of the studied region can be broadly divided into three, namely: area around the cameroun volcanic line (cvl), with moderate to active seismicity, the rest of the adjoining area of the gulf of guinea (west africa sub-continent) with less seismicity and the gulf of guinea region, which its earthquake sources has been categorized by the isc and the gcmt as well as other seismological organizations into: the north of ascension island, ascension island and central mid atlantic ridge with active seismicity. however, due to non availability of fms data, stress tensor inversion could only be carried out on the gulf of guinea region alone. the fms data of the studied zone is mainly characterized by normal, strike-slip, oblique and very few thrust fault mechanism. 4. methodology 4.1 seismicity in order to completely understand the seismicity pattern of the gulf of guinea and its implications on the sub sahara west african region, a total of one thousand and one hundred (1,100) numbers of earthquake events were collected from international seismological centre (isc), which comprised of historical and recent seismic data from the period of 1900-2016 and magnitude (mw) range of 3.0-7.1. figure 1 presents the seismicity pattern of the studied area. the corresponding epicenters are located inside a quadrangle from -10.0 0 n to 17.0 0 n in latitude and -25.0 0 e to 17.0 0 e in longitude, which represent a total area of nearly 14,193,792 km2. figure 1 summarizes the distribution of the earthquake data as a function of magnitude and depth. the epicentral map, indicate clusters of earthquakes in the gulf of guinea, within the area of longitude -25 0 e to -11.0 0 e and latitude -9.0 0 n to 4.0 0 n. the overall seismicity pattern shows a progression / migration of earthquakes from clustered zone in the gulf of guinea to the adjoining subsahara west african region. 4.2 quality assessment of the employed focal mechanism solutions the earthquake events of which the fault plane solutions were used in our stress tensor inversion (sti) analysis were recorded teleseismically around the world. the global centriod moment tensor (gcmt) catalogue contains two descriptions of moment tensor for every earthquake source. the first one contains the azimuths (jt, jb, jp) and plunges (at, ab, ap) of the t, b and p axes respectively. their associated eigenvalues is represented as (lt, lb, lp). the second description is in form of the six independent components of the matrix (m11,m22,m33,m12,m13, and m23) which represent the moment tensor (m) and their respective standard error estimate as follows: u11,u22,u33,u12,u13 and u23 the standard error estimates is made up of the standard error tensor denoted by u. in this study, we employed various statistical approaches that are useful tools in measuring the quality of the focal mechanism solutions employed. the first parameter we considered is the ƒclvd. this is the sign ratio of the amplitudes of the intermediate and largest principal moments, an indicator used to determine the similarity of a particular focal mechanism solution (fms) to a compensated linear vector dipole (clvd), which is same as earthquake produced by faulting occurring simultaneously along series of properly oriented non parallel surfaces or more similar to a double-couple solution (emergence of earthquake by slip on a planar sur5 tectonic stress regimes of the gulf of guinea face). sanchez and nuuez [2009] define ƒclvd statistic as: ƒclvd = (1) whenever a compensated linear vector dipole (clvd) nears ±0.5 the extreme value of ƒclvd, this implies that the orientation of the t, b and p axes are not conclusive. simultaneously, we also consider evaluating a relative error (erel) for the respective focal mechanism solutions employed in the stress tensor inversion analysis. the erel compares the relative size of moment tensor m and its standard error tensor u [davis and frohlich, 1995; frohlich et al., 1997 and sanchez and nuuez, 2009]. mathematical expression for erel is given below as: (erel) = = (2) the use of colon denotes the tensor scalar product, if a snd b are tensors, their respective elements are aij and bij, then a:b = si.jaijbij. we therefore consider (erel) as the norm of u divided by the norm of m. 0 ≤ erel ≤ 1. in practice, it is usually advisable to seek for values of erel as low or small as possible. the next parameter that we examined for the quality assessment of the employed fms is the nfree. this takes into consideration the numbers of moment tensor components of an earthquake source that is not equal to zero. for an instance, if a particular element mij of the moment tensor is equal to zero, the associated standard error uij is also equal to zero. hence, nfree is determined by counting the number of elements of u that satisfy the condition uij ≠ 0. this is an important factor for shallow earthquake sources, whereby the cmt constrains the m12 and m13 elements of the moment tensor to be exactly zero. the geometrical effect of restricting one of the p, t or b axes to the vertical and the two others are confined to the horizontal plane. however, constraining the p, t or b axes to the vertical in the case of pure double couples for shallow earthquakes is equivalent to having a perfect strike-slip, normal or thrust fault. out of the 463 fms reported by the isc and gcmt for the gulf of guinea region, only 104 satisfy the three aforementioned conditions. our computed values for ƒclvd for the gulf of guinea region range between -0.0070.275; (erel), varies between 0.096-1.00 and earthquake events that are made up of moment tensor component of nfree = 6 was also considered for the sti analysis. these are the criteria upon which the selected 104 fms data for this study were based upon. 4.3 fms data preparation and stress tensor inversion (sti) stress tensor inversion analysis was done by selection of one hundred and four (104) fms data on global centroid moment tensor (gcmt) catalogue dziewonski et al. [1981] and ekström et al. [2012] as compiled in table 1. statistical analyses of these data were prepared for: depth, magnitude and time which are depicted through histogram (figure 2). figure 3 represents the beach ball presentation of the focal mechanism solutions associated with the earthquake events in respective tectonic zones of the investigated region. respective beach balls were generated using rake software louvari and kiratzi [1997]. − b max( t . p l l l u :u m :m u112 +u222 +u332 +2u122 +2u132 +2u232 m112 +m222 +m332 +2m122 +2m132 +2m232 zone number of events depth (km) magnitude (mw) fms type min max avge min max avge nf ss tf ob romanche 36 4.0 33.2 18.6 6.1 7.1 5.2 27 9 chain / romanche 34 3.0 25.0 14.0 4.8 6.8 5.8 3 17 14 chacotte 10 10.0 23.2 16.6 4.9 5.6 5.25 5 5 ascension 24 08.0 33.0 20.5 4.8 6.5 5.65 9 6 1 8 table 1. shows distribution of earthquake focal mechanism solutions (fms) within subzones of the investigated zones of the gulf of guinea. the following acronyms are contained in the above table and are explained as follows: nf indicates normal fault, ss indicates strike−slip fault, tf indicates thrust fault and ob indicates oblique fault mechanism respectively. eluyemi et al. 6 figure 2. shows the statistical analysis of 104 focal mechanism solutions selected for sti analysis for the clustered zone of the gulf of guinea. the following histograms are presented: (a) depth (b) magnitude and (c) time. database of the focal mecha− nism solutions for the period of 1977−2016 used for the stress tensor inversion analysis. figure 3. map showing the selected 104 focal mechanism solutions in the study region. all focal mechanism solutions are shown by conventional beach−ball illustrations of global cmt solutions. the beach balls were divided into four subzones, associated to four fracture zones, based on the corresponding tectonic domain. the focal parameters of the fault plane solutions utilized in this study (supplementary) and the direction of the stress tensor were calculated and determined using the inversion algorithm of michael [1984, 1987] also known as michael’s method and zalohar & vrabec [2007] known as gauss method, simultaneously, in order to constrain the consistency of inferred results. inversion results of the 104 fault plane solutions are zone-wise tabulated in table 2. the operational principles of the gauss method is based upon the concept of best fit of stress tensor, with consideration for angular misfit between resolved shear stress and actual direction of movement of fault plane. secondly, it also takes into account the ratio between the normal and shear stress on fault plane. the optimal stress tensors for each homogeneous sub-system of faults are found by maximizing the object function, which is defined through a summation of the compatibility function for all the focal mechanism solutions. michael’s principle works upon a condition that must be satisfied by the input data. during the observed time interval, stress is assumed to be uniform in respective zones of investigation, earthquakes are assumed to be shear-dislocated on pre-existing faults and slips occur in the direction of resolved shear stress on fault plane. michael method accounts for determination of three principal stresses: maximum stress, intermediate stress and minimum stress, from statistical technique of bootstrapping re-sampling. orientations of the principal stresses s1 and s3 of the stress tensor inversion results derived from gauss and michael methods are illustrated in figure 4. the world stress map of the study region (gulf of guinea) heidbach et al. [2016] is presented in figure 5 and the geodetic gps data from africa to eurasia kreemer et al. [2014] movement of african plate towards the eurasia plate is indicated with dashed arrow line, in figure 6. 5. result 5.1 stress inversion and earthquake mechanisms: seismotectonic consideration in order to examine the homogeneity or variations in stress regime within the identified active seismicity zone (clusters of earthquake events) in the gulf of guinea (figure 1). methods of michael [1984, 1987] and gauss by zalohar and vrabec [2007] have been implemented. the results of stress tensor inversion for each of the mentioned four fracture zones are shown in table 2. the principal stresses and obtained orientations from michael and gauss methods are illustrated in figure 4. romanche fracture zone comprised of stress tensor inversion of 36 focal mechanism solutions which shows that the principal ex7 tectonic stress regimes of the gulf of guinea zone method used (σ1) (σ2) (σ3) ɸ ɸ avg variance std dev. azim (°) pln (°) azim(°) pln (°) azim (°) pln (°) romanche fault zone michael 135.9 11.3 -52.7 78.6 45.7 1.2 0.75 0.63 0.019 0.138 gauss 127.0 7.0 272.0 81.0 37.0 5.0 0.5 chain/romanche fault zone michael 159.9 87.8 -39.8 2.1 50.2 0.7 0.8 0.70 0.089 0.293 gauss 305 65.0 145.0 24.0 51.0 8.0 0.6 chacotte fault zone michael -71.8 66.5 137.8 20.6 43.8 10.6 0.44 0.52 0.084 0.289 gauss 226.0 76.0 339.0 6.0 71.0 13.0 0.6 ascension fault zone michael -172.6 19.6 0.1 70.0 96.4 2.3 0.82 0.76 0.100 0.316 gauss 2.0 86.0 189.0 4.0 98.0 0.0 0.7 table 2. result of stress tensor inversion for the four subzones of the studied region of the gulf of guinea using michael and gauss methods. eluyemi et al. 8 figure 4. stress tension inversion results of the four fault zones for the cluster of events therein using michael and gauss meth− ods. syntheses of stress pattern at the associated fault zones / clusters are depicted with symbols. the black convergent and divergent arrows indicate compression and extensional stress regimes. figure 5. world stress map release, 2016 edition showing the tectonic of the studied region in rectangular box. 3 types of tecton− ics have been identified within the studied region by wsm project: strike−slip (ss), normal (nf) and thrust (tf) [heid− bach et al., 2016]. the small red box indicated a thrust fault but with only two fms data, which were found on isc and gcmt data catalogues for the period of 1977−2016. perhaps an indicator to the development of a new tectonic domain, these were synthesized with the result of our study in figure 6. tensional axis s3 is along n-e direction. michael method revealed azimuthal, plunge and phi measurement of this zone, for principal extensional axis (s3) as follows: 45.7 0, 1.7 0 and f = 0.75. the dominant extensional axis (s3) measurement, using gauss method is as follows: azimuthal value of 37.0 0, plunge of 5.0 0 and f = 0.5 respectively, while the dominant compression axis s1 is along nw-se direction. michael method shows azimuth and plunge measurement for s1 as follows: 135.9 0, 11.3 0 while gauss method revealed azimuth and plunge for s1 as 127.0 0 and 17.0 0 respectively. chain-romanche fault zone is made up of stress tensor inversion of 34 focal mechanism solutions, michael method shows s3 with an azimuth of 50.2 0; plunge of 0.7 0 and f = 0.8 while gauss method shows, s3 with an azimuth of 51 0; plunge of 8.0 0 and f = 0.6. s3 tends towards north-east direction, charcot fault zone comprised of stress tensor inversion of 10 focal mechanism solutions which revealed that the zone is governed by nearly east-west extension s3. the dominant principal extensional axis (s3) measured in this zone using michael method is as follows: azimuthal value = 43.8 0, plunge = 10.6 0 and phi = 0.44. while gauss method showed azimuthal value of 71.0 0, plunge = 13.0 0 and phi = 0.6. ascension fracture zone is made up of stress tensor inversion of 24 focal mechanism solutions, michael method estimates s3 with azimuth of 96.4 0; plunge of 2.3 0 and f=0.82 0 while gauss method shows s3, with an azimuth of 98.0 0; plunge of 0.0 0 and f = 0.7. out of the four fracture zones, only romanche fracture zones is characterized by strike-slip (ss) stress regime while the rest of the fracture zones are characterized by extensional stress regimes only. 5.2 synthesis of stress tensor inversion result and world stress map project the obtained stress tensor inversion results of the present investigation have been supported by data of world stress map (wsm) project heidbach et al. [2016] (figure 5). the derived stress map from the result of the present study as it is, in figure 4 was synthesized with the existing global stress map of the studied region (figure 5). on a broader scale, two main tectonic stress regimes [strike-slip and extensional stress regime] have been identified that are primarily associated with seismic activities in the gulf of guinea. however, two of the existing stress regimes have been well indicated in this study. there is possibility of development of a new stress regime perhaps in its initial stage (figure 5). the red small rectangular box, contains just two focal mechanism (fms) 9 tectonic stress regimes of the gulf of guinea figure 6. vmap depicting the synthesis of main stress regime derived from stress tensor inversion associated with four fracture zones. the inputs from global stress map are included as indicated in legend. the symbols are explained in details in the text. yellow triangle denoted with vp represent the volcanic points; pairs of convergent arrows denoted by stizd stands for compression stress regime and divergent arrows denoted by stizc stands for extensional stress regimes are shown as in figure 9 are the final determination. extensional stress regime accounts for 72 per cent of the events out of the total data set. compiled gps observation in adjoining region (see text), the large dotted arrow marks on the right top corner of the figure indicate the velocity vector of the african plate with respect to the arabian and eurasian plate at the same scale. data belonging to thrust fault. as such, no stress tensor inversion could be done, due to limited availability of fms data. romanche fracture zone is characterized with strike slip stress regime and its stress orientation is well located, while chain-romanche, charcot and ascension fracture zones are characterized by extensional stress regimes respectively, each with different degrees of orientation. the outcome of our stress tensor inversion result and the wsm data map project of the same region have been synthesized together to generate a detailed and current seismotectonic map of the investigated region. a comprehensive explanation of the present day seismotectonic stress regimes of the gulf of guinea and its kinematic implication on its adjoining region is presented in figure 6. 6. discussion the study region is quite large enough and has significant implications for seismic hazard assessment of the adjoining continental region of sub-sahara west africa because of the presence of five regional fracture zones that are oceanic and continental by extension. the michael and gauss methods of stress tensor inversion analysis employed in this work provide reliable and stable results. the results of the two methods are relatively comparable in terms of stress axes orientation and stress regimes determination (table 2). the synthesized stress pattern from focal parameters and associated earthquake events of the seismogenic zones could be inferred. these seismogenic zones can be categorized into three major zones, namely: area around and along the cameroun volcanic line (cvl) or the west african rift zone, the rest of the adjoining region (west african continental crust) and the gulf of guinea region, (involving the central mid atlantic ridge, mid atlantic ridge, north of ascension island and the ascension island). seismicity around the cvl region is directly connected to the gulf of guinea region, since the region is the zone of weakness arising from the rifting and the separation of the south american plate from the african plate, therefore, build up of stresses from the gulf of guinea region are easily released along the cvl zone in form of earthquakes more than the adjoining areas. migration and succession of seismic events in linear pattern are evident towards the cvl and the adjoining area, around the west african continental crust, in conformity with gubin [1960] and nikonov [1976] from the clusters of the earthquake events within the gulf of guinea. it is obvious that tectonic stresses are being released on a regular basis within the studied zone (gulf of guinea). however, amidst and closer to the clusters of the seismic events, lies an array of submarine volcanic field, within the oceanic segment, which is obvious on the continental segment extending up to cameroun, sudan chad and algeria. historically, volcanic eruption along the cvl, on mount cameroun a subset of the continental segment of cvl took place in 1986, 1999 and 2000 [fomine, 2011; suh et al., 2003]. 7. conclusion we have determined the regional stress regime and its orientation in the gulf of guinea as well as its kinematic implications on the adjoining sub-sahara west africa region, which was not previously known. the result obtained from our investigation gives a better understanding to the tectonics stress regime of the gulf of guinea and a better insight to the work of the earlier researchers. since the adjoining region of the gulf of guinea (the west african sub region) is characterized by regional fracture/fault zones that originated from the gulf of guinea. the resultant effects from the gulf of guinea are easily propagated through these medium to manifest themselves in various ways, which can be described as follows: first of all, the investigated region lies on the lithospheric plate boundaries in between the south america and the african plates, there exists a plate boundary forces as well as potential energy and the ridge-push forces estimated around 2-3×1012 n per metre of the ridge length [lister, 1975; parson and ritcher, 1980 and coblentz et al., 1994]. as a result of this, stresses are being generated. the orientation of the generated stress on the gulf of guinea, which are mainly extensional stress regime became zones of weakness and tends to fracture as a result of action of forces per unit area (stress = f/a). these explain the origin of the multiple and regional fracture zones in the gulf of guinea (st paul, romanche, chain, charcot and ascension). interestingly, of all the investigated sub-fracture zones, only romanche fracture zone is made up of the strike slip regime, which can be explained in terms of plate boundary forces. the rest of the sub fracture zones are made up of extensional stress regime, which is as a result of simultaneous effect of ridge push and a plate boundary forces. for as long as the stress orientation remains in the same direction, as it is, in this study. it is expected that the processes of the fracturing continue, hence the aforementioned fracture zones traversed several thousand kiloeluyemi et al. 10 meters from mid atlantic ridge in the gulf of guinea onshore through west africa into the african plate. the current seismicity being experienced on the continental crust of the west african region could be explained not solely because of transmission of motion from either romanche or chain fracture zones in the gulf of guinea to the continental faults or an early stage in the development of plate boundary on the continent or deflection of motion from romanche or chain fractures to coastal boundary fault as theorized by earlier researchers burke [1971], blundel [1976] and yarwood and doser [1990] but also the seismicity of the west african region can be further explained as a result of release of built up stress in the gulf of guinea along the axial orientation of extensional stress regimes, which are towards and along the existing fracture zones in the gulf of guinea, up to the african continent. this is evident in the progression/migration of earthquake events from the gulf of guinea to the west african sub region (figure 1). the west african rift system, along the cameroun volcanic line (cvl) is mainly a result of ridge push forces exerted on the cvl from the mid atlantic ridge in the gulf of guinea and gravitational potential energy along the mountainous region on the continent in cameroun. we also observed a direct connection of the cvl to the mid atlantic ridge system through charcot and ascension fracture zones. the present extensional stress regime within the charcot subzones is oriented towards and along the west african rift system or the cvl. this implies that the west african rift system is still in its active stage of rifting. hence a linear progression and migration of earthquakes are observed from the mid atlantic ridge in gulf of guinea through the cvl up to chad region in (figure 1). results obtained from our work also suggest that the incessant eruptions of the cvl could be due to mechanical coupling between the gulf of guinea fracture zones (chain, charcot and ascension) and magmatic systems, due to stress release from large earthquakes on to the magmatic body, thereby perturbing the system and triggering volcanic eruptions. for as long as the orientation of stress from the gulf of guinea is towards the cvl, the cvl will remain an active volcano for years to come, unless the stress regime axial direction is rotated away from the cvl. all in all, this work also corroborates the hypothesis of blundell [1976], which suggested an early stage of development of a new plate boundary, possibly through fault propagation but not rifting processes, along the akwaipim fault zone (ghana) into the continent, as a result of active tectonism from romanche fracture zone to akwaipim fault zone. our work concludes that similar activities could be true of kandi regional fault line, on the shore of west africa, in between benin republic and nigeria up to hogga, a volcanic zone in algeria north africa. data and resources the global centroid moment tensor project database was searched using http://www.globalcmt.org/cmtsearch.html (last accessed on november 18th, 2016). the international seismological centre catalogue was searched using http://www.isc.ac.uk/iscbulletin/search/catalogue/ (last accessed was on october, 2nd, 2016). acknowledgements. eluyemi a.a. pays gratitude and appreciation to the world academy for sciences (twas) italy and council for scientific and industrial research (csir) india, for a csirtwas sub unesco ph.d fellowship. the academy of scientific and innovative research (acsir) india is acknowledged by eaa for availing the opportunity to carry-out ph.d. authors also acknowledge the international seismological centre, on-line bulletin, http://www.isc.ac.uk internatl. seismol. cent., thatcham, united kingdom, 2014 for enabling us access to recent and historic seismic data. authors hereby acknowledge the criticisms and loopholes identified and presented by the anonymous reviewer in contribution to the quality of this manuscript. those comments have sincerely improved the quality of this manuscript. much profound appreciation is hereby extended. funding. this work was fully supported by the council for scientific and industrial research (csir) india and the world academy for sciences (twas), italy [csir-twas, fr number: 3240275042]. 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(1968). south atlantic continental drift and the benue trough, tectonophysics 6 (4), 273-352. doi: https://doi.org/10.1016/0040-1951(68)90046-2 yamaji, a., k. sato (2006). distance for the solution of stress tensor invasion in relation to misfit angles that accompany the solutions, geophys. j. int. 167(2), 933-942, doi:10.1111/j.1365246x.2006.03188.x yarwood d.r., d.i. doser (1990). deflection of oceanic transform motion at a continental margin as deduced from waveform inversion of the 1939 accra, ghana earthquake, tectonophysics 172 (3-4), 341-349, doi: https://doi.org/10.1016/00401951(90)90040-f zalohar j., m. vrabec (2007). paleostress analysis of heterogeneous fault-slip data: the gauss method. j. struct. geol. 29, 1798-1810, doi: 10.1016/j.jsg.2007.06.009 13 tectonic stress regimes of the gulf of guinea *corresponding author: saurabh baruah, academy of scientific and innovative research (acsir) csir north east institute of science and technology (csir-neist) campus, jorhat-785006, assam india. geosciences and technology division, csir north east institute of science and technology, jorhat-785006, assam india email: saurabhb_23@yahoo.com © 2019 the istituto nazionale di geofisica e vulcanologia. all rights reserved eluyemi et al. 14 o n quantitative determination and m a p p i n g of seismic activity j . v . r i z n i c h e n k o earthquakes beneath our feet as well as stars iu the sky oeeur the more often, the less their magnitude is. the laws of their distribution according to the magnitude are similar for both. in this respect a view of the night sky recalls, even quantitatively, to a seismic activity map with magnitude denotations. let us determine the magnitude or the size of an earthquake by means of the value of the seismic energy e propagating through a spliere of a certain radius (say, ti = 10 km) surrounding the source. then between the value e and the number n of earthquakes of various classes k of the energy e = 10a' there exists a statistically estabhshed dependence n s== n (e) of a kind being sliown in fig. 1. tlùs graph is constructed in the doublé coordinate system log e, log n. i t represents an approximately straight hne for a wide range of energies from rather small values of about io2 joules to about 1018-1017 joules. the power number k for energy representation in joules e = 10x is the energy class of earthquakes. to precise, the class k comprises earthquake energies ranging from io*-0-5 to 10a'+0-5 joules. only for great values of e a remarkable deviation from the straight hne is observed, which is expressed in the strong decrease of n for the most disastrous earthquakes of the world. if the slope of the graph n versus e were nearly constant we should have an ability to compare the seismicity of various regions by comparing the " levels " of the graphs, which may be given by establishing the meaning of n for a fìxed k, for instance for k = 7 or k = 10. these figures n\k = 7 = ay or n\k = io = -̂10 could serve a measure of seismic activity. thus the cruciai question is avhether or not the slope of the dependence between n and e can be considered as a constant. the region of strong earthquakes is diseussed by gutenberg and richter, kawasumi and others ( 1 3 ). the region of weak earthquakes 2 2 8 t . v . r i z n i c h e n k o was thoroughly studied on the initiative of academician gamburtzev (4) during the last 3 or 4 years in the tadjik complex seismological expedition of the institute of the earth's physics (academy of sciences, iogn l o ? a , loga„ fig. 1. the energy-frequency of occurrence graph a log n 4 log e for k — 2 15 const n frequency of occurence a number of earthquakes of a given energy class k = log e (joules) beeng registered on a given territory s within a given time interval t. a seismic activity a7 refers to s = 100 km2, t = 1 year ; a10 refers to s = 10.000 km2, t = 1 year. u s s r ) and the seismological institute of the tadjik academy of sciences. the cliief participants of this study are i . l . nersesov and v . i . bune. the interpretation of a mass material reveals a perfect stability of a linear law for the dependence n = n (e) as it is shown in fig. 2. o n q u a n t i t a t i v e d e t e e m i n a t i o n a n d m a p p i n g , e t c . 2 2 9 the size of elipses on this graph corresponds to the values of the errors of n and e determinations. this graph has been constructed from many thousands of earthquakes being registered in the garin region (tadjikistan), in one of the most seismically active regions of the soviet union. n [by nersesov, riznichenkó) fig. 2. — k energy class of earthquake; e = 10a" joules n frequency of occurrence of earthquakes ranges of errors are shown. garin region, 1955-1957. the slope y = zi log n/a log e of the graph in the doublé log-coordinate system usually equals to about 0.43 for the values of k varing in v e r y large ranges from k = 2 to k = 16. i n fig. 3 the " occurrence graplis " for a number of regions, namely tadjikistan, kirgisia, ivazachstan in central asia; japan; new zealand (tonga-kermadek region); california ( u s a ) and the last one for the most disastrous earthquakes of the globe (by gutenberg and eicliter) are compared. 2 3 0 t. v . r i z n i c h e n k o i t is seen that the slopes of ali these graphs are nearly the sanie, except the most strong earthquakes of the globe with the class number k more than 16 or 17. k (by nersesov) fig. 3 energy ranges: e — io2 4io18 joules. 1 garm-stalinabad (tadjikistan); 2 kirgisia; 3 kazachstan; 4 j a p a n ; 5 new zealand; 6 california; 7 t h e earth. y fa const n refers to s = 100 km2, t — 1 year ( 1 joul = 1 . 0 0 0 5 1 x io7 erg). the material of observations of the tadjik complex seismological expedition demonstrates that for usuai normal seismic conditions the frequency of occurrence of earthquakes fìuctuates, and the degree ti of the " occurrence scatter " is approximately constant: r = anj ]rn, [1] where n is the observed number of earthquakes of a given class of enei'o n q u a n t i t a t i v e d e t e e m i n a t i o n a n d m a p p i n g , e t c . 2 3 1 gy, and a n is the. standard deviation of this number related to the sanie conditions of time and space is the dispersion). i t appeared also that for the normal seismic regime (excluding the periods of aftershocks and so on) the value of e is approximately 1. an analogous dependence of fluctuations is encountered in many branches of physics, forexample, the fluctuations of weak luminous fluxes in connection with their discrete quantum microstructure. the sense of the equation [1] for our reasoning becomes clear from a numerical example. if the number of the observed earthquakes is n = 10000, the relative error in establisliing the corresponding average frequency of occurrence is d^ = cr.v/a7.100 per cent = 1 per cent; if n = 100, then = 10 per cent and so on. a suffìciently exact determination of the average frequency of occurrence of earthquakes requires a suffìciently great number of observations, in fact no less than some dozens. weak earthquakes are frequent, and for them there is no difficulty in this respect. but difficulty arises in connection with strong earthquakes wliicb are rare. however the mean frequency of occurrence of strong earthquakes can be established with a certain stability if to take into account the general law n (e). certainly it requires a great number of observations of earthquakes for a wide range of energy including especially small values of energy. i t is important that such observations should not take too much time. thus the study of weak earthquakes is effective not only for them but for strong earthquakes as well. now we pass to the problem of the possible use of the frequency of earthquakes' occurrence for a quantitative representation of the regional distribution of seismic activity. i t is a along time already that seismologists are interested in the problem of a quantitative determination of seismicity. but the really constructive suggestions to the problem were made perhaps only beginning with 1953-1951. the first step was made in netherlands by koning (6). a little later it was done in some other way by bàth (7) in sweden, sponheuer (8) and toperczer (9) in germany, and others (3.10"12). i n ali these studies the quantitative representation of seismicity was based directly or indirectly on the summarized seismic energy e of earthquakes or on the square roots of the energy values (13'14). this value is known to be determined chiefly from strong earthquakes comprising the overwhelming part of the total seismic energy of earthquakes. but the strong earthquakes are rare, their numbers n are small and the estimate of seismicity obtained in this way must great2 3 2 t. v . r i z n i c h e n k o ly fluctuate. therèfore the results of these methods are usually rather unsatisfaetory as to the stability, exactness and detailed representation of the quantitative distribution pattern of seismicity. it is quite explainable that the quantitative determination of seismicity obtained with the help of earthquakes' occurrence and based mainly on weaker earthquakes, whose numbers n are great, would be much more stable. as a measure of seismicity we suggest " seismic activity " representing the frequency of occurrence n of earthquakes of a certain energy class k limited by a certain area s and time t of observations. this value will be designated by a . (15). for detailed seismic investigations a value a = a7 is conveniente for k — 7, s = 10u km2, t = 1 year; for regional investigations on a smaller scale, the values a = a10 for k = 10, s = 10.000 km2, and t = 1 year are quite suitable. the use of the frequency of earthquakes' occurrence for a quantitative determination of seismicity does not prevent from obtaining energy characteristics suggested by other authors (7"9' " ) ; moreover, it should be noted that these characteristics may be determined more stably if taking into account the occurrence law n (e), than if using direct calculations. thus the average density of the earthquakes' power (energy) flux may be estimateci from the formula co w = i enclk [2] 00 where n = dnlcìk, n — quantity of earthquakes, n being approximately equal to a number of earthquakes corresponding to the energy limits el = 10a'~0-5; eì = 10a' + 0'5. this formula can be used instead of that suggested by other authors (7"9) for the direct summation of earthquakes' energy w = l l e i > [ 3 ] where q is tlie total number of earthquakes at the area s for the time t. similarly the formula 00 £ = i) ' èndk , [4] • x based on the frequency of occurrence law allows to determine with a greater stability the so called " tectonic flow " or the " strain release " on q u a n t i t a t i v e d e t e e m i n a t i o n a n d m a p p i n g , e t c . 2 3 3 quantity (n'1 2) which is usually calculated by direct summing up according to the formula if the law n (e) for relative values of n is supposed to be flrmly established, the values of w and e from the formulas [2] and [4] can be fig. 4. determinaf.ion of the size of the averaging region to calcitiate the seismic activity a in accordante with the seismic waves energy attenuation. li tlie average dspth of earthquake focus. expressed as functions of the seismic activity a. the form of the law n (e) is certainly to be controlied and specified proceeding from observations for each region under consideration. w h e n mapping the seismic activity a by the suggested method it is necessary to use the observations of ali earthquakes for which a sufficiently reliable estimation of the frequency of occurrence is possible. h = 2 0 « m ai o n q u a n t i t a t i v e d e t e e m i n a t i o n a n d m a p p i n g , e t c . 2 3 5 to pass from the discrete pattern of the distribution of earthquakes' epicentres of various energy classes k to the smooth one, where the value of the seismic activity a is represented in isolines, a method of averaging (smoothing) was applied with a sliding two-step region. the idea of its construction is shown in fig. 4. the densities of epicentres on the areas of averaging si and s2 are multiplied by factors pl and p2, whicli decrease from the centre to the periphery. fig. 5 gives an example of a schematic map of the seismic activity for garm and stalinabad (the capital of soviet tadjikistan) regions of the tadjik ssr being drawn by the method described above. w e believe that the suggested method of the representation of seismicity which has evident advantages as compared with other indicated methods may serve as a basis for mapping the seismic activity not only of separate areas but of the whole globe provided the necessary observations conducted by joint efforts of the seismologists tliroughout the world are accumulated. abstract 1. the quantitative determination of seismicity being based on a direct summation of seismic energy of ali the earthquakes arising on a given territory within a given time interval is scarecely applied to the detailed mapping of seismicity: it leads to widely fluctuating values. the reason is, that the main part of the total seismic energy belongs to the strong earthquakes which occur rarely. 2. as a more stable value for the quantitative representation of the average level of the frequency of occurrence of numerous earthquakes in a wide range is suggested. this level may be determined by the number of earthquakes of a given energy class, ivhich relates to a unii space-time region. the possibility of such a determination is due to the fact of eocistence of an approximately stable dependence between the relative number of earthquakes and their energy. 3. in a highly seismic active garm-stalinabad region (tadjik ssr) many thousands of earthquakes, especially weak ones, have been studied by means of a net of high sensitive seismic stations. on this base a detailed seismic activity map of this region has been constructed which gives in isolines the frequency of occurrence of earthquakes on the territory under consideration. 23(5 y . v . r i z n i t c h e n k o •a. the author appeals to the seismologists of other countries to start on a systematic study of the frequency of occurrence of earthquakes of various energies, with the aim to secure the base for constructing the seismic activity map of the globe. such a map is of great importance to solve many questions of both pure geophysical and practical interest. riassunto 1. la determinazione quantitativa della sismicità che è stata basata sulla sommatoria diretta dell'energia sismica di tutti i terremoti che si verificano su un dato territorio entro un determinato intervallo di tempo, è scarsamente applicata alla preparazione di mappe dettagliate della sismicità: essa porta a valori molto variabili. la ragione è che la maggior parte dell'energia sismica totale appartiene ai terremoti forti che si verificano raramente. 2. si suggerisce, per un più certo valore, una rappresentazione quantitativa del livello medio della frequenza con cui si verificano numerosi terremoti (la determinazione quantitativa di cui sopra) per un largo intervallo. questo livello può essere determinato in base al numero di terremoti di una determinata classe di energia, che si riferisce ad una regione spazio unitario-tempo. la possibilità di una tale determinazione è dovuta al fatto dell'esistenza di una dipendenza quasi stabile tra il numero dei terremoti e la loro energia. 3. in una regione altamente attiva dal punto di vista sismico, quale quella di garm-stalinabad (tadjik ssr), sono state studiate molte migliaia di terremoti, specialmente quelli di forza debole, per mezzo di una rete di stazioni sismiche ad alta sensibilità. in base ai risultati ottenuti è stata costruita una mappa dettagliata dell'attività sismica di questa regione che dà in isolinee la frequenza con cui si verificano i terremoti nella zona in esame. 4. l'autore fa appello ai sismologi degli altri paesi per dare inizio ad uno studio sistematico della frequenza con cui si verificano i terremoti di varia forza, allo scopo di stabilire le basi per tracciare la mappa dell'attività sismica della terra. tale mappa è di grande importanza per risolvere diverse questioni di interesse sia puramente geofisico, sia pratico. on q u a n t i t a t i v e d e t e e m i n a t i o n a n d m a p p i n g , etc. 2 3 7 r e f e r e n c e s ( ! ) g u t e n b e r g , b., r i c i i t e r , c . , seismicity of the earth., princeton press, 1954. ( 2 ) k a w a s u m i , h . , on the energy law of occurrence of japanese earthquakes, « b u l l . earthq. res. », inst. t o k y o univ., 30. 4, 319-323, (1952). ( 3 ) byh3, b . h . , cpaenumeabho shepzemuneckan xapakmepucmuka ceiicmuuhoami mpex pauohoe tadoicmucmana. « crajihhasaflckoro, xahtckoro h tobhjib-jiophhckoro ». jxokji. a h tafl>khckoh cccp, 10, 3-12 (1956). ( 4 ) raaibypueb, r . a . , cocmohhue u nepcnekmuebi pabom e oó/iacmu npoeno3a 3emaemphcenuu. « eiojiji. cobeta no cencmonoriih npn npe3 ». a h cccp, 1, 7-14 (1955). ( 6 ) eyh3, b . h . , o/ibim ucnojib3oeanuh diiepiemuneckiix xapakmepucmuk npu u3yheimu ceùcmuhhocmii tadoicukuanana. « h3b. ota. ectecra. hayn a h ta«>khckckoh cccp, bbin. 23, 19-34 (1957). ( 6 ) koning, l . p. g., earthquakes in relation to their geographical distribution, depth and magnitude, « proc. kon. nederl. acad. wet. », 55, (1952). ( ' ) bath, m., seismicity of fennoscandia and related problems, « gerì. beitr. z. geophys. », 63, 3, (1953). ( 8 ) s p o n h e u e r , w . , erdbeben und tektonik in deutschland, « freiberger forscliungsbefte », h . c. 1953. (9) toperczer, m., zur deflnition der seismizitat. arch. meteorol., « geophys. und bioklimatol. », 5, 4, (1953). ( 1 0 ) t r a p p , e . , zur praktischen darstellung der seismizitat. « gerì. beitr. z. geophys.», 64, 2, (1954). ( u ) ritsema, a . r., the seismicity of the sunda are in space and time. « indonesia journ. for naturai science», 1-3, (1954). (12) amand, p . s., two proposed measures of seismicity. « bull. seism. soc. amer. », 46, 1, 27-45, (1956). ( 1 3 ) b e n i o f f , h . , earthquake and rock creep. « bull. seismol. soc. amer. », 41, 1, (1951). (11) — seismic evidence for crustal structure and tectonic activity. the crust of the earth (a symposium). « geol. soc. amer., special p a p e r » , 62, (ì955). ( 1 5 ) ph3hhlìehcko i o . b . , od u3ynemiu ceucmuueckoeo peyicuma. « h a b e c r i i u a h cccp, cep. reotj)h3 ». 9, (1958). articolo martinez.pdf key words paleoseismicity – slow active faults – fan morphology – alhama de murcia fault – betic cordillera 1. introduction in this work, we present a study of the el saltador alluvial fan, which is situated in the southeastern betic cordillera (fig. 1a-c), and is associated with an active mountain front created by the lorca-totana section of the alhama de murcia fault (fig. 2a,b). the distal area of this fan is affected by a strand of the fault that shows quaternary activity. the southeastern betic cordillera is located in the southeast of the iberian peninsula close to the area where the african and the eurasian plates converge. the convergence direction is nnw-sse with a relative velocity of 4 to 5 mm/yr (argus et al., 1989). since this low velocity is distributed in a broad area (meghraoui et al., 1996), the velocity of the active faults in this area is lower than 0.2 mm/yr (martínez-díaz, 1998). the instrumental seismicity is scattered over the region (sanz de galdeano et al., 1995) and has low magnitude (fig. 1a-c). however, several historical earthquakes with magnitude greater than 6.5 occurred in the central and southeastern betic cordillera (buforn et al., 1995). 775 annals of geophysics, vol. 46, n. 5, october 2003 effects of repeated paleoearthquakes on the alhama de murcia fault (betic cordillera, spain) on the quaternary evolution of an alluvial fan system josé j. martínez-díaz (1), eulalia masana (2), josé l. hernández-enrile (1) and pere santanach (2) (1) departamento de geodinámica, universidad complutense de madrid, spain (2) departamento de geologia dinàmica i geofísica, universitat de barcelona, spain abstract in this work we present a study of an alluvial fan system, which is affected by the quaternary activity of the leftlateral, reverse alhama de murcia fault (betic cordillera). paleoseismic studies in this area yield data that can be compared and correlated with the morphologic and tectono-sedimentary evolution of the alluvial fan. the spatial arrangement of the sedimentary alluvial fan units near the fault zone, shown in trenches, is controlled by the recurrent reverse, left-lateral coseismic events. we analysed the morphology of the drainage network using a 1:5000 scale orthoimage to identify and measure horizontal deflections along the fault. the channel pattern analysis allowed us to estimate the average horizontal slip rate of the samf for the last 130 ka. this value is 0.21 mm/a, which is slightly higher than the range of values obtained by trenching analysis for the last 30 ka, (0.06 to 0.15 mm/yr). the interpretation of the stratigraphic sequence exposed along the trench walls constrained the occurrence of at least two surface faulting earthquakes during the last 30000 years. the most recent event happened after the el saltador creek dissected the alluvial fan. the penultimate event occurred while the alluvial fan was still active. mailing address: dr. josé j. martínez-díaz, departamento de geodinámica, universidad complutense de madrid, ciudad universitaria, 28040 madrid, spain; e-mail: jmdiaz@geo.ucm.es the mountain fronts in the study area have experienced several phases of alluvial fan sedimentation and many of them are controlled by quaternary active faults. these faults appear to have controlled the history and modality of alluvial fan generation (harvey, 1990; silva et al., 1992a). until now, the relationship between faulting and quaternary depositional phases of the alluvial fans has been analysed only in terms of long-term tectonic influence (silva et al., 1992c; silva, 1994; martínez-díaz, 1998). no studies dealing with the effects of recurrent paleoseismic activity on alluvial fan deposits and morphology exist for this area. recent paleoseismic studies in this area using trenching data (hernández-enrile et al., 2000; martínez-díaz et al., 2001) yield data that can be compared and correlated with the data obtained from our study of the morphologic and tectono-sedimentary evolution of the alluvial fan. to achieve this correlation we first carried out an analysis of the tectono-sedimentary features of the fan using field data from natural outcrops and from two trenches dug on the alluvial fan surface across the fault trace. then we show a morphotectonic analysis using a digital elevation model built at the 1:5000 scale. estimates of horizontal movement across strike-slip and oblique-slip faults are typically difficult to obtain from trenching data. we analysed the morphology of the drainage using a 1:5000 scale orthoimage to identify and measure horizontal deflections along 776 josé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach fig. 1a-c. a) map showing the location of the study area (square) and the instrumental seismic activity (period 1930-2001) related with the convergence of the eurasian and african plates. the velocity vectors of plate convergence are taken from argus et al. (1989). the increase in velocity from west to east is produced by the increasing distance to the eulerian pole of the african plate. b) seismic activity with intensity > iv occurred in the eastern betic cordillera from 500 a.d. to 2000, projected on a map of the quaternary faults. seismic data from the instituto geográfico nacional. c) regional map showing the regional frame of the study area with the situation of the internal zone (metarmorphic units of the alboran domain) of the betic cordillera, and the mesozoic and tertiary tectono-sedimentary units of the external zone of the cordillera. c a b repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system fig. 2a,b. a) schematic tectonic map of the lorca-totana section of the alhama de murcia fault. this fault is a left-lateral, reverse fault composed of two ne-sw strands: the northern alhama de murcia fault (namf) nw-dipping, high angle reverse fault, and the southern alhama de murcia fault (samf) se-dipping, high angle left-lateral, oblique-slip fault. b) map of the quaternary sediments in the studied area. the largest middle-late pleistocene alluvial fans are related to the el saltador and colmenar creeks, and are cut by the samf and the namf. the small arrows represent the main drainage directions on top of the alluvial fan surface. s1-3 show the sampling sites were 13 samples from the upper layers of the alluvial fans were collected and dated (tables ii and iii). tc fluvial terrace of colmenar. 777 a b the fault, to estimate horizontal slip rate, and to understand the evolution of the fan after the fluvial incision which caused the alluvial fan surface to become inactive. the mean recurrence time of large earthquakes with surface expression on slow intraplate active faults situated near plate-tectonic boundaries may range between 104 to 105 years (scholz, 1990). the preservation of the geologic and geomorphic evidence of past earthquakes depends on relative rates of the surface processes versus tectonic deformation (wallace, 1986). for slow fault zones, the rates of geomorphic processes may exceed the tectonic deformation rates and the paleoseis-mic evidence, if preserved, is likely to be only stratigraphic (mccalpin, 1996, p. 17). in many cases, this makes the identification of fault traces difficult. as a general rule larger faults are associated with larger longterm cumulated deformation of the surface, with the pattern of surface coseismic deformation being in relationships with the fault geometry and kinematics. as a consequence, the analysis of the geomorphic elements and/or the sin-tectonic geologic strata can be used to constrain the geometrical parameters and slip type of the tectonic source. the main subject of this paper is to understand how the tectonic activity influenced the development of geomorphic features and how to use it to correctly interpreting the punctual information coming from trenches, and to do a preliminary estimate of the horizontal displacement of an left-lateral, oblique-slip fault with paleoseismic activity. 2. geological setting 2.1. structure of the lorca-totana area the southeastern betic cordillera is the area with the highest seismic activity in the iberian peninsula (fig. 1a-c). there are several quaternary faults > 50 km long making this area of particular interest for paleoseismic and seismic hazards studies. the longer faults are ne-sw oblique-slip and strike-slip faults. they are active under a nnw-sse shortening direction (montenat et al., 1987). smaller nw-se normal faults are also active during the quaternary. both sets of faults started its activity in the middle miocene and they controlled the geometry and evolution of the basins and ranges during the pliocene and the quaternary (montenat and ott d’estevou, 1996). the el saltador alluvial fan is situated along the southeast front of the la tercia range (fig. 2a,b). this mountain front is controlled by the alhama de murcia fault (bousquet and montenat, 1974), a ne-sw trending (n 45° to 65° e) leftlateral, oblique-slip quaternary fault, up to 80 km long. the nnw-sse compressive tectonic regime has caused the fault to move in a left-lateral, reverse fashion which uplifted the la tercia range and triggered the alluvial sedimentation along the front of the range (armijo, 1977; silva, 1994; martínez-díaz, 1998). recent paleoseismic studies performed along the alhama de murcia fault (fig. 2a,b) demonstrate the occurrence of at least two oblique-slip paleoearthquakes with magnitude > 6.5 and with surface rupture during the last 30.000 years (hernández-enrile et al., 2000; martínez-díaz et al., 2001). the alhama de murcia fault has been divided into four tectonic segments on the basis of differences in orientation, secondary fracture patterns and differences in quaternary slip (silva et al., 1992b; martínez-díaz and hernández-enrile, 1999). the el saltador alluvial fan is located along the 15 km long segment situated between the towns of lorca and totana (fig. 2a). in this area, the alhama de murcia fault consist of two principal nesw fault strands (fig. 2b). the north-western strand (namf) is a left-lateral, oblique-slip fault that dips steeply to the northwest and forms the se boundary of the la tercia range. the south-eastern strand (samf) is a left-lateral, oblique-slip fault that dips steeply to the southeast, separating in some places miocene conglomerate, marl and gypsum to the southeast from quaternary alluvial fan deposits to the northwest. the mountain front was created by the activity of the namf, but the samf has absorbed most of the shortening in this area since the messinian, and affects the upper layers of the larger pleistocene alluvial fans (martínez-díaz, 2002). 778 josé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach 2.2. quaternary sedimentation in the lorca-totana area since the pleistocene, the climate of the southeastern betic cordillera has been semiarid (amor and florschultz, 1964). under these conditions the sedimentation along the tectonically driven mountain front of the la tercia range is characterized by the superposition of alluvial fans (bousquet and philip, 1976; silva et al., 1992 a,b; baena et al., 1993). the development of alluvial fans all over the southeastern betics is characterized by a change from dominantly aggradational to dominantly dissectional dynamics. according to differences of calcrete crust development the aggradation phases appear to predate the würm phase and the dissection phases appear to be würm and holocene (harvey, 1984, 1990). early phases generated the fan surfaces with development of calcrete crusts, and later phases were dominated by dissection resulting in incision of alluvial channels (silva, 1994). in the lorca-totana area the quaternary deposits extend along the southern border of the la tercia range. they unconformably overlie the metamorphic rocks of the paleozoic basement and late miocene marine deposits. we recognize four sedimentary units repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system table i. quaternary alluvial units defined in the study area. units (label in figs. 2b and 3) sediments sedimentary environment age age criteria (o) no cartographic expression white gravels with oolitic limestone clasts proceeding from the external zone of the chain (30 km far to the north) and grey and brown silt beds. marginal system deposited during the transition from marine sedimentation to located alluvial sedimentation along the newly formed tectonic mountain fronts. late plioceneearly pleistocene regional sedimentary evolution (silva, 1994) b silts, sands, gravels and conglomerate with a number of calcrete levels. maximum observed thickness is ca. 20 m alluvial fans along the tectonic mountain fronts middle pleistocene regional sedimentary evolution (silva, 1994) c, d silts, sands, gravels and conglomerate with a number of well developed calcrete levels. alluvial fans along the tectonic mountain fronts. middle to late pleistocene radiocarbon dating (see table ii) e, f smaller sized shale, quartzite and dolomite clasts coming from the alpujárride and maláguide metamorphic units predominate in these fans. no calcrete levels. alluvial fans and fluvial terraces late pleistoceneholocene regional sedimentary evolution (silva, 1994), radicarbon and thermoluminiscense dating (see tables ii and iii) 779 defined by their lithology, relative position and deformation style (see table i and fig. 3). the outcrop extent of the quaternary alluvial units is controlled by the tectonic activity. the combined activity of the two fault strands has created a ne-sw trending topographic lowland that is filled with the quaternary sediments (fig. 2b). the outcrops of the pliocenelower pleistocene unit (o in fig. 3) and the lower to middle pleistocene unit (b in figs. 2b and 3) are restricted to the foot-wall. the upper layers of the middle to upper pleistocene unit (c, d in figs. 2b and 3) are affected by the samf but they spill over the fault zone. we have estimated the ages of these units by correlation with previously dated (using climatic criteria) analogue deposits of this region and by absolute dating. we dated silt samples taken from the uppermost two meters of alluvial fans and terraces. 6 samples using thermoluminiscense and 9 samples using radiocarbon methods (tables ii and iii). these samples were collected at the points labelled s1-3 in fig. 2b and their ages constraint the time of the deformations described in this work that affect the alluvial fans to the late pleistocene and holocene. 3. el saltador alluvial fan 3.1. sedimentation the el saltador fan is a middle-late pleistocene alluvial fan (martínez-díaz, 1998) that occupies an area of ca. 8 km2 (figs. 2b and 4a,b). this fan is entrenched by the el saltador creek. along the river banks of this creek is exposed a sedimentary sequence that consists of a facies association of debris flow sediments capped by mudflow horizons. channels filled with stream sediments are incised into the debris flows and mudflows. channel deposits are progressively more abundant in the upper part of the sequence. this type of sedimentation is observed in exposures of other fans belonging to the same depositional phase all over the area (silva et al., 1992a). the activity of the southern strand of the alhama de murcia fault (samf) affects the current fan morphology being expressed by a northfacing scarp, and has also partially controlled its sedimentary evolution. two trenches were dug across the fault scarp, and we analysed the tec780 josé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach fig. 3. stratigraphic column of the quaternary units differentiated in this work. it is represented the extent of relative outcrop of each unit. the units a and b are restricted to the downthrown block situated between the two main fault branches of the alhama de murcia fault. the southern strand (samf) starts its activity in the late miocene and affects holocene deposits of units e and f. the northern strand (namf) was active since early miocene and decreased its activity during the quaternary. all the units are labeled with the same letters in fig. 2b, with the exception of unit o that is absent on the map. it was only observed on the vertical banks of the creeks. the position of the samples dated with absolute methods is also represented (tables ii and iii). n a m f s a m f pliocene late miocene q u a te rn a ry p le is to c e n e e a rl y m id d le holocene l a te 1.64 0.7 0.13 0.01 age (ma bp) a o b c,d e,f murcia 1 and 2 murcia 3 and 4 colmenar 1 lor1-rc1 trm21 trm27 trm43 trm56 trm49 trm41 tr4-rc1,2 and 3 poorly developed calcrete levels well developed calcrete levels gravels with metamorphic pebbles gravels with miocene pebbles marls and gypsum silt silts, sands and conglomerates fault activity table iii. absolute corrected ages (in years before the present) obtained with thermoluminiscense method for 6 samples collected at site s3 (see fig. 2b). the values of beta, gamma and cosmic dose-rate assessments are shown. sample loc. unit/ betanrd beta + external beta gamma cosmic corrected tl fig. trench (gy) dose-rate dose-rate dose-rate dose-rate age 2b (gy/ka) (gy/ka) (gy/ka) (gy/ka) (ka) bp trm 21 s3 i/ tr3 81.4 ± 6.6 2.659 ± 0.204 1.685 ± 0.191 0.829 ± 0.071 0.145 ± 0.011 23.8 ± 3.1 trm 27 s3 d/ tr3 40.1 ± 3.2 1.885 ± 0.128 0.982 ± 0.112 0.709 ± 0.061 0.194 ± 0.013 17.2 ± 2.1 trm 43 s3 d/ tr3 42.2 ± 3.9 2.069 ± 0.145 1.134 ± 0.129 0.747 ± 0.066 0.188 ± 0.015 17.2 ± 2.3 trm 56 s3 i/ tr3 102.5 ± 5.4 2.841 ± 0.228 1.952 ± 0.217 0.766 ± 0.066 0.123 ± 0.012 25.4 ± 3.1 trm 49 s3 e/ tr3 95.1 ± 7.3 2.951 ± 0.227 1.938 ± 0.215 0.855 ± 0.072 0.159 ± 0.012 22.2 ± 3.2 trm 41 s3 f/ tr3 42.3 ± 3.2 2.326 ± 0.173 1.388 ± 0.158 0.795 ± 0.071 0.143 ± 0.013 14.5 ± 1.7 table ii. absolute ages obtained with radiocarbon method for 9 samples collected at sites s1, s2 and s3 (see fig. 2b). silty samples are ams dates and charcoal and shell are conventional method dates. measured c13/c12 were calculated relative to the pdb-1 international standard and the radiocarbon years before present ages were normalized to – 25 ‰. dates were done at beta analytic. sample loc. in material localisation c13/c12 conventional calibrated fig. 2b (‰) radicarbon age 2 age murcia 1 s2 silt colmenar terrace – 24.8 6350 ± 40 bp 5335-5240 b.c. 1.5 m beneath the surface murcia 2 s2 silt colmenar terrace – 25.3 8220 ± 50 bp 7410-7040 b.c. 2.7 m beneath the surface colmenar 1 s2 silt colmenar terrace – 21.0 2850 ± 70 bp 1215-830 b.c. 0.8 m beneath the surface lorc 1 – rc-1 s2 silt colmenar terrace – 22.6 3390 ± 40 bp 1760-1600 b.c. 1.3 m beneath the surface tr4-rc2 s3 shell unit a, tr-4 – 7.3 1350 ± 40 bp 640-720 a.d. pounding event tr4-rc3 s3 charcoal unit a, in tr-4 – 22.8 340 ± 40 bp 1450-1650 a.d. pounding event tr4-rc8 s3 silt unit b, tr-4 – 24.2 480 ± 40 bp 1410-1460 a.d. pounding event murcia 3 s1 silt silty layer 20 cm depth – 25.5 820 ± 40 1170-1280 a.d. south of samf murcia 4 s1 silt silty layer 45 cm depth – 24.0 1690 ± 50 245-450 a.d. south of samf repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system 781 josé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach fig. 4a,b. a) shaded relief map processed from cartographic data at the 1:5000 scale showing the el saltador alluvial fan together with the fault traces of the northern and southern alhama de murcia fault (namf and samf). to the right the longitudinal (e-f) and transversal (a-b and c-d) topographic profiles of the fan show different degree of dissection of the fan surface in the foot-wall and hanging-wall of the samf. h represents a relict of an old fan surface uplifted in the hanging-wall of the samf (upper dashed line in profile c-d). b) oblique view of the fan surface from the southeast showing the morphology of the fan surface in relationship with the geometry of the two fault strands, and the capture (arrow) experienced by the el saltador creek in the fault zone due to headward erosion triggered in the hanging wall by the uplift. 782 a b tono-sedimentary record exposed on the walls to study the sedimentary dynamics of the fan in the vicinity of the samf (fig. 5a,b). trench tr4 was dug in a stream channel while trench tr3 is located 20 m toward the east on a higher part of the fan surface (fig. 6a). both trenches expose the uppermost three meters of the fan, deposited prior to the incision of the main channel of el saltador creek. this incision caused sedimentation to shift farther downstream, initiating the formation of several small holocene fans situated 2 km to the southeast and left the el saltador fan surface abandoned (fig. 2b). the trenches also allowed us to study the dynamics of local sedimentation on the fan surface after the end of the fan activity. on the walls of the trenches we observed that the sedimentation of the youngest fan beds was active at the same time with the activity of the samf which raised the southern block during paleoseismic events and dammed the river courses, triggering the deposition of finegrained units. 3.2. trench tr3 we divided the alluvial deposits exposed in trench tr3 (fig. 5a) into 9 units using two criteria: i) abrupt lithological changes revealing a change in the sedimentary regime, and/or ii) the repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system fig. 5a,b. a) aerial photograph showing the location of the two trenches dug across the samf. the logs of the trenches are interpreted. we differentiated 9 sedimentary units. the units d, e, f, g, h, i and j are clearly affected by the movements of two strands of the samf (fa and fb). a is a pounded sedimentary unit that probably was deposited after the coseismic surface bending above the fault fa. at least two events (o and p) are recognized. absolute ages obtained from thermoluminiscence and radiocarbon methods are labeled in the logs of the trenches (see tables ii and iii). b) represents a tectono-sedimentary interpretation of the trenches, with 5 tectono-sedimentary units created by the paleoseismic activity of the samf. 783 josé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach fig. 6a,b. geomorphic map of the alluvial network on the el saltador fan mapped at the 1:5000 scale. a) complete drainage network along with main morphological features. darkest grey colour shows the relict topographic high in the hanging wall of the samf (see fig. 4a,b). intermediate grey colour shows the old el saltador creek channel before the capture triggered by the headward erosion in the hanging wall. topographic sections a and b show the asymmetric entrenching of the creek near the fault driven by the capture. light grey colour represents modern channels. b) the higher order channels are represented together with the fault trace. we measured the systematic left-lateral deflections of the channels crossing the samf. the histogram shows the component parallel to the fault of the deflections. only the grey bars were used to calculate the mean value of deflection. 784 a b existence of angular unconformities indicating the occurrence of surface coseismic deformation. the uppermost three meters of the fan consist of debris-flow deposits alternating with sheet flow deposits. the debris flows consist of matrix-supported gravels with a coarse, grey, sandy matrix. angular pebbles are typically 2 cm in size with some up to 40 cm, and are composed of phyllite, diabase, dolomite and quartzite. gravels are typically capped by mud flow deposits up to several cm thick. the sheet flow deposits consist of well sorted, loose, clast-supported gravels with coarse sandy matrix. criteria to determine the debris-flow nature of the deposits include the presence of boulders, matrix-supported structure, poorly sorted textures, absence of stratification, and lack of rounding and sorting of clasts (sharp and nobles, 1953; bull, 1977). on top of the debris flow deposits and in some sheet flow deposits there are mud flow horizons that present incipient calcic soils, generally orange or yellow in colour, with signs of root porosity, suggesting periods of non-deposition. the uppermost unit exposed in the trench (unit a) is a package of upward-thinning dark-brown silts. it contains charcoal and gastropod shells and local matrix-supported gravel. we interpret this deposit as being generated by temporary damming produced by the bending of the surface linked to the paleoseismic activity of the fault (fig. 5b). the present, active soil is developed on top of this unit. 3.3. trench tr4 trench tr4, a 2 m-deep excavation within a channel exposed two types of deposits (fig. 5a). the lower part of the trench exposed a fluvial unit of loose, clast-supported gravels with angular pebbles, commonly imbricated, and containing internal lamination. a brownorange soil has formed on the top this unit. this soil is covered by dark brown, thinning upward, clast-supported gravels that grade into matrix-supported gravels and finally to silts. this uppermost deposit is similar to the upper unit a of trench tr3 and we interpret it as representing the local and temporary pounding of the river feeding the fan, due to fault motion. in summary, the el saltador fan was built by the accumulation of three types of deposits during the latest aggradational phases: stratified fluvial sand and gravel, debris-flow deposits with gravel and silts, and mud flow deposits. upon a number of mud flows we observe paleosoils and incipient calcretes with different degrees of development. in the channel (trench tr4) we observe a predominance of fluvial deposits, and neither debris flow deposits nor paleosols are present in contrast to what we observed in trench tr3 at the same topographic level. however, the pounded unit in both trenches indicates that after alluvial fan deposition ceased, the sedimentary record is homogeneous in both of the trenches. 3.4. influence of oblique slip movement on the morphology of the el saltador fan the surface of the el saltador fan has been modified by agricultural activity since the 1950’s. however, the analysis of aerial photographs taken in 1956 (1:33.000) and the orthoimage at 1:5000 scale allows us to observe its morphology in detail. furthermore, we have constructed a digital elevation model at a scale of 1:5000 from topographic data at the same scale that enables us to observe, with a 1 m resolution, the relief on the fan surface from different point of view (fig. 4a). the fan is currently undergoing active headward erosion and el saltador creek is entrenched up to 20 m. figure 6a,b shows the paleo-drainage pattern active during the last aggradational phases as interpreted from the air photos. the channels have large and consistent width/height ratios. they resemble the alluvial fan channels acting alternatively as debris and fluvial channels described by beaty (1990) in active fan areas with similar climate and lithology. the el saltador fan is cut by the samf, which trends n.55°-60°.e (fig. 2b). it is a south-dipping left-lateral, reverse fault south side up. the digital elevation model (fig. 4a) shows the surface expression of the fault and repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system 785 its control of the drainage pattern and river behaviour. channels are highly entrenched on the southern block due to vertical incision on the hanging wall. a topographical longitudinal profile along the fan surface (fig. 4a profile e-f) shows a change in slope close to the fantoe that is interpreted as due to repeated fault movements. transverse topographic profiles, parallel to the fault and carried out on both sides of the fault, show different degrees of erosion in the footwall (profile a-b) and in the hanging wall (profile c-d). in the hanging wall, fluvial gravels are preserved on the top of a small topographic culmination (h in fig. 4a), 10 m above the current fan surface. we interpret this topographic high as a relict of an old fan surface raised by the fault activity. in addition, headward erosion in the hanging wall induced the channel shift of the el saltador creek due to a process of capture when it was flowing 10 m higher than the present creek bed (see arrow in fig. 4b). the drainage network on the fan surface (fig. 6a) shows some channels slightly deflected, possibly by left-lateral movement of the samf. the tectonic origin of these deflections is supported by martínez-díaz (2000) that in a regional study identifies that more than 70% of the alluvial channels crossing the samf show a left-lateral deflection. the number of data with high quality is inadequate to do a statistical analysis of the relation between the frequency of stream deflection, and the fault displacement for the lorca-totana section of the fault. however, we consider that they are good enough in the study area to analyse the fault movement affecting the el saltador fan. the mean value of the deflections measured on this alluvial fan is 26.7 m (fig. 6b). the entrenchment of some of the streams along the fault zone causes deflections that is much more large (number 8 of fig. 6b) and it were not considered in the calculation. in a few cases the deflection is right-lateral. these right-lateral deflections were not diverted by the fault activity, but rather followed the gouge (weaker) zone of the fault and/or they were forced by the original low angle between the channel and the fault, similar to those described along other active strike-slip faults (huang, 1993). the leftjosé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach fig. 7a,b. a) drainage network of the guadalentín river in the vicinity of the lorca-totana section of the alhama de murcia fault zone. the white arrow shows the point along the guadalentín river where a change in the fluvial pattern (from braided to meandering) is observed. this change coincides with a change of slope in the longitudinal profile of the river (b), and take place crossing ahama de murcia fault (amf). this profile was drawn using 1:10.000 topographic maps. nbf northbetic fault. black arrows show the deflection of a pliocene fluvial network due to the tectonic uplift of la tercia range. 786 a b lateral deflection is clearest on the better-preserved channels designated 1-7 on fig. 6b and it is also visible along the old el saltador channel (shaded zone in fig. 6a). the capture of the old el saltador creek by the present el saltador creek (arrow in fig. 4b) also produced an apparent deflection in the present creek larger than the deflection of the older creek. the asymmetry of the transverse profile of the el saltador creek valley close to the fault (profile b of fig. 6a) agree with an asymmetrical headward erosion of the valley triggered by the capture. only the deflection of the old saltador creek is considered as a tectonic feature. the influence of the holocene activity of the alhama de murcia fault in the quaternary alluvial network is also observed at a more regional scale. the uplift of the la tercia range driven by the reverse movement of the namf, produced the deflection of the old plioceneearly pleistocene fluvial network in the lorca basin (fig. 7a,b). in addition, an abrupt change in slope and a morphologic anomaly in the longitudinal profile of the guadalentín river occurs where the river crosses the alhama de murcia fault in the vicinity of lorca (fig. 7b). this change coincides with a shift of the river pattern from a braided pattern in the hanging wall of the alhama de murcia fault to a meandering pattern in the footwall. this very localised change of fluvial pattern indicates very recent relative vertical movements inducing longitudinal changes of the stream gradient (schumm and khan, 1972; bullard and lettis, 1993). 3.5. influence of the fault activity on the el saltador fan sedimentation drag associated with the reverse movement of the namf results in a southward tilting of the proximal deposits of the el saltador fan. since the youngest fan deposits are not in contact with this fault, it is difficult to deduce the age of its most recent earthquake activity. however, holocene alluvial terraces close to the fan apex are undeformed crossing the fault trace. the strong incision of the fans during the late pleistocene and holocene together with the deposition of the holocene fans far from the range front can be attributed to a reduction of the range uplift rate, possibly related to a decrease of the activity of the namf from the late pleistocene (as suggested in the geomorphologic study of silva (1994)), that was coeval with the paleoseismic activity of the southern strand (martínez-díaz et al., 2001). during the quaternary, this fault absorbed most of the nnw-sse shortening produced by the present stress field. this redistribution of slip produced a decrease of the slip rate of the namf and a decrease of the uplift rate of la tercia range that facilitated the entrenchment of the el saltador creek in the sediments of the alluvial fan. the upward reduction of the depositional energy observed in the stratigraphy of the el saltador fan showed by the fining upward sequence agrees with this process. the morphology and the sedimentary distribution of the youngest deposits of the medium and distal areas of the el saltador fan are conditioned by the recurrent paleoseismic activity of the samf. this control is observed on the wall of the trenches dug across the fault zone (fig. 5a). silva et al., (1992a) suggested that the raised southeastern block acted as a shutter ridge, which has been a barrier for sedimentation. however, the stratigraphy observed in the trenches and along the banks of the el saltador creek indicates that during deposition of the last 10 m of the alluvial fan deposits in the downthrown block, the deposits spilled over the southern fault. the structure observed in trench tr3 suggests at least two paleoseismic events due to the movement of the samf (faults fa and fb in fig. 5a). the fault fb cut the layers of units j and i. the fault fa bend the units e and f and produced pebble rotations in the gravels of unit f. no clear evidence of deformation is observed in units c and d, however, the natural slope of the alluvial fan surface is modified above the fault fa. this suggests a coseismic bending of the surface. the penultimate event occurred after the deposition of unit i and before unit f with a vertical displacement (adding the amplitude of the fold to the fault slip) of more than 3 m. due to this event, units g, h and part of i were eroded from the hanging wall. the last event occurred after the deposition of unit c and before the deposition of unit a, and produced about 1 m of vertical displacement exrepeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system 787 pressed as the bending of the topographic surface and the local obstruction of the drainage. the damming caused by this obstruction resulted in the deposition of unit a. trench tr3 shows an onlap on the buried fault scarp due to the infilling of the depression located at its foot-wall (fig. 5a). younger layers cover the fault zone. during the erosion of the hanging wall the fan debris and fluvial channels incised into the raised surface. if we extrapolate these observations of the trenches to the difference in thickness of the fan deposits, seen in a section from the hanging-wall to the foot-wall along the el saltador creek exposures (to the southeast of the fault only 2 m of alluvial deposits are preserved, while to the northwest the fan thickness exceeds 10 m), we infer that the tectonically driven process of uplift, erosion of the hanging wall, and sedimentation on the fault zone has been recurrent during the history of the fan. bearing in mind that the average recurrence period of paleoseismic events of this fault strand is over 10.000 years (masana et al., 2003), the interseismic periods are long enough to develop complete erosion-sedimentation cycles. below the unit a trenches tr3 and tr4 show different facies associations at the same topographic level. trench tr4, dug along a channel, shows fluvial deposits and trench tr3 shows fig. 8a-d. model showing sedimentary development of an active debris-fluvial channel in an oblique-slip fault zone. the channel dimensions and the morphology of debris sedimentation are taken from the description of beaty (1963, 1990) for the upper milner creek fan. the activity of the fault controls the drainage pattern and behaviour (erosional/depositional in areas undergoing tectonic uplift / subsidence). the ideal sections a, b and c are drawn parallel to the fault (in the hanging-wall) and show that the left-lateral and reverse movement on the fault zone causes the channel to produce vertical and lateral incision. section d is transverse to the fault and show the sedimentation of pounded deposits of unit a due to the obstruction of the drainage. 1.2 m tr1 tr1 3 m fresh debris pre-flood active channel paleosol horizont paleosoil level filled paleochannel new cut excavated by high water which folow debris deposition older fan deposits mud flow and paleosoil beds debris flow sediments tectonic movement of the hanging wall sense of relative migration of channel sedimentation fluvial sediments: sheet flow and channel deposits paleochannell filled with alluvial gravels and a silty paleosol horizon paleochannell filled with alluvial gravels and a silty paleosoil level paleosoil level e e ew w w tr3tr4 old alluvial sediments younger alluvial sediments transverse profile local damming sediment ns hanging wall hanging wall hanging wall hanging wall foot wall josé j. martínez-díaz, eulalia masana, josé l. hernández-enrile and pere santanach 788 c d a b debris flow, mud flow and fluvial deposits typically capped by paleosols. figure 8a-d shows the geometry of the sedimentation observed in a typical debris channel. the active channel may alternately play as a debris channel and as a fluvial channel. we consider that the channel cut the block that was uplifted by the fault. the interaction between the recurrent fault activity, which produced uplift in the fault zone, and the sedimentation of the channel may produce substantial differences in the sedimentary record at two closely spaced points. the vertical movement in the fault zone causes the channel to produce vertical incision. the left-lateral offset may induce the erosion to be concentrated on the western bank of the channel, causing sedimentation to migrate laterally. the facies observed in trench tr4 represent the record of the last fluvial sedimentation in the channel before the entrenchment of the el saltador creek. the facies association in trench tr3 represents the sedimentation of fluvial channel deposits alternating with the debris-flow and mud flow deposits. we could expect these alternating facies to occur in the areas of debris channel sedimentation abandoned by the westward migration of the channel, as we see here, due to the left-lateral movement of the fault (fig. 8c). when the fan was entrenched by the deep incision of the current el saltador creek, the channel activity on the fan surface ceased. since then, the channels on the fan surface were characterized by low water discharge as they were no longer connected with the range. the last coseismic reactivation of the fault induced obstruction of these small channels, and consequently, local damming and sedimentation (fig. 8d). 3.6. long and short term slip rates the paleoseismic studies carried out in the el saltador fan have demonstrated the occurrence of at least two surface faulting events during the last 35.000 years (martínez-díaz et al., 2001). quaternary striations on the fault plane, observed in the trenches and in natural outcrops, present a pitch ranging from 32° to 60°. these pitch angles confirm the reverse movement with left-lateral component. however, no markers have been identified in the trenches to quantify the horizontal slip rate of the fault. we use the horizontal deflection of the channels on the alluvial fan of 27 m, as deduced above, to estimate the horizontal slip rate of the samf. the el saltador fan formed at the end of the first quaternary depositional phase of silva (1994), during the middle pleistocene on the basis of climatic criteria. if the channels formed close to the middle-upper pleistocene boundary (130.000 years bp), the horizontal slip rate is ca. 0.21 mm/yr. the short-term horizontal slip rate during the last 30.000 years can be calculated using the pitch of the striations observed on the fault plane and the vertical slip rate (0.10-0.20 mm/yr) deduced from the trenches excavated on the el saltador fan (masana et al., 2003). it ranges from 0.15 mm/yr for a pitch of 32° to 0.06 mm/yr for a pitch of 60°. the comparison between longand short-term horizontal slip rates of the samf shows values of similar magnitude, even if a little smaller for that of the younger interval (0.06 to 0.15 mm/yr) with respect to the values averaged over the whole upper pleistocene-holocene (0.21 mm/yr). this rate is similar to the long-term slip rate estimations of (martínez-díaz, 1998) using fault displacements measured in upper miocene and pliocene rocks in the lorca-totana segment of the alhama de murcia fault. this is the first preliminary estimate of quaternary horizontal slip rate in the alhama de murcia fault. since the morphology of the study area was highly modified by the agricultural activity, the slip rates are derived from poorly constrained data, and more absolute ages are also necessary to diminish the uncertainty of the results. the stream deflections utilized in this study were measured in topographic maps of adequate scale for a good accuracy. however, more alluvial fans must be analysed in detail to obtain more data and to apply a statistical analysis to the complete alhama de murcia fault. 4. conclusions we studied the stratigraphy, sedimentology and morphology of a sequence of quaternary repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system 789 alluvial fans in relationship with the paleoseismic activity of the left-lateral, reverse alhama de murcia fault, with the aim of interpreting the tectonic control on the fan dynamics and the fault evolution. our results regarding fan dynamics can be summarized as follows: i) during the last 30 ka, the el saltador fan was built by a succession of stratified sands and gravels, debris-flow with gravels and silts, and thin horizons of mud flow deposits. the geometry of the sedimentation was controlled by the dynamics of debris channels and sheet flows. ii) the spatial arrangement of the sedimentary units near the fault zone, shown in trenches tr3 and tr4, is partly controlled by the recurrent reverse, left-lateral coseismic events that affected the dynamics of sedimentation in the active fan channels. the recurrent uplift of the hanging wall produced vertical incision of the stream channels and headward erosion that caused most of the fan deposits to be stripped away from the hanging wall. the recurrent eastward movement of the hanging wall produced a relative westward migration of the fluvial sedimentation in the channel. iii) the entrenchment of el saltador creek, triggered a gradual decrease of the geomorphic activity on the fan surface. the last coseismic reactivation of the samf induced obstruction of the small fluvial channels on the fan surface, and consequently, local pounding and sedimentation. our results regarding fault evolution can be summarized as follows: i) the channel pattern analysis allowed us to estimate the average horizontal slip rate of the samf for the last 130 ka. this value is 0.21 mm/yr, which is slightly higher than the range of values obtained by trenching analysis for the last 30 ka (0.06 to 0.15 mm/yr). this preliminary horizontal quaternary slip rate is the first estimate in the alhama de murcia fault. ii) the interpretation of the stratigraphic sequence exposed along the trench walls constrained the occurrence of at least two surface faulting earthquakes during the last 30.000 years. the penultimate event occurred while the alluvial fan was still active. the most recent event happened after the el saltador creek dissected the alluvial fan. aknowledgements paleoseismic data presented in this study were obtained within the projects: «active tectonic analysis of the murcia region and their application to seismic hazard assessment» amb97-0523, faust 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(1986): overview and recommendations, in studies in geophysics. active tectonics (national academy press, washington d.c.), pp. 266. repeated paleoearthquakes on the alhama de murcia fault and quaternary evolution of an alluvial fan system 791 aòlkjdòklaòakldjf ag_57.04.14_suresh_finalonline:layout 6 annals of geophysics, 57, 4, 2014, s0431; doi:10.4401/ag-6540 s0431 crustal velocity structure of the deccan volcanic province, indian peninsula, from observed surface wave dispersion gaddale suresh1,*, satbir s. teotia2, sankar n. bhattacharya1 1 india meteorological department, seismology division, new delhi, india 2 kurukshetra university, department of geophysics, kurukshetra, india abstract through inversion of fundamental mode group velocities of love and rayleigh waves, we study the crustal and subcrustal structure across the central deccan volcanic province (dvp), which is one of the world’s largest terrestrial flood basalts. our analysis is based on broadband seismograms recorded at seismological station bhopal (bhpl) in the central india from earthquakes located near west coast of india, with an average epicentral distance about 768 km. the recording station and epicentral zone are situated respectively on the northern and southern edges of dvp with wave paths across central dvp. the period of group velocity data ranges from 5 to 60 s for rayleigh waves and 5 to 45 s for love waves. using the genetic algorithm, the observed data have been inverted to obtain the crust and subcrustal velocity structure along the wavepaths. using this procedure, a similar velocity structure was also obtained earlier for the northwestern dvp, which is in the west of the present study region. comparison of results show that the crustal thickness decreases westward from central dvp (39.6 km) to northwestern dvp (37.8 km) along with the decrease of thickness of upper crust; while the thickness of lower crust remains nearly same. from east to west s-wave velocity in the upper crust decreases by 2 to 3 per cent, while p-wave velocity in the whole crust and subcrust decreases by 3 to 6 per cent. the pand s-wave velocities are positively correlated with crustal thickness and negatively correlated with earth’s heat flow. it appears that the elevated crustal and subcrustal temperature in the western side is the main factor for low velocities on this side. 1. introduction the deccan volcanic province (dvp) is the most remarkable continental flood basalts provinces of the indian peninsula and formed around 65 my, when this region passed over the reunion hotspot during its northward movement [duncan and pyle 1988]. the dvp has emerged as most interesting subjects of research particularly for its enormous size. the west coast and narmada-son lineament are the probable source regions from which the deccan lavas were poured out through tensional fractures [sen 2001]. the lava flowed great distances towards east and south to form the largest terrestrial flood cover extending nearly half a million square km between latitudes 16-24°n and longitudes 70-77°e (figure 1). the flood basalts are known as deccan traps. there are several studies in the past to evaluate the lithospheric velocity structure of the indian peninsula using body waves and surface waves from regional and teleseismic earthquakes. dube et al. [1973] used body wave records of observatories situated in the peninsular india from 40 earthquakes occurred in the koyna region (figure 1). bhattacharya [1981] inverted the surface wave group velocities along paths in the central india and obtained the model ip11. bhattacharya [1991] found that ip11 also satisfies the surface wave group velocity data across the northwestern part of the indian peninsula. furthermore, reviewing the surface wave studies in the indian peninsula, bhattacharya [1992] noted that the short period (0.6 to 2.5 s) surface wave data across the koyna region (in southern dvp) are satisfied by ip12 (figure 2); ip12 is same as ip11 except the top 1 km of latter is replaced by a deccan trap layer. singh et al. [1999] obtained velocity structure using surface waves recorded at regional distances from 1997 jabalpur earthquake in the central india (figure 2). from the same earthquake, rao et al. [2002] used body waves at regional distances and refined the velocity model in a grid search method through ray tracing. figure 2 summarizes the s-wave velocity structure of central india from these studies and shows that main difference is in the depth of the interface between layers of upper and lower crust. the crustal s-wave velocity model of rao et al. [2002] agrees with that of singh et al. [1999]; however, there are differences in moho depth and p-wave velocities article history received april 10, 2014; accepted june 10, 2014. subject classification: deccan volcanic province, surface waves, crustal structure. between these two models. the moho depth of singh et al. [1999] is nearly same as that of ip11 or ip12 [bhattacharya 1992]. the crustal structure of northwestern dvp was obtained by prajapati et al. [2011] using observed surface wave dispersion of rayleigh and love waves along a few wavepaths across this region. the mean wavepath of this study is shown in figure 1; we shall refer this region crossed by the wavepaths in northwestern dvp and studied by prajapati et al. [2011] as northwestern dvp. the authors also evaluated the crustal structure of the adjoining continental shelf. in the present study, we shall evaluate the velocity structure of crust and subcrust (a layer below the crust) of central dvp which is east of northwestern dvp (figure 1). we shall note the changes in the structure of dvp as we shift westward from central to coastal dvp region. study of these changes will improve our understanding of the geology, tectonics and geodynamics processes of dvp. 2. data and measurement of group velocity to select surface wavepaths across central-dvp, we consider the broadband records of the station bhopal (bhpl), which is located in the northern part of dvp and analyze earthquakes occurred in the koyna region in the west coast of india (figure 1). the station bhpl is located on hard exposed bed-rock and has low ambisuresh et al. 2 figure 1. major tectonic features of the deccan volcanic province (dvp). shaded area indicates the dvp. wavepaths from the epicentres of koyna earthquakes to bhopal (bhpl) have been shown in solid lines (present study) and are through central dvp. the dotted line is the mean wave path considered for evaluating the structure of northwest dvp [prajapati et al. 2011]. figure 2. s-wave velocity structure of the indian peninsula in various studies: singh-etal [singh et al. 1999], ip12 [bhattacharya 1992], rao-etal [rao et al. 2002], northwestern dvp [prajapati et al. 2011]. theoretical group velocity curves have been drawn in figure 3 for these structures. ip12 is modified from ip11 [bhattacharya 1981, 1992], whose top 1 km is replaced by a deccan trap layer. 3 ent ground noise. we consider well recorded surface waves from earthquakes of magnitudes between 4.2 and 5.4 (table 1). a three component velocity broadband seismogram of bhpl for the april 17, 2006, koyna earthquake (mb 4.4) is shown in figure 3 indicating well recorded surface waves which are prominent compared to the body waves. the hypocentral parameters of the earthquakes are determined using seisan analysis software [havskov and ottemoeller 1999] utilizing the phase data of regional seismological observatories. all the events were located using ip12 velocity model [bhattacharya 1992]. table 1 lists the hypocentral parameters of earthquakes considered; the wavepaths from all these events to the recording station at bhopal are through the central portion of dvp (figure 1). bhopal (23.241°n, 77.424°e, 520 m) is a part of national seismological network of india meteorological department (imd), equipped with three component broadband sensor, nanometrics trillium-240 (with flat 3 db velocity response from 240 s to 35 hz) and accelerometer tsa-400s (with flat 3 db acceleration response from dc to 225 hz) connected to the six channel 24 bit digitizer (taurus with trident) and internal time offsets are adjusted by gps receiver automatically. the station is connected to central receiving station in delhi and tsunami warning center at incois, hyderabad through vsat communication facilities for near real time transfer of continuous recorded data and to perform scheduled quality checks [dattatrayam et al. 2009]. the station is also equipped with broadband seismometer sts-2 connected to quanterra digitizer (q680lvg) [bhattacharya and dattatrayam 2000]. the average epicentral distance from koyna earthquakes to bhpl is about 768 km. the group velocities are obtained through frequency-time-analysis (ftan) as described in dziewonskei et al. [1969] and bhattacharya [1981, 1983]. for each event the 3-component 100 sps (samples per second) data of bhpl is (1) decimated to 5 sps, (2) changed to ground motion using the seismograph response and (3) converted to vertical, radial and transverse components. the vertical component is used for rayleigh wave and transverse component for love wave. in ftan, the seismogram is passed through narrow gaussian filter centered at each period. the time of the peak of the envelope to the amplitudes of the filtered seismogram gives the group arrival time (tg) and the group velocity = δ/tg, where δ is the epicentral distance. such filtering makes it easier to separate different modes. the error in hypocenter determination is minimized by locating the earthquakes using regional structure and using phase data from regional observatories [bhattacharya and dattatrayam 2000]. consequently, the error in epicenter is less than 5 km and the error in origin time is within 1 s; considering average epicentral distance of 768 km, these two errors cause an error less than 0.03 km/s in group velocity measurement from a single event. however, these errors are reduced by considering the mean of the group velocities from many events; crustal structure of dvp, indian peninsula event no. date (yyyy/mm/dd) origin time hh:mm:ss longitude (°e) latitude (°n) depth (km) magnitude used for* 1 1998/02/11 01:08:49.1 73.965 17.321 33.0 4.2 (mb) lq, lr 2 2000/09/05 00:32:43.1 73.790 17.332 10.0 5.4 (mb) lq, lr 3 2005/03/14 09:43:49.8 73.873 17.288 26.8 4.7 (mb) lq, lr 4 2006/04/17 16:39:59.4 73.805 17.083 11.8 4.4 (mb) lq, lr 5 2007/11/24 10:57:47.0 73.744 17.141 9.0 4.8 (mw) lq, lr 6 2008/09/16 21:47:13.3 73.734 17.287 15.0 5.0 (mw) lq, lr 7 2009/11/14 13:03:32.8 73.792 17.102 14.8 4.7 (mw) lq, lr 8 2009/12/12 11:51:24.8 73.613 17.112 9.7 5.1 (mw) lq, lr table 1. list of earthquakes used for evaluating group velocities. *lq= love wave, lr=rayleigh wave. figure 3. broadband waveform of bhpl from the april 17, 2006, koyna earthquake (δ = 7.01°, mb = 4.4, table 1, event no. 4). waveform starts at 16:44:55.78. although the amplitudes of body waves are weak but rayleigh and love waves are strong. an estimate for the uncertainty may be seen from the standard deviations of the group velocities. the group velocities are for periods ranging from 5 to 60 s for fundamental mode rayleigh wave and from 5 to 45 s for fundamental mode love waves (table 2, figures 4 and 5). because of this limited distance and moderate magnitude of usable earthquakes, no reliable surface wave dispersion measurements could be possible for longer periods. the standard deviations are generally higher for rayleigh wave than for love wave. perhaps this can be explained by the source mechanisms of the earthquakes, which caused amplitudes of love wave relatively stronger than those of rayleigh wave at the azimuth of the station from the sources. 3. comparison of observed group velocity data in figure 4 the observed mean group velocity data is compared with the theoretical dispersion curves for the previously evaluated models (figure 2) in the indian peninsula discussed in the introduction. the theoretical curves have been drawn following bhattacharya [1987]. these theoretical curves are close to the observed data between 25 and 50 s. for love wave, the dispersion curve for ip12 agrees with the observed data even at lower periods while that for the model of singh et al. [1999] is close to rayleigh wave data in this period range. at lower periods (5-25 s), the observed group velocities across central dvp are significantly higher than those of the dispersion curves of northwestern dvp; this implies a relatively higher wave speed structure in the upper crust for central dvp. at periods above 25 s, the rayleigh and love wave group velocities are only slightly more to that of northwestern dvp, which shows no significant change in lower crust and upper mantle shear velocity structure between them. however, this observation indicates that the crust and upper mantle velocities in central dvp are on average faster than those of northwestern dvp. 4. inversion and lithospheric structure the observed dispersion is a nonlinear function of the medium parameters viz., shear-, compressionalwave velocities, density and thickness of each layer. in order to obtain these model parameters through joint inversion of rayleigh and love wave group velocities, we apply a nonlinear inversion through genetic algorithms (ga) [lomax and snieder 1995, yamanaka and ishida 1996, suresh et al. 2008]. the ga is configured to suresh et al. 4 period (s) rayleigh waves love waves group velocity (km/s) s.d. (km/s) number of measurements group velocity (km/s) s.d. (km/s) number of measurements 5.000 3.064 0.034 8 3.356 0.024 8 5.741 3.074 0.051 8 3.365 0.017 8 6.591 3.105 0.042 8 3.407 0.026 8 7.568 3.156 0.037 8 3.428 0.022 8 8.689 3.123 0.035 8 3.461 0.025 8 9.976 3.143 0.035 8 3.469 0.024 8 11.454 3.127 0.069 8 3.458 0.021 8 13.151 3.102 0.046 8 3.468 0.033 8 15.100 3.079 0.051 8 3.458 0.041 8 17.337 3.102 0.051 8 3.484 0.019 8 19.905 3.143 0.105 8 3.529 0.042 8 22.854 3.168 0.143 8 3.569 0.077 8 26.240 3.363 0.078 8 3.629 0.050 8 30.128 3.484 0.106 8 3.673 0.051 8 34.592 3.636 0.068 7 3.769 0.102 7 39.716 3.797 0.136 4 3.998 0.113 5 45.600 3.900 0.150 3 4.079 0.132 5 52.356 3.989 0.173 3 60.113 4.097 0.092 3 table 2. observed group velocities across central dvp. 5 find acceptable solutions within a user defined search space. during this process, the solution at each generation is improved with misfit between observed and theoretical group velocities becoming less or equal to the preceding generation. here, the misfit { is considered similar to prajapati et al. [2011] and is given by where, di r is the absolute difference between the observed and theoretical group velocity of a given structure, vi r is the standard deviation of the observed group velocity at period ti (i = 1, 2, …, nr) for rayleigh waves. dj l and vj l are the corresponding values at period tj (j = 1, 2, …, nl) for love waves. the misfit { gives a measure of closeness of the theoretical curve to the observed data with respect to standard deviation. a model is acceptable if { < 1 and in such a case theoretd , ,max i r i r j l j l { d = vv = g crustal structure of dvp, indian peninsula figure 4. the theoretical group velocity curves for different structures (figure 3) of the indian peninsula are compared with the present mean observed data across central dvp. figure 5. observed mean group velocity data (open triangles) with ± one standard deviation. the theoretical curve has been drawn for the structure of central dvp (table 4). (1) ical curves of this model are within the limits of one standard deviation around each mean observation. we keep the search space extended down to the depth 100 km; this depth limit is due to limit of the longer periods of the observed data. while performing the inversion, we have evaluated the model parameters of the top 5 layers consisting of 4 layers of the crust and a layer of subcrust, whose base is fixed at 100 km depth as in prajapati et al. [2011]. below 100 km the structure is same as in ip11 [bhattacharya 1981]. such 4-layered crust has also been obtained by forward modeling of gravity data derived from surface measurements on the indian peninsula [arora et al. 2012]. the 14 variables in our ga search are: (1-4) thickness of top 4 layers of the crust, (5-9) vs of the top 5 layers and (10-14) vp/vs of top 5 layers. here, vp is p-wave velocity and vs is s-wave velocity. the search space of these 14 variables is wide around the model parameters of those of northwestern dvp (table 3). the density in each layer has been kept constant because it has the least effect on the dispersion curves and the density is considered in each layer as in northwestern dvp (table 3). the ga begins with a random initial population of k random models within solution limits. we consider a population of size k = 60 at each generation, where we create a new population with two elite members, 46 members through crossover and 12 members by mutation; the elite members replace the worst models in the current generation with the best individuals of previous generation, so that the best individuals are not lost [yamanaka and ishida 1996]. each ga operation gives best solution within 150 generations and the best solution is accepted. such operation is repeated making a list of 40 accepted solutions and the misfit value of all these solutions are between 0.74 and 0.83. the mean and standard deviations (sd) for each of varying parameters of all the 40 accepted solutions are obtained (table 4). the sd generally shows the error in the results. vp is obtained from vs and vp/vs for each solution; the mean and sd of vp are also obtained. this model with mean values is considered as the model for central dvp. the misfit for this model is 0.77; this misfit value shows that the theoretical curves are within the sd of the observed data. 5. discussion central dvp mostly lies in the central india. comparing present result with other studies for different areas in the central india [bhattacharya 1981, singh et al. 1999, rao et al. 2002] (figure 2), we find vs varies between 3.55 and 3.65 km/s in the upper crust and between 3.85 and 3.92 km/s in the lower crust; in the subcrust vs varies between 4.60 and 4.68 km/s. thus in the central india, the variation of vs around mean value is 2 per cent in the upper crust and these variations are 1 per cent in lower crust and subcrust. there is large varisuresh et al. 6 layer depth of top layer (km) thickness of layer (km) p-wave velocity (km/s) s-wave velocity (km/s) density (gm/cm3) 1. deccan trap 0.0 1.6 4.79 2.74 2.40 2. shallow crust 1.6 2.5 5.78 3.29 2.50 3. upper crust 4.1 12.7 6.12 3.54 2.60 4. lower crust 16.8 21.0 6.58 3.89 3.01 5. subcrust 37.8 62.2 8.09 4.57 3.30 layer thickness (km) vp (km/s) vs (km/s) density (g/cm3) 1. deccan trap 0.9±0.1 4.97±0.20 2.79± 0.04 2.40 2. shallow crust 3.3±0.3 6.07±0.07 3.38± 0.05 2.50 3. upper crust 14.8±1.3 6.44±0.10 3.65± 0.02 2.60 4. lower crust 20.6±1.3 6.83±0.20 3.87± 0.02 3.01 5. subcrust 60.4±0.2 8.37±0.08 4.68± 0.01 3.30 table 3. the crust and subcrustal structure for northwestern dvp down to 100 km [prajapati et al. 2011]. below 100 km the structure is same as that of ip11 [bhattacharya 1981]. table 4. crustal and subcrustal structure for central dvp down to 100 km. below 100 km the structure is same as that of ip11 [bhattacharya 1981]. thickness of crust = 39.6 ± 0.2 km. the value after ± shows corresponding standard deviation. 7 ation in the depth of lower boundary of the upper crust in different study areas of central india and this depth varies between 13.8 and 20.4 km; however, the moho depth varies only between 38.7 and 40.2 km and the crustal thickness of 39.6 km obtained in this study lies in this range. figure 6 shows the comparison of velocity structure of central dvp (table 4) and northwestern dvp (table 3). the crustal thickness is 39.6 km at central dvp and it decreases westward becoming 37.8 km at northwestern dvp. the crustal thickness decreases along with decrease of thicknesses of shallow and upper crust. the thickness of lower crust is nearly constant from central dvp (20.6 km) to northwestern dvp (21.0 km) and further westward to the continental shelf (20.9 km). in the continental shelf, prajapati et al. [2011] found radial anisotropy in the crust caused by extensional tectonic deformation; it was found that the mean of shand sv-wave velocities are close to s-wave velocity of the isotropic crust of the adjoining northwestern dvp. such radial anisotropy is not noted either in northwestern dvp or in central dvp. the observation of eastward decrease of trap thickness from 1.6 to 0.9 km is in agreement with other studies [harinarayana et al. 2007]. from central dvp in the east to northwestern dvp in the west, vs decreases by 2 to 3 per cent in the shallow and upper crust. westward decrease of vp is also seen by 3 to 6 per cent in all layers of the crust and subcrust. these differences are above the standard deviations of the evaluated velocities (table 4) and thus the changes noted are reliable. heat flow of central dvp is low compared to northwestern dvp [sen 2001, verma 2003]. in the peninsular india, sharma et al. [1991] also noted a significant decrease of subcrustal vp in the regions of high heat flow; from this correlation he inferred that low subcrustal vp is associated with elevated temperature of the subcrustal region. pei et al. [2004] has shown by quantitative analysis of the observed results that subcrustal vp is positively correlated with crustal thickness and negatively correlated with the earth’s heat flow. here we note such correlation with vp not only in the subcrustal regions but also in all layers of the crust (tables 3 and 4). the past volcanic activity was more on the west than on central part of dvp [sen 2001]; thus low vp in northwestern dvp also agrees with the past volcanic activity as found by pei et al. [2004]. in continuation of the study of sharma et al. [1991], we may infer that the elevated temperature of the crust and subcrust in northwestern dvp is the main factor for causing low velocity in this part of dvp. 6. conclusions we make the following inferences from the present study across central dvp and previous study across northwestern dvp [prajapati et al. 2011]. (1) the crustal thickness of central dvp is 39.6 km, which remains in the range between 38.7 and 40.2 km obtained in other areas of the central india. however, the thickness of upper crust differs significantly at other areas of the central india. (2) the crustal thickness decreases westward from central dvp (39.6 km) to northwestern dvp (37.8 km) along with the decrease of thickness of the upper crust. however, the lower crust has nearly the same thickness. (3) in a previous study, crustal radial anisotropy was found in the continental shelf adjoining northwestern dvp. such anisotropy is not noted in the continental part of dvp. (4) in the shallow and upper crust of dvp, vs decreases by 2 to 3 per cent from east to west, while vp in the whole crust and subcrust decreases by 3 to 6 per cent. the variations of vp and vs are positively correlated with the crustal thickness and negatively correlated with the earth’s heat flow. it appears that the elevated crustal subcrustal temperature in the western side is the main factor for low velocities on this side. acknowledgements. the first author is thankful to the director general of meteorology and head (seismology), india meteorological department for providing necessary facilities to carry out the study and permission to publish the paper. comments from anonymous reviewer were useful in improving the manuscript. references arora, k., v.m. tiwari, b. singh, d.c. mishra and i. grevemeyer (2012). three dimensional lithospheric crustal structure of dvp, indian peninsula figure 6. comparison of velocity structures of central dvp (solid thick line) and northwestern dvp (dashed line). for central dvp (present study), ± one standard deviation of the velocities and thickness are also shown by shaded areas. structure of the western continental margin of india constrained from gravity modelling: implication for tectonic evolution, geophys. j. int., 190, 131150. bhattacharya, s.n. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice convegni e congressi c o n s i g l i o n a z i o n a l e d e l l e r i c e r c h e c o m m i s s i o n e p e r l ' i d r o l o g i a s c i e n t i f i c a n e l l ' a g o s t o del 1 9 5 4 si t e r r à a r o m a l ' a s s e m b l e a d e l l ' u n i o n e g e o d e t i c a g e o f i s i c a i n t e r n a z i o n a l e della q u a l e f a p a r t e l ' a s s o c i a z i o n e i n t e r n a z i o n a l e di i d r o l o g i a . l a p a r t e c i p a z i o n e i t a l i a n a a l l a s e z i o n e di i d r o l o g i a v e r r à c o o r d i n a t a dalla c o m m i s s i o n e p e r l ' i d r o l o g i a scientifica, e m a n a z i o n e del consiglio n a z i o n a l e delle r i c e r c h e , con sede in r o m a , v i a della p u r i f i c a z i o n e , 23. i temi da t r a t t a r e s o n o i s e g u e n t i : 1) glaciologia e nevi t e m i l i b e r i . 2) erosione del suolo t e m i l i b e r i . 3| acque sotterranee: ai f a t t o r i geologici e i d r o g e o l o g i c i c h e i n t e r v e n g o n o n e l l a form a z i o n e delle r i s e r v e s o t t e r r a n e e con p a r t i c o l a r e r i g u a r d o al r i m p i n g u a i n e n t o a r t i f i c i a l e ; b i influenza dei lavori di s i s t e m a z i o n e del suolo sulla a c c u m u l a z i o n e delle a c q u e s o t t e r r a n e e ; c ) i n t e r p r e t a z i o n e delle v a r i a z i o n i dei l i v e l l i d ' a c q u a n e i pozzi e n e l l e s o r g e n t i ; d) b r e v e d e s c r i z i o n e delle a c q u e s o t t e r r a n e e e della l o r o util i z z a z i o n e n e i diversi paesi. 4) evaporazione: a.) n u o v e r i c e r c h e c o n c e r n e n t i l ' e v a p o r a z i o n e della s u p e r f i c i e dei l a g h i , della s u p e r f i c i e t e r r e s t r e , del suolo e a t t r a v e r s o l a vegetaz i o n e ; b) s t u d i o dei diversi f a t t o r i c h e i n f l u e n z a n o l ' e v a p o r a z i o n e e l o r o i m p o r t a n z a r e l a t i v a ; c) m e t o d i e s t r u m e n t i di m i s u r a . 5) deflussi: a) c o r r e l a z i o n e tra p o r t a t e e f a t t o r i m e t e o r o l o g i c i , s o p r a t t u t t o p r e c i p i t a z i o n i e t e m p e r a t u r e . f o r m u l e e m p i r i c h e , c u r v e e s b a n c h i sper i m e n t a l i : l o r o s t u d i o c o m p a r a t i v o e c r i t i c o e t e n t a t i v i di standardizz a z i o n e . a p p l i c a z i o n e a l l a p r e v i s i o n e delle p i e n e e d e l l e m a g r e ; b) s t u d i o delle c o r r e l a z i o n i i n t e r p e r i o d i c h e ( p e r i o d i : l ' a n n o , i l t r i m e s t r e , i l m e s e o p e r i o d i più c o r t i ) . i n p a r t i c o l a r e c o r r e l a z i o n i in4 2 8 convegni e c o n g r e s s i t e r m e n s i l i , c a r a t t e r i s t i c a etl a t t e n d i b i l i t à delle p r e v i s i o n i b a s a t e su queste c o r r e l a z i o n i ; c) c o r r e l a z i o n i f r a m a n t o nevoso e p o r t a t e : a p p l i c a z i o n e dalla p r e v i s i o n e d e l l e p o r t a t e ; d) s t u d i o p a r t i c o l a r e dei f a t t o r i i n f l u e n z a n t i i deflussi ( t e m p e r a t u r a v e n t o e u m i d i t à d e l l ' a r i a , r u g i a d a e p r e c i p i t a z i o n i o c c u l t e ) . nat u r a g e o l o g i c a dei t e r r e n i : p o s s i b i l i t à di c a r a t t e r i z z a r e u n b a c i n o a t t r a verso l a sua c a p a c i t à di r i t e n z i o n e . 6) laghi: a) m i s u r a della t e m p e r a t u r a e d e l l e c o r r e n t i n e i l a g h i n a t u r a l i e a r t i f i c i a l i . m e t o d i e s t r u m e n t i di m i s u r a . b i l a n c i o i d r o l o g i c o dei l a g h i ; b) c a p a c i t à r e g o l a t r i c e dei l a g h i e dei s e r b a t o i con p a r t i c o l a r e r i f e r i m e n t o a l l a l a m i n a z i o n e d e l l e p i e n e ; studi t e o r i c i e s p e r i m e n t a l i . 7) trasporto solido: t e m i l i b e r i . p o i c h é l e r i u n i o n i d e l l ' a s s o c i a z i o n e di i d r o l o g i a sono s e m p r e state c a r a t t e r i z z a t e da u n l a r g o c o n t r i b u t o s c i e n t i f i c o , a n c h e da p a r t e ital i a n a , sarà g r a d i t a la c o l l a b o r a z i o n e dei t e c n i c i e degli s t u d i o s i delle m a t e r i e c h e f o r m a n o oggetto dei t e m i p r e d e t t i , t a n t o p i ù i n consideraz i o n e del f a t t o c h e , t e n e n d o s i l a p r o s s i m a r i u n i o n e a r o m a , dovrà essere dato un m a g g i o r i m p u l s o a l l a n o s t r a p a r t e c i p a z i o n e . c o l o r o c h e d e s i d e r a n o p r e s e n t a r e m e m o r i e d o v r a n n o d a r n e comun i c a z i o n e a l l a s e g r e t e r i a della p r e d e t t a c o m m i s s i o n e p r e c i s a n d o i l tit o l o e l ' a r g o m e n t o c h e i n t e n d o n o t r a t t a r e . l a c o m m i s s i o n e dopo a v e r e s a m i n a t o e c o o r d i n a t o l e s i n g o l e proposte, d e c i d e r à sulla p o s s i b i l i t à della l o r o p r e s e n t a z i o n e al congresso. i testi d e l l e r e l a z i o n i a c c e t t a t e , a c c o m p a g n a t i da u n riassunto di 2 5 0 p a r o l e , d o v r a n n o p e r v e n i r e alla c o m m i s s i o n e stessa e n t r o il 15 m a r z o 1 9 5 4 . i testi stessi n o n d o v r a n n o s u p e r a r e l e 15 p a g i n e , di 3 . 0 0 0 l e t t e r e c i a s c u n a , figure c o m p r e s e . l e r e l a z i o n i devono essere p o s s i b i l m e n t e p r e s e n t a t e in l i n g u a francese o inglese. il presidente della commissione per l'idrologia scientifica p r o f . ing. marco visentini annals n.6/2003 ok 23/04 1247 annals of geophysics, vol. 46, n. 6, december 2003 key words historical seismology – earthquake catalogue completeness 1. introduction it is unlikely that many strong earthquakes (i0 ≥ 8/9, m ≥ 5.8) are still missing from italian catalogues: the latest such catalogue (cpti working group, 1999) lists 141 of them, and there are physical limits to the amount of energy that italian seismogenic structures can have released. unlikely, however, does not mean impossible. in principle at least, a few strong earthquakes could still be missing outright or lurking in disguise in the lower échelons of catalogues, their actual strength underestimated (there could also be some overestimated impostors in the upper échelons, but this is another story). how many, if any? a numerical answer could be worked out in many ways and lead to many results. it is reasonable to think that any results obtained would appear more or less convincing according to each user’s outlook. as a roundabout, even empirical but perhaps more convincing way to the same end, it was decided to single out some extreme cases of strong earthquakes that could reasonably be said to have been catalogued only «by chance», and to look at them closely. these earthquakes only narrowly avoided being left out of catalogues. was there some special reason for this to happen in these particular cases? and are they unique or do they share some common features, a pattern whose re-appearance in other, seemingly earthquakeless areas strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? viviana castelli istituto nazionale di geofisica e vulcanologia, sezione di milano, sede di ancona, italy abstract in principle, a few of the strong earthquakes (i0 ≥ 8/9, m ≥ 5.8) that affected italy in the past may still be missing from parametric catalogues or be listed there as lesser events, their actual strength unrealized. this seems a reasonable enough inference, given that some strong earthquakes were listed by catalogues quite by chance, from information drawn, mainly or even solely, from a single source. had this source been destroyed before catalogue compilers were able to consider it, or had they for any reason overlooked it, the earthquake it recorded could also have been missed or underestimated. this paper examines the two most peculiar italian cases of «single-source earthquakes» (1561 «vallo di diano?»; 1639 «amatrice?»). is all relevant information on each event really tied up in a single source? and if so, why? finally, are these cases unique or do they share any common features that could, by occurring elsewhere, act as markers for situations where forgotten earthquakes could still lurk undetected? mailing address: dr. viviana castelli, istituto nazionale di geofisica e vulcanologia, sezione di milano, sede di ancona, c/o centro assistenza protezione civile regione marche, strada statale cameranese, 60029 passo varano (an), italy; e-mail: castelli@mi.ingv.it 1248 viviana castelli could mark the occurrence of some as yet undetected seismic event? 2. what does «catalogued by chance» mean? it will be assumed in this paper that strong earthquakes can be said to have been catalogued «by chance» when: – all, most or the major part of the available information (as collected by standard studies and seismological compilations and summed up in a catalogue record) can be traced back to one «mother-source». – any other source known to studies/compilations is either derivative (i.e. based on the «mother-source») or – if original – makes only a minor contribution to general knowledge (e.g., it covers one or a few sites versus many sites covered by the «mother-source»; describes minor effects only, etc.). such situations do occur in italy and are quite normal, in some cases: for instance during the historical periods conventionally known fig. 1. «single-source» strong earthquakes listed in cpti working group (1999). 1249 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? table i. cpti working group (1999) «single-source» strong earthquakes. year month day hr min area study n. i0 lat. long. m points mcs 217 b.c. 06 etruria cfti 2 10 43.250 11.250 6.6 174 b.c. sabina cfti 1 10 42.250 12.670 6.6 100 b.c. picenum cfti 1 8-9 43.170 13.500 5.8 76 b.c. rieti cfti 1 10 42.400 12.870 6.6 17 reggio c.-sicily cfti 2 8-9 37.800 15.200 4.7 99 circello cfti 1 9-10 41.350 14.800 6.3 101 s.valentino in a.c. cfti 1 9-10 42.230 13.980 6.3 361 sicily cfti 1 10 37.500 14.000 6.6 374 reggio calabria cfti 1 9-10 38.100 15.650 6.3 375 benevento cfti 1 9 41.130 14.780 6.0 778 treviso cfti 1 8-9 45.670 12.250 5.8 951 rossano cfti 1 9 39.570 16.630 6.0 989 10 25 irpinia cfti 7 9 41.020 15.170 6.0 1223 gargano cfti 5 9 41.850 16.030 6.0 1273 potenza cfti 1 8-9 40.630 15.800 5.8 1361 07 17 19 30 ascoli satriano cfti 5 9 41.230 15.450 6.0 1414 vieste cfti 1 8-9 41.880 16.180 5.8 1561 08 19 14 10 vallo di diano cfti 30 9-10 40.520 15.480 6.3 1639 10 07 00 30 amatrice dom 27 10 42.636 13.252 6.6 as antiquity and early middle ages (loosely, up to 1000 a.d.). italy, however, is a country with a massive historical record. in the socalled high middle ages (or similar) – roughly the 1100’s-1400’s – it becomes more and more peculiar that major italian earthquakes are recorded only by one source. from the late 15th century onwards (loosely marking the start of what european historiography calls the modern age) a single-sourced, strong italian earthquake is something of a curiosity. medieval italy was already more literate than most other european countries, but in the modern period the production/preservation rates of public and private written records increase enormously all over europe, boosted by the invention of mechanical printing, the spread of literacy and the growth of bureaucracy. of course one has to make allowances for the loss of uncountable records and for the fact that only a small fraction of those actually written can be expected to have dealt with earthquakes. however, roughly from the 16th century onwards the historical seismologist is quite justified in expecting that most strong earthquakes be the subject of several independent written testimonies. any exception to this rule of thumb is remarkable enough in itself to be worth looking into more closely. 3. «single-source» strong earthquakes in the italian catalogues the parametric catalogue of italian earthquakes (cpti working group, 1999) lists 141 strong earthquakes with parameters derived from analytical studies giving references to sources. of these, 19 can be traced back to a single source (fig. 1; table i). are these cases normal, in their own timecontext? most (ten) occurred in the period conventionally known as antiquity (table iia), a remote time-frame in which scarcity and sparseness of information (mostly coming from literary fragments and epigraphs) are quite normal. the same applies to the three cases dated in the so-called early middle ages (table iib), given the scarcity of recording agencies, difficult communications, low literacy levels and other obstacles to good record-keeping that affect this historical period. a sadder but not really odder case is that of the four earthquakes which occurred in the socalled high middle ages (table iic). generally speaking, a comparatively large amount of records should be available for this period of italian history, with a most noticeable exception in peninsular southern italy, then the kingdom of naples. the most authoritative and comprehensive medieval source of information for this area (the royal neapolitan chancery records) was destroyed in 1943, before any systematic investigation of its potential earthquake record was made. it is now impossible to tell exactly how many earthquakes the neapolitan chancery papers originally recorded, apart from those few that were dealt with in such documents as were 1250 viviana castelli table iia-d. time-distribution and sources of «single-source» strong earthquakes. date area mother-source (*) (a) classical antiquity 217 b.c. etruria antipater (2nd century b.c.) quoted by cicero (1st century b.c.) 174 b.c. sabina titus livius (1st century b.c.) 100 b.c. picenum julius obsequens (4th-5th century a.d.) 76 b.c. rieti julius obsequens (4th-5th century a.d.) 17 a.d. sicily-calabria flegon of tralles (2nd century a.d.) 99 a.d. near circello (bn) memorial tablet (cil 9/1466) 101 a.d. near s. valentino in abruzzo citeriore (pe) memorial tablet (cil 9/3046) 361 a.d. sicily libanius of antiochia (4th century a.d.) 374 a.d. reggio calabria memorial tablet (année ep. 1913/227) 375 a.d. benevento symmachus (4th century a.d.) (b) early middle ages 778 a.d. treviso annales laureshamenses (9th-10th century a.d.), written in lorsch (mainz, germany) 951 a.d. rossano bartolomeo da rossano (10th-11th century a.d.) 989 october 25 irpinia leo cardinal of ostia (11th-12th century a.d. (or 990) a.d.), written in montecassino abbey (c) high middle ages 1223 gargano sarnelli (1680) 1273 potenza lost document from the neapolitan chancery, 1273 (filangieri, 1958) 1361 july 17 ascoli satriano villani (14th century) 1414 vieste lost document from the neapolitan chancery, undated (giuliani, 1768) (d) early modern period 1561 vallo di diano pacca (16th century) 1639 amatriciano tiberii (1639a,b) (*) for complete references see guidoboni (1989). 1251 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? occasionally transcribed, summarized or otherwise quoted in historical and/or seismological compilations produced before 1943. all table iic earthquakes being located in southern italy, it is to be expected that they are poorly documented. the earthquakes of 1561 and 1639 (table iid) are another story. they occurred in the socalled early modern age (1500’s-1700’s), a time-frame overflowing with potential recorders. professional printers and reporters thrived, cheap earthquake pamphlets were sold at market-stalls, red-tape letters were produced in triplicate. in such a garrulous age, why should two strong earthquakes attract such scant attention that only one truly relevant record of each was made? were they in fact less strong than we think they were? did most of the original records get lost? or was it all the fault of past earthquake compilers – whose work still forms the backbone of modern earthquake catalogues – for contenting themselves with retrieving only part of the potentially available records? or what else? 4. the earthquakes of 1561 (vallo di diano?) and 1639 (amatrice?) in present-day knowledge, both table iid events are some place’s «only destructive earthquake to date». in the 1561 case, the place is vallo di diano, a valley some 100 km southeast of naples, on the western side of the apennines. in the 1639 case, the place is the amatrice basin, on latium’s northernmost tip. when the earthquakes occurred, both areas belonged to the kingdom of naples, that was in its turn an appendage of the kingdom of spain. both were provincial backwaters, under feudal rule, and rather far from main thoroughfares and larger towns. both earthquakes were studied, independently and at roughly the same time, by two research teams working for the compilation of the parametric catalogues cfti (boschi et al., 1995, 1997, 2000) and nt4.1 (camassi and stucchi, 1997). the latest cpti99 catalogue has adopted cfti parameters for the 1561 earthquake and nt4.1 parameters for the 1639 earthquake. the alternative parameters produced by the available studies differ to varying extents. exploring these differences is not this paper’s aim. suffice to say that, whatever their reasons, they do not derive from using different sets of historical sources. in fact, the concurrent studies on both earthquakes rely on the same core of information, derived from single sources. a data set which is derived from a single source cannot avoid some taint, not so much of bias as of borrowing from a great historian, «the unavoidable fallacy of the point of view» (bloch, 1997). the accepted 1561 earthquake location was recently challenged by mucciarelli et al. (2000), on the grounds that the available data set could be biased by possible vagaries in the distribution of urban settlements at the time and/or by damage-enhancing site effects. as quite similar objections could doubtlessly be made in the 1639 case, both epicentral locations should be taken with a pinch of salt pending further consideration. hence the question marks. 4.1. basic knowledge to date on the 1561, vallo di diano (?) earthquake in 1561, a sizable chunk of the kingdom of naples was affected by two major seismic events within a month, with some lesser shocks in between. according to the «mother-source» (pacca 16th century, on which more anon), the first one, on 31 july 1561, affected the provinces of terra di lavoro, principato and basilicata (lying between the apennines and the tyrrhenian coast, naples and the calabrian borders). though pacca says it «weakened all buildings in those provinces ... so that they collapsed under the onslaught of the latter one», the only places he names are buccino, a small town some 100 km south-east of naples as the crow flies, which suffered heavy damage, and naples where the shock was felt. two other sources (unrelated to pacca, as far as one can tell) add that, on the same day and at roughly the same time, an earthquake fissured most buildings in avellino (bellabona, 1656) and caused panic in benevento (vipera, 1635). bellabona (1656) adds that lesser shocks were felt in 1252 viviana castelli fig. 2. the 1561 earthquake according to pacca (16th century): hd – heavy damage; d – damage; sd – slight damage; f – felt. fig. 3. another version of the 31 july 1561 event (pacca, 1563). hd – heavy damage; f – felt. 1253 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? avellino for some days after. benevento and avellino lie roughly midway between naples and buccino. evidence on them tallies with pacca’s tale as concerns date, time and subsidiary circumstances (a violent storm preceeding the earthquake by a couple of hours), so increasing the credibility of the whole picture. the second major event described in the «mother-source» occurred on 19 august 1561 and affected, we are told, principato and basilicata. here pacca (16th century) lists 29 severely damaged towns and villages, several of them close to buccino. buccino itself, however, is not mentioned (fig. 2). of course it seems unlikely that buccino was singled out for damage on 31 july and escaped untouched on 19 august (and vice versa). in fact, a later chapter of pacca (16th century) states that «in 1561 the earthquake hit buccino twice within a month». as it will be explained later, the pacca (16th century) description of the 1561 earthquake was never published. a shortened, rather garbled version, was however included in a printed work by the same author (pacca, 1563). it gives quite a different picture of the 31 july event (fig. 3) and, for the 19 august event, only a generic mention of «more damage in the vallo di diano». this version would be mirrored in a trickle of secondhand mentions of the 1561 earthquake to be found in 16th and 17th century historical compilations. 4.2. basic knowledge to date on the 1639, amatrice (?) earthquake the «mother-source» (tiberii, 1639a,b) describes three events which occurred on 7, 14 (or 8 and 15, according to what one makes of the original time-quotes) and 17 october 1639 (table iii). each of the earlier two ones purportedly caused heavy damage in a different set of sites inside the amatrice estate. this included the namesake town and 67 villages, and was a fief of prince alessandro orsini (massimi, 1958). tiberii (1639a) also reports damage in four sites not belonging to the amatrice estate (accumoli, montereale, poggio cancelli and roccasalli), without specifying which event was responsible for it. however, all the sites intable iii. 1639 earthquake effects according to tiberii (1639a,b). 7/8 october 14/15 october «at the same time» (?) 17 october tiberii amatrice amatrice accumulo (+) (1639a) campo tosto saletta rocca de salli (+) san martino corsenito poggio cancello (+) collalto casale monte reale (+) pinaca rocca (#) filetta torreto nescaia colle basso (#) s. lorenzo abbey (*) pasciano padarga santo iorio cantone colle moresco corva (#) forcella capricchio la leia tiberii all the above all the above all the above amatrice (1639b) recanati rieti (+), localities outside the amatrice estate; (#), unidentified localities; (*), possibly not a locality (as tiberii believed) but a church in amatrice (lupachino, 1669). 1254 viviana castelli volved are so close (fig. 4) that it seems unlikely that some were damaged by the first event while others, their close neighbours, remained untouched until a week later. tiberii (1639b) piles on colourful details of popular reactions and curious natural phenomena connected to the earthquake, describes one more event that caused further damage in amatrice on 17 october and winds up his tale adding that «at the aforesaid time» the earthquake «was felt in recanati [near the mid-adriatic coast] and caused fear in rieti». this piece of news is generally interpreted as proof that the main amatrice event was felt as far as the marche coast, though of course it could also conceivably be related to some other, otherwise still unknown, local earthquake. 5. the «mother-sources» 5.1. the 1561 key witness: colanello pacca according to present-day knowledge, the key-witness of the 1561 earthquake is one colanello (or niccolò agnello) pacca, a neapolitan university professor with interests ranging from philosophy to history and genealogy (table iv). the discorso del terremoto (pacca, 16th century), probably compiled between 1561 and 1580, was his most elusive work. as far as it has been possible to ascertain, the only known contemporary reference to this book is a self-quote: after briefly mentioning the 1561 earthquake in his storia del regno di napoli (pacca, 1563), fig. 4. the 1639 earthquake according to tiberii (1639a,b). 1255 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? pacca refers any reader wishing for more details to «my treatise on earthquakes». when pacca died, in 1587, the discorso was still unfinished. it remained unpublished and forgotten for centuries. at least, none of several 17th to mid-19th century. neapolitan literary and seismological compilations listing pacca’s works seems aware of its existence. it was first brought into comparative limelight in 1891 by giuseppe mercalli, who published a selection of 22 descriptions related to 16th century southern italian earthquakes (mercaltable iv. who’s who? colanello pacca. 1534 born in naples. 1557 graduates in humanities and medicine from naples university. publishes a latin philosophy primer. takes on the chair of logic at naples university. 1557/1562 works on 7th book of storia del regno di napoli a multiple-authors work. 1561 publishes another latin philosophy primer. starts writing discorso del terremoto. 1563 publishes 7th book of storia del regno di napoli. 1574 takes on the chair of aristotelian logic at naples university. 1580 works still on discorso del terremoto. 1582 takes on the chair of metaphysics at naples university. 1587 dies in naples. other known fields of interest: poetry, genealogy. references: toppi (1678), tafuri (1752), soria (1781), minieri riccio (1844) and nicolini (1966). fig. 5. the shadow of a real itinerary? narrative sequence of the sites affected by the 19 august 1561 event in pacca (16th century). 1256 viviana castelli li, 1891), out of a list of 350 earthquakes occurring «in all the world from its creation», which takes up about half the discorso (morelli, 1993). mercalli’s selection was further trimmed down – for no evident reason – by baratta (1901), who adopted only nine of the items mentioned by mercalli (1891). the 1561 earthquake has a special place in the discorso because, as the author recounts, «[it] gave me the idea of undertaking such a lengthy work [and] i resolved to get as many details as possible on it». in 1561 pacca was preparing a two-volume update of a standard multi-authored storia del regno di napoli (pacca, 1563). it was surely in his capacity of historian that he started collecting information on the earthquake. he claims his source was «a true report by trustworthy men». it is unclear who these people were: occasional travellers, envoys from the affected areas, or even government officers sent on a damage-assessing tour. the numbers pacca gives for casualties and affected buildings per locality are finite and believable-sounding («six» or «eight» or «twenty», not «hundreds» or «innumerable»). also interesting is that the narrative sequence in which the names of sites affected by the 29 august event are given seems to follow a real itinerary (fig. 5). the only exception is «san licandro», listed between calitri and san fele, in an area where no place of this name is to be found (fig. 5 lists some alternatives). all this leads us to believe that the tale was based on reliable sources: the likeliest would be an official report of some sort, subsequently lost or still hidden in the labyrinthine neapolitan archives or (naples was then a subject of the most catholic king) perhaps even in the spanish central archives of simancas. 5.2. the 1639 key witness: carlo tiberii carlo tiberii, proudly self-styled «a roman citizen», wrote two journalistic pamphlets on the 1639 earthquake (tiberii, 1639a,b). by sheer serendipity, a little information was gained on this person (table v). apparently the black-sheep of a well-to-do bourgeois family (acap, 16381639), tiberii made a living by writing comic plays and acting in them (bragaglia, 1958; mariti, 1978; franchi, 1988). at those times it was a precarious and demeaning job: tiberii aspired in fact to «something better» as he says in a self-vindicatory introduction to his last known play (tiberii, 1641). prince marcantonio borghese, to whom he dedicated the play, could be the influential patron that tiberii hoped would foster his social aspirations. whether these were fulfilled remains unknown: tiberii disappears from history after 1643, the printing date of his last work, a volume of poems. as tiberii does not figure in a census of 17th century. roman professional reporters (bulgarelli and bulgarelli, 1988), it is likely that the 1639 earthquake pamphlets were his only journalistic venture. it is unclear whether he wrote them using privileged information gleaned from contacts in the orsini or borghese households – marcantonio borghese was related by marriage to the orsini lord of amatrice (ugurgieri azzolini, 1649) – or whether he was simply hired by a roman printer-bookseller to spin a marketable story out of data collected some way or other. that the pamphlets were written from a roman point of view is shown by tiberii’s anticipation of the arrival of «better news» (tiberii, 1639a) and his mention of the tales told by newly arrived amatrice refugees (tiberii, 1639b). that he was no eyewitness is also shown by his poor grasp of local geography: for instance he mistakes «san lorenzo abbey» (the courtesy title of a church inside amatrice; lupachino, 1669) for a real abbey in the country. his lists of damaged sites show no trace of a conjectural real itinerary (fig. 6), contrary to pacca’s description of the 1561 earthquake. his lavish estimates of damage, escalating from «400 000 scudi» (tiberii, 1639a) to «one million gold» (tiberii, 1639b) are clearly made up for sensation’s sake, as the whole amatrice estate had been evaluated as worth 80.000 scudi as early as 1610 (ragioni ..., 1693). commercially, the pamphlets must have been a real success. copies of the first (tiberii, 1639a), printed in rome around mid-october, are preserved in public libraries of northern tuscany, the marches, abruzzo, naples and venice, implying a largish issue and a wide circulation. it was reprinted too, in perugia (minieri riccio, 1861) and florence (british 1257 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? table v. who’s who? carlo tiberii. ??? born (rome?). ??? possibly a jesuit pupil at the collegio romano. ??? in his teens, writes play hoggi corre quest’usanza (this is the fashion now). ??? becomes a member of the «academy of the hidden ones», a minor roman literary club. 1637 play li tre amanti burlati (the three hoodwinked lovers), dedicated to landgrave frederick of hesse. 1638 last will and testament of tiberii’s mother. small legacy to him, his brother is made universal heir. 1639 play escharistumerotos overo i contrasti d’amore (e. or love’s labours). march formally declares he lives apart from his father, and renounces to his inheritance for a lump sum of 100 scudi. oct/nov nuova, e vera relatione del terribile, e spaventoso terremoto [...] (two printings in rome; one each in perugia, florence and milan). 1640 writes new introduction to play hoggi corre quest’usanza, now about to be printed for the first time. biographical details. 1641 play hoggi corre quest’usanza printed, dedicated to marcantonio borghese prince of sulmona. 1643 la fama messaggiera. poetiche compositioni (fame the messenger. poems) printed in spoleto (umbria). ??? dies. references: acap (1638-1639), tiberii (1639a,b; 1641), mariti (1978) and franchi (1988). fig. 6. no shadow of a real itinerary. narrative sequence of the sites affected by the first two 1639 events in tiberii (1639a,b). 1258 viviana castelli library, 1988). the original roman issue of the second pamphlet (tiberii, 1639b) was printed at the end of october, probably in fewer copies. the only two known extant copies (one now missing, formerly in the roman archive of the company of jesus; another in the historical neapolitan society library) come not from the roman first printing but from a milanese reprint, issued around 6 november. the same text was also inserted in the milanese avvisi, a handwritten weekly newssheet. 6. are they really «single-source» earthquakes or not? 6.1. the 1561 earthquake: old sources, new data, perchance a new map? the earthquakes of 1561 and 1639 are «single-source» ones, in this paper’s wording and as far as the seismological tradition is concerned, because all or most of what’s known of them in seismological literature (and therefrom their catalogue parameters) can be traced to a single source. this does not strictly mean that seismological studies and compilations do not know of any other sources dealing with them, but rather that – except for the descriptions set down by pacca and tiberi – any other source listed by seismological studies and compilations in connection with the 1561 and 1639 earthquakes is either derivative, or else it adds only minor details to the whole amount of knowledge available. with this qualification in mind, in the case of the 1561 earthquake, apart from the 1561 «mother-source», i.e. the long handwritten description by pacca (16th century), the seismological tradition lists: – the abridged version by pacca (1563), reprinted in pacca (1591); it names only a few of the sites linked by pacca (16th century) to the 19 august event, but connects them instead with the 31 july event. – a few second-hand 16th and 17th century texts based on pacca (1563, 1591). – a handful of original sources, independent of pacca, each recording the effects of the 1561 earthquake in one locality not mentioned by pacca (maffei, 1564; seripando, 16th century; vipera, 1635; bellabona, 1656). just to stress how much a «forever in progress» thing historical earthquake research is, though maffei (1564) was first quoted in the mercalli (1891) compilation, its being an eyewitness account of the earthquake as felt in solofra was only realized while working on this paper. if one leaves the well-trodden path of seismological tradition to look for untapped sources, the findings are rather meagre. local municipal archives mostly lack contemporary records. in the local, sparse, mostly recent, often unreliable historiography, the remembrance of the 1561 earthquake in the vallo di diano and nearby areas is almost effaced by memoirs of the 1694 and 1857 earthquakes (i0 = x-ix) and the 1656 plague, responsible for a drastic population drop. positive findings include a couple of original records (table vi) and a scattering of sourceless – i.e. uncheckable, therefore unreliable – recent mentions of damage purportedly wrought by the 1561 earthquake in sites not named by the older sources (sansone and sansone, 1959; ricchetti, 1983; mattia, 1986). interestingly enough, some of this new evidence singles out the 31 july event as more significant than pacca made out. in tito – a site that, according to pacca (16th century) was damaged by the 19 august event – a 16th century memorial tablet dates the locally damaging event to 31 july «1560» (the latter hopefully being, in the light of all additional evidence, a carver’s mistake). sourceless recent evidence also refers to the 31 july event rather than to the 19 august one (fig. 7). 6.2. the 1639 earthquake: new (but hardly mappable) data to all intents and purposes, tiberi (1639a,b) remains the relevant source for anyone attempting a reconstructions of the 1639 earthquake. the only additional sources known to seismological tradition are some, yet to be identified, 17th century annali ecclesiastici by one scoglio, quoted by the bonito (1691) compilation as relating that «amatrice and villages were savaged by an earthquake in 1639». among the references of the 1639 studies in boschi et al. 1259 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? (1995, 1997, 2000) there are the almost identically-titled annali ecclesiastici della liguria (schiaffino, 17th century). a search for original 1639 records unearthed new evidence of damage in amatrice, not in the local municipal archive (lacking any records before 1750) or in the missing orsini feudal papers, but in those of the bishopric of ascoli, to which amatrice was subjected, and in local mementoes (acvap, 1652; memorial tablet, 17th fig. 7. the 1561 earthquake at the end of this study (i mcs). d – damage; f – felt; r – rockfall; ? – doubtful data). table vi. 1561 earthquake memorial tablets. location: tito (pz), church of the «congrega» location: polla (sa), facade of the castle, (destroyed by the 1980 irpinia earthquake) left of main gate. (1) communication by don nicola laurenzana to the author (2001); (2) bracco (1976). «iohannes villanus, oppido motu terrae concusso, fere prostrato, viis, plateis coenobis restituto adaucto, secum marchionatus titulo decorato, castro elegantius erecto, pollae posuit a[nno] d[omini] mdxc.» (2) giovanni villano, having rebuild and enlarged the town after the earthquake hit and almost destroyed it and its streets, squares and monasteries, having obtained the title of marquis and rebuilt this castle in a more elegant style put up this memorial in polla, in the year of our lord 1590. «domus hec noviter constructa fuit per venerabilem dom[inum] primum de suma, patronum, ad honor[em] s[anc]ti antonii de padua an[n]o divi 1568. que tota corruit a terremotu ultimo die iulii a[nno] d[omini] 1560.» (1) this house, utterly collapsed in the earthquake of the last (day) of july 1560 [sic], was built anew by the reverend lord and patron primo de suma, to honour the blessed st anthony of padua, in the lord’s year 1568. 1260 viviana castelli century; lupachino, 1669). more interesting, however, is the evidence from the nearby region of abruzzo, an area not previously known to have been affected by this earthquake. a contemporary diary (ciurci, 17th century) and a reliable 18th century historical compilation (antinori,18th century) state that the 1639 earthquake was strongly felt in the abruzzo provincial capital, aquila. in the aquila district «in a range of forty miles ... many of the highest buildings were thrown down» (antinori, 18th century). information on minor effects in ascoli piceno (marcucci, 1766) and leonessa (rossi da voltaggio, 1695) also came to light. the data about earthquake damage in the aquila district cannot be mapped unless more details come to light (this is, unfortunately, somewhat unlikely). however, the newly discovered evidence hints, clearly enough, that the present reconstruction of the october 1639 earthquake (see, for instance: monachesi and stucchi, 1997) could in fact account for no more than half of the actual near field. 7. some final considerations the 1561 and 1639 earthquakes are two strong enough events that attracted less contemporary attention than one would expect (though they weren’t by any means, «single-source» ones, apart from the specific sense meant here). why it was so, is not clear. in 1561, there were political and military matters to draw away the attention of potential witnesses. after an unlucky 1560 spanish expedition against djerba, western europe was tensely waiting for a summer large-scale attack by the ottoman fleet. the neapolitan kingdom was in a frenzy of military preparations, complicated by outbursts of plague and the rebellion of the valdensian communities of calabria. in july, the turkish pirate dragut destroyed the sicilian fleet and proceeded to pillage the adriatic coast (braudel, 1982). the most perfunctory browsing of contemporary newssheets shows that the whereabouts of the turkish ships was the main interest of all correspondents (see, for instance: asfi, 1542-1662). in 1639, on the contrary, there seems to be no such europeansize distraction. there could be more long-term reasons too. the examined cases occurred in quite similar circumstances, geographically and administratively. vallo di diano in 1561 and the amatriciano in 1639 were secluded rural areas, far from the main thoroughfares and cultural centres: this would make it harder for inside occurrences to become known abroad in a consistent way. in such situations, even strong earthquakes could pass unobserved, whereas even quite minor shocks would have many chances of being recorded, if felt in a large town such as naples, venice, rome or florence. another thing that could matter is that both areas were ruled by feudal families, i.e. mostly insulated from direct interference by the central government. where the state is less likely to interfere, state records are less likely to be produced, and as a rule, these are less liable to dispersion than private family records. of course things could have gone differently. there are quite a few cases of 17th century earthquakes located in various parts of the kingdom of naples (1638 in calabria, 1654 in sorano-marsica, 1688 in sannio and 1694 in irpinia-basilicata), that originated a large production of records, from pleas for economic help to damage surveys. in all these cases, large-scale searches of archival records were launched, something that did not happen in the 1561 and 1639 cases. whether this was because these particular earthquakes were never felt as important enough to warrant the time and expense required by such an investigation, or because it was always deemed that the «mother sources» gave information enough, it is a matter of speculation. in any case, had pacca (16th century) and tiberii (1639a,b) i) never been written, ii) been destroyed or iv) remained unknown to historical earthquake compilations (the background and backbone of modern parametric earthquake catalogues), the assessment of the 1561 and 1639 earthquakes would have been quite different. in fact, none of the 1561 and 1639 «additional» sources identified so far matches the «mothersources» for quantity and quality of data. in any case, it cannot be taken for granted that any of these additional sources would have come under the compilers’ scrutiny, had not the compilers’ 1261 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? interest first been stirred by the «mothersources». the 1561 earthquake as shown in fig. 2 (from pacca, 16th century) is rather different from that of fig. 3 (based on pacca, 1563). a tiberii-less 1639 earthquake would probably be located in aquila, rather than in the amatriciano. in conclusion, it can be assumed that the 1561 and 1639 earthquakes could well have been completely eclipsed owing to a multiple set of circumstances: occurring in marginal areas, being dealt with by one very detailed source, never having become «strategic» enough to warrant a thorough search. as far as a historian’s judgment can stretch, beside vallo di diano and amatriciano, there are many other corners of non-metropolitan italy where other such earthquakes could have been prevented from becoming general knowledge by the absence of a pacca or a tiberii, and remained undetected in the fringes of italian «earthquake consciousness». the area affected by the 31 october 2002 molise earthquake could perhaps be one of these. of course, the larger the area affected by its destructive effects, the harder for an earthquake to be completely forgotten. it is unlikely that very large and destructive earthquakes, such as those of 1456 (southern italy), 1693 (sicily) or 1703 (central italy) can have been blurred out of recognition, unless perhaps they occurred in the «hic sunt dracones» time-windows of antiquity and the early middle ages. it could be, however, much easier for earthquakes whose largest effects affected comparatively small areas, and especially so if such areas should happen to belong to those portions of the italian territory that were not a subject of foremost interest for any of the large-scale programmes of historical earthquake research launched in the past 30 years and including extensive archive investigation. acknowledgements this paper was first conceived as a joint venture with massimiliano stucchi, who provided crucial insights in its seminal stage and presented its first draft at the 1998 esc general assembly (tel aviv, israel). gratefully acknowledging his many generous contributions to this paper, the author claims full responsibility for any mistakes, inconsistencies and unsound ideas marring the final version. thanks to gregory quenet and a nameless reviewer for their suggestions and to romano camassi and emanuela ercolani for their patience with a mapinfo-challenged one. references acap (archivio capitolino) (1638-1639): archivio urbano, notary augustinus theolus, 42, 1639 october 5. acvap (archivio della curia vescovile di ascoli piceno) (1652): sacre visite, giulio gabrielli. asfi (archivio di stato di firenze) (1542-1622): mediceo del principato, 4148, avvisi di napoli e messina dal 1542 al 1622. antinori, a.l. (18th century): annali degli abruzzi, 22/1, biblioteca provinciale ‘s. tommasi’ of l’aquila, antinoriani mss (facsimile edition, bologna 19711973). baratta, m. (1901): i terremoti d’italia, torino (anastatic reprint, bologna 1979). bellabona, s. (1656): raguagli della città d’avellino del p[adre] bacc[elliere] fra scipione bellabona [...] nelli quali si dà notitia d’alcuni antichi luoghi de gl’irpini. si descrive la detta città, e sue grandezze, destruttioni, casi successi a coloro, che la dominarono [...], trani. bloch, m. (1997): apologie pour l’histoire ou le métier d’historien (nouvelle edition, armand colin/références, paris). bonito, m. (1691): terra tremante, napoli. boschi, e., g. ferrari, p. gasperini, e. guidoboni, g. smriglio and g. valensise (eds.) (1995): catalogo dei forti terremoti in italia dal 461 a.c. al 1980 (ing, roma-sga, bologna), pp. 974. boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (eds.) (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990 (ing, roma-sga, bologna), pp. 644. boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (eds.) (2000): catalogue of strong italian earthquakes from 461 b.c. to 1997, ann. geofis., 43 (4), 609-868. bracco, v. (1976): polla. linee di una storia, salerno. bragaglia, a.g. (1958): storia del teatro popolare romano, roma. braudel, f. (1982): la mediterranée et le monde mediterranéen à l’èpoque de philippe ii (librairie armand colin, paris). british library (1988): catalogue of seventeenth century italian books in the british library, 2, m-z, london. bulgarelli, s. and t. bulgarelli (1988): il giornalismo a roma nel seicento, roma. camassi, r. and m. stucchi (1997): nt4.1, un catalogo parametrico di terremoti di area italiana al di sopra della soglia di danno (nt4.1.1 version), (gndt, milano). 1262 viviana castelli ciurci, f. (17th century): familiari ragionamenti delli commentarii et annali dell’aquila, l’aquila, biblioteca provinciale ‘s. tommasi’, ms 49. cpti working group (1999): catalogo parametrico dei terremoti italiani (ing gndt sga ssn, bologna), pp. 92. filangieri, r. (1958): i registri della cancelleria angioinaricostruiti da r.f. con la collaborazione degli archivisti napoletani, 11 (1273-1277), napoli. franchi, s. (1988): drammaturgia romana. repertorio bibliografico cronologico dei testi drammatici pubblicati a roma e nel lazio, secc. xvii-xviii, 1, roma. giuliani, v. (1768): memorie storiche politiche, ecclesiastiche della città di vieste, napoli. guidoboni, e. (ed.) (1989): i terremoti prima del mille in italia e nell’area mediterranea. storia, archeologia, sismologia (ing, roma-sga, bologna), pp. 765. lupachino, n. (1669): vita di camillo orsini [...] descritta dal sig. giuseppe horologi nel 1565, con l’aggiunta in questa terza and ultima impressione, di alcune lettere della ser. repubblica di venetia [...] con elogij di tutti i suoi discendenti, fino al dì d’hoggi, bracciano. maffei, g.c. (1564): scala naturale, overo fantasia dolcissima. intorno alle cose occulte, e desiderate nella filosofia, venezia. marcucci, f.a. (1766): saggio delle cose ascolane e de’ vescovi di ascoli nel piceno, teramo. mariti, l. (1978): commedia ridicolosa: comici di professione, dilettanti, editoria teatrale nel seicento: storia e testi, roma. massimi, a. (1958): amatrice e le sue ville. notizie storiche, amatrice. mattia, g. (1986): tolve nella storia – dall’anno 1000 al 1935, no place. memorial tablet (17th century): eulogy of prince alessandro maria orsini’s intervention after the 1639 earthquake, amatrice, palazzo orsini, in situ. mercalli, g. (1891): i terremoti napoletani del secolo xvi ed un manoscritto inedito di cola anello pacca, boll. soc. geol. it., 10 (1891), 179-195. minieri riccio, c. (1844): memorie storiche degli scrittori nati nel regno di napoli, napoli. minieri riccio, c. (1861): biblioteca storico-topografica degli abruzzi composta sulla propria collezione da c.m.r. direttore della real biblioteca palatina di napoli, 1, napoli. monachesi, g. and m. stucchi (1997): dom4.1, un database di osservazioni macrosismiche di terremoti di area italiana al di sopra della soglia del danno, rapporto tecnico (gndt, milano-macerata), pp. 1052 (on line http://emidius.mi.ingv.it/dom/). morelli, s. (1993): una fonte inedita per lo studio della sismologia storica: il «discorso del terremoto» di cola anello pacca (1563), rassegna storica salernitana, xi, 247-259. mucciarelli, m., g. valensise, m.r. gallipoli and r. caputo (2000): reappraisal of a xvi century earthquake combining historical, geological and instrumental information, in papers and memoranda from the first workshop of the esc working group «historical seismology», 1-5 september 1999, macerata, italy, edited by v. castelli, 64-67. nicolini, f. (1966): saggio d’un repertorio biobibliografico di scrittori nati o vissuti nell’antico regno di napoli, banco di napoli, biblioteca del «bollettino» dell’archivio storico, 11, napoli. pacca, c. (16th century): discorso del terremoto, ms., biblioteca della società napoletana di storia patria di napoli, fondo sismico, 7/a3. pacca, c. (1563): della seconda parte del compendio dell’istoria del regno di napoli, il settimo libro del dottor colanello pacca napoletano, in compendio dell’istorie della città di napoli, edited by p. collenuccio, m. roseo, c. pacca and t. costo, venezia. pacca, c. (1591): della seconda parte del compendio dell’istoria del regno di napoli, il settimo libro del dottor colanello pacca napoletano, in compendio dell’istorie della città di napoli, edited by p. collenuccio, m. roseo, c. pacca and t. costo, venezia. ragioni a pro della serenissima vittoria montefeltria della rovere gran duchessa di toscana [...] sulla baronia di amatrice, sequestrata ad istanza del regio fisco per la pretesa devoluzione (1693). ricchetti, d.m. (1983): castelpagano, cosenza. rossi da voltaggio, a.m. (1695): vita del venerabile servo di dio b[eato] giuseppe da leonessa predicatore cappuccino della provincia di s. francesco, genova. sansone, g. and l. sansone (1959): la città di lioni e dintorni, lioni. sarnelli, p. (1680): cronologia de’ vescovi et arcivescovi sipontini, manfredonia. schiaffino, a. (17th century): annali ecclesiastici della liguria, genoa, biblioteca civica berio, ms m.r. 3.1.5. scoglio, * (17th century): annali ecclesiastici (in bonito, 1691). seripando, g. (16th century): giornale dal 1513 al 1562, naples, biblioteca nazionale "vittorio emanuele iii", ms ix.c. 42. soria, f. (1781): memorie storico critiche degli scrittori napoletani, napoli. tafuri, g.b. (1752): istoria degli scrittori nati nel regno di napoli, 3/2, napoli. tiberii, c. (1639a): nuova, e vera relatione del terribile, e spaventoso terremoto successo nella città della matrice, e suo stato, con patimento ancora di accumolo, e luoghi circonvicini, sotto li 7 del presente mese di ottobre 1639, con la morte compassionevole di molte persone, la perdita di bestiami d’ogni sorte, e con tutto il danno seguito fino al corrente giorno. con ogni diligenza, e certezza descritta da carlo tiberii romano, per memoria d’un caso così miserando, e lagrimevole, roma. tiberii, c. (1639b): nuova e vera relatione del terribile, e spaventoso terremoto sucesso nella città della matrice e suo stato, con patimento ancora di accumulo, e luoghi circonvicini, sotto li 7 del mese di ottobre 1639. con la morte compassionevole di molte persone, la perdita de bestiami d’ogni sorte, e con tutto il danno seguito fino al corrente giorno. con ogni diligenza, e certezza descritta da carlo tiberij, romano, per memoria d’un caso così miserando, e lagrimevole. aggiuntovi di più altri successi dalli 17 del sodetto mese 1263 strong earthquakes included «by chance» in italian catalogues: single cases or a hint of more? fin all’hora presente; dove s’intende le gran ruvine di monte reale, poggio, cancello e ancora con spavento nuovamente di recanati, e rieti nel detto tempo, roma e milano. tiberii, c. (1641): hoggi corre quest’usanza. comedia del signor carlo tiberii romano, il disprezzato accademico nascosto, ronciglione. toppi, n. (1678): biblioteca napolitana [...], napoli. ugurgieri azzolini, i. (1649): le pompe sanesi, pistoia. villani, m. (14th century): cronica, edited by g. porta, parma, 2 voll. vipera, m. (1635): chronologia episcoporum, et archiepiscoporum metropolitanae ecclesiae beneventanae, napoli. (received february 13, 2003; accepted august 4, 2003) linear models of dissipation whose q is almost frequency independent (*) m . caputo ( * * ) r i c e v u t o il 31 maggio 1966 summary. — l a b o r a t o r y experiments and field observations indicate that tlie q of many non f e r r o m a g n e t i c inorganic solids is almost f r e q u e n c y independent in the range 10' to 10~2 cps; although no single substance has been i n v e s t i g a t e d over the entire f r e q u e n c y spectrum. one of the purposes of this investigation is to find the analytic expression of a linear dissipative mechanism whose q is almost frequency independent o v e r large frequency ranges. this will be obtained by introducing fractional derivatives in the stress strain relation. since the aim of this research is to also contribute to elucidating the dissipating mechanism in the earth free modes, we shall treat the cases of dissipation in the free purely torsional modes of a shell and the purely radial v i b r a t i o n of a solid sphere. t h e t h e o r y is checked with the new values determined for the q of the spheroidal free modes of the earth in the range between 10 and 5 minutes integrated with the q of the railegh w a v e s in the range between 5 and 0.6 minutes. a n o t h e r check of the t h e o r y is made with the experimental values of the q of the longitudinal waves in an alluminimi rod, in the range between 10~5 and 10~3 seconds. i n both clicks the theory represents the observed phenomena v e r y s a t i s f a c t o r y . riassunto. • — 1 risultati delle, ricerche ili laboratorio e delle osservazioni in f e n o m e n i naturali indicano che il q di parecchi solidi inorganici non f e r r o m a g n e t i c i è indipendente dalle frequenze nell'intervallo io 2 , io7 cicli al secondo; per quanto nessuna sostanza sia stata studiata in t u t t o ( * ) this paper was presented at the 1966 annual meeting of a g u in w a s h i n g t o n d c . ( * * ) d e p a r t m e n t of geophysics, u n i v e r s i t y of british columbia, canada. 384 m. c a p u t o questo intervallo di frequenze. tino degli seopi della presente ricerca è quello di trovare l'espressione analitica di un modello di dissipazione lineare in cui q sia indipendente dalla frequenza in un vasto intervallo di frequenze. questo sarà ottenuto introducendo derivate di ordine frazionario nelle relazioni fra sforzo e deformazione. poiché uno degli scopi di questa ricerca è anche di contribuire ad una miglior comprensione dei meccanismi di dissipazione dell'energia nelle oscillazioni libere delia terra, in questa nota si applicherà la legge di dissipazione citata al caso delle oscillazioni torsionali libere di uno strato sferico. l a teoria esposta viene poi applicata allo studio dei valori di q osservati nelle onde di rayleigh e nelle oscillazioni sferoidali della terra. un'altra applicazione della teoria è fatta allo studio dei valori di q osservati nelle onde longitudinali di una sbarra di alluminio. in entrambe le applicazioni la teoria rappresenta in maniera soddisfacente i fenomeni osservati. introduction i n a homogeneous isotropic elastic field, the elastic properties of the substance are specified b y a description of the strain and stre sses iu a l i m i t e d portion of the field since the strains and stresses are linearly related b y t w o parameters which describe the elastic properties of the field. i f the elastic held is not homogeneous nor isotropic the properties of the field are specified in a similar manner b y a larger number of parameters which also depend on the position. these p e r f e c t l y elastic fields are insufficient models f o r the description of m a n y physical phenomena because t h e y do not allow to explain the dissipation of energy. a more complete description of the actual elastic fields is obtained b y introducing stress-strain relations which include also linear combinations of t i m e derivatives of the strain and the stress. t h e numerical coefficients appearing in the general linear combinations of higher order derivatives are called viscoelastic constants of higher order. elastic fields described b y elastic constants of higher order h a v e been discussed b y m a n y authors, [e.g. see k n o p o f f , 1954; capnto, 1966]. k n o p o f f studied the case in which the stress strain relations are of the t y p e (l"l tr„ = i ghi grs e„i+ '¿¡x ers + — [),m ghi g„ ehi + 2/um ers) [1] where xm (hid. flm are constant, he obtained a condition f o r these viscoelastic constants of higher order analogous to those existing f o r the l i n e a r models of d i s s i p a t i o n ' wii.lsl <3 is a l m o s t , etc. 3 8 5 perfectly elastic fields and also proved that in order to have a dissipative elastic field the stress-strain relations should contain time derivatives of odd order. a generalization of the relation [1] is p dm trs = ¿jm -j— 0 ' « g'" ors chi + 2flm crs] [2] w e can generalize [2] to the case when the operation j ^ is perforwhere one can also consider /,„ and ¡xm functions of position. dm it™ rncd with to as a real number 2 (see appendix) and also further b y substituting the summation with an integral as follows bi b2 f dz f dz tr. = i /i (r, z) --g»t (jrb ehi dz + 2 / /2 (r, z) — e„ dz. [3] «1 a 2 ¡j, is the radial coordinate in a spherical coordinate system. relations [1] and [2] are a special case of [3] — they are obtained b y setting p f 1 (»', «0 = <5 (« w) lm ? p /2 (r, z) = 2 " ! <5 (z — m) ¡xm i [4] where d(z—-in) are unitary delta functions. if at = q = p = 0, then we have the case of a perfectly elastic field: if q = p = 1 then we have a perfectly viscous field; if q = at — 0 and p = 1 then we have a viscoclastic field. dissipation in a plane wave in the simple case of a plane wave, assuming / = rjd{z — z0), the stress-strain relation [3] gives the following equation of motion . a 2 w s 2 « az° a 2 « p + a b v = 0 • [5] b y taking the laplace transform of [5] w e have 32 u a 2 u 386 m. c a p u t o and the nature of the motion depends on the roots of the following equation rj a2 p'o + q p* -f fj, a 2 = 0 . [6] the approximate solution of [6], neglecting the term in rj, which we assume to be small with respect to /u, is p2 = _ e the solution which takes into account the dissipation is \p°\zo a2 / 7t . . n \) p = x | p. i 1 + j ^ l ^ (cos s„ + » sin * . ) j [7] and the specific dissipation is s i n ^ , 0 . [8] solution of the equations of motion in spherical coordinates w e shall follow the method described in caputo [1966]; the operator oi introduced in that paper is ii oi = vil (r) — ¡9] here, according to the definition [3] of the stress-strain relation, these operators will be h il 0 1 = l f l { r ' z ) y p d z + 2 h r 02 = ft(r,g) — dz. [ 1 0 ] one can see that the method of solving the equations of equilibrium resulting f r o m the definition [1] of the stress-strain relation (see caputo 1966) can be applied also to the case when the estress-strain relation is [3]. l i n e a r models of d i s s i p a t i o n ' wii.lsl <3 is almost, etc. 387 t h e l a p l a c e transform 8 (<sv, s^) of the displacement vector s(sr, sy) — where r, q> are spherical coordinates, d colatitude, <p longitude —• is: sr sa -^i 1 »i à t s » s * + [11] s œ = s » sfc i ï s r « sin & ~ò <p r, where y„ are spherical harmonics n + 4 71 / j>>=< 2 w + l (n—k)\ \^ 2 jt ( « + -e) 2w + l ( 2 n — k ) ^ 1 ' 2 if 7c = 0 if7c = 1 , 2 , . . .,n [12] p w ( c o s ) ? ) c o s / f y p(*-»)(cosi?) cos ( 7 c — 9 ? , if7c = n + 1 , . . . , 2w 27t and jrj.n, b2,n, r3,n are solutions of the system a. dr 0,v + 2 0 . 4 y . ' ^ i . i i r m , a r j r r2 d r n d r , dpo = , d(p—po) , d ( dp„\ i 1 o x v — 0 dr1 d2 {rrs) dr dr1 + 02r r, d£2/r /• dr [13] d r o d ( p — p „ ) 0 2 dr 1 d2 r r2 n(n +1) ìtiqg v + 7 , — — i j r t f g ^ = 0 r dr2 v = r2 n(n-\-1) r r r drjr + 02r dr + 03r3 = 0 w i t h r2 dr v ' 2tt tc 00 -p0=j i j"(r—vo)e-ptsmêd(pâêdt. o o 0 388 m. c a p u t o v — vu is the perturbation of the gravitational potential arising f r o m the perturbation of the density field and f r o m the attraction of the density perturbation at the deformed interfaces. w e assume also that the integration in [3] can be interchanged with the integration of the laplace transformation and also that we can use formula [38] (see appendix at the end of the paper). then e.g. 0 , j o2 si e-*1 dt = i pz /2 (>•, z) dz j s, e~"1 dt . [14] 32 dissipation in the torsional oscillation of a shell w e shall discuss some models of dissipation, obtained b y specializing the functions fi {r, z) appearing in [3], in the free purely torsional and radial vibrations of a shell. the equation which governs the motions of the torsional modes in a non perfectly elastic shell of radiae and r2, assuming a stress-strain relation of the type [3], is: fi [r, z) dp 1 i>2 rs 1 3 ( 1 ì> s sin i r 5 r 2 r i •& \sin# j) •& dz [15] ì) r fz (r, z) d* d p 3 (s 5 r \ v dz = q y-s m 2 w e assume an earth model defined b y a liquid core and a homogeneous mantle, and assume also that the dissipation of energy due to the viscous interaction between the core and the mantle, is negligible i [caputo, 1966] and that — /2 ('", z) = 0, then the boundary condition is 3 r f n (r2) fn (n) f-n (r2) f-n (>-,) = 0 [16] f (r) = r-3!2 j (ra) ± » d r ± » + l/s l i n e a r models of dissipation' wii.lsl <3 is almost, etc. 389 where s, = j an jn+y2 (a r) + (— 1 ) » « a-n j-n-y2 (clv) j ^ [17] a2 i-2 = q-1 i /a («) dz is a solution of the laplace transform of [15]. j_„_i/a ( a » j and jn+i/2 ( a r ) are bessel functions, and p „ (cos # ) are legendre polinomials. the solutions of [16] determine the periods of free oscillation and also the q. without loss of generality [10] can be written f2 ( * ) = / * + fj,(e)p*de. a n interesting case arises when u (z) =fhà {z — z„ + e) equation [16] is then fj, + zo-e p* which gives p = i i p01 j1 + ^ pzos if z0 = 2 to (m integer) then we have |va p» i = « 1 'il q p = i and if to = 0 r /w 7r e . . n e cos — — t sm — 2 2 ll/2 jr e . . n e cos — — t sin — • 2 2 the specific dissipation function is therefore q-1 = ~ i y » r . 31 s i n e [18] [19] [20] [21] [22] [23] [24] 390 m. c a p u t o dissipation in the purely radial modes t h e study of the dissipation of energy in the purely radial modes, can be done as that of the torsional modes. t h e only difference is that in this case the forces which g o v e r n the periods of the modes are the elastic forces and also the gravitational forces. assuming a homo3/, geneous earth model of density p„ and — = 0 one can see that the i)r periods of the free modes and the dissipation are given b y ¡>i [25] q° >\ ( v 2 — 4 j.) = — x2 i fi (z) p'dz . 4 71 g qo a = ——-— and ® is a solution of tan x x 4 p. w i t h o u t loss of generality f1 can be w r i t t e n h u = x + j j1(z)rdz. a n interesting case arisen when fi («) = kà (z — zo + e) equation [25] is then [26] [27] [28] p° = 4a — ~ (à + 2/j pz°~e)= — |p.|«. = -\po\22 xa po = a + 2/i). * 2 g r o (a+ 2/1) — 4a erl p»' zo—e a, 4aà, + a + 2/j, r a + 2/i po zo-e [29] l i n e a r m o d e l s of d i s s i p a t i o n ' wii.lsl <3 is a l m o s t , etc. 3 9 1 i t s solutions are v = ± » i vo i 1 + a + 2 / ^ 0 (a+2//)ipo|21 which becomes for = 0 » + i i „ i j1 + — 1 — » 4 a a i p ° ~ £ 1 p — i i p0 i > -l -t. p0 > f x + 2/i (a + 2 f l ) \ pa i2} equation [31] can also be written p = ± < | p . | i l ' 2 ( a + 2 p ) and the specific dissipation is £a i 71 . . 71 cos £ — i sin — e q-1 = ¿i i v° . 71 s m e a + 2 ( i the ratio of the observed q's gives e «¡! (\ v q:1' l — 4 1 i v» i2 pox ly i poi i xx is therefore obtained from [34] b y substitution (a + 2 | ffo |e 0 sin e 7i [30] [31] [32] [33] [34] [35] gheclcs with the observations a v e r y extensive analysis of the attenuation of the rayleigh waves has been made b y ben-menahem (1964) who measured it from four great earthquakes f r o m observation of multiple circuits around the earth past one station. b y fourier analysis he obtained the specific attenuation factor in the period range between 300 and 40 seconds; f r o m the attenuation factor he computed then the specific dissipation. the discrete spectrum of the spheroidal oscillations of the earth, where the matter is both compressed and sheared, approaches the continuous rayleigh waves spectrum. i n order to extend the range where the q is known, we computed it also from the free spheroidal oscillations 392 m. c a p u t o of tlic earth of period between 5 and 18 minutes, using the 110 horns record f o the 1961 chilean quake recorded at u c l a . the record was subdivided in several intervals whose initial points where 4.5 and 13.5 hours apart; from the power spectral analysis of these intervals we obtained the decrease in energy and subsequently the q's b y means of formula [24] of caputo (1966). p r o m a preliminary analysis of these q's we can see that the agreement with the mathematical model of dissipation proposed in this paper, with e = — 0.15, is satisfactory. another check of this theory can be made using the q's obtained by zemaneek and rudnik (1961) for longitudinal waves in the period range between 103 and 10"s seconds. here again the agreement of the observed q's with the model of dissipation proposed in the present paper, with e — — 0..15, is satisfactory. w e plan to complete and publish soon the discussion of the above mentioned experimental results. appendix a generalization of the operation of differentiation with real order of differentiation, for a wide class of analytic functions, can be made as follows. l e t d* , u tx~" i i x > o ) o < z < l — ( e f ) = r(x + l—z) [36] ( o if ic = o r i n t e s e r i t f(t) is an analytic function, the operator [36] can be applied to the terms of its power series expansion; the, resulting series, if convergent, can be assumed to be the z order derivative of / (t). w e shall need to evaluate the laplace transform of derivatives of order z of a class of analytic functions. l e t the power series expansion of f(t), convergent in the interval (0, oo), be / it) = si aip [37] at = 0 if i < 0 and, diffei'entiating, d*iit) r ( i +1) . = e i a t i ^ t i ^ [ 3 8 ] l i n e a r m o d e l s of d i s s i p a t i o n ' wii.lsl <3 is a l m o s t , etc. 3 9 3 w e w a n t to p r o v e that f o r 0 < z < 1 ^ d 1 = v i f (t) a t. [38] t o obtain [38] w e shall assume that the l a p l a c e transform of a* f (t) d p exists, than w e h a v e dp i ^ f (i + 1 — z) o 0 and, assuming that it is possible to interchange the sum w i t h the integral 00 = p> s i at r (i +1) = ai i p e-rt dt = 0 so oo = pz j 2 «'• v e ~ f d t = pz j f (t) e-pt dt o n which proves [38]. r e f e r e n c e s ben-menahem a . , attenuation of seismic surface waves in the upper mantle 1964. caputo m., estimates of anelastic dissipation in the earth's torsional modes. " annali (li geofisica 1, 75-94, (1966). knopoff l., on the dissipative viscoelastic constants of higer order. " j. acoustic soc. a m . " , 26, 183-186, 1954. zemanek j. jr., rudnick i., attenuation and dispersion of elastic waves in a cylindrical bar. j. acoust, soc. am. " , 33, 1283-1288, (1961). l ' e r u z i o n e d e l l ' e t n a d e l f e b b r a i o m a r z o 1 9 1 7 gaetano p o n t e ti/nchr eruzioni dell lina. lo e r u z i o n i d e l l ' e t n a sono e f f u s i v e ed e s p l o s i v e ad un t e m p o p e r ò q u a l c h e volta s o n o s o l t a n t o e s p l o s i v e e in tal c a s o a v v e n g o n o q u a n d o il m a g m a , s o l l e v a n d o s i nel c o n d o t t o e r u t t i v o , r a g g i u n g e il f o n d o del c r a t e r e c e n t r a l e e v i e n e a c o n t a t t o con il t e r r e n o u m i d o : -i lui a l l o r a q u e l l a r e a z i o n e f r e a t i c a v i o l e n t a , c l i c va sotto il n o m e «li a t t i v i t à p l i n i a n a . ali a t t i v i t à p l i n i a n a può s e g u i r e un p e r i o d o pi il o m e n o lungo di e f f u s i o n e l a v i c a i n l e r c r a t e r i c a , c o m e a v v e n n e il f> lug l i o 1942 d o p o la b r e v i s s i m a e r u z i o n e l a t e r a l e del .'50 g i u g n o . a l l o r a la terrazza c r a t e r i c a c e n t r a l e fu invada da una c o n s i d e r e v o l e massa (li m a g m a c l i c sgorgò c o m e una i m m e n s a polla e fini col l a p p a r e la b o c c a i n t e r c r a t e r i e a da cui e r u p p e . nel 1911 a v v e n n e il c o l l a s s o di una p a r t e del lato ine del c o n o t e r m i n a l e e si f o r m ò una b o c c a elle fu d e t t a s u b t e r m i n a l e e e l l e , trovandosi ad un l i v e l l o più ba—o d e l l a t e r r a z z a c r a t e r i c a c e n t r a l e , e s t a t a , -in d a l l a sua f o r m a z i o n e , la più attiva e le sue e r u z i o n i , e s p l o sive o e f f u s i v e i n s i e m e , sono s t a t e i n d i c a t e c o m e s u b t e r m i n a l i onde d i s t i n g u e r l e d a l l e c e n t r a l i . l e e r u z i o n i più e a r a l l e r i s l i c l i e d e l l ' e t n a sono le l a t e r a l i : esse h a n n o un m e c c a n i s m o p a r t i c o l a r e c h e d i p e n d e d a l l a s t r u t l u ' a del vulc a n o la cui c o m p a g i n e e f o r m a t a di c o l a l e di lava p e r i c l i n a l i irregol a r m e n t e s o v r a p p o s t e ed a f f i a n c a t e . p o i c h é le c o l a l e p r e s e n t a n o d e l l e g a l l e r i e di s v u o t a m e n t o l a v i c o e d e l l e a n f r a t t u o s i t a il m a g m a trova in esso d e l l e faci l i vie di p e n e t r a z i o n e a p a r t i r e dal c o n d o t t o e r u t t i v o c e n t r a l e . le r e a z i o n i f r e a t i c h e c h e h a n n o luogo q u a n d o il manina r o v e n t e va p e n e t r a n d o nel t e r r e n o più o m e n o u m i d o d e t e r m i n a n o s c u o t i m e n t i e s p a c c a t u r e a p a r t i r e dal c u n e o igneo fino a l l a s u p e r f i c i c ; esse -i e s t e n d o n o spesso p e r c h i l o m e t r i sui fianchi del v u l c a n o e la lava v i e n e a s g o r g a r e nel posto ove la s p a c c a t u r a è m e n o p r o f o n d a . n e l l a d e s c r i z i o n e c h e f a r e m o d e l l a r e c e n t e e r u z i o n e a p p a r i r à c h i a r o q u e s t o i n t e r e s s a n t e m e c c a n i s m o d e l l e e r u z i o n i l a t e r a l i . i . ' e i u z i o m : o e i j . ' e t . n a b e i . i e h i u i a i o m a h z o 1 9 4 7 l ' e t n a ha p u r o avu to d e l l e e r u z i o n i e c c e n t r i c h e , m a esse avvenn e r o in t e m p o as.-ai r e m o t o , nel p o s i p l i o c e n e . q u a n d o la vasta s u p e r ficie o r a o c c u p a l a dal v u l c a n o era s o m m e r s a nel m a r e . a l l o r a qua e là nell a m p i o grullo p r e e t n e o a v v e n n e r o d e l l e p e n e t r a z i o n i di mailm a , a l c u n e d e l l e q u a l i , d a p p r i m a s u b m a r i n e , s u p e r a r o n o il l i v e l l o del m a r e e r u t t a n d o lave e d e t r i t i , c o m e il v u l c a n e l l o di p a t e r n o c l i c si e s t i n s e ben p r e s t o ed o r a , s m a n t e l l a l o d a l l ' e r o s i o n e , m o s t r a il s i l o n e c k . a l t r e i n t r u s i o n i r a g g i u n s e r o a p p e n a il l'ondale del g o l f o e cons o l i d a r o n o in m a s s e g l o b u l a r i con c r o s t a v e t r o s a , c o m e q u e l l e d e l l a r u p e di aci c a s t e l l o : a l t r e a n c o r a r i m a s e r o nascoste fra le m a r n e e le a r g i l l e p l e i s t o c e n i c h e c h e o r a , d e n u d a t e d a l l ' a b r a s i o n e del m a r e , a p p a i o n o sulla costa di aci t r e z z a c o m e dei p o l l o n i , d e l t i i « f a r a glioni dei c i c l o p i ». molti f u r o n o i t e n t a t i v i di e r u z i o n i eccelli l i d i e r i s p e t t o a l l ' a s s e a t t u a l e d e l i ' k l n a . i n s e g u i l o a q u e s t e p r i m e m a n i i c s t a z i o n i . m o l l e d e l l e (piali r e s t a n o n a s c o s t e sotto la massa del vulc a n o , a v v e n n e u n ' a c c e n t r a m e n t o d e l l ' a t t i v i t à e r u t t i v a p e r cui si p o t u t o f o r m a r e il g r a n d e v u l c a n o . le eruzioni ili un secolo. le e r u z i o n i dell l i n a m e g l i o o s s e r v a l e e d e s c r i t t e sono q u e l l o l a t e r a l i e s o p r a t t u t t o q u e l l e di qucsl u l t i m o s e c o l o . p o c h e e r u z i o n i s t o r i c h e dei secoli p r e c e d e n t i sono -tato d e s c r i t t e c m a n o a m a n o le più a u l i c h e d i m i n u i s c o n o di n u m e r o , non p e r c h è l ' r t n a a b b i a avuto p e r i o d i di m i n o r e a t t i v i t à nel p a s s a l o , ma solo p e r c h è la c r o n a c a ha r e g i s t r a l o q u e l l e di m a g g i o r e i m p o r t a n z a c s o p r a t t u t t o q u e l l e c h e h a n n o a r r e c a l o i m a g g i o r i d a n n i . l e eruzioni dell u l t i m o s e c o l o , c i o è a p a r t i r e d a l l a ci uy.ione del 1813, sono s i a l e 16 e sono a v v e n u t e a d i s i a n z a v a r i a di a n n i : 9 . 13, 1. io. i. 3 . 16, 2 . ] . 7 . .">, 11 e •> da q u e l l a dei 1942 a q u e s t a del 1947. la ( p i a n t i l a di lava e r u t t a l a non è stata m a i u g u a l e , c o m e non lo è s t a l a la d u r a l a . i n o l t r e d o p o una g r a n d e e r u z i o n e ne è seg u i t a , a n c h e a b r e v e d i s t a n z a , ' u n ' a l t r a più g r a n d e : cosi ali e r u z i o n e del i88(>, c l i c fu i m p o n e n t e per la ( p i a n t i l a di lava e r u t t a t a in i . g i o r n i , segui q u e l l a del 1892 d u r a l a sci m e s i . così p u r e ad e r u z i o n i non m o l l o grandi por q u a n t i t à di lava e r u l l a l a e p e r d u r a l a , sono, q u a l c h e v o l t a , s e g u i l e p i c c o l e e r u z i o n i , c o m e q u e l l a del 1942. d u r a l a 1:2 o r e . c h e s c o p p i ò i i a n n i d o p o q u e l l a del 1928 c l i c svolse in 2 4 g i o r n i e d i e d e in due t e m p i od in duo posti m i n o r lava d e l l e e r u z i o n i del 1911 e del 1923. i n o l i l e non è a v v e n u t o s e m p r e c h e criir . a i . i w o c o m i ; /.ioni c l i c h a n n o a v u t o h o c c l i c e f f u s i v e a b a s s a q u o t a e c h e sono t a l e pili c a l a m i t o s e p e r c h è p i ù v i c i n e ai c e n i l i a b i t a l i , a b b i a n o d a t o semi n e m a g g i o r q u a n t i t à di lava di q u e l l a s g o r g a l a a q u o t a piìi a l l a . p e r t a n t o p e n s i a m o c h e la non c o s t a n t e q u a n t i t à di lava e r u t t a t a , -ia r i s p e t t o a l l a d u r a l a c h e ali i n t e r v a l l o tra una e r u z i o n e e l ' a l t r a , m o -tri c o m e le forze e n d o g e n e , c h e d e t e r m i n a n o il s o l l e v a m e n t o del m a g m a dal p r o f o n d o c r o g i u o l o v u l c a n i c o , non d i p e n d a n o da fenomeni c h e a g i s c o n o con lilla c e r t a u n i f o r m i t à il i n t e n s i t à e con r i c o r renza di t e m p o . fenomeni che precedettero l'eruzione laterale del febbraio-marzo '17. d o p o la b r e v e e r u z i o n e l a t e r a l e del 3 0 g i u g n o 19-12 alla q u a l e e g u i . il .") l u g l i o , una v i o l e n t a a t t i v i t à p l i n i a n a c e n t r a l e , il ('.calere t e r m i n a l e r i m a s e l a p p a t o da una p o l e n t e c o r a z z a lavica c h e ne ridusse la p r o f o n d i t à a p o c h e d e c i n e di m e t r i e f o r m ò una p i a n e g g i a n t e t e r r a z z a , sulla (piale ora è f a c i l e p o t e r v i s c e n d e r e . d ' a l l o r a a t u t t o ii 194.") il c r a t e r e c e n t r a l e r i m a s e c a l m o ; s o l t a n t o f u r o n o s e m p r e ailive le f u m a r o l e s o l f o r o s e c h e da m o l l i anni a p p a i o n o sulle n e pareli i n t e r n e . i n v e c e la b o c c a s u b t c r m i n a l e . a p e r t a s i nel 191 i -ni dauco \ e e elle t r o v a i più bassa d e l l a terrazza c r a t e r i c a c e n t r a l e , è s t a l a s e d e di f r e q u e n t e a t t i v i t à e s p l o s i v a p i u t t o s t o m o d e r a l a con rade s b u f f a t e (li c e n e r e c h e d i n v e r n o h a n n o i n t o r b i d a t o la neve di quel v e r s a n t e del v u l c a n o . n e l l ' o t t o b r e del 191."i. nel e l t o r c nk d e l l a terrazza c r a t e r i c a c e n t r a l e si f o r m ò una b u c a di -pi o l o n d a i n c n l o del d i a m e t r o di c i r c a 1.1 m c h e non s e m p r e fu t r o v a l a l u m a n l e : c s a aveva p a r e l i v e l l i c a l i , quasi inori p i o m b o , per cui le f r a n e l ' a l l a r g a v a n o s e m p r e di p i ù . nel g e n n a i o 19-17 la b o c c a s u b l c r m i n a l e a c c e n t u ò la sua a t t i v i t à e s p l o s i v a , m e n t r e la b u c a d e l l a t e r r a z z a c r a t e r i c a c e n t r a l e , c l i c si era a n c o r a più a l l a r g a l a ed a p p r o f o n d i l a , c o n t i n u ò a r i m a n e r e c a l m a ( b i g . d. n e l l e p r i m e o r e d e l 6 f e b b r a i o a p p a r v e sulla b o c c a s u b l e r m i n a l c una f o n t a n a (li lava d a l l a q u a l e si s t a c c a v a n o d e l l e c o l i e l e g g e r e c h e , spirile dal v e n t o , g i u n s e r o (ino al m a r e e ne c a d d e r o -ni v e r s a n t e o i i e n t a l c del v u l c a n o in d i r e z i o n e di c i a n e , ove in un t r a t t o di un p a i o di c h i l o m e t r i , fino a s . l e o n a r d c l l o . a r r i v a r o n o quasi a c o p r i r e il s u o l o : esse e r a n o p i u l l o s l o p i a l l e , b o l l o s e ed a l c u n e l a r g h e fino a 8 c u i . il g i o r n o 1(1 a l l e o r e 2(1 il f e n o m e n o si r i p e t è più i n t e n s a m e n t e i / e i i u z i o n i : d e i . i . ' e t . n v d e i . i k i : ; » i ì a i o m a h z o 1 9 4 7 f i g . 1 e d u r ò c i r c a d u e o r e . m o l l o m a l c r i a l e lavico ili r i v e r s a l o a f o n t a n a e n e c a d d e t a n t o n e l l a b o c c a s u b l e r m i n a l e . da dove s c a t u r ì , da colm a r l a e c h i u d e r l a . k v i d e n t e m e n l e il m a g m a era s l a t o s o l l e v a t o dal -no c r o g i u o l o da f o r t e p i n t a fino a l l a p a r i e a l t a del c o n d o t t o eruttivo o v e , s e m b r a , trovasi u n a b r e v e d i r a m a z i o n e v e r s o la b o c c a u b t e r m i n a l e , d i f a l t i , i m m e d i a t a m e n t e d o p o la t a p p a t u r a di q u e s t a b o c c a , il m a g m a c o n t i n u ò a n c o r a a s o l l e v a r s i in d i l e z i o n e d e l c r a t e r e c e n t r a l e ove e n t r ò in a t t i v i t à la b u c a di s p r o f o n d a m e n t o . s ' i n i z i ò così un i n t e n s o p a r o s s i s m o e s p l o s i v o di t i p o s t r o m b o l i a n o , c i o è con forti d e t o n a z i o n i c h e fu ron o a v v e r t i t e a n c h e da c a t a n i a e con b r a n d e l l a m e n l i di lava c h e g i u n g e v a n o a n o t e v o l e altezza -ul c r a t e r e c e n t r a l e . dato c h e la b u c a d e l l a terrazza c r a t e r i c a c e n t r a l e -i trovava un c e n t i n a i o di m e t r i più a l t a d e l l a b o c c a s u b l e r m i n a l e . a p p a r e c h i a r o c l i c la s p i n t a e s e r c i t a l a ~u! m a g m a d a l l e forze e n d o g e n e non fo-se più tale da f o r m a r e getti a fontana a q u e l l a m a g g i o r e a l t e z z a e solo avv e n i v a n o e s p l o s i o n i d e t o n a n t i , d o v u t e ali a c c e n s i o n e di m i s c e l e di gas c o m b u s t i b i l i con l ' a r i a n e l l e c a v i t à s u b c r a t e r i c b e . così ai d u e getti di lava a f o n t a n a senza d e t o n a z i o n i , a v v e n u t i il 6 e il io f e b b r a i o a l l a b o c c a s u b l e r m i n a l e . seguì u n ' i n t e n s a a t t i v i t à e s p l o s i v a c e n t r a l e , d a n d o la e v i d e n t e d i m o s t r a z i o n e c l i c il m a g m a non fu p i n t o da p r e s s i o n e di g a , ma da a l t r e forze. d i f a t t i l e e s p l o s i o n i 68 g a e t a n o p o n t e c o m i n c i a r o n o q u a n d o il m a g m a , v e n e n d o a c o n t a n o con le r o c c e non c o e v e ed u m i d e d e l l e c a v i t à s u b c r a t e r i c h e d e l l a b u c a c e n t r a l e , dava luogo a l l a l'orinazione di gac o m b u s t i b i l i c h e a c c e n d e v a n o con detonazioni di diverga i n t e n s i t à a s e c o n d a il v a r i a r e d e l l a m i s c e l a <on l ' a r i a . p o c h i giorni d o p o si n o t ò da c a t a n i a l ' i n i z i o di u n a d e b o l e attivila esplosiva al l a base d e l l a b o c c a s u b l e r m i n a l e . il g i o r n o 2,~> febb r a i o il d o t t . cucuzza s i l v e s t r i , a s s i s t e n t e d e l l ' i s t i t u t o di v u l c a n o l o g i a , trovandosi s u l l ' o r l o del c r a t e r e c e n t r a l e c o n s t a t ò c h e le esplosioni d e l l a b u c a c e n t r a l e e r a n o n o t e v o l m e n t e d i m i n u i t e , m e n t r e appariva più f r e q u e n t e l ' a t t i v i t à esplosiva a l l a b a s e d e l l a b o c c a subt e r m i n a l e ove il g i o r n o d o p o s ' i n i z i ò l ' e r u z i o n e l a t e r a l e . sjmccuturu eruttiva ed inizio ilei!'eruzione laterale. c a t t i v i t à e s p l o s i v a d e l l a b u c a c e n t r a l e , d a p p r i m a f o r m i d a b i l e , a n d ò d e c r e s c e n d o a p p e n a i n c o m i n c i ò ad a t t i v a r s i l ' a p e r t u r a c h e -i era f o r m a t a in s e g u i t o agli s c u o t i m e n t i «opra d e t t i «otto il t a p p o >li lava c h e a v e v a , il io f e b b r a i o , c h i u s o la b o c c a s u b l e r m i n a l e . d a p p r i m a sgorgò d e l l a lava verso q u o t a 30ó0 e l'ormò d u e c o l a l e effimere e ^ • ' i / e i u z i o n e d e l l ' e t n a d e i . f e b b r a i o m a r z o 1 9 4 7 c l i c si a r r e s t a r o n o a p p e n a il m a g m a p o t è c o n t i n u a r e il i l o c o r s o s o t t e r r a n e o d i s c e n d e n t e i r a g l i s t r a l i d i r o c c i a l a v i c a e l e a i i f r a l l u o i l à d e i v e r s a n t e n o r d d e l v u l c a n o ( f i g . 2 ) . l a l a v a , m a n o a m a n o c l i c s i n c a n a l a v a i l a a u l i c h e c o l a l e n o n m o l l o p r o f o n d e e p i ù o m e n o u m i d e , c o n l a s u a e l e v a l a i c m p e r a t i u a . p r o v o c a v a e s p l o s i o n i ' f r e a t i c h e e d i c o n s e g u e n z a s p o s t a m e n t i d e l t e r r e n o c o s i d a f o r m a r e u n a s e r i e d i s p a c c a t u r e p a r a l l e l e e v i c i n e ; e s s e r a p i d a m e n t e s i e s t e s e r o i n d i r e z i o n e dei d u e l izzi e più o l t r e , s e m p r e sul fianco nord del v u l c a n o l i n o a « p i o t a 2 2 0 0 , a s u d d e l m . c a c c i a t o r e . i v i ~i a p r ì l a b o c c a e f f u s i v a d a c u i e r u p p e ii t o r r e n t e d i l a v a p r i n c i p a l e , c h e r a p i d a m e n t e si d i r e s s e v e r s o il s o t t o s t a n t e b o s c o c o p e r t o d i n e v e . a b b i a m o d e t t o c h e la s p a c c a t u r a i n c o r n i l i ! i«) ad a p r i r s i a l l a lia-c d e l l a b o c c a s u b l e r m i n a l e , ove t r a b o c c ò la p r i m a lava e c o n t i n u ò ad «'stendersi v c r o n o r d . nel p r i m o t r a t t o , p e r più di un c h i l o m e t r o fin p r e s s o le b o c c h e s a u s i i r r e . n o n si polo s c o r g e r e c h e una p i c c o l a inc r i n a t u r a l i l l a n o v e e l l e p<)i s c o m p a r v e con la n e v i c a l a d o l i " 8 m a r z o , m a in g i u g n o , d i s o i o l l a s i la n o v e , a p p a r v e e v i d e n t e una s e r i e di s p a c c a t u r e p a r a l l e l e e i r r e g o l a r i l a r g h e da p o c h i c e n t i m e t r i a d u e n i e l l i . v i posti più c a b l i la lieve si d i s e i o l s e u b i l o od a p p a r i v a il c r e p a c c i o , p r o f o n d o p a r e c c h i m e t r i , c o p e r t o p e r lo più ili t e r r a f r a n a l a . l e s p a c c a t u r e si e s t e n d e v a n o a n c o r a in d i r e z i o n e n o r d e dal al. g r i g i o in giù e r a n o c a l d i s s i m e , t a n t o c h e la novo a t t o r n o si e r a d i s c i o l t a por l a r g o i r a l t o . o n e s t o , e v i d e n t e m e n t e , e r a il t r a t t o d e l l a s p a c c a t u r a più -up c r f i e i a l e , t a l l i o c h e il g i o r n o 2 8 f e b b r a i o , in e g u i l o ad un r i g u r g i t o d u r a l o a p p e n a una n o t t e , t r a b o c c ò «iella lava e si f o r m a r o n o dei c o n c i t i m a m m i l l a r i d i p o s l i a b o t t o n i e r a ( f i g . 3 ) . a l t r i c o n c i l i di m a g g i o r e d i m e n s i o n e si l ' o r m a r o n o noli u l t i m o h a l l o «iella s p a c c a t u r a nei p r e s s i d e l l a b o c c a e f f u i v a pur o--i d i s p o s t i a b o t t o n i e r a . le borrite effusive e le colate laviche. l e r u z i o n e l a t e r a l e s i n i z i ò c o n d u e c o i a l i n e e f f i m e r e s u b l c r i n i n a l ì d i b r e v i s s i m a d u r a l a c h e s g o r g a r o n o d a d u o s p a c c a t u r e p o c h i m e t r i v i c i n e e c o r s e r o v e r s o n o r d s u l l a p a r t e a l l a d e l v u l c a n o . l a c o l a l i n a p i l i p i c c o l a e r a l u n g a c i r c a .uhi m . i a l t r a , « p i a s i p a r a l l e l a , e r a p i ù l a r g a e l u n g a c i r c a u n c h i l o m e t r o . l e d u e c o l a t e s p i c c a v a n o s u l l a n o v o s u l l a « p i a l e si d i s t e s e r o s e n z a « l i s e i o g l i o r l a . s u l l a f o t o g r a f i ; a e r e a l o d u e c o l a l e a p p a i o n o u n a c o m e il s e g n o d imi a c c e n t o e l ' a l l r ; c o m e u n a v i r g o l a . d a l a l a l a p i d i l a e l a i r a n i p i i l l i l à d o l l ' c l l l i i s o l a v i i •«>, i n « p i c i p o s t o n o n i f o r m a r o n o c o n c i l i e s p l o s i v i , m a t u t t o r a v i 68 g a e t a n o p o n t e a p p a i o n o ben dodici b o c c i l e i m b u l i i o r m ì nel cui l o n d o -i s c o r g e c h i a r a m e n t e il c a n a l e ove s c o r r e v a la lava c l i c , p o i , c o n t i n u ò il n o c o r s o -(•i t e r r a n e a m e n t e . t r a il m . g r i g i o ed i due pizzi si f o r m ò una l i e v e s b a v a t u r a di lava nel p r i m o i m p e l o d e l l a s p i n t a del c u n e o igneo di m a g m a c l i c a n d a va. a p r e n d o la s p a c c a t u r a in senso d i s c e n d e n t e ; 7 0 0 111 più giù dei due l i il. .'!. — e t n a , versante nord. in a l l o : colale effimere sublerminali !21 febbraio i : al e e n l r o : p a r r a i m a e rigurgiti '.21. 20 e 2ì! f e b b r a i o ) : a sini.-tra in b a s s o : bocca effusiva e colala p r i n c i p a l e . p i z z i , verso q u o t a 2311). t r a b o c c ò a l t r a lava c l i c si d i s t e s e per c i r c a 5 0 0 n i ; essa era larga p o c o m e n o di 5 0 m e si f e r m ò s u b i t o a p p e n a la s p a c c a t u r a c o n t i n u ò ad e s t e n d e r s i . a n c o r a 2 0 0 m più giìi si verificò a l t r a s b a v a t u r a e c i r c a mezzo c h i l o m e t r o a v a n t i , a (piota 2 3 0 0 . -i e b b e un q u i n t o p i c c o l o t r a b o c c o di lava c l i c f o r m ò una c o i a l i n a t (ti m e r a , lunga a p p e n a 100 i n , c h e non iti a c c o m p a g n a t a da e s p l o s i o n i , t a n t o c h e non -i f o r m a r o n o , c o m e n e l l e p r e c e d e n t i , c o n c i t i d e t r i t i c i . l a c o l a t a p r i n c i p a l e i n c o m i n c i ò a s g o r g a r e dal posto ove -i arr e s t ò la s p a c c a t u r a , c i o è v e r s o q u o t a 2 2 0 0 , a sud di i v i . c a c c i a t o r e , c i r c a un c h i l o m e t r o più giù d e l l a p r e c e d e n t e c o l a t i m i e f f i m e r a . la b o c c a effusiva d e l l a c o l a t a p r i n c i p a l e -i p r e s e n t a v a c o m e un e a n a l e . al p r i m o i m p e t o la lava g o r g o a b b o n d a n t e più a m o n t e pei i/uiu z i o m : ]ji:i.i,'[;inv ma. i ' e i u ì k a i o m a h z o ] < ) i 7 1111 i r a l l o di c i r c a 10(1 m e s p a n d e n d o s i da a m b o i lati della spaecal u r a ; poi r e s t r i n s e per più di m e t à il n o l e l l o e p r o c e d e t t e i r r e g o l a r m e n t e s e g u e n d o le m a g g i o r i d e p r e s s i o n i del t e r r e n o c con veloci là v a r i a . il p e r c o r s o t o t a l e d e l l a c o l a l a fu di c i r c a 6 km in 8 g i o r n i -ti di un d i s l i v e l l o di 1350 in. -\ncbe la l a r g h e z z a e la p o t e n z a v a r i a r o n o m o l l o da posto a p o s t o , ma ali i n g r o s o fu f a c i l e r i c o n o s c e r e c h e la -uà p o r t a t a fu d i gran lunga i n f e r i o r e a ([nella d e l l ' e r u z i o n e del 1 9 2 3 . c l i c si svolse n e l l o stesso v e r s a n t e e, in li) o r e . p e r c o r s e 7 k m sii t e r r e n o con la t c s a p e n d e n z a : c i o è in un t e m p o c i r c a 2 0 v o l t e min o r e di q u e s t a e r u z i o n e . 1 t e r r e n i c o p e r t i d a l l a c o l a l a e r a n o a r i d i sulla p a r t e a l l a ed i n c o m i n c i a r o n o ad a v e r i d a n n i ira q u o t a 1201) e 8 5 0 ove p r o s p e r a v a n o ubertosi v i g n e t i . sihi-cssioiic dei fenomeni eruttili. ( .oinc a b b i a m o d e l l o i e r u z i o n e l a t e r a l e e b b e i n i z i o . 011 l ' a p e r tura di una s p a c c a t u r a elle d a l l a b a s e d e l l a b o c c a s u b l e r m i n a l e . c i o è da q u o i a 3 0 5 0 c i r c a , si e s l e s e r a p i d a m e n t e , in p o c h e o r e , fino a (piota 2 2 0 0 per una e s t e n s i o n e c o m p l e s s i v a di c i r c a 6 k m in d i r e z i o n e nord f o r m a n d o un a r c o d e b o l m e n t e i n c u r v a l o verso ovest. nel p r i m o imp e l o dell e r u z i o n e , c o m e a b b i a m o d e l l o , -i e b b e r o d e l l e c o l a l e effim e r e : le p r i m e d u e , v i c i n e una a l l ' a l i l a , t r a b o c c a r o n o dal p r i m o t r a t t o d e l l a s p a c c a t u r a : ira il m . g r i g i o e i due p i z z i a p p a r v e una l i e v e s b a v a t u r a (li l a v a : un l e r z o r i g u r g i t o -i f o r m e v e r o q u o t a 2 3 1 0 : un ( p i a r l o , c h e fu una b a v a t u r a , poco più giù ed un ( p ù n t o a q u o t a 2 3 0 0 : a (piota 2 2 0 0 in fine -i a p r ì la b o c c a effusiva p r i n c i p a l e . questi l e n o m e n i a v v e n n e r o di n o t t e senza e s p l o s i o n i tali da s v e g l i a r e la pop o l a z i o n e del v e r s a n t e nord del v u l c a n o : solo p o c h e p e r s o n e c h e si t r o v a v a n o sveglie v e d e n d o b a g l i o r i nell o s c u r i l a c r e d e t t e r o d a p p r i m a c h e -i t r a t t a s s e d i n c e n d i o , ma (piando v i d e r o z a m p i l l a r e la l a v a , m a n o a m a n o s e m p r e più in b a s s o , d i e d e r o l ' a l l a r m e . i . e f i l u s s o l a v i c o , per q u a n t o a v v e n i s s e a fiotti d a l l a b o c c a e f f u s i v a p r i n c i p a l e , si m a n t e n n e c o s l a n l c nei p r i m i i giorni e p r o c e d e t t e tranq u i l l a m e n t e a c c o m p a g n a l o da d e b o l i s s i m e e p l o s i o n i e dal l a n c i o di p o c h i b r a n d e l l i di lava. i.a e r a del 2 8 f e b b r a i o fu ron o v i l i i nt ensi b a g l i o r i a m o l i l e della bocca e f f u s i v a . gli a b i t a n t i di kaiidazzo c r e d e t t e r o c h e si l'osse a p e r t a una nuova b o c c a , i n v e c e e r a a v v e n u t o un r i g u r g i t o d a l l a l e s s a spacf l l c a r i a n o l ' h . v r l i i l i o o l i o r i m a s o i o l ' i n n a t i t i l iu. i klna. bocca ellusiva d e l l ' e r u z i o n e late, , . . i i i 01 r 11 i 'ii m i p o r [ u l t a la ( i n a l a ( i d rate dal 21 iclihraio al iii marzo l ' m i . i l ' e r u z i o n e . kv ideili e m o n i e ora a v v e n t i l o un a u m e n t o d e l l a p o r t a l a d e l l a lava d e n t r o la s p a c c a tura e d i l a l l i da r a n d a z z o f u r o n o \isti b a g l i o r i più i n l e n i d e l l o a l t r e n o t t i . a n c h e nel t r a i l o più a l l o d e l l a s p a c c a t u r a . il r i g u r g i t o del 2 8 f e b b r a i o a v v e n n e verso q u o t a 2 3 0 0 p e r c h è il t r a i l o più b a s s o della s p a c c a i tira, l i m a i o c o p e r t o di s c o r i e , f u n z i o n ò da t u b o a n z i c h é da c a n a l e e q u i n d i non p o t è r i c e v e r e a l t r a lava più di (pianta n e c o n c n t i v a ia u a s e z i o n e . il g i o r n o 3 m a r z o l'efflusso l a v i c o i n c o m i n c i ò a d i m i n u i r e alla b o c c a e f f u s i v a . in p o c h e o r e la sua p o r t a l a si ridusse c o n s i d e r e v o l m e n t e ; il f r o n t e r a l l e n t ò la sua a v a n z a t a : non f u r o n o \isti più bagliori ed il g i o r n o 1 n o t t e s o m b r ò elio l ' e r u z i o n e fosse f i n i t a . i m p r o v v i s a m e n t e il ."> m a r z o a u m e n t ò n u o v a m e n t e l'efflusso lavico d a l i a s p a c c a t u r a ove a inolilo d e l l a b o c c a e f f u s i v a , verso (piota 2 3 0 0 , n c i l o -tosso posto del 2 8 f e b b r a i o , si m a n i f e s t ò un f o r t e r i g u r g i t o . la lava olio t r a b o c c ò sj s o v r a p p o s e -lilla c o l a t a effimera p r e c e l ' e r u z i o n e d e l l ' e t n a 1 ) e l f e b b r a i o m a r z o ] 9 4 7 6 5 eruzione d e l l ' e t n a febbraio-marzo1947 meno nwioenmo dui m s ctkuzzil.suiesib d e n t e , la s u p e r ò e s c i v o l ò giù fino a q u o t a 1 8 5 0 d o p o a v e r e p e r c o r s o c i r c a u n c h i l o m e t r o c m e z z o s u l l ' e r t o p e n d i o n e v o s o c h e g u a r d a r a n dazzo. v e r s o sera cessò il r i g u r g i t o , m a la lava c o n t i n u ò a fluire d a l l a b o c c a effusiva p r i n c i p a l e ; l a c o l a t a si s o v r a p p o s e sulla p r e c e d e n t e c , senza s u p e r a r n e gli a r g i n i , c o r s e p e r q u a l c h e c h i l o m e t r o fino al 10 m a r z o n o t t e q u a n d o fu c o n s t a t a t a la fine d e l l ' e r u z i o n e . dinamismo esplosivo e scuotimenti del suolo. il d i n a m i s m o di q u e s t a e r u z i o n e l a t e r a l e è stato d e b o l i s s i m o ; n o n f u r o n o m a i a v v e r t i t e e s p l o l i o n i d a i paesi p i ù v i c i n i n e m m e n o a l l o s c o p p i o , q u a n do, n e l l ' u l t i m o t r a t t o della s p a c c a t u r a v e r s o la b o c ca e f f u s i v a , s ' i m p i a n t a r o n o i c o n e t t i m a m m i l l a r i ; n è vi f u r o n o p r o i e z i o n i di c e n e r e c o m e n e l l e e r u zioni d e l 1 9 1 1 e 1 9 2 3 . avv e n u t e sullo stesso versante. a l s e c o n d o g i o r n o d e l l ' • , , , • i , « i o tehipo i e r u z i o n e t u t t o il t e a t r o e r u t t i v o a p p a r i v a t r a n q u i l l o e da p a s s o p i s c i a r o , c l i c è il c e n t r o a b i tato p i ù v i c i n o , solo di n o t t e si n o t a v a n o i b a g l i o r i d e l l a l a v a . non f u r o n o a v v e r t i l e scosse s i s m i c h e n è p r i m a , nè d u r a n t e l ' e r u z i o n e ; a n c h e gli s t r u m e n t i s i s m i c i d e l l ' o s s e r v a t o r i o di c a t a n i a , i m p i a n t a l i d a l l ' i s t i t u t o n a z i o n a l e di g e o f i s i c a , n o n r e g i s t r a r o n o a l c u n m o v i m e n t o , n e m m e n o q u a n d o f u r o n o a v v e n i t e l e f o r t i e s p l o s i o n i del c r a t e r e c e n t r a l e a v v e n u t e nei g i o r n i c h e p r e c e d e t t e r o l ' e r u z i o n e laici a l e . meccanismo dell eruzione. 1 f e n o m e n i osservali in q u e s t a e r u z i o n e m o s t r a n o in m o d o c h i a r o il m e c c a n i s m o d e l l e e r u z i o n i l a t e r a l i d e l l ' e t n a c o m e fu i n t u i t o da tifi c a l i a > 0 p o n t e m. g e i n m e l l a r o c c o m e è s i a l o d i m o s t r a l o da g . p o n t e in v a r i e p u b b l i c a z i o n i e c i o è c l i c il m a g m a e r u l l i v o s o s p i n t o d a l l e forze e n d o g e n e segue s e m p r e la via del c o n d o t t o e r u t t i v o c e n t r a l e . d i f a t t i in q u e s t a e r u z i o n e (piando a u m e n t ò la p i n t a d e l m a g m a , e b b e luogo la g r a n d e f o n t a n a di lava c h e si s o l l e v ò sulla b o c c a s u b l e r m i n a l e fino a r i e m p i r l a ed a t a p p a r l a . il m a g m a , in s e g u i l o , n o n t r o v a n d o a l t r e v i e , -i sollevò a n c o r a più in a l t o fino a r a g g i u n g e r e la gola d e l l a linea di s p r o f o n d a m e n t o del c r a t e r e c e n t r a l e e si f e r m ò , p o r c i l e la forza elio lo s p i n g e v a r e s t ò e q u i l i b r a l a d a l p e s o d e l l a c o l o n n a m a g m a t i c a . i.e v i o l e n t e e s p l o s i o n i olio ne s e g u i r o n o por lo reazioni s u p e r f i c i a l i d e l l a m a s s a r o v e n t e con il terr e n o . scossero la c o m p a g i n e t e r m i n a l e del v u l c a no e d e t e r m i n a r o n o la p e n e t r a z i o n e del m a g m a sotto il t a p p o di lava non del t u t t o i m p i e t r i l o della b o c c a s u b l e r m i n a l e da ( [ ( o d d o teit1po d o v e , nel p r i m o i m p e to, v e n n e r o i d u e t r a b o c c h i effimeri di l a v a . s o n o n fosse avventil a a l t r a p e n e t r a z i o n e f r a gli strati del v u l c a n o . i e r u z i o n e si s a r e b b e l o c a l i z z a t a in quel p o s t o . i n v e c e il c u n e o di m a g m a ohe p e n e t r a v a l a t e r a l m e n t e , a l i m e n t a t o dal t i n v a s a m e n t o c l i c a v v e n i v a dal c o n d o t t o p r i n c i p a l e , c o n t i n u ò a d i n c u n e a r s i in d i r e z i o n o p a r a l l e l a alili s t r a t i p e r i o i i i i a l i c h e via v i a si a n d a v a n o s p a c c a n d o . lo a p e r t u r e c h e -i f o r m a r o n o o r a n o m o l l o s u p e r f i c i a l i ed il flusso l a v i c o d i s c e n d e n t e , c h e c o i a v a c o m e in un c a n a l e , v e n n e a s g o r g a r e d a l p o s l o m e n o p r o f o n d o d e l l a s p a c c a t u r a . ivi la r e a z i o n e d e l l a m a s s a r o v e n t e con il t e r r e n o u m i d o e c o p e r t o di n e v e fu f o r t e , m a di b r e v e d u r a t a . le s c o r i e lanc i a t e d a l l e e s p l o s i o n i c h i u s e r o l ' u l t i m o t r a i l o d e l l a s p a c c a t u r a o v e , di c o n s e g u e n z a , il c a n a l e d i v e n n e un tubo por cui i"efflusso l a v i c o a l l a l ' e r u z i o n e d e l l ' e t n a d e l i ' e r b r a i o ' m v r z o 1 9 4 7 67 b o c c a e f f u s i v a si p r e s e n t ò p i u t t o s t o c o s t a n t e . i n f a t t i i s o v r a c c a r i c h i «li m a g m a (lei 2 8 f e b b r a i o e del 5 m a r z o n o n f u r o n o a v v e r t i l i alla b o c c a e f f u s i v a , i n a d e t e r m i n a r o n o r i g u r g i t i un poco più a m o n t e v e r s o «piota 2 3 0 0 , ove la s p a c c a t u r a era p i ù s u p e r f i c i a l e degli a l t r i p o s t i . d e l l e osservazioni «li g r a n d e i m p o r t a n z a eruzione d e l l ' e t n a f ebbraic-marzo 1947 hieuo (wcionnato dm m.s.ukuizosiiveu»! l ' a c c i a i n o qui r i l e v a r e c l i c d i m o s t r a n o in m o d o e v i d e n t e c o m e il m a g m a si fosse i n c a n a l a t o in senso d i s c e n d e n t e n e l l a spacc a t u r a a p a r t i r e d a l l a b o c c a s u b l e r m i n a l e : i d u r a n t e tutta la d u r a l a dell e r u z i o n e l a t e r a l e la m a g g i o r e q u a n t i t à «li esal a z i o n i p r i m a r i e ( c l o r u r i di s o d i o e p o t a s s i o ) venn e fuori dal p r i m o t r a i l o d e l l a s p a c c a t u r a ( q u o t a 3 0 5 0 ) . 2 " nel p r i m o imp e l o d e l l ' e r u z i o n e , m a n o a m a n o c l i c la lava p e n e t r a v a e l'ormava la spacc a t u r a . si a v e v a n o su q u e sta dei t r a b o c c h i di l a v a . 3 " q u a n d o a v v e n n e r o i rig u r g i t i del 2 8 f e b b r a i o e del 5 m a r z o nel p r i m o l r a l i o d e l l a s p a c c a t u r a ( 3 0 5 0 ili) a p p a r v e r o da k a n d a z z o b a g l i o r i v i v i s s i m i . 4 " i. i n t e n s i t à lum i n o s a d e l l a lava a l l a b o c c a effusiva fu r i s c o n t r a l a m e n o i n t e n s a «iella lava c l i c a p p a r v e a l l a b o c c a s u b l e r m i n a l e ali i n i z i o della e r u z i o n e l a t e r a l e . ultimo um?o (.mise probabili delle eruzioni etnee. il c r o g i u o l o p r o f o n d o d e l l ulna è e s a u r i b i l e , c o m e q u e l l o «li l u l l i i v u l c a n i d e l l a l ' e r r a . dal p l e i s t o c e n e ad oggi una i n g e n t e massa «li lava e di d e t r i t i è stala e r u t t a l a da q u e s t o g l a n d e v u l c a n o , c e r i o 110l e v o l m e n l e di più della sua m o l e , d a t o c l i c una c o n s i d e r e v o l e p a r t e di «lolrili «'• s t a l a p o l l a t a dagli a g e n t i a t m o s f e r i c i nel m a r e . o r a . p e r 6 8 g a e t a n o p o n t e <|iianlo l ' e t n a a b b i a a n c o r a una i n c a l c o l a b i l e riserva (li m a g m a , non sì s a p r e b b e c o m e s p i e g a r e c h e esso non trovi posto nel c r o g i u o l o c o n s i d e r e v o l m e n t e s v u o t a l o senza a m m e t t e r e un c o n t i n u o r e s t r i n g i m e n t o di esso sotto le s p i n t e d e l l a l i t o s f e r a . n e l p e r i o d o t e r z i a r i o il v u l c a n i s m o r a g g i u n s e il m a s s i m o s v i l u p p o c o m e è a m m e s s o da d i v e r s i geologi per via del f o r t e c o r r u g a m e n t o o r o g e n i c o , si p u ò q u i n d i p e n s a r e c h e la d i m i n u i t a a t t i v i t à d e i v u l c a n i a t t u a l i sia i n r e l a z i o n e c o n l e rid o t t e f o r z e t e t t o n i c h e . q u e s t e c o n s i d e r a z i o n i f a r e b b e r o c a d e r e q u a l siasi a l t r a ipotesi sulla c a u s a d e l l e e r u z i o n i . organizzazione per la difesa contro i danni delle eruzioni etnee. p a r e c c h i o t e m p o p r i m a c h e s c o p p i a s s e q u e s t a e r u z i o n e il d i r e t tore d e l l ' s t i t u t o v u l c a n o l o g i c o e t n e o aveva p r e s e n t a t o al p r e f e t t o d e l l a p r o v i n c i a di c a t a n i a un p i a n o p e r la difesa c o n t r o i d a n n i di una e v e n t u a l e e r u z i o n e . r i p r e s o i n e s a m e il p i a n o a p p e n a s c o p p i ò l ' e r u z i o n e , s ' i n c o n t r a r o n o d e l l e difficoltà e s o p r a t t u t t o q u e l l a di n o n a v e r e n e l l ' a e r o p o r t o di c a t a n i a un v e l i v o l o di r i c o g n i z i o n e p e r cui se n e d o v e t t e r i c h i e d e r e u n o al c o m a n d o m i l i t a r e di b a r i . 11 r i c o g n i t o r e c h e g i u n s e a c a t a n i a , iti v e r o , non e r a m o l t o a d a t t o per v o l i -u| vulc a n o . m a aveva a b o r d o un v a l e n t e p i l o t a , il t e n e n t e m o r e l l i , il q u a l e con i a i u t o del t e c n i c o dell i s t i t u t o , m . t a f f a r a , p o t è r i u s c i r e a d a r e g i o r n a l m e n t e e s a t t i l i l i e v i f o t o g r a f i c i del t e a t r o e r u t t i v o . m o l t o u t i l i r i u s c i r o n o le f o t o g r a f i e a e r e e f a t t e il g i o r n o 2 6 f e b b r a i o dal c o m a n d a n t e d e l l a e r o p o r t o di c a t a n i a , c a p i t a n o r e i n e r . il q u a l e , in a t t e s a del r i c o g n i t o r e , v o l l e a u d a c e m e n t e v o l a r e s u l l ' e t n a con un p i c c o l o c a c c i a . q u a n d o la c o l a t a i n c o m i n c i ò a d i n v a d e r e i t e r r e n i c o l t i v a t i il c o m a n d o di difesa r i m a s e i n c e r t o se dovesse t e n t a r n e il d e v i a m e n t o c o n l ' u s o di b o m b a r d e l a n c i a t e da t e r r a sui fianchi del t o r r e n t e l a v i c o nei pressi d e l l a b o c c a e f f u s i v a a l l o s c o p o di a r r e s t a r e l ' a v a n z a m e n t o del f r o n t e , m a f o r t u n a t a m e n t e l ' e r u z i o n e p r o c e d e t t e assai l e n t a m e n t e e cessò p r e s t o . caldina istituti) ! ulcanologico etneo dell unir. ghigno 1917. riassumo si espongono e si esaminano i fenomeni più notevoli elle hanno caratterizzato i eruzione etnea dei febbraio-marzo 1917. l e p i o g g e d e l l a l i b i a e l ' a t t i v i t a ' s o l a r e g . godoli 1. generalità. — sino ad una diecina di anni fa, le ricerche sulle relazioni fra l'attività solare ed i fenomeni meteorologici si basavano su metodi statistici che — in ultima analisi — si riducevano ad un confronto dei diagrammi. modernamente tali studi sfruttano l'analisi periodale dei diagrammi che permette di determinare, per gli intervalli di tempo considerati, l'ampiezza e la fase delle componenti. dal confronto delle oscillazioni elementari ottenute dall'analisi delle serie relative ai fenomeni solari con le oscillazioni delle serie relative ai fenomeni meteorologici, si mettono in evidenza le relative influenze (1). risultano evidenti i vantaggi di questo metodo di ricerca sui metodi più antichi. va però notato che, secondo quanto ha recentemente dimostrato picone, il problema dell'analisi periodale è determinato soltanto nel caso che la funzione del tempo, sottoposta ad analisi, sia soluzione di un'equazione differenziale l i n e a r e ordinaria omogenea a coefficienti costanti (2). inoltre per eseguire l'analisi occorrono serie lunghe ed omogenee. in casi particolari possono essere quindi ancora da consigliarsi i vecchi metodi statistici anche se una lunga esperienza ha dimostrato — in generale — la loro inadeguatezza specie nel caso dei caratteri pluviometrici. 2. scopo del presente lavoro. — è stato recentemente pubblicato da a. fàntoli, un vasto lavoro che raccoglie e rielabora tutti i dati pluviometrici della libia (3). sono raccolte le serie pluviometriche (intensità e frequenza) di 247 stazioni costituenti una rete che si estendeva dalla costa mediterranea sino al deserto libico. le diverse serie abbracciano periodi di tempo diversi. le più lunghe sono quelle di bengasi (1881-1941) e tripoli (1879-1948). le più corte abbracciano solo pochi anni. 126 c. co doli le serie hanno, in generale, diverse lacune dovute a vari fattori contingenti descritti nell'opera del fàntoli. alcune considerazioni aprioristiche consigliavano l'esame di questo materiale in relazione all'attività solare. a) l'ahhondanza delle serie pluviometriche può essere utile per fornire una media che, scevra da fattori contingenti, può rivelare con maggior chiarezza la presunta influenza dei fenomeni correlati. b) i caratteri generali si mantengono percettibili nelle piogge di tutto il nord-africa ( 4 ). questo fa supporre che l'influenza dei fattori locali sia trascurabile di fronte a quella di fattori più generali eventualmente legati all'attività solare. ci si tratta di una regione in cui il predominio dei fattori generali su quelli locali è già stalo confermato. le ricerche sulle variazioni del livello del lago victoria hanno infatti dimostrato che esse contengono il termine di chandler della polodia ( 5 ) ed il termine undecennale dell'attività solare (°). 3. metodo seguito. — \olendo eseguire una tale ricerca si imponeva il metodo da applicare. non potendosi infatti pensare di f a r e delle analisi periodali di serie in generale corte e spesso colme di lacune si è presentata l'opportunità di elaborare il materiale con i vecchi metodi stilistici. innanzi tutto si sono raggnippate le diverse stazioni in climi e, nell'ambito di uno stesso clima, in regioni. i meteorologi dividono la libia in sei climi : 1) clima marittimo (m) 2) clima steppico litoraneo(sl) 3) clima steppico continentale (sc) 4) clima altipiani (a) 5) clima predesertico (p) 6ì clima desertico (d) per la completa descrizione dei quali rimandiamo all'opera del fàntoli. qui basti accennare che il clima marittimo si estende per una fascia larga al massimo qualche chilometro lungo la costa libica, mentre gli altri si susseguono nell'entroterra secondo zone orientate approssimativamente lungo i paralleli. l e pioc.oe d e l l a l i b i a e l ' a t t i v i t à s o l a r e 127 i geografi dividono poi la libia in sette r e g i o n i : 1) regione tripolitana ( t ) 2) regione hainàda el liamra (hh) 3) regione fezzàn (f) 4) regione sirtica (s) 5) regione cirenaica (c) 6) regione inarmarica (m) 7) regione deserto libico (dl). qui la differenziazione risulta meno netta. però, diverse considerazioni di natura meteorologica, morfologica, ecologica, unite al fatto cbe tali regioni banno serbato la loro denominazione anche attraverso tanti eventi storici e sovrapposizioni etniche, conducono a consider a r l e separatamente. avviene così che uno stesso clima contiene più regioni e viceversa. l'influenza del sole sulle piogge della libia è stata studiata per gli ultimi quattro cicli di attività solare dato che tutte le serie, escluse quelle di bengasi e tripoli —• del resto già studiate dal fàntoli (t) —, sono contenute in questo periodo. gli anni di minima ( m ) e massima (m) attività solare sono indicati nel quadro seguente: sono state eseguite per tutte le stazioni di una stessa regione e di uno stesso clima le medie delle intensità ( i ) e delle frequenze (/) negli anni ni — 1, m, in f 1 ; m—1, m, m 1 e nelle triadi di minimo tm contenenti gli anni ni — 1, m, ni + 1 e nelle triadi di massimo 7 m contenenti gli anni m—1, m, m -f1. sono state quindi eseguite le differenze di queste medie nel senso massimo-minimo. per poter discutere il significato di queste differenze alla luce 1913 1923 1933 1944 m m 1917 1928 1937 1947 128 c . c o d h l i del loro vero valore, si sono determinate per ogni regione e per ogni clima le medie pesate dei valori assoluti degli scarti tm — ( m — 1) t m — t, „ — (ni + 1) t u — ( m — 1 ) t m — (m + l ) ove i simboli stanno ad indicare le medie a cui si riferiscono. 4. risultati conseguiti e discussione. —• i risultati sono raccolti nelle tabelle 1, 2, 3, 4 di immediata interpretazione. in tali tabelle i numeri f r a parentesi quadre indicano il numero di stazioni a cui l e medie si riferiscono. in ogni tabella sono riportati per il confronto, nelle ultime due colonne, gli scarti delle medie relative alle intensità ed alle frequenze calcolati col metodo descritto. per l a discussione consideriamo separatamente i dati relativi alle frequenze e quelli relativi alle intensità. intensità epoca — 1. — le differenze sono tutte negative e significativamente rilevanti per tutti i climi ad eccezione che per il clima d. questa anomalia può spiegarsi o considerando l'esiguo numero di stazioni — specie nell'epoca di minimo — o pensando che i caratteri di anormalità che si distinguono per l'instabilità e che sono strettamente connessi coi massimi di attività solare (8), in paesi tanto aridi si possano estrinsecare solo mediante un aumento delle precipitazioni. vedremo in seguito come la prima ipotesi sia più attendibile. per quanto riguarda le regioni, si ha che nella regione t la differenza f r a l a quantità di pioggia caduta durante il minimo di attività solare e quella caduta durante il massimo, raggiunge sempre valori elevati. qui le differenze sono tanto più significative essendo molto numerose le stazioni ed attendibili i dati. anche le regioni s ed m hanno differenze che si distinguono nettamente dagli scarti. l i : p i o g g e d e l l a l i d i a e l ' a t t i v i t à s o l a r e 129 tabella i — epoca — 1. minimo mahhimo muksimt -minimo ff f i f i f i f m t 3 1 0 . 8 5 4 . 5 2 2 7 . 9 3 7 . 0 — 8 2 . 9 — 1 7 . 5 1 8 . 4 5 . 2 |14] |14| [ 1 9 ] [ 1 8 ] 1 8 . 4 5 . 2 s 2 6 2 . 2 4 2 . 0 1 3 6 . 9 3 4 . 6 — 1 2 5 . 3 — 7 . 4 2 5 . 7 4 . 5 |4| [4] [5| [ 5 ] 2 5 . 7 4 . 5 c 3 9 4 . 4 6 4 . 7 2 6 9 . 8 4 9 . 8 — 1 2 4 . 6 — 1 4 . 9 4 8 . 1 8 . 7 1*1 14] [ 1 5 | [ 1 5 ] 4 8 . 1 8 . 7 m 2 3 7 . 0 3 7 . 6 1 1 8 . 3 2 1 . 5 — 1 1 8 . 7 — 1 6 . 1 3 1 . 4 6 . 9 [ 3 ] [3] m [7] 3 1 . 4 6 . 9 tei 3 0 7 . 5 5 2 . 1 2 1 5 . 0 3 8 . 6 — 9 2 . 5 — 1 3 . 5 2 3 . 5 5 . 7 1 2 5 ] [ 2 5 ] [46| [ 4 5 ] 2 3 . 5 5 . 7 t 3 5 3 . 8 4 8 . 5 2 5 7 . 8 3 3 . 2 — 9 6 . 0 — 1 5 . 3 4 7 . 5 5 . 3 [ 2 9 ] |29] [ 5 7 ] [ 5 3 ] 4 7 . 5 5 . 3 c 2 3 2 . 7 3 8 . 2 2 2 9 . 0 4 1 . 0 — 3 . 7 co <m* 4 1 . 6 7 . 5 [ 9 ] [8| [ h | [ 1 2 ] 4 1 . 6 7 . 5 m — — 7 2 . 2 1 6 . 6 — 1 8 . 6 2 . 2 |0| |0| [3| [3] 2 . 2 t o i . 3 2 5 . 1 4 6 . 3 2 4 5 . 5 3 3 . 9 — 7 9 . 6 — 1 2 . 4 4 6 . 7 5 . 5 [ 3 8 j [ 3 7 ] [ 7 1 ] [ 6 8 ] 4 6 . 7 5 . 5 s e t 2 2 5 . 0 3 0 . 0 1 5 9 . 4 3 1 . 0 — 6 5 . 6 + i o 3 2 . 5 3 . 7 |7| |7| [ 1 3 ] [ 1 0 ] 3 2 . 5 3 . 7 s 1 8 5 . 3 3 1 . 3 1 2 5 . 5 2 4 . 2 — 5 9 . 8 — 7 . 1 2 7 . 6 3 . 8 |3j ! 3 j [4j [4] 3 . 8 c 2 3 3 . 2 3 6 . 5 1 3 4 . 1 3 0 . 0 — 9 9 . 1 — 6 . 5 2 9 . 4 6 . 0 [2] [2] [4| [4] 6 . 0 m 1 8 8 . 3 3 0 . 0 1 0 3 . 6 1 9 . 0 — 8 4 . 7 — 1 1 . 0 3 5 . 1 5 . 8 11] m . [1| [ 1 ] 5 . 8 7 oi. 2 1 4 . 3 3 1 . 3 1 4 6 . 1 2 8 . 2 — 6 8 . 2 — 3 . 1 2 7 . 9 3 . 7 [ 1 3 ] [ 1 3 ] [ 2 2 ] [ 1 9 ] a t 3 1 2 . 0 3 7 . 3 2 1 5 . 0 3 0 . 8 — 9 7 . 0 — 6 . 5 3 8 . 0 4 . 4 [ 1 7 [ [ 1 6 ] [ 2 7 ] [ 2 4 ] 4 . 4 c 4 9 5 . 4 6 1 . 1 4 0 7 . 7 6 2 . 3 — 8 7 . 7 + l 2 7 9 . 8 6 . 8 [ 1 5 ] [ 1 6 | [ 3 5 ] [33| t o t . 3 9 8 . 0 4 9 . 2 3 2 3 . 8 4 9 . 1 — 7 4 . 2 — 0 . 1 5 0 . 2 5 . 8 [ 3 2 ] [ 3 2 ] [ 6 2 ] [ 5 7 ] p t 1 0 5 . 8 2 1 . 0 4 2 . 8 1 2 . 3 — 6 3 . 0 — 8 . 7 2 1 . 3 4 . 0 13] [3| |5| [6] c 7 0 . 5 2 9 . 0 5 6 . 3 3 2 . 0 — 1 4 . 2 + 3 . 0 2 7 . 4 9 . 4 11] [ 1 ] [1] [1) t c t . 9 7 . 0 2 3 . 0 4 5 . 1 1 5 . 1 — 5 1 . 9 — 7 . 9 2 2 . 5 3 . 1 [4] [4[ [6| [7] d h h 5 7 . 6 1 3 . 5 4 0 . 3 1 0 . 7 — 1 7 . 3 — 2 . 8 8 . 4 1 . 6 [2] [ 2 ] [8] [8] f 6 . 1 4 . 5 1 2 . 2 4 . 2 + 6 . 1 0 . 3 4 . 9 0 . 7 [2] [2] [5] [5] s 0 . 0 9 . 0 3 2 . 0 9 . 0 + 3 2 . 0 0 . 0 1 3 . 6 1 . 6 [ 1 ] [3] [3] [3] d l 2 . 5 4 . 5 6 . 7 3 . 5 + 4 . 2 — 1 . 0 5 . 8 0 . 5 [2[ [2] [41 [2) tot. 1 8 . 9 8 . 0 2 5 . 3 7 . 8 + 6 . 4 — 0 . 2 5 . 6 1 . 0 [7] [9] [ 2 0 ] [ 1 8 ] 130 c . godoli tabella i i — epoca 0. minimo i n a n i m o massimo 'minimo n ivi ĵ ioik i f ' f i f i f m t 3 4 5 . 4 4 3 2 2 4 4 . 8 4 4 . 7 — 1 0 0 . 6 4 1 . 5 1 8 . 4 5 . 2 [ 1 5 ] [ 1 5 ] [ 1 8 ] [ 1 7 ] s 1 0 5 . 2 2 2 . 0 1 3 1 . 7 2 8 . 0 4 2 6 . 5 + 6 . 0 2 5 . 7 4 . 5 [ 4 ] [ 4 ] [ 6 ] [ 5 ] 4 2 6 . 5 c 1 8 0 . 2 " 4 1 . 8 2 7 2 . 1 5 1 . 3 f 9 1 . 9 + 9 . 5 4 8 . 1 8 . 7 [ 1 1 1 [ 1 0 ] [ 1 4 ] [ 1 4 ] m 8 8 5 2 0 . 6 1 0 4 . 0 2 1 . 7 + 1 5 . 5 + 1 . 1 3 1 . 4 6 . 9 [ 5 ] [ 5 ] |6| m + 1 . 1 t o t . 2 2 9 . 3 3 6 . 9 2 1 8 . 9 4 1 . 1 — 1 0 . 4 4 4 . 2 2 3 . 5 5 . 7 [ 3 5 ] [ 3 4 ] [ 4 4 ] [ 4 3 ] 4 4 . 2 s l t 3 3 0 . 9 3 6 7 1 8 3 . 4 2 8 . 0 — 1 4 7 . 5 — 8 . 7 4 7 . 5 5 . 3 [ 4 4 ] [ 4 5 ] |67] [ 6 4 | c 1 4 6 . 8 2 9 . 4 2 3 4 . 4 4 1 . 3 + 8 7 . 6 4 1 1 . 9 4 1 . 6 7 . 5 [ 1 5 ] [ 1 5 ] [9] [9| m 6 4 . 0 2 8 . 5 4 9 . 9 1 3 . 0 — 1 4 . 1 — 1 5 . 5 1 8 . 6 2 . 2 [2] [ 2 ] [ 2 ] [2| t o t . 2 7 6 . 9 3 4 7 1 8 5 . 9 2 9 . 2 — 9 1 . 0 — 5 . 5 4 6 . 7 5 . 5 [ 6 1 ] [ 6 2 | [ 7 8 | [ 7 5 ] s e t 2 3 0 . 5 3 1 4 1 1 4 . 8 2 2 . 8 — 1 1 5 . 7 — 8 . 6 3 2 . 5 3 . 7 [ 9 ] [9] [ 1 7 ] [ 1 6 ] 3 2 . 5 s 6 2 . 7 1 5 . 3 8 7 . 8 1 9 . 2 + 2 5 . 1 4 3 . 9 2 7 . 6 3 . 8 [2| [3| [4| 1 0 5 . 6 |4| c 5 2 . 8 1 7 . 0 [4| 1 0 5 . 6 2 0 . 2 + 5 2 . 8 4 3 . 2 2 9 . 4 6 . 0 [ 2 ] [2| [41 [4] m 5 3 2 1 0 . 5 7 7 . 3 1 9 . 0 4 2 4 . 1 + 8 . 5 3 5 . 1 5 . 8 t o t . [ 1 ] [2] |1| |1| t o t . 1 6 8 . 5 2 4 . 0 1 0 7 . 8 2 1 . 6 — 6 0 . 7 — 2 . 4 2 7 . 9 3 . 7 [ 1 4 ] [ 1 6 | [ 2 6 ] [ 2 5 ] a t 3 6 6 0 3 6 . 6 1 6 5 2 2 6 . 4 — 2 0 0 . 8 — 1 0 . 2 3 8 . 0 4 . 4 [ 2 3 | [ 2 3 ] [ 3 2 ] [ 3 2 ] c 2 3 2 . 2 4 4 . 7 4 1 2 . 8 5 6 . 3 4 1 8 0 . 6 + 1 1 . 6 7 9 . 8 6 . 8 [ 2 3 ] [ 2 4 ] [ 3 4 ] [ 3 5 ] t o t . 2 9 9 1 4 0 . 7 2 9 2 . 8 4 2 . 0 — 6 . 3 4 1 . 3 5 0 . 2 5 . 8 [ 4 6 ] [ 4 7 ] [ 6 6 1 [ 6 7 ] p t 1 0 5 . 0 2 1 . 3 4 4 . 8 1 2 . 0 — 6 0 . 2 — 9 . 3 2 1 . 3 4 . 0 [3] [3] [^l m c 3 3 5 1 1 . 0 1 0 . 8 1 3 . 0 — 2 2 . 7 4 " 2 . 0 2 7 . 4 9 . 4 [ 1 ] [ 1 ] [ 1 ] |1| t o t . 8 7 . 1 1 8 . 7 4 0 . 5 1 2 . l — 4 6 . 6 — 6 . 6 2 2 . 5 3 . 1 w [ 4 ] |8] [ 8 ] d h h 6 5 0 2 1 . 0 4 5 . 3 1 1 . 8 — 1 9 . 7 — 9 . 2 8 . 4 • 1 . 6 [4] [ 4 ] [6| [6] f 2 7 . 1 5 . 5 1 . 9 1 . 6 — 2 5 . 2 — 3 . 9 4 . 9 0 . 7 [ 4 ] [ 4 ] [ 6 ] [6] s 8 1 . 7 1 1 . 5 1 3 . 5 7 . 2 — 6 8 . 2 — 4 . 3 1 3 . 6 1 . 6 [ 1 ] [21 [ 3 ] [4| d l 2 9 . 5 3 . 5 1 2 . 6 4 . 0 — 1 6 . 9 + 0 . 5 5 . 8 0 . 5 [ 2 ] [ 2 ] [3| [ 2 ] t o t 4 6 . 2 1 1 . 3 2 0 . 1 6 . 5 — 2 6 . 1 — 4 . 8 5 . 6 1 . 0 [ 1 1 ] [ 1 2 ] [ 1 8 ] [ 1 8 ] l e pioc.oe d e l l a l i b i a e l ' a t t i v i t à s o l a r e 1 3 1 tabella i i i — epoca + 1minimo m a usi mo mahkimo -minimo a l i i m a negione i f i f i f ' f m t 3 2 4 . 6 4 6 . 2 2 8 7 . 3 5 4 . 1 — 3 7 . 3 4 7 . 9 1 8 . 4 5 . 2 [ 1 7 j [ 1 7 ] [ 2 1 ] [ 1 9 ] 4 . 5 s 1 6 8 . 5 3 2 . 6 1 4 8 . 4 3 6 . 2 — 2 0 . 1 4 3 . 6 2 5 . 7 4 . 5 ]5j [5] [7] [7] c 3 0 7 . 7 6 1 . 3 3 4 8 . 5 6 9 . 7 f 4 0 . 8 4 8 . 4 4 8 . 1 8 . 7 [ 1 3 ] [ 1 3 ] [ 1 3 ] [ 1 3 ] 6 . 9 m 2 2 3 . 8 3 8 . 4 1 3 0 . 7 3 5 . 2 — 9 3 . 1 — 3 . 2 3 1 . 4 6 . 9 [ 7 ] [7] [7] [ 8 ] 5 . 7 t o t . 2 8 4 . 0 4 7 . 9 2 6 0 . 7 5 2 . 5 — 2 3 . 3 4 4 . 6 2 3 . 5 5 . 7 [ 4 2 ] [ 4 2 ] [481 [ 4 7 ] s l t 3 8 0 . 1 4 2 . 5 3 5 9 . 5 4 3 . 9 — 2 0 . 6 + l 4 4 7 . 5 5 . 3 [ 5 3 ] [ 5 2 ] [ 7 1 ] [ 7 3 ] 7 . 5 c 2 8 0 . 6 5 1 . 0 2 8 7 . 0 5 5 . 7 f 6 . 4 4 4 . 7 4 1 . 6 7 . 5 [ 1 4 ] [ 1 4 ] ] 1 0 ] [9] 2 . 2 m 1 2 8 . 6 3 3 . 6 5 4 . 5 2 0 . 0 — 7 4 . 1 — 1 3 . 6 1 8 . 6 2 . 2 [3] [3] [2] [2] 5 . 5 t o t . 3 4 9 . 4 4 3 . 9 3 4 3 . 5 4 4 . 6 — 5 . 9 f 0 . 7 4 6 . 7 5 . 5 [ 7 0 ] [ 6 9 ] [ 8 3 ] [ 8 4 ] s e t 2 2 5 . 4 2 8 . 9 2 4 5 . 0 3 6 . 7 + 1 9 . 6 + 7 . 8 3 2 . 5 3 . 7 [ 1 1 ] [ 1 2 ] [ 1 6 ] [ 1 9 ! 2 7 . 6 3 . 8 s 1 5 2 . 6 2 3 . 5 1 4 2 . 5 2 8 . 2 — 1 0 . 1 + 4 . 7 2 7 . 6 3 . 8 [3] [4] [5] [5] 6 . 0 c 1 7 7 . 5 3 2 . 6 1 2 9 . 2 3 4 . 7 — 4 8 . 3 + 2 . 1 2 9 . 4 6 . 0 [3] [3] [4| [4] 3 5 . 1 5 . 8 ivi 1 9 5 . 1 2 8 . 5 1 0 5 . 9 2 1 . 0 — 8 9 . 2 — 7 . 5 3 5 . 1 5 . 8 [2] [2] [11 [ 1 ] 3 . 7 t o t . 2 0 3 . 1 2 8 . 3 2 0 2 . 1 3 4 . 4 — 1 . 0 + 6 . 1 2 7 . 9 3 . 7 [ 1 9 ] [ 2 1 ] [ 2 6 ] [ 2 9 ] a t 3 1 5 . 2 3 6 . 8 2 9 9 . 8 4 6 . 0 — 1 5 . 4 + 9 . 2 3 8 . 0 4 . 4 [ 3 0 ] [ 2 8 ] [ 3 0 ] [ 2 8 ] 7 9 . 8 6 . 8 c 5 0 1 . 4 6 8 . 2 5 3 1 . 8 7 0 . 6 4 3 0 . 4 + 2 . 4 7 9 . 8 6 . 8 [ 3 2 ] [ 3 1 ] [ 3 5 ] [ 3 5 ] 5 0 . 2 5 . 8 t o t . 4 1 1 . 3 5 3 . 3 4 2 4 . 7 5 9 . 7 4 1 3 . 4 4 6 . 4 5 0 . 2 5 . 8 [ 6 2 ] [ 5 9 ] [ 6 5 ] [ 6 3 ] p t 111.1 1 6 . 5 1 1 0 . 8 2 3 . 1 — 0 . 3 4 6 . 6 2 1 . 3 4 . 0 [4] [4] [7] [7[ 2 7 . 4 9 . 4 c 1 2 0 . 9 3 5 . 0 — — — — 2 7 . 4 9 . 4 [ 1 ] [1[ [0] [0] 2 2 . 5 3 . 1 t o t . 1 1 3 . 1 2 0 . 2 1 1 0 . 8 2 3 . 1 — 2 . 3 4 2 . 9 2 2 . 5 3 . 1 [ 5 ] [5[ [7] [7[ d h h 3 0 . 8 1 2 . 2 2 9 . 9 1 0 . 5 — 0 . 9 — 1 . 7 8 . 4 1 . 6 [4] [ 4 ] [7] [7] 0 . 7 f 1 7 . 6 4 . 4 1 0 . 5 2 . 8 — 7 . 1 — 1 . 6 4 . 9 0 . 7 [5] [5] [6] [5] 1 . 6 s 0 . 2 2 . 0 2 8 . 0 8 . 0 + 2 7 . 8 + 6 . 0 1 3 . 6 1 . 6 [ 1 [ [2] [4] [4] 0 . 5 d l 3 . 5 4 . 0 1 2 . 3 5 . 2 4 8 . 8 + 1 2 5 . 8 0 . 5 [ 3 ] [2] ]4] [4] 1 . 0 t o t . 2 0 . 1 6 . 3 2 0 . 6 7 . 0 4 0 . 5 4 0 . 7 5 . 6 1 . 0 [ 1 3 j [ 1 3 ] [ 2 1 ] [ 2 0 ] 132 c. godoli tabella i v — triade. m i n i m o massimo massime m i n i m o (t p i ' « c l i m a negione i f « f i f i f m t 3 2 7 . 2 4 7 . 7 2 5 4 . 6 4 5 . 4 — 7 2 . 5 — 2 . 3 1 8 . 4 5 . 2 ] 4 6 ] [ 4 6 ] [ 5 8 ] [ 5 4 ] s 1 7 7 . 9 3 2 . 2 1 3 9 . 6 3 3 . 3 — 3 8 . 3 + 1 . 1 2 5 . 7 4 . 5 [ 1 3 | [ 1 3 ] |18| [ 1 7 ] e 2 7 0 . 0 5 4 . 5 2 9 4 . 9 5 6 . 5 + 2 4 . 9 + 2 . 0 4 8 . 1 8 . 7 [ 2 8 ] [ 2 7 ] [ 4 2 ] | 4 2 ] m 1 8 1 . 3 3 2 . 3 1 1 8 . 3 2 6 . 5 — 6 3 . 0 — 5 . 7 3 1 . 4 6 . 9 [ 1 5 | [ 1 5 1 [ 2 0 1 [ 2 2 ] t e l . 2 7 1 . 0 4 5 . 2 2 3 2 . 1 4 4 . 3 — 3 8 . 9 — 0 . 9 2 3 . 5 5 . 7 [ 1 0 2 ] | 1 0 1 [ [ 1 3 8 1 [ 1 3 5 ] s l t 3 5 6 . 9 4 1 . 8 2 6 9 . 3 3 5 . 6 — 8 7 . 6 — 6 . 2 4 7 . 5 5 . 3 | 1 2 6 | [ 1 2 6 ) [ 1 9 5 ] 1 1 9 0 ] e 2 1 6 . 5 3 9 . 5 2 4 9 . 9 4 5 . 5 + 3 3 . 4 + 6 . 0 4 1 . 6 7 . 5 [ 3 8 ] [ 3 7 ] [ 3 0 | [ 3 0 ] m 1 0 2 . 7 3 1 . 6 6 0 . 7 1 6 . 5 — 4 2 . 0 — 1 5 . 1 1 8 . 6 2 . 2 |5] [5] [7| [7] t o t . 3 1 7 . 8 4 1 . 0 2 6 0 . 5 3 6 . 3 — 5 7 . 3 — 4 . 7 4 6 . 7 5 . 5 [ 1 6 9 | 1 1 6 8 ] [ 2 3 2 | [ 2 2 7 ] s e t 2 2 7 . 0 3 0 . 0 1 7 2 . 7 3 0 . 3 — 5 4 . 3 0 . 3 3 2 . 5 3 . 7 [ 2 7 ] [ 2 8 ] [ 4 6 ] [ 4 5 ] s 1 4 2 . 4 2 3 . 4 1 2 0 . 4 2 4 . 2 — 2 2 . 0 ( 0 . 8 2 7 . 6 3 . 8 [8[ [ 1 0 ] [ 1 3 ] [ 1 3 ] e 1 5 7 . 8 2 9 . 2 1 2 2 . 9 2 8 . 3 — 3 4 . 9 — 0 . 9 2 9 . 4 6 . 0 m [ 7 ] [ 1 2 ] [ 1 2 ] m 1 5 7 . 9 2 1 . 6 9 5 . 6 1 9 . 6 — 6 2 . 3 — 2 . 0 3 5 . 1 5 . 8 |4[ [ 5 ] [3] [ 3 ] t o t . 1 9 5 . 7 2 7 . 7 1 5 2 . 3 2 8 . 4 — 4 3 . 4 |0 . 7 2 7 . 9 3 . 7 [ 4 6 j [ 5 0 ] [ 7 4 ] [ 7 3 ] a t 3 3 1 . 1 3 6 . 8 2 2 5 . 7 3 4 . 2 — 1 0 5 . 4 2 . 6 3 8 . 0 4 . 4 |70| [ 6 7 | [ 8 9 ] 1 8 4 ] e 4 1 1 . 7 5 8 . 7 4 5 1 . 1 6 3 . 1 + 3 9 . 4 4 . 4 7 9 . 8 6 . 8 |70] [ 7 1 ] [ 1 0 4 ] [ 1 0 3 ] t o i . 3 7 1 . 4 4 8 . 1 3 4 7 . 2 5 0 . 1 — 2 4 . 2 42 . 0 5 0 . 2 5 . 8 1 1 4 0 ] [ 1 3 8 ] | 1 9 3 ] [ 1 8 7 ] p t 1 0 7 . 7 1 9 . 3 6 8 . 6 1 6 . 0 — 3 9 . 1 — 3 . 3 2 1 . 3 4 . 0 1 i o j [ 1 0 ] [ 1 9 ] [ 2 0 ] c 7 4 . 9 2 5 . 0 3 3 . 5 2 2 5 — 4 1 . 4 — 2 . 5 2 7 . 4 9 . 4 |3] [3| (2) [2| t o ' . 1 0 0 . 1 2 0 . 6 6 5 . 3 1 6 . 5 — 3 4 . 8 4 . 1 2 2 . 5 3 . 1 [ 1 3 | [ 1 3 ] [ 2 1 ] [ 2 2 ] d h h 5 3 . 7 1 6 . 0 3 8 . 2 1 1 . 0 — 1 5 . 5 — 5 . 0 8 . 4 1 . 6 |10| f i o ] [ 2 1 ] [ 2 1 j f 1 9 . 0 4 . 8 8 . 0 2 . 8 — 11.0 — 2 . 0 4 . 9 0 . 7 [ 1 1 ] ' i h [ 1 7 ] [ 1 6 ] s 2 7 . 3 2 4 . 8 8 . 0 — 2 . 5 4 0 . 3 1 3 . 6 1 . 6 d l [3| [7[ [ 1 0 ] [ 1 1 ] d l 1 0 . 6 4 . 0 1 0 . 3 4 . 5 — 0 . 3 + 0 . 5 5 . 8 0 . 5 m |6] [ 1 1 ] [8] t c t . 2 9 . 1 8 . 5 2 2 . 0 7 . 1 — 7 . 1 — 1 . 4 5 . 6 1 . 0 i § ul [ 3 1 j [ 3 4 ] [ 5 9 ] [ 5 6 ] l e pioc.oe d e l l a l i b i a e l ' a t t i v i t à s o l a r e 133 invece dei cinque climi della regione c tre (sl, a e p) hanno differenze negative che praticamente rientrano negli scarti. epoca 0. — si mantengono molto più attenuate le caratteristiche dell'epoca — 1. in questa epoca scompare l'anomalia del clima d che anzi ora risulta uno dei più regolari. questo fatto spinge a ritenere casuale l'anomalia riscontratasi nell'epoca — 1. per quanto riguarda la regione t, le differenze sono, in rapporto agli scarti, più accentuate che nell'epoca — 1. per l a regione s si hanno differenze positive — che però rientrano negli scarti —in tutti i climi ad eccezione che nel clima d ove si ha una significativa differenza negativa. per la regione m si hanno differenze positive e negative che rientrano negli scarti. per la regione c, che anche nell'epoca — 1 non mostrava il regolare andamento, si lianno ora quattro valori positivi con significato per i primi quattro climi (m, sl, sc, a) mentre per il clima p la differenza rientra nello scarto. epoca + !• — qui le differenze rientrano generalmente negli scarti. fanno eccezione la regione m in cui si hanno significative differenze sempre negative, le regioni t e c che hanno differenze negative con significato rispettivamente nei climi m e sc e la regione s che ha nel clima d una differenza positiva abbastanza significativa. triade. — nella triade, lo studio dei caratteri delle differenze mostra l'evidente dipendenza da quelli che sono i caratteri nelle epoche già esaminate. molto valore ha, per la comprensione dei caratteri della triade, la considerazione del peso relativamente a cui entrano nella triade i dati delle epoche. frequenza in linea molto generale si può dire che l'andamento delle differenze per le frequenze segue quello per le intensità. nel caso delle frequenze, però, le variazioni sono molto minori e quindi esse rispecchiano soltanto quelle più accentuate delle intensità. riassumendo e concludendo, possiamo dunque asserire che l'andamento delle piogge libiche risente l'influenza dell'attività solare. 134 c . godoli nella regione t tale influenza è nettissima in tutti i climi mentre nelle altre regioni risulta meno cospicua. nella regione t e forse in quelle del clima d si hanno le minime piogge al massimo dell'attività solare, mentre nelle altre regioni tale andamento è sfasato di un anno. confermano le nostre conclusioni gli studi di eredia (°ì, it. clayton (1 0), r. de flotte de roquevaire ( n ) . fàntoli, invece, studiando il regime pluviometrico di rengasi e tripoli ( 7 ) giunge alla conclusione che non c'è nessuna relazione f r a l'attività solare e l e piogge della libia. essendo qui lo studio limitato a due stazioni, probabilmente il fattore tempo non ha compensato la mancanza del fattore spazio. firenze — osservatorio astrofisico di arcetri — centro di astrofisica del c.n.r. — febbraio 1952. riassunto nella presente nota si studiano le piogge della libia in relazione con il ciclo dell'attività solare. i caratteri pluviometrici mostrano una netta dipendenza dalla fase del ciclo. summary in the present paper are referrecl researclies on the rains of lybia in relation to the c y c l e of solar activity. the pluviometrich charaeteristics shoiv a neat dependence on the phase of the c y c l e . b i b l i o g r a f i a ( ! ) s . p o l i i e f . vercelli: relazioni fra vattività sohire e fenomeni yneteorologìci e climatici, con speciale riguardo ai lavori compiuti i n italia. l i n c e i . pro* b l e m i a t t u a l i di scienza e di c u l t u r a . q u a d e r n o n . 7 ( 1 9 4 8 ) . ( 2 ) m . picone: vedute matematiche sull'analisi dei periodi. r e n d . s e m . m a t . e f i s . m i l a n o , 1 9 ( 1 9 4 8 ) . ( 3 ) fàntoli: le piogge della libia. r o m a , u f f i c i o studi d e l m i n i s t e r o a . i . ( 1 9 5 2 ) . ( 4 ) fàntoli: quarantanni di piogge in libia. b o l l . g e o g r . n . 2 . t r i p o l i 1 9 3 2 . le pioc.oe della l i b i a e l ' a t t i v i t à s o l a r e 135 (®) t . nicolini: ii termine di chandler nelle variazioni ili livello del info victoria. n a p o l i , c a p o d i m o n t e . c o n t r . g e o f . 2, 9 ( 1 9 4 6 ) . (") london, air ministry, met. office geoph. meni. n. 2 0 ( 1 9 2 3 ) . q u a r t . j o u r n . r o y . m e t . s o c . a p r i l e 1 9 3 0 pag. 1 1 3 . m o . w e a . r e w . m a r » o 1 9 2 4 . ( 7 ) fìntoli: se esista una relazione fra il ciclo undecennale dell'attività solare e le piogge in libia. b o l l . g e o g . n . 4 . t r i p o l i ( 1 9 3 3 ) . ( k ) polli: andamento climatico stagionale e attività solare. t r i e s t e , istituto t a l a s s o g r a f i c o . pubi), n. 2 5 5 . (") peisino: l'attività solare e gli elementi meteorologici di collurania. c o l l u r a n i a t e r a m o , meni, e oss. 2, 1 ( 1 9 4 6 ) . ( 1 0 ) h. clayton: les varialions de la railiation solai re et le temps. meni, d e l'office nat. mat. de f r a n c e . p a r i s , 1 9 2 5 . ( " ) r . de flotte de roquevaire: reclierches sur les ladies solaires et la pluie à alger. c o n g r è s de l'eau, a l g e r . 1 9 2 8 . a p r o p o s de l'usage d u c h a m p m a g n é t i q u e terrestre p o u r l'étude d e s r a y o n s e o s m i q u e s o u de l'activité m a g n é t i q u e p i e e r e n o t ì l m a y a u d depuis plusieurs années déjà, on a cherché à détermi ner e x p é r i m e n t a l e m e n t la position du m i n i m u m d ' i n t e n s i t é de la composante nucléaire en fonct i on d e la l a t i t u d e , et rapidement des écarts considérables, v a r i a n t d'ailleurs avec la longitude, ont été coustatés e n t r e c e t t e position e t l ' é q u a t e u r géomagnétique, alors que la théorie prévoit leur coincidence. des h y p o t h è s e s diverses (x) ont été avancées p o u r r e n d r e c o m p t e de tels écarts; mais derniérement, p . bothwell a montré qu'ils coincidaient — a u moins d a n s les régions africaines — avec les anomalies du c h a m p m a g n é t i q u e t e r r e s t r e (z). or nous avions eu l'occasion (3) (4) d ' a t tirer l ' a t t e n t i o n des géomagnéticiens sur le fait que le c h a m p géomagnétique, actuellement utilisé p o u r ordonner les phénomènes magnétiques observés, é t a i t sans d o u t e inapte 4 remplir ce róle. certes, il a v a i t été légitime d ' a b a n d o n n e r les valeurs au sol pour u n tei usage d e v a n t l'évidence que les corpuscules responsables des phénomènes ne subissent p a s l'influence de ce c h a m p a u sol, souvent f o r t e m e n t déformé p a r des anomalies superficielles; mais n ' é t a i t c e p a s t o m b e r dans l'excès inverse que de lui s u b s t i t u e r le c h a m p géomagnétique, p u r e a b s t r a c t i o n m a t h é m a t i q u e n e conservant de l'analyse sphérique h a r m o n i q u e des données d'observation que vunique p r e m i e r t e r m e , alors que, seule, la f o n c t i o n m a t h é m a t i q u e c o m p r e n a n t tous les t e r m e s de c e t t e a n a l y s e p o u v a i t prétendre décrire de m a n i è r e approchée mais significative la réalité physique visée? de ceci, u n e p r e u v e en quelque sorte m a t h é m a tique est donnée p a r les r é s u l t a t s de l'analyse qui m o n t r e n t que l ' i m p o r t a n c e relative, p a r r a p p o r t au premier t e r m e , des a u t r e s t e r m e s est de 14.7% au sol, 5 . 4 % à 6.300 k m d ' a l t i t u d e , 1 . 8 % à 25.000 k m et 0 . 9 % à 57.000 k m : le c h a m p géomagnétique ne commence donc à bien représenter le c h a m p réél q u ' a u delà de 30.000 k m (*); a u x altit u d e s inférieures, il ne p e u t en f a i t pas plus p r é t e n d r e que les valeurs m a g n é t i q u e s a u sol ordonner effectivement les phénomènes observés, et c'est p o u r t a n t p e u t è t r e à ces altitudes, où l'intensité d u c h a m p commence à devenir b e a u c o u p plus forte, que la f o r m e réelle du c h a m p a u r a t o u t e son i m p o r t a n c e dans l ' é t u d e des phénomènes. de c e t t e conclusion t o u t e théorique, u n e p r e u v e expér i m e n t a l e é t a i t t e n t é e en m o n t r a n t que les coordonnées m a g n é t i q u e s (inclinaison e t déclinaison) qui o r d o n n a i e n t le m i e u x les phénomènes d ' a c t i v i t é m a g n é t i q u e — ce-là qui sont la m a n i f e s t a t i o n la plus directe d ' u n e cause corpuscolaire — é t a i e n t représentées p a r la conflguration réelle d u c h a m p vers 3.000 à 5.000 k m d ' a l t i t u d e , telle qu'ellc p e u t è t r e déduite de l'analyse de e . h . vestine (5). l a signification d ' u n tei r é s u l t a t risque c e p e n d a n t d ' é t r e pleine d'équivoques, e t sans d o u t e ne les avions-nous pas entiér e m e n t dissipées d a n s la p r é s e n t a t i o n q u e nous en avions f a i t e ([3] p. 91, [4], p. 51 ) ; (*) l e s 5 % d e s 6 . 3 0 0 k m e a u s e n t d e s d é f o r m a t i o n s p a r f o i s c o n s i d é r a b l e s d e s i s o c l i n e s ; a i n s i , d a n s les r é g i o n s s i b é r i e n n e s , les c o u r b e s q u i r e p r é s e n t e n t l a p r o j e c t i o n s u r la s u r f a c e d e l a t e r r e d e c e s i s o c l i n e s s o n t d i s t a n t e ® d e 600 k m d e s c o u r b e s r e p r é s e n t a n t les i s o c l i n e s g é o m a g n é t i q u e s e o r r e s p o n d a n t e s . 132 p.-n. mayaud c'est p o u r q u o i nous essaierons à n o u v e a u d ' e x p o s e r c e t t e signification (*). n o u s noterons d'aborri quo, si les géomagnéticiens — et après eux les cosmiciens — o n t cboisi le c h a m p g é o m a g n é t i q u e p o u r ordonner les phénomènes, c ' e s t qu'ils se t r o u v a i e n t d e v a n t u n problème tridimensionnél, à savoir le m o u v e m e n t des eorpuscules d a n s un espace à trois diinensions où existe u n c h a m p m a g n é t i q u e qui lui aussi est tridimensionnel, e t q u e ce c h a m p « géom a g n é t i q u e » leur é t a i t u n m o y e n commode de décrire le c h a m p tridimensionnel de la terre. l'idéal serait de prendre le champ magnétique tridimensionnel tei qu'il peut étre déduit en tout point de vespace à partir de vanalyse spliérique. mais à qui connaìt d'une part la complexité du calcul des t r a j e c t o i r e s d a n s le c h a m p d ' u n dipòle, il est é v i d e n t q u ' u n e teile solution a p p a r a ì t impossible à r e t e n i r p o u r les é t u d e s théoriques; et d'autre part c o m m e n t ordonner les observations faites à la surface de la t e r r e (espace à d e u x diinensions) p a r u n e q u a n t i t é tridimensionnelle (le c h a m p m a g n é t i q u e réel) (**)? sans d o u t e est-il justifié d'utiliser le c h a m p géom a g n é t i q u e p o u r des calculs théoriques, à condition toutefois de ne p a s vouloir chercher ensuite u n e coincidence précise e n t r e prévisions ainsi o b t e n u e s e t observations: u n e teile coincidence ne p e u t é t r e p u i s q u e le c h a m p réel n'est pas le c h a m p géomagnétique. mais u n e teile simpliflcation n ' é t a n t pas possible p o u r ordonner les o b s e r v a t i o n s elles-mèmes, il d e v i e n d r a nécessaire de s'or i e n t e r vers u n e solution i n t é r m é d i a i r e . p o u r en c o m p r e n d r e la portée, nous ferons d ' a b o r d u n e r e m a r q u e de vocabulaire: l'expression « c h a m p au sol » — comrne celle d e « c h a m p à u n e a l t i t u d e donnée » — est équivoque, car ce « c h a m p a u x sul » n ' e s t en f a i t q u ' u n e coupé à d e u x dimensions d a n s le c h a m p réel qui, lui, est tridimensionnel (*) n o u s v o u d r i o n s ioi r e m e r c i e r m r . e . t l i e l l i e r , q u i n o u s a a i d é p a r ses c r i t i q u e s à c l a r i f i e r n o t r e p e n s é e . (**) b i e n q u e le c h a m p g é o m a g n é t i q u e s o i t l u i a u s s i t r i d i m e n s i o n n e l , o n n e se h e u r t e p a s à l a m è m e d i f f i c u l t é d a n s s o n u t f l i s a t i o n ; p a r e x e m p l e , s u r la v e r t i c a l e d ' u n l i e u d e l a s u r f a c e d e la t e r r e , i n c l i n a i s o n e t d é c l i n a i s o n s o n t c o n s t a n t e s , elles n e le s o n t p a s d a n s le c a s d u c h a m p r é e l . c o m m e t o u t c h a m p (*); et pourrait-on a j o u t e r , le c h a m p g é o m a g n é t i q u e , considéré vis à vis d e la réalité p h y s i q u e existante n ' e s t lui-mème q u ' u n e teile coupé, c'est à dire que, à p a r l e r en t o u t e rigueur, il ne représente le c h a m p réel q u e sur la surface de la sphère de r a y o n infìni, concentrique à la terre. p a r c o n s é q u e n t , a u c u n de tous ces c h a m p s — a u sol, à u n e a l t i t u d e quelconque, ou g é o m a g n é t i q u e — ne p e u t suffir à ordonner les p h é n o m è n e s observés, bien que c h a c u n d ' eux r e p r e s e n t e le c h a m p réel sur la surface d ' u n e sphère donnée, ceci à l ' a p p r o x i i n a t i o n près de l ' a n a l y s e spliériq u e (*). i l a été commode, e t ce n ' é t a i t rien de plus que cela, d'utiliser la « coupé » rep r é s e n t e p a r le c h a m p a u sol, puis on lui a s u b s t i t u é celle que représente le c h a m p géom a g n é t i q u e ; m a i s e e t t e dernière solution n ' e s t plus suffisante à cause de la, finesse plus g r a n d e a t t e i n t e p a r les observations, et il devient necessaire d'utiliser une « coupé » intermédiaire e n t r e le c h a m p a u sol et le c h a m p g é o m a g n é t i q u e . le eritère qui peut p e r m e t t r e de f a i r e le choix de c e t t e coupé inermédiaire est simple; ainsi, lorsque la theorie m o n t r e q u e d a n s le c h a m p d ' u n dipòle tei p h é n o m è n e d e v r a i t avoir u n e intensité c o n s t a n t e à u n e l a t i t u d e donnée, la « coupé » la meilleure sera celle qui m a n i f e s t e r à le m i e u x c e t t e constance, e t on sera alors conduit p o u r ordonner le p h é n o m è n e à utiliser les « valeurs m a g n é t i q u e » à u n e altit u d e donnée. certes il ne se passe rien de special à une teile altitude-, la seule réalité p h y s i q u e agiss a n t e est le c h a m p réel d a n s tout l'espace, et la coupé ainsi f a i t e n ' e n donne qu'une image convèntionnelle. on p e u t dire d'elle qu'elle est l'image la plus commode pour ordonner les phénomènes, et l ' a v a n t a g e cert a i n qu'elle procure est de s u p p r i m e r des (*) i l f a u d r a i t e m p l o y e r p o u r é t r e e x a c t l ' e x p r e s s i o n « v a l è u r m a g n é t i q u e », e t n o n p a s « • c h a m p ». (**) on n o t e r à q u e c e t t e a p p r o x i m a t i o n n ' e s t p a s m o i n s b o l i n e q u e c e l l e l à m è m e qui e x i s t e p o u r le c h a m p g é o m a g n é t i q u e p u i s q u e l ' u n e e t l ' a u t r e s o n t celles d e l a mème a n a l y s e s p l i é r i q u e h a r m o n i q u e d e s mèmes d o n n é e s exp é r i m e n t a l e s . p a r c o n t r e , u t i l i s e r le c h a m p g é o m a g n é t i q u e a i l l e u r s q u ' à f i n t i n i ou à de g r a n d e s d i s t a n c e s d e l a t e r r e e s t i n t ' r o d u i r e u n e a p p r o x i m a t i o n supplémentaire. a i'ropos de l'usage du ciiamp magnetique t e r r e s t r e , etc. 133 faux problèmes provoqués p a r l'utilisation du champ g é o m a g n é t i q u e ; ou, t o u t a u moins faudrait-il, a v a n t d e chercher des explications aux é c a r t s observés e n t r e observations et champ géomagnétique, voir si ces écarts ne peuvent è t r e réduits p a r l'emploi d ' u n e autre « coupé » — le c h a m p géomagnétique, nous l'avons dit, n ' e s t en f a i t au p i a n des observations e t de la réalité q ' u n e « coupé » parmi d ' a u t r e s — plus r e p r é s e n t a t i v e du champ réel agissant. la complexité ainsi i n t r o d u i t e d a n s l'étude des p h é n o m è n e s qui d é p e n d e n t du c h a m p magnétique t e r r e s t r e est évidente; car ehaque observatoire ou lieu d ' o b s e r v a t i o n devrait se voir a t t r i b u e r des inclinaisons magnétiques multiples selon le t y p e de phénomène; et, de plus, il f a u d r a i t t e n i r compte de la v a r i a t i o n séculaire. mais aussi longtemps que l'on r e f u s e r à u n e telle complexité, il semble q u e l'on se h e u r t e r a i n é v i t a b l e m e n t à des effets parasites dans l ' i n t e r p r é t a tion des phénomènes, et ceci i n u t i l e m e n t . les conclusions précédentes a v a i e n t été obtenues à p a r t i r de resultata expérimentaux p r o v e n a n t des seules régions de h a u t e latitude; c ' e s t là seulement en effet q u ' a r rivent les corpuscules responsables des variations rapides d u c h a m p m a g n é t i q u e observées à la susface d a la t e r r e . les rayons cosmiques permettenti, semble-t-il, de conflrmer de telles conclusions en m o n t r a n t leui' v a l ; di t é p o u r les régions de l a t i t u d e moyenne ou équatoriale. p o u r cela, nous utiliserons les mesures mentionnées ci-dessus (2) e t r e p r e n d r o n s d'autres mesures (6). d a n s l'utilisation de telles données, qui souvent o n t été entreprises s u r t o u t p o u r déterminer l ' é q u a t e u r des rayons cosmiques, nous ne chercherons pas à d é t e r m i n e r en lui-mème un tei équateur; en effet d é t e r m i n e r vabscisse d ' u n minimum sur u n e courbe expérimentale est u n e opération s u j e t t e à t r o p d'erreurs. mais, donnant la m è m e i m p o r t a n c e a u x mesures faites à t o u t e l a t i t u d e , nous chercherons le système de coordonnées magnétiques, c'està-dire encore l ' a l t i t u d e à laquelle les valeurs magnétiques réelles p e r m e t t e n t d ' o b t e n i r la meilleure symétrie e n t r e les deux hémisphères pour l'intensité des rayons cosmiques en fonction de la l a t i t u d e , ou encore la meilleure homogénéité e n t r e différents méridiens p o u r u n m è m e hémisphère, en p a r t a n t du principe q u ' a v a n i t o u t a u t r e cause de variation de ce pliénomène, à l a t i t u d e m a g n é t i q u e égale doit correspondre i n t e n s i t à égale. nous emploierons s y s t é m a t i q u e m e n t comme abscisse la latitude m a g n é t i q u e a u lieu de vinclinaison bien que la notion de l a t i t u d e ne puisse en f a i t è t r e définie que pour le c h a m p du dipóle; u n e telle liberté est permise en t a n t que la relation m a t h é m a t i q u e (tg 1 = 2 t g 0 ) qui lie les d e u x q u a n t i t é s d a n s le c h a m p d ' u n dipòle et que nous conservons a r b i t r a i r e m e n t ici est u n e fonction u n i v o q u e et continue; et, p a r c e q u e c e t t e « l a t i t u d e » a l ' a v a n t a g e de fournir u n p a r a m è t r e d'espace qui est grossièrement en relation linéaire avec les distances le long des méridiens, ce que ne f a i t pas l'inclinaison on évite u n e concentration des points dans les régions polaires et leur dispersion dans les régions équatoriales. les l a t i t u d e s magnétiques à cliaque a l t i t u d e o n t été déduites des données de e . h . vestine (5). l a figure 1 représente la v a r i a t i o n de l'intensité de la c o m p o s a n t e nucléaire en fonction de la l a t i t u d e m a g n é t i q u e a u sol, à 300 k m , à 1.000 k m , 5.000 k m d ' a l t i t u d e e t d u dipòle. les échelles d ' i n t e n s i t é qui v a r i e n t avec chacun des ensembles de mesures sont indiquées. les cercles correspond e n t a u x l a t i t u d e s nord, les croix a u x lat i t u d e s sud. a) les doimées de d. c. rose (1), recueillies lor des voyages d u « l a b r a d o r » a u t o u r de l ' a m é r i q u e de n o r d e t de « l ' a t k a » d a n s l ' a n t a r c t i q u e avec aller p a r le pacifique e t r e t o u r p a r l ' a t l a n t i q u e , o n t élé utilisées de la manière s u i v a n t e : 1) é t a n t donné le très g r a n d n o m b r e de p o i n t s d o n t chacun correspond à douze h e u r e s d ' o b s e r v a t i o n , on a f a i t la m o y e n n e des intensités moyennes observées d a n s des aires géographiques limitées en gros p a r des méridiens de 4 degrés en 4 degrés et des parallèles de 2 degrés en 2 degrés; 2) p o u r les valeurs ainsi obtenues, on a mesuré la l a t i t u d e m a g n é t i q u e a u x différentes a l t i t u d e s ou au sol, corre»pondant au b a r y c e n t r e de c h a c u n des groupes de points contenus dans les aires précédentes; 3) on a f a i t alors à n o u v e a u la m o y e n n e des valeurs correspondant à des t r a n c h e s de latitudes m a g n é t i q u e s a l l a n t de 5 en 5 de134 p-n mayaud grès; 4) p o u r les l a t i t u d e s 20° n à 50° n, les é e a r t s à la m o y e n n e des intensités o n t été calculés et sont représentés sur les graphiques p a r des barres proportionnelles; cett e trancile de l a t i t u d e r e p r é s e n t e trois passages d a n s l ' a t l a n t i q u e n o r d e t u n passage d a n s le pacifique n o r d ; 5) les courbes t r a cées l ' o n t été à p a r t i r des seuls p o i n t s de l'hémisphère n o r d ; 6) e n t r e les l a t i t u d e s 10° n et 50° s, on a p o r t é les p o i n t s individuels o b t e n u s en 2); ces points représent e n t les d e u x passages de « l ' a t k a » à ces l a t i t u d e s (le t r a j e t r e t o u r p a r l ' a t l a n t i q u e est forme p a r l'ensemble de points — pet i t e s croix — qui sont très au-dessus de la courbe m o y e n n e d a n s le cas d u c h a m p a u sol, à 300 k m et à 1.000 k m ) . b) p o u r les données p . r o t h w e l l (2), on a mesuré la l a t i t u d e m a g n é t i q u e a u x différentes a l t i t u d e s on au sol pour c h a q u e point — c h a c u n d ' e u x correspond à six heures d'observation — , puis l ' o p é r a t i o n 3) précédente a été effectuée. les données utilisées correspondent, à d e u x voyages successifs d u « roxburgh» d a n s le sens s u d n o r d de cape town à s o u t h a m p t o n p a r la còte o u e s t de l ' a f r i q u e . chaque point d u grap h i q u e est la m o y e n n e de cinq à dix points observés. les courbes tracées l ' o n t été à p a r t i r des seuls points de l ' h é m i s p h è r e nord. g) p o u r les données m. oda, la l a t i t u d e m a g n é t i q u e a u x différentes a l t i t u d e s ou au sol a été m e s u r é e p o u r c h a q u e point — chacun d ' e u x correspond à 24 heures d'observation — . p o u r les p o i n t sud d o n t la l a t i t u d e est supérieure à 50°, de m è m e q u e p o u r t o u s les p o i n t s n o r d , on a p o r t é sur le g r a p h i q u e les p o i n t s individuels; pour les points sud d o n t la l a t i t u d e est comprise e n t r e 0 e t 50°, on a effectué l'opération 3). ces données correspondent géographiquem e n t à un t r a j e t d u « soja » depuis l'équat e u r vers 100° e de longitude j u s q u ' a u cercle polaire a n t a r c t i q u e vers 70° e en p a s s a n t p a r cape town et r e t o u r à peu près p a r la m è m e voie j u s q u ' a u j a p o n via sing a p o u r . sur ce graphique, chaque point eorr e s p o n d a n t à u n e m o y e n n e de m e s u r e s est o r d i n a i r e m e n t la m o y e n n e de 3 à 5 mesures individuelles, parfois b e a u c o u p plus. les courbes tracées l ' o n t été à p a r t i r des seuls p o i n t s de l ' h é m i s p h è r e s u d . sur c e t t e figure, on p e u t f a i r e les remarques suivantes. les mesures a, a u x latitudes moyennes .nord, se r e g r o u p e n t le mieux, selon les é e a r t s à la m o y e n n e fiaurés pour c h a q u e cercle, pour 1.000 ou 300 k m d'alt i t u d e ; t a n d i s que les m e s u r e s sud de l'aller de 1' « a t k a » r e s t e n t t o u j o u r s à l'intérieur des m a r g e s d ' é c a r t p a r r a p p o r t à la courbe m o y e n n e n o r d , l'anomalie d a n s l'intensité des ravons cosmiques, qui a p p a r a ì t aux lat i t u d e s équatoriales d a n s le cas d u champ d u dipòle pour les mesures d u retour de l'« a t k a », se développe e t semble exister aussi a u x l a t i t u d e s m o y e n n e s sud. les mesures b a p p a r a i s s é n t è t r e le m i e u x coordonnées q u a n t à la comparaison des d e u x hémisphères vers 1.000 k m d ' a l t i t u d e ; l'écart qui subsiste alors e n t r e 10° e t 20° est certain e m e n t b e a u c o u p moins i m p o r t a n t que celui qui a p p a r a ì t avec le c h a m p a u sol entre 20° et 50°; la question r e s t e r a i t posée de savoir si l ' a l t i t u d e de meilleur coordination v a r i e r a i t a v e c la l a t i t u d e , mais de nouvelles mesures seraient sans d o u t e nécessaires pour r é p o n d r e à c e t t e nouvelle question. dans les mesures g, les points n o r d r e s t e n t systém a t i q u e m e n t au-dessus des points sud, l'éc a r t é t a n t c e p e n d a n t le plus faible dans le cas d u c h a m p à 1.000 ou 5.000 k m . ce qui est sans d o u t e le plus r e m a r q u a b l e est que avec le c h a m p d u dipòle, l'inflexion (ou le m a x i m u m ) d e la courbe t r a c é e au travers des valeurs sud s ' a m o r c e dès 35° de latit u d e ; et ceci e n t r a ì n e r a i t é v i d e m m e n t une différence considérable d ' a v e c les mesures a e t b, à supposer que, après normalisation des intensités, on cherche à f a i r e u n e superposition des courbes a, b et c p o u r les c o m p a r e r ; de ce p o i n t de vue, les courbes c p o u r 1.000, 300 k m ou a u sol apparaissént b e a u c o u p plus normales. e n f ì n , la dispersion des points au-dessus des l a t i t u d e s 50° s correspond à u n e aire géographique relativ e m e n t r e s t r e i n t e ; elle semble donc due plus a u x rayons cosmiques eux-mèmes plutòt q u ' à des anomalies m a g n é t i q u e s . e n conclusion, les trois séries de mesures ici étudióes semblent bien confìrmer la conclusion o b t e n u e a n t é r i e u r e m e n t p o u r les seules h a u t e s l a t i t u d e s et p o u r u n a u t r e type d e r a y o n n e m e n t corpusculaire: le c h a m p du dipòle s ' a v è r e incapable d ' o r d o n n e r , a u x lat i t u d e s m o y e n n e s ou équatoriales, les ini n t c s i t e _i7o •irf-." i n t (70 90 . v • i n t 5 o o v i i \ . 130 . <10 \ "i n t _ 170 _ 90 *v-j"v-ì. v i v y int. .170̂ . 90 _ 130 . 110 90' 80" 6 0 ' 2 0 ' r ' 0* b intensive 150 . 140 130 . 110 \ 6 0 " e \ _ 100 i n t . 80 _ (so ® v v _ 1 0 0 * i n t . '•-•-•-è" i50_ 80 . 140 e \ a v \ 5 . n l. x \ int. _ 80 .150 _140 _1z0 .100 \ \ \ x _ 140 .120 _ 100 x\. 40" > v » r ® 0 • moyenne de mesures ì nord • m esure indìviduelle j x m oy«nne de mesures 1 5 j • m esure indìviduellc j u c inten site' _ 220 x . v x v . \ x \ i n t .220 . 2 0 0 x v \ x \ a \ x vx. .140 i n t . • . 2 2 0 ' x« • .120 _ 2 0 0 _ 1 8 0 . 1 6 0 _ (40 i n t . 2 2 0 x \ 2 0 0 x ' v v >: " x \ x \ \ •• x . \ -140 i n t . (• .220 . 120 .zoo _180 -k.0 _140 2 0 " 70" io" 40" *xx -r1z0 0' fig. 1 v a r i a t i o n e n f o n c t i o n d e l a l a t i t u d e m a g n é t i q u e a u s o l , à d i v e r s e s a l t i t u d e s o u g é o m a g n é t i q u e d e l ' i n t e n s i t é d e l a c o m p o s a n t e n u c l é a i r e d e s r a y o n s c o s m i q u e s o b s e r v é e s u r d i f f e r e n t e s r o u t e s m a r i t i m e s ; p o u r o b t e n i r le n o m b r e d ' i m p u l s i o n s e n r e g i s t r é e s p a r h e u r e , il f a u t r e s p e c t i v e m e n t m u l t i p l i e r l e s é e h e l l e s d ' i n t e n s i t é i n d i q u é e s s u r l e s d i f i é r e n t s g r a p h i q u e p a r 6 4 ( a ) , 100 ( b ) e t 3 2 (c). 136 p.-n. mayaud tensités observées de la composante nucléaire des rayons cosmiques. selon la terminologie proposée, la « coupé i n t e r m é d i a i r e » qui p e r m e t t r a i t d ' o r d o n n e r a u m i e u x d e tels phénomènes serait représentée p a r les valeurs réelles d u c h a m p m a g n é t i q u e vers 1.000 à 300 k m d ' a l t i t u d e . c e t t e a l t i t u d e d e m a n d e r a i t é v i d e m m e n t à é t r e précisée p a r des mesures effectuées sur les longitudes les plus diverses de p a r t et d ' a u t r e de l'éq u a t e u r . nous noterons seulement q u ' u n e comparaison e n t r e les a l t i t u d e s des « coupes intermédiaires » qui se révèlent les meilleures p o u r ordonner soit les phénomènes d ' a c t i vité m a g n é t i q u e soit ceux de rayons cosmiques ne doit p a s é t r e serrée de t r o p près. car d ' u n e p a r t , les r é s u l t a t s d ' a c t i v i t é m a g n é t i q u e utilisés d a n s n o t r e é t u d e précéd e n t e (3) p r o v i e n n e n t de moyennes annuelles, lesquelles r e p r é s e n t e n t p a r conséquent la superposition de multiples phénomènes, ce qui t e n d sans d o u t e à adoucir considérablem e n t les anomalies du c h a m p m a g n é t i q u e réel, t a n d i s que les r é s u l t a t s d e rayons cosmiques étudiés d a n s c e t t e n o t e p r o v i e n n e n t de mesures quasi instantanées-, e t d ' a u t r e p a r t , les r é s u l t a t s m a g n è t i q u e s sont le f r u i t d ' u n p h é n o m è n e secondaire, c'est-à-dire la superposition des c h a m p s m a g n è t i q u e s prod u i t s à clistance p a r des c o u r a n t s électriques qui sont eux-mèmes la conséquence d u phénomène p r i m a i r e que constitue l'arrivée des corpuscules, t a n d i s q u e les r é s u l t a t s cosmiques sont la conséquence p r e s q u e uniquement locale de l'arrivée des corpuscules. résumé tant dans vinterpretation de résultats magnètiques que dans celle de résultats de rayons cosmiques, des auteurs se sont souvent heurtés à des éearts importants entre les valeurs observées à la surface de la terre puis ordonnées à l'aide du champ du dipòle et les valeurs théoriques-, par exemple, l'intensité d'un phénomène donne, qui devrait selon la théorie étre constante sur un parallèle de latitude, se révèle en fait étre variable. on montre ici que vemploi d'un champ conventionnel, intermédiaire entre le champ du dipòle et le champ au sol, emploi qui avait permis antérieurement de rendre compie de tels éearts pour des résultats d'activité magnétique, permei d'en rendre compie également pour des résultats de rayons cosmiques. l'attention est ainsi attirée sur le fait que les coordonnées géomagnétiques devront peutétre étre abandonnées dans vinterpretation des phenomènes géophysiques. abstract as much in the interpretation of magnetie results as in that of cosmicrays results, some authors have frequenfly found theirselves confronted with important discrepancies between values as observed at eartli-level and subsequently ordinateci by means of the dipolefield, and theoretical values-, to talee an example, the intensity of a given phenomenon, which ought, theoretically, to be constant for a specific latitude, proves in point of fact to be variable. we wish to show here how the use of a conventional field, intumediate between the values of the dipole-field and these of the real field at earth-level, which previously enabled one to explain such discrepancies in the case of magnetie activity results, enables one also to explain similar discrepancies in cosmic-rays results. one's attention is thus directed toivards the fact that the geomagnetic coordinates may perhaps have to be abandoned in the interpretation of geophysical phenomena. riassunto tanto nell'interpretazione dei risultati magnetici che in quella dei raggi cosmici, alcuni autori si sono spesso trovati di fronte a rilevanti divergenze rispetto ai valori osservati al livello del suolo, in seguito superate con l'aiuto del camjw dipolo e dei valori teorici-, per esempio l'intensità di un dato fenomeno che, teoricamente, dovrebbe essere costante su una specifica latitudine, in pratica si rivela poi, variabile. in questo lavoro si dimostra che l'impiego di un campo convenzionale intermedio fra i valori del campo dipolo e quelli del campo reale al livello del suolo, impiego che aveva permesso, in precedenza, di spiegare tali divergenze nel caso di risultati di attività maa propos de l'usage du champ magnetique t e r r e s t r e , etc. 137 gnetica, permette di spiegare, altresì, tali divergenze per alcuni risultati relativi ai raggi cosmici. viene così attirata l'attenzione sul fatto che le coordinate geomagnetiche, nell'interpretazione di fenomeni geofisici, dovrebbero — forse — essere abbandonate. b i b l i o g r a f i e (!) r o s e , d . c . ; f e n t o n , k . b . ; k a t z t m a n j . e t simpson, j . a . , latitude effect of the cosmic ray nucleoli and meson components at sea level from the arctic to the antarctic, « c a n . j o u r n . of p h y s . », 3 4 , 9 6 8 9 8 4 ( 1 9 5 6 ) . ( 2 ) r o t h w e l l , p . e t q t j e n b y , j . , beport at varetina conference, j u n e 1957, t o b e p u b l i s h e d i n « n u o v o c i m e n t o », (le d e t a i l d e s m e s u r e s u t i l i s é e s d a n s c e t t e n o t e n o u s a é t é d i r e c t e m e n t c o m m u n i q u é paim i s s p . r o t h w e l l ) . (3) mayaud, p . n . , activité magnétique dans les régions polaires, « a n n . g e o p h y s . », 12, 8 4 1 0 0 ( 1 9 5 6 ) . (4) — terre adélie 1951-1952, « m a g n e t i s m o t e r r e s t r e » , f a s e . i l , a e t i v i t é m a g n é t i q u e d a n s l e s r é g i o n s p o l a i r e s , e x p é d i t i o n s p o l a i r e s f r a n f a i s e s , p a r i s , 1 9 5 5 . (5) vestine, e . h . , the geomagnetic field, c a r n e g i e i n s t i t u t i o n , 1947,' p u b l i c a t i o n 5 8 0 . (6) oda, m . , beport al varenna conference, j u n e 1 9 5 7 . lauciani_fc_corretto_layout 6 631 shakemaps during the emilia sequence valentino lauciani*, licia faenza, alberto michelini istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6160 1. introduction shakemap is a software package that can be used to generate maps of ground shaking for various peak ground motion (pgm) parameters, including peak ground acceleration (pga), peak ground velocity, and spectral acceleration response at 0.3 s, 1.0 s and 3.0 s, and instrumentally derived intensities. shakemap has been implemented in italy at the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology) since 2006 (http://shakemap.rm.ingv.it), with the primary aim being to help the dipartimento della protezione civile (dpc; civil protection department) civil defense agency in the definition of rapid and accurate information on where earthquake damage is located, to correctly direct rescue teams and to organize emergency responses. based on the shakemap software package [wald et al. 1999, worden et al. 2010], which was developed by the u.s. geological survey (usgs), the ingv is constructing shake maps for ml ≥3.0, with the adoption of a fully automatic procedure based on manually revised locations and magnitudes [michelini et al. 2008]. the focus of this study is the description of the progressive generation of these shake maps for the sequence that struck the emilia-romagna region in may 2012. at its core, shakemap is a seismologically based interpolation algorithm that exploits the available data of the observed ground motions and the available seismological knowledge, to produce maps of ground motion at local and regional scales. thus, in addition to data that are essential to derive realistic and accurate results, the fundamental ingredients for obtaining accurate maps are: the ground-motion prediction equation (gmpe), as a function of distance at different periods, and for different magnitudes; and realistic descriptions of the amplifications that the local site geology induces on the incoming seismic wavefield; i.e., the site effects. in its current version, shakemap relies on regional attenuation laws and local site amplifications based on the s-wave velocities in the uppermost 30 m (vs30) to generate its pgm maps. in this report, we start with a chronicle of the generation of the shake maps for the two strongest earthquakes of the sequence, and we conclude with a comment on the critical aspects of the procedure we adopted. 2. the may 20, 2012, ml 5.9 earthquake in this section, we present a concise description of the evolution of the shakemap determination for the may 20, 2012, ml 5.9 earthquake. (i) the automatic final earthquake location (origin time, 02:03:52 gmt; latitude, 44.89˚n; longitude, 11.27˚e; depth, 4.95 km) was available at 02:07:23; 4 min after the origin time. (ii) the manually revised location became available 20 min after the origin time, with a similar location, but slightly different depth (6.3 km). (iii) for the magnitude estimation, the first automatic determination, which became available within about 4 min from the origin time, was ml 5.9. the manual revision, which was available after 20 min, confirmed the same value. the first moment magnitude was available 1.5 h later, as mw 5.9. (iv) the first shake map based on the revised location and magnitude came out a couple of minutes after the final revised location, without any data, as it relied only on the epicentral information, the gmpe and the site effects. for technical reasons, the procedure to download the data failed. only 50 min from the origin time were the observed data included in shakemap. the data availability during the main events suffered from saturation of the near-source broadband recordings, as discussed in faenza et al. [2011]. also, faenza et al. [2011] stressed the importance of availing to the observed data to accurately reproduce the ground shaking experienced. in particular, the importance of the strong-motion data was indicated, which do not saturate at distances close to the epicenter, where source effects on ground shaking, which are hardly predicted by the gmpes, can strongly influence the near-source shaking. in contrast to what occurred with the l'aquila mainshock, this time the ingv strong-motion data were available. unfortunately, the event occurred in an area with poor spatial coverage, and the closest strong-motion station was located at ca. 50 km. moreover, to prevent errors in the magnitude and/or inter-event variability, shakemap article history received july 24, 2012; accepted september 19, 2012. subject classification: ground motion, surveys, measurements and monitoring, interments and techniques, seismological data. 2012 emilia earthquakes adopts a bias correction [worden et al. 2010] to match the observed data and the predicted ground motion. (v) based on the time domain moment tensor solution [scognamiglio et al. 2012, this volume], the scaling laws [wells and coppersmith 1994], and the geology, with the analysis of the active tectonic structures in the area and their orientation, the fault could be included the morning after the event, to better constrain the shaking in the epicentral area (see figure 1). (vi) the data of the rete accelerometrica italiana (ran; italian accelerometric network) maintained by the dpc became available only 10 days after the mainshock. we note that the inclusion of these data is relevant, as they increase the spatial sampling of the ground shaking near the lauciani et al. 632 figure 1. final shakemap for the may 20, 2012, ml 5.9 earthquake. event location and magnitude from ingv seismic center, peak ground motion data from ingv (red triangles) and ran (blue triangles). intensities expressed in terms of the mercalli modified scale (top left panel) and pga (top right panel). left bottom panel: gmpe as a function of distance. pga values of akkar and bommer [2010] (bottom left panel: solid red line, straight predictions; solid green line, bias corrected predictions). thin green lines, uncertainties resulting from the adopted relations and used as flagging/unflagging outlier thresholds. the bias factor is -0.47. other colors are assigned to data obtained from other networks. right bottom panel: uncertainty ratio pga map. the figure shows how much the map relies on real data (blue) or on estimation (white to red). 633 source – the strong motion station in mirandola lies ca. 10 km from the surface fault projection (see figure 1). 3. the may 29, 2012, ml 5.8 earthquake immediately after the may 20, 2012, ml 5.9 earthquake, a temporary seismic network was installed in the epicentral area. twenty-four seismic stations were installed by the ingv, and 36 more by others institutes (the istituto nazionale di oceanografia e di geofisica sperimentale [inogs], the dpc, the institut de physique du globe de paris [ipgp], and others) [see details in moretti et al. 2012, this volume]; all but 10 of these stations were stand-alone. these just-mentioned 10 stations were set to transfer the data in real-time to the ingv seismic center in rome, and six of them were provided with strong-motion recorders. the resulting spatial configuration of the deployed networks greatly increased the spatial coverage in the nearsource region, compared to the pre-sequence configuration. 3.1. the ground motion parameters from earthworm on may 4, 2012, a new system, advanced italian data acquisition for seismology (aida) [mazza et al. 2012, this volume], was implemented at the ingv as the primary tool rapid shakemap in emilia figure 2. final shakemap for the may 29, 2012, ml 5.8 earthquake. as for figure 1. the bias factor is -0.53. to monitor, analyze, save and distribute the italian national seismic network seismograms. this system is based on the earthworm software [johnson et al. 1995]. as mentioned above in the description of the shakemap for the may 20, 2012, earthquake (section 2), the procedure used to generate the maps suffered from a temporal delay due to two primary factors. first, the shakemap procedure waits until the manually revised event location is ready before starting the waveform download, and secondly, the procedure got intertwined with other procedures that were similarly requesting data. it was found that while this approach works satisfactory for medium-sized events, for strong events (or during seismic sequences) within the italian territory, it is severely affected by the slow-down caused by the delayed responses of the data wave server. thus, while following the experience of the may 20, 2012, earthquake, and while exploiting the earthworm system, we decided to activate the gmew (http://www.isti2.com/ ew/ovr/gmew ovr.html) module to determine the pgm parameters suitable for shakemap in real-time. specifically, the upgrade of the procedure was implemented in test-mode on a dedicated server, making the publication of shake maps possible a very few minutes from the earthquake occurrence. the new module prepares the data for the analysis by checking for gaps, and removes the mean, and then processes the traces in the frequency domain using fast fourier transform, and removes the instrument response. the gmew module calculates the acceleration, velocity, displacement and spectra responses for the three periods of 0.3 s, 1.0 s, and 3.0 s, with 0.5% damping, for all of the traces, and then it writes an xml file formatted for shakemap applications for stations readings. 3.2. shakemap evolution of the may 29 ml 5.8 earthquake the description of the evolution of shakemap for the may 29, 2012, ml 5.8 earthquake follows. (i) the automatic final earthquake location (origin time, 07:00:03 gmt; latitude, 44.85˚n; longitude, 11.06˚e; depth, 8.21 km) was available at 07:04:22; 4 min after the origin time. (ii) the manual revised location became available 19 min from the origin time, with similar coordinates, but 2 km deeper. (iii) for the magnitude estimation, the first automatic determination that was available at the same time after the origin time was ml 5.8. the manual revision confirmed the same value. the first moment magnitude was available was 1 h and 45 min later, as mw 5.7. (iv) the first shake map was calculated using the automatic final earthquake location and magnitude, using the new earthworm module to calculate the pgm parameters; it became available 4 min after the origin time on the dedicated server mentioned above. (v) the published shake map was calculated using the reviewed final earthquake location and magnitude, and it came out 19 min after the origin time on the public server. in addition, the temporary seismic network guaranteed good spatial coverage in the epicentral area (see figure 2). (vi) as for the may 20, 2012, ml 5.9 earthquake, based on the time domain moment tensor solution [scognamiglio et al. 2012], the scaling laws [wells and coppersmith 1994], and the geology, the fault could be included 2 h after the origin time. (vii) the ran data could be included in a shake map only after almost two weeks from the earthquake occurrence, when the data become available (see figure 2). 4. discussion in may 2012, a seismic sequence struck the emilia-romagna region, an area in the po valley that had already been hit by moderate-sized earthquakes in the past, and was known for its centuries-old seismic history [castelli et al. 2012, this volume]. in this study, we describe the progressive determination of shakemap as more information became available after the mainshocks of may 20 and 29, 2012. it is well known that inclusion of observed data is of fundamental importance in the calculation of shake maps. indeed, accurate quantifications of pgm near the epicenter that avail solely of the pgm prediction equations and the site-effect corrections are difficult and prone to macroscopic errors and bias [faenza et al. 2011]. moreover, for larger earthquakes that saturate the recordings of the velocimeters at and near the epicenter, the accuracy of the shake maps depends also on the prompt availability of strong-motion data. the may 20, 2012, ml 5.9 earthquake certainly did not have enough data to produce accurate maps of the pgm given the very poor station coverage in the epicentral area. figure 3 shows the improvement in the quantification of the ground shaking with the inclusion of the source model and new data. the comparison was done following the real temporal evolution of the maps available online, and it was quantified using differential pga maps. the top panel in figure 3 illustrates the role of the source model. it was calculated by subtracting the 'preliminary' shake map based on the ingv data without a source model from the one that included the source model. this first preliminary shake map stayed online for 1 day. figure 3 shows a different pattern in the nearsource shaking due to the adoption of the joyner-boore distance measure from the fault location. indeed, the point source approximation leads to an underestimation of the pga in near source of almost 8%g. the bottom panel of figure 3 quantifies the role of the near-source stations. it compares the pga based on the ingv data only (as previously mentioned, for the first weeks, the map relies only on ingv data; figure 1, red triangles) with the 'final' one available on line at the time. this last map is calculated using both the ingv and the ran data. figure 3 illustrates the importance of the near-source stations; in this case, only the ran station of mirandola was lauciani et al. 634 635 rapid shakemap in emilia figure 3. differential pga map of the shakemap for the may 20, 2012, ml 5.9 earthquake. top panel: the map is calculated as the difference between the map based on the ingv data with the source model and the 'preliminary' map calculated without the source model. bottom panel: the map is calculated as the difference between the map based on the ingv and the ran data, as the 'final' shake map. with the one based on the ingv data only. the map highlights the importance of the near-source mrn (mirandola) station, with an increase in the shaking of ca. 10%g in the west fault area. 11˚00' 11˚00' 11˚30' 11˚30' 12˚00' 12˚00' 44˚30' 44˚30' 45˚00' 45˚00' mantova verona padova rovigo reggio nell’emilia modena bologna ferrara differential map: ingv + ran data � ingv data pga(%g) �10 �8 �6 �4 �2 0 2 4 6 8 10 10˚30' 10˚30' 11˚00' 11˚00' 11˚30' 11˚30' 44˚30' 44˚30' 45˚00' 45˚00' mantova verona padova rovigo reggio nell’emilia modena bologna ferrara differential map: ingv data � ingv data without source model pga(%g) �10 �8 �6 �4 �2 0 2 4 6 8 10 close to the epicenter. this station strongly drives the shaking in the near source, with an increase of almost 10%g in the west fault area, which indicates an underestimation of the ground motion by the gmpe in the near-source area. fortunately, the installation of the temporary stations in the epicentral area provided fair coverage for the may 29, 2012, event. this increased accuracy is well expressed by the map of the uncertainties of figure 2 (bottom-right panel). there are several sources of uncertainties in shakemap calculations, including sparse station networks, fault finiteness, and the gmpe [wald et al. 2008]. the uncertainly map represents the ratio between the actual standard deviation (e.g., the standard deviation of each point of the shakemap grid) and the standard deviation of the gmpe. the uncertainties in shakemap follow a weight scheme, which depends on the source of the data [see worden et al. 2010, for details]. the intent of the bottom-right panels of figure 1 and figure 2 is to reveal the importance of the station spatial coverage in the calculation of the shake maps, and its improvement after the installation of the temporary stations. the maps follow a color-based scale, where the red areas are poorly constrained, the white areas have the uncertainties represented by the standard deviation of the gmpe, and the blue areas are better constrained and represent the seismic stations [wald et al. 2008]. figure 1 (bottom-right panel) shows that the shaking defined for the first event relies almost entirely on the gmpe and source model, while this condition is substantially changed for the may 29, 2012, ml 5.8 earthquake (figure 2, bottom-right panel). a matter of concern remains the persistent unavailability in the short term of the accelerometric data recorded by the ran. calculation of the shake maps is important for the emergency response, since they provide the 'first-cut' estimates of the impact of an earthquake. due to their nature and to the interpolations they rely upon, the shake maps cannot be considered as an instrument to be used much further than the initial estimation of the ground shaking. in this regard, despite many efforts and projects towards almost realtime data sharing, it is still impossible to access the ran data for fast shake-map estimations. the availability of the data (on request) two weeks after the earthquake occurrence definitely appears to be a little too long in a world where information is spread almost instantaneously through social networks, and there might be something more that should be done to improve this situation. comparing the previous experience in 2009 with the l'aquila sequence, the main changes in the shakemap procedure relate to data access, for the accelerometric stations. in addition, during the sequence itself, we were able to modify the procedure to calculate the peak parameters, further reducing the computational time and providing the possibility to disengage from the queues caused by the simultaneous data requests of other procedures. after the first event, the existing procedure that relied on requesting the waveform data as a full seed volume, which was then processed to extract the relevant pgm parameters, was replaced by the gmew module of earthworm. this does not require offline requests, and the parameters are determined on-the-fly from the incoming data streams and starting from the automatic earthquake location provided by earthworm. as a final comment, figure 4 shows the number of visits to the shakemap portal throughout the sequence. the increment in the public interest in shakemap during the strong events is clear. during the sequence, more than 200 shakemaps were calculated, 28 with 4 ≤ m < 5 and 7 with m ≥5. acknowledgements. we thank the editors of the special issue on the emilia sequence for the support in the writing and publication phases of our manuscript. references akkar, s., and j.j. bommer (2010). empirical equations for the prediction of pga, pgv, and spectral accelerations in europe, the mediterranean region, and the middle east, seismol. res. lett., 81, 195-206. castelli, v., f. bernardini, r. camassi, c.h. caracciolo, e. ercolani and l. postpischl (2012). looking for missing earthquake traces in the ferrara-modena plain: an uplauciani et al. 636 figure 4. number of visits to the shakemap portal, starting a week before the main event of the sequence. the majority of the visits were from within italy. 637 date on historical seismicity, annals of geophyics, 55 (4); doi:10.4401/ag-6110. faenza, l., v. lauciani and a. michelini (2011). rapid determination of the shake maps for the l'aquila main shock: a critical analysis, b. geofis. teor. appl., 52, 407-425. johnson, c.e., a. bittenbinder, b. bogaert, l. dietz and w. kohler (1995). earthworm: a flexible approach to seismic network processing, iris newsletter, 14, 1-4. mazza, s., a. basili, a. bono, v. lauciani, a.g. mandiello, c. marcocci, f. mele, s. pintore, m. quintiliani, l. scognamiglio and g. selvaggi (2012). aida – seismic data acquisition, processing, storage and distribution at the national earthquake center, ingv, annals of geophysics, 55 (4); doi:10.4401/ag-6145. michelini, a., l. faenza, v. lauciani and l. malagnini (2008). shakemap implementation in italy, seismol. res. lett., 79, 688-697. moretti, m., et al. (2012). rapid response to the earthquake emergency of may 2012 in the po plain, northern italy, annals of geophysics, 55 (4); doi:10.4401/ag-6152. scognamiglio, l., l. margheriti, f.m. mele, e. tinti, a. bono, p. de gori, v. lauciani, f.p. lucente, a.g. mandiello, c. marcocci, s. mazza, s. pintore and m. quintiliani (2012). the 2012 pianura padana emiliana seimic sequence: locations, moment tensors and magnitudes, annals of geophysics, 55 (4); doi:10.4401/ag-6159. wald, d.j., v. quitoriano, t.h. heaton, h. kanamori, c.w. scrivner and c.b. worden (1999). trinet 'shakemaps': rapid generation of peak ground motion and intensity maps for earthquakes in southern california, earthq. spectra, 15, 537. wald, dj., l, kuo-wan and v. quitoriano (2008). quantifying and qualifying usgs shakemap uncertainty, u.s. geol. surv. open file report 2008-1238. wells, d.l., and k.j. coppersmith (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, b. seismol. soc. am., 84, 974-1002. worden, c.b., d.j. wald, t.i. allen, k. lin, d. garcia and g. cua (2010). a revised ground-motion and intensity interpolation scheme for shakemap, b.. seismol. soc. am., 100, 3083-3096. *corresponding author: valentino lauciani, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy; email: valentino.lauciani@ingv.it © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. rapid shakemap in emilia << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice le s e s s e d e l l a g o d i c o m o p a r t e i p . caloi m. c. s p a d e a 1. come applicazione delle teorie sulle oscillazioni libere dei laghi esposte precedentemente ('), saranno studiate le sesse ordinarie del lago di como o lario. ecco i principali elementi morfometrici del lago di como. altezza sul livello del mare 111 198 profondità massima 111 414 larghezza massima ni 4400 larghezza media m 1940 a bellagio il lago si divide in due rami meridionali. p e r ottenere i d a t i necessari al calcolo della curva normale abbiamo usato la c a r t a batimetrica del lago alla scala di 1:50.000, t r a t t a dall'atlante dei laghi italiani della casa de agostini. l'equidistanza delle curve batimetriche è di ni 25, t r a u n a isobata e l'altra sono inoltre segnati molti p u n t i con profondità intermedie. i n questo lavoro è stato trascurato il ramo sud-ovest del lago; quello che, dalla linea d ' a c q u a di minima profondità che unisce menaggio alla p u n t a spartivento, raggiunge como. sono s t a t e praticate 42 sezioni normali alla linea di valle, f r a l'estremo nord e l'estremo sud (lecco). 2. è noto che la prima rigorosa teoria idrodinamica sulle sesse è d o v u t a a chrystal. per le ipotesi che sono a f o n d a m e n t o di questa teoria, oltre che ai lavori originali, rimandiamo alla prima p a r t e del citato lavoro sul lago di garda f1). qui ci limitiamo a riportare l'equazione, che di chrystal porta il nome, e che regge il moto libero (sesse] dei laghi: (p u 4 ri1 a (v) -— + u = 0 y ' dv2 q t[1] dove u si ritiene rappresentabile mediante la somma di u n a serie di semplici funzioni armoniche di t. questa teoria mostra che quando la larghezza e la f o r m a della sezione trasversale di u n lago varia con la profondità, p u r c h é 11011 in modo brusco, si può procedere ai calcoli introducendo due variabili, a e v. a è il prodotto dell'area s(x) di u n a sezione trasversale per la larghezza b(.r) di d e t t a sezione alla superficie del lago, m e n t r e r(x) è l'area della superficie del lago fra la traccia in superficie della sezione trasversale corrispondente a er, e u n ' a l t r a linea similare, scelta come riferimento. noi conteremo le x a partire da un estremo (l'estremo sud) del lago. secondo chrystal quindi lo studio delle oscillazioni di un lago può essere f a t t o ritenendo r e t t a la linea mediana del lago in superficie, uniforme la sua larghezza, r e t t a n golare u n a sua generica sezione trasversale, si che la sezione longitudinale del lago risulterà limitata da u n a curva i cui p u n t i hanno per ascissa e ordinata rispettivamente r e a . è questa la curva normale del lago. ne risulta che v è la distanza misurata lungo la linea mediana del lago ridotto e a è la profondità nel p u n t o v. i valori o t t e n u t i per le varie grandezze che rientrano nel calcolo: x, s(x), b(x), v(x), a(x), sono r i p o r t a t e nella tabella i. la curva normale (vedi fig. 1) ha f o r m a così complessa da render ardua, se non ini102 1'. caldi m. c. s p a d e a possibile, l'integrazione dell'equazione di chrystal [i], per mezzo dei metodi suggeriti dallo stesso. ci siamo serviti perciò del metodo dell'integrazione numerica della [1] suggerito da h i d a k a tenendo presenti le condizioni agli estremi corrispondenti a 2 0, 2 = 1. 3. metodo di h i d a k a . l a teoria di questo metodo è s t a t a esposta e, in p a r t e a m p l i a t a nel primo lavoro sulle sesse del v garda (4). posto z — , dove a è l'area totale del lago, h i d a k a (2) scrive la [1] sotto la f o r m a (p u 1 ~dz~+ ajz) u = °> pi con le condizioni ai limiti il (0) = 0 , u(l) = 0 [3] t a b e l l a i sez. x 103 • n i 8 (x) io 2 -in 2 b(x) 10 2 •in v(x) s i n g . io3 ni2 vx c o m p l e s s . io3 ni2 ( x ) 10 • m 3 0 0,375 0 0 0 0 0 1 1,375 235 11 500 500 25,85 2 2,375 1525 25.55 2150 2650 389,64 3 3,375 2965 30,75 3275 5925 911,74 4 4,375 2680 26.25 3475 9400 703,50 5 5,375 2595 20,75 2475 11875 538,46 6 6,375 5325 42.50 3750 15625 2263,12 7 7,375 3325 33.25 4250 19875 1105,56 8 8,375 3990 22 3025 22900 877,80 9 9,375 4730 27,50 3400 26300 130,75 10 10,375 5635 31,75 3575 29875 1789,11 ] 1 1 1.375 5070 28,50 3300 33175 1444,95 12 12.375 4340 26.50 2675 35850 1 150,10 13 13,375 2800 18 2500 38350 504,00 14 14,375 2605 20,25 1800 40150 527.51 15 15,375 7530 33,50 3100 43250 2522,55 16 16,375 7490 34,50 3750 47000 2584,05 17 17,375 9010 37 3725 50725 3333,70 18 18,375 9330 34,25 3500 54225 3195,52 19 19,375 9505 37,50 4175 58400 3564,37 20 20,375 8170 33,50 4100 62500 2736,95 21 21,375 7760 30,75 3500 66000 2386,20 22 22,375 5915 23,50 3000 69000 1390,03 23 23,375 4400 20 2475 71475 880,00 24 24,375 4100 19,50 2150 73625 799,50 25 25,375 5110 24,50 2475 76100 1398,95 26 26,375 4170 26 2450 78550 1084,20 27 27,375 2925 23,75 2750 81300 694,69 28 28.375 2810 20 2275 83575 562,00 29 29,375 2790 19,75 2125 85700 551,02 30 30,375 2585 22,25 2250 87950 575,16 31 31,375 1945 18,25 2225 90175 354,96 32 32,375 1065 14,75 1950 92125 157,09 33 33,375 1420 16,50 1725 93850 234,30 34 34,375 1870 16,50 1925 95775 308,55 35 35,375 1275 12 1600 97375 153,00 36 36,375 1440 13,50 1325 98700 194,40 37 37.375 1805 14,25 1 650 100350 257,21 38 38,375 1325 11,75 1375 101725 155,69 39 39,375 1355 12,75 1250 102975 172,76 40 40,375 985 13,75 1350 104325 135,44 41 41,375 500 8 1225 105550 40,00 42 42,375 0 0 825 106375 0,00 l e s e s s e d e l lago d i como 1 i»3 e dove a(z) u' dz, dove h i d a k a p r o v a che l'integrazione della [2], soggetta alle condizioni |3], è equivalente alla ricerca del minimo valore dell'integrale. p e r il calcolo dell'integrale [7] hidaka dà un procedimento che, permettendo di esprimere analiticamente a (z), consente u n a rapida integrazione. ma, nel nostro caso, la curva normale è così complicata, da dover dove u può scriversi ricorrere all'integrazione numerica, tenendo ,„ presenti le condizioni agli estremi corrisponu = v, z (1 — z) z\ [5] denti a 2 = 0, s = 1. " ci siamo limitati al caso m = 2. come l'eliminazione delle m + 1 costanti a„, a, . r i s u l t a d a l l a tabella i i i , il calcolo ha d a t o . .. am, conduce alla equazione dei periodi, g l i integrali i seguenti risultati: che si ottiene annullando il determinante dei coefficienti di a„ a» .. . a„, nelle equazioni: h = 40,6125848; p 19,8444314; / -, , 1 72 = 12,0871103; 7 , = 8,4690561; 0,4270210 [7 a] • 7 2 / u , + = 0 4. periodi: nel caso m = 1 l'equazione ( l p a) a0 + ( l a) a, + d e i p e l i o d i è : \ 6 ) \ l o / , / e, 1 + + 0 + ( i ' . ' m è ' 1 ) * ' 1 8 1 / 3 \ tenendo conto dei valori di 70, i u . . . la [8] + ( 3 5 74 a j a2 + = 0 diventa: 970873344 a2 — 28292376 a + 166667 = 0 194 p . caloi m. c. s p a d e a t a b e l l a ii sez. v a 1 — 2 ( l z f ( 1 — zf c 2 ( l —zy a(x) io"6 a z 1 0,00470 0,00002 0,99930 0,99062 0,00002 0,77369 0,00470 2 0,02491 0,00002 0,97509 0,95080 0,00059 0,51422 0,02021 3 0,05570 0,00310 0.94430 0.89170 0,00276 3,02718 0,03079 4 0,08836 0,00781 0,91 164 0,83109 0,00649 9,22530 0,03266 5 0,1 1 163 0,01246 0,88837 0,78920 0.00983 18,25577 0,02327 (i 0,14688 0,02157 0,85312 0.72781 0,01569 6,93291 0 , 0 3 5 2 5 7 0.18684 0,03491 0,81316 0,661 16 0,02308 20,87630 0,03996 8 0,21528 0,04634 0,78472 0,61578 0,02s54 32,51310 0,02844 9 0,24724 0,06113 0,75276 0,56665 0,03464 264,93307 0,03196 ]() 0,28085 0,07887 0,71915 0,51718 0,04079 22,79904 0,03361 ] ] 0,31 187 0,09726 0,68813 0,47352 0,04605 31,86961 0,03102 12 0,33701 0,11358 0.66299 0,43955 0,04992 43.40492 0,02514 13 0,36052 0,12998 0,63948 0,40893 0,05315 105,45635 0,02351 14 0,37744 0,14246 0,62256 0,38758 0.05522 104,68048 0,04694 15 0,40658 0,16531 0.59342 0,35215 0,05821 23,07585 0,02914 16 0,44183 0,19521 0,55817 0,31155 0,06082 23,53670 0,03525 17 0,47685 0,22739 0,52315 0,27368 0,06223 18,66695 0,03502 18 0,50975 0,25984 0,49025 0,24034 0,06245 19,54299 0,03290 19 0,54900 0,30140 0.45100 0,20340 0,06131 17,20080 0,03925 20 0,58754 0,34520 0,41246 0,17012 0,05873 21,45819 0,03854 21 0,62045 0,38496 0,37955 0,14406 0,05873 23.24197 0,03291 22 0,64855 0,42075 0.35135 0,12345 0,05194 37.36610 0,02830 23 0,67191 0.45146 0.32809 0,10764 0.04860 55,22727 0.02326 24 0,69213 0,47904 0,30787 0,09478 0,04540 56,78549 0,02022 25 0,71539 0,51178 0,28461 0,08100 0 ' 0 4 1 4 5 29,62936 0,02326 2(5 0,73842 0,54526 0,26158 0,06842 0,03731 34,41247 0,02303 27 0,76428 0,58412 0,23572 0.05556 0,03245 46,71148 0,02586 28 0,78566 0,61726 0,21434 0,04594 0,02836 50,46263 0,02138 29 0,80564 0,64905 0,19436 0,03778 0,02452 44,49929 0,01998 30 0,82679 0,68358 0,17321 0,03000 0.02051 35,65964 0,02115 31 0,84771 0,71861 0,15229 0,02319 0,01 166 46,93486 0.02092 32 0,86604 0,75002 0,13396 0,01794 0,01345 85,61971 0,01833 33 0,88226 0,77838 0,1 1774 0,01386 0,01079 46,15277 0,01622 34 0,90035 0,81063 0,09965 0,00993 0,00805 26,08977 0,01809 35 0,91539 0,83794 0,08461 0,08461 0,00716 39,21568 0,01504 36 0,92785 0,86090 0,07215 0,00521 0,00449 23,09671 0,01246 37 0,94336 0,88993 0,05664 0,0032 1 0,00286 11,1 1932 0,01551 38 0,95628 0,91447 0,04372 0.00191 0,00175 11,24028 0,01292 39 0.96804 0,93710 0,03196 0,00102 0.00096 5,55684 0,01176 40 0,98073 0.96183 0,01927 0,00037 0.00036 2,65800 0,01263 41 0,99224 0,98454 0,00776 0.00006 0,00005 1,47500 0,01151 42 1,00000 1,00000 0,00000 0,00000 0,00000 0,00000 0,00776 l e s e s s e d e l l a g o d i c o m o 195 t a b e l l a i i (seguito) s e z . a(z) i o 2 mz mz2 mz* mz1 i 0 , 3 6 3 6 3 0 , 0 0 1 7 1 0 , 0 0 0 0 0 8 0 , 0 0 0 0 0 0.00000 2 3 , 0 6 0 2 4 0 , 0 7 6 2 3 0 , 0 0 1 9 0 0 , 0 0 0 0 5 0 , 0 0 0 0 1 3 9 , 3 2 0 6 9 0 , 5 1 9 1 6 0 , 0 2 8 9 2 0 , 0 0 1 6 1 0 , 0 0 0 0 9 4 3 0 , 1 2 9 8 3 2 , 6 6 2 2 7 0 . 2 3 5 2 4 0 , 0 2 0 7 8 0 , 0 0 1 8 4 5 4 2 , 4 8 1 1 8 4 , 7 4 2 1 7 0 , 5 2 9 3 7 0 , 0 5 9 0 9 0 , 0 0 6 5 9 (i 2 4 , 4 3 8 5 1 3 , 5 8 9 5 3 0 , 5 2 7 2 3 0 , 0 7 7 4 4 0,01 137 7 8 3 . 4 2 1 6 9 15,58651 2 , 9 1 2 1 8 0 , 5 4 4 1 1 0 , 1 0 1 6 6 8 9 2 , 4 6 7 2 6 1 9 , 9 0 6 3 5 4 , 2 8 5 4 4 0 , 9 2 2 5 7 0 , 1 9 8 5 9 9 8 4 6 , 7 2 6 0 9 2 0 9 , 3 4 4 5 6 5 1 , 7 5 8 3 5 1 2 , 7 9 6 7 3 3 , 1 6 3 9 9 10 7 6 , 6 2 7 5 7 2 1 , 5 2 0 8 5 6 , 0 4 4 1 3 1 , 6 9 7 4 9 0 . 4 7 6 7 0 1 1 9 8 , 8 5 9 5 3 3 0 , 8 3 1 3 2 9 , 6 1 5 3 6 2 . 9 9 8 7 4 0 , 9 3 5 1 9 12 109.1 1997 3 6 , 7 7 4 5 2 1 2 , 3 9 3 3 8 4 , 1 7 6 6 9 1 , 4 0 7 6 4 13 2 4 7 . 9 2 7 8 8 8 9 , 3 8 2 9 6 3 2 , 2 2 4 3 4 1 1 , 6 1 7 5 2 4 , 1 8 8 5 2 14 4 9 1 , 3 7 0 1 7 1 8 5 , 4 6 2 7 6 7 0 , 0 0 1 0 6 2 6 , 4 2 1 2 0 9 , 9 7 2 3 5 15 6 7 , 2 4 3 0 3 2 7 , 3 3 9 6 7 1 1 , 1 1 5 7 6 4 , 5 1 9 4 4 1 , 8 3 7 5 5 16 8 2 , 9 6 6 8 7 3 6 , 6 5 7 2 5 1 6 , 1 9 6 2 7 7 , 1 5 6 0 0 3 , 1 6 1 6 8 17 6 5 . 3 7 1 6 6 3 1 , 1 7 2 4 7 1 4 , 8 6 4 5 9 7 , 0 8 8 1 8 3 , 3 8 0 0 6 18 6 4 . 2 9 6 4 4 3 2 , 7 7 5 1 1 16,70711 8 , 5 1 6 4 5 4 , 3 4 1 1 7 19 6 7 , 5 1 3 1 4 3 7 , 0 6 4 7 1 2 0 . 3 4 8 5 2 1 1 , 1 7 1 3 4 6 , 1 3 3 0 4 2 0 8 2 , 6 9 9 8 6 4 8 , 5 8 9 4 7 2 8 , 5 4 8 2 6 1 6 , 7 7 3 2 4 9 , 8 5 4 8 6 2 1 7 6 . 4 8 9 3 2 4 7 , 4 5 7 8 0 2 9 , 4 4 5 1 9 1 8 , 2 6 9 2 7 1 1 . 3 3 5 2 2 22 1 0 5 , 7 8 6 0 6 6 8 , 5 9 2 1 8 4 4 , 4 9 2 3 2 2 8 , 8 5 9 9 4 1 8 . 7 2 0 1 4 2 3 1 2 8 , 4 5 8 6 3 8 6 , 3 1 2 6 4 5 7 , 9 9 4 3 3 3 8 , 9 6 6 9 7 2 6 , 1 8 2 1 2 2 4 1 1 4 , 8 2 0 2 6 7 9 , 4 7 0 5 4 5 5 , 0 0 3 9 4 3 8 , 0 6 9 8 8 2 6 , 3 4 9 0 9 2 5 6 8 , 9 1 7 8 9 4 9 , 3 0 3 1 7 3 5 , 2 7 0 9 9 2 5 , 2 3 2 5 1 1 8 , 0 5 0 9 9 2 6 7 9 , 2 5 1 9 2 5 8 , 5 2 1 2 0 4 3 , 2 1 3 2 2 3 1 , 9 0 9 5 1 2 3 , 5 6 2 4 4 27 1 2 0 , 7 9 5 8 9 9 2 , 3 2 1 8 8 7 0 , 5 5 9 7 7 5 3 , 9 2 7 4 2 4 1 , 2 1 5 3 7 28 1 0 7 , 8 8 9 1 0 8 4 , 7 6 4 1 5 6 6 , 5 9 5 8 0 5 2 , 3 2 1 6 6 4 1 , 1 0 6 9 2 2 9 8 8 , 9 0 9 5 8 7 1 , 6 2 9 1 1 5 7 , 7 0 7 2 8 4 6 . 4 9 1 2 9 3 7 , 4 5 4 9 1 3 0 7 5 . 4 2 0 1 4 6 2 , 3 5 6 6 2 5 1 , 5 5 5 8 3 4 2 , 6 2 5 8 4 3 5 , 2 4 2 5 3 31 9 8 , 1 8 7 7 3 8 3 , 2 3 4 7 2 7 0 , 5 5 8 9 0 5 9 , 8 1 3 4 8 5 0 , 7 0 4 3 3 3 2 1 5 6 , 9 4 0 9 3 1 3 5 , 9 1 7 1 2 1 1 7 , 7 0 9 6 6 1 0 1 , 9 4 1 2 7 8 8 , 2 8 4 6 0 3 3 7 4 , 6 9 6 4 6 6 5 , 9 0 1 7 0 5 8 , 1 4 2 4 3 5 1 , 2 9 6 7 4 4 5 . 2 5 6 9 0 34 4 7 , 1 9 6 3 9 4 2 , 4 9 3 2 7 3 8 , 2 5 8 8 1 3 4 , 4 4 6 3 1 3 1 , 0 1 3 7 4 3 5 5 8 . 9 8 0 3 8 5 3 , 9 9 0 0 5 4 9 , 4 2 1 9 5 4 5 , 2 4 0 3 5 4 1 , 4 1 2 6 3 36 2 8 , 7 7 8 5 0 2 6 , 7 0 2 1 3 2 4 , 7 7 5 5 7 2 2 , 9 8 8 0 1 2 1 , 3 2 9 2 9 37 1 7 , 2 4 6 0 6 1 6 , 2 6 9 2 4 1 5 , 3 4 7 7 5 1 4 . 4 7 8 4 5 1 3 , 6 5 8 4 2 38 1 4 , 5 2 2 4 4 1 3 , 8 8 7 5 2 1 3 , 2 8 0 3 6 1 2 , 6 9 9 7 4 1 2 , 1 4 4 9 1 39 6 , 5 3 4 8 4 6 , 3 2 5 9 8 6 , 1 2 3 8 0 5 , 9 2 8 0 8 5 , 7 3 8 6 1 4 0 3 , 3 7 3 0 0 3 , 3 0 8 0 0 3 , 2 4 4 2 5 3 , 1 8 1 7 3 3 . 1 2 0 4 2 41 1 , 6 9 7 7 2 1 , 6 8 4 5 4 1,67147 1 , 6 5 8 4 9 1 , 6 4 5 6 3 4 2 0 0 0 0 0 4 0 6 1 , 2 5 8 4 8 1 9 8 4 , 4 4 3 1 4 1 2 0 8 , 7 1 1 0 3 8 4 6 , 9 0 5 6 1 6 4 2 , 7 0 2 1 0 1 x 7 t 196 p. caloi m. c. s p a d e a da cui: conseguono da u n a coppia delle t r e equazioni: à = 0,0081960 a2 = 0,0209451. ^ ^ la superfìcie del lago è: ( — ^v-] ^-o + ( " y — i i a + a = 10637,50 imi2, e g = 9,807 m/sec ( 1 \ __ + — m a 2 0 d a t a l ' u n i t a prescelta (m), si ha per le sesse uninodah e binodah rispettivamente: f i t . \ . ( 2 r \ ^ . -m a) + jj -il + ti = 39'" 18s = 24'" 35s ^ 6 / \ 1 0 / i periodi delle t r e sesse, q u a n t e ne con( 10 — ^ sente di determinare il caso m = 2, conseguono dalla equazione, ( _ _ j 2 ; \ + ( __ j j \ a t + 10 / ° 1 \ 10 (70 72 7, p0 1,-11 1,+ 2 7, 72 73 — 7') a3— , o , ( 7 0 7 2 7 1 2 ) + 1 . ( 7 0 73 + ^ 7 , + v" 35 / < r a 1 n 35 1 , 1 5 2 „ , 1 ci serviremo, per esempio delle equezioni: 7 x 7 3 ) + ^ 7 0 7 4 + ( 7 1 7 1 7 2 2 + 7 2 7 3 + x , , , 1 5 3 t i f i t 4 / 1 3 1 3 v n — u u ' u h 11) + ( 7 ( ) 0 / „ ; 5 5 ( ) , / 1 / ; \ , l 2 ft 53 1 1 \ 1 ' + \ 10 ~ 2 ] x 0 j j a = 0 ' ^ \ a [10] 2.100 " 30 60 / 10500 i l t , f i t a a t [9] i o l 2 l + l i o ~ v 1 4 . ^ che deriva dalla [6], eliminando le costanti ' + f ^ \ ~ 2 = 0 a0, au a2. \ 3 5 ) a0 tenendo conto dei valori di 7 0 , . . . 74, la poiché per la sessa uniuodale [9] diventa i = 0,00810718 con i valori di 7 0 ,7 1 ; 72,73, 7 79494222à3 8147989a2 + c h e % u r a n o n e u e [ 7 a l risolvendo si h a : + 1779580a — 952,38 = 0 é l = _ 1,0118485 ^ = + 0,8831617 . an a„ che risolta dà per a i valori: >-0 p e r m = 2 è: l x = 0,00810718; a2 = 0,019736; . . / ax a2 \ u = a0z (1 — z) 1 + 0 + z2 , a3 = 0,0746547. 1 a0 / i periodi per le sesse uninodah, binodah, da cui derivando rispetto a * e ricordando che trinodali, saranno rispettivamente: n e j du _ q sj i a . t, = 39nl 30s t2 = 25 m 19s t3 = 33 m l s . f " 4 ^ + 3 1 a ' nodi. la determinazione delle linee noda, ' [11] li delle oscillazioni libere si ha mediante l'an2 ( 1 1-1 z 1 = 0 du \ a j nullamento di — . ^ ^ , . , con i valori già calcolati di -.--e occorre dunque determinare prima l'esavremo 0 0 spressione di u. . , . , . a2 3,5326468 s 3 — 5,6850306 z2 + p e r m = 2 i valori dei r a p p o r t i ^ , j + ^ ^ ^ s _ 1 = qm l e s e s s e d e l lago d i como 197 dello t r e radici di questa equazione u n a sola è compresa t r a 0 e 1, (valori entro i quali può variare z); essa è: & = 0,-16798 l'uninodo corrisponde, quindi come risulta dalla tabella i i , alla sezione trasversale 16,747 e si t r o v a a k m . 17,122 dall'estremo nord del lago. il valore di a corrispondente alla sessa binodale è: a = 0,019736; la [10] dà perciò, — + 1,2128716 a " = — 5 , 3 4 8 7 2 4 4 -lo -lo per cui la [11] diviene: 21,3948976 z3 — 19,6847880 z2 + 0,4257432 z + 1 = 0 . delle t r e radici di questa equazione, u n a è negativa, le altre sono: = 0,2881859 2z2 = 0,827822 . le linee nodali della sessa binodale corrispondono perciò alle sezioni trasversali 10,236 per il binodo nord e 30,049 per il binodo sud, che distano dall'estremo nord rispettivamente k m 10,611 e k m 30,424. la sessa trinodale, finalmente, corrisponde a a = 0,0746547. dalla [10] otteniamo: a, a„ — = — 4,607241 ~ = + 4,186492 . ' 0 -1 0 la [11] allora diviene: 16,745968 z3 — 26,381199 z2 + + 11,214482 2 — 1 = 0 , le cui radici sono: 3^=0,12093 3z2=0,539988 aza = 0,914457. le linee nodali, dunque, corrispondono alle sezioni trasversali 5,264 per il trinodo nord; 18,770 per il trinodo medio; 34,938 per il trinodo sud e le rispettive distanze dall'estremo nord sono: k m . 5,639; k m . 19,145 e k m . 35,313. t a b e l l a i i i sez. u n i b i t r i — sez. u n i b i t e i sez. n o d a l e sez. n o d a l e sez. vi v2 % sez. vi vi % 0 1 i i 22 0,18213 1,16713 0,25533 1 — 0,98121 + 1,00161 — 0,94782 23 0,20859 — 1.1 1089 0,29421 2 — 0,90324 + 0 , 9 9 8 7 3 0,73674 24 + 0,23273 — 1.04148 — 0,32351 3 0,79289 + 0.96630 — 0,45424 25 + 0,26238 0,93656 0,34757 4 0,68643 0,89865 0,20357 26 0,29371 — 0,80467 — 0.361 18 5 0,61677 4" 0.83201 0,05333 27 + 0,33148 — 0.62155 0,36286 6 0,52041 40,70571 + 0,13119 28 + 0.36544 — 0.44052 0,35223 7 0,42363 4" 0.53189 + 0,28357 29 0 , 3 9 9 0 5 — 0,24602 — 0,33143 8 — 0,36198 40,39290 + 0,35881 30 + 0,43720 — 0,01223 0,29720 9 0,29929 + 0.22528 + 0,41308 31 + 0 , 4 7 5 5 5 + 0,24841 0,24997 10 0.24007 + 0,04094 + 0 , 4 3 9 8 3 32 + 0,51522 _l 0,50171 0.19695 1 1 0,19091 0,13281 + 0,43957 33 + 0,55324 40,74595 0,14048 12 0,15421 — 0.27337 + 0,42671 34 -f0,59258 1,04134 0,06638 13 0,12277 0.40256 + 0 , 3 9 8 7 3 35 + 0,62921 1,30577 + 0,00458 14 — 0 , 1 0 1 2 3 — 0,49320 0,37496 36 40,66097 + 1,53384 0,07029 15 0,06640 — 0,64300 40.32404 37 40,70224 4 1,84499 + 0.16044 16 0,02729 — 0,80727 40 , 2 4 9 3 5 38 + 0,73824 + 2,11558 + 0,24355 17 + 0,00903 — 0,95326 + 0,16458 39 0,77230 + 2,37392 + 0,32534 18 + 0,04178 1,07107 + 0,07974 40 + 0,81046 2,66592 + 0.42061 19 + 0,08007 — 1.39281 — 0,02362 41 + 0 , 8 4 6 3 5 + 2,94271 + 0,51325 20 + 0,11810 — 1,20576 — 0,12144 42 4 0,87130 + 3,13584 + 0,57925 21 + 0,15176 1,20356 — 0,19795 198 p . caloi m. c. s p a d e a distribuzione delle ampiezze riassunto le ampiezze sono date dalla formula: du c = nel caso m = 2 si h a : dz c = a0a\i a; z* + z(a: a„ a„ ' b i ì i p e r la sessa uninodale del lago di como avremo: c= a0 a j 3,5326 z3 — 5,6850z2 + 4,0237 2 — 1 [ per la binodale: c= a0 a ) 21,3949 z3—19,6848 z2—0,4257 z—1 i per la trinodale: c = .4„rt)16,7459z3—26,3812z2+ll,2145zfacendo variare z nelle t r e precedenti equazioni, si lui l ' a n d a m e n t o dell'ampiezza pelle tre sesse considerate. i risultati del calcolo sono contenuti nella c tabella i i i , dove r] = aa0 data la -particolare configurazione del lago di como, poiché il ramo di lecco ni presenta come una naturale continuazione della parte superiore del lago, abbiamo ritenuto che vinsieme costituisca un bacino con movimenti propri, sia pure influenzato dal ramo che fa capo a como. in questa prima parte delle ricerche concernenti le oscillazioni libere di questo caratteristico lago, ci siamo limitati alla determinazione dei periodi, dei nodi e delle ampiezze che ci si devono aspettare fra gera e lecco. naturalmente le ricerche verranno poi estese all'intero bacino. abstract in view of the particular configuration of lake como and as the lecco brandi appears to be a naturai continuation of the upper part of tlie lake, ice have viewed its ensemble as a basin with movcments of its own even if influenced by the branch which begins at como. during the present initiàl stage of research concerning the free oscillations of tliis interesting lake, ice have limited ourselves to the determination of periods, knots and amplitudes that should be expected between gera ami lecco. naturaui/, the research will subsequently be extended to the tohole basii). ci riserviamo di riprendere la ricerca con altri metodi che consentono di considerare il bacino lacustre nella sua interezza. b i b l i o g r a f i a f1) c a l o i p., le sesse del lago di garda. parte i, «annali (li geofisica» 1918. rehman_corrected:layout 6 annals of geophysics, 57, 2, 2014, p0212; doi:10.4401/ag-6398 p0212 influence of indian ocean high pressure on streamflow variability over southwestern australia saqib-ur-rehman iqra university, department of computer science, north nazimabad, karachi, pakistan abstract the seasonal and interannual variability of the southwestern australia streamflow was examined based on re-analysis dataset from the national centers for environmental prediction / national center for atmospheric research, plus monthly streamflow data from 17 meteorological stations obtained from the department of water of the australian water resource council (1976-2008). significant findings reveal that since the middle of the last century, the early winter (may-august) streamflow variability over southwestern australia has been associated with variability in the intensity of the indian ocean high pressure (iohp) system, as well as the zonal movement of the high pressure system across the indian ocean. of note, however, the emphasis in the previous studies has linked the ongoing winter drought in western australia to changes in sea level pressure, both on a local basis and over larger areas. we introduce objective indices for the area-weighted pressure of the iohp, and the iohp area-weighted latitude (iohlt) and longitude (iohln) positions. when the iohp is shifted to the east, there is less streamflow in the southwestern australian rivers that are examined in this study. the situation is reversed when the iohp is shifted to the west, when there is more streamflow in the southwestern australian rivers. the characteristics of the streamflow pattern are determined for the river catchment areas located around the coastal region over southwestern australia using principal component analysis. the first principal component taken into consideration describes 84.43% of the total variance. the multiple correlation between the loading of the first principal component scores of the streamflow and the iohp and iohln is 0.54, while the correlation between the southern oscillation index is 0.44. centers of action indices explain 29% of the streamflow variability, while the southern oscillation index explains only of the 19% streamflow variability. this discovery is of real importance for the definition of the physical mechanism of seasonal (may-august) streamflow variability of southwestern australian and the iohp. 1. introduction a nexus has been developed between the el niño southern oscillation (enso) activity and climate change throughout many parts of the world [e.g., ropelewski and halpert 1987, 1989, glantz et al. 1991, dettinger and diaz 2000]. rainfall and streamflow fluctuations have mainly been attributed to the enso activity across disparate spatio-temporal scales. the streamflow that is an integrator of the rainfall over a large area is, therefore, linked with the enso [western australian department of water 2009] activity. this suggests one important use of enso in forecasting streamflow. however, we argue here that indian ocean high pressure (iohp) is more important a factor to be considered in analyzing the variations in streamflow. exposure of the streamflow to the enso has been explored for a number of rivers [kahya and dracup 1993, simpson et al. 1993, dracup and kahya 1994, piechota and dracup 1996, amarasekera et al. 1997, piechota et al. 1997, uvo and graham 1998, wang and eltahir 1999, kahya and karabork 2001]. to develop a link between the enso and climate, several circulation indices have been used. previous studies that have investigated these enso effects have used a wide range of methods to categorize periods, such as el niño, la niña and neutral episodes. chiew and mcmahon [2003] explored the worldwide enso–streamflow teleconnection through the installation of a first harmonic to the 24-month el niño streamflow segments from 581 catchment areas across the world. recently, the ‘centers of action’ (coa) approach has been shown to be useful in investigations of a number of regional phenomena. winter rainfall in southwestern and western australia is significantly influenced by the iohp [rehman and iqbal 2011]. the early winter streamflow in the donnelly river in southwestern australia has closer correlation with the pressure and longitude position than with the enso [rehman and iqbal 2011]. interannual variability of the gulf stream northwall position has a closer correlation with the longitude position of the icelandic low pressure than with article history received august 16, 2013; accepted january 8, 2014. subject classification: el niño southern oscillation, indian ocean high pressure, indian ocean high longitude, southern oscillation index. the north atlantic oscillation [hameed and piontkovski 2004]. the variability of the transport of saharan dust to the tropical atlantic ocean in winter has a closer correlation with the latitude position of the azores high pressure than with the north atlantic oscillation [riemer et al. 2006] and the transport of dust to the caribbean islands in summer is governed by the longitude of the azores high pressure [doherty et al. 2008]. the main objective of the present study was to use the coa approach to analyze the influence of the iohp early winter (may-august) streamflow variability in the major rivers of the southwest coastal division of southwestern australia. multivariate statistical analysis was carried out using principal component analysis (pca) for important catchment areas of the rivers of the southwest coastal division. the pca describes the spatial and temporal variations in the streamflow patterns from 1976 to 2008. the study also compares the correlations of the enso and the coa indices with the streamflow. finally, we provide the physical justification through which the atmospheric circulation over the indian ocean and its spatial and temporal patterns are discussed, providing support for the claim raised in this study. 2. data description and methods the monthly streamflow data used in this study were obtained from the department of water of the australian water resources council. the dataset was developed using topography-resolving analysis methods applied to all available streamflow station data that passed a series of internal quality tests. the stations that have the longest records available, from 1976 to 2008, were used in this study (see table 1). the monthly mean gridded sea level pressure (mslp) data used in this study were obtained from the national centers for environmental prediction (ncep) re-analysis [kalnay et al. 1996] for calculating the objective coa indices for the monthly mean iohp, iohlt and iohln, as described by hameed and piontkovski [2004]. other global climate indices, such as, multivariate enso index (mei), were obtained from the ncep (www.cpc.ncep.noaa.gov). for easy comparisons between the rainfall and the streamflow, the streamflow data were normalized by dividing them according to their catchment area. to remove linear trends from interannual variation of the streamflow, we applied detrending processes, which allowed the removal of a linear trend of the original streamflow data. 3. center of action methodology an estimate of the influence of atmospheric pressure fluctuations on rainfall variability over australia can be attained through quantitative assessment of the fluctuations in the pressure and locations of the iohp. the pressure index ip of a high pressure system can be defined as an area-weighted pressure departure from a threshold value over the domain (i, j), as given in equation (1), and as suggested by santer [1988] and hameed et al. [1995]: where pij,dt is the slp at grid reference (i, j) as the mean over a time interval dt. in this case, the mslp values were taken from the ncep re-analysis, pt is the threshold slp value (for the iohp, pt = 1016 hpa,), zij is the latitude of the grid point (i, j). d = 1 if (pij,dt − pt )>0, and d = 0 if (pij,dt − pt ) <0. this threshold was selected based upon observations that values above 1016 hpa tend to isolate regions of local pressure maxima over subtropical and mid-latitude regions of the indian ocean and adjacent land areas. this ensures that the pressure difference is due to the high pressure system. the intensity is thus a measure of the anomaly of the atmospheric mass over the section (i, j). the domain of the iohp was chosen as 10° s to 45° s, and 40° e to 120° e. the domain of the iohp was chosen by examining the geographical ranges in the ncep re-analysis data over the period from 1976 to 2008. similarly, the latitudinal index was defined as in equation (2): and the longitudinal index i m,dt was defined in an analogous manner. 4. spatio-temporal analysis of streamflow variability using principal component analysis we examine the streamflow data using pca, which is a multivariate statistical technique that is useful to reduce the dimensionality of large numbers of variables, and to interpret data in an appropriate manner. we analyzed the streamflow data of all of the stations that had the longest records available, to find the seasonal and annual variability for the period of 1976 to 2008. our analysis focused only on the first principal component that cumulatively accounts for more than two-thirds of the variance, at approximate 84.43% of the total variance in the data. the rotation of the principal component is vital for precise interpretation in connection with domain shape, as described by richcos cos i p p , , , , p t ij ij t ji i ij t t j ij ij t i i j j 11 11 z d z d = d d d d == == ^ h // // cos cos i p p p p , , , , , t ij t t ij ij t ji i ij t t j ij ij ij t i i j j 11 11 z d z z d = z d d d d d == == ^ ^ h h // // rehman 2 (1) (2) 3 man [1993], while others have countered that this rotation is only appropriate in common factor analysis, and that domain shape has little influence on the principal components [legates 1993]. the varimax rotation is used on the first unrotated principal component score time series, to extract the maximum variance of the streamflow dataset of all of the stations included in this study (figure 1). the first principal components, both rotated and unrotated, are highly correlated; the correlation coefficient of the first unrotated and rotated principal component scores of annual streamflow data is 0.96. this leads us to believe that there is little different in the variance. figure 2 illustrates the strongest correlation pattern for each station with the first principal component loading. it should be noted that a strong positive correlation coefficient is observed for all of the river catchment areas in the southwest coastal division. we observe that the maximum decline occurs in the warren river, donnelly river, preston river, harvey river and swan coastal river, and along the busselton coast. the mean component score is calculated on the seasonal and annual basis to determine the maximum variance in the first principal component. the seasonality of the first component that is illustrated in figure 3 reveals that the positive phase of this principal component explains the seasonality, and the yearly mean score is highest in july and august, which means that the maximum decrease occurs in the winter season. figure 4 shows the annual observed and detrended streamflow time series of the principal component scores. the similar pattern of both of these time series shows that decreasing trends were seen in the southwest coastal division catchment areas from 1976 to 2008. the annual mean component scores of the time series (figure 4) has become more prevalent in the last 33 years; however, the decrease is particularly apparent from 1976 to 2008, except for the 1990s decade. a trend line was best effects of iohp on streamflow trends figure 1. streamflow stations of the southwest coastal division of australia that were used in this study (source: water resources management division). figure 2. correlation coefficients of the first unrotated versus rotated principal component of the annual streamflow, with the original streamflow data for all of the catchment areas that were used in this study. figure 3. first principal component. seasonal variation of the streamflow at the southwest coastal division for all of the catchment areas that were used in this study (from 1976 to 2008). figure 4. annual observed and detrended streamflow mean component score time series of the first unrotated principal component scores of the southwest coastal division that were used in this study. the subplots show the residual scores. fitted to the annual mean component scores, which described a statistically significant negative slope of −0.43. the residuals plot (figure 4) describes the approximation of the random errors. so the errors (residuals) show random behavior, which tells us that the model described the appropriate data attributes. on the contrary, if the errors (residuals) show seasonal behavior, we can see that the model does not fit the data appropriately. the rainfall decrease since the mid-20th century has been largely concentrated in the early part of the winter season, while the late winter rainfall (august-october) has not changed very much [ioci 2004, li et al. 2005]. the slope for the annual streamflow model during 19762008 (see figure 4) is −0.43. the slope of early winter (may-august) is found to be −1.0 (see figure 5), and the slope in the case of late winter (august-october) is −0.59 (figure 6). this significantly demonstrates that the most decreasing trend in the streamflow is observed during the early winter period. therefore, we direct our attention to the streamflow variability in the early part of the winter (may-august). this useful information can also be used in estimation studies. 5. impact of iohp and iohln on the streamflow variability the anomalies in figure 7 describe the 10 winters when the mean central pressure (mcp) of the iohp was at a minimum (more run-off observed in the southwest coastal division; in increasing order of the mcp of the iohp: 1977, 1988, 1981, 2007, 1991, 1996, 2003, 1985, 1986, 1978); and similarly, the 10 winters when the mcp of iohp was at a maximum (less run-off observed in the southwest coastal division; in decreasing order of the mcp of the iohp: 2002, 2000, 1987, 1980, 1979, 2004, 1997, 1982, 1989, 1993). however, this study reveals that the iohp influenced the regional climate of western australia, so we are concerned about the intensity of this high pressure across the indian ocean. we also analyzed the influence of the eastward (iohln) and southward (iohlt) shifts of the high pressure centers on the southwest coastal division runoff variability. the term iohp has been frequently used in many studies [e.g., hameed et al. 2011, rehman and iqbal 2011]. as shown in figure 8a,b, during the early winters when the mcp of the iohp was at a maximum, the component bar has the lowest mean principal component score, which shows that less streamflow occurred for the southwest coastal division. however, during the early winters when the mcp of the iohp was at a minimum, there is a tendency towards the highest mean principal component scores that quantify the streamflow in the southwest coastal division. the changes in the longitudinal position of the iohp have a dominant influence on the streamflow in the southwest coastal division [rehman and iqbal 2011]. the streamflow pattern described in figure 8c,d rehman 4 figure 5. early winter-time observed and detrended streamflow mean component score time series of the first unrotated principal scores of the southwest coastal division for all of the catchment areas that were used in this study. may-august. figure 6. late winter-time observed and detrended streamflow mean component score time series of the first unrotated principal scores of the southwest coastal division for all of the catchment areas that were used in this study. aso, august, september, october. figure 7. anomalous time series of the early winter mcp of the iohp (hpa/month-1) over the indian ocean (10°-45° s; 40°-120° e) from 1976 to 2008, with standard deviations (0.2 hpa month-1). may-august. 5 reveals that the southwest coastal division catchment area streamflow is significantly influenced by the zonal movement of the iohp. when the iohp shifted most towards the west, then the catchment areas had more streamflow. this situation was reversed when the iohp shifted most towards the east, when the catchment area streamflow was reduced. the highest seasonal mean streamflow principal component score for may-august (early winter) was approximately 55 units, and the center of high pressure was located the most to the west, with the longitudinal position of the iohp at 66.88° e. similarly, the minimum mean streamflow principal effects of iohp on streamflow trends station n° station id longitude (°e) latitude (°s) river basin catchment area (km²) 1 603003 117.217 34.699 603 denmark coast 241.89 2 603136 117.315 -34.867 603 denmark coast 502.4 3 603190 117.292 -34.704 603 denmark coast 56.32 4 604053 117.087 -34.889 604 kent river 1,806.04 5 605012 116.789 -34.906 605 frankland river 4,508.89 6 607004 116.459 -34.331 607 warren river 666.66 7 607144 116.347 -34.349 607 warren river 460.53 8 608151 115.785 -34.327 608 donnelly river 782.13 9 610001 115.055 -33.942 610 busselton coastal 443 10 610006 115.022 -33.796 610 busselton coastal 82.26 11 611111 115.948 -33.628 611 preston river 102.1 12 612001 116.263 -33.33 612 collie river 1,345.25 13 612014 116.276 -33.278 612 collie river 366.11 14 612034 116.163 -33.387 612 collie river 661.62 15 613007 115.948 -32.946 613 harvey river 13.57 16 616001 116.116 -31.734 616 swan coastal 514.66 17 616216 116.290 -31.973 616 swan coastal 590.91 figure 8. (a) the highest monthly streamflow occurred when the lowest monthly mcp of the iohp was observed, and vice versa. (b) the highest early winter streamflow occurred when the lowest early winter mcp of the iohp was observed, and vice versa. (c) the highest monthly streamflow occurred when the iohln shifted towards the west, and vice versa. (d) the highest early winter streamflow occurred when the iohln was shifted towards the west, and vice versa. may-august. table 1. stream gauging stations with locations, river basin names and catchment areas. a c db component score of approximately 11.45 units occurred when the center of the iohp was located the most to the east, with the longitudinal position at 73.12° e. 6. correlation analysis of streamflow in the southwest coastal division table 2 shows the correlations between the streamflow mean principal component scores and the circulation indices. the correlation between the iohp and the wintertime streamflow is −0.33, which is significant at the p <0.05 confidence level. similarly, the correlation between the wintertime streamflow mean principal component score and the iohln is −0.50, which is also significant at the p <0.05 confidence level. these negative correlations imply that when the iohp is shifted towards the east, there is less streamflow over the southwest coastal rivers, and vice versa. the correlation between the southern oscillation index (soi) and the streamflow is 0.45, which is significant at the p <0.05 confidence level. the correlation matrix for the coa indices and the enso indicators is shown in table 3. we note that the iohp and the iohln are mutually independent of each other, while the enso indicators are statistically significantly correlated with both the coa indices and the other enso indicators. an interesting question is whether the iohp and iohln provide independent information about the streamflow in the southwest coastal division catchment areas. therefore, we constructed a multilinear regression model for the winter-time streamflow (in mm), as in equation (3): streamflow = 15491 − 14.788i (iohp) − 5.247 (iohln) (3). with r2 for this regression as 0.298, this provides a significant improvement over the soi r2 of 0.193. hence the coa explains 29% of the streamflow variability, while the soi explains only 19% of the streamflow variability. the forecasts can benefit considerably from the management of the water resources systems, particularly in australia, where the streamflow variability is higher than elsewhere in the world [ropelewski and halpert 1987, chiew and mcmahon 2002, chiew et al. 2003]. we calculated the lag correlations between the september-december streamflow principal component scores versus the may-august iohp and soi in the 17 catchment areas of the southwest coastal division, which are statistically significant at the 95% confidence level. the lag correlation between the streamflow and the iohp is −0.45, while the correlation between the streamflow and the soi is 0.40. the enso hydroclimate teleconnection is particularly strong in australia, and it can be exploited to forecast the rainfall and streamflow for several months ahead [chiew and mcmahon 2002]. our calculations suggest that the iohp has a rehman 6 parameter mean principal component score for the southwest coastal division observed detrended time -0.31 0.00 iohp -0.33 -0.33 iohln -0.50 -0.57 mei -0.14 -0.18 niño1,2 -0.04 -0.10 niño3 -0.26 -0.28 niño3.4 -0.34 -0.34 niño4 -0.26 -0.22 soi 0.44 0.45 iohlt 0.50 0.54 iohp and iohln 0.54 (29%) 0.57 (31%) parameter parameter iohp iohln mei niño3 niño3.4 niño4 niño1,2 iohln 0.1400 mei 0.4450 0.1580 niño3 0.3520 0.2220 0.8700 niño3.4 0.3690 0.2500 0.8600 0.9040 niño4 0.4000 0.1220 0.8130 0.6840 0.8830 niño1,2 0.2280 0.1490 0.7670 0.8610 0.6190 0.3900 soi -0.4030 -0.3350 -0.8150 -0.7270 -0.8430 -0.8080 -0.4930 table 2. correlation matrix for the may-august (early winter) streamflow mean principal component scores for the southwest coastal division catchment areas. iohp, indian ocean high pressure; iohln, indian ocean high longitude; mei, multivariate enso index; niño1, 2, 3, 3.4, 4, el niño regions; soi, southern oscillation index; iohlt, indian ocean high latitude. table 3. correlation matrix between the may-august (early winter) streamflow mean principal components for the southwest coastal division catchment areas. for abbreviations, see table 2. 7 similar lag correlation and can be used for an additional forecast streamflow in southwest western australia. therefore, when the run-off–iohp has a strong connection, it can provide long lead-time forecasts in the southwest coastal division. 7. identification of the underlying factors in the relationship between the iohp and the streamflow variability the above correlation and regression analysis provided casual relationships, so next we examined the ncep observations and re-analysis data to determine the spatial patterns of the climate parameters associated with the extreme years of the ihop, as shown in figure 7. we provide physical justification using the ncep re-analysis climate data of the mslp and the vector winds. we also used monthly anticyclone frequency composite maps [rehman 2012] to describe the anticyclone spatial distribution over the indian ocean. the monthly historical australian rainfall data used in this study is based on a new high-resolution (0.25° × 0.25°) gridded dataset that has been developed by the australian bureau of meteorology [jones et al. 2009] to describe the spatial patterns of precipitation over southwest western australia. all of the climate parameters were used to identify and examine the circulation pattern influence on the early winter (may-august) runoff from 1976 to 2008. figure 7 describes the anomalous early winter time series of the mcp of the iohp. a standard deviation of ±0.2 of the iohp was used to define the anomalies of the 10 winters in which the iohp was the lowest from 1976 to 2008. in increasing order of the mcp of the iohp, these winters were for 1977, 1988, 1981, 2007, 1951, 1996, 2003, 1985, 1986 and 1978. for the 10 winters in which the iohp was the highest from 1976 to 2008, in decreasing order of the mcp of the iohp these were for 2002, 2000, 1987, 1980, 1979, 2004, 1997, 1982, 1989 and 1993. 7.1. rainfall spatial patterns composite analysis of the early winter (may-august) mean precipitation (figure 9a) showed distinctly different patterns for the 10 winters of the iohp minimum in southwest western australia. the seasonal mean rainfall across the western corner of southwest western australia appeared as high values, with the mean of 180 mm centered at 116° e and 32-33° s. the mean seasonal precipitation of the central and eastern parts was relatively low compared to the western part of southwest western australia. it has been established that the streamflow is the chief integrator of the rainfall, especially in these regions where the quantified run-off is dependent on the rainfall and where glacier systems do not intensify the peak run-off. we conclude that precipitation is dominant in the australian hydrological cycle. figure 1 describes the spatial distribution of streamflow catchments over southwestern australia. this situation indicates that a significant increase occurs in the early winter-time streamflow that is associated with the quantifying rainfall when the mcp of the iohp is at a minimum. this situation is reversed during the 10 winters with the iohp at a maximum (figure 9b), with the seasonal mean rainfall of 160 mm showing an almost 11% decrease for the same geographical coordinates; this pattern also persists over all of the region of southwest western australia. hence, a low (peak) run-off–streamflow in southwest western australia appears to be part of a rainfall pattern that affects the western corner and the surrounding region of southwest western australia. 7.2. mean sea level pressure spatial patterns the composite analysis (figure 10) of early wintertime (may-august) mslp pattern shows remarkably different patterns, in particular, the analysis indicates an inverse relation of iohp with the streamflow. the mean composite of the slp fields showed that during the 10 winters when the mcp of the iohp is minimum the highest streamflow occurs over the rivers of southeffects of iohp on streamflow trends figure 9. composite mean of the early winter (may-august) precipitation rate (mm month-1). (a) the 10 winters in which the iohp was at a minimum, from 1976 to 2008 (in increasing order of the mcp of the iohp: 1977, 1988, 1981, 2007, 1951, 1996, 2003, 1985, 1986 and 1978). (b) the 10 winters in which the iohp was at a maximum, from 1976 to 2008 (in decreasing order of the mcp of the iohp: 2002, 2000, 1987, 1980, 1979, 2004, 1997, 1982, 1989 and 1993). western australia. this means that for the 10 winters where the iohp is minimum (see figure 10a) the streamflow is found to be maximum. on the contrary, the case where the 10 winters show maximum iohp (figure 10b) the streamflow is found to have minimum value. however, during the 10 winters when the iohp was minimum, the mslp across the indian ocean and the adjacent land area had closed isobars and equivocal grid lines where values that were equal to or greater than 1023 hpa tended to isolate regions of maximum pressure that were positioned far from the australian continent, and centered at 20° s to 40° s and 80° e to 120° e over subtropical regions of the indian ocean. similarly, there was a band of high pressure in australia, and particularly in central australia. the mcp fell between 1016 hpa and 1018 hpa, with values >1020 hpa occurring less than 4% for the total study region. we note that the mcp of the iohp over the australian continent was approximately 1018.5 hpa and was centered over southwest western australia. the mcp of the isolated region of high pressure across the tropical and subtropical region over the indian ocean was around 1020 hpa. this high pressure region was situated around the eastern indian ocean, centered at 30° s, which is far from southwest western australia. our finding is consistent with hameed et al. [2011], who showed that when the center of the iohp is located most to the west across the indian ocean, southwest western australia receives more rainfall. thus, 75% of all of the mcp fell between 1016 hpa and 1020 hpa, and values greater than 1022 hpa occurred less than 8% of the time. the situation was reversed for the 10 winters when the iohp was at a maximum (when the minimum streamflow occurred over the rivers of southwest western australia). the mcp of the iohp was higher by up to 3 hpa, in-contrast to the lowest mcp of the iohp center over the indian ocean and the australian continent, with its dual pattern of high pressure system (figure 10b). a band of high pressure was positioned nearer to southwest western australia. the analysis indicates that the early winter-time streamflow is associated with the intensity as well as the zonal movement of the ihop, which is again consistent with hameed et al. [2011], who showed that when the mcp of the iohp was located the most to the east across the indian ocean, southwest western australia received more rain. the composite of the 10 high winters show weaker geostrophic westerly winds around 30° s to 33° s, whereby westerly disturbances provide most of the moisture to western australia [hope et al. 2006]. we observed that the mcp of the ihop over the australian continent was approximately 1020.5 hpa. another band of the high pressure system was positioned across the indian ocean, which covered a large part of the eastern and central indian ocean. the mcp of the isolated local pressure maxima across the indian ocean was around 1023.5 hpa. this second high pressure region was centered at 25° s to 40° s, and the subtropical belt across the indian ocean for this center is near to southwestern australia. thus, 75% of all of the mcp fell between 1019.5 hpa and 1023 hpa all of the time. the atmospheric conditions are important in the development of moist inflow from more tropical latitudes [wright 1974]. thus the may to july rainfall would be more strongly related to the slp. it is during this early part of the cool season (may-july) that the greatest rainfall decrease has been observed [ioci 2004]. 7.3. vector winds and moisture flux the composite analysis of surface vector wind distribution (figure 11) shows that the slp spatial patterns for early winter time, which were obtained in figure 10, is responsible for variation in surface vector wind over large part of the indian ocean. moreover, the wind circulation is associated with pressure fields that results in geostrophic balance. the vector winds at surface level are directed parallel to the closed isobars of the iohp centers. this rehman 8 figure 10. as for figure 9, but for the composite mean of the early winter (may-august) mean sea level pressure (mb month-1). 9 shows that for the 10 winters when the iohp was at a minimum, it is particularly evident that the polar westerlies moved towards southwestern australia and the southern flank of the australian continent. these westerlies were particularly strengthened (figure 11a), which resulted in an eastward shift of the westerlies responsible for the association between the rain-bearing fronts and the polar fronts. the westerlies across the polar belt over the indian ocean weakened and provided less moisture, and less streamflow occurred during this period. similarly, the situation was reversed for the 10 winters when the iohp was at a maximum, where weak westerly disturbances were weakened and moved towards southwest western australia (figure 11b) and the southern flank of the australian continent. the westerlies across the polar belt over the indian ocean weakened and provided less moisture, and less streamflow occurred during this period. this analysis also provides physical justification of the relationship between rainfall and moisture flux. as mentioned above, the large-scale circulation features, such as the slp distribution, vector winds and rainfall, are associated with the southwest western australia run-off. the wind is closely related and influenced by the advection of moisture on the southwest coastal division. the transportation of moisture to southwest western australia is associated with rainbearing fronts arriving from the indian ocean. investigations into the moisture transport and its variations provides knowledge of the dependence of the southwest western australia rainfall variability. during the 10 winters when the iohp was at a maximum, anomalies of 1000 hpa winds were off-shore, nearer the region of southwest western australia. similarly, they were onshore during the 10 winters when the iohp was at a minimum. 7.4. anticyclone activity anticyclones having slight equator-ward components of motion, and they are generally concentrated in the subtropics between 25° s and 40° s [sinclair 1994, jones and simmonds 1994]. we now address the issue of the run-off variability over southwest western australia using frequency analysis of the indian ocean anticyclones [rehman 2012], from the daily analyses obtained from the national centers for environmental prediction/ national center for atmospheric research. we examined the frequency counts using an automated scheme such that an anticyclone is considered to be present at a grid node on a given day when the slp becomes equal to, and exceeds, 1020 hpa. here we examined how the early winter-time streamflow was influenced by the mean anticyclone frequency. figure 12a shows the spatial frequency distribution of the may-august mean anticyclone frequencies across the indian ocean during the 10 winters when the mcp of the iohp was at a minimum, where the tropical region over the indian ocean was dominated by an anticyclone for around 20 days, which represents 80% of the time. figure 11a shows that there is a strengthening and coincident eastward (southward) shift of the polar westerlies, which results in more moisture associated with the polar front. this is consistent with allan and haylock [1993], who reported that southwest western australia winter rainfall anomalies can be associated with fluctuations in mid-latitude frontal systems, which result from modulation in the transient anticyclone trough south of australia. in contrast, during the 10 winters when the mcp of the iohp was at a maximum, the tropical region over the indian ocean was dominated by an anticyclone for around 26 days, which represents 87% of the time (figure 12b). the iohp strengthened the anticyclone frequency, as well as weakening wind anomalies along tasmania and southern australia and the tropical belt over the southern flank of the australian continent, thereby affecting the tropical ridge that drives winds and moisture onto the southwest coastal division. these findings clearly suggest that there is a strong connection between anticyclones and peak runoff over southwest western australia. this is consistent effects of iohp on streamflow trends figure 11. as for figure 9, but for the composite mean of the early winter (may-august) surface vector wind (m/s month-1). with england et al. [2006]. wind anomalies then develop during the ensuing months (march-may), which amplify to a pronounced large scale acceleration (deceleration) of the indian ocean mean anticyclones by wintertime ( june-august) during dry (wet) years. during wet years, anomalously low air pressure extends over much of the region, driving deceleration of the indian ocean subtropical anticyclone. in the immediate vicinity of southwest western australia, the winds are anomalously stronger onshore, which forces increased moisture transport into the region. 8. conclusions and discussion previous studies have identified increases in sealevel pressures in southwest western australia as the immediate cause of the ongoing drought in this region. the present study shows that consistent deficiencies in the streamflow with time evolution strongly influence the iohp and its zonal movement, which leads to severe drought. we have examined this relationship in terms of the dynamics of the iohp. specifically, we show that the east-west shifts in the position of this subtropical high pressure significantly influences the early winter streamflow in the southwest coastal division. when the iohp shifts to the west, the streamflow in southwest western australia increases, and vice versa. in addition, the streamflow in southwest western australia is negatively correlated with the area-mean of the iohp. the iohp and iohln of the iohp are not significantly correlated with each other. a significant increase occurs in the early wintertime streamflow associated with quantifying rainfall when the mcp of the iohp is at a minimum, when a low (peak) run-off–streamflow in southwest western australia appears to be part of a rainfall pattern that affects the western corner and the adjacent regions of southwest western australia. the mslp across the indian ocean and the adjacent land area has closed isobars and the westerlies across the polar belt over indian ocean are weakened and provide less moisture, with less streamflow occurring during this period. the westerlies across the polar belt over the indian ocean weaken and provide less moisture, with less streamflow occurring during the early winter period. on average, there is maximum (minimum) run-off for southwest western australia where extreme years are characterized by a progressive strengthening of the spatial distribution of the slp field during progression from january-april and may-august. a statistical model of the may-august streamflow in southwest western australia using the iohp and iohpln as independent variables is presented. this explains 29% of the observed streamflow variance, while the soi explains only 19% of the variance, from 1976 to 2008. the lag correlation between the streamflow and the iohp is −0.45, while the correlation between the streamflow and the soi is 0.40. it is of note that the winter streamflow in southwest western australia is much more sensitive to variations in the pressure and longitudinal position of the iohp than it is to fluctuations in the sea surface temperatures, as represented by the indian ocean dipole. acknowledgements. the data used in this study were obtained from the western australian department of water, and i thank jo gregory, data management officer water information for assistance. references allan, r.j., and m.r. haylock (1993). circulation features associated with the winter rainfall decrease in southwestern australia, journal of climate, 6, 1356-1367. amarasekera, n.a., r.f. lee, e.r. williams and e.a.b. eltahir (1997). enso and the natural variability in the flow of tropical rivers, journal of hydrology, 200, 24-39. chiew, f.h.s., and t.a. mcmahon (2002). global enso– streamflow teleconnection, streamflow forecasting and interannual variability, hydrological sciences journal, 47 (3), 505-522. rehman 10 figure 12. as in figure 9, but for the composite mean of the early winter (may-august) mean anticyclone frequency. 11 chiew, f.h.s., and t.a. mcmahon (2003). el niño/southern oscillation and australian rainfall and streamflow, australian journal of water resources, 6, 115-129. chiew, f.h.s., s.l. zhou and t.a. mcmahon (2003). use of seasonal streamflow forecasts in water resources management, journal of hydrology, 270, 135-144. dettinger, m.d., and h.f. diaz (2000). global characteristics of stream flow seasonality and variability, journal of hydrometeorology, 1, 289-310. doherty, o.m., n. riemer and s. hameed (2008). saharan mineral dust transport into the caribbean: observed atmospheric controls and trends, j. geophys. res., 113, d07211; doi:10.1029/2007jd009171. dracup, j.a., and e. kahya (1994). the relationships between u.s. streamflow and la niña events, water resources research, 30 (7), 2133-2141. england, m.h., c.c. ummenhofer and a. santoso (2006). inter-annual rainfall extremes over southwest western australia linked to indian ocean climate variability, journal of climate, 19, 1948-1969 glantz, m.h., r.w. katz and n. nicholls, eds. (1991). teleconnections linking worldwide climate anomalies: scientific basis and societal impact, cambridge university press, cambridge, uk. hameed, s., w. shi, j. boyle and b. santer (1995). proceedings of the first international amip scientific conference, monterey, california, wcrp 92, 221. hameed, s., and s. piontkovski (2004). the dominant influence of the icelandic low on the position of the gulf stream northwall, geophys. res. lett., 31, l09303; doi:10.1029/2004gl019561 hameed, s., m.j. iqbal, s. rehman and d. collins (2011). impact of the indian ocean high pressure system on winter precipitation over western australia and southwest western australia, australian meteorological and oceanographic journal, 61, 159-170. hope, p.k., w. drosdowsky and n. nicholls (2006). shifts in the synoptic systems influencing southwest western australia, climate dynamics, 26, 751-769. ioci, indian ocean climate initiative western australia (2004). 18th panel meeting sets agenda to broaden dialogue on climate change, bulletin no. 4; http://www.ioci.org.au/pdf/ioci_bulletin4.pdf. jones, d.a., and i. simmonds (1994). a climatology of southern hemisphere anticyclones, climate dynamics, 10, 333-348. jones, d.a., w. wang and r. fawcett (2009). high-quality spatial climate data-sets for australia, australian meteorological and oceanographic journal, 58, 233-248. kahya, e., and j.a. dracup (1993). u.s. streamflow patterns in relation to the el niño southern oscillation, water resources research, 29 (8), 2491-2503. kahya, e., and m.c. karabork (2001). the analysis of el niño and la niña signals in streamflows of turkey, international journal of climatology, 21, 1231-1250. kalnay, e., and coauthors (1996). the ncep/ncar 40-year reanalysis project, bull. amer. meteor. soc., 77, 437-471. legates, d.r. 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(2012). modelling the impact of indian ocean high pressure on western australia climate variability and streamflow, phd thesis, university of karachi, karachi, pakistan. richman, m.b. (1993). comments on: ‘the effect of domain shape on principal components analyses’, international journal of climatology, 13 (2), 203-218. riemer, r., o.m. doherty and s. hameed (2006). on the variability of african dust transport across the atlantic, geophys. res. lett., 33, l13814. ropelewski, c.f., and m.s. halpert (1987). global and regional scale precipitation patterns associated with the el niño/southern oscillation, monthly weather review, 115, 1606-1626. ropelewski, c.f., and m.s. halpert (1989). precipitation patterns associated with the high index phase of the southern oscillation, journal of climate, 2, 268-284. santer, b.d. (1988). regional validation of general circulation models, ph.d. thesis, university of east angelia, norwich, england. simpson, h.j., m.a. cane, a.l. herczeg and j.h. simpson (1993). annual river discharges in southeastern australia related to the el niño southern oscillation forecasts of sea surface temperatures, water resources research, 29 (11), 3671-3680. effects of iohp on streamflow trends sinclair, m r. (1994). an objective cyclone climatology for the southern hemisphere, monthly weather review, 122, 2239-2256. uvo, c.b., and n.e. graham (1998). seasonal runoff forecasts for northern south america: a statistical model, water resources research, 34 (12), 3515-3524. wang, g., and e.a.b. eltahir (1999). use of enso information in medium and long-range forecasting of nile floods, journal of climate, 12, 1726-1737. western australian department of water (2009). streamflow trends in south-west western australia, surface water hydrology series report no. hy32, 79 pp. wright, p.b. (1974). seasonal rainfall in southwestern australia and the general circulation, monthly weather review, 102, 219-32. *corresponding author: saqib-ur-rehman, iqra university, department of computer science, north nazimabad, karachi, pakistan; email: mathematician60@hotmail.com. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. rehman 12 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice cusano1_final:layout 6 annals of geophysics, 56, 4, 2013, s0441; doi:10.4401/ag-6446 s0441 seismic activity and thermal regime of low temperature fumaroles at mt. vesuvius in 2004-2011: distinguishing among seismic, volcanic and hydrological signals paola cusano1,*, simona petrosino1, paolo madonia2, cinzia federico2 1 istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy abstract seismological, soil temperature and hydrological data from mt. vesuvius are collected to characterize the present-day activity of the volcanic/hydrothermal system and to detect possible unrest-related phenomena. we present patterns of seismicity and soil temperature in the crater area during the period february 2004-december 2011. the temporal distribution of number and depth of volcano-tectonic earthquakes and the energy release are considered. hourly data of soil temperature have been acquired since january 2004 in different locations along the rim and within the crater. the observed changes of temperature are studied to establish a temporal-based correlation with the volcanic activity and/or with external forcing, as variations of the regional and local stress field acting on the volcano or meteorological phenomena. the comparison between seismic activity and temperature data highlights significant variations possibly related to changes in fluid circulation in the hydrothermal system of the volcano. the common continuous observations start just before a very shallow earthquake occurred in august 2005, which was preceded by a thermal anomaly. this coincidence has been interpreted as related to fluid-driven rock fracturing, as observed in other volcanoes. for the successive temporal patterns, the seismicity rate and energy release are characterized by slight variations accompanied by changes in temperature. this evidence of reactivity of the fumarole thermal field to seismic strain can be used to discriminate between tectonic and volcanic signals at mt. vesuvius. 1. introduction in outlining the state of a volcanic system and its evolution, several parameters have to be monitored and analyzed. in the last decades, the technological progress has allowed the acquisition of a huge amount of data of different nature (seismological, geochemical, volcanological, geodetic, etc.), with a dense spatial distribution inside an area of interest and on long time intervals (from several years to decades). among the seismic-derived parameters, seismicity rate and energy release play an important role in volcano monitoring since their temporal variations could be indicators of changes in the acting stress field. furthermore, useful insights into the source processes are provided by hypocenter locations. likewise, variations in the chemistry, temperature and mass rate of volcanic fluids (steam, water, co2) could be linked to important dynamic processes. since the fluids circulating in a volcanic area easily result from the mixing of hydrothermal, volcanic and shallow meteoric sources, it is essential to attribute the correct interpretation to the any observed anomaly. anomalies can be due to: (1) variations in the volcanic ‘deep’ source; (2) changes in soil permeability due to the local or regional stress field acting on the volcano; (3) modifications of the hydrological regime, with variable amounts of meteoric waters entering the volcanic edifice. recently multidisciplinary approaches have been introduced by researchers to characterize the activity of many volcanoes. d’auria et al. [2011] analyzed a multiparametric dataset of seismological, geodetic and geochemical data recorded at campi flegrei caldera (southern italy) in the period 1982-2010 and provided further constraints to quantitative models of the campi flegrei geothermal system. in particular, the authors highlighted the timing between the onset of the uplift and the occurrence of long-period (lp) and volcanotectonic (vt) events. moreover, basing on the results of this work, the authors have argued the improvement of microseismicity detection in the development of the monitoring networks. article history received august 29, 2012; accepted january 3, 2013. subject classification: surveys, measurements and monitoring, seismicity, fumarolic thermal regime, multidisciplinary data comparison, rest state definition. special issue: vesuvius monitoring and knowledge mauri et al. [2012] presented a groundwater flow model that highlights the existence of active volcanic structures cutting across the caldera floor of masaya volcanic complex (nicaragua). by using 5 years of selfpotential, ground temperature and soil co2 surveys, from 2006 to 2010, they obtained a multi-scale wavelet tomography showing that the shallowest groundwater flow mimics the topography and that uprising fluids are of hydrothermal origin. even if in the analyzed period the volcano was in a steady state, the adopted technique delineated the active subsurface structures of the caldera and permitted the monitoring of the hydrothermal system, whose fluctuations may precede important changes in the volcanic activity. in order to understand the magmatic-hydrothermal interactions that can lead to phreatic eruptions at meakandake volcano, shinohara et al. [2011] monitored temperature, composition and emission rate of the three fumarolic fields existing on the volcano, from 2003 to 2008. their observations allowed to characterize the temporal evolution of meakandake volcanic activity that preceded the 2006 phreatic eruption and its evolution. the authors were able to recognize a magmatic-hydrothermal system for each fumarolic field, identifying which of them was linked to the 2006 eruption. since volcanic hazard at mt. vesuvius is considered one of the most critical in the world, a long-lasting multidisciplinary volcanic surveillance program has been carried out during the last decades. monitoring allows to analyze and compare the shortand long-term evolution of different parameters. by studying long-time series of chemical and isotopic compositions of mt. vesuvius fumaroles from 1998 to 2010, caliro et al. [2011] identified the phenomena that occur on short-period, which appear to be linked to shallow processes in the hydrothermal system, and longtime variations associated to changes of volcanic activity. madonia et al. [2008] (hereinafter referred to as m08) compared shortand long-term variations (with timescales ranging from few days to several years) of geochemical parameters, (chemistry of groundwater and thermal regime of the fumarolic field) and seismic activity in the vesuvius volcanic system during the 19982005 period, interpreted as having common source mechanisms. in the present work we illustrate the progression of m08, after the updating of the seismological and geochemical data set from 2005 to 2011, for what concerns the correlations between seismicity and fumarolic activity. the relationships between seismicity and chemistry of groundwater are discussed by federico et al. [2013]. in the following we first describe the volcanic system and the data set, then, after a brief summary of the results obtained in m08, we show the analyses performed in the period february 2004-december 2011. finally we discuss the results on the basis of the 1999-2011 data set. 2. volcanological setting the somma-vesuvius volcanic complex is a stratovolcano, 1281 m high above sea level and 10 km wide. it presents a recent cone, vesuvius, located inside the pre-existing somma caldera. the volcanic complex is sited in the piana campana (southern italy), which is bordered by mesozoic carbonates. this structure, located between the tyrrhenian margin and southern apennines, is characterized by a general normal stress field. the main fault systems strike ne-sw and nwse and produced the graben-like morphology of the carbonate basement [santacroce 1987, patacca et al. 1990, acocella and funiciello 2006, di renzo et al. 2007]. the subsidence of this basement, that lays at a depth of about 2 km below mt. vesuvius, generates a wide gravimetric anomaly located in the central part of the campanian plain, defining an area named the acerra graben [scandone et al. 1991, marzocchi et al. 1993]. the structural depression is mainly filled by plioquaternary volcanic rocks and sediments. the somma-vesuvius edifice is interested by two main fault systems: (a) the ne-sw system involves the northwestern sector of mt. somma, where probably dips to dislocate the sedimentary basement; (b) the nw-se structures involve the northeastern sector of the somma edifice and extend to the southwestern sector of mt. vesuvius. eruptive fissures, aligned along the strike of the main faults and in e-w and n-s directions, affect the mt. somma edifice and the southern and western flanks of mt. vesuvius. the volcanism is controlled by the compressive tectonics determined by the adriatic slab dynamics [piochi et al. 2005]. the eruptive history of the last 20 ky is signed by at least 4 plinian eruptions: pomici di base (18-20 ky), mercato (8.0 ky), avellino (3.9 ky), and pompei (79 a.d). these main episodes were intercalated by sub-plinian eruptions (pomici verdoline, 16 ky; pollena, 472 a.d.; 1631 eruption), by minor strombolian and effusive activity, and by quiescent periods [arrighi et al. 2001, cioni et al. 2008 and references therein]. after 1631 a persistent open-conduit activity began, which terminated with the violent strombolian eruption of 1944 [santacroce 1987, arrighi et al. 2001]. since then the volcano has been characterized by hydrothermal activity with diffuse co2 degassing and low-temperature fumaroles in the crater area, thermal submarine emissions, low energy, and discontinuous seismic activity [chiodini et al. 2001, saccorotti et al. cusano et al. 2 3 2002, frondini et al. 2004, caliro et al. 2011]. these activities could be mainly reconducted to the hydrothermal fluid circulation inside the volcanic edifice. chiodini et al. [2001] hypothesize the presence of a deep hydrothermal reservoir at depths of roughly 2.5-5.0 km, within the carbonate sequence, underneath the crater area. this reservoir, characterized by the presence of nacl solutions or halite saturated brines, would be subjected to fluid overpressures whose discharge through fracturing rocks could explain the seismic crises. 2.1. the stress field acting on mt. vesuvius the seismicity at mt. vesuvius consists of vt earthquakes, which concentrate at depths shallower than about 4 km below sea level (b.s.l.) along the crater axis. the maximum of the hypocentral distribution is at a depth of 2-3 km, which corresponds to the transition between the collapsed (caldera) central sector of the volcanic edifice and the carbonate basement [saccorotti et al. 2002, scarpa et al. 2002, del pezzo et al. 2004]. strike-slip, normal and reverse dip-slip are the typical focal mechanisms associated to vesuvian earthquakes, with nodal planes mainly oriented along the nw–se and ne–sw directions [bianco et al. 1998]. two main orientations exist for the p (nne–ssw and ese–wnw) and t (ese–wnw and nne/n–ssw/s) axes [bianco et al. 1998, zollo et al. 2002]. the deepest events have an average stress drop between 1 and 10 mpa and are probably caused by the regional tectonic stress, which concentrates in the prefractured carbonate basement [del pezzo et al. 2004]. the shallowest earthquakes are characterized by stress drop up to 1 mpa and are likely triggered by an increase of pore fluid pressure generated by changes in the level of the hydrothermal aquifer, whose top is located at about 1 km b.s.l. beneath the crater [del pezzo et al. 2004]. the last relevant seismic crisis started in august 1999 and reached the maximum rate in october 1999, when more than 140 earthquakes were recorded. the strain release reached 5.6·109 joule and the maximum magnitude earthquake (md = 3.6) occurred at 07:41 ut on october 9, at about 4 km b.s.l. inside the carbonate basement, beneath the crater area. the magnitude of this event was the highest recorded since the last eruption of 1944. the event was generated by tectonic stress release along a pre-existing fracture system and had a stress drop of about 10 mpa [ventura and vilardo 1999, zollo et al. 2002, del pezzo et al. 2004]. the seismic crisis continued with lower-magnitude events and lasted until november 1999. after this sequence, the seismic activity decreased to a lower level. 2.2. the fumarolic field the fumarolized area of the 1944 crater is characterized by the presence of vents distributed between its rim and the bottom. fumaroles discharge fluids with different temperature and chemical composition. fumaroles located on the crater rim are mainly composed of water vapor and atmospheric gases at temperature lower than 75 °c [caliro et al. 2011]. fumarolic vents at crater bottom show discharge temperature of about 95 °c and a chemical composition with h2o and co2 as major components, followed by other species as h2, h2s, n2, ch4, co and he [caliro et al. 2011], whose origin is related to a high-temperature hydrothermal system located below the crater [chiodini et al. 2001]. 3. data description and methods we performed the comparison between fumarole temperature and seismological parameters by using data acquired since 1998. the seismic activity at vesuvius volcano has been monitored continuously by a permanent seismic network (figure 1) managed since 1972 by the osservatovesuvius seismic and fumarolic activity figure 1. map of the vesuvius area plotting the locations of earthquakes and stations of the seismic and geochemical surveillance networks. rio vesuviano (ov), now istituto nazionale di geofisica e vulcanologia (ingv). at present, the seismic network is composed by 10 analogical stations equipped with short-period geophones, of which 3 are threeand 7 single-component; 2 short-period, three-component digital stations; and 6 broadband, three-component digital stations. the employed sensors are 1hz velocimeter geotech s13 and mark le-3d, and broadband guralp cmg 40t and trillium 120p. the signals are acquired in continuous mode with a sampling rate of 100 hz and are telemetered to the acquisition centre in naples [giudicepietro et al. 2010]. currently 4 digital stations of the mobile seismic network operate at mt. vesuvius. they consist of lennartz marslite and nanometrics taurus dataloggers, equipped with three-component broadband sensors, guralp cmg-40t or lennartz le3d/20s. the data are locally stored at sampling rate of 125 hz or 100 hz. details are in castellano et al. [2012]. the historical seismic catalogue of mt. vesuvius contains the duration magnitude (md) for the vt earthquakes since 1972. magnitude is estimated at ovo station, located in the western sector of the volcano. hourly data of soil temperature were acquired in two low temperature (<100 °c) fumaroles, identified by the acronyms fc2 and cra (previously identified as vesa in m08), located at the bottom and on the northern rim of the crater, respectively (figures 1, 2). the acquisition periods were between february 2004 and january 2005 for fc2 and between april 2005 and september 2010 for cra; the latter went out of order in september 2010 and was replaced with a new station in july 2011. temperature was measured at 30 cm depth using a waterproof, 12 bit gemini tinytag plus 2 logger, equipped with an external measuring probe at a resolution of 0.06 °c. rainfall data until july 2007 are a courtesy of mt. vesuvius national park and were acquired in a meteorological station located few tens of meters apart of cra. the rest of the data series, until october 2010, was reconstructed from data acquired in the closest available station, located at pozzuoli, about 20 km west of mt. vesuvius (regional campanian agro-meteorological center, available on line at the url http://www.sito. regione.campania.it/agricoltura/meteo/agrometeo.htm), using a linear regression algorithm based on common data between the two stations (pozzuoli and vesuvius) for the period june 2003-july 2007. 4. summary of seismological and geochemical observations in 1998-2005 m08 analyzed both shortand long-term variations in geochemical and seismic parameters for the vesuvius volcanic-hydrological system in the period 1998-2005. they interpreted these variations as having common source mechanisms. 4.1. long-term trends in the period of observation, the salinity and dissolved carbon content of groundwaters indicate that the contribution of the deeper saline and gas-charged waters was higher from 2001 to 2003, while it significantly decreased in 1999 and after late 2003, with a major contribution of shallower cold and dilute water. in the latter period, the increasing contribution of the shallower water was also evidenced by a decrease in well temperature, while air temperature and rainfall remained steady. the number of seismic events and energy release also exhibited coeval variations. the seismic parameters showed a peak in correspondence of the october 1999 crisis, with deep and high-stress drop earthquakes, occurred in the pre-fractured carbonate basement in response to the regional tectonic stress release [del pezzo et al. 2004]. after the crisis, the seismic activity decreased to a very low level, with earthquakes characterized by shallow hypocenters and low-stress drop, mainly clustered within the volcanic edifice. this seismicity was mainly triggered by pore fluid pressure variations, in turn caused by the charge/discharge mechanism of the shallow aquifer or by perturbations of the local stress field. after august 2004, the number of earthquakes increased slightly. in the periods of higher seismicity, related to variations in the regional stress field or to pore fluid pressure, the chemistry of the aquifer resulted more affected by the shallow diluted water. this effect sugcusano et al. 4 figure 2. from top to bottom, clockwise: a) aerial view of the mt. somma-vesuvius complex (source of image: google earth) with the locations of fumarole temperature monitoring stations; b) picture taken from the north-western rim of the 1944 crater illustrating the position of fc2 station; c) picture taken from the top of mt. vesuvius illustrating the position of cra station. 5 gests the influence of variations in the strain field on the groundwater chemistry and can be due to local permeability changes (i.e., opening or closing of microcracks and fractures). 4.2. short-term trend on august 30, 2005, a very shallow earthquake (hypocenter at 300 m a.s.l., green star in figure 3a), with a magnitude value (md = 2.8) particularly high for the area, occurred during a phase of slight enhancement of seismicity. the location and the low stress drop of this earthquake suggested that it was triggered by the pressure increase in the fluids circulating close or within the central conduit. this hypothesis was also supported by the moment tensor analysis, which is consistent with a tensile crack opening along the direction orthogonal to the maximum stress axis of a shear failure. the opening could have been enhanced by the increase of pore fluid pressure that gradually lowered the cohesion, favoring a shear rupture. the pore pressure build-up could be due to the stress field acting locally on the upper portion of central conduit and would consequentially enhance the upwelling of thermal fluids toward the surface, leading to the thermal anomaly observed at the vesa station (now cra station) a few days before the earthquake. 5. seismological and geochemical patterns in 19992011 we have updated the monthly distribution of vt number and energy release presented in m08 (figures 3b and c, respectively). the data set refers to the temporal interval from january 1999 to december 2011. in the distribution calculation we have considered the earthquakes contained in the seismic catalogue for the ovo station (available at http://www.ov.ingv.it/ov/it/ banche-dati/186-catalogo-sismico-del-vesuvio.html). the energy temporal distribution is calculated from the gutenberg-richter relationship as function of duration magnitude md [lay and wallace 1995] (1) where the energy e is expressed in erg and the md values are extracted from the historical catalogue of the ovo station. for the whole data set, the long term trend of seismic parameters in the period 1999-2011 can be schematized as follows (figures 3b,c): 1) the seismicity rate and the corresponding energy level started to decrease after the seismic crisis of 1999. 2) the seismicity reached minimum values in may 2001-july 2004; from january 2000 to december 2005, the energy release decreased. 3) the seismicity rate slightly increased after august 2004. 4) the seismicity rate reached a relative maximum in summer 2006, and then returned to very low value. a similar but less evident behavior can be retrieved for the seismic energy. the observations 1-3 were validated in m08 by a change-point test [mulargia and tinti 1985], while a very stable trend for the seismicity rate after july 2008 has been detected with the updated data by using the kolmogorov-smirnov distribution [press et al. 1992]. in particular, we found a constant linear behavior with a rake of about 2° and a regression coefficient of 0.94. in figure 3a we show the depth of the vts as function of time. the hypocentral depths are obtained from a 3d probabilistic location procedure (nonlinloc) [lomax et al. 2000]. the corresponding epicenters are reported in figure 1. it can be seen that: 1) starting from 2001 the seismicity lacks deep (> 2.5 km) hypocenters. the depth is in the upper 0-2 km of the crust. 2) the decrease of deep seismicity culminates in 2005, when the very shallow earthquake of 30 august 2005 occurred. it is noteworthy the occurrence of an anomalous long period earthquake (quasi-monochromatic specvesuvius seismic and fumarolic activity figure 3. seismic parameters of mt. vesuvius vt earthquakes from 1999 to 2011: (a) source depth (km); (b) monthly distribution of number of events; (c) energy (erg). the reported data are extracted from ov historical catalogue for the period january 1999-december 2011. the size of each symbol in panel a is proportional to the magnitude. the blue star indicates the july 20, 2003, long period earthquake, the green star the august 30, 2005, earthquake and the red circle the october 9, 1999, md 3.6 earthquake. . .log e m9 9 1 9 d= +^ h trum picked at 3.9 hz) recorded by the ov seismic stations on july 20, 2003 [bianco et al. 2005]. this deep seismic event, located about 4 km b.s.l., has a probable hydrothermal origin. array analysis indicates the presence of p and s wave in the first 2 seconds of the signals [bianco et al. 2005]. it is indicated with a blue star in figure 3a and, obviously, it is not included in the calculation of vt distributions in figures 3b and c. continuous monitoring of soil temperature started at fumarole fc2, located at the bottom of the 1944 crater (figure 2). available data, ranging from february 2004 to january 2005, are presented in figure 4 together with daily values of rainfall amounts. as shown in the figure, the main feature of the signal is the buffering at ~ 94 °c, that is the water boiling temperature at the altitude of the crater bottom. the medium term (months) trend is affected by short lasting (days) cooling cycles mainly linked to rainfalls. the effect of rainfall on decreasing soil temperature depends on the amount of water effectively infiltrating in the soil, in turn related to air temperature, humidity and rainfall rate. once excluded hydrological modulations, the only evident anomalous feature is the huge temperature drop recorded from the beginning of december 2004 to the end of the observations ( january 2005), during which soil temperature halved to 45 °c. due to serious environmental issues in ensuring a continuous record from fumarole fc2, mainly due to the high corrosion rate of the sensors determined by the acidic gases emitted by the vents and the difficulties to safely access the crater bottom, a new monitoring point was established in april 2005 on the northern sector of the crater rim (cra in figure 2). figure 5 shows the hourly data of soil temperature acquired at cra station (orange line), their monthly running average (purple thick line) and the total monthly rainfall amount (green bars). three phases, labeled from (a) to (c) in the figure, each characterized by a different dynamic of the thermal signal, may be recognized in the studied period: (a) the first phase, from the beginning of observations in april 2005 to june 2006, was dominated by the thermal anomalies driven by the august 2005 shallow earthquake. a huge thermal anomaly, with a maximum of ~ 95 °c (the highest value of the entire observational period) was recorded several days before the earthquake, after which soil temperature diminished first rapidly and later with a more gentle slope to a minimum of ~ 20°c observed in december 2005. after, soil temperature rose up to a relative maximum of ~ 60 °c in february 2006, followed by a relative minimum of ~ 30 °c in june 2006, in complete counter tendency with respect to the normal seasonal cycle. (b) the second phase, characterized by a strong variability of the temperature, was followed by a period during which soil temperature, even if with some modulation induced by rainfall events, progressively rose up from 40 °c in june 2006 to ~ 50 °c observed in july 2007. it is noteworthy the negative spike recorded at the beginning of december 2006 with a minimum of less than 10 °c (the main minimum of the entire observational period), very similar to that observed on december 2004 at fumarole fc2 (see figure 4). (c) the third and last phase, from july 2007 to december 2011, was characterized by a more regular thermal signal with respect to the previous phases, with the absence of significant maxima and with relative minima modulated by rainfall. 6. discussion in figure 6 we present a synthesis of all the results obtained in this study together with the time variation of co2/ch4 ratio from caliro et al. [2011] useful to interpret our observations. in the period january 1999-december 2011, seiscusano et al. 6 figure 4. top: hourly temperature (orange line) measured at 30 cm depth at fc2 station and its 1 month-running average (purple thick line); bottom: monthly rainfall amounts (green bars). data refers to the period from february to december 2004. figure 5. top: hourly temperature (orange line) measured at 30 cm depth at cra station and its 1 month-running average (purple thick line); bottom: monthly rainfall amounts (green bars). (a), (b) and (c) indicate the three phases in which the studied period was subdivided (see text for details). data refers to the period from april 2005 to december 2011. 7 mic and fumarolic activities show coeval trends: – the pattern of number of earthquakes, strongly influenced by deep (deeper than 2.5 km b.s.l.) events linked to the october 1999 crisis, showed a fast decay toward a minimum in 2001. between the end of 2001 and the first months of 2002, caliro et al. [2011] observed the highest pressure in the hydrothermal aquifer. they found contemporary maxima in several markers of an enhanced contribution of fluids of deep (magmatic) origin, as the equilibrium pressure of carbon dioxide (pco2) and the ratios co2/ch4 (shown in figure 6), co2/h2o and he/co2. by the first half of 2002, these parameters indicated the occurrence of a new depressurization phase of the hydrothermal aquifer. – the constant depressurization trend was interrupted at the end of july 2003 in correspondence of an anomalous seismic event, interpreted as linked to the presence of hydrothermal fluids. striking anomalies in geochemical parameters followed. a detailed discussion about this point is reported in cusano et al. [2013] and federico et al. [2013]. – after the july 2003 anomalies, the depressurization trend regained with major evidence. a marked and sudden fall of fc2 fumarole temperature occurred between the end of 2004 and the first days of 2005. – the positive trend of seismicity proceeded toward a relative maximum, reached in the first half of 2006 and preceded by the shallow earthquake of august 2005. as already described in m08 and now confirmed by the temperature anomalies recorded at cra, this earthquake likely perturbed the steam flux in the crater rim. its effects were visible since the first half of 2006. – after this long series of seismic and geochemical anomalies, lasted five years (2002-2006), nothing more noticeable is observed in our data until the end of the observational period (end of 2011). from the results obtained on the whole dataset, we find a confirmation for the long-term trend common to seismic and geochemical parameters already identified in m08. we can hypothesize two main stages. the first one, lasting from 1999 to the end of 2001/beginning of 2002, is mostly dominated by the effects of the volcanotectonic stress on both seismicity and geochemical parameters, as described in m08. the second stage starts in 2002-03 and is characterized by the progressive depletion of these effects. in this stage, deep earthquakes, characterizing the most recent crises (august-november 1995, february-may 1996, january-april 1999 and august-november 1999) and interpreted by saccorotti et al. [2002] as due to the migration of the rupture front from the surface down to about 5 to 6 km b.s.l., practically disappear. conversely, the observed oscillations in the seismicity rate can be mainly ascribed to pore fluid pressure variations, as the characteristics of most earthquakes (i.e., shallow source depth and low energy) would suggest [del pezzo et al. 2004]. in a system close to the failure threshold, small variations of fluid pressure, lubricating the pre-existing fracture system, are able to eventually trigger seismicity [hill 1977, sibson 1996, saccorotti et al. 2002]. pore pressure variations can be caused by the charge/discharge mechanism of the shallow aquifer, by perturbations in the local strain field or by the increased outgassing of hydrothermal/magmatic systems. actually, gas discharge rate and composition of crater fumaroles claim for the enhanced output of hydrothermal/magmatic gases in the period 2000-2003 [caliro et al. 2011], thus confirming the role of fluids in generating seismicity in the shallow parts of the volcanic edifice (figure 3a). indeed, the shallow earthquake of august 2005 has been heralded by a sharp increase of the temperature of fumarole cra (figure 5 and m08), thus re-enforcing the idea that fluid pressurization can generate seismicity. analogously, the reduced output of deep fluids from crater fumaroles [caliro et al. 2011] since 2002, remarked by a decrease of soil co2 flux (figure 6), would have been provoked by a general depressurization of the volcano-hydrothermal system, as testified by a peculiar event occurred on july 2003 at about 4 km of depth. according to cusano et al. [2013], this lp earthquake was generated in a fluid-filled volume; a consequent pressure drop in the neighbour rock could have favored the circulation of shallower fluids, thus explaining several changes in chemical composition and saline content [federico et al. 2013]. moreover, the pressure drop could be responsible of some episodes of drastic soil temperature decrease, as observed at fc2 station in late 2004 and cra station in mid 2006. it is worth of note that this latter occurred in correspondence of the peak of seismicity recorded in 2006. vesuvius seismic and fumarolic activity figure 6. synthesis of the main results in the period 1999-2011. from top to bottom: time variation of co2/ch4 ratio (data from caliro et al. [2011], purple circled line), fc2 temperature (1 month running average, ruby red thick line) and cra temperature (1 month running average, orange thick line), number of total localizable vt (green bars) and number of events with hypocenters deeper than 2.5 km (acid green thick line). the gray star at the bottom identifies the july 20, 2003, long period seismic event. 7. conclusions this work indicates that, since 2002, both seismicity and soil temperature are mainly controlled by pore fluid pressure variations, as the characteristics of most earthquakes would suggest (i.e., shallow source depth and low energy). hot fluid circulation in the crater area varies during time as an effect of both meteorological causes (i.e., rainfall) and the local stress field. the decrease of the deep fluid output from the crater fumaroles, recorded since 2002, paralleled by a general de-pressurization of the volcano-hydrothermal system, could have induced some episodic fluctuations in the pattern of steam circulation in the vesuvius cone. pressure drops can induce vapor separation from hot water at temperature close to boiling, locally increase steam pressure and generate in turn rock failure, as observed in august 2005. on the other hand, rock fracturing and enhanced permeability would promote circulation of shallow (colder) fluids at greater depth, thus decreasing soil temperature, as occurred in late 2004 or in 2006, in correspondence of a relative increase of seismicity. a close relationship is confirmed between seismicity and hot fluid circulation, both playing a reciprocal role, and it appears probably related to the stress field acting on the volcano. we want to conclude by remarking the utility of long-lasting and multidisciplinary monitoring of quiescent volcanoes. the possibility of characterizing the behavior of the volcanic activity and understanding the dynamical processes that define the state of a volcano, such as mt. vesuvius, could deal with the lack of a statistics regarding the signals which lead to eruptions. acknowledgements. daniele andronico and guido ventura are fully acknowledged for their useful comments. references acocella, v., and r. funiciello (2006). transverse systems along the extensional tyrrhenian margin of central italy and their influence on volcanism, tectonics, 25 (2); 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(1987). somma-vesuvius, quaderni de “la ricerca scientifica”, cnr, 114 (progetto finalizzato geodinamica, monografie finali, 8), 251 pp., 4 tables. scandone, r., f. bellucci, l. lirer and g. rolandi (1991). the structure of the campanian plain and the activity of the neapolitan volcanoes (italy), j. volcanol. geoth. res., 48, 1-31; doi:10.1016/03770273(91)90030-4. scarpa, r., f. tronca, f. bianco and e. del pezzo (2002). high resolution velocity structure beneath mount vesuvius from seismic array data, geophys. res. lett., 29 (21), 2040; doi:10.1029/2002gl015576. shinohara, h., n. matsushima, k. kazahaya and m. ohwada (2011). magma-hydrothermal system interaction inferred from volcanic gas measurements obtained during 2003-2008 at meakandake volcano, hokkaido, japan, bull. volcanol., 73, 409-421; doi: 10.1007/s00445-011-0463-2. sibson, r.h. (1996). structural permeability of fluiddriven faultfracture meshes, j. struct. geol., 18 (8), 1031-1042. ventura, g., and g. vilardo (1999). slip tendency analysis of the vesuvius faults: implications for the seismotectonic and volcanic hazard assessment, geophys. res. lett., 26 (21), 3229-3232; doi:10.1029/ 1999gl005393. zollo, a., w. marzocchi, p. capuano, a. lomax and g. iannaccone (2002). space and time behavior of seismic activity at mt. vesuvius volcano, southern italy, bull. seismol. soc. am., 92 (2), 625-640; doi:10.178 5/0120000287. *corresponding author: paola cusano, istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy; email: paola.cusano@ov.ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. vesuvius seismic and fumarolic activity << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice g r a f i c o p e r i l c a l c o l o d e l p u n t o d i r u g i a d a m e d i a n t e l o p s i c r o m e t r o < *'» m . tenani 1 . — n e l l a l'orma di « s o i n t e r n a z i o n a l e dei b o l l e t t i n i m e t e o r o l o g i c i , l ' u m i d i t à d e l l ' a r i a è oggi e s p r e s s a d a l v a l o r e d e l p u n t o di r u g i a d a : e c i o è d a l v a l o r e d e l l a t e m p e r a t u r a t,i a cui l ' a r i a si t r o v e r e b b e s a t u r a q u a l o r a v e n i s s e r a f f r e d d a t a a p r e s s i o n e c o s t a n t e , p e r s e m p l i c e s o t t r a z i o n e di c a l o r e ( a d es. p e r r a d i a z i o n e ) , s e n z a m o d i f i c a r e i l r a p p o r t o t r a l a m a s s a d e l v a p o r e p r e s e n t e e l a m a s s a d e l l ' a r i a s e c c a : l a t e m p e r a t u r a t,i c o s ì d e f i n i t a è t u t t a v i a q u a s i i d e n t i c a a q u e l l a a cui a v v i e n e l a c o n d e n s a z i o n e q u a n d o in q u a l s i a s i m o d o l ' a r i a v e n g a r a f f r e d d a t a , p e r c h é n e l l a p r a t i c a di u n a t a l e d e t e r m i n a z i o n e l e v a r i a z i o n i d e l s u d d e t t o r a p p o r t o di m e s c o l a n z a s o n o t r a s c u r a b i l i . 2 . — c o n s e g u e n t e m e n t e l a d e t e r m i n a z i o n e di t,i p u ò e s e g u i r s i i n ] i r a l i c a i n d i f f e r e n t e m e n t e : o d e t e r m i n a n d o t(i d i r e t t a m e n t e c o n u n o p s i c r o m e t r o a c o n d e n s a z i o n e , il c l i c o f f r e l e b e n n o t e d i f f i c o l t à ; o d e t e r m i n a n d o , c o l l o p s i c r o m e t r o , l a t e n s i o n e o d e l v a p o r e n e l l ' a r i a e d e t e r m i n a n d o p o i , c o l s u s s i d i o di u n a t a v o l a d e l l e t e n s i o n i m a s s i m e e* d e l v a p o r e , l a t e m p e r a t u r a t,i p e r c u i — u n a t a v o l a di t a l g e n e r e è c o s t i t u i t a p r e c i s a m e n t e d a l l a c o l o n n a e d e l l ' o r d i n a r i a t a v o l a p s i c r o m e t r i c a r e l a t i v a a l l a d i f f e r e n z a p s i e r o m e t r i e a 0 ° , 0 , e d a l l a c o l o n n a c o n t e n e n t e l e c o r r i s p o n d e n t i t e m p e r a t u r e d e l t e r m o m e t r o b a g n a t o ( o a s c i u t t o ) ; o v v e r o r i c o r r e n d o a t a v o l e s p e c i a l i c h e f o r n i s c a n o t,\ i n f u n z i o n e d e l l a d i f f e r e n z a p s i c r o m e t r i c a a' e d e l l a t e m p e r a t u r a d e l t e r m o m e t r o b a g n a t o ( o a s c i u t t o ) . u s a n d o l o p s i c r o m e t r o , c o n v e r r à t e n e r s e m p r e p r e s e n t e c h e l a f o r m u l a s e m i e m p i r i c a c h e l e g a il v a l o r e di e a l l a d i f f e r e n z a p s i c r o m e t r i c a e a l l a t e m p e r a t u r a , v a r i a a l q u a n t o c o n l a p r e s s i o n e : a c o n t i f a t t i si o t t i e n e c h e , p e r q u o t e i n f e r i o r i ai 1 0 0 0 m e t r i , t e n u t o c o n t o d e i < *ì c o m u n i c a z i o n e p r e s e n t a l a al c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , , t e n u t o a r o m a il 1 7 1 8 g i u g n o 1 9 5 3 . 4 4 8 m . t e n a n i jr>a\s()ut3drejtij j'/cu. ndicvfe, i in /jiodo\ che -a tute tn di te te 'cfa/ yj/n{ a c cj\rri.^pon< ndìy\idud\ fra le curvi // ragià èia . i fs.;1 tem sicrbmetrìcdj 7.8}\ put. ] il ! grafico i vj/i bordo ' 1 0 m.fn., l —;—y , , tf>er/a , dei jrrh/ndliorfe, hunt.o idi | ru sim'stvj / cateto |a c yr. //-j|/c> ve\rti(̂ ^ i f 0/', ter/ricmetro , órfy^jtffo /te a f e à'fóirnn, qnsffdp il\vd/à pw j terny. bsyna )to di rugiedci \ + 16 peb letture [ escc i p/fssitne &tm\sfc arotrietficij i i t 7/c?<y<? j { twe/o \a 4 c//[ awfo <0^.5 sotto : ids cofy /<? /<7 j <//v/̂ /b/lt c/(6>|/<f pst'crnmeth'crf i — f c i i/*m fcutw '•f del bu'iko •o +y20,ydi/f?rer v; j/i '/ctfi e prossima \ preeisvrh 15*7). livide ty/ ! 5 ° " ' d 3 io* " " 15° d i f f e r e n z e p s i c r o m e t r i c h e f i g . 1 g r a f i c o p e r l a d e t e r m i n a z i o n e d e l p u n t o d i r u g i a d a . g r a f i c o p e r i l c a l c o l o d e l p u n ' i o 1)1 r u g i a d a m e d i a n t e lo p s i c r o m e t r o 4 4 9 l i m i t i di p r e c i s i o n e d e l l e o s s e r v a z i o n i , è p o s s i b i l e t r a s c u r a r e l e v a r i a z i o n i d e l l a p r e s s i o n e i n t o r n o a l s u o v a l o r m e d i o ; e c i o è s u p p o r r e c h e l a p r e s s i o n e a b b i a il v a l o r m e d i o c o r r i s p o n d e n t e a l l a q u o t a d e l l o p s i c r o m e t r o . a n a l o g h e a v v e r t e n z e d o v r a n n o t e n e r s i p r e s e n t i q u a n d o l a t e m p e r a t u r a è i n f e r i o r e a 0° c, q u a n d o c i o è v e n g a a v a r i a r e l a r e l a z i o n e t r a e* e ta. 3 . — u n v a l o r e a p p r o s s i m a t o di ta è a n c h e f a c i l e a c a l c o l a r s i s e n z a u s o d i t a v o l e , s e m p l i c e m e n t e : t o g l i e n d o d a l l a t e m p e r a t u r a d e l t e r m o m e t r o b a g n a l o l a d i f f e r e n z a p s i c r o m e t r i c a ; o v v e r o t o g l i e n d o d a l l a t e m p e r a t u r a d e l t e r m o m e t r o a s c i u t t o i l d o p p i o d e l l a d e t t a d i f f e r e n z a ; i l r i s u l t a t o risulta t a n t o p i ù e s a t t o q u a n t o p i ù p r o s s i m a a l l a c o n d e n s a z i o n e è l ' a r i a , e c i o è q u a n t o p i ù c r i t i c h e s o n o l e c o n d i z i o n i p e r l a f o r m a z i o n e d e l l a n e b b i a . 4 . — 11 n u o v o m e t o d o d i m i s u r a r e l ' u m i d i t à d e l l ' a r i a h a i n c o n t r a t o p a r t i c o l a r m e n t e f a v o r e f r a i n a v i g a n t i : s o p r a t t u t t o p e r c h é i l c o n f r o n t o d i 7',i c o n l a t e m p e r a t u r a s u p e r f i c i a l e d e l l ' a c q u a o f f r e u n m e t o d o c o m o d o e s i c u r o p e r g i u d i c a r e d e l l a p r o b a b i l i t à d i f o r m a z i o n e d e l l a n e b b i a . s e l a t e m p e r a t u r a d e l m a r e r i s u l t a i n f e r i o r e a t,i, t e n e n d o c o n t o d e l f a t t o c b e l a r o t t a si s v o l g a v e r s o r e g i o n i i n c u i i l c o n t a t t o a r i a a c q u a r i s u l t i f a v o r e v o l e o c o n t r a r i o a u n a d i m i n u z i o n e d i t e m p e r a t u r a d e l l ' a r i a d a l s u o v a l o r e a t t u a l e t a l v a l o r e t(1, il n a v i g a n t e a v r à u n a s i c u r a g u i d a p e r l a p r e v i s i o n e d e l l a n e b b i a o p e r e s c l u d e r n e l a f o r m a z i o n e : i l c h e a v v e r r à c e r t a m e n t e se l a t e m p e r a t u r a d e l m a r e 7's s a r à s u p e r i o r e a l p u n t o di r u g i a d a t d ; se t s > t(1 e t < t( ] è p r o b a b i l e c h e l a n e b b i a s c o m p a i a d o p o u n b r e v e p e r c o r s o s u l m a r e . n e i l u o g h i i n c u i l a n e b b i a è f r e q u e n t e u n c o s t a n t e c o n f r o n t o d i t,i c o n la t e m p e r a t u r a s u p e r f i c i a l e d e l m a r e è d i e s t r e m a u t i l i t à . 5. — t,i r a p p r e s e n t a m o l t e v o l t e p e l n a v i g a n t e l ' u n i c o m o d o di e s e g u i r e a t t u a l m e n t e u n a p p r e z z a m e n t o d e l l a b a s e d e l l e n u b i b a s s e : se n o i p e n s i a m o c h e essa si t r o v i a l l a q u o t a di c o n d e n s a z i o n e d e l v a p o r d ' a c q u a c o n t e n u t o n e l l ' a r i a a m b i e n t e i n s e g u i t o a u n s o l l e v a m e n t o a d i a b a t i c o , si t r o v a c h e l a b a s e d e l l e c o n d e n s a z i o n i s a r à a l l ' i n c i r c a a u n a q u o t a u g u a l e a l p r o d o t t o d e l l a d i f f e r e n z a ( i n g r a d i c) t r a t e t() p e r 1 0 0 1 2 5 m e t r i o, a p p r o s s i m a t i v a m e n t e , u g u a l e a l p r o d o t t o d e l l a d i f f e r e n z a p s i c r o m e t r i c a p e r 2 5 0 m e t r i . • 1 5 0 m . t e n a n i 6. — l a r e g o l a i n d i c a l a i n 3 p e r il c a l c o l o d e l p u n t o d i r u g i a d a , e s a t t i s s i m a al limite, q u a n d o l ' a r i a è s a t u r a , p e r c h é a l l o r a i l p u n t o di r u g i a d a è e s a t t a m e n t e u g u a l e a l l a t e m p e r a t u r a d e l t e r m o m e t r o b a g n a lo, d i v e n t a p e r ò r a p i d a m e n t e i n e s a t t a a l c r e s c e r e d e l l a d i f f e r e n z a p s i c r o . m e t r i c a . p o i c h é l e t a v o l e d e s t i n a l e a q u e s t o c o m p u t o s o n o p i u t t o s t o v o l u m i n o s e , p u ò e s s e r e u t i l e i n p r a t i c a u n p i c c o l o g r a f i c o c h e p e r m e i te l a p r e c i s i o n e d e l d e c i m o di g r a d o , p i ù c h e s u f f i c i e n t e n e l l a p r a t i c a ( n e i t e l e g r a m m i m e t e o r i c i t,i è a r r o t o n d a t o a l g r a d o , e p e r c i ò d e v e e s s e r e p e r l ' a p p u n t o c o n o s c i u t o c o n l a p r e c i s i o n e d e l d e c i m o d i g r a d o ) . in q u e s t o g r a f i c o l e o r d i n a t e s o n o l e t e m p e r a t u r e d e l t e r m o m e t r o bag n a t o , l e a s c i s s e s o n o l e d i f f e r e n z e p s i c r o m e t r i c h e d e t e r m i n a t e c o n p s i c r o m e t r i a fionda d e l g e n e r e c o n s i g l i a b i l e u s a t o s u l l e n o s t r e n a v i . l e i s o p l e t e t r a c c i a t e f o r m a n o t a n t i v i a l i l a c u i m e z z e r i a è sul p e r c o r s o d e l l e i s o p l e t e d e l tci c o r r i s p o n d e n t i ai g r a d i i n t e r i . u n p i c c o l o g r a f i c o m o s t r a c o m e , c o n u n a s q u a d r e t t a si p o s s a n o e s e g u i r e l e l e t t u r e d e l t,i q u a n d o s i a n o a s s e g n a t e l a t e m p e r a t u r a d e l t e r m o m e t r o b a g n a t o e l a d i f f e r e n z a p s i c r o m e t r i c a ( f r e c c i a ) . riassunto si presenta un grafico che permette (li determinare il plinto di rugiada mediante la differenza psicocrometrica e la temperatura del termometro bagnato. summary a grapli is presented for determining the deiv-point by means of the hygrometric difference and the temperature of the net thermometer. frequency dependent attenuation of seismic waves for delhi and surrounding area, india annals of geophysics, 58, 2, 2015, s0216; doi:10.4401/ag-6636 s0216 frequency dependent attenuation of seismic waves for delhi and surrounding area, india babita sharma*, prasantha chingtham, anup k. sutar, sumer chopra, haldhar p. shukla center for seismology, ministry of earth sciences, new delhi, india abstract the attenuation properties of delhi and surrounding region have been investigated using 62 local earthquakes recorded at nine stations. the frequency dependent quality factors qa (using p-waves) and qb (using swaves) have been determined using the coda normalization method. quality factor of coda-waves (qc ) has been estimated using the single backscattering model in the frequency range from 1.5 hz to 9 hz. wennerberg formulation has been used to estimate qi (intrinsic attenuation parameter) and qs (scattering attenuation parameter) for the region. the values qa , qb , qc , qi and qs estimated are frequency dependent in the range of 1.5hz-9hz. frequency dependent relations are estimated as qa = 52f 1.03, qb = 98f 1.07 and qc = 158f 0.97. qc estimates lie in between the values of qi and qs but closer to qi at all central frequencies. comparison between qi and qs shows that intrinsic absorption is predominant over scattering for delhi and surrounding region. 1. introduction the attenuation of seismic waves provides important information about the medium characteristics which is required for the determination of earthquake source parameters as well as for prediction of earthquake ground motions. attenuation of seismic waves is controlled by geometrical spreading, scattering due to inhomogeneities in the medium and damping. the attenuating property of a medium is described by the dimensionless quantity called quality factor q, which expresses the decay of wave amplitude during its propagation in the medium [knopoff 1964]. various studies have been done worldwide to understand the attenuation characteristics by estimating q using p-waves (qa), s-waves (qb) and coda waves (qc). aki and chouet [1975] gave the single backscattering model to estimate qc. the coda normalization method to estimate qb, was developed by aki [1980] and later on yoshimoto et al. [1993] extended coda normalization method to estimate qb also. the estimates of q have been found to be frequency dependent by several researchers worldwide [e.g., aki and chouet 1975, rautian and khalturin 1978, aki 1980, sato and matsumura 1980, roecker et al. 1982, hough and anderson 1988, masuda 1988, woodgold 1990, campillo and plantet 1991, sekiguchi 1991, takemura et al. 1991, mayeda et al. 1992, yoshimoto et al. 1993, gupta et al. 1995, gupta et al. 1998, mandal et al. 2001, sharma et al. 2007, sharma et al. 2008, sharma et al. 2009, sharma et al. 2011]. various methods have been developed to measure the relative contribution of intrinsic attenuation qi and scattering attenuation qs to the total attenuation. wu [1985] proposed a method for an estimation of the relative contribution of qs and qi from the dependence of total s-wave energy on hypocentral distance. frankel and wennerberg [1987] used the energy flux model of seismic coda to obtain the separate estimates of qs and qi based on coda amplitude and decay. hoshiba et al. [1991] developed a method based on monte carlo simulations of the temporal shape of the coda envelope. wennerberg [1993] provided the formulation to determine the contribution of qs and qi attenuation to the total attenuation. the objective of the present study is to understand the attenuation mechanism of medium beneath delhi and surrounding region, india by estimating q using different parts of the seismograms and to estimate the relative contribution of qi and qs in the region. the extended coda normalization method [yoshimoto et al. 1993] has been used to estimate the frequency-dependent relations for qa and qb. single backscattering model of aki and chouet [1975] has been used to estimate qc. wennerberg’s [1993] formulation has been used to estimate the relative contribution of qi and qs. 2. seismotectonics and geology of the area the delhi and surrounding area had active seismic history [tandon 1975, verma et al. 1995, mohanty 1997, article history received july 7, 2014; accepted february 6, 2015. subject classification: coda, intrinsic, scattering, attenuation, frequency. bansal et al. 2008, singh et al. 2010, bansal and verma 2012, prakash and srivastava 2012]. the first reported earthquake with intensity ix occurred in the delhi region on july 15, 1720 [tandon 1975, chandra 1992]. the estimated intensity of these earthquakes on the modified mercalli scale was found to be between vii and ix at delhi and its surrounding region, as indicated by the damage pattern. the earthquake of august 27, 1960, was another significant earthquake of magnitude 6.0 with its epicenter between delhi and gurgaon. the seismicity of delhi and surrounding region show maximum concentration of epicenters in north-south trending sonepat-sohna fault, west of delhi and at the tri-junction of delhi-haridwar ridge, delhi-lahore ridge and the axis of delhi folding. it has been indicated that there are numerous hidden faults in the thick alluvial deposits of the indo-gangetic plains. according to the seismotectonic studies of the region, haridwardelhi ridge, sohna fault, aravalli fault and moradabad fault are the prominent tectonic features in delhi and the surrounding areas [mohanty 1997, bansal and verma 2012, prakash and srivastava 2012]. the entire delhi and its surrounding area exhibit moderate seismicity and fall under seismic zone iv of the seismic zonation map of india [singh et al. 2010]. it is important to consider seismic factors for urban planning, industrialization, designing and construction of civil engineering structures. the rock formations exposed in the delhi area are mainly quartzite of the alwar series of the delhi supergroup which are 1500 million years in age and overlain by unconsolidated quaternary to recent sediments which are 1.65 ma old. the terrain is generally flat except for a low nne-ssw trending delhi ridge in the southern and central part of the area which consists of quartzite while the quaternary sediments, comprising the older and newer alluvium cover the rest of the area. the older alluvium comprises silt, clay with minor lenticular fine sand and kankar beds [choudhary et al. 1984]. the newer alluvium mainly consists of sand, silt and clay occurring in the older and active flood plains of the yamuna river. thickness of the alluvium, both on the eastern and western side of the ridge, is variable but west of the ridge, it is generally thicker nearly 300m [mohanty et al. 2009, bansal and verma 2012]. figure 1 shows the area of present study along with the seismological stations, earthquake locations and major tectonic features of the area. sharma et al. 2 figure 1. epicentral locations of earthquake events and seismological stations in and around delhi area. s. no. station with station code latitude (deg min) longitude (deg min) 1 ayanagar (ayan) 28°28.93 77°07.60 2 sohana (sona) 28°14.70 77°03.78 3 bhadurgarh (bhgr) 28°41.26 76°56.33 4 rataul (rtl) 28°49.93 77°20.51 5 bisrakh (bis) 28°34.26 77°26.34 6 kuldal (kudl) 28°08.65 76°29.35 7 asauara(asr) 28°45.35 77°46.33 8 unchagaon (ucg) 28°18.60 77°54.60 9 new delhi (ndi) 28°35.41 77°12.13 table 1. epicentral locations, origin time, date and depths of the events use in the present study. 3 3. methodology in the present study single backscattering model of aki and chouet [1975] is used to estimate qc. extended coda normalization method [yoshimoto et al. 1993] is used to calculate qa and qb and wennerberg formulation [wennerberg 1993] is applied to estimate qi and qs. these methods are described as below. single backscattering model the qc has been estimated using the single backscattering model proposed by aki and chouet [1975]. according to this model, the coda waves are interpreted as backscattered body-waves generated by numerous heterogeneities present in the earth’s crust and upper mantle. it implies that scattering is a weak process and outgoing waves are scattered only once before reaching the receiver. under this assumption, the coda amplitudes, ac( f,t), in a seismogram can be expressed for a central frequency f over a narrow bandwidth signal, as a function of the lapse time t, measured from the origin time of the seismic event, as [aki 1980]: (1) where s( f ) represents the source function at frequency f, and is considered a constant as it is independent of time and radiation pattern, and therefore, not a function of factors influencing energy loss in the medium; a is the geometrical spreading factor, and taken as 1 for body waves. the swapping of geometrical spreading factor and q could give rise to unreasonable values of these parameters. to minimize the risk to get unreasonable values an inversion method based on a parabolic expression of the coda-normalization equation has been developed by de lorenzo et al. [2013]. for various indian regions the attenuation properties are estimated by several researchers [gupta et al. 1995, sharma et al. 2007, sharma et al. 2008, mohanty et al. 2009, etc.] and in all the related studies carried out for various parts of indian subcontinent the geometrical spreading factor is considered to be unity. qc is the apparent quality factor of coda waves representing the attenuation in a medium. the above equation can be rewritten as (2) it is a linear equation with the slope from which qc is estimated. extended coda normalization method this method is based on the idea that coda waves consist of scattered s waves from random heterogeneities in the earth [aki 1969, aki and chouet 1975, sato 1977]. the spectral amplitude, ac( f,tc ), of the coda at a lapse time tc can be written as [aki 1980]: (3) where f is the frequency, ss( f ) is the source spectral amplitude of s waves, p( f,tc ) is the coda excitation factor, g( f ) is the site amplification factor and i( f ) is the instrumental response. the spectral amplitude of the direct s wave, as( f,r) can be expressed as (4) where riz is the source radiation pattern and a denotes the geometrical exponent which is taken unit value as explained in the previous section. qb( f ) is the quality factor of s waves, vs is the average s wave velocity and } is the incident angle of s waves. on dividing equation (4) by equation (3), taking the logarithm and simplifying, we get [yoshimoto et al. 1993]: (5) using a similar equation the quality factor for the p-waves can be obtained [yoshimoto et al. 1993]. (6) the quality factor for p waves can be obtained from the linear regression of versus r by means of least-squares method as done for s-waves. wennerberg formulation to estimate qi and qs wennerberg [1993] provided the formulation based on zeng et al. [1991] model to estimate qi and qs. according to zeng et al. [1991], we can write the observed value of qc in terms of qi and qs as below: (7) where, d(x) is , ~ is the angular frequency and t is the lapse time. assuming qd as the quality factor of direct wave evaluated in the earth volume ac f, tcq v = ss fq vp f, tcq vg fq vi fq v as f, rq v = riz ss fq vr -a exp q b fq vvs fr u zg f, }q vi fq v ln ac f, tcq v as f, rq vr u z r ! r =q b fq vvs fr + const fq v ln ac f, tcq v ap f, rq vr u z r ! r =q a fq vvp fr + const fq v q c 1 = q i 1 + q s 1 2d xq vq v ac f, tcq v ap f, rq vr u z r ! r 4.44 + 0.738q v 1 , x = q s ~t , ln ac f, tq vq v = ln s fq vq v q c fq v t q c fq v ac f, tq v = s fq vt-a exp q c ftq v attenuation for delhi, india equivalent to the volume sampled by coda waves, it can be written as [wennerberg 1993]: (9) if qc is measured as a function of lapse time t, qi and qs can be estimated using equations (7), (8) and (9), where qd is measured as a function of distance. data analysis earthquake data of 62 events with ml 2.0-4.9 recorded by the digital seismic telemetric network operated by india meteorological department in and around delhi. the data is recorded using short period sensors at 20 samples per second (sps), which limits the nyquist frequency 10 hz. figure 1 shows the epicentral locations of events and stations considered. locations of the stations along with the station codes are given in table 1. the vertical component of each seismogram have been filtered at five different frequency bands (1-2) hz, (2-4) hz, (4-6) hz, (6-8) hz and (8-10) hz using a butterworth band pass filter. on the filtered seismograms, the root-mean-square amplitudes of coda waves amplitude measurement starts at twice the travel time of the s-waves in a window length of 256 samples and lapse time window length of 30 seconds have been used to estimate qc. figure 2 shows one original and filtered seismogram recorded at sona station on june 7, 2006, and figure 3 shows the variation of ln(ac( f,t).t) with lapse time t along with the least-squares-fitted line for different central frequencies at sona for the same event. figure 4 represents the plot of qc values as a function of frequency obtained at 30 sec lapse time window. data used in the present study is analysed visually for the signal to noise ratio. the seismograms having signal to noise ratio less than two is not considered in the present study. table 2 shows the events selected for the present study. q s 1 = 2d xq v 1 q d 1 q c xq v 1 t y q i 1 = 2d xq v 1 q c xq v 1 + q d 2d xq v 1q v u z sharma et al. 4 figure 2. a sample of original and filtered seismogram at different frequency bands for sona station recorded on june 7, 2006. figure 3. the variation of ln(ac( f,t).t) with lapse time t along with the least-squares-fitted line for different central frequencies at sona for the event of june 7, 2006. figure 4. the plot of qc values as a function of frequency obtained at 30 sec lapse time window. (8) 5 attenuation for delhi, india s. no. year month day hr min sec latitude (°n) longitude (°e) magnitude (ml) depth (km) 1 2001 1 28 15 22 42.4 28.74 76.61 2.4 5 2 2001 4 28 13 28 29.3 28.62 76.55 2.3 4 3 2001 8 10 12 19 32.8 28.91 77.24 3.2 13 4 2001 8 12 21 9 30.2 28.96 77.69 2.7 26 5 2001 8 23 18 23 20.3 29.08 76.97 2.4 9 6 2001 9 14 14 31 29.9 29.22 77.23 2.1 15 7 2001 10 9 23 18 15.5 29.17 77.37 2.2 10 8 2002 10 22 21 43 12.9 28.68 77.11 2.7 15 9 2002 10 23 20 52 31.7 28.91 76.81 2.2 15 10 2002 10 30 5 10 10 28.77 77.06 2.1 23 11 2002 10 31 0 59 23.2 28.8 76.95 2.1 22 12 2002 11 6 2 12 0.5 28.97 76.84 2.9 10 13 2003 2 15 6 37 6.1 29.3 77.21 2.6 5 14 2004 5 3 16 55 5.7 29.25 76.46 3 5 15 2004 5 15 3 12 53.9 29.19 76.39 2.3 13 16 2004 5 22 14 45 55 29.14 76.46 2.6 18 17 2004 5 3 16 55 5.7 29.25 76.46 3 5 18 2004 5 15 3 12 53.9 29.19 76.39 2.3 13 19 2004 5 22 14 45 55 29.14 76.46 2.6 18 20 2004 8 22 22 47 53.6 28.44 77.38 2.1 12 21 2004 10 9 11 34 41.5 29.23 76.43 2.8 5 22 2004 12 18 15 39 16.9 29 76.62 2.7 4 23 2004 12 20 1 11 57.5 28.92 77.11 2.6 28 24 2004 12 23 19 21 1.7 28.56 76.69 2.5 6 25 2005 6 10 23 53 20.6 29.32 77.02 2.6 9 26 2005 6 13 17 31 48.6 28.3 76.19 2.2 5 27 2006 1 19 8 12 38.3 28.82 76.57 2.4 5 28 2006 3 15 5 5 54.5 28.91 76.7 2.3 17 29 2006 4 7 18 56 40.3 28.93 76.92 3.3 5 30 2006 4 10 14 11 58.7 29.12 76.64 3 5 31 2006 4 11 23 21 8.5 29.29 76.9 2.4 40 32 2006 4 11 23 26 23.7 28.83 76.81 2.7 10 33 2006 5 7 16 1 0.2 28.72 76.6 4.2 4 34 2006 5 11 7 0 10.4 28.67 76.66 2.7 15 35 2006 5 12 1 30 55.2 28.89 76.65 2.3 10 36 2006 7 9 2 30 34.1 28.91 76.74 2.5 25 37 2006 10 31 12 59 3.1 28.75 76.67 2.1 5 38 2006 12 9 18 52 4.3 28.95 76.64 2.8 10 table 2 (continues on next page). coordinates of the stations along with their station codes in and around delhi region. in order to estimate qa and qb, the rms amplitudes of pand s-waves have been taken from the filtered seismograms and normalized by the coda wave amplitude. figure 5 shows a plot of ln((as/ac)r) with respect to hypocentral distance r (km) and corresponding plots of s-waves at five different central frequencies for ndi. the slopes of the best-fitted lines are used to estimate qa and qb by using equations (5) and (6). the average velocities of 7.02 km/sec and 4.06 km/sec for p and s waves respectively, have been used in the present study [imd 2000]. afterwards, qs and qi are estimated according to equations (8) and (9) with the help of values of qc and qb calculated using single backscattering and coda normalization methods assuming the qb as quality factor of direct wave. 4. results and discussions in the present study the attenuation properties of the crust for delhi, india have been estimated. for this purpose, 62 local earthquakes recorded by 9 stations in delhi and surrounding area have been used. the estimated mean values of qa, qb, qc, qi and qs along with the standard deviation error at different central frequencies for the region considered in the present study are given in table 3. the average value of qc varies from 274 at 1.5 hz to 1656 at 9 hz. the average values of qa and qb vary from 77 and 156 at 1.5 hz. to 538 and 969 at 9 hz, respectively. the increase in q values with increasing frequency indicates the frequency-dependent nature of the q estimates in the region. in order to obtain the frequency-dependent relations, the estimated average qa, qb and qc values as a function of frequency are plotted in figure 6. the frequency-dependent relationships estimated for the region along with the standard deviation are: qa = (52±4) f (1.03±.07), qb = (98±7) f (1.07±.09) and qc = (158±9) f (0.97±.08). the small lateral variation found in the estimated q values may be attributed to the heterogeneities present in the resharma et al. 6 39 2006 12 23 5 9 32.1 28.31 76.05 2.4 10 40 2006 12 28 13 40 46.8 28.98 76.67 2 10 41 2006 12 30 18 31 52.4 28.3 76.09 2.5 5 42 2007 1 8 12 1 0.6 28.81 76.76 2.5 10 43 2007 1 23 1 48 26.1 28.33 76.82 2.3 20 44 2007 1 29 20 26 0.4 28.9 76.6 2.2 10 45 2007 2 27 20 37 58.5 29.24 77.26 2.5 10 46 2007 3 6 3 14 4.4 28.93 76.62 2.2 9 47 2007 3 12 5 51 26.9 28.6 76.96 2.2 13 48 2007 4 3 15 35 11.2 28.94 76.62 2.8 10 49 2007 5 14 7 22 47.9 28.92 76.64 3.2 5 50 2007 5 14 10 51 22 28.81 76.77 2.2 33 51 2007 5 16 12 56 7.9 28.78 76.64 2.3 18 52 2008 5 27 21 51 59.4 29.8 77.07 2.0 15 53 2008 8 29 3 43 27.1 28.8 77.34 2.3 15 54 2008 10 19 7 56 48.4 29 76.76 3.9 6 55 2008 10 28 20 29 48.1 28.8 76.52 4.9 10 56 2009 01 07 14 53 29.5 29.57 77.8 3.9 5 57 2009 02 01 18 32 9.6 29.5 77.88 2.8 10 58 2009 03 10 12 22 26.7 29.5 77.9 3.6 10 59 2009 03 14 2 22 54.7 29.27 77.2 4.2 10 60 2009 05 30 8 00 00 29.2 77.65 2.3 7 61 2009 06 17 12 07 31.7 29.53 77.82 4.2 11 62 2011 09 07 17 58 16.3 28.6 77.05 4.3 10 s. no. year month day hr min sec latitude (°n) longitude (°e) magnitude (ml) depth (km) table 2 (continued from previous page). coordinates of the stations along with their station codes in and around delhi region. 7 gion and difference in distances of the events from the recording stations. the estimate of qc is found to be higher than qb in this region. the effect of intrinsic and scattering attenuation combine in a manner that qc is more than qb as shown in figure 6. this supports the zeng et al.’s [1991] model which predicts the idea of coda enrichment over qb. according to wennerberg [1993] formulation, qc is separated in terms of scattering and intrinsic attenuation in the present study. the estimated qi values vary from 472 at 1.5 hz to 2525 at 9 hz. the estimated qi values vary from 232 at 1.5 hz to 1937 at 9 hz. it has been reported in literature and using laboratory measurements that coda-q is very close to qi [frankel and wennerberg 1987, matsunami 1991]. however, mayeda et al. [1992] have found that this observation is valid at higher frequencies while qc is intermediate between qi and qs. it has been observed attenuation for delhi, india c.f. qa qb qc qi qs 1.5 77±14 156±11 274±25 232±25 472±22 3 185±17 363±19 730±79 591±33 941±23 5 277±12 583±21 931±94 816±18 2040±25 7 382±19 764±16 1265±216 1096±16 2520±32 9 538±26 969±12 1656±91 1937±31 2525±46 table 3. values of different quality factors estimated at different central frequencies. figure 5. plot of ln((as/ac)r) with respect to hypocentral distance r (km) and corresponding plots of s-waves at five different central frequencies for ndi station. figure 6. plot of estimated average qa, qb and qc values as a function of frequency. from the present study that qc values lie in between qi and qs at all frequencies (table 3). a comparison between estimates of qi and qs in this study shows that intrinsic absorption is predominant over scattering for the frequency range (1.5 hz 9 hz) considered here. it has been found that the value of q0 (qc at 1 hz) varies from 47 to 200 and that of n varies from 0.70 to 1.10 for the active regions including parkfield [hellweg et al. 1995], friuli, italy [rovelli 1982]. singh et al. [2004] have estimated a relation q( f ) = 800f 0.42 for the indian shield region using the dataset of four earthquakes recorded in the distance range of 240-2400 km. using the accelerograms of the aftershocks of 2001 bhuj earthquake, bodin and horton [2004] have obtained a relation q( f ) = 790f 0.35 for the kachchh basin. the coda-based method used in this study gives q of a very shallow portion of the crust, while q estimates obtained by singh et al. [2004] and bodin and horton [2004] sample deeper in the crust. mohanty et al. [2009] have estimated coda wave attenuation for delhi using local earthquakes and obtained frequency dependent relationship as qc = 142f 1.04. the frequency dependent relationship obtained using coda waves in the present study as: (158±9) f (0.97±.0), is comparable to that of mohanty et al. [2009]. the study region of mohanty et al. [2009] is same but they have computed only qc for the region and we have extended the attenuation study by separating the total attenuation parameter in terms of intrinsic (qi) and scattering parameters (qs). for this purpose we have also estimated qb, qa and qb/qa, which represent the attenuation of seismic waves for delhi and surrounding region in a better way. figure 7 shows the comparison of present estimates of qc with some attenuation studies of india and worldwide, which in turn shows a similar trend for delhi capital area as other tectonic regions. this shows that the attenuation characteristics of seismic waves in the delhi region are similar to the seismically active regions of the world. in figure 7, if we compare the delhi qc with indian regions, it is clear that qc values lie close to koyna region at lower frequencies and match with qc values of kachchh region at higher frequencies. this may lead to the conclusion that crust of delhi and surrounding areas is less attenuative as compared to kachchh and koyna regions. using the aftershocks of 2001 bhuj earthquake and multiple lapse time window analysis, hoshiba et al. [1991], fehler et al. [1992] and ugalde et al. [2006] have shown that intrinsic absorption is predominant over scattering for all frequencies except for 1-2 hz in kachchh region. similarly for delhi and surrounding region we found that intrinsic absorption is dominating over scatsharma et al. 8 figure 7. comparison of present estimates of qc with some attenuation studies in india and worldwide. 9 tering. figure 8 shows the comparison of the ratio qb/qa estimated here at different frequencies with those of other tectonic regions worldwide. we note that qb/qa ≥ 1 obtained in the present study for the frequencies considered here is comparable with other regions of the world. mandal [2006] estimated the qb vs. qa relation for the kachchh rift zone using the sp converted phases on the accelerograms. he estimated that the ratio qb/qa lies in between 0.41 to 2.99 in kachchh region. also padhy [2009] estimated qb/qa ≥ 1 for bhuj region. to interpret the results, we compared our results with the laboratory measurements of qb and qa. vassiliou et al. [1982] have given general observations of qb and qa relations in sedimentary rocks. qb = qa for dry rocks, qb ≥qa for partially saturated rocks and qb ≤qa for fully saturated rocks. in our case, we obtained qb > qa, which shows that the region is comprised of partially saturated rocks or crustal pore fluids [de lorenzo et al. 2013]. according to figure 8, we analyze that delhi and surrounding area are comprised of partially saturated sediments. also, it is seen in figure 8 that at lower frequencies qb/qa valves are lower than bhuj region, but at frequencies between 5 and 6 hz, qb/qa are nearly equal to that of bhuj region of india and after that there is a decrease in qb/qa ratio. the bump at frequency 5-6 hz in this figure may correspond to the sediments present in the subsurface of the delhi region. it is known that if the qc values are lower than 200 then it depicts the seismically active region [aki and chouet 1975]. our results for delhi region show low q (qc is 158), which corresponds to high attenuation and is comparable with other seismically active regions of india and world. the area has a considerable thick layer of partially saturated sediments demonstrated by qb/qa > 1, due to which most part of the energy gets dissipated in the medium. 5. conclusions the present study is an attempt to understand the attenuation properties of delhi, india and surrounding region, india. for this purpose quality factors qa, qb, qc, qi and qs are estimated. the analysis shows their dependence on the frequency in the range from 1.5 hz to 9 hz in the region. power law relationships for the region along with the standard deviation are obtained as: qa = (52±4) f (1.03±.07), qb = (98±7) f (1.07±.09) and qc = (158±9) f (0.97±.08). the attenuation characteristics of coda waves in the delhi region are close to other similar and tectonically active regions of the world. the estimates of qc are found to be higher than qb in the studied region. this observation shows that the effects of intrinsic and scattering attenuation combine in such a manner that qc is more than qb. the qc is separated in terms of scattering and intrinsic attenuation parameter qi and qs. the qc estimates lie in between the estimates of qi and qs but are closer to qi at all frequencies. this is in agreement with the theoretical as well as laboratory observations/measurements. a comparison between qi and qs shows that intrinsic absorption is predominant over scattering in delhi and surrounding region. qb/qa ≥ 1 obtained for the frequency range of 1.5 hz to 9 hz shows that the area of the present study is mainly comprised of partially saturated sediments. the results of present study indicate high attenuation which also corroborates well with the regional geology of delhi and surrounding areas. acknowledgements. authors are thankful to the secretary, ministry of earth sciences, government of india for support and encouragement to do this study. authors are grateful to director general of meteorology, indian meteorological department (imd), new delhi for providing earthquake waveform data. the authors also acknowledge continued guidance and support of head, center for seismology, ministry of earth sciences. references aki, k. 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(1985). multiple scattering and energy transfer of seismic waves: separation of scattering effect from intrinsic attenuation, i, theoretical model, geophysical journal of the royal astronomical society, 82, 57-80. yoshimoto, k., h. sato and m. ohtake (1993). frequency dependent attenuation of p and s waves in the kanto area, japan, based on the coda normalization method, geophys. j. int., 114, 165-174. zeng, y., f. su and k. aki (1991). scattered wave energy propagation in a random isotropic scattering medium, i, theory, j. geophys. res., 96, 607-619. corresponding author: babita sharma, center for seismology, ministry of earth sciences, new delhi, india; email: babita_s@rediffmail.com. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. attenuation for delhi, india << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ag_57.04.14_spada_finalonline_layout 6 annals of geophysics, 57, 4, 2014, s0432; doi:10.4401/ag-6548 s0432 anomalous secular sea-level acceleration in the baltic sea caused by isostatic adjustment giorgio spada1,*, marco olivieri2, gaia galassi1 1 università di urbino, dipartimento di scienze di base e fondamenti (disbef), urbino, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di bologna, italy abstract observations from the global array of tide gauges show that global sealevel has been rising at an average rate of 1.5-2 mm/yr during the last ~150 years [douglas 1991, spada and galassi 2012]. although a global sea-level acceleration was initially ruled out [douglas 1992], subsequent studies [douglas 1997, church and white 2006, jevrejeva et al. 2008, church and white 2011] have coherently proposed values of ~1 mm/year/century [olivieri and spada 2013]. more complex non-linear trends and abrupt sea-level variations have now also been recognized. globally, these could manifest a regime shift between the late holocene and the current rhythms of sea-level rise [gehrels and woodworth 2013], while locally they result from ocean circulation anomalies, steric effects and wind stress [bromirski et al. 2011, merrifield 2011]. although isostatic readjustment affects the local rates of secular sea-level change [milne and mitrovica 1998, peltier 2004], a possible impact on regional acceleration has been so far discounted [douglas 1992, jevrejeva et al. 2008, woodworth et al. 2009] since the process evolves on a millennium time scale [turcotte and schubert 2002]. here we report a previously unnoticed anomaly in the long-term sea-level acceleration of the baltic sea tide gauge records, and we explain it by the classical post-glacial rebound theory and numerical modeling of glacial isostasy. contrary to previous assumptions, our findings demonstrate that isostatic compensation plays a role in the regional secular sea-level acceleration. while studying the regional patterns of the global sea-level acceleration (gsla) [olivieri and spada 2013], we noted that accelerations in secular tide gauge time series from the baltic sea tend to exceed, albeit slightly, those from records of comparable length and quality from elsewhere. tide gauges from this region are generally not exploited in gsla studies [church and white 2006, woodworth et al. 2009] because these are believed to be too strongly influenced by local circulation [jevrejeva et al. 2008] and possibly by gia. to confirm the existence of an anomalous acceleration, we have now compared the sea-level trends obtained from 60 validated baltic sea records (figure 1a, bs) with those shown by the global set of tide gauge time-series previously employed by douglas [1991, 1997] and obeying rigorous quality constraints in terms of record length and completeness. contrary to the former, the latter are not from regions that were deeply covered by ice at the last glacial maximum (~21,000 years ago) where isostatic disequilibrium is still causing crustal uplift, and consequently a relative sea-level fall of several millimeters per year [steffen and wu 2011]. we have augmented the global set with ten records of comparable length, recently shown [spada and galassi 2012] to minimize the contamination from glacial isostatic adjustment (gia) and the sensitivity to the model adopted to perform the gia correction. the resulting global set consists of 32 records (figure 1b, gl); it includes sites largely insensitive to gia and thus more suitable to constrain the long term sea-level changes driven by current climate variations. following douglas [1992], sea-level accelerations in figure 1 have been computed as twice the quadratic term of a second-degree best-fitting polynomial. the distribution of the accelerations obtained for sets bs and gl are shown by the histograms of figure 2a (here, we have found convenient to express accelerations in units of mmyc = mm/yr/century). by a student t-test with welch correction [welch 1947] we have verified that the weighed averages of the two distributions, shown figure 2b, are significantly different at the 95% confidence level. this supports the hypothesis that the sea-level accelerations for the bs tide gauges are, on the average, larger than those belonging to the gl set. according to our computations, the anomaly (i.e. the difference in the average sea-level accelerations for the two sets), can be quantified as na = (0.2 ± 0.2) mmyc. the anomaly is small, but it is not negligible compared to article history received april 17, 2014; accepted june 11, 2014. subject classification: sea-level change, glacial isostatic adjustment, rheology. gsla estimates so far obtained in the literature, which are close to 1 mmyc [olivieri and spada 2013]. the positive sign for na can be explained, in a straightforward way, invoking the physics of postglacial rebound [turcotte and schubert 2002]. in formerly glaciated regions like the baltic sea, relative sea-level has been gradually falling since the end of deglaciation ~8,000 years ago, as a direct consequence of the post-glacial uplift of the earth’s crust [steffen and wu 2011]. according to the classical post-glacial rebound theory for a newtonian viscous earth [turcotte and schubert 2002], after unloading, the free surface relaxes toward isostatic equilibrium following an exponential law. hence the rate of relative sea-level fall was larger immediately after the end of ice melting and it is now progressively increasing toward less and less negative values. at the bs tide gauges in figure 1a, this today implies an excess positive sea-level acceleration compared to distant sites, which are less (or not at all) influenced by the post-glacial rebound process; this explains why the observed anomaly na is positive. by more quantitative arguments, it is possible to predict the order of magnitude of the anomalous acceleration. for a simplified half-space model, the postglacial displacement u(t) of the earth’s free surface decays with time exponentially [turcotte and schubert 2002]. hence, the rate of vertical uplift v(t) scales with v(t) = −u(t)/xr , where t is time and xr is the maxwell relaxation time of the earth’s mantle [turcotte and schubert 2002]. since vertical uplift dominates the relative sea-level variation s(t) in formerly glaciated regions [steffen and wu 2011], neglecting gravitational effects spada et al. 2 figure 1. location of tide gauge instruments. (a) location of baltic sea (bs) tide gauges (number of records: 60, minimum record length: 100 years, average completeness: 92%). in (b) the global (gl) set is considered (32, 56 years, 58%). shades of blue in (b) depict the thickness of the ice sheets at the last glacial maximum according to the gia model ice-5g(vm2) [peltier 2004]. colors of squares and diamonds denote the local sea-level acceleration (1 mmyc = 1 millimeter per year per century). all time series and tide gauge station information are extracted from the psmsl database of annual rlr (revised local reference) records [woodworth and player 2003]. figure 2. distribution of observed sea-level accelerations. (a) sea-level accelerations observed for the baltic sea (bs, blue) and the global (gs, red) sets of tide gauges. (b) average sea-level accelerations for the two sets and their standard deviations. for bs and gl, the accelerations have average values of (0.8 ± 0.1) and (0.6 ± 0.1) mmyc, respectively. their difference defines the anomalous sea-level acceleration na = (0.2 ± 0.2) mmyc. 3 [peltier 2004] we can write s(t) = −u(t), which taking the second derivative gives a(t) = v(t)/xr , where a(t) is the local sea-level acceleration. we note that the rate of present day sea-level change and its acceleration are negatively correlated, namely r(t) = −a(t)xr. the relaxation time xr is obtained analytically in terms of the bulk properties of the earth and the spatial wavelength of the ice load [turcotte and schubert 2002]. assuming a characteristic uplift rate v ~10 mm/yr, representative of the baltic region [steffen and wu 2011], for xr= 4,400 years [turcotte and schubert 2002] (suitable for the size of fennoscandia) and a bulk mantle viscosity [haskell 1936] of 1021 pa · s, we obtain a ~0.2 mmyc at present time, consistent with the amplitude of the observed anomaly na in figure 2b. since a(t) scales with v(t), the anomalous acceleration is doomed to disappear as soon as the surface of the earth will reach, in a few millennia, a full isostatic compensation (v(t) = 0). to substantiate the order-of-magnitude estimates above, we have employed a realistic gia model to compute and visualize the “fingerprints” [milne and mitrovica 1998, mitrovica et al. 2001] describing patterns of present-day sea-level rates and acceleration associated with gia. here we have implemented model ice5g(vm2) [peltier 2004] for the melting history of the late-pleistocene ice sheets in the open source code selen [spada and stocchi 2007, spada et al. 2012]. our computations account for the horizontal migration of shorelines and for the impact of earth rotation variations on sea-level change, and assume a three-layer viscoelastic mantle with maxwell rheology with viscosities obtained by volume-averaging profile vm2 [peltier 2004]. the two fingerprints are shown in figures 3a and 3b, respectively; the second is obtained from the first by numerical time differentiation. we have verified that their global shape would not change perturbing the viscosity profile of the mantle within reasonable bounds, nor adopting alternative gia models, characterized by different assumptions about the earth’s viscosity profile and including a distinct time chronology for the melting of the late-pleistocene ice sheets [lambeck and johnston 1989, lambeck et al. 1989]. the gia fingerprint for the rate of sea-level change in figure 3a clearly reflects the direct effect of unloading, which dominates across the previously glaciated regions causing an enhanced sea-level fall. finer details of the map have been quantitatively explained in terms of “continental levering” (tilting of coastal regions due to ocean water loading) and “ocean syphoning” (migration of water from the far field of former ice sheets to fill the space left by the collapsing isostatic forebulges) [mitrovica and milne 2002]. this pattern is indeed well known since it is commonly employed to decontaminate the rates of sea-level variations observed at tide gauges from the effects of gia [douglas 1991, spada and galassi 2012]. however, the fingerprint of sea-level acceleration in figure 3b is shown here for the first time, since previous studies have assumed that gia has negligible effects on the gsla [douglas 1992, 1997, church and white 2006, jevrejeva et al. 2008, woodworth et al. secular sea-level acceleration and gia figure 3. gia sea-level fingerprints. gia fingerprints for the rate of sea-level rise (a) and its time-derivative (sea-level acceleration) (b), computed at present time for model ice-5g(vm2) [peltier 2004] and obtained by code selen [spada and stocchi 2007, spada et al. 2012]. in (a) the dots show the psmsl tide gauges, and the actual rates in polar regions far exceed the range of the color table. since relative sea-level is defined by the offset between the geoid height and the earth’s solid surface [milne and mitrovica 1998], these fingerprints are also defined across the continents. 2009, olivieri and spada 2013] and its importance has been overlooked. similarly to the correction for the rate of sea-level rise (figure 3a), that for the acceleration (3b) has a vanishing average across the oceans, and the mean value at the locations of the global set of psmsl tide gauges (see figure 3a) is effectively negligible (<10−2 mmyc), which justifies the assumptions in previous gsla studies. however, the regional impact of gia-induced sealevel acceleration is apparent, especially in areas which are still subject to significant isostatic disequilibrium like the baltic sea, the hudsons bay region, northern greenland and west antarctica. across the baltic sea, the gia acceleration amounts to ~0.1-0.2 mmyc, in agreement with the order of magnitude observed in this study and with the classical post-glacial rebound theory. the anomaly attains its largest amplitude (>0.3 mmyc) in west antarctica and in central canada. since century-long tide gauge observations from the hudsons bay region are not available, it is impossible to compare predictions for this region with instrumental data. the negative correlation between the two fingerprints in figures 3a and 3b is apparent and consistent with the extremely simplified post-glacial rebound theory discussed above. however, their ratio, which can be interpreted as the gia “relaxation time”, is not constant since the complex rheological layering of the gia model employed here implies a multi-exponential relaxation [peltier 2004, spada and stocchi 2007]. using directly the results from figure 3, for the baltic sea region we obtain xr ~5,000 years, in agreement with the classical haskell theory [haskell 1936]. in response to gia, tide gauge records located along the coasts of the baltic sea exhibit a small but significant long-term sea-level acceleration in excess to those in the far field of previously glaciated regions. the sign and the amplitude of the anomaly is consistent with the post-glacial rebound theory and with realistic numerical predictions of gia models routinely employed to decontaminate the tide gauges observations from the gia effects [peltier 2004]. model computations predict the existence of anomalies of similar amplitude in other regions of the globe where gia is still particularly vigorous at present, but no long-term instrumental observations are available to support their existence. we confirm that a gia correction for secular sea-level acceleration is not required in gsla assessments because its average value is vanishingly small at the locations of the psmsl tide gauges [douglas 1992]. nevertheless, here we have shown that gia is contributing significantly on a regional scale, and therefore it should be recognized as one of the processes responsible for local, long-term sea-level acceleration. acknowledgements. party supported by a research grant of dipartimento di scienze di base e fondamenti (disbef) of the university of urbino. all figures have been drawn using the gmt package [wessel and smith 1998]. florence colleoni is acknowledged for advice. the tide gauge data are obtained from the permanent service for mean sea level (psmsl) database (http://www.psmsl.org/). references bromirski, p.d., a.j. miller, r.e. flick and g. auad (2011). dynamical suppression of sea level rise along the pacific coast of north america: indications for imminent acceleration, j. geophys. res., 116, c07005. church, j.a., and n.j. white (2006). a 20th century acceleration in global sea-level, geophys. res. lett., 33, l01602. church, j.a., and n.j. white (2011). sea-level rise from the late 19th to the early 21st century, surv. geophys., 32 (4), 585-602. douglas, b. (1991). global sea sevel rise, j. geophys. res., 96, 6981-6992. douglas, b. (1992). global sea level acceleration, j. geophys. res., 97 (c8), 12699-12706. douglas, b. (1997). global sea-level rise: a redetermination, surv. geophys., 18, 279-292. gehrels, w.r., and p.l. woodworth (2013). when did modern rates of sea-level rise start?, global planet. change, 100, 263-277. haskell, n.a. (1936). the motion of a viscous fluid under a surface load, part ii, physics, 7, 56-61. jevrejeva, s., j.c. moore, a. grinsted and p.l. woodworth (2008). recent global sea level acceleration started over 200 years ago?, geophys. res. lett., 35, l08715. lambeck, k., and p. johnston (1989). the viscosity of the mantle: evidence from analyses of glacial rebound phenonena, in: i. jackson (ed.), the earth’s mantle: composition, structure, and evolution, cambridge univ. press, new york, 461-502. lambeck, k., c. smither and p. johnston (1989). sealevel change, glacial rebound and mantle viscosity for northern europe, geophys. j. int., 134, 102-144. merrifield, m.a. (2011). a shift in western tropical pacific sea level trends during the 1990s, j. clim., 24 (15), 4126-4138. milne, g., and j. mitrovica (1998). postglacial sea-level change on a rotating earth, geophys. j. int., 133, 1-10. mitrovica, j., m.e. tamisiea, j.l. davis and g. milne (2001). recent mass balance of polar ice sheets inferred from patterns of global sea-level change, nature, 409, 1026-1029. mitrovica, j.x., and g. milne (2002). on the origin of late holocene sea-level highstands within equatorial ocean basins, quat. sci. rev., 21, 2179-2190. olivieri, m., and g. spada (2013). intermittent sea-level spada et al. 4 5 acceleration, global planet. change. 109, 64-72. peltier, w. (2004). global glacial isostasy and the surface of the ice-age earth: the ice-5g(vm2) model and grace, annu. rev. earth pl. sc., 32, 111-149. spada, g., and p. stocchi (2007). selen: a fortran 90 program for solving the “sea level equation”, comput. and geosci., 33, 538-562. spada, g., and g. galassi (2012). new estimates of secular sea-level rise from tide gauge data and gia modeling, geophys. j. int., 191 (3), 1067-1094. spada, g., d. melini, g. galassi and f. colleoni (2012). modeling sea level changes and geodetic variations by glacial isostasy: the improved selen code; http://arxiv.org/abs/1212.5061. steffen, h., and p. wu (2011). glacial isostatic adjustment in fennoscandia a review of data and modeling, journal of geodynamics, 52, 169-204. turcotte, d.l., and g. schubert (2002). geodynamics, cambridge university press, cambridge. welch, b.l. (1947). the generalization of “student’s” problem when several different population variances are involved, biometrika, 34, 28-35. wessel, p., and w.h.f. smith (1998). new, improved version of generic mapping tools released, eos, 79, 579. woodworth, p.l., and r. player (2003). the permanent service for mean sea level: an update to the 21st century, j. coastal res., 19, 287-295. woodworth, p., et al. (2009). evidence for the accelerations of sea level on multi-decade and century timescales, int. j. climatol., 29, 777-789. *corresponding author: giorgio spada, università di urbino, dipartimento di scienze di base e fondamenti (disbef), urbino, italy; email: giorgio.spada@gmail.com. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. secular sea-level acceleration and gia << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice d'auria_final:layout 6 annals of geophysics, 56, 4, 2013, s0442; doi:10.4401/ag-6448 s0442 the recent seismicity of mt. vesuvius: inference on seismogenic processes luca d’auria*, antonietta m. esposito, domenico lo bascio, patrizia ricciolino, flora giudicepietro, marcello martini, teresa caputo, walter de cesare, massimo orazi, rosario peluso, giovanni scarpato, ciro buonocunto, marco capello, antonio caputo istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy abstract we have analyzed the recent seismicity of mt. vesuvius, with particular emphasis on the period 1999-2012. since 1972 continuous observations with electromagnetic seismometers allowed the compilation of a detailed earthquake catalogue for the station ovo. furthermore since 1999 another, more complete, catalogue for the station bke, closer to the crater, is available. the gutenberg-richter distribution of magnitudes shows a temporal decrease of the b-value since 1985, with current values close to 1.0. the temporal pattern of the strain release shows a non-stationary behavior with periods of increased release rates (as in 1989-1990, 1995-1996 and 1999-2000). the spatial distribution of the seismicity consists in two main seismogenic volumes, one with hypocenters clustered below the mt. vesuvius crater at depths mostly between 1-6 km, and another with hypocenters clustered within the gran cono volcanic edifice, with depths above the sea level. we compare the statistical properties of the seismicity occurring within these two volumes and their spatial and temporal patterns. moreover we analyze the statistical distribution of focal mechanisms for each volume. our results point to gravity-induced stresses as the source of the shallow seismicity and of a combined effect of crustal heterogeneities, regional stress and hydrothermal dynamics for the deeper seismicity. finally we discuss possible future developments of the seismic monitoring system in the light of the past and current seismicity. 1. introduction studying the seismicity of mt. vesuvius has an immediate application to the monitoring of this high risk volcano [orsi et al. 2003, de natale et al. 2006], but has also an intrinsic scientific interest in understanding the seismicity of a quiescent volcano. seismic observations at mt. vesuvius started in the second half of the 19th century on the behalf of luigi palmieri, director of the osservatorio vesuviano (from 1855 to 1903). after the last eruption of mt. vesuvius in 1944, the seismicity dropped to low rates (less than 50 events/year). in 1964 it was observed a marked increase in the background volcano-tectonic seismicity [giudicepietro et al. 2010]. currently, at the reference station ovo (figure 1), we observe occurrence rates usually higher than 100 events/year with peaks of more than 600 events/year (in 1996 and 1999). the permanent seismic network of mt. vesuvius (figure 1) consists of 8 short period 1c, 3 short-period 3c and 9 broadband stations [orazi et al. 2013]. this network is complemented by temporary stations and by a permanent seismic array. it records hundreds of seismic transients every year, both of natural and artificial origin. they are manually detected and classified on the basis of their type [iannaccone et al. 2001, esposito et al. 2013]. earthquakes are manually picked and, if there are enough seismic phases, they are also located. all the hypocentral parameters and their waveforms are stored in a relational database. 2. time series analysis currently two earthquake catalogues are available for mt. vesuvius. the first starts in 1972, ends in june 2012, and contains all the events detected by the ovo station (figure 1). this station operated almost continuously (> 99%) during the whole period. this catalogue consists in more than 10,500 events, with duration magnitudes (md) [vilardo et al. 1996] ranging from −0.4 to 3.6. in figure 2 we represent the gutenberg-richter (gr) distribution for this dataset. the experimental distribution has been analyzed using the sofware zmap [wiemer 2001] and the inferred b-value is 1.66 ± 0.03 while the magnitude completeness (mc) is 1.9 ± 0.1. in figure 3 we represent the temporal distribution of magnitudes, the event occurrence rate, cumulative article history received september 3, 2012; accepted march 1, 2013. subject classification: mt. vesuvius, volcano seismology, statistical seismology, volcano monitoring. special issue: vesuvius monitoring and knowledge benioff strain release [benioff 1951], the b-value and the mc. the most striking features are the 4 episodes (marked by the arrows in figure 3) of increased strain release rate, occurrence rate and event magnitude [bianco et al. 1999, capuano et al. 1999, de natale et al. 2001]. these episodes occurred in 1978-1980, 1989-1990, 1995-1996 and 1999-2000. during the last one, a m=3.6 event was recorded on october 9, 1999. this event, the strongest since the last mt. vesuvius eruption in 1944 [del pezzo et al. 2004, giudicepietro et al. 2010], and was widely felt in the mt. vesuvius area, reaching a macroseismic intensity of vi mcs at the epicenter [cubellis and marturano 2002]. the highest observed occurrence rate is about 700 events/year, in 1999. the temporal variation of the b-value and of mc has been computed on groups of 100 adjacent events, with an overlap of 25 events. it shows an overall decreasing trend over the whole period, in particular after 1985. this feature was already identified in previous studies [iannaccone et al. 2001, zollo et al. 2002b, de natale et al. 2004]. here we focus the attention on the period 19992012, when the b-value seems to be stable around a value of about 1.1. the temporal variation of mc shows a general decrease, starting from about 2.0, until 1995 and reaching currently values of about 1.3. this could be related to the transition from analog to digital recording systems [giudicepietro et al. 2010, orazi et al. 2013]. the second catalogue contains the events detected by the bke station (figure 1). this catalogue starts in 1999, ends in june 2012, and consists in about 10,000 events, with magnitudes ranging from −1.6 to 3.6. as for ovo, bke station operated almost continuously (> 99%) during this period. giudicepietro et al. [2010] pointed out by that the differences observed between the ovo and bke catalogues are related to the peculiar seismicity pattern of mt. vesuvius. since most of the earthquakes occur close to the crater axis, the smallest and shallowest earthquakes are recorded only by the station bke, which is about 1 km from the crater, while the station ovo is about 2.5 km away. d’auria et al. 2 figure 1. map of the vesuvius permanent seismic network. circles: short-period 1c stations; triangles: short-period 3c stations; diamonds, broadband stations. the vesuvius permanent array is indicated by a cross. stations bke and ovo are evidenced in red. structural features are taken from: vilardo et al. [1996], bianco et al. [1998], orsi et al. [2003]. figure 2. gutenberg-richter relationship for the ovo catalogue. the x-axis is the magnitude while the y-axis represents the cumulative number of events. triangles are the experimental data while the solid line is the theoretical relationship with a = 6.32 and b = 1.66. the line starts at mc = 1.9. 3 in figure 4 we show the gr distribution for the bke catalogue. the distribution seems to have at least two knees, suggesting that it results from the superposition of two different gr distributions. we have studied the observed distribution by comparing two alternative models. in the first (a) we hypothesize a simplified model: (1) where m is the magnitude and b* < b. this distribution is a simplified model of a statistically homogeneous earthquake population. before the magnitude completeness mc, the curve has a lower slope b *, because of the catalogue incompleteness. the actual b-value of the distribution is given by b. the definition of the parameter a* differs slightly from the commonly used parameter a of the gr distribution: it represents the intercept of the distribution at m = mc. in the second (b) we hypothesize that the observed data are modeled by a a generalization of the standard gr distribution. this distribution consists in the sum of two of the previous model distributions, hence possessing two distinct mc and b. using a non-linear probabilistic approach, we have fitted the data using these two alternative models. to compare the two models we have used the |2 to test the normality of residuals and the kolmogorov-smirnov test to check the goodness-of-fit. results (see table 1) show that both models provide a reliable fit even if, both tests, show that the double distribution (b) gives a better fit. this is also confirmed by the akaike information criterion (aic) [akaike 1974], which provides a helpful means for model selection. aic values for model b are lower compared to model a (see table 1). the difference in the aic values between the two model is 43.88, giving a negligible relative likelihood for model a compared to model b. this strongly supports model b, or in other words that the observed distribution results from the contribution of two distinct fault populations characterized by different statistical properties. in table 2 we report the best fit values for both models along with their confidence intervals computed using the approach of jackson and matsu’ura [1985]. in the model b, the first distribution (b1) has b = 1.17 ± 0.06 and mc = 0.0 ± 0.12 while the second (b2) has b = 1.45 ± 0.25 and mc = 1.91 ± 0.07. the higher uncertainty on the determination of the b-value of b2 is caused by the more limited number of data (earthquakes with higher magnitudes) which makes this parameter poorly constrained. in figure 4 we represent the two theoretical distributions b1 and b2 (dashed lines) and their sum (solid line) compared to the observed overall distribution. seismicity of mt. vesuvius figure 3. time series for the ovo catalogue. the first panel shows the event magnitudes while the second one the number of events for each year. the superimposed black histogram represents the number of events having m ≥ 2.0, which is the maximum value of mc in the bottom panel. the third represents the cumulative benioff strain release. arrows indicate episodes of increased release rate. the fourth panel shows the temporal variation of the b-value. vertical error bars indicate the 1 v uncertainty. the last panel shows the mc values with their 1 v error bars. figure 4. gutenberg-richter relationship for the bke catalogue. triangles are the experimental data. the dashed lines represent the two theoretical distributions while the solid line is their sum (see section 2 for details). the b-value of the first distribution is b = 1.17 ± 0.06 while mc = 0.0 ± 0.12. for the second distribution b = 1.45 ± 0.25 and mc = 1:91 _ 0:07. if if log n m m m m a b m m m m a b mc c c c 10 1 $ = ) ) )^ ^^h h h) in figure 5 we show the temporal distribution of the parameters for the bke catalogue. the event occurrence rate has a maximum in 1999 with about 2000 events/year. in the following years the rate dropped, reaching a minimum in 2004, with about 400 events/year. in the last 10 years the average occurrence rate at bke station is about 500 events/year. the strain release rate (figure 5) shows a marked increase during the 1999-2000 crisis. conversely, since 2002 it shows an almost stationary, low rate. as for the ovo catalogue, also the temporal variation of the b-value since 1999, does not show significant trends and is almost stationary with an average value of 0.7. the mc too, does not show relevant variations. 3. earthquake locations we have located the earthquake dataset using a probabilistic approach in a 3d velocity model [lomax et al. 2001]. the velocity model used in this work consists in a weighted average of three models [d’auria et al. 2008]. the first model comes from the active seismic experiment tomoves96 [lomax et al. 2001, zollo et al. 2002a], the second from a local earthquake tomography [scarpa et al. 2002] and the third from a regional 1d model [improta et al. 2000]. details about the merging procedure can be found in d’auria et al. [2008]. we have relocated events recorded from 1999 to 2000. among the whole traveltime dataset we were able to locate about 2,400 events having at least 5 phase pickings. all the events belong to the bke catalogue with about 43% of them belonging also to the ovo catalogue. their hypocenters are shown in figure 6. most of the epicenters are located within a radius of 3 km from the crater axis. depths are distributed from about 8 km up to an elevation of 1 km above the sea level (a.s.l.). it is interesting to note that the maximum event depth coincides with the inferred top of the mt. vesuvius magma chamber [auger et al. 2001]. in the two cross-section (figure 6) it can be seen that earthquakes are grouped in at least two clusters. the first spans mainly the interval between 1 and 5 km depth, while the second is located above the sea level. it should also be noted that earthquakes occurring within the shallow cluster have, generally, lower magnitudes (m ≤ 2.8) compared to the deeper events. hypocentral depths of the deeper cluster are consistent with values determined in previous works regarding the interval 1986-1999. in particular vilardo et al. [1996], using a dataset of 172 events, showed how during the period 1986-1994 the moment density distribution had two maxima, one at about 4 km depth and another just below the summit of the volcano [see vilardo et al. 1996, fig. 4c]. capuano et al. [1999] located about 200 events recorded in the period 1989-1995. they found a concentration of hypocentral depths between 2 and 6 km. bianco et al. [1999] analyzed the seisd’auria et al. 4 |2 p-value ks aic a 0.2289 0.351 -65.36 b 0.9477 0.188 -109.24 b b* mc a * a 1.504 ± 0.085 0.51 ± 0.10 1.54 ± 0.06 3.02 ± 0.14 b1 1.17 ± 0.06 0.00 ± 0.21 0.00 ± 0.12 3.72 ± 0.15 b2 1.45 ± 0.25 0.51 ± 0.37 1.91 ± 0.07 2.34 ± 0.12 table 2. b-value and mc for the two models. see section 2 for details. table 1. statistics on the two models tested (a and b). the first colums shows the p-value resulting from a |2 test. in both cases it shows that the normality of residuals can be accepted for a statistical significance of at least 5%. the second column shows the ratio between the critical values at a significance level of 5% and the retrieved kolmogorov-smirnov statistics. since both values are lower than 1 the goodness-of-fit is achieved by both models. the third column shows the aic value for the two models. figure 5. time series for the bke catalogue. the meaning of the symbols is the same as for figure 3. the superimposed black histogram represents the number of events having m ≥ 0.5, which is the maximum value of mc in the bottom panel. the arrow indicates the step in the cumulative strain release related to the october 9, 1999, (m=3.6) event. 5 micity during two seismic crises occurred at mt. vesuvius between august-october 1995 and march-may 1996. they located about 600 events showing that, their hypocentral depths, are distributed mostly between 1 and 4 km. finally, zollo et al. [2002b] relocated a dataset of 400 events recorded in the period 1989-1998, showing a concentration of hypocenters between 1 and 5 km depth. even if hypocenters occurring within the shallow cluster were already identified by vilardo et al. [1996] and bianco et al. [1998], most of the previous papers, dealing with hypocenter location at mt. vesuvius, focused mainly on the deep seismicity linked to seismic crises. 4. strain release in order to better delineate these two volumes we plot in figure 7 the spatial distribution of the strain release. panel a shows the strain release integrated along the vertical direction. it can be seen that most of the strain release occurs within an area of about 1 km2. in the panel b the strain is integrated along the ew direction, while in panel c along the ns direction. the cross sections clearly delineate the two seismogenic volumes. the first has a peak at a depth of about 2 km and extends between 1 and 7 km. the second has a peak between 0.5 and 1 km a.s.l. the isolated spike indicated by the arrows is relative seismicity of mt. vesuvius figure 6. earthquake hypocenters for the 1999-2012 interval. the top left panel shows earthquake epicenters for events whose location uncertainty (erh and erz) is less than 1 km. the size of simbols is proportional to the event magnitude (see the legend on the lower right). the plot on the left shows hypocenters projected on a ns cross-section, while the lower plot shows an ew cross-section. figure 7. cumulative strain release. in a we represent the strain release integrated along the vertical direction. in b the integration is performed along the ew direction while in c along the ns direction. the arrows point to a strain release spike related to the october 9, 1999, (m=3.6) event. the grids used in the plots are equally spaced with an interval of 500 m. to the october 9, 1999, (m=3.6) event, located at about 5.6 km depth. we have already analyzed, in figures 3 and 5, the temporal variation of the strain release rate. in figure 8 we represent the strain release as a function of both time and depth. in panel b it can be seen that, until 2002, the strain was released mostly by the deeper volume while, since then, most of the strain has been released within the shallow volume. panel c shows the strain release as a function of depth. this graphic shows unambiguously that a zone of very reduced strain release occurs between the shallow and the deep seismogenic volumes. the separation between the two volumes is located at a depth of about 1.0 km. moreover, the curve of panel c, has a clear asymmetry in the part below 1 km depth. it increases sharply and then slowly decays with depth. 5. b-value analysis to characterize the seismogenic processes occurring within the two volumes we have studied the spatial distribution of the b-value. in figure 9 we plot the gr distribution for events located since 1999, separately for the shallow cluster (triangles) and for the deep one (circles). the b-value for the shallow cluster is 1.01 ± 0.05, while for the deep is 1.26 ± 0.08. the respective mc are 0.9 ± 0.15 and 1.7 ± 0.1. the gr distribution for the bke catalogue (figure 4), discussed in section 2 can now be justified in terms of a superposition of two distributions related to two different seismogenic volumes. the shallow volume seems to correspond to the first d’auria et al. 6 figure 8. time-depth distribution of the strain release. in a we plot events in a time-depth graphic. symbol sizes are proportional to magnitude as for figure 6. in b we represent the strain release as a function of time and depth. in c we show the strain release as a function of the depth. the arrows in b and c point to the strain release spike related to the october 9, 1999, (m=3.6) event. figure 9. gutenberg-richter relationship for located earthquakes. triangles are experimental data for events located a.s.l. while circles are for the deeper events. the theoretical relationship for triangles has parameters b = 1.01 and a = 3.6 with mc = 0.9. for the circles the parameters are b = 1.26, a = 4.55 and mc = 1.7. figure 10. spatial distribution of the b-value. the two panels on the top represent the b-value distribution along an ew (left) and a ns (right) cross-section. the two cross-sections intersect on the mt. vesuvius crater. the bottom panels represent the uncertainty on the retrieved b-values. 7 distribution (having b = 1.17 ± 0.06 and mc = 0.0 ± 0.12) while the deep volume to the second distribution (with b = 1.45 ± 0.25 and mc = 1.91 ± 0.07). a more clear understanding of the spatial distribution of the b-value can be seen in figure 10. this figure shows the b-value along two orthogonal cross sections (ew and ns) intersecting on the mt. vesuvius crater. for each point we have computed the b-value and its uncertainty, using all the events located within a radius of 2 km from the point itself. points are regularly spaced on a 3d grid with a spacing of 0.5 km. only volumes containing at least 30 events have been used. the uncertainty in the b-value estimate increases for depths higher than 3 km because of the decreasing earthquake spatial density (figure 10). both the ew and the ns cross sections show similar patterns. above the sea level the b-value varies between 0.7 and 1. it increases with depth, showing a maximum value of about 1.5, at 1 km depth. below this depth, it decreases steadily, reaching a value of about 0.6 at 6 km depth. it should be noted that the b-value estimates are poorly reliable below 4 km (figure 10). the spatial distribution of the b-value seems to indicate a difference in the seismogenic processes between the two volumes and a variation within the deep volume itself. in particular this analysis confirms that the shallow volume is characterized, on average, by a lower b-value compared to the deeper volume. furthermore the correct determination of the spatial distribution of the b-value is fundamental in seismic risk studies [convertito and zollo 2011]. 6. focal mechanisms we have computed focal mechanisms for events having at least 8 p-wave polarities. this subset consists in 197 events, with magnitudes ranging from 1.8 to 3.6. focal mechanisms have been computed using the fpfit software [reasenberg and oppenheimer 1985]. in figure 11 we show the mechanisms projected on the horizontal plane and along two ns and ew cross-sections. they do not seem to have a clear preferential orientation, neither a predominant fault type. this can be seen more clearly in figures 12 and 13. the first figure shows the fault type classified on the basis of the rake angle (aki and richards [2002] convention). the panel a is for events located a.s.l. it shows a dominance of strike and reverse mechanisms. in panel b the histogram refers to events located below the sea level. in this case the mechanisms are almost equally distributed among all the fault types. in figure 13 we represent also the azimutal distribution of fault strike angles, separately for shallow and deep events. shallow events show a marked predominance of wnw direction, while for deep events seismicity of mt. vesuvius figure 11. focal mechanisms for 195 events. beach-balls are projected on the horizontal plane (top left ), on a ns cross-section (top right) and on a ew cross-section (bottom). the preferential direction is ene. both directions are compatible with the faulting pattern observed at regional and local scales (see structural features delineated in figure 1) [vilardo et al. 1996, bianco et al. 1998, orsi et al. 2003]. to better understand the relationship between the faulting pattern and the stress field we plot in figure 14 the tension, the neutral and the pressure axes of the focal mechanisms for shallow and deep events. in both cases the pattern is quite scattered. for deep events there seems to be a concentration of p axes in the se sector of the graphic. this direction is compatible with the regional stress field retrieved from gps measurements and borehole breakout data [barba et al. 2010, pierdominici et al. 2011]. 7. discussion the results presented in the previous sections highlight at least 3 important features of the recent seismicity at mt. vesuvius. the first is the presence of two main seismogenic volumes: one located between 1 and 7 km, and another between 0.5 and 1 km a.s.l. the strain release of the deep volume occurs mostly between 1 and 3 km (figure 8). the second is that the deep volume is characterized, on average, by a higher b-value compared to the shallow one (1.26 versus 1.01) (figure 9). however it should be noticed that the b-value varies with depth within this volume, ranging from 1.5 at 1 km depth to 0.6 at 6 km depth. this trend is similar to the strain release pattern (figure 8) which, for the deep volume, starts at 1 km depth, reaches a maximum at 2 km depth and then decreases, becoming almost null at 7 km depth. the similar patterns suggest the existence of a relationship between these two parameters and their link with the seismogenic processes. a similar pattern was also evidenced by del pezzo et al. [2004], which showed how the stress drop of shallow earthquakes (< 2.6 km) is lower (around 1 bar), while for deeper earthquakes (> 4 km) is higher (10 bar), reaching a maximum value of about 100 bar for the october 9, 1999, (m=3.6) event. the third observation is that the focal mechanism distribution is different within the two volumes. in the shallow volume it does not show a dominant orientation of p and t axes. this, together with the almost stationary strain release within this volume (see section 4), suggests that the contribution of gravitational stresses may be an important factor for the seismogenesis within this volume. actually, the concentration of shear stresses along the slopes of a relief, has been assessed using both analytical and numerical approaches [martel and muller 2000]. furthermore, the the low values of the lithostatic load, existing at shallow depth, favours the occurence of shear failure [russo et al. 1997]. in this case the orientation of p and t axes would d’auria et al. 8 figure 13. rose diagram showing the distribution of strike values of the focal mechanisms. the graphic on the top is for events located a.s.l. while the one on the bottom is for the other events. each sector is 15° wide and the outer edge of the graphics corresponds to a value of 10 events for the top and 30 for the bottom. figure 12. distribution of rake angles for the mechanisms of figure 11. in a we represents rake angles of events located a.s.l. while in b the same for the other events. 9 be strongly dependent on the topography, hence justifying the absence of a clear dominant orientation. to support this hypothesis we also notice that the measurement of ground deformation at mt. vesuvius have shown a long term trend of downward and radially outward motion [lanari et al. 2002]. this pattern has been interpreted as the result of a slow spreading of the volcanic edifice under the effect of the gravitational loading [borgia et al. 2005]. the clayey ductile layer identified by borgia et al. [2005] and responsible of the spreading roughly concides with the level of reduced strain release, separating the two seismogenic volumes. this suggests that it could be also related to observed volume of reduced strain release existing between 0.5 km a.s.l. and 1 km depth (figure 8). concerning the deep volume, various hypothesis about the seismogenic mechanism have been proposed. bianco et al. [1998] suggested a dominant role of the regional stress field, while de natale et al. [1998] proposed that a local crustal heterogeneity (in particular a central high rigidity body) plays a significant role in the seismogenesis. the analysis of focal mechanisms, indicates that both mechanisms could act simultaneously. the p axes distribution (figure 14) shows a prevalence in the se sector, even if with a significant scattering. this seems to suggest that, focal mechanism at mt. vesuvius, result from the superposition of a regional stress field and local perturbations induced by the complexity of the volcanic structures. however, these two factors are not able to justify the observed temporal patterns in the strain release (figures 3 and 5) and in the time-depth-strain relationship (figure 8). on other active volcanoes, like campi flegrei (see d’auria et al. [2011], and references therein), a causal link has been established between episodes of magmatic fluid injection in the hydrothermal system, and an increase in the seismicity. a connection between the mt. vesuvius hydrothermal system and seismogenesis has been already recognized by madonia et al. [2008] and caliro et al. [2011]. in particular, the latter work, points to a deep magmatic origin of fluids perturbing the shallow hydrothermal system, which feeds the mt. vesuvius crater fumaroles. they also recognized significant variations accompanying the 1999-2000 seismic crisis. in their model the hydrothermal systems host a high-temperature (t = 450°) reservoir of nacl rich brines, at depth approximately between 2.5 and 5 km. the position of the top of this reservoir is consistent with the area of highest strain release. the higher bvalues found at these depths (figure 10) are consistent with the presence of high pore pressure. in other volcanic system high b-values have been interpreted as a possible evidence of proximity to a magma chamber [power et al. 1998, wiemer et al. 1998]. however, at mt. vesuvius, there is no evidence of shallow magma chambers [zollo et al. 2002a]: a possible magma layer has been seismicity of mt. vesuvius figure 14. stereographic plots of tension (t), neutral (n) and pressure (p) axes for the focal mechanisms of figure 11. the graphics on the top represent data for events located a.s.l. while those on the bottom are for the other events. recognized only at about 8 km depth [auger et al. 2001]. from a seismological point of view, indirect evidences of the role of fluids is given by the analysis of short-term statistical properties of the seismicity. at a global scale the seismicity of mt. vesuvius has statistical features similar to tectonic earthquakes [bottiglieri et al. 2009]. however, godano et al. [1997] and zollo et al. [2002b], using different approaches, have shown that at a smaller time scale the seismicity of mt. vesuvius is clustered, showing a dominant tendency of earthquakes to occurr within seismic swarms. this is a common feature of the seismicity observed in volcanic areas [zollo et al. 2002b] and in general the occurrence of seismic swarms can often be explained in terms of pore pressure driven processes [yamashita 1999]. another indirect evidence of the role of fluids was given by pandolfi et al. [2006] which, using coda-wave interferometry, detected velocity variations preceeding and following the 1999-2000 crisis. they interpreted that variation as the effect of a fluid pressure increase before the crisis and its subsequent drop. all these evidences point to a tight link between the dynamics of the hydrothermal system and the spatio-temporal pattern of the seismicity. concerning the temporal decrease in the b-value between 1985 and 1999, zollo et al. [2002b] proposed that it could be related to a decrease in the pore pressure. they observed that the b-value decrease has been accompanied by a progressive decrease in the temperature of mt. vesuvius fumaroles. the increase of the seismicity, observed in 19781980, 1989-1990, 1995-1996 and 1999-2000, could be explained as the results of the injection of fluid batches, of magmatic origin, at the bottom of the hydrothermal system. the upward migration of these fluids can be the cause of the pore pressure increase and consequently of the seismicity. a similar mechanism has been observed in the recent dynamics of campi flegrei caldera [d’auria et al. 2011]. the vertical variation of the thermodynamical properties of the hydrothermal system could also explain the observed variations of the strain release, the stress drop and the b-value with depth. it may be justified by a decrease of the pore pressure with depth in parallel with the increase of the lithostatic load. 8. conclusions the study of the mt. vesuvius seismicity, since 1999, allowed to identify two separate seismogenic volumes. the first, shallower, is located above the sea level, within the mt. vesuvius volcanic edifice. this seismicity is characterized by low magnitudes and by a low bvalue (about 1.01). the temporal distribution of the strain release is quite stationary (figure 8) compared to the events located within the deeper volume which seem to occurr preferentially during seismic crises [vilardo et al. 1996, bianco et al. 1999, capuano et al. 1999, de natale et al. 2004]. we postulate that the seismicity within this volume is mostly driven by shear stressed induced by the gravity. the second, deeper, seismogenic volume, extends from about 1 up to 7 km depth. the characteristics of its seismicity point to heterogeneous stress field, resulting from the superposition of a regional field and (strong) local perturbations induced by density and rigidity variations at kilometric scales, occurring below the volcanic edifice. this stress field alone is not able, usually, to be a causative source of seismicity. with the support of direct (geochemical) and indirect (seismological) evidences, we postulate that, fluids within the hydrothermal system of mt. vesuvius, play a fundamental role in modulating the occurrence of earthquakes within this volume. the temporal modulation of the seismicity, or in other words, the occurrence of seismic crises at mt. vesuvius, is likely to be caused by the injection of fluids, from the magma chamber (at about 8 km depth) [auger et al. 2001], within the mt. vesuvius hydrothermal system. after the 1999-2000 crisis, until 2012, the seismicity of mt. vesuvius has been characterized by a stationary pattern, consisting mainly in the occurrence of low-magnitude earthquakes within the shallow volume. accurate 3d numerical modeling of the stress field together with a thermo-fluid-dynamical modeling of the hydrothermal system, could shed light on some still unresolved questions. the most important are the nature of the reduced strain release volume existing between 0.5 km a.s.l. and 1 km depth and the origin of the vertical variations in the seismicity patter observed below 1 km depth. our results also give some hints concerning future improvements of the seismic monitoring system. the difference between the mc for the whole bke catalogue and that for the located events indicates the necessity of further lowering the capability to locate small (m < 0) earthquakes. efforts in this direction have been already carried out, putting more stations close the epicentral area (gran cono), achieving a significative drop in the mc for events located at shallow depth [orazi et al. 2013]. a further improvement can be achieved by enhancing the signal/noise ratio. currently, a reliable approach to this problem, is to replace surface stations with borehole installations. another possibility is to complement, standard location procedures, with seismic array analysis. lastly, since the temporal and spatial variations of d’auria et al. 10 11 the b-value have shown to be important parameters: they should be included in the seismic monitoring system as routine analyses. acknowledgements. we are grateful to bruno massa for the useful discussion about the determination of the stress field and ada de matteo for revising the focal mechanisms dataset. we are also grateful to two anonymous reviewers, whose valuable suggestions and constructive criticisms, improved the manuscript. references akaike, h. 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(2001). a software package to analyze seismicity: zmap, seismol. res. lett., 72, 373-382. yamashita, t. (1999). pore creation due to fault slip in a fluid-permeated fault zone and its effect on seismicity: generation mechanism of earthquake swarm, pure appl. geophys., 155 (2-4), 625-647; doi: 10.1007/ s000240050280. zollo, a., l. d’auria, r. de matteis, a. herrrero, j. virieux and p. gasparini (2002a). bayesian estimation of 2-d p-velocity models from active seismic arrival time data: imaging of the shallow structure of mt. vesuvius (southern italy), geophys. j. int., 151, 566-582. zollo, a., w. marzocchi, p. capuano, a. lomax and g. iannaccone (2002b). space and time behavious of seismic activity and mt. vesuvius volcano, southern italy, bull. seismol. soc. am., 92 (2), 625-640. *corresponding author: luca d’auria, istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy; email: luca.dauria@ov.ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. d’auria et al. 12 << /ascii85encodepages false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 preface new analytical techniques for understanding volcanic systems, from magma generation to the emplacement of volcanic products the development of new biand tri-dimensional (2d, 3d) analytical techniques of volcanic materials represents a new frontier in the world of modern volcanology. these advances not only offer important and reliable information on, e.g., magma vesiculation, degassing, and crystallization processes but can also shade light on the transport and sedimentation processes in volcanic flows (e.g. pyroclastic density currents, volcanic clouds). hence, these novel approaches have the potential to contribute both to the understanding eruption dynamics towards volcanic hazard quantification. in this special issue, vol. 61, supplement to no. 6, 2018, of annals of geophysics, we present 4 contributions focused on the application of both 2d and 3d analytical techniques, addressing the following main topics: a x-ray microtomography morphological and textural analyses on volcanic particles in order to study how particle shape is influenced by their internal structures; a new freeware tool, partical shape analyzer (partisan), for 2d morphometric analysis, to study and archive results of particle shape analyses, opening the way towards an inter-group effort for a standardized 2d description of particle shape; textural and chemical characterization of crustal xenolithics embedded in lavas by combining different analytical techniques (xrf, sem-eds, two-dimensional observation and 3d imaging by synchrotron x-ray microtomography) to provide information on the lithology of the volcano basement and the process occurring during magma ascent; a multi-technique approach (field emission-scanning elecrotn microscopy, electron microprobe analysis, x-ray power diffraction and 3d imaging by x-ray micro-tomography) for understanding the formation mechanism of accretionary lapilli. we thank and congratulate all of the contributors for their valuable contributions, which show the potential of new emerging technologies in the understanding of volcanic processes. daniela mele dipartimento di scienze della terra e geoambientali, università degli studi di bari “aldo moro”, bari, italy fabio dioguardi british geological survey, the lyell centre, edinburgh, united kingdom mattia de’ michieli vitturi istituto nazionale di geofisica e vulcanologia, sezione di pisa, pisa, italy annals of geophysics, 61, 6 sup, 2018; doi: 10.4401/ag-8226 l e p r i n c i p a l i i p o t e s i s u l l ' i s o s t a s i a c.i c . aquilina s i n d a l t e m p o di n e w t o n si e r a n o t a t o il c o m p o r t a m e n t o a n o m a l o d e l l a d e v i a z i o n e d e l l a v e r t i c a l e n e l l e v i c i n a n z e d i m a s s e s u p e r f i c i a l i d i n o t e v o l i d i m e n s i o n i . b o u g u e r a v e v a s e g n a l a t o il f e n o m e n o e m a s k e l y n e a v e v a r i p r e s e l e c o n s i d e r a z i o n i d e l p r e c e d e n t e . s i e r a c o n c l u s o s u l l a esigua d e n s i t à d e l l e c o r d i g l i e r e r i s p e t t o a q u e l l a m e d i a t e r r e s t r e . m a a p r a t t , a r c i d i a c o n o d i c i t y d e e p , p o i d i c a l c u t t a , si d e v e ( 18551 l a i p o t e s i d e l l a p r e s e n z a di u n a superfìcie di compensazione ad u n a c e r t a p r o f o n d i t à sotto il l i v e l l o m e d i o d e l m a r e . teoria di pratt. s e c o n d o q u e s t o a u t o r e , c h e a v e v a a r a g i o n e c r i t i c a t o i l p r o c e d i m e n t o s e g u i t o d a e v e r e s t n e l c o m p e n s a r e l a d i f f e r e n z a t r o v a t a f r a l e l a t i t u d i n i a s t r o m i c l i e e g e o d e t i c h e d e l l e s t a z i o n i d i k a n l i a p u r e k a l i a n a , d e l l a t r i a n g o l a z i o n e i n d i a n a , e r a n e c e s s a r i o t r o v a r e u n a spieg a z i o n e p e r q u e s t o scarto. n o n r i p o r t e r e m o q u i i l r a g i o n a m e n t o , l e c o n c l u s i o n i , l e d i s p u t e , cose t u t t e c o n s a c r a t e a l l a s t o r i a di q u e s t o camp o d e l l a scienza e ci l i m i t e r e m o a r i p o r t a r e i c a r a t t e r i g e n e r a l i d e l l a t e o r i a e n u n c i a t a da q u e s t o scienziato. essa p u ò essere espressa n e l l a s e m p l i c e f o r m a seguente. a d u n a p r o f o n d i t à i p o t e t i c a d a t a , e d a c o n s i d e r a r s i c o s t a n t e , si t r o v a u n a s u p e r f i c i e al di s o p r a d e l l a q u a l e l e masse, p e r t i n i t à di a r e a , ( c h e su d i essa i n s i s t o n o ) s o n o i n ogni p u n t o u g u a l i . è c o m e a s s e r i r e c h e , l u n g o l a v e r t i c a l e , l e m o n t a g n e h a n n o s u b i t o u n a d i l a t a z i o n e , gli a v v a l l a m e n t i o c e a n i c i , u n a c o n t r a z i o n e . è v e r i f i c a t a d u n q u e l a legge di d i p e n d e n z a d e l l a d e n s i t à d i ciascun b l o c c o e l e m e n t a r e , v e d i fig. 1 , d a l l a sua a l t e z z a h sul g e o i d e o d a l l a sua p r o f o n d i t à p sotto l a s u p e r ficie m a r i n a . il d i f e t t o d i d e n s i t à s e g n a l a t o n e i b l o c c h i t i p o 1 , 2, 3, o l'eccesso d i essa i n q u e l l i t i p o 4, 5, 6, 7, r i s p e t t o a q u e l l a m e d i a c h e si r i s c o n t r a i n q u e l l i t i p o 8, h a t e r m i n e s u l l a s u p e r f i c i e di c o m p e n s a z i o n e . ( * ) c o m u n i c a z i o n e p r e s e n t a t a a l c o n v e g n o d e l l ' a s s o c i a z i o n e c e o f i s i c a i t a l i a n a , t e n u t o a r o m a i l 1 0 1 1 g i u g n o 1 9 5 2 . 3 6 8 c . a q u i l i n a il v a l o r e h, p r o f o n d i t à sotto il geoide della s u p e r f i c i e di compensazione, è a r b i t r a r i o e va fissato, n e i calcoli e v e n t u a l i , in m a n i e r a da p e r m e t t e r e i l r a g g i u n g i m e n t o di r i s u l t a t i c h e giustifichino l e a n o m a l i e d a l l e quali e b b e o r i g i n e l a t e o r i a stessa. h a y f o r d , a m m e t t e n d o l'ipotesi di u n a compensazione p e r f e t t a , secondo l a t e o r i a di p r a t t , iniziò l a r i d u z i o n e d e l l e osservazioni di tutta l a r e t e g r a v i m e t r i c a degli s t a t i u n i t i . p e r f a c i l i t a r e l e o p e r a z i o n i di calcolo costruì d e l l e t a v o l e di r i d u z i o n e cbe f u r o n o poi r i p r e s e e, p u r m a n t e n e n d o l o schema p r i n c i p a l e di esse, i n t e g r a l m e n t e r i f a t t e dai nostri g. cassinis, p. d o r è e s. b a l l a r i n . geoide superfìcie di queste t a v o l e sono, i n v e r o , q u e l l e c h e h a n n o reso agevole l a grande m o l e del l a v o r o di r i d u z i o n e d e l l e d e t e r m i n a z i o n i g r a v i m e t r i c b e , o p e r a z i o n e , a l t r i m e n t i , p r a t i c a m e n t e i m p o s s i b i l e a p o r t a r s i a compim e n t o . nel calcolo di queste r i d u z i o n i v a n n o t e n u t e n e l m a s s i m o conto l e approssimazioni c h e possono essere r a g g i u n t e sia n e l l a scelta d e l l o schema di r i p a r t i z i o n e in blocchi e l e m e n t a r i , n e l l e r e g i o n i prossime e l o n t a n e , sia n e l l a r i c e r c a dei v a l o r i di h e di p e q u i n d i degli accorgimenti da adottarsi p e r r e n d e r s i esenti dagli e r r o r i commessi p e r l'impossibilità di conoscere detti v a l o r i n e l l e r e g i o n i t o p o g r a f i c a m e n t e m e n o note, o da r i d u r l i al m i n i m o possibile. osserviamo (-1) che nell'ipotesi di p r a t t , considerata sotto il s u o aspetto fisico, l'uguaglianza d e l l e masse dei blocchi e l e m e n t a r i p o r t a ad u n a disuguaglianza d e l l e pressioni insistenti sulla superficie d i le p r i n c i p a l i i p o t e s i s u l i , ' l s o s t a s i a 3 6 9 c o m p e n s a z i o n e , m e n t r e s a r e b b e p i ù logico a t t e n d e r s i u n e q u i l i b r i o i d r o s t a t i c o , c h e p o r t a a l l ' u g u a g l i a n z a d e l l e pressioni. s i d o v r e b b e f r a l ' a l t r o , t e n e r p r e s e n t e c h e l ' a c c e l e r a z i o n e di g r a v i t à v a r i a con l a l a t i t u d i n e . è p e r a l t r o i n t u i b i l e c h e n e g l i s t r a t i p r o f o n d i si sia r e a l m e n t e s t a b i l i t o u n e q u i l i b r i o i d r o s t a t i c o ed i n o l t r e a l l ' a u m e n t a r e d e l l e prof o n d i t à , con l ' a u m e n t o d e l l a t e m p e r a t u r a , c'è da a s p e t t a r s i u n a dimin u z i o n e d e l l a viscosità n e l l e masse. i n o l t r e l'ipotesi del p r a t t , seguita con rigore, v u o l e b l o c c h i indip e n d e n t i , c o m p o r t a n t i s i i n d i p e n d e n t e m e n t e ciascuno dai suoi v i c i n i : è n e c e s s a r i o d u n q u e t r a s c u r a r e l e r e a z i o n i l a t e r a l i . s e si segue l'ipotesi di u n e q u i l i b r i o i d r o s t a t i c o è o v v i a m e n t e n e c e s s a r i o s u p p o r r e , ad u n a c e r t a p r o f o n d i t à , l ' e g u a g l i a n z a d e l l e p r e s s i o n i : ciò c o m p o r t a a n c h e l a necessità di l i m i t a r e l e r e a z i o n i l a t e r a l i a l l e sole p r e s s i o n i i d r o s t a t i c h e . u n ' a l t r a o s s e r v a z i o n e v a f a t t a , e d e l m a s s i m o i n t e r e s s e . s e n z a v o l e r e n t r a r e n e l m e r i t o d e l l a r i s p o n d e n z a o m e n o con l a r e a l t à geologica p r i m i t i v a d e l l ' i p o t e s i t r a t t a t a , c h e p e r a l t r o n o n sodd i s f a n o t o r i a m e n t e i p u n t i di v i s t a degli e m i n e n t i c u l t o r i di q u e s t o c a m p o , d o b b i a m o a m m e t t e r e c h e u n a r o t t u r a d e l l ' e q u i l i b r i o isostatico successiva, d o v u t a ad e r o s i o n e o s e d i m e n t a z i o n e , o ad a l t r o , n o n si sa c o m e possa v e n i r e c o m p e n s a t a . l o stesso p r a t t a m m e t t e v a , d ' a l t r o n d e , c h e l a sua i p o t e s i e r a a p p r o s s i m a t a . è d a o s s e r v a r e t u t t a v i a c h e s e g u i r e l'ipotesi significa a s p e t t a r s i la c o m p e n s a z i o n e i n a t t o n e l l e f o r m a z i o n i p i ù r e c e n t i ( 1 ). è n o t o r i o c h e si a v v i c i n a al v e r o i l c o n t r a r i o : n e l l e p i ù a n t i c h e p i a t t a f o r m e contin e n t a l i si r a g g i u n g e l a c o m p e n s a z i o n e , n e l l e r e g i o n i a n c o r a soggette ad a s s e s t a m e n t o è i n v e c e e s t r e m a m e n t e i m p r o b a b i l e che si abbia. ipotesi di airy. quasi c o n t e m p o r a n e a m e n t e a l l a p u b b l i c a z i o n e d e l l a m e m o r i a di p r a t t , riguardante la t e o r i a esposta, l ' a s t r o n o m o a i r y e s p r i m e v a conc i s a m e n t e il « suo s c h e m a » , di c o m p e n s a z i o n e . p a r t e n d o dall'osservaz i o n e c h e u n o spessore a n c h e n o t e v o l e d e l l a l i t o s f e r a n o n p o t e v a assic u r a r e a q u e s t a u n a s t a b i l i t à , p e r la viscosità d e l l e r o c c i e , e c h e q u i n d i e r a i l l o g i c o s u p p o r r e c h e l e escrescenze, r i g u a r d a n t i l e a l t u r e , f o s s e r o s o r r e t t e d a l l e r e g i o n i c i r c o s t a n t i , egli i m m a g i n a v a c h e i v a r i b l o c c h i , a f f o n d a n d o p i ù o m e n o n e l « s i m a » sottostante, p e r i l p r i n c i p i o di a r c h i m e d e , subissero a u t o m a t i c a m e n t e l a c o m p e n s a z i o n e . la d e n s i t à media d e l l e r o c c i e d o v e v a esser c o n s i d e r a t a c o s t a n t e 3 7 0 c . a q u i l i n a e così q u e l l a d e l f l u i d o m a g m a t i c o sottostante. questa c o n c e z i o n e perm e t t e v a l e n t i m o v i m e n t i v e r t i c a l i c h e in q u a l c h e m o d o p o r t a v a n o al susseguente r i s t a b i l i m e n t o d e l l a c o m p e n s a z i o n e c h e a v e v a s u b i t o u n a r o t t u r a , d o v u t a ad u n m o v i m e n t o o r o g e n e t i c o o ad a l t r o . n e l l a fig. 2 sono r a p p r e s e n t a t i s c h e m a t i c a m e n t e gli a f f o n d a m e n t i d e i v a r i b l o c c h i n e l sima: q u e l l i c o n t i n e n t a l i , 11. 1, 2 , 3, d i p i ù di q u e l l o c a m p i o n e , i n d i c a t o col n. 7, q u e l l i o c e a n i c i , n. 4, 5, 6, di m e n o . n a t u r a l m e n t e , c o m e si è p r i m a d e t t o , v a fissata l a d e n s i t à del s i m a ; v a fissata a n c o r a l ' a l t r a i n c o g n i t a : l a p r o f o n d i t à ii, r i s p e t t o al geoide, d e l l a s u p e r f i c i e di g a l l e g g i a m e n t o . questa h a p e r a l t r o notev o l e i n f l u e n z a s u l l a c o m p e n s a z i o n e e m a l a u g u r a t a m e n t e , n o n si h a n n o i n d i c a z i o n i c h e possano g u i d a r c i ad u n v a l o r e sia p u r e di l a r g o o r i e n t a m e n t o . f a c c i a m o p r e s e n t e c b e a n c h e n e l t r a t t a r e questa i p o t e s i , se a prim a v i s t a l a p o s s i b i l i t à d e l r a g g i u n g i m e n t o d e l l ' e q u i l i b r i o i d r o s t a t i c o , l e p r i n c i p a l i i p o t e s i s u l i , ' l s o s t a s i a 3 7 1 dei b l o c c h i e l e m e n t a r i , s e m b r a i n t u i t i v o , l a cosa n o n si p r e s e n t a p i ù così s e m p l i c e ad u n e s a m e a p p r o f o n d i t o . a p a r t e il f a t t o c h e i singoli b l o c c h i , p e r u n s o p r a v v e n u t o t u r b a m e n t o n e l l ' e q u i l i b r i o p r e c e d e n t e m e n t e r a g g i u n t o ( p e r e s e m p i o p e r erosione o s e d i m e n t a z i o n e ) d o v r e b b e r o v a r i a r e l a l o r o p o s i z i o n e v e r t i c a l e , senza s u b i r e l ' i n f l u e n z a d e i b l o c c h i v i c i n i , l e stesse leggi del galleggiamento d o v r e b b e r o r i s e n t i r e , n e l l a f a s e d i assestamento, d e l l e condiz i o n i t r a n s i t o r i e , a n c h e p e r i b l o c c h i v i c i n i . l e r e a z i o n i l a t e r a l i , i n o l t r e , p e r l a c o n v e r g e n z a d e l l e v e r t i c a l i , d o v r e b b e r o d a r l u o g o a r i s u l t a n t i v e r t i c a l i e, sotto l a l o r o i n f l u e n z a , i b l o c c h i n o n d o v r e b b e r o c o m p o r t a r s i da c o r p i p e r f e t t a m e n t e r i g i d i . c'è a n c h e da o s s e r v a r e c h e n o n è p r o b a b i l e c h e l e basi di questi b l o c c h i s i a n o p e r f e t t a m e n t e r i g i d e ed a c o n t a t t o d e l s i m a fluido. la c o n c e z i o n e di a i r y n o n p r e m e t t e a l c u n a i p o t e s i c h e possa ess e r e c o n t r a r i a a l l e c o g n i z i o n i g e o l o g i c h e e si l i m i t a ad a m m e t t e r e che l ' e q u i l i b r i o isostatico si r a g g i u n g e , m a n m a n o , p e r il p r i n c i p i o di a r c h i m e d e . t u t t o sta a v e d e r e c o m e si c o m p o r t a l a crosta t e r r e s t r e p e r p o t e r s i assoggettare a l l a r e a l i z z a z i o n e di u n v e r o e p r o p r i o gallegg i a m e n t o di b l o c c h i o, ciò c h e è l o stesso, di u n c o m p o r t a m e n t o schem a t i z z a b i l e i n q u e s t o m o d o . i n t a n t o n o n ripugna il p e n s i e r o di u n e q u i l i b r i o isostatico i n t e r r o t t o , p e r e s e m p i o da u n a s e d i m e n t a z i o n e s o p r a v v e n u t a , e q u i n d i m a n m a n o r i p r i s t i n a t o . r i m a n e n a t u r a l m e n t e da v e d e r e q u a l e sia i l c o m p o r t a m e n t o d e l l e r o c c e a l l e t e m p e r a t u r e ed a l l e pressioni d e l l e g r a n d i p r o f o n d i t à , p e r d u r a t e d e l l ' o r d i n e di m i l l e n n i e m i l l e n n i . e s p e r i m e n t i ed i n t e r p r e t a z i o n i si t e n t a n o p e r c e r c a r e in q u a l c h e m o d o di s o l l e v a r e u n v e l o del m i s t e r o i m p e n e t r a t o . g o g u e l ( 2 ì ad e s e m p i o , d i c h i a r a c h e l e m a t e r i e p i ù p l a s t i c h e , p e n e t r a n d o n e i v u o t i d e l l e r o c c e p i ù d u r e , n e m u t a n o c o m p l e t a m e n t e l e c a r a t t e r i s t i c h e elastiche. n e l l e sue l i n e e g e n e r a l i , d u n q u e , l'ipotesi di a i r y n o n è f u o r i d e l l e p o s s i b i l i t à r e a l i , l e concezioni s e m p l i f i c a t i v e dei calcoli, i n v e c e , c o m e ad e s e m p i o l ' i p o t e s i di costanza d e l l a d e n s i t à , p o r t a n o a conclusioni m e n o v e r o s i m i l i . ipotesi di vening-meinez. — p a s s i a m o o r a au'ipotesi e n u n c i a t a da v e n i n g m e i n e z , c h e si d i f f e r e n z i a d a l l e p r e c e d e n t i p e r c h é è u n sistema di c o m p e n s a z i o n e r e g i o n a l e c h e t i e n e c o n t o d e l l e r e a z i o n i l a t e r a l i d e i b l o c c h i v i c i n i . l e d u e p r e c e d e n t i i p o t e s i , q u e l l a di p r a t t e q u e l l a di 3 7 2 c . a q u i l i n a a i r y , i n f a t t i , i m m a g i n a n o u n a c o m p e n s a z i o n e s t r e t t a m e n t e verticale, l o c a l i z z a t a cioè al di sotto d e l l e m a s s e s u p e r f i c i a l i . l a c o r r e z i o n e r e g i o n a l e i n v e c e p r e s u p p o n e la l i t o s f e r a c o m e u n a p i a s t r a p i a n a , estesa i n d e f i n i t a m e n t e , e l a s t i c a , g a l l e g g i a n t e , in e q u i l i b r i o i d r o s t a t i c o , su u n o s t r a t o , i n f e r i o r e , p l a s t i c o , a d e n s i t à s u p e r i o r e . l e escrescenze t o p o g r a f i c h e s o v r a p p o s t e , al di s o p r a del l i v e l l o del m a r e , c a u s a n o u n a c e r t a flessione, a p p u n t o p e r i l l o r o peso, d e l l a piastra c b e è soggetta a l l e leggi d e l l ' e l a s t i c i t à . l a c o m p e n s a z i o n e dei « s o v r a c c a r i c h i t o p o g r a f i c i » a v v i e n e insomm a p e r l ' a f f o n d a m e n t o , ad essi p r o p o r z i o n a l e , d e l l a p i a s t r a n e l l a sott o s t a n t e massa p l a s t i c a a d e n s i t à s u p e r i o r e , t e n u t o o p p o r t u n o c o n t o p e r ò degli e f f e t t i l a t e r a l i . n e l l e r e g i o n i r i c o p e r t e d a l m a r e , n e l l e q u a l i si h a u n a t o p o g r a fia, r i s p e t t o al l i v e l l o m a r i n o , n e g a t i v a , si h a u n q u a l c h e cosa d i inv e r s o . s i h a cioè c o m p e n s a z i o n e p e r l a p r e s e n z a di masse p i ù dense s o l l e v a n t i s i , c o m e n e l caso d e l l ' i p o t e s i di a i r y , c h e p r e n d o n o il posto di q u e l l e m a n c a n t i , d e l l a p i a s t r a , a l l a r a d i c e di essa. il p r o b l e m a , n e i suoi a s p e t t i a n a l i t i c i , v i e n e i m p o s t a t o i m m a g i n a n d o u n a p i a s t r a i n d e f i n i t a , di o p p o r t u n o spessore, assoggettata ad u n a p r e s s i o n e d o v u t a a l l a d i f f e r e n z a d e l l e densità tra p i a s t r a e massa m a g m a t i c a , p e r l ' a f f o n d a m e n t o d e l l a d e t t a p i a s t r a , ed in p i ù , ad 1111 peso c o n c e n t r a t o c h e r i g u a r d a i l s o v r a c c a r i c o d o v u t o a l l a t o p o g r a f i a . il p r o b l e m a , di t e o r i a d e l l ' e l a s t i c i t à , r i s o l t o da h e r t z , dà un'espressione c b e è f u n z i o n e d e l l e c a r a t t e r i s t i c h e e l a s t i c h e d e l l a p i a s t r a . la d e f o r m a z i o n e d e l l a p i a s t r a dà l u o g o a l l ' a f f o n d a m e n t o d e l l a rad i c e d e l l o s t r a t o , i n c o r r i s p o n d e n z a d e l s o v r a c c a r i c o . questo a f f o n d a m e n t o v a r a p i d a m e n t e s m o r z a n d o s i m a n m a n o c b e ci si a l l o n t a n a dal p u n t o di m a s s i m o c a r i c o , m a , o l t r e p a s s a t o i l p u n t o di i n f l u e n z a n u l l a , se a n c o r a ci si a l l o n t a n a , si h a u n s o l l e v a m e n t o , con d e l l e successive o n d u l a z i o n i d e l l a s u p e r f i c i e i n f e r i o r e d e l l o s t r a t o , fino ad u n a spariz i o n e t o t a l e d e l f e n o m e n o . il d i a g r a m m a , c h e r a p p r e s e n t a q u e s t e d e f o r m a z i o n i , v i e n e p r e s o c o m e b a s e p e r l a r i p a r t i z i o n e d e l l a c o m p e n s a z i o n e , n e l senso o r i z z o n t a l e : si h a p e r t a n t o u n m a s s i m o a l d i s o t t o d e l l ' e l e m e n t o d i s t u r b a n t e e, m a n m a n o c h e da q u e s t o ci si a l l o n t a n a , si a r r i v a a l l a c o m p e n s a z i o n e n u l l a , i n c o r r i s p o n d e n z a d e l l a d e f o r m a z i o n e n u l l a . p e r s e m p l i f i c a r e , l ' a u t o r e n o n p r e n d e i n c o n s i d e r a z i o n e l e ondul a z i o n i successive al p u n t o d i a n n u l l a m e n t o d e l l a p r i n c i p a l e , e, ad espressioni p i ù complesse, sostituisce f u n z i o n i p i ù s e m p l i c i c b e d à n n o l e p r i n c i p a l i i p o t e s i s u l i , ' l s o s t a s i a 3 7 3 v a l o r i c o i n c i d e n t i , con q u e l l i d e l l e p r e c e d e n t i , i n p u n t i c a r a t t e r i s t i c i d e l d i a g r a m m a di d e f o r m a z i o n e . il p r o f . v e n i n g m e i n e z h a poi disposto la c o m p e n s a z i o n e n e l senso v e r t i c a l e i n m o d o da p o t e r t r a r r e i l m a s s i m o u t i l e d a l l e c i t a t e t a v o l e f o n d a m e n t a l i di g. cassinis, p . d o r è e s. b a l l a r i n l e q u a l i possono r a p p r e s e n t a r e , n e l sistema di c o m p e n s a z i o n e in esame, t a v o l e p e r « reg i o n a l i t à n u l l a ». lo stesso a u t o r e a f f e r m a c h e l a r i p a r t i z i o n e d e l l a c o m p e n s a z i o n e n e l senso v e r t i c a l e n o n r i p o s a su a l c u n a i n t e r p r e t a z i o n e fìsica p a r t i c o l a r e e questo a l l o scopo di p e r m e t t e r e i l f a c i l e uso di a l t r e t a v o l e c h e si a p p o g g i a n o su p r i n c i p i fisici d i v e r s i . d a l p u n t o di v i s t a geofisico l'ipotesi del v e n i n g m e i n e z a m m e t t e u n a p a r t e c i p a z i o n e r e g i o n a l e al p r o b l e m a d e l l a c o m p e n s a z i o n e a m m e t t e n d o il g a l l e g g i a m e n t o d e l l a p i a s t r a s o v r a s t a n t e sul m a g m a . s i h a i n f o n d o l ' a g g i u n t a , alla crosta t e r r e s t r e , di u n a t e n s i o n e c h e d o v r e b b e m a n i f e s t a r s i , n e l l e f o r m a z i o n i r e c e n t i , con u n a v a r i a z i o n e di densità, p e r la t r a z i o n e , ai m a r g i n i dei b l o c c h i c h e « scendono ». l o s g u a r d o g e n e r a l e d a t o a l l e v a r i e ipotesi m o s t r a di p e r se c o m e i n e f f e t t i c i a s c u n o s c h e m a t e n d a a g i u s t i f i c a r e l e a n o m a l i e c h e si incont r a n o . c h e l e m i s u r e f a t t e , p e r d i v e r s e v i e , accusano, con u n a legge g e n e r a l e . o r a s e m b r a f u o r di d u b b i o c h e u n a t a l e legge, se si r i f e r i s c e ad u n r a g g i u n t o e q u i l i b r i o , n o n sarà v e r i f i c a t a se n o n n e l l e r e g i o n i n o n p i ù p e r t u r b a t e da m o v i m e n t i , n e l l e q u a l i n o n si m a n i f e s t a n o p i ù f e n o m e n i sismici e così v i a . d ' a l t r a p a r t e il risultato d e l l e c o r r e z i o n i i n t r o d o t t e n e l l e d e t e r m i n a z i o n i g r a v i m e t r i c h e , v e r a m e n t e r i d u c e n o t e v o l m e n t e l e a n o m a l i e e si d i m o s t r a a n c o r a c h e l ' i n f l u e n z a dell'ipotesi a d o t t a t a n e l l a c o r r e z i o n e è assai p i c c o l a ( 1 ). questo f a t t o s t a r e b b e a d i m o s t r a r e c h e l ' i p o t e s i di u n a c o m p e n s a z i o n e isostatica ad o p p o r t u n a p r o f o n d i t à , n o n è f u o r i d a l l a r e a l t à . ora f e r m a r e s t a n d o l a p r e s e n z a d e l l e eccezioni n e l l e r e g i o n i n e l l e q u a l i si è l o n t a n i da u n e q u i l i b r i o d e f i n i t i v o , si p u ò asserire c h e semb r a l o g i c o a c c e t t a r e l'ipotesi di u n e q u i l i b r i o r a g g i u n t o , n e i casi ad e s e m p i o nei q u a l i ci si t r o v a d a v a n t i a v e c c h i e p i a t t a f o r m e c o n t i n e n t a l i . l ' i p o t e s i di u n assestamento n e l l a sua f a s e u l t i m a , n e i casi n e i q u a l i si h a da f a r e con r e g i o n i c h e p u r n o n essendo i n c o n d i z i o n i di sicura s t a b i l i t à , sono l o n t a n e d a l l a f a s e di t o r m e n t i sismici ecc. o r a n e l l e r e g i o n i c h e h a n n o r a g g i u n t o al c o m p l e t o l ' e q u i l i b r i o ( f o r s e nessuna) s e m b r a c o n v i n c e n t e a t t e n e r s i a l l ' i p o t e s i di a i r v se os55 c . a q u i l i n a s e r v i a m o c h e n o n r i p u g n a p e n s a r e ad u n a s t a b i l i z z a z i o n e , n e l senso v e r t i c a l e , a n c h e i m m a g i n a n d o i l b l o c c o che, con l a p r o f o n d i t à v a r i l e sue c a r a t t e r i s t i c h e f ì s i c h e fino a r a g g i u n g e r e q u e l l e del sima. e p o i c h é r a g g i u n t a la c o m p e n s a z i o n e n o n si d e b b o n o a v e r e r e a z i o n i l a t e r a l i ( a l t r i m e n t i s i g n i f i c h e r e b b e c h e l ' e q u i l i b r i o n o n è s t a t o r a g g i u n t o ) si p u ò ben p e n s a r e ad u n b l o c c o f o r m a t o da u n a p i l a di r o c c e c b e m a n m a n o con l a p r o f o n d i t à passano ad a v e r e l e c a r a t t e r i s t i c h e d e l sima. l a seconda ipotesi, la p i ù g e n e r a l e , c h e si r i f e r i s c e al sistema di c o m p e n s a z i o n e r e g i o n a l e p o t r e b b e p u r r i s p o n d e r e ad u n c r i t e r i o c o m e q u e l l o o r o r a esposto. o s s e r v i a m o a n c o r a c b e v e n i n g m e i n e z , con il suo sistema di c o m p e n s a z i o n e , n e l l ' a m m e t t e r e u n a s s e s t a m e n t o , i m p l i c i t a m e n t e , d o v r à a m m e t t e r e c b e l a p r e s s i o n e , d o v u t a al s o v r a c c a r i c o t o p o g r a f i c o f a c c i a p r e v e d e r e u n ' a z i o n e p r o p a g a n t e s i n e l m a g m a , c h e d u r e r à fino al r a g g i u n g i m e n t o d e l l ' e q u i l i b r i o isostatico. è o v v i o , p e r c e r t o , c b e la c o m p r i m i b i l i t à d e l fluido d e l q u a l e si t r a t t a d o v r à a s s o r b i r e p a r t e di q u e l l e p r e s s i o n i , m a , t e n u t o i l d o v u t o c o n t o d e l l e distanze, ed a m e n o di u n coefficiente di a t t u l i m e n t o del f e n o m e n o , ci a p p a r e v e r o s i m i l e c b e u n s o v r a c c a r i c o c o m e l ' h i m a l a j a , a d e s e m p i o , d e b b a d a r e , a l m e n o sotto i m a r i p i ù ricini, u n a s p i n t a v e r s o l ' a l t o secondo i l p r i n c i p i o di p a s c a l . il f e n o m e n o p o t r e b b e g i u s t i f i c a r e u n a m a g g i o r e c o m p r e s s i o n e e q u i n d i u n a m a g g i o r e d e n s i t à , d e l l e r e g i o n i s u b m a r i n e , cosa c h e ci a v v i c i n e r e b b e p e r i l p u n t o i n e s a m e e sotto q u e s t o aspetto, al c o n c e t t o t e o r i c o di d i s t r i b u z i o n e d e l l e d e n s i t à s u p p o s t e da p r a t t . c o n c l u d e n d o questi b r e v i c e n n i s u l l e i p o t e s i p r i n c i p a l i finora f o r m u l a t e s e m b r a logico s o t t o l i n e a r e l ' a p p o r t o v e r a m e n t e i m p o n e n t e d a t o d a v a r i i l l u s t r i scienziati c h e h a n n o t r a t t a t o i l p r o b l e m a sia sotto l o a s p e t t o t e o r i c o c h e sotto q u e l l o a p p l i c a t o , m a d ' a l t r a p a r t e , n é se l o s o n o nascosto gli stessi a u t o r i , l e difficoltà da s u p e r a r e sono t a l i c h e s a r e b b e a u s p i c a b i l e , a v e r e m o l t i a l t r i e l e m e n t i a d i s p o s i z i o n e p e r p o t e r p a r l a r e di t e o r i a c b e si a v v i c i n i di p i ù a l l a r e a l t à dei f a t t i . roma — istituto di geofisica mineraria della facoltà di ingegneria dell'università — giugno 1952. riassunto l'autore richiama le tre principali ipotesi, sull'isostasia, e cioè quella dovuta a pratt, quella di airy e quella di vening-meinez. nel farne il confronto e la critica propone, tra l'altro, di prendere in. l e p r i n c i p a l i i p o t e s i s u l i , ' l s o s t a s i a 3 7 5 considerazione le eventuali spinte, secondo il principio di pascal, dovute (die azioni dei pesi dei forti dislivelli topografici. svmmary the author revieics the three principal hypotlieses concerning isostasy; luimely those of pratt, airy and vening-meinez. in making the comparison and proposing a criticism, the principle of pascal concerning the possible effect of the iveights of the substantial topographical inequalities has been taken into consideration. b i b l i o g r a f i a ( ! ) lejay pierre r . p . , développemenls modernes de la gravimetrie. p a r i s , g a u t l i i e r , v i l l a r s 1 9 4 7 . ( 2 ) g'oguel j . : introduction à l'étude mecanique des déformations de l'écorce terrestre. p a r i s 1 9 4 3 1 9 4 8 . ( 3 ) c a s s i n i s g . d o r è p . b a l l a r i n s . , tavole fondamentali per la riduzione dei valori osservati della gravità. p a v i a 1 9 3 7 . ( 4 ) gulatee b . l . , gravity reductions and the figure of the earth. p . 3 2 3 g e r l a n d s b e i t r a g e z u r g e o p l i i s i k . l e i p z i g 1 9 3 8 , b a n d . 5 3 . ("') meinez-veninc f . a . , tables fondamentales pour la réduction isostatique regionid. b u l l e t i n g é o d é s i q u e , p . 7 1 1 , n . 6 3 , 1 9 3 9 . s u l l a r i d u z i o n e r e g i o n a l e e s u l m e t o d o d e l l e d e r i v a t e s e c o n d e i n g r a v i m e t r i a antonio m a r u s s i 1. i n a l c u n i r e c e n t i l a v o r i ( 1 ) sono s t a t i i l l u s t r a t i p a r t i c o l a r i m e t o d i p e r l a i n t e r p r e t a z i o n e d e l l e m i s u r e di g r a v i t à , c h e t e n d o n o a m e t t e r e i n r i s a l t o l e c o s i d e t t e a n o m a l i e l o c a l i , d o v u t e a d i s t r i b u z i o n i d i d e n s i t à l o c a l i z z a t e e p o c o p r o f o n d e , q u a l i s o n o p e r l o p i ù q u e l l e c h e i n t e r e s s a n o l a p r o s p e z i o n e g e o m i n e r a r i a ; q u e s t i m e t o d i si b a s a n o sos t a n z i a l m e n t e s u l l o s t u d i o d e l l e d i f f e r e n z e f r a i l v a l o r e ga d e l l a g r a v i t à i n u n p u n t o p0, e l a m e d i a ( o l e m e d i e ) gh dei v a l o r i g su u n o (o p i ù ) c e r c h i di o p p o r t u n o r a g g i o r a v e n t i pa c o m e c e n t r o , t r a c c i a t i s u l l a sup e r f i c i e e q u i p o t e n z i a l e s p e r p0. s i p r e s c i n d e q u i dai p r o c e d i m e n t i a d o t t a t i p e r ridurre a l l a s u p e r ficie e q u i p o t e n z i a l e x i v a l o r i d e l l a g r a v i t à e f f e t t i v a m e n t e o s s e r v a t i , n o t a n d o s o l o c h e l e c o n c l u s i o n i c h e s e g u o n o v a l g o n o o v v i a m e n t e p e r i l c a m p o p o t e n z i a l e d e l l a g r a v i t à m o d i f i c a t o i n seguitoa l l e i p o t e s i c h e s t a n n o a f o n d a m e n t o di t a l i riduzioni. a l c u n e f o r m u l e si p o s s o n o p e r ò a p p l i c a r e a n c h e q u a n d o l a s u p e r f i c i e s u l l a q u a l e si c o n s i d e r a l a d a t a d i s t r i b u z i o n e d e l l a g r a v i t à , è u n a s u p e r f ì c i e q u a l u n q u e , a n c h e ad e s e m p i o l a s u p e r f i c i e fisica d e l t e r r e n o ; i n t a l caso i v a l o r i d e l l a g r a v i t à d a c o n s i d e r a r e sono q u e l l i e f f e t t i v a m e n t e o s s e r v a t i , e l e c o n c l u s i o n i si r i f e r i s c o n o al c a m p o r e a l e d e l l a g r a v i t à . t r a l a s c i a n d o q u e s t e c o n s i d e r a z i o n i , s u l l e q u a l i n o n i n t e n d i a m o i n s i s t e r e q u i , i m e t o d i a c c e n n a t i d a n n o l u o g o p r e c i s a m e n t e : a) a l l a riduzione regionale, m e d i a n t e l a q u a l e ci si p r o p o n e d i m e t t e r e i n r i s a l t o l e a n o m a l i e l o c a l i , t o g l i e n d o al v a l o r e d e l l a g r a v i t à i n o g n i p u n t o , l a m e d i a dei v a l o r i d i q u e s t a l u n g o u n a c i r c o n f e r e n z a di r a g g i o p r e f i s s a t o a v e n t e i l p u n t o c o n s i d e r a t o c o m e c e n t r o ; b) al procedimento delle derivate seconde, c h e consiste n e l r i p e t e r e il c a l c o l o r i c o r d a t o o r o r a p e r u n a s e r i e di c i r c o n f e r e n z e a v e n t i i l p u n t o c o n s i d e r a t o c o m e c e n t r o , e r a g g i v i a v i a d e c r e s c e n t i . i l l i m i t e c u i t e n d o n o i v a l o r i così o t t e n u t i , d i v i s i p e r i l q u a d r a t o d e l r a g g i o dell a c i r c o n f e r e n z a r i s p e t t i v a , si a s s u m e a l l o r a u g u a l e a l l a d e r i v a t a sec o n d a — — d e l l a g r a v i t à n e l senso d e l l a v e r t i c a l e v. du* 1 5 6 a n t o n i o m \ r u s s i tu questa nota ci si p r o p o n e di m o s t r a r e c o m e in a m b e d u e questi m e t o d i i r i s u l t a t i p o s s a n o m e t t e r s i i n r e l a z i o n e con il s e c o n d o p a r a m e n t r o d i f f e r e n z i a l e (a„ ed i successivi ( a„h di o r d i n e 2s d e l l a g r a v i t à g s u l l a s u p e r f i c i e s c o n s i d e r a t a , v a l e n d o s i di a l c u n i r i s u l t a t i o t t e n u t i a n c o r a n e l 1 9 0 9 dal p i z z c t t i ( 2 ) ; e di i l l u s t r a r e poi il significato g e o m e t r i c o e m e c c a n i c o , in r e l a z i o n e a l l a s t r u t t u r a l o c a l e del campo di g r a v i t à , di tale p a r a m e n t r o . 2. p r i m a di p r o c e d e r e , r i c o r d i a m o p e r c i ò i r i s u l t a t i o t t e n u t i d a l p i z z c t t i . d e t t o g0 il v a l o r e di u n a f u n z i o n e g in u n p u n t o p„ del piano, gu la m e d i a a r i t m e t i c a dei v a l o r i elle la f u n z i o n e stessa assume n e i p u n t i d e l l a c i r c o n f e r e n z a di r a g g i o r c c e n i l o 7\„ si lia + + + [ i ] • • • +— gh + (-igog)v 2 2 4 2 . . . (2„_ 2)2 b ~ 2 2 4» . . . ( 2 n ) ' o v e i (/\2ll si i n t e n d o n o c a l c o l a l i i n p,„ e (a„n g)y. è u n v a l o r e c o m p r e s o f r a il m a s s i m o ed il m i n i m o di q u e l l i clic a s l l g assume nell ' a r e a c i r c o l a r e c o n s i d e r a t a . t a l e f o r m u l a si a p p l i c a s o l t a n t o , c o m e si v e d e , al caso del p i a n o , o d e l l e s u p e r f i c i s v i l u p p a b i l i ; e la sua estensione al caso d e l l e s u p e r fici q u a l s i v o g l i a n o n o n è i m m e d i a t a ; m a i n v e c e è a n c o r a v e r a , c o m e il p i z z c t t i ha d i m o s t r a t o , a n c h e p e r l e s u p e r f i c i q u a l s i v o g l i a n o , l a r e l a z i o n e , o v v i a p e r il p i a n o , l i m 4 ( , „ „ . ) r — 0 r' i n q u e s t o caso gli e n u n c i a t i p r e c e d e n t i si m o d i f i c a n o s o s t i t u e n d o l a c i r c o n f e r e n z a geodetica di r a g g i o r a l l a c i r c o n f e r e n z a d e l p i a n o . 3. r i c o r d a n d o ora l a d e f i n i z i o n e clic è stata data d e l l a r i d u z i o n e r e g i o n a l e , ben si v e d e c o m e questa possa essere espressa — n e l l ' i p o t e s i i n cui in p r i m a a p p r o s s i m a z i o n e l a s u p e r f i c i e a l l a q u a l e sono r i d o t t i i v a l o r i di g possa c o n f o n d e r s i con il p i a n o —• con l o s v i l u p p o p e r l e p o t e n z e p a r i di r d a t o d a l p i z z e t t i , i cui coefficienti sono espressi med i a n t e i secondi p a r a m e t r i d i f f e r e n z i a l i s u p e r f i c i a l i successivi di g; i quali v e n g o n o d u n q u e ad a s s u m e r e u n r u o l o i m p o r t a n t i s s i m o n e l l o stud i o d e l l e a n o m a l i e l o c a l i , d o v u t e a d i s t r i b u z i o n i di densità s i t u a l e a p i c c o l a p r o f o n d i t à ; f r a questi il secondo p a r a m e t r o d i f f e r e n z i a l e s u l l a r i d u z i o n e r e g i o n a l i , e s u l m e t o d o d e l l e d e r i v a t e s e c o n d e 1 5 7 (a., è poi q u e l l o p a r t i c o l a r m e n t e interessante, poiché esso può essere c a l c o l a t o a g e v o l m e n t e come il l i m i t e «lato (lai pizzetti o, p i ù f a c i l m e n t e ancora, p e r la regola dell'hospital, come (a» g)v. = 4 l i m . [ 3 ] fi —0 d (fi2) c a l c o l a t i p r e c i s a m e n t e i v a l o r i m e d i £„ su di u n a serie di circ o n f e r e n z e di raggio decrescente, e r i p o r t a t i i q u a d r u p l i dei v a l o r i ottenuti come o r d i n a t e i n u n sistema c a r t e s i a n o in cui l'asse d e l l e ascisse ahhia come p a r a m e t r o fi-, l'angolo che la tangente n e l l ' o r i g i n e a l l a c u r v a o t t e n u t a r a c c o r d a n d o i p u n t i r a p p r e s e n t a t i v i f o r m a con l'asse delle ascisse, ha come tangente il cercato. a l t r i p r o c e d i m e n t i p i ù r a p i d i , se p u r e meno precisi, possono ven i r e a p p l i c a t i p e r il calcolo n u m e r i c o di detto ( a 2 g ) 2 . 4. ciò premesso, v e n i a m o a studiare p i ù da v i c i n o il significato meccanico del secondo p a r a m e n t r o d i f f e r e n z i a l e di b e l t r a n i d e l l a grav i t à , su di u n a s u p e r f i c i e ^ che, p e r ragioni di semplicità, a m m e t t e r e m o essere s u p e r f i c i e e q u i p o t e n z i a l e o di l i v e l l o del c a m p o ; si i n t e n d e che, se i v a l o r i della gravità n o n fossero stati m i s u r a t i d i r e t t a m e n t e su questa, essi d o v r e b b e r o i m m a g i n a r s i ad essa r i d o t t i secondo i consueti p r o c e d i m e n t i c o n v e n z i o n a l i . p o t r e m o i n f i n e d e f i n i r e il secondo p a r a m e t r o d i f f e r e n z i a l e scrivendo in f o r m a v e t t o r i a l e [ 3 ] (a2 g ) v = d i v g r a d v g [ 4 ] in cui g r a d v g — grad g — grad gxvv è la c o m p o n e n t e s u p e r f i c i a l e su ^ del g r a d i e n t e di g. con v si è indicato il v e r s o r e della n o r m a l e a ^ ( d i r e t t o v e r s o l'alto), che c a r a t t e r i z z a l a congruenza r e t t i l i n e a delle n o r m a l i ; n e l m e n t r e con n i n d i c h e r e m o n e l seguito il v e r s o r e delle tangenti alle l i n e e di f o r z a del campo, p u r e d i r e t t o v e r s o l'alto, che c a r a t t e r i z z a invece la congruenza c u r v i l i n e a delle l i n e e di f o r z a . si ha d u n q u e n = v su —, e poi d n d v d 1' d i> [6] a essendo l ' o m o g r a f i a di b u r a l i f o r t i generalizzata, e (tv la sua compon e n t e s u p e r f i c i a l e (4). 1 5 8 a n t o n i o m \ r u s s i si ha dunque, p r e n d e n d o la divergenza di [ a ] : ( a 2 g ) s = a . g — h g r a d g x v — grad ( g r a d g x v ) = a „ ? — h ~ — ^ d v a v [ 7 ] • ™ t 2(o2 1 d g , , in cui f i = div v = — e la c u r v a t u r a media di (co e la g g da velocità a n g o l a r e di r o t a z i o n e della terra). si osservi ora che, n e l m e n t r e si ha o v v i a m e n t e , su dg dg — — = [ 8 ] d v d n non a l t r e t t a n t o può dirsi p e r le d e r i v a t e s e c o n d e ; ed i n f a t t i fd'"g d grad g — = grad ( g r a d g x o x v = — v • v + kc grad gx^ = l dvs dp 2 d grad g d grad g = —vv—— n-n 191 d f dp p e r c h é k c v = o v ; i n v e c e d~g d orad g — = g r a d ( g r a d g x « ) x n = p n n + a." a grad g x n = dn• d p = ~ + = [ 1 0 ] dv" d\in cui f = mod g r a d v lg g è la p r i m a c u r v a t u r a della linea di f o r z a . a sua volta, il calcolo di is.»g f o r n i s c e ( 4 ) à ì g = g(c + f'-) [11] dove c = h— 2k [ 1 2 ] è l a c u r v a t u r a di casorati di s (k n e è la c u r v a t u r a totale o gaussiana). t e n e n d o conto di ciò, al (a» g)v p u ò essere data u n a delle seguenti f o r m e ( a , g ) v = 2 g (hk + f--) + 2 o r h = dn2 1 1 3 1 la p r i m a in p a r t i c o l a r e ci dice che il ( a . d i f f e r i s c e dalla d e r i v a l a seconda della g r a v i t à secondo l a v e r t i c a l e , p e r t e r m i n i d i e d i p e n d o n o dalla c u r v a t u r a della s u p e r f i c i e e q u i p o t e n z i a l e e della linea s u l l a r i d u z i o n e r e g i o n a l e e s u l m e t o d o d e l l e d e r i v a t e s e c o n d e 1 5 9 di f o r z a , e che in g e n e r a l e non sono a l l a t t o t r a s c u r a b i l i . cosi ad esemp i o si a v r e b b e , p e r il c a m p o n o r m a l e d e l l a g r a v i t à ( c a m p o di somigliana) alla l a t i t u d i n e di 4 5 ° : d ~ g = — 1 4 , 4 9 5 x 1 0 ~ 1 5 c m ' sec"* , dn2 nel m e n t r e i t e r m i n i d e r i v a n t i dalla c u r v a t u r a del c a m p o a m m o n t a n o a 1 4 , 5 0 1 x io 5 cm"1 s e c , tanto c h e il a> s u p e r f i c i a l e della gravità r i s u l t a u g u a l e a 0 , 0 0 6 x io"5 cm'1 sec"2. 5. a l t r e f o r m u l e espressive, che possono t o r n a r e a v o l t e u t i l i , si h a n n o q u a n d o si consideri i n v e c e del c a m p o g della g r a v i t à , q u e l l o del suo l o g a r i t m o n a t u r a l e . l a c o n s i d e r a z i o n e del l o g a r i t m o della g r a v i t à , anziché della grav i t à stessa, presenta il vantaggio di a t t r i b u i r e al suo g r a d i e n t e l e dim e n s i o n i di u n a c u r v a t u r a ; ed a l l e sue d e r i v a t e seconde (od alla divergenza del suo g r a d i e n t e ) le d i m e n s i o n i di u n a c u r v a t u r a totale. ciò è m o s t r a t o c h i a r a m e n t e d a l l e f o r m u l e c h e s e g u o n o : grad lg g — f n' -— (h | n ( f o r m u l a di b r u n s ) \ g ì [ 1 4 ] in cui a ' è il v e r s o r e della n o r m a l e p r i n c i p a l e alla l i n e a di f o r z a . facili calcoli d a n n o i n o l t r e 1 j \ a n [ 1 5 ] g1 g dn \ d n e poi ( a j g g h = c + f"-+ " w d n trieste — istituto di topografìa e geodesia dell'univ. — marzo 1953. riassunto si, richiamano alcune ricerche di pizzetti, che consentono di ottenere, con procedimenti applicabili in pratica, la divergenza del gradiente superficiale della gravità (secondo parametro differenziale di beltrami della gravità in superficie). queste ricerche vengono messe 1 6 0 a n t o n i o m \ r u s s i iti relazione con i metodi della riduzione regionale e delle derivate seconde, che sono stati proposti recentemente dagli autori americani per le pratiche applicazioni, in gravimetria. nel caso in cui la superficie sulla quale si calcola tale divergenza sia di livello, il secondo parametro differenziale si esprime assai semplicemente mediante gli elementi geometrici e meccanici del campo. summary some formulae given by pizzetti are recalled, tvhich allow to obtain by practical procedures the divergence of tlie surface gradient of gravity (second differential parameter of delirami of surface gravityj. sudi researclies are related to the metliods of regional reduction and of the second derivatives, which have been recently proposed for practical applications by american authors. ìt henever the surface consiclered be a level surface of the potential fichi, the second differential parameter may be easily expressed by means of the geometrie and dynamic elements of the field itself. b i b l i o g r a f i a ( ! ) w . raymond grikfin, residuili gravity in tlieory and practice. g e o p h y s i c s , v o i . x i v , j a n u a r y , 1 9 4 9 . — thomas a . elkins, the second derivatives method of gravity interpretalion. g e o p h y s i c s . v o i . x v i , j a n u a r y , 1 9 5 1 . — j . j . jakosky, exploration geophysics, pag. 4 1 5 e segg., l o s a n g e l e s , 1 9 5 0 . ( 2 ) p . pizzetti, sul significato geometrico del secondo jyaramelro differenziale di una funzione soi>ra una superficie qualunque. r e n d . r . a c c . dei l i n c e i , v o i . x v i i i , 1 ° seni., 1 9 0 9 . ( 3 ) c. burali-forti e r . makcolongo, analisi vettoriale generale e applica. zioni. t r a s f o r m a z i o n i l i n e a r i , b o l o g n a , z a n i c h e l l i , 1 9 2 9 , pag. 227. ( 4 ) a . marbssi, fondamenti di geodesia intrinseca. m e m o r i a n . 7 d e l l a c o m m i s s i o n e g e o d e t i c a i t a l i a n a , m i l a n o , 1 9 5 0 . su alcune disposizioni elettrodiche interessanti processi elettrocinetici in terreni saturi d'acqua a. crea studiamo il fenomeno elettrogeosmotieo, nel caso stazionario di u n campione di terreno saturo d'acqua in cui si faccia fluire corrente elettrica mediante due elettrodi indefiniti, cilindrici non coassiali (v. fig. l a ) , o cilindrico l'anodo, piano il catodo e parallelo alle generatrici anodiche (v. fig. l i ) . supporremo che all'interno dell'anodo si eserciti artificialmente u n a pressione idraulica ha maggiore o uguale della pressione esistente al catodo (z/a). sia anodo che catodo si penseranno permeabili all'acqua, per esempio forellati. useremo i simboli seguenti: q = vettore di flusso idraulico; j = vettore densità di corrente elettrica ii — pressione idraulica nel p u n t o generico del campione; v potenziale elettrico nel p u n t o generico del campione; ha = pressione idraulica anodica hk = pressione idraulica catodica va = potenziale anodico vk — potenziale catodico a = conduttività elettrica del terreno i = corrente elettrica totale p = resistenza elettrica; k = coefficiente di permeabilità idraulica (darcy) ke = coefficiente elettroosmotico [a — keja = coefficiente mediamente invariantivo. § 1 l'equazione fondamentale del fenomeno (che congloba le legge di darcy ed helmholz), è q = — />' grad h + [x j [1] e le condizioni di continuità d\v~q = 0 , di v j = 0 . [1'] inoltre, d e t t o v (x, y) il potenziale elettrico, si avrà anche j = — a grad v [1"] applicando l'operatore divergenza alla [1] e tenendo conto delle [1'], si ottiene subito di® (grad h) = — — + = 0 2 ì) x2 ò y2 e dalla [1"], applicando ancora l'operatore divergenza e tenendo conto della seconda delle [1'] d i v j = o div (gradi + ^ j = 0 . [3] 200 a. crea alle [2] e [3] a n d r a n n o aggiunte le condizioni al contorno, come segue: per la [2]: r (x, y) = ha sull'anodo, h (x, y) = hk sul catodo, per la [3] v (x, y) = va sull'anodo, v (x, y) = vk sul catodo. [4] il problema, p e r t a n t o , è identico sia per la pressione idrostatica che per il potenziale elettrico: t r o v a r e u n a funzione armonica (p (x, y) (che può essere la pressione o il potenziale elettrico) che su anodo e catodo assuma valori costanti prefissati. consideriamo dapprima u n a circonferenza nel piano w, di centro nel p u n t o w = 0 e raggio arbitrario i?; essa a v r à equazione | w\2 = r2 ove | w | è il modulo di te; indicando con sopralineature i coniugati dei numeri complessi, potremo scrivere la precedente equazione: z — a z — a \ w\2 = w w = • = az — 1 a z — 1 \z\2 — 2ar(z) + a2 ~ a2 \z\2 — 2ar(z) + 1 ma z2 = x2 + y2, r (z) = x, quindi l'equazione precedente si scrive, dopo ovvie semplificazioni: (x_ af + y2 (ax — l)2 + a2y2 = r2 . [6] allo scopo di risolvere il problema di dirichlet ora posto, introduciamo i due piani complessi (z = x + iy) e w = u + iv), e t r a essi poniamo la corrispondenza subordinata dalla funzione complessa w = [5] (a costante reale > 1 ) , analitica nel piano « salvo nel p u n t o « = in cui essa ha u n a a singolarità polare del primo ordine. come è ben noto dalla teoria delle f u n zioni di u n a variabile complessa, la corrispondenza così posta f r a i piani (z) e (te) è generalmente conforme, nel senso che vengono conservati gli angoli f o r m a t i f r a loro da due archi di curva uscenti da un p u n t o del piano (z), nel passaggio al p u n t o e alle curve corrispondenti del piano (w); f a n n o eccezione le singolarità della funzione, e i p u n t i in cui si annulla la derivata della funzione. è altresì noto, dalla teoria delle f u n zioni armoniche, che una trasformazione conforme nel piano m u t a funzioni armoniche in funzioni armoniche. studiamo ora brevemente la corrispondenza definita dalla [5], mettendone in evidenza alcune proprietà fondamentali. l a [3] ridotta in f o r m a intera, può scriversi: x2 + y2 — 2«(1 — r2) a2 r2 x + 1r 2 a2r2 = 0 [6'] che è l'equazione di u n a circonferenza del piano (z = x-\-iy) di centro j^j , o j , e di raggio r (1—a2 1— r2a2 (si noti che ciò comporta, essendo a > 1 , r > j , che è la curva corrispondente, nel piano (z) alla generica circonferenza con centro nell'origine del piano (tv). si noti poi che alla circonferenza di raggio r = l e centro nell'origine del piano (tv) corrisponde la circonferenza di raggio 1 e centro nell'origine del piano z, qualunque sia a. alla circonferenza del piano (w) con centro nell'origine e raggio ti — corrisponde u n a circonferenza di raggio infinito, e con centro all'infinito nella direzione dell'asse delle x: cioè la r e t t a di equazione (come si ricava sempre dalla [6]) [18] su a l c u n e disposizioni e l e t t e o d i c i i e , ecc. 201 parallela all'asse y e a. distanza > 1 da esso. ~ a sia ora u n a r e t t a per l'origine del piano w, detto w0 = u0 + iv0 un suo punto, l'equazione di tale r e t t a può scriversi: iv = twn [8] z — a az — 1 w0 t (x — a) (ax — 1) + ay2 (ax — l)2 + a2ìf ( a 2 1 ) y = u0t , [9] (ax — .l)2 + a2 y2 eliminando t f r a le [9] e posto 0 = a otteniamo: (x — a) (ax — 1) + ay2 (a2 1 )y = a cioè a2 + 1 , a2 — 1 x2 + ìfx — l « + 1 = 0 a a che è l'equazione di u n a circonferenza di a2 + 1 „ a2 •— 1 centro | 1 + a2 2 a , a e di raggio , passante, qualunque sia a, 1 per i p u n t i (a, o) e — , o : inoltre, al variare di a, i centri di tali circonferenze hanno ascissa costante, quindi sono situati su u n a r e t t a che coincide con quella di equazione [7]. le circonferenze di equazione [6] e [10] sono m u t u a m e n t e ortogonali, come assicura il c a r a t t e r e conforme della corrispondenza, e come è del resto facile verificare direttamente; per ogni p u n t o del piano (z = x + iy) ne passa u n a ed u n a sola di ciascuna famiglia, eccetto che per i punti, singolari per la corrispondenza, (a, o), (—, o) per cui non passa nessuna delle circonferenze [6] e passano t u t t e le [10]. essendo t un p a r a m e t r o reale. la curva corrispondente nel piano (z) soddisfa alla relazione da cui, ponendo z = x + iy e separando il reale dall'im m aginario fig. 2 la fig. 2 mostra l'andamento dei sistemi di circonferenze [t3] e [10]. due § 3. con l'ausilio della corrispondenza studiata nel paragrafo precedente, mostreremo nel presente paragrafo come sia possibile trasformare il dominio compreso f r a i nostri elettrodi, pensato a p p a r t e n e n t e al piano (z), in u n a corona circolare nel piano (w). assumiamo l'asse delle ascisse passante per i centri delle circonferenze sezioni degli elettrodi e consideriamo come coordinate di u n p u n t o del piano (z) i rapporti f r a le coordinate effettive e il raggio della circonferenza catodica esterna (il che, ovviamente, corrisponde a scegliere tale raggio come u n i t à di misura), il che comporta che la circonferenza esterna avrà, nel nuovo sistem a di coordinate (« coordinate ridotte») raggio unitario. siano a ^ e a ^ (—1 < », < x2 < 1 ) le coordinate r i d o t t e delle intersezioni della circonferenza interna coll'asse delle ascisse. si è già visto (v' § 2) che la circonferenza di raggio unitario nel piano (z) corrisponde alla circonferenza unitaria nel piano (w); per conseguire il nostro scopo, basterà far vedere 202 a . c r e a che è possibile specializzare nella [6] a ed r, in modo che la circonferenza che ne risulta (corrispondente, come si è visto, a u n a circonferenza del piano (tv) con centro nell'origine) coincida con la circonferenza interna (anodo). all'uopo, intersechiamo la [6] con l'asse delle ascisse, e imponiamo che le ascisse dei p u n t i di intersezione coincidano con xx e x2 r i s p e t t i v a m e n t e : tali ascisse sono fornite dalle soluzioni dell'equazione: 2 a (1 — r2) a2 — r2 x2 — v x + = 0 111 1 — a2 r2 1 — a2 r2 ' j che sono d a t e d a : a (1 — r2) r(a2— 1) x ~ 1 — a2 r2 111 a2 r2 — 1 essendo poi a2 — 1 > 0, a2 r2 — 1 > 0 dovrà essere x, = xo = a ( 1 — r2) r (a2 —1) 1 — a2 r2 ~ 1 — a2 r2 ' a(l—r2) r(a2— 1) [12 a2 r2 a2 r2 — 1 r = 1 — x,x2 ± i (1— a^2) (1— x22) a = r l+x1xl+\(l—xì2) (1— x22) [13'] 1 — x1x2 + 1(1—^) (1 p e r t a n t o l'assunto è dimostrato: notiamo esplicitamente che alla circonferenza interna nel piano (z) corrisponde la circonferenza esterna n e l p i a n o (w). risolviamo ora il problema di dirichlet posto nel § 1. a questo scopo trasformiamo il dominio compreso f r a i due elettrodi secondo le modalità del § 3, risolviamo il problema di dirichlet per il dominio trasformato (corona circolare) e invertiamo la trasformazione. p e r quanto d e t t o alla fine del § 3, occorrerà considerare, nel piano (?<•), l'anodo esterno al catodo (v. flg. 3). jt ___ fi ) jjj 7 o *4k/|v " con qualche calcolo, è facile vedere che le [12] comportano le due equazioni: fa+ao «2 —2 (1+a^) a + (x2+xx) = 0 (xì—x1)r» — 2(l—x1x2)r + (x2—x1) = 0 [12'] che a m m e t t o n o le soluzioni 1 + x,x2 ± l/(l —xx2) (1 —x22) [13] nelle quali vanno assunti segni concordi. poiché deve essere, per ipotesi, a > 1, e le due soluzioni della prima delle [12'] sono positive e u n a inversa dell'altra, occorrerà scegliere nelle [13] il segno positivo, per cui è f i g . 3 ovviamente, per ragioni di simmetria, e per cose note, la funzione che risolve il nostro problema per la corona circolare è del tipo: q>(r) = a lg r + b con r distanza del p u n t o generico dall'origine, e a e b costanti da determinare con l'ausilio delle condizioni ai limiti. p e r la pressione idrostatica avremo h (r) = a lg r + b = ha (sull'anodo) ii (!) = b hk (sul catodo) e quindi e, con procedimento analogo per il potenziale elettrico v (r), v(r) = va~~jk l g r+vk x2 x1 e risulta anche r > 1 (anche le soluzioni della seconda delle [12'] sono f r a loro reciproche). lg-r tenendo conto che è / (x — a)2 + y2 r = w (ax-l)2-\-a2y2 su a l c u n e d i s p o s i z i o n i e l e t t r o d i c h e , eco come è facile vedere, e come è fisicamente ovvio, t a l e flusso catodico ha un a n d a m e n t o del tipo della tìg. 4, simmetrico rispetto all'asse dei centri degli elettrodi,massimo dove le superfiei elettrodiche sono più vicine, minimo nel p u n t o d i a m e t r a l m e n t e opposto. le linee di flusso nell'interno dei d u e elettrodi, sono le circonferenze [10]. è poi facile calcolare la corrente elettrica (per la [5]), otteniamo, nel piano (z = x-\-iy) 2 lg li (ax — l)2-\-a2y2 v(xv) = ~a ~ v k le { x ~ + v 1 [x,,j)~ 21gr {ax-l)2 +a2y2 ' k [14] che risolvono il nostro problema. n a t u r a l mente, (a) ed (r) h a n n o i valori forniti totale fluente f r a gli elettrodi: b a s t a ricoriik dalle [13']. dalle [14], passando in coordinate polari nel piano (z), con polo l'origine, o t t e n i a m o h(o,0) = h a — hk j ' / —2 a ti cos 0 + a 2 = 2 l g r g a2'/ — -2no cos(9 + 1 v(e,0) = [ 1 4 ' ] _ va — vk g 2 — 2 a g cosi? + a2 „ ~ 2 lg r g a2g2 — 2 « g cos (9 + 1 k da cui è facile risalire al flusso idrico catodico, m e d i a n t e la [1]: i n f a t t i = \ ì)q jq= 1 \ < > g / e = 1 a2-1 & (ha ha-) + k (ja ~ là lg r a2 — 2a cos •& + 1 [15] qk (0) = d a r e che, per la [1"], è (w h = — cr a (a2 —1) \ òo )o= 1 v l v k e quindi 1 gr a2 —2a coati + 1 i = i jk ds = (a) g(a2 — 1) (f^-ua) f lg 7? dfl 2a cos $ + 1 2 n a ( v a l g />' [16] d a cui la resistenza elettrica del dispositivo r = v v lg r 2 ti a [1200 a. crea § 5. studiamo ora il caso in cui ad un anodo cilindrico indefinito, parallelo alle generatrici dell'anodo (v. fig. 5). i)i conseguenza, per il flusso catodico, si ha a2 + 1 = %n cioè a = x0 + | x02 — 1 (dovendo essere a > 1). con tale valore di a, al catodo corrisponde nel piano (w) la circonferenza di centro nell'origine e raggio — < 1, ina t e r n a alla circonferenza t r a s f o r m a t a dell'anodo. allora, con procedimento p e r f e t t a m e n t e analogo a quello del § 4, si giunge alle relazioni h (x, y) h a h a 2 lg « lg (x-a)2+y2 [ax—l)2+a2y2 (x-a)2+y2 3 a v (x, y) = a , k lg — + v, [ , j i 2 lg a n {ax—l)2+a2y2 a [18] ìk (y ) = — òh 7>x x = x le equazioni fondamentali sono ancora quelle del paragrafo 1, con le stesse condizioni al contorno. risolveremo ancora il problema usando la rappresentazione del § 2: b a s t e r à f a r vedere che è possibile, trasformare, ancora mediante la [5], il dominio illimitato t r a t t a g g i a t o in fig. 4, in una corona circolare. usando come coordinate ridotte, il rapporto f r a le coordinate effettive e il raggio anodico, l'anodo, di raggio unitario e centro nell'origine si trasforma nella circonferenza di raggio unitario e centro nell'origine nel piano (w)-, per dimostrare l'asserto, basterà f a r vedere d e t t a x0 la distanza ridotta del catodo dell'origine, che è possibile determinare a in modo che la r e t t a [7] coincida col catodo; il che ovviamente comporta . lfl(«2-l) k(ha-hk) + kava~v«) lgfl (a2 — l ) 2 + 4a2ìf |-lg, che è massima per y — 0 (cioè nel punto in cui la normale al catodo per il centro dell'anodo interseca il catodo stesso; punto in cui le superfìci elettrodiche sono più vicine) decrescente al crescere di y in valore assoluto, nulla all'infinito. la fig. 6 m o s t r a l ' a n d a m e n t o ^ di qk lungo il catodo. f i g . 6 anche in t a l caso è immediato il calcolo della corrente totale / e della resistenza f. sempre tenendo presente la (1"), abbiamo <r(» ka.) ia (a2 1) lg a (a2 — l) 2 + 4 a2y2 e quindi i = i jk ds = (à) + =o 4ag(a2—1) f dy lg a 1 a k'j (a2—iy+ ±«y2tz a(va-vk) lg a lg « i ~ 2 no l> [20] [21] su a l c u n e d i s p o s i z i o n i e l e t t r o d i c h e , ecc. 205 conclusione. le disposizioni elettrodiche qui studiate per la prima volta riguardano s t r u t t u r e di elettrodi cilindrici ad assi paralleli di cui uno è incluso t o t a l m e n t e nell'altro, cilindri coassiali, eccentrici, elettrodi esterni l'un l'altro, piano-cilindrici, ecc. queste disposizioni polari possono ricorrere frequentemente nella prospezione geoelettrica utilizzante quali elettrodi fori di sonda t u b a t i anche parzialmente, ricorrono spesso nella elettrogeotecnica in processi eletta-osmotici bifilari introdotti dalla scuola russa (netushil-lomizé), sono dimostrative per quanto riguarda le caratteristiche di direzionabilità del nuovo processo dielettroiniezioni di leganti chimici. 36bilham.qxd 839 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words india – earthquakes – history 1. introduction throughout the invasions of different ethnic and religious entitites in the past two millennia the indian subcontinent has been known as hindoostan, hindustan or india in recognition of its unique isolation imposed by surrounding mountains and oceans. the northern, eastern and western mountains are the boundaries of the indian plate. the shorelines are the echoes of ancient plate boundaries. only in recent time have the separate nations of pakistan, india, and bangladesh subdivided the continental expression of the indian plate. in this article i shall use the term india to signify both the indian tectonic plate and the subcontinent of india. perhaps the most disappointing observation is that despite a written tradition extending beyond 1500 b.c. we know very little about indian earthquakes earlier than 500 years before the present, and records are close to complete only for earthquakes in the most recent 200 years. this presents a problem for estimating recurrence intervals between significant earthquakes, the holy grail of historic earthquake studies. certainly no repetition of an earthquake has ever been recognized in the written record of india and the himalaya, although great earthquakes in the himalaya should do so at least once and possibly as much as three times each millennium. the strain rate within the indian plate is observed to be less than 3 ns/yr (bilham and gaur, 2000) and the renewal time for earthquakes in the sub-continent may exceed many thousands of years, rendering it unlikely that earthquakes will have repeated during the time of written records. earthquakes in india and the himalaya: tectonics, geodesy and history roger bilham cooperative institute for research in environmental sciences (cires) and geological sciences, university of colorado, boulder, co, u.s.a. abstract the record of earthquakes in india is patchy prior to 1800 and its improvement is much impeded by its dispersal in a dozen local languages, and several colonial archives. although geological studies will necessarily complement the historical record, only two earthquakes of the dozens of known historical events have resulted in surface ruptures, and it is likely that geological data in the form of liquefaction features will be needed to extend the historical record beyond the most recent few centuries. damage from large himalayan earthquakes recorded in tibet and in northern india suggests that earthquakes may attain m = 8.2. seismic gaps along two-thirds of the himalaya that have developed in the past five centuries, when combined with geodetic convergence rates of approximately 1.8 m/cy, suggests that one or more m = 8 earthquakes may be overdue. the mechanisms of recent earthquakes in peninsular india are consistent with stresses induced in the indian plate flexed by its collision with tibet. a region of abnormally high seismicity in western india appears to be caused by local convergence across the rann of kachchh and possibly other rift zones of india. since the plate itself deforms little, this deformation may be related to incipient plate fragmentation in sindh or over a larger region of nw india. mailing address: dr. roger bilham, cooperative institute for research in environmental sciences (cires) and geological sciences, university of colorado, boulder, co 80309-0399, u.s.a.; e-mail:roger.bilham@colorado.edu 840 roger bilham in contrast, trench investigations indicate that faults have been repeatedly active both on the subcontinent (sukhija et al., 1999; rajendran, 2000) and within the himalayan plate boundary (wesnousky et al., 1999). the excavation of active faults and liquefaction features is likely to play an important role in extending the historical earthquake record of indian earthquakes in the next several decades. a feature of indian earthquakes for which numerical deformation data have recently been exhumed is that these data, once analyzed, have required substantial revision of earlier informed, but speculative, interpretations of the causal mechanisms of historic earthquakes. geodetic data have surfaced for the 1819, 1881, 1897 and 1905 earthquakes that have largely negated the conclusions of many learned articles. this obviously raises a cautionary flag: that conclusions concerning felt reports about earthquakes in history and prehistory have limited value in interpreting subsurface structure. i first give a brief overview of indian tectonics. i then describe catalogues and data that characterize indian earthquakes, and conclude with a number of case histories that discuss some of the important problems that have surfaced in studies of indian earthquakes, and that may be resolved by the discovery of further data. i conclude with a discussion of our current understanding of seismic hazard in india and the himalaya. 2. tectonic setting of india india is currently penetrating into asia at a rate of approximately 45 mm/yr and rotating slowly anticlockwise (sella et al., 2002). this rotation and translation results in left-lateral transform slip in baluchistan at approximately 42 mm/yr and right-lateral slip relative to asia in the indo-burman ranges at 55 mm/yr (fig. 1). because of complexities in the structural units at its northern, western and eastern boundaries these velocities are not directly observable across any single fault system. deformation within asia reduces india’s convergence with tibet to approximately 18 mm/yr (wang et al., 2001), and because tibet is extending east-west, convergence across the himalaya is approximately normal to the arc. arc-normal convergence across the himalaya results in the development of potential slip available to drive large thrust earthquakes beneath the himalaya at roughly 1.8 m/cy, hence earthquakes associated with, say, 6 m of slip cannot occur before the elapse of an interval of at least three centuries (bilham et al., 1997). slip across the 150-300 km wide plate boundary between asia and india in baluchistan is apparently partitioned between thrust and strike-slip components. for example, the 1931 mach ms = 7.3 earthquake was associated with 1 m of nw directed reverse slip on a fault that may have extended entirely through the crust. it was followed 4 years later by the ms = 7.7 strike-slip quetta earthquake on a subparallel fault less than 150 km nw of the mach event. the mach event slipped in a sense that effectively unclamped the subsequent quetta earthquake (ambraseys and bilham, 2003a). slip on the chaman fault further to the north in afghanistan in the past century, and possibly for a longer period, has been much less than 42 mm/yr according to seismic moment summation of observed seismicity (ambraseys and bilham, 2003b). although this may be the result of minor deformation in the northern afghan mountains, or unreported creep on the chaman fault, it is quite possible that the northern chaman fault system may be overdue for a large earthquake. slip in the indoburman ranges is also accompanied by strike-slip and thrust seismicity and although no recent large earthquakes have occurred on land, the north-south sagaing fault system is clearly strike-slip and the indo-burman ranges to its west the result of distributed east-west convergence. near the andaman islands slip is partitioned between thrust earthquakes to the west and beneath islands, and strike-slip faulting on the north andaman fault to their east (curray et al. 1979; 1982; ortiz and bilham, 2003) gps measurements in india reveal that convergence is less than 5 ± 3 mm/yr from cape comorin (kanya comori) to the plains south of the himalaya (paul et al., 2001). hence the indian plate should not be expected to host frequent seismicity. however, the collision of india has resulted in flexure of the indian plate (bilham et al., 2003). the wavelength of this flexure is of the order of 650 km and results in an approximately 450-m-high bulge near the central 841 earthquakes in india and the himalaya: tectonics, geodesy and history fig. 1. schematic views of indian tectonics. plate boundary velocities are indicated in mm/yr. shading indicates flexure of india: a 4 km deep trough near the himalaya and an inferred minor (40 m) trough in south central india are separated by a bulge that rises approximately 450 m. tibet is not a tectonic plate: it extends eastwest and converges north-south at approximately 12 mm/yr. at the crest of the flexural bulge the surface of the indian plate is in tension and its base is in compression. locations and dates of important earthquakes mentioned in the text are shown, with numbers of fatalities in parenthesis where known. with the exception of the car nicobar 1881, assam 1897 and bhuj 2001 events, none of the rupture zones of major earthquakes are known with any certainty. the estimated rupture zones of pre-1800 great earthquakes are shown as unfilled outlines, whereas more recent events are filled white. 842 roger bilham indian plateau, corresponding to the outer rise of an oceanic collision. normal faulting earthquakes occur north of the flexural bulge (e.g., possibly on 15 july 1720 near delhi) and deep reverse faulting occurs beneath its crest (e.g., the m = 6.3 21 may 1997 jabalpur earthquake). shallow reverse faulting occurs south of the flexural bulge where the indian plate is depressed (e.g., the m = 6.3 29 september 1993 latur earthquake, fig. 1). the indian plate is bent downwards by 4-6 km beneath the southern edge of the himalaya attaining depths of 18-20 km beneath the southern edge of tibet (fig. 1). stresses within the plate vary from tensile above the flexed neutralaxis to compressional below it. where no inplane end-loading prevails the position of the neutral axis lies theoretically half way through the thickness of the elastic plate. since in-plane stresses of the order of 500 bars exist (necessary to maintain the height of the tibetan plateau) this effectively means that the neutral axis rises above the plate south of the crest of the central indian bulge. the neutral axis descends into the plate just north of the bulge where it is initially flexed downward. the axis would descend to a path a little above half-way through the plate were it perfectly elastic, since the flexural stresses are much larger than the weak in-plane collisional stresses. however, normal-faulting in the upper surface of the plate near the ganges trough weakens the top surface of the plate thereby lowering the neutral axis, and plastic conditions near the base of the plate both raise the neutral axis, both thinning the effective elastic thickness and shifting the neutral axis to an unknown depth. eventually, when sufficient focal mechanisms are available from the descending plate, it may be possible to identify the location of the neutral axis from the absence of earthquakes near the axis, and from the difference in mechanisms above and below it. the presence of both flexural stresses and plate-boundary slip permits all mechanisms of earthquakes to occur beneath the lesser himalaya (fig. 1). at depths of 4-18 km great thrust earthquakes with shallow northerly dip occur infrequently that permit the northward descent of the indian plate beneath the subcontinent. earthquakes in the indian plate beneath these thrust events range from tensile just below the plate interface, to compressional and strikeslip at depths of 30-50 km (e.g., m = 6.6 20 august 1988, udaypur). a belt of microearthquakes and moderate earthquakes beneath the greater himalaya on the southern edge of tibet indicates a transition from stick-slip faulting to probable aseismic creep at around 18 km. this belt of microseismicity defines a small circle with radius 1695 km (seeber and gornitz, 1983; bendick and bilham, 2001). seismicity in tibet is largely shallow and is either normal faulting or strike-slip faulting. the flexural geometry of the indian plate is manifest as a standing wave fixed relative to southern tibet. stresses in the plate vary with time because the indian plate streams slowly though this flexural wave, bringing points within india towards, or away from, compressional or tensile failure. it is for this reason that the earthquakes that occur throughout central and northern india appear to have no distinctive spatial pattern. the flexural stresses are significantly larger than the in-plane stresses needed to sustain the elevation of the tibetan plateau, but their change with time is slow (mbar/yr). despite this their spatial change is large (up to 2 bars per km northeastward) and this results in an important imposed south-north spatial variation in stress (bilham et al., 2003). stress changes of less than 1 bar are known to trigger earthquakes. although stresses throughout most of ne india are everywhere close to failure, the triggering of earthquakes occurs partly from the movement of india through the flexural stress field, and partly from local stress perturbations caused by other tectonic, erosional or dynamic processes. 3. historical data sources and catalogues early earthquakes described in mythical terms include extracts in the mahabharata (≈ 1500 b.c.) during the kurukshetra battle (iyengar, 1994), and several semi-religious texts that mention a probable himalayan earthquake reputed to have occurred during the time of enlightment of buddha ca. 538 b.c. 843 earthquakes in india and the himalaya: tectonics, geodesy and history earthquake because it is alleged to have been followed by three years of aftershocks, but the absence of reports from other locations renders this of little value in estimating its rupture dimensions or magnitude. similarly the arrival of vasco de gama’s fleet in 1524 coincided with a violent sea-quake and tsunami that caused alarm at dabul (bendick and bilham, 1999). note that this portuguese port at latitude 17°34’ on the malabar coast is unrelated to debil above. this could have been a local event, but since it was not reported onshore it could have been the tsunami from a remote earthquake that occurred along the makran or gujarat coastlines. such accounts are thus of fragmentary value in quantifying earthquake locations and sizes. the emergence and disappearance of coastal tracts has sometimes been ascribed to earthquakes. a storm near cochin in 1341 caused an island to emerge, but inspection suggests this to be a common accretional feature of storms along the malabar coast (bendick and bilham, 1999). an island that sank in 1769 south of chittagong (oldham, 1883) may have undergone lateral spreading at the time of significant earthquake near there (seeber, pers. comm., 2003). in the mid 19th century some of these fragmentary data were collected successively in summaries of earthquakes by mallett, bairdsmith and oldham, but there followed more than a century of archival neglect when little archeological excavations in sindh and gujarat suggest earthquake damage to now abandoned harrappan cities. a probable earthquake around 0 a.d. near the historically important city of dwarka is recorded as a zone of liquefaction in archeological excavations of the ancient city (rajendran et al., 2003). the town of debal (dewal, debil, diul sind or sindi) near the current site of karachi was alleged to have been destroyed in 893 a.d. (oldham, 1883), but until recently accounts of its collapse and inundation were considered too vague to be taken seriously. rajendran and rajendran (2002) present a case that the destruction of debil was caused by an earthquake linked to the same fault system responsible for the 1819 and 2001 rann of kachchh earthquakes, however, ambraseys (2003) notes that oldham’s sources refer to daibul (dvin) in armenia, and that liquefaction 1100 years ago in western india must be attributed to a different earthquake. figure 2 shows the location of debil west of the indus delta in a 1690 map drafted by a.d. winter. other maps place it within the distributaries of the indus. yule and burnell (1903) describe debil’s 1000-year-long history, prior to its effective disappearance from accounts within a century of a second earthquake in its vicinity in 1668 (oldham, 1883; ambraseys, 2004). a single paragraph describes a massive earthquake in the kathmandu valley in 1255 (wright, 1877), which may have been a great fig. 2. maps in 1690 and 1740 show debil near the current location of karachi. other maps show it on a distributary of the indus. an earthquake occurred there in 1668 and another is alleged by thomas oldham (1883) to have occurred in 893 but the event he invokes occurred in an armenian town with a similar name (ambraseys, 2004). the city is last mentioned in the 18th century (yule et al., 1903) 844 roger bilham new information surfaced. the seismicity of the sub-continent has been summarized in compilations by chandra (1977), srivastava and ramachandram, (1985), rao and rao (1984) and by khattri (1987). recent interest in early earthquakes have engaged historians in india and elsewhere in a systematic search through urdu, arabic, tibetan, chinese, nepalese and european languages. two important publications summarize recent findings: iyengar and sharma (1998) report accounts in arabic, sanskrit and urdu sources and ambraseys and jackson (2003) provide new data from tibet and recently collated colonial records. data presented in these publications remain sparse but provide a skeletal framework of events on which to build a future quantitative assessment of historic indian earthquakes as new documents surface. a list of indian earthquakes is to be found in bapat et al. (1983) but this contains numerous entries that have been included uncritically from secondary sources, and for these reasons can be misleading. similarly, entries in the uncritical listing of dunbar et al. (1992) require careful evaluation before use. a useful and easily accessible compilation of information and resources for the study of indian earthquakes is a webpage maintained by stacey martin, http://asc-india.org/menu/gquakes.htm. relocated instrumental earthquakes are listed by engdhal et al. (1998). an important recent realization is that a sequence of significant earthquakes occurred throughout the west himalaya in the 16th century. the sequence started in kashmir in 1501, followed by two events a month apart in afghanistan and the central himalaya, concluding with a large earthquake in kashmir in 1555. the central himalayan 1505 earthquake may have been mw ≥ 8.2 based on its probable rupture area. it destroyed monasteries along a 500 km segment of southern tibet, in addition to demolishing structures in agra and other towns in northern india (jackson, 2002; ambraseys and jackson, 2003; bilham and ambraseys, 2004). a himalayan earthquake that damaged the kathmandu valley in 1668 is mentioned briefly (a single sentence) in nepalese histories but as with events in 1255 and 1408 no details are given (chitrakar and pandey, 1986). earthquakes in the 18th century are poorly documented. an earthquake near delhi in 1720 caused damage and apparent liquefaction but little else is known of this event (kahn, 1874; oldham, 1883). this event, from its location, could have been a normal faulting event, but because of the absence of damage accounts from the himalaya it may have been a himalayan earthquake. in 1713 a severe earthquake damaged bhutan and parts of assam (ambraseys and jackson, 2003). thirteen years later, in september 1737, a catastrophic earthquake is alleged to have occurred in calcutta. this is the most devastating earthquake to be listed in many catalogues of indian (and global) earthquakes but is actually a storm surge that resulted in numerous deaths by drowning along the northern coast of the bay of bengal. the handwritten ledgers of the east india company in bengal detail storm and flood damage to shipping, wharves, warehouses and dwellings in calcutta with an estimate of 3000 deaths by drowning (bilham, 1994). calcutta’s population at the time was approximately 30 000. a figure of 300 000 fatalities is often ascribed to this «fake quake» for which earthquake shaking was probably invoked in news reports as a metaphor for destruction, a possible description of the buffeting accompanying extreme wind velocities. the spire of st. annes church, calcutta, was blown down by these winds, but the masonry church survived. an approximate 10% increase in burials is recorded in its churchyard for 1737, an increase in deaths that year by fewer than two dozen. although the death-toll from drowning along the coast of southern bengal was presumably greater than the official estimates in calcutta, the fatality count of 300 000 is repeated only in accounts published in monthly magazines and newspapers in europe, and is not substantiated by official documents from any of the several administrative centers then functioning in bengal. india in the early 19th century was as yet incompletely dominated by a british colonial administration. remote administrators in distant parts of the india subscribed to newspapers and wrote verbose and sometimes extensive descriptions of their experiences which were typ845 earthquakes in india and the himalaya: tectonics, geodesy and history fig. 3. oldham, father and son, were both geologists in india. thomas oldham (left) compiled the first catalogue of indian earthquakes. richard (right) made definitive studies of individual earthquakes (1819, 1869, 1881 and 1897) in addition to identifying for the first time p-waves and s-waves, and the core of the earth. ically printed and circulated to each administrative outpost. an earthquake in india was something of a rarity and generated detailed letters from residents describing its effects. very often the same report would be copied verbatim from one newspaper and reported by another. few of the original letters have survived, but the earthquakes in kumaon in 1803, nepal in 1833 and afghanistan in 1842 were felt sufficiently widely to lead scientifically inclined officials to take a special interest in the physics and geography of earthquakes. mallett’s (1853-1855) global catalogues of earthquakes included several from india, with a special section devoted to the 1833 earthquake for which he discussed seismic propagation velocities. at about the time of the sequential publication of mallet’s global catalogue an army officer named baird-smith wrote a sequence of articles (1843a,b,1844) in the asiatic society of bengal summarizing data from several indian earthquakes and venturing to offer explanations for their occurrence. he was writing shortly after the first afghan war which had coincided with a major 1842 earthquake in the kunar valley of ne afghanistan (ambraseys and bilham, 2003b), which must have impressed him and others in the military service who were in nw india at the time. baird-smith’s accounts of other earthquakes include citations from his sources. the director of the geological survey of india, thomas oldham (1816-1878) published the first real catalog of significant indian events in 1883. his catalog includes earthquakes from 893 to 1869, and acknowledges the works of mallet and baird-smith. his important additions include verbatim textual extracts with references that permit verification and further work. his notes on some of the earthquakes form the first detailed studies of individual earthquakes in india. his son, richard. d. oldham (1858-1936), wrote accounts of four major indian earthquakes (1819, 1869, 1881 and 1897). he completed first his father’s manuscript on the 1869 silchar, cachar, assam earthquake which was published under his father’s name (oldham, 1884). he next investigated the mw = 7.9 december 1881 earthquake in the andaman islands, visiting and mapping the geology of some of the islands (oldham, 1884, 1885). he mistakenly located the event deep in the northern bay of bengal based largely on timing data from clocks in calcutta and madras. an analysis of the tsunami generatquake on the northern edge of the rann of kachchh close to what is now the india/pakistan border. the earthquake figures prominantly in lyell’s principles of geology (1830) as one of the first clear examples of geological uplift associated with an earthquake. oldham’s 1928 account refers to, but does not reproduce, baker’s map and profile from a leveling survey crossing the allah bund. this profile is key to quantifying the mechanism of the earthquake, and it is entirely due to its serendipitous discovery by oldham (1898) that we have access to it. the map had been accidently omitted in baker’s original 1846 publication by the editor. in a frontispiece to the geographical society of bombay in 1846 he apologizes for omitting the map and cross-section and promises to include the figure in subsequent issues, a promise that he failed to fulfill. oldham had discovered the map quite by accident when supervising a clean-up of the bombay office of the survey of india. in his discussion of the cause of the 1819 allah bund earthquake oldham speculates that the morphology across the natural dam measured by baker in 1846 was caused by subsurface faulting akin to that reported from japanese earthquakes in the early 20th century. assuming the surface morphology to be representative of co-seismic deformation during a single earthquake, baker’s 6 m crest-totrough observation is consistent with 11 m of slip on a north-dipping reverse fault terminating 0.5-2 km below the surface (bilham, 1999). however, recent geological studies in the region (rajendran and rajendran, 2001 and 2002) have raised the possibility that the observed morphology was a factor of two smaller than that reported by baker, and that its current elevation of < 3 m crest-to-base is caused partly by the 1819 event and partly by pre-1819 earthquakes. a difficulty in rejecting baker’s survey, a canal engineer of repute, is that he would have made vertical errors of less than a few cm in measuring topography over the 10 km width of the allah bund. thus an error of 23 m can be rejected. the cross-section that was intended to accompany baker’s account was drafted from a larger scale survey deposited with the sind government. the smaller version 846 roger bilham ed by this earthquake places it on the subduction zone west of car nicobar (ortiz and bilham, 2003). his account of the 1897 mw = 8.1 shillong plateau earthquake in assam (oldham, 1899) was exemplary, and according to richter provided the best available scientific analyses of available physical data on any earthquake at the time. in contrast to the care with which he investigated the geological, geodetic and geophysical aspects of the earthquake, oldham’s reports are thin on specific accounts of building damage which he felt were often exaggerated. despite the care with which he interpreted the intensity data available to him, his estimated intensities for the 1897 earthquake on a modified version of the rossi-forel scale are 1.5 to 3 intensity units too high in the epicentral region (ambraseys and bilham, 2003c). r.d. oldham’s accounts established a template for the study of earthquakes that occurred in india subsequently. the great earthquakes of 1905 kangra (middlemiss, 1910) and 1934 bihar/nepal (dunn et al., 1939) were each assigned to geological survey of india special volumes, but these never quite matched the insightful observations of oldham’s 1899 volume. investigations of the yet larger assam earthquake of 1950 were published as a compilation undertaken by separate investigators (e.g., poddar, 1953; ray, 1953; tandon, 1953). in many ways this proved to be the least conclusive of the studies of the 5 largest indian earthquakes 18191950. information available to indian authors on the effects of the earthquake were confined largely to a narrow corridor of information along the brahmaputra valley since access to tibet, burma, or the tribal regions south of the epicenter was unavailable. regrettably geologists did not make a thorough search for surface faulting in the epicentral region and geodesy near the epicenter was virtually non-existent. 4. uncertainties associated with the 6 june 1819 allah bund earthquake oldham wrote his account of the 1819 earthquake in kachchh in retirement in england (oldham, 1928). his monograph synthesized all the data available for the allah bund earth847 earthquakes in india and the himalaya: tectonics, geodesy and history of frontal fill and co-seismic slip could be tested with suitable excavations, or seismic profiles, of the northern edge of the bed of lake sindri. while excavations of sindri sedimentation might clarify the discrepancy between historic leveling and current morphology, the observation by rajendran and rajendran that two or more earthquakes caused incremental changes in the height of the allah bund requires downward revision of the 11 m estimate of coseismic slip to a more modest 5 m. any further reduction in the coseismic uplift of the allah bund can be rejected based on baker’s mapping of the elevation of the bed of the narra river since this would have been at river base-level before the earthquake, unaffected by previous earthquakes. the recent 26 january 2001 bhuj earthquake was associated with 3-6 m of slip (bendick et al., 2001). since this occurred on a 40 × 40 km rupture, and resulted in isoseismal intensity distributions throughout india similar to the 1819 earthquake (hough et al., 2002), it is tempting to assume that the two events had similar stress drops and local attenuation relationships, and somewhat similar geometry and magnitude. this would require the along-strike length of the allah bund earthquake to be shortened considerably below the > 100 km length first suggested by oldham and adopted by all later authors. in contrast, ambraseys and douglas (2004) favor a mw = 8.19 magnitude for this event, requiring rupture dimensions consistent with those inferred by oldham (1928). 5. himalayan earthquakes 1 september 1803 and 26 august 1833 these earthquakes occurred at the western and eastern ends of the inferred 6 june 1505 earthquake. the first of these events occurred during the opening battles of the 2nd war against the mahrattas. in late august 1803 a british army had laid seige to the fort and town of aligarh on the banks of the calini river (between the ganges and jumna) some 200 km from the himalaya. the commander of the british army, lt. general lake, writing to wellesley on 1 september indicates that the strength of the defences will require a one month seige. yet, not three published by oldham included a typographical error in the vertical scale, but it is unlikely that gross drafting errors would have been introduced. moreover, the accompanying map view of the river system is exact in many details compared to recent satellite photos suggesting that its execution was fastidious. several explanations can be invoked to reconcile the leveling data and current morphology. the first is that the uplift and subsidence morphology may have changed since the earthquake. for example, it is possible that baker’s measurements started at a lower vertical datum than that available to the rajendrans in 2000. according to burnes (1833) the footwall subsided by 1-3 m, with maximum subsidence near the scarp. burnes’s two handwritten accounts in the geological society of london describe slightly different views of the river cut through the allah bund in 1827 and 1828 that suggest it was evolving in response to the flood of 1826. currently the sediments of lake sindri slope upwards towards the southern edge of the allah bund. in the past 180 years sediments eroded from the front of the scarp, supplemented by sediments from the narra river in flood, would have filled any depression fronting the scarp along the northern shore of lake sindri resulting in a datum possibly 2 m higher than that available to baker. the rajendrans were unable to map vertical profiles northward into the sindh province of pakistan hence it may not have been possible to recover baker’s northern datum. a second possibility is to assume that the southern edge of the allah bund has now been eroded 1 km or more northward by monsoon winds and floods driving waves across the 3050 km wide fetch of open water to its south. in 1827 the crest of the scarp was fewer than 600 m from its southern edge. ablation of the crest of the allah bund may have also occurred although this is considered unlikely because rajendran and rajendran report the survival of surface geodetic monuments installed in 1860. the subsidence deformation profile, now buried beneath lake sindri, may in fact be better preserved than the uplift profile, and this, at some future date, may provide additional constraints of slip in the 1819 earthquake. the depth 848 roger bilham days later lake writes again to wellesley that they have successfully stormed the town with minor loss of life. in contrast to lake’s silence on the earthquake that occurred between the two letters, a member (thorn, 1818) of the besieging army describes violent shaking for 2 minutes at midnight accompanied by collapse several buildings. the earthquake appears in part responsible for the successful capture of the fort, either from damage to its walls or distress to inhabitants, although specific details are lacking. a summary of materials available for the 1803 event is recorded by ambraseys and jackson (2003) who assign it an approximate magnitude of mw = 7.5. this was later revised to mw = 8.09 by ambraseys and douglas (2004) using additional materials, who place it at the weatern end of the 1505 rupture. the 1833 erathquake almost exactly 30 years later occurred at the eastern end of the 1505 rupture. in contrast to the extensive damage reported from tibet in 1505, few accounts of damage have surfaced from tibetan sources for these two earthquakes, suggesting that they were significantly less severe than the 1505 event. the one exception to the apparent silence from tibet for the 1833 earthquake are accounts of damage from members of the nepal quinquennial tribute delegation returning from beijing who brought with them accounts of the increasing damage they encountered as they approached the northern nepal border (bilham, 1995). the ms ≥ 7.7 august 1833 earthquake near kathmandu consisted of three shocks (bilham, 1995). the first caused alarm and the second, 5 hours later, brought most people from their homes. the mainshock (mw = 7.69, ambraseys and douglas, 2004) occurred 15 minutes later causing widespread structural damage in india and nepal, but the combined loss of life in india and nepal was only 500 because most people were already in the open, alarmed by the two foreshocks. newspaper reports of these events are abundant as are scientific commentaries in journals in india and europe. the isoseismals from this earthquake suggest an epicentral region similar to, or at the western end of, the 1934 ms = 8.1 rupture, which together with the multiple shocks in the event, raises a number of interpretational difficulties. the earthquake did not affect western nepal and its magnitude is too small to have had much effect on releasing strain accumulated since the 1505 earthquake. however, had it occurred on the plate boundary «detachment fault» it could not have released much of the slip available to drive the larger magnitude 1934 earthquake a century later. since the 1934 earthquake is believed to have released up to 8 m of slip, and since potential plate-boundary slip is renewed at a rate of less than 2 m per century, the 1833 rupture would have had to occur on different fault systems or to have slipped on a small patch contiguous to the 1934 rupture. one possibility is that one or more of the three 1833 earthquakes occurred deep in the indian plate where both strike-slip and thrust faulting can occur, or that all three earthquakes were m ≥ 7.5 thrust earthquakes at the northern edge of the 1934 rupture zone, similar to those that have occurred in the past several decades in western nepal. 6. cachar 10 january 1869 this m > 7 earthquake occurred in the sylhet region (silchar) of what is now ne bangladesh. although numerous accounts of this earthquake were compiled by the oldhams the data are insufficient to estimate a causal fault or a precise magnitude. ambraseys and douglas (2004) estimate mw = 7.39. the most likely fault to be associated with this earthquake is the eastern extremity of the dauki fault, as hinted by godwin-austin (1869) who was undertaking firstorder triangulation in the region at the time. few first hand accounts of the event exist outside the covers of oldham (1884) but the occasional letter describing its effects surfaces. an example is reproduced below: «the earthquake has not been a single shock but has lasted, on and off, a month nay it is said some of the shocks have gone on rocking for five minutes by the watch till some people were literally sea sick. the bazaar at silchar (the capital of cachar) is the handsomest street anywhere east of calcutta and it has been engulfed. i.e. it has gone bodily down not at once but in a series of descents, some ten feet at a 849 earthquakes in india and the himalaya: tectonics, geodesy and history time. the river in silchar in the cold weather runs about 50 feet below the level of its banks which are only dried mud, and the country has been so rocked up and down till the river has cut its banks right down to its own level and the plain at silchar is all one debris with no particular river anywhere. the commissioner told me on monday last that it was officially reported that the only thing left standing at silchar was clarke’s bridge, and it was the most wonderful sight that ever was seen.» (clarke, 1869) the dangers of speculating on a causal fault or mechanism for the 1869 earthquake are highlighted by radical errors of a century of interpretations of the 1897 earthquake that were shown to be baseless once the geodetic signal was assessed in 2001. it is possible that enough of the 1869 geodetic survey network was in place prior to the earthquake to render its remeasurement even now of value. 7. the 31 december 1881 mw == 7.9 car nicobar earthquake this earthquake caused minor damage in the andaman island penal colony and generated a tsunami that was observed throughout the bay of bengal but not along the burmese coast. the tsunami did no damage around the bay of bengal where tide gauges recorded a maximum amplitude of 0.8 m (oldham, 1884). an analysis of five tide gauge records reveals that the earthquake was mw = 7.9 ± 0.2 and occurred on an east-dipping thrust fault below and to the west of car nicobar, an island at 9°n midway between the andaman and nicobar islands (ortiz and bilham, 2003). gps measurements at port blair indicate oblique convergence of the plate boundary (paul et al., 2001). the earthquake is believed to have occurred on the interface between the indian and andaman plates and the inferred mechanism of westward slip of the hanging wall slip is consistent with slip partitioning between the dipping subduction zone, and the strike-slip west andaman fault east of car nicobar. a feature of this earthquake is the inferred presence of a region of minor slip ne of the main rupture zone. this may have been a secondary earthquake triggered by the mainshock. its timing would have to have occurred within a few minutes of the mainshock for it to have produced the sea wave observed at port blair. local populations were concentrated in only two islands and therefore there is no corroboration of this inferred northern region of submarine faulting which occurred between them. it is probable that offshore corals may be of use in reconstructing an extended history of earthquakes in the andaman-nicobar islands. the island of car nicobar is believed to have been raised and tilted during the 1881 event. deformation models that do not include this uplift result in an inappropriate estimate of the observed tsunami runup on the island (ortiz and bilham, 2003). 8. 1897 shillong plateau earthquake the 1897 great assam earthquake (ms = 8.0) for more than a century was believed to have occurred on a thrust fault dipping gently to the north. some considered it to have been a himalayan basal thrust. we now recognize that the earthquake occurred on a reverse fault dipping steeply to the south. slip during the 1897 earthquake may have exceeded 16 m, resulting in 10 m uplift of the northern edge of the plateau. oldham clearly recognized the value of surface deformation as a quantitative measure of what happens in an earthquake, but the analytical tools to interpret these data were not to emerge for a further half century. in 1897 correspondence with the surveyor general, sydney burrard, oldham requested a geodetic re-survey of the shillong plateau. the work undertaken by j. bond covered only the southern half of the plateau and was considered by burrard (1898) to be inferior in accuracy to normal survey standards because numerous triangles did not close precisely. (a test of survey accuracy is whether angles in a triangle after correcting for spherical excess add up to 180°). we now know that these misclosures were probably due to postseismic adjustments in the epicentral region continuing after the earthquake. the 1897 displacement results available to oldham were ambiguous: either the plateau had bodily expanded and risen with no southward motion, or it had risen with850 roger bilham out strain and moved southwards by slip on the dauki fault bordering its southern edge. realizing this, oldham urged resurvey of the northern half of the plateau but he was destined never to see the data since it was completed in 1936, the year he died (davison, 1936). analyses of angle changes between 1869 and 1936 reveal that oldham’s instincts were correct. the fault that slipped in 1897 was a 110-km-long blind reverse fault beneath the northern edge of the plateau, dipping southward at 45° with 16 ± 5 m of slip between 9 and 39 km (bilham and england, 2001). we named this unmapped fault the oldham fault in his honour. the earthquake raised the northern edge of the plateau roughly 10 m. the causal fault is believed to have cut right through the lower crust but did not approach closer than 9 km to the earth’s surface. oldham (1899) photographed secondary faulting of up to 10 m at the western end of the plateau on the chedrang fault. the 1.6 km mean-height of the plateau surface appears to have been driven to its current position by reverse faults acting on both its northern and southern edges. three dissected terraces border the northern edge of the plateau that may be separated by active faults, but none have been mapped by geologists possibly due to the thick forest cover that makes access difficult. enigmatic aspects of this earthquake concern the uniqueness of the shillong plateau which permits contraction of the indian plate within 80 km of the himalaya convergence zone, thereby reducing the productivity of himalayan earthquakes. an uplift rate of 2.5 ± 1 mm/yr can be calculated from the current elevation of the plateau, and from the date of its initial elevation estimated from changes in sedimentation styles in northern bangladesh. this convergence requires a convergence rate of 4 ± 2 mm/yr, or approximately a factor 4 less than the india/tibet convergence rate (bilham and england, 2001). the only large historical earthquake known in the bhutan himalaya is the 1713 event described in ambraseys and jackson (2003) and the precise location of this event is far from certain. the southern edge of the shillong plateau is truncated by the dhauki fault. in order that the surface of the plateau be horizontal the dhauki fault must also act as a reverse fault, and this raises additional concerns. no historical earthquakes have been recorded on this fault, and many previous studies interpret the fault as a dextral strike-slip fault. although the fault may have slipped differently in the past there is little doubt that reverse slip is now the prevailing mechanism, and has been so for the past one or two million years. earthquakes beneath the plateau have thrust mechanisms parallel to the strike of the oldham fault at depths of more than 35 km. the 1869 cachar earthquake described by the oldhams may have occurred at the eastern end of the dhauki fault (godwinaustin, 1869; oldham, 1884). a recent review of instrumental records of the 1897 earthquake reveals its teleseismically derived magnitude to be ms = 8.0 (ambraseys, 2000) effectively the same as its geodetic seismic moment of m = 8.1 (bilham and england, 2001). a re-evaluation of oldham’s 1897 isoseismal intensity data supplemented by additional data from newspapers, diaries, books and government reports unavailable to oldham, reveal significantly reduced areas for contours of i > viii isoseismals, but similar areas for lower intensity shaking. the newly evaluated intensities include data from tibet and bhutan (ambraseys and bilham, 2003c). 9. kangra 1905 m == 7.8 earthquake occurring just 7 years after the 1897 assam earthquake, the kangra event found the geologists of india eager to map the details of the event. the earthquake had its oddities – in particular a prominent epicentral region of rossiforel shaking of intensity viii to x near kangra and dharmsala and an island of viii shaking almost 250 km to the se near dehra dun. this, and an artificially-inflated estimate for magnitude (richter rounded gutenberg’s calculated magnitude upward from m = 7.8 to m = 8; see ambraseys and bilham, 2000), led several investigators to assume that rupture may have extended more than 350 km along strike. although geodetic measurements existed along the probable southern edge of the rupture, no remeasurements were made after the earthquake except near the remote region of high ac851 earthquakes in india and the himalaya: tectonics, geodesy and history celerations near dehra dun. no horizontal deformation was detected and a vertical deformation signal, though discussed by many subsequent investigators, has recently been dismissed as an artifact of the leveling process (bilham, 2001). hence there is little evidence to believe that its rupture exceeded 200 km. first-order triangulation prior to the earthquake is limited to the southern edge of the inferred rupture zone and it appears not to have been re-measured since its initial measurement in 1845. an interpretation of a gps occupation of some of these points in 2001 is currently underway. intensities of this event are re-evaluated by ambraseys and douglas (2004). 10. discussion the above review of early earthquakes and case histories of some of the larger earthquakes, omits numerous smaller ones felt by individuals or communities. the larger ones form a patchy history that may be complete for the past 200 years, but which is certainly missing many large earthquakes before then. an important question is whether there is scant information on pre-muslim or medieval earthquakes because there were few events, or whether it is because we have no records of them. although this question cannot be answered from the historical record alone we may consider extreme scenarios as a guide to future searches to resolve the issue. aggravating our lack of knowledge of previous earthquake is the curious observation that none of the numerous earthquakes that have occurred in india and the himalaya in the past several centuries have produced surface ruptures, with the exception of secondary surface faulting in the 1897 earthquake (oldham, 1898), and surface fractures of the 1993 latur earthquake (seeber et al., 1996). in 1505 and 1892 surface faulting was observed at the surface along the pakistan/afghanistan border (ambraseys and bilham, 2003a) but no surface faulting has ever been reported in the himalayan and indo-burman plate boundaries, despite geological indications that surface rupture of the frontal faults has occurred in the past (wesnousky et al., 1999). the primary ruptures of the largest mid-plate events of the past two centuries, the 7.8 < m < 8.1 1819 allah bund, the m = 8.1 1897 shillong, the m = 7.3 1931 mach, and the m = 7.6 2001 bhuj earthquakes have all been on blind thrust faults, dipping at approximately 45°, terminating 1-9 km below the surface, and extending to the base of the crust. thus, although they have caused widespread destruction in the historical record, the geological manifestation of their passage is limited to secondary cracks and liquefaction phenomena that tell us little about their mechanisms. such knowledge about rupture geometries as we have obtained for these earthquakes, with the exception of the most recent, has been derived almost entirely from sparse geodetic data. the conclusion to be derived from this absence of surface ruptures in the subcontinent is that many historical earthquakes occurred on faults that are currently unmapped, and the corollary is that there may exist many hundreds of subsurface faults potentially awaiting re-activation for which we have no geological intelligence. the mechanisms of the numerous smaller shocks that appear in historical indian catalogues must be inferred from modern focal mechanisms in those same geographic settings. the inherent problem in doing this is that focal mechanisms in some parts of india, e.g., the himalayan foothills, vary with depth. surviving intensity data are rarely adequate to distinguish between deep and shallow shocks. 10.1. intensity and attenuation estimates of intensities for the two largest earthquakes of the past two centuries (1905 and 1897) have revealed that previous estimates of rossi-forel or modified mercalli intensity tend to exaggerate high intensity shaking by 1-3 intensity units (ambraseys and bilham, 2003c) whereas lower intensities (v-ii) are estimated with reasonable accuracy. the reason for this exaggeration is that the style of building construction suffers significant damage at intensities around vii-viii and that subsequent shaking produces somewhat imperceptible additional damage (ambraseys and bilham, 2003b). 852 roger bilham even quite recent intensity estimates can be suspect. for example, the 1989 udaypur earthquake in southern nepal resulted in both nepali (pandey and nicolas, 1989; dikshit and koirala, 1989) and indian (sinha, 1993) intensity and engineering damage studies. the resulting intensity contours show an abrupt jump of 1-1.5 intensity units at the nepal/india border where the two studies abut. the re-evaluation of the felt intensity reports for the 1833, 1897, 1905, 1934 and 1950 earthquakes on a common scale is an important priority, that has been partly completed by ambraseys and jackson (2003), since it may reveal the details of seismic hazards in intervening regions where future himalayan earthquakes are anticipated. currently more than three scales have been used to report these data. rossi-forel, modified mercalli and msk intensities, with caveats imposed by their specific inapplicability to indian building methods. in some areas acceleration damage can only with difficulty be distinguished from collapse caused by liquefaction-induced foundation failure. in 1897 regions of extensive liquefaction and catastrophic lateral spreading follow the banks of the main rivers and result in building damage from foundation collapse, rather than grades of shaking intensity. ambraseys and bilham (2003c) separated liquefaction observations from msk assignations based on shaking intensity lest they bias the areas of isoseismal contours. 10.2. himalayan recurrence interval the recurrence interval for great himalayan earthquakes remains conjectural since the historical record is probably incomplete even for the past 500 years. a summary of those events for which we have data is depicted in fig. 4a,b, although both the rupture area and the amount of slip are unknown for each of these events. the figure suggests that the western himalaya may have slipped in a sequence of events between 1501 and 1555, and that since then there have been relatively modest earthquakes, insufficient to release the 1.5-1.8 m per century of accumulating convergence revealed from geodetic measurements. the largest of the pre-1900 earthquakes, the 6 june 1505 kumaon/western nepal earthquake (jackson, 2002; ambraseys and jackson, 2003), may have exceeded mw = 8.2, and its recurrence now would result in a similarsized earthquake (9 m of slip along a 500-600 km rupture zone). damage in northern india was considerable during the 1505 event and it is likely that its recurrence would damage many of the large cities along the ganges and jumna rivers through shaking, and from the effects of extensive liquefaction. smaller seismic gaps are evident in kashmir, in sikkim and in assam for which the historical record is ambiguous or absent. assuming that seven to ten great ruptures permit the slip of the entire himalayan arc, and a recurrence interval of 500 years (≥ 9 m slip on 200-300 km long, 70-90 km wide, ruptures) we should anticipate m ≥ 8 earthquakes occurring every 50-70 years. insufficient earthquakes have occurred recently to match this estimate. two great earthquakes only that approach this severity have occurred in the past 200 years (1934 and 1950), and two others are known in the previous 300 years (kashmir, 1505, 1555). no great earthquake has occurred for 53 years. almost two thirds of the himalaya remain unbroken by recent earthquakes, suggesting that several seismic gaps may currently exist. finally, the summation of seismic moment from all known earthquakes since 1505 along the entire arc yields a slip rate less than 30% of that derived from the current geodetic slip rate (bilham and ambraseys, 2004). from these arguments we may form one of two conclusions: that one or more great himalayan earthquakes are overdue, or that our understanding of the way in which the northern plate boundary slips is flawed. the case for the imminent failure of a seismic gap is hampered by the absence of any well documented recurrence interval, or indeed any evidence for regular failure of the himalayan plate boundary. the absence of constraint permits the extreme view, for example, that failure occurs in clustered sequences, as may have occurred in the western himalaya 1400-1555. if indeed this sequence released accumulated displacements in the western himalaya five centuries ago, then a case can be made for the immediate recurrence 853 earthquakes in india and the himalaya: tectonics, geodesy and history fig. 4a,b. a) the himalaya describe a small circle that subtends an arc of approximately one radian symmetrically about the thakola graben in tibet. important mid-plate earthquakes named. microseismicity follows this small circle (from engdhal et al., 1998) and is negligible to its south where great earthquakes are located. b) the arcuate box is expanded (and straightened) and it shows time distance plot of approximate rupture areas of large earthquakes in the past eight centuries plotted along the arc (approximate transverse-mercator projection of linear transverse-km versus angular distance). one or more large earthquakes appear to be overdue in kashmir, kumaon and western nepal. we know of no earthquakes in sikkim, and the 1897 assam shillong earthquake may have reduced the slip potential in eastern bhutan. pre-1500 earthquakes are known with less certainty. trench studies have revealed slip on the frontal thrusts at the beginning of the 15th century at several locations west of dehra dun (senthil kumar, per. comm., 2004) and surface rupture on frontal thrusts in eastern nepal may correspond to the earthquake that destroyed kathmandu in 1255 (rockwell, pers. comm., 2004). a b 854 roger bilham of one or more 9 m slip events, based on the current convergence rate of 18 mm/yr. the region of the 1505 earthquake has been hitherto termed the central himalayan seismic gap by khattri and tyagi (1983) and khattri (1987). alternatively, the assumption that great earthquakes are essential features for plate boundary slip may be incorrect – the himalaya may fail in smaller events that incompletely rupture the plate boundary. these smaller events might be considered to be similar to the chichi earthquake of 1999 that ruptured through a midlevel segment of the accretionary wedge, rather than through a basal detachment. such events may accommodate convergence without translating the entire himalaya southward over india. the major 1833, 1885 and 1905 earthquakes (7.5 < mw < 7.8) may have been examples of these «out-of-sequence thrusts». one of the most troubling observations, that might be accounted for by out-of sequence thrusting or deep events, is that no recent himalayan earthquake has ever resulted in a recorded surface rupture. such ruptures have obviously occurred in recent geological time, on the main frontal thrusts for example (wesnousky et al., 1999), signifying either that recent earthquakes are anomalously small, or that the search for surface rupture may not have been exhaustive. if some, but not all, great earthquakes rupture the himalayan frontal thrusts we cannot hope to quantify the recurrence interval from these events using paleosismic fault-trenching methods. out of sequence thrusts cannot represent a steady-state condition for himalayan slip since it would not explain the geological observation of occasional slip on the basal thrust systems and main frontal thrusts. however, it is possible that excessive recent erosion of the himalayan foothills may have upset the uniform taper of the himalayan accretionary wedge such that adjustments are now underway that result in a predominance of high-level thrusting interspersed with infrequent basal thrusts. historical studies have an important role in distinguishing between these various scenarios, yet it is unlikely that we shall ever find a history that is complete acrossand alongthe himalaya, even near the kathmandu and kashmir valleys that have been administered continuously by a record-keeping population for the past thousand years. for this reason, trench investigations of faults and liquefaction features will be necessary to fill in the record. in practice, the subsurface record of strong-ground motion is complete, but its interpretation may be non-unique, suitable conditions may not exist everywhere for it to be recorded, and it is insensitive to small earthquakes whose recurrence may be quite damaging. 10.3. mid-plate recurrence intervals although numerous micro-earthquakes, and many damaging shocks have occurred in the past several centuries in india, the geodetic stability of the plate, and the absence of recent mountain ranges indicates that earthquakes should not recur repeatedly on the same fault during the written history of india. yet archeological observations in india suggest earthquakes may have repeatedly destroyed early settlements there, especially in westernmost india. rajendran et al. (1996) present evidence for reactivation of the fault causal to the latur earthquake. the town of latur itself, like many indian villages, is a mound city built on the ruins of previous cities. the occurrence of the m = 7.6 bhuj 2001 earthquake less than two centuries after the m ≥ 7.8 allah bund 1819 earthquake has been considered by some investigators to represent a short recurrence interval for earthquakes in mid-plate india. such an appraisal would be incorrect because the two events did not, of course, rupture the same fault, nor even the same fault system. the two earthquakes occurred on the ancient kachchh rift zone, an east-west fault system that can be traced structurally from near karachi to ahmedabad. in a study of the 1819 event it was concluded that contiguous future faulting might be anticipated, with specific concern that rupture to the west would create hazards for karachi (bilham, 1999). as it happened, rupture in 2001 occurred 2-4 rupture lengths to the east of the 1819 earthquake. hence there is a possibility that the entire kachchh rift may be converging. in fact geodetic data suggest that the rift north of the 855 earthquakes in india and the himalaya: tectonics, geodesy and history bhuj region may have converged by more than 1 m since 1856 (sri devi et al., 2003). should this be the case, additional large earthquakes may be anticipated both to the east and west of the 1819 and 2001 earthquakes. the observed geodetic convergence of the rann of kachchh by 9 ± 3 mm/yr is approximately 2-3 times larger that the entire geodetic convergence rate between northern and southern india (paul et al., 2001). two explanations for this have been proposed: one is that a 400 km wide continental «sindh flake» is in the process of fracturing from the ne edge of the indian plate (stein et al., 2002), the other is that the interconnected ancient rift systems of northern india define a small northern plate « the harappan plate» that allows a large triangle in ne india between the central himalaya and bhuj to converge with the main body of the indian plate to the south (bilham et al., 2003). support for either mechanism of plate fragmentation is weak, and future geodetic observations are needed to resolve the extent of plate deformation in ne india. 11. conclusions the tectonic setting of india’s collision with asia is now reasonably well characterized from recent seismicity and geodetic studies of relative motion at their plate boundaries. direct measurements across and within the himalaya reveal a locking line beneath the edge of the tibetan plateau and the absence of creep to its south (bilham et al., 1995, 1998, 2001), implying that the advance of the himalaya over the indian plate proceeds largely through the recurrence of great plate boundary earthquakes. earthquakes within the indian plate are attributable to the superposition of the nw compressional stress of collision, on the stresses arising from plate flexure. the depth and mechanisms of recent earthquakes reflect the sense of these combined stresses. a several millennia-long written record in india has revealed few major earthquakes prior to the past two centuries. this is partly due to the fact that extant records have yet to be searched rigorously for earthquakes, but is in part due to the corruption of potentially valuable records and their loss through fire, war and decay. despite their sparseness it is likely that documents on historical earthquakes will surface in tibetan, urdu and arabic records that will change current estimates of the significance of seismic gaps in the himalaya, and may change our understanding of earthquakes within the indian continent. our current understanding of himalayan earthquakes is such that we may calculate potential slip in several segments of the plate boundary, but we cannot estimate the timing of future events. making assumptions about the probable completeness of the historical seismic record we can estimate estimate minimum slip potential based on the time since the last known earthquake (bilham et al., 2001). this has moderate relevance to planning for future earthquakes. the eventual establishment of recurrence intervals for himalayan ruptures will require a combination of serendipitous historical studies and geological trench investigations of faulting and earthquake-induced liquefaction features. acknowledgements r.s. cox of the american philosophical society provided a copy of c.b. clarke’s letter concerning the silchar earthquake. i thank the organizers of the erice conference on historical earthquakes for inviting my participation. i also thank an anonymous reviewer for amusing and insightful comments on the first draft of this article. the study was funded by the national science foundation ear-0003449. references ambraseys, n.n. 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(1877): history of nepal (reprint, ranjan gupta, calcutta, 1966, pp. 271). yule, h. and a.c. burnell (1903): hobson-jobson: a glossary of colloquial anglo-indian words (new delhi, john murray), pp. 1021 (reprint, 1995, 320-321). annals n.6/2003 ok 23/04 1271 annals of geophysics, vol. 46, n. 6, december 2003 key words b-value – seismology – statistical methods – synthetic-earthquake catalogs 1. introduction the gutenberg-richter law (from now on gr law) (gutenberg and richter, 1954) is certainly one of the most remarkable and ubiquitous features of worldwide seismicity. in the most common form it reads n m a bmlog = -^ h8 b (1.1) where n is the number of events with magnitude m, and a and b are two constant coefficients. the scientific importance of the gr law is linked to its apparent ubiquity. it emerges in a variety of tectonic settings and depth ranges, in seismic catalogs ranging from a few months to centuries, and in natural as well as induced seismicity. this ubiquity makes the gr law very useful in seismic hazard studies, and very attractive from a theoretical point of view. in past years, many efforts were devoted to understanding the physical meaning of such a power law, but the conclusions are diverging. some authors (e.g., bak and tang, 1989; ito and matsuzaki, 1990), by assuming a presumed universality of the parameter b and the implicit scale independence in eq. (1.1), propose a model of self-organized criticality (soc) to explain earthquake occurrence. a review and new insights on the estimation of the b-value and its uncertainty warner marzocchi and laura sandri istituto nazionale di geofisica e vulcanologia, bologna, italy abstract the estimation of the b-value of the gutenberg-richter law and its uncertainty is crucial in seismic hazard studies, as well as in verifying theoretical assertions, such as, for example, the universality of the gutenberg-richter law. in spite of the importance of this issue, many scientific papers still adopt formulas that lead to different estimations. the aim of this paper is to review the main concepts relative to the estimation of the b-value and its uncertainty, and to provide some new analytical and numerical insights on the biases introduced by the unavoidable use of binned magnitudes, and by the measurement errors on the magnitude. it is remarked that, although corrections for binned magnitudes were suggested in the past, they are still very often neglected in the estimation of the b-value, implicitly by assuming that the magnitude is a continuous random variable. in particular, we show that: i) the assumption of continuous magnitude can lead to strong bias in the b-value estimation, and to a significant underestimation of its uncertainty, also for binning of ∆m = 0.1; ii) a simple correction applied to the continuous formula causes a drastic reduction of both biases; iii) very simple formulas, until now mostly ignored, provide estimations without significant biases; iv) the effect on the bias due to the measurement errors is negligible compared to the use of binned magnitudes. mailing address: dr. warner marzocchi, istituto nazionale di geofisica e vulcanologia, via d. creti 12, 40128 bologna, italy; e-mail: marzocchi@bo.ingv.it 1272 warner marzocchi and laura sandri in this frame, the fractal geometry distribution and the earthquake dynamics are the spatial and temporal signatures of the same phenomenon. on the other hand, several lines of empirical evidence dispute the universality of the gr law. in fact, in spite of a «first order» validity of the gr law with a constant b-value ≈ 1, significant spatial and temporal variations in the bvalue seem to take place (miyamura, 1962; healy et al., 1968; pacheco and sykes, 1992; kagan, 1997; wiemer and mcnutt, 1997; wiemer and wyss, 1997; wiemer et al., 1998; wiemer and katsumata, 1999). these variations are usually attributed to many processes, such as fault heterogeneity (mogi, 1962), the stress level imposed on rocks (scholz, 1968), and pore pressure variations. another important discrepancy of the hypothesis of the universality of the gr law is that the latter seems to hold only for a finite range of magnitudes (e.g., pacheco and sykes, 1992; pacheco et al., 1992; scholz, 1997; triep and sykes, 1997; knopoff, 2000). for example, kagan (1993, 1994) found that a gamma distribution better matches the requirement of a maximum seismic energy released by an earthquake (cf. wyss, 1973). in all the cases reported above the b-value should not be a constant in the earthquake catalog. the empirical validation of the «universality» hypothesis, as well as the identification of spatial and temporal changes, passes through the estimation of the b-value and its uncertainty. different formulas were proposed in the past (e.g., aki, 1965; utsu, 1965, 1966; shi and bolt, 1982; bender, 1983; tinti and mulargia, 1987) which take into account the unavoidable binning of the magnitudes in different ways. as a matter of fact, aki’s formulas, which consider magnitude a continuous random variable, are still the most used. implicitly, this means that, at least for instrumental measurements, the effect of the binning is considered negligible, and that all the formulas mentioned above give almost comparable estimations. in this paper, we review the most diffuse formulas so far proposed and investigate analytically and numerically on the main properties of such estimations. in particular, we estimate the reliability and the possible bias of the formulas as a function of the number of data and of the measurement errors in the magnitude. here, we do not consider the estimations provided through the least squares technique. in fact, even though this approach was still employed in many recent scientific papers (pacheco and sykes, 1992; pacheco et al., 1992; karnik and klima, 1993; okal and kirby, 1995; scholz, 1997; triep and sykes, 1997; main, 2000), the use of the least squares technique does not have any statistical foundation (e.g., page, 1968; bender, 1983). 2. estimation of b and bv t through the maximum likelihood technique in the first papers that described the maximum likelihood (ml) estimation (aki, 1965; utsu, 1965), the magnitude m was considered a continuous random variable (rv). if eq. (1.1) holds, the probability density function (pdf) of m is lnf m b 10 10 10 10 bm bm bm min max = ^ ^h h (2.1) where mmin and mmax are, respectively, the minimum and the maximum magnitude allowed. if mmax >> mmin, eq. (2.1) becomes m 10 b m m min.lnf b 10=^ ^ ]h h g (2.2) note that the passage from eq. (2.1) to eq. (2.2) requires that, in practice, the gr law holds for a range of magnitudes mmax – mmin ≥ 3. this assumption might be questionable in studying variations of the b-value in smaller ranges (e.g., knopoff, 2000). the ml estimation of eq. (2.2) consists of choosing the b-value which maximizes the likelihood function (fisher, 1950), that is ln b m10 1 thresh ) = -n t t^ `h j (2.3) where nt is the sampling average of the magnitudes, and mthresh is the threshold magnitude 1273 a review and new insights on the estimation of the b-value and its uncertainty which usually corresponds to the minimum magnitude for the completeness of the seismic catalog. hereinafter, the symbol ^ distinguishes the estimation by the true value of the parameter. we have also attached an asterisk to b* in order to distinguish it from another estimation which we shall introduce later. the uncertainty is estimated by (aki, 1965) n b b =v ) )t t t (2.4) where n is the number of earthquakes. a major contribution was provided later on by shi and bolt (1982) who provided a new formula to estimate the error of the b-value . b n n m 2 30 1 ( ) b sb i i n 2 2 1) = v n = ) ! t t t t ^ ` h j (2.5) where n is the number of earthquakes. compared to eq. (2.4), eq. (2.5) provides a reliable estimation also in the presence of possible (time and/or spatial) variations of the b-value (shi and bolt, 1982). as a matter of fact, the magnitude is not a continuous variable and it is not devoid of measurement errors. in practical cases, the uncertainties on the measured magnitudes lead to the use of «binned» magnitudes, i.e. the magnitudes are grouped by using a selected interval ∆m. for instance, for instrumental measurements, the magnitude interval used for the grouping is ∆m = 0.1; for magnitude estimation of historical events, the grouping can even be ∆m ≥ 0.5. in spite of such unavoidable binning, the «continuous» aki’s (1965) formulas (eqs. (2.3) and (2.4)) are probably the most employed in practical applications to estimate the b-value and its uncertainty (e.g., jin and aki, 1989; henderson et al., 1994; öncel et al., 1996; mori and abercrombie, 1997; utsu, 1999; hiramatsu et al., 2000; gerstenberger et al., 2001; klein et al., 2001; vinciguerra et al., 2001). in such cases, an implicit assumption is that the binned magnitudes can be considered a continuous rv. while this approximation can be strictly justifed only for ∆m → → 0, in practice it is usually assumed also for ∆m = 0.1 (instrumental magnitudes). in the following sections, we consider separately the effects due to the binning of the magnitudes and the measurement errors on the estimations of the b-value and its uncertainty. we remark that a wrong choice of the completeness threshold magnitude mthresh can also cause a significant bias, as shown by wiemer and wyss (2000). here, we do not deal with this issue, but it should be kept in mind in any practical analysis. 3. influence of the binned magnitudes in this section, we first study the biases introduced by the binning of the magnitudes in the estimation of the b-value and its uncertainty made through eqs. (2.3) and (2.4). then, we check through numerical simulations the reliability of some formulas reported in scientific literature which either assume the magnitudes as a continuous rv or take properly into account the binning of the magnitudes. 3.1. effects of the binned magnitudes on aki’s (continuous) formulas the use of eq. (2.3) produces a biased estimation of the real b-value, because of two factors: i) the average µ of a continuous rv with a power law distribution is different from the average of the same «binned» rv; ii) mthresh ≠ mmin. hereinafter we will call these biases θ1, and θ2, respectively. the influence of θ1 was estimated by bender (1983). she found that the sample average nt computed from binned data is systematically higher than the true value µ. this is due to the fact that the real (continuous) magnitudes in the interval mi − ∆μ/2 ≤ m < mi + ∆μ/2 are not symmetrically distributed around the central value mi. moreover, she showed that the bias θ1 is negligible for ∆m = 0.1, while it is very important for larger ∆m (for example, ∆m = 0.6), that might be necessary to evaluate the b-value for historical catalogs. 1274 warner marzocchi and laura sandri the bias θ 2 was considered by the first paper which addressed the use of the binned magnitudes (utsu, 1966), suggesting a slight modification of eq. (2.3). since the lowest binned magnitude, i.e. the threshold magnitude, contains all the magnitudes in the range mthresh − ∆μ/2 ≤ m < mthresh + ∆μ/2, then mmin = = mthresh − ∆μ/2 < mthresh (e.g., bender, 1983). then, eq. (2.3) becomes . ln b m m10 2 1 thresh = -n ∆ t t^ `h j: d (3.1) remarkably, this «corrected» formula was not largely employed. utsu himself, in a recent paper (utsu, 1999), reported eq. (2.3) to estimate the bvalue. a major exception is the works making use of zmap (e.g., wiemer and benoit, 1996; wiemer and mcnutt, 1997; wiemer and wyss, 1997, 2000) and aspar (reasenberg, 1994) codes, which estimate the b-value through eq. (3.1). yet surprisingly the correction reported in eq. (3.1) is rarely explicitly mentioned in the scientific literature (e.g., guo and ogata, 1997). as a matter of fact, we suspect that, in some cases, the correction reported in eq. (3.1) is used in the analysis without explicitly mention of it in the manuscript. for instance, gerstenberger et al. (2001) quoted aki’s (1965) formulas (eq. (2.3)) in the manuscript, but the code zmap written by one of the authors (see above) contains the «corrected» formula given by eq. (3.1). in any case, we argue that this correction could have not been largely employed also because the influence of the bias θ 2 is erroneously considered negligible. indeed, the difference between b)t and bt , respectively the «non-corrected» and «corrected» estimation, is certainly considerable because in a power law distribution the average µ is very close to the minimum value of the distribution m= + ln ln b m dm b 10 10 10 1 min b m m m min min $= =n 3 -#^ ] ^ h g h (3.2) in particular, 2 . ln b b m m m 10 2 thresh thresh 2 = = = + i n n ∆ ) m∆ t t t t^ ` `h j j: d (3.3) for ∆m = 0.1 (as for instrumental magnitudes), and .m 0 38thresh.-nt (obtained by eq. (3.2) with b = 1) we obtain θ2 ≈ 0.13. another crucial aspect, until now completely neglected, concerns the effect of the bias θ 2 on the estimation of the uncertainty given by eq. (2.4) and the «corrected» form which reads . n b b =vt t t (3.4) the positive bias θ 2 has two competitive effects. it leads to an increase in the estimated uncertainty; in fact, comparing eqs. (3.4) and (2.4) we obtain b 1b b 2 = +v i v)t t tt td n (3.5) therefore, > .bbv v)t tt t on the other hand, the bias θ 2 leads to an increase of the dispersion of the rv b)t around its expected value. by neglecting the bias θ 1, the true variance of b)t is lnb var. e .= ln e b b m m10 1 10 b thresh thresh 2 2 2 4 2 = = v n n n n n ) ) ) ) t t t t t t j l k kk _ ^ ` ` ` ^ n p o oo i h j j j h r t s s s s 9 v x w w w w c (3.6) where var ( nt ) is the variance of the rv nt . in the same way, we have . ln e b b e m m m m 10 1 2 2 10 b thresh thresh 2 2 2 4 2 $ $ = = + + v n n n n n ∆ ∆ lnb var. t t t t t t j l k k_ ^ ` ` ` ^ n p o o o i h j j j h r t s s s s s 9 v x w w w w c (3.7) 1275 a review and new insights on the estimation of the b-value and its uncertainty the validity of eqs. (3.6) and (3.7) deserves further explanations. in particular, these equations assume that e ( b)t ) = b) and e( bt) = b, respectively. if we take the expected value of taylor’s expansion around the true value µ of eqs. (2.3) and (3.1), we see that these assumptions hold only for small deviations of nt , i.e. for large datasets. numerical investigations have shown that the biases are negligible for datasets with 50 or more earthquakes. by comparing eqs. (3.6) and (3.7) we obtain b 1b b 2 2 = +v i v) t t td n (3.8) therefore, > .b bv v)t t from eqs. (3.5) and (3.8), we can conclude that the true dispersion of the rv b bv) )t t^ h increases more than the increase in the estimation of the uncertainty bv )t t . in other words, eq. (2.4) provides an underestimation of the true dispersion. 3.2. binned formulas after the correction suggested by utsu (1966), bender (1983), tinti and mulargia (1987) provided formulas to estimate the bvalue, by properly taking into account the grouping of the magnitudes. remarkably, besides very few exceptions (e.g., frohlich and davis, 1983), these formulas were almost ignored in subsequent applications. we argue that the reasons are mainly of a technical nature. bender’s (1983) formula, for example, can be solved only numerically. moreover, in her analysis she gave more emphasis to the bias θ1 introduced by the use of the continuous approximation (eq. (2.3)), concluding that the latter provides almost unbiased estimations of the b-value if the magnitude interval for the grouping is ∆m = 0.1. a definite improvement to the estimation of the b-value was provided by tinti and mulargia (1987). their formula reads 1 lnb p 10 = ln m∆ tm t ^ _ h i (3.9) where p m m 1 thresh = + -n ∆ t j l k k n p o o (3.10) and the associated asymptotic error is npln m p 10 1 b = v ∆ tm t t ^ h (3.11) where n is the number of earthquakes. in this case, we think the very scarce use of these formulas was probably due to some kind of crypticism of the paper. 3.3. numerical check in order to check the reliability of the formulas described above, we simulate 1000 seismic catalogs, for different catalog sizes. the magnitudes m are obtained by binning, with ∆m = 0.1 (as for the instrumental magnitudes), a continuous rv distributed with a pdf given by eq. (2.2); in other words, mi is the magnitude attached to all the synthetic seismic events with real continuous magnitude in the range mi – 0.05 ≤ m < mi + 0.05. in fig. 1a,b we report the medians of b)t , bt and btmt calculated in 1000 synthetic catalogs as a function of the number of data, for the case b = 1 and b = 2. to each median is attached the 95% confidence interval, given by the interval between the 2.5 and 97.5 percentile. from fig. 1a,b, we can see that the estimation btmt (tinti and mulargia, 1987) is bias free, also for a small dataset. as regards the continuous formulas, with and without correction (respectively eqs. (3.1) and (2.3)), we can see that the bias θ2 reported in fig. 1a,b is comparable to the theoretical expectation given by eq. (3.3). the corrected estimation bt is undoubtedly much closer to the real b-value. the slight underestimation of bt (much less than 1% of the real b-value) is due to the bias θ1 previously discussed (bender, 1983). therefore, at least for ∆m = 0.1, θ1 can be neglected (e.g., bender, 1983), but θ2 is certainly relevant. in order to evaluate the reliability of the estimations of the uncertainty, it is necessary to compare each estimation with the true dis1276 warner marzocchi and laura sandri fig. 2a,b. f test values (see eq. (3.12)) for the case of fig. 1a,b (see text). open and solid squares are, respectively, the f test values relative to the uncertainties given by eqs. (2.4) and (2.5). open and solid circles are the f test values relative to the same uncertainties calculated for bt instead of b )t (i.e. eq. (3.4)). asterisks are the f test values relative to the uncertainty calculated through eq. (3.11). the dotted line represents the critical value to reject the null hypothesis at a significance level of 0.05. a b fig. 1a,b. medians and 95% confidence bands of b)t (dashed lines), bt (dotted lines) and btmt (thicker solid line) from 1000 synthetic catalogs, as a function of the catalog size, for the cases b = 1 (a) and b = 2 (b). the solid thin line represents the true b-value. a b 1277 a review and new insights on the estimation of the b-value and its uncertainty persion of estimation of the b-value around its central value. in particular, we compare the dispersion of the b-value estimation around its average with the average of the estimated uncertainties, through the fisher test (e.g., kalbfleisch, 1979) = average of the square of the uncertainty variance of the estimation of the valuef b (3.12) the null hypothesis is that the two variances are the same. the results are reported in fig. 2a,b. the most interesting result is relative to the systematic rejection of the f test for the non-corrected case, given by eq. (2.4). this means that the use of the non-corrected estimation b)t (eq. (2.3)) leads to an underestimation of its real dispersion. this fact is very important because it might suggest variations in the b-value whereas they do not exist. note that the amplitude of this bias is correctly estimated by eq. (3.8). on the contrary, all the other estimations of the uncertainty do not have significant biases. it is important to note that the error estimated through eq. (2.5) (shi and bolt, 1982) is unbiased if the b-value is estimated through the corrected (eq. (2.3)) and non-corrected (eq. (3.1)) formulas. 4. effects of the measurement errors in order to take into account the measurement errors, we consider the «real» magnitude m + as a sum of two independent rvs m m= + f + (4.1) where m is the earthquake magnitude devoid of measurement errors distributed with a pdf given by eq. (2.1), and ε simulates the measurement errors distributed as a gaussian noise. the choice of the normal distribution has been discussed in detail by tinti and mulargia (1985), and rhoades (1996). in the case the two rvs, m and ε, are independent, the statistical cumulative distribution of m + can be obtained as follows (ventsel, 1983; tinti and mulargia, 1985; rhoades, 1996) d dg m f m f m m m m m 1 2 min max = f f 3+ + # #c ^ ^m h h r t s s s v x w w w (4.2) where g( m + ) is the cumulative distribution of m + , f1(m) is given by eq. (2.1), and f2 (ε) is n (0, 2vf ) distributed. by differentiating with respect to the variable m + , we obtain the pdf .exp$ $ +$ ln ln ln ln ln g m b b m b m m b m m b erf erf 2 1 10 10 10 2 1 10 2 10 2 10 2 10 max min bm bm 2 2 2 min max $= + + + v v v v v f f f f f + + + + c ^ ^ ^ ^ ^ m h h h h h r t s s s r t s s s ; v x w w w v x w w w e z [ \ ]] ]] ) _ ` a bb bb 3 (4.3) in the case mmax >> mmin, eq. (4.3) becomes .$ exp $$ ln ln ln ln m m b m m b erf b b g 10 2 10 1 10 10 2 1 10 2 2 1 min bm 2 2 min $ + + = v v v f f f + + + ^ ^ ^ ^ c h h h h m r t s s s ; v x w w w e z [ \ ]] ]] ) _ ` a bb bb 3 (4.4) the ml estimation of eq. (4.4) gives + 0= + ln ln ln ln ln exp ln b m b n m m b m m b erf 1 10 10 10 2 10 1 2 10 2 10 min min min i i i n 2 2 2 2 2 1 $ $ + + + + n v r v v v v v = f f f f f f + + / ! ^ ^ ^` ^ ^ ^ h h hj h h h r t s s ss r t s s ss v x w w ww v x w w ww z [ \ ] ] ]] ] ] ]] z [ \ ]] ]] _ ` a bb bb _ ` a b b b b b b b b (4.5) 1278 warner marzocchi and laura sandri fig. 3a,b. as for fig. 1a,b, but relative to the case with added measurement error to the magnitudes. the standard deviations of the measurement errors are σε = 0.1, 0.3, 0.5 (from left to right). a b 1279 a review and new insights on the estimation of the b-value and its uncertainty a b fig. 4a,b. as for fig. 2a,b, but relative to the case with added error to the magnitudes. the standard deviations of the measurement errors are σε = 0.1, 0.3, 0.5 (from left to right). 1280 warner marzocchi and laura sandri where +n / is the sample average of m + . the first three addenda represent the estimation of the bvalue without measurement errors (cf. eq. (2.3)), while the term inside the braces is the additional part which takes into account the measurement errors. 4.1. numerical check the contribution of the term inside the bracket in eq. (4.5) to the b-value estimation for different σε can be studied by adding measurement errors to the synthetic catalogs generated as described before. in fig. 3a,b we report the results for different σε. compared to the case without measurement errors (fig. 1a,b), the only observable difference is a global negative bias present in all the estimations for b = 2 and σε = 0.5. this can be due to the improper binning of ∆m = 0.1 in the presence of larger measurement errors (σε = 0.5). as regards the estimation of the uncertainties bvt t and bv )t t , we operate as in the previous case, by plotting the results of the f test (fig. 4a,b). the results are almost the same as those obtained for the case without measurement errors (fig. 2a,b). note that also in this case the use of eq. (2.4) leads to a significant underestimation of the uncertainty. 5. final remarks the purpose of this paper was to review the main concepts of the estimation of the b-value and its uncertainty, and to provide new insights on the reliability of different formulas reported in scientific literature. the detection and quantification of possible significant biases in such estimations is a crucial issue in many scientific applications, such as hazard studies and any attempt to test the constancy or the universality of the b-value. here we studied analytically and numerically the possible biases introduced by the use of the binned magnitudes and measurement errors on different formulas. we found that, in general, the influence of the measurement errors appears negligible compared to the effects of the binned magnitudes if improperly dealt with. in particular, we found that the most commonly used formulas (aki, 1965; see eqs. (2.3) and (2.4)), which assume the magnitude as a continuous random variable, produce a strong bias in the estimation of the b-value, and a significant underestimation of its uncertainty. this means that any spatial or temporal variation of the b-value obtained by eqs. (2.3) and (2.4) has to be regarded with a strong skepticism. on the contrary, we show how the same continuous formula with a small correction to take into account the binned magnitudes (eqs. (3.1) and (3.4)) drastically reduces the biases of the b-value and of its uncertainty. we also verified that other very simple formulas provided in the past (e.g., tinti and mulargia, 1987) do not have significant biases and they work very well also with a small dataset contaminated by realistic measurement errors. acknowledgements the authors thank stefano tinti for the helpful comments. references aki, k. 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(received march 25, 2003; accepted august 1, 2003) layout 6 1 annals of geophysics, 63, 3, gd327, 2020; doi:10.4401/ag-8180 m2 constituent of ocean tide loading displacements from vlbi cont14 hourly sessions kamil teke1 (1) hacettepe university, department of geomatics engineering, 06800, ankara, turkey article history: received april 16, 2019; accepted october 14, 2019 abstract several studies prove that ocean tide loading (otl) displacements can be observed with space geodetic techniques. in this study, the amplitudes and greenwich phase lags for each coordinate component, i.e., radial, west, and south of the principal lunar semidiurnal tide, m2 of otl displacements were estimated at the very long baseline interferometry (vlbi) sites of the 15 days long continuous vlbi campaign, cont14, carried out by the international vlbi service for geodesy and astrometry (ivs). in the estimation of the amplitudes and greenwich phase lags of the m2 tidal constituent, hourly vlbi station coordinate time series were used as observations derived through analyzing 1 hour vlbi sessions of the cont14 campaign. in the analysis of hourly sessions of the cont14 campaign, to derive accurate hourly station coordinates, troposphere delays estimated from daily sessions were reduced from the observations a priori to the analysis. the estimated amplitudes and greenwich phase lags of the m2 constituent of otl displacements were compared with the predictions the state-of-the-art ocean tide models, among others, fes2012 [lyard et al., 2006; carrère et al., 2012], fes2014 [carrère et al., 2016], and tpxo8 [egbert and erofeeva, 2002; egbert et al., 2010]. both the amplitudes and the phases between cont14 estimates and ocean tide models agree well for the m2 tide at all the sites and in most of the coordinate components. the rms misfits of the m2 tide of otl displacements in all coordinate components between cont14 and ocean tide models over coastal sites are found about two times larger than those of inland sites. this result confirms the modeling insufficiencies in shallow waters of ocean tide models which cause an accuracy restriction of otl displacement predictions around coastal regions. keywords: vlbi; troposphere zenith delays; cont14 hourly sessions; ocean tide loading displacements; m2 constituent. 1. introduction the seafloor pressure variations due to the ocean tide loading (otl) cause position and tidal frequencydependent harmonic displacements on the earth crust, the so-called otl displacements. otl displacements can be predicted by convolution software, e.g., olfg/olmpp [scherneck, 1991], spotl [agnew, 1996], nloadf [agnew, 1997], gotic2 [matsumoto et al., 2001], carga [bos and baker, 2005], loaddef [martens et al., 2016] which convolve the elastic load green’s function over the gridded values of global ocean tide models such as schw81 [schwiderski, 1980], tpxo8 [egbert and erofeeva, 2002; egbert et al., 2010], fes2012 [lyard et al., 2006; carrère et al., 2012], or fes2014 [carrère et al., 2016] for the whole oceans. green’s function formulates the elastic response of the earth’s crust to the surface load that depends on the angular distance to the load as well as earth structure [farrell, 1972]. the sparse spatial resolution of global tide models and modeling insufficiencies in shallow waters have been proved to considerably restrict the accuracy of otl displacements around coastal regions (up to ~150 km distance from the coast) [e.g. agnew, 1997; khan and scherneck, 2003; bos and baker, 2005; penna et al., 2008; yuan and chao, 2012] whereas tidal heights derived from global ocean models do agree within 2-3  cm in deep oceans (shum et al., 1997). further details and discussions on the calculation of otl displacements from ocean tide models and errors in the otl displacements propagated from ocean tide models are provided, e.g., in farrell [1972, 1973], pagiatakis [1990, 1992], scherneck [1991, 1993], agnew [1996, 1997], scherneck and bos [2002], and baker and bos [2003]. several studies prove that otl displacements can be observed with space geodetic techniques. in literature, two main approaches are proposed for resolving the tidal harmonic constituents from the observations of space geodetic techniques, static and kinematic. in the static approach also called harmonic parameter estimation approach [e.g., penna et al., 2015] real (in-phase) and imaginary (out-of-phase) components of otl displacements tidal constituents are included in the observation model along with daily station coordinates as part of daily solutions. then, the daily estimates of tidal constituents and their covariance information are stacked in a combined solution, e.g. using a kalman filter as often implemented in global navigation satellite system (gnss) analysis [e.g., schenewerk et al., 2001; dach and dietrich, 2001; allinson et al., 2004; king et al., 2005; thomas et al., 2007; yuan et al., 2013] and in several very long baseline interferometry (vlbi) analysis [e.g., schuh and moehlmann, 1989; sovers, 1994; haas and schuh, 1998; scherneck et al., 2000; petrov and ma, 2003]. yuan et al., [2013] estimated otl displacements 3d components of eight principal harmonics at semi-diurnal and diurnal tidal periods by means of analyzing the observations of globally distributed 456 continuous gps stations since 1996 till 2011. they used the precise point positioning technique [ppp, zumberge et al., 1997] on daily gps batches where the tidal constituents were treated as additional parameters. then, they combined the resultant daily estimates of tidal constituents with their variance-covariance matrices. among other results, yuan et al. [2013] found the accuracy of their lunar only constituent estimates, m2, n2, o1, and q1 in horizontal and vertical components better than 0.12 mm and 0.24 mm, respectively. analyzing more than 3 million vlbi observations of 3126 sessions observed from april 1980 to january 2002 and considering 40 stations, petrov and ma [2003] showed that the vlbi technique is capable of observing ocean tide loading displacements with the average accuracy of ~0.5 mm and ~1.7 mm in the horizontal and radial coordinate components, respectively. for 28 colocated sites petrov and ma (2003) found that vlbi has a better agreement in vertical displacement amplitudes at 8 principal tidal waves than gps [schenewerk et al., 2001] if compared to the got00.2 (ray 1999) ocean tide model values. in the kinematic approach, station coordinate time series and their formal errors are estimated at sub-daily intervals (usually in 1-2 hour batches) without reducing the otl displacements from the observations a priori to the parameter estimation. then, the amplitudes and phases of otl displacements tidal constituents are extracted through harmonic analysis of sub-daily station coordinate time series, thereby treating the station coordinates as independent tide gauge measurements. the kinematic approach is implemented in several gnss analyses, [e.g, by baker et al., 1995; dragert et al., 2000; khan and tscherning, 2001; khan and scherneck, 2003; vey et al., 2002; king, 2006; yun et al., 2007; melachroinos et al., 2008; vergnolle et al., 2008; penna et al., 2015; martens et al., 2016]. dragert et al., [2000] estimated the position time series of the coastal gps site holb (canada) using 4 weeks of continuous gps data for 3 hour and daily solutions. according to dragert et al. [2000], un-modeled parts of ocean loading displacements propagate to daily averages of station positions as minimal biases. besides, they found that introducing ocean loading corrections in daily gps solutions and constraining e.g. position to daily average, the vertical tidal motion propagates into the hourly troposphere delay estimates, thereby biasing the hourly zenith delay estimates by up to 1 cm. khan and tscherning [2001] estimated differential amplitudes of m2 and n2 between two gps stations (fair and chi3) from hourly solutions of standard relative gps positioning technique and found similar m2 and n2 differential amplitudes to those of the got99.2 [ray, 1999] ocean tide model. due to the fact that estimating troposphere zenith delay in hourly intervals absorbs and eliminates the loading signal [dragert et al., 2000; khan and tscherning, 2001] fixed troposphere zenith delays to the saastamoinen [1972] model in addition to daily l3 ambiguities in their hourly solutions. as a follow up study, khan and scherneck [2003] could also separate differential vertical and north-south amplitudes and phases of the kamil teke 2 m2 tide from zenith total delay (ztd) estimates through hourly solutions of gps observations at the sites, fair and chi3 in alaska during a period of 49 days and validated their results using those of the got99.2 ocean tide model. vey et al. [2002] investigated the impact of ocean tide loading corrections on gps ztd estimates by means of analyzing the observations of 3 hour sessions with the contribution of six gps stations over a period of 3 days and estimated ztd with and without introducing otl displacements using the csr4.0 ocean tide model [eanes and shuler, 1999]. by means of comparing the correlation between the otl vertical position displacements with the ztd differences of these two solutions as a rule of thumb, vey et al. [2002] concluded that 4.4 cm station height error would cause an error of 1 cm ztd. similarly, based on the vlbi observations of the cont11 campaign, teke et al. [2013] found about 1 cm zenith delay differences between the analyses of 24 hours and 2-hour sessions as well as the radial components of station positions differ by about 2 to 4 cm depending on the number of observations and their sky coverage in 2-hour sessions. the main purpose of this study is to investigate the possibility of estimating m2 constituent of otl displacements from the vlbi observations carried out during the 15 days long continuous vlbi campaign, cont14 and investigate the level of agreement of the estimated m2 constituent of otl displacements with the predictions of the ocean tide models fes2012 [lyard et al., 2006; carrère et al., 2012], fes2014 [carrère et al., 2016] and tpxo8 [egbert and erofeeva 2002; egbert et al., 2010], which were generated from global hydrodynamic simulations that assimilate tide gauge and satellite altimetry data. troposphere delay is the largest portion of error in the reduced observations (observed minus computed vector) of radio-frequency space geodetic techniques. relatively to the other geophysical effects, troposphere delays cannot be reduced from the observations at the desired level with the widely-used present troposphere delay models e.g. saastamoinen [1973], davis et al. [1993], böhm et al. [2006], and chen and herring [1997]. the original value of this study is reducing the relatively accurate troposphere delays, estimated through the analysis of 24hour sessions, from the observations of 1 hour sessions a priori to the parameter estimation. this contributes to the reliable estimation of sub-daily varying geodetic and geophysical parameters. for this study, among others the sub-daily varying geophysical parameter is selected as m2 tidal signal of otl displacements and the investigation is focused on the possibility of estimating this sub-daily parameter with such a small data set (about 360 positions for a station evenly distributed at hourly epochs covering 15 days) i.e. cont14 campaign. the reason of choosing the m2 tide as a metric for the success of observing the otl displacements from the 15 days long continuous vlbi observations, cont14 is that it has the largest amplitude among those of other principal semi-diurnal and diurnal tides of otl displacements that leads to more distinct comparisons with those of ocean tide models. in this study, cont14 hourly sessions were obtained by dividing each of the original 15 1-day sessions into 24 1-hour segments. an intrinsic parameter estimation strategy for the analysis of 1 hour vlbi sessions of the cont14 campaign was used by means of reducing the troposphere delays estimated from 24-hour sessions and using a cont14 specific terrestrial reference frame (trf) and earth orientation parameters (eop) series as a priori values. it is worth to note that this paper does not aim to attain better accuracies of m2 otl constituent from vlbi cont14 campaign than those of ocean tide models. the experimental work presented in this paper provides investigations into how reliable and accurate m2 tidal signals of otl displacements can be estimated from the sub-daily i.e. hourly vlbi station positions over 15 days. only m2 tide is estimated in this study and the station displacements caused by the remaining; long period, semi-diurnal and diurnal tides are calculated from fes2014 model and reduced from the hourly station coordinates a priori to the estimation. because the 15 days long sub-daily i.e. hourly station coordinate time series over cont14 campaign does not satisfy the minimum period of rayleigh criterion [foreman, 1977] to distinguish between neighboring frequencies. as an example, according to the rayleigh criterion to distinguish between m2 tide with a period of 12.42  hour and n2 tide (12.66 hours) at least 27.3 days of data is needed. in section 2 the vlbi cont14 campaign is briefly described, and then a detailed explanation of the analysis of vlbi hourly sessions is presented focusing on the reduction of the 24-hour session troposphere delays from the observations of 1 hour sessions a priori to the adjustment. in section 3 the estimation procedure of the m2 tide amplitudes and phase lags of otl displacements at vlbi sites of cont14 from the hourly station coordinate time series is introduced, and the level of agreement between the estimates of this paper and those of the fes2012, fes2014, and tpxo8 ocean tide models is presented. in section 4 the impact of a priori ocean tide model errors on the estimated m2 tide displacements is discussed in the scope of the analysis of 1 hour sessions over 15 days. 3 ocean loading m2 tide displacements from vlbi kamil teke 4 2. cont14 campaign and analysis of vlbi hourly sessions the international vlbi service for geodesy and astrometry [ivs, nothnagel  et al.,  2017; behrend,  2013; schuh and behrend, 2012] carries out continuous vlbi campaigns over two weeks with the state-of-the-art vlbi technology every three years. one of the scientific opportunities enabled by these campaigns is to address the discrepancies between ocean tide models and vlbi observations at principal diurnal, semidiurnal, and even ter-diurnal periods. in this study, the cont14 campaign is analyzed, which was continuously observed by the ivs from 6 (0  ut) to 20 (24  ut) may 2014 to provide the highest accuracy of geodetic parameters that vlbi can currently provide. the cont14 observation network consists of 17 vlbi stations located at 16 sites. homogeneous distribution of the network sites is provided as far as possible through selecting 6 sites in the southern hemisphere (figure 1). for further information on the cont14 campaign, e.g. scheduling, performing correlation, technical and scientific prospects, readers are referred to the web site https://ivscc.gsfc.nasa.gov. the geodetic vlbi observatories (stations) contributing to the cont14 campaign are listed in table 1. the 17 vlbi stations are separated into inland (7) stations and coastal (10) stations, depending on whether the station is located within 150 km from the coastline or not (table 1). although the vlbi stations at zelenchukskaya (russia) and matera (italy) are located within 150 km from the coastline, they are considered as inland stations in this paper due to the very low tidal amplitudes at the black sea and the mediterranean sea. in contrast to 24-hour session analysis, the main disadvantage of analyzing the observations of 1 hour vlbi sessions is that reliable station positions cannot be estimated due to the high correlation between troposphere delays and station positions [e.g. rothacher and beutler, 1998; teke et al., 2013]. to show the amount of shared variances (degree of linear relationship) between the zwd and radial coordinates estimated simultaneously from 1 hour sessions of the cont14 campaign, zwd from 24 hour sessions were subtracted from those of 1 hour sessions, δzwd1h-24h=zwd1h-zwd24h. then, pearson correlation coefficients were calculated between δzwd1h-24h and the radial coordinates of the stations for each 1 hour session. all negative correlations between zwd1h-zwd24h and radial estimates from 1 hour session analysis at the vlbi stations of cont14 are found between -0.74 and -0.93. the p-values of these correlations are below 0.05 indicating that they are all statistically significant. it is inferred figure 1. the vlbi stations contributing to the cont14 campaign. https://ivscc.gsfc.nasa.gov that, as a rule of thumb, 1 cm zwd1h variation propagates into the radial positions approximately 1.5 to 2.5 cm when 1 hour session is analyzed as well as these two parameters are simultaneously estimated (see e.g. figure 2 for kokee vlbi station and supplementary material for the other stations). according to teke et al. [2013] depending on the number and sky distribution of the observations over the hourly sessions, correlations between antenna radial positions and troposphere delays are varying. they also state that these correlations between these two parameters can be mitigated if homogeneously distributed adequate number of observations are carried out at each antenna and at each hourly session. consequently, the classical gauss-markov least squares adjustment cannot decorrelate these two parameters when they are estimated simultaneously in sub-daily e.g. 1 hour intervals. thus, troposphere delays and antenna coordinates propagate into each other and resulting in unreliable estimates. for example, the cyan and grey lines in figure 3 show the very noisy zwd and radial coordinate components estimated simultaneously from the analysis of 1 hour sessions at the vlbi station kokee and for other stations readers are referred to the supplementary material of this paper. to overcome this restriction, external troposphere slant delays δ𝐿 estimated from daily (24 hour) sessions were reduced from the observations of hourly sessions a priori to the adjustment and residual troposphere delays were not estimated. the troposphere delay model from davis et al., (1993) δ𝐿(𝛼,𝜀) = zhd 𝑚ℎ (𝜀)+zwd 𝑚𝑤 (𝜀)+ 𝑚𝑔 (𝜀)[𝐺𝑛 cos(𝛼)+𝐺𝑒 sin(𝛼)] (1) was used to compute the slant delays δ𝐿 so as to reduce them from the vlbi observations of hourly sessions. in equation (1) 𝛼 denotes the azimuth of the observation, 𝜀 the outgoing elevation angle from the local horizon, zhd the troposphere zenith hydrostatic delay, zwd the zenith wet delay, 𝑚ℎ the troposphere hydrostatic mapping 5 ocean loading m2 tide displacements from vlbi table 1. geodetic vlbi stations participating in the cont14 campaign with their geographical coordinates and tide-free ellipsoidal heights of vietrf13b [krásná et al., 2014]. the vlbi stations which are closer than 150 km distance from the nearest coastline are assumed as coastal stations. the vlbi sites are ordered according to the latitudes of the sites from north to south. observatory name country vlbi acronym latitude (°) longitude (°) ellipsoidal height (m) region (distance to the nearest coastline in km) ny ålesund norway nyales20 78.93 11.87 87.79 coast (2) onsala sweden onsala60 57.39 11.93 59.73 coast (1) badary russia badary 51.77 102.23 822.02 inland (1849) wettzell germany wettzell 49.14 12.88 669.56 inland (374) zelenchukskaya russia zelenchk 43.79 41.57 1175.43 inland (98) westford usa westford 42.61 288.51 87.19 coast (44) matera italy matera 40.65 16.70 543.80 inland (29) yebes spain yebes40m 40.52 356.91 989.17 inland (261) tsukuba japan tsukub32 36.10 140.09 85.14 coast (44) kokee park usa kokee 22.13 200.33 1168.72 coast (5) fortaleza brazil fortleza -3.88 321.57 23.48 coast (5) katherine australia kath12m -14.38 132.15 189.55 inland (227) hartebeesthoek south africa hart15m -25.89 27.68 1409.82 inland (482) yarragadee australia yarra12m -29.05 115.35 248.47 coast (47) warkworth new zealand wark12m -36.43 174.66 128.09 coast (3) hobart tasmania hobart26 -42.80 147.44 65.52 coast (8) hobart tasmania hobart12 -42.80 147.44 41.39 coast (8) function and 𝑚𝑤 the wet mapping function, 𝐺𝑛 and 𝐺𝑒 are the north and east total horizontal gradients, respectively. in equation (1), the formulation by chen and herring (1997) with c=0.0032 as a gradient mapping function 𝑚𝑔, was used. for all solutions of hourly sessions, the zhd were calculated at observation epochs with total surface pressure values measured at the vlbi sites [saastamoinen, 1972; saastamoinen, 1973; davis et al., 1985]. zwd were estimated at 20-minute intervals with relative loose constraints as 1.5 cm after 20 minutes from 24-hour sessions. note that in the analysis of these 24-hour sessions fes2004 ocean tide model [lyard et al., 2006] displacements are introduced to the a priori station coordinates (solution: case 1, see table 3 in section 4). then, these zwd estimated from 24 hour vlbi sessions were linearly interpolated to observation epochs of hourly sessions. zhd and zwd were mapped with the vienna mapping functions  1 [vmf1, böhm  et al.,  2006] to get the slant hydrostatic and slant wet delays. kamil teke 6 figure 2. the circles show the zwd1h - zwd24h versus hourly radial positions estimated simultaneously once for each 1 hour session of the cont14 campaign at kokee vlbi station. the correlation between zwd1h - zwd24h and radial positions is found as -0.80 at this station. figure 3. cyan and grey lines show the hourly zwd and the radial coordinate components estimated simultaneously from the analysis of 1 hour sessions at the vlbi station kokee, respectively. blue line illustrates the zwd estimated from 24 hour sessions. blackline shows the radial coordinate components estimated from 1 hour sessions when zwd from 24 hour sessions (blue line) are reduced from the observations a priori to the parameter estimation. similar to zwd, troposphere total north and east gradients at 1 hour intervals with relative loose constraints as 1 mm after 1 hour estimated from 24 hour vlbi sessions were linearly interpolated to the observation epochs of hourly sessions. then, azimuthally asymmetric troposphere delays were calculated through mapping these horizontal total north and east gradients to slant direction using the third term of equation (1) where the gradient mapping function by chen  and  herring  [1997] was used. finally, troposphere slant delays δ𝐿 from each vlbi observation (delay) of the hourly sessions were subtracted a priori to the parameter estimation. as previously mentioned, due to the small number and inhomogeneous sky coverage of observations as well as the weak global coverage of the stations, each hourly session was analyzed with a specific parameterization. the terrestrial reference frame (trf) catalogue, series of daily nutation offsets and hourly earth rotation parameters intrinsic to the cont14 campaign (covering only the time span and observations of cont14 campaign) were estimated from a series of global solutions, used as a priori values and held as fixed parameters in the analysis of hourly sessions. specific to each global solution the following parameterization was introduced: • cont14-trf catalog: a cont14 specific trf catalog from a global solution with the observations of cont14 (referred in this paper as cont14-trf) was estimated. in this global trf solution no-net-translation (nnt) and no-net rotation (nnr), trf datum conditions with respect to vietrf13b catalog [krásná et al., 2014] were introduced to the accumulated datum free normal equation system and station trf velocities were fixed to those of vietrf13b. the datum conditions were not imposed on the estimated coordinates of the vlbi stations tsukub32 and wark12m since vietrf13b coordinates of these stations are not available for the cont14 period. the reason for creating cont14 specific trf, cont14-trf is to eliminate the biases on estimated coordinates resulting from e.g. the inadequacies in the a priori trf coordinates and velocities as well as unmodeled episodic displacements. • cont14-eop-d series: daily earth orientation parameters for cont14 were estimated in a global solution (referred in this paper as cont14-eop-d) of which a priori nutation values were taken from the iers c04 08 corrections [bizouard  and  gambis,  2009] in addition to the iau2006 precession-nutation model [petit and luzum, 2010]. thus, vlbi only derived daily eop corrections intrinsic to cont14 were introduced to the a priori iers c04 08 series. the reason for a global solution of daily eop series, cont14-eop-d is to derive more accurate daily nutation parameters specific to the cont14 campaign. • cont14-erp-h series: the hourly earth rotation parameters (erp: polar motion coordinates and earth’s daily rotation phase angle, ut1-utc) for cont14 were estimated (referred in this paper as cont14-erp-h) where a priori daily nutation offsets are fixed to cont14-eop-d and a priori erp were taken from cont14-eop-d plus high-frequency corrections. in all hourly session analyses, interpolated values of the cont14-erp-h series to observation epochs were fixed. the parameterization for the analysis of the hourly vlbi sessions is as follows: the cont14 vlbi observations were analyzed using vienna vlbi and satellite software [vievs, böhm et al., 2018]. the parameters were estimated using the classical gauss markov least-squares adjustment method. the observations were not removed below a certain elevation angle and not down-weighted. source coordinates were fixed to icrf2 (international celestial reference frame 2, fey et al., 2009). the high-frequency variations of earth rotation parameters were modeled as recommended by the iers conventions 2010 [petit and luzum, 2010]. tidal and nontidal atmospheric loading [petrov and boy, 2004], as well as ocean tide loading based on the ocean model fes2014 [carrère et al., 2016], were introduced to each observation a priori to the adjustment. a priori station coordinates were not corrected for hydrological loading and non-tidal ocean loading. troposphere delays from 24-hour sessions were reduced from the observations a priori to the adjustment and residual delays upon the a priori zwd, and corrections to the a priori north and east gradients were not estimated. daily nutation offsets and hourly erp were fixed to cont14-eop-d and cont14-erp-h series, respectively. for each vlbi hourly session, clock synchronization errors as one offset and a rate of a polynomial for each clock with respect to a fixed clock and antenna coordinates with nnt/nnr conditions with respect to cont14-trf were estimated. analyzing cont14 hourly sessions were repeated 17 times, i.e., the same number as stations in cont14, using the same parameterization at each run with the difference that the fes2014 ocean tide model displacements were not introduced to the a priori coordinates of one of the vlbi stations and nnt/nnr datum conditions were not imposed on those station coordinates. at least by doing so the transformation defect of nnt/nnr trf datum constraints on the estimated hourly coordinates of the concerning station was recovered. to prevent singularity of the normal equation system solution of hourly sessions, as an 7 ocean loading m2 tide displacements from vlbi experience, the antennas of which the number of observations is less than 20, were excluded from the analysis. a posteriori variances (chi2) of 9 sessions out of 360 were found larger than 2. thus, these sessions were excluded from the hourly session set so the remaining 351 hourly sessions were considered for each run. through introducing the solid earth tide displacements, which can be modeled with an often-stated accuracy of around 1%, as well as pole tide, tidal and non-tidal atmosphere loading corrections to the a priori station coordinates, at the final point, otl displacements were assumed to be unveiled by the estimated hourly station positions (e.g. see figure 4 for the vlbi site, onsala in sweden). 3. m2 constituent of otl displacements from cont14 hourly sessions and assessment on the level of agreement with those of the selected ocean tide models the tidal frequency and position-dependent harmonic displacements on the earth crust, caused by the seafloor pressure variations due to the ocean tide loading (otl), δ𝑛,𝑘 for the 𝑘’th coordinate component (i.e. radial, west, or south) at a site, 𝑛 and at a particular time 𝑡 can be formulated with the harmonic function δ𝑛,𝑘 =�𝐴𝑛,𝑗,𝑘 cos(𝜒𝑗(𝑡)‒𝜑𝑛,𝑗,𝑘) (2) � where 𝜒𝑗 denotes the astronomical phase of the 𝑗’th tidal constituent which is computed from fundamental astronomical arguments [petit and luzum, 2010]. in equation (2), 𝐴𝑛,𝑗,𝑘 and 𝜑𝑛,𝑗,𝑘 are the amplitude and the phase lag with respect to the 𝑗’th tidal potential at greenwich meridian, respectively. the darwinian designations of the principal tides at semi-diurnal periods m2, s2, n2, k2, at diurnal periods k1, o1, p1, q1, and at longer periods mf, mm, and ssa describe more than 95% of the tidal signal [lambeck, 1988]. principal lunar semi-diurnal tide, m2 is the strongest tidal constituent that has a frequency of 2 cycles per lunar day (period: 12.42 hour) equals to twice the first doodson argument and zero times all of the others [doodson, 1921]. kamil teke 8 figure 4. hourly coordinate estimates of onsala60 (sweden) station when the ocean tide model corrections are not introduced to the coordinates a priori to the adjustment are depicted in black (hourly estimates). the differences in hourly estimates with respect to fes2014 ocean tide model displacements are shown in orange. the median formal errors of the hourly coordinate estimates of onsala60 station are 5.1, 3.2 and 4.5 mm in radial, east and north components, respectively. readers are referred to the supplementary material of this paper for the plots of the hourly otl displacement estimates of all cont14 stations. for the calculations of otl displacements from ocean tide models according to equation (2), the amplitudes and phase lags of principal tides were derived from “ocean tide loading provider”, a web-based facility provided by m.s. bos and h.g. scherneck (http://holt.oso.chalmers.se/loading, accessed on july 2017 for fes2012, fes2014 and tpxo8) that uses olfg/olmpp algorithm [scherneck, 1991] and the green’s functions using gutenberg-bullen standard earth model to determine the deformation due to point loads [farrell,  1972]. tidal coefficients were selected as not to correct for geocenter motion due to ocean tides. for all calculations of otl displacements from the tidal constituents, the sidelobes of the principal tides, caused by the nodal modulation of these tides with the 18.6-year period of the lunar node [mccarthy, 1996; scherneck, 1999; tamura 1987] were not introduced. instead, fully interpolated loading tide spectrum with 342 tidal constituents was computed by spline interpolation of the real (in-phase) and imaginary (quadrature) components of principal tides (hartmann and wenzel 1995) as recommended by petit  and  luzum  [2010]. matlab functions of vievs based on a fortran program, “hardisp.f” provided by petit and luzum [2010] were used. the cont14 hourly vlbi station coordinate time series δ𝑛,𝑘 were handled as observations to estimate the amplitudes (𝐴𝑛,𝑗,𝑘) and phase lags (𝜑𝑛,𝑗,𝑘) of the m2 constituent of otl displacements using the linearized form of equation (2) δ𝑛,𝑘 =�𝑅𝑛,𝑗,𝑘 cos 𝜒𝑗(𝑡) + 𝐼𝑛,𝑗,𝑘 sin 𝜒𝑗(𝑡) (3) � where 𝑛 is the station index, 𝑅𝑛,𝑗,𝑘 = 𝐴𝑛,𝑗,𝑘 cos 𝜑𝑛,𝑗,𝑘 and 𝐼𝑛,𝑗,𝑘 = 𝐴𝑛,𝑗,𝑘 sin 𝜑𝑛,𝑗,𝑘 are the real (in-phase) and imaginary (quadrature) parts of the 𝑗’th tide (m2) and the 𝑘’th coordinate component. note that no constraint was introduced on the estimated real and imaginary parts of the otl displacements m2 constituent. the real and imaginary parts were then converted to an amplitude and greenwich phase lag with the convention of phase lags taking negative values. the amplitudes, 𝐴𝑛,𝑗,𝑘 = �𝑅²𝑛 ,𝑗,𝑘 + 𝐼²𝑛 ,𝑗,𝑘 and the greenwich phase lags, 𝜑𝑛,𝑗,𝑘 = arctan (𝐼𝑛,𝑗,𝑘 / 𝑅𝑛,𝑗,𝑘) of the m2 tide, were computed from the direct estimates of 𝑅𝑛,𝑗,𝑘 and 𝐼𝑛,𝑗,𝑘, respectively. the errors of the amplitudes and phase lags were calculated from the one sigma formal errors of the estimated real and imaginary parts of the m2 constituent using the law of propagation of variances. the displacements due to long-period tides i.e. mf, mm and ssa as well as the semi-diurnal and diurnal tides other than m2 tide provided by the fes2014 model were reduced from the hourly station coordinates a priori to the estimation. the estimated amplitudes and phases of m2 tide, as well as their formal errors at the cont14 vlbi sites, are presented in table 2. depending on the number and quality of the observations per station, the formal errors of the estimated amplitudes of m2 tidal constituent were found similar, ranging from 0.04 to 0.30 mm in all coordinate components except for the vlbi sites hartebeesthoek and fortaleza with formal errors of the amplitudes as large as about 0.400.48 mm in radial and 0.20-0.35 mm in horizontal components. the formal errors of the phase angles of m2 tide are found mostly on the order of 0.4-9.6 degrees except for the vlbi sites at wettzell and hartebeesthoek that reach up to 12.7 degrees (south component wettzell) and 14.6 degrees (west component hartebeesthoek). the reason for the larger formal errors of the south component phase of wettzell and the west component phase of hartebeesthoek is that the amplitudes of these components are small. the best agreement of the estimated radial amplitudes of m2 tide with respect to those of ocean tide models were found at the sites onsala, badary, wettzell, zelenchukskaya, matera, tsukuba, and yarragade varying between 0.01 and 0.40  mm whereas the worst agreement is seen at ny-ålesund, yebes, and fortaleza sites with the radial amplitude differences of 1.48  mm with respect to fes2014, 0.93  mm and 1.50  mm with respect to tpxo8, respectively. at most of the stations, the estimated tangential amplitudes of m2 tide with respect to those of ocean tide models vary from 0.03 mm (at matera) to 0.75 mm (at fortaleza). it is worth to note that there is a large m2 tide radial amplitude difference of about 0.7 mm between the tpxo8 model and both fes2014 and fes2012 models at ny-ålesund site. when the greenwich phase lags of m2 tide are considered across the cont14 sites, the agreement between the estimates and the ocean tide models do agree within 1 to 5 degrees for most of the stations in all coordinate components. however, the largest m2 tide phase difference is found between the cont14 estimates and ocean tide models at hartebeesthoek site in west component with the value of about 27 degrees. to see the bar plots of the amplitudes and the phases of m2 constituent otl displacements derived from the ocean tide models with respect to the estimates of the cont14 campaign at all vlbi sites and in all coordinate components readers are referred to the supplementary material of this paper. 9 ocean loading m2 tide displacements from vlbi http://holt.oso.chalmers.se/loading from the phasor vector plots of the m2 tidal constituent of otl displacements (see figure 5 and the supplementary material of this paper) it can be seen that the agreement of the amplitudes and phases between cont14 estimates and ocean tide models do agree well for most of the sites and coordinate components except fortaleza. the amplitudes of the vector differences of the phasor vectors were used as a metric for assessing the level of agreement of the estimated amplitudes and phases of m2 tidal constituent derived from cont14 observations to those of the ocean tide models i.e. fes2014, fes2012, and tpxo8. the amplitudes of the vector differences were calculated from ⃒ 𝑑𝑛,𝑗,𝑘 ⃒ = ⃒ 𝐴𝑉𝐿𝐵𝐼(cos𝜑𝑉𝐿𝐵𝐼+𝑖 sin𝜑𝑉𝐿𝐵𝐼)𝑛,𝑗,𝑘−𝐴𝑀𝑂𝐷𝐸𝐿(cos𝜑𝑀𝑂𝐷𝐸𝐿 +𝑖 sin𝜑𝑀𝑂𝐷𝐸𝐿 )𝑛,𝑗,𝑘 ⃒ (4) where 𝑗 denotes to the tidal constituent i.e. m2, 𝑘 the coordinate component (in radial, west, and south directions) at the station 𝑛. the amplitudes of m2 tide phasor vector differences between fes2014, fes2012, tpxo8 ocean tide models and those estimated from the cont14 campaign are shown in figure 6. for most of the sites, the amplitudes of the vector differences in radial components of the m2 tide are up to two times larger than those of the tangential components. the amplitudes of the m2 tide phasor vector differences in all coordinate components are below 0.5 mm at most of the inland sites i.e. badary, wettzell, zelenchukskaya, kamil teke 10 stations radial west south a m2 [mm] φ m2 [°] a m2 [mm] φ m2 [°] a m2 [mm] φ m2 [°] nyales20 9.26±0.13 [7.78] 175.7±0.8 [178.5] 2.84±0.05 [2.57] -19.5±1.1 [-12.5] 1.52±0.05 [1.34] -27.5±1.9 [-18.1] onsala60 3.28±0.06 [3.67] -63.1±1.1 [-63.9] 1.47±0.04 [1.49] 89.8±1.7 [85.0] 0.67±0.05 [0.58] 115.8±4.1 [111.3] badary 0.86±0.08 [0.85] 85.5±5.5 [78.0] 0.63±0.06 [0.49] 149.6±5.9 [156.2] 0.42±0.07 [0.27] 81.2±9.6 [87.6] wettzell 5.44±0.06 [5.14] -72.20±0.7 [-71.3] 2.06±0.04 [1.91] 81.30±1.2 [77.6] 0.23±0.05 [0.39] -55.30±12.7 [-49.8] zelenchk 2.81±0.10 [2.64] -68.8±1.9 [-64.9] 0.77±0.08 [0.76] -2.7±5.9 [-3.0] 0.70±0.08 [0.74] -136.2±6.2 [-122.9] westford 7.65±0.15 [7.24] -167.9±1.1 [-167.4] 3.59±0.08 [3.40] -130.3±1.3 [-129.4] 1.85±0.09 [1.73] -22.5±3.0 [-15.8] matera 5.76±0.08 [5.41] -79.8±0.8 [-78.5] 1.24±0.06 [1.24] 72.3±2.7 [62.8] 0.93±0.07 [0.96] -81.2±4.4 [-78.6] yebes40m 14.19±0.10 [13.35] -87.3±0.4 [-87.4] 4.22±0.08 [3.91] 70.5±0.8 [69.0] 3.07±0.06 [2.88] -64.2±1.1 [-62.2] tsukub32 7.66±0.15 [7.57] 49.7±1.1 [50.2] 2.57±0.08 [2.35] -14.0±1.8 [-9.5] 1.90±0.06 [1.95] -80.4±1.9 [-76.1] kokee 11.97±0.29 [12.6] -120.6±1.4 [-121.3] 2.97±0.18 [2.49] 150.8±3.5 [159.0] 4.44±0.14 [4.09] 96.5±1.7 [97.3] fortleza 36.88±0.48 [35.36] 32.0±0.8 [34.0] 5.20±0.35 [4.45] 27.9±3.9 [20.7] 5.77±0.33 [5.15] 49.4±3.3 [52.1] kath12m 2.57±0.15 [3.02] 111.8±3.3 [112.6] 1.97±0.11 [1.60] 85.3±3.1 [87.5] 1.15±0.10 [1.19] 44.4±5.0 [47.3] hart15m 17.22±0.40 [16.66] -129.6±1.3 [-131.9] 0.78±0.20 [0.58] 69.2±14.6 [43.1] 1.84±0.26 [1.39] 79.7±8.1 [69.6] yarra12m 3.61±0.19 [3.48] 143.9±3.0 [146.4] 2.28±0.13 [2.24] -129.8±3.3 [-128.7] 2.52±0.14 [2.12] -161.20±3.3 [-154.1] wark12m 26.41±0.24 [25.63] 57.9±0.5 [56.6] 8.58±0.14 [8.27] -21.8±0.9 [-20.8] 5.70±0.20 [5.01] 14.6±2.0 [20.1] hobart26 10.38±0.18 [9.57] 150.7±1.0 [152.7] 4.26±0.13 [3.97] 104.7±1.7 [103.4] 1.10±0.16 [1.21] 107.3±8.2 [103.5] hobart12 10.20±0.18 [9.56] 151.30±1.0 [152.7] 4.36±0.14 [3.97] 101.3±1.8 [103.4] 1.06±0.17 [1.21] 109.8±9.3 [103.5] table 2. the amplitudes and phase lags of otl displacements m2 constituent with their formal errors estimated from cont14 hourly sessions at all contributing stations in millimeter and arc degrees, respectively. the fes2014 ocean tide model values are written in brackets. matera, and katherine as well as at the coastal sites onsala and westford. at ny-ålesund, yebes, yarragadee, and hobart (hobart12 and hobart26), the tangential amplitudes of vector differences are below 0.6 mm. however, at these sites, except yarragadee, the radial amplitudes of vector differences are larger relative to tangential components and range from 0.7 to 1.6 mm. the worst agreement between cont14 estimates and the ocean tide models in terms of m2 tide phasor vector differences is found at the site fortaleza varying in all coordinate components between 0.7 mm to 1.6 mm (see figure 6). 11 ocean loading m2 tide displacements from vlbi figure 5. phasor vectors of otl displacements m2 constituent with one sigma error ellipses at wettzell and matera vlbi sites. the formal errors of the estimated amplitudes and phase-lags are written to the top of each subplot, respectively. the horizontal axis represents the in-phase component, and the vertical axis out-of-phase component relative to the tidal potential at greenwich. the greenwich phase lag is zero along the positive direction in the horizontal axis and the phase angle increases counterclockwise. figure 6. the amplitudes of the m2 tide phasor vector differences between fes2014, fes2012, tpxo8 ocean tide models and those estimated from the cont14 campaign. 4. the impact of introducing different ocean tide models on the estimated otl displacements and the m2 tide when analyzing cont14 campaign the errors of the geophysical models, e.g. earth tides, pole tide, ocean loading, atmosphere loading, and hydrological loading, introduced a priori to the station coordinates, propagate into the residual estimates of station positions in the estimation process. to quantify the effect of ocean tide model errors on the estimated hourly positions, besides fes2014 (see case 1, in table 3), the otl displacements calculated from another ocean tide model, got00.2 [ray, 1999] (case 2, in table 3) is introduced to the station positions a priori to the analysis of hourly sessions. when the a priori ocean tide model is switched between fes2014 and got00.2, the level of change of the estimated otl displacements of the corresponding site is tested. a relatively older and less accurate model, got00.2 is selected to get more significant differences to fes2014. it is worth to note that for both of the hourly solutions (case 1 and case 2), the reduced external troposphere delays estimated from the 24 hour sessions are the same and fes2004 ocean tide model [lyard et al., 2006] is used for predicting the otl displacements in both of the analyses of 24 hour sessions. using the same observations and parameterization in the analysis of 1 hour sessions and only changing the a priori ocean tide model results in estimating the same position errors in magnitudes and directions. thus, as expected, all the errors in the estimated otl displacements are removed after having the differences, except those effects caused by the discrepancies of the ocean tide models (see e.g. figure 7). the differences of the otl displacement estimates reveal a tide-like harmonic behavior (see figure 7). the peakto-peak amplitudes of the differences of otl displacements range from 0.1 to 1 mm for all coordinate components of all stations while the medians of formal errors of the estimated displacements for all coordinate components are varying in 5 to 10 mm. this finding suggests that in the solutions of hourly vlbi sessions, the effects of changing a priori ocean tide model on the station coordinate estimates would not exceed 1  mm which is far below the magnitude formal errors of the hourly otl displacement estimates. to quantify the ocean tide model errors on the estimated amplitudes and phases of m2 tide, amplitudes of phasor vector differences are used as a metric. different ocean tide models introduced in various stages of the analysis, are shown in table 3 and explained in the below items. • to quantify the effect of using different a priori ocean tide models i.e. fes2014 or got00.2 in the analysis of 1 hour sessions the amplitudes of phasor vector differences of m2 tide between case 1 and case 2 are calculated (see table 3 and figure 9). • to get only the m2 tide otl displacements, in addition to the fes2014, the got00.2 displacements, except m2, are reduced from the estimated hourly position time series. if the fes2014 or the got00.2 models would contain errors, these tides would not be completely reduced at the end and would affect the estimated m2 tide amplitudes and phases especially due to the short time period of 15 days. the effect of reducing fes2014 or got00.2 tidal displacements (except m2) from the hourly position series, on the estimated m2 tide, is quantified through producing the amplitudes of the phasor vector differences between case 1 and case 3 (see table 3 and figure 9). kamil teke 12 table 3. introduction of different ocean tide models into various stages of the analysis. for correcting the a priori station positions in the analysis of 24 hour sessions for correcting the a priori station positions in the analysis of 1 hour sessions for reducing all the other tides, except m2, from the estimated otl hourly displacements case 1 fes2004 fes2014 fes2014 case 2 fes2004 got00.2 fes2014 case 3 fes2004 fes2014 got00.2 case 4 fes2014 fes2014 fes2014 • to quantify the effect of using different a priori ocean tide models i.e. got00.2 or fes2014 both in the analysis of 1  hour sessions and in the reduction of all the other tides, except m2, from the estimated otl hourly displacements, the amplitudes of phasor vector differences between case 2 and case 3 are calculated (see table 3 and figure 9). • when fes2014 or fes2004 ocean tide model displacements are introduced to the a priori station coordinates in the analysis of 24 hour sessions, two different sets of troposphere delays are estimated (e.g. see figure 8 and readers are referred to the supplementary material for the plots of the zwd differences at the other vlbi sites when different a priori ocean tide models are used in the analysis of 24  hour sessions). the level of influence of these two types of troposphere delays estimated from 24 hour sessions and reduced from the observations of 1 hour sessions, on the estimated m2 tide of 1 hour sessions is evaluated. thus, the estimated m2 tides are compared in terms of the amplitudes phasor vector differences between case  1 and case  4 (see table 3 and figure 9). the smallest variations in the amplitudes of the phasor vector differences of m2 tide are found between the results of case 1 and case 3 solutions. for most of the sites and coordinate components the amplitudes of the phasor vector differences between case 1 and case 3 are not larger than 0.1 mm, except the radial components of 13 ocean loading m2 tide displacements from vlbi figure 7. the differences of otl displacements in radial, east, and north coordinate components at kokee estimated from 1 hour sessions of cont14 when the a priori ocean tide model is switched between fes2014 and got00.2 (case 1 – case 2). readers are referred to the supplementary material for the differences of otl displacements at the other sites. figure 8. the zwd differences estimated from the analysis of 24  hour sessions at ny  alesund when a priori station coordinates are corrected with the ocean tide models; fes2014, fes2004 or got00.2. ny  alesund, warkworth, and hobart (hobart26) vlbi sites (see the red bars in figure  9). the phasor vector differences for most of the sites and coordinate components between case 1 and case 2 range within 0.1-0.2 mm. the phasor vector differences between case 2 and case 3 are a bit larger than those of case 1 and case 2. as an inference from these findings, when 15 days long, 1 hour vlbi sessions are considered, selection of ocean tide model used for correcting the station positions a priori to the analysis of hourly sessions as well as for reducing the other tides (except m2) from the position time series would make a difference of a few sub-millimeters, i.e. about 0.1 to 0.3 mm in the differences of m2 tide phasor vectors. the largest differences of m2 tide phasor vectors are found between the solutions of case 1 and case 4 in the radial components of the coastal vlbi sites. the amplitudes of the differences of m2 tide radial phasor vectors of these sites are found as 1.1 mm at ny alesund, 0.6 mm at onsala, 0.5 mm at kokee, 0.4 mm at fortaleza, 0.7 mm at katherine (assumed to be an inland site, not too far away from sea about 230 km), 1.4 mm at warkworth and 0.3 mm at hobart (hobart26). in the analysis of 24 hour sessions, the station displacements due to the change of a priori ocean tide models, i.e. fes2004 or fes2014, cause the estimated troposphere delays i.e. zenith wet delays (zwd) to vary in -2 and 2 mm (see e.g. figure 8 for the station ny alesund and the supplementary material for the other stations). root-mean-square of the estimated zwd differences (purple horizontal lines in figure 9) and the m2 tide phasor vector differences in radial components between case 1 and case 4 reveal a statistically significant strong positive correlation of 0.89. these findings suggest that the larger errors of ocean tide models (e.g. fes2014 and fes2004) at coastal sites relative to the inland sites, propagate into the troposphere delays estimated from 24 hour sessions, which in turn these errors in the troposphere delays cause larger differences in the m2 tide phasor vectors of otl displacements estimated from the 1 hour sessions. kamil teke 14 figure 9. the amplitudes of the estimated m2 tide phasor vector differences from the cont14 campaign between four different cases of parameterization (see table 3). the effect of using different a priori ocean tide models i.e. fes2014 or got00.2 in the analysis of 1 hour sessions are plotted as black bars (case 1 - case 2). the effect of reducing fes2014 or got00.2 tides, except m2, from the estimated otl hourly displacements are shown in red bars (case 1 - case 3). the effect of using different a priori ocean tide models i.e. fes2014 or got00.2 both in the analysis of 1 hour sessions and in the reduction of all the other tides, except m2, from the estimated otl hourly displacements are shown in grey bars (case 2 -case 3). the effect reducing the two types of troposphere delays, when fes2014 or fes2004 is introduced a priori in the analysis of 24h sessions, from the observations of 1  hour sessions is depicted as orange bars (case  1 -  case  4). root-mean-square (rms) of the differences of zwd estimates from the analysis of 24 hour sessions when a priori ocean tide model is used as fes2014 or fes2004 are plotted as purple horizontal lines. to assess the overall agreement between cont14 estimates and ocean tide models the root-mean-square (rms) of the amplitudes of the vector differences between observed and modeled phasor vectors across the stations for the 𝑗’th tidal constituent (m2) and the 𝑘’th coordinate component were calculated using (5) and are given in table 4. hereupon, the term “rms misfits” refers to the rms of the amplitudes of phasor vector differences between the cont14 estimates and those of ocean tide models i.e. fes2014, fes2012 and tpxo8 across all the stations. the rms misfits of the m2 tide in the radial component between cont14 and ocean tide models over coastal sites are found about 0.70-0.96 mm both from the solutions of case 1 and case 4 which is significantly larger than those of inland sites i.e. varying in 0.34-0.51 mm. the rms misfits in both radial and tangential components over coastal sites are found about two times larger than those of inland sites which is valid for both solutions of case 1 and case 4 (see table 4). when all sites are considered, the rms misfits of the m2 tide between cont14 and ocean tide models are within 0.66-0.81 mm in radial and 0.21-0.40 mm in tangential components. the best agreement between cont14 and the selected ocean tide models for m2 tide is seen in all the components of inland sites estimated from the case 4 solution (see table 3) with the rms misfits ranging in 0.10-0.41 mm. 5. conclusions in this study, hourly vlbi station coordinate time series derived from the analysis of 1 hour vlbi sessions of 15 days long continuous vlbi campaign, cont14 were used to estimate the principal lunar semidiurnal tide, m2 of otl displacements. only m2 tide is estimated in this study and the station displacements caused by the remaining; long period, semi-diurnal and diurnal tides were calculated from fes2014 model and reduced from the hourly station coordinates a priori to the estimation. intrinsic to this study, the troposphere delays estimated from 24-hour sessions were reduced from the observations of hourly sessions a priori to the analysis. 𝑅𝑀𝑆𝑗,𝑘 =� � ⃒ 𝑑𝑛,𝑗,𝑘 ⃒ 21𝑁 𝑚𝑖𝑠𝑓𝑖𝑡𝑠 � �⁼¹ 15 ocean loading m2 tide displacements from vlbi table 4. rms misfits of m2 tide calculated from equation (5) in mm between cont14 phasor vectors from the solutions of case 1, case 4 and those of ocean tide models (i.e. fes2014, fes2012, and tpxo8) over coastal (10 stations, see table 1), inland (7 stations otherwise, see table 1) and all stations. results from the case 4 solution is written in italics. case 1 case 4 radial west south radial west south coast cont14-fes2014 0.96 0.45 0.42 0.84 0.37 0.27 cont14-fes2012 0.92 0.45 0.41 0.70 0.32 0.22 cont14-tpxo8 0.89 0.46 0.40 0.89 0.39 0.25 inland cont14-fes2014 0.45 0.24 0.15 0.36 0.20 0.10 cont14-fes2012 0.46 0.23 0.15 0.34 0.19 0.10 cont14-tpxo8 0.51 0.25 0.16 0.41 0.21 0.12 all cont14-fes2014 0.81 0.39 0.35 0.70 0.32 0.22 cont14-fes2012 0.78 0.38 0.34 0.66 0.32 0.21 cont14-tpxo8 0.77 0.40 0.33 0.75 0.33 0.21 the estimated amplitudes and phase lags of the m2 constituent of otl displacements at each site are compared with the predictions of the considered ocean tide models, i.e., fes2012 [lyard et al., 2006; carrère et al., 2012], fes2014 [carrère et al., 2016], and tpxo8 [egbert and erofeeva, 2002; egbert et al., 2010]. the amplitudes of m2 tide phasor vector differences in all coordinate components between cont14 estimates and those of ocean tide models are found below 0.5 mm at the inland sites i.e. badary, wettzell, zelenchukskaya, matera, and katherine as well as at the coastal sites onsala and westford. the worst agreement between cont14 estimates and ocean tide models for m2 tide in terms of phasor vector differences is found at the site fortaleza where the values in all coordinate components are between 0.7 mm to 1.6 mm. this is due to the extremely humid weather at fortaleza which results in the unmodelled troposphere delays propagate into coordinate components in the least-squares adjustment. besides, the fortaleza vlbi site is located at about 5 km far away from the atlantic ocean and its m2 amplitude of otl displacements is about 36 mm in the radial component, the largest radial amplitude relative to the other cont14 vlbi sites. as an overall assessment, the rms of the amplitudes of m2 tide phasor vector differences between the cont14 estimates and those of ocean tide models i.e. fes2014, fes2012, and tpxo8 across all the stations, namely rms misfits are considered. the best agreement between cont14 and the selected ocean tide models for m2 tide is seen in all the components of inland sites with the rms misfits ranging in 0.1-0.4 mm. the rms misfits of the m2 tide of otl displacements in all coordinate components between cont14 and ocean tide models over coastal sites are found about two times larger than those of inland sites. this result confirms the modeling insufficiencies in shallow waters of ocean tide models which results in an accuracy restriction of otl displacements around coastal regions. the larger errors of ocean tide models (e.g. fes2014 and fes2004) at coastal sites relative to the inland sites, propagate into the troposphere delays estimated from 24 hour sessions, which in turn these errors in the troposphere delays cause larger differences in the m2 tide phasor vectors of otl displacements estimated from the 1 hour sessions. the larger discrepancies between the vlbi observed and model-predicted m2 tides at coastal sites may not only be resulted from the errors of ocean tide models at coastal regions but also additional errors can become from vlbi data and analysis. besides, hydrological loading and non-tidal ocean loading, which are not introduced a priori to the station coordinates in the vlbi analysis of 1 hour sessions in this study, would contribute to the increase in the misfits of phasor vectors between ocean tide models and vlbi at both coastal and inland sites. because, even these geophysical effects have periods much longer than one day, given the short time span of cont14 their contributions may not be negligible. to conclude, the level of agreement between the vlbi observed and model-derived otl displacements can be significantly increased through reducing the 24-hour session troposphere delays from the observations of hourly sessions. as an outlook to improve the estimated accuracies of semidiurnal and diurnal tidal constituents of otl displacements from the analysis of the observations of vlbi, further developments should be introduced to the present-day troposphere delay models as well as to the geophysical models for the station position displacements. besides, the number of vlbi observations should be increased. their temporal distribution over a session and sky coverage should be homogenized. acknowledgments. this work is supported by the scientific and technological research council of turkey (tübi̇tak), project number: 115y244. the author acknowledges the international vlbi service for geodesy and astrometry (ivs) for providing the observations of the cont14 campaign. the author is grateful to the anonymous reviewers for their comments which were helpful for improving the paper. references agnew, d. c. 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department of geomatics engineering, ankara, turkey e-mail: kteke@hacettepe.edu.tr © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved. 19 ocean loading m2 tide displacements from vlbi neural network multispectral satellite images classification of volcanic ash plumes in a cloudy scenario annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6638 neural network multispectral satellite images classification of volcanic ash plumes in a cloudy scenario matteo picchiani1*, marco chini2, stefano corradini3, luca merucci3, alessandro piscini3, fabio del frate1 1 tor vergata university, dipartimento di ingegneria civile e ingegneria informatica, rome, italy, 2 luxembourg institute of science and technology, environmental research and innovation department, belvaux, luxembourg, 3 istituto nazionale di geofisica e vulcanologia, roma, italy *picchian@disp.uniroma2.it abstract this work shows the potential use of neural networks in the characterization of eruptive events monitored by satellite, through fast and automatic classification of multispectral images. the algorithm has been de veloped for the modis instrument and can easily be extended to other similar sensors. six classes have been defined paying particular attention to image regions that represent the different surfaces that could possibly be found under volcanic ash clouds. complex cloudy scenarios composed by images collected during the icelandic eruptions of the eyjafjallajökull (2010) and grimsvötn (2011) volcanoes have been considered as test cases. a sensitivity analysis on the modis tir and vis channels has been performed to optimize the algorithm. the neural network has been trained with the first image of the dataset, while the remaining data have been considered as independent validation sets. finally, the neural network clas sifier’s results have been compared with maps classified with several interactive procedures performed in a consolidated operational framework. this comparison shows that the automatic methodology proposed achieves a very promising performance, showing an overall accuracy greater than 84%, for the eyjafjalla jökull event, and equal to 74% for the grimsvötn event. i. introduction fter volcanic activity, ash clouds affect the population and its economy at different levels, and represent a major hazard for air traffic due to the harming effects they have on aircraft engines. accurate and fast satellite monitoring of the phenomena's evolution is one of the most effective tools when managing the risk mitigation phase. in the late 1980s, prata demonstrated [1989] how ash particles of effective radius, a ranging between about 0.5 to 10 μm, can be identified by thermal infrared (tir) radiometers, since at shorter infrared wavelengths ash absorbs more radiation than water droplets or ice, which mainly absorb at longer infrared wavelengths. this behavior implies that the difference between the brightness temperature measured at 11 and 12 μm (btd) will be negative if the instantaneous field of view (ifov) area is contaminated by ash. on the contrary, the btd will generally be positive for meteoro1 annals of geophysics, fast track 2, 2014 logical clouds. different works studying the eruption phenomena characterization using different satellite data are based on this assumption [prata and kerkmann, 2007; corradini et al., 2008]. such studies highlighted the role of remote sensing as a primary source of information when retrieving ash particle properties such as effective radius, optical depth and the total column mass. moreover, sensitivity analysis highlighted the importance of ancillary information to constrain the output of radiative transfer models and showed that the accuracy of inversion methods is affected by errors even equal to 40% [corradini et al., 2008]. the ash retrieval accuracy is also constrained by the emissivity of the surfaces underlying the ash clouds, since ash is semitransparent in the tir spectral range. thus, the ash retrieval can generally achieve better results if ash clouds and their underlying surfaces are correctly classified [corradini et al., 2010]. to identify these classes, a “standard procedure” based on btd, vis channels, modis level 1 (l1) land/sea mask and modis level 2 (l2) cloud products is generally applied in a cascade procedure (see section ii). in this paper, we propose a novel classification approach based on neural networks (nn) to identify ash clouds and their underlying surfaces, which can recursively be applied in order to quickly obtain automatic and accurate maps. the suitability of the nn approach for the detection of ash cloud in conditions free from meteorological cloud, has already been proved [picchiani et al., 2011], and the detection was simplified by considering two main classes: “ashy” and “non-ashy” pixels in the modis images. here we propose to classify a more complex scenario, where “ash above sea” and “ash above meteorological clouds” classes are present, both playing an important role in the ash parameters retrieval. the nn algorithm has been tuned on moderate resolution imaging spectroradiometer (modis) images collected in overcast conditions during the 2010 eyjafjallajökull and 2011 grimsvötn eruptions. this test set represents a challenge in the identification of the ashy pixels. indeed, the complexity of the images characterized by meteorological formations at different heights and other issues like the presence of glaciers, if not correctly identified, may produce false alarms even with the “standard procedure”. therefore, the nn classifier is also trained to identify the other main surfaces in the image, defining a total of six classes labeled in the following manner: 1–ash above sea; 2-ash above meteorological clouds; 3–meteorological clouds; 4–sea surface; 5–ice surface;. 6–land. figure 1: rgb (channels 1-4-3) color composite of modis images acquired 6th, may 2010 (jd 126) during the ejafjallajökull eruption (coastline plotted in red), used to train the nn. ii. methodology the most common type of nn, known as feedforward multilayer perceptron (mlp) [bishop, 1995], has been chosen for this study. a mlp is composed of simple non-linear computational units called neurons, sorted in layers and connected by weighted links (synapses). the weights can be considered as parameters of a nonlinear regression computed through a process of error minimization between input in2 annals of geophysics, fast track 2, 2014 formation and computed outputs, referred to as the training phase. nn classifiers are good candidates for this kind of problem because: 1) non parametric approaches are not limited to prior assumptions on the distribution of input data, thus making the integration of different sources more effective; 2) an output is produced very quickly if the nn is correctly trained. with regard to the first reason, it is well-known that any classification algorithm, which has to reproduce a physical phenomenon, needs a representative dataset for the training phase and, usually, in real scenarios this requirement partially fails. this is due to outliers or spectrally non-pure pixels. in this respect, the mlp-nns have already proved to be more robust than parametric classifiers, such as bayesian ones [chini et al., 2008]. regarding the second reason for using nn classifiers, it is already well-known that in an emergency one of the principal requirements is speed, thus this immediate automatic classification of the image could be an effective solution. this approach can be a precious tool and significantly improve the retrieval of ash parameters, especially when the ash cloud moves from one image to the next and the surface under the ash changes (e.g., from sea to land or ice). in this study, the mpl-nn potentials have been exploited to classify three modis images collected on 8th, 12nd and 13rd of may 2010 (jd 128, 132 and 133 respectively) during the eyjafjallajökull eruption and one collected on 23rd may 2011 (jd 143) during the grimsvötn eruption. the dataset also includes another image of the eyjafjallajökull event acquired on 6th may 2010 (jd 126) which is used to train the nn (see fig. 1). the nn classifier is designed to replicate the same results as those obtained when applying the iterative “standard procedure”. the latter is the most suitable approximation to represent the real situation pictured by the sensor. therefore, all the modis images were first interactively classified, and then the results were used to train and evaluate the nn algorithm. the “standard procedure” uses the btd approach firstly to identify ash clouds and then to classify them by putting them into “ashy pixels” above sea and “ashy pixels” above meteorological clouds by intersecting the btd output with both the modis l1 land/sea mask and the modis l2 cloud products. the latter are also used to classify all the other meteorological clouds present in the satellite image. the classes “land” and “sea” are obtained by the intersection between the modis land/sea mask and the ash and meteorological cloud pixels previously classified. the result of this operation is subtracted from the pixels indicated by the land/sea mask in order to identify the “land” and “sea” classes. finally, the “ice” surfaces are identified by fixing an appropriate threshold on the modis band 7 radiance. as stated before, only one modis image has been used here for training, while the others have been used as independent validation sets. this approach was considered in order to assess the generalization capabilities of the nn and the statistical representativeness of the training/test samples. in particular, the validation on the grimsvötn eruption image represents a very strict test as the data is totally independent from the training set. one of the most critical issues in training complex nn is the selection of the inputs, since generally fewer inputs ensure better generalization capabilities [bishop, 1995]. consequently, a branch and cut approach has been applied in order to select the most significant modis channels from the whole set of bands. the selected modis channels are: 1 ÷ 7, 20 ÷ 23, 29, 31 and 32. this selection of input wavelengths optimizes also the retrieval approach of ash parameters based on nn [piscini et al. 2014]. 3 annals of geophysics, fast track 2, 2014 moreover, we have found that the inclusion of the modis l1 land/mask improves the generalization of the classifier. the final input space of the nn is therefore composed of 15 inputs, while the output space is composed of six classes provided by the standard procedure (see fig. 2). by applying a simple trial and error procedure, the best topological performance proved to be the combination of two hidden layers with 15 and 5 neurons, respectively. in the nn learning phase, to avoid the oversampling of particular classes, the same number of pixels was randomly extracted for each class. as a result, for each one of the six classes 2660 pixels where extracted from the jd 126 image. the nn was trained using an error back-propagation algorithm on the training set (equal to 75% of the pixel considered from the jd 126 image). using the cross validation technique the training was interrupted when the generalization performance evaluated on a different image subset (on the remaining 25% of pixels) degenerated. iii. results the nn’s performance has been evaluated by using the independent dataset of the modis images (jd 128, jd 132 and jd 133 from 2010 and jd 146 from 2011). the results are satisfactory with overall classification accuracy greater than 84% for the eyjafjallajökull validation cases, and equal to 74% for the grimsvötn image. figure 2 shows the classified maps for all the modis acquisitions obtained by the benchmark “standard procedure” and the nn approach respectively. the figure shows how the standard products and the nn classification are qualitatively compatible. the quantitative analysis of the automatic algorithm’s performance is given by the confusion matrixes for the validation dates presented in figure 3, where the target class refers to the “standard procedure” results and the output class to those of the nn's. the errors for each class are also given, as well as the commission and omission errors reported in the last grey column and row of each matrix [richards and 4 figure 2: standard procedure maps (upper panel). nn maps (down panel). the six classes are identified by color: red – ash above sea. cyan ash above meteorological clouds. yellow – meteorological clouds. blue – sea. white – ice. green – land. the coastline is plotted in black. volcano’s vents are marked by black stars. annals of geophysics, fast track 2, 2014 jia, 1999]. in observing the main matrices diagonally it can be noted that the majority of the pixels belonging to every class have been correctly classified, even if a few classification errors are still present. these mainly occur between spatially or spectrally adjacent classes. firstly, the “ice” class is sometimes identified by the nn where the “standard procedure” indicates meteorological clouds. as described in section ii, in the “standard procedure” the meteorological clouds are classified by using the modis l2 cloud product. in the event of semitransparent meteorological formation (thin clouds), the contribution of the underlying surface is not negligible and may strongly characterize the radiometric response. in these intermediate conditions pixel mis-classifications can occur. secondly, some errors occur on ash over sea pixels that are classified as ash over clouds (i.e. in jd128 case). again, this behavior seems to be due to the method used to define the classes in the “standard procedures”. indeed, to identify the ash over clouds and meteorological clouds, the modis l2 data are used to apply a cloud mask. this procedure does not ensure that one pixel identified as ash and not flagged as cloudy will not present mixed effects according to meteorological formations. on the contrary, it is reasonable that pixels identified as cloudy and free from ash, are partially contaminated by ash and thus classified as ash over cloud by the nn (i.e. in jd 132 case). finally, considering the spatial distributions of the nn classified features (i.e. ash clouds, glaciers, meteorological clouds) it has to be underlined that the misclassification errors are mainly localized on the edges of correctly classified objects, and that the spatial features of the “standard procedure” and the nn classifier images are accordance with each other. this is also the reason why the land class in jd 143 has very low classification accuracy: it is represented by a very small number of pixels distributed on the borders of other classes. iv. conclusions this experiment demonstrates the nn’s potentials in classifying eruptive event images. despite some mis-classification errors, the proposed automatic classifier shows good overall accuracy, greater that 84% for the eyjafjallajökull validation images. in the more challenging grimsvötn case, data achieves an overall accuracy equal to 74%, and a spatial distribution for the classified features consistent with the results obtained by the interactive and time consuming workflows. the trained nn's capability of immediately providing reliable classification maps of multispectral satellite images can be profitably used to improve ash retrieval parameters in an operational scenario and hereafter be applied to processing images of new eruptions, including those of other volcanoes in particular conditions (i.e. ash characteristics, latitude and season). furthermore, to improve the nn’s accuracy, additional data can be considered. however, this is not necessary if the training set is already statistically representative of the studied phenomena. future developments could enhance the nn’s potentials by improving its capability to detect other classes like “ash on land” and “desert dust” in different climate conditions. acknowledgemets the work of s. corradini was founded by the european union's seventh framework programme (fp7/2007-2013) through the project aphorism under grant agreement number 606738. references 5 annals of geophysics, fast track 2, 2014 [bishop, 1995] bishop c. m. (1995). neural networks for pattern recognition. oxford univ. press. [chini et al., 2008] m. chini, pacifici, f., et al. (2008). comparing statistical and neural network methods applied to very high resolution satellite images showing changes in man-made structures at rocky flats. ieee trans. geosci. remote sens., 46(6), 18121821. [corradini et al., 2008] corradini, s., spinetti, c. et al. (2008) mt. etna tropospheric ash retrieval and sensitivity analysis using moderate resolution imaging spectroradiometer measurements. j, atmosph. rem. sens., 2, 023550. [corradini et al., 2010] corradini, s., merucci, l. et al. (2010). volcanic ash and so2 in the 2008 kasatochi eruption: retrievals comparison from different ir satellite sensors. j. geophys. res., 115(d2), d00l21. [picchiani et al., 2011] picchiani, m., chini, m., et al. (2011). volcanic ash detection and retrievals from modis data by means of neural networks. atmos. meas. tech., 4, 2619-2631. [piscini et al., 2014] piscini, a., picchiani, m., et al. (2014). a neural network approach for the simultaneous retrieval of volcanic ash parameters and so2 using modis data. atmos. meas. tech., 7, 4023-4047. [prata, 1989] prata a. j. (1989). infrared radiative transfer calculations for volcanic ash clouds. geophys. res. lett., 16(11), 1293-1296. [prata and kerkmann, 2007] prata, a. j., and kerkmann, j. (2007). simultaneous retrieval of volcanic ash and so2 using msg-seviri measurements. geophys. res. lett, 34(5), l05813. [richards and jia, 1999] richards, j.a., jia, x. (1999). remote sensing digital image analysis: an introduction. new york: springer-verlag. jd 128 jd 132 jd 133 jd 143 figure 3: confusion matrices. the six classes are: 1 – ash above sea. 2 ash above meteorological clouds. 3 – meteorological clouds. 4 – sea. 5 – ice. 6 – land. 6 marzocchi_corretto_layout 6 561 daily earthquake forecasts during the may-june 2012 emilia earthquake sequence (northern italy) warner marzocchi*, maura murru, anna maria lombardi, giuseppe falcone, rodolfo console istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6161 1. introduction on may 20, 2012, at 02:03 utc, a magnitude ml 5.9 earthquake hit part of the po plain area (latitude, 44.89 ˚n; longitude, 11.23 ˚e) close to the village of finale-emilia in the emilia-romagna region (northern italy). this caused a number of human losses and significant economic damage to buildings, and to local farms and industry. this earthquake was preceded by an increase in the seismicity the day before, with the largest shock of ml 4.1 at 23:13 utc (latitude, 44.90 ˚n; longitude, 11.26 ˚e). it was then followed by six other ml 5.0 or greater events in the following weeks. the largest of these six earthquakes occurred on may 29, 2012, at 07:00 utc (ml 5.8), and was located 12 km southwest of the may 20, 2012, main event (latitude, 44.85 ̊ n; longitude, 11.09 ˚e), resulting in the collapse of many buildings that had already been weakened, a greater number of victims, and most of the economic damage (see figure 1). this sequence took place in one of the italian regions that is considered to be at small-to-moderate seismic hazard [gruppo di lavoro mps 2004]. earthquakes of the m6 class have occurred in the past in this zone [gruppo di lavoro cpti 2004], but with a much smaller time frequency with respect to the most seismically hazardous parts of italy. to date, it appears that the largest amount of damage was caused to structures that were not built according to the building code, which was established after the publication of the latest italian seismic hazard map [gruppo di lavoro mps 2004] and become effective only after the l'aquila 2009 earthquake. this highlights even more that our best defense against earthquakes is to build constructions according to a sound building code. having said that, we also argue that the building code is not the only defense that we can have against earthquakes, and that different kinds of mitigation actions, more directed to reduce human loss that damage, can be taken over shorter time intervals [e.g. van stiphout et al. 2010, jordan et al. 2011]. traditionally, seismic hazard maps forecast the expected ground motion in a time interval of 50 years or longer. the underlying earthquake occurrence process is presumed to be time independent [cornell 1968], and the mean seismic hazard rate is expected to remain constant through time. nonetheless, we know that the earthquake occurrence process has significant time variability in the seismic rate; these variations are much larger then would be anticipated with a pure time-independent process. the clearest of these variations is the time and space clustering of seismicity. an earthquake suddenly alters the dynamic conditions within the nearby fault systems, which can lead to future earthquakes over a time scale of days, and up to a few years. the use of time-dependent models for practical purposes has been introduced recently in italy [marzocchi et al. 2012a] to forecast seismicity over a 10-year time window. this model was requested by the civil protection authorities to select areas where building retrofitting is more urgent. this mitigation action comes from a decision of the italian government that provided funding at the beginning of 2010 for the reduction of seismic risk based on medium-term time scales (interventi per la prevenzione del rischio sismico). time variations of the seismic rate are more evident over the short-term (days to weeks), and in particular after a large earthquake. the modeling of these time variations and its possible use for practical purposes was discussed in depth by the international commission on earthquake forecasting for the italian civil protection [jordan et al. 2011], which was nominated by the italian government after the l'aquila 2009 earthquake. this commission had two main tasks: (i) to report on the current state of knowledge of short-term prediction and forecasting of tectonic earthquakes; and (ii) to indicate guidelines for the use of possible forerunners of large earthquakes to drive civil protection actions, including the use of probabilistic seismic hazard analysis in the wake of a large earthquake. the commission recommended the development of operational earthquake forecasting (oef), and of quantitative and transparent decision-making protocols that encompass article history received july 25, 2012; accepted august 21, 2012. subject classification: earthquake interactions and probability, operational earthquake forecast, etas. 2012 emilia earthquakes mitigation actions at different impact levels, with each one associated with the surpassing of a specific probability threshold. in essence, oef comprises procedures for gathering and disseminating authoritative information about the time dependence of seismic hazards, to help communities to prepare for potentially destructive earthquakes [jordan et al. 2011]. to date, seismologists have already developed time-dependent models based on earthquake clustering, and these have been used to track the evolution of aftershock sequences in real-time. here, we apply two versions of the epidemic type aftershock sequence (etas) model to the seismic sequence of the may-june 2012 emilia earthquakes (see figure 1a for the area considered), using the real-time earthquake data recorded by the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology) seismic network. the models were described by falcone et al. [2010] (and references therein) and lombardi and marzocchi [2010a] (and references therein). they are thus referred to here as the fcm and lm models, respectively. specifically, we consider the first three weeks of the emilia 2012 sequence (may 19 to june 12, 2012), when most of the largest earthquakes occurred (see supplementary information for maps). for both models, we show the preliminary results of the daily forecasts of the number of events with ml ≥2.5 and the daily occurrence probability of one or more events with ml ≥4.0. 2. short-term earthquake forecasts: the models the short-term forecasting models so far proposed are of four types: (i) the reasenberg and jones [1989] model, which is based on the temporal clustering of earthquakes and which was used to forecast the aftershocks of the octomarzocchi et al. 562 10˚24' 10˚24' 10˚36' 10˚36' 10˚48' 10˚48' 11˚00' 11˚00' 11˚12' 11˚12' 11˚24' 11˚24' 11˚36' 11˚36' 11˚48' 11˚48' 44˚36' 44˚36' 44˚48' 44˚48' 45˚00' 45˚00' 45˚12' 45˚12' a) 11˚00' 11˚00' 11˚12' 11˚12' 11˚24' 11˚24' 11˚36' 11˚36' 11˚48' 11˚48' 44˚48' 44˚48' 45˚00' 45˚00' 1.0e−08 1.0e−06 5.0e−06 1.0e−05 5.0e−05 1.0e−04 5.0e−04 1.0e−03 1.0e−02 1.0e−01 1.0e+00 ferrara mirandola bondeno cento vigarano camposanto poggio−renatico poggio−rusco finale−emilia novi−di−modena ostiglia occhiobello may 29 2012, 06:00 utc m5.5+ b) figure 1. (a) location of the events with ml ≥2.5 that occurred from january 1, 2009, in the area hit by the emilia 2012 sequence. stars, squares, circles, earthquakes with ml ≥5.0, 4.0 ≤ ml < 5.0, and 2.5 ≤ml <4.0, respectively. black, red, blue symbols, events that occurred in the period of january 1, 2009, to april 30, 2012, may 1 to may 28, 2012, and may 29 to june 12, 2012, respectively. red, blue rectangles, areas chosen for the real-time daily forecasting, before and after may 30, 2012, respectively (see text for details). (b) example of a spatial forecast. the map reports the density of the expected number of earthquakes with ml ≥5.5 per km2 in the 24 h starting from may 29, 2012, 06:00 utc. three black stars, earthquakes with ml ≥5.0; gray dots, all earthquakes with 4.0 ≤ ml < 5.0 that occurred before. red star, ml 5.8 earthquake that occurred during the forecasting time window. 563 ber 17, 1989, loma prieta earthquake (see lolli and gasperini [2003] for a retrospective application to the italian territory); (ii) the etas models [e.g., ogata 1988, ogata 1998, console and murru 2001, zhuang et al. 2002, console et al. 2003, wossner et al. 2011], which were used to forecast aftershocks in real-time after the l'aquila earthquake [marzocchi and lombardi 2009], and to set up a short-term forecasting model for the whole italian territory [murru et al. 2009, falcone et al. 2010, lombardi and marzocchi 2010a]; (iii) the short term earthquake probability model [gerstenberger et al. 2005], which was used to forecast the daily ground shaking in new zealand after the september 3, 2010, darfield earthquake; (iv) the agnew and jones model [agnew and jones 1991], which is mostly used to forecast mainshock occurrences, and which has been adopted by the california earthquake prediction evaluation committee. the reasenberg and jones [1989] model is based only on temporal distribution after a main shock, while the other three models also include the spatial distribution. the etas models are probably the most diffuse class of models for daily and weekly forecasts, as testified by the many that have been applied in different collaboratory for the study of earthquake predictability (csep) experiments in japan, california (usa) and italy [falcone et al. 2010, lombardi and marzocchi 2010a, ogata 2011, zhuang 2011]. for longer forecasting time windows, models mostly based on a single omori law decay can work reasonably well [see lolli et al. 2011 for a more elaborated model]. the etas models are based on simple physical components: a spatially variable seismic background that is characterized by a stationary poisson distribution in time, and radially symmetric triggering. the etas models are generically described by an equation of this form: (1) where m is the rate of magnitude m events expected in the location , at time t and of magnitude m, and mmin is the minimum magnitude considered. the first term represents the background varying with space (and not in time), while the summation takes into account the triggering effects of all the previous earthquakes, as a function of the distance, elapsed time, and magnitude of the triggering event. the last factor outside the square brackets is the frequency–magnitude law, which is supposed to be independent from space and time. once has been estimated, the probability p of one event at least in a specific time–space–magnitude window ω, is: (2) in our real-time application, we provide the daily probability for ml 4 events or larger, and for ml 5.5 events or larger, in the region shown in figure 1b. we adopt two different and independent versions of the etas-class models. the detailed descriptions of these were reported in falcone et al. [2010] (the fcm model) [murru et al. 2009, console et al. 2010a, console et al. 2010b] and in lombardi and marzocchi [2010a] (the lm model). both of these models have been submitted to the csep experiment that is now running in italy. since the csep experiment covers the whole of the italian area, the real-time application for aftershock sequences in small areas can require some optimization/modifications of the etas models. this optimization aims to account for the spatial variability of the completeness magnitude [see schorlemmer et al. 2010], the specific model parameters of the area considered [see, e.g., lolli and gasperini 2003], and the increase in mmin soon after the occurrence of a large shock [e.g. gasperini and lolli 2009]. in the following, we briefly describe these models, with emphasis on the differences among them and the modifications made with respect to the model used for the csep experiment that was carefully described in lombardi and marzocchi [2010a] and falcone et al. [2010]. for the lm model, equation (1) reads as: (3) where o is the poisson background seismic rate, is the spatial distribution of the background seismicity, a is the coefficient of the exponential magnitude productivity law, k is the productivity coefficient, c is the time constant of the generalized omori law, p is the exponent of the generalized omori law, d is a characteristic distance for triggering, ri is the distance with the i-th past earthquake, cdqc is a normalization factor for the spatial distribution, b is the parameter of the probability density function derived from the gutenberg-richter law. for the fcm model, equation (1) reads as: (4) where o is the average proportion of background events to the total number of events, is the average spatially variable rate of seismicity, ri and a have the same meaning as for equation (3), b is the b-value of the gutenberg-richter law, and the average triggering distance of the aftershock zone di is related to the magnitude of the triggering event, by the forecasting emilia earthquake sequence t1 , ,t x m x f m m f x x f t t g m m1 2 3i min i i:t min i i = + + m on ` ` ` ` ` ` j j j j j j : d/ x , ,t x mm^ h 1 exp , ,p t x m dtdxdm= mx ^ h' 1### t1 ,x t m x t t c k e r d e c e , 2 0 2 i p m m i:t i m m q dq m m i min i i min min = + + + m on b a c b -c ^ ^ ^ ^ ^ ^ ^ h h h h h h h ; 6 @ e / xn^ h xn^ h t1 , , 10 x t m x t t c k d r d 2 2 2 i p i:t i i i q b m m i min = = + + + m on ^ ^ ^ c ^ h h h m h; e/ equation [zhuang et al. 2004, 2005], . for both of the models, the set of free parameters are estimated by applying a maximum-likelihood procedure to the local seismicity observed before may 20, 2012. the parameters are reported in table 1. it is worth noting that the different parameterization adopted by the two models makes it difficult to compare the parameters, like the productivity k and the background rate. some important differences between the models are worth stating: (i) in the lm model, the function represents the spatial probability density function of background events, and it is estimated using the iteration algorithm developed by zhuang et al. [2002], while in the fcm model, represents the average spatially variable seismic rate of the area, and it is computed using the method introduced by frankel [1995]. moreover, the parameter o indicates the rate of background events in the whole area in the lm model, whereas it indicates the proportion of background events in the fcm model. (ii) the lm and fcm models assume different parametrizations for the dependence on the magnitude of the triggering earthquakes. specifically, for the lm model, the parameters a and c determine how the triggering capability and the characteristic distance depend on the magnitude of the triggering earthquake, respectively. for the fcm model, the dependence on the magnitude of the triggering earthquakes is included in the spatial response function through parameter a alone. (iii) for the fcm model first, the integral in equation (2) is replaced by the instantaneous rate calculated in the middle of the forecasting time window, considering the triggering effect of the earthquakes occurred before the time t, and without taking into account the contribution of unknown events that occurred inside. the model lm adds the contribution of unknown events that occurred during the forecast period to the triggering effect of all of the real events that occurred up to the start of the forecasting time window. the latter is computed as the median of the rates coming from 1,000 different simulated catalogs. (iv) for the lm model, the confidence interval reported in figure 2 is computed as the 15-th and 85-th percentiles of the rates obtained by the 1000 simulations [see also lombardi and marzocchi 2010b]. the 70-th confidence interval is expected to mimic the 1-v confidence interval. the confidence interval of the expected number of events for the fcm model is computed by assuming a poisson distribution of the number of events in each forecasting period. in this case, for large mean m, the distribution is well approximated by a normal distribution with standard deviation m0.5. then, for each forecast period, the confidence interval is computed as [m–m0.5, m+m0.5]. to cope with the strong variation of mmin immediately after a large shock, the lm model includes an empirical correction; in particular, instead of using the gutenberg-richter relationship obtained before may 20, 2012, the lm model uses the frequency–magnitude distribution that is observed before the time t of the start of the forecast. therefore, it is implicitly assumed that any discrepancy from a gutenbergrichter law will hold also for the next 24 h. usually, this empirical correction is useful for the first days immediately after the large shocks, when the variation in mmin is strong, and it becomes negligible as soon as mmin returns to the value observed before the start of the sequence. 3. daily earthquake forecasts for the emilia 2012 earthquake sequence the area considered for the forecasts is (10.90-11.80 ˚e) × (44.70-45.10 ˚n). during the evolution of the sequence, we changed the area considered once only (on may 29, 2012, immediately after the second-largest earthquake), to account for the east-west expansion of the sequence (see figure 1a; see also the maps in supplementary information). we provided daily earthquake forecasts in real-time for two threshold magnitudes (ml ≥4.0, and ml ≥5.5) from may 20, 2012, at 13:00 utc, a few hours after the first large event (ml 5.9; 02:03 utc), which occurred close to finale-emilia village. all of the forecasts were then regularly provided at 6:00 utc. in some cases, we also provided intermediate forecasts if something important was to happened. figure 1b shows the typical space–time forecast format. each forecast was based on the seismicity that occurred before 6:00 utc, and provided an estimation of the expected seismicity in the following 24 h. immediately after large events, as well as the time variation of the completeness threshold magnitude that was discussed in the previous section, the real-time seismic catalog might have other potential shortcomings, like low-quality magnitude estimations, an increase in the completeness magnitude, and some medium-to-high earthquakes missed in marzocchi et al. 564 parameter lm model fcm model o 2.7 ×10–3 day–1 (small area) 3.1 ×10–3 day–1 (large area) 0.43 k 0.034 dayp–1 0.12 dayp–1 d0 0.7 km 1.48 km q 1.5 (imposed) 2.02 c 7 ×10–3 day 7.08 ×10–3 day p 1.22 1.022 a 0.9 0.63 b 1.0 0.93 c 0.3 not applicable mmin 2.0 2.1 table 1. parameters of the two etas models estimated by the maximumlikelihood method (learning phase). d d 10 ( )i m m 0 / i min 2= a xn^ h xn^ h 565 real-time and included in the database only a few hours or days from their occurrence. this also happened during the l'aquila 2009 sequence, which caused a general underestimation of the forecasts in the first few days [marzocchi and lombardi 2009]. the same problem also appeared during the emilia 2012 earthquake sequence, as we will show later. figure 2 summarizes the evolution of the forecasts made with the fcm (left panel) and lm (right panel) models, including the confidence interval bands and the comparisons with the real number of data observed with magnitude ≥2.5. retrospectively, we ran the same models with the updated seismic catalogs. here, we do not attempt any rigorous tests for checking the consistency of the forecasts; nonetheless, figure 2 shows that the two models forecast the number of earthquakes with magnitude ≥2.5 well, with the notable exception of the days immediately after the two largest shocks (note that the confidence intervals of the models reported in figure 2 are expected to encompass about 65% to 70% of the data). retrospectively, we ran the same models with the updated seismic catalog (that was not yet the official catalog, which will be released after a few months). the revised forecasts of the lm model were closer to the real number of earthquakes observed, while the fcm model forecasts remained almost unaltered. this is due to the different strategies of the models to cope with incomplete seismic catalogs (see previous section). more importantly, these results highlight the importance of reliable real-time catalogs to provide reliable forecasts. we also note that both of the models performed well in terms of the spatial distribution of the earthquakes. all of the forecasting maps for ml ≥4.0 are reported in the supplementary information. for example, all of the ml ≥5.0 events occurred in parts of the grid that were characterized by higher spatial probabilities (see, for instance, figure 1b). the importance of reliable real-time seismic catalogs is clearly highlighted in figure 3, where we report on the temporal evolution of the earthquake probability of ml ≥4.0 for the whole area (figure 1a). the lm model has a significant variability between the forecasts made in real-time and the forecasts made using the updated seismic catalog. nonetheless, as also shown in figure 2, these differences are probably inside the natural variability of the process, and therefore this bias is unlikely to produce completely unreliable results, excluding in some cases the first days immediately after a large shock [see also woessner et al. 2011]. finally, the presence of two independent models poses the problem of how to condense the forecasts of each model in one single forecast. for now, we simply present the results of the two models separately. this choice has been made to highlight the epistemic uncertainty of the forecasts, but it is not yet satisfactory from a scientific point of view (see next section). 4. conclusions and perspectives the preliminary results shown here highlight some important issues that are worth mentioning: 1. both models forecast the number of earthquakes observed and the locations of the largest shocks reasonably well. we have not yet checked the consistency of the forecasts through statistical tests, but the plot of figure 2 shows that the number of forecast earthquakes matches well the number of aftershocks observed. this indicates that seismologists are ready to provide reliable information in real-time to decision makers in order to set up short-term risk mitigation actions. 2. to provide accurate forecasts, we need accurate realtime seismic catalogs. in particular, the reduction in the detecforecasting emilia earthquake sequence 05/19/12 05/24/12 05/29/12 06/03/12 06/08/12 10 −2 10 −1 10 0 10 1 10 2 time d ai ly n um be r of e ar th qu ak es w it h m >= 2. 5 observed fcm fcm revised confidence interval bands 05/19/12 05/24/12 05/29/12 06/03/12 06/08/12 10 −2 10 −1 10 0 10 1 10 2 time observed lm lm revised confidence interval bands figure 2. daily observed (black circles) and forecast number of events of ml ≥2.5 in the whole area. the two plots represent the forecasts made with the fcm model (left) and lm model (right). we report two forecasts: one that has been provided in real-time, and one that is a revised forecast using an updated seismic catalog. the black lines represent the 1-v confidence interval for the fcm model and the 70% confidence interval for the lm model applied to the revised seismic catalog (see text for details). vertical black dotted lines, times of occurrence of the two main shocks of the sequence (may 20, 2012, ml 5.9; may 29, 2012, ml 5.8). tion efficiency immediately after a large shock usually results in an underestimation of the earthquake forecasting models, as the triggering effects of all of these earthquakes cannot be considered in the forecast. this bias tends to decrease sharply after the occurrence of a large earthquake. a reliable real-time seismic catalog would allow us to produce daily forecasts every few hours; in this way, we might be able to better capture the variations in the seismicity that can occur during the day. 3. after a large shock, the completeness magnitude changes through time. these etas models need robust strategies to cope with this issue. to date, those who use these models sometimes use ad-hoc empirical strategies (see the lm model). more satisfactory and robust strategies are required to tackle this issue in the future. 4. an oef implies authoritative scientific forecasts. with different models, we argue that an authoritative forecast must incorporate all of the information available, to quantify the epistemic uncertainties among the models, and it should represent the 'best-available' model [cf. marzocchi and zechar 2011]. in the case of the 2012 emilia earthquake sequence, we simply report the output of the different models without making any attempt to provide a single forecast. in the near future we plan to: (i) increase the number of earthquake forecasting models, with the only restriction that each model to be included has to be under testing in the csep experiment in italy or elsewhere; and (ii) create an ensemble model by averaging all of the forecasts. this last step can be achieved by weighting of the forecasts of each model according to the performance of each model, using the csep testing phase [see marzocchi et al. 2012b]. in this way, the ensemble model will represent an average estimation of the forecasts. this approach has some similarity with the use of the logic tree in seismic hazard, where all of the branches are merged into one single hazard assessment (usually the average). with respect to this kind of averaging, here we have the advantage that we can to assign objective weights to each forecast [see marzocchi et al. 2012b], while the weight of the branches in the logic tree are usually assigned subjectively. acknowledgements. this work has been partially funded by the fp7 eu project reakt. references agnew, d.c., and l.m. jones (1991). prediction probabilities from foreshocks, j. geophys. res., 96, 11959-11971. console, r., and m. murru (2001). a simple and testable model for earthquake clustering, j. geophys. res., 106, 8699-8711. console, r., m. murru and a.m. lombardi (2003). refining earthquake clustering models, j. geophys. res., 108, 2468; 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(2011). next-day earthquake forecasts for the japan region generated by the etas model, earth planets space, 63, 207-216. *corresponding author: warner marzocchi, istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy; email: warner.marzocchi@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. forecasting emilia earthquake sequence << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice l ' a t t u a l e d e f o r m a z i o n e d e l l a c r o s t a t e r r e s t r e ( * ) silvio p o l l i premesse. — i n p r e c e d e n t i l a v o r i , i n d i c a t i n e l l a b i b l i o g r a f i a , è s t a l o m e s s o i n e v i d e n z a u n p r o c e d i m e n t o c h e c o n s e n t e d i m i s u r a r e i l e n t i m o v i m e n t i v e r t i c a l i d e l l a s u p e r f i c i e t e r r e s t r e i n d i p e n d e n t e m e n t e d a l l a v a r i a z i o n e d e l l i v e l l o m e d i o m a r i n o e d a g l i s p o s t a m e n t i v e r t i c a l i c o s t i e r i . i l m e t o d o d à p e r t a n t o m i s u r e c h e h a n n o u n g r a d o d i a s s o l u t i s m o s u p e r i o r e a q u e l l e c h e si o t t e n g o n o p r e n d e n d o c o m e sup e r f i c i di r i f e r i m e n t o p i a n i p a s s a n t i p e r c a p o s a l d i d e l l a c o s t a o p p u r e l i v e l l i m e d i m a r i n i . e s s o si b a s a s u l l a d e t e r m i n a z i o n e d e l l a d i f f e r e n z a f r a l e v e l o c i t à d e l m o v i m e n t o v e r t i c a l e m e d i o di t u t t i i m a r i e q u e l l o d i u n a d a t a s t a z i o n e m a r e o g r a f i c a . q u e s t a d i f f e r e n z a dà l o s p o s t a m e n t o v e r t i c a l e a s s o l u t o l i n s e n s o l a r g o ! d e l c a p o s a l d o m a r e o g r a f i c o n e l l ' i n t e r v a l l o di t e m p o c o n s i d e r a t o . q u a l e a p p l i c a z i o n e d i q u e s t o p r o c e d i m e n t o s o n o s t a t i c a l c o l a t i gli s p o s t a m e n t i v e r t i c a l i a t t u a l i d e i c a p o s a l d i c o s t i e r i p e r t u t t e l e s t a z i o n i m a r e o g r a f i c l i e di t u t t i i m a r i . 11 c a l c o l o è s t a t o eseg u i t o p e r p e r i o d i d e c e n n a l i n e l l ' i n t e r v a l l o 1 8 7 1 1 9 4 0 . p r e s e n t i a m o n e l l a t a b e l l a a l l e g a t a u n e s t r a t t o d e i v a l o r i r i g u a r d a n t i i l c i n q u a n t e n n i o 1 8 9 1 1 9 1 0 . 11 materiale. — s o n o s t a t e p r e s e i n c o n s i d e r a z i o n e t u t t e l e staz i o n i m a r e o g r a f i c h e c h e d i s p o n e v a n o d i r e g o l a r i s e r i e di r e g i s t r a z i o n i p e r a l m e n o u n p e r i o d o c o n t i n u o di 1 0 a n n i c o m p r e s o n e l l ' i n t e r v a l l o 1 8 9 1 1 9 4 0 . n e l c a s o d i d u e o p i ù s i t i v i c i n i e c o n r e g i s t r a z i o n i s i m i l i si s c e l s e q u e l l o p i ù r e g o l a r e e s i g n i f i c a t i v o . r i s u l t a r o n o c o s ì s e l e z i o n a t e 1 1 0 s t a z i o n i d i s t r i b u i t e l u n g o t u t t i i m a r i e c o m p r e s e t r a i 6 4 ° d i l a t i t u d i n e n o r d ( r o n n s k ì i r , f i n l a n d i a ) e d i 48" d i l a t i t u d i n e s u d ( d e s e a d o , a r g e n t i n a ) . m o l t o s c a r s e s o n o l e s t a z i o n i n e l l ' a f r i c a , s p e c i a l m e n t e m e r i d i o n a l e , e n e l l ' a m e r i c a d e l s u d . i n u t i l i z z a b i l i a l n o s t r o scop o s o n o l e b r e v i s e r i e m a r e o g r a f i c h e e s e g u i t e d a l l e d i v e r s e s p e d i z i o n i ( * ) c o m u n i c a z i o n e p r e s e n t a t a a l c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , t e n u t o a r o m a i l 1 7 1 8 g i u g n o 1 9 5 3 . l a t t u a l e deformazione della crosta t e r r e s t r e 5 5 7 p o l a r i a l a t i t u d i n i s u p e r i o r i ( i n v a l o r e a s s o l u t o ) a q u e l l e s o p r a i n d i c a t e . s i v e d r à p i ù a v a n t i q u a n t o p r e z i o s i p o t r e b b e r o e s s e r e i d a t i di q u e s t e z o n e . p e r e v i d e n t i r a g i o n i di c a l c o l o d e i v a l o r i m e d i è n e c e s s a r i o d i s p o r r e di u n a d i s t r i b u z i o n e u n i f o r m e d e l l e s t a z i o n i m a r e o g r a f ì c l i e l u n g o l e c o s t e di t u t t i i m a r i . p e r t a n t o l e s t a z i o n i v i c i n e f u r o n o r a g g r u p p a t e s e c o n d o z o n e c o s t i e r e o s e c o n d o m a r i , a t t r i b u e n d o a d o g n i z o n a i l v a l o r e m e d i o d i q u e l l i c o r r i s p o n d e n t i a l l e s i n g o l e l o c a l i t à . i v a l o r i m e d i d e g l i s p o s t a m e n t i v e r t i c a l i d e l l e z o n e c o n s i d e r a t e , i n c m p e r d e c e n n i o , figurano n e l l a stessa t a b e l l a a l l e g a t a . il procedimento di ricerca. — s e g n a n d o su d i u n g l o b o t e r r e s t r e 0 su d i u n a s u a r a p p r e s e n t a z i o n e p i a n a c o n u n s e g n o — l e z o n e i n f a s e d i a b b a s s a m e n t o e c o n u n s e g n o +q u e l l e i n f a s e d i s o l l e v a m e n t o , si v e d e c b e l e p r i m e s o n o p i ù f r e q u e n t i l u n g o u n a l a r g a f a s c i a e q u a t o r i a l e c o m p r e s a c i r c a f r a i 4 5 ° di l a t i t u d i n e n o r d e i 4 5 ° di l a t i t u d i n e s u d , m e n t r e l e s e c o n d e , c i o è i s o l l e v a m e n t i , s o n o p i ù n u m e r o s i f u o r i di t a l e f a s c i a , c i o è n e l l e d u e c a l o t t e p o l a r i s i t u a t e s o p r a e s o t t o 1 45" di l a t i t u d i n e n o r d , r i s p e t t i v a m e n t e s u d . q u e s t a v i s i o n e q u a l i t a t i v a è c o n f e r m a t a a n c h e q u a n t i t a t i v a m e n t e dai s e g u e n t i c a l c o l i n u m e r i c i . s i d e t e r m i n i , p a r t e n d o d a i d a t i d e l l a t a b e l l a , la m e d i a a r i t m e t i c a d e i v a l o r i d e g l i s p o s t a m e n t i ( c o n s i d e r a t i c o n il l o r o s e g n o a l g e b r i c o ) di t u t t e l e z o n e c o m p r e s e n e l l e d u e c a l o t t e p o l a r i s i t u a t e a n o r d e a s u d d e l l e l a t i t u d i n i di 4 5 ° n e di 4 5 ° s . si o t t e r r à il v a l o r e + 0 , 3 m m / d e c e n n i o , c o r r i s p o n d e n t e p e r t a n t o a d u n s o l l e v a m e n t o g e n e r a l e d e l l e t e r r e s i t u a t e n e l l e d u e c a l o t t e p o l a r i . d e t e r m i n a n d o i n v e c e , i n m o d o a n a l o g o , l o s p o s t a m e n t o v e r t i c a l e m e d i o di t u t t e l e r e g i o n i c o s t i e r e c o m p r e s e t r a i p a r a l l e l i di 4 5 ° s u d e n o r d , c i o è s i t u a t e n e l l a f a s c i a e q u a t o r i a l e , si a v r à i l v a l o r e — 0 , 2 m m d e c e n n i o , c o r r i s p o n d e n t e a d u n a b b a s s a m e n t o g e n e r a l e d e l l e z o n e c o n t i n e n t a l i c h e si t r o v a n o i n q u e s t a f a s c i a . r i p e t e n d o gli stessi c a l c o l i p e r l e z o n e e q u a t o r i a l i c o m p r e s e t r a le l a t i t u d i n i di . £ 5 5 ° , + 50", + 40", + 35", 30", e p e r l e c o r r i s p o n d e n t i c a l o t t e c o m p l e m e n t a r i si o t t e n g o n o i v a l o r i d e l l a s e g u e n t e t a b e l l i u a : spostamenti verticali in mm/ decennio (-(innalzamento, — abbassamento) z o n a l a t i t u d i n i l i m i t i t r a l a f a s c i a e q u a t o r i a l e e l e c a l o t t e p o l a r i z o n a + 5 5 ° 4.50" ± 4 5 » ± 4 0 " ± 3 5 " ± 3 0 " p o l a r e e q u a t o r i a l e + 1 , 7 0 — 0 , 2 3 + 0 . 5 1 — 0 , 2 4 + 0 , 3 2 — 0 , 2 0 + 0 , 1 2 — 0 , 1 8 + 0 , 0 9 — 0 , 2 8 + 0 , 0 9 — 0 , 3 3 5 5 8 s i l v i o p o l l i spostamenti verticali decennali dei caposaldi costieri (innalzamento, — abbassamento) 1 8 9 1 0 0 1 9 0 1 1 0 1 9 1 1 2 0 1 9 2 1 3 0 1 9 3 1 4 0 1 8 9 1 4 0 c m c m m i c m cni c m f e n n o s c a n d i a + 2 . 8 + 5 , 2 + 3,9 + 6 , 0 + 0,5 + 3 , 7 l e t t o n i a — 4 , 2 + 8 , 0 + 0 , 4 — 1 , 6 — -1-0,7 g e r m a n i a — 3 , 3 + 0 , 3 + 1,2 — 0 , 7 + 2 , 1 — 0 , 1 o l a n d a — 2 , 0 — 3 , 2 — 0 , 6 + 1 , 1 + 2 . 8 — 0 , 4 i n g h i l t e r r a + 0 , 1 — 2 , 4 — — 3 , 0 + 1 , 4 — 1 , 0 f r a n c i a w + 1 , 4 — 5 , 5 + 0 , 9 — 1 , 3 — 1 , 6 — 1 , 2 f r a n c i a s + 0 , 2 + 0 , 5 + 0 , 2 — 0 , 6 2 , 6 — 0 , 5 s p a g n a + 1 , 7 + 4 , 9 — 4 , 2 + 3 , 3 — + 1 , 4 p o r t o g a l l o — — + 4 , 3 + 4 , 4 + 2 , 5 + 3 , 7 a z z o r r e — — + 0 , 8 — 1 , 6 — — 0 , 4 c o r s i c a — — . + 0 , 1 — + 0 , 1 s a r d e g n a — — 0 , 9 0 — 2 , 1 — — 1 , 0 m . t i r r e n o — 0 , 2 — 0 , 3 — 1 , 1 — 0 , 9 + 0 , 6 — 0 , 4 s i c i l i a — + 0 , 9 — 0 , 5 — — + 0 , 2 m . a d r i a t i c o — 0 , 3 — 1 , 4 — 0 , 7 (1.6 — 1 , 6 — 0 , 9 c a n a l e s u e z — — — + 0 , 1 + 0 , 1 a l g e r i a + 0,8 (i.1) + 1 , 5 — 4 , 8 + 1 , 3 — 0 , 4 m a r o c c o — — — — — 0 , 7 — 0 , 7 c o s t a d ' o r o — 3 , 4 — 3 , 4 a r g e n t i n a — — + 1,2 2 , 5 — 0 , 2 — 0 , 5 u s a s — — — 2 , 8 — 1 , 6 3 , 6 — 2 , 7 u s a e — — 0 , 1 2 2 + 0 , 5 — 3 , 8 — 1 , 1 a l a s k a — — — 0 , 7 + 2 , 1 + 0 , 7 u s a w — — 1 , 6 + 1 , 3 — 1 , 4 — 1 , 1 — 0 , 7 c a n a d a e — + 0 , 9 + 0 , 3 + 0 , 4 + 0,3 + 0,5 c a n a d a w — — + 0 , 3 — 0 , 3 — 0 b a l b o a — — + 0,9 — 0 , 4 2 , 8 — 0 , 8 c r i s t o b a l — — + 1 , 7 + 2 , 5 + 1 , 5 + 1 , 9 h a w a i --— + 0,3 + 1 , 0 + 1 , 5 + 0 . 9 n u o v a z e l a n d a — — — 2 , 6 + 2 , 1 + 0 , 3 0 a u s t r a l i a + 1 , 4 — 2 , 8 + 3 , 5 + 2 , 1 — 0 , 7 + 0 , 7 m a d a g a s c a r — — — — + 0 , 6 + 0 , 6 a r a b i a + 0,2 + 1/) + 0 , 3 — — + 0 , 7 i n d i a + 1 , 4 — 0 , 5 0 0 + 1 , 0 + 0 , 4 b u r m a + 2 , 0 — 2 , 1 — 1 , 6 -1-2,4 + 3 , 4 + 0 , 8 i s . a n d a m a n e — 2 , 1 — 1 , 4 — 0 , 8 — — 1 , 4 i n d i e s e — — — — 0 , 8 — — 0 , 8 g i a p p o n e — 0 2 , 2 — 2 , 5 — — 1 , 6 f o r m o s a — ' — 1 , 2 + 1 , 2 — • 0 l ' a t t u a l e deformazione della crosta t e r r e s t r e 5 5 9 d a l l ' e s a m e d i q u e s t a t a b e l l i n a r i s u l t a q u a n t o s e g u e : 1 ) i n t u t t i i sei casi di d i v i s i o n e d e l l a s u p e r f i c i e t e r r e s t r e o r a c o n s i d e r a t i si h a s e m p r e u n s o l l e v a m e n t o d e l l a c r o s t a stessa n e l l e z o n e p o l a r i e d u n a b b a s s a m e n t o n e l l a f a s c i a e q u a t o r i a l e . 2 ) i l s o l l e v a m e n t o m e d i o p e r l e c a l o t t e p o l a r i è d e l l ' o r d i n e d i 0 , 3 m m / c l e c e n n i o , l ' a b b a s s a m e n t o e q u a t o r i a l e è d e l l o stesso o r d i n e . 3 | i l s o l l e v a m e n t o m a s s i m o ai d u e p o l i è di q u a s i 0 , 5 m m / d e c e n n i o , l ' a b b a s s a m e n t o m a s s i m o a l l ' e q u a t o r e r i s u l t a p u r e di q u a s i 0 , 5 m i n / d e c e n n i o . conclusioni. — a t t u a l m e n t e l a c r o s t a t e r r e s t r e s u b i s c e u n a d e f o r m a z i o n e c o r r i s p o n d e n t e a d u n a d i m i n u z i o n e d e l l o s c h i a c c i a m e n t o l u n g o l'asse p o l a r e . essa t e n d e ad a v v i c i n a r s i a l l a f o r m a s f e r i c a . l ' a u m e n t o d e l d i a m e t r o p o l a r e r i s u l t a di q u a s i 1 m m p e r d e c e n n i o , d e l l o stesso o r d i n e r i s u l t a la d i m i n u z i o n e d e l d i a m e t r o e q u a t o r i a l e . t u t t o c i ò p o t r e b b e e s s e r e i n r e l a z i o n e c o n l a d i m i n u z i o n e d e l l a p r e s s i o n e e s e r c i t a t a d a i g h i a c c i a i s u l l e c a l o t t e p o l a r i i n c o r r i s p o n d e n z a a l l a l o r o a t t u a l e n o t e v o l e f a s e di f u s i o n e . trieste — istituto talassografico — giugno 1953. riassunto sulla base dei dati mareografici di tutte le stazioni della terra si mette in evidenza come attualmente la crosta terrestre si deformi con tendenza a diminuire lo schiacciamento lungo l'asse polare. l'aumento del diametro polare risulta di quasi 1 mm per decennio, la diminuzione del diametro equatoriale è dello stesso ordine. su m mary on the basis of marinographical data from ali the stations of the earth, it is shown hoiv the terrestrial crust is actually cleforming with a tendency to diminish the sliortenirig along the potar axis. the increase in the polar diameter lias become about 1 nini for every ten years, and the decrease in the equatorial diameter is of the sanie order. 5 6 0 s i l v i o p o l l i b i b l i o g r a f i a polli s . : livello medio del mare nella livellazione di precisione. b o l l . s o c . adì". s e . n a t . , x l i i ( 1 9 4 6 ) , 5 2 5 6 , t r i e s t e . polli s . : il graduale alimento del livello marino determinalo per 30 porti del mediterraneo. b o l l . s o c . a d r . s e . n a t . , t r i e s t e , v o i . x l i i , 1 9 4 6 . polli s . : gli attuali movimenti verticali delle coste italiane. t e c n i c a i t a l i a n a , t r i e s t e , n . s „ 4 , 1 9 4 7 . polli s . : registrazioni dei bradisismi costieri. g e o f i s i c a p u r a e a p p l i c a t a , m i l a n o , v o i . x i i , f a s e . 3-4, 1 9 4 8 . polli s . : misura dei lenti movimenti verticali della superficie terrestre. r i m i . a s s . g e o f i s . i t a l . , 1 0 1 1 g i u g n o 1 9 5 2 . polli s . : gli attuali movimenti verticali delle coste continentali. a n n a l i di g e o f i s i c a , v o i . v . n . 4 , 5 9 7 6 0 2 , 1 9 5 2 . articolo galadini 1.pdf key words paleoseismology – active fault – holocene – central italy 1. introduction most of the available structural frameworks of the abruzzi apennines related to the present tectonic regime show that faults active during the late pleistocene-holocene are organised in two different fault sets roughly trending nw-se (barchi et al., 2000; galadini and galli, 2000; d'addezio et al., 2001; valensise and pantosti, 2001). paleoseismological data (e.g., pantosti et al., 1996) and the comparison between the active faulting framework and the distribution of the strong historical seismicity (m 6.5; e.g., from working group cpti, 1999) show that almost all the faults of the western set activated during the 2nd millennium a.d. (galadini and galli, 2000). the same data, in contrast, indicate the lack of recent activation for most of 793 annals of geophysics, vol. 46, n. 5, october 2003 mailing address: dr. fabrizio galadini, istituto di geologia ambientale e geoingegneria, area della ricerca di roma «tor vergata», via del fosso del cavaliere 100, 00133 roma, italy; e-mail: f.galadini@igag.cnr.it paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) fabrizio galadini (1), paolo galli (2) and marco moro (3) (1) istituto di geologia ambientale e geoingegneria, cnr, roma, italy (2) servizio sismico nazionale, dpc, roma, italy (3) istituto nazionale di geofisica e vulcanologia, roma, italy abstract paleoseismological analyses were performed along the campo imperatore fault (part of the gran sasso range fault system) in order to define the seismogenic behaviour (recurrence interval for surface faulting events, elapsed time since the last activation, maximum expected magnitude). four trenches were excavated across secondary faults which are related to the main fault zone. the youngest event (e1) occurred after 3480-3400 years bp; a previous event (e2) occurred between 7155-7120/7035-6790 years bp and 5590-5565/5545-5475 years bp, while the oldest one (e3) has a late pleistocene age. the chronological interval between the last two displacement events ranges between 1995 and 6405 years. the minimum elapsed time since the last activation is 800 years, due to the absence of historical earthquakes which may have been caused by the campo imperatore fault and based on the completeness of the historical catalogues for the large magnitude events in the last eight centuries. based on the length of the fault surficial expression, earthquakes with m 6.95 may be expected from the activation of the entire gran sasso range fault system. the effects of the fault activation were investigated through the simulation of a damage scenario obtained by means of the faces computer code, made by the national seismic survey for civil protection purposes. the damage scenario shows that the activation of the gran sasso range fault system may be responsible for an earthquake with epicentral intensity i0 10.5 mcs, with a number of collapsed buildings ranging between 7900 and 31100 and a number of damaged buildings ranging between 99 000 and 234 000. the investigated case defines, therefore, a high risk level for the region affected by the campo imperatore fault. the faults of the eastern set. the elapsed time since the last activation may be, in such cases, larger than 1000 years and for this reason these faults have been defined as silent by galadini and galli (2000). for example, in the case of the mt. morrone fault an elapsed time of about 1800 years has been hypothesized (galadini and galli, 2001). considering that the recurrence interval per fault in the abruzzi apennines is in the order of 1500-2500 years, a high level of seismic hazard may be related to the faults of the eastern set (e.g., peruzza and pace, 2002). the campo imperatore fault, which is part of the longer gran sasso range fault system, is one of the structures forming the eastern fault set. paleoseismological investigations therefore seem appropriate to ascertain the fault activity and to define the fault behaviour. this paper deals with paleoseismological investigations along the campo imperatore fault, made through the excavation and analysis of four trenches. investigations were made during 1999. the campo imperatore fault was partially investigated in the early 90s by means of two trenches along its westernmost portion (giraudi and frezzotti, 1995). we tried to increase the data set on this fault by further trenching in the central portion. preliminary results of the paleoseismological analysis along the campo imperatore fault have already been published in a short note (galli et al., 2002a). in the present paper, we intend to give a complete account of our paleoseismological analyses related to this fault. after a short introduction on the seismotectonic framework of the abruzzi apennines, the following sections will be dedicated to the description and interpretation of the three most significant trenches. implications of the analysis, in terms of fault behaviour (recurrence interval for surface faulting events, elapsed time, maximum expected magnitude) will be described in detail. a final section will be dedicated to the effects of the fault activation, in terms of damage in the territory surrounding the gran sasso range fault system. although the investigated fault affects a high mountainous region, numerous villages and towns with maximum 100.000 inhabitants are located in the hangingwall area and at a distance not further than 15 km from the fault surficial expression. the produced damage scenario will show that the activation of the investigated silent fault may have a destructive impact on a large portion of central italy. 2. active tectonic framework 2.1. the abruzzi apennines the abruzzi apennines have been affected by normal and normal oblique faulting during the quaternary (e.g., cnr-pfg, 1987), synchronous to the regional uplift (e.g., d’agostino et al., 2001) and the e-ward thrust migration in the adriatic area (e.g., patacca et al., 1990). quaternary normal fault activity has been documented in a number of papers dealing with neotectonics (e.g. cavinato, 1993; galadini and messina, 1994; carrara et al., 1995). the main quaternary faults generally border intermontane basins, filled by plio-quaternary lacustrine and alluvial deposits, showing geometric characteristics typical of the half-graben depressions. persistent activity during the late pleistocene-holocene can be derived from works dealing with paleoseismology (pantosti et al., 1996; galadini and galli, 1999; d’addezio et al., 2001) and active tectonics (barchi et al., 2000; galadini and galli, 2000). moreover, available seismic catalogues (e.g., working group cpti, 1999) show that the investigated area has been struck by six events with m > 6.5 since the 14th century (i.e. the 1349, 1456, 1703 january 14, 1703 february 2, 1706, 1915 earthquakes). the structural framework related to the present tectonic regime (fig. 1) shows that the abruzzi apennines are affected by two sets of nw-se to n-s trending normal faults, as reported in most of the available works on paleoseismology and active tectonics (barchi et al., 2000; galadini and galli, 2000; d’addezio et al., 2001; valensise and pantosti, 2001). paleoseismological and historical data suggest that almost all the faults of the western fault set activated during the 2nd 794 fabrizio galadini, paolo galli and marco moro millennium a.d., while there is no evidence of recent activation for most of the faults related to the eastern set (galadini and galli, 2000). the latter authors defined these faults (including the campo imperatore fault) as silent. 2.2. the gran sasso chain – campo imperatore plain the gran sasso range area is affected by three main fault systems (from west: assergi, campo imperatore and mt. cappucciata.-.mt. 795 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) fig. 1. seismotectonic framework of the investigated area. the investigated faults are reported in red. earthquake epicenters and the highest intensity datapoints distribution (hidd; size proportional to intensity, i > 8 mcs) have been taken from magri and molin (1984) for the 1456 earthquake, boschi et al. (1995) for the 1349 and 1706 earthquakes, monachesi and stucchi (1998) for the 1703 earthquake, molin et al. (1999) for the 1915 earthquake; all the epicenter locations have been derived from working group cpti (1999). mvef mt. vettore fault; nfs norcia fault system; lmf laga mts. fault; uafs upper aterno valley fault system; af assergi fault; cif campo imperatore fault; cvf mt. cappucciata-mt. san vito fault; cfcf campo felice-colle cerasitto fault; mavf middle aterno valley fault; opf ovindoli-pezza fault; mmf mt. morrone fault; ff fucino fault; mrpf mt. rotella-mt. pizzalto fault; acf aremogna-cinquemiglia fault; usfs upper sangro valley fault system. fabrizio galadini, paolo galli and marco moro fig. 2. view of the campo imperatore plain and of the southern slope of the gran sasso range; arrows mark the position of the bedrock scarp related to the main fault; trenches have been excavated across minor faults located in the large alluvial fan in the foreground (see next figures for further details). fig. 3a,b. a) plan map showing the faults of the gran sasso range area characterised by late pleistoceneholocene activity. b) geomorphological map of the mt. paradiso area and location of the trench sites. a b 796 san vito faults, respectively) showing evidence of late pleistocene-holocene activity (galadini and galli, 2000; fig. 1). we investigated the central fault, n120-130 trending and 21.5 km long, which bounds the campo imperatore plain (figs. 1, 2 and 3a,b). this basin is filled by glacial, fluvioglacial, fluvial and lacustrine deposits, mainly of late pleistocene-holocene age (fig. 3b; giraudi, 1994). in the central-western part of the gran sasso range, the campo imperatore fault is made of two parallel splays (fig. 3a). the southernmost one has been responsible for the displacement of late pleistocene-holocene cemented slope deposits and colluvial units (carraro and giardino, 1992), while the northernmost splay affects the slope and the main e-w divide of the chain, displacing the glacial cirque of mt. brancastello. this cirque fed a glacial tongue related to the lgm and its displacement is, therefore, subsequent to 22.600 years bp (age of the lgm in the central apennines, i.e. the campo imperatore stade by giraudi and frezzotti, 1997). the western portion of the fault affects the maone and venacquaro valleys, located at more than 1900 m a.s.l. (fig. 3a). in this part of the gran sasso range, the fault displaced glacial till, colluvial, alluvial and palustrine deposits of late pleistocene-holocene age (giraudi and frezzotti, 1995). impressive fault scarps can be detected in these deposits. paleoseismological investigations defined four displacement events in the last 18.000 years (giraudi and frezzotti, 1995). the most recent event occurred after 3490 ± 160 years bp. the fault is sealed by a number of colluvial units whose presence corroborates the conclusion by giraudi and frezzotti (1995) that the fault activity occurred before 1000 a.d. the recurrence interval defined through these paleoseismological analyses ranges between 2500-3000 and 60007000 years. the authors also defined a vertical slip rate of 0.67-1 mm/yr. 3. geomorphologic framework of the trench area the mt. paradiso alluvial fan (area of the paleoseismological investigation) is located in the central portion of the campo imperatore plain. the fan was built during six alluvial phases (fig. 3b). deposition can be chronologically constrained through the comparison with the adjacent fornaca alluvial fan (showing the same alluvial phases and radiocarbon dated by giraudi and frezzotti, 1997) and the case history on the late pleistocene-holocene alluvial deposition of the abruzzi apennines (frezzotti and giraudi, 1992; giraudi, 1994; giraudi and frezzotti 1997). according to giraudi and frezzotti (1997) the first building phase of the fornaca fan occurred before the formation of a paleosol dated at 31.500 ± 550 years bp, while the second phase occurred in the period close to the lgm, since the related deposits are sealed by a paleosol dated at 17.840 ± 200 years bp. based on the chronological constraints available for another large alluvial fan of the central apennines (majelama alluvial fan; frezzotti and giraudi, 1992), showing the same number of depositional phases defined for the mt. paradiso fan, the 5th and the 6th phases of fan building may be related to the boundary late pleistocene-holocene (about 12.000 years bp) and to 3800-3200 years bp. therefore, the 3rd and 4th alluvial phases occurred between the lgm and the beginning of the holocene and can be related to the resistatic episodes of the late glacial period. an evident bedrock fault scarp affects mt. paradiso (figs. 3b and 4a). its western prolongation is defined by a scarp in the unconsolidated gravels of the mt. paradiso fan, in the part deposited during the 2nd phase (fig. 3b; between 31.500 ± 550 years bp and 17.840 ± ± 200 years bp). the entire fault branch is about 1.6 km long and being located 2 km south of the main fault bordering the gran sasso chain, it probably represents a secondary normal fault dipping southwards. south of this fault, several scarps (shorter than 1 km, about 1 m high) can be detected on the mt. paradiso fan (fig. 3b), bounding small graben-like features on the top of the fan in the sectors built during the 1st (before 31.500 ± 550 years bp) and 2nd phases. the paleoseismological analysis was performed by means of trenches excavated across these scarps. paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) 797 4. paleoseismological analysis the main purpose of our work was to confirm the previous hypotheses on the late pleistocene-holocene activity of the campo imperatore area and to define the chronology of single displacement events. the main fault affecting the campo imperatore area is located along a carbonate slope (fig. 2). investigations along this fault cannot give significant chronological data with traditional approaches. for this reason, the analyses have been made through trenches excavated across the secondary faults described in the previous section. the identification of the trench sites has followed the traditional geomorphological criteria. the investigated short faults affect the top surface of the late pleistocene-holocene mt. paradiso alluvial fan and are easily detectable through aerial photographs and field surveys, since they have been responsible for the formation of scarps in unconsolidated deposits. some of the scarps are related to the activity of fault planes with a dip direction opposite to the drainage on the alluvial fan. therefore, the fault activity was responsible for damming minor streams and inducing deposition (of fine sediments too) in front of the 798 fabrizio galadini, paolo galli and marco moro fig. 4a,b. a) view looking ne of the area of trenches 3 and 4; the black arrows mark the northernmost scarp (facing south) while the white arrows mark the north facing scarp across which trench 3 has been excavated. b) view looking se of the scarp (white arrows) across which the trenches 1 and 2 have been excavated. a b scarps (fig. 4b). this is a general condition for the availability of datable material and for this reason three trenches (nos. 1, 2 and 3) were excavated across two n-sloping scarps, forming small sedimentary traps. trench 4 was excavated across a scarp formed by a fault plane dipping towards south (figs. 3b and 4a). we made this trench to define the displacement history preceding 31.500 ± 550 years bp. although the fault responsible for the formation of the scarp was clearly visible in the trench (fig. 5), no significant stratigraphic markers were found. the trench walls, indeed, only showed the gravels of the 1st phase alluvial fan and gave no evidence for correlating units of gravels across the fault. for this reason, this trench will not be described in detail in the next sections. close to trench 1, qualitative geomorphic correlation of the flanks of a stream across the scarp suggests that a dextral component (about 10 m) can be attributed to the fault. this component, however, cannot be precisely quantified due to the strong irregularity of the stream flanks. the horizontal displacement in the order of several metres is consistent with the relatively old age of stream formation, subsequent to 31.500 ± 550 years bp (age of the alluvial fan) but older than the colluvial deposits filling the incisions, uncovered in trench 2 (see below) and related to the upper late pleistocene. trench walls were analysed through the definition of stratigraphic units and their relationship with the shear planes and by reporting paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) fig. 5. photomosaic of the eastern wall of trench 4, excavated across the northernmost fault of the mt. paradiso fan (campo imperatore fault). the trench was dug in the gravels of the 1st alluvial fan phase (preceding 31.500 ± ± 550 years bp); units across the fault zone cannot be correlated and only a generic activity since the late pleistocene can be defined; arrows mark the main shear plane affecting the gravels. n s ground surface tr a ce o f th e m a in f a u lt 799 the information on graph paper. samples were collected for radiocarbon dating (table i) from a paleosol (cimp-1, -3, -4, -5, -6, -7, -8) or from colluviated material (cimp-9, -11, -12), which gives a minimum age for the deposition. the samples were analysed by beta analytic inc. (miami, florida) and calibrated by means of the intcal98 database (stuiver et al., 1998). in the following sections, we report the 1 calibrated results (68% of probability). however, table i also gives the 2 results, thus giving the possibility to define the related chronological constraints. chronological constraints were also derived from the late pleistocene-holocene stratigraphic framework of the abruzzi apennines, defined by a number of works dealing with paleoclimatic issues (e.g., frezzotti and giraudi, 1989, 1990; frezzotti and narcisi, 1989, 1996, giraudi, 1994, 1998a,b; giraudi and frezzotti, 1997). these authors have defined chronologically constrained (numerical and archaeological datings) depositional events (alluvial, colluvial, lacustrine, aeolic, ...) and pedogenetic phases in the investigated region since about 30.000 years bp. 4.1. trench 1 units 2 and 3 of fig. 6a,b represent the a and b-c horizons of an andisol, respectively. andisols in the central apennines (characterised by the typical blackish colour) formed from colluviated «neapolitan yellow tuff» (see frezzotti and narcisi, 1996, on this point). remnants of the parent material are evident within unit 3. the eruption (related to the volcanic activity of the phlegrean fields, close to neaples) was dated to about 14.120 years bp (date used in the sections dedicated to the paleoseismological analysis) on the basis of the varvesupported sedimentation rate (zolitschka and negendank, 1996) and at about 14.200 years bp on the basis of 14c chronological constraints (asioli et al., 2001; siani et al., 2001). six radiocarbon dates are available for unit 2 (table i) and one for unit 3 (7690-7605 years bp, 1 cal age). considering the climate history following the deposition of the «neapolitan yellow tuff», its pedogenesis probably began at the end of the younger dryas (at the boundary late pleistocene-holocene), i.e. after a long 800 fabrizio galadini, paolo galli and marco moro table i. radiocarbon dates of samples collected in the trenches excavated in the campo imperatore area. r radiometric technique; blc bulk/low carbon material procedure. sample lab. code analysis c13/c12 measured conventional calibrated age calibrated age sample beta ratio age bp age bp l bp 2 bp description cimp1 139363 r, blc –.23.1 ‰ 6790 ± 60 6820 ± 60 7690-7605 7750-7580 buried soil (silty sand) cimp3 139364 r –.24.1 ‰ 4470 ± 40 4480 ± 40 5285-5155/5145-5040 5300-4970 buried soil (silty sand) cimp4 139365 r –.24.1 ‰ 4760 ± 40 4780 ± 40 5590-5565/5545-5475 5600-5460/5370-5340 buried soil (silty sand) cimp5 139366 r –.23.9 ‰ 3530 ± 40 3540 ± 40 3870-3815./.3785-3730 3915-3700 buried soil (silty sand) cimp6 139367 r –.24.3 ‰ 3790 ± 40 3800 ± 40 4245-4140./.4115-4100 4290-4080 buried soil (silty sand) cimp7 139368 r –.24.1 ‰ 3230 ± 40 3240 ± 40 3480-3400 3560-3375 buried soil (silty sand) cimp8 139369 r –.24.1 ‰ 3970 ± 40 3980 ± 40 4510-4475/4445-4410 4530-4380 buried soil (silty sand) cimp9 139370 r, blc –.22.6 ‰ 6050 ± 100 6090 ± 100 7155-7120./.7035-6790 7240-6690 colluvium (silt) cimp11 139371 r –.23.6 ‰ 2340 ± 70 2370 ± 70 2465-2335 2720-2310/2230-2190 colluviated soil (silt) cimp12 139372 r –.23.9 ‰ 2250 ± 40 2270 ± 40 2340-2310./.2230-2190 2350-2290/2270-2155 colluviated soil (silt) 801 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) period of aridity not favourable to the action of the pedogenetic processes. units 4, 5 and 6 are made of gravels whose deposition can be related to debris flows (4, 5 in fig. 6a) or stream flows (6 in fig. 6a,b). two fault planes bounding a small graben can be detected in the eastern trench wall (f1 and f2 in fig. 6a). fault f1 was responsible for the displacement of units 6, 3 and 2, while f2 displaced units 6 and 3, being apparently sealed by unit 2 (fig. 6a). two displacement events were defined through the analysis of this trench. the youngest event (e1) was responsible for the displacement of the a horizon of the andisol (unit 2) across the fault f1. considering that the paleosol is buried and no evidence of contamination from the present soil (e.g., root penetration) was observed, the youngest age of the andisol (3480-3400 years bp) represents a reliable lower chronological limit for e1. in contrast, an upper chronological limit cannot be defined, since f1 is sealed by the present soil whose origin (parent material) is unknown. for this reason, we did not date samples from unit 1 and preferred to maintain the problem of the upper chronological limit of e1 unsolved. no estimate of the vertical offset can be provided, due to the irregular attitude of the unit 2 base and to the evidence of horizontal movements (see previous section). the latter may create apparent vertical movements in horizontally displaced irregular surfaces. an older event (e2) is indicated by the presence of thick unit 3 deposits between f1 and f2 (fig. 6a,b). this may suggest that the deposition of unit 3 occurred within the depression created fig. 6a,b. a) log of the e wall of trench 1 excavated across one of the faults affecting the mt. paradiso alluvial fan, campo imperatore fault (see fig. 3b for location); at least two events occurred after about 14.000 years bp (see text for explanation) can be detected through the paleoseismological analysis. b) view of the e trench wall in the sector affected by the fault f1. a b by the movement along the two faults and that the displacement event occurred slightly before the deposition of unit 3. the event may also be suggested by the erosional surface carved into the gravels of unit 6 in the footwall (south of f1), possibly related to the retreat of a paleoscarp before the unit 3 deposition. if we accept that the deposition of unit 3 occurred within the depression created by the fault movement during e2, this event has approximately the same age of the «yellow neapolitan tuff», i.e. about 14 120 years bp. alternatively, we may hypothesise that faulting occurred after the deposition of the tephra. the thickness of unit 3 increased in the lowered part due to colluviation from the uplifted footwall. this conditioned the pedogenesis, as a larger thickness of andisol formed in the lowered part of the fault (where also the bc horizon, i.e. remnants of the original tephra, are preserved). in such a case, e2 occurred after the deposition of the «yellow neapolitan tuff», i.e. after ca. 14120 years bp. the upper chronological limit for e2 would be represented by the oldest chronology of the a horizon (not affected by the same offset of the b-c horizon). e2 would have occurred, therefore, between 14 120 years bp and 5590-5565/5545-5475 years bp. we prefer this second hypothesis on the basis of the comparison with the succession of events defined in trench 2 (see next section) showing the occurrence of an e2 event after 71557120/7035-6790 years bp. it is probable, therefore, that the event e2 of trench 1 occurred after the date (7690-7605 years bp) reported in the log of fig. 6a,b, related to a phase of the pedogenetic history of the colluviated «yellow neapolitan tuff». the e2 event is also testified: 1) by the upper termination of fault f2, which does not cut the boundary between units 2 and 3, and 2) by the fact that in the sector close to f1, unit 3 fills a triangular area in fig. 6a, probably related to an open fissure which did not affect unit 2. the lack of unit 3 in the footwall (although it was present there, since the andisol formed also south of f1) and the irregular shape of the unit 3 base make the estimation of the vertical offset difficult. a rough evaluation of the minimum offset (related to e1 + e2) may be obtained by calculating the vertical separation between the most regular part of the unit 3 base and the base of unit 2 in the footwall of f1 (far from possible effects of paleoscarp retreat, fig. 6a. this estimate gives 0.63 m. if we accept the second hypothesis about the e2 event, the erosional surface carved into the gravels of unit 6 in the footwall (suggesting paleoscarp retreat processes) may suggest an event e3, which occurred slightly before the deposition of the «yellow neapolitan tuff», i.e. slightly before 14.120 years bp. this evidence is, however, too faint to ascertain the occurrence of an e3 event. 4.2. trench 2 the excavation uncovered a number of stratigraphic units made of carbonate gravels related to debris flow (2, 4, 6, 11, 12 in fig. 7a,b) and stream flow (8, 9, 10 in fig. 7a,b) episodes. the present soil (unit 1) is black and pulverulent. these two characteristics suggest that current pedogenesis is acting over a colluviated andisol. colluviation (also indicated by the presence of carbonate clasts within unit 1) probably occurred as a result of a crisis of the andisol during an arid late holocene period. this period is subsequent to the youngest date available for the andisol of trench 1 (3480-3400 years bp). available works with a paleoclimatic perspective (e.g., giraudi, 1995, 1997), define an arid period that began close to the above reported date and lasted until 2800-2700 years bp. deposition of the parent material of unit 1 probably occurred during this time interval. a colluvial origin can be hypothesized for the massive sandy silt with sparse pebbles of unit 3 (colluviation of a soil) and for the gravel of unit 7. a radiocarbon date is available for unit 3 (7155-7120/7035-6790 years bp), which defines a lower chronological limit for the deposition. unit 7 displays a faint stratification indicating a deposition over a gentle slope dipping northwards. the matrix is scarce and the deposit is prevalently clast supported. moreover it outcrops only in the area affected by the faults. in short, sedimentological fea802 fabrizio galadini, paolo galli and marco moro tures and the geometry of unit 7 indicate that it represents the product of scarp retreat following a displacement event along the fault zone. unit 5 is a quartzic loess; similar deposits have been found in other areas of the central apennines (frezzotti and giraudi, 1989, 1990) and related to aeolic deposition fed by the saharian region. frezzotti and giraudi (1989, 1990) attributed a specific paleoclimatic significance to the aeolic deposition. in particular, the loess deposition in the aremogna plain (about 70 km south of campo imperatore) has been related to a period between 16 000-15 000 years bp (age of the buhl-schlern stades in the alps, corresponding to arid and cold climatic periods) and 12 850 ± 200 years bp (radiocarbon age of a peat level subsequent to the loess deposition). the same chronological constraint has been defined for the quartzic aeolic deposit found in other areas close to the aremogna plain, i.e. mt. greco and matese mts. (frezzotti and giraudi, 1990). on this basis, we hypothesize that unit 5 was deposited between 15 000-16 000 years bp and 12 850 ± 200 years bp. further chronological constraints are available for units 10 to 12 which are part of the alluvial fan succession related to the late pleistocene, older than 31.500 ± 550 years bp (see section dedicated to the active tectonic framework). the eastern trench wall is affected by six fault planes. the shear planes f1 and f2 (fig. 7a) have been interpreted as the main fault zone, since they place in contact units of gravels which cannot be correlated across the investigated trench wall. this means that this fault zone is characterised by an offset larger than that affecting the units of the northern part of the trench, along the f3-f6 fault planes (fig. 7a). faults f3-f5 have been responsible for the displacement of units 4 to 9, while the fault f6 has also been responsible for the displacement of unit 3 and perhaps of unit 1 (present soil). although a lowering of the displaced units northwards is visible (suggesting that the displacements are characterised by a normal component), the different vertical offset (e.g., higher in unit 5 than in unit 8) along f5 and the 803 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) fig. 7a,b. a) log of the e wall of trench 2 excavated across one of the faults affecting the mt. paradiso alluvial fan, campo imperatore fault (see fig. 3b for location); three events occurred after 31500 ± 550 years bp can be detected through the paleoseismological analysis. b) view of the e trench wall; the arrows mark the fault f1. a b different sense of displacement observed along some surfaces (e.g., compare the offset of the unit 7 base with that of the base and top of unit 5; compare the offset of the unit 4 base with that affecting the top of the same unit) suggest that the units have also been horizontally displaced. however, the amount of horizontal offset cannot be estimated. three displacement events were detected through an analysis of the section reported in fig. 7a. the youngest faulting (e1) was responsible for the displacement of the base of the present soil, across the fault zone f1-f2. an open fissure formed between f1 and f2, which was filled by the material of unit 1. the displacement of the base of unit 1 may also have occurred along the fault f6. the vertical displacement (estimation of the vertical separation of the base of unit 1 across the fault zone) is 0.38 m. since unit 1 is forming over the colluvium derived from the andisol, its displacement occurred after the colluvial deposition. based on the characteristics of unit 1 defined at the beginning of this section, this deposition occurred between 3480-3400 years bp (youngest available age for the andisol) and 2800-2700 years bp. e1 occurred, therefore, after 3480-3400 years bp. a previous event (e2) is indicated by the tilting of units 3 to 6 between the faults f6 and f2. as a result of this deformation, subsequent to the deposition of unit 7 (see e3 event below), the stratification and the boundary of deposits with northern provenance (streams as well as the top of the alluvial fan dip southwards), are dipping northwards. as for the chronology of the displacement, e2 occurred after 7155-7120/7035-6790 years bp (1 ), age derived from a sample of unit 3. the deformation is sealed by unit 1 (present soil), as suggested by the evident unconformity between this unit and the older ones. the southernmost outcropping part of unit 2, indeed, seems to be involved in the warping of the older units. the deposition of the parent material of unit 1 occurred between 3480-3400 years bp (1 ) and 2800-2700 years bp and therefore the reported dates represent the upper chronological limit of e2. the lack of deposits which can be correlated across the entire investigated section prevents the possibility of defining the vertical displacement (fig. 7a). an estimation of the minimum vertical offset (comprising, however, the offset along f6, which may also record e1) can be obtained from the vertical separation of the furthest available points of the unit 5 base, which is one of the most regular surfaces available in the investigated section. this minimum offset is 0.80 m. a previous event (e3) is testified by unit 7, made of deposits which have been fed from the south, i.e. from a scarp formed in the area affected by the faults. unit 7 was interpreted as a colluvial wedge derived from the retreat of a scarp due to a displacement along f1-f2. the chronology of e3 is constrained by the age of units 9 and 5. the former is part of the alluvial succession which makes the mt. paradiso fan. since unit 9 is close to the top of the 1st phase alluvial fan and the deposits are preserved in the fault hangingwall (i.e. they are part of the youngest gravel unit of the 1st phase uncovered in the trench), it is probable that its deposition occurred close to 31.500 ± ± 550 years bp (age of the soil formed after this alluvial phase, according to giraudi and frezzotti, 1997). unit 5 was instead chronologically constrained (between 16.000-15.000 years bp and 12.850 ± 200 years bp) through the works by frezzotti and giraudi (1989, 1990). this means that more than 15.000 years passed between the deposition of units 9 and 5. the lack of deposition or very limited deposition during this long time interval is probably due to the end of operation of the old mt. paradiso fan system, and to the deposition of a younger alluvial fan (located elsewhere) with the post-lgm fluvioglacial phases (fig. 3b). the trench area experienced, in the same period, water flow within the incisions carved into the pre-31.500 ± 550 years bp fan. considering this kind of evolution, the new depositional activity (from unit 6 to unit 2) has probably been conditioned by the presence of a scarp damming an area affected by small streams (fig. 4b) and therefore creating the conditions for sedimentary traps. the presence of a scarp derived from the fault displacement and the subsequent deposition of unit 7 during the scarp retreat is also proved 804 fabrizio galadini, paolo galli and marco moro by the thinning of unit 5 towards south and by the end, with a lenticular geometry, of unit 6 towards the same direction (fig. 7a). since unit 7 is overlaid by unit 5 (south of f5), whose chronology is 16.000-15.000 years bp/12.850 ± ± 200 years bp, and deposited after unit 9, whose age is probably slightly older than 31.500 ± 550 years bp, we consider these chronological constraints as the e3 upper and lower chronological limits, respectively. due to the lack of correlative deposits across the fault zone, it is not possible to define the vertical displacement related to this event. 4.3. trench 3 trench 3 was up-to-2.7-m deep and was excavated across a scarp located north of that of trenches 1 and 2 (see fig. 3b for location). like the previously described cases, however, the scarp is sloping northwards. it is, therefore, related to a fault plane responsible for the lowering of the northern block. stratigraphic units are entirely made of gravels related to various alluvial facies (4, 5, 7, 8, 9 and 10 in fig. 8a,b) or to colluvial episodes fed from areas adjacent to that of deposition (2, 3, 6 in fig. 8a,b). unit 1 is the present soil forming over a colluviated older andisol; similarly to unit 1 of trench 2, colluviation of the parent material occurred between 3480-3400 years bp (1 cal age, youngest available radiocarbon date for the andisol) and 2800-2700 years bp. the present pedogenetic phase on the colluviated andisol began, therefore, after the reported ages. as for unit 3, it is characterised by pebbles strongly oriented parallel to the unit base. moreover, these deposits only outcrop in the sector affected by the faults. the geometry and the attitude of the pebbles suggest a local origin, related to a slope dipping northwards. all these aspects indicate that unit 3 represents a colluvial wedge due to the retreat of an n-dipping scarp formed during an episode of faulting. the eastern trench wall is affected by seven fault planes (f1 to f7 in fig. 8a) which were responsible for the displacement of the entire stratigraphic succession previously described. f1, f2 and f3 affect the gravels (units 6, 9 and 10) of the southern part of the trench and are sealed by unit 1, i.e. the present soil (f1), and by unit 3 (f2 and f3). unit 3 is, however, affected by fractures without vertical displacement along the upward prolongation of f2 and f3. the central sector of the trench is affected by the fault planes f4 and f5, to which the most recent activity can be related (displacement of units 3 and 1, fissure filled by unit 2). the fault planes display a high dip (70-80°). vertical components of the movement are evident along faults f1 to f3 and f6-f7. the different thickness of unit 6 across f3, the high dip of the fault planes, the whole geometry of the displacement (resembling a flower structure) suggest the probable occurrence of horizontal movements (fig. 8a). the amount of horizontal offset, however, cannot be estimated. three displacement events were detected through the analysis of the eastern trench wall. the youngest event (e1), is evident along the fault planes f4 and f5 and is indicated by the small offset (0.3 m) affecting the present soil (unit 1). the base of unit 1 is lowered in the sector between the two mentioned faults (fig. 8a). displacements of the unit 1 base were also detected along f7 and f6. as for f7, a horizontal component of the movement may be defined, since the base of unit 1 is lowered north of the fault plane, while units 4 to 8 are uplifted in the same sector (fig. 8a). the horizontal movements affecting units characterized by an irregular geometry and different attitude prevent the possibility of defining the amount of vertical offset. the lowering of the central part of the trench is also suggested by the larger thickness of the present soil in the faulted sector, probably due to post-faulting colluviation from the southern and northern part of the trench. considering that unit 1 may represent the pedogenesis of a parent material made of the andisol discovered in trench 1, and that the colluviation of the andisol occurred between 3480-3400 years bp and 28002700 years bp, faulting related to e1 is subsequent to a moment within the defined time span. a previous event (e2) is suggested by the formation of the open fissure between f4 and f5, filled by gravels without a sandy or silty fraction derived from the present soil or from the andisol. this aspect indicates that the displacement oc805 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) curred before 3480-3400 years bp/2800-2700 years bp. the lower chronological limit is less defined, since no direct dating is available for the displaced gravels. as reported in the previous sections, the gravels are related to an alluvial fan whose top soil has been dated at 31.500 ± 550 years bp (giraudi and frezzotti, 1997). the aerial photographs show that the investigated alluvial fan is everywhere characterized by its original form, representing the depositional top of the alluvial succession. this means that significant erosional episodes did not occur in the investigat806 fabrizio galadini, paolo galli and marco moro fig. 8a,b. a) log of the e wall of trench 3 excavated across one of the faults affecting the mt. paradiso alluvial fan, campo imperatore fault (see fig. 3b for location); three events occurred after 31500 ± 550 years bp can be detected through the paleoseismological analysis. b) view of the e trench wall in the area affected by the faults; the arrows mark the fault plane f4. b a ed area after the gravel deposition. the implication of this observation is that the age of the displaced gravels (close to the top of the fan) is probably slightly older than 31.500 ± 550 years bp. the lower chronological limit for e2 is, therefore, an age preceding (but close to) 31.500 ± ± 550 years bp. available data do not permit hypotheses about the correlation between the stratigraphic units exposed in the southern and northern sectors. it is not possible, therefore, to define the vertical offset related to this event. a previous event (e3) was responsible for the formation of the scarp whose retreat fed the deposition of the colluvial wedge defined as unit 3 in fig. 8a. this colluvial wedge was subsequently faulted, during e2, along fault f4. also in this case, due to the lack of correlation between the units in the southern part and those in the northern part, there is no possibility to define the vertical offset. the thickness of unit 3 defines, however, a significant amount of material derived from a pre-existing scarp. this suggests a vertical offset larger than that related to e1 (indicated by the small displacement of the unit 1 base). as for the chronology of e3, constraints are the same as e2, i.e. the event occurred after an age slightly preceding 31.500 ± 550 years bp and before 34803400/2800-2700 years bp. 5. summary of the displacement events the investigated area gave evidence for the occurrence of at least three coseismic displacement events (fig. 9). two of these events occurred during the holocene (i.e. e1 and e2 of trenches 1 and 2), while there is evidence for the occurrence of at least one event during the late pleistocene (e3 of trench 2). available data (fig. 9) indicate that all the trenches gave evidence for the youngest displacement (e1) after 3480-3400 years bp/.2800-2700 years bp. the upper chronological limit for the e1 occurrence is, however, paleoseismologically undefined. the penultimate seismic event (e2) is constrained at trench 1, between 14.120 years bp and 5590-5565/5545-5475 years bp. this large time span may be reduced if e2 of trench 1 is the same as e2 of trench 2. in such a case, the lower chronological limit is 7155-7120/70356790 years bp. e2 of trench 3 is definitely poorly constrained (between 31.500 ± 550 years bp and 3480-3400 years bp). it is not possible, therefore, to compare this event to the e2 events found in the other trenches. evidence of a late pleistocene seismic event is not conclusive in trench 1 (e3), where a retreating scarp may have been buried by the «yellow 807 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) fig. 9. chronological constraints for the displacement events paleoseismologically inferred along the faults of the mt. paradiso alluvial fan. neapolitan tuff». this suggests that e3 may have occurred slightly before 14.120 years bp. e3 in trench 2 is constrained in a large time span, between 31.500 ± 550 years bp and 12.850 ± 200 years bp. the faint evidence for e3 in trench 1 and the chronological uncertainty in trench 2 prevent a comparison of the two events. e3 is also recorded in trench 3 (one of the two events indicated as e2 and e3 in fig. 9). in trench 2, e3 was responsible for the formation of a scarp which fed a quite thick colluvial wedge made of gravel. in trench 3, the same process was caused by e3. possibly, the e3 events defined in trenches 2 and 3 are related to the same displacement, responsible for the formation of relatively high scarps in unconsolidated gravels. 6. fault behaviour 6.1. slip rate the paleoseismological trenches in the campo imperatore area were excavated across secondary faults. for this reason, data derived from the trenches made across the scarps on the mt. paradiso fan (campo imperatore fault) cannot give information on the vertical slip rate of the primary fault. in this case, therefore, we cannot give updated information on the slip rate. the value which will be considered is, therefore, 0.67-1 mm/yr, produced by giraudi and frezzotti (1995). 6.2. elapsed time since the last activation the paleoseismological analysis did not define precise upper chronological limits for the observed displacements. in the investigated case, the displacement also affects the present soil, due to faulting after 3480-3400 years bp/2800-2700 years bp. the lack of sealing units makes, therefore, the definition of the elapsed time since the last fault activation impossible. an indication about the elapsed time for the campo imperatore fault may be derived from the data published by giraudi and frezzotti (1995). the authors reported that the fault is sealed by some colluvial units; the defined stratigraphic setting seems to corroborate the authors’ conclusion about the lack of activation in the past 1000 years (based on the absence of historical earthquakes which may be consistent with the activation of the fault). this suggests that the last activation probably occurred in early historical times or in prehistorical times. the association of historical earthquakes with central apennine active faults suggests that no earthquakes of m > 6 may be related to the activation of the campo imperatore faults (e.g., galadini et al., 1999; barchi et al., 2000; galadini and galli, 2000; valensise and pantosti, 2001). available historical catalogues (boschi et al., 1995, 1997, 2000; camassi and stucchi, 1997; working group cpti, 1999) give reliable information on the occurrence of large magnitude earthquakes since the first centuries of the 2nd millennium a.d. recent studies on the completeness of the historical information for the largest magnitude events defined a low probability of lost historical information in the period 1200 a.d.present (stucchi and albini, 2000). on this basis, we define eight centuries as a sort of «conventional» minimum elapsed time for the campo imperatore fault. 6.3. recurrence interval radiocarbon dates permit hypotheses to be made on the time span between the defined displacement events. as shown by the stratigraphy of trenches 1 and 2, deposition during the holocene was conditioned by the presence of a scarp which dammed the course of minor streams. due to the continuous sedimentary succession, the e1 and e2 seismic events recorded in the trenches 1 and 2 possibly represent consecutive displacements. an evaluation of the time span between e2 and e1 can be obtained by defining an upper chronological limit for e1. it is highly probable that e1 did not occur after 1200 a.d. (see previous section). therefore, the maximum time span between e2 and e1 can be defined from the difference between the oldest age of the e2 occurrence (i.e. the oldest age of unit 3 in trench 2, 7155 years bp) and the upper chronological limit of e1 (i.e. 1200 a.d.). the maximum time span is 6405 years. 808 fabrizio galadini, paolo galli and marco moro the minimum time span can be obtained from the difference between the upper chronological limit of e2 (i.e. the youngest lower chronological limit of unit 2 in trench 1, 5475 years bp) and the oldest chronological limit of e1 (i.e. the oldest upper chronological limit of unit 2 in trench 1, 3480 years bp). the minimum time span between e2 and e1 is, therefore, 1995 years. based on the available ages, the time span between e2 and e1 is 1995-6405 years. this conclusion is consistent with the available data on the recurrence intervals of other faults in the abruzzi apennines (pantosti et al., 1996; galadini and galli, 1999; d’addezio et al., 2001). 6.4. expected magnitude we defined the maximun expected magnitude from the surficial fault length, by using the equation linking the surface rupture length (srl) with the moment magnitude (wells and coppersmith, 1994). the srl of the campo imperatore fault has been mapped in detail through the observation of aerial photographs and subsequent investigations in the field during the last decade. the definition of the length of the fault surficial expression in the gran sasso range is definitely complicated. indeed, three different segments (assergi, campo imperatore and mt. san vito-mt. cappucciata) can be defined and no data are available which define the relationship between the three faults during an earthquake. it is probable, however, that the impressive scarp formed along the mt. cappucciata-mt. san vito fault (15 km long) and the displacements of the late pleistocene deposits (reported in galadini and galli, 2000) are due to coseismic events with energy larger than that expected from the activation of such a short segment. moreover, the assergi fault is parallel and very close to the campo imperatore fault for about half of its length (figs. 1 and 3a). this geometry suggests a connection at depth between the two faults. on this basis we believe that the entire 38 km long structure made of the three fault segments is the surficial expression of a single seismogenic source. the expected magnitude is, therefore, 6.95 (wells and coppersmith, 1994). 7. damage scenario related to the gran sasso range fault as reported in the previous sections, although the investigated fault ruptured during the holocene, at present it cannot be related to any historical earthquake. it is interesting, therefore, to analyse the effects of the fault activation through the production of a damage scenario. thus, we tried to reproduce the intensity distribution generated by the investigated fault related to the maximum magnitude event expected from the activation of the gran sasso range source. the damage scenario was generated starting from a computer code implemented in the emergency management system (sige), developed by the seismic survey of italy for civil protection purposes (di pasquale et al., 1998). the computer code was recently updated (faces, fault controlled earthquake scenario, in galli et al., 2002b) to take into account the geometrical characteristics of the source. in short, the input data for faces are represented by the following source parameters: l fault length; az strike of the fault (0°-360°); h maximum depth of the fault rupture (hypocenter, kilometer); cc coordinates of the fault medium point at depth (hypocenter); d ratio of anisotropy of the attenuation law (0-1); azimuth of the minimum attenuation direction from the fault (0°-360°); r cutback of the m/i0 relationship for depth > 10 km (0-1). figure 10a,b shows the parameters used for the investigated fault. we assumed 10 km as the hypocentral depth of earthquakes potentially triggered by the faults. moreover, the medium point of the lowest portion of the fault area at depth defined the hypocentral coordinates. the values for d, and r were derived from the comparison between faces and the highest intensity datapoint distribution (hidd) of historical italian earthquakes and the geometry of the related seismogenic sources (galli et al., 2002b). we adopted = 350° simulating an along-strike directivity effect towards the w (nevertheless, = 190° may be used to simulate an eastward effect). even if the fault is located in the highest mountainous range of peninsular italy (the 809 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) highest peak exceeds 2.900 m a.s.l.), the derived earthquake scenario, also due to the large expected magnitude, is severe. it shows, indeed, an epicentral intensity of i0 10-11 mcs and an area of heavy damage (i 8, mcs) elongated more than 70 km in a wnw direction (fig. 10a,b). figure 10b summarises the analytical results gathered through faces in terms of col810 fabrizio galadini, paolo galli and marco moro fig. 10a,b. a) image of the intensity datapoint distribution related to the activation of the entire gran sasso range source, obtained through the application of the faces computer code (galli et al., 2002b) with the reported fault parameters; the dashed rectangle defines the seismic source, while the dashed curves limit areas with the same intensity; the star marks the location of the epicenter used in the program routine. b) analytical results of the damage scenario in terms of collapsed buildings, casualties, uninhabitable buildings, damaged buildings and homeless. a b lapses, people involved in building collapses and damage. in particular, the town of l’aquila would experience i 10 (mcs), with an estimated number of people involved in collapses ranging between 3000 and 11.000 (3500-15.000 in the entire region). total collapsed buildings range between 7900 and 31.100, while damaged buildings are 99.000-234.000. this scenario suggests that, due to the location in the hangingwall of the campo imperatore fault, the seismic risk can be considered very high for l’aquila. the risk is not less serious than that related to the towns of messina and reggio calabria in the area of the messina straits fault, or to the towns and villages of the fucino basin in the central apennines before the occurrence of the 1908 (m 7.2) and 1915 (m 7) earthquakes, respectively. in short, the future rupture of the campo imperatore fault will produce one of the most catastrophic earthquakes in modern italian history. 8. conclusions the paleoseismological analysis in the campo imperatore area was performed to collect data on the holocene activity of the fault bordering the gran sasso range. the main results of our study are: 1) number of events: three events have been identified along a secondary fault of the campo imperatore system: the youngest (e1) occurred after 3480-3400 years bp/2800-2700 years bp, a previous displacement (e2) occurred between 7155-7120/7035-6790 years bp and 5590-5565/ 5545-5475 years bp, the oldest event occurred slightly before 14.120 years bp. 2) recurrence interval: the time interval between the last two events ranges between 1995 and 6.405 years (e2-e1 interval). 3) minimum elapsed time: data on the elapsed time for the campo imperatore fault are sparse, due to the lack of reliably datable units sealing the investigated faults; for this reason the elapsed time has been fixed in eight centuries, on the basis of the available historical data showing no evidence of earthquakes which may be related to the investigated faults and on considerations about the completeness of the seismic catalogues for the largest magnitude earthquakes. 4) maximum expected magnitude: based on the length of the fault surficial expressions, and by means of the equations linking the surficial rupture length with the moment magnitude, the investigated fault may cause earthquakes with m 6.95. considering the silent behaviour of the fault during at least the past eight centuries and the related high level of seismic hazard which may be associated with it, a damage scenario due to the activation of the gran sasso range source has been produced. we used the faces code, specifically developed by the seismic survey of italy for civil protection purposes. results of the calculation showed that the activation of the mentioned source may be responsible for the collapse of 790031.100 buildings and the damage of 99.000234.000 buildings. the obtained results indicate that although the fault is located in a sparsely inhabited area (high mountainous environment), its activation may be responsible for destructive effects in plain areas located up to 20 km far from the fault surficial expressions, where villages and towns are built. this substantially means that (beside a high level of hazard related to the silent state of the faults) a high seismic risk level can be associated with the entire region affected by the gran sasso range source. available data and related implications therefore define a quite vast area of central italy to which preventive activities should be devoted more than to other italian tectonically active regions which experienced strong earthquakes during the 2nd millennium a.d. acknowledgements a. cittadini (cnr, istituto di geologia ambientale e geoingegneria, roma) helped us in the fieldwork. we are grateful to c. giraudi (enea-casaccia, roma) for the discussions about the chronological framework of the late pleistocene-holocene continental stratigraphy and to g. di pasquale and g. orsini (servizio sismico nazionale, roma) for the update of 811 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) faces and the discussions about the damage scenarios. references asioli, a., f. trincardi, j.j. lowe, d. ariztegui, l. langone and f. oldfield (2001): sub-millennial scale climatic oscillations in the central adriatic during the lateglacial: palaeoceanographic implications, q. sci. rev., 20, 1201-1221. barchi, m., f. galadini, g. lavecchia, p. messina, a.m. michetti, l. peruzza, a. pizzi, e. tondi and e. vittori (editors) (2000): sintesi delle conoscenze sulle faglie attive in italia centrale: parametrizzazione ai fini della caratterizzazione della pericolosità sismica, gndt, gruppo nazionale per la difesa dai terremoti, roma, spec. publ., pp. 62. boschi, e., g. ferrari., p. gasperini, e. guidoboni, g. smriglio and g. valensise (editors) (1995): catalogo dei forti terremoti in italia dal 461 a.c. al 1980 (ing, roma sga, bologna), vol. 1, pp. 974. boschi, e., e. guidoboni, g. ferrari, g. valensise and p. gasperini (editors) (1997): catalogo dei forti terremoti in italia dal 461 a.c. al 1990 (ing, roma sga, bologna), pp. 644. boschi, e., e. guidoboni, g. ferrari, d. mariotti, g. valensise and p. gasperini (editors) (2000): catalogue of strong italian earthquakes from 461 b.c. to 1997, ann. geofis., 43 (4), 609-868. camassi, r. and m. stucchi (1997): nt4.1, a parametric catalogue of damaging earthquakes in the italian area (release nt4.1.1), http://emidius.mi.ingv.it/nt/ consnt.html. carrara, c., g. dai pra and c. giraudi (1995): lineamenti di tettonica plio-quaternaria dell’area, in lazio meridionale, sintesi delle ricerche geologiche multidisciplinari, enea dipartimento ambiente, roma, 151-155. carraro, f. and m. giardino (1992): geological evidence of recent fault evolution. examples from campo imperatore (l’aquila-central apennines), il quaternario, 5, 181-200. cavinato, g.p. 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(1994): elementi di geologia del quaternario della piana di campo imperatore (gran sasso d’italia), atti tic. sci. terra, spec. ser., 2, 137-143. giraudi, c. (1995): sedimenti eolici, variazioni climatiche ed influenza antropica: considerazioni su alcune piane intermontane dell’appennino abruzzese, il quaternario, 8, 211-216. giraudi, c. (1997): le oscillazioni tardo-pleistoceniche ed oloceniche del lago effimero del piano di pezza (abruzzo-italia centrale), il quaternario, 10, 191-200. giraudi, c. (1998a): late pleistocene and holocene lake level variations in fucino lake (abruzzo central italy) inferred from geological, archaeological and historical data, in esf workshop palaeohydrology as reflected in lake-level changes as climatic evidence for holocene times, paläoklimaforschung, 25, 1-17. 812 fabrizio galadini, paolo galli and marco moro giraudi, c. (1998b): i laghi effimeri tardopleistocenici e olocenici di campo imperatore e del massiccio del gran sasso d’italia (abruzzo italia centrale), il quaternario, 11, 217-225. giraudi, c. and m. frezzotti (1995): paleoseismicity in the gran sasso massif (abruzzo, central italy), quat. int., 25, 81-93. giraudi, c. and m. frezzotti (1997): late pleistocene glacial events in the central apennines, italy, quat. res., 48, 280-290. magri, g. and d. molin (1984): il terremoto del dicembre 1456 nell’appennino centro-meridionale, enea report rt/amb, 8 (83), pp. 180. molin, d., f. galadini, p. galli, l. mucci and a. rossi (1999): sismicità della zona del fucino, in 13 gennaio 1915, il terremoto nella marsica, edited by s. castenetto and f. galadini, servizio sismico nazionale and cnr istituto di ricerca sulla tettonica recente, roma, 249-271. monachesi, g. and m. stucchi (1998): dom 4.1 an intensity database of damaging earthquakes in the italian area, http://emidius.mi.ingv.it/dom/home.html. pantosti, d., g. d’addezio and f.r. cinti (1996): paleoseismicity of the ovindoli-pezza fault, central apennines, italy: a history including a large, previously unrecorded earthquake in the middle ages (860-1300 a.d.), j. geophys. res., 101, 5937-5959. patacca, e., r. sartori and p. scandone (1990): tyrrhenian basin and apenninic arcs: kinematic relations since late tortonian times, mem. soc. geol. it., 45, 425-451. peruzza, l. and b. pace (2002): sensitivity analysis for seismic source characteristics to probabilistic seismic hazard assessment in central apennines (abruzzo area), boll. geofis. teor. appl., 43, 79-100. siani, g., m. paterne, e. michel, r. sulpizio, a. sbrana, m. arnold and g. haddad (2001): mediterranenan sea surface radiocarbon reservoir age changes since the last glacial maximum, science, 294, 1917-1920. stucchi, m. and p. albini (2000): quanti terremoti distruttivi abbiamo perso nell’ultimo millennio? spunti per la definizione di un approccio storico alla valutazione della completezza, in le ricerche del gndt nel campo della pericolosità sismica (1996-1999), edited by f. galadini, c. meletti and a. rebez (gndt, gruppo nazionale per la difesa dai terremoti, roma), 333-343. stuiver, m., p.j. reimer, e. bard, j.w. beck, g.s. burr, k.a. hughen, b. kromer, g. mccormac, j. van der plicht and m. spurk (1998): intcal98 radiocarbon age calibration, 24.000-0 cal bp, radiocarbon, 40, 1041-1084. valensise, g. and d. pantosti (editors) (2001): database of potential sources for earthquakes larger than m 5.5 in italy, version 2.0, ann. geofis., suppl. 44 (4), 797-964. wells, d.l. and k.j. coppersmith (1994): new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, seismol. soc. am. bull., 84, 974-1002. working group cpti (1999): catalogo parametrico dei terremoti italiani (ing, gndt, sga, ssn, bologna), pp. 92. zolitschka, b. and j.f.w. negendank (1996): sedimentology, dating and palaeoclimatic interpretation of a 76.3 ka record from lago grande di monticchio, southern italy, quat. sci. rev., 15, 101-112. 813 paleoseismology of silent faults in the central apennines (italy): the campo imperatore fault (gran sasso range fault system) òlakhjdfgòksgòkljsfòkljsdg un criterio di resistività pratico di trasformazione fra diagrammi apparente ottenuti con differenti dispositivi e , cassixis d a lungo t e m p o si discute sulla più conveniente configurazione geometrica dello stendimento da a d o t t a r e nei sondaggi elettrici verticali. p e r varie ragioni, del resto ormai ben note, la maggioranza degli operat o r i t e n d e ad usare il dispositivo schlumberger; in questo schema la distanza t r a le sonde del potenziale resta fissa m e n t r e vengono spostati gli elettrodi di corrente oppure, se la distanza t r a le sonde viene variata, la misura viene r i p e t u t a lasciando fissi gli elettrodi. i diagrammi teorici interpretativi vengono calcolati in maggioranza per il dispositivo schlumberger nella ipotesi che la distanza t r a gli elettrodi potenziometrici sia infinitesima; il vecchio metodo wenner però, è ancora molto diffuso; è assai utile, p e r t a n t o , poter adoperare i diagrammi ricavati con a m b e d u e le ipotesi e, poiché il calcolo è assai laborioso, passare da una serie all'altra di curve m e d i a n t e u n procedimento semplice e sufficientemente approssimato. a. belluigi f1) p a r t e n d o dall'espressione generale della resistività per il polo singolo, t r o v a la relazione seguente, che fornisce la differenza t r a la resistività osservata (oa) mediante u n rapporto y = 0 e quella ( o j che si otterrebbe con y 0 (dispositivo schlumberger) : 1 —.'/3 r q (d + x) — q (d) a — p = —— / dee l , a [ i ] dove d è la semidistanza t r a gli elettrodi di corrente e « è la semidistanza t r a le sonde del potenziamento (fig. 1). la correzione può essere ricavata dirett a m e n t e dalla [1] calcolando l'integrale per via grafica; questo procedimento è ovviam e n t e troppo laborioso ed occorre perciò cercare u n metodo più rapido. p e r il calcolo approssimato conviene sviluppare in serie la funzione integranda. p e r 1 2 : s o n d e d e l p o t e n z i a l e 3 4 : e l e t t r o d i d i c o r r e n t e fig. i 10 sviluppo di o ci limiteremo al termine di terzo grado, poiché è p r a t i c a m e n t e impossibile calcolare le derivate superiori alla seconda, e l'ultimo termine ci servirà solo a nostrare che i n generale è a b b a s t a n z a piccolo da potere essere trascurato. anche lo sviluppo di | l + "-j è stato limitato generalmente ai termini quadratix ci, perchè ^ può raggiungere al massimo 11 valore di 1/3; il termine di terzo grado, pari a 4 ( y j > vaio perciò circa 1/8. tenendo presente che le variazioni relative di resistività a p p a r e n t e nel passaggio da uno stendimento all'altro raggiungono difficilmente il 10%, si deduce che e f f e t t i v a m e n t e i termini di 3° ordine sono piccoli ed è senz'altro r. c a s s i x i s sufficiente tenerne conto solo per il calcolo del coefficiente della derivata prima, come è stato f a t t o . si ottiene così: q(d+x)-e(d) [, 1 „ , 1 „ 3 ) = o x + o + — p x3..; ( x x2 x3 = q x+ 3 l d:i 2 d* q 1 — ~d 2 p" 1 . . _ + t q d 0 1 o"'_ 3 d x* iii moltiplicando per dx, integrando e limit a n d o t r a — a e + a, i termini con le potenze pari si elidono e rimane: qa — so = + k 1 r 2 a r o 2 3 0 ^ 2 dn 2 — • 1 1 o" d3 1 2 d3 d ovvero, ponendo a = dy: q«—°o = 0 — 2 / 1 / 1 3 \ „ 1 2 ( 3 5 ' 15 ^ 1 3 y q nel caso in cui y ner) si ha: solo a grandi distanze, ove la precisione delle misure non può essere molto elevata. consideriamo il caso di ~ = — come d 3 valore massimo raggiungibile nella pratica degli stendimenti dei sondaggi elettrici ed esaminiamo perciò con la [3] il contributo di ciascuno dei due termini nella correzione totale da a p p o r t a r e ai valori di q per trasform a r e u n a curva « schlumberger » in una « wenner » e viceversa. è ovvio che, quando la c u r v a t u r a è nulla vi è solo da considerare il contributo del 1° termine; in questo caso può essere comodo a p p o r t a r e alla curva, anziché la correzione di ordinata (o), quella di ascissa (x): = = — 0,0747 d. y'd [2] (dispositivo wenh = oa — q0 = — 0,0747 ci • q + + 0,0198 dq" — 0,0007 d3 q" [3] la correzione può essere limitata al secondo termine. i coefficienti numerici dei termini successivi, i n f a t t i sono assai piccoli; come si vede, quello del terzo termine è circa il 4 % di quello del secondo. perciò, anche se la derivata terza ha un valore sensibile, l'influenza di tale termine diviene consistente ricordando inoltre che i diagrammi dei sondaggi elettrici vengono disegnati sul piano logaritmico, si h a che, per i t r a t t i rettilinei, b a s t a spostare la curva di u n a quant i t à costante lungo l'asse delle ascisse. dai segni dei termini della [3] si rileva anche che, quando le curve scende, la correzione è positiva (passando da q0 a ga); quando sale, la correzione è negativa, e cioè la curva « w e n n e r » si trova al disotto della schlumberger: è noto, difatti, che le curve del primo tipo sono più « a t t e n u a t e » di quelle del secondo. si n o t a ancora che, passando da ga a q0, la correzione per il secondo termine deve essere a p p o r t a t a sempre verso la p a r t e interna della curva. nei t r a t t i dove la c u r v a t u r a diviene sensibile, il secondo t e r m i n e non è più trascurabile e sorge il problema di come calcolare n" con esattezza sufficiente; difatti, se si sceglie un intervallo piccolo, gli errori di l e t t u r a portano una imprecisione sensibile nella determinazione delle c u r v a t u r e : se l'intervallo è grande, non si tiene conto della forma reale della curva. occorre u n a certa esperienza per la scelta dell'intervallo più opportuno. u n criterio può essere anche quello di eseguire l'integrazione grafica limitatamente a qualche p u n t o delle zone in cui la c u r v a t u r a è più forte. i n fig. 2 è riportato un diagramma teorico per il caso di 3 strati, calcolato per il dispositivo schlumberger (tratto continuo); i rapu n criterio pratico di t r a s f o r m a z i o n e fra diagrammi, ecc. 235 porti di resistività sono s t a t i scelti in modo che la curvatura, nella p a r t e inferiore della curva, fosse assai sensibile. la curva tratteggiata è quella ottenuta apportando, ai valori di n letti sul diagramma, la correzione calcolata con la [3] limitata al 2° termine. considerato uguale a zero. difatti, aument a n d o la distanza t r a gli elettrodi, la tensione misurata alle sonde si abbassa, fino a non poter essere più determinata con la precisione occorrente. p e r non spostare le sonde troppo frequentemente, la misura, per lo alcuni controlli eseguiti integrando graficamente la [1], hanno dimostrato che l'approssimazione raggiunta con la [3] è del t u t t o soddisfacente. i l procedimento descritto può essere aplicato utilmente, oltre che per trasformare le curve teoriche, anche per eliminare delle deformazioni talvolta piuttosto sensibili, negli stessi diagrammi sperimentali di campagna. come è noto, impiegando il disposici tivo schlumberger, il rapporto y = ^ non rimane costante e non può essere sempre stesso (1, viene r i p e t u t a con a notevolmente più grande. i n tal modo y può raggiungere il valore di e, talora, anche di ' . i n o 3 questi p u n t i di « riattacco » i valori misurati (li g con i due diversi a possono divergere sensibilmente. questo f a t t o h a due cause: la prima è che le condizioni superficiali nell'interno delle sonde sono variate; la seconda è che ci si discosta dall'ipotesi y = 0. l a differenza delle condizioni superficiali si corregge assai semplicemente, traslando parallelamente i vari t r a t t i di curva in modo da 236 r. cassi x i s portarli a coincidere. lo scarto causato dalla variazione di y, invece, si può correggere con la [2], calcolando i coefficienti per vari y. i n pratica b a s t e r à determinare le correzioni solo per i valori più grandi del rapporto, poiché già per y = — il coefficiente del pri-8 1 mo termine diviene 1/7 (li quello per y = e la correzione è perciò trascurabile. nella maggioranza dei casi, è sufficiente determinare la differenza di o nel p u n t o di riattacco e raccordare la curva corretta con il p u n t o del diagramma osservato nel quale i/ è sufficientemente piccolo. quando i « k » del diagramma sono elevati e il « riattacco » avviene proprio nei pressi della massima curvatura, cioè y è variabile quando g" è sensibile, occorre procedere con maggiore cautela per non deformare la curva si n o t a però dalla [2] che, a p a r i t à di d, la correzione d o v u t a al secondo termine è sensibilmente proporzionale ad y2. un criterio che garantisce una accuratezza notevole è quello di calcolare p e r mezzo delle [3], solo per il t r a t t o curvo, i diagrammi teorici di wenner, e di disegnarli sulle curve interpretative. confrontante questo t r a t t o di diagramma teorico con quello osservato, è più agevole eseguire il raccordo t r a il p u n t o d i « riattacco » e quello dove la correzione diviene trascurabile. la correzione ai valori misurati viene cosi d e d o t t a dir e t t a m e n t e dalle curve; essa deve essere eventualmente ridotta, se a < a0 = ' f7, nel hi assunto viene proposto un metodo approssimato per trasformare i diagrammi di resistività ottenuti con differenti dispositivi elettrodici, in quelli ricavati seguendo lo schema schlumberger, o viceversa. il criterio esposto, di applicazione assai rapida, si presta anche a risolvere il problema, che sorge durante le misure in campagna, di raccordare i vari tratti di curva osservata quando cambia la distanza fra le sonde del potenziale. abstract a rapid method is proposed to transfer the observed resistivity curves obtained with any electrode configuration into schlumberger diagrams. such procedure is of practical application even to solve the field problem of fitting the different sections of the diagrams, ivhen the dislance between the voltage electrodes changes. b i b l i o g r a f i a f1) belluini, a . , su una « exata quaestio » di gerarchia nei dispositivi dei sondaggi elettrici. « geofisica p u r a e a p p l i c a t a », x x i v , 1956. (2) — abaques de sondage éléctrique par la compagnie generale de oéophysìque. « geop l i y s i c a l p r o s p e c t i n g », 1955, s u p p l . 3. ( 3 ) a l f a n o , l . , studi sulla interpretazione dei sondaggi elettrici verticali. « r i v i s t a di geofisica a p p l i c a t a », 2, 1951. yokoyama_finalonline2:layout 6 annals of geophysics, 57, 5, 2014, s0541; doi:10.4401/ag-6404 s0541 krakatau caldera deposits: revisited and verification by geophysical means izumi yokoyama the japan academy, ueno park, tokyo, japan abstract one of the differences between volcanic craters and calderas is that the latter bottoms are flatways filled with caldera deposit with lower density in comparison to country rocks. the 1883 krakatau eruption affords us important knowledge on caldera formation even if it was not observed with modern sophisticated instruments. first, volcanic activities of the krakatau islands before and after the 1883 eruption are reexamined: previous suppositions involving a caldera-forming eruption of the proto-krakatau prior to 1883 proved to be unsupported by the bathymetric topographies and gravity anomalies on and around the krakatau islands. then, anak krakatau is interpreted as a parasitic cone of the main krakatau volcano. as supplementary knowledge to discussion of caldera deposits, the results of drillings at several calderas in japan and mexico are introduced. mass deficiency of the caldera deposit at krakatau caldera is estimated by the gravity anomaly observed there and converted to probable volume with suitable density. for quantitative examination of the subsurface structure beneath the krakatau complex, spatial distributions of seismic s-wave attenuation and vp/vs ratios have been already studied by temporary seismological observations and their results have been published. the high ratios of vp/vs observed approximately at krakatau caldera may be attributable to the caldera deposit that is low density and contain much water. as additional remarks, a zone having both the characters, s-wave attenuation and zones of relatively high vp/vs ratio, may be a probable magma reservoir centering at a depth of about 10 km. 1. introduction the 1883 eruption of krakatau volcano in the sunda straits, between java and sumatra islands, indonesia (inset in figure 1), and its destructive effects have been long disputed among many researchers. however, we still have unsettled problems in the field of volcanology such as identifying reliable distribution of the ejecta, their volume, modes of the explosions, origins of tsunamis, and subsurface structure left by the explosions. especially, in the study of caldera formation, the following two facts are still the obstacles to the above unsettled dispute to be ever challenged. a) the explosions progressed within dense explosion clouds and zero visibility impaired accurate scientific reports of the events. b) seismometers were not available around krakatau volcano at that time, and only a tide gauge, a pressure meter and magnetometers had been operated in batavia located about 150 km from the volcano. the present author will try to synthesize the results from recent geophysical observations on and around the krakatau islands, such as marine gravity surveys, measurements of s-wave attenuations and seismological tomography to determine vp/vs ratios, all relating to magma reservoirs or caldera deposits beneath krakatau caldera. 2. volcanic activities at the krakatau islands before and after the 1883 eruption 2.1. assumptions on geological structure of the pre-1883 krakatau complex verbeek [1885] and escher [1919] proposed an assumptive history of the krakatau complex and focused on a large central andesitic cone measuring about 15 km in basal diameter and probably 2000 m a.s.l. according to their account, this proto-krakatau erupted violently resulting in the formation of a caldera and later the three islands of krakatau, verlaten and lang were formed on its rim. the present author doubts their speculations for the following reasons. 1) generally, caldera-forming eruptions do not tend to occur from the same vent and the magma activity would migrate to another sites finally to form a composite caldera of larger dimension. as shall be discussed in the next subsection, and shown in figure 3, rakata, danan, perbuwatan and anak krakatau are equally the parasites of the 1883 krakatau volcano article history received september 2, 2013; accepted september 2, 2014. subject classification: krakatau caldera, parasitic cones, caldera deposit, drillings at calderas, s-wave attenuation, vp/vs ratio. which was the remnants of explosion activity of the proto-krakatau. its explosion center was located under the sea and reactivated in the 1883 eruption. 2) we do not find any trace of a larger caldera or depression around the krakatau islands in the bathymetric chart surveyed in 1868 (figure 1) which was reproduced from the report of wharton [1888]. in this figure, the bathymetric contours were newly added every 25 m intervals by the present author. in fact, the circular bathymetric contours around the islands do not suggest the remains of a large depression. we can infer from these topographies that the three islands existing prior to 1883 were situated on a submarine plateau of the remnants of the proto-krakatau. as shall be mentioned in subsection 4.1, we find no low gravity anomaly indicating the existence of a pre-1883 caldera. instead the anomaly at the periphery shows an increase toward the center of the islands (figure 2). based on these data, we infer that there existed the basement of proto-krakatau volcano until the 1883 eruption. an alternative scenario to the verbeek and escher model is as follows: on the proto-krakatau, no caldera had been formed, but periodic and destructive volcanic activities culminated and transformed this single landmass into the three islands or separate blocks (krakatau, yokoyama 2 figure 1. british admiralty chart of the krakatau area surveyed in1868. spot depths are given in fathom (1 fathom = 1.83 m), and isobaths are added every 25 m by the present author. r denotes rakata, d danan and p perbuwatan. an inset shows the krakatau islands in the sunda straits after the 1883 eruption and a square frame at the center of the main figure approximately indicates the areas of figures 2 and 3. 3 verlaten and lang islands) before the 1680 activity that was reported to have been at some parasitic craters and not so violent. the main vent may have been always located at the same post as the center of the 1883 eruption. subsequently, on krakatau island, a basaltic volcano (rakata) formed, followed by andesitic parasitic cones (danan and perbuwatan volcanoes). originally these three are all parasites of the proto-krakatau. the british admiralty chart reproduced in figure 1 shows the shape of the krakatau islands as of 1868. after 2 centuries of low activities since 1680, a caldera-forming eruption took place in 1883, and the three parasitic cones on krakatau island were partially or totally destroyed. then, 44 years later (1927), anak krakatau appeared at a “parasite gap” [yokoyama 2013] between ex-danan and ex-perbuwatan among the krakatau complex (figure 3). to discuss the subsurface structure of volcanic areas, gravity analyses are useful. yokoyama [1963] classified calderas from the standpoint of their subsurface structure into 2 types: high-gravity-anomaly type and low-gravity-anomaly type. to determine the basement structure of the krakatau complex, gravity surveys on and around the islands have been repeated as follows: first, yokoyama and hadikusmo [1969] measured gravity on the krakatau islands with a land gravimeter and obtained bouguer gravity anomalies in the potsdam system published in 1930. the results indicated funnel-shape caldera deposits of about 5 km3 in volume on the base of the gravity anomaly. these deposits, named “fallbacks” by yokoyama [1981] were produced by explosions and composed of the juvenile ejecta and lithic fragments of pre-caldera volcanic edifices. yokoyama [1987] supplemented the 1968 survey with the data obtained in 1982 and the final results were reduced to international gravity standardization net 1971 (igsn71) [morelli et al. 1974]. later, deplus et al. [1995] carried out marine gravity and bathymetric surveys, and confirmed a relative low gravity anomaly amounting to 20 mgal at the center of the caldera. they discussed the subsurface structure of the krakatau complex and analyzed the gravity anomaly by adopting various density models. their models revealed the geometry of the dense substratum of the proto-krakatau and the presence of a collapsed structure filled with low-density material beneath the caldera. the present author compiled the gravity data obtained during a period 1969~1995 on and around the krakatau complex and figured a tentative distribution of the bouguer anomaly as shown in figure 2. although this gravity anomaly map contains some ambiguity due to an insufficient number of gravity krakatau caldera deposits figure 3. topographic and bathymetric chart on and around krakatau. both the contours and isobaths are drawn every 50 m. a circle of broken line is an assumed caldera rim. b–b’ line is the datum line in figure 5b. ak denotes anak krakatau, r rakata, d danan, and p perbuwatan. figure 2. bouguer gravity anomaly in mgal on and around the krakatau islands. land anomalies are compiled from yokoyama and hadikusumo [1969] and deplus et al. [1995] and more than 40 gravity points have been occupied. marine anomalies are compiled from deplus et al. [1995]: ★ symbol: the krakatau cruise of 1985; broken line: the corindon cruise of 1983. h and l denote relatively high and low gravity anomalies, respectively. points, a relatively low anomaly is evident at the caldera area. the gravity anomaly decreases by 15 mgal, roughly from 75 to 60 mgal, caused by the 1883 eruption. as mentioned above, krakatau is a caldera of the low-gravity-anomaly type. the gravity distribution around the krakatau islands may have been faintly influenced by appearance of anak krakatau after 1927. it is noticeable that the low anomaly caused by the 1883 caldera is surrounded by a high anomaly increasing from 70 to 80 mgal. in all likelihood, the high anomaly was produced via the remains of the basement of the proto-krakatau. based on these results, we may conclude that the proto-krakatau had not formed a caldera and been destructed into the three remnants by violent eruptions long before 1680, and that the remaining topographies were roughly the same as observed in 1868 as shown in figure 1. a low gravity anomaly or its trace suggesting the pre-1883 caldera is not present in figure 2. if a pre-1883 caldera had been formed, we would have expected to observe a relatively low gravity anomaly larger than 20 mgal over an area of 5 km in diameter. the low gravity anomaly originated from the 1883 eruption overlaps the region where the high gravity anomaly due to the basement is located. as previously mentioned, rakata, lang and verlaten islands may be the remnants of the violent eruptions, and two acidic parasitic cones, danan and perbuwatan were added to rakata ultimately forming krakatau island before 1680. 2.2. relationship between krakatau caldera and anak krakatau although there were no eyewitnesses of the 1883 eruption, stehn [1929] believed the main vent of the 1883 eruption to be at the central and deepest point of the depression formed by the eruption. the craters of danan and perbuwatan (figure 1) were expelling ashes before the climatic eruption. the main central vent of the proto-krakatau volcano may have been in roughly the same location as the vent of the 1883 caldera mentioned above, while rakata, danan, perbuwatan and anak krakatau appear to be parasites formed in a circular pattern and branched from the main conduit which is connected to a deep magma reservoir. before the 1883 eruption, the main conduit was located under the sea. furthermore, this hypothetical main vent may have produced submarine explosions that would easily explain the occurrence of huge tsunami in this location. contacts between magma and seawater may have accelerated the strength of the eruptions but were not substantial enough to cause the large eruptions in 1883. the original motive force of the 1883 eruption should mainly be attributed to magmatic forces. the 1883 eruption produced a caldera measuring about 5 km in diameter with rather flat bottom as shown by a circle of broken line in figure 3 prepared by the volcanological survey of indonesia in 1970. later deplus et al. [1995: figure 7] compiled a detailed bathymetric chart of the krakatau area. in both the charts, caldera bottom is relatively flat ranging from 200 m to 250 m b.s.l. and the caldera walls are not so steep; for example, the steepest slope of the northern wall is measured as about 25 degrees that mean a gentle slope. this may suggest that the caldera wall is a funnel-shape as already suggested by yokoyama [1963]. anak krakatau is a parasite of krakatau volcano. we know empirically that parasites are monogenetic: that is to say, they would exert volcanisms for a cycle of the existing magmatic activity and we cannot anticipate its length of life. however, we cannot exclude the possibility that anak krakatau may grow up with magmatic evolutions and finally cause climatic explosions forming a caldera. such activities should develop to form a larger composite caldera combined with the present krakatau caldera. camus et al. [1987] discussed the petrologic evolution of the krakatau complex and emphasized periodicity of volcanic activity. an eruptive cycle culminating in the 1883 dacite, resulted from magmatic differentiation in the magma reservoir, and the cycle of anak krakatau has been characterized by a change of the eruption products from basaltic to dacitic in the 1981 eruption. however, they added that the more ancient cycles are not so clearly defined. the fact that the periodicity in magmatic differentiation was maintained by the 1883 eruption means that evolution of the magma in krakatau magma reservoir was not disturbed by the 1883 eruption. as mentioned above, no caldera was formed on the proto-krakatau, and the present author deduces that such magmatic evolution is not always related to caldera-forming eruptions and admits the possibility that dacitic magmas may cause devastating explosions. prior to discussion of the formation process of anak krakatau, parasitic eruptions shall be reviewed in general terms according to the work of de la cruzreyna and yokoyama [2011] and yokoyama [2013]. if we assume the presence of a point dilatation model beneath a volcano, surface deformations can be interpreted as the result of upward principal stresses while fractures at the surface are explained by both the theories of maximum stress and of maximum shearing stress. at an active crater, directly above the pressure source, only upward force is exerted. when magma drains back leaving conduits free of volcanic material, or when the blockaded conduit is not strong enough, yokoyama 4 5 the magma can break out the crater bottom. since the processes described fit the usual model of polygenetic eruptions, the theory of maximum stress is applicable. magma cannot always penetrate through a strongly solidified central crater. instead, under various conditions, low depth magma has a tendency to proceed obliquely toward the mechanically weakest point. by applying the theory of maximum shearing stress, we can access this situation. material mechanics shows that the critical stress is equal to a half of the value of horizontal differential stress and is represented as: 1/2 (vx − vz), (1) where vx and vz denote the principal stresses. and the maximum shearing stress occurs across a plane whose normal bisects the angle between the greatest and least principal stresses. in the following, we assume a simple model beneath a horizontal plane: a uniform expansion of a spherical pressure source p = p0 at depth d and with intensity a3p0 (figure 4). after some analytical treatments, the critical stress (1) proves to have a positive or negative maximum value corresponding to: r = ± √6 d/3 = ± 0.82 d, or d = 1.22 r (2) therefore, the hypothetical volcano body receives the maximum horizontal differential stress at radial distance r = ± 0.82 d, or at the point where the dip angle of the pressure source from the fracture point at the surface is 51°. consequently, the shear fracture should take place in a radial direction on the surface or along the slope theoretically we expect the maximum horizontal differential stress to exist at r = ± 0.82 d or diagonally opposite to the center of the volcano though declination i cannot be estimated quantitatively because some parameters in the above discussion are not determinable on the actual volcanoes. the particular conditions of each volcano may affect how these pairs of fracturing points react. when a parasitic vent is formed at one fracture point, usually the other pair point doesn’t fracture probably due to stress concentration at the former. twin parasites are very rare but we know some examples such as sakurajima, japan during the last three large eruptions [yokoyama 2013], fogo, the cape verde islands in 1951, hekla, iceland in 1970, and tyatya, kuril in 1973. whether magma breaks out from main crater or from parasitic vents depends on the condition of the existing main crater and strength of the material concerned. alternatively, the balance between magma pressure and rock strength at the sites may influence the outcome. compressive stress affects the strength of main craters at the summits and parasitic sites have both compressive and shear stresses. thus, rock strength plays an important role in the above determination. compressive rock strength is roughly in the order of 100 mpa and shear strength is roughly in the order of 10 mpa [jaeger 1964: tables 3 and 4]. the balance between upward compressive force against the strong compressive strength and shear force against the weak shear strength determines the location of the fracture. at present the balance cannot be estimated quantitatively because some parameters mentioned above are not accurately determinable. furthermore, based on past records, we cannot expect eruptions to exclusively follow summit or parasitic eruption model. parasitic eruptions have occurred simultaneously with summit eruptions on some volcanoes. in such cases, volcanic activities should be rather energetic. in case of krakatau caldera, the center of the 1883 eruption has been concluded to be at the deepest point among the three islands as shown in figure 3. the eruption site of anak krakatau in 1927 is located at a distance of about 3.5 km from the center. hence, the branch point from the main conduit is assumed to be roughly 4.3 km deep by equation (2), and is represented by the star symbol in figure 5b and the main conduit may be possibly connected with a magma reservoir. the dispositions of anak krakatau and its junction of the conduit are similar to those of disappeared parasites, rakata, danan and perbuwatan, and support the hypothesis of cyclic magma activity of the krakatau complex in sequence of basalt, andesite and dacite [camus et al. 1987]. krakatau caldera deposits figure 4. shear fracture model for formation of parasitic vents. p0: point source of dilatational pressure; s1 and s2: a pair of parasitic vents. 3. evidence of caldera deposits found by drillings at some calderas in japan and mexico from a geophysical standpoint, yokoyama [1963] classified calderas as highand low-gravity-anomaly types according to the density of the deposits: high-density deposits are basaltic lavas and mainly found at the caldera bottoms while low-density deposits are mainly silicic pyroclastic ejecta distributed around calderas and partly fallbacks at the caldera bottoms. the former are created by gravity subsidence while the latter origin is unsettled partly because we have scarcely experienced caldera formation of this type except the 1883 krakatau, the 1912 katmai and the 1991 pinatubo eruptions. krakatau caldera belongs to the latter type: its deposit is totally located below sea level, and no drilling results are available. more specific information about the kind of material and its configuration situated are necessary. the geology of krakatau caldera itself has not been well studied. however, we have obtained valuable data from some other calderas located above sea level by geological and geophysical methods and also by drilling operations. the results obtained by drillings at some calderas of the latter type in japan and mexico shall be mentioned in the following: kuttyaro, hokkaido: this is one of the largest calderas in japan, measuring 20 km in diameter with a caldera lake. yokoyama and tajima [1959] measured gravity on the frozen lake and found relative low anomalies amounting to about 40 mgal at the maximum. the gravity anomaly is attributed to the caldera deposit of low density and its depth is estimated at about 2 km at the center. nishida and yokoyama [1965] examined physical properties of the cores drilled at its center of the caldera. the drilling reached 1 km deep. those cores are generally porous and pumiceous and the majority is agglomerates partly welded tuff, and there is also a small quantity of andesite lava. nigori-gawa, hokkaido: this is a small caldera, 3 km in diameter. relative low gravity anomaly is 6 mgal at the maximum. ando [1983] and kurozumi and doi [2003] studied this caldera. since 1974, 37 exploratory drillings had been done within the caldera; 4 drillings reached depths over 3 km. on the basis of these results, they determined the structure of this caldera as to be a steep funnel-shape filled with fallbacks, and concluded that the origin of the caldera was explosions. aso, central kyushu: this is a large caldera measuring about 20 km in diameter, and there are active craters on the summit. gravity anomalies in and around the caldera observed by komazawa [1995] suggest that aso caldera is composed of 5 depressions, small and large in diameter. the relative low gravity anomaly was about 20 mgal. there have been several drillings, shallow and deep, within the caldera. shallow drillings near the northern and southern caldera rims detected the basements at around 400 m deep, and deeper drillings near the center of the caldera penetrated a thick layer of welded pyroclastic flow deposits at a depth of 1.5 km. these results indicate that aso caldera is funnel-shaped and its caldera deposit is composed of fallbacks. aira, southern kyushu: this caldera is located at the northernmost part of the kagoshima bay and measures about 20 km in diameter and gravity anomaly reaches about 40 mgal relatively low at the center. a drilling site is at the western shore of the island of sakurajima volcano that is a post-caldera cone roughly 3 km inside from the assumed caldera rim. the drilling reaches a depth of 810 m b.s.l. according to aramaki [1977], almost all drilled cores are composed of tuff, welded tuff and pyroclastic deposits. la primavera, mexico: this measures 13 km in diameter, and formed about 95,000 years b.p. since 1980, the comisión federal de electricidad, mexico has drilled 13 geothermal wells reaching depths of 668 to 2986 m around the center of the caldera. the caldera deposits are mainly pyroclastics (tala tuff ) and partly admixed with lithic tuff layers. beneath the caldera deposits, there are the basements of compact granite at a depth of about 3 km. yokoyama and mena [1991] determined the structure of the caldera by reviewing data from the drilling results and gravity anomalies composed of two relative lows amounting to 14 and 9 mgal. the caldera structures were assumed to be about 1.5 and 1 km deep, respectively. conclusively, the results of sufficiently deep drillings carried out at the above calderas verify that the caldera deposit is underlain with basement rocks and the basements prove to be possibly funnel-shaped. and there is no evidence that caldera-forming eruptions destroyed the basements which could collapse into magma reservoirs. caldera deposits tend to be composed of pyroclastic and lithic material of low density. furthermore it has been established by these studies that magma reservoirs don’t exist directly beneath the calderas of low-gravity-anomaly type. 4. geophysical studies of subsurface structure of krakatau caldera immediately after the 1883 eruption, bathymetry records taken around the islands were improved but geophysical studies of the 1883 eruption of krakatau volcano and of the subsurface structure of the newly formed caldera may have begun in 1980’s. as technology developed, various methods have been applied to study the subsurface structure of the krakatau complex and even now new projects are continuing. it is necesyokoyama 6 7 krakatau caldera deposits sary to review the past interpretations and to summarize the present developments of the studies. 4.1. gravity anomalies and volume of the caldera deposit in the previous subsection 2.1, the gravity anomaly observed on and around krakatau caldera is utilized to examine the subsurface structure of the pre-1883 krakatau complex. in this subsection, it shall be applied to estimate volume of the caldera deposit. verbeek [1885] and his successors had not paid attention to the caldera deposit in their studies. in the case of the krakatau caldera, the caldera deposit is located beneath the sea. by utilizing gravity anomalies in this region, the present author [yokoyama 1981] estimated the volume of the caldera deposit. according to the results of drillings at several calderas, caldera deposits are usually composed of pyroclastic ejecta and disrupted material of pre-caldera volcanoes (lithic fragments). such caldera deposits have low density in comparison with country rocks and have been labeled as “fallbacks” by yokoyama [1981] as mentioned before, and he proposed a subsurface model adopting the density contrast 300 kg/m3. it is true that assumptions of the density contrasts have much ambiguity. in the present paper, the distribution of bouguer anomalies is modified as shown in figure 2 on referring to the results of the marine gravity surveys carried out by deplus et al. [1995]. in the figure, two high anomalies exceeding 80 mgal coincide with verlaten and rakata islands, respectively, due to the basements of the proto-krakatau. a low gravity anomaly amounting to a maximum of −15 mgal is due to the caldera deposit of low density deposited by the 1883 eruption. in general, gravity anomalies may be interpreted differently according as different assumptions on density contrasts and distribution of source material, and therefore the results should have some ambiguities. actually deplus et al. [1995] discussed gravity data at the wider area including krakatau caldera and showed different models of the deeper structure of the caldera by adopting different assumptions. here, the present author is concerned only with the volume of the caldera deposit and later will take the results of drillings at various calderas into consideration. thus, the mass anomaly δm at the caldera area is related to gravity anomalies with the gauss’s theorem: where k2 denotes the gravity constant. in the present case, δm is graphically obtained at about 2 ± 0.5 × 1012 kg above a region covering approximately 20 km2. as introduced previously, drillings have revealed that caldera deposits are usually funnel-shaped and comprised of relatively low-density fallbacks resulting from vesiculations and fragmentations. uniquely exploded ejecta composed of juvenile and lithic material had also fallen into the 1883 krakatau caldera, mixing with seawater during the repeated explosions. these circumstances influence how density contrast between the basement and caldera deposits is estimated. estimation of densities of various material related to caldera formation is rather ambiguous. when seawater penetrates into the fallbacks, material density increases according to porosity of the deposit and accordingly the density contrast decreases more than usual. hitherto we have assumed densities of the basements and the deposits at land calderas as 2400 and 2100 kg/m3, respectively. when the dry deposit has a porosity of 10~20 %, a density contrast decreases to 150 kg/m3 on an average by saturation with water. consequently the bulk volume of the deposit may be calculated as (2 ± 0.5) × 1012 kg/ 150 kg/m3 = 13 ± 3 km3, though we have ambiguity of the density contrast. if this volume is converted to an inverted circular cone or funnel shape with a base covering the same area as the caldera (a circle of 5 km in diameter), its maximum depth is about 2 km as a rough standard. in this case, it is rather difficult for us to locate magma reservoirs by analyses of gravity anomalies observed on the krakatau islands because the anomaly due to the caldera deposit predominates over that caused by the possible presence of deeper magma reservoir. to discuss the deeper structure by utilizing gravity anomalies, gravity surveys should cover a wider area in order to detect anomalies of larger wavelength. to express the structure of krakatau caldera, we adopt figure 5 that is composed of two planes: (a) is a horizontal plane at a depth of 2 km deep while (b) is a vertical plane passing b−b’ line shown in figure 3. the caldera deposit of krakatau is schematically shown with a circle of 5 km in diameter in figure 5a and with a dotted part in figure 5b: the volume of the deposit is about 13 km3 and its configuration is schematically expressed in conical or funnel shape on consideration of the results of drillings discussed in section 3. 4.2. seismological observations searching for subsurface liquid parts seismicity in volcanic areas is closely related to the activities of the volcanoes and the corresponding subsurface structure. the 1883 eruption: the seismic activities that accompanied the 1883 eruption of krakatau were not instrumentally observed. yokoyama [2001] examined the 2 ,m k g x y dx dy 1 2r = ^ h## (3) yokoyama 8 figure 5. seismological parameters determined by the temporary observations. the epicenters and hypocenters analyzed by harjono et al. [1989] and jaxybulatov et al. [2011] are approximately plotted and denoted by solid and open dots, respectively. ak denotes anak krakatau. distribution of the vp/vs ratios are obtained by jaxybulatov et al. [2011] and that of the s-wave attenuations by harjono et al. [1989]. (a) distribution of vp/vs ratios at the horizontal plane at a depth of 2 km b.s.l. a circle of broken line is an assumed caldera rim. (b) distributions of vp/vs ratios and s-wave attenuations (striped parts) projected on a vertical plane shown by b−b’ datum line in figure 5a. dotted part: assumed caldera deposit shown schematically; star symbol: branch point of the vent for anak krakatau from an assumed main conduit. (a) (b) 9 relationship between the maximum distances of seismic perceptibility and the intensity in mm (modified mercalli’s scale) of the krakatau earthquakes felt at some towns along the sunda straits. he determined the magnitudes of the earthquakes occurring in the 1883 eruption to be no larger than magnitude 5 by examination of the magnetograms figured by magnetic needles at a magnetic observatory in jakarta, not seismograms. generally speaking, magnitudes of volcanic earthquakes accompanied with explosions are usually smaller than m 5, although volcanic earthquakes are occasionally enhanced into the range of m 7 by large volcanic eruptions such as m 7.0 in the 1912 eruption of katmai [abe 1992]. in case of the krakatau eruption accompanied with earthquakes of a maximum magnitude of m 5, volcanic energy may have dispersed mainly in the explosive state and have not contributed as much energy to the earthquakes during caldera formation. seismicity obtained by temporary seismological observations: some of the epicenters or hypocenters around the krakatau complex which were determined by harjono et al. [1989] during the summer of 1984 and by jaxybulatov et al. [2011] for 8 months of 2005~2006 are approximately plotted with solid and open circles, respectively in figure 5. distributions of seismometers and analyzing methods as well as seismometer units were not consistent between both surveys, however both could only place seismometers on land, not in the sea. thus, we may roughly get an idea about the seismicity around the krakatau complex. the data suggests that the caldera deposit doesn’t accumulate elastic energy. one of the characteristic properties of magma reservoirs is fluidity of their contents. in seismological observations around volcanoes, this property should be revealed in particular phenomena such as s-wave attenuations and high vp/vs ratios. harjono et al. [1989] set up 10 temporary seismic stations on the both sides of the sunda straits and one on anak krakatau, all being equipped with a single vertical seismometer. wave paths were examined from 14 local earthquakes occurring in the summer of 1984 and two bodies of shear-wave attenuation were detected near the krakatau complex. some of their results [harjono et al. 1989: figure 8b] are schematically reproduced in figure 5b: several zones of s-wave attenuation were detected beneath the krakatau islands, but all were located at the depths deeper than 2 km, and hence they don’t appear in figure 5a. most probable s-wave attenuating zones were detected at between 6~14 km deep as shown by striped zones in figure 5b, and further a voluminous zone was detected at a larger depth of about 22 km, extending eastwards. jaxybulatov et al. [2011] carried out temporary seismometric observations at 14 onshore stations on the krakatau islands and the coasts of java and sumatra. during about 8 months of 2005~2006, more than 700 local earthquakes were recorded, and by using the results, tomographic inversions for p and s wave-velocities and for the vp /vs ratio were performed. they determined the distribution of vp/vs ratio in a square of 25 km side centering krakatau caldera in the map, and to a depth of roughly 15 km. some of their results are also schematically reproduced in figure 5 where the epicenters or hypocenters are denoted by hollow circles. from the surface to a 4 km depth, they observed vp/vs ratios higher than 2.1 (according to them, a maximum of roughly 2.2) in the adjacent zone of the caldera, and interpreted this as an indicator of the presence of material with partially molten and/or high fluid content. we do not expect more than a scarce amount of molten magma to exist at such a shallow depth after a caldera forming eruption. on the other hand, one of the zones of s-wave attenuation located approximately 10 km deep beneath the krakatau islands shows a rather high value (1.94) of vp/vs: this zone may contain the magma reservoir that provoked the 1883 eruption. jaxybulatov et al. [2011] explained that their network configuration and the distribution of the events (the origins of shallow seismic activities all originated on the east side of the caldera) were not favorable for high quality tomographic imaging. considering this, we may deduce that the zone of high vp/vs ratio mentioned previously nearly coincides with the caldera deposit of krakatau complex. from another viewpoint, their network configuration was especially suitable to detect the caldera deposit. as mentioned in subsection 4.1, a subsurface model of krakatau caldera is created on the assumption that the caldera deposit has density difference of 150 kg/m3. the present author is inclined to attribute the local and anomalously high values of vp/vs ratio to the caldera deposit saturated with seawater. although the zone of the highest vp/vs ratio doesn’t strictly coincide with the caldera deposit zone shown by a dotted part in figure 5b, the above rationale may be legitimate considering the challenging situation in which high–quality tomographic imaging took place. an anomalously high vp/vs ratio higher than 2.1 has been reported by laboratory measurements. for example, carmichael [1982] mentioned that compacted “globigerina ooze” (a kind of slime) had the ratio of 2.23 under pressure of 512.0 kg/cm2. it is rather difficult for us to calculate with precision the actual state of fallbacks in the 1883 krakatau eruption because the eruption was so tremendously vikrakatau caldera deposits olent, effectively blocking visibility. eyewitnesses could not approach the event from the sea. however, we theorize that a large amount of ejecta composed of pyroclastics and lithic material fell chaotically into the caldera depression located in the sea during or after repeated gigantic explosions; if this is the case, the caldera deposits of krakatau may have kept a large amount of seawater and remain saturated even now. the origin of the caldera deposits continues to be unknown. at many calderas in japan and mexico, we have obtained much knowledge about caldera deposits by drillings in these locations as discussed in section 3: in these instances along with many others, caldera deposits are usually composed of low density fallbacks deposited in funnel-shaped craters. synthetic images of the subsurface structure of krakatau caldera are summarized in figure 5 as already discussed above: figure 5a is a horizontal section at a depth of 2 km, on which the shallow vp/vs ratios and all epicenters are projected but the area of s-wave attenuations are not because they are located deeper than 2 km. in this projection, the region of highest vp/vs ratio partly overlaps on the caldera area. as discussed above, the high value of vp/vs ratio may evidence the watery caldera deposit. figure 5b is a vertical section along b-b’ line and shows all the main geophysical features including a schematic profile of the caldera. it is noticeable that the region of the highest vp/vs ratio partly overlaps on the caldera deposit as well as in figure 5a. in this projection, anak krakatau is hypothetically interpreted as a parasite derived from a magma reservoir of the krakatau complex. harjono et al. [1989] determined several zones of s-wave attenuation, one of which was located at a depth about 9 km directly beneath the krakatau complex. on the other hand, jaxybulatov et al. [2011] detected a zone of relatively high vp/vs ratio roughly at the same zone of swave attenuation. this zone may be a possible magma reservoir. 5. conclusion the pre-1883 krakatau complex: we don’t know of any clear examples of repeated caldera-forming eruptions at the same vent though our observation period is not so long. the topographies and gravity anomalies observed on and around the krakatau islands suggest that there was no krakatau caldera before 1883. anak krakatau is a parasite: it is plausible that a shallow depth (~10 km) magma body supplied magmas to the krakatau complex, causing violent explosions in 1883. also, we can infer that anak krakatau branched from this main conduit at a depth of about 4 km where is likely to be shallower than the magma reservoir. this model agrees with the fact that the periodicity of magmatic differentiation in the magma reservoir was not disturbed by the 1883 eruption. caldera deposit: existence of caldera deposit is already affirmed by the results of drillings at some calderas in japan and mexico. the volume of the caldera deposit at krakatau was estimated to be 10 ± 3 km3 on the base of the gravity anomalies observed there by yokoyama [1981]. however this calculation depends on the accuracy in the choice of the density contrast between the caldera deposit and the country rocks: the present author has theorized the density contrast as to be 150 kg/m3 since the deposits have probably remained saturated and submerged in the sea. consequently the bulk volume of the deposit is calculated as 13 ± 3 km3. in 2005~2006 seismological tomographic imaging by jaxybulatov et al. [2011] detected anomalously high vp/vs ratios roughly at the location where the zone of caldera deposits have probably settled. magma reservoirs: at the krakatau area, both temporal observations of two characteristics, s-wave attenuation and vp/vs ratio, have been carried out by harjono et al. [1989] and jaxybulatov et al. [2011], respectively. the results indicate zones of s-wave attenuation at three depths, 5~10, 8~13 and 20 km, directly beneath the krakatau complex while zones of relatively high vp/vs ratio were detected at two levels, very shallow and roughly 10 km deep. the most probable zone of magma reservoir beneath the krakatau islands must be endowed with both the characters in common: that is the zone located at a depth of about 10 km. these are schematically shown in figure 5b: an assumed magma reservoir is labeled with a vp/vs ratio 1.94 on an swave attenuating zone. it may be plausible to speculate that this magma reservoir has the potential to have contributed to the 1883 caldera forming eruption with continuing influence on anak ktakatau since 1927. acknowledgements. the author owes much recent geophysical knowledge on the krakatau complex to h. harjono and his group for detection of magma bodies from anomalous shear waves, c. deplus and his group for gravity and bathymetry data, and k. jaxybulatov and his group for seismological tomographic results. the author expresses his gratitude to these groups. the author gratefully acknowledges the help of ms. sara tanaka who took some of her time to criticize the details of the present discussion. the author is thankfully benefited by the critical comments from anonymous referees on the earlier version of the manuscript. references abe, k. 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(2013). parasitic eruptions on sakurajima volcano, b. volcanol. soc. jpn., 58, 91-102. *corresponding author: izumi yokoyama, the japan academy, ueno park, tokyo, japan; email: iyokoyama@joy.ocn.ne.jp. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. krakatau caldera deposits << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice u n n u o v o t i p o d i p i r e l i o m e t r o d i a n g s t r o m a c o m p e n s a z i o n e e l e t t r i c a renato cialdea 111 q u e s t a n o t a v i e n e d e s c r i t t o un n u o v o t i p o di p i r c l i o m e t r o di angstrom da n i e o t t e n u t o a p p l i c a n d o la t e c n i c a d e l l a e v a p o r a z i o n e d e i m e t a l l i n e l v u o t o . u n a p i c c o l a s e r i e di s t r u m e n t i di q u e s t o t i p o da m e studiati e p r o g e t t a t i p e r i n c a r i c o r i c e v u t o dal d i r e t t o r e d e l l ' i s t i t u t o n a z i o n a l e di g e o f i s i c a p r o f . a. l o s u r d o , si t r o v a n o a t t u a l m e n t e in c o s t r u z i o n e n e l l e officine di q u e s t o i s t i t u t o e sono d e s t i n a t i agli o s s e r v a t o r i d e l l a r e l è g e o f i s i c a i t a l i a n a per il r i l e v a m e n t o si-tem a t i c o d e l l ' i n t e n s i t à d e l l a r a d i a z i o n e s o l a r e . c o m e è n o t o , i p i r e l i o m c t i i per la m i s u r a assoluta d e l l a r a d i a z i o n e t i p o a n g s t r o m ( ' ) , c o n s i s t o n o in due sottili l a m i n e t t e di m a n gallina a n n e r i t e , una d e l l e q u a l i v i e n e e s p o s t a a l l a r a d i a z i o n e , m e n t r e i a l t r a , o p p o r t u n a m e n t e s c h e r m a t a , v i e n e r i s c a l d a t a da u n a c o r r e n t e e l e t t r i c a , q u a n t o o c c o r r e p e r c h è l e due l a m i n e t t e a s s u m a n o la stessa, t e m p e r a t u r a . in q u e s t e c o n d i z i o n i , p o i c h é l e due l a m i n e t t e sono uguali e q u i n d i uguali le p e r d i t e di c a l o r e , la q u a n t i t à di c a l o r e r i c e v u t a p e r r a d i a z i o n e d a l l ' u n a è uguale a q u e l l a s v i l u p p a t a n e l l ' a l t r a p e r e l i c i l o j o u l e . la d i f f e r e n z a di t e m p e r a t u r a tra l e d u e l a m i n e v i e n e r i v e l a l a m e d i a n t e una c o p p i a t e r m o e l e t t r i c a , l e c u i d u e s a l d a t u r e sono a p p o g g i a l e , la p r i m a d i e t r o una d e l l e l a m i n e di m a n g a n i m i e la sec o n d a d i e t r o l ' a l t r a , con l ' i n t e r p o s i z i o n e di un s o t t i l i s s i m o foglio «li m i c a . il p i r c l i o m e t r o è q u i n d i f o r m a t o da d u e e l c m e n l i d i s t i n t i , ciascuno d e i q u a l i è c o s t i t u i t o da tre sottili l a m i n e t t e : la m a n g a n i m i a n n e r i t a , il f o g l i o di m i c a e u n a l a m i n a di r a m e c o n al c e n t r o la s a l d a t u r a d e l l ' a l t r o m e t a l l o d e l l a c o p p i a t e r m o e l e t t r i c a : c i a s c u n elem e n t o v i e n e poi b l o c c a t o a l l e e s t r e m i t à in un s u p p o r t o di e b a n i t e , con o p p o r t u n i c o n l a t t i e s e r r a f i l i p e r i n s e r i r e la l a m i n a di m a n g a n i m i nel c i r c u i t o d e l l a c o r r e l i l e di c o m p e n s a z i o n e e la c o p p i a t e r m o e l e t t r i c a nel c i r c u i t o del g a l v a n o i n e t r o . le l a m i n e t l c m e t a l l i c h e c h e c o s t i t u i s c o n o c i a s c u n e l e m e n t o , v e n g o n o in g e n e r a l e p r e p a r a t e con processi m e c c a n i c i di l a m i n a z i o n e ; ti) r e n a t o ( j a i . o e a c i ò l i m i t a il loro a s s o t t i g l i a m e n t o e r e n d e poco c o m p a t t o il c o m p l e s s o f o r m a l o d a l l e d u e l a m i n e t t e e «lai f o g l i o di m i c a i n t e r p o s t o . \d e v i t a r e questi i n e o n v e n i e n t i , elle e v i d e n t e m e n t e i n f l u i s c o n o a n c h e sulla p r e c i s i o n e d e l l e m i s u r e , h o e f f e t t u a l a la p r e p a r a z i o n e degli c l e m e n t i del p i r c l i o m e l r o a p p l i c a n d o la t e c n i c a d e l l ' e v a p o r a z i o n e dei m e t a l l i nel v u o t o , t e c n i c a c h e ha r a g g i u n t o in q u e s t i u l t i m i a n n i una g r a n d e p e r f e z i o n e . così -i p o s s o n o o t t e n e r e c l e m e n t i elle c o s t i t u i s c o n o dei c o m p l e s s i n o t e v o l m e n t e c o m p a t t i c o s t i t u i t i da l a m i n e t t e i cui s p e s s o r i , r e g o l a n d o o p p o r t u n a m e n t e l ' e v a p o r a z i o n e , si possono f a r e m o l t o più p i c c o l i di q u e l l i elle -i o t t e n g o n o p e r v i a m e c c a n i c a . c i a s c u n e l e m e n t o p r e p a r a l o con q u e s t a t e c n i c a , r i s u l l a c o m p o sto da un s o t t i l i s s i m o foglio di m i c a , o t t e n u t o per s f a l d a t u r a , sagom a t o c o m e n e l l a lig. i » ) : da una p a r t e vi è d e p o s i t a l o d e l l ' a l l u m i n i o (zona p u n t e g g i a t a lig. 1?;), elle v i e n e r i n f o r z a l o solo a l l e e s t r e m i tà con successivo d e p o s i t o d e l l o slesso m e t a l l o ( z o n a t r a t t e g g i a l a fig. l/>h sul c o r p o c e n t r a l e di q u e s t o e l e m e n t o v i e n e e f f e t t u a l o il d e p o s i t o di n e r o di b i s m u t o ( z o n a p u n t e g g i a l a fig. ifc); q u e s t a p a r t e a n t e r i o r e è il s e m i e l c i n c n t o c o m p e n s a t o r e . l ' a l t r a f a c c i a d e l l e l a m i n e t t e di m i c a viene c o p e r t a per m e t à con d e p o s i l o di a n t i m o n i o e p e r l ' a l t r a m e t à con b i s m u t o , c o m e è i n d i c a l o n e l l a fig. i r ) , ri ove la zona i l'ai l e a g i a t a r a p p r e s e n t a i a n t i m o n i o e q u e l l a p u n t e g g i a l a il b i m u l o : a n c h e da q u e s t a p a r t e le e s t r e m i t à d e l l a c o p p i a sono r i n f o r z a l e p e r a v e r e un buon c o n l a t l o e l e t t r i c o . i d u e e l e m e n t i v e n g o n o q u i n d i m o n t a t i in p a r a l l e l o su un s u p p o r l o di e b a n i t e : esso p r e s e n t a degli o p p o r t u n i c o n i a i i i . c o n s i s t e n t i i n d u e l a m i n e t t e di r a m e , p e r f e t t a m e n t e s o v r a p p o s t e . c h e s e r r a n o t r a l o r o gli e l e m e n t i . in lig. t i ) si vede lo schema d e l i e c o n n e s s i o n i p o s t e r i o r i p e r il c o l l e g a m e n t o con il g a l v a n o m e t r o ed in fig. 21)) di q u e l l e a n t e r i o r i elle si i n e r i s c o n o n e l c i r c u i t o c o m p e n s a t o r e . con q u e s t o m e t o d o si o t t i e n e c o s i un e l e m e n t o già p r o n t o , lullo c o m p a t t o e c h e p u ò e s s e r e m e s s o a p o s t o c o n g r a n d e s e m p l i c i t à ; c u n n u o v o t i p o i h p i k e i . i o m e t k » h i a n c s t k o m a c o m p e n s a z i o n i ; e l e t t r i c a t i f a c i l e q u i n d i una r a p i d a o s l i u i z i o n e . q u a l o r a c i ò si r e n d e s s e n e c e s s a r i o . c o m e i s o l a n t e s e r v e a n c h e in q u e s t i e l e m e n t i la m i c a , m a in essi la sua f u n z i o n e n o n è solo di i s o l a n t e , m a s o p r a l u t t o di supp o r t o . p e r o t t e n e r e le l a m i n e t t e di m i c a i l p i ù p o s s i b i l e u g u a l i , veniva a d o p e r a t o un a p p o s i t o s t a m p o a b i l a n c i e r e : -i p o t e v a n o co-i p r e p a r a r e in b r e v i s s i m o t e m p o un b u o n n u m e r o di l a m i n e t t e sagom a l e . di u g u a l e s p e s s o r e e di uguali d i m e n s i o n i : lo s p e s s o r e da m e a d o t t a t o a l l o s c o p o di a v e r e una sufficiente s o l i d i t à m e c c a n i c a , e di c i r c a io u. d o p o il t a g l i o si eseguiva un a c c u r a t o e s a m e por v e r i f i c a r e lo s p e s s o r e , la f o r m a e l ' o m o g e n e i t à d e l l e l a m i n e t t e . e s s e f i g . 2. v e n i v a n o inlino a c c u r a t a m e n t e s g r a s s a t e e p u l i t e p r i m a di e l l e l l u a r v i i d e p o s i t i m e t a l l i c i : p as s ate in una s o l u z i o n e di soda c a u s t i c a , l a v a t e poi in a c q u a d i s t i l l a t a , passate di nuovo in a l c o o l , s o t t o p o s t e a n c o r a ad un p r o l u n g a l o lavaggio in a e q u a d i s t i l l a t a , i n f i n e a s c i u g a l e e p r o n te così p e r d e p o s i t a r v i i m e t a l l i . p e r la m e t a l l i z z a z i o n e m e d i a n t e e v a p o r a z i o n e mi sono s e r v i l o , a d a t t a n d o l o al nuovo l a v o r o , di un a p p a r a l o c o s t r u i t o n e l l ' i s t i t u t o di f i s i c a d e l l a l n i v e r s i t à di r o m a . esso consisto in una c a m p a n a di v e t r o del d i a m e t r o di 18 em e n t r o la (piale si p u ò o t t e n e r e , con un g r u p p o di p o m p o a d i f f u s i o n e ad o l i o ed una p o m p a p r e p a r a t i n e rot a t i v a . una p r e s s i o n e i n f e r i o r e a io' m i n di m e r c u r i o . nella c a m pana sono posti gli e l e m e n t i da m e t a l l i z z a r e ed i f o r n e l l e l l i p e r la f u s i o n e dei m e t a l l i . questi -onn di d u e t i p i , secondo il m e t a l l o c h e -i d e v e e v a p o r a r e : per l ' a l l u m i n i o a d o p e r o c o m e f o r n e l l o un filo di t u n g s t e n o r i p i e g a t o ad l . a t t o r n o al (piale è avvolto un filo di alilir e n a t o c i a i . o e a m i n i o p u r o : p e r l ' a n t i m o n i o ed il b i s m u t o i n v e c e , i f o r n e l l i s o n o c o s t i t u i t i da una c o r t a s p i r a l e c o n i c a di filo di t u n g s t e n o , nel cui int e r n o v e n g o n o posti i p e z z e t t i del m e t a l l o da e v a p o r a r e : i m a c o r r e n t e e l e t t r i c a , p a s s a n d o nel filo di t u n g s t e n o , p r o v o c a la f u s i o n e e la c o n s e g u e n t e e v a p o r a z i o n e del m e t a l l o . q u e s t i f o r n e l l i v e n g o n o o p p o r t u n a m e n t e s c h e r m a l i in m o d o da l a s c i a r l i b e r o solo un p i c c o l o a n g o l o s o l i d o , e n t r o il q u a l e v e n g o n o posti gli e l e m e n t i da m e t a l l i z z a r e : u n o s c h e r m o c o m a n d a t o d a l l ' e s t e r n o fa si elle il m e t a l l o si d e p o s i t i sulle l a m i n e t t e s o l a m e n t e d u r a n t e u n t e m p o d e t e r m i n a t o . d u r a n t e t u t t a 1 o p e r a z i o n e si m a n t i e n e n e l l a c a m p a n a u n a p r e s s i o n e i n f e r i o r e ai io1' m m di m e r c u r i o . 1 n grosso r u b i n e t t o a v a l v o l a i n t e r r o m p e la c o m u n i c a z i o n e tra la c a m p a n a e l e p o m p e : in tal m o d o -i p u ò far rient r a r e l ' a r i a n e l l a c a m p a n a , las c i a n d o i n t e r a m e n t e ili efficienza lip o m p e a d i f f u s i o n e . l e e v a p o r a z i o ni p o s s o n o così e s s e r e e s e g u i t e i m a d o p o i a l t r a c o n u n i n t e r v a l l o di t e m p o di c i r c a u n ( p i a r l o d o r a . i n t e r v a l l o n e c e s s a r i o al r i e m p i m e n t o d ' a r i a d e l l a c a m p a n a , al l ' e v e n t u a l e e a m b i o dei f o r n e l l i e d e l l a s u p e r f i c i e da m e t a l l i z z a r e , a l l a c h i u s u r a d e l l a c a m p a n a , a l l a p r e p a r a z i o n e del vuoto p r e l i m i n a re f a l l o solo con la r o t a t i v a ed ali e s e c u z i o n e del v u o t o d e f i n i t i v o eseg u i l o con l e p o m p e a d i f f u s i o n e : in p o c h e o r e così -i possono e s e g u i r e t u t t e le o p e r a z i o n i i n e r e n t i a l l a m e t a l l i z z a z i o n e d e g l i e l e m e n t i . t ' o m o a b b i a m o visto n e l l a fig. 1 gli e l e m e n t i p r e s e n t a n o , n e l l e p a r l i m e t a l l i c h e , d e l l e s a g o m e , c h e d e v o n o essere m o l l o p r e c i s e c o m e d i m e n s i o n i e d e v o n o s o v r a p p o r s i e s a t t a m e n t e , sia d a l l a p a r t e del c o m p e n s a t o r e c l i c da q u e l l a d e l l a c o p p i a t e r m o e l e t t r i c a . p e r q u e s t a rag i o n e h o usato un p i c c o l o t e l a i o , c l i c b l o c c a le l a m i n e t t e di m i c a , p o s t e ira d u e s o t t i l i l a m i n e m e t a l l i c h e ; e ; s e p o r t a n o , p e r f e t t a m e n t e s o v r a p p o s t e , u n a , la s a g o m a d e l c o m p e n s a t o r e e l ' a l t r a , q u e l l a d e l l a c o p p i a ( f i g . 3 ) . p e r p o t e r e a v e r e poi u n a p i c c o l a s e r i e di e l e m e n t i c h e s i a n o siali p r e p a r a l i n e l l o stesso m o d o , il t e l a i o h a ( p i a t i l o p o s t i : c o s ì p e r ogni e v a p o r a z i o n e o t t e n g o q u a t t r o e l e m e n t i g e m e l l i . non è l a t o p o s s i b i l e a u m e n t a r e ((tic-io n u m e r o p e r non r i s e n t i r e l ' i n f l u e n z a d e l l a d i s t a n z a dal f o r n e l l o e s o p r a l u l l o l ' i n f l u e n z a d e l l a d i u n i f o r u n n u o v o t i p o d i p 1 r e l i o m e t r o d i a n g s t r o m a c o m p e n s a z i o n e e l e t t r i c a 7 3 m i t à di e v a p o i ' a z i o n e p r e s e n t a t a da e s s o : t a l e i n f l u e n z a fa e v i d e n t e m e n t e d i m i n u i r e ai b o r d i la q u a n t i t à di m e t a l l o e v a p o r a t o . p e r a v e r e poi i r i n f o r z i a l l e e s t r e m i t à , d o p o a v e r e d e p o s i t a t o il m e t a l l o p e r e v a p o r a z i o n e su tutta la s a g o m a , tiì es-a si d i s p o n e una m a s c h e r i n a c l i c l a s c i a s c o p e r t e solo l e e s t r e m i t à d e l l ' e l e m e n t o , d o v e , e s p o n e n d o l a u l t e r i o r m e n t e a l l ' e v a p o r a z i o n e , si o t t e n g o n o dei d e p o s i t i p i ù spessi. nella p a r t e p o s t e r i o r e poi si u=a u n ' a l t r a m a s c h e r i n a c h e l i m i t a il d e p o s i t o s o l t a n t o a mezza l a m i n a : q u e s t a m a s c h e r i n a v i e n e p r i m a posta su una p a r t e e c o s ì si o t t i e n e il s e i n i e l e m c n t o di b i s m u t o ed in s e g u i l o , r u o t a l a in m o d o da p o r t a r l a ali a l t r a p a r t e , si o t t i e n e , con una successiva e v a p o r a z i o n e , il s e m i e l e m e n t o di a n t i m o n i o . p o i c h é le m a s c h e r i n e s c h e r m a n o m e n o d e l l a m e t à della l a m i n a , si o t t i e n e u n a s o v r a p p o s i z i o n e d e i d u e m e t a l l i c h e f o r m a u n a s a l d a t u r a estesa 0 . 5 min nel senso d e l l a l u n g h e z z a . i n f i n e u n a u l t i m a m a s c h e r i n a v i e n e a d o p e r a t a p e r r i n f o r z a r e l ' e s t r e m i t à del s e m i e l e m e n t o di b i s m u t o o di q u e l l o di a n t i m o n i o . l a d u r a t a d e l l a e v a p o r a z i o n e p e r m e t t e di r e g o l a r e gli s p e s s o r i d e i d e p o s i t i . r i a s s u m e n d o , l e o p e r a z i o n i si susseguono i n q u e s t o m o d o (fig. 4 ) : ] ) p a r i e a n t e r i o r e : d e p o s i t o ili a l l u m i n i o : a g o n i e i n t i e r e ; 2 ) p a r t e a n t e r i o r e : d e p o s i t o di a l l u m i n i o : m a s c h e r i n a 2 . dep o s i t o solo sulle due e s t r e m i t à : 3 ) p a r i e p o s t e r i o r e : d e p o s i t o di a n t i m o n i o : m a s c h e r i n a 3 ; 4 ) p a r i e p o s t e r i o r e : d e p o s i t o di a n t i m o n i o : m a s c h e r i n a 4 , dep o s i t o solo sulla e s t r e m i t à ; 5 ) p a l l e p o s t e r i o r e : d e p o s i t o di b i s m u t o : m a s c h e r i n a 3 ; 6 ) p a r i e p o s t e r i o r e : d e p o i t o di b i s m u t o : m a s c h e r i n a 4 , d e p o sito solo sulla e s t r e m i t à . d o p o a v e r e e f f e t t u a l o i d e p o s i l i m e t a l l i c i , si passa a l l ' a n n e r i m e n t o d e l l a s u p e r f i c i e a n t e r i o r e . il p r o b l e m a d e l l ' a n n e r i m e n t o d e l l a s u p e r f i c i e si p u ò c o n s i d e r a r e c o m e u n o dei p i ù i m p o r t a n t i p e r q u e s t e r i c e r c h e : è noto i n f a t t i c h e una s u p e r f i c i e , p e r q u a n t o a s s o r b e n t e possa e s s e r e , r i f l e t t e s e m p r e p a r t e d e l l e n e r g i a i n c i d e n t e , s p e c i e nell i n f r a r o s s o : il coefficiente di r i f l e s s i o n e s e c o n d o le m i s u r e di c o b l c n i a ( ' ) p e r il n e r o f u m o si m a n t i e n e q u a s i c o s t a n t e dal v i s i b i l e fino a c i r c a 8 u , p e r ò esso è di c i r c a il 3 p e r c e n t o : il coefficiente di r i f l e s s i o n e p e r il n e r o di p l a t i n o , d e p o s i t a l o e l e t t r o l i t i c a m e n t e , a u m e n t a g r a d a t a m e n t e d a l l ' i p e r c e n t o nel v i s i b i l e fino al 3 p e r c e l i l o p e r l u n g h e z z e d o n d a di 2 4 ti. n e g l i u l t i m i a n n i p l u n d ( ) trovò c h e il n e r o di b i s m u t o d e p o s i t a l o p e r e v a p o r a z i o n e nel vuoto sotto una p r e s s i o n e ili 0 , 2 5 m i n . h a un c o e l i t f . n a t o c i u . h f . a o o g yj hl 11 : \\ j c ' r ! ' t n l i i l lij s i 0 0 0 0 j 0 f | i | f p | i | f 1 | t. j 1 • t t t = 1 m ' •j i.l ; p | i | ] ] f i g . 4 . f i c i e n l e di r i d e s s i o n e d e l l ' i p e r c e n t o p e r l u n g h e z z e d ' o n d a c o m p r e s e ira 0 , ó e 13 u . p e r q u e s t a r a g i o n e h o a n n e r i l o le l a m i n e col n e r o di b i s m u t o , a d o p e r a n d o a q u e s t o s c o p o una c a m p a n a di v e t r o c o l l e g a l a d i r e t t a m e n t e con la p o m p a r o t a t i v a : il b i s m u t o v i e n e posto in f o r n c l l c t t i del o l i t o l i p o f o r m a t o da filo di t u n g s t e n o a s p i r a l e : e s s o , e v a p o r a n d o , si d e p o s i t a l i l l e l a m i n e t t e p o s t e o r i z z o n t a l m e n t e . p e r lim i t a r e tale d e p o s i t o al c o r p o c e n t r a l e degli c l e m e n t i , v i e n e usala u n nuovo tipo di p 1 r e l i o m e t r o di angstrom a c o m p e n s a z i o n e elettrica 73 una m a s c h e r i n a del t i p o 7 ) fig. 4 , c l i c è fissata, c o m e le p r i m e , al s o l i l o t e l a i o ; essa p e r ò è un p o ' d i f f e r e n t e , m e n t r e le p r i m e s o n o d e l l e l a m i n e c l i c si s o v r a p p o n g o n o s e m p l i c e m e n t e al t e l a i o , q u e s t ' u l t i m a p r e s e n t a i n v e c e d e i r i a l z i c l i c si i n f i l a n o n e l l e sue s a g o m e , dim o d o c h é la m a s c h e r i n a v i e n e quasi a t o c c a r e la m i c a ; c i ò -i è reso n e c e s s a r i o p e r c h è tra l a m i c a e la m a s c h e r i n a v e r r e b b e a c r e a r s i un i n t e r v a l l o di c i r c a m e z z o i n n i , i n t e r v a l l o c l i c s a r e b b e sufficiente a perm e t t e r e la d i f f u s i o n e del n e r o di b i s m u t o a n c h e sulle e s t r e m i t à r i n f o r z a t e , tlalo c h e si o p e r a con p r e s s i o n e 11011 e c c e s s i v a m e n t e b a s s a ( c i r c a 0 , 2 5 m i n ili m e r c u r i o ) : r i d u c e n d o q u e s t o i n t e r v a l l o a pochi c e n t e s i m i eli m i n , posso così e l i m i n a r e q u e s t a difficoltà, c l i c , o l i r e a t u t t o , c r e e r e b b e un c o n l a t t o e l e t t r i c o c a t t i v o , p e r c h è il n e r o di iiim u t o è i s o l a n t e . ( ì l i e l e m e n t i così p r e p a r a t i v e n g o n o fi-sali ad un s u p p o r t o di e b a n i t e ( f i g . 2> ed il lutto p o t o in una c u s t o d i a m e t a l l i c a c o n o p p o r tuni d i a f r a m m i a n n e r i l i p e r e l i m i n a r e l ' e n e r g i a r i f l e s s a e d i f f u s a sec o n d o la b e n n o i a d i s p o s i z i o n e di a n g s t r o m . la m i s u r a col m e t o d o di c o m p e n s a z i o n e si e f f e t t u a c a l c o l a n d o i e n e r g i a t r a s f o r m a l a in c a l o r e per e f f e t t o j o u l e s u l l ' u n i t à di s u p e r ficie d e l l a i a m i n a . q u a n d o questa è c o s t i t u i t a da un m e t a l l o , la cui resistenza -i p u ò c o n s i d e r a r e e n t r o i l i m i t i del l ' a p p r o s s i m a z i o n e cons e n t i t a in q u e s t e m i s u r e c o m e c o s t a n t e , a l l o r a b a s t a c o n o s c e r e la res i s t e n z a r p e r u n i t à di lunghezza d e l l a l a m i n a ( c h e nel caso d e g l i a n g s t r o m è di m a n g a n i m i ) p e r o t t e n e r e l ' i n t e n s i t à d e l l a r a d i a z i o n e c o n la n o t a f o r m u l a : t=<) 21 — ù() a b dove : i i n t e n s i t à d e l l a r a d i a z i o n e in cai cm m i m i l o p r i m o i i n t e n s i t à d e l l a c o r r e n t e c o m p c n s a t r i c e r r e s i s t e n z a d e l l ' u n i t à di lunghezza d e l l a l a m i n a ti coefficiente di a s s o r b i m e n t o b l a r g h e z z a d e l l a l a m i n a . p o i c h é nel n o s t r o c a s o i! coefficiente eli t e m p e r a t u r a d e l l a r e i l e n z a d e l l a l a m i n a n o n -i può c o n s i d e r a r e t r a s c u r a b i l e , a l l o r a la int c n s i i à della r a d i a z i o n e p u ò essere c a l c o l a l a d e t e r m i n a n d o la p o t e n z a 7 6 m i n a t o c i a i . u k a e l e t t r i c a t r a s f o r m a t a in c a l o r e «u tutta la l a m i n a e f a c e n d o il q u o z i e n t e r i p e t l o a l l a s u p e r f i c i e t o t a l e d e l l a l a m i n a stessa. q u i n d i si h a : i v 1 = 0,24 60 as d o v e : a coefficiente di a s s o r b i m e n t o ; i n t e n s i t à d e l l a c o r r e n t e c o m p e n s a t r i c e i d i f f e r e n z a di p o t e n z i a l e ai c a p i d e l c o m p e n s a t o r e h s u p e r f i c i e d e l l a l a m i n a . ii l'atto di d o v e r f a r e la m i s u r a o l t r e c h e d e l l a i n t e n s i t à aneli.d e l l a t e n s i o n e ai c a p i d e l l a l a m i n a c o m p c n s a t r i c e , n o n r a p p r e s e n t a p r a t i c a m e n t e una c o m p l i c a z i o n e in q u a n t o un v o l t m e t r o di r e s i s t e n z a m o l t o e l e v a t a v i e n e s t a b i l m e n t e c o l l e g a t o c o n gli e s t r e m i d e l l a l a m i na c o m p c n s a t r i c e : la r e s i s t e n z a di d e t t o s t r u m e n t o si s c e g l i e t a l e da non i n t r o d u r r e s e n s i b i l i v a r i a z i o n i d e l l a i n t e n s i t à c l i c si legge sul m i l l i a m p e r o m e t r o i n s e r i t o nel c i r c u i t o c o m p e n s a t o r e . p e r a v e r e u n a i d e a d e l l a p r e c i s i o n e del m e t o d o , p o s s i a m o t r o v a l i i e r r o r e r e l a t i v o p a s s a n d o ai l o g a r i t m i e d i f f e r e n z i a n d o d i da _l di 1 dv 1 ds i a i v nr s p o i c h é i . i ed s p o s s o n o d i f f i c i l m e n t e e s s e r e m i s u r a t i e o r r e n t e m e n l i con una p r e c i s i o n e m a g g i o r e d e l l ' i per m i l l e , s u p p o n e n d o il coeffic i e n t e di a s s o r b i m e n t o d e t e r m i n a b i l e con la l e s s a p r e c i s i o n e , le ìniu r e a s s o l u t e d e l l a r a d i a z i o n e sono a f f e t t e da un e r r o r e di c i r c a il 0 . 1 p e r c e n t o . m a t a l e c o e f f i c i e n t e v i e n e d e t e r m i n a l o con u n a a p p r o s s i m a z i o n e di c i r c a il 0 , 5 p e r c e n t o , c i ò c h e fa s a l i r e l ' e r r o r e t o t a l ' r e l a t i v o a c i r c a l ' i p e r c e n t o . pi ima del m o n t a g g i o d e f i n i t i v o degli c l e m e n t i , v e n g o n o deto<m i n a t e le s e g u e n t i l o r o c a r a t t e r i s t i c h e : « ) s u p e r f i c i e d e l l a l a m i n a ; b) r e s i s t e n z a del c o m p e n s a t o r e ; e ) r e s i s t e n z a d e l l a e o p p i a t e r m o e l e t t r i c a ; d ) s e n s i b i l i t à , a l l a t e m p e r a t u r a a m b i e n t e di 2 0 " ( ' , . della c o p p i a t e r m o e l e t t r i c a ( f . e . i n . e c o e f f i c i e n t e di a s s o r b i m e n t o ) alla r a d i a z i o n e : e ) s e n s i b i l i t à , alla t e m p e r a t u r a a m b i e n t e di 2 0 " c . d e l l ' a p p u r a t o a l l a c o m p e n s a z i o n e . o l t r e a l l a u p e r f i c i e . c h e è una c o s t a t i l e d e l l ' e l e m e n t o , n e c e s s a r i a u n nuovo tipo di p 1 r e l i o m e t r o di angstrom a c o m p e n s a z i o n e elettrica 73 al c a l c o l o d e l l a r a d i a z i o n e , si m i s u r a n o le c a r a t t e r i s t i c h e e l e t t r i c h e c l i c sono u t i l i p e r p o t e r e a v e r e u n a serie di e l e m e n t i p e r q u a n t o poss i b i l i uguali tra l o r o . i n o l t r e h o a g g i u n t o l e m i s u r e d e l l a s e n s i b i l i t à d e l l ' a p p a r a t o a l l a r a d i a z i o n e ed a l l a c o m p e n s a z i o n e c h e m i p e r m e t l o n o di d e t e r m i n a r e la p r e c i s i o n e c o n s e n t i t a s p e r i m e n t a l m e n t e : h o c h i a m a t o p e r c i ò s e n s i b i l i t à d e l l ' a p p a r a l o a l l a r a d i a z i o n e , la l e . n i . d e l l a c o p p i a t e r m o e l e t t r i c a , q u a n d o su un e l e m e n t o ilei p i r c l i o m e t r o i n c i d a la r a d i a z i o n e di una l a m p a d a h e f n e r a l t e n e k , p o s t a a l l a distanza di un m e t r o ed a l l a t e m p e r a t u r a a m b i e n t e di 2 0 ° : essa si m i sura q u i n d i in v o l t / h e f n e r m e t r o : ed i n o l t r e ilo c h i a m a t o s e n s i b i l i t à alla c o m p e n s a z i o n e , la p o t e n z a e l e t t r i c a n e c e s s a r i a p e r p o t e r e a v e r e lo s p o s t a m e n t o di u n a d i v i s i o n e n e l g a l v a n o m e t r o c o l l e g a t o con la c o p p i a t e r m o e l e t t r i c a s e m p r e a l l a t e m p e r a t u r a a m b i e n t e di 2 0 " e q u i n d i , con o p p o r t u n e r i d u z i o n i , essa si m i s u r a i n e a l . / m i n u t o c m 2 v o ! t . l e c a r a t t e r i s t i c h e g e o m e t r i c h e sono state d e t e r m i n a t e c o n un c o m p a r a t o r e c o n la p r e c i s i o n e d e l m i l l e s i m o di m m : l e r e s i s t e n z e e l e t t r i c h e sono s t a t e m i s u r a t e c o n u n p o n t e di w h e a l s t o n e . d i c i a s c u n u p p o i i o i n f i n e h o d e t e r m i n a t o l a r e s i s t e n z a d e i c o n d u t t o r i del co nip e n s a t o r e : c i ò p e r c h è , m i s u r a n d o la d i f f e r e n z a di p o t e n z i a l e ai c a p i dei m o r s e t t i e s t e r n i del p i r c l i o m e t r o , p a r i e d e l l ' e n e r g i a s v i l u p p a t a per e f f e t t o j o u l e , v i e n e d i s s i p a t a in q u e s t i c o n d u t t o r i : essa p e r ò è m o l l o p i c c o l a , e c o m e v e d r e m o , m i n o r e d e l l ' ] p e r m i l l e di q u e l l i t o t a l e , e q u i n d i p u ò e s s e r e d e l t u t t o t r a s c u r a t a . g l i e l e m e n t i finiti, d o n o la d e t e r m i n a z i o n e d e l l e l o r o c a r a t t e r i s t i c h e e finché non sono c o l l o c a t i n e l l ' a p p a r a t o , v e n g o n o chiusi in fialette p r o t e t t r i c i di v e t r o . c o n q u e s t o nuovo l i p o di p i r c l i o m e t r o h o e l i m i n a l o in g r a n p a l l e i seg uent i i n c o n v e n i e n t i p r e s e n t a t i dal l i p o c l a s s i c o : 1 ) l ' e r r o r e s i s t e m a t i c o i n d i c a t o da a n g s t r o m ( ' ) stesso, c h e h a o r i g i n e in una d i s u g u a g l i a n z a di t e m p e r a t u r a tra la l a i n i n e l t a riscald a l a p e r r a d i a z i o n e e q u e l l a p e r e f f e t t o j o u l e : t a l e d i f f e r e n z a è dovuta al f a l l o c l i c la d i s t r i b u z i o n e d e l l a t e m p e r a t u r a d e n t r o le l a m i nelte n o n è u g u a l e , p o i c h é c s i s l c un g r a d i e n t e t e r m i c o s e c o n d o io s p e s s o r e di ciascun e l e m e n t o : questo eli e t t o è n o t e v o l e d a t o lo spessore r e l a t i v a m e n t e g r a n d e d e g l i e l e m e n t i del t i p o c l a s s i c o : 2 ) la n o t e v o l e d e l i c a t e z z a e f r a g i l i t à degli e l e m e n t i p r e p a r a l i p e r l a m i n a z i o n e ; la loro c o s t r u z i o n e i n f a t t i ed il l o r o m o n t a g g i o sono e s t r e m a m e n t e d e l i c a t i e c o m p l e s s i , p o i c h é -i t r a t t a di s t r i s c i o m e t a l l i c h e e di l a m i n e t t e di m i c a d e l l o spessore del c e n t e s i m o e elei m i l l e s i m o di m m . d e l l a l a r g h e z z a di un p a i o di min e d e l l a l u n m i n a t o i ( a l 1 :l: v ghezza (li un p a i o (li c u i . il loro m o n t a g g i o r i c h i e d e poi una c u r a p a r t i c o l a r e p o i c h é t u l l e e ire le l a m i n e t t e d e v o n o r i m a n e r e p e r f e t t a m e n t e a l l i n e a t e : 3) la p i c c o l a resistenza p r e s e n t a t a d a l l e l a m i n e t t e di m a n g a n i t i » : gli e l e m e n t i i n f a t t i c h e v e n g o n o a s s o t t i g l i a l i per via m e c c a n i c a , non p o s s o n o a v e r e , p e r e v i d e n t i r a g i o n i di s o l i d i t à , uno s p e s s o r e i n f e r i o r e al c o n t e i m o o al m a s s i m o a q u a l c h e m i l l e s i m o ili m i n : -i c o m p r e n d e così c h e la loro r e s i s t e n z a e l e t t r i c a r i s u l t a e s s e r e d e l l ' o r d i n e d e l (leonini di o h m , q u i n d i p o r la l a m i n e l l a c o m p c n s a t r i c e o c c o r r o n o d e l l o c o r r e n t i n o t e v o l i dell o r d i n e dell a m p è r e , c o r r e n t i c h e vi p r o d u c o n o d e l l e c a d u t o di p o t e n z i a l e dell o r d i n e del d e c i m o ili v o l t : c i ò significa elio, s u p p o n e n d o i uso di un a c c u m u l a t o r e ad un solo e l e m e n t o , più di novo d e c i m i dell e n e r g i a e l e t t r i c a v e n g o n o d i s s i p a l i n e l l e r e s i s t e n z e r e g o l a t r i c i d e l l a c o r r e n t e stessa. por d a r e m i a i d e a d e l l a r e g o l a r i t à di f u n z i o n a m e n t o di q u e s t o nuovo t i p o di pi lei i o n i e ! r o . r i p o r t o qui a t i t o l o di e s e m p i o a l e n i l i dati r e l a t i v i ad una e l i o di m i u r o d e l l a i n t e n s i t à d e l l a r a d i a z i o n e s o l a r e o e g u i l o con esso in r o m a : c o n t e m p o r a n e a m e n t e la r a d i a z i o n e v e n i v a m i u r a l a con un a l l i n o m e l r o c a m p i o n e di l i n k o e f e u s s n e r . lo m i u r e sono s l a t e e h c t l u a t e con un m i l l i a m p o r o m e t r o con 150 nia e con un v o l t m e t r o a 6 v o l t : le l e t t u r e e r a n o a f f e t t e da un e r r o r e d e l l ' i per m i l l e : la c o r r e n t e c o m p c n s a t r i c e e r a e r o g a t a da u n a p i l a a secco a d un solo e l e m e n t o . l ' i r o l i o m e l r o n u o v o t i p o e l e m e n t o il. 7 e l e m e n t o il. 8 s —0 , 4 13 c u t ' s = (1,457 o n r r 2 5 . 2 1 o h m r 2 2 . 1 8 o l m i s e n s i b i l i t à a l l a r a d i a z i o n e in il volt h o f n e r i n e t r o 0 . 1 3 0 . 1 8 s e n s i b i l i t à a l l a c o m p e n s a z i o n e in cai m i n u t o o n r -ti voli 0 . 0 0 1 5 0 . 0 0 5 6 nella s e r i e di m i s u r o d e l l a r a d i a z i o n e s o l a r e , per v a l o r i c o m presi tra 0 . 9 e 1.3 c a i . m i n u l o o i i r . ho c a l c o l a l o il r a p p o r t o tra i v a l o r i o l l e n u l i con il p i r o l i o m e l r o n u o v o t i p o e q u e l l i o t t e n u t i con i a l l i n o m e l r o c a m p i o n e : ho t r o v a t o così c l i c q u e s t o r a p p o r t o è r i m a s t o i n n i ( i \ < > t i p o i>r e > i k i : i , [ o n i i : n i o m v n g s t h ò m v i o m u ì n m / k i m : i : i . k t i ' i i ) c a 7 9 c o s t a n t e , con uno s c a r t o m e d i o del 2 p e r m i l l e , c h e r i e n t r a nei l i m i t i degli e r r o r i s p e r i m e n t a l i . roma isliiu!o nazionale ili geofìsica luglio 1947. ri i . w v / o sella /trescale nota è descrillo un nuovo tipo ili pircliometro ili ingslriim. i cui clementi compensatori sono siali realizzali (fall a. mediante evaporazione di metalli nel vuoto. onesto nuovo tipo è notevole per la precisione raggiunta e per la sua praticità. b i b l i o g r a f i a l ' i k . angstrom : !'h>.-. r o v . 1. li!1'!, p. 3 0 5 : vnnulen der l'iiy.-ik 67. 1899p. olio. ( 2 ) w . w . c o b l e m z : b u l l , oi' bureau of standard* i x . 1913. p. 283. ( 3 ) a . h . pl'und: f . i x . of sriont. i n s l r u m . 1. 1930. p. 397. (•') a n g s i r o m : m,-icorol. z r i i , l i . x x x i . 1911. p. 369. 37adams.qxd 859 annals of geophysics, vol. 47, n. 2/3, april/june 2004 key words earthquake location – macroseismic locations 1. introduction instrumental recording of earthquakes began only in the late nineteenth century. until then knowledge of earthquakes and their distribution depended entirely on the observing and interpreting of their felt effects. despite this limitation many creditable studies of seismicity were undertaken, an example being the map of global earthquake distribution of mallet in 1858 (fig. 1), which agrees well with present day knowledge, apart from omitting activity on the oceanic ridges, from where no felt information could be obtained. because of limitations in the design of early instruments, and the lack of knowledge of earth structure, the advent of instrumental recording did not immediately result in a major improvement in earthquake location, but rather re-evaluation of early instrumental earthquake locations: methodology and examples robin d. adams international seismological centre, thatcham, berkshire, u.k. abstract the difficulties of locating earthquakes in the early instrumental period are not always fully appreciated. the networks were sparse, and the instruments themselves were of low gain, often had inappropriate frequency response and recording resolution, and their timing could be unreliable and inaccurate. additionally, there was only limited knowledge of earth structure and consequent phase identification and propagation. the primitive zöppritz tables for p and s, with no allowance for the core, did not come into use until 1907, and remained the main model until the adoption of the jeffreys-bullen tables in the mid-1930s. it was not until the early 1920s that studies of hindu kush earthquakes revealed that earthquake foci could have significant depth. although many early locations are creditably accurate, others can be improved by use of more modern techniques. early earthquakes in unusual places often repay closer investigation. many events after about 1910 are well enough recorded to be re-located by computer techniques, but earlier locations can still be improved by using more recent knowledge and simpler techniques, such as phase re-identification and graphical re-location. one technique that helps with early events is to locate events using the time of the maximum phase of surface waves, which is often well reported. macroseismic information is also valuable in giving confirmation of earthquake positions or helping to re-assess them, including giving indications of focal depth. for many events in the early instrumental period macroseismic locations are to be preferred to the poorly-controlled instrumental ones. macroseismic locations can also make useful trial origins for computer re-location. even more recent events, which appear to be well located, may be grossly in error due to mis-interpretation of phases and inadequate instrumental coverage. a well converging mathematical solution does not always put the earthquake in the right place, and computer location programs may give unrealistically small estimates of error. examples are given of improvements in locations of particular earthquakes in various parts of the world and in different time periods. mailing address: dr. robin d. adams, international seismological centre, pipers lane, thatcham, berkshire, rg19 4ns, u.k.; e-mail: robin.adams1@btopenworld.com in a gradual refinement. particularly in the early days the locations could be unreliable, and instrumental information was used to refine macroseismic studies rather than replace them. even at the present time instrumental and macroseismic investigations can be used to supplement each other, and their combined use produces the best results. this paper summarises various location techniques and points out some difficulties associated with them that are not always appreciated. 2. macroseismic locations macroseismic location was the only technique available until the beginning of the twentieth century, and in some parts of the world is still the best available. some general points are obvious. for small events, say of magnitude 2 1/2 or less, the felt area will be small, and the epicentre must be close to anywhere where it was reported felt. for larger events the pattern of isoseismals may give the epicentre, as being close to the centre of the isoseismal of highest intensity. in some instances epicentres are very well controlled by felt effects and may be preferred to poor instrumental determinations. there can be complications, however, in areas above subduction zones, where lateral velocity variations can cause focussing of energy and consequent distortion of the felt pattern. in new zealand, for example, because of the shallowing of the zone of deep earthquakes southwards, intermediate depth earthquakes beneath the centre of the north island are often felt only lightly at the epicentre, with the highest intensities displaced southwards. there is an additional complication with the very largest earthquakes, say of magnitude 7 and greater, where the source dimension can be more than several tens of kilometres in extent. unless the source rupture is symmetrical the epicentre, above the point of initiation of rupture, may be displaced from the region of highest energy release identified by macroseismic reports. in such cases the epicentre has less significance. the spacing of isoseismals may also give an indication of focal depth. in general isoseismals from a shallow event will have a peak near the epicentre and fall off evenly with distance; those from deeper events will have inner isoseismals that are wider, with the outer isoseismals more closely spaced. many attempts have been made to quantify this effect, an early one being by kövesligethy (1906), with more recent relations given, for example, by musson and cecic (2002). epicentral intensity may give an indication of magnitude. various formulae have been developed for this applicable in various regions, see for example, ambraseys et al. (1994). it must be remembered that for many centuries these were the only techniques available for earthquake location. in some areas macroseismic techniques may now be calibrated by well-controlled instrumental locations, enabling some re-evaluation of earlier results. 3. location with networks basic principles some information about an earthquake’s location may be found from readings at a single station, such as its distance from the interval between different phases, and its approximate azimuth from analysis of horizontal motion. the character of the recording and possibly the identification of depth phases may also give an indication of focal depth. recent advances in digital processing allow these parameters to be determined more accurately from modern digital stations. for reliable location, however, it is desirable to compare arrival times of different phases at a network of stations. to determine the origin time and the three spatial co-ordinates of the focus at least four independent arrival times are needed from at least three stations, and for a good solution many stations should be used. computers were not generally available before the early 1960s, so before then it was usual to use graphical methods to locate earthquakes. in this method, the origin time is 860 robin d. adams 861 re-evaluation of early instrumental earthquake locations: methodology and examples f ig . 1. g lo ba l se is m ic it y m ap o f m al le t (1 85 8) b as ed o n m ac ro se is m ic r ep or ts o nl y. 862 robin d. adams found from the differences between phases at individual stations, and the travel times of observed phases is then used to determine distances to the stations. arcs corresponding to these distances are drawn on a map or globe, and adjustments made to give the best fit. graphical methods demonstrate well the principles involved, and in particular the necessity of having stations well distributed around the epicentre to obtain a reliable solution. mathematical analysis generally uses the method of least squares, proposed by geiger (1910). a trial origin is chosen and for each phase the time difference between the observed arrival and that expected from the adopted velocity model is calculated, and the origin parameters adjusted to minimise the sum of the squares of these «residuals». before the advent of computers this procedure could be carried out on mechanical calculators, but this was a laborious task and usually only a single iteration was performed. the difficulties of graphical solutions still remain when computer location is used, and the use of computers cannot overcome bad station geometry. if the station distribution is bad, the solution will still be bad and the formal errors large. depending on station geometry and choice of trial origin, the program might find a false location or even fail to converge. this method will do exactly what it is asked to do – it will minimise the sum of squares of residuals for the given velocity model. this is not always helpful. for example the program will try to overcome any deficiencies in the velocity model. in subduction zones with large lateral velocity variations, the computer may put the event in quite the wrong place, with an obligingly small but unrealistic error, with no indication that the event is misplaced. this is often the case in the south pacific. figure 2 shows an example from new zealand in which an intermediate-depth earthquake in the centre of a network of more than 20 stations was given an apparently well-controlled solution using the laterally homogeneous jeffreysbullen velocity model (k = 1.0 in fig. 2). this origin did not fit reported arrival times from stations outside new zealand, however, nor the felt effects. assuming a velocity in the deep earthquake zone 10% higher than normal gave a solution in which the epicentre was moved 60 km (k = 0.9) and the focal depth reduced by 50 km. adopting the new position removed all discrepancies and gave what was clearly a position closer to the true one, but from the mathematical point of view the initial determination was well controlled, with small errors, and there was no reason to doubt its validity (adams and ware, 1977). it must be stressed that the formal standard errors given by the least squares procedure are a measure of the consistency of the data with respect to the specific model used, and do not necessarily reflect physical errors. 4. difficulties in earthquake listings and location 4.1. general difficulties some sources of error are common to all earthquake catalogues and listings, including those from the pre-instrumental era. errors may be divided into omissions, spurious events and mis-location. 4.1.1. omissions in pre-instrumental times earthquakes were reported only from populated areas; note for example the lack of earthquakes in oceanic areas in early compilations of global seismicity (e.g., mallet, 1858). the increasing sensitivity of instruments now means that global coverage of detection is generally down to magnitude 4 1/2 at the international seismological centre (willemann, 1999), but detection thresholds are much lower in areas with close local networks. nevertheless, earthquakes may remain undetected if they occur at times of instrumental failure or excessive microseismic noise, or if their record is confused with that of another event. little can be done to rectify such omissions. earthquakes reported with errors in time or position may result in the omission of the true event. 863 re-evaluation of early instrumental earthquake locations: methodology and examples 4.1.2. spurious events errors in timing are a common source of spurious events; sometimes the event is also reported at the correct time, resulting in duplication. errors of minute, day, month, year or even century occur. minute and day errors are particularly common in recent instrumental listings and isc uses a program to seek these. a felt report can sometimes help resolve an ambiguity, but these too may be subject to error. different agencies can also locate the same event far enough apart to cause a «split» event. an example of such a three-way split is shown in fig. 3. three national agencies each used their own network to locate a small earthquake off central america at widely different positions; isc was able to combine the readings into a single event (adams and richardson, 1996). fig. 2. new zealand earthquake of 4 january 1975. stars show epicentres determined by usgs and by new zealand procedures using standard (k = 1.0), and modified (k = 0.9) velocity models. 864 robin d. adams on the global scale spurious events can be formed by chance mis-association of unrelated readings. the automatic «search» procedure at isc regularly found several hundred of these spurious events each month, which had to be removed from the files; occasionally, however, such spurious events would remain in the listings. improved procedures now reduce the chance of such mis-associations. mis-interpretation of phases can also result in spurious events. there is a tendency for small local networks to interpret arrivals from teleseisms as a local event. core phases from pacific earthquakes have been interpreted in europe as local readings, and a false local earthquake postulated. in an extreme case the large aperture seismic array in montana (lasa) during the early 1970s mis-interpreted steeply arriving core phases as direct p arrivals from earthquakes near the far limit of allowable distance, resulting in a ring of false events at distances near 110º (ambraseys and adams, 1986). figure 4 shows these events in cameroon, where by chance they could be associated with a line of volcanic activity. catalogues may also be contaminated with non-seismic events such as explosions, and other disturbances. a final source of false events arises simply from mistakes in copying information from other sources. such mistakes may propagate through many generations of catalogues. cases have occurred of transposition of latitude and longitude, and north-south and east-west confufig. 3. small stars show the locations given by three national agencies for a central american earthquake on 3 september 1992. large star shows position obtained by isc by combining all readings. 865 re-evaluation of early instrumental earthquake locations: methodology and examples sion. there can also be confusion between the order of day and month. 4.2. difficulties in early period of earthquake location it is sometimes difficult for present-day seismologists to appreciate the difficulties our early colleagues faced. there are fundamental recording difficulties arising from several sources. 4.2.1. sparse networks early networks developed slowly. in some areas of strong local activity, such as japan and california, regional networks grew reasonably rapidly after the development of instruments, but on a global scale the coverage was initially sparse, enabling only the largest events to be detected and located. nevertheless, the early global network of about 30 milne instruments set up by the british association for the advancement of science in the late 1890s was the first attempt to provide global coverage (fig. 5, milne, 1900), and provided early locations, albeit with limited sensitivity and precision. 4.2.2. instrumental difficulties early instruments were mainly insensitive and had inappropriate characteristics. for example, the milne seismographs were undamped, of low gain and slow recording speed. they were of intermediate period (about 12s) and were not good for recording body waves. 4.2.3. timing difficulties again, it is hard for present-day seismologists to appreciate the difficulty in obtaining accurate timing in the era before crystal clocks and radio transmission. it is not by chance that many early seismological stations fig. 4. false events in africa mis-located by lasa by interpreting core phases from pacific earthquakes. numerals give the proposed magnitudes. 866 robin d. adams were installed at astronomical observatories, for example, mount wilson in california and wellington in new zealand. here the observatory clocks could be rated by astronomical observations, but large errors could accumulate during cloudy periods. up till the 1960s marine chronometers remained one of the most reliable timing sources, in later periods checked against radio time signals. 4.2.4. lack of knowledge of seismic phases and travel times in early seismology knowledge of earth structure and earthquake location developed together, each helping to improve the other. the earliest travel-time tables in common use were those of zöppritz (1907), for p and s phases, with no allowance for the core. the existence of the core was proposed in 1910, and the possibility of deep events recognised in the early 1920s, but the zöppritz tables remained in common use until jeffreys and bullen developed their tables in the 1930s. the inner core was not discovered until 1936. thus early seismologists, even if they could pick arrivals from their records, lacked the knowledge of earth structure to enable them to interpret them with certainty. 5. data available there are several sources of data for reevaluating early earthquake locations. for global coverage the publications of the british association (baas) provide the fullest source. with their support milne published lists of phases recorded at his global network from 1899 onwards. these are generally referred to by the name of his home town in the isle of wight off the south coast of england as the «shide circulars». baas later published epicentral estimates for the period 1899-1917, after which this work was taken over by the international seismological summary (iss), originally set up in 1921 by the newly-formed international union of geodesy and geophysics. iss was reconstituted as the present international seismological centre (isc) in 1964. between them the bulletins of iss and isc remain the most complete source of seismic readings available for re-evaluation of global seismology (adams, 2002). other agencies also contributed to the collection and analysis of global earthquake information. the bureau central de séismologie (bcis) in strasbourg published global bulletins for 1903 to 1963, after which it concentrated mainly on european seismicity. successive governmental agencies in the united states have carried out global earthquake location since 1928; at present this is undertaken by the national earthquake information center of the us geological survey. many regional agencies also undertake some global analysis as well as the detailed study of the seismicity of their own region. the institute of physics of the earth in moscow and the japanese meteorological agency are foremost among these. an extremely valuable source of information on early earthquakes is the bulletins regularly published by networks and individual stations throughout the world. these often contain much more information than was submitted to international agencies, including later phases and details of amplitude and period of recorded phases, that are invaluable in the estimation of magnitude. the bulletins of the swedish network published by uppsala university are a particularly rich source of information. sadly, with the growth of modern technology and automated data exchange, such bulletins have now almost totally disappeared. 6. re-evaluation techniques some experience is required in identifying poor solutions in catalogues. obvious clues that suggest that an event warrants closer investigation are unusual positions, unusual groupings of stations, unsatisfactory residuals and discrepancy with felt reports. the first step is to re-assess the data. this involves looking at the given station readings 867 re-evaluation of early instrumental earthquake locations: methodology and examples f ig . 5. g lo ba l ne tw or k of s ei sm og ra ph s ta ti on s in st al le d by m il ne a bo ut 1 90 0. n um be rs g iv e ap pr ox im at e lo ca ti on s of d et ec te d ea rt hq ua ke s. to see if they could have been mis-interpreted, or if they might have been mis-associated from another event, or even if they contain systematic timing errors. phase mis-identification is a common error, sometimes simply confusion between p and s phases, but also gross mis-identifications, such as interpreting core phases as p arrivals from a fictitious event. it is also worthwhile searching for additional readings from station bulletins or other sources, and checking for any available felt information. if enough readings are available a computer re-location may be attempted; this may be especially relevant for earthquakes previously located only by graphical means. often, however, the quality and quantity of data are not enough for a computer solution to converge, and in such cases simple graphical methods can improve a poor solution. a technique that is useful in the re-interpretation of early earthquakes, particularly those recorded by milne instruments is to make use of the reported time of arrival of the maximum phase m of surface waves (ambraseys and adams, 1986). assuming that this travels at a velocity of about 3 km/s enables distances from stations to be calculated and locations estimated by graphical means. although the timing may not be known accurately, the slow velocity reduces corresponding errors in distance. an example is shown in fig. 6 for an earthquake in 1906, originally located by baas in the mediterranean off the coast of egypt. re-interpreting later arrivals at ten stations ranging in distance from 15º (helwan) to 69º (batavia) showed the event to be at a more usual location in the red sea. the arcs drawn in this figure show that such locations are not well determined by present standards, but a gross mis-locations has been corrected. an example of systematic re-location of early events in a given region is found in ambraseys and adams (2001) for earthquakes in central america. here a variety of techniques was used, with great reliance being given to macroseismic reports for early events. some instrumental information was available from 1898 onwards; at the beginning of the period this could only be shown to be consistent with the felt information, but later could more 868 robin d. adams fig. 6. solid star shows position of earthquake originally located off coast of egypt; arcs show re-location in red sea, using milne readings from stations shown by small stars. 0° 30° 60° 90° 60° 30° 0° 20 march 1906 ∆° helwan 15 beirut 15 bombay 30 kodaikanal 37 calcutta 42 shide 43 san fernando 44 edinburgh 50 irkutsk 65 batavia 69 869 re-evaluation of early instrumental earthquake locations: methodology and examples fig. 7. earthquakes in central american region for which positions re-located by ambraseys and adams (2001) were at least 500 km from those originally assigned. triangles show original positions, circles relocations. fig. 8. solutions of earthquake near south of france in 1990 with improved isc position about 50 km from that given by neic after re-interpretation of crustal phases. 870 robin d. adams confidently confirm the macroseismic position. the earliest event for which a reliable instrumental position could be determined was on 1 july 1907, in honduras. figure 7 shows events for which our re-determinations in this region were at least 500 km from earlier solutions. the quality of published instrumental locations improved with time, and particularly after the advent of the worldwide standard seismograph network in 1964 there were only a very few major discrepancies. an example of useful combination of instrumental and macroseismic information is given by ambraseys and adams (1993) for a damaging earthquake of magnitude 6 in cyprus on 10 september 1953. it was well recorded by stations world wide and given a well-determined position some 15 km offshore to the west of the island. the macroseismic information, however, placed highest intensities well inland. careful scrutiny of reported phase arrivals then revealed that many stations reported a second arrival about 10s after the initial onset. according to distance these had been variously interpreted as p*, pp, pp or pcp. when these later arrivals were analysed separately they established the existence of a second event of approximately the same size about 50 km from the first, in the area of highest reported intensity. failure to correctly identify crustal phases in local earthquakes can also result in significant mis-locations. figure 8 shows two solutions for an earthquake near the south coast of france. the first, obtained by neic, was calculated without the benefit of readings from the closest station, cadarache, and treating arrivals from the remaining stations as simple p. reinterpreting arrivals at the closest four stations as the crustal phase pg and that at the most distant as p* gave a much improved solution at a location some 50 km away. 7. future work an experienced analyst will learn to recognise signs that a particular solution may be in error, and to re-assess the data to give an improved result, but there is no easy way to improve early locations if the data are not adequate. in the earliest period each earthquake needs to be looked at individually, bearing in mind the limitations of knowledge available to the contemporary seismologists who carried out the original location. for the period when recordings are routinely better, it may be possible to undertake routine computer re-evaluation, but this will not necessarily reveal all deficiencies. the correct assessment of macroseismic information can also be used as an additional tool to control poorly determined instrumental locations and to resolve ambiguities. a combination of these techniques may be used to improve the reliability of early earthquake catalogues for use in tectonic studies and hazard analysis. references adams, r.d. (2002): international seismology, in international handbook of earthquake and engineering seismology. part a, edited by w.h.k. lee, h. kanamori, p.c. jennings and c. kisslinger (academic press, london), 29-37. adams, r.d. and w.p. richardson (1996): a view of south and central america from the international seismological centre, geofís. int., 35, 193-203. adams, r.d. and d.e. ware (1977): sub-crustal earthquakes beneath new zealand, locations determined with a laterally inhomogeneous velocity model, n.z. j. geol. geophys., 20, 59-83. ambraseys, n.n. and r.d. adams (1986): seismicity of west africa, ann. geophysicae, 4b (6), 679-702. ambraseys, n.n. and r.d. adams (1993): seismicity of the cyprus region, terra nova, 5, 85-94. ambraseys, n.n. and r.d. adams (2001): the seismicity of central america: a descriptive catalogue 18981995 (imperial college press, london), pp. 309. ambraseys, n.n., c.p. melville and r.d. adams (1994): the seismicity of egypt, arabia and the red sea: a historical review (cambridge university press, cambridge), pp. 181. geiger, l. (1910): herdbestimmung bei erdbeben aus den ankunftszeiten, nachr. königlichen ges. wiss. göttingen, 4, 331-349. kövesligethy, r. (1906): a makroszeizmikus rengések feldolgozása, math. és természettudományi értesítõ, 24, 349-368. mallet, r. (1858): fourth report upon the facts and theory of earthquake phenomena, report of 28th meeting of british association for the advancement of science (john murray, london), 1-136 (plate xi). 871 re-evaluation of early instrumental earthquake locations: methodology and examples milne, j. (1900): fifth report of the committee on seismological investigations, report of british association for the advancement of science (london), plate ii. musson, r.m.w. and i. cecic (2002): macroseismology, in international handbook of earthquake and engineering seismology. part a, edited by w.h.k. lee, h. kanamori, p.c. jennings and c. kisslinger (iaspei, academic press, san diego, u.s.a.), 807-822. willemann, r.j. (1999): regional thresholds of the isc bulletin, seismol. res. lett., 70, 313-321. zöppritz, k. (1907): über erdbebenwellen ii, laufzeitkurven, nachrichten der königlichen gesellschaft der wissenschaften zu göttingen, mathematisch-physikalische klasse, 529-549. volcanic so2 by uv-tir satellite retrievals: validation by using ground-based network at mt. etna annals of geophysics, fast track 2, 2014; doi: 10.4401/ag-6641   1 volcanic so2 by uv-tir satellite retrievals: validation by using ground-based network at mt. etna claudia spinetti1*, giuseppe giovanni salerno 1, tommaso caltabiano 1, elisa carboni 2, lieven clarisse 3, stefano corradini 1, roy gordon grainger2, pascal andre hedelt 4, maria elissavet koukouli 5, luca merucci 1, richard siddans 6, lucia tampellini 7, nicolas theys 8, pieter valks4 and claus zehner 9 1 istituto nazionale di geofisica e vulcanologia, italy, 2 comet, atmospheric, oceanic & planetary physics, oxford university, uk, 3 université libre de bruxelles, belgium, 4 german space agency, germany, 5 laboratory of atmospheric physics, aristotle university of thessaloniki, greece, 6 rutherford appleton laboratory, chilton, uk, 7 compagnia generale per lo spazio, italy, 8 belgian institute of aeronomie, belgium, 9 european space agency, italy *claudia.spinetti@ingv.it abstract mt. etna volcano in italy is one of the most active degassing volcanoes worldwide, emitting a mean of 1.7 mt/year of sulphur dioxide (so2) in quiescent periods. in this work, so2 measurements retrieved by moderate resolution imaging spectroradiometer (modis), hyper-spectral infrared atmospheric sounding interferometer (iasi) and the second global ozone monitoring experiment (gome-2) data are compared with the ground-based data from the flux automatic measurement monitoring network (flame). among the eighteen lava fountain episodes occurring at mt. etna in 2011, the 10 april paroxysmal event has been selected as a case-study for the simultaneous observation of the so2 cloud by satellite and ground-based sensors. for each data-set two retrieval techniques were adopted and the measurements of so2 mass and flux with their respective uncertainty were obtained. with respect to the flame so2 mass of 4.5 gg, modis, iasi and gome-2 differ by about 10%, 15% and 30%, respectively. the so2 flux correlation coefficient between modis and flame is 0.84. all the retrievals within the respective errors are in agreement with the ground-based measurements supporting the validity of these space measurements. annals of geophysics, fast track 2, 2014   2 i. introduction onitoring active volcanoes is crucial for social, environmental and economic aspects, especially in a densely populated area such as at mt. etna in italy. during the last 10 years, the surveillance of mt. etna ash cloud has seen a rapid growth due to technological advances that include the development of a warning system for paroxysmal events, real-time satellite observations and the forecasting of ash cloud movements [andronico et al., 2009; patane’ et al., 2013]. the european space agency projects ‘study on an end-to-end system for volcanic ash plume monitoring and prediction’ (smash and sacs2) are aimed at exploring the ability of operative polar-orbiting satellite instruments in the ultraviolet to thermal infrared (uv-tir) spectral range to retrieve quantitative measurement of volcanic so2 and ash cloud parameters. in particular, the modis on  board nasa terra and aqua satellites, iasi on board eumetsat metopa, and gome-2 on board eumetsat metop-a and b satellites have been selected for generating so2 satellite products. mt. etna is the test site of the smash project while eyjafjallajökull and grímsvötn in iceland are test sites of the sacs2 project [koukouli et al., 2014]. in this work, part of the validation activities are presented. in particular, the modis, iasi and gome-2 so2 retrievals are compared with the ground-based measurements obtained by the flame network of one of the short-lasting 2011 lava fountains episodes. ii. mt. etna 2011 lava fountains mt. etna is characterized by a constant degassing activity from its summit craters (3300 m a.s.l.) which produce volcanic plumes both from the summit and eruptive fractures that often open on the flanks of the volcano during eruptions. recently, etna has frequently shown powerful explosive activity, including episodic paroxysms. between 12 january and 15 november 2011, etna produced eighteen lava fountains from the new-south east crater. the eruptive episodes were characterized by paroxysmal phases, lasting from a quarter of an hour to three hours, preceded by mild strombolian activity and coupled with lava flows. each paroxysm formed an eruption column ranging in height from 5 to 11 km a.s.l.. although these eruptive episodes lasted just few hours, the volcanic clouds travelled thousand of km, mostly over the mediterranean sea and were tracked by polar orbiting instruments flying over the area [behncke et al., 2014]. in order to select the events for validation, eruptive episodes need to be simultaneously observed from both satellite and groundsensors. the most powerful and long-lasting lava fountains have been identified as measurable by space sensors. since flame is based on uv spectroscopy, those episodes occurred during the night time have been excluded. following these criteria, in the range of the eruptive sequence taking place in 2011, the 10 april event has been selected and presented hereafter as a case-study. the paroxysmal phase lasted about three hours, starting at 10.07 and ending at 12.56 gmt. the maximum ash cloud altitude estimated by ground video-cameras [scollo et al., 2014] was over 9 km a.s.l. at 11.18 gmt. m annals of geophysics, fast track 2, 2014   3 iii. ground so2 measurements since 2004, the daylight surveillance of mt. etna degassing is performed by the flame automated network that provides real-time and high-resolution so2 flux measurements [salerno et al., 2009]. the network consists of 10 uv scanning spectrometers spaced ~7 km apart and installed at an altitude of ~900 m a.s.l. on the volcano flanks. during the daylight data are transmitted to the istituto  nazionale  di  geofisica  e  vulcanologia  (ingv), where reliable so2 flux is automatically computed, providing near instantaneous information on degassing rates and plume location. during the 10 april event, flame acquired data from 10.05 to 12.57 gmt with an so2 flux peak of ~15300 t/d at 10.53 gmt. at 12.24 gmt a second flux peak of ~11600 t/d was recorded suggesting a new impulse of fresh volatilerich magma throughout the eruption. iv. space so2 retrievals iasi so2 products were generated by two retrieval schemes, one developed by the earth observation data group at the university of oxford (uniox) [carboni et al., 2012] and one developed by the atmospheric spectroscopy group of the université libre de bruxelles (ulb) [clarisse et al., 2014]. the uniox scheme retrieves simultaneously the so2 plume height and the so2 column amount together with the retrieval errors. the ulb scheme retrieves the height prior to the column amount. the modis so2 products have been generated by ingv [corradini et al., 2009] and by the rutherford appleton laboratory (ral) [poulsen et al., 2011]. the ingv product is retrieved on a pixel by pixel weighted least squares fit procedure using measured and simulated radiances, including the ice/ash correction. the ral product is based on the optimal-estimation scheme. in particular, the scheme simultaneously retrieves ash and so2 contents once the cloud height is fixed a priori. the 8.7 μm band has been used for both products since it is sensitive to tropospheric so2 clouds. the technique to generate the gome-2 so2 products was developed by the deutsches zentrum fur luftund raumfahrt (dlr) [rix et al., 2012]. the inversion algorithm is based on a differential optical absorption spectroscopy technique, assuming three fixed cloud heights of 2.5 km, 6 km and 15 km a.g.l.. v. space and ground comparison during the 10 april paroxysm, modis aqua detected the so2 plume at 12.30 gmt. figures 1 a and b show ingv and ral products: both detect two so2 plumes spreading toward se directions from the summit of mt. etna. the ingv product shows a 12% greater area than the ral product. in terms of the so2 column amount in the entire cloud area, the ingv product retrieves so2 values from 36% to 93 % higher with respect to ral. the maximum of ingv so2 value is 300 +/100 du while ral retrieved 210 +/20 du. the discrepancy is probably due to the assumptions adopted. the ral a priori cloud height setting could affect the retrieval. the ingv retrieval considers the effect of ice particles detected in the cloud. the difference between the two retrieval techniques also derives from the number of pixels retrieved since the ral product is one pixel for every two from ingv. annals of geophysics, fast track 2, 2014   4 figure 1: modis 10 april 12.27 gmt so2 column amount: a) ingv and b) ral. after the end of the paroxysm, iasi at 19.42 gmt captured the so2 cloud spreading south-east over the mediterranean sea (figures 2 a and b). figure 2: iasi 10 april 19.43 gmt so2 column amount: a) uniox and b) ulb. the ulb product shows a 4% greater area than the uniox one. in terms of so2 content, uniox values are 17% greater with respect to the ulb product. the maximum so2 content of the ulb product is 12 du compared to uniox so2 content of 14 +/2 du. also for iasi products, the differences between the two retrieval techniques are probably due to the retrieved cloud height: the higher the cloud height, the lower the so2 content. the day after the end of the lava fountain, both iasi and gome-2 captured the so2 clouds over the eastern part of the mediterranean sea (figure 3). the dlr product retrieves a maximum value of 2 du. ulb product shows a 11% greater area than the uniox product. in terms of so2 content, uniox values are 23% greater with respect to ulb product. the maximum so2 content of the ulb product is 6 du compared to uniox so2 content of 8 +/1 du. figure 3: gome-2 11 april 07.44 gmt dlr so2 column amount. as aforementioned, the products have different so2 content and acquisition times depending essentially on sensor and satellite characteristics. in order to compare the so2 products with the ground-based data, the so2 mass and the so2 flux have been calculated. so2 mass. the so2 mass emitted during the paroxysmal event has been obtained by reanalysis of the flame data. the total mass of the entire paroxysm is (4.5 +1.3, -1) gg. since the aqua satellite passed during the ongoing lava fountain, the so2 mass from the flame network was calculated from the start time of the event to the time of annals of geophysics, fast track 2, 2014   5 the aqua pass (12.27 gmt). for each of the satellite products, modis, iasi, and gome2, the so2 mass has been calculated as a cumulative mass and averaged over the cloud area. figure 4 shows the results of the comparison (dashed vertical lines indicate the event start and end). all the retrieved  masses are the same order of magnitude of ground so2 mass. the ingv, uniox, ulb and dlr masses are within the flame quoted uncertainties (grey area in figure 4). on 10 april, the ingv mass concurs well with flame since the masses distance is about 10% of the quoted uncertainties. the uniox and ulb mass distances from flame are about 15%. on 11 april, the dlr differs from flame by about 30% and ulb differs by about 25%. only ral underestimates the so2 mass by about 2 gg. in addition, figure 4 highlights the depletion of so2 in the cloud far away from the source (11 april gome-2 and iasi). figure 4: so2 mass comparison ground vs satellite measurements across mt. etna for the 10 april 2011 paroxysmal event. so2 flux. the 10 april so2 flux has been used to compare the modis ingv product with ground flame data, as the modis data was acquired during the paroxysm. the raw flame data were reprocessed in order to calculate the so2 flux assuming the same constant wind speed of 10 m/s and the same plume altitude assumed in the modis ingv product. this approach was adopted in order to compare the two fluxes. the modis so2 flux was computed by multiplying the integral of the columnar abundance along each plume transect orthogonal to the plume axis by the wind speed. from the computed so2 flux vs distance from the vent, the so2 flux time series was reconstructed, i.e. the so2 flux vs time of the emission. the correlation coefficient between modis and flame flux results in 0.84. a rather good agreement can be seen in figure 5. the differences are in the grey zone, where land surface temperature and emissivity uncertainty leads to significant errors in modis so2 retrievals [merucci et al., 2011]. figure 5: so2 flux comparison. the dashed vertical lines are the event start and end. the grey zone indicates where the modis retrieval is carried out on land. vi. conclusions a case-study was selected among the mt. etna 2011 lava fountain events in order to compare and validate the iasi, modis and gome-2 products generated in the frame of the european space agency projects smash and sacs2. for the 10 april event, the iasi ulb and uniox, gome-2 drl and modis annals of geophysics, fast track 2, 2014   6 ingv so2 mass values are rather well correlated with those of ground-based flame data, thus supporting the validity of such space measurements. small differences in the retrieval algorithm of the cloud area and so2 amount have been evidenced for iasi ulb and uniox, gome-2 dlr, while a greater difference was found between modis ingv and ral. results gathered in this work, though limited to a single case-study, suggest that satellite retrieval is a promising volcano monitoring tool for so2 volcanic cloud observation, providing day-night integration in the daylight ground-based so2 measurements records. acknowledgements we thank two anonymous reviewers for their comments that greatly improve the paper. references [andronico et al., 2009] andronico, d., spinetti, c., et al., (2009). observations of mt. etna volcanic ash plumes in 2006: an integrated approach from ground-based and polar satellite monitoring system. j. volcanol. geotherm. res., 180, 135–147. [behncke et al., 2014] behncke, b., branca, s., et al., (2014). the 2011–2012 summit activity of mount etna: birth, growth and products of the new se crater. j. volcanol. geotherm. res., 270, p. 10–21. [carboni et al., 2012] carboni, e., grainger, r., et al., (2012). a new scheme for so2 retrieval from iasi: application to the eyjafjallajökull eruption of april and may 2010. atmos. chem. phys., 12, 11417-11434. [clarisse et al., 2014] clarisse, l., coheur, p.f., et al., (2014). the 2011 nabro eruption, a so2 plume height analysis using iasi measurements. atmos. chem. phys., 14, 3095-3111. [corradini et al., 2009] corradini, s., merucci, l., (2009). retrieval of so2 from thermal infrared satellite measurements: correction procedures for the effects of volcanic ash, atmos. meas. tech., 2, 177–191. [koukouli et al., 2014] koukouli, m.e., clarisse, l., et al., (2014). intercomparison of metop-a so2 measurements during the 2010-2011 icelandic eruptions. ann. geophys., this issue. [merucci et al., 2011] merucci, l., burton, m., (2011). reconstruction of so2 flux emission chronology from space-based measurements, j. volcanol. geotherm. res., 206, 3-4. [patane' et al., 2013] patane', d., aiuppa, a., et al., (2013). insights into magma and fluid transfer at mt. etna by a multi-parametric approach: a model of the events leading to the 2011 eruptive cycle. j. geophys. res., 118, 1-21. [poulsen et al., 2011] poulsen, c.a., watts, p.d., et al., (2011). cloud retrievals from satellite data using optimal estimation: evaluation and application to atsr. atmos. meas. tech. discuss., 4, 2389-2431. [rix et al., 2012] rix, m., valks, p., et al., (2012). volcanic so2, bro and plume height estimations using gome-2 satellite measurements during the eruption of eyjafjallajökull in may 2010, j. geophys. res., 117, d00u19. [salerno et al., 2009] salerno, g.g., burton, m., et al., (2009). novel retrieval of volcanic so2 abundance from ultraviolet spectra. j. volcanol. geotherm. res., 181, 141-153. [scollo et al., 2014] scollo, s., prestifilippo, m., et al., (2014). eruption column height estimation of the 2011-2013 etna lava fountains, ann. geophys., 57, 2. marekova_finalonline:layout 6 annals of geophysics, 57, 5, 2014, s0545; doi:10.4401/ag-6556 s0545 analysis of the spatial distribution between successive earthquakes in aftershocks series elisaveta marekova plovdiv university “paisiy hilendarski”, department of experimental physics, plovdiv, bulgaria abstract the earthquake spatial distribution is being studied, using catalogs for different recent aftershock series. the quality of the available data, taking into account the completeness of the magnitude, is examined. based on the analysis of the catalogs, it was determined that the probability densities of the inter-event distance distribution collapse into a single curve when the data were rescaled. the collapse of the data provides a clear illustration of aftershock-occurrence self-similarity in space. 1. introduction ever since the observations of omori [omori 1894] temporal clustering of earthquakes has been seen as a remarkable feature of seismic appearance. spatial clustering has also been evaluated [kagan and knopoff 1980] and together with the omori and gutenberg-richter laws [gutenberg and richter 1965], it has become groundwork for probabilistic tools for defining time-dependent seismic hazard [ogata 1988, reasenberg and jones 1989, gerstenberger et al. 2005]. the catalogs of earthquakes, both contemporary and historical, allow drawing several important conclusions on the mechanisms of seismicity by describing of the inter-event properties, i.e. by separation of successive events in time and space. many studies have attempted to describe the type of distribution of these inter-event properties, using global and regional seismic catalogs [bak et al. 2002, corral 2004, davidsen and goltz 2004, davidsen and paczuski 2005, corral 2006b, touati et al. 2009]. the distribution d(δt) of the inter-time δt between two successive events is a suitable quantity for describing of the temporal organization of seismicity. in an analogous manner, the distribution d(δr) of the distances δr between epicenters for successive earthquakes provides a convenient tool for describing the spatial organization. both distributions have been the subject of much research in recent years [bak et al. 2002, mega et al. 2003, corral 2004, yang et al. 2004, corral 2005a, davidsen and paczuski 2005, lindman et al. 2005, lippiello et al. 2005, livina et al. 2005, corral 2006a, corral 2006b, saichev and sornette 2006, lippiello et al. 2007, bottiglieri et al. 2010, lippiello et al. 2012]. these distributions show universal behavior, independent from the considered region or the magnitude range, as described by corral [2004, 2006b]; saichev and sornette [2006]; davidsen and paczuski [2005]. e.g., for the region of south california, marked with intensive observations during the last decades, results have been reported, according to which both the inter-event distances or “jumps” between the earthquake epicenters [davidsen and paczuski 2005, corral 2006b], and the inter-event times, or return times [bak et al. 2002, corral 2004, corral 2006b], show statistical distributions in the form of a power-law. this is a manifestation of a complex spatiotemporal (self-) organization of seismicity [saichev and sornette 2006]. it should be noted that most of the authors focus on one of these properties only, i.e., either on the interevent separation distance or on return times. some argue, however, that the understanding of the kinematics of earthquake realization suggests a link between the inter-event properties [batac and kantz 2013]. while some authors report observing a universal law of return time distribution, obtained by means of rescaling data [corral 2004, 2006b], others adduce arguments that the spatial scale of the observations is important for the return times distribution [davidsen and goltz 2004, touati et al. 2009]. touati et al. [2009] describe the visible differences in the inter-event times distributions in regional and global earthquake catalogs: the histogram of the interevent times for the earthquakes in south california shows two different peaks, thus expressing the differences in the waiting times for the correlated (some aftershock series) and the independent events, while the article history received june 23, 2014; accepted september 24, 2014. subject classification: seismology, earthquake interaction and probability, general and miscellaneous. global statistic shows a single peak due to the overlapping sequences of different regions. these authors explain their results by means of an epidemic-type aftershock sequence (etas) model [kagan and knopoff 1981, ogata 1988, sornette and helmstetter 2002]. in batac and kantz [2013] an attempt is made to complete the analysis of touati et al. [2009], using the separation of the distributions of the epicentral distances depending on the corresponding time intervals between successive earthquakes. the analysis is performed in accordance with the fact that the spatiotemporal clustering is an established property of seismicity [omori 1894, kagan and knopoff 1980]. instead of making an attempt to find a fitting function for the inter-event distances and time distributions between successive earthquakes, the study puts an accent on the links between them by means of a simple procedure, whose parameters are derived from the data. the conditional distributions of the earthquake return times, depending on the spatial distribution of events, show that the earthquakes separated by short (long) distances are more likely to be separated by short (long) time intervals. this clearly demonstrates the clustering (separation) of the dependent (independent) events. the interesting thing here is that the different catalogs used by batac and kantz [2013], show behavior, already described in the etas model [touati et al. 2009]. 2. methodology and results the sequences of seismic events due to the realization of strong earthquakes (main shocks) and the socalled aftershock series (secondary shocks) are subject of extensive research in seismology. there are many secondary events immediately after a strong earthquake for a relatively short time period. the aftershock sequences are potential sources of information about the physical and mechanical properties of the medium in the focal zone and the processes going on in it. main characteristics of these sequences are their spatial location as well as the rate at which a sequence weakens with time. the main objective of this work is the study and analysis of the distribution of distances between successive earthquakes from different aftershock series. the parameters of the considered series are described in table 1. they include the time of realization and the magnitude of the main shock, as well as the chosen time interval, defining the duration of a series; the number of events in it and the minimum magnitude mc, over which the sequence is complete, i.e., there are no events omitted at registration. checking for completeness is carried out by the program zmap [wiemer 2001]. the table also includes the depth interval of the considered aftershock series (h). the time duration of the sequences is selected so that it does not exceed marekova 2 no. earthquake date period mmain shock mc number of events h [km] 1 st. elias,canada 28.02.79 28.02.79 30.03.79 7.1 3.0 174 0-19 2 joshua tree, california 23.04.92 23.04.92 28.06.92 6.1 3.0 234 0-18 3 landers, california 28.06.92 28.06.92 31.12.93 7.3 3.0 803 0-20 4 big bear, california 28.06.92 28.06.92 31.12.93 6.4 3.0 411 0-20 5 nahanni, canada 05.10.85 05.10.85 20.12.85 6.6 3.1 244 6-10 6 andreanof island ’86, alaska 07.05.86 07.05.86 15.06.88 8.0 4.8 245 18-67 7 andreanof island ’96, alaska 10.06.96 10.06.96 23.12.98 7.7 4.5 179 10-60 8 aegean ’81 19.12.81 19.12.81 20.06.83 7.2 3.2 261 0-35 9 aegean ’82 18.01.82 18.01.82 05.04.83 6.9 3.5 77 0-72 10 aegean ’83 06.08.83 06.08.83 12.06.84 6.9 3.6 100 1-47 11 albania 15.04.79 15.04.79 10.10.79 7.2 3.0 461 0-22 12 strazica, bulgaria 07.12.86 07.12.86 11.05.87 5.7 2.0 123 0-25 13 hector mine, california 16.10.99 16.10.99 01.11.00 7.1 3.0 529 0-18 14 northridge, california 17.01.94 17.01.94 26.09.95 6.7 3.0 451 0-23 15 greece ’81 24.02.81 24.12.81 08.10.82 6.6 3.2 575 0-47 16 friuli, italy 06.05.76 06.05.76 05.06.77 6.5 3.0 277 0-58 table 1. description of the studied aftershock series. mmain shock is the main earthquake magnitude; mc is the magnitude of completeness, h is the depth interval for earthquakes in series. 3 knopoff’s window, depending on the magnitudes of the major earthquakes [gardner and knopoff 1974]. a modern trend in recent times is to visualize a wide range of natural systems as networks. the networks are complex web-like structures, consisting of nodes and branches [albert and barabasi 2002]. such structures are also used for the description of seismic events and aftershock series in particular [baiesi and paczuski 2005]. all pairs of successive events are considered for the purpose in a wide spatiotemporal window. this approach is used to define appropriate metrics, which could determine the level of connection between the successive events, taking into account the established statistical properties of seismicity. this procedure allows identifying the aftershock groups by means of a quantitative measure. this paper offers a different approach. the spatial properties of several predetermined aftershock series are studied. only the successive events from the sequences are used for this purpose. the spatial distribution analysis was carried out following marekova [2014]. due to the usually higher relative and absolute error for the depth of earthquakes, only epicenters from seismicity catalogs were considered. the distances between the epicenters ( jumps) of the i−th earthquake and the previous in time event, (i −1), were calculated from the catalog. a fortran program was developed, intending to calculate distances between points on the earth’s surface. for this purpose, conversion of the geographic coordinates ({, m) into geocentric ({g, mg) was performed, in order to take into account the ellipsoid shape of the earth: {g = { + 0.00339466sin(2{) (1) mg = m (2) then the distance δ° (in degrees) was defined by the formula: cos(δ°) = sin{1 g sin{2 g + cos{1 g cos{2 g cos(m2 g − m1 g) (3) the indexes 1 and 2 were assigned to the coordinates of the pair of events from the series catalog. the results for δ° were converted into kilometers d, one angular degree being equal to approximately 111.1 km. in this way, a list of the spatial distances di between successive events is obtained. the range of change of the distances is divided into subintervals of length δd. the number of nj values for each subinterval is taken into account and its relative frequency is calculated: (4) where n is the total number of shocks in the aftershock series, j = 1, …, k − k is the number of subintervals into which each range for the distances is divided. figure 1 shows the graphs obtained for the distribution p(d), considering different aftershock series. it can be seen that there is a similarity in the curves but, the curves are scattered from one another. i find it more useful to describe the distribution of the inter-event distance by means of a probability density, p(δd), defined as (5) where prob denotes probability, and δd describes the bins over which the probability density is calculated. it is straightforward to estimate p(δd) from pj(d) as (6) where n is the total number of earthquakes. the probability density removes the dependence of the size distribution on δd, as well as extension of the spatial region under analysis. the curves show in figure 1 can be transformed into coordinates, rescaled as follows. the x-axis is divided by l and the y-axis is multiplied by l, i.e. (7) y = l* p(δd) (8) where l is the maximum distance, obtained for the investigated series. pre-selected series of aftershocks are used in the presented study. the duration and the spatial distribup d n n d j j =^ ^ h h prob p d d dd d d t t t1 # = +l ^ ^ h h p d d d n dp d n j jj t t t= =^ ^ ^ h h h x l dt = aftershocks spatial distribution figure 1. frequency distribution p(d) of the spatial distances versus distance value d, for the epicenters of the earthquakes listed in table 1. tion of the aftershock series are selected so that they correspond to the ones defined by the knopoff’s window one of the methods for selecting aftershocks. according to this method all events, which have occurred in the radius of r(m) km with respect to the main quake or for less time than the interval of t(m) days, are considered to be aftershocks of this particular quake. r(m) and t(m) depend on the magnitude of the main quake. the maximum distance l used in this study is obtained only for successive quakes in the series. the observed maximum values of l for the various series are in most cases smaller than those calculated by gardner and knopoff [1974] for outlining the aftershock region. the equation, describing the relationship between the magnitude of the main quake and the obtained maximum distances between successive quakes is: l(m) = l010 am, where l0 = 0.05±0.05 and a=0.46±0.06. this result complies with the result of bottiglieri et al. [2010]. the defined maximum distance l can be used as a characteristic dimension, related to the aftershock region, what has been done analogically in the above cited paper. the process of rescaling causes a change in the curves of figure 1, presented in figure 2. it can be seen that considering the inter-event distances distribution, the data collapses when rescaled. attempts for an approximation of the distance distributions between successive quakes from aftershock series by means of various functions have been made. curd et al. [2000], for example, suggest for this purpose gamma and weibull distributions. it is shown in marekova [2014] that distributions of the inter-event distances are successfully approximated by the so-called beta-distribution, using for this purpose data from regional catalogs. the beta distribution is a power function of the variable x and its reflection (1 − x) with shape parameters a > 0 and b > 0 (real numbers): (9) where b(a, b) represents the beta function, which appears as a normalization constant to ensure that the total probability integrates to unity; and 0 < x < 1. by approximation function (9) was used in the form: (9a) parameters in (9a) have the values: a = 4.47 ± 1.09 a = 1.17 ± 0.10 b = 3.02 ± 0.27 figure 2 also shows the model curve for the case of data approximation by gamma distribution. the function is applied in the present paper in the following form [corral 2005b]: (10) where a, b, and c are the parameters: a = 6.24 ± 0.38 b = 5.13 ± 0.57 c = 1.67 ± 0.14 in many publications this function appears to be appropriate for fitting the distributions of the time intervals between successive earthquakes [wang and kuo 1998, corral 2004, corral 2005b, corral 2006a, corral 2006b, hainzl et al. 2006, apostol et al. 2008, corral 2009, marekova 2012]. as it can be seen from figure 2, beta distribution describes the data better than gamma distribution. in gamma distribution the deviation of the real data from the model curve is within the big values of x − for x > 0.7; at smaller values of x the two curves do not differ significantly. for the same series the distribution of distances r between the main shock and the subsequent secondary earthquakes has also been studied. the series landers, big bear, nahanni, hector mine, andreanof island ’86 and friuli have been selected. all of them are characterized by a large number of events. after using the methodology described in this paper the results show that in case of mixing all distributions of the distances r, an analogical result is not obexpf x a bx bx1= -c-^ ^ ^h h h 1a-f x x xa 1 1-= b-^ ^h h 1a, f x b x x1 1 a b = b^ ^ ^ h h h marekova 4 figure 2. probability density for the rescaled data: x = δd/l; y = l*p(δd) from catalogs, listed in table 1. the green continuous line is a beta distribution fit and the red continuous line a gamma distribution fit. 5 served. the curves do not collapse not only in the case of frequency distribution p(r), but also after re-scaling (figures 3 and 4). in these cases lmax is the distance to the most remote aftershock in the corresponding series. series with different magnitude ranges have been selected for some of the aftershock sequences. mixing the data on the corresponding magnitude intervals, analogical to the one carried out in bottiglieri et al. [2010], lippiello et al. [2009], does not lead to the collapse of the data again (figures 5 and 6). aftershocks spatial distribution figure 6. probability density for the rescaled data: x = r /lmax; y = lmax*p(r) – r main shock-aftershock distance, for the selected earthquakes in various magnitude intervals. figure 5. frequency distribution p(d) versus main shock-aftershock distance r, for the selected earthquakes in varius magnitude intervals. figure 4. probability density for the rescaled data: x = r /lmax; y = lmax*p(r) – r main shock-aftershock distance, for the selected earthquakes. figure 3. frequency distribution p(d) versus main shock-aftershock distance r, for the selected earthquakes. therefore, following the idea of lippiello et al. [2009] and felzer and brodsky [2006] the present study has been complemented with a study of the spatial density of the aftershocks. linear density has been analyzed in this case, i.e., the number of aftershocks per unit of length. because of the difference in the way of formation of the series in this work and in the cited papers, a different approach is also used in this study. for the series, pointed out above the analysis has first been made for a number of time intervals of the aftershock activity for the whole interval, for the 1st day, for the first 10 days, for the first 100 days (figure 7). choosing a shorter interval, e.g. 30 minutes, as in lippiello et al. [2009] and felzer and brodsky [2006], leads to a great reduction of the number of quakes in the series, what makes the results unconvincing. it should be noted that all studied aftershock series comprise events with a definite threshold of completeness, which in most cases is mc = 3.0. the data from all time intervals can be approximated by a power law: p(r) = cr−dt where c is a constant, varying in dependence on the number of aftershocks. the values of the exponent dt change for the different sequences: landers from 2.1 to 2.3; big bear from 2.0 to 2.2; hector mine from 1.4 to 2.3; nahanni from 2.5 to 3.0; andreanof island ’86 from 1.5 to 1.8; friuli from 3.5 to 4.0. for some of the series fitting is carried out at r ≥ 10 km. besides, the data about some of the time intervals cannot be approximated by a power law e.g. for the 1st day for big bear and friuli. the other study of the above aftershock series is in dependence on the magnitudes of the earthquakes. magnitude intervals m[3.0; 4.0) and [4.0; 5.0) have been selected together with the data about the whole series, which are with mc ≥ 3.0. the data for m ≥ 5.0 have not been analyzed since their number is small. the only exception is the series of the earthquake andreanof island ’86. it has a minimum magnitude of completeness mc ≥ 4.8. the intervals in it are correspondingly m[4.8; 5.5) and [5.0; 6.5), together with the whole series [4.8; 8.0] (figure 8). in these cases the data about the linear density as a function of the distance “main shock secondary quake” can also be fitted by a power law with an exponent dm, and in most cases this is done at r ≥ 10 km. the values of the exponent dm change as follows: landers from 1.1 to 1.3; big bear from 2.2 to 2.3; hector mine from 1.9 to 2.5; nahanni from 2.6 to 3.0; andreanof island ’86 from 1.51 to 1.53; friuli from 2.6 to 3.2. this time again for two of the series the data cannot be fitted by the power law. these are big bear for m[4.0; 5.0) and andreanof island ’86 for [5.0; 6.5). aftershocks are defined by their cluster properties both in time and in space. temporal clustering follows the well-defined omori law. the spatial distribution of the aftershocks is more complex and not fully explained yet. on the one hand, it is clear that many of the aftershocks occur along the surface of the fault or in its immediate vicinity. aftershock grouping along or near the destruction caused by the main shock may reflect the local concentration of stress on asperities, serving to stop destruction and close the fault. on the other hand, aftershocks occur also away from the fault line due to various trigger mechanisms, the simplest of which is the stress transfer and the increase of the coulomb stress [stein 1999]. the investigation of linear density t(r), where r is the epicentral distance between each aftershock and its related mainshock, represents a useful tool to discriminate between triggering by static or dynamic stress [felzer and brodsky 2006, lippiello et al. 2009]. in these papers, t(r) decay asymptotically as r−µ, where µ � 1.4 to 1.9 [lippiello et al. 2009] and µ � 1.4 [felzer and brodsky 2006]. here i show that the linear density exhibits a power law behavior, but the obtained exponents are more scattered. the basic differences with this study and previous analyses is that aftershocks have been classified according to the mainshock magnitude and distances in a different manner. therefore it is necessary to carry out joint research on these properties of aftershock series, which will be the subject of future investigation. seismic events are complex spatial-temporal phenomena [turcotte 1993, sornette 1999, bak et al. 2002, pavlos et al. 2007, vecchio et al. 2008, sornette and werner 2009], since they occur in clusters and exhibit scale-invariant characteristics. the most typical description in such cases is the division of the earthquakes into foreshocks, main quakes and aftershocks. very often during the analysis of their properties in space and time power laws are used. nowadays widely used to describe such complex phenomena is the definition of the so-called fractal dimension [mandelbrot 1982]. the concept of fractal dimension takes central place in understanding the chaotic picture of seismicity, especially for defining the level of marekova 6 7 clusterization in the earthquakes’ spatial distribution. the value of the fractal dimension gives a quantitative measure of the process of grouping for the epicenters/hypocenters of the “main” events, both for the active regions [kagan and knopoff 1980, de rubeis et al. 1993, tosi et al. 2004, tosi et al. 2008] and for the aftershock regions [robertson et al. 1995, nanjo and nagahama 2000, nanjo and nagahama 2004]. in the works of bak et al. [2002] the structure of seismicity in space, time and in magnitude is revealed, placing all earthquakes on the same base without making difference between main and secondary quakes [bak et al. 2002, christensen et al. 2002]. thus seismicity is regarded as a single process. the above researchers propose a new scaling law, outlining the framework for assessing the likelihood of aftershocks spatial distribution figure 7. linear density versus main shock-aftershock distance r, for the selected earthquakes in various time intervals. earthquakes in a given region for a given threshold magnitude. the law shows that the seismic events occur in hierarchically related clusters, which overlap other spatially separated clusters for sufficiently big time intervals and regions. for sufficiently small regions and temporal scales only single related groups can be selected. the aftershocks are usually described as related events, which occur after a strong seismic event. this definition is widely used in order to classify the catalogs, containing seismic data, as catalogs of aftershocks and catalogs of main shocks. omori’s law is based on this classification of the earthquakes and it usually suggests that the aftershocks are due to mechanisms other than the ones that caused the main events. gardner and knopoff [1974] assume that an earthquake can be classified as an aftershock if it belongs to marekova 8 figure 8. linear density versus main shock-aftershock distance r, for the selected earthquakes in various magnitude intervals. 9 a cluster of related events; it can be regarded as a main quake if it belongs to a series of “isolated” random events with poisson statistics. however, nowadays a reliable scientific method for differentiating whether an earthquake is an aftershock or a main quake does not exist, since, according to this definition, the categorization depends on the considered scales for length, time and magnitude. the proposed law puts together the law of gutenberg-richter, omori’s law and the law describing the fractality of the fault systems. in many other publications after bak et al. [2002] when spatial-temporal properties are concerned, all seismic events are considered together [corral 2003, corral 2004, davidsen and goltz 2004, lindman et. al. 2005, corral 2005b, shcherbakov et al. 2005, corral 2006a, corral 2006b, saichev and sornette 2006, shcherbakov et al. 2006]. following the studies described above, i have considered the spatial distributions of both aftershock series and earthquakes from declusterized series [marekova 2014]. the purpose is to have the idea of considering seismic events as a single process developed in more than one way. 3. conclusions this paper presents an investigation of the distributions of distances between consecutive earthquakes from different aftershock series. it has been established that when these distributions are rescaled, all curves converge in one. the rescaled data are well fitted by a beta distribution. the obtained results are similar to those, reported in marekova [2014]. it can be noted that the distributions of the distances between successive earthquakes in declusterized series of regional seismicity [marekova 2014] and aftershock series, separated by the conventional spatiotemporal windows, are analogical. they are well approximated by beta distribution. it gives reason to consider that the spatial distributions of the earthquakes are similar irrespective of their separation in series of major (independent) and secondary (dependent) events. the differences can be seen only in the extension of the regions in which they are distributed. in the process of rescaling the distributions, as it can be seen from figure 2 and analogical figures in marekova [2014], these differences disappear. this provides a clear illustration of earthquake-occurrence self-similarity in space. considering the distance distributions it is important that the data collapse, which is a proof of a critical process, since only these exhibit this type of collapse, is known as scaling in critical phenomena. this analysis shows that earthquakes are a phenomenon of soc [bak et al. 1987, bak 1997, kanamori and brodsky 2004]. this is strikingly reminiscent of the collapse onto a single curve [varotsos et al. 2005, 2011] of the order parameter scaled distributions deduced from the seismicity in various regions worldwide when they are analyzed in a new time domain termed natural time [varotsos et al. 2001, sarlis et al. 2008], which has the privilege to extract the maximum information possible from a given time series [abe et al. 2005]. both in distance distributions for declusterized sequences, and in aftershock series there is no tendency for the real distributions to exceed the values given by the model curve; there is an excess of short inter-event distances. such a tendency appears when all earthquakes from regional catalogs are considered on the same base, without dividing them into foreshocks, main quakes and aftershocks. in most studied cases, regions where these points exist are relatively large in area. since during data selection aftershock series have not been removed, it is possible that the availability of an increased number of short distances between the earthquakes is due to them. in smaller sized regions, such an increased number of small distances is not observed, as the distances between the aftershocks in them and the distances between the independent events will be comparable. the division into series of independent and dependent events shows that they have similar spatial distributions. therefore the observed deviations could be due to some more specific manifestation of seismic activity, though they could also be explained by some disadvantages in the methodology of the study or the properties of the analyzed catalogs. the separate analysis of both the inter-event distances and times between successive earthquakes allows marking important properties of seismicity. such an analysis is made both for regional seismicity, wherein all earthquakes are placed on the same base and for declusterized sequences and aftershock series separately. the results will be complemented and extended if a joint study on space-time properties of seismic activity, similar to that in batac and kantz [2013] is carried out. references abe, s., n.v. sarlis, e.s. skordas, h.k. tanaka and p.a. varotsos (2005). origin of the usefulness of the natural-time representation of complex time series, phys. rev. lett., 94, 170601; 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(2001). a software package to analyze seismicity: zmap, seismol. res. lett., 72 (2), 373-382; doi: 10.1785/gssrl.72.3.373, http://seismo.ethz.ch/ staff/stefan/introtozmap6_online.htm. data sources for aftershock series: joshua tree, landers, big bear: http://quake.geo.berkeley.edu/cnss andreanof island 1986: http://giseis.alaska.edu/seis/ andreanof island 1996: http://quake.geo.berkeley.edu/cnss hector mine, northridge: http://www.scecdc.scec.org; or http://www.ncedc.org/anss/ st. elias, canada: horner [1983]. nahanni: cd from world data center-a, boulder, colorado all other data: from personal communication with colleagues. all original catalogs are included in electronic supplement: file aftershock series.xls. *corresponding author: elisaveta marekova, plovdiv university “paisiy hilendarski”, dept. of experimental physics, plovdiv, bulgaria; email: eligeo@uni-plovdiv.bg, eligeo@yahoo.com. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. marekova 12 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice o n d e s u p e r f i c i a l i a t t r a v e r s o i l b a c i n o d e l l ' a t l a n t i c o (*) p . caloi l . marcelli 1. —d o b b i a m o u n a b r e v e r e p l i c a ad a l c u n e o b i e z i o n i (*): a) l e o n d e da noi i n d i c a t e con l^ n e i l a v o r i p r e c e d e n t i ( 2 ' 3 ) n o n possono, i n nessun caso, essere c o n f u s e con ss, sss, sia p e r i t e m p i di t r a g i t t o , sia p e r i r e l a t i v i p e r i o d i . b) p e r l e r a g i o n i esposte n e l l a s e c o n d a n o t a s u l l ' a r g o m e n t o ( 3 ), n o n è c o n s e n t i t o a t t r i b u i r e l e stesse c a r a t t e r i s t i c h e e l a s t i c h e agli opposti r e t r o t e r r a d e l l ' a t l a n t i c o . c i ò n o n o s t a n t e , c o m e si è p u r e o s s e r v a t o n e l l a v o r o o r a c i t a t o , a c o n f e r m a d e l l a g r a n d e v e l o c i t à d e l l e o n d e l q , a n c h e n e l b a c i n o o r i e n t a l e d e l l ' a t l a n t i c o , o l t r e a l l a s t a z i o n e di l i s b o n a , c'è l a testimon i a n z a b e n p i ù i n c o n t e s t a b i l e di k e w , d o v e l e o n d e lq a p p a i o n o nett i s s i m e e m o s t r a n o u n a v e l o c i t à a n c o r a p i ù g r a n d e di q u e l l a r e g i s t r a t a ad o t t a w a , di 4,3 km/sec a l m e n o . ma c'è di p i ù . p e r q u a n t o c o n c e r n e l e r e g i s t r a z i o n i di t o r t o s a e b a r c e l l o n a , c o m e d e l r e s t o a b b i a m o già o s s e r v a t o ( 3 ), p e r u n eccesso di p r u d e n z a a b b i a m o , in un p r i m o t e m p o , posto l ' i n i z i o d e l l e i q d o v e ci p a r v e p i ù s i c u r o . u n o s g u a r d o a d e t t e r e g i s t r a z i o n i p r o v a p e r ò c b e l ' i n i z i o d e l l e o n d e stesse p o t e v a essere a n t i c i p a t o d i o l t r e u n m i n u t o . p e r b a r c e l l o n a , p. es., p o n e n d o l ' i n i z i o d e l l e 'lq c o m e da fig. 1 , l a v e l o c i t à di q u e s t e o n d e r i s u l t a di 4 , 1 k m / s e c , d e l l ' o r d i n e c i o è di q u e l l a o s s e r v a t a n e l l ' o p p o s t o r e t r o t e r r a d e l l ' a t l a n t i c o , a c o l u m b i a e a ott a w a , n o n o s t a n t e il p e r i o d o o s s e r v a t o a b a r c e l l o n a sia s o l t a n t o di 1 8 sec c i r c a . a n a l o g o il c o m p o r t a m e n t o a t o r t o s a , d o v e l e o n d e s u p e r f i c i a l i p o s s o n o essere a n t i c i p a t e di 62 sec, il c h e p o r t a l a l o r o v e l o c i t à a 4,2 k m / s e c ca. l e i n s u f f i c i e n t i r e g i s t r a z i o n i di j e r s e y ( d o v e f u n z i o n a n o s t r u m e n t i di p o c a s e n s i b i l i t à ) sono n e u t r a l i z z a t e d a l l e n i t i d e r e g i s t r a z i o n i di k e w , c h e si t r o v a presso a p o c o s u l l o stesso a z i m u t e a m a g g i o r d i s t a n z a , d o v e c o m e si è d e t t o ( 3 ) l e o n d e l q m o s t r a n o u n a e l e v a t a v e l o c i t à di p r o p a g a z i o n e , m a g g i o r e di q u e l l a o s s e r v a t a ad ott a w a . il f a t t o c h e a t o r t o s a e a b a r c e l l o n a p r e c e d a n o l e o n d e di m i n o r i*) c o m u n i c a z i o n e p r e s e n t a t a a l c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , t e n u t o a r o m a il 1 0 1 1 g i u g n o 1 9 5 2 . 3 9 8 p . c a l o ! l . m a r c e l l i [s. p e r i o d o , p u ò spiegarsi con l a t e o r i a dei f r o n t i g u i d a : l ' o n d a p r o v e n i e n t e d a l l ' a t l a n t i c o p r o segue i l suo t r a g i t t o sotto l o zoccolo c o n t i n e n t a l e , d o v e int a n t o nasce l ' o n d a a p e r i o d o m a g g i o r e , m a i n i z i a l m e n t e di m i n o r e v e l o c i t à , attesa l a elasticità r i d o t t a d e l l o s t r a t o cont i n e n t a l e . c) a p r o p o s i t o d e l l a p r e s u n t a s a l d a t u r a d e l l a c i n t a sismica m e d i t e r r a n e a con l e a z z o r r e , n o n si v e d e c o m e , se essa esiste, possa t e s t i m o n i a r e d e l l a e q u i v a l e n z a geologica di estese r e g i o n i q u a l i l ' e u r o p a , l ' a f r i c a e l ' a t l a n t i c o o r i e n t a l e . l a sismicità n e i pressi dell e a z z o r r e è d e l t u t t o d i v e r s a da q u e l l a d e l l a p e n i s o l a i b e r i ca. i n o l t r e , l a n o t e v o l e d e n s i t à degli e p i c e n t r i c h e si osserva l u n g o t u t t a l a d o r s a l e c e n t r a l e a t l a n t i c a , p a r a g o n a t a a q u e l l a d e b o l i s s i m a d e l l a f o s s a f r a l e a z z o r r e e la p e n i s o l a i b e r i c a , costituisce u n a p r o v a c h i a r i s s i m a d e l d i f e t t o di s a l d a t u r a f r a l e due zone s i s m i c h e d e l m e d i t e r r a n e o e d e l l a d o r s a l e a t l a n t i c a . d) non s o l a m e n t e l e d u e z o n e d e l l ' a t l a n t i c o h a n n o u n a h a t i m e t r i a c o m p a r a b i l e , m a quasi u g u a l e è a n c h e l a l o r o sismicità. e) l a t e o r i a di w e g e n e r è d i f f i c i l m e n t e sostenibile. n e l caso specifico, se l a s e p a r a z i o n e d e l l e a m e r i c h e si f o s s e v e r i f i c a t a l u n g o l a d o r s a l e a t l a n t i c a , l e e n o r m i f r a t t u r e così d e t e r m i n a t e , a v r e b b e r o d o v u t o 'ortadr 'v.y/ortnanng cai f i g . 2 o n d e s u p e r f i c i a l i a t t r a v e r s o i l b a c i n o d e l l ' a t l a n t i c o 3 9 9 p r o d u r r e sul l a t o o c c i d e n t a l e d e l l a d o r s a l e u n a s c a r p a t a , con p e n d ì o p i ù o m e n o a c c e n t u a t o ; m e n t r e a o r i e n t e i l f o n d o d e l l ' o c e a n o dov r e b b e r i s u l t a r e a l q u a n t o p i ù s o l l e v a t o . l a d o r s a l e , al c o n t r a r i o , giustifica i n p i e n o i l suo n o m e ed h a t u t t e l e c a r a t t e r i s t i c h e d'una m o n t a g n a s o m m e r s a . la m o r f o l o g i a n o n t e s t i m o n i a d u n q u e n é p e r l a r o t t u r a , n é p e r l ' a l l o n t a n a m e n t o . /j s e n o n p r o p r i o u n sottile s t r a t o c o n t i n u o , a l m e n o d e l l e isole s i a l i c h e possono b e n e s i s t e r e sul f o n d o d e l l ' a t l a n t i c o . u n t a l e s t r a t o , p e r ò , se esiste, dev'essere e s t r e m a m e n t e s o t t i l e : da u n o spessore di 4 0 k m ca. i n c o r r i s p o n d e n z a del sistema a l p i n o , esso d i v i e n e di 2 0 k m ca. p e r i l w u r t t e m b e r g , di 1 6 k m ca. p e r la r e n a n i a , p e r r i d u r s i a m e n o di 1 0 k m p e r l e isole b r i t a n n i c h e . a q u e s t o r i g u a r d o o s s e r v i a m o c h e i r i s u l t a t i d e l l e p r o s p e z i o n i sis m i c h e p e r r i f r a z i o n e , o t t e n u t i da m a u r i c e n. h i l l ( 8 ) i n t o r n o a l p u n t o 53° 5 0 ' n, 18" 4 0 ' w , si r i f e r i s c o n o ad u n a z o n a che n o n h a n u l l a a c h e v e d e r e con l a d o r s a l e a t l a n t i c a ; l a z o n a in q u e s t i o n e , i n f a t t i , si t r o v a ai m a r g i n i m e r i d i o n a l i del b a n c o di r o c k a l l e presso il b a n c o di p o r c u p i n e , v a l e a d i r e i n u n a p o s i z i o n e p r o s s i m a a l l o zoccolo cont i n e n t a l e (fig. 2). e b b e n e , i n questa zona, secondo l e v a l u t a z i o n i di h i l l , ad u n o s t r a t o di s e d i m e n t i sciolti di u n o spessore v a r i a n t e dai 1 9 0 0 ai 3 0 0 0 m , segue u n o s t r a t o c r i s t a l l i n o c a r a t t e r i z z a t o da u n a vel o c i t à di 5 , 0 0 km/sec p e r l e o n d e l o n g i t u d i n a l i , a v e n t e u n o spessore d e l l ' o r d i n e di 3,4-2,7 k m . a i m a r g i n i d e l l o zoccolo, l o s t r a t o contin e n t a l e si è q u i n d i n o t e v o l m e n t e assottigliato. è p e r t a n t o l o g i c o riten e r e che, p i ù ad o c c i d e n t e e a sud, d o v e l a p r o f o n d i t à d e l l ' o c e a n o è p r o s s i m a ai 5 0 0 0 m , t a l e s t r a t o si r i d u c a u l t e r i o r m e n t e e finisca con l o s c o m p a r i r e del tutto. g) la s c o p e r t a di m a t e r i a l i sialici in q u a l c h e p u n t o sul f o n d o dell ' a t l a n t i c o o r i e n t a l e , n o n è a f f a t t o a r g o m e n t o decisivo a f a v o r e dell'ipotesi sulla d i f f e r e n z a geologica f r a a t l a n t i c o o r i e n t a l e e a t l a n t i c o o c c i d e n t a l e . il geologo p o r t o g h e s e c. t e i x e i r a f a s u l l ' a r g o m e n t o u n a a n a l i s i s o t t i l e ( 4 ) e p r o v a l a p o s s i b i l i t à d e l l a p r o v e n i e n z a c o n t i n e n t a l e di tali m a t e r i a l i . r i g u a r d o a l l ' i p o t e s i citata, egli è e s p l i c i t o ed a f f e r m a c h e essa è i n s o s t e n i b i l e . lo stesso m a u r i c e e w i n g , a l c u n i l a v o r i d e l q u a l e e r a n o stati inv o c a t i a f a v o r e dell'ipotesi s u l l a d i f f e r e n z a geologica d e l l e z o n e a t l a n t i c h e ad est e ad ovest d e l l a d o r s a l e m e d i a n a , i n u n r e c e n t e studio, c o m p i u t o con f r a n k p r e s s ( 5 ), s u l l a s t r u t t u r a d e l l a crosta a l l a b a s e m/nr1tak1. (jvinea po«t m (afr) slato 01 ntw-york ( u b a) (inforno ir/wpara/mo.vorr/j f lori da i« bahama / fra u20'e // jo 'para/fc/o a 'orti j (uaa.) mi o f i g . 3 o n d e s u p e r f i c i a l i a t t r a v e r s o i l b a c i n o d e l l ' a t l a n t i c o 399 fa 4 0 2 p . c a l d i l . m a r c e l l i degli o c e a n i , così si e s p r i m e : « t l i e r e s u l t s f o r t h e a t l a n t i c a r e i n good a g r e e m e n t w i t l i t h e d a t a o h t a i n e d i n a r e c e n t seismic r e f r a c t i o n m e a s u r e m e n t m a d e 1 2 0 i n i l e s n o r t h w e s t of b e r m u d a , a n d offer strong evidence that the result of tliis single refraction measurement ivill be found to be typical of the entire ocean ». h) l a b a t i m e t r i a p r e s e n t a f o s s e e p i a n u r e s o m m e r s e di p r o f o n d i t à c o m p a r a b i l e ai d u e l a t i d e l l a d o r s a l e (fig. 3). s e d o b b i a m o r i t e n e r e v a l i d a l a t e o r i a isostatica (e t a l e è r i t e n u t a dai p i ù n o t i geodeti e geologi), s p e c i a l m e n t e n e l l o s v i l u p p o ad essa dato da p r a t t , n e l l ' i p o t e s i d e l l a d i f f e r e n z i a z i o n e geologica f r a l e d u e z o n e d e l l ' a t l a n t i c o , l'equil i b r i o isostatico s a r e b b e i m p o s s i b i l e n o n s o l a m e n t e f r a l e d u e z o n e d e l l ' a t l a n t i c o , m a , b e n p i ù , f r a l a z o n a o r i e n t a l e e i c o n t i n e n t i lim i t r o f i . 2. — in u n l a v o r o p r e c e d e n t e (3), a b b i a m o f a t t o u n t e n t a t i v o di d e t e r m i n a z i o n e d e l l o spessore d e l l o s t r a t o su cui poggia l ' o c e a n o a t l a n tico, v a l e n d o c i d e l l e c u r v e di d i s p e r s i o n e d e l l e o n d e di l o v e , r e g i s t r a t e s u l l e r i v e l i m i t a n t i l ' a t l a n t i c o ad est e ad ovest, o n e l l ' i m m e d i a t o ret r o t e r r a , i n o c c a s i o n e del t e r r e m o t o d e l l a d o r s a l e m e d i a n a a t l a n t i c a d e l 2 4 a p r i l e 1 9 4 7 . s u l l a b a s e d e l l e n o s t r e p r e c e d e n t i ricerche, a b b i a m o r i t e n u t o press o c h é t r a s c u r a b i l e l a p r e s e n z a d e l l o s t r a t o del « g r a n i t o » ad o r i e n t e e a d o c c i d e n t e d e l l a d o r s a l e m e d i a n a a t l a n t i c a . ciò ci h a p o r t a t o a s c e g l i e r e p e r l e d e n s i t à i s e g u e n t i v a l o r i : q o 3 , 0 ; q ì = 3 , 3 2 , c o n f o r m e m e n t e ai r i s u l t a t i dei c a l c o l i di b u l l e n p e r l o s t r a t o seguente i l « g r a n i t o » e i l s u b s t r a t o u l t r a b a s i c o . d a l l e c u r v e di d i s p e r s i o n e è risultato c o = 4 , 0 , <?i=4,5 km/sec , q u a l i v a l o r i d e l l a v e l o c i t à d e l l e o n d e t r a s v e r s a l i n e i d u e s t r a t i sov r a p p o s t i . p e r i d a t i a n a l i t i c i rimandiamo al n o s t r o secondo c o n t r i b u t o (3). o s s e r v i a m o q u i solo che, n e l p r e c e d e n t e l a v o r o , ci s i a m o l i m i t a t i a l l a t e o r i a s v i l u p p a t a da j e f f r e y s n e l caso di d u e m e z z i o m o g e n e i , a v e n d o ritenuto s u p e r f l u o r i c o r r e r e a l l a t e o r i a s u c c e s s i v a m e n t e estesa, •dallo stesso j e f f r e y s (b"7), al caso di u n m e z z o ( i l secondo) e t e r o g e n e o . a l l o scopo di d a r e c a r a t t e r e di c o m p l e t e z z a a l l a n o s t r a r i c e r c a , a b b i a m o f a t t o ricorso a n c h e a q u e s t a e s t e n s i o n e d e l l a t e o r i a di j e f o n d e s u p e r f i c i a l i a t t r a v e r s o i l b a c i n o d e l l ' a t l a n t i c o 399 f r e y s . in q u e s t o caso, l ' e q u a z i o n e d e l l a v e l o c i t à p e r l e o n d e di l o v e assume la f o r m a (°). tang k. a0 d= 1 + u l + — ì ( l a « « ) ( 5 + o , » ) + pl0 a c ( 2 fc l a\ a 2 ; / 8fe2 l" af + 0 ( f c l ) 3 j , [ 1 ] 2 jt d o v e a; = — , l è u n a l u n g h e z z a costante e gli a l t r i s i m b o l i h a n n o a il s i g n i f i c a t o espresso d a l l e [ 1 ] , [ 3 ] del n o s t r o p r e c e d e n t e l a v o r o ( 3 ). q u a n d o k l è i n f i n i t o i l mezzo i n f e r i o r e è o m o g e n e o e l a [ 1 ] si r i d u c e a l l a classica e q t i a z i o n e di v e l o c i t à d e l l e o n d e di l o v e . la s o l u z i o n e d e l l a [ 1 ] è stata p r o p o s t a a l b e n e m e r i t o i s t i t u t o n a z i o n a l e p e r l e a p p l i c a z i o n i del c a l c o l o . f r a l e i n f i n i t e r a d i c i posit i v e e n e g a t i v e d e l l a [ 1 ] , c o r r i s p o n d e n t i ad ogni v a l o r e del r a p p o r t o c/c0 f a n n o al n o s t r o caso q u e l l e qui r i p o r t a t e : 1 . 0 2 0 1 . 0 2 5 1 . 0 3 0 1 . 0 4 0 1 , 0 5 0 . 3 6 6 . 3 1 4 . 2 7 6 . 2 2 4 . 1 8 7 1 . 0 8 5 1 . 0 9 0 1 . 0 9 5 1 . 0 9 8 1 . 1 0 0 . 1 0 7 . 0 9 7 9 . 0 8 9 1 . 0 8 3 4 . 0 7 8 8 . c / c 0 1 . 0 0 5 1 . 0 1 0 1 . 0 1 2 5 k . 8 2 8 . 5 6 0 . 4 9 0 c/c0 1 . 0 6 0 1 . 0 7 0 1 . 0 8 0 k . 1 5 8 . 1 3 5 . 1 1 6 4 0 4 p . c v l o i • l . m a r c e l l i 2 51 è così p o s s i b i l e r a p p r e s e n t a r e g r a f i c a m e n t e c/c0 i n f u n z i o n e di d. d ' a l t r o n d e , se c è la v e l o c i t à di g r u p p o e c l a v e l o c i t à di f a s e , si h a ( 3 ) : cq co \ d l i . d 2 5 t [2] 11 coefficiente d i f f e r e n z i a l e n e l s e c o n d o m e m b r o d e l l a [ 2 ] p u ò essere d e d o t t o d a l l ' i n c l i n a z i o n e d e l l a c o r r i s p o n d e n t e c u r v a p e r c / c 0 ; ciò p e r m e t t e di c a l c o l a r e c/c0 c o m e f u n z i o n e di 2 5 t ^ ( f i g . 4) (*). i l e c u r v e di d i s p e r s i o n e d a n n o i l v a l o r e di c c o r r i s p o n d e n t e ad ogni l u n g h e z z a d'onda. noto c d a l l a c u r v a r a p p r e s e n t a n t e c/c0 si d d e d u c e il r e l a t i v o v a l o r e p e r 2 sr — e q u i n d i l o spessore d d e l l o s t r a t o . s i sa che, i n questo g e n e r e di m i s u r e , n o n è p o s s i b i l e r a g g i u n g e r e v a l o r i m o l t o a p p r o s s i m a t i , s p e c i e p e r ciò c h e si r i f e r i s c e a l l a l u n g h e z za d'onda. ecco, ad ogni m o d o , i v a l o r i o t t e n u t i p e r d: x d harvard 7 6 k m 1 7 k m 6 8 » 1 7 » 5 8 » 1 6 » 5 8 » 1 8 » scoresby sund 6 6 » 1 7 » 6 1 » 1 8 » 52 » 1 7 » 4 8 » 1 7 » lisbona 7 6 » 1 6 » 63 » 1 6 » 7 0 » 1 9 » 6 5 » 2 0 » halifax 9 2 » 2 0 » 1 0 1 » 2 4 » ( * ) n e l l a v o r o p r e c e d e n t e , n e l d a r e f o r m a d e f i n i t i v a a l g r a f i c o r i p o r t a t o i n fig. 3, si è f a t t o u n e r r o r e n e l l a v a l u t a z i o n e d e l l e o r d i n a t e ( e r r o r e s e n z a c o n s e g u e n z e a i fini d e l c a l c o l o ) . l a fig. 3 d e l s u d d e t t o l a v o r o , v a p e r t a n t o s o s t i t u i t a c o n l a fig. 5 . o n d e s u p e r f i c i a l i a t t r a v e r s o i l b a c i n o d e l l ' a t l a n t i c o 399 san juan kew obs. qltai la m e d i a dei 1 0 8 » 2 8 82 » 3 0 7 7 » 1 7 8 0 » 1 9 7 0 » 1 9 6 9 » 2 0 8 5 » 1 9 7 5 » 1 9 7 0 » 2 0 5 7 » 1 8 5 6 » 2 0 dei v a l o r i r i p o r t a t i è (l—\9 k m . nel l a v o r o p r e c e d e n t e , a v e v a m o o t t e n u t o d = 1 7 k m . i v a l o r i q u i r i p o r t a t i p r e s e n t a n o p e r ò c a r a t t e r e di m a g g i o r e unif o r m i t à . i n f a t t i , solo i v a l o r i d a t i da s a n j u a n s c a r t a n o s e n s i b i l m e n t e d a l l a m e d i a . non si è t e n u t o c o n t o dei v a l o r i t r a t t i d a l l e r e g i s t r a z i o n i di b e r m u d a , cbe, a n c h e questa v o l t a , sono stati e s t r e m a m e n t e p i c c o l i (e c i o è : p e r a = 4 0 k m , d = 9 k m ; x = 4 8 , d= 1 3 ; x = 3 9 , ci = 1 1 ; x = 34, d = 1 1 ; x = 35, d = 12). r e s t a n o così c o n f e r m a t i l ' o r d i n e di g r a n d e z z a o t t e n u t o p e r lo spessore d e l l o s t r a t o s o p p o r t a n t e l ' a t l a n t i c o e l a s t r e t t a a n a l o g i a c h e esso p r e s e n t a con l ' a n a l o g o s t r a t o s o p p o r t a n t e i l p a c i f i c o . u n ' u l t e r i o r e c o n f e r m a d e l l a v a l i d i t à di q u e s t o ris u l t a t o , da n o i o t t e n u to n e l 1 9 4 9 , è d a t a d a l già c i t a t o r e c e n t e l a v o r o di e w i n g e p r e s s ( 5 ) . i d u e studiosi amer i c a n i c o n c l u d o n o , ' t r a t t a n d o l ' a r g o m e n t o ' p e r u n a v i a d i v e r s a fig. 5 4 0 6 p . c a d d i • l . m a r c e l l i d a l l a n o s t r a : « no significant difference in the nature of the suboceanic basement of the atlantic and pacific has been found, s i n c e t h e v e l o c i t y of s h e a r w a v e s w a s c a l c u l a t e d t o 4 , 4 5 k m / s e c f o r b o t h oceans. p r e v i o u s l y r e p o r t e d d i f f e r e n c e s i n a t l a n t i c a n d pacific, v e l o c i t y f o r r a y l e i g h w a v e s of s o m e selected p e r i o d a r e n o w b e l i e v e d t o h e d u e p r i i n a r i l y to d i f f e r e n c e s i n t h e d e p t h of w a t e r p l u s s e d i m e n t i n t h e t w o o c e a n s » . c o n c l u s i o n i i d e n t i c h e a q t i e l l e da n o i p r e c e d e n t e m n t e o t t e n u t e , p e r qruanto r i g u a r d a i l f o n d o dei d u e oceani a t l a n t i c o e p a c i f i c o . p e r ciò c h e si r i f e r i s c e a l l a s p i e g a z i o n e d e l l e p r e c e d e n t i c o n t r a s t a n t i c o n c l u s i o n i t r a t t e s u l l a v e l o c i t à d i p r o p a g a z i o n e d e l l e o n d e di r a y l e i g h , a b b i a m o a v u t o m o d o di p r o v a r e ( 3 ) c h e esse v a n n o p r i n c i p a l m e n t e a t t r i b u i t e al f a t t o c h e i casi i n p r e c e d e n z a e s a m i n a t i si r i f e r i v a n o ad o n d e di r a y l e i g h c h e solo i n p a r t e i n t e r e s s a v a n o l ' a t l a n t i c o , n e l l o r o t r a g i t t o . roma —• istituto nazionale di geofisica — giugno 1952. riassunto in precedenti lavori abbiamo provato che il bacino dell' atlantico (almeno nella sua parte centro-settentrionale) si comporta, dal punto di vista della trasmissione delle onde superficiali, in maniera del tutto analoga al bacino del pacifico. si portano nuovi argomenti a favore di detta conclusione e si riassumono le testimonianze, favorevoli ai nostri risultati, tratte da lavori del geologo teixeira e dei geofisici eiving e press. si ripete il tentativo di determinazione dello spessore dello strato superficiale sotto l'atlantico applicando la teoria di jeffreys sulla propagazione delle onde superficiali trasversali, che considera il 2" strato eterogeneo. partendo dalle curve di dispersione delle onde l,, e prescindendo dallo strato del « granito », ritenuto mancante o di entità trascurabile, si trova uno spessore del sottostante strato pari a 19 km ca., a conferma di quello ottenuto precedentemente e dell'ordine di quello sopportante il pacifico. summary in preceding papers ice have proved that the basin of the atlantic ocean (at least in the central-northern part) behaves, from the point of view of the transmission of surface waves, in a way completely similar to the pacific basin. o n d e s u p e r f i c i a l i a t t r a v e r s o i l b a c i n o d e l l ' a t l a n t i c o 399 we state here new arguments in favor of this conclusion, and include a summary of the confirming results of the geologist teixeira and the geophysicists eiving and press. a new attempt has been made to determine the depth of the first layer under the atlantic. the theory of jeffreys, that considers the second layer to be heterogeneous, is applied to the propagation of surface transversai ivaves. starting from the curve of dispersion of the illave i.,, and leaving out of consideration the layer of « granite » considered to be missing or of negligible size, one finds a thickness of the underlying layer equal to about 19 km in confirmation with the value obtained previously, and of the same order as that supporting the pacific. b i b l i o g r a f i a ( ! ) rothè j . p . : la structure de l'atlanlique. « a n n a l i d i g e o f i s i c a » , i v , 1 ( 1 9 5 1 ) . ( 2 ) c a l o i p . , m a r c e l l i l . , p a n n o c c h i a g . : sulla velocità di propagazione delle onde superficiali in corrispondenza dell'atlantico. « a n n a l i d i g e o f i s i c a » , i i , 3 ( 1 9 4 9 ) . ( 3 ) c a l o i p . , m a r c e l l i l _ p a n n o c c h i a g . : ancora sulla velocità di propagazione delle onde superficiali per tragitti subatlantici... « a n n a l i d i g e o f i s i c a » , i i i , 2 ( 1 9 5 0 ) . (4) teixeira c . : a propos d'une hypothese sur la structure de l'océan atlantique. « b o l e l i n m u s e u m e l a b o r a t o r i o m i n e r a l o g i c o e g e o l ò g i c o » , u n i v e r s i d a d e l i s b o a , n . 1 8 ( 1 9 5 0 ) . ( ° ) ewinc m . a n d press f . : crustal structure and surface-uave dispersion. « b u l l . s e i s n i o l o g . s o c i e t y of a m e r i c a » , x l , o c t o b e r 1 9 5 0 . ( ° ) jeffreys harold: the effect on love waves of lleterogeneity in the loiver layer. « m o n t i l i . n o t . r o y . a s t r . s o c . » , g e o p h y s . s u p p l e m . , i i , 2 ( 1 9 2 8 ) . ( 7 ) jeffreys harold: the surface waves of earthqiwkes. « m o n t h l . n o t . r o y . a s t r . s o c . » , g e o p h y s . s u p p l . , i i i , % ( 1 9 3 5 ) . ( 8 ) hill maurice n . : seismic refraction shooting at a point i n t h e eastern atlantic. « a s s . d ' o c é a n o g r . p h y s i q u e , g e n . a s s e m b l y at b r u s s e l s » , a u g u s t 1 9 5 1 . r e p o r t s a n d a b s t r . of c o m m u n i c . , p . 5 5 . a n a l i s i r a d i o a t t i v a d e l l e r o c c e c o l m e t o d o f o t o g r a f i c o ( * ) g . imbò l . casertano 1. — i n (lue n o t e p r e c e d e n t i ( j ) è stato i n d i c a t o i l m e t o d o p e r c a l c o l a r e , dalle d i s t r i b u z i o n i s t a t i s t i c h e d e l l e p a r t i c e l l e « , r a c c o l t e su l a s t r e f o t o g r a f i c h e , i n f u n z i o n e della p r o i e z i o n e sulla l a s t r a del perc o r s o d e l l a p a r t i c e l l a n e l l ' e m u l s i o n e , n o n solo il r a p p o r t o i n peso delle c o n c e n t r a z i o n i di u r a n i o (cl,) r i s p e t t o a q u e l l e del t o r i o ( c j ] , ! , m a a n c h e i l c o n t e n u t o i n u r a n i o e t o r i o della r o c c i a i n e s a m e : presupp o n e n d o p e r ò r a g g i u n t e l e c o n d i z i o n i di e q u i l i b r i o r a d i o a t t i v o in o g n u n a d e l l e f a m i g l i e . o r a n e l l ' a p p l i c a r e i l m e t o d o a l a v e vesuviane si è voluto prescind e r e da q u a l s i a s i a m m i s s i o n e , e si è a r r i v a t o a r i c o n o s c e r e — a l m e n o i n l i n e a t e o r i c a — l a p o s s i b i l i t à di v e r i f i c a r e a d d i r i t t u r a l e condiz i o n i i n c u i si t r o v a n o v a r i e l e m e n t i d e l l e f a m i g l i e r a d i o a t t i v e ris p e t t o a q u e l l e c h e d o v r e b b e r o essere l e c o n d i z i o n i di e q u i l i b r i o . l a f o r m u l a [ 5 ] della s e c o n d a delle n o t e c i t a t e c o n s e n t e di r i c a v a r e , p e r spessori della sostanza e q u i v a l e n t i a percorsi n e l l ' a r i a m a g g i o r i di q u e l l o ( r ) d e l l ' e l e m e n t o c h e si c o n s i d e r a , il n u m e r o (n) di p a r t i c e l l e u emesse p e r c m 3 di sostanza in base al n u m e r o ( n ) di p r o i e z i o n i , c o n t a t e p e r c m 2 , maggiori di r, essendo il] il coefficiente di f r e n a m e n t o della sostanza r a d i o a t t i v a , e uu q u e l l o dell ' e m u l s i o n e . l a [ 1 ] , c o m e è stata estesa a l l e f a m i g l i e r a d i o a t t i v e in e q u i l i b r i o , può g e n e r i c a m e n t e e s t e n d e r s i a e l e m e n t i q u a l s i a s i r a d i o a t t i v i , della stessa f a m i g l i a o m e n o , e, n e l p r i m o caso, n o n n e c e s s a r i a m e n t e i n e q u i l i b r i o . n e l caso g e n e r a l e e v i d e n t e m e n t e a n c h e p e r iv devono assum e r s i valori c h e possono a n c h e n o n essere u g u a l i p e r i singoli elem e n t i . l a f o r m u l a , q u i n d i , d i v e n t a : [ 1 ] ( * ) comunicazione presentata al convegno dell'associazione geofìsica italiana, tenuto a r o m a il 17-18 giugno 1953. 32(1 0. i m b ò • l . c a s e r t a n o con la sommatoria estesa a tutti gli e l e m e n t i il cui percorso nell'aria (ri) è maggiore dell'equivalente in aria (r/u„) della proiezione (r). 2. — con le tracce contate su una lastra è possibile ricavare dalla [ 2 ] tante equazioni quante sono le proiezioni r c o n s i d e r a t e : le incognite risultano l e ni. r i s u l t a c h i a r o c h e è conveniente c e r c a r e di r i d u r r e il n u m e r o delle incognite, considerando proiezioni c h e sono m i n o r i di determinati v alori dei percorsi delle particelle a . d ' a l t r a parte la differenza f r a le proiezioni deve essere tale da consentire f a c i l m e n t e i l conteggio delle t r a c c e , con l a m i n o r e possibilità di conf u s i o n e : il che porta alla diminuzione del n u m e r o delle equazioni. s i ritiene conveniente il conteggio dei n u m e r i tii, rio, . . . . , n 1 0 di tracce aventi proiezioni maggiori rispettivamente di 0,60 u 2 /ìtiic 0,64 tio ^thc' ' ^ t h c essendo rxhc' = = c m percorso nell'aria delle p a r t i c e l l e a emesse dal thc. quindi il sistema da risolvere risulta il seguente: a^ t h c ' + binrac + c . / v a c a + dtnan + etn tha + acc = nl at + b, + <\ + l>, + fs — a„ + b, + c, = n.. a + b} = na a, + p 5 — n : > ac + b, = nn a. = il. a * = n * a0 — nv a o = " i o con i coefficienti a, b, c, d, e, f c h e si ricavano dalla [ 2 ] in base ai valori di r e di jj. ; ed essendo n i p ^ . ; ; a ^ca ' -^ an 5 ^tlia * ^ acc rispettivamente il numero di particelle u emesse per c m 3 di sostanza in tutto il periodo di esposizione, dal t o r i o c , dal r a d i o c ; dall'attinio a, dall'/vttinon, dal t o r i o a e d a l l ' a t t i m o c. 11 sistema surriportato però è indeterminato, pur sembrando di 10 equazioni e 6 incognite. l i m i t a n d o l o alle ultime 8 equazioni esso risulta risolvibile nelle tre incognite ^ x h c » ^ r a c e -^aca • ' v a " lori di queste, e con opportune considerazioni sulle condizioni di equilib r i o delle famiglie radioattive, si p o t r e b b e , con le altre due equazioni, e tenendo conto dei legami i n t e r c o r r e n t i f r a gli elementi delle f a m i g l i e dell'uranio e dello attinio, da una parte dedurre i valori di n , j\ e a , e, dall'altra, verificare l'esattezza o meno dell'ammissioni fatte. a n a l i s i radioattiva d e l l e rocce con i l metodo fotografico 3 1 7 3. — si può ancora considerare che il n u m e r o di particelle a emesse da ogni elemento della famiglia dell'attimo è, generalmente, trascurabile rispetto a quello delle particelle a emesse da ogni elemento, contenuto nella stessa roccia, della f a m i g l i a d e l l ' u r a n i o ; quindi in pratica può trovarsi a'a„ — ^acc = = < l u e s l 0 caso è possibile risolvere un sistema di 10 equazioni n e l l e tre incognite i v t h c , a pag/ e a ' j i ^ : m e n t r e però per la famiglia del t o r i o , l'esistenza o meno d e l l ' e q u i l i b r i o può direttamente dedursi dai valori di ^ thc e j^tha intende per gli elementi compresi f r a il t o r i o a e il t o r i o c'), nulla potrà dirsi per la famiglia dell'uranio. dal fatto poi c h e a ' ^ c c o m p a r e in tutte le equazioni, m e n t r e ^rac' c o m p a r e in sei, e a't],^ in due soltanto vi è da aspettarsi, per le tre grandezze, pesi sensibilmente diversi, ed e r r o r i medi più f o r t i per a f ^ e a'gaq» (in p a r t i c o l a r modo per la prima) che non per a'-j h c , . con i valori di n ottenuti per lave delle eruzioni vesuviane del 1631, 1855, 1872 e 1944 sono stati ricavati per n v a l o r i non solo compresi nei l i m i t i degli errori, ma a n c h e — com'era da aspettarsi — trascurabili rispetto a quelli di a ' x h c e a : il c h e h a autorizzato a risolvere il sistema n e l l e tre incognite a ^ h c > ^ r a c ' e ^tha • p e r q u e s t ' u l t i m a grandezza però sono stati dedotti e r r o r i medi molto forti, da raggiungere perfino —come nel caso dei campioni delle lave del 1855 e del 1872 —• il valore ricavato per l'incognita. p e r c i ò si sono ritenuti accettabili soltanto i valori calcolati per n e a'r a c, che si r i p o r t a n o nella t a b e l l a i . tabella i data 1 6 3 1 1 3 5 5 1 8 7 2 1 9 4 4 p e r poter risalire dal valore di arr;lqr alla quantità di r a d i o e quindi di u r a n i o presente, e da alla quantità di t o r i o , debbono essere f a t t e alcune ammissioni (-). nei casi in esame, oltre a supporre c h e le famiglie radioattive erano in e q u i l i b r i o n e i campioni di lava — prelevati n e l l ' i n t e r n o di colate delle date indicate — fino a l l ' a t t o della polverizzazione dei c a m p i o n i , si può ritenere che alla esposizione a , a (in seri 5 , 0 3 . 1 0 ® ( 1 7 , 2 0 + 0 , 3 1 ) . 1 0 5 ( 5 , 7 4 + 1 , 4 3 ) . 1 0 3 7 , 5 2 ( 2 5 , 7 4 + 2 , 4 2 ) ( 3 6 , 5 9 + 1 1 , 3 5 ) 5 , 9 8 ( 1 7 , 9 7 ± 0 , 7 6 ) ( 3 1 , 6 6 + 3 , 4 9 ) 5 , 0 3 < 4 3 , 8 4 ± 1 , 0 0 ) ( 3 0 , 1 5 ± 4 , 7 8 ) 32(1 0. i m b ò • l . c a s e r t a n o h p x h c j w ca c h x p c: — cn io ci m in (n o o o © © —' c; ©" +1 41 +i -4-1 cc cn irt >h cc n h co © co ^ r-̂ ct © © © t— -rf v© os r h o lo co © m <o ©̂ o" © © o" •h -h -h h © lo o © i— o o i o © m m ĉ o c © © ĉ © '© © i-t co © +1 +1 -fi +1 ov © co lo lo c-j cc lo —*" (-t cc ot r—« i—i "m +1 -h +1 lo lo io +1 +1 +1 p—i co t i o c c a e = i c c -c i +1 +1 +1 o n n « 4-1 +1 +1 +1 e co t i o i. co vĉ ti co f-f cô vo" m m n rr-h co rn n n ô io c-i -+ co io i -1c cc k ? co = o chiusura delle cassettine, c o n l e n e n t i l e lastre fotografiche a contatto con le sostanze polverizzate, non erano presenti l s emanazioni, e che, infine, tutte le quantità di emanazioni, formantisi nel corso dell'esposizione sono rimaste racchiuse n e l l e casseltine stesse. perciò il numero n * p . ( di atomi di r a d i o presenti per cin: ! è dato d a : ra = r a c t — rn [ 4 ] r a essendo l il tempo di esposizione àpa e rispettivamente le costanti di disintegrazione del r a d i o e del r a d o n . analoghe f o r m u l e possono ricavarsi per l ' u r a n i o e il t o r i o , solo c h e in quest'ultimo caso è da tener conto a n c h e del rapporto di ramificazione del t o r i o (.. dai valori del peso atomico degli elem e n t i e della densità della r o c c i a può ottenersi il tenore in uranio e t o r i o delle lave in e s a m e ; nella t a b e l l a i i sono riportali i n s i e m e ai valori ricavati con il metodo qui descritto, a n c h e quelli ottenuti, con il metodo esposto nella nota precedente, ammettendo l ' e q u i l i b r i o radioattivo n e l l e lave. nella t a b e l l a sono r i p o r t a t i ancora i rapporti in peso cy/c-pj, e un indice della radioattività complessiva data dal numero ( n ) di particelle (< raccolte per c m 2 di superficie e per sec aventi la proiezione sulla lastra del percorso nell'emulsione maggiore di 0,60 u2 ^ f l i c ' ' 4. — in conclusione nel f a r n o t a r e come i dati ricavati concordano, nel senso delle variazioni, con quelli trovali dal j o l v ( 3 ) si mette in evidenza che disponendo di un n u m e r o convenientemente elevalo di a n a l i s i r a d i o a t t i v a d e l l e rocce con i l m e t o d o f o t o g r a f i c o 3 1 9 t r a c c e — in modo da avere una migliore precisione nelle soluzioni del sistema [ 3 ] — è possibile ricavare direttamente, senza cioè alcuna ammissione « a priori », il numero di atomi di t o r i o c', r a d i o c' e a t t i n i o a disintegrati per cm 3 di sostanza n e l corso dell'esposizione. con questi valori poi si può calcolare il numero di atomi disintegrati di attinon, t o r i o a e a t t i n i o c e c o n t e m p o r a n e a m e n t e verificare le condizioni dei diversi elementi delle f a m i g l i e radioattive rispetto a quelle che sono le condizioni di e q u i l i b r i o . in condizioni meno favorevoli riesce possibile dedurre direttam e n t e il n u m e r o di atomi disintegrati di t o r i o c', t o r i o a e r a d i o c ' : è quindi possibile procedere a verifiche soltanto per la f a m i g l i a del t o r f o . i n generale, infine, riesce possibile ottenere valori attendibili del n u m e r o di atomi disintegrati di t o r i o c' e r a d i o c ' : per risalire al tenore in u r a n i o e t o r i o della sostanza si deve necessariamente ricorrere ad ammissioni sulle condizioni di e q u i l i b r i o di e n t r a m b e l e famiglie radioattive. napoli — osservatorio vesuviano — giugno 1953 riassunto in base alle formule date in note precedenti si ricava un sistema di equazioni che permette, dal numero di particelle a raccolte su una lastra fotografica e aventi le proiezioni del percorso nell'emulsione maggiori di determinate lunghezze, di calcolare il numero di atomi di t h c ' , r a c ' e a n a disintegrati per cm3 di sostanza in tutto il tempo dell'esposizione. si indica come da questi valori si può contemporaneamente dedurre il numero di atomi, di an, t h a e acc disintegrati e verificare le condizioni in cui si trovano i diversi elementi delle famiglie radioattive. si mostra poi come con lo stesso sistema in condizioni meno favorevoli è possibile dedurre il numero di atomi disintegrati di t h c ' , t h a e r a c ' , e verificare le condizioni della sola famiglia del torio; ovvero soltanto il numero di atomi disintegrati di t h c ' e r a c ' . nel primo caso il tenore in uranio e torio può ricavarsi senza fare alcuna ammissione; nel secondo invece bisogna farne soltanto per quel che riguarda la famiglia dell'uranio; e nell'ultimo, infine, è 32(1 0 . i m b ò • l . casertano necessario farne su entrambe le famiglie radioattive, con l'evidente conseguenza che i valori ricavati assumono significati diversi ili ognuno dei tre casi. summary on the basis of the formulai' given in a preceding note, one obtains a system of equations which permits, from the number of alpha particles registered oli a photographic piate, and from the projections of the paths in the emulsion greater than a cerlain de ter mi ned range, the calculation of the number of atoms of t h c ' , r a c ' , aiul a n a disintegrating per cm' of the substance diiring the total exposure lime. it is indicated, how, at the same time, one can deduce front these values, the number of atoms of a n , t h a , and acc disintegrating, and verifv the conditions in ichich one finds the various elements of the radioactive families. one then demonstrates hoiv, with the same system under less favorable conditions; it is possible to deduce the number of disintegrating atoms of t h c ' , t h a and r a c ' , and verifv the conditions of the single family of thorium, or else only the number oj disintegrating atoms of t h c ' and r a c ' . in the first case, the content of uranium and thorium can be determined without any approximations, in the second, it is necessary to malie approximations only for uranium family, and in the last approssimations must be made for both radioactive families, with the evident consequence that the values so calculated have a different significance in each of the three cases. b i b l i o g r a f i a ( 1 ) i m b ò e c a s e r t a n o , ann. geof., v o i . i l i , 4 7 5 , 1 9 5 0 e v o i . v , 1 , 1 9 5 2 . ( 2 ) nota in corso di stampa su ann. oss. ves^ serie vi, voi. i . (3) joly, phil. mag., 18, 577, 1909. note sopra un breve periodo sismico svoltosi nel territorio di montespertoli durante il mese di giugno 1 9 5 8 m. d e p a n f i l i s nel mese di giugno 1958, dal giorno 6 al giorno 12, avvennero nel territorio del comune di montespertoli, a sud-ovest di firenze, numerose scosse di terremoto che turbarono vivamente la tranquillità di quella popolazione. su richiesta delle autorità amministrative locali, il 24 giugno 1958 effettuammo un'indagine macrosismica nella zona ove i movimenti tellurici erano s t a t i avvertiti. nelle pagine seguenti esponiamo alcuni dati ricavati sul luogo, che p o t r a n n o essere utili per la conoscenza delle caratteristiche sismiche della zona di montespertoli. sulla base delle notizie ottenute nelle località ove le scosse furono avvertite e dei dati ricavati dalle registrazioni sismografiche degli osservatori ximeniano di firenze e di s. domenico in prato, si può dire che i movimenti sismici, che t a n t o allarme destarono nella popolazione di montespertoli e delle sue più vicine frazioni, furono t u t t i , eccetto uno, di modesta intensità e t u t t i , senza eccezioni, di propagazione macrosismica assai limitata. la massima distanza epicentrale alla quale la perturbazione tellurica più intensa, verificatasi alle 18h50m circa del 7 giugno, risultò ancora percettibile per le persone raggiunse i n f a t t i appena i venti chilometri. anche sotto la forma microsismica le onde telluriche ebbero una modesta propagazione: le registrazioni ottenute per le varie scosse furono infatti, secondo quanto ci risulta, soltanto quelle degli osservatori sismici predetti i quali, dalla zona epicentrale, distano appena, in linea d'aria, 20 e 25 chilometri rispettivamente. quali segni premonitori del periodo sismico in esame possono considerarsi alcuni rombi, piuttosto intensi, uditi ad intervalli di tempo più o meno lunghi in t u t t o il territorio di montespertoli nei giorni immediatamente precedenti il 6 giugno. essi peraltro, non essendo stati accompagnati da movimenti tellurici percettibili all'uomo, non destarono molta preoccupazione negli abitanti della zona, i quali, per la maggior parte, ritennero quei fenomeni acustici come dovuti a scoppi di mine. il periodo sismico vero e proprio s'iniziò alle 16h24m circa del 6 giugno 1958 con u n a scossa a v v e r t i t a a montespertoli e nelle sue frazioni più vicine con leggera intensità. ad essa fecero seguito, nella stessa giorn a t a , due altre scosse, pure leggere, verificatesi rispettivamente alle 20h21m ed alle 21h55m circa. d u r a n t e il 7 giugno le scosse, t u t t e susfultorie ed accompagnate o precedute da forti rombi paragonati dagli abitanti ad esplosion di mine, si susseguirono con rilevante frequenza, specialmente nelle ore pomeridiane, le più notevoh, avvertite da t u t t a la popolazione del comune di montespertoli e ben determinate nel tempo perchè registrate dai sismografi degli osservatori di p r a t o e di firenze, furono quelle avvenute nelle ore seguenti: l a _ 05" 09 m 17! 2 a — 14 09 37 3 a 18 50 30 4 a 19 04 14 5 a 20 46 25 6 a 20 51 15 7 a 20 51 45 m. d e p a x f i l i s la, terza, cioè quella delle 18h50m30s, raggiunse, nella ristrettissima area epicentrale, un'intensità di y-yi° grado mercalli e può considerarsi la scossa principale del periodo sismico; la sesta f u forte (v grado) e le altre di un'intensità pari al iv agrado mercalli. lo stato di viva preoccupazione già destato al m a t t i n o del 7 giugno negli abitanti di montespertoli, di monte albino e di tresanti, località ove più sensibili risultarono i movimenti tellurici, si trasformò in panico dopo la scossa delle 18h50m30s e t u t t i abbandonarono in massa le case sostando a lungo all'aperto pei' timore di qualche crollo. f o r t u n a t a m e n t e , dopo tale scossa, i movimenti sismici andarono diminuendo d'intensità, con l'unica eccezione di quello avvenuto alle 20h51m15s, e nessun danno si verificò negli edifìci. solo in località villa aliano, situata circa due chilometri e mezzo a sud-est di montespertoli, per effetto della scossa principale si aggravarono nelle m u r a t u r e di u n a f a t t o r i a alcune lesioni preesistenti. f u a p p u n t o in u n a ris t r e t t a area intorno a questa località, che t a l e scossa raggiunse un'intensità compresa f r a il v ed il vi grado mercalli. ivi è da ritenere con probabilità ubicato l'epicentro, cui p e r t a n t o possono essere approssim a t i v a m e n t e assegnate le coordinate geografiche 42°37'20"n e l l ° 0 o ' 5 0 " e . l ' u n i t a cartina mostra la propagazione macrosismica della scossa principale intorno all'epicentro. come da essa risulta, la zona d'intensità f o r t e (v grado) si estende per la maggior p a r t e f r a il torrente virginio, affluente della pesa, e il torrente pesciola, affluente deu'elsa. tale zona comprende la cittadina di montespertoli con le frazioni di monte albino e di tresanti ed ha all'incirca u n a f o r m a ovale con l'asse in direzione ne-sw. con sensibile intensità (iv grado la scossa f u a v v e r t i t a nelle frazioni di poppiano, s. quirico, montegufoni, lucignano, ortimino e cortina, t u t t e del comune di montespertoli. la isosista d'intensità leggera racchiude u n a zona avente ai suoi limiti certaldo, castelfìorentino, ginestra (frazione di lastra a sigila), cei'baia e monteiìridolfl. (frazione di s. caseiano), s. casciano, barberino val d'elsa. la isosista di i i grado, infine, da m011taione sale verso nord, passa alla destra dell'arno includendo carmignano, quindi piega verso est descrivendo un'insenatura che lascia fuori signa e l a s t r a a sigila; indi, dopo aver toccato la periferia meridionale di firenze, volge verso sud, passa ad est d ' i m p r u n e t a e di greve per tornare a montaione comprendendo poggibonsi e gambassi. l ' a r e a della zona ove la scossa si rese sensibile all'uomo risulta p e r t a n t o all'incirca di 900 chilometri quadrati, mentre quella ove essa f u sentita f o r t e m e n t e raggiunge appena u n a venticinquina di chilom e t r i quadrati. l'intensità alquanto forte nella ristrettissima zona epicentrale circondante la località di villa aliano e la modesta propagazione della scossa principale inducono a ritenere che la profondità ipocentrale non sia s t a t a rilevante. l'applicazione della formula dove / , ed / 2 sono le intensità espresse in gradi della scala mercalli alle distanze epicentrali rlt ed r2 conduce i n f a t t i ad un valore della profondità ipocentrale li compreso f r a i 3 e i 5 chilometri. non è forse inutile qui r a m m e n t a r e , che la zona a sud dell'arno, compresa f r a i fiumi elsa e pesa, zona nel cui centro trovasi montespertoli, non è n u o v a alle manifestazioni telluriche. dei diversi focolari sismici ad essa interni, alcuni (come quello di certaldo) sono di secondaria importanza ed i sismi da essi irradiati nel passato furono t u t t i di modesta entità; altri invece, come quello di castelfìorentino, generarono terremoti notevoli per intensità, anche se di propagazione relativamente non molto estesa. particolarmente i m p o r t a n t e per il comune di montespertoli, perchè corocentrico rispetto al suo territorio, è il terremoto d e l l ' i ! settembre 1812, il cui epicentro f u individuato f r a la pesa e il torrente virginio, pochi chilometri ad est di montesper®carmignano jcandicci ,monte,lupo trnpo/i bina fra impruneta ^js nonfagnana'^ / " p / i v '^nkfu/óni'^^^ confina *jj x / / gigliola' 'incigna orfmmoh ( ^ v l j ^ y v / (v °hohkjlbino/^ vài di phja l caifdnovo monfefìridm cashliiorentino taverne!/e monfaione ® cerfaldo barberino di / \ ® val d'elsa gambajii j. donato in poggio n o t e s o p r a u n b r e v e p e r i o d o s i s m i c o , e c c . 239 i s o s i s t e d e l t e r r e m o t o di m o n t e s p e r t o l i d e l 7 v i 1 9 5 8 (i6h50"30') fi km j. gimignano 240 m. d e p a n f i l i s toli. l a linea t r a t t e g g i a t a , nell'unita carta delle isosiste, mostra la posizione, secondo il b a r a t t a (*), della zona epicentrale di quel terremoto. i n tale zona, ove l'intensità f u di v i i i grado mercalli, rimasero rovinosamente colpite le frazioni di poppiano e di 8. quirico in collina. l'energia sismica decrebbe però rapidamente verso ovest sicché montespertoli, nonostante la sua vicinanza all'epicentro, non ebbe a soffrire sensibili danni. ancor più importante, per l'elevatissima sua intensità, è il violento terremoto del 18 maggio 1895. la sua area epicentrale, ove si ebbero crolli e danni gravissimi, f u , è vero, esterna alla zona limitata dai corsi della pesa e dell'elsa, m a le onde sismiche si propagarono v a s t a m e n t e all'intorno con fortissima intensità e montespertoli si trovò inclusa nell'area ove si verificarono lesioni sensibili agli edifici. non è stato possibile stabilire con sicurezza la causa dei recenti movimenti tellurici poiché essi, f o r t u n a t a m e n t e , si sono manifestati con un'energia così esigua da d a r luogo, come sopra si è detto, a pochissime registrazioni sismografiche. non è da escludere però che essi siano in relazione con l'accumulo, nel sottosuolo della zona, di ingenti quantità di gas, la cui esistenza, da tempo indeterminato, si manifesta con esalazioni naturali più o meno abbondanti e che, a d e t t a degli a b i t a n t i del luogo, si sono di molto accentuate d u r a n t e il periodo dei recenti moti sismici. abbiamo pot u t o osservare il fenomeno in u n p u n t o situato poco a nord di montespertoli, in u n a località a t t r a v e r s a t a da u n piccolissimo torrente, tributario del virginio, chiam a t o localmente, forte in relazione al gorgogliamento dell'acqua per effetto dell'emissione gassosa, « borro délvacquabolla ». ivi, al momento del nostro sopraluogo, l'emissione di gas dal terreno era assai copiosa e, in u n a larga pozza, l'acqua ribolliva t u m u l t u o s a m e n t e per lo sprigionarsi di gas dal fondo. si t r a t t a di un gas, o di u n miscuglio gassoso, incoloro, non combustibile né comburente e cosi irritante per le vie respiratorie che, inspirato anche in piccola quantità, provoca u n immediato senso di soffocazione. esalazioni gassose analoghe a quella prossima al « borro della acquabolla » si hanno in numerosi altri luoghi del comune di montespertoli e potrebbe essere interessante effettuare delle indagini per accert a r e la n a t u r a del gas e m a n a t o e la consistenza di eventuali suoi giacimenti nel sottosuolo. riassunto viene riferito su un periodo sismico svoltosi durante il mese di giugno 1958 nella zona del comune di montespertoli, a sud-ovest di firenze. della scossa principale, avvenuta alle 18h55'" circa (t.m.e.o.) elei 7 giugno, viene studiata la propagazione macrosismica e vengono assegnate le coordinate epicentrali. si fa un cenno sull'attività sismica manifestatasi nel passato nella zona di montespertoli. abstract the present is a report on a scismic period tchich tool: place in june 19-58 in the area of the commune of montespertoli, south-west of florence. it covers tlic studi/ of the macroseismic propagation of the principal shock wmcli took place at about 18 hrs 55 min. (e. c. m. t.) on june 7 and allots the epicentral coordinates. a reference is made to seismic activity recorded in the area montespertoli in the past. b i b l i o g r a f i a f1) bakatta m., i terremoti d'italia, f r . b o c ca, t o r i n o , 1901. 40vogt.qxd 907 annals of geophysics, vol. 47, n. 2/3, april/june 2004 in the best tradition of what could be called the jesuit school of seismology (e.g. udías and stauder, 1996), pierre gouin first disentangled complex sources to provide a chef d’oeuvre on ethiopia’s historical seismology (gouin, 1979). back in his native québec he offers us a second essay on a wholly different seismo-tectonic and historical setting. the province of québec has a sparse and low seismic activity, known mostly from the lowlands from the midst of the 17th century on; gouin describes it until 1925, mentioning a «tremor, … the first, in our opinion, of the seismic instrumental era in the province of québec», a rather late beginning indeed. this critical inventory of sources and events is published both in french (even pages) and english (odd pages). until mid 18th century it is based on unpublished or printed primary sources, partly of ecclesiastic origin, sometimes characterized by an «apocalyptic style» and «oratorical convulsions». from 1764 on, information, though often less reliable, is also found in early newspapers. a rigorous methodology is implemented with, for instance, critical comments on catalogues «too often based on secondary sources which are not completely reliable», by means of the example of smith’s catalogue (1962) from the dominion observatory, «the quasi-official catalogue of canadian earthquakes». indeed gouin’s interpretations are sometimes very different, and smith’s magnitude estimates do not escape criticism, since he used an «equation… valid for california, not for québec». while the first pages are introductory and methodological, a catalogue follows, supplying both places affected and some earthquake parameters, such as epicentral intensity (mm scale), epicentral location and magnitude. some methodological considerations are available here and there. gouin uses an arsenal of symbols. so his appraisal of the reliability of the information, a «personal evaluation», is emphasized, with letters «r for real, p for probable, p for possible, d for doubtful, f for false». gouin denounces widespread dogmatism, leading sometimes to astonishing and even ridiculous results. actually he writes «that there is no fixed rule of interpretation for these wordings [used to describe the intensity of earthquakes] and each case is different». while «all known pre-1764 documents were read and annotated», gouin insists on the fact «that the survey is not exhaustive», avoiding any triumphal attitude. review of pierre gouin trémblements de terre «historiques» au québec (de 1534 à mars 1925) identifiés et interprétés à partir des textes originaux contemporains by jean vogt strasbourg, france mailing address: dr. jean vogt, rue du docteur woehrlin 1, 67000 strasburg-robertsau, france. 908 jean vogt this review highlights three events with typical discrepancies in their interpretation. the 1663 sequence, lasting «a minimum of seven months» is the quebec event par excellence. for the main shock gouin proposes an intensity ≥ viii mercalli modified scale and a magnitude of 6.5 ± 0.5. however «the epicentral region is not perfectly known», the one given by smith (1962) being «doubtful». historical sources emphasize major seismo-geological effects. sharper disagreements appear for the 1665 event. former authors had proposed an intensity viii, and different magnitudes, but gouin states that «details are insufficient to determine intensity, an epicentral region and the magnitude». such problems also arise also with the 1732 earthquake. while an intensity ix had been proposed for montreal, for gouin the highest known intensity is no more than «vi ?». instead of an epicentre near montreal, gouin «places the epicentral region in the adirondacks». the bulk of gouin’s book is devoted to «commented documents», with a wealth of remarks on the preceding catalogue, and enriched by maps where the mentioned places are shown, while a complete geographical index is given in appendix 1. in this section gouin also discusses some false events, among which the oldest reported ones (1534 or 1535, 1623), with far-off effects of new england events (1638, 1668). many pages are devoted to the 1663 sequence with a number of systematically discussed testimonies, some of which are dealing with landslides and liquefaction. for one or another event we find more arguments against smith’s proposals. for 1665, according to gouin «the location of the epicentre is given with too much precision to be plausible». and for an event in 1768, relocated in the adirondacks, he says that «the determination of an intensity of vi [mm] and the location of an epicentre are not justified by the primary documents». of great interest is the discussion of the faroff and isolated testimonies from abitibi house post (1811) and even earlier from the hudson bay (1764, 1782). luckily gouin does not discard such minor events, of great seismo-tectonic interest; but in some cases (e.g., 1764 and 1811) he compares the historical evidence with that emerging from the instrumentally located epicentres between 1970 and 1984. long hours should be spent with gouin’s book, not only for its specific achievements, but also for his outstanding contribution to the methodology of historical seismology. references gouin, p. (1979): earthquake history of ethiopia and the horn of africa (international development research centre, ottawa), pp. 259. gouin, p. (2001): trémblements de terre «historiques» au québec (de 1534 à mars 1925) identifiés et interprétés à partir des textes originaux contemporains («historical» earthquakes felt in québec (from 1534 to march 1925) as revealed by the local contemporary literature) (guérin, montreal), pp. 1491. smith, w.e.t. (1962): earthquake of eastern canada and adjacent areas, 1534-1927, publications of the dominion observatory, ottawa, 26 (5), 271-301. udías, a. and w. stauder (1996): the jesuit contribution to seismology, seismol. res. lett., 67 (3), 10-19. layout 6 1 annals of geophysics, 63, 5, pa550, 2020; doi:10.4401/ag-8227 toward the development of a multi parametric system for a short-term assessment of the seismic hazard in italy nicola genzano1, carolina filizzola2, mariano lisi2, nicola pergola2, valerio tramutoli1 (1) school of engineering, university of basilicata, potenza, italy (2) institute of methodologies for environmental analysis of the national research council, tito scalo (pz), italy article history: received october 23, 2019; accepted january 2, 2020 abstract with the aim to develop a multi-parametric system devote to improving our present capability to assess seismic hazard particularly in the short-medium term, the preliminary step is to identify those parameters that have demonstrated their non-casual relation with earthquake occurrences and whose anomalous variations can be associated to the complex seismic process. since ‘80s, fluctuations of earth’s thermally emitted radiation, measured by satellite sensors in the thermal infrared (tir) spectral region, have been reported by several authors in some connection with the occurrence of earthquakes; they can be considered as one of the possible parameters to include within a multi-parametric system. in this paper, the robust satellite techniques (rst) approach has been exploited to highlight significant sequences of tir anomalies (sstas) possibly related to seismic events happened in italy in the period june 2004 december 2014. in particular, we evaluated the level of correlation between occurrence of earthquakes with m≥4 and rst-based tir anomalies using two different spatial resolutions of msg-seviri (meteosat second generation -spinning enhanced visible and infrared imager) images. results of the two rst analyses show that the 82% of the identified sstas does not disappear when downscaled spatial resolution seviri tir records were used. for both analyses, more than 60% of sstas is apparently in connection with the occurrence of m≥4 earthquakes; more than the 80% of them has tendency to anticipate the occurrence of earthquakes. although about the 40% of sstas is not apparently related to documented seismic activity (false positives), results of random tests (i.e. molchan’s error diagrams) indicate a non-casual correlation between sstas and earthquake occurrences. these results confirm that the parameter “rst-based satellite tir anomalies” is one of the possible candidates to be included in a multi-parametric system for time-dependent assessment of seismic hazard (t-dash). keywords: satellite thermal anomalies, earthquakes, correlation analysis, seismic hazard assessment, multi-parametric system. 1. introduction the occurrence of even destructive earthquakes in areas that until then were not considered at high seismic hazard, on the basis of probabilistic maps, highlighted the weakness of approaches such as the probabilistic seismic hazard analysis [psha; cornell, 1968], widely used for almost 50 years but never validated by objective testing [mulargia et al., 2017]. events of sumatra (26 december 2004, mw=9.1), kashmir (8 october 2005, mw=7.6), wenchuan (12 may 2008, mw=7.9), haiti (12 january 2010, mw=7.0), and tohoku (11 march 2011, mw=9.0) are only the most striking examples of understated seismic hazard [geller, 2011; kossobokov and nekrasova, 2012; stein et al., 2012; kagan and jackson, 2013] which caused heavy human and economic losses. the increasing awareness that seismic hazard cannot be assessed exclusively through such traditional approaches has progressively led to exploring new ways. integration of multi-parametric observations seems to be the direction that scientific community is recently taking in the attempt to improve our present capability to assess seismic hazard particularly in the short-medium term. building up a system based on continuously updated observations (i.e. timedependent), provided that they are adequately selected and analysed, may represent a hopeful research line to actually exploit accumulated knowledge and improve quality of seismic hazard. isolated attempts of multi-parametric approaches were done during specific conferences (e.g. the 2018 workshop on electro-magnetic studies of earthquakes and volcanoes; emsev 2018 website, 2018) or within the framework of international research projects such as eu-fp7 pre-earthquakes (pre-earthquakes project website, 2019) and istep (integrated search for taiwan earthquake precursors [tsai et al., 2006]). increased reliability and precision can be expected from multi-parametric t-dash (time-dependent assessment of seismic hazard [tramutoli et al., 2014]) systems. in fact, considering just the intersection of space-time volumes individually alerted by each parameter, a significant reduction of both, the (combined) probability of false alarms and alerted space-time volumes can be achieved. seismic hazard levels associated to specific space-time volumes can be then systematically updated on the basis of the specific alerted space-time windows and predictive capabilities of the considered parameters. however, the selection of suitable (chemical, physical, biological …) parameters to be integrated within such a t-dash system is not a trivial task. in principle, a candidate parameter suitable for integration in a t-dash system should: a) be selected on the basis of experimental observations and/or convincing physical models (e.g. scholz et al., [1973]; tronin, [1996]; freud, [2007a]; pulinets and ouzounov, [2011]; huang, [2011b]; tramutoli et al., [2013]) that could justify their appearance within the complex process of an earthquake preparation phase; b) be measurable with sufficient space-time continuity; in this context, satellite-based observations may have a great potential because they guarantee information over large areas (up to a global scale) in a quite continuous way. c) exhibit space-time transients (potentially related to eqs) identifiable through clear, scientifically founded and repeatable, data analysis techniques. in fact, even if a huge amount of scientific papers reports the appearance of variations of geophysical parameters (e.g. tronin, [2006] and reference therein; cicerone et al., [2009]) in some relation with the occurrence of seismic events, used methodologies and the definition itself of “anomaly” concept are not always clearly stated. d) show a non-occasional relation between space-time transients and earthquake occurrence, which should be preliminarily established by specific long-term (plurennial) correlation analyses. an actual correlation with earthquake characteristics (e.g. time, position, magnitude) can be appropriately evaluated only if long-term correlation analyses are performed. only from such analyses it will be possible to quantitatively characterize the predictive capabilities (in terms of successful forecasts, false positive rates and/or missed events) of each parameter with reference to specific regions, alerted space-time volumes, expected earthquake magnitudes, etc. good examples of such long-term correlation analyses are represented by studies concerning variations of the plasma frequency at the f2 peak fof2 of ionosphere during 1994–1999 [liu et al., 2006], ultra low frequency (ulf) geomagnetic signals over a 10-year period [han et al., 2014], ionospheric ion density recorded by demeter satellite during more than 6 years [li and parrot, 2013] and, more recently, earth’s thermally emitted radiation [eleftheriou et al., 2016]. e) provide an appreciable improvement of the eq forecast capability of the whole t-dash system. with all that in mind, in this paper we consider the earth thermally emitted radiation, measured by satellite sensors in the thermal infrared (tir) spectral region, as a suitable candidate for a t-dash system. since the ‘80s, several nicola genzano et al. 2 authors [gorny et al., 1988; qiang and dian, 1992; tronin ,1996; qiang et al., 1997; tronin et al., 2002; ouzounov and freud, 2004; choudhury et al., 2006; ma et al., 2010; tramutoli et al., 2015a, 2018a; and reference therein] have been reporting tir anomalous transients (hereafter “tir anomalies”) in some connection with the occurrence of earthquakes. different physical models have been invoked to explain their appearance before and even after [scholz et al., 1973] a seismic event. among them, it is worth mentioning the increase of greenhouse effect due to an anomalous degassing of optically active gases such as co2 and ch4 [qiang et al., 1991; tramutoli et al., 2001, 2005, 2013; tronin, 1996; zhang et al., 2010], the atmospheric warming after a massive radon emission (lithosphere–atmosphere– ionosphere coupling (laic) model; [pulinets and boyarchuk, 2004; pulinets, 2004, 2006; yasuoka et al., 2006; pulinets and ouzounov, 2011]), and the activation of positive holes in stressed rock volumes [ouzounov and freund, 2004; freund, 2007a, 2007b; freund et al., 2006, 2007; wu and liu, 2009; wu et al., 2012]. a quite long list of works [see detailed reviews in tramutoli et al., 2015a, 2018a; jiao et al., 2018] exists where different satellite-based approaches, many of which have been object of criticism, try to highlight tir anomalies in some relation with seismicity. these methodologies use satellite data such as those acquired by the advanced very high resolution radiometer (avhrr) carried on the national oceanic and atmospheric administration (noaa) satellites (e.g. tronin, [1996]), the meteosat visible and infrared imager (mviri) on the meteosat first generation (mfg) platforms [qiang et al., 1997], the japanese geostationary meteorological satellite (gms) [xu et al., 2000], the multifunctional transport satellite (mtsat-1r) [yang and guo, 2010], the chinese feng yun 2c [zhang et al., 2010] and 2e [xie et al., 2013], the moderate resolution imaging spectroradiometer (modis) aboard the earth observing system (eos) [e.g. ouzounov and freund, 2004], and the spinning enhanced visible and infrared imager (seviri) on board the meteosat second generation (msg) satellites [e.g. piroddi and ranieri, 2012] as shown in several quoted papers [e.g. tramutoli et al., 2001, 2005, 2009, 2015b, 2018b; filizzola et al., 2004; di bello et al., 2004; corrado et al., 2005; aliano et al., 2007, 2008a, 2008b; genzano et al., 2007, 2009a, 2009b, 2015; lisi et al., 2010,2015; pergola et al., 2010; eleftheriou et al., 2016], differently from other approaches, the rst [tramutoli, 1998, 2005, 2007] methodology is able to isolate residual tir variations which are potentially related to seismic events from normal tir fluctuations due to other causes. this is done following a well-defined procedure, based on a statistical definition of what a tir anomaly means, which has been reproduced also in independent studies [e.g. eneva et al., 2008; halle et al., 2008; blacket et al., 2011; okyay 2012; xiong et al., 2013; khalili et al., 2019]. the application of the rst technique to tens of earthquakes, occurred in four different continents and geo-tectonic contexts, with magnitudes ranging from 4.0 to 7.9, using both polar (noaa-avhrr, eos-modis) and geostationary (mfg-mviri, msg-seviri, goes-imager, mtsat-imager) satellite data, permitted us to find correlations between tir anomalies and seismic event occurrence, mainly selecting a space window based on the dobrovolsky et al. [1979] distance and a time window ranging from 30 days before to 15 days after a seismic event. at the same time, such longterm correlation analyses allowed us to highlight the appearance of spurious tir anomalies, for example, in case of quite rare events which are able to increase (for more than 1 day) measured tir signal due to an increase of surface temperature (e.g. extended forest fires) or emissivity (e.g. very large floods). however, it is expected that large-scale phenomena (e.g. earth degassing) continue to produce tir anomalies also in large size pixels, while small-scale events (e.g. forest fires) can give a decreasing contribution to the tir signal as the pixel size increases. in this paper, we evaluate the level of correlation between occurrence of earthquakes with m≥4 and rst-based tir anomalies using two different spatial resolutions of seviri images. in particular, the same tir seviri time series (from june 2004 to december 2014) is analysed at full seviri pixel resolution as well as at a downscaled spatial resolution (3 × 3 pixel box). results will be compared and random tests (molchan diagram) will be performed in both cases to assess the lack of randomness in the appearance of rst-based tir anomalies. 2. data analysis 2.1 rst methodology and retira index the method proposed by tramutoli et al. [2001], [2005] , called rst (robust satellite techniques) is mostly based on the general approach named rat (robust avhrr technique; tramutoli, [1998]). being the rat methodology exclusively based on satellite data at hand (no ancillary data are requested), it is intrinsically exportable on different 3 to develop a multi parametric system for short-term sha in italy nicola genzano et al. 4 satellite packages, reason why the original name rat was changed in the more general rst [tramutoli, 2005; 2007]. the rst approach is devoted to isolating possible anomalies from those signal variations, that are related to known, but also unknown, natural and/or observational factors [e.g. surface spectral emissivity, temporal variations of satellite view angle, etc.; see tramutoli et al., 2005], which can be responsible for “false alarm” proliferation. it is based on a multi-temporal analysis of a historical dataset of satellite observations, which were acquired in similar observational conditions (e.g. same month of the year, same hour of the day, same sensor, etc.), devoted to characterizing the measured signal (in terms of its expected value and variation range) for each pixel of the satellite image to be processed. in this way, space-time anomalies can be identified by comparing the measured signal, in the geographical position r at time t’, with a preliminarily computed signal behavior. in order to identify abnormal fluctuations of the measured signal (i.e. anomalies), the general index alice (absolutely llocal index of change of the environment)1 has been proposed for the first time by tramutoli [1998] and it is reported below: (1) where v(𝒓,𝑡') is the value of a variable v at location 𝒓, and at acquisition time 𝑡', 𝜇� (𝒓) and 𝜎� (𝒓) are respectively the expected value (usually the time average) and the standard deviation of v(𝒓,𝑡') computed on cloud-free records belonging to the selected dataset. by this way, the alice index provides a statistically well-based definition of local signal anomalies. moreover, the intensity of anomalies can be evaluated in terms of signal to noise ratio (s/n) because v(𝒓,𝑡')‒𝜇� (𝒓) represents the signal (s) to be investigated for its possible relation with studied phenomena and 𝜎� (𝒓) represents its natural and observational noise (n), as historically observed at the same site in similar observational conditions. in order to better characterize the analyzed signal a historical, homogeneous (same platform, period of the day, period of the year, etc.) and long-term dataset of satellite images should be constructed. in fact, selecting satellite images acquired in the same period of the year and in the same hour of the day (e.g. midnight), it is expected a reduction of the signal variability linked to the daily (diurnal variation of the temperature) and annual (seasonal variation of the temperature and emissivity, which is mainly related to the different vegetations coverage) solar cycle. moreover, in the case of studies on earth’s thermally emitted radiation, using only nighttime images, which are less influenced by effects related to soil-air temperature differences, normally higher during other hours of the day, and less sensitive to local solar exposition, further elements of variability of the signal (independent of the seismicity) can been reduced. in order to identify possible tir anomalies associated to seismic activity, the retira (robust estimator of tir anomalies) index has been used. the retira index belongs to the most general class of the alice indexes, it has been introduced for the first time by filizzola et al. [2004] and subsequently has been always applied in all rst applications [tramutoli et al. 2005, 2015b, 2018b; corrado et al., 2005; aliano et al., 2007, 2008a, 2008b; genzano et al., 2007, 2009a, 2009b, 2015; lisi et al., 2010; pergola et al., 2010; eleftheriou et al., 2016] devoted to the thermal monitoring of seismic areas. it can be computed as follows: (2) where: • 𝒓≡(x, y) represents location coordinates on a satellite image; • 𝑡' is the time of acquisition of the satellite image at hand, with 𝑡∊𝜏 where 𝜏 defines the homogeneous domain of satellite imagery collected in the same time-slot (hour) of the day and period (month) of the year; • 𝛥𝑇(𝒓,𝑡')=𝑇(𝒓,𝑡')–<𝑇(𝑡')> is the difference between the current (𝑡=𝑡') tir signal 𝑇(𝒓,𝑡') measured at location 𝒓, 1 since tramutoli [1998] the double l is used to make reference not only to a specific place 𝒓 but also to a specific time 𝑡'. ⨂�(𝒓,𝑡')= v(𝒓,𝑡')‒𝜇� (𝒓) 𝜎� (𝒓) ⨂��(𝒓,𝑡')= [𝛥𝑇(𝒓,𝑡')‒𝜇�� (𝒓)] 𝜎�� (𝒓) and its spatial average <𝑇(𝑡')>, computed in place on the image at hand, discarding cloudy pixels and considering only sea pixels, if 𝒓 is located on the sea, only land pixels, if 𝒓 is located over the land; • 𝜇𝛥𝑇(𝒓) and 𝜎𝛥𝑇(𝒓) are the time average and standard deviation values of 𝛥𝑇(𝒓,𝑡), at location 𝒓, computed on cloud-free satellite records belonging to the selected homogeneous dataset (𝑡'∊𝜏). the choice of such a differential variable 𝛥𝑇(𝒓,𝑡') instead of 𝑇(𝒓,𝑡') is expected to reduce possible contributions (e.g. occasional warming) due to day-to-day and/or year-to-year climatological changes and/or season time-drifts. as proposed by eleftheriou et al., [2016], a further reduction of signal variability due to particular meteorological conditions, such as the extensive cloudiness and the presence of an asymmetrical distribution of clouds within the analyzed scene, could be achieved selecting only the satellite images that are not affected by these conditions. in this study, the same criteria adopted in the study of eleftheriou et al., [2016] have been taken in account. briefly, land (or sea) portions of the tir images that are excessively covered by cloud coverage (i.e. ≥ 80% of the total land/sea pixels) or affected by the cold spatial average effect [aliano et al., 2008a; genzano et al., 2009a] have been removed from the datasets construction, and consequently excluded from the computation of the rst reference fields (i.e. time average and standard deviation). land (or sea) tir portions possibly affected by the cold spatial average effect are identified on the basis of the following expression 𝑇(𝑡') ≤ 𝜇𝑇 2𝜎𝑇 (where 𝑇(𝑡') is the tir spatial average computed only on cloud-free pixels of the scene acquired at time t’ and belonging to the same land/sea class, 𝜇𝑇 and 𝜎𝑇 are respectively the temporal average and standard deviation of 𝑇(𝑡'), which are computed using all tir images of the homogeneous dataset. in this work, with the aim to reduce the proliferation of tir anomalies due to sporadic and localized events (for example due to fires or to industrial accidents), and concurrently trying to preserve and emphasize tir anomalies with an appreciable spatial extension, such as tir anomalies possibly related to seismic activities, a new index called retirabox has been introduced. for each image, which represents the differential variable 𝛥𝑇(𝒓,𝑡'), a spatial filter has been applied. taking in account the schematic representation of figure 1, every three pixels starting from the pixel at position row r2 and line l2, the spatial mean value has been computed only if 5 out of 9 pixels within a region of 3 × 3 pixels centered on the considered pixel were not affected by clouds. as a result, a downgrade of the image spatial resolution has been obtained2. on this basis, the retirabox index can be computed as follows: (3) where: • 𝛥𝑇box(𝒓,𝑡') is the spatial average of 𝛥𝑇(𝒓,𝑡') within a region of 3 × 3 pixels centered at location 𝒓. it is computed only if 5 out of 9 pixels are clear or not close to clouds; • 𝜇𝛥𝑇box(𝒓) is the time average value of 𝛥𝑇box(𝒓,𝑡') at the location 𝒓 computed on cloud-free records belonging to the selected data set (𝑡'∊𝜏); • 𝜎𝛥𝑇box(𝒓) is the standard deviation value of 𝛥𝑇box(𝒓,𝑡') at the location 𝒓 computed on cloud-free records belonging to the selected data set (𝑡'∊𝜏). note that only cloud-free radiances have been taken in account in the rst elaborations. so, cloudy pixels, which have been identified by oca (one channel cloudy radiance detection approach; cuomo et al., [2004], and the first neighboring pixels around them, which can be partially affected by clouds (edge clouds) which are not identified by the cloud detection analysis, have been excluded. 2 the pixel size of seviri image over italian region is averagely 3.5 (longitude) x 4.5 (latitude) km2. after the application of the spatial filter the size of new images is ~ 10.5 x 13.5 km2. ⨂𝛥𝑇box(𝒓,𝑡')= [𝛥𝑇box(𝒓,𝑡') ‒ 𝜇𝛥𝑇box (𝒓)] 𝜎𝛥𝑇box (𝒓) 5 to develop a multi parametric system for short-term sha in italy 2.2 significant sequence of tir anomalies (ssta) and their possible relations with earthquake occurrence in this study, nighttime (i.e. time slot 00:00-00:15 gmt) images acquired by the sensor seviri (spinning enhanced visible and infrared imager) on board msg (meteosat second generation) satellite in the thermal infrared (tir) spectral range (i.e. ir10,8 channel at 9,80-11,80 μm) from june 2004 to december 2014 over the italian peninsula (top-left 47.07°n-6.08°e; top-right 47.7°n-22.4°e; bottom-right 35.9°n-18.05°e; bottom-left 35.6°n-4.9°e) have been organized in monthly reference datasets and used to compute the reference fields. both indexes, i.e. retira and retirabox, have been computed for all tir images belonging to the msg/seviri datasets, producing in this manner daily thermal anomaly map (tam) to be analyzed. in addition to thermal anomalies (tas) possibly related to seismic activities, other natural and/or observational conditions could generate a proliferation of tas in the tams, such as night-time cloud passages and errors in image navigation/co-location processes [aliano et al., 2008a]. in both cases, these tas have a tendency to vanish with the time and/or to be restricted in some geographical regions (i.e. along the coastlines). this particular character allows us to distinguish them from tas possibly related to an impending earthquake, which are spatially and temporally persistent [tramutoli et al., 2005]. like the previous rst applications to satellite thermal monitoring of seismic areas, a space-time persistence analysis has been carried out with the aim to qualify tas highlighted by the rst methodology as significant thermal anomalies [stas; eleftheriou et al., 2016; genzano et al., 2015; tramutoli et al., 2015b], which could be part of a significant sequence of thermal anomalies (ssta). so, each single location (𝒓) has been considered (at the time of observation 𝑡') to be part of an ssta, if it nicola genzano et al. 6 figure 1. schematic representation of the applied spatial filter. dashed lines represent the original full resolution seviri pixels; instead, continuous lines the downscaled resolution seviri pixels. every three pixels the spatial mean value has been computed only if 5 out of 9 pixels within a region of 3 × 3 pixels centered on the considered pixel were not affected by clouds. satisfies the following requirements: • relative intensity: the ta should have a value of the rst-based index3 ≥ k (in this study k=3.5); • spurious effects are discarded: portions (i.e. land or sea) of the tams affected by a wide cloudy coverage (in this study we considered useful only land/sea portions of the scene having a fraction of cloudy pixels less than 80%), navigation errors [filizzola et al., 2004] and known spurious effects [e.g. cold spatial average effect, in this study identified by this condition 𝑇(𝑡) ≤ 𝜇𝑇 2𝜎𝑇; see aliano et al., 2008a; genzano et al., 2009a, 2015; eleftheriou et al., 2016 for more details] are discarded from the subsequent analyses; • persistence in space-time domain: the existence of a group of stas which cover at least 150 km2 within an area of 1×1° around 𝒓 (spatial persistence) and reappear at least one more time in the 7 days preceding/following t (temporal persistence). the possible space-time relations among the appearances of sstas and time, location and magnitude of earthquakes4 occurred in the same space-time period have been evaluated applying empirical rules. taking in account both the existing genetic models up to now proposed to explain the origin of tas [see for example tramutoli et al., 2013] and the results reported in the previous studies [a summary is reported in tramutoli el al., 2015, 2018a], each single sta observed at the time t in the location (𝒓) will be considered possibly related to seismic activity if: • it belongs to a previously identified ssta; • an earthquake of m≥4 occurs 30 days after its appearance or within 15 days before (temporal window); • an earthquake with m≥4 occurs within a distance d, from the considered sta, where d=rd being rd=100.43m the dobrovolsky et al., [1979] distance (spatial window). on the basis of the previous discussion, it is assumed d=150km for m≤5. 3. results figure 2 reports a comparison between the tams obtained by the two rst analyses. in both cases, the tas that are not due to spurious effects, with a spatial extension and temporal continuity suitable for classifying them as stas belonging to an ssta, appear in the central part of italy on march 30 and march 31, 2009. looking at figure 2, it is possible to note that: • moving from tir images at full resolution to downscaled tir images, tas indentified by using the retira or the retirabox index with the same s/n ratio (i.e. s/n≥3.5) appear in the same regions (on march 30 over the eastern part of abruzzo/marche region and on march 31 over the central apennines). moreover, in the tams reporting the values of the retirabox index (i.e. tams on the right part of figure 2) a more diffuse presence of stas with an s/n≥4.5 can be noted. both circumstances indicate that the applied spatial filtering does not produce a significant loss of information, but it preserves and emphasizes (in terms of s/n) those stas with an appreciable spatial extension (i.e. ≥ 150 km2); • several earthquakes with magnitude greater than 4 (including the l’aquila earthquake of magnitude mw~6.3 happened on april 6, 2009) occurred inside the spatial window of 150km from the stas and in the temporal window march 28, 2009 april 23, 2009. by applying the aforementioned rules to all tams obtained by applying the rst methodology and computing both the traditional retira index and the new proposed retirabox index, in the period june 2004 december 2014, 33 sstas have been identified by using the retira index and 28 sstas using the retirabox index (table 1). it is possible to note that 27 of 33 sstas identified by using the retira index are also identified by using the retirabox index. the remaining 6 sstas disappear when retirabox is used because stas do not satisfy the requirements of relative intensity and/or persistence in space-time domain; on the contrary only 1 ssta has been identified by using retirabox but it is not is identified by using the traditional retira index. 3 the retira index in the case of seviri tir images at full resolution; retirabox in the case of seviri tir downscaled images using a 3 × 3 pixel box. 4 as reported in the iside (italian seismological instrumental and parametric database) earthquake catalogue (iside working group, 2016) from may 15, 2004 to january 30, 2015 in the area top-left 48.07°n-5.08°e, top-right 48.7°n-23.4°e, bottom-right 34.9°n-19.05°e, bottom-left 34.6°n-3.9°e 656 events of m≥4, 38 events of m≥5and 1 event of m≥6. 7 to develop a multi parametric system for short-term sha in italy nicola genzano et al. 8 figure 2. comparison between thermal anomalies maps (tams) obtained by the two rst analyses. on the left side, a significant sequence of thermal anomalies (ssta) identified by applying the rst approach on seviri tir images at full resolution, while on the right side the corresponding ssta identified by apply the rst approach on seviri tir images at the downscaled resolution. in both cases, tir anomalies (s/n≥3.5) are depicted in different colors according to the retira (or retirabox) index values and clouds are depicted in gray. contours in different colors correspond to different space/magnitude windows (see text) and stars correspond to earthquakes. significant tir anomalies (i.e. not due to spurious effects, with spatial extension and temporal continuity) appear in the central part of italy on march 30 and march 31, 2009, respectively 7 and 6 days before l'aquila earthquake (mw~6.3) happened on april 6, 2009. 9 to develop a multi parametric system for short-term sha in italy table 1. summary of the correlation analyses. id place period (from-to) rst and retira rst and retirabox ssta eqs mag (time lag fta) ssta eqs mag (time lag fta) 1 south italy (campania) aug 21-25,2004 y y 4 (+13) n 2 north-east italy dec 1-3, 2004 y n n 3 central italy (marche) dec 3-10, 2004 y y 4.2 (0), 4.5 (+1), 4.3 (+6), 4.2 (+14), 4.4 (-4), 4.3 (-4), 4.9 (-8), 4.4 (-8), n. 2 4.1 (-8) y y 4.2 (0), 4.5 (+1), 4.3 (+6), 4.2 (+14), 4.4 (-4), 4.3 (-4), 4.9 (-8), 4.4 (-8), n. 2 4.1 (-8) 4 north africa (tunisia) mar 24-26, 2005 y n y n 5 south italy (sicily) may 11-14, 2005 y n y n 6 north africa (algeria) aug 10-18,2005* aug 10-11,2005 y y 4.2 (+27) y* y 4.2 (+27) 7 south italy (calabria) aug 12-13,2005 y y 4.5 (+2), 4 (-9) y y 4.5 (+2) 8 north africa (tunisia) april 1-2,2006 y n y n 9 south italy (sicily/calabria) may 23-27, 2005 y y 4.2 (+7), 4 (+21), 4.5 (+30), 4.1 (-3) y y 4.2 (+7), 4 (+21), 4.5 (+30), 4.1 (-3) 10 sea of sicily jun 22-23, 2006 y n y n 11 north africa jun18-26, 2007 y y 4 (-9), 4.1 (-15) y y 4 (-9), 4.1 (-15) 12 south italy (apulia/calabria) jun 22-28, 2007 y y 5.2 (+12), 4 (+30) y y 5.2 (+12) 13 balkan region july 23-25, 2007* july 22-25, 2007 y y 4.1 (+16), 4 (+28), 4.2 (-5) y* y 4.1 (+17), 4 (+29), 4.2 (-4) 14 south italy aug 19 sept. 1, 2007 y y 4.1 (+34), 4 (-1) y y 4.1 (+34), 4 (-1) 15 south italy (sicily) jun 17-18, 2008 y y 4.2 (+16) y y 4.2 (+16) 16 south italy (calabria) aug 9-16, 2008 y n n 17 s. italy (st of messina) sep 9-15, 2008 y y 4 (+18) y y 4 (+18) 18 south italy (campania) jan 2-9, 2009 y y 4 (-3) n 19 central italy (abruzzo) mar 30-31, 2009 y y 4 (0), 4.5 (+6), 4 (+6), 6.3 (+7), 5 (+7), 4.8 (+7), 4.7 (+7), n. 2 4.3 (+7), n.3 4.2 (+7), n. 4 4.1 (+7), 4 (+7), 5.4 (+8), 4.9 (+8), 4.3 (+8), n.2 4.1 (+9), 5.2 (+10), 5 (+10), 4.2 (+10), 4.1 (+10), n.2 4 (+10), 4 (+11), 4.8 (+14), 4.1 (+15), 4.2 (+24), 4 (+24), 4 (-1), 4 (-2) y y 4 (0), 4.5 (+6), 4 (+6), 6.3 (+7), 5 (+7), 4.8 (+7), 4.7 (+7), n.2 4.3 (+7), n.3 4.2 (+7), n. 4 4.1 (+7), 4 (+7), 5.4 (+8), 4.9 (+8), 4.3 (+8), n.2 4.1 (+9), 5.2 (+10), 5 (+10), 4.2 (+10), 4.1 (+10), n.2 4 (+10), 4 (+11), 4.8 (+14), 4.1 (+15), 4.2 (+24), 4 (+24), 4 (-1), 4 (-2) 19bis balkan region mar 30-31, 2009 n y y 4.1 (+1), 6.3 (+7) 20 adriatic sea may 26-28, 2009 y y 4.8 (+26), 4.4 (+27), 4 (+28) y y 4.8 (+26), 4.4 (+27), 4 (+28) 21 sardinia jul 23-24, 2009 y n y n 22 central italy jul 24-25, 2009 y y 4 (+13), 4.2 (-2) y y 4 (+13), 4.2 (-2) 23 south italy jul 25-26, 2009 y n y n 24 north africa jul 23-29, 2009 y n y n 25 north africa feb. 28-mar 3, 2010 y n y n 26 ligurian sea may 25-27, 2011 y n y n 27 balkan region jul 15-20, 2011 y y 4.1 (-1) n 28 north africa (algeria) may 18-19, 2012 y y 4 (+16) y y 4 (+16) 29 balkan region sep 1-3, 2012 y n n 30 north africa sep 22-23, 2012 y y 4.6 (+18) y y 4.6 (+18) 31 balkan region sep 25-30, 2012 y y 4.4 (+13) y y 4.4 (+13) 32 south italy (sicily) apr 29 may 5, 2013 y n y n 33 north africa oct 4-6, 2013 y y 4.4 (+14) y n 33 ssta 20 y 13 n 60.6 % y 39.4 % n 28 ssta 17 y 11 n 60.7 % y 39.3 % n as announced in the previous paragraph, the possible relation between the highlighted sstas and occurred m≥4 earthquakes has been verified on the basis of pre-established rules. the long-term correlation analysis highlights that in both rst analyses more than 60% of sstas is in a space-time relation with the occurred earthquakes, with a remain ~40% of sstas apparently not related to documented seismic activity. moreover, looking at figure 3, it is possible to note that more than the 80% of sstas associable to earthquakes has tendency to anticipate the occurrence of earthquakes (more than the 50% of them appears only before earthquakes and not before and after). with the aim to reveal possible periodicities and characteristics between seismic events and the sstas identified by the rst analysis and the retirabox index in italy, superposed epoch analysis (sea) has been performed. here, the sea approach has been implemented by comparing the time of the occurrences of earthquakes associable to sstas (i.e. that satisfy the used empirical rules) with the time of the occurrences of first sta (fsta) appearances. here, clusters of earthquakes have been considered as daily single events and the temporal interval between earthquake occurrence and fsta has been aggregated on the basis of two day counts. moreover, in order to verify particular tendency of different types of earthquake (i.e. events due to normal faulting or due to reverse faulting), results of the sea analysis have been differentiated on the basis of earthquake focal mechanisms. the type of faulting of those earthquakes that show some space-time relations with identified sstas has been established consulting the italian cmt [centroid moment tensor; pondrelli et al., 2006] dataset for italy and the global cmt catalogue [dziewonski et al., 1981; ekström et al., 2012] and the european-mediterranean rcmt catalogue [pondrelli et al., 2002] for balkan and north africa regions. for the seismic events where moment tensor solution is not computed, the type of faulting has been established in accord with the local tectonic regime of the specific geographical region where earthquake has happened, on the basis of seismic source zone model zs9 proposed by meletti et al., [2008]. results of the sea analysis shown in figure 4 highlight that more than 45% of considered earthquakes occurs within a temporal window of two weeks after the first appearances of the significant thermal anomalies and that the 50% of them is due to normal faulting, which corresponds to the 66% of all events due to normal faulting. nicola genzano et al. 10 figure 3. distribution of the sstas compared to earthquake occurrences. for both analysis, more than the 80% of sstas associable to earthquakes has tendency to anticipate the occurrence of earthquakes. in order to verify the non-causality of achieved results, an adapted molchan error diagram approach [molchan 1990, 1991, 1997; molchan and kagan, 1992] has been performed. as proposed in the study of eleftheriou et al. [2016], molchan error diagram was implemented by plotting the fraction of missed earthquakes (i.e. apparently not preceded/followed by sstas) n(m) against the fraction of alerted space-time volume τ(m), which were computed as in the following expressions: number of eqs with magnitude ≥ m outside the correlation window (missed) 𝜈(m) = total number of eqs with magnitude ≥ m occurrred within the whole space × time investigated volume alerted space × time volume for eqs with magnitude ≥ m 𝜏(m) = whole investigated space × time volume figure 5 reports the molchan error diagrams for both long-term correlation analyses. the left side of figure 5 shows the results achieved taking in account sstas identified by using the retira index, while on the right side there are the results obtained considering the sstas identified by using the retirabox index. in addition, for classes of magnitude m≥4 and m≥55, confidence limit curves surrounding the diagonal of random guess have been computed and plotted. in this study, confidence levels of 95% are used to indicate “significant” deviation from random guessing based on the null-hypothesis [see kossobokov 2006 for more details]. taking in mind that a proliferation of missed events could occur, due to large and persistent meteorological clouds, as well as the data missing, which prevent from fully appreciating and monitoring with continuity the occurrence of all possible stas, looking at figure 5 it is possible to note that a non-casual correlation between sstas and earthquakes exists. in fact, by comparison with random guess line (i.e. diagonal of molchan diagram), both in the case of earthquakes 5 confidence limit is not computed for m≥6 because only 1 event does not allow us to attribute any statistical significance. 11 to develop a multi parametric system for short-term sha in italy figure 4. superposed epoch analysis (sea) between the sstas identified by the rst analysis and the retirabox index and the seismic events occurred in the investigated region and period. results have been differentiated on the basis of earthquake focal mechanisms (red indicates earthquakes due to normal faulting, blue earthquakes due to reverse faulting and green earthquakes due to not predominant faulting type, i.e. oblique). that follow the sstas (full circles/triangles) and in the case of earthquakes that follow or precede the sstas (empty circles/triangles), all considered strategies (i.e. various classes of earthquake magnitude and spatial correlation windows) are in the optimal strategy zone (i.e. below the random guess line). in both cases, achieved statistics do not reject the null-hypothesis of random guessing at the significant level 𝛼=5% (i.e. the accepted significance level for this kind of study). in fact, in this study it is 𝛼=14,8% in the case of analysis at full resolution and 𝛼=17,2% in the case of analysis at downscaled resolution. finally, although the measurement of the probability gain (g) is not trivial [see molchan, 1991], an attempt to estimate g has been done following the aki’s paper [aki, 1989]. results highlight that a probability gain, which was computed as g=(1-𝜈)/𝜏) up to 4,9 (for m≥5 earthquakes), can be achieved if only pre-seismic sstas identified by using the retira index are considered, and up to 6.26 for the same classes of earthquake magnitude but considering sstas identified by using the retirabox index. in comparison with results achieved by using the traditional retira index, when the retirabox is used to identify stas: • more than 81% of sstas survives; • more than 60% of sstas is apparently in connection with the occurrence of m≥4 earthquakes; • an improvement in terms of probability gain up to 6.2 as compared to random guess is achieved. 4. conclusion with the aim to define a multi-parametric system able to improve our capabilities to provide short-term (from days to weeks) seismic hazard forecasting, in this paper we have studied the potentiality and the capabilities of the 6 it should be noted that for magnitudes greater than 6 a higher probability gain is achievable, but the little data sample does not allow us any statistical significance to be attributed to it. nicola genzano et al. 12 figure 5. on the left side, molchan error diagram analysis computed for different classes of magnitude and ssta identified by applying the rst approach on seviri tir images at full resolution on the whole study period (2004–2014), while on the right side the corresponding molchan error diagram computed to ssta identified by applying the rst approach on seviri tir images at downscaled resolution. full circles and full triangles refer to earthquakes occurred only after the appearances of sstas (pre-seismic anomalies). empty circles and empty triangles refer to earthquakes occurred before or after the appearances of sstas (earthquake-related anomalies). red line and blue line indicate the 95% confidence limit curves surrounding the diagonal of random guess for earthquakes with magnitudes greater than 4 and 5, respectively. parameter “earth’s thermally emitted radiation measured by satellite sensors in the thermal infrared (tir) spectral region” to provide useful information about the seismic process. to this purpose, on the basis of 11 years of msg/seviri tir records collected over italian peninsula, we have evaluated the level of correlation between the occurrence of earthquakes with magnitude greater than 4 and satellite tir anomalies highlighted by robust satellite techniques (rst). moreover, for the first time, with the aim to reduce the proliferation of spurious tir anomalies, due to other causes independent from the seismic activity (e.g. extended forest fires or very large floods) an ad-hoc version of the retira index (i.e. retirabox) was presented. to evaluate its accuracy, the same tir seviri time series (from june 2004 to december 2014) has been analysed computing both the retira and the retirabox index. results of the two rst analyses have shown that the 82% (i.e. 27 of 33) of the identified sstas does not disappear when a downscaled spatial resolution seviri tir records was used. these circumstances confirm that the low spatial resolution is not a constraint to monitor possible preseismic thermal anomalies; in fact, large-scale phenomena (e.g. earth degassing) continue to produce tir anomalies also in larger size pixels. results of correlation analyses performed applying pre-established rules highlight that in both the rst analyses: • more than 60% of sstas is in a space-time relation with the occurred earthquakes, with a remaining ~40% of sstas apparently not related to documented seismic activity (false positives); • more than the 80% of sstas associable to earthquakes has tendency to anticipate the occurrence of earthquakes, with a 50% of them appearing only before earthquakes and not before and after the seismic events; • superposed epoch analysis indicates that more than 45% of earthquakes associable to sstas occurs within a temporal window of two weeks after the first appearance of the significant thermal anomalies. taking in mind the intrinsic technological limitation represented by the presence of meteorological clouds, which prevent from appreciating space-time continuity of observed thermal anomalies and consequently which can produce an overestimation of missed events, the results of molchan’s error diagrams show a precursory correlation in both performed analyses. moreover, moving from the analysis of tir seviri time series at full pixel resolution (i.e. sstas identified by using the retira index) to the analysis at a downscaled spatial resolution (i.e. sstas identified by using the retirabox index), an increase of probability gain has been obtained. finally, results achieved in this work confirm that satellite tir anomalies highlighted by the rst approach and the retira-based indexes provide useful information on seismic processes and oncoming earthquakes. although a long-term correlation analysis shows an intermediate number of false positives, on the basis of random test analysis (i.e. molchan diagram) we can conclude that the parameter “rst-based satellite tir anomalies” can be included in the framework of multi-parameter systems devoted to short-term earthquake forecast [tramutoli et al., 2014], which should be based on well-founded observational methodologies and data analysis techniques. so, thanks to the systematic integration of the short-term observations with the traditional approaches (e.g. based on seismological catalogues) devoted to defining a probabilistic assessment of the seismic hazard in the long-term, it could be expected an increase in terms of reliability and precision of seismic hazard assessment, which can be systematically updated on the basis of the specific alerted space-time windows and predictive capabilities of the considered parameters. acknowledgements. the authors wish to thank the aeronautica militare italiana for its support to have access to msgseviri data used in this work. references aki, k. 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(2010). wenchuan earthquake: brightness temperature changes from satellite infrared information. chinese science bulletin, 55(18), 1917–1924. http://doi.org/10.1007/s11434-010-3016-8. *co r r e s po n d i n g a u t h o r: nicola g e n za n o, school of engineering, university of basilicata, potenza, italy; e-mail: nicola.genzano@unibas.it © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 17 to develop a multi parametric system for short-term sha in italy r a d i o a t t i v i t à dt u n a l a v a e t n e a s t u d i o q u a n t i t a t i v o l . barbera, m . curatolo * , m . m . indovina addario, d . p a l u m b o , m . santangelo 1. introduzione. •— tu u n l a v o r o r e c e n t e m e n t e p u b b l i c a t o (*) a b b i a m o r i p o r t a t o i r i s u l t a t i (li a l c u n e m i s u r e di a t t i v i t à a e s e g u i t e su l a v e d e l l ' e t n a a p p a r t e n e n t i ad e r u z i o n i a v v e n u t e in e p o c h e d i v e r s e . l a t e c n i c a i m p i e g a t a è s t a t a q u e l l a d e l l e s e z i o n i l e v i g a t e s o v r a p p o s t e ad e m u l s i o n i n u c l e a r i , da m o l t i a u t o r i già d e s c r i t t a . p r o s e g u e n d o i n q u e s t o c a m p o di r i c e r c h e a b b i a m o t e n t a t o di m e t t e r e a p u n t o u n m e t o d o q u a n t i t a t i v o p e r l a d e t e r m i n a z i o n e d e l r a p p o r t o d e l l e c o n c e n t r a z i o n i di u r a n i o e t o r i o , cv c t h , n e i m a t e r i a l i a basso c o n t e n u t o r a d i o a t t i v o . a b b i a m o p u r e c e r c a t o d i s t u d i a r e il p r o b l e m a d e l l a d e t e r m i n a z i o n e d e l l ' e q u i l i b r i o p e r l a f a m i g l i a d e l l ' u r a n i o , i n rista d e l l ' i n t e r e s s e c h e q u e s t o p r o b l e m a p r e s e n t a i n r e l a z i o n e a l l e q u e s t i o n i r i g u a r d a n t i i processi di f o r m a z i o n e e di d i f f e r e n z i a z i o n e d e l l a crosta t e r r e s t r e . di r e c e n t e (-| s o n o stati s v i l u p p a t i a l c u n i c a l c o l i c h e p e r v e n g o n o a s e m p l i c i f o r m u l e di f a c i l e a p p l i c a z i o n e , atti a l l a d e t e r m i n a z i o n e , dispon e n d o di u n a statistica s u f f i c i e n t e m e n t e r i c c a dei v a l o r i dei p e r c o r s i d e l l e p a r t i c e l l e it, d e l l a c o n c e n t r a z i o n e n e l m i n e r a l e d i c i a s c u n a specie di n u c l i d e e m i t t e n t e p a r t i c e l l e <t. è p o s s i b i l e i n o l t r e , p a r t e n d o d a i s u d d e t t i c a l c o l i , r i l e v a r e l'esistenza o m e n o d e l l ' e q u i l i b r i o r a d i o a t t i v o d e l l e f a m i g l i e p r e s e n t i n e l m a t e r i a l e . i n q u e s t o p r i m o l a v o r o q u a n t i t a t i v o , d i s p o n e n d o di u n a s t a t i s t i c a su c i r c a 2 0 0 0 t r a c c e , n o i u s e r e m o p e r la d e t e r m i n a z i o n e di c r / c r b u n m e t o d o g l o b a l e , b a s a t o sui c a l c o l i s o p r a a c c e n n a t i , c h e p e n s i a m o possa a d a t t a r s i m e g l i o al caso d a n o i s t u d i a t o . 2. metodi di misura. — l a tecnica s p e r i m e n t a l e o r m a i classica consiste n e l l a e s p o s i z i o n e di l a s t r e p e r fisica n u c l e a r e al m a t e r i a l e e m i t t e n t e p a r t i c e l l e a , c h e n e l n o s t r o caso c o n s i s t e v a i n u n f r a m m e n t o d i ('*) istituto <li m i n e r a l o g i a , u n i v e r s i t à , p a l e r m o . 1 6 2 b a r b e r a c u r a t o l o i n d o v i n a a d d a r i o • p a l u m b o s a n t a n c e l o l a v a d e l l ' e t n a , a p p a r t e n e n t e ad u n a c o l a t a d e l l a e r u z i o n e d e l l ' a n n o 1 9 2 8 . d e l l o stesso c a m p i o n e e r a stata p r e c e d e n t e m e n t e d e t e r m i n a t a l ' a t t i v i t à specifica g l o b a l e ( 1 ) , a d i f f e r e n z a p e r ò di q u a n t o e r a stato f a t t o a l l o r a , a b b i a m o p r e f e r i t o s o v r a p p o r r e a l l a l a s t r a a n z i c h é u n a sezione l e v i g a t a d e l l a l a v a u n a g g l o m e r a t o di p o l v e r e d e l l a m e d e s i m a . è q u e s t o u n m e t o d o già i m p i e g a t o da d i v e r s i a u t o r i (:1), ( 4 ), ( 5 ) p e r o t t e n e r e u n o s t r a t o s u f f i c i e n t e m e n t e o m o g e n e o sia in s u p e r f i c i e c h e in p r o f o n d i t à . ( u n i n d i z i o del c o n s e g u i m e n t o di tale o m o g e n e i t à p u ò essere d e d o t t o d a l f a t t o che s u l l a l a s t r a esposta a b b i a m o r i l e v a t o u n a d i s t r i b u z i o n e s e n s i b i l m e n t e n o r m a l e d e l l e t r a c c e a). lo spessore d e l l o s t r a t o di p o l v e r e c h e c o p r i v a l ' e m u l s i o n e a v e v a v a l o r e m a g g i o r e del r a n g e m a s s i m o d e l l e « ; si p u ò q u i n d i n e i n o s t r i c a l c o l i c o n s i d e r a r e t a l e spessore c o m e i n f i n i t o r i s p e t t o al p e r c o r s o d e l l e a . l e l a s t r e u s a l e , i l f o r d c„ spessore 5 0 fi, s o n o state esposte p e r 5 1 g i o r n i . le m o d a l i t à di c o n s e r v a z i o n e d e l l e l a s t r e e l e c a r a t t e r i s t i c h e dei b a g n i di s v i l u p p o , fissaggio, ecc. sono q u e l l e d e s c r i t t e n e l l a v o r o cit a t o (!). l a s u p e r f i c i e esposta è stata d i v i s a i n 2 p a r t i c h e sono state esplor a t e i n d i p e n d e n t e m e n t e con due m i s c r o s c o p i d i v e r s i : sono s t a t e mis u r a t e l e p r o i e z i o n i o r i z z o n t a l i e q u e l l e v e r t i c a l i d e l l e tracce, l ' a p p r o s s i m a z i o n e e r a di 0,4 u p e r l e p r i m e , di 1 u p e r l e s e c o n d e : e p o i c h é a b b i a m o c o n s t a t a t o u n a b u o n a c o e r e n z a t r a i 2 g r u p p i di m i s u r e , abb i a m o in u l t i m o m e s c o l a t o tutti i d a t i ed eseguito i c a l c o l i sul tot a l e d e l l e tracce. p e r t r a s f o r m a r e le l u n g h e z z e d e l l e t r a c c e n e l l ' e m u l s i o n e , misur a t e al m i c r o s c o p i o , in cm di a r i a eq. si è t e n u t o conto del f a t t o r e di c o n t r a z i o n e d e l l a e m u l s i o n e , c h e n e l n o s t r o caso è r i s u l t a t o da u n a m i s u r a d i r e t t a 2,4, e d e l l a v a r i a z i o n e del coefficiente di r i d u z i o n e a r i a g e l a t i n a in f u n z i o n e d e l l a e n e r g i a r e s i d u a d e l l e p a r t i c e l l e . ques t ' u l t i m a c o r r e z i o n e è stala c a l c o l a l a in base ai dati s p e r i m e n t a l i rip o r t a t i da m. b o g a a r d t e b. k o u d i j s (r>). a q u e s t o p u n t o n o n ci s e m b r a s u p e r f l u o r i l e v a r e c h e s o l i t a m e n t e v i e n e c o n s i g l i a t o (4ì, (") di p r e n d e r e u n coefficiente di r i d u z i o n e ariag e l a t i n a costante e p r e c i s a m e n t e p e r l e c„ i l f o r d il v a l o r e 1 7 5 0 , inv e c e c o m e si r i l e v a d a l c o n f r o n t o tra l e c u r v e s p e r i m e n t a l i p e r c o r s o i n g e l a t i n a e n e r g i a e q u e l l e p e r c o r s o in a r i a g e l a t i n a (°), (81, il c o e f ficiente di r i d u z i o n e è u n a f u n z i o n e c r e s c e n t e d e l l a e n e r g i a r e s i d u a . r a d i o a t t i v i t à di u n a l a v a e t n e a s t u d i o q u a n t i t a t i v o 1 6 3 a s s u m e n d o p e r t a l e coefficiente il v a l o r e costante di 1 7 5 0 si int r o d u r r e b b e r o e r r o r i p e r eccesso d e l l ' o r d i n e del 1 0 % p e r l e t r a c c e c o r t e ed e r r o r i p e r d i f e t t o d e l l ' o r d i n e del 3c/o p e r l e p i ù l u n g h e . è p r o b a b i l e c h e c o r r e z i o n i d e l l o stesso g e n e r e d o v r e b b e r o essere a p p o r t a t e n e l l a v a l u t a z i o n e del p e r c o r s o d e l l e p a r t i c e l l e n e l c a m p i o n e ; qtiesto i n t r o d u r r e b b e c o r r e z i o n i n e l l e f o r m u l e a p p l i c a t e : di queste p e r ò n o n a b b i a m o p o t u t o t e n e r e c o n t o p e r la m a n c a n z a di d a t i s p e r i m e n t a l i . s i p u ò c o m u n q u e p r e v e d e r e c h e t a l i c o r r e z i o n i sarebb e r o s e n s i b i l i solo s u l l e t r a c c e p i ù c o r t e ; è p e r questa r a g i o n e , e p e r a l t r i m o t i v i di cui v e r r à d e t t o in seguito, c h e i c a l c o l i sono stati eseguiti e v i t a n d o di u t i l i z z a r e l e t r a c c e corte. 3. risultati. — p e r p o t e r a p p l i c a r e l e f o r m u l e [ v e d i b i b l . ( 2 ) ] è n e c e s s a r i o d e t e r m i n a r e , n e l l ' i p o t e s i a b b a s t a n z a f o n d a t a c h e gli e l e m e n ti a t t i v i p r e s e n t i s i a n o e s s e n z i a l m e n t e q u e l l i d e l l e f a m i g l i e d e l l ' u r a n i o e del i o r i o , il n u m e r o «li tracce p r e s e n t i n e l l a e m u l s i o n e a v e n t i perc o r s o o p r o i e z i o n e o r i z z o n t a l e ( i n cm d'a. eq.) m a g g i o r e di ri, essendo r r . . . . ri r,, i p e r c o r s i dei d i v e r s i g r u p p i di </. d e l l e d u e f a m i g l i e disposti in o r d i n e c r e s c e n t e . r i p o r t i a m o n e l l a t a b e l l a i i v a l o r i di ri c o r r i s p o n d e n t i agli elem e n t i a t t i v i sot.tosegnati a n c h e p e r c h é questi dati, c h e r i t e n i a m o i p i ù a t t e n d i b i l i ( 8 ), d i f f e r i s c o n o u n po' da q u e l l i r i p o r t a t i da a l t r i a u t o r i i n l a v o r i di questo g e n e r e . t a b e l l a i k i n c m u r i a 2,65 2.70 3,18 3,21 3,30 3,81 3,98 1,05 •1,28 1,68 1,73 5,0 3,64 6,91 8,57 ; e l e m e n t i d e l l a ; f a m i g l i a d e l l ' u 1» u „ ha p o un ita a ka e e l e m e n t i d e l l a f a m i g l i a d e l t h th ita th t h x t h c t u t h a t h c ' n e l l a t a b e l l a l i sono r i p o r t a t i i n u m e r i n di t r a c c e a v e n t i p r o i e z i o n i o r i z z o n t a l i c o m p r e s e n e l l ' i n t e r v a l l o c o r r i s p o n d e n t e s o p r a segnato. p e r r i d u r r e al m i n i m o gli e r r o r i di conteggio si sono p r e s e i n c o n s i d e r a z i o n e solo l e t r a c c e a v e n t i p r o i e z i o n e o r i z z o n t a l e m a g g i o r e di 12 b a r b e r a c u r a t o l o i n d o v i n a a d d a r i o • p a l u m b o s a n t a n c e l o 0,8 cm. d'a., n o n è p e r ò escluso c h e q u a l c u n a d e l l e tracce a v e n t e p r o i e z i o n e di q u e s t o o r d i n e sia a n d a t a p e r d u t a . t a b e l l a l i r 0,81—2,50 2,21—2,51 2,66—3,18 3,19—3,21 3,22—3,30 3, 1—3,84 3,85—3,98 3 , 9 9 — 4 , 9 5 n 1209 79 269 10 35 146 32 11 r 4,06—4,28 4,29—4,66 4,67—4,73 4,74—5,00 5,01—5,64 5,65—6,91 6,92—8,57 n 45 40 17 23 56 38 21 i n figura 1 ( t r a t t o c o n t i n u o , scala s e m i l o g a r i t m i c a ) è r i p o r t a t o i n o r d i n a t e i l n u m e r o di t r a c c e a v e n t i p r o i e z i o n e o r i z z o n t a l e magg i o r e d e l l a c o r r i s p o n d e n t e ascissa, n u m e r o r i c a v a t o dai d a t i d e l l a tad e l l e o r d i n a t e r e l a t i v e ad ascisse u g u a l i ai r a n g e s dei v a r i g r u p p i di « , gli a l t r i p u n t i sono i n t e r p o l a t i g r a f i c a m e n t e . 11 caso d e l l a distrib u z i o n e d e l l e l u n g h e z ze e f f e t t i v e p r e s e n t a m a g g i o r i difficoltà. inf a t t i s c a r t a n d o l e tracce a v e n t i p r o i e z i o n e o r i z z o n t a l e m i n o r e di 0,8 cm n o n si o p e r a s u l l e l u n g h e z z e u n ver o taglio p e r c h é evid e n t e m e n t e si t r a l a sciano, così f a c e n d o , q u e l l e t r a c c e c h e p u r a v e n d o p r o i e z i o n e or i z z o n t a l e m o l t o piccola, il c h e r e n d e d i f ficile l a l o r o i d e n t i f i b e l l a ii. i n detta figura sono esatti i v a l o r i fic. 1 r a d i o a t t i v i t à di u n a l a v a e t n e a s t u d i o q u a n t i t a t i v o 1 6 5 c a z i o n e , possono a v e r e l u n g h e z z a e f f e t t i v a r i l e v a n t e a causa di u n a l t o v a l o r e d e l l a p r o i e z i o n e v e r t i c a l e . ne v i e n e c h e i l n u m e r o di t r a c c e a v e n t i l u n g h e z z a e f f e t t i v a m a g g i o r e di 0,8 cm d'a. eq. sarà m o l t o super i o r e a q u e l l o d e l l e t r a c c e a v e n t i p r o i e z i o n e o r i z z o n t a l e m a g g i o r e di d e t l o v a l o r e . s i p u ò p e r ò p r e v e d e r e c h e l e t r a c c e a v e n t i l u n g h e z z a m a g g i o r e di 2 , 6 5 cm d'a. eq. s i a n o p r e v a l e n t e m e n t e da r i c e r c a r e t r a q u e l l e a v e n t i p r o i e z i o n e o r i z z o n t a l e m a g g i o r e di 0 , 8 cm essendo r e l a t i v a m e n t e r a r e l e p a r t i c e l l e << emesse q u a s i v e r t i c a l m e n t e . i n fig. 2 sono r i p o r tati i r i s u l t a t i d e l l e m i s u r e d e l l e l u n g h e z ze t o t a l i ; p e r q u a n t o d e t t o s o p r a il g r a f i c o è a t t e n d i b i l e solo a p a r tire da v a l o r i d e l l e ascisse n o n t r o p p o piccole, m e n t r e l ' o r d i n a ta c o r r i s p o n d e n t e a 0,8 cm del l'ascissa h a v a l o r e s e m p l i c e m e n t e indic a t i v o , m a n o t e v o l m e n te i n f e r i o r e al v e r o . n o t i a m o i n c i d e n t a l m e n t e a q u e s t o p r o p o sito c h e l a difficoltà di scelta del v a l o r m i n i m o p e r l e p r o i e z i o n i o r i z z o n t a l i r i d u c e not e v o l m e n t e l ' u t i l i t à dell o s p e t t r o d e l l e l u n ghezze e f f e t t i v e , s e b b e n e i c a l c o l i su t a l e s p e t t r o s i a n o m o l t o p i ù agev o l i c h e su q u e l l o d e l l e p r o i e z i o n i o r i z z o n t a l i . s i a v r e b b e r o i n f a t t i inev i t a b i l m e n t e d e l l e n o t e v o l i i n c e r t e z z e sul n u m e r o di t r a c c e c o r t e c h e sono q u e l l e c h e p i ù p e s a n o p e r p o t e r e a r r i v a r e a q u a l c h e c o n c l u s i o n e in m e r i t o a l l ' e q u i l i b r i o degli e l e m e n t i c o m p o n e n t i la f a m i g l i a dell ' u r a n i o . 4. interpretazione dei risultati. — nei c i t a l i l a v o r i ( 2 ) p e r i l caso p a r t i c o l a r e di u n a f a m i g l i a in e q u i l i b r i o , si p e r v i e n e a l l e f o r m u l e : cm di aria 1 6 6 b a r b e r a c u r a t o l o i n d o v i n a a d d a r i o • p a l u m b o s a n t a n c e l o . . . x re li p m r ) = — l x k ( r k 4 i + i n e * g(r) = xn\.i p (rk—r)" rv t'k [1] ( b i < r ^ r i + i ) [2] col seguente s i g n i f i c a t o dei s i m b o l i : h(r) , g(r) n u m e r o di t r a c c e p e r c n r di e m u l s i o n e e p e r secondo di esposizione a v e n t i r i s p e t t i v a m e n t e p r o i e z i o n e orizz o n t a l e o l u n g h e z z a t o t a l e n e l l a g e l a t i n a > r ; r1. ..ri.. rp; i c a m m i n i in a r i a , disposti in o r d i n e crescente, dei g r u p p i di a emesse dagli e l e m e n t i d e l l a f a m i g l i a ; n n u m e r o di a t o m i d e l l ' e l e m e n t o p a d r e p e r c m 3 di m i n e r a l e ; x c o s t a n t e di d i s i n t e g r a z i o n e d e l l ' e l e m e n t o p a d r e ; (i r a p p o r t o t r a il c a m m i n o n e l m a t e r i a l e e q u e l l o i n a r i a d e l l e « (supposto i n d i p e n d e n t e d a l l ' e n e r g i a ) ; coefficiente d o v u t o a l l e e v e n t u a l i b i f o r c a z i o n i . c o n o t t i m a a p p r o s s i m a z i o n e p = 1 p e r tutti gli e l e m e n t i della f a m i g l i a d e l l ' u r a n i o 2 3 8 , p e r q u e l l a del tor i o e s^ 1/3 p e r i l t h c , e 2/3 p e r i l t h c , 1 p e r t u t t i gli a l t r i e l e m e n t i . p a r t e n d o da q u e s t e f o r m u l e sono stati cos t r u i t i i g r a f i c i d e l l e figg. 3 e 4 n e i q u a l i sono r i p o r t a t i gli spett r i i n t e g r a l i t e o r i c i d e l l e p r o i e z i o n i orizz o n t a l i , fig. 3 (scala semi l o g a r i t m i c a ) e q u e l l i d e l l e l u n g h e z z e t o t a l i (fig. 4). in ent r a m b e l e figure l a lin e a i si r i f e r i s c e al caso di u n m a t e r i a l e fig. 3 c o n t e n e n t e solo t h i n 4 5 6 7 cm di aria r a d i o a t t i v i t à di u n a l a v a e t n e a s t u d i o q u a n t i t a t i v o 1 6 7 e q u i l i b r i o con i suoi d i s c e n d e n t i , l a l i n e a i i al caso del solo u 2 3 8 a n c b e esso in e q u i l i b r i o , la ii' si r i f e r i s c e a l l a p r e s e n z a del solo i o i n e q u i l i b r i o ed i n f i n e la ii" è n e l l ' i p o t e s i clic sia p r e s e n t e n e l m a t e r i a l e i l solo r a i n e q u i l i b r i o con i p r o p r i d i s c e n d e n t i . i d a t i sono stati calcol a t i p e r u n t o t a l e di 1 0 0 0 tracce p e r secondo e p e r c m 2 e solo p e r i p u n t i di ascisse u g u a l i ai r a n g e s dei v a r i g r u p p i di a d e l l e d u e f a m i glie, gli a l t r i p u n t i sono slati i n t e r p o l a t i g r a f i c a m e n t e . nei c a l c o l i c h e seg u o n o sono stati u t i l i z z a t i n a t u r a l m e n t e sol o i v a l o r i c a l c o l a t i in base a [ 1 ] e [ 2 ] e n o n a q u e l l i i n t e r p o l a t i . n e l l a l a v a in e s a m e il t h sarà q u a s i cert a m e n t e in e q u i l i b r i o con i suoi d i s c e n d e n t i p o i c h é l e v i t e m e d i e di q u e s t ' u l t i m i sono di m o l t o i n f e r i o r i a q u e l l a del t h . p e r q u a n t o riguarda l a f a m i g l i a d e l l ' u si pot r e b b e r o a v e r e i n v e c e d i f e t t i di e q u i l i b r i o in c o r r i s p o n d e n z a d e l l o 1 0 o d e l r a p e r r a g i o n i c h i m i c h e o fisiche; 11 r a p e r ò p u ò a n a l o g a m e n t e al t h esser c o n s i d e r a t o i n e q u i l i b r i o s e c o l a r e con gli e l e m e n t i c h e l o s e g u o n o n e l l a f a m i g l i a . a b b i a m o e f f e t t u a t o il c o n f r o n t o degli s p e t t r i s p e r i m e n t a l i (figg. 1, 2) con q u e l l i teorici (fig. 3, 4) l i m i t a t a m e n t e a q u e l l e p a r t i d e l l e d u e f a m i g l i e c h e possono s u p p o r s i i n e q u i l i b r i o , ossia, i l c h e è e q u i v a l e n t e , p e r v a l o r i d e l l e ascisse m a g g i o r i di 3 , 2 1 cm. i n d i c a n d o con qp (r) l e o r d i n a t e d e l l a l i n e a i, con y (ri, "/.' ir), l" (r) q u e l l e d e l l e l i n e e ii, ii' e ii", con f ( r ) l e o r d i n a t e d e l l a c u r v a s p e r i m e n t a l e , con a e ò l e c o n c e n t r a z i o n i i n u n i t à c o n v e n z i o n a l i di t h e di r a p r e s e n t i n e l l a l a v a , d o v r à e s s e r e : f ( r ) = a cp (r) + 6 z ( r ) ( 3 , 2 1 < r 8,57) [ 3 ] cm di aria f i g . 4 1 6 8 b a r b e r a c u r a t o l o i n d o v i n a a d d a r i o • p a l u m b o s a n t a n c e l o l a [ 3 ] d e v e essere v a l i d a sia p e r gli s p e t t r i i n t e g r a l i d e l l e l u n g h e z z e e f f e t t i v e (figg. 2 , 4) c h e p e r q u e l l e d e l l e p r o i e z i o n i (figg. 1, 3) (*). l a d e t e r m i n a z i o n e di ti, b è stata eseguita t e n e n d o c o n t o del sol o s p e t t r o d e l l e p r o i e z i o n i o r i z z o n t a l i che, c o m e è stato n o t a t o s o p r a , è da r i t e n e r e p i ù a c c u r a t o , e p o i c h é a causa di i n e v i t a b i l i e r r o r i di o s s e r v a z i o n e n o n è possibile t r o v a r e 2 v a l o r i p e r a e b t a l i c h e l a [ 3 ] v e n g a v e r i f i c a t a p e r t u t t i i v a l o r i r, si è p r e f e r i t o u s a r e p e r l a determ i n a z i o n e di a e b il m e t o d o dei m i n i m i q u a d r a t i . i n d i c h i a m o con cp; , xi , f-, , (i = 1,.... i l i i v a l o r i di ip (r), y (ri, /(r) p e r r = 3 , 2 1 ; 6 , 9 1 ; 8 , 5 7 , l o s c a r t o q u a d r a t i c o s(a, b) tra t r a la d i s t r i b u z i o n e t e o r i c a [ 3 ] e q u e l l a s p e r i m e n t a l e , è d a t o d a : s ( a ) 6 ) = v . ( a < p i + è i x i / 1 ) ! ì i v a l o r i a, b c h e r e n d o n o m i n i m o s o t t e n u t i r i s o l v e n d o i l s i s t e m a : 3 s " 3s h 0 = — = 2 2 , (p; (a rpi + b x, — /,) ; 0 = — = 2 ^ x, (a cfi + b x, — f i ) d a i ' db i sono, u s a n d o p e r f i i v a l o r i r i c a v a t i da t a b . i i : a — 2 , 8 6 = 0 , 4 i n fig. 1 la l i n e a t r a t t e g g i a t a r a p p r e s e n t a l a d i s t r i b u z i o n e t e o r i c a p a r t e n d o dai s u d d e t t i v a l o r i di a e b. c o m e si v e d e l ' a c c o r d o t r a l a d i s t r i b u z i o n e s p e r i m e n t a l e e q u e l l a teorica è m o l t o s o d d i s f a c e n t e ; p e r q u a n t o r i g u a r d a l o s p e t t r o d e l l e l u n g h e z z e totali l o s p e t t r o t e o r i c o ( l i n e a t r a t t e g g i a t a ) si a c c o r d a p u r e a b b a s t a n z a b e n e con q u e l l o sper i m e n t a l e . i n v e r i t à p e r q u e s t ' u l t i m o caso si a v r e b b e u n a c c o r d o mig l i o r e p r e n d e n d o p e r a u n v a l o r e l e g g e r m e n t e i n f e r i o r e ( c i r c a 2,61; c o m u n q u e questa p i c c o l a d i f f e r e n z a c a d e e n t r o gli e r r o r i s p e r i m e n t a l i . l e c o n c e n t r a z i o n i di t o r i o ed u r a n i o , c°tll , c y , c h e d a r e b b e r o l u o g o a l l e d i s t r i b u z i o n i t e o r i c h e d e l l e tracce c o m e i n figg. 3, 4 ( l i n e e (*) a t i t o l o di v e r i f i c a d e l l a a t t e n d i b i l i t à dei r i s u l t a t i s p e r i m e n t a l i è stato f a t t o u n c o n f r o n t o tra lo s p e t t r o d e l l e p r o i e z i o n i o r i z z o n t a l i e q u e l l o d e l l e p r o i e z i o n i v e r t i c a l i : c o m e e r a stato p r e v i s t o ( 2 ) i d u e spettri si a c c o r d a n o i n m a n i e r a s o d d i s f a c e n t e . r a d i o a t t i v i t à di u n a l a v a e t n e a s t u d i o q u a n t i t a t i v o 1 6 9 i e ii) possono essere c a l c o l a t e in gr/'cm3 di sostanza, m e d i a n t e l e f o r m u l e : 11 c o ^ i ^ i o m[4j l e c o r r i s p o n d e n t i c o n c e n t r a z i o n i c",„ , c"liu elle d a r e b b e r o l u o g o a l l e d i s t r i b u z i o n i l i ' e i i " sono q u e l l e stesse d i e c o m p e t e r e b b e r o all ' i o n i o e al r a r i s p e t t i v a m e n t e se essi f o s s e r o in e q u i l i b r i o con u di c o n c e n t r a z i o n e c"v . nel seguito a s s u m e r e m o q u e s t e c o n c e n t r a z i o n i c o m e u n i t à c o n v e n z i o n a l i p e r i l t l i , l'u, io, r a . i n d i c a n d o con a la s u p e r f i c i e d e l l ' e m u l s i o n e e s p l o r a t a i n c m 2 , con t i l t e m p o di e s p o s i z i o n e i n sec, d l a densità d e l m a t e r i a l e i n g r / e m 3 , l a c o n c e n t r a z i o n e assoluta di t h in gr/gr di m a t e r i a , è data d a : r _ a c ° t ' . t h —7 , a td [ 5 ] f o r m u l e a n a l o g h e si h a n n o p e r l e c o n c e n t r a z i o n i degli a l t r i elem e n t i . e v i d e n t e m e n t e p e r a p p l i c a r e la f o r m u l a [ 5 ] è n e c e s s a r i o p o t e r d e t e r m i n a r e i l v a l o r e di (.i, r a p p o r t o t r a p e r c o r s o d e l l e a n e l l a m a t e r i a e p e r c o r s o in a r i a . n e l l ' i p o t e s i di b r a g g e k l e e n i a n ( 4 ) si h a : d m a t l | ' a r i a " q m a , d o v e n l | ' n , a . = i « i | " " i 1 con «i p e r c e n t u a l e d e l l ' e l e m e n t o c o n s i d e r a t o c o s t i t u e n t e i l m a t e r i a l e e jf\ c o r r i s p o n d e n t e peso a t o m i c o . p e r la n o s t r a l a v a è stata calcol a t a ajimat p a r t e n d o dai d a t i di u n ' a n a l i s i c h i m i c a g e n t i l m e n t e f o r n i taci d a l p r o f . p o n t e u n i t a m e n t e al c a m p i o n e di l a v a ; p e r essa si h a : l | ' r a a . = 4 , 5 nel caso p r e s e n t e d a t a la n a t u r a del c a m p i o n e ( p o l v e r e ) si è p r e f e r i t o a p p l i c a r e la f o r m u l a [ 5 ] ai dati r i g u a r d a n t i l a stessa l a v a esposta in sezione l e v i g a t a , p e r la r a g i o n e c h e p e r q u e s t ' u l t i m a h a 1 7 0 b a r b e r a c u r a t o l o i n d o v i n a addario • p a l u m b o s a n t a n c e l o senso p a r l a r e di u n a densità e di u n a s u p e r f i c i e esposta. dai d a t i d e l l a v o r o già c i t a t o (*) e p r e n d e n d o p e r a e b i v a l o r i già d e t e r m i n a t i , si o t t i e n e : c t h = 1 , 5 8 . 10""' g r a m m i di t h p e r g r a m m o di l a v a , e n e l l ' i p o t e s i c h e l ' u r a n i o f o s s e i n e q u i l i b r i o : c „ = 0 , 7 . 1 0 ° gr/gr in q u e s t ' u l t i m a ipotesi d e l l ' e q u i l i b r i o d e l l a f a m i g l i a d e l l ' u , si h a : c d /c t b = 0 , 0 4 p a r t e n d o d a l l e f o r m u l e d a t e da 1. c u r i e (f l), c h e del r e s t o r i g u a r d a n o u n caso p a r t i c o l a r e r i s p e t t o a q u e l l e a p p l i c a t e s o p r a e v a l i d e s e m p r e solo p e r l a statistica d e l l e l u n g h e z z e t o t a l i , si o t t i e n e u n v a l o r e l e g g e r m e n t e d i v e r s o : cj c t b = 0 , 0 5 (**) p e r e s a m i n a r e o r a il p r o b l e m a d e l l ' e q u i l i b r i o si d o v r e b b e f a r e i l c o n f r o n t o d e l l a p a r t e di s p e t t r o s p e r i m e n t a l e di ascisse m i n o r i di 3 , 2 1 con l e c o r r i s p o n d e n t i p a r t i degli s p e t t r i t e o r i c i , cioè a n a l i t i c a m e n t e si d o v r e b b e c e r c a r e di d e t e r m i n a r e i coefficienti b, b', b" dell ' e q u a z i o n e : j (r) a cp (r) = 6 x (r) + b ' x' (r) + b " x" (r) ( r < 3 , 2 1 ) p o i c h é n e l caso d e l l a n o s t r a l a v a l a c o n c e n t r a z i o n e degli e l e m e n t i d e l l a f a m i g l i a d e l l ' u r i s p e t t o a q u e l l i del t h è i-isultata t r o p p o bassa, a b b i a m o a v u t o t r o p p o p o c h e t r a c c e c o r t e u t i l i z z a b i l i p e r p o t e r t e n t a r e u n a d e t e r m i n a z i o n e dei coefficienti, p e r c h é l e t r a c c e c o r t e sono in m a s s i m a p a r t e d o v u t e ad e l e m e n t i d e l l a f a m i g l i a dell'u. l ' a n d a m e n t o q u a l i t a t i v o p o t r e b b e s u g g e r i r e c h e n e l l a n o s t r a l a v a il r a (o l'io) n o n sia i n e q u i l i b r i o con l ' u m a c h e q u e s t o u l t i m o a b b i a u n a c o n c e n t r a z i o n e m i n o r e di q u e l l a c h e a v r e b b e se f o s s e in e q u i l i b r i o ; p e r ò a causa d e l p i c c o l o n u m e r o di t r a c c e c o r t e i m p u t a b i l i a questi e l e m e n t i , n o n (**) n e l caso p r e s e n t e è d i f f i c i l e c a l c o l a r e l ' e r r o r e sul r a p p o r t o u r a n i o t o r i o , si p u ò solo d i r e elle l ' a c c o r d o tra i 2 v a l o r i r i c a v a t i con m e t o d i d i v e r s i è b u o n o . r a d i o a t t i v i t à di u n a l a v a e t n e a s t u d i o q u a n t i t a t i v o 1 7 1 c r e d i a m o di a v e r e d a t i sufficienti p e r p o t e r e t r a t t a r e q u a n t i t a t i v a m e n t e il p r o b l e m a . n o t i a m o a q u e s t o p r o p o s i t o clie l a v a l u t a z i o n e dell'esistenza o m e n o d e l l ' e q u i l i b r i o d e l l a f a m i g l i a d e l l ' u r a n i o esige u n a conoscenza m o l t o esatta del r a p p o r t o t e o r i c o t r a i l n u m e r o di t r a c c e l u n g h e ed il n u m e r o di t r a c c e c o r t e ; d ' a l t r a p a r t e , q u e s t o r a p p o r t o è c e r t a m e n t e i n f l u e n z a t o , ed i n m i s u r a n o t e v o l e , d a l l a n o n p r o p o r z i o n a l i t à t r a i p e r c o r s i n e l m a t e r i a l e e n e l l a g e l a t i n a di p a r t i c e l l e d e l l a stessa e n e r gia. a g g i u n g i a m o c h e t a l e c o r r e z i o n e p u ò m a s c h e r a r e c o m p l e t a m e n t e l ' a n d a m e n t o e f f e t t i v o di q u e l l a p a r t e d e l l o s p e t t r o c h e c o r r i s p o n d e ai v a l o r i p i ù bassi del p e r c o r s o , e q u i n d i d a r e r i s u l t a t i e r r a t i p e r q u a n t o r i g u a r d a l ' e q u i l i b r i o . r i n g r a z i a m o i l p r o f . g. p o n t e p e r a v e r c i f o m i t i i c a m p i o n i da n o i s t u d i a t i e p e r a v e r c i c o m u n i c a l i i r i s u l t a t i d e l l ' a n a l i s i c h i m i c a che ci h a p e r m e s s o il c a l c o l o di !(•„,„, . i n o s t r i r i n g r a z i a m e n t i v a n n o p u r e r i v o l t i a l l a d i r e z i o n e g e n e r a l e del b a n c o di s i c i l i a c h e con u n a sua e l a r g i z i o n e h a c o n t r i b u i t o ili p a r t e a l l a spesa p e r l ' a c q u i s t o di un m i c r o s c o p i o . palermo — istituto di fisica dell'università — maggio 1953. riassunto nel presente lavoro vengono esposti i risultati di una ricerca sul contenuto di elementi delle due famiglie naturali dell'uranio e del torio in un campione di lava dell'etna. la tecnica impiegata è quella delle lastre sensibili per fìsica nucleare. si espoiìgono i criteri usati per la determinazione del rapporto tra il contenuto di uranio e quello di torio nel caso dell'equilibrio della famiglia dell'uranio. quest'ultima questione viene trattata e discussa dal punto di vista generale e analitico: si traccia un metodo per risalire dal confronto tra lo spettro sperimentale dai ranges delle tracce a in emulsione con quello teorico, alla esistenza o meno dell'equilibrio radioattivo. 1 7 2 b a r b e r a c u r a t o l o i n d o v i n a addario • p a l u m b o s a n t a n c e l o summary the results of a research on the content of elements of uranium and thorium family in a lava sample front mt. etna are presented. the nuclear emulsion tecnique has been used. the principles adopted for the determination of ratio of uranium to thorium content in the hypothesis of equilibrium are exposed. the last question is discussed front a general and analytical view, a procedure is shown in order to obtain, from a comparison of experimental spectrum of the alpha tracks in emulsion ivith theoretical spectrum, the eventual presence of radioactive equilibrium. b i b l i o g r a f i a ( ! ) l . b a r b e r a , m . c u r a t o l o , m . i n d o v i n a a d d a r i o , m . s a n t a n g e l o : a n n . g e o f . , 5, 6 0 3 ( 1 9 5 2 ) . ( 2 ) v a n stijvendael: b u l l . g e n . p h y s . n u c l . , n . 2 8 , 1 9 5 1 ; g . i m b ò l . casertano: ann. geof„ v , 1 9 5 2 ; d. palumbo: a n n . g e o f . , in corso (li s t a m p a . (3) r . coppens: b u l l . s o c . f r a n e . m i n . c r i s t i 73, 2 1 7 ( 1 9 5 0 ) . ( 4 ) h . yagoda: radioactive measurenients ivith nuclear emulsion (n. y . 1 9 4 9 ) . ( 5 ) e. picciotto: b u l l . c e n t r e p l i y s . n u e l é a i r e u n . l i b r e b r u x e l l e s , n. 33 ( 1 9 5 2 ) . s . m i i . o n e t a m b u r i n o , a . s t e l l a : n . c i n i , i x , 2 5 3 ( 1 9 5 2 ) . ( « ) m . bogaardt e b . k o u d i j s : p h y s . r e v . , 8 7 , 1 1 2 9 ( 1 9 5 2 ) . ( 7 ) h . v . buttlar e f . g . holtermans: g e o c . e t c o s m o c . a c t a , 2 , 13 ( 1 9 5 1 ) . ( 8 ) landolt-bornstein": zahlemverte und funktionen ans phvsik 6 auflage. ( 9 ) i . c u r i e : j o u r n . d e p h y s . e t r a d . , 1 1 , 6 3 3 ( 1 9 4 6 ) . layout 6 1 annals of geophysics, 63, 4, dm436, 2020; doi.10.4401/ag-8188 methodology to derive well-calibrated thermometers: a new glass-composition geothermometer for olivine-bearing glassy samples at one atmospher alessandro fabbrizio, václav špillar institute of petrology and structural geology, faculty of science, charles university, prague, czech republic article history: received april 29, 2019; accepted january 21, 2020 abstract this work describes the methodology used to develop a new glass-composition geothermometer by multiple linear least-squares regression analysis of experimental data available in literature. the geothermometer is applicable to natural olivine-bearing glassy samples quenched at one atmosphere. the calibration data set includes a total of 543 anhydrous experiments performed at one atmosphere which produced olivine coexisting with glass (± other phases). the accuracy of the geothermometer has been verified using an independent set of randomly selected one atmosphere anhydrous experimental samples external to the calibration data set. the new proposed equation reads: t(°c) = 1054.1±6.5 + (1458.0±23.5)(xmgo)liq – (267.1±45.8)(xcao)liq + (116.3±30.9)(xna2o + xk2o)liq, where the terms such as (xmgo)liq represent the cation fraction of mgo in the liquid. the new geothermometer is able to reproduce the calibration temperatures within a standard error of estimate (see) of ±23° c and to predict the temperatures for the test data set with a see of ±25° c. the proposed equation, when compared to previous models, gives the lowest systematic error. moreover, we mathematically demonstrate that the comparison of the slope and the intercept parameters of the regression line against the 1:1 line leads to correct evaluation of the model only when a regression of observed or measured (in the y-axis as dependent variable) vs. predicted or calculated (in the x-axis as independent variable) is used. keywords: olivine-glass geothermometer; linear regression; calibration; test data; equation; temperature; model. 1. introduction in volcanology it is often important to determine the temperature (t) at which lava flows erupt and lava lakes cool down. such information is crucial for understanding lava rheology and crystallization history, as well as for monitoring present day and for studying past volcanic eruptions [e.g., costa el al., 2013; de maisonneuve et al., 2012, 2013; oeser et al., 2018]. a geothermometer based on a regression analysis of experimental data (i.e., calibration data), to be useful, should give the lowest systematic error when used to predict t for other experimental data not used in the regression (i.e., test data). in the ideal case, slope and intercept of the regression line of the measured or observed (in the y-axis) vs. calculated or predicted (in the x-axis) temperatures should be one and zero, respectively. given that the slope and intercept parameters define a linear relationship between the measured and calculated temperatures, they can be used to test how far the model is from the ideal line 1:1, therefore, how much the model suffers for the systematic error. in the absence of testing of the regression model, the calculated temperatures may be affected by gross systematic error and in turn be meaningless. in this study, we describe the general methodology that should be followed in deriving the regression equations for geothermometers. we then employ this approach and formulate a new geothermometer based on anhydrous liquid composition in equilibrium with olivine ± other phases at one atmosphere. in order to test the ability of a new formulation to recover the t from olivine-saturated one atmosphere anhydrous experiments, we compare the new model with previous glass-composition geothermometers. moreover, we noticed that the evaluation of the model by comparison of the slope and the intercept of the regression line against the 1:1 line depends on which variable (predicted or observed data) is treated as dependent or independent variable. a short review of some previous work shows that there is no consensus on it, given that plots of predicted (in the y-axis) vs. observed (in the x-axis) are commonly used as well as plots of observed (in the y-axis) vs. predicted (in the x-axis) to estimate the slope and intercept. from a mathematical point of view, we show that only the slope and the intercept values estimated by plotting the observed (in the y-axis, dependent variable) vs. predicted (in the x-axis, independent variable) is correct and should be used to provide an appropriate interpretation of the model performance. 2. geothermometers based on glass composition in equilibrium with olivine despite the fact that basaltic compositions are predominant among volcanic rocks only few geothermometers applicable to liquids in equilibrium with olivine (± other phases) have been proposed in the last 30 years [helz and thornber, 1987; beattie, 1993; montierth et al., 1995; putirka, 2008]. the simplest models relate t to the concentration (wt%) of mgo in a liquid (mgoliq), and were calibrated using the experimental data obtained at 1-atm with specific bulk compositions [helz and thornber, 1987; montierth et al., 1995]. the thermometer of helz and thornber [1987]: t (°c) = 20.1mgoliq + 1014° c (1) was experimentally-calibrated explicitly for kilauean lavas using basaltic and picritic kilauean starting compositions. this geothermometer was calibrated over a temperature range of 1060° c to 1260° c, giving an uncertainty associated with the calculated temperatures of ±10° c, with the purpose to constrain the quench temperature of the drill core samples recovered from the kilauea iki lava lake. it has also been used to monitor changes in the lava temperature both at the vent [helz et al., 1991] and during its transport within the lava tube system [helz, 1993; cashman et al., 1994], and to analyze the thermal histories of recent eruptions at kilauea [helz et al., 1995]. subsequently, montierth et al. [1995] developed another empirical glass-composition-based geothermometer with the aim to obtain the quench temperature of any glassy mauna loa sample with an uncertainty of ±10° c: t (°c) = 23.0mgoliq + 1012° c (2) this equation was calibrated in the temperature range of 1110 to 1310° c using specific mauna loa compositions, which are slightly different with respect to those of kilauea, being richer in sio2 and poorer in tio2, feo, k2o, and p2o5 for comparable mgo contents [montierth et al., 1995]. these experimental calibrations (eq. 1 and 2) were developed for the specific bulk compositions and with the aim to obtain the quench temperatures solely for the lavas from the kilauea and mauna loa volcanoes. beattie [1993] developed a useful empirical thermometer solely based on the constraint that the activitycomposition relationships in the melt phases should allow equilibration temperatures to be calculated as accurately as possible. the combination of these activity models yields the following equation: alessandro fabbrizio et al. 2 t (°c) = [13603 + 4943×10-7(p×109 – 105)]/[6.26 + 2lndol/liqmg + (3) 2ln(1.5(clnm)) + 2ln(3(clsio2)) – nf] – 273.15, where p is a pressure in gpa, dol/liqmg = (0.666 – (-0.049(xmno)liq + 0.027(xfeo)liq))/((xmgo)liq + 0.259(xmno)liq + 0.0299(xfeo)liq), (clnm) = (xfeo)liq + (xmno)liq + (xmgo)liq + (xcao)liq + (xcoo)liq + (xnio)liq, (clsio2) = (xsio2)liq, nf = 3.5ln(1 (xalo1.5)liq) +7ln(1 (xtio2)liq), and the terms such as (xmno)liq represent the cation fractions of mno in the liquid. when predicting temperature for 1302 anhydrous olivine-saturated experimental data, the eq. (3) captures 94% of the experimental temperature variations with a standard error of estimate (see) of ±44° c [putirka, 2008]. later, to expand the applicability range of the early thermometers of helz and thornber [1987] and montierth et al. [1995], putirka [2008] developed other models calibrated on a database of 1536 olivine-saturated experimental data. the simple linear correlation between the temperature and mgo content (wt%) in the liquid becomes: t (°c) = 26.3mgoliq + 994.4° c (4) giving the see of ±71° c and capturing 84% of the experimental temperature variations. other empirical expressions considering additional compositional terms and the effect of pressure were developed with the aim to further reduce the error [putirka, 2008]: t (°c) = 754 + 190.6(mg#liq) + 25.52(mgoliq) + 9.585(feoliq) + 14.87(na2o + (5) k2o)liq – 9.176(h2oliq), and t (°c) = 815.3 + 265.5(mg#liq) + 15.37(mgoliq) + 8.61(feoliq) + 6.646(na2o (6) + k2o)liq – 12.83(h2oliq) + 39.16p, where mg#liq is a molar ratio calculated as mg#liq = (mgoliq/40.3)/(mgoliq/40.3 + feoliq/71.85), p is a pressure in gpa and the remaining terms are weight percentages of oxides in the liquid. the equations (5) and (6) give a see of ±51° c and ±60° c, respectively. the addition of different compositional parameters including water and of a pressure term have reduced the overall accuracy of the thermometer, when compared to the early works of helz and thornber [1987] and montierth et al. [1995], but have greatly expanded its applicability. the equations (4-6) are applicable to any volcanic rocks saturated at least with olivine over wider range of conditions: p up to 14.4 gpa, t = 729-2000° c, sio2 = 31.5-73.64 wt%, sum of alkalis (na2o + k2o) up to 14.3 wt%, h2o up to 18.6 wt%. however, the equations (4-6) have not been tested for their ability to predict t for the experimental data not used for the calibration. it is thus unclear to what extent these equations can be used to calculate reliable temperatures for natural samples. another thermometer developed by yang et al. [1999] and updated by putirka [2008] places further constraints on the compositional degrees of freedom requiring the liquid being in equilibrium exclusively with olivine, plagioclase and clinopyroxene: t (°c) = –583 + 3141(xsio2)liq + 15779(xal2o3)liq + 1338.6(xmgo)liq – (7) 31440((xsio2)liq(xal2o3)liq) + 77.67p, where the terms such as (xsio2)liq represent the mole fraction of sio2 in the liquid and p is the pressure in gpa. this equation is able to reproduce the temperature for the calibration data set (73 experimental data) with a see of ±19° c and r2 of 0.92, and those for 119 test data with a see = ±26° c and r2 of 0.75. the aim of this work is to define and elucidate the methodology of work that should be followed in order to calibrate and test new empirically derived thermometric equations. we achieve this task by presenting a new glass3 methodology to derive new thermometers alessandro fabbrizio et al. 4 composition geothermometer based on olivine-bearing anhydrous glasses synthesized at 1-atm that would be applicable to shallow magmatic systems, to determine and track the changes in temperature of olivine-bearing samples from pillow lavas, lava flows, and lava lakes. 3. methods 3.1 calibration range and component calculation procedure the new model is based on a regression analysis of 1-atm and olivine-saturated (± other phases) anhydrous experiments performed using both natural and synthetic compositions (supplementary material). experiments that crystallized olivine at 1-atm but performed by using model compositions lacking one or more main chemical compounds were not included in the database. the reason for this choice is that a model composition lacking tio2, feo or alkalis, etc., cannot be considered fully representative of a natural composition. consequently, the result of the regression could be biased by these compositions if they represent a large fraction of the database. the selected experiments cover the temperature and compositional range of 1021-1530° c, 37.1 ≤ sio2 ≤ 72.4 wt% and 0 ≤ na2o + k2o ≤ 14.3 wt%, respectively. the liquid compositions (wt%), with iron oxide considered as all feo, are recalculated as cation fractions following this procedure: first, the weight percentage of each oxide is divided by its molecular weight having only one cation (i.e., 60.08 for sio2, 50.98 for alo1.5). then, these cation proportions are converted to cation fractions by normalizing each oxide by the sum of the cation proportions [putirka, 2008]. original oxide percentages, not renormalized to 100 wt%, were used for calculating the cation fractions. it should be noted that the use of unnormalized or normalized analyses to 100 wt% gives the same result when the analyses are expressed as cation fractions. 3.2 data and regression strategies a total of 701 experimental data were collected from the literature and 20% of them, 140 data, were randomly chosen as a reserve to be used as a test data for the thermometer. multiple linear least-squares regression analysis was applied to the calibration data set (n = 561) and the experiments that yielded t estimates outside of a 2σ standard error of estimate (see) for the calibration data (n = 18) were considered suspect and excluded from the model. then, the remaining 543 experimental temperatures of the calibration data set, without the outliers, were regressed once again to obtain the final equation. basaltic compositions are the most representative for both calibration and test data, followed by basaltic andesite and andesite (figure 1). also, these compositions represent about 2/3 of both databases and the remaining 1/3 is represented by foidite, trachybasalt, basanite, trachyandesite, basaltic trachyandesite, phonotephrite, tephriphonolite, picrobasalt, trachyte, phonolite, rhyolite and dacite; in order of decreasing abundance. the most represented temperatures are those in the range of 1100-1250° c corresponding to about 75% of the experimental data for both calibration and test data sets (figure 1). the temperatures in the range of 1000-1100° c represent 14% of the experimental data in the calibration data set and 9% of those in the test data set, whereas 12% of the calibration data and 16% of the test data represent temperatures above 1250° c (figure 1). in this sense, we consider the test data set to be representative of the calibration data set in term of compositional variation and temperature range. for creating the model and to avoid over-fitting of the data the t-ratios (calculated as the ratios between the coefficients and their standard errors and thus representing a measure of the precision with which the regression coefficients are measured; e.g., rawlings et al. [1998]) were checked for all variables, looking for the t-ratios ≥ 5, and the f-ratio (a measure of how the overall result of the regression is statistically significant; e.g., rawlings et al. [1998]) was checked in order to achieve the maximum value similarly to the approach used in putirka et al. [2003]. next, the chosen variables were checked using partial-regression leverage plots to understand their importance in the calibration of the model. finally, when several models had been calibrated, they were compared based on their ability to predict temperatures for the test data set. we looked for the model having the lowest standard error of estimate (see) on temperature, and whose regression slope and intercept for measured vs. calculated temperatures most closely approached to one and zero, respectively. finally, the model errors were compared to the experimental temperatures and liquid compositions in order to exclude their mutual correlation. 4. results result of the regression is a new glass-composition thermometer applicable only to olivine-bearing glassy rocks, having the following empirical expression: t (°c) = 1054.1±6.5 + (1458.0±23.5)(xmgo)liq – (267.11±45.8)(xcao)liq + (8) (116.3±30.9)(xna2o + xk2o)liq, where the terms such as (xmgo)liq represent the cation fraction of mgo in the liquid. the equation (8) yields see of ±23 and ±25° c for calibration and test data, respectively, with corresponding r2 values of 0.91 and 0.90. the equation (8) shows the lowest systematic error; the addition of other compositional terms does not improve the result of the regression significantly. the slope (m) and the intercept (c) of measured vs. calculated temperatures closely approach the ideal values of one and zero, respectively, for the calibration data set (i.e., m = 1 and c = 4 × 10-10° c), whereas the regression line for the test data has a slope of 0.99 and an intercept of +12.4° c. given the strict adherence of both regression lines to the ideal 1:1 line, the equation (8) describes both data sets with high precision (figure 2). the regression line for the calibration data is nearly coincident with the ideal 1:1 line, whereas the regression line of the test data approaches the ideal line with an uncertainty of less than ±1° c in the temperature range of 1100-1250° c, it overestimates the temperature by 1.8° c at 1000° c, whereas it underestimates the temperature by 3.5° c at 1500° c. furthermore, the residuals of the test data are uncorrelated with t and with all individual liquid compositional parameters, indicating that there is no systematic error, that no variables are missing in the model, and that the model residuals represent random errors. 5 methodology to derive new thermometers figure 1. number of samples for the calibration (a, b) and test data set (c, d) as a function of rock composition (a, c) and experimental temperature (b, d). alessandro fabbrizio et al. 6 figure 2. calculated and measured temperatures are compared for the proposed new glass geothermometer using both the calibration (a) and the test (b) data sets. in panel (a) the regression line is nearly coincident with the 1:1 line. in panel (b) the solid line is the regression line and the dashed line is the 1:1 line. calibration data are for the liquids saturated with olivine ± other phases taken from: arndt [1977]; baker and eggler [1987]; baker et al., [1994]; barclay and carmichael [2004]; barr et al., [2009]; bartels et al., [1991]; bender et al., [1978]; chen et al., [1982]; delano [1977]; dunn and sen [1994]; elkins et al., [2003]; fram and longhi [1992]; gee and sack [1988]; grove and beaty [1980]; grove and bryan [1983]; grove et al., [1982, 1992, 2003]; grove and juster [1989]; herd et al., [2009]; ishibashi and sato [2007]; jurewicz et al., [1993]; juster et al., [1989]; kennedy et al., [1990]; kinzler and grove [1985]; longhi and pan [1988]; longhi et al., [1978]; mahood and baker [1986]; matzen et al., [2011, 2013, 2017]; mccoy and lofgren [1999]; medard et al., [2004]; medard and grove [2008]; meen [1987, 1990]; monders et al., [2007]; parman et al., [1997]; prissel et al., [2016]; sack et al., [1987]; scoates et al., [2006]; stamper et al., [2014]; stolper [1977]; thy et al., [1998, 2006]; tormey et al., [1987]; vander auwera and longhi [1994]; vander auwera et al., [1998]; veksler et al., [2007]; wang and gaetani [2008]; zhang et al., [2018]. test data are also the liquids saturated with olivine ± other phases and are taken from: arndt [1977]; baker et al., [1994]; barr et al., [2009]; bender et al., [1978]; delano [1977]; dunn and sen [1994]; gee and sack [1988]; grove and bryan [1983]; grove et al., [1982, 1992]; grove and juster [1989]; herd et al., [2009]; ishibashi and sato [2007]; jurewicz et al., [1993]; juster et al., [1989]; kennedy et al., [1990]; kinzler and grove [1985]; longhi and pan [1988]; longhi et al., [1978]; mahood and baker [1986]; matzen et al., [2011]; mccoy and lofgren [1989]; meen [1987, 1990]; parman et al., [1997]; prissel et al., [2016]; sack et al., [1987]; stamper et al., [2014]; stolper [1977]; thy et al., [1998, 2006]; tormey et al., [1987]; vander auwera and longhi [1994]; veksler et al., [2007]; wang and gaetani [2008]; zhang et al., [2018]. 5. discussion 5.1 how to evaluate a thermometer? as with any model fitted to the measured data a thermometer needs a criterion to evaluate its performance. for this purpose, the calculated or predicted temperatures, t(calculated), are plotted against the measured or observed values, t(measured); ideally to define a linear trend with zero intercept, c, and unitary slope, m. despite the fact that the order in which these two variables are assigned to the independent and dependent variables, respectively (i.e., the x and y axes), generally matters, it is used interchangeably in many studies [e.g., cf. putirka et al., 1996, 2003]. while the coefficient of determination, r2, remains unchanged, slope and intercept of the linear trend vary for the two possible types of projections [e.g., piñeiro et al., 2008]. as illustrated in table 1 for the thermometer of helz and thornber [1987] and for our proposed eq. (8)., the choice of an independent variable makes an important difference, which can produce both false positive and false negative impressions about the fit performance. in general, for any thermometer the measured temperature can be decomposed to a sum of the calculated temperature predicted by the thermometer and an error, ε: t(measured) = t(calculated) + ε. (9) the error in the eq. (9) encompasses the measured vs. calculated differences not only due to the temperature measurement error, but also due to other effects related to physical inability of the fitting expression to describe all variations of temperature with respect to intensive parameters. let us now explore the two possible linear relationships for the fit of measured vs. calculated temperatures. these two fits differ in the choice of the independent variable: 𝑇�(measured) = m1 × t(calculated) + c1, (10a) 𝑇�(calculated) = m2 × t(measured) + c2, (10b) where 𝑇� is a temperature estimated by the linear fit and the m1, m2 are the slope and the c1, c2 are the intercept coefficients. these coefficients are related to the variance and covariance of the measured and calculated data [e.g., kenney and keeping, 1962]. specifically, the slope coefficient equals to the ratio of the covariance of both measured and calculated temperature data sets to the variance of the temperature used as an independent variable (i.e., the one usually plotted on the x axis). for the two possible choices of the independent variable (eq. 10a and 10b) the expressions for the slope coefficients read [e.g., kenney and keeping, 1962]: 7 methodology to derive new thermometers thermometer r2 calculated as independent measured as independent m1 c1 (°c) m2 c2 (°c) h&ta, eq. 1 0.89 0.90 135 0.98 0.26 this study, eq. 8 0.90 0.99 12.4 0.91 113 ahelz and thornber, [1987]. equation numbers are referenced to this study. variables: r2 – coefficient of determination; m1, c1 – slope and the intercept of the fitted line taking t(calculated) as independent variable (eq. 10a); m2, c2 – slope and the intercept of the fitted line taking t(measured) as independent variable (eq. 10b). note that r2 is identical for both types of the fit. calculations performed for the test dataset. table 1. comparison of calculated vs. measured and vice versa linear fits for selected thermometers. (11a) (11b) where the functions cov(∙,∙) and var(∙) are the covariance and variance, respectively, and the eq. (11a) and (11b) differ in their denominator. the relationship between measured and calculated temperatures, eq. (9), can be inserted into both expressions for the slope coefficient, eq. (11), in order to eliminate t(measured) from the equations. assuming the model for t(calculated) is unbiased, that is, the mean value of the error ε approaches zero, and after a bit of algebra we arrive to: (12a) (12b) in the eq. (12a) for the slope coefficient m1 the calculated temperature is considered to be an independent variable, whereas in the eq. (12b) for the slope coefficient m2 it is the measured temperature. let us now assume an ideal unbiased model for the t(calculated). in such model, the error ε is a random variable which is uncorrelated to the temperature itself; in the opposite case the model would not be ideal as it could be modified to remove its residual temperature dependence. therefore, the covariance between the calculated temperature and the error ε approaches zero. the slope m1, as defined by the eq. (12a), thus approaches unity. the error ε itself, while it averages out to zero mean, has some nonzero and positive variance. the slope m2 (eq. 12b) is thus always less than unity. in the ideal model, the measured and calculated variables are expected to fit to a line with a unit slope. this condition is generically fulfilled only by the slope m1. therefore, if the unit slope and zero intercept of the fit of calculated vs. measured data are taken as a benchmark for the model performance, then the only statistically correct way is to use the calculated temperature as an independent variable (usually on the x axis). also, this result is intuitively correct since in the simple linear regression used to perform the fitting, only the dependent variable (usually the on the y axis) is assumed to be burden with an error, that is, t(measured) deviates by an error from the t(calculated). we employ the approach in which the calculated temperature is treated as an independent variable in the following section in order to compare the performance of individual thermometers. 5.2 evaluation and comparison of previous olivine-saturated liquid thermometers the calibration and the test datasets were also used to recover temperatures, t, for the models of helz and thornber [1987], beattie [1993], montierth et al. [1995] and putirka [2008]; see table 2 for comparison of the individual models. the model of helz and thornber [1987], i.e., the eq. (1), captures 89% of the experimental t variation (r2 = 0.89) with a see of ±33° c. a similar result is obtained when eq. (1) is applied to the test data set (r2 = 0.89, see = ±35° c). however, in both cases slopes and intercepts of the regression lines of t(measured) vs. t(calculated) (y vs. x) are very poor being 0.91 and 121° c for the calibration and 0.90 and 135° c for the test data sets, respectively (figure 3). the model of montierth et al. [1995], i.e., the eq. (2), gives a similar estimate of t, with r2 = 0.89 and see = ±30° c when using the calibration data and r2 = 0.89 and see = ±33° c when using the test data. however, it is affected by higher systematic error given the slopes and the intercepts are all worse with 0.80 and 240° c for the regression line of the calibration data, and 0.79 and 252° c for the test data, respectively (figure 3). the model of beattie [1993], i.e., the eq. (3), gives the best results with see = ±21° c and r2 = 0.92 for the calibration data, and see = ±24° c and r2 = 0.91 for the test data. it is also affected by high systematic errors, but at lower level compared to the other models, given the slope and intercepts of the regression lines are 0.95 and 56° c for the calibration data, and 0.94 and 77° c for the test data (figure 3). m1 = , cov(t(calculated),𝜀) var(t(calculated)) m2 = , cov(t(calculated),t(measured)) var(t(measured)) m2 = . var(t(calculated) + cov(t(calculated),𝜀) var(t(calculated))+2cov(t(calculated, 𝜀)) + var(𝜀) cov(t(calculated),t(measured)) var(t(calculated)) m1 = , alessandro fabbrizio et al. 8 9 methodology to derive new thermometers figure 3. calculated and measured temperatures are compared for the available glass geothermometers using both our calibration (a, c, e, g, i, m) and test (b, d, f, h, l, n) data sets. solid lines are regression lines and dashed lines are 1:1 lines. (a, b) equation 1 [helz and thornber, 1987]; (c, d) equation 2 [montierth et al., 1995]; (e,f ) equation 3 [beattie, 1993]; (g,h) equation 4 [putirka, 2008]; (i, l) equation 5 [putirka, 2008]; (m, n) equation 6 [putirka, 2008]. both data sets are liquids saturated with olivine ± other phases and are identical to those as in figure 2. the model of putirka [2008], i.e., the eq. (4), yields r2 = 0.89 and see = ±39° c. when applied to the test data, the eq. (4) yields again very similar values with r2 = 0.89 and see = ±43° c. however, the slopes and the intercepts of the regression lines are worst, being 0.70 and 353° c for the calibration data and 0.69 and 365° c for the test data (figure 3). a similar result is obtained for the model of putirka [2008], i.e., the eq. (5), with r2 = 0.84 and see = ±42° c for the calibration, and see = ±43° c and r2 = 0.87 for the test data sets, and with the slope and the intercept of the regression line 0.74 and 297° c for the calibration, and 0.74 and 292° c for the test data sets, respectively (figure 3). from all tested thermometers, the model of putirka [2008], i.e., the eq. (6), similarly to the model of beattie [1993], yields a good result when recovering t for the calibration (r2 = 0.87, see = ±28° c) and for the test (r2 = 0.88, see = ±28° c) data sets. however slope and intercept of the regression lines are still poor being 0.93 and 81° c for the calibration data, and 0.92 and 89° c for the test data (figure 3). in summary, the eq. (3) appears to predict t for both calibration and test data with an error in the range of 2124° c, and over 1200° c it tends to slightly underestimate predicted t, whereas it slightly overestimates t below 1200° c (figure 3). temperature estimates by the eq. (1) and (2) are less precise, with see between 33-35° c and 3033° c, respectively. predictions of temperature by the eq. (1) are underestimated over 1400° c and overestimated below 1400° c, those predicted by the eq. (2) are underestimated above 1200° c and overestimated below 1200° c (figure 3). the errors of t predicted by the eq. (4-5) are in the range of 39-43° c and these equations tend to underestimate the temperatures above 1175° c and above 1125° c for the eq. (4) and (5), respectively (figure 3). in addition (figure 3), predicted t are overestimated below 1175° c by the eq. (4) and below 1125° c by the eq. (5). the equation (6) tends to overestimate the temperatures below 1120° c and to underestimate the temperatures above 1120° c with a see of ±28° c (figure 3). despite the eq. (1-6) are able to capture a great fraction (84-92%) of the experimental temperature variation with relatively low standard errors in the range of 21-43° c, they are affected by strong systematic errors as shown by the slopes and the intercepts of the regression lines of measured vs. calculated temperatures (figure 3). their lower performance, compared to that offered by the new proposed equation (8), can be explained by the following reasons: (i) the eq. (1-2) proposed by helz and thornber [1987] and montierth et al. [1995] are calibrated on very specific bulk rock compositions from hawaiian volcanoes, i.e., kilauea lavas for eq. (1) and mauna loa lavas for eq. (2). consequently, when used to predict temperatures of rocks outside their compositional calibration range, their performance decrease. however, these thermometers are by far superior with respect to alessandro fabbrizio et al. 10 table 2. summary of the tested geothermometers. thermometer calibration dataset test dataset r2 see (°c) m c (°c) r2 see (°c) m c (°c) h&ta, eq. 1 0.89 33 0.91 121 0.89 35 0.90 135 montierthb, eq. 2 0.89 30 0.80 240 0.89 33 0.79 252 beattie (1993), eq. 3 0.92 21 0.95 56 0.91 24 0.94 77 putirka (2008), eq. 4 0.89 39 0.70 353 0.89 43 0.69 365 putirka (2008), eq. 5 0.84 42 0.74 297 0.87 73 0.74 292 putirka (2008), eq. 6 0.87 28 0.93 81 0.88 28 0.92 89 this study, eq. 8 0.91 23 1 4x10-10 0.90 25 0.99 12.4 ahelz and thornber, [1987]. bmontierth et al., [1995]. equation numbers are referenced to this study. variables: r2 – coefficient of determination; see – standard error of estimate; m, c – slope and the intercept of the line fitted to the calculated vs. measured data; taking calculated data as independent variable (x axis). any general thermometer when applied to their specific rock composition; (ii) the eq. (3) proposed by beattie [1993] and the eq. (4-6) derived by putirka [2008] are general thermometers developed with the aim to predict temperatures for the liquids in equilibrium with olivine in a broader compositional and p-t range. their low performance can be explained by their application to an anhydrous database of experiments performed solely at 1-atm with a narrower compositional range. also, their original calibration dataset included many data points representing simplified model systems lacking some of the main chemical components. in order to avoid fitting bias (for details, see the section on methods) these data points were excluded from our database which could have influenced the result of the regression. in summary, the eq. (8) seems to be a useful predictive tool for estimating the temperature of anhydrous silicate liquids, especially in the compositional range of basalts to andesites in equilibrium with olivine (±other phases) at one atmosphere. 5.3 applications the proposed new eq. (8) can be used as a geothermometer in particular for basalt, basaltic andesite, and andesite rocks erupted at 1-atm and temperatures in the range of 1100-1250° c. for this purpose, the eq. (8) represents a useful alternative to the other geothermometers being characterized by a lower systematic error. temperature measurements of the alae and makaopuhi lava lakes, hawaii, cover the range of 1140-1190° c [peck, 1978; wright and okamura, 1977; wright and peck, 1978]. a comparison of these temperatures may be done with the temperatures predicted via different geothermometers. apart from our equation, we choose to apply this comparison to the eq. (1-2) given they were designed specifically for the hawaiian volcanoes [helz and thornber, 1987; montierth et al., 1995], to the eq. (3) as it can be widely applied given it is based on activity-composition relations that tend to provide better extrapolations [beattie, 1993], and to the eq. (4) given it is similar to eq. (12) but its calibration is based on a wider compositional range [putirka 2008], using eq. (1-4) and eq. (8) in combination with the glass compositions representative of six different hawaiian volcanoes [garcia et al., 1989] we derive predicted temperatures which range from 1120 to 1195° c (figure 4, 23). in particular, the eq. (8) predicts temperatures in the range of 1145-1186° c which deviate less than 15° c compared to the temperatures predicted by the equations of beattie [1993], montierth et al. [1995], and putirka [2008]. higher differences (17-25° c) between the temperatures predicted by the eq. (8) and those predicted by the equation of helz and thornber [1987] are ascribed to the wider compositional database that has been used to calibrate our model. also, we note (table 3) that one glass composition from mauna loa gives an unrealistic temperature of 1308° c when calculated by the model of beattie [1993], whereas it is consistently in the range of 1166-1171° c when estimated by our equation and by the models of montierth et al. [1995] and putirka [2008]. the reason for this discrepancy is unclear. further, we employed the eq. (8) to calculate the temperatures of atlantic morb glassy rocks [schilling et al., 1983] and we compared the results with the temperatures calculated by the model of beattie [1993], i.e., the eq. (3), (figure 4, table 3). differences between the results of the two models are again lower than 15° c. the comparison with a thermometer of general application, such as that of beattie [1993], thus demonstrates the validity of our model for predicting temperatures of 1-atm magmatic systems. 6. conclusions our understanding of how a low pressure (1-atm) magmatic system, such as a lava flow or a lava lake, might look like, and how it might evolve, hinges on our interpretation of temperatures at which the magma body stagnates and partially crystallizes. silicate liquids in equilibrium with mineral phases provide an archive of these t conditions and hence the calculation of the temperature is crucial for deciphering the evolution of such magmatic systems. for a robust geothermometer, however, it is fundamental: (i) to be based on a large data set in order to ensure applicability to a wide range of liquid compositions; (ii) to be accompanied by the error estimates of both temperature recovery and prediction based on the calibration and testing (i.e., not used in the calibration process) data sets, respectively; (iii) to be evaluated using a regression of the observed/measured temperatures (dependent variable, y-axis) vs. predicted/calculated temperatures (independent variable, x-axis). 11 methodology to derive new thermometers alessandro fabbrizio et al. 12 figure 4. calculated temperature for glassy samples from hawaiian volcanoes (a) and the mid-atlantic ridge (b). in panel (a) circles are temperatures calculated by eq. (1), helz and thornber [1987]; squares are temperatures calculated by eq. (2), montierth et al. [1995]; downward triangles are temperatures calculated by eq. (3), beattie [1993]; diamonds are temperatures calculated by eq. (4), putirka [2008]; upward triangles are temperatures calculated by eq. (8), this study. in panel (b) squares are temperatures calculated by eq. (3), beattie [1993]; circles are temperatures calculated by eq. (8), this study. 13 methodology to derive new thermometers table 3. predicted temperatures for glassy rocks from the hawaiian volcanoes and from the mid-atlantic ridge. t (°c)a t (°c)b/δtc t (°c)d/δte t (°c)f/δtg t (°c)h/δt i hawaiian volcanoesl loihi 1169 1167/2 1151/18 1169/0 1174/-5 loihi 1166 1163/3 1149/17 1166/0 1171/-5 loihi 1159 1153/6 1140/19 1156/3 1160/-1 kilauea 1161 1150/11 1143/18 1159/2 1163/-2 kilauea 1163 1152/11 1144/19 1161/2 1165/-2 kilauea 1162 1150/12 1143/19 1159/3 1163/-1 kilauea 1159 1147/12 1139/20 1155/4 1157/2 kilauea 1178 1172/6 1159/19 1178/0 1185/-7 kilauea 1161 1148/13 1142/19 1159/2 1162/-1 kilauea 1161 1147/14 1141/20 1158/3 1161/0 kilauea 1167 1157/10 1147/20 1165/2 1169/-2 kilauea 1153 1139/14 1130/23 1145/8 1146/7 kilauea 1154 1141/13 1132/22 1147/7 1148/6 mauna loa 1175 1167/8 1156/19 1174/1 1180/-5 mauna loa 1170 1162/8 1151/19 1169/1 1174/-4 mauna loa 1158 1148/10 1135/23 1150/8 1152/6 mauna loa 1159 1145/15 1142/17 1159/0 1162/-3 mauna loa 1168 1308/-141 1149/19 1166/2 1171/-3 mauna loa 1160 1151/10 1141/19 1157/3 1161/-1 mauna loa 1171 1162/8 1152/19 1170/1 1175/-4 mauna loa 1186 1183/3 1168/18 1188/-2 1196/-10 mauna loa 1161 1149/12 1141/20 1157/4 1160/-1 mauna loa 1156 1145/11 1134/22 1150/6 1152/4 mauna loa 1186 1184/2 1167/19 1187/-1 1195/-9 mauna loa 1156 1146/10 1136/20 1151/5 1154/2 mauna kea 1167 1160/7 1146/21 1163/4 1167/0 mauna kea 1145 1132/13 1120/25 1133/12 1133/12 mauna kea 1169 1161/8 1149/20 1166/3 1171/-2 mauna kea 1161 1153/8 1141/20 1157/4 1160/1 mauna kea 1167 1158/9 1147/20 1164/3 1169/-2 mauna kea 1148 1137/12 1125/23 1139/9 1140/8 mauna kea 1166 1158/8 1147/19 1165/1 1169/-3 mauna kea 1164 1155/8 1145/19 1162/2 1166/-2 mauna kea 1163 1154/9 1144/19 1161/2 1165/-2 mauna kea 1168 1159/9 1148/20 1166/5 1170/-2 hualalai 1167 1158/7 1149/18 1166/1 1171/-4 hualalai 1178 1174/4 1160/18 1179/-1 1186/-8 hualalai 1168 1159/9 1149/19 1166/2 1171/-3 kohala 1161 1153/9 1142/20 1158/3 1162/-1 kohala 1154 1147/7 1133/21 1148/6 1150/-4 kohala 1152 1142/10 1135/18 1150/2 1152/0 t (°c)a t (°c)b/δtc morbm normal ridge 1184 1188/-4 normal ridge 1224 1235/-11 normal ridge 1188 1195/-7 normal ridge 1186 1185/1 normal ridge 1184 1184/0 normal ridge 1192 1193/-1 normal ridge 1173 1173/0 normal ridge 1194 1194/0 normal ridge 1159 1159/0 normal ridge 1176 1176/0 transition zone 1166 1162/4 transition zone 1185 1182/3 transition zone 1177 1173/4 transition zone 1208 1210/-2 transition zone 1173 1173/0 transition zone 1210 1215/-5 transition zone 1162 1166/-4 transition zone 1205 1208/-3 plume 1155 1149/6 plume 1189 1188/1 plume 1200 1202/-2 plume 1182 1185/-3 plume 1210 1219/-9 plume 1157 1157/0 plume 1217 1227/-10 plume 1210 1223/-13 atemperature calculated by eq. (8). btemperature calculated by eq. (3), beattie [1993]. ctemperature difference ta-tb. dtemperature calculated by eq. (1), helz and thornber [1987]. etemperature difference ta-td. ftemperature calculated by eq. (2), monthiert et al., [1995]. gtemperature difference ta-tf. htemperature calculated by eq. (4), putirka [2008]. itemperature difference ta-th. ltemperatures calculated for rocks from hawaiian volcanoes on the basis of the glass compositions reported in garcia et al., [1989]. mtemperatures calculated for rocks from the mid-atlantic ridge on the basis of the glass compositions reported in schilling et al., [1983]. compared to prior geothermometers in which the liquid is saturated with olivine (± other phases), our eq. (8) provides a better estimate of t, with standard error of estimate of ±23° c for the calibration data set, and of ±25° c for the test data set. compared to similar liquid geothermometers, eq. (8) has been tested on a relatively large data set (n = 140), that was not used in the calibration process. preferably it can be used to obtain the temperature of any anhydrous silicate liquid, with composition ranging from basalt to andesite, in equilibrium with olivine (± other phases) at 1-atm. under these conditions the predicted temperatures are characterized by a lower systematic error than those predicted by using the previous geothermometers. acknowledgements. this work was supported by the czech science foundation (project 18-01982s to a. fabbrizio) and by the center for geosphere dynamics (unce/sci/006 to v. špillar). mike baker, john beckett, and keith putirka are thanked for their useful comments on an early version of the manuscript. two anonymous reviewers are thanked for providing the official revision. references arndt, n.t. 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135, 355-372. zhang, y.s., t. hou, i.v. veksler, c.e. lesher and o. namur (2018). phase equilibria and geochemical constraints on the petrogenesis of high-ti picrite from the paleogene east greenland flood basalt province. lithos, 300, 20-32. *co r r e s po n d i n g a u t h o r: alessandro f a b b r i z i o, institute of petrology and structural geology, faculty of science, charles university, albertov 6, 12843 prague, czech republic; e-mail: alessandro.fabbrizio@natur.cuni.cz, +420221951525, © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 17 methodology to derive new thermometers mailto:alessandro.fabbrizio@natur.cuni.cz 41kitahara.qxd 909 annals of geophysics, vol. 47, n. 2/3, april/june 2004 the exhibition «documenting disasters: natural disasters in japanese history 17032003» was held at the national museum of japanese history located in sakura, from 8 july to 21 september 2003. this was a unique and innovative project, documenting a truly collaborative research between scientists and historians in japan. the project started in the summer of 2000, when our application was accepted by the national museum of japanese history. as a general director of this project, i firstly aimed at searching for a new approach to the study of historical disasters. japan has been struck by various and numerous natural disasters such as earthquakes, tsunamis, and volcanic eruptions, which were documented in a variety of ways, such as disaster maps, private diaries and monuments. we planned to collect these materials from all over the japanese islands, and to exhibit selected ones for the audience to deepen their knowledge about various types of the historical disasters of japan. here, introducing the exhibits, i examine whether they could make the audience easily understand the aim of the exhibition. 1. the committee and the exhibits the committee for the exhibition project comprised 24 members, 16 natural scientists, 6 historians, and 2 staff of the national museum faculty. the exhibition was composed of four sections, that is, tsunamis, earthquakes, volcanic eruptions, and recoveries from them, selected from the big historical disasters in edo period of japanese history. the entrance hall and the stairs leading to the courtyard were reserved for the exhibition, occupying more than 1500 m2. 2. galleries for special exhibits tsunami section (entrance hall and stairs) (fig. 1a). located at the subduction zone of oceanic plates, japan frequently experiences earthquakes, volcanic eruptions, and tsunamis, the earliest recorded disaster being that of the 684 hakuho quake. the number of tsunamis in japan totals 196 over a 1320 year period, averaging one event every 6.7 years, the highest rate of occurrence in the world. this figure is small in comparison with floods and earthquakes, anyway, it accounts for the low level of tsunami awareness among japanese people. fortunately, japan’s historical record, combined with scientific insights and technological advances, such as computer simulation, provides a rich body of information and perspectives on their hazards. review of exhibition on «documenting disaster: natural disasters in japanese history 1703-2003» by itoko kitahara kanagawa university, yokohama city, japan mailing address: dr. itoko kitahara, kanagawa university, yokohama city, japan; home address: aobadai 2-33-2b502 aoba-ku, yokohama city, 227-0062, japan; e-mail: itoko@kk.iij4u.or.jp 910 itoko kitahara earthquake section (first gallery for the special exhibits) (fig. 1b). earthquakes have been recorded in japanese historical documents for over 1300 years. in addition to the official chronicles written by the officers of the imperial court in the ancient capitals of nara and kyoto, diaries of noblemen and priests make frequent mention of past earthquakes and tsunamis. for this exhibition, we selected five historical earthquakes, between 18th and 20th centuries. the 1847 zenkoji quake and the 1858 hietsu quake represent typical inland earthquakes. the exhibition also presented detailed distributions of seismic intensity in the edo-tokyo area for the 1703 genroku quake, the 1855 ansei-edo quake (fig. 2), and the 1923 great kanto quake. we tried to make clear not only the seismological features of these earthquakes, but also the social attitudes toward these natural disasters among people in the past. volcano section (second gallery for the special exhibits). there are many volcanoes in the japanese archipelago. of the roughly 1500 active volcanoes in the world, 108 are in japan. this exhibition seeked to illustrate important features of volcanic disasters by focusing on the representative cases of the mounts fuji, unzen, and asama. mt. fuji is the largest volcano in japan. during a very large eruption that occurred about 300 years ago (1707), abundant ash and scoria were ejected over an area reaching as far as edo. mt. unzen in kyushu is another active volcano. it had been quiet for 192 years before it erupted again in 1990. mt. asama is yet another example of very active and dangerous volcano. its eruption in 1783 caused enormous disaster. this exhibition aimed at showing that the more the people are aware of the nature of volcanic eruptions, the less they will suffer from them. volcanic landscapes and hot springs are a pleasure to enjoy, but we must also have wisfig. 1a-d. a) stairs to the special exhibits: the banner shows the actual heights of the big tsunamis from the edo period to nowadays. b) entrance to the gallery on earthquakes. c) entrance to the gallery on recovery. d) the namazu-e, woodblock prints of the giant subterranean catfish, believed to be the source of earthquakes. a b c d 911 exhibition on «documenting disaster: natural disasters in japanese history 1703-2003» dom to foretell dangers and to mitigate their damage as much as possible. recovery section (third gallery for the special exhibits) (fig. 1c,d) relief to natural disaster victims is a recurring subject of administrative records throughout japanese history. but such records do not often provide detailed descriptions of recovery from the perspective of ordinary people. this exhibition seeked to demonstrate the ways by which the ordinary people have recovered from natural disaster. it focused on three major disasters from the edo period: the kisakata quake of 1804, the zenkoji quake of 1847, and the ansei-edo quake of 1855, and on the recent hanshin-awaji earthquake. 3. conclusions the year 2003 was the 80th anniversary of the great kanto earthquake, and the 300th anniversary of the genroku earthquake, which also struck the kanto area together with a tsunami. three more museums in the kanto area held exhibitions on these earthquakes. newspapers and magazines had a number of articles introducing these disaster exhibitions, and they seemed to be effective for drawing more than 40 000 people to our museum. anyway, we can say that this exhibition was successful. our memories of disasters fade away quite quickly, as we can see in the case of the great kobe earthquake occurred in 1995. no wonder people forget the historical disasters which occurred centuries ago. one of the purposes of the exhibition was to let people know more about the prevention measures for disasters. along with the scientific description of the disasters, these historical materials about individual efforts to reconstruct their own families, villages, and communities, probably gave more personal and sympathetic impression to the audience. even if there is no indication of definite ways to prevent disasters, exhibiting the historical materials could be effective and persuasive. reference documenting disaster, natural disasters in japanese history 1703-2003 (2003), catalogue of the exhibition, national museum of japanese history, sakura, pp. 167. fig. 2. the great ansei-edo earthquake (11 november 1855) in a section of a coeval painted scroll. s u a l c u n e a n o m a l i e d e l l a f r e q u e n z a c r i t i c a i o n o s f e r i c a d u r a n t e u n ' e c l i s s e d i s o l e guglielmo zanotelli l e eclissi di s o l e offrono a l c u n e p o s s i b i l i t à di s t u d i a r e l'andam e n t o della c o n c e n t r a z i o n e degli e l e t t r o n i l i b e r i n e l l a i o n o s f e r a in c o r r i s p o n d e n z a a l l e v a r i a z i o n i d e l l ' a g e n t e i o n i z z a n t e solare. q u e s t e p o s s i b i l i t à f u r o n o già n e l passato messe a profitto da diversi a u t o r i , specie p e r lo s t u d i o dello strato f 2 c h e è cpiello p e r c u i p e r m a n g o n o a n c o r oggi l e m a g g i o r i i n c e r t e z z e c i r c a l a c o r r e l a z i o n e degli e l e m e n t i i n t e r e s s a t i . ci s e m b r a p e r c i ò n o n privo di i n t e r e s s e di a p p o r t a r e con il p r e s e n t e l a v o r o un c o n t r i b u t o alla q u e s t i o n e , e s p o n e n d o i risultati dei sondaggi i o n o s f e r i c i eseguiti presso l a sede d e l l ' i s t i t u t o n a z i o n a l e <fi geofisica in r o m a nel corso dell'eclisse di sole del 25 f e b b r a i o 1952. in r o m a l ' e c l i s s e si presentava c o m e m o l t o p a r z i a l e , di grandezza 0.27 in d i a m e t r i al suolo, e 0,21 c i r c a alla q u o t a di 2 5 0 k m , c o r r i s p o n d e n t e ad lina o c c u l t a z i o n e m a s s i m a di poco più di un d e c i m o della s u p e r f i c i e solare. p u r t u t t a v i a i l suo influsso si è r i v e l a t o n o n trascurab i l e n e l l e r e g i s t r a z i o n i i o n o s f e r i c h e , sì da p e r m e t t e r c i di d e d u r r e alc u n e c o n s i d e r a z i o n i s u l l ' o r i g i n e del r i t a r d o con cui la d i m i n u z i o n e di c o n c e n t r a z i o n e e l e t t r o n i c a si presenta r i s p e t t o alla f a s e m a s s i m a dell ' e c l i s s e , e d e l l ' a p p a r e n t e v a r i a z i o n e del coefficiente di r i c o m b i n a z i o n e n e l corso della stessa. l ' a p p a r e c c h i a t u r a r a d i o e l e t t r i c a usata esplorava a u t o m a t i c a m e n t e nel g i r o di 2 0 " l a g a m m a di f r e q u e n z e c o m p r e s e f r a 1,7 e 18 me, e r e g i s t r a v a f o t o g r a f i c a m e n t e gli e c h i i o n o s f e r i c i . i v a l o r i d e l l e f r e q u e n ze c r i t i c h e / dedotte dalle registrazioni del g i o r n o dell'eclisse sono r i p o r t a t e in f u n z i o n e del t e m p o t n e l l a fig. 1 p e r l o strato f 2 c h e è q u e l l o c h e a n o i p a r t i c o l a r m e n t e interessa, m e n t r e con l a l i n e a trattegg i a l a si è i n d i c a t o l ' a n d a m e n t o m e d i o della f r e q u e n z a c r i t i c a /„ ricavata d a l l e r e g i s t r a z i o n i dei giorni i m m e d i a t a m e n t e p r e c e d e n t i e seguenti q u e l l o dell'eclisse. s i c c o m e ai fini d e l l ' i n t e r p r e t a z i o n e i n t e r e s s a osservare n o n t a n t o l ' a n d a m e n t o della f r e q u e n z a c r i t i c a p e r se stessa n e l corso dell'eclisse, q u a n t o in r e l a z i o n e ai v a l o r i m e d i della stessa n e i g i o r n i p r e c e d e n t i 3 6 8 c u c l i e l m o z a n o t e l l i e seguenti, si è costruito i l diagramma (fìg. 2) d e l l ' a n d a m e n t o della funzione (///o)2; stante la proporzionalità della concentrazione elettronica ionosferica al quadrato della frequenza c r i t i c a , tale f u n z i o n e equivale al rapporto n/na f r a le rispettive c o n c e n t r a z i o n i elettronic h e n al m o m e n t o dell'eclisse, ed n a media nei giorni precedenti e seguenti. con questo accorgimento si sono p r a t i c a m e n t e depurati i risultati dall'influenza della v a r i a b i l e altezza del sole per l e varie ore della giornata. l a curva tratteggiata nella stessa fig. 2 rappresenta l'area solare scoperta n e l corso dell'eclisse in percento dell'area totale del disco solare, per l a quota di 2 5 0 k m . 10 me \ i— i eclisse — i j i i 6 i i l 1 ^ / 6 1 i i i i 1 1 1 i 4 i i p * ! 10 h ! 12* t fig. 1 la frequenza critica / durante l'eclisse di sole in confronto con quella /» media nei giorni precedenti e seguenti l'eclisse. d a l l ' e s a m e della fig. 2 risulta che la concentrazione e l e t t r o n i c a scende al disotto del valore medio già un paio d'ore prima dell'inizio dell'eclisse, e segna un m i n i m o netto c i r c a 2 0 ' avanti il primo contatto, con una altezza virtuale di 2 3 0 k m . un nuovo m i n i m o si riscontra in corrispondenza alla fase massima dell'eclisse, ed un altro infine sul finire di questa in u n a posizione c h e però non può essere d e t e r m i n a t a con esattezza per la mancanza di alcuni punti della registrazione, m a localizzabile con certezza poco oltre l ' u l t i m o contatto. l'altezza virtuale in corrispondenza a questi altri m i n i m i di f r e q u e n z a c r i t i c a f u di 2 5 0 k m . s u a l c u n e a n o m a l i e d e l l a f r e q u e n z a c r i t i c a i o n o s f e r i c a ecc. ? 6 9 se ci r i f e r i a m o alla equazione c h e lega la concentrazione elettronica n con il numero q di elettroni prodotti per unità di volume e di tempo, e con quello r rispettivamente di elettroni eliminati, d n r n — = q — r, [1] d t essendo in corrispondenza ad un minimo dn/dt = 0 deve aversi per l ' e q u i l i b r i o q = r . [2] e s a m i n a n d o in fig. 1 la curva dell'andamento medio della frequenza critica nei giorni non di eclisse, si riscontra che in corrispondenza all'ora della fase massima tale curva decorre quasi orizzontale, ed ivi potrà supporsi che per la corrispondente concentrazione elettronica sia dn0/dt — 0 ; quindi qo = r0 [ 3 ] essendosi contrassegnate con l'indice zero l e grandezze in assenza di eclisse. v i e n e generalmente ammesso che il numero q di elettroni prodotti sia proporzionale all'area scoperta s del disco solare, e che gli elettroni siano sottratti alla ionosfera in virtù del processo di ricombinazione, per cui r = a n2, ove a è il coefficiente di ricombinazione. dividendo m e m b r o a m e m b r o le [ 2 ] e [ 3 ] si ottiene allora q» r a s a ns a /4 t = i = 7 7 • [ 5 ] •5» a0n0~ a„ fa p e r la fase massima della nostra eclisse è circa s/so = 0,89 e corrispondentemente n/n„ = 0,73 : per la precedente dovrebbe ammettersi che forse a / a 0 = l , 7 , cioè che il coefficiente di ricombinazione fosse aumentato rispetto al normale di circa il 7 0 % . è bensì vero che si potrebbe introdurre come causa di diminuzione della concentrazione elettronica oltre il processo di ricombinazione anche la cattura 3 7 0 c u c l i e l m o z a n o t e l l i degli elettroni da parte di molecole o di atomi ( 1 ) ; ciò e q u i v a r r e b b e a p o r r e r = <i n~ -jbn. t u t t a v i a anclie in questo caso si dovrebbero supporre diversi i valori dei coefficienti a , b, in presenza od in assenza di eclisse, per giustificare il valore trovato 0,73. non m a n c a n o certo le ragioni per spiegare la variazione di un coefficiente c h e in realtà si dovrebbe supporre costante, quale quello di ricombinazione. tuttavia le considerazioni c h e ora svolgeremo circa 1.0 eclisse h i i i i 0.8\ i i i s . , m y^nfro 0.6 i i i i i 1.0 0.8 9h 10h i i ah n s/so 12h t f i g . 2 • ti rapporto fra la concentrazione elettronica n durante l'eclisse ed media nei giorni precedenti e seguenti in confronto alla percentuale di superficie solare scoperta. il m i n i m o che presenta la funzione n/na in corrispondenza al finire dell'eclisse sembrano indurci a rivolgere la nostra attenzione piuttosto alla funzione q che non alla r. i n f a t t i si tratta in quest'ultimo caso di i n t e r p r e t a r e non solo il significato della diminuzione della concentrazione elettronica c h e si presenta al t e r m i n e dell'eclisse, ma a n c h e di spiegare l'origine di un r i t a r d o così notevole rispetto al massimo occultamento della superficie solare. si noti c h e tale ritardo si riscontra oltre che n e l l e nostre registrazioni, anche nei rilevamenti ionosferici di altri autori ( 2 ) ( 3 ) in corrispondenza di eclissi solari. p u r e n e l punto di m i n i m o c h e ora consideriamo deve verificarsi per l ' e q u i l i b r i o , almeno approssimativamente la [ 4 ] ; se si volesse però assumere ancora il n u m e r o degli elettroni prodotti c o m e proporzionale alla superficie solare scoperta, si dovrebbe prendere ormai uguale ad s u a l c u n e a n o m a l i e d e l l a f r e q u e n z a c r i t i c a i o n o s f e r i c a e c c . 3 7 1 uno il p r i m o m e m b r o di tale relazione, e quindi, ammettendo agente la sola ricombinazione, essendo ora n/n0 uguale a circa 0,6, si perverr e b b e ad un rapporto a/a0 — 2,7 circa. p o i c h é l'altezza virtuale dello strato f 2 desunta dalle registrazioni in corrispondenza a questo min i m o c o i n c i d e con quella per il m i n i m o centrale, a questo elemento non può p e r t a n t o attribuirsi la f o r t e variazione del coefficiente di r i c o m b i n a z i o n e . p e r renderci ragione di come possano svolgersi i fatti, riflettiamo che le cause di eliminazione degli elettroni dalla ionosfera sono in generale i n t e r n e a questa, anzi locali e ristrette attorno al punto considerato, c o m e appunto la ricombinazione degli elettroni, la cattura di questi da parte di molecole o da atomi, e simili. quale influsso pot r e b b e dunque avere su queste cause locali l'eclisse dopo un'ora circa dalla sua fase massima, e quando la superficie solare è di nuovo completamente s c o p e r t a ? i n qual modo potrebbe giustificarsi l a f o r t e variazione del coefficiente di r i c o m b i n a z i o n e per i due m i n i m i ora esaminati con un ipotetico effetto dell'eclisse? s e m b r a senz'altro più logico ammettere che il ritardo con cui si presenta la variazione della concentrazione elettronica sia da imputare alla stessa causa da cui la q deriva, causa esterna alla ionosfera ed estranea perciò ad ogni circostanza locale di questa. ciò equivale a supporre la q/q0 non coincidente con la s / s 0 ; o quanto m e n o ad identificarla con u n a s/sa ritardata di un conveniente intervallo di t e m p o : tuttavia se quest'ultima ammissione può valere a spiegare il m i n i m o c h e si riscontra sul finire dell'eclisse, di alcun ausilio ci sar e b b e per l'interpretazione di quel m i n i m o che nelle nostre registrazioni appare ancor prima dell'inizio di quella. s i a m o perciò n a t u r a l m e n t e orientati a m e t t e r e in relazione la q con q u a l c h e causa, capace di indurre la produzione di elettroni ionosferici, localizzata in zone esterne alla parte visibile del disco s o l a r e : l ' o c c u l t a m e n t o di tali zone da parte della luna potrebbe essere la causa dei due m i n i m i e s t r e m i ; p o t r e b b e altresì c o n t r i b u i r e , insieme con l ' o c c u l t a m e n t o di una porzione della superficie solare visibile, al min i m o centrale, rendendo ragione del valore sperimentalmente riscontrato n e l l a diminuzione di concentrazione elettronica, senza ricorrere ad un fittizio aumento del coefficiente di ricombinazione. questa conclusione cui siamo pervenuti ragionando sugli elementi tratti dalle nostre registrazioni viene corroborata dalle circostanze che si presentarono per le eclissi solari del 4 e 5 f e b b r a i o 1943, 1° agosto 1943 e 25 gennaio 1 9 4 4 ; tali eclissi f u r o n o studiate da wells e 3 7 2 c u c l i e l m o zanotelli s h a p l e y ( 2 ) e presentarono m i n i m i della f r e q u e n z a c r i t i c a all'inizio e sul finire dell'eclisse, c h e gli stessi autori r i t e n n e r o poter essere messi in relazione ad una c o n c o r r e n t e attività solare, e specialmente a zone luminose della corona. roma —• istituto nazionale di geo fisica — giugno 1953. riassunto si espongono i risultati dei sondaggi ionosferici eseguiti in roma durante l'eclisse parziale di sole del 25 febbraio 1952, dai quali ri sultarono tre minimi della concentrazione elettronica, uno prima dell'inizio dell'eclisse, uno in corrispondenza alla fase massima ed uno sul finire dell'eclisse stessa. si mostra che ammettendo la causa dei due minimi estremi come localizzata in zone esterne alla parte visibile del disco solare, si può dar ragione dell'altrimenti inesplicabile ritardo dell'ultimo minimo rispetto all'eclisse visibile, e si può interpretare nel contempo la forte variazione apparente del coefficiente di ricombinazione per gli elettroni ionosferici. summary the results of the ionospheric soundings carried out in rome during the partial eclipse of the sun of february 25, 1952 are presented here. three minima icere found in the electronic concentration; one before the biginning of the eclipse, one for the maximum pliase, and one at the end of the eclipse. it is shown, admitting that the cause of the tiro extreme minima is to he found in tlie external zone of the visible part of the solar disk, that one can explain the otlierivise inexplicable retarci of the last minimum with respect to the visible eclipse, and one can interpret, at the sanie time, strong apparent variation in the coefficient of recombination of the ionospheric electrons. b i b l i o g r a f i a ( ! ) cfr. s. n. ghosh, proc. nat. inst. sci. of india, 10, 333, 1944. (2) h . w . wells e a . h . shapley, terr. magn. 51, 401, 1946. ( 3 ) h . w . wells, journ. geoph. res. 57, 291, 1952. ganas_fc_correttoereimpaginato_ok_layout 6 mo 655 static stress transfer from the may 20, 2012, m 6.1 emilia-romagna (northern italy) earthquake using a co-seismic slip distribution model athanassios ganas1,*, zafeiria roumelioti2, konstantinos chousianitis1 1 national observatory of athens, institute of geodynamics, athens, greece 2 aristotle university of thessaloniki, department of civil engineering, laboratory of soil mechanics, foundations and geotechnical earthquake engineering, thessaloniki, greece annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6176 article history received july 28, 2012; accepted august 22, 2012. subject classification: seismology, stress transfer, slip distribution, italy. 2012 emilia earthquakes 1. introduction in may 2012, northern italy (figure 1) suffered two strong earthquakes that resulted in 26 deaths and widespread damage. the earthquakes struck in the emilia-romagna region to the north of the city of bologna. the first strong event occurred on may 20, 2012, at 02:03:52.0 utc, with an epicenter location at 44.889˚n and 11.228˚e determined by the istituto nazionale di geofisica e vulcanologia (ingv; national institute of geophysics and volcanology), a focal depth of 6.3 km, and a moment magnitude of m 6.1 (global centroid moment tensor catalog, cmt). the second event occurred on may 29, 2012, at 07:00:03 utc with an epicenter location of 44.851˚n and 11.086˚e (determined by ingv), a depth of 10.2 km, and a magnitude of mw 5.9. the distance between the epicenters was 11.9 km, which indicated some form of interaction between the two events, given also the short time interval (9 days) and the similarity in the focal mechanisms (thrust-type of faulting). the emilia-romagna area is underlain by a series of active thrust faults and related folds, which strike wnw-ese, parallel to the mountain front, and which dip shallowly towards the south-southwest. this region comprises a foreland basin formed by the downflexing of the crust because of the loading of the northern apennine thrust sheets, across which there is about one millimeter per year of active shortening at present [e.g., devoti et al. 2011, bennett et al. 2012]. in the present study, we provide evidence of earthquake triggering through stress transfer. first, we investigate a heterogeneous slip model for the may 20, 2012, earthquake, and subsequently we apply the slip model in static stress change calculations for both optimal planes for failure and the specific fault planes of the may 29, 2012, earthquake in northern italy. 2. methods 2.1. slip distribution of the may 20, 2012, m 6.1 emiliaromagna earthquake the distribution of slip on the ruptured plane of the m 6.1 emilia-romagna earthquake was investigated using empirical green's functions (egfs) [hartzell 1978] and a least-squares inversion scheme of source time functions (stfs) computed from regional broadband seismological data [mori and hartzell 1990, dreger 1994]. to briefly describe the egf method, we consider the mathematical representation of the far-field wave displacement of a point source, u(~) recorded at distance r and azimuth z: (1) where ~ is the angular frequency, is the moment rate function of the event, g is the green's function response of the medium along the wave path, which includes the effects of attenuation and geometrical spreading, r is the radiation pattern factor, and i is the response of the recording instrument. by deconvolving a smaller event, i.e., an event with a stf that can be considered negligible relative to the stf of the larger event, all three factors of g, r and i are empirically removed. stfs computed through the deconvolution procedure are then inverted to reveal the characteristics of the source process. all stfs are normalized to unit area, i.e., only the shapes and not the absolute amplitudes of the stfs are inverted. the source parameterization in the inversion procedure includes a rectangular fault surface, which is divided into a certain number of subfaults. a radially expanding rupture front with constant rupture velocity is then considered, with the slip confined to one of the nodal planes, as determined by the focal mechanism of the event exam( ) ( ) ( , , ) ( )u m g r r i$ $ $~ ~ ~ z ~= o ined. the fit to each observed stf is determined by summing the synthetics from different parts along the fault, which are positioned at equal intervals. these synthetics are appropriately delayed by taking into account the time needed for the wave to propagate from the hypocenter to the recording station, and the time delay due to the propagation of the rupture front. distances to the different subfaults are estimated using a half-space ray trajectory approximation. it is also assumed that the contribution from each subfault has the form of a boxcar function. the length of the boxcar represents the slip duration of each subfault, and it is fixed in each inversion. the optimal values for the rupture velocity and the dislocation rise time, which are also fixed in each inversion, are found by performing a series of inversions testing the parameter space. the subfault stfs (b) are related to the observed stfs (d) through a system of equations of the form: (2) where x is the time delay due to wave and rupture propagation, i is a station index, j is a subfault index, and w is a weight proportional to the fault slip. two constraints are imposed on the inversion procedure. the first is a positivity con( ) stf ( ) ( )d t t b t wi i j ij j j m x= = -/ ganas et al. 656 figure 1. regional map showing the locations of the stations used relative to the epicenters of the may 20, 2012, m 6.1 earthquake and the may 29, 2012, m 5.9 event. the may 20, 2012, aftershock was used as an empirical green's function. table 1. source parameters for the two strongest events of the may 2012 emilia-romagna sequence and the strong aftershock, which was used as empirical green's function. locations are from http://iside.rm.ingv.it and moment tensor related parameters are from the global centroid moment tensor solutions catalog (http://www.globalcmt.org/). egf, empirical green's function. event date time latitude longitude depth m mo plane 1 plane 2 p-axis t-axis (dd/mm/yy) (hh:mm:ss) (˚n) (˚e) (km) (×1018 nt×m) strike/dip/rake (˚) (˚) (pl/az) (pl/az) main i 20/05/12 02:03:52 44.889 11.228 6.3 6.1 1.66 96/35/68 302/58/105 12/21 73/249 egf 20/05/12 13:18:02 44.831 11.490 4.7 5.1 0.066 110/38/82 300/51/96 6/26 82/246 main ii 29/5/2012 07:00:03 44.851 11.086 10 5.9 0.767 100/30/81 290/60/95 15/17 72/214 657 straint, which allows only those subfaults with positive slip to contribute to the solution, and the second is a spatial derivative minimization constraint to smooth the resulting slip model. taking into account the two constraints, equation 2 can be rewritten in a matrix form as: (3) where s is the matrix of first spatial derivatives, and m is a constant controlling the weight of the smoothing equation. s b dw 0 $ m =9 9c c emilia-romagna: slip model and earthquake triggering figure 2. comparison of the egf displacement waveforms with the corresponding studied mainshock waveforms for the three components (top, ew; middle, ns; bottom, z) of station aqu. all of the waveforms were bandpass filtered in the frequency range 0.01 hz to 1 hz. figure 3. comparison of synthetic (dashed lines) and computed stfs at the ten stations shown in figure 1. synthetic stfs were derived using the slip distribution model of figure 4. the slip weight vector is obtained by standard least squares. the timing of slip on each subfault is controlled by the radially expanding rupture front, and each part is allowed to rupture only once. since the inversion yields slip weights and not absolute slip amplitudes, to derive the absolute slip at each subfault, uj, an independent estimate of the seismic moment, m0, is required. 2.1.1. data the data used in this study were obtained from the orfeus web page (http://www.orfeus-eu.org/) and were provided by mednet and the italian, german, austrian and slovenian national seismic networks. the distribution of the stations used in the inversion, relative to the emilia mainshock, is shown in figure 1. the selection of the stations was based on availability, as well as azimuth criteria, i.e., obtaining the best azimuthal coverage of the epicentral area without overweighting the azimuths where the data availability was greater. 2.1.2. application and results in the present study, an aftershock of m 5.1 that occurred on may 20, 2012, at 13:18:02 utm was identified as the most appropriate to be used in the study of the may 20, m 6.1 mainshock. the basic source parameters of these two events are shown in table 1. the original velocity broadband waveforms of both the studied mainshock and the egfs were integrated into the displacement and band-pass filtered between 0.01 hz and 1 hz, using a causal fourthorder butterworth filter. the egf waveforms were then deconvolved from the mainshock waveforms in the frequency domain, using a 1% water level (i.e., small values in the denominator were replaced by 0.01 times the maximum value of the denominator [clayton and wiggins 1976] to stabilize the process. an indicative example of the comparison of the egf waveforms with the mainshock waveforms is presented in figure 2. the computed stfs were normalized to unit area, and are shown in figure 3 (continuous lines). the stf shapes present significant variations with smaller duration and larger amplitude to the ese direction (e.g., bly, aqu), and vice versa. this phenomenon is indicative of the emergence of rupture directivity effects. thus, based on the stf shapes, it is concluded that the rupture on the fault plane of the may 20, 2012, m 6.1 event propagated from the hypocenter to the ese. in the inversion of the stf shapes, we assumed a planar fault model with dimensions 40 km × 25 km, discretized into 1 km2 subfaults. the dimensions of the fault model were chosen to be larger than those expected for an earthquake of m 6.1 [e.g. wells and coppersmith 1994], to allow for possible unilateral propagation of the rupture in any direction. the optimum values for the rupture velocity and the rise time were determined through iterative inversions in a grid of the parameter space (vr = 2.2 km/s, x = 0.45 s). these best-fit ganas et al. 658 figure 4. slip distribution model for the 2012, m 6.1 emilia mainshock. the contours are for 5 cm intervals of slip, beginning at 5 cm. black star, adopted hypocentral location. 659 values were used to obtain the final slip distribution model, for which we obtained a variance reduction of 94.5%. in figure 3, we present the comparison of the final synthetic stfs (dashed lines) with the computed ones (continuous lines). figure 4 shows a map of the resulting slip. the slip, averaged across the ruptured fault surface, is ca. 13.8 cm, while its peak value is as high as 59 cm. most of the slip appears below and to the ese of the hypocenter location (figure 4, star symbol), thus below ca. 6 km. the dimensions of the ruptured area (along strike × along dip) are approximately 17 km × 16 km, and the slip does not reach the ground surface (upper edge of the fault model is at 0 km). however, it must be noted that the latter result is subject to the error in the hypocentral depth; i.e., if the hypocentral depth is emilia-romagna: slip model and earthquake triggering figure 5. coulomb stress changes at 10-km depth associated with the may 20, 2012, 02:03 utc mw 6.1 earthquake. the palette of stress values is linear in the range −1 to +1 bar. green stars, the two mainshock epicenters. blue areas, unloading; red areas, loading. color scale in bar (1 bar = 100 kpa). open circles are consequent events after the may 20, 2012, mainshock. the source slip model is that of figure 4. (a) map and (b) vertical cross-section crossing the hypocenter of the may 29, 2012, event heading n10˚e (normal to fault strike of the may 29, 2012, event). green line, horizontal section of dcff distribution shown in figure 5a. d e p th shallower, the entire model will be shifted to shallower depth. the small slip concentrations close to the upper left corner of the fault model disappear when data from stations tue and mabi are excluded from the inversion. comparison of synthetic stfs derived with and without these small slip concentrations revealed that these small patches are needed to slightly improve the stf fit at stations tue and mabi in the time window of 2.5 s to 5 s. as this result is azimuth dependent and relative values are too small to be considered well resolved, we consider it as an artefact of our analysis. different sets of data and a near-fault of higher frequency are needed to clarify the existence of such scale irregularities in the slip distribution model. 2.2. stress transfer modeling earthquakes on fault planes can trigger subsequent earthquakes at short distances from the hypocenter by transferring static stresses [e.g., harris et al. 1995, gomberg ganas et al. 660 figure 6. coulomb stress distribution as in figure 5, but for low-angle, reverse receiver faults striking n100˚e with friction of 0.4. open circles, consequent events after may 20, 2012, mainshock. (a) map and (b) vertical cross-section. green line, horizontal section of dcff distribution shown in figure 6a. d e p th 661 et al. 2001, freed 2005, ganas et al. 2008, 2010, parsons et al. 2008]. we model stress transfer assuming that failure of the crust occurs by shear, so that the mechanics of the process can be approximated by the okada [1992] expressions for the displacement and strain fields due to a finite rectangular source in an elastic, homogeneous and isotropic half-space. we use the coulomb 3.3 software [lin and stein 2004, toda et al. 2005] to compute the static coulomb stress change by assuming a young modulus of 8 × 105 bar, poisson's ratio 0.25, and effective coefficient of friction n´= 0.4, which is closer to friction values for major faults [harris and simpson 1998]. we calculated the change in the coulomb failure function (cff, or coulomb stress), on target failure planes [reasenberg and simpson 1992], dcff=dx+nldvh (4) where dx is the co-seismic change in shear stress on the receiver fault, and in the direction of fault slip, dvh is the change in the normal stress (with tension positive), and n´ is the effective coefficient of friction. dcff is the coulomb stress change between the initial (ambient) stress and the final stress. moreover, we checked the robustness of our results by using n´ values from 0.1 to 0.8. in all cases, the spatial distribution of dcff values does not change significantly compared to the results obtained for n´ = 0.4. we interpret a positive value of dcff to indicate that a fault plane that occurred within this stress lobe has been brought closer to failure; when dcff is negative, the fault is brought further from failure (i.e., relaxed). 2.2.1. triggering of the may 29, 2012, earthquake: calculation of dcff on optimal planes we computed the dcff caused by the may 20, 2012, event on optimally oriented planes to regional compression (assuming a deviatoric stress magnitude of 200 bar; figure 5). for the compression azimuth, we adopted the orientation of the p-axis of the focal plane solution of the may 20, 2012, event (n21˚e; table 1). the calculation was carried out for seismogenic depths (in the 5-11 km range), including the depth of the may 29, 2012, event hypocenter (10 km). the target planes are similar in orientation to the may 20, 2012, fault plane; i.e., they strike e-w and dip either to the n or the s. we calculate the stress tensor on horizontal observation planes. dcff was sampled on a 40 km × 40 km grid, with 1 km grid spacing. the calculations consider an inclined rectangular dislocation that ruptured on may 20, 2012, according to our slip model (figure 4). figure 5 shows the distribution of the coulomb stress at a depth of 10 km (depth of may 29, 2012, hypocentre; ingv solution) for optimal planes to regional compression (n 21˚e). the static stress transfer in the area of the may 29, 2012, earthquake amounts to 0.61 bar to 0.74 bar. 2.2.2. triggering of the may 29, 2012, earthquake: calculation of dcff, on may 29, 2012, target (receiver) planes we also evaluated the capability of the source fault (i.e., the first mw 6.1 event on may 20, 2012, at 02:03 utc) to trigger earthquakes along faults of geometrical characteristics and kinematics identical to those suggested by the may 29, 2012, m 5.9, focal mechanism. the target (receiver) planes are modeled as left-lateral reverse faults, with slip models identical to those of the may 29, 2012, 07:00 utc event (strike/ dip/ rake: 100˚/ 30˚/ 81˚). we evaluated the amount of static coulomb stress that was transferred to the region of occurrence of the may 29, 2012, event (figure 6), by sampling cff on a horizontal section at 10 km depth on a 40 km × 40 km grid surrounding the source event epicenter, with a 1 km grid spacing. figure 6 shows the distribution of coulomb stress where the static stress transfer in the area of the may 29, 2012, earthquake amounts to 0.61 bar to 0.74 bar. 3. discussion and conclusions the slip distribution of the may 20, 2012, m 6.1 emilia-romagna earthquake was investigated through an inversion methodology of the stf shapes, to incorporate it into realistic coulomb stress transfer computations. the study of the may 20, 2012, earthquake source revealed that the area that ruptured was approximately 17 km × 16 km (along strike × along dip), with a mean slip of ca. 14 cm and a peak of 59 cm. the slip appeared mostly to the e-se of the adopted hypocenter location, and did not reach the ground surface. both triggering models predict that the hypocenter of the second mainshock (mw 5.9) is in the crustal region where static stress was transferred. the results considering target planes similar to the may 29, 2012, nodal plane (figure 6) resemble those on optimal planes (figure 5). this result is indicative that the may 29, 2012, event might have been triggered by the preceding may 20, 2012, m 6.1 earthquake due to static stressing by fault slip (figure 4). the loading stress levels are of the order of 0.61 bars to 0.74 bars (figures 5, 6). most first-event (i.e., post may 20, 2012) aftershocks occur inside the loaded areas of the crust (total 770 events; figures 5, 6). a few aftershocks occur inside these relaxed areas and within a distance of 2 km from the vertical crosssection that passes through the hypocenter of the second strongest event (figures 5b, 6b). their occurrence might be attributed to misfit of the coseismic slip model (figure 4), dynamic stress triggering [gomberg et al. 2001], or the location uncertainty of the catalog [catalli and chan 2012]. we infer that our stress change calculations have been adequately validated by the observed distribution of aftershocks. emilia-romagna: slip model and earthquake triggering acknowledgements. we thank marco anzidei and christodoulos kyriakopoulos for preliminary reports and information on the may 2012 italian earthquakes. we thank the reviewers c. chan and r. console for comments and suggestions. this work was financed in part by the general secretariat of research and technology (ministry of development) of greece. figure 1-3 were made using the generic mapping tools software [wessel and smith 1991]. we acknowledge the use of the ingv earthquake catalog. references bennett, r.a., e. serpelloni, s. hreinsdóttir, m.t. brandon, g. buble, t. basic, g. casale, a. cavaliere, m. anzidei, m. marjonovic, g. minelli, g. molli and a. montanari (2012). syn-convergent extension observed using the retreat gps network, northern apennines, italy, j. geophys. res., 117, b04408; 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*corresponding author: athanassios ganas, national observatory of athens, institute of geodynamics, athens, greece; email: aganas@noa.gr. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. ganas et al. 662 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice m i s u r a d e l l a r a d i o a t t i v i t à a t m o s f e r i c a c o n e m u l s i o n i n u c l e a r i e . m o r a s c h i n e l l i è n o l o e l l e l e s o s t a n z e r a d i o a t t i v e c o n t e n u t e n e l l ' a r i a si t r o v a n o i n g r a n p a r t e a n c o r a t e a l l e p a r t i c e l l e c h e c o s t i t u i s c o n o l ' a e r o s o l a t m o s f e r i c o . p e r l a m i s u r a d e l l a r a d i o a t t i v i t à a t m o s f e r i c a si r i c o r r e p e r c i ò s o v e n t e a l l a s e p a r a z i o n e di t a l i s o s t a n z e d a v o l u m i n o t i d i a r i a , e p o i a l l a m i s u r a d e l l a l o r o a t t i v i t à c o n u n o d e i m e t o d i c h e s o n o n o r m a l m e n t e i m p i e g a t i p e r l e s o s t a n z e s o l i d e . l a s e p a r a z i o n e p u ò e s s e r e o t t e n u t a s i a m e d i a n t e a s p i r a z i o n e d e l l ' a r i a a t t r a v e r s o filtri c h e t r a t t e n g o n o l e p a r t i c e l l e s o l i d e d i d i m e n s i o n i s u p e r i o r i a d u n c e r t o l i m i t e f 1 ) , sia c o n l a p r e c i p i t a z i o n e e l e t t r o s t a t i c a i n u n a s c a r i c a a d e f f l u v i o f r a u n e l e t t r o d o a p u n t a e u n o p i a n o ("l. di epiesto s e c o n d o m e t o d o f a u s o l ' a p p a r e c c h i o d i g . a l i v e r t i p e r l a m i s u r a q u a n t i t a t i v a d e l l a r a d i o a t t i v i t à d e l l ' a r i a ( 3 ) . l ' a p p a r e c c h i o c h e v i e n e q u i d e s c r i t t o è b a s a t o s u l l o s t e s s o p r i n c i p i o e i n u n c e r t o s e n s o si p u ò r i t e n e r e u n a m o d i f i c a di q u e s t o , r e s a p o s s i b i l e d a l l a r e c e n t e p r o d u z i o n e di l a s t r e f o t o g r a f i c h e a d a t t e p e r m i s u r e d i r a d i o a t t i v i t à . u n v o l u m e n o t o di a r i a , a s p i r a t o a v e l o c i t à c o s t a n t e a t t r a v e r s o l ' a p p a r e c c h i o , v i e n e p r i v a t o d e l l e p a r t i c e l l e s o l i d e a l l e q u a l i s o n o a n c o r a t e l e s o s t a n z e r a d i o a t t i v e . c o n il m e t o d o a l i v e r t i l ' e s a m e d e l l a r a d i o a t t i v i t à d e l d e p o s i t o o t t e n u t o v i e n e e s e g u i t o m i s u r a n d o c o n u n a c a m e r a di i o n i z z a z i o n e il d e c a d i m e n t o d e l l a s u a a t t i v i t à ; i n q u e s t o c a s o i n v e c e , p o n e n d o il d e p o s i t o i n c o n t a t t o c o n l ' e m u l s i o n e s e n s i b i l e d i u n a l a s t r a n u c l e a r e si h a u n a r e g i s t r a z i o n e d e l l e p a r t i c e l l e a e m e s s e d u r a n t e i l s u o d e c a d i m e n t o . d o p o l o s v i l u p p o d e l l ' e m u l s i o n e i l s u o e s a m e m i c r o s c o p i c o p e r m e t t e d i d e t e r m i n a r e l a q u a n t i t à e, a p p r o s s i m a t i v a m e n t e , a n c h e l a n a t u r a d e l l e s o s t a n z e d e p o s i t a t e . c o n q u e s t o m e t o d o si p o s s o n o o t t e n e r e p r a t i c a m e n t e s o l t a n t o v a l o r i r e l a t i v i d e l l a r a d i o a t t i v i t à a t m o s f e r i c a ; p e r ò q u e s t i s o n o i s o l i c h e i n t e r e s s a n o q u a n d o si v o g l i a s t u d i a r e l ' a n d a m e n t o d i t a l e r a d i o a t t i v i t à col t e m p o , o i n d i v e r s i l u o g h i . l a m i s u r a c o n s i s t e n e l c o n t a r e l e t r a c c e su u n a l i m i t a t a s u p e r f i c i e d e l l ' e m u l s i o n e , s c e l t a i n m o d o o p p o r t u n o . k. m o k a s c h i n e l l i l ' i m p i e g o (li l a s t r e n u c l e a r i p e r l a m i s u r a d e l l a r a d i o a t t i v i t à a t m o s f e r i c a p r e s e n t a a l c u n i v a n t a g g i r i s p e t t o a g l i a l t r i m e t o d i c o m u n e i u e n l e i m p i e g a t i . t r a t t a n d o s i di a t t i v i t à m o l t o p i c c o l e , l a p r o p r i e t à int e g r a t r i c e d e l l ' e m u l s i o n e p e r m e t t e di o t t e n e r e u n a m a g g i o r p r e c i s i o n e n e l l a m i s u r a . i n o l t r e , o s s e r v a n d o l ' e m u l s i o n e a l m i c r o s c o p i o , è f a c i l e r i c o n o s c e r e f r a l e t r a c c e q u e l l e p r o d o t t e d a l l a d i s i n t e g r a z i o n e d e l tii c' d i e h a n n o u n a l u n g h e z z a di 4 6 , 6 |t ( 4 ) , m e n t r e l e a l t r e a r r i v a n o a l m a s s i m o a 3 8 . 4 ti (ra c i . q u e s t o f a t t o p e r m e t t e ili v a l u t a r e a p p r o s s i m a t i v a m e n t e il c o n t e n u t o di e l e m e n t i d e l l a f a m i g l i a d e l t o r i o n e l l ' a t m o s f e r a . i n f i n e , v a n t a g g i o n o n t r a s c u r a b i l e , l ' a p p a r e c c h i o p u ò e s s e r e f a c i l m e n t e t r a s p o r t a t o d a u n l u o g o a l l ' a l t r o p e r r i l i e v i r a d i o a t t i v i , p o i c h é l e l a s t r e i m p i e g a t e n e l l a m i s u r a p o s s o n o e s s e r e c o n s e r v a t e p e r l o svil u p p o e l ' o s s e r v a z i o n e i n u n s e c o n d o t e m p o . apparecchio. — l ' a r i a da e s a m i n a r e v i e n e a s p i r a l a c o n u n a s p i r a t o r e , a z i o n a t o d a u n m o t o r i n o e l e t t r i c o o a m o l l a , a t t r a v e r s o u n t u l i o di m a t e r i a l e i s o l a n t e ( p l e x i g l a s ! d e l l a l u n g h e z z a di 2 5 c m e di s e z i o n e r e t t a n g o l a r e di 1 , 5 x 2 . 5 c m . n e l r a c c o r d o f r a q u e s t o t u b o e l ' a s p i r a t o r e è i n s e r i t o u n a n e m o m e t r o , c h e p e r m e t t e di d e t e r m i n a r e l a q u a n t i t à di a r i a f l u i t a a t t r a v e r s o l ' a p p a r e c c h i o . l u n g o il t u b o , a 5 e a 1 5 c m d a l l ' i n g r e s s o , s o n o p o s t i d u e p r e c i p i t a t o r i , c o s t i t u i t i o g n u n o da u n a fila di 5 p u n t e d i a c c i a i o c h e e n t r a n o a t t r a v e r s o f o r i p r a t i c a t i t r a v e r s a l m e n t e i n u n a p a r e t e , p e n e t r a n d p p e r 5 m i n n e l l ' i n t e r n o . di f r o n t e ad esse, n e l l a p a r e t e o p p o s t a è p r a t i c a t a u n ' a p e r t u r a di 2 , 5 x 2 , 5 c m n e l l a q u a l e v i e n e p o s t a u n a l a s t r i n a di o t t o n e , i n m o d o d i c h i u d e r l a e r m e t i c a m e n t e . s i v e d r à p i ù a v a n t i c o m e l a f u n z i o n e d e l s e c o n d o p r e c i p i t a t o r e , a v a l l e d e l p r i m o , sia q u e l l a di p e r m e t t e r e l a d e t e r m i n a z i o n e d e l l ' e f ficienza d e l p r i m o n e l l a p r e c i p i t a z i o n e d e l l e s o s t a n z e r a d i o a t t i v e , e di f o r n i r e c o s ì u n a t a r a t u r a d e l l ' a p p a r e c c h i o , i n d i s p e n s a b i l e p e r e s e g u i r e l e m i s u r e . l e l a s t r i n e di o t t o n e c h e f u n g o n o d a e l e t t r o d o p i a n o dei p r e c i p i t a t o t i s o n o fissate c o n u n a m o l l a , e p o s s o n o e s s e r e f a c i l m e n t e s o s t i t u i t e : di q u e s t e b i s o g n a d i s p o r r e u n a c e r t a r i s e r v a , i l cui n u m e r o d i p e n d e d a l p r o g r a m m a d i m i s u r e c h e si v o g l i o n o e s e g u i r e . l e l a s t r e f o t o g r a f i c h e v e n g o n o c o n s e r v a t e i n u n a s c a t o l a di l e g n o a t e n u t a di l u c e . q u a l s i a s i t i p o di e m u l s i o n e s e n s i b i l e a l l e p a r t i c e l l e a p u ò e s s e r e i m p i e g a t o : o t t i m e si s o n o m o s t r a t e l e c< e l e fe\ i l f o r d c o n e m u l s i o n e da 5 0 u e d i m e n s i o n i 2 , 5 x 1 2 , 5 c m . e s s e v a n n o p r e m i s u r a della radioattività a t m o s f e r i c a con e m u l s i o n i n u c l e a r i 5 6 3 d i s p o s t e n e l l a c a s s e t t a c o n l ' e m u l s i o n e r i v o l t a i n p o s i z i o n e f a c i l m e n t e r i c o n o s c i b i l e . q u a n d o si è o t t e n u t o i l p r e c i p i t a t o r a d i o a t t i v o su u n e l e t t r o d o , si p o r t a cpiesto n e l p i ù b r e v e t e m p o p o s s i b i l e , e c o m u n q u e s e m p r e u g u a l e , i n c o n t a t t o c o n l ' e m u l s i o n e , a l l a q u a l e si m a n t i e n e a d e r e n t e c o n u n e l a s t i c o d i g o m m a , o c o n u n a m o l l e t t a di a c c i a i o . s e l e m i s u r e v e n g o n o f a t t e i n c a m p a g n a , q u e s t a o p e r a z i o n e p u ò e s s e r e f a t t a m a n t e n e n d o l a s c a t o l a d e l l e l a s t r e i n u n a c u s t o d i a di p a n n o n e r o , n e l l a ( p i a l e si i n f i l a n o l e m a n i p e r e s e g u i r e t u t t e l e o p e r a z i o n i s e n z a e s p o r r e a l l a l u c e l e e m u l s i o n i s e n s i b i l i . p e r l a p r e c i p i t a z i o n e si a p p l i c a a g l i e l e t t r o d i d e l p r e c i p i t a t o r e u n a d i f f e r e n z a di p o t e n z i a l e di a l c u n e m i g l i a i a d i v o l t . l a v e l o c i t à di p r e c i p i t a z i o n e è t a n t o m a g g i o r e q u a n t o p i ù q u e s t o p o t e n z i a l e è elev a t o , e d è m a s s i m a q u a n d o si f a u s o d i t e n s i o n e c o n t i n u a c o n p u n t e n e g a t i v e a d es. u n d e p o s i t o s u f f i c i e n t e p e r f a r e u n a m i s u r a si o t t i e n e , c o n u n flusso di a r i a di 1 l i t r o / m i n e t e n s i o n e a l l e p u n t e di 1 0 k v o l t , c o n u n a p r e c i p i t a z i o n e d e l l a d u r a t a di 1 5 m i n u t i . i n c a m p a g n a il p r e c i p i t a t o r e p u ò e s s e r e a l i m e n t a t o c o n u n g e n e r a t o r e p o r t a t i l e a p i l e e o s c i l l a t o r e e l e v a t o r e di t e n s i o n e 1 ' . s e i n v e c e si lia a d i s p o s i z i o n e la c o r r e n t e a l t e r n a t a d e l l a r e t e , u n p i c c o l o a l i m e n t a t o r e , c o s t i t u i t o da ini t r a s f o r m a t o r e d e l t i p o i m p i e g a t o n e g l i i m p i a n t i p e r i n s e g n e l u m i n o s e d a 1 0 w a t t , 6 ^ 7 k v o l t , s e g u i t o d a u n d i o d o r a d d r i z z a t o r e (es. 1 b 3 f i v r e l e da u n c o n d e n s a t o r e di l i v e l l a m e n t o , p u ò f o r n i r e l a t e n s i o n e c o n t i n u a r i c h i e s t a . misura. — m e s s o in f u n z i o n e l ' a s p i r a t o r e , si c h i u d e i l c i r c u i t o c h e dà l a t e n s i o n e a l p r e c i p i t a t o r e p e r l a d u r a t a d e s i d e r a t a ; p o i si t o g l i e l ' e l e t t r o d o p i a n o e l o si a p p o g g i a a l l a l a s t r a f o t o g r a f i c a , l a s c i a n d o v e l o u n t e m p o s u f f i c i e n t e p e r p e r m e t t e r e il d e c a d i m e n t o c o m p l e t o d e l l ' a t t i v i t à del d e p o s i t o . p o i c h é l e s o s t a n z e r a d i o a t t i v e c o n t e n u t e n e l l ' a t m o s f e r a s o n o l i m i t a t e , a l m e n o q u e l l e n a t u r a l i , ai p r o d o t t i di dis i n t e g r a z i o n e d e l l e e m a n a z i o n i d i r a d i o e di t o r i o , b a s t a t e n e r c o n t o d e l p i ù l u n g o d e i p e r i o d i di tali s o s t a n z e , c h e è q u e l l o d e l th b, di 1 0 . 6 o r e . 11 c o m p l e t o d e c a d i m e n t o d e l th b e d e i s u o i p r o d o t t i s u c c e s s i v i in e q u i l i b r i o t r a n s i e n t e con esso, a v v i e n e p r a t i c a m e n t e in u n i n t e r \ a l l o di 4 0 -h 5 0 o r e . l e l a s t r e d e v o n o e s s e r e p e r c i ò l a s c i a t e a c o n t a t t o d e l l ' e l e t t r o d o a l m e n o d u e g i o r n i p r i m a d i p r o c e d e r e a l l o s v i l u p p o : q u e s t o v i e n e poi e s e g u i t o s e c o n d o l e i s t r u z i o n i d e l l a i l f o r d , e n o n o f f r e p a r t i c o l a r i d i f f i c o l t à . 5 6 4 e. m 0 r a s c h 1 n e l l i p e r c o n t a r e l e t r a c c e d e l l e p a r t i c e l l e a n e l l ' e m u l s i o n e si u s a u n m i c r o s c o p i o m u n i t o d i t a v o l i n o t r a s l a t o r e , c o n u n i n g r a n d i m e n t o d i 4-00 -t5 0 0 x. l a d i s c r i m i n a z i o n e d e l l e p a r t i c e l l e « d e l tli c' p u ò ess e r e f a t t a r i c o r r e n d o a l l a l o r o m i s u r a c o n u n o c u l a r e c o n t e n e n t e u n a s c a l a t a r a t a . n a t u r a l m e n t e c o l m i c r o s c o p i o si o s s e r v a n o l e p r o i e z i o n i o r i z z o n t a l i d e l l e t r a c c e ; l e p i ù i n c l i n a t e n e l l ' e m u l s i o n e v e n g o n o senz ' a l t r o e s c l u s e d a q u e s t a c l a s s i f i c a z i o n e a p a r t e . p e r ò si v e d r à p i ù a v a n t i c o m e i l n u m e r o d e l l e t r a c c e c o n c o m p o n e n t e o r i z z o n t a l e t a l e d a p o t e r e s s e r e a t t r i b u i t e a l tli c' s i a s u f f i c i e n t e p e r u n a v a l u t a z i o n e a p p r o s s i m a t a d e l c o n t e n u t o d i q u e s t o e l e m e n t o n e l d e p o s i t o . taratura dell'apparecchio. — l a t a r a t u r a p e r m e t t e d i s t a b i l i r e q u a l e p e r c e n t u a l e d i p o r t a t o r i r a d i o a t t i v i v i e n e s e p a r a t a d a l l ' a r i a c b e fluisce a t t r a v e r s o l ' a p p a r e c c h i o . q u e s t a p e r c e n t u a l e d i p e n d e d a l l e c o n d i z i o n i d i f u n z i o n a m e n t o , c i o è d a l l a t e n s i o n e u s a t a p e r l a p r e c i p i t a z i o n e e d a l l a v e l o c i t à di a s p i r a z i o n e d e l l ' a r i a ; l a t a r a t u r a d e v e p e r c i ò e s s e r e r i f a t t a o g n i v o l t a c h e v e n g o n o c a m b i a t i q u e s t i p a r a m e t r i . p o i c h é i n p r a t i c a è f a c i l e m a n t e n e r e c o s t a n t e la t e n s i o n e a l p r e c i p i t a t o r e , m e n t r e l a v e l o c i t à di a s p i r a z i o n e c h e d i p e n d e , f r a l ' a l t r o , a n c h e d a l l a p r e s s i o n e a t m o s f e r i c a , p u ò r i s u l t a r e d i v e r s a d a u n a m i s u r a a l l ' a l t r a , è c o n v e n i e n t e t r a c c i a r e p r e v e n t i v a m e n t e u n a c u r v a d i t a r a t u r a , c o m e s u g g e r i t o d a g . a l i v e r t i 3 . p e r u n d e t e r m i n a t o v a l o r e d e l l a t e n s i o n e a l p r e c i p i t a t o r e , si v a l u t a , c o l m e t o d o i n d i c a t o p i ù sott o , l a p e r c e n t u a l e d e i p o r t a t o r i r a d i o a t t i v i p r e c i p i t a t i x , i n f u n z i o n e d e l l a v e l o c i t à d i a s p i r a z i o n e y . e n t r o a m p i e v a r i a z i o n i d e l l a v e l o c i t à , q u e s t a f u n z i o n e è r a p p r e s e n t a t a d a u n a r e t t a , c h e p u ò e s s e r e t r a c c i a t a c o n l a m i s u r a di d u e s o l e c o p p i e d i v a l o r i di x e v . l ' o p e r a z i o n e d i t a r a t u r a v i e n e e s e g u i t a c o l l e g a n d o i n p a r a l l e l o i d u e p r e c i p i t a t o l i e i m p r e s s i o n a n d o d u e l a s t r e c o n i d e p o s i t i o t t e n u t i s i m u l t a n e a m e n t e . i l c o n t e g g i o d e l l e t r a c c e su u n a c e r t a z o n a d e l l ' e m u l s i o n e d e l l a s e c o n d a l a s t r a ( q u e l l a c o r r i s p o n d e n t e a l p r e c i p i t a t o r e a v a l l e ) , d a r à u n n u m e r o no di t r a c c e p e r u n i t à di s u p e r f i c i e i n f e r i o r e a l l e ti] d e l l a p r i m a l a s t r a . 11 c a l c o l o di x si e s e g u e n e l s e g u e n t e m o d o . s i a n i l n u m e r o d i p o r t a t o r i r a d i o a t t i v i p e r c m 3 d i a r i a ; i l p r i m o p r e c i p i t a t o r e n e d e p o s i t a nx = a l s e c o n d o p r e c i p i t a t o r e g i u n g e a r i a c o n t e n e n t e s o l t a n t o n(l-x) p o r t a t o r i p e r c m 3 : di q u e s t i n e v e n g o n o p r e c i p i t a t i n(l-x)x = kn>. d a l r a p p o r t o f r a q u e s t e d u e r e l a z i o n i si r i c a v a il v a l o r e di x : x = 1 — n2/n1 m i s u r a d e l l a radioattività a t m o s f e r i c a con e m u l s i o n i n u c l e a r i 5 6 5 l a c o s t a n t e k è u g u a l e p e r l e d u e l a s t r e se si h a c u r a d i s c e g l i e r e p e r l ' e s p l o r a z i o n e u n a s u p e r f i c i e d i e m u l s i o n e s i t u a t a i n p o s i z i o n e c o r r i s p o n d e n t e r i s p e t t o a l l e p u n t e d e i p r e c i p i t a t o r i . contenuto relativo di elementi della famiglia del torio. — m e d i a n t e l a m i s u r a d e l l a l u n g h e z z a d e l l e t r a c c e n e l l ' e m u l s i o n e è p o s s i b i l e c o s t r u i r e u n o s p e t t r o d e i p e r c o r s i c h e p e r m e t t e d i r i c o n o s c e r e g l i e l e m e n t i p r e s e n t i n e l d e p o s i t o e r i c a v a r e l a q u a n t i t à r e l a t i v a d i c i a s c u n o d i essi 7 . q u e s t a o p e r a z i o n e è a l q u a n t o l a b o r i o s a , p e r c h é l a l u n g h e z z a di o g n i t r a c c i a d e v e e s s e r e r i c a v a t a m i s u r a n d o l a s u a p r o i e z i o n e o r i z z o n t a l e e l a s u a p e n e t r a z i o n e n e l l ' e m u l s i o n e , m e t t e n d o a f u o c o s u c c e s s i v a m e n t e i l p r i m o e l ' u l t i m o g r a n o di essa, e c o r r e g g e n d o l a c o m p o n e n t e v e r t i c a l e p e r t e n e r c o n t o d e l l a c o n t r a z i o n e d e l l a g e l a t i n a d o p o l o s v i l u p p o . c o m e si è v i s t o , il m a g g i o r p e r c o r s o n e l l ' e m u l s i o n e d e l l e p a r t i c e l l e a d e l th c p e r m e t t e d i r i c o n o s c e r e l e l o r o t r a c c e e v a l u t a r e a p p r o s s i m a t i v a m e n t e i l c o n t e n u t o r e l a t i v o di thc', e q u i n d i di th c e th b, n e l d e p o s i t o . p o i c h é l ' a p p a r e c c h i o d e s c r i t t o s e r v e p e r m i s u r e r e l a t i v e d e l l a r a d i o a t t i v i t à d e l l ' a e r o s o l a t m o s f e r i c o , l a p e r c e n t u a l e di t r a c c e c o n c o m p o n e n t e o r i z z o n t a l e di l u n g h e z z a m a g g i o r e d i 3 8 , 4 u g i à p e r se stessa è s u f f i c i e n t e p e r s e g u i r e l ' a n d a m e n t o d e l r a p p o r t o f r a g l i e l e m e n t i d e l l e d u e f a m i g l i e n e l l ' a t m o s f e r a . l o stesso r a p p o r t o p e r gli e l e m e n t i p r e s e n t i n e l d e p o s i t o a l l a fine d e l l a p r e c i p i t a z i o n e , p u ò e s s e r e c a l c o l a t o d e t e r m i n a n d o p r i m a il n u m e r o d e l l e t r a c c e di th c' n e l l ' e m u l s i o n e . g . i m b ò e l . c a s e r t a n o 8 e j . h . p o o l e e c . m . e . m a t t e w s 9 h a n n o r i s o l t o i l p r o b l e m a di c a l c o l a r e i l n u m e r o d i t r a c c e d i u n d a t o p e r c o r s o c o n o s c e n d o il n u m e r o di q u e l l e c o n p r o i e z i o n e o r i z z o n t a l e m a g g i o r e d i u n c e r t o l i m i t e . i n q u e s t o c a s o , t r a t t a n d o s i d i u n d e p o s i t o r a d i o a t t i v o i n f i n i t a m e n t e s o t t i l e , t a l e c i o è d a n o n a s s o r b i r e p a r t e d e l l ' e n e r g i a d e l l e p a r t i c e l l e e m e s s e n e l l o s p e s s o r e s t e s s o d e l d e p o s i t o , i l c a l c o l o r i s u l t a p a r t i c o l a r m e n t e s e m p l i c e . s e 6 è l ' a n g o l o c o m p r e s o f r a l a d i r e z i o n e d i u n a t r a c c i a e l a s u a p r o i e z i o n e n e l p i a n o d e l l a g e l a t i n a , a l l ' o s s e r v a z i o n e m i c r o s c o p i c a v e n g o n o r i c o n o s c i u t e c o m e t r a c c e d e l th c' s o l t a n t o q u e l l e c h e p e n e t r a n o n e l l ' e m u l s i o n e c o n u n a n g o l o i n f e r i o r e a 3 4 ° 3 0 ' . i n f a t t i l a c o n d i z i o n e p e r l a d i s c r i m i n a z i o n e è : 4 6 , 6 c o s 9 > 3 8 , 4 da c u i 0 < 3 4 ° 3 0 ' 5 6 6 e. m o r a s c h i n e l l i s i a n' il n u m e r o d e l l e t r a c c e d i s c r i m i n a t e su u n t o t a l e n " di t r a c c e c o n t a t e ; l e t r a c c e d e l th c' s a r a n n o n' p d o v e p è i l r a p p o r t o t r a 2j-c e l ' a n g o l o s o l i d o c o n g e n e r a t r i c i 0 = 0 e 6 = 3 4 ° 3 0 ' : p = 1 , 7 6 . i l n u m e r o n"-n' d e l l e t r a c c e c o n l u n g h e z z a i n f e r i o r e a 3 8 , 4 u d e v e e s s e r e c o r r e t t o p e r t e n e r c o n t o d i q u e l l e t r a c c e c h e s f u g g o n o all ' o s s e r v a z i o n e p e r c h é h a n n o 0 u g u a l e o v i c i n o a 9 0 ° . p e r q u e s t a c o r r e z i o n e è s t a t o d e t e r m i n a t o s p e r i m e n t a l m e n t e u n f a t t o r e k = 1 , 1 5 . i l n u m e r o d e l l e t r a c c e d e l th c' s t a a q u e l l e d e l th c n e l r a p p o r t o c o s t a n t e 6 6 , 3 / 3 3 , 7 . i l th b e m e t t e u n a p a r t i c e l l a (3 n e l l a t r a s f o r m a z i o n e th b j l th c e s u c c e s s i v a m e n t e u n a p a r t i c e l l a a n e l l a t r a s f o r m a z i o n e th c v th c" o p p u r e th c th c' —v th d : p e r c i ò o g n i e l e m e n t o d e l l e d u e s e r i e si t r a s f o r m a i n e l e m e n t o s t a b i l e c o n l ' e m i s s i o n e di u n a s o l a p a r t i c e l l a a , a d e c c e z i o n e d e l ra a, c h e p o t r e b b e d a r e n e l l ' e m u l s i o n e d u e t r a c c e a v e n t i u n p u n t o di e m i s s i o n e c o m u n e . t r a c c e d i q u e s t o t i p o n o n s o n o s t a t e o s s e r v a t e n e l l e m i s u r e f a t t e f i n o r a c o n q u e s t o a p p a r e c c h i o , e a n c h e i n l a v o r i a n a l o g h i ( 7 , n ) è s t a t a n o t a t a l a l o r o m a n c a n z a . i l r a p p o r t o r f r a gli e l e m e n t i d e l l a f a m i g l i a d e l t o r i o ( th b -fth ci e q u e l l i d e l l a f a m i g l i a d e l r a d i o p r e s e n t i n e l d e p o s i t o a l l a fine d e l l a p r e c i p i t a z i o n e è d a t o d a : r — n'p(l + 3 3 , 7 / 6 6 , 3 ) / [ { n " — a") k + n' + n'p (1 + 3 3 . 7 6 6 . 3 1 ] p e r r i s a l i r e a l v a l o r e di q u e s t o r a p p o r t o n e l l ' a t m o s f e r a , b i s o g n e r e b b e s u p p o r r e , c o m e si f a g e n e r a l m e n t e , c h e g l i e l e m e n t i d e l l e d u e s e r i e si t r o v i n o s u l l ' a e r o s o l i n e q u i l i b r i o r a d i o a t t i v o f r a l o r o . d a i ris u l t a t i di r i c e r c h e e s e g u i t e i n q u e s t o i s t i t u t o c o n v a r i m e t o d i 1 0 , s e m b r a c h e q u e s t a c o n d i z i o n e n o n sia v e r i f i c a t a p e r l e s o s t a n z e a n c o r a t e a l l ' a e r o s o l e p r e c i p i t a t e e l e t t r o s t a t i c a m e n t e ; c o m e c o n s e g u e n z a il v a l o r e d i c a l c o l a t o r i s u l t e r e b b e a f f e t t o d a u n e r r o r e d i f f i c i l m e n t e v a l u t a b i l e . c o l g o l ' o c c a s i o n e p e r r i n g r a z i a r e i l p r o f . o. s p e c c h i a , d i r e t t o r e d e l l ' i s t i t u t o di f i s i c a d e l l ' u n i v e r s i t à d i p a v i a , p e r l ' i n t e r e s s e m o s t r a t o p e r q u e s t o l a v o r o , e l a p r o f . g . a l i v e r t i p e r i p r e z i o s i c o n s i g l i e l e d i s c u s s i o n i s u l l ' a r g o m e n t o . pavia — istituto di fisica dell'università — luglio 1953. m i s u r a d e l l a r a d i o a t t i v i t à a t m o s f e r i c a c o n e m u l s i o n i n u c l e a r i 5 6 7 riassunto viene descritto un metodo per la misura della radioattività atmosferica con l'impiego di lastre fotografiche per ricerche nucleari. 'le sostanze radioattive vengono separate dall'aria mediante precipitazione elettrostatica, su un elettrodo piano, il quale viene in seguito appoggiato sull'emulsione sensibile, per ottenere una registrazione delle particelle a emesse. l'esame microscopico dell'emulsione permette di determinare il valore relativo della radioattività atmosferica e di discriminare gli elementi della famiglia del torio da quelli della famiglia d<dl' uranio-radio. summary a method of measuring atmospheric radioactivity by the employmenl of nuclear emulsions is described. the radioaclive components are separated frolli lite air by electrostatic separation info an electrode piate, follouing ivltich, the piate is placed on the sensitive emulsion ili order to obtain a registration of the emitted alpha particles. from a microscopie examination of the emulsion one determines the relative amount of the atmospheric radioactivity and it is also possible to discriminate the elements of the thorium family front those of the uranium-radium family. b i b l i o g r a f i a ( m j . h . h a h l e y : n u c l e o n i c s 1 1 , 1 2 , ( l u g l i o 1 9 5 3 ) . (2) a . sella: r e n d . a c c . l i n c e i , x i , 5 8 , 2 4 2 , ( 1 9 0 2 ) . (••') g . a l i v e r t i : n . c i n i . , 8 , 2 3 3 , ( 1 9 3 1 ) ; n . c i n i . , 9 , 3 1 3 , ( 1 9 3 2 1 . m i j . 1 ' . l o n g c h a m p : j . p h y s . r a d . , 1 4 , 8 9 , ( 1 9 5 3 ) . ( 3 ) g . a l i v e r t i : n . c i n i . , 1 2 , 2 6 , ( 1 9 3 5 ) ; e . m o r a s c h i n e l l i : r e m i . i s t . l o m b . , 8 6 , 5 3 , ( 1 9 5 3 ) . ( 0 ) e . m o r a s c h i n e l l i : r e n d . i s t . l o m h . , 8 6 , 2 8 1 , ( 1 9 5 3 ) . ( " ) e . m o r a s c h i n e l l i : n . c i n i . , 1 0 , 1 3 4 1 , ( 1 9 5 3 ) . ( 8 ) g . i m r ò , l . c a s e r t a n o : a n n . g e o f . , 3 , 4 7 5 , ( 1 9 5 0 ) ; 5 , 1 , ( 1 9 5 2 1 . ( ! l > j . h . p o o i . e , c . m . e . m a t t e w s : s c i . p r o c . r o y . d u b l i n s o c . , 2 5 , 3 0 5 , ( 1 9 5 1 ) . ( i ° ) e . m o r a s c h i n e l l i : r e n d . i s t . l o m h . , 8 4 , 2 9 3 , ( 1 9 5 1 ) ; n . c i n i . , 1 0 , 1 3 4 1 , ( 1 9 5 3 ) . ( i l ) r . s i k s n a : a r k . g e o p h y s . , 1 ( 6 ) . 1 2 3 , ( 1 9 5 0 ) . ag_57.04.14_sadeghi_finalonline:layout 6 annals of geophysics, 57, 4, 2014, a0430; doi:10.4401/ag-6407 a0430 determining precipitable water in the atmosphere of iran based on gps zenith tropospheric delays elaheh sadeghi*, masoud mashhadi-hossainali, hossein etemadfard k.n. toosi university of technology, department of geodesy and geomatics engineering, tehran, iran abstract precipitable water (pw) is considered as one of the most important weather parameters in meteorology. moreover, moisture affects the propagation of the global positioning system’s (gps) signals. using four different models, the current paper tries to identify the best relationship between the atmospheric error known as zenith wet delay (zwd) and pw. for that matter, based on 54,330 radiosonde profiles from 11 stations, two different models i.e. linear and quadratic have been derived for iran. for analyzing the accuracy of these models, zwds of three permanent gps stations located in the cities of tehran, ahvaz and tabriz have been used. applying the aforementioned models as well as those already developed for europe and the u.s., pws are derived at the position of these stations in iran. further, in this research, root mean square error (rmse) and bias are the measures for selecting the optimal model. here, the bias and the rmse (between gps and radiosonde derived pws) for the proposed linear model for iran is 1.44 mm and 4.42 mm, and for quadratic model 2.18 mm and 4.74 mm respectively while, the bias and the rmse for bevis’ linear model is 2.63 mm and 4.98 mm and for emardson and derk’s quadratic models are 2.80 mm and 5.08 mm respectively. as such, it is observed that the bias of the proposed linear model for iran is 1.19 mm and 1.36 mm less than the bevis’ and emardson and derk’s models. in addition, the rmse of the proposed linear model is 0.56 and 0.66 mm less than the rmse of the later ones. this emphasizes that the estimation of the model coefficients must be based on regional meteorological measurements. 1. introduction water vapor plays an important role in atmospheric processes. this parameters is one of the most variable components of the earth’s atmosphere. its non-uniform distribution, which is due to the atmospheric phenomena above the earth surface, depends both on space and time. due to the limited spatial and temporal coverage of observations, estimates of water vapor are always shown with some uncertainties [emardson et al. 1998, jarlemark et al. 1998]. being a key element in various atmospheric phenomena such as precipitation, the amount of water vapor is estimated by applying integrated water vapor (iwv) and precipitable water (pw). in other words, iwv signifies the amount of water vapor at a specif ic direction and the pw is the height of the equivalent column of the liquid water represented by , where tw is the liquid water density [bevis et al. 1992]. the pw is used as one of the inputs for the numerical weather prediction models [vedel and huang 2003] as well as the pattern analysis and forecast of rain which is considered as the most important and fundamental hydrological cycle [seco et al. 2012]. in addition, the pw is also a useful tool for determining the trends in climate change [bevis et al. 1992]. the existing restriction in the water vapor analysis is a major source of error in shortterm (0-24 hours) forecasts of precipitation [bevis et al. 1992]. a linear power structure has been reported for the time variation of the pw [hogg et al. 1981, jarlemark et al. 1998]. the spatial structure of the pw analyzed by emardson et al. [1998] demonstrated the significance of local models in estimating the precipitable water. the ground-based gps stations have improved both spatial and temporal resolutions of measurements required for estimating the water vapor [rocken et al. 1991]. for this purpose, the propagation delay experienced by gps signals in their transition through troposphere is used. in other words, the delay in this part of the atmosphere is partially controlled by the existing gaseous water. askne and nordius [1987] highlighted the required linear relation between the wet part of tropospheric delay and the pw. over a decade later, emardson et al. [1998] revised this ratio for the scandinavian region. following years, the same technique was also applied for estimating the pw in near real time [karabatic et al. 2011]. due to their reasonable accuracy, the pws pw iwv wt= article history received september 16, 2013; accepted may 19, 2014. subject classification: precipitable water, zenith wet delay, radiosonde profiles, gps, modeling. were computed using gps in rain pattern analysis and forecast models [seco et al. 2012]. taking into account the zwds, the current research primarily intends to assess the necessity and generate a local (conversion) model for the accurate estimation of the pw in iran. since radiosondes are expendable instruments, their high cost often restrict the number of launches (normally once to twice a day at 00,00 and 12,00 in utc) at each station. it must be remembered that iran possesses more than 107 permanent gps stations hence; the possibility of computing the pw from zwds provides a unique opportunity for improving both spatial and temporal resolutions required for estimating the pw in this country. the paper analyzes the validity of the existing models for estimating the pw parameter through zwds in iran. the next section discusses the conversion models which have already been published in other parts of the world. the paper highlights the development process of these models as well as the pw computation through zwds. the third section introduces the most appropriate model to the climatic conditions of iran. for this purpose, the pws estimated through different conversion models are compared with the precipitable water directly extracted from the radiosonde data. the most appropriate model is then selected using the rmse and the bias of gps and radiosonde derived pws. 2. estimating precipitable water from zenith wet delays the zwd of the gps tropospheric signals is due existing gaseous waters in the atmosphere. taking π as the required model for converting the zwd to the pw, one can write: pw = zwd/π. to estimate zwd, a slant tropospheric delay (std) is computed along the propagation path of the gps signals. this parameter is the signal delay between a satellite transmitting the signal and a ground station. when the std is mapped from the line of sight to the zenith direction, it is called zenith tropospheric delay (ztd). the ztd is comprised of hydrostatic and wet components. the zhd is modeled using temperature and pressure at the observing site. the zwd is computed as the difference between the observed ztd and the zhd [jin and luo 2009]: (1) various models are available for computing the zhd [hopfield 1969, saastamoinen 1972, goad and goodman 1974]. for example, according to saastamoinen [1972]: (2) where p0, h0 and { are the surface pressure in hpa, orthometric height of the station in meters and the latitude of the observing site respectively. when no pressure and temperature sensors are available at the gps station, the data can be extrapolated from the nearest meteorological site. as the vertical temperature gradient is used to transfer the temperature, equation (3) can be used to transfer the pressure [karabatic et al. 2011]: where psyn, tsyn, hsyn, are the pressure, temperature and height at meteorological station respectively. parameter c is the temperature gradient and r is the gas constant. since, the zenith tropospheric delay is normally estimated through analyzing the gps measurements, a time series of zwd can be computed at each permanent gps station. using the zwd estimates as well as the conversion model, the pw amount can be calculated independently from the radiosonde data profiles. as the pw depends on the temporal and spatial variations of the atmosphere, the conversion model π is also not a constant parameter. moreover, the best estimate for this parameter is obtained when the climate condition is taken into account [emardson and derks 2000]. to compute pw from zwd, there are various existing models which are classified into two groups of linear and quadratic here. the following subsections introduce these models in further detail. 2.1. linear model the ratio between the zwd and the pw parameters [bevis et al. 1992] is as followed: here, rv is the specific gas constant of water vapor, that is 461.45 jkg−1k−1, and the atmospheric refractivity constants k3 and are approximately 3.7×105 k2mbar−1 and 17 kmbar−1 respectively. tm is the weighted average of the atmospheric temperature. the value of tm is calculated using equation (5). therefore, computation of this parameter at each point requires water vapor pressure and temperature along a vertical profile: zwd ztd zhd= 0.002277 . .cos zhd mhpa h p 1 0 0026 2 0 00000028 1 0 0 # # { = -^ h6 @ p p t t h h syn syn syn syn r g c = c^ he o 10pw zwd r k t kv m 6 3 2= = + l^ h6 @ k2l sadeghi et al. 2 (3) (4) 3 in the equation above, t is the temperature in kelvin, z is the vertical coordinate and ew is the water vapor pressure. further, this parameter is obtained using the following equation [holton 2004]: where lv is the latent heat of vaporization and is equal to 2.5×106 jkg−1 and td is the dew point temperature. to calculate the distance between any two consecutive recorded levels, after taking logarithm of the equation proposed in [nafisi et al. 2012] can be used: here pi is the pressure on level i in hpa, rd is the gas constant for dry air and is equal to 287.05 jkg−1k−1, local gravity g is a function of latitude and height of the station and is the mean virtual temperature (tv) between the two consecutive levels. the virtual temperature is defined as the one that a hypothetical system of dry air would have, in relation to the actual condition of (moist) air at the same density and pressure. this parameter is expressed as [wallace and hobbs 2006]: where mv and md are the molecular mass of water vapor and dry air respectively. their ratio is defined as follows [andrews 2010]: tm can be estimated as a linear function of the temperature at 2 meters high above the earth surface (ts) [bevis et al. 1992]: analyzing more than 8000 vertical profiles of radiosonde stations in order to compute the unknown coefficients ai, i = 0,1 in the united states, bevis et al. proposed the following functional relation between tm and ts: 2.2. quadratic model according to bevis et al. [1992], the conversion model π and the temperature behave similar to the parameter tm with respect to time. based on the same, emardson and derks [2000] later proposed the following functional relation between the conversion model π and the difference of surface and mean surface temperatures: where tδ is the surface temperature minus the mean surface temperature the mean surface temperature is computed by averaging the temperature obtained from radiosonde profiles. further, the zwd and pw estimates are required for computing the unknown coefficients ai,i = 0,1,2. based on that as well as using 120,000 radiosonde profiles in europe, emardson and derks [2000] proposed the following functional relation [emardson and derks 2000]: to compute the zenith wet delay from the radiosonde profiles, one has [rocken et al. 1995]: here, this integral is computed from the surface (position of the station) to the upper troposphere where t and ew are derived from radiosonde profiles. practically, since these parameters are not available in continuous form, the discrete form of the integral is used: in equation (15), n is the number of levels in a radiosonde profile. rocken et al. [1995], computed the pw using the following equation: where tw is the density of liquid water. this integral is also computed from the surface (position of station) to the upper most tropospheric layer in the zenith direction. and for a similar aforementioned reason, the discrete form of this equation was used [hagemann et al. 2003]: the pw, for each profile, can be calculated using the temperature and water vapor pressure derived from the radiosonde profiles. then, the pw and zwd for t t e dz t e dz m w w 2 = # # . .expe t r l t6 11 273 15 1 1 w d v v d = -^ h ; e` j lnz z g r t p p i i d v i i 1 1 =+ + c cm m tv t p m m tp e1 v d v w = -c m .m m r r 0 62197 d v v d = = t a a tm s0 1= + . .t t70 2 0 72m s= + a a t a t0 1 2 2= + +o o .t t t t= -o^ h 6.458 1.7 10 2.2t t102 25# #= -o o 10zwd k t k t e z z z w 6 2 3 2 ground top d= +l^ h# 10 2zwd t t k t t k e e z z 2 2 i n i i i i w i w i i i 6 1 1 1 2 1 2 3 1 1 = + + + + + = + + + + l ^ ^ h h = =g ge o / pw r t e dz1 w v w t = # pw r t t e e z z1 w v i i w i w i i n i i 1 1 1 1 1t = + + + + = +< <f f/ pw determination for iran (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) every profile can be used to calculate the conversion model π. thereafter, unknown coefficients as in equation (12) are computed using the corresponding surface and the mean surface temperatures derived from the radiosonde profiles. 3. results and discussion the proposed linear and quadratic models briefly introduced in sections 2.1 and 2.2 are computed first time for iran. for this purpose, the current study could use profiles of 11 radiosonde stations. the acquired data covers the time interval of 1996 to 2012. these profiles are obtained from radiosonde lunches which were made once or twice a day at 00,00 and/or 12,00 in utc. the corresponding profiles are available for download in the text format through http://www.esrl. noaa.gov/raobs/. existing gaps in the radiosonde data and the small number of lunching stations compared to the much larger density of the gps as well as the continuous data provided by these equipments emphasize on the necessity of developing a suitable conversion model for iran. figure 1 illustrates the position of the radiosonde and the gps stations in this country. 3.1. derived models in the current research, following section 2, the coefficients of equations (10) and (12) were locally computed using the data introduced in the previous section. this section briefly highlights the obtained results. 3.1.1. empirical linear model p in the equation (4) is only a function of the parameter tm. in order to compute this parameter in the proposed area of research, a polynomial model of degree one, similar to the equation (10), is used. here, 54,330 radiosonde profiles have been taken into account. further, the unknown coefficients of the proposed linear model are computed from the inputs given by the above radiosonde profiles. in other words, the inputs are the parameters tm and ts which are computed and derived respectively from the radiosonde data. here, tm is computed using equation (5). the obtained polynomial model is given in equation (18). the estimated standard deviations for the interception and slope parameters of this model are ±0.1 degree of kelvin and ±0.003 respectively. 3.1.2. empirical quadratic model in order to find a conversion model of the second type (see equation 12), the zwd and pw parameters are computed using the equations given in section 2.2. for this purpose, the radiosonde data introduced above are used. the same number of radiosonde profiles is also used for estimating the aforementioned polynomial model (see equation 18). the estimated model coefficients are given in equation (19) where the root mean square error stands at 0.09. moreover, the estimated standard deviations for the parameters of this model are ±0.001, ±0.0001 and ±0.00001 for zero, first and second order coefficients, respectively. 3.2. computing precipitable water from zenith wet delays in the current study, for analyzing the accuracy of the developed models, three gps stations located in climatologically different areas including tehran, ahvaz and tabriz were selected. these three cities, respectively, have semi-dry, wet and semi-wet climatic conditions. a comparison of pws estimated from zwds with those directly derived from radiosonde profiles helps analyze the accuracy of conversion models. in fact, pws estimated directly from the radiosonde data are used as benchmark for comparing the existing and the proposed models in this research. rmse and bias are used to select the optimum result. thereafter, the pw time series obtained from the gps zenith wet delays was analyzed using the least squares harmonic estimation technique. that analysis provided an insight into the frequency content of this time series and therefore could be considered as an independent approach for evaluating the proposed models. to analyze the obtained results, two distinct time . .t t75 39 0 7103m s= + 6.221 0.01491 6.73 10t t5 2#p = -o o sadeghi et al. 4 figure 1. position of the radiosonde stations (solid) and the permanent gps stations in iran (circles). the position of the igs stations tehn, ahvz and tabz is shown by a solid circles and names here. (18) (19) 5 intervals were taken into account with two signified low and high rainfall, respectively. in fact, the meteorological organization of iran reported the minimal precipitation at tehran, ahvaz and tabriz stations in january, july and september of 2010, 2012 and 2011, respectively. the maximum precipitations were reported for these stations in may, february and december of the same years, respectively. as such, the zenith tropospheric delay has been computed for every period using the gps measurements of the above stations. it is worth mentioning that the distance between the gps stations of this study and the radiosonde stations located in tehran, ahvaz and tabriz is 2.3, 36.4 and 11.5 kilometers. the height differences the gps and the corresponding radiosonde stations for tehran, ahvaz and tabriz are approximately 2.5, 1.0 and 35 meters repw determination for iran figure 2 (top to bottom). the rms and bias of the proposed as well as the existing models in the rainy months: i) linear model proposed in bevis et al. [1992], ii) linear model proposed in this research, iii) quadratic model proposed in emardson and derks [2000] and iv) quadratic model proposed in this research. figure 3 (top to bottom).the rms and bias of the proposed as well as the existing models in the dry months: i) linear model proposed in bevis et al. [1992], ii) linear model proposed in this research, iii) quadratic model proposed in emardson and derks [2000] and iv) quadratic model proposed in this research. spectively. charts 2 and 3 compare the rms and the bias of the models for the gps stations. the analysis of the obtained results shows that the models found for iran perform better than those given by equations (4) and (13). moreover, the rmse and the bias for the proposed linear model is less than the quadratic one both for dry and wet months on all of the above gps stations. figures 4 and 5 illustrate the degree of misfit between the proposed linear model and the pw parameters which are computed directly from the high resolution radiosonde profiles as well as the time series of the estimated precipitable water at the tehran station. these figures show the high and the low precipitations for may 2010 and january 2010, respectively. dots are the precipitable water computed from the radiosonde records. the frequency content of the pw time series derived from the gps zenith wet delays can be considered as an independent approach for the evaluation of the linear model of this research. the zwds of tehran, again a typical station, are used for computing the pw time series elements. this time series starts at 2010 and ends at 2012 (see figure 6) the frequency content of this time series can be analyzed using one of the existing frequency analysis techniques. here, the least squares harmonic estimation has been used [amiri-simkooei 2007] because, this method has been efficiently used for analyzing the frequency content of various time series i.e. gps observations [amiri-simkooei 2007], time series of total electron content [amiri-simkooei and asgari 2012], tidal data [mousavian and mashhadi-hossainali 2012] and coordinate time series for detecting slow slip events [mousavian and mashhadi-hossainali 2014]. the application of the least squares harmonic estimation suggests annual and seasonal frequencies in the above pw time series. in other words, the annual and seasonal frequencies are the first that approved by the hypothesis test of this method (see amiri-simkooei [2007] for further details). moreover, since the least squares harmonic estimation is sensitive to the length of the input data [mousavian and mashhadi-hossainali 2012], a detailed analysis of the frequency content requires a pw time series of much larger length. further, the existence of annual and seasonal frequencies in the pw time series indicates that the adopted linear model does not distort the expected behavior of the pw parameter. as a matter of fact, this can be considered as independent evidence on the applicability of the proposed linear model in the field of meteorology and climatology in iran. 4. conclusion as discussed, the pw is a useful tool for determining the climate change. it is also used for producing the rain, as the most important and fundamental sadeghi et al. 6 figure 4. time series of the estimated pecipitable water at station tehran in january 2010. figure 5. time series of the estimated precipitable water at station tehran in may 2010. figure 6. time series of pw based on the gps zwd and π calculated using linear model of iran during 3 years. 7 hydrological cycle, prediction models. however, restriction in spatial and temporal resolution of the pw estimates seems to be the major source of error which can significantly and reliably be sorted out through the gps-derived pw parameters. the rmse and bias of the proposed models are slighter than the existing ones. as the obtained results is valid for both dry and wet climatic conditions in iran, the application of these models seems proper for computing the pw from the zwds in this country. in addition, the empirical linear model investigated in this research show that the rmse and bias of the proposed model is less than the quadratic one. as such, the application of this model is also efficiently superior for computing the pw from zwds in iran. the accuracy analysis of the pw parameters obtained from the gps zenith wet delays and the linear model of this study, as well as their frequency analysis approves the reliability of the gps-derived precipitable water. therefore, the linear model which has been determined here can be used for estimating the pw parameters together with their direct estimates from radiosonde profiles. this improved both spatial and temporal resolutions of this parameter in the country. acknowledgements. we are grateful to professor ghaemi from the meteorological organization of iran, for his kind and invaluable help during this research. we, particularly, appreciate the meteorological organization for giving us an access to radiosonde profiles with dense pressure levels at the stations of this research. data and sharing resources radiosonde data used in this study were taken from the esrl radiosonde database, and can be downloaded from: http://www.esrl.noaa.gov/raobs zenith tropospheric delay of the igs was taken from the crustal dynamics data information system (cddis) database in the goddard space flight center of nasa, and can be uploaded via: ftp://cddis.gsfc.na sa.gov/gps/products/troposphere. references amiri-simkooei, a.r. (2007). least-squares variance component estimation: theory and gps applications, ph.d. thesis, delft university of technology, publication on geodesy, 64, netherlands geodetic commission, delft. amiri-simkooei, a.r., and j. asgari (2012). harmonic analysis of total electron contents time series: methodology and results, gps solut., 16 (1), 77-88. andrews, d.g. (2010). an introduction to atmospheric physics, 2nd ed., cambridge university press. askne, j., and h. nordius (1987). estimation of tropospheric delay for microwaves from surface weather data, radio sci., 22, 379-386. bevis, m., s. businger, t.a. herring, c. rocken, r.a. anthes and r.h. ware (1992). gps meteorology: remote sensing of atmospheric water vapor using the global positioning system, j. geophys. res., 97, 15787-15801. emardson, t.r., g. elgered and j.m. johansson (1998). three months of continuous monitoring of atmospheric water vapor with a network of global positioning system receivers, j. geophys. res., 103, 1807-1820. emardson, t.r., and h.j.p. derks (2000). on the relation between the wet delay and the integrated precipitable water vapour in the european atmosphere, meteorol. appl., 7, 61-68. goad, c.c., and l. goodman (1974). a modified hopfield tropospheric refraction correction model, paper presented at the agu fall annual meeting (san francisco, california, december 12-17), abstract: eos, 55, 1106. hagemann, s., l. bengtsson and g. gendt (2003). on the determination of atmospheric water vapor from gps measurements, j. geophys. res., 108 (d21), 4678. hogg, d.c., f.o. guiraud and w.b. sweezy (1981). the short-term temporal spectrum of precipitable water vapor, science, 213, 1112-1113. holton, j.r. (2004). an introduction to dynamic meteorology, 4th ed., 535 pp. hopfield, h. (1969). two-quartic tropospheric refractivity profile for correcting satellite data, j. geophys. res., 74 (18), 4487-4499. jarlemark, p.o.j., t.r. emardson and j.m. johansson (1998). wet delay variability calculated from radiometric measurements and its role in space geodetic parameter estimation, radio sci., 33, 719-730. jin, s., and o. luo (2009). variability and climatology of pwv from global 13-year gps observations, ieee transactions on geoscience and remote sensing, 47 (7), 1918-1924. karabatic, a., r. weber and t. haiden (2011). near real-time estimation of tropospheric water vapour content from ground based gnss data and its potential contribution to weather now-casting in austria, adv. space res., 47, 1691-1703. mousavian, r., and m. mashhadi-hossainali (2012). detection of main tidal frequencies using least squares harmonic estimation method, journal of geodetic science, 2 (3), 224-233; doi:10.2478/v10156-0110043-6. mousavian, r., and m. mashhadi-hossainali (2014). geodetic constraints on the period of episodic tremors and slips using ls-he method with application to pw determination for iran cascadia subduction zone, acta geophysica, 62 (3), 451-468. nafisi, v., m. madzak, j. böhm, a.a. ardalan and h. schuh (2012). ray-traced tropospheric delays in vlbi analysis, radio sci., 47, rs2020; doi:10.1029/ 2011rs004918. rocken, c., j.m. johnson, r.e. neilan, m. cerezo, j.r. jordan, m.j. falls, l.d. nelson, r.h. ware and m. hayes (1991). the measurement of atmospheric water vapor: radiometer comparison and spatial variations, ieee t. geosci. remote, 29, 3-8. rocken, c., t. van hove, j.m. johnson, f. solheim and r.h. ware (1995). gps/storm gps sensing of atmospheric water vapor for meteorology, j. atmos. ocean. tech., 12, 468-478. saastamoinen, j. (1972). atmospheric correction for the troposphere and stratosphere in radio ranging of satellites, in: the use of artificial satellites for geodesy, geophysical monograph series, vol. 15, edited by s.w. henriksen et al., agu, washington, d.c., 247-251. seco, a., f. ramírez, e. serna, e. prieto, r. garcía, a. moreno, j.c. cantera, l. miqueleiz and j.e. priego (2012). rain pattern analysis and forecast model based on gps estimated atmosphericwater vapor content, atmos. environ., 49, 85-93. vedel, h., and x.y. huang (2003). a nwp impact study with ground based gps data, danish meteorological institute, lyngbyvej 100, dk 2100 copenhagen, denmark. wallace, j.m., and p.v. hobbs (2006). an introductory survey, atmospheric science, 2nd ed., academic, san diego, california. *corresponding author: elaheh sadeghi, k.n. toosi university of technology, department of geodesy and geomatics engineering, tehran, iran; email: elaheh_sadeghi@mail.kntu.ac.ir, sadeghi_elaheh@ymail.com. © 2014 by the 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice sul significato della presenza di metano nelle manifestazioni esalative e idrotermali alfonso d i n o i 1 . p r e m e s s a . già da alcuni anni f . p e n t a f1) ha posto il problema del significato genetico da attribuire agli idrocarburi riscontrati in talune manifestazioni esalative ed idrotermali naturali. sull'argomento lo scrivente ha condotto uno studio, del quale col presente articolo dà in sintesi i risultati. è da notare, anzitutto, che lo studio in parola è stato condotto soltanto su basi chimico-fisiche. ciò comporta implicitam e n t e la necessità di u n controllo su altre basi. sono s t a t i esaminati alcuni equilibri schematici, allo scopo di studiarne l ' a n d a m e n t o in condizioni di pressione e t e m p e r a t u r a paragonabili a quelle che si presume caratterizzino le masse ignee più o meno profonde. è implicita, in questa fase dello studio,l'ipotesi semplificativa che i prodotti gassosi in seno ai fusi, o rifusi, siano chimicamente inerti rispetto alla massa silicatica. p e r quanto riguarda l'approssimazione dei calcoli si è ritenuto di non introdurre le correzioni per la « fugacità », conformem e n t e a quanto f a t t o da a. j. ellis (-) in u n recentissimo lavoro. ciò perchè il carattere p u r a m e n t e indicativo dell'indagine non giustifica la maggiore laboriosità dei calcoli che i coefficienti di fugacità comporterebbero. b a s t a pensare che i calcoli sono riferiti a miscugli ipotetici, aventi funzione di « modello » dei fenomeni studiati, per i quali è s t a t a assunta u n a composizione iniziale, la composizione, cioè, relativa a condizioni normali di pressione e t e m p e r a t u r a , che, a rigore, è arbitraria. d ' a l t r o canto, è anche da osservare che in concomitanza con le pressioni maggiori, sono in genere da attendersi anche temper a t u r e più elevate, per cui è presumibile che i coefficienti di fugacità non siano t a n t o discosti dall'unità da togliere ai risultati il loro valore, sia pure soltanto indicativo. p e r quanto riguarda poi l'aspetto emetico del problema, esso non h a importanza che in casi particolari, d a t a la scala geologica dei tempi che, in generale, interessano. sono da attendersi eccezioni solo nei processi vulcanici particolarmente veloci; non t a n t o nei fenomeni effusivi, poiché le elevate t e m p e r a t u r e della lava rendono elev a t e anche le velocità di reazione, quanto nei fenomeni esplosivi, che sono accompagnati da grandi e repentine variazioni di pressione e t e m p e r a t u r a . circa il campo delle t e m p e r a t u r e che interessano, un primo elemento di riferimento è formto dalle misure relative alle lave fuse. se la letteratura è sufficiente a fornire elementi circa le t e m p e r a t u r e delle lave effondenti ed appare legittima l'estrapolazione per i magmi estremamente superficiali, non a l t r e t t a n t o può dirsi per quelli profondi. comunque come valore massimo assumeremo quello di 1300 °c che a. r i t t m a n n (3) attribuisce al magma subcrostale. molto più basse sarebbero, viceversa, le t e m p e r a t u r e relative alle masse plutoniche propriamente dette. bowen, citato da wahlstrom (4), ad esempio, osserva che la temp e r a t u r a di granitizzazione in particolari condizioni può variare nell'intervallo 300650 °c; d'altro canto, r i t t m a n n segnala u n a t e m p e r a t u r a massima di 800 °c. 2 46 a . d i n o i roo eoo 300 400 soo eoo 700 soo 900 /h at-masfes^e / s o r f e w ù f i l a q f a z / o w /n o/-c//no/a sono /-/portate /e //-a^/on/ /no/o/-/ c/e/ me/ono e c/e// /a//-ape no to eyc////&/-/o per forze cona'/z/on/ c//prensione e /emperofora. f i g . 1 p e r quanto riguarda, poi, le pressioni è apparso opportuno limitare il campo ai valori compresi t r a quelli relativi alla libera atmosfera e alla profondità dell'ordine di grandezza della diecina di migliaia di metri, ossia, in cifre tonde, t r a 1 e 3000 atmosfere. a pressione atmosferica la dissociazione è molto spinta per t e m p e r a t u r e di 1000°k, pari a 727 °c; nelle d e t t e condizioni la frazione molare di equilibrio non raggiunge il valore di 0,14; per t e m p e r a t u r e superiori ai 1400°k, corrispondenti a 1127 °c, la dissociazione è p r a t i c a m e n t e completa. 2 . p i r o l i s i d e l m e t a n o . l a prima reazione schematica studiata è quella di pirolisi del m e t a n o secondo la cu, < y g + -il, le costanti di equilibrio relative alle varie t e m p e r a t u r e sono state riprese da g. egloff (5); i risultati dei calcoli sono rappresentati in lig. 1. la pirolisi del metano è favorita dalle alte t e m p e r a t u r e e dalle basse pressioni. p e r quanto riguarda le condizioni di t e m p e r a t u r a , la. dissociazione non si verifica praticamente più al disotto dei 600°k, pari a 327 °c. a questo valore, infatti, l'ent i t à della frazione molare di metano in equilibrio è maggiore di 0,94 alla pressione atmosferica, ed è prossima all'unità a 20 atmosfere. bisogna ancora aggiungere, però, che l'equilibrio alle alte t e m p e r a t u r e , è sensibilm e n t e influenzato dalle pressioni, per cui la frazione molare, almeno nel campo di t e m p e r a t u r e che ci interessa, è sempre superiore a 0,3 se la pressione supera le 500 a t m . 3 . r e a z i o n e f r a m e t a n o e a l t r i c o m p o n e n t i d e i g a s m a g m a t i c i . la decomposizione del metano può aver luogo anche secondo le seguenti reazioni cu, + 2 # 2 o v a p ^ z l co, + 4 i i , [ 1 ] c e , + i ? 2 o v a p 1 co + 3 h„ [ 2 ] c'-hj + co, < " 2 co : 2 ti, [ 3 ] d i queste è stato studiato l'andamento, nell'ipotesi che i sistemi iniziali siano mis u l s i g n i f i c a t o d e l l a p r e s e n z a 1)1 m e t a n o , ecc. 2 4 3 scugli stechiomètrici dei composti che figurano al secondo membro. si è potuto accertare che l'andamento quantitativo delle reazioni non è sostanzialmente diverso, almeno dal p u n t o di vista che qui interessa. poiché i gas magmatici sono formati in prevalenza da vapor d'acqua, e ciò sposta sensibilmente 1'equihbrio delle reazioni [l] e [2] nel senso della decomposizione del metano, lo studio della [1] è stato ulteriormente approfondito; è stato, cioè, esaminato il comportamento di u n a miscela di h20, c02 e h2 avente la seguente composizione iniziale (espressa mediante le frazioni molari) n„2o = 0 , 8 9 , nco2 = 0 , 1 0 , nhi = 0 , 0 1 . nei calcoli sono stati introdotti i valori delle costanti di equilibrio riportati da a. j . ellis, nel campo di t e m p e r a t u r a f r a 400 e 1000° k , m e n t r e per i valori 1200° e 1600° k si è proceduto al calcolo mediante le formule approssimate di ulicli. i risultati sono riportati in tabella 1 e rappresentati nelle fig. 2 e 3. è da avvertire, ancora, che la frazione molare del metano è s t a t a convenzionalment e r i t e n u t a trascurabile per valori inferiore a io7. ii calcolo dimostra che, per t e m p e r a t u r e inferiori a 500° k (corrispondenti a 227 °c) se la pressione supera qualche centinaio di atmosfere, il metano è praticamente indecomposto; inoltre, anche per basse pressioni, la frazione molare del metano indecomposto è superiore al 50 % del massimo valore possibile in relazione alla composizione iniziale; ciò corrisponde a un valore di nchi sempre superiore a 0,0015. p e r t e m p e r a t u r e comprese t r a i 500° e i 700° k, ossia per l'intervallo 227° -f427°c, la frazione molare del ch1 è ancora notevole, salvo che per pressioni di poche atmosfere. p e r t e m p e r a t u r e superiori ai 1000° k la frazione molare del ch} diminuisce sensibilmente, in particolare a 1600° k, corrispondenti a 1327 °c il metano praticam e n t e scompare fintanto che la pressione non supera le 500 atmosfere. p e r pressioni superiori a questo valore permane sempre u n a certa frazione indissociata, t a n t o più grande q u a n t o più elevata è la pressione. 4 . interferenza di altre reazioni. beazione del gas d'acqua e dissoclvzione del vapor d'acqua. volendo studiare u n modello di ulteriore approssimazione, è s t a t a presa in esame u n a reazione, per così dire secondaria, la quale, inserendosi nell'andamento della precedente ne influenza l'equilibrio. come reazione secondaria è s t a t a considerata la così d e t t a reazione del gas d'acqua. è stato cioè, preso in esame il sistema di equilibrio c02 + 4 ^ z l ce, + 2 e20 [1] c02 + e2 ^z^ co + h20 [4] t a b e l l a 1 r e a z i o n e : c02 + 4 ii2 oh4 + 2 e20 f r a z i o n i m o l a r i d i chi i n e q u i l i b r i o p e r v a r i e c o n d i z i o n i di p r e s s i o n e e t e m p e r a t u r a l a c o m p o s i z i o n e i n i z i a l e è : = 0 , 8 9 ; « c o 2 = 0 , 1 0 ; n h 2 — 0 , 0 1 p ( a t m o s f e r e ) 1 100 5 0 0 8 0 0 1 0 0 0 2 0 0 0 3 0 0 0 t = 5 0 0 ° k 0 , 0 0 1 6 0 , 0 0 2 4 0 , 0 0 2 5 oo 0 , 0 0 2 5 cv. 0 , 0 0 2 5 < ^ 0 , 0 0 2 5 < ^ 0 , 0 0 2 5 t = 6 0 0 ° k 0 , 0 0 0 1 0 , 0 0 1 9 0 , 0 0 2 2 0 , 0 0 2 3 0 , 0 0 2 3 0 , 0 0 2 3 0 , 0 0 2 4 t = 7 0 0 ° k t r a s c . 0 , 0 0 0 9 0 , 0 0 1 7 0 , 0 0 1 8 0 , 0 0 1 9 0 , 0 0 2 0 0 , 0 0 2 1 t = 1 0 0 0 ° k t r a s c . 3 . io-6 1. io-5 2 . io-5 3 . io-5 1 . io4 2 . io-4 t = 1 2 0 0 ° k t r a s c . t r a s c . 1 . io-6 4 . io-6 s.io-6 3 . io-5 6 . io-5 t = 1 6 0 0 ° k t r a s c . t r a s c . t r a s c . 2 . io7 3 . io-7 1. i o-6 2 . io-6 2 46 a . d i n o i soo 700 aoo eoo iooo rroo /eoo /soo moo /soo /eoo temperate/re assolute /sosaqf dflla qmz/ort/ cos + /n orc/mc/sd sono r/porfa/e /e fr&z/on/ n?o/or/ a'/ o / o eya/y/6no per isar/e cono'/. z/on/' e//epe//'//ir/o ; // s/si,?/no ao compos/'e-'/vie/n/'s/o/e g *o.s9; n^p --0./0;r,^ -o.ot x> s: 0 oor6 o.oors 0 li ^ 00005 4 00004 00025 0.002s £ c 0.0020 \ 0 ooo 16 0.0006 o.ooso ^ 0.00/5 0 • ì o oo io ^ 0.0005 roo 200 300 400 soo 600 700 eoo eoo 1<?00 pressioni in atmosfere /sorfqmf ùfua bfaz/om cos 4he = ch4 * sfyo /n ordinala sono rrpor/ole /e froz/o/?/ mo/a/y e// o//4 /o eou/v/ar/o ner rar/e conc//'?/o/7/ a"/' pressione e te/nfierotcra; /'/ s/'stemo aa co/nfios/t/one //>/z/o/e : n 089; n^p » o./o; n^ 0.0!. fig. 2 000/6 00012 0.0008 00004 fig. 3 s u l s i g n i f i c a t o d e l l a p r e s e n z a 1)1 m e t a n o , ecc. 94 i valori delle costanti d'equilibrio p e r la reazione [4] sono s t a t i d e s u n t i d a j . e g gert (6). l ' e f f e t t o della reazione [4] è o v v i a m e n t e quello di s o t t r a r r e c02 alla reazione [1], ostacolando in t a l modo la formazione del chil a t o n a l i t à t e r m i c a f a p r e v e d e r e questa azione t a n t o più i n t e n s a q u a n t o più elevata è la t e m p e r a t u r a ; d ' a l t r o canto t a l e azione è i n d i p e n d e n t e dalla pressione, perchè la [4] decorre senza variazione di vo lume. ora noi a b b i a m o visto quale sia il comp o r t a m e n t o della [1] alle a l t e t e m p e r a t u r e , e, in particolare, come il 6 7 / , sia instabile a 1600°k sotto pressioni r e l a t i v a m e n t e basse. nel delineato c a m p o di pressione e t e m p e r a t u r e la reazione del gas d ' a c q u a n o n a v r à altro r i s u l t a t o notevole che f a r comparire nel miscuglio gassoso il co. p i ù i n t e r e s s a n t e a p p a r e , quindi, l ' e f f e t t o della reazione [4], nei riguardi della stabilità del m e t a n o , sotto f o r t i pressioni. alle basse t e m p e r a t u r e , già s a p p i a m o com e la formazione del ce,, sia f a v o r i t a , m a n o n saranno d a p r e v e d e r e sensibili interferenze d a p a r t e della reazione del v a p o r d ' a c q u a , poiché essa è ostacolata in quelle condizioni di t e m p e r a t u r e . p e r queste ragioni si è r i t e n u t o o p p o r t u n o limitare lo studio dell'influenza della formazione del gas d ' a c q u a alle alte t e m p e r a t u r e e alle alte pressioni. i r i s u l t a t i dei calcoli, sempre nell'ipotesi che la composizione iniziale sia quella già d e t t a , sono r i p o r t a t i in tabella 2. d a l c o n f r o n t o dei d a t i r i p o r t a t i in t a bella 2 con quelli di tabella 1 segue che la reazione del gas d ' a c q u a a m p l i a verso pili alte pressioni il campo di instabilità del m e t a n o p u r senza v a r i a r e sostanzialmente il q u a d r o generale p r e c e d e n t e m e n t e delineato. a l t r a i n t e r f e r e n z a con la [1] sarebbe d a a t t e n d e r e p e r la dissociazione del v a p o r d ' a c q u a . viceversa u n rapido calcolo orient a t i v o , e f f e t t u a t o p e r condizioni di bassa pressione e a l t a t e m p e r a t u r a , che sono le più favorevoli alla dissociazione del v a p o r d ' a c q u a , h a p o r t a t o a stabilire che t a l e influenza, a causa del valore della c o s t a n t e di equilibrio di q u e s t ' u l t i m a reazione, è t a n t o m o d e s t a d a essere t r a s c u r a b i l e nei limiti di approssimazione assunti. i n conclusione possiamo dire che il quadro delineato circa la stabilità del m e t a n o in precedenza m e d i a n t e lo schema c02 + 4 il, 7 ce, + 2 ii2ompok r e s t a sostanzialmente i n a l t e r a t o a n c h e prendendo in considerazione la. reazione del gas d ' a c q u a e la dissociazione t e r m i c a del v a por d ' a c q u a . ne, d ' a l t r o canto, n u o v i elementi qualit a t i v i sono d a a t t e n d e r s i considerando come p a r t e c i p a n t e all'equilibrio generale a n c h e la dissociazione t e r m i c a del m e t a n o . 5 . d e d u z i o n i . vediamo ora (li t r a r r e qualche conclusione d a q u a n t o p r e c e d e n t e m e n t e esposto. t a b e l l a 2 s i s t e m a di r e a z i o n i c02 + 4 ii„ cti, + 2 ilo c02 + 1i2 —t co + 1i20 f r a z i o n i m o l a r i di cu4 in e q u i l i b r i o p e r v a r i e c o n d i z i o n i di p r e s s i o n e e t e m p e r a t u r a . l a c o m p o s i z i o n e iniziale del s i s t e m a è : h//oo = 0,89; •ìicot = 0,10: — 0,01 p ( a t m o s f e r e ) t = 1200° k t = 1 (300° ic 100 t r . t r . 500 9 . io-7 t r . 800 2 . io-6 t r . 1000 4 . io-6 1 . io-7 2000 2 . io-5 4 . io-7 3000 3 . io-5 9 . io-7 2 46 a . d i n o i p e r q u a n t o riguarda la possibilità di equilibrio del metano si dovrebbe dedurre elie essa è esclusa in ambiente caratterizzato da elevata t e m p e r a t u r a (intorno ai 1000 °c) e bassa pressione (prossima all'atmosferica). detto ambiente chimico-fisico pare identificabile, dal p u n t o di vista vulcanologico, con l'ambiente caratteristico del vulcanesirno di p i a t t a f o r m a . d e t t o vulcanesimo è, come noto, caratterizzato da u n ' a t t i v i t à prevalentemente effusiva, per cui i fluidi possono seguire, al graduale variare della pressione, u n a trasformazione chimico-fisica restando in equilibrio con l'ambiente. dalla lava, u n a volta p e r v e n u t a a giorno, può liberarsi u n miscuglio gassoso di composizione corrispondente alle condizioni di pressione e temper a t u r a determinate dalla lava f u s a e dalla pressione atmosferica. i n questo quadro si inserisce l'assenza di m e t a n o riscontrata da l. d a y e e . s. shepherd (7) nelle classiche ricerche dei gas emessi dal kilauea. sensibilmente diverso da quello ora considerato è l'ambiente chimico-fisico caratteristico dei vulcani di materiali incoerenti avendo questi, come è noto, u n ' a t t i v i t à caratterizzata da fenomeni prevalentement e esplosivi. poiché la fase esplosiva è p r e c e d u t a da una fase preparatoria, d u r a n t e la quale la pressione raggiunge valori elevati, se in questa fase le condizioni sono favorevoli alla stabilità del metano, la velocità del fenomeno esplosivo, il rapido abbassamento di t e m p e r a t u r a causato dall'espansione dei gas, e la conseguente diminuzione delle velocità di reazione, giustificherebbero la presenza di metano nei prodotti gassosi che si sprigionano nell'atmosfera. i n altri termini, in concomitanza con i fenomeni esplosivi, sarebbero da attendersi veri e propri fenomeni di « tempera » del sistema gassoso per cui esso conserverebbe u n a composizione prossima a quella relativa alla fase di alta pressione precedente l'esplosione. esaminiamo ora l'ambiente del plutonismo tipico; esso è caratterizzato da u n a t e m p e r a t u r a sensibilmente inferiore e da pressioni superiori rispetto ai valori caratteristici dell'ambiente vulcanico di p i a t t a f o r m a . circa la t e m p e r a t u r a sono da attendersi, secondo r i t t m a n n , valori massimi dell'ordine degli 800 °c. la pressione è in relazione alla profondità: per profondità dell'ordine della diecina di migliaia di metri la pressione sarà dell'ordine di grandezza di qualche migliaio di atmosfere. secondo quanto precedentemente esposto l'ambiente plutonico sarebbe compatibile con la stabilità del metano. infine, per quanto riguarda l'ambiente subvulcanico e ipoabissico, non possiamo stabilire u n quadro semplice; ciò perchè da caso a caso possono variare sensibilm e n t e gli effetti combinati delle diverse pressioni e t e m p e r a t u r e . conclusioni. quanto precede m e t t e in evidenza come gli idrocarburi contenuti dai fluidi termali non possano riguardarsi, come tali, prova della non juvenilità, in senso lato, dei fluidi stessi. è i n f a t t i lecito ritenere che solo in taluni casi tali idrocarburi possano essere interp r e t a t i con sicurezza come secondari, nel senso di non originali, del « m a g m a » e in questa ipotesi essi possono aver origine per u n a duplice via: u n a volta che fluidi termali abbiano abbandonato le masse ignee che li hanno generati, per reazioni chimiche in seno ai fluidi stessi e senza apporto di m a t e r i a da p a r t e dei terreni a t t r a v e r s a t i ; oppure per scambio di m a t e r i a con i terreni, per reazioni chimiche o fenomeni pur a m e n t e fisici o mediante ambedue i meccanismi. inoltre è da ritenere che, per particolari condizioni di pressione o temperatura, anche in seno alle masse ignee possono permanere indissociate q u a n t i t à di metano analiticamente rilevabili, e, in questo caso, l'idrocarburo sarebbe juvenile, ovvero originario della massa magmatica. r o m a (s. p i e t r o i n v i n c o l i ) , centro di studio 1per la geologia tecnica del c.n.b. presso vistituto di geologia applicata e di giacimenti minerari della facoltà d'ingegneria, università di roma, a g o s t o 1 9 5 8 . s u l s i g n i f i c a t o d e l l a p r e s e n z a 1)1 m e t a n o , ecc. 247 riassunto da alcuni anni è stato posto il quesito se la presenza di idrocarburi in seno a fluidi esalativi e idrotermali sia da interpretare come indizio di non « juvenilità » dei fluidi stessi. allo scopo di raccogliere qualche idea in proposito, lo scrivente ha studiato per via teorica alcuni equilibri chimici, cercando in un primo tempo di vedere quali condizioni di pressione e temperatura siano compatibili con la stabilità del metano, inquadrando, quindi, i risultati dello stìidio chimico-fisico nei vari ambienti « ignei » (magmatici in senso lato e classico). le conclusioni di detta analisi sarebbero che gli idrocarburi riscontrati in vari fluidi termali naturali non possono considerarsi prova di non juvenilità. ciò perchè in ambienti di alte pressioni e di temperature non molto elevate, quali quelle del plutonismo tipico, il metano non è completamente dissociato. la completa dissociazione del metano, viceversa, sembrerebbe verificarsi nell'ambiente del vulcanesimo di piattaforma. questo è caratterizzato, come è noto, da un'attività di tipo prevalentemente effusivo-, il tranquillo decorso di essa dovrebbe permettere ai fluidi termali di mantenersi in equilibrio termodinamico con l'ambiente, il quale, essendo caratterizzato da elevatissime temperature e pressioni decrescenti fino all'atmosferica, via via che il magma effonde, sarebbe incompatibile con la stabilità del metaao. abstract in the last few years the question has been raiscd if presence of hydrocarbons in exhalative and hydrothermal fluids can or cannot be considered as a suggestion of non " juvenile " origin of these fluids. with the aim of picking' up some ideas about tliis problem the autor studied theorically some chemical equilibrio, trying at first to see ivhat pressure and, temperature conditions are compatible with the stabilita of methane. afterwards, the result of this physico-chemical study ivere compared with the known conditions of the different igneous milieus (magmatic in the ioide and classical meaning). the conditions of sudi anàlysis would be that hydrocarbons found in various fluids do not exclude juvenilism; this because at high pressures and not very high temperatures, like these reigning in the typical plutonio conditions, methane is not completly dissociated. complete dissociation of methane, on the contrary, seems to take place in the conditions of continental plateau vulcanism, this one is characterised, as known, by an activity of a mostly effusive type; the activity proceeds quietly and should allow the thermal fluids to remain in thermadynamical cquilibrium with the medium. during the effusion, indeed, temperatures remain very high, while pressures decrease down to the atmospheric vàlue, as magma is flovoing; these conditions would therefore be incompatible with the stabilita of methane. b i b l i o g r a f i a (x) penta, f . , caratteristiche e genesi delle manifestazioni esala livo-idrotermali naturali, a t t i x l v r i u n i o n e della s i p s , n a p o l i , 1954. — vedi a n c h e penta, f . , e di noi, a., sul significato genetico degli idrocarburi nelle •manifestazioni esalative ed idrotermali. « c h i m i c a e i n g e g n e r i a », 1, r o m a , 195(5. a q u e s t e d u e n o t e si r i m a n d a p e r la t e r m i n o l o g i a , q u i , a d o p e r a t a . (2) ellis, a. ,t„ chemical equilibrilim in magmatic gases. « a m e r i c a n j o u r n a l of s c i e n c e » , 2 5 5 , 6, 416-431 (1957). (3) rittmann, a.. vulcani: attività e genesi. e d . p o l i t e c n i c a s. a., n a p o l i , 1944. (4) waiilstrom, e . e . , introduetion lo theoretical igneous petrology w i l e y \ s o n s i n c . , n e w y o r k , 1950.' (5) egloff, g., the reaction of pure hydrocarbons. « r e i n h o l d p u b . c o r p . », n e w y o r k , 1937. ( « ) e g g e r t , .1., trattato di chimica fisica ed elettrochimica. 6 a e d i z . i t a l i a n a , e d . l l o e pli, milano, 1949. (7) clarke, f . w . , the data of geocliemistry. « u. s. geologica! survey b u l l . », 770, 1924. 6668_saponaro_final:layout 6 annals of geophysics, 58, 1, 2015, s0113; doi:10.4401/ag-6668 s0113 the contribution of emca to landslide susceptibility mapping in central asia annamaria saponaro*, marco pilz, dino bindi, stefano parolai helmholtz center potsdam german research center for geosciences, potsdam, germany abstract central asia is one of the most exposed regions in the world to landslide hazard. the large variability of local geological materials, together with the difficulties in forecasting heavy precipitation locally and in quantifying the level of ground shaking, call for harmonized procedures to better quantify the hazard and the negative impact of slope failures across the central asian countries. as a first step towards a quantitative landslide hazard and risk assessment, a landslide susceptibility analysis at regional scale has been carried out, by benefitting of novel seismic hazard outcomes reached in the frame of earthquake model central asia (emca) project. by combining information coming from diverse potential factors, it is possible to detect areas where a potential for landslides exists. initial results allow the identification of areas that are more susceptible to landslides with a level of accuracy greater than 70%. the presented method is, therefore, capable of supporting land planning activities at the regional scale in places where only scarce data are available. 1. introduction earthquake-triggered landslides in rock and soil are a major geological hazard in the mountains of central asia. the region is located in the asia-india continental collision zone where the northward-moving indian plate indents the eurasian plate [molnar and tapponnier 1975]. the ongoing collision has resulted in high mountain topography which is subject to active deformation, contemporary faulting and frequent strong earthquakes [gubin 1962, burtman and molnar 1993, pavlis et al. 1997, sidorova 1997, perov and budarina 2000], that combine to give rise to widespread landslide phenomena including massive rock slope failures in both the pamir and tien shan mountains [e.g., gaziev 1984, havenith et al. 1999, abdrakhmatov and strom 2006, havenith et al. 2006, strom and korup 2006, havenith and bourdeau 2010]. in addition, much of the topography of the region is mantled by weakly consolidated sediments, being particularly prone to flowslides triggered by seismic shaking and/or heavy rainfall [gubin 1962, ishihara 1989, ishihara 2012]. within the last 60 years, central asia has seen a multiplication of its population from 18 million in 1951 to more than 53 million in 2010 [lutz 2010]. the increase of population in combination with the expansion of urban settlements towards landslide-prone slopes further contribute to the destructive impact of landslides due to relatively little investment in understanding the hazards and risks and an added lack of appropriate resources [rosenfeld 1994, petley 2012]. already in the past compelling evidence of the destructive power of such landslides in central asia has been shown (figure 1): landslides, mudslides and debris flows caused an extensive number of casualties (table 1) during, e.g., the 1911 m = 8.2 kemin earthquake in the kazakh/kyrgyz border region, the 1949 m = 7.4 khait and the 1989 m=5.5 gissar earthquakes in tajikistan, the 1946 m=7.5 chatkal and the 1992 m=7.3 suusamyr earthquakes in kyrgyzstan. besides, the 1911 m=7.6 sarez earthquake in tajikistan triggered a massive landslide, blocking the murgab river and forming the tallest river dam in the world. additionally, there are a multitude of landslides around the fergana valley, which straddles uzbekistan, kyrgyzstan and tajikistan. since this region is quite densely inhabited almost every year landslides lead to damage of settlements and infrastructure and loss of human lives. here and elsewhere, landslides can further trigger other transboundary hazards, such as lake outburst floods and the release of toxic and radioactive substances into river basins (particularly in the mailuu-suu area, north of the fergana valley), with the potential to adversely impact a significant number of people. during soviet union times several programs have been developed to deal with the high level of landslide hazard. already as early as 1924, the soviet government set up a special commission to direct landslide control article history received september 19, 2014; accepted october 13, 2014. subject classification: earthquake-triggered landslides, landslide susceptibility, central asia. measures along the south coast of the crimea [sheko 1983]. over the next 40 years, landslide observation stations were created in many parts of the soviet union. regular monitoring of endangered areas was conducted from the 1960s until the break-up of the soviet union at the beginning of the 1990s. these activities also included extensive field-based mapping of single landslides as well as detailed engineering-geological investigations and their relationships to ground water conditions and precipitation, resulting in a good understanding of local slope instabilities. the main goal of these investigations had been the timely warning of the population and, if necessary, their evacuation and resettlement. based on a 1978 decree of the soviet council of ministers and coordinated by the state committee on science and technology, homogeneous landslide hazard maps have been published for those areas of high landslide risk which had been of greatest importance for the soviet economy. however, after the fall of the soviet union in 1991, the possibilities for landslide investigations and monitoring are drastically reduced. even in central asian countries such as kazakhstan, that have greater resources to devote to hazard analysis, landslide surveys remain underfunded and adequate observation posts are lacking. furthermore, significant parts of the alsaponaro et al. 2 name location date volume trigger number of fatalities usoi landslide tajikistan 1911 2.2 bilion m3 sarez earthquake 54 people kaindy, ananevo rock avalanches kyrgyzstan 1911 15 million m3 both kemin earthquake 38, 0 people chatkal kyrgyzstan 1946 15 million m3 chatkal earthquake na khait rock avalance tajikistan 1949 75 million m3 khait earthquake 7200 people sharora landslide, okuli-bolo mudslide takijistan 1989 5 million m3, 40 milion m3 gissar earthquake 200, 70 people belaldy rockslide kyrgyzstan 1992 40 million m3 suusamyr earthquake 35 people table 1. list of the largest historical earthquake-induced landslides in central asia. figure 1. location map of the study area and some among the strongest past earthquakes and seismically-induced landslides. in particular, the 1911 sarez (tajikistan), the 1911 kemin (kyrgyzstan), the 1946 chatkal (kyrgyzstan), the 1949 khait (tajikistan), the gissar (tajikistan), and the 1992 suusamyr (kyrgyzstan) earthquakes are known to have triggered the 1911 usoi (tajikistan), the 1911 ananievo and kaindy (kyrgyzstan), the 1946 chatkal (kyrzgystan), the 1949 khait (tajikistan), the 1989 sharora and okuli-bolo (tajikistan), the 1992 belaldy (kyrgyzstan) landslides, respectively. 3 ready existing data (e.g., maps and reports) are not available anymore or their use is limited because of the loss of accompanying information related to methodology and data sources. thus, the available landslide hazard and risk analysis in central asia is outdated and there is the ultimate need of an updated analysis. as landslides exist in nature and, in many cases, cannot be prevented it is highly recommendable to decrease their impact and consequences by appropriate hazard mapping and by reducing the exposure and the vulnerability to natural hazards. with the appropriate preventative measures, significant loss of life and economic damage could be avoided. nadim et al. [2006] assessed landslide occurrence probabilities worldwide on the basis of morphological, geological, meteorological and seismological data. they clearly showed that all landslide hotspots are located in seismically active mountain ranges. however, their analyses suffered from significant shortcomings in the quality and resolution of the available global data sets as well as heterogeneous data. to this regard, studies carried out in the earthquake model central asia (emca) project can contribute to harmonized procedures to better quantify the hazard and the negative impact of slope failures across central asian countries. by exploiting new advances in geographic information system (gis) technology, together with concepts from bayesian statistics, and promoting the use of open-source tools, it is possible to identify areas where the potential for landslide activation exists. since an inventory of past landslides in the area is currently missing, the evaluation of landslide susceptibility with limited background information constitutes a constant challenge. the presented method is, therefore, capable of supporting land planning activities at the regional scale in places where only scarce data are available. thereon, we will give an overview on the investigation of landslide susceptibility and related contributing factors in kyrgyzstan, tajikistan and uzbekistan. topographic and tectonic causative factors are selected and weights for different categories of these factors are statistically determined for the preparation of landslide susceptibility maps for central asian countries. 2. data for landslide susceptibility analysis, factors considered among the most influential to slope failures occurrences in central asia are taken into account. in particular, slope gradient, slope aspect, profile curvature, geology, distance from faults, and seismic intensity are selected. an inventory of past landslides in kyrgyzstan is compiled and, therefore, used as a reference dataset. slope gradient, slope aspect and profile curvature for kyrgyzstan, tajikistan and uzbekistan are derived from the nasa released shuttle radar topographic mission digital elevation model, with a spatial resolution of 81.7 m [srtm 2004]. geology, distance from faults and seismic intensity maps covering the territories of kyrgyzstan, tajikistan and uzbekistan are digitized and hence converted into raster format with a cell-size equivalent to the srtm digital elevation model. 2.1. landslide locations a landslide inventory represents an essential ingredient in order to carry out landslide hazard analysis at the regional scale [guzzetti et al. 1999]. it helps in identifying locations of previous landslides in order to be able to predict future failures. for the purposes of this study, we considered only one type of mass movement, i.e., landslides occurring in soft materials. a selection of landslide sites is defined on the basis of published information [kalmetieva et al. 2009], and their distribution is mapped as point locations. 2.2. topographic factors: slope gradient, slope aspect and profile curvature the stability of a slope is known to be highly dependent upon the slope angle and its material properties [terzaghi and peck 1967]. the slope is presented in degrees ranging from 0° to 88° and divided into four bins with approximately the same number of features (quantile classification), 0°-6.6°, 6.6°-16.6°, 16.6°-27.5°, >27.5°. defined as the direction of maximum slope of the terrain surface, slope aspect is typically taken into consideration, although in some cases its importance has been questioned [guzzetti et al. 1999]. for the selected areas, a classification based on azimuth being divided into eight bins, north, north-east, east, south-est, south, south-west, west, north-west, has been carried out. curvature represents one of the topographic attributes which are also commonly included in a landslide susceptibility analysis [alalew et al. 2004]. in particular, the profile curvature defined as the second derivative of the slope with respect to the maximum steepness direction may help in understanding patterns of flows acceleration and deceleration, and therefore, erosion and deposition. for the selected area, profile curvature values have been classified (quantile classification) into four bins, -0.02507 to -0.00101, -0.00101 to -0.00005, -0.00005 to 0.00095, 0.00095 to 0.01891. 2.3. geo-tectonic factors: geology and distance from faults lithology plays an important role in landslide susceptibility studies because different geological units have different slope failure behaviours. for example, landslide susceptibility mapping in central asia landslides in loess materials have occurred in uzbekistan, while landslide-prone slopes in cretaceous rocks may be found in kyrgyzstan. for our study, geological information is obtained from “the geological map of central asia and adjacent areas” [tingdong et al. 2008], scaled 1:2,500,000; overall, the study area is covered by a range of different sedimentary formations, mostly dated to the quaternary, neogene, paleogene, cretaceous, jurassic, and triassic. igneous rocks related to the palaeozoic epoch are also present. based on this information, stratigraphic units are digitised and accordingly classified into paleozoic, mesozoic and cenozoic units. the presence of major lineaments is among the important factors governing the stability of slopes [varnes 1984]. tectonic structures form zones of weakness in rocks and might accelerate the process of slope failures. central asia is covered by a large number of active faults (e.g., talas-fergana fault). for our analysis, fault lines were derived from the 1:2,500,000 scale geology map and four-buffer zone maps (less than 1 km, 1 to 5 km, 5 to 10 km, greater than 10 km) were prepared in gis. 2.4. trigger mechanism: seismic intensity as the study area is widely and strongly affected by earthquakes, it is necessary to take seismic ground shaking, expressed through the observed macro-seismic intensity (msk 64), into account as a triggering factor for landslides. within the emca project, bindi et al. [2012] carried out an uniform assessment of the seismic hazard in central asia mainly guided by the observed seismic histories without any a-priori assumption on seismic zonation or on the model of time recurrence. the application of such an approach to cross-border catalogues and considering intensity prediction equations developed for the investigated area [bindi et al. 2011], allowed to obtain a systematic and homogeneous evaluation of the hazard as well as the evaluation of the probability of exceedance of any given intensity value over a fixed exposure time over the entire analyzed area of different central asian countries. for the territory of kyrgyzstan and tajikistan, their studies returned intensities of viii and ix as expected in the future; for uzbekistan, an intensity of viii is expected. the main advantage of this approach is a step towards a homogenized and updated seismic input. 3. method in this study, the weights-of-evidence method is used for generating a landslide susceptibility map. the weights-of-evidence method is a data-driven bayesian quantitative method used to combine evidences in support of a hypothesis [bonham-carter 1994]. the method offers a flexible way of testing the importance of various input factors to the potential of slope failure, providing a simple statistical and straightforward tool that allows the calculated weights to be interpreted. specifically, the influence of landslide potential factors (evidence) on the occurrence of landslides themselves (hypothesis) is assessed. weights for each landslide causative factor are calculated based on the presence or absence of landslides within the study area. suppose that a landslide potential factor occurs in the studied area, and that a number of known landslides occur preferentially within the factor, it is possible to indicate the probability of finding a landslide given the presence (f) or the absence (f−) of a factor, through the definition of conditional probabilities: (1) (2) where p{f|l} and p{f−|l} are the conditional probabilities of being and not being within the factor, given the presence of a landslide. the weights for a landslide potential factor are defined as: (3) the difference between positive and negative weights is known as the weight contrast (c = w+−w−) and provides a useful measure of the overall spatial correlation between a certain class of factor and the occurrence of landslides [bonham-carter 1994]. a positive c indicates that the causative factor is present at the landslide location, and its magnitude is a measure of the positive correlation between the presence of the causative factor and landslides. on the other hand, a negative c is used to assess the importance of the absence of the factor in landslide occurrence. factors with contrast values around 0 have no significant connection with the occurrence of landslides. in weights-of-evidence modelling, it is typically assumed that landslide potential factors are conditionally independent with respect to landslide occurrences. a detailed description of the methodology may be found on our previous work [saponaro et al. 2014]. p l f p f p l f p l p f p f lk = =" " " " " " , , , , , , ,p l f f f f f p p l p l p p lk = =" " " " " " , , , , , , lnw p f l p f l =+ " " , , .lnw p f l p f l =" " , , saponaro et al. 4 (4) 5 landslide susceptibility mapping in central asia fa ct o r / c la ss t o ta l c el ls l an d sl id e ce ll s fr ee f ro m la n d sl id es c el ls w + s 2 ( w + ) w − s 2 ( w ) c c / s( c ) sl op e gr ad ie nt (° ) 06. 6 56 0, 92 3 36 56 0, 88 7 -0 .9 70 0. 02 8 0. 09 8 0. 00 2 -1 .0 69 -6 .2 35 6. 616 .6 1, 14 4, 61 3 35 6 1, 14 4, 25 7 0. 60 8 0. 00 3 -0 .4 20 0. 00 3 1. 02 8 13 .2 40 16 .6 -2 7. 5 1, 19 3, 56 0 25 0 1, 19 3, 31 0 0. 21 3 0. 00 4 -0 .1 09 0. 00 2 0. 32 1 4. 01 7 > 2 7. 5 1, 02 5, 60 5 25 1, 02 5, 58 0 -1 .9 38 0. 04 0 0. 26 3 0. 00 2 -2 .2 02 -1 0. 80 1 sl op e as pe ct (° ) n ( 33 7. 5 – 22 .5 ) 35 3, 55 5 84 35 3, 47 1 0. 33 9 0. 01 2 -0 .0 41 0. 00 2 0. 37 9 3. 24 9 n e ( 22 .5 – 6 7. 5) 36 4, 99 2 57 36 4, 93 5 -0 .0 81 0. 01 8 0. 00 8 0. 00 2 -0 .0 89 -0 .6 42 e ( 67 -5 – 1 12 .5 ) 52 3, 16 4 84 52 3, 08 0 -0 .0 53 0. 01 2 0. 00 8 0. 00 2 -0 .0 61 -0 .5 24 se ( 11 2. 5 – 15 7. 5) 53 5, 06 9 84 53 4, 98 5 -0 .0 76 0. 01 2 0. 01 1 0. 00 2 -0 .0 87 -0 .7 47 s (1 57 .5 – 2 02 .5 ) 56 1, 14 8 67 56 1, 08 1 -0 .3 50 0. 01 5 0. 04 8 0. 00 2 -0 .3 97 -3 .0 85 sw ( 20 2. 5 – 24 7. 5) 55 2, 57 1 91 55 2, 48 0 -0 .0 28 0. 01 1 0. 00 4 0. 00 2 -0 .0 32 -0 .2 87 w ( 24 7. 5 – 29 2. 5) 60 2, 70 7 11 2 60 2, 59 5 0. 09 3 0. 00 9 -0 .0 18 0. 00 2 0. 11 1 1. 06 8 n w ( 29 2. 5 – 33 7. 5) 42 6, 81 7 88 42 6, 72 9 0. 19 7 0. 01 1 -0 .0 27 0. 00 2 0. 22 4 1. 95 4 pr ofi le c ur va tu re (1 /m ) -0 .0 25 07 – 0. 00 10 1 1, 02 8, 25 5 18 9 1, 02 8, 06 6 0. 08 2 0. 00 5 -0 .0 31 0. 00 2 0. 11 3 1. 31 0 -0 .0 01 01 – 0. 00 00 5 93 4, 31 1 17 6 93 4, 13 5 0. 10 7 0. 00 6 -0 .0 36 0. 00 2 0. 14 2 1. 61 7 -0 .0 00 05 – 0 .0 00 95 94 7, 91 4 13 0 94 7, 78 4 -0 .2 11 0. 00 8 0. 05 9 0. 00 2 -0 .2 69 -2 .7 57 0. 00 09 5 – 0 .0 18 91 1, 02 8, 23 2 17 2 1, 02 8, 06 0 -0 .0 12 0. 00 6 0. 00 4 0. 00 2 -0 .0 17 -0 .1 87 g eo lo gy (e ra ) c en oz oi c 1, 16 1, 72 2 20 3 1, 16 1, 51 9 0. 03 1 0. 00 5 -0 .0 13 0. 00 2 0. 04 5 0. 53 2 m es oz oi c 39 1, 10 0 35 1 39 0, 74 9 1. 66 8 0. 00 3 -0 .6 43 0. 00 3 2. 31 1 29 .7 93 p al eo zo ic 2, 38 2, 88 3 11 3 2, 38 2, 77 0 -1 .2 73 0. 00 9 0. 74 3 0. 00 2 -2 .0 16 -1 9. 53 4 d is ta nc e fr om fa ul ts (k m ) < 1 52 2, 53 4 31 52 2, 50 3 -1 .0 49 0. 03 2 0. 09 5 0. 00 2 -1 .1 44 -6 .2 18 1 – 5 1, 58 6, 75 2 20 0 1, 58 6, 55 2 -0 .2 95 0. 00 5 0. 15 9 0. 00 2 -0 .4 55 -5 .3 78 5 – 10 97 0, 33 0 20 5 97 0, 12 5 0. 22 1 0. 00 5 -0 .0 84 0. 00 2 0. 30 6 3. 64 2 > 10 83 4, 88 3 23 0 83 4, 65 3 0. 48 7 0. 00 4 -0 .1 85 0. 00 2 0. 67 1 8. 24 1 se is m ic in te ns it y (i ) i = v ii 1, 14 5, 95 8 55 1, 14 5, 90 3 -1 .2 61 0. 01 8 0. 25 8 0. 00 2 -1 .5 19 -1 0. 78 9 i = v ii i 97 3, 28 5 84 97 3, 20 1 -0 .6 74 0. 01 2 0. 14 9 0. 00 2 -0 .8 23 -7 .0 55 i = i x 1, 81 6, 46 2 52 8 1, 81 5, 93 4 0. 54 0 0. 00 2 -0 .9 50 0. 00 7 1. 49 0 15 .6 33 t ab le 2 .c la ss , c om pu te d w ei gh ts , v ar ia nc es a nd c on tr as t va lu es o bt ai ne d fr om t he a pp lic at io n of t he w ei gh ts -o fe vi de nc e m et ho d to t he ja la l-a ba d st u dy a re a, k yr g yz st an . 4. landslide susceptibility results in central asia and discussion this section examines where landslides are preferentially triggered by earthquakes within seismically active mountain belts with respect to geomorphological, geological, structural and seismo-tectonic factors. weights’ calculation has been carried out in the jalal-abad district in kyrgyzstan, where the distribution of landslide factors values is representative of existing relationships for the entire country of kyrgyzstan. a landslide sample of 1347 locations is used. specifically, 50% of the total number of locations are randomly selected in the study areas, and then used as the “training dataset” to calibrate the method. the remaining 50% of locations are used as the “test dataset” for validating results. the “training dataset” of landslides is overlaid with each landslide potential factor to calculate weights and the statistical parameters representative of existent spatial relationships, shown in table 2. as can be seen, the most noteworthy classes of parameters with a positive impact on slope instability are: slope gradient 6°-16.6°, northfacing slope aspect, mesozoic-aged lithologies, distance from faults greater than 10 km, and seismic intensity values (msk 64) equal to ix. furthermore, the highest contrasts (c/s(c)) values are found for the geology factor, while lowest ones are for slope aspect. thus, landslide factor maps are re-classified on the basis of most stable and unstable classes of factors. by combining previously calculated contrast values with re-classified factors maps, a landslide susceptibility zonation map has been calculated based on: where lsi indicates a landslide susceptibility index and cij represents the contrast for the i-th bin of the jth factor. the validity and accuracy of landslide susceptibility maps are typically ascertained with the help of successand prediction-rate curves in combination with the area under the curves [chung and fabbri 2003]. the curves provide information about the relationship between the percentage of area identified as being landslide susceptible and the actual landslide occurrences. landslide susceptibility results are hence cross-tabulated with landslide locations and presented as a cumulative frequency diagram. when comparing landslide test dataset with landslide susceptibility maps, 66.119% of landslides are found in the 20% of highest susceptibility classes, with an accuracy value equal to 0.801 (figure 2). in order to present a cross-border landslide susceptibility map for the territories of kyrgyzstan, tajikistan and uzbekistan, landslide factors are re-classified and weighted, following the above described procedure. in particular, values are normalized and classified into 10 equal-interval classes, ranging from 0.016 to 0.971 (figure 3). based on landslide susceptibility results in kyrgyzstan, high lsi levels are present in the southern region, precisely along the eastern border of fergana valley, where slope values mostly range from 0.0° to 16.6°, and the majority of past landslides are also distributed. in addition, high landslide susceptibility are recognized across the jalal-abad and the naryn regions, though the lack of landslide observations. this latter result might indicate the potential for landslide activation and, therefore, serve as input when estimating landslide risk. a general low level of landslide susceptibility can be observed for almost the entire territory of uzbekistan. this outcome is plausible given the presence of a conspicuous flat and desert area. exceptions are represented by the tashkent and the buhkara provinces which are characterized by high and medium landslide susceptibility, respectively (figure 3). in addition, evidences of past slope failures in the tashkent region provide credit to the calculated susceptibility level. with regard to the tajik territory, it can be seen that the eastern region is characterized by a larger potential to slope failures than the western one, in agreement with expected higher seismic intensity values. in particular, high levels of landslide susceptibility are expected in the central districts of tojikobod and nurobod, in the proximity to the devastating 1949 khait landslide (figure 3). it has to be remarked that the distribution of geological materials is not uniform throughout central asian countries. for example, a relatively lower level of landslide susceptibility is found in tajikistan with respect to kyrgyzstan. in fact, our proposed susceptibility lsi cij j n 1 = = / saponaro et al. 6 figure 2. accuracy assessment of landslide susceptibility model for the test database. (5) 7 model entitled mesozoic rocks as the most prone to landslides. however, in tajikistan the majority of rocks are paleozoic materials (in particular fractured igneous rocks) that, based on the calibrated model, are classified as “not influent”. additionally, the widespread presence of loess lithology both in tajikistan and uzbekistan should not be underestimated, considering connections to previous slope failures [evans et al. 2009, niyazov and nurtaev 2013]. the fact that both fractured igneous and loess materials have not been explicitly addressed in the analysis might reflect the relatively low susceptibility level found in these countries. 5. conclusions central asia is one of the most challenging places in the world where various natural hazards can heavily injury populations and resources. among these hazards, landslides pose a serious threat to human life and human facilities. a fundamental step for minimizing landslide risk is represented by the preparation of a susceptibility map. specifically, the map allows the identification of areas currently free of landslides with a potential for future slope instability. consequently, such a map can be used as a tool to support disaster management and planning activities at the regional level. due to remote conditions characterizing central asia, data are not easily accessible, or may be affected by different sources of uncertainty. to this regard, studies conducted within the emca project provide harmonized procedures to better quantify the hazard and the negative impact of slope failures across central asian countries. in particular, thanks to the hazard analyses carried out in the frame of emca, we presented an approach to evaluate the potential to seismically-induced landslides using statistical relationships between past landslides and the most significant seismotectonic, geological and morphological factors in central asia countries. the bayesian-based method is initially calibrated and cross-validated with an independent dataset in kyrgyzstan [saponaro et al. 2014], providing us with a landslide susceptibility model having an accuracy level greater than 70%. this level allows us to consider the model sufficiently reliable for planning purposes. afterwards, due to uniformity in geomorphological and tectonic factors characterizing all central asian countries, an extension of the model to the territories of uzbekistan and tajikistan is carried out. as part of future work, we plan the installation of seismic strong motion networks, together with the development of early warning systems in case of a potential landslide-inducing earthquake occurrence. besides, a more detailed investigation of existing relationships between quaternary rocks (in particular loess) and slope failures will provide specific insight for landslide susceptibility in tajikistan and uzbekistan. for this reason, a more detailed inventory of past landslides in under construction. the present work offers the unique value of providing the first attempts of transnational landslide susceptibility assessment, in which it is possible to identify areas with a potential for future seismically-induced slope instability. landslide susceptibility mapping in central asia figure 3. landslide susceptibility index (lsi) map for kyrgyzstan, tajikistan and uzbekistan calculated with respect to slope gradient, slope aspect, profile curvature, geology, distance from faults, and seismic intensity factors. normalized susceptibility values are shown. acknowledgements. this research presented in this paper has been carried out under the tien shan-pamir monitoring program (tiptimon) project, with the support of earthquake model central asia (emca) project. we thank u. borchert and a. motschmann for helping with data digitization, and the central asian institute for applied geosciences (caiag) for providing the landslide data. references abdrakhmatov, k., and a. strom (2006). dissected rockslide and rock avalanche deposits; tien shan, kyrgyzstan, in: s.g. evans, g. scarascia mugnozza, a. strom and r.l. hermanns (eds.), landslides from massive rock slope failure, springer, netherlands, 551-570. alalew l., h. yamagishi and n. ugawa (2004). landslide susceptibility mapping using gis-based weighted linear combination the case of tsugawa area of agano river, niigata prefecture japan, landslides, 1, 73-81. bindi, d., s. parolai, a. oth, k. abdrakhmatov, a. muraliev and j. zschau (2011). intensity prediction equations for central asia, geophys. j. int., 187, 327-337. bindi, d., k. abdrakhmatov, s. parolai, m. mucciarelli, g. grünthal, a. ischuk, n. mikhailova and j. zschau (2012). seismic hazard assessment in central asia: outcomes from a site approach, soil dyn. earthq. eng., 37, 84-91. bonham-carter, g.f. 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(1984). iaeg commission on landslides and other mass movements, landslide hazard zonation: a review of principles and practice, the unesco press, paris, 63 pp. corresponding author: annamaria saponaro, helmholtz center potsdam german research center for geosciences, potsdam, germany; email: asapon@gfz-potsdam.de. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. landslide susceptibility mapping in central asia << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice l a s p e d i z i o n e d e l l ' i . n . g . a k h a r t o u m i n o c c a s i o n e d e l l ' e c l i s s e t o t a l e d i s o l e d e l 2 5 / 2 / 1 9 5 2 (*) r . claldea i n q u e s t a p r i m a r e l a z i o n e r i f e r i r ò s u l l a o r g a n i z z a z i o n e d e l l e ric e r c h e s v o l t e a k h a r t o u m d u r a n t e l'eclisse t o t a l e di sole d e l 2 5 f e b b r a i o 1 9 5 2 . c e r t a m e n t e l e eclissi d i sole h a n n o u n a g r a n d i s s i m a i m p o r t a n z a d a l p u n t o di v i s t a geofisico, g i a c c h é p e r m e t t o n o d i a p p r o f o n d i r e l e c o n o s c e n z e s u l l e r e l a z i o n i t r a i f e n o m e n i s o l a r i e q u e l l i t e r r e s t r i . i n f a t t i , l e r a d i a z i o n i sia c o r p u s c o l a r i c h e e l e t t r o m a g n e t i c h e d e l s o l e p r o d u c o n o s u l l a t e r r a e n e l l ' a t m o s f e r a t e r r e s t r e e f f e t t i term i c i , l u m i n o s i e i o n i z z a n t i , e f f e t t i c h e possono i n f l u i r e i n m i s u r a p i ù o m e n o g r a n d e su t u t t i i f e n o m e n i geofisici d e l l a t e r m o d i n a m i c a , d e l l ' o t t i c a , d e l l ' e l e t t r i c i t à a t m o s f e r i c a e d e l m a g n e t i s m o t e r r e s t r e . l a l u n a p e r t a n t o , con i l suo passaggio d a v a n t i al disco s o l a r e , costit u i s c e u n o s c h e r m o c h e si sposta r a p i d a m e n t e ; i l s o l e così, p u ò essere c o n s i d e r a t o c o m e u n a s o r g e n t e o t e r m i c a t> l u m i n o s a o i o n i z z a n t e p i ù o m e n o o s c u r a t a . t a l e r a p i d o o s c u r a m e n t o p u ò f a r s p e r a r e di p o t e r s e p a r a r e l ' i n f l u e n z a d e l l a r a d i a z i o n e s o l a r e d i r e t t a da q u e l l a di t u t t i gli a l t r i f e n o m e n i c h e a v v e n g o n o s u l l a t e r r a . i n o l t r e , u n a eclisse t o t a l e di s o l e si p r e s e n t a c o m e o s c u r a m e n t o p a r z i a l e d e l l ' a t m o s f e r a , ciò c h e f a s o r g e r e a l t r i p r o b l e m i r e l a t i v i ai f e n o m e n i s e c o n d a r i di r i e m i s s i o n e d e l l a r a d i a z i o n e d i r e t t a . p u r t r o p p o fino ad o r a l e o s s e r v a z i o n i dei f e n o m e n i geofisici, ad e c c e z i o n e d e l l e n u m e r o s e m i s u r e n e l c a m p o i o n o s f e r i c o , d u r a n t e l e eclissi (') sono s t a t e s p o r a d i c h e e spesso c o n t r a d d i t t o r i e ; n a t u r a l m e n t e q u e s t o f a t t o d e r i v a i n g r a n p a r t e d a l l a n o t e v o l e i n f l u e n z a dei f e n o m e n i l o c a l i m e t e o r o l o g i c i s u l l e o s s e r v a z i o n i e f f e t t u a t e . questa eclisse, v i s i b i l e n e l s u d a n , i n u n c l i m a , c o m e v e d r e m o in seguito, e s t r e m a m e n t e secco, si p r e s e n t a v a p a r t i c o l a r m e n t e f a v o r e v o l e a l l e d e t t e o s s e r v a z i o n i . f u deciso p e r c i ò l ' i m p i a n t o di u n o s s e r v a t o r i o geofisico t e m p o r a n e o n e l l a z o n a di t o t a l i t à ; p e r q u e s t o scopo e l a b o r a i u n p i a n o di r i c e r c h e , che, d a t a l a n o t e v o l e l o n t a n a n z a ( * ) c o m u n i c a z i o n e p r e s e n t a t a a l c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , t e n u t o a r o m a i l 1 0 1 1 g i u g n o 1 9 5 2 . 4 1 0 h . c1all>ka d e l l a base scelta e l e difficoltà d e i t r a s p o r t i , f u r i d o t t o solo a q u a t t r o g r u p p i di o s s e r v a z i o n i : 1° a n d a m e n t o d e l l a t e m p e r a t u r a v i r t u a l e d e l c i e l o ; 2° s t u d i o d e l l a l u c e d i f f u s a d e l l ' a t m o s f e r a ; 3° m i s u r e d e l l e p r o p r i e t à e l e t t r i c h e d e l l ' a t m o s f e r a ; 4° m i s u r e del c a m p o m a g n e t i c o t e r r e s t r e . s i r e n d e v a i n o l t r e n e c e s s a r i a l a r e g i s t r a z i o n e di q u e s t i d a t i p e r u n c e r t o p e r i o d o , a l l o scopo di r i c a v a r e l ' a n d a m e n t o m e d i o d i u r n o n e l l a z o n a di t o t a l i t à , p e r p o t e r r i l e v a r e u n e v e n t u a l e e f f e t t o dell'eclisse. i n t a l m o d o si r i c a v a v a a n c h e l ' a n d a m e n t o d i u r n o d e l l e g r a n dezze e s a m i n a t e i n u n a z o n a e q u a t o r i a l e . r i f e r i r ò o r a i n p a r t i c o l a r e sui p r o b l e m i connessi con d e t t e r i c e r c h e e s u l l a p r e p a r a z i o n e degli a p p a r a t i a l l e s t i t i a questo scopo. temperatura virtuale del cielo. — p e r l o s t u d i o d e l l ' e q u i l i b r i o t e r m i c o d e l l ' a t m o s f e r a p u ò essere u t i l e sia l a conoscenza del r a f f r e d d a m e n t o d e l l ' a t m o s f e r a stessa d u r a n t e u n ' e c l i s s e t o t a l e t r a i l p r i m o , i l s e c o n d o e i l t e r z o c o n t a t t o , q u a n d o cioè l a r a d i a z i o n e d i r e t t a d e l sole d i m i n u i s c e e si a n n u l l a , sia l a conoscenza d e l r i s c a l d a m e n t o dell ' a t m o s f e r a stessa d o p o i l t e r z o c o n t a t t o , q u a n d o cioè questa r a d i a z i o n e r i a u m e n t a . u n a g r a n d e z z a c h e p u ò c a r a t t e r i z z a r e i l r a f f r e d d a m e n t o ed i l r i s c a l d a m e n t o d e l l a a t m o s f e r a , è l a t e m p e r a t u r a v i r t u a l e del c i e l o (2). p e r q u e s t o scopo possono essere a d o p e r a t i m e t o d i assol u t i ( p e r e s e m p i o p i r o e l i o m e t r i a c o m p e n s a z i o n e e l e t t r i c a del t i p o di a n g s t r o m ) o m e t o d i r e l a t i v i ( p e r e s e m p i o s o l a r i g r a f ì ) . o r a , d a t a la necessità di u n f u n z i o n a m e n t o c o n t i n u o ed a u t o m a t i c o d e l l ' a p p a r a t o , n o n p o t e i s c e g l i e r e u n m e t o d o assoluto. s c e l s i a l l o r a u n s o l a r i g r a f o t i p o g ò r c z y n s k i d e l l a k i p p e, c o m e s t r u m e n t o , u n g a l v a n o m e t r o h a r t m a n n e b r a u n , d o t a t o di u n a s e r i e di s h u n t p e r l e d i v e r s e s e n s i b i l i t à . luce diffusa dell' atmosfera. —• l'eclisse t o t a l e di sole p u ò consid e r a r s i c o m e u n ' o m b r a c i l i n d r i c a c b e i n v e s t a t u t t a l ' a t m o s f e r a e c h e s c o r r a s u l l a t e r r a n e l l a z o n a di t o t a l i t à . ora, dato c h e al c e n t r o di questa o m b r a l a i l l u m i n a z i o n e p r o d o t t a d i r e t t a m e n t e d a l sole è min i m a , p o i c h é l a c o r o n a e s t e r n a del sole è p o c h i s s i m o l u m i n o s a , l'eclisse p e r m e t t e l o studio d e l l a d i f f u s i o n e s e c o n d a r i a d e l l ' a r i a . i n f a t t i l a b r i l l a n z a d e l l ' a t m o s f e r a n e l c e n t r o d e l l ' o m b r a è d o v u t a solo a l l a ra-, d i a z i o n e d i f f u s a d a l l e r e g i o n i e s t e r n e a l l ' o m b r a e r i d i f f u s a da q u e l l e l a s p e d i z i o n e d e l l i . n . c . a k h a r t o u m i n t e r n e . i n o l t r e l ' i l l u m i n a z i o n e d e l l a zona d ' o m b r a r i s u l t a del t u t t o a n o r m a l e , d a t o c h e essa è d o v u t a e s c l u s i v a m e n t e ad u n c i l i n d r o lum i n o s o ( l e r e g i o n i e s t e r n e a l l ' o m b r a ) i n v e c e c h e ad u n f a s c i o p a r a l l e l o di l u c e ( i l l u m i n a z i o n e d i r e t t a del sole). p e r questa r a g i o n e e r a i n t e r e s s a n t e l o s t u d i o d e l l o stato di p o l a r i z z a z i o n e di t u t t a la v o l t a celeste. p e r p o t e r e s e g u i r e t a l e r i c e r c a f u n e c e s s a r i o p r e p a r a r e u n appar a t o c h e p e r m e t t e s s e l a m i s u r a i s t a n t a n e a d e l l o stato di p o l a r i z z a z i o n e di t u t t a l a v o l t a celeste, p o i c h é il f e n o m e n o è r a p i d a m e n t e v a r i a b i l e n e l t e m p o . t a l e appar a t o è il n e f o s c o p i o f o t o g r a fico a t r e c a m e r e , c h e sarà p i ù d i f f u s a m e n t e d e s c r i t t o in u n a n o t a s e g u e n t e ; esso consiste e s s e n z i a l m e n t e in tre a p p a r e c c h i f o t o g r a f i c i posti l ' u n o s o p r a l ' a l t r o su u n tel a i o m e t a l l i c o (fig. 1), cias c u n o dei q u a l i f o t o g r a f a u n a s e m i s f e r a m e t a l l i c a con l a s u p e r f i c i e r i f l e t t e n t e . in t a l m o d o , di t u t t a l a v o l t a celeste si f o r m a u n a i m m a g i n e v i r t u a l e n e l l ' i n t e r n o di c i a s c u n a s f e r a , i m m a g i n e c h e v i e n e poi r i p r e s a d a l l ' a p p a r e c c h i o f o t o g r a f i c o c o r r i s p o n d e n t e . g l i specchi e gli a p p a r e c c h i (nettarzeiss con o b i e t t i v o n o v a r f : 4 , 5 ) sono posti i n m o d o da m a s c h e r a r e i l m i n i m o a n g o l o s o l i d o possibile. ciascun o b i e t t i v o è d o t a t o di u n filtro di l u c e e di u n p o l a r o i d ; i p i a n i di v i b r a z i o n e (p.d.v.) di questi polar o i d sono posti a 60° l ' u n o d a l l ' a l t r o . in questo m o d o si o t t e n g o n o t r e f o t o g r a f i e s i m u l t a n e e di t u t t a l a v o l t a c e l e s t e : dai r i s p e t t i v i a n n e r i m e n t i di ciascuna z o n a si possono a l l o r a r i c a v a r e l ' i n t e n s i t à t o t a l e della l u c e d i f f u s a , il g r a d o di p o l a r i z z a z i o n e e la p o s i z i o n e del p.d.v. d e l l a comp o n e n t e p o l a r i z z a t a , g r a n d e z z e q u e s t e che d e f i n i s c o n o l o stato di po4 1 2 r . c i a l d e a l a r i z z a z i o n e . t a l e a p p a r a t o f u a p p o s i t a m e n t e c o s t r u i t o n e l l e o f f i c i n e dell'i.n.g. p e r questa occasione. i n o l t r e si a d o p e r a v a a n c h e u n n e f o s c o p i o f o t o e l e t t r i c o ( 3 ), costit u i t o da u n a s e m p l i c e l e n t e c h e p r o i e t t a u n a c e r t a zona fissa del c i e l o su u n a c e l l u l a f o t o e l e t t r i c a a s t r a t o di s b a r r a m e n t o : u n p o l a r o i d rot a n t e v i e n e i n t e r p o s t o t r a l a l e n t e e l a c e l l u l a . u n o s t r u m e n t o m i s u r a l a c o r r e n t e f o t o e l e t t r i c a e q u i n d i l ' i n t e n s i t à d e l l a l u c e d i f f u s a d a l l a zona e s a m i n a t a . d a l l a m i s u r a dei m a s s i m i e dei m i n i m i d e l l a c o r r e n t e si o t t i e n e il g r a d o di p o l a r i z z a z i o n e . elettricità atmosferica. — d u r a n t e l'eclisse t o t a l e di sole i n o l t r e v i e n e a m a n c a r e l ' a z i o n e i o n i z z a n t e d e l l a r a d i a z i o n e , c h e si r i s e n t e i n special m o d o n e l l a s t r a t o s f e r a . questa d i m i n u z i o n e p u ò i n f l u i r e s u l l e p r o p r i e t à e l e t t r i c h e d e l l ' a t m o s f e r a . si r e n d e v a così n e c e s s a r i o e f f e t t u a r e d e l l e m i s u r e e l e t t r i c h e e p r e p a r a i p e r t a n t o i seguenti a p p a r a t i : 1° d u e s o n d e r a d i o a t t i v e di p o t e n z i a l e al p o l o n i o ; 2° d u e d i s p e r s o r i t i p o e l s t e r e gei tei p e r la m i s u r a d e l l a cond u c i b i l i t à ; 3° q u a t t r o e l e t t r o m e t r i a q u a d r a n t e t i p o i.n.g. l e d u e s o n d e di p o t e n z i a l e d o v e v a n o essere poste a d u e q u o t e d i v e r s e , in m o d o c h e la v a r i a z i o n e del c a m p o con l ' a l t e z z a desse a n c h e i l v a l o r e m e d i o d e l l a c a r i c a spaziale. 11 s u p p o r t o d e l l e s o n d e e r a del t i p o a stilo, i s o l a t o in basso con u n grosso b l o c c o di e l e t t r i n a . 1 dis p e r s o r i e r a n o f o r m a t i da s f e r e m e t a l l i c h e del d i a m e t r o di 2 0 c m , p o s t e su stili isolati. la m i s u r a d e l l a c o n d u c i b i l i t à d o v e v a essere eff e t t u a t a col m e t o d o classico c a r i c a n d o ogni m i n u t o il d i s p e r s o r e a 5 0 1 0 0 v o l t p o s i t i v i o n e g a t i v i e l a s c i a n d o l o poi s c a r i c a r e p e r s e m p l i c e c o n d u c i b i l i t à e l e t t r i c a . t u t t i gli i s o l a t o r i e r a n o in e l e t t r i n a p r e p a r a t a a l l o scopo dall'officina dell'i.n.g., c h e p r o v v i d e i n o l t r e a l l a c o s t r u z i o n e dei p o r t a sonde, dei dispersori e degli stili. magnetismo terrestre. — la v a r i a z i o n e d e l l a i o n i z z a z i o n e degli a l t i s t r a t i d e l l ' a t m o s f e r a p o t e v a i n o l t r e i n f l u i r e sul c a m p o m a g n e t i c o t e r r e s t r e . a t a l e scopo si p r e v i d e u n a stazione m a g n e t i c a c o m p l e t a , c o m p o s t a di t r e v a r i o m e t r i del t i p o r u s k a . apparati accessori. — t u t t i gli a p p a r a t i ai q u a l i h o a c c e n n a t o c o m p o r t a n o dei g a l v a n o m e t r i e degli e l e t t r o m e t r i (in t u t t o sei strum e n t i ) , c h e h a n n o bisogno di u n a r e g i s t r a z i o n e f o t o g r a f i c a . p r e v i d i a l l o r a u n u n i c o r e g i s t r a t o r e f o t o g r a f i c o a p i ù tracce, ciò c b e p e r m e t l a s p e d i z i o n e d e l l ' i . n . c . a k h a r t o u m 4 1 3 t e v a u n o s p o g l i o piix r a p i d o d e l l e r e g i s t r a z i o n i ed u n a m a n o v r a b i l i t à p i ù s e m p l i c e . p o i c h é e r a n e c e s s a r i o u n o s c o r r i m e n t o r a p i d o (2 m m al m i n u t o , cioè 2 8 8 cm al g i o r n o ) si u s a v a u n f o r t e m o t o r e a m o l l a ; o c c o r r e v a poi u n a l a r g a b a n d a di c a r t a f o t o g r a f i c a d e l l a d i m e n s i o n e di 6 0 cm, ciò c h e p e r m e t t e v a la r e g i s t r a z i o n e di o t t o t r a c c e c o m p l e t a m e n t e s e p a r a t e l u n a d a l l ' a l t r a . f u così r e a l i z z a t o u n r e g i s t r a t o r e f i g . 2 f o t o g r a f i c o c o s t i t u i t o da u n a g r a n d e i n c a s t e l l a t u r a di 2 0 0 x 1 7 0 x 9 0 cm, i n t u b o di a l l u m i n i o , l e g g e r i s s i m a e t u t t a s m o n t a b i l e . un grande1 t a m b u r o di 6 0 x 3 0 cm e r a a z i o n a l o a f r i z i o n e da u n r o b u s t o m o t o r e a d o p p i a m o l l a con u n a c a r i c a di circa 1 8 ore. l ' i n c a s t e l l a t u r a (fig. 2) f o r m a v a d u e s c o m p a r t i m e n t i s e p a r a t i a t e n u t a di l u c e : u n o d e s t i n a t o a l l a c a r t a f o t o g r a f i c a , l ' a l t r o agli s t r u m e n t i ed a l l e l a m p a d i n e e l e t t r i c h e necessarie p e r la r e g i s t r a z i o n e . 11 t a m b u r o p e r la c a r t a f o t o g r a f i c a e r a addossato a l l a p a r e t e d i v i s o r i a s u p e r i o r e dotata di u n a l u n g a f e n d i t u r a o r i z z o n t a l e , sulla q u a l e gli s t r u m e n t i p r o i e t t a v a n o l ' i m m a g i n e dei filamenti r e t t i l i n e i di a l c u n e l a m p a d i n e e l e t t r i c h e . t u t t o i l registra4 1 4 r . c i a l d e a t o r e v e n i v a p o i c o p e r t o da u n g r a n d e t e n d o n e n e r o con u n a f i n e s t r a a n t e r i o r e d o t a t a di u n f o g l i o di c e l l u l o i d e rosso p e r l ' i s p e z i o n e agli s t r u m e n t i ed a l l a r e g i s t r a z i o n e ; s u l l a p a r t e s u p e r i o r e si t r o v a v a n o gli i s o l a t o r i i n e b a n i t e p e r i c o n t a t t i e l e t t r i c i dei g a l v a n o m e t r i e d e l l e l a m p a d i n e e l e t t r i c h e e gli i s o l a t o r i in e l e t t r i n a p e r i c o n t a t t i con gli e l e t t r o m e t r i . r i g u a r d o p o i a l l a a l i m e n t a z i o n e e l e t t r i c a , f u r o n o p r e v i s t e d e l l e b a t t e r i e di a c c u m u l a t o r i al p i o m b o a sei v o l t . organizzazione generale della spedizione. —t u t t i questi appar a t i d o v e v a n o c o s t i t u i r e l ' a t t r e z z a t u r a d e l l ' o s s e r v a t o r i o geofisico temp o r a n e o da i n s t a l l a r s i i n u n a z o n a di t o t a l i t à cioè a k l i a r t o u m , c b e e r a l ' u n i c a città i n v i c i n a n z a a l l a l i n e a di t o t a l i t à (8-9 k m ) ed al p u n t o di m a s s i m a d u r a t a dell'eclisse ( c i r c a 8 0 0 km). i n f a t t i l'eclisse t o t a l e di sole del 2 5 f e b b r a i o 1 9 5 2 h a a v u t o i n i z i o n e l g o l f o di g u i n e a ; l a sua z o n a di t o t a l i t à d e s c r i v e v a q u a s i u n a r c o p e r c o r r e n d o t u t t a l ' a f r i c a c e n t r a l e f i n o a t a g l i a r e i l s u d a n a n g l o e g i z i a n o p a s s a n d o a p o c h i c h i l o m e t r i da k l i a r t o u m , p o i , s f i o r a n d o p o r t o s u d a n t r a v e r s a v a il m a r rosso, d o v e si t r o v a v a i l p u n t o di m a s s i m a d u r a t a dell'eclisse, p r o s e g u i v a a t t r a v e r s a n d o l ' a r a b i a , e, s f i o r a n d o b a s s o r a a t t r a v e r s o l ' i r a n , passava n e l l ' a s i a c e n t r a l e r u s s a d o v e a v e v a t e r m i n e l'eclisse. i n o l t r e k l i a r t o u m p r e s e n t a v a u n c l i m a v e r a m e n t e f a v o r e v o l e a l l e n o s t r e r i c e r c h e . r i p o r t e r ò q u i i n f a t t i l e m e d i e m e n s i l i del m e s e di f e b b r a i o t o l t e d a l b o l l e t t i n o del s u d a n m e t e o r o l o g i c a l s e r v i c e : l d i u r n a del m a s s i m o 33,2 °c 1 d i u r n a del m i n i m o 1 6 , 3 » t e m p e r a t u r a m e d i a ' a l l e 0 8 , 0 0 » 11,00 f » 1 4 , 0 0 1 9 . 5 » 2 7 , 4 » 3 2 . 6 » t e m p e r a t u r a del t e r m o m e t r o b a g n a t o 1 0 , 9 °c 1 5 , 0 » 1 7 , 0 » u m i d i t à r e l a t i v a a l l e 0 8 , 0 0 » 11,00 « 1 4 , 0 0 3 0 % 21 % 1 7 % p i o v o s i t à m e d i a 0 m m l a s p e d i z i o n e d e l l ' i . n . g . a k h a r t o u m 4 1 5 ( a l l e 0 8 , 0 0 1 , 8 d e c i m i n e b u l o s i t à t o t a l e . . . . < » 1 1 , 0 0 1 , 5 » ( » 1 4 , 0 0 1 , 7 » n e b u l o s i t à n u b i sotto 2 5 0 0 m 0 d e c i m i l 1 % c a l m o v e n t o ' 8 9 % l < f < 3 ( 1 0 % f > 4 1 6 % < 1 0 k m i a l l e 0 8 , 0 0 • 9 9 % < 5 0 » i 1 °/o > 5 0 » ( 12 % < 1 0 » v i s i b i l i t à » 1 1 , 0 0 l 1 0 0 % < 5 0 » i ( 0 7o > 5 0 » i ( 7 % < 1 0 » ' » 1 4 , 0 0 1 0 0 % < 5 0 » ( 0 % > 5 0 » da questo q u a d r o si p u ò v e d e r e c o m e l e c o n d i z i o n i c l i m a t i c h e s i a n o v e r a m e n t e o t t i m e p e r l e suesposte r i c e r c h e : l ' u m i d i t à r e l a t i v a così bassa, l a n e b u l o s i t à così scarsa e l a p i o v o s i t à n u l l a , f a c e v a n o prev e d e r e u n a b u o n a r i u s c i t a d e l l e o s s e r v a z i o n i . i n f a t t i t u t t i i f e n o m e n i d e l l a t e r m o d i n a m i c a , d e l l ' o t t i c a e d e l l a e l e t t r i c i t à a t m o s f e r i c a sono p a r t i c o l a r m e n t e i n f l u e n z a t i d a l l a p r e s e n z a d e l v a p o r d ' a c q u a e dei f e n o m e n i di c o n d e n s a z i o n e ed e v a p o r a z i o n e . i n f i n e k h a r t o u m , con l a sua o r g a n i z z a z i o n e di città e u r o p e a , f a v o r i v a p a r t i c o l a r m e n t e t u t t a la p a r t e l o g i s t i c a d e l l a s p e d i z i o n e . è da t e n e r p r e s e n t e p u r e c h e l ' u n i o n e a s t r o n o m i c a i n t e r n a z i o n a l e a v e v a s c e l t o k h a r t o u m c o m e base p e r l e o s s e r v a z i o n i a s t r o n o m i c h e ; i n f a t t i b e n 1 8 s p e d i z i o n i di 1 1 n a z i o n i d i v e r s e f u r o n o o s p i t a t e d a l s u d a n a n g l o e g i z i a n o p e r questa occasione. p e r t a l i m o t i v i q u i n d i f u deciso di i n s t a l l a r e l ' o s s e r v a t o r i o n e l l a z o n a di k h a r t o u m . n e l l a r e l a z i o n e s e g u e n t e r i f e r i r ò , i n s i e m e al dott. p i e r o d o m i n i c i geofisico dell'i.n.g., s u l l ' i m p i a n t o di q u e s t o o s s e r v a t o r i o e sull'andam e n t o g e n e r a l e d e l l e r i c e r c h e e f f e t t u a t e . roma — istituto nazionale di geofisica — maggio 1952. 4 1 6 r . c i a l d e a riassunto l'a. riferisce sulla organizzazione scientifica della spedizione dell'i.n.g. inviata a. kliartoum per eseguire osservazioni di fenomeni geofisici in occasione dell'eclisse totale di sole del 25 febbraio 1952. in particolare si riferisce sugli apparati allestiti a questo scopo per la temperatura virtuale del cielo, per la distribuzione dello stato di polarizzazione su tutta la volta celeste, per le proprietà elettriche dell'atmosfera (potenziale e campo elettrico, carica spaziale media, conducibilità elettrica) e per il magnetismo terrestre. su m mary the author reports on the scientific organization of the i.n.g. expedition sent to khartoum to make geophysical observations during the total solar eclipse of february 25, 1952. mentioned in particular various types of apparatus for measuring the virtual temperature of the sky, for the distribution of the state of polarization of the sky, for the electric properties of the atmosphere (potentini and electric field, average spadai charge, electric conductivity) and for the terrestrial magnetism. b i b l i o g r a f i a ( ! ) e . mathias: traile (télectricité atmosphérique. p a r i s 1 9 2 4 . h . f . johnston: terrest. magn. 2 9 ( 1 9 2 4 ) p . 1 3 . .1. p . ault, s . j . mauchly, r . h . goddard: terrest. magn. 3 0 ( 1 9 2 5 ) . p . 1 2 5 . h . e . seemann: frank. jnst. ]. 2 0 0 ( 1 9 2 5 ) p . 6 2 9 . p . j . nolan, c . o'brolchain : roy. irish acad. proc. 3 8 ( 1 9 2 8 ) p. 1 . s . nakamura, e . h u k u s i m a : tohoku imp. univ. sci. reports. 1 9 ( 1 9 3 0 ) p . 2 8 3 . n . a . critikos: gerlands. be'itr. z. geophys. 5 0 ( 1 9 3 7 ) p . 1 . m . p i e r u c c i : a ' . cimento. 1 6 ( 1 9 3 9 ) p . 2 2 5 . j . q . s t e w a r t , c . d . m a c c r a c k e n : astrophys. j . 9 1 ( 1 9 4 0 ) p . 5 1 . e . o . hulburt: nat. geogr. soc. contrib. techn. pap. 2 ( 1 9 4 2 ) p . 7 8 . m . w . jones, a . a . giesecke: terr. magn. atmos. fjecl. 4 9 ( 1 9 4 4 ) p. 1 1 9 . o . h . g i s h : terr. magn. atmos. elect. 4 9 ( 1 9 4 4 ) p . 1 2 3 . r . cialdf.a: ricerca scient. e ricostr. 1 5 ( 1 9 4 5 ) p . 3 8 5 . j . rouch: c . r. acad. sci. paris 2 2 8 ( 1 9 4 9 ) p . 1 5 4 7 . ( 2 ) a . l o surdd: nuovo cimento. 5 f 1 9 1 3 ) p. 4 3 7 . ( 3 ) r . cialdea: ricerca scient. e ricostr. 1 7 ( 1 9 4 7 ) p . 6 3 3 . layout 6 1 annals of geophysics, 63, 4, se440, 2020; doi:104401/ag-8256 seismoelectric effect in lamb’s problem vadim v. surkov*,1,2, valery m. sorokin1 and аleksey k. yaschenko1 (1) pushkov institute of terrestrial magnetism, ionosphere and radio wave propagation of the russian academy of sciences, moscow, troitsk, russia (2) institute of physics of the earth, russian academy of sciences, moscow, russia article history: received june 30, 2019; accepted february 17, 2020 abstract seismoelectric effect is studied in the framework of classical lamb’s problem with impulse or timevariable mechanical action on an elastic porous half-space. radiation of elastic waves gives rise to pressure variation of groundwater fluid contained in pores and cracks. this causes the generation of telluric electric fields and currents due to the seismoelectric effect. a diffusion type equation is applied to describe the variations of the pore pressure and telluric electric field. particular emphasis has been placed on the properties of seismoelectric signals caused by rayleigh wave propagation since this wave has maximal amplitude at a considerable distance from the seismic source. for practical purposes and geophysical application, the co-seismic phenomena related to seismoelectric effect are examined in more detail. keywords: seismology, ground motion, waves and wave analysis, groundwater processes, fluid geochemistry. 1. introduction in the field of applied seismology the lamb’s problem [lamb, 1931] is of great utility especially in the modelling of seismic wave originated from earthquakes, explosions, meteorite impacts on the earth and etc. in the lamb’s problem, the source of acoustic waves is generally assumed to be an impulse mechanical action on the elastic halfspace. there are several types of such waves including longitudinal, transverse and surface waves, which can propagate in elastic media at different speeds. actual geophysical media consist of a great variety of rocks, which contains inhomogeneous inclusions such as quartz, calcite and dolomite as well as cracks, pores filled with groundwater and etc. the seismic wave propagation in the ground is accompanied by the deformation of the pore space which results in the pore vibrations, diffusion of underground pore fluid followed by the seismoelectric phenomena and weak perturbations of the geomagnetic field [e.g., surkov and hayakawa, 2014]. the co-seismic electromagnetic variations are usually observed during the passage of seismic waves through the ground-based magnetometers and antennas or buried electrodes [ivanov, 1940; martner and sparks, 1959; eleman, 1965; anisimov et al., 1985; iyemori et al., 1996]. there are two basic physical mechanisms of these coseismic phenomena: the seismoelectric effect in porous water-saturated media and the perturbation of the earth’s magnetic field due to the motion of the conductive ground [surkov et al, 2018]. the seismoelectric effect builds up as a result of electric charge separation caused by the electro-chemical processes in the electrolyte solution, which is contained in the underground fluid [frenkel, 1944]. the surfaces of cracks and pores can adsorb ions of certain polarity (more frequently the negative-charged ions) from the electrolyte solution whereas the underground fluid accumulates the excess ions of opposite/positive sign. the seismic wave induces the underground fluid motion, which results in the generation of electrokinetic currents due to entrainment of the positive ions with the fluid. the electrokinetic current closes through a conduction current flowing in the fluid and conductive rocks. notice that the seismoelectric effect manifests itself in electric field predominantly rather than in magnetic field. the co-seismic variations of the telluric electric potentials have been observed after the occurrence of earthquakes with magnitude 𝑀 ≥ 5 at short epicentral distances [nagao et al., 2000; mogi et al., 2000]. analysis of the experimental data has shown that there are two types of co-seismic electric variations measured with the electrodes, which were embedded in the ground. the oscillating perturbations arising at the time of seismic wave arrival can be interpreted in terms of the seismoelectric effect. the other type is the co-seismic variations gradually declining during 1 minute after earthquake occurrence. this attenuation effect has been assumed to be due to the changes in hydrologic status of the ground surface layer [nagao et al., 2000]. electric and magnetic perturbations have been observed after the izmit earthquake (𝑀 = 7.6) in turkey on august 1999 immediately upon arrival of the seismic p-wave at the magnetotelluric stations [honkura et al, 2002]. acoustic and electrokinetic phenomena in porous water-saturated media are related through a set of coupled equations for acoustic and electrodynamic parameters [frenkel, 1944; pride, 1994; pride and garambois, 2005; hu and gao, 2011]. when considering the seismic problem, the electroosmotic phenomenon can be neglected. in this case, the basic set of equations can be split into two different sets in such a way that one of these sets includes only the acoustic field variables, independently of the electric field variables. this means that the acoustic variables contained in the second set play the role of known sources of seismoelectric variations. the seismoelectric effect has been studied in more detail for the case of a harmonic acoustic wave propagating in an infinite homogeneous space [e.g., surkov et al., 2018]. more complex models of the seismic source are necessary in order to study the seismoelectric phenomenon associated with earthquake and explosion. the models need to take into account the impulse character of the seismic source and the influence of boundary conditions at the ground surface. in this study the seismoelectric effect is analyzed on the basis of solution of the classic lamb’s problem; that is, the problem of an impulse or transient impact on an elastic porous half-space. 2. perturbation of pore fluid pressure caused by the propagation of the acoustic wave the electrokinetic effect in porous water-saturated medium arises under influence of the pore fluid pressure gradient ∇𝑃�. the field of extrinsic force related to this effect is described by the vector e�� = ‒𝐶��∇𝑃�, where 𝐶�� is the streaming potential coupling coefficient. the total current density in the porous medium consists of the electrokinetic current density 𝜎e�� and the conduction current ‒𝜎∇𝜑, where 𝜎 is the mean conductivity of the porous medium and 𝜑 is the electric field potential. considering the steady-state distribution of the electric current in porous medium, the potential 𝜑 is described by the following equation ∇ · 𝜎 (e�� ‒ ∇𝜑) = 0. (1) we shall restrict our consideration to a homogeneous medium with constant 𝜎 and 𝐶��. thus equation (1) reduces to ∇²(𝜑 + 𝐶��𝑃� ) = 0. (2) the electric potential as well as the normal component of the total current density have to be zero at the boundary between the ground and the atmosphere. in addition, the pore fluid pressure has to be equal to atmospheric pressure on the ground surface. this requirement is replaced by the approximate boundary condition 𝑃� = 0 since the atmospheric pressure is small. in such a case the solution of equation (2) has a simple view (e.g., [surkov and hayakawa, 2014]): 𝜑 + 𝐶��𝑃� = 0. (3) vadim v. surkov et al. 2 the equations related the volumetric strain 𝜃 of porous medium and the variation of pore fluid overpressure 𝑃� (with respect to hydrostatic pressure) have been derived by [frenkel, 1944]. these equations include the second order time-derivatives which can be neglected in the low frequency region. in this approximation the equations are simplified and reduced to the diffusion-type equations: (4) where 𝐷 denotes the coefficient of diffusion of the fluid pressure perturbation in porous medium, 𝜂 stands for the viscosity of the pore fluid while 𝑛 and 𝑘� are the medium porosity and permeability, respectively. here we made use of the following abbreviations: (5) where 𝐾� is the bulk modulus of compressibility of pore fluid, 𝐾� is the bulk modulus of solid which forms a medium skeleton and 𝐾 is the bulk modulus of dry porous rock (the medium skeleton without the pore fluid). the porous medium is treated as a homogeneous elastic half-space bounded from above by the atmosphere. since the medium porosity is assumed to be small, the velocities of elastic waves and other medium parameters depend only slightly on the porosity. additionally, it is conjectured that the wave damping due to viscous friction in the fluid can be neglected. the volumetric strain in equation (4) is treated as a given function. in order to find this function we first consider the lamb’s problem for the elastic half-space. suppose the origin of coordinate system is located on the surface of the half-space while the z-axis is pointed vertically downward. the boundary surface is subjected to the normal force distributed on the surface. the normal component of the stress tensor 𝜎�� corresponding to this force is assumed to be a given function at 𝑧 = 0: (6) where 𝐹₀ and 𝑇 are the force amplitude and typical time of the force action, 𝑟₀ is the characteristic radius of the area subjected to the normal force, 𝑟 = (𝑟²+𝑦²)¹/² is polar radius and 𝛿 (𝑡) denotes dirac delta-function of time. notice that if 𝑟₀ → 0 then 𝑟₀ / (𝑟²+ 𝑟₀² )³/² → 𝛿(𝑟)/𝑟. in this extreme case we come to the known axially symmetric lamb’s problem on point normal force acting on elastic half-space. the cylindrical coordinates z, r are convenient to use since the normal force (6) is axially symmetrically distributed on the surface. the solution of the corresponding boundary-value problem for the elastic half-space enables one to determine the radial and vertical components of the displacements of the elastic medium [petrashen et al., 1950; brekhovskikh, 1980; nikitin et al., 2013]: (7) (8) here the following designations are used for laplace integrals (9) 𝜕𝑃� 𝜕𝑡 = 𝐷𝛻²𝑃� ‒ 𝛼 , 𝐷 = , 𝜕𝜃 𝜕𝑡 𝑛𝛽𝑘�𝐾� 𝜂 𝛼 = (𝐾� ‒ 𝐾)𝛽, 𝛽 = ,𝐾�𝐾� (𝐾� ‒ 𝐾)𝐾� + 𝑛𝐾� (𝐾�‒𝐾�) 𝜎�� = ,𝐹₀𝑇𝛿(𝑡)𝑟₀ 2𝜋(𝑟²+𝑟₀² )³/² ∞ ₀𝑢� = 𝐴� ℎ� �𝑞,𝑧,𝑡�exp�‒𝑞𝑟₀�𝑞j₁�𝑞𝑟�𝑑𝑞, ∞ ₀𝑢� = 𝐴� ℎ� �𝑞,𝑧,𝑡�exp�‒𝑞𝑟₀�𝑞j₀�𝑞𝑟�𝑑𝑞. 𝜀+𝑖∞ 𝜀‒𝑖∞ ℎ� = � ��2 + 𝜁²�exp�‒𝑧𝑞𝑠₂�‒2𝑠₁𝑠₂ exp�‒𝑧𝑞𝑠₁�� ,exp (𝑞𝑡𝐶�𝜁 )𝑑𝜁 𝑅(𝜁) 3 seismoelectric effect in lamb’s problem vadim v. surkov et al. 4 (10) where 𝑅(𝜁) = (2+𝜁²)² ‒4𝑠₁𝑠₂ denotes rayleigh function. here we made use of the following abbreviations (11) where 𝐶� and 𝐶� are the velocities of longitudinal and transverse elastic waves while 𝜌 is the undisturbed medium density. the volumetric strain is given by [landau and lifshitz, 1970]: (12) substituting equations (7) and (8) for 𝑢� and 𝑢� into equation (12), yields (13) where (14) taking into account the construction of the expressions for functions (13) and (14) we seek for the solution of equation (4) in the form: (15) substituting equations (13) and (15) for 𝜃 and 𝑃� into equation (4) we obtain the following equation for the unknown function 𝑝: (16) where (17) here the primes denote derivatives with respect to z. assuming the pore overpressure is zero at the surface z = 0 and taking into account that the solution of equation (16) has to be limited with z → � we obtain (18) substituting equations (17) and (18) for 𝐵 and 𝑝 into equation (15), we find the overpressure of the underground fluid in pores: (19) 𝜕𝑢� 𝜕𝑧𝛳 = �𝑟𝑢�� + . 1 𝜕 𝑟 𝜕𝑟 ∞ ₀𝛳 = 𝐴�ℎ� �𝑞,𝑧,𝑡�exp�‒𝑞𝑟₀�𝑞j₀�𝑞𝑟�𝑑𝑞, 𝜀+𝑖∞ 𝜀‒𝑖∞ ℎ� = � ��2 + 𝜁²�𝜁²exp�‒𝑧𝑞𝑠₂� .exp (𝑞𝑡𝐶�𝜁 )𝑑𝜁 𝑅(𝜁) 𝑞 𝛾² ∞ ₀𝑃� = � � � 𝑝�𝑞,𝑧,𝜁�exp�𝑞𝑡𝐶�𝜁�𝑑𝜁�j₀�𝑞𝑟�𝑑𝑞. 𝜀+𝑖∞ 𝜀‒𝑖∞ 𝑝˝ ‒ 𝜆²𝑝 = ‒ exp�‒𝑧𝑞𝑠₂ ‒𝑞𝑟₀�,𝐵𝑞³𝜁³(2+𝜁²) 𝐷𝑅(𝜁)𝛾² 𝐵 = , 𝜆²= 𝑞² + .𝛼𝐹₀𝑇 4𝜋²𝑖𝜌 𝑞𝜁𝐶� 𝐷 𝜀+𝑖∞ 𝜀‒𝑖∞ ℎ� = � ��2 + 𝜁²�exp�‒𝑞𝑧𝑠₂�‒2exp�‒𝑞𝑧𝑠₁�� ,𝑠₂exp (𝑞𝑡𝐶�𝜁 )𝑑𝜁 𝑅(𝜁) ∞ ₀𝑃� = 𝐵� � � exp�𝑞𝑡𝐶�𝜁‒𝑞𝑟₀�𝑑𝜁� 𝑞²j₀�𝑞𝑟�𝑑𝑞. 𝜀+𝑖∞ 𝜀‒𝑖∞ 𝜁²(2+𝜁²){exp(‒𝜆𝑧)‒exp(‒𝑧𝑞𝑠2)} 𝑅(𝜁)(𝑞𝐷𝜁‒𝐶�𝛾²) 𝐴 = , 𝑠₁ = �1 + 𝜁²�¹/² , 𝑠₂ = �1 + �¹/² , 𝛾 = ,𝐹₀𝑇 4𝜋²𝑖𝜌𝐶� 𝜁² 𝛾² 𝐶� 𝐶� 𝑝 = exp�‒𝑞𝑟₀�.𝐵𝑞²𝜁²(2+𝜁²){exp(‒𝜆𝑧)exp(‒𝑧𝑞𝑠2)} 𝑅(𝜁){𝑞𝐷𝜁‒𝐶�𝛾²} as is seen from equation (3) the distribution of the electric potential in the ground can be expressed through 𝑃� via 𝜑 = ‒ 𝐶��𝑃�. thus, we have derived the general solution of the above problem for the case of a pulsed seismic source. the solution of the more general problem for an arbitrary source can be derived by convolution of the above solution with the source function. we shall concern this problem in the fourth section. the contribution of the longitudinal and rayleigh waves to the variation of the pore fluid pressure is associated with the singularities of the integrand in equation (19) that are discussed in appendix. the function 𝑠₁ associated with the transverse wave does not enter equation (19). the reason is that the transverse wave does not create the volume deformation of the medium and thus it has not effect on both 𝑃� and the seismoelectric signal. however, in the high frequency range the shear horizontal (sh) wave can serve as a source of the seismoelectric effect even if 𝜃 = 0 (e.g., [han and wang, 2001; monachesi et al., 2018; gao et al., 2019]). in the theory, the electric signal produced by the sh-wave is proportional to a relative acceleration of the fluid phase with respect to the solid one [han and wang, 2001]. this source does not enter equation (4) for 𝑃� since this relative acceleration is proportional to the frequency squared and thus is small in the seismic frequency range (below or of the order of several hz). in the remainder of this paper we shall return to this question. it should be noted that the derived solutions are valid for the weakly porous homogeneous media where the dissipation and dispersion of acoustic waves can be neglected. in the more general case of dissipative porous watersaturated media, it is necessary to solve a set of coupled equations that includes both the acoustic and electrical parameters [frenkel, 1944; pride, 1994]. 3. variation of pore fluid pressure caused by the rayleigh wave in this section we seek for the variation of the pore fluid pressure due to the propagation of the rayleigh wave. the contribution of this wave to 𝑃� is defined by the poles of the integrand in equation (19). the coordinates of these poles are 𝜁 = ±𝑖𝜉. here 𝜉 = 𝐶�/𝐶� where 𝐶� denotes rayleigh wave velocity [lamb, 1931]. calculations of the corresponding pole residues are found in appendix. the result of these calculations is given by (20) here we made use the following abbreviations (21) the diffusion coefficient of the pore fluid pressure is directly proportional to the medium porosity 𝑛 which can vary within a very wide range. for example, the average porosity of the typical rocks are of the order of (0.2‒6)·10⁻³, 0.04 ‒ 0.3 and 0.2 ‒ 0.3 for granite, gneiss and tuff, respectively [mavko et al., 2009]. in order to estimate the diffusion coefficient and other parameters we use the following numerical values: 𝑛 = 0.1, 𝐾� = 2 gpa, 𝐾= 10 gpa and 𝐾� = 20 gpa whence it follows that 𝛽 = 0.714. the water viscosity coefficient varies within (0.6 ‒ 1.8)·10⁻³ pa·s in the temperature interval from 0° c to 40° c. the permeability coefficient of the actual rocks also varies in a very wide range from 10⁻¹⁴ to 10⁻⁹ m2 [teodorovich, 1958]. for the permeable and well permeable rocks at 𝑘� =10⁻¹² ‒10⁻¹⁰ m2 the diffusion coefficient in equation (4) is estimated as 𝐷 ≈ (0.008‒2.4)·10⁻² m2/s. using this numerical estimate one can derive the approximate expression for the integral in equation (20). at first, notice that the real part of the exponent of exponential function exp(‒𝑧𝑞𝜒) is smaller than unit under requirement that (22) 𝑞<𝑞₀ = 𝑧⁻¹�1+ �⁻¹/².𝑧²𝐶�² 4𝐷² 𝑃�,� = �𝑞²exp�‒𝑞𝑟₀�j₀�𝑞𝑟� re � �exp(‒𝑧𝑞𝜒)‒exp(‒𝑧𝑞𝑝₂)��𝑑𝑞.𝛼𝐹₀𝑇(2 ‒ 𝜉²)𝜉 4𝜋𝜌𝑄(𝜉) exp(𝑖𝑞𝑡𝐶�) 𝑞𝐷𝜉+𝑖𝐶�𝛾² ∞ ₀ 𝑄(𝜉) = 2‒𝜉² ‒ ‒ , 𝜒 = �1+ �¹/², 𝑝₁ = �1 ‒ 𝜉²�¹/², 𝑝₂ = �1 ‒ �¹/².𝜉² 𝛾² 𝑖𝐶� 𝑞𝐷 𝑝₂ 𝑝₁ 𝑝₁ 𝑝₂𝛾² 5 seismoelectric effect in lamb’s problem here 𝑞₀ is the characteristic scale of the function decrease. in order to estimate this parameter, we use 𝐶� = 1.4 km/s and the above range of values of the diffusion coefficient. then we get the following estimate: 𝑞₀ ≪ 𝑧⁻¹ under the requirement that 𝑧 ≫ 𝑧∗ = 2𝐷/𝐶� = 1.2·10⁻³ ‒ 0.34 m. in this approximation the first exponential function in the square brackets in equation (20) falls off more rapidly with 𝑞 than does the second exponential function. therefore, with these parameters the function exp(‒𝑧𝑞𝜒) in equation (20) becomes insignificant in this range of depths 𝑧. in addition, one can neglect the summand 𝑞𝐷𝜉 in the denominator of equation (20) under the requirement that 𝑧 ≫ 𝐷𝜉/𝐶�𝛾²) ≈ 2.6·10⁻⁴ ‒0.08 m. in practice, to measure telluric potentials the electrodes are buried in the ground for a few tens of cm so that both of the above conditions hold true. this approximation can be taken into account by setting formally 𝐷 = 0 in equation (20). as a result we obtain: (23) where 𝜇 = 𝑧𝑝₂ + 𝑟₀. performing integration in equation (23) yields [grandshteyn and ryzhik, 2007] (24) where 𝑄± = �𝜇² + �𝐶�𝑡 ± 𝑟�²�¹/². let us introduce a new variable 𝑥 = 𝑟 ‒ 𝐶�𝑡 in order to analyze equation (24) near the wave front and far away from the seismic source. by making assumption that |𝑥| ≪ 𝑟 and 𝜇 ≪ 𝑟, equation (24) can be simplified (25) where (26) let 𝑥₁ = 𝑥/𝜇 be a new dimensionless variable. taking derivative of this expression with respect to 𝜇 we get (27) now let us make some simple estimates to gain a better understanding of the application area of the results obtained above. for this purpose we consider an impulsive pressure source situated at some point in elastic pore space. after the source is switched on, the spherically symmetric p-wave and the diffusion of the pore fluid begin to propagate simultaneously from this point. the front radius of the seismic p-wave increases with time as 𝑟� ~ 𝐶�𝑡 whereas the size of area covered by the diffusion increases as 𝑟� ~ 2(𝐷𝑡)¹/² (e.g., [surkov and hayakawa, 2014]). from these relations it follows that at first the diffusion front propagates faster, that is, ahead of the seismic wave. the diffusion velocity decreases with time, so the p-wave catches up with the diffusion front at some distance 𝑟*. equating 𝑟� and 𝑟�, one can find this distance: 𝑟* ~ 2𝑧* ≈ 4 𝐷/𝐶� ≈ 0.2 ‒ 70 cm. this value is much smaller as compared to the seismic wavelength. this means that the process of the pore fluid diffusion is insignificant because the pore fluid overpressure is mainly controlled by the variations of the seismic wave parameters. 𝑃�,� = ‒ , 𝛼𝐹₀𝑇(2 ‒ 𝜉²)𝐶� 2⁵/²𝜋𝜌𝑄(𝜉)𝐶�² 𝑑² 𝑑𝜇² {𝐶�² 𝑡² ‒ 𝑟² ‒𝜇²+𝑄₊𝑄₋}¹/² 𝑄₊𝑄₋ 𝑃�,� = ‒ 𝑓(𝑥,𝜇) , 𝛼𝐹₀𝑇(2 ‒ 𝜉²)𝐶� 8𝜋𝑟¹/²𝜌𝑄(𝜉)𝐶�² 𝑓(𝑥,𝜇) = .�(𝜇² + 𝑥²)¹/² ‒𝑥�¹ /² (𝜇²+𝑥²)¹/² 𝑑² 𝑑𝜇² 𝑃�,� ≈ ‒ �𝑞² sin�𝑞𝑡𝐶��exp(‒𝜇𝑞)𝐽₀�𝑞𝑟�𝑑𝑞,𝛼𝐹₀𝑇(2 ‒ 𝜉²)𝐶� 4𝜋𝜌𝑄(𝜉)𝐶�² ∞ ₀ 𝑓(𝑥, 𝜇) = , 𝑔 = �𝑢�1‒𝑥₁² � + 𝑥₁�𝑥₁² ‒3��, 𝑢 = �1+ 𝑥₁² �¹/².3(𝑢 + 𝑥₁)¹/² 4𝑢⁵ 𝑔(𝑥₁) 𝜇⁵/² vadim v. surkov et al. 6 4. seismoelectric effect caused by the raleigh wave in order to estimate the rayleigh wave contribution to the seismoelectric effect, equation (25) for pore fluid overpressure needs to be substituted into equation (3) for telluric potential: (28) the strength of the telluric electric field due to rayleigh wave is derivable from a potential 𝜑 through 𝐄 = ‒∇𝜑. the most interesting is the horizontal component 𝐸� of the telluric field which dominates near the ground surface (29) where (30) the dimensionless dependences of the potential and the radial component of the electric field in a reference frame moving at rayleigh wave velocity are shown in figures 1a and 1b as functions of the dimensionless distance 𝑥₁. as is seen from these figures, the potential and electric field reach peak values of the order of 𝜑� and 𝐸� , respectively, in the region around the point 𝑥 ~ 𝜇. for the surface layer of the ground; that is, under requirement that 𝑟0 ≫ 𝑧𝑝2, one can assume that 𝜇 ~ 𝑟0. taking into account this approximation and substituting 𝐹₀ = 𝜋𝑟₀² 𝑃₀ into equations (28) and (29), we obtain (31) here 𝑟0 is the characteristic radius of the area subjected to the force 𝐹₀ while 𝑃₀ = 𝐹₀ / (𝜋𝑟₀² ) is the pressure caused by this force. 𝐸� = ‒ 𝐸� , 𝐸� = ,𝜑� 𝜇 𝑑𝑔 𝑑𝑥₁ = �𝑢𝑥₁�𝑥₁² ‒ 3� ‒ 𝑥₁⁴ + 6𝑥₁² ‒ 1�.𝑑𝑔 𝑑𝑥₁ 15(𝑢 + 𝑥₁)¹/² 8𝑢⁷ 𝐸� ~ .𝛼𝐶��𝐹₀𝑇(2 ‒ 𝜉²)𝐶� 8𝑟¹/² 𝑟₀³/² 𝑄(𝜉)𝜌𝐶�² 𝜑 = 𝜑�𝑔(𝑥1), 𝜑� = . 𝛼𝐶��𝐹₀𝑇(2 ‒ 𝜉²)𝐶� 8𝜋𝑟¹/² 𝜇⁵/² 𝜌𝐶�² 𝑄(𝜉) 7 seismoelectric effect in lamb’s problem figure 1. a model calculation of (a) the dimensionless telluric potential and (b) the radial component of the electric field measured near ground surface as functions of the dimensionless coordinate. the seismoelectric effect is resulted from rayleigh wave radiated by pulsed seismic source. the reference frame is used which moves at rayleigh wave velocity. a) b) to take one example, consider the electrokinetic effect resulted from the point explosion on the ground surface. the approximate dependence of the parameter 𝜉 = 𝐶� / 𝐶� on poisson coefficient 𝜈 (a coefficient of transverse strain) is given by [landau and lifshitz, 1970]: (32) for the actual solids the value of 𝜉 varies in the range from 0.874 to 0.955. considering the upper layer of sedimentary rocks, the typical values of the poisson coefficient are the following: 0.27 for large fragmental ground, 0.30 for sand and sandy loam, 0.35 for loamy soil and 0.42 for clay [sorochan, 1987]. the dependence of 𝛾 = 𝐶� / 𝐶� on 𝜈 is given by (33) by setting the poisson coefficient 𝜈 = 0.3 and substituting this value into equations (21), (32) and (33) we obtain that 𝜉 = 0.928, 𝛾 = 1.870 and 𝑄 = ‒1.314. the electrokinetic coefficient 𝐶�� can vary within a very wide range depending on the porosity, permeability, moisture saturation, concentration of mineral salts in the pore fluid and other parameters of the medium in which the measuring electrodes are located [jouniaux et al., 2000; vinogradov et al., 2010]. the explosion parameters are chosen to be 𝑃₀ = 0.1 gpa, 𝑇 = 0.1 s and 𝑟0 = 2 m. additionally we suppose that 𝐶�� = 10⁻⁶ ‒ 10⁻⁸ v/pa and 𝜌𝐶�² ≈ 𝐾 = 10 gpa. substituting these and above-mentioned values of the parameters into equation (31), we obtain that 𝐸� = 1 ‒ 100 mv/m at the distance of 𝑟=10³ km from the explosion site. typically, the natural electric noise amplitude in the ground is about 1‒10 𝜇v/m; that is, several magnitudes smaller than 𝐸�. this means that the seismoelectric signal produced by such kind of explosion can be detected at the background of the telluric noise. 5. seismic sources with arbitrary time dependence to this point the impulsive seismic source has been examined. now let us suppose that the pressure acting on the ground surface is described by an arbitrary source function 𝐺(𝑡). in such a case one must replace time 𝑡 with 𝑡‒𝜏, the parameter 𝑇 with 𝐺(𝜏)𝑑𝜏 and then make a convolution of the above solution with the function 𝐺(𝜏). hence the function 𝑓(𝑥, 𝜇) in the above solution has to be replaced by the following function (34) where 𝑥� = 𝑟‒𝐶�(𝑡‒𝜏) = 𝑥 + 𝐶�𝜏. by analogy with equations (29) and (30), we can thus write (35) where 𝑥1 = 𝑥� /𝜇 while 𝐸� and 𝑑𝑔/𝑑𝑥₁ are given by equations (28)-(30). by way of illustration let us first consider the pressure pulse described by a step function 𝐺(𝜏), which equals unity within the interval 0 ≤ 𝜏 < 𝑇 or zero as 𝜏 > 𝑇. in such a case it is suitable to perform integration in equation (34). as a result, we obtain (𝑡 > 𝑇) (36) 𝐸� = ‒ 𝐸� �𝐺�𝜏� ,𝑑𝜏 𝑇 𝑑𝑔 𝑑𝑥₁ ∞ ₀ 𝑓₁(𝑥, 𝜇) = ��(𝜇² + 𝑥²)¹/² ‒ 𝑥�¹/²‒�(𝜇² + 𝑥�² )¹/² ‒𝑥��¹/²�, 𝜕² 𝜕𝜇² 2 𝐶� 𝛾 = � �¹/².2(1 ‒ 𝜈) 1 ‒ 2𝜈 ∞ ₀𝑓₁(𝑥, 𝜇) =� 𝐺�𝜏�𝑑𝜏, 𝜕² 𝜕𝜇² �(𝜇² + 𝑥�² )¹/² ‒ 𝑥��¹/² (𝜇² + 𝑥�² )¹/² 𝜉 = .0.87 + 1.12 𝜈 1 + 𝜈 vadim v. surkov et al. 8 where 𝑥� = 𝑟 ‒ 𝐶� (𝑡‒𝑇) = 𝑥 + 𝐶�𝑇. taking derivatives of this expression with respect to 𝜇 and rearranging one can reduce equation (36) to the view: 𝑓₁(𝑥, 𝜇) =[𝑔₁(𝑥₁) ‒𝑔₁ (𝑥�/𝜇)]/(2𝐶�𝜇³/²), (37) where (38) by making calculations analogous to those given above, one can find the potential and the radial component of the electric field (39) (40) here as before the peak values of the telluric potential and horizontal component of the electric field are of the order of 𝜑� and 𝐸� = 𝜑� / 𝜇, respectively. the dimensionless potential 𝜑/𝜑� and electric field 𝐸� /𝐸� are shown in figures 2a and 2b as functions of the dimensionless distance 𝑥₁. the signals shown with lines 1 correspond to the value 𝑥� / 𝜇 = 0.1 and are similar to the ones displayed in figures 1a and 1b originated from the pulse source. the qualitative difference between the electric signals produced by the different sources becomes evident when considering lines 2 and 3, which correspond to the values 𝑥� / 𝜇 = 3 and 10, respectively. the graphs in figure 2 are more complicated in shape than the ones shown in figure 1. the individual peaks of the electric signals shown with lines 3 form two groups. one group is concentrated near the anterior front of the seismic wave whereas the other one near the posterior front. in the time interval between these groups, the electric signals are small because of small value of the pore pressure gradient. 𝑔₁(𝑥₁) = .𝑥₁² ‒ 𝑢𝑥₁ ‒ 1 𝑢³(𝑢 ‒ 𝑥₁)¹/² 𝜑 = 𝜑�[𝑔₁(𝑥₁) ‒𝑔₁(𝑥�/𝜇)], 𝜑� = ,𝛼𝐶��𝐹₀(2 ‒ 𝜉²) 16𝜋𝑟¹/² 𝜇³/² 𝜌𝐶�² 𝑄(𝜉) 𝐸� = 𝐸�[𝑔₂(𝑥₁) ‒𝑔₂(𝑥�/𝜇)], 𝑔₂(𝑥₁) = .3[𝑥₁(𝑥₁ ‒ 𝑢) (2𝑥₁ + 𝑢)]+ 𝑢‒2𝑥₁] 2𝑢⁵(𝑢 ‒ 𝑥₁)¹/² 9 seismoelectric effect in lamb’s problem figure 2. the same as in figures 1a and 1b but for the case of the seismic source describing by a step function. lines 1, 2 and 3 correspond to the values 𝑥� /𝜇 = 0.1, 3, and 10, respectively. a) b) the co-seismic electromagnetic phenomena have been frequently observed after earthquake occurrence away from the epicenter [e.g., surkov and hayakawa, 2014 and references herein]. it is usually the case that the far-field displacement components of the seismic wave have a shape largely similar to the damped oscillations. to illustrate this property of seismic wave, let us now approximate the source function 𝐺(𝑡) as follows 𝐺(𝑡) = exp(‒𝑡/𝑡�)sin(2𝜋𝑓𝑡), (41) where 𝑓 is the seismic wave frequency and 𝑡� denotes the characteristic decay time. substituting equation (41) for 𝐺(𝑡) into equation (35) and performing the numerical integration, one can find the electric field. a model calculation of the normalized radial component of the electric field and the source function 𝐺(𝑡) are shown in figure 3a,b with solid and dotted lines, respectively. in making these plots we have used the following numerical values of the parameters 𝑓 = 0.5, 0.8 hz; 𝑡� = 10 s; 𝐶� = 3 km/s; 𝑟₀ = 5 km and 𝑧 = 0. the zerotime reference in figure 3a,b corresponds to the time 𝑡 = (𝑟 ‒ 𝑟₀)/𝐶� of seismic wave arrival at the observation point. from figure 3, the typical frequency of the electric field vibrations is close to 𝑓 though the electric signal is phase shifted with respect to the seismic variations of the pressure or mass velocity, which are described by the function 𝐺(𝑡). such co-seismic geoelectric field changes after earthquakes occurrence have been observed by many researchers [e.g., eleman, 1965; nagao et al., 2000; honkura et al., 2002; balasco et al., 2014; romano et al., 2018]. 6. discussion and conclusion the general solution given by equations (3) and (19) shows seismoelectric effects associated with propagation of p-wave and rayleigh wave. the transverse wave cannot excite such effects in the low frequency region. the reason is that transverse waves such as the s-wave or love wave do not create a volume strain and thus they have not influence upon the pressure gradient of pore fluid. however, the observations have shown that the co-seismic electromagnetic signals can be excited by both pand s-waves [e.g., balasco et al., 2014; romano, 2019]. our rationale is that these co-seismic signals can be interpreted in terms of two main underlying mechanisms: (a) electrokinetic effect in porous medium [e.g., frenkel, 1944; martner and sparks, 1959; pride, 1994; hu and gao, 2011; surkov and pilipenko, 2015] and (b) a geomagnetic inductive (gmi) effect [e.g., knopoff, 1955; guglielmi, 1986; gorbachev and surkov, 1987]. the gmi perturbations during the seismic wave propagation across an observation site arise from the generation of electric currents in the conductive layers of the ground due to the vadim v. surkov et al. 10 figure 3. numerical modeling of the co-seismic signal caused by quasi-oscillatory seismic wave; (a) 𝑓 = 0.5 hz, (b) 𝑓 = 0.8 hz. the normalized radial component of electric field and the source function 𝐺(𝑡) are shown with solid and dotted lines, respectively. a) b) ground motion in the geomagnetic field. thus, one may assume that the gmi mechanism can explain the electromagnetic perturbations observed during the s-waves propagation. the main difference between the seismoelectric and gmi mechanisms is that the gmi disturbance should spread from a seismic wave front along a conductive crust in a diffusive way. depending on the ground conductivity the velocity of the gmi diffusion can even supersede the seismic wave velocity, thereby producing an “electromagnetic precursor” of a seismic wave front [surkov 1997]. such a kind of electromagnetic changes in both magnetic [honkura et al., 2002] and electric components [balasco et al., 2014; romano, 2018] before the arrival of seismic waves has been occasionally observed. however, in practice, it is difficult to conclude unambiguously which of those mechanisms prevails because of a large variability of the realistic crust parameters, such as porosity, permeability, conductivity and etc. [surkov et al. 2018]. in order to estimate the contribution to the co-seismic signal from the seismoelectric effect, one need information on the distribution of the streaming potential coupling coefficient 𝐶�� around an observation point. in this study, we have shown that the coefficient 𝐶�� has a much greater influence on the seismoelectric signals as compared with the effect of the fluid diffusion coefficient 𝐷. away from the seismic source, the rayleigh wave has the greatest amplitude as compared to other seismic waves, and thus it can provide a considerable contribution to the seismoelectric effect. analysis of the solutions has shown that the electric field perturbations are mainly concentrated near the seismic wave front and propagate in the medium at the speed of rayleigh wave. in the case of the pulsed source, the seismoelectric signal has a shape somewhat similar to a bipolar pulse unlike the seismic signal. as the seismic source time function resembles a step function, the seismoelectric perturbations are mainly grouped near the front area and in the vicinity of a trailing edge of the seismic signal, i.e. in those areas where the pressure gradient of the pore fluid has the greatest value. the seismoelectric effect caused by surface detonation has been estimated too. it was shown that for a pressure amplitude in the source of about 0.1 gpa, the electric perturbations in the ground can exceed the background telluric variations at distances up to thousand kilometers. modelling of the seismoelectric effect caused by a seismic wave coming from the earthquake focal zone has been performed by using the source function in the form of damped oscillations. the co-seismic electromagnetic signals are shown to be excited practically synchronously with the passage of seismic waves through an observation point though the electric signal is phase shifted with respect to the seismic signal, which is consistent with the observations. the amplitude of the seismoelectric signals has been shown to decrease with distance 𝑟 in the same fashion that the pressure and other rayleigh wave parameters fall off, i.e. inversely proportional to 𝑟¹/². however, the exact values of the amplitudes of the seismoelectric signals cannot be predicted in advance because of the uncertainty of the streaming potential coupling coefficient, which can vary widely depending on the porosity and permeability of actual rocks, the mineral composition of groundwater and other parameters of porous media. acknowledgements. this study was partly supported by the grant no. 18-05-00108 from rfbr. we appreciate useful comments of all reviewers. references anisimov, s.v., m.b. gokhberg, e.a. ivanov, m.v. pedanov, n.n. rusakov, v.a. troitskaya, and v.e. goncharov (1985). short period vibrations of the earth’s electromagnetic field due to industrial explosion, rep. ussr acad. sci. 281 (3), 556-559. balasco m., v. lapenna, g. romano, a. siniscalchi, t.a. stabile, and l. telesca (2014). electric and magnetic field changes observed during a seismic swarm in pollino area (southern italy), bull. seism. soc. america 104 (3), 1289-1298, doi:10.1785/0120130183. brekhovskikh, l.m., (1980), waves in layered media, second edition, applied mathematics and mechanics 6, translated by r. t. beyer, academic press, new york. eleman, f. 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(ussr) 8, 230-241. gao, y., d. wang, c. yao, w. guan, h. hu, w. zhang, p. tong, and q. yang (2019). simulation of seismoelectric waves 11 seismoelectric effect in lamb’s problem using finite-difference frequency-domain method: 2d shte mode, geophys. j. int. 216 (1), 414-438. gorbachev, l.p., and v.v. surkov (1987). perturbation of an external magnetic field by a rayleigh surface wave, magnetohydrodynamics 23, 117-125. grandshteyn i.s., and i.m. ryzhik (2007). table of integrals, series, and products, seventh edition, ed. by a. jeffrey and d. zwillinger, elsevier, academic press, p. 743. guglielmi, a.v., (1986). excitation of electromagnetic field oscillations by elastic waves in conducting body, geomagn. aeron. 27, 467-470. han, q., and z. wang (2001). time-domain simulation of sh-wave-induced electromagnetic field in heterogeneous porous media: a fast finite element algorithm, geophysics 66 (2), 448-461. honkura, y., m. matsushima, n. oshiman, and et al. (2002). small electric and magnetic signals observed before the arrival of seismic wave. earth planets space 54 (12), e9-e12. hu, h., and y. gao (2011). electromagnetic field generated by a finite fault due to electrokinetic effect, j. geophys. res. 116, b08302, doi:10.1029/2010jb007958. iyemori, t., t. kamei, y. tanaka, m. takeda, t. hashimoto, t. araki, t. okamoto, k. watanabe, n. sumitomo and n. oshiman (1996). co-seismic geomagnetic variations observed at the 1995 hyogoken–nanbu earthquake, j. geomagn. geoelectr. 48, 1059-1070. ivanov, a.g. (1940). seismoelectric effect of the second kind. proc. ussr acad. sci. ser. geogr. geophys. 4, 699-726. jouniaux, l., m.l. bernard, m. zamora, and j.p. pozzi (2000). streaming potential in volcanic rocks from mount pele´e, j. geophys. res. 105, 8391-8401. knopoff, e.l., (1955). the interaction between elastic waves motion and a magnetic field in electrical conductors, j. geophys. res. 60, 617-629. lamb, h. (1931), the dynamical theory of sound, 2nd edition, london, edward arnold & co. landau, l.d., and e.m. lifshitz (1970). theory of elasticity, volume 7 of a course of theoretical physics, pergamon press, the second english edition, 1970, 165 p. martner, s.t., and n.r. sparks (1959). the electroseismic effect, geophysics 24, 297-308. mavko, g., t. mukerji, and t. dvorkin (2014). the rock physics handbook: tools for seismic analysis of porous media, 2nd edn. cambridge university press, cambridge mogi, t., y. tanaka, d.s. widarto, e.m. arsadi, n.t. puspito, t. nagao, w. kanda, and s. uyeda (2000). geoelectric potential difference monitoring in southern sumatra, indonesia co-seismic change, earth planets space 52, 245-252. monachesi, l.b., f.i. zyserman, and l. jouniaux (2018). an analytical solution to assess the sh seismoelectric response of vadose zone. geophys. j. int. 213 (3), 1999-2019. nagao, t., y. orihara, t. yamaguchi, i. takahashi, k. hattori, y. noda, k. sayanagi and s. uyeda (2000). co-seismic geoelectric potential changes observed in japan, geophys. res. lett. 27, 1535-1538. nikitin i.s., a.v. philimonov, and v.l. yakushev (2013). rayleigh wave propagation at the oblique impact of a meteorite on the earth’s surface and their impact on buildings and structures, computer research and modeling, 5, no. 6, 981-992 (in russian). petrashen, g.i., g.i. marchuk, and k.i. ogurtsov (1950). on lamb problem for the case of half space, memoir of leningrad state university, mathematical series, issue 21, no. 55, 71-118 (in russian). pride, s. (1994). governing equations for the coupled electromagnetics and acoustics of porous media, phys. rev. b 50, 15678-15696. pride s.r., and s. garambois (2005). electroseismic wave theory of frenkel and more recent developments, j. eng. mech. 131, 898-907. romano, g., m. balasco, a. siniscalchi, a.e. pastoressa, v. lapenna (2018). robust analysis for the characterization of the seismo-electromagnetic signals observed in southern italy, ann. geophys. 61, doi: 10.4401/ag-7811. sorochan, e. a. (1985). handbook of the designer. bases, foundations and underground structures, 480 p. (in russian). surkov, v.v., (1997). the electromagnetic precursor of a seismic wave, geomagn. aeron. 37, 792-796. surkov, v.v., and m. hayakawa (2014). ultra and extremely low frequency electromagnetic fields, springer geophysics. surkov, v.v., and v.a. pilipenko (2015). estimate of ulf electromagnetic noise caused by a fluid flow during seismic or volcano activity, ann. geophys. 58(6), s0655, https://doi.org/10.4401/ag-6767. surkov, v.v., v.a. pilipenko, and a.k. sinha (2018). possible mechanisms of co-seismic electromagnetic effect, acta vadim v. surkov et al. 12 http://link.springer.com/bookseries/10173 http://link.springer.com/bookseries/10173 http://link.springer.com/bookseries/10173 geodaetica et geophysica 53, https://doi.org/10.1007/s40328-018-0211-6 2018. teodorovich, g.i. (1958). the doctrine of the formation of sedimentary rocks, leningrad, 572 p. (in russian). vinogradov, j., m.z. jaafar and m.d. jackson (2010). measurement of streaming potential coupling coefficient in sandstones saturated with natural and artificial brines at high salinity, j. geophys. res., solid earth 115, no. b12, doi:10.1029/2010jb007593. *co r r e s po n d i n g a u t h o r: vadim surkov, pushkov institute of terrestrial magnetism, ionosphere and radio wave propagation, russian academy of sciences, moscow, troitsk, russia and institute of physics of the earth, russian academy of science, moscow, russia; e-mail: surkovvadim@yandex.ru © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved 13 seismoelectric effect in lamb’s problem https://doi.org/10.1007/s40328-018-0211-6%202018 mailto:surkovvadim@yandex.ru s u l l e p r o p r i e t à e l a s t i c h e d e l m a n t e l l o i n t e r n o d e l l a t e r r a p . e . v a l l e introduzione. — l ' i p o t e s i c l i c lo s t r a t o d del m a n t e l l o della t e r r a sia s o s t a n z i a l m e n t e u n i f o r m e e ciie n e l l o s t r a t o la d i s t r i b u z i o n e della t e m p e r a t u r a si avvicini alla d i s t r i b u z i o n e a d i a b a t i c a , è stata r e c e n t e m e n t e a v v a l o r a t a dalle r i c e r c h e di f . b i r c l i f 1 ). q u e s t e r i c e r c h e h a n n o i n o l t r e posto in e v i d e n z a c h e i m a t e r i a l i c h e c o s t i t u i s c o n o lo s t r a t o d, p r e s e n t a n o , a p r e s s i o n e zero, u n ' e l a s t i cità t a n t o e l e v a t a da n o n p o t e r essere i d e n t i f i c a t i con nessuno dei c o m u n i s i l i c a t i . i n p a r t i c o l a r e v i e n e esclusa la p o s s i b i l i t à clic tali materiali possano essere i d e n t i f i c a t i con l ' o l i v i n a . l ' e l a s t i c i t à d e l l o strato d è p e r a l t r o c o n f r o n t a b i l e con la elastic i t à di a l c u n i ossidi c o m e mgo, al>0.\, tio->, la cui densità è i n o l t r e assai p r o s s i m a alla d e n s i t à dello s t r a t o a p r e s s i o n e zero, o t t e n u t a p e r e s t r a p o l a z i o n e dal m o d e l l o a di k . e . b u l l e n . questo f a l l o h a i n d o t t o f . b i r c l i a r i t e n e r e c h e lo s t r a t o d sia c o s t i t u i t o da ossidi di m a g n e s i o , s i l i c i o e f e r r o , in f a s i di a l t a press i o n e , a v e n t i u n a s t r u t t u r a a n a l o g a a q u e l l a degli ossidi s o p r a c i t a t i . n e l l o s t r a t o c si a v r e b b e r o g r a d u a l i c a m b i a m e n t i di f a s e o di m a t e r i a l e , o p p u r e , c o n t e m p o r a n e a m e n t e , di f a s e e m a t e r i a l e . l e r i c e r c h e di b i r c l i si b a s a n o sulla t e o r i a delle d e f o r m a z i o n i finite, sui dati di l a b o r a t o r i o di p . w . b r i d g m a n e sui v a l o r i d e l l e o n d e s i s m i c h e di h . j e l f r e y s ( 4 i . si a m m e t t e poi, in a c c o r d o con tutte le m o d e r n e d i s t r i b u z i o n i della densità n e l l ' i n t e r n o della t e r r a , c h e il c a m p o g r a v i t a z i o n a l e a b b i a , n e l l o s t r a t o d , un v a l o r e p r a t i c a m e n t e c o s t a n t e . l a v a l u t a z i o n e d e l l ' e l a s t i c i t à dello s t r a t o d a pressione zero, v i e n e e f f e t t u a t a da f . b i r c l i m e d i a n t e il c a l c o l o , a tale p ressio ne, del rapp o r t o 4 = v i i r , 3 [ 1 ] e in cui ks è l ' i n c o m p r e s s i b i l i t à a d i a b a t i c a , o la densità, n e t't l e v e l o c i t à d e l l e o n d e s i s m i c h e l o n g i t u d i n a l i e t r a s v e r s a l i , r i s p e t t i v a m e n t e . 3 7 4 p. e. valle tabella i h k m vl km/sec vt km/sec v g / c m 3 1000 1 2 0 0 1400 1 6 0 0 1800 2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0 2 8 0 0 11,42 11,71 11,99 12,26 12,53 12,79 13,03 13,27 13,50 13,64 6,36 6,50 6,62 6,73 6,83 6,93 7,02 7,12 7,21 7,30 4,68 4 , 8 0 4,91 5,03 5,13 5 , 2 4 5,34 5,44 5 , 5 4 5,63 a l l o scopo di o t t e n e r e m a g g i o r i i n f o r m a z i o n i sulle p r o p r i e t à elas t i c h e d e l l o s t r a t o d, i n q u e s t a n o t a , m e d i a n t e la t e o r i a classica dei s o l i d i , si c a l c o l a s e p a r a t a m e n t e i l v a l o r e delle v e l o c i t à d e l l e o n d e sis m i c h e l o n g i t u d i n a l i e t r a s v e r s a l i , a p r e s s i o n e z e r o e a l l a t e m p e r a t u r a della s o m m i t à d e l l o s t r a t o . v e n g o n o a l t r e s ì c a l c o l a t i i v a l o r i di a l c u n i p a r a m e t r i p e r i l c o n f r o n t o con i r i s u l t a t i di f . b i r c h . l a d i s t r i b u z i o n e d e l l a densità c o r r i s p o n d e n t e al m o d e l l o a di k . e . b u l l e n ( 2 3 ) e l e v e l o c i t à d e l l e o n d e s i s m i c h e di h . j e f f r e y s ( 4 ) c o s t i t u i s c o n o i d a t i di p a r t e n z a . questi dati sono c o n t e n u t i n e l l a tavalore del parametro di griineisen alla sommità dello strato d. — l o s t r a t o d si trova m o l t o p r o b a b i l m e n t e a d u n a t e m p e r a t u r a s u p e r i o r e a l l a t e m p e r a t u r a di d e b y e , il c u i v a l o r e m e d i o n e l l o s t r a t o può essere v a l u t a t o c i r c a 1 0 0 0 °c. o r a ad u n a t e m p e r a t u r a v i c i n a ai 1 0 0 0 °c i l c a l o r e specifico degli ossidi a c c e n n a t i n e l p r e c e d e n t e parag r a f o e dei s i l i c a t i è m o l t o p r o s s i m o al v a l o r e classico dei c r i s t a l l i i o n i c i f 1 ). p e r t a n t o l e v e l o c i t à i s o t e r m e delle o n d e e l a s t i c h e longitud i n a l i e t r a s v e r s a l i , sono date r i s p e t t i v a m e n t e d a l l e espressioni (r') b e l l a i . [ 2 ] n e l l e q u a l i l ' i n d i c e 1 c o n t r a s s e g n a i l v a l o r e delle grandezze alla somsulle proprietà elastiche del mantello interno della terra 375 mità dello strato. t a l e sommità verrà assunta, convenzionalmente, alla profondità di 1000 k m . l a velocità isoterma delle onde trasversali coincide con la velocità a d i a b a t i c a , m e n t r e p e r le onde longitudinali risulta « y t j [ 4 ] i n questa relazione a è la dilatazione termica, y il p a r a m e t r o di grùneisen e t la t e m p e r a t u r a assoluta. d a t o c h e « è dell'ordine di io"5 e y è compreso fra 1 e 2, tenuto conto c h e la t e m p e r a t u r a media nello strato d è quasi sicuramente i n f e r i o r e ai 5000 ° k , in prima approssimazione si potrà r i t e n e r e m s — («ili t l a [ 2 ] e la [ 3 ] possono essere scritte nella f o r m a v 0 ò c>t \ e / vi> a qi \ g / sostituendo in queste relazioni i valori di vi, vt e p contenuti n e l l a taliella i, si ottengono due sistemi di equazioni n e l l e incognite y 4 i, y , , , rispettivamente. l a soluzione dei due sistemi, effettuata col metodo dei m i n i m i quadrati, ha fornito yu = 1,421 + 0,006 [ 5 ] y l t = 1,169 + 0,c04 r i c o r d a n d o che il parametro di griineisen è dato da ( 5 ) y — i l l h a l i [ 6 ] 3 risulta yi = 1,253 [ 7 ] valore della densità dello strato d a pressione zero e alla temperatura della sommità dello strato. — questo valore può essere f a c i l 3 7 6 p. e. valle niente ottenuto dall'equazione di stato ( 5 ) p ~ pi = kn nella quale (2 t ) 1 ' 3 3 2 r c l /• i — 21" a (t — t ( ) r = yl.+ ì (vii — y 4 i) + [ 8 ] [ 9 ] 4 i r , , ó t'-j. i n base ai risultati del paragrafo precedente si h a r = 1,600 [ 1 0 ] posto quindi nella [ 8 ] p = 0, pi = 0,392 megabars, come risulta dal modello a di k . e . b u l l e n (2":l), klt2±ktb= 3,579 megabars, t — t i , o = (_•„ , risolvendo si ottiene co = 4,07 g/cms [11] valori delle velocità delle onde elastiche nello strato d a i>ressione zero e alla temperatura della sommità dello strato. — introducendo nella [ 2 ] e nella [ 3 ] i valori di y , i , y j t , v n , vu, £>, e e » , si ottengono le velocità delle onde elastiche, nei m a t e r i a l i c h e costituiscono lo strato d, a pressione zero e alla temperatura della somm i t à dello strato. i n d i c a n d o con t;tìi e v0t queste velocità, risulta voi = 9,67 km/sec vot = 5,59 km/sec i l cocfficienie di poisson a c il rapporto rr0 = 0,25 . il>0 = 51,8 (km/sec)2 r i c o r d a n d o poi che y q = costante, si ha yu = 1,44 [12] sono dati quindi da [ 1 3 ] [ 1 4 ] [ 1 5 ] nella fig. 1 viene mostrato l ' a n d a m e n t o delle velocità delle onde longitudinali e trasversali nello strato d, in funzione della densità. l e curve sono state calcolate m e d i a n t e la [ 2 ] e la [ 3 ] , m e n t r e i punti rappresentano i dati della t a b e l l a 1. sulle proprietà elastiche del mantello interno della terra 3 7 7 f i g . ] v e l o c i t à delle onde sismiche nello strato d del mantello della t e r r a in funzione della densità. 378 p. e. valle la funzione 1 — g"1 d 0 / d r nello strato d. — f . b i r c h h a mostrato che questa funzione, la quale è stata considerata p e r l a p r i m a volta da k . e . b u l l e n (°), può essere posta n e l l a f o r m a (-1) + « y t a + (a y t ) 2 b + a <i> t g~l c [ 1 6 ] dr \ 3p jt nella qtiale t è la differenza t r a il gradiente attuale di temperatura e i l gradiente adiabatico, g il c a m p o gravitazionale e a, b e c sono funzioni di p a r a m e t r i adimensionali. f i g . 2 l a funzione 1 — g~' a<i> 5 t nello strato d del m a n t e l l o della t e r r a . i l t e r m i n e più i m p o r t a n t e a secondo m e m b r o della [ 1 6 ] è i l primo, i l quale, in base alla teoria classica dei solidi, è espresso dalla relazione ( 5 ) 3k, p i r 3 e [ 1 7 ] nella fig. 2 è riportato l ' a n d a m e n t o di (3 kt/dp).r, in funzione della profondità, calcolato con i dati della t a b e l l a i m e d i a n t e la [ 1 7 ] , n e l l a qtiale si è posto 2 t = 3,2. l a figura c o n t i e n e i n o l t r e l'analogo andamento ottenuto da f . b i r c h dalla teoria delle deformazioni finite sulle proprietà elastiche del mantello interno della terra 379 e i v a l o r i di 1 — g'-ai>/ar c a l c o l a t i dallo slesso b i r c l i con l e velocità di h . j e f f r e y s , assumendo g = io3 cm/sec 2 . confronto con i risultati di f. birch. — i l c o n f r o n t o tra i valori di vari p a r a m e t r i c a l c o l a t i da f . b i r c h e q u e l l i o t t e n u t i nei p r e c e d e n t i p a r a g r a f i , v i e n e e f f e t t u a t o n e l l a t a b e l l a i i . l a v e l o c i t à delle o n d e l o n g i t u d i n a l i e trasversali a pressione zero c o n t e n u t e n e l l a t a b e l l a i i , s e b b e n e si r i f e r i s c a n o alla densità c o r r i s p o n d e n t e a l l a t e m p e r a t u r a della s o m m i t à d e l l o strato, n o n si r i s c o n t r a n o in nessuno dei c o m u n i s i l i c a t i (7"8). q u e s t o risultato c o n v a l i d a q u e l l o o t t e n u t o p r e c e d e n t e m e n t e da f . b i r c h m e d i a n t e la t e o r i a delle d e f o r m a z i o n i finite. p e r a l t r o i l v a l o r e r e g o l a r e del coefficiente di p o i s s o n trovato a p r e s s i o n e z e r o , m o s t r a c h e i m a t e r i a l i c h e c o s t i t u i s c o n o l o s t r a t o d non p r e s e n t a n o , a t a l e pressione, a l c u n a p r o p r i e t à e l a s t i c a e c c e z i o n a l e . r i m a n e p e r ò a p e r t a la q u e s t i o n e della c o m p o s i z i o n e e s t r u t t u r a dello s t r a t o z), s e b b e n e il s u g g e r i m e n t o di f . b i r c h c h e esso sia c o s t i t u i t o da ossidi di m a g n e s i o , s i l i c i o e f e r r o in fasi di a l t a pressione, costituisca u n a b a s e di r i c e r c a . tabella i i parametri relativi allo strato d a pressione zero * p a r a m e t r o e unità t e o r i a dei solidi e modello a t e o r i a delle d e f o r m a z i o n i finite ( b i r c h ) <!>„ ( k m / s e c ) 2 51.8 51 0 o ( g / c m 3 ) 4,07 4 ( m o d e l l o a) b u l k m o d u l u s ( m e g a b a r s ) 2,11 2,04 y„ ( a d i m e n s i o n a l e ) 1,44 1,47 13 ^ t d p't ( a d i m e n s i o n a l e ) 4,01 4,00 vnl ( k m / s e c ) 9,67 — vol ( k m / s e c ) 5,59 — c n ( a d i m e n s i o n a l e ) 0,25 — (in ( m e g a b a r s ) 1,27 — e o ( m e g a b a r s ) 3,17 — roma — istituto nazionale di geofisica — settembre 1953. ( * ) i valori ottenuti m e d i a n t e la teoria dei solidi si r i f e r i s c o n o alla t e m p e r a t u r a della s o m m i t à dello strato, m e n t r e i valori ottenuti da b i r c h m e d i a n t e la t e o r i a delle d e f o r m a z i o n i finite si r i f e r i s c o n o alla t e m p e r a t u r a m e d i a dello strato. 3 8 0 p . e . valle riassunto mediante la teoria classica dei solidi vengono calcolate le velocità delle onde elastiche, nello strato d dell'interno della terra, a ]>ressione zero e alla temperatura della sommità dello strato. si è ottenuta una velocità pari a 9,67 km/sec per le onde longitudinali e pari a 5,59 km/sec per le onde trasversali. nessuno dei comuni silicati presenta velocità così elevate. questo risultato è in eccellente accordo con le ricercha effettuate da f. birch. summary the velocities of elastic ivaves in the d layer of the earth at zero pressure and at the temperature of the top of the layer are calculated hy means of the classical theory of solids. these velocities are found to he 9,67 km/sec for longitudinal ivaves and 5,59 km/sec for shear ivaves. none of the experimental values of nave velocities in common silicates satisfies the requirements for layer d. this results are in excellent agreement with the results obtained hy f. birch. b i b l i o g r a f i a ( ! ) birch f _ elasticity and costiliilion of earth's interior. j . geopliys. res. 57, 227-286 (1952). ( 2 ) bullen k . e., the problem of the earth's density variations. bull. seism. soc. airi. 32, 19-29 (1942). ( 3 ) bullen k . e., introdnction to the theory of seismology. cambridge, university press (1947). ( 4 ) jeffreys h „ the time of p, s and s k s and the velocities of p and s. geopliys. suppl. 4, 498-533 (1939). ( 3 ) valle p . e., una equazione di stato per i solidi. annali di geofìsica, 6, 183-197 (1953). ( 6 ) bullen k . e., compressibility pressure hipotesis and the earth's interior. geophys. suppl. 5, 355-368 (1949). c) birch f., handbook of physical constants. geol. soc. ani. special papers (1950). ( s ) gutenberg b . (editor), internai constitulion of the earth. dover publications (1951). amato_corretto_layout 6 837 the ingvterremoti blog: a new communication tool to improve earthquake information during the po plain seismic sequence maurizio pignone1, concetta nostro1, alessandro amato1,*, emanuele casarotti2, claudia piromallo2 1 istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, italy 2 istituto nazionale di geofisica e vulcanologia, sezione roma 1, roma, italy annals of geophysics, 55, 4, 2012; doi: 10.4401/ag-6179 1. introduction the ingv sites that deliver information in quasi-realtime are well known. people often connect to the ingv home page (www.ingv.it), to the centro nazionale terremoti (national earthquake centre) page (http://cnt.rm.ingv.it) and to the italian seismic instrumental and parametric (iside) database (http://iside.rm.ingv.it) to obtain information about recent earthquakes. moreover, people look for more specific and detailed information on the historical earthquake catalog (http://cpti11.mi.ingv.it), the seismic hazard web pages (http://zonesismiche.mi.ingv.it), the ingvterremoti youtube channel (http://youtube.com/ ingvterremoti), the web-based macroseismic survey (http:// www.haisentitoilterremoto.it/) and others. for all of these sites, some of which are reached by hundreds of thousands, or even millions people in cases of strong earthquakes, people can find a lot of specific information on individual earthquakes, on the seismic history, and so on. however, a place where people could find updates and explanations on the ongoing activity was not available. when the po plain seismic sequence started on may 20, 2012, through the enormous number of hits on the website of the ingv, to the many phone calls, emails and tweets, we soon realized that the request for information was huge. there were 0.61 and 0.93 million visits and 12 million and 26 million accesses on may 20 and 29, 2012, respectively. this was not a surprise, of course, because also during the l'aquila 2009 sequence there was a similar situation. indeed, in the months after the april 6, 2009, event, also through the installation of the centro operativo emergenza sismica (seismic emergency operational centre) in l'aquila [moretti et al. 2011], we understood that the request from the public and local authorities for complete and authoritative information was higher than had been previously thought. in particular, in the 2009 emergency, and even more so in the 2012 emergency, as well as classical information on web sites, many people looked for information and updates on social network sites. for this reason, between 2009 and 2011, the ingv started to test different social media, such as youtube, twitter, facebook, and developed an application for the iphone, to release earthquake information. social media have proven to be very important for information sharing during crises [earle et al. 2011, bruns et al. 2012]. for all of these media, we observed relevant increases in the number of views and downloads corresponding to the important seismic events, when the attention was high [amato et al. 2012, nostro et al. 2012]. for this reason, in the days after the may 20, 2012, mainshock, we decided to open a new blog to provide quick updates and in-depth scientific information, such as articles on the ongoing seismic activity. providing timely information is particularly important when seismic sequences last for several weeks and are characterized by several m >5 events, as was the case of the po plain earthquakes. at the same time, we worked to provide fast, but scientifically sound, information, which was constantly updated and distributed throughout the territory, also to counter the bad information, and to fight rumors. 2. the ingvterremoti blog the blog was created using the well-known wordpress platform (http://ingvterremoti.wordpress.com). wordpress.com is a free hosting platform that makes it easy for anyone to publish online. it is not necessary to download software or manage a web server. wordpress.com has hundreds of themes, that include many for functionality and for widgets. for our blog, we chose the 'mystique' theme, a flexible theme that offers layout choices, many widgets, and support for social media. it was opened on may 21, 2012, initially for internal testing, and then open to the public on may 29, 2012. we decided not to allow comments on the blog, due to the huge amount of time needed to answer article history received july 31, 2012; accepted september 7, 2012. subject classification: seismology, educational, public issues, earthquakes, seismic awareness, seismic hazard. 2012 emilia earthquakes such questions and comments, which was not yet feasible for our present organization. it is our wish to open this section in the future. the blog contains both static and dynamic pages, the latter being the predominant part. the static section includes pages about the ingv (chi siamo/who we are), earthquakes in italy (i terremoti in italia), seismic monitoring (il monitoraggio sismico), and activities in an emergency (attività in emergenza). figure 1 shows the top part of the site, during the busiest day ( june 3, 2012). the official language is italian, as the blog is mostly devoted to people living in the italian region. it is our wish to create an english section in the future. the dynamic section contains mostly posts organized in different categories. for the po plain seismic sequence, we created a category named 'the po plain earthquakes in emilia' (i terremoti della pianura padana emiliana), with updates on the sequence, including, for example, maps, histograms, explanations, and videos. this part is divided into nine sub-categories, as given in table 1. in the two months after the start of the po plain seismic sequence, we published more than 80 posts about the sequence, to describe different activities carried out during the emergency, and the first scientific results obtained from preliminary data processing. as soon as better results were made available we promptly published in-depth articles. moreover, in this time interval, some relevant earthquakes hit other regions of italy, such as the may 28, 2012, ml 4.3 seismic event close to pollino (calabria), the june 6, ml 4.5 seismic event pignone et al. 838 figure 1. the top part of the ingvterremoti blog as it appeared on june 3, 2012, soon after the most clicked update was posted (857,785 clicks). 839 offshore of ravenna (emilia romagna), and another one on june 9, 2012, along the southern front of the alps, between belluno and pordenone (veneto). therefore, we decided to provide information about italian seismicity by opening new categories in the ingvterremoti blog (besides the one on the po plain sequence described above), such as: 'the pollino sequence' (la sequenza nel pollino), 'the italian seismicity' (sismicità italia), 'information' (informazione), and 'videos' (video). in the framework of the blog, we added a series of widgets that wordpress provides: search, calendar, link, categories, archives, recent posts, top posts, and pages, top clicks, twitter, flickr, tags, follow blog, rss. at the top of the home page of the blog, we added the 'social icons' relative to the ingvterremoti youtube channel, the flickr ingv archive, the ingvterremoti twitter feeds, and the rss of the blog. through the follow blog widget, which is a function that allows people to receive an e-mail as soon as a new posts is published, we had more than 1,000 followers (updated to september 1, 2012), which contributed to the disseminate of the information. 3. statistics soon after being published, the blog was found on the web by people looking for information about the po plain seismic sequence, and it was viewed by several thousand people. in the first two months (may 29 to july 29, 2012), we recorded more than six million contacts (6,403,843), most of which were concentrated in the first 15 days, with highs and lows in response to the relevant seismic events both for the po plain and for elsewhere in italy. on may 29, 2012, the first day of publication on line, the remarkable reactivation of the sequence with three earthquakes of magnitudes >5.0 in the morning led the number of contacts to over 200,000 for a few hours after the opening, even before the blog was linked to the ingv home page (http://www.ingv.it). in the following days, the number of visits increased continuously, also due to the link from the ingv home page. the peak in the number of visits, which was more than 857,000 in a single day, was registered on june 3, 2012, following a ml 5.1 earthquake in the po plain region. it is interesting to note that from june 2 (an italian holiday) to june 3, 2012, there was an increase of more than 700,000 contacts. since june 3, 2012, the number of contacts has been decreasing almost constantly, except for some relevant earthquakes that occurred in other regions of italy, as for the june 6 and june 9, 2012, seismic events (figure 2). from may 29 to june 29, 2012, the blog had a total of 5,700,000 visits, and the average daily number of contacts the ingvterremoti blog figure 2. the number of daily contacts during the first month (may 29 to june 29, 2012). stars: the may 31, 2012, ml 4.2 po plain event; the june 3, 2012, ml 5.1 po plain event; the june 12, 2012, ml 4.3 po plain event; the june 6, 2012, ml 4.5 event offshore of ravenna; and the june 9, 2012, ml 4.5 event along the alpine southern front, between belluno and pordenone. the po plain earthquake sub-categories (numbers of posts) the po plain earthquake sub-categories, in italian update on seismicity (52) aggiornamento sismicità animations (4) animazioni scientific deepening (2) approfondimento scientifico deformation (6) deformazione seismic hazard (1) pericolosità sismica mobile seismic network (1) rete sismica mobile geological surveys (5) rilievi geologici macroseismic surveys (4) rilievi macrosismici earthquake history (1) storia sismica table 1. the nine subcategories of 'the po plain earthquakes in emilia' (i terremoti della pianura padana emiliana) ingvterremoti blog. was about 190,000. during the second month ( june 30 to july 29, 2012) the blog had about 670,000 contacts, with the average number of daily visits of about 22,000. the peak in the number of visits, as more than 100,000 in a single day, was registered on july 9, 2012, following a ml 3.5 earthquake in the alban hills region, which was felt in rome. this huge difference of contacts between the first and the second month was due to the trend of seismicity in the po plain, which had a strong decrease in both the daily number of earthquakes and their magnitude. this trend in the number of daily visits during the two months of the blog shows the enormous demand for information as soon as an earthquake is felt by the population. looking at the statistics (figure 3a) of one of the static pages, 'seismic monitoring' (il monitoraggio sismico), during the first two months (may 29 to july 29), it is easy to note that a static page has a long life, even if it has some highs and lows and some peaks. the number of daily contacts of the post published soon after the june 3, 2012, ml 5.1 earthquake in the po plain region (figure 3b) had a peak in the number of daily visits (more than 857,000) and decreased very rapidly after a few days after its publication, because people lose interest in the news. looking at the statistics of other posts relating to some scientific insights (figure 3c), namely the video on 'the po plain buried faults' (le faglie sepolte in pianura padana), we pignone et al. 840 figure 3. (a) the number of daily contacts of the static page 'seismic monitoring' (il monitoraggio sismico), during the first two months (may 29 to july 29, 2012). (b) the number of daily contacts of the post published soon after the june 3, 2012, ml 5.1 earthquake in the po plain region. on that day, there was the peak in the number of visits, which was more than 857,000. (c) the number of daily contacts of the post 'buried faults' (le faglie sepolte), which was published on june 8, 2012. this post represents in-depth news. 841 note that when an earthquake happens, people seek both information about it and also in-depth news. indeed, since that post was published ( june 8, 2012) it has been seen by over 14,000 people: most of the readings are relative to a few days after it was posted, but there were also some readings on the days between july 7 and 17, 2012, with a peak on july 9, 2012, the day of the earthquake in the alban hills. the video on the buried faults has been viewed more than 60,000 times on the youtube channel. it is of note that 95% of the contacts are from italy, although there are 5% from foreign countries. the first three countries are: uk, germany and the usa. we note that most of the visits occurred through the site www.ingv.it (58%), and the remaining part from other sites, such as institutional sites, search engines, and social media. today, the blog has on average between 10,000 and 15,000 contacts every day. 4. concluding remarks this report emphasizes both the pressing request for qualified and comprehensible information by the general public, especially during the most critical stages of a long seismic sequence, and the quick responses by the ingv to such requests immediately after the may 20, 2012, earthquake in the po plain, as the research institute that is responsible for seismic monitoring of the italian territory. we believe that the availability of continuous and timely information is fundamental for the population living through an earthquake experience, as well as for the general public, and for this reason, this represents a crucial task for our institute. at the same time, scientifically correct information, that is fast and constantly updated, and is distributed throughout the territory can counter bad information, and can also fight rumors. indeed, according to crescimbene et al. [2012], to fight rumors and to disseminate accurate scientific information, it is essential to go in two directions: there must be the promotion of quality science journalism, and scientists must be trained to engage in communication and dissemination of scientific information. we are trying to work in this direction. the experience during the po plain sequence showed us that as well as the classical information released through our web sites, the blog structure is suitable for fast, constantly updated, and scientifically correct information. indeed, it allowed us easier and quicker publication of updates and results, and the use of a simpler language of communication. although we had to close the comments section due to the impossibility of answering the questions and requests during the emergency, we received many e-mails and interesting feedback, thanks to which we have modified and improved the blog. because of the innovative features of the blog, and because of its success that can be seen by the statistics on the number of accesses to its pages, the blog can be considered one of the first interactive tools of mass communication in italy between a great institute of scientific research and the general public. this blog not only provides timely updates of italian seismicity, but it also includes reports of ongoing seismic activity, and disseminates the basics of seismology, of which there is also a great demand from the public, as shown by the high number of contacts with the static pages of the blog. we also think the blog has had a positive effect on some media (newspapers, magazines, broadcasters and news agencies), whereby having qualified information directly from researchers of the ingv avoided inaccuracies due to the transfer chain of information (from source to end user, through the medium of the press agencies), and approximations and inaccuracies that can arise due to the distorted interpretation of hastily granted and gathered interviews. further research might include an in-depth analysis of audience behavior: to assess the effectiveness of a blog, the number of contacts is surely the main indicator, but nevertheless, other indicators can be taken into account (e.g., navigation strategies: how do user find/access the blog; how many posts do they read during each session; how do they move inside the blog; where do users come from: considering the area they come from – not only the country – to understand if the blog reaches a broader audience or if it is limited to the population shaken by the earthquakes; page views; time spent on each post; platform used to access the blog: computer vs. mobile). moreover, users can also be invited to respond to an ad-hoc survey to monitor their satisfaction and their further needs. both strategies can also provide useful material to further improve this blog. a broader research project might also include crossmedia comparisons, taking into account the whole set of social media used by the ingv (youtube, twitter, facebook, flickr, apple). future developments include the creation of new categories and new articles for other regions of italy, and in-depth scientific articles to explain the results of research activities. we also wish to increase the faq (frequently asked questions) section, and to add an english version of the blog. we also plan to find a way to implement the interaction with the public, through answering questions and comments. this blog was designed and published immediately after the earthquake of may 20, 2012, and it is an innovative tool to provide information to people, not only during a seismic sequence, but also 'in times of peace', thus contributing to the work of prevention, which remains the best defense against earthquakes and the only way to reduce their effects. acknowledgements. we thank all our colleagues who have contributed to this initiative, and who have written articles and provided scientific material. thanks to giuliana d'addezio, franco foresta martin and an anonymous reviewer for comments that improved this manuscript. the ingvterremoti blog references amato, a., l. arcoraci, e. casarotti, r. di stefano and the ingvterremoti team (2012). the ingvterremoti channel on youtube. annals of geophysics, 55 (3), 403-408; doi:10.4401/ag5546. bruns, a., j. burgess, k. crawford and f. shaw (2012). #qldfloods and @qpsmedia: crisis communication on twitter in the 2011 south-east queensland floods, brisbane, arc centre of excellence for creative industries and innovation; http://cci.edu.au/floodsreport.pdf crescimbene, m., f. la longa and t. lanza (2012). the science of rumors, annals of geophysics, 55 (3), 421-425; doi:10.4401/ag-5538. earle, p.s., d.c. bowden and m. guy (2011). twitter earthquake detection: earthquake monitoring in a social world, annals of geophysics, 54 (6), 708-715; doi:10.4401/ ag-5364. moretti, m., c. nostro, a. govoni, m. pignone, f. la longa, m. crescimbene and g. selvaggi (2011). l'intervento del centro operativo emergenza sismica in occasione del terremoto del 2009 a l'aquila, quaderni di geofisica, 92, ingv, 29 pp. nostro, c., a. amato, g. cultrera, l. margheriti, g. selvaggi, l. arcoraci, e. casarotti, r. di stefano, s. cerrato and the may 11 team (2012). turning the rumor of the may 11, 2011, earthquake prediction in rome, italy, into an information day on earthquake hazard, annals of geophysics, 55 (3), 413-420; doi:10.4401/ag5559. *corresponding author: alessandro amato, istituto nazionale di geofisica e vulcanologia, centro nazionale terremoti, roma, italy; email: alessandro.amato@ingv.it. © 2012 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. pignone et al. 842 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 foreword since the origin of humanity volcanism and human life have been strictly linked to each other. despite the hazards posed by volcanoes, humans have always found good reasons for settlement and development around them, at least in temperate zones, because of high soil fertility or for the presence of ore deposits and the abundance of volcanic rocks that are good building materials. evidence from archaeological excavations demonstrates that volcanic and related phenomena often have strongly conditioned human life, causing environmental changes, forcing people to abandon their settlements, and preparing the conditions for later recolonization and soil exploitation during phases of quiescence. the mediterranean region is one of the most impressive examples of this interaction, where the development of civilization has been repeatedly boosted and hindered, throughout prehistory and history. nowadays, as demonstrated by the 2010 eyjafjallajökull eruption (iceland), the impact of even moderate scale eruptions is amplified by the increasing vulnerability of modern society related to growing population, rising standard of living, settlement and industrialization of very exposed regions, and complex interdependencies in commerce, including transport and trade systems at a global scale. this special volume of annals of geophysics originates from the workshop explosive eruptions and the mediterranean civilization through prehistory and history, held in the island of ustica from 12 to 16 september 20171. its main goal was to promote cultural exchange and interaction among diverse disciplines, so to enhance our knowledge of the relationships between volcanism, environment and human communities, and exhibit and spreading the best practice of scientific culture dissemination about explosive volcanism. consistently with these intentions, the contents of the contributions presented at the workshop, most of which are included in this volume, address the following main topics: characteristics of explosive volcanic activity in the mediterranean basin; petrochemical and geochronological studies on the products of the eruptions; paleo-environmental reconstructions of ancient eruptions with evaluation of their impact on nature and human settlements; search for indicators of vulnerability and risks associated with volcanic activity, with particular attention to densely populated or touristic areas; characterization of volcanic rocks used as lithic tools and building materials in prehistoric and historical times, with archaeometric methods. we hope that the fruitful interdisciplinary debate developed during the workshop, with the participation of geologists, geophysicists, engineers, archaeologists and experts in scientific communication, can continue and extend thanks to the publication of these contributions. maria luisa carapezza sandro de vita franco foresta martin 1the abstracts of all the contributions presented at the workshop have been published in miscellanea ingv, no. 37, 2017, pp. 90. available online at: http://istituto.ingv.it/it/le-collane-editoriali-ingv/miscellanea-ingv.html. annals of geophysics, 62, 1, vo15, 2019; doi: 10.4401/ag-8320 angelini_corrected:layout 6 annals of geophysics, 57, 2, 2014, a0218; doi:10.4401/ag-6408 a0218 some remarks about lidar data preprocessing and different implementations of the gradient method for determining the aerosol layers federico angelini1,2,*, gian paolo gobbi2 1 enea, laboratorio diagnostiche e metrologia (upatprad-dim), frascati (rome), italy 2 cnr, istituto di scienze dell’atmosfera e del clima (isac), rome, italy abstract the determination of atmospheric aerosol layers from lidar returns is possible through automated algorithms. this product is useful, for example, in monitoring the boundary layer height (blh) as well as volcanic plumes. aerosol layers are usually detected using the gradient method, i.e. by finding the inflection points of the range-corrected backscattered signal. these points can be either calculated as the minima of the numerical derivative of the signal, or by zero-order digital wavelet transforms. since for low signal-to-noise ratios the numerical derivative is very prone to noise-induced fluctuations, a moving average is often performed. we demonstrate why this procedure should be avoided in the gradient calculation. finally, an alternative approach to the digital wavelet transform is proposed, giving the same results but lowering the computational times by about one order of magnitude. 1. introduction during the last years both research lidar and ceilometers networks have been set up worldwide [flentje et al. 2010b] to monitor the vertical structure of aerosol and clouds. more than 1000 ceilometers have been operating in europe in 2012, producing a very huge amount of data. although the raw signal contour plot for visual inspection is an important output of these systems, some quantities are calculated by automated algorithms (cloud bottom, cloud coverage). among them, a very important quantity is the boundary layer height (blh), often also called mixing layer height or mixing height, used as a first approximation of atmospheric dispersion capability. on the other hand, the retrieval of the blh is not straightforward since there are not direct methods for its measurement. a thorough discussion about the problems related to the determination of the blh can be found in seibert et al. [2000]. elastic lidars allow the determination of the blh under some assumptions that will be discussed later on. at the basis of this method, however, there is the assumption that the aerosol is produced at ground and dispersed by turbulence within the pbl. the blh is one of the most important quantities related to the atmospheric boundary layer, having many theoretical and practical applications. this quantity, in fact, is important in air pollution modelling, in aviation applications and evaluation of model performances. in spite of its importance, the definition of mlh is not straightforward. according to the cost710 action final report, “the practical determination of the mlh, and sometimes even its definition, is not trivial, and there are many associated practical and theoretical problems”. for our purposes, the following definition, from seibert et al. [2000] may be assumed: “the mixing height is the height of the layer adjacent to the ground over which pollutants or any constituents emitted within this layer or entrained into it become vertically dispersed by convection or mechanical turbulence within a time scale of about an hour”. because of the importance of the blh, many parameterization have been developed and implemented in numerical weather prediction models [arya 1981]. in fact, the turbulence in the pbl is a sub-grid process that cannot be explicitly resolved by the nwp, and must be somehow parameterized. the experimental determination of the blh is then important for the evaluation of the performances of the different parameterization employed in the numerical weather forecast models [bei et al. 2010, hu et al. 2010]. there are a few ways to identify the mixing layer article history received september 20, 2013; accepted march 25, 2014. subject classification: data processing, volcano monitoring, volcanic effects, atmosphere: composition and structure, instruments and techniques. from aerosol elastic backscatter signals. some employ the temporal variance of the signal [hennemuth and lammert 2006] or a threshold on the backscatter coefficient [melfi 1985], but the most used criterion is to search for inflection points in the backscatter coefficient profile. this method is referred to as the gradient method, proposed initially by endlich et al. [1979], and since then widely used to detect the mlh. in fact, the other methods are not suitable for automated detection by low power systems. moreover, while the variance method performs well under convective regimes, the detection of stable layers becomes hard, as witnessed by the fact that most works intentionally refer only to the unstable boundary layer [hennemuth and lammert 2006, lammert and bosenberg 2006, pal et al. 2010]. as a consequence, the gradient method is the most used for mlh detection, and much effort is done to make it more efficient. using the gradient method is also possible to infer the thickness of the entrainment zone [flamant et al. 1997]. unfortunately, however, the main drawback of this method is given by the possible presence of multiple aerosol layers, since strong gradients are found at the top of each layer. recent works [haeffelin et al. 2011] have demonstrated, through an in depth comparison among different algorithms, that the major uncertainty comes from the attribution of the blh to a layer than the determination of the layers itself. the best way to automatically attribute the blh to the right layer is still under investigation [angelini et al. 2009, di giuseppe et al. 2011, haeffelin et al. 2011, pal et al. 2013]. it is worth to sketch quickly that the results of the gradient method differ from a system to another also depending on the wavelength employed. in fact, the relative contribution of aerosols and molecules strongly depend on the laser wavelength, as discussed for example in haeffelin et al. [2011]. in any case, the importance of the detection of all the aerosol layers is a key step in every gradient-based algorithm and should then be performed at better and in the most efficient way. furthermore, the determination of aerosol layers is also useful in monitoring volcanic plumes, although because of their sporadic nature such observations are still less systematic than estimations of the blh. nevertheless, the development of ceilometer networks worldwide can make this technique very powerful in monitoring the status of volcanoes [sassen et al. 2007, flentje et al. 2010a]. 2. discussion of method and results strictly speaking, the gradient analysis should be performed on the backscatter coefficient, which represents the volume backscatter cross section of the total aerosol amount. however, this requires a calibration of the signal over the attenuated molecular backscattering profile. this is not convenient for automated retrievals and, moreover, the low power of the ceilometers coupled to the typical use of wavelengths in the infrared region does not allow this easily. anyway, the logarithm of the range-corrected signal (rcs) is a good alternative, being roughly proportional to the total backscatter coefficient, apart from the attenuation due to extinction. the lidar equation can be written in the form: where b represents the background signal and c the lidar constant, which takes into account a variety of factors (power of the emitted pulses, efficiency of the collection and detection system), and bm, ba represent the backscatter coefficient of molecules and aerosols, respectively, and t the atmospheric transmissivity given by: , being a the extinction coefficient, and where i represents either the molecules or the aerosols. the term o(z) should be introduced to take into account the fact that the overlap between the lidar beam and the telescope field of view is not usually uniform in space, but is instead a function of the distance. o(z) is typically zero in the close field, and reaches 1 at long distances when the laser enters completely in the receiver’s field of view, and the full overlap is achieved. this means that almost all lidars are blind at very short distances (from tens to hundreds of meters), and hence the detection of very low layers is sometimes impossible. this represents often an insurmountable problem when shallow pbls have to be detected, like during nighttime and under very stable atmospheric conditions in general. anyway, these problems lie outside the purposes of this paper, and o(z) can be considered as a constant. in practice, we will consider only the lidar equation after the full overlap. the range-corrected signal is usually calculated as: , so that the lidar equation becomes: since tm is a very smooth function, (at 855 nm, for standard atmosphere, it equals 0.987 between ground and 4 km), no influence on gradients is expected and it can be neglected. also ta may be generally neglected. in fact, while the backscatter coefficient may show rapid ( ) ( ) ( ) ( ) ( ) .rcs z c z z t z t zm a m a 2 2b b= +6 @ ( ) ( ) ( ) ( ) ( ) ( ) bs z o z z c z z t z t zm a m a2 2 2b b= + +6 @ 2( ) i r dr z 0 ai ( )t z e= # ( ) ( ) rcs z z s z b 2= angelini and gobbi 2 3 variations in case of superimposed layers, the transmissivity depends on the integral of the extinction coefficient, and usually it varies smoothly. since we are concerned in detecting the gradients of the signal, such approach actually works well. as an example, we can consider a typical optical thickness of 0.1 at 870 nm: the transmissivity is 0.82 and hence, neglecting the correction, this leads to an underestimation of about 20% at the top of the layer. moreover, t is a monotonic function and therefore neglecting it cannot introduce large errors in the detection of the inflection points. hence, we can neglect the transmission coefficients, and we can write, assuming an exponential distribution of the molecular density, and if c is expressed in opportune units: where n0 is the loschmidt constant at standard conditions (i.e. the molecular density in air), h the scale height for molecular density (~ 8 km), and represents the rayleigh backscatter volume cross section. for aerosol free atmosphere, ba=0 and the equation above becomes: since ~5 10-33 m2 sr-1 at 855 nm and n0 ~2.7 10 25 m-3, the first addend on the right becomes the of the order of −15, while within the boundary layer z/h spans from 0 to 0.5. hence, if expressed in rcs, the lidar attenuated molecular backscattering profile is approximately a straight line, with a small angular coefficient that makes ln(rcs) a good choice for gradient detection. another interesting feature of such quantity is that the slope of the attenuated molecular backscattering profile does not depend on the wavelength of the system. on the contrary, the aerosol contribution of course does. in presence of aerosols (ba >0 and dependent on wavelength) the rcs is still greater than in the case of pure molecular echo, at least as long as the aerosol extinction can be neglected. though this representation of the lidar equation is not convenient to retrieve the aerosol backscatter and attenuation, it is useful for the application of the gradient method. in spite of the definition, however, it should be noted that the determination of the inflection points by gradient method can be determined in a variety of ways, i.e. the direct numerical differentiation, the digital wavelet transform (dwt) or even through best fitting procedures with gaussian or error functions. for systems having a limited signal-to-noise ratio, the direct differentiation of the lnrcs may introduce unrealistic gradients. for this reason, spatial and temporal averages are introduced to improve the signal-tonoise ratio. 3. signal averaging both temporal and spatial averaging of the lidar signal are widely used to improve the signal-to-noise ratio. the problems related to this procedure is the lost of information of small-scale events, such turbulence or microphysics. orlanski’s classification of atmospheric processes [orlanski 1975] showed a continuous cascade of spatial and temporal scaled from planetary waves to turbulence. the processes that involve the pbl are quoted in table 1. from table 1 it is clear that different averaging will point out some patterns and will completely mask the processes at smaller scales. hence, suitable averaging windows must be carefully exploited to highlight the -( ) ( ),ln ln lnrcs d d n e ca h z 0b v x = + + r ` j d dv x r ( ) ( ) .ln ln lnrcs z d d n z h c0 v x = + r ^ h ` j on the application of gradient method to lidar data process temporal scale spatial scale scale definition nocturnal low-level jet inertial waves 1d 2h 20 200 km meso-b urban effects internal gravity waves 10 h 10 min 2 20 km meso-c deep convection short gravity waves 1h 2 min 0.2 2 km micro-a dust devils thermals wakes 10 1 min 20 200 m micro-b plumes roughness turbulence 1 minute 1 s < 20 m micro-c table 1. scales of atmospheric processes involved in the pbl. adapted from orlanski [1975]. d dv x r processes investigated. the average can be performed either using moving average or a block average. advantages and drawbacks of these approaches will be now discussed. the running average may be employed to improve the signal-to-noise ratio of the lidar data without reducing the resolution. this is true only apparently, since the running average may introduce artifacts such spurious maxima and saturation plateau around peaks (figure 1). of course, the points averaged may be weighted by suitable filters (i.e. binomial, gaussian), but in this case the effectiveness of the smoothing in removing noise lowers considerably. the other procedure to increase the signal-to noise ratio consists in lowering the temporal resolution performing a block average over the profiles. since the standard deviation of the background signal (ssd) can be thought as noise in lidar measurements, to understand the effect of temporal averaging on the spatial standard deviation of the signal between 6 and 7 km has been calculated for several temporal averaging intervals, spanning from 1 to 240 min. it has been found that with a 30 min-average, the ssd falls closer than 10% to its minimum value, and after 60 min, falls below 5%. these results are shown in figure 2. after 200 min the ssd raises slightly, likely because of changes in the overall status of the atmosphere. in the spatial dimension, the signal is often smoothed with range-dependent windows (from 50 to 200 m) and then the direct differentiation is performed on smoothed data. anyway, it is possible to demonstrate that the moving average smoothing is completely not useful for gradient calculation, since the resulting data are as noisy as in the case of no smoothing. to demonstrate that, let us consider a function f (k) the ±n-point window centered around the k-th point of the discrete domain of f. the smoothed function f ’ is obtained as an average over the considered window: at the point k+1, the smoothed function is obtained as: hence, the difference f ’(k) − f (k+1) becomes: the gradient is then expressed by the difference of two points, far 2n+1 points one from the other. the first effect of this operation is that only one point is used for the gradient calculation, strongly limiting the effectiveness of the aveaging. furthermore, the points used lay as far as the window width, causing the values of the gradient to become meaningless. this is visible in figure 3, where the red solid curve shows the gradient of the function smoothed over a moving average, compared to the dwt and a third algorithm that will be described hereafter. 4. wavelet algorithms algorithms based on the use wavelets are used to overcome this problem, being them especially effective for noisy data. these methods employ the convolution ( ) .n f k j2 1 1 1j n n + + +=-f =( )k 1+’ / ( ) ( 1) ... ( ) ( 1 ) ... ( ) ( 1 ) ( ) ( ) . n f k n f k n f k n f k n f k n f k n n f k n f k n 2 1 1 2 1 1 1 1 + + + + + + + + + + = + + + = ^ ^ h h ( ) .n f k j2 1 1 j n n + +=-f =( )k’ / angelini and gobbi 4 figure 1. the effect of running average on close peaks. around 5 a spurious peak (‘ghost’) appears although the original signal showed a minimum in this zone. these effect may affect both temporal and spatial averages, for example when small clouds are detected. figure 2. effects of time averaging on the noise. at 30 (60) min. the 90% (95%) of the lowest standard deviation on the signal is found. the ssd is the average of seven different profiles obtained throughout the day, for different days. 5 product of the function and a haar wavelet (also known as d2, the 0-th order daubechies wavelet) and have been abundantly described in the literature [brooks 2003, de haij et al. 2007, morille et al. 2007]. anyway a short description of the wct will be given. the haar wavelet is defined by the equation: it is characterized by a step-like appearance (figure 4), and the convolution of such function with the signal shows maxima in correspondence of inflection points. the advantage of this technique is that, being based on an integral rather than a derivative, it is less sensitive to noise, and hence it is widely used to analyze low power lidar profiles. the dilation of the wavelet (i.e. the size of its non null part) can be varied within the characteristic scales of the signal variations, so that the average of these multiple wavelets allow the so-called “multi scale approach” which is useful for better discriminate the noise-induced inflection points. the dilation of the specific algorithm implemented spans from 90 to 360 m. the main drawback of this method is that no determination is possible in that region closer to the ground than the half dilation of the largest wavelet. for our algorithm, in particular, no layer detection is possible below 180 m, which is definitely too high for most nocturnal layers. anyway, it must be noted that this problem usually arises in parallel to the lack of overlap in the close field and in many cases it does not extend really the blind zone of a lidar. the dwt is then compared with a threshold, and all the maxima exceeding this threshold are stored. the same is done for the numerical derivative. in order to take into account a temporal consistency of the layers, three consecutive profiles are grouped (representing hence a lapse of time of 45 min), all their inflection points are joined together and sorted by increasing height. then, the consecutive points whose distance exceeds a season-dependent threshold are considered belonging to different layers. such clustering allows to detect the mean height and the thickness of each layer. this latter is intended as twice the standard deviation of a k 01 1-, ( ) , , h k a a a k otherwise 1 1 0 02 $= z [ \ ]] ]] on the application of gradient method to lidar data figure 3. upper panel: signal (green) and smoothed signals using different window sizes. bottom panel: the gradient of the signal as calculated bt dwt, derivative on moving average and derivative on block average. in all cases, the multi-scale approach was the same, spanning from 9 (15) to 24 points of dilation with an increment of 3 each time. each resulting curve is the average of the six profiles obtained at different dilation, as explained for example in de haij et al. [2007]. the vertical lines represent the top of the aerosol layers detected when the functions in the bottom panel exceed a threshold at its maxima. the threshold in this case is set at 0.05. figure 4. the effect of the block average over the windows w1 and w2 and the dwt on the function f. the heights of the points belonging to the layer. since clusters containing only one point are excluded, this method lets one exclude some isolated spurious layers. the average values of the dwt or the first derivative maxima relative to all the points of each layer are also computed, since those may be included among the parameters to be considered for the attribution. anyway, the computation of the dwt is quite time consuming, since the product of the wavelet for the function (even though only the window of finite values of the wavelet can be considered) must be calculated for all the points of the function. this requires a big amount of operations, although this is not usually prohibitive for modern computers. however, if a lot of profiles must be analyzed and a multiscale approach is adopted, a quicker algorithm is certainly desirable. here we will illustrate a different algorithm to calculate the gradient, which is as robust as the dwt, but decreases the computation time of about one order of magnitude. it will be referred to as dba, since it is based on the derivative of box averaged data. let us now consider the same window sizes used for the moving average and the multiscale approach in the dwt. if the function f is now averaged over disjoint windows (the spatial resolution is reduced by a factor of 2n+1), the difference of the function between two contiguous points becomes similar to the wavelet transform. the scheme is presented in figure 4: if the function f is averaged over the two windows w1 and w2 and then the difference is computed, the result is the same of the convolution of the wavelet function h defined with the same dilation. after the calculation of the gradient with the box average, an interpolation is necessary to retrieve the same resolution of the signal. this is also necessary to compare gradients at different dilations. both linear interpolation and nearest neighbor approximation have been used, and the results do not change appreciably. at the end, as usually done with dwt, the average of the different windowed gradients is calculated. this multiscale approach helps refining the calculation of the gradient maxima, and almost the same results of the dwt are obtained. to evaluate the usefulness of the method, the algorithm has been implemented in matlab environment, and the computation times of both techniques have been considered. for the dilations used (15, 18, 21, 24 points) the ratio between the average computation times was about 10. this means that ‘jumping’ the moving window, especially at larger scales, does not worsen the results of the wavelet transform. the computation times of the dwt and the dba for 8 whole days of measurements, consisting in 96 different ceilometers profiles per day have been calculated. the ratio of the mean calculation times is about 10. if the smallest dilations are discarded, this ratio raises again, since the larger the windows the less calculations are required for the block average. 5. conclusions the lidar range-corrected signal (rcs) is widely employed in automated analysis for the detection of the atmospheric boundary layer height (blh), and, more in general, for detection of aerosol layers, including clouds. in this paper it has been demonstrated that ln(rcs) is a convenient quantity for the application of the gradient method, since in absence of aerosol it behaves as a straight line with small slope. the unhelpfulness of moving average has been demonstrated in gradient detection, since the cancellation of most terms makes the derivative extremely noisy and poorly meaningful. finally, an algorithm based on the derivative of block-averaged data (dba) for aerosol layer detection is compared with the classical digital wavelet transform. this algorithm is built starting from the strong analogy between the derivative of block-averaged data and the haar wavelet transform, but allows to save about the 90% of operations when detecting the inflection points: this aspect is helpful in the automated processing of large datasets. the dba algorithm can be applied to other kinds of analysis, wherever saving computational time is often mandatory. angelini and gobbi 6 figure 5. computation times, in seconds, for multi scale dwt and dba, using linear interpolation (green) and ‘nearest neighbor approximation’ (red). the ratio between the mean computation times is 10.91 for the first case and 10.56 for the latter. this is dependent on the dilations used, since the time save for larger windows is higher than for small windows. 7 references angelini, f., f. barnaba, t.c. landi, l. caporaso and g.p. gobbi (2009). study of atmospheric aerosols and mixing layer by lidar, 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environ., 34, 10011027. *corresponding author: federico angelini, enea, laboratorio diagnostiche e metrologia (upatprad-dim), frascati (rome), italy; email: federico.angelini@enea.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. on the application of gradient method to lidar data << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice gadirov&eppelbaum_rev_layout 6 annals of geophysics, 58, fast track 3, 2015 1 density-thermal dependence of sedimentary associations calls to reinterpreting detailed gravity surveys vagif g. gadirov1, lev v. eppelbaum2,* 1 oilgas scientific research project institute, baku, azerbaijan 2 tel aviv university, faculty of exact sciences, department of geosciences, tel aviv, israel abstract the modern gravimetric equipment allows to register very small effects of gravity field changes and can be applied solving different geological, geophysical and environmental problems. however, sometimes insufficient calculation of various kinds of geological noise complicates effective application of detailed gravity field analysis. one of such factors is the temperature regime over and outside different buried objects of investigation. in this paper temperature changes in a subvertical zone over a hydrocarbon deposit and outside its contour are analyzed. the integrated density change and corresponding gravity effects are calculated for the muradkhanly oil deposit situated within the south-east part of the middle kura depression (central azerbaijan). calculation of these effects on the basis of densitytemperature data correlation analysis could significantly improve the microgravity field examination over the hydrocarbon deposits. the similar correction procedure may be applied also over underground water horizons and some environmental targets. 1. introduction development of a new modern gravimetric and variometric (gradientometric) equipment (permitting registering earlier inaccessible small anomalies and improving the observation methodology) and creation of new methodologies for gravity data processing and interpretation have triggered an increase of microgravity methodology application in environmental and economic minerals geophysics [eppelbaum 2011]. however many kinds of noise complicate the precise gravity field analysis (the detailed block-scheme of disturbances arising in detailed gravity surveys is presented in figure 1). one of such disturbances is the densitythermal dependence (see “dependence on other physical parameters” in figure 1). let us consider the density-thermal effect on example of gravity field examination over a hydrocarbon deposit. the rocks composing sections of hydrocarbon deposits (hd) are characterized as a rule by inhomogeneity and variety of petrophysical characteristics. it is known that in hd different physical-chemical processes exist which frequently generate subvertical zones with distinct physical characteristics over hd [berezkin et al. 1978, berezkin et al. 1982, gadirov 2009, gadirov and eppelbaum 2012]. such zones usually outline vertical migration of fluid flows from the deposits. registered geophysical anomalies (including gravity and magnetic effects) over hydrocarbon structures are explained by physical properties (for instance, density and magnetization) change and specific distribution of petrophysical properties which were traced for many hundreds of meters over hd occurrences. analysis of numerous geological-geophysical data testifies to relation of some geophysical anomalies with hd; these anomalies did not observe over the “empty” structures. various researchers note those inhomogeneous zones over hd distinct from the surrounding media by their magnetic, density, thermal and other physical characteristics [berezkin et al. 1978, seifullin 1980, foote 1996, maksymchuk et al. 2012]. a presence of specific columnar (subvertical) zone with lowered density over hd is a reason of majority of observed negative gravity effects. we must note that existence of subvertical zone over hd has been discussing from the 70-s of the 20th century till the present, and morphology of these zones is explained from different points of view [e.g., berezkin et al. 1982, roslov et al. 2009, meger’ya 2011]. some similar effects (but more weak) are observed in hydrogeological and geoenvironmental investigations [e.g., somerton 1992, singhal and gupta 2010, eppelbaum et al. 2014]. porosity and density of hd collectors do not change, but hydrocarbons extrude water from the article history received september 27, 2014; accepted november 19, 2014. subject classification: temperature, density, subvertical zone, gravity anomaly. porous space that leads to density decrease in the subvertical zone over hd. at the same time, quantitative estimation of corresponding physical properties changes in this zone has not been conducted. some parameters of magnetic susceptibility and temperature changing in subvertical zone over hd were analyzed in [gadirov 2009, gadirov and eppelbaum 2012]. in this investigation we suggest to utilize temperature data for estimation of rock density changing in subvertical zone over hd. this investigation was carried out for the example of the muradkhanly deposit (description of this deposit was given in eppelbaum and khesin [2012]) situated within the south-east part of the middle kura depression, on the western slope of sabirabad-kurdamir zone of highs (azerbaijan). this deposit was discovered in 1971 when firstly in azerbaijan oil reserves were found in volcanogenic associations of upper jurassic (in interval of 3798-3761 m). this discovering served a foundation for carrying out wide-ranging drilling in this area. in these boreholes numerous measurements of temperature have been performed. the temperature data were carefully studied and the results are employed in this investigation. 2. analysis of temperature effects over hydrocarbon deposit for studying thermal characteristics of geological associations in the mutadkhanly and adjacent areas are constructed graphs of temperature changing versus depth both within deposits and without their contours (figures 2a and 2b, respectively). table 1 shows some gadirov and eppelbaum 2 figure 1. noise affecting microgravity investigations (modified and enlarged after eppelbaum, [2010]). 3 key lithological, density, porosity and thermal data for the muradkhanly area. thermal values as a whole increase with depth due to geothermal gradient, but some quasi-chaotic distribution was also revealed in the area. by other words, in some boreholes at one depth various temperatures were observed, and the same temperatures were observed at different depths. nevertheless linear relationships of temperature versus depth with sufficiently high coefficients of correlation r were obtained (r for the graph presented figure 2a is computed as 0.951 and for the graph shown in figure 2b as 0.927). comparison of the obtained regressions indicates that temperature values versus depth over hd increase more rapidly than without the deposit contour. with depth increase the difference between temperatures observed within the deposit contour and without it is also increase (figure 3). it is known that temperature increase causes volume expansion of physical bodies (including geological associations) that leads to decreasing their density [eppelbaum et al. 2014]. however, values of rock thermal expansion is so small that to detect it in hd areas practically is not possible. deep well sections in the muradkhanly area indicate that oil deposit was discovered at the depth about 3000 m in the arched uplift, and at the depth of 4000 m in the western continuation of the gravity anomalies depend on temperature geological age lithology depth, m vertical thickness, m average density, kg/m3 porosity n, % temperature increment in contour zone of deposit dt, °c coefficient of volume temperature expansion (b ·10−5, 1/°c) * decreasing rock density over deposit calculated using eq. (5), dv, kg/m3 q sandstones, clays, sand 0-400 400 2050 22 5.03 2.9 0.72 q1, n 22 sandstones, sands, siltstones, limestone 400-2000 1200 500 2130 25 13.03 2.9 2.53 pay section n 12 clays, sandstones, siltstones 2000-2200 200 2235 18 14.03 2.7 2.18 n1, n 21 sandstones, clays 2200-2750 500 50 2210 15.9 17.03 2.9 2.52 p–3+n 1 1 limestone, clays 2750-3400 650 2300 16.2 20.03 2.7 2.94 p–2 clays, calcareous sandstones 3400-3900 500 2340 11.2 23.03 2.9 2.89 dvaver = 2.45 kg/m3 table 1. some geological-petrophysical characteristics of rocks composing section of the muradkhanly deposit. * bw = 59 ·10 −5, 1/°c (bw is the volume thermal expansion of water). figure 2. graphs of the temperature dependence on the depth in oil deposit muradkhanly: (a) inside the deposit contour, (b) outside the deposit contour. h is depth, and r is the correlation coefficient. structure. the temperature difference at the depth of 4000 m between the arched uplift and western part consists of about 23° (figure 3). applying equation (1) (see below) and accepting value of volume thermal expansion b as 2.9 · 10−5 1/°c, and density as 2340 kg/m3 (see table 1) we obtain dv ≈ 2 kg/m3. it will be very hard to detect such rock density changes by any geophysical method in the central part of the deposit concerning to the peripheral part (even in the case of their identical stratigraphy and lithology). however, presence of fluids in geological sections sharply changes the physical pattern. for instance, volume thermal expansion of water under temperature of 50-80°c (bw = 59 · 10 −5 1/°c) almost in 20 times exceeds the volume thermal expansion of sandstone. it is known that water in geological section fills porous space in the majority of geological rocks. consequently we cannot consider volume thermal expansion of water and mineral skeleton separately since water expansion in the porous rock space will create corresponding pressure to the solid shell of the rocks. in this case porous pressure will increase and probably a zone of abnormally high layer pressure will be generated. analysis of pressures in boreholes at muradkhanly area has shown that there is not a clear relationship between the pressure distribution and hd location in this area. the abovementioned facts indicate that temperature change in the studied areas is accompanied by volume and density change by constant mass. a relationship between the density and temperature is well-known [e.g., somerton 1992]: where v0 and vt are the targets densities at temperatures 0 and t°c, respectively, and b is the coefficient of volume thermal expansion of targets. bodies with density vt1 at temperature t2 will have density vt2 : based on equation (2) we can determine the difference in body densities depending on temperature changes: or where dv dt is target density change by corresponding temperature change for dt interval, and vt1 is the target density at temperature t1. equation (4) can be applied for a homogenous body. geological rocks occurring in in the depth interval of 0–10 km as a rule consist of solid part, liquid (water) and gaseous phase filling the rock porous space. taking into account that contribution of gaseous phase is usually negligible, let us present changes of solid phase density and water, respectively as: and where dvr dt and dv w dt are the changings (increments) of rock and water density, respectively depending on temperature changing dt; br and bw are the coefficients of volume thermal expansion of rock and water, respectively. the volume occupied by liquid (water) may be estimated from use coefficient of porosity n. then solid phase will occupy (1 − n) part of the volume unit. total density change of solid skeleton and water may be presented as: (7) after substituting equations (5) and (6) to equation (7) we receive: (8) equation (8) enables us to calculate density decrease over hd relative to it peripheral part (outside the deposit contour). we must note that denominator in equation (8) is close to one and we can neglect it. for estimation of the temperature effect the welldvdt = 1 + b dt b dt vt1 . dvdt r = 1 +brdt brdt vt1 r ,. dvdt w = 1 +bwdt bwdt vt1 w,. dvdt =dvdt r 1 nq v +dvdt w n. dvdt = 1 + br +bwq vdt vt1 r br 1 nq v +vt1 w bw n! $dt . vt = v0 1 +bt 1 , vt2 . vt1 1 + b t2 t1q v 1 . vt1 -vt2 = vt1 -vt1 1 + b t2 t1q v 1 = 1 +b t2 t1q v b t2 t1q v vt1 . gadirov and eppelbaum 4 figure 3. graph of changing of temperature versus depth within the contour of muradkhanly deposit. (1) (2) (3) (4) (5) (6) 5 studied muradkhanly deposit (middle kura depression, azerbaijan) was selected. this deposit was comprehensively investigated by numerous drilling operations and geophysical methods and may be considered as polygon for testing different methodologies. the temperature data processed for the muradkhanly area enabled us to estimate the density change in this area. application of equation (8) indicates that decreasing rock density in the contour part of the deposit relative to the peripheral part averagely consists of 2.45 kg/m3. practically the density decrease exceeds this value since besides thermal volume expansion, extending liquid phase also produces some rock expansion. 3. calculation of gravity effect caused by thermal regime peculiarities how this nonsignificant rock density decrease could influence to amplitude and behavior of gravity anomaly? for this aim it is necessary to compute the direct calculation of deficit mass over the oil pool affecting the gravity field. we will develop a model of gravity deficit mass over hd in the subvertical zone with the example of the muradkhanly oil deposit. as was aforementioned the oil deposit in this area was discovered at the depth of 3000 m on the arch and consisted of more than 4000 m on the wings. in the oil-bearing part of the geological structure with a dimension 3 × 4 km the total volume of the deposits consists approximately 42 · 109 m3. in this case the nonsignificant density decrease (−2.45 kg/m3) could cause to deficit of mass in this area as several tens of million tons. in this concrete example the deficit of mass is: m = ov = 42 · 109 · 2.45 kg = 102.9 · 109 kg = = 102.9 · 106 tons ≈ 108 tons. which gravity effect may produce such a mass? we will conduct some simple calculations. let’s assume that the “missing” mass is concentrated in the middle of the geological section as a sphere (figure 4). then the gravity effect from this target may be calculated using known expression: where g is the universal gravity constant, m is the mass, h is the depth of sphere occurrence, and r is the distance from the sphere center to the point of gravity effect calculation. let us compute the gravity effect in points x0 and xl disposed directly over the sphere and over the boundary of deposit, respectively: the difference dx between points x0 and xl consists of 0.122 milligal (1 milligal = 10−5 m/s2). in fact, taking into account that our calculation was applied to “missing mass”, we must use this value with an inverse sign: −0.122 milligal. the distribution of gravity effect due to the geothermal phenomenon between the x0 and xl (xl here was assumed as 2000 m) is presented in figure 5. today when we speak about microgal (1 µgal = 10−8 m/s2) anomalies [eppelbaum 2011], the aforementioned value (122 microgal) may have an important role in gravity field examination in oil and gas bearing areas. performed 3d modeling of gravity field have shown that for real situations the estimations of gravity effects decrease in 1.25-1.4 times, but and for these cases the gravity effects are significant and considerably exceed the errors of the bouguer gravity determination in field conditions. calculation of temperature effect will enable to receive more exact gravity anomalies and to avoid some mistakes by geophysical data analysis not only in hydrocarbon geophysics, but also in hydrogeological, environmental (e.g., karst localization) and other investigations. dg = gm h2 1 = 0.22 mgal 1 mgal = 10-5 m/s2q v dgxl(xl = 1500 m) = gm r3 h = 0.098 mgal g j. dg = gm r3 h , gravity anomalies depend on temperature figure 4. model for computation of gravity effect of “deficient” mass in the subvertical zon over deposit. 4. conclusions it was shown increasing temperature over hydrocarbon deposit only for several degrees causes small decreasing of this zone rocks comparting with it peripheral part (about 2.45 kg/m3). however, even such nonsignificant density changing by sufficient vertical zone thickness (several thousand meters) over hd creates considerable gravity effect several tenth parts of milligal that can play a significant role for detection and localization of hydrocarbon deposits including inefficient and deep-seating ones. thus, the detailed gravity analysis over the hydrocarbon structures, as well as in hydrogeological and environmental studies should be accompanied by thermal data examination. acknowledgements. the authors would like to thank two anonymous reviewers who thoroughly reviewed the manuscript, and which valuable suggestions were very helpful in preparing this paper. references berezkin, v.m., m.a. kirichek and a.a. kunarev (1978). application of geophysical methods for direct searching oil and gas deposits, nedra, moscow (in russian). berezkin, v.m., g.a. bannikova and i.s. mazurova (1982). revealing ring subvertical inhomogeneties by by gravity data by oil and gas searching, applied geophysics, 103, 100-104 (in russian). eppelbaum, l.v. (2010). archaeological geophysics in israel: past, present and future, advances of geosciences, 24, 45-68. eppelbaum, l.v. (2011). review of environmental and geological microgravity applications and feasibility of their implementation at archaeological sites in israel, international journal of geophysics, id 927080, 1-9; doi:10.1155/2011/927080. eppelbaum, l.v., and b.e. khesin (2012). geophysical studies in the caucasus, springer. eppelbaum, l.v., i.m. kutasov and a.n. pilchin (2014). applied geothermics, springer. foote, r.s. (1996). relationship of near-surface magnetic anomalies to oil and gas producing areas, aapg memoir 66: hydrocarbon migration and its near surface expression, 111-126. gadirov, v.g. (2009). results of application of gravitymagnetic surveys for prognosis oil&gas deposits in the kura depression of azerbaijan, geophysics (moscow), 2, 51-56 (in russian). gadirov, v.g., and l.v. eppelbaum (2012). detailed gravity, magnetics successful in exploring azerbaijan onshore areas, oil and gas journal, nov. 5, 110 (11), 60-73. maksymchuk, v., r. kuderavets, i. chobotok and v. tymoschuk (2012). high-resulution surveys for oil and gas fields searching in nw part of the carpathian fore deep, trans. of 2nd intern. conf. alpine-petrol 2012, krakow-poland, 83-84. meger’ya, v.m. (2011). searching and prospecting of hydrocarbon deposits associated with geosoliton degassing of the earth on the basis seismic imaging, geophysics (moscow), 1, 67-74 (in russian). roslov, yu.v., n.n. efimova, a.n. kremlev and a.d. pavlenkin (2009). seismic features of fluid flow and associated with it deposits, geophysics (moscow), 2, 26-30 (in russian). seifullin, r.c. (1980) relationship of thermal and magnetic anomalies over hydrocarbon deposits with natural electric fields, oil&gas geology and geophysics, 8, 42-43 (in russian). singhal, b.b.s., and r.p. gupta (2010). applied hydrogeology of fractured rocks, springer. somerton, w.h. (1992) thermal properties and temperature-related behaviour of rock-fluid systems, elsevier. *corresponding author: lev v. eppelbaum, tel aviv university, faculty of exact sciences, department of geosciences, tel aviv, israel; email: levap@post.tau.ac.il. © 2015 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. gadirov and eppelbaum 6 figure 5. a distribution of gravity effect between the x0 and xl (l = 2000 m). << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy 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s o n o misur a r e con a p p r o p r i a t e a p p a r e c c h i a t u r e . p r o b l e m a f o n d a m e n t a l e n e l l o s t u d i o dei b r a d i s i s m i è q u e l l o d e l l a l o r o m i s u r a . esso a p p a r e p e r ò i n d e t e r m i n a t o , n e l senso c h e i l p i a n o r i s p e t t o al q u a l e d o v r e b b e r o essere r i f e r i t i gli s p o s t a m e n t i v e r t i c a l i , c i o è i l l i v e l l o m e d i o m a r i n o v a r i a anch'esso col t e m p o . l e m i s u r e ad esso r i f e r i t e n o n p o s s o n o d a r c i , g e n e r a l m e n t e , n e s s u n a i d e a sull ' e f f e t t i v o s p o s t a m e n t o v e r t i c a l e d e l c a p o s a l d o . s e u n p u n t o d e l l a costa m a n t i e n e p e r u n l u n g o i n t e r v a l l o di t e m p o l a stessa q u o t a r i s p e t t o ai successivi l i v . m . di q u e l m a r e , ciò s i g n i f i c a s o l a m e n t e c h e l a vel o c i t à d e l l o s p o s t a m e n t o v e r t i c a l e d e l l a costa è u g u a l e a q u e l l a del l i v . m . m . e ciò c o s t i t u i r à u n f a t t o e c c e z i o n a l e , p e r c h é g e n e r a l m e n t e i d u e m o t i n o n s a r a n n o u g u a l i m a d i v e r s i , sia d i v a l o r e c h e di segno. l e m i s u r e a t t u a l m e n t e e s e g u i t e h a n n o t u t t e c a r a t t e r e r e l a t i v o : o si r i f e r i s c o n o ad u n p u n t o l o n t a n o s u p p o s t o fisso, o p p u r e si r i f e r i scono al l i v . m . d i u n d e t e r m i n a t o m a r e c o n s i d e r a t o i n v a r i a b i l e . i n r e a l t à , n é l ' u n o n é l ' a l t r o sono f e r m i , m a s u b i s c o n o col t e m p o spostam e n t i v e r t i c a l i c o n t i n u i , senza t e n e r c o n t o dei c o n t e m p o r a n e i m o v i m e n t i o r i z z o n t a l i d e l c a p o s a l d o t e r r e s t r e . i l t r a s c u r a r e q u e s t o c o n c e t t o di r e l a t i v i t à dei d u e m o t i p u ò p o r t a r e a g r a v i e r r o r i , c o m e q u e l l o di r i t e n e r e i n f a s e di s o l l e v a m e n t o u n a costa c h e i n r e a l t à sta abbassandosi. il p r o b l e m a a m m e t t e p e r ò d e l l e s o l u z i o n i c h e p o s s o n o d a r e c o n s u f f i c i e n t e a p p r o s s i m a z i o n e i v a l o r i a s s o l u t i d e g l i s p o s t a m e n t i v e r t i c a l i ( * ) c o m u n i c a z i o n e p r e s e n t a t a al c o n v e g n o d e l l ' a s s o c i a z i o n e g e o f i s i c a i t a l i a n a , t e n u t o a r o m a i l 1 0 1 1 g i u g n o 1 9 5 2 . s i l v i o p o l l i (li c a p o s a l d i costieri. la p i ù s e m p l i c e è q u e l l a c h e p e r m e t t e di det e r m i n a r e p e r ogni p u n t o t e r r e s t r e u n a q u o t a r i s p e t t o ad u n d a t o p i a n o f o n d a m e n t a l e e l e successive sue v a r i a z i o n i . ciò si p u ò o t t e n e r e colleg a n d o u n o d e i d u e m o t i r e l a t i v i , e p r e c i s a m e n t e q u e l l o del l i v . m . i n . , n o n ad u n d a t o c a p o s a l d o t e r r e s t r e m a a l l a m e d i a d e l l e q u o t e di t u t t i i c a p o s a l d i m a r e o g r a f i c i . il c h e e q u i v a l e a r i f e r i r e i l l i v . m . m . al c e n t r o d e l l a t e r r a e s u p p o r r e t r a s c u r a b i l e l a v a r i a z i o n e d e l r a g g i o m e d i o t e r r e s t r e . 2. il procedimento generale. —• d a l l e r e g i s t r a z i o n i m a r e o g r a f i c h e di stazioni poste l u n g o l e coste di t u t t i i m a r i , si d e t e r m i n a n o i l i v . m . m . a n n u i r i f e r e n d o l i a c a p o s a l d i l o c a l i . p e r a v e r e u n l i v . m . l o c a l e m e n o i n f l u e n z a t o da cause di b r e v e p e r i o d o , c o n v e r r à d e d u r l o da u n i n t e r v a l l o p i ù l u n g o , p e r es. d i 1 0 a n n i . q u e s t o l i v . m . si a t t r i b u i s c e all ' i s t a n t e c e n t r a l e d e l l ' i n t e r v a l l o di t e m p o c o n s i d e r a t o . p e r es. si p o t r à c o n s i d e r a r e i l l i v . m . d e g l i a n n i 1 8 9 6 1 9 0 5 e a t t r i b u i r l o a l l ' i s t a n t e 0 d e l 1° g e n n a i o 1 9 0 1 . i n m o d o a n a l o g o si p o t r à d e t e r m i n a r e i l l i v . m . p e r il d e c e n n i o successivo e a t t r i b u i r l o a l l ' i s t a n t e 0 d e l 1° g e n n a i o 1 9 1 1 . l a d i f f e r e n z a f r a i d u e l i v . m . r a p p r e s e n t e r à i l v a l o r e d e l l o spostam e n t o r e l a t i v o t e r r a a c q u a , p e r q u e l l e l o c a l i t à . v i s a r a n n o d e l l e z o n e c o s t i e r e i n f a s e di s o l l e v a m e n t o ed a l t r e i n f a s e di a b b a s s a m e n t o . s u p p o n e n d o l a v a r i a z i o n e del r a g g i o m e d i o t e r r e s t r e di v a l o r e t r a s c u r a b i l e rispetto a q u e l l o d e l l e v a r i a z i o n i d e l l e singole z o n e c o s t i e r e si a v r à c o m p e n s a z i o n e f r a i m o v i m e n t i o p p o s t i . f a c e n d o a l l o r a l a m e d i a d e l l e v a r i a z i o n i di t u t t e l e r e g i o n i c o s t i e r e d e l l a t e r r a , c o r r i s p o n d e n t i a d u n o stesso d e c e n n i o , si o t t e r r à u n v a l o r e n u l l o , p o s i t i v o , o n e g a t i v o s e c o n d o c h e i l l i v . m . g e n e r a l e è r i m a s t o s t a z i o n a r i o , o h a s u b i t o u n i n n a l z a m e n t o , o p p u r e u n a b b a s s a m e n t o i n q u e l d e c e n n i o c o n s i d e r a t o . l e r e c e n t i ricerche s u l l e v a r i a z i o n i dei l i v . m . m . h a n n o messo in e v i d e n z a c o m e i l i v . m . di t u t t i i m a r i s u b i s c a n o a t t u a l m e n t e u n grad u a l e a u m e n t o d e l l ' o r d i n e di 1 , 5 cm p e r d e c e n n i o , rispetto ai capos a l d i costieri. questo a u m e n t o a v v i e n e i n s t r e t t a d i p e n d e n z a con u n a p r o g r e s s i v a f u s i o n e d e l l e m a s s e g l a c i a l i p o l a r i e a l p i n e . esso n o n è c o s t a n t e n e i successivi i n t e r v a l l i di t e m p o m a v a r i a p r i n c i p a l m e n t e in f u n z i o n e d e l l e c o n d i z i o n i c l i m a t i c h e g e n e r a l i . queste d i p e n d o n o a l l a l o r o v o l t a d a l l a a t t i v i t à p e r i o d i c a e n o n p e r i o d i c a d e l s o l e . nel d e c e n n i o 1 9 3 1 4 0 l ' a u m e n t o g e n e r a l e è stato di 1 , 9 cm. a v e n e z i a s. m a r c o , n e l l o stesso d e c e n n i o , si è a v u t o u n i n n a l z a m e n t o di 4 , 9 cm. non p o t e n d o a m m e t t e r e c h e solo a v e n e z i a p e r 1 0 anni cons e c u t i v i il l i v . m . m . sia stato s e m p r e p i ù a l t o c h e n e l l e r e g i o n i v i c i n e , m i s u r a d e i l e n t i m o v i m e n t i v e r t i c a l i d e l l a s u p e r f i c i e t e r r e s t r e • 1 3 5 si d e d u c e c h e i l suolo, a l m e n o i n q u e l l a zona r i s t r e t t a , d e v e essersi a b b a s s a t o p r o p r i o di 3,0 cm. il c h e e f f e t t i v a m e n t e è a v v e n u t o , c o m e si è p o t u t o c o n s t a t a r e m e d i a n t e l i v e l l a z i o n i g e o m e t r i c h e r i f e r i t e a p u n t i l o n t a n i s u l l a t e r r a f e r m a . i l f e n o m e n o p e r d u r a da m o l t i d e c e n n i e c o n t i n u a t u t t o r a . esso p o t r e b b e spiegai-si q u a l e c e d i m e n t o d e l s u o l o d o v u t o a c o s t i p a z i o n e d e l m a t e r i a l e t r a s p o r t a t o e d e p o s i t a t o dai fiumi e d a l m o t o o n d o s o . 3. le variazionii decennali generali del livello medio marino. — p e r l a d e t e r m i n a z i o n e dei successivi l i v . m . g e n e r a l i sono stati consid e r a t i i l i v . m . di 1 1 0 stazioni m a r e o g r a f i c h e . dai l i v . m . a n n u i di ciascuna s t a z i o n e si d e d u s s e r o q u e l l i d e i d e c e n n i 1 8 6 6 7 5 , 1 8 8 6 9 5 , . . . . 1 9 2 6 3 5 e 1 9 3 6 4 5 , c h e si a t t r i b u i r o n o agli i s t a n t i 0 d e l 1° g e n n a i o j 3 7 1 . 1 8 8 1 , . . . . 1 9 3 1 e 1 9 4 1 . l e d i f f e r e n z e f r a i l i v . m . degli anni c e n t r a l i 1 8 7 1 e 1 8 8 0 , 1 8 8 1 e 1 8 9 0 , 1 9 3 1 e 1 9 4 0 d a n n o l a v a r i a z i o n e dec e n n a l e r i s p e t t o al c a p o s a l d o l o c a l e . p e r d a r e , n e l c a l c o l o d e l l a m e d i a , l o stesso peso a t u t t e l e reg i o n i d e l l a t e r r a , l e s t a z i o n i c o n s i d e r a t e sono state r a g g r u p p a t e in 3 8 z o n e costiere, d i s t r i b u i t e u n i f o r m e m e n t e , c o m p a t i b i l m e n t e con la l o r o d i s p o n i b i l i t à , l u n g o t u t t i i c o n t i n e n t i . p e r c i a s c u n a di esse si d e t e r m i n a r o n o l e v a r i a z i o n i m e d i e d e c e n n a l i . s i o t t e n n e r o così i v a l o r i c h e si p r e s e n t a n o n e l l a t a b e l l a a l l e g a t a . i l v a l o r e m e d i o p e r u n o stesso d e c e n n i o , d e d o t t o da q u e l l i di t u t t e l e z o n e c o s t i e r e , r a p p r e s e n t a l a variazione generale decennale, dà cioè l ' i n n a l z a m e n t o o l ' a b b a s s a m e n t o m e d i o di t u t t i i m a r i d e l l a t a b e l l a . ii segno -fi n d i c a i n n a l z a m e n t o d e l l i v e l l o m e d i o , e i l segno -—i n d i c a l ' a b b a s s a m e n t o . l a v a r i a z i o n e d e c e n n a l e g e n e r a l e n o n è costante. i l v a l o r e m e d i o d e l s e t t a n t e n n i o 1 8 7 1 1 9 4 0 è di 1 , 1 cm. è stata m a s s i m a n e l l ' u l t i m o d e c e n n i o ( 1 9 3 1 1 9 4 0 ) . è stata n e g a t i v a , si è a v u t o cioè u n abbassam e n t o del l i v . m . g e n e r a l e , n e l l ' i n t e r v a l l o 1 8 8 1 1 8 9 0 . 4. la determinazione dei movimenti verticali costieri. — r i p o r t i a m o d a l l a t a b e l l a i v a l o r i d e l l e m e d i e g e n e r a l i dei l i v . m . m . : variazioni decennali generali del liv.m.m. in cm. ( f i n n a l z a m e n t o , — a b b a s s a m e n t o ) 1 8 7 1 1 8 8 0 1 8 8 1 1 8 9 0 1 8 9 1 1 9 0 0 1 9 0 1 1 9 1 0 1 9 1 1 1 9 2 0 1 9 2 1 1 9 3 0 1 9 3 1 1 9 4 0 1 8 7 1 1 9 4 0 + 0 , 9 1 , 4 + 1 , 6 + 0 , 2 + 0 , 7 + 0 , 9 + 1 , 9 + u m i s l ' r a d e i l e n t i m o v i m e n t i v e r t i c a l i d e l l a s u p e r f i c i e t e r r e s t r e 4 3 7 essi d a n n o d u n q u e l a v a r i a z i o n e m e d i a d e l l i v . m . m . p e r ciascun d e c e n n i o d e l p e r i o d o 1 8 7 1 1 9 4 0 . l a d i f f e r e n z a t r a questi v a l o r i e q u e l l i di u n a d a t a l o c a l i t à d a r à i l v a l o r e d e l s o l l e v a m e n t o o dell'abbassam e n t o a s s o l u t o di q u e l c a p o s a l d o , cioè i l v a l o r e d e l suo s p o s t a m e n t o v e r t i c a l e i n d i p e n d e n t e m e n t e d a l l a v a r i a z i o n e d e l l i v . m . m . 5. precisione delle misure. — la d e t e r m i n a z i o n e d e l b r a d i s i s m o d i p e n d e d u n q u e d a d u e m i s u r e : da q u e l l a d e l l i v . m . di u n a s t a z i o n e e da q u e l l a di t u t t i i m a r i . l a p r i m a d e t e r m i n a z i o n e p u ò essere m o l t o p r e c i s a , sia c h e si usi l a m e d i a d e d o t t a d a l l e altezze o r a r i e , sia c h e si a d o p e r i l a m e d i a di t u t t e l e a l t e e l e basse m a r e e ( i n g e n e r a l e i d u e m e t o d i d a n n o d i f f e r e n z e t r a s c u r a b i l i , d e l l ' o r d i n e di 1 / 1 0 di m m ì . l a p r e c i s i o n e , i n ogni caso, è di q u a l c h e d e c i m o di m m e p u ò a r r i v a r e ad 1 / 1 0 di m m p e r i l i v e l l i m e d i d e c e n n a l i . m i n o r e p r e c i s i o n e si h a n e l l a d e t e r m i n a z i o n e d e l l i v . m . m . di t u t t i i m a r i . e ciò s p e c i a l m e n t e p e r l ' a r b i t r a r i e t à d e l l a scelta e del r a g g r u p p a m e n t o d e l l e stazioni n e l l a d e t e r m i n a z i o n e d e l l i v . m . m . g e n e r a l e . è e v i d e n t e c h e n o n è o p p o r t u n o f a r e la s e m p l i c e m e d i a di t u t t e l e stazioni d i s p o n i b i l i , c o n v i e n e i n v e c e det e r m i n a r e l ' a u m e n t o m e d i o p e r ogni r e g i o n e c o s t i e r a e f a r e l a m e d i a di questi v a l o r i , scelti anch'essi i n m o d o c o n v e n i e n t e . r i t e n i a m o c h e u n l a v o r o a c c u r a t o possa d a r e l ' a u m e n t o m e d i o d e c e n n a l e di t u t t i i m a r i con l a p r e c i s i o n e di 1/2 m m . p e r l o s p o s t a m e n t o v e r t i c a l e assol u t o dei c a p i s a l d i m a r e o g r a f i c i la p r e c i s i o n e r i s u l t a , p e r q u a n t o si disse, di q u a l c h e d e c i m o di m m . 6. considerazioni generali. — si n o t i n o l ' a l t o g r a d o di p r e c i s i o n e c h e si p u ò r a g g i u n g e r e con q u e s t o p r o c e d i m e n t o n e l l a d e t e r m i n a z i o n e degli s p o s t a m e n t i v e r t i c a l i costieri e l ' i m p o r t a n z a c h e p u ò a s s u m e r e u n a c o n t i n u a e r e g o l a r e r e g i s t r a z i o n e m a r e o g r a f ì c a sia n e l l a r i c e r c a o c e a n o g r a f i c a c h e in q u e l l a geofìsica. q u a l e i m m e d i a t a a p p l i c a z i o n e p r a t i c a del p r o c e d i m e n t o i n d i c a t o sono stati d e t e r m i n a t i gli a t t u a l i m o v i m e n t i v e r t i c a l i d e l l e coste cont i n e n t a l i . i r i s u l t a t i di questa e l a b o r a z i o n e f i g u r a n o s e p a r a t a m e n t e n e l l a r e l a z i o n e c h e segue a questa n o t a . trieste — istituto talassografico — giugno 1952. 4 3 8 s i l v i o p o l l i riassunto si indica, nn procedimento che consente di misurare i lenti movimenti verticali della superficie terrestre mediante determinazioni mareografiche di livelli medi marini. esso vale tanto per zone ristrette che vaste. si applica direttamente alle regioni costiere e mediante una livellazione di precisione alle zone interne. esso si basa sulla determinazione della differenza fra le velocità del movimento medio verticale del livello di tutti i mari e quello di una determinata stazione. tale differenza dà lo spostamento verticale assoluto della costa nell'intervallo considerato. dai dati di tutte le stazioni mareografiche della terra, raggruppati per decenni e per regioni costiere, si determinano, per il periodo 1871-1940, le variazioni decennali medie sia delle regioni costiere che quelle generali di tutti i mari. in questo periodo di tempo l'aumento medio del livello marino è stato di 1,1 cm per decennio. il metodo consente di misurare spostamenti verticali dei caposaldi costieri con una precisione che è dell'ordine di pochi decimi di mm per decennio. summary a procedure is demonstrated that permits the calculation of slow vertical movements of the eartli's surface by means of marinografie det er minati on s of the mean ocean levels. this procedure is valid for large zones as w eli as for those more restricted. results obtained from the coastal regions cali be applied to the inland regions by means of a precise correlation. this method is based on the determination of the difference between the velocity of the mean vertical movement of ali the seas and that of a designated station. sucli a difference gives the absolute vertical displacement of the coast in the region considered. from the data from ali the marine stations of the earth, grouped, in ten-year intervals, and further by coastal region, the mean ten-yearly variation, of some coastal regions from the mean of ali the oceans for the period 1871-1940, lias been calculated. in this period of time the mean increase of the level of the sea has been 1.1 cm per ten years. the method permits the measurement of vertical displacements of the coastal level with a precision that is on the order of a few tentlis of a millimeter for a ten-year interval. m i s u r a d e i l e n t i m o v i m e n t i v e r t i c a l i d e l l a s u p e r f i c i e t e r r e s t r e 4 3 9 b i b l i o g r a f i a a s s o c . ocean. p h y s , monthly and annual mean lieights oj sca-level. u n i o n g e o d . g e o p h . i n t e r n . , p u b i . s c i e n t . n . 5 ( l i v e r p o o l , 1 9 4 0 ) e n . 1 0 ( o s l o , 1 9 5 0 ) . polli s . , livelli medi, capisaldi di livellazione e ampiezze delle maree nel porto di trieste. m e n i . 2 5 3 , c o m i t . t a l a s s o g r . i t . d e l c . n i t . , 1 9 3 8 . polli s . , ii graduale aumento del livello marino determinato per 30 porti del mediterraneo. b o l l . s o c . a d r i a t i c a d i s e . n a t . , t r i e s t e , v o i . x l i i , 1 9 4 6 . polli s . , il graduale aumento del livello del mare a venezia, trieste e pola. g e o f i s i c a p u r a e a p p l i c a t a , m i l a n o , v o i . i x , f a s e . 1 2 , 1 9 4 6 . polli s . , livello medio del mare nella livellazione di precisione. b o l l . s o c . a d r i a t i c a d i s e . n a t . , x l i i ( 1 9 4 6 ) , 5 2 5 6 , t r i e s t e . polli s . , gli attuali movimenti verticali delle coste italiane. t e c n i c a i t a l i a n a , t r i e s t e , n . s , 4 , 1 9 4 7 . polli s . , registrazione dei bradisismi costieri. g e o f i s i c a p u r a e a p p l i c a t a , m i l a n o , v o i . x i i , f a s e . 3 4 , 1 9 4 8 . federico_final:layout 6 annals of geophysics, 56, 4, 2013, s0447; doi:10.4401/ag-6453 s0447 groundwater geochemistry of the mt. vesuvius area: implications for volcano surveillance and relationship with hydrological and seismic signals cinzia federico1,*, paolo madonia1, paola cusano2, simona petrosino2 1 istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy 2 istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy abstract geochemical data obtained between 1998 and 2011 at the mt. vesuvius aquifer are discussed, focusing on the effects of both the hydrological regime and the temporal pattern of local seismicity. water samples were collected in a permanent network of wells and springs located in the areas that are mostly affected by the ascent of magmatic volatiles, and their chemical composition and dissolved gas content were analyzed. as well as the geochemical parameters that describe the behavior of groundwater at mt. vesuvius, we discuss the temporal distribution of volcano-tectonic earthquakes. the seismological data set was collected by the stations forming the permanent and mobile network of the istituto nazionale di geofisica e vulcanologia osservatorio vesuviano (ingv-ov). our analysis of seismic data collected during 1998-2011 identified statistically significant variations in the seismicity rate, marked by phases of decreasing activity from october 1999 to may 2001 and increasing activity from august 2004 to mid-2006. the water chemistry shows peculiar patterns, characterized by a changeable input of co2-rich and saline water, which must be related to either a changing stress field or an increased input of co2-rich vapor. the water chemistry data from 1999 to 2003 account for both higher fluid pressure (which induced the seismic crisis of 1999 that peaked with a 3.6-magnitude earthquake in october 1999) and the increased input of co2-rich fluids. the highest emission of co2 from the crater fumaroles and the corresponding increase in dissolved carbon in groundwater characterize the phase of low seismicity. the termination of the phase of intense deep degassing is associated with a change in water chemistry and a peculiar seismic event that was recorded in july 2003. all these seismic and geochemical patterns are interpreted according to temporal variations in the regional and local stress field. introduction fluids circulating in volcanic edifices are attracting increasing interest from scientists, mostly because their role in triggering flank instability, phreatic explosions, and eruptions has been documented in dozens of cases worldwide [day 1996, reid 2004, thomas et al. 2004]. internal pressurization of magmatic volatiles and hydrothermal systems can lead to dramatic steam blasts and, eventually, to eruptions [hill et al. 2002]. the fluid pore pressure can also be changed by external mechanisms, such as variation of the stress field or, more generally, variation of the porosity/permeability of volcanic rocks. static or dynamic stress changes, which have a clear expression in seismicity, exert a significant control on fluid patterns and eventually on the ascent of melts [newhall et al. 2001, hill et al. 2002, mortimer et al. 2011]. the reciprocal roles of tectonics and magmatic/hydrothermal activity, and their influence in determining the so-called crises, is still under investigation [roeloffs et al. 2003, gottsman et al. 2007]. monitoring of groundwater in volcanic systems is useful for predicting changes in the tectonic and volcanic stress field [newhall et al. 2001, hurwitz and johnston 2003, biagi et al. 2004, koizumi et al. 2004]. changes in water level in some wells were observed prior to the 2000 eruption of mt. usu, japan [shibata and akita 2001], and is a frequent characteristic in eruptions of mt. vesuvius [bertagnini et al. 2006]. mt. vesuvius hosts a shallow aquifer which receives magmatic/hydrothermal gases mostly in correspondence of the main tectonic lineaments [federico et al. 2002]. some changes in water and dissolved gas composition have been recorded in concomitance with the seismic crisis that occurred in 1999 [federico et al. 2004, madonia et al. 2008]. the present study aimed at elucidating the roles of both the stress field and the magmatic/hydrothermal degassing in changes in water chemistry in the vesuvian shallow aquifer. we report on the composition of major ions, temperature, and dissolved gas conarticle history received august 7, 2012; accepted december 17, 2012. subject classification: water chemistry, seismicity, hydrology, volcanic surveillance. special issue: vesuvius monitoring and knowledge tents (which were partly reported by madonia et al. [2008]) measured between 1998 and 2011 at selected springs and wells located on the lower flanks of mt. vesuvius. temporal variations of water chemistry can be attributed to either changes in porosity/permeability and fluid circulation or the input of co2-rich vapor, and provide insights into the present rest period. finally, we compare the variations of the geochemical parameters with those of the seismic activity. study area geological summary with a radius of ca 10 km, the somma-vesuvius complex (1281 m a.s.l.) in southern italy is a polygenetic volcanic complex composed of an old stratovolcano (somma), which is mainly preserved on the northern sector of the edifice, and the younger cone of mt. vesuvius (figure 1). the somma-vesuvius complex developed within a wide gravimetric anomaly located in the central part of the campanian plain (a depression bordered by tertiary and mesozoic carbonate massifs) named the acerra graben [scandone et al. 1991, marzocchi et al. 1993]. this anomaly is related to the subsidence of the carbonate basement lying about 2 km beneath mt. vesuvius. the stratovolcano developed at the intersection of two regional tectonic fault systems (running nw–se/nnw–sse and nne–ssw/ne– sw). in addition to these two regional structures there are also local eruptive fractures aligned in the e–w and n–s directions inside the somma caldera and on the southern flank of the volcano [bianco et al. 1998]. the volcanic activity of the volcanic complex has been characterized by the alternation between pyroclastic eruptions (separated by quiescent periods) and open conduit phases, characterized by strombolian and effusive activity [santacroce et al. 1994, cioni et al. 1998]. the somma volcano mainly consists of lava flows (k-rich basalts and latites) and minor pyroclastics (strombolian scoria fall deposits) and was active between 25 and 18 ka bp [santacroce 1987, ayuso et al. 1998]. four plinian eruptions between 18 ka bp and a.d. 79 caused the summit to collapse and gave rise to the formation of a multistage summit caldera (i.e., the somma caldera) [santacroce 1987, andronico et al. 1995, principe et al. 1999, de vivo and rolandi 2001]. after the sub-plinian eruption in 1631, an open conduit phase lasted until the 1944 eruption, since when the volcano has been in a state of weak volcanic-hydrothermal activity characterized by diffuse co2 degassing and low-temperature fumarolic activity in the crater area, thermal submarine features, and low seismic activity [aiuppa et al. 2004, del pezzo et al. 2004, frondini et al. 2004, caliro et al. 2011]. seismicity the natural vesuvian seismicity mainly consists of volcano tectonic (vt) earthquakes [chouet 1996], which are characterized by clear pand s-wave packets and a high-frequency content, mostly peaking in the band 5-15 hz. they are usually located within a volume centered along the crater axis, at depths shallower than federico et al. 2 figure 1. map of the mt. vesuvius area showing the locations of earthquakes and measuring points of the seismic and geochemical surveillance networks. 3 about 4 km below sea level (b.s.l.). the transition zone between the volcanic edifice and the carbonate basement, at 2-3 km b.s.l., coincides with the maximum of the spatial distribution of the hypocenters [saccorotti et al. 2002, scarpa et al. 2002, del pezzo et al. 2004]. assumptions about the source dynamics for the vesuvian vt earthquakes involve failures in the brittle rocks, with strike-slip and normal/reverse dip-slip focal mechanisms. a wide variety of nodal plane orientations is indicated, but overall the directions are nw–se and ne–sw [bianco et al. 1998, ventura and vilardo 1999]. the p and t axes are mainly assumed to be present along the nne–ssw and ese–wnw directions and along the ese–wnw and nne/n–ssw/s directions, respectively [bianco et al. 1998, ventura and vilardo 1999, zollo et al. 2002]. according to del pezzo et al. [2004], the deepest events are associated with average stress drop between 1 and 10 mpa and are mainly caused by the release of regional tectonic stress in the prefractured carbonate basement. on the other end, the shallowest earthquakes are characterized by stress drop of up to 1 mpa and are probably triggered by increasing pore fluid pressure caused by changes in the hydrothermal aquifer, which is located beneath the crater at about 1 km b.s.l. madonia et al. [2008] revealed two statistically significant variations in the seismicity behavior between 1998 and 2005: in may 2001 and july 2004. two phases corresponding to these variations can be identified. the first is relative to the 1999 crisis. the highest-magnitude (3.6) earthquake since the last eruption (in 1944) occurred during this crisis, at 07:41 ut on october 9, 1999. this earthquake was located about 4 km beneath the crater area, inside the carbonate basement. several studies have demonstrated that this event was generated by tectonic stress release along a pre-existing fracture system [ventura and vilardo 1999, zollo et al. 2002, del pezzo et al. 2004]. this was followed by a sequence of low-energy earthquakes, and the seismic activity decreased up to may 2001. low levels of seismicity were observed until july 2004, when a slight increase in the seismicity rate marked the beginning of the second phase. hydrogeology two main aquifers exist in the vesuvian area [corniello et al. 1990, celico et al. 1998]: (1) a deep carbonate aquifer hosted in the buried mesozoic series beneath the campanian plain and recharged by precipitation falling on the apennines, and (2) a shallower volcanic aquifer (the vesuvian aquifer) hosted in fractured lavas and coarse-grained pyroclastic deposits of the somma-vesuvius complex. as generally observed in stratovolcanoes, water at mt. vesuvius circulates in several overlapping water bodies separated by impermeable fine-grained pyroclastic layers. the transmissivity values of these aquifers range from 10–4 to 10–1 m2·s–1 [celico et al. 1998], with the highest values (10–2 to 10–1 m2·s–1) being found on the southern flank of the volcano. the exchanges between the deep and shallow water bodies are easier in this area due to both the volcanic cover being thinner and the presence of fractures in the carbonate basement, which facilitate upward water circulation. the numerous studies concerning the chemistry of somma-vesuvius groundwaters have clarified the mechanisms of gas–water–rock interactions, highlighting the role of volcanic co2 in controlling rock leaching and water chemistry [caliro et al. 1998, celico et al. 1998, federico et al. 2002, aiuppa et al. 2005, caliro et al. 2005]. analysis of stable isotopes in dissolved gases (co2 and he) have demonstrated that magmatic volatiles are actively transported by groundwaters flowing along the main faults and fractures, trending nw– se and ne–sw, which affect both the volcanic edifice and the sedimentary basement [federico et al. 2002]. moreover, a systematic contrast in the chemical compositions of the groundwaters flowing on the southern and northern sectors of the volcanic edifice has been demonstrated, with the former being typically characterized by higher outlet temperatures, total dissolved solids, and dissolved co2 contents. these findings clearly indicate that the gas supply is lower on the northern sector of the volcano than on the southern one, which could be ascribed to a structural-geological control of water circulation. according to federico et al. [2002], carbonate groundwaters flowing from the apennines to the tyrrhenian sea would interact with the central conduit system, thus becoming heated and gas-charged by the ascending hot fluids sustained by deep magma degassing. further south these co2-charged carbonate groundwaters may represent a co2 source for the shallow volcanic aquifer, for example in the torre annunziata area, where the carbonate aquifer lies at a depth of only 500 m [celico et al. 1998]. in an alternative proposed model the northern walls of the somma caldera represent an impermeable barrier to water infiltration, thus forcing groundwaters to flow southward [federico et al. 2002, caliro et al. 2005], in which case the groundwaters would dissolve co2 mainly in the crater area [caliro et al. 2005]. methods the seismological monitoring of mt. vesuvius is performed by ingv-ov personnel using a permanent seismic network that has been active since 1972. currently the monitoring network consists of 10 stations hydrology and seismicity on mt. vesuvius equipped with short-period geophones, of which 3 are three-component stations and 2 are short-period threecomponent digital stations. moreover, six broadband three-component digital stations are present in the crater area. the installed sensors are 1-hz velocimeters (geotech s13 and mark le-3d) and broadband sensors (guralp cmg 40t and trillium 120p). seismic data are continuously acquired at 100 sample·s–1, transmitted to an acquisition center in naples, and stored on hard disks. more information is available in giudicepietro et al. [2010]. the mobile seismic network of ingv-ov has installed four digital stations at mt. vesuvius. marslite and lennartz m24 devices equipped with three-component broadband sensors (guralp cmg-40t or lennartz le-3d/20s) operate on the volcano. the signals are acquired in situ at a sampling rate of 125 or 100 hz. further technical details are available elsewhere [castellano et al. 2012]. a seismic catalogue of mt. vesuvius, which contains the occurrence time and the duration magnitude (md) values of the vt earthquakes, has been compiled since 1972. the relation between the magnitude and the seismic trace duration was calibrated by del pezzo et al. [1983] for vesuvian vt earthquakes recorded on the vertical component of the ovo station, located in the western sector of the volcano (figure 1). figure 2 shows the monthly distribution of the number of vt earthquakes based on this catalogue. the vesuvian vt earthquakes included in the catalogue were localized using a 3d probabilistic algorithm (nonlinloc) [lomax et al. 2000] over the 3d velocity model inferred by scarpa et al. [2002]. the hypocenter depth as a function of the time is reported in figure 2. the hydrogeochemical parameters of the vesuvian aquifer have been monitored since 1998. the first hydrogeochemical monitoring network consisted of 10 private wells that were mainly used for irrigation and 2 springs (figure 1). currently the network consists of six wells and two springs, while temperature is recorded hourly at selected sites (data not shown). temperature, ph, eh, and alkalinity were determined by sampling with conventional field instrumentation; laboratory determinations were carried out at ingv, palermo, following the procedures described by federico et al. [2002]. the concentrations of major ions were measured by applying ion chromatography to filtered (cl–, no3 – , and so4 2–) and filtered and acidified (na+, k+, ca2+, and mg2+) samples, with an analytical uncertainty of <5%. the dissolved gases in water samples were measured after equilibration in a host gas (ar) and extraction, following the procedure described by capasso and inguaggiato [1998]. analyses were performed using a gas chromatograph (perkin-elmer 8500), equipped with 4-m carbosieve ii columns and two detectors (hot wire and flame ionization), with ar as the carrier gas. the analytical uncertainty was <5%. results the number of vt earthquakes peaked at the time of the october 1999 seismic crisis. the earthquakes belonging to this crisis occurred in the prefractured carbonate basement in response to a release of the regional tectonic stress and are characterized by deep locations and large reductions in stress drop values [del pezzo et al. 2004]. the effects of this crisis lasted until the beginning of 2002, after which the seismic activity decreased to very low levels. the hypocenters clustered within the volcanic edifice at depths in the range 0-2.5 km diminished dramatically, and those deeper than 2.5 km practically disappeared. moreover the stress drop assumed low values. as asserted by many authors [saccorotti et al. 2002, del pezzo et al. 2004], it is likely that most of these vt earthquakes were caused by variations of the pore fluid pressure. these variations could be induced by the local stress perturbations themselves or by charging and discharging mechanisms of the shallow aquifer. on july 20, 2003, an anomalous low-frequency earthquake with a quasimonochromatic spectrum in the frequency band of 3.5-4 hz was recorded by all of the stations of the ingv-ov seismic network. it was located at about 4 km b.s.l., and bianco et al. [2005] reported that it was probably of hydrothermal-volcanic origin. a slight seismic anomaly began in august 2004, after which the number of vt earthquakes began to increase, reaching a relative maximum in 2006 before returning to very low values. this period included an earthquake (on august 30, 2005) at the very shallow depth of 300 m a.s.l. using a combination of seismological and geochemical methods, madonia et al. [2008] federico et al. 4 figure 2. hypocentral depths of vesuvian vt earthquakes as functions of time (upper panel) and the monthly number of vt earthquakes (lower panel). 5 modeled the source mechanism of this vt earthquake as the superposition of tensile cracking and shear failures. the crack opened along the direction orthogonal to the maximum stress axis of the faulting, due to an increase of the pore fluid pressure. this increase could in turn be caused by the variation of the local stress field that enhanced the upward migration of thermal fluids. the 2005 year was also characterized by the absolute minimum of deep seismicity. as shown by federico et al. [2002, 2004], the waters circulating through the vesuvian aquifers are characterized by large spatial and temporal variations in chemical composition (figure 3). in general, these waters are rich in hco3 – and their alkali content (here represented by the k+ content) is strictly controlled by the co2-driven mechanism of rock dissolution. nevertheless, a variable enrichment in either cl– or so4 2– with respect to the volcanic rock is observed. sample 14 shows the lowest relative content of so4 2–, while its relative content of cl– is higher than that in the host rock. as suggested by federico et al. [2002], the southern sector of the vesuvian aquifer is characterized by the local ascent of brine-type warm fluids along faults. among collected samples, well 14 is the most saline (cl = 2000 mg·l–1 on average) and, together with spring 13, it is the warmest, which suggests that it is contaminated by hot cl-rich brines. the enrichment in so4 2–, which is mostly observed in samples (namely sites 13, 19, and 29) collected in the torre annunziata–torre del greco area (where s-rich gas manifestations have been identified along the coast), can be attributed to either the oxidation of hydrothermal s-bearing minerals (i.e., pyrite) or the dissolution of a h2s-bearing gas phase by groundwater during circulation at depth [federico et al. 2002]. the enrichment of so4 2– in a few of the wells can be ascribed to the contribution of fluids related to human activities, in particular to the use of so and n-rich fertilizers or rural sewage, as suggested by federico et al. [2004]. this effect is evident in samples 47 and 19, which also have higher no3 − contents (see appendix). during the monitored period the vesuvian groundwater exhibited compositional variability, which can be quantified as so4 2–/hco3 − and cl−/so4 2– ratios (figure 3a). moreover, significant changes in water salinity are observed at some sites. figures 4-7 plot the time trends of some significant parameters at selected sites during the entire monitored period. figure 4 displays the time trends of the water temperature measured at sites 13, 14 and 29. a common feature is the small (<1°c) and slow increase in temperature after the 1999 earthquake, with a marked decrease of as high as 4°c occurring after 2003 or 2006 (spring 13). temperature variations are paralleled by changes in chemical parameters, as shown in figures 5-7. figure 5 shows the time trends of hco3 − contents, cl−/so4 2– ratios (for sites 13 and 29), dissolved co2 content (for site 29), and water temperature. the time trends of estimated pco2 for the hydrothermal system, computed hydrology and seismicity on mt. vesuvius figure 3. a) cl−-so4 2−-hco3 − triangular diagram. b) cl−-so4 2−-k+ triangular diagram. the average composition of lava and scorias from belkin et al. [1998] and the point representative of seawater are plotted for comparison. concentrations are expressed in units of mg·l−1. figure 4. time trends of water temperature at wells 29 and 14 and spring 13. arrows indicate phases of increases (red) and decreases (blue) in temperature. the time trend of pco2 values, computed for the hydrothermal aquifer by caliro et al. [2011], is plotted as a shaded area. the histogram shows the monthly number of seismic events. from chemical data of fumarole fc2 by caliro et al. [2011], are shown in figure 5 as a shaded area. at well 29, the above-described time trends of temperature follow the changes in dissolved co2 content fairly well, with both dramatically decreasing after 2004. the hco3 − concentration exhibits a relative minimum coinciding with the november 1999 earthquake, and then increases along with co2 until 2002-2003, when a longlasting decreasing trend begins, with an overall decrease of about 30%, which is approximately paralleled by changes in the cl−/so4 2– ratio. from 2006 onward, while cl−/so4 2– ratio remains steady, the hco3 − content continues decreasing. similarly, the hco3 − content, temperature, and cl−/so4 2– ratio at site 13 follow very similar trends in the months before and after the 1999 earthquake. thereafter, while cl−/so4 2– starts decreasing in 2003 before remaining steady from 2006 onward, hco3 − and temperature start long-lasting decreasing trends in 2004-2005 that stop in 2009-2010, with an overall decrease in the hco3 − content of almost 50%. the time trends of hco3 − contents at two other co2-rich sites from the torre annunziata area, namely wells 6 and 14 (figure 6), confirm the above-described general pattern, characterized by relative decreases in 1999, followed by marked increases soon thereafter and new long-lasting declines from 2003 or 2004. the chemical variations observed at well 19 (figure 7) in the torre annunziata area are worth noting. during 1998-2003 its hco3 − content shows time trends very similar to those for the other described sites. a sharp negative peak in hco3 − was detected in mid2003, paralleled by comparable positive peaks in the no3 − content and so4 2–/hco3 − ratio, which last only a few months. the hco3 − finally declines after 2007, which is still ongoing. figure 8 illustrates the time trends of some geochemical parameters measured at a spring (yield <0.1 l·min–1) located on the northern flank of the volcano (olivella spring; figure 1). this site shows significant modifications in the water and dissolved gas chemistries and ph at the beginning of the investigated period, corresponding to the seismic sequence of october 1999, which have been attributed to the stress-induced input of acidic volcanic gases – essentially co2 and h2s [federico et al. 2004]. an overall increase in hco3 − is evident after the november 1999 earthquake, paralleled by a simultaneous slight decrease in ph (about 0.2 ph units federico et al. 6 figure 5. a) from top to bottom: time trends of the cl−/so4 2− ratio (triangles), dissolved co2 content (open circles), hco3 − content (squares), and water temperature (filled circles) at well 29 (torre del greco). b) from top to bottom: time trends of the cl−/so4 2− ratio (triangles), hco3 − content (squares), and water temperature (filled circles) at spring 13 (torre annunziata). arrows indicate phases of increases (red) and decreases (blue) in the plotted parameters. the time trend of pco2 values, computed for the hydrothermal aquifer by caliro et al. [2011], is plotted as a shaded area. the histogram shows the monthly number of seismic events. figure 6. time trends of hco3 − contents at wells 6 and 14. arrows indicate phases of increases (red) and decreases (blue) in hco3 − contents. the time trend of pco2 values, computed for the hydrothermal aquifer by caliro et al. [2011], is plotted as a shaded area. the histogram shows the monthly number of seismic events. figure 7. from top to bottom: time trends of the so4 2−/hco3 − ratio, no3 − content, and hco3 − content at well 19. the time trend of pco2 values, computed for the hydrothermal aquifer by caliro et al. [2011], is plotted as a shaded area. the histogram shows the monthly number of seismic events. 7 on average) and a marked decline in the no3 − contents (from 60 to 20 mg·l–1), which represents a marker for the shallower polluted endmember. the compositional change appears particularly evident after 2003. finally, time trends of dissolved ch4/co2 molar ratios at selected sites (where ch4 is detectable) – namely sites 1, 31, and 48 – are shown in figure 9, in which the co2/ch4 ratios measured at fumarole fc2 are plotted as a shaded area [caliro et al. 2011]. we can observe relatively high ch4/co2 ratios at wells 1 and 31 in 1999 (and low co2/ch4 ratios at fumarole fc2), followed by very low values during 2000-2001, when co2 progressively increased to ch4 at the crater fumarole [caliro et al. 2011]. since 2002, progressively higher ch4/co2 ratios have been measured in dissolved gases, remaining almost steady during the entire analyzed period. this coincides with the decrease in co2 concentrations at the fumaroles in the crater area. general discussion according to previous studies, water circulation on mt. vesuvius occurs in different overlapped water bodies that are characterized by different temperatures and salt contents. higher salinity and cl content characterize the deep-circulating and gas-charged fluids (partly contaminated by brines in the southern and western sectors), whereas the shallower endmember has a typical bicarbonate-richer composition [federico et al. 2002, 2004]. where the brine contribution is small (i.e., at well 6), the water salinity is almost exclusively controlled by the extent of the co2-driven rock-leaching and, ultimately, by the amount of interacting co2. therefore, time changes in the water chemistry at the sampled sites can be reliably ascribed to changes in the mixing proportions of the deeper and shallower endmembers and to the input of co2. the mixing proportions of the deeper and shallower endmembers are reliably related to the water flow rate within the aquifers, which in turn results from the variation of pressure gradients and/or permeability, according to the darcy’s law, which describes water flow in porous media. in a volcano-hydrothermal setting we can expect variations of fluid pressure to be controlled by magma degassing and/or the heat and vapor supply to hydrothermal systems [day 1996, reid 2004, thomas et al. 2004]. additionally, changes in pore pressure can be attributed to changes in the pore space volume, due to stress or self-sealing processes [kim et al. 1997, lazear 2009, mortimer et al. 2011]. as suggested by madonia et al. [2008], the crustal deformation that caused intense seismic activity in 1999 could also have changed the fluid circulation, as suggested by water chemistry data (figures 5-7). analysis of the entire data record reveals a clear-cut compositional difference between the first 5 years of observations and the later period. in general, the salinity, temperature, and co2 and hco3 − contents of the wells were highest before 2003, since when (and especially after 2005) there has been a systematic decrease in these parameters characterizing the vesuvian aquifer. we suggest that the higher cl−/so4 2– ratios measured during 1998-2003 at wells 13 and 29, paralleled by higher temperatures and hco3 − and co2 contents, are indicative of a higher fluid pressure resulting from both the preseismic crustal deformation and the magmatic gas release corresponding to the 1999 seismic crisis. it is worth noting that the fluids emitted from fumaroles in the crater area indeed display the highest magmatic he and co2 contributions for typical hydrothermal gases (ch4 and h2o) from 2000 to 2003 [caliro et al. 2011]. the shortlived and moderate decreases in salinity and hco3 − contents observed in the few months of 1999 when the largest number of seismic events occurred [madonia et al. 2008] could be ascribed to fracture reopening and an increase in permeability caused by the intense seishydrology and seismicity on mt. vesuvius figure 8. from top to bottom: time trends of the ph, hco3 − content, and dissolved co2 content at olivella spring. the histogram shows the monthly number of seismic events. figure 9. time trends of ch4/co2 molar ratios in the dissolved gas phase at selected sites (namely wells 1, 31, and 48). the time trend of co2/ch4 molar ratios, measured at the crater fumarole by caliro et al. [2011], is plotted as a shaded area. the histogram shows the monthly number of seismic events. micity. as demonstrated by both field data and numerical modeling, the increase in permeability should favor the circulation of shallow fluids at greater depth [lazear 2009], thus supporting that the shallow water endmember predominated over the deeper one. after 2003 there was a clear-cut change in water circulation at several sites – namely 6, 13, 19, and 29, that marks an increasing contribution of the shallower clpoorer endmember relative to the deeper saline one. interestingly, a dramatic change was recorded at well 19 in 2003, where a peak in the no3 − content (which probably reflects the contribution of shallower polluted water) is paralleled by a decrease in the hco3 − content. mid-2003 was also characterized by the occurrence of an anomalous low-frequency seismic event. cusano et al. [2013] classified this event as a long period (lp) earthquake. the spectral and coda envelope analyses provided evidence of the dominance of sustained lowfrequency oscillations, typical of the long-period seismicity (fluid-filled crack resonance). the presence of fluids at the depth of the vesuvian lp event (about 4 km b.s.l.) support the conceptual geochemical model of the volcanic system of mt. vesuvius depicted by caliro et al. [2011], in which the brine sources were positioned just shallower than the lp hypocenter. on these grounds and taking into account the variations in the groundwater chemistry described in the present work, we suggest that the lp seismic event that occurred in summer 2003 could have caused a pressure decrease that was sustained until that moment by the degassing of magmatic volatiles (mostly co2), as observed by caliro et al. [2011]. the pressure drop could have increased the circulation of shallower fluids into a deeper aquifer, thus explaining the change in chemical composition and the decrease in the saline contents of fluids at the monitored sites. while the water composition indicates the dominant contribution of shallow water since 2003, the temperature and/or co2 and hco3 − concentrations at some wells (namely wells 26, 13, 14, and 29) remained high until 2005-2006. this would support inputs of water vapor and co2 into the shallow aquifer, and we cannot exclude that this is also an effect of a reduced confining pressure in the hydrothermal aquifer caused by the 2003 event. otherwise, as observed in figure 8, an increase in hco3 − content from 2003 at olivella spring is paralleled by a slight reduction in ph (of about 0.2 units), which supports a subsequent higher input of co2. the time trends of ch4/co2 ratios measured at selected wells (where dissolved ch4 is detectable, at >10–5 cc·l–1 stp) are compatible with the co2/ch4 ratios measured at the crater fumarole [caliro et al. 2011], and support that the contribution from the hydrothermal vapor has been greater than that from the magmatic co2-richer gases since 2003. additionally, the ch4/co2 ratios in groundwater can vary due to a reduced flux of deep gases, since these gas species have different solubilities in water. indeed, partial dissolution of gases in water can modify the original gas composition as an effect of the specific solubility coefficients and the relative amount of residual gas, according to a rayleigh-type fractionation [federico et al. 2002, caracausi et al. 2003, capasso et al. 2005]: (1) where subscripts liq, gas, and in indicate the liquid phase, gas phase, and initial condition, respectively, kh,ch4 and kh,co2 are henry’s constants for the gas species ch4 and co2, and mco 2 is the number of moles of co2. given the large difference in henry’s constant between ch4 and co2 (39,900 and 1667 atm·mol·mol –1, respectively, at 25°c [wilhelm et al. 1977]), the exponent in equation (1) is <0. therefore, a dramatic reduction in the fraction of the residual gas (i.e., ) due to the progressive dissolution in water can be expected to increase the ch4/co2 ratios. the lower salinities, temperatures, and co2 contents from 2006 onward clearly indicate a reduced input of volcanic volatiles and lower fluid pressure in the volcanic aquifer. conclusions the chemical analyses of water and dissolved gas samples obtained during 1998-2011 indicate that the geochemical behavior of the vesuvian aquifer is strictly controlled by variations in the input of deep-seated volcanic gases and in the stress field. the changes in the stress field, which were responsible for the seismic crisis of 1999, and the almost simultaneous increased input of co2-rich vapor, significantly affected the recorded composition and temperature of the groundwater. indeed, an increase in fluid pressure characterized the period from 1998 to 2003, inducing a change in water circulation and the predominance of a deeper water endmember over the shallower one in the vesuvian aquifer. moreover, the groundwater temperature and dissolved carbon contents (in terms of both hco3 − and co2) are clearly indicative of inputs of co2 and steam until 2005-2006. the termination of the phase of intense deep degassing, as observed in the crater area and in groundwater, is associated with a pressure decrease evident in the water chemistry and paralleled by a peculiar seismic event that was recorded in july 2003. the recent observations of low salinity, temperature, federico et al. 8 co ch co ch m m k k ,,, , co in co gas inliq h ch h co k k 2 4 2 4 1 , , h ch h co 2 2 4 2 4 2 $$= 6 6 6 6 @ @ @ @c c c c em m m m o m m 1 ,co in co 2 2 % 9 and dissolved carbon contents in groundwater provide strong evidence for reduced pressure in the volcano-hydrothermal system at mt. vesuvius. the variations of specific geochemical parameters can therefore be 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of usu volcano, japan, geophys. res. lett., 28, 17991802. thomas, m.e., n. petford and e.n. bromhead (2004). the effect of internal gas pressurization on volcanic edifice stability: evolution towards a critical state, terra nova, 16, 312-317. ventura, g., and g. vilardo (1999). slip tendency analysis of the vesuvius faults: implications for the seismotectonic and volcanic hazard assessment, geophys. res. lett., 26 (21), 3229-3232; doi:10.1029/1999 gl005393. wilhelm, e., r. battino and r.j. wilckock (1977). low pressure solubility of gases in liquid water, chem. rev., 77, 219-262. zollo, a., w. marzocchi, p. capuano, a. lomax and g. iannaccone (2002). space and time behavior of seismic activity at mt. vesuvius volcano, southern italy, bull. seismol. soc. am., 92 (2), 625-640; doi:10.1785/ 0120000287. *corresponding author: cinzia federico, istituto nazionale di geofisica e vulcanologia, sezione di palermo, palermo, italy; email: c.federico@pa.ingv.it. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. hydrology and seismicity on mt. vesuvius appendix federico et al. 12 si te 6 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p 11 /9 8 16 .9 5. 9 -2 70 14 2 25 2 83 58 20 9 < 0. 1 36 99 0 17 70 5. e -0 1 10 60 1/ 99 16 .8 5. 9 -1 30 13 6 24 2 83 57 20 2 < 0. 1 39 10 10 17 70 1. e + 00 10 90 3/ 99 16 .4 6. 1 -9 0 14 1 25 8 81 57 21 0 0. 5 39 85 0 17 10 < 0. 00 01 52 0 10 /9 9 16 .7 5. 8 -1 20 14 4 25 2 90 59 16 8 0. 5 41 99 0 16 60 5. e -0 1 12 00 11 /9 9 16 .9 5. 9 -2 80 14 9 24 3 89 64 17 0 0. 3 34 99 0 17 10 7. e -0 1 94 3 3/ 00 16 .8 6. 1 -2 75 14 6 25 0 84 59 16 8 4. 3 36 99 0 17 30 n. m . n. m . 5/ 00 16 .8 5. 9 -2 00 15 1 25 9 89 56 16 5 1. 9 31 10 40 17 90 2. e -0 1 99 0 7/ 00 17 .1 5. 8 -1 80 13 9 24 0 92 67 18 1 1. 5 44 10 30 17 70 2. e -0 1 12 30 10 /0 0 16 .7 5. 7 -2 10 13 6 24 1 92 52 14 9 < 0. 1 34 10 00 16 80 n. d. 15 20 11 /0 0 16 .6 5. 8 -1 60 13 4 23 5 85 54 17 1 < 0. 1 38 10 20 17 40 4. e -0 1 12 40 1/ 01 16 .5 6. 0 n. m . 13 7 25 1 75 57 16 8 0. 2 36 96 0 16 90 5. e -0 2 74 0 3/ 01 16 .6 5. 9 n. m . 14 4 24 2 78 71 17 0 < 0. 1 35 10 10 17 40 6. e -0 4 97 0 4/ 01 16 .7 5. 9 -5 0 13 7 23 7 84 55 15 5 3. 0 40 97 0 17 00 5. e -0 1 95 0 5/ 01 17 .1 5. 7 24 13 4 23 0 88 59 14 8 0. 2 34 10 30 17 70 7. e -0 2 11 90 6/ 01 17 .1 5. 9 57 13 5 23 4 88 57 17 4 0. 2 45 97 0 17 00 < 0. 00 01 83 0 8/ 01 16 .7 5. 9 -8 5 13 8 24 1 89 62 17 9 5 45 99 0 17 50 n. m . n. m . 9/ 01 16 .6 5. 9 60 14 4 24 3 89 66 17 1 4. 3 46 10 00 17 60 n. m . n. m . 10 /0 1 16 .7 5. 9 -3 4 14 4 23 9 93 66 16 5 3. 7 47 10 20 17 70 4. e -0 2 85 0 11 /0 1 16 .6 5. 8 25 14 3 23 7 90 64 16 9 < 0. 1 35 10 10 17 50 < 0. 00 01 45 0 12 /0 1 16 .7 6. 0 15 13 4 24 1 84 64 16 0 < 0. 1 33 10 00 17 10 n. m . n. m . 1/ 02 16 .7 5. 9 -1 30 13 6 23 4 81 60 17 7 < 0. 1 59 97 0 17 20 1. e -0 1 10 30 2/ 02 16 .7 6. 1 30 14 1 23 6 81 58 14 4 < 0. 1 39 10 10 17 10 6. e -0 2 10 30 3/ 02 16 .4 5. 9 -2 00 14 2 24 8 83 61 17 3 < 0. 1 43 98 0 17 30 2. e -0 1 68 0 4/ 02 16 .8 5. 8 -3 6 14 4 23 8 89 59 18 7 < 0. 1 35 97 0 17 20 3. e -0 1 83 0 5/ 02 16 .9 5. 9 -3 8 14 3 23 6 86 58 16 8 < 0. 1 59 10 10 17 60 5. e -0 1 87 0 6/ 02 16 .9 6. 0 -2 0 14 9 24 9 87 62 17 4 < 0. 1 47 99 0 17 60 1. e -0 1 82 0 7/ 02 17 .5 5. 9 -2 00 14 9 24 8 87 62 17 1 < 0. 1 48 99 0 17 50 5. e -0 1 85 0 9/ 02 17 .4 5. 9 -1 80 13 9 23 6 80 56 16 8 < 0. 1 36 10 10 17 30 6. e -0 1 82 0 9/ 02 16 .7 5. 9 -1 80 13 8 23 5 78 55 16 8 < 0. 1 37 90 0 16 10 8. e -0 1 85 0 12 /0 2 16 .3 5. 8 -9 0 13 8 23 9 79 55 14 6 < 0. 1 39 10 10 17 10 4. e -0 4 71 0 1/ 03 16 .5 0 5. 6 -1 10 14 1 23 6 81 52 16 3 3 33 10 00 17 00 1. e -0 1 77 0 3/ 03 16 .9 5. 8 -1 20 15 8 24 8 86 57 17 0 < 0. 1 44 10 20 17 80 2. e -0 4 76 0 4/ 03 16 .9 5. 9 -1 50 15 3 24 3 85 55 21 0 < 0. 1 37 98 0 17 60 1. e -0 4 83 0 5/ 03 17 .3 5. 9 -2 10 13 9 24 8 84 53 17 2 < 0. 1 36 99 0 17 20 1. e -0 4 71 0 t ab le a 1 (c on ti nu es o n ne xt p ag e) . 13 hydrology and seismicity on mt. vesuvius 6/ 03 17 .6 n. m . -1 80 14 4 25 2 87 55 17 4 < 0. 1 38 99 0 17 40 2. e -0 3 67 0 7/ 03 17 .5 6. 0 -1 90 14 0 24 8 85 56 18 2 < 0. 1 47 99 0 17 50 n. m . n. m . 8/ 03 17 .3 6. 0 -1 70 13 6 24 1 89 54 19 0 < 0. 1 58 99 0 17 60 3. e -0 2 82 0 9/ 03 17 5. 9 -1 10 13 5 24 3 91 60 17 2 0. 19 60 95 0 17 10 3. e -0 1 86 0 10 /0 3 16 .9 5. 8 -2 50 13 8 24 7 87 54 16 3 < 0. 1 57 95 0 17 00 3. e -0 1 10 00 12 /0 3 16 .6 5. 9 -1 20 13 5 23 9 83 60 17 4 < 0. 1 37 96 0 16 90 3. e -0 1 90 0 01 /0 4 16 .5 5. 5 -2 10 13 2 23 4 82 58 16 5 < 0. 1 49 93 0 16 50 6. e -0 1 80 0 4/ 04 16 .7 5. 7 -2 90 13 8 24 4 82 58 17 6 4. 34 38 96 0 17 00 3. e -0 1 80 0 5/ 04 17 5. 9 -1 40 13 4 24 1 84 58 17 6 < 0. 1 47 94 0 16 80 6. e -0 2 78 0 6/ 04 16 .9 n. m . -1 10 13 5 24 5 87 60 18 1 < 0. 1 44 96 0 17 20 5. e -0 1 86 0 8/ 04 n. m . n. m . n. m . 13 7 24 3 89 59 19 2 < 0. 1 56 95 0 17 30 5. e -0 1 79 0 9/ 04 16 .9 5. 8 14 0 14 2 25 4 90 61 20 1 < 0. 1 72 95 0 17 70 4. e -0 1 77 0 10 /0 4 16 .8 5. 9 -8 5 12 7 24 6 79 63 17 8 < 0. 1 61 95 0 17 10 5. e -0 1 84 0 11 /0 4 16 .7 5. 9 -8 5 13 8 24 3 83 59 17 2 < 0. 1 42 96 0 17 00 8. e -0 1 84 0 3/ 05 16 .8 5. 8 -1 00 13 8 25 4 86 50 17 6 < 0. 1 46 95 0 17 00 3. e -0 1 85 0 4/ 05 16 .7 5. 9 43 12 7 22 7 81 58 16 5 < 0. 1 38 96 0 16 50 2. e -0 1 76 0 6/ 05 16 .9 5. 8 -1 80 13 7 24 4 91 64 19 5 < 0. 1 54 94 0 17 30 4. e -0 1 73 0 7/ 05 16 .9 5. 9 -1 60 13 6 24 1 91 63 20 2 < 0. 1 67 92 0 17 20 4. e -0 4 65 0 8/ 05 16 .9 n. m . n. m . 13 7 24 5 89 62 18 9 < 0. 1 67 94 0 17 30 4. e -0 1 76 0 10 /0 5 16 .8 n. m . n. m . 13 5 24 3 88 63 18 9 < 0. 1 44 95 0 17 10 5. e -0 1 77 0 11 /0 5 16 .4 5. 7 n. m . 13 4 24 1 86 63 17 2 < 0. 1 41 96 0 17 00 5. e -0 1 79 0 3/ 06 16 .7 n. m . n. m . 14 1 24 2 88 62 20 7 < 0. 1 50 94 0 17 30 7. e -0 1 82 0 6/ 06 17 5. 7 n. m . 13 9 24 7 90 65 21 7 < 0. 1 10 1 88 0 17 40 4. e -0 1 71 0 2/ 07 16 .7 n. m . n. m . 14 1 24 4 93 66 22 5 < 0. 1 38 94 0 17 50 n. m . n. m . 6/ 07 n. m . n. m . n. m . 13 8 23 9 95 70 21 8 < 0. 1 41 95 0 17 50 5. e -0 1 82 0 2/ 08 16 .8 5. 9 -2 00 14 0 24 5 92 63 21 4 < 0. 1 40 95 0 17 40 3. e -0 1 84 0 5/ 08 16 .9 n. m . -5 0 14 2 24 8 99 71 23 5 < 0. 1 35 95 0 17 80 n. m . n. m . 10 /0 8 17 .0 5. 9 -2 40 12 7 21 8 86 63 22 6 < 0. 1 56 87 0 16 40 n. m . n. m . 7/ 09 17 .0 6. 0 -2 20 13 5 24 5 94 63 23 2 < 0. 1 48 94 0 17 60 n. m . n. m . 4/ 10 17 .1 n. m . -1 70 12 7 21 8 92 67 19 1 < 0. 1 47 92 0 16 60 n. m . n. m . 7/ 10 17 .5 5. 9 n. m . 13 0 23 7 92 63 22 1 < 0. 1 37 95 0 17 30 3. e -0 1 76 0 si te 6 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 1 (c on ti nu ed fr om p re vi ou s pa ge ). federico et al. 14 si te 1 3 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) 5/ 98 22 .5 6. 0 -9 0 17 9 32 8 10 5 18 8 36 0 20 .6 24 2 10 90 24 90 11 /9 8 23 .2 6. 0 -1 00 18 5 35 0 10 1 16 7 46 0 < 0. 1 27 5 10 60 25 90 1/ 99 22 .1 6. 2 -7 0 17 5 32 7 10 1 18 0 39 0 3. 2 27 3 10 40 24 90 3/ 99 22 .7 5. 9 -1 10 18 0 33 0 10 1 17 4 42 0 0. 4 26 2 92 0 23 90 5/ 99 23 .2 5. 9 -2 60 17 0 33 3 95 16 6 36 0 0. 2 25 6 90 0 22 90 10 /9 9 23 .3 6. 7 -1 10 17 3 34 2 88 16 4 34 0 3. 5 23 1 10 10 23 20 11 /9 9 23 .3 6. 0 -2 50 15 9 32 1 91 14 9 32 0 6. 8 21 9 95 0 22 40 3/ 00 22 .6 6. 0 -2 70 17 1 30 7 92 16 9 34 0 5. 0 24 7 99 0 23 20 5/ 00 23 .0 6. 0 -1 60 17 9 32 6 96 15 7 38 0 3. 1 25 5 99 0 23 60 7/ 00 23 .2 6. 0 -6 0 17 8 32 4 10 4 18 6 40 0 3. 1 26 8 10 50 25 00 10 /0 0 23 .1 6. 0 -2 00 20 0 36 0 11 2 17 9 42 0 < 0. 1 28 1 10 70 26 20 11 /0 0 22 .9 5. 9 -1 80 17 1 30 6 96 18 0 38 0 1. 8 27 4 10 50 24 50 1/ 01 22 .3 6. 0 n. m . 16 7 34 0 95 22 3 39 0 < 0. 1 26 5 10 40 24 80 3/ 01 22 .4 6. 0 n. m . 18 5 32 5 10 5 19 1 39 0 < 0. 1 28 0 10 70 25 60 4/ 01 22 .8 6. 0 -3 5 17 7 32 5 10 7 19 2 39 0 < 0. 1 26 3 10 80 25 30 5/ 01 23 .0 6. 0 34 18 1 32 6 10 9 19 1 40 0 < 0. 1 28 2 10 30 25 20 6/ 01 22 .8 5. 93 44 18 4 33 5 10 8 18 6 43 0 5. 2 29 3 10 40 25 80 7/ 01 23 .2 5. 99 -2 20 20 6 36 2 11 1 19 2 46 0 6. 7 31 0 11 00 27 40 8/ 01 23 .8 5. 74 -1 00 21 6 36 5 11 6 18 4 47 0 5. 0 31 2 11 00 27 60 9/ 01 23 .2 5. 93 -1 00 21 5 37 2 11 7 19 4 49 0 4. 3 32 4 11 10 28 30 10 /0 1 23 .3 5. 93 -1 00 22 7 38 6 12 3 19 8 48 0 1. 2 31 3 11 30 28 60 11 /0 1 22 .9 5. 95 55 20 5 35 3 12 0 20 0 44 0 < 0. 1 32 9 11 20 27 70 01 /0 2 n. m . 5. 8 -9 0 21 7 39 0 11 9 23 3 47 0 < 0. 1 34 1 11 60 29 40 02 /0 2 22 .9 5. 8 -7 0 22 0 37 6 11 2 20 6 48 0 30 .4 32 3 11 60 28 80 03 /0 2 22 .9 5. 9 -2 00 22 7 40 0 11 9 22 4 50 0 < 0. 1 36 0 11 30 29 60 04 /0 2 23 .2 5. 9 -1 50 21 4 38 6 11 6 23 0 45 0 < 0. 1 33 5 11 40 28 70 05 /0 2 23 .2 6. 0 -2 0 22 4 38 4 11 9 20 7 46 0 < 0. 1 31 4 11 70 28 70 06 /0 2 23 .5 6. 1 -6 22 3 38 2 11 2 21 6 46 0 < 0. 1 31 7 11 20 28 30 07 /0 2 23 .9 6. 1 -1 90 23 1 39 0 11 4 21 0 47 0 < 0. 1 32 5 11 60 29 00 08 /0 2 23 .7 6. 0 -3 50 23 1 39 3 11 2 19 4 47 0 < 0. 1 33 4 11 50 28 80 09 /0 2 23 .7 6. 0 -1 44 22 8 38 9 11 0 19 8 47 0 < 0. 1 33 2 10 90 28 20 10 /0 2 23 .8 5. 9 -1 40 23 3 39 8 11 2 20 1 47 0 < 0. 1 33 2 11 40 28 90 12 /0 2 23 .5 6. 1 -5 9 22 7 38 1 11 1 19 8 45 0 < 0. 1 33 1 11 40 28 40 12 /0 2 23 .5 5. 9 12 21 4 37 8 10 7 19 5 41 0 < 0. 1 29 5 11 30 27 30 01 /0 3 23 .1 5. 9 -4 3 20 2 34 3 10 0 20 8 41 0 < 0. 1 31 8 11 20 27 00 03 /0 3 23 .3 6. 0 -8 3 22 5 35 6 10 4 20 3 41 0 < 0. 1 31 2 11 10 27 30 04 /0 3 23 .2 6. 0 -1 90 21 7 35 1 10 2 20 7 43 0 7 31 8 11 00 27 30 05 /0 3 23 .7 n. m . -1 40 19 6 36 0 98 21 2 41 0 4 30 6 11 20 27 00 t ab le a 2 (c on ti nu es o n ne xt p ag e) . 15 hydrology and seismicity on mt. vesuvius si te 1 3 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) 08 /0 3 24 .1 6. 1 -1 10 20 5 38 0 10 4 19 7 43 0 3 31 4 11 10 27 40 09 /0 3 24 6. 1 -2 00 19 9 36 7 11 5 20 2 44 0 6 31 0 10 90 27 20 10 /0 3 23 .8 6. 1 -9 3 20 3 36 9 10 8 22 0 48 0 7 29 2 11 20 27 90 11 /0 3 23 .6 6. 0 -9 0 21 4 38 7 10 5 19 8 45 0 6 32 0 11 30 28 00 12 /0 3 23 .7 6. 0 -6 2 20 8 38 5 10 0 20 5 42 0 5 30 4 11 40 27 60 01 /0 4 23 .0 5. 9 -2 00 20 5 37 3 10 4 20 5 42 0 5 29 9 11 40 27 40 03 /0 4 23 .3 5. 8 -3 0 18 4 33 5 95 21 4 39 0 0 27 2 10 90 25 70 04 /0 4 23 .3 5. 8 -3 0 18 5 33 0 93 21 2 37 0 6 27 6 11 20 25 80 05 /0 4 23 .7 6. 0 -2 7 18 0 32 4 89 21 0 34 0 2 26 8 10 90 25 00 06 /0 4 23 .6 n. m . -8 2 18 4 33 7 94 20 4 37 0 19 28 1 10 90 25 60 08 /0 4 n. m . n. m . n. m . 20 3 36 0 97 20 0 40 0 2 29 7 11 00 26 50 09 /0 4 24 .1 6. 1 -9 0 20 3 36 8 96 19 1 40 0 2 30 0 10 90 26 40 10 /0 4 23 .9 6. 0 -7 0 17 1 33 0 75 17 9 29 0 16 25 3 10 40 23 40 11 /0 4 23 .5 5. 9 -3 8 16 9 31 0 79 18 0 30 0 12 23 2 10 50 23 20 12 /0 4 23 .5 5. 9 -1 8 17 7 32 2 86 19 3 34 0 4 26 1 10 50 24 20 01 /0 5 23 .5 n. m . -3 7 18 1 32 2 86 18 6 34 0 4 26 5 10 20 24 00 03 /0 5 23 .2 6. 0 -6 0 16 2 31 6 78 17 9 30 0 7 23 4 98 0 22 50 04 /0 5 23 .3 5. 8 70 15 7 28 8 74 16 9 29 0 9 21 4 99 0 21 80 05 /0 5 23 .2 5. 9 -3 .5 17 2 31 4 80 18 1 31 0 < 0. 1 23 8 10 10 23 00 06 /0 5 23 .4 5. 9 -9 2 18 3 33 0 85 18 6 34 0 < 0. 1 26 3 10 20 24 00 07 /0 5 23 .8 5. 9 -3 6 17 9 32 6 84 17 9 34 0 < 0. 1 24 8 10 10 23 60 08 /0 5 23 .5 n. m . n. m . 19 3 34 6 91 18 9 36 0 < 0. 1 27 1 10 50 25 00 10 /0 5 23 .7 n. m . n. m . 17 7 32 0 81 17 7 32 0 < 0. 1 25 2 10 20 23 40 11 /0 5 23 n. m . n. m . 18 2 33 0 86 18 2 33 0 < 0. 1 25 4 10 30 23 90 12 /0 5 23 .1 n. m . n. m . 16 7 29 3 79 17 7 31 0 < 0. 1 22 8 96 0 22 10 01 /0 6 22 .4 n. m . n. m . 17 5 30 5 83 18 3 32 0 < 0. 1 25 1 98 0 23 00 03 /0 6 22 .5 n. m . n. m . 18 3 31 9 86 18 4 35 0 < 0. 1 26 2 98 0 23 70 06 /0 6 22 .7 5. 9 n. m . 16 1 29 4 80 18 6 31 0 < 0. 1 22 8 98 0 22 30 10 /0 6 22 .3 5. 8 -4 0 16 1 29 4 80 18 6 31 0 < 0. 1 22 8 98 0 22 30 02 /0 7 21 .5 n. m . n. m . 14 5 25 9 68 16 6 25 0 < 0. 1 19 2 91 0 19 90 06 /0 7 22 .0 5. 9 -2 0 13 6 24 2 63 15 4 22 0 < 0. 1 16 5 89 0 18 70 02 /0 8 21 .7 5. 8 -3 5 12 8 23 0 60 13 8 20 0 < 0. 1 15 5 91 0 18 24 05 /0 8 21 .2 6. 8 -1 50 11 6 21 3 52 13 0 19 0 < 0. 1 13 7 77 0 16 10 10 /0 8 21 .4 5. 8 -2 30 10 7 19 0 47 11 8 17 0 < 0. 1 11 9 72 0 14 70 07 /0 9 20 .7 5. 9 -2 00 88 16 7 41 10 8 13 0 < 0. 1 97 66 0 12 90 04 /1 0 19 .7 5. 8 n. m . 88 16 3 55 15 4 18 0 < 0. 1 13 2 73 0 15 00 07 /1 0 20 .0 5. 6 n. m . 10 3 19 6 59 16 5 22 0 < 0. 1 16 4 78 0 16 90 07 /1 1 20 .4 5. 9 40 14 2 24 7 68 18 1 28 0 < 0. 1 21 2 90 0 20 20 t ab le a 2 (c on ti nu ed fr om p re vi ou s pa ge ). federico et al. 16 5/ 98 23 .3 6. 3 -1 70 53 0 41 0 76 0 62 0 20 00 < 0. 1 88 40 60 84 70 0. 01 0 50 0 11 /9 8 23 .3 6. 4 -2 60 54 0 41 5 77 0 62 0 22 00 2. 4 86 43 40 89 80 0. 50 0 89 0 1/ 99 23 .2 6. 4 -7 0 56 0 41 3 81 0 59 0 20 20 7. 1 88 43 40 88 10 0. 18 0 74 0 3/ 99 23 .1 6. 4 -9 0 53 0 43 3 76 0 61 0 19 70 0. 7 82 39 10 83 00 0. 19 0 10 10 5/ 99 23 .5 6. 4 -2 60 53 0 41 3 72 0 59 0 18 30 5. 9 87 40 80 84 70 0. 08 0 68 0 10 /9 9 23 .2 6. 4 -1 50 51 0 42 9 83 0 55 0 18 80 1. 7 82 42 90 85 50 0. 13 0 73 0 3/ 00 22 .6 6. 5 -2 00 52 0 42 0 79 0 41 0 21 00 21 .0 11 4 38 20 81 80 0. 07 0 90 0 5/ 00 22 .9 6. 9 -2 50 54 0 41 4 83 0 40 0 20 80 6. 8 11 3 37 70 79 90 0. 19 0 11 80 7/ 00 23 .4 6. 4 -2 00 53 0 40 1 77 0 67 0 19 80 17 .0 10 5 45 90 91 20 0. 00 1 66 0 10 /0 0 23 .0 6. 3 -1 30 56 0 42 2 79 0 51 0 18 80 < 0. 1 77 45 90 89 70 0. 23 0 10 30 11 /0 0 22 .3 6. 3 -8 0 51 0 38 7 79 0 55 0 19 80 < 0. 1 77 44 50 88 60 0. 45 0 10 90 1/ 01 21 .8 6. 6 n. m . 56 0 35 2 80 0 73 0 21 10 1. 2 71 42 90 86 60 0. 35 0 86 0 3/ 01 22 .1 6. 5 n. m . 53 0 40 6 78 0 57 0 18 10 1. 2 80 43 00 84 20 0. 43 0 82 0 4/ 01 22 .1 6. 4 -9 0 51 0 39 2 84 0 61 0 18 60 7. 3 95 43 40 86 40 0. 34 0 80 0 5/ 01 22 .4 6. 2 -8 0 50 0 39 2 79 0 57 0 19 10 0. 8 71 46 20 88 90 0. 33 0 96 0 6/ 01 23 .1 6. 4 -5 8 51 0 38 1 81 0 68 0 20 40 5. 0 84 46 20 91 20 0. 26 0 10 40 7/ 01 23 .7 6. 3 -9 0 54 0 40 5 80 0 70 0 20 10 32 95 46 20 91 80 0. 09 0 11 20 8/ 01 23 .3 6. 5 -8 0 54 0 40 8 83 0 73 0 20 20 21 85 46 10 92 20 0. 06 0 11 00 9/ 01 22 .9 6. 3 -6 0 55 0 40 4 82 0 68 0 20 40 20 84 44 30 90 00 0. 64 0 10 60 10 /0 1 22 .9 6. 3 -6 0 55 0 39 8 81 0 64 0 20 50 6. 0 83 44 10 89 40 n. m . n. m . 11 /0 1 22 .9 6. 2 -4 0 53 0 38 3 86 0 47 0 18 60 < 0. 1 93 41 70 83 70 n. m . n. m . 12 /0 1 23 .1 6. 1 -8 0 55 0 41 5 81 0 65 0 20 10 < 0. 1 97 44 40 89 70 n. m . n. m . 01 /0 2 22 .7 6. 3 -6 0 53 0 40 0 82 0 65 0 19 70 < 0. 1 98 44 90 89 60 0. 05 2 91 0 02 /0 2 22 .6 6. 0 -5 0 55 0 39 2 81 0 62 0 18 90 < 0. 1 10 3 45 80 89 30 0. 14 3 91 0 03 /0 2 22 .8 6. 1 n. m . 56 0 39 6 80 0 58 0 19 50 < 0. 1 94 45 90 89 70 0. 09 9 11 80 04 /0 2 23 .2 6. 4 -3 0 55 0 39 4 81 0 68 0 20 00 11 .8 92 44 20 89 40 0. 06 1 70 0 05 /0 2 23 .3 6. 4 -1 50 56 0 39 4 80 0 64 0 19 10 < 0. 1 91 48 30 92 20 0. 04 7 65 0 06 /0 2 23 .4 6. 5 -1 00 58 0 41 9 80 0 72 0 20 10 < 0. 1 85 46 60 92 70 0. 06 9 67 0 07 /0 2 23 .4 6. 5 -1 50 53 0 39 5 75 0 70 0 19 50 24 83 47 10 91 10 0. 05 1 67 0 08 /0 2 23 .8 6. 7 -1 10 54 0 39 2 78 0 61 0 19 50 < 0. 1 83 46 90 90 30 0. 05 9 69 0 09 /0 2 23 .5 6. 4 -1 30 54 0 39 6 78 0 62 0 19 40 < 0. 1 89 45 20 88 80 0. 17 0 77 0 10 /0 2 23 .3 6. 4 -1 50 55 0 41 1 77 0 57 0 19 30 22 88 44 40 87 60 0. 16 4 81 0 12 /0 2 20 .8 6. 4 -7 0 54 0 38 8 79 0 53 0 18 10 < 0. 1 93 43 80 85 30 0. 29 0 67 0 01 /0 3 20 .8 6. 1 -7 0 55 0 39 2 76 0 53 0 18 40 10 92 44 40 86 00 0. 31 0 68 0 si te 1 4 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 3 (c on ti nu es o n ne xt p ag e) . 17 hydrology and seismicity on mt. vesuvius 03 /0 3 n. m . n. m . n. m . 54 0 40 2 73 0 59 0 19 50 12 92 42 70 85 80 0. 39 1 79 0 04 /0 3 23 .4 6. 5 -1 80 59 0 40 3 79 0 68 0 21 60 11 7 95 42 70 89 80 0. 04 6 81 0 06 /0 3 23 .5 n. m . -2 20 56 0 43 1 78 0 72 0 18 80 < 0. 1 88 47 40 92 00 0. 16 5 62 0 07 /0 3 23 .1 6. 3 -1 50 54 0 40 8 77 0 68 0 19 50 < 0. 1 95 47 10 91 50 n. m . n. m . 08 /0 3 23 .7 6. 4 -2 00 55 0 42 1 83 0 67 0 19 50 < 0. 1 99 45 90 91 20 0. 11 7 79 0 09 /0 3 22 .8 6. 5 -1 10 55 0 42 2 79 0 69 0 20 60 < 0. 1 91 46 20 92 20 0. 11 7 77 0 10 /0 3 21 .9 6. 5 -1 10 54 0 41 7 81 0 63 0 19 30 22 .3 12 1 46 70 91 20 0. 61 77 83 0 12 /0 3 22 .6 6. 4 -1 40 55 0 41 8 76 0 67 0 19 30 < 0. 1 10 8 46 60 91 00 0. 16 0 76 0 01 /0 4 22 .2 6. 3 -1 80 53 0 40 5 76 0 67 0 19 30 < 0. 1 10 1 45 60 89 50 n. m . n. m . 03 /0 4 22 .2 6. 3 -1 50 53 0 41 3 73 0 67 0 18 50 < 0. 1 12 4 46 10 89 30 0. 67 1 71 0 04 /0 4 21 .4 6. 3 -1 70 51 0 40 5 74 0 67 0 19 60 < 0. 1 11 0 45 10 89 10 0. 08 9 89 0 05 /0 4 22 .5 6. 5 -1 60 53 0 40 5 79 0 72 0 19 30 < 0. 1 11 3 46 90 91 90 0. 09 2 68 0 06 /0 4 23 .0 6. 7 -1 20 54 0 41 3 78 0 69 0 18 70 < 0. 1 11 2 47 40 91 30 n. m . n. m . 09 /0 4 22 .8 6. 6 -1 40 56 0 42 8 78 0 62 0 20 50 < 0. 1 10 1 44 50 89 80 0. 05 0 75 0 10 /0 4 22 .4 6. 4 -1 50 56 0 42 7 83 0 57 0 19 70 < 0. 1 11 2 43 40 88 10 0. 20 7 76 0 11 /0 4 22 .2 6. 3 -1 10 53 0 40 9 75 0 55 0 19 40 < 0. 1 96 42 70 85 40 0. 23 5 62 0 12 /0 4 22 .0 6. 4 -1 10 53 0 40 7 78 0 55 0 19 40 < 0. 1 88 42 90 85 90 0. 06 6 65 0 01 /0 5 22 .0 n. m . -8 0 53 0 41 0 78 0 59 0 18 90 < 0. 1 11 5 42 70 85 80 n. m . n. m . 03 /0 5 22 .4 6. 2 -1 40 53 0 40 8 78 0 49 0 18 30 < 0. 1 10 6 42 70 84 00 0. 48 1 74 0 04 /0 5 22 .3 6. 2 2. 0 52 0 39 8 75 0 58 0 18 50 < 0. 1 93 43 20 85 00 0. 20 8 61 0 05 /0 5 22 .3 6. 2 -1 40 51 0 39 7 74 0 55 0 18 50 < 0. 1 89 42 10 83 40 n. m . n. m . 06 /0 5 22 .6 6. 4 -1 30 55 0 40 4 79 0 64 0 19 80 < 0. 1 89 44 60 89 20 0. 09 7 52 0 07 /0 5 22 .7 6. 5 -1 40 51 0 38 7 72 0 62 0 19 80 < 0. 1 89 41 50 84 60 0. 13 7 59 0 08 /0 5 22 .6 n. m . n. m . 54 0 41 6 78 0 66 0 19 90 < 0. 1 87 44 20 89 00 0. 43 0 84 0 10 /0 5 22 .7 n. m . n. m . 55 0 41 8 78 0 57 0 20 60 < 0. 1 95 41 60 86 20 n. m . n. m . 11 /0 5 21 .4 6. 3 n. m . 54 0 41 4 78 0 59 0 19 40 < 0. 1 94 42 70 86 30 0. 43 0 70 0 12 /0 5 22 .3 6. 2 n. m . 57 0 42 2 81 0 59 0 20 60 < 0. 1 85 42 70 88 10 0. 18 4 72 0 01 /0 6 21 .8 6. 3 n. m . 52 0 38 0 73 0 54 0 19 10 < 0. 1 99 39 90 81 70 0. 28 6 74 0 03 /0 6 22 .3 6. 4 n. m . 54 0 39 7 76 0 55 0 20 00 < 0. 1 10 6 40 60 84 00 n. m . n. m . 10 /0 6 22 .8 6. 4 -8 0 54 0 41 2 79 0 59 0 18 90 < 0. 1 89 44 60 87 70 0. 14 3 77 0 02 /0 7 22 .0 n. m . n. m . 54 0 40 7 78 0 57 0 19 90 < 0. 1 84 42 80 86 60 n. m . n. m . 06 /0 7 22 .5 6. 0 -2 00 55 0 41 0 78 0 67 0 19 60 < 0. 1 91 44 70 89 30 0. 30 7 80 0 si te 1 4 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 3 (c on ti nu ed fr om p re vi ou s pa ge ). federico et al. 18 11 /9 8 21 .0 6. 6 14 0 19 5 36 1 11 9 13 6 33 1 74 19 6 12 10 25 00 1/ 99 20 .9 6. 6 13 0 21 2 38 6 13 0 11 0 32 6 84 21 0 11 30 24 60 3/ 99 16 .9 7. 1 17 0 20 2 35 4 11 4 93 30 7 72 18 3 10 90 23 00 5/ 99 21 .1 6. 7 90 19 3 36 0 10 6 97 26 6 78 18 9 10 20 22 00 10 /9 9 20 .8 6. 7 18 0 20 7 36 8 12 2 99 29 4 11 2 19 5 11 50 24 40 11 /9 9 20 .7 6. 6 23 0 20 3 38 4 12 8 11 0 29 9 10 7 19 1 11 40 23 90 3/ 00 20 .0 6. 6 11 0 21 9 38 1 12 1 97 28 3 77 19 1 11 70 24 40 5/ 00 20 .9 6. 7 14 0 21 7 38 0 11 8 93 28 4 73 18 4 11 70 24 10 7/ 00 21 .3 6. 7 20 0 20 6 36 4 12 9 10 7 29 6 74 19 1 12 30 24 70 10 /0 0 20 .8 6. 7 16 0 21 6 38 5 13 2 92 30 0 76 19 1 12 40 25 10 11 /0 0 20 .7 6. 5 18 0 21 4 37 1 13 2 97 32 8 79 20 8 12 80 25 90 1/ 01 19 .2 6. 8 n. m . 20 3 41 4 13 1 12 0 32 3 71 19 1 12 40 25 70 3/ 01 20 .0 6. 5 n. m . 21 8 37 1 13 0 11 2 31 0 76 20 1 12 30 25 60 4/ 01 20 .8 6. 5 15 0 21 5 37 5 12 9 91 30 0 72 18 2 12 30 24 90 5/ 01 21 .2 6. 6 15 0 20 4 36 2 12 9 11 0 31 4 98 19 3 12 50 25 20 6/ 01 21 .0 6. 5 26 0 22 0 37 7 13 7 11 1 32 7 73 19 9 12 50 25 60 7/ 01 21 .4 6. 5 21 0 22 8 39 6 14 0 10 3 33 1 80 20 1 12 60 26 00 8/ 01 22 .5 6. 6 19 0 22 5 38 9 13 3 10 7 32 0 54 19 5 12 90 25 80 9/ 01 20 .9 6. 5 23 0 23 2 39 6 14 2 12 2 34 1 60 20 5 12 70 26 30 10 /0 1 20 .9 6. 5 18 0 24 2 40 4 14 8 12 2 34 1 72 19 1 12 80 26 50 11 /0 1 20 .3 6. 6 19 0 22 6 39 7 14 2 11 4 33 9 77 21 6 13 10 26 80 12 /0 1 20 .2 6. 6 11 0 22 1 37 9 13 0 10 4 32 2 65 20 7 12 80 25 80 01 /0 2 20 .6 6. 3 14 0 21 6 37 9 13 5 11 1 33 8 77 22 2 12 90 26 30 02 /0 2 20 .1 6. 5 24 0 22 9 38 5 12 8 10 2 30 8 70 19 2 12 40 25 20 03 /0 2 20 .0 6. 1 n. m . 22 6 39 6 13 1 10 7 33 4 61 21 7 12 30 25 70 04 /0 2 20 .4 6. 5 -6 0 22 7 39 5 13 0 10 5 33 0 47 21 3 12 50 25 60 05 /0 2 21 .0 6. 6 15 0 22 2 37 9 13 3 10 0 31 9 73 19 5 12 70 25 50 06 /0 2 21 .3 6. 6 17 0 23 1 38 9 13 1 10 2 32 0 76 19 4 12 50 25 60 07 /0 2 21 .5 6. 5 15 0 23 7 40 0 13 4 10 6 32 7 75 19 6 12 70 26 10 08 /0 2 21 .1 6. 6 16 0 23 5 39 4 13 0 98 32 9 76 20 6 12 70 26 10 09 /0 2 20 .4 6. 6 11 0 23 2 39 1 13 1 98 33 4 74 20 6 13 10 26 40 10 /0 2 20 .8 6. 5 14 0 23 5 39 0 12 8 96 31 7 69 19 5 13 20 26 20 12 /0 2 20 .9 6. 5 n. m . 23 1 36 5 13 1 11 7 32 0 72 20 4 13 10 26 20 12 /0 2 19 .8 6. 5 12 0 22 7 39 3 13 0 92 29 1 62 18 1 12 80 25 30 01 /0 3 19 .2 6. 2 12 0 22 4 37 5 12 5 93 30 0 71 19 9 12 60 25 20 03 /0 3 n. m . n. m . n. m . 21 1 33 5 10 3 93 23 9 12 2 21 0 10 30 22 40 04 /0 3 20 .8 6. 7 15 0 19 2 31 0 10 1 99 24 5 13 6 22 2 92 0 21 20 05 /0 3 20 .8 6. 5 16 0 16 4 30 1 87 86 18 9 13 1 19 3 87 0 19 40 si te 1 9 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) t ab le a 4 (c on ti nu es o n ne xt p ag e) . 19 hydrology and seismicity on mt. vesuvius 06 /0 3 20 .9 n. m . 16 0 16 0 28 7 86 94 18 6 13 6 19 0 85 0 19 10 07 /0 3 21 .4 6. 6 16 0 15 3 27 8 85 10 3 19 7 12 8 19 2 85 0 19 00 09 /0 3 21 6. 5 n. m . 17 9 32 6 10 7 11 1 26 1 10 4 20 6 10 40 22 30 10 /0 3 20 .3 6. 6 13 0 19 0 33 3 11 8 11 2 24 6 85 17 9 11 30 22 80 11 /0 3 18 .4 6. 5 18 0 19 8 35 1 11 5 11 9 28 0 93 20 2 11 60 24 00 12 /0 3 18 .4 6. 5 12 0 20 0 36 0 11 0 11 3 26 8 85 19 0 11 50 23 70 01 /0 4 18 .5 6. 3 11 0 19 3 35 0 10 9 11 0 27 0 82 19 0 11 40 23 40 02 /0 4 n. m . 6. 8 14 0 19 2 34 6 11 1 11 0 26 8 75 18 9 11 50 23 30 03 /0 4 19 .8 6. 8 14 0 20 5 36 3 11 6 11 2 28 5 68 19 0 11 60 23 80 04 /0 4 17 .6 6. 6 12 0 21 0 36 9 11 8 10 7 29 6 68 19 8 12 00 24 05 05 /0 4 17 .6 6. 6 50 21 1 37 3 12 2 10 8 29 2 73 19 6 12 00 24 50 06 /0 4 21 .8 6. 5 90 21 3 38 8 12 4 62 30 0 66 19 6 11 90 24 10 08 /0 4 n. m . n. m . n. m . 21 9 39 0 13 0 10 7 31 7 71 20 5 12 30 25 40 09 /0 4 21 .6 6. 6 10 22 4 40 1 13 0 10 8 32 6 80 21 2 12 60 26 10 10 /0 4 21 .3 6. 5 90 22 9 40 7 13 9 10 8 32 2 76 20 8 12 70 26 20 11 /0 4 19 .6 6. 4 12 0 22 6 40 0 13 0 10 8 32 6 78 20 6 13 00 26 40 12 /0 4 19 .6 6. 5 12 0 22 6 40 3 13 4 10 7 32 7 78 20 7 13 00 26 50 01 /0 5 19 .3 n. m . 70 22 4 40 3 13 0 10 1 30 8 78 19 9 12 70 25 80 03 /0 5 19 .4 6. 4 90 22 3 40 4 13 4 89 30 0 74 19 5 12 60 25 40 04 /0 5 19 .8 6. 4 12 0 22 4 39 7 13 0 10 8 30 9 76 19 6 12 60 25 70 05 /0 5 20 .9 6. 4 10 0 21 6 38 5 12 8 10 4 29 6 74 19 1 12 60 25 30 06 /0 5 21 .6 6. 4 14 0 22 2 39 9 13 2 10 8 32 6 76 20 4 12 60 25 90 07 /0 5 21 .9 6. 6 20 0 22 4 40 1 13 4 11 1 32 6 71 20 4 12 60 26 00 08 /0 5 22 .5 n. m . n. m . 22 2 39 7 13 4 11 7 33 9 81 21 2 12 60 26 30 10 /0 5 20 .8 n. m . n. m . 22 7 40 4 13 7 11 3 33 8 81 21 5 12 70 26 50 11 /0 5 20 .1 6. 4 n. m . 23 1 41 3 14 0 11 2 33 9 80 21 3 12 80 26 60 12 /0 5 18 .2 6. 5 n. m . 23 7 40 4 14 1 11 3 35 3 79 21 2 12 60 26 60 03 /0 6 19 .4 6. 5 n. m . 23 0 39 0 13 6 10 7 33 6 79 20 4 12 60 26 10 06 /0 6 21 .2 6. 3 n. m . 24 4 43 3 15 0 12 3 38 9 10 0 24 6 12 60 27 90 10 /0 6 20 .8 6. 5 10 0 24 0 42 0 15 0 12 1 36 9 89 23 0 13 40 28 10 02 /0 7 19 .1 n. m . n. m . 23 6 41 0 14 3 11 4 33 9 82 21 1 13 20 27 20 06 /0 7 21 .8 6. 5 10 0 21 2 36 3 12 5 10 8 30 4 62 19 7 11 60 24 10 02 /0 8 19 .7 6. 8 13 0 21 3 37 1 12 3 10 5 29 8 68 19 7 11 70 24 20 05 /0 8 20 .3 6. 9 12 0 20 5 35 5 11 7 99 27 0 57 18 5 11 50 23 20 10 /0 8 20 .7 7. 0 12 0 20 3 34 8 11 3 97 27 7 70 18 3 11 40 23 10 04 /1 0 19 .2 6. 6 20 0 16 7 29 0 10 4 93 22 9 68 16 8 98 0 20 00 07 /1 0 21 .5 6. 6 n. m . 18 2 32 6 10 0 83 23 4 69 17 0 10 40 21 00 07 /1 1 22 .3 7. 0 21 0 19 4 34 0 10 9 89 25 6 80 18 4 10 80 22 20 si te 1 9 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) t ab le a 4 (c on ti nu ed fr om p re vi ou s pa ge ). federico et al. 20 5/ 98 21 .7 7. 0 -3 0 16 3 22 5 68 73 15 3 4. 4 24 1 73 2 16 60 2. 2e -2 56 11 /9 8 21 .5 7. 1 -6 04 15 8 22 9 65 68 18 0 2. 2 24 8 73 8 16 90 2. 8e -2 57 1/ 99 21 .4 7. 3 -1 5 16 2 23 4 73 79 19 1 2. 5 27 2 75 0 17 60 4. 8e -2 44 5/ 99 21 .5 7. 4 -4 0 15 1 21 3 69 58 14 8 1. 4 23 0 62 8 15 00 2. 0e -1 41 10 /9 9 20 .9 7. 0 12 0 16 2 21 7 63 82 14 9 3. 7 22 6 10 07 19 10 6. 9e -2 43 11 /9 9 21 .8 6. 8 12 0 15 0 21 7 74 77 16 3 2. 9 23 5 68 3 17 00 2. 7e -2 60 3/ 00 19 .7 7. 1 11 0 16 0 22 0 67 75 15 4 8. 5 23 2 70 2 16 10 4. 1e -2 44 5/ 00 21 .2 7. 5 70 16 6 23 1 68 68 16 1 6. 2 23 9 72 0 15 70 n. m . n. m . 7/ 00 21 .4 7. 2 55 15 1 21 1 70 83 16 4 7. 5 23 4 73 2 16 60 n. m . n. m . 10 /0 0 21 .2 7. 1 70 16 8 21 9 68 68 14 6 4. 1 22 8 72 0 16 10 2. 2e -2 67 11 /0 0 21 .3 6. 9 10 0 15 6 23 2 70 72 16 5 3. 8 24 7 74 4 16 70 4. 2e -2 54 1/ 01 20 .3 6. 9 n. m . 15 5 22 7 65 71 16 6 3. 1 23 7 72 3 17 30 3. 8e -2 83 3/ 01 21 .3 6. 9 n. m . 16 6 22 6 74 83 17 6 3. 8 25 1 78 1 17 70 n. m . 78 4/ 01 21 .4 6. 8 70 16 1 22 9 80 80 15 8 3. 8 22 7 78 7 17 10 6. 3e -2 12 7 5/ 01 21 .5 7. 0 90 15 2 21 3 74 79 16 9 2. 9 25 0 73 8 16 80 6. 4e -2 76 6/ 01 21 .5 6. 9 10 0 16 0 21 8 74 72 16 9 5. 5 24 7 70 2 16 50 n. m . n. m . 7/ 01 21 .5 6. 7 15 0 16 2 22 3 70 73 16 5 9. 2 24 1 72 0 16 60 n. m . n. m . 8/ 01 22 .0 6. 8 70 15 9 22 2 72 73 15 6 0. 6 23 0 72 6 16 40 n. m . n. m . 9/ 01 21 .5 6. 9 20 0 16 6 22 3 75 78 17 2 4. 3 24 5 74 1 17 00 n. m . n. m . 10 /0 1 21 .6 7. 0 11 0 14 9 21 7 67 70 16 3 10 .4 23 6 73 8 16 50 9. 6e -3 70 01 /0 2 21 .8 6. 7 90 15 3 21 1 66 74 17 6 < 0. 1 26 2 71 7 16 60 n. m . n. m . 02 /0 2 20 .6 6. 7 11 0 15 9 21 4 65 72 14 8 7 22 9 68 3 15 80 5. 4e -2 61 03 /0 2 21 .1 6. 7 n. m . 15 7 23 2 70 69 18 2 < 0. 1 25 9 71 1 16 80 2. 3e -2 62 04 /0 2 21 .5 6. 8 70 16 3 22 9 70 81 18 3 < 0. 1 25 7 74 7 17 30 3. 4e -2 16 5 05 /0 2 21 .9 6. 9 11 0 15 9 21 1 67 65 16 4 < 0. 1 23 6 72 6 16 30 n. m . n. m . 06 /0 2 21 .8 6. 9 60 16 2 22 5 68 77 17 4 < 0. 1 23 6 72 3 16 70 4. 9e -2 69 07 /0 2 21 .5 6. 9 -5 16 4 22 4 66 74 15 4 < 0. 1 23 0 71 7 16 30 8. 7e -2 62 09 /0 2 21 .4 6. 9 -2 5 15 4 21 1 60 67 15 9 < 0. 1 23 4 70 5 15 90 1. 4e -1 59 09 /0 2 21 .1 7. 3 -5 0 15 1 20 9 60 66 15 5 < 0. 1 23 1 70 2 15 70 7. 1e -2 45 10 /0 2 21 .2 7. 0 -3 0 16 3 22 0 65 67 15 9 < 0. 1 23 9 70 8 16 20 1. 4e -1 62 12 /0 2 21 .0 7. 0 40 15 5 21 5 63 66 13 8 < 0. 1 22 1 78 1 16 40 1. 2e -1 52 12 /0 2 20 .9 6. 9 50 15 8 21 8 64 69 14 2 < 0. 1 22 0 73 2 16 00 8. 6e -2 60 01 /0 3 21 .7 6. 8 20 16 6 21 5 65 71 15 6 3 24 7 72 3 16 50 1. 2e -1 61 03 /0 3 21 .3 7. 2 80 17 5 21 3 64 63 16 3 < 0. 1 25 0 68 6 16 20 8. 5e -2 58 04 /0 3 21 .6 6. 9 -9 0 17 0 21 1 64 69 18 1 1 26 9 67 2 16 40 6. 7e -2 56 05 /0 3 21 .5 n. m . 70 15 3 21 6 63 69 15 7 1 24 9 65 3 15 60 6. 0e -2 63 07 /0 3 21 .8 7. 0 60 15 2 21 4 61 68 15 3 < 0. 1 24 3 64 1 15 30 n. m . n. m . si te 2 9 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 5 (c on ti nu es o n ne xt p ag e) . 21 hydrology and seismicity on mt. vesuvius 08 /0 3 21 .8 7. 0 80 14 2 20 4 62 72 14 2 0 23 5 65 3 15 10 3. 3e -2 56 09 /0 3 21 .3 6. 8 n. m . 14 0 20 7 63 69 13 3 3 23 9 62 8 14 80 1. 9e -2 66 10 /0 3 20 .9 7. 0 13 0 14 2 20 7 60 64 12 3 < 0. 1 21 1 64 7 14 50 1. 9e -2 63 11 /0 3 21 .4 7. 0 60 14 8 21 5 60 67 14 0 3 24 1 63 7 15 10 8. 6e -3 58 12 /0 3 21 .1 7. 0 30 14 6 21 3 59 69 13 4 < 0. 1 23 1 67 1 15 20 < 0. 00 1 54 01 /0 4 20 .7 6. 7 50 13 6 20 5 59 69 13 1 4 23 1 63 4 14 70 n. m . n. m . 03 /0 4 21 .0 6. 2 -3 0 14 3 20 7 59 69 12 3 4 24 2 63 7 14 90 2. 9e -2 59 04 /0 4 20 .8 7. 0 30 14 4 20 9 59 69 14 6 2 24 7 65 3 15 30 2. 9e -2 60 05 /0 4 21 .1 7. 0 40 14 3 20 4 63 75 14 4 0 23 9 64 7 15 10 2. 9e -2 59 06 /0 4 21 .3 n. m . 10 14 3 21 1 60 68 13 5 0 23 0 64 7 14 90 1. 2e -2 55 09 /0 4 20 .9 7. 0 40 14 3 21 1 60 71 14 8 0 25 7 64 1 15 30 < 0. 00 1 23 10 /0 4 20 .7 7. 1 30 13 0 20 6 50 68 10 8 0 23 9 63 7 14 40 3. 3e -1 33 11 /0 4 20 .3 6. 9 70 13 9 20 6 57 67 13 9 4 23 8 63 7 14 90 < 0. 00 1 29 12 /0 4 20 .1 7. 3 30 14 0 20 7 60 69 13 8 2 24 5 63 1 14 90 6. 6e -1 10 03 /0 5 20 .3 6. 9 70 14 0 21 1 60 63 13 2 0 23 6 60 7 14 50 3. 8e -3 39 04 /0 5 19 .5 7. 3 30 13 9 21 4 60 73 14 5 0 24 0 60 7 14 80 < 0. 00 1 17 05 /0 5 20 .8 6. 8 -2 13 6 20 4 59 70 13 3 < 0. 1 23 4 60 4 14 40 n. m . n. m . 06 /0 5 21 .1 7. 3 80 14 3 21 3 58 66 13 6 < 0. 1 24 0 60 7 14 60 2. 9e -3 23 07 /0 5 21 .1 6. 7 90 13 4 20 1 58 70 13 6 < 0. 1 24 1 58 0 14 20 3. 9e -3 14 08 /0 5 20 .5 n. m . n. m . 13 5 20 3 58 69 13 5 < 0. 1 24 1 58 0 14 20 4. 9e -3 10 10 /0 5 20 .4 n. m . n. m . 13 5 20 3 58 69 13 5 < 0. 1 24 1 58 0 14 20 n. m . n. m . 11 /0 5 18 .1 6. 9 n. m . 13 2 20 0 58 69 12 8 < 0. 1 23 7 59 2 14 20 9. 3e -3 30 12 /0 5 19 .6 7. 6 n. m . 13 9 20 4 60 71 14 0 < 0. 1 25 4 57 6 14 40 3. 6e -3 10 01 /0 6 19 .7 7. 6 n. m . 13 1 19 5 59 73 13 1 < 0. 1 24 4 57 6 14 10 2. 8e -3 12 03 /0 6 19 .5 7. 4 n. m . 13 4 19 5 56 67 13 3 < 0. 1 24 3 57 6 14 10 < 0. 00 1 20 06 /0 6 20 .2 7. 0 n. m . 13 5 20 6 61 74 13 5 < 0. 1 25 2 59 8 14 60 5. 5e -3 18 10 /0 6 19 .7 6. 9 15 0 n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . 2. 3e -2 42 02 /0 7 19 .6 n. m . n. m . 13 5 20 0 58 72 13 6 < 0. 1 24 8 57 0 14 20 2. 8e -3 25 06 /0 7 20 .4 7. 6 30 13 2 19 8 59 76 15 5 < 0. 1 23 3 57 0 14 20 3. 3e -3 14 02 /0 8 19 .6 7. 6 -1 0 12 8 19 3 58 75 16 1 < 0. 1 22 8 56 4 14 10 4. 2e -3 16 05 /0 8 19 .7 7. 6 10 0 12 4 18 7 52 69 12 4 < 0. 1 22 8 52 5 13 10 2. 8e -3 10 10 /0 8 19 .5 7. 4 70 12 0 18 1 50 64 12 0 < 0. 1 22 0 51 2 12 70 4. 0e -3 12 07 /0 9 19 .8 7. 4 80 11 2 17 7 48 62 11 2 < 0. 1 20 4 50 6 12 20 n. m . n. m . 04 /1 0 19 .7 7. 4 15 0 10 6 16 6 45 54 11 2 < 0. 1 19 7 46 7 11 50 5. 9e -3 16 07 /1 0 22 .2 7. 1 10 7 17 3 43 55 11 3 < 0. 1 19 2 45 8 11 40 7. 8e -3 24 07 /1 1 19 .3 7. 1 14 0 10 8 17 0 41 51 11 1 < 0. 1 18 0 45 1 11 10 4. 0e -3 20 si te 2 9 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 5 (c on ti nu ed fr om p re vi ou s pa ge ). federico et al. 22 5/ 98 19 .0 7. 0 -9 0 30 9 28 5 19 1 14 1 38 3 5. 8 17 3 16 50 31 40 0. 00 01 13 0 11 /9 8 19 .1 7. 0 -9 0 31 3 28 1 18 7 13 3 41 1 2. 7 16 7 16 30 31 20 0. 00 01 17 0 1/ 99 18 .4 6. 9 -9 0 30 0 26 9 17 8 12 4 44 8 0. 0 18 3 15 90 31 00 0. 30 00 14 0 3/ 99 20 .1 7. 0 -1 00 31 0 28 2 18 3 14 1 45 6 1. 2 17 8 15 90 31 40 0. 00 50 80 5/ 99 20 .2 7. 0 -1 20 28 5 27 0 16 8 13 4 39 5 0. 2 18 5 13 80 28 30 0. 00 04 13 0 10 /9 9 19 .7 7. 2 -1 10 30 3 27 7 17 5 12 8 41 2 0. 3 17 7 16 80 31 50 0. 00 30 12 0 11 /9 9 20 .7 6. 8 -9 0 29 7 27 5 18 4 15 4 37 8 0. 2 16 4 15 60 30 70 0. 04 00 23 0 3/ 00 20 .5 7. 0 -9 0 30 2 27 7 17 2 13 5 39 1 8. 7 17 7 16 20 31 50 n. m . n. m . 5/ 00 20 .4 6. 9 -8 0 31 8 28 9 18 2 14 1 40 3 1. 2 17 6 16 50 31 40 < 0. 00 01 17 0 7/ 00 19 .8 6. 8 -9 0 29 0 27 0 18 6 13 9 38 2 3. 2 17 3 15 40 29 80 < 0. 00 01 12 0 10 /0 0 18 .8 6. 7 -1 10 30 1 28 6 19 0 12 2 38 3 < 0. 1 17 6 16 50 31 10 < 0. 00 01 17 0 11 /0 0 20 .0 6. 8 -9 0 30 0 26 2 18 7 13 7 38 4 < 0. 1 17 8 17 00 31 50 n. m . n. m . 1/ 01 19 .7 6. 9 n. m . 28 8 25 6 17 7 13 2 38 3 < 0. 1 16 5 16 30 30 60 n. m . n. m . 3/ 01 19 .6 6. 8 n. m . 30 6 27 1 19 2 15 9 41 5 < 0. 1 18 5 16 50 31 50 < 0. 00 01 20 0 4/ 01 20 .6 7. 0 -9 0 29 8 26 3 18 7 12 7 41 4 2. 0 16 9 17 10 32 10 0. 05 18 0 5/ 01 20 .4 6. 8 -5 0 28 1 26 6 18 9 13 4 41 2 0. 8 20 8 15 80 30 70 < 0. 00 01 18 0 6/ 01 20 .3 6. 8 -1 0 29 1 27 8 19 9 15 1 42 3 1. 8 21 1 15 80 31 40 n. m . n. m . 7/ 01 20 .5 6. 7 -1 10 30 9 28 0 20 0 15 9 43 2 6. 7 20 0 16 90 32 80 n. m . n. m . 8/ 01 20 .7 6. 9 -7 0 28 7 28 6 20 0 13 0 38 1 7. 3 24 0 16 40 31 70 < 0. 00 01 12 0 9/ 01 19 .9 6. 8 -5 0 30 4 27 1 18 5 14 8 38 5 4. 3 17 5 16 80 31 50 < 0. 00 01 12 0 10 /0 1 19 .8 6. 8 -4 0 28 2 29 1 19 4 14 8 40 8 4. 3 23 4 15 20 30 80 < 0. 00 01 13 0 11 /0 1 19 .9 6. 7 -6 0 31 5 28 1 20 1 14 1 41 6 < 0. 1 22 1 16 90 32 60 < 0. 00 01 15 0 12 /0 1 20 .0 6. 9 -7 0 29 2 25 9 18 1 13 9 38 7 < 0. 1 18 6 16 60 31 00 n. m . n. m . 1/ 02 20 .3 6. 7 -7 0 28 5 26 3 17 9 14 0 37 3 < 0. 1 17 4 16 80 30 90 0. 01 14 0 2/ 02 19 .9 6. 8 -3 0 32 0 28 1 19 5 13 8 40 7 < 0. 1 19 0 16 80 32 10 < 0. 00 01 13 0 3/ 02 20 .1 6. 8 30 5 26 9 18 8 13 8 41 8 < 0. 1 19 9 16 80 32 00 0. 1 20 0 4/ 02 20 .0 6. 9 -8 0 29 4 27 7 18 7 14 5 37 4 < 0. 1 19 4 16 50 31 20 0. 1 21 0 5/ 02 20 .1 6. 9 -6 0 29 8 27 3 19 5 13 6 40 6 < 0. 1 20 4 17 10 32 20 < 0. 00 01 16 0 6/ 02 20 .1 6. 8 -3 0 n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . 0. 01 12 0 si te 3 1 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 6 (c on ti nu es o n ne xt p ag e) . 23 hydrology and seismicity on mt. vesuvius 7/ 02 19 .9 6. 9 -8 0 n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . 0. 3 13 0 8/ 02 19 .9 6. 9 -8 0 n. m . n. m . n. m . n. m . n. m . n. m . n. m . 17 00 n. m . 0. 4 13 0 9/ 02 19 .5 7. 0 -8 0 31 5 26 4 17 9 12 1 37 0 < 0. 1 16 6 16 90 31 00 0. 5 14 0 10 /0 2 19 .8 6. 9 -1 10 30 5 27 8 18 2 15 3 41 3 < 0. 1 18 2 16 90 32 10 0. 15 17 0 12 /0 2 19 .7 7. 0 -9 0 31 1 26 7 18 5 12 5 33 8 < 0. 1 14 9 17 20 31 00 0. 5 15 0 12 /0 2 19 .6 7. 0 -1 10 30 6 26 6 18 1 12 0 35 5 < 0. 1 15 9 17 50 31 40 0. 3 11 0 1/ 03 19 .0 6. 8 -8 7 30 5 25 6 17 4 11 8 35 0 < 0. 1 16 0 17 60 31 30 0. 4 14 0 2/ 03 n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . 0. 4 13 0 3/ 03 19 .4 7. 0 -4 0 32 5 21 3 17 7 12 0 35 9 < 0. 1 19 8 16 40 30 40 0. 2 14 0 4/ 03 19 .8 6. 8 -9 0 29 6 27 5 18 6 12 9 39 8 3 21 5 16 70 31 70 0. 2 13 0 5/ 03 19 .6 n. m . -1 00 28 6 28 5 17 8 10 5 39 0 < 0. 1 19 8 17 00 31 40 0. 00 03 11 0 6/ 03 20 .3 n. m . -9 0 28 3 29 1 17 6 11 7 39 0 < 0. 1 20 6 17 00 31 60 < 0. 00 01 13 0 7/ 03 n. m . n. m . n. m . 30 2 29 4 19 5 12 1 36 8 < 0. 1 19 5 16 50 31 20 n. m . n. m . 8/ 03 n. m . n. m . n. m . 26 4 27 1 17 7 10 3 30 8 < 0. 1 19 0 16 20 29 30 n. m . n. m . 9/ 03 n. m . n. m . n. m . 26 5 26 7 19 1 11 0 35 8 2 17 3 15 74 29 40 0. 5 13 0 10 /0 3 n. m . n. m . n. m . 30 6 27 7 18 9 11 5 33 2 < 0. 1 14 1 16 90 30 50 n. m . n. m . 11 /0 3 n. m . n. m . n. m . 30 9 27 6 18 2 14 0 38 9 < 0. 1 17 1 17 10 31 80 0. 4 13 0 12 /0 3 n. m . n. m . n. m . 31 1 28 6 17 2 12 5 36 7 < 0. 1 16 3 16 90 31 20 0. 7 15 0 1/ 04 18 .6 7. 10 -1 70 29 6 27 2 16 8 12 2 34 5 < 0. 1 15 4 16 50 30 10 0. 6 15 0 2/ 04 n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . n. m . 0. 5 15 0 3/ 04 19 .5 7. 20 -1 30 29 8 27 9 18 0 12 9 35 5 < 0. 1 17 7 16 60 30 80 0. 4 15 0 4/ 04 19 .6 6. 80 7 29 4 26 9 17 3 12 5 36 6 9 16 3 16 00 30 00 0. 8 14 0 5/ 04 19 .4 6. 9 -1 50 29 6 26 9 17 2 11 9 41 7 < 0. 1 24 16 20 29 10 0. 6 16 0 6/ 04 19 .9 n. m . -1 10 29 3 27 2 17 5 11 9 32 5 < 0. 1 14 3 16 80 30 00 < 0. 00 01 15 0 8/ 04 n. m . n. m . n. m . 29 0 26 6 17 1 11 9 35 6 < 0. 1 15 8 16 00 29 60 0. 5 12 0 9/ 04 19 .1 7. 0 -1 80 29 5 27 3 17 2 11 5 35 6 < 0. 1 16 0 16 60 30 30 0. 6 13 0 10 /0 4 19 6. 9 -1 00 29 9 26 9 18 9 11 4 36 5 < 0. 1 16 6 16 40 30 40 0. 6 14 0 11 /0 4 19 .1 6. 9 -1 00 29 9 27 2 17 4 12 3 35 3 < 0. 1 16 7 16 90 30 70 0. 5 15 0 1/ 05 19 .1 n. m . -1 30 29 9 27 8 17 9 11 6 35 3 < 0. 1 15 8 16 50 30 30 0. 6 14 0 si te 3 1 d at e (m o/ yr ) t (° c ) ph e h (m v ) n a+ (m g/ l) k + (m g/ l) m g2 + (m g/ l) c a2 + (m g/ l) c l− (m g/ l) n o 3− (m g/ l) so 42− (m g/ l) h c o 3− (m g/ l) t d s (m g/ l) c h 4 cc /l s t p c o 2 cc /l s t p t ab le a 6 (c on ti nu ed fr om p re vi ou s pa ge ). federico et al. 24 5/ 98 13 .2 8. 0 11 0 50 72 22 78 36 56 52 40 0 73 0 n. m . 11 /9 8 14 .1 7. 7 18 0 52 75 20 70 37 60 57 40 0 74 0 8 1/ 99 10 .7 8. 3 60 52 74 23 65 38 61 60 35 0 69 0 7 3/ 99 9. 2 8. 2 19 0 52 75 22 63 37 60 63 32 0 66 0 4 5/ 99 12 .8 8. 0 12 0 53 73 24 79 38 58 60 34 0 69 0 7 10 /9 9 13 .7 6. 8 -1 50 55 78 24 81 38 64 64 41 0 77 0 19 11 /9 9 12 .7 8. 1 24 0 51 80 27 68 38 64 62 37 0 72 0 3 3/ 00 10 .8 8. 2 23 0 54 70 21 60 44 52 57 31 0 62 0 4 5/ 00 12 .7 7. 9 13 0 53 76 22 78 37 57 60 32 0 64 0 3 7/ 00 13 .4 7. 9 21 0 51 73 25 76 36 60 62 43 0 77 0 7 10 /0 0 13 .5 7. 7 16 0 56 78 25 72 34 56 61 41 0 76 0 7 11 /0 0 13 .3 7. 8 17 0 50 72 22 66 36 51 60 39 0 70 0 9 1/ 01 9. 3 8. 3 n. m . 52 60 20 61 32 42 48 35 0 67 0 2 3/ 01 12 .2 8. 1 n. m . 50 72 24 71 34 51 50 37 0 68 0 3 4/ 01 12 .4 8. 0 15 0 47 69 23 69 32 40 46 40 0 69 0 8 5/ 01 13 .2 7. 9 18 0 47 69 22 67 34 42 53 40 0 70 0 7 6/ 01 13 .5 8. 0 28 0 49 72 22 74 41 44 59 41 0 73 0 n. m . 7/ 01 13 .7 8. 0 14 0 54 76 23 75 37 46 53 43 0 75 0 n. m . 8/ 01 13 .8 7. 8 24 0 53 76 24 75 37 45 53 43 0 75 0 n. m . 9/ 01 13 .6 7. 9 23 0 55 76 25 74 36 49 53 43 0 77 0 5 10 /0 1 13 .8 7. 8 33 0 55 75 24 67 33 45 51 41 0 73 0 4 11 /0 1 11 .4 8. 0 43 0 54 73 22 68 36 45 53 38 0 70 0 5 12 /0 1 7. 2 7. 9 25 0 54 68 20 68 42 47 60 37 0 68 0 n. m . 01 /0 2 8. 4 7. 9 29 0 51 68 21 69 40 49 60 37 0 69 0 4 02 /0 2 9. 7 8. 2 19 0 52 69 20 64 41 43 57 38 0 69 0 n. m . 03 /0 2 9. 1 8. 1 16 0 51 71 22 66 36 38 54 36 0 66 0 3 04 /0 2 10 .7 8. 0 11 0 53 70 23 66 37 36 54 38 0 68 0 n. m . 05 /0 2 13 .2 8. 0 14 0 54 71 23 64 39 42 57 38 0 70 0 5 06 /0 2 13 .2 7. 9 17 0 56 76 23 74 44 43 56 38 0 71 0 4 07 /0 2 13 .6 7. 8 16 0 57 76 23 73 43 43 61 38 0 72 0 4 08 /0 2 13 .9 7. 9 18 0 54 75 22 69 43 46 62 39 0 72 0 6 09 /0 2 12 .5 7. 9 13 0 52 72 21 63 41 38 59 38 0 68 0 n. m . 10 /0 2 13 .6 7. 8 18 0 51 71 22 69 40 38 55 39 0 70 0 3 12 /0 2 11 .6 8. 3 11 0 57 67 21 65 35 38 52 37 0 67 0 5 12 /0 2 8. 3 8. 2 30 49 67 19 59 35 34 52 35 0 63 0 n. m . 01 /0 3 9. 7 8. 1 17 0 51 67 20 62 40 35 55 36 0 65 0 4 03 /0 3 13 .2 7. 9 18 0 58 73 21 58 39 58 60 43 0 76 0 9 04 /0 3 13 .0 7. 6 25 0 57 73 22 84 49 49 73 39 0 75 0 4 05 /0 3 13 .3 n. m . 21 0 55 74 22 87 39 45 59 44 0 78 0 4 o li ve ll a d at e (m o/ yr ) t (° c ) ph e h (m v ) n 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice ag_57.04.14_nisi_finalonline_layout 6 annals of geophysics, 57, 4, 2014, s0438; doi:10.4401/ag-6584 s0438 origin of the gases released from the acqua passante and ermeta wells (mt. amiata, central italy) and possible environmental implications for their closure barbara nisi1,*, orlando vaselli2,3, franco tassi2,3, javier de elio4, marcelo ortega4, juan caballero4, daniele rappuoli5, luis felipe mazadiego4 1 cnr-igg istituto di geoscienze e georisorse, pisa, italy 2 università di firenze, dipartimento di scienze della terra, firenze, italy 3 cnr-igg istituto di geoscienze e georisorse, firenze, italy 4 universidad politécnica de madrid, escuela técnica superior de ingenieros de minas y energía, madrid, spain 5 unione dei comuni amiata-val d’orcia, castiglion d’orcia (siena), italy abstract the mt. amiata volcano (tuscany, central italy) hosts the second largest geothermal field of italy. its sw and ne sectors are characterized by the presence of several co2-rich (>95% by vol.) gas discharges. an intense hg mining activity had taken place from the 19th century up to the end of the ’70s, particularly close to abbadia san salvatore, during which two drillings (acqua passante and ermeta) intercepted a co2-rich gas fertile horizon. the related gases are emitted in the atmosphere since 1938 and 1959, respectively, causing severe concerns for the local air quality. in this work the results of a geochemical and isotopic survey carried out on these gas emissions from march 2009 to january 2014 are presented. co2 fluxes from both the two wells and soil from an area of about 653,500 m2 located between them were measured. the two wells are emitting up to 15,000, 92 and 8 tons y-1 of co2, ch4 and h2s, respectively, while the computed soil co2 output was estimated at 4,311 ton y -1. the spatial distribution of the co2 soil flux suggests the presence of preferential patterns, indicating sites of higher permeability. since the local municipality is evaluating the possibility to plug the ermeta vent, a temporarily closure should first be carried out to test the possible influence of this operation on the diffuse soil degassing of deep-originated co2 in the surrounding area. this implies that diffuse soil gases should carefully be monitored before proceeding with its definitive closure. 1. introduction the occurrence of co2-rich gas discharges is typical of active, quiescent and recent volcanic activity [e.g. allard et al. 1991, burton et al. 2013 and references therein]. natural subsurface fluid reservoirs accumulating co2 produced by magma degassing and/or thermo-metamorphic reactions of carbonates are indeed a relatively common geological feature, particularly in areas affected by subduction processes or lithospheric thinning where volcanic [e.g. gerlach 1991] and/or geothermal [e.g. brombach et al. 2001, chiodini et al. 2004a, 2005] systems occur. carbon dioxide released from these reservoirs is discharged into the atmosphere through soil diffuse degassing and/or punctual gas manifestations [e.g. chiodini et al. 1999, 2010, rogie et al. 2001, minissale 2004]. in the peri-tyrrhenian italy, a region characterized by geothermal systems (e.g. larderello, mt. amiata) and quaternary (roman co-magmatic province) and present (e.g. vesuvius, phlegrean fields, aeolian islands) volcanic activity related to crustal thinning generated by the opening of the tyrrhenian sea [e.g. zitellini et al. 1986, bartole 1995, mauffret et al. 1999, peccerillo 2003, 2005, peccerillo and lustrino 2005], a large amount of deep-originated co2 is discharged from gas vents and diffuse soil emissions, and is present as dissolved inorganic carbon in ground waters [e.g. minissale et al. 1997a, chiodini et al. 1998, 2004b, gambardella et al. 2004, minissale 2004]. it has been estimated that >25.000 ton co2 day -1 are naturally discharged by the tuscan-roman and campanian degassing systems (trds and cds, respectively) [chiodini et al. 2004a]. in these areas, co2-rich (>90 % by volume) gases discharged from gas vents located in subcircular-shaped craters and associated to fissures, fracarticle history received may 5, 2014; accepted august 15, 2014. subject classification: central italy, amiata volcano, hydrothermal gases, co2, environmental impact. tures and structural lows (e.g. sinkholes), have caused fatal accidents, which have involved animals and human beings [e.g. carapezza et al. 2003, vaselli et al. 2006a, tassi et al. 2009]. locally, co2 accumulation in air may be favored since this gas is relatively heavy (gas density of co2 and air being of 0.001977 and 0.001280 g ml -1, respectively). as a consequence, when the climatic conditions are characterized by low wind, cloudy weather and cold air temperature, the morphologically depressed areas hosting these gas vents may become deadly traps. such co2-rich gas discharges are commonly accompanied by the presence of other toxic, dangerous and inflammable gases such as h2s and/or ch4, whose contents can be up to a few percentages [e.g. minissale et al. 1997a, minissale 2004]. the mt. amiata volcanic edifice is located in southern tuscany (figure 1). it represents the biggest and youngest volcano (~200,000 years old) of this region and hosts the second largest geothermal field of italy after larderello. its south-westernmost (e.g. selvena, banditella and zancona; minissale et al. [1997b], frondini et al. [2009]) and east-northernmost (e.g. bagni san filippo and pietrineri; minissale et al. [1997b], frondini et al. [2009], tassi et al. [2009]) flanks are characterized by the presence of several co2-rich (>95% by volume) gas discharges, mostly recognized during mercury (cinnabar) exploitation in the eastern part of this volcanic complex. in 1938 and 1959 two wells, named acqua passante (1,048 a.s.l.) and ermeta (1,079 m a.s.l.), were drilled close to the town of abbadia san salvatore (figure 2) in the volcanic cover at the depth of 116 and 298 m, respectively. it is worth to point out that, despite the large number of mining galleries, none of them was intercepted during the drilling activity. currently, gases from these wells are discharged at the height of about 6 m above the ground, as two chimneys were built to minimize the disagreeable effects for the people approaching these areas due to the presence of co2 and h2s, the latter having the typical smell of rotten eggs. once the hg mining activity was terminated, the owner of the mining concession (e.n.i.: national agency for hydrocarbons, agip division) produced a series of documentation for the reclamation of the hg extraction and processing areas. finally, in 2008, an agreement between e.n.i. and the municipality of abbadia san salvatore was signed and, consequently, the ownership of the reclamation passed to the public agency [e.g. vaselli et al. 2013]. in the agreement the ermeta and acqua passante wells were also mentioned, since the local municipality was in charge to: i) periodically monitor the discharged gases; ii) efficiently maintain the chimneys and iii) propose alternative strategies to reduce the impact of the discharged gases released to the atmosphere. the acqua passante well is located close to a picnic area along the main road that leads to the top of mt. nisi et al. 2 figure 1. geological sketch map of mt. amiata and location of the study area. legend: (1) town; (2) fault; (3) third volcanic complex: the ermeta olivine-latite lava flow: 209±0.29 ka; (4) second volcanic complex: porphyritic rhyodacite dome and lava flow, from 300± 0.04 ka to 190± 0.23 ka; (5) first volcanic complex, upper unit: trachydacite: from 318± 0.08 ka to 217± 0.01 ka; (6) first volcanic complex, lower unit: trachydacite: from 305± 0.005 ka to 241± 0.07 ka; (7) clay and sand: early pliocene; (8) ligurian units: jurassic-middle eocene; (9) tuscan nappe: late triassic-oligocene (modified after brogi et al. [2010]). 3 amiata, where skiing and trekking facilities are present. that of ermeta is close the homonymous church (built in 1296) that is an important tourist and religious site. in this paper we present the results of a geochemical and isotopic survey (from march 2009 to january 2014) carried out on the gases discharged from these two chimneys. the co2 flux from both the two vents and those from the soil carried out in july 2011 in an area of 653,550 m2 located se of the ermeta chimney (figure 2), was measured by using the hot-wire anemometer and accumulation chamber method [baubron et al. 1990, 1991], respectively. the chemical composition of the gas discharges located close to the study area [tassi et al. 2009] was reported for comparison. the main goals of this study were those to: i) investigate the genetic relationships between the well gases and those naturally discharging nearby and ii) verify the possible implications related to the closure of the ermeta and acqua passante wells by the local municipality to reduce the impact of the discharged gases on the local population and tourists. 2. geological setting and mining activity the mt. amiata area (southern tuscany, central italy; figure 1) is characterized by a volcanic activity that lasted ca. 100,000 years, the related deposits, dominated by a trachydacitic composition, being emplaced between 300 and 190 ka [ferrari et al. 1996, cadoux and pinti 2009]. the “residual” volcanic activity of this edifice related to the emplacement, at 6-7 km below the sea level, of a magmatic body in the pliocene, produced two geothermal reservoirs, which are feeding a number of co2-rich gas manifestations and ca-so4-rich thermo-mineral waters, mainly located in ne and sw sector of the volcanic edifice [minissale et al. 1997b, frondini et al. 2009, tassi et al. 2009]. presently, two geothermal plants are operating at bagnore and piancastagnaio (figure 1). the geodynamic setting of the mt. amiata geothermal area is dominated by extensional structures developed from the middle miocene to the quaternary, whilst collisional structures occurred from the cretaceous to the early miocene [e.g. brogi et al. 2010 and references therein]. the anomalous heat flux related to the pliocene magmatic body, likely caused the emplacement of hg ore deposits [tanelli 1983, klemm and neumann 1984]. in the recent past, mt. amiata was one of the biggest hg producers in the world [e.g. rimondi et al. 2011, vaselli et al. 2013 and references therein]. the mt. amiata hg district is mainly distributed in the central-eastern and south-western sectors of this volcanic complex, although the principal mining activity was located in the municipality of abbadia san salvatore (figure 1). the extraction and metallurgy of cinnabar (hgs) was aimed to produce metallic mercury from historic times until the middle seventies, when most of the hg mines in the world were shut down due to environmental concerns. in the second half of the last century several wells, including those of ermeta and acqua passante, were drilled for mining exploration close to abbadia san salvatore (figure 2). it is worthwhile to mention that some of the mining galthe co2-rich gases from the acqua passante and ermeta wells (mt. amiata, italy) figure 2. location of the i) ermeta and acqua passante wells and ii) soil diffuse ϕco2 measurement sites. leries were affected by dangerous co2 emissions, occasionally causing fatalities, and co2 is still abundantly present in some of them [vaselli et al. 2006a, 2011]. both the ermeta and acqua passante wells are located in the volcanic cover and that of ermeta is situated few hundreds meters from the ermeta olivine-latite lava flow that marked the latest event of the volcanic history of mt. amiata [e.g. ferrari et al. 1996]. 3. material and methods 3.1. sampling and analysis of gases the ermeta well is located to the east of abbadia san salvatore whilst that of acqua passante is about 1 km ne on a fly of bird from ermeta (figure 2). from the two wells, 18 gas samples were collected from march 2009 to january 2014. the well gases were collected using a 40 cm long tygon® tube inserted into an artificial opening (25 mm in diameter) located at the base of the two ~6 m high chimneys (whose diameters are 300 mm) and connected to a pre-evacuated 60 ml glass thorion-tapped flask filled with 20 ml of a 4n naoh and 0.15 m cd(oh)2 suspension [vaselli et al. 2006b and references therein]. the analysis of the inorganic gases stored in the headspace of the sampling flasks (n2, o2, h2, he, ch4, ar and ne) was carried out with a shimadzu 15a gas-chromatograph equipped with a 10 m long 5a molecular sieve column and a thermal conductivity detector (tcd). the alkaline solution, separated from the solid precipitate by centrifugation, was used for the determination of co2, as co3 2-, by titration (metrohm basic titrino) with a 0.5n hcl solution. the solid cds was oxidized to so4 2with h2o2 and analyzed for the determination of h2s by ion-chromatography [montegrossi et al. 2001]. analytical error was <5%. the 13c/12c isotopic ratio of co2 (expressed as d 13c-co2 ‰ v-pdb) was measured after the purification and extraction of co2 by using the two cryogenic traps method [vaselli et al. 2006b]. the analytical error and the reproducibility were ±0.05‰ and ±0.1‰, respectively. the determination of the 3he/4he ratios (expressed as r/ra, where r is the 3he/4he measured ratio and ra is the 3he/4he ratio in the air: 1.39*10-6; mamyrin and tolstikhin [1984]) was carried out at the university of rochester rare gas facility using a vg 5400 rare gas mass spectrometer by using the method described in poreda and farley [1992]. the measured 3he/4he ratios were corrected for the addition of air on the basis of the 4he/20ne ratios measured by mass spectrometry, assuming that 20ne had an atmospheric origin [craig and lupton 1976, sano and wakita 1988]. analytical error for r/ra determination was ≤0.3 %. 3.2. measurements of the co2 flux from the chimneys and diffuse soil the co2 flux of the two chimneys was determined by inserting into the chimneys (whose section had an area of 0.07 m2) a delta ohm model hd2103.2 hot-wire anemometer probe. co2 flux records were set to acquire the data from august 5 to september 5, 2011, during which the co2 flux was measured every 5 minutes and was calculated on the basis of the discharged gas velocity and temperature inside the pipelines. the co2 flux values were normalized at 20°c. the temperature of the ermeta chimney was ranging between 26.8 and 29°c, while that of acqua passante was between 16.9 and 22.5°c. the survey of diffuse soil co2 flux ( july 2011) covered an area of 653,550 m2 with 301 evenly distributed measuring sites and was carried out in the se sector of the ermeta chimney (figure 2). all the measurements were carried out where soils developed on the mt. amiata volcanic products, avoiding those areas where the volcanic rocks were cropping out. to measure the co2 flux from the soil, a direct and dynamic method that follows the principle of the socalled accumulation chamber [baubron et al. 1990, 1991] was used. this method, initially used to determine soil respiration [i.e. kanemasu et al. 1974, parkinson 1981], has been widely applied for measuring diffuse soil co2 flux in volcanic and geothermal [e.g. tonani and miele 1991, chiodini et al. 1996, 1998, 2000, 2003, 2004a, pancioli et al. 2009, tassi et al. 2009] and solid waste disposal landfills [e.g. raco et al. 2006, 2008, moretti et al. 2007]. the instrument consisted of: i) an inverted chamber equipped with a system to mix the air in the chamber headspace (volume = 3.06 × 10-3 m3 and area height = 100 mm), ii) an infra-red (ir) spectrophotometer with licor li-820 sensor (measuring range of 0-20,000 ppm, accuracy of 4% of reading), iii) an analogue-digital (ad) converter and iv) a palmtop computer (pc). in order to prevent damages to both the pump and the co2 detector, a magnesium perchlorate desiccant trap was placed between the outlet fitting of the accumulation chamber and the inlet of the ptfe filter to adsorb soil humidity. once the accumulation chamber was positioned on the ground a low-flux pump (20 ml s−1) continuously convoyed the soil gas from the chamber to the ir sensor. to minimize the disturbance effects due to changes of barometric conditions, the soil gas was reinjected into the chamber. the dcco2/dt values were computed in the field with the pc connected with the ir through the ad. the soil {co2 was calculated on the basis of the measured co2 concentration increment inside the chamber over time (dcco2/dt), according to the following equation: nisi et al. 4 5 the co2-rich gases from the acqua passante and ermeta wells (mt. amiata, italy) table 1. chemical (in mmol/mol) and isotopic (carbon: as d13c-co2 vs. v-pdb ‰; and helium as r/ra) composition of the ermeta and acqua passante gases and of the bubbling pools and dry vents (pn, sp ho, po, pg and ma). (*): data from tassi et al. [2009]. name date co2 h2s n2 ch4 ar o2 h2 he d 13c-co2 r/ra ermeta mar-09 986 0.30 2.8 10 0.014 0.16 0.074 0.00064 n.d. 0.33 ermeta jul-09 982 0.77 14 2.8 0.048 0.057 0.078 0.00097 -3.39 ermeta oct-09 978 0.62 15 5.7 0.025 0.004 0.68 0.0013 -3.39 ermeta dec-09 986 0.56 4.2 8.9 0.038 0.23 0.12 0.0012 -3.33 ermeta apr-09 986 0.36 5.1 7.7 0.056 0.16 0.21 0.0014 -3.35 ermeta jul-10 987 0.33 5.5 6.9 0.053 0.19 0.12 0.00089 -3.48 0.32 ermeta oct-10 987 0.40 6.2 5.8 0.045 0.16 0.24 0.00055 -3.13 ermeta nov-10 987 0.33 6.3 5.6 0.049 0.16 0.21 0.00069 -3.27 ermeta apr-11 989 0.58 4.6 5.5 0.032 0.14 0.54 0.00091 -3.27 ermeta jun-11 987 0.42 5.6 6.7 0.037 0.15 0.55 0.00075 -3.41 ermeta oct-11 983 0.77 6.1 9.5 0.045 0.14 0.44 0.00074 -3.45 ermeta dec-11 987 0.34 5.6 6.6 0.056 0.19 0.25 0.00066 -3.09 ermeta mar-12 988 0.55 5.3 5.1 0.052 0.21 0.34 0.00039 -3.33 ermeta apr-12 989 0.45 5.2 5.0 0.046 0.21 0.28 0.00031 n.d. ermeta jul-12 988 0.52 5.6 5.0 0.056 0.15 0.30 0.00028 -3.36 ermeta jan-13 989 0.72 5.1 4.9 0.052 0.19 0.26 0.00018 -3.27 ermeta jul-13 989 0.81 5.0 4.6 0.061 0.22 0.27 0.00011 -3.32 ermeta jan-14 989 0.66 5.5 4.8 0.036 0.16 0.22 0.00016 acqua passante mar-09 987 0.34 6.4 5.9 0.029 0.11 0.080 0.00069 -3.31 0.29 acqua passante jul-09 977 0.33 12 11 0.043 0.73 0.069 0.0022 -2.92 acqua passante oct-09 988 0.30 7.5 4.0 0.020 0.01 0.12 0.00063 -3.38 acqua passante dec-09 988 0.39 6.1 5.0 0.032 0.16 0.085 0.00087 -3.33 acqua passante apr-09 988 0.22 6.7 4.5 0.063 0.22 0.076 0.00061 -3.31 acqua passante jul-10 988 0.22 6.4 5.2 0.060 0.20 0.087 0.00055 -3.47 0.40 acqua passante oct-10 989 0.32 5.9 4.4 0.053 0.13 0.18 0.00087 -3.26 acqua passante nov-10 988 0.36 6.1 4.9 0.056 0.11 0.22 0.00065 -3.13 acqua passante apr-11 989 0.59 5.2 5.0 0.036 0.14 0.39 0.00086 -3.13 acqua passante jun-11 987 0.37 5.7 6.0 0.041 0.16 0.34 0.00061 -3.11 acqua passante oct-11 985 0.66 6.4 7.8 0.030 0.12 0.33 0.00066 -3.39 acqua passante dec-11 986 0.30 6.3 6.6 0.061 0.27 0.14 0.00044 -3.25 acqua passante mar-12 989 0.40 5.5 5.0 0.061 0.24 0.26 0.00035 -2.84 acqua passante apr-12 989 0.41 5.4 5.1 0.053 0.19 0.23 0.00025 n.d. acqua passante jul-12 989 0.45 5.3 4.9 0.055 0.18 0.25 0.00024 -3.33 acqua passante jan-13 989 0.56 5.1 4.6 0.056 0.22 0.25 0.00015 -3.21 acqua passante jul-13 989 0.63 5.1 4.8 0.050 0.19 0.31 0.00013 -3.35 acqua passante jan-14 989 0.58 5.3 4.9 0.054 0.18 0.26 0.00019 name id co2 h2s n2 ch4 ar o2 h2 he d 13c-co2 r/ra polla nera* pn 980 0.54 11 8.7 0.025 0.0047 0.0020 0.0031 spuntone* sp 972 0.55 16 11 0.032 0.0027 0.00003 0.0045 -2.87 hole* ho 982 0.61 11 6.0 0.021 0.0020 0.0014 0.0033 -2.61 poggio all'olivo* po 983 0.95 8.6 7.2 0.018 0.0038 0.00043 0.0026 -3.24 polla grigia* pg 976 0.79 13 11 0.029 0.0028 0.0034 0.0039 mammellone* ma 978 0.81 14 7.5 0.022 0.0021 0.00057 0.00086 -2.63 0.13 {co2 = cf × (dcco2/dt) (1) where the proportionality factor (cf ) between dcco2/dt and {co2 was determined by laboratory tests through an artificial soil, during which an interval of controlled {co2 values (from 10 to 10,000 g m -2 day-1) was measured (at least 6 measurements were repeated for each {co2 value). the cf factor was then computed as the slope of the linear best-fit line of {co2 vs. dcco2/dt. 4. results 4.1. chemical and isotopic (d13c-co2 and r/ra ) compositions of gases from wells, bubbling pools and dry vents the chemical and isotopic (d13c-co2 and r/ra values) compositions of the ermeta and acqua passante gases and, for comparison, those of six bubbling pools and dry gas vents (namely pn, sp ho, po, pg and ma; see tassi et al. [2009] for their location), located at about 5 km ne of abbadia san salvatore, are reported in table 1. the ermeta and acqua passante wells discharged gases having a similar composition, being both dominated by co2 (up to 989 mmol mol -1), with significant concentrations of n2 (up to 14 and 12 mmol mol -1, respectively), ch4 (up to 10 and 11 mmol mol -1, respectively) and h2s (up to 0.81 and 0.66 mmol mol -1, respectively). hydrogen concentrations ranged from 0.069 (acqua passante, july 2009) to 0.55 mmol mol-1 (ermeta, june 2011), while those of noble gases, i.e. ar, he and ne, were up to 0.061, 0.00022, and 0.00003, mmol mol-1, respectively. the chemical composition of the bubbling pools was basically identical to that of the two wells, with the only exception of h2, whose concentration in the well gases was up to 3 orders of magnitude lower than in the wells. helium (r/ra) and carbon (d13c-co2) isotopic compositions in the wells were in a narrow range, from 0.32 to 0.40 and from −3.48 to −2.84‰ vs. v-pdb, respectively. the helium isotopic ratio for the natural gas discharges was only available for ma, whose value was moderately lower (r/ra=0.13) that those of the ermeta and acqua passante gases, while those of carbon in co2 were, on average, slightly more positive (between −3.24 and −2.61 ‰ vs. v-pdb). 4.2. co2 flux from the chimneys and diffuse soil the database of the co2 flux measurements from the ermeta and acqua passante chimneys consisted of more than 9,000 measurements (not reported) and ranged from 8.86 to 11.06 (mean value: 10.05±0.45) mol sec-1 and from 0.89 to 2.51 (mean value: 1.32±0.17) mol sec-1, respectively. the calculated co2 output from ermeta was ≈13,000 ton y-1, whereas that of acqua passante clustered around 1,800 ton y-1. the spatial distribution of the soil diffuse co2 flux measurements was partly dictated by the rough and intensely vegetated territory. the measured {co2 values (301 data), ranging from 0.07 to 3.42 mol m-2 day-1 with an average value of 0.38 mol m-2 day-1 (table 2), showed a normal logarithmic behavior with a 0.05 significance level (figure 3), indicating that the ln({co2) values had a modal density distribution. mean, median, minimum, maximum {co2 values and standard deviation are reported as ln-values in table 3. 5. discussion 5.1. origin of gases the chemical composition of the ermeta and acqua passante gases is basically consistent with that shown by the gas discharges located ne of mt. amiata, which are likely fed by a hydrothermal system consisting of two aquifers at distinct depth (500-1,000 m and >3,000 m, respectively) and temperature (up to 230 and 350°c, respectively) [calamai et al. 1970, gianelli et al. 1988]. the d13c-co2 values (table 1) are slightly heavier than the typical range of mantle co2 [rollinson 1993, hoefs 1997, ohmoto and goldhaber 1997], suggesting that co2 has a twofold origin, i.e. mantle degassing and thermometamorphic reactions acting on limestone [minissale et al. 1997a, frondini et al. 2009]. this feature is common to the great majority of the gas emissions from the peri-tyrrhenian central italy (tuscan-roman degassing system, trds; chiodini et al. [2004a]). in this region, a significant mantle contamination by crustal fluids released from the subducted adriatic plate occurs [martelli et al. 2004, frondini et al. 2009], as testified by the relatively low r/ra values (<0.4; table 1). thermogenic processes involving organic material buried in sedimentary formation are the most likely source for the high n2/ar ratios and ch4 and h2s concentrations measured in gases from both the two wells and the nearby gas discharges. the relatively high h2 concentrations characterizing the ermeta and acqua passante gases, significantly higher than those of the mt. amiata gas discharges (figure 4), are likely resulting by the corrosion of the internal iron tubing of the wells, which is favored by the presence of co2and h2s-rich gases. it is likely that this alteration process allows air to enter the chimneys. this would also explain the ar enrichment shown by the wells gases with respect to the naturally discharges ones (figure 5). 5.2. spatial variation and output of soil diffuse {co2 in order to better constrain the total {co2 and visualize its spatial distribution, the co2 iso-flux map is reported in figure 6, which was drawn by means of the nisi et al. 6 7 the co2-rich gases from the acqua passante and ermeta wells (mt. amiata, italy) d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 ju l-1 1 71 59 23 47 51 61 0 0. 38 ju l-1 1 71 70 23 47 51 41 0 0. 74 ju l-1 1 71 65 23 47 51 21 0 0. 64 ju l-1 1 71 70 23 47 50 86 0 0. 15 ju l-1 1 71 59 73 47 51 61 0 0. 21 ju l-1 1 71 70 73 47 51 41 0 0. 34 ju l-1 1 71 65 73 47 51 21 0 0. 45 ju l-1 1 71 62 23 47 51 01 0 0. 49 ju l-1 1 71 60 23 47 51 61 0 0. 22 ju l-1 1 71 71 23 47 51 41 0 0. 76 ju l-1 1 71 66 23 47 51 21 0 0. 27 ju l-1 1 71 62 73 47 51 01 0 0. 41 ju l-1 1 71 60 73 47 51 61 0 0. 59 ju l-1 1 71 59 73 47 51 36 0 0. 50 ju l-1 1 71 66 73 47 51 21 0 0. 35 ju l-1 1 71 63 23 47 51 01 0 0. 46 ju l-1 1 71 61 23 47 51 61 0 0. 23 ju l-1 1 71 60 23 47 51 36 0 0. 77 ju l-1 1 71 67 23 47 51 21 0 0. 40 ju l-1 1 71 63 73 47 51 01 0 0. 33 ju l-1 1 71 61 73 47 51 61 0 0. 28 ju l-1 1 71 60 73 47 51 36 0 0. 27 ju l-1 1 71 67 73 47 51 21 0 0. 17 ju l-1 1 71 64 23 47 51 01 0 0. 47 ju l-1 1 71 62 23 47 51 61 0 0. 39 ju l-1 1 71 61 23 47 51 36 0 0. 38 ju l-1 1 71 68 23 47 51 21 0 0. 29 ju l-1 1 71 64 73 47 51 01 0 0. 28 ju l-1 1 71 59 23 47 51 56 0 0. 26 ju l-1 1 71 61 73 47 51 36 0 0. 24 ju l-1 1 71 68 73 47 51 21 0 0. 71 ju l-1 1 71 65 23 47 51 01 0 0. 57 ju l-1 1 71 59 73 47 51 56 0 0. 32 ju l-1 1 71 62 23 47 51 36 0 0. 18 ju l-1 1 71 69 23 47 51 21 0 0. 91 ju l-1 1 71 65 73 47 51 01 0 0. 65 ju l-1 1 71 60 23 47 51 56 0 0. 25 ju l-1 1 71 62 73 47 51 36 0 0. 54 ju l-1 1 71 69 73 47 51 21 0 0. 40 ju l-1 1 71 66 23 47 51 01 0 0. 77 ju l-1 1 71 60 73 47 51 56 0 0. 46 ju l-1 1 71 63 23 47 51 36 0 0. 87 ju l-1 1 71 70 23 47 51 21 0 0. 45 ju l-1 1 71 66 73 47 51 01 0 0. 14 ju l-1 1 71 61 23 47 51 56 0 0. 55 ju l-1 1 71 63 73 47 51 36 0 0. 52 ju l-1 1 71 70 73 47 51 21 0 0. 75 ju l-1 1 71 67 23 47 51 01 0 0. 71 ju l-1 1 71 61 73 47 51 56 0 0. 42 ju l-1 1 71 64 23 47 51 36 0 0. 26 ju l-1 1 71 71 23 47 51 21 0 1. 65 ju l-1 1 71 67 73 47 51 01 0 0. 35 ju l-1 1 71 62 23 47 51 56 0 0. 84 ju l-1 1 71 64 73 47 51 36 0 0. 44 ju l-1 1 71 61 73 47 51 16 0 0. 18 ju l-1 1 71 70 73 47 50 91 0 0. 45 ju l-1 1 71 59 73 47 51 51 0 0. 44 ju l-1 1 71 65 23 47 51 36 0 0. 27 ju l-1 1 71 62 23 47 51 16 0 0. 12 ju l-1 1 71 71 23 47 50 91 0 0. 35 ju l-1 1 71 60 23 47 51 51 0 0. 54 ju l-1 1 71 65 73 47 51 36 0 0. 35 ju l-1 1 71 62 73 47 51 16 0 0. 32 ju l-1 1 71 61 73 47 50 86 0 0. 60 ju l-1 1 71 60 73 47 51 51 0 0. 24 ju l-1 1 71 66 23 47 51 36 0 0. 26 ju l-1 1 71 63 23 47 51 16 0 0. 22 ju l-1 1 71 62 23 47 50 86 0 0. 49 ju l-1 1 71 61 23 47 51 51 0 0. 46 ju l-1 1 71 66 73 47 51 36 0 0. 15 ju l-1 1 71 63 73 47 51 16 0 0. 23 ju l-1 1 71 62 73 47 50 86 0 0. 86 ju l-1 1 71 61 73 47 51 51 0 0. 38 ju l-1 1 71 67 23 47 51 36 0 0. 25 ju l-1 1 71 64 23 47 51 16 0 0. 53 ju l-1 1 71 63 23 47 50 86 0 0. 21 ju l-1 1 71 62 23 47 51 51 0 0. 64 ju l-1 1 71 67 73 47 51 36 0 0. 20 ju l-1 1 71 64 73 47 51 16 0 0. 29 ju l-1 1 71 63 73 47 50 86 0 0. 57 ju l-1 1 71 62 73 47 51 51 0 0. 23 ju l-1 1 71 68 23 47 51 36 0 0. 41 ju l-1 1 71 65 23 47 51 16 0 0. 66 ju l-1 1 71 64 23 47 50 86 0 0. 22 ju l-1 1 71 63 23 47 51 51 0 0. 22 ju l-1 1 71 68 73 47 51 36 0 0. 89 ju l-1 1 71 65 73 47 51 16 0 0. 39 ju l-1 1 71 64 73 47 50 86 0 0. 27 ju l-1 1 71 63 73 47 51 51 0 0. 65 ju l-1 1 71 69 23 47 51 36 0 0. 31 ju l-1 1 71 66 23 47 51 16 0 0. 30 ju l-1 1 71 65 23 47 50 86 0 0. 45 ju l-1 1 71 64 23 47 51 51 0 0. 31 ju l-1 1 71 69 73 47 51 36 0 1. 04 ju l-1 1 71 66 73 47 51 16 0 0. 47 ju l-1 1 71 65 73 47 50 86 0 0. 36 ju l-1 1 71 64 73 47 51 51 0 0. 26 ju l-1 1 71 70 23 47 51 36 0 0. 72 ju l-1 1 71 67 23 47 51 16 0 0. 41 ju l-1 1 71 66 23 47 50 86 0 0. 43 t ab le 2 ( co nt in u es o n pa ge s 8 an d 9) . s am pl in g da te , g eo gr ap hi ca l c oo rd in at es ( in u t m ) an d ϕ c o 2 va lu es ( in m ol m -2 da y1 ) . nisi et al. 8 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 ju l-1 1 71 65 23 47 51 51 0 0. 43 ju l-1 1 71 70 73 47 51 36 0 0. 29 ju l-1 1 71 67 73 47 51 16 0 0. 38 ju l-1 1 71 66 73 47 50 86 0 0. 57 ju l-1 1 71 65 73 47 51 51 0 0. 35 ju l-1 1 71 71 23 47 51 36 0 0. 39 ju l-1 1 71 68 23 47 51 16 0 0. 53 ju l-1 1 71 71 23 47 50 86 0 0. 57 ju l-1 1 71 66 23 47 51 51 0 0. 38 ju l-1 1 71 61 73 47 51 31 0 0. 31 ju l-1 1 71 68 73 47 51 16 0 1. 93 ju l-1 1 71 69 73 47 50 86 0 0. 32 ju l-1 1 71 66 73 47 51 51 0 0. 26 ju l-1 1 71 62 23 47 51 31 0 0. 47 ju l-1 1 71 69 23 47 51 16 0 1. 01 ju l-1 1 71 68 23 47 51 01 0 0. 45 ju l-1 1 71 67 23 47 51 51 0 0. 24 ju l-1 1 71 62 73 47 51 31 0 0. 36 ju l-1 1 71 69 73 47 51 16 0 0. 45 ju l-1 1 71 68 73 47 51 01 0 1. 25 ju l-1 1 71 67 73 47 51 51 0 1. 12 ju l-1 1 71 63 23 47 51 31 0 1. 23 ju l-1 1 71 70 23 47 51 16 0 0. 46 ju l-1 1 71 69 23 47 51 01 0 0. 27 ju l-1 1 71 68 23 47 51 51 0 0. 11 ju l-1 1 71 63 73 47 51 31 0 0. 92 ju l-1 1 71 70 73 47 51 16 0 0. 88 ju l-1 1 71 69 73 47 51 01 0 0. 23 ju l-1 1 71 68 73 47 51 51 0 0. 40 ju l-1 1 71 64 23 47 51 31 0 0. 33 ju l-1 1 71 71 23 47 51 16 0 1. 76 ju l-1 1 71 70 23 47 51 01 0 0. 36 ju l-1 1 71 69 23 47 51 51 0 0. 63 ju l-1 1 71 64 73 47 51 31 0 0. 71 ju l-1 1 71 61 73 47 51 11 0 0. 27 ju l-1 1 71 70 73 47 51 01 0 0. 64 ju l-1 1 71 59 73 47 51 46 0 0. 34 ju l-1 1 71 65 23 47 51 31 0 0. 59 ju l-1 1 71 62 23 47 51 11 0 0. 26 ju l-1 1 71 71 23 47 51 01 0 0. 24 ju l-1 1 71 60 23 47 51 46 0 0. 26 ju l-1 1 71 65 73 47 51 31 0 0. 11 ju l-1 1 71 62 73 47 51 11 0 0. 27 ju l-1 1 71 61 73 47 50 96 0 0. 24 ju l-1 1 71 60 73 47 51 46 0 0. 32 ju l-1 1 71 66 23 47 51 31 0 0. 56 ju l-1 1 71 63 23 47 51 11 0 0. 18 ju l-1 1 71 62 23 47 50 96 0 0. 13 ju l-1 1 71 61 23 47 51 46 0 0. 26 ju l-1 1 71 66 73 47 51 31 0 0. 53 ju l-1 1 71 63 73 47 51 11 0 0. 61 ju l-1 1 71 62 73 47 50 96 0 0. 41 ju l-1 1 71 61 73 47 51 46 0 0. 44 ju l-1 1 71 67 23 47 51 31 0 0. 46 ju l-1 1 71 64 23 47 51 11 0 0. 17 ju l-1 1 71 63 23 47 50 96 0 0. 37 ju l-1 1 71 62 23 47 51 46 0 0. 58 ju l-1 1 71 67 73 47 51 31 0 1. 12 ju l-1 1 71 64 73 47 51 11 0 0. 38 ju l-1 1 71 63 73 47 50 96 0 0. 42 ju l-1 1 71 63 23 47 51 46 0 0. 46 ju l-1 1 71 68 23 47 51 31 0 0. 59 ju l-1 1 71 65 23 47 51 11 0 0. 17 ju l-1 1 71 64 23 47 50 91 0 1. 21 ju l-1 1 71 63 73 47 51 46 0 0. 24 ju l-1 1 71 68 73 47 51 31 0 0. 51 ju l-1 1 71 65 73 47 51 11 0 0. 07 ju l-1 1 71 64 73 47 50 91 0 0. 58 ju l-1 1 71 64 23 47 51 46 0 0. 29 ju l-1 1 71 69 23 47 51 31 0 0. 27 ju l-1 1 71 66 23 47 51 11 0 0. 18 ju l-1 1 71 65 23 47 50 91 0 0. 49 ju l-1 1 71 64 73 47 51 46 0 0. 20 ju l-1 1 71 69 73 47 51 31 0 1. 07 ju l-1 1 71 66 73 47 51 11 0 0. 47 ju l-1 1 71 65 73 47 50 91 0 0. 45 ju l-1 1 71 65 23 47 51 46 0 0. 52 ju l-1 1 71 70 23 47 51 31 0 0. 30 ju l-1 1 71 67 23 47 51 11 0 0. 83 ju l-1 1 71 66 23 47 50 91 0 0. 27 ju l-1 1 71 65 73 47 51 46 0 0. 34 ju l-1 1 71 70 73 47 51 31 0 0. 39 ju l-1 1 71 67 73 47 51 11 0 0. 79 ju l-1 1 71 66 73 47 50 91 0 0. 40 ju l-1 1 71 66 23 47 51 46 0 0. 24 ju l-1 1 71 71 23 47 51 31 0 0. 13 ju l-1 1 71 68 23 47 51 11 0 0. 32 ju l-1 1 71 67 23 47 50 91 0 0. 39 ju l-1 1 71 66 73 47 51 46 0 0. 47 ju l-1 1 71 61 73 47 51 26 0 0. 35 ju l-1 1 71 68 73 47 51 11 0 0. 52 ju l-1 1 71 67 73 47 50 91 0 0. 43 ju l-1 1 71 67 23 47 51 46 0 0. 38 ju l-1 1 71 62 23 47 51 26 0 0. 63 ju l-1 1 71 69 23 47 51 11 0 0. 20 ju l-1 1 71 68 23 47 50 91 0 0. 34 ju l-1 1 71 67 73 47 51 46 0 0. 53 ju l-1 1 71 62 73 47 51 26 0 0. 20 ju l-1 1 71 69 73 47 51 11 0 0. 27 ju l-1 1 71 68 73 47 50 91 0 0. 20 t ab le 2 ( co nt in u ed fr om p re vi ou s pa ge ). s am pl in g da te , g eo gr ap hi ca l c oo rd in at es ( in u t m ) an d ϕ c o 2 va lu es ( in m ol m -2 da y1 ) . 9 the co2-rich gases from the acqua passante and ermeta wells (mt. amiata, italy) t ab le 2 ( co nt in u ed fr om p re vi ou s pa ge ). s am pl in g da te , g eo gr ap hi ca l c oo rd in at es ( in u t m ) an d ϕ c o 2 va lu es ( in m ol m -2 da y1 ) . d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 d at e u t m -w e st (w g s8 4) z on e 32 u t m -n o r t h (w g s8 4) z on e 32 ϕ c o 2 ju l-1 1 71 68 23 47 51 46 0 0. 29 ju l-1 1 71 63 23 47 51 26 0 1. 44 ju l-1 1 71 70 23 47 51 11 0 0. 23 ju l-1 1 71 69 23 47 50 91 0 0. 32 ju l-1 1 71 68 73 47 51 46 0 0. 18 ju l-1 1 71 63 73 47 51 26 0 0. 41 ju l-1 1 71 70 73 47 51 11 0 0. 20 ju l-1 1 71 69 73 47 50 91 0 0. 72 ju l-1 1 71 69 23 47 51 46 0 0. 22 ju l-1 1 71 64 23 47 51 26 0 0. 63 ju l-1 1 71 71 23 47 51 11 0 1. 05 ju l-1 1 71 70 23 47 50 91 0 0. 17 ju l-1 1 71 59 73 47 51 41 0 1. 01 ju l-1 1 71 64 73 47 51 26 0 0. 27 ju l-1 1 71 61 73 47 51 06 0 0. 71 ju l-1 1 71 69 23 47 50 86 0 0. 30 ju l-1 1 71 60 23 47 51 41 0 0. 18 ju l-1 1 71 65 23 47 51 26 0 0. 26 ju l-1 1 71 62 23 47 51 06 0 0. 26 ju l-1 1 71 64 23 47 50 96 0 0. 70 ju l-1 1 71 60 73 47 51 41 0 0. 30 ju l-1 1 71 65 73 47 51 26 0 0. 42 ju l-1 1 71 62 73 47 51 06 0 0. 64 ju l-1 1 71 64 73 47 50 96 0 0. 59 ju l-1 1 71 61 23 47 51 41 0 0. 24 ju l-1 1 71 66 23 47 51 26 0 0. 66 ju l-1 1 71 63 23 47 51 06 0 0. 86 ju l-1 1 71 65 23 47 50 96 0 0. 54 ju l-1 1 71 61 73 47 51 41 0 0. 54 ju l-1 1 71 66 73 47 51 26 0 0. 51 ju l-1 1 71 63 73 47 51 06 0 0. 44 ju l-1 1 71 65 73 47 50 96 0 0. 23 ju l-1 1 71 62 23 47 51 41 0 0. 27 ju l-1 1 71 67 23 47 51 26 0 0. 35 ju l-1 1 71 64 23 47 51 06 0 0. 45 ju l-1 1 71 66 23 47 50 96 0 0. 29 ju l-1 1 71 62 73 47 51 41 0 0. 28 ju l-1 1 71 67 73 47 51 26 0 0. 31 ju l-1 1 71 64 73 47 51 06 0 0. 36 ju l-1 1 71 66 73 47 50 96 0 0. 19 ju l-1 1 71 63 23 47 51 41 0 0. 15 ju l-1 1 71 68 23 47 51 26 0 0. 53 ju l-1 1 71 65 23 47 51 06 0 0. 19 ju l-1 1 71 67 23 47 50 96 0 0. 21 ju l-1 1 71 63 73 47 51 41 0 0. 27 ju l-1 1 71 68 73 47 51 26 0 0. 35 ju l-1 1 71 65 73 47 51 06 0 0. 26 ju l-1 1 71 67 73 47 50 96 0 0. 11 ju l-1 1 71 64 23 47 51 41 0 0. 15 ju l-1 1 71 69 23 47 51 26 0 0. 32 ju l-1 1 71 66 23 47 51 06 0 0. 10 ju l-1 1 71 68 23 47 50 96 0 0. 24 ju l-1 1 71 64 73 47 51 41 0 0. 27 ju l-1 1 71 69 73 47 51 26 0 0. 30 ju l-1 1 71 66 73 47 51 06 0 0. 52 ju l-1 1 71 68 73 47 50 96 0 0. 37 ju l-1 1 71 65 23 47 51 41 0 0. 15 ju l-1 1 71 70 23 47 51 26 0 0. 14 ju l-1 1 71 67 23 47 51 06 0 0. 60 ju l-1 1 71 69 23 47 50 96 0 0. 55 ju l-1 1 71 65 73 47 51 41 0 0. 39 ju l-1 1 71 70 73 47 51 26 0 0. 61 ju l-1 1 71 67 73 47 51 06 0 0. 36 ju l-1 1 71 69 73 47 50 96 0 0. 20 ju l-1 1 71 66 23 47 51 41 0 0. 23 ju l-1 1 71 71 23 47 51 26 0 0. 58 ju l-1 1 71 68 23 47 51 06 0 0. 65 ju l-1 1 71 70 23 47 50 96 0 3. 42 ju l-1 1 71 66 73 47 51 41 0 0. 82 ju l-1 1 71 61 73 47 51 21 0 0. 20 ju l-1 1 71 68 73 47 51 06 0 0. 36 ju l-1 1 71 70 73 47 50 96 0 0. 38 ju l-1 1 71 67 23 47 51 41 0 0. 31 ju l-1 1 71 62 23 47 51 21 0 0. 29 ju l-1 1 71 69 23 47 51 06 0 0. 90 ju l-1 1 71 71 23 47 50 96 0 0. 50 ju l-1 1 71 67 73 47 51 41 0 0. 52 ju l-1 1 71 62 73 47 51 21 0 0. 43 ju l-1 1 71 69 73 47 51 06 0 0. 31 ju l-1 1 71 61 73 47 50 91 0 0. 71 ju l-1 1 71 68 23 47 51 41 0 0. 77 ju l-1 1 71 63 23 47 51 21 0 0. 19 ju l-1 1 71 70 23 47 51 06 0 0. 14 ju l-1 1 71 62 23 47 50 91 0 0. 39 ju l-1 1 71 68 73 47 51 41 0 0. 48 ju l-1 1 71 63 73 47 51 21 0 0. 35 ju l-1 1 71 70 73 47 51 06 0 0. 43 ju l-1 1 71 62 73 47 50 91 0 0. 32 ju l-1 1 71 69 23 47 51 41 0 0. 64 ju l-1 1 71 64 23 47 51 21 0 1. 32 ju l-1 1 71 71 23 47 51 06 0 0. 26 ju l-1 1 71 63 23 47 50 91 0 0. 30 ju l-1 1 71 69 73 47 51 41 0 0. 97 ju l-1 1 71 64 73 47 51 21 0 0. 67 ju l-1 1 71 61 73 47 51 01 0 0. 28 ju l-1 1 71 63 73 47 50 91 0 0. 43 ju l-1 1 71 70 73 47 50 86 0 0. 30 ju l-1 1 71 68 23 47 50 86 0 0. 30 ju l-1 1 71 68 73 47 50 86 0 0. 30 ju l-1 1 71 67 23 47 50 86 0 0. 37 ju l-1 1 71 67 73 47 50 86 0 0. 23 log-normal kriging method [e.g. krige 1951, matheron 1970] using the isatis© software package of geovariances. the processing of the geostatistical data indicates that the spherical model with a nugget effect is the best model to describe the spatial variability of {co2. the variogram parameters have a range of 115 m, a sill of 0.12 and a nugget of 0.2 {co2 mol m -2 day-1. the iso-flux map shows that the diffuse {co2 is generally low (average value of 0.38 mol m-2 day-1), although values >0.6 and up to 3.42 mol m-2 day-1 were occasionally measured at nw and sw and e, respectively, of the studied area (figure 6). since the co2 flux values tended to be log-normally distributed, the whole frequency distribution was analyzed by using a gaussian (normal) distribution in the complete data set (figure 3), and the ln({co2) data were processed according to the method proposed by sinclair [1974]. the sinclair procedure allows to choose threshold values between anomalous and background data using probability graphs. on a log-probability plot, a single log-normal population results as a straight line, whereas a curve with an inflection point describes the theoretical distribution of two overlapping log-normal populations and n overlapping log-normal populations result on a curve characterized by n-1 inflection points. generally speaking, the log-probability plot shows a straight line for the co2 flux data related to the ermeta area (figure 3). nisi et al. 10 figure 3. probability plots of ϕco2 values obtained by applying the partitioning method of sinclair [1974, 1991]. circles show the measured soil diffuse ϕco2 (in mol m -2 day-1). surveyed surface (m2) number of measurements mean median minimum maximum std. dev. 653,550 301 -0.95 -0.97 -2.63 1.23 0.56 table 3. surveyed surface (in m2), number of measurements and statistical parameters (mean, median, minimum, maximum and standard deviation) of the ϕco2, expressed as ln(ϕco2) (mol m -2 day-1). figure 4. h2s–ch4–h2 ternary diagram for the ermeta and acqua passante gases and the mt. amiata gas discharges (by tassi et. al. [2009]). figure 5. n2–he–ar ternary diagram for the ermeta and acqua passante gases and the mt. amiata gas discharges (by tassi et. al. [2009]). asw: air-saturated-water (asw). 11 on the basis of this reasonable consideration, the data set can be modeled with a population that can be considered as the “local co2 flux” (average value of 0.38 mol m-2 day-1) and comparable with the mean biological co2 flux (~0.11 mol m -2 day-1; e.g. raich and schleisinger [1992]). the respective statistical parameters of the considered population are reported in table 4. the total co2 output for the ermeta area was calculated by applying the sichel’s t-estimator (mi) [david 1977], whose value was derived by multiplying mi times the area covered by the population (table 4). in the same way, the central 95% confidence intervals of the total co2 output were used to calculate the uncertainty of the population. according to the sichel’s t-estimator (mi), the total amount of co2 released through diffuse degassing is estimated to be 11.81 ton day-1. 5.3. computation of the amount of dangerous gases released from the ermeta and acqua passante wells since their construction and their relationships with the soil diffuse gases the impact of the hydrothermal gases released to the atmosphere from the two chimneys is of critical importance for the municipality of abbadia san salvatore since disagreeable effects for people approaching these areas, such as odor of rotten eggs, are clearly felt. furthermore, locally lethal gas accumulation in air may occur as testified by the presence, though rare, of dead animals found in proximity of the wells. our data show that the chemical composition of the well gases in the last 5 years had no significant variations and the co2 flow rate from the two chimneys has remained fairly constant during the period of observation. consequently, we may reasonably assume that the amount of co2 has not significantly changed since the construction of the ermeta and acqua passante wells, i.e. 54 and 75 years ago, respectively. this allows us to compute that since then the two wells have emitted up to 850,000 tons of co2 in the atmosphere. this calculation can be extended to h2s and ch4 on the basis of the measured mean values of the h2s/co2 (ermeta = 0.0005 and acquapassante = 0.0004) and ch4/co2 (ermeta =0.0063 and acquapassante = 0.0056) ratios, obtained by the 5 years of periodic sampling collection of the well gases. thus, the total amount of h2s and ch4 emitted from the two chimthe co2-rich gases from the acqua passante and ermeta wells (mt. amiata, italy) figure 6. map of soil diffuse ϕco2 (in mol m -2 day-1) for the area close to the ermeta well. red dashed lines refer to preferential alignments (faults or fractures) where highest co2 flux values were measured. population number of measuraments co2 flux population mean ln (ϕco2) flux value (mi ) total diffuse co2 output (ton/day) 95% confidence interval (ton/day) 1 301 local background -1.076 11.81 13.07-10.84 table 4. estimated parameters of partitioned populations and derived total co2 output calculated according to sinclair [1974] and sichel’s t-estimator (mi ) [david 1977]. neys is of about 430 and 5,150 tons, respectively, the ermeta well discharging slightly less than one order of magnitude the amount of h2s and ch4 released by that of acqua passante. it is worth noting that the sum of co2 released on a year basis from ermeta and acqua passante wells was up to twice the amount of co2 emitted from bubbling pools and dry vents computed in the surrounding areas of mt. amiata, such as: acqua borra at castelnuovo berardenga (siena) (5 ton y-1; vaselli et al., unpublished data), lo spuntone, il palazzo and buca alle colline at campiglia d’orcia (siena) (≈ 370 ton y-1 at each site; vaselli et al. [2006a], tassi et al. [2009]), bossoleto at rapolano (siena) (4,380 ton y-1; van gardingen et al. [1995], vaselli et al. unpublished data), pienza (5,475 ton y-1; giuli et al. [1997]) and ambra river (7,300 ton/year; cuccoli et al. [2006]). a similar comparison was not computed for h2s and ch4 since in most cases the co2 flux was determined by open-path ir laser measurements [belotti et al. 2003], which were related to the gas discharges from various vents, contributing to the calculated amount of co2. no specific investigations were performed to highlight possible differences in the h2s/ch4 ratios from each emission discharging from a certain site. if present, the calculated amount of h2s and ch4 might be affected by a large error. the generally low co2 soil flux values measured in se the ermeta well seem to indicate that the diffuse gas emission from the soil is mainly related to soil respiration. plant roots and soil microbial activities are expected to produce a relatively homogeneous distribution of the diffuse soil co2 when a soil cover is present, likely modulated by seasonal patterns of moisture availability, land use and biological cycles [e.g. almagro et al. 2009, elío et al. 2013 and references therein]. as previously mentioned, the studied area is generally characterized by a well-developed vegetation and no measurements were performed where the volcanic rocks were outcropping. nevertheless, the spatial distribution of the co2 fluxes (figure 6) shows the occurrence of preferential nw-se and nne-ssw oriented alignments characterized by relatively high co2 flux values, which might be related to weakness zones, such as fractures and/or faults. as a first approximation, these sites may suggest the presence of gases fed by the deep-seated reservoir. in this respect, an isotopic survey of the co2 soil gases is required to assess whether these zones can be regarded as the preferential pathways for the uprising of deep-originated co2. the carbon isotopic signature of the ermeta gas (−3.5 v-pdb ‰), as well as that of acqua passante, is indeed significantly different with respect to that associated with a biogenic source (~ −20 v-pdb ‰; schoell [1980]). to further support this hypothesis a geostructural survey is required to verify the correspondence between the geochemical and geological signals. 5.4. is the closure of the ermeta and acqua passante wells feasible and safe? since 2008, the municipality of abbadia san salvatore has become the owner of the mining concession from where cinnabar was extracted and in agreement with the authorities of the tuscany region it was appointed to i) monitor the gases discharged from the ermeta and acqua passante wells and ii) actuate prevention measurements to minimize such emissions to the atmosphere. as previously mentioned, the two wells are located in the proximity of tourist areas and the reduction of their gas discharges is expected to provide benefits in terms of safety and healthy conditions. presently, ~15,000 tons of co2, along with 8 and 92 tons of h2s and ch4, respectively, are emitted on a yearly basis from the ermeta and acqua passante chimneys and they are related to the discharge of a natural subsurface co2 reservoir intercepted during hg prospection campaigns few kms away from abbadia san salvatore. the ermeta well contributes for >85% of the total co2 discharge. the amount of co2 is more than three times higher than that released by diffuse soil degassing released from an area of 653,550 m2 located se of the ermeta chimney, this value being of 4,310 ton y-1. according to the aforementioned agreement, the municipality of abbadia san salvatore would intend to proceed with the closure of one of the wells and that of ermeta was thought to be considered as the most suitable since it is i) discharging a larger quantity of gas, ii) located more far away from the main roads and iii) surrounded by a vegetation (and soil) cover (figure 2), similar to that where {co2 fluxes were directly measured suggesting that the diffuse soil co2 can be assumed to be only affected by soil respiration, e.g. no gas discharges are present at the surface. nevertheless, one of the most critical points is to understand and recognize which are those sites that might possibly be affected by potential leakage or micro-seepage of co2 [e.g. klusman 2013, elío et al. 2013, nisi et al. 2013 and references therein] as gases in deep reservoirs are buoyant and tend to vertically or laterally migrate following the most permeable pathways in response to pressure. several authors have proposed buoyancy of gas (micro)bubbles [e.g. macelvain 1969, davidson 1982, klusman 1993, klusman and saeed 1996] as a potential process of vertical migrations of gases and gas overpressuring underground, as that expected once the closure of the ermeta well will occur, plays an important role [klusman 1997]. in this respect, nisi et al. 12 13 faults and fractures, may then vehiculate the co2-rich gases upwards [e.g. etiope 1999, brown 2000 and references therein]. as pinpointed by klusman [2013], the rapidity of transport is another important parameter to be taken into account. it appears to be realistic the fact that the weakness zones, individuated by the direct measurements of the co2 soil fluxes (figure 6), can likely be affected by an increase in the co2 flux, being these areas characterized by a higher permeability. this would theoretically imply the possibility to recognize those areas from where a higher probability of co2 leakage is expected in the case of a gas overpressure by applying a relatively simple, though effective, method such as that of the accumulation chamber. owing to the lack of specific investigations addressed to a detailed geostructural study in the surrounding areas of the ermeta well, to tentatively reduce the discharge of co4, ch4 and h2s we highly recommend the municipality of abbadia san salvatore to solely temporarily close the ermeta well by using n2-filled air-bags before a definitive cementation. although it is difficult to predict the side effects deriving by such an action, we may speculate that an increasing gas flow rate could be expected. furthermore, no indications are also presently available about the timing before a modification of the soil diffuse co2 is to be recorded. this implies that a geochemical and isotopic monitoring program has to be designed and mainly focused on the diffuse soil co2 with particularly reference to the recognized weakness zones. in the case of increasing activity in terms of release of soil co2, the tube closing being temporary, previous conditions could easily be restored by re-opening the ermeta well. thus, currently there are no sufficient clues to definitively shut down the ermeta well. a similar approach should be considered if the closure of the acqua passante well is to be considered. 6. conclusions the municipality of abbadia san salvatore is intending to reduce the impact related to the emission in the atmosphere of co2(h2s,ch4)-rich gases by two wells: acqua passante and ermeta, built in 1938 and 1959, respectively, during hg prospection campaigns and located in the eastern part of the mt. amiata volcanic system. the easiest solution to avoid the gas discharge is to close down the wells, the ermeta drilling being the one to be tested first. in this work we have determined that the ermeta and acqua passante drillings are emitting up to 15,000 ton y-1 of co2 and 92 and 8 ton y -1 of ch4 and h2s, respectively. these gases can be regarded as the discharge of a natural subsurface co2-rich reservoir. the origin of these gases is similar to those naturally released from dry vents and bubbling pools located few kilometers away from the two wells, with the exception of h2, whose higher content is apparently related to corrosion processes of the iron tubing of the chimneys as the deep-seated gases flow through them. we have also estimated the co2 output from soil (4,310 ton y -1 in an area of about 653,500 m2) close to the ermeta well and determined by the accumulation chamber method. relatively high co2 fluxes were recognized along preferential alignments, possibly associated with the presence of faults or fractures (figure 6). standing the absence of detailed geostructural investigations nearby ermeta, we strongly recommend a temporarily closure of the ermeta well by means of n2-filled air-bags before proceeding with a definitive cementation. afterwards, an appropriate geochemical and isotopic monitoring activity of the soil gas is required to verify the presence of leakage and microseepage of co2. acknowledgements. many thanks are due to the municipality of abbadia san salvatore that partly financially the present study. the authors are grateful to tom darrah for providing the helium isotopic ratios of the ermeta and acqua passante gases. the personnel of west system is thanked for their help in the installation of the hotwire anemometer in the acqua passante and ermeta wells. f. frondini and two anonymous reviewers are kindly acknowledged for their suggestions and comments to an early version of the manuscript. references allard, p., j. carbonnelle, d. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice de martino_corrected:layout 6 annals of geophysics, 57, 2, 2014, s0213; doi:10.4401/ag-6431 s0213 gps time series at campi flegrei caldera (2000-2013) prospero de martino*, umberto tammaro, francesco obrizzo istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy abstract the campi flegrei caldera is an active volcanic system associated to a high volcanic risk, and represents a well known and peculiar example of ground deformations (bradyseism), characterized by intense uplift periods, followed by subsidence phases with some episodic superimposed mini-uplifts. ground deformation is an important volcanic precursor, and, its continuous monitoring, is one of the main tool for short time forecast of eruptive activity. this paper provides an overview of the continuous gps monitoring of the campi flegrei caldera from january 2000 to july 2013, including network operations, data recording and processing, and data products. in this period the gps time series allowed continuous and accurate tracking of ground deformation of the area. seven main uplift episodes were detected, and during each uplift period, the recurrent horizontal displacement pattern, radial from the “caldera center”, suggests no significant change in deformation source geometry and location occurs. the complete archive of gps time series at campi flegrei area is reported in the supplementary materials. these data can be usefull for the scientific community in improving the research on campi flegrei caldera dynamic and hazard assessment. 1. introduction the campi flegrei (cf) caldera, which includes the western part of naples, is an active volcano associated to a high volcanic risk [ricci et al. 2013]. cf caldera is located in the southern part of the plio-quaternary campanian plain, where there are also others volcanic systems (ischia, procida and somma-vesuvius). their volcanism is related to the spreading of the tyrrhenian basin, the collaps of the western margin of the appennine chain and campania plain graben formation [finetti and del ben 1986, luongo et al. 1991a, patacca and scandone 2007, vezzani et al. 2010]. the cf caldera last eruption, occurred in 1538, gave rise to mt. nuovo [di vito et al. 1987]. many authors assign the most remarkable events in the cf volcanic history to the eruptions of the campanian ignimbrite and neapolitan yellow tuffs (nyt) calderas occurred respectively at 40 and 15 ka. since the nyt eruption, the volcanic activity have been confined inside the caldera boundaries [rosi and sbrana 1987, deino et al. 2004, fedele et al. 2008, isaia et al. 2009, arienzo et al. 2011]. cf are known worldwide to be a site of notable slow vertical ground movements (bradyseism), testified by many submerged roman ruins which are found along the coast of pozzuoli gulf, at nearly 14 m below sea level. these archeological ruins, as well as historical and recent observations, indicate that such phenomena have been active during the last 2000 years. sinking has been then dominant process in historical times, but phases of uplift are also documented mainly from the well known evidence on the serapeo columns (roman market in pozzuoli, the central part of the caldera) [günther 1903, parascandola 1947, morhange at al. 1999, 2006]. the most recent uplift episodes occurred in 19701972 and 1982-1984 with a cumulative vertical displacement of about 3.5 m at the town of pozzuoli, partially recovered successively. in addition to notable ground movements, the 1982-84 crisis was characterized by intense seismic activity with the occurrence of about 15,000 shallow earthquakes, with a maximum magnitude of 4.0. [corrado et al. 1977, barberi et al. 1984, berrino et al. 1984, de natale et al. 1991, luongo et al. 1991b]. after 1984, small uplift episodes occurred in 1989, 1994, 2000 with maximum amplitude ranging between 1 and 7 cm [del gaudio et al. 2010]. from 2005, an inflation period has been detected [troise et al. 2007] and, with increasing rates over time, is still ongoing. the overall uplift from 2005 to date is about 23 cm. the dense urbanization of the cf area, makes the monitoring of geophysical and geochemical parameters, linked to the eruption precursors, essential for an assessment of volcanic hazard and risk mitigation. ground deformation is an important volcanic precursor, because it is linked to magma overpressure and migration. article history received october 23, 2013; accepted february 12, 2014. subject classification: geodesy: measurements and monitoring, gps, time series, campi flegrei, ground deformation. the osservatorio vesuviano (ingv-ov), department of the istituto nazionale di geofisica e vulcanologia, is in charge of monitoring the neapolitan volcanic area. the monitoring system consists of several permanent networks, which provide seismological, geodetic and geochemical data. continuous geodetic measurements are made using networks of tiltmeters, tide gauges and gps stations. in particular, the permanent gps network, called nevocgps (neapolitan volcanoes continuous gps), provides stations 3d position over time [bottiglieri et al. 2010, vilardo et al. 2010, tammaro et al. 2013]. with this paper we are providing gps time series of the north, east and vertical components of 14 stations of the nevocgps network operating at cf area from 2000 to date. the paper is organized as follows: in section 2, we describe the data recording and data processing of nevocgps network; in section 3, we present the time series of gps stations in cf area, section 4 is dedicated to the discussion, and finally section 5 contains the conclusions. 2. the nevocgps network the nevocgps network consists of 30 permanent stations located in the neapolitan volcanic district and surrounding area (figure 1). all stations have been equipped with data transmission channels and with antennas to reduce multipath interference effect (choke-ring design). due to the high urbanization of the area and the considerable logistical constraints on siting (undisturbed view of the sky for clear signal reception, power connection, data transmission and instruments protection), most of the stations are monumented on the roof of stable buildings through three-dimensional forced centering devices for precise antenna set up. in some cases, concrete pillars anchored on bedrock are adopted. at present, 14 stations of the nevocgps network are operating at cf area (figure 1). some stations have been active since 2000 (table 1). de martino et al. 2 station first observation (decimal year) time span interval (year) acae 2000.1 13.5 arfe 2000.7 12.9 baia 2000.1 13.5 frul 2008.1 5.5 ippo 2000.0 13.6 lico 2003.2 10.4 mafe 2010.5 3.1 moru 2007.8 5.8 nisi 2009.3 4.3 quar 2000.0 13.6 rite 2000.4 13.2 solo 2009.1 4.5 strz 2008.0 5.6 vica 2009.0 4.6 figure 1. map of nevocgps network. red triangles indicate gps permanent stations. the box on the left shows cf area and the stations used in this paper. table 1. first observation and time span interval of the gps stations used in this paper. 3 2.1. data recording every day, 24 hours of observed dual-frequency gps data, with 30 seconds sampling rate and 15 degrees cut-off angle, are downloaded from each site to the osservatorio vesuviano [tammaro et al. 2013]. the data quality have to be quickly and accurately tested before their processing and storage in a database. the osservatorio vesuviano uses an in-house automatic application (seta) for a quality check. it makes use of freeware programs, e.g. teqc software, [estey and meertens 1999], to translate from the binary receiver format to the standard rinex format, perform a quality check of the data and transfer the data files in an archive. all the procedures for remote stations managing, raw data downloading, rinex files generation and data quality control work automatically [tammaro et al. 2013]. 2.2. data processing data processing is performed by the bernese processing engine (bpe) of the bernese gps software v. 5.0 [dach et al. 2007] in double difference mode. the data are processed on a daily basis using the ultra-rapid international gnss service (igs) products [dow et al. 2009], in a fully automated processing chain. once the igs final orbits and earth rotation parameters (erps) become available, the data are reprocessed on a weekly basis. the elevation cut-off is set at 15° and the igs absolute phase center variations for the satellite and receiver antennas are modeled. independent baselines are selected taking into account the criterion of maximum common observations. the ambiguity resolution is based on the quasi ionosphere free analysis [mervart et al. 1994]. daily coordinates of the stations are estimated, together with the troposphere, in the final ionosphere free l3 solution. the dry part of the troposphere is modeled using the dry-niell a-priori model and estimating the troposphere zenith delay parameters every hour at each site using the wet-niell mapping function [niell 1996]. the daily and weekly normal equations (neqs) are saved for subsequent combination. in order to get high-precision results, the utilization in the processing of homogeneous gps products (orbits and erps), same correction models (e.g. antenna phase center variations) and precise and stable reference frame is essential. during the time span (2000-2013) different improvements were introduced by the igs into the processing strategies and modeling of global gps solutions and different reference frames were adopted. since 2000, the igs has used its own realizations of the successive itrf releases as reference frames for its products. in november 2006, a reference frame based on itrf2005 [altamimi et al. 2007], called igs05, was adopted by the igs, simultaneously with the switch from relative (igs_01.pcv) to absolute antenna phase center calibrations (igs05.atx) [schmid et al. 2007]. because of growing velocity propagation errors and of jumps having affected the positions of many reference stations, igs05 has become obsolete. so, a new reference frame based on itrf2008, called igs08, was designed and officially adopted by the igs starting with gps week 1632 (april 17, 2011). furthermore, an updated set of absolute antenna calibrations (igs08.atx) was adopted [rebischung et al. 2012]. these changes have made the previous igs products inhomogeneous and inconsistent over time, introducing discontinuities into the time series of gps station positions. in order to get homogeneous results, all the gps data of the nevocgps network collected before november 2006, have been reprocessed using orbits and erps obtained by gps-pdr reprocessing project of the global reference network based on absolute antenna phase center calibrations [steigenberger et al. 2006]. station specific discontinuities due to the antenna calibration updates from igs05.atx to igs08.atx are calculated with latitude-dependent correction models proposed by [rebischung et al. 2012]. also discontinuities associated with equipment changes (e.g., due to the replacement of antennas and/or tracking receivers, change of the radome on the antenna, firmware updates, etc.) are estimated. all discontinuities are introduced in the station information file as antenna eccentricities, and, after a final neqs combination, we generate the updated time series of station coordinates. the geodetic datum is realized by three no-net translation conditions imposed on a set of six igs08 gps time series at campi flegrei station north (mm) east (mm) up (mm) cagl 12725m003 -2.1 2.6 -3.5 gras 10002m006 2.5 3.3 0.9 mate 12734m008 2.5 -3.4 2.2 not1 12717m004 -2.2 -0.8 0.2 wtzr 14201m010 0.9 2.3 -3.5 zimm 14001m004 -1.4 -4.0 3.7 rms / component 2.2 3.2 3.0 rms of helmert transformation 3.3 mm table 2. residuals from helmert transformation between igs08 and our solution for the reference stations. reference stations (minimum constraint solution), which are included in the processing. table 2 shows the difference between the igs08 coordinates and our estimated values for the reference stations. residuals demonstrate the consistency of estimated coordinates with igs08 at level of 3.3 mm in total rms after the helmert transformation. 3. gps time series the estimated igs08 horizontal components (north and east) of the time series primarily reflect a rigid motion of the eurasian plate with an average rate of about 2.5 cm/yr in the ne direction. therefore, to eliminate the global tectonic motion and highlight the volcanic deformation, we define a local reference frame based on the nevocgps reference station enav, located on the limestones of sorrento peninsula, outside the volcanic area, and with long time span of measurements and constant horizontal velocities [de martino de martino et al. 4 clockwise from top left: figure 2. gps weekly time series of north component for cf stations relative to enav station. uncertainties are not shown here, but are reported in data files (supplementary material). curves have been arbitrarily shifted for clarity. see figure 1 for the location of the gps stations. figure 3. gps weekly time series of east component for cf stations relative to enav station. uncertainties are not shown here, but are reported in data files (supplementary material). curves have been arbitrarily shifted for clarity. see figure 1 for the location of the gps stations. figure 4. gps weekly time series of vertical component for cf stations. uncertainties are not shown here, but are reported in data files (supplementary material). curves have been arbitrarily shifted for clarity. see figure 1 for the location of the gps stations. 5 et al. 2011, tammaro et al. 2013]. we simply remove the enav horizontal gps velocities (16.5 mm/yr and 21.3 mm/yr, in the north and east component respectively) from all gps time series of the cf area, while the vertical (up) components were untouched. the deformation signal does not have significant changes on timescales shorter than a week, so we use weekly combined solutions. as an indicator of realistic uncertainties of the weekly positions, we use the rms of daily solutions with respect to the weekly solution, the so-called repeatabilities [steigenberger et al. 2012]. the mean rms values are less than 1mm and 3 mm in the horizontal and vertical components, respectively. figures 2, 3 and 4 show the final gps weekly position time series for the stations at cf from 2000 to july 2013. in the supplementary material, we report the complete archive in ascii format of all the gps time series used in this paper. 4. discussion gps data at all the continuous recording stations from 2000 to 2013 are shown in figures 2, 3 and 4 displaying the whole evolution of the subsidence and uplift. during this period, the rite station, located at the center of pozzuoli, has the largest associated uplift, of about 18.5 cm (figure 5). the acae, arfe, solo, strz stations (figure 4), located within about 3 km from rite, have smaller vertical displacements from 6 cm (arfe) up to 17 cm (acae). the other stations, located over 3 km from rite, have vertical displacements from less than 1 cm (mafe, frul) up to 6 cm (ippo). figures 2 and 3 show that the solo, acae, ippo, frul and mafe stations have horizontal displacements in ne direction. the rite, vica, lico, moru, strz and rite sites have horizontal displacements in nw direction. nisi has a displacement towards se and baia in sw direction. only quar has a horizontal displacement mainly to north. as mentioned in the introduction, cf are known worldwide to be a site of notable slow vertical ground movements (uplift and subsidence) and, as noted in several papers [e.g. de natale and pingue 1993, de natale et al. 1997], the shape of vertical ground deformation pattern is remarkably constant, independent from the total amount of displacement. this characteristic behavior suggests that the position of the source is always the same. the precise modeling of displacement sources generally requires a dense geodetic data sampling, therefore it is possible to use continuous gps measurements to extract basic information about source geometry and depth. during period 2000-2013, we have identified, in the rite time series, 7 episodes of uplift with time span greater than 0.23 years and/or a speed greater than 18 mm/yr. figure 5 shows the horizontal displacements observed during the selected 7 uplift period. simple geometrical considerations (crossing point of the displacement vectors for each uplift) suggest a stable source location. therefore, for each uplift episode, we jointly invert the horizontal and vertical displacements measured at gps stations. we have used dmodels software package [battaglia et al. 2013a,b], which provides matlab functions and scripts to invert gps data for spherical pressure source in a homogenous, isotropic, elastic, flat half-space. the seven sources obtained by the inversion results (0.4 < |2 <1.1) are concentrated in an area (south of rite station, figure 6) of 130 × 260 m at depth ranging between 2400 and 3000 m. although the real conditions might be much more complex, it seems plausible to assume a constant position of the source of ground deformation. nevertheless, it is unequivocal that the location of the axis of the pressure source is remarkably constantly, although the source depth and shape can change [de natale and pingue 1993, de natale et al. 1997]. therefore, simulations with more complex sources in heterogeneous structures are needed. gps time series at campi flegrei figure 5. gps weekly time series of vertical component for station rite: (a) 2000-2006 time span; (b) 2007-2013 time span. the gray box indicate the selected seven uplift periods (up1, up2…,up7). 5. conclusion in this paper we have presented the part of the nevocgps network that includes cf caldera. we have described data recording and data processing of the complete dataset of gps time series at cf caldera from 2000 to july 2013. the cf caldera appears to be in a constant motion both in uplifting and in dowlifting, and nevocgps network has been able to measde martino et al. 6 figure 6. the vectors show the magnitude and direction of displacement of each continuous gps station during the selected 7 uplift periods in figure 5. for each period have shown the position of the source obtained by means joint inversion of horizontal and vertical displacements, and uplift map. 7 ure uplift and subsidence during period 2000-2013. simple geometrical considerations, and jointly inversion of the horizontal and vertical displacements measured at gps stations seem to suggest a stable source location during the time interval from 2000 to 2013. nevertheless, complex models with one or more sources and heterogeneous half-space are needed. the complete dataset of gps time series at cf area from 2000 to july 2013 is reported in the supplementary materials. these data can be useful for the scientific community in improving the research on cf caldera dynamic and hazard assessment. acknowledgements. the authors thank giuseppe brandi, andrea d’alessandro, mario dolce, santa malaspina and claudio serio, who have taken care of the maintenance of nevocgps network. we are grateful to the laboratorio di geomatica e cartografia (ingv-ov) for providing the dtm (digital terrain model) of the area. this research benefited from funding provided by the italian presidenza del consiglio dei ministri dipartimento della protezione civile (dpc). scientific papers funded by the dpc do not represent its official opinions and policies. references altamimi, z., x. collilieux, j. 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america, special paper 469, 1-58. vilardo, g., r. isaia, g. ventura, p. de martino and c. terranova (2010). insar permanent scatterer analysis reveals fault re-activation during inflation and deflation episodes at campi flegrei caldera, remote sens. environ., 114, 2373-2383; doi:10.1016/j.rse.2010. 05.014. *corresponding author: prospero de martino, istituto nazionale di geofisica e vulcanologia, sezione di napoli, osservatorio vesuviano, naples, italy; email: prospero.demartino@ov.ingv.it. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. de martino et al. 8 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 64, 3, se331; 2021; doi:10.441/ag-8471 special issue empowering communities for non-structural seismic risk mitigation: the central role of communication” foreword this special issue of “annals of geophysics” concerns the dissemination of knowledge on the prevention of damage mainly due to non-structural elements during earthquakes and its practical application at houses, schools and offices by common citizens, companies and institutions. the seismic capacity of buildings and other civil engineering structures and infrastructures are object of regulations for design and construction, and in some cases also the design, fabrication and mounting of electrical and mechanical equipments. consequently, even in strong earthquakes many collapses of buildings and infrastructures are avoided. however, with few exceptions, design procedures do not aim at avoiding seismic vibrations from being transferred to the structures, but enable the structures to resist to the effects of those vibrations. this means that non-structural elements, including contents, furniture, personal objects and working devices, will be subject to shaking, with the potential to cause damage, injuries and loss of life. therefore, the reduction of damage due to non-structural elements has a relevant role in the reduction of material and human damage due to earthquakes, especially in developed societies, where other measures and policies aiming at the reduction of structural damage are enforced. actions to reduce non-structural damage require the participation and involvement of the public in general, at home and at work places. thus, the transference of knowledge and raising awareness of the general public regarding seismic risk and the potential contribution of common citizens to reduce it, is critically important. this was the aim of the european union (humanitarian aid & civil/protection) echo funded project knowrisk (“know your city, reduce seismic risk through non-structural elements”). the project sought to facilitate access to the local community to specialist knowledge on non-structural seismic risk protection solutions. scientific knowledge was turned into practical knowledge to be used by citizens and to engage communities in disaster risk reduction. several tools, such as guides, portfolios, serious games, videos, and augmented reality were designed and disseminated to communicate, teach and help different sectors of society, namely the population in general, students and selected stakeholders, in order to reduce seismic risk associated with damage due to non-structural elements. in the three participant countries different type of stakeholders such as schools, engineers, architects, facility managers, historical downtown citizens, civil protection officers, etc., gave their contribution to the project in different ways, and this was crucial to develop specific communication techniques and tools. this special issue of annals of geophysics discusses the relevance of risk communication to common citizens, students and the business sector, and contains a set of papers that provide an overall view of the project results and two guest papers that point out how important is the communication for the reduction of seismic risk. there are three key points that the knowrisk project posed at the core of its risk mitigation action and communication strategy: “move, protect, secure”. they were highlighted throughout the whole process, from research to action, and were turned into a simple three words slogan to guide stepwise to the increase in safety. “move” refers to simply rearranging items that might fall, “protect” refers to valuable and fragile items, “secure” refers to anchoring tall or loose items. the increase in safety goes along with the needs for higher level of expertise: from simple interventions that can also be carried out by householders to retrofits that absolutely requires specific skills. papers by ferreira et al. and by solarino et al. identify the non-structural elements most commonly damaged by earthquakes with greater impact, and present communication tools to address citizens, students and the building sector, in order to turn research into preventive actions. thorvaldsdottir et al. develop guidelines for risk reduction based on an extensive portfolio of photos and assessment of non-structural earthquake damage in residences in south iceland. those guidelines can be applied not only to residences but also to offices, industrial facilities, hospitals and government services. ferreira et al. extend the scope from housing to business by means of engaging stakeholders of this sector to develop knowledge on the reduction of non-structural damage in office buildings, shops, warehouses and other facilities, as well as communication of results. in the knowrisk project, a large emphasis was placed on engaging young people, as it is one of the most effective means of risk mitigation in the long run as they seed the roots that shape the future. musacchio et al. describe this vision and systematize a protocol to communicate seismic risk in italian schools, comprising design, test and assessment, while piangiamore et al. present students’ point of view concerning non-structural risk mitigation. solarino et al. describe an approach to risk mitigation that stands on the ability that tools such as serious games have to instill in young people a proactive attitude. lastly, falsaperla et al. describe how crucial is dissemination within a risk communication strategy. risk communication is also a key instrument in the studies reported by kouskona et al., that apply the risk game tree and its formation for the purpose of policy selection considering not only disaggregation of earthquake-related direct and cascading effects, but also risk communication. knowrisk is an attempt to engage stakeholders in non-structural risk mitigation in europe, nevertheless dey et al. present the need for a widespread dissemination of information on seismic hazard preparedness and action through community participation in north east india, the highest seismic hazard zone in whole india. references m. a. ferreira, f. meroni, r. azzaro, g. musacchio, r. rupakhety, b. bessason, s. thorvaldsdottir, m. lopes, c. s. oliveira, s. solarino (2021). what scientific information on the seismic risk to non-structural elements do people need to know? part 1: compiling an inventory on damage to non-structural elements, ann. geophys, this issue, doi:10.4401/ag-8412 s. solarino, m. a. ferreira, g. musacchio, r. rupakhety, h. o’neill, s. falsaperla, m. vicente, m. lopes, c. s. oliveira, (2021). what scientific information on the seismic risk to non-structural elements do people need to know? part 2: tools for risk communication, ann. geophys, this issue, doi:10.4401/ag-8439 s. thorvaldsdottir, b. bessason, r. rupakhety (2021). non-structural earthquake risk management for residential buildings, ann. geophys, this issue, doi:10.4401/ag-8451 m. a. ferreira, c. s. oliveira, m. lopes, f. mota de sá, g. musacchio, r. rupakhety, d. reitano, i. pais (2021). using non-structural mitigation measures to maintain business continuity: a multi-stakeholder engagement strategy, ann. geophys, this issue, doi:10.4401/ag-8559 g. musacchio, e. eva, m. crescimbene, n. a. pino, l. cugliari (2021). a protocol to communicate seismic risk in schools: design, test and assessment in italy, ann. geophys, this issue, doi:10.4401/ag-8533 g. l. piangiamore, s. falsaperla, e. eva, g. musacchio (2021). seismic risk communication: let’s students show their own way, ann. geophys, this issue, doi:10.4401/ag-8396 s. solarino, m. a. ferreira, g. musacchio, e. eva (2021). playing games for risk prevention: design, implementation and testing of serious games in recent european projects upstrat-mafa and knowrisk, ann. geophys, this issue, doi:10.4401/ag-8436 s. falsaperla, g. musacchio, m. a. ferreira, m. lopes, c. s. oliveira (2021). dissemination: steps towards an effective action of seismic risk reduction for non-structural damage, ann. geophys, this issue, doi:10.4401/ag-8394 kouskouna vsiliki, g. sakkas, i. cecic, v-i. tsimpidaros, s. sakkas, g. kaviris, a. tertulliani (2021). earthquake induced crises: game tree approached risk communication and lessons learnt, ann. geophys, this issue, doi:10.4401/ag-8405 c. dey, s. baruah, b. k choudhury, t. chetia, s. saikia, a. sharma, m. k. phukan (2021). living with earthquakes: educating masses through earthquake awareness: north east (ne) india perspective, ann. geophys, this issue, doi:10.4401/ag-848479 2 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8412 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8412 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8412 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8439 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8439 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8439 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8451 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8451 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8451 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8533 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8533 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8533 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8396 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8396 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8396 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8436 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8436 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8436 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8394 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8394 https://www.annalsofgeophysics.eu/index.php/annals/article/view/8394 mário lo p e s ceris, instituto superior técnico, university of lisbon, lisbon, portugal, gemma m u s acc h i o ingv, amministrazione centrale roma, italy mónica amaral f e r r e i ra ceris, instituto superior técnico, university of lisbon, lisbon, portugal, carlos s o u s a o l i v e i ra ceris, instituto superior técnico university of lisbon, lisbon, portugal 3 u n a s o l u z i o n e d e l p r o b l e m a d e l l e r e g i s t r a z i o n i m e c c a n i c h e enrico m e d i il p r o b l e m a di dover s o s t i t u i r e l e r e g i s t r a z i o n i f o t o g r a f i c h e , con r e g i s t r a z i o n i di t i p o m e c c a n i c o è stato a f f r o n t a t o e risolto con v a r i m e t o d i ( g a l v a n o m e t r i r e g i s t r a t o r i p e r p u n t i , cellule f o t o e l e t t r i c h e ecc.). q u i si e s p o n e b r e v e m e n t e u n artificio s p e r i m e n t a l e c h e , p e n s o , possa p o r t a r e n o t e v o l i v a n t a g g i e risolvere in m a n i e r a e l e g a n t e e pratica il p r o b l e m a p r o p o s t o . bisogna t e n e r e p r e s e n t e a tale fine a l c u n e c o n d i z i o n i , alle q u a l i d e v e s o d d i s f a r e il dispositivo, p e r e s e m p i o : la sicurezza del f u n z i o n a m e n t o , la c o n t i n u i t à a n c h e p e r o r e e p e r giorni e p e r m o l t i g i o r n i talvolta, la s e m p l i c i t à , la r a g i o n e v o l e e c o n o m i a d e l l ' i m p i a n t o e del f u n z i o n a m e n t o , l a f e d e l t à della r i p r o d u z i o n e , la p r o n t e z z a e a l c u n i a l t r i c a r a t t e r i , c h e q u i n o n è essenziale r i c o r d a r e . s o p r a t t u t t i p e r ò sta la c o n s i d e r a z i o n e c h e l'uso d e l l ' a p p a r e c c h i o r e g i s t r a t o r e n o n deve r e c a r e d i s t u r b i a l l ' a p p a r a t o f o n d a m e n t a l e , n é i m p o r r e m u t a m e n t i di esso. u n artificio m o l t o u s a t o è q u e l l o di r i c o r r e r e alle c e l l u l e fotoe l e t t r i c h e . la l u c e riflessa dallo s p e c c h i e t t o d e l l o s t r u m e n t o investe u n a m a g g i o r e o m i n o r e s u p e r f i c i e della cellula e q u i n d i q u e s t a e m e t t e u n a c o r r e n t e p r o p o r z i o n a l e o, c o m u n q u e f u n z i o n e della d e v i a z i o n e a n g o l a r e dello s p e c c h i e t t o . la c o r r e n t e di cellula ( m e d i a n t e amplific a z i o n e o no) va allo s t r u m e n t o r e g i s t r a t o r e a p e n n a . u n o degli i n c o n v e n i e n t i di tali sistemi è d a t o dalla l i m i t a t e z z a della s u p e r f i c i e d e l l a cellula c h e n o n p e r m e t t e di r a c c o g l i e r e la l u c e riflessa l e cui deviazioni s i a n o n o t e v o l i ; n é è s e m p r e possibile, p e r e v i t a r e ciò, m e t t e r e la cellula t r o p p o vicina allo s t r u m e n t o . i l sistema p r o p o s t o è il s e g u e n t e : la s o r g e n t e di l u c e è costituita da u n filamento r e t t i l i n e o verticale, il cui fascio l u m i n o s o (secondo l e o r d i n a r i e disposizioni) i n c i d e sullo s p e c c h i e t t o dello s t r u m e n t o , a t t r a v e r s o u n a l e n t e convessa, posta n e l l e i m m e d i a t e v i c i n a n z e dello s p e c c h i e t t o stesso: il sistema o t t i c o è r e g o l a t o i n m o d o c h e l ' i m m a g i n e r e a l e del filamento l u m i n o s o ( o 502 e n r i c o m e d i della f e n d i t u r a ) c a d a sulla s u p e r f i c i e di u n o s p e c c h i o concavo, il cui c e n t r o di c u r v a t u r a si trova n e l l a posizione o c c u p a t a d a l l o s p e c c h i o dello s t r u m e n t o . la l u c e riflessa d a l l o s p e c c h i o concavo f o r m a u n a i m m a g i n e r e a l e dello s p e c c h i e t t o , localizzata sullo s p e c c h i e t t o stesso e la sua estensione e posizione sono i n d i p e n d e n t i d a l m o v i m e n t o e dal p u n t o di i n c i d e n z a del p e n n e l l o l u m i n o s o sullo s p e c c h i o concavo. p e r r a g i o n i di p r a t i c i t à si f a in m o d o c h e q u e s t a i m m a g i n e r e a l e dello s p e c c h i e t t o cada u n poco f u o r i dello s t r u m e n t o : il c o m p o r t a m e n t o del sistema è c o m e s o p r a . f i g . 1 dove si f o r m a l ' i m m a g i n e r e a l e sopra n o m i n a t a , si p o n e la cellula f o t o e l e t t r i c a . i n q u e s t e c o n d i z i o n i la s u p e r f i c i e della cellula illumin a t a è s e m p r e la stessa, e la e s t e n s i o n e della cellula n o n h a a l c u n a i m p o r t a n z a , p e r c h é s e m p r e , q u a l u n q u e sia lo s p o s t a m e n t o a n g o l a r e e il m o t o d e l l ' i m m a g i n e r e a l e della s o r g e n t e sullo specchio concavo, il fascio d i l u c e r i t o r n a sulla cellula. r e s t a solo da osservare che, e f f e t t i v a m e n t e , con la r o t a z i o n e , la p r o i e z i o n e o r t o g o n a l e dello s p e c c h i o d i m i n u i s c e e q u i n d i , a n c h e la sua g r a n d e z z a a p p a r e n t e . si r i s p o n d e , c h e , p e r gli o r d i n a r i s t r u m e n t i , l e v a r i a z i o n i a n g o l a r i n o n sono tali da r e n d e r e s e n s i b i l m e n t e notevole tale p r o i e z i o n e se n o n p e r v a l o r i del secondo o r d i n e . u n a s o l u z i o n e bel p r o b l e m a belle r e g i s t r a z i o n i m e c c a n i c h e 5 0 3 in ogni m o d o a n c h e a q u e s t o i n c o n v e n i e n t e v i e n e posto r i p a r o , c o m e è d e t t o poi. risolto il p r i m o p u n t o r i m a n e q u e l l o f o n d a m e n t a l e : m o d u l a r e la i n t e n s i t à della luce in f u n z i o n e della p o s i z i o n e del p e n n e l l o luminoso. sullo s p e c c h i o concavo è posta u n a sagoma leggera di m a t e r i a l e 11011 r i f l e t t e n t e ( c a r t a , m e t a l l o ) di f o r m a o p p o r t u n a ( s u p p o n i a m o p e r o r a t r i a n g o l a r e ) , in m o d o c h e nella p a r t e c e n t r a l e (posizione di zero) l ' i m m a g i n e r e a l e della sorgente r e t t i l i n e a , sia p e r m e t à s c h e r m a t a p e r m e t à riflessa d a l l o s p e c c h i o concavo. sulla cellula i n c i d e u n certo flusso luminoso. se il p e n n e l l o si sposta verso d e s t r a , si h a u n a dimin u z i o n e del t r a t t o riflesso p r o p o r z i o n a l e allo s p o s t a m e n t o ; se verso sinistra si ha u n a u m e n t o p r o p o r z i o n a l e allo s p o s t a m e n t o stesso. il flusso l u m i n o s o sulla cellula risulta q u i n d i m o d u l a t o d a l l e posizioni della l u c e sullo specchio concavo, p u r r e s t a n d o c o s t a n t e la superficie di essa i l l u m i n a t a : v a r i a solo la i n t e n s i t à di i l l u m i n a z i o n e . si v e d e i m m e d i a t a m e n t e come si p u ò s a g o m a r e lo s c h e r m o p e r c o r r e g g e r e i d i f e t t i a c c e n n a t i s o p r a o p p u r e p e r v i n c o l a r e la illumin a z i o n e di cellula m e d i a n t e u n a qualsiasi f u n z i o n e della posizione d e l l o s p e c c h i e t t o dello s t r u m e n t o . questa possibilità è di u n a u t i l i t à g r a n d i s s i m a nella geofisica e in molli a l t r i c a m p i di ricerca. 504 e n r i c o m e d i t u t t o il sistema p u ò essere i n f i n e t a r a t o p e r p u n t i ed a v e r e l a curva di c o r r i s p o n d e n z a c h i a r a m e n t e n o t a . la c o r r e n t e di uscita della cellula va al sistema r e g i s t r a t o r e . di u n t i p o di sistema r e g i s t r a t o r e speciale, d i n u o v a c o n c e z i o n e , p a r l e r ò in u n a p r o s s i m a n o t a . p e r e v i t a r e c h e l e d i m i n u z i o n i d i i n t e n s i t à della l u c e della sorg e n t e possano p o r t a r e e r r o r i sulla r e g i s t r a z i o n e , si usa l ' a r t i f i c i o di p o r r e n e l l e i m m e d i a t e p r o s s i m i t à dello s p e c c h i e t t o dello s t r u m e n t o u n secondo s p e c c h i e t t o , di e g u a l i d i m e n s i o n i , fisso. q u e s t o secondo è o r d i n a r i a m e n t e c o p e r t o d a u n piccolo s c h e r m o , c h e , mosso da u n m o v i m e n t o d i o r o l o g e r i a , a i n t e r v a l l i d e t e r m i n a t i , lo lascia s c o p e r t o , o c c u l t a n d o lo s p e c c h i o m o b i l e . la l u c e riflessa d a l l o s p e c c h i o fisso c a d e e s a t t a m e n t e n e l l a p o s i z i o n e c e n t r a l e d i zero. è così possibile t r a c c i a r e u n a l i n e a di zero, c h e p e r m e t t e di c o r r e g g e r e l e fluttuazioni d o v u t e alla s o r g e n t e l u m i n o s a . i l sistema descritto è s t a t o r e a l i z z a t o con u n o s p e c c h i o c o n c a v o di otto c e n t i m e t r i di altezza, t r e n t a d i l u n g h e z z a e c i n q u a n t a centim e t r i d i r a g g i o di c u r v a t u r a . si c o m p r e n d e c h i a r a m e n t e c o m e , i n v e c e di tino s p e c c h i o concavo, sia possibile u s a r e u n a l e n t e convessa d i d i a m e t r o o p p o r t u n a m e n t e g r a n d e , m e t t e n d o lo s t r u m e n t o e la cellula s i m m e t r i c a m e n t e posti al d o p p i o della d i s t a n z a f o c a l e , r i s p e t t o alla l e n t e e s a g o m a n d o la scherm a t u r a della s u p e r f i c i e r i f r a n g e n t e . c o m e è a n c h e c h i a r o c h e l e p o s i z i o n i della s o r g e n t e e della imm a g i n e s o n o s t a t e scelte con il c r i t e r i o di r i d u r r e al m i n i m o le a b e r r a z i o n i e o t t e n e r e i m a s s i m i r e n d i m e n t i ; m a l e r e c i p r o c h e posizioni possono a n c h e essere c a m b i a t e e n o n l a v o r a r e i n t o r n o al c e n t r o di c u r v a t u r a o al d o p p i o delle d i s t a n z e focali. roma — istituto nazionale eli geofisica — ottobre 1952. riassunto è proposto un dispositivo che permette di far cadere il fascio di luce riflesso dallo specchietto di uno strumento, costantemente sulla stessa superficie di una cellula fotoelettrica. il sistema è realizzato con uno specchio concavo; in prossimità del centro di curvatura di esso si trovano lo strumento e la cellula fotoelettrica. lo specchio concavo è opportunamente e parzialmente schermato u n a s o l u z i o n e del p r o b l e m a delle r e g i s t r a z i o n i m e c c a n i c h e 505 con ima sagoma, che modula l'intensità della luce riflessa in funzione della posizione dell'immagine della sorgente sullo specchio roncavo e quindi in funzione della deviazione dello strumento. summary an arrangement has been proposed which permits the heam of light reflected from the mirror of an instrument to fall constantly on the same surface of a photoelectric cell. the system consists of a concave mirror; the instrument and the photoelectric celi are placed near the center of curvature of this mirror. the concave mirror is opportunely and partially shielded with a forni which modulates the intensity of the reflected light as a function of the position of the image of the source on the mirror, and thus a function of the deviation of the instrument. ag_57.03.14_index+preface_proofs_corrected_layout 6 annals of geophysics preface geophysical monitorings at the earth’s polar regions the following collection of papers is dedicated to geophysical research and experiments conducted in the polar regions of the earth, during a period centered around the last third international polar year (ipy, 2007-2009) and extending into succeeding years. the issues celebrating the ipy are intended to stimulate interest in the physical processes occurring at the polar regions, with the involvement of all the nations that collaborate and play key roles in these remote areas. one of the main reasons for the great interest in polar science is that almost all the early signs of observable changes at planetary scale may be recognized primarily at the earth’s poles. this is one of the aspects that characterizes the polar regions as a natural and still undisturbed laboratory; an ideal place where international communities can improve the knowledge of mechanisms that are still incompletely understood. every individual discovery contributes in some way to the final value of the research; the following papers are a representative selection of the geophysical activities conducted in the arctic and antarctic regions. the four papers span a range of scientific topics, from volcanology, and seismology to the physics of the upper atmosphere. rather than exhausting the myriad topics and sub-topics relevant to geophysics, the following papers describe methods of performing geophysical research in remote regions, and summarize the effort made in sharing technical and human resources, as well as the common desire to develop strategies for data collection, dissemination, and sharing. it is probably unnecessary to state that polar regions are very extreme areas in terms of the external conditions in which humans, machines, and instruments must operate. even a simple task, performed at a very low temperature and under very harsh environmental conditions, may require extreme effort, and special care is thus required in planning appropriate methodologies. in the following papers the reader can find descriptions of the field work performed, the logistic and scientific problems faced, the solutions found, and the principal breakthroughs made, in a sort of ideal journey into geophysical research in the earth’s polar regions. guest editor domenico di mauro istituto nazionale di geofisica e vulcanologia, rome, italy annals of 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice layout 6 1 annals of geophysics, 63, 3, se328, 2020; doi:10.4401/ag-8366 short-term seismic activity in vrancea. inter-event time distributions bogdan felix apostol*,1 and liviu cristian cune2 (1) national institute for earth physics, călugăreni street, 12 magurele, ilfov, romania (2) national institute of physics and nuclear engineering, magurele, ilfov, romania article history: received october 3, 2019; accepted november 14, 2019 abstract short-term seismic activity in vrancea is analyzed by means of the inter-event magnitude distributions (distributions of the next seismic event following a conditional seismic event). this seismic activity in vrancea is fitted by omori-type time-power laws, with possible correlations over a range of 20−25 days, for magnitudes m < 4−5. such temporal patterns can be employed in seismic hazard and risk estimation, providing the data set is statistically significant. keywords: inter-event time distributions; next-earthquake time-magnitude distributions; omori law. 1. introduction the general goal of the statistical analysis of short-term seismic activity, especially the inter-event time distributions (next-earthquake, inter-occurrence, waiting time distributions), is to reveal possible regular patterns of earthquake occurrence (see, for instance, bak et al., 2002, lindman et al., 2005, corral, 2004, 2005, and, especially, de arcangelis et al., 2016 and references therein). needless to say, such information is of relevance for seismic hazard and risk estimation. short-term earthquake forecasting models were put forward [reasenberg and jones, 1989, 1994; ogata, 1988, 1998; gerstenberger et al., 2005; omi et al., 2016, de arcangelis et al., 2018], where universal scaling laws [bak et al., 2002, de arcangelis et al., 2016] have been discussed, possibly with a limited validity [lindman et al., 2005], clustering and time-decreasing sequences governed by omori-type power laws, or stochastic point-processes [ogata, 1988, 1998]. we report herein upon similar statistical distributions for short-term seismic activity in vrancea, focusing especially on their temporal behavior over short times. a general statistical analysis of the seismic activity in vrancea has been reported previously [apostol, 2006a, 2019]. a geometric-growth model has been introduced for energy accumulation in the focal region by apostol [2006a]. making use of this model, time, energy and magnitude distributions of earthquakes have been derived, in particular the well-known magnitude distribution law 𝛽e‒𝛽𝑀. these results have been applied to a set of 1999 earthquakes with magnitude m≥3, which occurred in vrancea between 1974 and 2004. this statistical analysis provided the value 𝛽=1.89. the analysis has been updated recently to 3640 vrancea earthquakes with m≥3, corresponding to the period 1981-2018 [apostol, 2019]. 2. general considerations upon temporal distributions of earthquakes it is generally accepted that earthquakes may conveniently be divided into background earthquakes and earthquakes accompanying main seismic shocks, as aftershocks and foreshocks. the background earthquakes are characterized by a mean recurrence time, depending on their energy (magnitude). the mean recurrence time has been interpreted as a failure time in a brownian-motion model [matthews et al., 2002]. poisson-like random processes may also be employed for the temporal distributions of the background earthquakes, as for rare events. they exhibit the well-known gutenberg-richter magnitude distribution 𝛽e‒𝛽𝑀, where m denotes the magnitude, which is well documented for various types of seismic activities, periods and regions. a typical value 𝛽 =1.17 is provided for parameter 𝛽 by an isotropic point-like seismic focus model [apostol, 2006 a,b], though different values are also known, like, for instance, 𝛽 =1.89 for vrancea seismic region [apostol, 2006a]. similarly, data given by bullen [1963], indicate 𝛽 =1.38 for a large set of global earthquakes. variations in these parameters were reported, depending on the magnitude threshold [godano et al., 2014]. the value 𝛽 =1.89 corresponds to a geometric-growth model of an approximate faulting geometry for vrancea focal region [apostol, 2006 a,b]. also, tectonic-style variability for this parameter has been reported for various seismic regions [wiemer and wyss, 2002; giulia and wiemer, 2010]. background earthquakes represent a distinct seismic activity over long times. when a main shock is present, the seismic activity over short time is dominated by accompanying seismic events. their temporal distributions are power laws of omori type [omori, 1894; utsu, 1961], like 1/t, where t is time measured with respect to the main shock, both in the future, as for aftershocks, and in the past, as for foreshocks (usually with a cutoff time). it is assumed, on theoretical grounds, that the accompanying seismic activity is symmetric under time reversal with respect to the main shock [papazachos, 1978; kagan and knopoff, 1978; jones and molnar, 1979], although, in practical realizations, the past and future wings of these distributions are often unbalanced [kazemian et al., 2015]. the pair distributions used here for our next-earthquake analysis are symmetric by their definition. the assumption that the analysis of the foreshock activity may shed light upon the occurrence of the main shock is questionable [sornette et al., 1992; sornette and sammis, 1995; sornette 1998; helmstetter and sornette, 2002, 2003; felzer et al., 2004; lippiello et al., 2012a, 2017; seif et al., 2018]. omori laws may arise from a self-replication phenomenon underwent by an original, exponential generating distribution [apostol, 2006c], the aftershocks and foreshocks being distributed by a gutenberg-richter law in magnitude, as suggested previously [vere-jones, 1969]. generating exponential distributions for the accompanying seismic activity have also been used to analyze bath’s law [bath, 1965; console et al., 2003; helmstetter et al., 2004]. if the accompanying seismic activity is viewed as a relaxation process [apostol, 2006 a, b], its time distribution can be written as (1) where t is the time measured with respect to the moment of the main shock and 𝛼 is a positive-valued coefficient of relaxation. the distribution given by equation (1) is symmetric in the parameter 𝛼 and the statistical variable |t|. therefore, we may assume that the parameter 𝛼 is also a statistical variable, and we may set 𝛼 = 𝛾r, where gamma is a positive constant and r is a statistical variable. this corresponds to an etas model [epidemic-type aftershock sequence, ogata 1988, 1998; zhuang et al., 2011] with a continuous branching range, governed by r. therefore, we have a two-variable distribution p, which can be written as (2) whence, by integrating with respect to r, we get (3) 𝑑𝑝 𝑑|𝑡| ~𝑒 ‒𝛼|𝑡| 𝑑2𝑃 𝑑𝑟𝑑|𝑡| ~𝑒 ‒𝛾𝑟|𝑡| 𝑑|𝑡| 𝑑𝑝 ~∫ 𝑑𝑟 � 𝑒‒𝛾𝑟|𝑡| = 𝛾|𝑡| 1 � ₀ bogdan felix apostol et al. 2 which is omori power law. this is another derivation of omori law, which is equivalent with the derivation that makes use of the self-replication process described by apostol [2006c]. omori’s law represents only a particular pattern in the short-term seismic activity accompanying a main seismic shock, the seismic activity over a short-time scale being more complex. the scarcity of data regarding the foreshocks (i.e. the poor statistics) and the difficulty of identifying the aftershocks may render omori’s law of little practical use. in general, it is still missing a practical, reliable method of disentangling the truly accompanying seismic activity from other, background seismic activity, which may have not a direct relationship with the main shock, especially for the long tail of omori’s law (declustering problem). in addition, the shorttime seismic activity may exhibit a more complex pattern, of a multiple-branch character, indicating a superposition of omori-type distributions. the short-time seismic activity may be described by a set of independent statistical variables, like, for instance, magnitude, time, location, etc., in a stochastic point-process approach, correlations included. such short-term earthquake forecast models have been introduced in the fundamental works of ogata, 1988, 1998. a parameter-decoupling scheme was used in the short-term sequenceclustering forecast model proposed for california [gerstenberger et al., 2005; reasenberg and jones, 1989, 1994; omi et al., 2016; de arcangelis et al., 2018], which introduced a probability 𝑃(𝑀,𝑡,𝑟)~𝑒‒𝛽𝑀 � � for earthquakes characterized by magnitude m, occurrence time t, and location r, and written own this probability like a product of gutenberg-richter magnitude distribution and omori-type distributions, where tc and rc are time and spatial cutoff parameters, respectively (for an isotropic spatial distribution). such a probability law is fitted to empirical data, and employed for short-term prediction of daily rates of earthquakes. for a particular earthquake sequence the evolution parameters may be updated in real time, and employed for the next-day prediction. 3. next-earthquake correlation functions in view of the complexity of the statistical behavior of the short-term seismic activity, more general approaches may be convenient. such approaches should be of more practical relevance, without resorting too much to patterns established statistically on general sets of data. from the practical standpoint the most relevant question in short-term earthquake forecasting seems to be “what happens next?”. the general approach described herein is focused on this main question. it is based on inter-event time distributions employed for statistical analysis of short-term seismic activity. suppose that an earthquake occurs at time t0 and the next one occurs at some time t measured with respect to t0. then we may define a distribution p(t) of these next earthquakes, and determine it from a set of relevant statistical data. once determined, it may be used for estimating the time probability of occurrence of the next earthquake, based on the principle “what happened will happen again”. characteristic scale time or size, or correlation range could be identified from the statistical analysis of such functions, providing the statistical set of data is large enough, which may shed light on the statistical patterns of a seismic activity. the statistics is rather poor, in general, precisely for that range of characteristic parameters where the estimation of the seismic hazard and risk is most interesting, like, for instance, for high values of magnitude m. the inter-event time distribution approach is applied here to earthquakes occurring in vrancea, in order to illustrate its predictive capabilities and limits. temporal correlations exhibited by inter-event time distribution functions are described by gamma distributions, which imply power laws multiplied by exponentials [corral, 2004, 2005]. magnitude correlations have been demonstrated by a more elaborate model of dynamic scaling [lippiello et al., 2008, 2012b; see also stallone and marzocchi, 2019]. also, deviations from gamma distributions are reported [bottiglieri et al., 2010; lippiello et al., 2012c]. in general, the correlations involved in the short-term seismic activity are similar with those pertaining to accompanying seismic activity, which makes reasonable an omori-type law for the former distributions. 4. brief characterization of vrancea earthquakes the seismic area that is analyzed here is vrancea, one of the most active intracontinental seismic areas in europe. the earthquakes occur here within a volume with lateral extent ~ 30×70 km2 in the depth range 60-180 km. the rate of the seismic moment release is about 1.2 x 1012 nm/yr [apostol, 2006 a,b]. vrancea seismic region 1 𝑡 � +𝑡 1 (𝑟� +𝑟)² 3 analysis of inter-event distributions bogdan felix apostol et al. 4 exhibits mainly middle-depth earthquake (~80 km to ~200 km), and occasionally crustal, or surface, small earthquakes. strong earthquakes occur sometimes in vrancea, as, for instance, m=7.4, march 4, 1977, depth 94 km, or m=7.1, august 30, 1986, depth 131 km. nine earthquakes with magnitude m>7 have been recorded in vrancea in the past two centuries. although several tectonic models, spanning a large range of geodynamic scenarios, have been advanced to explain the characteristics of this undercrustal seismicity, the nature and mechanisms for earthquake generation are still subjects of debate. the statistical analysis of the associated distribution functions may provide information on the time and size patterns of earthquake occurrence if the statistical data set is large enough. a set of 1999 earthquakes with magnitude m≥3, occurred in vrancea between 1974 and 2004 has been analyzed by using magnitude distribution of the type 𝛽e‒𝛽𝑀 and exceedance rate, with 𝛽 =1.89 and a total seismicity rate 1/t0=104.21 per year, on average [apostol, 2006a]. the basic data set used for the statistical analysis reported here (magnitude step δm=0.1) includes 3640 earthquakes with (moment) magnitude m≥3 recorded between 1981 and 2018 (38 years) [romplus catalog, 2018]. the magnitude distribution of these vrancea earthquakes has been analyzed by using the gutenberg-richter law [apostol, 2019], with 𝛽≅2.1 and 1/t0≅104.92 [error ≅10%, apostol, 2019]. 5. inter-event time distributions for vrancea for the sake of simplicity we neglect here the geographical and depth distribution of the earthquakes in vrancea, because, on one hand, these earthquakes do not exhibit a large variability with respect to location coordinates, and, on the other hand, their inclusion may reduce considerably the size of the data set. therefore, the generic parameter used in the analysis of the inter-event time distributions is the magnitude m. seismic activity of the intermediatedepth source of region vrancea (romania) is analyzed here in terms of inter-event time-magnitude distributions. the data considered are seismic events with moment magnitude 3.0 ≤ m (3640 events), which occurred during about four decades (since 1981 to 2018, 38 years). for this time interval and magnitude threshold, the romplus catalog has been used [romplus catalog, 2018]. the seismic events with m≥3 are distributed in figure 1 on the inter-event time (panel a); the corresponding probabilities p(t) (in %), calculated from the frequency of the events, are shown in figure 1 panel b (time is measured in days on the abscissa). we have taken all the earthquakes with magnitude m≥3 and have read from data how many days elapsed from each such earthquake to the next earthquake with magnitude m ≥3 (a next-earthquake pair distribution). we can see that the rate of occurrence per day of the next earthquake follows a power-law time dependence similar to omori’s law, over a time window of about 25 days. the distribution is fitted with the law a/(b+t), where a=1066.45, b=1.15 and t denotes the time (coefficient of determination r=0.96; the coefficient of figure 1. inter-time distribution of seismic events in vrancea 1981 − 2018 (3640 events, m0, m ≥ 3, panel a) and probabilities p = p (t) (in %, panel b). the fitting curve in panel a is 1066.45/(1.15+t) (determination coefficient 0.96). determination is defined as 𝑅² = 1 ‒ �(𝑑𝑖 ‒ 𝑓𝑖)² / �(𝑑𝑖 ‒ 𝑑�)² where 𝑑𝑖 are the data, 𝑑� is the mean value of the data and 𝑓𝑖 are fit values). the mean time for p(t) is cca 5.89 days, and the variance is 𝜎=9.55 days. we note the presence of the characteristic time b, which indicates a deviation from the gamma distribution, as pointed out recently [lippiello et al., 2012c]. a fitting function ~ , 𝑡 > 1, 𝛼 =0.25, has been used previously by apostol et al. [2008], for vrancea, m ≥3, time interval 1974-2004 (1999 earthquakes), with similar results. in a refined fit a decreasing exponential can be included, to account for the small values of the distribution tail. the data in figure 1 have also been fitted with 𝐴𝑒‒𝛾𝑡 / 𝑡𝛼, where a≃683.3, 𝛼≃0.12, 𝛾≃0.22. all these fits are of the same high quality. to check whether the results are biased by the aftershock sequences of the strongest seisms of the investigated time period (four earthquakes with moment magnitude ≥ 6.0), two shorter time intervals have been considered: 1991-2018, avoiding the aftershock sequences of three events with m>6.0 (occurred on august 30, 1986, m=7.1, may 30, 1990, m=6.9, and may 31, 1990, m=6.4), and 2005-2018, when no earthquake larger than 5.6 moment magnitude occurred. the results show very similar forms of next-earthquake probabilities of occurrence, in all three cases (a=881.85, b=1.25, r=0.94 for 1991-2018 and a=492.6, b=1.16, r=0.93 for 2005-2018). the probabilities p(m, t) for m0≥3 for vrancea are shown in figure 2 for 3≤m<4, 4≤ m<5, 5≤m<6, and 6≤m (panels a, b, c and d, respectively). first, we note that the inter-event distributions p(m, t) for vrancea exhibit a characteristic decrease in time, with the highest probability of next-earthquake occurrence in the same day as the reference earthquake, at least for small magnitudes (m<5). then, we note the decreasing maximum values of these probabilities ∼ 22.7 for 3≤m<4, ∼ 2.75 for 4≤m<5, while the probability p(m, t) vanishes practically for m≥5. this result agrees with the magnitude correlations discussed recently for conditional probability distributions [lippiello et al., 2008, 2012b; stallone and marzocchi, 2019]. 1 𝑡𝛼 5 analysis of inter-event distributions figure 2. inter-event time distributions p = p (m, t) (in %) for vrancea 1981-2018 for m0 ≥ 3 and 3 ≤ m < 4 (panel a), 4 ≤ m < 5 (panel b), 5 ≤ m < 6 (panel c) and 6 ≤ m (panel d). bogdan felix apostol et al. 6 figure 4. same as in fig. 3 for 4 ≤ m0 < 5. figure 3. the inter-event time (conditional) probability p = p (m, t | m0) (in %, vrancea) for 3 ≤ m0 < 4 and 3 ≤ m < 4 (panel a), 4 ≤ m < 5 (panel b), 5 ≤ m < 6 (panel c) and 6 ≤ m (panel d). also, it is worth noting that p(t) and p(3≤m<4, t) are similar, obeying, omori-type power laws, at least for short times, while the distributions become gradually irregular, exhibiting large fluctuations on increasing magnitude above m=4-5. the statistics becomes poor for higher magnitude (m>5), as expected. a correlation time of 20-25 days can be estimated, after which the probabilities decrease appreciably (below 1), as well as a size correlation of cca m=4-5, above which the distributions acquire very small values, and are very irregular. the conditional probabilities including the magnitude m0 of the former earthquake p(m, t | m0) are shown in figures 3-5 for 3 ≤ m0 < 4 (figure 3), 4≤m0<5 (figure 4) and 5≤m0<6 (figure 5). the first observation is that distributing the time-magnitude events with respect to the former earthquake magnitude m0 does not change practically the characteristic time-decreasing behavior of the next-earthquake activity for small magnitudes. it can be seen in figure 2 (panel a) and figure 3 that p(m, t) and p(m, t ⃒ 3≤ m0 <4) are very similar, while considerable differences appear for p(m, t ⃒ 4≤ m0 <5), even for small magnitudes 3≤m<4. this reflects again the size correlation m=4−5, and indicate large fluctuations for higher-magnitude distributions. higher-order correlation functions (as well as higher-magnitude analysis, or enlarging the magnitude gap δm = 1) reduce considerably the statistical set, thus exhibiting a poor confidence. 6. conclusions the inter-event time distributions are analyzed here for short-term seismic activity in vrancea. the main distribution function is the time-magnitude probability p(m, t ⃒ m0) of the next earthquake of magnitude m occurring at time t elapsed from the occurrence of the former with magnitude m0 (inter-event time distribution). it may be viewed as a two-point correlation function, or a conditional probability. this distribution is obtained by analyzing 3640 earthquakes with (moment) magnitude m ≥ 3 recorded in vrancea over the last 38 years [since 1981 to 2018, romplus catalog, 2018]. the results show a characteristic time-decreasing behavior of the 7 analysis of inter-event distributions figure 5. same as in figs. 3 and 4 for 5 ≤ m0 < 6. occurrence of the next earthquake, at least for small magnitudes. correlations are identified for lowermagnitude earthquakes (m, m0<4−5), extending roughly over 20-25 days. statistics becomes poor for stronger earthquakes (m>5), preventing any confident estimation. the results of the present report show that the interevent time distributions can be employed for estimating the short-term seismic hazard and risk for moderate earthquakes (m<5) in vrancea. for instance, from figure 2 we read the probability 0.047 for occurring an earthquake with magnitude between 4 and 5 the same to the next day after one with magnitude m ≥3 has occurred. in general, the approach can be useful for those seismic activities, periods and regions of interest, characterized by rich statistics (as, for instance, crustal, or surface, moderate earthquakes). data and sharing resources: http://www1.infp.ro/realtime-archive acknowledgements the authors are indebted to dr. luminita angela ardeleanu for enlightening discussions and to the anonymous referees who made valuable comments and suggestions. this work was partially carried out within the program nucleu 2019, funded by romanian ministry of research and innovation, project number pn19080102. references apostol, b. f., (2006)a. a model of seismic focus and related statistical distributions of earthquakes. rom. reps. phys. 58, 583. apostol, b. f., (2006)b. a model of seismic focus and related statistical distributions of earthquakes, phys. lett., a357, 462-466. apostol, b. f., (2006)c. euler’s transform and a generalized omori’s law, phys. lett., a351, 3, 175-176. apostol, b. f., (2019). statistical seismology. niep internal report, apoma mg, magurele. apostol, b. f., l. c. cune, and m. apostol (2008). scaling and universal power laws in time series of seismic events. roum. j. phys. 53, 3-4, 593-600. bak, p., k. christensen, l. danon, and t. scanlon (2002). unified scaling law for earthquake, phys. rev. lett., 88, 17, 178501. bath, m. 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(1969). a note on the statistical interpretation of båth’s law, bull. seismol. soc. amer., 59, 4, 1535-1541. 9 analysis of inter-event distributions wiemer, s. and m. wyss (2002). mapping spatial variability of the frequency-magnitude distribution of earthquakes, adv. geophys. 45, 259-302. zhuang, j., m. j. werner, s. hainzl, d. harte and s. zhou (2011). basic models of seismicity: spatiotemporal models. community online resource for statistical seismicity analysis, doi:10.5078/corssa-07487583 (http://www.corssa.org). *corresponding author: bogdan felix a po s to l, national institute for earth physics, călugăreni street, 12 magurele, ilfov, romania e-mail: apostol@infp.ro © 2020 the istituto nazionale di geofisica e vulcanologia. all rights reserved bogdan felix apostol et al. 10 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice layout 6 special issue: memovolc preface uncertainty quantification in lava flow hazard modeling and real-time source term provision the idea of this special issue comes out of discussions at the workshop on “uncertainty quantification in lava flow hazard modeling and real-time source term provision” held at the istituto nazionale di geofisica e vulcanologia (ingv) in catania (italy) during february 2017. the workshop was supported by the memovolc network of the european science foundation, and was attended by delegates from italy, france, uk, germany, spain, and the usa. at the workshop, researchers expert in lava’s physical properties, satellite volcano hot spot detection, lava flow modeling, and effusive crisis response were brought together to share experiences and perspectives on the current capabilities in application of remote sensing and modeling to better forecast hazards during effusive eruptions. in addition, the workshop led to a community-wide agreement that many scientific uncertainties still need to be resolved in model-based hazard assessments, and this requires support through the creation of a special working group. this special issue consists of 15 papers focused largely on the conclusions of the memovolc workshop, as follows. giordano presents a review of recent advances in the field of magma rheology. the results and approaches focus on multiphase (i.e. melt+bubbles+crystals) rheology of natural systems, and are applicable to the emplacement of lavas, dykes, and sills, as well as to magma eruption dynamics during explosive eruptions. calvari gives an overview of diverse lava flow surfaces, morphologies and structures in a framework of their associated eruptive parameters. this is done to guide preliminary, but prompt, hazard evaluations that can be applied during the initial phases of effusive volcanic crises at basaltic volcanoes. kolzenburg et al. discuss new data on the rheological evolution of a crystalizing, high-mg, basalt from piton de la fournaise volcano. the data presented serve to expand the existing modest based on experimental databases that consider the non-equilibrium rheology of lavas, and are intended as a step towards better understanding underlying rheological processes and their link to flow dynamics. pick et al. investigate limits of the dual-band method when used to derive estimates of a lava flow size and temperature when represented as a sub-pixel feature in infrared satellite images. the satellite measurement situation is simulated through an indoor analog experiment, using a steady heat source, which is observed by thermal cameras measuring at the wavebands commonly used by satellite sensors. ganci et al. constrain the lava volume emitted at mt etna during the 17-25 may 2016 eruption using pre-eruptive and post-eruptive digital elevation models obtained from satellite images acquired by the pléiades constellation, which provides very-high spatial resolution images in tri-stereo mode. the total volume estimated from dem differencing bounds that derived from thermal data obtained from other satellite-based sensors. coppola et al. use the mirova system to measure heat radiated by the growing lava field during the 2014-2015 eruption at holuhraun (iceland). the results suggest that the mir-derived radiant flux essentially mimics the overall trend of lava discharge rates, with only a annals of geophysics, 62, 2, vo232, 2019; doi: 10.4401/ag-8370 minor influence due to the emplacement style and evolving eruptive conditions. ramsey et al. perform a detailed flowgo-based modeling study of lava flows emplaced at tolbachik volcano during the 2012-2013 and the 1975-1976 eruptions. the results show that incorporating a two-component emissivity model linked to the fraction of molten lava and cooled crust is an important factor for model accuracy and is especially critical for large, high effusion rate flows. filippucci et al. numerically model cooling and dynamics of lava flowing in a rectangular channel driven by gravity. a far field thermal boundary condition allows the assumption of constant temperature or constant heat flow as boundary conditions to be overcome; providing more realistic results. zago et al. evaluate the accuracy and robustness of the gpusph particle engine, based on the smoothed particle hydrodynamics (sph), method. the results of benchmark tests of growing complexity are reported in terms of correctness and performance, highlighting the benefits and drawbacks derivred from the use of sph to simulate lava flows. lev et al. provide a primer on modern analog lava flow experiments in the laboratory. the authors discuss several open questions about lava flow emplacement and the ways in which future improvements in experimental methods, such utilization of three-phase suspensions and materials with complex rheologies and imaging of flow interiors. tsepelev et al. present two-dimensional numerical models of steady-state lava flows to analyze the development of the lava crust depending on the heat flux into the atmosphere from the lava surface, lava viscosity, discharge rate, and lava front propagation rate. cappello et al. demonstrate the potential of the integrated use of satellite remote sensing techniques and lava flow modelling during the 2017 eruptive activity of mt etna. this combined approach provides insights into lava flow field evolution by supplying detailed views of flow field construction that were useful for more accurate and reliable forecasts of eruptive activity. tarquini et al. test the new mrlavaloba probabilistic code for the simulation of lava flows during the emplacement of the holuhraun 2014-2015 lava flow (iceland). the simulations highlight that small-scale morphological features in the pre-emplacement topography can strongly influence the estimates of the area likely to be inundated and the thickness of the final lava flow field. harris at al. focus on satellite-based surveillance of volcanic hot spots coupled with modeling to allow ensemble-based approaches to crisis response. the authors complete benchmark tests on an effusive crisis response protocol aimed at delivering product for use in tracking lava flows during the april–may 2018 eruption of piton de la fournaise. hajian et al. present two machine learning approaches to classify volcanic activity using multivariate geophysical data, namely the decision tree and k-nearest neighbour methods. the models are implemented using a data set recorded at mount etna (italy), over the period 1 january 2011 – 31 december 2015; a period which encompassed lava fountain events and intense strombolian activity. the editors are grateful to fabio arzilli, giuseppe bilotta, costanza bonadonna, arturo buscarino, annalisa cappello, oryaelle chevrel, antonio costa, mattia de' michieli vitturi, alicia felpeto, gaetana ganci, mike james, james kauahikaua, einat lev, michele mangiameli, francesco marchese, marco neri, matthew patrick, simon plank, cristina proietti, nikola rogic, scott rowland, elie saikali, ricardo sanchez, arianna soldati, peter styles, andrea tallarico, alessandro vona, and klemen zakšek for their careful reviews of the submitted manuscripts. ciro del negro andrew harris sonia calvari ingv | sezione di catania lmv – université clermont auvergne articles (final order) 1) advances in the rheology of natural multiphase silicate melts: import for magma transport and lava flow emplacement daniele giordano 2) understanding lava flow morphologies and structures for hazard assessment sonia calvari 3) equilibrium viscosity and disequilibrium rheology of a high magnesium basalt from piton de la fournaise volcano, la reunion, indian ocean, france stephan kolzenburg, daniele giordano, andrea di muro, donald b. dingwell 4) assessment of dual-band method with indoor analog experiment leonie pick, valerio lombardo, klemen zakšek 5) 3d lava flow mapping of the 17–25 may 2016 etna eruption using tri-stereo optical satellite data gaetana ganci, annalisa cappello, vito zago, giuseppe bilotta, alexis herault, ciro del negro 6) monitoring the time-averaged discharge rates, volumes and emplacement style of large lava flows by using mirova system: the case of the 2014-2015 eruption at holuhraun (iceland) diego coppola, sara barsotti, corrado cigolini, marco laiolo, melissa anne pfeffer, maurizio ripepe 7) the influence of emissivity on the thermo-rheological modeling of the channelized lava flows at tolbachik volcano michael s. ramsey, magdalena oryaelle chevrel, diego coppola, andrew harris 8) far-field boundary conditions in channeled lava flow with viscous dissipation marilena filippucci, andrea tallarico, michele dragoni 9) preliminary validation of lava benchmark tests on the gpusph particle engine vito zago, giuseppe bilotta, annalisa cappello, robert anthony dalrymple, luigi fortuna, gaetana ganci, alexis hérault, ciro del negro 10) analog experiments of lava flow emplacement einat lev, elise rumpf, hanna dietterich 11) crust development inferred from numerical models of lava flow and its surface thermal measurements igor tsepelev, alik ismail-zadeh, yulia starodubtseva, alexander korotkii, oleg melnik 12) satellite-driven modeling approach for monitoring lava flow hazards during the 2017 etna eruption annalisa cappello, gaetana ganci, giuseppe bilotta, alexis herault, vito zago, ciro del negro 13) modeling lava flow propagation over a flat landscape by using mrlavaloba: the case of the 2014–2015 eruption at holuhraun, iceland simone tarquini, mattia de' michieli vitturi, esther jensen, gro pedersen, sara barsotti, diego coppola, melissa anne pfeffer 14) validation of an integrated satellite-data-driven response to an effusive crisis: the april–may 2018 eruption of piton de la fournaise andrew j.l. harris, magdalena oryaëlle chevrel, diego coppola, michael s. ramsey, alexis hrysiewicz, simon thivet, nicolas villeneuve, massimiliano favalli, aline peltier, philippe kowalski, andrea di muro, jean luc froger, lucia gurioli 15) classification of mt etna (italy) volcanic activity by machine learning approaches alireza hajian, flavio cannavò, filippo greco, giuseppe nunnari eruption products of the 1883 eruption of krakatau and their final settlement annals of geophysics, 58, 2, 2015, s0220; doi:10.4401/ag-6529 s0220 eruption products of the 1883 eruption of krakatau and their final settlement izumi yokoyama the japan academy, ueno park, tokyo, japan abstract firstly the volume of pyroclastic ejecta during the 1883 eruption of krakatau is re-examined. to revise the volume of flow deposits, the author basically follows verbeek’s observation while to estimate the fall deposits, as the last resort, the author assumes that volume ratios fall/flow are common to similar caldera eruptions, and the ratios determined by the calderaforming eruptions of novarupta and pinatubo are applied to the krakatau eruption. verbeek’s estimation of the total volume of ejecta, 12 km3 is revised to 19 km3. this is significantly different from the volume of disrupted volcano edifice, 8 km3. such a result does not support the predecessors’ hypothesis that calderas are formed by collapses of volcano edifices into magma reservoirs in replacement of the total ejecta. through the discussion on the volume estimation of volcanic ejecta on and around krakatau, the author recognizes that such estimation should be originally very difficult to attain enough accuracy. much importance of “caldera deposits” to post-eruption settlements of the ejecta is emphasized. in relation to caldera formation, mechanical stability of a cavity in the crust is discussed. lastly, upon the basis of subsurface structure, especially caldera deposits, a structural image of krakatau caldera is presented. 1. introduction we have experienced caldera-forming eruptions four times since the 19th century, tambora, sumbawa in 1815, krakatau, sunda straits in 1883, novarupta, alaska in 1912, and pinatubo, luzon in 1991. the tambora eruption was exceptionally violent but not purposefully observed. sigurdsson and carey [1989] carried out field studies there and clarified that the eruption was a complex series of events for about 10 days developing its explosive phases. this eruption was rather exceptional in its sequences while the other three were observed more or less at scientific levels of those days and have been well studied until now. in the present paper, the 1883 eruption of krakatau shall be discussed partly in the light of the last two caldera-forming eruptions although there exist many calderas formed in geological ages. the 1883 krakatau eruption has been studied by several distinguished researchers of the 20th century. some of them have kept a conception that calderas were formed by collapses of volcano edifices into magma reservoirs in replacement of erupted magmatic material by reason of the paucity of lithic material around the calderas, and this conception presupposes equality of volume between discharged magmatic ejecta and disrupted volcano edifices. in order to verify the equality, we have to estimate the volume with reasonably high accuracy not in an order of magnitude: generally such estimation is impossible even now. the difficulties in studies of the 1883 eruption of krakatau are as follows: (a) the eruption occurred in the midst of the sea and was accompanied with the dark of huge explosion clouds, and under no instrumental observations except a few accidental ones: a tide gauge and a pressure meter at batavia located at about 160 km distance from the volcano. consequently the sequence of this eruption or caldera formation has been not fully clarified. (b) at that time, surveys of pyroclastic ejecta under the sea had been inexperienced. around krakatau islands, the new eruption deposits suffered from effects of the sunda current: it was important to make a survey as soon as possible after the eruption to minimize post-depositional erosions. verbeek [1885] may have preferred an immediate survey to an accurate one. at present, an overall estimation of pyroclastic deposits in the sea area produced by the 1883 eruption is no more possible because of post-depositional erosions for more than 100 years, to say nothing of the land deposits. thus, only verbeek’s observational data are uniquely available but have not been re-examined. we should recognize utility of the past estimation and promote the discussion on the 1883 eruption. the topographic changes of krakatau and its surarticle history received march 19, 2014; accepted april 8, 2015. subject classification: krakatau caldera, volume of the ejecta, pyroclastic-fall and -flow, caldera deposit, magma reservoir. rounding islands, surveyed immediately after the 1883 eruption by judd [1888, fig. 11] are shown in figure 1. these changes were caused by deposition of volcanic ejecta and explosive destructions. according to judd, many of the topographic discrepancies at the southern limit of krakatau were due to the imperfection of the old survey. this suggests difficulty in re-occupying the survey spots on the sea and in volume estimation of the submarine ejecta by using such charts. after verbeek [1885], several authors have repeated volume-estimation of pyroclastic ejecta from the 1883 eruption of krakatau by their own manners. their results shall be summarized in section 3.3. about one month after the eruption, verbeek started surveys of the volcanic ejecta in the sea by the techniques of those days. when we take the effects of post-depositional erosions in the sunda straits into consideration, we should appreciate the merit of verbeek’s early survey though his method was not necessarily complete. 2. volume estimation of pyroclastic-flow deposits of the 1883 eruption 2.1. verbeek’s volume estimation of pyroclastic ejecta verbeek [1885] did not explicitly distinguish fall deposits from flow deposits. to estimate the total amount of ejecta around krakatau islands, he divided the area into two districts at and around the newly formed caldera; one is a circular area of 15 km in radius (figure 2a) and the other is a wider area surrounding the former and extending toward the n and the sw to the maximum distance of about 1300 km (figure 2b). in the former, pieces i ~ iv, and rings a ~ c, he compared the two bathymetric charts surveyed before and after the eruption, and in the latter, rings d, e, f, and h, yokoyama 2 figure 1. chart of krakatau and the surrounding islands, from the chart surveyed immediately after the eruption [from judd 1888, fig. 11]. the shaded areas show the form of the islands drawn in the old chart. dotted lines show sandbanks and lines of breakers. “danan” is added by the present author. figure 2. division maps of verbeek’s survey of ejecta in and around the sunda straits. (a) survey divisions of the inner area of krakatau islands: a simplified map of verbeek’s figure [1885, fig. 4]. pieces i ~ iv are within a circle of 15 km radius around krakatau islands, and rings b, c and d successively continue to ring a. ⓢ and ⓒ denote steers and calmeyer reefs, respectively. a hatched area is the caldera depression; verbeek’s method is not applicable there. (b) survey divisions in the outer area of krakatau islands. the innermost circle (r = 50 km) includes rings a, b, c and pieces i ~ iv shown in figure 2a. (a) (b) 3 and rectangle g, triangles k and l, he roughly presumed thicknesses of the new deposits on the bases of the reports from the inhabitants and navigating ships in these areas. he actually surveyed the depth changes within radial distance of 15 km from the center of the caldera (pieces i ~ iv). on the other hand, he did not survey in the area outside of piece iv, but assigned presumed thickness for each division as shown in table 1. in the table, the thickness of deposits discontinuously decreases (0.2 to 0.02 m) between divisions c and d (50 km from the center). under such prearranged condition, the present author assumes that the deposits within a circular area of 50 km in radius (or pieces i ~ iv and rings a ~ c) are mainly pyroclastic-flow deposits including a small quantity of pyroclastic-falls while in the outer area, almost all the deposits are pyroclastic-falls. such distinction is one of the last resorts as far as we follow verbeek’s estimations, but may be not much unreasonable. in the following, verbeek’s results on pyroclastic-flow obtained only in a circular area of 15 km in radius (pieces i ~ iv) shall be re-examined, and the fall deposits in other rings (a ~ c) remain the same as verbeek’s estimation. and the deposits in divisions (d ~ l) are regarded as pyroclastic-falls, but the total volume of the fall deposits shall be re-estimated by analogical method with the other caldera-forming eruptions on land (novarupta and pinatubo) in section 3.2. 2.2. revision of verbeek’s method to estimate pyroclastic-flow deposits verbeek [1885] estimated volume of the ejecta by comparing the depths of the two charts of this area; namely, the blommendal chart of 1874 and the hydrograaf chart after the 1883 eruption as shown in figures 3a and 3b, where the isobaths are added every 25 m by the present author. verbeek presented the sea depths before and after the eruption together on a sheet as shown in verbeek’s atlas [1885, fig. 4]. the present aukrakatau eruption products division a b c d e f g h k l thickness (in meter) 1 0.3 0.2 0.02 0.05 0.01 0.001 0.005 0.005 0.005 table 1. thickness of pyroclasic deposits in division a ~ l shown in figures 2a and 2b after verbeek [1885, pp. 133 and 134]. figure 3. british admiralty charts surveyed before and after the 1883 eruption of krakatau. in both the figures, the isobaths are added every 25 m by the present author. (a) chart surveyed in 1854. spot depths are in fathom (1 fathom = 1.83 m). ⓢ and ⓒ denote the locations of steers and calmeyer reefs, respectively. p perbuwatan, d danan, r rakata. (b) chart surveyed in 1886. the central region collapsed to a depth of 250 m at the maximum. (a) (b) thor does not utilize this figure but compare both the depths in meshes of 1 km square as shown in figure 4. 2.2.1. plumbings in the 19th century verbeek [1885] measured thicknesses of the marine deposits of the krakatau eruption by comparing the two bathymetric charts around krakatau islands, surveyed before and after the eruption; he relied on bathymetries by british and dutch navies. originally he had to directly measure thicknesses of the deposits, but such measurements require time, and hence he may have wished to obtain the result as early as possible to reduce the effects of post-depositional erosion by the current of the straits. this method is valid in case of thick deposits, several meters thick at least. for successful comparisons of two bathymetric charts surveyed at different periods, accurate re-locations of previous measuring points should be very desirable. in 1880s, re-locations were rather doubtful, especially in remote areas. we should take appropriate errors into consideration when we treat with verbeek’s results. the above comparison of bathymetric charts is not applicable to the area where the seabed was destructed by the eruption. it is concluded that his results contain some errors due to incomplete methods and inaccurate charts, particularly one surveyed before the eruption. verbeek [1885] did not describe any details of his bathymetries. here, bathymettic methods in the 19th century shall be reviewed: echo-soundings were first adopted after 1920. in the 19th century, soundings at shallow sea were made usually by using a cable with a plumb of heavier than 7 pounds (3.2 kg). the accuracy of plumbings depends on verticality of plumb cables that is disturbed by the current thereabouts. around krakatau a few weeks after its eruption, the plumb of 3.2 kg weight may have penetrated the upper part of ejecta deposits, approximately 0.5 m in depth. by this method, it may have been rather difficult to measure the thickness of the deposits with high accuracy. at present, we can measure it directly by scuba core-samplings as used by mandeville et al. [1996] or by seismic profilers and side-scan sonar as used by camus et al. [1991]. in those days, ships usually determined their positions by sextants, and its accuracy depended on visibility of landmarks around the ships. as shown in figure 1, all krakatau islands drastically changed their shapes missing many remarkable topographies those were previously favourable targets of positioning. in sum, the overall errors in the comparison of the two surveys around krakatau islands consisted of those of soundings and those of positioning, and may have reached possibly more than a few meters. here, we will follow verbeek’s method to re-estimate the volume of the 1883 ejecta, i.e. comparisons of the two charts, before and after the eruption. his data were published in his book of 1885, and fortunately we can effectively refer various data of the 1883 eruption and the related problems to a book compiled by simkin and fiske [1983]. 2.3. revised volume of the pyroclastic-flow deposits verbeek may have obtained the differences between the two soundings on the charts. in figure 2a, an area hatched by verbeek [1885] was depressed by the eruption. this is the most important part in our discussion, but the resurveys could not be realized because the bottom topographies changed by the explosions. he grouped the central area of deposits into four pieces (i ~ iv), assigning thicknesses of the deposits for each piece as shown in table 2. in the present paper, to examine verbeek’s estimations, the author follows his method in principle and use similar two charts surveyed in 1854 and 1886, before and after the 1883 eruption, by british admiralty and referred by wharton [1888, plts. xxxii and xxxiii]. the charts are reproduced in figures 3a and 3b where the depths are in fathom (1 fathom = 1.83 m) and the isobaths are added every 25 m by the present author. strictly speaking, the two charts are not the same as verbeek’s ones. on this point, judd [1888] yokoyama 4 figure 4. distribution of thickness of volcanic ejecta, newly deduced from figures 3a and 3b. dotted lines show sandbanks and lines of breakers. the broken line in the central part contours an isobath of 150 m after the 1883 eruption. a circle of 15 km radius corresponds to that of verbeek’s figure (figure 2a). a mesh is 1 km × 1 km. ○—: < 0 (thin deposits); no numerals: 0~5 m (deposits of a few meter thick); 1: 6~15 m, roughly around 10 m thick; 2: 16~25 m, roughly around 20 m thick 20 m; 3: 26~35 m, roughly around 30 m thick; 4: > 35 m. 5 stated that the english chart was surveyed in 1854 and the dutch one in 1874, but surveying officers of both the navies made additions and corrections to both the charts from time to time. therefore, these two charts should be actually the same as verbeek’s ones. the old chart has less numbers of plumbings and the depths in the two charts (figures 3a and 3b) do not always correspond with each other. several depths in the old chart were omitted from the new chart probably because of their uncertainty. it is difficult to draw the isobaths automatically. to improve scatterings and to get smooth distribution of the sea depths, the author newly contoured isobaths every 25 m (about 14 fathoms) by interpolations. of course, these isobaths have some ambiguity, and drawings of every 10 m contours are impossible because of paucity and ambiguity of the measurements. in order to estimate the depth changes objectively, both the charts (figures 3a and 3b) are divided into meshes of 1 km square, and the median value in each mesh is determined as the average depth, and then the corresponding depths of the two charts are compared in each mesh. thus, the changes in depth of the sea bottom due to the new deposits or the bottom displacements are represented in figure 4 where mark ○— indicates a change smaller than ±5 m, numbers 1, 2, 3, and 4 indicate changes of around 10, 20, 30, and 40 m, respectively. as mentioned above, around krakatau islands, the pyroclastic-flow deposits are distinguished from the pyroclastic-fall deposits at a boundary of 50 km radius from the center of the explosion. the flow-area is divided into 4 pieces i ~ iv and 3 rings a, b and c as shown in figure 2a. verbeek’s estimations at each division are revised as follows: pieces i ~ iv: the volume of deposits in the 4 pieces is estimated on the base of figure 4 and the results are tabulated in table 2; piece ii is not uniform in thickness of the deposits, and divided into two by thickness. piece iv includes the depression of the caldera and a triangular part (hatched part in figure 2a), of which area is about 38 km2. we cannot apply verbeek’s method to this depression. nevertheless verbeek [1885] assigned 6 m as the thickness of the ejecta in all piece iv. he did not pay any special attention to the depression that, in the present writer’s opinion, should be the most important part of the krakatau area after the eruption. we cannot exclude the possibility that many lithic fragments deposit there. its bottom of 200 m deep covers an area of about 11.8 km2 in figure 5. in table 2, verbeek divided piece iv into three parts, all 6 m thick deposits. the area of the caldera depression (hatched area in figure 2a) is 38 km2 and here the author assumes thickness of the caldera deposits to be 50 m. this is not so overestimated considering that the maximum thickness of the deposits is 40 m in figure 4. thus verbeek’s estimates are revised and modified as shown in the last column of table 2. the revised volume of pieces i ~ iv is converted into dense rock equivalent (dre) as 10.8 km3 × 2.1/2.6 = 8.7 km3 dre. in the table, piece iv covers the caldera where there should be “caldera deposits” composed of fallbacks, i.e. juvenile ejecta and lithic fragments: it is imkrakatau eruption products unit surface (km2) thickness (m) volume (km3) piece i 54 20 1.08 piece ii 45 198 30→ 20 30→ 10 0.90 1.98 piece iii 36 40→ 35 1.26 piece iv 30 38 (caldera area) 306 6→ 20 6→ 50 6→ 10 0.60 1.9 3.06 total σ 707 10.8 ± 3 (bulk) 8.7 (dre) ring a (r =15~22.5 km) 883 1 0.883 ring b (r =22.5~40 km) 3434 0.3 1.030 ring c (r = 40~50 km ) 2826 0.2 0.565 total σ 7143 2.48 ± 0.5 (bulk) 2.0 (dre) grand total σς 7850 10.7 (dre) table 2. revised values of verbeek’s estimation in an area of 50 km in radius from the centre of the explosion. the deposits are mostly pyroclastic flows. underlined values are revised ones. “caldera area” in piece iv is the same as a hatched area in figure 2a. possible for us to know their partitions at present. in the future, we shall get knowledge of the deposits by drillings at the spot. the caldera deposits shall be referred to in section 4.1. 2.3.1. errors of verbeek’s volume estimation pieces i ~ iii: verbeek assigned thickness of the ejecta to 20, 30 and 40 m for the three pieces, respectively. in other words, he measured these depths with a unit of 10 m and the error is ± 5 m. thus, the error in estimation of the volume should be: (area of pieces i ~ iii) × (error in thickness) = (error in volume): (54+243+36) km2 × (± 5 m) = ± 1.7 km3 ≈± 2 km3. hence the total volume of pieces i ~ iii is expressed as (5.2 km3 ± 2 km3) in bulk. this error originates from only errors in measurement of the thickness, and varies with the locality. 2.3.2. comments on the pyroclastic ejecta in the innermost region (a) the central area among the three islands was depressed remarkably reaching deeper than 200 m b.s.l. to this caldera area, verbeek’s method is not applicable because the old depths were lost. nevertheless, verbeek [1885] assigned thickness 6 m to the all areas of piece iv including the collapsed part, or the caldera depression in figure 2a. the present author cannot understand the grounds for estimation of 6 m thick, and anew assigns thickness 50 m to the central area considering the maximum thickness of the deposits shown in figure 4, as mentioned above. verbeek did not take thought of caldera deposits. (b) danan has been assumed to be the main vent of the caldera forming explosions by some authors. however, the center of the existing depression is more probable as the explosion center because danan was located outside the depression in figure 6, and erupted as a parasite of pre-krakatau volcano [yokoyama 2014] before and during the 1883 climactic explosions. (c) a large amount of ejecta was deposited roughly 40 m thick at the maximum off the nw coast of verlaten (sertung) island in figures 1 and 4. stehn [1929] actually investigated the same deposition on the island. this means that the pyroclastic flows deposited on these areas after the large tsunami passed over there. (d) scarcely the sea depths changed at the s of rakata island within the limits of surveying errors. this means that the large explosions were directed mainly northand northwestward. (e) reefs of steers and calmeyer: before the 1883 eruption, the two reefs were below the sea level as shown in figure 3a, and after the eruption, they were observed above the sea between lang and sebesi islands as in figures 1, 3b and 4. these are located in piece ii as marked by letters and in figures 2a&c&s yokoyama 6 figure 5. a part of indonesian chart around krakatau islands surveyed in 1970 by kepala dinas hidro-oseanografi. spot depths are in meter. the isobaths of 200 and 250 m in the central part are drawn by the present author. sertung and rakata kcl are another names of verlaten and lang, respectively. ⓢ and ⓒ near the crossing of the longitude and latitude lines denote steers and calmeyer reefs, respectively. a anak krakatau, b bootsmans rots. figure 6. topographic and bathymetric chart on and around krakatau: a part of indonesian chart around krakatau islands surveyed in 1940. the isobaths are drawn every 50 m and the contour intervals every 20 m. ak anak krakatau, r rakata, d danan, p perbuwatan. a circle of broken line shows the assumed caldera rim. 7 and 5. the changes in depth around these reefs were not so much, or less than 25 m. originally these reefs were below the sea level because, on a chart (scale 1/500 000, 1970) shown in figure 5, steers and calmeyer are 4.3 m and 6 m below the sea level, respectively. according to judd [1888], the ejecta deposited temporarily on both the reefs during the eruption, had been completely washed away in a few months by the action of the sea waves. in atlas of verbeek [1885, fig. 4] or simkin and fiske [1983, p. 180, plt. 4], steers stood 3 m above sea level and calmeyer did 6.5 m. then, the ejecta are supposed to have deposited during the eruption on these reefs, 7.3 m and 12.5 m thick, respectively. these thicknesses are agreeable to those given in figure 4. this means that the large tsunami on august 27 traversed this area before the pyroclastic ejecta deposited on these reefs. in other words, first the gigantic explosions occurred causing the large tsunami and strong airwaves, and later the pyroclastic-flows deposited on and around these reefs. also this suggests importance of post-depositional erosions in the present discussion. rings a, b and c (figure 2a): we unwillingly cannot re-examine the data because verbeek [1885] left the unique thickness of the deposits of 1, 0.3 and 0.2 m, respectively. these thickness were not actually measured but probably may have been reported by the inhabitants on the islands. eventually, adding the volume of rings a, b and c (2.0 km3 dre) to the volume of pieces i ~ iv, the revised volume of the pyroclastic-flow deposits amounts to 10.7 km3 dre as shown in table 2. in the revised results, it is noticeable that about 80 % of the flow-deposits remain within 15 km distance from the center of the eruptions. 2.3.3. supplements to verbeek’s estimation of the flow deposits (a) volume of disrupted parts of rakata island: volume of disrupted volcanic edifices and sea bottom by 1883 eruption was not estimated by verbeek, but is also important to draw a balance sheet between the volume of volcanic ejecta and disrupted volcanic edifices. topographic maps of rakata island before the 1883 eruption were poor, and there were the three peaks on the island: rakata (809 m), danan (449 m) and perbuwatan (120 m). by the eruption, the northern 2 ⁄ 3 of the island disappeared and rakata decreased its height to 803 m. the volume of disrupted parts of rakata island above the sea level is calculated on the old and new topographic maps (figures 3a and 6) as about 2.9 km3. (b) volume of topographic depression beneath the sea level: the 1883 eruption formed a depression between rakata and verlaten islands in addition to disruption of 2/3 rakata island, and almost simultaneously these depressions were filled with mixture of the ejecta and sea water during the volcanic paroxysms, and after the 1883 eruption, a depression of funnel shape with maximum depth 250 m was left among the three islands (figure 5). this is krakatau caldera. the topographic depression under the sea level as of 1940 is shown in figure 6 where probable caldera boundary is shown with a circle of broken line. intensive explosions of danan volcano may have caused eastward extension of the central depression. volume of the topographic depression is estimated at 5.1 km3 on the charts of figures 3a and 6. this is “caldera depression”. in the case of krakatau, the volume of disrupted parts of rakata island above sea level (about 2.9 km3) is not included in the count. (c) deposits on the three islands in the central area: verbeek [1885] did not pay particular attention to volume of the ejecta on the three islands. he assigned thickness of the ejecta on the three islands extrapolating the depth changes measured at the adjacent sea in figure 2a where the average thickness of pieces ii, iii and iv covering the three islands is about 20 m in table 2. considering that thickness of the ejecta on the three islands is about 20 m on the average based on stehn’s report [1929], we may assume the volume of deposits on the three islands as about 0.7 km3 dre. this value was eventually included in verbeek’s calculation. 3. volume of pyroclastic-fall deposits of the 1883 eruption 3.1. verbeek’s estimation of pyroclastic-fall deposits originally volume estimation of fall deposits from volcanoes is problematic, especially at volcanoes in the midst of the sea. in the discussion of the 1883 eruption of krakatau, we expect a large amount of pyroclastic-falls considering the anomalous “bigness” of the eruption [simkin and fiske 1983, p. 23] such as (1) a large volume of flow deposit, (2) energy coupled into the airwaves and tsunami, (3) the maximum eruptive column height: 50 km, (4) long duration of climatic eruption: about 23 hours. as already discussed in section 2.1, according to verbeek’s data given in table 1, the ejecta in the area from ring d to triangle l, covering the sea and lands of 5 × 105 km2 in area (figure 2b), are deemed as pyroclastic-falls. this area is the outside of 50 km radius from the center of the caldera. he assigned 0.001~0.02 m to thickness of the deposits at each division (table 1) as far as 1400 km distance, and obtained the total amount of the ejecta in this area as 3.6 km3 bulk (table 4). this is extremely rough estimation in comparison with that of flow deposits. his data based on the reports from the inhabitants and navigating ships at scattered and distant spots around the volcano. krakatau eruption products to convert the bulk volume to dre volume, we need density of fall deposits. sarna-woicicki et al. [1981, fig. 339] obtained density distribution of the may 18, 1980, fall deposits from mt. st. helens along axis of downwind lobe: the density of air-fall deposits varies between 0.5~0.1×103 kg/m3 with the distance from the volcano. here, we adopt density 0.3×103 kg/m3 for the fall deposits. thus verbeek’s result of pyroclastic-fall deposits is modified to 0.4 km3 dre (table 4) and the total volume of the flows and falls amounts to 12.2 km3 dre. 3.2. estimation of fall-deposits after the analogy of caldera-forming eruptions at novarupta and pinatubo volcanoes when we try to revise the above verbeek’s estimation, it is clear that we can no more collect systematic data of the pyroclastic falls of the 1883 eruption. here, to re-estimate the fall deposits, as the last resort, we assume that volume of pyroclastic-fall deposits is proportional to that of pyroclastic-flow deposits in the caldera forming eruptions provided that these eruptions would occur by similar mechanisms and processes. the volume of pyroclastic-fall deposits of the 1883 krakatau eruption shall be speculated after the analogy with well-studied caldera-forming eruptions, novarupta, alaska in 1912 and pinatubo, the philippines in 1991. these two volcanoes are similar to krakatau in explosivity or magnitude of eruptions and properties of magmas (andesite ~ dacite ~ rhyolite), and both the two calderas are filled with lithic-rich caldera deposits. their eruptions were later than krakatau, and have been studied much more with modern knowledge and methods, and furthermore both they are on land: the last is absolutely advantageous to estimate volume of the pyroclastic ejecta. there are many papers discussing their eruptions and transport processes of eruption deposits in detail. some particulars of the three eruptions shall be comparatively discussed and are listed in table 3. novarupta: hildreth [1983] estimated volume of the ash-flow tuff at 11~15 km3 bulk, that is converted to about 8.2 km3 dre, and fierstein and hildreth [1992, appx. 1] did that of the tephra falls at 6.8 km3 dre. hilddreth [1983] assumed 5% of the total ash falls and flows to be lithic material that derived from the former ground. this lithic material fills the caldera depression that was formed by explosive excavations. and further he assumed the caldera deposits to be about 1 km3 in volume, a half of the original ground. thus, the original ground was approximated to an inverted cone (funnel-shape) of 2 km deep and 2 km in top diameter. in other words, about a half of the caldera deposits is reasonably presumed to be lithic fallbacks at novarupta. pinatubo: the climatic eruption culminated with lithic-rich facies in the pyroclastic-flow deposits and resulted in formation of a caldera of 2.5 km in diameter. scott et al. [1996] estimated volume of the pyroclasticflow deposits issued in various directions from the crater at 2.1~3.3 km3 dre by the valley-cross-section method. yokoyama 8 volcano krakatau 1883 novarupta∗ 1912 pinatubo∗∗ 1991 rocks andesite ryolite, andesite, dacite dacite (a) magma volume of pyroclasticflow deposit (km3 dre) 10.7 8.2 2.1~3.3 (b) magma volume of pyroclasticfall deposit (km3 dre) 10.7 × 0.8 = 8.6 6.8 1.6~2.0 (b) / (a): fall-flow ratio ∗∗∗ 0.8 0.7 total volume of ejecta (km3 dre) 19.3 14~16 mean 15 3.7~5.3 mean 4.5 volume of disrupted volcanic edifices (km3) 3 (a.s.l.) 5 2.5 + 1.6 = 4.1 incl. dilation volume of topog. depression (km3) 5 (b.s.l.) 0.7 1.1 dia. of caldera surface (km) 5 2 2.5 duration of climatic eruption (hour) 23 11~16 3~5 max. column height (km) 50 26 34~40 table 3. comparison among the three caldera-forming eruptions. (∗): after hildreth [1983] and fierstein and hidreth [1992]; (∗∗): after paladio-melosantos et al. [1996] and scott et al. [1996]; (∗∗∗): (b) / (a) is assumed to be 0.8. 9 paladio-melosantos et al. [1996] distinguished tephra layers into a, b, c and d. c-layer is the most voluminous, 98% of the total volume, amounting to 1.6~2.0 km3 dre. scott et al. [1996] also made a comparative discussion between pinatubo and novarupta eruptions, and independently assumed the caldera structure of pinatubo to be funnel-shape of a large depth (6~8 km b.s.l.) and full of lithic material: it is noticeable that the concept of “caldera deposits” was entertained in the discussions of both novarupta and pinatubo calderas. here we refer to the two large eruptions to check their relationships, if any, between volume of pyroclastic-flow deposits and pumice-fall deposits. if these are correlated, we may apply the correlation to the krakatau eruption. in table 3, the ratios of volume dre of pyroclastic-fall to that of pyroclastic-flow (fall/flow ratio) in the eruptions of novarupta and pinatubo are roughly 0.7~0.8. such similar ratios may be not always accidental. if the ratios are approximately common to the eruptions of similar magmas and magnitudes, we may use the ratio to estimate volume of pumice-fall deposits of the third eruption of which volume of pyroclastic-flow is known. we apply the ratio 0.8 to the eruption of krakatau. then volume of the krakatau pyroclastic-fall is approximately estimated at 8.6 km3 dre that is comparable to that of novarupta (6.8 km3). the fall deposits of krakatau may have been distributed over the wider area than novarupta considering the difference in their column heights. the total volume of the ejecta from krakatau is revised to (10.7+8.6) km3 = 19.3 km3 dre as shown in table 4. in the table, original verbeek’s estimation of the flow and fall deposits are converted to 11.8 km3 dre and 0.4 km3 dre, respectively. assuming that the present re-estimation is correct, the difference is mainly due to underestimation of the pyroclastic-fall deposits. the present re-estimation of pyroclastic-fall deposits is approximate but may be more reasonable than verbeek’s estimate because the latter depended on scanty data. verbeek [1885] commented on the errors of his estimation (18.2 km3 bulk in total) as “the figures everywhere used for thickness can be hardly be considered too large, but is most likely too small. the possible inaccuracy of this calculation, i think, is not more than 2 or at most 3 km3.” strictly speaking, we cannot argue the errors in verbeek’s estimation because we cannot evaluate the accuracy of the two charts used in his calculations. the present result, 19.3 km3 at the minimum, seems not so unreasonable comparing with 15 km3 of the 1912 novarupta eruption in table 3. 3.3. volume estimations of eruption products in the 1833 krakatau eruption by various authors after verbeek, volume estimations of eruption products in the 1883 krakatau eruption have been carried out, sometimes as a by-product of fundamental researches. here, only the results in the volume estimation of pyroclastic-flows and -falls, and disrupted parts, by various authors shall be mentioned: (1) verbeek [1885] first carried out re-plumbing about one month after the 1883 eruption. this is a merit of his work if we consider post-depositional erosions. he did not discriminate between flows and falls, but in the present paper, these two are classified according to the distance from the eruption center (table 1). thus his estimation of the total volume of the ejecta amounts to 18.2 km3 bulk or 12.2 km3 dre (table 4). (2) archibald [1888] estimated volume of ash falls collecting reports of the dust falling on 8 ships traversing the indian ocean assuming its thickness to be 0.005 m and the area represented by these ships to be 2.8×106 km2 (1700 km × 1700 km; cf. figure 2b). then he got the total amount of ash fallout as 14.4 km3 bulk, or ×0.3/2.6 = 1.7 km3 dre. (3) yokoyama [1981] determined volume of the total ejecta as 101 km3 in order of magnitude based on verbeek’s estimation (18.2 km3 bulk) and in consideration of its ambiguity. also he deduced volume of “caldera deposit” as 9.3 km3 bulk from gravity anomalies, and later yokoyama [2014] revised it to 13±3 km3 bulk by new knowledge on vp/vs ratios [jaxybulatov et al. 2011]. (4) self and rampino [1981] carried out field investigation on krakatau islands to correlate the eruption sequence with the stratigraphy of pyroclastic deposits. they evaluated the total volume of eruption deposits as 18~21 km3 bulk or 9~10 km3 dre, made up as folkrakatau eruption products verbeek [1885] this paper bullk dre (km3) dre (km3) pyroclastic-flow (2.5+ 12.1) × (2.1/2.6) = 11.8 revised 10.7 pyroclastic-fall 3.6 × (0.3/2.6) = 0.4 ∗newly estimated 8.6 sum 18.2 12.2 19.3 table 4. revised volume of the pyroclastic deposits on and around krakatau islands; (∗) analogical estimation in section 3.2. lows: 12 km3 bulk of pyroclastic deposits and 8.5 km3 bulk of ash fall deposits. (5) sigurdsson et al. [1991] collected scuba cores of the sediments in the area between krakatau islands and sebesi island, mainly around steers and calmeyer reefs, and newly constructed a bathymetric chart based on their 1990 survey, and a contour map of the changes in bathymetry. they followed verbeek’s manner and determined finally the total volume of the eruption products as 9.0 km3 dre, and caldera depression as 8.9 km3 bulk. (6) mandeville et al. [1996] studied the stratigraphy of submarine pyroclastic deposits within a 15 km radius of the caldera collecting land-based and scuba cores, and estimated the volume of flow deposits as 12.4 km3 dre and fall deposits as 2.3 km3 dre: the total volume amounts to 14.7 km3 dre. (7) in the present paper, verbeek’s estimate of volume of the flow deposits is revised to 10.7 km3 dre following his procedures. the volume of pyroclastic fall is assumed to be 80% of that of flow deposits on the analogy of the two caldera-forming eruptions: it is 8.6 km3 dre. finally the total amount of pyroclastic material is obtained as 19.3 km3 dre. and the volume of disrupted volcano edifice is reckoned at 8 km3 bulk. in a summary, the total volume of the ejecta from the 1883 krakatau eruption calculated by various authors ranges over 9.0, 9.5, 12.2, 14.7 and 19.3 km3 dre, as summarized above. the results are rather diverse because the above authors based on the different assumptions and the different data. after all, it may be concluded that the total volume of the krakatau pyroclastic products including the flows and falls, widely ranges 9~19 km3 dre, being roughly comparable to novarupta and larger than pinatubo, and that the volume of disrupted volcano edifice is roughly 8~9 km3 bulk according to some authors. now we may say that we must be content with the volume estimation with accuracy of order of magnitude in the discussion of ejecta volume from the 1883 krakatau eruption. in no distant future, we shall be able to estimate the volume of volcanic ejecta as mass flux at the level of volcano vents by remote sensing techniques in place of the present manner of searching the ejecta around volcanoes after eruptions. 4. subsurface structure of krakatau caldera and settlement of the eruptive products 4.1. caldera deposits of krakatau caldera generally caldera depression is defined as a vacant space formed by disruption of volcano edifices during violent explosions of pyroclastic ejecta. in the case of krakatau caldera, the caldera depression was formed by disruption of 2/3 parts of pre-existed krakatau island and excavation of the seabed to a depth of 250 m. the present author [1981, 2014] discussed subsurface structure of the caldera by the gravity anomalies observed there, and assumed “caldera deposits” of low density immediately beneath the caldera depression, and estimated their volume at roughly 13 km3 bulk. van bemmelen [1929, 1949] expressed his conception that disrupted volcano edifices collapsed into magma reservoir in replacement of the discharged material by magmatic eruptions. to accept this idea, the following two assumptions should be satisfied: (1) the volume of disrupted volcano edifice (“caldera depression”) should be approximately equal to that of discharged magma, and (2) lithics of the volcano edifice scarcely remain at the surface around the caldera: in the following, we discuss the problems on krakatau caldera. firstly, to verify assumption (1), both the volume should be similar in the exact numerals, not in the order of magnitude. the total volume of pyroclastic material of the krakatau eruption is estimated at 19.3 km3 dre in table 4, and that of disrupted volcano edifice is reckoned as (2.9+5.1) = 8 km3 in section 2.3: the ratio of (volume of ejecta) / (volume of disrupted volcanic edifice) is (19.3 / 8). here, we check the cases of novarupta and pinatubo calderas: in table 3, the ratios are (15/5.7) and (4.5/5.2), respectively. in short, van bemmelen’s idea is narrowly valid in the pinatubo eruption though clearly invalid in the krakatau and the novarupta eruptions. hence we may conclude that the assumption proves not always to hold good. secondly, as for assumption (2), verbeek [1885] reported that the ejecta mainly consisted of newly erupted magmatic material and fragments of the old volcano edifices, and that the latter volume amounted to 5% of the total. his estimation 5% may have been very rough admitting that the estimation method has not been established even now. stehn [1929] observed a lithic part of 16 m thick of 40 m columnar section at the nw part of lang island. we can find other higher percentages in the recent literatures: 13 % at the shore of lang island [mandeville et al. 1996]. simkin and fiske [1983, p. 135, fig. 32] introduced the investigation report carried out by u.s. steamship juniata around krakatau islands within 2 weeks of the catastrophic eruption: the crew prepared the first post-paroxysm map of krakatau, dated september 10, 1883, in which we see several small islands formed of the fragmental deposits near verlaten island. verbeek started his survey at the end of september and did not register such large amount of deposits. the delay of 2 weeks in the start of the surveys may be rather significant from the viewpoint of afterdepositional erosions. hence, the present author doubts validity of 5 % reported by verbeek [1885]. yokoyama 10 11 we may assume the caldera deposits to be composed of juvenile pyroclastic ejecta and crushed precaldera rocks (lithic material): all these are fallbacks, and the latter may have been due to explosivity of the caldera-forming eruption. admitting the volume equivalence between the caldera depression and the total erupted magmas, it is necessary to locate the magma reservoir and to explain the mechanism how the reservoir cavity was filled with the disrupted material. the validity of van bemmelen’s assumptions should depend on the possibility of settlement of the eruption products into magma reservoirs. 4.2. magma reservoir beneath krakatau volcano and mechanical stability of a cavity in the earth crust williams [1941] supported van bemmelen’s idea and established the 1883 krakatau eruption as the typical example in collapses of a pre-existing volcanic edifice that was disrupted by large-scale eruption of silicic magma derived from a shallow magma reservoir. “shallow magma reservoirs” may be a rather difficult conception in the 21st century except “magma stops” between magma reservoirs and vents at some basaltic volcanoes. beneath krakatau caldera, searching for liquid zones, harujono et al. [1989] detected several zones of strong attenuation of seismic s-waves and jaxybulatov et al. [2011] determined the distribution of high vp/vs ratio of seismic waves; yokoyama [2014] assumed a probable magma reservoir at a zone of about 10 km deep, characterized commonly by s-wave attenuation and high vp/vs ratio as shown in figure 7. here, mechanical stability of a magma reservoir with a cavity produced by magma ejection shall be examined: a cavity in the deep crust cannot exist against lithostatic pressure. if we assume compressive strength of the shallow crust to be 100 mpa, a cavity cannot exist at depths deeper than about 3 km in average. in other words, a magma reservoir at a depth deeper than 3 km should be crushed according as the magma is discharged from the reservoir. it is doubtful how the disrupted volcanic edifices can reach such magma reservoirs during formation of calderas. in the present case, the cataclysm is assumed to start from a magma reservoir, not from a surface volcano. when a magma reservoir discharges magma partly or totally through a conduit leading to the volcano, the reservoir should be crushed partly or totally under crustal load when it is located at deeper than 3 km. here, for the sake of simplicity, we assume a spherical magma reservoir at a depth of 10 km, and concentric crushing force f to act at the center of the reservoir. timoshenko and goodier [1970] discussed “force at a point in an infinite solid”, and their results are approximately applicable to the present problem. beginning from laplace’s equation, they finally gave vertical stress v acting along the axis r = 0 of cylindrical co-ordinates (r, i, z) where the origin is the center of the cavity, as (1) assuming poisson’s ratio o = 0.25, we get: vz,r=0 = 0.186 f/z 2 (2) qualitatively speaking, the reservoir should be crushed by lithostatic pressure according as the magma is discharged, and the effect of crushes of the reservoir (crushing force f) decreases rapidly in proportion to z2. in other words, the crushing effect does not reach the surface when the reservoir is located at a sufficient depth. if a magma reservoir were located at shallow depths, shallower than 5 km, the crustal strength is much weaker than the deeper parts and cover rocks of the reservoir may be easily exploded. on krakatau caldera, yokoyama [2014] assumed depth of the caldera deposits to be about 2 km (figure 7) and hence a shallow reservoir should be rather near to the caldera bottom. anyway disrupted volcano edifices cannot settle at a magma reservoir in replacement of the discharged magma but may settle as caldera deposits. 8 1 1 2 3f z,z r 0 2v r o o= += ^ ^ h h" , krakatau eruption products figure 7. assumptive caldera deposits and magma reservoir projected on a vertical plane along b—b’ line in figure 6. dotted part schematically shows caldera deposits. distribution of the s-wave attenuations (striped parts) was obtained by harjono et al. [1989] and that of the vp/vs ratios by jaxybulatov et al. [2011]. a colored circle on the assumed magma reservoir shows the total amount of the pyroclastic ejecta (19 km3 dre) discharged by the 1883 eruption. 4.3. a model of krakatau caldera viewed from its subsurface structure williams [1941] discussed general features of calderas and their origin but did not refer to their subsurface structure. thus, he had no idea about caldera deposits. yokoyama [1963] studied subsurface structure of some calderas and classified them into two: high-gravityanomaly type and low-gravity-anomaly type. the gravity anomalies are due to density of the caldera deposits which have been verified by drillings at some calderas on land in japan and mexico [yokoyama 2014]. at krakatau caldera, yokoyama [1981] analyzed low gravity anomalies observed on the islands and later deplus et al. [1995] discussed the inner structure of the caldera analyzing gravity anomalies observed in a larger area around krakatau islands. yokoyama [2014] interpreted the gravity anomalies to be due to mainly the caldera deposits of low density and estimated their volume at 13 ± 3 km3. the krakatau caldera depression (5.1 km3) is about 200 m deep b.s.l. and full of seawater. beneath the caldera bottom, there exist caldera deposits that are composed of fallbacks of exploded materials (lithics and juveniles) and are soaked in seawater. the watery deposits are verified by anomalous values of vp/vs ratio as high as 2.2 roughly around krakatau caldera determined by jaxybulatov et al. [2011]. on the other hand, harjono et al. [1989] detected several seismic s-wave attenuating zones beneath the krakatau islands. a vertical configuration of krakatau caldera obtained by yokoyama [2014] is reproduced in figure 7 where the most probable zone of magma reservoir must be endowed with both the seismic-wave characters in common. this model has been achieved by contributions from several authors as mentioned above. the total volume of the ejecta discharged by the 1883 eruption (19.3 km3 dre) is projected as a sphere on the assumed magma reservoir. 5. concluding remarks the estimation of volume of pyroclatic ejecta from the 1883 eruption of krakatau originally carried out by verbeek [1885] is revised in this paper principally based on his data. as a result, his estimation of the flow deposits proved to be acceptable while the fall deposits to be much underestimated. its revision in the present paper is based on the assumption that the volume ratio, fall/flow is similar among caldera-forming eruptions, deduced from the 1912 eruption of novarupta and the 1991 eruption of pinatubo. as a result, the volume of pyroclastic fall is evaluated to be similar to that of the flow deposits. originally volume estimation of volcanic ejecta with reasonable accuracy is very difficult, especially on island volcanoes. in the case of the 1883 krakatau eruption, we must be content to get results with accuracy of the order of magnitude, and so van bemmelen’s assumption that the dense rock equivalent of the total ejecta derived from a caldera-forming eruption is the same as the volume of the resulting caldera depression, proved not necessarily to hold good. as for the final settlement of the disrupted volcanic materials, some papers have proposed that the materials settled in magma reservoirs in replacement of the discharged magma by reason of volumetric equality between both the materials. however, the equality is not always approved as discussed in the present paper and is not the necessary condition for caldera formations. on the other hand, the caldera deposits of krakatau caldera are confirmed as well as some other calderas, and its magma reservoir is conjectured to be located at a depth of about 10 km from a geophysical standpoint. the paucity of lithic fragments among the krakatau ejecta can be semi-quantitatively attributed to their settlement into the caldera deposits. the eruption products of the 1883 krakatau eruption are discussed on their origin, volume and settlement on the base of the two examples of calderaforming eruptions that occurred under our eyes. if the existence and the origin of “caldera deposits” are approved, we may define “caldera” of low-gravityanomaly type, from the viewpoint of subsurface structure as a large crater, such as larger than 2 km in diameter, with flat bottom, underlain by caldera deposits and accompanied with a large quantity of pyroclastic deposits around the volcano. acknowledgements. the above discussion is indebted to the data originally provided by verbeek [1885]. the present author had a chance to read this priceless book at the volcanological survey of indonesia, bandung, and thanks dr. s. siswowidjoyo and prof. a. sudradjat for their kind cooperation. the author’s knowledge on plumbing in old times owes to dr. m. tsuchide of the hydrographic department of japan, to whom the author is grateful. critical and helpful comments by anonymous referees are gratefully appreciated. references archibald, e.d. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice s t r u t t u r a g e o l o g i c o s i s m i c a d e l l ' e u r o p a c e n t r o m e r i d i o n a l e , d e l l ' i t a l i a e d e l m e d i t e r r a n e o c e n t r o o c c i d e n t a l e , q u a l e r i s u l t a d a r e c e n t i r i c e r c h e c o m p i u t e i n i t a l i a (*) plktro caloi i n questa relazione mi p r o p o n g o di r i a s s u m e r e q u a n t o è stato f a t t o , negli u l t i m i a n n i , da ricercatori italiani sul p r o b l e m a delle car a t t e r i s t i c h e geologico-sisiniche del c o n t i n e n t e europeo. non si t r a t t a q u i n d i di u n a sintesi di lavori interessanti esclusivamente l ' i t a l i a , m a u n riassunto delle ricerche c o m p i u t e da i t a l i a n i sulle p r o p r i e t à sismic h e d e l l ' e u r o p a , a n c h e se tali ricerche r i g u a r d a n o come è ovvio in m o d o p a r t i c o l a r e l'italia. sono stati presi in considerazione soltanto lavori basati sull'int e r p r e t a z i o n e di registrazioni sismiche, a n c h e se i n t e g r a t i con considerazioni macrosismiche. non h o preso in esame lavori dedicali esclusivamente a l l ' e l a b o r a z i ó n e dei dati macrosismici. distribuzione geografica delle scosse esaminate. — p e r ciò che concerne la d i s t r i b u z i o n e geografica delle scosse esaminate d i r ò che esse r i g u a r d a n o il v e n e t o (in n u m e r o di 4) ( 1 2 j , l'alto a d r i a t i c o (3) (3), la valle p a d a n a (1) (4), la toscana (4) (""), il lazio (1 0ì, l ' a b r u z zo (2ì (11"12), lo j o n i o •— presso le coste calabre (1 3) — (1) e presso le cosle greche — (1) (1 4), il basso t i r r e n o (2) (lr>10) e — f u o r i d'italia — la svizzera (2) (1 7), il w u r t t e m b e r g (13) (17), n o n c h é t e r r e m o t i extra-europei, s t u d i a t i nei loro riflessi sulla costituzione del c o n t i n e n t e e u r o p e o , come il t e r r e m o t o del t u r k e s t a n del 2 n o v e m b r e 1946 e della dorsale m e d i a n a atlantica del 24 a p r i l e 1947. non m i s o f l e r m e r ò sulla sismicità — intesa come f r e q u e n z a e intensità delle scosse — delle zone sismiche interessate dai t e r r e m o t i oggetto dei lavori q u i sintetizzati : l'esame di tale caratteristica esula dai limiti di questa b r e v e relazione. qui si vuole solo a c c e n n a r e come, dallo studio dei t e r r e m o t i citali, si siano p o t u t i t r a r r e alcuni elementi, atti a p r o p o r r e — con b u o n a (*) c o m u n i c a z i o n e presentata al c o n v e g n o della « c o m m i s s i o n s é i s m o l o g i q u i e u r o p é e n n e » (stoccarda, 22-27 s e t t e m b r e 1952). 31 p i e t r o caldi s t r u t t u r a g e 0 l 0 g i c 0 s i s m 1 c a dell'europa centro-meridionale 509 a p p r o s s i m a z i o n e — a l c u n e c o n c l u s i o n i sui v a l o r i d e l l e velocità d e l l e o n d e l o n g i t u d i n a l i e trasversali, sulla s t r a t i f i c a z i o n e p r o f o n d a del cont i n e n t e e u r o p e o , sulle r a d i c i d e l l e a l p i e degli a p p e n n i n i , sul meccan i s m o di p r o d u z i o n e dei t e r r e m o t i p e r ciò c h e r i g u a r d a in p a r t i c o l a r e la zona d e l l e a l p i , l ' a l t o a d r i a t i c o e la zona degli a p p e n n i n i . velocità di propagazione delle onde longitudinali e trasversali. — sono stati r i v e l a t i , in m o d o c h i a r o , t r e t i p i d i s t i n t i di o n d e l o n g i t u d i n a l i e t r a r s v e r s a l i , c o r r i s p o n d e n t i a t r e diversi mezzi di p r o p a g a z i o n e : il mezzo u l t r a b a s i c o , i n i z i a n t e il m a n t e l l o t e r r e s t r e , il mezzo basaltico (o p e r o d i t i c o , o d e l g a b b r o ) e il mezzo del « g r a n i t o ». n o n è esclusa l'esistenza di o n d e i n t e r e s s a n t i a l t r i mezzi di diversa n a t u r a geologica; m a , allo stato a t t u a l e delle r i c e r c h e , n o n è d a t o di p o t e r t r a r r e n e s s u n a c o n c l u s i o n e al r i g u a r d o . i n d i c a n d o con pn, p*, pg le o n d e l o n g i t u d i n a l i i n t e r e s s a n t i i t r e mezzi n e l l ' o r d i n e d e t t o e con sn, s*, sg le c o r r i s p o n d e n t i o n d e trasversali, ecco i v a l o r i d e l l e c o r r i s p o n d e n t i velocità n e i t r e mezzi cons i d e r a t i : v l ' n k m l n e c v , . * v p g v s a v h * v»g regione veneta e p i c e n t r o c o n s i g l i o : 4 6 ° 6 \ 2 n ; 1 2 ° 2 6 \ 5 e 8 , 0 0 + 0 , 0 1 6 , 6 1 + 0 , 0 4 5,7 + 0 , 0 4 4 , 4 1 + 0 , 0 2 3,64 3 , 3 6 + 0 , 0 2 5 alto adriatico 44«18',1 n ; 1 3 ° 2 9 \ 6 e 7 , 7 7 + 0 , 0 8 — 4 . 8 0 + 0 , 0 1 3 regione toscana 4 4 ° 4 \ 4 n ; 11°38',7 e 7 , 8 6 + 0 , 0 4 — 5,2 —5,4 4,34 | 0,02 3 , 0 1 + 0 , 0 4 44°14',0 n ; 10°i2",2 e 8 , 1 6 + 0 , 0 6 6 , 6 3 + 0 , 0 8 5 , 2 5 + 0 . 0 0 4 , 4 4 + 0 , 0 2 3 , 5 7 + 0 , 0 4 3 , 0 6 + 0 . 1 regione abruzzese 4 2 ° 3 0 \ 8 n ; 13»19',6 e 8 , 1 9 + 0 , 0 4 6,38-1-0.05 5 , 4 6 + 0 , 1 8 4 , 3 9 + 0 , 0 2 3 , 0 1 + 0 , 0 2 regione laziale 4 2 ' 1 \ 8 5 n ; 13°4",55 e 8,09 5,39 4,40 — 3,05 regione calabro 38°41',4 n ; 1 6 » 4 7 \ 7 e 7 , 9 4 + 0 , 1 8 6 , 9 6 + 0 , 4 1 — 4 , 5 7 + 0 . 0 8 4 , 0 0 + 0 , 0 9 — u n a ricerca sul t e r r e m o t o del basso t i r r e n o d e l 16 m a r z o 1941 h a messo in e v i d e n z a u n n o t e v o l e a n t i c i p o n e i t e m p i di r e g i s t r a z i o n e delle o n d e d i d i l a t a z i o n e e di d i s t o r s i o n e n e l l e s t a z i o n i p i ù vicine a l l ' e p i c e n t r o . a n a l o g h e osservazioni sono s t a t e f a t t e n e l l o s t u d i o d e l 510 p i e t r o caloi t e r r e m o t o p r o f o n d o d e l l e isole l i p a r i d e l 13 a p r i l e 1938, s t u d i o c h e è stato r i p r e s o r e c e n t e m e n t e da caloi e giorgi. v e d r e m o p i ù o l t r e c h e t a l e n o t e v o l i s s i m a a n o m a l i a p u ò t r o v a r e l a sua s p i e g a z i o n e n e l l a nat u r a v u l c a n i c a della zona. u n ' a l t r a c o n s i d e r e v o l e a n o m a l i a c h e r i s u l t a d a l l ' o s s e r v a z i o n e d e l l a t a b e l l a r i p o r t a t a consiste n e l l ' e l e v a t o v a l o r e a s s u n t o dalla velocità d e l l e o n d e trasversali d e s t a t e d a l t e r r e m o t o d e l l ' a l t o a d r i a t i c o del 30 nov e m b r e 1934, i n u n a z o n a c a r a t t e r i z z a t a da t e r r e m o t i di s p r o f o n d a m e n t o . u n ' a n a l o g a a n o m a l i a è stata messa in e v i d e n z a da p . e . v a l l e p e r lo j o n i o , dove, c o m e p e r l ' a l t o a d r i a t i c o , ad u n a leggera d i m i n u z i o n e della velocità delle o n d e pn f a r i s c o n t r o u n s e n s i b i l e a u m e n t o della velocità delle o n d e sn. se i n d i c h i a m o con à e il le c o s t a n t i di l a m é e con p la d e n s i t à del mezzo, n e l t e n t a t i v o di s p i e g a r e d e t t a anom a l i a , p . e. v a l l e è c o n d o t t o alla r e l a z i o n e (1 3) \ m j d q \ j dk\ il p a c h e deve v a l e r e affinché n e l p a s s a r e d a l c o n t i n e n t e al m e d i t e r r a n e o si osservi u n a d i m i n u z i o n e della velocità delle o n d e pn ed u n a u m e n t o della velocità d e l l e o n d e sn. s e m p r e in t e m a di velocità di p r o p a g a z i o n e delle o n d e sismiche, in u n a m p i o s t u d i o su diciassette t e r r e m o t i c e n t r o e u r o p e i è s t a t o possibile t r o v a r e che, n e l l o s t r a t o di 20 k m di « g r a n i t o » — c o s t i t u e n t e la p r i m a s t r a t i f i c a z i o n e della crosta p e r l ' e u r o p a c e n t r a l e — l e o n d e l o n g i t u d i n a l i e trasversali d i r e t t e si p r o p a g a n o con velocità crescenti con la p r o f o n d i t à , secondo le leggi (1 7) vpe = 5,530 (1 + 0,00158 . h) ; vsg = 3,275 (1 + 0,00158 . h) p e r a = 0,23. stratificazione della crosta terrestre. — gli spessori degli s t r a t i , c o s t i t u e n t i la crosta t e r r e s t r e , sono stati o t t e n u t i con m e t o d i b a s a t i o sulla p r o p a g a z i o n e delle o n d e spaziali, o sulla d i s p e r s i o n e delle o n d e s u p e r f i c i a l i o sulla v a r i a z i o n e d e l f a t t o r e vv—v. le p r i m e d e t e r m i n a z i o n i r i g u a r d a n o lo spessore dello s t r a t o delle o n d e pg i n c o r r i s p o n d e n z a delle p r e a l p i c a r n i c h e e d e l cansiglio. si sono o t t e n u t i , i n diverse d e t e r m i n a z i o n i , v a l o r i v a r i a b i l i f r a 40 k m s t r u t t u r a g e 0 l 0 g i c 0 s i s m 1 c a d e l l ' e u r o p a c e n t r o m e r i d i o n a l e 5 1 1 e 34 k m e si è concluso c h e lo spessore dello s t r a t o delle pg, in corris p o n d e n z a del sistema a l p i n o o r i e n t a l e è d i 35 kjn. ca. si è poi p a s s a t i a l l ' i t a l i a c e n t r a l e e p r e c i s a m e n t e a l l ' a p p e n n i n o tosco-romagnolo e alle a l p i a p u a n e . lo spessore dello s t r a t o del « gran i t o » è r i s u l t a t o , r i s p e t t i v a m e n t e , d i 25 km e di 30 km. i n c o r r i s p o n d e n z a del g r a n sasso, d e t e r m i n a z i o n i successive h a n n o p o r t a t o ad u n v a l o r e di 25 k m , c h e s e m b r a essere l o spessore m e d i o del sistema degli a p p e n n i n i . a l t r e r i c e r c h e f a n n o r i t e n e r e c h e , i n c o r r i s p o n d e n z a della v a l l e p a d a n a lo spessore dello s t r a t o del « g r a n i t o » sia a l q u a n t o m i n o r e ( d e l l ' o r d i n e di 10 k m ) . n e l 1943 f u p r o v a t o , p e r la p r i m a volta c h e , i n u n a vasta zona o l t r e il sistema a l p i n o , c o m p r e n d e n t e a l m e n o p a r t e della svizzera e il w i i r t t e m b e r g (1 7), la p r i m a s u p e r f i c i e di d i s c o n t i n u i t à n e l l ' i n t e r n o d e l l a crosta t e r r e s t r e si trova ad u n a p r o f o n d i t à di circa 20 k m v a l o r e s u c c e s s i v a m e n t e c o n f e r m a t o d a a l t r i r i c e r c a t o r i , q u a l i r o t i l e , p e t e r s c h m i t t e w a n n e r . il v a l o r e o t t e n u t o n e l 1943 p r o v a v a cbe la s u p e r f i c i e di discontin u i t à f r a « g r a n i t o » e b a s a l t o e r a , p e r la r e g i o n e c o n s i d e r a t a , a l q u a n t o p i ù sollevata di q u a n t o n o n fosse fino allora r i t e n u t o . u l t e r i o r i d e t e r m i n a z i o n i , c o n d o t t e con m e t o d i b a s a t i sulla dispersione d e l l e o n d e s u p e r f i c i a l i , h a n n o c o n f e r m a t o p e r l ' i t a l i a u n v a l o r e m e d i o di 26 k m p e r lo s t r a t o del « g r a n i t o », m e n t r e tale s t r a t o , n e l l a d i r e z i o n e j o n i o s p a g n a ( i n t e r e s s a n t e g r a n p a r t e del m e d i t e r r a n e o ) h a u n v a l o r e m e d i o di 15, n e l l a d i r e z i o n e j o n i o a d r i a t i c o di 17 k m , lo spessore dello s t r a t o del « g r a n i t o » n e l l a d i r e z i o n e t u r k e s t a n r o m a è d e l l ' o r d i n e di 18 k m (1s). ai m a r g i n i del c o n t i n e n t e , d a l l a t o dell ' a t l a n t i c o , tale s t r a t o s a r e b b e p r e s s o c h é m a n c a n t e (2 0). r i s u l t a v a e v i d e n t e c h e la s u p e r f i c i e l i m i t a n t e lo s t r a t o del g r a n i t o n o n poteva essere p a r a l l e l a alla s u p e r f i c i e della t e r r a : essa a p p a r i v a s p r o f o n d a r s i in c o r r i s p o n d e n z a dei sistemi di m o n t a g n e . con u n m e t o d o c h e n o n è qui il caso di r i f e r i r e , n e l 1940 ( 2 2 ì f u i n f a t t i messo in e v i d e n z a c h e la p r i m a s u p e r f i c i e di d i s c o n t i n u i t à t e n d e a sollevarsi q u a n d o si p r o c e d e d a l l ' i n t e r n o del s i s t e m a a l p i n o verso sud e verso n o r d . i n c o r r i s p o n d e n z a del c e n t r o del sistema a l p i n o ( t i r o l o ) , si è t r o v a t o la p r o f o n d i t à di 40 k m , p r o f o n d i t à c h e ai m a r gini m e r i d i o n a l i (cansiglio) si r i d u c e a 34 k m . r e s t a v a così p r o v a t a l'esistenza d e l l e « r a d i c i » d e l l e m o n t a g n e . t a l e esistenza è stata con512 p i e t r o caloi f e r m a t a da u n o s t u d i o successivo di g u t e n b e r g p e r la s i e r r a n e v a d a ( c a l i f o r n i a ) e, d i r e c e n t e , — con il m e t o d o di caloi su a c c e n n a t o — d a di f i l i p p o e m a r c e l l i p e r gli a p p e n n i n i . p e r il sistema a l p i n o , lo s t r a t o del « b a s a l t o » (o « g a b b r o ») è r i s u l t a t o p i u t t o s t o piccolo, d e l l ' o r d i n e di 11 km, m e n t r e s e m b r a assum e r e valori a l q u a n t o p i ù g r a n d i in c o r r i s p o n d e n z a degli a p p e n n i n i . natura delle scosse all'ipocentro e profondità ipocentrali. — p e r q u a n t o si r i f e r i s c e al m e c c a n i s m o di p r o d u z i o n e dei t e r r e m o t i s t u d i a t i , va s u b i t o d e t t o c b e esso si è esplicato secondo t r e diversi m o d e l l i : frattura ( m o v i m e n t o del suolo d e l l o stesso segno in q u a d r a n t i opposti), sprofondamento ( d i l a t a z i o n i e n t r o u n a c i r c o n f e r e n z a di raggio d e t e r m i n a t o , c o m p r e s s i o n i all'esterno), sollevamento ( c o m p r e s s i o n i n e l l ' i n t e r n o di u n a c i r c o n f e r e n z a di d a t o raggio, d i l a t a z i o n i all'esterno). i l p r i m o m o d e l l o ( f r a t t u r a con m u t u o s c o r r i m e n t o di strati) è s t a t o osservato i n t u t t o il sistema a l p i n o , dove esso s e m b r a p r e d o m i n a r e : l ' e s e m p i o p i ù c a r a t t e r i s t i c o è stato f o r n i t o d a l t e r r e m o t o del cansiglio del 18 o t t o b r e 1936 (1), d o v u t o a f r a t t u r a p r o v o c a t a da u n a c o p p i a di f o r z e i n c l i n a t e , g i a c e n t e in u n p i a n o p u r e i n c l i n a t o . i t e r r e m o t i d e l l ' a l t o a d r i a t i c o finora s t u d i a t i p r e s e n t a n o i n v e c e il m o d e l l o c a r a t t e r i s t i c o dei t e r r e m o t i di s p r o f o n d a m e n t o . a q u e s t o rig u a r d o , è i n t e r e s s a n t e osservare il p a r a l l e l o n e i f e n o m e n i b r a d i s i s m i c i della r e g i o n e . è n o t o i n f a t t i c h e , i n ère geologiche passate, f u m o l t o a c c e n t u a t o il b r a d i s i s m o n e g a t i v o i n t u t t a l ' i s t r i a e la d a l m a z i a ; e dell ' e n t i t à di q u e s t o b r a d i s i s m o si h a n n o c h i a r e t e s t i m o n i a n z e ( f o r m a f r a s t a g l i a t i s s i m a della costa a d r i a t i c a o r i e n t a l e , g r a n n u m e r o di isole presso l e coste, ecc.). l ' a p p e n n i n o p r e s e n t a i n v e c e il f e n o m e n o c o n t r a r i o . lo s t u d i o dei t e r r e m o t i d e l l ' a p p e n n i n o tosco-romagnolo, della g a r f a g n a n a , del m o n t e a m i a t a e, p i ù r e c e n t e m e n t e , del g r a n sasso d ' i t a l i a , h a messo c h i a r a m e n t e in luce il c a r a t t e r e di u n s o l l e v a m e n t o di strati. la geologia c o n c o r d a con q u e s t e c o n s t a t a z i o n i ; f r a l ' a l t r o , l ' e s a m e d e l l e f r o n t i di a l c u n e f a g l i e t e s t i m o n i a a f a v o r e dei s o l l e v a m e n t i , q u a l i ad e s e m p i o q u e l l o del s i r e n t e e q u e l l o della c a t e n a del g r a n sasso verso c a m p o i m p e r a t o r e . n e l t i r r e n o m e r i d i o n a l e i t e r r e m o t i s t u d i a t i r i e n t r a n o i n v e c e n e l m o d e l l o c a r a t t e r i s t i c o delle f r a t t u r e . q u i p e r ò il f e n o m e n o p r e s e n t a u n interesse a sé, t r a t t a n d o s i di t e r r e m o t i di o r i g i n e p r o f o n d a . a p r o p o s i t o della p r o f o n d i t à i p o c e n t r a l e , va osservato che i terr e m o t i i n t e r e s s a n t i il sistema a l p i n o , in senso lato, r i g u a r d a n o esclus t r u t t u r a g e 0 l 0 g i c 0 s i s m 1 c a d e l l ' e u r o p a c e n t r o m e r i d i o n a l e 513 s i v a m e n t e la crosta t e r r e s t r e ; m o l t i f r a essi o r i g i n a n o a piccole prof o n d i t à i p o c e n t r a l i . di 17 t e r r e m o t i i n t e r e s s a n t i d e t t a zona, 7 h a n n o a v u t o p r o f o n d i t à i n f e r i o r i ai 10 k m , 6 i n f e r i o r i ai 20 k m ed 1 solo e b b e o r i g i n e alla b a s e della crosta t e r r e s t r e . i t e r r e m o t i d e l l ' a l t o a d r i a t i c o s e m b r a n o o r i g i n a r e d a m a g g i o r i p r o f o n d i t à : p r o b a b i l m e n t e ai l i m i t i della crosta t e r r e s t r e . le scosse che avvengono n e l l ' a m b i t o degli a p p e n n i n i o r i g i n a n o invece, q u a s i e s c l u s i v a m e n t e , a piccola p r o f o n d i t à , i n f e r i o r e ai 10 k m . mi s e m b r a di e s t r e m o i n t e r e s s e a q u e s t o r i g u a r d o , d a r e il d o v u t o r i s a l t o a n c h e a l l ' a l t r o p a r t i c o l a r e c h e d i s t i n g u e la sismicità d e l l ' a d r i a tico da q u e l l o della c a t e n a , ad esso p a r a l l e l a , degli a p p e n n i n i : l'opposto m e c c a n i s m o d a cui o r i g i n a n o i t e r r e m o t i n e l l e d u e z o n e cont i g u e : agli s p r o f o n d a m e n t i in p r o f o n d i t à d e l l ' a l t o a d r i a t i c o , f a n n o ris c o n t r o i s o l l e v a m e n t i s u p e r f i c i a l i della c a t e n a a p p e n n i n i c a . s e m b r a q u a s i c h e alle c o m p r e s s i o n i l a t e r a l i d e t e r m i n a t i d a i p r i m i c o n s e g u a n o i s o l l e v a m e n t i causati d a i secondi. a m e n o c h e l e q u a l i fiche di causa ed e f f e t t o n o n d e b b a n o essere i n v e r t i t e . a d ogni m o d o , s i a m o in p r e s e n z a di u n a s p e t t o g r a n d i o s o d e l l ' o r o g e n e s i a n c o r a in a t t o n e l l a r e g i o n e i n q u e s t i o n e , c o m e è c o n f e r m a t o d a m o l t i a l t r i indizi di c a r a t t e r e geologico. c o n s i d e r a z i o n i a p a r t e m e r i t à la sismicità del basso t i r r e n o . q u i m o l t i t e r r e m o t i o r i g i n a n o a g r a n d i p r o f o n d i t à : q u e l l o del 16 m a r z o 1941 ad u n a p r o f o n d i t à di circa 85 k m (1 5) e q u e l l o delle isole l i p a r i del 13 a p r i l e 1938 alla p r o f o n d i t à di 300 k m circa (1 6). la z o n a si p r e s e n t a n o t e v o l m e n t e a n o m a l a d a t u t t i i p u n t i di v i s t a : geologico, geodetico, sismico. a n o r d e a s u d della sicilia, o l t r e c h e sull'isola stessa, sono n u m e r o s e le t r a c c e di v u l c a n i s m o attivo o s p e n t o : l'isola d i ustica, d ' o r i g i n e v u l c a n i c a , il g r u p p o i n s u l a r e delle l i p a r i di r e c e n t e o r i g i n e v u l c a n i c a , con i d u e v u l c a n i attivi d e l l o s t r o m b o l i e dell'isola di v u l c a n o , u n v u l c a n o s p e n t o t r o v a s i a sciacca, p a n t e l l e r i a è d ' o r i g i n e v u l c a n i c a : t u t t a la zona i n s o m m a , senza c i t a r e i v u l c a n i m a g g i o r i , è stata spesso t e a t r o d i e r u z i o n i v u l c a n i c h e sottom a r i n e , r i v e l a t e da t e r r e m o t i e da m a r e m o t i . a b b i a m o a c c e n n a t o alla g r a n d e a n o m a l i a sulla p r o p a g a z i o n e d e l l e o n d e s i s m i c h e p r o v o c a t e dai t e r r e m o t i p r o f o n d i citati e r i v e l a t a d a l n o t e v o l e a n t i c i p o n e l l a r e g i s t r a z i o n e d e l l e o n d e pn ed sn n e g l i osserv a t o r i vicini alla zona e p i c e n t r a l e . p o i c h é le o n d e trasversali d a n n o d e b o l i r e g i s t r a z i o n i n e l l a zona, si è p o r t a t i a r i t e n e r e c h e l e m a g g i o r i velocità n o n siano da a t t r i b u i r e 514 p i e t r o caldi ad u n a m a g g i o r e r i g i d i t à del m e z z o , b e n s ì ad u n d i f e t t o di d e n s i t à da collegarsi alle e n o r m i r i s e r v e di m a t e r i a l e v u l c a n i c o . u n c o n t r i b u t o in q u e s t o senso p u ò d a r e p u r e la sottigliezza dello s t r a t o s u p e r f i c i a l e del « g r a n i t o » c b e , n e l l a z o n a , s e m b r a p r e s s o c h é m a n c a n t e . s t r e t t a m e n t e collegata alla n o t e v o l e a n o m a l i a sismica c o n s i d e r a t a sta il f a t t o c b e la r e g i o n e i n p a r o l a è s e d e di u n a cospicua a n o m a l i a g r a v i m e t r i c a p o s i t i v a . roma —• istituto nazionale eli geofisica — ottobre 1952. riassunto si riassumono i risultati principali ottenuti daricercatori italiani, negli ultimi quindici anni, nello studio delle caratteristiche geologichesismiche della penisola italiana, del mediterraneo centrale e di una vasta zona comprendente la svizzera settentrionale e il wurttemberg, nonché di zone marginali atlantiche. i. — lo spessore dello strato delle onde p g per le alpi orientali, meridionali (consiglio e prealpi comiche) è risultato di circa 35 km. ii. — lo spessore dello strato delle p * , per la stessa zona, è dell'ordine di 10-15 km. iii. — le alpi presentano « radici »: nella porte centrale del sistema alpino, tali radici sembrano raggiungere i 40 km. iv. — in corrispondenza dello val padana lo spessore del « granito » sembra diminuire sensibilmente. v. — lo strato delle pg torna od ispessirsi in corrispondenza dell'appennino. vi. — anche il sistema degli appennini presenta « radici ». lo spessore di tale sistema, di circa 25-30 km in corrispondenza ilei suo asse centrale, tende ad assottigliarsi verso l'italia meridionale: in corrispondenza della sicilia sembra assai sottile. vii. — lo spessore medio dello strato del « granito » per il mediterraneo centro-occidentale risulta di 15 km circa. vili. — in una vasta zona. dell'europa centrale, comprendente la svizzera settentrionale e il wurttemberg, lo strato del « granito » presenta uno spessore di soli 20 km, come risulta dallo studio di 13 terremoti interessanti la zona, compiuto sul 1943. tale spessore tende ad aumentare procedendo verso il sistema alpino. ix. — nella stessa zona, di cui al n. vili, la velocità delle onde longitudinali e trasversali dirette segue le seguenti leggi: s t r u t t u r a g e 0 l 0 g i c 0 s i s m 1 c a d e l l ' e u r o p a c e n t r o m e r i d i o n a l e 515 f p g = 5,530 (1 + 0,00158 . li) ; vsg = 3,275 ( 1 + 0,00158 . li} , per li variabile da 0 a 20 km. x. — 'la velocità di propagazione delle onde p g nell'italia peninsulare risulta, in media, di 5,4 km/sec; alquanto inferiore a quella osservata per l'europa centrale di 5,7 km/sec in media. xi. — per ciò che riguarda la profondità ipocentrale, questa presenta. in genere piccoli valori nel sistema alpino, dove, per altro, si possono verificare terremoti con profondità fino a 30 km circa. nell'appennino, le profondità ipocentrali sono quasi esclusivamente comprese fra 0 e 10 km: molto piccole quindi. nel tirreno meridionale, invece, oltre ai terremoti d'origine vulcanica con ipocentro prossimo alla superficie, si verificano terremoti di notevole profondità ipocentrale, variabile fra 100 e 300 km. xii. — un interesse particolare presenta la natura fisica delle scosse all'ipocentro. si distinguono —a questo riguardo — tre zone: la zona delle alpi, dove la distribuzione superficiale delle compressioni e delle dilatazioni è, generalmente, quella propria delle f r a t t u r e , a zona del medio adriatico, caratterizzata da violente scosse dovute a sprofondamento di strati e la zona degli appennini, dove si verificano, quasi esclusivamente terremoti presentanti il modello proprio dei sollevamenti. le zone dell'adriatico e della catena degli appennini, che corrono geograficamente parallele l'una all'altra, sembrano quindi animate, in profondità, da movimenti in senso contrario. sarà interessante stabilire se esistono, tra questi movimenti, rapporti di causa ad effetto. xiii. — la zona del basso tirreno è caratterizzata da scosse del tipo a frattura. essa è f o r t e m e n t e a n o m a l a sia per il vulcanismo in atto, sia per i terremoti profondi di cui è sede, sia per il notevole anticipo con cui le onde longitudinali e trasversali — destate da questi terremoti — vengono registrate nelle stazioni sismiche circonvicine: queste anormalità trovano pure riscontro in lina notevole anomalia gravimetrica positiva. xiv. — i risultati sopra riassunti non hanno naturalmente carattere definitivo: essi possono subire modifiche più o meno profonde con l'affinarsi dei metodi di ricerca. 516 p i e t r o caldi svmmary we summarize here the principal results obtained by italian researchers in the last 15 years from the study of the geologie and seismic characteristics of the italian peninsula, covering the centrai mediterranean and a vast zone which includes northern switzerland and wurttemberg, and also border zones of the atlantic. i. — the thickness of the layer of the p g waves for the eastern alps and for the southern alps (consiglio and prealpi comiche) is about 35 km. ii. — the thickness of the layer of the p * waves for the same zone is on the order of 10-15 km. iii. — the alps have « roots»: in the centrai part of the alpine system such roots seem to reach to about 40 km. iv. — in comparison with the val padana the thickness of the « granite » seems sensibly less. v. — the layer of p g becomes again thick under the apennines. vi. — also the apennines system has « roots ». the thickness of this system of mountains, on the order of 25-30 km in comparison ivith its centrai axis, tends to diminish toward southern italy: in comjyarison with sicily it seems rather tliin. vii. — the mean thickness of the layer of granite for the centralwestern mediterranean is about 15 km. vili. — in a large zone of centrai europe, including nortliern switzerland and wurttemberg, the layer of « granite » has a thickness of only 20 km, as computed from the study of 13 earthquakes pertaining to the zone. in 1943. this thickness tends to increase in the direction of the alpine system. ix. — in the same zone (that of vili) the velocities of the direct longitudinal and transverse ivaves folloiv the following law: vpi = 5,530 (1 + 0,00158 . h) ; vsq = 3,275 ( 1 f 0,00158 . li) , for h variable from 0 to 20 km. x. — the velocity of propagation of the p g ivaves in peninsular italy is, on the average, 5,4 km/sec; rather less than that observed for centrai europe of about 5,7 km/sec. xi. — as regards the hypocentral depth one has, in general, small values in the alpine system where, nevertheless, one can ascertain earthquakes with depths up to about 30 km. in the apennines the s t r u t t u r a g e 0 l 0 g i c 0 s i s m 1 c a d e l l ' e u r o p a c e n t r o m e r i d i o n a l e 5 1 7 hypocentral clepths are almost exclusively in the range from 0 to 10 km, thus very small. in the southern tirreno instead, in addition to earthquakes of volcanic origin with the hypocenter close to the surface, one finds earthquakes of notable hypocentral depth, variable from 100 to 300 km. xii. — of particular interest is the physical nature of the shocks at the hypocenter. for this purpose one distinguishes three zones: the alpine zone, where the surface distribution of the compressions and dilatations is generally that properly of the fractures; the centrai adriatic zone characterized by violent shocks due to sinking of the layers; and the zone of the apennines where one finds almost exclusively model earthquakes of the raising kind. the zones of the adratic sea a n d the apennines chain, which run geographically parallel to each other, seem to be brought about, as regards the depth, by opposite movements. it would be interesting to establish if there exists a relation of cause to effect between these movements. xiii. — the zone at the base of the tirreno is characterized by shocks of the fracture type. it is slrongly a n o m a l o u s either because of volcanic activity, or because of the deep earthquakes of which it is the seat, or because of the notable advance with which the longitudinal and transverse waves, set in motion by these earthquakes, are registered in the nearby surrounding seismological stations. this anomalous behavior even finds confirmation in a notable positive gravimetrie anomaly. xiv. — the results summarized above are naturally not of a definite character and they can be modified more or less seriously with refinements in the metliods of research. b i b l i o g r a f i a ( ! ) c a l d i p., ricerche su terremoti ad origine vicina. scosse del consiglio dell'ottobre 1936-xvi. la r i c e r c a s c i e n t i f i c a , ix, 7-8 (1938). ( 2 ) c a l d i attività sismica in italia nel decennio 1930-1939. le m o n n i e r , fir e n z e (1942). ( 3 ) c a l o i p., il terremoto adriatico del 30 novembre 1934. b o l l . s o c . s i s m o l . i t „ xxxv (1937). ( 4 ) caloi p_, epicentro e profondità ipocentrale del terremoto di salò del 24 gennaio 1943. i s t i t u t o v e n e t o di soc., lett. e d art, civ (1944-45). 518 p i e t r o c a l d i ( 5 ) c a l o i p . , il terremoto dell'appennino tosco-romagnolo dell'il febbraio 1939-xvii. la r i c e r c a s c i e n t i f i c a , x (1939). ( ( 1 ) c a l o i p . , caratteristiche sismiche dell'appennino tosco-romagnolo. la ricerca s c i e n t i f i c a , xi (1940). ( 7 ) r o s i n i e . , il terremoto della garjagnana del 15 ottobre 1939. la r i c e r c a s c i e n t i f i c a , xi (1940). ( s ) d i f i l i p p o d , il terremoto del monte amiata del 19 giugno 1940. la ricerca s c i e n t i f i c a , xii (1941). ( ° ) g i o r g i 1 1 , il terremoto del monte amiata del 16 ottobre 1940. la ricerca s c i e n t i f i c a , xii (1941). ( 1 0 ) d ì f i l i p p o d , il terremoto di cervara di roma dell'8 settembre 1941. b o l l . s o c . s i s m o l . it., x l (1942). ( n ) d i f i l i p p o d , il terremoto di teramo del 29 gennaio 1943. a n n a l i «li g e o f i s i c a , ii (1949). ( 1 2 ) d i f i l i p p o d . • m a r c e l l i l . , uno studio del terremoto del gran sasso d'italia del 5 settembre 1950. a n n a l i di g e o f i s i c a , i v (1951). ( 1 3 ) v a l l e p . e . , contributo allo studio delle caratteristiche sismiche del mediterraneo centro-orientale. a n n a l i di g e o f i s i c a , i (1948). ( 1 4 ) f e s t a c . v a l l e p . e . , una valutazione dello spessose dello strato del « granito » nel mediterraneo centro-occidentale. a n n a l i di g e o f i s i c a , i (1948). ( l r ' i d i f i l i p p o d . , studio microsismico del terremoto del basso tirreno del 16 marzo 1941. b o l l . s o c . s i s m o l . it., xxxix (1941). ( 1 0 ) c a l o i p . g i o r g i m , studio del terremoto delle isole lipari del 13 aprile 1938. a n n a l i di g e o f i s i c a , i v (1951). ( 1 7 ) c a l o i p , caratteristiche sismiche fondamentali dell'europa centrale. b o l l . s o c . s i s m o l . i t , xl (1942). ( l s ) p e r o n a c i f „ determinazione del valore medio dello spessore dello strato di granito per i tragiiti turkestan-europa. a n n a l i di g e o f i s i c a , i i i (1950). ( 1 b ) m a r c e l l i l . p a n n o c c h i a g , terremoto della cresta mediana atlantica del 24 aprile 1947. a n n a l i di g e o f i s i c a , i, 4 (1948). ( 2 0 ) c a l o i p . m a r c e l l i l. p a n n o c c h i a g , sulla velocità di proimgazione delle onde superficiali in corrispondenza dell'atlantico. a n n a l i di g e o f i s i c a , i i , 3 ( 1949). — i d , ancora sulla velocità di propagazione delle onde superficiali in corrispondenza dell'atlantico. a n n a l i di g e o f i s i c a , iii, 2 (1950). ( 2 1 ) v a i l e p . e , sulla struttura della crosta terrestre nel mediterraneo centrooccidentale. a n n a l i di g e o f i s i c a , i v (1951). ( 2 2 ) c a i . o i p , sidla velocità di propagazione delle onde p* e sullo spessore dello strato del granito nell'europa centrale. la r i c e r c a s c i e n t i f i c a , xi, 11 (1940). singh 8.0_revfc_layout 6 annals of geophysics, 57, 6, 2014, a0657; doi:10.4401/ag-6596 a0657 shift of effective lightning areas during pre to post period of solar cycle minimum of 2008-2009 as determined from schumann resonance studies at agra, india birbal singh*, devbrat pundhir rbs engineering technical campus, department of electronics and communication engineering, bichpuri, agra, india abstract employing a set of 3-component search coil magnetometer, schumann resonance studies have been in progress at agra (geograph. lat. 27.2°n, long. 78°e), india since 01 april, 2007. we have analysed the data for two periods; first from 01 april, 2007 to 31 march, 2008 (period-i), and then from 01 march, 2011 to 29 february, 2012 (period-ii) which correspond to pre and post periods of solar cycle minimum of 2008-2009. from the diurnal variation of first mode intensity and frequency, we study the seasonal variations of global thunderstorm activity, effective source distance and level of lightning during both the periods. we show that world thunderstorm activity shifts to summer in the northern hemisphere as the effective source distance approaches close to the observer, and the level of intense lightning shifts from the month of july, 2007 in period-i to august, 2011 in period-ii. this is supported by lightning imaging sensor (lis) satellite data also. a possible explanation in terms of increasing solar activity is suggested. 1. introduction the phenomenon of schumann resonance (sr) occurring in the earth-ionosphere cavity as a result of resonance between direct and round the globe propagation of extremely low frequency (elf) waves radiated from lightning discharges was first predicted by schumann [1952] and experimentally verified by balser and wagner [1960]. the sr appears in the form of standing waves at the frequencies of 8, 14, 20, 26... hz which are known as sr modes. owing to its widespread applications in the studies of global thunderstorm activities, ground surface temperature, lower region of the ionosphere, and forecast of monsoon etc. extensive research work has been done in this field and a detailed description of the early work is presented in an excellent monopgraph by nickolaenko and hayakawa [2002]. some very interesting recent results which have come out of the extensive morphological and varying geophysical condition studies have been presented by many workers. for example, price and melnikov [2004] have studied the diurnal, seasonal, and inter-annual variations in the sr phenomenon and have found three dominant maxima in the diurnal cycle related to lightning activity in south-east asia, africa, and south america. further, the largest global lightning activity occurs during the northern hemisphere summer ( jja), with the southern hemisphere summer (djf) having the least lightning around the globe. greenberg and price [2007] have compared the elf data on the ground with the optical transient detector (otd) data on satellite and found discrepancies in lightning activities between the asian and south american centers. to understand the relative strength of the different source regions they calculated the attenuation rate for the electromagnetic waves in sr band in both the northsouth and east-west direction reaching their station. they found that the simulated magnetic field relative amplitude based on otd observations showed a peak at 2000 ut corresponding to american source 40% larger than african and asian sources, whereas in the east-west direction the african peak at 1400 ut was 80% larger than the american peak and asian peak was not visible. hayakawa et al. [2005] and nickolaenko et al. [2006] have reported anomalies in sr bands as they found increase in the intensity of fourth harmonic due to earthquake which were interpreted in terms of superposition of direct elf signals from a distant thunderstorm source in america with those scattered from a conducting disturbance in atmosphere over the epicenter of earthquake. williams and satori [2007] have studied the solar radiaarticle history received may 30, 2014; accepted december 26, 2014. subject classification: schumann resonance, thunderstorm activity, effective lightning areas. tion-induced changes in ionospheric height and sr waveguide on different time scales. they find that, in general, order of magnitude changes in radiation are needed to cause relative changes in ionospheric height as large as 10% as in the case on both the diurnal and 11-year time scales. on day-night asymmetry of the earth-ionosphere cavity and solar terminator effects nickolaenko and hayakawa [2002] concluded theoretically that the amplitude variations to be expected at the sunrise/sunset termination line are practically undetectable. figure 7 of nickolaenko et al [2006] demonstrates that the lens-like “terminator” pattern is pertinent to the global thunderstorm activity itself. this is why a uniform earth-ionosphere cavity provides the “terminator effect” similar to observations (in the ut) at an orbitary observatory. the confusion with the ionosphere local height occurred due to closeness of the start-stop times of the planetary lightning activity with the sunrise and sunset moments at the middle east and central europe. similar inference was drawn also by penchony et al. [2007] who observed that the sr field variations are governed primarily by the variations in source intensity and source-receiver geometry and that the effect of the day-night asymmetry in the ionosphere is secondary. in contrast to the above, melnikov et al. [2004] presented observational evidence for the sunrise/sunset terminator effect based on multistation sr observations with hourly time resolution and found steep increase/decrease of sr amplitude during terminator times. a similar result was found also by ondraskova et al. [2007] at modra observatory, slovakia for the first four modes of electric field component. the later results were supported also by the observations of satori et al. [2007a] who found jump-like increase of sr amplitude between the local ionosphere and surface sunrise times and sharp decreases between the local surface and ionospheric sunset times. the sr phenomenon is due to contribution from both the global and local thunderstorm activities. in order to separate the local contribution from the global one sentman and fraser [1991] have suggested a technique which has been applied by many workers [pechony and price 2006, nickolaenko and hayakawa 2008, nickolaenko et al 2011]. it is shown that local modulation function depends primarily on source-receiver distance geometry and it can be separated with the technique suggested by sentman and fraser [1991]. ondraskova et al. [2009, 2011] have studied the decrease of sr frequencies and changes in effective lightning areas toward the solar cycle minimum of 2008-2009 using the data from modra observatory slovakia. the extraordinary fall in the first mode frequency was interpreted in the light of the suggestion made by satori et al [2005] that the reduction in x-ray radiation causing decrease in the ionospheric conductivity may be attributed responsible for the observed effect. they further calculated the effective thunderstorm areas from the monthly mean diurnal frequency range (dfr) of the electric field component. they found that the difference in the northern and southern hemisphere summer areas not only declined with the decreases in solar activity but almost vanished during the deep solar minimum of 2008-2009. semi-annual variation in the areas dominates in the years of the deep solar minimum. in our recent paper [tyagi et al. 2013, hereafter paper i] we have studied the characteristics of global thunderstorm activities extracted from sr data for the period of 01 april, 2007-31 march, 2008. in the present paper we extend some of the studies made earlier in paper i especially those related to variation of global thunderstorm activities and compare the results. then, we consider the first mode frequency and intensity of the magnetic field data to study the effective source distance and level of lightning for both the periods 01 april, 2007-31 march, 2008 (period-i) and 01 march, 2011-29 february, 2012 (period-ii) which are pre and post periods of solar cycle minimum of 2008-2009 and compare the results. 2. experimental set up and method of data processing the details of experimental set up and method of data processing are not available in paper i. however, these details are available in our another paper [singh et al. 2014]. here, we mention the details briefly as follows;we are using a 3-component search coil magnetometer (lemi-30) which has been procured from lviv center of institute of space research, ukraine. the search coils are buried under-ground in a relatively noise free area in agriculture fields of the bichpuri campus of r.b.s. college, agra which is located 12 km west of agra city in rural area. the three sensors are oriented in geographical north-south (x-component), east-west (y-component), and vertical (z-component) directions and each of them has following specifications; the lemi-30 system has sampling rate of 256 hz and sends these samples to dedicated pc. the lemi-30i softsingh et al. 2 dimension : 870 mm (length), 85 mm (diameter) frequency range : 0.001-30 hz measuring range : ± 200 nt transformation factor 0.001-1hz : 20×f mv/nt 1-30 hz : 20 mv/nt auxiliary output gain : 20 db magnetic noise level at 0.01-10 hz : ≤ 20pt/hz1/2-≤0.04 pt/hz1/2 mains interference rejection : > 60 db 3 ware in the pc takes average of each four samples simply by summing and dividing so that a binary file containing data at the sampling rate of 64 hz is stored in the pc. the recorded data on pc in amplitude-time may be seen in frequency-time (dynamic spectra) by performing spectral analysis using fft available in matlab with 1024 words of data length (temporal resolution=16 sec, frequency resolution=0.06 hz) at a time. the power spectral density (psd) of the input signal is evaluated using welch spectral technique [welch 1967] which uses averaged modified periodograms. the psd are prepared for each one hour data (230400 data points) and a hamming window of 1024 data points with sliding of half the window is used to compute the modified periodogram of each segment. in this method the spectrum obtained per hour is the average of 450 spectra with 1024 data points. the welch method is closely related to the method of complex demodulation described by bingham et al [1967] and hence a separate method to extract sr modal frequencies in the analysis of data is not applied. an example of dynamic spectrum and corresponding psd obtained from the data recorded by the x-component of the sensors on 18 february, 2012 is presented in figure 1a and 1b respectively. the psd shows amplitude in db which is converted in intensity (in nt2/hz) and the results corresponding to figure 1b is shown in figure 1c. in figure 1a the time is shown in ut which is related with local time lt=ut+5.5hrs. here, it may be noted that figure 1a resembles in many respect with figure 1a of paper i especially the distribution of spikes in the coloured figure. however, these two figures are different because they were observed on different days, and the differences are indicated by psds also. since the time of measurement is same during which a passenger train passes every day, similar spikes appear in both the figures. the detailed examination of the quality of the sr data shows that due to local noises including the movement of trains on a railway track (about 300 m away in east-west direction) the x-component is affected least, the y-component moderately, but the zcomponent severely. hence, we use x-component data mostly in our analysis. however, we use y-component data also wherever necessary by applying careful selection of the data. in order to further process the data we write a program in matlab to extract hourly information about the peak frequencies and intensities (determined by the program itself ) of three modes and arrange them in separate columns in excel. in this way, we prepare a time series for the frequencies and intensities of the sr data for 24 hours each day and then for 12 months from 01 march, 2011 to 29 february, 2012. this time series is used to deduce various characteristics of sr phenomenon observed at agra station. 3. lightning imaging sensor (lis) data while electromagnetic radiations emitted by lightning form an important tool for the study of global lightning, another very useful advanced space based tool is the optical transient detector (otd) which allows us to understand the spatial and temporal distribution of lightning around the globe, christian et al. [2003] have given a detailed description of instruments and their characteristics and measurements used to construct lightning climatology maps demonstrating the geographical and seasonal distribution of lightning activity for the globe. however, since otd satellite stopped working prior to the beginning of measurements at agra, we have used data for this paper from lightning image sensor (lis) available on the website: http://thunder.nsstc.nasa.gov/data/query/distribution/html collected on tropical rainfall measurement mission satellite (trmm) with the following coverage; the data have been used for both the periods of 01 april, 2007 to march 31, 2008 and 01 march, 2011 to 29 february, 2012. it may be noted here that both the electromagnetic (sr) and otd/lis measurements suffer shift of effective lightning areas figure 1. (a) frequency-time spectrogram of the ulf/elf data recorded on 18 february, 2012 at agra showing four schumann resonance lines (b) power spectrum density (psd) of the data in figure 1a and (c) corresponding intensity-frequency variation. location : sub-tropical latitude range : ± 40° longitude range : ± 180° spatial resolution : 3-6 km temporal resolution : orbit due to some drawbacks. the problem with sr measurements lie in a too high global rate of strokes of about 100 events per second. with the basic 8 hz frequency of sr we have about 12 independent pulses arriving at an observer during the single period of this frequency. it is obvious that signals will overlap severely making impossible their separate processing. the main problem in otd observations is that an observer does not know what is going on outside the frame of optical device at any particular time. furthermore, there are ‘invisible’ strokes, i.e. the discharges whose optical emissions do not reach the top of the cloud [nickolaenko and hayakawa 2002]. in contrast to the above, a space sensor capable of mapping both inter cloud and cloud-tocloud lightning discharges from geostationary orbit during day and night with a spatial resolution of 10 km and a detection efficiency of 90% has been developed. in addition, this sensor, which is called the lightning mapping sensor (lms) can monitor storms on a continual basis. the combination of modern solid state mosaic focal planes with extensive on-board signal processing in the lms provides a powerful technique for the detection of weak background-contaminated signals and permits the detection of lightning during the day [christian et al. 1989]. such data could be very helpful in defining the source diameter of prominent lightning regions. however, the capabilities of lightning detection from geostationary orbit are limited by several factors which include visible area limited to a nominal field of view, resolution depends on geographical position and drops with distance from nadir, earth observation is interrupted and detection efficiency reduces if a spinning satellite is used, light scattering in clouds limits the location to 8-10 km etc. [finke and hauf 2002]. hence, we have employed the lis data only in this paper. as far as sr observations are concerned we have mentioned in section 2 how movements of trains on nearby tracks produce spikes in our data whose intensities are not as significant as to appear in the psds of the sr data. especially, the use of north-south (x-component) component of the data permits us to process the sr characteristics without much problem. in general, the x-components contain more power than y-components due to the fact that energies generated by lightning from nearby thunderstorm center (asian source) and those from american source transmitted along east-west direction are higher than those transmitted in the north-south direction. the raw temporal data show sporadic spikes in amplitude due to passage of trains but no saturation effect is observed. the noises arising from other sources (electrical and atmospheric noises) are sometimes serious and do affect the quality of data. however, they are occasional and due precaution is taken while processing the data. 4. observational results 4.1 monthly distribution of thunderstorm activities in paper i, we have made a correlative study between the first mode sr intensity observed at agra and lightning imaging sensor (lis) data for the period 01 april, 2007-31 march, 2008 (period i). here, we make a similar study for the period 01 march, 2011-29 february, 2012 (period ii). both the results are shown in figure2a,b. the dark histograms show the lis data whereas the light dark histograms show the sr intensity. the intensity variations shown in both parts of the figure are integrated intensity of the first three modes because a modal sr intensity depends not only on the activities of the source but also on source-observer geometries. from a general scanning of the data we find that the intensity of the second and third modes constitutes approximately 40 percent of the total intensity of the three modes. the consideration of integrated intensity may effectively neutralize the geometrical factors. it may be noted from figure 2 that peak thunderstorm activity occurred in the month of july during period i whereas the same occurred in the month of august during period ii.the correlation coefficients between the sr intensity and lis data for the two periods are 0.81 and 0.70 respectively. in section 3, we have indicated the short falls in the measurements of both the sr and otd data which may be responsible for the low correlation coefficient. the correlation coefficients are low but not poor possibly because both the sr and lis data are influenced more by south-asian thunderstorm center (which is just 3000 km from the observing stations) in spite of the fact that both the measurements respond to global phenomena. further, looking at the two panels of figure 2 we find that the lis data in the upper panel corresponding to period i are 3-5 times larger than those in the lower panel corresponding to period ii, whereas the magnitude of sr intensities during the two periods are opposite to each other i.e. the intensities are 2-3 times larger in period ii than these in period i. these anomalies in sr intensity and lis data will be discussed later. 4.2 effective source distance and level of lightening the dependence of frequency range, i.e. the maximum value minus its minimum value (df1=f1 max– f1 min) has been used for establishing the effective size of zone occupied by the world wide thunderstorm activity from the sr records [nickolaenko and rabinovicz 1995, nickolaenko et al. 1998]. the diurnal variation in df1 is related to movement of the lightening region along the equator from one continent to another around the globe. the physical background is based on the discontinuity of f1 when the point source crosses the nodal singh et al. 4 5 line. when the source becomes wider the discontinuity turns into the abrupt transition from small to great frequency values. the variation in df1 vanish when the lightening sources uniformly cover the globe. a detailed description of these results are given in section 4.1.5 and 4.2.2 of nickolaenko and hayakawa [2002]. here, it may be emphasized that df1 is connetcted with the source size only when measured in the ez field component. it reflects the daily alternations of the source distance when measured in the horizontal magnetic field. however, the above results are based on measurement of vertical electric field component and one has to measure the peak frequency in the <|e(f )|2> spectra to evaluate the source width [nickolaenko and hayakawa 2002, satori et al. 2009, ondraskova et al. 2011]. the reason for why electric field measurement is more important than magnetic field measurement for the above purpose may be understood from figure 3 in which power spectra for both e and h fields in the vicinity of the first mode over the distance-frequency plane is presented (the point source is considered). this figure has been received with the courtesy of prof. a.p. nickolaenko. although the figure is in black and white the results are clearly evident. the left frame shows the power spectra of the vertical electrical field ez, and the right frame shows the dynamic spectrum of the horizontal magnetic field hφ. the obvious distinction of these spectra is the node around the 10 mm distance in the ez-field. the horizontal magnetic field component has the “tilted” maximum here. the tilt is conditioned by the influence of higher resonance modes, and the same influence makes the nodal line asymmetric. as a result, the f1 frequency in the ez power spectrum has the discontinuity at d=10 mm. since we do not have electric filed measurement setup at agra station we do not use df1 to evaluate source width during shift of effective lightning areas figure 2. (a) correlation between the global distribution of thunderstorm activities deduced from sr data at agra for the period of 01 april, 2007 31 march, 2008 with lightning image sensor (lis) data on the satellite (b) the same as above but for the period of 01 march, 2011 29 february, 2012. the two periods under consideration using magnetic field data. however, some of the parameters of sr magnetic field data such as f1 and intensity i1 can be used to derive effective source distance (from f1) and, after words, the effective level of lightning activity (from i1, and effective source distance d). for this purpose we follow the equation [nickolaenko and hayakawa 2002], f1(d)=ao+a1d+a2d 2+a3d 3+……………………...........(1) here d is the source-observer distance in mm, ao= 6.71336, a1=0.181732, a2=-0.008612582, a3=-0.0001477123. this equation can be used to evaluate peak frequency from the known source distance. if we rotate the coordinate axes of the graph obtained from above and plot the same data in the form d(f1) the polynomial fit provides [see nickolaenko 2014] d(f1)=bo+b1f1+b2f1 2+b3f1 3+b4f1 4 ……………………(2) where bo=207049, b1=-110796, b2=22229.7, b3=-1982.03, b4=66.2696. we can use this equation to evaluate the source distance from the experimental peak frequency. since the real source width is non-zero, this distance should be regarded as effective one. the results of these calculations are presented in graphical forms for the two periods in figure. 4a to 4c. in the two panels of figure 4a are shown the seasonal variation of first mode sr frequency of the north-south component (xcomponent) of the horizontal magnetic field and corresponding source-observer distance derived from equation (2) for period-i (upper panel) and period-ii (lower panel) respectively. the range of frequencies shown on the ordinate of the panel of period-i is higher than that on the ordinate of panel-ii. an explanations for the decrease in the first mode frequency in periodii and increasing trend with the months will be given in discussion section. here, we see that in both the periods the summer thunderstorms shift towards the northern hemisphere (nh) summer (may-august) as the minimum source-observer distances reach around 12.4 mm and 9 mm respectively. annual drifts of these sources in the two periods are approximately 4 mm and singh et al. 6 figure 4(a). seasonal variation of first mode sr frequency of the north-south component (xcomponent) of the horizontal magnetic field for period-i (upper panel) and period-ii (lower panel). the ordinate on the right shows the corresponding source-observer distance. figure 4(b). seasonal variation of source-observer distance derived from observed first mode frequency for the two periods. figure 3. power spectra of the electric filed e (left) and magnetic field h (right) in the vicinity of the first mode sr over the distance-frequency plane (received with the courtesy of prof. a.p. nickolaenko).. 7 1.3 mm corresponding to 36° and 12° respectively. the seasonal variations of the effective source-observer distance derived from the first mode sr frequency for the two periods are separately presented in figure 4b. it should be noted that the obtained estimates of the distances to the source in the two periods are more than 10 mm (for period-i) and less than 10 mm (for periodii) respectively. clearly visible are the seasonal drift of world thunderstorms to the nh summer. satori [2003] has carried out long-term monitoring of the ez component of the sr parameters (peak amplitude and peak frequencies) and provides very useful information on seasonal motion of the global lightning activity [see also hayakawa et al 2010]. the variations of average intensity of the first mode sr corresponding to north-south components for the two periods are shown in the upper and lower panels of figure 4c. the intensity data are the same as shown in figure 2. however, they are shown here to be examined in the light of the variation of source-observer distances presented in figure 4b. here, we see that the intensities are higher in the summer months (may-august) when the source-observer distances are lowest indicating that nearer the sources larger are the intensities. an interesting point to be noted here is that the level of maximum intensity corresponding to intense lightning exists in the months of july in period-i whereas the same exists in the months of august in period-ii. these months of largest sr intensities are supported by lis data also (see figure 2). hence, we can say that there is a shift of maximum lightening areas from july in period-i to august in period-ii. there is an increase in the intensities in the month of october in period-i also but it is not supported by lis data which are less than those in the month of july. the intensities fall in the southern hemisphere (sh) summer (novemberjanuary) because the thunderstorms move away from the observer as shown by the increasing trend in source-observer distance. 5. discussion as per nasa report the solar cycle minimum of 2008-2009 was extraordinarily deep and the sun was quietest ever seen in almost a century. the x-ray radiation, which ionizes the earth’s atmosphere and is responsible for the conductivity profile of the ionosphere, was by an order of magnitude lower than the solar cycle minimum studied previously [onraskova et al. 2011]. in figure 5 we show the variation of total number of m and x classes of flares occurred between the years 2006 and 2012. these flares are shown by hatched and open portions of histograms respectively. as it may be seen from the figure, the number of the two classes of flares decreases towards the solar cycle minimum such that there is no flare of either class in 2009. then the number increases as the solar cycle increases. the solar flare data are obtained from noaa national geophysical data center on the website: www.ngdc.noaa.gov. it is well known that according to classical schumann formula, the eigen-frequencies are given by the expression fn=7.5 √n (n+1) which are 10.6, 18.4, 26… hz for a spherical earth-ionosphere cavity. however, the real (experimental) values are 8, 14, 20… hz. the deviations are caused by the influence of the finite conductivity of the ionosphere [nickolaenko and hayakawa 2002]. from figure 1 it may be seen that peak frequencies of the different modes do not coincide with their corresponding customary frequencies but are lower. for example, the fundamental mode looks to be near 7.5 hz (instead of closer to 8 hz), the third at 19 hz rather than 20 hz, and the fourth at 25 hz (instead of 26 hz). a possible reason for the low frequencies seems to be the fact that since the period of observation corresponds to the increasing phase of solar cycle minimum of 2008-2009, the effect of deep minimum activity still continues shift of effective lightning areas figure 4(c). seasonal variation of first mode intensity for the two periods. figure 5. variation of m and x classes of solar flares between the years 2006 and 2012. under which the modal frequencies are influenced. it may be mentioned here that ondraskova et al [2011] have reported similar decrease in the frequencies of all the four modes from their observations at modra observatory from solar cycle maximum of 2001-2002 to solar cycle minimum of 2008-2009. satori et al [2005] have also reported a similar decrease in the frequencies. from figure 4a we see that the peak frequencies of the first sr mode are much lower in period-ii than those in period-i. the peak frequencies are lower than that of customary frequency of 8 hz in period-i also. reductions are also seen in peak frequencies of sr data obtained at shillong station in india for the months of july and august 2007 and march and may, 2008 (data obtained with the courtesy of dr. b.m. pathan, indian institute of geomagnetism, mumbai, may, 2010). recently, ondraskova et al. [2009, 2011] have reported a significant decrease of about 0.30 hz in the fundamental sr frequency from observations at modra observatory in slovak republic during the latest solar cycle having minimum around 2008-2009. this extraordinary fall of the fundamental mode frequency is attributed to the unprecedented drop in the ionizing radiation in x-ray frequency band, although the patterns of the daily and seasonal variation remain unchanged. similar decreases in the first mode sr frequency have also been reported by satori et al. [2005] for the previous 11 year solar cycle. further decreases in frequency in period-ii but having an increasing trend with the month may be interpreted in terms of the minimum solar cycle effect and increasing period of current solar cycle (2009-2020). according to satori et al. [2005], changes of the x-ray radiation dominate the variations in the conductivity profile within the upper characteristic layer (90-100 km portion of e-region). the decrease of this conductivity by up to one order of magnitude over the solar cycle is responsible for the observed sr frequency decrease by several tenths of hz. although, there is a controversy that this is the intermediate region where electron conductivity is substituted by the ionic one. therefore knee models of the conductivity profiles are used while modelling sr [e.g. mustak and williams 2002]. in general two possibilities have been suggested for the reduction in frequencies of the first mode; (i) variation in median distance between the source and the observer, and (ii) ionospheirc modification. peak frequency changes to the extent of 10-20 percent have been observed which are attributed to the uncertainties arising from spatial distribution of lightning sources exciting the sr modes [de 2007]. de et al [2009] have also observed large diurnal variation in the peak frequency of the first mode and found similar variation in the data obtained at moshiri in japan. model calculations have shown that a uniform decrease in the reference ionospheirc height reduces the resonant frequencies and q factors (i.e. it simultaneously increases the wave attenuation). specifically, it is found that a shift from 8 to 7.6 hz in the peak frequency of the first mode is caused by a global reduction of ionospheirc height by approximately 5 km. however, such reduction will be observed in all the modes, and the effect will be observed globally. since measurements differ from place to place it is concluded that the deviations observed experimentally are conditioned by the displacement of the global thunderstorms rather than by modification of the global ionospheirc profiles [nickolaenko and hayakawa 2002]. the long-term observations of sr carried out by ondraskova et al [2007] have also confirmed that variation in peak frequency of the lower sr modes can be attributed mainly to the source-observer distance effect. the intensity variation during the two periods (see figure 2) is influenced by solar activities. since the period i corresponds to close to solar cycle minimum of 2008-2009, the intensities are lower. in contrast, the period ii corresponds to increasing solar cycle, the intensities are higher. these results are very well supported by the results of earlier workers. for example, kulak et al. [2003] have analyzed six years of sr data at a mid-latitude station which included the periods of minimum to maximum (1995-2001) of solar cycle 23. the results of the analysis and modelling have shown that first sr frequency increases from 7.75 hz at minimum to about 7.95 hz at maximum while the global mean attenuation rate at 8 hz varies from 0.31 db/mm at minimum to about 0.26 db/mm at maximum. recently, koloskov et al. [2013] have reported the results of long period (20022012) horizontal magnetic field sr observations conducted at ukrainian antarctic station “academician vernadsky” and shown that both the sr first mode frequency and horizontal component’s intensity follow the trend in variation of solar activity. the lis data relate to cloud formation and lightning flashes near the earth’s atmosphere and are also influenced by solar activities. a detail discussion on this topic will follow later. a possible explanation for shifting of the peak thunderstorm activity from the month of july, 2007 in period-i to the month of august, 2011 in period-ii may be given in terms of galactic cosmic ray (gcr) variation with solar activity. many workers [tinsley and yu 2004, siingh and singh 2010, siingh et al. 2011, williams and mareev 2014] have reviewed the cosmic raymediated cloud microphysics on the 11 year solar cycle. it has been mentioned that in all specific mechanisms, increased gcrs during solar minimum are linked with increased cloud condensation singh et al. 8 9 nuclei (ccn) and ultimately with increased cloudiness, and (in the atmospheric electrical context) with increased cloud water in the mixed phase region of moist convection [williams et al. 2002]. this has been supported by other workers also [pierce and adams 2009, erlykin and wolfandale 2011].it is now a wellestablished fact that gcr flux is reduced as solar activity increases. this is because the increased solar magnetic flux associated with enhanced solar wind during high solar activity periods deflects the gcrs away from the heliosphere and hence from entering the earth’s atmosphere. the reduced levels of gcrs modulated by increasing solar wind can significantly decrease cosmogenic ionization resulting in decreased conductivity and thus increase electric potential. any significant change in electric field in the region of cloud formation can affect cloud formation, cloud movement etc. through a number of processes not yet understood [lakshmi 2008]. the possible long term correlation between gcrs and low cloud cover (lcc) could be explained considering the influence of solar activity [erlykin et al. 2009]. the enhanced solar irradiance causes a rise of the mean surface temperature and results in enhanced vertical convection. from below 3 km warm air rises to greater heights and causes the lcc to decrease and median cloud cover (mcc) to increase setting a change in vertical gradient of the electric field. thus, during high solar activity periods the decrease in gcrs causes a decrease in lcc, although based on this argument no casual connection between gcrs and lcc could be established. keeping the above arguments in view, it is possible that the intense level of lightning in the month of july in period-i is influenced by the increasing solar activity in period-ii and shifted to the month of august in period-ii. since the increased solar activity during the period-ii is responsible for decreased cloud formation, this would result in reduction in lightning activities and hence a reduction in lis data as found in figure 2 during period-ii. however, this explanation is tentative and a detailed explanation for the fall of lis data during period-ii is under study. in all the results presented in figure 4, we have adopted a single source model due to nickolaenko and hayakawa [2002] assuming that the global lightening can be approximated by a single source migrated in longitude around the earth. this is not a good model for the real global lightening source, which exists in three well recognized continental zones, none of which ever really goes to sleep over the course of 24 ut hours. hence, a model incorporating the three sources is better and this has been recognized by nickolaenko et al. [1998]. however, it is also far from the ideal because the source parameters depend (although slightly) on the particular field site. applications of otd/lis data also does not provide one-to-one correspondence to observations. and also we do not know spatial distributions of strokes [a.p. nickolaenko, personal communication, september 2014]. 6. conclusion employing a set of 3-component search coil magnetometer and analysing the x-component data, we study the variation of first mode sr intensity and frequency during two periods of 01 april, 2007 to 31 march, 2008 (period i) and 01 march, 2011 to 29 february, 2012 (period ii) which correspond to pre and post periods of solar cycle minimum of 2008-2009. using the intensity data, we first study the monthly distribution of global thunderstorm activity and then using both the frequency and intensity data we study the effective source distance and level of lightning during both the periods. we show that the world thunderstorm activity shifts to summer month in the northern hemisphere summer as the effective source distance approaches close to the observer, by 12 and 9 mm respectively in both the periods. the level of lightening in different months of the two periods are supported by the lis data with correlation coefficients of 0.81 and 0.70 respectively. the level of intense lightning lies in the month of july, 2007 in period-i which shifts to august, 2011 in period-ii. this shifting is attributed to increasing solar activity in period-ii due to which cloud formation and movement are reduced and hence the effective lightning also is delayed. acknowledgements. the authors are highly grateful to the department of science and technology, government of india, new delhi for the financial support in the form of major research project. the authors also thank mr. ajay singh for typing the manuscript and help with the figures. references balser, m. and c. a. wagner (1960). observation of earth-ionosphere cavity resonances, nature, 188, 638-641. bingham, c., m.d. godfrey and j.w. turkey (1967). modern techniques of power spectrum estimation, ieee trans. audio and electroacoust, au-15, 56-66. christian, h.j., r.j. blakeslee and s.j. goodman (1989). the detection of lightening from geostationary orbit, j. geophys. res: atmospheres (1984-2012), 94, 13329-13337. christian, h.j., r.j. blakeslee, d.j. boccippio, w.l. boeck, d.e. buechler, k.t. driscoll, s.j goodman, j.m. hall, w.j., koshak, d.m. mach and m.f. stewart (2003). global frequency and distribution of lightshift of effective lightning areas ening as observed from space by the optical transient detector, j. geophys. res., 108, doi: 10.1029/ 2002 jd002347. de, s. s. 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 2, dm212, 2022; doi:104401/ag-8746 o p e n acc e s s addressing the challenges of making data, products, and services accessible: an epos perspective michèle marti*,1, florian haslinger1, silvia peppoloni2, giuseppe di capua2, helen glaves3, irina dallo1 (1) swiss seismological service (sed) at eth zurich, switzerland; michele.marti@sed.ethz.ch, florian.haslinger@sed.ethz.ch, irina.dallo@usys.ethz.ch (2) istituto nazionale di geofisica e vulcanologia, rome, italy silvia.peppoloni@ingv.it, giuseppe.dicapua@ingv.it (3) british geological survey, keyworth, nottingham, u.k. hmg@bgs.ac.uk article history: received november 3, 2021; accepted february 22, 2022 abstract novel measurement technologies, additional sensors and increasing data processing capacities offer new opportunities to answer some of the currently most pressing societal and environmental questions. they also contribute to the fact that the available data volume will continue to increase. at the same time, the requirements for those providing such data rise and the needs of users to access it. the epos delivery framework aims to support this endeavour in the solid earth domain by providing access to data, products, and services supporting multidisciplinary analyses for a wide range of users. based on this example, we look at the most pressing issues from when data, products, and services are made accessible, to access principles, ethical issues related to its collection and use as well as with respect to their promotion. among many peculiarities, we shed light on a common component that affects all fields equally: change. not only will the amount and type of data, products, and services change, but so will the societal expectations and providers capabilities. keywords: epos delivery framework; access; ethics; communication; user; provider 1. introduction climate change, volcanic eruptions, pandemics, financial crises, or large earthquakes; the problems societies must deal with nowadays are often complex and multifaceted. consequently, they demand sophisticated, multidisciplinary approaches that explore and create unprecedented solutions. in a world where more and more data is generated and archived, data repositories and the analysis of their holdings serve an important role in solving such complex problems. combining data from diverse sources in new and innovative ways is a critical element of developing these solutions, and the framework allowing such data access are of particular importance. nonetheless, establishing these frameworks to ensure data is findable, accessible, interoperable, and reusable (fair) is also mailto:michele.marti@sed.ethz.ch mailto:florian.haslinger@sed.ethz.ch mailto:irina.dallo@usys.ethz.ch mailto:silvia.peppoloni@ingv.it mailto:giuseppe.dicapua@ingv.it mailto:hmg@bgs.ac.uk michèle marti et al. 2 a difficult task as the european commission emphasizes: “[…] attention needs to be paid to the extremely challenging task of developing fair data frameworks across disciplines and for interdisciplinary research” [european commission expert group on fair data, 2018]. the european plate observing system (epos), the research infrastructure (ri) for the solid earth domain in europe1, is one such framework. it is a multidisciplinary, distributed research infrastructure that facilitates the integrated use of data, data products and services, and facilities from across the european solid earth science community. epos aims to support multidisciplinary analyses for a wide range of users including scientists, authorities, disaster managers, or stakeholders from the private sector. to this end, information in different formats coming from various institutions must be collected, processed, and exposed for future use. this results in a transnational dataset that is openly available for the purposes of studying geological phenomena without the limitations of national or institutional borders. while such a delivery framework brings obvious advantages, it also faces numerous challenges. this contribution is based on scientific evidence, as far as available, and on professional observations and experiences in the domain of data, products, and service provision for a diverse audience. despite focusing on the epos delivery framework, our observations are equally relevant for any other context where a complex framework has been established to offer multidisciplinary data access. each offer needs a provider. within the epos delivery framework, the thematic core services (tcs)2 take on this role and provide access to data, products, and services, both through the common integrated core services central hub (ics-c)3 as well as through their own tcs-specific data portals and services. the ics-c thereby serves as the interoperable cross-domain discovery and access platform for all epos assets. despite their role, the tcs do not necessarily own the data but facilitate its access. the data is collected or generated at the level of international, national or regional institutions with different backgrounds and governance structures. in consequence, defining commonalities for offering data, products, and services and ensuring its (inter-)operability in the longterm comes with a set of potential issues. this includes among others challenges with respect to legal compliance, interoperability, policy, and governance. most of the data currently made available by the tcs is not research data in the sense that it has been collected in the framework of a specific research project. it is rather ‘generic’ in the sense that it has not been collected for a narrowly defined purpose and is therefore of interest for a wide range of potential use cases. examples for such generic data available from the epos tcs’s include geological maps, geodetic ground motion data, geophysical satellite observations, seismological waveforms, or geomagnetic field observations. although providing such a variety of data based on a set of common standards is already highly demanding, it is not the end of the story. when being made available additional new challenges arise, in particular from a societal perspective. a significant proportion of the data made accessible through the epos delivery framework can be used to support hazard and risk assessments that form the basis for decisions on mitigation actions. therewith, it has a direct or indirect societal value that goes beyond a pure scientific approach. ethical and communication issues are already of relevance in the scientific discourse but become even more important when society as a whole is potentially affected. this paper looks at the most pressing issues from when data, products, and services are made accessible, to access principles, ethical issues related to its collection and use as well as with respect to their promotion. for improved clarity and readability, we will use the term ‘assets’ to include everything that is made available through epos including datasets, metadata, products, services, guidelines etc. while in principle there is a difference between provider and producer of such assets, we will use the term ‘provider’ unless the distinction between provider and producer is relevant. 1 https://www.epos-eu.org/ 2 https://www.epos-eu.org/tcs 3 https://www.epos-eu.org/integrated-core-services https://www.epos-eu.org/ https://www.epos-eu.org/tcs https://www.epos-eu.org/integrated-core-services making assets accessible: an epos perspective 3 2. assets’ provider perspective in this section we introduce some issues of relevance and concern to providers of assets within epos. 2.1 asset providers in the epos architecture within the epos architecture (figure 1), assets are made available by the tcs both on their own access platforms as well as via the ics, in particular the central ics-c platform. the tcs in turn receive much of the assets from contributing institutions in their realm, the ‘national research infrastructures and data centres’ (nri) layer in the epos architecture. these institutions are of various types and legal settings: academic research institutions like universities, national research organizations, governmental entities such as geological or geodetic surveys, other national public entities with a service mandate, and other potentially mixed constructions. what they have in common is that they are governed by relevant national legislation, which may be quite different from country to country. these institutions are the primary providers of assets, from raw observation data like seismic waveforms or geodetic displacement to highly complex scientific products like seismic source models or geodetic strain maps, to give just a few examples. some data products, for example the european seismic hazard and risk models, are produced and provided by collaborations between institutes from different countries, and the legal organization of these collaborations can take very different forms, from non-existing to project-based consortia to proper legal entities. within many tcs, important roles are taken by international organizations or initiatives, either with a european focus like orfeus, emsc and efehr in seismology, eurogeosurveys in geology, euref in geodesy, or even global ones like intermagnet in geomagnetism. these institutions are usually rooted in their scientific domain community, with governance structures that may exist since and potentially have evolved over decades. building the various tcs in a way that properly integrates that variety of institutions with due respect of these legacies is one of the successes of epos over the last ten years. it was achieved through intense discussions on organizational models and governance issues, finding compromises in the interpretation of roles and mandates, and retaining a pragmatic view of the final goal to improve access and use of the wealth of available assets. one important aspect in all tcs is that, while the assets are provided by a multitude of diverse contributing institutions, the services where those assets are then collected and made available are hosted and operated by some specific ‘service provider’ institutions. these service providers are again organized differently in the various epos tcs, but they are the interfaces that provide assets to the ics-c and may also provide the same or other assets to other entities. figure 1. the epos architecture (concept). the tcs and ics (-c and -d) together make up the ‘epos delivery framework’ (from an epos scientific and technical description, internal document, 2018). michèle marti et al. 4 2.2 origin and status of assets the diversity in the organizational and legal setup of the asset providers also means that the legal status of assets may differ, following the specific governing regulations. this may for example affect ownership, intellectual property, usage conditions and licensing, responsibilities and boundary conditions for management and curation, or compliance with the general data protection regulation [eu, 2016]. similar legal status issues affect the access services, not only the (assets) content. while in this work we focus on the assets’ providers within the tcs and nri, the ics-c (and likely future distributed components of the ics, ics-d) are likewise hosted and operated by specific institutions and thus fall under specific organizational and legal regulations. interestingly, only a rather small part of the assets that are currently available within the epos delivery framework result from specific (academic) scientific studies. for many tcs, most of their assets are generated either in the context of regulatory mandated monitoring of natural or anthropogenic geohazards, for mapping and monitoring georesources, or for more general surveillance services. while one of the stated purposes of the data collection, but also more generally any ‘asset generation’ normally is to be available and usable also for scientific studies, initial collection or generation of the assets is usually not targeted towards a specific scientific objective. this may pose a challenge in the context of the often used reference to ‘research / scientific data’ in the discussion on fair data [e.g. wilkinson et al., 2016] and open science [e.g. unesco, 2021], as for example the boundary between ‘public data’ or ‘governmental data’ and ‘research data’ is not really clear. 2.3 challenges in asset provision and curation throughout the asset life cycle, providers face various challenges that will appear in different forms depending on their specific setting and the characteristics of their assets. these challenges are also closely linked to implementation of the fair data principles and adoption of open science standards and policies. without claiming completeness, some of the significant issues that have emerged from various discussions within epos are: the availability and practical usability of standardized formats and protocols, tracking and recording of processing parameters, determining adequate granularities of data elements for application of unique persistent identifiers, the type of identifiers to use and how to generate them, defining and implementing appropriate mechanisms to support attribution (in particular for assets combining input from different sources), application and applicability of licenses (ownership/ipr and usage conditions), or addressing reproducibility of data selection by users (versioning). another family of issues relates to the long-term (sustainable) preservation of assets and their continuous curation, starting with selection of a suitable repository – including the question whether it should or even has to be ‘certified’ [coar, 2021]. one of the main concerns for long-term curation are still costs, and ‘value for money’ discussions on what assets to keep for how long (assuming that not all can be kept for eternity) and at what price still lack guidance, while it is even not clear where such guidance should come from and who should ultimately make those decisions. a general observation seems to be (offered without proof or reference) that over time many long-term data archives have proven useful and valuable for novel scientific analysis, while ‘lost’ data is greatly mourned. two further issues connected to data provisioning are worth mentioning: (i) thus far epos has been concerned mainly with standard and well-established assets (at least within their communities), there is little discussion or activity regarding the so-called ‘long-tail of science’, i.e., assets arising from one-time, particular / exotic activities that are difficult to access and not readily available for reuse. these assets are also often in danger of getting lost over time, in particular if they are originally deposited in connection with a researcher’s ‘private’ repository / website. (ii) ‘citizen science’ and ‘internet of things’ data are starting to become of significant interest for solid earth sciences. apart from established processes of specific data collection, e.g., for earthquake felt (macroseismic) reports submitted by the public, there is to date little structured development towards standardized uptake and curation of such data within the epos delivery framework. due to its comprehensive community representation, epos is well positioned to help advance these issues in the future, especially in partnership with those organizations that are seeking to raise the profile of citizen science such as the group on earth observations [geo-citsci, 2020]. epos provides a useful platform for coordination among asset providers in its community, and in particular may enable and strengthen the connection with other european or international initiatives that work on addressing these challenges in an even broader context, such as the european open science cloud (eu/eosc), the intemaking assets accessible: an epos perspective 5 grated core services (isc), the international research data alliance (rda)4 or the committee on data (codata)5 of the international science council. as a participating institution of geo, the group on earth observations6 (), epos is also well positioned to interact with and benefit from these discussions, e.g., the results of the data sharing and management working group (geo, 2015), now part of the 2020-2022 geo work program foundational task ‘geoss data, information and knowledge resources [geo, 2020]. 3. assets user perspective epos aims to provide fully open and easy access to homogeneous high-quality assets from many solid earth science disciplines such as seismology, volcanology, geodesy, and geology. depending on the nature of the assets, potential users face a variety of challenges when seeking to (re-)use them both independently and in combination with those from other disciplines and domains. these challenges relate to data accessibility including the permitted re-use of the data and products as specified through the licensing or policies applied by the data provider/ creator, as well as the specific nature of the data as defined in the associated metadata and supporting information e.g., data formats, standards. users of epos assets are working both across the solid earth domain and with other relevant disciplines with the aim of addressing a range of research questions, including those associated with societal benefits and challenges such as climate change and fulfilling the un sustainable development goals7. these users require a variety of asset types that can be readily accessed, integrated and utilized for their specific applications but they often face a number of barriers when accessing and re-using those assets. 3.1 overcoming the technical barriers to data re-use many initiatives already exist with the specific goal of addressing the challenges encountered by users seeking to re-use the diverse range of assets that have been generated both from research activities and delivered from continuous monitoring programs. for example, the rda brings together experts to address topics, both technological and sociological, that act as barriers to the sharing and re-use of research data. these challenges are associated with both locating and accessing the assets and using it in a combined form that requires integration of assets from multiple different providers. assets from the solid earth domain are heterogeneous and highly fragmented having originated from a wide range of sources and delivered in a variety of different formats that can be based on either common standards or a proprietary output such as that delivered by certain types of environmental sensors. appropriate re-use of these assets requires that they are described using established metadata standards, and the necessary supporting information must also be available to ensure that the users can both understand the asset and use it appropriately (see also section 4). to support this multidisciplinary re-use, the epos delivery framework aims to provide integrated assets from a range of distributed research infrastructures within the solid earth domain (see previous section), through the implementation of common standards for metadata and data. 3.2 navigating the cultural, legal, and social barriers to data re-use in addition to the technical barriers faced by users, a range of guidelines, policies, and legislation at different levels from the institutional to the regional level introduce an added layer of complexity that makes it difficult for users to access and/or re-use assets. however, these mechanisms for managing and supporting reuse of assets also ensure they are used appropriately and that the originators receive the necessary credit. 4 https://www.rd-alliance.org/ 5 https://codata.org 6 www.earthobservations.org 7 https://sdgs.un.org/goals https://www.rd-alliance.org/ https://codata.org http://www.earthobservations.org https://sdgs.un.org/goals michèle marti et al. 6 licenses allow the creator or the provider of the asset to define the rules for its re-use, which can limit the types of users that can access it and restricts the acceptable purposes for which it can be used. restrictions are imposed on access and reuse for various reasons such as where release could compromise national security, it is commercially sensitive or contravenes specific legislation e.g., gdpr. users can also be discouraged from using individual datasets if they are unclear about the permitted usage even where they are fully accessible and available for reuse. as noted by labastida [2020], all data and metadata should be made available with a clear and accessible license to facilitate reusability irrespective of whether it is fully open or subject to certain restrictions. to address these issues, open access licenses such as the open data commons8 and creative commons suite of licenses9 have been developed, which are increasingly used by data providers to clearly define the permitted re-use of individual datasets and other resources even where there are limited restrictions imposed. the most widely adopted license for open access, including by the epos research infrastructure, is the creative commons by attribution (cc by 4.010) that allows users to make unrestricted use of a dataset on the condition that the originator is appropriately credited. one major argument for making cc by the ‘standard license’ within epos is that it requests attribution, i.e., the use of a such licensed asset in any further publication needs to be referenced, and while the numbers of individual citations is still a key impact indicator in the academic community it supports this form of metrics. discussions are ongoing, however, on the appropriateness of the cc by license, mainly in two contexts. one issue concerns the support for open science that may call for even less restrictive licensing of information, as for example suggested in the factsheet on open science published by the creative commons initiative [creative commons, 2017] that argues for putting information in the public domain (e.g., by using a ‘cc0’ declaration or a public domain mark). the second issue regards the general applicability of a license to specific assets. a fundamental condition for licensing is the existence of intellectual property rights (ipr) on an asset, and there is evidence that at least raw observational data, and possibly also standardized derived products, do not carry such ipr. a study from data.europe.eu [european commission, 2021] suggests that this non-licensability of assets may be overcome by introducing appropriate ‘terms of use’ for the services through which the assets are made accessible. mechanisms such as licensing are used to underpin the implementation of various policies for sharing and re-use of data, including those that foster open access which is increasingly being mandated at the national and regional level. for example, the european commission open data directive11, which has been in force since 2019, laid the foundations for improved user access to public sector data. this has since been followed up by the eu strategy for data [2021] that advocates making data as widely available for re-use whilst ensuring that all “rules for access and use of data are fair, practical and clear”. these actions are an illustration of the role played by the relevant national and regional government agencies in fostering and implementing open access policies, which enables greater re-use of data for a diverse range of applications. in addition to these policies and legislation that mandate open access for selected research outputs, there are also an increasing number of initiatives that are producing guidelines and best practices that aim to encourage and facilitate data re-use. probably the most notable of these are the fair data principles [wilkinson et al., 2016] that were intended to “enhance the reusability of data holdings” by making them findable, accessible, interoperable and re-usable (fair). implementation of these principles by data providers (see previous section) has facilitated re-use and made data more widely accessible for a range of users since these principles focus not only on individual users but also on making the data directly discoverable and accessible by machines. by encouraging adoption of these fair principles throughout the solid earth domain, the epos research infrastructure has succeeded in reducing data fragmentation and maximized opportunities for its discovery and reuse. ensuring that epos data services are fair compliant allows users to discover and access a range of multidisciplinary data for a variety of applications both within the solid earth domain and beyond. however, as noted by bailo [2020], the epos research infrastructure actually predates the concept of fair but it is still fully compliant with these principles having already identified the requirements of users for fully accessible and interoperable data from the solid earth domain. 8 https://opendatacommons.org/licenses/by/ 9 https://creativecommons.org/about/cclicenses/ 10 https://creativecommons.org/licenses/by/4.0/ 11 https://digital-strategy.ec.europa.eu/en/policies/legislation-open-data https://opendatacommons.org/licenses/by/ https://creativecommons.org/about/cclicenses/ https://creativecommons.org/licenses/by/4.0/ https://digital-strategy.ec.europa.eu/en/policies/legislation-open-data making assets accessible: an epos perspective 7 4. ethical challenges ethics is a key topic for the development of modern scientific and technological activities, while respecting the needs and requirements of the scientific communities and societal stakeholders [evers, 2001; allea, 2013; agu, 2017]. ethical compliance influences the quality of data, results produced, and services provided: they are only as good as the values used to acquire/develop them. ethics affects personal and collective conduct and thereby shapes relationships and their outcomes. finally, ethics matters because research institutions/networks/organizations and their employees have a societal responsibility [marti et al., 2019], since they belong to a community of people (stakeholders, partners, and society) and scientific and technological data, products, and services may have a direct or indirect impact on people’s life. this implies that science and technological activities have to consider ethics to develop their full potential. 4.1 has epos ethical responsibilities? assuring excellent science and technological activities (by applying updated knowledge and the best methods and instrumentations) within epos requires not only dedicated skills but also a high level of awareness with respect to the specific roles and responsibilities of scientists, technicians, and administrative supporting units especially when managing complex issues affecting data life-cycle management [epos-ip, 2018] and science-society interfaces [isc, 2005]. the operational choices of epos operators and scientists cannot be considered completely neutral with respect to the decisions of target audiences and the consequences that epos products and services can have for stakeholders and partners. ethical responsibilities of epos derive from the fact that operators and scientists within the epos delivery framework and in the nri provide data and services which are helpful for hazard monitoring and assessment, risk mitigation, and geo-resources management. in addition, ethical issues often arise in relation to conflicts of interest, conflicting requirements, multiple objectives and diverging priorities, as well as different stakeholders including public-private sector relationships. ethical issues involving the working environment and relationships among colleagues in order to achieve common goals, as well as publication ethics, are further important topics for the success of the activities carried out within epos. 4.2 potential ethical issues in epos some of the practical issues related to epos services that can emerge are: inadequate or missing protection of institutional and personal scientific data; misuse of data and technological information [ec, 2016], also in the context of, for example, terrorism or other criminal purposes; misleading hazard and risk communication, with particular reference to crises and emergency phases [cauzzi et al., 2018]; negative societal impact at the interface between civil protection officers and scientists, and other societal stakeholders. important and specific ethical concerns also rise when considering the two-way communication between individuals and epos services, such as citizen science activities for observations of natural and anthropogenic phenomena [sanz et al., 2014; lee et al., 2020], as well as potential conflicts between industrial needs and developments and societal interests. especially important in this regard, is the objective and impartial role of research infrastructures in presenting scientific data and results, where such conflicting interests exist, keeping in mind the principle of impartiality for public good (referring to all those elements, information, and service provision for the benefit or well-being of the public, including individual and collective safety, knowledge development, and health). 4.3 ethical background from epos-ip in this perspective, the ethics working group, established in the framework of the project epos-ip (implementation phase), had already identified the following macro-categories to synthetically map ethical issues in epos: – protection of personal data: activities involving collecting or processing personal data – misuse/abuse of data: activities having potential for terrorist or criminal abuse – communication and societal impact: communicating science vs. communicating risk – impartiality for public good michèle marti et al. 8 these four macro-categories were associated with three strategic actions characterizing the epos activity plan, namely: fostering open science, contributing to risk mitigation, and enhancing cooperation with the private sector. 4.4 a survey on ethics for epos-sp in order to investigate the epos community’s perception of the ethical and social implications of its scientific and technological activities related to data and service provision, a survey through an ad hoc online questionnaire was carried out in the european project epos-sp (sustainability phase) [di capua and peppoloni, 2021]. the questionnaire, filled out anonymously, had also the aim of being a means to make participants (all involved in the tcs) more aware about issues related to ethics in their professional activities and in the science-society interface. the topics of the questionnaire concerned research integrity/science ethics and truthful conduct of research [steneck et al., 2010], data ethics [tranberg et al., 2018], open access policies [erc, 2017], service accessibility and related issues like traceability, authorization, user profiling, data misuse, societal commitments of scientists/technicians, science, hazard and risk communication, public institutions-private sector relationships, conflicts of interests, and dissemination of scientific knowledge to the public. the most relevant results of the survey [di capua and peppoloni, 2021] are the following: 1) the questionnaire was filled in by a low number of epos tcs’s representatives (only 18). this highlights that ethical issues are considered important from an ideal point of view, but also perceived as something theoretical that doesn’t refer to one’s own activities. this could also be due to a limited ability to refer ethics to real cases or behaviours. 2) responsibility of scientists towards society is generally recognized as important, but it is not necessarily considered as a primary duty (figure 2). 3) differences between science communication and hazard/risk communication (that are multidisciplinary activities) were recognized by the majority of respondents, who agree that risk communication can have legal implications for communicators in cases such as the l’aquila earthquake in 2009 (italy) or more recently in new zealand with respect to a deadly volcano eruption [cocco et al., 2015; lewis, 2021; mutter, 2021]. 4) there is a large consensus that scientists are science advisors and not decision makers. they have to provide relevant hazard and risk related data to society, and the general public should have open access to scientific information. figure 2. results of the “ethical” questionnaire about scientists’ responsibilities and duties towards society (in di capua and peppoloni 2021). y-axis: number of respondents; x-axis: five-level likert scale to investigate the agreement/disagreement with the sentence below each graph, ranging from 1=disagree to 5=fully agree. making assets accessible: an epos perspective 9 5) personal data protection policies are necessary for epos operators and scientists. the majority of respondents agree on open access for data, data products, metadata (metadata should be fully open access), services for data visualization and analysis, and services for data processing and modelling generating new data products to all stakeholders, particularly to scientists, government agencies, students, and teachers. some concerns were expressed about data, products, and services released to private companies, especially because in many cases those companies are reference stakeholders for scientific/technological activities, so “there is always [a] risk when taking commercial obligations to contractors who are not indifferent to the delivered product’s content”, “most industrial partners keep the produced data for business needs and/or interest” (quotes of participants) or public-funded data should be shared with private industry or commercial undertakings (figure 3). this implies that ethical issues related to relationships between scientists and private companies should be carefully considered in future epos activities. open data should be provided together with easy-to-understand description of the contents and instructions/recommendations to assist users (see section 4 for more details). issues on how to guarantee open access to data, needs to be deepened, in order to clarify if and when user registration/authentication and/or authorization and/or profiling should be adopted. user registration is generally considered the best method. there is a wider agreement about the need for epos to collect information concerning the purpose the user wants to accomplish by having access to data and services. epos should ensure access related to relevant scientific products to national authorities, civil protection agencies and local authorities: this should be considered an ethical obligation where assets are relevant for mitigation and risk management decisions. 6) epos data are considered subject to be misused, particularly to feed scientific controversies and media scoops. in any case, this should not affect the open access to them and could be avoided or mitigated through ad hoc regulations. 7) impartiality for public good regarding data and data products is fundamental for always acting in the public interest. firstly, public research organizations committed to monitoring activities and secondly governmental authorities should be in charge of ensuring this principle. 8) conflicts of interests compromise individual and institutional reputation and credibility, but for respondents, they are not so frequent and inevitable in the epos service management. in any case, they can be mainly solved by adopting appropriate codes of conduct, by increasing the ethical awareness of operators and scientists, or by declaring potential and current conflicts in advance to the respective institutional governing offices. 9) epos should consider developing tools to improve the understanding of scientific content to non-expert audiences. a communication strategy would help with this (see section 4). figure 3. results of the “ethical” questionnaire about public funded data-private companies’ topic (in di capua and peppoloni 2021). y-axis: number of respondents; x-axis: five-level likert scale to investigate the agreement/ disagreement with the sentence below each graph, ranging from 1=disagree to 5=fully agree. michèle marti et al. 10 regarding the point 3) above, ethical challenges related to risk and hazard communication using the data, products, and services allocated by epos, are discussed in more detail in the next section. 5. communication challenges when providing access to assets, external communication is an integral component of making them known and reachable to the outside world. when provided in a framework such as epos, additional internal communication measures are required to coordinate these efforts. developing and maintaining communication activities in such a context poses several challenges that can be grouped into the following areas: 1) promoting access 2) assisting users 3) taking responsibility 4) communicating internally the findings of this section are mainly based on the insights gained through twelve semi-structured interviews with tcs representatives, user feedback retrieved from three online surveys with users of already operating tcs, and the ethical questionnaire introduced in section 3 [dallo and marti, 2020; di capua and peppoloni, 2021]. despite this focus, we assume the communication challenges that we identify within the epos delivery framework to be of relevance in any other context where assets are provided to different audiences. up to now, the tcs’ target their offers towards predominantly professional users, mainly originating from the scientific community in the solid earth domain (figure 4). this target audience is highly motivated and trained to figure 4. current target audiences of the tcs. the main target audience is the “within solid earth science community”. the total percentage of this main target audience was used to normalize the proportion of the other groups (dark colours). making assets accessible: an epos perspective 11 work with the assets offered. they are also willing to invest time to read documentations and explore different access options. the results of the user surveys indicate that the tcs currently operating cover the needs of this target audience well. however, the majority of the tcs mentioned that they aim to reach a broader audience including institutional, private and commercial user groups as well as the general public in the midto long-term. this brings new challenges in all the aforementioned four areas as explored hereafter. 5.1 promoting access now and even more in the future, access to assets within the epos framework must be promoted [orlecka-sikora et al., 2020]. the value and benefit of an offer is highly defined by its demand. a data repository, for example, can be as comprehensive as it may be, but if no one knows about it then its value is decreased. up to now, tcs are mainly challenged in promoting their assets to their core target audience, which is the scientific community. however, they are also trying to reach other target audiences: “[..] but, of course, the overarching goal is to serve not only scientific audiences, but also institutional audiences, if they are interested, for example municipalities, governments, industry partners” (quote of a participant). when aiming to reach a larger audience, providers should, before investing in additional promotion measures, first elaborate the following question: are the assets relevant for the envisioned user groups? from a user perspective, assets are relevant when they serve to advance their studies, be it for a scientific project, a professional mandate or a personal interest. providers should orient their offers to the needs of users, not only considering the current situation, but also looking ahead assessing what could become of relevance for different user groups in the future. this assessment should then guide investment decisions and promotion measures. what might be a highly valuable source of information for scientists is probably not of interest for other stakeholders. in consequence, available resources should in this case be invested in promoting and improving the access rates of current target audiences. from a providers’ perspective, however, there is also a normative or even compulsory approach. some information should or has to be available for specific target groups, because they must rely on it to make certain decisions. for example, civil engineers depend on the access to seismic hazard assessments for designing earthquake-resistant structures. civil protection agencies could benefit from accessing certain data sets but would need to be made aware of this resource. whereas from such professional target groups it can be assumed, when aware, that they have the necessary skills and knowledge to access it, higher requirements apply in other cases as a recently published article emphasizes. after the volcanic eruption at white island (nz) in 2019, it is debated whether the alert bulletin produced and published by a scientific institution had been sufficiently accessible and understandable for different user groups including tour operators frequently visiting the island [lewis, 2021]. providers are therefore prompted to increase their efforts if they see a need or are legally obliged to enhance the access to certain information for dedicated user groups. besides raising awareness for the existence of certain information, dedicated training may be offered to support users in accessing and using it. in consequence, before promoting any access, providers should first consider the relevance of their offers for different user groups. secondly, before promoting it and raising expectations, they should ensure that their offers are accessible, which leads us to the next area of concern. 5.2 assisting users as emphasized by the fair principles [wilkinson et al., 2016], assets must be accessible. from a communication perspective, this is more than supporting users in finding and accessing the data, it is also about availability of explanatory material, usability, product design, and training. such comprehensive support becomes more costly the larger the envisioned target audience gets and therewith poses a big challenge to providers. surveys of the users of existing seismology services (emcs, efehr, and orfeus) reveal that the majority finds them useful, trustful, and reliable. this is a positive outcome, indicating that the needs of the users predominantly working for research institutions are currently well met. however, the interviews with the tcs representatives as well as the ethical survey also disclosed their worries with respect to future use and user groups. a main concern raised in the ethical survey was “[...] that the data, products or services could be misused by individuals or groups, and michèle marti et al. 12 in turn, lead to ethical issues” (quote of a participant), which might include drawing misleading scientific conclusions, to support conspiracy theories or as a basis for media scoops. in addition, it is unclear for many tcs how to best deal with potential needs or requests from users from other domains such as those in the insurance or tourist sectors. the majority of the tcs representatives agree that these concerns should not lead to any access restriction. we would like to add that it should also not restrict communications, on the contrary, providers should be eager to seek new and innovative ways to carefully embed their assets through the use of suitable communication tools. to this end, as also recognized in the ethical survey, an interdisciplinary collaboration between trained professionals and it developers, communication specialists and graphic designers is advantageous. in addition, it is strongly recommended to test products and services with the targeted user groups before they are launched [marti et al., 2020]. another option is to co-produce products or services with future user groups or offer dedicated training e.g., in the form of webinars. elaborating a dedicated communication strategy and policy will further support institutions with the handling of user requests as well as drive future updates and developments of communication measures. the implementation of such a holistic view on data, product, and service provision requires dedicated resources, a real challenge for most providers, especially in the longer-term. nonetheless, it is essential to consider appropriate communication measures to satisfy user needs and ensure that they actually benefit from an offer. it is also a question of responsibility as discussed in the next section. 5.3 taking responsibility providers and users do not only have different needs, they also take different perspectives on responsibility issues. providers are expected to establish and maintain access to the assets while users are required to respect access and copyright regulations. technical measures such as user authorization or legal statements are one component to ensure responsible access [manteigueiro et al., 2020], another is to provide open, clear, and comprehensive information supporting users in taking their share of this responsibility. again, the more users access certain assets the greater the value of the information dealing with responsibility aspects becomes. whereas professional users may be well aware of the limitations of a specific data set, related uncertainties or copyright restrictions, less trained users could be unaware. structuring content hierarchically starting from a very generic level and then going into more detail is an established pathway to guide users and address different levels of expertise and knowledge [shou-bin dong, 2004]. in addition, important information from a providers’ perspective should not be hidden somewhere, e.g. in the disclaimer, but should appear prominently for all users. a particular communication challenge was emphasized by tcs providers related to the access of real-time or up-to-date data, in particular, when such data can be used to inform risk management. besides legal and ethical considerations as discussed in sections 1 and 3, it is also challenging with respect to communication. users may need 24/7 support for such access and explanatory material related to such information must be designed even more carefully. further, not all tcs are legally allowed or have the mandate to communicate real-time information: “[..] we cannot provide services dealing with real time data because we are not a national agency, you know, providing alerts. so, we have to keep in mind this kind of approach, let’s say nearly real-time and just for scientific targets”(quote of a participant). 5.4 communicating internally the exchange of information between providers only needs to be considered when assets are offered within an institutional framework such as epos. in this case, providers must talk to each other to find common ground. with respect to communications, providers should take advantage of their network and exchange best practices. they are often facing similar challenges and, having tried different approaches to finding a solution, could potentially benefit from their mutual experiences. this is nicely summarized by one of the tcs representatives who noted: “so, if the question is related to [my tcs], to me the main challenge is the coordination of these broad domains, and then show that it works. because we are not really used to working together. we didn’t do that in the past. it is something new that we are learning right now” (quote of a participant). making assets accessible: an epos perspective 13 6. conclusions complex problems such as climate change, pandemics, or financial crises that severely impact society require comparable solutions. in a data-driven world, providing access to comprehensive, multidisciplinary data repositories to tackle such major challenges is indispensable. this is exactly what the epos delivery framework aims to deliver for the solid earth domain. although the importance of this undertaking is obvious, its implementation from the provision to the actual use of assets presents many challenges that must always be addressed from two very different perspectives: that of the provider and the user. in this contribution, we have discussed the most pressing issues that may materialize when providing access to assets. at the very beginning, access to assets must be established. this is particularly challenging when, as in the epos context, data from diverse institutions are assembled. it not only requires consent on a technical level, but also from a legal and governance perspective. in addition, it is not a one-time effort, it needs constant maintenance to ensure operability and user satisfaction. this, in turn, demands constant investments in infrastructure and trained personnel. this will not become an easier task in the future, with a steadily growing amount of data to be handled as more and more sensors are deployed, for example, as part of the internet of things. with the increasing amount of data and possibilities to provide and handle it, further issues arise. one of the most important being the ethics of data provision and use, which is already of relevance when gathering the data. a survey with data, product, and service providers within the epos delivery framework shows a theoretical awareness for ethical issues. nonetheless, most struggle to apply this knowledge to their practical work and in consequence do not implement any mitigation measures. we therefore see a clear need to invest in training to help providers to become more aware of ethical considerations in their day-to-day activities. however, it would be too short-sighted to only focus on the providers. there are reasonable fears that assets could be misused, for example to inspire scientific controversies or provoke media scoops. there is a clear agreement among current providers in the epos delivery framework that such fears should not lead to an access restriction or denial. however, additional efforts are needed to implement access in such a way that the conditions are ubiquitous for users. in addition, explanatory material as well as support options should be provided on a best effort basis to guide users and help them to make informed decisions. implementation of common access to assets combined with other user needs requires additional user support from both providers and the regulatory framework. this fuels the already omnipresent discussions within epos about the best use of available resources, and will gain even more importance the larger the potential target audience becomes. currently, the scientific community is the main user, but the providers stated objective in the mid to long-term is to reach a larger, more diverse audience. this is important from a societal and ethical perspective when a significant proportion of the assets can be used to assess hazard and risks, and thereby contribute to the design of appropriate mitigation measures. a larger audience means that more people and institutions can potentially benefit from accessing the epos delivery framework. however, it also requires a significant investment in supportive measures to facilitate access for these novel users, who are less experienced, and thus reduce the possible inappropriate use of the data provided. regardless of the path the epos delivery framework takes, constant readjustments on all levels will be needed to accommodate not only the changing nature of the assets, but also the evolving user needs, and our understanding of the social and ethical context. this requires the will and openness for internal debates within the epos community as well as regular interactions with current and future user groups. funding. this contribution has been supported by the epos-sp project, which has received funding via the european union’s horizon 2020 research and innovation programme under grant agreement n° 871121. acknowledgement. the insights of the section 4 were gained through twelve virtual semi-structured interviews with representatives of the tcs and the results of user surveys launched for three already operational tcs (dallo & marti, 2020). the interviews were conducted on zoom or skype and lasted about one hour. the user surveys were linked to the website of the tcs and sent to the members via email. this research was approved by the ethics committee of eth zurich (ek 2020-n-76). michèle marti et al. 14 references agu (2017). the responsibilities and rights of scientists. position statement. american geophysical union, 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rocca-serra, m. roos, r. van schaik, s.-a. sansone, e. schultes, t. sengstag, t. slater, g. strawn, m.a. swertz, m. thompson, j. van der lei, e. van mulligen, j. velterop, a. waagmeester, p. wittenburg, k. wolstencroft, j. zhao and b. mons (2016). the fair guiding principles for scientific data management and stewardship, sci. data, 3, 160018, https://doi. org/10.1038/sdata.2016.18. *co r r e s po n d i n g a u t h o r: michèle m a rt i, swiss seismological service at eth zurich, 8092 zurich, switzerland, email: michele.marti@sed.ethz.ch https://www.earthobservations.org/open_eo_data.php https://www.earthobservations.org/open_eo_data.php https://www.earthobservations.org/documents/gwp20_22/geo-citsci.pdf https://council.science/wp-content/uploads/2017/04/science-and-society-2005.pdf https://council.science/wp-content/uploads/2017/04/science-and-society-2005.pdf https://council.science/wp-content/uploads/2017/04/science-and-society-2005.pdf https://doi.org/10.1016/j.pgeola.2020.07.010 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https://wcrif.org/guidance/singapore-statement https://wcrif.org/guidance/singapore-statement https://wcrif.org/guidance/singapore-statement http://dataethics.eu https://dataethics.eu/wp-content/uploads/dataethics-uk.pdf https://dataethics.eu/wp-content/uploads/dataethics-uk.pdf https://www.unesco.org/en/natural-sciences/open-science https://doi.org/10.1038/sdata.2016.18 https://doi.org/10.1038/sdata.2016.18 mailto:michele.marti@sed.ethz.ch editorial_final:layout 6 annals of geophysics, 57, 1, 2014, e0191; doi:10.4401/ag-6532 e0191 annals of geophysics: ad majora fabio florindo1, francesca bianco2, paola de michelis1, simona masina3,4, giovanni muscari1, francesco caprara3, anna grazia chiodetti3, edoardo del pezzo2, stefano gresta1,5 1 istituto nazionale di geofisica e vulcanologia, via di vigna murata 605, 00143, rome, italy 2 istituto nazionale di geofisica e vulcanologia, sezione “osservatorio vesuviano”, via diocleziano 328, 80124, naples, italy 3 istituto nazionale di geofisica e vulcanologia, via donato creti 12, 40128, bologna, italy 4 centro euro-mediterraneo sui cambiamenti climatici, 40128, bologna, italy 5 università di catania, dipartimento scienze biologiche, geologiche e ambientali, corso italia 57, 95129, catania, italy annals of geophysics (issn: 1593-5213; from 2010, 2037-416x) is a bimonthly international journal, which publishes scientific papers in the field of geophysics sensu lato. it derives from annali di geofisica (issn: 0365-2556), which commenced publication in january 1948 as a quarterly periodical devoted to general geophysics, seismology, earth magnetism, and atmospheric studies (figure 1). the journal was published regularly for a quarter of a century until 1982 when it merged with the french journal annales de géophysique to become annales geophysicae (issn: 0755-0685) under the aegis of the european geophysical society. in 1981, this journal ceased publication of the section on solid earth geophysics, ending the legacy of annali di geofisica. in 1993, the istituto nazionale di geofisica (ing), founder of the journal, decided to resume publication of its own journal under the same name, annali di geofisica. to ensure continuity, the first volume of the new series was assigned the volume number xxxvi (following the last issue published in 1982). in 2002, with volume xlv, the name of the journal was translated into english to become annals of geophysics and in consequence the journal impact factor counter was restarted. starting in 2010, in order to improve its status and better serve the science community, annals of geophysics has instituted a number of editorial changes including full electronic open access, freely accessible online, the possibility to comment on and discuss papers online, and a board of editors representing asia and the americas as well as europe. to celebrate this major change to the modern online format, the first volume of the new annals of geophysics was produced as a historical volume in which we published a set of selected papers by eminent authors who, in different ways and in different contexts, have provided us with important benchmarks in the development of geophysics [e.g., aki et al. 2010, båth 2010, gutenberg and richter 2010, lehmann 2010]. all of these papers were published before 1977 in annali di geofisica, as the journal was then called. in march 2013, sixty-five years after the journal foundation and three years after the relaunch of annals of geophysics as an online open-access journal (we use open journal systems 2.4.1.0, which is open source journal management and publishing software developed, supported, and freely distributed by the public knowledge project), which certainly contributed to its international success, we proceeded with a partial reeditorial figure 1. first issue of annali di geofisica, 1948. organization of the journal itself. in addition to “research articles”, annals of geophysics now publishes several other categories of papers including “scientific news”, “data and experiment description”, “review articles”, “letters”, and “fast track” papers. data and experiments descriptions is a kind of paper in which data collected during important experiments or field surveys (seismology during eruptions, aftershocks of main earthquakes having particular impact and so on) are described and put in sharing with the scientific community. the journal itself can act as data repository. the latter two categories are among the most recent addition and allow scientists to quickly publish the preliminary results of their findings. the first issues including “letters” and “fast track” papers were volume 56, 2013, on atmospheric composition validation and evolution acve 2013 [e.g., richter et al. 2013] and volume 55 (4) on the emilia (northern italy) seismic sequence of may-june, 2012: preliminary data and results, which was entirely dedicated to this dramatic seismic sequence [anzidei et al. 2012] (figure 2). the journal is also active in providing assistance to conveners of important national and international meetings and workshops, for the publication of papers therein presented, and will publish special volumes on specific topics of broad interest to the scientific community. recent special issues include volume 55 (3), 2012, on geoethics and geological culture. reflections from the geoitalia conference 2011 [peppoloni and di capua 2012] with a collection of papers presented during the ‘geoethics’ scientific session at the 8th italian forum of earth sciences that was held in september 2011 in turin, italy, and volume 55 (6), 2012, entitled 5th european repeat station (magnete) workshop rome, italy 2011 [de michelis and duma 2012] which contains a subset of the scientific results presented at the 2011 magnete symposium. one of the next goal will be to re-organize the publication of special volumes focused onto scientific topics that are generally considered as top-arguments in the fields of geophysics and volcanology. as part of the journal’s reorganization we have appointed four section editors covering four main research areas (1. seismology, volcanology, geodesy and geochemistry; 2. geomagnetism, paleomagnetism and solar terrestrial physics; 3. atmospheric physics and chemistry; 4. physical oceanography and climatology), 22 new associate editors, and a new editor-in-chief. like all peer-reviewed journals, annals of geophysics has a board of associate editors comprising leading national and international experts on the various section topics of the journal. the associate editors provide the section editors and the editor-in-chief with advice on choosing referees, assist in reviewing papers themselves, undertake book reviews, and may contribute editorials to the journal. the section editors need to ensure both that they have the right balance of expertise available to them for the papers that the journal is receiving, and that the role of associate editor is spread across the community. since 2002 the number of citations has been steadily increasing (figure 3) and in 2012 the impact factor increased to 1.138, placing annals of geophysics in a respectable position among the geochemistry and geophysics subject category of isi web of science. none of this progress would have been possible without the commitment of authors, reviewers, and previous editors, and we are grateful for all of the effort they have expended on the journal. for further information about the journal’s scope, submission formats, and review and production procedures, please see http://www.annalsofgeophysics.eu. we look forward to handling your paper. editorial 2 800 700 600 500 400 300 200 100 0 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 years nu m be r of c ita tio ns figure 2. cover of the annals of geophysics fast track issue on the emilia (northern italy) seismic sequence of may-june, 2012. figure 3. number of citations per year (source: web of science). 3 references aki, k., m. bouchon, b. chouet and s. das (2010). quantitative prediction of strong motion for a potential earthquake fault, annals of geophysics, 53 (1), 81-91; doi:10.4401/ag-4665. anzidei, m., a. maramai and p. montone (2012). preface, annals of geophysics, 55 (4), 515; doi:10.4401/ ag-6232. båth, m. (2010). teleseismic magnitude relations, 53 (1), 69-80; doi:10.4401/ag-4664. de michelis, p., and g. duma (2012). preface, annals of geophysics, 55 (6), 1051; doi:10.4401/ag-6313. gutenberg, b., and c.f. richter (2010). magnitude and energy of earthquakes, annals of geophysics, 53 (1), 7-12; doi:10.4401/ag-4588. lehman, i. (2010). on amplitudes of p near the shadow zone, annals of geophysics, 53 (1), 13-15; doi:10.44 01/ag-4589. peppoloni, s., and g. di capua (2012). foreword and acknowledgements, annals of geophysics, 55 (3); doi:10.4401/ag-6100. richter, a., m. weber, j. p. burrows, j.-c. lambert and a. van gijsel (2013). validation strategy for satellite observations of tropospheric reactive gases, annals of geophysics, 56, fast track 1, 1-10; doi:10.4401/ ag-6335. © 2014 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. editorial << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice 1 annals of geophysics, 65, 2, dm208, 2022; doi:10.4401/ag-8756 o p e n acc e s s the epos research infrastructure: a federated approach to integrate solid earth science data and services massimo cocco*,1, carmela freda2, kuvvet atakan3, daniele bailo1, kauzar saleh contell2,4, otto lange5, jan michalek3 (1) istituto nazionale di geofisica e vulcanologia, rome, italy (2) epos eric, rome, italy (3) university of bergen, bergen, norway (4) eth zurich, switzerland (5) utrecht university, utrecht, the netherlands article history: received november 15, 2021; accepted march 24, 2022 abstract the european plate observing system (epos) is a research infrastructure (ri) committed to enabling excellent science through the integration, accessibility, use and re-use of solid earth science data, research products and services, as well as by promoting physical access to research facilities. this article presents and describes the epos ri and introduces the contents of its delivery framework. in november 2018, epos eric (european research infrastructure consortium) has been granted by the european commission and was established to design and implement a long-term plan for the integration of research infrastructures for solid earth science in europe. specifically, the epos mission is to create and operate a highly distributed and sustainable research infrastructure to provide coordinated access to harmonized, interoperable and quality-controlled data from diverse solid earth science disciplines, together with tools for their use in analysis and modelling. epos relies on leading-edge e-science solutions and is committed to open access, thus enabling a step towards the change in multidisciplinary and cross-disciplinary scientific research in earth science. the epos architecture and its delivery framework are discussed in this article to present the contributions to open science and fair (findable, accessible, interoperable, and reusable) data management, as well as to emphasize the community building process that supported the design, implementation and construction of the epos ri. keywords: research infrastructure; solid earth science; open science; fair data management; eric massimo cocco et al. 2 1. introduction the term research infrastructure (hereinafter ri) refers to the facilities, resources and services that are used by the research communities to conduct research and foster innovation in their fields [eu regulation no 1291, 2013]. ris may include major scientific equipment (or sets of instruments, including distributed observational sensor networks), knowledge-based resources such as collections, archives and repositories of scientific data, but also e-infrastructures like data processing and computing systems, communication networks, and any other tools that are essential to achieve excellence in research and innovation1. since a few decades ris are playing a key role of enablers of scientific research and promoters of innovation in all scientific domains, as they trigger the generation and provision of high-quality scientific data and new technologies supporting science at the frontiers of knowledge. ris are implemented through different organizational models either as central facilities and laboratories for on-site experiments and measurement sessions, or as centers for the coordination and management of geographically distributed observing systems and laboratories, remotely accessible computational resources and data repositories. ris are nowadays assuming an ever increasingly central role in supporting “open science” by fostering the sharing of scientific data and products. open science represents a specific and novel approach to the scientific process based on cooperative work and effective ways to share knowledge and scientific findings by using digital communication technologies and new collaborative tools [european union, 2015]. the key role of ris in supporting the digital transformation of science in general and the open science movement specifically is closely related to their capacity of enabling new forms of research and collaboration as well as providing solutions to share and distribute data and scientific products [research data alliance, 2014]. in the context of data sharing, “openness” has an extended meaning, as the paradigm of open access demonstrates. the extended significance of openness relies on the fact that research data are a prominent part of the research process if compared to open access publications [kunst and degkwitz, 2018], the latter being only one, certainly important, stage of the data lifecycle [sinaeepourfard et al., 2016]. among the different motivations for sharing data, the traceability of research results and the re-use of scientific data for new research projects and investigations assume a particular value. moreover, the key requirements for successful re-use of data are findability, accessibility and interoperability, aspects that are covered by the so-called fair principles (findable, accessible, interoperable, reusable) [boeckhout et al., 2018; mons et al., 2017; wilkinson et al., 2016 and 2018]. fair data management is one of the current objectives for ris in europe and a global challenge requiring well-defined principles and shared practices. the ris play a key role to achieve these goals by fostering research data management to support open science [cocco et al., 2019]. in this article, we present and discuss a specific ri: the european plate observing system (epos, www.epos-eu.org). epos is a ri dealing with solid earth science and a landmark in the esfri (european strategic forum on research infrastructures, www.esfri.eu) roadmap. solid earth science encompasses the study of the planet earth’s solid surface (both on land and offshore) and its interior. it is dedicated to the understanding of the structure and the functioning of the earth and its dynamics. solid earth science brings together many diverse disciplines such as geology, seismology, geodesy, volcanology, geomagnetism as well as chemistry and physics as they all apply to the workings of the earth. as such, accessing data from the independent observing systems from these diverse disciplines is challenging, because of difficulties in finding and using the relevant data, as well as integrating them with other research data, which is essential to understand the earth’s complex dynamic processes. by improving and facilitating the integration, access, use, and re-use of solid earth science data, research products, services and facilities, epos is developing a holistic, sustainable, multidisciplinary research platform to provide coordinated access to harmonized, interoperable and quality-controlled data from diverse solid earth science disciplines, together with tools for their use in analysis and modelling. specifically, the epos mission is to create and operate a distributed and sustainable research infrastructure that integrates the diverse european research infrastructures for solid earth under a common framework. epos, relying on new e-science solutions, gives open access to solid earth science data enabling a step change in multidisciplinary and cross-disciplinary scientific research in many, diverse fields of earth science. indeed, solid earth scientists have a long-lasting tradition in data acquisition, collection, quality-control and standardization. they are also the key actors for curating and implementing metadata and services for qualification, 1 https://ec.europa.eu/info/research-and-innovation/strategy/strategy-2020-2024/our-digital-future/european-researchinfrastructures_en http://www.epos-eu.org http://www.esfri.eu https://ec.europa.eu/info/research-and-innovation/strategy/strategy-2020-2024/our-digital-future/european-research-infrastructures_en https://ec.europa.eu/info/research-and-innovation/strategy/strategy-2020-2024/our-digital-future/european-research-infrastructures_en epos data integration in solid earth science 3 storage and accessibility of research data. epos represents the collaborative framework where joining efforts and resources, sharing existing experiences and skills of domain scientists and e-scientists in order to develop a federated approach for integrating research data and scientific products to ensure data management and interoperability through e-science innovation. the epos ri has been envisioned and it is managed to accomplish this mission. it is important to emphasize that the epos vision goes beyond data sharing. rather, data integration is also envisioned to enable the use of different aggregated data sets through services for (data) visualization, analysis, and processing. this requires both a technological challenge, to build and operate the novel epos platform where interoperability is achieved, and a community building action to engage the different scientific communities through novel thematic services, and to coordinate data generation and preservation in solid earth science. the epos data integration system will allow the full exploitation of the value of solid earth science data, breaking down existing silos and establishing the cooperation necessary for sustainable data management in a digital era. in this article, we also describe and discuss the epos federated approach to integrate scientific data and foster fair data management in solid earth science contributing to open science. 2. the epos architecture epos is a complex enterprise that can be successfully run only taking duly into account what has been built and learned in the previous phases of its lifecycle. indeed, since its conception and design phase (2002-2009), epos is managing its complexity relying on a shared and effective roadmap toward construction in which the scientific concept and the architecture design are evolving consistently with the original epos vision and mission. since the preparatory phase (2010-2014) the epos architecture has been planned and designed in such a way as to allow the enterprise to work as a distributed, sustainable research infrastructure. the decision to build and operate a single research infrastructure for the whole solid earth science has been motivated by the awareness of the added value of data integration, and it has been strategic for the design and the implementation of the epos architecture during the implementation phase (2015-2019). the distinctive feature of the architecture design relies on assembling key elements belonging to four complementary categories (see figure 1) mapping the data generation, qualification, integration and provision: a) national research infrastructures (nris) b) thematic core services (tcs) c) integrated core services (central hub, ics-c and distributed, ics-d) d) epos eric executive coordination office (epos eric eco). the rationale behind this architecture design is to guarantee the effective engagement of the communities providing the required skills for the data and service provision, while ensuring the coherency with and the effectiveness of the legal, governance, financial and technical frameworks in epos. the epos delivery framework (figure 1) is composed of the tcs, ics and epos eric eco, and it is further described later in this article. the national research infrastructures (nris) characterise the framework where data are generated, processed, analysed, and archived. they represent the foundation that is covered by the epos ri and its delivery framework. they also represent the key players of the epos conception phase and the reference framework where data providers have been engaged in epos since its preparatory phase (2010-2014). most of the national research infrastructures engaged in epos are deployed for the monitoring of areas prone to geo-hazards and the surveillance of national territory including areas used for exploiting geo-resources. providing access to data, scientific products and information to different stakeholders requires effective solutions and harmonization with national and international priorities and strategies [atakan et al. 2022, this volume]. epos is responding to this need, and it has been designed to harmonize data and metadata and to ensure that new data products are accessible to both researchers and users across wider society. high-quality data require an effective and sustainable management over the entire data lifecycle. with this perspective, the thematic core services (tcs) have been established in epos since the implementation phase (2015-2019). the tcs2 represent the community-specific integration and they bring in the governance framework 2 https://www.epos-eu.org/tcs https://www.epos-eu.org/tcs massimo cocco et al. 4 that is needed to ensure the data and service provision (i.e., access to data, data products, services and software – ddss) within the epos delivery framework. currently, nine tcs (see figure 2), are formally established through the signature of a consortium agreement among different research organizations, and they are providing implemented and validated services that are integrated in epos. these are seismology, near fault observatories, gnss data & products, volcano observations, satellite data, geomagnetic observations, anthropogenic hazards, geological information and modelling, and multi-scale laboratories. one more scientific community, the tsunami data and services, recently joined epos as candidate tcs and it is presently discussing its governance structure and implementing its data and service provision [babeyko et al., 2022, this volume]. the ics has been implemented as the virtual environment where users can find, access, visualize and download data and data products through the novel epos data portal3 (see figure 3). the ics is composed of the central hub (ics-c) and the distributed core services (ics-d). the latter represents the virtual research environment that provides access to services for the direct analysis of aggregated datasets and for developing computational earth science applications and virtual research environments dedicated to solid earth science. the ics-c prototype has been implemented during the implementation phase (2015-2019) and it is currently under extensive testing for entering in the operational phase in 2023. operating the ics-c implies succeeding in making the different data, data products and services provided by tcs interoperable and accessible to users for fostering multidisciplinary research in solid earth science [bailo et al. 2022, this volume]. the ics-d has been designed during the implementation phase (2015-2019) and a prototype is currently under implementation [spinuso et al., 2021] also through related european commission projects coordinated by the epos community (such us the cheese project4). the ics central hub (ics-c) is hosted in france (brgm) and uk (ukri-bgs) and the hosting design has also involved geus (denmark). epos has been implemented according to a governance model in which the eco and the ics-c belong to epos eric and are located inside the eric perimeter, as illustrated in figure 1. the executive 3 https://www.epos-eu.org/dataportal 4 https://cheese-coe.eu figure 1. main elements of the epos architecture: nris, tcs and ics (ics-c & ics-d) form the epos functional architecture, coordinated by epos eric, and designed to ensure the epos data and service provision. the integrated core services central hub (ics-c) and the executive coordination office (eco) belong to the epos eric legal subject. https://www.epos-eu.org/dataportal https://cheese-coe.eu epos data integration in solid earth science 5 coordination office (eco), hosted in italy at the istituto nazionale di geofisica and vulcanologia, represents the legal seat of epos eric (see next section for further details). the eco, guided by the executive director, ensures the administration, management and day-to-day operation of the eric and the coordination of the epos delivery framework. 3. the epos delivery framework solid earth science data represent the knowledge base to unravel the physical and chemical processes that control earthquakes, tsunamis, volcanic eruptions, and ground instability, as well as processes driving tectonics and earth surface dynamics. understanding how the earth works as an interconnected system of these processes is critically important to modern society. indeed, society needs natural resources to support home life, industry, and business and it needs security in the face of natural hazards. solid earth science offers answers about how to maintain a safe, prosperous, and habitable planet, hence potentially addressing many of the 17 united nations sustainable development goals (un-sdg). the key challenge is to make the enormous wealth of available solid earth data openly accessible to promote crossand multi-disciplinary science for society. to tackle this challenge epos has proposed a federated approach to integrate solid earth science data according to the architecture described above. this federated approach relies on the construction of the thematic core services (tcs), where data and metadata are generated, quality-controlled and standardized, as well as of the integrated core services (ics) where data and metadata can be integrated and used. the epos delivery framework is composed of the data integration system represented by the tcs and the ics (hereinafter, tcs-ics system) as well as epos eric (see figures 1 and 3). figure 2. thematic core services currently integrated in epos massimo cocco et al. 6 the tcs represent the key component of the epos distributed ri and the novel solution to build and operate the epos federated approach for data integration. establishing the tcs is much more than linking national nodes by grouping nris. tcs represent the community governance of the data generation and management, ensuring participation, the sharing of the epos mission, the coordination of data and service providers according to agreed data policies and access rules, as well as the community building to tackle scientific challenges and innovation. the tcs are established through the signature of a consortium agreement, where the signatories are research institutions or organisations. the consortium takes the responsibility of the tcs governance and oversees the services and the data provision to epos through designated organisations (service providers); consequently, a legal agreement (collaboration agreement) is signed between epos eric and the service provider. the service providers bring together data providers through the shared epos data policy. therefore, the tcs represent the new collaborative framework for contributing to the operation and development of the epos ri. the pan-european dimension of the epos delivery framework is demonstrated by the number of countries and research organizations participating to the dataand service provision and to the tcs governance: 25 countries are involved in epos through nearly 140 research organizations. among them 65 research organizations and 4 international organizations are formally engaged in the tcs consortia, belonging to 21 countries out of the 25 involved in epos. the data and service provision is currently ensured by 35 service providers, which are making available nearly 198 validated data, data-products and services (ddss elements) provided through 238 services (figure 4). if we look at the details of the data and services delivered by tcs to epos, it is possible to appreciate that they are not only composed of services, existing or new, that support already accessible data and products, but that they also cover data and products which are not yet accessible to users. it is illuminating here to emphasize again the relevance of the integrated use of different datasets (see figure 4). at this point it is useful to briefly describe the contents of the data and products provided by each of the nine tcs formally established so far. figure 3. sketch showing the anatomy of the tcs-ics system: the data portal through the graphic user interface (gui) is the access point to ics-c for users. established tcs are illustrated in the figure. epos data integration in solid earth science 7 figure 4. histogram with ddss (data, data products, services, software) provision per tcs, including the tsunami candidate tcs. tcs seismology is providing access to data, data products and services for seismic waveforms, seismological products (such as information about recent and historical earthquakes, among many others) as well as seismic hazard and risk products [haslinger et al., 2022, this volume]. three international research organizations participate to the tcs: namely, orfeus (observatories & research facilities for european seismology, http://www.orfeus-eu.org) [cauzzi et al., 2021; haslinger et al., 2019] and its european strong-motion [lanzano et al., 2021] integrated data archive and service infrastructure (eida) [strollo et al., 2021] in charge of coordinating the access to waveform data, the emsc (european and mediterranean seismological centre, https://www.emsc-csem.org) [mueller et al., 1980] in charge of coordinating the integration and the access to seismological products and information about the seismic events in the european region and beyond and efehr (european facilities for earthquake hazard & risk, http://www.efehr.org/) [danciu et al., 2017] created over the last decade also with support from epos, providing open access [cotton et al., 2021] to data, models, tools and expertise to assess seismic hazard and risk in europe. tcs near-fault observatories (nfos) is providing access to multidisciplinary data, including borehole (seismological, geodetic and geochemical) data, and scientific products to study the anatomy of active faults in europe [festa et al., 2018]. by providing access to multidisciplinary data and products from different solid earth disciplines, such as geodesy, geochemistry, seismology and geology, the nfos will foster scientific progress in understanding seismogenic processes, ground shaking and earthquake forecasting. the tcs nfo currently involves six observatories, located across europe and the mediterranean region, in areas of elevated seismic hazard, providing highly accurate information on the mechanics and the geometry of active fault systems [chiaraluce et al., 2022]. it also includes a geo-hazard supersite (the marmara sea and istanbul) included in the geo work program (group on earth observations5, www.geo-gsnl.org) [ryan et al., 2014]. in addition, these on-site observatories can be considered as a natural laboratory to collect a broad range of signals related to the physical and chemical processes causing earthquakes and faulting, essential to model the causative processes down to very small spatial and temporal scales. these scientific data are also used to implement and operate innovative early warning systems for the rapid detection of earthquakes, real-time estimation of earthquake magnitude, and notification of expected 5 https://www.earthobservations.org/documents/gwp20_22/gsnl.pdf http://www.orfeus-eu.org https://www.emsc-csem.org http://www.efehr.org/ http://www.geo-gsnl.org https://www.earthobservations.org/documents/gwp20_22/gsnl.pdf massimo cocco et al. 8 shaking. this community has been established in epos and the data and services delivered by this tcs were not previously available in a structured, integrated, form. tcs gnss data and products (gnss) is providing access to data, metadata, scientific products and services to foster research on geodesy and the earth’s surface motion in different timescales [fernandes et al., 2022]. the tcs gnss is also coordinating the archiving of relevant gnss data, metadata and data products, promoting best practices for gnss station operation, data quality control and data management, maintaining and developing gnss data products and open-source software [menut et al., 2019]. in particular, the tcs gnss has developed the open-source glass platform to allow users to store and disseminate data and metadata from an integrated network of gnss stations operating in europe. the planned activities are performed in close partnership with the geodetic gnss community represented by euref, the iag sub-commission for the european reference frame (http://www.euref.eu). tcs volcano observations (vo) integrates the available multidisciplinary data (seismic, geodetic, electromagnetic, geochemical, and environmental data) collected on european volcanoes and their geodynamic surroundings [spampinato et al., 2019; puglisi et al., 2022]. the tcs vo provides scientists access to a portfolio of data, scientific products and services as well as coordinates the trans-national access to volcano observatories and facilities to foster scientific research on volcanic processes and related hazards. the tcs vo also includes two geo-hazard supersites (the italian and the icelandic volcanoes) declared in the geo work program (group on earth observations4, www.geo-gsnl.org). the data and the products provided by the tcs vo are also used to assess volcanic hazard and to support governmental and private decision-making. tcs satellite data (sd) uses satellite measurements to foster research on earth’s ground deformation phenomena and their causative processes, such as earthquakes, landslides and volcanic activity [manunta et al., 2019]. it develops, harmonizes and integrates these satellite measurements into services and products that can be exploited by the solid earth science community. the tcs sd will allow users to discover and download satellite data products generated over selected areas, including time series, and to remotely process satellite datasets. satellite data products are mainly generated through radar techniques from the esa sentinel-1 constellation of the copernicus program. the tcs sd is collaborating with the european space agency (esa) and its geohazards thematic exploitation platform (https://geohazards-tep.eu, gep) [foumelis et al., 2019], a user-driven environment specifically designed for the advanced exploitation of earth observation data. gep is using cloud processing facilities to enable the community to tailor different ways of visualizing data, in a user-friendly way. the tcs sd is also coordinating the interactions with national space agencies to foster the use of satellite data. tcs geomagnetic observations (geom) coordinates magnetic observatories and provides usable and easy access to geomagnetic data, taking advantage of the enormous amount of high-resolution data and large datasets that were made possible with the beginning of the digital era [flower et al., 2017]. access to accurate and integrated geomagnetic data is vital to the well-functioning of navigation systems, the interpretation of tectonic plates’ movement, and the accuracy of european mapping agencies, among other important applications. the tcs geom aims to consolidate the geomagnetic community and break down barriers to data access, by modernizing data archival and distribution and creating new services for magneto-telluric data and geomagnetic model. tcs anthropogenic hazards (ah) provides data on the human-induced seismicity in the form of “episodes” [orlecka-sikora, 2017; lasocki et al., 2022]. each episode consolidates geophysical data provided by public research infrastructures and industry on anthropogenic activities in a time-limited frame, thus allowing researchers to evaluate the temporal evolution of the seismic hazard throughout time. episodes are collected and organized in the episodes platform (https://tcs.ah-epos.eu) [orlecka-sikora et al., 2020] to foster both research and training on induced seismicity and hazards related to the exploration and exploitation of geo-resources. through this workspace, users can process the open access episodes, tailor the data visualization and analysis according to their needs with custom packages and collaborate with other researchers. tcs-ah coordinates the integration and the access to facilities, datasets, and scientific products on anthropogenic hazards. tcs geological information and modeling (gim) develops and integrates the information and data infrastructures operated by the international community, such as the european geological surveys [urvois et al., 2020; 2022]. shared reference datasets on geological maps, boreholes (vertical shafts constructed for water, oil, or gas extraction), 3d and 4d geological models, mineral resources, geohazards, and groundwater will be made accessible to the users through the epos data portal. by providing virtual access to borehole data and catalogues of geological maps, users can retrieve consolidated information concerning geological data and scientific products. the tcs gim is also coordinating the sharing of new data and information produced by researchers ensuring quality control http://www.euref.eu http://www.geo-gsnl.org https://geohazards-tep.eu https://tcs.ah-epos.eu epos data integration in solid earth science 9 and standardization, thus further engaging the geological community in the epos delivery framework also in collaboration with eurogeosurveys (https://www.eurogeosurveys.org). tcs multi-scale laboratories (msl) preserves and harmonizes experimental laboratory data that are representative of the physical and chemical processes acting at different scales [elger et al., 2022; willingshofer et al., 2021]. the tcs msl also coordinates trans-national access (tna) to over ninety laboratories affiliated with eleven institutes in eight european countries. they operate world-class experimental apparatuses, such as electron microscopes, deformation testing machines, and paleomagnetic measurements, divided into four sub-domains of the solid earth science: (i) analogue modeling of geologic processes allowing researchers to model the tectonic processes and analyze the properties of materials; (ii) rock and melt physical properties fostering research on earthquake mechanics and volcanic processes; (iii) paleomagnetic and magnetic measurements that provide records of the strength and polarity reversals of the earth’s magnetic field; (iv) geochemical and microscopy data, which facilitate the study of elements and minerals’ composition and the study of the rocks age. the tcs msl is establishing a collaborative network of european experimental laboratories for solid earth science. the tcs multi-scale laboratories will make available, through the epos portal, data, products, and software from several types of laboratory infrastructures while offering trans-national access (tna) to the newest and most advanced solid earth laboratory facilities in europe. a dedicated tna portal will be implemented in the ics-c [see wessel et al., 2022]. the multidisciplinary breadth of the data and service provision ensured by tcs corroborates the scientific impact of the epos ri and the community building behind the epos federated approach. the success in making all these data and scientific products interoperable and accessible through the ics-c in a fair compliant way [bailo et al., 2020] further confirms the importance of promoting the integrated use of these data to users and stakeholders. figure 5 illustrates the integration of data, metadata and services provided by tcs through the ics-c as well as the different stakeholders categories envisioned in epos. this figure depicts the centrality of the tcs-ics system as the engine of the epos delivery framework as well as the importance of providing access to distributed core services (namely, the ics-d) to fully exploit the multidisciplinary data and scientific products (see also figure 4) made available through the ics-c. worthy of note is the vision to engage scientists from other earth science domains to foster cross-disciplinary research, with particular attention to environmental science [kutsch et al, 2016; sorvari et al., 2016]. 4. epos eric an eric (european research infrastructure consortium) is a specific legal form that facilitates the establishment and the operation of ris with pan-european dimension. by granting the status of eric to a ri, the european commission acknowledges the new legal subject entitled to govern the ri. the establishment of an eric is a long-lasting process, which usually starts during the preparatory phase together with the design of the ri. the decision body of an eric is its general assembly composed of members representing national authorities and funding agencies. epos decided to adopt the eric model to govern and coordinate the delivery framework at the beginning of the preparatory phase in 2010. this decision has been then implemented and finalized during the two key stages of epos ri lifecycle, namely the preparatory phase (2010-2014) and the implementation phase (2015-2019). indeed, the european commission granted the eric status to epos on october 30, 2018. the main task of epos eric is to provide an effective governance framework to drive the integration and coordination of the tcs and to build and provide governance for the ics. epos eric has been established to design and implement a long-term plan for the integration of research infrastructures for solid earth science in europe. as anticipated above, the legal seat of epos eric, the executive coordination office (eco), is hosted in rome (italy) in the headquarter of the istituto nazionale di geofisica e vulcanologia (ingv). currently, epos eric is composed of thirteen country members (belgium, denmark, france, greece, iceland, italy, norway, poland, portugal, romania, slovenia, the netherlands, united kingdom) and one observer (switzerland). according to the epos architecture and the epos eric governance model, there are two elements included within the epos eric perimeter (figure 1) representing assets of epos eric: the eco and the ics-c. these two elements represent the capacity of epos eric to operate and govern its delivery framework. from a financial perspective, the costs for operating the eco and the ics-c are supported through the provision of host contributions by hosting countries, namely italy for the eco, and france and uk for the ics-c. host contributions are necessary to run the eco and the ics-c without financially impacting on the epos eric cash flow provided through the membership fees from the members and the observer of the eric. therefore, from a governance perspective https://www.eurogeosurveys.org massimo cocco et al. 10 the commitments of eric country members to provide host contributions to host and operate the eco and the ics-c strengthen the long-term sustainability of the epos ri. from a legal perspective, the partnership agreement signed by epos eric and the hosting organizations transforms the agreed financial and technical frameworks into an effective governance background for operating the epos ri. thematic core services (tcs) represent the foundation of the epos delivery framework. they are located outside the epos eric perimeter, but within the perimeter of the epos delivery framework (figure 1). this because the tcs have their own governance structure operated by the involved research organizations and scientific communities. from a technical perspective, each tcs is linked to epos eric through the interoperability layer (figure 5) built to establish the tcs-ics system. from a legal perspective, the tcs are linked to epos eric through dedicated collaboration agreements for the data and service provision and for their governance and coordination. according to the epos eric statutes, the costs for operating the tcs, as defined in the tcs cost-book, are only partially supported by the eric and mainly at national level by the involved research organizations. this is an essential element of the epos eric sustainability plan [see saleh contell et al., 2022, this volume], because tcs operation relies on in-kind resources provided by research organizations owning the nris as well as on further inkind contributions provided by national authorities (eric country members). presently epos is in a peculiar stage of the ri lifecycle, namely the transition from the implementation to the operational phase. the latter is referred to as the epos pilot operational phase (epos pop) and it is the stage in which epos eric is called to test both the readiness of the tcs-ics system and the robustness of its delivery framework to allow a successful transition to the operational phase in 2023. 5. discussion epos is a distributed ri of pan-european dimension, as corroborated by the number of countries (25) and research organizations (≈140) participating to its integration plan. the dimension of the epos distributed ri requires the adoption of an effective and shared governance to harmonize the national strategies and priorities with figure 5. the tcs-ics system representing the epos delivery framework. epos data integration in solid earth science 11 the epos needs and requirements [see atakan et al, 2022, this volume] while ensuring an added value for sharing resources and strengthening the sustainability of the delivery framework. the epos eric statutes have been implemented to regulate the governance and financial frameworks, according to the eric regulation (council regulation (ec) no 723/2009 of 25 june 2009), strengthening the epos eric role. the recognition of the epos role by the national authorities (eric country members) and the confirmation of their commitments to support the epos ri is essential to tackle the challenge of ensuring the long-term sustainability of the epos ri [saleh contell et al. 2022, this volume]. the participation of national and international ris to the epos integration plan has fostered their inclusion in the national roadmaps for ris, thus ensuring the provision of national funds to support their operation. this has certainly represented one of the main impacts of epos on the participating communities. the community building has been a key factor for creating the conditions to integrate nris and establish the tcs and the ics. the pro-active participation of solid earth scientists and e-scientists to the design, implementation, and construction of the epos delivery framework (that is, the novel tcs-ics system) represents a further positive impact of epos on its scientific community. it is worth noting that the co-design approach involving domain scientists and e-scientists to implement the epos ri has been adopted since the preparatory phase and it is continuing nowadays. this is also a key indicator of the wide perspective of epos contributions to multidisciplinary research in earth sciences as well as to cross-disciplinary collaborations in environmental science. epos is indeed cooperating with other ris in the environmental domain under the envri (environmental research infrastructures, https://envri.eu) [petzold et al., 2019] umbrella to support the sharing of data in earth sciences, fostering fair data management and cross-disciplinary research. the epos integration plan has still ample space for further developments, as for example promoting the use of data and services to generate new scientific products to be shared and made accessible through the epos data portal after quality-control and standardization. this corresponds to transforming users into data product providers, which directly contribute to foster open science and the culture of data sharing coherently with fair data management. these actions have to be coordinated by the thematic communities (tcs), because they are in charge of ensuring quality control and the adoption of shared standards for data and metadata. according to the epos vision and mission, the scientific community is the key enabler of innovation and scientific progress effectively supporting open science. the impact of the epos ri can be also measured in terms of the added value for science and society. the impact on science is corroborated by the size and diversity of the epos data and service provision, determining the contents of the epos delivery framework. it will be further assessed and, hopefully, confirmed by the number of users that will have access to the epos services and the number of applications and scientific products that will be shared through the epos platform when it will become operational. the added value for society is ensured by the use of solid earth science data for geo-hazards assessment and risk mitigation and by the engagement of the data and service providers formally committed to carry out the surveillance of the national territory from natural and anthropogenic events. this implies the necessity for epos to consider the ethical dimension of providing access to scientific information and services to stakeholders and society at large [see martì et al., 2022, this volume]. epos is not unique in the global landscape of research infrastructures. there are similar initiatives in australia (auscope, https://www.auscope.org.au) and united states (earthscope, 2003-2018, https://www.earthscope.org and earthcube https://www.earthcube.org/) which are dealing with the sharing of data and services in solid earth sciences. what makes epos unique is its federated approach involving different european countries, diverse national landscapes, and numerous data and service providers. moreover, the epos vision and mission to share and integrate data, fostering fair data management, is a current paradigm worldwide, placing epos in a global perspective in data science. indeed, the epos community is adopting concrete actions to make fair data principles a practice [bailo et al., 2020; bailo et al. 2022-a; bailo et al, 2022, this volume; cocco et al., 2019] and, importantly, to make fair data management sustainable for solid earth science in europe. 6. conclusions the european plate observing system (epos) is a distributed research infrastructure (ri) whose vision is to ensure sustainable and universal use and re-use of multidisciplinary solid earth science data and products fostering state-of-the-art research and innovation. consequently, the epos mission is to establish and underpin sustainable https://envri.eu https://www.auscope.org.au https://www.earthscope.org https://www.earthcube.org/ massimo cocco et al. 12 and long-term access to solid earth science data and services integrating the diverse european research infrastructures under a common federated framework governed by epos eric. the design and implementation of the epos integration plan has required a decade (2010-2019) in which the epos community has shared the vision and worked together to accomplish the mission. considerable advances in information technology and e-sciences have made the integrated approach possible by easing the access to the wealth of data and products available across solid earth science and related fields. during the past decade an enormous, highly skilled, workforce has been engaged in epos. the success of any ri relies on the engagement of highly motivated and skilled people as well as on the capacity to manage the human resources to have the right people in the right place, something which is a key element to tackle the challenge of sustainable operation of the ri. preserving their enthusiastic engagement is a priority action to deal with long-term sustainability. this article has presented and described the epos ri, emphasizing the successful outcomes of the preparatory and implementation phases, and has discussed the current construction stage that will yield to the operational phase in 2023. accessible datasets will bring novel cross-fertilization of ideas and lead to innovative research that is the key to foster a science for society. through the integration of data, models, and facilities epos will allow the earth science community to make a step change in developing new concepts and tools for key answers to scientific and socio-economic questions concerning geo-hazards and geo-resources as well as earth sciences applications to the environment and to human welfare. the work done to make the data and the services provided by tcs interoperable and integrated in the novel platform represented by the ics represents a good practice in data science to promote open science and make fair data management practicable. this makes epos visible and well positioned in the european and global landscape of fair data management. we have also presented the epos architecture and the delivery framework discussing the proposed federated approach to govern data integration. we have pointed out the strength of the approach and the impact on solid earth science, emphasizing the successful community building that is supporting the current epos construction. these results corroborate that the operation of the epos ri is viable. ensuring the long-term sustainability of a pan-european research infrastructure such as epos, is the current challenge that has to be tackled to plan and execute the operational phase for the next decades. acknowledgements. we wish to thank the epos eric service coordination committee composed of tcs and ics representatives, who are working hard to make the epos ri operational, as well as to the it board for the continuous efforts to ensure ri progress toward operation. we also wish to thank the executive coordination office whose staff is fully committed to build and operate epos eric. special thanks to all the people who worked actively in epos in past years, in particular to torild van eck and russ evans who passed away leaving us their heritage of humanity in this professional endeavor. we wish to thank helle pedersen for chairing the service coordination board and carine bruyninx for chairing the implementation phase council during the implementation phase. we wish to thank barbara angioni for her support in the graphic design and for the figures. this work has received funds from the ec project epos sp (contract number 871121) funded in the framework of the horizon 2020 program. references atakan k., m. cocco, b. orlecka-sikora, r. pijnenburg, l. ottemöller t. langeland, j. michalek, c. rønnevik c., olszewska d., górka-kostrubiec b., et al. 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data and facilities in epos: updates on services for the analogue modelling community in the tcs multi-scale laboratories, egu21-16301, copernicus meetings, https://doi.org/10.5194/egusphereegu21-16301. *co r r e s po n d i n g a u t h o r: massimo co cco, istituto nazionale di geofisica e vulcanologia, rome, italy, email: massimo.cocco@ingv.it https://meetingorganizer.copernicus.org/egu2019/egu2019-19201.pdf https://doi.org/10.1162/dint_a_00129 https://doi.org/10.1785/0220200413 https://doi.org/10.5194/egusphere-egu2020-5131 https://doi.org/10.1038/sdata.2016.18 https://doi.org/10.5194/egusphere-egu21-16301 https://doi.org/10.5194/egusphere-egu21-16301 mailto:massimo.cocco@ingv.it microsoft word ag_56.ft1.2014_casadio_online_.doc annals of geophysics, 56, fast track 1, 2013; 10.4401/ag-6535 the atmospheric composition validation and evolution workshop (acve2013) recommendations stefano casadio1,2, angelika dehn1, thorsten fehr1, bojan r. bojkov1 1esa-esrin, frascati, italy 2serco-ideas, c/o esa-esrin, frascati, italy i. introduction uring the last 18 years, the european space agency (esa) has provided the scientific community with a large amount of valuable atmospheric composition data produced by a series of instruments, starting with gome [burrows et al. 1999], onboard the ers-2 satellite (1995-2011), and followed by the gomos [kyrola et al. 2004], mipas [fischer et al. 2008], and the sciamachy [bovensmann et al. 1999], all flying on-board the envisat satellite (20022012). these observations will be continued by the sentinel-5 precursor, sentinel-4 and sentinel-5 and extended the earthcare and adm missions for aerosols and clouds. the quality assessment of atmospheric parameters retrieved from esa satellite instruments has been, and continues to be, performed through the joint effort of the scientific community and esa. the calibration and validation of atmospheric products (cal/val), for both measured radiances (level-1) and retrieved atmospheric parameters (level-2), is a continuous process that starts well before the launch of the satellite, and continues after the decommissioning of the platform. in this context, a large number of activities are promoted by esa with the support of the mission advisory groups (mags) during the pre-launch phases of the missions, and the quality working groups (qwgs) during the post-launch phases of the missions. the knowledge gained by esa’s cal/val activities is made publicly accessible to the atmospheric community through scientific and technical publications, as well as through a series of dedicated workshops and conferences sponsored by esa. the series of workshops dedicated to the cal/val of atmospheric products started with the “pre-launch workshop on the atmospheric chemistry validation of envisat (acve)” in 2001 [acve 2001], followed by the “envisat validation workshop” in 2002 [envisat validation 2002]. two years afterwards, in 2004, esa organised the second “atmospheric chemistry validation of envisat (acve– 2) [acve-2 2004], followed by the third “atmospheric chemistry validation of envisat (acve– 3)” in 2006 [acve-3 2006]. after each cal/val workshop, esa was provided by the participants with a comprehensive list of comments, suggestions and recommendations, critical for the successful operations of esa missions [snoeij et al. 2004, snoeij et al. 2007]. cal/val recommendations are also provided to esa in occasion of conferences with broader scopes. an important and recent example is represented by the atmos conference, organised by esa and held in bruges (belgium) on 18-22 june 2012 [atmos2012]. in particular, the recommendations regarding spectroscopy studies, the reprocessing capabilities, and the maintenance/enhancement of the validation infrastructure are important contributions to the cal/val activities. apart from specific recommendations to the ers-2 and envisat missions, a number of general considerations were raised during past acves. the most relevant recommendations are summarised below: d annals of geophysics, 56, fast track 1, 2013; 10.4401/ag-6535 • improve communication between level-1 and level-2 development teams; • provide information about data processor changes to users in a transparent way; • document and announce well in advance (whenever possible) all changes in processors, algorithms and initialisation files; • product user manuals should be well written, including detailed format and content description; • auxiliary files should be available and documented; • a well-defined validation reference set should be processed completely after each processor upgrade and analysed before reprocessing the complete mission. these recommendations have all been fulfilled by esa during the envisat mission lifetime, and are now guidelines for future missions. in march 2013, esa organised the “atmospheric composition validation and evolution (acve2013)” [acve 2013] workshop with a much wider scope including: 1) future missions and evolution [e.g., castelli et al. 2013]; 2) instrument calibration and level-1 products; 3) methods and instruments for validation [e.g., garcia et al. 2013]; 4) algorithms and products [e.g., raspollini et al. 2013]; 5) ozone [e.g., waymark et al. 2013]; 6) co, co2 and ch4 [e.g., del bianco et al. 2013]; 7) minor trace gases [e.g., papandrea et al. 2013]; 8) clouds, aerosols and water vapour; 9) methods and instruments for thermodynamic variables. scientists and cal/val experts have been provided with the opportunity to present latest results from their on-going research, including the evolution of algorithms and new validation methods, not restricted to esa missions. 125 participants from 14 countries were registered with the large majority from esa member states and a significant participation from the usa and canada. a set of recommendations for the evolution of the esa’s activities, resulting from the workshop discussions, are reported in the next paragraphs. the recommendations have been grouped in the following seven categories: calibration and level-1 products, spectroscopy, product development/evolution, algorithms and products review process, validation infrastructure, data utilisation, and evolution of validation needs. esa will carefully assess and prioritise the acve2013 recommendations, noting that a number of recommendations are already being addressed or are planned to be considered in the near future. in the next paragraphs, recommendations are identified with the ‘[recx]’ label, where x is an identification number with no reference to any priority ranking. calibration and level-1 important considerations on pre-flight activities have been raised. firstly, it was recommended to focus during the development phase on intensive instrument on-ground calibration activities, along with the provision of the related detailed documentation and the maintenance of the instrument models [rec1]. secondly, it was stressed that the end-to-end performance tests of the complete data processing system should be accomplished well before the satellite launch [rec2]. spectroscopy spectroscopy plays a fundamental role in the atmospheric remote sensing and the necessity of establishing a consistent spectral database is evident [rec3]. in particular, there is a strong need for support to the development of a homogenous ozone cross-section dataset across wavelength ranges [rec4]. in addition to ozone, supporting new spectroscopy activities on water vapour and methane in the swir (short wave infra-red) range, impacting the co retrievals around 2.3 μm, have been recommended [rec5]. product development and evolution establishing a program for new satellite products is needed, if possible coordinated among esa, the european commission (ec) and the national space/funding agencies [rec6]. esa should continue to coordinate the product developments of sentinel-5p (s5p), sentinel-4 (s4) and sentinel-5 (s5) [rec7], taking into account the individual performances of the different instruments. for future developments, a special focus needs to be on the absorption annals of geophysics, 56, fast track 1, 2013; 10.4401/ag-6535 cross-sections used in the retrievals for the interpretation of differences between products [rec8] and multi-missions calibration aspects. as follow on activity to the acve, esa organised an inter-instrument workshop the “eo level-1 lessons learned” in june 2013 covering degradation and pointing aspects. a specific recommendation from this workshop is to create a level-1 quality working group (qwg) across instrument families to enhance the information and analysis exchange. the necessity to create a priority list for the level-1b evolution tasks, as identified by the instrument qwgs, as well as to make it available to the science community for comments and suggestions [rec9] was recognised. the need to establish new mission performance criteria for existing products (e.g. envisat, ers-2) has been identified. these new criteria should be superseding pre-launch requirements, based on evolving user requirements and scientific developments, and should be used for prioritizing evolution activities [rec10]. this could be achieved by redefining the performance criteria for products through the revision of existing requirements as a joint effort from esa, qwgs and the user communities. in order to optimise the verification and validation process of new envisat/ers product releases, well defined verification and validation datasets need to be provided to the cal/val community [rec11]. in addition to the qwgs, it should also involve teams outside esa funded projects, i.e. through national and international activities. this activity would facilitate the establishment of a joint cycle for reprocessing and re-analysis within the atmospheric composition community [rec12]. algorithms and products review process the acve2013 community recommends the establishment of a review process for evolution new algorithms and new mission activities [rec13]. validation infrastructure an essential aspect for future esa cal/val activities consists in securing the existing cal/val infrastructure and establishing an entity taking over the ownership and the responsibility [rec14]. in parallel, new developments in validation infrastructure should be supported. in particular, the improvement of airborne max-doas systems [rec15], the creation of total carbon column observing network (tccon) [toon et al. 2009] sites characterised by different illumination/albedo conditions [rec16]. the use of mini-spectrometer instruments in the network for the detection of atmospheric composition change (ndaac) [rec17] shall be facilitated by providing support to the development and operation of instruments targeting air quality (aq). further investigating options to support "mini-interferometer" developments and operations for high-precision greenhouse gas measurements were addressed. this should be done in conjunction with the implementation of systems supporting the validation of minor trace gases (e.g. hcho, bro, oclo) and of tropospheric ozone [rec18]. the identification of reference validation sites for both past and future esa missions [rec19] should be addressed by revisiting the reference site concept with ceos and cvi for atmospheric composition and specifically air quality (aq) validation. in addition to aq instrumentation, the support to stratospheric balloon “anchor” measurements in the post-limb sounding era is essential in order to address the montreal protocol to understand the trends in the stratosphere, as well as for the assessment of the remaining limb observation systems [rec20]. esa cooperation and coordination with national environmental agencies and the european environment agency (eea, http://www.eea.europa.eu/) for satellite validation [rec21] shall be undertaken. the need to establish a legal framework for air quality monitoring that goes beyond the current regulations was also highlighted. this would require more sophisticated measurement protocols and instrumentation that can also be used for satellite validation [rec22]. a web portal covering esa and non-esa atmospheric validation activities, offering links to cal/val information and datasets, and allowing for the direct involvement of the international scienannals of geophysics, 56, fast track 1, 2013; 10.4401/ag-6535 tific community should be implemented, establishing links with non-esa cal/val teams, in particular regarding infrastructure [rec23]. evolution of validation needs a clear need to develop validation activities for aq products in "challenging" conditions has been identified during the acve2013 to be documented in an “air quality validation plan”. in particular, areas with strong pollution gradients, different cloud conditions, variable terrain types and slopes, and high aerosol loads should be considered for the installation of validation tools [rec24] [see e.g. richter et al. 2013]. in addition, the validation of background profile information, used in nadir retrievals and relative to the challenging conditions indicated above, is also required [rec25]. as a result the need to establish ground based instrument networks in polluted areas [rec26] was highlighted. it is recommended that esa facilitates and co-organises validation campaigns with a focus on aq satellite product challenges. support for the development and operation of instrument developments targeting aq (e.g., mini-spectrometers [tzortziou et al. 2012], aircore [karion et al. 2010]) and the implementation of a european minispectrometer/maxdoas network is needed. the necessity for the improvement of an effective characterisation of observation errors for satellite and correlative validation datasets has been identified [rec28]. data utilisation satellite data distribution is the core activity for a space agency, and improvements in this context are fundamental for the success of any satellite mission. the acve2013 community recommends to establish a “data product utilization advisory group” that should provide esa with feedback and advise on data accessibility and on prioritisation of developments [rec29]. a pressing need highlighted during the workshop is the harmonisation of the data format for all specific instruments, possibly adopting an established, selfdescribing, easy readable format [rec30]. this could be achieved by providing all operational esa atmospheric composition products in community accepted formats such as netcdf-4 with the netcdf climate and forecast (cf) metadata convention (http://cf-pcmdi.llnl. gov/). on the data dissemination, a coordinated access to esa, european and national data centres is urgently needed [rec31]. generally users need to have easy access to data, avoiding complicated registration procedures. [rec32]. conclusions the acve2013 provided an important and valuable forum for cal/val and algorithm scientists to present and discuss their results with the atmospheric composition satellite remote sensing community. in addition, it gave the opportunity to highlight and discuss a broad range of issues related to the satellite missions directly with esa. on the esa side, the intense exchange with the expert users is of highest priority to identify problems and to find opportunities to resolve them with the recommendations by the community, as well as to prepare future developments. esa will carefully assess and prioritise the recommendations and defining activities accordingly. a number of recommendations are already being addressed, e.g., first steps to a level 1b working group have been implemented, the existing ceos cal/val portal is being extended to cover atmospheric composition and a number of system developments are on-going or under preparation. the mission end-to-end data quality improvement and evaluation loop is a process open to the community. several mechanisms are already in place, e.g., the esa helpdesk, qwgs and workshops. in the near future, esa is committed to facilitate direct contacts between cal/val entities, through regular acve workshops in the coming years. references acve (2001). pre-launch workshop on the atmospheric chemistry validation of envisat (acve), 16-18 may 2001, estec, the netherlands, http://esamultimedia.esa.int/conferences/01 c13/index.html. annals of geophysics, 56, fast track 1, 2013; 10.4401/ag-6535 acve-2 (2004). atmospheric chemistry validation of envisat (acve– 2), 3-7 may 2004, esa-esrin, frascati, italy, http://earth.esa.int/envisat/workshops/acve 2/presentations/. acve-3 (2006). atmospheric chemistry validation of envisat (acve– 3), 4-7 december 2006, esa-esrin frascati, italy, http://earth.esa.int/workshops/acve3/. acve2013 (2013). atmospheric composition validation and evolution (acve2013) workshop, 13-15 march 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice piochi_corr.qxd editrice compositori the attached version of the paper by m. piochi, l. pappalardo and g. de astis, «geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition», fully replaces the one published in annals of geophysics, 2004, vol. 47, n. 4, pp. 1485-1499. i.r. 1485 annals of geophysics, vol. 47, n. 4, august 2004 key words lithosphere – asthenosphere – campanian province – tyrrhenian basin – geochemistry 1. introduction the campanian plain is located between the western margin of the apennine chain and the eastern border of the tyrrhenian abyssal plain (fig. 1a,b). since the pliocene the plain has been affected by intense extensive tectonic and magmatism (e.g., scandone, 1979; peccerillo and manetti, 1985; di girolamo, 1987; doglioni, 1991). at the present time the campanian plain is bordered by conjugate ne-sw and nw-se fault systems formed during the opening of the tyrrhenian sea. the volcanic districts of campi flegrei, procida and ischia, and the strato-volcano of somma-vesuvius developed on ne-sw structure lines (see beccaluva et al., 1991, for a review). campanian volcanoes are among the most studied volcanoes in the world. the many papers published in the last decade (e.g., civetta et al., 1991a,b, 1997; civetta and santacroce, 1992; belkin et al., 1993; santacroce et al., 1993; d’antonio and di girolamo, 1994; cioni et al., 1995; orsi et al., 1995; ayuso et al., 1998; d’antonio et al., 1999a,b; de vita et al., 1999; marianelli et al., 1999; pappalardo et al., 1999, 2002; piochi et al., 1999; del moro et al., 2001; somma et al., 2001) provide a wide data base of chemical and isotopic analyses. however the meaning of petrochemical variations mailing address: dr. monica piochi, osservatorio vesuviano, istituto nazionale di geofisica e vulcanologia, via diocleziano 328, 80124 napoli, italy; e-mail: monky5@ov.ingv.it geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition monica piochi, lucia pappalardo and gianfilippo de astis osservatorio vesuviano, istituto nazionale di geofisica e vulcanologia, napoli, italy abstract a spatial variation in chemical and isotopical composition is observed between the volcanoes belonging to the campanian comagmatic province. at a given mgo content, magmas from volcanic islands (procida and ischia) are enriched in ti, na, depleted in la, ba, rb, sr, th, k contents, and shows lower lree/hfse (e.g., la/nb = = 1-2), lower sr-pb isotopic ratios and higher nd isotopic ratios with respect to magmas from volcanoes located inland (campi flegrei and somma-vesuvius). the observed compositional variations are explained involving two different mantle sources in the genesis of the magmas erupted in this region: a deeper asthenospheric mantle source, from which the tyrrhenian magmas also derived and a lithospheric mantle source enriched by slabderived fluids. the contribution of the enriched-lithospheric mantle became more pronounced moving from the tyrrhenian abyssal plain through the italian peninsula where it dominates, likely in response to the thickening of the lithosphere observed under the peninsula. 1486 monica piochi, lucia pappalardo and gianfilippo de astis fig. 1a,b. a) southern italy map showing location of the campanian volcanic area and tyrrhenian abissal plain. bold line is the cross section in fig. 6; b) geological and tectonic sketch map of the campanian plain showing distribution of sedimentary sequences and campi flegrei, ischia, procida and somma-vesuvius volcanic rocks (modified from orsi et al., 1996). a b 0 km 10 shown by the erupted products is not completely clear, and a debate exists about the genesis and evolution of campanian magmas and its geodynamic setting (see turi and taylor, 1976; vollmer, 1976; hawkesworth and vollmer, 1979; cortini and hermes, 1981; vollmer et al., 1981; peccerillo and manetti, 1985; ellam et al., 1989; beccaluva et al., 1991; d’antonio et al., 1996; ayuso et al., 1998; peccerillo, 1999, 2001; somma et al., 2001). an open question about the nature of the most mafic magmas is whether they represent deep asthenospheric mantle magmas or reflects shallower imprinting acquired in the lithospheric mantle or in the crust. to shed light on this problem, our paper compares the available com1487 geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition positional data (major-, trace-elemental, and isotopes) from rocks erupted from campanian volcanoes both situated inland (somma-vesuvius and campi flegrei) and offshore (procida and ischia), with that from magmas erupted on the thyrrenian abyssal plain. in this way, the regional chemical and isotopic variations of magmas going from east to west can be investigated in relation to the deepening of the crust mantle boundary (ferrucci et al., 1989) and the increase in lithospheric thickness (cella et al., 1998) observed moving from tyrrhenian basin towards the apennine chain. we assume that if significant modification of the deep mantle magmas occurs en route to the surface and/or by the crust, then such a process should be revealed by comparing the geochemical and isotopic signatures of offshore and inland volcanic complexes. 2. geological setting the campanian volcanoes are part of the potassic-rich italian belt which developed on the eastern side of the tyrrhenian sea whose abyssal plain is composed of morb-like basaltic rocks (beccaluva et al., 1990). they constitute the campanian comagmatic province that is distinguished from the roman one on the basis of some important geochemical differences in the outcropping volcanics (duschenes et al., 1986; beccaluva et al., 1991). the two provinces are separated by a lithospheric discontinuity corresponding to the 41° parallel (fig. 1a,b) dividing two different mantle domains (savelli and wezel, 1979; lavecchia, 1988; spadini and wezel, 1994; bruno et al., 2000). the crustmantle boundary beneath the campi flegrei and somma-vesuvius has been identified at 25-30 km, whereas it rises moving westward, being at less than 25 km beneath ischia (corrado and rapolla, 1981; ferrucci et al., 1989) and less than 10 km in the central tyrrhenian basin (duschenes et al., 1986; gueguen et al., 1997). the nature of the lower crust is not known although the correlation with the southern apennine structures (finetti and morelli, 1973; schutte, 1978; scandone, 1982) and petrological studies on xenoliths (pappalardo et al., 2002) suggest that it should be a hercynian basement. the upper crust consists of a succession of triassic to cretaceous limestones and dolomites overlain by miocene arenaceous and/or flysch sediments, or by pyroclastic rocks (d’argenio et al., 1973; ippolito et al., 1975) cropping out in the apennine chain. this succession, which is displaced down to 4000 m depth in the campanian plain (d’argenio et al., 1973; ippolito et al., 1975; d’argenio et al., 1987), has been drilled at depths of about 2.4 km beneath the sommavesuvius (brocchini et al., 2001) and has been identified by seismic profiles at depths of more than 3-4 km in the gulf of napoli and pozzuoli (finetti and morelli, 1974; bruno et al., 1998). the campi flegrei, and the ischia and procida islands are volcanic fields of the phlegraean volcanic district characterized by several monogenic vents that produced both low-energy explosive and effusive eruptions. high-volume ignimbrite eruptions also occurred in the area at least from 200 kyr bp, some of these eruptions generated large caldera structures at campi flegrei and ischia (e.g., rosi and sbrana, 1987; vezzoli, 1988; orsi et al., 1996; de vivo et al., 2001). volcanism is broadly coeval at campi flegrei and ischia where the onset of volcanic activity is dated before 80 kyr and 150 kyr and lasted up to 1532 a.d. and 1302 a.d., respectively (alessio et al., 1971; gillot et al., 1982). volcanic activity occurred from about 80 kyr up to 14 kyr at procida (see rosi et al., 1988; beccaluva et al., 1991). somma-vesuvius is a strato-volcano that experienced lava flows and small-to-high scale energy eruptions; the oldest somma-vesuvius volcanic products are dated at 0.4 myr (brocchini et al., 2001) and the last eruption occurred in 1944 a.d. at the present time, campi flegrei, ischia and somma-vesuvius are the site of widespread fumaroles and thermal springs, as well as seismic activity. furthermore, bradyseismic phenomena, characterized by ground movements and seismic activity affected campi flegrei in 1972, 1984 and 2000 a.d. 3. petrological setting the campanian volcanic rocks belong to the ks series of appleton (1972) and range in composition from shoshonites to trachy-phonolites. 1488 monica piochi, lucia pappalardo and gianfilippo de astis the latter are the most widespread products. at somma-vesuvius milddly to highly undersaturated rocks belonging to the hks of appleton (1972), ranging in composition from alkalibasalt to phonolite, also occur. the campanian rocks have nearly aphyric to strongly porphyritic texture: generally the porphyriticity is higher in somma-vesuvius with respect to phlegraean rocks, and in ischia with respect to the campi flegrei and procida rocks. in the ks evolved rocks k-feldspar, plagioclase, fe-rich diopside, magnetite and biotite represent typical minerals. in the least evolved ones, k-feldspar and biotite are absent, olivine and mg-rich diopside join the other phases reported. apatite is the most common accessory crystal. the least evolved hks rocks from somma-vesuvius contain olivine, plagioclase and fe-rich and mg-poor diopside. leucite is the most common mineral in the most evolved rocks. garnet and phlogopite are very common accessory phases. major and trace element and sr-nd-pb isotope variations within the campanian rocks have been attributed to complex evolutionary processes involving magmas chamber refilling, magmas mixing and crustal assimilation by mantle-derived magmas in a multi-depth magmatic system (e.g., cioni et al., 1995; ayuso et al., 1998; d’antonio et al., 1999a; pappalardo et al., 1999, 2002; piochi et al., 1999; del moro et al., 2001). in particular, the existence of both deep and shallower crustal reservoirs has been proposed in several studies of the campanian volcanoes (belkin et al., 1985; cortini et al., 1985; piochi et al., 1999; marianelli et al., 1999; pappalardo et al., 2002; lima et al., 2003; de astis et al., 2004). inside the phlegraean volcanic district, the deeper reservoir was tapped by a regional fault system during eruptions extruding the least evolved magmas that mingled during ascent with magmas evolving at shallower depth (de astis et al., 2004; and references therein). compositional differences have been detected in volcanic rocks from ischia island with respect to those from campi flegrei and somma-vesuvius (e.g., turi and taylor, 1976; cortini and hermes, 1981; paterne et al., 1988; vezzoli, 1988; pappalardo et al., 1999), although their meaning has not been thoroughly investigated. the source of magmastism has been located in a mantle variably enriched in incompatible elements, radiogenic sr and unradiogenic nd (e.g., hawkesworth and vollmer, 1979; peccerillo and manetti, 1985; beccaluva et al., 1991; d’antonio et al., 1996; ayuso et al., 1998; d’antonio et al., 1999b; peccerillo, 1999, 2001). debate exists about the agent of enrichment that should be i) mantle-derived fluids in a intra-plate tectonic setting (e.g., cundari, 1980; vollmer, 1989) or ii) fluids or melts released by an undergoing oceanic slab (e.g., di girolamo, 1978, 1987; hawkesworth and vollmer, 1979; peccerillo and manetti, 1985; peccerillo, 1990, 2001; beccaluva et al., 1991; serri et al., 1993; d’antonio et al., 1996) which modify a oib-type (e.g., beccaluva et al., 1991) or a morb-type mantle (e.g., d’antonio et al., 1996). recently, the discovery of xenoliths of crustal origin in the least evolved 87sr-enriched rocks from the campi flegrei strongly contributed to highlight both the role of crustal contamination in magmas composition and the existence of a homogeneous mantle source beneath the phlegraean area (pappalardo et al., 2002). the nature of xenoliths allows recognition of at least two crustal levels of contamination the deeper of which located in the hercynian basement. following cecchetti et al. (2001), this depth is about 10-15 km and is similar to that proposed by belkin et al. (1985), belkin and de vivo (1993), marianelli et al. (1999), lima et al. (2003) and by zollo et al. (1996) for somma-vesuvius. crustal contamination has been proposed also at somma-vesuvius on the basis of mineral chemistry, fluid inclusion and isotope data (e.g., savelli, 1968; fulignati et al., 1995, 1998; gilg et al., 1999, 2001; del moro et al., 2001; pappalardo et al., 2004). 4. data presentation 4.1. geochemistry major and trace elements have been published in previous papers (civetta et al., 1991a,b, 1997; civetta and santacroce, 1992; orsi et al., 1992, 1995, 1996; caprarelli et al., 1489 geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition 1993; belkin et al., 1993; santacroce et al., 1993; d’antonio and di girolamo, 1994; cioni et al., 1995; ayuso et al., 1998; d’antonio et al., 1999a,b; de vita et al., 1999; marianelli et al., 1999; pappalardo et al., 1999, 2002; piochi et al., 1999; del moro et al., 2001; somma et al., 2001); the database is available on request from the authors. figure 2 shows selected major and trace element variation diagrams versus sio2. these diagrams, although sometimes scattering mainly as consequence of different analytical procedures fig. 2. selected major and trace elements variation diagrams versus sio2. source of data: civetta et al. (1991a,b, 1997); civetta and santacroce (1992); orsi et al. (1992, 1995, 1996); caprarelli et al. (1993); belkin et al. (1993); santacroce et al. (1993); d’antonio and di girolamo (1994); cioni et al. (1995); ayuso et al. (1998); d’antonio et al. (1999a,b); de vita et al. (1999); marianelli et al. (1999); pappalardo et al. (1999, 2002); piochi et al. (1999); del moro et al. (2001); somma et al. (2001). symbols: × = lava ejecta in solchiaro deposits (procida); full diamonds: procida samples; full triangles = ischia samples; open squares = campi flegrei samples; open circles = vesuvius samples; ∗ = lithic clasts in campi flegrei deposits. 1490 monica piochi, lucia pappalardo and gianfilippo de astis fig. 3a,b. a) ree patterns for selected campanian magmas. normalization on the basis of nakamura (1974); b) trace elements distribution for selected campanian magmas. normalization on the basis of pearce (1983). symbols as in fig. 2. dashed area indicates the intra-plate component following thorpe et al. (1984). a b (icp or xrf) and crystal contents (see previous section), display increasing na2o content and decreasing p2o5, tio2, mgo, and fe2o3 contents. k2o increases up to about 60 wt% of sio2 and then decreases; mno decreases up to 60 wt% of sio2 then became strongly scattered. the most mafic compositions (mgo > > 9wt%) are represented just by some rocks from procida. la, ce, rb, y, zr, nb and th increase by factors up to 2 from mafic to silic compositions; sc, cr, co ni, eu, v show an opposite trend. ba is firstly strongly scattered in the range 45-55 wt% of sio2 content and then decreases, whereas in the same range, sr is initially constant and then decreases. some samples from somma-vesuvius, related to a single eruption, define a separate array. the most evolved rocks from campi flegrei and ischia show a steep enrichment in some incompatible elements (e.g., th, ree). magmas erupted at somma-vesuvius, define a trend depleted in ti, mg, fe, p, ca, v and enriched in al, nb, rb, th, k, na with respect to the campanian trends (fig. 2). procida, campi flegrei and ischia magmas show chemical similarity, although, at a given sio2 content, the ischia magmas are enriched in ti, na, nb, yb and depleted in ba, sr, th, eu (fig. 2). however, in the least evolved rocks (mgo > 6 wt%) from each volcanic district display reasonably constant ratios between elements with similar degree of incompatibility. for example (fig. 2), la/ce, th/zr and, nb/zr are 0.5 ± 0.1, 0.06 ± 0.04, 0.14 ± 0.06, respectively. some variations could be related to different analytical procedures and analytical error. ree and other trace element distributions are reported in fig. 3a,b, where campanian magmas of similar degrees of evolution (3.8 < mgo < 5 wt%) are compared. the only exception in these spiderdiagrams are the campi flegrei rocks with mgo contents less than 3.8 wt% that pappalardo et al. (2002) suggest are representative of the least-contaminated magmas. ree and trace element patterns are roughly sub-parallel to each other with no difference between products from each district. generally, all selected samples show enrichment in lrees and other incompatible trace elements (rb, k, th, sr, ba), and minor enrichment in more compatible elements such as hree and y. negative nb and eu anomalies characterize the campanian rocks, although their amplitude decreases toward the less evolved samples. mafic rocks from procida define a less enriched pattern, have lower ree and incompatible trace element contents coherently with their more primitive nature, and no detectable eu anomaly. 4.2. isotope geochemistry 87sr/86sr and 143nd/144nd ratios are variable and depict a trend from procida towards the 1491 geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition crustal-derived xenoliths recovered in campi flegrei rocks (fig. 4a). magmas erupted at ischia are isotopically distinct from that of somma-vesuvius and campi flegrei: sr-isotope composition ranges from 0.7060 to 0.7068 at ischia and from 0.7068 to 0.7089 at sommavesuvius and campi flegrei. 87sr/86sr ratios also vary from 0.7051 to 0.7060 in the procida volcanic rocks. nd-isotope composition ranges from 0.51245 to 0.51265 in the ischia rocks and from 0.51230 to 0.51265 at campi flegrei and somma-vesuvius. 143nd/144nd values range from 0.51255 to 0.5127 in rocks from procida. 5. discussion the absence of «primary» magmas compositions at campi flegrei, ischia, and sommavesuvius prevents knowing directly the nature of the mantle source. most of the mafic rocks produced in the campanian region have much lower cr, ni and mgo contents with respect to those indicated by perfit et al. (1980) (500-600 ppm, 300 ppm and 6 wt%, respectively), to have been in equilibrium with a peridotite magmas source. some «primitive» compositions have been recognized in the lava ejecta on procida island and on the basis of their high mgo, cr and ni contents (10-11 wt%, 426-610 ppm, 134-233 ppm, respectively) they were considered nearly primary magmas (d’antonio et al., 1996, 1999b). the features of the majority of the campanian volcanic rocks are characterized by high sio2 contents and high porphyritic indicis, and the absence of mantle-derived xenoliths. these features suggest that the primary magmas underwent shallow-level differentiation processes after segregation from their mantle source. the isotopic variations reported above, not associated with fractional crystallization (fc) process in closed reservoirs, have to be produced by interactions of parental magmas with crustal rocks en route to the surface or by addition of crustal materials to the source. therefore, as we are interested in identifying the nature of the source, we will discuss the rock features inherited by shallow-level processes and then deal with the source related features. 5.1. shallow level magma evolution most of the chemical variations in campanian rocks are consistent with a differentiation mechanism involving the fractionation of the observed mineral phases (fc) within closed magmatic systems. the trend defined by sr and eu respect to the silica content (fig. 2) and the existence of eu anomaly in the most evolved rocks (fig. 3a,b) are consistent with the role of feldspar (the main crystal phase) crystallization during magmas evolution. the fc process is supported by the constancy of the sr-isotopic ratios in rocks from some eruptive events at ischia (civetta et al., 1991b; piochi et al., 1999), campi flegrei (pappalardo et al., 1999) and somma-vesuvius (ayuso et al., 1998 and referfig. 4a,b. selected lile/hfse diagrams for campanian rocks with mgo > 3.8%. symbols: full crosses = tyrrhenian samples (beccaluva et al., 1990; gasperini et al., 2002). other symbols as in fig. 2. a b 1492 monica piochi, lucia pappalardo and gianfilippo de astis ences therein). on the other hand, variations of the isotopic compositions of magmas with time have generally been related to mixing processes between distinct parental magmas batches reaching a shallow reservoir (cioni et al., 1995; d’antonio et al., 1999b; pappalardo et al., 1999; piochi et al., 1999). new studies on campi flegrei (pappalardo et al., 2002), on somma-vesuvius (del moro et al., 2001; pappalardo et al., 2004) and ischia (piochi et al., 1999) eruptions have also highlighted that the variability of the sr-isotope ratio can be explained by crustal contamination in a multidepth magmatic system. in these cases, magmas show an interaction with the wall-rocks because they contain crustal xenoliths (campi flegrei) and hornfelsed clasts (somma-vesuvius) with sr isotope ratios > 0.708 and sometimes show chemical and mineralogical disequilibria. moreover, pappalardo et al. (2002) showed that the sr-isotopic ratios increase in the younger campi flegrei products as their residence time in the magmas chamber increases. furthermore, crustal interaction is indicated by the negative correlation described by campanian volcanics and crustal xenoliths on the classical isotope diagram of sr and nd (fig. 5a). finally, crustal contamination for the campanian magmas is suggested by the la/nb ratio which is always > 1 (thompson et al., 1984) and positively correlated to the sr isotope ratio. calculation based on the quantitative ec-afc (energy conservation-assimilation fractional crystallisation) approach that accounts for mass and energy conservation (bohrson and spera, 2001; spera and bohrson, 2001) indicates that the isotopic variations relative to the magmas feeding eruptions at campi flegrei and somma-vesuvius are explained by contamination with less than 40% of crustal rocks (pappalardo et al., 2002, 2004). despite the evidence of crustal interaction, there is a wealth of papers which suggest the existence of a «crucial» sr-isotopic value below which we can assume to deal with «mafic rocks». a correlation is observed between the sr-isotopic ratios of rocks and their crystallization depth at somma-vesuvius (mastrolorenzo et al., 2003; pappalardo et al., 2004). olivines and pyroxenes from somma-vesuvius rocks crystallized at depths >10 km (belkin et al., 1985; cortini et al., 1985; belkin and de vivo, 1993; marianelli et al., 1999; lima et al., 2003) have sr-isotopic ratios lower than 0.7074 (civetta and santacroce, 1992; ayuso et al., 1998, and references therein). these values are significantly different from those of the late-crystallized feldspars (crystallization depth < 5 km; belkin et al., 1985; cortini et al., 1985; belkin and de vivo, 1993; lima et al., 2003), with 87sr/86sr values around 0.7075-7 (civetta et al., 1991a; civetta and santacroce, 1992; cioni et al., 1995; ayuso et al., 1998, and references therein). moreover, clinopyroxene cumulitic rocks from somma-vesuvius and campi flegrei (ayuso et al., 1998 and references therein), display 87sr/86sr below 0.7065. pappalardo et al. (2002) exclude that the isotopic variations at campi flegrei are related to in situ radioactive growth, because a much longer time than that of the magmatic system (ca. 60 kyr) would be required to produce the observed isotopic variability, and indicate that petrological variation not reflect mantle melting processes and/or mantle source(s) heterogeneity because the constancy of ratios between elements with similar degrees of incompatibility of the products. the authors, on the basis of a correlation between sr-isotopic ratios and age, and of sr-isotopic similarity between the least radiogenic rocks and the mantle xenoliths from vulture, have linked the least contaminated magmas erupted at campi flegrei with those erupted before the campanian ignimbrite eruptions characterized by a sr-isotopic ratios in the range 0.7067-0.7073. on the basis of these studies, we can choose as the least contaminated campanian rocks those showing sr-isotopic ratios lower than 0.70735. we can compare them with the mafic rocks from procida and ischia, which have 87sr/86sr widely below this limit and geochemical features akin to more primitive rocks. 5.2. primary magma compositions and the nature of mantle sources the possibility to discriminate the role of the shallow evolutionary processes (in particular crustal contamination) allow some representative mafic or near-mafic rocks to be investigated fur1493 geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition ther although they have variable mgo contents (3.8-10-11 wt%). these rocks that approach or mirror the «original» geochemical and isotopical composition are those reported in figs. 3a,b, 4a,b and 5a-c. in the following, the campanian rocks we have chosen will be compared with the transitional morb-type basalts erupted in the contiguous sector of tyrrhenian basin (vavilov basin, sites 655 and 651 of odp leg. 107 and dsdp site 373, leg. 42; beccaluva et al., 1990; gasperini et al., 2002) and with the intraplate basalts erupted on the opposite margin of the sea basin on the sardinia coast (site 654). the comparison among most mafic rocks from volcanic islands and from volcanoes located inland, as regards the major, ree and several trace elements pattern (figs. 2 and 3a,b) reveal some significant differences. at a given sio2 content, ischia and procida magmas are enriched in ti, na, depleted in la, ba, rb, sr, th, k contents, and show lower lree/hfse (e.g., la/nb = 1-2) with respect to the campi flegrei and somma-vesuvius magmas. furthermore, lile/hfse ratios for rocks characterized by mgo exceeding 3.8% increase moving west to east through the campanian volcanic area (fig. 4a,b). this trend is highlighted if we consider data (beccaluva et al., 1990; gasperini et al., 2002) for morb-like magmas erupted in the contiguous sector of tyrrhenian basin (vavilov basin, sites 655 of odp leg. 107 and dsdp site 373, leg. 42) that is also reported in fig. 4a,b. the least contaminated campanian samples define a unique trend when plotted in the 87sr/86sr versus 143nd/144nd diagrams (fig. 5a). procida shoshonitic basalts have the lowest sr a b c fig. 5a-c. a) 143nd/144nd, b) 208pb/204pb, c) 206pb/204pb versus 87sr/86sr diagrams for selected campanian magmas. symbols: open crosses = etna samples (tanguy et al., 1997); full crosses = tyrrhenian samples (beccaluva et al., 1990; gasperini et al., 2002); field for the vulture mantle xenoliths (a) from downes (2001) and hypothesized (c) on the basis of considerations from downes (2001) and dunai and baur (1995). other symbols as in fig. 2. 1494 monica piochi, lucia pappalardo and gianfilippo de astis ratios (0.7050) and highest nd (0.5128) ratios in the campanian magmas, whereas the campi flegrei and somma-vesuvius rocks have the highest sr ratios (0.7073) and the lowest nd ratios (0.5126). ischia magmas plot between these two compositions (fig. 5a). the sr-isotopic differences are highlighted in 87sr/86sr versus 206pb/204pb diagrams (fig. 5b,c). as in fig. 4a,b, in these isotope-isotope diagrams (fig. 5a-c) we have also drawn the field for morb-like magmas erupted in the contiguous sector of tyrrhenian basin (vavilov basin, sites 655 of odp leg. 107 and dsdp site 373, leg. 42) interpreted as derived from an upwelling asthenospheric mantle (beccaluva et al., 1990) probably having a plume-like imprint (gasperini et al., 2002). in the sr-nd isotope diagram (fig. 5a), the primitive magmas erupted at procida plot between the campi flegrei and somma-vesuvius magmas and the tyrrhenian magma field. this is supported also by sr-pb trends (fig. 5b,c) where the relationship between the tyrrhenian and the campanian magmas is highlighted. therefore, we consider the tyrrhenian basalts as end-members for the isotopic trend of fig. 5a-c. as we have excluded from the discussion the crustally contaminated rocks, we assume that the other end member is also located in the mantle domain. the trace element patterns of the campanian magmas (fig. 3a,b) are similar to intraplate-magmas with respect to immobile elements zr, hf, ti, y, yb and, at to a lesser extent, nb but they are enriched in low ionic potential elements. recent studies (kostoula et al., 1999; downes, 2001) suggest a lithospheric origin for the mantle xenoliths recovered in the volcanic products from vulture volcano located to the east of the campanian plain. these xenolith samples are geochemically and isotopically similar to the campanian magmas (fig. 5a-c). on this basis we suggest that the other end member could be sub-continental enriched lithospheric mantle. in particular, the enrichment in the low ionic potential elements, especially rb and ba, the small negative anomaly at nb, and the ratio between incompatible elements (e.g., th/yb > 2) should be generated by the interaction with subduction related fluid as also suggested for the vulture xenoliths (downes, 2001). following peccerillo and manetti (1985) the higher k-enrichment of the most somma-vesuvius rocks with respect to the other campanian rocks should be the consequence of melting at greater depths. trace element distributions and isotope compositions of the volcanic rocks from the islands (higher ti and p content and lowest sr-pb isotope ratios) compared with those from the mainland volcanoes, indicate that the asthenospheric contribution is more evident in the offshore volcanoes (procida and ischia) and it tends to vanish moving inland (campi flegrei and sommafig. 6. trace elements distribution for procida mafic magmas normalized with respect to sample 654 from western margin of tyrrhenian sea (data from beccaluva et al., 1990), to samples 655 from vavilov basin (data from gasperini et al., 2002) and to samples from sardinia mafic volcanism (data from lustrino et al., 2000). 1495 geochemical and isotopical variations within the campanian comagmatic province: implications on magma source composition vesuvius) where the lithospheric contribution becomes dominant. this can be a consequence of the progressive drop of the lithosphere-asthenosphere boundary moving from the tyrrhenian basin towards the appennine chain. its depth varies from 25 km under the tyrrhenian basin, to 65 km beneath the thyrrenian coast of italy and finally up to a depth of 100 km under the apenninic chain (cella et al., 1998). to test this hypothesis we compared the primitive procida magmas (mgo = 11 wt%) with the anorogenic intraplate tholeiitic magmas erupted on the other border of tyrrhenian sea (mgo = 7 wt%), near the sardinia coast (site 654), in the vavilov basin (site 655) and on the sardinia island (fig. 6). the similarity between procida and sardinia mafic rocks, also in term of sr, nd and pb isotope ratios (see lustrino et al., 2000), as well as the similarity in immobile elements (i.e. sr, nb, zr, sm, yb) contents shown by procida magmas and tyrrhenian magmas, evidence that moving radially away from the basin toward its eastern (italian peninsula) and western (sardinia island) borders, the contribution of the asthenospheric morb-type mantle decreases in favour of that of the «infig. 7. interpretative cross section from tyrrhenian sea towards the campanian margin as indicated in fig. 1a,b. chemical and isotopical variations moving from the tyrrhenian basin to the campanian comagmatic province and to sardinia are shown. 1496 monica piochi, lucia pappalardo and gianfilippo de astis traplate» lithospheric mantle. furthermore, moving away from the tyrrhenian basin the contribution of subduction increases. 6. conclusions a spatial variation in chemical composition has been recognised among the volcanoes belonging to the campanian comagmatic province (fig. 7). when the role of low-p evolution processes has been evaluated, the most primitive (not contaminated by crustal assimilation) rocks erupted from procida and ischia islands have: i) lower lile/hfse ratios with respect to those erupted at campi flegrei and somma-vesuvius. these ratios are insensitive to partial melting and fractionation processes and can be considered to be representative of the magmas source; ii) lower sr, pb isotopic ratios and higher nd isotopic ratios with respect to that of campi flegrei and somma-vesuvius. these compositional variations can be explained assuming that both asthenospheric and «intra-plate» lithospheric mantle sources are involved in the genesis of the «primitive» magmas erupted in the campania comagmatic region. the asthenospheric mantle, from which tyrrhenian magmas were extracted, represents the deeper source. moving from the tyrrhenian abyssal plain to the italian peninsula the contribution of enriched-lithospheric mantle became more pronounced in response to the increasing lithospheric thickness that magmas must cross en route to the surface (fig. 7). the involvement of the lithosphere may be explained by geotherms uprising related to the asthenospheric upwelling. this hypothesis is supported by the presence of a low-velocity body underneath somma-vesuvius (de natale et al., 2001) which extends from 15 km down to the upper mantle (35 km) and represents the presence of a partially molten body into the lithosphere. acknowledgements we thank c. hawkesworth, an anonymous reviewer and b. de vivo that provided very helpful comments, greatly improving the manuscript. we are also grateful to those people, although opposing the progression of our research, have contributed to increase our interest in volcanological and geochemical studies. references alessio, m., f. bella, g. belluomini, g. calderoni, c. cortesi, m. fornaseri, m. franco, f. improta, a. scherillo and b. turi (1971): datazioni con il metodo c-14 di carboni e di livelli humificati (paleosuoli) intercalati nelle formazioni piroclastiche dei campi flegrei (napoli), rend. soc. it. mineral. petrol., 27 (2), 305-317. appleton, j.d. 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e-mail: patane@ct.ingv.it key words mt. etna 2001 eruption – seismicity – fault plane solutions – dike intrusion 1. introduction the study of seismic activity is a powerful tool for understanding both the inner structure of a volcano and its eruptive behavior. over the last 30 years, seismic data collected at mt. etna volcano have provided information on both the dynamics and structure of the volcano. the most evident consideration regarding the volcano’s behavior before, during and after an eruption originates from the analysis of spacetime distribution and fault plane solutions of earthquakes. a general observation is that summit eruptions are not preceded by a seismic crisis; whereas lateral eruptions are usually preceded by a 1-2 day seismic crisis (e.g., gresta and patanè, 1987; ferrucci and patanè, 1993). in such a case, only the smallest and shallowest events have generally occurred close to the eruptive fissure, the largest shocks being spread over much larger areas than the eruptive ones (e.g., gresta and patanè, 1987). an example of such behavior was given during the onset of the december 1985 and the october 1986 february 1987 eruptions. the two eruptive fractures opened on the eastern side of the volcano, in valle del bove and valle del leone calderas, respectively. 600 domenico patanè et al. simultaneously with their opening, two shocks (both of magnitude 3.8-4.0) occurred, about 7 km away on the northeastern side, along the provenzana-pernicana fault system (gresta, 1995). also the strongest (m d = 4.5) earthquake, recorded during the swarm accompanying the onset of the 1991-1993 eruption, occurred several kilometers from the eruptive fissure (ferrucci and patanè, 1993; bonaccorso et al., 1996). these case histories disclose that the seismic activation of fault systems at mt. etna often occurs as a response to accommodate deformations induced by the magma injection. further interpretation on the eruptive dynamics may be taken from fault plane solutions (fpss) and space-time migration of hypocenters, during the months preceding eruption onsets. for example, the 1991-1993 flank eruption was preceded by a change in the local stress field orientation (e.g., patanè et al., 1994; bonaccorso et al., 1996; cocina et al., 1998). it was suggested that an upward migration of magma along the nnw-sse structural trend and its intrusion into the upper crust was responsible for the rotation of the local stress field observed from fpss of shocks at depths less than 10 km (bonaccorso et al., 1996; barberi et al., 2000; patanè and privitera, 2001). seismicity that occurred before and during the 1991-1993 eruption along the seismogenic ne-sw structural trend, was considered to be the brittle response to magma intrusion along the nnw-sse volcano-tectonic system. in fact, the occurrence of seismic swarms in the southwestern sector of the volcano along ne-sw trending structures has frequently been observed both before and during eruptions. such swarms may be related to magma movements through the upper crust and/or inside the volcanic edifice. the same mechanism was observed in january 1998, during an intrusive event (bonaccorso and patanè, 2001), when, however, no lateral eruption occurred. finally, a similar seismic swarm occurred between 20 and 24 april, 2001, three months before the july-august eruption. this paper focuses on the july 12-18, 2001 seismic swarm which was one of the strongest (both in number and in cumulative energy of the shocks) recorded in the last 20 years at mt. etna volcano. during 2001, a total number of 37 seismic stations of the local permanent network, run by the istituto nazionale di geofisica e vulcanologia, sezione di catania (ingv-ct) were operating, eight of which equipped with three component seismometers (fig. 1). the network provided good data for the location of earthquakes with m d 1.5, enabling us to characterize the kinematics of the shallow magma intrusion and the opening of the eruptive fissure. the aim of this paper is to characterize and interpret the space-time evolution and kinematics of the seismicity preceding and accompanying the july 17 august 9, 2001 lateral eruption. 2. pre-eruptive seismic and volcanic activity after the 1991-1993 major lateral eruption, activity at the summit craters of mt. etna (fig. 1) resumed in 1995 with a series of lava fountaining episodes. in particular, from the middle of 1998, there were quiet lava effusions and several spectacular sequences of explosive paroxysms (research staff of ingv-ct, 2001). seismic activity revealed a change in the dynamic conditions acting on the volcano in late 2000, about 8 months before the onset of the lateral eruption. on november 5, a seismic swarm occurred on the southern flank of mt. etna at a depth of about 8-10 km. this event marked the beginning of a significant increase in the cumulative strain release curve (bonaccorso et al., 2003). from november 2000 to april 2001, seismic activity gradually increased both in number and average magnitude of events. the epicenters were spread over a wide area covering most of the southern and eastern part of the volcano (bonaccorso et al., 2003). on april 20-24, 2001 a seismic swarm occurred with more than 200 events (max m d = 3.6). the hypocenters were distributed in the south-southwest upper part of the volcano, in the depth range 5-10 km, clearly defining a ne-sw alignment (bonaccorso et al., 2003). from june 6, a series of eruptive episodes characterized by strong strombolian activity and lava overflows occurred at the south east crater (sec). by late july 12, a paroxysmal eruption, one of the most violent in the whole 601 seismological constraints for the dike emplacement of july-august 2001 lateral eruption at mt. etna volcano, italy series, was accompanied by the resumption of seismicity in the southern upper flank of the volcano, heralding the opening of non-eruptive (dry) ground fractures. a further paroxysmal eruptive episode at the sec occurred in the early morning of july 17. a few hours later, the highest part of a fracture originating from the southern base of the sec fig. 1. map of the central part of mt. etna volcano showing the location of seismic stations (white squares: 1-component stations; black triangles: 3-component stations) of the permanent ingv-ct network. ground fractures (white line) that opened between july 12 and 17, and lava flows emplaced from july 18 to august 9 are also shown. vdb indicates the location of the valle del bove caldera. 602 domenico patanè et al. became eruptive (fig. 1). this fracture gradually propagated downslope and bent south, producing mild strombolian activity and a lava flow that extended sse. in the following hours, a second part of the fissure became eruptive north of la montagnola (a large pyroclastic cone 2700 m a.s.l.) giving rise to a major lava flow that spread southward. in the early morning of july 18 at about 00:20 (all hours are quoted as gmt), further seismic events accompanied the opening of a third eruptive vent located south of la montagnola, at about 2100 m a.s.l. over the following days, explosive activity occurred and lava flows advanced slowly to the south. a total of six eruptive centres were active simultaneously along the complex fracture system. the eruption abated within two weeks and stopped completely on august 9, 2001 (research staff of ingv-ct, 2001). 3. the july 12-18, 2001 swarm 2694 earthquakes with m d 1 were recorded from july 12 (at 21:44) until the end of the eruption, but the great majority of them (2645 events) occurred before the onset of the magma outflow. about 800 earthquakes were recorded in the first 15 h of the swarm. during this stage, a warning was issued and alert messages were sent to local and national civil defence departments. after this initial phase, the hourly number of shocks showed a fluctuating decrease until july 18, but the cumulative strain release gradient was almost constant (fig. 2). the highest magnitude observed during the swarm was 3.9, while only 62 earthquakes had m d 3.0. accurate digital picking of pand s-phases was performed (s-phases were picked only at three-component stations). earthquakes were located using the program hypoellipse (lahr, 1989) to take into account the difference in elevation of the seismic stations and to accurately determine the spatial pattern of hypocenters in the volcanic cone. the 1d velocity model used was derived from hirn et al. (1991), with the sea level assumed as the reference (zero) elevation. due to the low magnitude and shallowness, most of the events have good signal to noise fig. 2. earthquake hourly rate and related strain release of the july 12-18 seismic swarm. 603 seismological constraints for the dike emplacement of july-august 2001 lateral eruption at mt. etna volcano, italy b a c fig. 3a-c. epicentral maps and hypocentral w-e cross sections of the best constrained earthquakes of the july 12-18 swarm, in three continuous sub-periods. the three sub-periods a, b and c are selected on the basis of major difference observed in t s -t p time evolution: a) from the beginning of the swarm (july 12, 21:44) to july 13, 05:00; b) from july 13, 05:01 to july 14, 10:00; c) from july 14, 10:01 to the end of the swarm (july 18, 21:00). only earthquakes (and the related errors) located within the 2 km wide zone (evidenced in grey on the maps) are plotted in the cross sections. vdb = valle del bove caldera. the black star in (a) indicates the location of the largest (m d = 3.9) shock of the swarm. the eruptive fractures are shown in (c). 604 domenico patanè et al. ratio mainly at the closest stations. moreover, first p-wave arrivals were often masked by the high tremor amplitude and/or the coda waves of a previous event. in order to better examine the space-time foci migration, we selected the location data set by using the following criteria: number of observations 10, erh £ 1 km, erz £ 1.4 km, rms £ 0.35 s, gap £ 180°. the resulting data set consists of 337 events. a detailed analysis of t s t p , at the three closest threecomponent stations was also performed, to better constrain the locations. on the basis of major difference observed in t s -t p values, we selected three sub-periods to plot the foci distribution (fig. 3a-c): i) from the beginning of the swarm until 05:00 of july 13; ii) from july 13, 05:01 to july 14, 10:00; iii) from july 14, 10:01 until the end of the swarm (july 18, at 21:00). we focus our attention on the earthquakes that occurred in a two kilometer wide east-west elongated area where the main eruptive fissures developed (see fig. 3a-c). in the first period (fig. 3a), hypocentral depths ranged from 0 to 3 km b.s.l. a major cluster was found on the highest part of southern slope, aligned about nnw-sse, from east of la montagnola to the base of the summit craters. a second smaller cluster occurred at 0-2 km depth beneath the valle del bove. it is noteworthy that the strongest event of the whole sequence occurred ca. 5 km away from the main cluster (fig. 3a), on the volcano’s southwestern flank at a depth of ca. 0.5 km a.s.l. in the second period (fig. 3b), seismicity along the nnw-sse alignment clustered east of la montagnola and was slightly deeper (1-3 km b.s.l.). two other crustal volumes became seismically active to the west and to the east of the main earthquake cluster, respectively. the former was located in the sw flank of the volcano (depths ranging between 1 km a.s.l. and 2 km b.s.l.), and the latter was located on the southern rim of the valle del bove (depth ranging between 0 and 3 km b.s.l.). in the third period (fig. 3c), events migrated slightly westward and hypocenters clustered at shallower depths (from 2 km a.s.l. to the sea level). earthquakes located to the west and to the east of the eruptive fractures maintained almost the same spatial features as observed during the second period; the only remarkable difference was the occurrence of shocks in the easternmost sector at depth of 3-6 km b.s.l. at 00:20 on july 18, a m d = 3.0 shock occurred in coincidence with the opening of an eruptive vent at 2100 m a.s.l. thereafter, the earthquake activity decreased both in number of events and in magnitude. afterwards, the only relevant seismic phenomenon was the enhanced amplitude of volcanic tremor that accompanied the eruption until the end. 4. focal mechanisms the earthquakes used to compute fault plane solutions (fpss) were characterized by high frequency content, sharp first arrivals and clear s-phases at the nearest stations. therefore, a double couple source was assumed for these earthquakes. the analysis was performed using the fpfit algorithm (reasenberg and oppenheimer, 1985) to plot first motion data and to evaluate nodal planes and orientation of pand t-axes. the good azimuth coverage of the investigated area constrained single focal solutions for most of the events. a set of about 300 fpss was obtained. in this study, we considered 47 of them, selected on the basis of rigorous criteria (number of polarities 10, number of polarity discrepancies £ 2, focal planes uncertainty < 20°, unique and unambiguous solution). in order to improve the reliability of the solutions, available s-wave polarization at 3-component stations were jointly inverted with p-wave polarity data, using the probabilistic approach by zollo and bernard (1991) and de natale et al. (1991). results indicated that most of the earthquakes had dominantly strike-slip rupture mechanisms. in several cases the strike-slip mechanisms have a remarkable normal component. dip-slip mechanisms were also observed, mostly along the nnw-sse alignment, in the seismogenic volumes located in the eastern sector. it is also noteworthy that a small number of reverse dipslip mechanisms was found. figure 4 shows some examples of fpss representative of the data set. we also analysed spatial distribution of the horizontal projection of the p-axes (fig. 4). p-axes directions were mostly distributed 605 seismological constraints for the dike emplacement of july-august 2001 lateral eruption at mt. etna volcano, italy fig. 4. azimuth distribution of the horizontal projection of p-axes of 47 earthquakes selected from the swarm. black circles mark axes with a dip of 0-30°, white circles mark axes with 31-60°, and grey circles represent nearly vertical axes (61-90°). the rose diagrams, related to the areas a, b and c, show the cumulative number of azimuth directions. some representative fpss examples of the data set are also shown. first motion data are projected onto lower hemisphere. the comparison between theoretical (thin line) and experimental (thick line) s-wave polarization, measured at three-component stations, is also shown on the fpss. sub-horizontally, showing that transtensive and strike-slip faults dominated the whole data-set. normal dip-slip mechanisms, about nnwsse aligned (b in fig. 4), were also observed in the area near the main seismic cluster. a rose diagram shows a prevalence of p-axes with azimuth n330°-350°e in this area. similar features were also observed in the eastern sector (area c of fig. 4). in the western sector (area a of fig. 4), the p-axes were oriented ne-sw, coherent with ground deformation data that indicated a similar orientation of the displacement vectors for this sector of the volcano (research staff of ingv-ct, 2001). 5. discussion and conclusions the seismological findings reported above disclose that brittle fractures accompanied the final stages of the dike intrusion driving the july-august 2001 lateral eruption at mt. etna. at the beginning, only the basement rocks fractured, then also the volcanic pile. moreover, seismic data may help to constrain the mechanism of the intrusion. the south-eastern flank of mt. etna is one of the weakest zones of the volcano where the emplacement of dikes during previous eruptions has occurred (e.g., bonaccorso et al., 1996; bonaccorso, 2001). 606 domenico patanè et al. the hypocenters of the july 12-18 swarm were located mainly along the nnw-sse structural trend, at a depth between 0 and 3 km b.s.l. the deepest events were located east of the eruptive fractures, while the shallowest were located along these fractures (see figs. 1 and 3a-c). overall, this swarm (fig. 3a-c) may represent the response of both the basement and the volcanic pile to the stress induced by the upward propagation of the dike. in particular, dike emplacement at depths ranging from 1 to 3 km b.s.l. occurred in the volcano basement, where the westernmost part of a high velocity (and rigidity) body is located (e.g., chiarabba et al., 2000; laigle et al., 2000; patanè et al., 2002). it should be noted that the thickness of the volcanic pile, which is characterized by poor mechanical properties, ranges from a few tens of meters in the peripheral areas of the volcano to about 2000 m below the summit craters (rust and neri, 1996). the sudden foci depth variation that occurred on july 14 (fig. 5) is indicative of the intersection of the upward-propagating dike with the volcanic pile. moreover, we believe that earthquakes located as far as 6 km east and west of the main seismic cluster (fig. 3a-c) were caused by stress transfer induced by compression of the host rocks in response to the jerky dike inflation. it has been observed worldwide that major tensile cracks and dikes are generally aligned in the direction of regional maximum compressive stress (e.g., hill, 1977; nakamura, 1977; shaw, 1980), which for mt. etna area is oriented ca. n-s (lanzafame et al., 1997; cocina et al., 1998; patanè and privitera, 2001; among others). thus the july 2001 eruptive fracture system is in good agreement with the direction of the regional maximum compressive stress. similar evidence holds for the trend of the swarm seismicity. it is worth recalling that magma intrusion is accompanied by different types of focal mechanisms (sometimes at short distances from each other), as observed for example at hawaii (thurber and gripp, 1988) and at mt. usu (matsumura et al., 1991). this is also the case at mt. etna in 2001. strike-slip mechanisms dominate in the intrusive volume, and some transtensive and normal mechanisms are also present (area b in fig. 4). the observed normal faulting mechanisms may be explained by local stress from vertical dike propagation (resulting in sub-vertical p-axes), whereas horizontal fig. 5. time upward migration of earthquake foci, and comparison with the eruptive phenomena. 607 seismological constraints for the dike emplacement of july-august 2001 lateral eruption at mt. etna volcano, italy propagation and/or the geometric complexity (several digitations) of the dike, may explain the observed strike-slip and transtensive rupture mechanisms (sub-horizontal p-axes). a complex dike geometry is suggested by field evidence, specifically, discrete linearly distributed vents, rather than a unique long eruptive fracture. finally, the evidence of a unique stress source is suggested by ground deformation results (bonaccorso et al., 2002) and is well supported by the axi-symmetric orientation of p-axes for the two «peripheral» seismic volumes (areas a and c of fig. 4). this stress source is the nearvertical, n-s trending dike emplaced a few kilometres south of the summit region, prior to the onset of eruption. acknowledgements we wish to thank enzo boschi for his continuous encouragement. massimo cocco and an anonymous reviewer are acknowledged for their useful suggestions. references barberi, g., o. cocina, g. neri, e. privitera and s. spampinato (2000): volcanological inferences from seismic strain tensor computation at mt. etna volcano, sicily, bull. volcanol., 62, 318-330. bonaccorso, a. 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(received august 27, 2002; accepted may 15, 2003) ag_56.05.13_hao_shuji_finalonline:layout 6 annals of geophysics, 56, 5, 2013, a0566; doi:10.4401/ag-6308 a0566 influences of the state of ionospheric background on ionospheric heating effects hao shuji1, li qingliang2, che haiqin2, yang jutao2, yan yubo2, wu zhensen1,*, xu zhenwen2 1 xidian university, department of physics, xi’an, china 2 china research institute of radiowave propagation, qingdao, china abstract according to the well-performed ionospheric heating experiments at arecibo in the low latitudes as well as at tromsø in the high latitudes, the large-scale modification effects are simulated under an assumption of equivalent conditions, i.e., with the same effective radiative power and the same ratio of the heating frequency fhf to the critical frequency of ionospheric f region fof2. the findings are extensively exploited to verify the validation of our model by comparison to the experimental results. further, a detailed study is carried out on the influences of the background electron density gradient as well as the ratio of fhf to fof2 on heating effects. conclusions are drawn as follows: under certain conditions, a smaller electron density gradient of background ionospheric f region leads to a better ionospheric heating effect; during over-dense heating, the heating effects are enhanced if the ratio of fhf to fof2 increases, which is slightly limited by the resultant elevation of the reflection height. however, there might be a better ratio range with small values of the ratio of fhf to fof2, e.g., [0.5, 0.7] in the current study. finally, we analyzed how to select heating parameters efficiently under adverse conditions so to obtain relatively effective results. 1. introduction as an active ionospheric research, modification of the ionosphere by powerful high frequency (hf) waves is to date of great scientific interest and rapidly developed in the last several decades since they provide an effective tool for investigations of the earth environment as well as for understanding various plasma phenomena. the heating experiments were successfully performed firstly at platteville, colorado [utlaut 1970], almost simultaneously at arecibo [gordon and showen et al. 1971], in the early 1970s, and subsequently at other ionospheric heating facilities such as tromsø, sura, haarp (the high frequency active auroral research program) and spear (space plasma exploration by active radar). a wide variety of fascinating phenomena are observed, some of which are associated with the development of parametric instabilities and ionospheric turbulence, including enhancement of electron temperature [mantas et al. 1981], generation of plasma oscillations and field-aligned irregularities [rietveld et al. 2003, tereshchenko et al. 2006, wright and dhillon 2009], excitation of artificial radio and optical emission [frolov et al. 1999, pedersen and carlson 2001, kosch et al. 2007]. among the main phenomena, renewed attention is paid to the wave-plasma interaction process leading to nonlinear density perturbations at different scales. large-scale (kilometer size) irregularities are best interpreted in terms of thermally driven self-focusing of the incident hf electromagnetic wave [duncan and sheerin 1985]. small-scale (meter size) field-aligned striations are generally explained as differential plasma heating from enhanced langmuir wave collisional dissipation or hf scatter from the induced plasma oscillations [blagoveshchenskaya et al. 1995]. concerning the nature and origin of langmuir wave, a popular view is the theory of thermal parametric (resonant) instability developed at the upper hybrid resonance altitude [das and fejer 1979]. only the o-mode propagating in the vertical or nearly vertical direction reaches the resonance region, and the x-mode is always reflected from the region lying below the resonance one. in the previous heating experiments, field-aligned irregularities, electron density and anomalous absorption all occurred only during o-mode heating [robinson et al. 1998]. recently experimental evidence of the excitation of strong small-scale fieldaligned irregularities was presented due to an x-mode hf pump wave radiated in the direction of magnetic zenith [blagoveshchenskaya et al. 2011]. in most of studies, the hf-induced electron density profile modification is observed only by a few percent. xu et al. [2009] presented evidence of a density article history received february 21, 2013; accepted june 11, 2013. subject classification: ionospheric heating, ionospheric background, heating effects, heating parameters, electron density gradient. depletion by 4-10% with the heating facility at tromsø with an effective radiated power perf = 90 mw and heating frequency fhf = 4.04 mhz. isham et al. [1987] reported a decrease of electron density by 3-5% using the heating facility at arecibo with perf = 100 mw and fhf = 5.1 mhz. duncan et al. [1988] first reported direct observations of large electron-density depletions of about 30% with perf = 60 mw and fhf = 3.175 mhz. such large depletions were observed reproducibly only after local midnight in the winter ionosphere for ionospheric densities critical only for the lowest pump frequency available at arecibo. the state of the background ionosphere is found to be an important factor influencing the observed effects. the resultant heating effects and their magnitudes may be different due to discrepancies of ionospheric background and heating parameter settings in specific research processes. therefore, researchers undertook a series of theoretical investigations by simulation of the factors influencing the strength of ionospheric heating effects. the basic theory of ionospheric heating was systematically discussed by gurevich [1978] who used a set of continuity equation, momentum equation and energy equation. shoucri et al. [1984] considered the dependences of heating effects on perf and fhf, respectively. hansen et al. [1992] modeled the large-scale hf-induced ionospheric modifications reported by duncan et al. [1988]. mingaleva and mingalev [1997] and mingaleva et al. [2008] investigated the range of values for fhf to get the best heating effects in midor low-latitude regions, it is fhf ≈ 0.8 ~ 0.86 fof2 during the day, and fhf ≈ 0.9 ~ 0.96 fof2 at night. in order to perform a detailed study on diversities of heating effects in different areas, an investigation is carried out on the influences of the background electron density gradient as well as the ratio of fhf to fof2 on heating effects in section 2. then our final conclusions and proposals for future work are presented in section 3. a theoretical model of ionospheric heating will be built in appendix a. 2. analysis of simulation results 2.1. comparion of simulations with experimental results in order to verify the numerical model described in appendix a, simulation parameters are selected to be the same with those in the heating experiments listed in table 1. density depletions and temperature enhancements are found at the reflection height for both cases at arecibo as well as at tromsø. in our simulation, the density depletions n1/n0 are about 3.4% at arecibo. the maximum temperature in the depletions increases about 28.4% which is consistent with the experimental results at 20-30%. for the case at tromsø, we found the density depletion up to 6.12% and the temperature enhancement reaching 70.6% which also are in accordance with the measured enhancement of 60-120%. 2.2. analysis of factors influencing heating effects in the f-region, the heating effects have a close relationship with energy absorption via various electron heating channels. besides the linear ohmic heating process, the various nonlinear processes with different thresholds, characteristic times, and saturation mechanisms may also be excited, depending on the incident power and the state of background ionosphere [duncan et al. 1988]. thus, with the values of the pump frequency and perf selected, the geomagnetic dip and the state of ionospheric background (mainly described by the electron densities and their gradients) have a remarkable impact on heating effects (variation of electron density), which will be dealt with in section 2.2 separately. 2.2.1. influences of perf and the dip angle on heating effects influences of system’s perf and the geomagnetic dip on heating effects have already been specified in the previous calculations, which will be briefly discussed here. we hence compare the results of heating experiments performed at tromsø and arecibo (see table 1) and find that the two experiments were carried out under differhao shuji et al. 2 site geographic coordinate fhf (mhz) fof2 (mhz) fhf/fof2 h0 (km) te1/te0 at h0 n1/n0 at h0 arecibo 68.8°w, 18.3°n 5.1 7.44 0.69 200 20% ~ 30% 3% ~ 5% tromsø 19.2°e, 69.6°n 4.04 4.99 0.81 217 60% ~ 120% 4% ~ 12% table 1. parameters of the heating experiments carried out at arecibo [isham et al. 1987] and tromsø [xu et al. 2009], where h0 refers to the reflection height of hf radio wave in the ionosphere. 3 ent heating conditions (e.g., different perf and fhf/fof2), leading to different heating effects. in order to study the influence of perf and fhf/fof2 on the heating effects and compare the difference of the heating effects under the same conditions at different radar sites, we theoretically simulate the heating effects assuming the same heating conditions (i.e., perf = 100 mw and fhf/fof2 = 0.81) at tromsø, arecibo, and guangzhou. under the equivalent conditions with perf = 100 mw and fhf/fof2 = 0.81 during daytime, heating effects are simulated for the cases at tromsø, arecibo, and guangzhou (113°e, 22°n), respectively. the background parameters derived from the iri and msis model at tromsø and arecibo are the same as those mentioned in section 2.1. the relative variation profiles of electron density are plotted in figure 1. heating effects are found to be quite comparable between arecibo and guangzhou, with density depletions of 4.20% and 4.27%, respectively. however, the effects are appreciably stronger at tromsø with a density depletion of 6.47%. comparison between the simulations above in section 2.1 and the results shown in figure 1 indicates that when the reflection altitude h0 remains invariable, the increase of fhf/fof2 = 0.81 or perf of the heating system leads to an enhancement of heating effects. for example, with fhf/fof2 increased from 0.69 to 0.81 at arecibo, the density depletion ranges from 3.4% to 4.2%; with the perf enhanced from 90 mw to 100 mw at tromsø, the depth of electron density depletions increases slightly from 6.12% to 6.47%. it must be pointed out that the numerical comparisons of the density modification at different locations have limited guiding significance. the regional difference between highand mid-latitudes and the time difference at similar latitudes are not taken into account. the ionospheric heating experiments were easily carried out after sunset at arecibo but mainly done during daytime at tromsø for lack of high enough electron density during night [rietveld et al. 2002]. at arecibo, the large depletions were observed reproducibly even after local midnight in the winter ionosphere. 2.2.2. influences of the electron density gradients on heating effects in order to study the impact of electron density gradients on the density modification in ionospheric heating experiments, we select different background density profiles deduced from two ionograms illustrated in figure 2 at different times of a day and keep the other parameters fixed in our simulation. the density profiles are labeled as density mode a and b (both derived from guangzhou ionospheric observatory of crirp; ionogram a and b were deduced at 15:31 lt in october 5, 2010, and 16:13 lt in october 17, 2010, respectively), respectively, as shown in figure 3. the relative variations of electron density with height caused by heating are shown figure 4. the reflection height of hf radio wave h0 is about 243 km for the selected density models shown in figure 2. from figure 3. we can see ∇na<∇nb for the case of h > h0 and ∇na>∇nb for h < h0. as illustrated in figure 4, a larger density gradient leads to a smaller density depletion and temperature increment, which implies that the density gradient is an adverse factor for ionospheric heating experiments. the interpretation can be more easily understood in terms of electron density scale height h [white and the influence factors of heating effects figure 1. comparisons of relative variation profiles of electron density at arecibo, tromsø and guangzhou under equivalent conditions with perf = 100 mw and fhf/fof2 = 0.81 indicated in the insert. figure 2. the selected background ionograms (the critical frequencies of ionograms a and b are 16.6 mhzand 15.2 mhz, respectively; in order to derive an entire electron density profiles of o-mode and x-mode from ionogram a, extend the ionogram traces reasonably using extrapolation). chen 1974], which is inversely proportional to ∇n (1) the electric field of o-mode near the reflection altitude behaves as a standing wave and undergoes an amplitude swelling. the swelling factor of the incident power density just below the reflection height [kantor 1972] is (2) the standing wave pattern near the reflection height has a significant relationship with the possible excitation of parametric processes and thus the hf wave energy absorption. under certain conditions, anomalous absorption will play an important role. a smaller electron density gradient, which means a larger swelling factor, will lead to a larger heating effect. 2.2.3. influences of the ratio of fhf to fof2 to on heating effects the background ionosphere is controlled by solar activity and its critical frequencies vary in a large range in a day, especially during high solar activity. as a comparison, the heating parameters can remain constant in certain periods due to limits of heating conditions or experimental setup. in order to investigate the influence of the ratio of fhf to fof2 on heating effects and compare results under different conditions, we do the simulations as follows: (1) assume fhf as a constant and set fof2 different values in a day; (2) keep fof2 as a constant and change fhf. here the heating effects at guangzhou with perf = 200 mw are simulated. fhf is equal to 6.5 mhz and fof2 ranges from 7 mhz to 15.5 mhz in the first case while fof2 remains fixed at 15.2 mhz and fhf ranges from 6 mhz to 15 mhz in the second case. the results are plotted in figure 5 and figure 6 respectively. the electron density depletion n1/n0 and temperature increment te1/te0 has a close relationship with the ratio of fhf to fof2 (see figure 5). the critical frequency fof2 is inclined to change quickly in a day but the hf frequency fhf remains constant and smaller than fof2 all the time. the heating effects are getting better as fhf/fof2 approaches to 1. as fof2 increases (hence the ratio fhf/fof2 decreases), the reflection height h0 firstly shows a drop until fhf/fof2 = 0.65 and then shows a rise. they influence heating effects together, resulting in an appreciable density disturbance under the condition of low fhf/fof2, as shown by the local maximum at fhf/fof2 ≈ 0.65. these findings are quite helpful for the selection of fhf. a similar result can be found more clearly in figure 6, where a maximum of n1/n0 occurs when hao shuji et al. 4 figure 3. the selected background electron density profiles deduced from two measured ionograms (figure 2). figure 4. relative variation profiles of electron density (left) and electron temperature (right) caused by heating with different background density gradients. h f f z np 2 2 1 2 2 = -c m .a c h7 2 /1 3~ = ` j (a) (b) 5 fhf/fof2 ranges form 0.5 to 0.7. when fof2 is constant, the reflection height and fhf/fof2 varies in phase with fhf. their increases have a positive and a negative impact on heating effects, respectively. unlike the first case shown in figure 5, the reflection height shown in figure 6 covers a larger range and thus has a more effective impact on variations of electron density. with fhf/fof2 less than 0.5, the reflection height plays a dominant role and the increase of fhf leads to a decrease of n1/n0. as fhf increases up to a certain value fhf/fof2 becomes dominant and n1/n0 begins increasing. with a further increase of fhf/fof2 larger than 0.7, the reflection the influence factors of heating effects figure 5. influences of the ratio of fhf to fof2 on heating effects with a constant fhf and different values of fof2. the solid line with asterisks represents the density depletion n1/n0 (left) and temperature increment te1/te0 (right), the broken line with squares represents h0. figure 6. same as figure 5. but for a constant fof2 (=15.2mhz) and different values of fhf. figure 7. relative variation profiles of electron temperature and density caused by heating with different values of ratio fhf/fof2. the observations are indicated with asterisks and the fitted curve with solid line. (a) density depletion (a) density depletion (b) density depletion (b) temperature increment (b) temperature increment (a) temperature increment height shows its dominance on heating effects again. the temperature increment shows a similar trend. the results imply that there might be a better ratio range with small values of the ratio of fhf to fof2, which is supported further by an experimental example. i.e., the experiment study of influences of the ratio of fhf to fof2 on heating effects was conducted using the eiscat facility on august 17, 2009. the facility radiated o-mode waves at the frequency fhf =4.04 mhz with perf = 120 mw in the following mode: 8-min radiation 4-min pause in the experiment. the results (the electron temperature) were dealt with using average value of each period, which are shown in figure 7. when the value fhf/fof2 is small, the variation of the electron temperature is small, when fhf is close to fof2, the variation of the electron temperature is relatively large, but meanwhile there is a peak at fhf/fof2 = 0.74. the electron density perturbation shows a similar trend.the experimental result shows that there was a better ratio range with a small fhf/fof2, which can be used during the ionospheric heating to obtain relatively effective results. 3. conclusions in this paper, we have simulated the heating effects in the lowand high-latitude under the equivalent conditions stemming from the well-known ionospheric heating experiments at arecibo as well as at tromsø, respectively. the findings have been extensively exploited to verify the validation of our model by comparison to the experimental results. further, the analysis has been carried out on the influences of the dip angle, the background electron density gradient and the ratio fhf/fof2 on heating effects. finally we come to draw the following conclusions: (1) under certain conditions, a smaller electron density gradient of background ionospheric f region leads to a better ionospheric heating effect. when the background electron density suffers a rapid change, the altered gradient affects heating effects. (2) during over-dense heating, the heating effects are enhanced if the ratio of fhf to fof2 increases, which is slightly limited by the resultant elevation of the reflection height. however, there might be a better ratio range with small values of the ratio fhf to fof2, e.g., [0.5, 0.7] in the current study. according to the simulation results in the paper, several measures can be taken to improve heating effects under unfavorable background conditions. when the height of ionospheric f region rises up, we may increase the perf of the heating facilities to increase the incident power density of the heated volume. when the ionospheric critical frequency shifts rapidly, we may make use of a short duty cycle and switch pump frequencies swiftly. if the capacity of the available heating system is limited, we may choose matched times with low ionospheric height, critical frequencies or gradient of electron density to obtain relatively effective results. we considered the influences of the ratio fhf/fof2 and the reflection height together which have been neglected by the previous theoretical investigations. during the experiments, especially at noon during high solar activity, unfavorable background conditions are often encountered without choice, in which we can adjust heating parameters adaptively. acknowledgements. we are grateful to li qiang for useful suggestions. the authors are appreciated to the eiscat scientific association. eiscat is an international association supported by china (crirp), finland (sa), japan (nipr and stel), norway (nfr), sweden (vr) and the united kingdom (nerc). this work is supported by the nsfc with grant no. 41004065. references banks, p.m., and g. kockarts (1972). aeronomy, part b, academic press, newyork. blagoveshchenskaya, n.f., a.d. andreev et al. 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(1970). an ionospheric modification experiment using very high power, high frequency transmission, j. geophys. res, 75 (31), 6402-6405. white, p.b., and f.f. chen (1974). amplification and absoption of electromagnetic waves in overdense plasmas, plasma physics, 16, 565-587. wright, d.m., r.s. dhillon et al. (2009). excitation thresholds of field-aligned irregularities and associated ionospheric hysteresis at very high latitudes observed using spear-induced hf radar backscatter, ann. geophysicae, 27 (7), 2623-2631. xu bin, wu jun, wu jian and wu zhensen et al. (2009). observations of the heating experiments in polar winter ionosphere, chinese j. geophys., 52 (2), 322341 (in english). *corresponding author: wu zhensen, xidian university, department of physics, xi’an, china; email: wuzhs@mail.xidian.edu.cn. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. the influence factors of heating effects appendix a: theoretical model according to the basic theory of ionospheric heating, microscopic modification of the electron temperature and density in the f region can be modeled by fluid equations [meltz et al. 1974, gurevich 1978]. the three physical equations involved in our simulation model are the momentum equation, the continuity equation and the energy equation. a.1. the momentum equation considering four species of singly charged ions and electron (o+, no+, o2 +, e) and neglecting the inertia term, the momentum equation can be written as follows: (a1) where the subscripts a,b refer to the different types of charged particles (o+, no+, o2 +, e), n the neutral particles. pa = nakbta is the pressure of species a, and na, ma, ta, va are density, mass, temperature and drift velocity, respectively. g is the acceleration of gravity and kb is boltzmann’s constant. the collisional rate oan and oab represent collisions of species a with neutral particles and species b, respectively. summing equation (a1) over the charged species, the collisional terms among charged particles are deleted completely, and the electromagnetic force terms vanish with the hypotheses of quasi-neutrality and ambipolar diffusion. introducing a mass-weighted ionic collisional term miyin [hinkel et al. 1992] and the diffusion coefficient d (a2) and assuming the three species ions have the identical temperature ti then the momentum equation in the direction of the geomagnetic field can be written as (a3) where yen and yin are collisional frequencies of electron and ions with neutral particles, respectively, and i is the geomagnetic dip. a.2. the continuity equation the main ion constituent in the f-region is atomic o+, which does not recombine with electrons directly [rishbeth and garriot 1969]. instead, o+ undergoes an ion-atom interchange reaction with neutral particles (mainly n2 and o2) and the resultant molecular ions combine with electrons, known as the b-type recombination, which has no dependence on electron temperature (a4) when ignoring the density variations of n2, o2 and o+ during the heating process, the enhancement of electron temperature has little effect on the production of no+ and o2 +. therefore, the continuity equation takes the form: (a5) where k1 = 4.2×10 −7(300/te ) 0.85 cm3/s and k2 = 1.6×10−7(300/te ) 0.55 cm3/s are dissociative recombination coefficients, q0 is electron production rate in the equilibrium state (∂/∂t = 0) without external electric fields. a.3. the energy equation the energy equation can be written as follows: (a6) equation (a6) is a time-dependent expression for electron temperature including the effects of convection, compression, conduction, heat production and loss. the left terms are the change rate of electron energy, the convection, and compression rate. the first term on the right side is the heat conduction rate. the heat conduction across the magnetic field can be ignored and the parallel thermal conductivity ke [banks and kockarts 1972] is (a7) where, nn represents the densities of neutral particles and the transfer cross-section of mean momentum for corresponding neutral particles [banks and kockarts 1972]. a complete theoretical description of the absorption of the hf wave shf involves ohmic heating due to deviative absorption, linear mode conversion to electrostatic waves, parametric processes and langmuir collapse, and the slow-down of super thermal electrons accelerated by the mode-converted waves. the exact amount of energy deposited in the ionospheric electrons in this region is therefore difficult to calculate accurately from first principles. here following the simple absorption model proposed by hansen [hansen 1990], we utihao shuji et al. 8 , v v cos n d n k t t n m g n ie b e i e n e e d = + + +a a a ^ h8 b/ ,vt n q k n k n n nno o e e 0 1 2 e e2 $ 2 2 d= + -+ +^ ^h h . . ev cm s k k t n n q t 1 3 22 10 7 7 10 e e e e / n n 4 2 5 5 2 1 1 1 # # $ $ $ = + d^ h/ 0 v v v v v e b g v v p n q q n m n m n mn n #d+ + = ! a a a a a a a a a a a a a a b a b b a^ ^h h/ ,m n n d m mm 1 1 i in i n i e i n i ini e e y y y y = = + ^ h/ o n n n, o o o o n n , o o oo o o e e 2 2 2 2 " " " " + + + + + + + + + + + + + + . v vk n t t n t n t t k k s s l 2 3 b e e e e e e e e e e b hf 0 d d d d 2 2 + =+ + + -^ h 8 b 9 lize a gaussian profile placed near the reflection layer. s0 and l are the electron heating rate from other sources (mainly the solar radiation) and electron cooling rate, respectively. it is widely accepted that the dominant electron cooling processes involve elastic collisions between electrons and neutrals, rotational and vibrational excitation of molecular species, fine structure transitions in atomic oxygen, and collisions between electrons and ions. the updated estimates of inelastic cross sections associated with cooling rate calculations can be found [pavlov 1998, pavlov and berrington 1999, gustavsson et al. 2010]. electron and ion temperatures in the background ionosphere are specified with the iri-2012 model, and the atmosphere density and temperature are derived from the msis-90 model. in our calculations the time step is 0.01 second and the space step is 1/sini km. the simulation processes are as follows: solve equation (a3) to calculate ve according to the background parameters (te0, ne0, etc.); calculate variations of electron temperature te1 in the equation (a6) and then update te0 and the recombination and loss rates; obtain variations of electron density ne1 with equation (a5), then update ne0 and the corresponding terms; finish the current calculation loop and step into the next loop until the end of heating. the subscript e in electron density will be omitted for convenience. the influence factors of heating effects << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.1000 /colorconversionstrategy /leavecolorunchanged /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage -1 /imagememory 1048576 /lockdistillerparams true /maxsubsetpct 100 /optimize false /opm 1 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice zhang_correctedok_layout 6 annals of geophysics, 56, 3, 2013, r0329; doi:10.4401/ag-6309 r0329 development of a new seismic-data acquisition station based on system-on-a-programmable-chip technology zhang qi-sheng1,4, deng ming1,4,*, guo jian2,3, luo wei-bing3, wang qi1, feng yong-qiang1 1 china university of geosciences, beijing, china 2 institute of geology and geophysics, chinese academy of sciences, beijing, china 3 beijing geophysics technology co. ltd., beijing, china 4 key laboratory of geo-detection, china university of geosciences, beijing, and ministry of education, beijing, china abstract there has been considerable development of seismic detectors over the last 80 years. however, there is still a need to further develop new earthquake exploration and data acquisition systems with high precision. in particular, for china to keep up with the latest technology of these systems, it is important to be involved in the research and development, instead of importing systems that soon fall behind the latest technology. in this study, the features of system-on-a-programmable-chip (sopc) technology are analyzed and used to design a new digital seismic-data acquisition station. the hardware circuit of the station was developed, and the analog board and the main control data-transmission board were designed according to the needs of digital seismic-data acquisition stations. high-definition analog-to-digital converter sequential digital filter technology of the station (cascade integrator comb filter, finite impulse response digital filter) were incorporated to provide advantages to the acquisition station, such as high definition, large dynamic scope, and low noise. a specific data-transmission protocol was designed for the station, which ensured a transmission speed of 16 mbps along a 55m wire with low power consumption. synchronic acquisition was researched and developed, so as to achieve accuracy better than 200 ns. the key technologies were integrated into the sopc of the main control data-transmission board, so as to ensure high-resolution acquisition of the station, while improving the accuracy of the synchronic acquisition and data-transmission speed, lowering the power consumption, and preparing for the follow-up efforts to tape out. 1. introduction in over the last 80 years, the development of seismic exploration instruments has gone through five stages: the electron tube (optical spot recorders), transistors (analog tape recorders), conventional digital seismographs (digital tape recorders), 16-bit telemetry seismography, and 24-bit telemetry seismography [song et al. 2012]. at the beginning of the 21st century, ion and sercel launched the iv system and the 400 series of fully digital seismographs, respectively, which were recognized in the industry as sixth generation seismographs [song et al. 2012]. the development of seismic instruments cannot be separated from the continuous development of seismic-data acquisition methods, combined with the latest technology available at the time. currently, seismograph development is closely related to and mutually reinforces the progress of electronic technology, computer science, seismic exploration methods, intelligent control, network technology, signal processing, and other disciplines. as early as the 1970s, sercel in france launched the sn338 digital seismograph with its single-station, single-channel, data-acquisition station [huang and yu 1994]. at present, the most advanced seismic field-data acquisition systems internationally are multi-channel seismographs, which are mainly represented by: the fully digital platform of system four (upgraded in 2007 to scorpion), with the firefly acquisition system, launched by ion in the usa; the 428xl acquisition system of sercel in france; and aram aries of geo-x in canada. the maximum number of channels of the input/ output system four has reached 30,000, and it can be used in conjunction with multi-component digital detectors, to achieve full-wave seismic-data acquisition. a cable-less connection for this was achieved with firefly, which significantly reduces the transportation costs in the field. then the 428xl has a real-time channel capacity of up to 100,000, thus guaranteeing the highest resolution data while having more channels. from these data, it can be speculated that seismographs article history received february 23, 2013; accepted may 13, 2013. subject classification: seismic-data acquisition station, seismic prospecting, sopc, high precision synchronization, digital filter, data transmission. designed outside of china have reached a high level of channel capacity and connection patterns, and are advancing towards high-resolution, high-density seismographs with more diversified connection patterns. from a global perspective, 50% dependence on foreign oil can be seen as a security warning, but in 2011, china's dependence on foreign oil exceeded 55%, which is a threat to energy and economic security [mazza et al. 2012, qi 2012]. therefore, china should strengthen domestic oil and gas resource prospecting to contribute to national stability, sustainable development, and building of a more prosperous society. the most widely used and powerful geophysical method in oil and gas exploration is reflection seismology [wood and gettrust 2002, morandi and ceragioli 2002]. as oil and gas exploration techniques have developed, china has entered a very difficult stage of oil and gas exploration; namely, exploration of the pre-cenozoic marine residual basins [wu et al. 2011]. this will require oil and gas resources to be sought in complex geological areas, such as buried hills and their internal structures, and high and steep structures on mountainsides. not only will exploration depth increase, but surface and subsurface conditions will also become more complex, and hence seismographs will face new demands and challenges. there is thus an urgent need to develop new types of high-precision seismic-data acquisition systems that are suitable for our national conditions. in recent years, with national attention being placed on research and development of geophysical instruments, the number of research units engaged in seismic-data acquisition and recording systems has grown. bgp inc. of the china national petroleum corporation (group) and tsinghua university have developed the ‘es109 new seismic-data acquisition and recording system’, with a capacity of 2,000 channels in a single tape track, and a total channel capacity of 20,000 [wang 2010]. the institute of geology and geophysics of the chinese academy of sciences has designed a new digital seismograph and a sea seismograph with broadband and double cabins, which have low cost and low power consumption, yet are still very powerful and can achieve 10,000-channel data transmission using reliable methods. jilin university used an ethernet relay solution to achieve a single station 8-channel seismic-data acquisition and recording system, with a transmission distance of 100 m, and a transfer rate of 16 mbps. this has now passed the indoor tests and seismic focus experiments [zhang 2007]. the university of science and technology of china has adopted long-distance ethernet physical layer technology and has customized transfer protocol programs to achieve a peak transfer rate of 40 mbps [xie 2009]. the chengdu university of technology has achieved wireless telemetry with a 48-channel, high-resolution, shallow-layer, digital seismograph based on wireless communication technology, which has enhanced seismograph construction capacity in mountain and foothill areas. in recent years, although china’s research and development capabilities for seismic exploration instruments have been increasing, the vicious cycle of ‘import fall behind import again fall further behind’ over 30 years of economic reform has created a wide gap in the research and production of digital seismic-data acquisition systems in china, compared to that in more developed countries. the achievement of high-resolution, high-precision, synchronized data-acquisition and realtime high-speed data transmission at low power (less than 200 mw for each station) is still the aim of seismograph developers. therefore, there is a very important practical significance in the study of the key technologies of digital seismic-data acquisition stations, and in the development of such a station for which china owns the independent intellectual property rights. 2. system-on-a-programmable-chip technology and telemetric digital seismic-data acquisition stations the nature of telemetric digital seismic-data acquisition stations led to the introduction of system-ona-programmable-chip (sopc) technology. in turn, the advantages of sopc have greatly enhanced the performance of telemetric digital seismic-data acquisition stations. 2.1. system-on-a-programmable-chip technology sopc technology was first proposed by altera corporation in the usa in 2000, together with the corresponding development software, quartus ii. sopc is a system on a chip that is based on field programmable gate array (fpga) solutions, which integrates the processor, memory, input/ output ports, low-voltage differential signaling, clock and data recovery, and other components, as well as other functional modules required by the users of a programmable logic device, to construct a programmable chip system [astarloa et al. 2005, zhang et al. 2012]. it has a flexible design that can be trimmed, is scalable and upgradeable, and has hardware and software system programmability [zhang et al. 2012]. sopc is an integration of programmable logic-device technology and application-specific integrated circuit technology, which represents the direction of future development in the semiconductor industry. this is because it has the advantages of being a single chip with micro-encapsulation, low-power consumption, and high programmability, and it is easy to develop and tape-out. therefore, with the development zhang et al. 2 3 of electronic design automation technology and the continuous improvement of large-scale programmable devices, sopc technology has already been widely used in many fields [zhang et al. 2012]. in the present study, sopc technology was used to design a new digital seismic-data acquisition station. the details of the key technologies involved in designing the telemetric digital seismic-data acquisition station based on sopc are explained thoroughly here. 2.2. components of telemetric digital seismographs telemetric digital seismographs are composed of the detector, data-acquisition stations, power stations, interconnected stations, and a central station (instrument cart) [zhang et al. 2012]. the system structure is shown in figure 1. there are high-speed fiber-optic connections between individual interconnected stations, as well as between these stations and the central station. special four-core cables for seismic exploration are used for the connection between the power station and the acquisition station. the detector and the acquisition station together constitute the front end of the acquisition device, while the interconnected stations and power station provide the power-over-ethernet mode of power supply to the connected data-acquisition station. these also serve as relay devices for data collection, to achieve high-speed, high-fidelity uploading for bulk data, while the central station is responsible for monitoring the work of the entire acquisition system. 2.3. overall framework of a new telemetric digital seismic-data acquisition station based on system-on-a-programmable-chip technology data-acquisition stations are one of the most basic components of a telemetric digital seismograph. the overall framework of the data-acquisition station used in the present study is shown in figure 2. cs5373 plus sopc are used to achieve high-precision conversion and high-speed digital transmission. the earthquake input signal bandwidth is 2 khz dc. a nios processor generates commands, and an analog-to-digital converter (adc) controller controls the cs5373 to perform the analog-to-digital conversion, thereby generating a serial data stream at 512 kbps. the sigma-delta (r-d) serial datastream has a low sampling accuracy and a large sampling rate. to improve accuracy, high-resolution adc digital filtering techniques must be introduced. after passing through a r-d converter, a cascade integrator comb (cic) filter (designed by the authors of the present study) is used for low-pass filtering for the r-d serial datastream. after the data has passed through the cic filter, the data rate is reduced, but the quantified noise spectral density remains unchanged after sampling. after this, finite impulse response (fir) digital filtering, infinite impulse response (iir) digital filtering, and high-precision synchronization are carried out, and the data are transmitted to the power station using the seismic data acquisition based on sopc figure 1. block diagram of a telemetric digital seismic instrument. ps, power station; as, acquisition station; is, interconnection station. figure 2. the overall structure of the data-acquisition station with the cs5373 subsequent digital filter, which was independently designed. low-voltage differential signaling data-transmission module. the data-acquisition station that we developed has the following features: (1) high-resolution analogto-digital conversion, and subsequent cic, fir, and iir digital filters, which are suitable for geophysical data acquisition; and (2) data-acquisition station core technology integrated into a single-chip sopc. after industrialization, this can easily be customized into a monolithic integrated chip, for which independent intellectual property rights can be obtained. power consumption after customization can be further reduced, while the performance and integration can also be improved. 3. key technologies of the new telemetric digital seismic-data acquisition station 3.1. hardware circuit design of the data-acquisition station the hardware circuit of the data-acquisition station based on sopc consists of two stacked circuit boards: (1) an analog board is used for the acquisition of seismic-wave data received by the detector, after which analog signal conditioning, high-precision analog-todigital conversion, and direct-current-to-direct-current (dc-dc) conversion are carried out, and (2) a master data-transmission board is used to control the modules in every unit on the analog board and the transmission of the data flow. the analog board block diagram of the data-acquisition station is shown in figure 3. the analog switch used an adg733 chip, with the signals selected by the detector or the calibration signals from the cs5373 passed through the passive filter module, and then through the cs3301 programmable gain differential amplifier for programmable magnification (the magnification can be set to 1, 2, 4, 8, 16, 32 or 64 times). finally, the signal is sent to the cs5373 high-precision r-d adc to be digitized. as the power station in the telemetric digital seismic-data acquisition station design supplies power in power-over-ethernet mode, when the power station supplies power to multiple data-acquisition stations, the voltage from the power station steadily decreases. depending on the location of the data-acquisition station, its voltage varies from 22 v to 48 v, while the data-acquisition station analog board needs ±2.5 v and 3.3 v. the pwb4803md type dcdc chips convert the input voltage to 3.3 v, after which the lt1161 and lt1962 convert the 3.3 v to -2.5 v and +2.5 v, as required. the master data-transmission board block diagram is shown in figure 4. it consists primarily of the ep3c5e144i7 sopc chip and the epcs4 configuration chip. as for data transmission, four pairs of low-voltage differential signaling interface cables and two ms10232nl network-isolation transformers are used. the r2 and t1 are used as a pair of transceivers, while the r4 and t3 are used as another pair of transceivers. the intermediate taps of (r2, t3) and (t1, r4) are extracted, and have common-mode voltage, v1 and v2. the power station uses power-over-ethernet mode power supply, such that the common-mode voltage difference between v1 and v2 is 22 v to 48 v, which is related to the position of the data-acquisition station. it passes through the bridge rectifier circuit, r, to identify the positive and negative voltage, and it is then sent to the analog board for dc-dc conversion. the analog board and the master digital transmission board have digital and power interfaces for docking, to provide the information exchange and the power supply. 3.2. digital filtering techniques of the high-resolution analog-to-digital converter the high-resolution digital-filtering technology of the adc is one of the difficulties encountered when designing telemetric seismic-data acquisition stations. the flow chart of the design of the filter implementation is shown in figure 5. it is used mainly to complete the sampling filtering for the 1-bit r-d stream from the zhang et al. 4 figure 3. block diagram of the analog board of the data-acquisition station. figure 4. block diagram of the master data-transmission board. 5 cs5373 modulator, and to improve the resolution while reducing the sampling rate. geophone signals after cs5373 modulation have an output of 512 kbps bit r-d stream. as the input and output of the cic filtering use two's-complement, conversion modules are used to convert this into binary two's complement (0 is converted to 11, 1 is converted to 01). the binary two's complement is then passed through the fifth-order cic filter (as the decimation rate of the cic is 128, the sampling rate of the cic output is 4 khz), the cic compensation filter (as the decimation rate of this cic is 2, the sampling rate is 2 khz), the half-band fir filter (the decimation rate of the fir is 2) and the third-order iir high-pass filter. finally, 24-bit output data with a sampling rate of 1 khz are sent to the first-in, first-out buffer of the sopc. (1) (2) the transfer function of the fifth-order cic filter design is shown in eq. (1). it can be seen from equation (1) that it is a five-cascade cic decimation filter, and that the single-stage cic filter might be a concatenation of the integrator and a comb-like device. the single-stage filter impulse response is shown in equation (2), which demonstrates that the single-stage cic filter can be achieved using a memory unit and an adder, without a multiplier. this provides a significant saving of logical resources within the sopc (fpga). as the fifth-order cic filter has a certain degree of attenuation within its pass band, it needs a cic compensation filter for pass-band compensation. after passing through the cic decimation filter and the compensation filter, the data still need double downsampling, which can be realized through a half-band fir filter. the half-band fir is a symmetric fir, with an intermediate design factor of 0.5, and the coefficients of the other odd terms of 0. owing to the symmetry of the coefficients, the actual nonzero coefficients are only a quarter of the total filtering coefficients, which greatly decreases the logical resources and power consumption within the sopc (fpga). to filter out dc and low-frequency signals, at the final level of the high-resolution adc of the data-acquisition station, a 0.3% fs, thirdorder high-pass iir filter was designed. 3.3. the dedicated data-transmission protocol of the data-acquisition station to achieve reliable, high-speed data transmission for the data-acquisition station, a dedicated data-transmission protocol based on the characteristics of the seismic-data acquisition has to be designed. this mainly involves two problems: encoding of the data transmission between the data-acquisition stations, and design of the data-frame protocol. manchester coding is used for data-transmission encoding, where a synchronization clock can be extracted from the signal to reduce the peripheral circuit design and power consumption. in addition, glitches that occur easily in data edges when using conventional encoding can be avoided. the fpga coding module converts 8-bit parallel data into 16.384 mbps serial data through the manchester coding. the decoding program uses the oversampling approach, with a sampling frequency 8-times that of the data transmission, to detect whether data transitions occur within the effective transition time, and then to complete data sampling, decoding, data shift, synchronization code determination, and byte synchronization. after manchester decoding, serial binary data can be transmitted. however, to effectively transmit data, the problems of frame synchronization and byte synchronization must also be solved. at the receiving end, upon receipt of the preamble code 01011111, one frame of data reception begins. the first part of this frame is the header data, which contains the data length of this frame. the receiving end then receives the data in accordance with the data length defined in the header, and then restores the data into parallel data in units of bytes. in the data-frame protocol design, the characteristics of seismic-data acquisition are combined, and the data frame is divided into 51 units; as there are 816 bytes, each unit contains 16 bytes (as shown in table 1), within which there are the unit header, command, status, and cyclic redundancy check code as one byte each, whereby the data comprise 12 bytes. the first unit is the header segment, where the header data includes the data length of the frame, and the other 50 units are data units used seismic data acquisition based on sopc figure 5. acquisition of high-definition ad transmission digital filter by the data-acquisition station. hb fir, half-band fir filter; cf, compensation filter; hp iir, high-pass iir filter. 127 01127 n # or n # 2 , , 0 h n n 1 0 =^ h ' h z z z 1 1 1 128 5 = -^ h ` j for communication between the power stations or between a power station and a data-acquisition station. the relay synchronous acquisition and asynchronous communication of the data-acquisition station are shown in figure 6. the seismic-data acquisition station has its own data transmission characteristics. it uses synchronous acquisition for the main power station to send commands to the data-acquisition station, for the secondary power stations to send commands, and for the data and status information of the data-acquisition station to be uploaded to the secondary power station; however, it uses asynchronous transmission for sending data and status information from the secondary power station to the main power station. in synchronous acquisition, the main power station sends out ready-made null-data frames with no data acquisition, and each data-acquisition station performs the appropriate action according to the command in the data frame, or writes four 24-bit sample data into the corresponding cell during the acquisition period, sets the unit status word to 1, sets the command byte to 0x0f, and finally re-calculates and writes the unit cyclic redundancy check code. in the asynchronous transmission direction, no change is needed for the data frame, and it only re-encodes the data received and sends them out; in this direction, the acquisition station is only a relay between the power stations, and the data frames can pass through quickly. 3.4. high-precision synchronous data acquisition synchronous data acquisition implies that all of the data-acquisition stations will collect data from each detector at the same time, and will transmit the collected data completely and systematically to the cable acquisition unit (power station), and that the synchronization accuracy is one of the main performance indicators of the seismograph. the main and secondary power stations send out synchronous data-acquisition commands to control the collection station to achieve high-precision synchronous data acquisition. when the collection station receives data from the main power station by synchronous data acquisition, it decodes the data frame. if it is a synchronous data-acquisition command frame, time is counted while it is transmitting the data frame to the next data-acquisition station, until the data-acquisition station receives the synchronous acquisition command frame sent from the power station in asynchronous communication mode. if at the nth collection station, the time counted is tn, then: (3) (4) (5) (6) where the time used in the synchronous data-acquisition command frame in the synchronous data-acquisition mode is sn; the time used in the asynchronous transmission mode is an; the time used for re-transmission from the power station is ts; and the time difference between the synchronous data-acquisition and asynchronous transmission is pn. in the process of synchronous acquisition and asynchronous transmission, the transmission times consumed in the cable are almost identical; therefore, pn can be regarded as the processing time difference between the different transmission modes of the data-acquisition station. if for the ith synchronous data-acquisition station, the processing time is ti, and for asynchronous transmission, the processing time is ti', when there are m data-acquisition stations, (7) (8) where ti, ti' and ts can all be obtained through actual measurements, and a list can be established for each data-acquisition station. therefore, in the future, when the acquisition command frame is received, sn can be delayed to achieve synchronous data acquisition. previously, all of the acquisition times of the data-acquisition station were the moment when they received the synchronous acquisition command frame from the power station. for hardware circuit testing, two power stations with 15 data-acquisition stations were used, and the distances between the data-acquisition stations were 25 m to 55 m. the synchronization accuracy between the data-acquisition stations was always better than 200 ns. zhang et al. 6 figure 6. schematic diagram of synchronous data-acquisition and asynchronous communication of the data-acquisition station. s t t t t2 1 n i i i n m n s 1 = + = + l^c h m/ p t tn i i i n m 1 = = + l^ h/ s p t t2 1 n n n s= + -^ h p t s t2n n n s+ = + p s an n n= t a s tn n n s= + + unit header command status data crc 1 byte 1 byte 1 byte 12 bytes 1 byte table 1. cell format of data frames in an acquisition station. crc, cyclic redundancy check code. 7 4. collection station performance indicators following tests and analyses in the laboratory and the field, the data-acquisition station developed has the following performance indicators: (1) full-scale input: better than 1.6 v rms at 0 db and 25 mv rms at 36 db; (2) acquisition station power consumption: 260 mw (3) acquisition station interval: up to 55 m; (4) high-pass filtering: 3 hz; (5) low-pass filtering: 0.8 fn (digital filter); (6) sampling rate: 1 khz; (7) word length: 24 bits; (8) power supply: dc 18-72 v, long-distance supply with signal transmission cable; (9) dynamic range: 120 db at 0.4 khz bw (1 khz sampling rate); (10) self-harmonic distortion: -105 db; (11) cmrr: > 102 db; (12) stop-band attenuation > 120 db (above the nyquist frequency); (13) noise (3-200 hz): 450 nv rms at 0 db; (14) data-transmission speed: 16 mbps; (15) synchronization accuracy: better than 200 ns; (16) operating temperature: −20 °c to +70 °c. 5. conclusions and discussion in the present study, a new type of digital seismicdata acquisition station was designed, based on the sopc technique. through the research and development program, the following technical aspects were explored: (1) acquisition-station-dedicated data-transmission protocol. the acquisition station data-frame format was defined. synchronous data-acquisition, digital transmission, and asynchronous data transmission functions were designed according to the actual needs of seismic-data acquisition, such that low-power and 16 mbps high-speed data-transmission on a 55-m cable can be achieved. (2) high-resolution adc digital filtering technology. according to the features of r-d bit stream from the cs5373 modulator output, subsequent cic, fir and iir hardware filters were designed. (3) high-precision synchronous data acquisition. asynchronous data-acquisition problems due to digital transmission delay of the data-acquisition stations were calibrated. the data-acquisition station synchronization accuracy was better than 200 ns. (4) the high integration level of the hardware circuit. in addition to a small proportion of analog circuits and an adc circuit, all of the digital integrated circuits and the core technologies of the digital seismic-data acquisition stations were integrated into a single-chip sopc, to facilitate the subsequent tape-out. the main advantages of our project are reflected in the above characteristics (2) to (4). the sopc used in this study was a 65-nm cycloneiii series chip. if the digital circuits were rapidly transplanted into the current 28-nm sopc, the power consumption would be further reduced. meanwhile, to produce a highly integrated single chip for the analog-to-digital circuit, a future research area should be an in-depth study of high-precision r-d modulators, and on how to integrate these with the current core technology for the tape-out. thus, while single-chip integration has been achieved, the performance of the data-acquisition station can still be further enhanced while significantly reducing power consumption. acknowledgements. this study is supported by the national natural science foundation of china (no. 41204135), the national “863” program of china (no. 2012aa061102), and the fundamental research funds for the central universities of china (no. 2011yyl108). references astarloa, a., u. bidarte, j. lazaro, a. zuloaga and j. arias (2005). multiprocessor sopc-core for fat volume computation, microproc. microsyst., 29, 421-434. huang, x.l., and j.s. yu (1994). numerical analysis for the characteristics of sn338 digital seismic instrument, chin. j. geophys., 37, 597-602. mazza, s., a. basili, a. bono, v. lauciani, a.g. mandiello, c. marcocci, f. mele, s. pintore, m. quintiliani, l. scognamiglio and g. selvaggi (2012). aida–seismic data acquisition, processing, storage and distribution at the national earthquake center, ingv, annals of geophysics, 55 (4), 541-548. morandi, s., and e. ceragioli (2002). integrated interpretation of seismic and resistivity images across the «val d’agri» graben (italy), annals of geophysics, 45 (2), 259-271. qi, y.y. 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(2010). key problem research on data transmission of large-scale seismic acquisition and recording system, ph.d. thesis, tsinghua university, beijing, china. seismic data acquisition based on sopc wood, w.t., and j.f. gettrust (2002). new developments in deep-towed seismic acquisition, oceans conference record. ieee, 1139-1142. wu, z.q., s.g. wu, s.y. tong, h.s. liu and y.b. zhang (2011). a study on seismic acquisition basic on marine carbonate hydrocarbon exploration in the southern yellow sea, chin. j. geophys., 54, 1061-1070. xie, m.p. (2009). key technologies of large-scale seismic survey system design, phd thesis, hefei, university of science and technology of china. zhang, l.x. (2007). study on data transmission techniques based on relay ethernet in seismic exploration using vibroseis, phd thesis, changchun, jilin university. zhang, q.s., m. deng, j.l. cui and q. wang (2012). research and development of one novel distributed digital seismic acquisition station, adv. inform. sci. service sci., 4, 184-190. *corresponding author: deng ming, china university of geosciences, beijing, china; email: dengming@cugb.edu.cn. © 2013 by the istituto nazionale di geofisica e vulcanologia. all rights reserved. zhang et al. 8 << /ascii85encodepages false /allowtransparency false /autopositionepsfiles false /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /warning /compatibilitylevel 1.3 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false 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>> >> setdistillerparams << /hwresolution [2400 2400] /pagesize [595.000 842.000] >> setpagedevice